(239a) Waste-to-Energy and Chemical Recycling of Mixed Plastic Waste. Economic and Environmental Optimisation of the Northern Italian Supply Chain

1. Problem

Plastics are the world's largest synthetic consumer product, and their increasing demand poses a challenge in terms of end-of-life management (recycling, energy recovery, landfilling), an issue that needs to be addressed in the transition to a circular economy. The same resilient properties that lead to high annual production, reaching 390 million tonnes in 2021 (Plastic Europe, 2022), also create a major environmental problem in terms of end-of-life management, as significant fractions of plastic waste that are not properly disposed of most often end up in landfills or leak into the ocean, with harmful consequences (Li et al., 2022). The largest fraction of plastic waste is made up of plastic packaging; only a limited fraction can be mechanically recycled due to downcycling, and the residual fraction, called mixed plastic waste (MPW), which is often landfilled, can be turned into a resource according to a circular economy approach. The challenges in managing MPW are due to variability in its composition, and incineration can help reduce the amount of waste that needs to be landfilled and generate electricity. Pyrolysis and gasification represent potentially attractive emerging technologies for MPW management (Solis and Silveira, 2020). In this study, the waste-to-energy pathway is represented by incineration with electricity generation, and the chemical recycling pathways are represented by gasification, producing syngas from which electricity is generated, and pyrolysis, generating a pyrolysis oil to be transported to a refinery for further processing. In this context, it is important to explore the potential of waste-to-energy technologies and chemical recycling for the management of mixed plastic waste and to assess them based on the economic feasibility and environmental impact. Supply Chain (SC) optimisation can be used to evaluate both the optimal selection of MPW treatment technologies and the optimal distribution of material flows between the different stages of the SC. This study aims to propose a decision-making tool for MPW supply chain management through a multi-objective, spatially explicit Mixed integer linear programming (MILP) framework. The model aims at finding the trade-offs between economic optimisation (in terms of maximising the annual gross profit) and environmental impact (in terms of minimising the net GHG emissions of the supply chain), focussing on Northern Italy.

2. Methodology and expected results

The problem is formulated as an annualised spatially-explicit MILP modelling framework. The spatially-explicit nodes are described in the model through a set n comprising 47 provinces, 12 sorting centres, 24 incinerators and 3 refineries. Each node is characterised by its exact geographic coordinates. Gasification plants can be installed only in proximity of incineration plants; similarly, pyrolysis plants can be placed either close to incineration plants or refineries. Both waste-to-energy (i.e. incineration) and chemical recycling (i.e. pyrolysis and gasification) technologies are considered as treatment options. The stages of the MPW SC are divided into the transport of plastic packaging waste (PPW) from provinces to sorting centres, where the PPW share that can be mechanically recycled is separated from the residual part, i.e., the MPW. From the sorting centres, the MPW fraction is transported to a treatment plant (incineration, gasification, pyrolysis), whose outputs are: electric power in the case of incineration and gasification, and oil in the case of pyrolysis plants, the latter one to be transported to a refinery for further processing. Road transport is considered as the only transport option since it reflects current practice for plastic waste in Northern Italy.

The modelling framework aims at maximising the economic performance of the overall SC from the perspective of the final treatment plants (in terms of gross profit) and minimising the environmental impact of the supply chain (in terms of net greenhouse gas emissions).

The decision variables in the optimization problem are:

• the PPW flowrates distribution from the provinces to the sorting centres;
• the MPW flowrates distribution from the sorting centres to the treatment plants;
• the selection of technology, plant size and location of the treatment plants;
• the pyrolysis oil flowrate distribution from a pyrolysis plant to a refinery.

The modelling framework provides a decision tool for the optimal choice of the treatment technology for MPW in terms of both economic performance and environmental impact and can provide different trade-offs between the conflicting objectives.

3. Conclusions

This study proposes a multi-objective mixed integer linear programming framework for the Northern Italian supply chain of mixed plastic waste management. The model optimisation outcomes provide quantitative information on the SC in terms of economic and environmental impact performance, and provide the optimal selection of the treatment technology under different scenarios. Results show that, when aimed for economic optimisation, the selected MPW treatment technology is exclusively the incineration, being the technology that has the highest gross profit, while in the case of environmental optimisation, the selected technology is pyrolysis, due to lower GHG emissions of the technology.

References

1. Li, et al., 2022, Expanding Plastics Recycling Technologies: Chemical Aspects, Technology Status and Challenges, Green Chemistry Journal, 1463-9262

Plastics Europe (2022). Plastics – The facts 2022. [Accessed 29/03/2023] (Available at: https://plasticseurope.org/knowledge-hub/plastics-the-facts-2022).

105. Solis, S. Silveira, 2020, Technologies for chemical recycling of household plastics – A technical review and TRL assessment, Waste Management, 105.