Materials Characterization Paper 

In Support of the

Proposed Rulemaking – 

Identification of Nonhazardous Secondary Materials That Are Solid Waste 

Coal Combustion Residuals - Coal Fly Ash, Bottom Ash, and Boiler Slag 

April 5, 2010 

==================================================================== 

1. Definition of CCRs 

Coal combustion residuals (CCRs) are formed during coal-burning processes in power plants and 

industrial boilers. Coal combustion produces various forms of CCRs that are categorized by the 

process in which they are generated. The CCRs that can be used as ingredients in the 

manufacture of portland cement include:1

 

• Fly ash: Exhaust gases leaving the combustion chamber of a power plant entrain particles 

during the coal combustion process. To prevent fly ash from entering the atmosphere, power 

plants use various collection devices to remove it from the gases that are leaving the stack. 

Fly ash is the finest of coal ash particles. 

• Bottom ash: With grain sizes ranging from fine sand to fine gravel, bottom ash is coarser 

than fly ash. Utilities collect bottom ash from the floor of coal burning furnaces used in the 

generation of steam, the production of electric power, or both. The physical characteristics of 

the residuals generated depend on the characteristics of the furnace. 

• Boiler Slag: Boiler slag consists of molten ash collected at the base of cyclone and 

pulverized coal boilers. Facilities cool boiler slag with water, which then shatters into black, 

angular pieces that range in size from course sand to fine gravel and have a smooth 

appearance. 

2. Annual Quantities of CCRs Generated and Used 

(1) Sectors that generate CCRs: 

• The coal-fired power industry, represented by NAICS sector 221112 – Fossil Fuel 

Electric Power Generation, is the largest generator of CCRs. Other industries, 

such as commercial boilers and mineral and grain processors that use coal as a 

fuel source, also produce small quantities of CCRs. 

(2) Quantities and prices of CCRs generated: 

• In 2008, the coal-fueled electric power industry generated approximately 72.4 

million tons of coal fly ash, 18.4 million tons of bottom ash, and 2.0 million tons 

of boiler slag. 

• In 2003, the price per ton of concrete-quality fly ash, freight on board (FOB) (i.e., 

costs net of shipping or transport costs), ranged from $0-$45 (ACAAa). 

• In 2003, the price per ton of bottom ash ranged from $3-$8 FOB (ACAAa). 

 1

 This paper focuses on CCRs used primarily as an ingredient in cement manufacture; see the Materials 

Characterization Paper for Coal Refuse for further information on mining rejects and combustion applications. 
 Coal Combustion Residuals 

• The price of boiler slag could not be determined at this time; as with many small 

markets, it may be that most transactions involving boiler slag are private. 

(3) Trends in generation of CCRs: 

• The coal-fired power industry is the largest generator of CCRs in the United 

States. Coal-generated electricity supplied approximately 45 percent of the 

electricity consumed in the United States in 2009.2 Since electricity demand is 

projected to increase in coming years, and coal will continue to be an important 

fuel source, the quantity of CCRs produced and available for beneficial reuse may 

also increase (USDOE 2008, p.10). While other industries that use coal as a fuel 

source in commercial or industrial boilers (e.g., mineral and grain processors) also 

produce CCRs, these quantities tend to be small, and increases or decreases in 

their production will not have a significant impact on the overall trend in CCR 

production. 

3. Uses of CCRs 

(1) Ingredient and combustion uses of CCRs: 

• Fly ash, bottom ash, and boiler slag can be added to the raw material feed in 

clinker manufacturing to contribute specific required constituents, such as silica, 

alumina, and calcium, in the final cement composition. 

• Fly ash with high unburned carbon content can also be reburned in cement kilns 

for energy recovery at the same time as it provides ingredient value. 

• Public comments from the American Coal Ash Association (ACAA) in response 

to the Advanced Notice of Proposed Rulemaking (ANPRM) indicate that fly ash 

is a particularly useful fuel because of its high minerals and carbon content (Goss 

2009). 

