Reburning involves the injection of a secondary fuel above the main firing zone of pulverized coal-fired utility boilers to produce a reducing zone which acts to reduce NOx to molecular nitrogen. Overfire air is added above the reburn reducing zone to complete the combustion. Bench scale evaluations of the process carried out in a plug flow furnace at 23 KW have indicated that NOx reductions of up to 70 percent can be achieved depending on a number of process variables. The dominant variables include the initial NOx level that is to be reduced, the reburning fuel type (pulverized coal type or natural gas), and the residence time and temperature in the reducing zone. The reburning process has been combined with the injection of calcium-based sorbents (limestone) to investigate the potential for combined NOx and SOx reduction.

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