Final Report: Reactive Scrubbing for Mercury Removal and StabilizationEPA Contract Number: EPD04011
Title: Reactive Scrubbing for Mercury Removal and Stabilization
Investigators: Broderick, Thomas E.
Small Business: ADA Technologies Inc.
EPA Contact: Manager, SBIR Program
Project Period: March 1, 2004 through August 31, 2004
Project Amount: $69,986
RFA: Small Business Innovation Research (SBIR) - Phase I (2004) RFA Text | Recipients Lists
Research Category: SBIR - Waste , Hazardous Waste/Remediation , Small Business Innovation Research (SBIR)
The current state-of-the-art mercury control technology for medical and hazardous waste incinerators is the adsorption of mercury onto activated carbon, either by injecting powdered activated carbon into the flue duct upstream of a particulate control device, or by passing the flue gas through carbon beds. For the past 3 years, ADA Technologies, Inc. (ADA) has been engineering a system using wet gas scrubbing as an alternative method for controlling mercury emissions from point sources such as kilns and incinerators. ADA calls this new patent-pending mercury control technology “reactive scrubbing.” Besides being a highly efficient process, this alternate technology concentrates the captured mercury as granular mercuric sulfide, a feature not possible with carbon-based sorbents.
As part of Phase I, ADA estimated the costs for implementing a reactive scrubbing system and found that reactive scrubbing is very cost competitive with an activated carbon system for certain sized facilities. In one case study, a carbon injection system would cost 28 percent more than a reactive scrubber system on an annual basis.
The reactive scrubbing system is a two-step process in which vapor-phase mercury is removed from the gas stream by an oxidizing scrubber liquor forming mercuric oxide solids. The mercuric oxide fines subsequently are removed in a scrubber liquid blowdown bleed, and transferred to a secondary reactor vessel in which the mercuric oxide is converted to mercuric sulfide, a highly insoluble solid form that has been shown to pass the Toxicity Characteristic Leaching Procedure (TCLP) test for leachable mercury. The reactive scrubbing principles have been used for the treatment of an ambient temperature (85°F) industrial process gas stream saturated with mercury vapor. A commercial reactive scrubber was sold by ADA to an industrial client and now is in commercial service at the client’s facility in Nevada.
The goal of this research project was to broaden the application of ADA’s reactive mercury scrubbing technology to address treatment of hot gas streams from hazardous and medical waste incinerators. This new application for the reactive scrubbing technology presents a new set of challenges because gas streams from incineration sources typically are at an elevated temperature and have lower mercury concentrations compared to those conditions previously tested. Gas temperatures exiting a hazardous waste incinerator facility can vary considerably, ranging from 120 to 340°F. Mercury concentration in the gas stream also is highly variable, depending greatly on the type of waste materials handled by the facility. The U.S. Environmental Protection Agency published final rules in 1997 that established emissions limits for nine pollutants, including mercury, for medical waste incinerators at 550 μg/sdscm. Mercury emissions standards for existing and new hazardous waste incinerators are even more stringent, at 130 μg/sdscm and 45 μg/sdscm, respectively. ADA has demonstrated the principles of reactive scrubber on gases with mercury concentrations as low as 150 μg/Nm3 with impressive results. These results illustrate the wide range of operation for the reactive scrubbing technology.
During Phase I, ADA focused on optimizing each of the two steps of the reactive scrubber system as it applies to incinerator exhaust gas: (1) the removal of mercury from the gas stream in a packed-column scrubber, and (2) the subsequent conversion of the captured mercury to mercuric sulfide. At the start of this research project, ADA designed and built a pilot-scale packed bed reactive scrubber column for laboratory testing. Parameters of interest investigated with the pilot-scale scrubber were liquid-to-gas ratio in the scrubber, the concentration of reagent in the scrubber solution, temperature of the gas stream, mercury removal as a function of packing height, and the composition of the feed gas to the scrubber. The effect of each of these variables was examined in terms of mercury removal efficiency and reagent consumption. Tests showed that the reactive scrubber technology is a very efficient process, capable of reducing mercury concentrations to nondetectable levels within the first foot of packing. These results strongly suggest that it is possible to improve the design of the reactive scrubbing system using smaller equipment, which can result in a significant cost savings. The temperature of the gas stream did not affect the mercury capture ability of the technology, but it did increase reagent consumption by thermally degrading the active chemical in the scrubber solution. The presence of carbon monoxide and hydrogen chloride did not adversely affect scrubber performance or increase reagent consumption.
In the second step of the reactive scrubber system, scrubber liquids generated during the scrubber tests were treated with a series of chemicals to stabilize both soluble and insoluble mercury fractions in the scrubber solution. Mercury in the liquids was converted to the very stable mercuric sulfide form. Three treatment methods were tested during Phase I. One of these methods reduced the amount of soluble mercury in the liquid from 18,000 ppb to less than 10 ppb. Solids from the treated solutions were separated from the liquids and subjected to the standard TCLP. In all cases, the mercury solids had less than 100 ppb leachable mercury, meeting the mercury TCLP limit of 200 ppb.
The final Phase I activity examined the costs associated with reactive scrubbing for mercury control. In this task, a credible cost estimate for a reactive scrubber system designed for a commercial-scale incinerator was prepared. ADA contracted CH2M HILL to develop a reactive scrubber system cost model used to project capital equipment costs, operating and maintenance costs, chemical costs, and utilities for two sizes of reactive scrubbing systems; 1,500 acfm and 5,000 acfm. The cost model then was used to estimate the cost of implementing the reactive scrubber technology at two operating incineration facilities.