feasibility study for developing an energy park at hartsfield …€¦ · ·...

Feasibility Study for

Developing an Energy Park at

Hartsfield Jackson Atlanta International Airport

Final Report

Engineers for a Sustainable World – Georgia Institute of Technology

May 2012

2

Preface The city of Atlanta is striving to be one of the nation’s top tier sustainable cities.

Implementation of sustainable initiatives at Hartsfield Jackson Atlanta International Airport

(ATL) is essential to meet these goals. ATL seeks to fulfill the goals and objectives outlined in

the Sustainable Management Plan and the City of Atlanta’s Zero Waste Zone initiative.

As part of these initiatives, ATL has proposed the creation of a sustainable ‘Energy Park’

that would divert grease, compostables, yard trimmings, and FW from ATL and the Southern

Crescent community from landfills and turn them into energy and useful products. Determining

the best use of the products and the energy to supplement the Airport’s energy needs is the

desired end goal of this proposal. The designated location for the Energy Park is a 39 acre site

near the airport. The first stage of the design process is to evaluate the monetary, environmental,

and social costs and benefits of building the Energy Park.

As a first step towards the development of the proposed Energy Park, ATL has requested the

Engineers for a Sustainable World (ESW), a nonprofit student organization at the Georgia

Institute of Technology, to evaluate the costs and benefits of multiple technology options for the

Energy Park. Components of the Energy Park that are considered in this report include

anaerobic digestion (AD), enclosed composting, a combined heat and power plant, biofuel and

food crop production in greenhouses, and biodiesel generation. Also included is an analysis of

outsourcing some of these processes. An energy balance analysis has been performed for each

process to show the net energy gain of the proposed system.

We hope that this report sheds some light on different aspects of such a complex system and

helps the decision makers at ATL and the City of Atlanta in their sustainable initiatives and

strategic plans.

Soheil Shayegh

ESW-GT Vice President and Project Coordinator School of Industrial & Systems Engineering, Georgia Institute of Technology, Atlanta

3

Acknowledgements This research project would not have been possible without the support of many people.

ESW wishes to express its gratitude to its academic supervisor, Dr. Wayne Whiteman who was

abundantly helpful and offered invaluable assistance, support, and guidance. Deepest gratitude

is also due to Dr. Valerie Thomas, Dr. Steven Van Ginkel, and Dr. Spyros Pavlostathis from

Georgia Institute of Technology; without their knowledge and assistance, this study would not

have been successful.

Our thanks and appreciation also goes to Mr. Michael Cheyne, Director of Asset

Management and Sustainability, who trusted ESW-GT with such a significant project, and Ms.

Sharon Douglas for providing us with data and information from the airport. We also thank Ms.

Jean Pullen from the City of Atlanta who willingly introduced us to this project and the airport.

4 Table of Contents

Table of Contents Waste Management at ATL ............................................................................................. 9 1.

Introduction ............................................................................................................. 10 1.1.

Current Waste Statistics at ATL ............................................................................. 10 1.2.

Waste Collection Methods ...................................................................................... 11 1.3.

Disposal Procedures and Costs ............................................................................... 11 1.4.

Challenges associated with onsite waste management ........................................... 12 1.5.

Effective solutions for waste management at other airports ................................... 13 1.6.

Chicago Airports ............................................................................................................ 13

Los Angeles International Airport ................................................................................. 13

Oakland International Airport ........................................................................................ 13

London Gatwick Airport ................................................................................................ 14

Recommendations ................................................................................................... 14 1.7.

Training programs ......................................................................................................... 14

Self-Sorting Systems ..................................................................................................... 14

Waste Separation with Compostable Packaging ........................................................... 15

References ............................................................................................................... 16 1.8.

Anaerobic Digestion ....................................................................................................... 17 2.

Introduction ............................................................................................................. 18 2.1.

Anaerobic Digestion Parameters............................................................................. 19 2.2.

Maintaining anaerobic conditions .................................................................................. 19

pH value ......................................................................................................................... 19

Composition of the FW ................................................................................................. 19

Loading and CH4 production rates ................................................................................. 19

Solids retention time ...................................................................................................... 19

Operating Temperature .................................................................................................. 20

Classification of AD systems .................................................................................. 20 2.3.

Number of reactors used in series: ................................................................................. 21

Method of introducing the feed into the reactor: ........................................................... 21

Plug Flow Reactor (PFR)............................................................................................... 21

Single Stage ................................................................................................................... 22

5 Table of Contents

Double stage .................................................................................................................. 22

TPASBR and MTSASBR .............................................................................................. 23

CSBR ............................................................................................................................. 24

SEBAC .......................................................................................................................... 24

CSTR ............................................................................................................................. 24

DRANCO ...................................................................................................................... 25

UASB ............................................................................................................................. 25

Sizing and Methane Output .................................................................................... 26 2.4.

Conclusion .............................................................................................................. 26 2.5.

References ............................................................................................................... 28 2.6.

Combined Heat & Power ............................................................................................... 31 3.

Introduction ............................................................................................................. 32 3.1.

Reciprocating Engine CHP Systems ....................................................................... 33 3.2.

Gas Turbine CHP Systems...................................................................................... 35 3.3.

Cost Analysis .......................................................................................................... 36 3.4.

Stage 1: Qualifications ................................................................................................... 36

Stage 2: Feasibility Analysis at a High Level ................................................................ 36

Stage 3: Development .................................................................................................... 37

Stage 4: Procurement ..................................................................................................... 37

Stage 5: Operations and Maintenance ........................................................................... 37

Installation and O&M Costs ................................................................................... 37 3.5.

Appendix ................................................................................................................. 39 3.6.

References ............................................................................................................... 42 3.7.

Composting Systems ...................................................................................................... 43 4.

Introduction ............................................................................................................. 44 4.1.

Composting for ATL............................................................................................... 45 4.2.

Preprocessing ................................................................................................................. 45

Processing ...................................................................................................................... 48

Static Aerated Pile Composting .............................................................................. 48 4.3.

Equipment and Building Input ....................................................................................... 48

Labor Input .................................................................................................................... 48

6 Table of Contents

In-Vessel Composting ............................................................................................. 49 4.4.

Cost Considerations ....................................................................................................... 50

Cost Estimation .............................................................................................................. 50

Post Processing .............................................................................................................. 50

References ............................................................................................................... 51 4.5.

Offsite Composting ........................................................................................................ 52 5.

Introduction ............................................................................................................. 53 5.1.

Offsite Composting Options ................................................................................... 53 5.2.

Only paper waste is sent to the offsite composting facility ........................................... 54

All compostable waste could be redirected to an offsite composting facility ............... 54

Net Present Value (NPV) Analysis ......................................................................... 55 5.3.

Biofuel from Paper Waste .............................................................................................. 56 6.

Introduction ............................................................................................................. 57 6.1.

Hydrolysis, Fermentation, Distillation, Ethanol ..................................................... 57 6.2.

Gasification, Fischer-Tropsch synthesis, Diesel/Gasoline...................................... 59 6.3.

Conclusion .............................................................................................................. 60 6.4.

References ............................................................................................................... 61 6.5.

Greenhouse ..................................................................................................................... 62 7.

Introduction ............................................................................................................. 63 7.1.

Algae ....................................................................................................................... 63 7.2.

Cultivation of Algal Biomass ........................................................................................ 63

Algal Harvesting ............................................................................................................ 63

Cost analysis .................................................................................................................. 64

Conclusion ..................................................................................................................... 64

Camelina ................................................................................................................. 66 7.3.

Yield .............................................................................................................................. 66

Duckweed ............................................................................................................... 67 7.4.

Potential Uses ................................................................................................................ 67

Energy ............................................................................................................................ 67

Challenges ...................................................................................................................... 67

Yield .............................................................................................................................. 68

7 Table of Contents

Hydroponica ............................................................................................................ 69 7.5.

How it works .................................................................................................................. 69

Hydroponics Cost Efficiency ......................................................................................... 70

Density ........................................................................................................................... 70

Crop Harvest per Year ................................................................................................... 70

Temperature ................................................................................................................... 71

Light ............................................................................................................................... 71

Diseases ......................................................................................................................... 72

Other factors .................................................................................................................. 72

Vegetable Analysis ........................................................................................................ 72

Conclusion and Summary .............................................................................................. 76

Aquaponics ............................................................................................................. 78 7.6.

References ............................................................................................................... 81 7.7.

Biodiesel from Cooking Oil Waste ................................................................................ 83 8.

Introduction ............................................................................................................. 84 8.1.

Types of biodiesel ................................................................................................... 84 8.2.

Sources of biodiesel ................................................................................................ 84 8.3.

Applications of biodiesel ........................................................................................ 84 8.4.

Advantages of biodiesel .......................................................................................... 84 8.5.

Biodiesel Production ............................................................................................... 85 8.6.

Batch Process ................................................................................................................. 85

Ultrasonication ............................................................................................................... 85

Cost Comparison of Processes ................................................................................ 87 8.7.

Batch Process ................................................................................................................. 87

Continuous Process ........................................................................................................ 87

Ultrasonication ............................................................................................................... 88

References ............................................................................................................... 89 8.8.

Energy Analysis ............................................................................................................. 90 9.

Introduction ............................................................................................................. 91 9.1.

Waste Characterization ........................................................................................... 91 9.2.

Description of Proposed Energy Park ..................................................................... 92 9.3.

8 Table of Contents

Energy Production Train ......................................................................................... 93 9.4.

Gas Turbine CHP System .............................................................................................. 95

Reciprocating Engine CHP System ............................................................................... 96

Heat and Power Requirements of the Energy Park ................................................. 98 9.5.

Biodiesel Conversion Unit ...................................................................................... 99 9.6.

Waste-to-Energy Comparison ............................................................................... 100 9.7.

Conclusions ........................................................................................................... 100 9.8.

References ............................................................................................................. 101 9.9.

Conclusion & Recommendations ............................................................................. 102 10.

Upstream waste management programs ............................................................ 103 10.1.

Waste Treatment at the Energy Park ................................................................. 103 10.2.

Collective Treatment ................................................................................................... 103

Chain Treatment .......................................................................................................... 104

9 Waste Management at ATL

Waste Management at ATL 1.

Team members:

Inthirai Somasuntharam PhD student, Biomedical Engineering

Anna Mazzolini 2nd

year undergraduate student, Nuclear Engineering

Haley Cole 1st year undergraduate student, Industrial Engineering

Laurynas Bileisis 2nd

year undergraduate student, Mechanical Engineering


Introduction 1.1.As the first step towards proposing a sustainable approach for waste management at the

airport, this chapter deals with analyzing the input waste as well as understanding how waste is

handled at the airport which includes collection, separation, and processing activities. This

chapter extracts and summarizes useful information from reports provided to our organization by

the Airport on waste characterization, current waste management practices, and waste

disposal/recycling costs. It also addresses best sustainable waste management practices and

efforts from airports around the world towards accomplishing similar goals to that of ATL.

Finally, this section includes recommendations to the airport that contributes to sustainable waste

management initiatives. These recommendations closely follow the waste hierarchy of reduce,

reuse, recycle, energy recovery, and disposal.

Current Waste Statistics at ATL 1.2.ATL generates an estimated 25,500 tons of waste per year [1], averaging about 70 tons a

day. The majority of the solid waste generated is municipal solid waste (MSW), which consists

of ordinary garbage, including recyclable materials. Error! Not a valid bookmark self-

reference. shows the categories of MSW in 2005 and their composition. In 2005, food waste

(FW) accounted for 28.4% of the solid waste coming from the airport; recyclables accounted for

41%, with the largest component being paper (22%). Non-recyclables (Plastics, Rubber, Wood,

etc.) accounted for 31%. Although the FW% is significant, a large component of the FW is

melted ice and leftover beverages. It is unclear exactly what percentage of FW is actually water.

Yard trimmings are composted onsite, although they are minimal because of the impervious

surfaces at the airport. The organic fraction of MSW (OFMSW) accounts for about 60% of total

waste.


In 2005, approximately 7% of MSW (majority being paper and cardboard) was recycled

from about 40% recyclables. In 2011, ATL recycled over 1,400 tons of office paper and

cardboard, which accounted for about 95% of the recycled materials last year, with plastic

materials accounting for 4% and aluminum 1%. Similar to 2005, the percent recycled was 6.2%.

In addition, ATL generated around 60,000 gallons of cooking grease in 2005. However,

recent data from 2010 suggests that only 30,000 gallons were recycled from concessions and

picked up by a vendor. From the recycled cooking grease, 22,700 gallons were used for biofuel

production while about 7,300 were determined not suitable for biofuel production.

Currently, waste from the restaurants is combined with waste from the rest of the airport.

Any international waste is handled in Concourse E and is treated separately. Construction and

demolition waste is also handled independently, usually by a contractor.

Waste Collection Methods 1.3.Waste generated inside the airport along the concourses is collected multiple times per hour.

Bags of waste are collected from each gate's electric compactors. Waste from these electric

compactors is collected in larger carts, called "gooses". When full, airport staff carries the

compressed waste to large trash compactors located outside of each concourse. These are later

emptied and collected by Waste Pro, a company contracted by the airport through Atlanta

Airlines Terminal Corporation (AATC) to sort the waste and manage recyclables.

Waste originating from the aircraft cabins is put in dumpsters and is not compacted. This

waste consists primarily of newspapers, food containers, FW, magazines, and aluminum cans.

Cardboard is collected in different compactors on concourses A, D, and E and is processed

separately and handled by WastePro.

Food Waste 28%

Other Nonrecyclables 31%

Other Recyclables 2%

Cardboard 8%

Paper 22%

Recyclable Plastic 5% Glass

3%

Metal 2%

Recylables 41%

Figure 1. Categories of waste as percent composition of MSW (From table 2.6 [1])


The compactors are emptied on a routine schedule, once per evening, generally between

1:00AM and 5:00AM. In total, there are approximately 10 trash compactors, 5 cardboard

compactors (each ranging from 30-34 cubic feet), and 40 dumpsters (each ranging from 4-8

cubic feet) that are serviced by Waste Pro at varying intervals throughout the week.

Disposal Procedures and Costs 1.4.MSW disposal costs include landfill tipping fees, collection, and hauling costs. When

transporting the waste, hauling costs can be reduced by utilizing transfer stations, which use

large capacity trucks for more efficient transportation to remote landfills. There is a transfer

station located within five miles of the airport which would minimize increased hauling costs

should landfills near the airport reach full capacity.

In 2005, the cost of disposal for the 23,047 tons of waste generated at ATL was $1,072,212,

with an average disposal cost of $46.52 per ton. The recycling costs for the 658 tons of

cardboard and paper was $42,000, with an average recycling cost of $63.83 per ton. Table 1

shows a detailed description of disposal costs. If the FW were removed through anaerobic

digestio, based on the 28% composition of MSW being FW and 23,047 total tons of waste

generated per year, an estimated 6,450 tons of waste reduction could potentially save about

$300,000 to the airport using $46.52/ton as the estimated savings in the cost of disposal (not

considering the capital and running costs for anaerobic digestion).

Table 1: 2005 Annual Solid Waste Disposal and Recycling Costs for ATL (from Table 4.1 [1])

Annual Cost

Solid Waste Collection Locations

Compactors from T/A/B/C/D, Terminal Open Tops $531,516

Front End Dumpsters at Terminal and Concourses $254,823

Delta Cabin Service, Delta Midfield, Gate B2 and Terminal North $285,873

Subtotal $1,072,212

Recycle Collections and Rebates

Recycled Cardboard $57,330

Recycled Paper $1,180

REBATES (Cardboard) ($16,355)

REBATES (Paper) ($154)

Subtotal $42,001

Total $1,114,213

Challenges associated with onsite waste management 1.5.The current options for disposal of solid waste include landfills, transfer stations,

incinerators, composting facilities, and energy and material recovery facilities. The majority of

ATL’s waste goes to landfills due to several reasons: extra costs to recycle as outlined in Error!

Reference source not found. and Error! Reference source not found.; limited space for

separate recycling containers; compaction and contamination of recyclable materials at the

terminals.


Table 2: Estimated Annual Costs for Source Separation Recycling Program (from Table 6-4 [1])

Capital Costs (collection bins, storage dumpsters, annual capital costs) $87,500

Annual Operation and Maintenance Costs (Labor, educational and promotional

materials, equipment maintenance, O&M contingency)

$125,000

Costs for averaged recovery scenarios between 10% and 30% (disposal of recycled

materials and non-recyclable materials)

$1,023,275

Table 3: Estimated Annual Costs for an Onsite Materials Recovery Facility (From table 6.6 [1])

The State of Georgia Department of National Resources has regulatory authority regarding

ATL's waste management practices. By using companies to dispose of waste, ATL avoids this

regulatory burden by deferring it to the contracted company. Any entity involved with

transporting or collecting solid waste must acquire a permit and operate under the requirements

of state regulation.

Effective solutions for waste management at other airports 1.6.With growing numbers of passengers passing through airports, waste produced by airports

has been increasing steadily. For example, waste production at the UK airports is currently

growing at 3% per annum [2]. According to an interview with B. McGuinness conducted by

Pitt, Brown et al. [2], the three main sources of solid waste in airports are office waste, tenant

concessionaires, and aircraft waste. Office waste is largely composed of recyclable paper. In

fact, according to Pitt, Brown et al. [2], the majority of waste produced by airports is recyclable.

In order to save time and resources to sort waste, many airports have adopted waste receptacles

that have three compartments for paper, glass, and tin. This waste is then resorted by a recycling

center before being sent off. The effectiveness of these passenger receptacles in saving time,

however, is directly proportional to the passenger participation.

Chicago Airports

With tenant concessionaires as the second leading source of waste generated, FW is a major

issue for airports. Some airports, such as Chicago O’Hare and Chicago Midway, the 2nd

and 27th

busiest airports in the US respectively, have instituted composting programs to decrease the

amount of waste sent to a landfill. Between these two Chicago airports’ pilot composting

programs, 200 tons of compostable materials ranging from leftover food to cardboard avoid

landfills every year [3].

Los Angeles International Airport

The most effective means of waste management involves an integrated approach, combining

various methods of waste disposal and reduction. In 2001, the 3rd

busiest airport in the US, the

Los Angeles International Airport (LAX), launched a pilot program aimed at using FW for

Capital Costs (Building, site improvements, utility truck, front end loader, fork lift, bag

breaker, conveyer, balers, scale, other)

$2,809,000

Annual Operation and Maintenance Costs (Supervisor, operators, laborers, utilities, fuel,

insurance, equipment maintenance, other)

$1,230,000


electricity generation. The plan incorporated digesting waste to produce methane gas that could

be converted to electricity. Additionally, wastewater from the process would serve as irrigation,

and residual biosolids would be used to fertilize soil [4]. The pilot study found that digesting the

FW and disposing of residual biosolids was economically productive due to the high value of the

gas produced by the digestion. The pilot program can therefore be considered a success, leading

to economic gains for the airport [5].

Oakland International Airport

Since 2004, Oakland International Airport has incorporated a program involving the

composting of pre-consumer FW (coffee grounds, trimmings, etc) generated during the

preparation of food at various concessions throughout the airport. The FW collected is

transported to a local composting facility, and following several months of processing, converted

to a high-nutrient fertilizer. This, in turn, is then sold to local golf courses, wineries, and organic

growers. The City of Oakland also passed an ordinance prohibiting the use of polystyrene

containers in food service. The Oakland Airport is using this as an initiative to develop plans for

the collection of post-consumer waste as compostable material in the future [6].

London Gatwick Airport

London Gatwick Airport, the UK’s 2nd

busiest airport, developed an environmental policy

that involves the periodic review and auditing of a number of action plans relating to the

management and disposal of third party waste. These plans were implemented to help the airport

meet its long term goals of 70% recycling, zero waste diversion to landfills, and reducing waste

production per passenger. As stated in their most recent waste action plan for 2009-2011 [7], in

order to meet such long term goals, the Gatwick airport officials hope to impose measures that

encourage segregating biodegradable waste to divert and treat through anaerobic digestion or

even in-vessel composting. This waste action plan, which is designed to deal with short to

midterm length goals, was created with the idea of gradually fostering the airport towards the

accomplishment of their long-term environmental policy goals. Some of these measures include

simple measures such as development of guidelines on disposal and airport waste management

procedures, implementing and conducing annual reviews to assure sustainable waste

management practices, and deploying terminal bins for customer self-sorting by certain projected

deadlines.

Recommendations 1.7.

