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There are a wide variety of gaseous and liquid fuels used in gas turbines – and it would be economically and ecologically unwise not to utilize that potential. Fuel flexibility is evidently the industry’s trending topic. But as technology becomes more sophisticated, so do customers’ requirements.
By Barbara Simpson
“What it all boils down to is creating the perfect flame,” says Martin Urban. Though his statement sounds like a philosophical axiom, or maybe a pop lyric, what he is really referring to is the challenge of burning a variety of gaseous and liquid fuels with the best possible energy extraction and the least possible emissions. “Remember the Bunsen burner from your high school chemistry class? Initially, the flame is bright yellow, with a lot of soot, but it’s very stable. Once you open the air supply, the flame turns blue – a sign that it’s a very clean, homogeneous flame with evenly distributed heat and low emissions.” Burning fuel is not that difficult. But burning any fuel with consistently low emissions is a science, and one that Urban and his team have perfected: “The clue is the optimal mix of air and fuel.”
As Vice President for Global Product Development for Distributed Generation at Siemens, it is Urban’s responsibility to create specifications, designs, and models so that the industrial and aero derivative gas turbines used in small and medium-sized power generation packages can operate in a wide range of challenging as well as changing conditions. They can be applied in local solutions such as urban power supply, or industrial power generation for the oil and gas industry, or temporary solutions such as emergency power supply in disaster areas, or for bridging energy supply shortfalls prior to the installation of larger, more permanent energy infrastructure. The unifying challenge of all these scenarios: The engines need to run on whatever fuel is available.
“That yellow flame is called a diffusion flame,” explains Urban. “Twenty years ago, diffusion was the prevalent combustion technology. However, it produced too much emission and was inefficient.” Premixing the fuel with air provided a remedy to both predicaments. The result was a clean, blue flame in the optimal thermal range. Nowadays, all advanced systems operate with premix combustion technology to achieve low emissions and high efficiency. For stability reasons, some gas turbines still use diffusion flame components, e.g., to assist with the ignition of difficult fuels. “Fuel flexibility means the ability to burn a wide range of fuels – but also to control their emissions,” he says. As it turns out, the quality and types of fuels can differ enormously.
Fuel flexibility means the ability to burn a wide range of fuels – but also to control their emissions.
Martin Urban, Vice President for Global Product Development for Distributed Generation at Siemens
On offshore oil platforms, small gas turbines such as the popular Industrial RB211 are used to generate the electricity needed for operations. Gas fuel for oil and gas applications can come from a variety of sources. Operators will have access to pipeline-quality gas, but will also use wellhead gas or process gas. The chemical composition of the associated gas extracted from an oil well changes over the years. Even if an oil producer starts out with a clear understanding of how the fuel is composed, there is no telling exactly how high the carbon dioxide, ethane, or propane levels – to name just a few of the possible ingredients will be in five or ten years’ time. And then there are so-called upset conditions, in which rapid fuel switchover might, for example, mean an instantaneous switch from a primary gas source with 2 percent of carbon dioxide to a back-up source with 25 percent of carbon dioxide. From one moment to the next, complying with emissions regulations will become a lot more difficult.
In the old days, whenever the fuel quality changed, burners would have to be exchanged to suit the new conditions. With today’s requirements of fast fuel switchover, combustion technology needs to be smart and react to changing parameters almost in real time. The key to built-in flexibility that endures over time is hardware that is tolerant of the changing demands and a control system that intelligently fine-tunes the valves and air inlet based on the real-time data transmitted from the numerous data points. The real intelligence lies in premixing and swirling the perfect ratio of fuel and incoming air to create the ideal conditions for the energy conversion. “Today, we can offer customers much more in terms of fuel flexibility than we could just a while ago,” says Urban. “This flexibility relates to the range of the Wobbe index, i.e., the change in composition of a gas, as well as to the speed at which it changes.”
In order to optimize the premix process, a lot of research and testing has gone into improving the geometry of the burner. There are numerous fuel valves, edges to make the incoming air swirl, and the variable ventilation slits that adjust the volume and the speed of air influx. Comparing veteran diffusion burners with state-of-the-art third-generation dry low emission (DLE) burners like the one used for the SGT-600, SGT-700, and SGT-800, one cannot help but notice that burner innovation has come a long way.
