bioenergy conversion lecture notes

B Y

JEAN DE DIEU IYAKAREMYE

UR/CAVM

[email protected]

BIOENERGY CONVERSION

Chap.1 : BIOMASS AS A SOURCE OF ENERGY

Introduction:

Bioenergy is any renewable energy produced from living organism

Biomass is a vital source of energy for the poor and underdeveloped countries for cooking and heating of food. There are three types of energy crops.

Types of energy crops

Annual Energy crops (Grains and Grasses) :

The crop residues of cotton, maize etc can be used as bio fuel.

Grains like sorghum, wheat and maize contain high amount of starch which can be converted in to fuel alcohol to run the petrol and diesel engine.

If we have surplus land and surplus food production then we can divert a part of our land into fuel crops.

To day the prices of essential food commodities are increased this because USA has diverted much of its food production land into the production of jatropha which is a fuel crop to produce bio diesel.


Silviculture Energy farms (Trees)

Energy farming in Rwanda can be practiced in the areas where food crops cannot be cultivated like: the bordering areas of swamps, slopes of rocky mountains and other unproductive lands having lot of gravels and stones.

Even in the cultivated lands, a small part of the land can be allocated for native species of trees like Alnusand eucalyptus.


Aquatic Energy Farms (both land based and open Kivu lake)

Sewage water from cities forms the source of biomass production like algae and water hyacinth. Water hyacinth can be harvested and anaerobically digested to produce methane gas.

Addition of CO2 to the system is required for higher biomass production. Attempts should be made to grow water born plants in the stagnant waters of ponds and lakes.

Open banks of Kivu lake can be considered for the development of certain giant tree species.


There are 3 factors that influence the amount of energy in Rwanda can derive from energy farms. They are:

Land and water availability for energy farms

Productivity of biomass species

Economics of bio fuel produced in the farms

ENERGY PLANTATIONS

When land plants are grown purposefully for their fuel value by capturing the solar radiation. It is called energy plantation.

Plants are considered as solar energy storage devices. Energy plantation by design are managed and operated

to provide substantial amount of usable fuel continuously through out the year at competitive costs with other fuels.

This energy plantation can be converted and released in to heat with high temperature like fossil fuels like coal and petroleum.

The suitable calorie plants should be grown in agro forestry plantations to fill the gap of energy problem.

Energy plantations

Factors influence energy plantations are: Soil conditions, and species of vegetation, land availability, Design and management of energy plantation

Energy plantations

Soil conditions and species of vegetations.

Agricultural land cannot be fully diverted for calorie crops.

Hence, identify suitable land and climate for calorie crops which needs a few cm of top soil to retain soil moisture and a moderate growing season.

Lands are available in Rwanda which needs a survey. The next requirement is choosing the species of calorie crop for a particular zone.

The species must capture maximum solar energy in small area of its exposure. More number of trees should be planted per unit area. There should be little space between tree to tree.

The geometry of the leaves bearing branch should be such that maximum surface area faces the sun.

Energy plantations

Design and management of energy plantation

Rwanda has to develop its energy plantation based on its demand of fuel.

Energy plantation has to be developed such that the new planting and harvesting should exactly coincide with the fuel requirement of the country.

It is found that a typical calorie crop gives 12000 to 24000 trees/hectare. It yields the biomass of 17 to 25 dry tons/ha/year.

The harvesting period of the crop vary from one to three years. Planting of tree should be made in such a way that harvesting must give the annual demand of the country.

Advantages of Energy plantations

Storage

Economics

Air pollution

Ash content

Ecological condition

CO2 balance of the earth


Storage

Solar energy as such cannot be stored.

If we want to store in the form of photovoltaic based electricity we need costly battery cells.

If we want to store solar thermal in hot boxes, it also cost considerable amount.

Even then there is problem of energy loss. Energy plantation store solar energy in the form of wood. There is no problem of energy loss.

It is not a costly solution. There is flexibility for the user in the form of harvesting period and storage of woods.


Economics The cost involved in the energy plantation is less than the cost

involved in purchasing the fossil fuels like petrol and diesel. The cost of processing wood into charcoal (amakara) is less

compared to the refining of crude mineral oil in to kerosene or petrol.

The cost of bio fuel depends only upon the cost of land and the period of maturity of calorie crop. If the period of maturity of fuel plant is less then the cost of production of fuel is comparatively less.

The cost of production of bio fuel can be reduced by adopting multiple crops and increasing the population density of the energy plantation.

Under optimized conditions, the cost of bio fuel is less than the cost of fossil fuels.


Air pollution

Wood and other plant residues contain less than 0.1% sulphur content. Therefore, it produces very less SO2 production during combustion of bio fuel in cooking and other operations.

Fossil fuels like petrol and diesel produce more SO2

during its combustion.


Ash content

Wood and other plant residue produce more ash during their combustion.

The ash contains more plant nutrients. It can be directly used as fertilizer for the crop growth.


Ecological condition

Energy plantation can be raised in arid, semi arid and in unproductive rocky mountain slopes having mostly gravel stones and very little soil.

These unproductive land tracks are turned into green belts.

This causes more evapotranspiration and soil conservation.

This leads to more conducive ecological balance.


CO2 balance of the earth

Combustion of biomass produce large amount of CO2

to the atmosphere. However, it will not affect the CO2 balance in the atmosphere because plant consumes more CO2 during its growth stage than the CO2 it releases during its combustion.

Combustion of fossil fuels releases more CO2 in to the atmosphere and takes more O2 from atmosphere for burning than the biomass.

Hence compared to fossil fuels, biomass does not affect CO2 balance of the atmosphere.

