Optimizing biomass combustion in a 130 t/h grate boiler: Assessing gas-phase reaction models and primary air distribution strategies

Abstract

Grate-firing technology is widely used for biomass combustion for heat and power production due to its good fuel flexibility. However, grate boilers often suffer from high unburnout, low efficiency, and high emissions, and require optimization to improve their performance. The distribution of primary air along the grate length significantly affects combustion and pollutant formation behaviors in the furnace; however, this effect is often under-investigated and poorly quantified. In this study, combustion of corn stover in a 130 t/h grate boiler is investigated experimentally and numerically. To establish a reliable baseline computational fluid dynamics (CFD) model and provide valuable reference for modeling and optimizing this industrial grate boiler, this study first investigates the applicability of the commonly used gas-phase reaction models. The finite rate/eddy dissipation model is found to outperform other gas-phase reaction models such as eddy dissipation model and eddy dissipation concept, which well predicts the temperature and shape of the flame in both the kinetics-controlled primary combustion zone and the diffusion-controlled burnout zone and shows the best agreement with the measured and site observations. Then, the study simulates and quantifies the effects of different primary air distribution modes (uniform, staged and refined-staged) on combustion performance of the boiler. Compared with the uniform primary air distribution, both the staged and refined-staged primary air distribution show an obvious increase in the char burnout ratio, from 76.5 % to 87.8 % and 95.5 %, respectively. Furthermore, the CO concentration at the furnace outlet is remarkably reduced, from 3178 ppm to 2766 ppm and 1917 ppm, respectively. This demonstrates that reasonable air supply in the latter two primary air distribution cases facilitates complete combustion in both the fuel-bed and freeboard. Moreover, in the latter two cases, the temperature distribution in the freeboard becomes more uniform, and the local oxygen-rich regions are eliminated. These are beneficial in suppressing the NO formation, as the emission of which decreases from 172 mg/m3 (at 11 % O2) to 161 mg/m3 and 156 mg/m3, respectively. This work provides important theoretical guidance for the accurate modeling and optimization of grate-firing boilers as well as the design of new grate boilers.