Pennsylvania State University
Materials Research Laboratory
University Park, Pennsylvania 16802
Pennsylvania Department of Environmental Protection - District Mining Operations
5 West Laurel Boulevard
Pottsville, Pennsylvania 17901-2454
Pennsylvania Department of Environmental Protection - Bureau of Mining and Reclamation
P. O. Box 8461
Harrisburg, Pennsylvania 17105-8461
The beneficial use of coal ash in mine reclamation and mine drainage pollution abatement projects in Pennsylvania has greatly increased and improved during the past ten years due to regulatory incentives developed by the Pennsylvania Department of Environmental Protection (DEP) (formerly Department of Environmental Resources - DER), and due to significant innovation and cooperation in technology development and coal ash utilization by the coal industry, the power production industry, and researchers from universities, industry and government agencies including DEP. Prior to 1986 a separate Waste Management Permit was required by DER for the use of coal ash in backfilling and reclaiming a surface mine site, even if the ash placement area was within the boundaries of a Surface Mining Permit. In order to eliminate the dual permitting requirements and promote mine reclamation, policies and procedures were developed to authorize the Bureau of Mining and Reclamation (BMR) and District Mining Operations (DMO) of DEP to issue permits for the use of coal ash in reclaiming active and abandoned pits and other abandoned mine lands, providing that these areas are included within Surface Mining Permit boundaries, and that the ash placement activities are conducted in accordance with the Solid Waste Management Act (Act 97 of 1980) and the regulations administered by the DEP Bureau of Waste Management and Land Recycling (BWM).
While coal ash was previously handled as a type of residual waste in 25 PA Code Chapter 75 of DER regulations, specific new regulations for the beneficial use of coal ash were promulgated by the Environmental Quality Board in 1992 and are set forth in Sections 287.661 through 287.666 of 25 PA Code Chapter 287 (see Pa. Bulletin, Vol. 22, No. 27, July 4, 1992). Some key elements of these regulations concerning physical and chemical requirements, ash placement requirements and ranges of acceptable beneficial uses of coal ash will be briefly described in later sections of this paper. The result of these regulatory developments is that a significant portion of the coal ash generated in Pennsylvania annually, no longer goes to landfills, slurry impoundments or ash monofill piles/structures; instead it is beneficially used in mine reclamation and mine drainage pollution abatement projects throughout the Anthracite and Bituminous Coal Regions. This utilization is a win/win scenario with significant environmental, social and economic benefits.
Approximately 10 million tons of coal ash are produced or disposed of in Pennsylvania annually. According to the Pennsylvania Electric Association (PEA) 4.5 to 4.8 million tons of coal ash are produced from the combustion of 38 to 39 million tons of coal by Pennsylvania's coal fired power plants (Biden, 1997). The Anthracite Region Independent Power Producers Association (ARIPPA) reports that in 1995 approximately 3.4 million tons of coal ash were produced from the combustion of 5.6 million tons of anthracite culm in circulating fluidized bed cumbusters, while approximately 1.5 million tons of coal ash were produced from the combustion of 2.3 million tons of bituminous coal waste, for a total of 4.9 million tons of coal ash from Pennsylvania's cogeneration plants. In addition to the coal ash accounted for by the PEA and ARIPPA statistics above, it is estimated from DEP permit file records, that approximately 400,000 tons of coal ash is transported annually to sites within the Anthracite and Bituminous Coal Regions for disposal or beneficial use from a variety of sources including coal fired utilities from other states.
In 1995 approximately 6.75 million tons of coal ash were beneficially used for backfilling, acid mine drainage pollution prevention and abatement, and reclamation activities within the operations associated with District Mining Operations in Pennsylvania. The amount increased in 1996 and will probably continue to increase as other coal ash sources are beneficially utilized within the state of Pennsylvania.
The magnitude of abandoned mine lands problem in Pennsylvania is enormous. Inventories of abandoned mine lands maintained by the DEP Bureau of Abandoned Mine Reclamation (BAMR) show that there were 175,000 acres of unreclaimed abandoned mine land in Pennsylvania before the enactment of the Federal Surface Mining Control and Reclamation Act (SMCRA) in 1977. DEP water quality records demonstrate that acid mine drainage (AMD) is the most serious water pollution problem in Pennsylvania, and that as of 1971, more than 2,500 miles of Pennsylvania's streams had been polluted by coal mine drainage (DER, 1971; Hornberger, et al 1990). In the preamble to DEP remining regulations, it was stated, "In all likelihood, government funded reclamation of abandoned mine lands will not solve the estimated $15 billion in environmental problems caused by past mining in the Commonwealth." (PA Bulletin, Vol. 15. No. 26, June 29, 1985, p. 2379).
Federal and state funding of abandoned mine reclamation and AMD pollution abatement projects has been significant and highly successful to date, but the major portion of the work remains to be completed. For example, BAMR cost records show that construction cost alone for 2 categories of projects completed between 1980 and 1996 exceed $202 million for 935 completed projects including mine fires, mine subsidence control, AMD abatement and mine hazards; while the estimated costs of sites not reclaimed exceeds $987 million for 2,455 sites in the same categories. In 1967, the Commonwealth of Pennsylvania issued bonds to establish a $500 million Land and Water Conservation and Reclamation Fund, and in 1970, $200 million of that was appropriated for the elimination of land and water scars created by past coal mining practices though a program called Operation Scar Lift. Recently, the last of those funds have been used by DEP. Since 1977, Federal funds from tonnage fees paid by active mine operators under SMCRA requirements are collected and distributed to state programs by the U.S. Office of Surface Mine Reclamation and Enforcement (OSM). From 1980 through 1996, OSM allocated approximately $426 million to Pennsylvania for abandoned mine reclamation projects administered by BAMR. Pennsylvania's share accounts for approximately 18% of the total funds allocated throughout the United States, and the funds may expire in 2004.