Training programs

Tenants within an airport facility can include airlines offices, rental car agencies, restaurants,

newsstands, and gift stores. Beginning a tenant education program can be an essential first step

to reducing the amount of waste and increasing recycling at the airport. Providing tenants with

the necessary materials needed for a successful recycling program and continually delivering

training to employees and managers can be instrumental in fostering a productive recycling

mindset at the airport.


In conjunction, the airport can implement a biannual program aimed at helping tenants to

dispose of some of their more bulky materials safely and effectively. The tenants can use these

events to break apart these bulky materials into disposable wood, metal, and plastics for

recycling. These events would only happen twice a year but would facilitate the idea that the

airport is encouraging more effective means of reducing waste at the airport.

Self-Sorting Systems

The airport has two main options to deal with sorting recyclables. The first is a system that

would promote sorting by the consumers, tenants, and airport staff. In Error! Reference source

not found., the estimated costs of a source separation system are outlined. Because of the large

amounts of recyclable materials coming out of the central passenger terminal, introducing

separate waste containers to encourage self-sorting could save the airport time and money in the

long run. No extra personnel would be required to pick up the sorted waste, as the airport

already employs workers to pick up trash from the terminals.

A second option would be an onsite materials recovery facility. Waste would be taken to

such a facility for sorting by workers. Unlike the source separation discussed above, an onsite

materials recovery facility would require a separate building and various equipment, thus

incurring greater capital costs, as shown in Error! Reference source not found.. In comparing

both options, the source separation system, if carried out properly, would be cost-efficient to the

Airport in the long run.

Waste Separation with Compostable Packaging

An ATL pilot program started in January of 2012 implemented a new process in which the

restaurants in the airport disposed of FW independently of other waste. This program

successfully separates FW from other concourse waste at its origination, allowing separation

from the source of waste rather than post-disposal sorting to extract the food components.

This pilot, if continued and implemented with compostable packaging, allows the back of

the house waste from the airport to be easily separated from other waste, facilitating the fueling

of any potential onsite anaerobic digesters or composting. Compostable packaging would create

a convenient opportunity to dispose of all compostable food separately, without the consumers

needing to separate their remaining food from its packaging. If the restaurants and kiosks were

required to use only compostable packaging, the airport could provide separate FW containers

that would provide an easy opportunity for self-sorted compostable materials to be collected and

sent directly to the anaerobic digesters.


References 1.8.1. Camp Dresser & McKee (CDM). 2006 Waste Characterization Report. Tech. Final ed. Atlanta,

Georgia. Print.

2. Pitt, M., A. Brown, et al. "Waste management at airports." Facilities. 2002. 20 (5/6): 198-207

3. Hilkevitch, Jon. "Green Initiatives Cleared for Takeoff at O'Hare, Midway Airports." Chicago Tribune.

01 Nov. 2011. Web. 15 Mar. 2012. <http://articles.chicagotribune.com/2011-11-01/news/ct-met-green-

airports-1101-20111101_1_alternative-fuels-solar-power-composting-program>.

4. Onorato, Danielle. "Los Angeles Airport Creates Energy from FW." Waste360. 01 July 2001. Web. 30

Mar. 2012. <http://waste360.com/mag/waste_los_angeles_airport>.

5. Hernandez, Gerald, Redd Kenneth, Wendy Wert, Min An, and Tim Haug. "Los Angeles Digesters

Produce Energy from Airport Food Residuals." BioCycle Jan. 2002. Print.

6. Composting at Oakland International Airport. Issue brief no. 17. Airports Council International, July

2007. Web. 15 Mar. 2012.

<http://www.aci.aero/aci/aci/file/ACI_Priorities/Environment/case%20study%20OAK_Food%20comp

osting.pdf>.

7. Waste Management Action Plan. Tech. London Gatwick Airport, 2009. Web.

<http://www.gatwickairport.com/Documents/business_and_community/Misc/Waste_Action_Plan_Fina

l.pdf>.

17 Anaerobic Digestion

Anaerobic Digestion 2.

Team members:

Angela Phung 2nd

year undergraduate student, Environmental Engineering

Jean Mullaney 3rd

year undergraduate student, Environmental Engineering

Gabe Park 1st year undergraduate student, Mechanical Engineering


Introduction 2.1.Anaerobic digestion (AD) is a biological process that occurs in an enclosed reactor without

oxygen [19]. It occurs naturally in anaerobic niches such as marshes, sediments, wetlands, and

the digestive tracts of ruminants and certain species of insects [4]. Generally four main reactions

occur during the entire process of the anaerobic digestion to methane: hydrolysis, acidogenesis,

methanogenesis, and acetogenesis. Although AD can be considered to take place in four stages

as shown below, all reactions occur simultaneously and are interdependent [2]. In the hydrolysis

process, complex organic solids are hydrolyzed by bacteria through secretion of extracellular

enzymes. Carbohydrates, proteins, and fats are converted to simple carbohydrates, amino acids,

and fatty acids. The acid forming process, which includes acidogenesis and acetogenesis,

involves the conversion of soluble carbon formed during hydrolysis into eventually acetic acid

i.e. ‘acetogenesis’ and H2 by facultative and anaerobic bacteria. The pH drops, and other

fermenting bacteria convert the organic acids to acetic acid and H2. The last process is

methanogenesis which is the bacterial conversion of acetic acid and H2 into CH4 and CO2 by

strict anaerobes called methanogens [19].. Figure 2 shows a diagram of the stages of anaerobic

digestion.

Figure 2. Four main stages of anaerobic digestion from [21]

Two products come out of anaerobic digestion: biogas and a stable sludge. The biogas

normally consists of 50-70% CH4 and 30-50% CO2 [26] which can be combusted to provide heat

for the digesters or used for generating electricity. In the sludge, only small quantities of organic

matter and cellular protoplasm remain [19] and can be used as soil fertilizer [2].

AD has been conducted for centuries all over the world, but in order to have a consistent and

reliable AD process, it is important that the FW loaded into the AD be constant in both quality

and quantity. Plastics or other non-biodegradables should not be fed to the AD. Some fat, oil, or

grease (FOG) is allowed, but usual high rate AD systems do not treat FOG. If the feedstock

contains non-biodegradable materials,, it can cause technical difficulties with the equipment and

cause poor compost quality.


Anaerobic Digestion Parameters 2.2.There are many important parameters that affect the anaerobic digestion of FWs.

Maintaining anaerobic conditions

One of the parameters includes maintaining anaerobic conditions so that methanogenesis can

occur. Anaerobic conditions are easily maintained since bacteria readily consume all the

oxygen. Reactors must be well sealed which allows the biogas to be collected for energy

conversion and eliminates odor and methane emissions during the AD process [4].

pH value

The pH value of the reacting material is a pivotal factor in the AD of FW. The importance

of the pH is due to the fact that methanogenic bacteria are very sensitive to low pH [2].

Alkalinity or pH buffering capacity is important to resist changes in pH. High carbohydrate

containing wastes will generate a low pH while protein containing wastes will raise the pH as the

ammonia released from protein degradation will help buffer against pH changes.

Composition of the FW

It is important to know if the composition of FW is variable throughout the year. As stated

above, a high carbohydrate containing waste may need added alkalinity to maintain a constant

pH. If the moisture content is high and pure FW is used, traditional AD technologies can be

used. There are new technologies that digest MSW including all biodegradable material such as

paper. This may be the best option if no sorting is desired or if the paper is contaminated with

FW. Reactors that digest FW and MSW are discussed below.

Loading and CH4 production rates

Organic loading rate (OLR) is a measure of the biological conversion capacity of the AD

system and is stoichiometrically related to the CH4 production rate. Typical AD can treat 10-20

kg of COD/m3 of reactor volume, where COD is the chemical oxygen demand - a measure of

how much oxygen is needed to combust or release the energy in the waste. Theoretically, 1 kg

COD yields 0.35 m3 of CH4. Operating an AD at an OLR higher than designed can result in a

low pH as acidogenic bacteria convert the FW to organic acids and eventually a ‘stuck’ digester

which means the pH is too low to support methanogenic growth (Figure 3).

Figure 3. Effect of overloading an anaerobic digester [2]

Solids retention time

The solids retention time (SRT) in AD refers to the time the feedstock stays in the digester.

In a completely mixed system, the SRT is inversely proportional to the OLR. It is determined by

the average time needed for decomposition of the organic material. The longer the substrate is

kept under proper reaction conditions, the more complete its degradation will be. However, the


rate of the reaction decreases with longer residence times, indicating that there is an optimal

retention time that will achieve the benefits of digestion in a cost effective way.

DeN

Operating Temperature

Operating temperature is the most important factor determining the performance of AD

because it affects the growth rate of the microbial consortia. Despite the fact that they can

survive a wide range of temperatures, bacteria have two optimum ranges of temperature, defined

as mesophilic and thermophilic temperature optimum. Mesophilic digesters have an operating

temperature range of 25-40°C and thermophilic digesters have an operating temperature range of

50-65°C. The rate of an AD process can be characterized by gas production, bacteria growth,

and substrate degradation rates (Figure 4). Mesophilic reactors don’t need to be heated as much

while thermophilic reactors need to be heated, yet can have higher OLRs (i.e. smaller reactors)

and produce pathogen-free sludge.

Figure 4. Rate of AD Process vs. Temperature from [2]

Classification of AD systems 2.3.There are many different technologies on the market that are used for AD of the organic

fraction of FW and MSW. These systems differ based on the design of the reactor, operating

parameters, and the nature of the feedstock. Digestion of wastes containing low-solids content

(<15% total solids, (TS)) is called “wet digestion” while “dry digestion” is for wastes containing

25-30% TS.

When the feedstock used is the organic fraction of MSW (OFMSW), both systems apply and

have been proven successful. In both cases, water may need to be added in order to lower the

content of TS. The “dry digestion” system generally requires smaller and therefore less costly

digesters due to the lower water content, but feedstock pretreatment and mixing costs are higher.


Number of reactors used in series:

Single stage digester: All reactions take place in one reactor and environmental

conditions are maintained at levels that suit all types of bacteria. Therefore, operating

conditions for all stages are not optimal.

Multi-stage digesters physically separate the hydrolysis and acidogenesis reactions in

different reactor vessels. These systems can be more efficient since conditions can be

optimized for each reaction.

Method of introducing the feed into the reactor:

Continuous flow reactors have feed and discharge flows in a continuous or semi-

continuous manner. This is the most common form. These reactors can handle high

OLRs since conditions for the microbes are relatively constant. The flow coming out of

the reactor (the effluent) has the same composition as the reactor contents.

Batch reactors are loaded and allowed to react for a certain period (usually two weeks)

before settling the solids, decanting, and loading again with fresh waste. These reactors

are larger and have longer SRTs since conditions are not constant for the microbes.

However, effluent quality can be very good due to the long SRTs and allowing the solids

to settle before decanting the effluent [2].

Types of anaerobic digesters

Plug Flow Reactor (PFR)

A plug-flow reactor is arranged as one long reactor or many short reactors in a tube bank

with no radial variation in reaction rate (concentration), only concentration changes down the

length of the reactor [17]. An ideal plug-flow reactor has perfect mixing in the radial direction

and no mixing in the axial direction [8]. It is generally less technically complex than other

reactors. However, the wastes must be mixed and inoculated with the digested product [24].

Plug-flow reactors require high solids content. These systems can handle things such as rocks

that must be screened out with low solids systems. High solids processes can also handle higher

organic loading rates and generally uses less water when compared to low solids processes.

DRANCO, Kompogas, and Valorga are all single-stage thermophilic reactors (Valorga

sometimes uses mesophilic) with a retention time of 14-21 days [25]. These reactors have an

advantage in that they can handle all types of biodegradable MSW including paper. They also

don’t have any moving parts inside which allows for low maintenance costs.

The DRANCO reactor is a vertical plug-flow reactor. Matter to be digested is added to

the top and sludge is extracted from the bottom. Gravity aids this process [25].

The Kompogas process is a horizontal plug-flow reactor that best handles wastes with a

solids content of 23%; any lower, heavier particles may accumulate on the bottom; any

higher, the flow of materials may be affected [25].

The Valorga process utilizes a vertical cylindrical reactor. Undigested material enters

through an entry point on the bottom and moves upward and around a baffle until it


discharges through an exit point on the top. Biogas is injected at the bottom of the

reactor to provide mixing. These injectors have added maintenance associated with them

[25]. One mesophilic Valorga Plant in the Netherlands takes in food and garden waste

and produces CH4 varying from 210 to 290 m3 per Mg of volatile solids. This variation is

most likely due to differences in feedstock [9].

A PFR operating off manure and FW (egg breakage waste, fish breading waste, crab meat

trimmings, and ravioli sauce waste) had a COD reduction of 68%. The FW substrate mixture

and the digester influent had CODs of 223 ± 24 g/L and 84±15 g/L, respectively. CH4

production was 0.37±0.05 m3 CH4/kg VS [12].

Single Stage

Single stage reactors are more conventional in that they use one reactor for both the

acidogenic phase and the methanogenic phase. They can be divided into classifications of low

solids and high solids [25].

Low solids reactors are very simple. Usage is well-established for use in the wastewater

industry. A common reactor that is single stage is the completely stirred tank reactor

(CSTR). For the solids percentage to be maintained, water must be added. This results

in higher reactor volumes and more energy required for heating[25].

High solids reactors have higher biogas yield when compared to low solids processes.

They require less water than low solids processes. An example of a high solids single

stage reactor is the plug-flow reactor. High solids reactors can generally handle higher

organic loading rates when compared to low solids processes [25].

Experiments conducted using single stage methods with feedstocks of general FW, potato

waste in a CSTR under thermophilic conditions, pineapple peel, and barley waste with combined

kitchen waste had a methane productivity of 0.35–0.44, 0.65–0.85, 0.67, and 0.36 L/g VS,

respectively. [11].

Double stage

Double stage processes separate acidogenesis and methanogenesis via physical barriers.

Generally, two reactors are used. The first reactor is for liquefaction or hydrolysis and

acidogenesis. The reaction rate in this tank is limited by the rate of hydrolysis of large polymers

of carbohydrates, proteins, etc. The second tank is used for methanogenesis and is limited by the

rate of microbial growth. This two reactor system allows for the different tanks to me optimized

for different processes.

Double stage processes may also be classified by high-solids and low-solids. There are

many similarities between low-solids processes that are single stage and double stage. The high-

solids processes of single stage and double stage also share similarities [25]. Double stage

experiments using cheese whey, fruit and vegetable waste, and general FW had methane yields

of 0.55 m3 /kg CODremoved, 0.320 m

3/kg COD, 0.21 m

3/kg VSadded, respectively. Double stage


processes are more suitable for substrates with low pH, low buffering capacity, and high

concentrations of ammonia [11]. One study attempted to compare energy yields of single and

double stage reactors. Their findings showed the single stage reactor yielded slightly more

biogas (13.4%) than the double stage reactor. However, they state that optimization of the first

stage of the double stage process could improve biogas yield [18].

TPASBR and MTSASBR

Huge advances in thermophilic anaerobic digestion in the last decade have made these

systems the most efficient in terms of methane output (Figure 5). The food composition in

typical temperature-phased anaerobic sequencing batch reactors (TPASBR) is mostly sewage

sludge and FWs. Thus it is seen as a promising approach to the digestion of OFMSW. Due to

the organic composition of OFMSW, it will have high water content. Lab scale testing has been

done at the Slovak University of Technology in Bratislava that tests different types of food for

biogas yield and biodegradability. Results indicate that the thermophilic anaerobic process is

suitable for treating kitchen wastes [16]. A comparison study of co-digestion of FWs and

sewage sludge assesses the practicality of using TPASBR, using mesophilic two-stage anaerobic

sequencing batch reactors (MTSASBR) as a control. The most significant difference between

the two is the temperature at which they operate. Thermophilic anaerobic digesters typically

operate within the ranges of 50-65°C while mesophilic anaerobic digesters operate within the

ranges of 25-40°C [14]. The temperature of the thermophilic anaerobic digester in this particular

experiment is controlled using a water bath circulator, and a built in water jacket. The co-

substrate TPASBR system was able to obtain 44% VS removal. An experiment revealed that the

optimum ratio of sewage sludge to FW is 60:40 [14]. The result of this experiment verifies that

TPASBR enhances the treatment of OFMSW, at the same time, producing more biogas.

The advantages of using a thermophilic digester are shortened retention times and higher

loading rates resulting in higher methane production. In fact, a finding from a study of anaerobic

digestion of olive mill wastewater shows that the yield coefficient of methane is 28% greater in

thermophilic processes than in mesophilic processes [3]. The disadvantages are it is sensitive to

toxins and environmental changes. While the TPASBR seem to be the way to go, there are

benefits that come with the MTSASBR. They are more attractive to commercial plants because

they require much less investment costs, and they are also easier to operate and maintain.

Another advantage of MTSASBR is that it is much more stable because they tolerate greater

changes in the environment. As a result of the temperature of MTSASBR being lower than

TPASBR, the process has a longer retention time and lower biogas production [2].


Figure 5. Anaerobic Digester by Anamix from [1]

CSBR

A study is done at the Ohio State University, which use the continuously stirred batch

reactor (CSBR) to assess the feasibility of methane yield from a variety of wastes such as FOG

and FPW. It mixes sewage sludge with processing wastes in order to achieve the highest

efficiency. Four liter reactors were used with a magnetic stirrer to continuously mix the

substrate. Reactor temperatures were 35and 52°C. The higher temperature produced a higher

methane yield [26]. Fat, oil, and grease are suitable feedstock for CSBR due to their high levels

of carbon, resulting in higher methane yield. In a batch reactor, the feed is loaded and left to

react for around two weeks. Batch reactors must be bigger than other types of reactors due to its

high retention time [26]. Of the FWs tested in this experiment, expired creamer had the highest

COD content of 732 g/L. FWs generally have a higher COD content than sludge or manure. It

is shown from this experiment that co-digestion of food-wastes with dairy manure can result in

an increase of methane yield of up to 5.2 times compared to pure manure [26].

SEBAC

The sequential batch anaerobic composting (SEBAC) process progresses through three

stages for conversion of MSW to methane and requires at least two reactors, representing start-

up and completion stages. In Stage 1 the putrescible fraction of MSW (mainly paper, yard

waste, and FW) is coarsely shredded (to about 10 cm), placed into the reactor, and moistened and

inoculated by recycling leachate from Stage 3. Leachate recycle also removes inhibitory

organics produced in Stage 1 by depolymerization and fermentative reactions. In Stage 2 the

fermentation is active and balanced and thus operated in the batch mode. Stage 3 allows for

completed conversion of degradable particulates and also serves as an inoculum for start-up of

Stage 1 and conversion of acids pumped out of Stage 1 via leachate recycle [6].

CSTR

This type of a digester is characterized by the provision for mixing the digester contents

either continuously or periodically. Mixing can be mechanical, hydraulic or pneumatic, with the


latter being effected by compression and sparging of biogas. Gas mixing is preferred in large

digesters. The treatment efficiency of a CSTR is further enhanced by heating the digester

content with a proper temperature control system. The CSTRs are operated semicontiniously or

continuously, that is the wastewater is fed either periodically (semicontinuously) or continuously

to the digester. By insulating the digester and mixing the contents, it is possible to install

digesters of very large capacities, for example up to 5000 m3. CSTRs based on anaerobic

process have HRTs in the range of 15-20 d. They are particularly suited to animal wastes such

as piggery waste, dairy cattle manure, and silage waste [21].

DRANCO

The DRANCO technology consists of a thermophilic, one-phase anaerobic fermentation

step, which is followed by a short aerobic maturation phase. If necessary, mesophilic operation

can be applied for specific waste streams. During the anaerobic digestion phase, the organic

material is partially converted into biogas in an enclosed reactor for about 20 days. The digested

sludge is extracted from the digester and stabilized aerobically. The final product is a

hygienically safe and stabilized product. Combining anaerobic digestion and aerobic composting

produces energy, shortens the aerobic composting step and prevents wastewater production.

Wastewater can also be prevented by drying the sludge with the lost heat of the gas engines or by

adding shredded green waste to the sludge in the composting step. The advantages of the

DRANCO process include dry anaerobic digestion, vertical digester with conical outlet, one-

phase digestion, and mixingby gas injection in the digester.

Some characteristics of the process include a digester loading rate of 10 to 20 kg COD/m³

reactor per day; a temperature range of 48 to 57°C, if thermophilic, or 35 to 40°C, if mesophilic;

a retention time of 15 to 30 days; a biogas production of 100 to 200 m³ of biogas per ton of

waste; and electricity production of 220 to 440 kWh per ton of waste. This technology is suited

to OFMSW obtained through mechanical separation; biowaste and other source-separated

organic waste streams; dewatered sewage sludge; and other organic waste streams, including

non-recyclable paper, market waste, FW, and industrial waste [27].

