Approach predicts reaction rate in combustion of ammonia over unprecedented temperature range

Applying a high-level kinetic model that accounts for tunneling effects, which result from low-probability events with particles penetrating classical energy barriers, University of Florida researchers have accurately predicted rates of reaction between ammonia—an emerging, carbon-free alternative to fossil fuels—and hydroxyl radicals over a large 200 to 2,500 kelvin temperature range.

As reported this month in The Journal of Chemical Physics, the theoretical approach reliably tracks the removal of hydrogen from ammonia, a process critical to ammonia combustion. Thermal rate constants, calculated from first principles, agreed well (within 5% to 20%) with experimental values, using calculated thermochemistry that also agrees well with Active Thermochemical Tables.

T. Lam Nguyen and John F. Stanton say their investigation of the ammonia-hydroxyl reaction demonstrates the predictive power of the High-Accuracy Extrapolated Thermochemistry (HEAT) ab initio protocol, used with semi-classical transition state theory (STCST). Integrating HEAT with STCST enabled accurate results over the entire temperature range.

From 200K to 2,500K, calculated rate constants increased a hundred-thousand-fold. Quantum-tunneling effects peak at lower temperatures in the studied range—where rates depart significantly from the classical Arrhenius formula for predicting the temperature dependence of reaction rates—and decreased to about 10% at 1,000K.

Proponents of ammonia as a zero-carbon, liquid transportation fuel and for energy storage—the focus of research and demonstration projects around the world—point to several advantages. For example, ammonia can power internal combustion engines as well as serve as the hydrogen source in fuel cells. Also, existing infrastructure for fuel handling and distribution can be adapted to accommodate ammonia. While nitrogen oxides may form during ammonia combustion, these can be eliminated with existing selective catalytic reduction systems, according to Nguyen and Stanton.

Source: “High-level theoretical study of the reaction between hydroxyl and ammonia: Accurate rate constants from 200 to 2500 K,” by Thanh Lam Nguyen and John F. Stanton, The Journal of Chemical Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4986151 .