# REVIEW OF HANDLING AND USE OF FGD

MATERIAL

CARRC Topical Report

Prepared for:

Mr. Robert Patton

U.S. Department of Energy National Energy Technology Laboratory

626 Cochrans Mill Road PO Box 10940, MS 922-273C Pittsburgh, PA 15236-0940

U.S. Department of Energy Coopera tive Agreement No. DE-FC26-98FT40321

Prepared by:

Tera D. Berland Debra F. Pflughoeft-Hassett

Bruce A. Dockter

Kurt E. Eylands

David J. Hassett

Loreal V. Heebink

Energy & Environmental Research Center University of North Dakota

PO Box 9018 Grand Forks, ND 58202-9018

April 2003

# REVIEW OF HANDLING AND USE OF FGD

MATERIAL

CARRC Final Report

Prepared for:

Mr. James Roewer

Utility Solid Waste Activities Group 701 Pennsylvania Avenue Northwest

Fifth Floor Washington, DC 20004-2696

CARRC Members

U.S. Department of Energy Coopera tive Agreement No. DE-FC26-98FT40321

Prepared by:

Tera D. Berland Debra F. Pflughoeft-Hassett

Bruce A. Dockter

Kurt E. Eylands

David J. Hassett

Loreal V. Heebink

Energy & Environmental Research Center University of North Dakota

PO Box 9018 Grand Forks, ND 58202-9018

2003-EERC-04-04 April 2003DOE DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United

States Government. Neither the United States G overnment, nor any agency thereof, nor any of their employees makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not n ecessarily constitute or imply its endorsement, recommendation, or favoring by the United States G overnment or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

ACKNOWLEDGMENT

This report was prepared with the support of the U.S. Department of Energy (DOE) National Energy Technology Laboratory Coop erative Agreement No. DE-FC26-98FT40321. However, any opinions, findings, co nclusions, or recommendations e xpressed herein are those of the authors(s) and do not necessar ily reflect the views of DOE.

The Authors gratefully acknowledge the input of CARRC members, with special thanks to Ms. E. Cheri Miller, Tennessee Valley Authority , who provided insightful review and comment.

EERC DISCLAIMER

LEGAL NOTICE This research report was prepared by the Energy & Environmental Research Center (EERC), an agency of the Univ ersity of North Dakota, as an account of work sponsored by DOE. Because of the research natu re of the work performed, neither the EERC nor any of its employees makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or re presents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or othe rwise does not necessarily consti tute or imply its endorsement or recommendation by the EERC.i TABLE OF CONTENTS

LIST OF FIGURES ................................................................................................................ .......iii

LIST OF TABLES................................................................................................................. ........iii

INTRODUCTION ................................................................................................................... ....... 1

BACKGROUND ..................................................................................................................... ....... 1

DISTRIBUTION OF FGD SYSTEMS .......................................................................................... 3

COMPARISION OF FGD SYSTEMS........................................................................................... 3 Wet FGD Systems................................................................................................................ ....... 4

Limestone-Forced Oxidation .................................................................................................. 4 Limestone-Forced Oxidation/Organic Acid ........................................................................... 5

Lime Dual Alkali Process ....................................................................................................... 5 Magnesium-Promoted Lime ....................................................................................................... 5

Seawater Processes ............................................................................................................. .... 5 Sodium Scrubbing Process ..................................................................................................... 5 Ammonia Scrubbing .............................................................................................................. . 6 Dry FGD Systems................................................................................................................ ....... 6 Lime Spray Drying .............................................................................................................. ... 7 Duct Sorbent Injection (DSI).................................................................................................. 7 Furnace Sorbent Injection (FSI).............................................................................................. 7 Circulating Fluidized Bed...................................................................................................... . 7

FGD MATERIAL CHARACTERISTICS ..................................................................................... 7 Chemical and Mineralogical Composition ................................................................................. 8 FGD Material from Wet FGD Systems .................................................................................. 8 FGD Material from Dry FGD Systems..................................................................................... 11 Physical and Engineering Pr operties of FGD Material............................................................. 12 FGD Material from Wet FGD Systems ................................................................................ 12 FGD Material from Dry FGD Systems................................................................................. 12

FGD AS A MULTIPOLLUTANT CONTROL TECHNOLOGY ............................................... 13 Background on Mercury Emission ........................................................................................... 13 Background on Regulations...................................................................................................... 14 Control Technologies........................................................................................................... . 16

Human Risk ..................................................................................................................... ..... 16

HANDLING OF FGD MATERIAL............................................................................................. 17

Continued . . .ii TABLE OF CONTENTS (continued)

FGD MATERIAL UTILIZATION............................................................................................... 18

ECONOMIC ISSUES ASSOCIATED WITH FGD UTILIZATION .......................................... 21

DISPOSAL PRACTICES AN D REQUIREMENTS ................................................................... 23

FUTURE FGD SYSTEM INSTALLATIONS AND FGD MATERIAL MANAGEMENT....... 23

SUMMARY........................................................................................................................ .......... 25

REFERENCES ..................................................................................................................... ........ 25

FGD BY-PRODUCT GENERATED BY STATE, 1994..............................................Appendix A

FGD SYSTEMS AND MATERIALS – DEFINITIONS ..............................................Appendix B

FGD SYSTEMS AND MATERIALS – DEFINITIONS ..............................................Appendix Ciii LIST OF FIGURES

1 Comparison of wet–dry and regenerable FGD systems.........................................................4

2 Diffraction of spray dryer material.......................................................................................13

3 FGD material utilization applications ..................................................................................19

LIST OF TABLES

1 Typical Chemical Comp osition of FGD Residues.................................................................9

2 Chemical Analysis of FGD Gypsum......................................................................................9

3 Chemical Analysis of Spray Dry Scrubber Material............................................................11

4 Gypsum Production in the United States .............................................................................18

5 Potential Uses of Dry FGD Material....................................................................................211 REVIEW OF HANDLING AND USE OF FGD MATERIAL

INTRODUCTION

Flue gas desulfurization (FGD) material is one of the “four large-volume wastes from the combustion of coal by electric utility power plants” as defined by the U.S. Environmental Protection Agency (EPA) (Federal Register; Code of Federal Regulations, Part 261, Title 4). FGD material is produced from the captu re or scrubbing of sulfur dioxide (SO 2 ) emissions using a sorbent. FGD technologies were developed and refined to aid coal-fired powe r plants in achieving emission reductions of SO 2 mandated in the 1990 Clean Air Act Amendments (CAAA).

In 1987, the American Coal Ash Association (ACAA) reported that 14.2 million tons of FGD material was produced in the United Stat es. In 2000, that number nearly doubled to 26 million tons. In 1987, only about 1% of the F GD material produced was utilized, but in 2000, nearly 20% of the annua l production was utilized. Despite the large increase in utilization, FGD material still remains a vastly underutilized, va luable material. Emission control regulations generally have a direct impact on coal com bustion byproduct (CCB) production, and while EPA “encourage[d] the utilization of coal combustion byproducts and support[ed] State efforts to promote utilization in an envir onmentally beneficial manner,” fe deral actions are expected to impact the volumes of FGD mate rial produced annually in the Un ited States and potentially some management options for CCBs including F GD material. In fact, the annual production of FGD material is predicted to increase significantly in coming years as the number of FGD systems is expected to double in the next 7 years in response to new emission regulations (Government Panel, 2001). This production incr ease in FGD material, coupled with future federal guidance on the disposal and mine pl acement of CCBs, including FGD material, may have a significant impact on utility CCB mana gers and others invol ved in CCB management.

The purpose of this report is to provide in formation on the state of the FGD production; advantages and disadvantages of various FGD systems and materials; chemical and physical

characteristics of FGD material s; current practices for handli ng, disposing, and utilizing these materials; and other information for those interested in CCB management.

BACKGROUND

Emissions from electric generating facilities, such as SO 2 , NO x, and mercury, can cause detrimental impacts to human health and the environment. Although concentrations are low, electric generation facilities account for the majority of SO 2 and mercury emissions in the United States. The major health concerns associated with exposure to hi gh ambient concentrations of

SO 2 include breathing difficulty, respir atory illness, and aggravati on of existing cardiovascular disease. In addition to health impacts, SO 2 leads to acid deposition in the environment. This deposition causes acidification of lakes and stream s and damage to tree foliage and agricultural crops. Furthermore, acid deposition accelerates th e deterioration of buildings and monuments (Srivastava and Jozewicz, 2001). Atmospheric merc ury concentrations are usually low and of2 little direct concern; however, atmospheric mercur y contributes to the mercury loading in surface

waters where it can bioaccumulate in fish and an imal tissue in the form of, methylmercury (EPA, 1998).

