Constructing of Fixed Dome Biogas Plant Video

Constructing of Fixed Dome Biogas Plant Video

The video shares the process of constructing a fixed dome biogas plant, the initial charging process of the digester with water and dung. The video intends to demystify the technology and help new entrepreneurs or individuals in setting up biogas plants for domestic application.

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Purification of Biogas

Purification Technologies for Biogas Generated by Anaerobic Digestion

Q. Zhao, E. Leonhardt, C. MacConnell, C. Frear and S. Chen

Biogas is produced in many different environments, including in landfills, sewage
sludge and during anaerobic degradation of organic material. Biogas is comprised of
methane (CH4, about 45-75% by volume), carbon dioxide (CO2, 25-55%), and other
compounds including hydrogen sulfide (H2S, present in concentrations from several
hundred to a couple of thousand parts per million), water, and other trace gas
compounds. Methane is a powerful greenhouse gas if emitted into the atmosphere,
but can also represent a valuable renewable energy source, with the potential to
reduce GHG emissions when it is collected and substituted for fossil fuels.

Background

Biogas can be used directly to generate power, but the large volume of CO2 reduces
the heating value of the gas, increasing compression and transportation costs and
limiting economic feasibility to uses that occur at the point of production.
Purification allows for a wider variety of uses, either for heat and electricity, or for
vehicle fuels. For use as a fuel, purification to remove carbon dioxide (CO2) and
hydrogen sulfide (H2S) is required, because H2S corrodes vital mechanical
components within engine generator sets and vehicle engines if it is not removed.

Purified biogas provides reductions in GHG emissions as well as several other
environmental benefits when used as a vehicle fuel. Biogas emits less nitrogen
oxide, hydrocarbon and carbon monoxide than gasoline or diesel, and engines
fueled by purified biogas are quieter than diesel engines. Refueling with biogas
presents fewer environmental risks than refueling with gasoline or diesel, because it
can be done at small units located at an owner’s home or business, minimizing the
potential impacts if leaks or spills occur. Potential negatives include the high cost
($3-6/GJ) to upgrade the biogas, reduced driving range for vehicles dependent on
specialty fuel, and less cargo space due to biogas storage.

Feasible biogas purification technologies exist for large-scale sewage and biowaste
digesters, and the technologies for upgrading biogas, compressing, storing and
dispensing biomethane are well developed. If cost-effective methods for upgrading
biogas could be developed for the farm-scale, biogas purification could provide
dairy farmers with revenue to complement (or replace) electrical power sales. This
is especially critical in the Pacific Northwest, where low power rates have prevented
cost competitive power from farm-scale anaerobic-digesters, limiting total dairy-
derived power.

Engine conversion to accommodate biogas also represents a potential barrier, but
because biogas has the same properties as natural gas, it can be easily used by
vehicles which are configured for natural gas. Worldwide, there are about 10,000
biogas driven cars and buses, plus an additional 3.8 million natural gas fuelled
vehicles (representing 0.5% of the world vehicle stock), mainly in Argentina, Brazil,
Pakistan, Italy, India and the U.S. (ENGVA, 2004).

To help develop appropriate biogas purification technologies for farm-scale
anaerobic digesters, Washington State University evaluated various methods for
removing acidic impurities, and developed and tested absorption tower
technologies for application to a farm-scale anaerobic digester. In addition, Western
Washington University has begun the process of building a full-scale pilot system.
This pilot system will purify biogas from Vander Haak Dairy (Lynden, Washington),
and sell it to Airporter Shuttle/Belair Charters for use by buses running along the
Interstate 5 “Green” corridor from the Seatac airport south of Seattle to Ferndale,
north of Bellingham. This is a new project for the diary industry, fuel users, and the
community, as there is currently only one operational
transportation facility in North America, at the Hilarides Dairy in Lindsay, California,
which began operation in the summer of 2009.

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APPLICATION OF BIOGAS TECHNOLOGY AS A RENEWABLE ENERGY SOURCE AND ENVIRONMENTAL FRIENDLY TECHNIQUE TO MANAGE SOLID WASTE | Research Paper

APPLICATION OF BIOGAS TECHNOLOGY AS A RENEWABLE ENERGY SOURCE AND ENVIRONMENTAL FRIENDLY TECHNIQUE TO MANAGE SOLID WASTE

H.M.C.K.Bandara, K.D.N.Weerasinghe, G.Y.Jayasinghe Faculty of Agriculture, University of Ruhuna, Sri Lanka

ABSTRACT

Biogas technolog y can be play a vital role in supplying of energy demand whileutilizing solid waste as a raw material for the gas generation. This research was carried out to find out possible avenues to produce Biogas from Agricultural waste such as paddy straw and market garbage.The objectives of the study were to (1) analyze the chemical composition of market garbage and paddy straw, (2) find the composition of market garbage and (3) assess the gas production pattern in biomethantion of market garbage and paddy straw. Two
Sri Lankan batch type digesters were used with a volume of 5m3 and total weight of the market garbage and paddy straw filled in to the digester were 3580 kg and 871 kg respectively. Cow dung and Urea were added to the paddy straw digester as inoculum s and to balance the C: N ratio in to 30:1. Carbon, Nitrogen, Potassium and Phosphorous in raw material were analyzed. Daily gas generation and pressure indicate by the manometer in each digester were measured. Result revealed that dry matter percentage in market garbage and paddy straw were 19.27% and 85.69% respectively. The Carbon, Nitrogen, Potassium, Phosphorus and C: N ratio of market  garbage was 34.66%, 1.57%, 1.66%, 0.311% and 22.07 respectively. In paddy straw,
it was 46.8%, 0.69%, 0.098%, 0.16% and 67.82 respectively. Total gas production from market garbage and paddy straw were 164.20 m3 and 156.20 m3 during the 228 days and 166 days respectively. From the result it could be concluded that market garbage enhances with higher Nitrogen, Potassium, Phosphorous values in comparison to paddy straw. Market garbage digester shows higher gas production with a long lag phase. Average daily gas production from Paddy straw digester could be sufficient for cooking purposes of
an average household (approximate ly 3.82 cooking hours).

Keywords:
- Paddy straw, Market garbage, Biomethantion, Digesters, Renewable energy

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Biogas Plant Notes Chapter 12| Pending technical issues

Pending technical issues

Several tests have been made by CAMARTEC. The results of these tests have not yet been put into practice, but might already be of interest to the reader.

Position of the Outlet

As reported above, the slurry is found in layers of different TS-content. Until now, the outlet pipe has been placed near the bottom, because it is known that digested slurry is heavier than the fresh substrate. At the same time, liquid slurry which hardly produces any gas remains for a long time in the plant. On the other hand, active sludge is driven out from the plant relatively early. Hence, the retention time of the viscous slurry is reduced. When the outlet pipe is placed higher up in the liquid zone, unproductive liquid overflows and active sludge remains longer in the plant. CAMARTEC has not yet decided if the volume of the plant could be reduced when the outlet pipe will be higher or if two alternative outlet pipes would be provided. For the time being, the present models remain valid.

Standpipe for Gas Release in Case of Scum

In one case, where pigs have been fed with husks, a heavy layer of scum had been developed. Most of the gas escaped through the inlet pipe because it could not penetrate the scum. It has now been proposed to place a perforated plastic pipe vertical in the centre of the digester, to allow the gas to pass the scum. It might still be necessary to take out the scum from time to time. For easier opening and closing of the lid, a rubber sealing and wedges of steel would be advisable.

Pre-Heater for Water

In institutions, or when there is a surplus of gas, pre-heating of water might be a reasonable way of utilizing the gas. The gas burns continuously and heats the water to 30-400C. This water is used for washing or for boiling. A long-lasting and reliable apparatus has not yet been developed.

Slurry Lifting Device (Pressure Booster)

In case of an insufficient natural slope, the slurry could be lifted up with the help of water pressure from a main. Slurry flows in a pressure tank which is connected to a water pipe with sufficient pressure. The slurry inlet valve will be closed when the tank is filled. Then the water cock will be opened. The hydraulic pressure transports the slurry to a higher level from where it flows by gravity to the field. When the slurry becomes watery, the water cock will be closed and water from the pressure tank will be drained off. Then the slurry valve is opened again for re-filling. The system is valid when water is supplied free or at a low price from the main. The volume of the pressure tank depends on the intervals of distributing the slurry.

Fig.42: Stand-pipe for gas release

In case of heavy scum formation, a perforated 4"-plastic pipe ( 1 ) is placed in the centre of the digester to allow the gas to pass through the scum. Thus, removing of scum will be only necessary in longer intervals. It is held at the bottom in a concrete foot (2) and fixed at the top with a timber cross (3). The pipe can be inserted, when necessary, even after the plant is in operation (4).