UASB

Up-flow anaerobic sludge blanket (UASB) reactors are usually used in wastewater

treatment. In addition to usage in wastewater treatment they are used in the beverage, brewery,

food and tannery industries [13]. In UASB reactors, the active microbial biomass form very

dense granules, which are highly settleable. As a result, very high concentration of active

biomass is achievable per unit working volume of the digester. UASBs are operable at high

COD loading rates with adequate treatment. The feed enters through the bottom of the reactor

and flows upward. After passing through the active granular sludge, the treated wastewater

passes through a gas-liquid-solid separation device. This device separates solids (granules) from

the liquid effluent and also separates gas bubbles form the effluent. Only the liquid effluent

flows out of the reactor while the solid sludge settles back in the reactor and the gas is collected

in the gas collector [21]. A diagram of the reactor is shown in Figure 6.


Figure 6. Up-flow anaerobic sludge blanket (UASB) reactor [21]

Sizing and Methane Output 2.4.From the waste characterization report, the total MSW is 25,000 tons/yr. The FW% is

28.4% which yields 19 tons/day. At a solids content of 27.1%, this yields 5.2 dry tons of

FW/day. Assuming 1.3 g COD/g FW and some conversions, about 6.2 million grams of FW as

COD is produced per day. Dividing by an estimated volumetric COD removal rate of a reactor

(12 g COD/L-day) yields a reactor volume 531 m3. Based on the theoretical ration of 0.35 L

CH4/g COD, the methane output would be 2.2 million L CH4/day or about 112 m3/ton of FW.

Conclusion 2.5.From the reactors listed above, we are not able to precisely determine the cost of a digester.

The type, size, and operating and maintenance costs of a digester will depend on the quantity of

waste and whether pure FW or MSW is to be treated. For pure FW, we recommend the CSTR

due to its simplicity. The only pre-treatment that would be required before sending the waste to

a CSTR reactor is liquefaction of the FW.

We were able to get some preliminary cost data by comparing FW digestion at the airport

with digestion of animal waste where there is a lot of data. We also contacted a few suppliers to

get quotes. Based on published reactor cost data for pig and cow manure digestion, we scaled up

these numbers to airport’s FW output. We estimate a complete CSTR system (pretreatment, gas

handling, etc.) to treat the airport’s waste to be < $2.5 million and this number has been

confirmed by BIOferm Energy Systems. We also received a quote from Southeastern Tank.

For the tank, foundation, and cover, the estimated cost is $220,000 for a tank 34’ in diameter and

24’ in height. Additional costs to this tank quote would be engineering fees, pretreatment,

liquefaction, mixing, and gas handling which would seem to be minor compared to the cost of

the tank, so we think the system cost would be much less than $2.5 million. A $2.5 million will

translate in $380 per tons of FW.


For a UASB, the reactors require low solids, such as municipal solids. The SEBAC reactor

may produce ammonia too quickly which will affect the pH because of its buffer capacity. With

the SEBAC reactor, the pH will need to be monitored closely or else the low pH will inhibit

methanogens from producing methane. With double stage reactors, one of the phases would

include an acid phase reactor which is similar to composting. Whether the reactor will be

mesophilic or thermophilic will depend on the FW. Even though thermophilic reactors have a

faster detention time and produce more methane, it may not be cost efficient to continue heating

the reactor at thermophilic temperatures.

For treating OFMSW in its entirety, including paper, we recommend the DRANCO or the

Valorga technologies. We received a quote from Dranco and are awaiting a quote from Valorga.

Dranco was recommended by the Lettinga Associate Foundation which is the leader in the

construction of UASBs. Valorga has more installations than Dranco. More study is needed to

decide which system is better. Dranco estimates to treat 25,000 tons of MSW/year, their reactor

cost would be 6 million Euros or $7.8 million. Since this system is ~3-4 times more expensive

than treating pure FW, this price may only be justified if paper wastes can’t be easily separated

from the FW or if the paper can’t be recycled due to contamination by FW. The estimated height

of their reactor is 30 meters.


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31 Combined Heat & Power

Combined Heat & Power 3.

Team members:

Richard Dowdell 4th


Elena Joy 2nd


Sarina Basu 4th

year undergraduate student, Industrial Engineering

Warren Johnson 4th



Introduction 3.1.Combined heat and power (CHP) systems are an efficient way of generating both electric

power and thermal energy (heat) from a single fuel source such as biogas. A typical CHP system

will produce 30 units of electricity and 45 units of steam given 100 units of fuel. Whereas a

separate power plant and boiler would require 154 units of fuel to produce the same heat and

power as the CHP system; this efficiency is illustrated in Figure 7.

Figure 7: Conventional Generation compared to Combined Heat and Power [4]

There are five major types of CHP systems: gas turbine (GT), micro-turbine, reciprocating

engine (RE), steam turbine, and fuel cells. Of these five types of CHP systems, all but fuel cells

can be fueled directly by biogas. The advantages and disadvantages of CHP systems capable of

running on biogas are outlined below as well as in Tables 1 and 2 in the appendix [4].

GT CHP systems can be used for applications requiring between 500 kW and 250 MW. GT

have high reliability, low emissions, high-grade heat, and the turbine requires no cooling.

However, they require high gas pressure; the output is dependent on ambient temperatures. At

90°F the output would be at 95% of the maximum. They have poor efficiency at low loading.

However, natural gas can supplement to increase efficiency. Micro-turbine CHP systems are

similar to gas turbines with sizes ranging from 30 kW to 250kW but are generally more costly

and less efficient than gas turbines. RE are most commonly used in applications requiring less

than 5 MW. They have high power efficiencies with variable loading, fast start-up, and can

operate at low gas pressure. RE do have higher maintenance costs due to the number of parts in

the systems. Steam turbine CHP systems are used for power outputs between 50 kW and 250

MW. They have high overall efficiency, long working life, and high reliability. Also, the power

to heat ratio can be varied so that you could provide more heat to a given system if needed.


However, steam turbines require up to one day for startup. With all of these technologies, gas

pretreatment would be required. A high-pressure water scrubbing system is necessary to

removal hydrogen sulfide at a cost of $0.06/MMBtu of biogas.

Out of the four technologies, we recommend either the GT or RE depending on power needs

at ATL. The RE will be most useful in situations where the required heat and power varies over

time. However, we assume that a CHP unit will be running continuously and the best use of the

heat is to heat the digester. If there is enough heat to make the digester thermophilic, we would

be able to reduce the size of the digester. The GT would be best if the power need was constant

at >10 MW. The biogas input could be supplemented with natural gas to achieve this output.

The relative benefits of the units as well as a cost analysis of each are discussed below.

Reciprocating Engine CHP Systems 3.2.The RE, commonly known as the piston engine, consists of at least one or more pistons

enclosed in a tight-fitting cylinder (Figure 8). An imbalance of pressure is caused at one end of

the cylinder (usually by a controlled explosion) that forces the piston in the other direction. The

free end of the piston is connected to a shaft that converts the pressure into rotational motion in

the form of torque. The more cylinders the more complicated the timing of the mechanics, but

the tradeoff generally produces more power than a similar mechanism with fewer pistons. An

RE would require a small crew of operators and mechanics.

Figure 8: Ignition Cycle of an RE [2]

The RE unit to be used in the CHP for ATL would need to run on biogas consisting mostly

of methane. According to the Environmental Protection Agency, the RE proves to be the best

suited. The RE is capable of a power efficiency of up to 40%, an effective electrical efficiency

of up to 80% and a power to heat ratio of roughly 1:2. REs have short startup times of merely 10

seconds as opposed to the range of 10 minutes to an hour used by a GT.

REs range in output from a few kilowatts to over 5000 kW and can be split into two basic

categories: spark ignition which uses lighter fuel and compression ignition which uses heavier

fuel. Though diesel (compression ignition) engines are the most popular of these, emissions

concerns have increased the number of natural gas burning (spark ignition) engines. The total


installed costs range from between $1,130 to 2,700/kW. Maintenance may be less expensive due

to the similarities between reciprocating and automobile engines.

The four sources of heat recovered from REs are from the exhaust gas (30-40% of energy

input), engine jacket cooling water (30%), lube oil cooling water, and turbocharger cooling. The

recovered heat is in the form of low-pressure steam or hot water. If effectively harnessed, this

means roughly 70-80% of the engine’s energy input can be recovered in the outputs of electricity

or heat. Because exhaust gases are kept above temperatures that would allow condensation in

the engine components, the best use of engine exhaust is to heat water, which will in turn allow

for easier and lengthier transmission of the heat. Lube oil cooling and single-stage after-cooling

will also be integrated into the creation of hot water or steam.

Reciprocating engines are rated into three categories: low, medium, and high speed with

respective rpm ranges of 58-275, 275-1000, and 1000-3600. The rich-burn spark ignition engine

only operates in the high-speed range and can produce a power output ranging from 0.01 to 1.5

MW. The lean-burn spark ignition engine only operates in the medium and high-speed range

and produces 1.0-6.0 MW and 0.15-3.0MW, respectively. Dual-fuel and diesel engines

significantly increase power output on the order of 4 times that of the other choices per speed

range and at low speeds produce between 2 and 65MW.

The use of a turbo-charging system uses hot exhaust air to power a turbine connected to the

intake that will increase pressurization in the cylinders and boost engine output (Figure 9). This

increases electrical output and decreases heat output. Most engines above 300kW include turbo

charging to increase electrical output.

Figure 9: Turbo Charging System [1]


The single downfall of reciprocating compression engines is the output of emissions. If the

energy park is subject to strict NOx emissions regulations, the engine can be tuned to produce

lower emissions. Using high-energy ignition technology, the engines can be operated in a lean-

burn state that uses a higher air-to-fuel ratio to produce the same output. The tradeoff comes in

lower emissions of NOx, which can be doubled when an engine is tuned for maximum instead of

lean-burn efficiency. NOx, CO, and VOCs are emitted while particulate matter and SOx levels

are dependent on the fuel used. Spark ignition engines have a max NOx output of 150 ppmv,

which is 1/3rd

of diesel engines. CO and VOCs are the result of incomplete combustion and can

be mitigated with use of a finely tuned injection and combustion system. By controlling

temperature, pressure, and the fuel to air ratio, the combustion process can be brought under

control to produce a maximum overall efficiency of roughly 80%. Still, the natural gas or biogas

variants produce around 1/20th

the particulate emissions of the diesel engine variants. The three-

way catalytic converter, often seen in automobiles, is best at removing CO, NOx, and VOCs from

the engine exhaust, especially when combined with a lean-burn engine system (Figure 10).

Figure 10: Catalyitic Converter [3]

Gas Turbine CHP Systems 3.3.GTs apply the Brayton thermodynamic cycle which includes three components: a gas

compressor, a combustion chamber, and an expansion turbine (Figure 11). In the gas

compressor, atmospheric air is first compressed then mixed with fuel, in our case biogas, heated

in the combustion chamber, and finally expanded in the expansion turbine, generating electricity.

Lower ambient air temperature corresponds to lower compressor power usage because the air is

less dense. The power produced comes from the final stage, but the compressor in turn

consumes power. Thus, for the highest efficiency, the temperature of the air at the compressor

inlet should be as low as possible while the temperature of the gas in the expansion turbine inlet

should be at the maximum possible. This creates a larger driving force to generate electricity.


Although the cost of the gas turbine CHP increases when technological advancements are

applied to increase the pressure and temperature inside the chamber, the efficiency of power

created is increased to the point that the overall system provides a net economic benefit.

Figure 11: Gas Turbine CHP System

The heat from the expansion turbine exhaust can be used in various ways. If the fuel is pre-

heated with a regenerator from the exhaust, fuel consumption is decreased. If a heat recovery

steam generator (HRSG) is added, the steam created may be used to generate electricity through

a steam turbine. This system is called a combined-cycle gas turbine. The steam or hot water

created can be also used for any other application needed.

The type of GT pertinent to this project is the aero derivative version that is essentially a

stationary jet engine. These are suited for stationary power and the largest maintain a capacity of

40-50 MW though they can be much smaller. Though initially more expensive than their

industrial counterparts, power efficiency is greater to the point that for the airport’s use, they are

the more cost-effective option among GTs.

Cost Analysis 3.4.

Stage 1: Qualifications

Generally according to the EPA, if the cost of power is $0.07 kWh, the site is a good

candidate for hosting a CHP system

Stage 2: Feasibility Analysis at a High Level

At this stage, we designed a CHP system based on:

● Capacity

● Thermal application

● Operation

● Size of the system

● Complexity of the system


Stage 3: Development

There are two ways of developing a CHP projects:

1. Develop the project internally which is considered a traditional approach also known as

design-bid-build. This approach maximizes the economic return and is the best choice

for this project.

2. Purchase a “turnkey” project by hiring a third party to do all the measurements and

applications. In this case the airport will have minimum control over the plans or the

system.

Stage 4: Procurement

Potential problems in this stage are:

● Zoning laws

● Site permitting

● Utility agreements with Georgia Power

There are other considerations such as will the staff be in-house employees that have been

trained to perform typical CHP functions? Or, will the CHP unit be operated by a 3rd

party?

Stage 5: Operations and Maintenance

Once the CHP system has been implemented, the typical O&M costs vary depending on

which machinery is used and the size of the system.

Installation and O&M Costs 3.5.The designed system was sized to be a 260 kW plant for food waste only according to a 3.1

MMBtu output of CH4 from the anaerobic digester. This size was confirmed by BIOFerm

Energy Systems; they estimate a size of 250 kW using food waste. In order to determine the

costs of a plant this size, a linear regression was used between two examples from the EPA - one

for a 100 kW plant and one for a 500 kW plant. Figure 12 displays a plot of the installation cost

and annual maintenance cost per kW along with the respective linear equations for sizing.

Figure 12: Linear regression of Installation and Maintenance Costs

y = -0.675x + 2277 y = -1.50E-05x + 2.35E-02

0

0.005

0.01

0.015

0.02

0.025

$1,900.00

$2,000.00

$2,100.00

$2,200.00

$2,300.00

0 100 200 300 400 500 600

Main

ten

ance

($/kW

h)

Inst

all c

ost

($

/kW

)

System Capacity (kW)

Cost/kW Maintenance


Additional cost breakdown of the installation is displayed in Table 4 in the appendix where

the total installed cost of the system would be approximately $550,000 and the annual

maintenance would be approximately $44,640/year. Performing a Net Present Value (NPV)

analysis for a 10-year period gives the total cost of $407,000 or about $13.2 per tons of FW. If

we assume electricity price of $0.1/kWh the total benefit of generating electricity from a 260 kW

plant will be around $200,000 per year or $25.4 per tons of FW and therefore, the net benefit will

be $12.2 per tons of FW.


Appendix 3.6.Table 4: Summary of Installation and Maintenance Costs

Installation Costs

Nominal Capacity 100 kW 500kW

Costs ($/kW)

Gen Set Package $1,000 $880

Heat Recovery $110 $240

Interconnect/Electrical $260 $60.00

Total Equipment $1,370 $1,180

Labor/Materials $340 $300

Total Process Capital $1,710 $1,480

Project and Construction Management $200 $180

Engineering and Fees $200 $180

Project Contingency $70 $60

Project Financing (interest during construction) $30 $40

Total Plant Cost $2,210 $1,940

Maintenance Costs

Cost $/kWh 0.022 0.016

Design

Design Size (kW) 260

260 kW System Size Installation Cost

Design Cost ($/kW) 2101

Installation Cost $546,390

260 kW System Size Maintenance Cost/year

Design Cost ($/kWh) 0.0196

Design Maintenance (per yr) $44,640


Table 5: Summary of CHP Technologies [4]

CHP system Advantages Disadvantages Available sizes

Gas turbine High reliability.

Low emissions.

High-grade heat

available.

No cooling required.

Require high-pressure

gas or in-house gas

compressor.

Poor efficiency at low

loading.

Output falls as ambient

temperature rises.

500 kW to

250 MW

Microturbine Small number of

moving parts.

Compact size and

lightweight.

Low emissions.

No cooling required.

High costs.

Relatively low

mechanical efficiency.

Limited to lower

temperature cogeneration

applications.

30 kW to 250 kW

Spark ignition (SI)

reciprocating engine

Compression ignition

(CI) reciprocating engine

(dual fuel pilot

High power efficiency

with part-load operational

flexibility.

Fast start-up.

Relatively low

investment cost.

Can be used in island

mode and have good load

following capability.

Can be overhauled on

site with normal

operators.

Operate on low-pressure

gas.

High maintenance costs.

Limited to lower

temperature cogeneration

applications.

Relatively high air

emissions.

Must be cooled even if

recovered heat is not

used.

High levels of low

frequency noise.

< 5 MW in DG

High speed (1,200 RPM)

≤4MWapplications

Low speed (102-514

RPM) 4-75 MW

Steam turbine High overall efficiency.

Any type of fuel may be

used.

Ability to meet more

than one site heat grade

requirement.

Long working life and

high reliability.

Power to heat ratio can

be varied.

Slow start up.

Low power to heat ratio.

50 kW to 250 MW

Fuel Cells Low emissions and low

noise.

High efficiency over

load range.

Modular design.

High costs.

Low durability and

power density.

Fuels requiring

processing unless pure

hydrogen is used.

5 kW to 2MW


Table 6: Summary Table of Typical Cost and Performance Characteristics by CHP Technology [4]

Technology Steam

Turbine1

Recip. Engine Gas Turbine Micro-

turbine

Fuel Cell

Power efficiency

(HHV)

15-38% 22-40% 22-36% 18-27% 30-63%

Overall efficiency

(HHV)

80% 70-80% 70-75% 65-75% 55-80%

Effective electrical

efficiency

75% 70-80% 50-70% 50-70% 55-80%

Typical capacity

(MWe)

0.5-250 0.01-5 0.5-250 0.03-0.25 0.005-2

Typical power to

heat ratio

0.1-0.3 0.5-1 0.5-2 0.4-0.7 1-2

Part-load ok ok poor ok good

CHP Installed costs

($/kWe)

430-1,100 1,100-2,200 970-1,300

(5-40 MW)

2,400-3,000 5,000-6,500

O&M costs

($/kWhe)

<0.005 0.009-0.022 0.004-0.011 0.012-0.025 0.032-0.038

Availability near 100% 92-97% 90-98% 90-98% >95%

Hours to overhauls >50,000 25,000-50,000 25,000-

50,000

20,000-

40,000

32,000-

64,000

Start-up time 1 hr - 1 day 10 sec 10 min - 1 hr 60 sec 3 hrs - 2 days

Fuel pressure (psig) n/a 1-45 100-500

(compressor)

50-80

(compressor)

0.5-45

Fuels all natural gas,

biogas, propane,

landfill gas

natural gas,

biogas,

propane,

oil

natural gas,

biogas,

propane,

oil

hydrogen,

natural

gas, propane,

methanol

Noise high high moderate moderate low

Uses for thermal

output

LP-HP steam hot water, LP

steam

heat, hot

water,

LP-HP steam

heat, hot

water,

LP steam

hot water, LP-

HP

steam

Power Density

(kW/m2)

>100 35-50 20-500 5-70 5-20

NOx ( lb/MMBtu)

(not including SCR)

Gas 0.1-.2

Wood 0.2-.5

Coal 0.3-1.2

0.013 rich burn 3-

way cat.

0.17 lean burn

0.036-0.05 0.015-0.036 0.0025-.0040

lb/MWhTotalOutput

(not including SCR)

Gas 0.4-0.8

Wood 0.9-1.4

Coal 1.2-5.0.

0.06 rich burn 3-

way cat.

0.8 lean burn

0.17-0.25 0.08-0.20 0.011-0.016

* Data are illustrative values for typically available systems; All costs are in 2007$ 1For steam turbine, not entire boiler package


References 3.7.1. B., Taylor. "Turbos in Car Engines - Engineerography Blog." Turbos in Car Engines - Engineerography

Blog. Engineerography Blog, 2 Apr. 2009. Web. Mar. 2012.

<http://engineerography.com/2009/04/turbos-in-car-engines/>.

2. "Gas-Fired Reciprocating Engines." AMO Industrial Distributed Energy:. U.S. Department of Energy.

Web. Mar. 2012. <http://www1.eere.energy.gov/manufacturing/distributedenergy/gas_fired.html>.

