Effect of inner and outer secondary air ratios on ignition, C and N conversion process of pulverized coal in swirl burner under sub-stoichiometric ratio

Abstract

This study proposes changing the inner and outer secondary air ratios (RSA) under sub-stoichiometric (sub-SR) conditions to optimize the burn-out performance and reduce the NOx emissions of a swirl burner. The ignition and combustion characteristics, NOx formation, and the effect of RSA on the carbon and nitrogen conversion processes under sub-SR (SR = 0.85) were investigated via thermal tests carried out in a 0.5 MWe pulverized coal-fired furnace. The fuel-rich flow in the center of the furnace was further rearward than the fuel-lean flow. The pulverized coal burnt rapidly after entering the furnace. The average percentage of unburned carbon was less than 15% in the x/d = 3.5 (x is the axial direction distance, and d is the diameter of the burner) section away from the burner. The N to NO conversion at the beginning of combustion (x/d < 3.5) was effectively suppressed in the swirl burner under sub-SR conditions. The NO reduction reaction almost completely stopped after x/d > 11.5. RSA mainly affected the combustion characteristics and NOx generation characteristics of pulverized coal by changing the degree of air staging at different positions in the furnace. Larger RSA values meant a greater amount of oxygen involved in the early combustion. This accelerated the early burn-out of pulverized coal and the release of elements, increasing NOx emissions. This study recommends RSA values between 0.7 and 1, that the over-fire air (OFA) nozzle be positioned behind the near burner region (x/d > 3.5) for the combustion of bituminous coal to minimize NOx emissions and unburned carbon.