Plants proposed for energy plantation

S.No Name of the

species

Yield, m3/ha/year Calorific value,

Kcal/kg

1 Eucalyptus 7 to 10 4800

2 Sesbania Species 20 ------

3 Casuarina 7 to 10 4950

4 Prosopis Juliflora 5 to 6 High

5 Leucaena

Leucocephala

30 to 40 4200 to 4800

Plants proposed for energy plantation

Sweet potato

Irish potato

Water hyacinth

Jatropha

Simplified Biomass Conversion Process

Detailed biomass conversion process

Process and its definitions

SN

Process Description

1 Drying Decreasing the moisture content by mechanical or thermal means

2 Briquetting Densification by compression under heat and pressure

3 Pelletization Densification and cutting into standards pellet size under pressure

4 Combustion Burning with the liberation of heat

5 Pyrolysis Thermal decomposition of organic matter in the absence of air

6 Carbonization Formation of charcoal by heating in an oxygen free atmosphere

Process and its definitions

SN Process Description

7 Liquefaction Conversion to an oil by reaction with synthesis gas and an alkaline catalyst under high temperature and pressure

8 Hydrolysis Chemical decomposition through addition of water

9 Aerobic fermentation Decomposition of organic matter in an enzymatically controlled air

10 Anaerobic fermentation Biochemical decomposition in an enzymatically controlled and oxygen-free atmosphere

11 Biophotolysis Production of hydrogen in a chemical form through solar energy

12 Hydrogeneration Extraction of oxygen from cellulose in the presence of steam and carbon monoxide at high pressure with moderate temperature

13 Hydro-gasification Conversion to a gaseous state in the presence of hydrogen

14 Catalytic gasification Conversion to a gaseous state by using a catalyst in an inert gas atmosphere

15 Chemical reduction Change in chemical composition through heat and pressure

Chap 2. Biogas generation technology

Biogas, a mixture containing 55-65 percent methane, 30-40 percent carbon dioxide and the rest being the impurities (H2, H2S, and some N2), can be produced from the decomposition of animal, plant and human waste.

It is a clean but slow burning gas and usually has a colorific value-between 5000 to 5500 kcal/kg. It can be used directly in cooking, reducing the demand for firewood.

Moreover, the material from which the biogas is produced retains its value as a fertilizer and can be returned to the soil.

It is not only the excreta of the cattle, but also the piggery Waste as well as poultry droppings are very effectively used for biogas generation

Biogas generation technology

A few other materials through which biogas can be generated are algae, crop residues (agro-wastes), garbage kitchen wastes, paper wastes,. sea wood, human waste, waste from sugarcane refinery, water hyacinth etc., apart from the above mentioned animal wastes.

Any cellulosic organic material of animal or plant origin which is easily biodegradable is a potential raw material suitable for biogas production.

Biogas generation technology

Biogas is produced by digestion. Digestion is the biological process that occurs in the absence ofoxygen and in the presence of anaerobic organisms at ambient pressures and temperatures of 35-70°C.

The container in which this digestion takes place is known as the digester.

Anaerobic digestion

Biogas technology is concerned to microorganisms. These are living creatures which are microscopic in size and are invisible to unaided eyes.

These are different types of microorganisms. They are called bacteria.

Fungi, virus etc. Bacteria again can be classified into two types:beneficial bacteria and harmful bacteria.

Compost making production of biogas, vinegar, etc., are examples of beneficial bacteria.

Bacteria causing cholera, typhoid, diphtheria are examples of harmful bacteria. This type of bacteria which causes disease both in animals and human beings is called pathogen.

Anaerobic digestion

Bacteria can be divided into two major groups based on their oxygen requirement.

Those which grow in presence of oxygen are called aerobic bacteria

while the others grow in absence of gaseous oxygen are called anaerobic bacteria.

When organic matter undergoes fermentation (process of chemical change in organic matter brought about by living organisms) through anaerobic digestion, gas is generated.

This gas is known as biogas. Biogas is generated through fermentation or bio-digestion of various wastes by a variety of anaerobic and facultative-organisms.

Facultative bacteria are capable of growing both in presence and absence of air or oxygen.

Aerobic digestion

Aerobic and anaerobic fermentation can be used to decompose organic matter.

Normally aerobic fermentation produces CO2, NH3 and small amounts of other gases along with a decomposed mass and evolution of heat.

Anaerobic fermentation produces CO2, CH4, H2 and traces of other gases along with a decomposed mass.

Aerobic fermentation is used when the main aim is to render the material hygienic and to recover the plant nutrients for reuse in the fields. The residue is C, N2, P, K and other nutrients.

In a biogas plant the main aim is to generate methane and hence anaerobic digestion is used. Here the complex organic molecule is broken down to sugar, alcohols, amino acids by acid producing bacteria. These products are then used to produce methane by another category of bacteria.

Uses of biogas

The biogas technology offers an appropriate technique to convert non-conventional energy into conventional energy.

The gas thus produced is a neat, combustible and pollution-free fuel.

The technology is simple and appropriate for both rural and urban areas .

The plant can be maintained and operated by the housewife with little training education.

Uses of biogas

Cooking fuel

Fuel for lighting

Fuel for motive power to replace diesel oil

Enriched organic manure of agriculture and aquaculture

Manure for mushroom growing

Manure for seed coating

Use of light to trap insects at the farm

To treat human excreta and for pollution control

The slurry which is the byproduct is used to improve the soil retention capacity and as organic manure

Development of Fixed dome Biogas plant

This digester was originally developed and is used in China.

It runs on a continuous batch basis. It can digest plant, human and animal wastes.

It is usually built below ground level, hence it is easy to insulate in a cold climate.

The digester can be built from bricks, cement and sand. The variable pressure inside the digester was found to cause no problems in China in the use of the gas.

The bottom of the gas plant is flat concrete, the digester has cylindrical wall and the dome is a semi sphere with the brick masonry.

Designing of Fixed dome type Biogas Plant

The fixed dome plant has four important components:

Digester (fermentation chamber)

Gas storage chamber;

Plant dome; and

Displacement chambers (Inlet & outlet).

Theoretical equations for designing a biogas plant

1. D : H : : 1.75 : 1

2. D /H = 1.75 3. D = 1.75 Hd

4. D for the smallest size plant should not be less than 2 mt.

5. Pmax = 90 cm of water column (approx. 900 Kg/m2)

6. Vg = 33% of the daily gas production (corresponds to 8 hours gas production)

7. Vi = Vo

8. Hm is fixed at 60 cm (600 mm) of waste column (assuming density of

slurry equal to the density of water for Janata plant design) for Janata

plant of 2 to 6 cum capacity. 9. Average gas production per day for 50-60 HRT= 0.04 cum/Kg of fresh

cattle dung.

10. The minimum Hp should be 7cm (7,0 mm) for the smaller size

Janata plant.

