The prime cycle consists of eight air-cooled gas turbine generator units, with a 12:1 compressor pressure ratio and 1315°C firing temperature (that is, the temperature at the inlet of the first-stage rotor). The prime cycle generates two-thirds of the total electrical power output. The gas turbine exhaust temperature is 640°C. The bottoming cycle includes eight heat-recovery steam generators (HRSG) and two steam turbines. The HRSGs extract thermal energy from the gas turbine exhaust stream. The steam is supplied to steam-turbine generators, which contribute about one-third of the total power output. At full capacity, coal feed is required at a rate of 455 t/h or 2.8 x 10$ t/yr at 70% capacity factor. At this rate, an average of 7650 t of coal would be required at the site each day; as coal storage requirements are estimated at full capacity factor, a 3-day live storage stock pile would contain 22,950 t of coal, and a 57-day reserve storage would contain 435,000 t of coal. The average storage density of utility coal in live storage is 700 kg/m^ and reserve storage density averages 865 kg/m^. The assumption of 9.15- m-high active storage and 15.25-m-high reserve storage yields an area of about 36 x 10^ m^ devoted to coal storage and handling. Figure 3.8 depicts the major environmental pollutants that arise from the impurities in the fuel and water used by the plant. The major impurities in the fuel are the sulfur and nitrogen compounds and incombustible ash. In the gasification step, almost all of the ash is separated from the coal, so it does not show up as a potential air pollutant. Similarly, about 93% of the sulfur is removed from the process stream by the combined Alkazid-Claus process. Another 1.3% is disposed of in the scrubber sludge from the wet limestone scrubber. The remaining sulfur appears as SO2 in the stack gas from the various plant flues and exhausts. NO2 control is effected by removing most of the NH3 from the fuel gas streams before combustion. Water used for plant cooling is also a source of environmental pollution. As the water is evaporated, concentrations of dissolved solids increase. This brackish "blowdown” water must be specially handled, to avoid pollution of local water systems. The cleanup system must remove enough sulfur as elemental sulfur so that the sum of the SO2 emitted from incinerator flues and the SO2 emitted from the power plant flues will be no greater than allowed by EPA standards. The carbonyl sulfide (COS) formed in the gasifier and entering with the raw gas is almost completely hydrolyzed to hydrogen sulfide (H2S) before entering the Alkazid plant. Five percent of the H2S removed by the Alkazid plant will not be converted to elemental sulfur in the Claus plant. The tail gas is treated in the Wellman-Lord plant, which allows 90% of the sulfur to be recycled back to the Claus process. The Alkazid plant removes 95% of the H2S entering with the raw gas. Only 5.6% of the total sulfur is emitted to the atmosphere as SO2• Nitrous oxides are formed by combustion of ammonia, and to a limited extent by oxidation of N2 gas diluent. To meet the emission standards, 0.14 kg of NO2 may be emitted to the atmosphere per million kJ of gaseous fuel.
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