The physical properties of Coal Combustion Residuals (CCR) present many challenges to utility owners and environmental professionals. The waste material (commonly referred to as Coal Ash) is most often captured and stored in open ponds or impoundments, which allow for unrestricted stormwater infiltration, resulting in saturated CCR.
The Dangers of Saturated Coal Combustion Residuals (Coal Ash)
When the fine particle portion of CCR is saturated, the material can look like a solid on the surface; but when vibrations are applied, the ash has the tendency to fluidize.
When saturated CCR is excavated during an impoundment closure, the vibrations of heavy machinery (i.e. bulldozers and track-hoes) have been known to cause catastrophic fluidization of the CCR, leading to loss of equipment and even loss of human life.
Managing and Safely Removing CCR
Wet CCR, if placed in a landfill, can cause shifting or even collapse of the landfill over time. To mitigate CCR landfill failures, CCR typically must meet geotechnical stability criteria as determined by the site’s engineer. Unconfined compressive strength and compaction are two parameters that are often used to ensure the stability of CCR that is placed in a landfill for permanent disposal.
To stabilize saturated CCR, it is critical to remove the water that is contained in the pore spaces of the CCR or to manipulate the properties of the water. Stabilization of CCR material can be accomplished in several ways.
- Gravity-draining and air-drying (flipping)
- Mechanical dewatering (drainage trenches)
- Chemical treatment (SAPs, cement)
- A combination of any of these methods
So what's the best method for your operation?
The 3 Best Methods for Stabilizing CCR
As mentioned, there are a few good ways to improve CCR stability in order to meet minimum landfill requirements. These include:
- Air-drying, gravity drying, flipping
- Cement solidifiers
- Superabsorbent Polymers
Let’s look at a few of their advantages.
1) Gravity Draining (Flipping)
Flipping of CCR Material is a simple, old school means for eliminating free liquids and increasing compressive strength of saturated CCR material via gravity and the sun’s energy. Heavy equipment (i.e. front-end loader) is used to pick up wet ash and “flip” the material over repeatedly until sufficient pore water has drained out. Flipping can be a cost effective method for CCR stabilization if there is plenty of land available for staging the wet CCR material, and time to reach the target moisture content.
Due to the heterogeneous nature of ponded CCR material, particle sizes can range from coarse to fine, with the fine CCR having a much higher affinity for pore water. Fine CCR material is far more challenging to dewater with manual flipping than is coarse ash. Great variation in CCR particle sizes may occur in adjacent pockets of CCR.
It’s impossible to predict the texture and therefore the water retention capacity of CCR until it is excavated. Coarser material may dry sufficiently using manual flipping alone, while finer CCR often requires a more sophisticated approach.
2) Concrete for Coal Ash Stabilization
In situations in which air drying is insufficient for solidifying liquid waste, cement and other pozzolanic materials are widely utilized to stabilize CCR. When mixed with water, cement reacts to form rigid, stable bonds with the solid waste material, often resulting in very high compressive strength. While cement stabilization is appropriate in many situations, there are instances in which the physical characteristics of the ash impede the performance of cement. Very fine CCR with very high water content and very high organics content can render cement stabilization very inefficient and time consuming.
Curing times for cement, under ideal conditions, often take up to 24 hours. But when working with very fine fly ash, with a high level of saturation, cement stabilization of CCR can require days to weeks. When the project deadline will not allow for the luxury of waiting weeks for CCR to stabilize, specialty absorbents may be required.
3) Superabsorbent Polymers for Fast CCR Stabilization
Superabsorbent polymers, designed to absorb 300-350 times their own weight in water (sawdust = 10x for comparison), are a specialized absorbent technology, that utilizes a basic osmotic reaction. SAPs react with CCR pore water instantaneously, allowing the solidification of saturated coal ash in minutes (depending on volume). The SAP absorbs the water into its internal polymer network, forming a “hydrogel” that does not release the liquid even under pressure and vibration. The texture of the resulting treated material is similar to that of a dry, stackable soil with no free liquids and suitable for transportation.
Example: 10,000 tons of CCR would typically require 50 or less tons of SAP to eliminate all free liquid
The superior absorbency of SAPs allows for the stabilization of saturated CCR material with dosage rates often at 0.5% or less (by weight). The ultra-low dosage requirements of SAPs allow for minimal amendment blending as well as the minimal impact from the soft hydrogel texture to the overall compressive strength of the stabilized ash material.
As an engineered absorbent technology, SAPs should be applied sensibly, such as in pond locations that contain excessive volumes of fine ash material that can be difficult to fully stabilize with manual flipping and cementitious materials. SAPs, when not carefully applied, can easily be overdosed, which results in excessive reagent costs.
Ultimately, each job will come with its own set of requirements. In these cases, experience is the best teacher. We are happy to consult on strategies for waste and CCR disposal methods. If you're struggling with a job, reach out in the comments below. You can also reach us or schedule a meeting by pressing the Contact Us button below.
