A new approach to the modelling of the evolution of different chemical species from coal during its thermal decomposition under variable heating rate regimes was developed. The approach, which can be based on experimental data or the multiple-reaction model (MRM) with distributed activation energies, uses an Nth order reaction model where the pre-exponential factor and the apparent activation energy are functions of the rate of heating. A comparison of the predictions of the new model with the MRM was carried out. The new approach significantly cuts down the computation time with almost no loss of accuracy. The functional group pyrolysis model was examined and the parameters for the new Nth order approximation obtained considerably simplifying pyrolysis modelling. In addition, the article shows how to obtain estimates for certain parameters, which characterize chemical processes modelled by the MRM.