Investigation on the correlation between AAEMs transformation and reactivity synergy during the co-combustion of biochar and coal char

Co-combustion synergy behavior was caused by the coupling effect of AAEMs in different chemical forms but co-combustion synergy mechanism was currently unclear. In the study, co-combustion reactivity and synergy behavior of blends of rice straw char (R) and coal char (C) and blends of pretreated R (washed by NH4Ac solution) and pretreated C were investigated. AAEMs transformation during co-combustion was analyzed based on experimental and simulation methods and co-combustion synergy mechanism was proposed. The results show that as temperature increased from 500 to 800 °C, synergistic effect firstly decreased and then increased with an extreme point at 600 °C. For pretreated blends, transformation of HCl-soluble K in R to H2O-soluble K could catalyze the residual semi-char reaction, leading to synergistic effect. KCl in R has higher catalytic and migration activity compared to active Na/Ca in C, therefore synergistic effect was generally improved when KCl was present in blends. Thermodynamic simulation demonstrated that KCl volatilized significantly at temperature ranging from 700 to 800 °C. Therefore, synergistic effect increased with temperature increasing from 600 to 800 °C. Formation of stable Na–Ca–Si combeite reduced SiO2 transformation into liquid and covering C surface, and thus cause an increased synergistic effect at 700–800 °C.