Stack gas desulfurization

Citric acid is used in soft drinks, candies, wines, desserts, jellies, jams, as an antioxidant in frozen fruits and vegetables, and as an emulsifier in cheese. As the most versatile food acidulant, citric acid accounts for about 70 percent of the total food acidulant market. It provides effervescence by combining the citric acid with a biocarbonate/carbonate source to form carbon dioxide. Citric acid and its salts are also used in blood anticoagulants to chelate calcium, block blood clotting, and buffer the blood. Citric acid is contained in various cosmetic products such as hair shampoos, rinses, lotions, creams, and toothpastes. More recently, citric acid has been used for metal cleaning, substituted for phosphate in detergents, for secondary oil recovery, and as a buffer/absorber in stack gas desulfurization. The use of sodium citrate in heavy-duty liquid laundry detergent formulations has resulted in a rapid increase in the use of citric acid. 

Adaptability. In the future the market for low sulfur fuel oil may become less attractive for several reasons. Among these are increasing use of nuclear energy, installation of stack gas-desulfurization processes, and increased availability of low sulfur fuels. Should this occur it would be desirable to put to alternative use any facility installed today to desulfurize fuel oil. At such a time, the market could revert to the traditional U.S. pattern in which high sulfur residual oil is a low value material, and there would be a consequent economic incentive to convert it to lighter products. 

Future Applications. If nuclear power becomes more prevelant, or if stack gas-desulfurization processes are brought to the point of economic application, the market for low sulfur fuel oil may shrink and the price will decrease. Under either of these circumstances we would revert to the classic U.S. position in which high sulfur vacuum residue is a marginal product and a candidate for conversion to lighter materials. 

TABLE 3.8 The Chemical Details of Examples of Stack Gas Desulfurization Processes"... 

Limestone (CaCC>3) dissolution is an important phenomenon in stack gas desulfurization processes using limestone slurry to absorb SC>2 and produce CaSC>3/CaS04 waste solids (1). The rate of dissolution directly determines the need for excess limestone and interacts strongly with SC>2 removal and scale-free operation in the absorber. There is a need to know the dependence of dissolution rates on both solution composition and the type and grind of limestone. This paper presents a mass transfer model and... 

Toprac, A.J., and Rochelle, G.T., "Limestone Dissolution in Stack Gas Desulfurization Processes - Effect of Type and Grind", presented at AIChE Annual Meeting, New Orleans, November 8-12, 1981, in press, Env. Prog. 

There is worldwide and increasingly the discharge of the sulfur dioxide with die use of fossil fuel. Especially, the discharge in the Asia region increases recently, but the environment countermeasure has been retarded for the problem of economical efficiency. Though in the advanced nation, the limestone gypsum method is mainly used as stack gas desulfurization facility, it is the method in which both cost of equipment and operational cost are expensive. 

Tamura, Z 1970, Stack Gas Desulfurization Method by Activated Carbon, paper pre.sent-ed at Second International Clean Air Congress of the International Union of Air Pollution Prevention Association, Washington, D.C., Dec. 6-11. 

More recently, citric acid is being used for metal cleaning, in detergents substituting phosphate, as plasticizers in its ester forms, for secondary oil recovery, and as a buffer/ absorber in stack gas desulfurization. The use... 

Battersea A pioneering flue-gas desulfurization process, operated at Battersea power station, London, from 1931 until the station was closed. The flue-gases were washed with water from the River Thames whose natural alkalinity was augmented by chalk slurry. One of the problems of this process was cooling of the stack gases, which caused the plume to descend on the neighborhood. 

The elemental sulfur is removed by conventional technology. The gases are purified by the Lurgi Rectisol process which uses a low temperature methanol wash to remove H2S, COS and CO2. The acid gas stream is then passed to a Stretford unit which is preferred to the Claus unit because of the high percentage of carbon dioxide in the stream. Sulfur in the stack gas would be removed by conventional flue gas desulfurization techniques and the sulfur would then remain as sulphite sludge and not be recovered as elemental sulfur. 

Thus, it is evident that as of the early 1980s, the panacea for stack gas treatment was yet to be realized. Difficulties in treating stack gases have provided incentives at the other end of the combustion cycle, namely, in the treatment of the fuels prior to combustion. Various means for treating coal are described in detail in the entry on Coal and desulfurization of petroleum fuels is described under Petroleum. 

Figure 21. Three locations for SCR in a power plant. B, boiler APH, air preheater ESP, electrostatic precipitator FGD, flue-gas desulfurization S, stack.

Muela, C.A., Menzies, W.R., Brna, T.G., "Stack Gas Reheat— Energy and Environmental Aspects," 1979, Proceedings Symposium on Flue Gas Desulfurization—Las Vegas, Nevada, March 1979, Vol. II, EPA-600/7-79-167b (NTIS PB 80133176), 1161-78. 

Absorption is a process that relies on a solvent s chemical affinity with a solute to dissolve preferably one species into another. It is widely proposed for CO2 separation where a solvent, generally, monoethanolamine (MEA) or a solid absorbent like lithium zirconate is used to dissolve CO2, but not the other components of a flue gas stream. C02-rich solution is typically pumped to a regeneration column, where CO2 is stripped out from the solution and the solvent recycled for a new batch of flue gas. The absorption equipment should be placed after the flue gas desulfurization-step and before the stack. Optimal conditions for absorption are low temperature and high pressure, making this the best location for absorption to occur. In addition, most solvents are easily degraded by compounds such as fly ash, other particulates, 80, (SO2, SO3) and (NO, NO2), so the absorption step must take place after electrostatic precipitation and desulfurization. In a typical absorption process, the C02-lean flue gas is either emitted to the atmosphere or possibly used in other applications e.g. chemical production). 

Sulfur dioxide represents a high percentage of the sulfur oxide pollutants generated in combustion. The removal of the sulfur dioxide from the combustion gases before they are released to the stack is essential, and a considerable number of procedures exist for flue gas desulfurization (FGD). These procedures may be classified as wet or dry (Mokhatab et al., 2006 Speight, 2007, 2008) depending on whether a water mixture is used to absorb the sulfur dioxide or whether the acceptor is dry. 

Flue gas desulfurization (FGD) Removal of the sulfur gases from the flue gases (stack gases) of a coal-fired boiler—typically using a high-calcium sorbent such as lime or limestone three primary types of flue gas desulfurization processes commonly used by utilities are wet scrubbers, dry scrubbers, and sorbent injection. 

Char produced from bituminous hvc coal contains too much sulfur to be used as pulverized fuel without sulfur removal procedures. Sulfur removal from the char seems preferable to stack gas cleanup. The desulfurization of char will not be discussed in this paper but a promising process has been developed. 

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