|A large Hyundai shovel operates on the surface of cured dense slurry at the Matra power facility’s impoundment in Hungary, attesting to the compressional strength and environmental stability of the end-product. The shovel excavates cured slurry from around the perimeter for use in building up the levee of the 15-tiered, 150-foot-high impoundment. Photo courtesy: NAES|
By Dale Timmons
The Environmental Protection Agency’s (EPA) newly enacted Coal Combustion Residuals (CCR) rules and proposed Effluent Limitations Guidelines (ELG) will significantly impact waste management practices in the coal-fired power industry. The new rules will regulate fly ash settling ponds out of existence; regulate the location, design, operation, and closure requirements for impoundments; and impose new requirements for wastewater.
Traditional “dry ash” management techniques satisfy the rules’ proposed requirements, but they suffer from inherent technical deficiencies and pose prohibitive costs.
The Circumix™ Dense Slurry System (DSS) technology, developed by GEA EGI Ltd. of Hungary and represented exclusively by NAES Corporation in North America, mixes wastewater with CCRs to produce a stable product with near-stoichiometric use of water. Once cured, the slurry exhibits low hydraulic conductivity, high compressional strength, no discharge of fly ash transport water, little or no fugitive emissions, and enhanced metals sequestration, thereby achieving the goals of the CCR and ELG rules.
The EPA has also imposed stricter standards for air emissions with the Mercury and Air Toxics Standards (MATS). As with the proposed CCR and ELG rules, the vast majority of toxic metals targeted by MATS originate from coal-fired power plants. The EPA recognized that many processes designed to remove metals from gaseous emissions result in a transfer of the metals to other effluents, which is one reason it proposed the ELG rule.
Suffice it to say, the CCR, MATS, and proposed ELG rules are requiring owners and operators of coal-fired power plants in the U.S. to make pivotal decisions regarding future operations at these plants and how best to address the regulatory changes.
Dry Ash Management
Power plants face a number of challenges when converting to an alternative ash management system because few options are available. Conventional practice is commonly called “dry ash” management, which is misleading. So-called dry ash management for transport and disposal to an impoundment or landfill typically involves the addition of 20 to 25 percent water to suppress dust. Once the wetted ash is transported and disposed of, it is typically spread and compacted using heavy equipment. Additional water is often added using sprinklers or water trucks to control dust and improve compaction.
Traditional dry ash management typically involves handling and moving the ash multiple times, with each transfer adding more risk of dust release. To address this, the new CCR rules impose stringent controls on fugitive dust at impoundments. Even after ash is spread and compacted, it can easily be mobilized by wind if allowed to dry. It also exhibits relatively high hydraulic conductivity, which translates into high rates of leachate production.
Traditional dry ash management also poses a major expense. The costs of transferring the ash to ash/water mixing facilities, together with the capital and operating costs of the facilities themselves, are high. Truck transport, road construction and maintenance, fuel management, heavy equipment operation and maintenance, continual dust suppression, lighting and security at the disposal site, plus associated labor further reduce the appeal of dry ash handling. Lastly, the continual operation of trucks and heavy equipment significantly increases safety risks.
Dense Slurry System Ash Management
A dense slurry system (DSS) offers a safer, less expensive alternative to dry ash management while producing a product with improved environmental performance. DSS is a high-intensity mixing process that combines plant wastewater with CCRs to produce dense slurry that is easily pumped to an impoundment or landfill. The process maximizes the availability of reactive ions in the ash and optimizes the use of wastewater.
Dense slurry produced by the DSS process displays a consistency of 50 to 60 percent solids by weight with a density of about 1.3 g/cm3, which is maintained to within 1 percent. This is thick enough to minimize free water but thin enough to allow pumping to a distance of over 6 miles using centrifugal pumps.
Once discharged, the slurry hardens in 24 to 72 hours and substantially cures in about a month. The cured product exhibits low hydraulic conductivity, high compressional strength, no fly ash transport water discharge, little or no fugitive dust, and enhanced sequestration of contained metals. These properties meet the performance requirements specified in the new CCR rule and the proposed ELG.
Electron Microprobe Image of a No-Lime Sample
DSS is currently used at eight power plants – seven of them in Europe and one in the U.S. Two more plants are being built or commissioned – one in Europe and one in India – that will use the technology. Circumix DSS systems have processed over 60 million cubic yards of dense slurry into environmentally stable end products, primarily using flue gas desulfurization (FGD) water and other plant wastewater as the stabilizing medium.
