Recent Innovations from Gas Turbine and HRSG OEMs

Demand for natural gas–fired power plants is perhaps more intense than it’s ever been. That means a big market to serve, and new opportunities for innovation. The major manufacturers are all working hard to stay ahead of the curve. 

There is no hotter market in power generation than gas.

According to the Energy Information Administration (EIA), the U.S. is projected to add just under 60 GW of new generating capacity between 2013 and 2017. More than half of that—and fully three times as much as the next-closest technology, solar—will be natural gas–fired.

If you suspect numbers like that have manufacturers of combined cycle power plant technology excited, you’d be correct.

The rapidly growing market is not growing in a vacuum, however. Though the U.S. may be adding almost 35 GW of gas-fired capacity by 2017, it’s also adding at least 15 GW of wind and solar. That means the gas fleet of the future needs to be ready to back up large amounts of intermittent generation, and that’s a role manufacturers are moving briskly to fill. Numerous advances in technology that will allow combined cycle plants to start faster, ramp faster, respond more rapidly to grid fluctuations, and do it all more cleanly and efficiently are in development or have just been introduced to the market.

Reduced Emissions

One of the challenges of backing up intermittent generation with gas is that this operational mode can significantly increase emissions. This occurs for several reasons. First, operating gas turbines at low loads produces higher levels of CO and NOx. Conventional combined cycle plants typically need to be brought up to full power in phases to allow the rest of the plant to heat up safely. Waiting out these low-load hold points dramatically increases overall emissions.

Second, rapid changes in turbine output disrupt fuel and selective catalytic reduction (SCR) equilibrium. The additional pilot fuel required during load changes causes increased NOx production, and when turbine load is changing, maintaining accurate ammonia injection in the SCR is more challenging: Too little means increased NOx out the stack; too much means ammonia slip.

The major turbine manufacturers such as Siemens and General Electric (GE) have recently introduced fast-starting plant technology that is designed to address the first problem (such as the Siemens Flex-Plant used at the Lodi Energy Center in California, a 2012 POWER Top Plant). Improvements to heat recovery steam generator (HRSG) design enable such plants to start up very quick and avoid low load holds that increase emissions.

Addressing transient SCR emissions, however, requires operational changes in addition to design adjustments. Siemens is introducing a solution it calls Clean-Ramp, which is designed to be integrated into the Flex-Plant solution (Figure 1).

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1. Tight targets. NRG Energy’s El Segundo Energy Center near Los Angeles, a Siemens Flex-Plant 10, incorporates Clean-Ramp technology to meet the area’s stringent emissions controls. Courtesy: NRG

This technology changes how the gas turbine is controlled so that the emissions control system can accurately predict changes in turbine exhaust when a load change is requested. The exhaust molar flow rate is calculated based on factors such as combustion airflow, fuel flow, historical performance, and so on. This information is used to predict NOx emissions, and the system then adjusts the ammonia injection flow rate accordingly. This allows the plant to stay at baseload emission levels even when the load is changing. Siemens claims this allows a plant to ramp continuously at rates above 30 MW/minute while keeping NOx emissions under 2 ppm.

GE has developed a similar product in its GEN II SCR control. This solution pairs a Rapid Response plant and GE’s OpFlex Startup Ammonia Control to reduce overall startup emissions. GEN II measures specific equipment and emissions parameters and, using model-based control technology, controls the ammonia to the SCR to reduce emissions and ammonia slip.

Retrofits

New plants aren’t the only ones benefitting from new technology. With a large number of older gas-fired plants seeing increased run time with the fall in gas prices, manufacturers are offering upgrades that allow these projects to capture increases in output and efficiency.

GE has been offering its Advanced Gas Path (AGP) upgrade solution for several years to increase the output, efficiency, and availability of its workhorse 7F line. It recently expanded this offering to its 9E and 9F turbines. The AGP solution involves improved blade aerodynamics and better sealing, as well as advanced materials and improved cooling technologies to allow higher operating temperatures. The physical improvements are paired with OpFlex model-based control software to deliver additional performance improvements.

Alstom recently rolled out its MXL2 upgrade package for its line of GT13 turbines. The MXL2 upgrade consists of a completely new blade design to boost aerodynamic efficiency in the compressor and turbine, optimized sealing and tighter clearances, improvements to the combustor, and enhanced cooling design (Figure 2).

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2. Ready to roll. Alstom’s MXL2 turbine is designed to improve power and efficiency on legacy systems. Courtesy: Alstom

Alstom says the upgrade will improve the power and efficiency of legacy turbines, as well as stretch maintenance and inspection intervals. The upgrade offers two modes of operation: M (for maximum output and efficiency) and XL (for extended life). Operating modes can be switched with the press of a button, allowing generators to increase output when market demand is high but reduce stress on components during periods of reduced need. (For more on mitigating the effects of new operating modes, see “Managing the Changing Profile of a Combined Cycle Plant” in this issue.)

