Integration of CO₂ air capture and solid oxide electrolysis for methane production

This work studied the production of substitute natural gas (SNG) from CO2 captured from the atmosphere followed by co-electrolysis with H2O in solid oxide electrolyzer cells (SOEC) and downstream catalytic methane production.Over the coming 20 years, Denmark is on a track to remove fossil fuels from all sectors of the energy system except for transportation.

In the recently published Energikoncept 2035 [1], the Danish grid operator, Energinet.dk lays out a scenario based on 72 % wind power and 21 %biomass and waste in the electricity grid mix. In this scenario, biogas and electrolysis gasses are projected to be used for production of process heat, peak-load power generation and on the longer term to replace hydrocarbons in the most energy intensive parts of the transportation sector; especially aviation.

As a prerequisite for the scenario, no biomass can be imported to enhance the supply of combustible resources. In such an energy system, technologies for production of CO2 neutral hydrocarbons for easy storage and use in the existing infrastructure; especially in the natural gas grid; may be of great value.

The studied technology fulfills those demands.The main goal of the work was to design a plant and develop a thermodynamic model of the plant operation, enabling analyses related to selection of operating parameters; analysis and optimization of internal heat recovery and integration between the main technological subsystems.Finally to identify the main areas of technological development through economic analyses.The work included experimental work on an example of a system for capture of CO2: the humidity swing (HS) system, qualitatively evaluating the H2O uptake and CO2 desorption characteristics of the sorbent material, especially in relation to the supply of H2O to the sorbent.

It was found that H2O supplied in the gas phase resulted in slow uptakes and desorption rates of CO2 whereas supplying liquid water to the sorbent resulted in fast desorption in the first hours, after which the rate dropped sharply.A method was developed and used to characterize the impurities present in CO2 stream from the HS system in addition to the temperature vacuum swing (TVS) system under development by Climeworks Ltd.

The method relied on adsorption of impurities on a filter consisting of nickel-yttria-stabilised-zirconia (Ni/YSZ), similar to the material used in the fuel electrodes of SOECs followed by elemental analysis by glow discharge mass spectrometry. The method had a sub-ppm detection limit. Across the tested systems, a range of elements known to be detrimental to solid oxide cell (SOC) operation were detected in the range from tens of ppb to 20 ppm.The SNG plant was modelled using the process integration software package PRO/II alongside the design process, and a series of minor studies using PRO/II and thermodynamic analysis software FactSage® aided the design process.

This included studying a long range of questions such as alternative strategies for CO2 compression; the structure of the methanation plant; and the risk of carbon formation in both SOEC and methanation reactors, etc. The model was based on a thermodynamic 0-dimensioal model of the electrolyzer sub-system,developed to technological specifications from the thermodynamic SOEC model published bySun et al. [2] This model was used for a study of operating parameters, and two design cases were identified for the full plant based on these results.

The two cases both operated at 80atm, and had SOEC operating temperatures of 850 °C and 600 °C. The area specific resistance(ASR) of the SOECs were extrapolated to high pressure and low temperatures based on data for standard DTU Energy Ni-YSZ based cells, and the pressure dependency of the individual cell processes.

With the full plant model finished, the potential for internal recovery of surplus heat was analyzed, and a network of heat exchangers synthesized in order to minimize the requirements for external heating and cooling services. Based on the process flow sheets and the heat exchanger network, the dimensions and costs of the equipment of the plant were calculated and additional cost components such as installation of equipment, land use, labor costs,operation and maintenance, etc. were estimated according to standard methods.The plant had a yearly production capacity of 575,000 Nm3 of SNG with a methane content above 98.5 % which resulted in a Wobbe index of 49 MJ/Nm3 which is sufficient for injection into the natural gas grid.

The SOEC stack power was around 700 kW, and the plant operated a tan energy efficiency of 65 % (HHV) and 58 % (LHV).An economic analysis based on guidelines from the Danish energy agency and standard methods was conducted accounting for interest rates, taxes, depreciation etc. at a minimum acceptable rate of return set to the minimum of 4 %.The economic analysis resulted in SNG production prices of 1.88 €/Nm3 and 2.94 €/Nm3 basedon an electricity price of 18.6 €/MJ, a price of process heat at 120 °C of 11.9 €/MJ and a price of cell area of 0.23 €/cm2.

The main cost drivers were identified as the capital costs of the SOEC and air capture systems and the heat exchanger network. For operating costs, the electricity price had a significant impact, whereas the dependency of the SNG price on the heat price was minor.The technical issues where discussed in separate chapters interspersed by chapters documenting the modelling and design process.

Finally, a comprehensive discussion at the end treats the technical issues of the plant in the light of the economic analysis.