Sandia researchers focus on soot, furans, oxygenated hydrocarbons

Researchers in Sandia’s Combustion Research Facility (CRF) are developing the understanding necessary to build cleaner combustion technologies that will in turn reduce climate impact.

Their work focuses on understanding the oxidation chemistry of organic carbon species critical to many processes, including those that control emissions of toxic combustion by-products that contribute to climate change. Researchers expect this to work to benefit geosciences, astrophysics, and energy applications.

“Soot released from combustion sources is of global concern, as it causes premature deaths, global warming, and hydrological changes,” says Olof Johansson (8353). “In addition, furans and other large oxygenated hydrocarbons are highly toxic and very frequently observed in combustion emissions.”

Their research was published in the July 2016 issue in the Proceedings of the National Academy of Sciences in a paper titled “Formation and emission of large furans and oxygenated hydrocarbons from flames.” The paper is coauthored by Olof, Hope Michelsen, Paul Schrader, Matthew Campbell (all 8353), and Farid El Gabaly (8342); Tyler Dillstrom, and Angela Violi, University of Michigan; Matteo Monti, Stanford University; and Denisia Popolan-Vaida, Nicole Richards-Henderson, and KevinWilson, Lawrence Berkeley National Laboratory.

Understanding furans, oxygenated hydrocarbons

Oxygenated hydrocarbons are molecules that contain oxygen in addition to carbon, hydrogen, and various other elements present during combustion. Many of these molecules are toxic pollutants. The molecules may influence cloud formation and have a significant climate impact if they end up on soot particles released from combustion sources.

Olof says there are many significant studies on these species, but the precise chemical formation pathways and their links to carcinogenic polycyclic aromatic hydrocarbon (PAH) molecules are unknown.

“We show that oxygen adds to PAH molecules via reactions that involve OH and O₂ to generate these large oxygenated hydrocarbons,” Olof says. “The molecular sites where oxygen is added are targets for additional reactions that can lead to formation of a five-member ring containing four carbon atoms and an oxygen atom, which is known as a furan.”

Furans are particularly toxic and carcinogenic. The work shows that furans are the main group of large oxygenated molecules formed during combustion.

Combustion sources of furans include biomass burning, cigarette and pipe smoke, waste incineration, electronic waste recycling, and volcanic activity. Previous studies have shown that soot’s ability to absorb and hold water is greatly enhanced by the presence of oxygen on the surface of soot particles. Understanding how oxygen becomes incorporated into PAH species and soot particles during combustion is a key step in designing technologies that can mitigate the release of large oxygenated hydrocarbons.

Hope Michelsen (8353) says understanding the mechanisms leading to formation and destruction of hazardous combustion byproducts is the key to controlling their formation and emission.

“Soot particles have a very short life cycle in the atmosphere compared to greenhouse gases, such as carbon dioxide and methane,” Hope says “Still, soot particles are projected to be second only to carbon dioxide when it comes to anthropogenic climate impact. Hence, developing the understanding necessary to build cleaner combustion technologies that reduce the climate impact of soot would have almost immediate effects.”

Spawning additional research

The present work is intended to provide a guidebook to the oxygen chemistry and importance of different oxygenated functional groups. The work may assist other researchers in interpreting data from soot measurements by providing the masses of oxygenated species formed during different combustion conditions. The current work might also aid in designing new experiments as it provides information on molecular structures.

“Hopefully, our work will spark new ideas among our colleagues,” Olof says. “One important outcome of the present study, which we think may advance the work done at the CRF, is that large oxygenated species need to be considered for the hydrocarbon growth chemistry under many combustion conditions.”

The present research can be viewed as a continuation of the work performed by Craig Taatjes (8353) and his colleagues when they were the first combustion scientists to detect small enols in flames.

“Our work shows that enols larger than those Craig and colleagues were able to detect play an important role as intermediate species on the chemical route toward furans,” says Olof.

One of the most intriguing challenges of this research is laying out the transition from gas-phase molecules to solid particles as they first form in a combustion environment, Olof says.

“Measurements in combustors are very difficult because there are few diagnostic techniques that do not perturb the combustion chemistry, and the techniques available do not provide all of the necessary information,” he says. “Implementation and interpretation of experiments can be challenging. Modeling, on the other hand, is also challenging because the combustion systems are large and complex. Using a close combination of measurements and modeling allowed us to uncover new chemical mechanisms.”

After the researchers revealed the existence of oxygenated carbon species, the next step was to know their arrangement — to determine what molecular species were actually being formed.

“The process is comparable to having a number of letters but not knowing what word they came from because there can be several ways to combine them,” says Farid. “This riddle can be resolved with information about how certain letters are connected to others.”

In terms of chemistry, this means that experimental evidence of the types of chemical bonds between carbon, hydrogen, and oxygen species was needed, Farid says. The researchers used a model to predict what chemical species could be formed but the experimental confirmation came from X-ray photolelectron spectroscopy (XPS) measurements performed by Farid in his materials physics laboratory. The XPS instrument uses X-ray light to produce photo-emitted electrons from atomic levels inside carbon and oxygen from collected soot samples. These electrons carry information about what bonds carbon and oxygen have between them and with hydrogen. The bonding information revealed that the theoretically predicted furan molecular structure was in fact being generated.