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