Study on intrinsic reaction kinetics of coal char oxy-fuel combustion based on general surface activation function model

•Developed a variable kinetic model—general surface activation function model (GSAFM).•GSAFM's reaction mechanism function is 1−X, which is highly generalizable.•GSAFM outperforms fixed kinetic models in predicting intrinsic reaction rate (IRK).•CO2 replacement of N2 in O2/N2 reduces IRK of coal char by decreasing active sites.•Effects of CO2 and coal rank on IRK of char oxy-fuel combustion were compared. Investigating the intrinsic reactivity differences of coal char under O2/CO2 and O2/N2 atmospheres is crucial for oxy-fuel combustion boiler design and operation. However, existing fixed kinetic models suffer from uncertain parameters and limited generality. This paper proposes a variable kinetic model without uncertain parameters, the general surface activation function model (GSAFM). Compared to three commonly used fixed kinetic models—the general model (GM), the random pore model (RPM), and the modified autocatalytic model (MACM)—GSAFM demonstrates superior predictive performance for intrinsic reaction rates R(X) of oxy-fuel combustion and O2/N2 combustion of two different coal ranks obtained from isothermal thermogravimetric experiments. GSAFM unifies variable activation energy mean with fixed activation energy values, and its reaction mechanism function, 1 − X, contains no uncertain parameters, making it independent of coal rank, reaction atmosphere, and conversion ratio range, thus exhibiting strong generality. The study results indicate that replacing N2 with CO2 causes only a slight change in coal char's activation energy, but significantly reduces the pre-exponential factor, namely, the number of active sites. This reduction is likely due to CO2 and O2 competing for the same active sites, leading to a decrease in R(X) of coal char. For low-rank coal char under 21% O2/79% CO2, R(X) initially exceeds that of high-rank coal under 21% O2/79% N2 but eventually falls below it as the reaction progresses. This indicates that the reduction in R(X) due to CO2 replacing N2 has a more significant impact in the later stages of the reaction than the increase in R(X) due to the lower coal rank. The GSAFM proposed in this study provides a new perspective and methodology for investigating the intrinsic reaction kinetics of coal char oxy-fuel combustion.