3. Motavalli, Jim. "The 20-Year Fight to Get the Lead Out." The Daily Green. 17 Dec. 2007. Web. Mar.

2012. <http://www.thedailygreen.com/living-green/blogs/cars-transportation/lead-gasoline-catalytic-

converters-pollution-461219>.

4. United States Environmental Protection Agency. www.epa.org. Dec. 2008. Web. Feb. 2012.

<http://www.epa.gov/chp/documents/catalog_chptech_full.pdf>

43 Composting Systems

Composting Systems 4.

Team members:

Richard Dowdell 4th


Elena Joy 2nd


Sarina Basu 4th


Warren Johnson 4th



Introduction 4.1.Composting is the aerobic bio-degradation of organic materials under controlled conditions.

The result of composting is a rich material that can be used for agriculture, horticulture, or

erosion control. There are three major types of composting: windrow composting, static aerated

pile, and in-vessel composting.

Windrow composting involves spreading organic material into long piles which are turned

by heavy equipment in order to maintain even decomposition. Piles are generally 4-8 feet in

height and 14-16 feet in width. The relative simplicity of windrow composting makes it a prime

candidate for processing high volumes of materials, particularly yard trimmings, on larger

amounts of land. Moisture may be added in dry climates or a cover installed over the pile to

prevent evaporation. Onsite leachate must be managed, as well as blowing debris and odor. In

order to avoid debris affecting airline safety, the piles will have to be covered or enclosed in a

shelter.

Static aerated pile composting maintains composting by forcing air through a pile (Figure

13). Little or no mechanical turning is required as in windrow composting. The use of bulking

agents such as shredded paper or wood chips are required to enhance the flow of air, thus this

may be a good disposal option for the paper waste at ATL. Piles can be covered with heavy

plastic and the airflow can be pulled through the pile and then passed through a bio-filter to

reduce the odors of the decomposition. Because there is little to no mechanical agitation it is

more important to keep the particle size homogeneous thus a material grinder may be necessary.

Figure 13. Static Aerated Composting [5]


In-vessel composting occurs within a rigid enclosed container varying in size from 55-gallon

drums to a silo (Figure 14). Conditions inside the container must be closely regulated such as

temperature, moisture, and aeration to minimize leachate and odor production. Materials can be

ready in as little as 7-14 days though further curing is required. They have a high capital cost but

require less labor and land than static piles or windrows.

Figure 14. How Vertical In-Vessel Composting Works

Because ATL does not want to hire any more laborers and due to the complexity of odor

control, windrow composting is not an efficient method. However, static aerated pile

composting and in-vessel composting are both reasonable choices and each has their own pros

and cons.

Composting for ATL 4.2.The main goal of the composting facility at the airport is to create a high quality product that

can be used in ATL’s greenhouses. No matter which type of composting vessel is used, the

process is separated into three distinct sections: preprocessing, processing, and post processing.

Preprocessing

Preprocessing can be done in one of two ways. Unsorted material can bypass the

preprocessing step and go directly to the processing stage. Though this option saves time


initially, skipping the preprocessing stage will result in a higher cost during the processing stage

and will yield a lower quality of product. The other option is to remove all non-compostable

items from ATL’s MSW. This preprocessing procedure incurs minimal cost and little labor,

especially when the materials are paper and other OFMSW.

Preprocessing paper simply means shredding the paper. This can be done with a small

household or office shredder or a large-scale shredder can be purchased. According to the

collected data, the airport accumulates 14.2 tons of paper per day. For this sizeable amount, a

large shredder should be purchased.

Suggested Shredder Specifications:

Brand: Intimus S16.50 Paper Shredder

Capacity: 350-450 sheets at one time

Shred Size: 1/4 inches

Input Width: 20 inches

Practical Capacity: 1800 pounds per hour

o 2000 lbs = 1 ton

o 14.2 tons = 28,420 lbs

o 1800 lbs = 1 hour

o 28,420 lbs per day = 15.78 hours per day

Weight: 1600 lbs

Dimensions (W” × D” × H”): 47.25 × 76.75 × 61

Accessories: Plastic Bags, Shred Box, Oiler

Table 7: Suggested Shredder Costs

Cost of Shredder $30,000

Shredder Bags (100 count) $90

Shredder Box $356

Automatic Oiler $499

Total $30,945

To sort yard trimmings, a laborer spreads out the incoming material and visually identifies

“large” pieces of wood, shrubbery, or other greens. The laborer manually picks these pieces out

and breaks them down into smaller pieces that can be more easily composted.

Preprocessing also involves all of the incoming material (with the exception of shredder

paper) to be reduced in size. After the laborer has taken care of the visually large pieces of yard

trimmings, all the material should be processed in machines called hammer mills and tub

grinders. This reduction in size is crucial for woody material mixed with other yard trimmings

because wood decomposes at a much slower rate and can therefore delay the development of the

compost end product.

A hammer mill is a steel drum containing a vertical or horizontal rotating shaft on which

hammers are mounted (Figure 15). The hammers swing on the ends of a cross or are fixed to the

central rotor. The rotor spins at a high speed inside the shaft while the input material is fed into a


feed hopper. The material is crushed by the hammer bars and is thereby shredded and expelled

through screens in the shaft of chosen size. An issue with a normal hammer mill is the lining

wears out quickly when handling large amounts of material. The airport is a large-scale client

and should therefore purchase a screen-less hammer mill. A screen-less hammer mill uses

airflow to separate small particles from larger ones. This type of hammer mill is designed to be

more reliable and more energy efficient than a regular hammer mill, which are two qualities that

are fundamental to the airport’s goal.

Figure 15. Screenless Hammer mill [3]

The final part of the preprocessing stage is to check the carbon to nitrogen ratio. All

organic matter (yard trimmings) is made up of large amounts of carbon and lesser amounts of

nitrogen. The best compost requires the best carbon to nitrogen ratio. A common rule of thumb

is that green vegetation is high in nitrogen and brown vegetation is high in carbon. For fresh or

“wet” yard trimmings, it is common to add paper into the mixture. Table 8 shows some C-N

ratios for some common materials.

Table 8: Estimated Carbon-Nitrogen Ratios

Shredded Cardboard 350:1

Leaves 60:1

Shredded Newspaper 175:1

Pine Needles 80:1

Wood Chips 400:1

Garden Waste 30:1

Grass Clippings 20:1

Weeds 30:1

FW 15:1

The ideal ratio of carbon to nitrogen in compost is (25-30):1. Since most ingredients do not

have this ideal ratio, a mixture must be created. High carbon to nitrogen ratios may be lowered


by adding grass clippings and FW while low carbon to nitrogen ratios may be raised by adding

paper, dry leaves, or wood chips.

Processing

The second stage of the composting procedure is processing. There are a plethora of

techniques to choose from but given the airport’s needs and constraints, the two most reasonable

choices are static aerated piles or in-vessel composting.

Static Aerated Pile Composting 4.3.In aerated static pile composting, waste is mixed together in a large pile instead of in rows.

To aerate the pile, layers of loosely piled bulking agents (paper) are added so that air can pass

from the bottom to the top of the pile. There is no turning necessary to complete the composting

process.

In a more advanced technique, the piles can be placed over a network of pipes that deliver

air into or draw air out of the pile. For the bio-degradation to work evenly throughout the whole

pile it is very important that the temperature is even throughout the entire pile. Air blowers are

activated by a timer or by a temperature sensor [2]. When it is necessary to withdraw air from

the pile, the air can be filtrated with a bio-filter that is made from the finished compost [2]. The

bio-filter removes air pollution, thus also reducing odor. Using this method, the piles can be

fairly large which means they occupy less area.

The ideal weather for this method is a warm, arid climate. This is potentially problematic

since Atlanta is such a humid area. For this reason, aerated static piles are often located under a

shelter of some sort. During cold months, the core of the pile will retain its warm temperature.

However, aeration might be more difficult in the cold because this method involves passive air

flowing rather than active turning. Some aerated static piles are placed indoors with proper

ventilation [1].

Whenever a compost facility is implemented, odor control is always a concern. At a high

population area such as the airport, minimizing odor is extremely significant. Odor control can

be achieved by simply filtering the air pulled through the piles by the blowers through a bio-filter

made of already completed compost.

Equipment and Building Input

An aerated static pile composting facility requires an aeration system, including blowers,

pipes, aeration control, and temperature sensors. The aeration system can be as simple as

perforated pipes on the ground, or pipes embedded in the floor. Walls can be used for

containment. Other options include breathable fabric membranes or plastic sheets with holes [6].

Labor Input

Labor is required for mixing the material, loading and unloading the aerated composting

system, and monitoring temperatures. On an average yearly basis, relatively efficient large-scale

composting operations require one labor-hour for every 6 tons of incoming material, or

approximately one full-time worker per 10,000 tons, year in and year out [6]. Table 9 shows the


estimated cost for a static aerated pile. We assume a 4% annual interest rate to calculate the total

cost over 10 years period.

Table 9: Estimated Static Aerated Pile Costs [6]

Tons per year

OFMSW Paper only

14,000 5,000

Installation Cost

General $ 75,000 $ 15,000

Site Work $ 150,000 $ 50,000

Paving $ 455,000 $ 150,000

Concrete $ 273,000 $ 100,000

Leachate System $ 100,000 $ 100,000

Equipment $ 487,500 $ 200,000

Total $ 1,540,500 $ 615,000

O&M Cost per Year

General $ 24,375 $ 8,000

Fuel/Supplies $ 23,000 $ 10,000

labor $ 100,000 $ 30,000

Repairs/Maintenance $ 40,000 $ 10,000

Electricity $ 6,500 $ 2,500

Total $ 193,875 $ 60,500

Approximate Cost Over 10 Years

Total cost over 10 years $ 3,307,000 $ 1,166,000

Cost per ton over 10 years $ 21.5 $ 21.2

In-Vessel Composting 4.4.In-vessel composting is the most controlled form of composting. It takes the original waste

and wood mixture and encloses it in a normally cylindrical tank. In a horizontal vessel, the ends

are left slightly open while the entire tube slowly rotates providing ample aeration for the

bacteria in the pile. Vertical in-vessel composting consists of upright tanks where the inputs are

poured into the top and the finished compost pours out from the bottom. The tanks vary in size

but the most efficient allow a throughput of roughly 10 tons per day with a retention time of 4-14

days. Heat produced by the bacterial processes at the bottom of the pile is drawn up through to

the top of the tank creating a chimney effect. This creates a temperature differential from the

bottom to the top of the tank from 40-70°C. This heat buildup at the top of the tank effectively

kills the pathogens in the new material that has just been dumped in.

Aside from the space saved by building in the vertical direction, vertical composting has the

added benefit of using gravity to move the material through the process rather than some other

method that requires machinery and thereby adds costs. The new waste entering the top goes

through the superheated portion of the tank and then moves through to the second portion where


the lower heat is just enough to facilitate the natural breakdown of all the solid waste and wood

particles. As it passes into the third and final stage, the compost is in its final breakdown stages

and producing the most heat. The fresh air drawn up through the bottom accelerates the final

composting processes and readies the lower level for removal to make space for the next input.

This process doesn’t require bio-filtration, temperature or air control, or agitation. With the

near lack of moving parts, operating and maintenance costs are very low. Because the entire

process is sealed off, temperature and smell is automatically controlled and maintained.

Cost Considerations

ATL does not have to worry about land acquisition. In terms of equipment procurement

there will be a need for composting vessels, loading equipment, conveyors, air supply

equipment, and odor control equipment. Operation and maintenance labor will need to be

included as well as electrically powering the operation and treating the water supply.

Cost Estimation

Annual operation and maintenance is $200 per ton of processed waste. Fees for equipment,

maintenance, and fuel are estimated at around $450,000 per year. Given these assumptions, the

total cost of In-vessel composting would be around $192 per ton for a 10-year period.

Post Processing

The final stage of the composting procedure is post processing. This stage is optional;

however, it is often implemented to refine the compost into the end user’s specifications. The

same equipment used in the preprocessing stage can be used in the post processing stage. The

main purpose of the post processing procedure is to get the composted product into a uniform

size. Simply run the finished product through the hammer mill. The result is the finished

compost product.


References 4.5.1. "Aerated Static Pile Composting." EPA. Environmental Protection Agency, 3 Nov. 2011. Web. 06 Apr.

2012. <http://www.epa.gov/osw/conserve/rrr/composting/static.htm>.

2. "Biosolids Technology Fact Sheet In-Vessel Composting of Biosolids." National Service Center for

Environmental Publications. Web. 06 Apr. 2012.

<http://nepis.epa.gov/Exe/ZyNET.exe/901U0V00.txt?ZyActionD=ZyDocument>.

3. "Hammer Mill Function." Web. 06 Apr. 2012. < http://diqky.blogspot.com/2009/11/hammer-mill-

function.html >.

4. Haaren, R.“Large scale aerobic composting of source separated organic wastes: A comparative study

of environmental impacts, costs, and contextual effects.” Earth Engineering Center, Columbia

University, <http://www.seas.columbia.edu/earth/wtert/sofos/haaren_thesis.pdf>

5. "Compost System and Training." Compostwerks.com. Web. 05 Apr. 2012.

<http://www.compostwerks.com/O2Compost_Compostwerks_Info_Packet.pdf>.

6. "Integration of Renewable Energy on Farms - Composting - Aerated Static Pile."

Composting-Aerated Static Pile. Integration of Renewable Energy Farms, 31 Mar. 2008. Web. 06 Apr.

2012. <http://www.farm-energy.ca/IReF/index.php?page=aerated-static-pile-ataglance>.

52 Offsite Composting

Offsite Composting 5.

Team members:

Sanjay Arora PhD student, Public Policy

Pratik Irudayaraj 4th



Introduction 5.1.The scope of ATL’s Energy Park is a waste processing facility that, in the end, will

contribute to the functioning of an all-season green house. We understand that ATL will pursue

a number of objectives, one of which includes an educational and community outreach program

that will introduce the sustainability agenda to the public. Consequently, an offsite approach to

the Energy Park (or some components of the Energy Park) may provide limited utility given this

social mission. However, an offsite analysis offers one basis by which to compare the costs of

operating onsite facilities vis-à-vis alternative arrangements procured through market-based

channels. Here, we focus on one particular option of interest: an offsite analysis of composting

with a large-scale, local composting facility.

The offsite analysis focuses on two different approaches, the first of which assesses a

scenario wherein the airport sends its paper-based waste offsite for composting and all FW is

directed to an onsite AD. The second scenario looks at sending all OFMW offsite. This second

approach assumes that no FW is sent for AD. In either case, in the analyses below, we assume

that the airport collects and separates compostable waste onsite and designates certain dumpster

or containers for pick-up by an offsite composting vendor. We anticipate that costs incurred for

the pick-up of waste for offsite composting will be redirected from Waste Pro.

Offsite Composting Options 5.2.We have consulted with an offsite composting facility to develop a general pricing strategy,

though we emphasize that these estimates are tentative. The offsite compositing facility works

with individual clients to determine tailored pricing schemes. We understand ATL would pay

roughly $1000 per month to have an 8 yd3

container (1,616 gallons) picked up 5 five times a

week. We assume that the airport’s purchasing power may afford a $1000 per month disposal

fee per 8 yd3 container, even if the container is emptied 7 days a week.

From our prior consultation with the composting vendor, we learned that 35 gallons of waste

weighs approximately 135 pounds. Thus, an 8 yd3

container holds about 3.1 tons of waste (Table

11). At 25,000 tons /year, 70% compostable, 3.1 tons/pickup, 5 pickups/week, 4.5 weeks/month,

and $800/month, that’s ~$200K/year for the offsite composting option. However, it is uncertain

if 3.1 tons of paper can be compacted to fit an 8 yd3 dumpster.


Table 10: Calculating the tons of waste held in a 8 cubic yard container

Pounds of waste per gallon 3.9

Gallons in cubic yard 202

Pounds of waste per cubic yard 779

Pounds of waste in 8 cubic yard container 6,232

Tons of waste in 8 cubic yard container 3.1

Only paper waste is sent to the offsite composting facility

In this analysis, we assume the airport generates 5,000 tons of paper waste (Table 11).

Table 11: Offsite composting analysis sending only paper waste to an offsite vendor

Cost to dispose one 8 cu. yd container per month $1000

Tons per day generated – paper 14.2

Number of containers needed for paper waste per day 5

Total cost for containers – paper $5,000

Per year (12 months) $60,000

All compostable waste could be redirected to an offsite composting facility

Here, we assume the airport generates 23,000 tons of waste per year, 60% of which is

OFMW (Table 12).

Table 12: Offsite composting analysis sending only all compostable waste to an offsite vendor

Cost to dispose one 8 cu. yd container per month $1000

Tons per day generated – OFMW 37.8

Number of containers needed for all waste per day 13

Total cost $13,000

Per year (12 months) $156,000


Net Present Value (NPV) Analysis 5.3.Using the estimated costs from both offsite analyses presented above, we perform two NPV

analyses (see Table 13). Note that the NPV was obtained using an inflation factor of 4% which

is slightly higher than the national average of 3.4%.

Table 13: NPV analysis of two scenarios

Paper only OFMW

Year 0 $ 60,000 $ 156,000

Year 1 $ 57,692 $ 150,000

Year 2 $ 55,473 $ 144,231

Year 3 $ 53,340 $ 138,683

Year 4 $ 51,288 $ 133,349

Year 5 $ 49,316 $ 128,221

Year 6 $ 47,419 $ 123,289

Year 7 $ 45,595 $ 118,547

Year 8 $ 43,841 $ 113,988

Year 9 $ 42,155 $ 109,604

Year 10 $ 40,534 $ 105,388

NPV $ 546,654 $ 1,421,300

In conclusion, over a ten year period, we estimate that the airport will incur approximately

$9.9 per ton to send all paper-based waste to an offsite vendor for composting while it would

cost about $9.2 per ton to send all OFMW to the same offsite vendor.

56 Biofuel from Paper Waste

Biofuel from Paper Waste 6.

Team member:

Christopher Capocasale 1st year undergraduate student, Mechanical Engineering


Introduction 6.1.While energy yields tend to be lower than other feedstocks for biofuel production,

alternative feedstocks such as paper waste are very attractive, primarily because the feedstock

itself is a waste that has a disposal cost. Unlike crops grown for biofuel production, the

feedstock does not take its own environmental toll. Additionally, paper waste does not have

competing uses, unlike food crops such as corn. The primary barrier to using paper waste as a

feedstock lies in the molecular structure of lignocellulosic materials; the sugars contained within

the cellulose of paper waste are not accessible without pre-treatment. As a result, industrial use

of lignocellulose as a feedstock is still being developed. Figure 16 shows three processes close

to commercialization as designated by the flow of the red arrows.

Figure 16. Processes close to commercialization;(ETBE = ethylbenzene; BTL = Biomass To Liquid fuel [1].

Before any of these possible routes may be taken, pretreatment is necessary to remove lignin

and hemi-cellulose and reduce the crystallinity of cellulose. Hydrolysis involves the

saccharification of cellulose molecules resulting from pre-treatment [1]. Either enzymes or

dilute acid may be used to catalyze the reaction. Acid hydrolysis may be cheaper in the short

run, but maintenance costs due to corrosion are high. Enzymatic hydrolysis is generally

considered the most cost-effective process, giving high yields with low maintenance costs. An

alternative process involves gasification, carried out by heating without combustion of organic

material in the presence of specific oxidants. CO, H2, and CO2, or syngas, are its products.

Syngas may then be fed into a Fischer-Tropsch (FT) reactor that converts the syngas into a wide

range of liquid hydrocarbons, including diesel, or even used as a fuel itself.

Hydrolysis, Fermentation, Distillation, Ethanol 6.2.Production of ethanol through hydrolysis first requires pre-treatment to maximize energy

efficiency by breaking down cell structure, making the cellulose of the feedstock – rigid, dense

polymers of glucose – accessible [2]. Once the pre-treatment is complete, hydrolysis is carried

out, converting the accessible cellulose into sugars. For large-scale operations, catalysis by


enzymes is the most cost-effective due to low maintenance costs compared to acid hydrolysis.

After hydrolysis, fermentation of sugars is carried out, followed by distillation.

Unfortunately, the effectiveness of pretreatment processes is highly dependent upon the

nature of the feedstock. Most of the current research in commercial lignocellulosic bioethanol

production is for woodchip feedstocks. While paper wastes especially show high potential as a

sustainable feedstock due to high cellulose content and low lignin content, processes for

conversion of paper waste to ethanol are only beginning to be carried out on a large scale. In the

rest of this section, only pre-treatment through dilute sulfuric acid pretreatment will be discussed.

