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

Abstract

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 μm are kinetics−diffusion-controlled at temperatures below ca. 1100 °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 °C for 3.5–5.5 μm particles to around 1350 °C for 13.5–20 μm 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.

Modeling Potassium Capture by Aluminosilicate, Part 2: Coal Fly Ash

“…Recently, Zhu et al . and Chen et al published models for capturing potassium with aluminosilicates under PF boiler conditions.…”

“…Recently, Zhu et al 18 and Chen et al 19 published models for capturing potassium with aluminosilicates under PF boiler conditions. To account for the additive deactivation and equilibrium limitation on alkali conversion, two competing reactions for additive were assumed and their rate coefficients were fitted to experimental data for potassium conversion.…”

“…Aluminosilicate additives have attracted lots of scientific and industrial interests for capturing alkali salts. − Kaolin and coal fly ash (CFA) are among the most investigated aluminosilicates. The short residence time in PF boilers demands very fine particles of fuel and additives, so additive particles of micrometer’s size should be suspended in a gas environment containing vaporized alkali to simulate PF boiler conditions in experiments.…”

Special Issue in Memory of Professor Mário Costa

“…They found that the chemical model can be successfully applied to predict the released gas composition along a grate-fired fuel bed using a stochastic reactor network. Sheng et al develop a transient one-dimensional (1D) single-particle model to describe kaolin reacting with KOH vapor under conditions pertinent to pulverized fuel (PF)-fired boilers. The model addresses the vapor diffusion and reactions within the particle as well as the reactivity change mainly as a result of the sorbent deactivation.…”

“…They found that the chemical model can be successfully applied to predict the released gas composition along a grate-fired fuel bed using a stochastic reactor network. Sheng et al 4…”