• ACAA’s comments also indicate that an electric generating facility in Wisconsin 

uses ash containing high amounts of carbon (20 to 30 percent carbon) as a 

substitute for coal (Goss 2009). 

(2) Non-combustion uses of CCRs:

• Fly ash can be used in a number of non-combustion applications. It’s most 

common, and most high-value, use is as a supplementary cementitious material in 

concrete (i.e., a substitute for, or amendment to, portland cement in concrete 

mixes). In this application, fly ash can provide certain material advantages such 

as greater workability, higher strength, and increased longevity in the finished 

concrete product. Fly ash is also used as a partial substitute for virgin dirt, sand, 

or gravel in structural fill, or as a substitute for portland cement in waste 

stabilization (ACAA 2006).

• Bottom ash can be used to offset virgin sand and gravel in applications such as 

structural fill, road base, and concrete (ACAA 2006).

 

2 http://www.eia.doe.gov/cneaf/electricity/epm/table1_1.html

2 
 Coal Combustion Residuals 

• Boiler slag can be used to offset virgin sand and gravel in applications such as 

structural fill and blasting grit (ACAA 2006). 

(3) Quantities of CCRs landfilled: 

• In 2008, 42.3 million tons of coal fly ash, 10.4 million tons bottom ash, and 0.34 

million tons boiler slag were landfilled, representing roughly 58 percent, 56 

percent, and 17 percent of generation, respectively. 

(4) Quantities of CCRs stockpiled/stored:

• The American Coal Ash Association (ACAA) estimates that between 100 million 

and 500 million tons of fly ash have accumulated in U.S. landfills since the 1920s, 

when the disposal of fly ash in landfills began. It is unclear, however, how much 

of this fly ash is available for beneficial use. 

• The recoverability of fly ash depends significantly on the manner in which it is 

disposed. Fly ash disposed of in a monofill or holding pond likely would be 

suitable for beneficial use because it has generally not been commingled with 

other materials. Unfortunately, the ACAA is unaware of data indicating how 

much of the 100-500 million tons of stockpiled fly ash is deposited in monofills or 

holding ponds 

• The recovery of landfilled ash for beneficial reuse does not appear to be widely 

practiced. Although a portion of the fly ash located in stockpiles across the U.S. 

may be available for beneficial use, a recent analysis of the market for fly ash 

found that approximately 49 percent of the 72.4 million tons of fly ash generated 

in 2006 was beneficially used (ACAA). Therefore, even if large quantities from 

stockpiles were made available, it is unlikely that the quantity re-used would 

increase significantly. In other words, the demand for fly ash appears to be met by 

existing production. 

• Certain electric utilities, however, have pursued programs to recover ash for use 

of its residual unburned carbon in power plants. Examples include: 

o Wisconsin Energy Corporation began reburning ash at its Pleasant Prairie 

Power Plant in 2000 and expanded reburning to include ash recovered 

from landfills in 2001 after receiving approval under Wisconsin’s first 

Environmental Cooperative Agreement. The patented process can use 

either dry high-carbon fly ash directly from the company’s older power 

plants, or use moist high-carbon ash from WEC’s power plants, stockpiles, 

existing landfills, and remediation projects. According to its 2006 

Performance Report, WEC reburned approximately 11,023 short tons of 

recovered landfill ash in 2002 (WEC 2006a, p.69). 3

o In 2001, Western Kentucky Energy's Coleman station in Hawesville, 

Kentucky, inaugurated an ash-recovery program developed by staff at the 

University of Kentucky Center for Applied Energy Research (CAER). The 

CAER researchers developed a system to remove fly ash from holding 

 

3

 The report indicates that between 2003 and 2006, the facility did not reburn any recovered coal ash from its 

landfill. During these years, however, the facility continued to reburn coal ash generated by power plants. 

3 
 Coal Combustion Residuals 

ponds and extract carbon to create a reburning fuel or several other 

products (BNET 2002, pp. 1-2). 

