Effect of char gasification on NOx formation process in the deep air-staged combustion in a 20Â kW down flame furnace

Deep air-staged combustion tests of Datong (DT) bituminous coal were carried out in a 20 kW down flame furnace (DFF) with the burner stoichiometric ratio (SR) ranging from 1.200 (unstaged) to as low as 0.696 (deep staged). The experimental results shown that the concentration of CO reach as high as 120,000 ppm (12 vol.%) and the NOx decrease to nearly zero in the reducing zone under deep staging conditions of SR = 0.696, which was never observed before. Thus, the extent of CO formation (i.e. char gasification) and the NOx reaction mechanism under deep staging condition were studied in order to understand the combustion process of coal. This paper presents a refined numerical simulation for reproducing the profiles of CO and NOx along the DFF under deep staging condition. The comparison between simulation and experimental results prove the reasonability of refined kinetic parameters of char gasification. The enhancement of char gasification by CO2 is proposed and validated. With the simulated CO profile in the DFF confirmed by experiment, the NOx profile could be further analyzed. The discrepancy of simulated NOx profile in the reducing zone (i.e. fuel-rich zone) indicates that there are some undetected nitrogenous species and undiscovered NOx transfer mechanism regardless of the consistence of final NOx emission between simulation and experiment. It is supposed by us that a majority of NOx immersing in high level of CO in the reducing zone is mainly transferred into undetected nitrogenous species (excluding HCN and NH3) which is then rapidly oxidized into NOx once the remaining oxygen is injected into the DFF.