New Turbines

Whether intended for new plants, retrofits, or repowering, gas turbine techology continues to evolve. Updated models of several workhorse designs are debuting this year.

GE introduced its steam-cooled H-class turbines more than 10 years ago, designs that have become a staple in the company’s lineup. This year, GE is rolling out two new air-cooled H-class turbines, the 9HA and 7HA. The 9HA.02 offers 592 MW of output at better than 61% efficiency in 1 x 1 combined cycle mode, and can reach full output in under 30 minutes (Figure 3). In simple cycle, it puts out 470 MW at 41% efficiency. The 9HA has a 14-stage compressor, a 16-chamber dry low-NOx combustor, and a four-stage air-cooled hot gas path.

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3. Big air. GE’s new 9HA air-cooled turbine offers up to 592 MW in combined cycle mode. Courtesy: GE

The smaller 60-hertz 7HA offers up to 486 MW at greater than 61% efficiency in 1 x 1 combined cycle mode and can reach full output in as little as 10 minutes. Both turbines are designed to be installed considerably faster than previous models through the use of modularized and preassembled components.

Mitsubishi Hitachi Power Systems (MHPS) is also rolling out an air-cooled update to its turbine line with the 60-hertz M501JAC (Figure 4). MHPS’s steam-cooled J-series turbines, which operated at temperatures of 1,600C, were introduced in 2011 and have been deployed mostly in Asia, with several plants coming online in 2013 and 2014.

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4. Evolution. Mitsubishi Hitachi Power Systems is upgrading its J-series line of large-frame gas turbines, like the one shown here, with the air-cooled M501JAC. Courtesy: MHPS

The M501JAC adds an optimized air-cooled combustor from the M501GAC model and offers output of up to 450 MW in combined cycle mode at better than 61% efficiency. The cooling holes in the turbine are also optimized for reduced gas temperatures. The M501JAC offers improved operational flexibility, such as by shortening the starting time while maintaining the same level of performance as the M501J. First shipments are expected in 2015.

New Approaches to Simple Cycle

Not all of the action is in combined cycle. MHPS is developing an approach to simple cycle turbine generation that could potentially equal or exceed combined cycle generation in efficiency. The technology, which is currently being commercialized for release later this year, is called AHAT, or advanced humid air turbine. AHAT takes a simple cycle turbine and uses humidified compressed air for combustion. The combustion air is cooled by water atomization, compressed in the compressor, and then passed through a humidification tower. The humidified air is then heated in a heat exchanger using the turbine exhaust before entering the combustor. The water vapor in the exhaust is then recovered and returned to the humidifier.

The method is similar to steam injection but adds far more water to the combustion process. MHPS has been developing the technology since 2000. A pilot project using an MHPS H-50 turbine was launched in 2010, and the company plans to commercialize it this year. The H-50 turbine with AHAT outperformed the same turbine in combined cycle mode, achieving 70 MW output at 50.6% efficiency. MHPS believes efficiencies above 60% are achievable with larger turbines.

MHPS is also developing a related retrofit product called Smart AHAT, which involves adding significant steam injection to a combined cycle arrangement, with AHAT’s water recovery system added to the exhaust.

New HRSG Technology

HRSG manufacturers have also been working to meet the demand for more flexible operations. NEM USA’s DrumPlus design is engineered to combine the advantages of drum-type HRSGs with the responsiveness of once-through design.

In the DrumPlus, the drum is replaced by a knock-out vessel with external separator bottles. The smaller drum has a relatively thin wall and is thus subject to lower thermal stresses with changes in output. The reduced volumes of both steel and water give the DrumPlus the dynamic capabilities of once-through steam generation, as well as the increased lifetime. These lower stresses eliminate the need for hold points on the gas turbine during startup, which allows faster startups, more cold starts, and more rapid load changes. DrumPlus HRSGs are able to handle 10-minute startups with no reduction in life. The El Segundo plant shown in Figure 1 employs a DrumPlus HRSG design (Figure 5).

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5. Fast starts. NEM USA’s DrumPlus HRSG design, shown here at NRG’s El Segundo Energy Center, allows for fast starts and rapid cycling with no reduction in life expectancy. Source: POWER/Tom Overton

Alstom is also offering HRSG designs for increased cycling. The Alstom OCC approach employs reduced header thickness–to–tube thickness ratio, single-row harps, and finned tubes with no bends. These changes reduce thermal stress by reducing areas of temperature difference and adding thermal flexibility to areas of the HRSG that will experience rapid changes in temperature with faster cycling.

Nooter/Eriksen is developing several options for increased cycling and faster startup. These include the use of stronger materials, multi-drum designs, and thinner drum walls.

Looking Ahead

This highly competitive market is sure to continue evolving. The demands from increased renewable generation are certain to increase pressure on the gas turbine fleet to become even more flexible and responsive, both in upgrades to existing plants and in new plants yet to be constructed. Whatever your role, these are definitely “interesting times” for natural gas. ■

Thomas W. Overton, JD is a POWER associate editor (@thomas_overton, @POWERmagazine).