Modeling of CH4-assisted SOEC for H2O/CO2 co-electrolysis
Introduction
Clean and sustainable energy technologies are urgently needed to address the fossil fuels-related energy crisis and environmental problems such as global warming, air pollution and acid rain. Renewable energies like solar energy and wind energy can hopefully meet our requirements. However, they are restricted in time and space and not reliable for instantaneous supply of energy [1], [2], [3], [4]. Therefore, effective energy storage is critical for renewable energy applications.
Solid oxide electrolysis cell (SOEC) is a high temperature electrochemical cell suitable for converting excess renewable power to fuels [5]. The produced fuel can be later converted back into electrical power via fuel cells when the renewable power is insufficient. Compared with low temperature electrolyzers, the electrical energy requirement of SOEC is relatively low as a significant part of energy input to SOEC is heat [1]. In addition, the high operating temperature of SOEC enables the use of non-noble metal catalyst, leading to lower cost of the system. SOECs are capable of co-electrolyzing CO2 and H2O to produce syngas (H2 and CO mixture), which can be further processed for gaseous or liquid fuel generation using Fischer-Tropsch (F-T) reactor [6], [7], [8], [9], [10], [11], [12], [13]. Becker et al. [9] developed a model for high temperature SOEC co-electrolysis for syngas production and subsequent conversion to liquid fuels by F-T process. They also evaluated the economics of production plant considering variations in electricity feedstock costs and operating capacity factors. Stempien et al. [12] further analyzed the thermodynamics of the combined SOEC and F-T processes. They proposed an optimized system that achieved overall efficiency of 66.67%. Chen et al. [13] integrated the high-temperature CO2H2O co-electrolysis and low temperature F-T synthesis in a single tubular unit and reached 11.40% of CH4 yield with an overall CO2 conversion ratio of 64.1%. Chen et al. [10] modeled this one-step system and identified optimal operating conditions. The combination of SOEC co-electrolysis and F-T process offers an alternative way of utilizing the captured CO2 for fuel synthesis using excessive renewable power.
For widespread application of SOEC, its electrical energy consumption needs to be further reduced as the quality of electricity (i.e. exergy) is high. Recent studies have demonstrated that by supplying low cost fuel to the anode of SOEC (termed as fuel-assisted SOEC) for steam electrolysis could significantly reduce the operating potential of SOEC thus greatly reduce the electrical power consumption [14]. Despite of preliminary modeling study on fuel-assisted SOEC for H2O electrolysis, the current literature is lacking detailed modeling of fuel-assisted SOEC for syngas production by CO2/H2O co-electrolysis which is very different from steam electrolysis due to the more complicated reaction processes.
To fill the research gap, a 2D mathematical model is developed for an axisymmetric-tubular CH4-assisted SOFEC (CH4-SOFEC) for H2O/CO2 co-electrolysis. For SOECs exposed to more than one gas, oxygen partial pressure model is adopted as suggested by Stempien et al. [15] The model is validated with the experimental data for CO2/H2O co-electrolysis. Parametric simulations are conducted to understand the effect of fuel assisting on the performance of SOEC and the interplay of different physical/chemical processes.
Section snippets
Model assumption and calculation domain
The 2D numerical model of tubular SOEC is developed by coupling governing equations of electrochemical reaction, chemical reactions, ionic/electronic charge transport, mass transport and momentum transport. In the literature, Luo et al.’s work [16] on syngas production by co-electrolysis provides detailed experimental setup and operating conditions, such as the cathode inlet gas composition, the operation temperature, the thickness of SOEC components, etc. In their study, the current–voltage (I
Model evaluation
In this section, the modeling results of current–voltage characteristics are compared with experimental data for model validation. The model tuning parameters can be found in Table 3. The simulation results and experimental data are compared in Fig. 3. Small difference between the modeling results and experimental data is achieved. In the subsequent parametric simulation, the operation voltage, inlet gas flow rate and inlet gas composition are purposely varied to evaluate their effects on SOFEC
Conclusions
A multi-physics model including electrochemical reaction, chemical reactions, ion/electronic charge transport, mass transport and momentum transport is developed to characterize the performance of a methane assisted SOEC for H2O/CO2 co-electrolysis. For comparison, a multi-physics model with same structure parameters is also developed to characterize the performance of an air assisted SOEC for H2O/CO2 co-electrolysis, which is validated by comparing the simulation results with experimental date
Acknowledgment
This research was also supported by a grant of SFC/RGC Joint Research Scheme (X-PolyU/501/14) from Research Grant Council, University Grants Committee, Hong Kong SAR.
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