Flue Gas Desulfurization Technologies for Coal …

1 Babcock & Wilcox1 AbstractThe control of sulfur dioxide emissions from thermalpower plants is examined in light of the recent advancesmade in developing commercial processes for this appli-cation. Beginning with a discussion of some of the morerecent developments in the conventional wet and dryscrubbing Technologies , the paper provides a descriptionof the results of the recent full-scale demonstrationprojects conducted on the lower capital cost furnace andduct sorbent injection Technologies Limestone Injec-tion Multistage Burner (LIMB) and Coolside, addition, the results of large pilot-scale research anddevelopment activities on the related Limestone InjectionDry Scrubbing (LIDS) and SOx-NOx-Rox Box (SNRB)processes are included. The paper concludes with a dis-cussion of the economics of each of the processes basedon awareness of environmental concerns hasbeen increasing in recent years as economists forecastexplosive growth around the world.

2 Such predictions high-light the importance of minimizing the impact of increasedair, water, and solid waste pollutants. One area that hasreceived a considerable amount of attention is the con-cern about the potential for acid rain that results from thegeneration of sulfur dioxide (SO2) and nitrogen oxides(NOx) during the combustion of fossil fuels. Particularlynotable are the programs on flue gas Desulfurization (FGD) Technologies that have been ongoing in a number of coun-tries for several years. Seeking to improve the effective-ness of SO2 emission control, FGD research and develop-ment has progressed to the point that an array of processesare available to cover a broad range of site-specific, tech-nical, and economic modern FGD development received sporadicattention between the 1920s and the 1950s, broader-based,concerted efforts began in the 1960s and continue throughthe present. Within these last few decades, wet scrubbingwith lime or limestone* slurries has come to be the domi-nant commercial FGD technology.

3 Worldwide, there arecurrently 678 FGD systems operating on a total capacityof about 229 GWe.[1] Approximately 79% of the units,representing 199 GWe of capacity, are based on lime orlimestone wet scrubbing. About 18% of the units, or about25 GWe, utilize either sodium-based or lime slurry (spray)dry scrubbing. The remainder use various regenerable pro-cesses or sorbent injection Technologies of one form the United States, coal-fired utility boilers have beenthe focus of much of the effort on emission control, sincethey represent a major source of SO2 emissions. Nationalstandards for the control of SO2 from utility boilers werefirst established in 1970, but were only applied to newly* In this paper, limestone refers to calcitic limestone (calcium carbonate),CaCO3, and lime to calcitic hydrated lime, Ca(OH)2. Reference is also made todolomitic hydrated lime, Ca(OH)2 MgO. The reader is reminded that calcina-tion of CaCO3 produces quicklime, CaO, also commonly referred to as calcinedlime, which in turn is slaked to become calcitic hydrated lime.

4 Dolomitic lime-stone, CaCO3 MgCO3, undergoes similar reactions to form analogous dolo-mitic compounds. The dolomitic hydrated lime can be further hydrated toCa(OH)2 Mg(OH)2 under pressure. Although this material was used in somerelated studies, no specific references to it are included in this Gas Desulfurization Technologiesfor Coal-Fired Power PlantsPresented by Michael X. Jiang at theCoal-Tech 2000 International ConferenceNovember 13-14, 2000 Jakarta, IndonesiaBR-1709 Paul S. NolanThe Babcock & Wilcox CompanyBarberton, Ohio, & Wilcoxconstructed units. Since many boilers built before theseregulations continued to operate, and even now have asubstantial number of years of useful life remaining, therewas a need to address emissions in a more comprehensivemanner. This was accomplished when the Congresspassed the Clean Air Act Amendments (CAAA) of into law on November 15, 1990, the Acid RainProvision of the CAAA effectively required SO2 emissioncompliance in two phases for coal-fired boilers rated at25 MWe or more.

5 Phase I, applying to 110 boilers that areamong the largest utility sources, required emission lev-els of no greater than 1075 ng SO2/J ( lb/106 Btu) byJanuary 1, 1995, with provision for a two-year extensionfor implementing 90% removal Technologies under mostconditions. With the addition of those units planned orunder construction to meet the requirements, there willbe a total of 366 FGD units in the operating on ap-proximately 125 GWe of capacity. Of these,approximately 75% are either lime or limestone wet scrub-bers, 15% dry scrubbers, and 5% other Technologies . Theremainder are planned units for which the technology hasnot yet been II of the CAAA effectively required boilers 25 MWe and larger to have their emissions capped at 516 ngSO2/J ( lb/106 Btu), and that these reduced emissionsbe effective January 1, 2000. A four-year extension wasavailable for implementing selected clean coal technolo-gies. The end result of the CAAA is a nationalemission limit of x 106 t ( x 106 ton) of SO2 byJanuary 1, 2000.

