Calcium sulfite system

Sulfur dioxide can be removed from power plant exhaust gas by a scrubber s tem. One common method involves the reaction of SO2 with calcium oxide (lime) to form calcium sulfite S02(g) + CaO( ) CaS03 ( ) Unfortunately, scrubber systems are expensive to operate, and the solid CaS03 is generated in large enough quantities to create significant disposal problems. 

However, the largest mass mean particle size reported thus far for the CaS03 l/2H20 system is only about 32 microns. The objectives of the present work were to demonstrate a method of achieving improved calcium sulfite particle size (by use of the DDO configuration) while showing the industrial practicality of this crystallizer configuration. The particular industrial case... 

Lime Reactor The efflnent from the EDV system is pumped to an agitated reactor vessel where the EDV efflnent is reacted with lime to form calcium sulfite and active sodium species via the following reactions ... 

The other type of salt precipitating in the column is calcium based. Elimination or reduction of calcium ions in the liquor is critical if the temperature in the system drops. A larger amount of precipitate was observed in the pilot plant when overnight temperature dropped to about 60°F. Most of these salts returned to solution after the system was reheated to operating temperatures. This relationship between the temperature and precipitation must be taken into consideration in the design and operation of a full-scale plant. As indicated by the analysis shown in Table V, these salts are believed to be primarily calcium sulfite and sulfate. 

The goal of this research was to improve activity coefficient prediction, and hence, equilibrium calculations in flue gas desulfurization (FGD) processes of both low and high ionic strength. A data base and methods were developed to use the local composition model by Chen et al. (MIT/Aspen Technology). The model was used to predict solubilities in various multicomponent systems for gypsum, magnesium sulfite, calcium sulfite, calcium carbonate, and magnesium carbonate SCU vapor pressure over sulfite/ bisulfite solutions and, C02 vapor pressure over car-bonate/bicarbonate solutions. 

The LCM was accurate within 20 percent error in SO- vapor pressure for the 50°C data, as well as data at 35, 70, and 90°C. The solid line shows the general trend of the LCM predictions. Figure 4 plots the apparent equilibrium constant as a function of ionic strength for a calcium sulfite/bisulfite system at 25, 50, and 60 C... 

The controlling chemical reactions for the lime/limestone wet scrubbing SO2 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. 

Following the initial steps of hydration (Reaction 1) and formation of calcium sulfite (Reaction 2), removal of SO2 in the lime or calcium hydroxide system depends on the formation of calcium bisulfite by reaction of suspended calcium sulfite with sulfur dioxide and water (Reaction 3). 

The control of sulfite scaling requires that a minimum amount of free hydroxide ion be recirculated to the scrubber therefore, fresh additive (lime or slaked lime) is added in the reaction tank external to the scrubber where calcium sulfite is formed (Reaction 4). An amount of calcium sulfite equivalent to the SO2 removed (or the fresh Ca(OH)2 added) is conveyed from the system to a pond or vacuum filter, and the remainder is recycled to continue the removal process. 

The reactions in which sulfite is oxidized to sulfate (Reaction 8) and soluble bisulfite is converted to a insoluble calcium sulfite (Reaction 9) account for the waste products as well as the regeneration of the solid calcium sulfite reactant that is recirculated to the scrubber. The ratio of calcium sulfite to calcium sulfate found in the air pollution control system solid waste depends on the extent to which these reactions go to completion. 

Calcium sulfite and calcium sulfate scaling in the system can be a problem for the lime/limestone wet scrubber systems. Scaling occurs when the solutions are supersaturated to a point where heterogeneous crystallization (crystallization on foreign surfaces such as the scrubber walls, overfiow pots, marbles) takes place, resulting from nucleation. The ratios of the products of the activities (A) of Ca and S04 " or to their solubility product constants Kgp) as a measure of the degree of supersaturation are ... 

Experimental work with lime scrubbing has shown that sulfite scaling occurs in the scrubber bed when free hydroxide is introduced. By proper control of the pH of the spray slurry (less than 10) entering the scrubber, calcium sulfite scaling will be prevented within the scrubber. In the calcium carbonate system, the buffering action of the carbonate-bicarbonate couple (Reaction 5) maintains a system pH between 5 and 6 thus sulfite scaling is not encountered. 

The Air Quality Control Systems (AQCS) using lime/limestone wet scrubbing have three basic types of chemical process equipment (1) scrubbers, (2) reaction tanks, and (3) solid-liquid separators, in addition to several auxiliary pieces of equipment such as pumps, demisters, and reheaters. The SO2 in the flue gas is transferred into the liquid in the scrubber, the sulfur in the liquid is converted to solid calcium sulfite, and calcium sulfate in the reaction tanks and solid calcium sulfite and sulfate are separated from the liquid and disposed from the solid-liquid separators such as clarifiers, vacuum filters, and ponds. 

With magnesium-, sodium-, or ammonium-based sytems, the bisulfite and sulfite salts are all soluble at all proportions in the presence of sulfurous acid. Even magnesium sulfite, with a solubility of about 1.25 g/100 mL, cold, is about 160 times as soluble as calcium sulfite at the same temperature and its solubility increases with temperature. So liquor preparation with these sulfite salts is easier, whether for acid sulfite, bisulfite, or NSSC pulping conditions, and even for experimental tests under alkaline conditions. For ammonium-based systems, ammonium hydroxide is contacted with a sulfur dioxide gas stream for liquor preparation. Magnesium-based systems use a magnesium hydroxide slurry to contact the sulfur dioxide gas stream. Sodium-based systems normally employ sodium carbonate lumps in a sulfiting tower, in a method similar to that used for NSSC liquor preparation. Sodium hydroxide may also be used if available at low cost. 

