The different processes that are followed in India to make these wastes into a useful fuel are discussed. .

V. Combustion
Agricultural residues-fired thermal power plants of small capacity are becoming popular with agro-processing industries. Thus, sugar mills use bagasse (bi-product, wastes of sugar mills) for meeting their energy needs and rice mills bum husks. Combustion involves burning of biomass fuels in the presence of air to generate heat energy. Main products of combustion are ash and fuel gases. Usually fuels are burnt on hot furnace. The heat generated is used to drive steam engines and also for irrigation & thrashing purposes.

VI. Pyrolysis
When wood and agricultural residues are heated in the absence of air they breakdown into a complex mixture of liquids involves heating organic matter at a high temperature of 540-1100oC in the absence of air for several hours. The earthen kilns or pit kilns or brick kilns or portable steel kilns are used for this process of which portable steel kilns make good charcoal.

Charcoal has a high calorific value, easy to transport, store and distribute and more efficient in burning and has a characteristic feature of creating less pollution. From eight tonnes of wood, around one tonne of charcoal can be made. The gases that are produced during the process of pyrolysis can be converted or synthesized into methanol and liquids which are used as fuels. Depending on the temperature, the degradation stages of pyrolysis are given in Table 3.

Sl. No	Temperature	Product
1	0-170	Evaporation of moisture
2	170-270	Evolution of carbon monoxide and carbon dioxide
3	270-400	Evolution of methanol
4	400-500	Charcoal with optimum carbon content

VII. Destructive Distillation
The destructive distillation is carried out in long steel retorts. Only the wood wastes such as branches, trunks of trees are used as raw materials. The process involves decomposition of wood at high temperatures in the absence of air. At an initial temperature of 230°C, the moisture is evaporated and then the temperature is raised to 370°C and maintained I for 6 hours. At the end of this period, wood is converted to charcoal which is cooled in the absence of air for 48 hours. After cooling, they are spread in open sheds, two days for drying and is ready for supply to consumers. The vapours of the volatile matters that are formed during the distillation process subsequently condensed to tar, methanol, acetic acid, methyl acetate, oil and gas. The uses of these distillation products are detailed here.

1. Charcoal -Apart from its use as a fuel, charcoal is used as a source of carbon in making carbon-di-sulphide.
2. Methanol -It is used as a solvent and an antifreeze for automobiles.
3. Acetic acid -It is used as a raw material for the manufacture of acetic anhydride, sodium acetate, cellulose acetate, ethyl acetate, butyl acetate etc.
4. Methyl acetate -is a solvent used in paint industry.
5. Tar -A product of tar known as "pitch" is used as rubber softener.
6. Oils -They are used as solvent and insecticide.
7. Gases -Gases are used as fuel for heating wood distillation and fuel for boilers.

VIII. Gasification


Gasification is a process of degradation of carbonaceous material (wood wastes) under controlled air or pure oxygen at a high temperature of l000°C. As a result of gasification high amount of gases are produced. Biomass gasification is done in gasifiers designed m various ways. Types of gasifiers are generally classified based on the physical conditions of the feed stocks in the gasifiers. Fixed bed gasifier, stired bed gasifier, tumbling bed gasifier, fluidized bed gasifier etc. are in use.

In course of gasification, a number of chemical reactions takes place. As soon as the biomass is ignited four distinct zones are set-up in the gasifier-unit. Biomass when introduced into the gasifier, it enters into drying zone where the temperature is 200-400°C. The products of this zone are vapours of tar, organo-chemicals, and liquid oils.

After drying zone it enters into pyrolysis zone, temperature of 400- 750°C. Pyrolysis results in char, organic liquids and some gases. At 750-l000°C, mainly gases like carbon-di-oxide, carbon mon- oxide, hydrogen, methane etc. are produced which is called as gasification zone. The oxidation zone which is at l000-l400oC also produces gases like nitrogen, carbon-di-oxide, hydrogen etc. When the process of oxidation is over, with the steam treatment, ash, an inert material is formed.

The gasification process, thus, ultimately results in a number of gases which are in mixture. This mixture of gases referred to as producer gas, the composition of which is given below.

Carbon monoxide -20-22%,
Hydrogen -15-18%,
Methan -2-4%,
Carbon-di-oxide -9-11%
Nitrogen -50-53%.

Producer gas somewhat burnt like a natural gas which Ican be used as a fuel for engines.
IX. Anaerobic Bio-gasification
Degradation of organic matter in the absence of air to methane and carbon-di-oxide is called anerobic biogasification. In villages, cattle manure is used as a fuel for cooking purpose. So, preparation of biogas has become popular among rural people. Biogas can be utilized for cooking, lighting, operating diesel engines, water pumps etc. As cattle dung is the main raw material for the production of biogas, this method has been given importance. The main advantages of the biogasification are given here.

A. Advantages
The biogas (also called as gober gas) production can be started by constructing a permanent structures (tanks) at a convenient place at home. It can be manufactured with least maintenance. Initial investment is also cheap. Gober gas contains 60% methane and 40% carbon-di-oxide. The digested manure contains 1.5-2% nitrogen and other soil nutrients, which can be used as an organic fertilizer. The biogas can be easily purified to methane enriched fuel gas of high calorific value. The process of conversion of raw animal dung as well as some agricultural residues to biogas is detailed as follows.

B. Process
The conversion carbon-di-oxide and methane takes place by bacteria. The biogas from cattle dung requires an optimum temperature of 35°C. The biomass is kept in closed tanks where the temperature is 35-38°C and the conversion process is carried out anaerobically (in the absence of air). The digestion time varies from 15-30 days. On an average biogas generated from cattle dung is composed of 60% methane and 40% carbon-di-oxide.

When the cattle dung is used as feed stock, the biogas plant is 10 be filled with a homogenous slurry made from a mixture of fresh dung and water in a ratio of 1:1. Subsequent digestion changes results in the production and accumulation of gas in the empty space at the upper portion of the tank. The gas accumulated presses the slurry digester and displaces it into the outlet displacement chamber. In other words, the slurry level in the digester falls where as in the outlet chamber, it starts raising. The slurry from the outlet displacement chamber can be used as organic fertiliser. The gas outlet pipe will be connected to the gas utiliser points by pipelines.

C. Feasibility
Many developing countries are encouraging for installation of biogas plants to meet the demand for fuel. India is one of the pioneer country in biogas technology where biogas research and plant construction has been caused out over the past 30 years. In most of biogas plant, cattle dung is used for gas production. Though, biogas is used mainly for cooking and lighting purposes, there are many other advantages also. It can be used in internal combustion engines to power water pump and electric generators. The most economical benefits are minimizing environmental pollution and meeting the demand of energy for various purposes. In general, five cattle produces enough dung to generate about 2m3 biogas to meet the demand of cooking and lighting for a family of four-five people.

Next : Biomass converting process - Alcoholic Fermentation.