Coal Ash Drying with Superabsorbent Polymers

A Step-by-Step Guide for Minimizing Reagent Quantities and Treatment Costs

Superabsorbent polymers (SAPs) are the most efficient available technology for eliminating free liquid and stabilizing saturated coal ash.  However, without the proper planning steps, SAPs can be severely over-applied which often leads to unexpected treatment costs and/or underperformance.   

The following steps are critical for the successful stabilization of coal ash with superabsorbent polymers: 

Step 1

Establish Design Parameters in Lab and in Field

Determine Optimal SAP Dosage Rate in Treatability Lab

Coal ash properties will vary from site to site (often significantly).  As a result, the amount of SAP needed to stabilize ash can vary from 0.3% to over 1.0% (by weight).  For a 10,000-ton quantity of coal ash, the potential variance in SAP dosage rate could result in over $100,000 in additional, unexpected reagent costs. 

Determine Additional Stabilization Reagent Dosage Rates in Lab

For sites that require treated ash to meet specific compaction requirements, the addition of quicklime or cement (together with SAP) can be necessary.  Curing times of quicklime and cement typically are shorter when used with an SAP.  SAP has also been demonstrated to result in a reduced quicklime/cement demand. 

A simple, fast, and low-cost way to determine the appropriate dosage rate of SAP for a specific coal ash site, is to submit samples of saturated ash to Zappa-Stewart’s Hickory, NC treatability lab. Our lab will test various dosage rates and SAP products to identify the minimal amount of SAP recommended for full-scale field application. A few simple tests in the lab can save days of blending and hundreds of thousands of dollars in treatment costs. 

Optimize Ash Processing Parameters with Field Trial

A field-scale trial allows contractors to confirm the lab-recommended SAP dosage rate and optimize additional site parameters like SAP hydration speed, mixing and loading procedures prior to beginning full-scale operations at the site.

How to Conduct a Superabsorbent Polymer (SAP) field trial: 

• Identify a defined coal ash treatment zone 

• Estimate weight of wet coal ash contained in the treatment zone 

• Determine the amount of SAP required to stabilize the ash in the treatment zone, based on the pre-determined SAP dosage rate 

• Blend coal ash and SAP in a mixing container (roll-off box or similar) or natural containment zone for 15-30 min or until free liquid has been eliminated from the ash.  

Step 2

Examples of Pre-Project Cost Evaluation

Using SAP with or without other additives can provide a cost-effective method for rapidly reducing free liquid and high moisture content in saturated coal ash materials. In addition to reducing the problem associated with excess moisture, SAP can increase the efficiency of the excavation and mixing process, as well as the overall cost of excavation, transport and disposal.

Examples of Cost Evaluation:

The following items demonstrate how SAP can be used to reduce the amount of quicklime and cement additives and the time required for drying coal ash:

1. For 10,000 Cubic Yards (CY) or 9,450 dry tons of coal ash at a moisture content of 40 to 50 percent will typically require 7 to 10 percent of quicklime or cement additives.
2. Assume 8-percent of quicklime or cement is added to stabilize the partially saturated coal ash. Probable cost of additives at $116 per ton = $87,000
3. Cost of processing for 2 days, assume 16 hours at $250 per hour for equipment and labor = $4,000
4. Using a 0.2 percent dosage rate of SAP will reduce the amount of additives (quicklime or cement) by approximately 5 percent = $54,750 reduction in cost of additives. Cost of 0.2 percent or 18.9 tons of SAP at $1,800 per ton = $34,020 cost of SAP. Net reduction in cost of additives and SAP = $20,730 of savings for 10,000 CY of saturated coal ash.

Step 3

Construct Contained Ash Mixing Zones to Eliminate Infiltration of Groundwater 

A contained treatment area for mixing ash with SAP is important in eliminating the influx of groundwater into the treated material. The SAP dosage rate depends on the amount of water trapped in the ash, therefore—once the dosage rate is determined—any additional water that enters the treatment zone will require additional SAP to fully stabilize the ash. A containment area allows the contractor to exert control over ash stabilization and consistently predict the quantity of SAP needed. 

Step 4

Mix SAP and Load Treated Ash 

• Apply and mix SAP with a track hoe, rotary mixer, or similar system. 

• Common SAP dosage rate is 0.2% - 1.0% (by weight) for CCR material 

• Reaction time is 15 min to an hour in most cases. 

• Load stabilized ash in dump trucks. 

Step 5

Waste Transportation and Placement in Lined Landfill 

• Site-specific requirements will determine the type of landfill that will accept the final, stabilized waste CCR. 

• The water retention functionality of SAP prevents the leaching of water out of the waste matrix during transport to the final disposal facility. 

• Final stabilized material can be placed in a lined landfill. 

We hope this guide helps you manage your operation's efficiency the next time SAPs enter the mix. Be sure to check out our Coal Ash Treatment White Paper for even more details on proper SAP use on-site. For questions or more information on our SAP solutions, please reach out using the Contact Us button below. 

For more information, contact us today!