Presentation on theme: "Site is on the upper end of the Watts Bar Reservoir just north of Interstate 40. The coal-fired generating station was constructed in the 1950’s. The."— Presentation transcript:
Site is on the upper end of the Watts Bar Reservoir just north of Interstate 40. The coal-fired generating station was constructed in the 1950’s. The generating station draws cooling water from the Emory River and discharges to the Clinch River which in turn enters the Tennessee River. Slack water from the Watts Bar Dam envelopes the mouth of both the Clinch and the Emory River. Prior to the ash spill the main channel of the Emory River at the Site is approximately 30-feet deep.
Kingston Power Plant Ash Storage Clinch Emory River
Originally, ash was sluiced wet from the plant to a basin constructed in the Watts Bar Reservoir The basin is contained by a clay dike that is raised three times to eventually impound 60-feet thickness of ash Starting in the 1980’s, accumulating ash was dredged and stored in an engineered storage cell constructed on top of the filled basin By late 2008, the storage cell reached 60-feet height and covered approximately 90 acres
Bottom ash removed in sluice trench Fly ash settled in Ash Pond Water quality obtained in stilling basin Ash pond solids hydraulically dredged to ash storage cell Emory River Ash Storage Cell Plant Water Intake Ash Pond Stilling Basin Sluice Trench
Early morning of December 22, 2008 a dike failure near the northwest corner of ash storage results in static liquefaction of nearly 2/3 of the stored ash The flowing ash water mixture flows into the Emory River moving both upstream and downstream When the flow stopped, the main channel of the river was filled with ash The Emory flows around the ash in a shallow area of the Watts Bar Impoundment
Main channel filled with over 30 feet of ash Emory River flowing around the ash in shallow impounded water of Watts Bar Reservoir Ash Storage Cell
Fly ash is the fine residue from the burning of coal that escapes with the combustion gas and is removed by air treatment equipment With bottom ash it is 10% by weight of the coal at Kingston The majority of fly ash is within the size range of 100 to 1 micron (fine sand and silt particle size) The particles are spherical with some particles forming as hollow spheres known as cenospheres Because of the cenospheres the solid specific gravity may be lower than for earth minerals (as low as 2.0)
Fly Ash consists primarily of oxides of silicon, aluminum iron and calcium (85-90% by weight). Magnesium, potassium, sodium titanium, and sulfur oxides make up most of the remaining weight with traces of heavy metals. Specific gravity of fly ash at Kingston in the 2.2 -2.4 range Source: Federal Highway Administration
Grain Size of Ash in Dredge Pipeline from Pilot Test SILT sizesSAND Sizes
Time Critical Removal was to open Main Thalweg of the Emory River Non-Time Critical work on land and in embayments separated from Time Critical by a landside rock berm. Opening river channel to approximate original contours was goal of removal It was recognized that further dredging may be required in later stages of the ash recovery after completion of the Time Critical Removal
Upland Rock Berm Segment 1: Ash Filled Full Channel Segment 2: Full Channel but With Some Flotation Water Segment 3: Underwater Rock Berm to Prevent Further Downstream Movement. Segment 4 & 5: Thinning Ash Thickness with Flotation Water
It was considered time critical that the main channel be open by no later than Spring 2010. Avoid impact from the Spring flood season of 2010. Besides the logistics of dredging and dewatering the ash, the logistics of removing the ash from the site had to be resolved to complete the removal. It was estimated that the time critical work involved 3.0 million cubic yards of ash.
Experience at the site was already good with hydraulic dredging for building the ash storage cell It was believed that the ash would settle out quickly in a rim ditch removal method as was used to build the ash storage cell The production rate using mechanical dredging was slower with the same level of manpower and equipment The existing ash pond and sluice ditch were available for hydraulic operation
Pilot testing of concept to hydraulically dredge to a Rim Ditch started late March 2009 and ran through July 2009 Trans Ash of Cincinnati Ohio operated (3) Ellicott 370 10-inch pump swinging ladder dredges with booster pumps Rim Ditch 1,800-ft. long, 40-ft. wide and 10-ft. deep Solids delivered to Rim Ditch at 4,000 dry tons per day Total flow in Rim Ditch of 8,300 gpm (10 MGD)
Dredges had twenty foot, twenty-five foot and thirty foot ladders
Rim Ditch prior to start of Pilot Dredging Ball Field Area Sluice Trench Rim Ditch
Average percent dry solids to the ditch 8.4% Flocculent Settling Rate 14 in/hour Solids Content after 12 hours of quiescent settling 65% Rim Ditch removal rate of 90% when the flow was under 10,000 gpm With higher flows more solids to the Sluice Trench Constant agitation of the Rim Ditch by backhoe results in a thickened solids content of 30-35% in the water column of the ditch.
10-inch pump dredges were too small to complete the work within the timeline goals Substantial problems with plugging from trees and other debris Ash is very abrasive and results in heavy maintenance impact on operation. Three 10-inch dredges moved on average 4,000 cubic yards per day Problems with dredges from fast river currents At end of pilot 375,000 cubic yards removed
Competitive bidding to dredge at full-scale operation defined as completing 1.5 million cubic yards to open the main channel by February 28, 2010. Required production rate of approximately 12,000 cubic yards per day 6-days per week Competitive bidding for the work with Sevenson Environmental selected for the work. Dredging equipment initially on site was one 20-inch cutterhead dredge and one 14-inch cutterhead dredge Later supplemented with a 16-inch cutterhead dredge
20-inch River Dredge average 470 cubic yards per hour 16-inch dredge average 230 cubic yards per hour 14-inch dredge average 160 cubic yards per hour Rim ditch is operated at twice its hydraulic capacity (on average 20 MGD) Substantial solids carryover to sluice trench and ash pond
Hydraulic StructurePercent of Solids Retained Rim Ditch55% Sluice Trench11% Ash Pond34%
Rim Ditch operated at 20 MGD 100% above its hydraulic capacity determined during the pilot test Efficiency of the Rim Ditch was reduced from the pilot operation and the fly ash pond was required to store ash With the use of a coagulent, the stilling basin continued to produce acceptable water for discharge (permit limit average TSS of 29 ppm) Using a dredge to remove ash from the fly ash pond and cycle back to the Rim Ditch provided little improvement of performance
Ash Pond filled at rate of approximately 4,000 yd 3 /day (approximately 1/3 of the daily in-river dredge volume) February 1, 2010 1.5 million cubic yards removed under full scale dredging Ash Pond removal was undertaken in January 2010 to develop new pond capacity to complete Emory River removal goal Goal of opening river channel is obtained with very little reserve capacity in ash pond