Gas Pressure Equalizer

When there are several biogas plants scattered. over the compound of an institution or a bigger farm, the pressure will be different because of the friction losses and different gasplant designs. A standardized instrument is to be developed which can be installed in such places.

Source:http://www.greenstone.org

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Biogas Plant Notes Chapter 11 | Operation and maintenance

Operation and maintenance

Operation of the Biogas Plant

If operational short-comings are often reported, the set up of the system is not appropriate to the farmer. The main task of a biogas engineer is to design and construct a user-friendly biogas unit. Operation and necessary maintenance must be logical to the user and should not be a burden to the ones attending the plant. A well designed biogas unit is easy to maintain. The ease of maintenance ensures constant attention by the farmer. Nevertheless, even with a perfect design, a minimum of daily care is needed to receive a proper service from the unit.

The clay sealing of the lid must stay moist. Therefore, the lid must be covered with water all the time. In order to reduce evaporation and prevent mosquito breeding, machine oil can be added on the surface of the water. Mineral oil pollutes ground water and should only be used in small quantities and with care. From time to time the water above the lid must be checked and refilled when necessary. A small control opening in the topmost cover makes control much easier and therefore more likely. When water is controlled, possible leakages at the lid are also detected.

Once in a while the expansion chamber should be cleaned In order to avoid solids assembling in the corners and thus, reducing the gas storage capacity.

The plant must be fed regularly in order to achieve regular gas production. The substrate should be free of stalks and other impurities in order to avoid scum formation and blockage of the inlet and outlet pipes. After removing straw and waste fodder from the dung, it should be mixed sufficiently with urine or water to avoid separation of solid and liquid material inside the digester. Every day the liquid from the urine chamber must be transferred into the mixing chamber. How much liquid, i.e. how often the urine chamber needs to be emptied per day, depends on the amount of dung. As a rule of thumb 1 kg of dung requires 1 l of liquid. The user is to be advised by the BES-staff.

Chopping of the fodder into pieces of 3-5 cm length saves fodder grass and reduces the amount of stalk mixing with the dung on the floor.

In dry environments the amount of urine might not be enough to obtain the required mixing rate. Water must then be added. However, It is better to give more water to the cows instead of adding water to the plant. Beside producing more urine the cows will be in better health and produce more milk.

The overflowing slurry should move away from the outlet. Otherwise It can block the overflow and the gas pressure might increase until it escapes through the inlet pipe or blows off the water trap. Therefore, the outlet and the slurry canal must be cleaned. This must be part of the daily routine of cleaning the stable and feeding the plant. The problem becomes less, if a proper slope is maintained and the slurry canal is shaded off from direct sunshine.

The slurry distribution system must be cleaned and slurry directed towards the plants for fertilization. If this is not done, the biogas plant does not suffer, but the farmer will waste valuable manure.

Fig.40: Cleaning the overflow point at the expansion chamber

Maintenance of Toilets

Well designed toilets do not require any maintenance except for cleaning the inlet and the floor of the cubical. Once a toilet is soiled and starts smelling badly, It is very difficult to again achieve cleanliness. Clean water should be used without disinfectants so as not to kill the methane bacteria in the plant. Soap water, from time to time, can be tolerated.

Regular Maintenance of Appliances

Psychologically, the stove should be regarded as a kitchen ware and not as a fire place. There is no maintenance needed besides keeping it clean like other kitchen vessels and utensils.

The lamp needs cleaning of the glass screen in order to have bright light all the time. Cleaning should be done only if necessary to avoid shocks on the lamp that can destroy the gas mantle. Gas mantles of lamps only have a certain life time and ought to be replaced frequently. They are fixed with a string to the nozzle and the replacement is easy and does not require any skill. Used mantles are radioactive. Therefore, dust or pieces of the broken mantle should not come in contact with foodstuff. Children should be protected from inhaling the dust when they are around. Hands and working place should be cleaned with water after replacement of the mantle.

Disturbance of the System

Trouble shooting becomes necessary if the customer, this is the owner or the user of the gasplant, complains about insufficient service or any nuisance caused by the plant. There are three sources for possible complains:

-Insufficient gasplant performance

-inadequate amount or kind of feeding material

-too high expectations on the service of a biogas unit

The latter is very difficult to deal with, because the fault was done when persuading the farmer with wrong promises. Serious information about possibilities and limitations of a biogas plant are the only way to have content customers. On the other hand, the farmer might have exaggerated the amount of feed material available to him. This can be the case when the animals are taken out for grazing when zero-grazing was expected. The actual amount of dung must be checked in order to distinguish between short-comings in gasplant performance or underfeeding of the gasplant. A well functioning plant produces between 35 to 40 litres of biogas per kg of fresh cattle dung, depending on fodder, temperature and retention time. In rainy or colder seasons the gas production may drop to 60-70% of the normal rate.

Interruption of Gas Production

"There is not enough gas" is by far the most common problem mentioned by the user. When the cause is found, normally the remedy is easy, except for scum problems. The following steps will help to find the reason for the gas shortage quickly:

Ask the user if gas supply is only less or if it stopped completely. Check the information given by the user concerning the appliances complained about. Ask if the problem occurred suddenly or gradual and when it was noticed first. If the problem occurred suddenly, a technical fault is very likely. If gas supply dropped gradually, one may guess that there is something wrong with the performance of the plant. This might be caused either by unsuitable properties of the feeding material or by inadequate feeding practices.

Check If there is gas in the plant. If the slurry level In the outlet chamber is high and slurry at the overflow is fresh, there is gas production. If the gas pressure is high but no gas reaches the point of use, there must be a blockage somewhere.

If there was no discharge of slurry because of not enough pressure inside the plant, there might be a leakage. Ask or observe if there is smell of gas in the kitchen. Check the lid for bubbles. Check the valves and than the Joints for leakages by applying soaped water to it. If no leakage is found, close the main valve and wait one day for gas pressure building up. If gas is produced, which can be seen if bubbles come up at the outlet or inlet pipe, but pressure does not build up, there must be either a leakage which opens up by increased pressure or a crack in the dome below a certain slurry level. A crack in the dome is the worst of all cases. The plant must be emptied and cracks must be repaired.

If there is no gas production at the plant, observe the smell of the slurry. If it smells sour, the fermentation process has been disturbed. Wait some time (maximum 4 weeks) without feeding the plant or feed with material from an other stable. If gas production does not start by itself again, the plant must be emptied and refilled with fresh material. Such a break down of fermentation is very rare and rarely happens with cattle dung, except in case of animal diseases treated with high doses of antibiotics. In most of the cases, gas is produced but can not arrive at the place of consumption.

If gas is produced but not available at only some of the stoves, lamps or other appliances, there is a blockage in the piping system or the Jet. If Jets are clean, there might be a water blockage in the piping system just before the appliances. Ask the customer, if gas was flickering before it finally went off. In this case, check If there is a water trap at the lowest point of the piping system. If not, change the pipe line or place a water trap. If there is once water in the pipe, there will be always water in the pipe. Reconstruction is the only solution.

The Problem of Scum

If there is heavy gas release from the inlet but not enough gas available for use, scum is most likely the reason. Often the gas pressure does not build up because of the continuous release through the inlet. Slurry does not overflow for weeks. There is the danger of blocking the gas pipe by rising scum because of daily feeding without equivalent discharge. The lid must be opened and scum is to be taken out by hand.

Fig.41: Scum formation in a fixed dome plant

The scum may prevent the gas from reaching the gas outlet pipe (1). Instead, the gas will form large bubbles below the scum (2) from where the gas escapes through the inlet pipe (3). If the gas cannot escape, it might also burst the brickwork structure. Therefore, unsuitable dung should not enter the plant. Suitable dung should be filled in fresh. Stalks and other fibrous material should be sorted out and be stored directly on the compost heap. Dried dung should be thoroughly mixed with urine or water before entering the plant.

Straw, grass, stalks and even already dried dung tends to float to the surface. Solid and mineral material tends to sink to the bottom and, in the course of time, may block the outlet pipe or reduce the active digester volume. In proper mixed substrate there is no such separation because of sufficient friction within the paste-like substance.

