(525c) Enviro-Economic Assessment of Electrochemical Routes to Syngas Production

Authors: 
Bernardi, A., University of Padova
Rodriguez-Vallejo, D. F., Imperial College of Science, Technology and Medicine
Guillén-Gosálbez, G., Imperial College London
Chachuat, B., Imperial College London
The general consensus is that we have reached the point where global CO2emissions and atmospheric levels need inflection, despite our ever-growing needs in energy. A promising technology entails capturing CO2, either from a concentrated source or directly from the atmosphere, and reacting it with a sustainable H2 stream to produce liquid fuels [1,2]. This may either be achieved directly, or using syngas as an intermediate. This work is concerned with the latter (indirect) route, which can rely on proven syngas conversion technology into methanol (MeOH), DME and Fischer-Tropsch (FT) liquids [3].

Of the alternative routes to sustainable syngas production, our assessment focuses on electrochemical conversion, which presents a very high electricity-to-syngas efficiency and offers the necessary flexibility to operate with intermittent energy sources (e.g., wind and solar) [4-6]. But despite being at a high level of technology readiness for scale-up [7], it remains unclear whether these electrochemical systems would be economically viable or environmentally sustainable [8].

A common distinction is made between high-temperature electrolysis (operated at 800-1000°C), e.g. carried out in a solid oxide electrolytic cell (SOEC); and low-temperature electrolysis (operated below 100°C), e.g. carried out in a proton exchange membrane (PEM) electrolyzer or gas diffusion electrodes (GDEs). An SOEC may either be fed with pure H2O or CO2, or operated with a mixture of H2O and CO2 in co-electrolysis mode. It presents higher diffusivity, reaction rates, and electricity-to-syngas efficiency compared to low-temperature electrolysis, but the temperature must remain steady and uniform across an SOEC stack to prevent mechanical failure. On the other hand, milder conditions in a low-temperature electrolysis system promote a longer lifetime and lower the costs. Currently, PEM electrolyzer are preferred in water electrolysis applications, whereas GDEs are better suited to CO2 electrolysis due to the low CO2 low solubility and mass transport in aqueous systems.

Herein, we conduct an enviro-economic assessment of both high- and low-temperature electrolysis for syngas production. We use steam reforming of natural gas (SRNG) as a reference in our assessment, and also consider hybrid processes whereby only CO or H2 is produced electrochemically in combination with a (reverse) water-gas shift reactor to adjust the syngas composition. We specifically target two syngas compositions: a ratio H2:CO=1, which is optimal for DME production; and a ratio H2:CO=2, for MeOH and FT synthesis.

Our assessment relies on detailed process simulations in Aspen Plus to describe the interplay between transport phenomena, chemical and electrochemical reactions for each technology. We embed these detailed models into a recent data-envelopment analysis (DEA) methodology [9] in order to identify the technologies that are efficient in terms of syngas production cost and associated GHG emissions. We then look at the technologies closer to the efficient frontier and target improvements that would make them efficient, yet without violating the underlyling thermodynamic limits of the process.

SRNG yields the cheapest syngas given current market prices of natural gas, and it is therefore on the efficient frontier despite carrying a high environmental burden. The capital cost of most electrocatalytic systems is only slightly higher than that of SRNG, and it has even been argued that the costs of SOEC and PEM electrolyzers may drop significantly in the near future. Regardless, the main contributor to the cost of syngas using electrochemical systems is, despite their high efficiency, the electricity price itself. But electrolyzers are found to be efficient when using, e.g., electricity from on-shore wind farms, which incurs significantly less GHG emissions than SRNG. Therefore, electrochemical systems could become economically viable upon introducing a carbon taxation or accounting for the social cost of carbon. Our current investigations are aimed at integrating both syngas production and conversion technologies into the enviro-economic assessment.

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