An improved model for the estimation of gasification rate constants using CO2 and steam as gasifying agents is presented in this work. The theoretically deduced model only considers the micropore surface area and catalyst (alkali) content of the raw feedstock as inputs for the gasification rate estimation. The main advantage compared with other models is its simplicity in determining the rate constant with only one regression parameter, which is a result of introducing, for the first time, theoretical limits to the activation energy of the main gasification reactions (steam gasification and Boudouard reactions). Experiments were carried out in a setup that reduces the most common problems associated with laboratory gasification procedures, such as misinterpretation of a maximum gasification rate, adverse effects on the char properties during the pyrolysis phase, and interparticle diffusion. Statistical analysis showed that the model fitted well with experimental data, since the determination coefficient was higher than 0.96 for 13 coal samples with a broad range of ash contents (6.4% to 74.8%). The new model gives an accurate mixing rule that is useful in determining gasification rates of different feedstock mixtures. The new concepts presented in this research can be used for simulating and determining suitable process conditions for industrial gasifiers operating with different feed mixtures. (C) 2016 Elsevier B.V. All rights reserved.