Use of Coal Combustion Products for Reclamation

This article was prepared by Paul F. Ziemkiewicz and Jeff Skousen, WVU Professor of Soil Science and Land Reclamation Specialist. It appeared in the spring 2000 issue of "Green Lands" magazine.

About 105 million tons of coal combustion products (CCP) were produced by American power generating utilities in 1997 (ACAA, 1998). Of that total, about 2 million tons were used in reclamation applications. Twenty years ago, almost all coal ashes were either bottom ash or fly ash. With the shift to new emission control technologies at power plants, large volumes of new products are being generated. Many of these new products are not suited to traditional ash applications (such as cement additions), so other uses have been investigated. Filling of mine voids has the potential to dispose of substantial quantities of CCPs, and state and federal policies encourage the beneficial use of CCPs. Beneficial uses in mining include acid drainage control, subsidence control, and soil reconstruction. States such as Pennsylvania and West Virginia have developed policies that define and regulate beneficial use of CCPs for coal mine reclamation. These successful policies will be summarized.

Coal combustion products are grouped into four main classes: 1) Class F, 2) Class C, 3) Fluidized Bed Combustion, and 4) Flue Gas Desulfurization. Class F and C ashes are produced in large coal boilers where pulverized coal is injected as fuel. These two ash types still comprise the bulk of CCPs produced in the U.S. They are distinguished by their free lime (CaO) content. Class F ashes have less than 10% lime, while Class C ashes have more than 10% lime. Nearly all ashes produced by coal boilers in the eastern U.S. are Class F, while those burning western U.S. coal are typically Class C. Table 1 shows typical chemical compositions for both Class F and Class C ashes.

Fluidized Bed Combustion (FBC) ashes and Flue Gas Desulfurization (FGD) solids (also sometimes referred to as sludges) result from relatively new, clean coal technologies. Both use lime or limestone (CaCO₃) to generate CaO to capture sulfur oxides in the boiler exhaust gas stream. FBC ashes are produced when high sulfur coal and coal waste materials (gob, coal partings or binders, black shales, etc.) are burned with limestone in a fluidized bed boiler. Sulfur oxides are precipitated as gypsum (CaSO₄) along with unreacted lime in a strongly alkaline ash (typically 25 to 30% free lime). Flue Gas Desulfurization solids are produced when lime or limestone slurries are injected into the exhaust gas downstream of the boiler. Sulfur oxides are precipitated either as gypsum or calcium sulfite (CaSO₃). Some utilities combine FGD solids with fly ash to increase the solids content, so FGD solids may or may not contain fly ash. In either case, sulfites may then be converted to gypsum by forced oxidation.

Currently, 25 million tons of FGD solids are produced each year with 9% of that total being beneficially used in reclamation. The remainder is landfilled. FGD solids normally have little inherent lime. However, they are often amended with lime (CaO) for solidification, but if not amended, they have the consistency of a thin paste.

This report will only discuss the first six scenarios since soil reclamation is an agricultural application.

Table 1. Typical composition of Class F and C ashes as defined by ASTM (1997).

Parameter	Class F	Class C
SiO₂	54.9%	39.9%
Al₂O₃	25.8%	16.7%
Fe₂O₃	6.9%	5.8%
CaO	8.7%	24.3%
SO₃	0.6%	3.3%
Moisture content	0.3%	0.9%
Loss on Ignition (LOI)(@750C)	2.8%	0.5%
Available alkalies as Na₂O	0.5%	0.7%
Specific gravity	2.34	2.67
fineness, retained on #325 mesh sieve	14%	8%

Mine environments are complex, and any given mine will contain zones of high groundwater flux and nearly stagnant areas. Mine groundwater can be oxidizing or reducing. Reducing conditions are often found in saturated zones, while unsaturated zones tend to be oxidizing. Some metals and anions of elements tend to be more soluble under reducing conditions.

Mine groundwater also varies according to its acidity/alkalinity. Many mine waters, particularly in the eastern U.S., are slightly to strongly acidic with significant concentrations of iron, aluminum and manganese. These ions are more soluble in acid conditions and alkalinity from CCPs can be used to neutralize acid mine drainage. The resulting metal hydroxides formed through neutralization will scavenge trace elements, such as arsenic and zinc, from the water.

