Effects of water content and diluent pressure on the ignition of aqueous ammonia/ammonium nitrate and urea/ammonium nitrate fuels

Two aqueous monofuels composed of aqueous solutions of ammonia/ammonium nitrate and urea/ammonium nitrate have been suggested as feasible nitrogen-based hydrogen carriers. Such synthetic fuels can serve as long-term energy storage media, providing back-up power generation for intermittent renewable energy sources. The effects of diluents such as helium and water on the thermal behavior of these fuels are mostly unexplored. Experimental results indicated the fuels' autoignition temperature increases with diluent content due to the heat sink effect of the inert water and helium. Simulations were performed using an updated mechanism to study the reaction pathways leading to thermal autoignition. Isocyanic acid underwent hydrolysis generating NH₃ and most of the CO₂. Ammonia was oxidized by either NO₃ or NO₂ to form NH₂ and either HNO₃ or HONO. Nitric acid reacted with HONO producing N₂O₄ and most of the H₂O. Molecular nitrogen was mostly produced by the termination reaction between NH₂ and NO. Sensitivity analyses indicated the ignition of these fuels is promoted by the generation of NH₂ from NH₃ by high oxidation NO_x, and inhibited by the reduction of NO₃ and NO₂ to NO₂ and NO, respectively. This work demonstrates for the first time the effect of aqueous ammonia/ammonium nitrate water content on its auto-ignition temperature and the crucial role of isocyanic acid hydrolysis in aqueous urea/ammonium nitrate pre-ignition pathways.