Theory and Modeling

Combustion theory and modeling are deeply rooted at Sandia starting in the late 1970s when Bob Kee, Jim Miller, and Tom Jefferson developed Chemkin to predict flame properties, such as burning velocities, extinction limits, flammability limits, and pollutant generation and destruction. Since then, this group has developed BAC-MP4 and G2Q quantum chemical methods and a comprehensive mechanism for nitrogen chemistry in combustion; predicted product distributions in functionalization chemistry of fullerenes; and identified the reaction between two propargyl radicals as a critical cyclization step in the formation of polyaromatic hydrocarbons in combustion.

Today, Sandia's combustion chemistry theory and modeling research focuses primarily on pollution control, particularly of nitrogen oxides and soot. To be able to control pollution, we must first understand how pollutants are produced in combustion processes. This understanding requires much basic chemical information. At Sandia, our combustion chemistry theory and modeling techniques provide such information. For example, electronic structure theory provides the detailed potential energy surfaces of elementary chemical reactions and the thermochemistry of unstable free radicals. Dynamics theory uses these potential energy surfaces to yield rates and product distributions of the elementary reactions. We use all of this information to develop chemical kinetic models of combustion, which we then use to simulate macroscopic experiments and industrial processes. The techniques we use range from calculating the details of molecular electronic structure and dynamics to modeling the chemical kinetics of flames.

Recently, Stephen Klippenstein and Jim Miller, in collaboration with Struan Robertson of Molecular Simulations, Inc. of Cambridge, England, completed an extensive theoretical investigation of the reaction between ethyl (C₂H₅) and molecular oxygen. The analysis consisted of using a combination of electronic-structure theory (both a G2-like approach and the B3LYP density functional method), variational transition-state theory to calculate RRKM rate coefficients, and solving the time-dependent, chemically activated master equation to characterize the rate and product distribution over wide ranges of temperature and pressure. The most interesting and important result of the analysis was the prediction of three distinct temperature regimes for the kinetics: At low temperature (T < 575 K), the deficient reactant (C₂H₅) decays exponentially, and the products (C₂H₅O₂ or C₂H₄ + HO₂) are functions of temperature and pressure. In the transition regime, (575 K < T < 700 K), C₂H₅ decays biexponentially, and the production of bimolecular products (C₂H₄ + HO₂) increases rapidly with temperature as the rate coefficient drops (corresponding to equilibration of the stabilization reaction). At high temperature (T > 700 K), the rate coefficient is independent of pressure.