Hydrocarbon fuels - also called fossil fuels - have been the main source of energy for the transportation and other sectors for more than a century. However, their rapidly increasing consumption and consequent depletion of reserves clearly show that the end of the ‘Fossil fuel age’ is not very far off. Besides, these fuels are the chief contributors to urban air pollution and a major source of green house gases - considered to be the prime cause behind the global climate change.Biofuels are renewable hence they can supplement hydrocarbon fuels, assist in their conservation, reduce GHGs as well as mitigate their adverse effects on the climate resulting from global warming. The term biofuels is usually intended to imply fuels sourced from biomass that are used for transportation purposes.
Biofuel include methane, producer gas, alcohols, esters and other chemicals made from cellulose biomass. Biofuels such as bio-ethanol and biodiesel have their own specific advantages.
Bio-ethanol, produced from substrates containing sugar, starches and cellulosic biomass, is an established oxygenate and transport fuel with many advantages:O It is an octane boosterO Being an oxygenate, it improves the combustion characteristics of gasolineO Reduces harmful emissions such as carbon monoxide, hydrocarbons and particulate matterO Readily blends with gasolineO Readily biodegradable, while a oxygenate such as MTBE is not
Biodiesel is produced mainly from vegetable oils and fats. Biodiesel, is commercially available in several countries from the esterification of vegetable oils including rape seed, soybean, sun flower and Jatropha oils as well as from tallow and waste cooking oils. O It can be used for greening waste landsO It is produced using a proven and low-capital-cost technology, hence can be readily implementedO It is biodegradable, non-toxic and free from sulfur.O It has properties that help reduce carbon monoxide, hydrocarbons, particulate matter and O It readily blends with diesel
Biofuels offer many benefits. They are beneficial for the environment because they add fewer emissions to the atmosphere than petroleum fuels on a per kilometer traveled basis, and they often utilize waste biomass resources that currently have no value and require disposal. Unlike petroleum, which is a non renewable natural resource, biofuels are renewable and inexhaustible sources of fuel, assuming the feedstock is produced in a sustainable fashion. Where energy crops are grown domestically, or other biomass sources are readily available for conversion, biofuels can reduce a country’s dependence on the vagaries of imported oil price fluctuations and uncertain supplies. The use of biofuels could therefore help to strengthen the energy security and boost a nation’s economy should crude oil prices reach and maintain levels above around $50/barrel.
All petroleum-derived fuels suitable for transport vehicles are compounds containing predominantly carbon and hydrogen atoms. Other constituent elements generally regarded as undesirable contaminants such as tetraethyl lead have been added in the past to modify fuel properties so as to reduce the tendency of the fuel to ‘knock’ during combustion in an internal combustion engine. Other chemical additives such as nitro methane have been added to specialty fuels to improve power output.
Liquid biofuels differ chemically from fossil fuels in that they contain oxygen in addition to the carbon and hydrogen atoms. As with fossil fuels, they may also contain other elements, notably nitrogen and once again this is generally regarded as an undesirable impurity.
For biofuels there is some discrepancy between their specifications and the measured octane or cetane number since the tests were originally developed for hydrocarbon fuels. A direct comparison between the octane and cetane numbers for fossil fuels and biofuels may therefore be misleading when compared in terms of engine performance. However, the measures can be used indicatively to show the suitability of the fuels for various applications.
Bioethanol and biodiesel are the two most common types of biofuel currently used around the world. This is due to good availability of suitable feed stocks, a relatively good understanding of conversion technologies and the opportunity for practical implementation by being able to blend the biofuels with petroleum-based gasoline or diesel.
The potential for biomass-derived products as lubricants is also gaining interest. Good varieties of oil seed rape have been bred for the good lubricity characteristics of the oils and have long been used in aircraft. They are also biodegradable, have low toxicity and therefore tend to be more environmentally acceptable than mineral oil, which can cause adverse effects on soils and plants when poorly disposed of.
With rising world prices of crude oil and petroleum products and increasing dependence on imports, developing countries is becoming more and more vulnerable in the matter of energy security. Biofuels will mitigate this vulnerability and other adverse effects of use of fossil fuels. In addition, harnessing of large areas of arable land resources for plantation of suitable energy crops will promote sustainable development and employment, mainly in rural areas.
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.
Nearly all organic substrates have potential of significant energy generation via the process of anaerobic fermentation. There are several factors which must be taken into consideration to operate the digester based on alternate feed materials effectively. The factors which effects the biogas production in cattle dung holds good for this material also. So to make use of alternate material for biogas generation it is essential to control the environmental and operational factors. A wide variety of plant wastes as well as crop residues in the farm, terrestrial and aquatic species have been studied for their potential for biogas generation. The characteristics of the plant wastes and cattle dung are quite different therefore, anaerobic digestion of plant wastes need additional requirements for maintaining environmental and operational parameters. Some of basic requirements for crop wastes for biogas production are summarized as below.
Developments over the last couple of decades, however, would seem to signal that the wheel is almost coming full circle with the increasing use of biomass being seen as imperative and in the larger interests of mankind. The driving forces behind such a move are briefly recounted and the recent technological developments particularly in the field of biomass conversion are evaluated.
The end of fossil fuel based economy is in sight and the Biomass based economy has begun. Biomass based systems are the only energy generating systems which have the combined benefits of renewability, decentralization and availability on demand without need for separate storage. In some case, they may mean waste recycling or captive power generation as well. In other cases, it is necessary to take note of existing site specific uses of biomass to avoid competition with manure/fodder needs.Agriculture yields enormus wastes every year, capable of partly supplementing coal. Biomass in the form of agro residue and industrial waste is available in all geographical locations of developing countries. Power plants set up in rural areas using biomass will help in the development of rural areas. Biomass based power plants will increase the commercial value of agro-residues and this will induce the farmers not only for biomass collection but also for effective utilization of the barren and uncultivable land for energy plantations.Biomass, as a fuel, has been in use for centuries all over the world. But, over the last five decades, with the conventional sources of energy playing a dominant role, biomass more or less became a fuel of the poor in the developing countries. However, of late, biomass, as a valuable renewable energy source, is attracting the attention of energy planners in both the developed as well as developing countries. Biomass fuels have several advantages as well as problems. The advantages of using biomass as compared to fossil fuels and nuclear power are numerous. More importantly, we do not have to worry about its availability as they are produced locally almost everywhere. They are generally available in sufficient quantities and have less economic value at present. Some of the benefits include CO2 neutrality, improved SOx and NOx, good water and soil quality, biodiversity, landscape, job creation, rural rehabilitation, etc.As concern for environmental protection and climate change increases, the importance of biomass as a viable fuel source for power generation is attracting greater attention. In these days of economic sustainability, technologies for biomass usage for power generation purely on commercial basis are being developed around the world.These biomass fuel could also pose problems. Their availability at the field and mill site requires elaborate coordination and management. They also cause transport problems due to their low bulk density. In spite of these problems, the prevailing economic conditions in many developing countries provide good opportunity to look into these issues seriously and solve these problems so that wide spread use of biomass fuel takes place. 
High rate anaerobic digestion technology
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