Production development and utilization of Zimmer Station wet FGD by-products. Final report. Volume 4, A laboratory study conducted in fulfillment of Phase 2, Objective 1 titled. Inhibition of acid production in coal refuse amended with calcium sulfite and calcium sulfate - containing FGD solids

Abstract

Control of S02 emission from coal combustion requires desulfurization of coal before its combustion to produce coal refuse. Alternatively, gaseous emissions from coal combustion may be scrubbed to yield flue gas desulfurization (FGD) by-products that include calcium sulfite (CaSO3∙0.5H2O or simply CaS03). Acid production in coal refuse due to pyrite oxidation and disposal of large amounts of FGD can cause environmental degradation. Addition of CaS03 and CaS03-containing FGD to coal refuse may reduce the amounts of oxygen and ferric ion available to oxidize pyrite because the sulfite moiety in CaS03 is a strong reductant and thus may mitigate acid production in coal refuse. In Chapter 1, it was shown that CaS03 efficiently scavenged dissolved oxygen and ferric ion in water under the conditions commonly encountered in a coal refuse disposal environment. In the presence ofCaS03, the concentration of dissolved oxygen in water exposed to the atmosphere declined to below 0.01 mg L"1 at pH <8.0. In Chapter 2, it was demonstrated that CaS03 prevented a pH drop in coal refuse slurry when 0.2 gCaS03 was added to a 2% fresh coal refuse slurry every three days. Calcium sulfite also inhibited acid leaching from fresh coal refuse in bench-scale columns undermore » controlled conditions. During the initial 13 weeks of leaching, the total amounts of titratable acidity, soluble H\ Fe, and Al from CaS03-treated refuse (6.4 gin 50 g fresh coal refuse) were only 26%,10%, 32%, and 39% of those of the control columns, respectively. A combination of CaS03 with CaC03 or fly ash enhanced the inhibitory effect of CaS03 on acid leaching. Calcium sulfite-containing FGD which combined CaS03, CaC03, fly ash, and gypsum showed a much stronger inhibitory effect on acid leaching than CaS03 alone. This combination effect was partially due to the positive interaction of CaS03 with CaC03 and fly ash on inhibition of acid leaching. In Chapter 3, CaS03-containing FGD was found to inhibit acid leaching from both fresh and aged coal refuse in large scale columns under simulated field conditions. During 39 weeks of leaching, the reduction of leachate acidity and Fe concentration and the increase ofleachate pH were significant (p <0.05) for the 22% FGD treatment with a linear response to increasing FGD rates (0%, 5.5%, 11%, and 22%). I conclude that CaS03 and CaS03-containing FGD have the ability to inhibit acid production in coal refuse and the inhibitory effect shown in this experiment is likely to occur under field conditions. Thus, the research results present a potential new method for mitigation of acid production in coal refuse and another beneficial utilization of FGD by-products.« less