The CAAA were designed to reduce SO 2 and NO

x air emissions. No mercury control policies from electric facilitie s are specified in the CAAA. Mercury emission reductions reflecting lower mercury release than availa ble from coal are due to emission control technologies for other pollutants, such as SO 2 and NO x, that can also incidentally capture mercury. Currently, emission control technologies are fairly broadly applied for control of particulate, SO 2 , and NO

x from coal-fired power plants. U.S. utilities generally employ one of two strategies to control SO 2 in the flue gas stream: 1) F GD units or 2) compliance fuels. Particulate control is typically accomplished by the use of fabric filters (also called baghouses or electrostatic precipitators [ESPs]). NO

x is typically cont rolled through low-NO

x burners, overfire, selective catalytic reduction (SCR), selective noncat alytic reduction (SNCR), or a combination of these technologies. Particulate NO

x control technologies w ill not be discussed in this report except in relation to the issue to SO 2 control. Other methods including fluidized bed combustion (FBC) units can also control SO 2 . Many western coals and some eastern coals are naturally low in sulfur and can be used to meet SO 2 compliance requirements. Utilities may also physically clean or wash all or part of the fuel prior to combustion. Physical coal cleaning at the mine, transshipment point, or power plant is qu ite widespread in the United States not only because it reduces emissions, but also because an increase in steam generator efficiency is often possible if the fuel feedstock can be made more homogeneous. Utilities ma y also blend coals of

different sulfur contents to achieve a mix that is in compliance with applicable regulations. Most utilities, approximately 70%, utiliz e compliance fuel to achieve the SO 2 emission levels currently mandated. According to the U.S. Department of Energy (DOE, 1997), FGD scrubbing unit utilization is estimated to cost $322 per ton of SO 2 removed and is the most expensive compliance method, although costs vary widely across regions. Also, according to DOE, modifying a high-sulfur bituminous coal-fired pl ant to burn lower-sulfur subbituminous coal is estimated to cost $113 per ton of SO 2 removed and is the least expensive method. Although FGD scrubbing units are the most expensive complian ce method, FGD can be cost-effective. The type of compliance method implemented needs to be car efully evaluated for th e specific coal-fired plant.

Approximately 22% of utilities in the United States utilize FGD systems to achieve the currently mandated SO 2 emission levels. This percentage is expected to double in the next 7 years in response to emission regulations (G overnment Panel, 2001). FGD systems can be classified as either wet or dry systems, and both systems produc e a byproduct generically

referred to as FGD material. FGD material is a high-volume byproduct included by EPA as one of the four high-volume CCBs in its 1993 RCRA (Resource Conservation and Recovery Act). The four high-volume CCBs (fly ash, bottom as h, boiler slag, and FGD ma terial) are designated

as solid wastes under RCRA Subtitle D as a re sult of RCRA. In the second phase (Government Panel, 2001) of the evaluation, EPA supported this determination.

Nearly 26 million tons of FGD material was generated in the United States in 2000, but less than 20% of that production was utilized according to the ACAA statistics on “Coal Combustion Product Production and Use” (ACAA, 2001) . These statistics do not differentiate3 between FGD material generated from wet or dry FGD systems, but a review of existing FGD systems indicates that wet FGD systems are the preferred control technology, accounting for over 80% of the total FGD capacity worldwide (IEA, 2000).

DISTRIBUTION OF FGD SYSTEMS

Coal-fired power plants in the United Stat es and Japan began using FGD equipment in the early 1970s. Western Europe fo llowed in the 1980s. In the 1990s, the application of FGD became more widespread and today includes countri es in central and eastern Europe, Asia, and elsewhere. Today, there are approximately 1140 coal -fired power plants in the United States. The states generating the bulk of the FGD material in 1996 were Texas, Kentucky, and Indiana, accounting for almost 50% of total FGD mate rial produced (Kalyoncu and Matos, 1997). An illustration of FGD generation by state is included in Appendix A.

According to the IEA Coal Research, the most widely used FGD systems in the world are wet scrubbers using calcium-based sorbents (IEA, 2000). Initially, these systems produced nonusable residues; however, systems produci ng a salable byproduct are now preferred. Wet FGD systems are currently installe d on about 25% of the coal-fired utility generating capacity in the United States, representing about 15% of the number of coal -fired units (M cDonald, 2000). Spray dry scrubbers and sorbent in jection installations are growing in use in the United States and Europe, especially on small units. Despite their potentially high-value byproducts, these processes have achieved only limited use. Figure 1 illustrates the number of wet and dry systems

in the United States and worldwide.

Other types of SO 2 removal technologies, such as FBC systems, can be effective. FBC is a dry process that is primarily in stalled on smaller-scale industria l systems because FBC units are effectively installed as a separate system. The system mixes the coal with a sorbent such as

limestone or other bed material. The coal and bed material mixture is fluidized during the combustion process to allow complete combusti on and removal of sulfur gases. The ash is collected in the flue gas usi ng a baghouse or ESP, and the bed residue is removed from the

bottom of the boiler.

COMPARISION OF FGD SYSTEMS

FGD technologies use an alkaline slurry to absorb SO 2 in the flue gas, producing sodium or calcium–sulfur compounds. According to Sr ivastava (2001), commerc ially available FGD technologies can conventionally be classified as once-through and regenerable, depending on

how the sorbent is treated after it has sorbed SO 2 . In one-through technologies, the SO 2 is permanently bound by the sorbent. In regenerable technologies, the SO 2 is released from the sorbent during the regeneration step and may be further processed to cr eate sulfuric acid, elemental sulfur, or liquid SO 2 . Both the once-through and re generable technologies can be further classified as wet or dry, depending on if the reagent is wet or dry when it leaves the absorber. New FGD processes, especially those combining SO 2 and NO

x removal, are continually being developed and are disc ussed further, later in this report.4 Figure 1. Comparison of wet–dry and regenerable FGD systems.

Wet FGD Systems

Wet FGD systems are the most widely used FGD technology. The use of a widely available and inexpensive sorbent (limest one), production of a usable byproduct (gypsum, calcium sulfate dehydrate, CaSO 4 2H 2 O), reliability, availability, and most importantly, the efficiency achievements that can be as high as 99% are the incentives behind the popularity of this technology especially at large-scale utilities. In 2000, 87% of FGD systems in the world were wet systems; this number is expected to increase even more as emission regulations are adopted. Although capital costs are higher with this technology, operating costs are lower, making this system very cost effective. Al so, the production of a valuable gypsum byproduct contributes toward reducing operating costs. We t limestone scrubbers, which produce a mixture of calcium sulfate/sulfite and fly ash, have a smaller share of the market compared to the gypsum-producing processes. The disposal costs of the byproducts are di scouraging the use of this technology in countries wher e disposal sites are unavailable or regulation prohib its it. In the United States, several plants have converted th eir wet lime/limestone waste plants to gypsumproducing facilities, and this trend is expected to continue (Soud, 2000).

Limestone-Forced Oxidation

The limestone-forced oxidation process, wh ich minimizes scaling problems in the absorber, is the preferred FGD technology worldw ide. This technology i nvolves recirculating a slurry of limestone and water th rough absorbers where it absorbs SO 2 in the flue gas. Air is bubbled through the calcium sulfat e–sulfite slurry to form gypsum. The byproduct dewaters easily and results in less wet waste volume. Th is process is a proven technology that has been used extensively in the United St ates and can achieve nearly comp lete oxidation. There are more than 10 active suppliers availabl e for this process. The process performs best with medium- to5 high-sulfur coals and does not respond quickly to load change. Economically, this process uses a low-cost reagent and consumes between 1.6%–1.8% of gross power (Miller, 2002).

Limestone-Forced Oxidation/Organic Acid

This process is the same as the limestone-for ced oxidation process described above except that it adds an organic acid to the slurry to incr ease the limestone dissolution rate which results in a higher SO 2 removal (Miller, 2002).

Lime Dual Alkali Process

This process uses sodium sulfate solution as an absorbing agent that is recirculated through the spray tower to remove SO 2 . The spent solution is mixed with lime in a separate process loop simultaneously forming a calcium sulfite sludge while regenerating the spent sodium sulfite solution. Advantages of this process are lowe r corrosion and erosion and less scaling and plugging. Disadvantages include the high cost of lime and soda ash reagents. Currently, there is no supplier for this technology in th e United States (Miller, 2002).

Magnesium-Promoted Lime

Either a magnesium-enhanced lime (typically 5%–8% magnesium oxide) or dolomitic lime (typically 20% magnesium oxide) is used in this process. This slurry is more reactive than limestone slurry but is al so more expensive (Srivast ava, 2000). The lime slu rry is fed to the spray tower to form a calcium sulfite sludge, which also contains a small amount of magnesium sulfite.

If forced oxidation external to the absorber is used, the quality of the gypsum will be improved. Commercial-grade gypsum produced from this process is brighter than that from conventional wet systems, resulting in a higher commercial va lue (Srivastava, 2000). Advantages to this system include a lower liquid-to-gas ratio and reduction in scaling and plugging. It is estimated

that there are from three to five s uppliers for this process (Miller, 2002).

Seawater Processes

The seawater processes use the natural al kalinity of seawat er to neutralize SO 2 . The chemistry of the process is similar to the limestone-forced oxidation chemistry except the limestone comes completely dissolved with se awater and the chemistr y does not involve any dissolution or precipitation of solids. The sulfate is completely dissolved in seawater so as a result, there is no waste product to dispose of (Srivastava, 2000). This process has achieved

limited use.