Consequently, a very large portion of Pennsylvania's abandoned mine land problems must be resolved by DEP and the active mining industry through remining operations, where operators receive regulatory incentives to mine and reclaim previously mined areas. The beneficial use of coal ash in mine reclamation and AMD pollution abatement activities at these remining sites and other areas authorized by 25 PA Code Chapter 287 is a very significant component in solving these problems. In addition, DEP will continue to authorize and promote the beneficial use of coal ash in abandoned mine reclamation and AMD pollution abatement projects in areas not covered by mining permits through coordination and innovation among BMR, DMO, BAMR, and BWM programs. The body of this paper features existing and proposed examples of a variety of coal ash reclamation and AMD abatement projects throughout the Anthracite and Bituminous Coal Regions of Pennsylvania, plus provides a description of the physical and chemical properties of coal ash, the key regulatory requirements and the range of beneficial uses of coal ash in Pennsylvania.
Ash Properties: Physical and Chemical
Coals by the very nature of their formation contain foreign mineral matter incorporated into their matrices. As the coal is combusted, this mineral matter transforms to fly ash and is thermally altered to forms, many of which are by themselves chemically very reactive or can which can be chemically activated. The term used to describe this behavior is "pozzolanic." Fly ash has found many uses which are based upon both the bulk chemical and mineralogical makeup of the ash and upon the physical size distribution and shape of the ash. Table 1 summarizes the more important chemical and physical characteristics of fly ashes that enhance their usefulness.
In addition, the processing and handling/storage of fly ashes can have a significant impact upon their pozzolanic reactivity. Likewise, the combustion process through which the coal is processed will significantly impact the chemical and physical properties of the fly ash. Figure 1 summarizes the thermal alteration that occurs for common coal minerals for combustion in fluidized bed combustors and in pulverized coal combustors. In the former combustion process, thermal alteration of the mineral assemblage at approximately 800o C mainly results in the dehydroxylation of the clays into chemically very reactive "meta-" forms. When coupled with calcium oxide, anhydrite readily reacts to form a plaster-ettringite cementitious system which will continue to react with the meta-clays to form C-S-H which will result in long stability of the solidified product. In contrast, the pulverized coal (PC) combustion process which occurs at approximately 1450oC results in nearly an identical mineralogical assemblage, minus the lime from the bed support, but with the exception that the clays will be converted into a glass; it is this glass that possesses the pozzolanic character that is responsible for its cementitious reactivity. PC fly ashes exhibit a chemical reactivity that is directly correlated to the calcium content of the ash.
Pulverized Coal Fly Ash
The bulk chemical composition of fly ashes can generally be correlated with the rank of the coal that was the source of the ash. Western lignitic and sub-bituminous coals are in general more calcium rich when contrasted to eastern bituminous and anthracites which tend to be more silica and alumina rich. Figure 2 summarizes these trends showing the variation in lime content a variety of coals. McCarthy et al. (1990) presented analytical results for a range of eastern and western coals also supporting this observation, Table 2.
The American Society for Testing of Materials (ASTM, 1980) recognizes two classifications of pulverized coal fly ash based upon the relative concentrations of silica alumina and iron oxide which correspond to a high- and low-lime content fly ash, Class C and F, respectively.
The bulk chemical composition and, subsequently, the resulting phase assemblage in this range of fly ashes reflect the different paragensis of the coal, both temporally and spatially. Because of the differences between low- and high-rank coals, a significant difference exists in the mineralogical content of the derived fly ashes. The higher-rank coals with typically low CaO content consist of a much simpler phase assemblage of crystalline phases and a larger proportion of an alumina substituted silica-rich glassy phase. In contrast, the lower-ranked coals consist of a complex array of crystalline phases numbering as many as 15 to 20, including a glassy phase that is more alumina-rich and silicia-poor (Scheetz et al., 1985). Table 3 contrasts the more commonly identified crystalline phases in both ashes.
Fluidized Bed Combustor Ash
The use of atmospheric pressure and pressurized fluidized bed combustors (FBC) has increased and is likely to continue to play a role in power generation because of their reduced SOx emission characteristics. These types of combustors operate at temperatures significantly lower than conventional pulverized coal power facilities; 800oC vs. 1450oC. In their operation, the fuel is introduced into a bed of granulated limestone or dolomite which is suspended in a stream of forced air. The bed materials constantly circulate within the closed loop facility where the coal is burned and the heat extracted. The ash derived from these facilities is composed of largely calcined limestone, the mineral assemblage from the coal minerals and the reaction by-products between the sulfur dioxide emissions from the coal and the thermally altered bed materials. Construction of FBC facilities as co-generation technology in which the fuels are derived from old coal processing wastes is a maturing application of the technology. Table 4 details the bulk chemical compositions for FBC ash derived from burning high-BTU coal, from anthracite derived culm and from bituminous derived gob, both from Pennsylvania.
