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Char oxidation is often modeled using an mth order intrinsic reaction rate in conjunction with an effectiveness factor (η) to account for intraparticle diffusion of gas species. This approach involves the use of a general modulus (M_T) and using the first-order curve of η vs M_T. This method was originally referred to as the general asymptotic solution. It has been suggested that a simple Langmuir rate equation is more suitable for modeling the effects of pressure on char reactivity. Therefore, several methods of developing general moduli for the Langmuir rate expression are shown. The general asymptotic solution is most accurate as M_T approaches the limits of zero and infinity. However, in the intermediate range of M_T (0.2 < M_T < 5), the general asymptotic solution exhibits errors of up to −17% error in spherical coordinates and −24% error in Cartesian coordinates. A correction function was constructed to improve the …