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Solar-Thermal Production of Hydrogen and Graphitic Carbon via Methane Decomposition


Current hydrogen and carbon production technologies emit massive amounts of CO2 that threaten Earth’s climate stability, especially as demands for these materials continue to grow. Compared to alternative clean hydrogen production technologies, solar methane pyrolysis has lower energy requirements, produces carbon materials of commercial interest, and provides higher process efficiencies. In this work, a new solar-thermal methane decomposition process involving flow through a fibrous carbon medium to co-produce hydrogen gas and high-value graphitic carbon product with zero CO2 emissions is presented and thoroughly investigated. A 10 kWe custom-designed and built solar simulator is used to instigate the methane decomposition reaction with direct irradiation in a custom solar reactor. In contrast to prior work on solar methane pyrolysis, the present process reaches steady-state thermal and chemical operation from room temperature within the first minute of irradiation due to localized, direct solar heating of fibrous medium. Additionally, the present approach provides enhanced thermal transfer and efficiency, delivers graphitic carbon product in an easy to handle and extract form, and prevents undesired carbon deposition within the reactor that would otherwise lead to process interruption. These aspects are ongoing challenges reported in prior literature. In contrast to similar methane decomposition prior work that reports production of amorphous carbon black, this work produces high-quality graphite with production rates that are order(s) of magnitude higher. Parametric variations of methane inlet flow rate (10-2000 sccm), solar power (0.92-2.49 kW), operating pressure (1.33-40 kPa), and medium thickness (0.36-9.6 mm) are thoroughly presented, with methane conversions as high as 96% and graphite Raman D/G peak ratios as low as 0.06. A pathway to process scale-up for continuous production is presented by implementing a roll-to-roll processing method, which was effective in achieving continuous processing with methane conversion enhancements up to 1.5 times higher. The optical design of the solar reactor was then optimized using a secondary concentrator, by which solar-to-chemical efficiencies increased by up to 62% to reach a maximum demonstrated efficiency of 6.5%. Realizing this process at scale would avoid emissions of 10 kg of CO2 per kg of H2 and 5 kg of CO2 per kg of graphite.

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