By Russell Ray, Managing Editor
Four years after more than one billion gallons of coal ash slurry spilled from a storage pond at the Kingston Power Plant in eastern Tennessee, environmental groups are still waiting for the U.S. Environmental Protection Agency to finalize tougher standards for handling and storing coal ash.
Their wait may soon be over.
Industry observers expect the EPA to issue a rule this year expanding the oversight of bottom ash management and disposal at U.S. power plants.
The proposed rules would require coal-fired power plants to eliminate wet ash handling and phase out surface impoundments, or ponds, within five years. Anticipating tougher standards, most power producers have already studied the cost of converting to dry bottom ash systems and are bracing for the regulatory changes. The cost of compliance could exceed $20 billion industrywide, according to a 2010 EPA study.
|Clyde Bergemann’s patented DRYCON™ technology is a mechanical conveyor that conveys and cools bottom ash without the use of water. Photo courtesy of Clyde Bergemann|
“The overwhelming majority of utilities have done some level of technology investigation and definitely a budget study,” said Kevin McDonough, director of sales Americas for United Conveyor Corp. “Almost all of them have gone that far, so that they understand what technical options are out there and, of course, the approximate cost associated with it.”
The potential market for dry bottom ash conversions is significant. Less than 1 percent of the nation’s coal-fired plants are equipped with dry bottom ash systems, said Ron Grabowski, vice president of Business Development at Clyde Bergemann. More than 90 percent of bottom ash systems remain wet.
“Most likely you’re going to have to be a zero discharge plant,” Grabowski said. “If you’re using water to move around your bottom ash, you can’t discharge it.”
In 2010, the EPA offered two proposals to regulate the handling and disposal of coal ash. The first option calls for classifying coal ash as a special waste regulated by the Resource Conservation and Recovery Act (RCRA) Subtitle C hazardous waste provisions. Under the second option, coal ash disposal would be regulated under RCRA’s Subtitle D nonhazardous waste provisions.
In either case, wet ash handling would be eliminated and surface impoundments would be phased out within five years.
“We don’t think they’re going to reclassify bottom ash to be a hazardous material,” Grabowski said. “I think they recognize there are beneficial uses for ash as a byproduct. They simply want to persuade plants to eliminate the discharge of water.”
In December 2008, a 40-acre coal ash storage pond at Tennessee Valley Authority’s Kingston plant in Harriman, Tenn., failed. The earthen wall collapsed, spilling more than 1 billion gallons of coal ash slurry, damaging 40 homes and contaminating the Emory and Clinch Rivers. The cleanup costs for TVA: About $1 billion.
“Since that time, TVA has responded aggressively to wet-to-dry conversions and has converted wet system at both Kingston and Bull Run to dry,” McDonough.
The incident prompted the EPA to pursue a new rulemaking, but the agency delayed issuing a final rule after intense political pressure from utilities, coal-mining companies and coal ash recyclers who fear classifying coal ash as a hazardous waste would stigmatize their products.
Meanwhile, Congress has proposed legislation that would pre-empt EPA’s proposed rules by granting states the authority to regulate coal ash disposal through the use of permitting programs. Critics say the legislation would keep states in control and stymie the EPA’s effort to promulgate new rules for the disposal of coal ash, also known as coal combustion residuals (CCR).
|In addition to water elimination, DRYCON™ will increase boiler efficiency, reduce power consumption and reduce maintenance. Photo courtesy of Clyde Bergemann|
According to a report by the non-partisan Congressional Research Service, the legislation provides states too much discretion in adopting a permit program or applying federal standards for disposal of coal ash.
“EPA would have no authority to compel states to adopt and implement the program according to provisions in the proposed amendments to RCRA,” the report found.
Coal-fired power plants have three options for the disposal of coal ash. Dry ash can be disposed in landfills. According to the EPA, more than 30 percent of coal combustion waste from power plants is disposed in dry landfills. Coal ash is, of course, stored in ponds, which account for 20 percent of coal ash disposal. About 40 percent of coal ash is recycled and used in a wide range of industrial applications.
More than 300 coal-fired plants in the United States dispose of coal ash in on-site landfills, according to an EPA report. Nearly 150 plants use off-site commercial landfills for coal ash disposal. Nearly 160 U.S. plants use coal ash ponds for disposal.
The size of coal ash disposal units can range from modest to very large, with some ponds covering 1,500 acres or more.
Meanwhile, power producers using ponds to store coal ash have important choices to make. They have several options and solutions to choose from as they prepare to comply with stricter federal regulation.
Power producers seek solutions from companies like Clyde Bergemann and United Conveyor Corp., two of the leading suppliers of dry ash handling systems. What follows is a description of the technologies available to power producers.
Coal-fired power plants have four basic options to upgrade their existing wet bottom ash system. The first two options not only eliminate the use of an ash pond but also eliminate the need of a wet impounded bottom ash hopper. The other two options keep the wet impounded bottom ash hopper but eliminate the ash pond.
What follows is a brief description of each, listed in the typical order of power plant preference:
Convert to a dry bottom ash system
Clyde Bergemann’s DRYCON technology is a mechanical conveyor that conveys and cools bottom ash without the use of water. With the successful wet to dry conversion of two 650-MW units in Florida, the industry is now seeing the benefits of this technology. Dry bottom ash handling provides the most benefits over all other bottom ash technologies. In addition to water elimination, DRYCON™ will increase boiler efficiency, reduce power consumption and reduce maintenance.
The advantages of using this system are: Increased boiler efficiency, reduced maintenance, reduced power consumption and complete water elimination. The disadvantages: A 20- to- 30-day boiler outage and a direct path from under the boiler is required.
