CFD analysis of the effects of co-firing biomass with coal

Computational Fluid Dynamics (CFD) analysis of the effects of co-firing biomass with coal is presented in this study. Coal/biomass co-firing is a complex problem that involves gas and particle phases, along with the effect of the turbulence on the chemical reactions. The CFD analysis includes the prediction of volatile evolution and char burnout from the co-pulverized coal/biomass particles along with the simulation of the combustion chemistry occurring in the gas phase. The mathematical models consist of models for turbulent flow (RNG k-ε model); gas phase combustion (two-mixture fractions/PDF model); particles dispersion by turbulent flow (stochastic tracking model); coal/biomass particles devolatilization (two competing rates Kobayashi model); heterogeneous char reaction (kinetics/diffusion-limited rate model); and radiation (P-1 radiation model). The coal used is a Canadian high sulfur bituminous coal. The coal was blended with 5-20% wheat straw (thermal basis) for co-firing. The effect of the percentage of biomass blended with coal on the flow field, gas and particle temperature distribution, particles trajectories and gas emissions (CO₂ and NO_x) are presented. One important result is the reduction of NO_x and CO₂ emissions when using co-combustion. This reduction depends on the proportion of biomass (wheat straw) blended with coal.