Methods of generating energy from biomass   

The methods of generating energy can be split in two different groups. There are the dry processes and the wet processes.
The dry processes are: Combustion and Pyrolysis
The wet processes are: Anaerobic Digestion, Gasification and Fermentation

Combustion

The most obvious way of extracting energy from biomass, the technology of direct combustion is well understood, straightforward and commercially available. Combustion systems come in a wide range of shapes and sizes burning virtually any kind of fuel, from chicken manure and straw bales to tree trunks, municipal refuse and scrap tyres. Some of the ways in which heat from burning wastes is currently used include space and water heating, industrial processing and electricity generation. One problem with this method is its very low efficiency. With an open fire most of the heat is wasted and is not used to cook or whatever. One method of improving this in developing countries is to build stoves out of mud and scrap iron.

Pyrolysis

A wide range of energy-rich fuels can be produced by roasting dry woody matter like straw and woodchips. The process has been used for centuries to produce charcoal. The material is pulverised or shredded then fed into a reactor vessel and heated in the absence of air. Pyrolysis can also be carried out in the presence of a small quantity of oxygen ('gasification'), water ('steam gasification') or hydrogen ('hydrogenation'). One of the most useful products is methane, which is a suilable fuel for electricity generation using high-efficiency gas turbines.

Anaerobic Digestion

Biogas is produced when wet sewage sludge, animal dung or green plants are allowed to decompose in a sealed tank under anaerobic (oxygen-free) conditions. Feedstocks like wood shavings, straw and refuse may be used, but digestion takes much longer. Each kilogram of organic material (dry weight) can be expected to yield 450-500 litres of biogas. The residue left after digestion is a potentially valuable fertilizer or compost. Fermentation: Ethanol (ethyl alcohol) is produced by the fermentation of sugar solution by natural yeasts. Suitable feedstocks include crushed sugar beet and fruit. Sugars can also be manufactured from vegetable starches and cellulose by pulping and cooking, or from cellulose by miling and treatment with hot acid. After about 30 hours of fermentation, the brew cantains 6-10 per cent alcohol, which can be removed by distillation as a fuel.

Gasification

This process, usually using wood produces a flammable gas mixture of hydrogen, carbon monoxide, methane and other non flammable by products. This is done by partially burning and partially heating the biomass (using the heat from the limited burning) in the presence of charcoal (a natural by-product of burning biomass). The gas can be used instead of petrol and reduces the power output of the car by 40%. It is also possible that in the future this fuel could be a major source of energy for power stations.

Fermentation

If the biomass used is (or can be converted into) mostly sugar, then yeast can be added. The fermentation that follows produces alcohol which is a very high energy fuel that makes it very practicle for use in cars. This has been tried succesfully in Brazil.