Direct combustion of biomass.
Thermochemical processing to upgrade the biofuel. Processes in this category include pyrolysis, gasification and liquefaction.
Biological processing. Natural processes such as anaerobic digestion and fermentation which lead to a useful gaseous or liquid fuel.

The immediate ‘product, of some of these processes is heat - normally used at place of production or at not too great a distance, for chemical processing or district heating, or to generate steam for power production. For other processes the product is a solid, liquid or gaseous fuel: charcoal, liquid fuel as a petrol substitute or additive, gas for sale or for power generation using either steam or gas turbines.

COMBUSTION
The technology of direct combustion as the most obvious way of extracting energy from biomass 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.

Combustion of wood can be divided into four phases:
Water inside the wood boils off. Even wood that has been dried for ages has as much as 15 to 20% of water in its cell structure.
Gas content is freed from the wood. It is vital that these gases should burn and not just disappear up the chimney.
The gases emitted mix with atmospheric air and burn at a high temperature.
The rest of the wood (mostly carbon) burns. In perfect combustion the entire energy is utilised and all that is left is a little pile of ashes.

Three things are needed for effective burning:
high enough temperatures;
enough air, and
enough time for full combustion.

If not enough air gets in, combustion is incomplete and the smoke is black from the unburned carbon. It smells terrible, and you get soot deposited in the chimney, with the risk of fire. If too much air gets in the temperature drops and the gases escape unburned, taking the heat with them. The right amount of air gives the best utilisation of fuel. No smell, no smoke, and very little risk of chimney fires. Regulation of the air supply depends largely on the chimney and the draught it can put up.
Direct combustion is the simplest and most common method of capturing the energy contained within biomass. Boiling a pan of water over a wood fire is a simple process. Unfortunately, it is also very inefficient, as a little elementary calculation reveals.
The energy content of a cubic metre dry wood is 10 GJ, which is ten million kJ. To raise the temperature of a litre of water by 1 degree Celsius requires 4,2 kJ of heat energy. Bringing a litre to the boil should therefore require rather less than 400 kJ, equivalent to 40 cubic centimetres of wood - one small stick, perhaps. In practice, with a simple open fire we might need at least fifty times this amount: a conversion efficiency no better than 2%.
Designing a stove or boiler which will make rather better use of valuable fuel requires an understanding of the processes involved in the combustion of a solid fuel. The first is one which consumes rather than produces energy: the evaporation of any water in the fuel. With reasonably dry fuel, however, this uses only a few percent of the total energy. In the combustion process itself there are always two stages, because any solid fuel contains two combustible constituents. The volatile matter is released as a mixture of vapours or vaporised tars and oils by the fuel as its temperature rises. The combustion of these produces the little spurts of pyrolysis.
Modern combustion facilities (boilers) usually produce heat, steam (used in industrial process) or electricity. Direct combustion systems vary considerably in their design. The fuel choice makes a difference in the design and efficiency of the combustion system. Direct combustion technology using biomass as the fuel is very similar to that used for coal.  Biomass and coal can be handled and burned in essentially the same fashion. In fact, biomass can be “co-fired” with coal in small percentages in existing boilers. The biomass which is co-fired are usually low-cost feedstocks, like wood or agricultural waste, which also help to reduce the emissions typically associated with coal. Coal is simply fossilized biomass heated and compressed over millions of years. The process which coal undergoes as it is heated and compressed deep within the earth, adds elements like sulphur and mercury to the coal. Burning coal for heat or electricity releases these elements, which biomass does not contain.

