Modelling Approach

In the following text, the calculation of GWP of the WtE plant is described in detail. All the calculations were done using a computational model designed in Microsoft Excel. The calculation procedure itself consisted of several steps. To be able to assess GWP credits, the amount of energy recovered from the treatment process had to be determined. In this case, a techno-economic model of a WtE (TE model) thoroughly described in [17] was used and is briefly discussed further in the text. The values of GWP burdens were determined from the basic combustion equations and the amount of the products (CO2). Both of this information is highly influenced by the waste composition and its properties such as the content of fossil-based carbon and calorific value of the waste., A specialised tool called JUSTINE, available at the workplace of the authors, was used to satisfy the requirement for high-quality data estimation. This tool is further described in [28]. After gathering both GWP credits and GWP burdens, a simple balance was performed and the net GWP obtained by the approach is summarised in [17]. Using this method, the resulting GWP of thermal treatment of RES without distinguishing its components can be readily determined. However, the paper aims to determine the GWP of biowaste component of RES.

To obtain such information, a concept of marginal change was proposed in this contribution. This concept is widely applied in economics. We then speak about marginal cost. The marginal cost concept has been employed in the field of waste management in a complex [15], where the impact of RES components on WTE plant economy has been investigated. However, the authors are not aware of any application of this concept for GWP analysis.

Figure 3 hows the steps required to apply such an approach in this case:

**Figure 3.** Calculation concept using the marginal change approach to evaluate GWP related to processing components of residual waste (RES) in waste-to-energy (WtE).


$$\text{GWP}\_{\text{COMP}} = \text{GWP}\_{\text{BIO}} = \frac{\text{GWP}\_{\text{REFERSINE}} - \text{GWP}\_{\text{ALTERNAIVE}}}{\text{m}\_{\text{MARCINAL}}}.\tag{1}$$

Such a component-specific GWP calculation is a base for WtE process evaluation, according to system boundaries displayed in Figure 4.

**Figure 4.** System boundaries for biowaste treated with other components of residual waste (RES-BIO) in WtE plants.

The case study considered in this paper is adjusted to suit current European conditions and is mainly focused on the Czech Republic. The input waste data, technological advancement, infrastructure availability (e.g., heat distribution network), and WtE plant specifications were chosen accordingly:

