By: Steve Blankinship, Associate Editor
The rush to gas in the 1990s was driven by cheap natural gas pricesgenerally in the range of $2/MMBtu. Today’s gas rush is happening with gas prices four times as high and is being driven by electricity demand growth at a time when environmental opposition is also sidelining many coal plant projects. A complicating factor is that the time required to build coal and nuclear plants is simply too long for them to fill the gap between demand and capacity. But the next wave of gas-fired plant construction will differ from the last in many ways.
A Siemens SGT6-5000F turbine being assembled in Hamilton, Ontario, Canada. Photo courtesy Siemens.
Power Engineering posed questions to three major gas turbine original equipment manufacturers. Speaking for GE Energy is Mas Fukumoto, gas turbine marketing executive. Representing Mitsubishi Heavy Industries is John Adams, senior vice president of new product operations. And responding for Siemens is David Boyce, manager, marketing and strategy, Americas. (A fourth OEM, Alstom, was unable to participate.)
Power Engineering: Unlike the last rush for gas, this one is predicated on high natural gas prices. How have your gas turbine offerings changed since the late 1990s to meet this new demand for additional gas-fired capacity?
Mas Fukumoto, GE Energy
Fukumoto: A key factor shaping the power generation market is lingering uncertainty about the use of coal for future power plant projects due to environmental concerns, escalating prices and longer cycle times. These factors are increasing the interest in gas turbines with their lower environmental emissions, higher efficiency, better cost certainty, short cycle times and greater operating and fuel flexibility.
In response to these diverse market requirements, GE has developed both heavy duty and aeroderivative gas turbines with greater capability to support dual fuel and alternate fuel sources. A wide range of operational flexibility enhancements enable customers to effectively use equipment for peak and cycling applications. The relative ease and speed of installation also give natural gas solutions an advantage when it comes to meeting emergent and urgent power demand.
A few examples are the LMS100, an aeroderivative turbine capable of a 10-minute start for lower megawatt applications; the 50 hz 9FB heavy-duty turbine, which improves upon the output and efficiency of our successful F-class technology; the newly-announced 7FA gas turbine with 10-minute startup capabilities; and the 7EA gas turbine, which offers liquid fuel flexibility capabilities.
John Adams, Mitsubishi Heavy Industries
Adams: Higher energy costs undoubtedly call for design efforts to improve efficiency. We have successfully focused on increasing the already high efficiency of our advanced G technology. Performance measurement executed during the commissioning of our latest 50 and 60Hz upgraded units (the M501G1 and M701G2G, respectively) have demonstrated efficiencies in excess of contractual values. It is worth mentioning that 1:1 50Hz combined cycle efficiency in excess of 59.3 percent has been measured at commercial plants. This number is very significant considering that 60 percent efficiency numbers are frequently announced in the industry but have rarely if ever been demonstrated.
In addition to high efficiency, Mitsubishi has focused on continuous improvement of G machine durability and reliability. It would be a partial success to increase efficiency if the availability and startup reliability do not match the high standard. The G fleet’s equivalent availability factor of 96 percent and starting reliability exceeding 99 percent clearly testify that the machines will operate when needed.
These achievements have led a number of large utility fleets to opt for the newer G machines because the highest efficiency and high reliability will result in better operating profiles. Their forecast involves the M501G1 being the first machine to start and the last to shut down.
David Boyce, Siemens
Boyce: In 2002, Siemens addressed rising natural gas prices by taking the proven technology of the SGT6-5000F, with its fast-start capabilities, and integrating it into the Flex-Plant 10 and 30 power plant solution. By utilizing the SGT6-5000F engines’ fast-start capabilities in the Flex-Plant solutions, there is a substantial decrease in the amount of fuel required to start the power plant, thus resulting in a fuel cost savings, as well as providing more dispatchable power quicker, reducing start-up emissions while maintaining high rates of efficiency (FP10, 48 percent net efficiency and FP30, 57+ percent net efficiency). These designs also allow for extremely flexible operation depending on how the plant is called upon.
PE: The new rush will also demand that gas units be more flexible than their predecessors, meaning they will be called upon for a wide range of cycling capability as well as baseload and peaking. How are your new gas turbines being designed to meet such flexibility requirements?
Fukumoto: GE is an industry leader in developing fuel and operating flexibility for our gas turbines. For example, in December 2007 we announced 10-minute start capability for our 7FA gas turbine. We’ll roll out simple cycle capabilities first, then combined cycle. With a start-up NOX under 9 ppm, this turbine will be well suited to cycling and peaking service.
