Analysis of an Ammonia/Ammonium Nitrate Fuel’s Thermal Decomposition By Mass Spectrometry and Thermogravimetric Analysis

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.