• Wisconsin Energy Corporation also recovers landfilled ash for sale as a 

construction material. Under the Environmental Cooperative Agreement and 

Wisconsin regulations for beneficial industrial byproduct use, the company 

recovered 31,967 short tons (706.293 cubic feet) of coal ash from its P4 landfill 

and sold it as a base material to replace stone or gravel under roads, parking lots, 

and buildings. Since 2002, more than 93,696 short tons have been recovered 

(WEC 2006a, p.70). 

 Exhibit 1: Overview of Generation and Use of CCRs in 2008 

Annual Quantity Used as 

Ingredient 

Annual 

Quantity 

Generated Fuel 

Cement 

Ingredient 

in Kilns 

Annual 

Quantity 

Landfilled 

Annual 

Quantity in 

Other Uses 

Total 

Quantity 

Stockpiled as 

of 2006 

Commodity 

-----------------------------Short Tons -------------------------

Coal fly ash 72.4 million Undetermined 3.2 million 

42.31 

million 

26.89 

million 

100 – 500 

milliona

Bottom ash 18.4 million 0 0.61 million 

10.36 

million 7.43 million Undetermined 

Boiler slag 2.0 million 0 0 million 0.34 million 1.66 million Undetermined 

Sources: 

Unless otherwise noted, data are from American Coal Ash Association. (ACAA), 2008, Coal Combustion 

Product (CCP) Production and Use Survey, 

Notes:

a. Personal communication with Dave Goss, ACAA, November 27, 2006. 

4. Management and Combustion Processes for CCRs 

(1) Types of units using CCRs:

• When used in clinker manufacture, fly ash, boiler slag, and bottom ash are fed 

into cement kilns with other raw material feed. 

(2) Sourcing of CCRs:

• Although electric utility companies produce ash at their coal-fired power plants, 

most utilities do not handle, dispose of, and/or sell the ash that they produce. 

There are approximately 40 to 50 commercial ash marketing firms operating 

throughout the United States, in all states except Hawaii. In addition to 

commercial ash marketing organizations, certain coal-burning electric utility 

companies have formal ash marketing programs of their own. Most coal-burning 

electric utility companies currently employ an ash marketing specialist whose 

responsibility it is to monitor ash generation, quality, use or disposal, and to 

interface with the ash marketers or brokers who are under contract to the utility 

companies (Turner Fairbank Highway Research Center). 

4 
 Coal Combustion Residuals 

(3) Processing of CCRs: 

• Use of CCRs as a raw feed in cement kilns does not require processing. 

However, kilns do require feed of consistent quality, quantity, and composition in 

order to ensure that the feed mix in the kiln is appropriate. In particular, levels of 

key metals (e.g., iron and alumina) in CCRs are carefully calibrated with other 

ingredients to ensure that the final cement product has the correct mineral and 

metal content. 

• Due to its pozzolanic properties, stockpiled class C fly ash may harden over time 

if exposed to moisture. (Class F fly ash does not exhibit this characteristic). 

Hardened class C fly ash must be crushed prior to beneficial use but is typically 

limited to use as an aggregate in concrete. Class F fly ash does not loose reactivity 

over time and can be recovered from stockpiles for use in concrete, but it is 

unclear to what extent stockpiled Class F fly ash can be used in cement kilns, or 

whether it requires processing prior to use. 

• Bottom ash may require stockpiling for a short period of time (at least 1 or 2 days) 

to allow excess water to drain prior to beneficial use. Ponded bottom ash 

reclaimed from a lagoon should be stockpiled and allowed to drain for a 

somewhat longer time period, perhaps up to 1 or 2 weeks, depending on the 

amount of rainfall. After size reduction, bottom ash can be screened to produce 

different size ranges, if desired. (Turner Fairbank Highway Research Center). 

