|The Biogas/Biofertilizer Business Handbook (Peace Corps, 1982, 186 p.)|
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).