Improved GT starts reduce gas consumption, satisfy dispatch requirements

2015 CCJ Best Practices Awards: Best of the Best

Gila River Power Station, a 2200-MW facility located in the Sonoran Desert at Gila Bend, Ariz, has four combined-cycle power blocks that cycle daily—each having two GE 7FA gas turbine/generators (GT) and one GE D11 steam turbine/generator (ST). EthosEnergy Group operates the plant for owners Sundevil Power Holdings LLC, Gila River Power LLC, and Tucson Electric Power Co; Doug Cantrell is the plant manager.

In a typical year, the GTs ring up more than 1200 starts, which must be efficient and effective to meet expectations. When a unit is dispatched, the control room operator’s (CRO) primary goal is to start the gas and steam turbines and ensure key performance indicators are achieved. Startup goals include the following:

      • Minimize fuel gas use during hot, warm, and cold starts.

      • Minimize the time to meet dispatch while operating equipment within design parameters.

To satisfy these goals, the engineering team worked closely with operations to improve the control schemes for GT and ST starts. When reviewing opportunities for improvement, the engineers’ challenge is to ensure that all modifications improve startup efficiency and the ability of the operators to effectively conduct the start.

Solution. The Gila River team approached the challenge by implementing process changes to improve GT starts. Enhancements to the startup procedure were implemented to satisfy the owners’ goals of reducing gas consumption, meeting required dispatches, insuring against safety and environmental issues, and operating equipment within design constraints. Modifications included the following:

      • Startup sequence automation was increased to avoid human errors (Fig 1). Another mod limits GTs to Mode 1 operation until steam conditions are met. When the unit initially goes into Mode 3 exhaust CO is elevated. By preventing the gas turbine from reaching Mode 3 prematurely, the control logic maintains CO levels within permit limits (100 lb/hr during startup). The logic uses pre-selected load control up to the firing-temperature transfer point of Mode 3 to ensure CO emissions are held in check while minimizing startup time.

Gila BOB Fig 1

      • A startup calculator is used to determine the GT start time based on predicted operating conditions, megawatts required for dispatch, and required dispatch time. Temperature-decay curves were plotted to provide expected reheat bowl temperature for cold and warm starts and HRSG HP steam temperature/pressure for hot starts. The DCS captures the last dispatch event, and based on the next dispatch time, the temperature decays are used to predict the amount of time necessary to reach desired steam conditions and dispatch load. The time directly correlates with the amount of fuel necessary to reach Mode 6Q as well as back-calculating a startup time.

      • Created a startup “snap shot” (Fig 2) to allow the control room operators to understand if they have met critical startup parameters—such as fuel burned to Mode 6Q and “dispatch met,” as well as the time dispatch was achieved. The operators have a recorded document of the performance of the latest startups as an immediate reference.

 Gila BOB Fig 2

      • The GT fuel curve schedule was increased from six to 12 points and changed to operate in a rich PM1 split schedule. The units were tuned to allow them to operate within environmental parameters (predominantly CO) at reduced output (extended turndown). Tuning of the GTs provides the capability to operate at 65 to 70 MW. The need to shut down and then restart the GT can be avoided. With the GTGs in extended turndown, the power block can be quickly brought to full load in 17 minutes (Fig 3).

Gila BOB Fig 3

Gila BOB Fig 4, 5Results. Changes to the startup procedure have made significant improvements in the facility’s ability to start power blocks quickly and effectively. These improvements have resulted in the following:

      • Gas usage for a 1 × 1 hot start from flame to Mode 6Q, which averaged 600-million Btu, was reduced by 33% to less than 400 million Btu (Fig 4).

      • Reduced missed dispatches from three per month to less than one per month during critical summer months (Fig 5).

      • Extended turndown operation of the power blocks has improved the economics of overnight operation, resulting in fewer starts to meet morning peak operations.

 

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