 John Martin, Panda Brandywine general manager |
Panda Energy’s gas-fired Brandywine Plant is located near Washington, D.C. in Brandywine, Md. The 230 MW cogeneration facility, which entered commercial operation on October 31, 1996, features two GE Frame 7EA gas turbines, two Nooter Eriksen HRSGs, and a GE condensing steam turbine. Average annual heat rate is 8,250 Btu/kWh. Average annual capacity factor is only 29 percent, which reflects the plant’s typically daily dispatch as a qualifying facility (QF) under a 25-year power purchase agreement with the local utility, PEPCO. Equivalent availability factor is 99.39 percent on a 12-month rolling average basis, which places it near the top of the ORAP rankings for similar plants.
Brandywine’s first big challenge began before the plant was built. “We’re on a peninsula, which means that most of the water is supplied by deep-well taps into the aquifer,” says John Martin, general manager. “When the locals started hearing about millions of gallons of water a day, they became very concerned. We worked closely with a state agency – the Power Plant Research Program – to study the feasibility of bringing treated effluent water into the facility for cooling.” This ultimately lead to a combination permit for water access: one permit for an average of 64,000 gpd of groundwater for boiler makeup and other auxiliary uses, and a second for effluent from a publicly owned treatment works (POTW) in Charles County, Md. Panda constructed a 17-mile pipeline to bring tertiary treated effluent to the plant for cooling tower makeup.
Dispatch levels with one gas turbine are 99 MW and 117 MW (seasonally adjusted). With two gas turbines, the levels are 199 MW and 230+ MW. Because the dispatched output often doesn’t coincide with minimum heat rate, Brandywine’s greatest impacts on heat rate are associated with start-ups and shutdowns, management of the low-pressure steam system, and minimizing use of auxiliary equipment.
The original operating procedures, for example, called for a stepwise steam turbine (ST) ramp-up, with one or both combustion turbines (CT) at or slightly above the spinning reserve level of 8 MW when the ST was synchronized. This was followed by step changes in the CT output, and corresponding step changes on the ST as steam pressure increased. Because the CTs operate most efficiently at higher loads, the procedure has been refined over time to ramp the CTs to their maximum output based on the dispatch level as soon as possible, and to quickly ramp the ST as steam pressure steadily increases. Typical CT output at ST synchronization is 20 MW instead of 8-10 MW. Upon ST synchronization, CT controls are set to automatically ramp up to the prescribed dispatch level at the predetermined ramp rate, and the ST is loaded up correspondingly.
Maintaining stack temperature at a minimum level has helped achieve optimum heat rates as well. Originally, during startups, Brandywine started the cogen facility and ramped it to the prescribed dispatch level, followed by start-up of the steam host, an inside-the-fence distilled water plant. “Using this procedure, LP steam usage was limited to deaeration, and LP drum pressures would climb to 50-90 psig depending on whether we were in single or two CT operating mode,” says Martin. “Stack temperature would climb as heat was wasted into the atmosphere. The standing procedure now is to start the distilled water plant as soon as the LP steam pressure has reached 20 psig. This maintains maximum circulation in the LP boiler and keeps stack temperature at minimum levels, 180-190 F.”
The gas turbines rely on a dry low-NOx combustion system to meet regulated NOx emission levels of 9 ppm on gas and 54 ppm on No. 2 fuel oil. “During the first two years of operation, the turbines typically ran at levels well below 9 ppm, in the 5 to 7 range,” says Martin. “After our first set of combustion inspections, however, when we started the units back up, NOx emissions were above 9 ppm. Over the course of 2-4 weeks of operation, the levels dropped back below 9 ppm.”
Although the OEM couldn’t identify a definitive cause for the anomaly, Panda decided to have the fuel nozzles flow-tested – to tolerances at least twice as good as the OEM recommendations – during the next set of combustion inspections. “On a post-CI basis, NOx levels were right back where they were before the outage,” says Martin. “Had we not done the flow-testing, we believe the levels would have been higher. Other than the fuel nozzle testing, there’s not much else we did during the combustion inspection that would have affected the low-NOx combustion system. We considered the dilution holes in the combustion liners, but the size of these holes wouldn’t have changed over a period of 2 to 4 weeks.”
Maintenance outages at Brandywine are coordinated around the condition of the turbines and the status of the PJM market. “We prefer fall outages over spring outages because of the increased availability of maintenance resources in the fall, and because we remain available to take advantage of increased dispatch opportunities in the spring created by reduced capacity in PJM when other generators are performing seasonal maintenance,” says Martin.
After each outage, Panda Brandywine designates one gas turbine as the lead unit and the other as the secondary unit. In 2001, for example, the lead unit had 266 starts and the secondary unit had 181 starts. Major maintenance events are typically the pivot point for changing the lead status. “For example, the A unit had a combustion inspection in May 2001, at which time it became the secondary unit and B became the lead unit,” explains Martin. “Based on our forecast, this will allow us to maximize the number of starts on B until its November 2002 hot gas path inspection. Our target is approximately 500 starts between major maintenance events, compared to the OEM-recommended 400 starts, and this has been achieved on each of the CT’s first two maintenance events.” Borescope inspections are used to confirm the CT’s condition and corroborate the determination to stretch the maintenance interval to 500 starts.
As with many combined-cycle power plants operating in today’s competitive markets, cyclic operation has lead to some problems at Brandywine. To address various superheater support bundle failures, Panda converted 1/2-inch plate to thicker 3/4-inch plate and/or installed four supports where previously there were only two. For GT exhaust diffuser cracking, which has been found during each of the four combustion inspections to date, Brandywine simply makes repairs part of the work scope for the next outage.
“Like most plants, we need to convince our people that we’re running a business, not just a power plant,” says Martin. “To help convey this message, we get everyone involved in the dollar side of the business. In addition to performance incentives, which keep attention focused on availability and the income side of the business, we also get everyone involved on the expense side as well. Maintenance technicians, for example, are active in defining and developing budgets for their individual areas of responsibility.”
