A high-density ammonia storage/delivery system based on Mg(NH3)6Cl2 for SCRSCR–DeNOxDeNOx in vehicles

In this paper, we present a new benchmark for the automobile selective catalytic reduction of NOxNOx: Mg(NH3)6Cl2. This solid complex releases ammonia upon heating and can be compacted into a dense shape which is both easy to handle and safe. Furthermore, the material has a high volumetric ammonia density of up to 93% of that of liquid ammonia. This provides a long lasting ammonia storage (≈20000km of driving per 6.2 L Mg(NH3)6Cl2 for an average medium-sized vehicle). The controlled thermal decomposition of Mg(NH3)6Cl2 was demonstrated. A small reactor with a volume of 785 mL was filled with ≈260g of Mg(NH3)6Cl2 yielding a bed density of 331 kg/m3. The reactor was coupled to a buffer with a free volume of roughly 200 mL. A heating wire wrapped around the outside of the reactor supplied the heat-energy. A mass-flow controller was used to simulate a varying NOxNOx signal. It was demonstrated that it was possible to control the desorption using a simple ON–OFF controller with the buffer pressure as the control variable. Approximately 99% of the ammonia contained in the salt could be desorped and dosed, while maintaining the 5 bars used as the set-point pressure. The low density was improved by compressing the Mg(NH3)6Cl2 powder to a density of 1219 kg/m3, which is very close to the theoretical crystal density of 1252 kg/m3. Temperature programmed desorption showed that the ammonia could easily be desorped by heating the densified material. Stoichiometric calculations have shown, that compared to the current choice of ammonia delivery for mobile DeNOxDeNOx (thermal decomposition of a 32.5% wt/wt aqueous urea solution), the high-density Mg(NH3)6Cl2 compound weighs 2.8 times less and takes up 3.1 times less space. This makes Mg(NH3)6Cl2 ideal for use as an ammonia storage compound in both diesel and lean-burn gasoline-driven automobiles.