We’re also focused on increasing turndown capabilities. We recognize that a growing number of power providers are faced with decisions about the financial and environmental cost of frequent startups. If we can give them the ability to turndown equipment in off-peak time periods, we give them the maneuvering room they need to participate in ancillary markets and to meet their own power needs on a cyclical basis. Our OpFlex portfolio of operational flexibility enhancements, which include upgrades around startup, turndown and fuel flexibility, will play an increasing role in how we help our customers succeed.
Adams: Thorough consideration to operational flexibility is required for the continuously changing market. In addition to the high cycling duty, it is becoming very common to experience changes in gas composition that can easily disrupt the stability of the unit.
Upgrades to the F and G class turbines include significant improvements in emission capabilities. New ultra-low NOX combustion systems were developed with 9 ppm and 15 ppm NOX for F and G classes, respectively. These systems are also coupled with a proven self-tuning combustion dynamics system, allowing stable operation of the units even under changing gas composition as high as +/- 5 percent Wobbe Index.
The upgraded designs have also evolved toward faster load rates while maintaining tighter emission regulation. Additionally, more robust designs have allowed longer operating periods and more frequent start/stop cycles between maintenance outages. This cycling capability has been demonstrated at our in-house verification power station, where the industry steam- cooled pioneer, the M501G gas turbine, has accumulated 1,865 starts during its 10 years of operation.
Boyce: Siemens has developed the Flex-Plant 30 which integrates the fast-start technology of the SGT6-5000F into its existing highly efficient 2×1 60 Hz F-class combined cycle product with a triple-pressure reheat bottoming cycle. The focus of this plant is to utilize a proven product designed primarily for continuous-duty operation and expand its flexibility to span continuous-to-intermediate-duty operating regimes economically. For peaking- to intermediate-duty operation, Siemens has developed the Flex-Plant 10 which also integrates the fast-start technology of the SGT6-5000F into combined cycle application. The Flex-Plant 10 consists of the SGT6-5000F with 10-minute start capability (baseload in 12 minutes) with 9 ppm NOX, advanced single-pressure non-reheat HRSG with conventional SCR; bedplate-mounted non-condensing steam turbine; air-cooled heat exchanger condenser with full-capacity steam bypass system; and air-cooled generators for gas and steam turbines.
Both plants address more than just the need for flexible power. They also address the need to minimize emissions and water consumption, reduce start-up fuel and maximize plant efficiency with minimized plant complexity.
PE: Discuss what you offer as retrofits to existing simple and combined cycle NG units to allow them to meet the demands of today’s power market?
Fukumoto: Responding to the power industry’s need for fuel flexibility solutions, GE Energy recently introduced OpFlex Wide Wobbe, which provides customers with the ability to operate continuously across a range of natural gas-derived fuels of varying compositions while maintaining performance and still have reliability and operating flexibility. The newest OpFlex offering can be used where natural gas fuel heating values fluctuate. Wide Wobbe automatically adjusts control parameters based on actual gas turbine performance and environmental conditions, allowing the unit to continue operations within emissions constraints.
With rising demand for natural gas and growth in LNG imports, the frequency and magnitude of variations in fuel composition and heating value are expected to increase. OpFlex Wide Wobbe was designed specifically with this in mind and allows customers to operate their units within +/- 5 percent variation from a baseline Wobbe Index.
Wide Wobbe is the latest addition to GE’s OpFlex technology program that provides operational flexibility enhancements for gas turbines. The program also includes OpFlex Turndown, OpFlex Peak Fire, OpFlex Auto Tune and OpFlex Cold Day.
The increased focus on energy efficiency and cleaner technology is forcing customers to consider repowering. GE has demonstrated capabilities in placing new cogeneration and combined-cycle gas turbines for customers who previously used non-GE equipment. Several of these customers have benefited from additional reliability as well as process efficiency improvements.
In addition, GE continues to advance its dry low NOX combustion technology to meet increasingly stringent emissions regulations and has developed uprate options for a wide range of gas turbines that offer output and efficiency increases.
Adams: For Mitsubishi Heavy Industries, extensive validation of new components is a fundamental step in the design and validation process before commercial offering. This philosophy also applies to retrofits of upgraded features to earlier machines. Several improvements that include more efficient blade and vane profiles, improved materials, coatings and other modifications have been applied to previous generation G machines following extensive validation at our in-house power station in Japan. These retrofits have resulted in substantial extension of parts life intervals, improved performance, lower turndowns, reduced maintenance costs and lower life cycle costs.
