Process for the enhanced capture of heavy metal emissions

Details Process for the enhanced capture of heavy metal emissions Process for the enhanced capture of heavy metal emissions

1998-04-08

2001-06-19

Wong, Edna (Department: 1741)

Chemistry: electrical and wave energy

Processes and products

Processes of treating materials by wave energy

C204S157150, C204S157300, C204S157510, C075S670000, C588S256000

Reexamination Certificate

active

06248217

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to preventing air emissions of heavy metal species e.g., mercury (“Hg”), from a variety of different sources (e.g., combustion sources, coal combustion, incineration, chemical process industry and others). More specifically, it relates to using ultraviolet (“UV”) radiation to oxidize Hg, typically in the elemental form in exhausts, and furthermore, in the vapor phase, to its ionic forms which are less volatile (such as oxides or chlorides). In addition, the present invention relates to the absorbing, adsorbing or entrapping a substance with a sorbent material. In another aspect of the invention, the process may be used to form a sorbent-metal complex by employing a sorbent precursor and then irradiating with ultraviolet radiation to oxidize the complex which enhances capture and binding of the mercury species.
BACKGROUND OF THE INVENTION
Toxic and heavy metals are one of the most problematic classes of contaminants due to their ubiquity and toxicity. Heavy metals represent a significant source of pollution when released into the environment. They are present in fossil fuels and ores, for example, and are released into the environment via airborne emissions during industrial processing of these materials, e.g., during incineration, or leach into soils and groundwater from ash and other residues when these materials are landfilled. Heavy metals from all sources present a major environmental concern.
Heavy metals are present in fossil fuels such as coal, oil and natural gas, in biomass, in ores and in wastes. Heavy metals are volatilized in the hot regions of process units such as boilers, incinerators or furnaces used for waste disposal, energy generation or metal recovery. Subsequently, as the gases are cooled, less volatile metal species (e.g., cadmium and lead) condense onto particles of ash entrained in the gas stream, while more volatile metals (e.g., arsenic and mercury) remain in the gas phase, where they end up as airborne emissions.
Heavy metals include, for example, arsenic, beryllium, lead, cadmium, chromium, nickel, zinc, mercury and barium. Most of these metals are highly toxic to humans and animals. Metal-contaminated wastes often also contain organic contaminants. Thus, treatment technologies for treating wastes contaminated with toxic metals preferably should be effective for treating organic waste as well.
Toxic metals may enter a combustion system in many physical and chemical forms, for example, as constituents of a hazardous or municipal solid waste to be incinerated or as trace quantities in coal. Once introduced into a combustion environment, a metal may undergo transformations to different phases as well as to different chemical species depending upon combustion conditions and the presence of chlorine and other reactive species. Also, at combustion temperatures, metals may be vaporized and then undergo nucleation to form a submicron aerosol, or the metal vapor may condense onto existing particles. These resulting primary particles, formed by nucleation or condensation, have been observed to have a diameter of approximately 0.02 Fm. Through growth by condensation of vapor or by coagulation with other particles, these particles ultimately may have a diameter of from about 0.02 Fm to about 1.0 Fm in the flue gas. For example, in one study on hospital waste incineration, a bimodal distribution in the flue gas was observed, and the particles having a diameter between about 0.1 Fm and 0.2 Fm accounted for 7% to 74% of the lead, 62%-77% of the cadmium, and 20%-80% of the zinc in the total particulate phase. (Kauppinen, E. l. and Pakkanen, T. A., “Mass and Trace Element Size Distributions of Aerosols Emitted by a Hospital Refuse Incinerator”,
Atmos. Environ.,
24A, 423 (1990).