In a given underground mine, one might encounter acid/oxidizing, acid/reducing, alkaline/oxidizing and alkaline/reducing conditions. Care must be taken to ensure that CCPs are matched to zones which take advantage of their beneficial properties and which minimize their exposure to conditions that will mobilize toxic element concentrations.

The CCPs can have permeable or impermeable properties. At one end of the spectrum, bottom ashes have the hydraulic conductivity of gravel, while most fly ashes have the hydraulic conductivity of a silt-textured, soil-like material. Class F ashes tend to be more permeable than Class C ashes due to the texture differences and some Class C ashes have a high enough lime content to form very weak cements. At the opposite extreme, FGD solids and FBC ashes have very low hydraulic conductivity and are virtually impermeable like strong cement.

Nearly all CCPs contain soluble and insoluble salts. If exposed to water, soluble salts in the ash and attached to its surface will dissolve in the water. On the other hand, sulfate salts and calcium or magnesium carbonates in the ash may not be dissolved because their concentration may already be high in the water. It is not unusual to find mine waters that are already saturated with respect to gypsum or calcium carbonate. In such cases, little or no net dissolution will occur. Care should be taken that CCPs containing substantial amounts of soluble salts are not placed in areas where significant groundwater flux occurs.

The quality of CCPs must also be tested when used in mine applications according to West Virginia’s coal ash policy (13 Jan 1998). For example, beneficially used CCPs must pass the USEPA’s Test Methods for Evaluating Solid Waste, SW-846, Method 1311 (Toxicity Characteristic Leaching Procedure or TCLP) for non-organics. They must also have at least 0.5% alkalinity (calcium carbonate equivalent) and be applied at a rate needed to treat any acidity that could be generated by the acid-producing rock. The latter is calculated by the following formula:

The West Virginia ash policy calls for a 10% safety factor. Hence, the total is multiplied by 1.1.

As an example, assume 1000 tons of waste rock containing 1% S is to be neutralized with limestone (100% NNP). So the equation shows that about 34.5 tons of the alkaline material (100% limestone in this case) would be required to neutralize this 1000 tons of rock with the 10% safety factor.

Under Pennsylvania’s Certification Guidelines for Beneficial Uses of Coal Ash (30 Apr 1998), beneficial ash applications include:

Pennsylvania also requires leaching tests prior to approval of beneficial uses for CCPs. Extracts from the USEPA’s Test Methods for Evaluating Solid Waste, SW-846, Method 1312 (Synthetic Precipitation Leaching Procedure or SPLP) are evaluated prior to approval of beneficial use. Table 2 summarizes the concentrations of elements by West Virginia and Pennsylvania test methods.

The Maryland Department of Natural Resources Power Plant Research Program and the Maryland Department of the Environment initiated a project in 1995 to demonstrate the use of CCPs for acid mine drainage abatement in an underground mine (Rafalko et al., 1999). The strategy was to completely fill the mine voids and replace mine water with CCP grout. The demonstration occurred at the Frazee Mine on Winding Ridge, near Friendsville, Maryland. The mine was abandoned in the 1930s and has produced acid drainage for decades.

Table 2. Comparison of West Virginia and Pennsylvania standards for CCP leachate concentrations.

By filling the mine voids, the grout was intended to minimize contact between groundwater and pyrite remaining in the mine. A grout was developed consisting of solid phase CCPs with acid mine water used for slurry makeup water. The grout was injected into both dry and inundated portions of the mine.

The grout consisted of FGD material and Class F fly ash from Virginia Power Company’s Mount Storm power plant and FBC ash from Morgantown Energy Associates’ Morgantown power plant. The FGD material, containing mostly calcium sulfite and calcium sulfate and no free lime, was used as an inert filler. The Class F ash was used as a pozzolon, while the FBC ash was used as the cementing agent. The grout contained approximately 60% fresh FBC (<24 hours old), 20% FGD, and 20% Class F fly ash. The FBC ash arrived from the power plant containing about 15% moisture. The final design mix yielded 8 inches of spread using ASTM PS28-95, and a 28-day unconfined compressive strength of 520 pounds per square inch (psi) as determined by ASTM C39-94.

