Hydrogen production using middle-temperature solar thermal reforming.

Middle temperature solar thermal methanol steam reforming reactors have been shown to be very effective at efficiently producing hydrogen (Liu et al. 2009).  Higher solar flux values lead to an increase in methanol conversion as more energy is available to drive the reaction.  The reactors decomposed over 90% of the injected methanol at a solar flux of 580 W/m2 with an injection rate of 3.0 kg/h and 750 W/m2 with an injection rate of 4.3 kg/h. The volumetric concentration of hydrogen found in the product gas was between 66–74%: within 1% of the theoretical maximum hydrogen concentration. Hydrogen was produced in a 3:1 ratio with CO2, with trace amounts of CO, CH3OH and H2O also found in the product gas. The maximum hydrogen yield produced per mole of methanol was 2.90 mole, which was 0.10 mol less than the maximum theoretical yield of 3.00 mol per mol of methanol. The observed thermochemical efficiency of 30–50% is competitive with other high-temperature thermochemical processes.­—Tim Fine  
Liu, Q., Hong, H., Yuan, J., Jin, H., Cai, R., 2009. Experimental investigation of hydrogen production integrated methanol steam reforming with middle-temperature solar thermal energy. Applied Energy 86.2, 155–162. 

  Lui and colleges at the Chinese Academy of Sciences’ Institute of Engineering Thermophysics examined the effect of solar radiation and mole ratio of water/methanol on the reactivity and hydrogen yield in a methanol steam reformer. The mole ratio of water to liquid methanol was set from 1 to 2.5. The reactor laden with Cu/ZnO/Al2O3 was driven by solar energy at 150–300º C.

The study showed that increased solar flux values raised the reactor temperature and increased methanol conversion. Methanol conversion rates were found to be higher than 90% for solar flux values of 580 W/m2 and 750 W/m2. More than 40% thermochemical efficiency can be achieved with two different mass flow rates. The observed 3.0 kg/k injection rate had a maximum thermochemical efficiency of 46%; the 4.3 kg/h injection rate was had a maximum thermochemical efficiency of 50%. However, the thermochemical efficiency of the reactors decreased above 580 – 630 W/m2, indicating that more solar energy was being lost through heat radiation from the reactor. The study found that hydrogen concentrations of up to 66–74% were found using solar driven methanol steam reforming, which is larger than the 58–63% concentrations produced by methanol decomposition. It is important to note that the maximum theoretical hydrogen concentrations obtainable for each technology are 75% and 66% respectively. With a solar flux of about 600 W/m2, the hydrogen yield ranged from 2.56–2.90 mol per mol of methanol, which is very close to the maximum theoretical hydrogen yield of 3.0 mol per mol methanol. Solar thermal methanol reforming can produce a hydrogen yield 70% greater than solar methanol decomposition because hydrogen is obtained from the water as well as the methanol.

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