With pure and fresh cattle dung there is no scum problem. Floating layers will become a problem when husks are part of the fodder. This is often the case in pig breeding. Before installing a gasplant at a piggery, the kind of fodder and consequently the kind of dung, must be checked to ensure If it is suitable for a biogas plant. It might be necessary to grind the fodder into fine powder. The user must be aware of this and the occurring costs before deciding on a biogas unit. The problem is even bigger with poultry droppings. The kind of fodder, the sand the chicken pick up, and the feathers falling to the ground make poultry dung the most difficult substrate. In case of doubt, no gasplant should be build.

Scum can be avoided by stirring, but....

Stirring must be done for 5 minutes every hour, throughout day and night to avoid scum formation. This can only be assured by automatic mechanisms and should not be expected from workers attending the stable. For simple gasplants, stirring is not a viable solution against scum formation.

Scum can be broken by stirrers, but....

Scum is not brittle but very filthy and tough. Scum can become so solid after only a short time, that It needs heavy equipment to break it. It remains at the surface after being broken up. To destroy it by fermentation, it must be kept wet. Either the scum must be watered from the top or pushed down into the liquid. Both operations demand costly apparatus. For simple gasplants, stirring is not a viable solution for breaking the scum.

The only solution for simple biogas plants to avoid scum is by selecting suitable feed material and by sufficient mixing of the dung with liquid before entering the plant.

Trouble with Feeding the Plant

When there is a problem of charging dung and urine to the plant, there is a blockage at the inlet pipe. The problem might be caused by straw or grass and could be solved by thorough poking. If this happens more often, there might as well be scum blocking the inlet pipe from below. By entering a stick or a pipe into the inlet one may find out where and of which nature the blockage will be. Don't be surprised to find stones or other trash in the pipe which had been placed there by playing children. If it feels sandy, there is a heavy accumulation of soil below the inlet pipe. In this case, open the lid and scrap the sand off With a dipper or steel shovel. Only in serious cases the plant needs to be emptied.

Faults at Appliances

It is surprising to see, how much complaints arise because equipment was not kept clean. Often food has dropped into the- burner head but sometimes dust or cinders block the Jets of burners or lamps. Normally, blockage of the jet is removed with help of a fine wire or needle. Only when blockage occurs in short intervals, the jet must be dismantled and cleaned. The rubber tube is to be disconnected and freed from dust by blowing through.

If a lamp starts to loose its brightness, it is very likely that dust particles have blocked the nozzle. Again, the nozzle must be unscrewed and cleaned.

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Biogas Plant Notes Chapter 10 | Use of gas

Use of gas

General

As biogas burns with an open flame, the place of gas consumption should be ventilated but free of draft directly at the flame. The flame will be more stable. For lighting, the lifetime of lamp mantles will also be prolonged. Sensitive equipment like refrigerators or incubators should be situated where they can be controlled.

Biogas can be used like any other combustible gas, e.g. LPG. Each gas has its own properties which must be observed for efficient combustion. The main influencing factors are:

-gas/air mixing rate

-flame speed

-ignition temperature

-gas pressure

Compared to LPG, biogas needs less air per cubic metre for combustion. This means, with the same amount of air more gas is required. Therefore, gas jets are larger in diameter when using biogas. About 5.7 litre of air are required for total combustion of 1 litre of biogas, while for butan it is 30.9 litres and for propan 23.8 litres.

The flame speed is lower with biogas than with LPG. Therefore, speed of gas at the burner heads must be reduced. This can be achieved by conical orifices, but normally, the bottom of the cooking pot functions as a speed breaker for the flame.

The ignition temperature of biogas is higher than of diesel. Therefore, when biogas is used in engines, ignition spark plugs are required or partly diesel must be added to the gas (dual fuel) to run the engine. Slow turning diesel engines (approx. 2000 RPM) suit biogas better than fast turning Otto-engines (above 5000 RPM).

The efficiency of using biogas is 55% in stoves, 24% in engines but only 3% in lamps. A biogas lamp is only half that efficient than a kerosene lamp. The most efficient way of using biogas is in a heat-power combination where 88% efficiency can be reached. But this is only valid for larger installations and under the condition that the exhaust heat is used profitably. The use of biogas in stoves is the best way of exploiting the energy of farm household units.

Gas Stoves

All gas burners follow the same principle. The gas arrives with a certain speed at the stove. This speed Is created by the given pressure from the gasplant in the pipe of a certain diameter. By help of a jet at the inlet of the burner, the speed is increased producing a draft which sucks air into the pipe. This air is called primary air and is needed for combustion. Therefore, it must be completely mixed with the biogas. This happens by widening the pipe to a minimum diameter, which is in constant relation to the diameter of the Jet. By widening the pipe further the speed of the gas again is reduced. This diffuse goes over Into the burner head. The cone of the diffuse and the shape of the burner head is formed in such a way as to allow the gas pressure to equal everywhere before the gas/air mixture leaves the burner through the orifices with a speed only slightly above the specific flame speed of biogas. For final combustion the gas needs more oxygen which is supplied by the surrounding air. This air is called the secondary air.

Fig.37: Gas stoves

Biogas from a fixed dome plant arrives with a pressure of 50-60 cm W.C. on an average. The ideal stove (1) has a diffuse cone (2) and an air breaking ring (3). For manufacturing in a normal ironmonger workshop, the shape of the stove is slightly simplified (4). Important dimensions are the diameter of the Jet (5), the length of the air intake holes measured from the end of the jet (6), the length of the mixing pipe (7) and its diameter (8),, the number and diameter of the [lame port holes (9). Increasing the height of the burner head ( 10) makes good for the missing diffuse cone. The top lid (11) is loosely laid on the burner head. It projects 1 cm over the rim (12). The air-intake holes of the mixing pipe (13), made by two drillings of Ø 8 mm (14), should be at the side as to avoid food dropping into them. The burner is screwed to a steel frame out of L 50·3 (15). Brackets welded on the top of it (16) secure the proper distance between the bottom of the pot and the flame port.

If combustion is perfect, the flame is dark blue and almost invisible in daylight. Stoves are normally designed to work with 75% primary air. If too little air is available the gas does not burn fully and part of the gas escapes unused. With too much air supply the flame cools off and thus, prolonging the cooking time and increasing the gas demand.

Manufacturing Stoves

Gas stoves are relatively simple appliances which can be manufactured by most blacksmiths or metal works. Gas stoves of mild steel may corrode if the hydro-sulphur content in the biogas is high. This is often the case when biogas is produced from human excrete or pig dung. Therefore, high quality steel or cast iron is advantageous. Clay burners are widely used in China and have proved to render good service. For experimental use in schools, stoves can be made from used food-tins.

When manufacturing stoves in an ironmongers workshop, the shape of the burner must be simplified. This is justifiable because the methane content and the gas pressure are changing. Full adaptation is not possible for that reason. A standard design used for biogas delivered from fixed dome plants has been developed.

When it happens that all the flames are torn off the flame port and ignition becomes impossible the flow speed must be reduced. This can be done by reducing the volume of the gas/air mixture by partly closing the air intake holes.

The stove itself, i.e. the stand for the pots, needs consideration as local food habits have high influence on the design. The stand must be strong to allow stirring of even thick foods like "ugali", rice or stew.

Modification of LPG Stoves

LPG stoves can be modified to fit the properties of biogas. The efficiency will often not be as good as with a genuine biogas stove. Hence, the geometry of the burner will not be known exactly, modification remains subject to trial and error. The easiest way is to close the primary air inlet completely 'and then widen the Jet according to the wanted heat supply. The air intake might then again be opened little by little. When lighting the burner about half of the orifices should bear flames. After a pot is placed on the fire, all orifices are ignited.

The jets of LPG burners can be widened with a drill. It is better to go step by step instead of spoiling the burner by opening the jet too far. For example, an original 1.2 mm jet should be widened in the first step to 1.4 mm only, in the second step to 1.6 mm until it gives the wanted result. If there is no vice at hand, the drill can even be used without a drilling machine when jets are of soft brass metal.

Gas Lamps

Although biogas lamps have proved not to be economical compared to kerosene lamps, they are often the major reason for wanting a biogas plant rather than the clean and smokeless cooking fuel.

The principal of a gas lamp is similar to that of the stove. With a stove, the burning gas heats a pot. In a lamp, the burning gas heats a mantle until it glows brightly. The secret behind a lamp is to adjust the flame in such a way that the hottest part of the flame exactly matches the form of the mantle. Proper air mixture and appropriate size of the mantle play the biggest roles. The methane content of biogas sometimes changes. Therefore, brightness of the light will also change.

Local production of lamps is far more problematic in design and more complicated to manufacture than producing efficient stoves. Trial and error provides the best method in most cases. There are several lamps available that could be imported from India, China, Kenya, Brazil or Italy. The Patel outdoor lamp (Pate Crafters, Bombay/India) has proved to be the most expensive but also the best serving model. It has an air mixing chamber where outside air gets pre-heated before combustion.