Theoretical equations for designing a biogas plant

Where

Pmax = Maximum allowable gas pressure inside the plant

C = Rated gas production in Cumor the plant Yg = Daily gas yield / unit from-manure for a given H

D = Diameter of the Digester = Diameter of the “Gas

storage chamber”

H = Total height of the digester including gas storage chamber Hd = Height of the effective digester (fermentation chamber)

Vd = Volume of digester (fermentation' chamber)

Hg = Height of the "Gas storage chamber"

Vg = Volume of “Gas storage chamber”

Hf = Height of the plant dome Fd = Daily feed (slurry) to the plant in Lts

HRT = Hydraulic Retention Time

Vi = Volume of inlet displacement chamber

Vo = Volume of outlet displacement chamber

Hm = Height of the displacement chamber l =Length of the displacement chamber (Inlet or Outlet)

b = Breadth of the displacement chamber (Inlet or outlet)

Hp = Vertical distance (gap) between the ceiling of the Janata plant

dome where the gas outlet pipe is fixed and the lower end of the discharge opening

Digester Design

The D and H of the Janata plant has to be designed by trial and error in

accordance with the assumption 3 & 4. The total digester of the fixed

dome plant consists of :-

a. Fermentation chamber or the effective digester of the plant and b. Gas storage chamber.

Both (a) and (b) above are integrated part of each other as explained

in the section of special plant components of a fixed dome plant.

Digester Design

a. Design of the fermentation chamber

Vd = Fd X HRT__________________i D2

Vd = ---- X Hd ___________________ii

4

4Vd Hd = ----- ___________________iii

D2

b. Design of the gas storage chamber

Vg = 33% of C

Vg = 0.33 C __________________iv

D2Hg

Vg = -------- __________________v

4

4 Vg

Hg = ------- __________________vi

D2

Total Height of the digester

H = Hd + Hg _______________________vii

Digester Design

If the D/H is not correct as per assumption, take a new value of D (not less than 2 mts.) for fresh calculation. This procedure for calculation should be followed till the D/H is close to 1.75 (or D = 1.75H)

Displacement chamber design

There are two displacement chambers in the fixed dome plant – Inlet

displacement chamber (IPC) and Outlet displacement chamber (ODC)

The IDC is the integral part of the Inlet tank and the ODC is the integral

part of the Outlet tank. The combined volume of both the displacement

chambers (IDC & ODC) is equal to the total volume of the "gas storage

chamber" (GSC) so that the slurry displacement due to biogas

accumulation in the latter is GSC may be accommodated equally in the

former i.e. IDC and ODC.

Displacement chamber design

Therefore Mg = Vi + Vo ______________________ viii

Vg = 2 Vi = 2 Va ___________________ ix

HM = 90 – Hg _____________________ x

Note : For family size fixed dome plants of 2,3,4 & 6 cum capacity, the

value of Hm is fixed as 61 cm (610 mm) for the sake of standardization

in the design; therefore the equation (X) does not hold good for these

four sizes of fixed dome plants

Vi = Vo = Hm x l x b ____________________ xi

By fixing appropriate value of either L or b, the value of other parameter

or the displacement chambers can be calculated from equation (xi)

Plant Dome Design

Hp = 70 mm (minimum as per assumption) __________ xii

Ht = D/45 _______________ xiii

Ht = (Hp + Hm) __________ xiv Select value of Ht as the greater of the two calculated from equations

(xiii) and (xiv).

Note: a. From the structural point of view, the minimum height of the dome

(without reinforcement) should be kept as D/4. 5.

b. Ht should always be kept more than Hm to avoid any dogging of gas

outlet pipe due to slurry getting in to it, when there is no gas stored

in the “Gas storage chamber,". c. For the smallest family size fixed dome plant, the minimum practical

value of Hp would be 70 mm (or 7 cm)

Parameters Affecting Biogas Production

CHAPTER 3: THERMAL GASIFICATION OF BIOMASS

GASIFIERS

Gasification means converting the solid biomass into gaseous fuel without leaving any solid carbonaceous residue. Gasification is carried out by the following two processes:

Heating the biomass with limited air or oxygen.

Heating at high temperature and high pressure in presence of steam and oxygen.

Gasifier

It is an equipment to convert various types of biomasses like wood, agricultural wastes and maize cobs into gases.

It is a chemical reactor in which various physical and chemical reactions takes place.

Gasifier is a air tight closed container. There is partial supply of air into the system. Biomass kept in the container is ignited. It burns under

partial supply of air. Biomass gets dried, heated, pyrolysed, partially oxidized and

reduced as it flows through the gasifier. The gas produced in the gasifier is a clean burning fuel. It is

called producer gas

Gasifier

The calorific value of the gas is 950 to 1200 Kcal/m3. The composition of the producer gas is as follows:

S.no Composition of producer gas Percent by volume base

1 Carbon mono oxide gas 18 to 22%.

2 Hydrogen 13 to 19%.

3 Methane 1 to 5%.

4 Heavier Hydrocarbons 0.2 to 0.4%.

5 Carbon di oxide gas 9 to 12%.

6 Nitrogen 4.5 to 5.5%.

7 Water vapour 4%

Gasifier

Uses of producer gas :

Petrol engines can be run completely with producer gas.

Diesel engine can be run with both diesel and producer gas as dual fuel engine. The producer gas can save the diesel consumption of the engine by 0 to 80%.

Producer can be burnt directly in the industrial burners.

Advantages of gasifiers:

It is easy to operate the gasifier.

Its maintenance is easy

It is sturdy in construction.

It is reliable in operation.

Use of producer gas

Gasifier

Types of gasifiers according to the air inlet and gas outlet are as follows:

Up draught gasifier

Down draught gasifier

Cross draught gasifier

Gasifier

Up draught gasifier

Up draught gasifier

Air enters below the combustion zone.

Producer gas leaves near the top of the gasifier.

This type of gasifier is easy to build and operate.

The gas produced has no ash content but it contains tar and water vapour because of the fact that the gas is passing through the unburnt fuel wood.

Up draught gasifier are suitable for tar free fuels like charcoal.

Down draught gasifier or concurrent moving bed gasifer

Down draught gasifier or concurrent moving bed gasifer

Air enters at the combustion zone through the air tuyers. Combustion of wood takes place in front of the air tuyers. Heat produced during combustion pyrolysis the wood fuel and the hot

producer gas is passed downwards. Producer gas leaves near the bottom of the gasifier through the grate. The volatile material and the tar produced during the combustion moves

down to the reduction zone where they are further heated to become hydrocarbons and produce ash. The gas produced is nearly clean gas.

