GAS CONCENTRATION PROFILES AND NOX FORMATION IN CIRCULATING FLUIDIZED-BED COMBUSTION

Experiments were conducted to understand competing NOx formation and d estruction reactions better in circulating fluidized bed combustion. B oth longitudinal and lateral gradients were measured for NOx, O-2, CO and CH4 during combustion of solid fuels of differing reactivity in a CFBC pilot unit 152 mm square by 7.3 m tall. Fuels tested were anthrac itic, bituminous, subbituminous and lignitic coals and petroleum coke. In all cases there were significant lateral and longitudinal gradient s of gaseous species. Profiles of NOx were correlated to a large degre e with profiles of oxygen and local char concentration, which in turn were coupled to the CFB hydrodynamics. High partial pressures of oxyge n at the base of the unit and in the char-lean core favour NOx formati on reactions, while low O-2 partial pressures in the solids-rich annul us and towards the top of the unit favour NOx reduction. When NOx form ation and destruction are limited to the char, profiles are readily in terpreted. The addition of volatile nitrogen makes the profiles consid erably more complex. Limestone used for sulfur capture can augment or reduce overall NOx formation, depending on the percentage volatile mat ter of the fuel. (C) 1997 Elsevier Science Ltd.