6 This represents a x 106 t ( x 106ton) reduction from the x 106 t ( x 106 ton) emis-sion level in 1980. (It is noted that, while FGD is oneoption available for utilities to comply with Phase ICAAA regulations, some utilities are switching to theuse of lower sulfur coals and other fuels. The extent towhich this is a viable alternative in Phase II will be deter-mined by supply and demand considerations.)The actual selection and application of any FGD tech-nology for a specific site is the result of a careful exami-nation of both technical and economic aspects. While it isgenerally recognized that high SO2 removal efficiency hasbeen responsible for the general popularity of wet and(spray) dry scrubbers, recent advances in developing lowercapital cost sorbent injection processes have renewed in-terest in these FGD Technologies . Moderate to high levelsof removal have been demonstrated, indicating that thesetechnologies can offer attractive alternatives over abroader range of conditions than originally TechnologiesBabcock & Wilcox (B&W) has been an active partici-pant in the development, demonstration, and commercial-ization of many of these Technologies .

7 With wet scrubbersales of 18,588 MWe and dry scrubber sales of 2,740 MWe,the company is one of the major worldwide suppli-ers of FGD systems. In addition, B&W has participatedin four sorbent injection projects (one 5 MWe and three~100 MWe demonstrations) sponsored by the Depart-ment of Energy (DOE) and the Environmental Pro-tection Agency (EPA). It is from this perspective that thebalance of this paper seeks first to provide the reader witha background appreciation of some of the more importantfeatures of the conventional wet and dry scrubbers offeredby B&W today. This forms the basis from which the devel-opment of the lower capital cost sorbent injection technolo-gies proceeded. The paper goes on to describe the results ofthese efforts as the Technologies begin to be accepted as ScrubbingAs noted earlier, lime and limestone wet FGD systemsare the mainstay of SO2 emission control throughout theworld. In the , passage of the Clean Air Act in 1970promoted trials of various types of systems.

8 It was notlong, however, before the utilities and major system sup-pliers gravitated toward wet scrubbers in which SO2 re-moval is accomplished by recirculating an aqueous slurryof lime or limestone in an absorber vessel to effect inti-mate contact with the flue inherent simplicity, the availability of limestone,and the high removal efficiencies required by the lawquickly advanced the popularity of this type of occurred in spite of the fact that the early systemsoften relied on redundancy to overcome difficulties re-sulting from scale formation in the absorbers. In addition,they tended to incorporate a design that produced a thixo-tropic, waste sludge that was difficult to dewater to morethan about 60% solids. The net effect was that the costs ofthese first-generation systems tended to be higher than theyotherwise might have state-of-the-art systems offer significantly im-proved performance compared to the first-generation FGDsystems.

9 Much of this is attributed to engineering designsdeveloped to conform better with fundamental processchemistry. The largest single improvement has been thedevelopment of sulfite oxidation control. Scale formationin the early systems tended to occur as the result of un-controlled crystallization of the naturally oxidized prod-uct calcium sulfate (CaSO4 2H2O [gypsum]) from therecirculating slurry. The blocky gypsum crystals typicallyrepresented 15 to 50 mol % of the absorbed SO2 and, whenintermingled with those of unoxidized calcium sulfite(CaSO3 1/2H2O) platelets in the slurry, were responsiblefor much of the difficulty in dewatering. For limestonesystems, blowing air into the slurry to force oxidation tonear 100% provides seed crystals that minimize scaling,while at the same time producing more homogeneous slur-ries that dewater to concentrations in excess of 90% these reasons, the Limestone Forced Oxidation (LSFO)system has become the preferred technology prime benefits of scale control derived from forcedoxidation are greater scrubber reliability and in the design and operation of these wet sys-tems has risen to the point that a number of utilities in and Canada are now specifying and/or buying singleabsorber systems, with no redundant absorber towers, tosatisfy their compliance requirements.

10 Figure 1 shows theprimary components of the absorber towers currently be-ing offered by B& B&W scrubber systems are currently operatingon high sulfur coal plants, producing gypsum for wall-board. FGD byproduct gypsum has also been used as anagricultural soil amendment and in cement & Wilcox3 Figure 1 B&W wet FGD absorber if the gypsum is not sold, the enhanced dewateringcapability makes the process attractive in congested areasbecause the gypsum is a stable landfill material that re-quires less area for waste variation of LSFO FGD systems used by a few utili-ties is the inhibited oxidation system. In this process,emulsified sulfur or sodium thiosulfate is added to thescrubber liquor to prevent oxidation to calcium sulfate,thus acting as a scale control agent. With low oxidationlevels, the growth of larger calcium sulfite crystals pro-duces enhanced dewatering benefits similar to those inthe fully oxidized system.