Owned and operated by South Carolina Electric & Gas Co.

For many utilities, repowering a 50-year-old coal-fired plant has become an attractive alternative to making expensive environmental upgrades to such a plant reaching the end of its useful life. Repowering is especially attractive if the plant’s existing water and air permits can be “recycled.” South Carolina Electric & Gas Co. was able to do just that at its Urquhart Station a few years ago. By repowering the plant’s older two units and cross-connecting them with Unit 3, the utility boosted Urquhart’s overall efficiency and operating flexibility. Creative engineers found a way to squeeze every dollar of savings from this plant.

By Skip Smith, Project Manager

South Carolina Electric & Gas Co. (SCE&G), a subsidiary of Scana Corp. (Columbia, S.C.), built the Urquhart Station’s original three coal-fired units—two 75-MW GE steam turbines and another rated at 100 MW—in the early 1950s. The plant (Figure 1) is located in Beach Island, S.C., on the Savannah River near Aiken City.

1. Urquhart Station.

By 1999, environmental pressures, rapidly escalating maintenance costs, and service territory load growth were straining SCE&G’s generating resources. The solution: Invest $228 million and repower the older two units to create a modern, combined-cycle facility based on GE 7FA gas turbines (GTs). The other GTs at the site are three 15-MW peakers installed in the 1960s and a 42-MW GE LM6000 simple-cycle peaker installed in 1999.

The repowered plant is an intermediate service plant, operated during times of high load demand. During the summer and winter months, the plant is generally operated 10 to 16 hours per day. For the first full year of operation, GT-1 produced 541,000 MWh and GT-2 produced 520,000 MWh. The plants operated 4,668 and 4,544 hours, respectively.

Beyond the addition of a badly needed 300 MW, the repowering project promised other benefits:

• It would allow SCE&G to retain Urquhart’s well-trained workforce.
  
• It would allow retention of the plant’s water discharge permit.
  
• By effecting a significant drop in NOx, SO2, and particulate emissions from Urquhart, the project would delay the need for SCE&G to install a selective catalytic reduction system on one of its other coal-fired plants (Figure 2).
  

2. Repowering benefits.

Upon commissioning of the new combined-cycle plant, Units 1 and 2 were taken out of service. Unit 3—rated at 100 MW—was kept on-line. As a result, the new plant has an output of 550 MW.

Duke/Fluor Daniel was selected as the EPC contractor for the repowering project in April 2000. Site preparation began that spring, and construction began in September 2000 immediately following receipt of an air permit for the plant. First fire was in March 2002, and the plant was declared commercial on June 1, 2002—ahead of schedule.

Greater operating flexibility

The new Urquhart Station has a 1 x 1 combined-cycle configuration. Each GE 7FA exhausts into a Nooter/Eriksen (St. Louis, Mo.) heat-recovery steam generator (HRSG). Steam conditions are 1,440 psig/1,000F superheat and 1,000F/366 psig reheat. The HRSGs are single-pressure units configured as follows: reheater-HP superheat 2-HP superheat 1-HP evaporator-HP economizer-Unit 3 feedwater heating.

One of the plant’s unique features is the cross-connection of the steam turbines for improved operational flexibility. In fact, both 75-MW steam turbines can be operated with a single gas turbine under steady-state conditions (Figure 3). These cross ties include hot and cold reheat connections for the reheat steam turbines. The reheat cross connection was predicated on an innovative design by Duke/Fluor Daniel and GE. The flexibility of this “semi-plant” mode of operation provides for more timely and economic dispatch of the facility.

3. Flexibility.

The design also allows the cold start of both steam turbines by one gas turbine. A cascading steam bypass from the HRSG to the condenser using the cold and hot reheat lines allows each GT to continue operating when both steam turbines are out of service. As part of the project, the steam turbines and generators of Units 1 and 2 were refurbished and upgraded to make them compatible with combined-cycle operation.

Another unique feature of the facility design is the use of steam generated by the HRSGs of repowered Units 1 and 2 to heat HP feedwater for the existing Unit 3 steam turbine (Figure 4). This not only increases the operational efficiency of Unit 3 but also the operational flexibility of the overall plant.

4. Steam use.

Other areas of the plantthat were upgraded included the fuel oil backup system for the gas turbines and the chiller for their inlet air. The latter step, which boosted output of the GTs by up to 20 MW, entailed the installation of inlet coolers from Caldwell Energy and Environmental Inc. (Louisville, Ky.). The plant’s 1.2-million-gallon oil tank can supply both gas turbines for five 16-hour operating days.

Control upgrades included a new distributed control system (a Westinghouse Ovation), the addition of a GE Mark VI system for controlling the Unit 3 steam turbine, and new electro-hydraulic controls for the steam turbine control valves. Balance-of-plant upgrades weren’t overlooked either: Higher-capacity demineralized water system and condensate pumps were added to complement the existing water treatment system (Figure 5).

5. Control system.