The ash is collected at the bottom. The gas produced is passed through the ash pit.

Hence it contains less tar and more ash. These gasifiers are most suitable to wood and agricultural residues.

This gasifiers are used to generate electricity up to 10 KW and beyond which there is geometrical limitation of the gasifier and gas quality.


In a cross-flow gasifier the feed moves downwards while the air is introduced from the side, the gases being withdrawn from the opposite side of the unit at the same level of air entry.

A hot combustion/gasification zone forms around the entrance of the air, with the pyrolysis and drying zones being formed higher up in the vessel.

Ash is removed at the bottom and the temperature of the gas leaving the unit is about 800-900˚C: as a consequence this gives low overall energy efficiency for the process and a gas with high tar content.

In this type of gasifier, the path length of travel of air into producer gas is very less. The gas contains more tar.

This type of gasifiers is not commonly used.

Reactions taking place in a gasifier

C + O2 CO2 + 393800 kJ/kg mol (Combustion)

C + H2O CO + H2 – 131400 kJ/kg mol (Water gas)

CO + H2O CO2 + H2 + 41200 kJ/kg mol (water shift reaction)

C + CO2 2CO – 172600 kJ/kg mol (Boudouard reaction)

C + 2H2O CO2 + 2H2 – 78700 kJ/kg mol

C + 2H2 CH4 + 75000 kJ/kg mol (Methane reaction)

Reactions in a downdraft gasifier

Gasifier

In a gasifier, the composition of the producer has produced depend upon the following aspects:

Temperature distribution within the gasifier

Average gas residence time in the gasifier

Mode of air supply in to the gasifier

Heat loss to the surrounding of the gasifier

Gasifier

Reasons for purifying and cooling the producer gas before admitting into the engine cylinder.

The generated gas contains varying amount of moisture, dust and char particles and tar

The gases are also at high temperature

Gasifier

Various processes carried out in a producer gas cooling and cleaning train are:

Removal of larger char particles

Scrubbing of gas to remove the tar

Cooling the gas

Removal of moisture

Removal of fine dust particles

Fluidized Bed Gasifier

Fluidized bed gasifier is most versatile and any biomass (including sewage sludge; pulping effluents etc.) can be gasified using this type of gasifier.

It provides a means of burning any combustible material from wet sewage sludge to refuse, with high efficiency and with minimal pollution.

At the heart of a fluidized bed combustor is a hot bed of inert particles which are held in suspension'fluidised'-by an upward current of air (refer Fig.).

The calorific value of biomass is not a constraint. Besides being highly efficient because of high heat release rates as well as effective heat transfer resulting from rapid mixing and turbulence within the fluidized bed, fluidized bed gasifier can handle biomass with high ash content (for example rice husk).


Fluidized bed generally contain either inert material (sand) or reactive material (limestone or catalyst).

These aid heat transfer and provide catalytic or gas clearing action.

The bed material is kept in fluid state by the rising column of the gas.

Normally the operating temperature of the bed is maintained within the range of 750-950°C, so that the ash zones do not get heated to its initial deformation temperature and this prevents clinkering or slagging.

Advantages of fluidized bed gasifier Fuel flexibility and type of fuel with calorific value ranging from 800 to

8000 kcal/kg can be used. Good heat storage capacity. The bed has a very high heat storage

capacity to always ensure combustion. Quick start up. High combustion efficiency. High output rate. Consistent rate of combustion. Usage of fuel with high moisture content. Rapid response to fuel input changes. Because of the low temperature combustion, corrosion caused by alkali

compounds in ash significantly reduced. Require much less boiler plan area than a stoker. Uniform temperature throughout the furnace volume. Reduced emission of harmful nitrous oxide. Sulphur dioxide emission can be reduced to acceptable level with less

expense. Operation is as simple as that of an oil fired boiler.

Fuel cleaning and cooling system of fluidized bed gasifier


The gasifier produces the producer gas.

It is hot and contain lot of impurities like ashes.

Hence there is a need to clean and cool the fuel. Admission of hot fuel into the engine results in loss of power produced by the engine.

Hence; the gas has to be cooled. The impurities present in the fuel gas will be detrimental to the operation of engine. Hence, it has to be cleaned


The following has to be removed from the gas:

Removal of larger char particles

Scrabbing of gas to remove tar

Cooling the gas

Removal of moisture

Removal of fine dust particles

BIOMASS

Pyrolysis of biomass

Pyrolysis is a general term for all processes of heating or partial combustion of biomass to produce secondary fuels and chemical products. The input may be wood, biomass residues, municipal waste or coal. The products are gases, condensed vapours as liquids, tars and oils and solids residue as char (charcoal) and ash.


Traditional charcoal making is pyrolysis adapted to produce charcoal in which the vapours and gases are not collected.


A charcoal kiln is a device which converts wood (cut into sizes of 200 mm diameter and length in the order of 500-1000 mm) into charcoal.

There are three types of kilns to produce charcoal. They are:

o Earth kiln

o Brick kiln

o Metallic Kiln

Earth kilns

Earth kilns are heaps of wood covered with a layer of herbs, grass, leafy biomass and toped with earth to act as insulation and to control draft (air flow).

While the kiln is operation, the mass of wood shrinks due to devolatilization and this has to be made up by covering the heap with fresh mud to isolate it from the ambient air entering into the earth kiln.

Earth kilns can be above ground or in a specially dug pit. As a safety measure, the surrounding area must be cleared

and several barrels of water placed in the vicinity. Inspection of kilns is needed every 2-3 hours and clear days.

More frequently inspection is needed in windy or rainy days.

Earth kilns have a efficiency of 25% and give charcoal yield above 15% by weight of the biomass input used.

The types of earth kilns built above the ground level are horizontal earth kiln, vertical earth kiln and parabolic mud walled kiln.

Brick kilns

It give much higher yields (20%) and efficiencies (35%) in comparison to earth kilns.

This is because of much superior control of the air to flue ratio to give the right extent of devolatilization.

Metallic kilns These are internally fired batch type. The simplest of these is the drum kiln, which is nothing

but an oil barrel (200 liters capacity) converted into a kiln by cutting out the bottom, making four holes at top, two bug holes and two large holes of 150 mm, Wood layers are packed vertically, leaving a small space in the centre, which is filled with leaves and twigs for lighting the fuel.

