Effect of O2/CO2 (Oxy-Fuel) Combustion on Particulate Matter Emission During Pulverized Biomass Combustion: Comparison with Air Combustion

Burning biomass in an O2/CO2 (oxy-fuel) atmosphere is an effective means of capturing CO2 from power plants, yet substituting the inert N2 diluent with a CO2-rich gas modifies the combustion environment and can influence the formation of particulate matter (PM). Two fuels were tested, namely redwood powder (RW, a Ca-rich and K-poor biomass) and cotton stalk powder (CS, an alkali-rich biomass), which were combusted in a high-temperature drop tube furnace in both air (21% O2/N2) and oxy-fuel (30% O2/CO2) at 900–1300 °C. It should be noted that the air and oxy-fuel atmospheres differ simultaneously in O2 concentration (21% vs. 30%) and in the diluent gas (N2 vs. CO2), so the reported differences reflect the combined effect of the selected oxy-fuel atmosphere rather than the influence of CO2 alone. The PM10 concentration and its size distribution were recorded in real time with an ELPI+, and the elemental composition of the particles was characterized by SEM-EDS. In most, though not all, conditions, oxy-fuel burning increased PM10 and, most notably, enhanced the submicron PM1 fraction. The bimodal size distributions showed that oxy-fuel strengthened the submicron mode while leaving the coarse-mode position (≈7 μm) largely unchanged, and SEM-EDS showed a strong K–Cl enrichment of the submicron particles for the alkali-rich CS under oxy-fuel. The particle concentrations reported here are measured mass concentrations in mg/Nm3 (dry gas, normalized to 0 °C and 101.3 kPa) and are not emission factors per unit mass of fuel. The promotion of PM1 is interpreted as a plausible consequence of the higher gas and particle temperatures together with the altered release and condensation of alkali species in the CO2-rich environment; these temperatures were not measured directly and the mechanism is therefore proposed as an explanation rather than demonstrated experimentally. Even though oxy-fuel combustion offers a practical means of CO2 capture, it may intensify fine-PM emission, a concern that should be taken into account when designing and operating oxy-fuel biomass boilers, especially for alkali-rich and chlorine-bearing fuels.