From this summary, it is clear that in the FBC facilities that burn high-BTU fuels, substantial amounts of the ash is composed of lime from the bed materials which are maintained high to act as a sulfur removal media. The two wastes burning ashes, however, exhibit much different chemistries. The bituminous gob burner most nearly approximates the high-BTU combustor ash reflecting the high sulfur content of the gob. By contrast, the culm burner sees very low sulfur and does not require such high amounts of lime. In this latter case, the bed inventory is principally composed of granulated rock. As an example, facilities in the anthracite fields of Pennsylvania have been reported to operate on a 2500 BTU/pound fuel for which 200 tons per hour are introduced into the combustor and 160 tons per hour of ash are removed (Martin, 1993).
The mineralogy of the FBC ashes is dominated by the limestone in the bed materials from those systems where high lime is added. The bed mineralogy also reflects the interaction of the decompositon products of coal sulfur with the lime to form anhydrite. Also, the mineralogy will be significantly affected by the post-combustion handling and storage conditions. Table 5 summarizes typical minerals identified in the various types of FBC ash.
The mineralogy is as important to the formation of cementitious grouts from these ashes as is the bulk chemistry. The bed inventory for those systems introducing sufficient amounts of lime contain mostly reaction products between CaO and SOx to form anhydrite (CaSO4); atmospheric moisture and carbon dioxide to form portlandite (Ca(OH)2) and calcite (CaCO3), respectively; and with anhydrite and aluminate-rich altered clays from the mineral matter of the coal to form, in the presence of moisture, ettringite (Ca6 Al2(SO4)3.32H2O). Quartz is ubiquitous in all of these ashes from both the rock and as a major impurity in the coal.
The culm burners possess a relatively simple phase composition of crystalline materials; simply quartz. Although significant amounts of other materials are present, they are not in the form of crystalline materials and also not as glasses, but as x-ray amorphous, dehydroxylated clays, in the form of meta-illites.
Fly vs. Bottom Ash
Both PC and FBC coal burners produce residual ash both as coarse bottom ash and as the more conventional fine fly ash. Many facilities separate their ash and treat it as two waste streams; many merely mix the ashs and treat the mixture as a single waste stream. The relative proportions of fly ash to bottom ash in a puliverized coal combustion unit is approximately 80/20. In fluidized bed combustors that burn high-BTU fuels this ratio is also maintain but can vary to as much as 60/40 to 40/60 for those units burning mine refuse.
The size distribution from fly ashes typically ranges from a few microns to as large as 200 microns with the median diameters in the 20 to 60 micron range. Bottom ash represents agglomerates and hence are much coarser. As an example, Table 6 is a typical screen analysis for a combined ash from an FCB unit composed of a ratio of 60% fly ash 40% bottom ash.
From the stand point of chemical reactivity, the bottom ash is inert, mainly attributable to its relatively low surface area. The relative inertness finds applications of this component of the ash stream as traction
materials, lightweight aggregate in concretes and as aggregates in road sub-base construction. In the remainder of this paper, the use of the terms coal ash and flyash is interchangeable and typically includes some proportion of bottom ash combined with the flyash.
Most sites where coal ash is beneficially used in mine reclamation and AMD pollution abatement in the Anthracite and Bituminous Coal Regions will have to comply with the provisions of four Pennsylvania laws: the Solid Waste Management Act (P.L. 380, No. 97., 35 P.S. § 6018.101 et seq.), the Surface Mining Conservation and Reclamation Act (P.L. 1198, 52 P.S. § 1396.1 et seq.), the Clean Streams Law (P.L. 1987, 35 P.S. § 691.1 et seq.) and the Coal Refuse Disposal Act (P.L. 1040, 52 P.S. §30.51 et seq). Pursuant to these laws, the Pennsylvania Environmental Quality Board has promulgated numerous chapters of regulations contained in the Pennsylvania Code under Title 25. Environmental Protection. These regulations typically contain subchapters or sections which set forth the requirements for approval of permits and licenses, environmental protection performance standards, and procedures employed in inspection, enforcement and civil penalty assessment activities. The regulations most relevant to the use of coal ash in mine reclamation and AMD abatement are in 25 PA Code, Chapter 287 (specifically, Subchapter H. Beneficial Use). In addition, sections of the coal mining regulations in 25 PA Code, Chapters 86 to 90, are applicable, as are sections of DEP's water quality and air quality regulations.
In an effort to reduce regulatory program redundancy and simplify procedures for permitting and monitoring coal ash placement activities at mine sites, several key documents were developed in 1986 and revised by DEP since then. These documents are a Memorandum of Understanding on Coordination of Policy and Procedure between BMR and BWM, a BMR Program Guidance Manual section on Fly Ash/Bottom Ash Disposal at Active Mine Sites, and two mining permit modules which incorporate the requirements of 25 PA Code Chapter 287 and BWM policy and procedures within the mining permit. Module 25 is completed for the placement of coal ash at surface mine sites, and Module 27 is completed if the coal ash is used as a soil additive to enhance revegetation as part of the mine reclamation plan. In addition to the contents of these modules, any site-specific detailed requirements for ash placement and the monitoring of coal ash quality, ground-water and surface-water quality are set forth in the special conditions of the permit.