The kiln needs continuous monitoring and shaking every 15-20 minutes. After about 90 minutes, the wood should be refilled. This should be done three times.

White smoke indicates charcoal formation, while black smoke or clear exhaust indicates that excess combustion is in progress.

Materials needed

200L of oil tank

1m. 4in (Ø) of fiber stone tube

5 pieces of brick

Red brick

Bambou tree

Those materials can be easily found in many rural areas of

Rwanda

Horizontal drum kiln

Vinegar wood(byproduct)

Energetic of a traditional earth kiln

The kiln is a wood stock in the ground firmly converted by earth with holes for the escape of flue gases.

About 5 tones of wood give about 1.05 tonnes of charcoal. The average charring temperature is 4000C.

The kilning operation takes place in about 96 hours.

When the volume of wood in the kiln shrinks continuously, earth has to be added to prevent exposure of the fuel to the atmosphere. The theoretical heat output is taken as


Q = McCc

= 1050 x 7200

= 75.60 x105 kcal.

Mass of input biomass used in earth kiln is 5 tonnes, that is 5000 Kg.

Mc - Mass of charcoal = 1.05 tonnes 1050 Kg

Cc - Calorific value of charcoal = 7200 Kcal/Kg.

Q - Heat content charcoal produced = 75.60 x105 kcal.


The main losses in the kiln are unburnt volatiles and thermal energy of the exhaust gas

The heat losses can be reduced in two ways

Using the gas as such after cooling and cleaning to do mechanical work (IC engine or for heating (burners).

Condensing the gas: The flue gas from the earth kiln lose its heat to the atmosphere without any use.

GASIFICATION

Gasification is pyrolysis adapted to produce fuel gases

Gasifiers are designed to produce maximum amount of producer gas rather than char, volatiles, and ashes. Gasifiers are also special case of pyrolysis to produce producer gases.

Vertical-top loading pyrolysis devices are usually considered the best. The fuel products are more convenient clean or transportable than the original biomass.

GASIFICATION

The efficiency of the pyrolysis unit:

Summary of operating conditions needed for Pyrolysis plant

Input biomass should be cleaned so as to remove soil and metals which are non combustible in the pyrolysis unit.

Biomass should be dried but complete dry material is avoided with gasifiers.

Biomass should be chopped to small size to be used in pyrolysis plant.

Air/Biomass fuel ratio during combustion is a critical parameter affecting both the temperature and the type of product.

Pyrolysis units are most easily operated below 6000C. Higher temperatures of 600 to 10000C, need more sophistication, but more hydrogen will be produced in the gas.

Summary of operating conditions needed for Pyrolysis plant

Below 6000C there are generally four stages in the distillation process: 100 to 1200C: Input biomass material get dried within the

pyrolysis plant.

120 to 2750C: Output gases produced are N2, CO and CO2; acetic acid and methanol.

280 to 3500C: Exothermic reactions occur, driving off complex mixtures of chemical (ketones, aldehydes, phenol, esters), CO2, CO, CH4, C2H6 and H2. Certain catalysts, e.g. ZnCl, enable these reactions to occur at power temperature.

Above 3500C: All volatile materials s are driven off, a higher proportion of H2 is formed with CO, and carbon remains as charcoal with ash residues.

Pyrolysis by gasification products

S.No Type of output obtainedfrom pyrolysis

Yields per 1000 kgdry wood

Percent of yield inpyrolysis

1 Charcoal 300 Kg Mass yield 25 to35%

2 Gas (Combustion 10465 kJ/m3) 140m3 (NTP) Mass yield 80% ingasifiers

3 Methyl alcohol 141 Condensed vapoursmass yield is 30%maximum.

4 Acetic acid 531

5 Esters 81

6 Acetone 31

7 Wood oil and light tar 761

8 Creosote oil (tar) 121

9 Pitch 30 Kg Mass yield 3%

Difference between Direct Combustion and Gasification of Wood

S.No Particulars Combustion by earth kiln

Gasification by pyrolysis plant

1. Principal products H2O, CO2,

traces of CO

CO, H2, CO2, traces of

CH4, C2H2, etc.

2. Permissible moisture in wood Up to 30% Up to 15%

3. Wood feed surface area to volume ratio,1/m

Up to 30%

150-1700

Up to 15%

500-1000

4. Grate heat release rate, MW/m2 0.10-0.20 0.25-1.0

5. Overall excess air factor for small sized systems, kg air/kg fuel

2-5 0.2-0.5 kg/kg for gasification 1.0-1.2 kg/kg of combustion

6. Overall thermal efficiency,% 30-80 50-75

7. Overall mechanical efficiency, % 10-15 16-20

8. Mechanical power production rate.

Steam engines

Diesel and Petrol engines

9. Processing steps Moisture removal

Moisture removal, size reduction, cleaning, cooling and compression of product gas.

10. Sensitivity of system to mixing of wood with other biomass fuels like basks

Not very sensitive

Extremely sensitive to types of woods at times gasification may be impossible

11. Particulate carbon in exhaust gas High Low

CHAPTER 5:ETHANOL (ALCOHOL) FROM BIOMASS

Selection of Raw Material:

Alcohol is one the renewable energy source. It is a fuel to run petrol engines.

It can be used for lighting.

Any material which is capable of being fermented by enzymes can serve as a source for alcohol production

Ethanol from biomass

The three basic types of raw materials for ethanol production are: SACCHARINE (sugar containing) materials in which the carbohydrate

(the actual substance from which the alcohol is made) is present in the form of simple, directly fermentable six and twelve carbon sugar molecules such as glucose, fructose, and maltose. Such materials include sugar cane, sugar beets, fruit (fresh or dried), citrus molasses, cane sorghum, and skim milk.

STARCHY MATERIALS that contain more complex carbohydrates such as starch that can be broken down into the simpler six and twelve carbon sugars by hydrolysis with acid or by the action of enzymes in a process called malting. Such materials include corn, grain sorghum, barley, wheat, potatoes, sweet potatoes and so on.

CELLULOSE MATERIALS such as wood, wood waste, paper, straw, corn stalks, corn cobs, cotton, etc., which contain material that can be hydrolyzed with acid, enzymes or otherwise converted into fermentable sugars called glucose.