Modification of Kerosene Pressure Lamps

Kerosene pressure lamps (petromax, anchor, butterfly and others) are available in most countries. They can be modified and there is no need to import special biogas lamps. Instead of 0,09 l kerosene 0,186 m³ biogas is consumed per hour. To modify a pressure lamp the workshop must be equipped with a lathe. In principal, the jet is widened and a new mixing pipe is mounted. The gas is connected via the original pump opening.

Other Appliances

Biogas can be used for various activities and requirements common in the project region. Refrigerators and chicken heaters are the most common. There are individual cases of using biogas for coffee roasting, bread baking or sterilization of instruments. If the properties of biogas are observed, there is no limitation to its utilization.

Fig.38: Biogas appliances

(1) Gaslamp made by CAMARTEC from aluminium vessels and a glass mantle from a kerosene lamp. The lamp worked well but was never mass produced. (2) Gaslamp from Italy, (3) from Germany and (4) from Brazil. (5) Big cookstove for institutions, schools etc., developed, built and exported by CAMARTEC. There should always be a smaller stand-by stove in institutional kitchens (6) Household stove from KIE, Kenya (7) Coffee roaster for 1-3 kg of coffee placed on a tube burner or ordinary kitchen stove. (8) Gas refrigerator modified for biogas by changing the jet and the air-intake holes. (9) The "Detroit" chicken hedger with temperature regulation with a floppy cap (a). (10) A room heating radiator, mainly used in chicken houses or piglet styes. A ceramic plate causes heat radiation.

Fig.39: Modification of pressure lamps

For modification, the head of the lamp ( 1 ) is to be taken off after removing the wig. Because the flame port (2) might break, unscrew it and save it. Remove the kerosene adjustment screw (3) from the U-pipe in' the head Either close the hole by hard soldering or cut the screw so short that it only closes the hole but does not disturb the gas-flow inside the U-pipe.

Look at the head from below and widen the hole (4) beside the flame port socket to Ø 15 mm in order to receive the new mixing pipe. Then enlarge the two notches at the rim (5) of the head from each side to 15 mm width. This will help to replace and remove the head easily. The head is now modified.

For modification of the gas intake system, unscrew the brass jet (6) which is fixed at the top of the standpipe (7). The jet's opening could be widened to Ø 1 mm but a stainless steel jet (8) is much more durable when using biogas. This will be made from a massive rod of Ø 8 mm and should have a thread of M7 · 1. Now unscrew the standpipe and you will find a long needle (9) with a tiny pin (10) at the top. The pin moves up and down, cleaning the jet when the handle of the main valve is turned (11). Unscrew the pin holder (10a) from the needle and widen the thread at the head of the needle with a drill of Ø 0.9 mm to 5 mm depth (12). Insert a steel pin 0 0.9 mm in there (13) or cut the drill in half and use the drill itself as the new pin for cleaning the new 0 1 mm-jet. Press the head of the needle with pliers to fix the pin (14). Take the old pin holder and cut its thread and the pin off and place the remaining shaft over the new pin (15) in order to give better guidance to the needle when it moves up and down inside the jet.

The top of the standpipe will be provided with a thread M10·1 (16) to receive the new mixing pipe which will extend the existing standpipe by 60 mm. Fix the jet to the standpipe and screw the standpipe in its former position. Make sure that the cleaning pin is turned up when replacing the jet with the standpipe (17).

The new mixing pipe (18) is made from Ø 18 mm brass rod. The lower part is turned to Ø 13 mm with an inner thread of M10·1. Four air holes of 0 6 mm cross the pipe. Their rims are bevelled. The upper part is turned to Ø 10.3 mm outside and 7.85 mm inside, where a stainless steel pipe (19) of Ø 8 mm outside and Ø 6 mm inside is pressed in. At the lower end of the mixing pipe there is a 3 mm flange to prevent uncontrolled air supply to the flame (20). After screwing the mixing pipe to the standpipe the inner parts are modified.

Remove the pump at the kerosene tank (21 ) and remove the inner valve with a screw driver. Either produce a new cap with hose connection nozzle and thread M20·0,8 mm or fix a hose connection to the existing cover cap (22). Make the joint gas-tight by soldering. Use teflon sealing tape for placing the cap on the nipple.

Assemble all parts, fix a new mantle (23), connect the gas (24) and light the lamp (25).

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Biogas Plant Notes Chapter 9 | Use of slurry

Use of slurry

General Properties of Digested Organic Matter

Digested substrate has almost no smell and is more liquid than undigested dung. These facts are the result of the same process which leads to the production of biogas, the transformation of long carbon chains (cellulose, alcohol and organic acids) into short carbon molecules such as CH4 and CO.. As part of the total carbon content of the substrate is transferred into biogas, the carbon/nitrogen ratio becomes more narrow. Only if the C/N ratio of a manure is narrower than that of the soil, one may talk of nitrogen supply by the manure.

Nitrogen is a major nutrient required for the plant growth. Nitrogen from organic manure has to be extracted by bacteria from large organic molecules and transformed into smaller inorganic water soluble compounds before plants can use it. This transformation is called mineralisation. During the digestion process in a biogas plant, part of the organic nitrogen is mineralized to ammonium (NH.+) and nitrate (NO3-) and thus, may be taken up by the plants immediately. The short term fertilizer value of the dung is doubled while the long term fertilizing effects are cut by half. Under tropical conditions the short term value is of greater importance because rapid biological activities degrade even the slow degrading manure fraction in relatively short time.

If ammonia is not dissolved in water it may escape as gas into the air. Therefore, digested slurry has to be kept moist or covered by soil to preserve its fertilizer value. The best way is to bring it immediately in liquid form to the roots of the plants. An other possibility is to compost the slurry together with other organic material. During composting ammonia is bound again in organic form by bacteria and does not evaporate.

Many chemical processes take place at the same time which need different attention. But in general, two rules must be followed for preserving the plant nutrients of both the undigested dung and the digested slurry:

-avoid long storage times and

-keep manure moist, cool and covered.

To avoid long storage times it is better to clean the stable twice a day instead of every second day. It is also better to use the slurry directly on permanent crops like fodder grass, trees or vegetables than on annual crops like maize or millet.

The Slurry Disposal

Clever and realistic planning of the site of slurry utilization is the key to an economical biogas unit. Insufficient slurry disposal leads to blockage of the outlet and rising gas pressure Inside the fixed dome plant. The volume-of digested slurry is about twice as much as that of fresh dung. Slurry manure must reach the crops without loosing too much of its fertilizer value. Whenever possible, slurry should be distributed directly to the crop by gravity. The best way Is using irrigation channels for slurry distribution. To prevent loss of nitrogen, the manure pits and slurry distribution channels should be covered or placed under shade of plants. A compromise must be found between shading the manure and not disturbing the flow of the slurry by roots growing into the canal. Shading by fodder grass Itself might be less troublesome than by trees.

Fig.33.: The nitrogen-cycle in nature

If fertilizing follows the cropping pattern, space for a liquid storage tank of sufficient size must be provided beside the point of overflow to bridge the time of no fertilizer use. Similar space would be needed if composting of slurry is envisaged.

The point of overflow can be extended nearer to the field to allow sufficient slope when the expansion chamber is shaped like a canal. There is also the possibility of arranging the compost heap parallel to this expansion canal. Slurry might then be taken out at convenient spots for pouring over the compost.

Use of Liquid Slurry

It is more Important to use the slurry instead of propagating complicated and labour intensive systems of optimum manure utilization. Any use of slurry is a good use. Therefore, whenever possible, slurry should be used in liquid form immediately after leaving the overflow of the biogas plant. A minimum slope of 2,5% is required for short distance distribution. Slope is to be increased for longer distances and in dry areas. Distribution of liquid slurry needs management Uncontrolled distribution may create swamps or thick layers of dried slurry sealing off the roots of crops or trees from necessary oxygen supply.

The most labour saving slurry utilization is for fodder grass. It should be encouraged when controlled fertilizing is unlikely. The fodder is planted near the stable where it is used. The gasplant is near to keep slurry distribution channels short.

Fig.34: Slurry distribution by gravity

Distribution channels need a minimum slope of 2,5% (1), in dry areas 5X may be required. The slurry is dumped amongst the fodder grass when cleaning the channels (2). Slurry flows mainly to recently harvested areas (3). Slurry is spread on vegetable filelds higher than the outlet and near the plant by buckets (4), The stable in the background fulfils the minimum requirements.

