Understanding low temp combustion

By Michael Padilla

Photography By Dino Vournas

Friday, September 15, 2017

Fundamental chemical knowledge will aid in predicting oxidation chemistry

Could mean more efficient engines

Hydrocarbons come in all shapes and sizes in nature, and their oxidation reactions can be quite complex. A fundamental understanding of this important area of chemistry requires careful and often very challenging experiments, along with state-of-the-art theoretical calculations.

A team of researchers at Sandia’s Combustion Research Facility were up to that challenge, pushing the limits of what’s possible with experiments and theory to unravel the complexity of hydrocarbon oxidation.

By conducting experiments in engine-relevant conditions using novel equipment and methods, the team produced new fundamental chemical knowledge. This new knowledge will aid in modeling the combustion of traditional fuels and proposed new fuels, facilitating the development of future efficient engines. Detailed fuel chemistry can be used to optimize performance in emerging engine designs.

According to principal investigator Leonid Sheps (8353), this approach is most fruitful in low-temperature combustion, below about 1,000 K, where oxidation chemistry dominates fuel reactivity.

“In this ‘autoignition’ regime, fuels with different molecular structure can react to form different products and at vastly different rates,” he says. “This is one promising strategy for engine optimization. In contrast, in the high-temperature regime the kind of chemical reactions that dominate combustion are much less sensitive to the exact identity of the fuel.”

The underlying goal of the work is to understand how the molecular structure of hydrocarbons affects their oxidation chemistry in general, and to develop ways of modeling the combustion of other fuel candidates with predictive accuracy.

Leonid worked with former Sandia postdoctoral researcher Ivan Antonov, who served as the lead author, in publishing a paper titled, “Pressure-Dependent Competition among Reaction Pathways from First- and Second-O2 Additions in the Low-Temperature Oxidation of Tetrahydrofuran,” in The Journal of Physical Chemistry A. Coauthors included Judit Zádor, David Osborn, Craig A. Taatjes (all 8353) and former Sandia postdoctoral researchers Brandon Rotavera and Ewa Papajak.

"We are producing critically important knowledge that cannot be obtained in other ways.”

This study targeted the autoignition of tetrahydrofuran (THF), a prototypical 5-membered ring cyclic ether, allowing the team to study the effects of cyclic structure (which include a certain degree of rigidity, as well as ring strain) and an ether functional group (O atom in the ring) on the autoignition mechanism.

The team conducted the research at elevated temperatures and pressures — approaching engine-relevant conditions to better understand the autoignition chemistry of ethers in important environments.

“Experiments at high pressures are extremely challenging,” Leonid says. “There is a lack of experimental data and conclusions extrapolated from low-pressure work are often unreliable. We are producing critically important knowledge that cannot be obtained in other ways.”

In principle, autoignition can occur through myriad coupled chemical reactions, many of which have complex temperature- and pressure-dependence. This study revealed the autoignition mechanism of THF by narrowing down all such possible processes to about a dozen or so key reactions that dominate THF oxidation. The work showed how the molecular structure of THF was responsible for selecting these key reactions. Such mechanistic insights increase the overall understanding of oxidation chemistry and allow researchers to make predictions about other fuel compounds.

High-pressure experimental reactor created to achieve desired sample conditions

The researchers relied on several innovations in their experimental approach, from the chemical reactor created for the research to the detection and analysis method. “Our Sandia-developed experimental capabilities are unique in the world,” Leonid says.

The team used a high-pressure flow reactor, capable of maintaining precisely controlled experimental conditions, free from effects of turbulence or wall reactions. They coupled this reactor to a newly developed novel mass spectrometer with a 100-fold increase in sensitivity over traditional approaches. This high sensitivity is needed in high-pressure experiments to detect elusive, fleeting chemical intermediates that are present at vanishingly small concentrations but have an outsized influence on the chemistry in question.

“This helped provide an in-depth look at the chemistry ‘as it happens.'"

The work was conducted by flowing a mixture of reactants (THF, oxygen and a small quantity of Cl2 – a photolytic radical source) in an inert buffer gas through the chemical reactor at strategically chosen temperatures, pressures, and mixture compositions. Short laser pulses were used to rapidly create Cl radicals within this mixture, which rapidly abstracted H atoms from THF.

This process resulted in a nascent population of hydrocarbon radicals, thus mimicking the initial stages of THF combustion. The THF-yl radicals then reacted with O2 and underwent a series of chemical reactions that collectively make up THF oxidation. Throughout the reactions, the team continuously monitored the chemical composition of the reacting mixture by photoionization mass spectrometry.

Microsecond time resolution

“This technique provided the time histories of nearly all important chemical species at once with microsecond time resolution,” Leonid says. “This helped provide an in-depth look at the chemistry ‘as it happens’ as opposed to collecting final products for later analysis, which would leave much to guesswork as to how exactly these products formed.”

The team also used isotopically labeled reactants, in which some of the H atoms in THF were replaced by D atoms, which are chemically similar but have a different mass. By analyzing the mass of the reaction products and radical intermediates, the team was able to identify which H or D atom moved during various reaction steps and thus track which way several key chemical species were formed.

The team continues to investigate the oxidation chemistry of diverse hydrocarbon compounds with a focus on the effects of molecular structure including size, degree of branching, and various functional groups. Further work includes expanding the range of chemical systems to alkanes, alkenes, ketones, other ethers, and esters to fully understand the link between molecular structure and reactivity.

The work, sponsored by the Basic Energy Sciences division of the Office of Science, DOE, is part of a joint effort between Sandia and Argonne national labs and benefits from close collaboration with other experimentalists and theoreticians, both at Sandia and Argonne.