Ethanol and methanol are alcohols and are highly flammable. They can be made from plant sugars or plant fibres. Alcohol is produced by feeding plant materials into large heated tanks called digesters. Inside the tanks, chemicals or yeasts are added to change the plant materials into alcohol. The alcohol is extracted, purified, and prepared for use as a fuel.

Both ethanol and methanol make excellent fuels for cars and trucks. In fact, ethanol is used in the engines of Formula 1 racing cars. It burns very cleanly, and delivers more power than gasoline. Many service stations now sell fuels that contain a blend of gasoline and an alcohol, usually ethanol.

Methanol and ethanol can be deadly poisons, especially in the amounts used to make transportation fuels. Methanol is especially toxic. Even small amounts breathed in as fumes or accidentally swallowed can cause blindness, severe liver damage, and death.

Biogas

Most mammals-humans included-produce a flammable gas called "biogas" as they digest their food. Bacteria living in their digestive systems produce methane as they break down cellulose present in the food. Biogas is also produced in bogs and wetlands where large amounts of rotting vegetation may accumulate. Biogas consists mostly of a gas called methane, which is the same as "natural gas", commonly burned in our furnaces and barbecues. Biogas can be used instead of natural gas for heating and cooking.

Humans have learned to duplicate this process in large tanks called biogas generators. To start the process, shredded plant materials and animal wastes are mixed with water in the biogas generator. Many kinds of naturally occurring bacteria arrive with the shredded plant material. The tank is then sealed so no air can get in. Within days, a special kind of bacteria in the tank will begin to produce biogas. These bacteria are known as "methanogenic", because they produce methane, the main ingredient in biogas. The biogas forms bubbles in the mixture, and collects at the top of the tank. It is piped to a large balloon-like bag where it is stored until needed.

Eventually, the production of biogas in the generator starts to slow down. The mixture of water and manure is replaced with a fresh supply to start the process again. The old material is unable to produce any more biogas, but still contains large amounts of plant material and other organic matter. It is dried to form a rich black soil, and is spread on fields as a fertilizer.

Another source of biogas is landfills. At the landfill site, large mounds of garbage are buried under the surface. Bacteria break some of the garbage down and can produce large amounts of biogas. This is sometimes collected and burned to heat buildings near the landfill. Biogas can contain traces of hydrogen sulphide (H2S) gas, particularly in the case of landfill gas. Care must be taken to deal safely with this gas because H2S can be fatal in small amounts.