Analysis of an ammonia/ammonium nitrate fuel's thermal
decomposition by mass spectrometry and thermogravimetric analysis

Bar
Mosevitzky, Gennady E. Shter, Gideon S. Grader^*,

*Corresponding
author: grader@technion.ac.il

Wolfson
Department of Chemical Engineering, Nancy and Stephen Grand Technion Energy
Program, Technion-Israel Institute of Technology, Haifa 3200003, Israel

Research
on renewable synthetic fuels has become a hot-topic in recent years. This is
due to the long-term energy storage capabilities of chemical bonds and their
potential compatibility with current energy infrastructure. Specifically,
nitrogen-based fuels offer a carbon-free solution to wide scale implementation
of renewable energies. Therefore, the inherent chemistry involved in the
utilization of these fuels for stationary and mobile power generation is of
prime interest. Unlike ammonium nitrate, ammonia suffers from unstable
combustion characteristics. Therefore, adding ammonium nitrate to ammonia
combustion may stabilize the process. However, while ammonia's gas-phase
reaction mechanism is well studied, ammonium nitrate's is
poorly understood. Furthermore, reports on their decomposition
interactions are lacking and occasionally upright conflicting.

In
this work, aqueous ammonia/ammonium nitrate solutions were
tested under inert/oxidative atmospheres in a mass spectrometry
(MS)/thermogravimetric analysis (TGA) coupled system. Gas-phase kinetics
simulations were utilized to explore the process
chemistry. A typical experimental profile included three separate endothermic
processes detected via weight loss (TGA), rate of mass loss (DTG) and heat loss
(DTA) profiles (Figure 1). Applying a peak separation algorithm on the DTA signal
enabled the identification of 6 separate peaks (Figure
2). These results were accompanied by concurrent mass
spectrometer readings (Figure 3). Utilizing these tools, the effects of the
ammonia to ammonium nitrate ratio and the utilized atmosphere on the thermal
decomposition of the solutions will be presented. The
implications of these results on the current understanding of ammonia and
ammonium nitrate gas-phase chemistry and their interactions will
be discussed.

Figure
1. Measured (a) DTA thermogram and (b) TGA/DTG curves for stoichiometric (φ=1) AAN under Ar/O₂.
The onsets of the three detected endothermal processes detected are denoted by grey dashed lines and numbered 1-4 in both
subfigures.

Figure
2. Measured DTA thermogram for stoichiometric (φ=1) AAN under Ar-O₂ flow
plotted against the detected peaks and their sum.

Figure
3. Species signals in arbitrary units over the decomposition process of
stoichiometric (φ=1)
AAN under Ar-O₂ flow as a function of the sample temperature.