Prior to injection, the grout was subjected to the Toxicity Characteristic Leaching Procedure for non-organics. None of the element concentrations in the leaching solution exceeded their respective regulatory limits for characterization as a hazardous waste.

During fall 1996, more than 5,600 cubic yards of grout were injected into the mine. The original design was for 3,900 cubic yards, but additional void space was encountered in the mine and also grouted. During the injection, it became apparent that the Frazee Mine was larger and more complex than originally determined during the mine characterization phase. As a result, the mine was not completely filled and the mine continues to produce acid mine drainage.

The mine’s discharge pH remained around 3.0 during and after grout injection, while Ca, Na, and K concentrations increased by nearly an order of magnitude (Aljoe, 1999). Sulfate, Cu, Ni, Zn, and Cl all nearly doubled with both Ni and Zn in excess of water quality discharge limits. Both Ni and Zn had exceeded water quality limits prior to injection. Two years after injection, however, concentrations of both Ni and Zn were at or slightly above pre-injection levels (Table 3).

In September 1997, nine bore holes were drilled into the Frazee Mine to recover grout material. The bore hole locations targeted wet and dry sections of the mine. The grout samples from core drilling were submitted to the laboratory for testing of density, permeability (hydraulic conductivity), and unconfined compressive strength. In general, the grout showed little sign of in-situ weathering and displayed good mine roof and pavement contact. Grout cores after one year yielded permeabilities between 1.89x10^-6 and 6.02x10^-8 cm/sec. Grout from one core matched the target compressive strength in the 28-day laboratory test. The other grout cores all had approximately twice the strength achieved in the laboratory after 28 days.

The behavior of calcium and sulfate after injection was significantly different than that of acidity, iron and aluminum. Calcium concentrations increased three to six times and remained at these levels for more than 16 months after injection. Sulfate levels remained at about twice the pre-injection level. These increases in calcium and sulfate are due to the dissolution of these ions from the injected FBC and FGD materials. Trends in sodium, potassium and chloride concentrations were similar to those of calcium. It is likely that their elevated concentrations result from some grout dissolution.

Prior to injection, the grout itself was subjected to a TCLP. The results showed that arsenic and barium were found at levels of 0.13 and 0.11 mg/L, respectively. Post-grouting quality of the mine discharge did not detect these constituents (the detection limit for arsenic in the mine water was 0.2 mg/L, but the detection limit for barium is one order of magnitude below the TCLP result). The data showed that with the exception of a short term increase in Ni and Zn, no high levels of elements were leached from the ash even though the ash is dissolving due to acid attack.

The grout was placed under nearly worst case conditions in this application. There was insufficient grout to neutralize acid in the mine water, so the grout was subjected to continuous weathering by pH 3.0 water. Further, the flow of this water through the mine was unhindered. The objectives of such mine grouting projects are to occlude voids and eliminate mine drainage. This project, however, represents a case where this objective was not achieved and the grout was subject to a high flow, chemically aggressive mine water.

In December 1996, Metikki Coal Corporation began injecting a mixture of non-fixated (non-hardening and low lime content) FGD solids, acid mine drainage metal precipitates (floc), and fine coal refuse into its underground coal mine near Redhouse, Maryland. Materials are mixed in a specially designed building with slurry water. The slurry is injected into the mine at about 15% solids content. There is some unreacted lime in both the FGD and the floc, which would dissolve in the thin slurry. The mixture is injected into inactive sections of the mine and to date about 320,000 tons of this mixture have been injected. The slurry is injected at the low point in the mine and displaces an otherwise acid mine pool. The FGD solids are not fixated, so they are not expected to solidify. On the other hand, since the slurry is placed in the low point of the mine and well below regional drainage, the mine pool is expected to be relatively stagnant. Thus, stratification of water above the slurry mixture is likely to occur with minimal mixing. Water has been sampled and analyzed both prior to and after injection of the slurry and these data are summarized in Table 4. Chloride was expected to be the most sensitive ion as the FGD solids have between 10,000 and 30,000 mg/L Cl. Since chloride is an anion and extremely soluble, it has been monitored closely. Maryland set a discharge limit of 860 mg/L on chloride.

