BIOMASS AS ENERGY SOURCE -I
Posted June 23, 2008on:
Biomass is plentifully available in the rural regions. It is already being used by the rural people as a major source of energy, mainly in cooking food, which constitutes almost 50% of the total energy consumption. Assuming that there are about 140 million households in rural India, and assuming that each family uses annually about 3 tonnes of biomass as fuel, one comes to the figure of about 400 million tonnes of biomass utilised annually only for domestic cooking.
Engineers and energy scientists generally think only of the calorific value of fuels and of fuel use efficiency. But there is also a third dimension to fuel use, and that is the pollution arising due to burning of biomass. As cooking is done within the confines of a house, the pollution caused by cooking fires is generally not taken very seriously.
But according to statistics published by the World Health Organisation, annually about 500,000 women and children die prematurely in India due to air pollution caused by cooking fires in rural households. Considering the fact that almost 70% of our population is rural, giving the rural women a cleanly burning biofuel is a major task, which is unfortunately not tackled by any of our major research centres.
There are many options for providing a clean and economical burning biofuel. The biomass that is currently available to villagers is free of cost.
One way of tackling this problem is to redesign the cooking devices in such a way that they burn the biomass more cleanly, so that the pollution caused by them is reduced. This is achieved by providing the fuel with sufficient air, so that it burns completely, reducing automatically the carbon monoxide and the particular matter in the fuel gases. Another strategy is to design a stove in such a way that waste of heat is avoided and a major part of the heat generated by the burning biomass is transferred to the pot. This results in higher fuel use efficiency, requiring the user to burn less fuel. Pollution is naturally reduced if the amount of fuel is reduced. Both the strategies are combined in modern improved cook stoves.
However, in practical terms, both the strategies often fail, because the fuel that is used in the laboratory while designing the stove differs from the fuel that the rural housewife actually uses. In a laboratory experiment, one normally uses good quality firewood, that has been properly dried and cut into pieces of adequate size. In contrast to this, the fuel used by the rural housewife consists of stalks of plants like cotton, maize, safflower, arhar, or of bushes growing in the vicinity, maize cobs, dung cakes, rhizomes of sugarcane, etc.
The traditional cookstove is designed to burn such material and therefore, the housewife often finds that the improved cookstove emits more smoke and soot than her traditional stove, comparatively. Standardisation of fuel is, therefore, another strategy that is considered in the context of using biomass as cooking fuel. The easiest way of standardising woody biomass is to cut it into uniform, small pieces called chips. Highly efficient and non-polluting stoves can be designed to burn these chips, but unfortunately not much effort has been made in this direction in India.
The second and traditional method of converting a non-standard fuel into standard one is to char it into charcoal. It is the volatile matter in biomass that gives rise to the particulate matter in the flue gases. In the process of charring, the volatiles are removed from the biomass to leave only the carbon and non-combustible matter behind. Therefore, when charcoal burns, it burns cleanly, without producing any smoke or soot. However, the traditional method of producing charcoal is itself highly polluting, because the volatiles are released into the atmosphere in this process. Sophisticated technologies are now available for charring, in which the volatiles are burned in the process of charring itself, to produce the heat required in the process.
Agricultural waste is an ideal source of charcoal. When one harvests any crop, one generally harvests only grain, fruits, pods, tubers or rhizomes. This constitutes only about 30 to 40% of the total biomass. This means that about 60 to 70% of the total agricultural biomass, or almost 600 million tonnes, is the waste biomass produced annually in India. A small part of it is used as fodder for cattle, but the rest is just wasted.
The standardised Sarai cooker, a stove-and-cooker system, can cook the meal for five persons, using just 100 g of our char briquettes. About 15,000 households in Maharashtra are already using the Sarai cooker.