Flue gas desulfurization FGD

The wet method of flue gas desulfurization brings the flue gases containing finely crushed limetstone into contact with water. Both these materials are required in large quantities — both are cheap and usually readily available. The overall sequence of chemical steps can be summarized by Equation 8.2  

The particulate solid product, gypsum (CaS04-2H20), is then collected. This is commercially useful because it can be manufactured into plasterboard for use by the building industry. Alternatively, it can be disposed of safely as waste in landfill. 

Four steps have been identified in controlling the rate of the overall chemical changes. 

Absorption of SO2. Gases are brought into contact with water droplets to form sulfurous acid, which then dissociates  

2 Oxidation of HSOs. The kinetics of the oxidation of the HS03 ion have not been completely agreed, but the reaction is believed to be catalysed by the small amounts of transition metal ions (catalytic, redox reaction), which are always present, at least in trace amounts, in the aqueous phase. This step is  

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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). 

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. 

In 1983 there were 116 flue-gas desulfurization (FGD) systems in service, representing 47 gigawatts-electric of power generation capacity (66). As of 1992, more than 150 coal-fired boilers in the United States operated with FGD systems. The total electrical generating capacity of these plants has risen to 72 gigawatts (67). FGD processes are classified into (/) wet-throwaway, (2) dry-throwaway, (J) wet-regenerative, and (4) dry-regenerative processes (68). 

These scrubbers have had limited use as part of flue gas desulfurization (FGD) systems, but the scrubbing solution flow rate must be carefully controlled to avoid flooding. When absorption is used for VOC control, packed towers are usually more cost effective than impingement plate towers (discussed later). 

Three major compliance options for SOj emissions available to utilities using coal-fired boilers are to switch fuels, purchase/sell SO, allowances, or install flue gas desulfurization (FGD) technologies. Costs, availability, and impact on boiler operation must be considered when evaluating switching to low-sulfnr coal or natural gas. As more utilities enter the free market to purchase SO, allowances, prices will rise. Therefore, to minimize costs and, at the same time, meet environmental standards, power producers should continuously monitor the tradeoffs among these three options. 

A novel Double Draw-Off (DDO) ciystallizer has been designed in order to improve the particle size distribution in the precipitation of CaS03 V 20 simulated Flue Gas Desulfurization (FGD) liquor. The effects of DDO ratio and residence time on the mean particle size were studied. Industrial conditions were maintained in all experiments as far as practical. Significant improvement in mean particle size was achieved. The performance of an actual industrial DDO ciystallizer (DuPont) for gypsum ciystallization was reported. 

Plant description. Two nearly identical 430-Mw(e), western, conventional pulverized-coal-utility boilers (referred to as plants A and D) were tested. Both units use tangentially fired burners and burn low-sulfur 200-mesh coal of heat content approximately 27 000 J/g. Both units are equipped with cold-side electrostatic precipitators (ESP) of design efficiency of 99.5% or greater, and a modern flue-gas desulfurization (FGD) system consisting of four verticle spray towers. 

Rai, D., Zachara, J. M., Moore, C. A., McFadden, K. M. Resch, C. T. 1989. Field Investigation of a Flue Gas Desulfurization (FGD) Sludge Disposal Pit, EPRI EA-5923. Electric Power Research Institute, Palo Alto, CA. 

Recovered sulfur supply predictions depend on explicit assumptions or scenarios concerning the development of specific fuels and the production of sulfide ores. They also depend on a second set of assumptions with respect to sulfur pollution control regulations, the means by which these will be met, and the recursive impact of the controls on the production scenarios. For example, given uncertainties surrounding regenerative flue gas desulfurization (FGD) processes, including the sale of sulfur products and concern over process reliability, utilities have been emphasizing throw-away techniques. As new control standards are implemented the disposal... 

Of these, 72 plants (37,279 MWe) were reported to be using low sulfur coal to meet (the then expected) air pollution control regulations. An additional 61 plants (28,601 MWe) were considering flue gas desulfurization (FGD) while one (330 MWe) was undecided. 

While the development of flue gas clean-up processes has been progressing for many years, a satisfactory process is not yet available. Lime/limestone wet flue gas desulfurization (FGD) scrubber is the most widely used process in the utility industry at present, owing to the fact that it is the most technically developed and generally the most economically attractive. In spite of this, it is expensive and accounts for about 25-35% of the capital and operating costs of a power plant. Techniques for the post combustion control of nitrogen oxides emissions have not been developed as extensively as those for control of sulfur dioxide emissions. Several approaches have been proposed. Among these, ammonia-based selective catalytic reduction (SCR) has received the most attention. But, SCR may not be suitable for U.S. coal-fired power plants because of reliability concerns and other unresolved technical issues (1). These include uncertain catalyst life, water disposal requirements, and the effects of ammonia by-products on plant components downstream from the reactor. The sensitivity of SCR processes to the cost of NH3 is also the subject of some concern. 

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. 

Equilibrium calculations are useful in the design or operation of a flue gas desulfurization (FGD) facility and provide the necessary foundation for complex process simulation (e.g., absorber modeling) (3). Since S02 absorption into FGD slurries is a mass transfer process which is primarily limited by liquid phase resistance for most commercial applications, the solution composition, in terms of alkaline species, is very critical to the performance of the system. Accurate prediction of solution composition via equilibrium models is essential to establishing driving forces for mass transfer, and ultimately in predicting system performance. 

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... 

S02 Direct or indirect oxidation or reduction SO3, SO. orS Called Electrochemical Flue Gas Desulfurization (FGD)... 

Fig. 5.17. A scheme of a power plant equipped with both selective catalytic reduction (SCR) and flue gas desulfurization (FGD). APH air preheater ESP electrostatic precipitator.

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