Convert to a Submerged Scraper Conveyor (SSC) semi dry system
An SSC can reduce water usage but not eliminate it. In most cases, power plants that can be fitted with a SSC can also be fitted with a DRYCON unit.
The advantages of converting to this system are: Reduced power consumption, reduced maintenance and low water consumption. The disadvantages: It’s not a dry system, boiler efficiency will not increase, it needs a direct path from under the boiler and a 20- to- 30-day outage is required.
Divert ash slurry to a Remote Submerged Scraper Conveyor
Clyde Bergemann’s patent pending ASHCON technology is also a semi dry system. Its major advantage is that is can be installed remotely from the boiler to intercept bottom ash slurry and dewater it without the use of an ash pond. Because of its low height, in most cases, the existing bottom slurry pumps can be reused.
The advantages: No outage is required, reduced power consumption, allows reuse of slurry pumps and the wet bottom ash hopper is unaffected. The disadvantages: It’s not a dry system, there is no increase in boiler efficiency and the wet bottom ash hopper is unaffected.
Divert ash slurry to traditional Dewatering Bins
Though this is a viable option, it is typically the least desired. Dewatering bins are a 40-plus year old technology and reviewed as a last resort when pond elimination is being considered. Many plants with dewatering bins have reached the end of their expected life cycle. Thirty-year-old dewatering bins can be worn and structurally unsound. In this situation plants investigate a bottom ash upgrade rather than replacement.
The advantages: Reduced power consumption, no outage is required and the wet bottom ash hopper is unaffected. The disadvantages: It’s not a dry system, slurry pumps may need to be modified, there’s no increase in boiler efficiency and additional slurry pumps may be needed to pump the slurry up the tall height of the new dewatering bins.
United Conveyor Corp. supplies dry bottom ash systems to utilities seeking wet-to-dry conversion technologies.These retrofit projects have been driven by many factors, including the anticipation of formal EPA regulations imposing new requirements on coal ash ponds and power plant effluent water quality.
|This continuous dewatering and recirculation system from United Conveyor Corp. was recently installed at a plant in South Carolina. The technology combines the benefits of a recirculation system and the proven technology of a submerged flight conveyor. Photo courtesy of United Conveyor|
In each instance, the focus has been on eliminating bottom ash sluice water from the plant wastewater balance by utilizing technologies that eliminate or minimize water from the existing bottom ash conveying systems or ones that provide closed-loop conveying and dewatering systems that result in Zero Liquid Discharge.
UCC’s Continuous Dewatering and Recirculation (CDR) system with Remote Submerged Flight Conveyors (SFC) is a preferred wet-to-dry conversion option for installations that have physical limitations underneath the boilers and seek to minimize costly outage-related activity.
The CDR system was the technology of choice for a South Carolina utility that recently invested in new wet bottom ash hoppers, as it combines the benefits of a recirculation system and the proven dewatering technology of a UCC Submerged Flight Conveyor (SFC).
The CDR system is designed to receive existing sluice conveying lines and divert the bottom ash slurry to a remote dewatering conveyor outside of the boiler house. Material is collected, dewatered and then discharged into a load-out bunker or secondary transfer conveyor in a condition that is favorable for transport to and compaction in a dry landfill.
After completing a fine particulate settling phase, the sluice water is then pumped back to the boiler house to complete a closed-loop, zero-liquid discharge system. The CDR system has been designed to address the complexities of a bottom ash water balance, considering multiple flow sources, intermittent conveying cycles and variable flow rates. This conversion option is highly favorable when considering capital cost and maintaining plant availability, as this can be implemented with no outage.
The SFC system dewaters the ash to the required percentage and is delivered to the marketer for beneficial use. It does not have any sticks or other foreign material when dredging it out of the old pond.
The patented UCC PAX Pneumatic Ash Extractor is a preferred wet-to-dry conversion option for installations that have physical limitations under the boiler and whose existing bottom ash hoppers may be in need of repair after years of operation.
The PAX system was the technology of choice of a North Carolina utility that preferred a conversion solution that removed water as a conveying medium. For this technical alternative, bottom ash is collected dry in a refractory-lined hopper under the boiler. Percolating air cools the ash to help complete combustion of unburned material and protection of ancillary equipment, resulting in greater boiler efficiency. As the ash cools, it is fed and crushed into a pneumatic conveying line and transported to a storage bin or transfer station.
Submerged Flight Conveyor System
UCC’s SFC is a proven bottom ash system and a cost-effective solution when long-term life cycle costs are a major decision factor and when existing bottom ash hoppers may be in need of repair. The SFC collects ash from the boiler into a water-filled trough where it quenches and cools the ash.
Horizontal flights move the ash continuously through the trough and up a dewatering ramp where it is then discharged into a load-out bunker or secondary transfer conveyor. Overflow water from the SFC trough is cooled and recirculated to complete the ZLD system. Numerous utilities have successfully implemented the SFC technology, which has been the industry standard on new units for the past few decades.
Vibrating ash conveying (VAX) is United Conveyor’s 100% dry solution technology. Vibrating conveying provides continuous removal of ash under the boiler, increasing boiler efficiency. The catch and throw motion of the vibrating deck moves ash from under the boiler to a crusher, where it is fed to a secondary conveyor. Cooling air is forced up from under the vibrating deck, combusting any unburned bottom ash.
For more information on coal ash handling solutions, review the archives of Power Engineering magazine at www.power-eng.com.
Power Engineerng Issue Archives
View Power Generation Articles on PennEnergy.com