Ash aerosol formation from oxy-coal combustion and its relation to ash deposit chemistry

Abstract

Ash aerosol and ash deposit formation during oxy-coal combustion were explored through experiments in a self-sustained 100kW rated down-fired oxy-fuel combustor. Inlet oxidant conditions consisted of 50% inlet oxygen with CO2 (hereafter denoted as OXY50 conditions). A Berner low pressure impactor (BLPI), a scanning mobility particle sizer (SMPS), and an aerodynamic particle sizer (APS) were used to obtain size segregated ash aerosol samples and to determine the particle size distributions (PSD). A novel surface temperature controlled ash deposition probe system that allowed inside and outside deposits to be separated was used to collect the ash deposits. The ash aerosol PSDs given by the BLPI and those produced by SMPS/APS were consistent with each other. Data suggested that oxy-coal combustion under these conditions did not change the formation mechanisms controlling the bulk ash aerosol composition, but it did increase the formation of ultra-fine particles initially formed through metal vaporization, due to increased vaporization of silicon at the higher combustion temperature. The smaller particles contained within the deposits had higher Si and lower Na and S concentrations under OXY50 conditions than for air combustion. Moreover, the ash aerosol composition for particle sizes less than 2.4μm was related to the composition of the inside deposits. A higher Na in the ash aerosol resulted in higher Na in inside deposits with comparable absolute Na concentrations in both those aerosol particles and those inside deposits particles. The contribution of S and Si to the inside deposits showed that S in the vaporization modes together with Si in the ultrafine vaporization mode, contributed significantly to the composition of the inside deposits. These results provided direct evidence that prediction of the chemistry of the initial deposit layer (but not of the bulk deposits) required knowledge of the size segregated chemistry of the ash aerosol.