Comparison of Particle Size Distributions and Elemental Partitioning from the Combustion of Pulverized Coal and Residual Fuel Oil

Abstract

U.S. Environmental Protection Agency (EPA) research examining the characteristics of primary PM generated by the combustion of fossil fuels is being conducted in efforts to help determine mechanisms controlling associated adverse health effects. Transition metals are of particular interest, due to the results of studies that have shown cardiopulmonary damage associated with exposure to these elements and their presence in coal and residual fuel oils. Further, elemental speciation may influence this toxicity, as some species are significantly more water-soluble, and potentially more bio-available, than others. This paper presents results of experimental efforts in which three coals and a residual fuel oil were combusted in three different systems simulating process and utility boilers. Particle size distributions (PSDs) were determined using atmospheric and low-pressure impaction as well as electrical mobility, time-of-flight, and light-scattering techniques. Size-classified PM samples from this study are also being utilized by colleagues for animal instillation experiments. Experimental results on the mass and compositions of particles between 0.03 and > 20 microns in aerodynamic diameter show that PM from the combustion of these fuels produces distinctive bimodal and trimodal PSDs, with a fine mode dominated by vaporization, nucleation, and growth processes. Depending on the fuel and combustion equipment, the coarse mode is composed primarily of unburned carbon char and associated inherent trace elements (fuel oil) and fragments of inorganic (largely calcium-alumino-silicate) fly ash including trace elements (coal). The three coals also produced a central mode between 0.8- and 2.0-micron aerodynamic diameter. However, the origins of these particles are less clear because vapor-to-particle growth processes are unlikely to produce particles this large. Possible mechanisms include the liberation of micron-scale mineral inclusions during char fragmentation and burnout and indicates that refractory transition metals can contribute to PM < 2.5 microns without passing through a vapor phase. When burned most efficiently, the residual fuel oil produces a PSD composed almost exclusively of an ultrafine mode (approximately 0.1 micron). The transition metals associated with these emissions are composed of water-soluble metal sulfates. In contrast, the transition metals associated with coal combustion are not significantly enriched in PM < 2.5 microns and are significantly less soluble, likely because of their association with the mineral constituents. These results may have implications regarding health effects associated with exposure to these particles.

Evaluation of elemental carbon as a marker for diesel particulate matter

“…The ability of molecular marker source apportionment models to distinguish diesel exhaust from biomass smoke, meat smoke, cigarette smoke, coal fly ash, and residual oil fly ash is independent of their EC composition since these sources can be distinguished by their organic compounds composition (Schauer et al, 1996;Schauer and Cass, 2000) and by their trace metals composition (Huffman et al, 2000;Linak and Miller, 2000). However, if estimates of diesel particulate matter are sought from EC measurements alone, then the contribution of EC from these other combustion sources must be properly addressed to avoid incorrectly attributing their elemental carbon emissions to diesel particulate matter.…”

Combustion Aerosols: Factors Governing Their Size and Composition and Implications to Human Health

“…162,163 When the residual oil is burned more efficiently under conditions typical of a utility boiler, the carbon content is lower and the PM is almost entirely in the ultrafine (condensation) mode. 164 The transition metals in residual oil combustion ash are in the form of sulfates rather than sulfides or oxides.…”

On trimodal particle size distributions in fly ash from pulverized-coal combustion

“…refractorylined vertical tube. Details are described elsewhere [15]. Pulverized coal (4 kg/h) was metered from a screw feeder and carried by transport air through a fuel injector into the combustor.…”