Without any need to specifically recite numerous pages of regulations, policies, permit modules and conditions on the physical and chemical requirements for the beneficial use of coal ash at mine sites, the fundamental physical requirements for the conventional placement of coal ash are summarized in the following four items, and the chemical requirements are shown in Table 7:
1) The ash must be delivered to the site within an acceptable moisture range, as determined by laboratory compaction tests [Standard Proctor(ASTM D 698) or Modified Proctor (ASTM D 1557)]. Figure 3 graphically depicts the relationship between optimum moisture content and the maximum laboratory dry density that can be achieved.
2) Site preparation and operations at the mine site must provide a minimum separation distance of 8 feet above ground-water level for ash placement. Similar separation or isolation distances are required between the ash placement deposit and adjacent mine highwalls, bottom-rock and other consolidated rock features at the mine site. These separation distances are all designed to maintain the ash in a relatively "high and dry" environment, away from significant ground-water flows and saturated conditions. Best available on-site materials may typically be used to achieve these separation zones, including mine-spoil and some coal refuse materials.
3) The coal ash must be compacted in layers or lifts not exceeding two feet thickness. Coal ash deposits of greater than 50 foot total thickness have been permitted in abandoned surface mine pits, wherein the 2 foot - compacted lifts of ash are placed and compacted with conventional surface mining equipment including bulldozers and rubber-tired loaders, although sheeps-foot rollers have been used occasionally.
4) Following the completion of ash placement, the coal ash must receive a top cover, typically 4 foot of soil material. The final grading of this cover material must achieve slopes equal to or greater than 3% in order to prevent ponding of surface water and promote positive drainage.
Table 7 summarizes the chemical analysis requirements for ash, leachate and groundwater samples. The regulations require the coal ash to have a natural or conditioned pH between 7 and 12.5 at the source. The regulations and permit modules also require the ash to be analyzed for all of the metals from aluminum to zinc shown in Table 7, except Calcium and Manesium, although no numerical limits are specified. The leachate and goundwater must be analyzed for the parameters shown in Table 7, wherein specific parameters not required for groundwater or leachate analysis are shown by cross-hatching. The numerical limits for leachate parameters shown in Table 7 are based upon allowable multiples of drinking water standards, as set forth in Module 25.
From 1986 through 1996, thirty (30) permits were issued by the Pottsville District Mining Office of the Department of Environmental Protection for the conventional placement of coal ash at anthracite coal surface mining sites. At least 12 of these sites are closely related to FBC ash production from the 9 operating cogeneration plants with the Anthracite Region. It is estimated that the five bituminous Region DEP District Offices in DMO have issued a total of 50 permits in the same 10 year time period, for conventional ash placement types of beneficial coal ash uses.
After at least 10 years of monitoring and inspection activities by the BMR/DMO program, DEP has not detected any significant off-site water pollution from the use of coal ash in mine reclamation, and has maintained an even longer record of ground-water and surface-water monitoring data for some ash types. DEP is now cautiously and carefully considering the approval of some beneficial uses of coal ash in some mine reclamation and AMD pollution abatement projects where the conventional ash placement requirements may be waived or modified in order to resolve site-specific reclamation logistics problems or demonstrate new technology, without compromising environmental quality. These special projects may be evaluated and approved by DEP through the use of site-specific agreements and no-cost contracts, or through the use of one-of-a kind Waste Management Demonstration Permits until the new technology and site-specific monitoring data are thoroughly evaluated to determine if more widespread use of these new approaches is justified.
For example, DEP has recently entered into an agreement and issued a Waste Management Demonstration Permit to authorize the slurry emplacement of FBC ash from an anthracite cogeneration plant into an adjacent water-filled abandoned surface mine pit, in order to reclaim the site of a recent drowning fatality, and save up to $28 million in backfilling and other reclamation costs (See the later section on the Shen Penn site for more detail). This proposed project will place wet ash in a wet environment in contrast to the conventional placement of relatively dry ash in a relatively dry environment. Two additional Waste Management Permit Applications are currently under review for different sites. One proposes to slurry FBC ash to a dry pit (wet to dry), and the other proposes to place relatively dry FBC ash below mine pool levels in another abandoned water-filled pit (dry to wet).
Range of Beneficial Uses
More than 4000 years ago the Romans had recognized the beneficial use of certain volcanic ashes when mixed with lime. They widely exploited the pozzolanic properties of the volcanic ash as a cementitious binding material in structural buildings, many of which are still in service to this day. The term "fly ash" can be traced to ca. 1930 where it was first used in the electric power industry. The first comprehensive data set on the use of fly ash, in this case in concrete, in North America was reported by Davis et al. 1937. The first major practical application in the United States was the construction of the Hungry Horse Dam by the Bureau of Reclamation in 1948 (Malhotra et al. 1990).
The United States generated, in 1993 approximately 88.5 million tons of combined fly and bottom ash on an annual basis with approximately 22% being used (ACAA, 1993). The largest usage of fly ash,~14%, is in portland cement and concrete. Fly ash has two principal applications: as an inexpensive substitute for the more costly portland cement, up to about 35% replacement, and as a reactive mineral admixture in portland cement and concrete formulations. In the former application, fly ash is also used in regions of the country where excessive summer time temperatures cause over heating of mass concrete pour because of it slightly slower hydration reaction and hence lower heats of hydration. In addition, the spherical nature of fly ash particles assists in improving the rheologic properties of concrete by acting like tiny ball bearings, which improve particle packing and reduce the water demand of the formulation (Earle, 1997).
