# A Review of Literature Related to the Use of Spray Dryer Absorber Material
Production, Characterization, Utilization Applications, Barriers, and Recommendations

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# A Review of Literature Related to **the Use of Spray Dryer Absorber Material**  Production, Characterization, Utilization Applications, Barriers, and Recommendations

**1014915**

Technical Report, September 2007

Cosponsor

University of North Dakota Energy & Environmental Research Center Coal Ash Resources Research Consortium 15 North 23 rd Street, Stop 9018 Grand Forks, ND 58202-9018 Project Manager L.V. Heebink**DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES**

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONS ORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

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Copyright © 2007 Electric Power Research Institute, Inc. All rights reserved.iii 

## CITATIONS

This report was prepared by

University of North Dakota Energy & Environmental Research Center Coal Ash Resources Research Consortium 15 North 23 rd Street, Stop 9018 Grand Forks, ND 58202-9018

Principal Investigators L.V. Heebink T. Buckley D. Pflughoeft-Hassett D. Hassett

This report describes research sponsored by the El ectric Power Research Institute (EPRI) and the University of North Dakota (UND) Energy & E nvironmental Research Center (EERC) Coal Ash Resources Research Consortium (CARRC).

The report is a corporate document that should be cited in the lite rature in the following manner:

A Review of Literature Related to the U se of Spray Dryer Absorber Material: Production, Characterization, Utilization Applic ations, Barriers, and Recommendations . EPRI, Palo Alto, CA and UND EERC CARRC, Grand Forks, ND: 2007. 1014915.v 

## REPORT SUMMARY

This report is a compilation of an extensive l iterature review focused on spray dryer absorber (SDA) systems and their by-products. It includ es background information on the current SDA production and use statistics, characterization data on SDA materi al reported in the literature, an overview of the current commercial and potential uses of SDA material reported in the literature, identification of barriers to th e increased use of SDA material, and the authors’ conclusions and recommendations based on the literature reviewed. This report is intended to assist utility ash managers, ash marketers, regulators, and r esearchers in understanding the issues surrounding SDA material utilization so that they can move from disposal to utilization.

**Background** SDA systems are a semi-dry SO 2 control technology that curren tly make up about 12% of the flue gas desulfurization (FGD) systems at U.S. po wer plants. SDA systems have typically been employed on small-to-moderate size units that burn low-sulfur coal in the western United States, where water supply is limited. Th e material produced by SDA syst ems primarily consists of calcium sulfite commingled with varying amounts of fly ash and unspent lime. Because most units in the United States do not pre-collect fly ash, the SDA material often contains a high percentage of it. SDA material presents a unique challenge for use applications, and as a result most coal-fired power plants currently dispo se of SDA material. The use of SDA systems is expected to grow along with wet FGD systems ov er the next 10 years as a result of increased requirements for SO 2 control, heightening the need fo r increased utilization options.
**Objectives**

• To summarize current informati on in the literature relativ e to SDA material production, characteristics, and uses

• To rank the potential of different uses for SDA material

• To provide recommendations for maintainin g/increasing the use of SDA material in the future

**Approach** The authors performed a detailed survey of the U.S. and European literature on the use of SDA material in a variety of applications. They summarized information on SDA material characteristics, production, and use, with partic ular emphasis on the bene fits and disadvantages of the higher fly ash content of the SDA materia l in the United States. Based on the review, the authors developed a prioritized li st of potential applications to advance the use of SDA material along with a set of recommenda tions for further research.vi **Results** According to American Coal Ash Association (ACAA) statistics, in 2006 U.S. utilities produced about 1.5 million short tons of SDA material and used only about 0.14 tons, or a little over 9%. Mining applications represented by far the largest type of application, accounting for 85% of the use. Other uses included concrete and concrete products, flowable fill, roadways, waste and soil stabilization, and agriculture. Key attributes of SDA material that must be considered in developing applications are the material’s inhere nt variability, its high sulfur content, and its potential for expansion.
Based on the literature review, the authors ranked the SDA material for use in various applications:

• High potential: cementitious products (suc h as masonry, flowable fill, and synthetic aggregate) and mining applications

• Moderate potential: cement replacement in concrete, engineering applications, agriculture, soil stabilization, and wet FGD sorbent

• Low potential: binder material, cement ma nufacture, hazardous waste fixation, marine applications, mineral wool, sulfu ric acid production, and wallboard Barriers inhibiting the use of the material were identified and include the following:

• Inconsistent terminology used to define the material

• Lack of understanding of the material

• Limited data on environmental and health effects

• Inconsistent guidelines on beneficial use

• Economics

**EPRI Perspective** Maintaining and increasing the utilization rate of coal combustion products (CCPs) is a primary strategic goal of EPRI’s CCP Use research pr ogram. SDA materials currently have a low utilization rate, and their volume is expected to significantly increase over the next several years. Understanding SDA material char acteristics and developing incr eased management options are key components of the overal l CCP use research strategy.

**Keywords** Flue gas desulfurization products Spray dryer absorber material Coal combustion products Utilizationvii 

## ABSTRACT

Coal-fired power plants account for the majority of sulfur dioxide (SO 2 ) emissions in the United States. Legislative actions in the United States and elsewhere have been responsible for most industrial SO 2 controls, resulting in the installation of flue gas desulfurization (FGD) systems. In the United States, approximately 85% of FGD sy stems are wet, 12% are spray dryer absorber (SDA) systems, and 3% are dry in jection systems. This report is a compilation of an extensive literature review on SDA systems and their byproducts.

Most SDA systems in the United States collec t fly ash (40–75%) along with SDA material, producing spherical, glassy fly ash particles coat ed by and intermixed with fine crystals of calcium/sulfur reaction products. Depending on their source, SDA materials can vary widely in their physical, chemical, and mineralogical pr operties, and the successful utilization of SDA material is highly dependent on these properties. Characterization data cited in the literature are included in this report.

In the literature reviewed, a number of current commercial and potential uses of SDA material were identified, including agriculture, binde rs, cement manufacture, cement replacement in concrete, civil engineering, flowable fill, fixati ng agent for waste, marine applications, masonry, mineral wool, mining applica tions, soil stabilization, sulfu ric acid production, synthetic aggregate, wallboard, and wet FGD sorbent. Many of the commercial uses are being successfully implemented in Europe but are slower to enter th e marketplace in the United States. Applications currently commercial in the United States include agriculture, concrete, concrete products, flowable fill, mining applications, soil stabili zation, structural fills and embankments, and synthetic aggregate. Potential uses in the resear ch and development stage were rated as high-, moderate-, or low-potential commercial applications. High-potential applications for the U.S. market are estimated to be those that take adva ntage of the presence of the fly ash component of the SDA material, can tolerate relatively hi gh sulfur content, a nd either have limited susceptibility to expansion or reduce expans ion potential in the production process. These applications fall into two categories: cem entitious products and mining applications.viii Barriers inhibiting the use of the material were identified and include the following:

• Inconsistent terminology used to define the material

• Lack of understanding of the material

• Limited data on environmental and health effects

• Inconsistent guidelines on beneficial ash use

• Economics

Based on the information obtained in the resear ch, the authors recommend the following to maintain existing commercial markets and develop new ones for SDA materials produced in the United States:

• Work within existing organizations such as A STM International and the American Coal Ash Association to develop and put into use a ppropriate terminology and definitions for SDA materials.

• Develop an understanding of the impact of compositional variability on the performance characteristics of SDA materials.

• Develop an understanding of the oxidation pr ofiles of SDA materials , and evaluate the impacts of oxidation on product performance.

• Educate potential users, regulatory representa tives, and other stakeholders about SDA materials.

• Address quality, compositional, environmental, and performance criteria in research, development, and demonstration efforts.ix 

## NOMENCLATURE

ACAA American Coal Ash Association AASHTO American Association of State Highway and Transportation Officials AFBC atmospheric fluidized-bed combustion Ca 6 Al 2 (SO 4 ) 3 (OH) 12 · 26H 2 O ettringite CaCl calcium chloride CAIR Clean Air Interstate Rule CaO calcium oxide, lime Ca(OH) 2 calcium hydroxide, hydrated lime, portlandite, unreacted lime CARRC ® Coal Ash Resources Research Consortium ® Ca 3 Si(CO 3 )(SO 4 )(OH) 6 · 12H 2 O thaumasite CaSO 3 calcium sulfite CaSO 3 · 1⁄2 H 2 O hannebachite, calcium sulfite hemihydrate CaSO 4 calcium sulfate, anhydrite CaSO 4 · 2 H 2 O gypsum, calcium sulfate dihydrate CCP coal combustion product CDF controlled density fill CHP combined heat and power CLSM controlled low-strength material DM Deutsche Mark DOE U.S. Department of Energy DSI dry sorbent injection ECOBA European Coal Combustion Products Association EERC Energy & Environmental Research Center EPA U.S. Environmental Protection Agency EPRI Electric Power Research Institute ERKOM “Waste Management of R esidues from Coal-Fired Power Plants and Waste Incineration Plants” (known as ERKOM from the German title) ESP electrostatic precipitator EU European Union FBC fluidized-bed combustion FGD flue gas desulfurization FSI furnace sorbent injection FSS fixated scrubber solids HCl hydrogen chloridex LIMB limestone injection multistage burner LOI loss-on-ignition MnDOT Minnesota Department of Transportation MW megawatt NO

x nitrogen oxides NR not reported OSU Ohio State University OVwG Oberverwaltungsgericht: Administrative Court of Appeals RCRA Resource Conservation and Recovery Act SCS Stearns, Conrad and Schmidt SDA spray dryer absorber SEM scanning electron microscopy SO 2 sulfur dioxide SO 3 sulfur trioxide TCLP toxicity char acteristic leaching procedure TGA thermogravimetric analysis UND University of North Dakota VAW Vereinigte Aluminium-Werke AG, Lünenxi 

## CONTENTS

**1 INTRODUCTION ..................................................................................................................** 

**.. 1-1**  Background .....................................................................................................................

...... 1-3 Development and Status of FGD Systems in the United States ...................................... 1-3 FGD Systems............................................................................................................... 1-4 Wet Calcium-Based FGD Systems ......................................................................... 1-4 Dry and Semidry FGD Systems .............................................................................. 1-4 Projected Future SDA Installations in the United States ................................................ 1-10
Current SDA Material Production and Use Rates ............................................................... 1-12 Projected Future SDA Production in the United States .................................................. 1-15

**2**

**RESULTS AND DISCUSSION OF THE LITERATURE REVIEW.......................................... 2-1** 

Definitions of SDA Material ................................................................................................... 2-1
Characterization of SDA Material.......................................................................................... 2-2 Physical Properties........................................................................................................... 2-3 Qualitative Physical Properties .................................................................................... 2-3
Particle Size................................................................................................................. 2-3 Specific Surface Area .................................................................................................. 2-5 Bulk Density................................................................................................................. 2-5 Specific Gravity............................................................................................................ 2-7 Optimum Moisture Content and Maximum Density ..................................................... 2-8 Unconfined Compressive Strength .............................................................................. 2-9 Permeability ............................................................................................................... 2-10 Mineralogical and Chemical Composition ...................................................................... 2-11 Chemical Analyses .................................................................................................... 2-13 pH .............................................................................................................................

. 2-17xii Current and Potential Uses of SDA Material....................................................................... 2-19 Agriculture ....................................................................................................................

.. 2-19
Binders ........................................................................................................................

... 2-23
Cement Manufacture...................................................................................................... 2-23 Cement Replacement in Concrete ................................................................................. 2-24 Civil Engineering............................................................................................................. 2-25
Flowable Fill..................................................................................................................

.. 2-28 Fixating Agent for Waste ................................................................................................ 2-29 Marine Applications ........................................................................................................ 2-30 Masonry........................................................................................................................

.. 2-30
Mineral Wool................................................................................................................... 2-31 Mining Applications......................................................................................................... 2-31
Soil Stabilization ............................................................................................................. 2-32
Sulfuric Acid Production ................................................................................................. 2-33 Synthetic Aggregate ....................................................................................................... 2-33
Wallboard ......................................................................................................................

. 2-35
Wet FGD Sorbent........................................................................................................... 2-36

**3**

**SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS................................................. 3-1** 

SDA Production and Material Characteristics ....................................................................... 3-1
Utilization of SDA Material....................................................................................................

. 3-2 High-Potential Applications............................................................................................... 3-4
Cementitious Products................................................................................................. 3-4 Masonry .................................................................................................................. 3-4
Flowable Fill ............................................................................................................ 3-5 Synthetic Aggregate................................................................................................ 3-5 Mining Applications...................................................................................................... 3-5 Moderate-Potential Applications....................................................................................... 3-6 Cement Replacement in Concrete............................................................................... 3-6 Engineering Applications ............................................................................................. 3-6 Agriculture.................................................................................................................... 3-7
Soil Stabilization........................................................................................................... 3-7
Wet FGD Sorbent ........................................................................................................ 3-8xiii Low-Potential Applications ............................................................................................... 3-8 The following applications are expected to have low utilization potential. Predictions for utilization rates have not been made. .................................................. 3-8
Binder .........................................................................................................................

. 3-8
Cement Manufacture ................................................................................................... 3-8 Fixating Agent for Hazardous Waste Sludge............................................................... 3-8 Marine Applications ..................................................................................................... 3-9
Mineral Wool................................................................................................................ 3-9
Sulfuric Acid Production............................................................................................... 3-9
Wallboard..................................................................................................................... 3-9
Barriers to SDA Material Utilization....................................................................................... 3-9
Conclusions....................................................................................................................

..... 3-11
Recommendations .............................................................................................................. 3-11

**4**

**REFERENCES ....................................................................................................................**

**... 4-1** 

**A BULK CHEMICAL COMPOSITION – REPORTED AS OXIDES – OF SDA**  **MATERIAL REPORTED IN LITERATURE REVIEWED.......................................................... A-1** 

**B**

**TOTAL ELEMENTAL COMPOSITION OF SDA MATERIAL REPORTED IN LITERATURE REVIEWED .......................................................................................................**  **B-1**xv 

## LIST OF FIGURES

Figure 1-1 Terms Used to Refer to SDA Material in the Literature ........................................... 1-2 Figure 1-2 Diagram of the SDA Process Showing the Fly Ash Precollection Option and the Solids Recycle [2]......................................................................................................... 1-6 Figure 1-3 ACAA Definitions of Dry FGD Material [19] ........................................................... 1-12 Figure 1-4 Dry Scrubber FGD Material Use in the United States In 2006. Of 1,488,951 Short Tons Produced, 136,639 Short Tons of Dry Scrubber FGD Material (9.18%) Was Beneficially Used [22] .............................................................................................. 1-13 Figure 1-5 Spray Dry Absorption Product Use in Europe (EU 15) in 2004. Of 466,278 Short Tons Produced, 373,684 Short Tons (80.1%) Was Beneficially Used [23] ............ 1-14 Figure 2-1 ASTM International Definitions [28] ......................................................................... 2-1xvii 

## LIST OF TABLES

Table 1-1 Typical SDA Operation Material Distribution [8] ....................................................... 1-6 Table 1-2 SDA Units in the United States a

................................................................................ 1-8 Table 1-3 Dry FGD Plants in Europe [16, 17] ......................................................................... 1-10 Table 1-4 U.S. SDA Market Projections [18]........................................................................... 1-10 Table 1-5 Details of U.S. SDA Market Projections [18]........................................................... 1-11 Table 2-1 Particle-Size Distribution and Mean Mass/Particle Size of SDA Material Reported in Literature Reviewed, μm................................................................................. 2-4 Table 2-2 Specific Surface Area of SDA Material Reported in Literature Reviewed, m 2 /g........ 2-5 Table 2-3 Bulk Density of SDA Material Reported in Literature Reviewed ............................... 2-6 Table 2-4 Bulk Density of SDA Material Specified as Aerated or Poured or Loose; Packed or Compacted; and Tapped or Settled Reported in the Literature Reviewed........ 2-6 Table 2-5 Specific Gravity of SDA Mate rial Reported in Literature Reviewed .......................... 2-7 Table 2-6 Optimum Moisture Content and Maximum Dry Density of SDA Material Reported in Literature Reviewed........................................................................................ 2-8 Table 2-7 Unconfined Compressive Strength of SDA Material Reported in Literature Reviewed, psi..................................................................................................................

. 2-10 Table 2-8 Permeability Coefficient of SDA Material Reported in Literature Reviewed, cm/sec.........................................................................................................................

..... 2-11 Table 2-9 Typical Composition of SDA Material Containing Fly Ash as Noted in the Reviewed Literature, wt% ................................................................................................ 2-12 Table 2-10 Typical Range of Main Components of SDA Material with Precollection of Fly Ash, wt% ...................................................................................................................

. 2-13 Table 2-11 Composition (wt%) of SDA Material Overall and as a Function of SO 2 Separation Efficiency. Adapted from Kolar [17] ............................................................... 2-14 Table 2-12 Summary of Bulk Chemical Composition, Reported as Oxides, of SDA Material Containing Fly Ash Reported in Literature Reviewed, wt%................................ 2-15 Table 2-13 Summary of Bulk Chemical Composition, Reported as Oxides, of SDA Material with Precollection of Fly Ash Reported in Literature Reviewed, wt% ................. 2-16 Table 2-14 Summary of Total Elemental Composition of SDA Material Containing Fly Ash Reported in Literature Reviewed, ppm ..................................................................... 2-17 Table 2-15 Summary of Total Elemental Composition of SDA Material with Precollection of Fly Ash Reported in Literature Reviewed, ppm............................................................ 2-20 Table 2-16 pH Values of SDA Material Reported in Literature Reviewed............................... 2-20xviii Table 3-1 Commercial Applications for SDA Material in the United States and Europe........... 3-2

Table A-1 Bulk Chemical Composition of SDA Materials Containing Fly Ash Reported in Literature Reviewed .......................................................................................................... A-2 Table A-2 Bulk Chemical Composition of SDA Materials with Precollection of Fly Ash Reported in Literature Reviewed....................................................................................... A-5 Table B-1 Total Elemental Composition of SDA Materials Containing Fly Ash Reported in Literature Reviewed ...................................................................................................... B-3 Table B-2 Total Elemental Composition of SD A Materials with Precollection of Fly Ash Reported in Literature Reviewed....................................................................................... B-71-1 

# 1

**INTRODUCTION**

The use of flue gas desulfurization (FGD) technologies to reduce sulfur dioxide (SO 2 ) emissions from flue gases at coal-fired power plants gained prominence in the 1980s in the United States. FGD systems are currently used on approximate ly 22% of U.S. coal-fired power plants. A variety of FGD system types are used globally, a nd the types of FGD systems used in the United States are summarized in a 2003 En ergy & Environmental Research Center (EERC) report [1].

The FGD material that is the focus of this re port is termed spray dry absorber material or SDA material. The system is referred to as a spra y dry absorber (SDA). An SDA system captures SO 2 from the flue gas by use of slaked lime slurry, wh ich is sprayed into the flue gas, dried by the heat of the flue gas, and collected in a part iculate control device. SDA systems may follow a particulate control device that collects the fly ash or the fly ash may intermingle with the lime slurry and be collected in combination with th e SDA material. Recycle of the combined solid may be used to improve sorb ent utilization. Alkaline fly ash such as that generated from subbituminous coals and some lignite coals will sorb SO 2 gases. As noted by Redinger, the use of fly ash precollection is common in Europe but is not common in the United States. SDA systems are considered efficient and reliable and have a lower capital cost than wet FGD systems [2]. Operating costs for SDA systems are higher than wet FGD systems but the water usage is lower. The resulting product currently has a low utilization rate in the United States.

The product produced from an SDA system is a dr y FGD material commonly referred to as SDA material or dry FGD material. Other terms are also used to refer to this material, and a number of these were identified in a previous literature search on the topic of sulfite-rich FGD materials [3]. These terms are noted in Figure 1-1. This re port will use the term “S DA material” throughout, except where the generic term “dry FGD material” was used in the review literature. In the United States, SDA material typically contains fly ash, so the authors have elected to note precollection of fly ash or the le vel of fly ash content when known.Introduction

1-2 • Spray/spray-dr y/dryer/drier ab sorber/atomization/absorption (S DA, SAV) sludge, ash, material, product, byproduct, by-product, end-product, waste, or residue

• Semidry absorber/atomization/absorption (SDA) sl udge, ash, material, product, byproduct, byproduct, end-product, waste, or residue

• Spray absorption process (SAP) sl udge, ash, material, product, byproduct, by-pro duct, end-product, waste, or residue

• Spray dryer (SPD) residue

• Spray dryer by-product (SDB)

• Calcium spray dryer/drier ash, material, produc t, byproduct, by-product, end-product, waste, or residue

• Lime spray dry/dryer/drier (LSD) ash, material, pr oduct, byproduct, by-product, end-product, waste, or residue

• Advanced SO 2 control (ASC) ash, material, product, by product, by-product, end-product, waste, or residue

• Sulfite-rich flue has desulfurization sludge, ash, material, product, byproduct, by-product, endproduct, waste, or residue

• Sulfite sludge

• Scrubber residue or sludge

• Dry flue gas desulfurization sludge, ash, materi al, product, byproduct, by-product, end-product, waste, or residue

• Dry scrubber sludge, ash, material, product, bypr oduct, by-product, end-product, waste, or residue

• Nonoxidized flue gas desulfurizat ion sludge, ash, material, produ ct, byproduct, by-product, endproduct waste, or residue

• Flue gas desulfurization sludge, ash material, prod uct, byproduct, by-produc t, end-product, waste, or residue (with no reference to the FGD process)

• Spent slurry

**Figure 1-1 Terms Used to Refer to SDA Material in the Literature** Introduction

1-3 Coal-fired power plants are currently evaluating op tions to comply with U.S. regulations that will require reductions of emissions of air toxics and acid gases. Responses are expected to result in an increase in SO 2 emission controls and a subsequent increase in the volumes of FGD products produced in the United States. SDA syst ems, already being used by coal-fired power plants primarily in the western United States, w ill be one option that power plants may install, especially where water resour ces are limited. While all types of FGD material production are likely to increase, the potential in crease in production volumes of S DA materials is a subject that raises the issue of materials management because of the current low utilization rate in the United States. With goals of 50% uti lization of coal combustion pr oducts (CCPs) set by the U.S. Department of Energy (DOE) and the American Coal Ash Association (ACAA) to be achieved

by 2010, additional high-volume production of one or more materials with limited potential for utilization in the current market threatens to o ffset the great strides to increase CCP utilization in the United States. The first step in determining th e best options for utilization of SDA material is to identify current commercial applications a nd any potential applicat ions that have been investigated or demonstrated.

