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Coal Combustion Byproducts [CCBs] are presently regulated as Solid Waste [Subtitle D] under the Resource Conservation Recovery Act [RCRA]. Such classification promotes beneficial use by enders-users i.e. mitigating excessive liability. According to the US Environmental Protection agency [EPA], about 131 million tons of coal combustion residuals -- including 71 million tons of fly ash, 20 million tons of bottom ash and boiler slag, and 40 million tons of flue gas desulfurization (FGD) material -- were generated in the U.S. in 2007. Of this, approximately 36% was disposed of in landfills, 21% was disposed of in surface impoundments, 38% was beneficially reused, and 5% was used as minefill. Stringent regulation, as Subtitle C (Hazardous Waste), would impose a perceived liability upon end-users; greatly reducing beneficial use opportunities. Mandatory use of synthetic liners - would not have prevented dike wall failure and fails to consider inherent engineering characteristics of CCBs.
If increased regulation translates into improved methods of placement [optimal compaction], better monitoring [via monitoring wells with periodic reporting], and enhanced site management [capitalizing upon concrete-like behavior of coal combustion by-products], then such stronger regulation can be justified. But, if greater regulatory scrutiny imposes a bureaucratic burden on operating coal-fired power plants without understanding characteristics of these by-products, future spills, leaks and dike failures will continue. For example, should USEPA mandate synthetic liners - a costly approach - the inherent behavior of coal combustion products to achieve liner-like permeability between 10-5 to 10-7 cm/sec will have been ignored (1).
Composition - Pozzolanic Chemistry
Fly Ash - Chemical Compostion
Particulate control devices (e.g. electro-static precipitators, bag-houses) remove fly ash particulates form flue gas for subsequent collection and beneficial use and/or disposal. The following tabulation shows the significant percent components [percent by weight] of CaO, SiO2 and Al2O3.
| Component | Bituminous | Subbituminous | Lignite |
| SiO2 (%) | 20-60 | 40-60 | 15-45 |
| Al2O3 (%) | 5-35 | 20-30 | 20-25 |
| Fe2O3 (%) | 10-40 | 4-10 | 4-15 |
| CaO (%) | 1-12 | 5-30 | 15-40 |
Flue Gas Desulfurization [FGD] - Chemical Composition
SO2 is an acid gas and thus the typical sorbent slurries or other materials used to remove the SO2 from the flue gases are alkaline. The reaction taking place in wet scrubbing using a CaCO3 (limestone) slurry produces CaSO3 (Calcium Sulfite). When FGD were first introduced this so called 'FGD sludge' was ponded. But some FGD systems go a step further and oxidize the CaSO3 to produce marketable CaSO4 - 2H2O or gypsum.
Air Pollution FGD Chemistry
The following chemical reactions depict the formation of final end-products containing CaSO3 .1/2H2O and CaSO4 .2H2O
Some FGD systems employ 'forced oxidation' to convert the CaSO3 (Calcium Sulphite) to produce marketable CaSO4 · 2H2O (gypsum):
Blending of FGD Residue with Fly Ash - Use as Liner and Embankment
The following tabulation shows the final composition of CaSO3, CaSO4 and Fly Ash. When blended together (i.e. in a pugmill) the resultant material can be landfilled to achieve in-situ pozzolanic reactants and behavior.
Major components of FGD scrubber material and Fly Ash from different coal types and scrubbing processes (percent by weight).
A relationship between mineralogical composition and strength has been developed (1). The sum of SiO2 + Al2O3 + Fe2O3 divided by CaO varies linearly with Unconfined Compressive Strength [UCS]; where constituents are expressed as % by weight and UCS expressed as thousands of psi. Direct linear extrapolation predicts UCS ranging from 6500 - 17,900 psi; however, extrapolation exceeded data points by order-of-magnitude. Applying highly conservative order-of-magnitude reduction yields reduced UCS predictions ranging from 650 to 1790 psi. These raw FGD sludge and Fly Ash blends adjusted UCS values support using this resultant material as dike embankment. Adding Portland cement would increase strength - supporting such load-bearing application and justifying laboratory - demonstration studies.
