ch11Flue Gas Desulfurization Systems.docx

1、ch11Flue Gas Desulfurization SystemsFlue Gas Desulfurization SystemsTimothy W. Devitt P.E.Vice-PresidentPEDCo Environmental, Inc.Cincinnati, Ohio11.1 DESCRIPTION OF CONTROL DEVICEFlue gas desulfurization (FGD) is the process of removing sulfur oxides, primarily SO2, from combustion gases. In wet pro

2、cess FGD systems, flue gases are contacted with an absorbent in a vessel called either an absorber or a scrubber. The SO2 reacts with the absorbent or dissolves into the solution to produce a slurry or liquid that contains dissolved or solidified sulfur compounds. In all operating systems, water is

3、used to dissolve or suspend the reacting chemicals, although a number of dry absorption systems are currently being commercialized. The advantages of dry scrubbing include potential cost and energy savings and relative ease of operation compared with wet FGD systems. Dry scrubbing, however, requires

4、 higher chemical consumption and may be restricted to low-sulfur coals. Because most of the commercial dry scrubbing installations are under construction or in the planning phase, there is a lack of data on full-scale systems. Therefore, this review is limited to wet FGD systems.Wet FGD processes ar

5、e grouped into two general categories, regenerable and nonregenerable, depending on whether the sulfur is separated from the absorbent as waste. Nonregenerable processes produce a sludge that requires disposal in an environmentally sound manner. Regenerable processes include additional steps to conv

6、ert the sulfur into by-products such as liquid sulfur dioxide, sulfuric acid, or elemental sulfur. Six basic wet FGD systems are used in this country, primarily to treat flue gas from large, coal-fired boilers. Of these, the lime scrubbing, limestone scrubbing, sodium carbonate scrubbing, and dual a

7、lkali systems are classified as nonregenerable. The magnesium oxide scrubbing and Wellman-Lord systems are classified as regenerable processes. Table 11-1 lists the major regenerable and nonregenerable systems and indicates the number of systems that are operational, under construction, or planned f

8、or application on both utility and nonutility boilers.Because most operating FGD systems are nonregenerable (a trend that is likely to continue because of their generally lower capital and operating costs), only the regenerable systems are reviewed here. Furthermore, only the wet lime- and limestone

9、-based systems are discussed, although many of the design, operation, and maintenance considerations are similar for other systems.TABLE 11-1 Distribution of FGD systems by process type.ProcessUTILITY BOILER FGD CAPACITY (MW)1.2ProcessINDUSTRIAL BOILER FGD CAPACITY (SCFM)3.4OperationalUnder Construc

10、tionPlannedTotalOperationalUnder Construction PlannedTotalAqueous carbonate/spray drying100100Ammonia164,000 154,000318,000Citrate605605Caustic211,000211,000Dual alkali1,2018422,043Caustic waste stream665,000665,000Lime8,8012,0606,84117,702Citrate142,0005142,0005Limestone11,4377,63717,25436,328Dual

11、alkali1,480,0001,480,000Limestone/alkaline fly ash1,4801,480Dual alkali (concentrated)489,070512,0001,001,070Lime/alkaline fly ash2,6131,4004,013Dual alkali (dilute)233,400233,400Lime/limestone20475495Lime40,00030,00070,000Lime/spray drying1101,0601,8132,983Lime/spray drying72,70072,700Magnesium oxi

12、de724724Limestone55,00055,000Sodium carbonate9253301,9003,155Sodium carbonate2,467,000504,5002,971,500Sodium carbonate/spray drying440440Sodium hydroxide1,140,30012,0001,152,300Wellman Lord1,5405342,074Sulf-x score10,00010,000Process not selected6,6506,650Process not selected124,000124,000Total28,18

13、714,28535,77578,247Total5,464,7703,041,2008,505,9701M. Smith, et al., EPA Utility FGD Survey: October-December 1980. Vol. 1. EPA-600/7-81-012a, PEDCo Environmental, Cincinnati, Ohio, October 1979. 2Capacities are reported as equivalent scrubbed capacity (ESC); the summation of effective scrubbed flu

14、e gas in equivalent MW based on the percent of the flue gas scrubbed by the FGD system(s). 3J. Tuttle, et al., EPA Industrial Boiler FGD Survey: First Quarter 1979. EPA-600/7-79-067b, PEDCo Environmental, Cincinnati, Ohio, April 1979. 4For approximate comparison purposes 1 MW may be considered 3000

15、scfm. 5The citrate process FGD system on St. Joe Zincs G. F. Weaton unit was included in both the utility and industrial report since it supplies power to the utility grid as well as steam for the industrial plant. This unit should be deleted from one or the other sections when estimating total proc

