Storing Coal Combustion Residuals in Ponds and Landfills for Beneficial Use

Coal is still a significant source of electricity in the United States, providing 30% of the country’s power—second only to natural gas (1). Burning coal to heat water that powers steam turbines produces coal combustion residuals (CCR), a byproduct that is typically disposed or beneficially used. In 2014, at least 46 million tons of coal ash were beneficially used, accounting for about 40% of all CCR produced. The rest is sent to landfills or specialized ponds for disposal (2).

Today, due to a regulatory amendment by the U.S. Environmental Protection Agency (EPA) in 2015, there are more opportunities to reuse CCR. In this article, we will discuss beneficial uses of CCR, how to create safe storage areas for CCR, and explain how to later harvest CCR from those storage areas.

Beneficial uses of CCR

Manufacturers can beneficially use CCR in place of virgin materials removed from the earth in a variety of products and materials, conserving natural resources. Over the years, EPA have implemented rules that encourages the beneficial use of CCR, which can have positive environmental, economic, and product benefits (3). Reusing CCR can also lower greenhouse gas emissions, reduce the cost of coal ash disposal, and improve the strength and durability of the products using the coal ash.

In most cases CCR is encapsulated, which means binding the coal ash in products like concrete, roofing materials, bricks, or wallboards. Encapsulation limits the ability of the coal ash to escape into the environment, protecting our water supply and local ecology from heavy metals that are toxic such as arsenic, lead, and mercury. More than 13 million tons of fly ash is used in the production of concrete products. Another 11 million tons is used in gypsum panels, or drywall.

When environmental risk of unencapsulated CCR (“loose particulate, sludge, or other unbound form”) can be demonstrated as “comparable to or lower than” use of standard materials and products the USEPA encourages CCR use. A little under a third of coal ash is put into unencapsulated use. The most common unencapsulated CCR use is processing and placement in structural fills and embankments (4).

For the most part, the use of CCR in concrete and wallboard products as well as regulated unencapsulated constructions have been deemed appropriate by the EPA because the environmental and human health impact is similar or lower than non-CCR products. While some power plants have arranged for the retrieval of CCR for beneficial use, most of the byproduct is sent to storage facilities like landfills or ponds.

The safe storage of CCR

Coal combustion residuals must be stored in specially designed ponds or landfills as set by the final CCR rule. These rules define design criteria for CCR storage including location restrictions, liner design, structural integrity requirements, and groundwater monitoring. The liner is especially important as it is responsible for preventing contaminants in CCR from leaching from the storage unit into the groundwater. The composite liner is a system that consists of two components: a geomembrane and a two-foot layer of compacted soil (5).

The geomembrane requirements are set by Title 40 section 257.70(b) or (c). If a high-density polyethylene (HDPE) geomembrane is used, for instance, then it should be 60-mil thick and be installed in direct and uniform contact with the compacted soil or lower-liner component. There are also related factors to consider, such as the structural integrity of the surface impoundment or landfill. The liner can have surface features like spikes or studs that contribute to slope stability or even drainage performance.

Once the storage system has been successfully constructed and it has begun accepting CCR, operators have to consider their long-term plan. For the most part, a storage unit accepts waste until it has reached its design limits at which point it is scheduled for closure. However, in some cases, it is possible to harvest CCR from storage for further use.

Harvesting CCR from storage for beneficial uses

Other byproducts of burning coal have uses that can be economically and environmentally beneficial such as fly ash. This material contains silicon dioxide as well as aluminum, iron, and calcium oxides. Each of these compounds can be useful in the creation of some products, enabling manufacturers to substitute fly ash in place of virgin materials. Calcium oxide, for example, is used in plasters, cement, and concrete to improve the strength, durability, and workability of each material.

By using fly ash instead of materials taken from the earth, manufacturers can reduce their environmental impact. There are also economic benefits such as reduced costs associated with coal ash disposal, increased revenue from the sale of coal ash, and savings from using coal ash in place of other, more costly materials.