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Technical Briefs

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

[+] Author and Article Information
Irina Vishnevetsky

Solar Research Facilities Unit, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israelirina.vishnevetsky@weizmann.ac.il

Michael Epstein

Solar Research Facilities Unit, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israelmichael.epstein@weizmann.ac.il

Tareq Abu-Hamed

Solar Research Facilities Unit, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israeltareq.abu-hamed@weizmann.ac.il

Jacob Karni

Solar Research Facilities Unit, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israeljacob.karni@weizmann.ac.il

J. Sol. Energy Eng 130(1), 014506 (Jan 07, 2008) (5 pages) doi:10.1115/1.2807215 History: Received September 21, 2006; Revised June 19, 2007; Published January 07, 2008

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 0.12g of boron, water mass flow rate of 0.11gmin, carrier gas flow rate of 100cm3min at total atmospheric pressure with steam partial pressure of 0.550.95bar 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.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic representation of the test setup: TC, thermocouple; PT, pressure transmitter; MMWP, micrometering water pump; FC, mass flow controller; FM, mass flow meter with low (l) and high (h) spans; GC, gas chromatograph; and HA, hydrogen analyzer

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Figure 2

The main parameters of amorphous boron powder hydrolysis versus time (0.15g B, 0.13g∕minH2O); (1) temperature in the upper part of Pyrex bead layer (TC5), (2) temperature in the reaction zone (TC4), (3) pressure in reactor (PT), (4) hydrogen flow rate from Eq. 3, (5) hydrogen flow rate from Eq. 4 (here, τ1 and τ2 are times of beginning and completion of the fast stage)

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Figure 3

The main characteristics of the fast reaction stage as a function of steam to boron (PH2O0∕B) ratio; (1) hydrogen peak width at half of its height (PW) (min), (2) peak height (PH) (maximum value of hydrogen flow rate) per gram B, (l/min g), (3) ratio of hydrogen peak to its width per gram boron (PH/PW) (l∕min2g), and (4) fraction of boron reacted during the fast stage (%)

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Figure 4

Sample weight relative changes after full tests (the line is the best curve fit)

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Figure 5

Structure and oxygen content of amorphous boron powder with crystalline inclusions before (a) and after (b) test

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