Ethanol production from different sources

UNIT OPERATIONS FOR ETHANOL (ETHYL ALCOHOL) CH3CH20H

CLEANING

The raw material is cleaned with water to remove mud and dirt sticking on the outer surface.

PEELING

Manual Peeling

The outer skin is removed manually by peeling.


Chemical Peeling

Peeling can also be done by chemical treatment. The material is put in hotwater with sodium hydroxide and then it is washed off by water under pressure to remove the outer skin.

CUTTING

After peeling the material is manually cut into chips using knife.

CRUSHING

The cut material is ground by mea ns of a hammer mill.


COOKING

The crushed starchy material is cooked for one hour at 2-3 atmospheric pressure to gelatinize the starch present. In its native state, starch consists of microscopic partly crystalline granules in which the amylose and amylopectin molecules are arranged in complex folded and ;stratified manner. At ambient temperature these granules are practically insoluble in water and not very susceptible in enzymatic hydrolysis. However, when treated with water the starch granules swell and gradually ruptured. The amylose and amylopectin molecules unfolding and dispersing into solution. This process is referred as gelatinization of starch.


HYDROLYSIS

Hydrolysis is the enzymatic reaction that converts starch to sugars Hydrolysis is the chemical reaction involving water and another substance in which the water molecule is ionized, and the compound hydrolyzed to split.

It may be done by acid or enzymatic hydrolysis. Acid hydrolysis is the one step process, in which the feedstock is mixed with a mild solution of sulphuricacid. Enzymatic hydrolysis is the process in which the material is mixed with an enzyme solution.


FERMENTATION

To the hydrolysed material, yeast is added and aeration is given for yeast multiplication. Fermentation is the process of conversion of sugars to enthanol and carbon di oxide.

The Ethyl alcohol is produced in plenty from biomass

Fermentation

The chemical equations of conversion of starch to Ethyl alcohol by fermentation are as follows:

Enzyme

C6H10O5 + nH2O nC6H12O6

Starch Dextrose

Enzyme

(C12H22O11)n + H2O 2C6H12O6

Maltose Dextrose

Yeast

(C6H12O6)n 2C2H5OH + 2CO2

Dextrose Ethyl alcohol Carbon dioxide

Fermentation

Fermentation temperature varies from 20 to 30°C. Fermentation process is completed in 50 hours. Alcohol content of the fermented beer is 10 to 20% depending upon the source of biomass and alcohol tolerance of yeast.


DISTILLATION

Distillation is a separation process of two or more liquids in solution that is based on the relative volatilities and takes advantages of the different boiling temperature. The fermented liquid mass is subject to distillation.


CONDENSATION

The distillate is condensed with cold water in a condensation unit obtain ethanol. Again redistillationis carried out to obtain ethanol with high concentration.

Uses of ethanol

Engine fuel: Ethyl alcohol can be used as an additive to the petrol in the automobile engines. There should be no traces of water in alcohol. Absolute alcohol is used for this purpose is called power alcohol. It is not sufficiently volatile to give proper starting in cold weather. It is therefore mixed with petrol. A mixture of 20% of power alcohol and 80% petrol has been used as engine fuel in many countries as a substitute of petrol. This mixture is called Gasohol.

Organic chemicals

Solvent

Drugs plastic, lacquers, polishes plasticizers, perfumes

Liquid detergents, sprays

Flow chart for preparation of alcohol from sweet potato

CHAPTER 6: METHANOL PRODUCTION

Raw Materials

Methanol production from wood involves two stages of thermo chemical transformation.

The first stage involves gasification of the wood to produce synthesis gas.

The second stage is the actual synthesis step which converts synthesis gas to methanol. This step is proven commercially available process. Gasification has been the bottleneck to commercialization. Considerable research and development is required to make the use of wood more economic than the use of natural gas. About 2.25 kg of natural gas is required to produce 4 litre of methanol. Similarly 9 kg of dry wood is required to produce 4 liter of methanol

METHANOL PRODUCTION

Physical properties of Methanol

Methanol is a clear, colorless liquid that freezes at -73°C and boils at 65°C.

It is excellent antifreeze, as it is completely miscible with water.

Its octane rating is high and it burns cleaner than petrol in an internal combustion engine.

METHANOL PRODUCTIONMethanol production from natural gas:

Natural gas is used to produce synthesis gas which is a mixture of CO and H2. This mixture is reacted under pressure of 100-200 Kg/cm2 at high temperatures of 250-330°C to yield methanol in the presence of a suitable catalyst:

CO + 2H2 CH3OH

The methanol-containing gases leaving the converter after the reaction are cooled to condense the methanol, and the unreacted gases are recycled. The methanol is then purified by distillation. Only a small amount of by product is obtained, including dimethyl ether and higher alcohols. Natural gas has been used because of cost, convenience and efficiency.

METHANOL PRODUCTION

Methanol production from Biomass:

Any carbon source can be used to make synthesis gas, It is possible to generate mixture of CO and H2

from coal, wood (including wood wastes) peat or urban solid wastes.

A drawback to any biomass-to-methanol scheme is the size of the plant required and the amount of raw materials needed. The minimum practical plant size (about 200 tones/day of methanol) required about 500 tones/day of oven-dry wood or equivalent biomass. Such volumes of biomass are not usually available at a given site without extensive gathering and transportation costs

METHANOL PRODUCTION

Gasification of wood

S.No

Action Reactions

1 Drying(1000–2000C)

Moist wood and heat Dry woodand water vapour

2 Pyrolysis(200 –5000C)

Dry wood and heat char + CO +CO2 + H2 + CH4 +tar andpyroligneous acids

3 Gasification>(5000C+)

Char + O2 + H2O CO + H2 + CO2

METHANOL PRODUCTION

Composition of final raw gas:

S.No Elements Composition of rawgas,%

1 H2 18.0

2 CO 22.8

3 CO2 9.2

4 C H4 2.5

5 Hydrocarobons 0.9

6 O2 0.5

7 N2 45.8

METHANOL PRODUCTIONGas purification and Shift Conversion

The raw gas is purified to remove all gases except H2

and CO.

This mixture is reacted with water so that the final gas mixture contains 2 : 1 ratio of H2 and CO. During the reaction with water, additional CO2 is formed.

It should be removed before synthesis. The raw gas is allowed to cool at 32˚C and it is compressed at a pressure of 7 Kg/cm2.

