(267f) Cofiring of Coal and Biomass Under Oxycombustion Conditions: NOx Formation and PM Characterization

The cofiring of coal with biomass is a promising method under consideration for power generation as a means to reduce net CO₂ emissions and to meet renewable energy standards. Furthermore, if carbon capture and sequestration (CCS) technology is applied to a cofired system, a net removal of atmospheric CO₂ may be realized, since the CO₂ removed by the biomass is not released back into the atmosphere. In this work, the particulate matter and NOx produced from the cofiring of coal and biomass is investigated under both conventional air-fired and oxy-combustion conditions. Oxy-combustion is advantageous since a concentrated stream of CO₂ is inherently produced for CCS. Experiments are performed in a 35 kW, horizontally-fired combustion facility, utilizing pulverized subbituminous coal and various biomass materials. When air is replaced with a mixture of 30% O₂ and 70% CO₂, and no overfire oxygen is utilized, oxy-fuel combustion results in a 20% reduction in NO production. The oxygen concentrations of the primary and secondary streams are systematically varied, while holding constant the total O₂ and CO₂ flow rates, and a minimum emission rate of NO is measured when the primary and secondary O₂ concentrations are both equal to 30% O₂. A further reduction in NO is measured when cofiring with sawdust, primarily due to the reduced nitrogen content of the fuel. The effects of cofiring and oxy-combustion on the fly ash size distribution and the size-fractionated composition are discussed. In addition, the interaction between coal and biomass fly ash in the high-temperature zone and the potential for increased deposition and fouling/slagging is explored.