Biogas is really no more dangerous than other fuels such as wood, gasoline, or bottled gas. But just as these fuels have their ways of being dangerous, so does biogas. Face it; anything that can cook meals and fuel an engine can also burn people.
Certain precautions should be observed in the operation of biogas systems. Biogas can be explosive when mixed with air in the proportion of one part biogas to 8-20 parts air in an enclosed space. This situation can occur when a digester is opened for cleaning, when biogas is released to repair a gas storage tank, or when there is a gas leak in a poorly ventilated room. In such cases, avoid sparks, smoking, and open flames.
A biogas leak can be smelled if the hydrogen sulfide has not been removed from the biogas. It smells like rotten eggs. No one should go inside large digesters unless they have a companion on the outside who can get them out in case they need help. Although the methane and carbon dioxide of biogas are not poisonous, a person may stop breathing if there is too much biogas and not enough oxygen in the air they are trying to breath.
Never allow negative pressure in a biogas system. Negative pressure occurs when the force created by the weight of the gases outside the biogas system is greater than the force inside the system. In normal operations the pressure inside the system should always be greater. How much greater should always be measured on a pressure gauge (see Diagram 14).
Negative pressure will pull air into the biogas system and the mixture of biogas and air might explode. If that does not happen, the oxygen in the air will kill the biogas bacteria and the gas production rate will drop. The only time the danger of negative pressure usually becomes a real possibility is when a person wants more gas from a digester than it can produce or there is an unnoticed gas leak.
When biogas is used at pressures below one column inch of water as measured on a pressure gauge, it is very likely that the flame will go out. Even though there is not much gas left in the system, biogas will continue to come out. Then the possibility for a spark or flame causing an explosion in the room or negative pressure pulling air into the biogas system causing an explosion in the system, becomes real (Maramba, 1978).
When opening a biogas digester for cleaning or repairing, do not use candles or smoke cigarettes. For light inside the digester, use a flashlight or have a person standing outside reflect sunlight off a mirror.
Make frequent smell checks for gas leaks in plastic pipes, Joints, clamps, and gate valves. Rats have been known to bite holes in plastic pipes. Stoves and gas mantle lamps should be placed with fire safety in mind. Special care must be taken in buildings with grass roofs to make sure that gas lamps are a good distance from the roof.
If the rotten egg smell of biogas is noticed in a room, immediately open doors and windows in order to get rid of the trapped gas before looking for the leak. On no account should anyone smoke cigarettes in the room. In case of fire in a house or engine room, shut the gas off at the gate valve just after the gas storage tank to keep biogas from feeding the fire.
When using any kind of gas, light the match first, then open the gas valve. If the valve is opened first and gas is allowed to flow without being lit for any length of time, large amounts of gas can escape and any flame might ignite a fireball.
Children must be taught not to play with fire close to biogas systems, in case there are any gas leaks which could cause a fire or explosion (A Chinese Biogas Manual, 1976).
Brass gate valves and pipes used in biogas systems must be of a lead-free type. The hydrogen sulfide in biogas will destroy lead, which will cause gas leaks.
The following flame arrester suggestion is adapted from the Guidebook on Biogas Development. A flame arrester is a safety device that should be added to every gas line. It is usually placed either just after the gate valve at the digester and just before the gas stove or stationary engine. Its purpose is, in case of fire, to prevent the flame from travelling down the gas pipe into the gas storage tank or digester and causing an explosion.
The arrester can be a ball or roll of fine mesh copper wire (iron and steel would rust) inserted into the gas pipe. It is sometimes not realized that this forms a barrier to the free and full flow of gas. It is recommended that the flame arrester be placed in a length of pipe of slightly larger diameter than the gas pipe. For a 0.5 inch pipe use a 0.75 inch arrester pipe; for a 1.0 inch pipe use a 1.25 inch arrester pipe.
It is very important that if a digester is built underground, that it is built in a place that never floods. If an above ground digester is built in an area that sometimes floods, make sure that the openings into the digester are above the high water mark. If a digester is built in an area that does have floods, safety measures should be taken in advance so that the gas can escape in case the digester and/or the gas storage tank are flooded. Failure to do so could result in dangerous, uncontrolled release of biogas and if the digester is a plastic bag, it could float up and away. An upside-down "T" pipe should be placed at the highest vertical point in the gas pipe line above the gas outlet from the digester. A vertical pipe and a gate valve should be joined to the stem of the upside-down "T" pipe. The gate valve can then be opened to release the biogas if a flood threatens to cover either the digester or the gas storage tank.
The following is a list of safety measures that should be read with great care before a biogas system is built.
1) Regularly check the whole system for leaks.
2) Provide ventilation around all gas lines.
3) Always maintain a positive pressure in the system.
4) The engine room floor must be at or above ground level to avoid the buildup of heavier- than- air gases.