Sodium Scrubbing Process

The primary absorbing material for this wet system is a sodium sulfite solution that is recirculated through the sp ray tower to remove SO 2 in the flue gas stream. Sodium carbonate is used as the reagent. Like the lime dual alkali pro cess, the advantages of this process are lower corrosion and erosion and less scaling and plugging. A disadvantage is the high cost of soda ash6 reagents. This system has been proven at seve ral locations in the United States; however, there currently are no suppliers of this technology (Miller, 2002).

Ammonia Scrubbing

This process is similar to ot her wet technologies except it us es highly reactive ammonia as a reagent. Ammonium hydroxide reacts with SO 2 to form ammonium sulfite, which is oxidized

to ammonium sulfate. This technology can provide SO 2 removal greater than 95%. This process is also capable of removing other acid gases su ch as sulfur trioxide and hydrogen chloride (Srivastava, 2000).

Marsulex Environmental Technologies (MET) is the major promoter of this technology. Alstom and Lurgi also have a version of this technology (Miller, 2002). According to MET (Walsh, 1999), this technology work s best with power plants that match the following profile:

• High fuel cost

• Low load factor

• Proximity to navigable water or good rail access for pet coke, ammonia, and ammonium sulfate transportation

• Preferably in location with high ammonium sulfate prices

MET’s patented technology claims it enables the use of low cost , 5%–7% sulfur fuels without the associated sulfur penalty.

The greatest benefit of this technology is that its high-value byproduc t, ammonium sulfate,

can be sold as fertilizer. It is important to note that the value of this product will depend largely on the market and product quality has been an issu e of concern in the past. MET suggests that for every ton of ammonia utilized, approximately f our tons of ammonium sulfate is generated (Walsh, 1999). MET’s technology is currently inst alled on the Dakota Ga sification Plant in Beulah, North Dakota. Two additional installations are planned for the Santee Cooper Facility in

South Carolina and the Mildred Lake Oil Sands Facility in Alberta, Canada (Marsulex, 2001)

Dry FGD Systems

Dry FGD systems are the second-most wide ly used FGD technology. This technology is

considered efficient and reliable and has lower capital costs th an wet scrubbers. However, its operating costs are higher because of the use of a more expensiv e sorbents. In addition, most byproducts from this technology cannot be utilized a nd need to be disposed of because they are usually made of a mixture of calcium sulf ite, calcium sulfate, and fly ash (Soud, 2000).7 Lime Spray Drying

The process consists of mixing hot flue gas in a spray dryer ve ssel with a mist of finely

atomized fresh lime. The SO 2 in the hot flue gas reacts with the sorbent as the water in the slurry evaporates. The dried solids are collected in th e bottom of the vessel in a particulate control device. This technology is most of ten used for power plants that are 550 MW or larg er that burn low-to-medium-sulfur coal, between 0.4% and 2%. An average of 90% SO 2 removal can be achieved (EPA, 2002b). The resulting FGD material is frequently referred to as spray dry ash or

material and is a calcium sulfite-rich material.

Duct Sorbent Injection (DSI)

DSI is intended to enable SO 2 control directly in the flue gas duct between the air preheater and the particulate control device. Hydrated lime is typically used as a sorbent, and occasionally sodium bicarbonate is used (Srivastava, 2000).

Furnace Sorbent Injection (FSI)

In the FSI system, a dry sorbent is injected directly into the furnace in the optimum temperature region above the flame. Because of the high temperature (approximately 1000°C), sorbent particles decompose and become porous solids with high surface areas (Srivastava, 2000).

Circulating Fluidized Bed

In this system, a dry sorbent, most often Ca(OH) 2 , is contacted with hum idified flue gas in a fluidized bed. The combustion fluidized bed pr ovides a long contact time between the sorbent and flue gas has because the sorbent passes thro ugh the bed several time s (Srivastava, 2000). The solids are collected in an ESP or baghouse. A la rger particulate collector may be needed to maintain the required particulate emission leve ls compared with a non-circulating sorbent.

This process is not widely used in the United States, and is primarily used in Germany for units ranging from 50 to 250 MW (Srivastava, 2000).

FGD MATERIAL CHARACTERISTICS

According to an IEA Coal Research (Clarke, 1993) study, differences between mineralogical, chemical, and phys ical characteristics of FGD ma terial, from both similar and different technologies, can be attributed to a combination of the following factors:

• Composition of the coal feedstock and sorbents

• Combustion conditions • Composition and mineralogy of the fly ash • Relative amounts of fly ash, unreacted sorbent, and desulfurization products • Reagent ratios8 Chemical and Mineralogical Composition

FGD Material from Wet FGD Systems

Wet FGD systems frequently use calcium-bas ed sorbents and pr oduce either wet FGD material (unoxidized wet FGD ma terial or sulfite-rich wet FGD material) or FGD gypsum (from forced-oxidation systems). These materials have similar bulk chemical compositions but have different mineralogical compositions. The chemi cal composition of wet FGD material depends largely on the sorbent used for desulfurization and the proportion of fly ash collected with the FGD residues. In most wet FGD systems, FGD residues are collected as a separate byproduct stream (Clarke, 1993). Since flue gas contains sulfur primarily as SO 2 , the initial material formed is calcium sulfite (CaSO 3 ). To generate calcium sulfate, the material must be oxidized. This can be done in the scrubber system with a process cal led in situ forced oxidation. In this process, excess air is added to the system to oxidize th e calcium sulfite to calcium sulfate as shown

below:

CaCO 3 + SO 2 → CaSO 3 +CO 2 → CaSO 3 + 1⁄2O 2 CaSO 4 [Eq. 1]

If it is desired or advantage ous to oxidize the FGD material as a slurry outside of the scrubber, this can be easily don e by passing either oxygen or ai r through a slurry acidified to around pH 4.5. The pH adjustment is necessary as the oxidation proceeds through the bisulfite (Ca[HSO 3 ] 2 ). It is necessary to form the bisulfite through acidification because of the extremely low solubility of the calcium sulfite. Calcium bisulfite is much more soluble, and because the ex situ oxidation occurs in solution, the much higher solubility of the bisulfite increases the reaction

rate.

Both wet FGD material and FGD gypsum are primarily crys talline in their morphology. FGD gypsum is composed of finely divided cube - or rod-shaped crystals ranging from 1–250 μm (Coal Research Establishment [CRE], 1992). Th e purity of FGD gypsum typically ranges from 96%–99% (Coclough and Carr, 1991). Wet sulfite-ric h FGD material is composed primarily of calcium sulfite hemihydrate (hannebachite).

Table 1 lists the typical bulk chemical com position for wet FGD material from systems in which fly ash is removed before the scrubbe r. The chemical compositions of bituminous, subbituminous, and lignite fly ash are included for comparison purposes.

In some systems, fly ash is used as part of the sorbent. High-calcium fly ash can be used but has been reported to be more abrasive than limestone. When fly ash is used as part of the sorbent, the bulk chemistry and mineralogy of the FGD material will reflect the chemistry and mineralogy of the fly ash based on the percentage of fly ash used.

The major and trace element compositions of several samples of FGD gypsum from Germany, Japan, the United Kingdom, and the United St ates are listed in Ta ble 2. Most of the major and trace elements leave the FGD system in the gypsum or wet sulfite-rich FGD material. Al, As, Ca, Fe, Pb, Sb, Si, and Ti are the elemen ts mainly left in the gypsum. Between 10% and 40% of Al, As, Cr, Cu, F, Fe, Ni , Sb, Sc, Si, Sm, Ti, U, V, and W leave the plant in the wet9 Table 1. Typical Chemical Composition of FGD Residues

Wet

Sulfite-rich

FGD Material FGD Gypsum Fly Ash (bituminous) Fly Ash (subbituminous) Fly Ash (lignite)

SiO 2 0.1–7.4 0.1–6.3 43–64 30–45 12–49

Al 2 O 3 0.1–6.0 0.1–5.1 11–32 17–21 6.3–16

Fe 2 O 3 0.1–6.0 0.1–5.7 3.5–25 3.9–6.5 3.7–14

CaO 38–52 27–32 0.7–20 17–30 16–34 MgO 1.3–6.1 1.0–4.9 0.6–1.6 3.4–6.4 3.8–11

Na 2 O 0.1–0.9 0–0.6 0.2–4.0 0.2–8.4 0.6–6.1

K 2 O 0–0.6 0–0.6 0.5–3.0 0.3–1.0 0.4–2.1

SO 3 2- 54–63 44–46 0.1–11 1.1–5.0 0.4–30 Typical Solids Content 5–10 5–10

Sources: Smith, 1992; CARRC Database.

