Capture effect of K and Pb by kaolin during co-firing of coal and wheat straw: Experimental and theoretical methods

Abstract

To reduce the emission of Pb in coal combustion and K in biomass combustion respectively, kaolin was applied widely as the in-furnace sorbent. In this study, effect of kaolin on K and Pb simultaneously in co-firing of coal and wheat straw (WS) was investigated in a lab-scale bubble fluidized bed. Relative distribution of the two elements was respectively compared with the conditions of the individual fuel combustion. Mechanism of reactions among coal, WS and kaolin was investigated via sample analysis and theory calculation. Experimental results indicated that kaolin could exert negative effect on the release of the two elements by transferring them into fly ash and bottom solid and the effect was promoted with the increasing temperature. During co-firing, sulfation reaction of K was superior to the capture reaction of K by kaolin at low temperatures while adsorption of K was slightly enhanced compared with the theory value based on results of the two individual fuels. Oppositely, the capture effect of Pb in co-firing was impaired to some extent. Lastly, result of DFT (Density Functional Theory) calculation proved that existence of KCl inhibited the adsorption of PbCl2 on metakaolin by occupying the optimum adsorption site and reducing the adsorption energy at high temperatures.