Decarbonizing energy will help avoid further CO2 emissions and provide a more sustainable energy solution in the near to medium-term future (Rodat et al. 2009). Solar decomposition technology takes carbon based energy sources such as natural gas and fossil fuels and converts them to carbon-black and H2 There is a variety of different types of solar decomposition reactors in development. This study examined the effects of temperature, methane concentration, and residence time on methane decomposition in a 10 kW solar chemical reactor prototype. The study found that temperature and residence time significantly affect both methane conversion and H2 yield. Higher temperatures cause the decomposition reaction to go to completion, eliminating the ethane, ethane and acetylene byproducts. An increase in residence time—the amount of time the methane spent in the reactor—also resulted in a higher methane conversion and H2 yield.—Tim Fine
Rodat, S., Abanades, S., Sans, J., Flamant, G., 2009. Hydrogen production from solar thermal dissociation of natural gas: development of a 10 kW solar chemical reactor prototype. Solar Energy 83, 1599—1610.
Sylvain Rodat and colleagues at the Processes, Materials and Solar Energy Laboratory calculated the effects of temperature, methane concentration, and residence time on methane decomposition in a 10 kW solar chemical reactor prototype. The reactor was insulated with three layers of different insulation materials to retain heat. The functional parts of the prototype consisted of three double graphite tubes. Each tube consisted of two tubes, one inside the other, through which a mixture of argon and methane gases was pumped. The gas entered through the inner tube and exited through the outer tube. Because of the way the solar reactor was designed, the outer tube is slightly hotter. This prevents the carbon generated by methane decomposition from depositing on the pipe. The reactor retained 60% of the solar energy as ambient heat to drive the reaction. Thirty-five percent of the energy was lost through the walls of the reactor and the remaining 5% was lost through the gas flow.
Increasing the concentration of methane pumped into the reactor was found to have no significant effect on either the percent decomposition of methane or the H2 yield. This indicates that the efficiency of the reactor can be greatly increased by increasing the concentration of methane present in the gas source. The temperature inside the reactor was found to have positive correlation with the efficiency of the reactor. Increasing the temperature from 1670 K to 1740 K resulted in a 22% increase in methane conversion and a 3% increase in the H2 yield. Increasing residence time of the gas also increased the efficiency of the reactor. Increasing the residence time from 12 ms to 35 ms increased the methane conversion by 36% and the H2 yield by 40%. The three aspects examined—residence time, methane concentration, and temperature—are basic components of a solar thermal decomposition reaction suggesting that these results may be useful in the development of other solar reactors.