BIOMASS AND WASTE CONVERSION TECHNOLOGIES

The once abundant conventional sources of energy like coal and oil are shrinking. Their escalating costs, coupled with problems of pollution, have necessitated a fresh look at alternatives. One such alternative is biomass. The term 'biomass' includes all plant life, trees, agricultural residues, bush, grass, algae and can be extended to livestock droppings which, of course, are derived from plants. Biomass may be obtained from forests in a planned or unplanned fashion or from agricultural lands. The entire organic content of biomass can be converted into usable forms of energy. The most obvious example is firewood, one of the most widely used fuels in the developing countries. Biomass potential Every year, plants convert about 200 billion tones of carbon into terrestrial and aquatic biomass through photosynthesis. The equivalent of the products of synthesis is 3000 billion giga joules. This is about 10 times the total energy being presently consumed in the world annually. One seventh of the world's total energy comes from biomass and this huge and potentially renewable resource is still left largely untapped. Potentially, organic residues can be utilized for a variety of purposes - fuel, fertilizer, feed, building materials, industrial chemicals and other products, medicinal and pharmaceutical formulations. keeping in view the importance of energy supply on decentralized basis for agro-industrial processing and agriculture, the primary residues need to be used as a source of energy and the secondary and tertiary leftovers for other purposes. Complete recycling will also help in reducing pollution resulting from biodegradation of organic materials. Therefore, the main objectives of residue management are two-fold: (i) use of residues as a source of energy and materials; and (ii) minimization of environmental pollution resulting from inefficient utilization. Biomass utilization for many major applications calls for its conversion into an adaptable fuel as a first step. The utilization of biomass as a source of energy involves thermo chemical or biochemical conversion. It can be used either by direct combustion in boilers, furnaces, cooking stoves, etc. or its utilization via gasification by thermo chemical or biochemical routes. For biomass to be able to substitute or supplement conventional fossil fuels, gasification is more appropriate. Biochemical conversion of biomass The products of biochemical conversion mainly include biogas from animal dung, sewage, and liquid fuel from fermentable sugars. Animal dung and agro-residues Anaerobic fermentation is a method of biochemical conversion of low lignin biomass in the presence of adequate moisture to produce methane rich gas called biogas. The calorific value of gas containing 60% of methane is 22 MJm-3. Cattle dung and large number of other agro-residues have been found suitable for anaerobic digestion in biogas plant. The biogas produced is a clean fuel for domestic cooking. Besides, it is used for illumination when burnt in silk mantle. It is also used as fuel for substantial replacement of diesel in engines for motive power and generating electricity. Biogas plants mitigate the drudgery of rural women, reducing the pressure on forest and recycling human waste by linking toilets with biogas plants, thereby improving sanitation in rural areas. Liquid waste (sewage) In recent years, increased urbanization in developing countries has given rise to a phenomenal increase in the quantity of sewage. Indiscriminate disposal of this wastewater results in pollution of ground water river systems, estuaries, lakes and land. The waste water can be used for the production of sewage gas which essentially consists of methane and CO2 with traces of hydrogen, nitrogen and hydrogen sulphide. It has relatively low calorific value which is more than that of gobar gas. It can successfully be used in diesel engine since its octane rating (110) is same as that of gobar gas. The presence of CO2 lowers the calorific value but increases the knock resistance about methane. It is estimated that the quantity of biogas generated per day by a sewage treatment plant is approximately 0.35 cft per capita of city's population. For example, it is calculated that a sewage treatment plant generating 5 million cft per day can yield energy equivalent to nearly 20 million litres of petrol per day. Collection, storage and utilization of sewage gas are economically justified only when the treatment works are large enough. The whole system has to be kept under pressure to avoid the formation of explosive mixture of gas and air. The gas becomes violently explosive in mixture of 1 vol. of gas to between 5-14 vols. of air, and at higher dilutions, gas burns freely. Also, the gas should be passed through scrubbers to remove unwanted constituents viz. CO2, H2S and water vapour. Fermentable sugars The use of biomass in alcohol manufacture is not a new discovery. In the 1930's and during the second world war, many countries converted waste agricultural products to alcohol for use in automobiles, as 10-15% blends with gasoline or as the entire fuel. Traditionally, alcohol for industrial use is manufactured from molasses. Grains, excess grapes or other fruits, potatoes, starchy roots like cassava, mahua flowers and palm juice may be used depending on their value in relation to human and animal foodstuffs. In countries like Sweden, Finland and Canada, which have flourishing pulp industries, the sulphite liquor, with its content of 2-3% of fermentable sugars constitutes a suitable raw