We’re pleased to share a blog report below from Stavros Michailos, Mohammed S. Ismail and Lin Ma of Energy 2050 at the University of Sheffield, on Lin’s Flexible Funding 2020 project, Evaluation of different CCUS systems based on the MCFC technology for decarbonising the power generation sector:
Carbon capture utilisation and storage (CCUS) is pivotal for delivering net-zero GHG emissions and strategically significant to the UK economy. The study details the performance of three CCUS systems based on MCFC via multiscale modelling, i.e. multiphysics modelling of the MCFC unit and process modelling. A distinct advantage of the MCFC over other CO2 capture technologies is that, instead of absorbing energy (electricity), it generates it. Mass and energy balances have been established in COMSOL Multiphysics® and Aspen Plus software. Based on the simulations, a levelised cost analysis has been undertaken to appraise the capture cost of the investigated technologies. The MCFC concentrates CO2 that can be stored or further utilised. Figure 1 is a visual representation of the scenarios examined in the study by means of simplified block flow diagrams (BFD).
Multiphysics model of MCFC
A two-dimensional MCFC multiphysics model has been created to inform the process model of the entire system. Figures 2 displays the profiles at a typical cell potential (0.7 V) for the temperature, CO2 concentration along the cathode channel and H2 concentration along the anode channel. The temperature is almost uniform across the various components of the fuel cell with slightly increased temperatures at the anode side of the fuel cell (Figure 1a). Figure 1b shows that the concentration of CO2 (mol/m³) expectedly decreases from the inlet to the outlet, with very low amount of CO2 at the cathode electrodes where the cathodic half reaction takes place. This observation also applies to H2 concentration in the anode side of the fuel cell (Figure 1c).
Figure 3 shows that the cell voltage and power density increase with increasing current density after 3500 A/m² (Figure 3(a-b)); this is attributed to the availability of amount of CO2 that is higher than that what is needed for the half reaction at the anode electrode. However, before 3500 A/m², the fuel cell performance appears to slightly improve with decreasing CO2 concentration; this seems to be due to availability of amount of oxygen that is substantially higher than what is needed to complete the half reaction at the cathode electrode. Evidently, the carbon capture factor, which is the ratio between the flow rate of CO2 concentrated at the anode and the flow rate of CO2 available in the flue gas, substantially decreases with increasing CO2 concentration in the flue gas.
Aspen Plus has been utilised to model the electrolyser and the Fischer-Tropsch (FT) plant. In particular, for the FT plant, CO2 and H2 are fed to a reverse water gas shift reactor to convert CO2 to CO. The produced syngas is sent to the FT reactor, which generates a mixture of light gases, syncrude oil and water. After proper separation a part of the unreacted gases and light hydrocarbons are sent to a CHP unit modelled as combined cycle turbine and the rest is recycled to the FT reactor. The syncrude oil is fractionated to naphtha, kerosene and diesel cuts. The CO2 that is not utilised is sent for compression and permanent storage. Figure 4 presents the basic mass balances of the Scenarios 2 and 3.
Conclusions and future work
The study revealed that MCFC is an interesting CO2 capture technology with prospects to play a role in the mid-term for decarbonising scenarios. The stand-alone case, i.e. scenario 1, with certain technology improvements can raise the competitiveness of the MCFC. The multiphysics model of the MCFC has been built and used to inform the process model on the amount of CO2 concentrated at the anode of the MCFC at a typical operating cell potential. The parametric study shows that the MCFC performance in terms of power density improves with decreasing temperature, increasing CO2 concentration in the flue gas and increasing H2 concentration in the fuel mixture. Conversely, the CO2 capture factor was found to increase with increasing operating temperature, decreasing CO2 concentration in the flue gas and decreasing H2 concentration in the fuel mixture. In the future, experimental testing of the MCFC in the Translational Energy Research Centre (TERC) will enable the improvement of the models developed in the current project.