The primary cost of the process shown in Figure 17 is that of enzymatic hydrolysis by

cellulase. Fortunately, for the cellulose-rich MSW that the airport will be producing, sugar yield

percentages are relatively high even for low concentrations of cellulase, above 80% for

cardboard and mixed paper (Figure 18).

Figure 17: Conversion of MSW to fuel ethanol [3]

Figure 18: Sugar yields (as Glucan-to-glucose) from enzymatic hydrolysis. Cellulose is a β-1,4-glucan [1]

Finally, according to compositions of cardboard and mixed paper wastes obtained from the

same study, the National Renewable Energy Laboratory's theoretical ethanol yield calculator, 99

gal/dry ton of cardboard may be produced and 12 gal/dry ton of mixed paper may be produced.


Assuming mixed paper to contain newspaper, magazines, office paper, and other paper, up to

1,364,551 gallons of ethanol per year may be produced, using numbers from Waste

Characterization Report [5].

While commercialization of the processes discussed is only beginning, the technologies

discussed show great potential. Combined with minimal costs of feedstock from the airport, this

process shows potential for profitability.

Gasification, Fischer-Tropsch synthesis, Diesel/Gasoline 6.3.Gasification is a multi-step process that any carbonaceous material may undergo (Figure

19). It involves partial combustion, usually at temperatures around 700°C where volatile

components react with one of several oxidants to form syngas, consisting mostly of CO, CO2 and

H2, and heat, while part the material remains solid. Air, steam, and oxygen, or some mixture of

these is generally used as an oxidant [1]. While oxygen is the most advantageous choice, it is

prohibitively expensive. Generally, some mix of oxygen and air is used. Steam, due to an

increased presence of hydrogen, may be used to create methane gas, which is not desirable for

FT synthesis. The primary advantage of gasification is that it allows production of syngas from

any carbonaceous material with only a slight variation in the process. In addition, gasification is

a commonly used industrial process and, as such, is well documented. While gasification

processes are many and varied, the most reliable gasification system for large-scale operations is

the fluidized bed gasifier [1], offering much quicker gasification than other processes.

Figure 19: Typical gasification-FT synthesis scheme [1]

The primary drawback of gasification processes is the necessity of gas cleaning. Residues

from the gasification reactions are present in the syngas, forming contaminant tars. One of

several methods, including scrubbers, may be used to cleanse the syngas of tars. A final cleaning


step is also often used to cleanse the syngas of inorganic impurities. These cleaning steps

optimize the efficiency of the FT synthesis reactions that are to follow.

Once clean syngas is obtained, the FT synthesis process builds up carbon chains from the

gas through a set of highly exothermic reactions, producing a wide set of synthetic fuels, the

majority of which may be used as diesel fuels, shown in Figure 20.

Figure 20: Theoretical FT synthesis distribution [1]

Conclusion 6.4.Both processes have their hurdles; bioethanol production from MSW requires exploration of

processes that are relatively unknown, while gasification also presents significant, but more

familiar, technological hurdles. While the gasification-FT synthesis route offers significant

savings with regards to GHG and other emissions, the ethanol production route would be a move

in a new direction [4]. Either process, due to the low feedstock cost, is very attractive, offering

either diesel fuels or bioethanol as a product. Market conditions, as well as possible uses of

product fuel within the energy park, should be considered in deciding upon a desired product.


References 6.5.1. Øyvind Vessia. “Biofuels from Lignocellulosic Material in the Norwegian Context 2010: Technology,

potential, and Costs.” Trondheim. 20 December 2005. <http://www.zero. no/transport/biodrivstoff/hva-

er-biodrivstoff/andregenerasjons-biodiesel/biofuels-from-lignocellulosic-material.pdf>.

2. Verma, Amrita, Santosh Kumar, and P.K. Jain. "Key Pretreatment Technologies on Cellulosic Ethanol

Production." Journal of Scientific Research 55 (2011): 57-63. Print.

3. Jian Shi, Mirvat Ebrik, Bin Yang, and Charles E. Wyman. “The Potential of Cellulosic Ethanol

Production from Municipal Solid Waste: A Technical and Economic Evaluation.” April 2009. Web.

<http://www.ucei.berkeley.edu/PDF/EDT_015.pdf>.

4. “Well-to-Wheels Analysis of Future Automotive Fuels and Powertrains In the European Context.”

Version 2c, March 2007. Web. <http://ies.jrc.ec.europa.eu/uploads/media/WTW_Re port_010307.pdf>.

5. Camp Dresser & McKee (CDM). 2006 Waste Characterization Report. Tech. Final ed. Atlanta,

Georgia. Print.

62 Greenhouse

Greenhouse 7.

Team members:

Fernando Ponce 3rd


Rahul Mody 2nd


Olivia Pan 2nd


Jiawei Li 1st year undergraduate student, Civil Engineering

Jiaye Pan 1st year undergraduate student, Civil Engineering

63 Greenhouse

Introduction 7.1.In this chapter different crop alternatives are evaluated to grow in greenhouses. Three

energy crops are considered and compared with growing non-energy crops using hydroponics

technology.

Algae 7.2.Algae are an attractive biofuel because of their fast growth rates and improved land use

efficiency when compared with ground-based crops. Converting algal biomass into energy

requires the following three distinct chronological processes: cultivating the algae, harvesting the

biomass, and converting the biomass into usable fuel.

Cultivation of Algal Biomass

The efficiency of algae biodiesel production is contingent on lipid productivity and the

growth rate of algae cells. The development of high lipid content appears to be triggered during

conditions of nutrient limitation, particularly nitrate. During these stressful periods, algae shift

biosynthetic pathways and produce lipids called triacylglycerols (TAGs), which accumulate in

the cytoplasm for the purpose of energy and carbon storage. However, intentionally cultivating

algae in stressful conditions inhibits cell division, which can lead to decreased overall lipid

productivity. Numerous cultivation systems designed to generate algal biofuel feedstocks have

been proposed, some are:

1. Wastewater Treatment Ponds

2. Raceway Ponds

3. Photobioreactors (PBRs)

Wastewater HRPs represent an inexpensive, low-technology approach to algal cultivation.

However, limited solar exposure resulting from increased operating depths results in lower cell

densities than are achievable with raceway ponds and PBR systems. Flat panel and tubular PBR

systems generate higher biomass concentrations, produce less evaporative losses, and prevent

contamination when compared with open ponds. It is also important to consider that the high

operating and construction cost of PBR systems may limit their application for large-scale

production of low value products, such as biofuel feedstocks. Providing elevated concentrations

of CO2 to algal cultures increases growth rates and it has been suggested that algal cultivation

operations be constructed in close proximity to coal-fired power plants, such that flue gas could

be used as a CO2 source.

Algal Harvesting

Harvesting algae is difficult because of their small size, low specific gravity, and negative

surface charges, which produce stable algal suspensions throughout the water column.

Techniques for recovering algal biomass are classified as either primary or secondary harvesting.

Primary harvesting refers to the separation of the algal cells from their growth media and can be

64 Greenhouse

achieved by sedimentation or with flotation technology. Primary harvesting is suitable for

anaerobic digestion, while algae biodiesel production requires much drier algal paste. Therefore,

careful evaluation of available harvesting technologies is necessary when considering algal

biofuel processing.

Figure 21. Algae cultivation and conversion to biofuel.

Cost analysis

A land, cost, and resource analysis is shown in Table 14.

Table 14: Approximate Costs [1]

Conclusion

In the use of algae there are many challenges to face. First, much of the current model data

is derived from literature mainly because this is new technology that hasn’t been fully developed

yet. Therefore, experimentally verified data will be more meaningful, but is not widely

available, so money would have to be spent on research. Secondly, the harvesting and extraction

65 Greenhouse

process unit operations possess high degree of uncertainty. Thirdly, there are many possible

combinations of process technology and configuration not currently modeled that would need to

be researched. Finally, the sustainability of the project (e.g. water and resource requirements) is

also an issue that needs to be taken into account.

The lower density of algal biomass present in wastewater ponds systems suggests that

anaerobic digestion is the most appropriate energy pathway. As these systems produce less

biomass than raceway ponds and PBR systems, it is unlikely that wastewater ponds will be used

for large-scale algae cultivation, so this one can be discarded as an option for the airport project.

Instead, these systems represent a low-technology, community-scale approach to algal biofuels.

If an algae pathway is taken, the most recommended option would be photobioreactors (PBR),

because even though they are more expensive than raceway ponds, as shown in the cost analysis

table, this technology is more suitable for large scale production than the later one.

66 Greenhouse

Camelina 7.3.Camelina is a new crop put in practice to produce biofuel. It is well tested at Montana State

University (MSU) and the biofuel it produces has gone through the test by United States Air

Force's Alternative Fuels Certification Division. Camelina plants are heavily branched, growing

to heights of one to three feet. They become woody as they mature. It is a short-seasoned (85-

100 day) crop, best adapted to cooler climates where excessive heat during flowering does not

occur. Camelina is a cool season crop that needs to be planted early to attain maximum yields.

In the case of Atlanta, the best season to plant camelina would be winter.

Camelina sativa was evaluated in a multi-species oilseed trial at the MSU Agricultural

Research Center in 2004. This trial included nine oilseed crops (sunflower, safflower, soybean,

rapeseed, mustard, flax, crambe, canola, and camelina). Evaluation parameters included input

costs, production costs, harvest costs, and yield. The net value of the crop was determined

utilizing current oil and meal prices. Camelina sativa was not the highest yielding crop but it

was the most economical crop to produce due to minimal input requirements. For example,

canola seed costs approximately $0.075/lb while Camelina sativa costs 0.015-0.025/lb [4].

Camelina biodiesel has been produced and evaluated by commercial biodiesel

manufacturers including Core IV, Wyoming Biodiesel, Peaks and Prairies, and Great Northern

Growers. Studies have shown camelina-based jet fuel to reduce net carbon emissions from jets

by about 80%. Camelina biodiesel performance appears to be equal in value and

indistinguishable from biodiesel produced from other oilseed crops such as soybean.

Yield

Camelina is well suited for marginal soils. According to MSU research, camelina has a

lower break-even cost than wheat and canola. The seed can be broadcast or drilled at 2.5 to 3

lbs/acre. The seeds are dense and small at 345,000 to 465,000 seeds per pound. Yields vary

depending on soils and rainfall. Camelina will average 1,800-2,200 lbs/acre under 16-18 inches

of rainfall. In other dryland research trials, camelina yields averaged 1,000-1,700 lbs/acre.

Yields drop with less rainfall and increase when using irrigation. MSU suggests including

camelina in a three- or four-year crop rotation [5], however if the need for crop rotation is due to

low nutrient content in the soil, camelina may be grown continuously if FW is used as a

fertilizer.

67 Greenhouse

Duckweed 7.4.Duckweed is very simple, lacking an obvious stem or leaves. They consist of a small

'thalloid' or plate-like structure that floats on or just under the water surface, with or without

simple rootlets. The plants are highly reduced from their earlier relatives in Araceae, a family of

flowering plants. Duckweed is an important high-protein food source for waterfowl and also is

eaten by humans in some parts of Southeast Asia. As it contains more protein than soybeans and

five to six times more starch than corn, it is sometimes cited as a significant potential food source

especially as a fish feed [6].

Potential Uses

The plants can provide up to 99% nitrate and phosphorus removal from wastewater streams.

They grow rapidly – doubling in mass about every 2-3 days. For these reasons they are touted as

water purifiers of untapped value. The Swiss Department of Water and Sanitation in Developing

Countries, SANDEC, associated with the Swiss Federal Institute for Environmental Science and

Technology, asserts that as well as the food and agricultural values, duckweed also may be used

for waste water treatment to capture toxins and for odor control, and, that if a mat of duckweed is

maintained during harvesting for removal of the toxins captured thereby, it prevents the

development of algae and controls the breeding of mosquitoes. Finally, duckweed cover may

also be used to conserve water by reducing evaporation rates [7].

Energy

Duckweed also has potential as a fuel due to high starch and low lignin content. While the

harvesting of the energy stored in Duckweed is not as simple as that from grains, duckweed has

much greater energy potential than most conventional biological energy sources. This potential

is increased due to duckweed’s extremely high growth rates. Growth may be accelerated by over

40% by increasing the CO2 concentration from 450-550 ppm. Exhaust gas from the CHP unit

could be used as this CO2 source.

After harvesting the duckweed, it is first homogenized using a poly-phasic separator. The

homogenized duckweed is then fed into a gasifier, where it is oxidized at high temperature,

producing char and the product gas, known as syngas, which consists mainly of CO, H2, and

CO2. Syngas may be burned directly as a natural gas or subject to various upgrading processes,

while the char may be used reused to fuel the gasifier.

Challenges

In order to maximize growth rates, still water environments with high nitrogen content are

necessary. The effluent from an anaerobic digester or ATL itself would be ideal for cultivating

duckweed. Duckweed requires still water with less than 1 m/s velocity [7]. To suppress algae

growth, odour, and mosquito breeding, complete surface cover is essential. Thus, it is

recommended that only 1/10th

of the surface area be harvested on a daily basis. In addition,

duckweed cultivation is water-intensive which may not be an issue if the wastewater from the

ATL were used as the source of water. Water level and acidity must be controlled. Finally, high

concentrations of fatty acids, oil, and grease are known to negatively affect duckweed growth by

http://en.wikipedia.org/wiki/Waste_water_treatment




http://en.wikipedia.org/wiki/Mosquitoes

68 Greenhouse

inhibiting nutrient uptake [7]. The optimum growth rate for duckweed may be realized at

temperatures between 25-31°C. However, as shown in Figure 22, duckweed shows a major

decrease in growth rate at temperatures <17° and >35°C [7].

Figure 22. Growth Rates [7]

Yield

With a doubling rate in optimal conditions of about 30 hours, duckweed reproduces nearly

twice as fast as most conventional crops. Duckweed has a potential yield of 13-38 tons/ha-year

[7]. According to Dr. Anne-Marie Stomp at North Carolina State University, duckweed yields

are on the order of 100 tons/ha-year. Dr. Stomp and Dr. Lam at Rutgers University have been

working on the conversion of the starch in duckweed to ethanol. Dr. Stomp stated that starch

from duckweed currently can’t compete with corn starch for the production of ethanol. Yet, it

could be sold as a protein source for $1,200/ton.

CH4 production using duckweed is a different story. Using estimated nitrate concentrations

in ATL’s FW of 1-5% and nitrate assimilation rates of 120-590 mg nitrate-N/m2

of duckweed

growing area/day, we estimate that from 21 to 504 acres would be needed for duckweed

production which would produce 0.3 to 8.4 billion L CH4/yr if the duckweed was fed into the

AD. Furthermore, in this scenario, nutrients would be retained and recycled in the system and

nitrate and phosphorus concentrations would build up to the point that they could be precipitated

out and sold as the fertilizer, struvite (NH4MgPO4). Since the products of this system are not

meant for human consumption, it may be very easy to divert ATL’s domestic wastewater stream

to the production of duckweed and AD.

69 Greenhouse

Hydroponica 7.5.Hydroponics is the method of growing plants in water without soil. Hydroponics systems

have been featured in Times Magazine as the World’s Best Invention and provides up to 20

times the yield of normal crops with 8% of the water typically used [28]. Plants are grown in

gravel, clay, or no soil at all [9]. Nutrients are dissolved in water and are pumped to the plants.

This system gives you higher product yield, healthier plants, better taste, longer shelf life and

less cost per pound of produce in comparison to conventional field gardening [22]. ATL could

either dedicate the entire greenhouse space to growing produce, or split the space to also

cultivate biofuel algae, camelina, or duckweed.

Using local organic hydroponics gardening, ATL could directly sell premium, freshly-grown

produce to restaurant vendors located in airport terminals. The Airport would offer higher

quality vegetables at a lower price and still make a profit because of the cost efficiency of

hydroponic gardening. In an optimum scenario, hydroponics gardening gives 100 times field

yield in produce. Additionally, ATL would gain prominence in the community for embracing

local, urban farming and supporting sustainability initiatives.

How it works

In hydroponics, crops are grown in a medium, such as sand, gravel, Perlite, or Vermiculite,

for the roots to hold onto, or no medium at all. In media based systems, roots are periodically

soaked with a nutrient solution dissolved in water that is pumped to all the plants. This nutrient

solution is a selected blend of chemicals in the correct proportions for plant food. Table 15

shows the 15 elements considered essential for plant growth [12].

Table 15: Selected crops for plant growth

Element Atomic Symbol Atomic Weight % of dry plant tissue

Carbon C 12.01 45

Oxygen O 16.00 45

Hydrogen H 1.01 6

Nitrogen N 14.01 1.5

Potassium K 39.10 1.0

Calcium Ca 40.08 0.5

Magnesium Mg 24.30 0.2

Phosphorus P 30.97 0.2

Sulfur S 32.06 0.1

Chlorine Cl 35.45 0.01

Iron Fe 55.85 0.01

Manganese Mn 54.94 0.005

Boron B 10.81 0.002

Zinc Zn 65.38 0.002

Copper Cu 63.55 0.0006

Molybdenum Mo 95.94 0.00001

70 Greenhouse

Many blends of hydroponic plant food are available depending on whether the vegetables

grown are vegetative (spinach, lettuce) or fruit bearing (tomatoes, peppers). Nutrient

concentrations need to be lower when the plants are bearing fruit. A typical gallon of plant food

costs $15 and a large bag of medium (12 gallons) will cost $20-50 depending on the medium.

Hydroponics Cost Efficiency

Of course it sounds a little ridiculous that hydroponics can increase plant yield by 100%, so

we’ll review the significant factors individually. These factors are density, crop harvests per

year, temperature, light, and diseases [14], [20].

Density

The most important and easily changed factor in increased yield with hydroponics is how

close the plants are placed, or plant density. It is simple to increase density by (1) growing plants

closer together, (2) eliminating walk space in fields, (3) training plants to grow vertically, (4)

growing plants in layers.

An acre is 43,560 ft2 and yields an approximation of 30,000 heads per crop. This means

each plant is spaced 12 inches away from each other. For cucumbers and tomatoes, the plants

are normally spaced 4-6 feet apart. In hydroponics, the plants could be trained to grow vertically

and spaced 24 inches apart, increasing density by a factor of 4-6. Then if we make use of all

three dimensions and have vertical gardens and grow in layers, we can increase density even

more [20].

For some of the more popular vegetables, the density increase factors are listed in Table 16.

Table 16: Density increase for selected crops

Crop Density Increase Factor

Lettuce 8

Cucumbers 6

Tomatoes 4

Crop Harvest per Year

The second most significant factor to increase yield, is to increase the number of harvests

per year (Table 17). Many field crops have one harvest per year, while other crops like broccoli

and carrots have two harvests per year while radishes and bean sprouts have multiple harvests

per year. Crops grown outdoors have limited harvests because of the frosts in the spring or fall

that kill baby plants, therefore many field crops are limited to one harvest per year. However

hydroponics crops are grown indoors. The plants can be grown all year long and growth cycles

can mature without a threat of frost. For example, if a crop takes 90 days to mature, you can

have 4 times the yield from four rounds of 90 day crops [20]. T

71 Greenhouse

Table 17: Yield for selected crops

Crop Crop harvests

per year

Increased yield factor

with hydroponics

Bean Sprouts 26 2-3

Radishes 12 2-3

Lettuce 8 4-8

Beets 6 3-6

Peppers 5 2-5

Tomatoes 4 2-4

Parsnips 3 2-3

Temperature

The environmental temperature of the plant is quite significant to plant growth and

influences the number of days different plants will take to finish germinating and emerge. The

data below is from the University of California Department Of Vegetable Crops and explains the

days it takes for different plants to emerge at different temperatures (Table 18). The data was

originally taken in ºC and I have converted that to ºF (written in parenthesis).