PYROLYSIS
Pyrolysis is the simplest and almost certainly the oldest method of processing one fuel in order to produce a better one. A wide range of energy-rich fuels can be produced by roasting dry wood or even the straw. The process has been used for centuries to produce charcoal. Conventional pyrolysis involves heating the original material (which is often pulverised or shredded then fed into a reactor vessel) in the near-absence of air, typically at 300 - 500 °C, until the volatile matter has been driven off. The residue is then the char - more commonly known as charcoal - a fuel which has about twice the energy density of the original and burns at a much higher temperature. For many centuries, and in much of the world still today, charcoal is produced by pyrolysis of wood. Depending on the moisture content and the efficiency of the process, 4-10 tonnes of wood are required to produce one tonne of charcoal, and if no attempt is made to collect the volatile matter, the charcoal is obtained at the cost of perhaps two-thirds of the original energy content.
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 suitable fuel for electricity generation using high-efficiency gas turbines.
With more sophisticated pyrolysis techniques, the volatiles can be collected, and careful choice of the temperature at which the process takes place allows control of their composition. The liquid product has potential as fuel oil, but is contaminated with acids and must be treated before use. Fast pyrolysis of plant material, such as wood or nutshells, at temperatures of 800-900 degrees Celsius leaves as little as 10% of the material as solid char and converts some 60% into a gas rich in hydrogen and carbon monoxide. This makes fast pyrolysis a competitor with conventional gasification methods (see bellow), but like the latter, it has yet to be developed as a treatment for biomass on a commercial scale.
At present, conventional pyrolysis is considered the more attractive technology. The relatively low temperatures mean that fewer potential pollutants are emitted than in full combustion, giving pyrolysis an environmental advantage in dealing with certain wastes. There have been some trials with small-scale pyrolysis plants treating wastes from the plastics industry and also used tyres - a disposal problem of increasingly urgent concern.

GASIFICATION
The basic principles of gasification have been under study and development since the early nineteenth century, and during the Second World War nearly a million biomass gasifier-powered vehicles were used in Europe. Interest in biomass gasification was revived during the “energy crisis” of the 1970s and slumped again with the subsequent decline of oil prices in the 1980s. The World Bank (1989) estimated that only 1000 - 3000 gasifiers have been installed globally, mostly small charcoal gasifiers in South America.
Gasification based on wood as a fuel 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.

SYNTHETIC FUELS
A gasifier which uses oxygen rather than air can produce a gas consisting mainly of H2, CO and C02, and the interesting potential of this lies in the fact that removal of the C02 leaves the mixture called synthesis gas, from which almost any hydrocarbon compound may be synthesised. Reacting the H2 and CO is one way to produce pure methane. Another possible product is methanol (CH3OH), a liquid hydrocarbon with an energy density of 23 GJ per tonne. Producing methanol in this way involves a series of sophisticated chemical processes with high temperatures and pressures and expensive plant, and one might wonder why it is of interest. The answer lies in the product: methanol is that valuable commodity, a liquid fuel which is a direct substitute for gasoline. At present the production of methanol using synthesis gas from biomass is not a commercial proposition, but the technology already exists, having been developed for use with coal as feedstock - as a precaution by coal-rich countries at times when their oil supplies were threatened.

FERMENTATION
Fermentation of sugar solution is the way how ethanol (ethyl alcohol) can be produced. Ethanol is a very high liquid energy fuel  which can be used as the substitute for gasoline in cars. This fuel is used successfully in Brazil. Suitable feedstocks include crushed sugar beet or fruit. Sugars can also be manufactured from vegetable starches and cellulose by pulping and cooking, or from cellulose by milling and treatment with hot acid. After about 30 hours of fermentation, the brew contains 6-10 per cent alcohol, which can be removed by distillation as a fuel.
Fermentation is an anaerobic biological process in which sugars are converted to alcohol by the action of micro-organisms, usually yeast. The resulting alcohol is ethanol (C2H3OH) rather than methanol (CH3OH), but it too can be used in internal combustion engines, either directly in suitably modified engines or as a gasoline extender in gasohol: gasoline (petrol) containing up to 20% ethanol.
The value of any particular type of biomass as feedstock for fermentation depends on the ease with which it can be converted to sugars. The best known source of ethanol is sugar-cane - or the molasses remaining after the cane juice has been extracted. Other plants whose main carbohydrate is starch (potatoes, corn and other grains) require processing to convert the starch to sugar. This is commonly carried out, as in the production of some alcoholic drinks, by enzymes in malts. Even wood can act as feedstock, but its carbohydrate, cellulose, is resistant to breakdown into sugars by acid or enzymes (even in finely divided forms such as sawdust), adding further complication to the process.
The liquid resulting from fermentation contains only about 10% ethanol, which must be distilled off before it can be used as fuel. The energy content of the final product is about 30 GJ/t, or 24 GJ/m3. The complete process requires a considerable amount of heat, which is usually supplied by crop residues (e.g. sugar cane bagasse or maize stalks and cobs). The energy loss in fermentation is substantial, but this may be compensated for by the convenience and transportability of the liquid fuel, and by the comparatively low cost and familiarity of the technology.