5) The engine room roof must be vented at its highest point to allow lighter-than-air gases to escape. This is also true for greenhouses that have biogas digesters, engines, or burners in them.
6) The engine exhaust pipe must be extended so that the dangerous and deadly exhaust gases are released outside the building.
7) Metal digesters and gas storage tanks must have wires to lead lightning to the ground.
8) Gas lines must drain water into condensation traps.
9) No smoking or open flames should be allowed near biogas digesters and gas storage tanks, especially when checking for gas leaks.
Methane, the flammable part of biogas, is a lesser danger to life than many other fuels. However, in the making and using of an invisible fuel, dangerous situations can arise unexpectedly and swiftly--such as when a gas pipe is accidently cut. On the other hand, precaution can be exaggerated. When cars first appeared on the roads, a man waving a red flag came first. Remember the ABC's: Always Be Careful (Fry, 1974).
Health hazards are associated with the handling of night soil and with the use of sludge from untreated human excrete as fertilizer.
In general, published data indicate that a digestion time of 14 days at 35 C is effective in killing (99.9 per cent die-off rate) the enteric bacterial pathogens and the enteric group of viruses. However, the die-off rate for roundworm (Ascaris lumbricoides) and hookworm (Ancylostoma) is only 90 per cent, which is still high. In this context, biogas production would provide a public health benefit beyond that of any other treatment in managing the rural health environment of developing countries.
Bottlenecks, considerations, and research and development
Bioconversion of organic domestic and farm residues has become attractive as its technology has been successfully tested through experience on both small- and large-scale projects. Feeding upon renewable resources and non-polluting in process technology, biogas generation serves a triple function: waste removal, management of the environment, and energy production. Nevertheless, there are still several problems (14, 19, 20) that impede the efficient working of biogas generating systems (Table 5).
TABLE 5. Considerations Relating to Bottlenecks in Biogas Generation
|
Aspect |
Bottlenecks |
Remarks |
|
Planning |
Availability and ease of transportation of raw materials and processed residual products |
Use of algae and hydroponic plants offsets high transportation costs of materials not readily at hand. Easily dried residual products facilitate transportation. |
|
Site selection |
Nature of subsoil, water table, and availability of solar radiation, prevailing climatic conditions, and strength of village population need to be considered. |
|
|
Financial contraints: Digester design; high Transportation costs of digester materials; installation and maintenance costs; increasing labour costs in distribution of biogas products for domestic purposes |
Use of cheap construction materials, emphasizing low capital and maintenance costs and simplicity of operation; provision of subsidies and loans that are not burdensome. |
|
|
Necessity to own or have access to relatively large number of cattle |
Well-planned rural community development, ownership and biogas distribution schemes necessary. |
|
|
Social contraints and psychological prejudice against the use of raw materials |
Development of publicity programmes to counteract contraints compounded by illiteracy; provision of incentives for development of small- scale integrated biogas systems. |
|
|
Technical |
Improper preparation of influent solids leading to blockage and scum formation |
Proper milling and other treatment measures (pre- soaking, adjustment of C/N ratio); removal of inert particles: sand and rocks. |
|
Temperature fluctuations |
Careful regulation of temperature through use of low-cost insulating materials (sawdust, bagasse, grass, cotton waste, wheat straw); incorporation of auxiliary solar heating system. |
|
|
Maintenance of pH for optimal growth of Methanogenic bacteria C/N ratio |
Appropriate choice of raw material, regulation of C/N ratio and dilution rate. Appropriate mixing of N-rich and N-poor substrates with cellulosic substrates. |
|
|
Dilution ratio of influent solids content |
Appropriate treatment of raw materials to avoid stratification and scum formation. |
|
|
Retention time of slurry |
Dependent upon dilution ratio, loading rate, digestion temperature. |
|
|
Loading rate |
Dependent upon digester size, dilution ratio, digestion temperature. |
|
|
Seeding of an appropriate bacterial Population for biogas generation |
Development of specific and potent cultures. |
|
|
Corrosion of gas holder |
Construction from cheap materials (glass fibre, clay, jute-fibre reinforced plastic) and/or regular cleaning and layering with protective materials (e.g., lubricating oil). |
|
|
Pin-hole leakages (digester tank, holder, inlet, outlet) |
Establishment of "no leak" conditions, use of external protective coating materials (PVC, creosotes |
|
|
Occurrence of CO2 reducing calorific value of biogas |
Reduction in CO2 content through passage in lime-water |
|
|
Occurrence of water condensate in gas supply system (blockage, rusting) |
Appropriate drainage system using condensate traps |
|
|
Occurrence of H2S leading to corrosion |
On a village scale, H2S removed by passing over ferric oxide or iron filings |
|
|
Improper combustion |
Designing of air-gas mixing appliances necessary |
|
|
Maintenance of gas supply at constant pressure |
Regulation of uniform distribution and use of gas; removal of water condensate from piping systems; appropriate choice of gas holder in terms of weight and capacity |
|
|
Residue utilization |
Risks to health and plant crops resulting from residual accumulation of toxic materials and encysted pathogens |
Avoid use of chemical industry effluents; more research on type, nature, and die-off rates of persisting organisms; minimize long transportation period of un-dried effluent |
|
Health |
Hazards to human health in transporting night soil and other wastes (gray-water) |
Linkage of latrine run-offs into biogas reactors promotes non-manual operations and general aesthetics |
|
Safety |
Improper handling and storage of methane |
Appropriate measures necessary for plant operation, handling, and storage of biogas through provision of extension and servicing facilities |
Rural communities using the integrated system are appropriate examples of recycled societies that benefit from low-capital investments on a decentralized basis and such communities are attuned to the environment. The technology thus seeded and spawned is, in essence, a populist technology based on "Nature's income and not on Nature's capital."
