Researchers from the US Department of Energy’s Argonne National Laboratory and the Technical University of Denmark have synthesized more than a decade’s worth of combustion studies to create a new overarching model of how nitrogen oxides are produced.

Even though the chemical processes that govern formation and destruction of NO_x in combustion processes has been the subject of extensive research over the last four decades, there are still unresolved issues that may limit the accuracy of engineering calculations and thereby the potential of primary measures for NO_x control. In a review published in Progress in Energy and Combustion Science, the team reports its current understanding of the mechanisms that are responsible for combustion-generated nitrogen-containing air pollutants.

A large amount of work has been devoted to improve the understanding of NO_x formation mechanisms in combustion and to develop control strategies to reduce emissions. Restrictions on NO_x emissions that used to apply only to large combustion systems such as power plants are now implemented on ever smaller scales …

The objective of the present work is to establish and evaluate a state-of-the-art, non-optimized chemical kinetic model for homogeneous nitrogen chemistry in combustion. It is based on the work on nitrogen chemistry reported over the last decades, drawing also on recent advances in the knowledge of thermochemistry and reaction rates from theoretical work. The different mechanisms for formation and consumption of NO are discussed and the key reaction steps are evaluated. Different subsets of the model are validated against experimental data and the predictive capability of the mechanism is assessed.

To limit the scope of the review, we emphasize nitrogen chemistry in combustion of light hydrocarbons, mostly CH₄, and/or light fuel-nitrogen species (HCN, NH₃, HNCO) at atmospheric or sub-atmospheric pressure. Oxidation of heterocyclic nitrogen compounds, which may be formed in combustion of more complex fuels, or of energetic materials such as nitroalkanes is not addressed in the current work.

—Glarborg et al.

NO_x production is one of the main concerns for engine companies. Our understanding of how these pollutants are produced in different engine environments has deepened dramatically.Argonne chemist Stephen Klippenstein, co-author

A wide array of different chemical interactions occur within the mixture of fuel and air in an engine, and the new model identifies several different routes to NO_x formation.

In one pathway, called prompt NO (nitrogen monoxide), atmospheric nitrogen combines with carbon to form an intermediary of one carbon and two nitrogen atoms, which eventually combine with oxygen to form nitrogen monoxide. In another pathway, called thermal NO, nitrogen monoxide is produced directly from nitrogen and oxygen. In a third, called fuel NO, a compound of nitrogen, carbon and oxygen forms the intermediary step on the way to nitrogen monoxide.

Trying to put together these pathways to create a model that accurately reproduces experimental observations has always been a bit of a guessing game. However, because so many scientists from around the world are contributing information about different segments of the larger picture, we’re closer than ever before to a model that truly represents reality.

—Argonne chemist Branko Ruscic, co-author

According to Klippenstein, one of the main characteristics of the combustion process—temperature—makes a big difference in the quantity of NO_x produced.

The temperature affects the lifetimes of the molecules in the mix. Being able to accurately model and predict the behavior of some extremely short-lived molecules is crucially important to determining the pathways of the reaction. If you can run your engine at a lower temperature, you can avoid the formation of much of the NO_x.

—Stephen Klippenstein

Another factor in the combustion process that significantly affects NO_x is the richness of the fuel mixture. Engines that run richer will have molecules with more methyl groups, Ruscic said, which tend to promote the formation of NO_x.

The research was funded, in part, by the US Department of Energy’s Office of Science.

Resources

• Peter Glarborg, James A. Miller, Branko Ruscic, Stephen J. Klippenstein (2018) “Modeling nitrogen chemistry in combustion,” Progress in Energy and Combustion Science Volume 67, Pages 31-68 doi: 10.1016/j.pecs.2018.01.002