3.2.2. Cumulative Exergy Composite Curves

To compare different scenarios with the MSW treatment system, the current waste practices in the Czech Republic [95] are used for comparison with the MBCT system in this study. The data used in this study are based on Table 6, assuming both scenarios use waste treatment technologies with similar efficiencies. Figure 10 illustrates the scenarios, showing the distribution of the MSW.

**Figure 10.** Workflows for MSW treatment under comparison.

The exergy liabilities and the exergy profit for the individual waste treatment processes are plotted in composite curves, as shown in Figures 11 and 12. The steps taken to draw the composite curves are explained below:


Based on Figures 11 and 12, the cumulative exergy liability for Scenario 2 is much higher than that of Scenario 1. However, the exergy profit for Scenario 2 (0.91 MW) is slightly higher than that in Scenario 1 (0.89 MW). This is specifically thanks to the chemical conversion process. This shows that the practices in Scenario 2 yield a somewhat higher exergy profit, but they also require a great deal more exergy.

According to Table 6, although incineration has a higher exergy profit per unit waste, a greater amount of waste is sent to the chemical conversion in Scenario 2, making its exergy profit surpass that of Scenario 1. It can be shown that the chemical conversion process has the highest specific exergy liability due to the requirement of steam for pulping and the other conversion processes. Nevertheless, the exergy profit for the chemical conversion is still highest due to the high exergy asset

level of the levulinic acid and that of the char as a secondary fuel. It is worthy of note that the specific exergy profit of landfill is lower, although its specific exergy liability is the lowest because the useful exergy asset level of the process is very low (see Table 6).

**Figure 11.** Composite Curves for the exergy liabilities of the MSW treatment under comparison.

**Figure 12.** Composite Curves for the exergy profit of the MSW treatment under comparison.

It can also be seen that incineration has the highest specific exergy liability for Scenario 1, but it also has the highest specific exergy profit, as the products have a high amount of useful exergy content. The composite curves are useful to compare process systems and pinpoint the processes with a high exergy liability or exergy profit, allowing mitigation actions to be targeted to improve the waste treatment practices.