Fodder grass should be cut when it is only 80 cm high. Slurry is led always to the freshly cut area. Per cow 500 m² of fodder grass are a guiding figure. Fodder grass is a permanent crop which makes the installation of a permanent distribution system advisable. The main distributor could be constructed even in concrete or laid out with concrete slabs. Branch canals every 2 m would allow equal fertilizer supply. Covered channels would be the best.

Where gravity distribution is not possible, liquid slurry must be carried to the plants by buckets or on specially adapted wheelbarrows. Wheelbarrow transport needs passable pathways.

Use of Slurry for Compost

The preparation of compost is best if distribution by gravity is not possible. Investment and labour input are reasonable and the nutrition value of the manure is preserved. Composting is a form of storing the slurry over some time without loosing too much nitrogen. Compost is also a method of increasing the amount of organic manure which stabilizes the soil structure. Compost is superior to liquid slurry for long-term improvement of soil fertility. Compost releases its nutrients slowly and therefore, is applied in few but larger doses over the year.

In principal, compost is, prepared in alternate layers of liquid slurry and fibrous agricultural residues. Compost should be made in heaps instead of pits because air is required to promote the rotting process. Compost should be kept in shade and should never dry out. A less optimal but utilized compost is better than propagating the ideal method which will not be applied by the farmer. The compost dam which is regularly poured over with slurry and sometimes turned over, is a reasonable compromise.

When compost is prepared, it is advisable to shape the expansion chamber of the biogas plant like a canal running parallel to the compost hear.

Fig.35: The slurry-cart

A wooden wheelbarrow ( 1 ) developed by CAMARTEC is modified to serve as a slurry transport cart. It is heavy but stout and has almost no metal parts which could corrode. The lid (2) prevents the slurry from spilling over. At the front it is held by a slot (3) at the longitudinal slat. The slurry is either dumped into a distribution system or taken out from the wheelbarrow by buckets.

Fig.36: Compost preparation

Compost consists of slurry and fibrous organic residues, like grass, leaves, and straw. It has a total solids content of 50%. The compost heap must be turned over several times during the 6 months ripening period, The inside temperature of a good compost heap is 60-70°C. The rotting process demands air. Therefore, the compost heap should be narrow and above the ground. The ideal compost heap is roofed and set in alternating layers of residues and slurry ("Indore" method) (1). In connection with an expansion canal of a biogas plant, the compost dam (2) is an appropriate compromise. Dumping 01° residues starts at the far end. The compost dam "grows" to the front and is poured over regularly with slurry (3). After some weeks it is turned over to the side (4).

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Biogas Plant Notes Chapter 8 | The sanitary biogas unit

The sanitary biogas unit

General

Sanitary Biogas Units are installations where the gasplants have been built in order to treat the waste of latrines. Human faeces are the main digestion material. Additional feeding with animal dung or kitchen waste is possible. Hygienic latrines have to fulfill the following requirements:

-no handling of human excrete by man; even accidental touch shouId be avoided

-no access of flies to undigested excrete

-no worms may escape from the latrine pit

-no bad odour and no indecent appearance

Important design criteria concerning hygiene and construction quality must be observed. Main planning criteria are the expected sanitary conditions which depend on frequency of use, frequency of cleaning, and safe slurry disposal. Slurry should be used for fertilizing trees or shrubs but not vegetables. The slurry may also drain into a soak pit. Energy and manure provision are of lesser importance but should be optimized whenever possible.

Construction of Toilets

Toilets connected to a simple fixed dome biogas plant should be latrines where a minimum of water is used for cleaning. Flushing toilets are not suitable for connection to biogas plants of less than 30 m3 digester volume because of the danger of diluting the slurry and thus reducing the retention time.

The toilet chamber is connected to a vent pipe which passes the roof. It is placed outside, if possible not shaded, and is painted black as to heat up for better draft.

Fig.32: Construction details of toilets

The toilet floor (a) has a groove serving as the toilet pan. The highest slurry level (b) and the lowest slurry level (c) depend on the gas storage requirements of the biogas plant. There are a few but very important details to be observed: (1) The inlet consists of a piece of pipe of 6" diameter placed absolutely vertical in order to avoid soiling the sides. (2) The bottom rim of the inlet piece is separated from the lower system in order to prevent worms from crawling out of the toilet. (3) The inlet piece ends always above the highest slurry level. (4) Below the inlet piece is a chamber of larger surface in order to avoid floating feces piling up in the pipe. Feces should drop directly into the slurry but never on parts of the structure which are normally above the slurry level. On the other hand, the dropping chamber should be as small as possible in order to release fresh feces as quickly as possible into the biogas plant. This is important for avoiding bad odour and for producing the biogas there, where it can be collected and utilized, which is the inside of the dome. (5) The down pipe is straight and at least of the same diameter as the inlet piece (6). A vent pipe passes above the roof (7)

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Biogas Plant Notes Chapter 7 | Construction of cattle stable

7. Construction of cattle stable

General

The stable should be near the house in order to keep the pipeline from the gasplant to the kitchen short. The fodder trough must be easily accessible. Water for the animals should be near the trough. The milking place lies at the highest floor level of the stable, opposite to the biogas plant. The roof should not drain its rainwater onto the biogas plant.

The stable should allow easy collection of dung and urine to charge the biogas plant. Whenever possible, the existing stable should be kept and modified if necessary to fulfill the above requirements. In many cases a new concrete or tiled floor will do. A new stable should be constructed in cases the farmer wishes to modernize his live stock or if adaptation would demand intolerable compromises.

The size of the stable depends on the number of animals and whether they are freely moving or tied-up. A stable suitable for a biogas unit might require more space than the existing one. Later extension should also be possible without changing or affecting the biogas plant.

The Principles of Design

The ideal cattle stable is a zero-grazing unit with separation of milk cows, heifers and calves, roofed or non-roofed exercising area, separate milking stand, restricting fodder trough and solid floor with urine channel. The fodder chopper, the squeegee and the dipper are essential tools for operation of the stable.

A stable connected to a biogas plant has to fulfill the following indispensable criteria:

-solid floor with urine drain in order to collect dung and urine without soiling

-dung and urine collection point lies higher in level than the inlet of the biogas plant in order to avoid laborious handling and losses of substrate

-animals are fed from a trough with neck restriction in order to avoid spreading of too much of fibrous material on the floor which will then enter the plant.

Additional favourable criteria are:

-stable is roofed in order to avoid rain water washing away too much of urine and dung

-resting and exercising area are separated in order to limit the area of dung dropping and to keep animals clean

-calves, heifers and milk cows are separated because of their different sizes to allow an optimal design of the stable

-there is a separate milking stand in order to improve hygienic conditions and optimize milk production by undisturbed milking.

Fig.29: Principle design of the cattle shed.

The stable is divided into a resting area ( 1 ) and an exercising cum feeding area (2). Calves' boxes (3) and the milking stand (4) are at the far end. Milking cows are placed adjacent to it (5). The urine drain (6) ends in the urine chamber (7). The dung and mixing chamber (8), which is the inlet of the biogas plant, are placed beside. The fodder trough passes along the feeding area (9); a water trough (10) is provided between, Calves have smaller troughs (11). The chopping block is at the centre of the trough (12). A bar passes over the necks of the cattle in the sleeping area (13) to force the cows to move back when getting up to drop dung. The necks of the cattle are also restricted at the trough to prevent them from scattering fodder to the floor. This can be done by a bar (14) or by narrow standing poles (15). The floor of the stable is concreted (16) or laid out with concrete tiles (17). The dung is pushed into the mixing chamber daily with the help of the squeegee (18). Urine is taken by a dipper (19) and mixed with the dung before entering the biogas plant. There is a verge of timber (20) or concrete (21) at the end of the sleeping box.

The Floor

The condition of the floor influences most the operation of the gas plant. Smooth stable flooring with appropriate slope encourages and simplifies daily cleaning of the stable. The floor should be even without holes. It should not be slippery but plain and slightly rough. There is a 2% slope of the floor into the urine drain and of the urine drain into the urine chamber. The urine drain is shaped one sided at the lowest end of the slope floor. Its corner is bottle curved.

Normally, floor is of 10 cm concrete (mixture 1: 2: 4) on stone bedding. Anti-termite chemicals should be spread on the stone bedding where applicable. The concrete is firmly rammed or vibrated. A good solution are concrete tiles of 15~15~5 cm in size and 1: 2: 4 by mixture. They are laid into solid sand bedding without joints. The tiles should be cured for at least one week by keeping them moist and cast in steel moulds in order to maintain exact rectangular shape to avoid unwanted wide Joints.

