Possibly the biggest obstacle to stopping climate change is the world’s fossil fuel based economy. Economic growth means increased fossil fuel consumption and the accompanying rise in greenhouse gas emissions. Solar cracking presents a way to continue to use fossil fuels without generating the greenhouse gases such as CO2 and CO that cause global warming. While solar cracking reactors are still in their prototype phase, the results generated by the 5 kW reactor tested in this study show great promise; the reactor generated a methane conversion ratio of 98.8% and a hydrogen yield of 99.1% (Maag et al. 2009). Furthermore, the results indicate that increasing the amount of methane pumped into a reactor as a fraction of the total gas could increase the energy efficiency of the reactor beyond the maximum 16.1% solar-to-chemical energy conversion rate observed. Tim Fine
Maag, G., Zanaganeh, G., Steinfeld, A., 2009 Solar thermal cracking of methane in a particle-flow reactor for the co-production of hydrogen and carbon. International Journal of Hydrogen Energy 34, 7676–7685.
G. Maag and colleges at the Department of Mechanical and Process Engineering in Zurich tested a 5 kW Solar partial-flow solar chemical reactor in a solar furnace over a 1300 – 1600 K range. The reactor was equipped with a continuous flow of methane laced with µm sized carbon black particles. The effect of flow rate on reactor efficiency was examined.
The key to solar thermal cracking is the heating of the target feedstock. This study found that the carbon-black particles injected into the methane acted as a catalyst, as they amplified the radiative heat transfer: converting the light into heat that is absorbed by the methane. In doing so, the carbon black increased the rate at which the reaction proceeded, improving the efficiency of the reactor. Higher concentrations of methane led to increased gas temperature and cooler reactor walls because the higher concentration of methane absorbed more light before it reached the reactor walls. There appears to be a trade-off between reaction completion—how much methane is converted—and the efficiency of the reactor. The reaction completion decreases when the concentration of methane is increased. While the temperature of the gas increased with a higher concentration, the energy per molecule went down, resulting in a lower conversion ratio. The solar-to-chemical energy conversion efficiency increases with an increase in methane concentration: more energy is being absorbed by the gas instead of the reactor walls, resulting in a higher energy conversion efficiency. Modeling simulations suggest that using pure methane could increase the efficiency of the reactor by a factor of 2–4.