Auto-Ignition of a Carbon-Free Aqueous Ammonia/Ammonium Nitrate Monofuel: Effect of the Equivalence Ratio

Bar Mosevitzky, Wolfson Department of Chemical Engineering, Technion - Israel Institute of Technology, Haifa, Israel
Gennady E., Wolfson Department Of Chemical Engineering, Technion - Israel Institute Of Technology, Haifa, Israel
Gideon S., Wolfson Department Of Chemical Engineering, Technion - Israel Institute Of Technology, Haifa, Israel

Storage of renewable energies is of prime importance due their inherent fluctuating energy output. Chemical fuels offer high energy densities and are easy to transport, making them a desirable energy storage vector. While renewable hydrogen can be easily produced via water electrolysis, its low energy density and incompatibleness with current infrastructure prevents its implementation. Both carbon- and nitrogen-based synthetic fuels can be used to store the hydrogen in an efficient manner.

A previously suggested nitrogen-based monofuel, AAN is an aqueous solution of ammonium hydroxide and ammonium nitrate. The thermal autoignition of AAN has been previously studied, but only at a stoichiometric ratio. In the case of AAN, ammonium nitrate acts as the net oxidizer and ammonium hydroxide as the reducer. Therefore, AAN produces nitric acid and ammonia when heated. Thus, it is possible to define the equivalence ratio (ϕ) of AAN as the ratio between its ammonia and nitric acid moles divided by the stoichiometric ratio.

A combined differential thermal/barometric analysis (DTA/DBA) system was used to study the thermal autoignition of AAN at 0.6≤ϕ≤12. As the equivalence ratio was increased the measured autoignition temperature (AIT) rose, the ignition peak value was higher and the autoignition itself was delayed (Fig. 1).

 Figure 1. The autoignition process for AAN at different equivalence ratios as observed in DTA curves vs. time starting from a temperature of 250ºC.

Kinetic gas phase calculations using the CHEMKIN-PRO^™ software package were performed to simulate the experimental thermal autoignition process. The resulting AIT values were in good agreement with the experimentally measured ones. Rate-of-production and sensitivity analyses were performed to identify both the main and rate-limiting reactions leading to the thermal autoignition of AAN at rich (ϕ=1.4), stoichiometric (ϕ=1.0) and lean (ϕ=0.6) conditions. The core results of this study will be presented and their implication for catalytic ignition purposes will be discussed.