Speciation of Arsenic in Feed Coals and Their Ash Byproducts from Canadian Power Plants Burning Sub-bituminous and Bituminous Coals

Abstract

The arsenic species in the feed coals and ash byproducts from seven Canadian power plants (including one with a fluidized-bed combustor) that were burning local sub-bituminous and bituminous coals with sulfur contents in the range of 0.30−3.5 wt % have been examined using As X-ray absorption fine structure (XAFS) spectroscopy. The feed coals can be grouped based on their contents of arsenic associated with pyrite (As/pyr) and as As3+ and As5+ (arsenate) species. The arsenic species in sub-bituminous feed coals with low sulfur (0.22−0.38 wt %) and arsenic (1.6−2.2 mg/kg) contents consist of ∼50% As3+ and ∼50% As5+, whereas those with moderate sulfur (0.50 wt %) and arsenic (3.63 mg/kg) contents consist of 84% As/pyr, 7% As3+, and 9% As5+. In bituminous feed coal with low sulfur (0.40 wt %) and arsenic (4.39 mg/kg) contents, the arsenic speciation consists of 34% As/pyr, 12% As3+, and 54% As5+, and for those with high sulfur (2.60−3.56 wt %) and arsenic (54−84 mg/kg) contents, it consists of 77%−82% As/pyr and 18%−23% As5+. The bottom ash produced from sub-bituminous feed coals with low sulfur and arsenic contents consists of 10%−20% As3+ and 80%−90% As5+, and for moderate sulfur (0.50 wt %) and arsenic (3.63 mg/kg), the arsenic speciation consists of 5% As/pyr, 10% As3+, and 85% As5+ as arsenate. For bituminous feed coals with low sulfur and arsenic contents, the bottom ash is entirely As5+, whereas for coals with high sulfur and arsenic contents, the bottom ash consists of 10%−15% As3+ and 85%−90% As5+; and for the fluidized-bed combustor, the bottom ash is entirely As5+ arsenate. The species of arsenic in fly ash from sub-bituminous and bituminous coals are mostly arsenate (As5+), possibly in part incorporated in the glass matrix, and remains the same for coarse- and fine-grained electrostatic precipitator (ESP), baghouse, and stack-emitted ashes. The only difference between the ESP and baghouse fly ash is the higher amount of crystalline arsenates in the hopper fly ash. Neither the sulfur content nor the pyrite content of the feed coal seems to influence the speciation of arsenic, because virtually all of the arsenic in fly ash samples from high-sulfur coal is in the form of arsenate (As5+). However, arsenic (mostly as As5+) in these fly ashes is found to be very surface-enriched, because the amount measured by XPS decreases from >3 wt % to <0.8 wt % in the first few atomic layers. The presence of stable calcium or transition-metal iron hydroxyl arsenate hydrate [(M2+)2Fe3(AsO4)3(OH)4·10H2O] complexes, as determined by X-ray diffractometry, in the fly ash produced from high-sulfur/pyrite feed coals indicates that some of the arsenic might be captured by calcium and iron compounds.