Geotechnical Properties
Geotechnical properties of typical calcium sulfite FGD scrubber material.
| Geotechnical Property | Dewatered | Stabilized | Fixated |
| Shear Strength - Internal Friction Angle |
20° | 35° - 45° | 35° - 45° |
| Permeability (cm/sec) | 10-4 to 10-5 | 10-6 to 10-7 | 10-6 to 10-8 |
| 28-Day Unconfined Compressive |
- | 170 - 340 | 340 -1,380 |
| Strength (kPa) (lb/in2) | - | 25 - 50 | 50 - 200 |
The geotechnical properties listed above represented dewatered FGD sludge (i.e. vacuum filter, centrifuge), stabilized FGD sludge (i.e. blended with fly ash) and fixated FGD sludge (addition of CaO or Portland cement to fly ash - sludge blend). Synthetic liner-like permeabilities of 10-7 cm/sec of less can be attained by (a) blending ash with FGD or (b) adding Portland cement or CaO to the blend. Based on in-situ field results, when 6.6 - 10 % Portland cement was added to blend, permeabilities from 10-7 - 10-9 cm/sec were attained (1). Since the internal angle of friction [O] ranged from 35 - 45 o for the blend with or without Portland cement or CaO, this material should behave like a cohesive soil and could be used as embankment material. For instance, addition of Portland cement or CaO could form a stable dike wall for CCBs surface impoundment (2).
Physical Properties of FGD Residue and Fly Ash - Retrofitting Surface Impoundments
The particle size of FGD residue and fly ash shows this blend could be used as a grout material to stabilize existing CCB surface impoundment dike walls. When used as grout, the blend must be ability to penetrate between the interstitial soil spaces.
Fly Ash - Particle Size
To be used in cement or concrete applications (i.e. grout) fly ash should conform to ASTM C618 - either as Class C or F - depending on their chemical composition. 75% of the ash must have a fineness of 45 µm or less, and have a Carbon content, measured by the loss on ignition (LOI), of less than 4%. In the U.S., LOI needs to be under 6%. Since not all fly ashes meet ASTM C618 requirements, this makes it necessary that fly ash used in concrete needs to be processed using separation equipment like mechanical air classifiers or similar separation equipment.
FGD Residue - Particle Size
The tabulation shown below indicates that FGD residue most closely resembles a silt-like soil, having a fine-grained consistency of less than 0.074 mm size. (3).
Typical particle sizes of FGD scrubber material.
| Property | (Unoxidized) Calcium Sulfite |
(Oxidized) Calcium Sulfate |
| Particle Sizing (%) | ||
| Sand Size Silt Size Clay Size |
1.3 90.2 8.5 |
16.5 81.3 2.2 |
| Specific Gravity | 2.57 | 2.36 |
Fly Ash and FGD Residue Blend - Particle Size - Suitable for Grouting Retrofit
Adding fly ash and FGD residue would yield a final material whose size would be less than 0.074 mm - suitable as a grout material even with fine-grained Portland Cement addition. Grouts formed from fly ash and FGD residue have been demonstrated as grout material in Maryland (2). CCB retention ponds are similar to earthen dams; strengthening dike walls of these dams often employ slurry cutoff walls. Grouting existing soil dike wall would be about 90% less costly then slurry cutoff wall (4).
Residue Management - Placement - Landfill Methodology
The inherent pozzolanic-like behavior of lime-laden CCBs enables achieving improved geo-technical properties i.e. strength, permeability. Sound engineering practice of CCB placement recognizes the relationship between achieving desired Geo-Technical Property and Optimal Moisture Content while maintaining adequate 'Water of Solubilization' (or % Final Solids) to ensure reacting Pozzolanic Constituents with Free Available Lime Adhering to these principles requires Proper Placement Control Management - Field Compaction and Water Addition. Improved field placement (i.e., compaction, addition of dust suppression water) of CCB could increase density and reduce permeability to decrease leachate rate through the buried CCB. Achieving liner-like permeabilities, by capitalizing upon CCB's inherent characteristics and applying Proper Placement Control, achieves cost savings of 65% over traditional disposal methods e.g. synthetic liners (1). USA Regulatory officials should consider incorporating these principles into residue management recommendations. Recognition and implementation of these principles would confirm that CCBs can be properly managed - to alleviate concerns - providing a cost-effective approach to future regulatory control.
Achieving liner-like permeabilities represent demonstrated technologies. Achieving low permeabilities and enhanced compressive or bearing strength are known and recognized methodologies. Using CCBs as embankment material requires additional demonstration.
