One of the most important models for gas-solid reactions with structural changes is the modified grain model. In this model, mass conservation leads to a series of complicated nonlinear coupled partial differential equations. There is no exact or approximate solutions for these complex equations, and their numerical solution is very tedious and time consuming with some convergence problems. In this work, a new mathematical approach (quantized method) has been applied for the solution of the modified grain model equations. This method gives simple approximate expressions for quick prediction of the conversion-time behavior. The results of this work have been compared with existing numerical solutions and show good accuracy. Moreover, this solution technique for the first-order volume reaction model shows very good accuracy. Thus, this method can be used for quick estimation of kinetic parameters from experimental data. Reactions with various structural changes have been considered by this technique successfully.