Municipal Waste

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Municipal solid waste can be converted to energy by thermal or biochemical technologies. The commercial thermal technology is combustion to produce steam for industry, hot water for district heat, or electricity for the grid. MSW may be burned as received in specially designed facilities on a hearth (typically 25-50 metric tons/day per unit) or on inclined grates (typically 250-1000 metric tons/day per unit). In addition, solid fuel can be produced mechanically. This refuse-derived fuel (RDF) can be burned in a specially designed boiler on a traveling grate (typically 400-1000 metric tons/day). RDF is also used as a substitute for coal, preferably in a grated boiler but typically in a suspension-fired boiler. Limited research is developing new technologies that produce fuel gas or liquid transportation fuels or new forms of RDF such as densified and powdered RDF. The former can be stored for longer periods and marketed to multiple users; the latter can be used as a substitute for oil in boilers designed for heavy oil. Current commercial combustion facilities are 65-75% efficient in producing steam (fossil fuel facilities are 88-90% efficient) mainly due to moisture and inorganic content. In contrast, a low moisture and low ash waste such as paper or dry wood has a calculated efficiency of 89%.

Biochemical conversion technologies use organisms to produce specific fuels. For example, a complex consortium of organisms found in nature will reduce cellulose and other carbohydrates to simple sugars, organic acids, and finally biogas. Currently, commercial biochemical conversion is limited to the recovery of biogas from landfills and sewage treatment. Future research could complete the development of technology to produce biogas or ethanol from municipal waste or selected components.

Recent measurements of the energy content of as-received MSW indicate that approximately 11.6 mJ/kg (5000 Btu/lb) of energy is contained in each pound of MSW [up from 10.4 mJ/kg (4500 Btu/lb) in recent years].

Municipal solid waste management can conserve energy by remanufacture of specific components into new materials. Certain inorganic components (steel, aluminum, and glass) can be separated by source separation of mechanically, magnetically, by density, size, shape, electrostatics, etc.) and recycled into new materials. Certain organic components (specific grades of clean paper and specific plastics) can be recovered and remanufactured. Since high purity is necessary for materials to be recycled, source separation is the preferred recovery technique. Research is required to economically recover components other than steel from mixed waste. The amount of energy conserved is different for different commodities. For example, the recycling a pound of aluminum conserves 242 mJ/kg (100,000 Btu/lb) while recycling a pound of glass conserves about 3.8 mJ/kg (1500 Btu/lb) [10], In addition, the conservation of energy within a commodity may be different. For example, recycling old newsprint into new newsprint conserves about 4.6 mJ/kg (2000 Btu/lb) while recycling cardboard into new cardboard saves about 2.3 mJ/kg (1000 Btu/lb).

Currently there are approximately 150 waste-to-energy facilities in the United States. They vary from mass-burn facilities where MSW is burned as received, through facilities that prepare a fuel for use in existing boilers, to those that recover recyclables. Over 90% of the energy generated from these facilities is from the burning of waste. There are a large number of recycling programs and 50 or more materials recovery facilities (MRF). Based on GAA, EPA, and DOE databases, currently about 0.29 X 1018 joules (0.28 x 1015 Btu) of primary energy is produced by burning, about 0.16 X 1018 joules (0.15 x 1015 Btu) 0.15 quad is conserved by recycling, and about 0.05 quad from landfill gas (LFG) is being recovered from about 100 landfills. About 80% of the energy produced from burning is electricity, with the remainder being steam or hot water for industry or district heating. The recycling mostly conserves electricity in aluminum production. About 80% of LFG is used to produce electricity, while the rest is used as boiler fuel or cleaned to pure methane for use as substitute natural gas.

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