• An example of the recovery process used by Wisconsin Energy Corp. includes: 

o Identifying a disposal site that contains the byproducts (typically fly ash 

and bottom ash) and removing a portion of the byproduct; 

o The removed materials are crushed, screened, and periodically sampled in 

accordance with ASTM D2234;4

o Analyzing a sample of the portion of the byproducts to determine the loss 

on ignition of the portion of the byproducts; 

o Either fly ash or bottom ash or a mixture of both is added in a fine particle 

condition to the furnace of a pulverized coal boiler in a small proportion to 

the pulverized coal fed to the furnace. The ash is burned with the 

pulverized coal. The proportion of coal ash is preferably in the range of 1 

percent to 3.5 percent, by weight, of the pulverized coal. 

o Introducing the portion of the byproducts along with pulverized coal into a 

pulverized coal furnace if the portion of byproducts has a loss on ignition 

greater than or equal to a predetermined loss on ignition value (typically 

greater than or equal to 1 to 5 percent); and 

o Burning the portion of the byproducts in the furnace with the pulverized 

coal to render the byproducts into a commercially valuable fly ash and 

bottom ash having very low loss on ignition, typically lower than 3 

percent (WEC 2003, p.7; WEC, p.1).5 

 

4

 During a pilot project to reprocess landfilled combustion products that was conducted by Wisconsin Energy 

Corporation in July and October 1998, samples were collected every 30 minutes from the transfer point where the 

ash fell onto the stacker conveyor during the entire operation per the ASTM standard, and a composite sample was 

prepared for every 5,000 tons processed and tested. 

5

 Landfill ash recovery operations at the Western Kentucky Energy (Coleman Station) involves the following 

activities. Ash first must be recovered from the pond or landfill and then separated into its fundamental components: 

5 
 Coal Combustion Residuals 

• Public comments received in response to the ANPRM indicate that there are at 

least four patented processes for removing unwanted carbon from fly ash, 

allowing the processed material to be used as an ingredient in concrete. In 

addition, the resulting carbon stream can be re-introduced into a boiler as a fuel 

(Goss 2009). 

(4) State status of CCRs use as ingredient: 

• In the state of Wisconsin, Wisconsin Energy Corporation has an agreement with 

the state to burn recovered landfill ash at their power plants. 

• At this stage, we have not identified any other states that have specifically given 

beneficial use designation to the use of CCRs in clinker manufacture, or that 

prohibit fly ash use in clinker, but we have not performed an exhaustive 

investigation of state activities and regulations. 

5. CCR Composition and Impacts 

(1) Composition of CCRs:

• The chemical properties of fly ash are influenced to a great extent by those of the 

coal burned. There are basically four types, or ranks, of coal, each of which varies 

in terms of its heating value, its chemical composition, ash content, and 

geological origin: anthracite, bituminous, subbituminous, and lignite. The 

principal components of bituminous coal fly ash are silica, alumina, iron oxide, 

and calcium pxide, with varying amounts of residual, unburned carbon, as 

measured by the loss on ignition. Silica, alumina, and calcium are also the 

primary ingredients in portland cement, along with iron oxide. Lignite and 

subbituminous coal fly ashes are characterized by higher concentrations of 

calcium and magnesium oxide and reduced percentages of silica and iron oxide, 

as well as a lower residual carbon content, compared with bituminous coal fly 

ash. Very little anthracite coal is burned in utility boilers, so there are only small 

amounts of anthracite coal fly ash. Fly ash contains trace metals, such as 

vanadium, zinc, copper, chromium, nickel, lead, arsenic, and mercury.6

• Bottom ash and boiler slag are composed principally of silica, alumina, and iron 

oxide, with smaller percentages of calcium and magnesium oxides, sulfates, and 

other compounds. The composition of the bottom ash or boiler slag particles is 

controlled primarily by the source of the coal and not by the type of furnace. 

Bottom ash or boiler slag derived from lignite or sub-bituminous coals has a 

higher percentage of calcium oxide than the bottom ash or boiler slag from 

anthracite or bituminous coals. (Turner-Fairbank Highway Research Center). 

• All of these materials provide effective substitutes for most or all of the raw 

materials in cement, but the relative levels of different oxides must be carefully 

managed to ensure that the kiln product meets industry specifications. 

