Modeling the Capture of KOH Vapor with Kaolin under Conditions of Pulverized Fuel-Fired Boilers

Injecting Al-Si-based sorbents, typically kaolin, to capture alkali vapors is an effective technology to mitigate alkali-related operation problems of pulverized fuel (PF)-fired boilers. Aimed at evaluating the sorbent performance with a numerical approach, a transient one-dimensional single-particle model was developed for kaolin particle capturing KOH vapor under the conditions prevailing in PF-fired boilers, which fully addresses the intraparticle vapor diffusion and reactions and the reactivity change as a result of the sorbent deactivation. The model was validated with literature data from the experiments on an isothermal entrained flow reactor and proven to reasonably describe kaolin capturing KOH vapor covering a wide range of reaction conditions. The model was employed to numerically investigate the effects of the temperature and sorbent deactivation, particle size, and K/(Al + Si) molar ratio on the K-capture performance. It was found that the reactions of sorbent particles of around 3.5-5.5 mu m are kinetics-diffusion-controlled at temperatures below ca. 1100 degrees C and the diffusion effects are enhanced with an increasing particle size. Sorbent deactivation at higher temperatures reduces K capture but looses the diffusion limitation on K capture for larger particles. The Arrhenius increase of reaction reactivity and sorbent deactivation together with the diffusion effects determine the optimal temperature window for kaolin capturing KOH vapor, which shifts from around 1200 degrees C for 3.5-5.5 mu m particles to around 1350 degrees C for 13.5-20 mu m particles. The modeling study suggests using smaller particle sorbents for achieving high K-capture performance but also supports using larger size sorbents because of the fitness of their temperature windows with high-temperature conditions of PF-fired boilers.