Table 2. Chemical Analysis of FGD Gypsum*

A B C D E F G H I J K L M Nat.
Trace Elements, ppm

As <1 1 1 1 1 1 1 1 1 2 1 1 1 <1

B <5 <5 11 10 100 10 11 4 29 <5 21 11 24 9

Ba 12 9 7 6 6 <1 <1 <1 <100 13 <1 6400 200 4

Cd <1 <1 <1 <1 <1 <1 <1 <1 <1 2 <1 <1 1 <1

Cr 4 5 10 3 4 4 4 6 4 5 9 24 3 1

Cu 64 9 12 3 5 3 <1 <1 5 7 <3 3 3 7
Hg <3 <3 1 1 <1 <1 <1 <1 <1 <3 <1 <1 <1 <3

Mn 10 11 28 49 32 15 3 17 8 94 10 35 15 4

Mo <1 <1 <1 <2 <1 <3 <3 <3 <2 <1 <3 <3 <2 <1

Ni 19 4 6 4 7 <3 <3 <3 5 17 <3 <3 3 2

Pb 1 2 7 3 <2 4 1 12 2 3 1 47 28 <1

Se 3 2 15 10 11 8 9 12 3 <1 4 6 <1 <1

V 8 16 12 12 12 10 10 10 10 21 12 10 10 27

Zn 10 8 3 <1 7 5 3 5 8 16 5 20 8 5 * A–I: Germany, J: Japan, K–L: USA, M: UK, Nat.: natural mineral gypsum sample. Source: CRE, 1992.

sulfite-rich FGD material. The am ounts of major, minor, and tr ace elements included with the FGD material are highly dependent on the amount of ash also included. If ash is excluded, the

contribution of trace elements in the FGD materi al is limited to those that are air toxic constituents and are present in the flue gas either as extremely fine particulates or vapors. Some elements, especially the more volatile ones, are removed mainly in the wastewater (Meij, 1989). It is recognized that the mercur y content of FGD materials is pr imarily determined by the species produced in the combustion system . Oxidized mercury, likely present as mercuric chloride, is highly retained in FGD systems, while elem ental mercury mostly passes through the FGD10 system. The Energy & Environmental Research Ce nter (EERC) (Hassett and others, 1997) noted that some trace elements were present in m easurable quantities in FGD gypsum produced in pilot-scale ex situ oxidation of wet FGD materi al. These elements were mercury, chromium, and chlorine (present as chloride), and although they were present in measurable amounts, it is important to note that the mercury and chromium were present at very low levels, with leachate concentrations less than the prim ary drinking water standard leve ls. Chloride was present at 110 ppm. Although it was known from previous analyses that trace elements we re present in the wet FGD material, it was anticipated that the oxidati on process used would faci litate the removal of contaminants. In follow-up work, solid materials, reaction waters, and leacha te were all analyzed

in an effort to determine the fate of these tr ace constituents relative to the FGD gypsum produced using the air oxidation process. Du ring the oxidation of the wet FGD material, many of the trace elements partitioned to the liquid phase becaus e of recrystallization purification as well as lowering of solution pH. Certain tr ace elements were present in measurable concentrations in the produced FGD gypsum, but in leaching experiments, only a small percentage of the total was released to the leachate. Mercury behaved differently than other trace elements in that it retained

its association with the FGD throughout the oxidation process. It is not known what the mechanism of mercury association is, but desp ite the dissolution–oxidation–recrystallization process that the material undergoes, mercury is associated with both the pre- and post-oxidation material. The concentrations remaining are, fo r the most part, immob ile under water leaching. Findings of trace element partitio ning and fate in the ex situ oxi dation of wet FGD material are

summarized below.

• Potentially problematic trace elements, includi ng heavy metals and chloride appear to partition into the liquid phase during the oxidation process, likely from recrystallization purification of the product as well as pH adjustment to the 4.5 necessary for air

oxidation.

• Concentrations of many of the potentially problematic trace elements are not especially high.

• Mercury sorbed or otherwise captured in wet FGD systems may tend to remain

associated with the solids if ex situ ox idation is used for the production of gypsum.

• Trace elements remaining in the scrubbe r sludge at measurable or potentially problematic levels have been found to be leach ed at concentrations at or below primary drinking water standards.

• The degree of dewatering that can be acco mplished will account for concentrations of trace elements in the produced gypsum that are inversely pr oportional to the efficiency of dewatering. This is because of the partit ioning of the trace elements into the liquid phase during the oxidation process.

Although the use of in situ forced oxidation ap pears to be a superior alternative for the production of gypsum without the bother of the a dditional solution chemistry required by an air oxidation system, the material produced in a for ced-oxidation system will likely be of a lower overall purity with respect to total calcium sulfate content si nce unreacted calcium hydroxide11 will be present in its original form and trace elements trapped in the scrubber material will remain there in original concentrations. These are favorably partitioned into the liquid phase during air oxidation, esp ecially heavy metals.

FGD Material from Dry FGD Systems

Like wet FGD materials, the chemical co mposition of spray dryer material residues depends on the sorbent used for de sulfurization and the proportion of fly ash collected with the

FGD residues. Table 3 lists th e major and trace element com positions of some spray dryer materials. Spray dryer FGD materials consist of fly ash entrained with reacted and unreacted

Table 3. Chemical Analysis of Spray Dry Scrubber Material

A B C D E Major Elements,wt%

SiO 2 7.9 10.6 8.7 47.1 32

Al 2 O 3 4.7 4.5 4.1 24.8 15.3

Fe 2 O 3 2.2 4.4 1.9 10.1 3.8

CaO 1.9 1.5 1.0 1.6 24.1 MgO 37.8 35.6 33.6 4.8 2.9

Na 2 O 0.3 0.3 0.2 0.9 1.67

K 2 O 0.4 0.6 0.3 2.6 0.52

CO 2 17.0 20.1 10.6 –

SO 3 2- 30.0 17.4 18.8 –

SO 4 2- 10.5 7.0 24.6 1.2

Cl - 2.2 0.9 3.0 <0.1 C (org.) 0.2 0.2 0.1 1.0
Trace Elements, ppm

As 5 11 10 110

B 100 115 150 200

Ba 465 270 4000 1500

Cd 1 3 7 <3

Cr 40 50 34 145

Cu 47 43 46 230 Hg <1 <1 <1 1

Mn 270 420 170 780

Mo 3 5 <3 22

Ni 32 36 35 170

Pb 28 110 57 205

Se 4 5 7 7

V 60 65 70 280

Zn 94 380 130 370
A–C: spray dry scrubber residues, D: bituminous fly ash, E: MN FGD. A–D Source: CRE, 1992; E Source: Solem-Tishmack, 1993.12 sorbent. The average particle size ranges from 20–40 μm. The fly ash in dry FGD materials has similar particle size, density, and morphology to those of conventional fly ashes, but FGD materials have lower bulk densities (Thompson a nd others, 1988). The difference in bulk density

is due to variations in the chem ical and mineralogical characteris tics of the reacte d and unreacted sorbent. Dry FGD materials contain higher con centrations of calcium and sulfur and lower concentrations of silicon, aluminum, and iron th an fly ash. The principal reaction product of FGD is hannebachite (calcium sulfite hemihydrate) (Solem-Tishmack, 1993):

CaO (lime) + SO 2 (g) + 1⁄2 H 2 O → CaSO 3 · 0.5H 2 O [Eq. 2]

Under more oxidizing conditions , gypsum may also form:

CaO (lime) + SO 2 (g) + 1⁄2 O 2 + 2H 2 O → CaSO 4 · 2H 2 O [Eq. 3]

Unused sorbent remains as portlandite (calci um hydroxide) in the residuals (Solem-Tishmack, 1993).

Dry FGD materials can vary widely in th eir properties depending on their source. Thompson and others (1988) collected dry FGD ma terials from different plants that used different types of boilers and burned different types of coal. They f ound the residuals had a moderate to wide range of vari ability in their chemical, physica l, and mineralogical properties. The amount of unused sorbent (portlandite) varied the most by a factor of 14. This variation depended on whether or not th e plant recycled its residuals.

Physical and Engineering Pr operties of FGD Material

FGD Material from Wet FGD Systems

The physical properties of wet FGD materials va ry significantly depe nding on the relative proportions of sulfate and sulfite from each syst em (Clarke, 1993). The average bulk density of gypsum depends on the particle shape with approximately 1200 kg/m 3 for blocky crystals and roughly 600 kg/m 3 for needlelike crystals. Wet sulfite-rich FGD material historically was termed “scrubbing or FGD sludge” because of its thixot ropic properties. The term “sludge” is less commonly used by the coal ash industry. Thixotropi c properties enable it to stiffen in a relatively short time on standing but, upon agitation or mani pulation, to change to a very soft consistency or to a fluid of high viscosity; the hardening process is completely reversible. Wet sulfite-rich FGD material is difficult to dewater; however , its physical properties can be modified by “fixation” or “stabilization” with fly ash, portland cement, or other additives.

FGD Material from Dry FGD Systems

The overall physical and handling properties of the residues from most spray dryer materials are similar to fly ash. The bulk density of spray dry system residues, with only a small proportion of ash, is about 600 kg/m 3 compared to 1000 kg/m 3 for fly ash (Jons, 1987; CRE,13 1992). Particle-size distributions of these materials indicate that most mate rials can be classified as silt-size range. The bulk density of residues fr om spray dryer FGD systems varies from 780 to 1250 kg/m 3 (Perri and others, 1988). An example of an x-ray diffraction of spray dryer material can be found in Figure 2 (Weber and others , 1993). The hannebachite is the sulfite.

