Air Pollution Control Equipment Services, Coal, Emissions

Don’t Let Startup Emissions Slow You Down

Issue 12 and Volume 117.


New combustion turbine (CT) simple and combined-cycle plants face new challenges in permitting because of more stringent startup emission limits. Recent permits issued in a variety of jurisdictions have included limits on startup emissions, both in mass per start and rolling annual startup mass limits for VOC, NOx and CO. The growth of renewable energy such as wind and solar, the retirement of coal units and low natural gas prices have increased demand for new CT power plants. Permitting agencies are demanding both lower emissions overall and during start-up, ramping and shut-down. The increased use of heavy duty/frame CTs for peaking applications create additional challenges. Startup and shutdown durations and emissions rates vary widely depending on CT model and other plant equipment and design choices. The emission rates predicted during permitting will become federally enforceable permit limits the plant must live with for the life of the unit. Predicting emissions, if done right, minimizes the impact of startup emission limits on the operation of the unit.

It was once common for permits to exempt operating units from meeting numerical emission limits during periods of startup and shutdown. Over the last five years, however, EPA has, through both its policy and guidance documents and comments on draft permits, maintained that such transient emissions are part of normal operation, are predictable and may not be exempted from New Source Review (NSR) permitting requirements. In addition, several state regulatory bodies are demonstrating a clear focus on transient emissions. Thus, any new or modified generating facility can expect to receive emissions and other limits specific to startup and shutdown periods.

Similar to steady-state emission limits, startup and shutdown-related limits are determined on a case-by-case evaluation and must represent either Best Available Control Technology (BACT) or the Lowest Achievable Emission Rate (LAER), whichever applies. The basis for mass emission limits may be hourly, per event or both. The permit should clearly define the beginning and end of each type of event. It will likely limit both the amount of time allowed for each event as well as the number of events that may occur per year. These different types of limits effectively cap the unit’s potential emissions on both a short and long-term basis.

Plant startup emissions depend on how quickly each piece of equipment can overcome its startup limitations. It is very important for an engineer to understand the complex interaction of individual equipment limitations to predict total plant startup times. A plant moving swiftly through each equipment’s startup duration results in lower startup mass emissions. The following is a list of major contributors to the startup durations of a conventional combined cycle plant:

  • Auxiliary Boiler – It can reduce the HRSG and steam turbine (ST) startup times by providing warming steam to HRSG Drums, ST pre-warming, BOP pipe warming, steam turbine seal steam supply, etc.
  • HRSG drum ramp rate – A drum is required to build pressure to supply the steam to the ST. A faster ramp rate results in earlier introduction of steam to the ST.
  • Steam Turbine – Its limitations to take acceptable steam conditions drives the ST startup time. The hold points and pre-warming requirements are manufacturer dependent but often times, contribute to the largest startup time duration.
  • Terminal Desuperheaters – It can shorten the startup process by allowing elevated steam temperature out of the HRSG and provide any desired steam temperature to the ST. This minimizes CT hold points.
  • Bypass Valves – Sizing of these valves will determine how fast a hot restart can occur.
  • Catalyst performance – The gas temperature influences the emission reduction performance of the catalyst.

Simple cycle units, especially with heavy duty/frame CTs have the added challenge of requiring hot Selective Catalytic Reduction (SCR). These SCRs present a host of unique performance issues such designing for dilution air to cool the gas and proper gas distribution. The performance of the SCR during startup must be addressed when permitting the facility.

Stronger, more stringent emission limits including startup and shutdown are an established trend that will continue. Accurate defensible predictions will assure that the permit limits are achievable. Predicting CT simple and combined cycle startup emissions requires the understanding and integration of applicable regulations and equipment startup times, limitations and emissions. A detailed analysis early will help accelerate the permit process, equipment selection, design and commissioning. Permit applications that are based on a thorough analysis to reduce the risks of application revisions and delays. Do it right the first time and startup emissions won’t slow you down.

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