Auto-Ignition Kinetics of Ammonia at Intermediate Temperatures and High Pressures


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The anxiety over global greenhouse gas emissions has intensified the demand for the development and use of CO2-neutral energy technologies. Ammonia is now attracting attention as a carbon-free energy carrier, because it has good energy density (22.5 MJ/kg) and can be easily liquefied (about 10 bar at 298 K). In addition, ammonia is produced according to the Haber-Bosch process, which makes it one of the most widely-produced inorganic chemical in the world. It could also be produced with renewable energy sources such as wind and solar energy using P2X technology.

As a potential fuel for applications in gas turbines and gas engines, ammonia is less reactive than most hydrocarbons and its ignition and combustion characteristics are not yet well understood. A major part of the previous research has focused on the ammonia oxidation at high temperatures or low pressures [1-3], while ignition measurements for pure ammonia or ammonia mixed with other gaseous fuels (such as hydrogen or methane) at high pressures and low-to-intermediate temperature is rare.

Rapid compression machines (RCMs) are regarded as an important experimental apparatus for investigating auto-ignition behavior at low-to-intermediate temperature conditions, which are quite relevant to the application in internal combustion engines and gas turbines [4,5].

In this study, autoignition properties of NH3/O2 and NH3/H2/O2 mixtures have been studied in a RCM at pressures from 20 to 60 bar, temperatures from 950 to 1150 K, and at equivalence ratios from 0.5 to 2. The effect of hydrogen-ammonia ratio in fuel has been also investigated.

It is observed in the experiments that a higher H2 mole fraction in fuel improves the reactivity of the mixture. When the mixtures contain 20% H2 in fuel (xNH3:xH2=0.8:0.2), fuel-richer mixtures have shorter ignition delay times, while for the mixtures with 1% H2 the equivalence ratio dependence is reverse. With 5% H2 in fuel, the stoichiometric mixture presents the shortest ignition delay time. In the mixtures without hydrogen, leaner mixtures show higher reactivity. Numerical simulations were performed based on the literature mechanism from Klippenstein et al. [6] and Glarborg et al.[7]. The model predicts well the ignition delay times for the mixtures with 20%, 1% H2 and pure ammonia. For the mixtures with 5% H2 the model underestimates the ignition delay time. Kinetic analysis further demonstrates that the reactions involving H2NO and the two channels of the reaction NH2 + NO are dominant for the ammonia auto-ignition in the investigated temperature range, and the reactions involving H2O2 are important for ammonia-hydrogen blends.

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