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Soot, organics and ultrafine ash from air- and oxy-fired coal combustion
Andersen, M., N. Modak, C. Winterrowd, Chun Wai Lee, W. Roberts, J. Wendt, AND Bill Linak. Soot, organics and ultrafine ash from air- and oxy-fired coal combustion. Proceedings of the Combustion Institute. Elsevier B.V., Amsterdam, Netherlands, 36(3):4029-4037, (2017). https://doi.org/10.1016/j.proci.2016.08.073
LOI, OC, and EC analyses of size classified fly ash samples from air- and oxy-combustion of a bituminous coal in a 10 W entrained flow furnace operated at realistic stoichiometric ratios indicate that although the total fly ash carbon was always acceptably low (<2%), the ultrafine fly ash (<0.6 um) from both air- and oxy-combustion consists largely of carbonaceous material. This observation is contrary to many previous studies that focused primarily on the vaporization, nucleation, and growth of volatile and semivolatile metals. The effect of oxy-combustion conditions is first to increase the ultrafine carbonaceous material and then to decrease the ultrafine carbonaceous material, as the percentage of inlet O2 is increased and CO2 decreased. Much of the LOI present in the ultrafine ash consists of OC, believed to contain significant amounts of oxygenated hydrocarbons. During oxy-coal combustion, changes in both the ultrafine OC and the EC can be explained by the competing effects of CO2 and O2 on the flame stand-off distance and soot oxidation, respectively. CO does appear to track the observed LOI trends of the ultrafine fly ash (<0.6 µm). However, CO does not follow the LOI trends of the bulk ash. Comparison of the OC data between air and oxy combustion suggest that any factor that affects flame attachment and delayed ignition may also affect the emissions of hazardous organic air pollutants. Coal type may be an important factor for organic HAP emissions, requiring further study.
This paper is concerned with determining the effects of oxy-combustion of coal on the composition of the ultrafine fly ash. To this end, a 10 W externally heated entrained flow furnace was modified to allow the combustion of pulverized coal in flames under practically relevant stoichiometric ratios (SR = 1.2-1.4), but with the ability to maintain constant residence times (2.3 s). Experiments were conducted using a pulverized bituminous coal under air-combustion and three oxy-combustion inlet oxygen conditions (28, 32, and 36%). Size-classified fly ash samples were collected, and measurements focused on the composition of the total and ultrafine (<0.6 µm) fly ash produced, in particular the soot, elemental carbon (EC), and organic carbon (OC) fractions. Results indicate that although the total fly ash carbon, as measured by loss on ignition, was always acceptably low (<2%) with all three oxy-combustion conditions lower than air-combustion, the ultrafine fly ash for both air-fired and oxy-fired combustion conditions consists primarily of carbonaceous material (50-95%). Subsequent analyses of the carbonaceous component on particles <0.6 µm by a thermal optical method showed that large fractions (52-93%) consisted of OC rather than EC, as expected. This observation was supported by thermogravimetric analysis indicating that for the air, 28% oxy, and 32% oxy conditions, 14-71% of this material may be OC volatilizing between 100 and 550 °C with the remaining 29-86% being EC/soot. However, for the 36% oxy condition, OC may comprise over 90% of the ultrafine carbon with a much smaller EC/soot contribution. These data were interpreted by considering the effects of oxy-combustion on flame attachment, ignition delay, and soot oxidation of a bituminous coal, and the effects of these processes on OC and EC emissions. The results presented here carry practical implications for electric utility particulate emissions and black carbon emission inventories for air combustion because submicron particles are collected with reduced efficiency in electrostatic precipitators commonly used by the industry. These results also suggest that flame attachment and ignition delay may be important parameters that affect emissions of organic hazardous air pollutants (HAPs) from a bituminous coal. Further study of organic HAP emissions from sub-bituminous coal is important to understand the effect of coal type on these emissions.
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