FGD AS A MULTIPOLLUTANT CONTROL TECHNOLOGY

Background on Mercury Emission

The operation of coal-fired power plants resu lts in the generation of CCBs that already contain mercury; however, that mercury is pres umably bound mercury that is relatively insoluble and nonleachable (EPA, 2002). The type of mercur y that is of greates t concern is mercury emitted into the air. Although mercury is emitted from coal-fired power plants in very low concentrations, as a group, coal-f ired boilers represent the larges t unregulated source of mercury emissions to the environment, accounting fo r about one-third of the total anthropogenic emissions (EPA, 1997). Further assessment based on EPA’s 1999 Information Collection Request (ICR) indicates that appr oximately 40% of the 75 tons of mercury contained in the coal burned in the United States is captured in ash and scrubber residues and 60% is emitted to the atmosphere (Kilgroe, 2000; Chu and others, 2000). In 1999, EPA estimated that the electric utility industry emitted 43 tons of mercury from 1149 units at 464 coal-fired power plants through both air and solid emissions. Projec tions for 2010 without regulations include approximately 60 tons of emission from 1026 units at 426 coal-fired plants (Duclos, 2001).

Figure 2. Diffraction of spray dryer material.14 Increased control of mercury emissions from coal-fired power plants may change the amount and composition of CCBs. Such changes may increase the potential for release of mercury to the environment from either landfi lling or utilization. Mercur y volatilization from CCBs in landfills and/or surface impoundments is expected to be low because of the low temperatures and relatively small surface area involved. For mercury control technologies using dry or wet FGD scrubbers, the byproducts are typically alkalin e and the acid le aching potential of mercury from these byproducts is expect ed to be minimal (EPA, 2002a). EPA (2002b) predicts that future policy for mercury cont rol will designate activated carbon injection downstream of the combustion process.

Background on Regulations

As previously mentioned, there are no specific regulations for mercury for electric utilities. The CAAA of 1990 did require EPA to regulate hazardous air pollutants (HAPs) using a

maximum achievable control tec hnology approach, commonly referr ed to as a MACT approach. MACT standards were put in place for any sources that emitted 10 tons or more of a particular pollutant. Utilities, on the other hand, were a source category to be treated separately. EPA was required under the CAAA to perform further st udies on HAPS for electri c utilities. The two reports that came out of this directive were the Mercury Report to Congress and the Utility Report to Congress. Based on the conclusions of these reports, EPA determined it necessary to regulate mercury emitted from electric utilities . Thus, there were no immediate or direct regulations for mercury and utilitie s, but the process indirectly led to mercury regulations for this particular source category. The form of regulation is still being debated (Pavlish, 2002). However, EPA has developed a five-pronged appr oach to controlling mercury emissions from power plants (EPA, 1998):

• Increasing public access to information concerning mercury emissions from power plants

• Seeking to reduce risk to highly expo sed populations through public information

• Making maximum use of the reductions in mercury emissions that can be achieved from controlling other pollutants

• Encouraging the development of merc ury emissions monitoring and control technologies

• Further research to increase understanding of the nature and fate of mercury emissions from power plants so as to fo rm a basis for future controls

Most recently, on February 14, 2002, President Bush announced his administration’s

initiative to address concerns about power plant emissions. The Cl ear Skies Initiati ve (CSI) cuts (by 70%) power plant emissions of the three major air pollutants: nitrogen oxides, sulfur dioxide, and mercury. NO

x emissions will be cut by 67%, from current emissions of 5 million tons to a cap of 2.1 million tons in 2008 and 1.7 million tons in 2018. SO 2 emissions will be cut by 73%,

from current emission of 11 million tons to a ca p of 4.5 million tons in 2010 and 3 million tons15 in 2018. The Bush administration anticipates that the national cap on mercury emissions will cut emissions by 69% from current emissions of 48 tons to a cap of 26 tons in 2010 and 15 tons in 2018. To accomplish these cuts, the CSI uses a market-based approach which will “... cut pollution further, faster, cheaper, and with mo re certainty, using a ‘cap-and trade’ program, replacing a cycle of endless litigation with rapi d and certain improvement s in air quality... Saves as much as $1 billion annually in compliance costs that are passe d along to American consumers, improves air quality, and protect the reliability a nd affordability of electricity. Use the model of

our most successful clean air law—the 1990 Clean Air Act’s acid rain program—and encourage the use of new and cleaner pollution control technologies” (Palmisano and Burnett, 2002). It should be noted that this plan has been fiercely scrutinized beca use it does not, in many people’s view, reduce emissions enough, starts too late and, therefore, has a lower monetized benefit.

Further regulations are scheduled to be propos ed in late 2003 and should be final by late 2004, with a compliance date of late 2007. These re gulations will affect all new and existing units. EPA has not yet determined the level of cont rol that will be required . However, there are at least five bills pending in the 107th Congress that would require regulation of mercury emissions from coal- and oil-fired utility boilers. Based on EPA’s extensive ICR test program, it is expected that dry FGD systems on boilers firi ng bituminous coal and wet FGD systems preceded by fabric filters on boilers firing bituminous coal ar e likely to be able to meet the predicted 90% control requirement by the end of th e decade (Weilert and others, 2002).

In addition to federal regulations, states ha ve begun developing their own emission control regulations and, in some cases, id entified specific control efficien cy requirements. These states

are as follows:

• Connecticut

• Maine

• Massachusetts

• Minnesota • New Hampshire • New Jersey

• Wisconsin

The Connecticut Department of Environmenta l Protection (2002) anticipates a reduction of

8900 tons of SO 2 emissions after 2003 in addition to a reduction of baseline NO

x emission by 30%, or 3483 tons, per year. Massachusetts has a dopted regulations that reject the use of “averaging,” meaning companies owning more th an one plant cannot ha ve a high-polluting facility just because they operate a clean plan t in another part of the state. Massachusetts regulations apply to NO

x , SO 2 , and CO 2 emissions, and a mercury sta ndard is expected to be proposed in 2006 (Commonwealth of Massachutts , 2001). According to Weilert and others (2002), New Hampshire has proposed a 50% reducti on in emissions from el ectric utility boilers by 2003 and 75% by 2005. Wisconsin has proposed a phase-in of mercury reductions over the next 15 years, beginning with 30% reduction by 2006 and extending to 50% reduction by 2011 and 90% by 2016 for utility boilers only (Weilert and others, 2002).16 Control Technologies

Currently, U.S. utilities make use of vari ous technologies to reduce mercury emissions; however, these technologies are not specifically designed to re duce mercury emissions (EPA, 2002). Technologies that are used to reduce particulate matter, SO 2 , and NO

x also reduce mercury emissions with varying levels of effectiveness (EPA, 2002). Some of these technologies involve the installati on of specialized equipment, wh ile others rely on modification of operating procedures or the use of specific fuels. Because ofthe large number of U.S. fossilfueled facilities and the various regulatory re quirements, there is a tremendous variety of emission control equipment provided and inst alled by many different suppliers and vendors (EEI, 1993).

Because the chemical species of mercury emitte d from coal-fired power plants vary from plant to plant, no single contro l technology removes all forms of mercury. There remains a wide

variation in the end costs of cont rol measures for utilities and th e possible impact of such costs on utilities. Preliminary estimates of national control costs for util ity boilers (based on pilot-scale data) are in the billions of dollars per year (EPA, 1997).

As new mercury control regulations are a dopted, more innovative t echnologies are being studied to address mercury extrac tion and removal from coal-fired flue gas. With more than 200 of the largest coal-fired power pl ants in the United States utilizing FGD systems, these systems still remain the most popular method of mercur y removal. Although these systems were designed primarily to remove SO 2 , wet scrubbers also entr ain oxidized mercury in the scrubber water that can reach 600 ppb (Duclos, 2001). FGD systems have not been shown to remove reduced mercury, and since the mercury species emitted varies by coal, FGD systems will not provide mercury reductions for some coals.

To optimize mercury removal using FGD, inve stigations have been initiated to oxidize mercury from the elemental form (Hg 0 ) to the oxidation form (Hg +2 ) so it can be removed in existing FGD systems. One example of this research was reported by Babcock & Wilcox and McDermott Technology, Inc., at the March 19, 2002, National Energy Technology Laboratory (NETL) Mercury Workshop and is noted on the DOE NETL Web site (DOE NETL, 2002).

New fine particulate cont rol technologies, such as the EERC’s ADVANCED HYBRID TM
technology (Miller and others, 2002), may also o ffer enhanced mercury control. The EERC is currently evaluating the ADVANCED HYBRID TM technology for mercury capture under funding

from DOE NETL.

Human Risk

Much is still unknown about the relationshi p of power plant emissions and mercury

concentrations in humans. Concentrations of merc ury in air are usually lo w and of little direct concern; however, atmospheric mercury contribu tes to the mercury loading in surface waters

where it can bioaccumulate in fish and animal tissue in the form of, methylmercury (EPA, 1998).17 The most direct threat of mercury exposure to humans occurs by eating contaminated fish. Exposure to high levels of mercury has been associated with serious neurological and developmental effects in humans. The effects can include subtle losses of sensory or cognitive ability, tremors, inabili ty to walk, convulsions, and death. Be cause a developing fetus may be the most sensitive to the effects from methylmerc ury, women of child-bearing age are regarded as the population of greatest interest (EPA, 1998).

