12 Wet Scrubbing of Sulfur Oxides from
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Flue Gases M. R. GOGINENI, W. C. TAYLOR, A. L. PLUMLEY, and JAMES JONAKIN Combustion Engineering, Inc., Windsor, Conn. 06095
The controlling chemical reaction for the lime/limestone wet scrubbing SO removal systems is calcium sulfite plus sulfur dioxide to form calcium bisulfite. Methods of preventing both calcium sulfite and calcium sulfate scaling are available. The marble bed is a good vapor-liquid contactor with a tray efficiency of 90-95%. The reaction tanks were designed using rate constants determined by Combustion Engineering, Inc. Full-scale C-E air pollution control systems that have been installed on utility steam generators consist of five furnace injection and three tail-end systems. Although many uses for the waste products produced in the air pollution control systems are being studied, land-fill and impounding are currently being used for waste disposal. 2
*"phe need for removing S 0 from stack gases of oil- and coal-fired combustion equipment has been and is being emphasized by the stringent emission limitations established by the Environmental Protection Agency and other government agencies. The EPA standards are 1.2 lbs/10 Btu for coal and 0.8 lb/10 Btu for oil. More stringent requirements in certain districts have been passed. An example is Clark County, Nev. which limits S 0 emissions for a 1,500,000 kW steam-generating unit to 0.15 lb/10 Btu or one-eighth of the EPA requirement. To meet these requirements a high percent of S 0 must be removed from stack gases produced from the combustion of most oil and coal fuels. Figure 1 is a nomograph for determining the percent of S 0 removal required for different fuels to meet a given standard. Even for the so-called low sulfur fuels it can be seen that in many cases S 0 removal systems will be required to meet emission standards. 2
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135 In Pollution Control and Energy Needs; Jimeson, Robert M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
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POLLUTION CONTROL AND ENERGY NEEDS
There are several ways to classify processes for the removal of S 0 from stack gases: wet or dry, recovery or non-recovery, and absorption, adsorption, or catalytic oxidation. Except for catalytic oxidation, each of these has been described by others in this volume. In reviewing these processes and many others, Combustion Engineering (C-E) decided that wet hme/limestone scrubbing without recovery of sulfur was worth developing because of its simplicity and low cost.
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REQUIRED REMOVAL, EMISSION RATES % , TO MEET EPA LB/MBTU STANDARDS COAL OIL
Figure 1.
Percent of S0 removal required for different fuels 2
The development of C-Es air pollution control system started in 1964 with the construction of a small pilot facility in our laboratories. A second pilot application on a Detroit Edison Co. unit in 1966 and 1967 followed. Table I lists the full-scale installations (1) that have been sold to date by C-E. In a move to accelerate further development, a large laboratory prototype unit of 12,500 cfm capacity was constructed and began operations in early 1970 (2). The controlling chemical reactions for the lime/limestone wet scrubbing S 0 removal systems have been established. In both the lime and limestone systems, the principal absorption reaction is calcium sulfite plus sulfur dioxide to form calcium bisulfite. Methods of preventing both calcium sulfite and calcium sulfate scaling are presented. 2
In Pollution Control and Energy Needs; Jimeson, Robert M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
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GOGINENI ET AL.
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Wet Scrubbing of Sulfur Oxides
Chemistry of S0 Removal System 2
The possible reactions taking place within the wet lime/limestone S 0 removal system have been studied. Our conclusions relative to the controlling reactions are based on consideration of theoretical equations in light of operating experience in both field and pilot systems. Because there is considerable difference in operating conditions required to provide adequate S 0 removal in the absence of scale or deposit formation when utilizing lime or limestone as additives, the chemical reactions of the systems are treated separately. The essential reactions governing these systems are: 2
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Calcium hydroxide or lime system reactions CaO + H 0 +± Ca(OH) 2
(1)
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Ca(OH) + S 0
