Final Report: Reactive Scrubbing for Mercury Removal and StabilizationEPA Contract Number: EPD05049
Title: Reactive Scrubbing for Mercury Removal and Stabilization
Investigators: Broderick, Thomas E.
Small Business: ADA Technologies Inc.
EPA Contact: Manager, SBIR Program
Project Period: April 1, 2005 through June 30, 2006
Project Amount: $225,000
RFA: Small Business Innovation Research (SBIR) - Phase II (2005) 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 4 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-patented 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. In the Phase I program, 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 size facilities. In one case study, a carbon injection system would cost 28 percent more than a reactive scrubber system, annually.
The reactive scrubbing system is a two-step process in which vapor-phase mercury is removed from the gas stream by contact with scrubber liquid containing an oxidant chemical. The reaction between elemental mercury and the oxidant forms mercuric oxide solids. The mercuric oxide fines are subsequently removed in a scrubber liquid blowdown bleed and transferred to a secondary reactor vessel, where the mercuric oxide is converted to mercuric sulfide, a highly insoluble solid form that has been shown to pass the Toxic 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.
In the Phase II program, ADA focused on three technical objectives and had a fourth objective to build a transportable reactive scrubber system to promote the demonstration and licensing of the technology to commercial clients. ADA also secured a commercial partner for marketing and constructing commercial reactive scrubber systems. Tri-Mer Corporation signed a licensing agreement for promoting the reactive scrubber technology. Together with Tri-Mer, ADA has developed a product specification document for commercial scrubber systems. The document incorporated many of the improvements investigated during the Phase II program. Tri-Mer is actively marketing the reactive scrubbing technology through environmental and pollution control journals. ADA continues to seek out companies with mercury emission issues that would benefit from the use of the reactive scrubber technology.
ADA’s first technical objective on the Phase II project addressed improvements to the original reactive scrubber design. The purpose of this objective was three-fold—to enhance the system’s functionality and reliability, to reduce the overall size of the system components, and to reduce the capital costs of a reactive scrubbing system. A laboratory-scale reactive scrubber was built and used extensively throughout the Phase II program for conducting scrubber tests. Phase I test results showed that the vapor-phase mercury reaction with the oxidant chemical is extremely fast and prompted an investigation into the use of venturi scrubbers as an alternate liquid/gas contactor. Additional tests were conducted with a hybrid system using a venturi in series with a shorter packed bed. In this configuration, the venturi removes the majority of the gas-phase mercury, and the packed bed acts as a mercury-polishing unit.
Mercury solids generated in the reactive scrubber are continuously removed from the scrubber liquid to maintain steady-state solids loading. Through early development with the commercial scrubber unit, ADA demonstrated that a hydrocyclone was the preferred method for removing mercury solids in the scrubber liquid. Tests conducted with surrogate solids slurries consisting of fine copper solids allowed ADA to generate performance curves for the hydrocyclone operated at different liquid pressures to determine the optimal operating conditions for the device.
Mercury solids separated from the scrubber liquid by the hydrocyclone are converted to mercuric sulfide in a secondary stabilization process. The second objective of the Phase II project advanced the state-of-the-art of the stabilization reaction process in terms of the reaction kinetics of the various chemical treatment steps, the chemistry mechanisms involved in the stabilization steps, and the stability of the mercury sulfide over a wide range of sulfide ion concentrations and pH environments. In another study, ADA assessed the long-term stability of the mercury sulfide solids using the advanced University of Cincinnati leaching protocol.
Early in the development of the reactive scrubber, the mercury capture efficiency for several oxidants was evaluated and sodium hypochlorite proved to be the most effective oxidant for this application. The ability to monitor and control hypochlorite concentration in the scrubber liquid, however, needed to be addressed so that the scrubber can be operated properly. As the third technical objective, ADA investigated two proprietary methods for monitoring oxidant level in the scrubber liquid. One of these methods was incorporated into the design of our pilot-scale scrubber system.
In the final objective, ADA designed and constructed a fully integrated pilot-scale reactive scrubber system for demonstrating the applicability of reactive scrubbing for prospective clients. The design of the demonstration reactive scrubber system was defined by the product specification document developed on this project and encompasses the enhancements developed during the Phase II project. The pilot-scale reactive scrubber system is constructed on a single transportable skid that can be mobilized to prospective client sites.
