Fibre
Fuel
Most
green plants have large amounts of a stiff material called cellulose. Cellulose is one of the main ingredients in wood, and
is extracted for use in paper-making. Green plants manufacture cellulose from sugars, which they make during photosynthesis.
Because cellulose is made from sugar, it contains a lot of stored chemical energy, energy that originally came from the sun.
This chemical energy can be released as heat when wood is burned. Wood has been used as a fuel far longer than any fossil
fuel. In some countries, wood is still the main fuel for heating and cooking. In places where wood is scarce, other forms
of plant fibre are burned. Grass, peat, and even cow manure can be used as a fuel, but these materials make very smoky fires!
As long as these materials are allowed to grow back as soon
as they are used, they are considered a source of renewable energy. However, if too much wood is harvested too rapidly or
in a way that damages the soil or other parts of the ecosystem, severe environmental problems can result
Ethanol and Methanol
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.
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.
Biogas is actually a mixture
of gases, usually carbon dioxide and methane. It is produced by a few kinds of microorganisms, usually when air or oxygen
is absent. (The absence of oxygen is called "anaerobic conditions.") Animals that eat a lot of plant material, particularly
grazing animals such as cattle, produce large amounts of biogas. The biogas is produced not by the cows themselves, but by
billions of microorganisms living in their digestive systems. Biogas also develops in bogs and at the bottom of lakes, where
decaying organic matter builds up under wet and anaerobic conditions. Besides being able to live without oxygen, methane-producing
microorganisms have another special feature: They are among the very few creatures that can digest cellulose, the main ingredient
of plant fibres. Another special feature of these organisms is that they are very sensitive to conditions in their environment,
such as temperature, acidity, the amount of water, etc
Biogas is a Form of Renewable Energy
Flammable biogas can be collected using a simple tank, as shown here. Animal manure is stored
in a closed tank where the gas accumulates. It makes an excellent fuel for cook stoves and furnaces, and can be used in place
of regular natural gas, which is a fossil fuel.
Biogas is Not New
People have been using biogas for over 200 years. In the days before electricity, biogas was drawn from
the underground sewer pipes in London and burned in street lamps, which were known as "gaslights." In many parts
of the world, biogas is used to heat and light homes, to cook, and even to fuel buses. It is collected from large-scale sources
such as landfills and pig barns, and through small domestic or community systems in many villages. Biogas is considered to
be a source of renewable energy. This is because the production of biogas depends on the supply of grass, which usually grows
back each year. By comparison, the natural gas used in most of our homes is not considered a form of renewable energy. Natural
gas formed from the fossilized remains of plants and animals-a process that took millions of years. These resources do not
"grow back" in a time scale that is meaningful for humans.
