Next month the print edition of Fuel Processing Technology will feature a paper entitled “Auto-ignition of a carbon-free aqueous ammonia/ammonium nitrate monofuel: a thermal and barometric analysis.” This title is provocative. First, what is this idea of a fuel composed of a mixture of ammonia and ammonium nitrate (AN)? If ammonia is a good fuel, is it made better with the addition of ammonium nitrate? Second, why is it aqueous? Is the presence of water a feature or a bug? Third, what is a monofuel and why is this term used when the fuel is a mixture of two molecular species? And finally, why is the paper ultimately about auto-ignition?
Before addressing these questions, some background information is in order. The authors of the paper, Bar Mosevitzky, Gennady Shter, and Gideon Grader are part of the Grader Research Group at Technion Israel Institute of Technology. The eponymous Grader received a doctorate in chemical engineering from the California Institute of Technology in 1986 and spent the early part of his career working on superconductors at Bell Labs in New Jersey. He moved to Technion in 1989 and conducted research on ceramic materials as they relate to superconductivity and other topics. He became interested in catalysis in the mid-2000s and then, as reflected in a paper he co-authored in 2007, in catalysis for methanol oxidation. His interests continued to evolve and eventually expanded to include hydrogen generation and ceramics.
Then, in 2012, Grader and three colleagues published “Corrosion of aluminum, stainless steels and AISI 680 nickel alloy in nitrogen-based fuels” in the journal Materials and Corrosion. The abstract’s first sentence signaled a major new research direction for the Grader Research Group: “Nitrogen-based compounds can potentially be used as alternative non-carbon or low-carbon fuels.”
Eleven papers on various aspects of nitrogen-based fuels followed. A culmination of sorts arrived with last spring’s publication in Angewandte Chemie of “Nitrogen-based fuels: a power-to-fuel-to-power analysis,” and last fall’s publication in Applied Energy of “The nitrogen economy: economic feasibility analysis of nitrogen-based fuels as energy carriers.” (The Angewandte Chemie paper was covered in an Ammonia Energy News post in March.)
This publication history shows that from the outset the Grader Group defined the scope of their interest as “nitrogen-based fuels” rather than simply ammonia. As detailed in the Angewandte Chemie paper, the list of such fuels includes “AN and AN-based compositions, aqueous hydroxylammonium nitrate, ammonium dinitramide, and aqueous AN with ammonium hydroxide or urea.” The specific ammonia derivatives profiled in the paper are aqueous ammonium hydroxide urea (AHU), aqueous ammonium hydroxide ammonium nitrate (AAN), and aqueous urea ammonium nitrate (UAN).
A key reason for considering nitrogen-based fuels beyond ammonia is safety. The Fuel Processing Technology paper notes that while “renewable hydrogen generation by water electrolysis is a possible large scale energy storage solution,” hydrogen transport “requires specialized infrastructure and involves inherent safety hazards.” In the Applied Energy paper, Grader and his co-authors assert that ammonia itself “is much more toxic than gasoline or methanol, with dangerous health effects for humans depending on the exposure time and dose. For this reason the utilization of ammonia as a transportation fuel has been hampered.” (It should be noted that comparison of the toxicity ammonia to that of gasoline involves a complex set of considerations that do not inevitably lead to the conclusion that ammonia is “much more toxic.” Relevant information in this regard is included in a previous post in the Ammonia Energy News. It should also be remembered that toxicity is just one component of a fuel’s risk profile.) By contrast, certain ammonia-derived fuels – especially those that can be mixed with water – have a benign safety profile. For example, “an aqueous AN solution is chemically stable and non-explosive, thus safe to transport, handle, and store,“ according to the Angewandte Chemie paper.
Grader defines a monofuel as a fuel that contains “the oxidizer as well as the reducer in the same solution. Consequently, no external oxidizer such as air is required for . . . combustion.” The stoichiometrics of this can be seen in the chemical equation for oxidation of the monofuel AAN, in which no externally sourced oxygen is required for the reaction to proceed to pure diatomic nitrogen and water:
3 NH4NO3 + 2 NH4OH + 4H20 → 4 N2 + 11 H20
(NH4NO3 is ammonium nitrate. NH4OH is ammonium hydroxide, which by convention is how ammonia in aqueous solution is denoted since each molecule of NH3 associates itself with a molecule of H2O to form NH4OH before it dissociates into NH4+ and OH- ions.)
None of the Grader papers directly discuss the advantages of a monofuel, but the potential positives of a fuel that can give up its energy without the need for outside air are readily apparent. All modern heat engines – including combustion turbines and internal combustion engines – require precisely controlled fuel-air mixtures to optimize power output and fuel efficiency while minimizing pollutant generation. Clearly many aspects of engine design could be simplified if the fuel-air mixing process could be eliminated and the stoichiometrically perfect ratio of oxygen was already embodied in the fuel itself. In an email Grader spoke to one dimension of such simplification, writing, “there is no need to compress the air/fuel mixture before combustion, and one can generate considerably higher pressures in the combustion chamber – since the pressure is regulated at the outlet (back pressure regulator).” Fuel delivery, he says, can be accomplished with a high-pressure pump. “These liquid pumps consume much less energy than gas compressors since unlike gases the liquids are incompressible, (no compression work losses).”
Finally, the Grader Group’s focus on auto-ignition relates to the importance of this concept in the function of heat engines generally. Auto-ignition is either the phenomenon that makes the engine work (as in diesel engines) or a phenomenon that causes problems if it is not controlled (as in spark-ignition engines and combustion turbines). Without disavowing interest in internal combustion engines, the Grader papers are evidently oriented toward combustion turbines – the presumed method of choice for converting electricity stored as a chemical fuel back into electricity.
Grader and his colleagues clearly see nitrogen-based fuels as a key element of a sustainable energy future. The Angewandte Chemie paper concludes with these words: “We showed that a nitrogen economy, where renewable hydrogen is chemically stored on abundant nitrogen in the form of a nontoxic and safe nitrogen-based alternative fuel, is energetically feasible, and that novel nitrogen-based fuels are comparable on an energy-return basis to existing carbon-based fuels. Incorporating nitrogen-based fuels as part of a future energy mix will enrich and ‘fertilize’ our energy portfolio.”