16、ess capacities for the industrial-utility marketplace as a whole.The Lime Scrubbing Process The first step of the process is preparation of a lime slurry. Lime (CaO) is reacted with water in a slaker 石灰消和器 to produce a slurry of calcium hydroxide Ca(OH)2 and water, according to the following reactio

17、n: CaO + H2O Ca(OH)2 + heat. This material is diluted 无力的,冲淡的with recycled water to produce a scrubber slurry containing about 15 to 20% solids.The lime slurry is added at a regulated rate to a recycle tank that contains the scrubbing slurry. Large pumps circulate the scrubbing slurry through the sc

18、rubber or absorber vessel. In the absorber, the slurry droplets are brought into contact with the flue gas containing SO2, and the SO2 passes from the gas into the droplets driven by a concentration gradient. The dissolved SO2 reacts with calcium from the lime according to the following reactions:Ca

19、(OH)2(s) Ca(OH)2(aq) Ca + 2OH 2OH + CO2 CO + H2OCO + CO2 + H2O 2HCOCa2+ + CO CaCO3CaCO3(s) CaCO3(aq) Ca2+ + COSO2(g) SO2(aq) + H2O H+ + HSOHSO H+ + SOCa2+ + SO CaSO3(aq)Ca2+ + SO + 1/2H2O CaSO3 1/2H2O(s)CaSO3 + H+ Ca2+ + HSOH+ + HCO H2CO3 CO2(g) + H2OCaSO3 1/2H2O + 3/2H2O + 1/2O 2 CaSO4 2H2OThese re

20、actions take place to some extent in every time scrubbing system; similar reactions occur in limestone systems. Such factors as ionic concentrations, pH, temperature, and retention time in the reaction tank influence the speed and completeness of the various reactions. The net effect, however, is re

21、moval of SO2 from the flue gas, depletion of calcium hydroxide in the slurry, and generation of insoluble calcium sulfite and calcium sulfate. The calcium ions used in the reactions are replenished by dissociation of calcium hydroxide. The use of an insoluble alkaline reagent is the primary differen

22、ce between the lime and limestone slurry scrubbing systems and the so-called clear solution scrubbers (e.g., double alkali scrubbers), where the predominant reactive alkali (sodium) is more soluble. Forcing most of the sulfite/sulfate precipitation to occur in the reaction tank requires a certain am

23、ount of retention time to permit the slurry to reach chemical equilibrium before it is recirculated to the scrubber. For this purpose, a reaction tank, sometimes an integral part of the scrubber vessel, is usually sized with a retention time of between 5 and 10 minutes. The calcium sulfite and calci

24、um sulfate that are formed in the reaction tank must be removed from the system. This is accomplished by bleeding a stream from the recirculating slurry to a thickener or clarifier. The calcium sulfite and sulfate settle out and are removed from the underflow of the clarifier in a slurry of about 30

25、% solids. The overflow, a clear liquid of less than 1% solids, is brought back to the system for reuse in the process. The sludge from the underflow is disposed of by one of several means. One method of disposal is to pump the sludge to a pond. In this case the sludge is pumped from the thickener un

26、derflow (or occasionally directly from the reaction tank) to a large pond, where the solids settle. Clear water is recycled from the pond, as necessary, to keep the pond from overflowing. Return of all this clear liquor from the pond to the process is called a “closed-loop” operation.Another method

27、of disposal is to dewater the sludge in a vacuum filter, chemically stabilize it by the use of generic or proprietary processes, and dispose of it in a landfill n.垃圾掩埋法, 垃圾. When vacuum filtration is used, the filtrate is returned to the system to create closed-loop operation.The Limestone Scrubbing

28、 ProcessLimestone slurry preparation is the only step that is significantly different from the lime scrubbing process. Lumpsize limestone is pulverized v.研磨成粉to -200 to 300 mesh in a ball mill and slurried with water. The fine size is needed because dissolution of CaCO3 is a slow reaction, the rate

29、being directly related to the particle surface area and therefore to the particle size. For further limestone dissolution, the scrubbing slurry is maintained at an acidic pH, in contrast to lime-based systems, in which scrubbing slurry is slightly alkaline. The overall process reactions are the same

30、 as for the lime slurry scrubbing process, except the reactant species is CaCO3, not Ca(OH)2.Figure 11-1 illustrates a typical limestone process. The lime process is similar except a lime slaker replaces the ball mill of the limestone process.Other wet process FGD systems operate similarly. The SO2

31、is scrubbed from the gases in a contactor, and reaction products are removed in a separate vessel. Several excellent texts and reports are available that describe the various systems, their chemistry, and status of commercialization; these are listed in Section 11.8.11.2 DESIGN PROCEDURESProper system design is crucial 极重要的 to successful operation of an FGD system. Although this is true for all air pollution control systems, it is especially important for FGD systems. In contrast to m