Unfortunately, flue gas cleaning equipment used in combustion systems is least efficient in capturing particles having diameters in the submicrometer size range. For example, electrostatic precipitators are used in many coal-fired combusters and typically exhibit the lowest collection efficiency for particles less than 1 Fm in diameter. Particles in these size ranges potentially pose a greater health threat than larger particles since they penetrate deeper into the lungs where the toxic materials come into contact with the blood. This potential adverse health impact of metal emissions from combustion devices is an appropriate incentive to investigate new methods and technologies for metal removal from waste gas streams. Furthermore, the United States Environmental Protection Agency (US EPA) has begun to regulate toxic metal emissions from combusters pursuant to Title III of the 1990 Clean Air Act Amendments which specifically lists eleven metals and their compounds as air toxics.
In an attempt to control such toxic metal emissions, researchers have proposed several control methods using various bulk solid sorbents to chemically adsorb various metals thereby reducing their discharge in particulate form into the atmosphere.
One such method includes combusting a metal contaminated waste in a fluidized bed of sorbent. (Ho, T., Chen, J., Hopper, J. and Oberacker, D., “Metal Capture During Fluidized Bed Incineration of Wastes Contaminated with Lead Chloride”,
Combust. Sci. and Technol.,
85, 101 (1992)). Other proposed methods include injecting a sorbent into the high temperature region of a combustion device (Scotto, M., Petersen, T. and Wendt, J., “Hazardous Waste Incineration: The In-Situ Capture of Lead by Sorbents in a Laboratory Down-Flow Combuster”, 24th
International/Symposium on Combustion,
the University of Sydney, Sydney, Australia (1992), and passing a metal vapor at high temperatures through a packed bed of sorbent (Uberoi, M. and Schadman, F., “High Temperature Removal of Cadmium Compounds Using Solid Sorbents”,
Environ. Sci. Technol.,
25, 7, 1285 (1991); and Uberoi, M. and Schadman, F., “Sorbents for Removal of Lead Compounds from Hot Flue Gases”,
AlChE Journal,
36, 2, 307 (1990)). However, none of these methods adequately control the emission of mercury.
Mercury emissions from combustion sources have been a great concern (Chu, P. and Porcella, D. B.
Water, Air, Soil Pollut.
1995, 80, 135-144; Krishnan, S. V., Gullett, B. K. and Jozewicz, W.
Environ. Sci. Technol.
1994, 2(Y((Y), 1506-1512). Unlike most other heavy metals that are emitted in particulate forms, mercury has been reported to be released mainly in the elemental form in the vapor phase. Data for waste incinerators show the fraction from 10% to 90% depending on the waste composition and operating conditions (Lindqvist, O.
Waste Management
&
Research.
1986, 4, 35-44; Bergstrom, J. G. T.
Waste Management
&
Research,
1986, 4, 57-64; Reimann, D. O.
Waste Management
&
Research,
1986, 4, 45-56; Hall, B., Schager, P. and Lindqvist, O.
Water, Air Soil Pollut.,
1991, 56, 3-14; Livengood, C. D., Huang, H. S., Mendelsohn, M. H. and Wu, J. M.
PETC=s
10
th Annual Coal Precapture, Utilization and Environmental Control Contractors Conf.,
July 1994). Data for coal-fired power plants show a higher fraction, in some cases even over 95% (Meij, R.
Water, Air, Soil Pollut.
1991, 56, 21-33; Larjava, K., Laitinen, T., Vahlman, T., ArtMann, S., Siemens, V., Broekaert, J. A. C. and Klockow, D.
Intern. J Environ. Anal. Chem.,
1992, 49, 73-85; Morency, J. R.
PETC=s
10
th Annual Coal Precapture, Utilization and Environmental Control Contractors Conference,
July 1994, Vogg, H., Braun, H., Metzger, M. and Schneider, J.
Waste Management
&
Research,
1986, 4, 65-74). Vapor phase elemental mercury is negligibly captured in typical air pollution control devices. Once emitted into the atmosphere, mercury may undergo various biological processes in the atmosphere to form even more toxic mercury species such as methyl mercury. It may also bioconcentrate in vegetation and fish. The consumption of these produce and fish leads to adverse health effects in human beings and predator animals (Seigneur, C., Wrobel, J. and Constantinou, E.
Environ. Sci. Technol.,
199

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