Still some CO2 is formed in the raw gas. It has to be removed. In the first stage, hot potassium carbonate solution reduces the CO2 concentration to 300 ppm

METHANOL PRODUCTION

Gas purification and Shift Conversion

In the second stage, Mono Ethanoloamine (MEA) reduces the CO2 content to 50 ppm.

The gas is allowed to pass through a cryogenic system so as to remove the residual CO2, water vapour, methane, hydro carbons and nitrogen.

Thus we get a purified gas of 44% H2 and 56% CO. Further processing is required to make 2 : 1 ratio of H2

and CO.

Then the gas is compressed to 28 Kg/cm2 m apart of CO2 react with water vapor in the presence of iron catalyst and produces hydrogen.

The final gas contains 2: 1 ratio of H2 and CO.

METHANOL PRODUCTION

METHANOL PRODUCTION

Synthesis gas:

The synthesis gas is compressed to 140 to 280 Kg/cm2 and then passed methanol synthesis reactor.

In the reactor, 95% of the gas is converted to methanol by using a zinc chromium catalyst. The unreacted gases are separated for recycling to produce methanol.

METHANOL PRODUCTION

Uses of methanol

Methanol can be used as a fuel in engines. It is better than petrol because of high octane number of the fuel.

Methanol is converted in to formaldehyde.

It is also used as a solvent.

It is also used as a intermediate chemical in many processes.

CHAPTER 8: VERMICULTURE As the name indicates, vermiculture means artificial raring or

cultivation of worms (earth worms) and the technology is the scientific process of using them for the betterment of human beings.

Vermi-compost is the excreta of earth worms, which is rich in humus.

Farm waste, house waste and non toxic solid and liquid industrial waste etc can be converted in compost by earth worms. Earth worms eat them and pass it through their body and in the process convert it into vermi compost.

Thus, it is understandable that earth worms not only convert garbage into gold but keep the environment health.

Earth worms are natural bio reactors that do not require fossil fuel to be operated.

Conversion of garbage by earth worms into compost and the multiplication of earth worm are simple processes and can be easily handled by any layman in the village.

Earth worms

Earth worms are tiny creature which like dark moist environment. It breaths through skin.It cnot tolerate solar light and if exposed, may die in few minutes. It may live for 4 to 5 years. The optimum temperature for breeding is 14ºC to 27º C. The grown up weighs about one gram and produce one gram excreta daily.

Under ideal circumstances, the worm’s population can double his size in a month.

Californian red worms get coupled regularly every 7 days depositing one cocoon each one.

Under ideal circumstances, can produce 20 new worms from each cocoon.

The new worms will get their sexual maturation at the age of two months, and they'll get coupled every 7 days for the rest of their life.

Earth worms

Earth worms Vermiculture unit in ISAE

Earth worms

Preparation of Bed

First of all 1.5 m wide and 6.6 m long shed made of bamboo/ wood and straw is erected.

It should be high enough to human entrance for watering.

Under this shed dried grass of 10 cm thickness are spread on the ground, there after a layer of 6 inches of well rotten FYM is placed over it.

This layer needs thorough watering then it is kept as such for 48 hours.

Earth worms

Now some earth worms (1000 worms/m²) are put into it, about 20 cm thick layer of garbage, cow dung, farm waste or city waste except pieces of glasses, metal and plastics are placed and watering is done above it.

It needs watering every day to remain sufficiently moist (50-60%). During dry season 2 watering, per day are required. After 30 days garbage is turned up and down and again kept on watering for next 30 days.

Earth wormsPrecautions for Compost making Moisture level in the bed should not exceed 50 – 60 % water

logged in the bed leads to anaerobic condition and change in pH of medium hampers normal activities of worms leading to weight loss and worm biomass and population,

Temperature of bed should be within the range of 20 – 30 degree centigrade;

Worms should not be injured during handling, Bed should be protected from predators like red ants, white ants,

centipedes and others like toads, rats, cats, poultry birds etc. Frequent observation of culture bed is essential as accumulated

casts retards growth of worms. Space is the criteria for growth and establishment of culture;

space required is 2 square metes per 2000 worms with 30 – 45 cm of bed.

Earth worms find it difficult to adapt themselves in new environment hence addition of inocular as a bait from earlier habitat helps in adaptation to new site of raring.

VermicompostingCompost Harvesting

After two months of composting a sheet of plastic is spread on open ground and the decomposed garbage is transferred on it. It is kept open on sunny day for about 5 -6 hours without watering.

The earth worms are sensitive to heat, light and dry condition, hence they will settle down inside for protection. Now the upper layer is removed. This is vermin-compost.

One may use a sieve to separate earth worm from the compost. It is observed that 40% of the garbage that are fed to earth worm converted into compost and the population of earth worm is doubled. One can erect another shed and thus repeat the process further for more production of compost.

The species that is used for conversion of garbage is called Eiseniafoetede, essentially a surface feeds.

Vermicomposting

Use of Vermicompost The final product (vermicompost) can be used directly on

farms and in the pots. The vermi-compost is 5 times richer in N , 7 times richer in P, 11 times richer in K, 2 times in Mg, 2 times richer in Ca and 7 times richer in actionomy-cites than the ordinary soil; Besides it contains valuable vitamins, enzymes and hormones like gibberellins. It can be used in following doses:

For horticulture crops 100 – 200 gm per tree For forest tree 100 – 200 gm per tree For raising crop 2 tones per ha For kitchen garden and pots 50 gm per pot

Vermicomposting

Earth worm can directly be placed near the stem / trunk of horticultural trees @ 100 -200 worms per tree and 10 – 15 litres of water per hectares in the field that has sufficient moisture all year.

CHAPTER 9AEROBIC AND ANAEROBIC BIOCONVERSION

SYSTEMS

Composting Methods From time immemorial it has been known that decaying organic

wastes improve the fertility of the soil. The accumulation of raw sewage is increasing in every city in

addition to city refuses like garbages. In the farm, animal wastes and crop residues are accumulating. However, as raw material these agricultural and city refuses cannot

be used as organic manures because of high C/N ratio and having highly resistant. lignin like materials, other cellulose and hemicelluloses.

Varying factors influence the maturity of the composts and also the rapidity and nutrient status of the composts.

Different methods of biodegradation and composting are available for adoption depending on the source of material available and other facilities present in the farm.

