Boron Hydrolysis at Moderate Temperatures: First Step to Solar Fuel Cycle for Transportation

Abstract

Boron hydrolysis reaction can be used for onboard production of hydrogen. Boron is a promising candidate because of its low molecular weight and relatively high valence. The oxide product from this process can be reduced and the boron can be recovered using known technologies, e.g., chemically with magnesium or via electrolysis. In both routes solar energy can play a major role. In the case of magnesium, an intermediate product, magnesium oxide, is formed, and its reduction back to magnesium can exploit solar energy. The boron hydrolysis process at moderate reactor temperature up to 650°C, potentially suitable for use in vehicles, has not been sufficiently studied so far. This paper addresses the operational requirements using an experimental setup for investigating the hydrolysis reaction of metal powders exposed to steam containing atmosphere. The output hydrogen is measured as a function of temperature in reaction zone, steam partial pressure, and the different steam to metal ratio. Test results obtained during the hydrolysis of amorphous boron powder in batch experiments (with 0.1–2g of boron, water mass flow rate of 0.1–1g∕min, carrier gas flow rate of 100cm3∕min at total atmospheric pressure with steam partial pressure of 0.55–0.95bar abs) indicate that the reaction occurs in two different stages, depending on the temperature. A slow reaction starts at about 300°C and hydrogen output increases with reactor temperature and steam partial pressure. The fast stage starts as the reactor temperature approaches 500°C. At this temperature, the reaction develops vigorously due to higher reaction rate and its strong exothermic nature. The fast stage is self-restrained when 50–60% of the loaded boron is reacted and 1.5–1.8 SPT L H2 per 1g of boron is produced. Raising the temperature before the steam flow starts during the preheating period above 500°C increases the hydrogen yield at the fast stage. Then, the reaction continues for a long time at slow rate until the hydrogen release is terminated. The duration of the fast step decreases sharply with the increase of the steam to boron ratio.