material for the production of alcohol. In recent times, Brazil has been converting sugarcane, directly into alcohol and the produces from cane and cassava about 2-8 billion gallons of alcohol. The good deal of research work has been carried out in developing countries on the enzymatic production of ethanol from cellulose hydrolysate in batch and continuous systems employing free cells, recycled cells and also immobilized cells. A yeast strain which is tolerant to high concentrations of glucose and ethanol was also developed. The new technique of fixing yeast cells on an inert support and using it in continuous production of ethanol at various dilution rates has also been developed. It has been applied successfully to both biogas hydrolysis and cane molasses. Other studies include the use of thermo-tolerant yeasts and bacterial species. The cell recycle system has been run with 530 and 300 litre reactors and is ready to be scaled up; this technique has steady operational stability of nearby 400 hours, while the immobilized system has been run for more than 75 days without any loss of activity . The utilization of alternate fuels like ethanol, methanol and biogas as substitutes to petrol, diesel and kerosene for vehicular and stationary combustion engines has been in progress in the various Engines Laboratories in developing countries. After extensive trials, an' optimum ' blend has been perfected which gives improved engine performance (6-8% more power), lesser consumption (3-5%), lesser exhaust emission (10-30%, exhaust HC and CO), reduced carbon deposit and cooler engine operation as compared to that obtained with gasoline. Thermochemical conversion of biomass The products of thermo chemical conversion include combustible gas and a variety of chemicals besides energy from feed stocks by the action of heat. Fuel wood and crop residuesThe rural household sector accounts for nearly 75% of total energy consumption. About 90% of this energy is consumed for cooking activities alone. Fuel wood, crop residues and animal residues meet an overwhelming proportion of rural energy demand providing 85-90% of the household sector energy. Surplus biomass from industries The other important application of thermo-chemical combustion of biomass surplus is for power generation through optimum co-generation for surplus power from biomass produced in sugar mills, paper mills, rice mill, etc. and biomass combustion based power generation. Pyrolysis is essentially the burning of organic matter in the absence of air at temperature 400- 700° C. Although it is an endothermic process, yet it yields a number of hydrocarbon gases and liquids such as carbon monoxide, methane, butane, wood tar and a variety of other chemicals which have very high energy content. Gasification by contrast is carried out at much higher temperatures (1000-1100° C) by blowing a jet of air or oxygen into a fire zone at the bottom. Gasification yields combustible gas useful for running engines producing heat and even electricity. Urban waste Incineration became popular in the beginning but the huge volume of useless waste gas and fine ash along with the liberation of highly noxious and corrosive chloride, nitride and sulphurous gas made this system very costly due to the operation of pollution control standards. Physical sorting was another alternative which came into vogue. It divides the garbage into three fractions: the bulk which consists of vegetable wastes and inorganic matter like clay, earth, bones and the like are converted into compost for use in vegetables, fruit trees and other field crops. The second fraction consists of paper and plastics. These are shredded, dried and pressed into briquettes or pellets which have the same heating value as wood or a medium grade coal. This Refuse Derived Fuels (RDF) is finding a ready market in Europe for industrial and even domestic use. The third fraction consists of mainly scrap metals, used batteries, some heavy plastic products, stones and the like. These are separated by magnetic and metal separators. The energy content in the garbage, particularly in the RDF is only of marginal importance therefore, this practice has remained only on social grounds due to savings of land, reduction in the garbage transport cost and an almost complete absence of environmental pollution. Oil crisis during 1979 gave new impetus to technologies like pyrolysis and gasification of urban waste. Methanol production rather than power from fuel gases generated by pyrolysis or gasification can change the economics of the process radically. The energy that can be recovered from garbage may be of marginal relevance to the rich nations, but it is of central, even pivotal importance to the poor nations. According to an estimate if the garbage generated by a city of 3500 tones a day, is gasified or pyrolysed, it will, on the basis of its calorific value and the energy recovery efficiency, yield around 18,000 tones of methanol a year. Industrial and agro-industrial waste Many industries such as tannery, sago and other units discharge effluents which cause environmental pollution. Such of these effluents which are biodegradable can be converted to energy besides mitigating pollution. For example biogas can be extracted from effluents and the thermal needs of the factory and even power can be producing to meet the electrical requirement of the units. As detailed above there is now a renewed interest in biomass as a significant energy source for a wider range of reasons, such as concern over the Greenhouse effect, energy security and socio-economic benefits.