(487f) Effects of Particle Size on Nitric Oxide Emissions During Coal/Biomass Combustion Under Air-Fired and Oxy-Fuel Conditions

An important concern facing regions that are dependent upon coal for electricity is the adoption of renewable energy portfolio standards. Recognizing that many of these regions do not possess solar and wind resources sufficient to meet renewable energy targets, biomass co-firing may play an important role in future power generation. Co-firing may also result in reduced criteria pollutant emissions, such as NOx and SO2, from coal-fired power plants. When introducing biomass into a coal power plant boiler, the combustion characteristics and emissions can change depending not only on the type of biomass, but also on processing such as size reduction and drying. In this work, flame stability and the dependence of NO emissions on biomass cofiring ratio, moisture content, and particle size is investigated experimentally and numerically. Numerical simulation is employed to provide a detailed analysis of the combustion zone and to better understand these effects. 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. Since new or retrofit coal-fired plants may employ oxy-combustion technology as a means to carbon capture, biomass cofiring under oxy-combustion conditions is also investigated. Flame stability and NO emissions are measured under oxy-combustion and air-fired combustion conditions.

Experiments are performed in a 35 kW, horizontally-fired combustion facility, utilizing pulverized subbituminous coal and various biomass materials. Biomass particle size is varied by sieving and co-fired with coal. The moisture content in the biomass is also varied. Experimental results show that NO emissions do not scale with the reduction in fuel-bound nitrogen. In flames with high percentages of biomass cofiring and decreased secondary swirl, flame lift-off and increased NO emissions was observed at reduced power outputs. Other experimental studies where a constant thermal input was maintained and the cofiring percentage was varied up to 50% biomass, resulted in a relatively constant NO emission. The effect of biomass cofiring on flame stability, and thus NOx emissions, is complicated and dependent upon many factors, including fuel particle size and composition, nitrogen and moisture content, feeding configuration, and burner hydrodynamics, including flow field and particle interactions.