Abstract
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.
Highlights
Accepted: 22 May 2021The continuous and ubiquitous use of fossil fuels as an energy source has raised serious concerns around the depletion of these resources and their associated greenhouse gas emissions
Speaking, when O2 is concomitantly fed with methane, air regeneration is a suitable method to compensate some of the energetic requirements of methane cracking, as it is an exothermic reaction, unlike endothermic steam regeneration [74]
Steam regeneration did not change the morphology of the bed, and contributed to more hydrogen production, which could be of significant interest
Summary
The continuous and ubiquitous use of fossil fuels as an energy source has raised serious concerns around the depletion of these resources and their associated greenhouse gas emissions. Hydrogen production for fuel has taken several pathways that include steam methane reforming (SMR), partial oxidation of hydrocarbons, methane cracking, coal gasification, and water electrolysis [1,2]. Most of these processes are CO/CO2 emitting, except methane cracking or electrolysis when powered by solar energy [3]. This review recalls the basics of conventional methane cracking, using solid metallic and carbonaceous catalysts, with the relevant catalyst issues It highlights the main studies on solar methane cracking, which could be a promising pathway for CO2 free hydrogen production. Reaction kinetics modeling, carbon product characteristics, and the main influencing factors affecting methane pyrolysis are discussed
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