HANDLING OF FGD MATERIAL

Because many utilities in the United States dispose of wet sulfite-rich FGD material in ponds or lagoons, pipeline transportation is most effective (Moore, 1992). A moisture content of <10% is desired for transport because moisture contents of 12%–15% can make the material sticky and clog up systems, and a moisture conten t of <5% can create dust (Clarke, 1993).Wet

sulfite-rich FGD material is sometimes stabilized or fixated by addition of fly ash, cement, or

other additives. The stabilized material is easier to handle and transport because the stabilization result reduces or eliminates the thixotropic properties of the materials. Stabilized wet sulfite-rich

FGD material has been used for liners and feedlot surfaces (Butalia and others, 1999).

Wet sulfite-rich FGD material may be dewa tered through filtering. This filtered material may be referred to as filter cake. After dispos al in wet conditions, such as lagoons or ponds, FGD material may be dewatered by “dry-stacking” which allows excess water to naturally migrate

from the FGD material.

Handling FGD gypsum can be difficult becau se the material is abrasive, sticky, compressible, and considerably finer (<0.2 mm ) than natural gypsum (CRE, 1992). Because of the difference in mixed natural gypsum a nd FGD gypsum, only limited quantities of FGD gypsum could be used to replace or supplement natural gypsum in older gypsum wallboard factories. New gypsum wallboard plants have been designed to accommodate the finer FGD

gypsum.

The bulk physical properties of dry FGD materi als are similar to fly ash; therefore, they must be handled similarly. While the bulk appear ance and properties may be similar to fly ash, dry FGD material is primarily cr ystalline in its morphology, and fl y ash is primarily glassy or amorphous. As a result, flow char acteristics of dry FGD material may vary significantly from fly ash. Some powders may require conditioning to tran sport. The typical moisture content of the conditioned residues is about 10%. The majority of dry FGD materials can be transported by rail, road, water, or pipeline (Clarke, 1993); however, it is suggested that residues discharged directly from the spray dryer FGD unit are best transf erred using mechanical conveyors (Klimek and others, 1987).

Materials discharged directly from the spra y dryer FGD system are best transferred using mechanical conveyors and may be transferred using a pneumatic system. These materials may require conditioning prior to transport to minimize dusting. The typical moisture content of these materials is 10%. Conditioning with water aids compaction of residues and may display thixotropic properties (CRE, 1992).18 FGD MATERIAL UTILIZATION

Record levels of construction activity in No rth America and the recovery of most Asian economies triggered significant expansion in the gypsum industry during 1999 (The Roskill Consulting Group, 2000); however, the U.S. gypsum industry experienced a decline in both production and consumption in 2000 (Olson, 2000). Br uce (2002a) suggests the U.S. decline could be attributed in part to the globalizatio n of the industry. It is important to note that although the overall gypsum indu stry experienced a downturn in 2000, the production and consumption of synthetic gypsum increased larg ely because of the construction of wallboard plants specifically designed to us e the material as a feedstock.

The primary source of synthetic gypsum is fr om forced oxidized FGD systems. Synthetic gypsum is becoming a very viable material an d has gained widespread acceptance as a raw material in the production of gypsum wallboard and cement. These two markets account for in excess of 80% of all natural gypsum and virtua lly all synthetic gypsum consumption in North

America.

During the next several year s, the use of mined gypsum ma y decline significantly in the United States as greater quanti ties of synthetic gypsum are pr oduced. The U.S. FGD market is expected to grow by 5000 MW annually over the next 7–8 years as a consequence of the newly introduced legislation (Global Gypsum, 2002). This large increase in production has made it possible for other companies to move into the ma rket and for existing companies to expand into regions that were previously untapped. It is im portant to note that al though more power plants are producing FGD gypsum, there are also increas ingly frequent reports of shortages. Today, synthetic gypsum represents about 18% of the g ypsum used in the United States (Bruce, 2002a), and some forecasts predict this percentage will increase to 30% by 2005 (Global Gypsum, 2001).

FGD gypsum is primarily produced in North America, Europe, and Japan; however, output is spreading to less developed countries where it will become an increasingly important source of material in the future (T he Roskill Consulting Group, 2000) . Based on the ACAA Coal

Combustion Product Production an d Use Statement, 25,652,994 tons of FGD material was produced in the United Stat es in 2000, and 4,824,727 tons (nea rly 19%) was utilized. As illustrated in Table 4, synthetic gypsum accounted for more than 15% of the total domestic gypsum supply in 2001. Synthetic gypsum includes FGD gypsum from coal-fired power plants and by-product gypsum from other sources.

Table 4. Gypsum in the United States, million tons

1997 1998 1999 2000 2001

Crude 18.6 19.0 22.4 19.5 18.8 Synthetic a 2.7 3.0 5.2 5.2 6.1

Calcined b 17.2 19.4 23.3 2.1 17.7

a Data refer to amount sold or used, not produced.

b From domestic crude. Source: U.S. Geological Survey (USGS) (2002).19 FGD materials have been successfully utilized in a wide variety of applications. According to the USGS (2002), 95% of the gypsum consumed in the United States is used for wallboard products, building plasters, and the manuf acture of portland cement. The USGS overall utilization rate (2002) is comparable to ACAA ( 2001) statistics illustrated in Figure 3. It is important to note that ACAA’s data do not indicate if the F GD materials used are wet FGD material, FGD gypsum, or dry FGD ma terials; however, it is reas onable to assume that wet FGD material is generally stabilized prior to use. The thixotropy of wet FGD material limits its

usefulness.

FGD materials have the potential to be beneficially reused in numerous applications if the economics are in place. It is reasonable to assu me that FGD gypsum is the primary FGD material represented by the use figures. Only gypsum can be used in wallboard and cement. FGD gypsum and sulfite-rich FGD materials would likely be us ed preferentially in ag ricultural applications.

Although FGD materials have the potential to be used in a variety of applications, many of those applications never reach the commercial phase for a number of reasons, including economics and government regulations. The follow ing nonexhaustive list of current commercial utilization practices for FGD materials was derived from a variety of sources, including symposium proceedings, Internet searches, and conversations with expe rts in the industry:

• Wallboard (regular, Type X, sheathing, soffit board, water-resistant gypsum, backing board, lath, veneer base, coreboard, gypsum liner board, predecorated, foil backed,board for manufactured housing, wood chip boards)

Figure 3. FGD material u tilization applications.20 • Structural fills

• Mining applications – Neutralization or encapsulat ion of acid-producing materials

– Barrier to acid mine drai nage formation/transport

– Alkaline amendment to ne utralize acid-producing rock – Subsidence control in underground mines – Filling underground mine voids to control acid drainage – Pit filling to reach approximate original contour in surface mines

– Soil amendment – Synthetic soil component

• Portland cement (Primarily used as a grinding aid and set retarder)

• Plaster

• Agriculture

– Acid soil neutralization clay breakdown and as a calcium and sulfur source

– Sodic soil reclamation – Carbon sinks in forestry

– Feedlot surface stabilization – Crop storage pads

• Floor underlayment

• Glass making

• Pigments and/or fillers in paper, plastic, food, pharmaceutical, fertilizer, pesticide, and

herbicide

Products developed and based mainly on a-hemihydrate including the following:

• Self-leveling floor screeds of varying strengths (very co mmon in Europe, particularly Germany, for heat and sound insulation)

• Double floor systems as cavity floor and raised access floor (“computer floor”)

• Mining mortars (consolidation, embankment, rock stabilizers)

• Tunnel mortars

• Construction products (adhesives, thin-layer systems, toppings)

• Molding plasters21 Magnesium hydroxide markets include the following:

• Acid neutralization

• Fuel additives • Feedstock for refactories, magnesium metal, and the chemical industry

Pyrite coal spoil and gypsum sludge can re sult in valuable products, namely, sponge iron

and lime.

Table 5 lists the potential uses of FGD gypsum and dry FGD material. It is important to note that some applications listed are not commercially available at this time.

Table 5. Potential Uses of Dry FGD Material High Potential Moderate Potential Low Potential Structural Fill Cement production Gypsum/wallboard Grout/Mine Backfill Cement replacement Metals extraction

Stabilized Road Base Soil stabilization Synthetic Aggregate Sl udge stabilization Lightweight Aggregate Mineral filler Mineral Wool Agricultural use Brick Production Ceramic products

Liner material

ECONOMIC ISSUES ASSOCIAT ED WITH FGD UTILIZATION

According to the Combustion 2000 Project (2 001), the technical challenges of producing commercially usable by-product gypsum have mo stly been solved, and the operating changes required to use these materials in commercial applications are becoming relatively well established. The area that remains a significant ch allenge is structuring successful relationships between producers and consumers. Ultimately, econom ic issues are the driving force that will determine the level of utilization. However, a by-product that is less expensive than a raw material will not automatically be sought after for industrial use. The factors which govern its desirability as a product are much more diverse.

