3.2.1. Municipal Solid Waste Exergy Profit Evaluation

To investigate the potential of resource recovery from MSW via integrated mechanical biological chemical treatment (MBCT), the case study in [84] is used. The exergy profit of the MSW is determined based on the selected downstream processes: material recovery, pulping, recovery of chemicals, and anaerobic digestion for biogas generation. In Figure 9, the selected potential process for MBCT for MSW, adapted from [84], is shown. The mass balances between the input and output are presented in Figure 9.

For this case study, the Municipal Solid Waste (MSW) is chosen as the output stream from a private household. The equation to calculate the specific exergy of MSW (adapted from [93]), which is a function of its heating value, is presented below:

$$\rm{EX}\_{\rm{MSW}} = 376.461 \times \rm{C} + 791.018 \times \rm{H} - 57.819 \times \rm{O} + 45.473 \times \rm{N} - 1536.24 \times \rm{S} + 100.981 \times \rm{Cl} \tag{16}$$

In Equation (16), EXMSW is the specific exergy of MSW (kJ/kg), C is the carbon content in %, H is the hydrogen content in %, O is the oxygen content in %, N is the nitrogen content in %, S is the sulfur content in %, and Cl is the chlorine content in %. All of the element percentages should be determined on a dry ash-free basis. The composition data of MSW are presented in Table 5, retrieved from [84].

**Figure 9.** Mechanical biological chemical treatment (MBCT) of Municipal Solid Waste (MSW), with mass balances between input and output,), adapted from [84].


**Table 5.** Data of Municipal Waste, from [84].

To evaluate the exergy profit of the MSW stream, it is imperative to evaluate the exergy performances of the treatment processes. Based on the diagram above, the treatment processes

are recycling, landfill, chemical conversion and anaerobic digestion (AD). The exergy profit of the MSW is calculated based on the implementation of Equations (2), (3) and (8) for the MSW case:

$$\text{EX}\_{\text{liabilities, MSW}} = \sum\_{i} \text{EX}\_{\text{input},i} \tag{17}$$

$$\mathbb{E}\mathbb{X}\_{\text{asset, MSW}} = \sum\_{j} \mathbb{E}\mathbb{X}\_{\text{output}, j} \tag{18}$$

$$\text{EX}\_{\text{profit, MSW}} = \text{EX}\_{\text{asset, MSW}} - \text{EX}\_{\text{liability, MSW}} \tag{19}$$

Table 6 shows the exergy data needed for various waste treatment processes. The superscripts in the table reflect the data sources and the contexts.


**Table 6.** Exergy calculation data for the MSW case study.


The exergy asset of the MSW stream is evaluated as the cumulative useful exergy of the secondary products after the waste treatment processes; i.e., the products after the incineration, landfill, recycling, AD and chemical conversion. The exergy liability is calculated by summing the cumulative exergy to be input to the waste treatment processes (see Table 6). The detailed calculation steps are presented in Part 3 of the Supplementary Materials.

Table 7 shows the calculation results, assuming a basis of 1 t/h of MSW produced. The original exergy of the MSW stream is calculated using Equation (16), which is a function of its heating value. The net exergy profit represents the exergy of the MSW stream after it passes through the potential downstream treatment via the MBCT system.

**Table 7.** Calculation results for MSW stream, on the basis of 1 t/h of MSW produced.


According to the calculation results, the exergy of the MSW stream is 6.90 MW, showing that it has high potential as a fuel. The above MBCT system shows that there is an exergy profit for the MSW stream (0.906 MW), due to the potential useful products. This is a significant chemical conversion section, as the exergy assets of the secondary products are high. The levulinic acid has a high exergy value, and the char also has potential as a fuel. Despite the recycling process requiring high exergy, it is only applied to the plastic recovery for the recyclables. The small amount of plastic in the recyclables (8.05%) reduces the useful exergy as well as its exergy liability.
