A new method to understand nitrogen oxides (NOx) production pathways in different fuel types with the help of computer analysis with laser-based NOx concentration measurements has been discovered by the researchers at KAUST. Long before climate change focused global attention on vehicle carbon emissions, regulators were clamping down on NOx because of its detrimental effect on air quality. Controlling nitrogen-oxide emissions continues to be a real challenge, with ever-tightening restrictions on automobiles, trucks, and aviation. The researchers are continually looking at emerging fuels and trying to develop chemical kinetic mechanisms to predict NOx formation.

There exists interest in using alcohols as renewable, lower emission fuels. It has been observed that alcohol flames generally produce lower concentrations of NO emissions and the cause of these reductions is attributable to a number of mechanisms. This work, therefore, investigates the relative contributions to total NO formation in alcohol-fueled flames, relative to comparably sized alkane flames.

In the latest study, the team examined NOx emissions from alcohol fuels, such as ethanol. These renewable fuels have gained attention because they are potentially carbon-neutral, but they also produce lower NOx emissions than conventional fossil fuels. Nitrogen oxides can be produced by various pathways, each of which dominates a different stage of combustion. By understanding these pathways and their relative importance, it becomes easier to develop NOx mitigation techniques.

The researchers used planar-laser-induced fluorescence (PLIF) to measure NOx production throughout the flame, comparing alcohol fuels with an alkane fossil fuel surrogate. The technique helped the team differentiate nonthermal NOx production pathways, which dominate early in the flame, from thermal NOx production pathways, which mainly occur in the high-temperature post-flame region.

These types of direct measurements are pretty difficult to pull off. But they are important because they directly show the differences in the nonthermal NO formation between the two fuel classes. As much as 50 percent less nonthermal NO was produced in the alcohol flames, the researchers observed. They also developed an algorithm to further pinpoint the specific NOx pathway contribution differences between alcohol and alkane fuels.

Alcohol fuels definitely have applications in internal combustion engines, where alcohols - ethanol in particular - are added to gasoline. There is a lot of potential to use fuel blends to mitigate NOx. The researchers are continuing to refine their mechanistic understanding of NOx production, validating their models using laser-based diagnostics, to predict, understand and eventually mitigate NOx emissions.