Turquoise Hydrogen from a Multi-Phase Pyrolysis Reactor

Most hydrogen produced today is made via steam-methane reforming, a mature production process in which a methane source, such as natural gas reacts with steam to produce hydrogen, carbon monoxide, and a relatively small amount of carbon dioxide. Subsequently, in what is called the “water-gas shift reaction,” the carbon monoxide and steam are reacted using a catalyst to produce carbon dioxide and more hydrogen. In a final process step called “pressure-swing adsorption,” carbon dioxide and other impurities are removed from the gas stream, leaving essentially pure hydrogen. 

It is known that natural gas pyrolysis allows to produce hydrogen without any hydrocarbon gas emissions such as CO and CO2. Instead, the only products of the process are the hydrogen gas itself, and solid carbon, the latter can be sold to various industries for incorporation into specialized materials.  Technoeconomic studies on natural gas pyrolysis further show that the process is indeed economical if the carbon byproduct is sold. 
The best catalysts for natural gas pyrolysis are based on solid nickel, however the solid carbon product quickly covers the surface and deactivates the catalyst. In 2019, a study published in Science showed that continuous stable pyrolysis could be achieved if the metal catalyst was a liquid. In this scenario, natural gas is bubbled through a molten metal reactor and hydrogen escapes out the top. The carbon that is formed floats on top of the liquid metal owing to the difference in density, hence maintaining the cleanliness of the catalyst. Furthermore, the fact that the carbon “floats” on top of the metal means that extraction of the carbon from the reactor is possible, paving the way for a stable continuous natural gas pyrolysis reaction. 
Despite being able to separate carbon, the rate of the pyrolysis reaction is significantly slower in the molten metal reactor compared to the solid nickel catalyst.
To address this challenge, we developed a multi-phase reactor where solid and liquid phases based on nickel exist in equilibrium. The presence of the solids in the reactor improved the hydrogen output by over 50% compared to the single-phase molten metal case.

Figure 1: A multi-phase tubular pyrolysis reactor where natural gas is bubbled from the bottom, carbon accumulates on top of the multi-phase melt, and turquoise hydrogen leaves from the top. 

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