Coal Ash Management

When coal is burned to produce electricity, it generates several byproducts. Coal combustion byproducts, or CCBs, are generically referred to as “coal ash.”  Fly ash, bottom ash and slag are all common terms to describe different types of coal ash. Coal ash is mainly made up of rocks, minerals and other non-combustible natural materials mixed in with the coal when it is mined from the earth.  In some cases, the ash might include nitrogen or sulfur compounds captured by air emissions-control equipment. Depending on the plant and process, these air emissions compounds may be collected and managed separately or co-mingled with coal ash.

Xcel Energy generating stations consume about 30 million tons of coal a year, yielding about 2.5 million tons of coal ash annually. Learn more about how we manage coal ash today at Xcel Energy power plants.

Each of Xcel Energy’s coal-fired generating stations is unique in the amounts and types of ash produced and how it is reused and/or disposed. Learn more about current ash management practices at individual Xcel Energy power plants.

Coal ash may be stored temporarily on-site at the power plant prior to reuse or permanent disposal. We collect the majority of our coal ash from plant equipment and temporarily stored in dry-storage silos. In some cases, we use water as an efficient means to convey ash material through a pipeline to a temporary holding pond or wet-storage bins. When temporary storage is full, the ash is removed, dewatered if necessary, and hauled away by truck for beneficial reuse or permanent disposal.

Most of our ash holding ponds are constructed as below-grade basins, similar to an in-ground swimming pool. A notable exception is the Sherburne County Generating Station (Sherco) where some ash ponds are substantially above grade or ground-level. Ash ponds that are above grade are subject to state dam-safety inspection programs.

EPA Assessment

In 2009, the U.S. Environmental Protection Agency (EPA) asked utilities across the country to submit detailed information on company-owned coal ash surface impoundments. Xcel Energy provided EPA with information on its 21 surface impoundments used to manage coal ash, as well as other surface impoundments, such as storm-water containment ponds. 

As part of EPA’s national coal ash pond safety assessment program, the agency inspected the Sherco Plant’s ponds in 2009.  After a rigorous inspection, the ponds were awarded the highest possible rating for design, safety and integrity. 

The recycling or beneficial reuse of coal ash offers many environmental, engineering and
economic benefits, such as:

• Improving the strength and durability of concrete and grouts
• Avoiding the need to construct new landfills
• Reducing greenhouse gas emissions by eliminating the need for virgin raw materials
• Reducing manufacturing and production costs

The U.S. Environmental Protection Agency and other federal agencies encourage the reuse of coal ash through a framework of engineering standards, material specifications, procurement guidelines, and other programs. Many uses of coal ash are also further regulated by state and local governments.

Throughout our system, we try to recycle coal ash whenever possible for beneficial reuse, such as in concrete products, roadbed material, soil stabilization, and engineered fill material. Other uses include manufactured aggregate, flowable fills and grouts, and grit for roofing shingles and sand-blasting. Some ash also is recycled for use in power plant air emissions-control equipment. Rather than using raw material, such as limestone, coal ash can be used as a sorbent to capture sulfur dioxide (SO2) from stack emissions. We strive to ensure that the coal ash we produce is only provided to third parties who will responsibly manage the product for beneficial reuse and in a manner consistent with federal, state and local requirements.

In 2005, Xcel Energy was the co-recipient of a Coal Combustion Products Partnership (C2P2) Award from the EPA for expanding the use of fly ash for soil stabilization in pavement and real estate development projects. In this application, fly ash is blended with weak on-site soils to stabilize or "strengthen" the soils.  The fly ash/soil mixture hardens like cement and provides the sub-grade strength and durability necessary for pavement or building support.  This is a lower-cost, lower-impact alternative to traditional construction methods where soil is removed and quarried aggregate is imported.

About half of the ash we produce annually is beneficially reused.  Further expansion of ash reuse is limited primarily by economic factors, such as local market demand, price and availability of competing raw materials, and transportation costs.

When our coal ash cannot be reused, our facilities dispose of the ash in regulated, engineered landfills. Some of our generating stations have on-site landfills and others dispose of ash at permitted, off-site commercial landfills. The design of each facility is tailored to local geologic, hydrologic and climatic conditions, as well as to comply with state and local regulatory requirements.