The floor of the calves pen is preferably of wooden boards raised 30 cm above the concrete floor leaving 2,5 cm slats between for faeces to be pushed between in order to keep the place dry and clean.

The Feeding Trough

Well designed fodder troughs prevent too much waste fodder from entering the gasplant. The feeding trough must fit the anatomy of the animal. A trough which is too small increases work for feeding, too big a trough increases waste of fodder. The trough should have rounded bottoms and corners for easy reach of fodder. A drain pipe for cleaning and drainage of rain water should also be provided.

There are different ways to prevent the cows from scattering fodder from the trough to the floor where it could mix with the dung and enter the biogas plant. One alternative is vertical poles of 1,50 m height and 20 cm free space between. These are erected in front of the trough. The poles should be rounded and smooth so as not to cause friction at the cows neck. The distance from the inner side of the trough and the outer side of the poles should not exceed 16 cm in order to allow the cow to reach the full width of the trough. The other, cheaper and easier method, is to place a bar above the cow's necks to restrict movements of their heads. In both cases, the outer wall of the trough is shaped in such a manner that animals do not push fodder off the trough while eating. Troughs may or may not be placed under the roof of the stable. Wooden troughs wear quickly.

Compartment Walls

All walls should be strong and smooth. Movable wooden bars used for closing of door openings are not recommended because they might be lifted Off by the animals. Normal wooden doors with solid hinges are more appropriate. Built-in wooden poles have to be protected by burning their surface when surrounded by mortar or concrete.

Sleeping Boxes

Sleeping boxes should be clean and dry. Therefore, for defecating, the cow has to get In a position in which she can not soil the floor. A 15 cm raise of the floor above the level of the exercising area prevents the cow from entering the box In reverse. A 15 cm high timber or concrete verge forces the cow to lay fully inside the box.

The neck bar allows the cow to lay inside the box in full length but if she gets up for defecating she has to step back dropping dung and urine outside the sleeping box. The floor should be of concrete or tiles in order to avoid too much of sand or soil being collected with the dung. If bedding is wanted or required, it should neither be of straw nor of sawdust but dried slurry could be used instead. To train cows using the sleeping box, requires tying them up for the first few nights after being placed in the new stable.

Dung and Urine Chamber

When cleaning the stable, dung is pushed into the dung or mixing chamber, which is actually the inlet chamber of the biogas plant. Urine and water collects in the urine chamber from where it is taken out with the help of a dipper and mixed with the dung in the dung chamber. The mixed substrate is then released into the biogas plant.

The urine chamber is in fact a storage and dosing tank designed for the capacity needed to get the right TS-content for the slurry. If rain or washing water exceeds the required amount, the urine chamber overflows into the slurry distribution channel. If too little urine gets collected, which might be the case in dry places or seasons, the urine chamber has to be filled with water to the required amount.

The Roof

The roof should at least cover the sleeping boxes, the calves pen and the milking stand. In case with no sleeping boxes, part of the exercising cum resting area should be covered. If the roof covers the total area of the stable it should be 3 m high in order to allow ventilation and sunshine. The roof should not drain on the biogas plant.

Fig.30: Cattle foot bath. If cattle are regularly out grazing, they should pass a cleaning foot bath before entering the stable in order to prevent too much soil from entering the gasplant. A drainage pipe would allow easier cleaning.

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Biogas Plant Notes Chapter 6 | Construction of the biogas plant

6. Construction of the biogas plant

General

The overflow of the biogas plant must be higher than the slurry bed or the slurry distribution channel. The inlet must be lower than the stable floor. The biogas plant should be so far from trees that roots will not grow into its brickwork. It should not be in areas where heavy machinery move frequently. Biogas plants are not meant to be a playground, still they should be safe for children and animals.

A gasplant of a rural biogas unit is standardized and preferably a fixed dome plant. Once the decision for standardization is made, modifications are only allowed in order to join existing local structures. The plant itself is not to be changed.

The size of the plant depends on the substrate available. In practice its volume is chosen according to the number of cattle or pigs and their stabling. In case of doubt, the energy demand may also be considered. The biomethanation process is rather hardy and robust and does not require defined loading rates. Therefore, it is possible to consider only a few standard digester volumes. The standard volumes of digester and gasholder have to be estimated in each project area according to gas production rates and general gas consumption patterns.

Larger gasplants have longer retention times and, therefore, higher gas production rates. Nevertheless, the amount of daily fed substrate has more influence on gas production than the volume of the digester. In case of doubt, criteria used are the investment costs and security of gas supply. Larger gas plants have higher gas storage capacity.

The most common size in the Arusha Region is the 16 m³-plant which can provide gas for cooking and lighting for a normal family. The 12 m³-plant is reserved for places of little gas demand, e.g. small families, or where ground temperature is above 24ºC and therefore, retention time could be less. 30 and 50 m³-plants provide gas not only for household use but fuel for big institutional kitchens and special appliances like refrigerators, incubators, hatching heaters or power engines, etc. Structural drawings for the standard plants are to be found in the Appendix.

The Principle Design

The standard fixed dome plant has a half-bowl spherical shape with flat bottom and a top opening. The outer walls rest on a foundation ring beam. The floor has no static function. The upper part of the sphere is separated from the lower part by a joint, called the "weak ring". Gas tightness of the upper part is achieved by a crack-free structure and a gas-tight inner surface plaster.

The Inlet pipe is connected to the spot of dung disposal in the stable. The outlet pipe connects the digester with an expansion chamber of reduced spherical shape. The overflow of the expansion chamber - really the final outlet of the gasplant - leads to the slurry disposal system, i.e. the distribution channel, storage tank or compost pit.

Fig.14: Principle of statics of fixed dome plant

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Biogas Plant Notes Chapter 5 | The agricultural biogas unit

5. The agricultural biogas unit

General

Most faults in biogas units are caused by planning mistakes. Siting of the biogas plant and layout of the biogas unit is as Important as the construction Itself. A good biogas plant at the wrong place Is a useless installation. Similarly, filling a plant with unsuitable material will result In an unproductive unit. Careless planning of the site may require unnecessarily additional structures or cause further labour input. Thorough inspection and assessment of site are preconditions for a profitably functioning biogas unit. This is especially true when using standardized structural elements;

Survey of Site

BES staff first check the proposed feed material for its suitability. After observing the overall environment of the farm, the master plan of the biogas unit is made on the spot In cooperation with the persons having decision making power at the farm. The technician must check if the space Is sufficient and must take the levels of the proposed structures. Planning the utilization of slurry is probably the most Important point to be discussed at the first site meeting with the farmer. The cost of the slurry distribution system should be calculated and made known to the farmer before starting the construction.

To allow swift construction work, access for transport and place for storage of material and excavated soil must be clear before starting. The farmer must be informed about providing approx. 500 1 of water per day during the building period for masonry and concrete construction. There must also be agreement as to which building materials are to be provided by the farmer and which quality requirements are to be observed.

The BES staff writes a report about the findings and assessments and gives reasons for decisions made at site. This is required to keep colleagues at the BES headquarter informed. Such records are also helpful In case of customers trying to save money by constantly complaining about the plant's performance.

Tools and equipment

There are three essential tools which are given to the farmer because they are part of the biogas unit:

-The dipper to scoop urine and water from the urine chamber into the mixing chamber and to take out and pour slurry in case of compost preparation. Several designs have been tested. The most durable solution was found to be a dipper made from a Ø 6" plastic pipe and a 1,30 m long wooden handle. The handle passes through both rims of the pipe and is fixed with a nail to the upper rim. Dippers from metal proved to corrode quickly and handles fixed on a shaft broke within a short time.

-The squeegee is used to clean the stable floor with only a little water, pushing the urine into the urine chamber and the solids into the mixing chamber.

-The chopping block is needed to chaff the fodder with a panga (machete). Chaffed grass Is eaten completely by the animals without leaving the stems or allowing them to be tossed out of the trough where they would mix with the dung and might block the biogas plant. The chopping block can be a standing solid log of wood but it is better to use the wood across the fibre to avoid the knife getting stuck. Truck tires have proofed to be an elegant solution Instead of wooden blocks because the knife Jumps up by itself when chaffing the grass. Mechanical chaff-cutters are, of course, an even better solution.

As farmers are not aquatint with this kind of equipment, It is best to provide these in order to stress the importance for adequate operation of the biogas unit.

