Improvement of NOx formation model for pulverized coal combustion by increasing oxidation rate of HCN

Abstract

The NOx formation simulation during pulverized coal (PC) combustion has been studied for several decades. In these studies, the global reaction rate expressions for N intermediates (HCN and NH3) proposed by De Soete were frequently used. Using De Soete’s expressions, the calculated reaction rates of HCN and NH3 in the typical temperature range of 1750–2000K for PC combustion show that HCN has obviously lower conversion rate to NO (or oxidation rate of HCN, HCN+O2→NO+⋯) than that to N2 (HCN+NO→N2+⋯) as well as obviously lower conversion rate to NO than NH3 does (NH3+O2→NO+⋯). This is completely in contradiction with the well-recognized conclusions in the publications. Thus, in this study, the oxidation rate of HCN is recommended to be increased by about 13 times according to related literature data. The modified NOx model was integrated into coal combustion model to simulate the NOx concentration distribution for three coals (two sub-bituminous coals and one lignite) in a down-fired autothermal test furnace. The input kinetic data of coal devolatilization and char combustion for coal combustion model were all obtained from the experiments in a drop tube furnace, which ensured the good agreements of gas temperatures and compositions between prediction and measurement in the test furnace. According to the simulated results, it was nearly impossible to accurately reproduce the NOx concentration profile in pc furnace using the original De Soete’s HCN oxidation rate, since the NOx concentrations in the high-temperature flame zone were greatly underestimated. Nevertheless, a satisfactory agreement for the NOx concentration distributions between prediction and measurement could be obtained using the increased HCN oxidation rate, if proper char surface area (S) values were chosen. Via using different S values, it was found that the S leading to the best simulation results for three coals were all about 17.5 times higher than those measured by Hg porosimetry (SHg), but they could not correlated well with that measured with low-temperature N2 adsorption (SBET). This indicated that SHg was a better surface basis for normalizing char reactivity with NO than SBET for different coals in the NOx simulation under the conditions of PC combustion.