Wet Scrubber System

EPA Grant Number: R827649C005
Subproject: this is subproject number 005 , established and managed by the Center Director under grant R827649
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: Center for Air Toxic Metals® (CATM®)
Center Director: Groenewold, Gerald
Title: Wet Scrubber System
Investigators: Mann, Michael D. , Evenstad, Dean N. , Galbreath, Kevin C. , Toman, Donald L. , Zygarlicke, Christopher J.
Institution: University of North Dakota
EPA Project Officer: Chung, Serena
Project Period: October 15, 1999 through October 14, 2002
Project Amount: Refer to main center abstract for funding details.
RFA: Center for Air Toxic Metals (CATM) (1998) RFA Text |  Recipients Lists
Research Category: Targeted Research

Objective:

One of the primary goals of CATM is establishing a fundamental understanding of the fate of air toxic metal emission generated by the combustion of coal or other alternative fuels and the development of emission control strategies. The EERC has constructed and currently uses the CEPS as a well-controlled system for researching the impacts of toxic metal species formation resulting from combustion. The addition of a wet scrubber to the CEPS will allow CATM to test the effect that a wet FGD system has on emission control and metal transformations with regard to key elements like mercury, chlorine, and fluorine. In addition, the availability of a wet scrubber system will allow novel methods aimed at enhancing mercury removal in a FGD system to be tested.

The goal of the proposed project is to develop a further understanding of the potential to capture toxic trace metal species by wet-scrubbing the flue gas generated by the combustion of selected coals. Trace metal species resulting from the introduction of sorbent chemical reagent material for sulfur capture will also be evaluated. Specific objectives necessary to attain this goal include the following: 1) design a wet scrubber system for flue gas sulfur dioxide capture to incorporate into the overall emission control system for the CEPS; 2) procure, construct, and shake down the wet scrubber system components; and 3) conduct limited testing to identify and quantify the concentrations of trace metal species introduced and captured utilizing a wet scrubber system as part of an overall flue gas cleanup system.

Approach:

The current status of wet scrubber systems being used by the energy industry and utilities for burning coal or other alternative fuels was evaluated to allow design of an appropriate system. The system was selected, designed, and installed to allow representative chemical reactions to occur. Special consideration was given to the wall effects and other impacts related to the small size of the scrubber, and measures to counteract these effects were taken. The system was designed to be incorporated into the existing CEPS system.

Progress

A wet limestone scrubber has been designed for installation on the CEPS. During the design effort, careful consideration was given to methods to ensure that good gas-phase mass transfer would be obtained. A spray tower was chosen since it represents the most widely used FGD system. The proposed design consists of a 3-inch-diameter absorber tower with two stages (see figure below). A narrow angle full-cone spray nozzle will be used to introduce the absorbing solution into each stage. The absorber tower sump will also serve as the absorber recirculation tank. This tank will be used to adjust the chemistry of the absorbent prior to circulating it through the tower and/or bubbling reactor. Control of the chemistry is also critical for simulating a full- scale scrubber, and provisions have been made to accommodate chemistry control in the absorber recirculation tank.

Therefore, a second mode of contacting is proposed. After flowing countercurrent to flue gas, the absorbent would drain into a second contacting vessel equipped with spargers similar to some of the advanced FGD units (Pure Air or Chiyoda) that utilize bubbling reactors rather than spray towers. The flue gas would be routed such that it could flow through the spargers to initiate contact with the absorbent in the bubbling reactor and then flow to the spray tower or to bypass the bubbling reactor and flow directly to the spray tower. The absorbent, likewise, would be piped so that it could pass through both the spray tower and the bubbling reactor or the bubbling reactor alone. This would allow operation of the spray tower and bubbling reactors either alone or in series. This allows both contacting methods to be investigated. In addition, should the spray tower prove to be inefficient at removing SO2 because of poor mass transfer caused by wall effects in the absorber tower, the second bubbling reactor absorber can be used to ensure SO2 levels comparable to industry could be obtained.

The wet scrubber is designed to operate over a fairly wide range of conditions. A gas velocity of 4 to 8 ft/sec should be achievable with the L/G ranging from 25 to 200 gal/l000 acfm. Either forced, natural or inhibited oxidation should be achievable. Other variables that can be controlled include chemistry (pH, [Cal, [Mg], [Cl], etc.), and flue gas injection temperature. The inlet gas concentrations can also be adjusted to investigate the impacts of a specific component on scrubber control through spiking of the CEPS flue gas.

