A NOVEL APPROACH TO THE HANDLING AND 

DISPOSAL OF COAL COMBUSTION PRODUCTS

Alex W. Hohne

Synthetic Materials, LLC, 6009 Brownsboro Park Blvd, Louisville, KY 40207

KEYWORDS: fly ash, ash, CCP, gypsum, FGD gypsum, synthetic gypsum, dewatering, 

belt filter

ABSTRACT

Recently, there has been a movement to more rigorously regulate and eventually 

eliminate surface impoundments and wet stacks of coal combustion products (CCP’s). 

The Environmental Protection Agency, in reaction to the Tennessee Valley Authority 

(TVA) Kingston release of ash into the Clinch and Emory Rivers, has requested 

information on the structural integrity of all 300+ impoundments currently being operated 

at coal fired power plants in the United States. 

Concern over our future ability to permit new surface impoundments or expand existing 

impoundments is growing. The liability posed by the continued use of existing 

impoundments is also a serious concern to many utilities. The risk must be understood 

and weighed against the cost of converting CCP disposal to dry stacking or even landfill

disposal. This paper puts forth a proven cost effect approach, developed by Synthetic 

Materials. It allows disposal of the two major CCP’s, coal ash and gypsum. This 

approach enables the elimination of higher risk wet-stacking and an economically 

advantageous conversion to dry stacking of CCP’s.

BACKGROUND

A growing volume of CCP’s are recovered for beneficial use in manufacturing and 

construction each year. Dry storage (landfills), wet stacking or surface impoundments 

are the most common disposal practices for CCP’s that are not beneficially used. The 

risk associated with wet stacking has most recently been magnified by the structural 

failure of a fly ash wet stack at TVA’s Kingston Fossil Pant. Additional regulation is 

expected as a result. The additional regulation, at both Federal and State levels, will 

make wet stacks and surface impoundments more difficult and expensive to permit and 

maintain. Construction of new or expanded impoundments or wet stacks in high risk 

areas will likely be severely restricted.

Historically, the cost to construct and install necessary infrastructure has been a major 

driving factor behind selection of the disposal method. In a typical coal fired power plant 

with flue gas desulphurization (FGD), it is necessary to install a dry ash handling system 

and a gypsum dewatering system to implement dry disposal of the CCP’s generated. 

These combined facility costs to process the CCP’s generated is significant, in many 

2009 World of Coal Ash (WOCA) Conference - May 4-7, 2009 in Lexington, KY, USA

http://www.flyash.info/
cases requiring over $100 million in capital as well as significant ongoing O&M costs. 

Conversion of existing plants to dry CCP collection can result in significant generating

unit downtime that reduces utility revenue. The cost of conversion may be offset by 

reduced disposal costs and potential revenue from the marketing of a portion of the 

CCP’s collected. However, when life of facility cost is considered, this is rarely a net 

positive revenue situation for the utility. 

Managing existing CCP surface impoundments may present a sizable liability in the 

future. The additional regulation and oversight expected will encourage alternatives to 

impoundments and wet stacks to be more carefully considered. The legacy of the 

existing impoundments and wet stacks represents a sizable liability until they can be 

completely closed. Acceleration of closure may be desirable to reduce the overall risk 

profile for the utility

APPROACH

The preferred and most environmentally responsible method of managing CCP’s is 

beneficial reuse. Fly ash is most commonly used as a high-performance substitute for 

Portland cement or as a raw material utilized in clinker production. Cements blended 

with fly ash are widely accepted in almost all applications. Building material applications 

for fly ash range from grouts and masonry products to cellular concrete. Many asphaltic 

concrete pavements also contain fly ash. Geotechnical applications include soil 

stabilization, road base, structural fill, embankments and mine reclamation. Fly ash also 

serves as filler in wood and plastic products, paints and metal castings.1

FGD gypsum is used in almost thirty percent of the gypsum panel products 

manufactured in the United States. It is also used in agricultural applications to treat 

undesirable soil conditions and to improve crop production. Other FGD materials are 

used in mining and land reclamation activities.2 FGD gypsum use is becoming more 

common in Portland cement production, replacing natural gypsum as a set regulator.

Implementation of the 1993 Clean Air Act has greatly increased the scrubbing of flue 

gas emissions from coal fired power stations. This has subsequently increased the 

production of FGD gypsum. As FGD production increases in the United States, the 

challenge becomes the proper disposal of the remaining CCP’s in a cost effective and 

environmentally acceptable way. 

Historically, the management of each of the CCP’s has been considered individually. 

Dry fly ash collection and handling systems have been installed to allow for the 

beneficial reuse of ash and to avoid disposal in impoundments. Gypsum dewatering 

systems are installed to allow for the processing and sale of gypsum cake to wallboard 

and cement manufacturers. In areas with limited or no marketability for the CCP’s, the 

 

1

American Coal Ash Association (ACAA), “ACAA – Promoting & Advancing Coal Combustion Products”, 

http://acaa.affiniscape.com/displaycommon.cfm?an=1&subarticlenbr=40, (accessed March 10, 2009)

2

Ibid, http://acaa.affiniscape.com/displaycommon.cfm?an=1&subarticlenbr=41
expense to install these systems has been difficult to justify when ponding or wet 

stacking is an available option. 

Wet stacking or impoundments are not without their own problems. Waste water 

impoundments are rigorously designed structures, which require close control of 

construction activities. The impoundments require full time technical oversight to 

properly construct and maintain the facility function and safety factor. Likewise, 

requirements for monitoring are expected to increase, especially for older 

impoundments. Common repairs to breaches or leaking wet stacks require significant 

cost to investigate and repair.

