When coal is burned to produce electricity, it generates several byproducts. Coal combustion residuals, or CCRs, are generically referred to as “coal ash.” Fly ash, bottom ash and slag are all common terms that 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 equipment used to control air emissions. Depending on the plant and process, these compounds from air emissions may be collected and managed separately or co-mingled with coal ash.
Our generating plants consume more than 25 million tons of coal a year, yielding approximately two million tons of coal combustion residuals annually. Each coal-fueled plant is unique in the amount and type of ash produced and how it is reused or disposed. Nearly all of the coal ash from our operations is permanently disposed in a dry form. The Sherburne County Generating Plant is the only facility that permanently manages wet coal ash.
Coal ash may be stored temporarily on-site at the power plant prior to reuse or permanent disposal. Our current practice is to collect the majority of coal combustion residuals from plant equipment and temporarily store it 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, de-watered as necessary, and hauled away by truck for beneficial reuse or permanent disposal.
Today, most of our ash holding ponds or impoundments are constructed as below-grade basins, similar to an in-ground swimming pool. A notable exception is the Sherburne County Generating Plant (Sherco) where some ash ponds are above grade or ground-level. The ponds are designed like dams, and are constructed and operated under a state permitting program that includes periodic safety inspections, monitoring and reporting.
In 2009, the U.S. Environmental Protection Agency (EPA) began a process to assess and inspect coal ash surface impoundment facilities around the country. Xcel Energy provided EPA with information on our facilities used to manage coal combustion residuals, as well as other surface impoundment facilities, 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:
The U.S. Environmental Protection Agency (EPA) and other federal agencies encourage the reuse of coal combustion residuals through a framework of engineering standards, material specifications, procurement guidelines and other programs. Many uses of coal ash also are further regulated by state and local governments.
Our current practice is to recycle coal ash throughout our system 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 emission-control equipment. Rather than using raw material in control equipment, such as limestone, coal ash can be used as a sorbent to capture sulfur dioxide from stack emissions. We currently do everything possible 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. The addition of equipment to control emissions of nitrogen oxides, sulfur dioxide and mercury are further limiting reuse, resulting in the increase of disposal of coal ash in on-site and commercial landfills.
In February 2014, the EPA released its evaluation on the beneficial use of fly ash in concrete and gypsum wallboard. EPA concluded that using encapsulated coal combustion residuals in concrete and wallboard is appropriate and protective of human health and the environment.
When coal ash cannot be reused, Xcel Energy's current practice is to permanently dispose of ash in regulated, engineered landfills. Today some of our power plants 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 Plant (Sherco) is unique in that some of the on-site ponds are used for permanent disposal of coal ash rather than temporary storage. The ponds are designed like dams, and are constructed and operated under a state permitting program that includes periodic safety inspections, monitoring and reporting, as well as pond closure and post-closure monitoring requirements. The ponds also are lined to protect groundwater. When full, they are capped and then de-watered using pumps. This process converts the ponds into dry-landfill disposal facilities. Sherco's ash management system is designed to be "zero discharge", which means that all water used to sluice ash to the ponds is collected and then reused in industrial processes at the plant.
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:
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 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 ongoing 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.
Our current practices help to ensure that we construct and operate temporary and permanent storage ponds 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. Through a framework of facility-specific permits and operating plans, state and local regulatory authorities review the design of these facilities and associated monitoring systems.
Today we use monitoring wells to confirm the design and operating practices of our ash disposal facilities 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 landfills, impoundments and 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.
The U.S. Environmental Protection agency regulates the disposal of coal ash under the Coal Combustion Residuals Rule, published in April 2015. The following are documents that Xcel Energy is required to provide under the new rule:
Unstable Area Report
Run-On and Run-Off Plans
Closure and Post Closure
Post Closure Plan
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