Pre-Stripper Sulfite-Induced Coal Fly Ash Leaching – Part 1: Leaching of Iron(II) and Iron(III) as Monitored by Spectrophotometry

ABSTRACT

CO₂ capture (PCCC) process. However, sources of Fe for such degradations are poorly characterized. One aspect of the PCCC process which is poorly researched is the interaction of sulfur dioxide (SO₂) gas in the flue gas (FG) with coal fly ash (CFA) and how this can affect amine degradation. Understanding that the SO₂ would be dominantly present in the anionic pH speciated forms (bisulfite (HSO₃^–_(aq)) and sulfite (SO₃^2–_(aq))) as a result of caustic scrubbing or amine-based desulfurization and that ammonia is commonly present either in original FG or as an amine degradation product, CFA leaching of Fe by ammonium sulfite ((NH₄)₂SO₃) was experimentally examined under different pH (~ 4 to 10) and temperature (50 °C to 90 °C) conditions to simulate various pre-stripper sections within an amine solvent based SO₂ and carbon dioxide (CO₂) capturing process such as at the Boundary Dam Unit 3 carbon capture and storage (BD3 CCS) facility. The results suggest that SO₂ and its pH-speciated aqueous forms can leach Fe, and thereby catalyze oxidative degradation of common amines used in capturing acidic contaminants from post-combustion FG. With the addition of (NH₄)₂SO₃ (0.25 mol ⋅ L^–1), the effect of SO₃^2– on the leaching of iron(III) (Fe³⁺) and iron(II) (Fe²⁺) was apparent and distinctly different from the effect of pH per se. Thus, irrespective of the amine type, SO₂ and its pH speciated forms can generate Fe ions to catalyze the degradation. Still, SO₃^2–-induced Fe leaching occurred more dominantly in acidic solutions that simulate the pre-scrubber and amine-based SO₂ capture conditions (pH ≤ 6) and less in alkaline aqueous conditions simulating amine-based CO₂ capture (i.e., pH of ~ 8 to 10). The work does not seem to support an effect of SO₂ on CFA leaching of Fe under pre-stripper conditions as the principal factor in catalyzing oxidative degradation of amines used in CO₂ capture, even though the association with pH depression could still make it significant. At pH = 8, the total leached Fe ions averaged only ≤ 0.2 mg ⋅ L^–1 and ≤ 1.0 mg ⋅ L^–1 from CFA obtained from flue gas cooler inlet and the caustic polisher in the SO₂ loop, respectively. Yet, in an integrated amine solvent SO₂ and CO₂ capturing process as employed at BD3 CCS, the effect is expected to be more significant with respect to amine degradation within the SO₂ capture process.

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