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Show '" . 1 1 • Injection of flue gas and steam using an eductor driven by the steam to compress the flue gas ~ or • Injection of flue gas using an eductor driven by natura) gas to compress the flue gas. Injection of flue gas using a compressor also would be feasible but the capital cost of the compressor and the energy required to drive the compressor would offset much of the benefits of FIR compared to conventional FGR. The operating costs of FIR are dependent on the cost of the steam for steam injection or the cost to compress the flue gas for flue gas injection. The fuel gas pressure is a key parameter establishing the operating characteristics of the FIR system. Without modifications to the burner nozzles, the use of FIR could be limited due to the maximum allowable burner pressure. To avoid exceeding the full load burner pressure, FIR would be limited to loads below about 50 percent. The use of FIR above 50 percent load may require a fe-design of the fuel nozzles. In tum, this may affect fuel/air mixing, flame stability, NOx emissions, and other combustion-related parameters. It is beyond the scope of this paper to project FIR perronnance under those conditions. To implement FIR with flue gas injection using the existing burners, the burner fuel pressure would establish the energy required to compress flue gas and the perronnance of an eductor used to compress the flue gas. For burners having relatively high fuel gas pressures of 15 to 20 psig (typically found in utility boiler applications): an eductor could supply only a relatively small amount of flue gas and direct injection of stearn would be required to provide relatively high levels of FIR. For burners using a low fuel gas pressure in the range of 5 to 8 psig (frequently found in industrial boiler applications) an eductor could be effective in reducing the cost of FIR. If sufficient natural gas pressure is available (80 to 100 psig) , the natural gas could be used in the eductor to compress the flue gas and mix it with the fuel. Alternately, steam could be used in the eductor to compress the flue gas. This would allow the flue gas to offset a portion of the steam injected into the fuel, reducing the cost of FIR compared to direct steam injection. In general, flue gas injection would provide a lower cost compared to steam injection. However, direct steam injection would be preferred in special applications such as facilities having a source of waste steam, for boilers with low capacity factors, or for boilers needing only a small reduction in NOx emissions. Since relatively low pressure steam is required for injection into the fuel, low pressure turbine extraction steam or waste steam that otherwise would be sent to a condenser could be utilized to minimize the operating cost of the FIR process. With steam injection, FIR generally will be cost-effective in reducing NOx emissions on boilers with relatively low capacity factors. FIR also can be cost-effective for very small boilers, where the capital costs of low NOx burners or other conventional control techniques can be prohibitively expensive. FIR also has the benefit that a large capital expenditure is not required. This can be attractive to industrial facilities who are interested in reducing capital expenditures. An application where FIR is particularly attractive is for boilers ~ heaters, and other combustion devices equipped with natural draft burners. Conventional FGR cannot be used without major modifications to the combustion system. A054E562.T |
