September 1, 1995, through August 31, 1996

Project Title: REMOVAL OF COAL-BASED VOLATILE TRACE ELEMENTS:  MERCURY AND SELENIUM

ICCI Project Number: 95-1/2.4A-1

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

Other Investigators: Dr. S. Behrooz Ghorishi and Dr. K. Raghunathan, Acurex Environmental Corporation

Project Manager: Dr. Ken Ho, ICCI

ABSTRACT

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

This research project examined the interaction of sorbents with mercury and selenium and investigated the effect of process conditions on their capture. This research was a continuation of an earlier effort for ICCI (ICCI project number: 94-1/2.4A-1). Tests were conducted in three tasks: Task I studied mercury capture in a bench-scale fixed-bed reactor in a simulated combustion environment. Various types of sorbents, and capture conditions were examined in order to optimize reduction of mercury emissions. Of interest was the effect of SO₂ and temperature on the capture of mercury by sorbents. Two different classes of sorbents were used; activated carbons and calcium-based sorbents. In task II, also in a bench-scale fixed-bed reactor, fly ash produced during combustion of high sulfur Illinois coal was evaluated with respect to its mercury reducing capabilities. Two different species of mercury, mercuric chloride (HgCl₂) and elemental mercury (Hg), were examined. This task, performed as a preliminary investigation, explored the potential utilization of Illinois coal combustion residues as mercury sorbents. Task III investigated the in-flight capture of mercury and selenium in a pilot-scale combustion of Illinois coal. This task examined the high temperature/short gas-phase residence time removal of mercury and selenium using dry injection of calcium-based sorbent (lime).

EXECUTIVE SUMMARY

The Air Pollution Prevention and Control Division (APPCD) of the United States Environmental Protection Agency (EPA) has proposed co-funded research in conjunction with the Illinois Clean Coal Institute (ICCI) to evaluate the reduction of volatile trace elements, especially mercury and selenium, by dry sorbent injection. Title III of the Clean Air Act Amendments (CAAA) places limitations on emissions of various air toxics. Among these, mercury and selenium, both present in Illinois coals, pose the most serious challenge to control technologies because of their high volatility. For coal-fired utilities, reduction of these metals 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 and selenium emissions. Since both these metals are present in Illinois coals, the proposed research will provide control strategies for utilities using Illinois coals in reducing air toxics emissions.

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 and selenium in a simulated combustion environment. For mercury, 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 will potentially allow the optimization of the process for mercury capture in coal combustors using Illinois coal. For the high temperature/in-flight mercury and selenium removal, the proposed research tested the sorbent already in use for acid rain control technology (lime) and quantified its capability in capturing mercury and selenium.

This research project was performed over a twelve-month period and consisted of three tasks: Task I involved bench-scale testing of different sorbent types and capture conditions to identify optimal removal conditions for mercury from flue gases. Fly ash 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 reducing properties of fly ash generated from burning high sulfur coal. Task II results will potentially provide utility industries using Illinois coals strategies for reducing emissions of mercury, and means by which utilize Illinois coal combustion residues. In task III, the combined interaction of mercury and selenium with calcium-based sorbent (lime) in a high temperature/short gas-phase residence time environment was investigated.