Fortunately, there is a method to allow co-disposal of varying proportions of fly ash 

and/or gypsum that is practical, requires less capital, fewer additional personnel, and 

utilizes proven technology. The SYNMAT process can produce a material which is more

suitable for safe disposal in a landfill, while eliminating the risk associated with 

impoundment design and construction. 

Dewatering gypsum using horizontal belt filters is common in the utility industry. The 

technology and equipment application are well documented. The challenge is to adapt

the application of standard dewatering techniques to fly ash. An effective method has 

been developed by Synthetic Materials which allows the ash and gypsum to be 

dewatered simultaneously. The primary equipment utilized for this process is a modified 

horizontal belt filter system that is properly engineered to meet the hydraulic and 

material handling requirements for a combined gypsum/fly ash feed stream. 

In the process, synthetic gypsum slurry is delivered to the first feedbox on the horizontal 

vacuum belt filter. Following gypsum cake formation, fly ash slurry, at approximately 

20% solids, is distributed on top of the gypsum cake through a secondary feed box. The 

gypsum cake acts as a filter media for the fly ash slurry, allowing the fly ash to be 

dewatered on top of the gypsum cake. The combined cake discharges to a common 

conveyance for mixing.

Full scale operation of this system has demonstrated that a minimal gypsum layer of

1/16” is sufficient to assure adequate dewatering of fly ash while maintaining high fly 

ash solids retention on the gypsum cake. Fly ash moistures as low as 12% have been 

recorded while maintaining normal capacity processing rates on the filter system. 

The results will vary slightly for each site. Through trial and laboratory analysis, 

Synthetic Materials is able to model and calculate the expected results based upon the 

characteristics of the fly-ash and gypsum to be managed at a particular site.
Figure 1

Figure 1 represents a simplified process flow diagram. The gypsum slurry can either be 

fed to the belt filter directly from the absorber or concentrated through hydrocyclones. 

The gypsum slurry is fed onto the horizontal belt filter through a distribution box. A 

vacuum is applied to the filter belt to dewater the gypsum. After initial cake formation, 

concentrated fly ash slurry is delivered to the top of the gypsum cake through a second 

distribution box. Depending on the existing system, it may be necessary for the fly ash 

to first be concentrated through thickeners to establish a minimum density of 20% 

solids. The arrangement of the multiple distribution boxes is shown on the photo in 

Figure 2. 
Figure 2

The fly ash slurry is deposited on top of the gypsum cake and is dewatered with the 

gypsum cake as the belt advances as shown in Figure 3. The characteristics of the 

combined cake will vary based on the thickness of the gypsum cake and the mass flow 

and materials characteristics of the fly ash in relation to the gypsum. The resulting 

mixed and dewatered product can be easily stored, handled, transported and deposited 

in a landfill. In addition, the fly ash can be mechanically separated from the gypsum 

and the CCP’s managed separately. The dewatered product is shown in the photo in 

Figure 4.
Figure 3

Figure 4
The advantages to this process are significant. Using properly engineered tankage and 

auxiliary equipment, the SYNMAT process will allow the simultaneous dewatering of 

concurrent gypsum production, concurrent fly ash production, and the incorporation of 

dredged fly ash and/or gypsum production from existing surface impoundment. 

Additionally, a properly designed facility can simultaneously dewater gypsum and ash 

for landfill disposal while also dewatering gypsum for beneficial reuse applications. 

The costs associated with this system are considerably less than those associated with 

the design and installation of the typical dry ash collection systems and silos required to 

handle the fly ash separately from the gypsum. Also, O&M costs are reduced by 

managing both CCP’s through a single facility. Utility downtime associated with the 

installation of the system may be considerably less as the only downtime required is to 

tie-in for the gypsum slurry and the ash slurry lines.

Mitigation/ remediation of existing fly ash and gypsum impoundments can be achieved 

through dredging and utilizing a separate feed system to deliver the dredged material to 

the proper location on the filter system. This recovery system could be either part of the 

permanently installed system or comprised of portable systems utilizing the same 

operating concepts.

TESTING

Proof of concept testing has been accomplished and continues at Synthetic Material’s 

Cumberland City, TN dewatering facility using Class F fly ash from the Cumberland

Fossil Plant. Gypsum cake thickness was varied and monitored, as was the mass feed 

rate of the fly ash slurry.

Since the Cumberland Fossil Plant is equipped with a dry fly ash handling system, ash 

was brought in by truck and slurried in an agitated tank. This fly ash slurry was then 

applied to the gypsum cake on a 58 square meter horizontal vacuum belt filter system. 

Free moisture of less than 20% was achieved in the dewatered fly ash product. Full 

scale development testing is underway that will result in an optimum facility design for 

all operating parameters. 

Design of the process is also impacted by the physical properties of the gypsum and the 

fly ash. No testing has been accomplished with Class C fly ashes. The plant water 

balance must be considered as part of the process design and evaluation. As 

previously indicated, the application and costs of the system will be site specific due to 

logistic considerations as well as for variable process parameters.

CONCLUSION

The preferred utilization for CCP’s is in a beneficial use application. Due to increased 

production and market saturation, 100% beneficial reuse is rarely achievable and 
utilities must plan for disposal when the market is not available. There continues to be a 

need to handle CCP disposal at the utility plant site in an efficient, environmentally 

sound, and cost effective manner. It is likely that additional regulation will make wet 

disposal of fly ash and gypsum more difficult and much more expensive. Expansion or 

construction of new impoundments will become environmentally and economically 

unfeasible.

Synthetic Materials has demonstrated an innovative application of technology that 

provides an economically feasible alternative for the operation of dry stack or landfill 

disposal of CCP’s. By co-dewatering ash and gypsum, a capital cost reduction of over 

50% may be realized and significant O&M cost reductions can be achieved by 

eliminating the primary source of risk.