Session: 17-03: Symposium Steinfeld - Solar fuels via an external energy addition

Paper Number: 138336

138336 - Low-Carbon Hydrogen Production via Oxidant-Assisted Catalytic Methane Pyrolysis 

Abstract: 

Hydrogen is known as a clean energy carrier due to its zero direct emissions when combusted or used in a fuel cell. Indirectly, however, its current manufacture emits 8-12 kg CO₂/kg H₂ through the combination of methane steam or autothermal reforming followed by water-gas shift. On the other hand, methane pyrolysis produces hydrogen by decomposing methane in a non-oxidative environment, while also producing solid carbon that can either find value in the market or be sequestered in a much more stable and manageable form as compared to CO₂. Additionally, compared to an alternative H₂ production method such as water decomposition via electrolysis, methane pyrolysis requires only about 1/4 of its thermodynamic energy input. One of the aims of this project was to produce carbon in the form of CNTs that, beyond their high value, also offer the possibility of higher carbon production per active site as opposed to graphitic or amorphous carbon. In fact, their vertical alignment, large aspect ratio, and weak adhesion to the catalyst site provide an opportunity for mechanical removal through abrasion during fluidization.

Catalytic approaches applied to methane pyrolysis allow to produce high-quality carbon forms, such as carbon nanotubes (CNTs), but are strongly limited by catalyst deactivation. Herein, we propose a variant of methane pyrolysis called oxidant-assisted methane pyrolysis that integrates oxidant co-feeds to enhance the production and recovery of solid carbon. While so far oxidant-assisted pyrolysis has found uses in biomass, oil, and coal, its utilization in natural gas projects is uncommon. Incremental amounts of certain oxidants, namely CO₂, H₂O, and O₂, were co-fed with a constant methane flow into a fluidized bed of catalyst particles composed of Fe supported on Al₂O₃, while tracking the amount of solid carbon produced. The carbon production was increased in the case of CO₂ and H₂O co-feed at certain concentrations of the oxidant. No increase in carbon production was measured in the case of O₂ co-feed. The beneficial role of the oxidants can be attributed to two factors: (i) the selective oxidation of amorphous carbon, whose formation competes with the growth of CNTs and whose presence deactivates the catalyst, (ii) the prevention of Fe₃C formation, which is less catalytically active than the Fe phase. The addition of oxidants increased the production of carbon without impacting its quality, favoring the formation of carbon shells that encapsulated the catalyst particles. These shells were easily dislodged through the mechanical friction generated under the fluidization regime, allowing them to be conveyed out of the reactor. Continuous operation was demonstrated for co-feed concentrations that resulted in the largest carbon production, resulting in significantly prolonged activity with higher carbon and hydrogen yields as compared to pure CH₄.

Presenting Author: Marco Gigantino Stanford University

Presenting Author Biography: Marco Gigantino earned his PhD at ETH Zurich under the supervision of Professor Aldo Steinfeld. He's currently a postdoctoral scholar at Stanford University, working in both the Mechanical and Chemical Engineering Departments with Professor Arun Majumdar and Matteo Cargnello on various projects related to decarbonization and carbon sequestration.

Authors:

Marco Gigantino Stanford University
Henry Moise Stanford University
Arun Majumdar Stanford University
Matteo Cargnello Stanford University