5.1.3. Effect of Varying CO2 Capture Ratio

Figure 13 shows the change of the electrical, cogeneration, and exergy efficiencies with carbon capture ratio. For the steam methane-based system, the profile of electrical efficiency is almost flat, whereas the exergy and cogeneration efficiency continuously decrease with increasing capture ratio from 80% to 95%. The exergy efficiency is reduced by 1.1% and the cogeneration efficiency by 4.06%. This trend is attributed to the fact that as the capture ratio decreased, heat consumption in the CCU decreased, leading to exergy and cogeneration efficiency. Note that a CO2 capture ratio below 80% in the methane-based system causes the formation of dry ice due to the lower temperature of LNG. Although the formation of dry ice can be managed by temperature control, it is not considered in this study. In the case of the methanol-based system, the electrical efficiency, cogeneration efficiency, and exergy efficiency decrease from 48.06%, 63.32%, and 45.12% to 47.35%, 54.03%, and 42.56%, respectively, with increasing carbon capture ratio from 70% to 95%. The higher decrease rate of electrical efficiency in the methanol-based system than that in the methane-based system is mainly attributed to the increase in power consumption of the CO2 compressor and NH3 compressors for CO2 liquefaction. In addition,

as the capture ratio increases, the liquefied CO2 ratio proportionally increases because the total emitted CO2 is fixed, as presented in Table 3. In methanol-based systems, the amount of available heat is relatively small compared to that in the methane-based systems, and thus, fuel consumption increases faster than in the methane-based systems as capture rates increase.
