The Tennessee Valley Authority (TVA) is known for its large fleet of coal-fired plants. With TVA’s renewed emphasis on nuclear power and gas-fired generation, the organization will soon fulfill its new goal: “to be one of the nation’s leading providers of low-cost cleaner energy by 2020.” Construction of the 880-MW John Sevier Combined Cycle Plant puts TVA one step closer to achieving that goal.
The Tennessee Valley Authority (TVA) is a federal utility that provides electricity to seven southeastern states. Electricity is produced by a system of 29 hydroelectric dams, 56 units at 11 coal-fired plants, three nuclear plants, and 11 gas-fired plants, for a total of 33,700 MW of installed capacity. The coal-fired plants produced about 51% of the electricity generated in 2010. That mix of resources will change in the future with TVA’s new integrated resource plan that states: “any needed capacity will be replaced with low-emission or zero-emission electricity sources, including renewable energy, natural gas, nuclear power and energy efficiency.”
The integrated resource plan includes closing uneconomical coal plants. In August 2010, TVA President and CEO Tom Kilgore unveiled plans to idle or retire nine units at several of TVA’s older coal plants, including two units at the four-unit, 704-MW John Sevier Fossil Plant (JSF), located near Rogersville, Tenn. In April 2011, TVA announced additional plant retirements, including the remaining two units at JSF that will be “idled” at the end of this year; TVA will “control, convert, or retire” those remaining two units no later than the end of 2015. In total, the plan calls for retiring about 2,700 MW of coal-fired capacity across the TVA system by the end of 2017.
At the same time, TVA has been busy upgrading its system with new gas-fired plants. The John Sevier Combined Cycle Plant (JSCC) is the fifth combined cycle gas plant TVA has bought or built in the past five years, for a total capacity of about 4,000 MW. TVA, which used little natural gas to produce electricity only five years ago, now owns gas-fired plants at Caledonia, Magnolia, and Southaven in Mississippi and at Lagoon Creek near Brownsville, Tenn., in addition to its fleet of combustion turbines located at several of its coal-fired sites. Kilgore noted during a May 4 conference call with the financial community, “We started [the move to gas-fired plants] long before low gas prices, so that just shows you that a balanced portfolio really does work.”
A More Efficient Option
JSCC, located adjacent to JSF, was approved by the TVA Board in June 2009 in response to a ruling by the U.S. District Court for the Western District of North Carolina regarding control of interstate emissions. The court set an accelerated schedule (compliance by Jan. 1, 2012) for TVA to install emissions control equipment on all four units at JSF. TVA determined that the short compliance schedule combined with an extended conversion shutdown was much more costly than shuttering two units, idling the remaining two units, and building a new gas-fired combined cycle plant.
Reliable electricity production from the new plant is essential because it, according to a TVA spokesman, “anchors the eastern portion of TVA’s power system, the absence of generation would increase the risk of power supply disruptions and reduce overall power system reliability.”
In April 2008, URS was contracted to perform engineering and procurement for a combined cycle conversion project in Gleason, Tenn. Then, in spring 2009, that project was deferred and the engineering and material were redeployed to the JSCC project.
In March 2010, Kiewit Power Constructors Co. (KPC) was awarded a contract for construction of JSCC with a June 2012 completion target date. The air permit was received on Apr. 17, 2010, and construction officially began when the first pile was driven into the ground two days later.
The $790 million JSCC began commercial service on Apr. 30, a month ahead of schedule and about $30 million under budget. “The John Sevier Combined Cycle Plant is a testament to TVA’s commitment to provide cleaner, lower-cost generation,” said Bob Deacy, senior vice president, Generation Construction, the organization within TVA responsible for the construction project. “Credit for this great success goes to the workers and the project management team for implementing the construction plan with a tireless eye toward quality work while beating our commitments for timing and budget.”
Innovation and Technology
The configuration of the plant is 3 x 1: three General Electric 7FA.04 dual-fuel combustion turbines, the first of this new generation to be shipped by the manufacturer; three Nooter/Eriksen (N/E) heat recovery steam generators (HRSGs), including bypass dampers that allow the combustion turbines to operate in simple cycle mode; and a single Toshiba 400-MW steam turbine generator. Each HRSG is configured with a selective catalytic reduction system, carbon monoxide catalyst, and duct burners.