The goal of the project was to assess the current state of the knowledge regarding the characterization and utilization of SDA materi al and make recommendations on how to improve the use of the material. The pr oject focused primarily on products produced from SDA systems; however, the report does cont ain limited information on other FGD processes and products.

This report provides background information on how SDA systems function, descriptions of the variability of SDA materials, and information on the current production and use statistics. This background information provides a framework fro m which to evaluate the information on utilization applications summarized from the literature.



### Background

**Development and Status of FGD S ystems in the United States** 

Currently, coal-fired power plants account for the majority of SO 2 emissions in the United States. Health concerns, including breathing difficulty, respiratory illness, and aggravation of existing cardiovascular disease, are associated with e xposure to high ambient concentrations of SO 2 . The emission of SO 2 from coal-fired power plants can also lead to acid deposition in the environment.

In an effort to address health and en vironmental concerns related to SO 2 in ambient air, legislation has been enacted to regulate most industrial SO 2 emissions. In the United States, major regulations in clude the Clean Air Act Amendments of 1970, 1977, and 1990. The 1990 amendment required a permanent 10-million-ton re duction (to almost half the 1980 level) in SO 2 emissions between 1980 and 2010. On March 1 0, 2005, the U.S. Environmental Protection Agency (EPA) issued the Clean Air Intersta te Rule (CAIR), which will permanently cap emissions of SO 2 and nitrogen oxides (NO

x ) in the eastern United States. SO 2 emission regulations have also been proposed in many ot her industrialized nations . Most members of the European Economic Community are regulated, and Canadian laws are similar to those in the United States.Introduction

1-4 Coal-fired power plants generally employ one of two strategies to control SO 2 emissions: 1) burncompliance fuels or 2) install FGD systems. Compliance fuels can be obtained by burning lowsulfur coal (coals with sulfur content below 2% by weight), blending low- and high-sulfur coals, and washing coal. Most modern power plants, pa rticularly plants built after 1978, are required to have an FGD system. A variety of FGD systems ar e in use and others are in various stages of development. Commercialized FGD processes in clude wet, semidry, and completely dry processes. Regardless of type, FGD processes typi cally use a calcium- or sodium-based alkaline sorbent. The sorbent is injected in the flue gas in a spray chamber/vessel or directly into the duct. The SO 2 is adsorbed, neutralized, and/or oxidized by the alkaline sorbent into a solid compound, either calcium or sodium sulfite or sulfate. The solid is remove d from the flue gas stream using downstream equipment.

FGD Systems

For a typical coal-fired power plant, FGD sy stems will remove ~90% or more of the SO 2 in the flue gas. According to EPA, approximately 85% of the FGD systems installed in the United States are wet, 12% are SDA systems, and 3% are dry injection systems [4]. The following is a brief description of wet and dry FGD systems.

Wet Calcium-Based FGD Systems

In a wet FGD system, flue gas is ducted to a spray chamber/vessel (absorber) where an aqueous solution of sorbent is injected into the flue ga s. The most popular type of sorbent used is limestone, but lime can also be used. A portion of the water in the solution is evaporated and the waste gas stream becomes sat urated with water vapor. SO 2 dissolves into the solution droplets where it reacts with the alkaline particulates. The resulting wet FGD material falls to the bottom of the spray chamber/vessel, where it is collected [4].

Wet FGD systems are the most popular technology to control SO 2 emissions because they use a widely available and inexpensive sorbent (l imestone), can produce FGD gypsum (a usable product) when using forced oxidati on, are reliable, and can achiev e efficiency up to 99% [1]. Several types of wet FGD processes are ava ilable and include limestone-forced oxidation, limestone with natural or inhibited oxidation, l ime with or without buffers, lime dual alkali, magnesium-promoted lime, a seawater process, a sodium-scrubbing process, and ammonia scrubbing [1, 5].

Dry and Semidry FGD Systems

There are four types of dry/semidry FGD system s: 1) SDA (semidry) systems; 2) duct sorbent injection (DSI) (dry); 3) furnace sorbent injection (FSI) or limest one injection multistage burner (LIMB) (dry); and 4) fluid ized-bed combustion (FBC) (dry). From 1980 to 1992, 7200 megawatts (MW) of dry FGD systems were installe d at electric utilities in the United States and have mainly been applied to units burning low-sulfur coals. Of the 43 electric utility dry FGD system installations in the United States, the majo rity lie west of the Mississippi River [5, 6]. The technology is also used in western Europe.Introduction

1-5 Dry FGD systems like DSI, FSI, and LIMB inject powdered sorbent directly into the furnace, economizer, or downstream ductwork. The resulting dry product is removed using particulate control equipment such as an el ectrostatic precipitato r (ESP) or baghouse/fabric filter. The flue gas is generally cooled prior to entering the pa rticulate control device. Water can be injected upstream of the absorber to enhance SO 2 removal [4]. Atmospheric fluidized beds use a sorbent such as limestone or dolomite to capture sulfur released by the combustion of coal. Jets of air suspend the mixture of sorbent and burning coal during combustion, converting the mixture into a suspension of red-hot particles that flow like a fluid [7].

The SDA, or semidry, process consists of four operations: sorbent preparation, the spray dryer absorber (also referred to as a chamber or vessel), particulate collection, and product management. An alkaline sorbent is delivered to th e power plant in covered railroad cars, trucks, or river barges and then stored in a suitable c ontainer to which is it usually pneumatically conveyed. The most popular sorbent is lime (calcium oxide, CaO pr ecalcined) or calcium hydroxide (hydrated lime, Ca(OH) 2 ), but a select number of system s use sodium carbonate. The sorbent is mixed with water, then classified to prevent any large grit par ticles from going to the spray dryer absorber, where they can cause or ifice plugging in the spray nozzles or rotary atomizer. The resulting aqueous slurry is sprayed into the hot flue gas in the spray dryer absorber in a cloud of fine droplets. The residen ce time is sufficient to allow the SO 2 and other acid gases such as sulfur trioxide (SO 3 ) and hydrogen chloride (HCl) to react simultaneously with the sorbent and for the water to evaporate. A diagra m of the SDA process is shown in Figure 1-2. In this figure, a fly ash precollection ESP is show n as an option. In Europe, fly ash precollection is common for SDA systems, allowing the fly ash stre am to be utilized without being mixed with the SDA material. In the United States, it is not common for the fly ash to be precollected, so the fly ash and SDA sorbent combine and pass into the particulate control device as shown in the figure. It is also typical that a portion of the SDA material is recycled back into the SDA, also indicated in the figure.

The resulting materia l is a dry powder product, whic h is a calcium sulfite (CaSO 3 · 1⁄2 H 2 O, or hannebachite) rich material, referred to as SDA material in this report. A small portion of the dry product is collected at the bottom of the absorber , but the bulk of the material is collected in either an ESP or a fabric filter/baghouse. The SDA material may contai n up to 75% fly ash by mass, depending on the location of the SDA instal lation. The distribution of materials collected from the bottom of the absorber and from the particulate control device (ESP or fabric filter/baghouse) in a typical SDA operation is give n in Table 1-1 [8]. Since there is a certain amount of unreacted lime in the SDA material, mo st SDA systems recycle part of their products

(SDA material and fly ash) back into the feed slurry to increase lime utilization and take advantage of the inherent alkalin ity of some fly ashes [9, 10, 11].Introduction

1-6 B&W Proprietary & Confidential Dynegy / S&L 1/10/06 6 2006 Western Fuels Symposium 6 H 2 O H 2 O CaO SDA

Recycle Solids Disposal / Utilization BH / ESP 

## SDA Process

ESP

Fly ash Pre-Collection

**Figure 1-2 Diagram of the SDA Process Showing the Fly Ash Precollection Option and the Solids**  **Recycle [2]**

**Table 1-1 Typical SDA Operation Ma**  **terial Distribution [8]**

**Material Bottom of Drying**  **Chamber (%) Particulate Control**  **Device (%)**
Fly Ash 70 72

Calcium Sulfite 13 15

Calcium Sulfate 7 9

Unreacted Lime 5 1

Water 5 3Introduction

1-7 The advantages of SDA systems over wet scrubbing include:

1. Less costly construction materials typically made of mild steel, thus lower capital costs.
2. Dry products that do not require the use of e xpensive handling equipment or a wastewater
stream.
3. Fewer unit operations requiring less space, making SDA a good choice for retrofit.
4. Flexibility of the feed system, allowing immediat e feed control of sorbent to follow boiler load.
5. High reliability.
6. Less sensitive and simpler pr ocess chemistry [5, 9, 12, 13].

7. Removal of SO 3 from flue gas.

Perhaps the greatest disadvantage of SDA systems is the higher cost of lime sorbents used in relation to the limestone used for wet scrubbing [9, 12, 13]. In addition, SDA systems produce a product that is difficult to sell and is often disposed of. SDA ma terial is typically disposed of in a manner similar to fly ash.

SDA systems are the second most popular FGD technology. SDA systems are used mostly for relatively small-to-medium-capacity boilers (4 0–500 MW) that burn low- to medium-sulfur coals. Currently, there are 26 SDA units in oper ation on coal-fired power plants in the United States. These units are shown in Table 1-2. SDA units in the United States were previously reported by Beidleman and Hilbe rt [14] and Soud [15]; however, multiple current sources were used to develop this updated table.1-8**Table 1-2 SDA Units in the United States** **a Nameplate** **Capacity (MW)** **Utility Plant and Unit** **with FGD System** **City State By Plant** **By FGD Unit** **In-Service Date** **Design Coal Sulfur (wt%)** **Sorbent Designed SO** **2 Removal** **(% Efficiency) Coal** **Type**AntelopeValleyUnit 1Beulah ND 900 450 1984 1.20 Lime Up to 90.0 LigbAntelopeValleyUnit 2Beulah ND 900 450 1986 1.20 Lime Up to 90.0 LigBasic Electric Power CooperativeLaramie RiverStation Unit 3Wheatland WY 1650 550 1982 0.50Lime/AlkalineFly Ash85.0 SubcDuke EnergyEast Bend Unit2Rabbit Hash KY 669 669 1981 5.20 Lime 99.0 BitdEast Kentucky Power Cooperative,Inc.H.L. SpurlockUnit 2Maysville KY 1118 525 1981 3.60 Lime 90.0 BitGrand River Dam AuthorityCoal-FiredComplex Unit 2Chouteau OK 1010 520 1986 1.50Lime/AlkalineFly Ash85.0 SubGreat River Energy Stanton Station Stanton ND 188 188 1982 0.70 Lime 70.0 LigKansas City Power & LightHawthornStationKansas City MO 594 594 2001 0.33Lime/AlkalineFly Ash88.0 SubMarquette Board of Light and Power Shiras 3Marquette MI 40 40 1983 0.50 Limestone 80.0 SubOtter Tail Power Company Coyote Station Beulah ND 400 400 1981 0.80Lime/AlkalineFly Ash70.0 LigPacifiCorpWyodak PowerPlantGillette WY 365 365 1986 0.80Lime/AlkalineFly Ash75.2 SubPlatte River Power AuthorityRawhideEnergyStationWellington CO 294 294 1984 0.30Lime/AlkalineFly Ash80.0 SubSierra Pacific PowerCompanySCValmy Unit 2 Valmy NV 521 267 1985 0.50 Lime 70.0 BitIntroduction1-9**Table 1-2 (continued) SDA Units in the United States** **a Nameplate** **Capacity (MW)** **Utility Plant and Unit** **with FGD System** **City State By Plant** **By FGD Unit** **In-Service Date** **Design Coal Sulfur (wt%)** **Sorbent Designed SO** **2 Removal** **(% Efficiency) Coal** **Type**South Carolina Electric & GasCompanyCope Station Cope SC 430 430 1996 1.90 Lime Up to 98.0 BitTri-State Generation &Transmission AssociationCraigUnit 3Craig CO 1274 446 1984 0.70 Lime 85.0 SubSunflower Electric PowerCorporationHolcombUnit 1Holcomb KS 360 360 1983 1.00Lime/AlkalineFly Ash80.0 SubSpringervilleUnit 1Springerville AZ 1560 380 1985 0.70Lime/AlkalineFly Ash61.3 SubSpringervilleUnit 2Springerville AZ 1560 380 1985 0.70Lime/AlkalineFly Ash61.3 SubTucson Electric Power CompanySpringervilleUnit 3Springerville AZ 1560 400 2006 0.70Lime/AlkalineFly Ash—SubCherokeeUnit 3Denver CO 715 151—0.40Lime/AlkalineFly Ash70.0 BitCherokeeUnit 4Denver CO 715 351 2003 0.40Lime/AlkalineFly Ash70.0 BitHaydenUnit 1Hayden CO 446 184 1998 0.40Lime/AlkalineFly Ash85.0 BitHaydenUnit 2Hayden CO 446 262 1999 0.40Lime/AlkalineFly Ash85.0 BitXcel EnergySherco Unit3Becker MN 2400 900 1987 0.90Lime/AlkalineFly Ash72.3 SubaMultiple current sources were usedto develop this update table.bLignite.cSubbituminous.dBituminousIntroduction

1-10 In nine European countries, 49 dry FGD plants we re reported [16]. The pl ants are categorized by country in Table 1-3.

**Table 1-3 Dry FGD Plants in Europe [16, 17]** 

**Country Sites with Dry**  **FGD Plants Dry FGD**  **Production**

**(approx.) (short tons/year)**  **Year Reported**  **% of FGD Units Using SDA** 

**Process**

Austria 4 50,000 1995 41.7 in 1993
Denmark 2 83,200 1997 37.3 in 1993
Czech Republic 8 140,000 1998 NR a
Finland 4 32,000 1999 NR
Germany 20 378,000 1996 7.0 in 1993
Italy 1 10,000 2000 NR

Poland 5 500,000

– 600,000 2000 NR
Spain 1 5,000 1999 NR
Sweden 4 10,000 1999 100

a Not reported.

**Projected Future SDA Installa tions in the United States** 

The Babcock & Wilcox Company tracks projected fu ture SDA installations in the United States (see Table 1-4) [18]. Details indicating new capac ity versus retrofit inst allations, generating unit size, coal type, geographic loca tion, and reagent system are pr ovided in Table 1-5. The numbers of projected SDA installations s hown in Tables 1-4 and 1-5 are plan ned projects that have been announced, but many factors could change these pla nned installations. Additionally, installations of circulating dry scrubber-type systems, produc ing a product expected to be similar to SDA material, are projected to increase.

**Table 1-4 U.S. SDA Market Projections [18]** 

**Time Horizon**

**1**

– **5 Years (2007**

– **2012) 5**

– **10 Years (2013**

– **2017)**
**Projected SDA**

**Installations # Gen. Units Gen. Capacity (MW)**  **# Gen. Units Gen. Capacity (MW)**

New Capacity 14 7,050 3 1,850
Retrofits 28 11,350 16 7,400Introduction

1-11 **Table 1-5 Details of U.S. SDA Market Projections [18]** 

**1**

– **5 Years (2007** 

– **2012) 5** 

– **10 Years (2013** 

– **2017) Time Horizon**  **New Capacity Retrofits New Capacity Retrofits**
**Generating Unit Size (MW)**

>800 2 3 1

500 to 800 7 7 3 6

250 to 500 1 8 8

100 to 250 2 7 1

<100 2 3
**Coal Type (sulfur content, %)**
Lignite 1

Subbituminous 13 18 3 16

Bituminous – western 3

Bituminous – eastern 7
**Geographic Location by EPA Region**
1 (ME, NH, VT, MA, RI, CT) 2
2 (NY, NJ) 3
3 (PA, WV, VA, MD, DE) 2
4 (KY, TN, NC, SC, GA, AL, MS, FL) 2
5 (OH, IN, IL, MI, WI) 1 10 14
6 (AR, LA, OK, TX, NM) 3 2 1
7 (IA, MO, KS, NE) 4 2 1
8 (CO, UT, WY, MT, ND, SD) 4 7
9 (CA, NV, AZ) 2
10 (AK, WA, OR, ID)
**Reagent Stream**
Fly ash recycle 11 26 3 16
Lime only 3 2Introduction

1-12 

### Current SDA Material Pr **oduction and Use Rates**

ACAA reports the yearly production and use of CCP s in the United States, including statistics on FGD products from wet and dry systems. SDA mate rial is reported with a ll other products from dry FGD systems, with the exception of FBC sy stems, in an FGD dry scrubber material category. The ACAA definitions of dry FGD material an d FGD dry scrubber material are included in Figure 1-3 [19].

• 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 main constituents are calcium sulfite and dr y fly ash, along with minor quantities of calcium sulfate. Sodium-based sorbent syst em main constituents are sodium sulfite and dry fly ash along with minor quantities of sodium sulfate. Dry FGD materi al is being used in construction, engineering, and agricultural applications; however, most of the material is stored in landfills.

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

**Figure 1-3 ACAA Definitions of Dr**  **y FGD Material [19]**

The production of dry scrubber FGD material has been reported as a separate category in the annual ACAA CCP Production and Use Survey sin ce the 2002 statistics were released in 2003 [20]. Since then, the reported production has va ried from a low of 935,394 short tons in 2002 to a high of 1,829,830 short tons in 2004 [20, 21]. ACAA reported that 1,488,951 short tons of dry FGD material was produced in the United Stat es in 2006. Of that, 136,639 short tons (or 9.18%) was beneficially used, which is the lowest pe rcentage reported by ACAA’s survey [22]. Figure 1-4 illustrates the major markets for dr y scrubber FGD material in the United States as reported by respondents to the 2006 annua l ACAA CCP Production and Use Survey.

The use application categori es reported for dry scrubber F GD material in the ACAA annual survey since 2002 statistics have included concre te/concrete products/grout , cement/raw feed for clinker, flowable fill, stru ctural fills/embankments, ro ad base/subbase/pavement, soil modification/stabilization, mineral filler in asphalt, mining applications, waste stabilization/solidification, agriculture, aggreg ate, and miscellaneous/other. However, the categories vary annually because of variations in respondents to the annual ACAA survey and added SDA units or unit closures (e.g., Xcel En ergy’s Riverside Unit 7 went out of service in 2004). ACAA typically notes a utilit y response rate of 54%–60% for its annual Production and Use Survey.Introduction

1-13 **Figure 1-4 Dry Scrubber FGD Material Use in the United States In 2006. Of 1,488,951 Short Tons**  **Produced, 136,639 Short Tons of Dry Scrubber FGD Material (9.18%) Was Beneficially**  **Used [22]**

SDA material production and use figures are incor porated into the overall reporting category of dry FGD material by ACAA, and quantities of SDA material produced and used have never been reported separately by ACAA. Using the 2004 and 2005 production figures for dry FGD material, and an estimate of sodium-based dr y FGD material production based on sodium-based sorbent use predictions for 2007, and taking into consideratio n the ACAA response rate, the authors estimated SDA annual production of appr oximately 3.6 million tons. Using the data in Table 1-2 and production figures provided by sever al industrial power pl ants, it was estimated that 350–400 tons of SDA material is pr oduced for each MW of capacity. Using the approximation of 350–400 tons of SDA material produced per MW unit si ze multiplied by 9556 MW of existing SDA units indicated that ~ 3.3–3.8 million tons of SDA material is likely produced annually in the United States. The auth ors estimate is significantly higher than the ACAA production figures indicate; how ever, the ACAA statistics are ex pected to reflect the use profits for dry FGD materials even though not all util ities are responding.

