(73e) Ash Partitioning and Ultrafine Aerosol Formation Mechanism for Air and Oxy-Combustion of Coal, Biomass and Blends

Rice husk is one of the important biomass resources for power plants since it is abundant in all rice producing countries. Utilization of rice husks when combined with carbon capture and sequestration (CCS) is one feasible solution for ‘Bio-energy with carbon capture and storage’ (BECCS), which can reduce the CO₂ emissions in the atmosphere. However, the formation of pollutant emissions (like PM1) during combustion process is not well understood yet, especially for oxy-combustion. Hence this paper will focus on the ash partitioning and formation of ultrafine aerosols for rice husks and their blends with coal.

In this work, mechanisms governing the formation of ash aerosol arising from air and oxy-combustion of coal, biomass, and a coal/biomass blend were compared. The three fuels (Utah Sufco coal, rice husks aided by natural gas, and the coal/rice husk blend) were burned in a 100 kW down-fired oxy-fuel combustor (OFC). Two conditions were considered: 1) air combustion; 2) oxy-combustion with 70% O₂, 30% CO₂ in the oxidant gas (denoted as OXY70). The adiabatic flame temperatures for all three fuels were similar for air and for OXY70. The real-time iso-kinetically sampled ash aerosol particle size distributions were determined using electric mobility (SMPS) and light scattering (APS) techniques. Furthermore, the size segregated aerosols were sampled by a low pressure impactor (BLPI), and their compositions were determined by EDS. A novel coagulation model concerning the formation of submicron particles was applied here to explain the formation of accumulation modes for coal and rice husks cases. In this model, it is assumed that the coagulation of nucleus occurs inside a diffusion layer surrounding the burning char. For OXY70 conditions, the measured submicron aerosol particle size distribution was consistent with a coagulation model in which coagulation was complete within 1 radius from the coal particle surface but further at 3 radii from the rice husk particle surface. For air combustion this distance was reduced by 80% for both fuels. Aerosol size segregated compositions suggest that alkali metals can be significantly scavenged from ultrafine ash aerosol by coarse aluminosilicates particles under high temperatures.