The Catalyst Review June 2024 - 12

Experimental Abstracts
Halogen-Mediated Methane Pyrolysis for CO2
Commercial production of hydrogen relies primarily
on reforming low-cost fossil resources involving
processes with high associated greenhouse gas
emissions. Emission-free hydrogen is currently
unavailable at competitive prices and will remain
until serious economic barriers are implemented
to reduce carbon dioxide emissions. Meanwhile,
alternative uses of hydrocarbon feedstocks are
being explored as hydrogen sources. One potential
approach involves pyrolysis, whereby a hydrocarbon
is thermochemically decomposed into hydrogen
and solid carbon. However, such a process requires
substantial amounts of heat for the endothermic
reaction to proceed. Herein, the authors report results
for a pyrolysis process in which methane is reacted
with sufficient quantities of halogens such that the
partial oxidation reaction to form solid carbon is
autothermal, with higher reaction rates and higher
equilibrium conversion than with methane alone.
The pyrolysis production process developed by these
workers (Figure 1) facilitates the decomposition of
relatively low-cost methane or other hydrocarbon
resources by eliminating two of the significant
hurdles associated with this reaction: (1) the transfer
of heat into a very high-temperature reactor and (2)
the thermodynamic limits on methane conversion.
Thus, methane is reacted with a halogen, Y2
, under
halogen-limited conditions to form, ideally, solid
carbon, hydrogen, and hydrogen halides (HY). Using
appropriate amounts of fluorine, chlorine, or bromine
allows the reaction to be autothermal or exothermic
so that no external heat needs to be added to the
reactor. Chlorine and bromine were found to be the
preferred halogens due to their safe and widespread
use in large-scale industrial processes.
Using a heat-integrated pyrolysis reactor designed
by BASF for high-temperature methane pyrolysis,
the authors overcame the temperature limitations
by using autothermal halogen-mediated pyrolysis,
wherein no external heat addition is required. Instead, preheated halogen is introduced at the bottom of the reaction zone into a rising countercurrent
stream of methane preheated by the carbon (Figure 2). The oxidation reaction exotherm maintains the reaction zone temperature, producing the gasphase
products, hydrogen and hydrogen halide, which leave the reaction zone and are cooled by the countercurrent down-going solid carbon stream fed
continuously at the top of the reactor. To efficiently produce solid carbon and hydrogen via this pathway, chlorine requires the least amount of oxidant
recycled per unit of hydrogen produced and uses commercially proven recovery technologies.
The CO2-free hydrogen cost of production for the halogen-mediated methane pyrolysis process is estimated to be $2.2−$2.4/kg H2
that for water electrolysis ($3.5/kg H2
H2). Qi J and McFarland E, (2024). Energy Fuels, https://doi.org/10.1021/acs.energyfuels.3c04780.
12
The Catalyst Review
June 2024
, which is lower than
) and comparable to conventional methane reforming combined with carbon capture and sequestration ($2.4/kg
-Free Hydrogen Production
Figure 1. Schematic representation of halogen-mediated methane pyrolysis. For the overall
reaction, CH4
halogen and 0 < m < 1 for halogen recovery by oxidation of the hydrogen halide, HY. Methane and a
limiting amount of halogen are combined and reacted at a temperature above approximately
900 °C to produce hydrogen, solid carbon, and hydrogen halide (HY). The products separated, and
the hydrogen halide is converted to recover halogen and either hydrogen (via electrolysis) or water
(by oxidation with O2
+ m/2O2
C + (2 - m)H2 + mH2, with m = 0 for the electrochemical recovery of
).
Figure 2. Heat-integrated autothermal halogen-mediated pyrolysis reactor. The methane feed flows
upward in the reactor and is preheated by the relatively slow-moving countercurrent bed of solid,
leaving the high-temperature reaction zone. Preheated halogen is introduced just below the reaction
zone, contacting and reacting exothermically with the methane, maintaining the reaction zone at
the required high temperature for high conversion (~1100 °C), where the decomposition proceeds to
completion. The high-temperature hydrogen and hydrogen halide products continue moving upward
countercurrent to the recycled solid carbon and exit the reactor after being cooled by preheating the
down-going solid stream.

https://www.doi.org/10.1021/acs.energyfuels.3c04780

The Catalyst Review June 2024

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