**3. System Simulation and Assumptions**

The process simulation and heat and mass balance calculations were carried out using the ASPEN HYSYS process simulator. The heat and mass balances obtained from the converged simulation are utilized for exergy and energy calculations in a separate spreadsheet. The Peng–Robinson equation of state is used for different units/sub-systems including steam reforming, HT-PEMFC, and CO2 liquefaction system [29,58]. A "Gibbs reactor" model, which considers the condition of the Gibbs free energy of the reacting system being at a minimum at equilibrium to calculate the product mixture composition [59], is used to simulate the reformer and WGS reactor, whereas the "conversion reactor" model is used to simulate the combustor [50]. There is no separate module to simulate a fuel cell in ASPEN HYSYS. Therefore, in the present study, the HT-PEMFC is modeled as a "conversion reactor" and a "splitter," which attain a conversion ratio equal to the hydrogen utilization factor [8,9]. For simplicity, the carbon capture unit using MEA is modeled as a "splitter" in ASPEN HYSYS, which can be considered as a black box assumption. In a black box assumption, the complex process for capturing CO2 is modeled by one single box, which has two outlet streams of differing composition from a single inlet stream. Although the black box model enables estimating the minimum work or heat required for CO2 capture, the calculated value for capture is generally a little different from the actual work and heat reported in the literature [30,41]. The concept for the black box model of the CO2 capture has been used in several studies [30,41,60]. To obtain information about the energy and exergy consumption per kg of captured CO2, the values reported in reference [37] are used in the present work.

The concentration of H2, CO2, and CO in the anode inlet stream can affect performance of HT-PEMFC. Andreasen et al. investigated the variation of HT-PEMFC performance by the feeding mixture of H2, CO (0%, 0.15%, 0.25%, 0.5%, 1%), CO2 (0%, 25%) to emulate methanol or methane reformate gas. Results reveal that increasing both CO and CO2 concentration decreases output voltage. In total, eight cases of experiments, 1% CO and 25% CO2 case and 0.5% CO and 25% CO2 case show the first and second lowest output voltage, approximately 0.645 and 0.652 V, respectively, at 0.2 A cm−<sup>2</sup>

and operating temperature of 160 ◦C [61]. Other experimental studies show similar results. Devrim et al. evaluated the combined effect of CO and CO2 in anode inlet stream and results show that no significant performance degrade due to the addition of only CO2 into H2, however addition of CO in H2 and CO2 mixture increase degrade of performance. H2, CO2, CO (75%, 24%, 3% and 75%, 24%, 1%) mixtures show output voltage of approximately 0.629 and 0.634 V at 0.2 A cm−<sup>2</sup> and operating temperature of 160 ◦C [62]. It was reported that the impact of the CO presence (up to 2.0%) at higher operating temperature (160 ◦C and above) and lower current densities (below 0.3 A cm−2) is very low [63,64]. Therefore, in this study, the performance degrade due to CO contents is neglected and output voltage of HT-PEMFC is fixed as 0.637 V [56].

Since the amount of electricity consumption vary with several operation modes of ship, it is assumed that the target ship operates with constant average shaft power, namely 475 kW for simplicity. Further, a current density of 0.2 A cm−<sup>2</sup> is assumed for constant power generation. Noteworthy, lower current densities lead to higher electrical efficiency, and require a larger cell area. However, considering the feasibility check and comparison for ship application of the methane-, methanol-based system is main purpose of this study, the above assumptions are deemed reasonable.

The general assumptions used in the modeling of the integrated energy system are as follows:

