| Description |
Aerosol particulates are one of a variety of products generated by coal combustion. The objectives of this study were to elucidate ash particulate formation during oxy-fuel pulverized coal combustion compared to O2/N2 combustion. Oxy-fuel coal combustion conditions provide exhaust gas with a high concentration of CO2 versus CO2/N2 due to the recycled flue gas. Understanding fine particle formation is important for predicting emissions and understanding potential deposition. The hypothesis for CO2 affect the particulate formation is that this high CO2 concentration reduces the vaporization of refractory oxides from combustion according to the reaction: MOn(s) + CO(g) <---> CO2(g) +MOn-1(g). This research experimentally investigated particulate formation in a laboratory laminar flow drop tube furnace with well controlled combustion conditions under different flue gas scenarios. Ash particulate formation has been studied as a function of temperature, coal type, and gas phase conditions, namely, CO2 versus N2. Two bituminous coals, Utah Skyline and Illinois #6, and one sub-bituminous coal, Powder River Basin (PRB) black thunder were reported. During the experiments, the furnace temperature was set at 1373 K and 1500 K to study the effect of the combustion temperature on particle size distributions (PSDs). A single particle model was developed for coal char to predict char particle temperatures and to illustrate the temperature differences between conditions. A Scanning Mobility Particle Sizer (SMPS) and an Aerodynamic Particle Sizer (APS) were utilized for ash PSDs in size ranges between 14.3nm to 20 microns. In addition, particles were collected on an eleven-stage, Berner Low Pressure Impactor (BLPI) for elemental analysis using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). It is found that temperature was the driving mechanism for increasing the amount of ultrafine formation. Increases in predicted particle temperature yielded an increase in mass. As a second order effect, the combustion in an O2/CO2 environment yielded smaller masses of ultrafine particles than combustion in an O2/N2 environment. EDS results supported theSMPS/APS data as well and showed that, as expected, the coarse composition did not change significantly for the coals, but the ultrafine compositions were dependent upon the silicon, for Utah, and calcium, for PRB. |