The Sherburne County Generating Station (Sherco) differs in that some of the on-site ponds are designed and permitted for permanent disposal of coal ash rather than temporary storage. The entire disposal pond is lined to prevent leakage into groundwater, capped when full and then dewatered using pumps. Through this process, the ponds are converted into a modern dry-landfill disposal facility. Sherco's ash management system also is designed so that all water that comes into contact with the coal ash, including all ash transport water and storm water, is collected and recycled for use in industrial processes at the plant.

Landfill design and operating practices

Today’s ash disposal facilities rely on synthetic and/or natural materials, such as manufactured plastic liners, low permeability clays or soils, or special geologic formations to contain the ash and prevent migration of contaminants to groundwater. We also incorporate prudent operating practices to minimize the potential for precipitation to come in contact with the ash.  These operating practices include:

• Minimizing the landfill working face to prevent exposure to precipitation
• Grading the site to control storm-water runoff
• Capping the landfill when full

A well-engineered and constructed cap is the key to long-term environmental performance for any landfill.  The purpose of a cap is to shield the landfill from precipitation, much like an umbrella. The cap provides long-term groundwater protection after closure by preventing precipitation from flowing down through the waste material in the landfill, where it could pick up contaminants along the way.

Landfill closure

Landfill closure is conducted under state approved plans that include post-closure care and monitoring.  Although post-closure care requirements vary by facility and jurisdiction, this activity typically involves on-going inspections, maintenance and groundwater monitoring to ensure the landfill cap is performing as designed and natural vegetation has been re-established to protect the cap from weathering and long-term erosion.  We also ensure that deed notifications and property restrictions are in place, so in the event of a future property sale, the new owner is aware of the landfill and legal obligations to maintain the cap’s integrity.

We construct and operate on-site storage ponds where ash is temporarily stored and on-site landfills where ash is permanently disposed in a manner that is protective of groundwater and the environment. The design of each facility is tailored to local geologic, hydrologic and climatic conditions, as well as applicable state and local regulatory requirements.  The design of these facilities and associated monitoring systems are reviewed by state and local regulatory authorities through a framework of facility-specific permits and operating plans.

Monitoring wells

Our ash disposal facilities use monitoring wells to confirm our design and operating practices are effective. The number and location of wells is determined as part of the facility permit or operations plan, and the system is tailored to site-specific geology and hydrology. A small facility may have as few as three wells, whereas a large facility may have a dozen wells or more. Wells also are often placed to monitor groundwater in and around storage ponds. Frequency of monitoring varies, depending on state or local requirements, but at a minimum occurs semi-annually. Monitoring results are reported to state or local authorities as required by the facility monitoring plan or permit.

For the first time, EPA is proposing to regulate coal ash disposal at the federal level. Xcel Energy supports the development of federal regulations for coal ash disposal under Subtitle D of the Resource Conservation and Recovery Act (RCRA-D). We support the D-Prime alternative discussed in the EPA proposal, which would regulate ash as a non-hazardous industrial waste, for several reasons:

• Coal ash has been determined to be non-hazardous based on well-established scientific criteria designed to protect human health and the environment.
• The EPA has confirmed this on multiple occasions.
• The states where we have coal ash landfills and ponds regulate our operations with programs that include permitting and operating plans, inspections and requirements for closure. Given the different climates and geologies of these states, the local engineers, geologists and hydrologists that implement our current state regulatory programs are most qualified to make decisions around such things as landfill liner design. EPA should establish suitable national RCRA-D standards and then work with the few states that may be lagging behind to ensure standards are implemented
• The D-Prime alternative would allow well designed and operated coal ash impoundments, such as the facility at our Sherco Generating Station, which received the highest-possible safety rating during a recent EPA assessment, to continue operating until the end of their useful lives. The D-Prime alternative would require ash ponds that cannot meet necessary environmental performance standards to close.
• Regulating coal ash as a hazardous waste would have significant negative environmental and economic consequences. It would become nearly impossible to dispose of or reuse the material and would lead to increased costs to our customers with no significant environmental benefit.