*3.3. Incineration with Energy Recovery*

While composting and fermentation processes use SEP-BIO (separation is done by producers/citizens), WtE handles biowaste present in residual waste (RES-BIO).

Figure 7 shows the resulting GWP of biowaste treatment in the WtE plant as a function of percentage utilisation of heat production. The horizontal axis displays the ratio between heat and power production during cogeneration. If the value is equal to 100%, the plant is heat-oriented, and it maximises the export of thermal energy into the network still working as a combined heat and power plant. Maximum steam goes through the extraction valve of the turbine. On the other hand, 0% indicates a strictly power-oriented plant, where no heat for export is produced, and electricity generation is maximised. All the steam flows through the condensing stage of the turbine.

It can be observed that the overall environmental effect of biowaste utilisation is in this case, always positive. This can be explained as follows: The values of GWP burdens are of the same value for both cases—before and after biowaste diversion. That is because biowaste component of RES does not contain any fossil-based carbon and therefore does not participate in GHG production during oxidisation of waste. On the other hand, GWP credits are related to BIO share. When an amount of biowaste with a heating value of approximately 4.6 GJ.t−<sup>1</sup> is removed, both exported heat and electricity decrease and therefore fewer credits from fossil fuels substitution are obtained. The highest overall GWP value of <sup>−</sup>272 kg(CO2)eq.twaste−<sup>1</sup> is achieved when all the available energy is exported as heat. If all the energy is exported as electricity, the GWP credits are more than twice lower at <sup>−</sup>115 kg(CO2)eq.twaste<sup>−</sup>1. Please note that although the result of the calculation of Equation (1) is positive, a positive environmental impact of biowaste treatment in WTE is desired, and therefore, the value is considered to be negative. The values of both GWP burdens and credits produced per year are summarised in Table 1 for strictly heat-oriented and power-oriented plants.

**Figure 7.** GWP as a function of percentage utilization of heat produced in WtE—the total balance for RES and contribution of its component biowaste (RES-BIO).



GWP is sensitive to waste composition, WtE plant efficiency, and fuel mix for power. The result for electricity-oriented operations is similar to other works published. For example, total emissions of CO2, eq. with a positive sign (burdens prevail over credits) for WtE operated as a power plant have been reported in [7]. On the other hand, previous investigations into WtE plants with a high share of heat delivery are missing, since only a few countries and regions have district heating systems with sufficient demand on heat, compared to required WtE capacity. The future potential for centralised systems of heat delivery in Europe has been analysed by the Heat Roadmap Europe initiative (see [29]).

The result of GWP of biowaste can also be compared with GWP of RES, considering it as a mixture of various components. GWP of RES treatment in the WtE plant as a function of percentage utilisation of heat production is also shown in Figure 2. Although the lower heating value of biowaste is low, biowaste is a component of RES, which offers significant credits for WtE operation and RES incineration.

The standard operating mode for a typical WTE plant in the Czech Republic, considering the seasonal fluctuations of heat demand, would be approximately 75% heat production in CHP. The structure of GWP burdens and credits of biowaste processing in a WtE plant operating in this mode is shown in Figure 8.

**Figure 8.** GWP impact of biowaste (SEP-BIO) incineration in WtE with heat production with CHP 75% of the maximum.

The values in this figure were obtained by applying Equation (1); therefore, they express the difference between the reference and alternative situation. In this case, the net value of GWP is equal to <sup>−</sup>225.2 kg(CO2)eq.twaste−1. The GWP burdens are close to zero because biowaste component of RES does not contain any fossil-based carbon and therefore no additional GHG is produced. The GWP credits resulting from heat production are equal to <sup>−</sup>196.4 kg(CO2)eq.twaste−<sup>1</sup> and from power production <sup>−</sup>36,4 kg(CO2)eq.twaste<sup>−</sup>1. The GWP burdens resulting from transport are estimated at 7.6 kg(CO2)eq.twaste<sup>−</sup>1.

## *3.4. Biowaste Treatment Methods Comparison and Discussion*

Based on the data above, the environmental performance of the three previously discussed ways of biowaste treatment can be compared in Figure 9.

