Effects of moisture and its input form on coal combustion process and NOx transformation characteristics in lignite boiler

Abstract

Effects of moisture content in lignite (MCL) and its input form (IFMC), i.e. liquid water or water vapor, on coal combustion behavior and NOx transformation characteristics were studied in this paper. For this, 6 cases were numerically investigated within a previously validated 660 MW lignite-fired pulverized boiler model, where coal properties were defined by simplified coal compositions method. Among cases, MCL and IFMC were artificially altered while the relative contents of other coal components remain unchanged. Results show that when MCL is increased as H2O(l), coal combustion process deteriorates heavily and thus results in decreases in combustion temperature and fuel utilization rate. Meanwhile, the weakened in-furnace reductive atmosphere leads to a monotonic increase of NOx emission with liquid MCL increasing. Besides, IFMC is found to greatly affect NOx emission, and an increased proportion in water vapor tends to significantly lower the final NOx emission. Based on this, the operating cost of pre-drying system is correlated to that of the post-furnace denitrification process. Considering the potential widespread use of high moisture lignite in future power plants, these findings can be of great theoretical help in guiding the practical operation of lignite boilers.