Oxy-coal combustion enables carbon capture and storage (CCS) through the utilization of oxygen to produce a flue gas that is primarily CO2. The flue gas can be recycled back to the combustion chamber to control temperatures and act as the primary fuel carrier. These changes to the combustion environment can impact flame structure and formation of NOx.
Although oxy-coal plants alone will not contribute to renewable energy targets, they may be able to if they are operated with biomass cofiring. When the flue gas is sequestered, CO2 is effectively removed from the atmosphere because the CO2 consumed by the biomass during growth is not released back into the atmosphere. In addition, the concentrations of SOx and mercury within the boiler can be reduced by cofiring.
However, biomass fuels are widely variable in composition, particle size, and nitrogen content, which can make utilization of these fuels challenging. In this work, a numerical study was conducted for cofiring of pulverized coal and sawdust under air-fired and oxyfuel conditions to investigate the effects of cofiring on flame length and nitric oxide (NO) formation. Previous experiments have shown an increase in nitrogen conversion to NO when cofiring under both air-fired and oxyfuel combustion, despite the fact that the sawdust cofired had less fuel-bound nitrogen. Computational Fluid Dynamics (CFD) is used to determine the cause of the increased NO conversion and to identify differences between air-fired and oxyfuel cofired flames. The simulations reveal that cofired flames have longer volatile-flame regions (the flame envelope), and this length is influenced by the increased volatile fraction and particle size associated with the biomass. Flame length theory for turbulent, non-premixed gaseous diffusion flames was found to be useful in interpreting the observed results in both air-fired and oxyfuel combustion. Large biomass particles that are not entrained in the near-burner region breakthrough the flame envelope, and this was shown to be detrimental to controlling NO formation. During oxy-cofiring combustion, particle breakthrough occurs at smaller diameter, leading to increased nitrogen conversion to NO when compared to air-fired conditions. This is a direct result of a decreased flame envelope length and elevated oxygen concentrations.