Sulfite FGD

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]. 

The degree to which FGD scrubber material is treated influences its physical properties. Basic physical properties include solids content, moisture content, specific gravity, and wet and dry density [67]. When dewatered, the calcium sulfite FGD sludges become a soft filter cake with a solids content typically in the 40-65% range. Calcium sulfate FGD sludges can be dewatered much more easily and may achieve solid contents as high as 70-75% after dewatering [67]. 

Dewatered and unstabilized calcium sulfite FGD scrubber sludge consists of fine silt-clay sized particles with approximately 50% finer than 0.045 mm. It has... 

Flue gas desul- - Fixated or stabilized calcium sulfite FGD scrubber furization material has been used as an embankment and... 

Flue gas desulfurization (FGD) scrubber material - In 1996,0.8 million metric tons (0.9 million tons) of calcium sulfate FGD scrubber material were used to produce wallboard and approximately 0.06 million metric tons (0.07 million tons) of this material were used as feed material for cement production - In 1996,0.04 million metric tons (0.05 million tons) of primarily fixated calcium sulfite FGD scrubber material were used for structural fill. Also, approximately 0.11 million metric tons (0.12 million tons) were used for road base construction 66-80... 

Absorption Processes. Most flue gas desulfurization (FGD) systems are based on absorption of the sulfur dioxide into a n on regen erabi e alkali-salt solvent. Sulfur absorbed using n on regen erabi e solvents is not recovered and the alkali sulfite—sulfate produced presents a disposal problem. 

Fersona A process for stabilizing the calcium sulfite/sulfate waste from FGD processes, so that it may be used for landfill. The waste is mixed with ferric sulfate waste from another process (e.g., metallurgical leaching) to form sparingly soluble basic sodium ferric sulfates. Developed in the 1970s at the Battelle Columbus Laboratories, OH, under contract with Industrial Resources. See also Sintema. 

Benson et al. (2) showed that high Mg ion concentration also inhibits nucleation during the precipitation of calcium sulfite hemihydrate from simulated FGD liquors. 

The CSD studies of CaS03 l/2H20 mentioned earlier all used the MSMPR configuration. In low solids systems, e.g. sulfite from FGD, the DDO crystallizer configuration is useful to increase particle size (3)(4)(8)(9)(10). These studies demonstrated that the mean size from a continuous crystallizer can be significantly increased using the DDO configuration. 

As stated earlier in this paper, FGD wet scrubbers can produce either calcium sulfite (the typical product) or calciiun sulfate. The DDO crystallizer is advantageous for either product. The following industrial case history describes the production of calcium sulfate dihydrate (or gypsum) product fi om an industrial in-plant weak sulfuric acid liquor using a DDO crystallizer configuration. 

The Bio-FGD process converts sulfur dioxide to sulfur via wet reduction (10). The sulfur dioxide gas and an aqueous solution of sodium hydroxide are contacted in an absorber. The sodium hydroxide reacts with the sulfur dioxide to form sodium sulfite. A sulfate-reducing bacteria converts the sodium sulfite to hydrogen sulfide in an anaerobic biological reactor. In a second bioreactor, the hydrogen sulfide is converted to elemental sulfur by Thiobacilh. The sulfur from the aerobic second reactor is separated from the solution and processed as a sulfur cake or liquid. The process, developed by Paques BV and Hoogovens Technical Services Energy and Environment BV, can achieve 98% sulfur recovery. This process is similar to the Thiopaq Bioscrubber process for hydrogen sulfide removal offered by Paques. 

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. 

An important technology for removal of S02 is Flue Gas Desulfurization (FGD), carried out in units known as scrubbers. Most scrubbers contact the flue gas with a slurry of lime or limestone to capture the sulfur oxides and produce a sludge containing calcium sulfite and calcium sulfate. However, disposal of sludge is another environmental problem, and some scrubbers include oxidation to convert all the calcium sulfite to sulfate (gypsum), which can be used for wallboard manufacture. Fluidized-bed combustion units add a sulfur... 

There is a positive effect of the SCR unit on downstream equipment such as flue gas desulfurization, which produces gypsum. In the absence of a SCR unit, water from a FGD unit contains hydroxyl amine disulfonic acid which has been formed from NO2, water, and SO2. This compound acts probably as an inhibitor for the oxidation of calcium sulfite into calcium sulfate [129]. 

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

A considerable amount of interest has developed in utilizing dry-sorbent injection as a means of controlling SO2 emissions from coal-fired industrial and utility boilers.(l-7 ) A dry sorbent FGD system is one in which an alkaline material is injected into the boiler flue gas as a dry powder or aqueous slurry the sorbent reacts with SO2 to form a dry product containing sulfates and sulfites. The mixture of spent material and fly ash is removed from the gas stream by means of an electrostatic precipitator or baghouse filter. The spent sorbent is usually disposed of, but in some cases it may be regenerable or have commercial end uses.(2)... 

In 1979, the Department of Energy initiated support of a number of fundamental projects in the chemistry of flue gas desulfurization. Five chapters in this book report the initial results of this effort. The topics of fundamental work include thermodynamic properties, activity coefficients in aqueous solutions, sulfur and NOx chemistry, and sulfite oxidation kinetics. The results provide the foundation for quantitative understanding of the FGD processes. 

Commercially, the most important application is the use of wet scrubbing with limestone for flue gas desulfurisation (FGD) at electricity generating stations. For example, a 2000 MW station operating on a 2 % sulfur coal, at a 70 % load factor, requires about 300,000 tpa of limestone to remove 90 % of the oxides of sulfur. The majority of generating stations fitted with FGD use limestone, and about half of those oxidise the calcium sulfite produced to gypsum (CaS04 -2H20) [12.18]. Some stations use lime (see chapter 29), while others use a variety of other processes [12.3]. 

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]. 

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. 

Concerning the catalytic effect of the metal ions mentioned above, research has been focused on Co, Mn " or Cu [1>5]- Little and contradictory data is available about the effect of Fe " [2,3,6,7,9]. Target of the present project has been, to examine the role of Fe and the role of Fe " combined with Mn, as well as the influence of the pH-value in sulfite/bisulfite oxidation. Provided a better understanding of this subject, the performance of S(IV) oxidation in FGD could be optimised by choosing absorption additives with appropriate catalyst concentration or by adding catalyst, to achieve the optimum concentration for maximum oxidation yield, as already suggested by Gmelin [3]. 

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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