Chloride concentrations remain well below the Maryland limit of 860 mg/L, averaging about 120 mg/L. This is, however, above the pre-injection level of 3 mg/L. Other than a 30% increase in sulfate concentrations, the injection has had little effect other than to increase the alkalinity in the mine pool. This has caused the pH to increase from about 3 to 4.5, while Al and Fe have both dropped substantially. Prior to discharge, mine water is treated in a high-density lime treatment system and discharged through a polishing pond to the NPDES monitoring point. Trout are successfully raised in the polishing pond.

Table 3. Summary of pre- and post-injection water quality at the Frazee Mine, Friendsville MD.

Case Study 3. Clinton County, PA. Surface Mine Grouting and Capping for AMD Control.

Between 1974 and 1977, a 37-acre surface coal mine was mined and reclaimed in Clinton County, Pennsylvania. Pyritic pit cleanings and refuse were buried in the backfill, producing severe acid mine drainage. The pyritic material was located in discrete piles or pods within the backfill. The pods and initial contaminant plumes were identified using geophysical techniques confirmed by drilling.

Three approaches were taken to abate acid mine drainage. On one area of the mine, a direct injection of an FBC ash grout into and around the pyritic pods was conducted. On a second area, the affected area was capped with FBC ash. On a third area, a combination of the first two approaches was conducted. The first approach was initially attempted at every acidic pod. If the pod was unable to accept the grout, the area directly above the pod was capped to minimize contact between surface water and pyritic waste. In several cases, the area around the non-receptive pod was grouted to divert groundwater flow. The project has been described in detail by Schueck et al., 1996.

For performance monitoring, 42 wells were drilled on and adjacent to the site. Well locations were guided by the results of geophysical mapping techniques. Wells located on the site were drilled through the spoil to the pit floor, while wells located adjacent to the site were drilled to the unmined lower split of the Lower Kittanning coal seam. The initial drilling confirmed the locations of the pods previously identified by geophysical methods.

Table 4. Water quality data from Metikki FGD underground injection. All values in mg/L.

Pressure grouting resulted in reductions of acid mine drainage. Acidity from the pods was reduced by 23 to 52%. Significant reductions in trace metal (Cd, Cu, and Cr) concentrations from 42 to 88% were also observed. Wells down gradient of the grouted pods exhibited 16 to 37% reductions in mean concentrations of the common acid mine drainage parameters. The exception was sulfate, which remained unchanged. Significant trace metal reductions were also noted in down gradient wells.

Where a surface cap of FBC ash was applied, results were mixed. Decreased infiltration from the cap may have abated some of the acid mine drainage in the upper portion of the pod, but the lateral flow of water along the pit floor was sufficient to create acid mine drainage. Wells down gradient of capped pods displayed significant reductions in mean concentrations of acid mine drainage parameters (29 to 34%).

Where both grouting and capping were employed, significant decreases in mean concentrations (42 to 64%) of acid mine drainage parameters were measured. The data suggested a reduction in acid mine drainage production within the pod and reduced migration of mine drainage down gradient of the pods.

The pods, which were treated by injection and capped, produced the most favorable results, followed by injection only. Capping alone produced the least favorable results. The combined approach inhibits contact among water, oxygen and pyrite by limiting infiltration and diverting lateral flow around the pods. Injection limits contact via lateral flow, but vertical infiltration may not be inhibited.

Although percent reductions in mean concentrations varied, concentrations of acid mine drainage parameters generally decreased by 30 to 40% and the reduction of trace metals was typically higher. This is significant given that only 5% of the site was grouted. Any change in water quality is expected to be permanent due to the pozzolanic nature of the FBC grout. It was known that the entire site generated acid mine drainage and there was no intention of eliminating acid mine drainage production. The objective was to prove the effectiveness of the FBC in reducing pollutant loads discharging from the site, while evaluating the potential for decreasing concentrations of toxic elements. Table 5 summarizes pre- and post-CCP monitoring data at well T-34, which is down gradient of a section of the mine that had been capped and grouted with FBC ash.