The European Coal Combustion Products Asso ciation (ECOBA) reports the production and use of CCPs in Europe (EU 15). EU 15 refers to th e fifteen countries that formed the European Union until the end of April 2004. These countries include the United Kingdom, France, Belgium, Luxembourg, the Netherlands, German y, Italy, the Irish Republic, Denmark, Greece, Portugal, Spain, Sweden, Finland, and Austria . Production of SDA material in 2004 was 463,000 short tons, down from 540,000 in 2003 [23]. Of this, 41% of SDA material produced in 2004 was beneficially used in nonmining applic ations, which are broken down into generalIntroduction

1-14 engineering fill, flowable fill, plant nutrition, and other uses. An additional 39% of the SDA material produced was used for mine reclamatio n and restoration purposes, for a total of 80% SDA product utilization. Therefore, 20% was di sposed of [23]. The beneficial use and mine reclamation and restoration applicat ions are compiled in Figure 1-5.

**Figure 1-5 Spray Dry Absorption Product Use in Europe (EU 15) in 2004. Of 466,278 Short Tons**  **Produced, 373,684 Short Tons (80.1%) Was Beneficially Used [23]**

ECOBA lists several specific current uses for SDA material in Europe [24, 25, 26]:

• As a component of mining mortar for stabilizing unde rground cavities

• As an addition in the production of sand–lime bricks

• In the production of cement cl inker in a special clinker pr oduction method (Müller-Kühne Process)

• As a sorbent in a wet FGD process in power plants

• As a sulfur fertilizer in agricultureIntroduction

1-15 In May 2004, ten Accession Countries joined the ex isting EU 15, resulting in an EU comprising 25 member states. The Accession Countries were C yprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia, and Slove nia. SDA material is produced in anthracite and lignite coal-fired plants in Poland. From anthracite, 93,304 thousand short tons of SDA material was produced, with 98% utilized in mi ning and beneficial landfill/land reclamation in 2000. In addition, 1,134,240 thousand short tons of fly ash + SDA material was produced, with a 100% utilization rate. App lications included buildi ng materials, bricks and ceramics, roads, and mining. From lignite, 424,389 thousand short tons of fly ash + SDA material was produced, with 100% utilization in beneficial landfill/land reclamation [27].

**Projected Future SDA Producti on in the United States** 

Based on the current production figures, estim ated production rate, and MW of SDA scrubbed units in the United States, it is estimated that up to 7 million tons of additional SDA material could be produced from SDA units installed in the next 5 ye ars, making the total annual production as high as 10 million tons, nearly 300 % of the current total, by 2012. With an estimated potential additional 9250 MW of SDA scrubbing added between 2012 and 2017, it is estimated that up to an additi onal 3.5 million tons of SDA mate rial could be generated annually in that time period. If all the projected SDA units are installe d as noted in Tables 1-4 and 1-5, and no existing units are taken off line, th e total SDA material production could reach 13–14 million tons annually in 10 years. This estimate reflects an increase of 400%–500%.2-1 

# 2

**RESULTS AND DISCUSSION OF THE LITERATURE REVIEW** 

Technical reports, journal arti cles, patents, conference proceedings, book chapters, and news articles were assembled and reviewed to prepare this report. A multitiered search approach was used to obtain literature including sources that we re the most readily available and those that required a more in-depth search. The search r esulted in a significant number of documents containing references to dry FGD materials includ ing SDA materials. A tiered approach was also used for the review of the literature obtained. Researchers used knowledge about these materials to choose the documents for review. The literature reviewed included information on the characteristics of SDA material and commercial and potential utilization applications for SDA material. An effort is under way to add all of the gathered references to the EERC FIRST SEARCH online database located at www.undeerc.org/carrc /firstsearch/.



### Definitions of SDA Material

ASTM International [28] and ACAA [19] ha ve developed and published terminology and definitions related to CCPs including FGD ma terials. These terms and definitions do not currently include SDA system or material de finitions; however, the definition of dry FGD material provided by both organizations is inclus ive of SDA material that contains fly ash. The ACAA definition of dry FGD material (see Figur e 1-3) includes materi als that would be produced in dry FGD systems utilizing sodium-b ased sorbents, but the ASTM International definition (see Figure 2-1) includes only calciu m-based materials. SDA systems utilize lime slurry, so these materials are ca lcium-based materials. Strictly applying the ASTM International and ACAA definitions for dry FGD material (which includes fly ash as a component of the material), material from an SDA system using fly ash precollection would not be defined as a dry FGD material; however, the term SDA material is u sed here to refer to materials that do and do not contain a fly ash compone nt to be consistent with the literature reviewed.

• 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. ASTM equivalent terms are dry FGD ash and lime spray drier ash.

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

**Figure 2-1 ASTM International Definitions [28]** Results and Discussion of the Literature Review

2-2 

### Characterization of SDA Material

SDA materials can vary widely in their phys ical, chemical, and mineralogical properties, depending on their source. The fo llowing factors affect both the qua ntities and characteristics of SDA material:

• Composition of the coal feedstock (ash content, sulfur content, heating value)

• Combustion conditions

• Sorbent type

• SO 2 uptake efficiency (Ca/S ratio)

• Fly ash collection location and efficiency

• Composition and mineralogy of the fly ash

• Recirculation rate

• Load level

• Stoichiometric (sorbent) ratio

Regardless of the type of process used to scr ub the flue gas, all FGD products include spent sorbent as sulfites or sulfates plus unreacted sorb ent. The quantity of the sorbent used is usually proportional to the sulfur content of the coal burne d but is also a result of the percent of SO

x recovery desired and system opera ting parameters [29]. The calcium sulfite content is dependent on the SO 2 removal efficiency [17].

Because fly ash commonly makes up a large prop ortion of the SDA material (40%–75%), the overall physical properties and morphology of most SDA materials ar e similar to fly ash; therefore, handling properties are similar to fly ash. Most U.S. systems collect the fly ash and SDA material together in an ESP or baghouse/fab ric filter, producing spherical glassy fly ash particles coated by and intermixed with fine crys tals of calcium/sulfur reaction products [10, 12]. The fly ash particles provide reac tion sites for the rem oval of sulfur and, consequently, become coated with reaction products. Th e result is a dry, free-flowing pow der with particles smaller and finer than fly ash [10]. However, if the SDA mate rial is collected as a separate product stream, then it will appear as a fine, dusty powder with an off-white color. The major difference between SDA material and conventional fly ash can be att ributed to the higher calc ium and sulfur content of SDA material. SDA materials ar e finer and more caustic, have a higher heat of hydration, and produce more alkaline leachate when co mpared to conventional fly ash [13].

Dry FGD materials from plants that used different types of boilers and burned different types of coal were collected, and it was found that the products had a moderate to wide range of variability in their chemical, physical, and mineralogical prope rties [30]. The amount of unused sorbent (portlandite, Ca(OH) 2 ) varied the most, by a factor of 14. This variation depended on whether or not the plant recycled its products . This example shows that the physical and chemical properties of dry FGD materials can be hi ghly variable from plant to plant and that an analysis of the material should be conducted pr ior to beneficial use, particularly when the material has the potential to expand or will contact water.Results and Discussion of the Literature Review

2-3 The following is an analysis of the physical pr operties, chemical charact eristics, and mineralogy of SDA material, based on the literature reviewe d. It is important to note that the properties and characteristics of the SDA materials are affect ed by varying amounts of fly ash present, which was not always reported.

**Physical Properties**

The physical properties of SDA material are important because th ey affect storage, handling, disposal, and engineering uses. Uniformity of th e material and the abilit y to pack tightly are especially important for some applications.

SDA materials are generally free-flowing to aver age-flowing, but higher moisture content can affect flowability and storage problems. Both qualitative and quantita tive physical properties can affect transfer and transport. The quantitative physical properties that can affect transfer and transport include particle size, bulk density, specific gravity, a nd temperature. Disposal is affected by optimum moisture and maximum density, unconfined compressive strength, and permeability [12].

Qualitative Physical Properties

Qualitative physical properties of SDA material incl ude abrasiveness, hygroscopicity, tackiness, corrosivity, and tendency to aerate. The abrasive nature of SDA material has been demonstrated through reported wear on handling equipment. SDA material has also been found to have hygroscopic tendencies due to potential hydration reactions of the lime, calcium sulfate, and calcium sulfite fractions of the ma terial. Evidence of its tacky natu re, if it does absorb water, has been demonstrated by solids buildup at the elbow s of pneumatic equipment. If wetted, it can stick and build up on mechanical conveyors. When wet, a moderate corrosivity exists. In addition, it has the tendency to aerat e and retain air. This makes it easy to convey pneumatically but limits the speed at whic h it can be conveyed [12, 13].

Particle Size

The particle-size distribution of a material is characterized by the proportion of particle sizes within a series of specific size intervals. Particle-size distribution is important because many engineering parameters are related to the variation of particle si ze of a material. The fineness of CCPs is also an important characteristic beca use high-surface-area CCPs are generally more chemically activated which, in turn, may be of im portance in evaluating utilization options [31]. The exposed surface area of a given volume will be greater for the smaller CCP particles.

The particle size of SDA material is very fine and tends to be sm aller than conventional fly ash. Most SDA materials can be classified as in th e silt size range (1/256–1/16 mm, or 3.9–62.5 μm). SDA material particle-size dist ributions are relatively uniform [13]. Particle sizes depend on both combustion and collection system designs [32].Results and Discussion of the Literature Review

2-4 The particle-size distribution and/or mean ma ss/particle size of SDA materials have been reported in numerous publications. A compilatio n is shown in Table 2- 1. The reported particlesize distribution ranged from 1 to 400 μm, w ith 1–70 μm more typical. The reported mean mass/particle size range is 1–45 μm.

**Table 2-1 Particle-Size Distribution and Mean Mass/Part**  **icle Size of SDA Material Reported in Literature Reviewed, μm** 

**Particle-Size Distribution Mean Mass/Particle Size Source (s)**
NR 16–45 [2, 30]
NR 13.5 [33]
20–40 NR [1]

5–60 a 10–15 a [16]
2–74 NR [29]
1–70 NR [31,34 b ]
NR 4–30 [17]
NR 2–30 [10]

1–400 7.4–18.0 c [35 b ]

NR 20–28 [12 b ]
18–60 24–29 [13]

NR 20–25 c [36]

a Precollection of fly ash; 3%–10% fly ash content.

b Literature review reported within source.

c Reported as mean diameter.

Conventional fly ash mean mass/particle si ze has been reported as 35–55 μm [10], 25–40 μm [36], and a mean of 45 μm at one plant [12, 13].

It was reported that 78%–80% of the SDA materi al was in the fraction finer than 40 μm, while 57%–78% of conventional fly ash was in this finer fraction [12, 13]. Similarly, four SDA material samples tested ha d greater than 80% of particles by weight finer than 25 μm, indicating very fine particles [37, 38]. Kolar reported that 85%–100% of the SDA material was less than 60 μm, where 60 μm was the maximum particle size ob served with precollection of fly ash [17]. It was reported that the SDA material from Basin Electric Power Cooperative pilot plant was a very fine-grain, powdery material, similar in particle size to the fly ash normally produced at a coal-fired power plant [39].Results and Discussion of the Literature Review

2-5 Early work performed for a University of Te nnessee thesis was summarized in another report [40]. The SDA samples tested contained a much smaller percentage of particles less that 2.5 μm in diameter than conventional fly ash. The particle- size distribution analysis also showed that the highest percentage of particles in the SDA material was found to be in the range of 4–10 μm in diameter. The thesis was not availa ble to researchers for review; therefore, the data could not be reviewed in complete context.

Specific Surface Area

The specific surface area of FGD samples tested by the Ohio State University (OSU) corresponded with particle size, indicating mo stly nonporous materials [3 7, 38]. Specific surface area ranges are a function of the fly ash content in the SDA material [17]. The specific surface area of SDA materials reported in the reviewed literature (0.2–16 m 2 /g) is shown in Table 2-2.

**Table 2-2 Specific Surface Area of SDA Material**  **Reported in Literature Reviewed, m 2**  **/g**

**Specific Surface Area Source (s)**
1.64 – 7.47 [2, 30]

9.49 ± 3.82 [37]
~9.4 [38]
1.6 – 7.5 [34 a ]
0.2 – 3.39 [35 a ]

1.5 – 16 [17]

a Literature review reported within source.

Bulk Density

Density is defined as the mass (or weight) per uni t volume of a material. The reviewed literature had several means of reporting bu lk density as compiled in Ta bles 2-3 and 2-4. Included with bulk density ranges are aerated, poured, packed, a nd tapped bulk density ranges. Bulk density measurements are used for different purposes, depending on the intended use of the material. Overall, the range of reported bul k density measurements was 400–1760 kg/m 3 (25–110 lb/ft 3 ).

The fly ash in dry FGD materials has similar particle size, pa rticle density, and morphology to those of conventional fly ashes, but dry FGD ma terials have lower bulk densities [30]. The difference in bulk density is due to variations in the chemical and mineralogical characteristics of the reacted and unreacted sorbent. The range in bul k density has been attributed to the fly ash content [17]. Contrary to Dawson et al. [30], Klimek et al. indicat ed that the bulk density of SDA material is higher than the bulk density of conventional fly ash because SDA material has a larger fine fraction (78%–80% finer than 40 μm for SDA material vs. 57% –78% for fly ash) [13].Results and Discussion of the Literature Review

2-6 The bulk density of fly ash has been reported as 1000 kg/m 3 (62 lb/ft 3 ) [41, 42]. The loose, tapped, and compacted bulk densities were simila r for fly ash and SDA material containing fly ash ranging from 30–90 lb/ft 3 (480–1440 kg/m 3 ) overall for the conventional fly ash [36].

**Table 2-3 Bulk Density of SDA Material Re**  **ported in Literature Reviewed**

**Bulk Density Source(s)**
630 kg/m3 (39 lb/ft3) [38]
780–1250 kg/m3 (49– 78 lb/ft3) [32]
960–1440 kg/m3 (60– 90 lb/ft3)a [11]
700 kg/m3 (44 lb/ft3)b [16]
400–1100 kg/m3 (25– 69 lb/ft3) [17]
600 kg/m3 (37 lb/ft3)b, c [41, 42]

a Reported as density.

b Precollection of fly ash; up to 20% fly ash content.

c Small portion of fly ash.

**Table 2-4 Bulk Density of SDA Material Specified as Aerated or Poured or Loose; Packed or**  **Compacted; and Tapped or Settled Reported in the Literature Reviewed**

**Aerated or Poured or**

**Loose Bulk Density Packed or Compacted**  **Bulk Density Tapped or Settled**  **Bulk Density Source(s)** 

480–960 kg/m 3 (30–60 lb/ft 3 ) NR 784–1250 kg/m 3 (49–78 lb/ft 3 ) [2]
580–960 kg/m 3 (36–60 lb/ft 3 ) NR 720–1250 kg/m 3 (45–78 lb/ft 3 ) [34 a ]
585–962 kg/m 3 (37–60 lb/ft 3 ) 784–1250 kg/m 3 (49–78 lb/ft 3 ) 780–1250 kg/m 3 (49–78 lb/ft 3 ) [30]
580–790 kg/m 3 (37–50 lb/ft 3 ) 780–1020 kg/m 3 (49–64 lb/ft 3 ) b NR [12, 13]
550–680 kg/m 3 (34–42 lb/ft 3 ) 710–760 kg/m 3 (44–47 lb/ft 3 ) NR [13 a ]
480–1040 kg/m 3 (30–65 lb/ft 3 ) 730–1680 kg/m 3 (45.5–105 lb/ft 3 ) 640–1040 kg/m 3 (40–65 lb/ft 3 ) [35 a ]
980–1460 kg/m 3 (61–91 lb/ft 3 ) NR NR [31]
480–640 kg/m 3 (30–40 lb/ft 3 ) 1280–1760 kg/m 3 (80–110 lb/ft 3 ) 720–1040 kg/m 3 (45–65 lb/ft 3 ) [36]

a Literature review reported within source.

b Refers to packed (tapped) bulk density.Results and Discussion of the Literature Review

2-7 Specific Gravity

Specific gravity is defined as the ratio of weight in air of a given volume of solids at a stated temperature to the weight in air of an equal volume of distilled water at the same temperature (usually 20 ° C). Specific gravity is often used as a method of comparison for engineering materials. This differs from bulk density because only the solid fraction of the material is used; the void fraction is not considered.

Specific gravities reported in th e reviewed literature as shown in Table 2-5 range from 2.088 to 2.84. A specific gravity range of 1.5 to 3.1 was reported for fly ash in a previous literature review [31].

**Table 2-5 Specific Gravity of SDA Material Re**  **ported in Literature Reviewed**

**Specific Gravity Source(s)**

2.29

– 2.80 [2, 30]

2.088

– 2.560 [33]

2.3

– 2.8 [34 a ]

2.29

– 2.80 b [12, 13]

2.48

– 2.84 [31, 39]

2.50

– 2.71 c [41]

a Literature review reported within source.

b Used ASTM International D854 (Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer).

c Precollection of fly ash; up to 20% fly ash content.

Early work performed for a University of Te nnessee thesis as summarized in another report indicated that average specific gravities of lig nite-fired calcium-based SDA material increased as a function of unreacted sorbent content [40]. Th e specific gravity of compacted fly ash was 2.52 and for compacted SDA material was 3.14–3.71. The th esis was not available to researchers for review; therefore, the inconsistenc y of the data from other availa ble data could not be evaluated.Results and Discussion of the Literature Review

2-8 Optimum Moisture Cont ent and Maximum Density

The moisture content of a CCP is a measure of the amount of water present in the voids in the CCP and is expressed as a weight percentage of to tal dry weight. The natural moisture content is a function of the deposition environment of the CCP and must be determined experimentally for each individual CCP. The natural moisture content of a CCP must be known to calculate the quantity of water that must be added or remo ved to bring the CCP to its optimum moisture content for compaction.

The optimum moisture content of a CCP is related to the maximum density obtained by compaction in the laboratory. The values of moisture content vers us dry density are plotted to form a compaction curve. As indicated by the curv e, density is dependent on moisture content. The highest point on the compaction curve co rresponds to the maximum dry density and optimum moisture content [31]. Calculations to determine optimum moisture content do not generally allow for the water that will be c onsumed by the formation of secondary hydrated phases such as ettringite in the use application. Many of these ph ases contain up to 50% moisture or even higher. The rate of water addition used by engineers in these calculations can lead to incomplete formation of cementitious phases and la ter expansion caused by delayed ettringite formation. As shown in Table 2-6, the optimum mois ture content range reported in the reviewed literature was 10%–63% and the maximum dry density reported in the reviewed literature ranged from 790 to 1860 kg/m 3 (49–116 lb/ft 3 ).

**Table 2-6 Optimum Moisture Content and Maximum Dr**  **y Density of SDA Material Reported in Literature Reviewed** 

**Optimum Moisture Content Maximum Dry Density Source(s)**

40%

– 68% a 833

– 1056 kg/m 3 (52

– 66 lb/ft 3 ) b [38]

28%

– 63% 790

– 1300 kg/m 3 (49

– 81 lb/ft 3 ) [43]

16%

– 60% 880

– 1630 kg/m 3 (55

– 102 lb/ft 3 ) [34 c ]

16%

– 38% 1140

– 1670 kg/m 3 (71

– 104 lb/ft 3 ) [13, 30]

10%

– 54% 977

– 1860 kg/m 3 (61

– 116 lb/ft 3 ) [35 c ]

18%

– 54% 977

– 1630 kg/m 3 (61

– 102 lb/ft 3 ) [12, 13 c ]

18%

– 54% NR [31]
19% 1610 kg/m 3 (101 lb/ft 3 ) [44]

18%

– 54% 980

– 1460 kg/m 3 (61

– 91 lb/ft 3 ) [40]

30%

– 32% d 1240

– 1350 kg/m 3 (77

– 84 lb/ft 3 ) d [41]

a Weight of water to weight of solids.

b Optimum density instead of maximum density.

c Literature review reported within source.

d Precollection of fly ash; up to 20% fly ash content.Results and Discussion of the Literature Review

2-9 Observations in the literature re viewed include the following:

• Results indicated that the optimum moisture content of the materials increased and the maximum dry density of the compacted material decreased with an increase in the unreacted lime content of the material [40].

• SDA materials had optimum moisture contents higher than the corre sponding fly ashes and increased with increasing sulfu r content. In contrast, dens ity values at optimum moisture decreased as the sulfur content increased [10].

• Data show that the maximum density is not pa rticularly sensitive to slight variations in the water content in the vicinity of the optimu m. Most FGD products can be considered lightweight materials with compacted densities lower than those of a typical natural soil. Lightweight fills impose smaller loads on th e natural soils upon wh ich they are placed, resulting in less settleme nt in the underlying soils and l ess likelihood that the soils will fail [38].

• Optimum moisture contents of 18%–54%, with an average of 28%, were reported in a previous literature review. Within that sam e report, the project samples yielded optimum moisture contents of 16%–38%, with an average of 28.5%. These moisture contents represent the amount of moisture added to the dry ma terial even though common practices base the optimum value on moisture content determina tions performed after compaction. Moisture content determinations performed during this study used a drying temperature of 110 ° C, causing hydrated water to be included in the de termination. It requires a temperature of over 205 ° C to dehydrate calcium su lfate completely [13].

The natural moisture content of SDA material has been reported at 1%–5% [31] and with a range of <0.1%–13.2% in previously reviewed literature [35], while the residual moisture of SDA material has been reported to be about 2% [45]. The values reported for optimum moisture content for compaction indicate th at a substantial amount of mois ture should be added to SDA materials in order to obtain maximum density [12].

