Understanding the Effect of Different Biomass Ash Additions on Pyrolysis Product Distribution, Char Physicochemical Characteristics, and Char Gasification Reactivity of Bituminous Coal

In this study, two typical biomass ash additives, rice straw ash (RSA), high K and high Si and cotton stalk ash (CSA, high K–Ca–Na and low Si), were adopted as natural and effective catalysts to systematically investigate the influence of biomass ash type on pyrolysis product distribution, char physicochemical characteristics (i.e., chemical forms and concentrations of alkaline and alkaline earth metals (AAEMs), pore structure, and carbon structure), and char gasification reactivity of bituminous coal. The results of pyrolysis product distribution show that RSA additive resulted in an increase of tar yield and decrease of char yield during coal pyrolysis, whereas CSA additive led to a decrease of tar yield and increase of gas yield in coal pyrolysis. The RSA additive had little impact on the gaseous product yields of coal pyrolysis, but the CSA additive obviously enhanced H2, CO, and CO2 yields of coal pyrolysis and slightly inhibited the CH4 yield of coal pyrolysis. The results of char physicochemical properties demonstrate that RSA and CSA additives were conducive to increasing active AAEM content and specific surface area of coal char and decreasing order degree of the coal char carbon structure. Moreover, the CSA additive showed a more significant positive effect on the coal char physicochemical properties. The results of gasification reactivity indicate that biomass ash additives stimulated coal char gasification, and the CSA additive showed a more prominent promotion effect, in accordance with the positive effect of biomass ash additives on active AAEM, pore structure, and carbon structure of coal char. The RSA and CSA additives resulted in the activation energy reduction of coal char gasification by 13.95 and 57.99 kJ·mol–1, respectively.