Studies on the Effect of Swirl on Nitric-Oxide Formation in Combustion—a Selected Review

Swirling gas/coal combustion are frequently encountered in combustion facilities. To reduce pollutant formation from combustion is an important problem in environment protection. Most previous studies were based on controlling the chemical reaction, air staging, or reburning. Some studies on the effect of swirl were reported, but contradictory results were obtained. This paper is a review of experimental and numerical studies on nitric-oxide formation in swirling methane-air and pulverized-coal combustion conducted by the present authors. The experimental and simulation results indicate that as the swirl number increases, the thermal NO at first increases and then decreases, whereas the total NO and fuel NO at first decrease and then increase. There is an optimal swirl number at which the total NO emission is minimal. The increase of swirl number causes at first the reduction of turbulence and then a slight increase of turbulence. It leads to at first the temperature rises and then the temperature falls in the near-inlet region. The thermal NO is more affected by the temperature and the fuel NO is more affected by the turbulent mixing. These research results are helpful for engineering application to reduce pollutant emission from combustion to surrounding environment.