Establishing essentially symmetrical combustion plus apparent improvement in burnout and NOx emissions within a down-fired furnace by rearranging its W-shaped flame into a sidewall-dominated pattern

Abstract

Asymmetric combustion, which is mainly incurred by the guiding effect of the short and asymmetric upper furnace, appears universally in down-fired furnaces equipped with a conventional front/rear wall-dominated W-shaped flame pattern (labeled as the FRW-flame pattern). It generally favors the occurrence of heat deviation, poor burnout, and high NOx emissions. Aiming at eliminating asymmetric combustion and solving these incidental problems from the mentioned fountainhead, a sidewall-dominated W-shaped flame solution (labeled as the SW-flame pattern) was proposed and then trialed in a 600 WMe supercritical down-fired furnace to replace the original FRW-flame pattern. Industrial-size measurements were performed at normal full load for not only uncovering the asymmetric combustion characteristics and furnace performance with the FRW-flame pattern but also validating the subsequent numerical simulations. Numerical simulations before and after the SW-flame application were carried out for achieving two functions: (i) deepening the understanding about asymmetric combustion and explaining the poor furnace performance incurred by the original FRW-flame pattern; and (ii) confirming the SW-flame solution in establishing essentially symmetrical combustion and improving comprehensively the furnace performance. For the FRW-flame pattern, asymmetric combustion (clearly higher gas temperatures in the rear-half side) plus a severely deflected flow field developed in the furnace, where the downward coal/air flow in the front-half side penetrated much further while proceeded mainly in the low-temperature near-wall zone. A combination of (i) the much poorer combustion status in the front-half part, (ii) great differences of the downward coal/air penetration and the fuel-lean coal/air flow trajectories in the front/rear half parts, and (iii) the sufficient oxygen levels in the local high-temperature zones strengthened by asymmetric combustion, finally developed the poor furnace performance with carbon in fly ash of 8.5 % and NOx emissions of 1467 mg/m3 at 6 % O2. As the FRW-flame pattern was replaced by the SW-flame pattern, an essentially symmetrical combustion form with a symmetrical flow field was established in the furnace. The symmetrical downward coal/air flows penetrated sufficiently in the lower furnace. The fuel-lean coal/air trajectories were symmetrically redirected in the high-temperature furnace central part. The local high-temperature zones below arches and in the furnace central part were homogenized by the symmetrical combustion and were filled with low O2 levels. Consequently, the furnace performance was greatly improved, with the calculated carbon in fly ash reduced from 7.74 % to 5.17 % and NOx emissions dropped from 1355 to 745 mg/m3 at 6 % O2 (i.e., reducing NOx emissions by 45 %).