Advantage of Illinois Coal for FGD Removal of Mercury

Abstract

An investigation was conducted to characterize and modify mercury speciation in Illinois coal combustion flue gas so that a mercury control strategy can be implemented in conventional flue gas desulfurization (FGD) units. Mercury is readily volatilized during coal combustion and leaves the high-temperature zone as gas-phase elemental mercury (Hg⁰). As the flue gas is cooled, a portion of the Hg⁰ is converted, primarily to mercuric chloride (HgCl₂), in the presence of catalytically active surfaces. Unlike Hg⁰, HgCl₂ is highly soluble in water and has a high affinity for alkaline sorbents; it can, therefore, be easily removed in wet scrubbers and spray dryers (FGD units). This study specifically examined the effect of injection of Illinois coal combustion residues (ICCRs) on the conversion of Hg⁰ to HgCl₂ in coal combustion flue gases. Tests were conducted in two tasks. Task I studied Hg⁰ oxidation using a fixed-bed, bench-scale reactor in a simulated Illinois coal combustion environment. Various types of ICCRs were examined to determine active residues in the oxidation of Hg⁰. A scrubber sludge and two bottom ash samples were determined to be active Hg⁰ catalysts. The most active ICCR (a bottom ash sample) was chosen for Task II. Task II was performed in two subtasks. In Subtask I, the active bottom ash sample was injected in a pilot-scale combustor while burning natural gas doped with appropriate amounts of Hg⁰, hydrogen chloride (HCl), nitrogen oxides (NO_X), and sulfur dioxide (SO₂). Injection of this bottom ash sample did not contribute to the background gas-phase oxidation of Hg⁰, indicating that in-flight oxidation of Hg⁰ in the pilot-scale combustor may be mass transfer limited. In Subtask II, a representative Illinois coal was combusted in the pilot-scale combustor. As with most bituminous coals, the combustion flue gas of this coal was dominated by oxidized forms of mercury. Additional Hg⁰ was doped into this flue gas, and significant oxidation of Hg⁰ was observed. It was determined that the fly ash generated from combustion of this coal is very active in oxidation of Hg⁰. Additional pilot-scale tests were performed to investigate the activity of the Illinois coal fly ash. It was determined that packed-bed flow modes, such as those observed in baghouses, are required to catalytically oxidize Hg⁰ using this Illinois coal fly ash.