Fig.12: Necessary equipment to operate a biogas unit

(1) Dipper made of a piece of plastic pipe with a wooden handle coming through. (2) Squeegee made from wooden boards with tyre rubber clamped between and wooden handle coming through both the boards and the rubber lip. (3) Chopping block or (4) used truck tyre for chaffing of fodder grass. The tyre leans against the trough and may be rolled to the place of use.

Principles of Layout

A biogas unit is a considerable investment. It should not be looked at as a temporary structure.

The agricultural biogas plant belongs to the stable. Without any exception. The distance to the kitchen is of secondary importance. With fixed dome plants, there is no practical limitation to the length of gas pipes, except for the cost. As a matter of principle, sustainability has first priority over cost reduction. This means that everything must be arranged in such way that it is less work to feed the plant than not.

On sloping ground, the stable lies higher than the biogas plant. On flat grounds the floor of the stable might be elevated in order to allow dung and urine to enter the plant by gravity. Handling of slurry demands high labour input and can be avoided by proper planning. The outlet of the biogas plant Is directed towards, or drains into, the fields. Overflowing slurry should never be allowed to accumulate on neighbour's or public ground. The biogas unit must be functional even when attendance and maintenance is poor. The owner has the final decision, but he often can not oversee the consequences of a decision. Beware of false compromises!

Fig.13: Position of gasplant to stable floor The ideal situation is a sloping ground, falling from the stable via the gasplant to the crop plantation (1). On horizontal ground (2), it might be necessary to lift the floor of the stable (H).

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Biogas Plant Notes Chapter 4 | Biogas extension work

4. Biogas extension work

General

The following chapter describes extension work within the framework of a project dealing exclusively with biogas units. But most of the points are also relevant when biogas extension is promoted within more general development programmes.

Target Group

The target group of a rural biogas extension programme are farms having at least 50 kg of cattle dung (or 35 kg of pig droppings) available per day' which means they have at least three milk cows or 10 adult pigs fully stable bound, or nine heads of local cattle half stable bound There are several conditions to be fulfilled before a farmer of the target group becomes a customer:

-He has to have enough income to buy a plant or repay a loan.

-He must be educated enough to understand the system.

-He must know about biogas and its suitability for his individual case.

-He must have easy access to sufficient water.

-He has no real fuel alternatives.

Standardization

Standardization means to define exactly and restrictives the materials, measurements and methods off the work. Given standards must be clear and universalIy adabtibIe.

Technical standardization is needed because it can not be expected that an artisan or farmer will fully understand the essentials of a biogas unit. Biogas plants are easy to construct but difficult to comprehend totally. Artisans must be trained to precisely observe all details and methods of construction. This is especially important for extension programmes that aim at handing-over the construction activities to the private sector, where permanent quality control is difficult. It should also be mentioned, that management training of artisans is needed to improve the efficiency of the enterprise and thus the quality of workmanship).

A farm benefits from a biogas plant if the plant works trouble free, gas and slurry are used profitably and operation of the plant is comfortable and easy. In fact feeding the plant must be less labour intensive than not feeding the plant. This user-oriented approach leads to a standardized biogas plant, if possible, connected to a standardized stable which are integrated as much as possible into the existing farm economy.

Strategy

The final goal of the extension project is to have independent artisans who construct standardized biogas units on demand of independent farmers against appropriate payment. connection to zero-grazing units is favoured.

Each biogas extension project starts by building demonstration units at selected farms which might be fully subsidized. The farmers must be willing to cooperate with the biogas extension service by allowing potential customers to visit their installations. It is most important that those farmers maintain and utilize their gasplants well.

After sufficient number of demonstration units have been installed, further biogas units are only constructed on demand and against full payment. Payment is usually done in 2 to 3 instalments. It is helpful to have standardized procedures for application, payment and realization of the construction (see sample of forms in the appendix).

When a potential customer first comes to the Biogas Extension Office he is given general informations including a price list. He is asked to file a written request which describes his farm and the proposed site of construction. Then the site is visited by BES-staff, assessments are made, technical details are worked out and a fixed price is given to the farmer.

After a contract agreement has been signed and a 50% down payment has been made, the biogas unit will be erected by trained private contractors under supervision of BES-technicians. When the construction is finished, the plant has been filled. appliances are connected and the final payment has been made, the unit is legally handed over to the customer. The customer receives a user manual, detailed explanations about plant operation, gas and slurry utilization and maintenance in his specific case. The customer is also given a set of tools and equipment for cleaning the stable, chopping the fodder and handling the slurry.

The main points of instruction are:

-to keep the overflow free from slurry

-to check the water trap from time to time, especially when there is no gas available for consumption

-to clean the burner regular like other cooking vessels

-to poke from time to time the inlet and outlet pipe, especially if substrate does not enter the plant

-how to change the mantle of the gas lamp

-the meaning of the slurry level in the expansion chamber

-where to turn to in case of problems the farmer cannot solve himself

The biogas unit is then visited once a month until the persons attending the plant are acquainted with the daily routine work and the utilization of gas and slurry.

Advertisement

As a principle. the farmer should decide freely wether he wants to have a biogas unit or not. Therefore, advertisement means mainly information and awareness building. Image cultivation is also a part of the publicity work. The biogas unit is presented as a clever way of running a modern farm unit. Well operated biogas units are the best advertisement.

Fig.10: A Biogas sticker used by CAMARTEC for advertising

Special Requests

Besides the standard applications for biogas plants there will always be special requests for individual solutions reaching the Biogas Extension Service. Special requests often demand individually designed units which differ either in size or proposed utilization of gas or slurry. The BES has to keep planning and supervising capacity for such services' because they often are requested by VIPs (Very Important Persons) who are important for the support of the biogas programme.

Research and Development

In addition to standardization, research and development needs will arise from the project activities. New ideas have to be tried out which will disturb the standardized routine. To minimize problems and preserve the standard of quality of construction, innovations and modifications should be restricted to a few that really improve the performance of the biogas unit or eliminating severe short-comings.

As for CAMARTEC, the most important research was the development of the weak-ring and the strong-ring. Tests on reducing the requirements for gas-tight plaster are under way. Own appliances have been developed and others from outside have been tested. A bench-scale test has been carried out to define the flow of slurry inside the digester more exactly.

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Biogas Plant Notes Chapter 3 | Explanation of terms

3. Explanation of terms

Biogas

Biogas is produced by bacteria during digestion or fermentation of organic matter under airless condition (anaerobic process). The gas consists mainly of CH₄ and CO₂. This mixture of gases is combustible if the methane content is more than 50%. Biogas from animal dung contains approx. 60% methane.

Fig.2: The big-chemical process of anaerobic digestion The different groups of bacteria responsible for fermentation live in an interacting eco-system. Each type of bacteria depends on others. The fermentation time is shortest when populations of different bacteria are adequately balanced.

Slurry

In practice, the term slurry is used for the digester content or the digested substrate flowing out of the plant. In digesters observed by CAMARTEC, slurry is found in different conditions inside the digester:

-a light and rather solid fraction, mainly straw or fibrous particles, which float to the top forming the scum

-a very liquid, watery fraction remaining in the middle layer of the digester

-a viscous fraction below which is the real slurry or sludge

-heavy solids, mainly sand and soil particles which rest at the bottom.

Slurry separates less if the feed material is homogeneous and the TS-content is high.

Fig.3: Slurry condition inside the CAMARTEC digester (1) Settlement of sand and soil. (2) Viscous slurry or sludge, having a TS-content of 6-7%. (3) Liquid slurry fraction, having a TS-content of 12%. (4) Floating scum, having a TS-content between 15 and 50 %. (5) Biogas.

Biogas Technology

Biogas Technology includes everything which is needed to produce and utilize the products of anaerobic digestion which are biogas and manure. Beside energy and fertilizer. other benefits of biogas technology are improved sanitation and environmental protection. The conditions to produce biogas are:

-digestable substrate, i.e. organic matter plus water

-a vessel where the substrate is not in contact with air

-a digestion temperature between 15ºC and 35°C

-a retention time longer than 30 days to allow the bacteria to produce the biogas. (The retention time is considerably reduced in industrial high-tech plants).

If methane producing bacteria are already present in the substrate (e.g. in dung from ruminants), biogas production begins within 3 to 5 days. At the farm site. biogas plants are filled slowly and gas production is used only after the plant has been filled completely. If there are problems with certain substrate starting the gas production, 20% of cattle dung should be mixed in the first filling as a starter.