Table 18: Days for seed to emerge at different temperatures (temperatures in degrees centigrade)

Crop 0

(32)

5

(41)

10

(50)

15

(59)

20

(68)

25

(77)

30

(86)

35

(95)

40

(104)

Asparagus --- 52.8 24.0 14.6 10.3 11.5 19.3 28.4

Bean , lima --- --- --- 30.5 17.6 6.5 6.7 --- ---

Bean, snap --- --- --- 16.1 11.4 8.1 6.4 6.2 ---

Cabbage --- --- 42.0 16.7 9.7 6.2 5.0 4.5 4.6---

Carrot --- 50.6 17.3 10.1 6.9 6.2 6.0 8.6 ---

Cauliflower --- --- 19.5 9.9 6.2 5.2 4.7 --- ---

Celery --- 41.0 16.0 12.0 7.0 --- --- --- ---

Corn --- --- 21.6 12.4 6.9 4.0 3.7 3.4 ---

Cucumber --- --- --- 12.0 6.2 4.0 3.1 3.0 ---

Eggplant --- --- --- --- 13.1 8.1 5.3 --- ---

Lettuce 49.0 14.9 7.0 3.9 2.6 2.2 2.6 --- ---

Okra --- --- --- 27.2 17.4 12.5 6.8 6.4 6.7

Onion 135.8 30.6 13.4 7.1 4.6 3.6 3.9 12.5 ---

Parsley --- --- 29.0 17.0 14.0 13.0 12.3 --- ---

Parsnip 171.7 56.7 26.6 19.3 13.6 14.9 31.6 --- ---

Pea --- 36.0 13.5 9.4 7.5 6.2 5.9 --- ---

Pepper --- --- --- 25.0 12.5 8.4 7.6 8.8 ---

Radish --- 29.0 11.2 6.3 4.2 3.5 3.0 --- ---

Spinach 62.6 22.5 11.7 6.9 5.7 5.1 6.4 --- ---

Tomato --- --- 42.9 13.9 8.2 5.9 5.9 9.2 ---

Turnip --- --- 5.2 3.0 1.9 1.4 1.1 1.2 ---

Watermelon --- --- --- --- 11.8 4.7 3.6 3.0 ---

Light

Light, especially in multi-layered hydroponics systems are very important. A 50% reduction

of light stunts plant growth by 20%. Plants prefer light between 360-760 nanometers. Plant

72 Greenhouse

photosynthesis is especially responsive to blue 430 nanometer and red 660 light. Plant

germination, flower, and stem growth is also responsive to red light 660-735 nm light. Therefore

for young plants and leafy vegetables, metal halide lights ($11-$45) could be used to increase

growth. For fruits and flowering plants, high pressure sodium lights ($20-$45) could be used

[14], [20].

Diseases

Most plant diseases are soil-born microbes. As hydroponic plants are grown in media or

water, 100% of the soil-born plant illnesses are eliminated. In addition, no soil-born pests or

weather damage harms the plant since the plants are grown indoors in a controlled environment.

In outdoor crops, there is an estimated 33% loss in crop harvest due to insects, birds, mammals,

weeds, and pathogens. This loss rises to 40% without pesticides. However, water borne diseases

and root rot from over watering are especially damaging to hydroponic systems if the systems

are not managed correctly. Growing medium should always be sanitized between each crop to

eliminate carryover diseases [9], [14].

Other factors

Other significant effects that influence hydroponic crop yields in comparison to field crop

yields are CO2 level, fertilizer, pollination, and adequate water supply. By controlling these

factors in an indoor setting, each factor increases the yield by 1-2 times [8]. Other factors

include: relative humidity, pH, oxygen received by the roots, night temperatures, hours of light,

etc [20]. Yield increase factors are shown in Table 19. By multiplying all these factors together,

the field yield can easily exceed 100 times by using hydroponics [20].

Table 19: Factors increasing the total yield

Factor Yield increase factor

Plant density 4-16

Crops per year 2-8

Crop variety 2-4

Carbon dioxide level 1-2

Fertilizer 1-2

Light level 1-2

Pollination 1-2

Temperature 1-2

Water 1-2

Others 1-2 each

Vegetable Analysis

As previously explained, hydroponic gardening has highly promising potential and

efficiency to maximize produce yield. Grown without external environmental stresses and larger

water content, hydroponics produce has a longer shelf life, fresher taste, and healthier plant.

73 Greenhouse

This produce can then be sold to vendors and used locally in the 63 different restaurant vendors

at the Airport.

Of these restaurants, the ones listed in Table 20 are most likely interested to be consumers

for produce locally grown at the airport. These restaurants are dedicated to fresh ingredients,

nutritious meals, or prepare their dishes fresh daily. However, note some of the listed restaurants

are franchises and have policies which they must follow regarding the third party vendors they

buy produce from.

Table 20: Possible vegetables for selected restaurants at the airport

Restaurant Possible Vegetable interests

Asian Chao Sugar snap peas, bok choy, broccoli, ginger, onion

Atlanta Bread Company Lettuce, cucumber, avocado, tomatoes

Au Bon Pain Lettuce, tomatoes, onion, ginger, cucumber, carrot

Bistro Del Sol Lettuce, cabbage, carrot

Blues and Brews Lettuce, potatoes

Charley’s Steakery Lettuce, potatoes, tomato, cucumber,

Chick-fil-A Potatoes, lettuce

Famous Famiglia Tomatoes, garlic

Great Wraps Lettuce, potato, tomatoes, carrots, cucmber

Moes Tomatoes, Avocado, lettuce, Cucumber, pepper

Nature’s Table Tomato, lettuce, avocado, cucumber

One flew south Lettuce, tomato, avocado, cucumber, ginger

Sweet Water Draft House and Grill Potatoes, tomatoes, lettuce, onion

Wall Street Deli Cucumbers, onion, tomato, avocado, lettuce

Wolfgang Puck Lettuce, cucumber, tomato, carrots

The top vegetables used are lettuce, tomatoes, and potatoes. However, hydroponic

gardening is also good for cucumber, onions, bell pepper, garlic, spinach, and sugar snap peas.

Table 21 shows America’s most popular vegetables [18], [24].

74 Greenhouse

Table 21: Most popular vegetables in the US ([11], [13], [15], [16], [19], [21], [25], [26])

Vegetable Retail

price

(cents/lb)

Days

needed to

germinate

Days

need to

mature

Total

days

Plant

spacing

(inches)

Yield

for a

100 ft

row

Height

of

plant

in feet

US

consumption

in pound per

person per

year

Beans 69 7 50 57 4 48 2 1.1

Bok Choy 89 4 45 49 8 150 1 .5

Cabbage 29 4 65 69 12 150 1 5.4

Carrots 35 6 60 66 2 75 1 8

Cucumber 33 3 55 58 18 400 2 4.4

Lettuce 69 3 70 73 10 120 1 21.2

Onions

(green)

133 6 70 76 2 50 1 .5

Onions

(white)

56 6 100 106 4 100 1 12.4

Peas 369 6 65 71 4 64 2 .1

Peppers

(green)

59 8 60 68 12 600 3 3.4

Potatoes 28 9 90 99 10 240 2 47.2

Potatoes

(sweet)

49 7 100 107 12 240 2 4.8

Spinach 79 5 45 50 4 10 1 .8

Tomatoes

(cherry)

139 6 55 61 18 500 3 08

Tomatoes

(regular)

49 6 90 96 18 1000 10 15.6

Table 22 shows the production rate of some of these vegetables if grown hydroponically.

The first column is the produce. The second column is the number of plants grown on each acre

in the thousands. To understand the scale, remember that an acre is 43,560 ft2, so if there is a 44

in that column that means there is one plant/ft2, and 174 means 4 plants/ft

2. This relates to the

density of each plant.

The third column is the retail value for the crop. The fourth column is the number of harvest

rounds per year and the fifth column is the yield/acre-year. In comparison, the sixth column

compares the yield per acre per crop in traditional farming. This takes data from the previous

table, by scaling up the yield from a 100ft row to the entire acre, and dividing by 2000

pounds/ton. The last row is the number of grams of edible good grown in a day on every square

meter. This number is useful to estimate the growing area required to support the restaurant

vendors in the airport.

75 Greenhouse

Table 22: Most popular vegetables from Hydroponics analysis

Vegetables Plants per

acre

(thousands)

Retail

value for

crop

(thousand

dollars)

Number

of

harvest

rounds

per year

Yield

per

acre

per

year

Tons per acre

per crop

(projected)

Grams of

edible food

grown per

square meter

per day

Beans 392 43 6.4 200.8 31.4 123

Bok Choy 98 87 7.4 365 49 224

Cabbage 44 19 5.3 172.8 32.7

Carrots 1568 69 5.5 542 98 333

Cucumber 19 38 6.3 365.5 58.1 224

Lettuce 63 43 5.0 156.8 31.4 96

Onions

Green

1568 174 4.8 313.8 65.3 193

Onions

White

392 77 3.4 225 65.3 138

Peas 392 309 5.1 215 41.8 132

Peppers

Green

44 58 5.4 163.4 49 162

Potatoes 63 35 3.7 231.3 62.7 142

Potato

(sweet)

44 51 3.4 178.3 52.3 110

Spinach 392 10 7.3 47.7 6.5 29

Tomatoes

(cherry)

19 202 6.0 434.4 72.6 267

Tomatoes

(regular)

19 142 3.8 552.1 145.2 339

In Table 23, we have a comparison study in yield between crop yield in the field and

hydroponically grown produce in facilities from California and Abu Dhabi studies.

Table 23: Yield comparison ([17])

Vegetables Conventional

Field Yield

in tons per

acre per crop

Hydroponics

yield in tons

per acre per

year

Beans 2.4 128

Cabbage 10.8 207

Cucumber 54 414

Lettuce 10.8 31.4

Tomatoes

(regular)

19 142

As we can see, hydroponics creates more produce than conventional farming. Hydroponics

uses less water and less fertilizer, but creating the system in the beginning is expensive. You are

essentially building a high tech green house. On the other hand, ATL has the advantage in that it

has an available market for its produce via the ATL’s restaurant vendors.

76 Greenhouse

While there are a variety of hydroponic systems and kits available online. In Atlanta, the

company Pod Ponics has refitted shipping containers and grows lettuce using hydroponics. Each

shipping container grows 1.5 acres worth of lettuce. As they are already working with the ATL,

perhaps they can give advice for systems to buy and ways to set up multi-layered hydroponic

systems. Pod Ponics has perfected their system of hydroponics such that they guarantee the

exact amount of lettuce available to vendors [22].

Online, large scale kits and systems are priced at $300,000 dollars for a half acre system.

This includes testing equipment, growing supplies, piping, pumps, growing trays, stands, air

circulation, heaters, computerized environmental controls, electronic panels, cooling systems,

vent doors, and the structure itself [10]. Structures can be built to handle heavy snow loads and

high winds while maximizing the amount of light entering. Some models on the market have the

strength to withstand 90 mph winds and 12 lb snow load [10].

Another reputable model is the VertiCrop system which was mentioned in the Time

Magazine as 2009’s World’s Greatest Invention. This system has been used at England’s

Paignton Zoo to produce 11,000 heads of lettuce (and red chard and other vegetables) every 3-4

weeks to cut animal feed costs by over $150,000 annually and act as an educational tool for the

public. This computerized system uses a conveyor system to automatically irrigate plants. A

200 m2 unit costs $412,000. However VertiCrop has not finished creating a system for root

crops or big tomatoes [27], [28].

Conclusion and Summary

While the benefits have been clearly spelled out in this proposal and are very notable, the

disadvantages must be addressed as well. While hydroponics saves cost, saves resources (water

and nutrient use), maximizes yield and produces higher quality produce, the system has an

expensive upfront cost and will have an electricity bill to run the hydroponics system. Growing

medium and nutrients will also need to be bought continuously. Another popularly voiced

disadvantage to hydroponics is maintenance. While some systems need a lot of daily

maintenance, many systems (such as VertiCrop) are automated and need no maintenance at all.

The tradeoff is cost. Another disadvantage to keep in mind is that while hydroponics eliminates

all soil borne diseases and pests, waterborne diseases and pathogen attacks because of over

watering and over dampness will damage the entire plant system, so sanitation and watering

plants at the correct levels is important. Again, a computer automated system can take care of

the watering levels.

Therefore, the only disadvantage not canceled out by an automated computer controlled

hydroponics system is the cost. As explained in this proposal, the hydroponics systems create an

incredible amount of produce by maximizing crop yield and quality. However, looking at the

retail price of the crops and tons produced, the Airport could make a profit by matching the need

and selling directly to the restaurants located in the airport, as well as to the restaurants in the

Atlanta community. Since hydroponic gardening maximizes space so efficiently, in actuality

only a small amount of space would be needed to fill the need of airport restaurants. The

additional space could be saved for other sustainability projects, development, or more produce

77 Greenhouse

to sell to the community. Built on current technology, the hydroponics vegetable garden could

be the most feasible options for the green house space for the ATL.

78 Greenhouse

Aquaponics 7.6.Another process that Dr. Van Ginkel spoke about at the ATL sustainability fair is

aquaponics. Aquaponics merges aquaculture (fish farming) and hydroponics (growing food

without soil) into one system (Figure 23). The only regular inputs to the system are fish feed,

oxygen, and base for pH adjustment. The fish feed supplies food for the fish and the fish waste

provides food for the plants. The fish waste comprises solids which are removed via

clarification, and ammonia which is nitrified to nitrate which is taken up by the plants. The two

systems work well together since the same water is cleaned and recycled which reduces water

consumption.

Figure 23. Aquaponics – the fish tanks are under the shelter at the top of the picture and the hydroponic tanks are

the long rectangular shapes.

If ATL chooses AD or composting to treat their FW, there will be a lot of nutrients passing

through the system that can be used to grow vegetables hydroponically. However, an alternative

method for treating ATL’s FW and even all of the MSW is to feed it to black soldier fly larvae

(BSL). The cultivation of BSL has been conducted only at small scales. However, a current

project at Georgia Tech called ArkFab (www.arkfab.gatech.edu) plans to use BSL and

earthworms to treat 0.5 tons FW/day which is about 1/40th

of ATL’s FW. BSL are ravenous

eaters of waste including anything compostable such as paper. We estimate that if fully utilized,

BSL could convert 0.5 tons FW into 0.1 tons of BSL pupae which can be used as fish feed after

rendering to remove the chitin [30]. This BSL yield of 0.2 lb larvae/lb FW is remarkable in the

wastewater treatment industry since most microbial processes yield about 0.4 lbs of microbial

sludge/lb of waste treated. The chitin could be sold as well. The key is to have an ongoing

supply of larvae and eggs to immediately and evenly inoculate the FW as it arrives. Some

researchers have proposed a conveyor belt such that fresh FW comes in on one belt and meets

with digested FW containing BSL eggs. An important thing to note is that pupae are self-

79 Greenhouse

harvesting. In the wild, BSL naturally climb to a high point to pupate. At this stage, they are

done eating and lives as adults with the sole intention of breeding. Current models of larvae

harvesting just have the larvae climb up an incline and drop into a bucket or trough.

We intend to build ArkFab next month at Truly Living Well’s Wheat St. Farm site just east

of downtown next to the Martin Luther King Jr. Center. The purpose of ArkFab is to help help

cure food deserts in the inner city of Atlanta. According to the U.S. Department of Agriculture

(USDA), a food desert is considered an area where people don’t have access to healthy food

either due to low income or mobility. Using USDA’s food desert locator, we determined that

within a 1 mile radius of the Martin Luther King Jr. Center, 7,778 residents live in a food desert,

4,036 children in the community have difficulty obtaining healthy food, and 705 seniors in our

community also have difficulty obtaining healthy food (Figure 24).

Figure 24. The food desert within 1 mile of the Martin Luther King Jr. Center.

At ArkFab, we will initially use commercial fish feed as we test the BSL system. We also

intend to use duckweed as a fish food. Table 24 shows our balance sheet after the 1st year of

operation which is modeled after [31, 32]. ArkFab will be enclosed in a 30’ by 66’ greenhouse.

This is a very small area for producing a very large amount of healthy food. It can be shown that

the total benefit will be around $450 per tons of FW in a 10-year project.

80 Greenhouse

Table 24. Expected costs and revenue after the 1st year of ArkFab

Costs (1st year) Amount

Greenhouse $5,000

UVI aquaponic system $18,000

Mushroom Facility $5,000

Startup costs $2,000

Student Stipends $10,000

Revenue (1st year) Amount

Tilapia (2,266 lbs/yr; $5/lb) $11,328

Lettuce (41,000 heads/yr, 1 lb/head, $3/lb) $121,654

Balance (1st year) Amount

Costs $40,000

Revenue $132,982

Return on Investment 232%

81 Greenhouse

References 7.7.1. Aden, Andy and Ray, Davis. "Algal Biofuel Pathway Baseline Costs." National Renewable Energy

Laboratory (2011). Web. Mar.12 2012 <http://www.nrel.gov/biomass/pdfs/algal_biofuels.pdf>

2. Wiley, Patrick, J. Elliott Campbell, and 2 McKuin^1. "Production of Biodiesel and Biogas from Algae:

A Review of Process Train Options." Water Environment Research (10614303), 83.4 (2011): 326-338.

3. THAYER, ANN. "Moving Forward on Algal Biofuels." Chemical & Engineering News, 88.29 (2010)

4. "The Scoop on Camelina Sativa." Biobased Products Institute - Montana State University (2008). Web.

Mar. 2012 <http://www.montana.edu/biobased/projects/CamelinaInfo.html>

5. Pilgeram A.L., Sands D. C., Boss D., Dale N., Wichman D., Lamb P., Lu C., Barrows R., Kirkpatrick

M., Thompson B., and Johnson D. L."Camelina sativa, A Montana Omega-3 and Fuel Crop" Issues in

new crops and new uses. (2007). Web. Mar. 2012

<http://www.hort.purdue.edu/newcrop/ncnu07/pdfs/pilgeram129-131.pdf>

6. Walsh, D. " Duckweed a possible solution to energy needs, researchers say " press of Atlantic City

(May 3, 2010). Web. Mar. 2012

<http://www.pressofatlanticcity.com/news/press/cumberland/article_4ae59e12-56f1-11df-a5d8-

001cc4c002e0.html>

7. Iqbal, S. "Duckweed Aquaculture" Dept. of Water & Sanitation in Developing Countries, SANDEC,

Swiss Federal Institute for Environmental Science & Technology, EAWAG, Ueberlandstrasse 133, CH-

8600 Duebendorf, Switzerland, (March 1999). Web. Mar.2012

<http://www.eawag.ch/forschung/sandec/publikationen/wra/dl/duckweed.pdf>

8. "Advanced Nutrients." Hydroponics Gardening. Web. 08 Apr. 2012.

<http://advancednutrients.com/hydroponics/articles/hydroponics-gardening/hydroponics-tips-for-

increasing-yield.php>.

9. "Advantages and Disadvantages of Hydroponics." Crescent Girls' School. Web. 08 Apr. 2012.

<http://www.crescent.edu.sg/ipw/2000/sec2/22g1-hydroponics/ad_dis.htm>.

10. "CropKing Green House Package: NFT System." CropKing. CropKing. Web. 8 Apr. 2012.

<http://www.cropking.com/sites/CropKing.com/files/pdfdocs/NFTPricing.pdf>.

11. D. Newcomb, 'The Backyard Vegetable Factory', Rodale Press, 1988.

12. F.B. Salisbury, C.Ross, "Plant Physiology", Wadsworth, 1969, p.194.

13. Fruit & Vegetable Facts & Pointers', United Fresh Fruit and Vegetable Association, Alexandria, Va.

14. "GreenCoast Hydroponics - Information." GreenCoast Hydroponics, Hydroponic Systems. Web. 08

Apr. 2012. <http://www.gchydro.com/information_introhydro.asp>.

15. Growing Vegetables in Home Gardens', USDA, USGPO.

16. Gurney's 1990 Spring Catalog.

http://www.nrel.gov/biomass/pdfs/algal_biofuels.pdf

http://www.eawag.ch/forschung/sandec/publikationen/wra/dl/duckweed.pdf

82 Greenhouse

17. H.M. Resh, 'Hydroponic Food Production', Woodbridge Press, 1987.

18. "Hydroponic Plants: What to Grow." Hydroponic Gardening Simplified. Web. 08 Apr. 2012.

<http://www.hydroponics-simplified.com/hydroponic-plants.html>.

19. J.F.Harrington, Dept of Vegetable Crops, UC Davis, Agricultural Extension Leaflet, 1954.

20. "No Food = No Problem!" Hydroponics Gives 100 times Field Yields. Web. 08 Apr. 2012.

<http://www.androidworld.com/prod26.htm>.

21. O.A. Lorenz, D.N. Maynard, 'Knott's Handbook for Vegetable Growers', 2nd ed., John Wiley & Sons,

1980.

22. "PodPonics: The Future of Farming." PodPonics: The Future of Farming. Web. 09 Apr. 2012.

<http://www.podponics.com/>.

23. "Restaurants." Shop, Dine and Explore. Hartsfield-Hackson Atlanta International Airport. Web. 08

Apr. 2012. <http://www.atlanta-airport.com/Passenger/Shop/default.aspx>.

