Waste-to-energy
Waste-to-energy is the process in which waste is used to generate useful energy –electricity, heat or both. This is possible (and convenient) when the heat generated by burning the waste is high enough to warrant satisfactory combustion conditions and make available enough energy to overcome losses and auxiliary consumption: in practice, a lower heating value of at least 4 megajoules per kilogram. Waste-to-energy is the offspring of waste incineration, which was originally introduced to sterilize and reduce the volume of waste by combusting it in a furnace. Modern waste-to-energy plants allow the export of energy, with very low environmental impact. The plant comprises four basic sections: waste combustor, recovery boiler, flue gas treatment and steam cycle. The design of the combustor varies widely with the waste characteristics: physical state (solid vs. liquid), size distribution, heating value, ash and moisture content, etc. Municipal solid waste (MSW) typically is burned on a moving grate, where it is kept 20-30 minutes until it is completely combusted. The hot gases generated in the combustor go through the recovery boiler to generate steam, which is used directly as heat carrier or sent to a steam turbine to produce power. Flue gases are treated by adding reactants called sorbents and by filtering the particulate matter. A modern, large plant treating half-million tons of municipal solid waste per year can generate more than 400 million kWh per year, meeting the electricity needs of more than 150,000 families.
Another method of converting waste to energy is biomethanation where putrescible waste is anaerobically digested to create biogas which consists mainly of methane, a high-fuel value gas.
Further reading
Waste to Energy systems, Integrated Waste Services Association
