(333f) Particle Size Distributions Arising from Vaporized Components of Coal Combustion Fly Ash - a Comparison of Theory and Experiment

It has been found that the ash aerosol particle size distribution from pulverized coal combustion contains several modes. Those modes lying below ~0.6 microns have been designated “vaporization” modes, since they are formed by ash constituents that vaporized in the hot part of the flame and subsequently nucleated and coagulated both in the furnace and in the cooled, dilution sample probe that was used to collect them. The modes greater than ~0.6 microns are designated “fragmentation” modes, since they result from ash particles that are released during the coal char oxidation portion of the coal combustion process. This paper focusses on predictions of the “vaporization” ash particle size modes only, and attempts to compare the measured particle size distributions from a test furnace to those predicted by a sectional particle coagulation model. The prediction of PM₁ (that is particles with a diameter < 1 micron) from coal combustion is important because these particles are difficult to capture, they contribute significantly to heat transfer “fouling”, and they cause air pollution, visibility loss and human health effects.

In our work, a 100kW rated down-fired pilot scale combustor was used to compare coal ash aerosol formation under two atmospheres: 1) using 27% O₂ in the inlet oxidant gas (denoted as OXY27); 2) with 70% O₂ in the inlet oxidant gas (denoted as OXY70). Particle size distributions were obtained using a cooled dilution sampling probe, together with an on-line electric mobility/light scattering analyzers and low pressure impactors. The latter also allowed size segregated composition to be determined. Data showed that there were two vaporization modes with one mode below 0.1 micron and 1 mode (an accumulation mode) of around 0.3 micron. It was hypothesized that the former mode was caused by nucleation/coagulation in the sample probe and the latter by nucleation/coagulation inside the furnace. Theoretical calculations used a sectional form of the General Dynamic Equation that has been made solvable by a computer code, MAEROS2, previously described by Gelbhard and Seinfeld (1979). That code was modified to allow changes in gas specific volume during the process, and the number of ash nuclei was calculated from the mass of vaporized ash measured, and an estimate of the initial nuclei size. Initially, none of our calculations led to predicted accumulation modes greater than 0.1 micron, suggesting that our estimation of the initial number concentration was too low. Consideration of the compositions of the submicron particles, led to the hypothesis that the volume of gas in which coagulation occurred was probably very much smaller than that of the entire flue gas at a given length along the furnace. Rather, the volume of interest consisted of a film of gas immediately surrounding each burning char particle. Following this line of attack, it was possible to bring the theoretical predictions into line with experimental data. Subsequent modeling allowed both the temporal evolution of the ash aerosol in the furnace, and the nucleation and coagulation within the sample probe to be more accurately predicted. These results tended to support existing mechanisms involving chemical reduction, followed by vaporization and then subsequent oxidation, nucleation and coagulation in the particle neighborhood.