In Latin America, a customer requested dual-fuel capability – the ability to switch over between gaseous and liquid fuels during operation – with the proviso that he did not intend to use oil as a simple back up fuel, but rather as a main fuel for the first two years of operation. This choice enabled him to immediately start generating electricity, and hence revenue, to invest in the continuation of this project and the building of a gas pipeline.
Usually, liquid fuel is a back up option for a couple of days, should the gas supply be temporarily interrupted. Liquid fuel is injected into an already hot engine and seamlessly feeds the combustion process. “We barely get 8,000 continuous operating hours on liquid fuel from the field,” explains Urban. “But we have accelerated product validation: systems that test our products as if they had been running for one year straight.” However, any fuel flexibility challenge such as this one will require a holistic approach. While the burner architecture is always the core element, all other parts of the gas turbine need to be considered, too.
This kind of flexible operation, perfectly adapted to regional conditions and fuel specifications, has become possible due to real-time sensor data collection and advanced machine learning programs. “We continuously measure the combustion process at the Service Data Center in Nuremberg. What kind of vibrations can be detected inside the combustion chamber? What are the temperatures in which part of the gas turbine? Which exhaust gases are produced in the end?” explains Urban. Based on this continuous data stream, Siemens can decide when to advise customers to adapt the operational mode to get the best efficiency or adjust the parameters to suit new fuel requirements. He adds: “Nowadays, we optimize the combustion process in real-time. Using gas chromatography, we can track each change in the chemical composition of the fuel and assist in creating a stable combustion process, maximizing efficiency while remaining emission-compliant at all times.” This operational data, which is collected around the world, teaches machine learning programs to become the most intelligent control system imaginable.
In Alaska, a customer wants to make sure that the fuel he uses will provide the desired fuel flexibility not only in test conditions but also in real ambient conditions, meaning temperatures as low as –60 degrees centigrade. “The operational demands placed on the combustion system in this setting are completely different than at 20 degrees centigrade,” says Urban. “Thanks to the intelligent control system, we can validate these operating conditions by profiting from comparable experience, for instance, in Siberia.”
In Germany, a large refinery was looking to fuel its gas turbine with waste gas that contained a high level of hydrogen – 15 percent. Usually, this gas would have been flared off, wasting its calorific value. Using this byproduct for energy generation not only saves the money otherwise spent on buying fuel, it also avoids the environmental fee associated with burning waste gas. Coke oven gas, which results from steel making and contains an array of further impurities in addition to hydrogen, can also be used, although its calorific value is very low.
The challenge of burning hydrogen is not only that it is highly corrosive but also that its flame speed is very high. In order to protect the burner hardware, the flame speed has to be lower than that of the incoming fuel; otherwise the flame is sucked backward and might auto-ignite the fuel supply. “For our standard burner in the SGT-600, SGT-700, and SGT-800, we have just solved the problem and are now able to add up to 45 percent hydrogen to the fuel mix,” Urban proudly relates. “The secret to this success lies in the tip of the burner, a sophisticated system that almost looks like a sieve, and could only be produced through additive manufacturing. The hydrogen can be injected in tiny jets at very high velocity, beating the flame speed.”
Fast fuel switchovers, fuel flexibility with DLE, dual-fuel capability, and using hydrogen as a gas turbine fuel – what has made this tremendous innovation boost in fuel flexibility possible over the last years? “There have been advances in materials, aerodynamics, the combustion process, and manufacturing. We have seen enormous leaps in numeric simulation and in testing and validation through digital technologies and additive manufacturing,” says Urban. “But it’s also the combined technology know-how that we harness here at Siemens from the aero-derivative gas turbine portfolio and the industrial gas turbine portfolio, which has been complemented by oil and gas know-how from Dresser-Rand.”
Martin Urban expects innovation cycles to accelerate even faster in the future. The accuracy of mixing precision, the precise simulation of combustion processes, and the adoption of rapid prototyping processes all contribute to creating very favorable conditions for further improvements in gas turbine technology and going beyond the next efficiency or flexibility frontier. Indeed, now that additive manufacturing has eliminated many design and production constraints, the engineer’s playground seems to have expanded in all directions. The areas of application are multiplying, as the recent technological breakthrough not only has inspired the engineers’ imagination but has also encouraged customers to dream bigger.
Barbara Simpson is a business and science journalist based in Zurich.
Picture credits: Siemens AG
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