Numerical evaluation of heat transfer and NOx emissions under deep-air-staging conditions within a 600 MWe tangentially fired pulverized-coal boiler

Abstract

Numerical evaluation of heat transfer and NOx emissions was conducted within a 600MWe tangentially fired pulverized-coal boiler configured with the deep-air-staging combustion technology, composing of concentric horizontal bias staged-air (CFS) burners and opposing tangential separated over-fire air (SOFA) burners. Refined char burning model considering gasification reactions and NOx model were implemented and validated with on-site measurement. Bell-shaped heat flux distributions with peaks severely deviating from the furnace center line appeared in the primary combustion zone, while heat flux distribution was uniform in the burnout zone. The results revealed that the larger the feed angle of CFS air, the more uniform the heat flux distribution with lower heat flux peaks. Concurrently, NOx emissions decreased by 12% from 250 to 220mg/m3 at 6% O2 and the optimum value occurred with CFS injection angle fixed at 15°. When the SOFA ratio was increased from 10% to 40%, distinct NOx reduction (levels from 358 to 174mg/m3 at 6% O2) associated with more uniform heat flux distributions was achieved. Owing to the intense turbulence mixing of the opposing tangential jets with the upward flow, saddle-shaped heat flux curves with slightly higher peaks appeared in the burnout zone as SOFA ratio was 40%.