Despite less than total success at acid mine drainage abatement, the investigators concluded that injection grouting is a viable acid mine drainage abatement technique worthy of application on sites with certain criteria. The technique would be most appropriate at reclaimed surface mines where the spoil is net alkaline, but where improper placement of acidic materials (pit cleanings or refuse) resulted in an acidic discharge. In addition to reclaimed sites, FBC ash is recommended on active surface mines and refuse disposal sites as a preventative measure. FBC ash can be directly applied to or mixed with refuse and pit cleanings to create monolithic structures capable of diverting water away from pyritic materials.

Table 5. Pre-grouting mean water quality at the Clinton County, PA spoil site capped with FBC ash. In addition, an FBC ash grout was used to isolate pyritic pit cleanings from groundwater. All values in mg/L.

Case Study 4. Chaplin Hill Mine, WV. FBC Ash for Pit Floor Sealing and Surface Capping

At the Chaplin Hill Coal Mine near Morgantown, WV, a series of surface mine pits were treated with FBC ash to control acid mine drainage. Pits in the same geological sequence had historically produced acid mine drainage due to a pyritic pit floor and pyritic units within the overburden. In 1991, the company adopted the practice of laying a 1-ft-thick layer of FBC ash on the pit floor and compacting it prior to backfilling. In addition, another 1-ft lift of FBC ash was placed on the graded spoil and compacted prior to topsoil application. All pits thus treated have not generated acid mine drainage and do not require water treatment. Table 6 summarizes the water quality from pits completed prior to FBC ash application and after ash application. The data indicate elimination of acid mine drainage with no significant increase in toxic element concentrations.

Table 6. Summary of pre- and post-CCP application water quality at the Chaplin Hill Mine, Morgantown WV. The data are for samples taken and analyzed by Anker Energy Corporation and reported to the state of West Virginia. All values in mg/L.

A project investigated by Paul et al. (1996) introduced 150 tons of FBC ash into a two-million-gallon pond of pH 2 mine water. The pond was carefully monitored during and after the dose of FBC ash. Iron and aluminum precipitated and the pH rose. Metal concentrations in the water fell about an order of magnitude. No toxic metal contamination from the ash was detected. The same result was observed for arsenic, which can be mobilized by acidic conditions even though the solubility of arsenic decreases very little as water is neutralized. This experiment indicates that acidified mine waters already containing toxic metals can be precipitated by FBC ash treatment of the water. The ash effectively neutralized the acid and metals in the water, but also scavenged other metals from the water as metal hydroxides formed and precipitated.

The use of CCPs in reclamation has been beneficial in some settings, neutral in others, and harmful in some other settings. While each setting and CCP form a unique set of circumstances requiring individual analysis and evaluation, several generalizations can be made.

Non-fixated FGD materials contain almost no neutralization potential and are presently not very useful in mine land reclamation. The non-fixated materials typically exhibit a high permeability, as well. However, fixated FGD contains excess alkalinity with low permeability. Fixated FGD materials can be useful in acid mine drainage abatement, subsidence control, and used as a barrier material to encapsulate acidic materials or seal pit floors on surface mines. Both materials can contain high chloride levels that are concentrated in the FGD units.

Aljoe, W. 1999. Hydrologic and water quality changes resulting from injection of CCB grout into a Maryland underground mine. Pp 66-1 to 66-12. In. Proceedings, 13^th International Symposium on Use and Management of Coal Combustion Products, Volume 3 (TR-111829-V3), January 1999, Orlando, Florida.

American Coal Ash Association, Inc. 1998. Coal Combustion Product (CCP) Production and Use – 1997 in the USA. Alexandria, VA.

American Society for Testing and Materials. 1997. Standard Specification for Coal Fly Ash and Raw Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete. ASTM C 618-97.

Paul, B., S. Chaturvedula, and B. Newton, "Use of coal combustion by-products for reclamation: environmental implications. Pp. 137-142. In: Proceedings, Coal Combustion By-Products Associated with Coal Mining, Interactive Forum, 29-31 October 1996. Lexington, KY.

Rafalko, L., and P. Petzrick. 1999. The Western Maryland coal combustion by-product/acid mine drainage initiative, the Winding Ridge Project. Pp. 70-1 to 70-16. In. Proceedings, 13^th International Symposium on Use and Management of Coal Combustion Products, Volume 3 (TR-111829-V3), January 1999, Orlando, Florida.

