techno-economic analysis of a biogas …sirius-c.ncat.edu/asn/afps/ajp/ajp-isotpand09/proc09...

Proceedings of the Second International Seminar on Theoretical Physics & National

Development, 5-8 July, 2009, Abuja, Nigeria

280

TECHNO-ECONOMIC ANALYSIS OF A BIOGAS PLANT FOR AGRICULTURAL

APPLICATIONS; A CASE STUDY OF CONCORDIA FARMS LTD, PORTHARCOURT†

Torbira-Lenee, Mtamabari Simeon

Department of Mechanical Engineering, University of Nigeria, Nsukka, Nigeria.

e-mail:[email protected]

Abstract

A techno-economic analysis of generating biogas using a fixed dome

digester coupled with a solar collector through a heat exchanger has been

studied for Concordia Farms Limited. This gas when generated from

organic waste on the farm could replace power-generating plant in the farm

and save the huge cost (in naira) consumed by the private power plant in

generating energy for the farm. Mathematical computations have been

made to optimize different analysis, namely; organic waste generating

capacity of the farms, volume of digester suitable for the farm, energy

requirements/needs of the farm, available energy sources of the farm and its

biogas generating potentials. The design criteria for thermal heating of an

active, fixed-dome type biogas plant is presented with the effects of heat

exchanger and collector panel incorporated in the thermal analysis.

Increasing the flow rate of the working fluid between the heat exchanger

and the collector loop can optimize the thermal efficiency. The economic

analysis takes into account, capital and maintenance costs, life of the

project, priced and unpriced benefits of owning a biogas plant. Priced

benefits involves cost valuation (in naira) of the various fuels used e.g. fuel

wood, kerosene, PMS, diesel and time and labour etc. which becomes the

cost saved/avoided by owning a biogas plant. The benefit – cost ratio,

internal rate of returns and net present values, cost-payback and energy

† African Journal of Physics Vol. 2, 280-299, (2009)

ISSN: PRINT: 1948-0229 CD ROM:1948-0245 ONLINE: 1948-0237



281

payback of the investment are also computed to establish the viability of the

proposed biogas project.

1.0 INTRODUCTION

The energy crisis in the early 70’s caused economic problems for many

countries that depend on imported oil and gases. With the high cost and instability

in the price, non-renewability of petroleum products as well as the growing

environmental concern (global warming) on burning of fossil fuels, the need for a

renewable and more environmentally friendly fuel has become imperative. The

exploitation of new energy sources and the adoption of new energy conversion

technologies became necessary towards reduction of enormous organic waste

generated especially in the integrated farms and providing an alternative,

environment compatible, cheap source of renewable energy for such farms – in

Nigeria. Huge quantities of organic waste running into several hundreds of tons

are generated in integrated farms a year. At the same time, these farms spent huge

sums of money on electricity bills, operating private power generating plants, fuel

wood, kerosene, etc. to meet energy needs of farm.

Biogas (also called “Marsh gas”), a by-product of anaerobic decomposition

of organic waste has been considered as an alternative source of energy. Wiley

(1996) noted that the common raw materials for biogas generation are often

defined as “waste materials”, e.g. animal manure, sewage sludge and vegetable

crop residues, all of which are rich in nutrients suitable for the growth of

anaerobic bacteria.

The interest in the present paper is therefore to produce biogas from animal

dungs generated on the this farm that can be used as a cheap, renewable source of

energy on the farm. It is also the aim of this work to compare the cost of owning a

owning a biogas plant by the farm with that of buying fossil fuels.

2. METHODOLOGIES AND MATERIALS

This project was conducted by using a triangulation method consisting of:

literature review, background research/case studies and direct interviews. The

literature review and background research provides an initial overview of biogas.

These sources described what biogas is, how it is produced, and how it could be

used. The literature review transcribed what studies have been done in reference



282

to biogas and current projects using biogas technology. To increase the validity of

the project, only recent journal articles were reviewed.

Background research and case studies were reviewed and will serve as a

comparison to the potential Concordia farms project and provide information as to

the size, capacity, and type of biogas plant that would best suit Concordia farms

limited. Interviews were conducted to several local farm workers about organic

wastes including farmers, farm manager, and farm equipment

operators/maintenance workers, and marketers. The verbal interview questions

were reviewed and passed by Concordia farms Office of Research Ethics.

Approval from the Research Ethics office was needed to interview the manager

and farmers. Interview participants were selected from criteria, which were based

on the proximity of the participants to the farm, and the volume of wastes that

could be generated. Maximum waste could be collected from such farm as

compared to slaughters’ wastes. Participants were contacted directly.

