Is Gas Getting Too Hot to Handle?

With ever-increasing demands for fast ramping and flexibility, natural gas–fired plants are grabbing a bigger share of the generation pie. But uncertainty about future prices and concerns about overreliance on a single fuel are dampening enthusiasm during what may be the most exciting time for gas ever. Natural gas is hot—but will generators and the market get burned?

In a series of standing-room-only sessions in Track 2: Gas Turbine/Combined Cycle Power Plants at ELECTRIC POWER 2013, an array of speakers from generating companies, manufacturers, and service providers reported on best operation and maintenance practices and new options in gas-fired power generation, particularly cycling existing plants and building distributed and combined heat and power plants (Figure 1).

1. Full house. Attendees in the gas track learned about meeting the challenges of the gas market. Source: POWER

The Challenges of Frequent Cycling

With the rapidly expanding role of wind in the generation mix, the ability to ramp up and down quickly is a gas-fired plant’s biggest advantage. Rapid cycling, however, can come with hidden costs, explained Douglas Hilleman of Intertek. Too often, plant owners make assumptions about these costs without drilling down into the details, even though small changes can reap big returns in more profitable operations. Not knowing the true costs of cycling operations can leave a substantial amount of money on the table. “It’s not just fuel,” Hilleman said, “it’s the latent costs.”

Hilleman reported on Intertek’s cycling analysis for Xcel Energy’s peaking plant in Riverside, California. A full understanding of cycling costs allowed Xcel to tailor its dispatch plans to the true costs, and do it without capital investments or hardware modifications. While a comprehensive analysis of cycling operations can be costly, the returns are big enough that it can pay for itself in a few months. In the case of the Riverside plant, simply reducing the ramp rate by 33% saved thousands of dollars in maintenance costs.

When a plant owner needs to shift gears in a changing market, careful analysis is key, reported Brian Eskra of Power Engineers Collaborative. Eskra described a study performed for a client in Southern California that needed to retire its steam electric plants and replace them with combined cycle plants in order to meet the demands of the CAISO market and changing regulations in the state. Recognizing that intermediate and peaking generation offered the best market niche was only the start, Eskra said. Analysis of the various options, market demands, and the returns on investment led to development of a 3 x 1 block configuration utilizing Mitsubishi Heavy Industries D/E class turbines.

Scott Polemus of LTSA Options described ways that plant owners can optimize their long-term service agreements (LTSAs). The market has changed in the past few years, and it’s worth reevaluating and renegotiating existing LTSAs for cost savings and/or increased benefits. Owners can restructure agreements to provide for more cost and risk sharing, Polemus explained. With the growing role of and demands on gas-fired plants, it’s time to determine if an LTSA is still serving a plant’s needs.

Even when a plant is optimized for its market, however, there’s no guarantee it’s going to stay that way: Wear and tear from frequent cycling and ramping, along with changes in weather and fuel properties, can reduce a plant’s operating windows. That’s why regular tuning is important, explained Donald Gauthier, senior technical lead for Power Systems Manufacturing (PSM). These kinds of changes will increase turbine dynamics, and running with high dynamics can be very expensive, whether the costs result from lean blowouts (LBOs) or accelerated equipment wear.

While manual tuning will work, automated tuning systems are preferable because of faster response and reduced maintenance. PSM’s AutoTune system offers one solution for 7FA units. AutoTune automatically manages combustion dynamics to reduce the risk of LBO and improve emission control, while maximizing power output.

In combined cycle plants, however, proper tuning extends beyond the turbine. Rapid cycling also places demands on the heat recovery steam generator (HRSG), explained Tony Thompson, chief technology officer for Vogt Power International—even when the HRSG is carefully designed for it. Operating profitably and efficiently under such conditions requires the ability to monitor the impact on HRSG component integrity and knowing whether replacement or repair is most cost-effective. Vogt offers an online monitoring system that records and analyzes operational data to calculate life cycles for critical components.

Small Power: When Size Matters

The fall in natural gas prices has renewed interest in distributed generation (DG) and combined heat and power (CHP) resources. Small turbines are ideal for these applications because of their flexibility, efficiency, and low maintenance requirements, Uwe Schmiemann, marketing manager for Solar Turbines, told attendees.

The need for small, decentralized power plants is likely to rise as the growth in renewable generation continues to outstrip transmission capabilities, Schmiemann said. Their rapid start and agile load-following capabilities, as well as the ability to operate with little or no on-site manpower, can efficiently manage localized grid fluctuations resulting from increased amounts of behind-the-meter renewables.

DG and CHP also showed their value last year during Hurricane Sandy, when facilities with their own generation were able to keep the lights and heat on when the grid went down and surrounding areas were without power for weeks. In addition, Schmiemann noted, the shift to small gas turbines from older fuel oil systems offers substantial emissions benefits for institutions such as universities and hospitals that have long generated their own power.

CHP is also growing in usage in industry as lower gas prices offer economic benefits over grid power, as well as a reduction in emissions. As an example, Schmiemann described a facility that was able to cut its overall CO2 emissions by 60,000 tons by installing a 15-MW CHP system.

Daniel Loero of GE Aero Energy described DG and CHP applications for GE’s venerable LM series turbines. More than 2,000 are in service worldwide. Loero told attendees about a new plant at a wind farm in Kansas that uses three LM units to support 1.2 GW of wind generation. The turbines are able to even out the wind farm’s output and compensate for short-term fluctuations.

Where very rapid response with high efficiency is necessary, gas-fired engines can be ideal, said Mark Harrer, business development manager for Wärtsilä North America. The key advantage of multi-engine plants is that generation output can be adjusted by turning individual engines on and off as needed rather than ramping the plant as a whole up and down. This allows each engine to operate at peak efficiency even when the plant is generating only a portion of its total capacity, Harrer said. Wärtsilä’s newest engines have simple cycle efficiencies greater than 46%. In addition, they can maintain stable output across wider ranges of ambient temperatures and altitudes than gas turbines. The engines can reach full power in 5 minutes, shut down, and be back at full power 10 minutes later.

This offers another benefit, Harrer explained. In competitive markets, the ability to respond rapidly to price spikes can be very lucrative. In one example he showed, the ability to respond immediately to a $3,000/MWh price spike returned $895,000 in revenue.

Thomas W. Overton, JD is POWER’s gas technology editor.

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