Applications developed for the purpose of utilizing large volumes of fly ash have been explored in Pennsylvania in association with the reclamation of abandoned mine lands. These applications include structural fills where unconsolidated ash is being place into abandoned surface mines simply as a fill. Several variants of this application are permitted in Pennsylvania as demonstration projects were an ash slurry will be placed into a large water filled surface mine to displace the water. A similar demonstration will end-dump ash onto a staging platform from which it will be placed into standing water in a surface mine. Other demonstration projects are utilizing the ash, in the form of a cementitious, alkali-activated grout, as an infiltration capping material on reclaimed mine sites (Fontana, 1993; Scheetz et al. 1993; Scheetz et al. 1995). The technology has been used to pressure inject grout into reclaimed mines sites as a means of placing the infiltration barrier between moving meteroic waters and buried mine spoils (Schuek et al 1993; Schuek et al. 1994). Several of these applications will be discussed in more detail in later sections. Other grout applications have demonstrated the feasibility of its use as a potential replacement for compacted clays in pond and wet land bases, as a grouting medium for pressure injection into deep mines for closure and in mine sealing applications. As a structural fill it has been used in brownfield reclamation of old industrial sites and as a monolithic cementitious fill against strip mine high walls to alleviate fall hazards (Scheetz et al. 1994; Scheetz et al. 1995). Several marine applications are under consideration as well. FBC-based grouts are being evaluated for "floating" docks, as substrates for re-establishing oysters in Chesapeake marine estuary, as artificial reefs and as fill materials in composite structural piling for marine use.
Fly ash, when used alone, and as a fill material accounts for about 2.8% of the total ash usage. As a fill material, compacted fly ash typically has a density of 1120-1520 Kg/m3 as contrasted to typical soils at 1600-2080Kg/m3when used as backfill for walls, retaining structures and bridge abutments (Meyers, et al 1976). Fly ash in some areas is also used as daily cover for municipal solid waste landfills to isolate and encapsulate successive lifts of waste.
The second most extensive category for the utilization of fly ash is as a material in the construction of roadways and associated peripheral projects. Fly ash finds usage in embankment soil stabilization, subgrade base course material aggregate filler, as a bituminous pavement additive and as a mineral filler for bituminous concrete. Of the total ash product, this category accounts for about 2.3% of the ash.
The remaining ~5% of the fly ash is utilized in applications where much smaller volumes are consumed. Some of these applications include its incorporation into building block and brick were it serves as a means of reducing consumption of more costly portland cement or because of it lower density as a cost savings in transportation of finished products. For example, a fly ash brick weighs about a third less than a conventional clay fired brick (Reidelbach, 1970).
Pulverized coal fly ash has for many years been directly applied to fields as an agricultural soil amendment. At least one FBC facility is known to adjust the amounts of bed materials during the summer months in order to enhance the lime content of its ash which in turn is sold as an agricultural liming agent. A more extensive agricultural application of fly ash is as a mineral source in the design and fabrication of "perfect" artificial soils that are tailored for specific groups (Baker et al. 1991,ASTM D 5435). These soils were developed in Pennsylvania and permitted as a General Processing Permit for use in reclaiming abandoned mine lands.
Examples of Current and Proposed Beneficial Uses of Coal Ash in The Anthracite and Bituminous Coal Regions of Pennsylvania.
Anthracite Coal Region Examples
The most prevalent beneficial use of coal ash in the anthracite coal region (anthracite region) of Pennsylvania is the utilization of coal ash for backfilling and reclamation of active and abandoned pre-act strip mines, and the surface related subsidence of abandoned deep mines. The amount of coal ash utilized beneficially in the anthracite region should increase to even higher levels in the near future as more and more coal fired utilities utilize their coal ash beneficially and close out landfill and waste dump disposal areas.
Foster Wheeler Co-Generation Plant Site
Surface Mine Reclamation
The site is located in Mount Carmel Township, in Northumberland County and the overall site consists of two separate coal refuse reprocessing permits which encompass approximately 250 acres of coal refuse banks, and approximately 50 acres of pre-act strip mine pits and spoil. The pre-act strip mine pit area was the site of a prior fatal accident, when a woman became disoriented and fell off the highwall. Initial operations on the site began in early 1990 with the reprocessing of coal refuse banks and the commencement of the utilization of coal ash for backfilling the fatality related pre-act strip mine pit area, and the entire pre-act strip mine pit area was backfilled to approximate original contour by the end of 1996. Overall, approximately 30 acres of pre-act pit and spoil area and approximately 20 acres of coal refuse banks or deposits have been backfilled, regraded and reclaimed since the operation has been initiated. The remaining acreage should be reclaimed within the next 15 years and some additional pre-act pit and spoil, and coal refuse bank reprocessing areas may be added to the project in the near future.
