Combustion in large boilers: Design and operating effects on efficiency and emissions

Abstract

Combustion process modifications in contemporary large boilers are constrained by a number of important design and operating considerations such as steam temperature,ash fusion, and material limitations. Plant efficiency gains obtainable by further increasing combustion efficiency are in most cases negligible. However, depending upon standard operation, a large potential exists for increasing overall efficiency by lowering excess air levels, while at the same time decreasing pollutant emissions. Dramatic increases-in fuel prices have now made investment in such improvements more attractive than it was in the past. Reduction of excess air level can also effect reductions in emssions of NOx and SO3, and even of particulates, through equipment and operating improvements. Pollution originates fromthe fuel, from incomplete combustion or through combustion processes. These various forms of pollution can be controlled in varying degrees during the combustion process by wet ash handling, SO3 conversion minimization or mixing and temperature control in the case of combustibles and nitric oxide. NOx reduction techniques, applied to a large gas-fired utility boiler, resulted in NOx emissions of 33 ppmV or less at power outputs up to 355 Mw.