Experimental Evaluation of Indirect Heating Tubular Reactors for Solar Methane Pyrolysis

Solar thermal pyrolysis of natural gas is studied for the co-production of hydrogen, a promising energy carrier, and Carbon Black, a high-value nano-material, with the bonus of zero CO2 emissions. A 10 kW multi-tubular solar reactor (SR10) based on the indirect heating concept was designed, constructed and tested. It is composed of an insulated cubic cavity receiver (20 cm side) that absorbs concentrated solar irradiation through a quartz window by a 9 cm-diameter aperture. The solar concentrating system is the 1 MW solar furnace of CNRS-PROMES laboratory. An argon-methane mixture flows inside four graphite tubular reaction zones each composed of two concentric tubes that are settled vertically inside the cavity. Experimental runs mainly showed the key influence of the residence time and temperature on the reaction extent. Since SR10 design presented a weak recovery of carbon black in the filter, a single tube configuration was tested with an external plasma heating source. Complete methane conversion and hydrogen yield beyond 80% were achieved at 2073K. Hydrogen and carbon mass balances showed that C(2)H(2) intermediates affect drastically the carbon black production yield: about half of the initial carbon content in the CH(4) was found as C(2)H(2) in the outlet gas. Nevertheless, the carbon black recovery in the filtering device was improved with this new configuration. Data are extrapolated to predict the possible hydrogen and carbon production for a future 50 kW solar reactor. The expected production was estimated to be about 2.47 Nm(3)/h H(2) and 386 g/h carbon black for 1.47 Nm(3)/h of CH(4) injected.