Mechanism of preheating modification on NOx emission reduction during peak shaving of different coal ranks

Abstract

During peak shaving operations, coal-fired power plants are often associated with unstable combustion and increased NOx emissions. In this study, a preheated modification device for pulverized coal was employed to reduce NOx emissions during peak shaving. The conversion characteristics of char-N, tar-N and NOx precursors (NH3 and HCN) have been extensively elucidated. Moreover, the mechanism for NOx reduction during preheating combustion has improved. The results indicate that in high-rank coals, char-N is the dominant nitrogen-containing component (>50 %), and the relatively high thermal stability limits its reduction to N2. The release of preheated tar mainly occurs in low-rank coals with loads less than 40 %, corresponding to temperatures of approximately 660 °C–725 °C. Moreover, the main NOx precursors in low-/high-rank coals are NH3 and HCN, respectively. After preheating, the fuel is activated, and 75 %–96 % of the fuel nitrogen can be removed because of the pre-denitrification mechanism. The NOx emissions can be maintained within 335 mg/m3 (6 % O2) over the load range of 25 %–110 %. However, N2O increases rapidly at loads below 60 % (<980 °C). Furthermore, the conversion of fuel-N to NOx increases and then decreases with increasing load, with inflection points at 80 % and 40 % loads for high- and low-rank coals, respectively.