Series of questions were asked regarding where the waste goes currently,

farms sources of energy, farm’s cost on energy, energy needs of the farm and

whether they would be willing to donate their organic waste if a biogas plant is

built on the farm for biogas generation, and the sludge used as manure in

agriculture. The collected data was taken and assessed to determine extra amounts

of organic waste needed for the biogas plant. The economic feasibility of the

biogas plant was conducted with all data collected. This was be followed by a

discussion, recommendations and alternatives for the feasibility of this project.

This method of triangulation attempts to use the most recent and innovative

technologies to minimize potential operational and start-up problems. This

method also emphasizes the benefits a biogas operation would have on the local

community and Concordia farms limited

The farms have the following number of animals and poultry as sources of

dung generation for biogas; 3500 – Birds, 400 – Pigs, 200 – Sheep, 300 – Cows

The type, quantity, and cost of energy consumed per month by the farm are as

follows;

- Fuel wood 20,000kg/month N128, 000.00

- Kerosene 7000 litres/month N330, 750.00

- Diesel 10,000 litres/month N542, 750.00

- P.M.S 10,000 litres/month N514, 000.00

- Charcoal 3500kg/month N8, 750.00



283

3. OBJECTIVE OF THE STUDY

To provide an alternative source of energy to the farm hence reduce its over

dependency on fossil fuels

-To produce a cheap, environmentally friendly energy for the farm use

To convert the huge organic waste generated on the farm into useful energy

hence enhancing good farm hygiene and reducing expenses on fossil fuels

To increase farm outputs and reduce inputs

To integrate biogas technology

4. LITERATURE SURVEY

Biogas is produced by decomposition of biomass and animal wastes, human

excreta, sewage sludge and vegetable residues and poultry wastes by decomposer

organisms like bacteria under anaerobic (airless) condition. This process is

favoured by warm, wet and dark conditions. This involves chemical and

biological processes known as “anaerobic fermentation”, but “digestion” is

often used in anaerobic conditions, that lead to methane production.

Biogas consists of 70% methane [CH4] and 29% carbon dioxide [CO2],

and 1% of hydrogen sulphide [H2S], nitrogen [N2], and some hydrogen [H2]. It

has a calorific value of 20Mj/m3. Biogas is generated from the slurry [50% water

and 50% dung] at an average temperature of about 35OC by chemical waste and

biological process called anaerobic fermentation. The optimum temperature for

maximum production of biogas from slurry is about 37oC. The quantity of gas

production depends on the nature of dung used. The optimum temperature of

maximum production is achieved after a number of days, referred to as retention

period, after feeding the slurry into the digester of the system. The production of

gas starts only after the retention period. Supplying thermal energy to the system

by external means, i.e. by heating slurry using either passive or active method,

can reduce the length of the retention period.

The anaerobic digestion of organic material is a very complicated

biochemical process, involving hundreds of possible intermediate compounds and

reactions, each of which is catalyzed by specific enzymes or catalysts. However,

the overall chemical reaction is often simplified to:

Organic matter anaerobic CH4 + CO2 + H2 + NH3 +H2S…………(1)

Digestion



284

In general, anaerobic digestion is considered to occur in the following stages:

The hydrolysis phase – liquefaction or polymer breakdown.

Acid formation phase

Methane formation.

In a biogas plant, all the three phases occur simultaneously and if only one

phase dominates, production of methane is seriously affected.

There are three main types of biogas plants suitable for integrated farms –

the fixed dome digester plant, the floating drum digester and the plastic covered

ditch. In this work, the fixed dome digester was used. Biogas has many

applications in integrated farms some of which are:

a). Biogas serves as a cooking fuel for farmers.

b). Biogas is used for lighting purposes on the farm.

c). Biogas lamps are use to warm birds and animals.

d). It is also possible to power an internal combustion (IC) engine that may

be found on the farms setting.

5. RESULTS AND DISCUSSIONS

5.1 Energy Audit and sizing of digester

The various forms of energy consumption per month and cost distribution account

were analyzed in this sub-section. The energy audit computed in table2 below is

based on fossil fuels used by the farms. From this table, Concordia farms utilize

1,218,111.25 Mj of fossil fuel per month (14,617,335 Mj per year) at a huge cost

of N1, 524,250.00 per month, (N18,291,000.00 per year).Table1 show heating

values of fuels used in the farm

Table 1: Heating vales of some fuels

Fuels Heating values (kj/kg) Heating values (Mj/kg)

Kerosene (paraffins) 46250 46.25Mj/kg

Fuel wood 12, 000 12 Mj/kg

Charcoal 9000 9 Mj/kg

Diesel (AGO) 46,000 46 Mj/kg

Motor petrol 46,800 46.8Mj/kg

[EASTop & McKonkey (1999)]



285

Also, in order to design a digester of an appropriate and suitable capacity

of 681.3 M3 for the farm, the influent in (Kg) generated from livestock on the

farm were computed as seen in table3 and Fig.1 used in sizing the digester.

