Modeling K-containing Gases Captured By Adsorbents And Its Impacts On The Formation Of Submicron Particles

K-containing gaseous species in high temperature flue gas from biomass combustion produce submicron particles through homogeneous nucleation and heterogeneous condensation during the flue gas cooling,which is the key issue requiring to be focused on in the process of biomass fuel utilization.Addition of solid adsorbents is an effective measure to control K-containg species transforming into particles through chemical reaction and physical adsorption.Investigation on the process and its influencing factors of adsorbents capturing K-containing species in high temperature flue gas is helpful to provide theoretical basis for controlling particulate matter formation in practical combustion process in the boiler.Therefore,to simulate the one-dimensional plug flow system,a physical condensation model,one-step and two-step global reaction kinetic models of adsorbents capturing K-containing species in high temperature flue gas were established.The rationality of models and their kinetic parameters were verified by comparing the model calculations with experimental measurements in the literature.The models were then applied to investigate the influence of flue gas temperature and K concentration on isothermal capture of K-containing species by kaolin and coal fly ash,and compare the accuracy between the one-step and two-step global reaction models.The physical condensation model and two-step reaction kinetic model were used to numerically investigate kaolin and coal fly ash capturing K-containing species and subsequent K transformation during flue gas cooling in a one-dimensional plug flow,and explore the effects of the addition amount and particle size of adsorbents on K-containing species transforming into submicron particles through homogeneous nucleation.One-step and two-step global reaction kinetic models were applied to investigate the isothermal reaction process of adsorbents capturing K-containing species.The results showed that the simple global kinetic models with their kinetic parameters are capable to describe the performance of kaolin and coal fly ash in capturing KOH,KCl,K₂SO₄ and K₂CO₃ under a wide range of temperature and K concentration and the influences of reaction conditions.Within the studied range of K concentration（50–750 ppmv）and flue gas temperature（1073–1573 K）,the abilities of kaolin and coal fly ash capturing various K-containing species increase with the increases of K concentration and flue gas temperature.Both adsorbents captured KOH and K₂CO₃,present the same patterns and have the same kinetic rates.The capabilities of the two adsorbents capturing various K-containing gases are:KOH>KCl>K₂SO₄,and the adsorption capability of kaolin is much stronger than coal fly ash when capturing the same K-containing species.One-step global reaction kinetic model,considering only the reaction process,predicts with relatively larger errors at lower flue gas temperature and higher K concentration.In contrast,two-step model,which takes both reaction process and adsorbents deactivation caused by product high-temperature melting into consideration,can more accurately predict the amount of K captured by the two adsorbents under the given conditions.The comparison of model prediction performance indicated that the two-step reaction kinetic model is more reasonable.Numerical investigation on adsorbents capturing K-containing species and subsequent K transformation during flue gas cooling in a one-dimensional plug flow showed that the amount of adsorbents added has a significant effect on the transformation of K-containing species into particulate matter.Under the given initial K concentration,the amount of K captured by kaolin and coal fly ash,K nucleated through homogeneous nucleation and condensed on adsorbents through heterogeneous condensation all decrease with the increase of adsorbents addition.Among them,the effect of adsorbent addition amount on homogeneous nucleated K is the most obvious.There is always a critical K/（Al+Si）mole ratio for kaolin and coal fly ash capturing various K-containing species.When the K/（Al+Si）ratio in flue gas is less than the critical value,the migration and transformation of K-containing species to particulate matter in flue gas are completely eliminated.The numerical study on the effect of adsorbent particle size showed that changing the size of adsorbent particles does not affect the amount of K captured by chemical reaction when ignoring the diffusion effect.The amount of K condensed on the adsorbent particles through heterogeneous condensation decreases with the increase of particle size,and correspondingly,the amount of K homogeneously nucleated to transform into submicron particles increase with the increase of adsorbent particle size.Both finally turn to be stable.The heterogeneous condensation of K-containing species on the surface of adsorbent particles with smaller particle sizes can significantly reduce the amount of submicron particles generated by homogeneous nucleation,which was more obvious for coal fly ash with relatively poor reactivity.