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Figure 1. Generic we-limestone FGD process flow diagram
utilizing an open spray tower. Home Features Article E-mail to a Friend
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# Important concepts of wet-limestone flue gas desulfurization

BRAD BUECKER

Introduction

Many utilities in the United States are installing flue gas desulfurization (FGD) systems to reduce sulfur dioxide emissions from coal-fired boilers. The majority of these systems will be based on wet-limestone technology due to the reliability of the process and the abundance of the scrubbing reagent, limestone, in many areas of the country. This article outlines a number of fundamental principles of wet-limestone scrubbing, which will hopefully prove useful to plant personnel who might be asked to plunge into unfamiliar territory. Important, and in some cases critical, aspects of operation include ensuring proper sulfur dioxide removal at all times, maximizing reagent utilization, and minimizing scale buildups on scrubber components.

Chemistry fundamentals A generic flow diagram of a spray-tower, wet-limestone scrubber is illustrated in Figure 1. The process is a classic example of an aqueous acid-base chemistry reaction applied on an industrial scale, where alkaline limestone slurry reacts with acidic sulfur dioxide. Sulfur dioxide is first absorbed into the liquid phase as it contacts the slurry sprays. Eq. 1: SO 2 +H 2 

### ↔ H 2 SO 3 Some theoretical chemists argue that true H 2 SO 3 does not exist, and that SO 2 retains its molecular
character and is surrounded by water molecules. However, when SO 2 is added to water, the pH
drops, which suggests that Equation 1 is correct and that the following dissociation reaction is

accurate.
Eq. 2: H 2 SO 3 

### ↔ H + +HSO 3 - 

### ↔ H + + SO 3 -2

Another argument for the formation of H 2 SO 3 , and its dissociated products bisulfite (HSO 3 - ) and

sulfite (SO 3 -2 ) ions, comes from the fact that the
principal component of limestone, calcium carbonate (CaCO 3 ), is only slightly soluble in water but will dissolve almost completely in well-designed
scrubbing systems.
Eq. 3: CaCO 3 + H + 

### → Ca +2 + HCO 3 -
Equations 1, 2, and 3 combined illustrate the simplified, but fundamental, scrubbing process.
Eq. 4: CaCO 3 + 2H + + SO 3 -2 

### →

Ca +2 + SO 3 -2 + H 2 O + CO 2 

### ↑
Extremely important is constant measurement of the alkalinity in the scrubber module or modules, since too much alkalinity will waste limestone while lean alkalinity will impair SO 2 removal. The technique universally
employed in wet scrubbers is continuous pH monitoring. For the system outlined in Figure 1, and many like it, a pH range of 5.6-5.8 is very common. Another very important principle with spray systems is the liquid-to-gas (L/G) ratio, where the units of measurement are typically gallons-per-minute (gpm) of slurry flow to 1,000 actual cubic feet per minute (ACFM) of flue gas flow. A general L/G ratio in the scrubbers I have worked with has been 120-to-1 or thereabouts, but this value will vary depending upon scrubber configuration. In the absence of any other reactants, calcium and sulfite ions precipitate as a hemihydrate, where water is actually included in the crystal lattice of the byproduct.
Eq. 5: Ca +2 + SO 3 -2 + 1⁄2H 2 O CaSO 3 ·1⁄2H 2 

### ↓
Oxygen in the flue gas greatly influences chemistry, as aqueous bisulfite and sulfite ions react with oxygen to
produce sulfate ions (SO 4 -2 ).

Eq. 6: 2SO 3 -2 + O 2 

### → 2SO 4 -2

Approximately the first 15 mole percent of sulfate ions co-precipitates with sulfite to form calcium sulfitesulfate hemihydrate [(0.85CaSO 3 ·0.15CaSO 4 ) ·1⁄2H 2 O]. Any sulfate above the 15 percent mole ratio
precipitates with calcium as gypsum.
Eq. 7: Ca +2 + SO 4 -2 + 2H 2 O 