Unconfined Compressive Strength

Unconfined compressive strength is usually determined using ASTM International D2166, Unconfined Compressive Strength of Coh esive Soil, or ASTM International D1633, Compressive Strength of Molded Soil–Cement Cylinders. The two procedures are similar except that ASTM International D6133 assumes there is no deformation of the sample during compression and uses its original dimension to calculate unit compressive strength. Results reported in the reviewed literature are summarized in Table 2-7.

Typically, the samples continued to gain strength between 28 and 56 days of curing. It was hypothesized that the most probable reason for the wide range of strengths is the difference in chemical composition [13].Results and Discussion of the Literature Review

2-10 It was noted that for low-sulfur SDA materials, a false set may occur as a result of the high calcium and low sulfur content, r esulting in disruptive expansion reactions [10]. Strengths were found to increase with curing time, with values be ing higher after 28 days than after 10 days [38]. The lowest unconfined co mpressive strength values closely resemble low-strength soil– cement mixtures, and the highest values represent low-strength concrete [40].

**Table 2-7 Unconfined Compressive Strength of SDA Material Reported in Literature Reviewed, psi** 

**1-Day 7- Day 21- Day 28- Day 56- Day Maximum Source**

NR NR NR 51

– 88 NR NR [38]

60 a NR NR 2700 NR NR [46]

NR 280

– 4690 NR 20

– 790 140

– 1650 NR [34]

NR 78

– 1780 NR NR NR NR [10]

NR NR NR NR NR 12

– 3000 [13 b ]

NR 41

– 536 81

– 2250 c 50

– 1411 72

– 1775 NR [40 b ]

a Used ASTM International C109 (Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or [50-mm] Cube Specimens).

b Literature review reported within source.

c Separate sample set.

Scanning electron microsc opy (SEM) confirmed that the pres ence of unreacted CaO or Ca(OH) 2 increases the firmness of the SDA materials. The available moisture dissolves some of the calcium compounds, which then recombine in a pozzolanic reaction with aluminosilicates contained in the fly ash, forming high-specific-volum e compounds [40].

Permeability

Permeability is defined as the rate of flow through a material. Permeability coefficients, typically reported in cm/sec, describe flow through a unit area under a unit hydraulic gradient. Hydraulic gradient correlates the forces causing water to fl ow and the forces resisting flow. A material is considered permeable if it has interconnected pores, cracks, or other passageways through which water or gas can flow. The range of permeability coefficients, or hydraulic conductivities, of fly ash compacted to its maximum dry density is from 10 -7 to 10 -4 cm/sec for bituminous fly ashes, 3 × 10 -6 to 1 × 10 -5 cm/sec for subbituminous ashes, and 1 × 10 -7 to 9 × 10 -6 cm/sec for lignite ashes [31]. Permeability coefficients for SDA mate rials from the reviewed literature compiled in Table 2-8 ranged from less than 9 × 10 -10 to 6.5 × 10 -3 cm/sec.Results and Discussion of the Literature Review

2-11 **Mineralogical and Chemical Composition**

Successful engineering applicati ons depend heavily on the mineralogical properties of SDA materials. SDA material has been described as a combination of s pherical glassy fly ash particles coated by and intermixed with fine crystals of calcium/sulfur reaction products [13]. Three SDA materials (with up to 20% fly ash) produced in Europe were described as dominated by fine, easily crushed spheres composed of finely aggregated crystals of calcium sulfite hemihydrate [41]. They hypothesized that the spheres were probably formed from the atomized slurry droplets, suggesting rapid water evapor ation as the slurry meets the hot SO 2 -laden gases. There was also evidence of calcite, gypsum, clay mineral, and quartz, usually coated with a fine layer of sulfite. An SDA material tested consis ted of 46% amorphous glassy material, 26.6% hannebachite, 17% mullite, 5.5% hydr ated lime, and 3% quartz [47].

**Table 2-8 Permeability Coefficient of SDA Material**  **Reported in Literature Reviewed, cm/sec**

**Permeability Coeffi cient Source(s)** 

10 -9

– 10 -6 [34 a ]

9 × 10 -10

– 9.7 × 10 -5 [34]

3.1 × 10 -9

– 2.7 × 10 -7 [30]

10 -7

– 6.5 × 10 -3 [35]

3.1 × 10 -9

– 6.8 × 10 -7 [13]

<9 × 10 -10

– 9.7 × 10 -5 [13 a ]

10 -7

– 10 -6 [11, 31, 40]

a Literature review reported within source.

The chemical composition of S DA material depends on the sorben t used for desulfurization, the proportion of fly ash collected with th e FGD product, coal sulfur content, SO 2 removal, and other factors. Dry FGD materials contain higher c oncentrations of calcium and sulfur and lower concentrations of silicon, alumin um, and iron than fly ash. The p rincipal reaction product of dry FGD is hannebachite (calci um sulfite hemihydrate):

CaO (lime) + SO 2 (g) + 1⁄2H 2 O = CaSO 3 · 1⁄2H 2 O **Eq. 2-1**

Under more oxidizing conditions, gypsum (cal cium sulfate dihydrat e) may also form:

CaO (lime) + SO 2 (g) + 1⁄2O 2 + 2H 2 O = CaSO 4 · 2H 2 0 **Eq. 2-2**Results and Discussion of the Literature Review

2-12 Sulfite-to-sulfate ratios range from 2:1 to 3:1 [10]. Unreacted sorbent remains as portlandite (Ca(OH) 2 ) in the residuals. At ambient temperature and moist conditions, calcium sulfite (CaSO 3 ) will slowly oxidize to calcium sulfate (CaSO 4 ), and calcium hydroxide will be converted to CaCO 3 by CO 2 in the air. SDA material from lime -based systems has similar chemical composition to stabilized wet FGD material [10].

The major constituents of Basin Electric Power Cooperative’s pilot SDA material were found to contain 14.6% CaSO 3 and 8.9% CaSO 4 [39]. The resulting sulfite /sulfate ratio was 1.64.

Unreacted lime concentrations were 1.6%, and CaCO 3 was 2.1%. Fly ash comprised 71.5% of the material, while the reaction products and unreacted sorbent were the remaining 28.5%.

The weight ratio of fly ash to sorbent-derived material can vary between 0.7 and 8, depending on the composition of the coal [45] and the proportion of the fly ash that is removed before the scrubber. Without precollection, the fly ash pr oportion will be 70%–85% by weight, producing a spherical grain. When fly ash precollection is util ized, a filter separation efficiency of >80% will produce an SDA material with <30% fly ash, and very good filters (>99%) will produce contents as low as 1%–4% [17]. If fly ash is recovered befo re the SDA system, then the product consists mainly of irregular clusters of sulfite crystals, fo rming agglomerates up to 50 μm across [17, 45].

Examples of the composition of SDA material containing fly ash are shown in Table 2-9.

**Table 2-9 Typical Composition of SDA Material Cont**  **aining Fly Ash as Noted in the Reviewed Literature, wt%** 

**Material From Low**  **Sulfur Coal From Low**  **Sulfur Coal From High**  **Sulfur Coal Five**  **Samples European Coal** 

Fly Ash 75 75 40 12–29 20–85

Calcium Sulfite (CaSO 3 · 1⁄2H 2 O) 16 13 38 28–44 9–47

Calcium Sulfate (CaSO 4 ) a 6 6 15 6 1.7–17

Unreacted Lime (Ca(OH) 2 ) 2 4 5 10–29 1–15

CaCO 3 NR c NR NR 15–33 4.5–13.7

CaCl (Calcium Chloride) NR NR NR NR 0.8–6.3

Moisture (Free Water) 1 2 2 NR NR
Source(s) [36] [10] [10] [38, 48] [49]

a Mixed hydrates.

b Includes some CaCO 3 .Results and Discussion of the Literature Review

2-13 As shown in Tables 2-10 and 2-11, precollection of fly ash does change the composition of the final SDA material. The impact of SO 2 collection efficiency on SDA material composition is shown in Table 2-11.

Chemical Analyses

Bulk chemical compositional data of SDA materi als as reported in the reviewed literature are compiled in Appendix A. Conventionally, majo r/minor components of CCPs are reported as oxides. A typical report may include a weight percent value for SiO 2 , Al 2 O 3 , Fe 2 O 3 , CaO, SO 3 , MgO, Na 2 O, K 2 O, P 2 O 5 , TiO 2 , BaO, MnO 2 , SrO, moisture content, and loss-on-ignition (LOI). These data are summarized in Table 2-12 for SDA material containing fly ash and in Table 2-13 for SDA material with precollection of fly ash.

**Table 2-10 Typical Range of Main Components of SDA Ma**  **terial with Precollect ion of Fly Ash, wt%** 

**Main Components Typical Range a**  **Range**
Fly Ash/Lime Inerts 3

– 10 <8
Calcium Sulfite (CaSO 3 ) 55

– 70 17

– 68
Calcium Sulfate (CaSO 4 ) 5

– 15 3.5

– 29
Unreacted Lime (Ca[OH] 2 ) 2

– 10 0.5

– 15

CaCO 3 5

– 15 5

– 13

CaCl 2 • n H 2 O 1

– 4 0.8

– 9.5 b
Moisture (Free H 2 O) 1

– 3 NR
Source [16] [49]

a 0.7%–2% sulfur coal and precollection of fly ash with efficient ESP.

b Reported as CaCl.Results and Discussion of the Literature Review

2-14 **Table 2-11 Composition (wt%) of SDA Material**  **Overall and as a Function of SO 2**  **Separation Efficiency. Adapted from Kolar [17]** 

**SO 2**  **Separation Efficiency a**  **Main Components Range with and**  **Without Fly Ash 90% 85% 80% 75% 70%** 
Fly Ash/Lime Inerts 0–85 15.4 17.0 19.5 21.0 22.8

Calcium Sulfite (CaSO 3 ) 15–75 33.6 35.8 39.8 40.5 41.5

Calcium Sulfate (CaSO 4 ) 2–30 5.0 5.3 5.9 6.0 6.15

Unreacted Lime (Ca[OH] 2 ) 0–25 38.1 33.4 25.3 22.4 18.8

CaCO 3 1–30 NR NR NR NR NR

CaCl 2 · 4 H 2 O 1–15 b 4.9 5.5 6.5 7.1 7.75
Moisture (Free H 2 O) 1–4 3.0 3.0 3.0 3.0 3.0

a Prerequisites: SO 2 =1600 mg/m 3 ; fly ash=1000 mg/m 3 ; and HCl=160 mg/m 3 (all standard temperature and pressure, dry).

b Reported as CaCl 2 · n H 2 O.Results and Discussion of the Literature Review

2-15 **Table 2-12 Summary of Bulk Chemical Composition, Re**  **ported as Oxides, of SDA Material Containing Fly Ash Reported in Literature Reviewed, wt%** 

**Parameter Range Sources**

SiO 2 6

– 46 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52]

Al 2 O 3 4

– 44 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52]

Fe 2 O 3 1

– 44 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52]

CaO 0.2

– 52 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52]

SO 3 0

– 32 [2, 14, 33, 34, 35, 40, 44, 49, 50, 51, 52]
MgO 0.1

– 14 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52]

Na 2 O 0.1

– 46 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52]

K 2 O 0.1

– 6.37 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52]

P 2 O 5 0.03

– 1.2 [2, 14, 31, 33, 35, 40, 44, 50, 51, 52]

TiO 2 0.2

– 1.19 [2, 14, 30, 33, 35, 40, 44, 49, 50, 51, 52]

BaO 0.39

– 0.85 [44, 50]

MnO 2 0

– 0.12 [2 a , 14, 44, 50]

SrO 0.11

– 0.46 [2, 44, 50]

Moisture <0.1

– 13.2 [31, 33, 35, 40, 44, 50, 51, 52]

LOI or C 0.19

– 20.5 [14, 31, 35, 44, 50]

Unaccounted 1.7

– 6.2 [40, 51, 52]

a Mn 3 O 4

When reviewing compositional data on CCPs, it is important to understand that reporting of major/minor components as oxides is merely a reporting convention and is not necessarily indicative of the actual chemical forms in the as h. One important example of this is found in the reported calcium oxide concentration. In fly ash or bottom ash, the calcium is usually present as a component of the glassy phase with other el ements and is not present as primarily CaO as reported. In FGD materials, calcium is likely a ssociated with sulfur and is present as calcium sulfite or calcium sulfate . When calcium oxide is present as lim e in any CCPs and is exposed to water by sluicing or storage in a pond situation, th at lime will be converted to calcium hydroxide or hydrated lime. However, the bulk composi tional data are useful in evaluating CCPs for various use applications because of the volumin ous comparative historical data, empirical evaluations, and comparison with other tests and standards.Results and Discussion of the Literature Review

2-16 Additional chemical parameters were reported as a weight percent of the evaluated SDA material in the reviewed literature. These included CaSO 3 , CaSO 3 · 1⁄2H 2 O, CaSO 4 , CaSO 4 · 2H 2 O,

**Table 2-13 Summary of Bulk Chemical Composition,**  **Reported as Oxides, of SDA Material with Precollection of Fly Ash Reported in Literature Reviewed, wt%** 

**Parameter Range Source(s)**

SiO 2 1.4

– 11.1 [41 a , 45, 49]

Al 2 O 3 0.8

– 5.4 [41 a , 45, 49]

Fe 2 O 3 0.4

– 4.4 [41 a , 45, 49]

CaO 32.9

– 60 [41 a , 45, 49]

SO 3 2

– 30 [41 a , 49]
MgO 0.4

– 1.9 [41 a , 45, 49]

Na 2 O <0.1

– 0.3 [41 a , 49]

K 2 O <0.2

– 0.6 [41 a , 49]

TiO 2 0

– 0.2 [45, 49]

LOI or C 1.3

– 2.1 [45]

a May contain up to 20% fly ash content.

CaCO 3 , CaCl 2 , SO 4 , organic carbon, total sulfur, free lime, hydroxide, Ca(OH) 2 , Cl, Ca as available CaO, CO 2 , SO 2 , SO 3 -2 , SO 4 -2 , CO 3 -2 , Cl - , loss at 500°C, and loss at 900°C. Loss at 500°C determines free moisture and moisture of hydration; at 900°C, other compounds such as carbonates decompose to oxides.

The total elemental composition of SDA materials as reported in the reviewed literature are compiled in Appendix B. The highly variable data for SDA material containing fly ash are summarized in Table 2-14.

The trace metal content of SDA ma terial with precollecti on of fly ash is, as a rule, lower than that of fly ash and comparable to that of soil [16]. The limited data ava ilable in the reviewed literature for SDA material with precollection of fly ash are summarized in Table 2-15. The narrow range of concentrations for each element is within the range for SDA material containing fly ash.Results and Discussion of the Literature Review

2-17 pH

Kost et al. suggest that the pH of dry FGD pr oducts depends primarily on the sorbent used and secondarily on the FGD technology [37]. They conc luded that the high pH values of most FGD samples were due to the presence of oxides and hydroxides of Ca and Mg. When these products are exposed to water and CO 2 , they will convert to carbonat es by carbonation reactions and the pH will decrease. The pH values re ported in the reviewed literature, shown in Table 2-16, ranged from 9 to 13.

**Table 2-14 Summary of Total Elemental Composition of S**  **DA Material Containing Fly Ash Reported in Literature Reviewed, ppm** 

**Element Range Source(s)**
Aluminum (Al) 10,000

– 230,000 [12, 13, 30, 31, 35, 37, 46, 53, 54]
Antimony (Sb) 0.8

– 29 [12, 13, 30, 31, 35, 55, 56]
Arsenic (As) 0.4

– 1200 [12, 13, 17, 30, 31, 35, 37, 44, 46, 47, 54, 55, 56]
Barium (Ba) 0.76

– 12,000 [12, 13, 30, 31, 35, 44, 46, 54, 55, 56]
Beryllium (Be) 0.7

– 63 [12, 13, 30, 31, 35, 37, 46, 54, 55, 56]
Boron (B) <10

– 1460 [12, 13, 30, 31, 35, 37, 44, 46, 47, 50, 54, 55, 56]
Bromide (Br) 0.3

– 21 [31, 35]
Cadmium (Cd) 0.01

– 70 [12, 13, 17, 30, 31, 35, 37, 44, 46, 49, 54, 55, 56]
Calcium (Ca) 7100

– 401,000 [12, 13, 30, 35, 37, 46, 53, 54]
Cesium (Cs) 1

– 22 [35]
Chloride (Cl) <0.1

– 10,200 [35]
Chromium (Cr) 3

– 1000 [12, 13, 30, 31, 35, 37, 44, 46, 49, 54, 55, 56]
Cobalt (Co) <0.5

– 172 [12, 13, 17, 30, 31, 35, 37, 46, 54, 55, 56]
Copper (Cu) 3

– 655 [12, 13, 30, 31, 35, 37, 44, 46, 47, 49, 55]
Fluoride (F) 0.4

– 1000 [31, 35]
Iodine (I) 0.1

– 0.6 [35]
Iron (Fe) 6300

– 367,000 [12, 13, 30, 31, 35, 37, 44, 46, 53, 54, 55, 56]
Lead (Pb) <0.3

– 800 [12, 13, 17, 30, 31, 35, 37, 44, 46, 49, 54, 55, 56]
Lithium (Li) 15.1

– 530 [12, 13, 30, 35, 37, 46, 54]
Magnesium (Mg) 3000

– 151,300 [12, 13, 30, 31, 35, 37, 46, 53, 54]Results and Discussion of the Literature Review

2-18 **Table 2-14 (continued) Summary of Total Elemental Composition of S**  **DA Material Containing Fly Ash Reported in Literature Reviewed, ppm** 

**Element Range Source(s)**
Manganese (Mn) 24.5

– 1432 [12, 13, 30, 31, 35, 37, 46, 47, 54, 55, 56]
Mercury (Hg) <0.001

– 10 [12, 13, 17, 30, 31, 35, 44, 46, 49, 55, 56]
Molybdenum (Mo) <0.018

– 514 [12, 13, 30, 31, 35, 37, 46, 47, 54, 55, 56]
Nickel (Ni) 1.4

– 460 [12, 13, 17, 30, 31, 35, 37, 44, 46, 47, 49, 54, 55, 56]
Phosphorus (P) 21

– 2200 [37, 46, 54]
Potassium (K) 1600

– 9300 [12, 13, 30, 35, 37, 46, 54]
Rubidium (Rb) 48

– 530 [35]
Selenium (Se) <0.4

– 760 [12, 13, 17, 30, 31, 35, 37, 44, 46, 47, 50, 54, 55, 56]
Silicon (Si) 22,000

– 157,200 [12, 13, 30, 35, 37, 46, 54]
Silver (Ag) <0.024

– 8 [12, 13, 30, 31, 35, 44, 46, 54, 55, 56]
Sodium (Na) 710

– 240,000 [12, 13, 30, 35, 46]
Strontium (Sr) 30

– 13,000 [12, 13, 30, 31, 35, 37, 46, 54]
Sulfur (S) 3000

– 170,000 [37, 53, 54]
Thallium (Tl) 0.1

– 42 [12, 13, 17, 30, 31, 35, 46, 55, 56]
Tin (Sn) 0.01

– 962 [12, 13, 30, 31, 35]
Titanium (Ti) 1050

– 6700 [12, 13, 30, 35, 46]
Uranium (U) 0.8

– 140 [12, 13, 30, 35]
Vanadium (V) 0.4

– 950 [12, 13, 30, 31, 35, 37, 46, 44, 54, 55, 56]
Zinc (Zn) <6

– 9000 [12, 13, 30, 31, 35, 37, 44, 46, 49, 54, 55, 56]Results and Discussion of the Literature Review

2-19 

### Current and Potential **Uses of SDA Material**

When SDA material was first pr oduced, there were no obvious u ses for it because of the novelty of the process, a lack of applications for th e main component calcium sulfite, the undesirable calcium chloride content, a wide range in chem ical composition, and th e low volume per location [17]. The literature reviewed contained numerous references to current commercial and potential uses of SDA material. Many of the uses have comme rcial potential but are still in the research and development phase. The following is a summary of current commercial and potential uses of SDA material noted in literature reviewed. It should be noted that this summary is not exhaustive and only represents uses reported in the reviewed literature.

