Experimental study of powder bed behavior of sodium alanate in a lab-scale H2 storage tank with flow-through mode

Chemical hydrogen storage in complex hydrides offers the potential of high gravimetric storage densities compared to intermetallic hydrides, and is therefore a promising technology for mobile applications. The main challenge for mobile application is still the required high refuelling rate of the hydrogen storage tanks. Since hydrogen is bonded by an exothermal chemical reaction in complex hydrides, appropriate storage tanks require high heat transfer rates for the cooling of the tank. Hydride tanks that are state of the art rely on an indirect cooling and are additionally equipped with e.g. finns, foams, etc. to improve the heat transfer rate. For the present study, an improved laboratory tank, which allows for indirect as well as direct cooling by excess H2 gas (flow-through mode), has been designed and built. This laboratory tank is filled with 87 g of NaAlH4 (doped with 2 mol% CeCl3) and equipped with 8 thermocouples as well as two pressure sensors. Experimental results presented in this paper show a significant influence of the cooling by gaseous excess H2 on the flow-directional temperature profiles at the part of the reaction bed close to the inlet. Considering the overall conversion, this influence is rather small due to the low heat capacity flux (ρcp)H2. Furthermore, it is shown that changes in material properties, attributed to the effects of heat and mass transport as well as intrinsic reaction kinetics, can be measured and assessed by the temperature and pressure sensors. After about 10 complete charging and discharging cycles, the initial permeability K of the bed has decreased by 50% to 1.6·10−12 m2. In the same time, the initial thermal conductivity has increased by a factor of 1.3 to values reported in literature (0.67 Wm−1 K−1) and remains constant during further cycles. Additionally, it is observed that the reaction rate of the second absorption step improves, even after a total of 36 cycles.

► Reactor design allowing for investigation of direct heat transfer to excess H2.
► Lab-scale reactor equipped with 8 thermocouples and 2 pressure sensors for detailed study.
► Significant densification of lab-scale powder bed has been observed with cycling.
► Influence of densification on conductivity, permeability and reaction rate measured and assessed.
► H2 capacity of NaAlH4 doped with CeCl3 proven stable over 36 cycles and densification of powder bed.