1. Introduction
In the traditional sense, renewable sources of energy are those that can be replenished by nature, such as hydropower, wind power, solar power, and biomass. Municipal solid waste (MSW) re- fers to the materials discarded in urban areas, including predomi- nantly household waste with sometimes the addition of commercial wastes, collected and disposed by the municipalities. MSW contains a significant fraction of paper, food waste, wood and yard trimmings, cotton, and leather, and is a source of biomass. Materials derived from fossil fuels, such as plastics, rubber, and fabrics, are also found in MSW. The U.S. Environmental Protection Agency considers MSW a renewable energy resource because the waste would otherwise be sent to landfills. The U.S. Department of Energy includes MSW in renewable energy only to the extent that the energy con- tent of the MSW source stream is biogenic . The non-renewable portion of MSW has to be either separated or accepted as part of the fuel, and practically all the wastes in MSW after mate- rial recovery and recycling are treated as renewable.
Waste-to-energy (WTE) processes recover the energy from the waste through either direct combustion or production of combustible fuels in the forms of methane, hydrogen, and other synthetic fuels.
Incineration and gasification are the two primary WTE technologies that have been used successfully throughout the world. It is estimated that about 130 million tonnes of MSW are combusted annually in over 600 WTE facilities worldwide, pro- ducing electricity and steam for district heating and recovered metals for recycling. WTE incineration has long been accepted as a solid waste management option, complement- ing landfilling and composting. The advantages and limitations of the major MSW disposal technology options, landfilling, com- posting, and incineration, are compared in Table 1. Incineration of MSW in WTE facilities prevents the possible aqueous and gaseous pollution associated with landfilling and provides a source of reliable, renewable energy. As a proven, environmentally sound technology, WTE has been used extensively in Europe and devel- oped countries in Asia such as Japan and Singapore.