In this context, the differential mass conservation equations were used to describe typical noncatalytic gas-solid reactions, such as biomass chars gasification, and an approximate method was applied to solve these equations. The methodology developed in this work was based on an approximate method for decoupling the gas conservation equation. With this strategy, the calculation of gas concentration and solid conversion at any time and position during reaction could be converted to the solution of two coupled algebraic equations. Thereby, a new mathematical model, modified quantize model (MQM), was constructed to describe the gasification rates of biomass chars according to a nonlinear relationship between the porosity and reaction surface of biomass chars when pore diffusion effect is considered. Carbon conversion rates predicted by the MQM were in closer agreement with the experimental data than those predicted by the Rafsanjani model (QM) and discrete random pore model (DRPM). Using the MQM, the gasification process of biomass chars in CO2 at different temperatures was analyzed.