Numerical optimization of separated overfire air distribution for air staged combustion in a 1000 MW coal-fired boiler considering the corrosion hazard to water walls

Abstract

Multigroup separated overfire air (SOFA) has been widely applied in large-scale coal-fired boilers for air staged combustion to reduce NOX emissions. However, unreasonable flow distribution in multigroup SOFA nozzles would cause boiler performance degradation, including decreased coal burnout rate, severe gas temperature inhomogeneity, and risks of high-temperature corrosion. In this study, six SOFA distribution modes including the reference case based on the field test were numerically investigated for operation optimization of a 1000 MW double-tangential circular coal-fired boiler. The combustion characteristics including flow field, temperature distribution, residual char particles, and gas species were comprehensively analyzed. Meanwhile, particular attention was paid to the reducing atmosphere near water walls of the upper furnace for ameliorating the problem of high-temperature corrosion. The numerical results show that SOFA distribution significantly determines the gas flow field in the burnout zone, which subsequently influences coal combustion behavior and boiler performance. The pagoda mode is suggested to be optimum for sufficient penetration of SOFA in the upper furnace, which features gradually decreasing air induced from SOFA nozzles along with the furnace height. Compared with the baseline scenario, the burnout rate is increased by about 0.2% due to the enhanced mixing of SOFA and combustibles, and the NOX emission is slightly decreased by about 8 mg/Nm3 in the pagoda mode. Besides, better temperature homogeneity is obtained at the furnace exit, which helps improve the heat transfer performance of downstream heat exchangers. Moreover, the reducing atmosphere near the water walls of the upper furnace is significantly weakened in this mode, contributing to the alleviation of the corrosion hazard to water walls.