What happens during a wildfire? The short answer is: much more than we know.

As wildfires burn through trees, grasses and other vegetation, heat from the flames set off thousands of chemical reactions at once. Molecules break apart, recombine and transform into new compounds, releasing gases, soot and other carbon-rich particles into the air.

 

Charred vegetation after burning in a wildfire is a product of combustion.

While scientists can see the products of these reactions, what happens in between is much harder to understand. Many of the chemical reactions that drive combustion take place so quickly that they are nearly impossible to observe.

That’s where Jas Shahanand comes in.

Shahanand, a sixth-year PhD student in the Paul M. Rady Department of Mechanical Engineering at CU Boulder, is finding ways to capture and identify some of these fleeting molecules, giving researchers a closer look at the hidden chemistry of combustion.

He believes the work, recently published in the American Physical Society’s PRX Energy journal, could be the foundation for more accurate wildfire emissions models, improved climate prediction and even cleaner fuel sources in the future.

“These chemical reactions are often very generalized because they occur in a black box. But there’s actually a lot of good information hidden in those intermediate processes," said Shahanand. “My goal is to analyze those processes from a gas-based perspective to help provide a deeper understanding of how combustion chemistry really works.”

Tiny chemical thieves

The key to Shahanand’s research lies in a type of tiny molecule called radicals.

 

Sixth-year PhD student Jas Shahanand working in the laboratory.

Radicals, created by heat and other high energy environments, act as the driving force behind combustion. They come with an unpaired electron, making them extremely unstable and reactive. 

When they react with other stable molecules, they steal an electron to create a pair. This sets off a chain reaction of molecular-level theft—new radicals crashing into molecules and robbing electrons over and over again.

Eventually, these reactions facilitate molecular growth, where gas-phase chemistry proceeds to particle-phase. This can especially be seen during wildfires, when gases like methane or ethylene transform into char and soot.

Understanding these “radical” reactions can tell us a lot about different forms of combustion, from how engines work to interstellar and planetary behavior. But according to Shahanand, they are very short-lived.

“We’re talking nanoseconds,” Shahanand said. “It’s very difficult to run experiments and watch them propagate at that speed.”

To combat this, Shahanand and his team in the Particulate Chemistry and Diagnostics Lab, led by Professor Hope Michelsen, began studying a specific kind of radical: resonance-stabilized radicals.

Unlike normal radicals, the unpaired electron in these molecules is spread out across multiple atoms, allowing them to be more stable and live longer. This gives researchers a much better chance at detecting and studying them, revealing chemical behavior that has largely remained unknown.

“We’ve always known the chemical pathways, but determining which radicals are causing the reaction has always been based on educated guesses,” said Shahanand. “Now, we can confirm those chemical species in our experiments and create models that identify and characterize them for future tests.”

The “fingerprints” of the future

One particular experiment, conducted by Shahanand and his team at Lawrence Berkeley National Laboratory’s Advanced Light Source in Berkeley, California, analyzed chemical reactions when lignin—a material found in plants and trees—is heated without oxygen during a process called pyrolysis.

Lignin pyrolysis is widely known for generating aromatic compounds called substituted anisoles, which have potential to be transformed into a renewable biofuel. But the process can also produce unwanted byproducts such as char and tar.

 

The process of pyrolysis, starting with lignin and ending with biofuels or char, tar and other particles.

Shahanand and his group discovered that by slightly modifying anisole molecules, they were able to drastically increase the amount of radicals produced under natural lignin pyrolysis conditions.

He says this finding could one day help scientists “control” these chemical reactions, allowing them to better produce useful compounds while limiting undesirable ones.

“If we want to better fight wildfires or produce clean, cost-effective fuel, we need to have a fundamental understanding of the chemistry,” Shahanand said. “We need to know which reactions are good for our goals and which reactions create bad byproducts. To do that, we need to know what’s actually happening inside of the reaction, which is where our research comes into play.”

They also are the first researchers to create a complete “fingerprint” for some radicals they detected that are especially relevant to combustion. Much like a fingerprint can identify a person, these results can help researchers identify a specific molecule or radical. 

Previously, scientists only had a reference curve covering part of the spectrum. Shahanand and his team filled in the gaps, producing a complete set of data across the full range and giving researchers a clearer way to identify the radical in future experiments.

Going forward, Shahanand says he wants to continue studying all types of chemical reactions and the radicals behind them. His next work will examine chemicals and conditions more akin to what takes place when an actual forest is burning.

But he believes his research has the power to impact many other aspects of society, as well.

“Maybe we want to explain why certain warming effects exist in different ecosystems. Or maybe understand how chemistry facilitates combustion in space or other planets with different atmospheres,” Shahanand said. “Whatever it is, it all comes back to these fundamental ideas.”