Kinetically Controlled Solar-Thermal Decomposition of Hydrocarbons via Direct Irradiation of a Porous Absorber

A novel solar-driven process is demonstrated for emission-free synthesis of ethylene and hydrogen via the kinetically controlled pyrolysis of alkanes. In this process, concentrated light from a 10 kWe xenon bulb with a spectral output that closely mimics the sun directly irradiates a thin, porous absorber through which alkane feedstocks are flowed. As the alkane feedstocks flow through the high-temperature absorber, they are subsequently decomposed. Owing to the limited residence time of the feedstocks in the heated medium, the progression of the decomposition reaction is constrained, resulting in a kinetically-limited process that can yield various products including desirable intermediates. The resulting product stream from the decomposition of the feedstock consists of a mixture of hydrocarbons and hydrogen, with graphitic layers of carbon deposited on the absorber. This work analyzes the ability to tune process parameters (incident heat flux and feedstock flow rate) to selectively maximize yields of species along the kinetic decomposition pathway of the two simplest alkanes (methane and ethane). The decomposition of these alkane feedstocks is assessed via an infrared laser absorption spectroscopy technique that facilitates measurement of methane, acetylene, ethylene, and ethane mole fractions. Time-resolved simultaneous measurements of these four hydrocarbons provide a means to monitor decomposition progress and also infer the concentration of the other remaining major product in these reactions, hydrogen, through a molar balance. A parametric analysis of the process was conducted by varying feedstock flow rates and incident heat fluxes on the porous absorber to assess the effects on product yields. Through careful selection of the process parameters, a maximum steady-state ethylene mass fraction of nearly 45% was achieved during the decomposition of ethane at relatively low heat fluxes, whilst a hydrogen yield of approximately 80% was achieved during the decomposition of methane at higher heat fluxes and longer residence times.