**Agriculture**

It is widely know that gypsum (CaSO 4 · H 2 O) has been used to improve soil conditions for centuries. Gypsum has been shown to improve water retention characteristics, increase infiltration rates, reduce soil crusti ng, and mitigate salinity and sodicity in alkaline soils or arid and semiarid regions [58]. Studies on the use of FGD gypsum compare it to natural gypsum in terms of the benefits when app lied to agriculture soil. Although FGD gypsum is widely used in agriculture, other FGD products, including SDA mate rial, contain very l ittle mineral gypsum; therefore, their suitability for agriculture applicat ions is different and shou ld not be compared to gypsum. ACAA reported 168,190 short tons of FGD gypsum and 846 short tons of dry FGD material were used in agricultu re applications in 2006, down dr astically from 19,259 short tons in 2005, possibly because of variations in r espondents to the ACAA survey [22, 59]. ECOBA reported 36,376 short tons of SDA material used for plant nutrition in 2004 [23]. The low solubility of calcium sulfite (CaSO 3 · 1⁄2H 2 O) in SDA material makes it a poor source of calcium and sulfur for agriculture use. The sulfites may be harmful to plants, by producing hydrogen sulfide gas under anaerobic conditions, unless placed far in advance of planting [60]. However, the substitution of alkaline su lfite-rich dry FGD products for c onventional liming materials in agriculture is a potential use for these products , as noted in the research studies summarized below. It is important to note that there are inst ances in the literature of agricultural use of SDA material in Europe, but the authors focused primarily on literature from the United States.Results and Discussion of the Literature Review

2-20 **Table 2-15 Summary of Total Elemental Composition of**  **SDA Material with Precollection of Fly Ash Reported in Literature Reviewed, ppm** 

**Element Range Source(s)**
Arsenic (As) 4.8

– 11 [41 a ]
Barium (Ba) 270

– 4000 [41 a ]
Boron (B) 100

– 150 [41 a ]
Cadmium (Cd) 0.9

– 6.9 [41 a , 49]
Chromium (Cr) 34

– 60 [41 a , 49]
Copper (Cu) 43

– 80 [41 a , 49]
Lead (Pb) 28

– 110 [41 a , 49]
Manganese (Mn) 170

– 420 [41 a ]
Mercury (Hg) <0.3 [41 a , 49]
Molybdenum (Mo) 2.7

– 5.4 [41 a ]
Nickel (Ni) 32

– 80 [41 a , 49]
Selenium (Se) 3.9

– 6.5 [41 a ]
Vanadium (V) 60

– 70 [41 a ]
Zinc (Zn) 94

– 380 [41 a , 49]

a May contain up to 20% fly ash content.

**Table 2-16 pH Values of SDA Material Reported in Literature Reviewed** 

**pH Source(s)**

9.7

– 12.8 [2]

11.8

– 12.5 [37, 57]
9.3 [47]

9

– 13 [34 a , 41 b ]

12.4 [53]

11.7

– 12.4 [39]

a Literature review reported within source.

b Precollection of fly ash; up to 20% fly ash content.Results and Discussion of the Literature Review

2-21 Calcium sulfite phytotoxicity is a concern when applying SDA material to agriculture soils, as evident below in the literature cited. Strate gies for reducing toxicity are also examined. Decreased maize growth was reported in an acid soil when levels of an FGD product containing CaSO 3 exceeded 2 g/kg in a soil initially at pH 4.2 [61]. Toxicity from volatile sulfur compounds and increased aluminum toxicity were suspected, but the relative contribution of each could not be quantified. The SO 3 -2 ion is thermodynamically unstable in the presence of oxygen. It as been reported that rapid CaSO 3 oxidation to CaSO 4 is expected in the oxidi zing atmosphere of most agronomic soils, and additional reactions that generate SO 2 may occur with CaSO 3 addition to low-pH media [62]. The oxidation of sulfite to sulfate at near-neutral to alkaline pH values is extremely slow at atmospheric pressure. The fact that oxidation is relatively rapid in soils leads to the conclusion that other mechanisms may play a crucial role. It is likely that in an agricultural soil, fungi, bacteria, and microfauna may aid in the ultimate oxidation of sulfite to sulfate through metabolic processes. Sulfite and SO 2 both are phytotoxic, but to xicity effects may be temporary, subsiding when most SO 3 -2 has been oxidized to SO 4 -2 [58]. Research by others indicated that raising the soil pH or allowing oxi dation of sulfite to su lfate to occur are two effective strategies for reducing calcium sulfite phytotoxicity [63]. The toxic effects resulting from calcium sulfite use in soils were related to low soil pH and can be considerably diminished by applying an acidity-neutralizing amendment su ch as hydrated lime. In addition, the oxidation of sulfite to sulfate in water and soil systems can occur in approximately 3–5 weeks. Others also demonstrated that both sulfate a nd sulfite FGD products can be used to reduce toxicity in acidic subsoil layers. Although short-term phytotoxicity was observed in the sulfite-ric h sample, the addition of small amounts achieved the goal of mitig ating soil aluminum t oxicity and increasing plant growth. This suggests that application of th e sulfite-rich sample or si milar materials to soils in autumn may improve crop growth the followin g growing season [58]. The lower toxicity of sulfite in alkaline environments is likely due to the reduced concentrati ons of bisulfite ions, which are pH-dependent and maximum at below pH 5. Bisulfite combining with atmospheric oxygen is the mechanism used in much of the forced oxida tion found in the production of gypsum from sulfite scrubber material.

The impact SDA material has on the environm ent and plant growth was studied by the University of Georgia and by Argonne National Laboratory. The University of Georgia study showed the incorporation of SDA material into native soils had no impact on germination rates of corn, soybeans, and cotton [47]. Concerning the effect on the elemental composition of plant tissues, arsenic, selenium, molybdenum, boron and, to some extent, calciu m increased within all crops grown in Year 1. Tissue concentrations of manganese and sodium decreased, and nickel, lead, iron, and copper levels were not affect ed. Leachate salinity and soil salinity rose immediately (from 0.2 to 2.9–3.3 dS/m) after the applica tion and remained elevated over the two years of the study. The leachate pH was not affected by the applic ation of SDA material, but the application did produce a stable increase in so il pH (from 5.5 to 8.1). Arsenic, selenium, and boron tended to accumulate in the plants. Boron emerged in the leachate, indicating possible impacts to groundwater quality. Plant growth expe riments using soil treated with SDA material were conducted at Argonne Nationa l Laboratory. Three different soil types and soybean and corn crops were tested, using a comb ination of SDA material with so il at 0%–4% by weight. For both corn and soybeans, growth decreased as the am ount of SDA material increased. Soybeans grown in soil with 4% SDA material exhibited stunted growth when compared to the control crop. For corn, a similar reduction in grow th was noticed, although not to the same degree as for soybeans. Boron concentration in the leaves of the corn and soybeans grown in soil treated with SDAResults and Discussion of the Literature Review

2-22 material were found to be up to 20 times greater than those found in the leaves of the control grown in soil containing no SDA material [64].

The use of SDA material as a liming agent has been studied. The British Coal Corporation analyzed the use of a European SDA material as a liming agent [41]. Pure limestone has a liming or neutralizing value (calcium ox ide content equivalent) of 56, a nd the SDA material tested had a neutralizing value of 25. The study concluded that if an appropriate amount of the SDA material were added to achieve adequate liming, th e recommended upper limits for trace element concentrations in soils after sewage sludge appl ication would not be exceeded. The use of CCPs (including SDA material) as compared to CaCO 3 as a liming agent in strongly acidic soils was studied, and it was concluded that CCPs functioned similarly to CaCO 3 and that they had very few adverse effects on soil enzyme activ ities compared with those of CaCO 3 [53]. In order of effectiveness for increasing soil pH were F BC ash > LIMB ash > SDA material > fly ash. Because CCPs contain CaO and MgO, the study concl uded that it seems likely that the ability of CCPs to act as liming agents may be related to their content of these oxides. For example, the high initial alkalinity associated with the application of CCPs to soil was reduced by conversion of CaO to CaCO 3 in soils exposed to atmospheric CO 2 . The results of this study also provide information on the use of soil enzyme activities as indicators of soil quality. They clearly demonstrate that caution is required in such u se because the relationships between soil qualities are complex and measurements of the changes in enzyme activities without reference to proper controls can be misleading.

Several commercial uses of SDA material in fertilizer applications were identified in Europe; however, the use of SDA material as a fertilizer has not reached the commercial stage in the United States.

Many soils throughout northern Europe are e xperiencing a growing sulfur deficiency, and therefore, fertilizer companies ar e increasing the amount of sulfur in commercial fertilizers. The application of SDA material as fe rtilizer for sulfur deficiency is now permitted in Germany, Denmark, and Austria, as described below [16].

SDA material with low fly ash content has b een used as a sulfur fertilizer since 1994 in Denmark, and it has been used as an admixture to and spread with liquid manure. The Lanskontoret for planteavl (Agricultural Research In stitute) has carried out tests that indicate that SDA material, collected without fly ash, is su itable for use on farm soils, and the Danish Ministry of Environmental Prot ection has given permission for th is application, with certain conditions. In commercial applications, the residu es are mixed with dolomit ic lime to prevent dusting and to allow application of the two materials simultaneousl y. It is necessary to apply the two materials together because the fertilizing poten tial of the SDA material is too small to make separate application viable. Dolomitic lime contains magnesium and acts slowly and continuously. The SDA material can also be mixed with liquid manure, agricultural lime, or wastewater sludge [65].Results and Discussion of the Literature Review

2-23 A three-year VGB Group research project unde r the “Waste Managemen t of Residues from Coal-Fired Power Plants and Waste Incineration Plants” (known as ERKOM) research program yielded results that gained approval of SDA ma terial as a fertilizer in Germany in 1999 by the Scientific Advisory Board of the Federal Minist ry of Agriculture [66, 67 ]. The minimum sulfur content of a fertilizer of this type is to be 10%, which is achieved by SDA material when the major amount of fly ash is precollected before the SO 2 removal. The German Ministry of Food, Agriculture, and Forestry has sp ecified SDA material as a sulfur–calcium fertiliz er in legislation. The SDA material was also licensed in Austri a based on the research results in Germany. The total cost of the VGB Group resea rch project of approximately 0.5 million Deutsche Mark (DM) has produced annual savings for the operato rs in the region of 2.8 million DM [66].

**Binders**

A novel binder for interior plas ters is described in U.S. Pa tent 5,522,928 using a calcium sulfite component, water, and a finely divided residua l material component comprising primarily calcium sulfite. SDA material is called out speci fically as the calcium sulfite component [68]. Kolar summarizes a multiphase binder described in German Patent DE 3 82 16 57 C2 [17]. SDA material is mixed with fly as h, and precalcination, oxidation, a nd calcination are carried out. A salable binder is produced by adding additives and grinding. The use of the product as a flooring binder is considered promising, and other potential applicatio ns include use in insulating building materials and raw materials for double fl oor plates where it may be substituted for cement products. Binder applications have not r eached commercialization in the United States.

**Cement Manufacture**

SDA material has been used commercially to manufacture cement in Germany. A German coalfired power plant treats SDA material using a fluidized-bed process. Pelletized anhydrite is produced that can be used as a substitute for natural anhydrite in industrial processes such as cement manufacture [69]. Two anhydrite production processes using SDA material are the FläktDorr-Oliver Process and the Vereinigte Aluminium-Werke AG, Lünen (VAW) Process [17]. Another process used dry FGD material to manu facture cement, once u tilized in the MüllerKühne Process at WSZ Wolfener S.u.Z. in Bitterfeld in Germany [16].

U.S. Patent 4,470,850 claims that a dry FGD materia l (likely SDA material) can be used in the place of fly ash and gypsum as a solidification regul ator in the production of fly ash cement [70]. The essential reaction product in the dry FGD ma terial that functions as the solidification regulator is the calcium sulfite hemihydrate. Although this applica tion appears to be technically feasible, no dry FGD material has been reported as used for cement manufacture in the United States since 2003 [71].Results and Discussion of the Literature Review

2-24 **Cement Replacement in Concrete**

Research has been conducted on the use of SDA material as a concrete admixture. Below is a summary of the research studies found in the l iterature reviewed, which consisted primarily of European sources. It is not known if the limited am ount of literature pertai ning to commercial or demonstrated use of SDA material as a concrete admixture is due to tech nical constraints or a lack of market development. ACAA reported 9660 short tons of dr y scrubber FGD material used in concrete, concrete produc ts, and grout in 2006 [22].

Research carried out at KEMA, Netherlands, has investigated the use of SDA material as a partial replacement for sand and cement in concre te [45, 72]. The SDA ma terials tested had no precollection of fly ash and cont ained a mixture of about 70% fly ash and 30% sorbent reaction products. For initial comparativ e tests, 20% of the portland cement in the concrete was substituted by SDA material. Except for a slight retardation in setting times, concretes in which cement was partly substituted by SDA material showed strength and durability performances comparable to or superior to reference c oncretes. No destructive ettringite (Ca 6 Al 2 (SO 4 ) 3 (OH) 12 · 26 H 2 O) reactions were observed in tests carried out with an e xposure time of two years. The presence of chloride was noted in some SDA material s, restricting their app lication in reinforced concretes [45]. The study concluded that the compositions could be applied successfully to concrete products, but that fu rther verification under real conditions was required. Long-term tests were still being carried out when the report was published.

A demonstration project was carried out by the Technical University of Denmark to assess the use of SDA material as a cement and concrete admixture [73]. Three precast, reinforced concrete front elements containing 20 and 30 wt% SDA material were manufactured. Additives, such as superplasticizers and an air-entraining agent, we re added to some specimens. After a period of 1 year, corrosion tests indicated a low probability of corrosion for most of the specimens. The results suggest that when SDA material is used as an admixture in mortar to replace part or all of the usual fly ash, increased comp ressive strength is achieved. The grain size of the SDA material has some influence on strength. Progressive substi tution of cement by SDA material will result in gradually decreasing strength. During this proj ect, it was found that th e mineral thaumasite (Ca 3 Si(CO 3 )(SO 4 )(OH) 6 · 12H 2 O) formed under certain conditions. Because thaumasite greatly reduces the strength of cement and concrete, the project was abandoned.

A review of studies in Europe by Kolar indicate d that nonreinforced concrete might be a more suitable application for SDA materials than reinforced concrete [17]. Chloride poses a risk of corrosion of steel reinforcement. Concrete pi pes produced containing SDA material in Austria had swelling attributed to the formation of ett ringite. Prevention of this was not successful. Additionally, the product was not frost-resistant. It was concluded that these factors limited the use of SDA material-containing concretes in buildi ng to solid or hollow bricks for interior walls or backing. Dry storage of the bricks would also be necessary.Results and Discussion of the Literature Review

2-25 The use of dry FGD material as a raw material for cellular or gas concrete was studied in Denmark and Poland [16]. Cellular concrete is made of cement and/or white lime and ground quartz sand and steam-hardened at 180°–200°C ( 356°–392°F), resulting in a porous structure. There are over 20 factories for the production of cellular concrete in Poland, many of which use fly ash as a raw material. Testing has concluded that the addition of approximately 20% dry FGD material is positive and that it can be used for cellular concrete manufacturing. However, the market incentive is low, and further development work is required. Earlier work in Denmark used up to 30% SDA material, resulting in problems including chlorine corrosion in the autoclaves and thaumasite formation [17].

Two sources of SDA material and one source of limestone FSI product were tested for partial cement replacement in concrete. The products were substituted for 30 wt% Type I cement. Three product concrete mixtures, one fly ash concrete mixture, and one control concrete mixture were prepared. One SDA sample met the ASTM In ternational C618 criteria except for SO 3 content, which was higher than the crit erion. The other SDA material a nd the limestone FSI product did not meet ASTM International C618 criteria for silica, alumina, iron oxides, SO 3 , and LOI. However, for all samples, their fineness and po zzolanic activity indices me t ASTM International C618 criteria. In addition, the autoclave expa nsion test results were within the ASTM International C618 limits. The concrete mixture s made with SDA material achieved a higher compressive strength than that of the con trol mixture at all tes ting ages (3, 7, 28, and 90 days). The SDA mixtures had longer setting tim es than the LIMB mixtures and controls [34].

In one project, portland cement c oncrete incorporated with 10% SDA material was used on a test section of road pavement [46]. Prior to the fi eld placement, laboratory specimens were made using 5% and 10% SDA material. Laboratory and field results indicated that compressive strengths increased with increased percent of S DA material for all test ages (7, 14, 21, and 28 days). A comparison of the laboratory and field concrete mixes shows a similar 28-day compressive strength.

**Civil Engineering**

Dry FGD material, including SDA material, mixed with fly ash has been used commercially in civil engineering in Europe and has been a topi c of study in research proj ects. Civil engineering applications including landfill c onstruction, embankments, structural fill, and road base have been documented in the literature. ECOBA reporte d 61,729 short tons of SDA material used for general engineering fill in 2004 [23]. ACAA reported 249 short tons of dry scrubber FGD material used in the road base/subbase/pavem ent category but no use in the structural fill/embankment category in 2006 [22].

In Nordic countries, there are six blending stations at coal-fired power plants blending dry FGD material and fly ash, sometimes with the addition of cement. These bl ends are referred to in the literature generically as stabilisate product. 1 An example is Cefill (or Cefyll), produced by the Swedish company Cementa AB, which was deve loped in the 1980s and has been producing Cefill since at coal-fired combined heat and power (CHP) power plants in Västerås, Sweden.

1 This is similar to the process used in the United States whereby wet FGD material is mixed with fly ash to produce fixated scrubber solids (FSS) or Pozzotec.Results and Discussion of the Literature Review

2-26 Three qualities of Cefill are typically produced with 30%–70% dry FGD material, 30%–70% fly ash, and 0%–8% cement. Compressive streng ths vary from 5 to 30 MPa (725–4350 psi) and water permeabilities vary from 10 -10 to 10 -12 cm/sec. A similar stabili sate product called REALIT is produced at coal-fired power pl ants in Dürnrohr, Austria [16].

A few commercial examples of the use of Cefill and REALIT as a capping material are provided by Bengtsson [16]. Cefill has frequently been used as a bottom- and top-capping material for hazardous waste such as metal ore mine residues and as a vertical seal for preventing horizontal groundwater flow contamination. Many other project s have been carried out in Sweden based on the tight and flexible nature of the cured Cefill material. REALIT was used to seal off a sludge deposit.

The construction of road dams, filling of foundation cavities and cable trenches, and sealing of waste disposal sites using Cefill are applications that have be en abandoned in Denmark but are being intensively pursued in Sweden and Finland [17].

In Europe, stabilisate products have been u sed extensively as a base course material in applications such as storage areas, coal yards, road banks, parking lots, and noise protection walls. An example is a 2-km-long road built at a coal-fired CHP station in Finland using a 1-mthick layer of a 50% dry FGD material and 50% fly ash without cement addition covered with 0.5–1.5-m fly ash and a 0.24-m layer of crushed stone [16]. This test road has been tested annually for important engineeri ng parameters. Results have b een very satisfactory, showing high frost resistance. REALIT was used in the base liner of a squeezed sewage sludge disposal site in Austria [74].

A common European application of stabilisate products is for landscaping and land reclamation [16]. Examples include fill for reclamation of des troyed land and old/closed quarries, fill in an area behind a new quay in a harbor, and reclamation of lignite pit mines. It is common practice to recultivate the top surface of reclaimed land wi th a layer of soil for planting trees and grass.

In addition, utilization of a stabilisate product is under consideration in the Czech Republic as embankment material for roads and railways.

The fine-grain Dutch SDA material is suitable for use in the stabil ized layer of road construction, whereas the artificially prepared grains (lump s) did not meet Dutch standard technical requirements [45]. However, research in Denmar k has shown that problems may occur with swelling during application as a f oundation material [65]. The expa nsion of several types of FGD materials was measured, and it was concluded that compacts containing SDA material, alone and in combination with fly ash, expanded by less than 0.2% after a year of exposure, with no evidence of structural deterioration after that time [41].

The use of SDA material in a truck ramp for vehicles to unload trash [57]. Difficulties in achieving uniform conditions during construction were experienced; howe ver, no problems with performance and no evidence of failure were reported.Results and Discussion of the Literature Review

2-27 Dawson et al. evaluated literature on the potential fo r SDA material to be used as structural fill and concluded that all but the nonreactive SDA mate rial has potential to be used as a structural fill [10]. For reactive SDA material, compress ive strengths in excess of 2000 psi can be achieved, whereas low-reactive SDA material strengt hs greater than 100 psi can be expected.

Swell properties of the material and the strengt h that can be achieved when compacted are factors in the application of FGD materials for engineering purposes. Swelling primarily occurred in FGD samples containing free lime an d occurred in two episodes [38]. The first episode occurred almost immediately upon water addi tion and is attributed to hydration reactions such as the hydration of lime (CaO) to portlandite (Ca[OH 2 ]) and of anhydrite (CaSO 4 ) to hemihydrate (CaSO 4 · 1⁄2 H 2 O). The second swelling episode usua lly began after about 10 days and is thought to be a result of the formation of secondary minerals such as ettringite. The strengths achieved varied when FGD samples were compacted at optimum water content.

The use of SDA material for cured compacted products suitable for use as landfill materials, embankments, roadbase compositions , and similar applications is described in U.S. Patent 4,354,876 [8]. It is specifically stat ed that the material used is obtained from a li me-based dryscrubbing FGD operation containing fly ash. The ma terial is mixed with water and compacted under sufficient load to achieve at least 70% of the laboratory dry dens ity. The resulting cured compacted products have compressive strengths of at least ~25 psi a nd permeabilities of less than 1 × 10 -5 cm/sec. The process of the invention is designed to take full advantage of the unique self-bonding capabilities of th e material. The SDA material may need to be conditioned for this use. Cementitious additives may be re quired for unreactive to moderately reactive SDA materials.

