(573d) A Techno-Economic Analysis of Sustainable Hydrocarbon Fuel Production By Co-Electrolysis of CO2 and H2O Using Solid Oxide Electrolysers

In the ever growing global population, fossil fuel supplies are falling short of sustaining the increasing energy demand. At the same time, a vast amounts of CO₂   is being generated, much of it from the use of conventional fossil fuels, which in turn has a strong climate effect. Carbon capture and geologic sequestration are the most commonly used techniques for reduction of CO₂, the techniques which still face challenges associated with effectiveness and the financial penalties. It is apparent that alternatives to carbon capture and storage (CCS) are needed, especially those capable of using captured CO₂beneficially, i.e. by converting it into valuable products.

Production of synthetic liquid hydrocarbon fuels from syngas by co-electrolysis of captured CO₂ and H₂O provides an alternative solution to CCS. The co-electrolysis of CO₂ and H₂O is based on solid oxide electrolyser cells (SOECs) which can be considered as the reverse of solid oxide fuel cells (SOFCs). With electricity being input to SOECs, CO₂ and H₂O are converted to a mixture of CO and H₂, i.e. syngas, within the cells, alongside the production of oxygen. Syngas can be then used to produce liquid fuels in well-established chemical processes such as the Fisher-Tropsch process. To this end, co-electrolysis-to-liquid-fuels process can potentially offer a sustainable route for producing carbon-neutral fuels. Numerous studies have addressed technical development and operation of SOEC stacks^[1], as well as modelling of the co-electrolysis and liquid fuel conversion process^[2]. Still, techno-economic study into deploying large scale production plants was much less considered.

This study investigates the possibility of producing Hydsynthetic fuels at a competitive market price from the co-electrolysis process and analyses challenges associated with integration into existing infrastructure, as well as possible supply chain routes of distributing the fuels, all with a view to inform decision makers and to screen technology development directions. To this end, a supply chain network model was developed to select the most suitable technologies involved in the co-electrolysis process (from CO₂ capture to electricity input and fuel production/distribution) and assess the most cost effective route for the production of synthetic fuels. The analysis accounts for variability in prices of CO₂generated from different processes, as well as prices of electricity from different sources, such as geothermal, hydroelectricity and wind. The model includes economic factors along the whole supply chain, i.e. costs of distribution, transportations, taxations and subsidies. Two downstream technologies for fuel production are considered: 1) the Fisher-Tropsch process, and 2) methanol synthesis.

A 100MW SOEC plant is chosen as the base case study for the research. Cost estimations of facility for co-electrolysis is based on the literatures ^[2-5]and include costs for production of syngas (i.e. SOEC plant), production of diesel (i.e. Fisher-Tropsch reactor) and methanol (i.e. methanol synthesis reactors and crude methanol distillation columns). The proposed supply chain network model is then solved as mixed integer programming (MIP) problem with respective sensitivity analysis to identify limitations on the price of the fuels.

A sensitivity analysis was first done to understand the dependence of the fuel price on the electricity and CO₂ prices. The result shows that the electricity cost is the key effect on the fuel price, which accounts for 80% of the diesel and 77% of methanol production costs. From our study, electricity cost has to be under $38/MWh for diesel production and under $52/MWh for methanol production to be competitive with the current market price which is in the range of $2.72 and $4.32 per gallon for diesel and in the range of $0.6 and $1.25 per gallon for methanol. If subsidies were introduced at $1 per gallon, the new electricity prices would be under $41/MWh for diesel and $78/MWh for methanol. Compared to electricity, the variation of CO₂price has a less significant impact.

Under the existing electricity and CO₂ price ranges (which reflect electricity from different energy sources and CO₂ captured from different emitters), our result shows that the cost for diesel production ranges from $1.43 to $66.09 per gallon and for methanol production from $0.69 to $14.44 per gallon. The best scenario found is the co-electrolysis process coupled with hydroelectricity at a cost of $26/MWh and CO₂from either the hydrogen production or fertiliser production at a cost of $5/tonne. This gives fuel production cost for diesel at $1.43 and methanol at $0.69 per gallon.

Further investigations are continued to include the effects of the economic cost of processing the oxygen stream into a saleable product and the effects of SOEC cell degradation on fuel production price. Future studies will also look at the potential and possible routes for commercialisation of this technology in the near future.

  References

[1] X. Sun, M. Chen, S. H. Jensen, S. D. Ebbesen, C. Graves, M. Mogensen, Thermodynamic analysis of synthetic hydrocarbon fuel production in pressurized solid oxide electrolysis cells, International Journal of Hydrogen Energy 37 (2012) 17101-17110.

[2] W.L Becker, R. J. Braun, M. Penev, M. Melaina, Production of Fisher-Tropsch liquid fuels from high temperature solid oxide co-electrolysis units. Energy 47 (2012) 99-115.

[3] Q. Fu, C. Mabilat, M. Zahid, A. Brisse, L. Gautier, Syngas production via high-temperature steam/CO₂co-electrolysis: an Economic Assessment. Energy& Environmental Science 3 (2010) 1382–1397.

[4] A.A.C.M. Beenackers, G.H Graaf, Comparison of two-phase and three-phase methanol synthesis process. Chemical Engineering and Processing (1996) 413-427.

[5] P.L. Spath, D.C. Dayton, Preliminary Screening – Technical and Economic Assessment of Synthesis Gas to Fuels abd Chemicals with Emphasis on the Potential for Biomass-Derived Syngas. National Renewable Energy Laboratory. (2003).