### →

CaSO 4 ·2H 2 O 

### ↓
Control of this chemistry offers interesting challenges, particularly in spray towers that have internal devices to enhance gas-liquid contact. Experience has shown that operation in either a completely oxidized state (no calcium sulfite-sulfate hemihydrate in the scrubbing slurry) or a completely un-oxidized state (no gypsum in the slurry) minimizes scaling of internal scrubber components. Deposit formation on scrubber internals and subsequent gas flow restrictions may cause unit de-rates and even forced outages if gas flow is severely
restricted. An often critical factor regarding the choice of oxidized or non-oxidized byproduct involves the handling characteristics and commercial value of the solid. Calcium sulfite-sulfate hemihydrate is a soft material that tends to retain water. It has little value as a chemical commodity. For this reason, many scrubbers are equipped with forced-air oxidation systems to introduce additional oxygen to the scrubber slurry. A properly designed oxidation system will convert all of the liquid sulfite ions to sulfate ions. Sulfate, of course, precipitates with calcium as gypsum, which forms a cake-like material when subjected to vacuum filtration. Typically, 85-90 percent of the free moisture in gypsum can be extracted by this relatively simple mechanical process. High-purity, dried synthetic gypsum has become a favorite material of wallboard
manufacturers.

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Figure 2. A n original first-generation FGD design. Home Features Article E-mail to a Friend
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# Important concepts of wet-limestone flue gas desulfurization

BRAD BUECKER

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Early scrubber design A first generation scrubber is outlined in Figure 2. As can be seen, each module of this multiple module system had two scrubbing zones, the first being a venturi section with spray nozzles. The purpose of the venturi section is intuitively obvious in that the design increases gas velocity and induces eddies, thus enhancing gas-liquid mixing. Only a portion of the SO 2 came out in the venturi zone, so
the system included a second spray zone, the absorber. Note the tray in this second section. Trays, or in some cases layers of packing, were standard items in first- and second-generation scrubbers. The concept is that the material forces both the gas and the liquid to take a torturous path as they pass each other, thus enhancing contact and reducing L/G requirements. Unfortunately, these mixing devices, and packing in particular, served as excellent sites for deposition, which would then restrict gas flow through the tower. A second generation system, a Research-Cottrell Double-Loop® scrubber from the 1980s, is illustrated in Figure 3. Whole papers could be written about this particular technology, but one of the key points is that the upper section (absorber) of each of the two scrubbing towers in this particular system contained a two-foot layer of honeycombed plastic packing.
A close look at the schematic shows that reacted product was only taken from the quencher of each module. Thus, the pH range for this loop was typically maintained at 5.2-5.5 for better limestone utilization, while the pH range in the upper loop was set at 6.0-6.2 for enhanced SO 2
removal. This scrubber was originally designed with forced oxidation in the quenchers for gypsum production, but with only natural oxidation in the absorbers. As it so happened, just enough oxygen entered the absorbers to produce sulfate at a concentration near or slightly above CaSO4 saturation levels. Thus, as gypsum precipitated, one of the favorite settling sites was the absorber packing. The resulting constant increase in differential pressure required weekly low-load cleanings, with packing removal and high-pressure cleaning or replacement on a semi-annual basis. Implementation of a gypsum seed recycle program considerably mitigated the scaling
difficulties. Since then, installation of an upper loop forced-air oxidation system and high efficiency spray nozzles have allowed replacement of the packing with a single tray.