In addition to achieving compliance with the new ELG and CCR rules, DSS offers numerous additional advantages:
- Combined stabilization of ash and wastewater
- Reduction of water use by 80 to 90 percent compared to traditional practice
- Zero discharge of transport water
- Significant reduction of plant-wide wastewater
- Low hydraulic conductivity (10-6 to 10-10 cm/sec)
- High compressional strength
- Enhanced metals sequestration
- No risk of liquefaction or spills associated with liquefaction
- Significant reduction of leachate volume
- Significant reduction of fugitive dust emissions
- Enhanced land-use efficiencies from elevated disposal facilities
- Reduced energy consumption
Electron Microprobe Image of a Lime-Added Sample
Several variables contribute to low hydraulic conductivities in the cured product, including particle size distribution, particle shape, water chemistry, and ash chemistry. The mixing process results in close packing of the ash particles upon discharge. The chemistry of the ash and water determine the type of crystal growth that takes place in the interstitial spaces between ash particles upon curing.
Performance Enhancement of Slurry Products
NAES has found in recent testing that variations in the amount of water used to make the slurry can impact processing parameters of that slurry. It has also demonstrated that small quantities of additives, where indicated, can dramatically improve product performance.
The compressional strength and hydraulic conductivity of cured DSS products depend largely on the chemical reactivity of the fly ash contained in the slurry. This reactivity in turn depends on several variables: type of fuel, emission controls used, type of boiler, and combustion temperature, among others.
As dense slurry cures, hydrated mineral crystals grow in the spaces between ash particles, including the following:
Ettringite 60% Bound Water
Allite 32% Bound Water
This interstitial crystal growth sequesters water, entrains small particles, and inhibits fluid flow. In addition, the crystals act as an adhesive that binds ash particles together, resulting in greater compressional strength. This process – the same that occurs in the curing of concrete – is a desired outcome of DSS. (For reference, most concrete contains about 25 percent bound water.)
Although DSS has been used extensively in Europe and at one plant in the United States for decades, plant-specific testing is still required to establish the proper blend of solid waste products and wastewater for optimal environmental performance. While performance-enhancing additives are available, all of the DSS facilities currently in operation process ash that is sufficiently reactive on its own.
The ash produced by some power plants in the United States, however, exhibits little or no reactivity. Where this is the case, additives may be used to increase compressional strength and reduce hydraulic conductivity. Typically, 2 to 3 percent active lime is enough to achieve adequate solidification.
Case Study: PRB Coal Ash
For example, NAES tested samples of Powder River Basin (PRB) coal ash to determine their performance relative to DSS. The samples contained over 20 percent CaO, but only 0.14 percent of it was chemically active.
Figure 1 shows an electron microprobe image of cured slurry product made using 60 percent PRB fly ash and 40 percent water. (Note the regions where ettringite crystals have formed.) After six weeks of curing, the low reactivity of the ash resulted in very little cementation. The cured product exhibits a porosity of about 50 percent, as evidenced by the dark regions of empty space in the image. After curing, the sample showed compressional strength of 48,263 Nm-2 (7 psi) and the hydraulic conductivity measured 3 x 10-5 cm/sec.
To find out how the PRB slurry performance could be improved, NAES prepared another sample – this time using 50 percent fly ash, 2.5 percent active lime, and 47.5 percent water by weight – and allowed it to cure for six weeks. In figure 2, the cured product shows a significant reduction in porosity compared to the no-lime sample – about 6 percent porosity in the lime-added product versus 50 percent in the no-lime product. The reduction in hydraulic conductivity of the lime-added sample – 3.4 x 10-6 – represents about one order of magnitude. The compressional strength increased by 97 percent to 1,296,214 Nm-2 (188 psi).
Sequestration of Water
Mineral growth that takes place during curing sequesters significant quantities of water. This is important because the EPA’s preferred options under the proposed ELG prohibits discharge of fly ash transport water under any circumstance. Disposal facilities that use the DSS process have achieved zero discharge of transport water by reprocessing leachate to produce more dense slurry.
To assess how much water is sequestered in the DSS curing process, NAES tested ash samples from the Matra Power Plant near Budapest, Hungary, the ‘flagship’ of DSS facilities. Using a slurry of 60 percent fly ash and 40 percent water by weight, NAES prepared samples with 2.5, 5, and 10 percent active lime added, as well as a control sample without added lime, to correlate the amount of water sequestered with the concentration of lime.
The samples were molded into 4-inch plastic tubes wrapped with geotextile fabric at the base to allow leachate to drain out of the slurry. The captured leachate was periodically poured back through the curing product. The samples and drained water were maintained in a closed system to prevent evaporation of water.
