Computational modelling of co-firing of biomass with coal under oxy-fuel condition in a small scale furnace

Utilization of biomass as a co fired fuel has proved to be a significant option for the mitigation of Green House gas emissions. In this study, a computational fluid dynamics modelling of the co-combustion of pulverised Russian coal with highly volatile biomass called Shea meal having different blending ratio has been considered in a 0.5 MWth combustion test facility (CTF) for air and CO2-rich environment by using a commercial CFD tool called AVL Fire version 2009.2 coupled with the user defined subroutines for the aerodynamics of irregular shaped biomass particle, devolatilisation and char Combustion modelling. Results are validated comparing against the experimental data to examine the effects of co-firing under different operating conditions and a reasonably good agreement was observed with the differences in range of 5–10% with the experimental data. Operating conditions were varied with the recycled ratios (RR) (68%, 72% and 75%) and the biomass sharing (20% and 40%). Results are presented in terms of temperature distributions, CO2 concentrations, and the effects of co-firing ratio under different oxy-fuel combustion. With the increase of biomass sharing from 20% to 40%, the volume of the flame increases, but the flame stability and the peak temperature decreases due to higher volatile content in the biomass. The relationship of the peak and mean flame temperature and furnace exit temperature with the normalized total flow (NTF) and normalized O2 (NO) are presented. The heat transfer is significantly manipulated by the biomass sharing with different recycled ratio. The working range for the co-firing of 20% biomass sharing suggested that optimum recycled ratio (RR) for flame temperature and radiative heat transfer to be closely matched with air-firing found at 71%, but for 40% sharing, a value of RR70% may be needed. Finally, unburned carbon in ash (CIA) is predicted and improved burnout is observed in oxy-fuel cases with higher biomass sharing. This study highlights the possible effects of varying the fuel on ignition environment and heat transfer characteristics of a small scale furnace, emphasizing the minor reform that may be needed when transforming to higher biomass sharing co-firing