A test road used SDA material in the paveme nt layers, the pavement , the subbase, and the embankment [46]. SDA material proved useful; es pecially as replacement for common earthen borrow material 2 in embankments. The granular and sel ect granular borrow material met or exceeded Minnesota Department of Transporta tion (MnDOT) specificat ions. Environmental testing showed the placement had no adverse impact on groundwater quality; however, barium in the SDA material–common borrow mixture slightly exceeded Minnesota drinking water limits. Although the placement was successful from an engineering and env ironmental standpoint, it did not offer economic advantages because additional costs were incurred for mixing and transporting the SDA material. The SDA material mi ght have an economical advantage if lime or lime kiln dust was required for borrow stabilization.

In Kansas, SDA material (containing about 80% fl y ash) has been mixed with dry economizer ash, moistened with water, and used to create a lining layer (~1.25 m thic k) for a landfill site. The conditioned SDA material was spread in layers about 0.5–0.75 m thick and compacted to give a layer with a permeability of <10 -6 cm/sec. Wet boiler slag was brought separately to the landfill and encapsulated in the conditioned SDA material [65].

2 Earthen borrow is sand, gravel, or other material used for grading.Results and Discussion of the Literature Review

2-28 **Flowable Fill**

Flowable fill is a material that flows like a liquid, is self-leveling, requires no compaction or vibration to achieve maximum density, and harden s to a predetermined strength [28]. Controlled low-strength material (CLSM), controlled density fill (CDF), and infill are names used to describe flowable fill that co mprises a blend of cement, fly ash, sand, and water. ACAA and ECOBA have reported commercial uses for dry sc rubber FGD material (9843 short tons in 2006) and SDA material (45,195 short tons in 2004), resp ectively, in flowable fill applications [22, 23]. Research conducted to date indicates that SDA ma terial can be an effective component in the production of flowable fill.

The potential for SDA material to be used in flowable fill as a replacem ent for conventional fly ash was examined. The design mixes consisted of varying amounts of SDA material, cement, lime, admixtures, and water. The mixes were te sted in the laboratory for flowability, unity weight, moisture content, unconfined compressiv e strength, erodiblity, set time, penetration, and long-term strength characteristics. Tests were co nducted for up to 90 days of curing. The study concluded that flowable fill containing SDA ma terial can be an econom ical alternative to conventional materials. SDA flo wable fill mixtures tested gain ed good strength (400 psi obtained in 1–2 days for standard flowable fill and 400 ps i obtained in 1/3–6 hours for quick-set flowable fill) and had excellent placeability. The authors suggested, especially for high-cementitiouscontent flowable fill, that long -term strength tests be conducted to estimate the pot ential for later excavation. Furthermore, chemical reaction and mechanisms that ac celerate initial set time need to be studied. Long-term strength tests for more than 1 year are needed, a nd full-scale field tests would be valuable. Resilient modulus, stress–st rain behavior, freeze–th aw, swell potential, and corrosivity characteristics also need to be studied [75].

Others mixed SDA material with an additional fly ash and stabilized the mixture by adding about 3 wt% lime kiln dust [76]. The st abilized product was eith er used as a flowable fill or left in a storage yard for several years. Samples extrac ted from these sites were analyzed using x-ray diffraction and scanning electron microscopy. The analytical resu lts show the formation of thaumasite, ettringite, and an intermediate phase with varying chemical composition of calcium, aluminum, silicon, and sulfur. Ettringite and thaumasite are not present in CCPs but are secondary minerals formed from th e reaction of the CCPs with wate r. Most of the thaumasite formed in the system was grow ing directly from the gypsum ma trix. Thaumasite was also growing in the void space. Thaumasite grown in the system occur as short, stubby crystals. Ettringite crystals, on the other hand, grow in isol ated pockets when the conditions of a saturated lime environment are available.Results and Discussion of the Literature Review

2-29 **Fixating Agent for Waste**

SDA material, and other alkaline-rich FGD materia ls have the potential to solidify or fixate wastes. Alkaline FGD materials may be useful to stabilize metals in acidic hazardous wastes that have reduced solubilities at higher pH, such as cadmium, iron, manganese, zinc, copper, and cobalt [34]. Waste sludge stabiliza tion is similar to soil and road base stabilization; however, since the sludges generally have a high moistu re content and low solids content, more FGD material needs to be used to fixate waste. Resea rch studies that have evaluated the use of SDA material as a fixating agent for waste are summarized here.

The great fineness of SDA materia l, especially with low fly ash content, yields a high water retention capacity, which could be used for the thickening of sewag e sludge and river or harbor silt. Some applications have been successful; however, negative experiences have occurred in Denmark because calcium hydroxide appear s to cause early hardening [17].

In one study, different waste materials (cadmi um and chromium plating precipitation sludges, waste oil digestion sludge, and a sedimentation slu rry from an aluminum can reclamation center) were mixed with SDA material [77]. In all mix designs, a ridged structur al material evolved as a result of the expansive and pozzolanic reactio ns occurring from the wetted SDA material. Structural and physical characteristics and le aching were evaluated. The cadmium plating waste– SDA material mix exhibited a 7-day unconfined compressive strength of up to 5.221 MPa (759 psi) and the chromium plati ng sludge–SDA material mix re ached 7.587 MPa (1103 psi). The SDA material mixed with water exhibited a 7- day unconfined compressive strength up to 5.035 MPa (762 psi). The unconfined compressive st rength for weak concrete is 10.346 MPa (1504 psi). The raw and waste materials fixated with SDA material were leached with acetic acid and with deionized water. The SDA material di d not leach above Resource Conservation and Recovery Act (RCRA) regulations. The cadmium waste and aluminum can-processing waste heavy metal leaching characteris tics were reduced from hazardous to nonhazardous by fixation with SDA material, as was chromium plating w aste at mass ratios of 1:1, 2:1, and 25:1 (SDA material to dry chromium waste). The oil sludge was not affected by SDA material fixation with regard to inhibiting leaching.

A study on the leachability of SDA material from Argonne National Laborator y concluded that if proper fixation techniques can be developed, SDA ma terial has the potential to be used as an impounding agent in the codisposal of chemical wastes containing lea d, cadmium, and other elements whose leachability decreases with increasing pH [64].

A laboratory study conducted at the Western Resear ch Institute evaluated the ability of FGD materials to stabilize the orga nic and inorganic constituents of hazardous wastes. Two sources of atmospheric fluidized-bed combustion (AFBC) material and two sources of SDA material were used in this study, and four types of hazardous waste streams were obtained including separator sludge, mixed metal oxide–hydroxide waste, meta l-plating sludge, and cr eosote-contaminated soil. Each product was mixed with each hazardou s waste, allowed to equilibrate, and then leached using the toxicity char acteristic leaching procedure (T CLP). It was found that chromium was leached from both SDA materia l-stabilized mixtures. However, the products tested can be used to stabilize the cadmium found in the metal oxide–hydroxide hazardous waste. Mineralogy tests were performed on a number of products and hazardous waste mixtures. Quartz andResults and Discussion of the Literature Review

2-30 ettringite were the most do minant mineral phases; others in cluded gypsum, portlandite, and calcite [34].

This application has not reached comm ercialization in the United States.

**Marine Applications**

It has been suggested that SDA material could be used in marine applicat ions such as artificial reefs and offshore sea defenses, although ACA A does not report this as a separate use application. The literature review ed, and summarized here, showed positive results when reefs and blocks were prepared using SDA material and placed in a seawater environment. All studies showed that artificial reefs and blocks could provide a favorable habitat for marine life without adverse impacts to the environment. Tests ca rried out in the United Kingdom by the Coal Research Establishment show that blocks cont aining SDA material, exposed to both tidal and totally submerged conditions, showed no evidence of surface friability or cracking [41]. Block weights generally increased with time to levels compatible with water adsorption values found in laboratory tests. The strength of blocks contai ning a proportion of portland cement and fly ash increased compared with those containing onl y SDA material, but all specimens behaved adequately. Similarly, the Marine Sciences Resea rch Center conducted an investigation on the preparation and evaluation of bl ocks prepared from a variety of CCPs and placed in a seawater environment. Testing indicated that properly desi gned reef blocks possessed adequate structural integrity for marine environments, exhibited no adverse environmental effects, and functioned acceptably as habitat for marine life [10, 78]. Du mp blocks from SDA material and fly ash mixtures bonded with 7.5% portland cement were produced. It was indicated that the blocks could be used for artificial reef constr uction. The compressive strength was 5 Nm/m 2 , and very low leaching was observed [45].

**Masonry**

SDA material has shown promis e for use as a raw material to manufacture masonry products; however, only one reference to the commercial manufacture of masonry was found in the literature. Based on results by KEMA and on furt her testing in Germany, considerable amounts of dry FGD material were used in German and Dutch sand–lime brick manufacturing [16]. However, the manufacturing of sand–lime bricks seems to have ceased (as of the writing of that report). Detailed information of the applicati on is limited, both for competitive reasons and because of the risk of unfavorable publicity re garding the use of “waste products” in a highquality application. ACAA reported 9660 short tons of dry scrubber FGD material used in concrete, concrete products, and grout in 2006 [22].

International patent WO 82/00819 and U.S. Patent 4,377,414 describe a method to use SDA material in the production of shaped cementitious products such as pellets, bricks, tiles, and blocks [79, 80]. The method uses a closely cont rolled compaction process. First, an SDA material containing fly ash is uni formly contacted with a critical amount of water and then immediately compacted at a crit ical compaction ratio to prov ide a manageable green body in which the fly ash particles are posi tioned with respect to one anothe r so that the in terstitial spaces are sufficient to accommodate th e volumetric changes in the ce mentitious materials without anyResults and Discussion of the Literature Review

2-31 deleterious expansion of the produc t. The product is then cured in as little as two days at 120°– 180 ° F (49°–82°C).

Evaluation of the use of SDA material in sa nd–lime bricks has been carried out in the Netherlands by KEMA [45, 81]. Sand–lime bricks are made of fine white lime and high-quartz sand molded, pressed, and steam-cured in autoclaves at 175°–214°C (347°–417°F). About 3000 sand–lime bricks were manufactu red and tested. The test bric ks were manufactured with mixtures of SDA material and fly ash replacing about 20% of the sand in the conventional bricks. The quality of the bricks containing S DA material was satisfactory in comparison to conventional bricks, with higher compressive st rength and splitting-tensile strength. The results were good with respect to compressive strength and splitting-tensile strength, porosity, and absorption coefficient with cap illary action of water. No efflorescence was found when only moderate amounts of sodium sulf ate (<0.4 wt%) or potassium su lfate (<2.2 wt%) we re present. The carbon content of the fly ash must be limited to <7 wt%. The oxidation rate of calcium sulfite to calcium sulfate is extremely low, particu larly in air. SDA materials have also been used in the manufacture of sand–lime bricks in Germany [69]. It has been noted that laboratory tests showed that the best results were obtained by replacing 50% of the sand with SDA material containing 70% fly ash [17]. For the pressing of so lid bricks, it is necessary to reduce the SDA material proportion to 20%.

**Mineral Wool**

SDA material can be used to manufacture mineral wool used in insulation and ceiling tiles, as described in the research study conducted by the EERC [82]. SDA ma terial with a high level of fly ash was used to prepare mine ral wool in a pilot-scale cupola furnace. The SDA material was formed into balls, melted at 2600°–3000°F ( 1430°–1650°C) in the cupola, and fiberized by means of a compressed airstream. The mineral wool product was easily fabricated and appeared to be comparable in fiber diameter to that of commercial mineral wool . There were no reported commercial uses of this applic ation in the United States, a nd ACAA does not report this as a separate use application.

**Mining Applications**

One large-volume use for alkaline, dry FGD materi al is in reclamation of acidic minespoils. Dry FGD material could be used to ne utralize the spoil acidity and reestablish the vegetative cover to stabilize soils and reduce erosion [83]. It has been indi cated that SDA material could potentially be used in a grout mix design to fill mine voids [34]. ECOBA reported 181,881 short tons of SDA material used for mine recl amation and restoration in 2004 [23]. Mining applications are the largest reported use of dry scrubber F GD material in the United States. ACAA reported 115,696 short tons of dry scrubber FGD material u sed in mining applications in 2006 [22].Results and Discussion of the Literature Review

2-32 Brendel et al. summarized the fin dings from an EPRI study entitled Advanced SO 2 Control ByProduct Utilization: Laboratory Evaluatio n, EPRI CS-60443 [34]. In this study, grout mixes were prepared with cement and FGD materials (including SDA material) at various proportions with sufficient water to achi eve a flow in the range of 5–35 s econds. Three mix designs were created with SDA material including 3:1, 1:1, an d 1:2 (cement to SDA material) ratios. Data show increased strength gain from 7 days [2.3–16.1 MPa (334–2335 psi)] to 28 days [4.5– 26.1 MPa (652–3785 psi)]. Data show that more cement does not always improve strength.

SDA material has been used as a mine fill in th e United States and Europe. Instances cited in literature are described below.

A former limestone room and pillar mine in Suga r Creek, Missouri, is being stabilized using up to 700 tons of SDA material per day. The SDA mate rial is mixed with water to create a slurry that is injected into the min e through 10-inch-diameter cased boreholes drilled through 160 feet or more of overburden. Because this particular SDA material c ontains ammonia, ventilation during mine stabilization was pr ovided by constructing steel reinfo rced shotcrete walls between selected pillars to co ntrol airflow. As of Septem ber 2002, over 71,000 tons of the SDA material/water slurry had been injected into the mine [84].

In North Dakota, SDA material (containing about 75% fly ash) has been mixed with wet bottom ash to a water content of 20%–25% and used to backfill an old lignite strip mine. The mine was lined with clay (0.5–1.5 m thick) and capped with a clay layer (~1.5 m thick), overburden, and top soil [65].

In Germany and Poland, SDA material has been used successfully in the min ing industry as filler in packing and backfill operations and as an a dditive in mining mortars [16, 69]. SDA material has been used in the backfilling of gravel pits without bottom sealing in Austria. Mining mortar from SDA material and fly ash was used in 1990 in Germany for the construction of underground retaining walls, for backfilling and c onsolidation, and for special uses. The use of SDA material for filling abandoned tunnels in lime, ore, and coal mines has been officially sanctioned and approved by German court de cisions (OVwG (Oberv erwaltungsgericht: Administrative Court of Appeals) Saarl ouis, File 1-W 125/89; 1 F 17/89) [17].

**Soil Stabilization**

SDA material has physical and che mical characteristics similar to those of a lime–fly ash mix and, thus, has the potential to be used in soil stabi lization. The ability for the SDA material to form ettringite without swelling is key to its engineering performan ce. It also needs to be durable enough to withstand potential damage due to freezing and thawing and wetting and drying action. ACAA reported 299 short tons of dry scru bber FGD material used in soil modification and stabilization in 2006 [22]. So il stabilization with SDA material has been documented in only a few laboratory and field demonstration projects.Results and Discussion of the Literature Review

2-33 OSU performed laboratory experiments to characte rize the engineering properties of silty clay stabilized with SDA material and FSI products. Tests were conducted to evaluate compressive strength, permeability, swelling potential, compr essibility, and leachate composition. These tests were performed on compacted samples of SDA material–soil mixtures at the optimum moisture content. The FGD materials tested substantially improved the strength and stiffness of soil, which was dependent on the mix ratio. All stabilize d soil mixtures developed strength of at least 0.69 MPa (100 psi) by 28 days. Generally, the permeab ility of the stabilized soil decreased with time as a result of the chemical reaction occurrin g within the mixture. Every sample showed an increase in volume over time due to ettringite formation. Consolidated test results showed low compressibility of stabilized soils. The con centration of heavy metals in the leachate was considerably lower compared to EPA drinking water standards [34].

ICF Northwest Inc. performed tests on a variety of FGD materials, including SDA material, mixed with A-7 clay, with and without addi tional lime. Soil had a liquid limit of 28.5 and a plastic limit of 24. The mixtures consisted of a) 90% soil, 7% FGD material, and 3% lime; b) 90% soil and 10% FGD mate rial; and c) 90% soil, 7% fly ash, and 3% lime. The 28-day compressive strength of SDA material mixed wi th lime was 4.34 MPa (630 psi) and without lime was 2.83 MPa (410 psi) [34].

**Sulfuric Acid Production**

The Müller-Kühne process is based on Müller’s tests to produce sulfuric acid from anhydrite and Kühne’s idea to produce cement at the same time by adding carbon (coke), clay, and sand. As early as 1985, investigations were started to substitute natural raw materials with products such as SDA material [17]. Dry FGD material was pro cessed into sulfuric acid in the Müller-Kühne Process at WSZ Wolfener S.u.Z. in Bitterfeld in Germany until recently [16]. No information was found to indicate why this process is not currently used.

No instances of sulfuric acid pr oduction research in the United St ates were noted in the reviewed literature, and ACAA does not report this as a separate use application.

**Synthetic Aggregate**

The production of synthetic aggregate using SDA material has been demonstrated in several countries on a commercial level. However, other commercial attempts have not been as successful or were only demonstrated on a labor atory scale. Production methods tend to use mixtures of fly ash and SDA material, either pell etized or briquetted. Mo st synthetic aggregates are lighter than natural aggregates, and are su itable for the manufacture of lightweight precast products such as roofing tiles, masonry blocks, or as a concrete and asphalt paving material. Although synthetic aggregate was not reported as an application for SDA material by ACAA in 2006 [22] or ECOBA in 2004 [23], numerous instan ces of commercial use a nd research projects were noted in the reviewed literature.Results and Discussion of the Literature Review

2-34 In the United States, a manufacturing plant is being operated at the Birchwood Power Facility by Universal Aggregates, LLC. The plant is tu rning CCPs, including SDA material, into 1,667,000 tons of lightweight aggregate annually [85]. The process used by Universal Aggregates, LLC, is described in U.S. Pa tent 6,054,074 [86]. The method mixes a sulfurcontaining CCP, recycle fines cont aining calcium hydroxide, an al uminum-containing material, and water to produce an agglomerated product. Th is is then combined with curing fines that contain calcium oxide and is cured. The cured ma terial is screened to separate dry fines, which are recycled, and the aggregate, which is used as a product. CCPs used in the method can include wet FGD material with fly ash, dry FGD material such as SDA material, wet lime kiln dust with dry lime kiln dust, or FBC material with fly as h. Specifications are pr ovided for the various materials. Further investigation of the end pr oduct is being conducted by the Research and Development Department of CONSOL Energy ( CONSOL R&D) with Universal Aggregates, LLC, under a DOE cooperative agreement. The object ive is to conduct a systematic study of the durability of manufactured aggregates using a variety of CCPs, including SDA material, with different chemical and physical properties. Th e manufactured aggregates and aggregate product materials were tested under different freeze– thaw, wet–dry, and long -term natural weathering conditions [33].

Sherburne EnviRock initiated production of manu factured, lightweight aggregate intended for use in the concrete masonry bl ock market in 2001. A year late r EcoBlend, a concrete mineral admixture also intended for u se in concrete masonry, was ad ded to production. SDA material containing fly ash is used as the primary feedstock [87].

A study performed by ICF Northwest Laborator y to produce syntheti c aggregate from five different FGD materials, including SDA ma terial, using two production processes: 1) agglomeration followed by sinteri ng and 2) briquetting followed by CO 2 environment curing. The SDA material and FSI materia l produced the strongest briquett es, with an average strength of 10.71 and 7.43 MPa (1554 and 1077 psi), respectively [34].

SDA material was used as an aggregate in asphaltic concrete pavement in a test road. Type 31B base course and Type 41A wear course mixes were made in the lab with 5% SDA material and 5.5% or 5.8% asphalt, respectively. The asphalt cont ent needed to be decreased to meet MnDOT air void specifications. However, no conclusion s can be drawn from these data, because it was difficult for the batch operators to meter the amount of SDA material and other aggregates used. An addition of 5% of SDA material caused a decrease in both stability and percent air voids compared to the control mixes. It was noted that less compactive effort was required to achieve the 95% of maximum density when using the S DA material containing b ituminous material [46].

Synthetic aggregate production using the Aard elite process has been used commercially to manufacture synthetic aggregate. Aardelite is a proprietary technology owned by Danieli Corus but licensed to Aarding Lightwei ght Granulates B.V. in the Netherlands. The technology and equipment are sold by Aarding Lightweight Gran ulates B.V. to power plants. To date, four Aardelite plants have been built [88]. To make Aardelite, SDA material can be mixed with water and pelletized [65]. The pellets ar e then embedded in fly ash, so the final pellet has a core of SDA material surrounded by fly ash. The composite pellets are hardened with steam at 90 ° C and size-graded by sieving. The four plants use a different embedding material. However, according to a company representative, there are no commercial plants using SDA material in operationResults and Discussion of the Literature Review

2-35 [89]. Aardelite pellets can be used as raw materia l in a variety of buildi ng applications including the following [88]:

• Masonry (building) blocks

• Ready mix concrete

• Prefabricated concrete elements

• Concrete piles

• Bitumen-bonded asphalt for road construction

• Paving stones

It has been suggested that these a ggregates could also be used in structural concrete. However, research carried out in Denmark by ELSAM indicat es that the pellets do not have sufficient strength for this application [65]. When mixed with fly ash, the SDA material can form the mineral thaumasite, which has poorstrength characteristics. Low- grade aggregates and pellets may be suitable for use in road b ase and as filler in asphaltic concrete.

