Pre-Stripper Sulfite-Induced Coal Fly Ash Leaching – Part 2: Leaching of Iron and Other Transition Metals as Monitored by Inductively Coupled Plasma Mass Spectrometry

ABSTRACT

This is Part 2 of a series of investigations on understanding the impact SO₂ in flue gas (FG) has on an integrated amine solvent based sulfur dioxide (SO₂) and carbon dioxide (CO₂) capturing process such as practiced at the Boundary Dam Unit 3 carbon capture and storage plant. It follows the Part 1 article on the effect of SO₂ on coal fly ash (CFA) leaching of iron and extends it to other transition metals (TMs) that may potentially cause oxidative degradation of amines used in the process. The results confirm the findings in Part 1 that, apart from pH on its own, SO₂ and its pH-speciated aqueous forms affect the leaching of TMs and potentially catalyze degradation of amines used to capture acidic contaminants from post-combustion FG. The sulfite-induced leaching of TMs occurred primarily under acidic conditions that simulate pre-scrubber and amine-based SO₂ capture conditions (pH ≤ 6). This occurred either exclusively (e.g., for iron) under acidic conditions, or additionally also in alkaline solutions – substantially for copper when at 50 °C, but only moderately for vanadium and titanium (and copper when at 90 °C). The alkaline leachates that represent conditions of the post-combustion CO₂ capture process are as follows for a caustic chimney tray fly ash (CCTF) sample: copper (Cu) > iron (Fe) > vanadium (V) > titanium (Ti) > nickel (Ni) > chromium (Cr) > manganese (Mn); and for a flue gas cooler inlet fly ash (FGCIF) sample: V > Fe > Mn > Ti > Cu > Ni > Cr. However, TMs leached more under the acidic conditions (pH 4 to 6) that better represent the SO₂ capture process, and the leaching order is as follows for CCTF: Fe > Cu > V > Ti > Mn > Cr > Ni; and for FGCIF: Fe > Mn > Ti > V > Ni > Cu > Cr.

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