The impacts of different profiles of the grate inlet conditions on freeboard CFD in a waste wood-fired grate boiler

The common combustion disturbances in the fuel bed in a grate-fired boiler pose a great challenge to accurate modelling of the fuel bed conversion. Therefore, it is difficult to describe the lengthwise profiles of the combustibles leaving the fuel bed into the freeboard accurately. This paper investigates how different fuel bed models or grate inlet profiles will affect the Computational Fluid Dynamics simulation of combustion in the freeboard in industrial grate boilers. Two grate inlet conditions, which are very different in the profiles along the grate but yield the same total mass, momentum, species and energy fluxes into the freeboard, are used in the freeboard simulation of a 13 MWth grate boiler, respectively. The key findings from the comparison between the simulation results and measurement data are as follows. Firstly, the fuel bed model or grate inlet condition accounting for the realistic lengthwise biomass conversion pattern can be used reliably for the simulation of a grate boiler and simulation-based boiler optimisation. Secondly, the impacts of different fuel bed models or different profiles of the grate inlet conditions are virtually restricted only to the vicinity of the fuel bed or in the primary combustion chamber. After the secondary combustion chamber in which up-flowing gas is mixed well with the secondary and tertiary air jets, the simulation results are close to each other. For more general and reliable applications, a comprehensive bed model resolving the mixing, reactions, heat and mass transfer in the fuel bed, is needed and under development.