(31a) Modeling and Techno-Economic Assessment of an Air-to-Syngas Process

Integrating carbon capture with utilization is one of the most promising pathways to pursue carbon neutrality^1–3. Particularly, valorizing CO₂ by integrating Direct Air CO₂ Capture (DACC) with renewably-driven carbon electrolysis offers a promising pathway to overcome the high capture costs of DACC. More immediately, syngas production via integrated DACC-electrolysis routes can become economically feasible by 2050 as CO₂ electrolysis (CO₂ER) systems are well-suited to produce CO as a main product and H₂ as a byproduct. In this contribution, we study the integration of hydroxide-based DACC with a low-temperature CO₂ER and with conventional Reverse Water Gas Shift (RWGS) to identify improvement opportunities for DACC-Utilization (DACCU) in general and DACC-electrolysis in particular. We define the syngas H₂:CO molar ratio to be 2 and the production rate of syngas to be 2.2 Mt-syngas/yr, and we assume hydrogen supply from Alkaline Water Electrolysis (AWE) in both routes due to its higher technological maturity level. We present a verified Aspen Plus model of a hydroxide-based DACC plant and a custom-built model for CO₂ and water electrolysis to enable the assessment of the studied routes techno-economically. Herein, we refer to the integrated DACC with RWGS and with CO₂ER pathways as DACC-AWE-RWGS and DACC-AWE-CO₂ER, respectively.

Our findings indicate that DACC requires energy inputs of 8.58 and 8.55 GJ/t-CO₂ when integrated with RWGS and CO₂ER, respectively, in line with current literature estimates^4,5. In addition, we calculate the power consumption of CO₂ electrolysis to be 6.56 MWh_el/kg-syngas and of H₂O electrolysis to be 6.01 and 9.68 MWh_el/kg-syngas for DACC-AWE-RWGS and DACC-AWE-CO₂ER, respectively. Such values, in addition to a referenced RWGS energy consumption value, are used to estimate the total energy consumption, and thus cost, of the two routes. Our assessment demonstrates that DACC-AWE-RWGS is a more carbon-efficient and a less energy-consuming pathway than the current state of the DACC-AWE-CO₂ER route. However, in terms of marginal energy-associated CO₂ emissions, we find DACC-AWE-CO₂ER to outperform DACC-AWE-RWGS, although at an insufficient emissions difference that would drive the shift from the conventional RWGS to the emerging CO₂ER. In addition, our CAPEX and OPEX calculations provide a lower total syngas cost of $1.44/kg-syngas via DACC-AWE-RWGS compared to that of DACC-AWE-CO₂ER ($1.53/kg-syngas). Further, our sensitivity analysis suggests that the H₂ production cost and the electricity price to be the main drivers in both pathways, and in that order. Moreover, our future scenario analysis compares three potential technology improvement scenarios and defines research targets for DACC-AWE-CO₂ER to compete economically with DACC-AWE-RWGS. Finally, we end with a discussion on the integration of renewable electricity with the investigated pathways.

References:

1. Mertens, J., Breyer, C., Arning, K., Bardow, A., Belmans, R., Dibenedetto, A., Erkman, S., Gripekoven, J., Léonard, G., Nizou, S., et al. (2023). Carbon capture and utilization: More than hiding CO2 for some time. Joule 0. 10.1016/j.joule.2023.01.005.
2. IPCC (2022). Climate Change 2022: Mitigation of Climate Change: Summary for Policymakers (Cambridge University Press) 10.1017/9781009157926.001.
3. International Energy Agency (September 19). Putting CO₂ to Use (IEA Publications).
4. Keith, D.W., Holmes, G., St. Angelo, D., and Heidel, K. (2018). A Process for Capturing CO₂ from the Atmosphere. Joule 2, 1573–1594. 10.1016/j.joule.2018.05.006.
5. Zeman, F. (2007). Energy and Material Balance of CO2 Capture from Ambient Air. Environ. Sci. Technol. 41, 7558–7563. 10.1021/es070874m.