As shown in the hydration curves for the four mixes (Figure 3), water is rapidly sequestered during curing. The mix with 2.5 percent active lime sequestered 90 percent of the free water in 15 days. Samples with higher active-lime concentrations sequestered the same amount of water in five days or less.
NAES also found that as the thickness of accumulated slurry product increases in an impoundment or landfill, so does the amount of water sequestered. As dense slurry impoundments accumulate more slurry, the amount of leachate produced thus declines over time because the water that infiltrates has more time to react as it percolates through the curing product. These continuing reactions enhance the performance of the impoundment over time by progressively reducing hydraulic conductivity and increasing compressional strength.
In active impoundments and landfills that receive dense slurry, evaporation removes significant quantities of water before it can infiltrate the impoundment. The hydration reactions that occur during curing, coupled with evaporation, result in zero discharge of fly ash transport water.
A Commercially Operating DSS Impoundment in Hungary
The active ash disposal impoundment at the Matra Plant, which began operation in 1998, consists of 15 tiers, each 10 feet thick, of solidified Type F ash that has been pumped to the impoundment from the plant as dense slurry. The 150-foot high impoundment covers an area of 314 acres at its base and 122 acres at the top. The established tiers have been planted with fruit trees.
The top of the impoundment is divided into six smaller enclosures separated by dikes. When an enclosure is full, discharge is transferred to an adjacent enclosure. Cured dense slurry from the perimeter of the full impoundment is then excavated and used to construct the dike for the next tier.
To prevent interruptions in plant operations caused by lack of disposal space, at least two of the multiple smaller impoundments at the top of the facility are always made available to receive dense slurry. The impoundment poses no risk of liquefaction of ash products or catastrophic failure (e.g., inundation of the surrounding community) because the compressional strength of the contents ranges from 5,000 to 11,000 lbs/ft2. Hence, there have been no slope failures or other incidents requiring remedial action since operations began. All leachate is returned to the plant for use in DSS processing, making this a zero-discharge facility for both transport water and leachate.
|The tiered and elevated DSS impoundment at Matra Power Plant in Hungary is planted with fruit trees. Inset: The cured dense slurry from the impoundment perimeter is used to construct a dike for newly discharged slurry on the top level. Photo courtesy: NAES|
The physical and chemical properties of ash and water vary from plant to plant, so these materials must be tested at each site to determine the best ‘recipe’ for stabilizing CCRs. NAES conducted testing at numerous locations using a pilot-scale dense slurry processing system.
Prior to the pilot test, samples of combustion products and wastewater are analyzed to determine their chemistry and particle size distribution. The pilot-scale system is then used to process a range of promising ‘recipes.’ Each recipe is allowed to cure for 90 days before the samples are collected for testing.
Data collected during slurry processing includes rheology parameters (yield stress and rigidity), water content/flow dynamics, energy consumption, mix ratios, and water stoichiometry. Cured samples may be analyzed for the following:
- Compressional strength
- Porosity and hydraulic conductivity
- Bulk chemistry
- Moisture and density
- Electron microprobe analysis
- Leach performance
The data collected, along with plant information, are used to determine system capacity, slurry pumping requirements, and impoundment/landfill design. They are also used to estimate probable leachate production and environmental performance of the stabilized product.
The CCR and ELG rules are closely related and interdependent. Design changes at coal-fired power plants that affect the quantity and chemistry of generated wastewater also affect the transportation, management, composition, beneficial reuse options, and disposal of combustion products. These changes in turn affect the design, operations, monitoring, and closure requirements for impoundments into which CCRs are deposited. They also influence decisions regarding the management and fate of CCRs in existing impoundments.
In terms of environmental protection, operational safety, and financial risk, DSS has proven itself altogether superior to “dry ash” management. It not only meets the requirements of CCR and ELG but yields a product with outstanding environmental performance:
- Hydraulic conductivity that is substantially lower than that resulting from traditional “dry ash” management as described in the proposed ELG
- 80-90 percent less consumption of water compared to traditional ash sluicing
- Stabilization of wastewater (including FGD water) used to produce the dense slurry
- Zero discharge of transport water
- Zero discharge of leachate if reused for dense slurry production
- Enhanced sequestration of contained metals
- Reduced risk of groundwater contamination
- Reduced or eliminated risk of dust generation
- High compressional strength
In addition, the tiered, elevated disposal facilities typically used with DSS enable more efficient use of disposal space. Piping the slurry to these impoundments reduces or eliminates the use of heavy equipment and its attendant safety and environmental risks. More to the point, DSS processing eliminates ash sludge liquefaction, and with it the risk of dike failure and catastrophic releases.
Dale Timmons is a egistered geologist and Business Development Program Manager with NAES Corporation.