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Negative emissions technologies will play an important role in preventing 2 °C warming by 2100. The next decade is critical for technological innovation and deployment to meet mid-century carbon removal goals of 10-20 GtCO 2 /yr. Direct air capture (DAC) is positioned to play a critical role in carbon removal, yet remains under paced in deployment efforts, mainly because of high costs. This study outlines a roadmap for DAC cost reductions through the exploitation of low-temperature heat, recent U.S. policy drivers, and logical, regional end-use opportunities in the United States. Specifically, two scenarios are identified that allow for the production of compressed high-purity CO 2 for costs ≤$300/tCO 2 , net delivered with an opportunity to scale to 19 MtCO 2 /yr. These scenarios use thermal energy from geothermal and nuclear power plants to produce steam and transport the purified CO 2 via trucks to the nearest opportunity for direct use or subsurface permanent storage. Although some utilization pathways result in the re-emission of CO 2 and cannot be considered true carbon removal, they would provide economic incentive to deploying DAC plants at scale by mid-century. In addition, the federal tax credit 45Q was applied for qualifying facilities (i.e., producing ≥100 ktCO 2 /yr).

In this work, a novel structure of a hydrogen-membrane reactor coupling HI decomposition and CO2 methanation was proposed, and it was based on the adoption of silica membranes instead of metallic, according to their ever more consistent utilization as nanomaterial for hydrogen separation/purification. A 2D model was built up and the effects of feed flow rate, sweep gas flow rate and reaction pressure were examined by CFD simulation. This work well proves the feasibility and advantage of the membrane reactor that integrates HI decomposition and CO2 methanation reactions. Indeed, two membrane reactor systems were compared: on one hand, a simple membrane reactor without proceeding towards any CO2 methanation reaction; on the other hand, a membrane reactor coupling the HI decomposition with the CO2 methanation reaction. The simulations demonstrated that the hydrogen recovery in the first membrane reactor was higher than the methanation membrane reactor. This was due to the consumption of hydrogen during the CO2 methanation reaction, occurring in the permeate side of the second membrane reactor system, which lowered the amount of hydrogen recovered in the outlet streams. After model validation, this theoretical study allows one to evaluate the effect of different operating parameters on the performance of both the membrane reactors, such as the pressure variation between 1 and 5 bar, the feed flow rate between 10 and 50 mm3/s and the sweep gas flow rate between 166.6 and 833.3 mm3/s. The theoretical predictions demonstrated that the best results in terms of HI conversion were 74.5% for the methanation membrane reactor and 67% for the simple membrane reactor. [ABSTRACT FROM AUTHOR]

• MR coupled with carbon capture, can produce ultrapure, clean hydrogen. • Zeolite 13X packed beds can mitigate carbon footprint of SMR process significantly. • Development of pinholes and oxidation of Pd remain significant challenges. • MR can produce environmentally friendly hydrogen at moderate operating conditions. In this work steam methane reforming (SMR) was carried in a palladium-based membrane reactor and its performance was evaluated in terms of methane conversion, hydrogen recovery, hydrogen purity, and CO and CO 2 selectivity. The membrane reactor was found to produce ultrahigh purity hydrogen with methane conversions as high as 40% at moderate operating conditions and in the absence of sweep gas. Permeation tests using pure hydrogen and inert gases indicate that the membrane reactor is highly selective toward hydrogen. The SMR reactions were performed at 673 K and operating pressures ranging from 100 to 400 kPa. A gas hourly space velocity of 2,600 hr-1 and a steam-to-carbon ratio of 3.5 were used. Methane conversion increased from approximately 23% at 100 kPa to 42% at 400 kPa while a maximum hydrogen recovery value of 43% was achieved at 400 kPa. Furthermore, the purity of the recovered hydrogen was >99.999%. The carbon capture experiments performed in this work consist of dehydrating the retentate stream and redirecting it to a zeolite 13X packed bed before analyzing the stream via mass spectrometry. The carbon capture studies reveal that approximately 5.96 mmol CO 2 (262.25 mg of CO 2) can be captured per gram of 13X. This value indicates that approximately 80% of the produced CO 2 via the SMR, can be captured before the sorbent bed is completely saturated. SEM-EDS, XRD, and XPS techniques were used to characterize the crystal structure and morphology of the membrane surface. These studies reveal that the surface of the membrane underwent significant oxidation during the SMR reaction. This oxidation is only limited to a few nanometers within the top surface of the palladium. [ABSTRACT FROM AUTHOR]