Combustion of carbonaceous solid fuels was modeled using a number of methods, and the employed models were further improved by thoroughly studying the physical and chemical interactions between carbonaceous solid fuels and oxidizers. Simulation accuracy was improved by using special techniques to reduce the fitting errors of earlier models; e.g., errors in the isothermal coal char combustion rate model were reduced by modifying the well-established random pore model, with increased model flexibility generally considered vital for improvement. Thermogravimetric analyses were performed for four categories of carbonaceous fuels: semi-anthracite coals, bituminous coals, sub-bituminous coals, and biomass. Scanning electron microscopy imaging was employed to understand correlations between the fuel structure and model parameters. The obtained results were used to confirm the hypotheses of the used models, and a general model of carbonaceous solid fuel combustion, termed "flexibility-enhanced random pore model", was established, improving correlation coefficients from 0.7 to 0.98 and decreasing deviations from 20 to 3%.