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 degrees C, synergistic effect firstly decreased and then increased with an extreme point at 600 degrees 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 degrees C. Therefore, synergistic effect increased with temperature increasing from 600 to 800 degrees 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 degrees C.