Numerical Modelling of Solid Biomass Combustion: Difficulties in Initiating the Fixed Bed Combustion

Bio mass is a renewable energy which has become an important fuel to produce thermal energy or electricity. It has no adverse effect on environment as it reduces carbon dioxide emissions. Biomass combustion is a difficult phenomenon due to its properties with less carbon, more volatile matters and lower calorific value. Fixed bed combustion is a useful technique for thermal conversion of solid biomass fuels as this can fire up a broad range of fuels with different properties and requires less fuel preparation. But due to complex solid combustion mechanism and inadequate knowledge on process the development of such combustion system is limited. Numerical modelling of this combustion system has some advantages over experimental analysis. But developing a complete model for this type of combustion system is a challenge. Due to its characteristic properties, modelling of biomass combustion has to overcome many difficulties. One such problem is initiating the combustion in numerical modelling. The usual way to model biomass combustion is to divide the system in to two phases namely solid phase and gas phase. Modelling the solid phase of this combustion system requires the major effort. In this work, a detailed analysis of difficulties occurred in initiating the biomass combustion has been presented. A complete three-dimensional numerical model is used for transient analysis of fixed bed biomass combustion. The bed is considered as a porous medium and solid phase of the combustion is characterised by using sub models with several variables in a commercial CFD code.