CFD investigation of pelletization effect on co-firing coal with wheat straw

Biomass co-firing in existing coal-fired power plants has been proven successful and practised in many installations worldwide as a greener alternative to coal. Attempts to overcome the low efficiency of transporting raw biomass have resulted in global wood pellets market growth in the past decade. This research studies the effect of biomass densification on biomass co-firing with coal. A three-dimensional CFD model was used to simulate the co-firing of coal and wheat straw. The model was verified by comparing the simulation results and experimental data presented in the literature. The experimental and predicted results were found to be in good agreement and exhibit the same general trends in terms of the gas species (O-2, CO, CO2, and H2O) concentrations. The effect of pelletization on biomass particle combustion was then investigated by considering particle shrinkage due to compression during pelletization. The baseline case is for uncompressed particles having particle density of 600 kg/m(3), whereas the two cases of densified particles have particle densities of 800 kg/m(3) and 1000 kg/m(3). Results show that, when compared to the baseline case, compressed particles experience slower volatilization and surface reaction rates, and hence a greater percentage of unburnt carbon. Larger wheat straw particles (>894 mu m) have different aerodynamic interaction compared to smaller particles in terms of residence time and particle trajectory. CFD results also show that compressed particles would exhibit lower NO emission, and NH3 as the intermediate product from biomass volatile-N has contributed to the reduction in NO emission during the volatilization process.