Combustion chamber of an SGT5-4000F gas turbine.Photo courtesy Siemens.
Boyce: Depending on the application and specific customer need, Siemens offers a portfolio of service solutions to optimize plant flexibility. This portfolio includes, but is not limited to, diagnostics and monitoring, HRSG retrofits and plant sequencing and control improvements.
PE: Tightening reserve margins in many parts of the United States are intensifying the need for fast-start gas turbines. How are your product offerings meeting this need?
Fukumoto: In December, GE announced a 10-minute start capability option for our new 7FA turbines. Equipped with this technology, the gas turbine can achieve 70 MW of output in 10 minutes, while still maintaining 57 percent efficiency, thus providing excellent coverage in the energy markets.
A key feature of this fast-starting turbine will be its emissions profile, with startup under 9 ppm for NOX and CO. Having the ability to connect to the grid in 10 minutes without excessive emissions will provide a tremendous amount of flexibility for customers with peaking needs and those who want to participate in the ancillary markets.
Adams: Fast-start capabilities need to be coupled with expectations of parts durability, reliability and reduced fall out. They also affect the base load efficiency due to increased clearances. Very aggressive startup and loading ramps can negatively affect repair and maintenance costs, not to mention that excessive thermal and dynamic loading can cause catastrophic failure with the associated forced outages. Our upgraded designs have incorporated modifications that allow a conservative increase in acceleration and faster load rates. This faster startup and loading capability, coupled with high startup reliability, produces reliable reserve margins.
Boyce: Siemens is able to meet the need for fast-start gas turbines utilizing our full load Berlin (Germany) test bed to achieve breakthrough advancements in the existing SGT6-5000F technology with fast-start capabilities. This means 150 MW in 10 minutes without imposing a maintenance impact for the start. The Berlin test bed is a unique tool that allows validation of new technologies. The test bed is not connected to the grid but can be run at full power using a large water brake. Therefore, the facility is also used to evaluate the impacts of non-standard operation. Siemens then took the fast-start technology a step farther by putting this proven technology into combined cycle application, allowing the plant to produce more power faster, meet environmental compliances sooner, reduce startup emissions and reduce overall plant costs.
Our SCC6-5000F 2×1 Flex-Plant 30 is a highly efficient (57+ percent net) 590 MW plant, which can be started in half the time of a traditional F-class plant. The SCC6-5000F 1×1 Flex-Plant 10 is a 275 MW plant with 48 percent net efficiency that can generate 150 MW within 10 minutes. So whether a customer is looking for simple cycle or combined cycle power plants, Siemens has been able to create a product to meet their need using proven technologies.
PE: How have emerging markets for capacity payments and ancillary services to cope with narrowing reserve margins affected the products you sell?
Fukumoto: These new markets have led us to really focus on turndown capabilities, faster start-up options and operational flexibility enhancements. Customers who may have previously focused solely on efficiency may now have additional requirements. The key for us is to understand the particular needs of each customer and project and help them choose the best capabilities to meet their needs.
Our focus is to allow customers to play in all ancillary markets without sacrificing performance or costs. As I previously noted, we enhanced our 7FA gas turbine to play in all markets very effectively.
Ancillary services are key financial tools to drive market competition and encourage proper siting of new generation for transmission relief as well as capacity. The existing technology has the capability to meet these criteria, meaning fast start, regulation up and down and VAR support, with simple attention to modifying controls for regional needs.
Adams: The emerging markets for capacity payments and ancillary services have increased the demand for Mitsubishi products and services by providing our customers with one of the most reliable and durable machines on the market. Our units’ ability to consistently deliver during startup provides great financial benefit and peace of mind.
It is not uncommon to find that advanced gas turbines display high efficiency but perform poorly in terms of reliability. Mitsubishi’s G fleet is an exception, incorporating a large variety of advanced features, such as steam cooling, while exhibiting a fleet-equivalent availability factor of 96 percent and startup reliabilities exceeding 99 percent.
Boyce: Using the proven technology of the SGT6-5000F in both simple cycle and combined cycle operation, Siemens is able to achieve a product portfolio that will fit with any customer need. This simplified portfolio allows for operation from peaking to continuous load with the use of proven technology. Siemens also has a strong service history with this line of products and is able to support the needs of customers from the generation side as well as the service side.
PE: The amount of wind capacity in several North American regions is increasing the need for ancillary services and capacity support for intermittent renewables. That means more gas-fired backup. How are you meeting this need?