Research initiated in Denmark has examined us ing the self-hardening properties of SDA material to form pellets [36]. Additives, such as portland cement or hydrated lime, may be added to SDA material with a low reactivity so that the pellets acquire adequate strength and density. Pellets (5–15 mm) were manufactured in the laboratory us ing a continuous roll m ill pelletizing machine, and cured in a water-saturated environment. Despite encouraging results, the demonstration work did not result in the commerci al production of aggregates.

It is possible to produce artificial gravel and chippings by mixing SDA material with 10%–20% water and compacting it in a pelletiz ing plant to a density of 2 t/m 3 and curing. A 5% addition of cement is required for low-fly ash SDA materials. Th e artificial gravel and chippings can be used as a substitute for natural gravel or chippings for cement or bitumen-bound road surfaces and cement-bound building brick. Development work in Denmark has been discontinued because of long setting times, reduced frost resistance, a nd an increase in volume due to thaumasite formation [17].

**Wallboard**

The use of SDA material in wallboard has b een demonstrated in Denmark by DURACON ApS [65]. The process uses the same production facilitie s as other fiber–cement boards that are fireresistant. The boards consist of a mixture of SDA material (~40%), fly ash (~10%), recycled paper (10%), cement, and additives. The finished pa nels should be cured in an autoclave. The raw materials are low in cost, minimizing production costs compar ed with similar wallboards. The panels are strong enough, with sufficient st iffness, for normal handling and working, and fire resistance tests show good performance. Th e product was undergoing full-scale testing in 1992, with a potential daily produc tion capacity of about 4000 m 2 of board. Instances of this product reaching full commercial implementation were not found.Results and Discussion of the Literature Review

2-36 Kolar also reported that several authors have described the production of wallboard made of SDA material, fly ash, fibers, and portland cement or lime hydrate [17]. The board is autoclaved and dried. The boards are claimed to be suitable for interior applications in residential buildings, hospitals, schools, and industrial buildings.

ACAA has not reported dry scrubber FGD material use in wallboard applications since dry scrubber FGD materials have been categorized.

**Wet FGD Sorbent**

Dry FGD material is used as a sorbent in th e wet FGD process in both Germany and Denmark; although it has not reached commerc ialization in the United States [ 16, 22]. The excess alkali in the dry FGD material serves as a sorbent for the SO 2 in the flue gas, and the sulfite in the material is simultaneously converted to commercial-grade gypsum. A prerequisite for this use is a very low fly ash content. It has been repor ted that the gypsum quality has improved when operating on dry FGD material as compared to using limestone powder and typically contains 98%–99% calcium sulfate dihydrate (gypsum) [16].3-1 

# 3
**SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS** 

An extensive literature review was pe rformed that provided information on:

• SDA systems and their operation and the impact s of system components and operation on the SDA material composition.

• The production and use statistics for SDA material produced in the United States and Europe.

• SDA material characteristics.

• Commercial and potential uses of SDA material.

The literature review brought into focus several issues evident to producers and users of SDA material that have an impact on the status of this material in commercial markets. These issues are summarized here and include a list of barriers to the commer cial utilization of SDA material.



### SDA Production and Mate **rial Characteristics**

SDA systems have two primary configurations. In the United States, the most common system configuration has the spray dryer chamber located so the flue gas is combined with the sorbent before it is introduced into the pr imary particulate collec tion system. In this configuration, the fly ash and dry SDA material are mixed in the spray dryer chamber, and in cases where the fly ash is alkaline, the fly ash actually serves as a sorbent so the fly ash particles are coated with calcium sulfite/sulfate. Recycle of this mixture is fr equently used to optimize sorbent utilization. Variables in this system include the amount of fresh lime sorbent that is introduced, the fly ash characteristics, the recycle rate, and the spent sorbent–fly ash ratio.

The typical European SDA system configuratio n includes a fly ash precollection unit that removes the fly ash from the flue gas before it ente rs the spray dryer chamber. The variables in this system configuration are fewer resulting in a more consistent material between plants

. The SDA material that results from these two pr imary system configurations have obvious compositional differences. The systems with fly ash precollection produce an SDA material that is composed primarily of calcium sulfite, calcium sulfate, and other minor calcium-based compounds. The systems that incorpor ate the fly ash stream into th e sorbent stream in the spray dryer chamber produce SDA materials that are composed of these same calcium compounds with fly ash that may or may not be coated with calcium sulfite or sulfate, depending on the alkalinity and composition of the fly ash. Even without acc ounting for varying chemical composition of the fly ash, it is apparent that SDA materials pr oduced from these two c onfigurations will beSummary, Conclusions, and Recommendations

3-2 significantly different in composition performance . In European literature, SDA materials are frequently referred to as dry FGD material. This term is used more generically in the United States but does include SDA material. Unfortunatel y, in most of the literature reviewed, the system configuration was not d escribed nor was the percentage of fly ash, if present, noted. However, the variability of the materials investigat ed and/or used is evident in the results of the literature review by the variability of the co mposition and properties reported. Wide variability for these materials was docume nted for every physical property for which literature was found and for the chemical composition to include the trace element concentrations. Although mineralogical characterizations we re limited, it is expected that similar crystalline phases would be present in all types of SDA materials, but that those with fly ash present would also contain a significant amorphous phase. On review of the charact erization data from the literature review, it was noted that varying conclusi ons were drawn by individual aut hors. The variability of samples used in the studies likely contributed to the dive rgence in conclusions. Another factor that likely played a role in the differences was the methodol ogy used to evaluate specific parameters. This serves as a caution in relating literature data a nd conclusions as represen tative of SDA materials without a clear understanding of the specific material(s) referred to in the literature.



### Utilization of SDA Material

As described in Section 2 under “Current and Po tential Uses of SDA Mate rial,” SDA material is commercially used in a variety of applications in the United States and Europe. A review of the ACAA production and use survey results also provid es an indication of util ization in the United States. These applications are noted in Table 3-1.

**Table 3-1 Commercial Applications for SDA Materi**  **al in the United States and Europe**

**European Commercial Applications U.S. Commercial Applications** 
Agriculture (e.g., sulfur fertilizer) Agriculture
Flowable fill Concrete, concrete products, and flowable fill

Raw material for cement manufacture Soil stabilization
Raw material for sulfuric acid prod uction Structural fills/embankments
Wet FGD sorbent Synthetic aggregatea
Mining applications

a Although not listed in the 2006 ACAA statistics [ 22], this is a current SDA material use.Summary, Conclusions, and Recommendations

3-3 The utilization profiles for SDA material in the Unite d States and Europe are different at least in part because of the different chem ical composition resulting from the inclusion of fly ash in the United States, while in Europe, SDA material t ypically does not includ e a fly ash component. Additionally, the more stringent European dispos al regulations encourage utilization of CCPs, and likely encouraged utilization of SDA materi als. Some commercial applications, such as mine fill and wet FGD sorbent, may have less stringe nt composition or quality control criteria.

Based on the commercial applicatio ns reported and research on utili zation of SDA material, a list of applications was assembled, ranking applicati ons as having high, moderate, and low potential for the U.S. market relative to the technical ac hievability of the applications. Key technical issues that were considered in ranking these applications were:

• Variability of SDA materials – Most U.S. cu rrent and projected SDA systems incorporate some fly ash into the final SDA material. The level of fly ash present and the specific chemistry of that fly ash results in a variabili ty in the chemical composition of the final SDA material, and that is expected to r esult in a variability in the physical and engineering performance of the material for many of the use applications noted.

• Sulfur content – Since SDA is used to remove sulfur gases from flue gas, the total sulfur content of SDA materials can be significantly higher than fly ash. For some applications, this elevated concentration of sulfur may li mit the use. Additionally, most of the sulfur present in SDA material is present both as sulf ite and as sulfate, with sulfite being the prevalent form. For some applications, the poten tial for oxidation of the sulfite to sulfate could limit use of SDA materials and needs to be considered.

• Potential for expansion – SDA material that c ontains fly ash has the potential to expand, especially when exposed to water. The com position of SDA materia ls containing fly ash includes the primary building blocks for et tringite formation, a key secondary hydrated phase that can cause expansion if it forms in finished products or final placements. The pH of SDA material is not considered conduc ive to ettringite fo rmation, but free lime present in SDA material may provide enough al kalinity to allow ettringite formation to occur. Expansion needs to be considered in any use applications for SDA material. While expansion can be predicted by use of appropriate tests, many tests currently available for determining expansion potential may not be adequate to predict field or full-scale performance. This is due to many factors, but laboratory studies indi cated that expansion may be taking place into material voids. A simple test based on density changes of materials that will with great certainty predict expansion under laboratory conditions has been developed by the EERC [90].

Economic feasibility was not used in this ranking process, but in order for any material to be commercially viable in a utili zation application, the economics need to be advantageous and should be considered in feasibility investiga tions. For some applica tions, the environmental suitability of SDA materials, or any other CCP or industrial resource, may need to be evaluated. Environmental performance of SDA materials was out side the scope of this literature review, but the issues associated with use applications that may result in impacts to human health and the environment are prevalent in public reports and technical literature.Summary, Conclusions, and Recommendations

3-4 **High-Potential Applications**

High-potential applications for the U.S. market are estimated to be those that take advantage of the presence of the fly ash component of the SDA material, can tolera te relatively high sulfur content, and have limited susceptibility to ex pansion or reduce expansion potential in the production process. These applications fall into two categories: 1) cementitious products and 2) mining.

Cementitious Products

ACAA’s 2005 utilization rate for dry scrubbing materials in concre te and concrete products was ~14,000 tons, or ~1% of the 2005 U.S. production. It is unlikely that this util ization rate includes significant quantities of sodium-based dry scr ubber materials because of the technical and performance issues associated with high levels of available alkali in materials used for formulation of concrete. Therefore, it is assumed that the 2005 1% utilization rate refers almost exclusively to SDA material. Anecdotal evidence sugg ests that this utilization is likely primarily in concrete products such as masonry. ACAA al so reported that nearly 10,000 tons of dry scrubbing materials were used for flowable fill in 2005 (0.7% of the amount produced) [59]. It is also assumed that the bulk of the dry scrubber mate rial reported in flowab le fill is also likely SDA material. Literature and anecdotal evidence indicated that some of these cementitious products or applications have specific requirements for the SDA material to be used most effectively. It is predicted that these use applications have high potential for growth as more SDA materials become available. Based on the prediction of 13–14 M tons of SDA material to be produced annually in the United States by 2017, it is estimated that the utilization rate should be maintained at the current approximate 2% level and could reach 5% or higher of production if industry develops technical and performance da ta and markets the product effectively. This equates to about 260,000 to 700,000 tons annually. Loca tion of material and markets will have a definitive impact on market development and pene tration. Much of this gr owth is anticipated after 2012 as installations increase in number.

Masonry

Although SDA material has been shown to be an effective component of masonry products, most commonly concrete blocks, the SDA material may need to be conditioned prior to use, and cementitious additives may be required for unreactiv e to moderately reactive SDA materials. Use of alkaline activation and the elevated-temperat ure curing of these products has been shown to result in good performance of the SDA material in these products.Summary, Conclusions, and Recommendations

3-5 Flowable Fill

The basic physical and engineering propert ies (moisture density, compressive strength development, and permeability) of reactive and low-reactivity SDA materi al indicate that these materials should be able to perform acceptably as flowable fill material. Research conducted to date supports this claim and shows that SDA ma terial can be an economical alternative to conventional materials; however, more research is needed on nonreactive SDA materials. It is recommended that more long-term tests be cond ucted to test long-term strength, stress–strain behavior, freeze–thaw properties, sw ell potential, and corrosivity.

Synthetic Aggregate

SDA material is currently being used to manuf acture synthetic aggregate in the United States; however, there is also evidence that some synthetic aggregate-manufacturing processes developed to use SDA material have not had commercial success. Successful aggregate production from SDA material indi cates the potential for use in c oncrete block, brick, and other shaped compacted product production. Much like the masonry application, it may be necessary to activate the SDA or use an elevated-te mperature curing to facilitate good product performance.

Mining Applications

SDA material has been used co mmercially as a mine fill. Additi onally, SDA material has been shown to be capable of neutralizing the spoil acidity and reestablishing vegetative cover to stabilize soils and reduce erosion. However, ex cessive application on mine spoils did cause excessively high pH and cementation. The technical issues associated with the use of SDA material in mine ap plications can be addressed through an understanding of the experience already reported in the literature, characterizatio n of the material, and careful attention to the methods for applications and quantities of ma terial applied. Statistics from 2005 indicate that approximately 8% of dry scrubber materials produced in the Unite d States were used in mining applications [59]. This represente d by far the largest current use of dry scrubber materials. It is reasonable to assume that much of the dry scrub ber materials included in the 2005 statistics were SDA materials simply because of the very l imited number of other dry scrubber systems currently being used as compared to SDA syst ems reported. Since projected SDA installations are expected to be in the Midwest and western Un ited States, it is likely that mining applications of SDA materials will remain a good utilizati on candidate and will increase proportionately to the amount of material produced in the wester n states. Contingent on rules currently under development by the Office of Surfac e Mining, it is anticipated that th e utilization rate of 8% can be readily maintained and could be expected to increase to 15% during the period from 2007 to 2012. During that period, most of the new installa tions will be in west ern states, where mine placement is expected to be economically feasible. Between 2012 and 2017, this use application could continue to increase but on a more limi ted basis because later SDA installations are anticipated to be in the Midwest and the advant age of location is not expected to favor mine reclamation as much as western inst allations. It is estimated that a maximum utilization rate of 20% could be realized by 2017. This would equa te to 2.6–2.8 million tons annually, a more than 10-fold increase over current utilization volumes.Summary, Conclusions, and Recommendations

3-6 **Moderate-Potential Applications**

Cement Replacement in Concrete

SDA material should be evaluate d for suitability as a cement replacement in concrete, especially as groups like ASTM International and the Am erican Association of State Highway and Transportation Officials (AASHT O) move toward performance-based specifications as opposed to prescriptive specifications. Research has shown that except for retardation in setting times, concrete in which cement was partially substituted by SDA material showed strength and durability performance comparable to superior to traditional concrete. This application is most promising when SDA material and fly ash are coll ected together with a high percentage of fly ash, as is common in U.S. installations. The technical issues that require some continued evaluation can be readily addressed using protocol s similar to the ones developed and used for fly ash and other supplementary cementitious ma terials. It will be possible to determine the appropriateness and amounts of specific SDA materia ls for use in concrete, and there is technical evidence that it can offer good performance. Much like fly ash qualification for use in concrete, the producer or potential marketer will need to take the responsibi lity of evaluating their specific material, developing the market, and educating th e users. This process will require time, so although the market has high potential to be technical ly successful, it is exp ected to take time to develop that market.

The potential for large volumes of SDA material to be used in this application will likely be dependent on a number of factors, including potential reduc tions in the availability of fly ash for use in concrete, which is expected to be impacted by the installation of mercury control technologies that use activated carbon and comb ine the activated carbon w ith the fly ash stream. If even some of the current concrete-quality fly ash being produced b ecomes unsuitable for use in concrete, there may be a mark et need for alternate materials. Of course, if activated carbon is used and mixed with fly ash on a unit or plant wh ere the fly ash is incorporated into the SDA material, the SDA material may not meet the required specifications for use in concrete.

Current data on dry FGD materia l utilization do not separate use in concrete or concrete products. It is estimated that only very low qua ntities of SDA material (<0.5%) are currently being used for cement replacement. This utilizati on rate is not expected to increase significantly, but at the predicted production rate for 2017 (1 3–14 M tons/year), a use rate of 0.5%, or 65,000– 70,000 tons, would be significant for the overall use of the material.

Engineering Applications

SDA material has been used commercially in engineering applications in Europe and with limited success in the United States. ACAA reporte d 2666 tons of dry FGD material were used for structural fills and embankmen ts in the United States in 2005 [59]. This utilization rate of approximately 2% was likely primarily SDA mate rial and indicates that SDA can be successfully utilized in engineered fills. This utilization rate would be expect ed to be maintained as additional SDA materials are generated.Summary, Conclusions, and Recommendations

3-7 Agriculture

In Europe, SDA material is typically collected separately from fly ash, whereas in the United States, SDA material and fly ash are often intermi ngled. This difference c ould be the reason that SDA material is used commercially as a sulfur fertilizer in Germany, Denmark, and Austria, but not in the United States. There are unanswered engineering and environm ental questions as well. The potential for SDA material to be used as a liming agent or for soil amendment has received mixed results depending on the pH of the soil, crops planted, amount of SDA material used, and whether the SDA material was blended with any ot her material. The leaching of boron could be a potential environmental concern. ACAA reported ~ 19,000 tons of dry scrubber material used in agricultural applications in 2005, nearly 1.5% of the total produc tion of ~1.4 million tons [59]. This represents the second larges t current use of dry scrubber mate rial. It is likel y that the dry scrubber material used in agricu ltural applications was SDA mate rial because the sodium-based dry scrubber materials have potenti al detrimental effects on many cr ops and soil types. An effort is currently under way in the United States to demonstrate the effectiveness of FGD gypsum for use as an agricultural soil amendment. Even t hough SDA material contains fly ash and calcium sulfite, which makes it different from FGD gypsum, the potential for SDA material to be used in agricultural applications may be enhanced by the current efforts on FGD gypsum use. The agricultural use of calcium sulfite, which is toxic to plants under certain conditions, appears to be less problematic than initially expected. Under normal soil pH a nd with a limited supply of air for oxidation, it would be expected that the oxidation of sulfite to sulfate, which is not toxic to plants, would be extremely slow. However, in fi eld studies, this conversion occurs rapidly. This anomaly is likely due to microbial transformations of sulfite to sulfate. It has been demonstrated that simply placing the sulfite-rich material in the agricultural soils a few weeks in advance of planting mitigates any detrimental effects of sulfite. There is moderate potential for SDA material to be used in agricultu re in the western United States, where FGD gypsum is expected to be less available. The trace element concentr ations of SDA materials are likely one technical and regulatory issue that will need to be a ddressed. Utilization rates are predicted to be maintained at near 1% based on the increased production.

Soil Stabilization

SDA material has physical and ch emical characteristics that ar e similar to a lime–fly ash mix and, thus, has the potential to be used in soil stabi lization even though mixed results have been found in the expansion potential of SDA material used as a road foundation. The ability for the SDA material to form ettringite without swelling is key to its engineering performance. It also needs to be durable enough to withstand poten tial damage due to freezing and thawing and wetting and drying action. Soil stab ilization with SDA material has been documented in only a few laboratory and field demons tration projects, and no commer cial uses were found in the reviewed literature even though marketers indicat ed the successful use of specific SDA materials in soil stabilization in certain regions of the Unit ed States. There is the potential for the technical issues related to the use of SDA material in soil st abilization to be addressed, so this application has moderate potential. Currently, there is only anecdotal evidence of SDA material being used for soil stabilization in the Un ited States, but if the technical i ssues are adequately addressed, it is projected that the utilization rate coul d reach 0.5%–1.0% of the production rate.Summary, Conclusions, and Recommendations

3-8 Wet FGD Sorbent

Dry FGD material has been used commercially as a sorbent in the wet FGD process in Germany and Denmark. Since there is alkalinity still availa ble in SDA material, it is possible that it could be a good candidate for use in wet FGD systems in the United States, likely in inhibited or natural oxidation systems. The technical issues w ould need to be addressed, and the actual use would likely be dependent on locations of an SDA producer and wet FGD system. Currently, there is no SDA material being used in this applic ation, but it could reach a utilization rate of up to 1% or greater by 2017. This application warran ts investigation to eval uate the impact of the presence of fly ash in the SDA material, and it is likely that this application has the highest potential for a region where an SDA material pro ducer and wet FGD system user are in close proximity.

**Low-Potential Applications**

The following applications are expected to ha ve low utilization poten tial. Predictions for utilization rates ha ve not been made.

Binder

SDA material used as a binder for interior plaste rs was noted in U.S. and German patents. The use of SDA material as a flooring binder is considered promising, and other potential applications include use in in sulating building materials and ra w materials for double-floor plates where it may be substituted for cement products. This is a pro mising low-volume, high-value application.

Cement Manufacture

Although SDA material has been us ed commercially to manufactur e cement in Germany, it has not reached the demonstration phase in the United States. A process has been patented in the United States to use dry FGD material in pl ace of fly ash and gypsum in the production of cement; however, it is not anticipated that the process will reach commercialization.

Fixating Agent for Hazardous Waste Sludge

Research has shown that SDA material has the pote ntial to solidify or fixate hazardous wastes if proper fixation techniques can be developed. The m echanism for this is likely due to the excess alkaline materials present in SDA material. Although SDA mate rial particles are generally coated with sulfite and sulfate, the grindi ng action of a pug mill w ould likely activate the material by exposing alka line surfaces through abr asion of particles.Summary, Conclusions, and Recommendations

3-9 Marine Applications

Research conducted to date has shown that SDA material can be used in marine applications such as artificial reefs and offshore sea defenses. SDA materials exposed to marine environments appear to show no evidence of surface friability or cracking and are a favorable habitat for marine life. The economical and e nvironmental feasibility of this application should be further evaluated.

