The low heat requirement of this process greatly reduces one of the largest obstacles to the large-scale implementation of solar hydrogen decomposition technologies. The low heat requirement lowers the energy required to obtain the hydrogen, which means that any power plants built using this process may be smaller than their high temperature counter parts. Smaller plants will cost less and have the potential to increase the hydrogen production capacity of an area: several low temperature reactors can be built on the area required a single high temperature reactor. The decrease in reactor costs and size associated with low temperature methanol reformation may help hydrogen become a viable power source
With the impending shortage of fossil fuels and the concerns over climate change, many new alternative sources of power are being considered. Solar hydrogen technologies promise to help alleviate the world economy’s dependence on fossil fuels, many of the new solar hydrogen technologies currently being tested use very high temperatures to obtain hydrogen. A new study demonstrates the potential feasibility of solar hydrogen production at temperatures as low as 150–300º C (Hong et al. 2009). The study examined a new process involving the steam reforming of methanol using light as the energy source. Methanol conversion ratios of 90% and 98% were achieved with a reactor temperature of 220–280ºC. The hydrogen obtained with the process reached purities of 99.99% with hydrogen recovery rates between 80% and 90 %. —Tim Fine
Hong, H., Liu, Q., Jin, H., 2009. Solar Hydrogen Production Integrating Low-Grade Solar Thermal Energy and Methanol Steam Reforming. Journal of Energy Resources Technology 131, 012601–012611.
Hui Hong and colleagues at the Chinese Academy of Sciences tested a new process for producing hydrogen using low temperature steam reforming of methanol. The process was made up of three parts: a tracking parabolic trough concentrator with a concentration ratio of about 30–100, a middle and a low temperature solar reformer laden with Cu/ZnO/Al2O3 catalysts, and a pressure swing adsorption (PSA) unit. The PSA unit was used to extract the hydrogen from the products of the solar reformer. However because the PSA unit works at 35 – 40ºC the product gases need to be cooled. The heated cooling water used to cool the product gas was fed into the preheater – the feedstocks are heated before being injected into the reformer –, thus increasing the efficiency of the process.
The conversion of methanol increased with an increase in the average solar energy, reaching a conversion rate of more than 90% at 580 W/m2. The heat to chemical conversion efficiency peaked at 45%, and a heat to chemical conversion efficiency of over 40% was achieved with the average solar energy ranging from 580 W/m2 to 720 W/m2. At average solar energy values above 720 W/m2, the heat to chemical conversion efficiency started to decrease. This indicates a higher radiance heat loss from the reactor. The 35–46% energy conversion efficiency obtained in this study is competitive with the 30% energy conversion efficiency found in high-temperature solar reforming of natural gas.
Because the process investigated in this study uses a solar reformer, it has a higher H2 yield than conventional methane reformation: H2 is obtained from H2O in addition to the feedstock. Both methanol and water are decomposed to produce H2 and GHGs, the majority of which were CO2. Because of the high efficiency of the PSA at extracting pure H2 from the product gas, the CO2 produced can easily be captured for sequestration.