Experimental And Modeling Study On Kaolin Capturing Alkali-containing Vapors Under Suspension Reaction Conditions

Combustion is an important way for bio-energy production.Suspension-firing boilers are increasingly used for production of power and heat from biomass.However,biomass combustion releases a large amount of K-containing vapors to transform into submicron ash particles through homogeneous nucleation or heterogeneous condensation during the flue gas cooling,which lead to an increase in particulate matter emissions and probably cause the problems including ash deposition,corrosion of heat exchanging surfaces and deactivation of SCR catalyst.Addition of solid sorbents to capture the alkali vapors through chemical reaction is an effective measure to reduce the conversion of alkali vapor into submicron particles.Therefore,in the present work,experimental and modeling methods were employed to systematical investigate a typical sorbent,kaolin,capturing various alkali vapors under suspension reaction conditions.The effects of different reaction parameters,including types of alkali vapors,reaction temperature,molar K/(Al+Si)ratio,properties of kaolin and sorbent deactivation,on K-capture level and K-conversion were studied so as to provide theoretical and experimental support for the use of solid sorbents in power plant boilers to control particulate matter emissions and mitigate ash deposition and corrosion.The experiments of kaolin capturing KOH vapor show that the K-capture level and Kconversion increase with increasing the reaction temperature.As the molar K/(Al+Si)ratio is increased,the K-capture level increases rapidly and then decreases slightly while the Kconversion decreases continuously.The addition of kaolin with a K/(Al+Si)ratio of ca.0.5 can balance the high K-capture performance and low cost of the sorbent usage.Due to the diffusion limitation on the K-containing vapor,the K-capture performance of small-sized kaolin particles is better than that of large-sized particles.Moreover,it is found that kaolin has the similar performance for capturing Na OH vapor and KOH vapor under a wide range of experimental conditions.The experiments of kaolin capturing KCl vapor indicate that the change of the reaction temperature has little effect on the K-capture level and K-conversion.The trend of K-capture performance with increasing the K/(Al+Si)ratio is similar to that for KOH capture,that is,as the K/(Al+Si)ratio is increased,the K-capture level increases and then decreases,and the Kconversion continuously decreases.The addition amount of kaolin that satisfies the K/(Al+Si)ratio of ca.0.35 can balance the high economic performance and K-capture performance.Meanwhile,it is found that the K-capture level and K-conversion for KCl capture are significantly lower than those for KOH capture under the same conditions,and the optimal temperature windows and optimal amount of sorbent addition for KOH capture and KCl capture are different.A mathematical model is established for kaolin capturing alkali vapors under suspension reaction conditions.The model validation and numerical study demonstrate that the model established and its kinetic parameters determined can reasonably describe the performance of kaolin capturing KOH and KCl vapors covering a wide range of reaction conditions and the patterns of the reaction conditions affecting the K-capture performance.The numerical investigation of using the model shows that the reactions of kaolin particles of around 3.5 μm–5.5 μm are kinetics-diffusion controlled at temperatures below ca.1100 ℃ and the diffusion effects are enhanced with increasing the particle size.However,sorbent deactivation at higher temperatures reduces the K-capture level but looses the diffusion limitation on K-capture level for larger particles.The reaction reactivity of kaolin capturing alkali vapors and sorbent deactivation together with the diffusion effects determine the optimal temperature window for kaolin capturing KOH vapor,which shifts from ca.1200 ℃ for 3.5 μm–5.5 μm particles to ca.1350 ℃ for 13.5 μm–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 the high temperature conditions of industrial suspension combustion boilers.