24. "The Best Vegetables to Grow Hydroponically." LIVESTRONG.COM. Web. 08 Apr. 2012.

<http://www.livestrong.com/article/217578-the-best-vegetables-to-grow-hydroponically/>.

25. S.E. Gebhardt, R.H. Mattews, 'Nutritive Value of Foods', USDA Home and Garden Bulletin #72,

USGPO, 1981.

26. 'Supply Guide', United Fresh Fruit and Vegetable Association, Alexandria, Va., 1989, compiled by

C.Magoon.

27. "Valcent Takes Vertical Farming to the Zoo | Fast Company." FastCompany.com. Web. 09 Apr.

2012. <http://www.fastcompany.com/blog/ariel-schwartz/sustainability/valcent-brings-vertical-farming-

zoo>.

28. “VertiCrop.” Verticrop. <www.verticrop.com>

29. "What Is Hydroponics?" Simply Hydroponics. Web. 08 Apr. 2012.

<http://www.simplyhydro.com/whatis.htm>.

30. Newton, L., Sheppard, C., Watson, D.W., Burtle, G., Dove, R. 2005. Using the black soldier fly,

Hermetia illucens, as a value-added tool for the management of swine manure. Animal and Poultry

Waste Management Center, North Carolina State University, Raleigh, NC.

31. Bondari, K. and Sheppard, C. 1987. Soldier fly, Hermetia illucens L., larvae as feed for channel

catfish, Ictalurus punctatus (Rafinesque), and blue tilapia, Oreochromis aureus (Steindachner).

Aquaculture and Fisheries Management, 18, 209-220.

32. Rakocy, J. 2006. Recirculating Aquaculture Tank Production Systems: Aquaponics—Integrating Fish

and Plant Culture. SRAC Publication No. 454

83 Biodiesel from Cooking Oil Waste

Biodiesel from Cooking Oil Waste 8.

Team members:

AliReza Khoshgoftar

Monfared

PhD student, Electrical Engineering

James Bourn III

Keith Wells


Introduction 8.1.Biodiesel is one of the most promising of the alternative fuels, which include bioethanol and

hydrogen. It is produced from vegetable oil using fairly simple chemistry. It is non-toxic and

rapidly biodegrades. It can be produced from either new or waste oil or offers good performance

together with a number of environmental benefits compared to ordinary petroleum diesel.

The conversion of waste vegetable oils (WVO) to biodiesel is a very cost friendly solution,

and the fact that it is made from renewable resources lowers our dependence on foreign oil and

helps our own economy. Biodiesel has also been shown to help reduce CO2 emissions, which

can help the global warming crisis. Biodiesel reduces net CO2 emissions by 78% compared to

petroleum diesel. This is due to biodiesel’s closed carbon cycle - the CO2 released into the

atmosphere when biodiesel is burned is recycled by growing plants, which later could be

processed into fuel.

Types of biodiesel 8.2.There are two different types of WVOs:

Brown grease: Brown grease is from stovetops and deep fryers. It is very thick, dark,

and filled with impurities. Brown grease production in the U.S./yr is 4-48 lbs/person and

3,000-24,000 lbs/restaurant. In terms of the biodiesel market, brown grease has little to

no value, although it could go to AD.

Yellow grease: Yellow grease is made up of the oils and animal fats that are used and

produced during cooking and deep-frying (Morea, Sept. 12). Its production range is 3-21

lbs/person-yr and 2,000-13,000 lbs/restaurant-year. This vegetable oil waste can be used

for rendering and has been established as a valuable commodity in the biodiesel market.

(http://www.nrel.gov/vehiclesandfuels)

Sources of biodiesel 8.3.Biodiesel has many sources. It can be made from virgin oil, produced mainly from soybeans

and rapeseed, waste vegetable oil, and animal fats like lard and tallow. Biodiesel can even be

made from the fatty acids found in algae.

Applications of biodiesel 8.4.As a fuel, biodiesel is very versatile; it can be applied to several different areas. An engine

that is designed to burn diesel fuel can be modified to burn pure biodiesel, known as B100. The

same diesel engine can burn an 80% diesel 20% biodiesel mixture with no modification at all,

known as B20. This is advantageous as it will help to keep emissions down and slightly increase

mileage. Pure Biodiesel, B100, can also be used as a replacement for home heating oil.

Advantages of biodiesel 8.5.One of the major differences between biodiesel and diesel is that biodiesel contains less

energy per gallon, but it burns more completely, making up for some of the loss due to the lower

energy density. Another advantage to biodiesel is its greater lubricating property than other

http://www.nrel.gov/vehiclesandfuels


fuels. This can reduce wear and prolong the life for engine parts that require the fuel as a

lubricant, such as fuel injectors. Biodiesel also burns cleaner; tests have shown a reduction in

unburned hydrocarbons, carbon monoxide and sulfur emissions, but a very slight increase in

nitrogen oxides. The flash point of biodiesel, the temperature at which the vapors above the fuel

become flammable, is about 100°C higher than that of diesel fuel. The higher flashpoint makes

biodiesel safer to handle and transport. A negative attribute of biodiesel is its high cloud point.

The cloud point is the temperature when a liquid starts to thicken and gel, this makes it difficult

to use in a regular internal combustion engine (Advantages of using Biodiesel, 2008). In

addition, biodiesel is also an effective solvent (Storing Biodiesel Fuels, 2007).

Biodiesel Production 8.6.

Batch Process

There are several ways to produce biodiesel from WVO. The most widely used way is the

batch process. The incoming oil must be filtered to remove any solids while the levels of free

fatty acids and water are being monitored; too much of either can cause problems later in the

production process. The catalyst, lye (NaOH), is mixed with the methanol, and then the

alcohol/catalyst mix is charged into a closed reaction vessel. Then the oil is added. The system

from here on is totally closed to the atmosphere to prevent the loss of alcohol through

evaporation. The vessel is then heated to between 55-70º C in order to speed the reaction up.

After 1-8 hours of mixing/reacting the reacted liquid is separated, the heavier glycerin

byproducts will sink, either by gravity or by centrifuge. After separation, the excess alcohol is

distilled away or evaporated and recycled. The unused catalyst is recovered from the glycerin

byproduct and is recycled (Blair, 2005). The glycerin is stored as it is a valuable commodity.

Then the biodiesel is washed with warm water to remove any residual impurities.

Ultrasonication

Ultrasonication is a newer way to produce biodiesel. The methanol and catalyst are pre-

mixed like in the batch process; this mixture is combined with the oil. The mixture is usually

heated to 50-60°C, while being hit with ultrasonic sound waves. The sound waves cause a

phenomenon called cavitation, where bubbles are randomly created and imploded with incredible

frequency. These cavitations provide both enough mixing and the needed activation energy for

the transesterification (the conversion of fatty acids to biodiesel) to take place in a much shorter

time than the batch process. Then, the reacted mixture is phase separated, washed and stored as

before (Hielscher,). This process has several advantages: as it is a continuous process, it is much

more energy efficient than the batch process and it takes much less time to create an equivalent

amount of biodiesel (Gogate, Kelkar, and Pandit, 2005).


Figure 25: Biodiesel Conversion Using Ultrasonication

One of the newest ways to produce biodiesel uses methanol in a supercritical state. A

substance in a supercritical state is neither a liquid nor a gas, but still retains properties of both.

In order to reach a supercritical state, the substance must be at a super high temperature and

pressure. For methanol, it must be over 650°F and 5000 psi. One of the main advantages is that

this process is incredibly fast. It is able to produce similar amounts of oil as the batch process in

as little as 6 minutes. Another advantage to using supercritical methanol is that the reaction is

completely spontaneous, meaning no catalyst is needed. The third advantage may be the

greatest; this process is much more tolerable of excess water and free fatty acids in the feedstock.

That means that it can accept and successfully react a much wider range of feedstocks as the

quality doesn’t need to be as high (Hegel, Mabe, Pereda, Brignole, 2007). There are negative

attributes to this process - startup costs would be huge as the equipment needs to be able to high

huge temperatures and pressures.


Cost Comparison of Processes 8.7.

Batch Process

The batch process is the most widely known biodiesel conversion process because a

rudimentary system is relatively inexpensive, and easy to operate. This makes the batch process

ideal for the people who only wish to produce enough fuel for their own use. However, the

versatility of the “batch process” easily allows for a production increase, making it the perfect

biodiesel conversion process for small companies.

Startup-cost: The main benefit of the batch system is the extremely low start-up cost

compared to other, more complex systems. A pilot plant consisting of 3 tanks, 2 pumps and the

respective hoses and fittings can cost well under $2000. Larger tanks and more powerful pumps

can be used to increase the production capacity while still keeping the price well under $5000.

Production costs are also more inexpensive than other biodiesel systems. Prices for legal permits

and insurance are the same or lower than other processes that require higher temperatures and/or

pressures. Raw material prices are generally fixed, but companies usually give a discount when

buying in large quantities.

Maintenance: Maintenance for the batch system is minimal. It requires replacing parts that

malfunction, which are inexpensive to purchase and replace.

Upgradability: Upgrading the system to increase the production capacity is as simple as

increasing the tank size and/or adding more tanks. However, as more and more tanks are added,

space does become a concern.

Efficiency: A downside of the batch process is the low efficiency compared to other, more

expensive systems. One can expect efficiencies between 85-90% for the typical batch process.

A methanol recovery system can also be implemented to recover almost 85% of the methanol

used in the process.

Personnel Requirements: Because of the volatility of the chemicals used and the

temperature required for transesterification, personnel must be present at all times. An employee

is expected to use the pumps to transfer the WVO/biodiesel between tanks, add specific amount

of chemicals, and control the temperature of the heating element. Training for personnel is

relatively straightforward, as most of the work being done is simple. Personal responsibility is

more essential to the job than any unique skill set.

Continuous Process

This process is very similar to the batch process, but has a few major differences. These

differences include a methanol recovery system and more tanks for mixing and settling

Startup-cost: This process is more costly compared to the batch process. Though no single

part of the process is overly expensive, purchasing the quantity of tanks to setup a proper process

can get costly. Although the startup costs are higher in this process, the cost of operation is

rather low. The methanol recovery system featured in this process will save on the purchase of

raw methanol and therefore lower production.


Maintenance: Maintenance costs are low due to the simplicity of the system and with most

of the parts available at any local hardware store.

Safety: The process uses corrosive materials; therefore protective gloves and possibly a

face shield are required at some point during the process.

Upgradability: This process is very upgradable. Parts such as the mixing components,

pipes, pumps, and filters can all be upgraded at any time when convenient to increase

productivity.

Efficiency: The efficiency can be split into two sub‐categories; yield and time/capacity.

For yield, the process overall has a percent yield of around 85%. The time for a single batch will

be equal to that of the regular batch process; however when multiple batches are in process much

more can be produced than the original process in a day.

Personnel Requirements: This process needs slightly higher requirements. To keep up

with the process, the operator will need to be able to effectively transfer multiple batches at a

time and orchestrate them in the proper sequence. This process does not require any prior

knowledge of the system to operate and mostly involves the opening and closing of valves. Only

simple process training would be required to educate one to effectively run the process.

Ultrasonication

Start-up Cost: Startup costs for an ultrasonic setup can be very large, as much as $100K.

A combination of lots of expensive mechanical and electrical equipment drives the cost up. The

operating cost to run an ultrasonic process is much smaller than one would think at first. While

being a very complicated process with lots of complex machinery, it is very energy efficient and

the process requires little labor for operation.

Maintenance: Maintenance on an ultrasonic operation could be very great. There is a lot

of complex equipment running at high temperatures and pressures that would require a certified

technician to repair. Other processes that have more simple components require only basic skills

to repair.

Upgradability: The ultrasonic process is difficult to upgrade. Once the process is set in

place it would need to be disassembled and put back together with new part and components.

Some of the more important parts such as the ultrasonic transducer could be used for a greater

capacity, if it was capable.

Efficiency: Ultrasonication, while being expensive, is very efficient. The percent yields

from the process are often greater than 98%. Ultrasonication is also capable of accepting a

greater variety of raw materials, meaning the WVO does not have to be filtered as extensively.

The ultrasonic process is much faster and more energy efficient than most other processes

available today. Due to the unique method of driving the transesterification process,

ultrasonication uses half the energy of the average batch process. It is also much faster; the

process can produce the same amount of biodiesel product in less than a quarter of the time.

Personnel Requirements: The stress level for the employees in an ultrasonic plant would

be much lower than in other production plants. The system is mostly automated and does not

require a major input from the operators.


References 8.8.1.Albergaria, Nikolas, et al. 2009. Biodiesel Process Analysis for EPOCA, An Interactive Qualifying

Project Report. WORCESTER POLYTECHNIC INSTITUTE. 2009.

2.Balfour, Senators. Georgia Biodiesel Assembly. Senate Bill 568.

3.Hartwig, Erika and Moore, Travis . 2006. An Assessment of the Restaurant Grease Collection

and Rendering Industry in South Carolina. Department of Energy, The SC Energy Office.

2006.

4.Wisner, Robert. 2009. Biodiesel Tax Credit. June 2009.

90 Energy Analysis

Energy Analysis 9.

Team members:

Kelly Nicholson PhD student, Chemical Engineering

Brandon Gardiner Masers student, Building Construction

David Repp Masers student, Civil Engineering

Kari Beasley Masers student, Industrial Engineering

Ashwin Ravi Sankar PhD student, Chemical Engineering

91 Energy Analysis

Introduction 9.1.The purpose of this Energy and Carbon Analysis is to characterize the proposed Energy Park

in terms of a mass and energy balance. The energy analysis seeks to provide a low level

calculation of the heat and power the Energy Park can generate based on available waste inputs

and also to answer the critical questions of whether or not the system is self-sustainable. Is the

Energy Park predicted to generate enough power and heat to run its individual units or is the

energy balance net negative? The carbon analysis compares waste incineration to the tons of

carbon emitted to the atmosphere by diverting waste to the Energy Park. The nature of the

analysis as a first look at mass and energy flow rates necessitates the use of many simplifying

approximations which are detailed throughout the report.

Waste Characterization 9.2.ATL completed a comprehensive Waste Characterization Report in 2007 based on a study of

2006 generation levels. Assumed waste inputs and compositions are based primarily on this

report. The airport typically generates over 23,000 tons of municipal waste (garbage not

including hazardous wastes) annually. Only 3% of that waste is recycled even though up to 40%

is likely recyclable. Most of the solid waste is generated at the Central Passenger Terminal

Complex (terminal and six concourses), including concessionaires, restrooms, administrative

offices, ticketing counters, and airplane cabin waste. The annual cost to the airport for waste

disposal is slightly >$1M [1, 2].

The Waste Characterization Report determined the composition of the MSW using both

visual and partial sorting approaches. Waste was assigned to 21 categories such as corrugated

cardboard and FW. Representative waste containers were sampled to determine the relative

volumes of waste in each category present in the container. Density factors were applied to each

category of waste to estimate the composition of airport waste by weight. Table 1 provides the

relative tonnage of food, paper, plastic, and other waste generated by the airport that can be

considered the material inputs to the Energy Park [2].

Table 25: ATL Municipal Waste Categorization [2, 3]

Waste Type weight % tons/yr

FW 28.4 7,100

paper (cardboard, newspaper, office paper, other paper) 48 12,000

plastics 12.4 3,100

other (wood, aluminum cans, steel cans, glass, etc.) 11.1 2,775

total (approximate)

25,000

gal/yr

waste cooking oil

23,000

The FW was noted to contain large amounts of melted ice, water, and other beverages [2].

The Los Angeles World Airports (LAWA) and Los Angeles Environmental Engineering

Division determined that samples of airport FW contained 73% moisture and 27% TS of which

92 Energy Analysis

94.9% were volatile (organic) solids [4]. It is assumed that ATL FW is equivalent to that of

LAWA. ATL concessionaires used over 60,000 gallons per year of cooking oil, but only 30,000

gallons were collected and recycled, of which only 23,000 gallons were actually suitable for

conversion to biodiesel. Thus, this report considers only 23,000 gallons/year of waste cooking

oil to be available for conversion to biodiesel in the Energy Park. However, improved collection

and recycling efforts at the airport could increase this substantially.

This report also does not consider conversion of plastics or “other” waste input streams for

processing in the Energy Park. The airport is currently working to improve recycling initiatives

which may resolve these waste streams. Thus, the total MSW input to the Energy Park is 19,100

tons/year and 23,000 gallons/year waste cooking oil.

Description of Proposed Energy Park 9.3.Figure 26 depicts the overall Energy Park flow sheet. Only major feed and output streams

are shown. The proposed Energy Park has five unit operations: transport, AD, CHP,

composting, biodiesel generation, and plant production (vegetables/energy crops). Of these, the

AD and CHP units are considered the Energy Production Train because the energy viability of

the park depends on the electricity and heat produced from these units being sufficient to sustain

operations of all other units. Sorting of the waste through mechanical or automated means is not

considered as a separate unit operation because much of this could be accomplished through

adjustments to airport waste collection practices and policies.

In the Energy Production Train (AD and CHP), FW is digested by bacteria in a low oxygen

environment producing biogas and sludge or organic residue. The biogas which is largely CH4

and CO2 is fed to the CHP to generate electricity and heat (as hot water or steam). This heat

stream is used to keep the AD system at the required temperature and also to heat the waste

cooking oil in the biodiesel conversion process. The electricity is used to run the Energy Park

mechanical equipment.

Paper waste is combined with the heavy organic residue from the AD process in a

composting unit. Here, the organic materials are bio-degraded under aerobic conditions to

produce nutrient-rich compost to be either sold or used for ATL landscaping needs. Waste

cooking oil, considered to be yellow grease, is converted to biodiesel in the biodiesel conversion

unit. The biodiesel can be either sold or used in airport vehicles. Glycerin is a heavy by-product

of the biodiesel conversion process and has uses in the food and personal products industry.

Currently the glycerin market is flooded and so this product will likely be viewed as a waste

stream. [5]

A myriad of technologies for each unit operation was reviewed with the exception of waste

transport. The following technologies were considered for this energy analysis:

AD: single stage continuously stirred reactor (CSTR) under mesophillic reaction

conditions

CHP: gas turbine with steam heat recovery system and reciprocating engine with hot

water heat recovery system

93 Energy Analysis

Composting: aerated static pile

Biodiesel: ultrasonication with biodiesel purification using dry-settling

Figure 26: Proposed Energy Park Unit Operations

Energy Production Train 9.4.Although, in principle, all of the organic waste streams can be digested in a Dranco or

Valorga unit, it was decided to divert paper waste directly to the composting unit and the waste

cooking oil to a separate biodiesel conversion unit. This is likely because FW is more easily

digested and will reduce the size of the digester. Also, there are examples of AD units

processing FW currently in operation in the United States. For example, in 2010 the East Bay

Municipal Utility District’s (EBMUD) Main Wastewater Treatment Plant digests 150 tons of

FW/week and expects to double production [6].

94 Energy Analysis

The AD productivity characteristics used in this analysis were modeled after EBMUD’s

bench-scale test of FW conversion to CH4 in a mesophillic with and SRT of 10 days [7]. Actual

CH4 production rates will, of course, depend on reaction conditions and the chemical

composition of the FW, but this approximation is useful at this stage of the design process. CH4

production rates can also be boosted by increasing the reactor residence time, but this may

increase operational costs. The following are the key AD productivity characteristics assumed

for this study:

CH4 Production Rate: 9,500 ft3/dry ton FW loaded to reactor

CH4 Production Rate: 2,600 ft3/day/1000 ft

3 digester volume

Residuals: 31 wt % of solids fed to reactor

Biogas Composition: 60% CH4, 40% CO2 (consistent with EBMUD’s 59% CH4 finding

FW Composition: 73% H2O, 27% TS, 95% VS which is consistent with EBMUD

Flow Rates: 1621 lbs/hr total FW, 416.9 lbs/hr VS

Figure 27 shows the overall mass balance for the AD system for solids and biogas streams.

The water balance is not shown. The key energy demands of the system include heating the

slurry to 35°C, grinding the FW to pulp consistency, and separating the heavy residual solids

from water in the rotary press. The power requirements for these tasks are discussed below. The

AD system is an efficient process for producing CH4 from organic waste, particularly FW. Only

31% of the organic solids fed to the reactor exits as organic sludge or residual waste which can

be further processed into nutrient rich compost. The CH4 production rate is 3123 ft3/hr. This is

the key input to the CHP system.

