Two subbituminous coals from the Powder River Basin were fired in a series of tests in a pilot-scale combustor to study the mechanisms governing the formation of fly ash and deposition under low-temperature fouling conditions. ASTM tests, chemical fractionation, and CCSEM with image analysis were used to determine the distribution of inorganic constituents in the two coals. The measured distribution of mineral matter in these coals was non-random in nature. Most of the pulverized coal particles (80% by area) contained no mineral grains larger than 1.6 {dollar}mu{dollar}m in diameter, while nearly 80% of the mineral area was contained in only 2% of the coal particles. Approximately 30% of the dominant mineral grains (quartz, aluminosilicate, and pyrite) were excluded from the coal matrix. Upon combustion, the organically associated elements (Ca, Mg, Al, and Fe) react with the included mineral grains to form silicate and aluminosilicate species. The organically associated elements in coal particles not containing mineral matter coalesce to form Ca-rich fly ash particles. An ash formation model was demonstrated that used the measured distribution of inorganic constituents to adequately predict both the size and composition distributions of the fly ash generated from the two low-rank coals.; The Ca-rich ash particles that deposit on the probe surface react with gas-phase SO{dollar}sb2{dollar} to form a layer of sulfate. No sulfation of calcium silicate or calcium aluminosilicate particles was observed. In situ emission FTIR spectroscopy was also used to monitor the deposition of sulfates, silica, and silicates on the probe surface as a function of time. The emission spectra suggested the presence of sulfate species as early as 5 minutes into the combustion tests.
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