Gas Production

The gas production potential of a certain substrate is high when organic matter content is high and the C/N ratio ranges from 20: 1 to 40: 1. The speed of the gas production depends further on the physical properties of the substrate and the temperature (optimum at 35°C). Dry and fibrous material takes longer to digest than fine-structured and wet substrate. Favoured total solid (TS) contents of the undigested substrate are between 7% and 11% which is approximately reached if dung is mixed with an equal volume of water or urine. A healthy digestion process shows a pH of 7.0 (neutral stage of substrate).

Biogas Plant

A biogas plant consists of the digester and the gas storage space. A continuous gas plant is charged and discharged regularly, e.g. every day. A batch-plant is filled once and emptied only after the material has been digested. A normal farmers biogas plant is a continuous plant with automatic discharge at the overflow.

Fig.4: Relation of gas production and retention time

The daily gas production (8p) is measured in litre of biogas produced by l kg of total solids (TS) added per day. The total solids content of fresh cattle dung is 15-25 %. The retention time (RT) is the calculated period of days the substrate remains in the biogas plant before it reaches the overflow. The gas production per day depends on the slurry temperature.

Curve (1) is taken from different sources at 30°c, mainly from India. Curve (2) shows results from field research by UNDARP/BORDA in India on floating drum plants at a temperature of 27°C. Curve (3) is the average gas production with CAMARTEC fixed dome plants at 24°C average digester temperature. he points (4) show some selected samples of CAMARTEC plants of average performance, recorded during the BORDA Biogas Survey 1988. Performance is defined as daily gas production per square root of the total solid content of the daily fed substrate times the active digester volume (8p·(TS·VD)^(-0,5)).

Biochemical problems are rare, even in simple gasplants. Technical problems may occur with immature designs and unsuitable, i.e. scum forming, feed material. There are three well performing and mature designs available which are suitable for farm households:

-The fixed dome plant

-the floating drum plant

-the plastic covered ditch.

In most large scale extension programmes fixed dome plants have been chosen for dissemination because they are long lasting and cheaper than the floating drum plant. Fixed dome plants need the least maintenance of all other types. But building them requires great care in design and workmanship. Once they are constructed well, they are robust and of reliable performance.

The size of the digester depends on the required digester volume (VD) which is found by multiplying the wanted retention time (RT) with the volume of daily fed substrate (VS). In fixed dome plants, the active digester volume is defined by the digester volume below the zero-line, minus half the expansion chamber volume below the overflow line.

The gasholder volume (VG) depends on the daily gas production and the pattern in which the biogas is used. If gas consumption is regular and equally distributed over day and night and from day to day, gas storage space can be small. Irregular and rather concentrated gas consumption demands larger gas holder.

Experimental biogas plants for schools can be made out of 4 kg paint-tins (0 17,5 cm) and 2 kg milk powder tins (0 15 cm). The gas valve of such a floating drum model is made by a U-pipe filled with water. For gas release, the water is drained off and must be re-filled for closing the valve again.

Fixed Dome Plant

In fixed dome plants the gas is stored in the upper part of the rigid digester structure. Fixed dome plants are sometimes called "Chinese" or "hydraulic" digesters. The accumulating gas needs room and pushes part of the substrate into an expansion chamber, from where the slurry flows back into the digester as soon as gas is released. The volume of the expansion chamber is equal to the volume of gas storage. Gas pressure is created by the difference of slurry levels between the inside of the digester and the expansion chamber. The main building material is plastered brickwork.

Fig.5: Small-scale biogas plants for rural areas in tropical countries

(A) Fixed dome plant. The gas collects in the upper part of the digester(1) and displaces the slurry into the expansion chamber(2).

(B) Floating drum plant. The gas collects in a floating steel gas holder (3) which rises according to the volume of gas production.

(C) Plastic covered biogas plant. The gas is collected under an inflating plastic cover (4). A wooden roof (5) protects the plastic against sunlight and increases the gas pressure by its weight.

Fig.6: System of the fixed dome plant

The digester (1) is filled via the inlet pipe (2) up to the bottom level of the expansion chamber (3). The level of original filling is called the zero line. The gasplant is closed by a gas-tight lid (4). Under the airless (anaerobic) condition, biogas is produced. When the gas valve (5) is closed, biogas collects in the upper part of the digester, called the gas storage part (6). The accumulating gas displaces part of the slurry into the expansion chamber. When the expansion chamber is full, slurry overflows into the slurry drain for use as manure. When the main valve (9) is opened, the gas escapes off the gas storage part until the slurry levels inside the digester and inside the expansion chamber balances. The gas pressure "p" depends on the prevailing difference of the slurry levels ( 10).

The substrate is filled daily so that slurry flows out daily at the time when large amount of gas is stored. Regular gas consumption requires smaller gas storage space. Consequently, the zero-line will rise. While daily feeding of the plant continues, gas is released before the slurry reaches the overflow level. The slurry level rises also when there is gas leakage. The level in the expansion chamber at zero gas pressure indicates the level of the zero line. The volume of slurry above the zero line inside the expansion chamber is equal to the gas storage space.

Fig.7: Different models of fixed dome plants Fixed dome plants originate from China and were built already before 1960. Several variations with or without a removable cover at the top have been developed. (1) Biogas plant from Chengdu/China; (2) Janata Plant from India; (3) Dheenbandhu Plant of AFPRO from India; (4) Modified BORDA plant from Cankuso in Burundi..

Biogas Unit

The terminus "biogas unit" should underline the importance of integrated planning when applying biogas technology. The biogas unit describes the total package offered to the farmer in connection with biogas extension work. The main components are: The biogas plant itself, the stable, the toilet, the slurry storage pit, the slurry distribution canals, the gas piping system, the appliances and the tools to handle the substrate. In individual cases other components could as well be part of the biogas unit, for example, rain water tanks, fish ponds. compost pits, demonstration fields, gas generators or engines with their attachments, etc., etc. one may distinguish between agricultural biogas units and sanitary biogas units.

The big-latrine is the centre part of a sanitary biogas unit. The septic tanks of big-latrines are designed as integrated fixed dome biogas plants. Sanitary aspects, i.e. rather maintenance-free but clean toilets, are more important than a high gas production.

Fig.8: Principal lay-out.(A) Agricultural Unit; (B) Sanitary unit

(1) Biogas plant; (2) Cattle stable; (3) Toilet; (4) Slurry distribution system; (5) Fodder grass or vegetable plantation; (6) Shrub or tree plantation; (7) Hedge between public area and slurry area; (8) Gas pipe; (9) Place of gas consumption; (10) Dung and urine collection chamber; (11) Fodder trough; (12) Chaffing block; (13) Urine drain (14) Sleeping boxes; (15) Milking stand; (16) Calves' box; (17) Exercising area, separated for cows and heifers.

Biogas Appliances

Biogas Appliances are pieces of equipment for utilizing the energy of the gas. Either special biogas appliances are used or LPG equipment is adapted. Biogas is mainly used in stoves for cooking and in gas lamps for lighting. Frequently, refrigerators and incubators, coffee roasters, baking ovens and water heaters, chicken or piglet heaters, Power engines for milling or generating electricity are fuelled with biogas.

Biogas Extension Service

The biogas extension service (BES) comprises of the organization, the staff and the logistic needed to work for the extension of biogas technology. The BES might be a governmental body, a non-governmental voluntary or commercial organisation or a development project of international cooperation. Normally, the costs for the superstructure of the extension work are not included in the price the gasplant owner has to pay. Because of the benefits for the society as a whole it is justifiable to cover the cost of the superstructure from public funds.

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Biogas Plant Notes Chapter 2 | Why biogas ?

2. Why biogas ?

There are several alternatives to solve a farmers energy problem and there are different ways of manure management on a farm. Biogas might not be the best solution for all problems but it is one method to take care of many aspects. The biogas unit is a system in which the three components biogas plant, animal production and fodder grass plantation form a natural cycle.

Each of the three parts has d,rect benefits to the farmer and his economy:

-The animals generate income by supplying milk and meat.

-The gasplant provides comfort and saves expenditure by supplying clean cooking and lighting fuel.

-The fodder grass plantation creates sustainability by protecting the soil against erosion. Fodder plantation gives most profit from a small patch of land and often is less labour intensive than cutting fodder grass outside the farm. Beside the fodder grass, vegetables and fruits benefit the use of digested slurry as fertilizer.

Biogas is Just a clever way of exploiting nature without -destroying it. Biogas optimizes farm economy. Biogas Plants support self-reliance and fit in concepts of sustainable development.

Fig.1: The cycle of organic matter and the benefits of an agricultural biogas unit The animals provide dung to the biogas plant, the gasplant provides manure to the fodder plantation and the plantation provides feed to the animals. If enough water is given to the animals, no additional water is required for the biogas plant.

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