SUMMARYA computer code for the modelling of turbulent reactive¯ows with heat transfer has been parallelized and applied to the simulation of a utility boiler. The code is based on the numerical solution of the densityweighted averaged form of the governing equations for mass, momentum and energy conservation, and transport equations for scalars associated with the turbulence and combustion models. The k-e model and the chemical equilibrium approach are used. The turbulent¯uctuations are accounted for in the calculation of the mean properties by means of a presumed joint probability density function for the mixture fraction and the fraction of radiative heat loss. The discrete ordinates method is used for radiation modelling. The governing equations are solved using the ®nite volume method. The parallelization is carried out using the domain decomposition approach and the message-passing MPI library. The paper is divided into two parts. This part is concerned with the description of the model and the parallel implementation, while the model evaluation and the analysis of the parallel performance are presented in Part II ( pp. 727±736).