Table 3: Calculated Influent/day for Sizing the Farm Digester

Kinds Population Discharge per

day (kg)

TS value of

fresh

discharge

(% by wt)

Total influent

for each kinds

(Kg)

Cow 300 10 16 6000

Chicken 3,500 0.10 20 875

Pig 400 6 20 6000

Sheep 200 1.5 20 750

Total Influent generated on farm/day 13,625

With a hydraulic retention time (HRT) of 40 days, and Total influent (Q) of

13,625Kg, the digester volume was determined using the formulae

0.8V=Q HRT (1000Kg=1M3)……………………………………………(2)

From equation (1.2), the digester size is computed to be 681.3 M3



286

Table 4: Biogas Energy Audit of the farm

Material Quantity (kg) Gas yield per

day (M3)

Total yield of biogas/

per day (M3)

Cattle dungs 3000 0.36 1,080

Sheep wastes 300 0.10 30

Pig droppings 2400 0.25 600

Poultry droppings 350 0.0112 3.92

Total volume of biogas yield per day 1,713.92

With 6,050kg of organic waste, 1,713.92 (M3) of biogas will be yielded

per day. This indicates that in one month, a total of 1,713.92 x 30 = 51,417.6 M3

of biogas will be generated in the farms. Since 1M3 of biogas is equivalent to

Fig.1: Calculated Dimensions of the cylindrical shaped biogas digester



287

0.4kg of diesel, 0.6kg of petrol, 3.5kg of fuel wood, and 0.8kg of charcoal and

0.5kg of kerosene. One can say that generating 51,417.6M3 of biogas in a month

is equivalent to buying.

20,567.04kg of diesel/month = N1, 601,027.066 cost/month

30,850.56kg of petrol/ month = N2, 938,148.57

179,92.16kg of fuel wood/month = N1, 151,757.824

41,134.08kg of charcoal/ month = N102, 835.20

25,708.8kg of kerosene/ month = N2, 666,097.778

which is far more than the quantities Concordia farms purchase per month as

reflected in the energy analysis.

Also, a total of 51,417.6 m3 x 6,300 Kcal /m

3 = 323,930,880 kcal of

energy will be available to the farm in a month. 323,930,880 kcal = 3.2393088 x

1011

cal = 1.355909881 x 1012

joules = 1,355,909.881Mj of energy. This amount

of energy is far more than the calculated 1,218,111.25 Mj of energy consumed on

the farm per month.

So, the biogas generation prospects of the farm can actually meet the energy

needs of Concordia farms. This amount of energy can be utilize in cooking,

lightening, heating, warming etc. on the farm

5.2 Thermal analysis of the biogas plant

Result obtained from calculations reveal a thermal efficiency of 25 %.This

value shows poor efficiency of the heating system of the plant.It is obvious that

the various heat losses to the ambient and ground is responsible for the value

obtained.There is a significant decrease in thermal efficiency by the unglazing

effect due to reduced solar flux at the absorber of collector plate caused by

covection. The losses should be a minimum. The expression for thermal

efficiency is given below

)3......(..........)()(/)()/()exp(1

))(/)(

tITTCamUtFatNAat

tNAtITTCm

asossLc

csossst



288

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 300

200

400

600

800

1000

1200

1400

1600

1800

2000

PTS [%]

Vb

[M3]

As seen in figure 2, the volume of biogas generated increases as percentage total

solid increase.In this research work,the total volume of biogas generated per day

stand at 1713.92 M3 and from the graph one can easily determine the average

PTS value to be 25.5 % .A marginal increase in PTS results in a geometrical

increase in the volume of biogas produced.

Figure 2: Graph of percentage total solid vs volume of biogas generated



289

0 2000 4000 6000 8000 10000 12000 140000

100

200

300

400

500

600

700

Q [Kg]

Vd

[M3]

10 20 30 40 50 60 70 80 90 1000

200

400

600

800

1000

1200

1400

1600

1800

HRT [days]

Vd

[M3]

Figure 3 Graph of digester volume Vd vs substrate Q at HRT of 40 days

Figure 4: Variation of Vd with HRT at substrate value of 13,625 Kg



290



291



292

As seen from result above, iron rod, iron wire and wood consume the highest

amount of construction cost of the digester plant representing 41.14%. Other

major costs are; sand & chippings, cement, and labour while digester accessories

gulp the list cost.

Cost payback time

Payback=capital cost/annual energy cost savings

Payback=5,954,100/18,214,175 = 0.33 years.

