Sulfur dioxide control system

Emissions control systems play an important role at most coal-fired power plants. For example, PC-fired plants sited in the United States require some type of sulfur dioxide control system to meet the regulations set forth in the Clean Air Act Amendments of 1990, unless the boiler bums low sulfur coal or benefits from offsets from other highly controlled boilers within a given utiUty system. Flue-gas desulfurization (FGD) is most commonly accomphshed by the appHcation of either dry- or wet-limestone systems. Wet FGD systems, also referred to as wet scmbbers, are the most effective solution for large faciUties. Modem scmbbers can typically produce a saleable waUboard-quaUty gypsum as a by-product of the SO2 control process (see SULFURREMOVAL AND RECOVERY). 

Incineration of the tail gas and conversion of all sulfur compounds to sulfur dioxide, followed by one of the sulfur dioxide control systems. 

The third class of control systems may use any of the sulfur dioxide control systems among those used commercially are the Haldor Topsoe (5, 80), Wellman-Lord (27, 07), and Chiyoda (27, 07, 87) systems. The circumstances are generally highly favorable for recovery processes that produce a stream of concentrated sulfur dioxide, since this can be recycled to the Claus plant. The application of processes that produce sulfuric acid or solid wastes will be dictated only by peculiar local circumstances. 

Qince the first large sulfur dioxide control system was installed at the Battersea plant in London, it has taken almost 50 yrs for calcium-based scrubbing technology to become commercially acceptable. In 1926, the 125 MW coal-fired Battersea power plant was equipped with a spray packed tower and final alkaline wash section which removed more than 90% of the sulfur dioxide and particulate (I). Thames River water provided most of the alkali for absorption, and about 20% was made up from lime addition. The process operated in an open-loop manner, returning spent reagent to the Thames. 

For the next 20 yrs no full-scale development work was performed in this area. In fact, during the mid-sixties, there were several steps backward when initial U.S. sulfur dioxide control systems started up and failed. For example, in the boiler injection of limestone followed by wet scrubbing, problems resulted from boiler and preheater pluggage rather than flue gas scrubbing. 

Since the utility industry represents the major market for sulfur dioxide control systems, it was necessary to develop a simple system which would not require a lot of attention, be inexpensive to operate, have moderate capital requirements, and not take effort away from their power producing function. Calcium-based scrubbing processes meet all of these requirements. In addition, the calcium reagents are inexpensive and form relatively insoluble reaction products which can be disposed of in sanitary landfills and slurry ponds. 

Satriana (2) provides a summary of the development of flue gas treatment technology. The first commercial application of flue gas scrubbing for sulfur dioxide control was at the Battersea-A Power Station [228 MW(e)] in London, England, in 1933. The process used a packed spray tower with a tail-end alkaline wash to remove 90 percent of the sulfur dioxide and particulates. Alkaline water from the Thames River provided most of the alkali for absorption. The scrubber effluent was discharged back into the Thames River after oxidation and settling. A similar process was also operated at the Battersea-B Power Station [245 MW(e)] beginning in 1949. The Battersea-B system operated successfully until 1969, when desulfurization efforts were suspended due to adverse effects on Thames River water quality. The Battersea-A system continued until 1975, when the station was closed. 

Huang, H., Allen, J.W., and Livengood, C. D., 1988, Combined Nitrogen Oxides/Sulfur Dioxide Control in a Spray-Dryer/Fabric-Filter System, ANL/ESD TM-8, Argonne National Laboratory, Argonne, IL, November. 

Pollutants. The problems posed by ak pollutants are very serious. Within a museum, measures can be taken to remove harmful substances as efficiently as possible by means of the installation of appropriate filter systems in the ventilation equipment. Proposed specification values for museum climate-control systems requke filtering systems having an efficiency for particulate removal in the dioctyl phthalate test of 60—80%. Systems must be able to limit both sulfur dioxide and nitrogen dioxide concentrations <10 /ig/m, and ozone to <2 /ig/m. ... 

Formation of emissions from fluidised-bed combustion is considerably different from that associated with grate-fired systems. Flyash generation is a design parameter, and typically >90% of all soHds are removed from the system as flyash. SO2 and HCl are controlled by reactions with calcium in the bed, where the lime-stone fed to the bed first calcines to CaO and CO2, and then the lime reacts with sulfur dioxide and oxygen, or with hydrogen chloride, to form calcium sulfate and calcium chloride, respectively. SO2 and HCl capture rates of 70—90% are readily achieved with fluidi2ed beds. The limestone in the bed plus the very low combustion temperatures inhibit conversion of fuel N to NO. ... 

J. Lanier and co-workers, "Sulfur Dioxide and Nitrogen Oxide Emissions Control in a Coal-Eked MHD System," ASME Winter Annual Meeting Adanta, Ga., Dec. 1979. 

J. B. Rosenbaum and co-workers. Sulfur Dioxide Emission Control hy Hydrogen Sulfide Eeaction inMqueous Solution—The Citrate System, PB221914/5, National Technical Information Service, Spriagfield, Va., 1973. 

Sulfur dioxide reduction to achieve required emission levels may be accomplished by switching to lower-sulfur fuels. Use of low-sulfur coal or oil, or even biomass such as wood residue as a fuel, may be less expensive than installing an SO2 control system after the process. This is particularly true in the wood products industry, where wood residue is often available at a relatively low cost. 

TTte most cost-effective methods of reducing emissions of NO are the use of low-NO burners and the use of low nitrogen fuels such as natural gas. Natural gas has the added advantage of emitting almost no particulate matter or sulfur dioxide when used as fuel. Other cost-effective approaches to emissions control include combustion modifications. These can reduce NO emissions by up to 50% at reasonable cost. Flue gas treatment systems can achieve greater emissions reductions, but at a much higher cost. 

Lime kilns dust emissions are controlled by baghouse systems. Kiln fuels can be selected to reduce SO, emissions however, this is not normally a problem, since most of the sulfur dioxide that is formed is absorbed in the kiln. 

One way to control gaseous pollutants like SO2 and SO3 is to remove the gases from fuel exhaust systems by absorption into a liquid solution or by adsorption onto a solid material. Absorption involves dissolving the gas in a liquid while adsorption is a surface phenomenon. In each case, a subsequent chemical reaction can occur to further trap the pollutant. Lime and limestone are two solid materials that effectively attract sulfur dioxide gas to their surfaces. The ensuing chemical reaction converts the gaseous pollutant to a solid nontoxic substance that can be collected and disposed or used in another industry. 

Most industrial operations today now have pollution control systems, like the one shown in this chemical plant, to reduce the levels of sulfur dioxide and oxides of nitrogen released to the atmosphere. (Maximilian Stock Ltd./Photo Researchers, Inc.)... 

When this is done, the calcium hydroxide is included in the melting furnace slag, and the unspent calcium carbide is either used or oxidized in the melting furnace. Little testing has been done to determine the actual fate of the sulfur. Most of it may be included in the slag, but it may also be emitted to the air as sulfur dioxide, or, for foundries with wet emission control systems, it may be dissolved in the water. 

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