LM6000-powered combined cycle with OTSG proves fast, flexible, resilient

In a power world that typically views bigger as better, aero engines often are not considered when it comes to planning new generation. However, given the growing demand for flexible fuel-based generation to fill in around intermittent renewables, they may fit in your future plans.

The goal of this article is to assist you in decision-making by examining the beneficial operating characteristics of a recently completed 1 × 1 combined cycle equipped with a 10-minute-start aero gas turbine (GT), a once-through steam generator (OTSG), a single-pressure steam turbine (ST), and conventional balance-of-plant equipment. Dave Tateosian, PE, principal of California-based Clean Power Consulting Powers was the project manager, as well as construction and commissioning manager during the latter stages of the project, for owner/operator Pasadena Water & Power Dept.

The equipment configuration was selected to meet the owner’s need for a reliable, efficient, and dispatchable thermal resource capable of starting quickly and providing the operational flexibility to address the variability of renewable resources. The combined cycle has the following characteristics:

      • Fast, offering the 10-min starting capability typically associated with a peaking unit. Full combined-cycle output and efficiency generally can be achieved from a cold start in less than two and a half hours (absent any water-chemistry restrictions).

      • Flexible, offering the simplicity of a peaking unit, the efficiency and emissions performance of a combined cycle, a wide operating range, and the ability to respond quickly to requested load changes.

      • Resilient, offering full gas-turbine output even after a steam-turbine or boiler trip. Plus, restart of the steam plant while the gas turbine continues to operate over its full operating range.

Air-permit impacts on plant design. The combined cycle was a replacement for an existing steam boiler plant with a gross output of 71 MW. Air permit considerations drove a decision that the new unit have a maximum gross output of 71 MW, limiting plant output and heat rate somewhat. Example: The single-pressure OTSG’s stack temperature was higher than what might normally be used (about 315F at full load) to restrict output to 71 MW; the unit’s full-load heat rate of 7800 Btu/kWh would be reduced with a lower stack temperature. Duct firing was not included in the plant design because of the output ceiling.

For this unit, air emissions had to meet combined-cycle standards whether the unit was operating in the simple- or combined-cycle mode. This posed additional design challenges for these reasons:

      • The variation in exhaust-gas temperature entering the SCR, depending on whether heat was being extracted for steam production during combined-cycle operation or the exhaust gas was being cooled by use of tempering air during simple-cycle operation.

      • The variation of exhaust-gas mass flow through the SCR, depending on whether tempering air was being added during simple-cycle operation or not being added during combined-cycle operation.

Were this a greenfield project, consideration might have been given to a two-pressure design with benefits of slightly greater output and reduced heat rate. However, capital cost would have increased given the need for low-pressure (LP) feedwater and steam circuits and associated equipment.  

Given that the benefit of increased electrical output and efficiency occurs primarily at high loads, an evaluation of the expected operating profile would be warranted to assess the cost benefit of adding an LP system in a unit of this size.

Operational experience. To illustrate the unit’s “fast, flexible, and resilient” capabilities, a summary is presented below for the following evolutions: cold start, warm start, and OTSG trip and restart. Design and operational details are provided in the upcoming 3Q/2017 issue of the COMBINED CYCLE Journal, scheduled to mail just before Christmas.

Cold start. When the unit starts up cold, the gas turbine ramps to achieve the desired plant load setpoint. The closed and open cooling-water systems are started at the same time to support GT operation; hence, plant net output is lower than GT output because of the auxiliary load.

Once there is sufficient exhaust heat to vaporize the 19% aqueous ammonia, the reagent is injected into the OTG to react with the SCR catalyst and reduce stack emissions to permitted levels. A tempering-air fan starts to prevent overheating of the SCR catalyst.

The circulating-water and condenser-vacuum pumps, and the auxiliary boiler, are placed in service to pull vacuum, sparge the condenser, and begin reducing dissolved-oxygen levels in the condensate. Additionally, the condensate pumps are used to recirculate condensate through the polisher to meet water-chemistry requirements.

Once the recirculating condensate chemistry is acceptable, and the OTSG has warmed up to the minimum required startup temperature, the feedwater flow control valve is opened. Feedwater flow gradually ramps up and steam production begins.

When the steam lines are warmed up and the steam-turbine inlet steam conditions are met, the turbine is started, paralleled, and loaded. Typically, it is operated VWO (valves wide open). The ST follows the GT and the plant control system adjusts gas-turbine output so unit net load matches the target.

A warm start proceeds more quickly than a cold start because water chemistry typically is closer to being in-spec. The primary constraint is the time to warm the OTSG (and the significant catalyst mass within). Steam-turbine metal temperature is another consideration. Typically, full load can be achieved within about 70 minutes—or about half the time needed to reach full load on a cold start.

OTSG trip, restart. As part of unit testing, the OTSG was tripped manually with the unit at full load. Steam-turbine load decayed over the next 10 minutes and the generator breaker opened. Steam was bypassed to the condenser to reduce steam pressure to the point where the steam drains could be opened and the OTSG prepared for restart.

Absent heat removal through the steam system, SCR inlet temperature increased and the tempering-air fan started automatically to control catalyst temperature.

The OTSG was restarted; subsequently, the steam turbine. The ST was back to full load within 50 minutes of the OTSG trip. During this time, the GT continued to operate unaffected by the steam-plant upset, delivering approximately 85% of the total plant capacity.

Experience with this unit to date confirms its relative ease of operation. Startups and shutdowns are straightforward, akin to a simple-cycle unit. The ability to make rapid and frequent load changes without having to manage drum water levels eases the burden on the operator.

Further, the ability to effectively separate the steam plant from the gas turbine provides both operational flexibility and the capability to keep a significant part of the plant’s generating capacity in service where other combined cycles might have lost all their generating capacity because of a steam-plant trip cascading into a unit trip.

Although the plant performance metrics presented in the CCJ article are for a specific site and equipment arrangement, the overall conclusions hold true—in general. Today many aero and some frame gas turbines offer 10-min-start capability—some even faster starting. While the 10-min start is critical in offering some services—such as peaking and non-spin reserve—the flexible and resilient attributes of this type of unit remain when using a gas turbine with a longer start time.

The bottom line: Combining the capabilities of a 10-min-start GT with an OTSG results in a unit that can start quickly, operate in the simple- or combined-cycle mode, and accommodate steam-plant upsets by allowing the GT to produce power while the steam turbine is restored to operation. This is an optimal mix of capabilities to address today’s dynamic electricity demands.

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