“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.
Diffusion and premix combustion
“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.