Pennsylvania Power and Light Company Sites
Surface Reclamation of Deep Mine Subsidence Areas
These sites are located in Porter Township, in Schuylkill County and involve the beneficial use of coal ash for backfilling and reclamation of surface related subsidence (crop falls) of abandoned deep mines. This utilization was initiated by the Pennsylvania Power and Light Company, a utility company currently operating in and around the anthracite region. PP&L typically produces 1.1 million tons of coal ash annually from the combustion of approximately 10 million tons of bituminous and anthracite coal in its pulverized coal combustors. The utility initially contracted with local coal operators for the beneficial use of coal ash for backfilling of abandoned pre-act and active strip mines, but then investigated the subsidence or crop fall related reclamation proposal. The subsidence or crop fall related areas represent a significant safety problem within the anthracite region. They consist of subsidence areas with almost vertical highwalls of various depths, and represent significant problems because of their size, depth, location and numbers, and also the fact that there is no material available to backfill them; as the material from the crop fall area has subsided into the deep mine. Current funding associated with the reclamation of abandoned mine lands does not allow these crop fall areas to be reclaimed through that type of funding. Therefore, this project may be the only way these crop fall areas will ever be reclaimed.
Panther Creek Partners Generation Plant Site
Surface Mine Reclamation and Acid Mine Drainage Pollution Abatement and Prevention
The site is located near Nesquehoning Borough, in Carbon County and is another example of the backfilling and reclamation related beneficial use of coal ash, which also incorporates the beneficial use of coal ash for acid mine drainage pollution abatement and prevention. The overall site consists of four separate coal refuse bank reprocessing areas and covers approximately 420 acres. This site is unique in that the area is located outside of the anthracite coal reserves, and also unique in that any acid mine drainage produced by the pre-act deposited coal refuse banks impacts a stream that would otherwise never be susceptible to acid mine drainage pollution.
This generating plant utilizes a fluidized bed combustion technology and in turn produces a coal ash with a high pH and some cementatious quality. This coal ash is mixed with the coal refuse as the coal refuse banks are reclaimed, and is also utilized as a soil additive and capping medium for reclamation purposes, and for controlling the acid producing potential of the coal refuse banks. The coal ash's other physical and chemical properties are also utilized in order to add moisture holding capability to the medium being used as final cover for the site, and to limit surface water infiltration by way of utilizing the coal ash's cementatious quality for membrane permeability purposes. Initial results from the beneficial uses of coal ash on the site look promising, but additional data and work must be completed before any definite conclusions can be made.
Shen Penn Site
Surface Mine Reclamation
This site is a large water filled pre-act strip mine pit near the borough of Shenandoah, in Schuylkill County. This proposal is classified as a demonstration project, as the placement of coal ash in or within eight (8) feet of groundwater is currently not permitted by current regulations. This project site consists of a water filled pre-act pit, over 120 acres in size and over 350 deep. Water depth within the pit varies from 10 feet to over 230 feet, and the site has been associated with two fatal accidents. The cost of backfilling and reclaiming the pit by conventional means has been estimated at up to $28 million. This cost is more than the entire average yearly budget of the Pennsylvania BAMR, therefore the probability of the pit being backfilled through state or federal funding is unlikely. Initially two types of placement techniques were tested within a small water filled impoundment, and although both the dry placement and slurry placement techniques proved acceptable, the slurry placement technique was chosen for simplicity and cost effectiveness. Significant testing, research and monitoring measures have been incorporated into the demonstration project to insure the safe placement of the coal ash. These measures involve the monitoring of 18 (eighteen) separate points for water quality information, and the monitoring of the coal ash in place for a number of physical properties to insure structural stability of the coal ash after placement. The project also is set up on a phase schedule to insure that the slurry emplacement technique is effective in varing water depths without causing phase separation of coarse and fine grained particles or groundwater pollution. The phasing also allows adjustments to be made to the plan if any problems occur during any stage of the project.
Bituminous Coal Region Examples
The site is a permitted 100 acre reclaimed surface strip mine located in Clearfield County, in north central Pennsylvania. Water as it leaves the site typically has a pH in the range of 2.2 to 2.7. Normally ground water in the area has a pH greater than 6. Surface mining had occurred on the site beginning in the 1950's through the end of 1985. After mining had stopped, the pit was back filled with a mixture of overburden and toxic spoils originally removed from the site. The toxic spoils were handled according to existing regulations and were evenly distributed across the site. The site was reclaimed to near original contours, covered with topsoil, seeded and partially replanted with trees. Hydrogeologic studies of the site reveal no apparent ground water contribution to the water balance for the site. Allowing for evaporation, rain water percolates though the spoils almost quantitatively and in approximately 2 days passes through the site with the majority of the water exiting the site via a single seep.
Capping of the site with an impermeable barrier will prevent water from encountering the acid producing spoil, stopping the formation of acid mine drainage. The project is being conducted under an amendment to an active PA DER surface mining permit #17793044. The cementitious grout was placed, first in a proof-of-concept plot of 11 acres and is now being placed in 10 acre plots. Five hundred thousand (500,000) tons of grout have been placed since September of 1992, which represents 75% of the project. Placement has occurred throughout the year including winter months. For this project, FBC ash is being activated. Water is metered onto the formulation and roller compacted in six inch lifts to a total thickness of 36 inches. The most important property of the capping material, because of the large volume of materials needed, (est. 700,000 tons) was its cost. The use of a high value added material such as Portland cement would be cost prohibitive. A preliminary analysis indicated that using alkali activated cements based on coal fly ash as the capping material would make the capping process economically competitive with land filling the ash.