Mineral Wool

The production of mineral wool from SDA material is technically feasible if the SDA material contains a high percentage of fly ash. However, th e market is not demanding this process at this time.

Sulfuric Acid Production

The use of SDA material in the production of sulf uric acid may be technically feasible; however, the authors believe this is not an economical beneficial use option.

Wallboard

Using SDA material as a source of gypsum in trad itional wallboard is not a promising utilization option. However, research conducted in Denmark s howed that SDA material, when mixed with fly ash, cement, and other additives and cured in an autoclave can produce good-quality fiber– cement board. The economics of this application need to be considered.



### Barriers to SDA Material Utilization

The European and U.S. production a nd utilization statistics for SDA material clearly indicate that SDA materials are currently underutilized, especia lly in the United States, even though this report summarized a number of commercial appli cations and several appli cations that have high potential to become commercial. The literature revi ewed brought into focus the barriers that exist and limit the use of SDA material in the United States. The barriers identified by the authors are:

• Inconsistent Terminology Used to Define the Material – In the literature reviewed, there was a marked inconsistency with regard to the terms used to describe SDA materials and FGD materials in general. The discrepancies we re so broad that, in some cases, the authors could not determine the specific product to which the reports referred. Inconsistent terminology makes it difficult for those in th e industry, particularly government entities, to define the material and its potential uses prope rly. In any area, a well-defined vocabulary is the cornerstone of effective communication, and th is is essential in technical fields. This lack of consistent terminology is a barrier to both the commercialization and to the research and development of SDA material utilization.Summary, Conclusions, and Recommendations

3-10 • Lack of Understanding of the Material – The successful manageme nt of SDA material requires a thorough understanding of the engineering and chemical properties of the material. Although a number of references were identified th at considered the ch aracterization of the material as it relates to potential uses, the engineering and chemical properties of specific materials need to be investigated further. As with other CCPs, it is difficult to generalize the properties of the material because of differences in coal type, combus tion system, collection process, and management. As previously note d, SDA materials exhibit a variability that results from the system configuration and the percentage of fly ash present in the final SDA material. Potential uses that appl y to one type of ma terial may not be appr opriate for others. The natural oxidation of sulfite to sulfate in SDA material is documented and yet the impact of this oxidation process on product performan ce is not well-defined and needs to be considered in evaluating utilization applications.

• Limited Data on Environmental and Health Effects – Although all SDA materials encountered in this literature review meet regulatory limit s for classifica tion as nonhazardous wastes, there are still concerns about surface water and groundwater contamination by runoff, seepage, and leachate during disposal or use applications. Many potential uses for SDA material fall under th e general category of land applica tion, which raises questions about the potential for the material to affect the environment and/or human health.

• Inconsistent Guidelines on Beneficial Ash Use – Many state rules apply to fly ash, bottom ash, and boiler slag utilization; however, pr oducts from FGD systems are relatively new in comparison to other products, and specifications have not been written that deal specifically with SDA material. In European countries wh ere regulations have been adopted, most progress has been made with regard to SDA material utilization.

• Economics – Economic factors are an overriding issue in utility ash management decisions. Currently, the potential to produce revenue from the sale of SDA material is limited; therefore, most utilities find it more economically feasible to dispose of the material rather than dedicate resources (i.e., employees and infras tructure) to utilize it. The prices received for SDA material are simply too low to justify a large financial commitment to SDA material marketing. In some countries in Europe, in creasing landfill taxes have driven the development of SDA material applications.Summary, Conclusions, and Recommendations

3-11 

### Conclusions

The literature assembled and reviewed provide d a good representative cross section of the technical information available on the utilizati on of SDA material in the United States and Europe. The following conclusions were develope d based on the information assembled from the review:

• SDA materials exhibit a broad variability based on the SDA system configuration, the fly ash content of the SDA material, the composition of the fly ash in the SDA material, and the use of optional sorbent recycle.

• The presence of significant levels of calcium sulfite and the natural oxidation of sulfite to sulfate has the potential to af fect the material performance in utilization a pplications and products.

• European SDA materials, frequently referred to as dry FGD material in the European literature, generally do not incorporate fly ash into the final SDA material, and documented commercial utilization of SDA material in Europe is higher than in the United States.

• U.S. SDA systems typically incorporate fly ash into the final SDA material and are most commonly used in coal-fired units where alkaline ash is produced so the fly ash can also act as an SO 2 sorbent.

• U.S. SDA material utilization rates are lower th an European utilizati on rates, likely because of higher variability of the U.S. material, incorporation of fly ash into the material, and current regulations in Europe that pr omote industrial r esource utilization.

• Numerous utilization applicati ons have high to moderate potential for commercialization in the United States, but technical, environmenta l, and economic evaluations will likely be needed before these materials can be successfully introduced into the markets identified.



### Recommendations

As U.S. coal-fired power plants inst all additional FGD systems to reduce SO 2 emissions in coming years, it is anticipated that a number of pl ants will elect to use SDA systems, resulting in an associated increase in the volume of SDA material produced in the United States. Although disposal is currently the predominant management option for SDA material in the United States, the potential exists that viable commercial options can be developed for this material. In order to optimize SDA material use in existing commercial applications and develop the potential commercial options in the United States, technical, environmental, and economic evaluations will be required. The following recommendations present an outline for the CCP industry to maintain existing commercial markets and develop new markets for U.S. SDA materials:

• Work within existing organizations such as ASTM International and ACAA to develop and put into use appropriate terminology and definitions for SDA materials.

• Develop an understanding of the impact of compositional variability on the performance characteristics of SDA materials.Summary, Conclusions, and Recommendations

3-12 • Develop an understanding of the oxidation pr ofiles of SDA materials and evaluate the impacts of oxidation on product performance.

• Educate potential users, regulatory representa tives, and other stakeholders about SDA materials.

• Address quality, compositional, environmental, and performance criteria in research, development, and demonstration efforts.4-1 

# 4

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# A

**BULK CHEMICAL COMPOSITION**

– **REPORTED AS OXIDES** 

– **OF SDA MATERIAL REPORTED IN LITERATURE REVIEWED** Bulk Chemical Composition–Reported as Oxides–of SDA Material Reported in Literature ReviewedA-2**Table A-1 Bulk Chemical Composition of SD** **A Materials Containing Fly Ash Repo rted in Literature Reviewed** **Source [2] [14] [30] [31] [33] [34] [35] [40] [44] [49] [50] [52]** **No. of Samples** **3 2**–**5 7 Unknown 2 Unknown Up To 7 Of** **24 References For Each** **Unknown 1 Unknown 1 1 Units** **wt%, Dry Basis** **wt%, Dry Basis** **wt% % wt%,** **Dry Basis % % % wt% wt% wt%** **wt%, Dry** **Basis**SiO238.78–46.26 22.05–33.10 16–26 21.1–32.4 24.05–30.22 16–32 6–68 21.1 26.52 10–40 32.0 21.5Al2O317.51–20.57 9.84–18.26 6.3–13 8.5–23 11.52–15.89 6–23 4–44 8.5 8.26 6–15 15.3 10.7Fe2O32.83–3.63 2.55–5.82 1.6–7.7 4.2–6.8 2.21–3.79 2–8 1–44 6.8 4.91 1.5–5 3.8 6.4CaO 13.12–18.34 21.59–29.27 18–31 19.7–32.2 25.66–34.13 18–32 0.2–52 18.1 27.16 20–28 24.1 28.4SO33.35–10.07 5.96–20.05——7.92–11.98 6–22 0–32 3.4 16.48 1–10 13.0 24.3MgO 1.34–1.47 2.18––4.33 0.65–4.6 5.8–6.0 0.89–3.90 0.6–6 0.1–14 5.8 4.62 0.5–1.5 2.9 0.73Na2O 1.02–1.39 0.49–1.52 0.9–46 2.7–4.7 0.13––1.82 0.9–5 0.2–28 4.7 2.60 0.1–0.4 1.67 0.25K2O 0.83–0.97 0.36–0.50 0.2–0.84 0.37–0.5 0.43–1.10 0.2–0.8 0.1–6.37 0.5 0.87 0.5–1.5 0.52 0.81P2O50.68–1.18 0.15–1.10—0.4 0.03–1.01—0.2–1.2 0.4 0.09—0.43 0.27TiO20.54–0.70 0.41–1.16 0.27–0.92—0.57––1.19—0.5–1.0 0.5 0.41 0.2–0.8 0.67 0.45BaO————————0.85—0.39—MnO20.03a0.00–0.12——————0.09—0.10—SrO 0.11–0.19———————0.46—0.44—Moisture———1–5 1.22–1.71—<0.1–13.2 1.3 1.10—0.55 0.61LOI or C—0.19–6.90—0.8——0.32–20.5—1.64—1.41—Unaccounted———————1.7—6.2CaSO3————11.89–17.96b——14.6————Bulk Chemical Composition – Reported as Oxides – of SDA Material Reported in Literature ReviewedA-3**Table A-1 (continued) Bulk Chemical Composition of SD** **A Materials Containing Fly Ash Repo rted in Literature Reviewed** **Source [2] [14] [30] [31] [33] [34] [35] [40] [44] [49] [50] [52]** **No. of Samples** **3 2**–**5 7 Unknown 2 Unknown Up To 7 Of** **24 References For Each** **Unknown 1 Unknown 1 1 Units** **wt%, Dry Basis** **wt%, Dry Basis** **wt% % wt%,** **Dry Basis % % % wt% wt% wt%** **wt%, Dry** **Basis**CaSO3· 1⁄2 H20——————13–59—————CaSO4——————6–43 8.9————CaCO3——————2.1 2.1————Ca(OH)2————8.5–25.0c—4–21————Free Lime———————1.6————Ca as AvailableCaO——————0–5.4—————SO4——12–22—————————SO2——————0.1–20.5——5–25——CO2—1.77–7.54————<0.1–15d—————Hydroxide——0.45–8.2——0.5–10 <0.1–9.5—————Cl—————————0.5–4——aMn3O4bBased on total sulfur content in ashcBased on thermogravimetric analysis(TGA) or lime index measurementdCarbonate, as CO2Bulk Chemical Composition

– Reported as Oxides

– of SDA Material Reported in Literature Reviewed

A-5 **Table A-2 Bulk Chemical Composition of SDA Materials**  **with Precollection of Fly Ash Reported in Literature Reviewed** 

**Source [41] [45] [49]**

**No. of Samples**  **3 a**  **4 Unknown**

**Units wt%,**  **Dry Basis wt% wt%** 

SiO 2 7.9

– 10.6 1.4

– 11.1 <2.5

Al 2 O 3 4.1

– 4.7 0.8

– 5.4 <1.5

Fe 2 O 3 1.9

– 4.4 0.4

– 1.8 <0.7

CaO 33.6

– 37.8 32.9

– 46.9 35–60

SO 3 17.4

– 30.0

— 2–17
MgO 1.0

– 1.9 0.4

– 1.7 <0.8

Na 2 O 0.2

– 0.3

— <0.1

K 2 O 0.3

– 0.6

— <0.2

TiO 2 —0.0

– 0.2 <0.1

LOI or C

—1.3

– 2.1

—
Na 2 O/K 2 O

—0.2

– 0.8

—
CaSO 3 · 1⁄2 H 2 0

—50.0

– 62.0

—
CaSO 4 · 2 H 2 O

—4.5

– 10.0

—
CaCO 3 —0.5

– 12.5

—
CaCl 2 —0.6

– 7.2

— Ca(OH) 2 —2.4

– 18.9

—
Free Lime <0.5

– 4.0

— —
SO 4 7.0

– 24.6

— —
SO 2 — —9

– 40

Total Sulfur 9.3

– 15.7

— —
CO 2 10.6

– 20.1

— — C (org.) 0.1

– 0.2

— —
SO 3 –2

—29.7

– 38.8

—Bulk Chemical Composition – Reported as Oxides – of SDA Material Reported in Literature Reviewed

A-6 **Table A-2 (continued) Bulk Chemical Composition of**  **SDA Materials with Precollection of Fly Ash Reported in Literature Reviewed** 

**Source [41] [45] [49]**

**No. of Samples**  **3 a**  **4 Unknown**

**Units wt%,**  **Dry Basis wt% wt%** 

SO 4 –2

—2.5

– 6.9

—
Cl 0.93

– 3.0

—0.5

– 6

Cl
–

—0.4

– 4.6

—
CO 3 –2

—0.3

– 7.5

—

a May contain up to 20% fly ash.B-1 

# B

**TOTAL ELEMENTAL COMPOSITION OF SDA MATERIAL REPORTED IN LITERATURE REVIEWED** Total Elemental Composition SDA MaterialReported in Literature ReviewedB-3**Table B-1 Total Elemental Composition of SDA Materials Cont** **aining Fly Ash Reported in Literature Reviewed Source [12, 13] [17] [30] [30] [** **31] [35] [37] [44] [46] [47] [ 49] [50] [53] [54] [55] [56]** **No. of Samples 8 1 a** **8 8** **(by AA) Unknown Unknown 13 1 1 1** **Frequent Conc.** **c 1 1 13 1** –**19 1 Units mg/kg mg/kg mg/kg mg/kg mg/k** **g mg/kg mg/kg mg/kg mg/kg mg/kg mg /kg mg/kg mg/kg mg** **/kg mg/kg mg/kg**Aluminum (Al) 33,300–66,700—33,300–66,700—18,000–110,000 18,000–230,000 10,000–86,000—92,100———24,000 10,000–86,000——Antimony (Sb) 5.6b–29—5.6b–29—7.5–8 0.8–20————————3.81–6.19 3.8Arsenic (As) 3.6–48 0.4–380 <12–22b3.6–48 14.2–47 2.3–1200 6.9–165 28.7 4 68.9———6.9–165.5 13.9–82.0 13.9Barium (Ba) 190–3090—190–6060—100–2000 20–12,000 0.76 3200————100–2400 139 139Beryllium (Be) 1.4–3.1—0.8–3.1—4–63 0.94–63 0.7–9.6—3————0.7–9.6 2.19–2.24 2.2Boron (B) <10–230—<10–230—500 10–1300 292–948 1460 650 42.1 1352—292–948 982–1329 1329Bromide (Br)————0.3–21 0.3–21——————————Cadmium (Cd) 0.2b–0.63 0.1–5 <0.4–5.8 <0.2–0.63 0.5–70 0.01–70 1.1–40.7 0.88 <4 0.1——1.1–40.7 0.55–1.54 0.55Calcium (Ca) 14,300–223,000—126,200–223,000——7100–360,000 178,000–401,000—159,400———245,000178,000–401,000——Cesium (Cs)—————1–22——————————Chloride (Cl)—————<0.1–10,200——————————Chromium (Cr) 36–210—36–210—27–130 3.6–1000 11.4–56.3 14 62 [Cr(VI): <6]—3——11.4–56.3 25.3–42.8 35Cobalt (Co) 9.9–26 <0.5–62 9.9–26—4.8–80 4.8–172 8.0–20.8—<20——8.0–20.8 13.4 13.4Copper (Cu) 20–160—20–160—8.1–170 7.1–655 13.3–73.2 30 61 111 3——13.3–139.0 71.8Fluoride (F)————0.4–1000 0.4–1000——————————Iodine (I)—————0.1–0.6——————————Iron (Fe) 10,900–53,700—10,900–53,700—10,000–367,000 6300–367,000 19,000–51,000—29,300———36,000 15,000–51,000 24,825 24,825Lead (Pb) <0.3–19 4–550 <16–44b<0.3–14 4.4–150 3.1–800 6.5–139 16.3 <20—5——6.5–139.0 18.6–59.57 21.1Lithium (Li) 18–42—18–42——48–530 15.1–90.5—110————15.1–90.5——Magnesium (Mg) 3880–27,700—3880–27,700——3000–22,000 3800–151,000—17,900———11,000 3800–151,300——Manganese (Mn) 55–680—55–680—45–630 24.5–1432 43–501—680 200———43–501 558 558Mercury (Hg) <0.05–0.39 <0.1–10—<0.05–0.39 BDL–0.5 <0.005–2.5—0.631 <0.02—<0.1———<0.001–2.553 0.015Molybdenum (Mo) 2.7–514—2.7–514—0.5–33 0.5–110 <0.02–50.2—<20 32.8———<0.018–50.2 13.2 13.2Nickel (Ni) 17–110 1.4–125 17–110—13–460 1.8–460 12.4–58.9 13 <20 64.7 2——12.4–58.9 12.9–30.3 27Total Elemental Composition SDA MaterialReported in Literature ReviewedB-5**Table B-1 (continued) Total Elemental Composition of SDA Materials Cont** **aining Fly Ash Reported in Literature Reviewed Source [12, 13] [17] [30] [30] [** **31] [35] [37] [44] [46] [47] [ 49] [50] [53] [54] [55] [56]** **No. of Samples 8 1 a** **8 8** **(by AA) Unknown Unknown 13 1 1 1** **Frequent Conc.** **c 1 1 13 1** –**19 1 Units mg/kg mg/kg mg/kg mg/kg mg/k** **g mg/kg mg/kg mg/kg mg/kg mg/kg mg /kg mg/kg mg/kg mg** **/kg mg/kg mg/kg**Phosphorus (P)——————21–1752—2200————21–1752——Potassium (K) 1760–7020 1760–7020——1700–9300 1600–5000—4700————1600–5000——Rubidium (Rb)—————48–530————————Selenium (Se) <0.4–7.3b3–34 71b–283 <0.4–7.3b4.7–20 0.6–760 4.9–23.0 5.4 <4 29.2—7.6 4.9–23.0 9.15–18.2 18.2Silicon (Si) 72,800–120,000—72,800–120,000——70,000–99,000 22,000–137,000—157,200————22,000–137,000——Silver (Ag) 1.6b–5.8—1.6b–5.8—0.2–0.5 0.04–8—0.20 <4————<0.024 1.18–1.97 1.18Sodium (Na) 6540–34,000—6540–34,000——710–240,000——13,500———————Strontium (Sr) 184–3040—184–3040—84–2500 30–13,000 153–1370—4300————153–3166——Sulfur (S)——————42,000–163,000—————300042,000–170,000d——Thallium (Tl) <18 <10 <18—20–30 0.1–42——<1————0.47 0.47Tin (Sn) <19–349—<24–349—30–36 0.01–962—————————Titanium (Ti) 1610–5360—1610–5360——1050–6700——3700———————Uranium (U) <5–140—<25–140——0.8–30——————————Vanadium (V) 72–180—72–180—0.4–610 0.4–950 22.5–83.9 49.7 86————22.5–83.9 56 56Zinc (Zn) 22–110—<6–79—12–330 12–9000 52–266 25 70—15——52–237 48.9–50 50aHigh fly ash.bIndicates less than 5 times the detection limit.cMultiplying or dividing by 2 can give the range.dCombination of LECO and ICP concentrations.B-7 **Table B-2 Total Elemental Composition of SDA Material**  **s with Precollection of Fly Ash Reported in Literature Reviewed** 

**Source [41] [49]**
**No. of Samples 3 a**  **Frequent Conc. b** 
**Units mg/kg mg/kg**
Arsenic (As) 4.8–11

— Barium (Ba) 270–4000

— Boron (B) 100–150

— Cadmium (Cd) 0.9–6.9 1
Chromium (Cr) 34–50 60
Copper (Cu) 43–47 80
Lead (Pb) 28–110 100
Manganese (Mn) 170–420

— Mercury (Hg) <0.3 <0.1
Molybdenum (Mo) 2.7–5.4

— Nickel (Ni) 32–36 80
Selenium (Se) 3.9–6.5

— Vanadium (V) 60–70

— Zinc (Zn) 94–380 120

a May contain up to 20% fly ash.

b Multiplying or dividing by 2 can give the range.Electric Power Research Institute 3420 Hillview Avenue, Palo Alto, California 94304-1338 • PO Box 10412, Palo Alto, California 94303-0813 USA 800.313.3774 • 650.855.2121 • askepri@epri.com • www.epri.com Export Control Restrictions Access to and use of EPRI Intellectual Property is granted with the specific understanding and requirement that responsibility for ensuring full compliance with all applicable U.S. and foreign export laws and regulations is being undertaken by you and your company. This includes an obligation to ensure that any individual receiving access hereunder who is not a U.S. citizen or permanent U.S. resident is permitted access under applicable U.S. and foreign export laws andregulations. In the event you are uncertain whether you or your company may lawfully obtain access to this EPRI Intellectual Property, you acknowledge that it is your obligation to consult with your company’s legal counsel to determine whether this access is lawful. AlthoughEPRI may make available on a case-by-case basis an informal assessment of the applicable U.S. export classifi cation for specifi c EPRI Intellectual Property, you and your company acknowledge that this assessment is solely for informational purposes and not for reliancepurposes. You and your company acknowledge that it is still the obligation of you and your company to make your own assessment of the applicable U.S. export classifi cation and ensure compliance accordingly. You and your company understand and acknowledge your obligations to make a prompt report to EPRI and the appropriateauthorities regarding any access to or use of EPRI Intellectual Property hereunder that may be in violation of applicable U.S. or foreign export laws or regulations.

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