Following Emerson’s completion of a combustion turbine purge credit retrofit project in early 2019, NAES Corporation has reduced average hot startup time at Middle River Power’s Tracy combined-cycle power plant by approximately 30%. The plant’s ability to more quickly generate megawatts is expected to result in an additional $170,000 in annual revenue.
The Tracy plant, located 80 miles southeast of San Francisco, is owned by Middle River Power and operated by NAES. In response to fluctuations in renewable energy generation, the 323-MW, 2×1 combined cycle plant starts frequently—typically 250 to 300 times per year.
Prior to the retrofit project, the plant was required to perform a 20-minute, fresh air pre-start purge cycle to remove potentially explosive residual fuels from the combustion system. The purge cycle extended overall startup times, putting the plant at greater risk of a delayed or failed start and missing its power purchase agreement (PPA) dispatch obligations and potentially triggering substantial financial penalties.
To reduce that risk, NAES took advantage of a change to the National Fire Protection Association’s NFPA 85: Boiler and Combustion Systems Hazard Code, which gave them the option to eliminate the pre-start purge by modifying fuel and monitoring systems. While replacing steam turbine controls and the excitation system at the Tracy plant with its Ovation automation technology, Emerson also made the National Fire Protection Association-compliant mechanical modifications to the fuel train and related changes to the combustion turbine and duct burner control logic. Emerson scope included mechanical, electrical and control system design, as well as installation and commissioning.
Since the project’s completion, the plant has been able to maintain an indefinite purge credit between dispatches. Eliminating the pre-start purge has reduced hot startup time by approximately 20 minutes, or 30%. Plant management estimates that this additional time online, at load, could translate into $170,000 in additional annual revenue. Reducing startup time may also help the Tracy plant avoid the expense of PPA failed-start penalties which could be in excess of $300,000 per incident. There have been no failed starts since project completion. Within 2 weeks of project completion, one failed start was already avoided due to the ability to re-start after a trip and still meet the dispatch.
There are other expected economic and operational benefits as well. Reducing heat recovery steam generator thermal stress is expected to extend the life of the unit by deferring maintenance costs for three to six years, which is beyond the timeframe of the utility’s current PPA. The project is expected to result in a return on investment in two to three years.