Energy Recovery from Waste

Table of Contents

Increasing urbanization, industrialization and development activities result in generation of enourmous quantities of wastes in the urban and industrial areas. In India, each year about 30 Mt of municipal solid waste and about 4 400 million cubic meter of liquid waste are generated.

Most wastes that are generated find their way into land and water bodies without proper treatment, causing evere water pollution. They also emit greenhouse gases like methane and carbon dioxide and add to air pollution. The problems caused by solid and liquid wastes can be significantly reduced through environment-friendly waste-to-energy technologies that will allow treatment and processing of wastes before their disposal. These measures would reduce the quantity of waste, generate substantial energy and reduce pollution.

Waste-to-Energy Technologies

Anaerobic Bigestion / Biometanation: In this process, the organic fraction of the waste is segregated and fed into a closed container, biogas digester. In the digester, the waste undergoes biodegradation in the presence of methanogenic bacteria under unaerobic conditions, producing methane-rich biogas and effluent. The biogas can be used either for cooking / heating or for generating motive power or electricity through duel-fuel or gas engines, low-pressure gas turbines, or steam turbines. The sludge from anaerobic digestion, after stabilization can be used as a soil conditioner. It can even be sold as manure depending on its composition, which is determined mainly by the composition of the input waste.

Combustion / Incineration: In this process, wastes are directly burned in presence of excess air at high temperature of about 800 °C, liberating heat energy, inert gases, and ash. The net yield depends upon the density and composition of waste; the percentage of moisture and inert material in it (which adds to the heat loss); ignition temperature; size and shape of its constituent materials, etc. Combustion results in transfer of 65 % - 80 % of heat content of the organic matter to hot air, steam and hot water. The steam generated, can be used in steam turbine to generate power.

Pyrolysis / Gasification: Pyrolysis is a process of chemical decomposition of organic matter brought about by heat. In this process, the organic material is heated in the absence of air until the molecules thermally break down to become a gas comprising smaller molecules (known collectively as syngas). Gasification can also take place as a result of partial combustion of organic matter in the presence of restricted quantity of oxygen or air. The gas so produced is known as producer gas. The gases produced by pyrilysis mainly comprise carbon monoxide (25 %), hydrogen and hydrocarbons (15 %), and carbon dioxide and nitrogen (60 %). The next step is to 'clean' the syngas or producer gas. Thereafter, the gas is burned in IC engine or turbine to produce electricity.

Landfill Gas Recovery: The waste dumped in a landfill becomes subjected over a period of time, to anaerobic conditions. As a result, its organic matter slowly volatilizes and decomposes, leading to production of 'landfill gas', which contails a high percentage of methane (about 50 %). Typically, production of landfill gas starts within a few months of disposal of wastes, and generally continues for 10 years or even more, depending upon the comp[osition of wastes and availability of mioisture. As the gas has a calorific value of 4 500 kcal/m³, it can be used as a source either for direct heating / cooking or to generate power through IC engine or turbine.

Plasma Arc: Plasma arc technology is relatively new technology for disposal of wastes, particularly, hazardous and radioactive wastes. This technology is now being seen as an attractive option for disposal of municipal wastes as well. Besides generating energy, plasma arc technology ensures near complete destruction of waste. Therefore it has an edge over the combustion processes descibed earlier. The major advantages of plasma arc technology are:

  • Compared to combustion / incineration, it creates much less atmospheric pollution.
  • In techno-economic terms, oxides of nitrogen and sulphur are not emitted during normal operations because the system works in absence of oxygen.
  • Toxic materials becomes encapsulated and are therefore much safer to handle than the toxic ash left by combustion / gasifier processes.

Despite these advantages, however, plasma arc technology is constly and has not been tried in India.

Waste-to-Energy Development

Advantages: The major advantages of waste-to-energy projects are:

  • The quantity of waste is reduced by 60 - 90%, depending on the waste and technology adopted.
  • With less wastes to dispose of, the demand for landfill is reduced, saving scarce land in cities.
  • Cost of transportation to far-away landfill site id reduced.
  • Environment pollution is reduced.
  • Energy is obtained.
  • The slurry produced by biometanation technology serves as a good fertilizer.

Limitations: Waste-to-energy is a new concept in India. The technologies, particularly for biomethanation of urban wastes, are usually imported; thus increasing costs. Treatment of urban wastes for energy recovery requires wastes to be properly segregated. However, segregated waste is generally not available at the plant site, and this hinders the setting up of waste-to-energy plants.

Costs: 

Cost of waste-to-energy plants
TechnologyCost in crore/MW
Biomethanation 
 Industrial waste6 - 7
 Urban waste8 - 9
Gasification / Pyrolysis9 - 10
Cambustion / Incineration9 - 10

Financial Support: From ₹ 20 lakh/MW to ₹ 3 crore/MW depending upon type of waste & technology utilized.

Potential: 3 600 MW from the urban, municipal and industrail wastes. With the economic development, the potential is likey to be 5 200 MW by 2017.

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