Fukumoto: The intermittent nature of wind is creating a need to have power available quickly when the wind isn’t blowing. Natural gas-fired turbines are a proven, reliable and “to scale” means of meeting this quick demand. Ramp rate (meaning MW/minute) is a key differentiator for wind-farm generation. Depending on the gas price, the technology selection can range from simple-cycle frame gas turbines, to aeroderivatives, to storage units. GE is able to offer the alternative technology and controls capabilities to integrate these solutions.
Adams: In addition to improvements in gas turbine technology to quickly respond to short notice demand of gas fired generation, Mitsubishi Heavy Industries is developing high efficiency hybrid systems that combine fuel cells and gas turbines that can operate on natural gas. Small-scale products in the range of 75 kW are currently in operation in Japan and will lead the implementation of larger-sized combined systems.
Boyce: The increasing concern for the environment is the primary driver for the rise in wind energy. Siemens has addressed the need for cleaner, more efficient gas-fired backup with the SGT6-5000F, which offers fast-start capabilities of 150 MW in 10 minutes and baseload GT power in 12 minutes. This results in lower fuel consumption on startup, 9 ppm NOX control by way of an advanced ultra low NOX system coupled with conventional SCR technology if necessary, thus significantly reducing startup emissions and complying with stricter air requirements.
When this technology is used in combined cycle application, the plant also minimizes water consumption by utilizing air cooled components. The Siemens portfolio of plant solutions offers environmentally sound alternatives to fill the gap in renewable energy.
Editor’s note: Watch for our upcoming June issue, which features an industry roundtable discussing latest utility-scale steam turbine technology.
Modeling Natural Gas Combustion Dynamics
By David Wagman
An ongoing challenge for natural gas turbine manufacturers is to simultaneously achieve low NOX emissions and flame stability. The balancing act depends on running a lean fuel mixture during combustion. Too rich a mixture and the unit risks exceeding air quality compliance standards. Too lean a mixture and the turbine can suffer loss of ignition.
“Gas turbines run near the edge of lean blowoff where the flame is always on the hairy edge,” said Bernie Rosenthal, CEO of Reaction Design. The California-based company, founded in 1997 by two MIT professors, markets software that helps turbine designers understand the chemical reactions that occur when natural gas is burned in a combustion turbine. Some 350 commercial customers are currently using Reaction Design’s Chemkin product, whose original use was to measure rocket engine combustion chemistry.
Chemkin is based in part on a fuel chemistry database, which consists of fundamental chemical descriptions. By altering variables such as catalysts, temperature and pressure, the model can predict not only what types of emissions are formed when the natural gas burns, but where in the combustion process emissions will emerge and when.
Natural gas is a fairly basic molecule, Rosenthal said, and the prediction can be fairly straightforward. The models grow in complexity, however, as synthetic and alternative natural gas sources are burned.
“As you add combinations of fuels, the effect becomes less well understood,” he said. Mixing natural gas with syngas can create effects not seen before. Those unseen effects drive an interest to understand what is happening during combustion.
Rosenthal said he sees Chemkin as a tool to fill a knowledge gap that exists for turbine designers and engineers.
“One potential game-changing innovation is the idea that you can map your geometry and CFD (computational fluid dynamics) results to an equivalent reactor network,” he said. Chemists use different reactor models as a way to understand what is happening in the combustion chamber. The models allow for detailed snapshots of mixing, flame, recirculation and post-flame segments within the chamber.
“You can start to see when emissions are created and flame stability with regard to regions where certain chemical reactions take place,” he said.
The chemical analysis has potential application in pulverized coal combustion boilers as well as in gas turbines. In the case of coal, unburned hydrocarbons and particulates are primary issues. With carbon regulation seen as almost inevitable, the ability to predict how much carbon dioxide will be produced through combustion is important.
Rosenthal said many basic fuels are already well understood, but only to the extent that we know how natural gas is being burned. “There’s a myth that we really understand what’s going on at the chemical level” in gas turbine combustors. A great deal of empirical understanding exists when it comes to combustion chemistry, but relatively little scientifically-based knowledge is available. With new fuels (such as syngas and landfill gas) experimental data can be gathered on an ever-widening universe of fuel options, but typically at the expense of time and money.
“There is a real interest on the part of turbine manufacturers to understand those dynamics with computer modeling,” Rosenthal said. Software such as that developed and marketed by Reaction Design may help accelerate the process of understanding the implications of combusting fuels in turbines.