Beneficial use of coal ash can reduce greenhouse gas emissions when used in concrete, serve as an agricultural amendment to improve soil quality, and buffer acid mine drainage to mitigate environmental impact.
Across the United States, over seven hundred storage units (i.e., landfills and surface impoundments) hold the remnants of America’s coal-burning past. These coal combustion products (CCPs), also known as coal combustion residuals (CCRs), are the byproducts of generating electricity from coal combustion. Decades of coal dependency have left the United States with 1.5 billion tons of CCPs in storage units. These CCPs, like coal, can be mined and repurposed for our benefit. With new federal regulations, operators or owners of old ash storage units have been given the question of whether to retrofit the storage unit or reuse the ash.
In 2015, the U.S. Environmental Protection Agency (EPA) implemented the first federal regulations for active coal ash storage units. This regulation, known as the 2015 Coal Ash Rule, required active landfills and surface impoundments containing coal ash to report on controls such as groundwater and structural monitoring and determine a method of closure. Then, in 2024, changes to the 2015 Coal Ash Rule were completed, requiring some legacy surface impoundments (i.e., surface impoundments at coal combustion plants closed by October 19, 2015) to comply with nearly all requirements of the 2015 rule. Facilities directed to comply had two options: 1. closure in place and 2. closure by removal. Beneficial use options satisfy the closure by-removal option, offering an opportunity to recycle the CCP and meet federal regulations.
CCPs exhibit various properties and can be sold into markets based on material characteristics. For example, some CCPs are pozzolanic, making them ideal for cementitious materials, while others have a high percentage of calcium sulfate, which is beneficial as an agricultural amendment. The reused CCPs fall into two categories: encapsulated and unencapsulated beneficial use. Encapsulated uses, such as concrete, can reduce greenhouse gas (GHG) emissions, while unencapsulated uses, such as soil amendments, can be used for sustainable agriculture. This article highlights innovative examples of beneficial use of CCPs that aim to reduce environmental impact and enhance resilience.
Encapsulated use of CCPs involves binding the ash to a solid structural matrix, which minimizes the mobilization of ash materials into the environment. When CCPs are encapsulated in materials like concrete, the leaching of heavy metals poses no increased environmental threats and produces durable and cost-effective products. Federal regulation and research promote encapsulated use to embrace sustainability concepts.
Studies indicate that using encapsulated CCPs in concrete, cement, and bricks can significantly lower GHG emissions. This is primarily because the production of Portland cement, a key component of concrete, is energy-intensive and generates substantial carbon emissions. Substituting CCPs for Portland cement can reduce these emissions by up to 40 percent. Additionally, CCPs may help support bacteria in self-healing concrete, enabling it to repair cracks up to 0.8 mm wide independently. These innovative uses of CCPs not only enhance the durability of construction materials but also reduce the carbon footprint of traditional building practices.
CCPs are designated as unencapsulated when used in an unbound or loose particulate form. When applied at appropriate rates, CCPs as soil amendments can improve acidity, texture, moisture content, and concentration of nutrients in soils. However, in loose particulate form, unencapsulated uses require engineering controls to limit runoff and leaching of metals. While the use of CCPs in this form is sustainable, regulations may limit ash as structural fill. In contrast, flue gas desulfurization gypsum (FGD gypsum) has been supported in agriculture. The EPA has assessed its environmental risks and concluded it poses no concerns when applied appropriately, with guidelines ensuring it serves a new purpose while meeting environmental standards.9
FGD gypsum, a type of CCP, serves as an agricultural amendment by providing nutrients and limiting phosphorus runoff. A recent Ohio study found that FGD gypsum helps inhibit phosphorus runoff, which can decrease harmful algae blooms. CCPs also offer a sustainable alternative to natural gypsum, reducing the need for extraction.
In addition, the alkaline properties of CCPs are effective in remediating acid mine drainage (AMD), a problem that arises when mining minerals are exposed to water and air, degrading streams by clogging them with sediment, lowering the pH, and altering the water chemistry. Beneficial use of alkaline CCPs to mitigate the impacts of AMD has been explored in case studies from Ohio and Indiana, among others. Studies from Ohio and Indiana show that alkaline CCPs can raise the pH of AMD, reducing metal dissolution and alleviating its harmful effects. By using CCPs for mine reclamation, we can restore these areas while extending the life cycle of coal.
As coal dependency shrinks and climate change impacts become apparent, the materials we previously regarded as waste may offer a solution. With more regulations, harvesting ash from storage units is becoming more cost-effective but requires careful management. Reusing CCPs can produce solutions for resilient communities and be protective of the environment. To fully realize these benefits, it is essential to implement standard methods and controls for the beneficial use of CCPs.
Proper characterization of CCPs is essential for identifying their safe uses and minimizing environmental risks. The American Society for Testing and Materials (ASTM) standards E3355 and E3183 provide comprehensive guidelines for characterizing and harvesting CCPs. Evaluation often involves treatability studies and Leaching Environmental Assessment Framework (LEAF) analyses to assess leaching potential. ASTM E3355 details procedures for characterizing CCPs, while E3183 covers harvesting methods from active and inactive units, including water management, contingency planning, and erosion control. Following these standards ensures the safe and beneficial use of CCPs, extending the life cycle of coal ash.
Burning coal has affected our planet’s climate. However, coal ash can be repurposed to reduce GHG emissions and restore the environment. By understanding CCP’s properties and protecting communities, the beneficial use of CCPs can help create a greener future.
