Direct Burning

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Direct Burning (Incineration) Biomass can be burned in incinerators for heat, mixed with coal and used directly for heating, or converted to electricity in power plants. Conventional incinerators directly burn biomass to heat water and generate steam that runs a steam turbine. In modern incinerators, biomass is first converted into producer gas, which can then be burned inside gas turbines. In some instances, the waste heat from the gas turbine may be used to drive a secondary steam turbine, converting more of the fuel energy into electricity in what are commonly referred to as combined heat and power (CHP) units. Finally, the waste heat from these secondary plants is hot enough for hot-water and space heating that would otherwise be lost to the atmosphere. According to the US Department of Energy, domestic capacity for biomass generation utilizing combustion could reach 20-30 GW by the year 2020. About four-fifths of all biomass energy used today comes from burning wood and wood scraps. The remainder is from crops, garbage, landfill, and alcohol fuel. Materials used for direct incineration are usually scrap wood and sawdust produced by timber companies, solid wastes and garbage, and landfill gases. Most energy generated by biomass fuel is used locally. Burning trash has the advantage of producing electricity and heat, and also reducing the volume of trash. The main problem with incinerators is that many of the pollutants associated with combustion remain and are released to the atmosphere. Some newer technologies burn these materials 16 The word “municipal” implies anything that is operated and controlled by elected local city or county officials. Figure 6-6 Composition of municipal solid waste in the U.S, (a) by type, and (b) by sector. RLT refers to rubber, leather, and textiles. Source: US EPA 2001 Facts and Figures. U.S. Waste by Type (2001)Paper37% Yard Trimming12% Other3%Wood 6% RLT 7% Plastics11% Glass5% Metals8% Food Wastes11% U.S. Waste by Sector (2001)Residential32% Commercial33% Other 14%Construction debris 21% (a) (b) Figure 6-5 Municipal solid wastes constitute a major source of energy. Source: National Renewable Energy Laboratory, Photographic Information Exchange 134 at much higher plasma temperatures. Plasma is gas heated (usually by passing strong electric currents through it) to temperatures around 10,000 oC, causing electrons to be stripped off the atoms. At these temperatures, hydrocarbons, PCBs, and other toxins break down, allowing them to be burned completely. The residues are mixed with soil and harden into inert and harmless rocks suitable for road gravel. Incineration accounts for 10-15% of all garbage generated in the US and for only a tiny one-tenth of one percent of all electricity supplies.17 Biomass has lower calorific values as compared to fossil fuels. Table 6-3 compares the calorific (heating value) of several biomass fuels with fossil fuels.


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