The enormous increase in the quantum and diversity of wastematerials generated by human activity and their potentially harmful effects on the general environment and public health, have led to an increasing awareness about an urgent need to adopt scientific methods for safe disposal of wastes. While there is an obvious need to minimize the generation of wastes and to reuse and recycle them, the technologies for recovery ofenergy from wastescan play a vital role in mitigating the problems. Besides recovery of substantial energy, these technologies can lead to a substantial reduction in the overall waste quantities requiring final disposal, which can be better managed for safedisposal in a controlled manner while meeting the pollution control standards.Waste generation rates are affected by socio-economic development, degree of industrialization, and climate. Generally, the greater the economic prosperity and the higher percentage of urban population, the greater the amount of solid waste produced. Reduction in the volume and mass of solid waste is a crucial issue especially in the light of limited availability of final disposal sites in many parts of the world. Although numerous waste and byproduct recovery processes have been introduced, anaerobic digestionhas unique and integrative potential, simultaneously acting as a waste treatment and recovery process.Waste-to-Energy Conversion PathwaysA host of technologies are available for realizing the potential of waste as an energysource, ranging from very simple systems for disposing of dry waste to more complex technologies capable of dealing with large amounts of industrial waste. There are three main pathways for conversion of organic waste material to energy – thermochemical, biochemical and physicochemical.Thermochemical ConversionCombustion of waste has been used for many years as a way of reducing waste volume and neutralizing many of the potentially harmful elements within it. Combustion can only be used to create an energy source when heat recovery is included. Heat recovered from the combustion process can then be used to either power turbines for electricity generation or to provide direct space and water heating. Somewaste streams are also suitable for fueling a combined heat and power system, although quality and reliability of supply are important factorsto consider.Thermochemical conversion, characterized by higher temperature and conversion rates, is best suited for lower moisture feedstock and is generally less selective for products. Thermochemical conversion includes incineration, pyrolysis and gasification. The incineration technology is the controlled combustion of waste with the recovery of heat to produce steam which in turn produces power through steam turbines. Pyrolysis and gasification represent refined thermal treatment methods as alternatives to incineration andare characterized by the transformation of the waste into product gas as energy carrier for later combustion in, for example, a boiler or a gas engine.Biochemical ConversionThe bio-chemical conversion processes, which include anaerobic digestion and fermentation, are preferred for wastes having high percentageof organic biodegradable (putrescible) matter and high moisture content. Anaerobic digestion is a reliable technology for the treatment of wet, organic waste. Organic waste from various sources is composted in highly controlled,oxygen-free conditions circumstances resulting in the production of biogas which canbe used to produce both electricity and heat. Anaerobic digestion also results in a dry residue called digestate which can be used as a soil conditioner.Alcohol fermentation is the transformation of organic fraction of biomass to ethanol by a series of biochemical reactions using specialized microorganisms. It finds good deal of application in the transformation of woody biomass into cellulosic ethanol.Physico-chemical ConversionThe physico-chemical technology involves various processes to improve physical and chemical properties of solid waste. The combustible fraction of the waste is converted into high-energy fuelpellets which may be used in steam generation. Fuel pellets have several distinct advantages over coal and wood because it is cleaner, freefrom incombustibles, has lowerash and moisture contents, is of uniform size, cost-effective, and eco-friendly.Factors affecting Energy Recovery from wasteThe two main factors which determine the potential of recovery of energy from wastesare the quantity and quality (physico-chemical characteristics) of the waste. Some of the important physico-chemical parameters requiring consideration include:*.Size of constituents*.Density*.Moisture content*.Volatile solids / Organic matter*.Fixed carbon*.Total inerts*.Calorific valueOften, an analysis of waste to determine the proportion of carbon, hydrogen, oxygen, nitrogen and sulfur (ultimate analysis) is done to make massbalance calculations, for both thermochemical and biochemical processes. In caseof anaerobic digestion, the parameters C/N ratio (a measure of nutrient concentration available for bacterial growth) and toxicity (representing the presence of hazardous materials which inhibit bacterial growth), also require consideration.Significance of Waste-to- Energy (WTE) PlantsWhile some still confuse modern waste-to-energy plants with incinerators of the past, the environmental performance of the industry is beyond reproach. Studies have shown that communities that employ waste-to-energy technology have higher recycling rates than communities that do not utilize waste-to-energy. The recovery of ferrous and non-ferrous metals from waste-to-energy plants for recycling is strong and growing each year. In addition, numerous studies have determined that waste-to-energy plants actually reduce the amount of greenhouse gases that enter the atmosphere.Nowadays, waste-to-energy plants based on combustion technologies are highly efficient power plants that utilize municipal solid waste astheir fuel rather than coal, oil ornatural gas. Far better than expending energy to explore, recover, process and transport the fuel from some distant source, waste-to-energy plantsfind value in what others consider garbage. Waste-to-energy plants recover the thermal energy contained in the trash in highly efficient boilers that generate steam that can then be sold directly toindustrial customers, or used on-site to drive turbines for electricity production. WTE plants are highly efficient in harnessing the untapped energy potential of organic waste by converting the biodegradable fraction of the waste into high calorific value gases like methane. The digested portion of the waste ishighly rich in nutrients and is widely used as biofertilizer in many parts of the world.Waste-to-Energy around the WorldTo an even greater extent than in the United States, waste-to-energy has thrived in Europeand Asia as the preeminent method of waste disposal. Lauding waste-to-energy for its ability to reduce the volume of waste in an environmentally-friendly manner, generate valuable energy, and reduce greenhousegas emissions, European nations rely on waste-to-energy as the preferred method of waste disposal. In fact, the European Union has issued a legally binding requirement for its member States to limit the landfilling of biodegradable waste.According to the Confederationof European Waste-to-Energy Plants (CEWEP), Europe currently treats 50 million ton of wastes at waste-to-energy plants each year, generating anamount of energy that can supply electricity for 27 million people or heat for 13 million people. Upcoming changes to EU legislation will have a profound impact on how much further the technology will help achieve environmental protection goals.A Glance at Feedstock for Waste-to-Energy PlantsAgricultural ResiduesLarge quantities of crop residues are produced annuallyworldwide, and are vastly underutilised. The most common agricultural residue isthe rice husk, which makes up 25% of rice by mass. Other residues include sugar cane fibre (known as bagasse), coconut husks and shells, groundnut shells, cereal straw etc. Current farming practic
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