Desulfurization, flue gas

In the 1970s, environmental controls began to include the removal of oxides of sulfur from the flue gas of power stations burning fossil fuels. While lime and limestone-based processes are by far the most widespread desulfurization technology, magnesia can also be utilized in flue gas desulfurization (FGD) however, it only has a minor presence in the United States compared with the use of lime and limestone based scrubbers. 

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R. W. Coughlin, R. D. Siegel, and C. Rai, eds., AICbE Sjmp. Ser. 70, (137) (1974). Contains four papers on flue gas desulfurization, four papers on coal desulfurization, and three papers on petroleum desulfurization. 

AICbE Sjmp. Ser. 68, (126) (1972). Contains four papers on flue gas desulfurization and two on NO control. 

Sulfur Dioxide Processing, Repriuts of 1972—1974 Chem. Eng. Prog, articles, AIChE, New York (1975). Contaius thirteen papers on flue gas desulfurization, two on SO2 control iu pulp and paper, one on sulfuric acid tail gas, one on SO2 from ore roasting, and two on NO from nitric acid. 

Flue Gas Desulfurization. The system Mg(OH)2 S02 H20 is employed in the scmbbing process for removing SO from flue gases (see SuLFURREMOVAL AND RECOVERY) (87). The equihbria involved in scmbbing has been studied in detail (147). 

Flue Gas Desulfurization and SulfuricMcidProduction Uia Magnesia Scrubbing U.S. Environmental Protection Agency, Technology Transfer, EPA 625/2-75/077, Washington, D.C., 1975. 

One method for using sodium alumiaate to desulfurize flue gas containing sulfur dioxide is described (45). This procedure led to a process where aluminum sulfate [10043-01-3] could be generated as a by-product of flue gas desulfurization (46). 

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). 

Utihties that reduce emissions below the number of allowances they hold may trade emissions credits on the open market. Owners of plants affected by Phase I regulations can also petition the EPA for a two-year extension for meeting Phase I emissions if they have selected a control option capable of reducing SO2 emissions by 90% or more, such as is capable by flue-gas desulfurization. Owners of these units can receive bonus allowances for 1997—1999 if they have operated at SO2 emissions below 0.52 kg/10 kj (1.2 lb/10 Btu) of fuel heating value input. 

Other Uses. Other uses include intermediate chemical products. Overall, these uses account for 15—20% of sulfur consumption, largely in the form of sulfuric acid but also some elemental sulfur that is used directly, as in mbber vulcanization. Sulfur is also converted to sulfur trioxide and thiosulfate for use in improving the efficiency of electrostatic precipitators and limestone/lime wet flue-gas desulfurization systems at power stations (68). These miscellaneous uses, especially those involving sulfuric acid, are intimately associated with practically all elements of the industrial and chemical complexes worldwide. 

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. 

Minor and potential new uses include flue-gas desulfurization (44,45), silver-cleaning formulations (46), thermal-energy storage (47), cyanide antidote (48), cement additive (49), aluminum-etching solutions (50), removal of nitrogen dioxide from flue gas (51), concrete-set accelerator (52), stabilizer for acrylamide polymers (53), extreme pressure additives for lubricants (54), multiple-use heating pads (55), in soap and shampoo compositions (56), and as a flame retardant in polycarbonate compositions (57). Moreover, precious metals can be recovered from difficult ores using thiosulfates (58). Use of thiosulfates avoids the environmentally hazardous cyanides. 

Minor and potential new uses for ammonium thiosulfate include flue-gas desulfurization (76,77), removal of nitrogen oxides and sulfur dioxide from flue gases (78,79), converting sulfur ia hydrocarbons to a water-soluble form (80), and converting cellulose to hydrocarbons (81,82) (see Sulfur REMOVAL AND RECOVERY). 

Calcium carbonate is finding increasing use in flue gas desulfurization. This appHcation by a variety of engineering processes traps the sulfur—oxygen compounds produced in the combustion of coal (qv) (see Coal conversion process Exhaust contdol, industrial Sulfurremoval and recovery). 

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). 

A. D. Randolph and D. Etherton, Study of Gypsum Crystal Nucleation and Growth Rates in Simulated Flue Gas Desulfurization Eiquors, EPRI Report CS1885, Electric Power Research Institute, Palo Alto, Calif., 1981. 

Minerals Limestone Fillers Paper coatings Flue gas desulfurization Kaolin Gypsum Alumina Precious metals hheration... 

Fig. 30-2. Cutaway drawing of a flue gas desulfurization spray tower absorber. Source CE Power Systems, Combustion Engineering, Inc.

Today s major emissions control methods are sorbent injection and flue gas desulfurization. Sorbent injection involves adding an alkali compound to the coal combustion gases for reaction with the sulfur dioxide. Typical calcium sorbents include lime and variants of lime. Sodium-based compounds are also used. Sorbent injection processes remove 30 to 60% of sulfur oxide emissions. 

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). 

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