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

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The densities of the krypton number in a methane/air flame were measured and simulated. (a) An image of a flame of the same spatial size as (b). (B) Experimental cut-out (left) and simulated (right) krypton-number densities throughout the flame. (c) Radial profiles of krypton population density at multiple heights above the hearth (HABs). Error bars for the measurements are represented by gray shaded areas. Total time for 2D data collection: 2 hours. Credit: Matthew J. Montgomery et al., science progress (2022). DOI: 10.1126 / sciadv.abm7947

A new X-ray technology to measure temperatures in a combustion flame could lead to cleaner biofuels.

Understanding the dynamics combustion Biofuels — fuels made from plants, algae, or animal waste — are essential to building clean, efficient biofuel engines. An important driver of these dynamics is temperature.

Scientists from the US Department of Energy’s (DOE) Argonne National Laboratory, Yale University and Pennsylvania State University have refined and used an X-ray technique to measure temperatures in a superheated, soot-borne flame. Such measurements have historically been difficult. The new technology could help reduce emissions from biofuel engines. The study was published in science progress.

The need to improve biofuels

Reducing emissions of greenhouse gases and other pollutants from fossil fuel combustion will require major changes to energy systems. The US Energy Information Administration reports that there are more than one billion fossil-powered vehicles worldwide, predicting that the conventional vehicle fleet will peak in 2038.

Advanced and cleaner biofuels can help reduce pollutants in the meantime. This is especially true for aircraft, ships and other heavy vehicles that are still difficult to electrify with current technologies.

But the development of new combustion systems for advanced biofuels Not an easy task. The main barrier was the accurate measurement of temperatures in the flames from the combustion of biofuels. Temperatures are a critical input into the models that researchers use to simulate flames and their emissions.

“Temperature has a significant impact on chemical reaction rates in flames,” said Alan Castingren, an Argonne physicist who was one of the study’s authors. “If the models don’t have accurate temperatures, they probably don’t predict the chemistry correctly. Better combustion models allow researchers to design better combustion systems—whether internal combustion engines or electricity generation systems.”

X-ray temperature measurement and krypton atoms

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

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

Then, at Argonne’s Advanced Photon Source (APS), a DOE Office of Science user facility, researchers bombarded the flames with high-energy X-rays. In response, the krypton atoms released 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-rays emitted. This enabled the researchers to map the presence of krypton atoms and determine their density throughout the flame. Next, 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.

The key to the experiment’s success was the use of ultra-bright X-rays in APS. X-ray beams generated by facilities such as APS are much more intense and more focused than those generated in laboratories.

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

Many ways to apply technology

While researchers have refined their X-ray technology using a methane Flame, methods can be applied to measure temperatures in other flames, including those from biofuel combustion. This could help improve the accuracy of models used to simulate flames in biofuel combustion systems. More powerful models could enable discovery of new ways to power aircraft engines, gas turbines and other power generation systems so that they are more efficient and with lower emissions.

“Imagine converting aircraft from standard fuels to sustainable jet fuel,” said Robert Tranter, a senior chemist at Argonne and author of the study. “You need to understand the effect this switch has on the combustion properties of the engine to make sure it is working properly. Physical testing of new fuels in a real-world engine is very expensive. Accurate combustion models can check the fuel to help determine when to perform those tests.”

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

This technique can even be applied outside the scope of combustion research. It can support any laboratory experiments that require accurate temperature measurements in hostile environments.

“We always run into different systems in which researchers need accurate temperature measurements,” Tranter said. “We are open to cooperating with them.”

In addition to Kastengren and Tranter, the authors are Matthew J.Montgomery, Yale; Hyungok Kwon, Penn State; Lisa de Pfeverley, Yale; Travis Sykes, Argonne; Yuan Xuan, Penn State and Charles S. McNally, Yale.

Putting the gas under pressure

more information:
Matthew C. science progress (2022). DOI: 10.1126 / sciadv.abm7947

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