FGD

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). 

Fig. 11. Limestone FGD system. Reproduced by permission of the American Institute of Chemical Engineers (67).

When the Clean Air Act of 1990 was signed into law, electric utiUties were requited to estabUsh plans and initiate projects to comply with that Act s Tide IV. Each utihty had to evaluate how the various commercial and emerging clean coal systems fit into the utiUty s technical and business environment resulting in strategies to utilize fuel switching and wet throwaway FGD processes almost exclusively (38,85,86). 

Other problems that can be associated with the high dust plant can include alkaH deterioration from sodium or potassium in the stack gas deposition on the bed, calcium deposition, when calcium in the flue gas reacts with sulfur trioxide, or formation and deposition of ammonium bisulfate. In addition, plugging of the air preheater as weU as contamination of flyash and EGD wastewater discharges by ammonia are avoided if the SCR system is located after the FGD (23). 

Flue gas desulfurization may be carried out using either of two basic FGD systems regenerable and throwaway. Both methods may include wet or dry processes. Currently, more than 90% of utility FGD systems use a wet throwaway system process. Throwaway systems use inexpensive scrubbing mediums that are less costly to replace than to regenerate. 

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. 

Although FGD processes, originally referred to as scrubbing SO, from flue gas, have been available for many years, installations in the United States were quite limited until passage of the Clean Air Act of 1970. Even then, installations were usually limited to new facilities because existing plants were exempt under the law. 

Projects in the CCT program demonstrated innovative applications for both wet and dry or seniidry FGD systems. The wet FGD systems, which use limestone as an absorber, have met or exceeded the 90 percent SO, removal efficiency required to meet air quality standards when burning high-sulfur coal. The di"y or semidry systems use lime and recycled fly ash as a sorbent to achieve the required removal. 

In wet FGD systems, fine gas exiting from the particulate collector flows to an absorber. In the absorber, the flue gas comes into contact with the sorbent slurry. The innovative scrubbers in the CCT program featured a variety of technologies to maximize SO, absorption and to minimize the waste disposal problems (sludge). 

Reported plant applications of a.c. impedance and electrochemical noise are rare, but include stainless steels in terephthalic acid (TA) plant oxidation liquors , nuclear fuel reprocessing , and fluegas desulphurisation (FGD) scrubber systems . 

The air and flue gas management system includes the various baffles, dampers, fans, dust collectors, scrubbers, and other FGD and emission control equipment, and the boiler flues and stack (chimney). 

Technology PF + fgd, NO, etc. IGCC and CCGT supercritical PF + fgd CCGT + co2 +co2 capture capture Nuclear Hydro Wind Turbines Biomass IGCC PV and Solar thermal... 

PF, pulverised fuel fgd, flue gas desulphurization IGCC, integrated gasification combined cycle ... 

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. 

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