Biogas generated from locally available waste material seems to be one of the answers to the energy problem in most rural areas of developing countries. Gas generation consumes about one-fourth of the dung, but the available heat of the gas is about 20 per cent more than that obtained by burning the entire amount of dung directly. This is mainly due to the very high efficiency (60 per cent) of utilization compared to the poor efficiency (11 per cent) of burning dung cakes directly.
Several thousand biogas plants have been constructed in developing countries. A screening of the literature indicates that the experience of pioneering individuals and organizations has been the guiding principle rather than a defined scientific approach. Several basic chemical, microbiological, engineering, and social problems have to be tackled to ensure the large-scale adoption of biogas plants, with the concomitant assurances of economic success and cultural acceptance. Various experiences suggest that efficiency in operation needs to be developed, and some important factors are: reduction in the use of steel in current gas plant designs; optimum design of plants, efficient burners, heating of digesters with solar radiation, coupling of biogas systems with other non-conventional energy sources, design of large-scale community plants, optimum utilization of digested slurry, microbiological conversion of CO2 to CH4, improvement of the efficiency of digestion of dung and other cellulosic material through enzyme action and other pre-digestion methods, and anaerobic di gestion of urban wastes
We may summarize some of the research and development tasks that need to be undertaken as follows.
In basic research:
a. Studies on the choice, culture, and management of the micro-organisms involved in the generation of methane.
b. Studies on bacterial behaviour and growth in the simulated environment of a digester (fermentation components: rate, yield of gas, composition of gas as a function of variables - pH, temperature, agitation - with relation to substrates - manure, algae, water hyacinths).
In applied research:
a. Studies on improving biogas reactor design and economics focusing on: alternative construction materials in stead of steel and cement; seeding devices; gas purification methods; auxiliary heating systems; insulator materials; development of appropriate appliances for efficient biogas utilization (e.g. burners, lamps, mini tractors, etc.).
b. Studies for determining and increasing the traditionally acknowledged fertilizer value of sludge.
c. Studies on quicker de-watering of sludge.
d. Studies on deployment of methane to strengthening small-scale industries, e.g., brick-making, welding, etc.
In social research:
a. Effective deployment of the written, spoken, and printed word in overcoming the social constraints to the use of biogas by rural populations.
b. Programmes designed to illustrate the benefits accruing to rural household and community hygiene and health.
c. Programmes designed to illustrate the need for proper management of rural natural resources and for boosting rural crop yields in counteracting food and feed unavailability and insufficiency.
d. On-site training of extension and technical personnel for field-work geared to the construction, operation, maintenance, and servicing of biogas generating systems.
e. Involvement and training of rural administrative and technical personnel in regional, national, and international activities focusing on the potentials and benefits of integrated biogas systems.
Table 6 shows a number of the benefits of biogas utilization, set against the related drawbacks of presently used alternatives.
|
Present problems |
Benefits of Biogas |
|
Depletion of forests for firewood and causation of ecological imbalance and climatic changes |
Positive impact on deforestation; relieves a portion of the labour force from having to collect wood and transport coal; helps conserve local energy resources |
|
Burning of dung cakes: source of environmental pollution; decreases inorganic nutrients; night soil transportation a hazard to health |
Inexpensive solution to problem of rural fuel shortage; improvements in the living and health standards of rural and village communities; provides employment opportunities in spin-off small-scale industries |
|
Untreated manure, organic wastes, and residues lost as valuable fertilizer |
Residual sludge is applied as top-dressing; good soil conditioner; inorganic residue useful for land reclamation |
|
Untreated refuse and organic wastes a direct threat to health |
Effective destruction of intestinal pathogens and parasites; end-products non-polluting, cheap; odours non-offensive |
|
Initial high cost resulting from installation, maintenance, storage, and distribution costs of end-products |
System pays for itself |
|
Social constraints and psychological prejudice to use of human waste materials |
Income-generator and apt example of self-reliance and self- sufficiency |