Concentrated solar-thermal methane pyrolysis in a porous substrate: Yield analysis via infrared laser absorption

Abstract

In situ compositional analysis via laser absorption spectroscopy is used to characterize the output of a concentrated solar-thermal methane pyrolysis process over a range of test conditions. The novel thermo-chemical process involves directed radiation of a simulated solar source, focused onto a porous carbon felt substrate through which methane flows and decomposes to produce primarily hydrogen gas and graphitic carbon, along with residual hydrocarbons. A mid-infrared laser diagnostic setup has been developed to measure the residual concentrations of light hydrocarbons in the pyrolysis product gas downstream of the reaction zone, with real-time on-line monitoring capability. The infrared absorption sensor utilizes two interband cascade lasers (ICLs) to probe the fundamental C-H stretch vibrational bands, targeting (1) the R(15) manifold of the ν3 band of CH4 as well as one R(14) transition of the ν9 band of C2H4 near 3.16 μm and (2) the RQ3 line cluster of C2H6 near 3.34 μm, respectively. A compositional analysis based on carbon and hydrogen balances is developed to characterize the hydrogen and carbon yields of the pyrolysis process using laser absorption measurements and deposited carbon weight. A range of test conditions are examined with variations of flow rate and solar flux, demonstrating real-time monitoring of product gas composition and sensitivity to operating conditions in variable and transient conditions.