A Hybrid Windowless Dual Tube Solar Reactor for Continuous Volumetric Natural Gas Dissociation

Abstract

Solar natural gas thermal dissociation, producing both hydrogen and solid carbon, appears as a promising way for progressive decarbonization of the world energy mix. Three main challenges remain to be tackled: carbon deposition issue, continuous round the clock operation of the solar reactor with an intermittent energy resource, and technology scale-up. The present work proposes a new windowless scalable solar reactor enabling volumetric gas-phase (i.e., not at the walls) methane cracking with possible hybridization. CFD simulations demonstrate the advantage of the new concept as compared to conventional tubular reactors. The proposed system uses a hot inert gas to generate a high temperature fluid zone similar to a combustion flame where the methane decomposition occurs. Based on the numerical model developed, a sensitivity study is carried out in order to optimize the reactor design enabling both high methane conversion and low carbon deposition. For methane flow-rates of 4.6 × 10 −6 kg.s −1 and 2.3 × 10 −6 kg.s −1 (argon flow-rate of 9.3 × 10 −5 kg.s −1), methane conversion reached 46 and 73% for maximum fluid zone temperatures of 1676 and 1750 K, respectively. Furthermore, electric hybridization is investigated in order to highlight the potential for day and night continuous operation. Indeed, since an inert carrier gas is used as heat transfer medium, it can be heated by different energy sources to maintain the thermal power input during low irradiation periods. Thus, the solar flame concept that we previously patented proves to be of interest for practical implementation to introduce solar heat into high temperature processes.

Methane Cracking for Hydrogen Production: A Review of Catalytic and Molten Media Pyrolysis

“…Although solar energy allows reaching very high conversions without catalysts, reactor blockage by carbon deposit growing on hot surfaces may be a serious obstacle in gas phase pyrolysis. Methane cracking in a solar flame was proposed by Rodat et al [119,120] in order to ensure a volumetric pyrolysis reaction that should avoid wall reaction and subsequent carbon deposition leading to reactor clogging. Another worthy novel technique for methane decomposition could also be cracking in molten media.…”

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

“…Methane pyrolysis is generally assumed as a first order chemical reaction, directly proportional to methane concentration [49]. Thus, the reaction rate can be represented as follows, assuming it follows an Arrhenius law: (17) where r is the reaction rate, k is the kinetic constant, A is the pre-exponential factor (s -1 ), E a is the activation energy (J/mol), and T is the operating temperature (K).…”

“…Concentrated solar energy (CSE) for solar-driven methane pyrolysis allows keeping the advantage of hydrogen production with zero CO 2 emissions. Furthermore, CSE seems worth investigating in methane cracking since high temperatures are easily reachable [17][18][19][20]. (1) Over the last few decades, solid catalysts have been used to lower the activation energy of the reaction and to increase the reaction rate at moderate temperatures (below 1000°C).…”

CFD Simulation of a Hybrid Solar/Electric Reactor for Hydrogen and Carbon Production from Methane Cracking

“…Alternatively, the use of metal- [33,34] or carbon-based catalysts [35][36][37][38] appears as an option to reduce the operation temperature to around 1273 K. However, solar direct normal irradiance (DNI) variations caused by weather changes may hinder the process continuity. A hybrid solar/electric reactor appears as a possible solution for continuous and stable processing under fluctuating solar irradiation conditions [39,40]. Methane pyrolysis in a novel hybrid solar/electric reactor was considered in this work.…”