Energy payback time

Payback=1,218,111.25/1,355,909.88 = 0.9years

Figure 5: Cost Distribution of 681.3 m3 Biogas Plant



293

5.3 Economic Valuation of Firewood and Charcoal

Use of firewood for cooking by a farm has negative effective on the

density of forest area in the locality, which in turn affects the microclimate of the

area and thus the society. Therefore, economic price of firewood has to be higher

for society than to an individual resulting into higher economic rate of returns on

the investment.

It is yet to be declared a single value for fuel wood that would reflect the

social cost or benefit of it. Some authorities have treated firewood as non-traded

goods and value it at lower than the financial price. Others value it at a percent

higher than the financial price. Still, other authorities have taken economic price

of firewood as 20 percent higher than the financial price.

5.4 Economic Valuation of Kerosene, p.m.s and Diesel

It is easier to arrive at the economic value of kerosene/PMS/Diesel as it is

readily marketed and the money value of subsidy in it can be calculated. In

Nigeria, petroleum products are refined locally and imported from oversees – for

imported goods; payment is made in US dollars. Assuming that the official

exchange rate between Nigeria’s Naira and the US dollars would fully reflect the

true economic value of goods traded with these currencies, the border price paid

by Nigeria is taken as the economic price of these products, while the cost of

production is the economic price when locally refined. About 10 percent is added

to this price to reflect the economic cost involved in transportation and handling

of kerosene/diesel/PMS within the country.

5.5 Economic Valuation of Labour

The use of biogas results in the saving of unskilled labour time. A wage

rate for unskilled labour has to be reduced by a factor that would reflect the cost

of large-scale farming. Gautam used a factor of 0.65 to arrive at the economic

wage rate of an unskilled labour (Gautam, 1988).

5.6 Valuation of Slurry

Slurry is valued for its content of soil nutrients, particularly N.P.K. As all

chemical fertilizers in Nigeria is imported, the economic values of N, P and K are

calculated at the international market price of N. P and K fertilizers.



294

5.7 Investment Cost

The guarantee fee (if any) and service charge taken by biogas builders

should be deducted from the total investment, as they are only transfer of

payments. The subsidy (if any) should be included as part of the investment cost.

The total expenditure actually incurred for construction activities should be

reduced by a factor to reflect the true economic cost of materials and labor used in

construction. Gautam used the weighted average construction factor of 0.76 in the

case study referred above.

It is seen from the above that the economic cost of goods and services

used for biogas plant installation become lower than the costs used for financial

analysis. Also, the benefits of biogas use are valued at higher rate for economic

analysis than the financial analysis. Therefore, any plant that proves to be

financially viable to an individual user will still be viable at higher rate of return

from the economic or social point of view.

6.0 CONCLUSION

The choice of owning a biogas plant depends on; (1) the availability of

sufficient organic wastes which serves as raw material or input. (2) The energy

needs or requirements of the environment, it is to be installed. The volume of the

biogas plant will also depend on the amount of waste generated within the

locality and the amount of energy needed for consumption.

In the case study farm, we find out that the waste generation per day runs

into several thousands kilograms. This greatly influences the biogas digester

volume designed for these farms. Also, the amount of energy consumed per

month and resultantly per year runs from a million per day to several millions

mega joules per year.

In the thermal analysis, the instantaneous efficiency of the biogas plant

was used for the design of the active biogas system with a given heat capacity

(Ms Cs). From the economic point of view, the net cash flow of a 681.25m3 active

biogas plant without subsidy is positive in the first year. This indicates that

without subsidy, a user can still invest to get a positive return on investment. This

is not beyond the investment capacity for a commercial or large-scale or

mechanized farmer. Though there is still need for subsidy to encourage this

technology. Another factor notice in the economic feasibility is the higher benefit

of biogas plant use in terms of petrol, diesel and kerosene saved. This suggests

that the biogas plant may not be viewed as profitable if these savings is not used



295

for generating more income by ploughing back these savings into the farming

business. Also, the biogas will be profitable if the labour saved is used for

generating income for the farm and the farm must attach values to all other

benefits of the biogas plant such as leisure, clean home stead/farm stead, and

better health.

Further more, the profitability of investment in biogas will increase with

the increase in the price of firewood, kerosene, diesel, etc. in the future.

So far, we have analyzed the organic waste generation of the case study

farms, its energy requirements, and we have compared its biogas generation

prospects with energy requirement. The economy studies also reveal the viability

of a project of installing an active biogas plant in Concordia farms Limited.

Biogas is a potential renewable energy source for rural Nigeria. Taking

biogas generation as a farm base activity, the energy requirements of these farms

can be meet.

From these analyses, I come to the conclusion that the designed biogas

plant will be suitable for this farm.

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