ANAEROBIC DIGESTION
Nature has a provision of destroying and disposing of wastes and dead plants and animals. Tiny micro-organisms called bacteria carry out this decay or decomposition. The farmyard manure and compost is also obtained through decomposition of organic matter. When a heap of vegetable or animal matter and weeds etc. die or decompose at the bottom of back water or shallow lagoons then the bubbles can be noticed rising to the surface of water. Some times these bubbles burn with flame at dusk. This phenomenon was noticed for ages, which puzzled man for a long time. It was only during the last 200 years or so when scientists unlocked this secret, as the decomposition process that takes place under the absence of air (oxygen). This gas, production of which was first noticed in marshy places, was and is still called as ‘Marsh Gas’. It is now well known that this gas (Marsh Gas) is a mixture of Methane (CH4) and Carbon dioxide (CO2) and is commonly called as the ‘Biogas’. As per records biogas was first discovered by Alessandro Volta in 1776 and Humphery Davy was the first to pronounce the presence of combustible gas Methane in the Farmyard Manure in as early as 1800. The technology of scientifically harnessing this gas from any biodegradable material (organic matter) under artificially created conditions is known as biogas technology.

Anaerobic digestion, like pyrolysis, occurs in the absence of air; but in this case the decomposition is caused by bacterial action rather than high temperatures. It is a process which takes place in almost any biological material, but is favoured by warm, wet and of course airless conditions. It occurs naturally in decaying vegetation on the bottom of ponds, producing the marsh gas which bubbles to the surface and can even catch fire.
Anaerobic digestion also occurs in situations created by human activities. One is the biogas which is generated in concentrations of sewage or animal manure, and the other is the landfill gas produced by domestic refuse buried in landfill sites. In both cases the resulting gas is a mixture consisting mainly of methane and carbon dioxide; but major differences in the nature of the input, the scale of the plant and the time-scale for gas production lead to very different technologies for dealing with the two sources.
The detailed chemistry of the production of biogas and landfill gas is complex, but it appears that a mixed population of bacteria breaks down the organic material into sugars and then into various acids which are decomposed to produce the final gas, leaving an inert residue whose composition depends on the type of system and the original feedstock.

BIOGAS
is a valuable fuel which is in many countries produced in purpose built digesters filled with the feedstock like dung or sewage. Digesters range in size from one cubic metre for a small ‘household’ unit to more than thousand cubic meters used in large commercial installation or farm plants. The input may be continuous or in batches, and digestion is allowed to continue for a period of from ten days to a few weeks. The bacterial action itself generates heat, but in cold climates additional heat is normally required to maintain the ideal process temperature of at least 35 degrees Celsius, and this must be provided from the biogas. In extreme cases all the gas may be used for this purpose, but although the net energy output is then zero, the plant may still pay for itself through the saving in fossil fuel which would have been needed to process the wastes. A well-run digester will produce 200-400 m3 of biogas with a methane content of 50% to 75% for each dry tonne of input.

Digestors - outside view.  Digestor from inside.

Biogas plant with integrated gas holder. Biogas plant with separate gas holder.