Product constancy and reliably are the primary f actors associated with the use of synthetic gypsum. Synthetic gypsum must consis tently meet stringent quality parameters to be utilized in wallboard and other manufacturing. Two main area s that have received attention as process improvements are additives and forced oxid ation. Additives are employed to improve SO 2 removal by increasing the liquid-phase alkalinit y. Forced oxidation of sulfite to sulfate (eliminating major dewatering problems) is an important major improvement in wet FGD systems. This has led to the development of the production of commercial-grade FGD gypsum (Combustion 2000 Project, 2001). Parameters that ar e most important to product quality include free moisture, purity, soluble salts, pH, partic le size, and surface area. The free or surface moisture of synthetic gypsum usually ranges fr om 6% to 25%. Assumi ng the synthetic gypsum22 meets particle-size requirements and does not contain any harmful impur ities, a higher-purity synthetic gypsum will allow a lower-weight boar d to be produced without sacrificing strength (Henkels and Gaynor, 1997). Solu ble salt impurities are common in natural and synthetic gypsums. The four soluble salt ions typically tested for are magnesium, potassium, sodium, and

chloride. Chloride is the most common contam inant in FGD gypsum. Sol uble salts found in FGD gypsum can often be washed away.

Because the gypsum industry tends to have a much more exaggerated cycle than other

industries within the constructi on industry, the commitment of th e gypsum supplier is critically important to encourage a partners hip with consumers. Commitment is seen in numerous forms, including supply reliab ility because building materials production plants operate very consistently, thus making supply interruptions difficult to manage; product quality to ensure impurities are controlled to acceptable levels; desi rability of the loca l market region; and competitive advantage over competing materials.

Supply trends are essential to gypsum economics. Bruce (2002b), suggests that the biggest reason for the recent drop in the price for regul ar gypsum board has been the huge increase in capacity that has been put in place over the last four years. Bruce also suggests in a more recent article (2002a) that the industry has experienced shortages of s ynthetic gypsum. These shortages

could be a result of the industrie s’ peak periods, where an excess capacity is needed to keep up with surges in demand. Despite the increase in wallboard produc tion, some regions in the United States still remain in short supply and other regions have a severe surplus.

A critical element in the economic considera tion of gypsum markets, natural or synthetic, is transportation costs. Gypsum wallboard, whic h is made from gypsum, and cement of which gypsum is a component, are bulk items sensitive to handling and transportation costs. For this reason, location of the gypsum source and manuf acturing plant, distance between the two facilities, and mode of transpor t are critical. Two sepa rate sources of info rmation have been quoted as identifying completely different methods of transpor tation as most practical. One source stated that rail is by far the most economi cal alternative for transporting high volumes of a coal by-product from the plant site to an indust rial location. A second s ource stated that any transportation distance under 100 m iles is commonly and most idea lly performed by truck. This

would allude to the fact that each method of tran sportation is site-specifi c and entirely dependent upon the type of by-product produced and the form of utilization intended for that by-product. Cement plants have traditionally been located on or adjacent to limestone or dolomite deposits and are usually located within 100 to 150 miles of their markets. Wa llboard plants located in the United States are generally within the vicinity of major urban areas. For the most part, the natural gypsum for these industries comes from those mine d closest to their plants. For several gypsum operations, the economics have been such that wall board plants are located on or adjacent to the mine (Combustion 2000 Project, 2001); however this scenario ha s changed with the new megaplants that are located near the sy nthetic gypsum suppliers (Miller, 2002).23 DISPOSAL PRACTICES AND REQUIREMENTS

Currently, EPA classifies F GD material as a solid waste under RCRA Subtitle D. This classification places the responsibil ity to regulate disposal of F GD material on in dividual states. In the United States, regulations concerning se lecting a disposal site and construction and operation of containment areas vary significantly between states. Collins (1992) lists the following parameters that generally play a role in development of state regulations:

• Sufficient distance above local groundwater table

• Soil conditions not prone to settling

• Siting away from limestone quarries, wate rways, floodplains, wellheads, aquifers, or underground mines

• Suitable topography

For the utility, it is also important to consid er the proximity of the disposal site to the power plant. Transportation costs can be significan t, and utilities generally desire to minimize disposal costs.

The following disposal options have been used for wet FGD material (Smith, 1992):

• Hydraulically conveyed and ponded

• Dewatered and stacked • Interbedded with fly ash • Stabilized with lime, cement, and/or fly ash

Most U.S. utilities dispos e of FGD material using ponding or lagooning techniques. Appendix B provides an illustration of FGD dispos al practices by state. Wet FGD material is frequently thixotropic, which is th e property of a material that enables it to stiffen in a relatively short time on standing, but upon agitation or manipul ation, to change to a very soft consistency or to a fluid of high viscosity, the pr ocess being completely reversible.

FGD material disposal is likely to be impacted by rules for utility landfills/surface impoundments that are currently being developed by the EPA Office of Solid Waste. The final rule is scheduled for comple tion in March 2004. Discussions with industry re presentatives

indicate some concern that the EPA rule for utility landfills/surface impoundments will require the phasing out of wet disposal si tes. Wet FGD material would then need to be dewatered or stabilized prior to final disposal.

FUTURE FGD SYSTEM INSTALLATI ONS AND FGD MATERIAL MANAGEMENT

The evolution of the electric power industry toward more competition has led utilities to delay capital expenditures for pollu tion control equipment as long as possible. Phase I of the24 CAAA affected only 10% of the coal-burning electric utilities (Kal yoncu, 1997). Although FGD

installations have not been as popular as originally pl anned, FGD technology has made considerable progress in SO 2 removal efficiency, reliability, and capital and operating costs during the past decade. Still, to meet stronge r emission limits under Phas e II of the CAAA, many utilities are expected to continue the cost-e ffective strategy of fuel switching. In a 1997 document discussing the impacts of the CAAA on elec tric utilities, DOE es timated that only 12 to 20 gigawatts of capacity woul d be scrubbed to comply w ith Phase II of the CAAA. The estimate was based, in part, on the fact that a number of utilities that had originally planned to install FGD systems had either deferred installati on or canceled them in favor of fuel switching or purchasing allowances. The Bush Administ ration’s CSI, announced early in 2002, provides a basis for an alternate prediction. The CSI re quires significant addi tional reduction of NO

x and

SO 2 emissions and the first-ever national cap on mercury emissions. It has been predicted that the number of FGD systems is expected to doubl e in the next 7 years in response to emission regulations (Government Panel, 2001). Wet FGD control of SO 2 has the added advantage of removing oxidized mercury from the flue gas, so for some power plants, installation of wet FGD will provide multipollutant control to meet CSI SO 2 and mercury emission caps. The use of wet FGD for this type of multipollutant emissi on control will depend on the mercury species currently produced at a given power plant and future coal sources because the mercury species in the emissions are coal-specific. The extent of applicability of merc ury removal technologies appropriated with FGD systems will depend on th e nature and level of control dictated by mercury emissions regulations. Based on curre nt information on mercury emission control technologies, FGD systems are attractive fo r several reasons: 1) control of both SO 2 and mercury, 2) demonstrated reliabi lity, and 3) relative cost-eff ectiveness. The disadvantages include 1) the generation of hi gh volumes of FGD material for management, possibly requiring disposal; 2) concern for mercury release from the FGD material under management scenarios; and 3) the specificity for contro l of oxidized mercury. These issues will need to be evaluated by individual companies considerin g the use of FGD for either SO 2 emission control or

SO 2 /mercury emission control.

Installation of FGD systems on currently nonscrubbed units will result in a large volume of FGD material for management. If predictions noted above are correct, the volume of FGD material could increase to 50+ million tons annually. If all new FGD systems were wet systems designed to produce FGD gypsum, the 25+ million tons of FGD gypsum produced would likely meet or exceed U.S. demand for gypsum, base d on current gypsum pro duction figures (USGS, 2002). U.S. gypsum wallboard ma nufacturers, cement manufacturer s, and other industry/uses currently use nearly 20% of the FGD material pr oduced in the United States (Figure 3). While these use applications have the potential to utilize much greater volumes of FGD gypsum,

barriers to these uses will need to be addres sed. These barriers include the competition with mined gypsum, quality issues, potentia l transportation issues, and others reported to the barriers

to utilization of coal combus tion/desulfurization by-products (Pflughoeft-Hassett and others, 1999). One area of potential increased use of FGD gypsum is in agricultural applications. FGD gypsum is currently used as an agricultural so il amendment (Figure 3), but before FGD gypsum

from SO 2 /mercury emission control will be readily accepted in agricultural applications, the issue of potential for mercury release from this FGD gypsum needs to be investigated. In the assessment of management options for multipolluta nt control FGD materia l/gypsum, the fate of the mercury needs to be considered.25 As utilities consider options for SO 2 emission control for future installations, it will be important to follow the current EPA activities re lated to guidelines for CCB disposal and mine placement. These guidelines will impact the cost of new CCB disposal faci lities which will be required if the predicted volumes of FGD material are produced a nd may impact the potential for FGD material to be placed in mine settings , a potential high-volume application for FGD

material and other CCBs.