Figure 27: Mesophillic CSTR Anaerobic Digester Solids and Biogas Mass Balance

Both gas turbines with steam heat recovery and reciprocating engine with hot water heat

recovery systems were considered as viable CHP technologies. The role of the CHP system is to

combust the biogas produced in the AD unit, generate electricity, and recover any waste heat for

use in Energy Park processes. Key performance indicators for a CHP technology are the Total

95 Energy Analysis

CHP Efficiency (unit energy output/unit energy input), the Power/Heat Ratio (electrical energy

output/heat energy output), and Electric Heat Rate (energy input/power output). Other details

include how the waste heat is recovered, i.e., as hot water or steam.

In this analysis, the performance parameters and heat recovery details were adapted from

system operating conditions listed in the EPA Catalog of CHP Technologies [8]. The system

parameters in the Catalog were obtained both from manufacturer specifications and industry

publications for commercially available systems. Heating rates were based on natural gas fuel

which contains more CH4 than biogas and, thus, produce more power per unit fuel input.

Because of this, electricity and heat outputs should be considered upper limits for basic systems.

Performance can be improved using additional technologies such as turbocharging or turbine gas

inlet preheating, but only simple systems based on the EPA Catalog were used here.

The biogas was assumed to have a higher heating value (HHV) of 805 Btu/ft3 biogas based

on 60% CH4 and 40% CO2 where CH4 has a HHV of 1000 Btu/ ft3 CH4. Note that the volumetric

ratio of biogas to CH4 was calculated to be 1.243 ft3 biogas/ ft

3 CH4 that leads to the 804.5 HHV

for biogas. Combining the biogas flow rate from the AD unit with the assumed HHV gives 3.1

MMBtu/hr of fuel energy input to the CHP unit. Better AD productivity or supplementing

biogas with another fuel like natural gas will boost this number. This is the maximum amount of

energy available for conversion to electricity and process heat.

Gas Turbine CHP System

The GT was sized based on the smallest system listed in the EPA catalog of approximately 1

MW electricity production [8]. This is much larger than what the Energy Park system can

generate, and an actual system will likely be less efficient than the model 1 MW example since

efficiencies typically improve as capacities scaled upward. However, this analysis is useful to

approximate the scale of the potential power and heat produced in the Energy Production Train.

In a gas turbine CHP system, the biogas and air are compressed then mixed and combusted. The

hot exhaust gases are expanded in a turbine which drives an electric generator. The exhaust gases

are then used in a boiler to generate steam. The calculated mass balance for the system is shown

in Figure 28. Key performance values used in determining the mass and energy balance for the

GT include the following:

Electric Heat Rate, HHV: 16,047 Btu/kWh

Total CHP Efficiency, HHV: 66.3%

Power/Heat Ratio: 0.47 Btu electricity/Btu heat

Steam Enthalpy: 1006.1 Btu/lb (150 psig saturated steam)

Complete Combustion of Biogas: 11.271 lb exhaust gas/lb biogas (exhaust gas

composition: 70% N2, 18% CO2, 12% H2O)

96 Energy Analysis

Figure 28: CHP Technology: Gas Turbine with Steam Heat Recovery System

The GT can generate approximately 195 kW electricity and 1400 lbs/hr of 150 psig steam

(1.4 MMBtu/hr) which could be used to heat the AD. GT technology is usually used for systems

larger than 1 MW which requires five times more fuel energy input than what is calculated to be

available from the AD unit [8]. Thus, GTs are likely not appropriate unless energy crops are

cultivated or more waste from nearby communities are used.

Reciprocating Engine CHP System

Similarly, the mass and energy balances were determined for a CHP system with a RE and

hot water recovery. This process was sized based on a 100 kW spark ignition system in the EPA

Catalog [8]. In a CHP arrangement, a biogas and air mixture are forced into a combustion

chamber, compressed, and ignited. The rapid expansion of gases drives an electric generator.

Heat from the engine jacket and exhaust gases are recovered via heat exchange with water. This

hot water can then be utilized for process heating needs in the Energy Park. Figure 29 shows the

calculated mass and partial energy balances for this system. Key performance parameters used

in the calculation include the following:

Electric Heat Rate, HHV: 12,000 Btu/kWh

Total CHP Efficiency: 79%

Power/Heat Ratio: 0.6 Btu electricity/Btu heat

Heat recovered from exhaust gases: 23% of fuel input energy

Heat recovered from engine jacket: 28% of fuel input energy

Complete Combustion of Biogas: 11 lb exhaust gas/lb biogas (exhaust gas composition:

70% N2, 18% CO2, 12% H2O)

97 Energy Analysis

Figure 29: CHP Technology: Reciprocating Engine with Hot Water Recovery System

A RE can generate 260 kW of electricity and 1.6 MMBtu/hr of hot water for use in the

Energy Park. This system has a much higher Total CHP Efficiency than the gas turbine

arrangement, meaning that it does a better job of converting the available fuel energy into useful

power and process heat. Less energy is vented to the atmosphere as exhaust gas. Overall, this is

the better choice for CHP technology for the Energy Park.

Utilizing the RE technology, an energy balance for the Energy Production Train is shown in

Figure 30. The heating value of the FW was assumed to be 2561 Btu/lb wet or 9958 Btu/lb dry

solids [9]. Approximately 75% of the FW energy is converted to biogas which is then converted

via the RE to electricity (21%), steam (38%), and exhaust gas waste heat (16%), leaving 25% in

the compost. The upper limit on the overall efficiency of the Energy Production Train in

converting FW energy to electricity and usable heat is about 60%. This does not include heat

and power requirements of the Energy Park, namely heating the AD slurry and powering

equipment. From another perspective, 84% of the FW is converted to useful products with only

16% as waste heat.

Figure 30: Energy Production Train Energy Balance on Carbon Streams with Reciprocating Engine CHP

Technology (Normalized to Input FW Stream)

98 Energy Analysis

Heat and Power Requirements of the Energy Park 9.5.The Energy Park produces heat and power in the Energy Production Train. However, the

equipment requires some heat and power to sustain the process. The largest loads include

maintaining the AD slurry at temperatures suitable for digestion, grinding the FW to pulp,

grinding the paper waste and air blowers for the composting unit, separating water from heavy

solids in the AD system before sending the solids to compost, and heating of the waste cooking

oil in the Biodiesel Conversion unit. Other loads include transporting the waste to the Energy

Park, sorting of the waste, and purifying the biogas after the AD system before combustion in the

CHP system.

Because full sizing of equipment and determination of the loads are beyond this work,

comparisons are made to a 2011 study completed by Banks et al. in which the energy and mass

balances for a similar Energy Production Train were tracked for 426 days in 2007-2008 [10]. In

this unit, domestic FW (95.5%), commercial FW (2.9%), and grass cuttings (1.6%) were fed to

an AD to produce biogas and digestate. The biogas was sent to a CHP unit, and the digestate

was pasteurized for use on local farms. The overall capacity of the plant is about half of the size

of the proposed ATL Energy Park based on the plant’s FW throughput of 3,936 metric tons over

426 days. Although some of the operating parameters are different such as a higher operating

temperature of 42°C, lower solids loading rate, and larger biogas conversion rate, the key loads

are similar: raw waste shredder, macerators, feed pumps, compressor pumps, CHP and boiler

feed water pumps, belt press, air filtration and minor ancillary equipment such as conveyors. In

the study, 30% of the gross electrical output was used to power the plant and 30% of the

recovered heat in the CHP unit was used for process heating in the plant. Since the 2011 study

and the proposed Energy Park involve similar processes and similar scales of FW throughput,

these heat and power requirements are reasonable to apply to the proposed Energy Park Energy

Production Train as a first approximation.

However, the 2011 study did not include Biodiesel Conversion and Compost conversion

units. Some equipment for these includes grinders for the paper waste, an air blower for aerating

the compost, and heating mechanisms for the waste cooking oil. Scenario 1 for composting

assumes that only paper waste and sludge from the AD units are the mass input streams available

to the composting unit. Based on a 30 horsepower (hp) hammer mill for the paper waste, 23 kW

electricity will be required for the tonnage of paper waste supplied to the composting unit [11] .

An air blower will require only a few kW at most and is a minor consumer. Scenario 2 for

composting assumes that all compostable MSW is used. For a waste input stream to the

composting unit of approximately 20,000 tons/yr, the size of the hammer mill increases to

approximately 40 kW based on a 50 hp machine [11]. This would consume about 15% of the

electricity output from the reciprocating engine CHP calculated in this analysis.

Heating requirements for the biodiesel process will require further study, but is not expected

to be encumbering since the volume of waste cooking oil is much less than the volume of FW

and water in the AD unit which only requires 30% of the heat produced in the CHP unit.

99 Energy Analysis

Biodiesel Conversion Unit 9.6.The airport concessionaires generate 23,000 gal/yr of waste cooking oil. Using

ultrasonication and dry settling is suggested to process the waste cooking oil into biodiesel and

glycerin. Biodiesel as an alternative fuel has numerous benefits including less smoke

production, fewer particulate emissions, lower CO emissions, a higher octane number, and

biodegradability [12]. Traditionally the biodiesel conversion process has used batch reactors that

are limited in the degree of mixing that can be achieved between the oil and the catalyst/alcohol

reactants. These mass transfer limitations have led to lower yields. However, sonochemical

reactors enhance mass transfer and interphase mixing. This not only improves biodiesel yields,

but can reduce the severity of operating conditions.

Several studies of the ultrasonication of waste cooking and vegetable oils have been

completed focused on determining optimum operating conditions [12, 13]. These studies have

yielded conversion rates in the range of 85-99%. However a detailed mass and energy balance

for the biodiesel conversion process requires more understanding of the composition of the waste

cooking oil, in particular free fatty acid content and triglyceride composition. Therefore, only a

mass balance based on 100% conversion of the waste cooking oil is provided in Figure 31 using

typical reaction conditions of a 6:1 molar ratio of methanol to oil and 1% potassium hydroxide

(KOH) catalyst concentration [14]. Because an energy balance is not appropriate at this time,

market values for each of the inputs and outputs are provided to give an idea of the “value-

added”. Biodiesel prices were taken from the January 2012 edition of the Clean Cities

Alternative Fuel Prices Report which reports both regional and average prices for biodiesel for

the U.S [15]. The average value was used. Prices for the other inputs and outputs were taken

from ICIS.com which provides historical prices for many commodity chemicals [16].

Waste Cooking Oil: $0.26/lb

Biodiesel: $4.20/gal

Glycerin: $0.36/lb

Methanol: $2.53/gal

KOH: $0.48/lb

Figure 31: Value Added Comparison for Biodiesel Unit for Typical Reaction Conditions

100 Energy Analysis

Although the waste cooking oil input is given a price here, ATL could consider this stream

to be free to the Biodiesel Conversion unit. The price shown is, rather, what the airport could

expect to get from selling the oil on the market. The oil, however, would likely need to be

filtered and cleaned for sale. Based on Figure 31, the value added from converting waste

cooking oil to biodiesel is large. Again, these numbers assume 100% conversion whereas typical

conversions of waste cooking oil are more on the order of 80-90%. This analysis should be

combined with a detailed accounting of the cost of the equipment outside the scope of this

analysis.

Waste-to-Energy Comparison 9.7.Another common use for solid waste is incinerating it to run a steam turbine and generate

electricity. These processes are only 15% efficient at converting input chemical energy to

electrical energy compared with about 30% for reciprocating engines [8, 17]. If all of the paper

and FW from the airport were combusted, about 11.3 MMBtu/hr of energy would be released.

This is based on the waste having a HHV of 1036 Btu/lb waste, calculated from the EPA’s Waste

Reduction Model for 1,950 tons/yr corrugated containers, 3,375 tons/yr newspaper, 7,100 tons/yr

food scraps, and 5,375 tons/yr mixed paper [18]. Converting this to electricity at 15% efficiency

generates about 500 kW of electricity. In theory, the waste heat could also be recovered.

In this scenario, however, about 1.3 tons/hr of carbon are emitted to the atmosphere (based

on 0.47 lb C/dry lb FW and 0.90 lb C/lb paper waste) [19, 20]. If carbon emissions from

composting are assumed to be zero in the Energy Park, approximately 0.06 tons/hr carbon are

emitted. While more power can be produced by incinerating the waste, emissions are a concern.

Since the Energy Park scenario is predicted to produce enough heat and electricity to sustain

itself with much fewer emissions from a carbon standpoint, the Energy Park is the preferable

option. Of course this does not include costs of construction, or labor.

Conclusions 9.8.The proposed ATL Energy Park is predicted to be self-sustainable from an energy balance

perspective, generating 260 kW of electricity and 1.6 MMBtu/hr of hot water heat (based on the

reciprocating engine CHP system scenario). Other AD/CHP plants consume about 30% of this

power and heat to run the load requirements of the plant, leaving 70% of the power and heat

available for other uses. Other demands will include heat and power to run the Composting and

Biodiesel Conversion units, but these are not expected to be high. The overall Energy

Production Train is predicted to be approximately 60% efficient in converting FW energy to

electricity and heat energy with only 16% released as waste exhaust gas energy and the balance

embodied in the heavy organic residuals sent to compost.

101 Energy Analysis

References 9.9.1. Airport, H.-J.A.I., Energy Park Information, 2011, Hartsfield-Jackson Atlanta International Airport:

Atlanta. p. 3.

2. Airport, H.-J.A.I., 2006 Waste Characterization Report, 2007, Hartsfield-Jackson Atlanta International

Airport: Atlanta.

3. General Questions to Hartsfield-Jackson Atlanta International Airport.

4. Hernandez, G., et al., Los Angeles digesters produce energy from airport food residuals. BioCycle,

2002. 43(1).

5. Lei, W.L. Focus: High inventory weighs on Asia refined glycerine market. ICIS news, 2012.

6. Kerr, D. EBMUD turns food scraps into electricity. OaklandNorth, 2010.

7. District, E.B.M.U., Anaerobic Digestion of FW: Final Report, 2008, U.S. Environmental Protection

Agency Region 9. p. 1-62.

8. Agency, U.S.E.P. and C.H.a.P. Partnership, Catalog of CHP Technologies, 2008.

9. Themelis, N.J., Y.H. Kim, and M.H. Brady, Energy recovery from New York City solid wastes. ISWA

Journal: Waste Managment and Research, 2002. 20: p. 223-233.

10. Banks, C.J., et al., Anaerobic digestion of source-segregated domestic FW: Performance assessment

by mass and energy balance. Bioresource Technology, 2011. 102(2): p. 612-620.

11. Stedman, Hammer Mills.

12. Hingu, S.M., P.R. Gogate, and V.K. Rathod, Synthesis of biodiesel from waste cooking oil using

sonochemical reactors. Ultrasonics Sonochemistry, 2010. 17(5): p. 827-832.

13. Singh, A.K., S.D. Fernando, and R. Hernandez, Base-Catalyzed Fast Transesterification of Soybean

Oil Using Ultrasonication. Energy & Fuels, 2007. 21(2): p. 1161-1164.

14. Van Gerpen, J., et al., Biodiesel Production Technology, 2004, National Renewable Energy

Laboratory.

15. Energy, C.C.U.S.D.o., Clean Cities Alternative Fuel Price Report, 2012. p. 1-17.

16. ICIS, Indicative Chemical Prices from 28 August 2006 issue of Chemical Market Reporter, 2006,

Reed Business Information Limited.

17. Morris, J., Recycling versus incineration: an energy conservation analysis. Journal of Hazardous

Materials, 1996. 47(1–3): p. 277-293.

18. Agency, U.S.E.P. Waste Reduction Model (WARM). 2012.

19. Zhang, R., et al., Anaerobic Phased Solids Digester Pilot Demonstration Project: Biodegradability

and Soil Amendment Potential ofAnaerobically Digested Residues, 2010, U.C. Davis.

20. Yen, H.-W. and D.E. Brune, Anaerobic co-digestion of algal sludge and waste paper to produce

methane. Bioresource Technology, 2007. 98(1): p. 130-134.

102 Conclusion & Recommendations

Conclusion & Recommendations 10.


In this chapter, we summarize the main findings and provide recommendations based on our

cost and energy analysis. These recommendations can be divided in two main parts:

Upstream waste management programs 10.1.These are initiatives aim to reduce waste generation and disposal at the airport terminals and

concessionaires. These programs include:

Training programs

Self-Sorting Systems

Compostable Packaging

These recommendations are mainly discussed in the first chapter of this report. They are

important in a sense that if fully implemented, can significantly reduce the need for additional

investment in the Energy Park. However due to the complex nature of interactions between

airport, tenants, and passengers inside the airport, the effect of implementing such initiatives are

not easy to predict. These programs will vary depending on the final MSW disposal method.

Waste Treatment at the Energy Park 10.2.A second set of recommendations are concerned with the idea of developing an Energy Park

next to the airport where the waste from the airport and potentially neighboring residential areas

can be collected and processed to produce useful energy and offset a portion of the airport energy

demand. There are two main feasible approaches to waste management at the Energy Park:

Collective Treatment

In this scenario, it is assumed that compostable waste is being collected without further

separation and therefore the proposed methods treat waste as a combination of paper, food, and

other compostable materials. In this case, composting can be done onsite or offsite. In case of

onsite composting, the recommended method would be static aeration composting. Static

aeration composting provides cost effective composting without the use of large turning

machinery as would be needed in standard windrow composting. Odor from the composting

process can also easily be controlled by using a negative pressure system and filtering the air

through already processed compost with a 95% reduction of the odors. Composting would take

place in large concrete bins with pipes below the compost or perforated flooring where a

negative pressure could be used to pull air through the compost pile thus aerating it increasing

the rate of composting. Over a 10 year period the average cost to compost a ton of waste would

be around $21/ton. However, if the airport sends all compostable waste to the offsite vendor the

cost will be ~ $10/ton. Therefore offsite composting is economically beneficial in the long run.

Another common use for solid waste is incinerating it to run a steam turbine and generate

electricity. Since this process emits about 20 times more GHG than the Chain Treatment, it is

not recommend as a sustainable option.


Chain Treatment

Chain treatment starts from separation of waste based on paper, cooking oil, and other

compostable materials. Here are our recommendations for each type of waste:

I) Paper waste:

Paper can be composted offsite with the cost of around $10/ton or onsite with the cost of

around $21/ton. Alternatively, paper waste can be used in a Gasification-to-Diesel process

which will contribute as fuel input to CHP unit. This is a preferred way of treating paper

which needs further investigation.

II) Cooking oil waste:

The airport concessionaires generate 23,000 gal/yr of waste cooking oil. Using

ultrasonication and dry settling is suggested to process the waste cooking oil into biodiesel

and glycerin.

III) FW and OMFW:

The proposed process for waste treatment consists of three stages: AD, CHP, and

greenhouses.

Based on estimated composition and consistency of the FW, the CSTR or the DRANCO

technology would be the best choice for the AD. The size of reactor will be estimated to be

about 540 m3 with the CH4 output of around 105 m

3 per hour. The only pre-treatment that

would be required before sending the waste to the CSTR reactor is liquifying the FW. The

cost of AD treatment would be around $380 per tons of input FW.

The recommended form of CHP system is a RE due to the small size and ease of use.

With the estimated outputs of the anaerobic digester gas turbine CHPs which were the other

suggested CHP system for use at the Energy park would have to be supplemented with

additional natural gas due to the larger base size of gas turbine CHP’s. With a biogas input

of 3.1 MMBtu/hr a reciprocating CHP system would output 260.2 kW and 1.6 MMBtu/hr of

heat energy in the form of hot water. A reciprocating engine that could handle an output of

260.2 kW would have a net estimated benefit of $12.2 per tons of input FW.

As for the greenhouse, different energy crops were evaluated but none of them seem to be

at commercial scale yet. Duckweed and struvite may be viable options to increase power

production and recover nutrients, respectively. Similarly, aqua- and hydroponics systems can

be operated with nutrients from the FW and create an incredible amount of produce. Since

hydroponic gardening maximizes space so efficiency, only a small amount of space would be

needed to fill the need of airport restaurants. The additional space could be saved for other

sustainability projects, development, or more produce to sell in the community. This system

can use the exhausted water from AD and CHP. However, an additional step to purify the

water before it feeds into the hydroponic system might be required. If Aquaponics system is

adopted it can generate up to $450 per tons of input FW in revenues.


In conclusion and through this preliminary study we can state that the main cost of waste

treatment at the energy Park comes from the Anaerobic Digestion which can be offset by the

benefit from generating electricity in CHP plant and growing fresh vegetables in the

greenhouse. Therefore, the net value of investment in this project is tightly dependent on the

choice of AD system and also the appropriate technology employed for the greenhouse.