FINAL TECHNICAL REPORT

September 1, 1996, through August 31, 1997

Project Title: REMOVAL OF ILLINOIS COAL-BASED VOLATILE TRACE MERCURY

ICCI Project Number: 96-1/2.4A-1

Principal Investigator: Dr. Brian K. Gullett, U.S. EPA

Other Investigator: Dr. S. Behrooz Ghorishi, Acurex Environmental Corporation

Project Manager: Dr. Ronald Carty, ICCI

ABSTRACT

Mercury, present as traces in Illinois coal, is readily volatilized during coal combustion. Mercury is the most volatile among various trace metals, and major portions of it can pass through existing particulate control devices. A sorbent that can react with this metallic species can effectively shift the metal from the vapor phase to the particulate (sorbent) phase, facilitating its removal. Past research has identified activated carbons and calcium-based sorbents to be effective in reducing mercury emissions.

This research project examined the interaction of sorbents with mercury and investigated the effect of process conditions on its capture. This research was a continuation of an earlier effort for ICCI (ICCI project number: 95-1/2.4A-1). Tests were conducted in two tasks. Task I studied mercury capture in a bench-scale, fixed-bed reactor in a simulated coal combustion environment. Two types of sorbents (an activated carbon and calcium hydroxide), and capture conditions (bed temperature and mercury concentration) were examined in order to optimize reduction of mercury emissions. Of interest was the effect of sulfur dioxide (SO2, 1000 ppm range) and hydrogen chloride (HCl, 50 ppm range) on the capture of mercury species (elemental mercury and mercuric chloride) by the sorbents. The presence of HCl and SO2 in the simulated flue gas drastically enhanced the Hg0 capture capability of a thermally activated carbon. This enhancement might have been effected through reaction of HCl and SO2 with this sorbent and creation of active chlorine and sulfur sites.

In Task II, also in a bench-scale, fixed-bed reactor, residues produced during combustion of high sulfur Illinois coal in four different full-scale power plants were evaluated with respect to their mercury capture capabilities. Two different species of mercury, mercuric chloride (HgCl2) and elemental mercury (Hg0), were examined. This task explored the potential utilization of Illinois coal combustion residues as mercury species sorbents. Among the Illinois coal combustion residues, scrubber sludge (calcium sulfate type) samples exhibited considerable (40%-50%) HgCl2 removal capabilities. Statistical calculations revealed that this activity is related to the presence of calcium in these samples. A coal fly ash sample obtained from an Illinois coal power plant (designated as 3PF) was the only combustion residue sample that exhibited both Hg0 and HgCl2 removal capability. The characteristics of this sample resembled those of a mixture of a typical fly ash and calcium sulfate.

EXECUTIVE SUMMARY

The Air Pollution Prevention and Control Division (APPCD) of the United States Environmental Protection Agency (EPA) has conducted co-funded research in conjunction with the Illinois Clean Coal Institute (ICCI) to evaluate the reduction of volatile trace mercury by dry sorbent injection. Title III of the Clean Air Act Amendments (CAAA) places limitations on emissions of various air toxics. Among these, mercury present in Illinois coals, poses the most serious challenge to control technologies because of its high volatility. For coal-fired utilities, reduction of this metal is critical in complying with the emission standards set forth by the CAAA. Injection of dry sorbents under suitable conditions is a possible option for controlling mercury emissions. The research described here developed control strategies to reduce air toxics emissions in utilities using Illinois coals.

The objectives of the proposed research were to conduct bench-scale studies to identify cost-effective sorbents and examine effects of process parameters to develop optimum reaction conditions for removing mercury in a simulated combustion environment. Control technology experiences gathered at our laboratories and from Municipal Waste Combustors (MWCs) were used to identify sorbents and capture conditions. The parametric investigation conducted in our laboratories with these sorbents allows optimization of the process for mercury capture in coal combustors using Illinois coal. The research was performed over a twelve-month period and consisted of two Tasks.

Task I involved bench-scale testing of different sorbent types and capture conditions to identify optimal removal conditions for mercury from coal combustion flue gases. It was determined that the presence of acid gases (sulfur dioxide and hydrogen chloride) drastically enhance elemental mercury capture in a thermally activated carbon and significantly inhibit mercuric chloride capture in calcium hydroxide. Temperature and mercury concentration were two important sorption parameters. Combustion residues produced from burning high sulfur coals may have intrinsic properties capable of reducing mercury emissions. As part of this research in Task II, we examined mercury capture properties of different residue samples generated during combustion of high sulfur Illinois coal. Scrubber sludge (calcium sulfate type) samples exhibited considerable mercuric chloride removal capabilities. Statistical calculations revealed a strong positive correlation between the amount of calcium in the residue samples and mercuric chloride uptake. Higher percentages of calcium in these samples (such as scrubber sludge) led to a higher capture of mercuric chloride. On the other hand, the statistical calculations showed a strong negative correlation between mercuric chloride capture and the amount of aluminum, silicon, and iron in the residue samples. Higher percentages of these elements in the samples (such as fly ashes) led to a lower capture of mercuric chloride. Few combustion residue samples exhibited measurable elemental mercury capture. Statistical calculations, based on these limited results, indicated a strong positive correlation between concentration of bromine (Br) in the residue samples and elemental mercury capture. One may hypothesize that this capture can be effected through formation of Hg-Br bonds. Task II results may provide utility industries using Illinois coals strategies for reducing emissions of mercury, and means by which to utilize Illinois coal combustion residues.