Demonstration Program - Landfill and Surface Impoundment Embankments
Considering the inherent engineering properties of CCBs justifies using this material to form surface impoundment dike walls. Approximately 27.5 million tons of CCBs are retained in surface impoundments. Preventing failure of these dike walls represents a primary issue for discussions between the electric utility industry and regulators. A demonstration program, based on laboratory and bench-scale testing, would indicate industry willingness to address future requirements in a cost-effective manner.
Laboratory and bench-studies could include: Triaxial testing, determination of Angle of Internal Friction, and Slope Stability Analyzes. Additive studies could include: varying percentages of CaO and Portland Cement. These studies would also investigate using FGD and fly ash and grout material - with and without additives.
Demonstration Programs could be conducted for new landfill and surface impoundment embankment stability. Demonstration Programs should be conducted for using fly and FGD sludge as retrofit grout material to strengthen existing embankments [landfill] and dike wall [surface impoundments]. Based upon their inherent characteristics suitable engineering properties should be attained at cost-effective results.
The electric utility industry with their trade and research organizations are urged to commit to conducting such programs - showing a 'good faith' effort to cooperate with regulatory and addresses recent CCB disposal upsets.
Staff Training
During execution of demonstration programs, operating staff should participate to gain field knowledge regarding proper land disposal management. The use of in-situ methods to optimize compaction and percent water of solubilization would be learned in the field. Encountering and adjusting for unexpected geo-technical behavior also enhances staff learning.
Besides learning from demonstration programs, upsets from CCB land disposal projects offer additional 'lessons learned' opportunity. For example, an ash monofill experienced leachate piping back-up and overflow. Lime was added to the ash to immobilize Lead and Cadmium. This lime/ash material pre-maturely reacted to form a concrete-like pozzolanic material that reduced leachate collector pipe opening - causing a back-up and overflow. This upset condition, causing excessive groundwater levels, could have avoided by applying the publicized, peer-review, engineering placement principles and methodology. Operators of CCB landfill facilities are urged to apply this methodology (5).
Discussion
An engineering approach, reflecting demonstrated technology and recognizing coal combustion products' chemical and geotechnical properties, should be embraced by the electric utilities with coal-fired power plants. Commitment to develop and implement this approach would curb excessive regulatory requirements and allay public concerns. Electric utilities should capitalize upon the industry-wide knowledge and submit to U.S. EPA as regulatory approaches are being developed. Application of CCBs engineering characteristics in land disposal projects would provide a cost-effective approach to pending regulatory negotiations. The electric utility industry and their trade-research organizations are urged to consider this engineering perspective in dealing with governmental agencies, elected and appointed officials and public media groups.
References - Specific Citations
(2) Wattenbach, H.L.; "An Evaluation of Free-Lime Containing By-products to Produce CCB Grouts for Use in AMD Abatement"; 1999 International Ash Utilization Symposium; University of Kentucky, Paper No. 20
(3) Zilly, R.G. (ed.); Handbook of Environmental Civil Engineering; Van Nostrand Reinhold Co., New York; 1975
(4) RS Means; Building Design Construction Data; Kingston MA; 2006
(5) Goodwin, R.W.; "Avoiding Ash Landfill Operating Mistakes"; Energy Pulse.net; Mar. 27, 2003
References - General
www.flyash.info/2009/189-fitzgerald2009.pdf
Rusch, K.A. "Development of CCB Fill Materials for Use as Mechanically Stabilized Marine Structures"; CBRC Project Number: CBRCM11; Subcontract No. 98-166-LSU; August 30, 2002
Science Applications International Corporation; "Technical Background Document on the Efficiency and Effectiveness of CKD Landfill Design Elements; EPA Contract 68-W4-0030, draft July 18, 1997
USDOE;" Clean Coal Technology - Coal Utilization By-Products'; Technical Report No. 24; August 2006
Berger, E. and Fitzgerald, H.B.; "Use of Calcium-Based Products to Stabilize Ponded Coal Ash Techniques and Results"; 2009 World of Coal Ash, Lexington, KY, May 4 - 7, 2009
(1) Goodwin, R.W.; Combustion Ash/Residue Management An Engineering Perspective; Noyes Publications/William Andrew Publishing; Mill Road, Park Ridge NJ 1993 (ISBN: 0 8155 1328 3) (Library of Congress Catalog Card No.: 92 47240)
http://www.netl.doe.gov/technologies/coalpower/
cctc/topicalreports/pdfs/Topical24.pdf
