Clean fuels that emit no greenhouse effect gas are requested in various fields especially of the global warming issue. The authors have proposed a hydrogen generation system using ammonia as a liquid fuel for fuel cells. The system consists of a cracking reactor, a heat exchanger and an ammonia separator to remove residual ammonia.
It have been clarified that ammonia by 13 ppm or more causes an output decrease in polymer electrolyte fuel cells. In this study, the cracking efficiencies of ammonia were investigated with two kinds of and two shapes of catalysts to review the optimum conditions of the system. The conditions were decided by analyzing hydrogen concentration and residual ammonia concentration for some cracking temperatures and space velocities.
At the cracking temperature of 500°C, the cracking efficiency of Ru/Al2O3 catalyst has shown sufficient results. At 800°C, the difference by the catalyst shapes was not seen and the residual ammonia concentration could be maintained at 13 ppm or less for about 75 minutes. Since the cracking efficiency of the honeycomb shape of the catalyst was better than that of the pellet shape, the cracking temperature of the honeycomb shape could be by about 100°C lowered.
From these results, it is found that the ammonia was cracked at 700°C using the honeycomb catalyst with Ru/Al2O3. Because the cracking temperature is lowered at 700°C by using the honeycomb catalyst, the system becomes compact and easy to be installed on-board in the vehicle. Although further improvements of energy efficiency and safety are needed, the liquid ammonia can be one of clean fuels for fuel cells in vehicles and also household electric generation systems.