A new method for simulating the combustion of a large biomass particle—A combination of a volume reaction model and front reaction approximation

Combining a volume reaction model and front reaction approximation is proposed to simulate the combustion of a large biomass particle. Two intraparticle processes—drying and char oxidation—are simplified as front reaction because they are transport controlled. The other intraparticle process—pyrolysis—is described as the volume reaction because it is controlled by both heat transfer and kinetics. A new numerical method based on the basic mechanism of the process is applied to mitigate oscillations of the solution of the front reactions. To compare the calculation results with the experimental results presented in the literature, combustion of cubic wood particles between 5 and 25 mm is chosen to test the new method. Drying, pyrolysis, char oxidation, vapor condensation, shrinkage of the process, heat transfer via conduction, diffusion, convection, radiation and mass transfer via diffusion, and convection inside particle are taken into account. Finite volumes attached to solid materials are used to discretize the domain and explicit method with variable time step is used to calculate the process. A program was written and the calculation showed that the conversion of a particle is almost independent of computational mesh from 10 cells on. However there is significant instability in the mass loss rate curve when the number of cells is less than 20. Predictions for different particle sizes, furnace temperatures and moisture contents were compared with measurements and they agree reasonably well. The results highlight the significance of pyrolysis kinetics on prediction. Thus, the front reaction model of pyrolysis assuming a constant reaction temperature of 773 K is sometimes inadequate. The proposed method also showed that moisture content and pyrolysis reactivity significantly affect the thickness of devolatilizing fuel.