New technique for measuring temperatures in combustion flames could lead to cleaner biofuels

Science Advances (2022). DOI: 10.1126/sciadv.abm7947″ width=”800″ height=”530″/>

Measured and simulated densities of krypton number in a fouled methane/air flame. (A) A photograph of the flame dimensioned to the same spatial scale as (B). (B) Image plots of experimental (left) and simulated (right) krypton number densities throughout the flame. (C) Radial profiles of krypton number density at several heights above the burner (HAB). Error bars for measurements are represented by shaded regions. Total 2D data collection time: 2 hours. Credit: Matthew J. Montgomery et al, Scientists progress (2022). DOI: 10.1126/sciadv.abm7947

A new X-ray technique for measuring temperatures in combustion flames could lead to cleaner biofuels.

Understanding the combustion dynamics of biofuels – fuels made from plants, algae or animal waste – is essential to building clean and efficient biofuel engines. Temperature is an important factor in this dynamic.

Scientists from the US Department of Energy’s (DOE) Argonne National Laboratory, Yale University and Penn State University have refined and used an X-ray technique to measure temperatures in a Extremely hot flame laden with soot produced by combustion. Such measures have always been difficult. The new technique has the potential to help reduce emissions from biofuel-powered engines. The study was published in Scientists progress.

A need to optimize biofuels

Reducing emissions of greenhouse gases and other pollutants from burning fossil fuels will require major changes in energy systems. The U.S. Energy Information Administration reports that there are more than one billion fossil-fuel vehicles worldwide, which projects that the conventional vehicle fleet will peak in 2038.

Cleaner-burning advanced biofuels can potentially help reduce pollutants in the meantime. This is especially true for aircraft, ships and other heavy vehicles which remain difficult to electrify with current technologies.

But developing new combustion systems for advanced biofuels is no easy task. A major hurdle has been the accurate measurement of temperatures in the flames produced by burning biofuels. Temperatures are critical inputs in the models that researchers use to simulate combustion flames and their emissions.

“Temperature has a big influence on chemical reaction rates in flames,” said Alan Kastengren, an Argonne physicist who was one of the study’s authors. “If models don’t have accurate temperatures, they’re probably not predicting chemistry correctly. Better combustion models allow researchers to design better combustion systems, whether internal combustion engines or power generation systems.

Measuring temperatures with X-rays and krypton atoms

Measuring flame temperatures is surprisingly difficult. Researchers have previously used lasers and other devices to assess the flames. However, the soot particles present in the flames can interfere with their ability to measure temperature.

X-rays are largely unaffected by soot particles, so another possibility is to use X-ray beams for flame analysis. The Argonne, Yale, and Penn State researchers used and refined a technique known as X-ray fluorescence. The technique involved several steps. First, they introduced a small amount of krypton gas into a flame composed of air and methane (a main component of natural gas). This is a standard flame used by laboratories around the world in combustion research. Krypton is an element with extremely low reactivity, so it does not alter the chemistry of the flame.

Then, at the Advanced Photon Source (APS) at Argonne, a DOE Office of Science user facility, researchers bombarded the flame with high-energy X-ray beams. In response, the krypton atoms emitted X-rays with a unique amount of energy in a process called fluorescence. The team then used an X-ray spectrometer to detect the energy of the X-ray fluorescence emitted. This allowed the researchers to map the presence of krypton atoms and quantify their density throughout the flame. Then the team calculated the temperatures in different parts of the flame, using an equation known as the ideal gas law that relates temperature and density.

One of the keys to the success of the experiment was the use of APS’ ultra-bright X-ray beams. X-ray beams generated by facilities such as APS have much greater intensity and much more focused beams than those created in laboratories.

“A lab-scale X-ray source is a bit like a light bulb. X-ray beams go out in all directions,” Kastengren said. “With synchrotrons, the X-ray beams all go in the same direction. This makes it much easier for us to effectively use the beam to measure interactions with the flame.”

Several ways to apply the technique

While the researchers refined the X-ray technique using a methane flame, the methods can be applied to measure temperatures in other flames, including those produced by burning biofuels. This can help improve the accuracy of models used to simulate flames in biofuel combustion systems. More robust models have the potential to discover new ways to operate aircraft engines, gas turbines and other power-generating systems so that they are more efficient and produce fewer emissions.

“Imagine airplanes switching from standard fuel to sustainable aviation fuel,” said Robert Tranter, senior chemist at Argonne and author of the study. “You need to understand the impact of this switch on the combustion properties of the engine to make sure it is working properly. Physical testing of new fuels in a real engine is very expensive. Accurate combustion models can filter fuels for help determine when to do these tests.”

More broadly, X-ray methods can advance the understanding of fundamental aspects of combustion, supporting a wide range of research areas. For example, they can inform efforts to develop systems that burn hydrogen to produce energy. They can aid research into using flames to create silicon nanoparticles, which have potential applications in medicine, batteries and other fields.

The technique can even be applied beyond combustion research. It can potentially support all laboratory experiments requiring accurate temperature measurements in harsh environments.

“We always come across different systems where researchers need accurate temperature measurements,” Tranter said. “We are open to collaborating with them.”

In addition to Kastengren and Tranter, the authors are Matthew J. Montgomery, Yale; Hyunguk Kwon, Penn State; Lisa D. Pfefferle, Yale; Travis Sikes, Argonne; Yuan Xuan, Penn State and Charles S. McEnally, Yale.

Put gas under pressure

More information:
Matthew J. Montgomery et al, In Situ Temperature Measurements in Fouled Methane/Air Flames Using Synchrotron X-ray Fluorescence of Seeded Krypton Atoms, Scientists progress (2022). DOI: 10.1126/sciadv.abm7947

Provided by Argonne National Laboratory

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Kevin A. Perras