During the Phase I project, ADA demonstrated the ability of the reactive scrubbing technology to handle gases over a wide temperature range (ambient to 350ºF) and inlet mercury concentrations (150 to 2,000 µg/m3) typical for incinerator offgas. Tests showed that the scrubbing process is very efficient and easily reduces mercury emissions to less than 50 µg/m3. In fact, the reaction between hypochlorite and vapor-phase elemental mercury was found to be extremely fast and prompted ADA to investigate the use of a single-stage venturi as an alternate liquid-gas contactor in the Phase II program. Parametric tests showed that mercury removal performance for the venturi scrubber is strongly dependent on the liquid to gas (L/G) ratio. Venturi tests conducted at an L/G ratio of 35-pound liquid/pound of gas reduced gas-phase mercury levels by 80 percent. More impressive mercury reductions on the order of 90 percent were seen when the venturi was operated at an L/G of 70. Additional tests were conducted with a hybrid scrubber system using a venturi in series with a packed-bed column. In this configuration, the venturi can be operated at lower L/G ratios, which reduce the operational cost of this type of system. Mercury removal efficiencies for the hybrid system were greater than 95 percent when operating the venturi and packed-bed scrubbers at a reasonable L/G ratio of 11.
The ability to control solids loading in the scrubber liquid was accomplished using a hydrocyclone device. The advantages of using this type of solids separation device are its compact size, low equipment cost, and the ability to purchase these units constructed entirely of plastic materials that can tolerate the corrosive nature of the scrubber solution. Performance of the hydrocyclone was found to be a function of the liquid pressure drop through the device. Tests done with a surrogate slurry prepared with copper fines showed that the hydrocyclone was able to remove nearly 100 percent of the copper particles for particle size ranging from 40 microns down to 2.5 microns.
Oxidant level in liquids is typically determined using an Oxidation-Reduction Potential (ORP) probe. The response from an ORP probe, however, is not linear concerning oxidant concentration, making this approach unacceptable for controlling oxidant level in the reactive scrubber. For this project, ADA investigated two novel methods for measuring oxidant level in scrubber liquids. Details of these methods are proprietary and cannot be disclosed at this time. An exciting feature of these methods is that the response is linear with oxidant concentration allowing these methods to monitor and control the oxidant level in the scrubber liquid. The electrochemical method has been demonstrated to function as an online oxidant monitor and has been incorporated into ADA’s pilot-scale scrubber system. This technology has broad commercial applications, including oxidant manufacturing and oxidant monitoring for pools and spas.
The secondary stabilization process is an innovative process for treating the scrubber liquids from the reactive scrubber. In this process soluble and particulate forms of mercury in the scrubber liquid are converted to mercuric sulfide for disposal. The Phase II project provided the necessary funding to examine the treatment process in detail. Several changes were made to the protocol that resulted in using fewer chemicals and reduced the cost of the treatment process. An important study was undertaken on this project to understand the relationship that sulfide ion content and pH have on the formation of mercury polysulfide compounds. With this new knowledge, the treatment protocol was modified to ensure that the mercury sulfide produced in the treatment process is very stable and leach resistant. Leachable mercury results for the mercury sulfide material are consistently less than 10 ppb, using the TCLP, and meet the 25 ppb Universal Treatment Standard (UTS) for mercury. Long-term leaching tests with the mercury sulfide material also were conducted using the University of Cincinnati protocol. Results from these tests showed the mercury sulfide to be stable over a wide pH range of 4 to 12, with leachable mercury numbers at or below the 25 ppb UTS level.
The conclusions from the Phase II project are:
- Performance of the venturi gas-liquid contactor was found to be a function of L/G ratio and the inlet mercury concentration. The venturi was able to reduce mercury concentration below 50 µg/Nm3 for inlet mercury concentration ranging from 500 to 2,000 µg/Nm3 but high L/G ratios were required. A venturi operated at reasonable L/G ratios was able to reduce mercury emissions by 70 percent.
- A hybrid scrubber system was able to achieve a high degree of mercury capture when operated at relatively low L/G ratios. In this configuration, the venturi removes the majority of the gas-phase mercury, and the packed bed acts as a mercury-polishing unit.
- The hydrocyclone device was found to be very efficient for controlling mercury solids in the scrubber liquid. Particle capture efficiency was found to be a function of pressure drop through the unit and type of pump used to feed the hydrocyclone.
- ADA has developed two proprietary technologies to monitor and control oxidant concentration in the scrubber liquid. Both methods have a linear response as a function of oxidant concentration, a feature that is not possible with the standard ORP technology.
- Mercury solids produced by the enhanced secondary stabilization process were found to be very leach resistant, with less than 25 ppb leachable mercury.
- Mercury solids produced by the enhanced secondary stabilization process were subjected to long-term leaching tests per the University of Cincinnati protocol. The leachable mercury levels met the UTS standard for tests conducted at pH levels ranging from 4 to 12.