Modern spray towers and basic operational issues Many modern systems greatly minimize scale formation by their design alone. Spraying technology has improved for scrubber applications, and open spray towers are now becoming popular, although designs from Wheelabrator and Babcock & Wilcox still employ a tray. Nozzle design and alignment are critical in these systems, as droplet size and spray patterns must be optimized to provide the best contact and to prevent channeling of the flue gas. (An excellent article on spray nozzle types may be found in one of the references.) One important aspect of spray nozzle design is that plugging may be quite common if the slurry feed piping and/or circulating pump volutes are lined. Linings can fail, where the pieces of failed material
collect in the nozzles. Limestone reactivity is, and always has been, an important issue in spray tower scrubbers. If the limestone reacts poorly in the system, overfeed is required for adequate SO 2 removal, and this results in excess
reagent usage. Limestone costs for large scrubbers can approach $1 million dollars/year, so loss of reagent due to poor utilization can be rather expensive. Factors that influence utilization include limestone grind size, quality and reactivity, residence time of the reagent within the scrubbing system, and performance of slurry separation devices. Limestone grind size has always been critical with regard to FGD efficiency. When I first started in the industry, some specifications called for 70 percent ground limestone passing through a 200-mesh screen, as determined by laboratory tests. However, it quickly became apparent that a finer grind was necessary to ensure the maximum scrubbing efficiency. The guideline evolved to, in many cases, 90 percent through a 325-mesh screen. The common method for grinding limestone is to mix the raw material with water in large ball mills, which are horizontal, rotating, rubber-lined cylinders containing thousands of iron balls. The continuous action of the balls crashing against one another grinds the stone, and typically, the raw slurry from the ball mills still contains too many coarse particles, so the discharge is routed through a bank of hydrocyclones, or hydroclones for short, whose general outline is shown in Figure 4. The slurry enters these stationary devices in a tangential opening near the top. The design imparts a centripetal action to the slurry, forcing lighter and more reactive limestone particles through the overflow to the scrubber, while heavier particles exit via the underflow and are routed back to the ball mills. Hydroclones are designed with vortex finders, somewhat like orifice plates, on the overflow and underflow lines, such that change-out of the finders can be utilized to adjust particle size exiting each port. However, it is important to initially size hydroclones properly as based on volumetric flow rate. Limestone reactivity also is important with regard to scrubber operation. The chemical makeup of the reagent has a large influence on scrubber efficiency. In general, limestones containing 94 percent or more calcium carbonate are very reactive, given of course that they are ground properly. However, not all power plants are located near such good sources. Often, a stone may contain greater than 90 percent total carbonate, but where 10 percent or more exists as dolomite (MgCO 3 ·CaCO 3 ), in which the magnesium is
bonded with calcium in the dolomite crystal lattice. While pure MgCO 3 dissolves quickly in scrubber solutions,
and provides liquid alkalinity, dolomite is rather non-reactive, and tends to pass through the FGD system
untouched. Utilities that do not have access to high-purity limestone might need to enhance slurry reactivity with additives. One choice in the past was the addition of supplemental magnesium salts. Magnesium compounds, other than dolomite, generally dissociate quickly in scrubber solutions, whereupon the magnesium increases the solution alkalinity and enhances the transfer of SO 2 from the initial liquid phase to a reaction product
with calcium. A popular organic additive is adipic acid (hexanedioc acid), as shown in Figure 5, or the less refined material dibasic acid (DBA), which is a mixture of adipic, glutaric, and succinic acids. Like magnesium, these chemicals increase the liquid phase alkalinity, where concentrations of a few hundred parts-per-million (ppm) often are sufficient to achieve suitable results. An important point that must be considered is whether the chemicals are needed to improve SO 2 removal or improve limestone utilization.
The former is obviously critical, as the plant must meet regulatory requirements. Regarding the latter issue, economic value is the important criteria. The cost of the additives must be balanced against the cost of limestone that would become more reactive via the chemical supplement. An issue of importance that remains with spray tower systems is particulate removal via the mist eliminators. Slurry particles entrain with the flue gas and would flow out of the scrubber without some type of protection. Excess particulate emissions foul downstream components and cause opacity difficulties at the stack. The tried-and-true method for particulate removal continues to rest with mist eliminators (demisters), which normally consist of chevron-vane packing, situated at the top of scrubber towers. As the flue gas passes through the vanes, the particles impinge on the blades and stick. Regular washing with clear water is an absolute requirement to prevent severe buildups of solids on the vanes. A rule-of-thumb range for wash rates is 0.5-1.5 gpm per square foot of packing area. Washing does not have to be continuous, but it must be regular. If the wash water flow rate or frequency is insufficient to flush the particles, they will eventually convert to a hard scale, as the material continues to react with the flue gas flowing past.

