Investigation of the ammonia cracking kinetics over Ru/Al2O3 using conductive reactor internals

NH3 cracking is receiving growing attention for the exploitation of ammonia as H2 carrier. Of the different aspects of this process that require in depth study, the choice of the optimal catalyst is arguably one of the most important, being the process kinetically controlled. Literature agrees that ruthenium is the most active metal for low temperature ammonia cracking, however, no consensus on the kinetic expression has been reached due to the high sensitivity of the reaction mechanism to the specific reaction conditions. For this reason, it is of utmost importance to work in conditions as close as possible to the industrial ones to derive a reaction rate useful for practical reactor design. This is challenging due to the strong endothermicity of the reaction that leads to non-uniform temperature profiles in concentrated kinetic tests, and the impact of temperature gradients in kinetic studies is well known in the literature. In this study, the activity of a commercial Ru/Al2O3 catalyst in the form of small spherical particles was investigated by feeding pure ammonia in the 2500–20’000 Ncc/h/gcat range at up to 3 bar-a as well as by cofeeding reaction products. Despite working with undiluted catalyst and pure ammonia in an integral reactor up to ≅ 100 % NH3 conversions, the inclusion of a thermally conductive aluminium POCS reactor internal enabled to achieve almost isothermal conditions, resulting in a LHHW rate expression able to fit the whole experimental data set with a MPE less than 5 %.