Numerical modeling of coal/biomass co-firing

Biomass co-firing within existing infrastructure of pulverized coal utility boilers is viewed as a practical near-term means of encouraging renewable energy while minimizing capital requirements, maintaining the high efficiency of pulverized coal boilers and reducing the emissions. Numerical investigation of coal/biomass co-firing is presented in this study. Co-combustion of biomass and coal is a complex problem that involves gas and particle phases, along with the effect of the turbulence on the chemical reactions. The transport equations for the continuous phase (gas) and discrete phase (spherical particles) are solved respectively in the Eulerian and Lagrangian frame of reference. The mathematical models used for co-pulverized coal/biomass particles combustion consist of models for turbulent flow (RNG k-ε model); gas phase combustion (two mixture fractions/PDF model: one mixture fraction is used for the fuel (char) and the second for the volatiles); 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 to 20% wheat straw (thermal basis) for co-firing. The effect of the percentage of biomass blended with coal on the velocity field, temperature distribution, particles trajectories and pollutant emissions at the exit of the furnace is presented in this paper. One important result is the reduction of NO and CO₂ emissions when using co-combustion. This reduction depends on the proportion of biomass (wheat straw) blended with coal.