|Courtesy: City of Tallahassee|
Owner/operator: City of Tallahassee, Florida
Known for its progressive, pro-sustainability policies, the City of Tallahassee recently repowered a 30-year-old conventional steam plant unit, turning it into a new 300-MW facility. The utility redesigned the Arvah B. Hopkins Generating Station, Unit 2 as a 1 × 1 combined cycle plant in order to improve efficiency, switched the primary fuel from oil to natural gas, and thereby reduced fuel costs and emissions. The plant’s flexible design even will enable expansion to a 2 × 1 configuration when additional capacity is needed in the future.
Some people describe Tallahassee as Florida with a Southern accent. Located in the Florida Panhandle next to the Georgia border, Tallahassee is both the state capital and home of Florida State University.
On June 18, the U.S. Conference of Mayors (USCM) honored the City of Tallahassee by awarding it the prestigious 2011 City Livability Award. This first-place designation is awarded annually by the USCM for exemplary leadership and innovation, and recognizes the selected cities as ensuring the highest quality of life for residents.
“This award brings national attention to our city and it shows what we’ve known all along—that Tallahassee is a great place to live,” said Mayor John Marks. “It demonstrates once again that Tallahassee is at the forefront, not only for our steadfast commitment to sustainability and environmental stewardship, but also for pioneering programs, award-winning parks, top-rated schools, world-renowned research facilities, and active civic involvement.”
The City of Tallahassee’s can-do attitude was exemplified by the municipality’s recent decision to take its 30-year-old power plant and update it into a more efficient and economical combined cycle system through the Hopkins Unit 2 Repowering Project.
Fast-Tracking the Conversion Project
The original Arvah B. Hopkins Generating Station, Unit 2—a 230-MW (summer net rating) gas/No. 6 oil–fired conventional steam plant—was installed in 1977 and was one of the two largest plants in the City of Tallahassee’s fleet before repowering. The city’s decision to convert Unit 2 into a gas-fired combined cycle facility was based on a study produced by Sargent & Lundy (S&L), which served as the city’s engineer in the project. S&L’s study determined that the repowered project would significantly reduce unit heat rate and cut air emissions.
“The reduction in heat rate and the corresponding annual fuel cost savings were so compelling that the city commission voted not only to move up the planned conversion by two years but also complete the conversion on a fast-track basis,” said Bock Yee, S&L’s project manager.
In addition, the S&L study concluded that the Hopkins Unit 2 steam turbine would accommodate the installation of two F class gas turbines and triple-pressure heat recovery steam generators (HRSGs), allowing this conversion to be completed in two phases to match the city’s load demand. The first phase of the conversion would be to construct a 1 x 1 combined cycle; the second phase would be conversion to a 2 x 1 arrangement. A supplementary-fired HRSG was also selected, to provide peaking capability.
City officials determined that S&L’s previous experience related to repowering projects would help to make the Hopkins repowering project successful. The city assumed responsibility for construction management, commissioning, and start-up, while S&L provided technical and field support. This teaming arrangement also contributed significantly to the project’s success.
Modifications Performed to Repower the Plant
Yee shared three examples of important interface areas that required modifications for reuse.
Controls. The original Hopkins 2 was designed with a conventional cascading bypass system and was equipped with nonmodulating steam turbine intercept valves. Yee pointed out that it would have been too expensive to replace these valves with ones capable of modulating. Thus, the system could not be used for turbine start-up in the same manner as a typical bypass system designed for a new combined cycle.
“Here’s how the repowered Hopkins is started: The cascading bypass system warms up steam piping initially,” he said. “Prior to steam roll, high-pressure and cold-reheat bypass steam are diverted to the high-pressure sky vent, and the reheater is evacuated.”
Controls are arranged this way because the nonmodulating intercept valves could not control overspeed if reheat pressure were to increase during the steam turbine roll. Cold reheat steam is admitted to the reheater during the turbine roll at a controlled and increasing rate dictated by turbine acceleration and speed control. Control system modifications were made to accommodate the various start-up modes required.
