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 100 kW 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 2.4 μm 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.

Relationship between submicron ash aerosol characteristics and ash deposit compositions and formation rates during air- and oxy-coal combustion

“…The focus is on effects of changes of coal composition, and of oxidant composition on the characteristics and composition of the fouling deposits formed, and on the ash aerosol formed during the air-and oxy-firing of pulverized coal. In contrast to previous work [4,5], special efforts were made here to relate both the size segregated ash aerosol compositions and the ash aerosol particle size distributions not only to the compositions of the deposit layer, but also to the rates of deposition of the sticky deposit layer. It is hoped the results can be used in simulations requiring predictions of deposition rates and deposit qualities under air and oxy combustion conditions.…”

“…The outer deposits are too loose to allow reproducible rate data to be obtained for practical coal combustion situations. Second, the methodology used to obtain the data to be presented is briefly recapped, although details can be found elsewhere [4,5]. Third, the effects of air and oxy combustion of various coals on the composition of deposits and on the size segregated composition of the ash aerosol are explored.…”

“…However, it is hoped that deposition rates in the well defined laminar regime of this apparatus can be extrapolated to the turbulent aerodynamic regime of practical units through simulation and mathematical modelling. A more complete description is provided by Zhan et al [4,5].…”

“…The deposit probe has been described in detail by Zhan et al [4,5]. The coupon on which ash deposits is of stainless steel, and has a wall temperature that is controlled through an adjustable air flow and a thermocouple placed on the coupon wall.…”

“…An important feature of this work is to relate the ash aerosol to the ash deposits. The ash aerosol was sampled through Port 9 using an isokinetic dilution [8] and additional dilution as described in previous papers [4,5]. A preseparator cyclone removed the larger particles upstream of a Berner low pressure impactor (BLPI) which, with EDX, was used to obtain the size segregated composition of the as aerosol.…”