Investigation of Mechanisms for the Formation of Ash Deposits for Two Powder River Basin Coals

Abstract

Two subbituminous coals from the Powder River Basin were fired in a pilot-scale combustor to study ash deposit formation under low-temperature fouling conditions. The two coals were chosen as they are similar in composition, but behaved differently when fired in a utility boiler. This study focuses on the mechanisms governing the formation of the initial deposit layer and is limited to deposition times of approximately 3 hours.

Both the elemental composition and phase composition of the fly ash and deposits were examined to gain insight into the deposition process. The dominant phase found in the fly ash from both coals was a calcium-rich phase formed from organically bound Ca, Fe, Al, and Mg in the coals. SEM analyses of both the surface and cross sections of fly ash samples showed little evidence for sulfation during the approximately 1.5s residence time in the furnace. However, ASTM analyses of the deposits showed significant sulfur enrichment. X-ray maps of the deposit cross sections revealed that the Ca-rich fly ash particles contained rings of sulfur, evidence of in situ sulfation. None of the silicate or aluminosilicate ash particles incorporated in the deposit showed evidence of sulfation, suggesting that the dominant mechanism for sulfation was the reaction of gas-phase sulfur species with the Ca-rich fly ash particles, and not condensation of sulfate species. However, a thin layer of sodium sulfate was present on the surface of the probe, probably due to condensation of sodium hydroxide followed by reaction with gas phase sulfur containing species to form the sulfate.

The particle capture efficiencies were similar for both coals, but varied significantly with temperature. However, the deposits formed from Coal A contained a larger fraction of the Ca-rich ash particles and had a higher concentration of sulfur than the Coal B deposits. The increased propensity of the Coal A deposits to undergo sulfation is consistent with the observation that Coal A caused more severe deposition problems when fired in a utility boiler.