Shueck, J., M. DiMatteo, B. Sheetz, and M. Silsbee. 1996. Water quality improvements resulting from FBC ash grouting of buried piles of pyritic materials on a surface coal mine. In. Proceedings, Seventeenth Annual West Virginia Surface Mine Drainage Task Force Symposium, 2-3 April 1996, Morgantown WV.

	Maximum Acceptable Leachate Concentrations (mg/L)
State:	West Virginia	Pennsylvania
Test Method:	TCLP	SPLP
Al		5.0
Sb	1	0.15
As	5	1.25
Ba	100	50
Be	0.007
B		31.50
Cd	1	0.13
Cr	5	2.5
Cu		32.5
Fe		7.5
Pb	5	1.25
Mn		1.25
Hg	0.2	0.05
Mo		4.38
Ni	70	2.5
Se	1	1.00
Ag	5
Tl	7
Zn		125
SO4		2500
Cl		2500

		EPA
RCRA	TCLP	Drinking	Pre-CCP	Post-CCP
Element	Limit	Water)	n=18	n=15
Sb	1	0.006	<0.2	<0.2
As	5	0.05	<0.2	<0.2
Ba	100	2	0.029	<0.02
Be	0.007	0.004	<0.02	<0.02
Cd	1	0.005	<0.02	<0.02
Cr (6+)	5	0.1	0.03	0.04
Pb	5	0.015	<0.02	<0.02
Ni	70	0.01	0.62	1.13
Se	1	0.05	<0.5	<0.5

Al			37	56
Ca			25	223
Cl			2.3	7.3
Co			0.3	0.5
Cu			0.08	0.25
Fe			67	67
Mg			26	32
Mn			2.7	2.8
K			0.9	13.3
Zn			1.4	2.3
Na			1	8
SO4			564	1182

RCRA Element	TCLP Limit	EPA Drinking Water	Pre-CCP injection Tons added 0	Post-CCP injection Tons added 51,716
Sb	1	0.006	<0.05	<0.05
As	5	0.05	<0.025	<0.025
Ba	100	2	0	0.03
Be	0.007	0.004	<0.002	<0.002
Cd	1	0.005	<0.002	<0.002
Cr (6+)	5	0.1	<0.007	<0.007
Pb	5	0.015	<0.025	<0.025
Hg	0.2	0.002	na	na
Ni	70	0.01	0.14	0.19
Se	1	0.05	na	na
Ag	5		<0.002	<0.002
Tl	7	0.002	<0.13	<0.13

Al			0.4	1.3
Ca			224	541
Cl	860		2.2	200
Co			0.1	0.14
Cu			0	0.01
Fe			39	34
Mg			50	84
Mn			2.7	4.8
K			7.4	10.2
V			<0.005	<0.005
Zn			na	0.27
Na			77	79
SO4			830	1346

RCRA Element	TCLP Limit (mg/L)	EPA Drinking Water	Pre-CCP n=7	Post-CCP n=14
Sb	1	0.006
As	5	0.05	0.18	0.04
Ba	100	2	0.03	0.04
Cd	1	0.005	0.130	0.006
Cr (6+)	5	0.1	0.43	0.04


Other Ions
Al			425	28
Ca			76.4	42.7
Cu			1.84	0.08
Fe			1193	124
Mg			87	14
Mn			63	50
Zn			7.5	0.6
Na			1.3	3.7
SO4			5513	430

		EPA
RCRA	TCLP	Drinking	Pre-CCB	Post-CCB
Element	Limit	Water
Sb	1	0.006	0.94	0.40
As	5	0.05	1.28	<0.1
Ba	100	2	<0.1	<0.1
Be	0.007	0.004	0.96	<0.1
Cd	1	0.005	<0.1	<0.1
Cr (6+)	5	0.1	0.0001	0.0001
Pb	5	0.015	0.72	<0.1
Hg	0.2	0.002	<0.0005	<0.0005
Ni	70	0.01	1.16	<0.1
Se	1	0.05	1.29	<0.1
Ag	5		<0.1	<0.1
Tl	7	0.002	2.68	1.21

Al			36	<0.1
Ca			450	750
Fe			4	<0.1
Mg			296	450
Mn			47	0.2
SO4			2022	1500