Mechanistic investigation into particulate matter formation during air and oxyfuel combustion of formulated water-soluble fractions of bio-oil

This paper reports a systematic study on the formation of particulate matter with diameter of <10 µm (i.e., PM₁₀) during the combustion of two formulated water-soluble fractions (FWSFs) of bio-oil in a drop-tube-furnace (DTF) at 1400 °C under air or oxyfuel (30%O₂/70%CO₂) conditions. FWSF-1 was an organic-free calcium chloride solution with a calcium concentration similar to that in the bio-oil. FWSF-2 was formulated from the compositions of major organics in bio-oil WSF, doped with calcium chloride at the same concentration. The results suggest that similar to bio-oil combustion, the FWSF combustion produces mainly particulate matter with diameter of between 0.1 and 10 µm (i.e., PM_0.1–10). Since there are no combustibles in the organic-free FWSF-1, the PM is produced via droplet evaporation followed by crystallization, fusion and further reactions to form CaO (in air or argon) or partially CaCO₃ (under oxyfuel condition). With the addition of organics, FWSF-2 combustion produces PM₁₀ shifting to smaller sizes due to extensive break up of droplets via microexplosion. Sprays with larger droplet size produce PM₁₀ with increased sizes. The results show that upon cooling CaO produced during combustion in air can react with HCl gas to form CaCl₂ in PM_0.1. The predicted PSDs of PM₁₀ based on the assumption that one droplet produces one PM particle is considerably larger than experimentally-measured PSDs of PM₁₀ during the combustion of FWSFs, confirming that breakup of spray droplets takes place and such breakup is extensive for FWSF-2 when organics are present in the fuel.