New scrubbing technologies Several other scrubbing technologies have been gaining in popularity. One of these is the Chiyoda CT-121TM process (Chiyoda Corporation, U.S. licensee, Black & Veatch Corporation), in which the incoming flue gas is quenched and then injected via drop-down tubes into a tank containing the limestone slurry. The clean gas bubbles out of solution, flows through riser tubes to the outlet plenum, and then exits to the stack. Sparger immersion (a function of unit load) is adjusted by raising or lowering the slurry depth in the tank. As 

## Energy-Tech Magazine 6/30/2014

http://www.energy-tech.com/article.cfm?id=21789&PageNum=2 2 / 6Sparger immersion (a function of unit load) is adjusted by raising or lowering the slurry depth in the tank. As with spray towers, pH is continuously monitored to control limestone feed to the vessel. Also, forced oxidation to produce gypsum is a normal procedure. Although this system produces fewer effluent particulates than a spray tower, mist eliminators are still a requirement on the outlet gas stream. Other immersion types are available, the most notable of which are the Alstom FLOWPAC scrubber and the Double Contact Flow Scrubber (engineered by Advatech, LLC, a joint company of the URS Corporation and Mistubishi Heavy Industries, Ltd.) FLOWPAC is, to some extent, similar to the CT-121 process in that the flue gas passes through a slurry bath, but with vigorous air injection from below that helps to mix the gas and slurry and also to oxidize the byproduct. The Advatech process utilizes a slurry fountain with the gas introduced from below, such that it contacts slurry in both a co-current and counter-current manner. All three of these technologies have been selected for use in the United States, and will be examined further in a future Energy-Tech article.

References S. Blankinship, "Looking for a Good Scrubbing: Today's FGD Technology"; Power Engineering, pp. 19-24, September 2005. B. Buecker, "Gypsum Seed Recycle in Wet-Limestone Scrubbers"; unpublished paper presented at the Utility Representatives FGD Conference, June 10-12, 1986, Farmington, N.M. Weilert, C., Basel, B., and P. Dyer, "Startup and Initial Operation of the Wet Limestone FGD Retrofit at V.Y. Dallman Station Units 31 & 32"; from the 2001 Conference Proceedings Power-Gen International, Dec. 11- 13, 2001, Las Vegas, Nev. Pagcatipunan, C., and R. Schick, "Maximize the Performance of Your Spray Nozzle System"; Chemical Engineering Progress, pp. 38-44, December 2005. R. Rhudy, Project Manager, FGD Optimization Workbook, Report TR-111118, Electric Power Research Institute, Palo Alto, Calif., July 1998.

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Figure 3. O utline of Double-Loop® sy stem. Home Features Article E-mail to a Friend
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Figure 4. General schematic of a hy droclone. Home Features Article E-mail to a Friend
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Brad Buecker recently joined the Nalco Company as an Industry Technical consultant, after having served as an air quality control specialist and plant chemist for Kansas City Power & Light. He has previous experience as a chemical cleaning services engineer, a water and wastewater system supervisor, and a consulting chemist for Burns & McDonnell Engineers. He also served as a results engineer, flue gas desulfurization (FGD) engineer, and analytical chemist for City Water, Light & Power, in Springfield, Ill. Buecker has written many articles on steam generation, water treatment, and FGD chemistry, and he is the author of threebooks on steam generation topics published by PennWell Publishing, Tulsa, Okla. He has an AA in preengineering from Springfield College in Illinois and a BS in chemistry from Iowa State University. He is a member of the ACS, AIChE, ASME, and NACE. He can be reached at editorial@woodwardbizmedia.com.



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Figure 5. The general structure of adipic acid. Home Features Article E-mail to a Friend
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Brad Buecker recently joined the Nalco Company as an Industry Technical consultant, after having served as an air quality control specialist and plant chemist for Kansas City Power & Light. He has previous experience as a chemical cleaning services engineer, a water and wastewater system supervisor, and a consulting chemist for Burns & McDonnell Engineers. He also served as a results engineer, flue gas desulfurization (FGD) engineer, and analytical chemist for City Water, Light & Power, in Springfield, Ill. Buecker has written many articles on steam generation, water treatment, and FGD chemistry, and he is the author of three books on steam generation topics published by PennWell Publishing, Tulsa, Okla. He has an AA in pre-engineering from Springfield College in Illinois and a BS in chemistry from Iowa State University. He is a member of the ACS, AIChE, ASME, and NACE. He can be reached at editorial@woodwardbizmedia.com.



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