Each of the net results of individual treatment method is negative, which means all of the abovementioned methods are beneficial from an environmental point of view, thus saving GHG.

The least credits are obtained by biowaste composting, which is also considered as a less investment-demanding method. The values of GWP for treatment in anaerobic digestion plants and WtE plants are comparable—depending on the operational mode of WtE. If the WtE plant is mostly heat-oriented, its environmental performance is more favourable than treatment by fermentation. However, if the WtE plant is strictly power-oriented, it generates fewer GWP credits than an anaerobic digestion plant.

There is a worldwide trend towards environmentally friendly waste management, with an effort to reduce the consumption of primary raw materials. This trend is known as the circular economy, and the EU, in particular, is very active in supporting circularity principles in waste management.

One of the significant achievements of EU legislation is the gradual implementation of the circular economy package. Since an increase in the share of municipal waste recovered materially to 65% by 2035 is obligatory for EU member states, WtE appears as a less important part of the system. Instead, separate collection of various fractions of MSW is stipulated. Reduction of RES is anticipated at the same time. In this respect, the separation of biowaste is becoming more and more popular and common. The two basic treatment methods of separated biowaste are composting and fermentation. The easiest and at the moment, the most widespread method, is composting. When compared with fermentation, it has fewer requirements for technical equipment and is less demanding on both capital and operational costs. From an environmental point of view, based on the obtained data, composting is a less-favourable method of biowaste treatment than fermentation, as worse GWP results suggest. The separation of biowaste as a single component is connected with the requirement for additional infrastructure such as specialised biowaste containers and collection, which increases the price. A collection of biowaste as a component of RES and its subsequent incineration with energy recovery in WtE plant can help avoid the extra expense. The simple GWP evaluation and related energy production-related analysis showed that the environmental impact of this method heavily depends on the operational mode of the plant. Based on the calculations performed, strictly power-operated WtE plants using normal steam parameters perform environmentally worse than anaerobic digestion plants. However, environmental performance improves with increased heat production in CHP. The more waste heat is used to export heat, the better results are obtained. The operational mode, CHP, proved to be the decisive parameter for the environmental performance of the WtE plant.

Considering these results, biowaste treatment as a component of RES incinerated in WtE showed the most significant environmental potential and should not be excluded from the range of choices of biowaste treatment methods during waste management planning. The results of this small case study correspond with the results in [12] with WtE proving to be more environmentally-friendly under certain conditions and at the current state of technological development. While composting and fermentation methods are currently more favoured (recycling) than waste incineration with energy recovery for biowaste streams, the contribution of WtE is also significant when heat is positively utilised.

The presented result is subject to boundary conditions. The figures presented are based on data for the Czech Republic. The most important aspect is the composition of RES and energy mix, which could be country-specific (see Appendix A). The extent of variation is in accordance with previous studies, where comprehensive sensitivity analysis was done (e.g., in [8]).

The need for sustainable energy production through MSW treatment is also highlighted in the study [30]. However, it should be pointed out that material products from composting and fermentation have the potential to provide nutrients (especially phosphorus) and organic matter to supply the soil. This additional environmental benefit with a view to the conservation of resources cannot be provided by WtE use of biowaste. A more detailed study further exploring both environmental and economic aspects of biowaste treatment in chosen plants should be conducted and then reviewed. The research presented in this paper confirmed that the potential of biowaste treatment can be environmentally beneficial and must be further explored.

The results of the calculations and, especially the methodology of marginal change, can also be further used in more detailed stages of waste management planning, e.g., when solving so-called reverse logistic problems, which are tools used for the detailed description of waste streams and complex waste management systems planning [19]. The methodology could be applied to other components of RES, which can provide input data for the reverse model, where components of RES are considered in detached problems. For example, plastics treatment chains are hot candidates for further investigation and optimisation due to recent unfavourable changes in the secondary material market.

**Figure 9.** Biowaste treatment methods' comparison, SEP-BIO and RES-BIO alternatives.