Condenser. The heat-rejection capability of the original condenser was evaluated for use in the repowered configuration. Yee pointed it was important to “keep in mind that extractions for the feedwater heaters, now abandoned in place, had to be capped, increasing flow to the condenser.” Engineers said the existing condenser was adequate for the duty, but that a few modifications were needed. They included:
- Redistribution of the heat source to eliminate hot spots.
- Rerouting steam turbine low-point drains.
- Replacing the first feedwater heater in the condenser neck with the bypass diffuser.
Steam Turbine. The steam turbine was evaluated to determine what modifications and/or operating limits were required to ensure its suitability for the repowered plant (Figure 1). Overall, 13 areas were studied, including these four:
- Verifying the mechanical integrity and permissible operating limits, considering all loads.
- Defining the temperature limits of all turbine components, including design limitations.
- Determining the maximum flow through all turbine sections to the condenser—this was to be done to verify the unit’s ability to handle the repowered steam flow with feedwater heater extraction nozzles capped.
- Defining the start-up requirements, procedures, and operational limits of the repowered turbine, including ramp limits for cold, warm, and hot starts.
|1. Steamy side. As part of the repowering project, a heat recovery steam generator (HRSG) was added to a new combustion turbine to produce steam to power the plant’s existing steam turbine, making it a true repowering project. The plant has provisions to add a second combustion turbine and HRSG in the future. Courtesy: City of Tallahassee|
The repowered Hopkins Unit 2 has a couple of additional unique features, Yee explained. For example, a bypass stack was installed ahead of each HRSG to permit simple cycle operation in the unlikely event of a steam turbine/generator failure. This bypass stack does not have a typical damper. To minimize leakage, a solid steel plate is used to direct hot gas to the HRSG during normal operation. When needed in the event of a steam turbine/generator failure, the plate can be removed from the stack and placed in front of the HRSG in the transition duct to block the gas flow to the HRSG.
Profile of the New Plant
The S&L project team provided their expertise as far as knowing what controls, systems, and other equipment to reuse, modify, relocate, or abandon in place. Because solutions vary from site to site due to differences in design criteria, margins, and other factors, it was important that the engineering firm had the experience to deal with a wide variety of challenges.
The new and modified equipment installed at the repowered plant included, but was not limited to, the following:
- New HRSGs (Nooter/Eriksen Inc.)
- New gas turbines (GE Energy)
- New digital control system (Emerson Process Management/Ovation)
- Modified existing condenser (Westinghouse Electric Corp.)
- Modified existing steam turbine (Westinghouse Electric Corp.)
- New selective catalytic reduction technology (Vector Systems Inc.)
- New bypass stack (Braden Manufacturing LLC)
- New critical piping/P91 piping (BendTec)
The plant is located on 230 acres of land seven miles west of Tallahassee. The area, which is covered with oaks and pines, provides a haven for a variety of wildlife, including birds, deer, alligators, and even a community of beavers.
Protection of the environment is a prime concern of the city’s electric utility and the plant employees. Chemists in the plant’s laboratory perform more than 40 tests on water every day, including tests for metal contaminants, mineral precipitates, and acidity. A separate environmental monitoring facility also provides independent oversight and verification of all environmental safeguards.
Realizing the Vision of a Leaner, Cleaner Plant
The first phase of the repowered Hopkins Unit 2 has run well since being declared commercial on June 1, 2008, according to City of Tallahassee General Manager Robert E. McGarrah, Manager of Power Production Triveni Singh, and Plant Manager Clark Sheehan. The repowered unit was completed on time, without any lost-time accidents, and 8% under the $156 million budget.
The repowered unit has met all expectations regarding efficiency goals and environmental performance. As a result of the efficiency gains, the city projects $12 million to $24 million per year in fuel savings for its electricity customers, depending on the cost of natural gas.
— Angela Neville, JD, is POWER’s senior editor.