Currently, hydrogen is mainly generated by steam methane reforming, with significant CO2 emissions, thus exacerbating the greenhouse effect. This environmental concern promotes methane cracking, which represents one of the most promising alternatives for hydrogen production with theoretical zero CO/CO2 emissions. Methane cracking has been intensively investigated using metallic and carbonaceous catalysts. Recently, research has focused on methane pyrolysis in molten metals/salts to prevent both reactor coking and rapid catalyst deactivation frequently encountered in conventional pyrolysis. Another expected advantage is the heat transfer improvement due to the high heat capacity of molten media. Apart from the reaction itself that produces hydrogen and solid carbon, the energy source used in this endothermic process can also contribute to reducing environmental impacts. While most researchers used nonrenewable sources based on fossil fuel combustion or electrical heating, concentrated solar energy has not been thoroughly investigated, to date, for pyrolysis in molten media. However, it could be a promising innovative pathway to further improve hydrogen production sustainability from methane cracking. After recalling the basics of conventional catalytic methane cracking and the developed solar cracking reactors, this review delves into the most significant results of the state-of-the-art methane pyrolysis in melts (molten metals and salts) to show the advantages and the perspectives of this new path, as well as the carbon products’ characteristics and the main factors governing methane conversion.

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Methane pyrolysis is a transitional technology for environmentally benign hydrogen production with zero greenhouse gas emissions, especially when concentrated solar energy is the heating source for supplying high-temperature process heat. This study is focused on solar methane pyrolysis as an attractive decarbonization process to produce both hydrogen gas and solid carbon with zero CO2 emissions. Direct normal irradiance (DNI) variations arising from inherent solar resource variability (clouds, fog, day-night cycle, etc.) generally hinder continuity and stability of the solar process. Therefore, a novel hybrid solar/electric reactor was designed at PROMES-CNRS laboratory to cope with DNI variations. Such a design features electric heating when the DNI is low and can potentially boost the thermochemical performance of the process when coupled solar/electric heating is applied thanks to an enlarged heated zone. Computational fluid dynamics (CFD) simulations through ANSYS Fluent were performed to investigate the performance of this reactor under different operating conditions. More particularly, the influence of various process parameters including temperature, gas residence time, methane dilution, and hybridization on the methane conversion was assessed. The model combined fluid flow hydrodynamics and heat and mass transfer coupled with gas-phase pyrolysis reactions. Increasing the heating temperature was found to boost methane conversion (91% at 1473 K against ~100% at 1573 K for a coupled solar-electric heating). The increase of inlet gas flow rate Q0 lowered methane conversion since it affected the gas space-time (91% at Q0 = 0.42 NL/min vs. 67% at Q0 = 0.84 NL/min). A coupled heating also resulted in significantly better performance than with only electric heating, because it broadened the hot zone (91% vs. 75% methane conversion for coupled heating and only electric heating, respectively). The model was further validated with experimental results of methane pyrolysis. This study demonstrates the potential of the hybrid reactor for solar-driven methane pyrolysis as a promising route toward clean hydrogen and carbon production and further highlights the role of key parameters to improve the process performance.

Biodiesel, an alternative for conventional diesel, can be produced from renewable resources such as vegetable oils. One of the most crucial characteristics of biodiesel fuel is oxidation stability, which significantly influences the stability of biodiesel during prolonged storage. In fact, biodiesel is less resistant against oxidation than the conventional diesel. In order to transcend this problem, additives are employed. This work uses the Rancimat technique to examine the effects of various antioxidants additives on the oxidation stability of different produced biodiesel. The oxidative stability of biodiesel produced from rapeseed, palm, sunflower and waste cooking oil were evaluated after assuring that the properties of used oils and produced biodiesel complied with the ASTM requirements. Palm biodiesel was selected since it showed the highest oxidation stability of 1.32 h. The experimental findings demonstrated that the two antioxidants investigated (phenolhydrazine and thiamine hydrochloride) are boosting oxidation stability, however phenolhydrazine is providing a higher oxidation stability than thiamine hydrochloride. The optimal amount of phenolhydrazine was found to be 2550 ppm and resulted in an oxidation stability of 6 h. This great enhancement of oxidation stability (454%) ascertained that phenolhydrazine is a strong antioxidant to be utilized in order to enhance shelf life of natural source‐derived biodiesels. The mentioned antioxidant was not yet studied in literature which confirms the novelty of our work.

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