Rationale:

A current critical issue regarding mercury control is the upstream mercury species, because conventional wet scrubbers are currently under consideration as a possible method for gaseous mercury emission control. Gas-liquid contact time, gas diffusivity, and interfacial surface area are the limiting factors. Factors to increase or decrease these effects are already well characterized for commercial systems. The potential exists for reduced capture if mercury concentrations are highly elevated because of recycling of streams within the process. However, little if any of the mercury in gaseous, elemental, insoluble form is captured, even in the presence of aggressive oxidizing agents. Field data from nine process configurations compiled through DOE's project Phase I Assessment of Toxic Emissions from Coal-Fired Power Plants showed emission factors for mercury as high as 11 lb/l012 Btu, even for systems utilizing advanced wet scrubbers. Percent penetration for mercury ranged from less than 25% to 50% for systems with wet scrubbers. Current information, based on using the latest mercury speciation methods to sample at several full-scale scrubbers, suggests that wet scrubbers effectively capture almost all of the soluble oxidized mercury (e.g., HgC12) and capture little if any of the elemental mercury. Therefore, if wet scrubbers are to be considered an effective control method, the mercury must be in oxidized form before it reaches the scrubber. Several research projects are currently under way outside of the EERC to convert all of the mercury to oxidized form to facilitate better mercury control with scrubbers. This demonstrates the need to understand and be able to predict the mercury transformations that occur in the upstream combustion system.

A related issue regarding the transformation of mercury in scrubber systems is, what happens to the oxidized mercury that is captured? Although oxidized mercury capture in scrubber system components may meet an immediate need for reduced air emissions, the critical question is whether another problem is created. The final fate of the mercury is not complete until the stability of mercury in disposed or used scrubber products has been determined. At present, it is not yet known whether the oxidized mercury is chemically or thermally stable in the disposed or reused scrubber products. Most of the oxidized mercury forms captured are volatile, especially mercuric chloride, and thus appear to be highly susceptible to mobilization into the environment through desorption into the atmosphere or leaching into surface or groundwater. Several modes of transfer into the environment could occur upon direct disposal during wastewater and sludge treatment before disposal or during processing of commercial scrubber by-products such as gypsum. The potential for growing concern may exist about the fate of mercury captured from coal-fired power plant stack emissions as wet scrubbers are discussed in terms of mercury capture. The final fate of mercury in the environment is an important question for any mercury control method. These issues must be resolved if mercury capture in wet scrubbers is to become a long-term solution.

Supplemental Keywords:

RFA, Scientific Discipline, Air, Toxics, Waste, Chemical Engineering, air toxics, Environmental Chemistry, Chemistry, HAPS, Incineration/Combustion, Engineering, EPCRA, 33/50, Engineering, Chemistry, & Physics, Environmental Engineering, ambient air quality, emission control strategies, trace metal emissions, Chlorine, control, mercury, emission control technologies, air pollution control, flue gas, Sulfur dioxide, metallic emissions, analytical chemistry, airborne metals, mercury & mercury compounds, Mercury Compounds, air scrubbers, wet scrubber system, trace metals, flue gases

Progress and Final Reports:

  • 2000
  • 2001
  • Final

  • Main Center Abstract and Reports:

    R827649    Center for Air Toxic Metals® (CATM®)

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R827649C001 Development And Demonstration Of Trace Metals Database
    R827649C002 Nickel Speciation Of Residual Oil Ash
    R827649C003 Atmospheric Deposition: Air Toxics At Lake Superior
    R827649C004 Novel Approaches For Prevention And Control For Trace Metals
    R827649C005 Wet Scrubber System
    R827649C006 Technology Commercialization And Education
    R827649C007 Development Of Speciation And Sampling Tools For Mercury In Flue Gas
    R827649C008 Process Impacts On Trace Element Speciation
    R827649C009 Mercury Transformations in Coal Combustion Flue Gas
    R827649C010 Nickel, Chromium, and Arsenic Speciation of Ambient Particulate Matter in the Vicinity of an Oil-Fired Utility Boiler
    R827649C011 Transition Metal Speciation of Fossil Fuel Combustion Flue Gases
    R827649C012 Fundamental Study of the Impact of SCR on Mercury Speciation
    R827649C013 Development of Mercury Sampling and Analytical Techniques
    R827649C014 Longer-Term Testing of Continuous Mercury Monitors
    R827649C015 Long-Term Mercury Monitoring at North Dakota Power Plants
    R827649C016 Development of a Laser Absorption Continuous Mercury Monitor
    R827649C017 Development of Mercury Control Technologies
    R827649C018 Developing SCR Technology Options for Mercury Oxidation in Western Fuels
    R827649C019 Modeling Mercury Speciation in Coal Combustion Systems
    R827649C020 Stability of Mercury in Coal Combustion By-Products and Sorbents
    R827649C021 Mercury in Alternative Fuels
    R827649C022 Studies of Mercury Metabolism and Selenium Physiology