Power extends power plant ash pond life through community project
By F. Clarke Hobson, Jr., Duke Power Co., and Michael L. Hammons, Trans-Ash.
Transfer of 300,000 cubic yards of combustion byproducts
cuts costs at a church construction site and adds 6 years to
an ash settling basin?s life
Members of a North Carolina church will move into their expanded church facility sooner than expected. An arrangement to provide much needed low cost landfill material made the project possible thanks to a cooperative effort of the New Covenant United Methodist Church?s building committee, Duke Power Co., and contractor Trans-Ash of Cincinnati, Ohio.
A 28-acre building site is being developed (Figures 1a and 1b) with 300,000 cubic yards of coal combustion by-products from Duke?s Riverbend steam station near the Lucia, N.C., site of the church. The material is being removed from an active primary ash settling basin, or pond, without interrupting power production. This application of coal ash is providing significant savings for the church over conventional fill materials and extends life for the power plant?s primary ash settling basin by about six years.
The first step in this process was to lower the basin?s water level approximately three feet to facilitate ash removal. In addition, floating sediment barriers were installed around both discharge towers to control suspended solids in the power plant?s discharge to the Catawba River.
Specialized earth moving equipment began taking ash from the active primary settling basin in August 1994. The excavated material is being transferred to the church site at the rate of 2,500 cubic yards to 3,000 cubic yards per day. At the end of October, 1994, more than 100,000 cu yd of ash had been transferred. The entire 300,000 cubic yards of material was scheduled to be in place by the end of 1994.
Options for pond life extension
Duke staff members began studying options for extending the storage life of the primary ash settling basin back in the 1980s. They considered several alternatives (Table 1). One possibility not detailed in the table was to increase basin capacity by raising the crest elevation of the ash dikes that form the basin. This was ruled out because the basin?s entire perimeter would need to be raised.
Dredging ash from the primary settling site to an ash dredge basin (known as a OcellO), allowing time for the ash to dewater and then removing it for placement in a nearby landfill, also was considered. However, local regulations prohibit construction of new landfills along the Catawba River watershed near the Riverbend Steam Station.
Current N.C. solid waste regulations allow the location of new landfills beyond the watershed protection limits, but a new landfill must have synthetic liners and a leachate collection system. Cost of construction for this choice would be approximately $350,000/acre.
Another option, an existing ash landfill at another coal-fired station operated by Duke Power Co., also was considered as a potential destination for Riverbend?s ash. One major drawback for this option involved a one-way haul distance of 25 miles.
The church site development project is the culmination of efforts by many parties dating back to the late 1980?s. From 1989 to 1993, approximately 150,000 cubic yards of ash from the Riverbend power plant had been used in developing three other pieces of property near the plant. Many valuable lessons were learned about the collection, transporting and placement of the ponded ash during that time.
These projects used ash that had been dredged from the primary ash settling basin and moved into an ash dredge basin. The material was allowed to dewater over a period of time. An ongoing effort to control the amount of moisture in the ash was costly and time consuming, but was necessary to facilitate the placement and compaction of the ash at a fill site.
The dredge holding basin used a stone and fabric drain system to control moisture. Problems with this design ranged from loss of ash due to overdrainage to water retention. The latter resulted when fine material clogged the filter media. There also was the increased cost of having to handle the material twice?dredging to the holding pond, then excavating and removing ash to the fill site.
Regulations and project background
There were a number of changes on the regulatory front during the early 1990?s. Duke environmental personnel and other concerned parties worked with N.C.?s Department of Environmental Management to formulate permits and regulations to benefit the use of coal combustion byproducts and still protect the environment. In those years, watershed regulations were being promulgated and threatened to have a significant impact on the siting of new landfills. Other issues, including environmental liability and future costs, served further to slow the siting process. Such sites were seen as the most likely alternative for the Riverbend power plant?s ash utilization challenge.
In 1991, when two church organizations combined to form the New Covenant United Methodist Church, engineers explained the advantages of using a low cost fill to church committees and neighbors of the proposed expansion site. Duke Power Co. ash management, environmental and corporate communications personnel, along with an ash marketer, also held numerous meetings to be sure that everyone was properly informed of the benefits of using coal combustion byproducts here. Negotiations, engineering and site assessment work continued for nearly two years.
In the end, perseverance paid off. Because some engineering and permitting had already been completed, the project began after final approval in mid-1994.
Contractor personnel set about preparing the active primary ash settling basin for ash removal, and clearing and grubbing of the property was completed in August. With the necessary permits in hand and the site engineering complete, the first ash was moved onto the site in late August. Work continued at a rapid pace with the anticipation of removing the 300,000 cubic yards by the end of 1994.
Benefits all around
All parties involved have benefitted from the project. The church now is able to develop a site that might not have been economically possible with conventional fill material, the ash and the utility benefits by avoiding future high disposal costs, and the contractor had an opportunity to exhibit its expertise in a new market. The utility also is able to use a large volume of coal combustion byproducts in a way that benefits the local community.
Numerous regulators, contractors, engineers and members of the general public have visited the site to examine the engineering and the factors that make the project an environmentally sound application. END
Details of River Bend Station
Riverbend Steam Station is a fossil-fueled power plant consisting of four active coal-fired units, three retired coal-fired units and four gas turbine units in Gaston County, N.C. Two units that began commercial operation in 1952 have a generating capacity of 94 MW each and two units that began commercial operation in 1954 brought the generating capacity of the station up to 454 MW. The plant?s gas turbine units began commercial operation in 1969. They have a generating capacity of 34 MW each, which brings the total generating capacity of the power plant to 590 MW.
The Riverbend plant site consists of 572 acres that includes 9 acres of plant yard, 13 acres of coal storage, 9 acres of switchyards, a 33-acre primary ash settling basin and an ash dredge basin with a surface area of 5 acres.
Ash generated by the station is sluiced approximately 3,100 feet through pipelines to the ash settling basin. The primary basin functions as the storage facility and has a total capacity of 1,437,000 tons. This basin drains to a secondary settling basin through a reinforced concrete discharge tower and pipe. The secondary basin functions as a polishing pond to further improve water quality and its water storage capacity is 390 acre-feet. Water is discharged from the secondary settling pond to the Catawba River impoundment (known as Mountain Island Lake) through another reinforced concrete discharge structure and pipe. The discharge is regulated by a National Pollutant Discharge Elimination System (NPDES) permit that is issued under the Clean Air Act by the North Carolina Department of Environment, Health and Natural Resources.
As of August 1994, the remaining ash storage capacity of the primary ash settling basin was 110,000 tons. Coal consumption projections indicated that the primary ash settling
basin would reach full capacity by mid- to late-1996.