Reducing NO_x Emissions for a 600 MW_e Down-Fired Pulverized-Coal Utility Boiler by Applying a Novel Combustion System

Abstract

A novel combustion system was applied to a 600 MWe Foster Wheeler (FW) down-fired pulverized-coal utility boiler to solve high NOx emissions, without causing an obvious increase in the carbon content of fly ash. The unit included moving fuel-lean nozzles from the arches to the front/rear walls and rearranging staged air as well as introducing separated overfire air (SOFA). Numerical simulations were carried out under the original and novel combustion systems to evaluate the performance of combustion and NOx emissions in the furnace. The simulated results were found to be in good agreement with the in situ measurements. The novel combustion system enlarged the recirculation zones below the arches, thereby strengthening the combustion stability considerably. The coal/air downward penetration depth was markedly extended, and the pulverized-coal travel path in the lower furnace significantly increased, which contributed to the burnout degree. The introduction of SOFA resulted in a low-oxygen and strong-reducing atmosphere in the lower furnace region to reduce NOx emissions evidently. The industrial measurements showed that NOx emissions at full load decreased significantly by 50%, from 1501 mg/m3 (O2 at 6%) to 751 mg/m3 (O2 at 6%). The carbon content in the fly ash increased only slightly, from 4.13 to 4.30%.

Optimization of Air Distribution to Reduce NOx Emission and Unburned Carbon for the Retrofit of a 500 MWe Tangential-Firing Coal Boiler

“…Various studies of the optimization of SOFA to improve boiler performance have been reported in the literature based on experiments and/or computational fluid dynamics (CFD). Increasing the SOFA ratio and optimizing its detailed distribution were found to be effective in reducing NOx emission for various boiler types [6][7][8][9][10][11][12][13]. However, the change in reaction stoichiometry can also significantly alter the combustion and heat transfer characteristics.…”

Numerical investigation on the influence of nozzle–organization–mode of split burner on flow field distribution and combustion characteristics of a 300‐MWe subcritical down‐fired boiler

“…However, it is found that four kinds of down‐fired boilers are all suffered from various problems in practical operations, such as late ignition, poor combustion stability, heavy slagging, poor burnout, asymmetric combustion, especially for high NO x emissions . For down‐fired boilers with split burner, various solutions were developed to deal with these problems, such as burning pulverized coals to improve ignition and combustion stability, positioning fuel‐lean coal/air flow nozzle far from the furnace central part to weaken slagging, inclining downward wall‐air jets below furnace arches to prolong coal residence times for higher burnout, and performing comprehensive combustion retrofits to reduce NO x emissions …”

Experimental‐ and numerical‐simulation research on the inner–secondary‐air ratio in a 600‐MW e down‐fired boiler

“…However, the introduced down‐fired boilers tend to function poorly when burning this type of coal in China, resulting in asymmetric combustion, late ignition, severe slagging, unstable combustion, insufficient burnout (typically with the unburnt carbon in fly ash of approximately 10%) and high NO x emission levels (around 1500 mg/m 3 at 6% O 2 ). Various solutions were proposed to solve these problems, such as adjusting the burner location and furnace parameters to avoid asymmetric combustion, changing burner operation models to mitigate slagging, retrofitting the combustion configuration to enhance the ignition and burnout, and adjusting air distribution to reduce NO x emissions. The regulation of air‐staging combustion in down‐fired boilers to simultaneously achieve significant reductions in NO x emissions, stable combustion, and good burnout is challenging.…”