The permeability of the cementitious cap was desired to be low (< 10-7 cm/sec), although a higher permeability could be offset to a certain extent by using a thicker cap. The cap was required to be strong enough to support a wheeled vehicle, approximately 400 psi. The capping material was required to be resistant to chemical attack by the acidic sulfate rich waters present on the site. The capping materials were also required to be resistant to the more alkaline conditions present on the site once the capping process was complete. The capping material needed to set within 6-8 hours to prevent damage by unexpected rainfalls. Finally, the capping material should not contribute to the release of any undesirable metals.
Strength in the grouts develops rapidly and continues to increase with prolonged curing time. Laboratory testing at 515 days after mixing indicates that this trend is continuing. Strengths in excess of 3500 psi are not uncommon. Similarly, water permeability continues to decrease as a function of curing time, showing a value of about 10-7 m/sec at 515 days that is comparable to clays caps.
Leaching experiments (Zhao 1994) were conducted on specimens of the various formulations outlined above following an MCC-1 leaching protocol (US DOE/PNL, 1980) originally developed to predict the performance of radioactive waste forms over hundreds of years. The leaching test lasted for 90 days. After leaching analysis showed that Ag, Cd and Sb were below laboratory detection limits of 20 ppb, Cr and Pb were at the detection limits. As, B, and Se were above detection limits but well below regulatory limits. In all cases the metal concentrations in the leachates were below the 25 times drinking water standard required by PA Module 25 regulations.
The MCC-1 leach tests demonstrated that the mechanism for dissolution was diffusion controlled for the FBC fly ash grouts. The principal crystalline hydration product in the grouts prepared with lime activation was ettringite and for the grouts prepared with an alkali hydroxide activator the matrix material was gypsum. Both of these phases hydrate to produce a matrix pore fluid that has pH values near neutrality. Reactions in the FBC ash grout are controlled by the dissolution of a neutral salt, gypsum. Hence, the pore liquids possesses a pH value of between 7 and 8.
Fran Contracting Site
In-Situ Encapsulation of Buried Pyritic Materials by Pressure Injection Grouting
The site is a 37 acre surface coal mine in Clinton County, Pennsylvania, that was mined and reclaimed between 1974 and 1977. Mining was by mountain top removal and reclmation was to approximate original contour. The Lower Kittanning coal seam was present in two splits separated by 10 to 20 feet of under clays. Only the uppersplit was mined, leaving the thick underclay as pavement. The coal was overlain by black shale capped by a sandstone unit. The black shale is generally pyritic and acid producing. Infiltrating precipitation is the only source of groundwater. Acid mine drainage (AMD) discharges developed soon after reclamation and were first noted after a fish kill in 1978. The discharges (surface and underground), estimated to average 35 gpm, destroyed five miles of native trout streams. The operator was unable to maintain treatment facilities and bonds were forfeited. During the mining operation, the operator segregated pyrite rich pit cleanings and imported a small quantity of tipple refuse for disposal by burial.
Several geophysical mapping techniques were used to define conditions within the backfill. These techniques included electromagnetic terrain conductivity (EM), magnetometry, and very low frequency (VLF). EM was used to map the location of the AMD plume throughout and off the site. The piles of buried refuse and pit cleanings were located with magnetometry. VLF was used to map bedrock fracture zones beneath and adjacent to the site (Schueck, et al 1994).
Forty two monitoring wells were drilled on and adjacent to the site. These wells were located using the results of the combined geophysical mapping. This initial drilling effort confirmed the location of the pods of refuse and pit cleanings identified by magnetometry. Water samples collected from the monitoring wells confirmed that the pods of pyritic materials were sources of severe, localized AMD production.
The basis for the research effort was that if the pyritic pods of material could be isolated from infiltrating precipitation and oxygen, then the localized AMD production would be abated sufficiently for stream recovery to occur. Although the black shale which is the bulk of the spoil is thought to be acid forming, no attempt was made to abate this source of AMD.
A grout composed of FBC ash mixed with water was selected for use as the encapsulation medium for several reasons. When mixed at a water to solids ratio of 0.5, an unconfined compressive strength of 1920 psi develops after 20 days. The fine grained nature of the ash allows it to be pumped under pressure to fill small voids. The beneficial use of the FBC ash eliminated landfill costs. This reduction in avoided costs allowed the ash to be delivered to the site at no cost to the project. Thus, project costs were reduced by about $50,000.00.
The source of the ash was Fort Drum, New York. Ash characterization was completed at the Penn State Material Research Lab (Zhao, 1995). Chemical analysis of the ash is as follows: A1203 - 12.51%, CaO - 38,03%, SiO2 - 23.91%, SO2 0 16.02% the ash was tested using the EPA's Toxicity Characteristic Leaching Procedure (TCLP). All leachate analysis of the ash fell within the established guidelines of the TCLP.
Only those pods of pyritic material identified with magnetometry were targeted for grouting as shown on Figure 4. The grout injection wells were installed on 10 foot centers using 2 1/2 inch perforated, schedule 40 PVC casing. The grout was mixed on site and injected using a 600 psi grout pump. Grout injection occurred during the summers of 1992 and 1993. The amount of grout accepted by the wells ranged from less than 0.3 yds.3 to 83 yds.3 Approximately 4500 yds.3 of grout were used on this project. Because of a clayey matrix, some of the pods of refuse did not accept the pressure injected grout. In this case the material overlying the pod was excavated and a fly ash cap was pooled into the excavation. This was done to eliminate contact through infiltration, although it would not prevent contact of the pyritic material with groundwater moving laterally through the site.
