Experimental comparison of solar methane pyrolysis in gas-phase and molten-tin bubbling tubular reactors

Abstract

This study experimentally compares solar methane pyrolysis in gas phase and molten tin for hydrogen and solid carbon production. Molten media are expected to facilitate carbon separation and to enhance solar-to-gas heat transfer. Effects of temperature (1000–1400 °C), gas feed flow rate (0.5–1.0 NL/min), and methane molar fraction in the feed (0.1–0.5) on methane decomposition were investigated in a tubular solar reactor. Methane conversion in molten tin was significantly lower than in gas phase at all temperatures (e.g., 64% vs. 92% at 1300 °C). It was justified by the negligible catalytic activity of tin, the small gas-liquid surface contact (bubble diameter ∼5 mm) and the reduced bubble-tin contact time, in comparison with gas-phase pyrolysis (e.g., 0.83 s vs. 0.5 s for 0.5 NL/min gas feed at 1300 °C). For both routes, a temperature increase (1000–1400 °C) improved methane conversion (gas phase: 2–98%, molten tin: 0–91%). Increasing gas feed flow rate (0.5–1.0 NL/min) decreased conversion (gas phase: 93-85%, molten tin: 64-48% at 1300 °C) by reducing space-time in gas phase (0.83–0.42 s) or by inducing bubbles coalescence in molten tin. Increasing methane molar fraction in the feed generally decreased methane conversion but energy efficiencies were enhanced. The carbon produced in gas-phase pyrolysis was carbon black powder (50–100 nm particle size) and carbon sheets in molten tin.