By Douglas J. Smith, IEng
PORTLAND GENERAL ELECTRIC (PGE) serves more than 730,000 retail customers in northwest Oregon and wholesale customers throughout the western United States. The utility has a generating capacity of 2,000 MW with a mix of hydroelectric, coal-fired and gas-fired plants. Despite this capacity it is a net importer of power for on-peak power periods.
To produce additional power, PGE decided to take advantage of the 5 percent overpressure margin on the existing steam turbine at their 240 MW combined-cycle Coyote Springs Power Generating Plant in Morrow, Oregon. The plant has a GE MS7001FA combustion turbine, a Deltak HRSG and a nominal 80 MW steam turbine.
Installation of duct burners at Coyote Springs power plant.
PGE awarded a contract to an engineering firm, HRST, Inc., to evaluate the suitability of adding duct firing to the existing unfired heat recovery steam generator (HRSG).
After evaluating the existing HRSG for the new operating conditions, HRST recommended a maximum heat input for the burner to prevent future flow assisted corrosion damage associated with the increased flows in the tubes and headers and to minimize high tube metal temperature. The review included modeling the HRSG performance using test data as a benchmark, determining material temperature limits for casing and pressure parts, and determining flow limits at the existing design pressure for steam separation, circulation and safety valves.
The burner presented a number of challenges: There was very little margin between the temperature design limitations of the HRSG and the unfired operating temperature. In addition, the position of the burner had to be carefully engineered so the additional heat would not adversely affect susceptible areas.
To satisfy the temperature constraints, the burner was positioned at the front of the inlet transition duct and relatively close to the combustion turbine. Although in this application there was no need to control the flow, most retrofits of this type would require some means of controlling the turbine exhaust flow.
PGE awarded Coen Company a contract for the design, supply and installation of a single element duct burner. Installation of the burner was completed in April 2001. With the burner set near the front of the duct, the mechanical integrity and strength of the assembly had to be strong enough to survive the turbulence of the gas turbine’s exhaust.
Unfortunately, the turbulence and pressures in this area were much higher than originally anticipated by the supplier and the structural design failed within weeks of installation. Coen then redesigned the burner with a larger header with a center support.
Since October 2001, when the redesigned burner was installed, it has operated without any problems. According to Dan Turley, project manager at the Coyote Springs Plant, the burner was inspected during the March 2002 maintenance outage and no problems were identified with any of the burner components. Since that time the burner has operated for more than 1,000 hours without any problems.
Turley states that a nominal 4 MW of additional steam turbine power generation has been realized with the retrofit installation. “The burner has become a nice addition to the plant which allows it to produce a few additional MW’s when displacement power costs are high,” says Turley.
Overall gas-side pressure loss increased less than 0.3 inches W.C. and there has been no noticeable impact on the performance of the gas turbines. The only emissions effect noticed was on carbon monoxide (CO). Without the duct burner the CO typically runs at 1 to 2 ppm and with the duct burner the unit runs 3 to 4 ppm. In addition, a stack test indicated no detectable VOC with or without the duct burner. As a result there were no permit changes necessary for the duct burner installation since the CO limit in the permit is 15 ppm on an eight-hour average.
The approximate installed cost of the duct burner, including engineering costs, was less than $500,000. If the burners are operated for approximately 6,000 annual hours of operation, and assuming $50/MWh on peak and $29/MWh off-peak, the simple payback for the duct burner is approximately 2-3 years. Currently the burner is operated as needed, depending on the cost of power. Many days it is put into service in the morning and shutdown overnight without any significant problems.
When additional generating capacity is desired, a duct burner is an economical alternative to an additional gas turbine HRSG train or auxiliary boiler. Duct burners can react quickly to load changes and are inherently more efficient than auxiliary boilers since the combustion air is the gas turbine’s high temperature exhaust gas.