Utilizing CO2 to produce H2O and O2 is critical for sustained manned missions in space, and supports both NASA’s cabin Atmosphere Revitalization System (ARS) and ln-situ Resource Utilization (ISRU) concepts. For long-term missions beyond low Earth orbit where resupply of consumables is significantly more difficult and costly, open-loop ARS can reduce the effectiveness of consumables recovery. The Bosch process has the potential to achieve complete loop closure for 100% O2 recovery; however, it has several limitations, including reactor fouling and low single-pass efficiency. NASA MSFC has been developing an innovative Bosch system comprising a Reverse Water Gas Shift (RWGS) reactor and a downstream Carbon Formation reactor that would significantly improve the overall O2 recovery.
The technical objective of this effort was to design, fabricate, and demonstrate an ultra-compact, Microlith®-based RWGS reactor with an embedded preheat section that can achieve equilibrium product distribution with minimal power requirement at high throughputs. Key characteristics required were equilibrium-limited CO2 conversion at high throughputs, high CO selectivity, low CH4 selectivity, rapid transient response, high heat transfer characteristics to minimize power requirement, minimal pressure drop, and the capability to operate at sub-ambient pressures.
Reverse water gas shift reaction is endothermic and heat-transfer-limited. Thus, for conventional RWGS reactors with poor heat transfer characteristics, high residence time is required to achieve desired equilibrium-limited CO2 conversion, resulting in an oversized and bulky reactor. Lower heat transfer characteristics of competing technologies also result in power-intensive reactor design. Moreover, lack of temperature uniformity across the catalyst surface results in poor CO2 conversion and CO selectivity compared to equilibrium-predicted performance.
Precision Combustion Inc.’s RWGS reactor prototype was based on its patented Microlith technology. The Microlith technology uses a series of very short-channel-length substrates that provides a high catalytic conversion efficiency while minimizing boundary layer buildup, resulting in remarkably high heat and mass transfer coefficients in comparison to competing technologies. The Microlith substrate also provides about three times higher geometric surface area over conventional monolith reactors with equivalent volume and open frontal area. The mechanical and thermal durability of Microlith-based catalytic substrates have also been rigorously demonstrated.
The RWGS reactor prototype delivered to NASA comprises a heating element integrated with a Microlith-based feed preheat section and a Microlith-based, short-contact-time RWGS catalyst. The inherent millisecond contact time, low boundary layer, and high thermal conductivity of the Microlith substrate lead to a catalytic reactor system with more uniform temperature and concentration profiles, resulting in near equilibrium product selectivity — characteristics that are significant contributors to improved catalyst performance. The Microlith-based reactor is much smaller, lighter, more effective, and efficient, and is expected to be more durable than competing microchannel or pellet-based technology.