FACTORS INFLUENCING COMPOSTING

In nature there are many internal and external factors affecting the rapidity of compost making. The nature of material to be composted influences greatly than the other factors.

AEROBIC AND ANAEROBIC BIOCONVERSION SYSTEMS

Composition of waste materials:The important factor in the type of waste material to be converted into compost is its nutrient composition. Generally the microorganisms act on these nutrients for biodegradation. The carbon nitrogen ratio of the waste material influences significantly. The rate of decomposition of organic matter of the waste depends on the amounts of carbon and nitrogen present. The C:N ratio of the biomass is in the order of 30:1 and it is an ideal ratio for the organic wastes to undergo biodegradation. However, in nature many of the organic wastes available are having a wider C:N ratio than this. The microbes utilizes the carbon of wastes as energy source and nitrogen for their body cell build up.


Composition of waste materials During this process the C:N ratio is narrowed down. Because

the carbon is degraded to gaseous carbon dioxide whereas N remains in the compost itself as part of microbial tissues. This process proceeds slowly and depending on the C:N ratio the time taken varied widely. Wider the C:N ratio greater the time it takes for getting a finished compost.

When we use wider C:N ratio materials like rice straw, wheat straw, sugarcane trash, leaf litter coir pith etc., it is advisable to mix these wastes with either FYM slurry or biogas slurry or green manure or legume green leaf manure or even limited application of nitrogenous fertilizers to narrow down the C:N ratio, i.e. optimum for quicker degradation.


Composition of waste materials

The time required for composting is reduced by the addition of the above materials.

Carbon: Phosphorus ratio optimum for better microbial activity is 100:1. Whenever waste materials having wider C:P ratio are to be composted it is recommended to apply phosphate fertilizer to narrow down the C:P ratio for quick degradation of the material and for conservation of nutrients. Compositions of some of the waste materials available in and around farm area are given in Table 8.1

In addition to C, N and P other nutrients like K, Ca, Mg, Na, S, Fe, Zn, Co etc are also needed for the microbial degradation. However, these nutrients are present in the waste materials to the required extent and have not created problem of external addition for composting.


Size of the waste materials:

The size of waste materials must be reduced to small pieces of about 5 cm to 10 cm length to provide higher surface area when the microbes can easily attack on these materials.


Moisture content: There must be optimum moisture for quick degradation of the

organic wastes. In general organics contain sufficient amount of moisture.

While mixing for composting wet and dry materials can be mixed together to get a balanced moisture.

Moisture to the level of field capacity is optimum. If the materials contains higher moisture we can mix it with soil or saw dust to reduce the moisture per cent. When the moisture content is in excess adequate per cent.

When the moisture content is in excess adequate turning can be given at intervals to reduce the moisture content and to increase aerobic decomposition.

About 40 to 50 per cent moisture is ideal to be maintained.


Aeration:

Adequate supply of gaseous oxygen is necessary specially for aerobic decomposition.

In the absence of oxygen anaerobic decomposition will dominate with the survival of anaerobic microorganisms.

Aeration is an important factor for proper decomposition.

Turning of the compost/manure pits is carried out for providing aerobic decomposition.

Type Source Nutrient content (%)

N P2O5 K2O

Animal

Oil cake

Crop

Weeds

Industrial wastes

Cattle dungCattle urineSheap and goat dungNight soilHuman urineLeather wasteHair and wool wasteHorn and hoof wastes

CastorCoconutCotton seedGroundnutNeemNigerMahuaRapeseedSafflowerSesame

Rice strawWheat strawSorghumPearl milletMaizePulsesSugarcane trashBanana stem

Water hyacinthPartheniumIpomoeaLegume weeds

Coir pithPressmudSeaweed residueCotton mill wasteBagasseSaw dustSewage sludge

0.400.800.651.51.27.012.312.5

5.83.26.57.85.24.82.65.17.86.2

0.580.490.400.650.591.600.350.11

1.791.431.312.00

0.261.151.501.400.250.251.50

0.150.020.500.80.20.10.17.5

1.81.82.81.71.01.80.81.82.22.0

0.230.250.230.750.310.150,040.26

0.280.290.350.17

0.052,400.500.600.120.202.00

0.200.700.200.50.30.20.31.2

1.01.72.11.41.41.31.81.02.01.2

1.661.282.172.501.312.000.500.43

0.430.740.551.70

0.841.984.201.200.40-1.50


Temperature:

During decomposition of organic waste considerable amount of heat is developed. Initially mesophilicmicroorganisms dominate in the decomposition with temperature below 40˚C. As the temperature increases subsequently above 40˚C the thermophilicmicroorganisms dominate.

If the moisture is maintained at optimum level the temperature will not rise above 50˚C. At higher temperatures the pathogenic organisms and weed seeds are destroyed.


Reaction:

In general, the reaction of the composting medium will be near neutral.

It may fall down to slight acetic range during decomposition due to release of organic acids by microbes.

The pH may rise to slight alkaline range when the maturity of the compost is attained. When the pH rises to alkaline range there is the possibility of loss of nitrogen in the form of ammonia volatilization.

Therefore, the pH is to be maintained at slightly acidic to near neutral level.


Micro Organisms:

In the early stage of composting fungi and acid producing bacteria dominate. It is the mesophilic stage.

As the temperature increases above 40˚C thermophilicbacteria, actinomycetes and fungi dominate.

Mesophilic organisms act on readily degradable carbohydrates and proteins. Actinomycetes degrade water soluble fractions.

Thermophilic bacteria attack on protein, lipids, and hemicelluloses. Thermophilic fungi acts on cellulose and lignin.

Depending upon the type of organic waste materials to be degraded the appropriate micro-organisms are inoculated.


Blending materials:

When the organic waste to be composed has a wide C/N ratio, then the material is to be supplemented with a narrow C/N ratio materials like leguminous plants, water hyacinth, biogas slurry etc.

Depending upon the necessity urea or ammonium sulphate or other nitrogenous materials can also be added.

When wet materials are to be composed, they are mixed with dry materials like dry soil also to reduce moisture and to absorb the released ammonia. Phosphate is also added for hastening the composting process.

METHODS OF COMPOSTING

There are:

Indore method of composting farm wastes

Bangalore Methods of Composting

Sugarcane trash composting

Biodegradation of weeds

The students will be able to explain these different methods of composting

THANK YOU VERY MUCH

bioenergy conversion lecture notes

Engineering