Advertisement

<< Prev 1 of 5 Next >>

Figure 1. Generic we-limestone FGD process flow diagram

utilizing an open spray tower.

Home Features Article E-mail to a Friend

E-mail Us

October 2008 Go to Page 1 2 3 4

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: SO2+H2↔H2SO3

Some theoretical chemists argue that true H2SO3

does not exist, and that SO2 retains its molecular

character and is surrounded by water molecules.

However, when SO2 is added to water, the pH

drops, which suggests that Equation 1 is correct

and that the following dissociation reaction is

accurate.

Eq. 2: H2SO3↔H

++HSO3-↔H++ SO3-2

Another argument for the formation of H2SO3,

and its dissociated products bisulfite (HSO3

-) and

sulfite (SO3

-2) ions, comes from the fact that the

principal component of limestone, calcium

carbonate (CaCO3), is only slightly soluble in water but will dissolve almost completely in well-designed

scrubbing systems. 

Eq. 3: CaCO3 + H+→ Ca+2 + HCO3

-

Equations 1, 2, and 3 combined illustrate the simplified, but fundamental, scrubbing process.

Eq. 4: CaCO3 + 2H+ + SO3

-2 → Ca+2 + SO3-2 + H2O + CO2↑

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 SO2

 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 + SO3

-2 + 1⁄2H2O CaSO3

·1⁄2H2↓

Oxygen in the flue gas greatly influences chemistry, as aqueous bisulfite and sulfite ions react with oxygen to

produce sulfate ions (SO4

-2). 

Eq. 6: 2SO3

-2 + O2 → 2SO4-2

Approximately the first 15 mole percent of sulfate ions co-precipitates with sulfite to form calcium sulfitesulfate hemihydrate [(0.85CaSO3

·0.15CaSO4) ·1⁄2H2O]. Any sulfate above the 15 percent mole ratio

precipitates with calcium as gypsum.

Eq. 7: Ca+2 + SO4

-2 + 2H2O → CaSO4

·2H2O↓

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. 

Go to Page 1 2 3 4

Energy-Tech Magazine 6/30/2014

http://www.energy-tech.com/article.cfm?id=21789 1 / 6
Advertisement

<< Prev 2 of 5 Next >>

Figure 2. A n original first-generation FGD design.

Home Features Article E-mail to a Friend

E-mail Us

October 2008 Go to Page 1 2 3 4

Important concepts of wet-limestone flue gas

desulfurization

BRAD BUECKER

Page 2 of 4

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 SO2 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 SO2

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 SO2 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 (MgCO3·CaCO3), in which the magnesium is

bonded with calcium in the dolomite crystal lattice. While pure MgCO3 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 SO2 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 SO2 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 / 6
Sparger 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. 

Go to Page 1 2 3 4

Energy-Tech Magazine 6/30/2014

http://www.energy-tech.com/article.cfm?id=21789&PageNum=2 3 / 6
<< Prev 3 of 5 Next >>

Figure 3. O utline of Double-Loop® sy stem.

Home Features Article E-mail to a Friend

E-mail Us

October 2008 Go to Page 1 2 3 4

Important concepts of wet-limestone flue gas

desulfurization

BRAD BUECKER

Page 3 of 4 <

Advertisement

Go to Page 1 2 3 4

Energy-Tech Magazine 6/30/2014

http://www.energy-tech.com/article.cfm?id=21789&PageNum=3 4 / 6
<< Prev 4 of 5 Next >>

Figure 4. General schematic of a hy droclone.

Home Features Article E-mail to a Friend

E-mail Us

October 2008 Go to Page 1 2 3 4

Important concepts of wet-limestone flue gas

desulfurization

BRAD BUECKER

Page 4 of 4

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 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.

Advertisement

Go to Page 1 2 3 4 -EndEnergy-Tech Magazine 6/30/2014

http://www.energy-tech.com/article.cfm?id=21789&PageNum=4 5 / 6
<< Prev 5 of 5 Next >>

Figure 5. The general structure of adipic acid.

Home Features Article E-mail to a Friend

E-mail Us

October 2008 Go to Page 1 2 3 4

Important concepts of wet-limestone flue gas

desulfurization

BRAD BUECKER

Page 4 of 4

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.

Advertisement

Go to Page 1 2 3 4 -EndEnergy-Tech Magazine 6/30/2014

http://www.energy-tech.com/article.cfm?id=21789&PageNum=4 6 / 6