Experimental characterization of granular sodium borohydride under varying operational conditions across flow regimes

Sodium borohydride (NaBH 4 ) is increasingly considered as an alternative fuel for maritime vessels due to its relatively high energy density. When stored in dry solid form, it is a granular material, similar to coal, starch, and iron ore. As NaBH 4 is historically used in the chemical industry in aqueous solutions, virtually no details regarding its behaviour as a solid granular material are known. Therefore, after determining particle properties such as size, shape, and density, this study characterises granular NaBH 4 in three flow regimes using three experimental setups. Ring shear tests are used for the quasi-static regime, ledge tests for the dense flow regime, and rotating drum tests characterise both dense and gaseous flow, depending on the rotational speed. Various operational conditions, including temperature, humidity, time consolidation, and handling stresses, are taken into account. Experimental results demonstrate that above a threshold temperature and humidity, NaBH 4 readily absorbs moisture from ambient air but remains free-flowing for most scenarios. However, time consolidation can transform this free-flowing material into a very cohesive substance. While this cohesiveness is reversible, requiring minimal agitation, the transformation from free-flowing to cohesive is accelerated by elevated moisture contents and a reduced particle size. Additionally, handling stresses were found to have minimal effect on the flow behaviour and characteristics of NaBH 4 . These findings are ultimately used to derive implications for the design of handling and storage equipment for NaBH 4 , enabling its use as an alternative fuel for maritime vessels. • Dry NaBH 4 is free-flowing, but is affected by moisture and time consolidation. • Time consolidation increases cohesion and reduces the flowability of NaBH 4 . • The effects of time consolidation on NaBH 4 are reversible with minimal agitation. • Elevated moisture content accelerates time consolidation effects. • The impact of moisture on the flowability of NaBH 4 is stronger for smaller particles.