Biogas Reforming Combined with Co-Electrolysis inside AC:DC Operated Solid Oxide Electrolyser Cells

Abstract

Upgrading biogas into added-value fuels and chemicals is seen as one of the main pathways to reduce carbon dioxide emissions from the energy sector. In particular, green methanol represents a strategical product due to its high versatility in the transportation and chemical industry. Biogas reforming is a key step in the upgrading process and it can be integrated with Power-to-X technology. The reforming can be conducted using Solid Oxide Electrolysers (SOEs). Furthermore, the outlet syngas composition can be adjusted and optimized for methanol production, via the additional production of hydrogen provided by the simultaneous steam electrolysis.This research focuses on analysing the effects of Dynelectro’s patented AC:DC operation [EP3719171A1, WO2020/201485A1, EP3947779A1] applied to biogas-fed SOE cells. The principle of this novel operating condition is to impose an alternating current over a direct current, which for SOE translates into reversing the polarization of the cell and having a regular switch between fuel cell mode and electrolysis mode. When performing steam electrolysis, the benefits of AC:DC operation are already proven to consist in reduced degradation, due to impurities removal, and enhanced temperature control during dynamic operations [Skafte, 2022]. When the SOE cell is fed with a composition of methane, carbon dioxide and steam, the main issue to solve is the high energy requirement from the steam reforming process and the steam electrolysis performed simultaneously. In fact, both reactions are endothermic at SOE operating conditions (750°C and ambient pressure), with an enthalpy difference of respectively 225 kJ/mol and 248 kJ/mol. In this regard, the AC:DC operation can provide additional heat, because of the polarization switch to fuel cell mode, and avoid the temperature drop within the SOE. Moreover, it is possible to tune the rate of co-electrolysis and biogas reforming.A mathematical model is developed to simulate biogas reforming in an AC:DC operated SOE cell. The model includes mass transport, electrochemistry, catalytic reactions and energy balance. The composition of biogas is assumed to be mainly methane and carbon dioxide. By operating both in DC and AC:DC, this gives a first indication of the extent of the expected benefits. To validate the model, laboratory experiments are performed, where single SOE cells are operated in an imitated biogas compositions and the outlet gas is analysed using gas chromatography. These experiments include current-voltage (i-V) curves and electronic impedance spectroscopy (EIS) analysis, which can be used to measure the performances of such operations by observing the cell voltage and the degradation rate.The results from ordinary direct current operation and Dynelectro’s AC:DC operation are compared in terms of syngas composition and operating voltage. The optimal composition of syngas for methanol synthesis, consisting of a ratio of hydrogen to carbon monoxide of approximately 2 and low traces of methane, is achieved. Reduced methane content in the outlet gas is shown when operating the SOE cells with AC:DC current, compared to DC operations. The latter shows a CH4 molar fraction of approximately 1%, while the former allows the CH4 concentration to go below this level, depending on the AC:DC parameters. It is found that the rate of methane and carbon dioxide conversion is dependent on the frequency and duty cycle of the AC:DC operation. The achievement of stable conditions during these operations leads to an optimized biogas to syngas conversion, and a simplified overall biogas to methanol process.