The entire John Sevier facility required a complex environmental permit as a result of the need to incorporate the combined cycle plant into existing permits for the adjacent coal plant. JSF’s National Pollution Discharge Elimination System permit is shared with the new combined cycle plant and was revised to include one additional outfall. With the exception of sharing the intake structure and electric transmission lines, almost all aspects of the JSCC design are independent of the existing coal plant.
Because each of the three HRSGs is outfitted with a bypass stack for simple cycle operation and a standard stack used during combined cycle operation, careful accounting of air emissions between the coal plant and the combined cycle plant became more complex. Each emission source across the two plants is tracked in a ledger to demonstrate compliance with the requirements of the shared air permit (Figure 1).
|1. Stack-sharing system. The new John Sevier Combined Cycle Plant shares a site emissions permit with the four-unit John Sevier Fossil Plant. The process of identifying emissions sources requires a continuous accounting of which stacks are being used: the HRSG stacks (on the left) when running in combined cycle mode, the bypass stacks (center) when operating in simple cycle mode, and the coal-fired plant stacks (not shown, but on the far right). Courtesy: Kiewit Power Constructors Co.|
In early April 2010, KPC met with Barnhart Crane & Rigging (Barnhart) and N/E to strategize how to set the 36 HRSG tube bundles. In the past, three cranes were required to successfully flip, tail, and set a bundle. Barnhart developed a unique tilting frame requiring only two cranes to flip the bundles and therefore expedite the bundle installation process. This unique approach eliminated the need for craftsmen to work beneath the suspended load and simultaneously increased the crew’s productivity, from three bundles to five bundles set per day.
Transportation of the steam turbine generator from storage in Memphis, Tenn., to the job site was a significant feat given its size and weight. The team called on Barnhart to develop the complex transportation plan. First, the equipment sailed 917 miles from Memphis to Knoxville, over about 13 days. Once it arrived on land, a specialty vehicle with 28 dollies and 224 tires, with a combined weight of 1,334,000 pounds—including the weight of four pulling/pushing tractors—was used to move the steam turbine generator to the site. The convoy, with an overall length of 344 feet, took four days to arrive on site from Knoxville.
Emphasizing Teamwork, Safety, and Quality
Jobsite safety was of the utmost importance for those working on the JSCC project. Simply stated, the team’s motto was “nobody gets hurt.” The TVA/URS/KPC team worked more than 3.1 million man-hours without a single lost time accident. The jobsite experienced only five Occupational Safety and Health Administration (OSHA) recordable injuries, 69 first aids, and the lowest OSHA recordable injury rate of all active construction projects within TVA’s fossil fleet.
Craft engagement was the key to a successful safety culture, and the Tennessee Valley Trades and Labor Council stepped up to the challenge. Job hazard analyses and SLAM (stop, look, assess, manage) cards were required for every operation, no matter how small. Jobsite management established an open door policy for anyone and everyone to voice safety concerns from start to finish.
By doing it “right the first time,” the team was able to adhere to the aggressive schedule and successfully build a quality project for TVA. John Sevier was completed with only 96 nonconformity reports written and two welds rejected out of 1,616 tested (of 2,960 total welds) for a 0.12% reject rate. The team established quality hold points for all operations to maintain uniformity. Furthermore, everyone was involved in pre-activity meetings prior to starting an operation to ensure that all concerns and specifications were addressed.
The team successfully overcame numerous design challenges as well. After relocation of the Gleason project, the engineering team verified that the equipment previously purchased for that project would meet the new heat balance parameters and incorporated the new combustion turbine generators into the design. Ultimately, the team was able to use 50% of the original project design. Other challenges included using auger cast piles in lieu of traditional driven piling, grouping pumps and equipment to minimize the number of ancillary buildings, and eliminating the need for cooling water make-up pumps.
— Dr. Robert Peltier, PE is POWER’s editor-in-chief.