EFFICIENT, FAST-START GENERATION: LM6000-powered combined cycle with OTSG proves fast, flexible, resilient

By Dave Tateosian, PE, Clean Power Consulting Partners

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 (sidebar).

This 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.

OTSG versus HRSG

The table compares operational capabilities of a combined cycle equipped with an OTSG to one with a conventional drum-type heat-recovery steam generator. Note: In this table, “combined cycle” denotes operation when gas-turbine exhaust heat is used to produce steam—irrespective of whether that steam is used for power generation or not. For example, the steam turbine may be offline with steam bypassed to the condenser.

Principal plant equipment

The combined cycle profiled in the main text consists of the following equipment:

LM6000PG gas turbine with water injection for NOx control, a SPRINT power augmentation system, and a mechanical chiller for inlet-air cooling. Supplier: GE.

Single-pressure once-through steam generator equipped with CO and SCR catalysts and an ammonia injection system for NOx emissions control. Also, a continuous emissions monitoring system. Supplier: Innovative Steam Technologies Inc.

Condensing steam turbine. Supplier: Shin Nippon Machinery Co (Japan).

Deaerating condenser, titanium-tubed, with full steam bypass capability. Supplier: Maarky Thermal Systems Inc.

Wet cooling tower uses recycled water to reject heat from the steam cycle and auxiliary equipment. Supplier: Cooling Tower Depot Inc.

Fuel gas compressors, two 100%-capacity units. Supplier: Kobelco/Kobe Steel Ltd.

Circulating-water, condensate, and feedwater pumps, two each, 100%-capacity. Supplier: ITT Inc, Goulds Pumps.

Condensate polisher, pre-coat type, full flow. Supplier: Graver Technologies.

Open cooling-water system uses circulating water to reject heat from the closed cooling-water system and inlet-air chiller, three 50%-capacity pumps. Because the chiller represents about 50% of the thermal load and is not always required, three half-size pumps are optimal, providing sufficient redundancy and flexibility.

Closed cooling-water system provides cooling for the GT and ST oil and generator coolers.

Electric auxiliary boiler.

Additional balance-of-plant equipment includes air compressors, water sampling system, fire protection systems, and chemical feed system for water chemistry control.

Operational experience

To illustrate the unit’s “fast, flexible, and resilient” capabilities, plant-performance trends are provided for the following evolutions:

Cold start (Fig 1).
Warm start (Fig 2).
OTSG trip and restart (Fig 3).
Cold start with steam-turbine trip and load changes (Fig 4).

The terms used in the illustrations are defined this way:

Plant output, net MW is the combined output of the gas and steam turbine/generators less the unit auxiliary load.

Gas-turbine, gross MW is the GT generator output.

Steam-turbine, gross MW is the ST generator output.

SCR inlet temp is the temperature of the exhaust gas upstream of the SCR inlet.

Tempering air running gives fan on/off status.

Cold start

When the unit starts up cold, the gas turbine ramps to achieve the desired plant load setpoint—51 MW for this particular start. 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.

Warm start

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.

Cold start with ST trip, load changes

The combined cycle entered commercial operation in late 2016. A typical day’s operation in response to system dispatch is illustrated by the timeline below. On this day, the unit also experienced an ST trip, and its recovery.

0632 hours. Cold start, dispatch to 19 MW.
0655. Load increased to 44 MW (simple cycle); steam plant readied for startup.
0827. The ST was paralleled and its load increased. Gas-turbine load decreased to maintain a steady combined-cycle output of 44 MW.
1208. Unit load increased to 55 MW.
1325. Steamer tripped, GT went to full load and the unit operated at 52 MW.
1401. ST was paralleled.
1408. Unit load returned to 55 MW.
1541. Unit load reduced to 53 MW.
1727 to 2130. Several load changes between 20 and 66 MW to accommodate system dispatch requirements.
2213. GT shutdown initiated.
2218. GT offline.
2219. OTSG trips.
2220. ST offline.

Wrap-up

Experience with this unit 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 above 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. CCJ

About the author

Dave Tateosian, PE, principal, Clean Power Consulting Partners, was the owner’s project manager, as well as construction and commissioning manager during the latter stages of the project. He acknowledges the assistance of the Pasadena Water & Power plant staff—in particular, Robert Picou—in the preparation of this article. GE, ARB, POWER Engineers, and Stantec are recognized for their contributions to the project’s success.