Sulfur dioxide control

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). 

T. D. Chatwin and N. Kikumoto, eds., Sulfur Dioxide Control in Pyrometallurgy, Metallurgical Society of AIME, Warrendale, PA, 1981. 

Sulfur Dioxide Control. Andersen 2000 Inc., http //www.crownandersen.com/ Sulfur.html... 

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. 

Spray drying has become increasingly important in recent years as an alternative to wet scrubbing for sulfur dioxide control. In the spray dryer the sulfur-containing flue gas is contacted with a fine mist of an aqueous solution or a slurry of an alkali (typically Ca(0H)2 or soda ash). The sulfur dioxide is then absorbed in the water droplets and neutralized by the alkali. Simultaneously, the thermal energy of the gas evaporates the water in the droplets to produce a dry powdered product. After leaving the spray dryer the dry products, including the fly ash, are removed with collection equipment such as fabric filters or electrostatic precipitators. 

Coastal utilities have been major consumers of products derived from imported crudes. East coast utility fuels have been based on Venezuelan and Middle East crudes while the West coast has obtained much of its fuel from Indonesia. There are a number of reasons why it would be difficult to convert these plants to coal firing. Auxiliary facilities such as storage areas, rail sidings, and unloading and conveying equipment are no longer in place to handle coal. It is even more significant that the land on which these facilities were located has been sold or used for other utility purposes. As a result, scrubbers could not be installed at these sites to allow for sulfur dioxide control. 

As noted earlier, an accurate selection of sources requiring control can best be achieved through the application of the Air Quality Display Model for each AQCR of interest. It is estimated that the total one-time cost of such an exercise at a level of accuracy necessary to predict source sulfur dioxide control requirements for each of the 245 AQCR s would be about 10 million with an additional annual maintenance cost of 500 thousand to allow for emission inventory changes. For comparison. 

Babcock Wilcox. Nitrogen oxides control. Chapter 34, Sulfur dioxide control. Chapter 35. In Steam, Its Generation and Use, 40th Ed. The Babcock and Wilcox Company, 1992 35-1-35-15. 

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. 

The pollution control industry has made significant technical contributions to the sulfur dioxide control field. As we gain more experience, process reliability will increase and operating and investment costs will decrease. Customer acceptance will improve when it is realized that pollution control equipment permits greater flexibility in fuel selection and possibly reduced fuel expenses. 

Further work is needed to understand the role of water in the sulfation mechanism more fully and to extend the kinetic studies to the reduction and regeneration reactions outlined above. The potential advantages of a process using dolomite in a closed cycle for sulfur dioxide control are sufficiently great to warrant continued effort. 

The sulfur gas produced by burning coal can be partially removed with scrubbers or filters. In conventional coal plants, the most common form of sulfur dioxide control is through the use of scrubbers. To remove the SO2, the exhaust from a coal-fired power plant is passed through a mixture of lime or limestone and water, which absorbs the SO2 before the exhaust gas is released through the smokestack. Scrubbers can reduce sulfur emissions by up to 90%, but smaller particulates are less likely to be absorbed by the limestone and can pass out the smokestack into the atmosphere. In addition, scrubbers require more energy to operate, thus increasing the amount of coal that must be burned to power their operation. 

Kirchgessner, D.A., and Jozewicz, W., Enhancement of reactivity in surfactant-modified sorbents for sulfur dioxide control, Ind. Eng. Chem. Res., 28(4), 413-418 (1989). 

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. 

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