*5.1. Energy and Exergy Analyses*

The steam reforming systems presented in this study are evaluated in terms of energy, exergy efficiency of the overall systems, and exergy destruction rate of each component. For the energy efficiency, electrical and cogeneration efficiencies are utilized. Electrical efficiency, cogeneration efficiency, and exergy efficiency at the base condition presented in Table 3 for the methane-based and methanol-based systems are compared in Figure 5. Electrical efficiencies of 39.53% for the methane-based system and 47.53% for the methanol-based system are obtained. It can be interpreted that the methanol-based system uses the feed energy input (LHV basis) more efficiently than the methane-based system for 475 kW of net electricity generation. Noteworthy, the overall electricity consumption of the methane-based and methanol-based systems are 18.51 and 30.97 kW, respectively, as indicated in Figure 6. The higher electricity consumption in the methanol-based system is mainly attributed to electricity consumption of the compressors (Comp-4 and -5) in the ammonia refrigerant cycle, whereas the methane-based system, which uses the cold energy of LNG for CO2 liquefaction, does not consume additional electricity. Regarding the cogeneration efficiency, the methane-based system has higher value, 63%, than that of the methanol-based system, 55.86%, and this can be explained by Figure 7. As can be observed in Figure 7, the methane-based system consumes a higher amount of overall energy (LHV basis) than the methanol-based system and generates more available heat for 475 kW of net electricity generation. The higher value of heat generated can be explained by the fact that the steam methane reforming reaction is more endothermic than the steam methanol reforming reaction.

To evaluate the performance of the components, the exergy destructions of each component in the methane-based and methanol-based systems at the base condition provided in Table 3 are calculated. It is observed that there is a total exergy destruction of 690.68 kW for the methane-based system and 591.16 kW for the methanol-based system. The exergy destructions in each system are broken down to the component level and the results are presented in Figures 8 and 9. The HT-PEMFC, reformer-combustor, and CCU have the largest percentages of total exergy destruction. For the

methane-based system, the HT-PEMFC is the component having the highest exergy destruction with 365.5 kW (51.5%), followed by the reformer-combustor with 145.76 kW (20.6%) and CCU with 86.03 kW (14.0%). The exergy destructions in the methanol-based system follow a similar trend, having the highest exergy destruction in the HT-PEMFC with 342.34 kW (57.9%), reformer-combustor with 145.76 kW (20.6%), and CCU with 85.74 kW (14.5%). Exergy destruction of the HT-PEMFC and reformer-combustor mainly results from high irreversibility of the chemical reaction [54]. Exergy destructions of CCUs are calculated by assuming 1529 kW/kg CO2 [37], which are caused by the unavoidable heat exergy required for MEA regeneration, and therefore, are dependent on the amount of generated CO2 in the overall process. The larger total exergy destruction in the methane-based system is mainly attributed to the larger exergy destruction in the reformer-combustor. This relies on the fact that the steam methane reforming reaction is operated at a higher temperature condition of 700 ◦C (combustion temperature of 800 ◦C) instead of 200 ◦C (combustion temperature of 300 ◦C) in the steam methanol reforming reaction.

**Figure 5.** Electrical, cogeneration, and exergy efficiencies of the methane-based and methanol-based systems for 475 kW of net electricity generation.

**Figure 6.** Break-down of electricity consumption of the methane-based and methanol-based systems for 475 kW of net electricity generation.

The other reasons explaining why the total exergy destruction is higher in methane-based systems than in methanol-based systems is that higher exergy destruction occurs in the heat exchangers (mainly HEX-1, HEX-3, and HEX-5), Mixer-1. For the heat exchangers, the larger exergy destruction derives

from the larger temperature difference between cold and hot streams. This is because fuel in cryogenic temperature (LNG) is supplied in the methane-based system and streams from the reformer and combustor have inherent temperature. For Mix-1, more exergy destruction in the methane-based system than that in the methanol-based system is generated from mixing of streams with larger temperature difference. Therefore, reducing the temperature differences in these heat exchangers and Mix-1 by optimization can effectively reduce the system exergy destruction [71]. The exergy destruction generated in the WGS reactor, which is additionally equipped for the methane-based system to lower the CO fraction, is another reason.

**Figure 7.** Overall energy consumption and generation of the methane-based and methanol-based systems for 475 kW of net electricity generation.

**Figure 8.** Break-down of exergy destruction for the methane based-system. Total exergy destruction 690.68 kW. (Unit names: Exergy destruction, kW; exergy destruction ratio, %).

**Figure 9.** Break-down of exergy destruction for the methanol based-system. Total exergy destruction 591.16 kW. (Unit names: Exergy destruction, kW; exergy destruction ratio, %).