Water quality was monitored on approximately a monthly basis, April through November, from 1990 through 1996. A series of one-tailed t-tests were computed for the water quality variables sampled from the monitoring wells. The p<=0.05 level of significance was used as the rejection level of the null hypothesis for all tests.
Grouting operations involved 5% of the total area of the mine site. Twenty pods were grouted across the site. The total surface area affected was 2 acres. As a result of the grouting effort, the most common mine drainage parameters, such as iron, manganese, aluminum and acidity, showed an overall reduction in concentration of approximately 40% as shown on Table 8. Trace metals such as cadminum, copper, chromium, and aresenic were present in relatively high abundance in the AMD prior to grouting as a result of secondary leaching. The overall decrease in the concentrations of these trace metals was approximately 65 to 90%. Nowhere within the mine sites was an increase in trace metals observed with the exception of barium. Increases in barium concentrations were noted to be associated with the supernatent associated with the grout. Barium levels returned to background levels soon after grouting was completed.
Deep and Surface Mine Reclamation Project
Bark Camp is an abandoned site in Clearfield, County, PA, where extensive coal mining on leased Commonwealth of Pennsylvania property occurred. The responsibility for the environmental consequences of the mining has reverted to the Commonwealth. The challenges at this site are acid discharges from deep mining, an accumulation of refuse from coal processing that occurred on the site and an exposed high wall as the result of surface mining. Bark Camp Run flows through the site and above this site supports a native fishery. Reports indicate that below the site the condition of the stream is severely degraded.
This site offers a very desirable demonstration site for the implementation of the fly ash-grout technology. The site contains an AMD seep which can be monitored to follow the outcome of the restoration. The Bark Camp site has been approved by the State of Pennsylvania as an ash utilization site by way of a no-cost contract.
The restoration of the Bark Camp site will require the remediation of the high wall, encapsulation of the tipple refuse, reclamation of Bark Camp stream, along with the construction of several holding ponds. The high wall at Bark Camp consists of approximately 11,000 feet of exposure extending 40 feet to a bench and then to an additional 60+ feet above this bench to the hill side. Tipple refuse estimated at approximately 40,000 tons of coal screening materials and other refuse is present on the site plus additional refuse material that was originally used to fill in the Bark Camp creek valley for construction of the tipple site. These screenings can potentially act as the source materials for further AMD development if not adequately isolated from infiltrating ground waters. The tipple refuse will be added in lifts to cementitious grouts as the grouts are being placed against the high wall in order to stabilize and recontour the slope. The refuse will be placed so that it is intimately encapsulated in the grout thus keeping it in direct contact with a material with neutralization potential and rejecting water from contacting the refuse. At the termination of the placement against the high wall, the slope will be covered with soils and planted. To date, approximately 100,000 tons of grout have been placed, soil applied and revegetated. Finally, the open void space in the deep mines will be closed by pressure injecting a fly ash-based cementitious grout. The Bark Camp #1 mine is extensive, stretching for over six miles with a wide lateral expanse. In contrast, Bark Camp #2 mine is much smaller extending for nearly a mile and a half with much less lateral expansion.
Studies show that from 6 - 10 million tons of grout will be required to complete the restoration of the Bark Camp Site. These volumes are so large that the use of ordinary construction materials is cost prohibitive. However, many alumino-silicate materials, which are produced in very large volumes as waste products, are available. The most readily available is fly ash produced from the combustion of coal in utility power plants. In this approach, the latent pozzolanicity of the fly ash is activated by a source of alkali or alkaline earth material (typically also a waste product).
Several approaches for using cementitious materials based on coal combustion by-products in the remediation/reclamation of acid producing sites have been demonstrated. Schueck, et al. (1993), Scheetz et al. (1993) and Zhao (1995) have reported on the use of ashes from fluidized bed combustion in the remediation of tipple refuse piles. In this approach the naturally cementitious ashes were pressure grouted in the refuse piles. In this case by-product alkaline earth additions were used to activate the latent pozzolonic potential of the coal ashes.
The Bark Camp Project will continue at the site for 5 - 10 years before restoration is complete.
Recycling and beneficial use of fly ash for reclamation, construction and other related areas is currently on the increase in both Pennsylvania and nationwide. If all the ash currently being disposed of in pre-existing landfills was recycled or beneficially used, the potential savings to utilities, in real non-PUC (Public Utility Commission) supported dollars, is in the 160 - 170 million dollar range. Furthermore, these utilities would save, in the same dollars, between 230 - 290 million dollars from the construction of new landfill capacity for each year's production of fly ash that they divert to recycling or beneficial uses. These real and potential cost savings and other related areas have opened up and helped develop new and important discussions, proposals and projects pertaining to the recycling and beneficial use of fly ash.
In conclusion, it makes sense for government and industry to promote the recycling and beneficial use of fly ash, since these practices represent a cost savings and benefit to State and Federal Government, utilities, consumers and the general public, by way of reduced disposal costs, increased no-cost reclamation of abandoned mine lands, and the abatement of water pollution.
The authors acknowledge the assistance, advice and support of the following DEP staff in the preparation of this paper and the development of the beneficial use projects described herein:
Sharon Hill, Hydrogeologist, DMO; Ernie Giovannitti, Director, BAMR; Bill Pounds, Division Chief, BWM; and Bob Dolence, Deputy Secretary, Mineral Resources Management.
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