Two Stage NOx Control on a Coal Fired Utility Boiler

New England Power has recently installed a two-stage NOx control system on Unit 3 at its Salem Harbor generating station. Unit 3 is a coal-fired unit rated at 155 MW. The unit is front wall-fired, and has a divided furnace with 16 unequally spaced burners. This firing configuration results in uncontrolled NOx levels of nominally 700 ppm (1.0 Ibl MMBtu). Retrofit NOx controls consist of 10w-NOx burners (LNB) and an overfire air (OFA) system supplied by Riley Stoker and a selective non-catalytic NOx reduction (SNCR) system. The SNCR system, supplied by Nalco Fuel Tech, utilizes urea in a multi-level injection configuration. NOx reduction performance of the LNB, OF A and SNCR systems was characterized both independently and in combination during the program. LNB tests showed that NOx reductions of nominally 20 percent were achieved with the burners alone. The use of OFA at design levels provided NOx reductions ranging from 42 percent at full load to 4 percent at minimum load relative to the LNB baseline. Ash carbon levels doubled when the OFA system was operated at design conditions at loads above 110 MW. The SNCR system provided full load NOx reductions of 33 percent relative to the LNB/OFA baseline of 0.55 Ib/MMBtu, at a molar NINO ratio of 1.3. Ammonia slip for these conditions was 2 ppm.