 

carbon, silicates, and high-density, iron-rich materials. A coarse carbon-fuel product is recovered by density 

separation using concentrating spirals. A fine carbon-fuel product is recovered with flotation cells. 6

 Listed by relative frequency. See USEPA, March 15, 1999. 

6 
 Coal Combustion Residuals 

(2) Impacts of CCRs use:

• In clinker manufacture, CCRs partially offset the need for virgin silica, iron, and 

alumina sources. Using CCRs in the cement kiln can reduce the unit consumption 

of virgin feed stock materials, which may reduce the overall emissions associated 

with materials extraction and processing, since CCRs generally require less preprocessing than the virgin materials they replace. 

• With respect to emissions from the cement kiln itself, to the extent CCRs have a 

lower emission profile than the virgin materials they replace, overall emissions 

will be reduced. However, CCRs may have higher emissions of pollutants such as 

mercury than virgin silica, iron and alumina sources when burned in the kiln. 

• The specific lifecycle impacts of CCR use as a raw material in clinker production 

are not evaluated here because of uncertainties in lifecycle scenario development. 

For example, it is difficult to determine the replacement ratio between CCRs and 

other raw feed materials in clinker production. Thus, the correct quantity of 

material to be modeled is unclear. In addition, CCRs may substitute for a 

combination of virgin raw materials and other secondary materials (e.g., blast 

furnace slag, foundry sand, cement kiln dust, etc.); the choice of material often 

depends on location-specific factors such as the proximity of material sources to 

the cement kiln and relative availability of different materials. Avoided upstream 

impacts depend heavily on the specific material being displaced in the lifecycle 

scenario. 

• When fly ash with a high unburned carbon content is introduced to the cement 

kiln during clinker manufacture, fuel supply may be reduced to accommodate the 

additional energy provided by the carbon in the fly ash (Bhatty, Javed et al. 2001). 

• One health risk issue is currently gaining attention in the use of fly ash in highheat applications such as cement manufacture. When exposed to elevated 

temperatures (approximately 2,750 degrees Farenheit) in a cement kiln, laboratory 

experiments have found that mercury is readily released from fly ash (PflughoeftHasset et al. 2007). At this time, the level of mercury in fly ash has not been 

considered significant enough to create a health risk. However, as coal utilities 

increasingly employ mercury capture technologies, some facilities may 

implement technologies that result in fly ash with much higher mercury content 

that is not suitable for use in cement manufacture. Other, more expensive 

technologies would ensure that most or all fly ash is separate from mercury 

capture; at this point it is not clear how much, if any, fly ash would be affected by 

mercury capture technology. 

• Combustion of stockpiled CCRs to produce a low loss on ignition (LOI) coal ash 

has several potential advantages: 

o Removal, recovery, and characterization of landfilled CCRs so that 

recovered CCRs may be put to beneficial use. Characterization includes 

energy content, sulphur content, moisture, and trace metals content for 

energy and air emissions analyses. 

o Preservation of licensed landfill space. 

o Preservation of coal reserves by recovering heat from reclaimed CCRs. 

7 
 Coal Combustion Residuals 

o Production of low LOI coal ash that may be substituted for portland 

cement thereby reducing the need for portland cement and reducing air 

emissions from its production process. 

o Reduced need for limestone quarries (due to the use of ash as a substitute 

for portland cement) and landfill sites (WEC, p.2).7

 

7

 Recovery of landfilled ash may require extra screening. For example, during the removal of ash from a landfill in 

Wisconsin in 2000, the removed materials were crushed, screened and periodically sampled in accordance with 

ASTM D2234. It was found that over 99 percent of the material removed consisted of coal combustion products, 

with foreign materials consisting of lost items from landfill operators (e.g., soda cans, safety ribbon, gloves, etc.) 

(WEC, 2006, p3). 

8 
 Coal Combustion Residuals 

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 Coal Combustion Residuals 

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10