(357e) Economically Optimal CO2 Utilization Via Flexible Chemical Production Facilities

Hydrogen production via electrolysis powered by renewable energy is emerging as a viable alternative to fossil fuel reforming due to the proliferation and resulting cost reduction of renewable generation and electrolysis technologies as well as production incentives provided by policy such as the Inflation Reduction Act [1]. This “green” or “renewable” hydrogen can be used as a carbon-free fuel for power generation or heating while also being used as a feedstock for other lower carbon intensity chemical production [2]. Toward a broader view on sustainability, carbon dioxide (CO₂) capture and subsequent sequestration or utilization will likely be necessary to meet emissions reduction targets while still producing indispensable commodities such as steel and cement [3]. At the same time, many essential societal functions are enabled by carbon-containing molecules. Prominent examples include urea, methanol, and fuel for long-haul air transportation. Urea accounts for over 50% of global nitrogen fertilizer applied to crops and is made by combining ammonia and CO₂ [4]. Methanol produced from renewable hydrogen can be used to displace conventionally produced methanol as polymer production feedstock and can also act as a carbon-neutral fuel. Renewable methanol can be produced through direct hydrogenation of CO₂ [5]. Aviation is responsible for 3% of global CO₂ emissions and is difficult to electrify using batteries or renewable hydrogen due to energy density requirements. One approach to sustainable aviation fuel (SAF) is to combine hydrogen and CO₂ via either water gas shift followed by Fischer-Tropsch synthesis (WGS-FT) or through a methanol-to-olefins (MTO) pathway [6].

In this work, we optimize the production economics of urea, methanol, and SAF considering only CO₂, water, air, and intermittent renewable energy as feedstocks. We define production facility superstructures for each commodity which include commercially established chemical production processes as well as storage technologies for energy and/or chemical intermediates to facilitate the use of intermittent renewables. We develop a combined optimal design and scheduling framework which minimizes the production cost of each commodity for a given annual capacity of CO₂ to be utilized. This is achieved by optimizing the selection and size of constituent technologies as well as their hourly operating schedules (e.g., production rates and storage inventories) in response to yearlong hourly resolution time-series data for renewable generation. This optimization approach enables systematic consideration of the unique capital investment, energy intensity, and dynamic flexibility (e.g., operating range and ramp rates) of each technology required to produce a given commodity.

We perform a case study for CO₂ utilization for a 50,000 mtCO₂/y to 1,000,000 mtCO₂/y scale range in five regions throughout Minnesota. These regions represent different renewable generation potentials and industries from which CO₂ is emitted. We optimize the production cost of urea, methanol, and SAF while also determining production cost sensitivity to feedstock CO₂ price, capital investment costs for renewable generation and electrolysis, and process flexibility for production of final products or intermediates (e.g., ammonia, WGS syngas). We compare the optimal renewable chemical production costs with market price histories and projections for urea, methanol, and aviation fuel. This comprehensive assessment allows determination of the best CO₂ utilization pathway in the near-term. It also allows for the identification of underlying technological improvements which are critical to the economical and sustainable CO₂ utilization in producing essential commodities.

References

[1] Inflation Reduction Act of 2022, H.R.5376 (2022). https://www.congress.gov/bill/117th-congress/house-bill/5376/text

[2] Clark et al. (2023). Opportunities for green hydrogen production with land-based wind in the United States. Energy Conversion and Management, 296, 117595.

[3] Fu et al. (2022). Research progress on CO2 capture and utilization technology. Journal of CO₂ Utilization, 66, 102260.

[4] Mao et al. (2024). Green urea production for sustainable agriculture. Joule. doi.org/10.1016/j.joule.2024.02.021

[5] Mucci et al. (2023). Cost-optimal Power-to-Methanol: Flexible operation or intermediate storage?. Journal of Energy Storage, 72, 108614.

[6] Grahn et al. (2022). Review of electrofuel feasibility—cost and environmental impact. Progress in Energy, 4(3), 032010.