There have been some fouling problems in the systems caused by pitch-containing small fibers and calcium sulfite. Improved pretreatment and membrane cleaning procedures are the keys to the better performance of the systems. 

Forced oxidation in flue gas desulfurization (FGD) systems converts calcium sulfite (CaSO. H O) to calcium sulfate, or... 

Gladkii(16) at the State Scientific Research Institute of Industrial and Sanitary Gas Cleaning at Moscow did work on the three-phase calcium sulfite slurry oxidation system, finding that the liquid phase oxidation (pH 3.6-6) is first order with respect to the sulfite species. He pointed out, on the basis of pH versus time data from his semi-batch reaction, that the slurry oxidation had different periods in which either reaction kinetics or solid-liquid mass transfer controlled the oxidation rate. He also presented an omnibus empirical correlation between pH, temperature, and the liquid phase saturation concentration of calcium sulfite solution for predicting the slurry oxidation rate. The catalytic effect of manganese... 

At the Shawnee Test Facility, major emphasis has been placed on the use of adipic acid in conjunction with forced oxidation of calcium sulfite to calcium sulfate, since this system results in better sludge dewatering properties and reduced waste solids disposal costs. Furthermore, the more tightly closed liquor loop,... 

In any within-scrubber-loop forced oxidation system, irrespective of whether it is additive promoted or not, the possibility exists for calcium sulfite blinding of limestone because the recirculated slurry lacks the solid CaSC>3 crystal seeds. 

The sulfate co-precipitates with the calcium sulfite, resulting in a mixed crystal (or solid solution) of calcium-sulfur salts. Gypsum is not formed. The relatively high sulfite concentrations in the solution prevent soluble calcium concentrations from reaching the levels required to exceed the gypsum solubility product, and the system operates unsaturated with respect to calcium sulfate. 

Unfortunately, there are many problems associated with scrubbing. The systems are complicated and expensive and consume a great deal of energy. The large quantities of calcium sulfite produced in the process present a disposal problem. With a typical scrubber, approximately 1 ton of calcium sulfite per year is produced per person served by the power plant. Since no use has yet been found for this calcium sulfite, it is usually buried in a landfill. As a result of these difficulties, air pollution by sulfur dioxide continues to be a major problem, one that is expensive in terms of damage to the environment and human health as well as in monetary terms. 

The burning of pulverized coal in electric power plants produces sulfur dioxide (SO2) gas emissions. The 1990 Clean Air Act and its subsequent amendments mandated the reduction of power plant SOj emissions [66-70]. The Best Demonstrated Available Technology (BDAT) for reducing SOj emissions is wet scrubber flue gas desulfurization (FGD) systems. These systems are designed to introduce an aUcahne sorbent consisting of lime or limestone in a spray form into the exhaust gas system of a coal-fired boiler. The aUcaU reacts with the SOj gas and is collected in a liquid form as calcium sulfite or calcium sulfate slurry. The calcium sulfite or sulfate is allowed to settle out as most of the water is recycled [66-80]. 

Dewatered flue gas desulfurization (FGD) scrubber material is most frequently generated as calcium sulfite, although some power plant scrubbing systems have the forced oxidation design, resulting in a calcium sulfate (or by-product gypsum) material. Calcium sulfite FGD scrubber material is oxidized to sulfate and used for road base, while the calcium sulfate FGD scrubber material is frequently used for wallboard or as a cement additive [66-80]. 

If coal or oil is the fuel source, the FGD control technologies result in the generation of solid wastes. Wet lime/limestone scrubbers produce a slurry of ash, unreacted lime, calcium sulfate, and calcium sulfite. Dry scrubber systems produce a mixture of unreacted sorbent (e.g., lime, limestone, sodium carbonates, and calcium carbonates), sulfur salts, and fly ash. 

In either the lime/limestone or sodium sulfite/lime scrubbing processes, the hydrated calcium sulfite and calcium sulfate can be of some environmental concern when the issue of disposal arises. This has, more than anything else, promoted efforts to develop alternate dry scrubbing procedures for the removal of sulfm dioxide. And the dry systems have the additional advantage of reducing the pumping requirements necessary for the wet systems. 

In one type of electrostatic precipitation system, the SO2 produced by the burning of fossil fuels is reacted with lime (CaO) to produce solid calcium sulfite (CaSOs) ... 

In the dual-alkali process, a recycled alkaline solution of sodium salts is the scrubbing liquid. The scrubber effluent is treated with slaked lime to precipitate insoluble calcium sulfite and calcium sulfate, while regenerating the alkalinity of the solution. Then, the calcium salts are thickened and filtered from the recycled solution. This system requires only a small make-up of sodium alkali and produces a smaller amount of solids for disposal than a simple lime scrubbing system. 

Forced oxidation is not required for scale control with lime systems (Gogineni and Mau-rin, 1975) since sulfate scaling is controlled either by the naturally occurring seed crystals or by co-precipitation of sulfate with sulfite as described in the Basic Chemistry section. Forced oxidation may also be used to oxidize calcium sulfite when alkaline fly ash is the primary source of alkali. The large commercial FGD system at the Colstrip Power Station in Montana, which uses a mixture of fly ash and lime as sorbent, has consistently produced an efflu-... 

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