SUMMARY

FGD technology is well established and can be used to achieve the SO 2 emission reductions required in the CSI. Additionally, much work has been and continues to be done on

the utilization of FGD material s. The use of FGD gypsum in place of natural gypsum in wallboard manufacture, cement production, and ot her existing products has been demonstrated and commercialized. This has been accomplished b ecause most of the technical challenges of producing commercially viable FGD gypsum have been solve d, and the operating changes required to use these materials in commercial applications are becoming relatively well established. An area that remains a significant ch allenge in increasing the utilization of FGD gypsum is structuring successful relationships between producers and consumers. Other FGD materials are still in the process of being inve stigated for use in va rious applications, and technical challenges may require further investigation and demons tration in order to lead to commercialization. The issue of mercury associated with FGD material and its potential release to the environment is part of investigations fu nded by EPA, DOE, and indus try; results of these studies may impact management of FGD material in the future.

Federal regulations are currently driving fu ture emission control for coal-fired power plants, but based on the informa tion currently available, it seem s apparent that additional FGD systems will be installed on existing power pl ants. The increased produ ction of FGD material will make it increasingly important to optimi ze utilization. As was recommended in the 1993 EERC Report to DOE on the Barrie rs to Utilization of Coal Combustion/Desulfurization ByProducts by Government and Commercial S ectors (Sondreal and others, 1993), procurement guidelines should be implemented at the federal level to encourage the use of CCBs, including FGD material. The CCB industry need s to be vigilant in following government actions in order to identify and take advantage of opportunities to effectively main tain or perhap s increase the

current FGD material utilization ra te of 20%. All levels of government and industry need to work together in order to achieve optimum u tilization of CCBs, including FGD material.

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# APPENDIX A

FGD BY-PRODUCT GENERATED BY STATE, 1994

# APPENDIX B

STATE TOTAL FGD BY-PRODUCT BY METHOD OF DISPOSAL, 1994

# APPENDIX C

FGD SYSTMES AND MATER IALS – DEFINITIONSFGD SYSTEMS AND MATERIALS – DEFINITIONS

ACAA prepared the following definitions of FGD materials based on American Society for Testing and Materials (ASTM) terminology. Th is list was taken from a draft document dated July 10, 2001, and ASTM has not formally adopted all changes to these definitions.

Dry FGD – an FGD system in which calcium- or sodium-based sorbents, usually hydrated lime, are introduced to the flue gas. Dry FG D systems use less water than wet systems, usually remove fly ash and su lfur dioxide simultaneously, a nd generate a dr y by-product. Spray dryer systems are the most common design. In a spray dryer, slaked lime slurry is sprayed into the flue gas, and the resulting by-pr oduct, dried by the heat of the flue gas, is collected in a particulate c ontrol device with th e fly ash. Other dry systems inject dry sodium sorbent directly into the boiler exha ust duct. The by-product of a dry FGD system is referred to by various names that incl ude dry FGD ash, dry FGD material, and dry

scrubber material.

Dry FGD material – the product that is produced from dry FGD systems and consists primarily of calcium sulfite, fly ash, portlandite (Ca[OH] 2 ), and/or calcite. Lime-based sorbent system dry FGD material mainly consis ts of calcium sulfite and dry fly ash, along with minor quantities of calcium sulfate. Sodi um-based sorbent systems mainly consist of sodium sulfite and dry fly ash along with mi nor quantities of sodium sulfate. Dry FGD material is being used in construction, engin eering, and agricultural ap plications; however,

most of the material is stored in landfills.

Dry sodium injection

– see dry FGD

.

Ex situ oxidation (wet FGD) – forced oxidation that occurs outside of the scrubber and is used to produce FGD gypsum.

FGD by-products – the term for the by-products from wet and dry FGD systems. See wet
and dry FGD material

.

FGD gypsum – gypsum formed from an oxidizing and calcium-based FGD process. Also, a precipitated gypsum formed through th e neutralization of sulfurous acid (H 2 SO 3 ) in FGD processes at coal-fired power plants. This gyps um can vary in purity, which is defined as the percentage of CaSO 4 ·2H 2 O, and generally is over 94% for use in wallboard manufacturing. The less pure gypsum can be stockpiled (gypsum stacking), placed in ponds or captive landfills, or ut ilized in agriculture or cons truction. The nearly pure or pure FGD gypsum is utilized beneficially. The pur e FGD gypsum is manufactured to meet the specifications of wallboard-manufacturing companies and is used for wallboard manufacturing, for cement production, and as plasters. Large quantities of FGD gypsum are produced and utilized.

FGD material

– a product of an FGD process typically using a high-calcium sorbent such

as lime or limestone. Sodium-based sorbent a nd high-calcium coal fly ashes are also used in some systems. The physical nature of th ese materials varies from a wet thixotropicsludge to a dry powdered material, depending on the process. The wet thixotropic sludge is usually from a lime-based reagent wet scr ubbing process and is predominantly calcium sulfite. It is the end product of dewatering equi pment such as vacuum filters or centrifuges, although it can be the end product of a sedimentation pond. This dewatered end product is usually stabilized by mixing wi th lime and fly ash or othe r materials for disposal in landfills. There are systems where the e nd product is not dewatered but is highly

concentrated in solids as the underflow from a thickener. It is then mixed with fly ash and another material and pumped to a surface impoundment for disposal. The wet product from limestone-based reagent wet scrubbing pro cesses is predominantly calcium sulfate dihydrate, which is gypsum. This material read ily dewaters, and there are systems in use where the slurry is transporte d to a pond and construction equi pment is used to excavate and stockpile the gypsum. The production of commercial-grade FGD gypsum used for wallboard manufacturing usually re quires forced oxidation in the scrubbers or external to the scrubbers and dewatering by filtration equipm ent such as vacuum filters or centrifuges and sedimentation ponds. The dry material fro m dry scrubbers that is captured in a baghouse along with fly ash consists of a mixture of sulfites and sulfates in addition to fly ash. This powdered material is referred to as dry FGD ash, dry F GD material, lime spray dryer ash, lime spray dryer, or lime spray dryer residue.

FGD material dry scrubbers – the dry powdered material from dry scrubbers that is collected in a baghouse along with fly ash and c onsists of a mixture of sulfites, sulfates, and fly ash.

FGD products – another term for the by-products from wet and dry FGD systems.

FGD sludge – another name for scrubber sludge we t FGD material or filter cake. See wet

FGD material

.

Filter cake – the material produced by filtering e quipment such as vacuum filters for dewatering wet FGD material. See wet FGD material

.

Fixated FGD material – a designed mixture of dewate red FGD sludge (also known as scrubber sludge or filter cake) which is prim arily calcium sulfite w ith either a low-lime (Class F) fly ash and lime with a high-lime fl y ash (Class C) or with a low-lime fly ash and a cementitious material such as cement kiln dust, lime kiln dust, or fluidized-bed combustion ash. The designed mixture is produced in a mixing facility sometimes referred to as a sludge treatment plant, and transpor ted by belt conveyor to an area where it is stockpiled for a number of hours or days to unde rgo an initial chemical set. The stockpiled

material is then excavated and loaded ont o trucks or other earthmoving equipment for placement as a fill in beneficial use applicatio ns or for placement in a landfill for storage or disposal, where it undergoes a further chemical set. After placement, the fixated material forms a stable, monolithic mass of low permeability.

Fixated scrubber sludge

– another name for fi xated FGD material.Flue gas desulfurization – removal of gaseous sulfur di oxide from boiler exhaust gas. Primary types of FGD processes are wet scru bbers, dry scrubbers, and sorbent injection. Sorbents include lime, limestone, sodium-bas ed compounds, and high-calcium coal fly

ash.

Forced oxidation

– a process employed to supply addi tional air in wet FGD systems, resulting in a predominantly calcium sulfate dihydrate (gyps um) by-product with improved storage characteristics as well as greater commercial potential.

In situ oxidation (wet FGD) – a process in which both SO 2 absorption and oxidation are

carried out within the scrubber.

Lime spray dryer ash – the residue from a spray dr yer FGD system. The resulting byproduct is dried by the heat of the flue gas and collected in a particulate control device with the fly ash. See dry FGD material

.

Lime spray dryer residue – another name for lime spray dryer ash.

Scrubber – any of several forms of chemical/ physical devices that remove sulfur compounds formed during coal combustion and es pecially from coal-f ired power plants.

See wet FGD

.

Scrubber sludge – another name for wet FGD material. See wet FGD material

.

Spray dryer

– a type of dry FGD system. See dry FGD

.

Stabilized FGD material – another name for fixated FGD material. See fixated FGD

material

.

Wet FGD – an FGD system which uses a wet scr ubber to introduce an aqueous solution of either slaked lime (calcium hydroxide) or li mestone (principally calcium carbonate) into the flue gas in a spray tower. The sorbent reacts with or oxidizes the sulfur dioxide in the flue gas and converts it to a by-product that is referred to as scrubber sludge, scrubber material, or wet FGD material.

Wet FGD material

– the by-product of wet FGD processes or systems. It is composed primarily of water, calcium sulfite/sulfate solids, and small quantities of fly ash. It has the consistency of a sludge when allowed to settle in a pond or when the water is removed by filtering equipment such as vacuum filters. It is commonly refe rred to as scrubber sludge. Depending on the composition of the injected lim e or limestone, some by-products will also contain magnesium sulfite and/or sulfate and possibly traces of bari um sulfite or boron in addition to some trace

metals.