Biomass Fuels
PLANT YOUR OWN POWER
The world's largest industrial use of solar energy is in agriculture. This
is due to the unique ability of plants to convert 0.02% of incident solar
radiation into stored chemical energy by the process of photosynthesis. The
plants absorb water and carbon dioxide from the soil and the air, and use
sunlight to convert these materials into carbohydrates such as sugars and
starches. The light energy is converted into the chemical energy bonding the
carbohydrate molecules.
Only the wavelengths of sunlight between 0.3 and 0.7 microns are used for
photosynthesis, compared with the light from 0.3 to 1.1 microns wavelength
which can be used by a silicon photoelectric cell. If the plant is eaten by
an animal the chemical bonds of the molecules are broken by oxidation and
energy is released to power the animal's body. The oxidation of plant matter
in an animal is analogous to burning fuel, which is the simplest way to use
the energy of plants to produce heat, drive machinery, etc.
If a plant decays rather than being eaten, its stored energy is released largely
as heat. An estimated thousandth part of total plant matter produced each
year is deposited in conditions which do not favour decay or complete energy
loss, and in these conditions the material undergoes the first stages of fossilization.
When coal, oil or natural gas are burned the same chemical bonds are being
broken as if the original plants had been burned, and the energy produced
is originally the energy of photosynthesis.
WOOD
In 1950 wood fuel still contributed 4% of
the total world energy supply. Of the earth's original 4.8 billion hectares
of forest, 1.2 billion are now under management to produce a regular timber
crop and another 2 billion are still virgin forest (the rest has been cleared
for agriculture, cities, transport, etc). A hectare of mixed forest with trees
of differing ages and varieties will supply about 7.4 tonnes of usable timber
per year, a pine forest 10 tonnes per hectare per year, and a tropical rain
forest 59 tonnes per hectare per year. If all the carbon in the 175 x 109
dry tons' of organic matter produced by the biosphere annually were burned
as fuel at 75% efficiency, it would supply about 500 x 1012 kWh, roughly 10
times the world's energy consumption for 1970. As a fuel, wood has a calorific
value of 4.2 kWh/kg.
The most effective way to use plants as a fuel is by burning them. Good combustion
can release a greater proportion of the calorific value of plants than any
other secondary conversion of their energy. Wood-burning appliances such
as heating stoves and cookers can be obtained with efficiencies of at least
75%, although none are made in Britain because of our lack of forests,
and traditional supplies of coal. The most efficient of these appliances
have secondary combustion chambers where the hydrogen and carbon monoxide
released by the burning wood are burned to it crease the total energy obtained.
Wood can also be used to fire boilers for industrial use, as in a furniture
factory in High Wycombe, Buckinghamshire, where a boiler is fuelled by wood
offcuts and the steam drives a generator to run the machinery of the factory.
Wood and other plant materials can be converted into more conventional liquid
and solid fuels such as ethyl alcohol and charcoal (usable as replacements
for petrol and smokeless fuel respectively). These fuels were widely used
during the war in timberproducing countries. During the Second World War the
forester Edward Lundh doubled the effective annual wood fuel harvest in Sweden
by improving forestry practices and eliminating waste.
Buses, boats and tractors were fitted with wood gas generators which produced
a mixture consisting largely of carbon monoxide, carbon dioxide, methane and
hydrogen which was fed directly to the engine as a petrol substitute. Wood
was also distilled to give motor lubricants, converted into cellulose cattle
feed and, when fortified with soya flour and yeast, sold as a meat surrogate.
These uses of wood ceased after the war because they were expensive compared
with conventional sources of materials, but they showed how many of the requirements
of a conventional energy-using system could be met by a renewable fuel.
PEAT AND DUNG
Two other fuels of vegetable origin deserve mention; these are peat and dung.
Peat is used in Ireland, Germany and the USSR and there are even power stations
fuelled by it. Peat production worldwide is 35 million tonnes per year. The
great advantage of peat is that it is a renewable resource, and a managed
bog will regenerate every hundred years. Dried dung cakes are used in underdeveloped
countries as a source of fuel for cooking and space heating. This is a practice
which is harmful agriculturally and medically because the land is deprived
of essential manure and the smoke from the burning dung causes eye disease.
But as long as no other source of fuel is available the burning of dung is
unavoidable.
ALGAL CULTURE
The wish to gain higher yields per hectare of biomass has encouraged research
into microscopic singlecelled plants such as Chlorella (a type of algae)
which can give yields up to 86 tonnes per hectare. These plants can then
be converted into charcoal, combustible gases and combustible liquids by
pyrolysis (heating to 500°-900°C in a closed container without air).
A considerable amount of energy is required to dry the water out of the organic
material, and the energy input could be higher than the value of the fuel
produced. Some researchers suggest that the higher plants could give yields
as high as algae, given suitable growing conditions. Reports of trials in
West Bengal of the Sharder process of hydroponic culture (using organic nutrients
rather than chemical solutions) quote an annual yield per hectare per year
of 2,000 tonnes, although the system has been tried only on a very small
scale. This technique could be used with such high yielding crops as sugar
beet to produce large quantities of fuel. Although beet could be converted
into alcohol as a fuel it would also be ideally suited for conversion into
methane gas by the method described in *****.
