Desulfurization characteristics of limestone slurry with added organic acid

Abstract

In this study, a desulfurization experiment was performed in a bubbling reactor to investigate the effects of adding organic acids, such as formic, acetic, propionic, lactic, oxalic and adipic acid, to limestone slurries. The limestone slurries added these organic acids were referred to as FAS, AAS, PAS, LAS, OAS, and ADS, respectively. Flue-gas desulfurization characteristics varied with the acidity of organic acid additives. The T99 of slurries increased with pKa of the organic acids because slurry pH was high in the early phase of absorption which increased the time to reach the slurry pH range (4.1–4.6) required for the T99 end. During SO2 absorption, the sulfate ion fraction was ≥0.5 in PAS, AAS, and ADS, with pKa > 4, but <0.5 in LAS and FAS with pKa < 4. PAS, AAS, and ADS supported the oxidation of sulfite ions, whereas LAS and FAS suppressed it. [H+] × [A−] increases with decreasing pKa of organic acids. Therefore, [H+]slurry and [A−]slurry in the slurry may be engaged in the oxidation of sulfite ions; consequently, increasing [H+] × [A−] inhibited the oxidation of sulfite ions. The sulfate ion fraction affected gypsum dehydration following completion of SO2 absorption. After drying at 60 °C, LAS and FAS (sulfate ion fraction < 0.5) formed only gypsum, whereas PAS, AAS, and ADS (sulfate ion fraction ≥ 0.5) formed both gypsum and anhydrite. Accelerated oxidation of sulfite ions with sulfate ion fraction ≥ 0.5 during slurry drying after reaction completion produced a sulfuric acid solution with a high sulfate ion concentration, which reduced the temperature of gypsum dehydration.