Experimental and modelling evaluation of an ammonia-fuelled microchannel reactor for hydrogen generation

PhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2015In this thesis, ammonia (NH3) decomposition was assessed as a fuel processing technology for producing on-demand hydrogen (H2) for portable and distributed fuel cell applications. This study was motivated by the present lack of infrastructure to generate H2 for proton exchange membrane (PEM) fuel cells. An overview of past and recent worldwide research activities in the development of reactor technologies for portable and distributed hydrogen generation via NH3 decomposition was presented in Chapter 2. The objective was to uncover the principal challenges relating to the state-of-the-art in reactor technology and obtain a basis for future improvements. Several important aspects such as reactor design, operability, power generation capacity and efficiency (conversion and energy) were appraised for innovative reactor technologies vis-à-vis microreactors, monolithic reactors, membrane reactors, and electrochemical reactors (electrolyzers). It was observed that substantial research effort is required to progress the innovative reactors to commercialization on a wide basis. The use of integrated experimental-mathematical modelling approach (useful in attaining accurately optimized designs) was notably non-existent for all reactors throughout the surveyed open literature. Microchannel reactors were however identified as a transformative reactor technology for producing on-demand H2 for PEM cell applications. Against this background, miniaturized H2 production in a stand-alone ammonia-fuelled microchannel reactor (reformer) washcoated with a commercial Ni-Pt/Al2O3 catalyst (ActiSorb® O6) was demonstrated successfully in Chapter 3. The reformer performance was evaluated by investigating the effect of reaction temperature (450–700 °C) and gas-hourly-space-velocity (6 520–32 600 Nml gcat -1 h-1) on key performance parameters including NH3 conversion, residual NH3 concentration, H2 production rate, and pressure drop. Particular attention was devoted to defining operating conditions that minimised residual NH3 in reformate gas, while producing H2 at a satisfactory rate. The reformer operated in a daily start-up and shut-down (DSS)-like mode for a total 750 h comprising of 125 cycles, all to mimic frequent intermittent operation envisaged for fuel cell systems. The reformer exhibited remarkable operation demonstrating 98.7% NH3 conversion at 32 600 Nml gcat -1 h-1 and 700 °C to generate an estimated fuel cell power output of 5.7 We and power density of 16 kWe L-1 (based on effective reactor volume). At the same time, reformer operation yielded low pressure drop (