title: Comparison of centralized and decentralized municipal plastic waste management producing value-added resources: economics, environmental impacts and optimisation
Creators: name: Biakhmetov, Bauyrzhan
Publisher: name: University of Glasgow
Year: 2025

description: Plastic waste is a global issue that severely threatens the environment if not managed properly. Municipal plastic waste is widely treated in unsustainable ways such as landfill and incineration that generally do not contribute to the circular economy or to the principles of the UN’s sustainable development goals (SDGs). Mechanical recycling is not able to handle all municipal plastic waste generated due the technological limitations. Chemical recycling, specifically pyrolysis, is considered an alternative solution or supplementary to mechanical recycling because of its potential to recover fuels and chemicals from non-recycled municipal plastic waste.

In this study, large-scale centralized and small-scale decentralized diesel and hydrogen production from NMPW (non-recycled municipal plastic waste) using pyrolysis-based thermochemical conversion technologies were compared in terms of environmental footprint and economic feasibility, specifically focusing on GWP (Global Warming Potential) and NPV (Net Present Value). Glasgow was chosen as the case city for this study. LCA (Life Cycle Assessment) was applied to evaluate the GWP of all systems. The results showed that centralized systems had lower GWP compared to decentralized systems, despite their greater transportation distances. The GWPs of diesel production for centralized and decentralized systems were 801 and 1,345 kg CO₂-eq per tonne of NMPW, respectively. Hydrogen production, however, had much higher GWPs of 7,110 and 7,990 kg CO₂-eq per tonne of NMPW for centralized and decentralized systems, respectively. The end use of diesel produced has a greater carbon footprint than the end use of hydrogen. The carbon saving from the displacement of fossil hydrogen was two times higher than that from diesel displacement. After considering the end use of products and displacement, the net GWP of large-scale hydrogen production is 2,496.53 kg CO₂-eq per tonne of NMPW, which is better than the net GWP of small-scale diesel production (2,766.3 kg CO₂-eq per tonne of NMPW) and close to the net GWP of large-scale diesel production (2,114.44 kg CO₂-eq per tonne of NMPW).

After completing the assessment of the environmental footprint, in Cost-Benefit Analysis (CBA), the economic feasibility of centralized large-scale and decentralized smallscale diesel and hydrogen production systems from NMPW was compared by defining their NPVs. Across all scenarios, only centralized large-scale diesel production, with and without carbon capture and storage, exhibited total positive net present values (£22,240,135 and £24,449,631, respectively), indicating their economic feasibility. The decentralized small-scale hydrogen production system with carbon capture and storage yielded the lowest net present value result (-£2391) per tonne of treated non-recycled municipal plastic waste. Particularly, the production of diesel and hydrogen from non-recycled municipal plastic systems, with carbon dioxide emissions to the environment, demonstrated better economic performance than the same systems capturing and storing carbon dioxide, attributable to its higher capital and operational expenditures. Also, sensitivity analysis revealed that the fuel sales price and OPEX had the most significant impact on the net present values.

In the MOO (Multi-Objective Optimization) study, 900 diesel and hydrogen-producing scenarios from NMPW were developed, and the data thus generated was then used for inventory analysis to calculate their GWPs and NPVs. After that, the long short-term memory recurrent neural network was applied to define temporal dependencies and dynamics of systems, which was integrated with Monte Carlo simulations of variables to expand scenarios from 900 to 700,000 and to predict their GWPs and NPVs. Finally, a Pareto front was derived from the GWPs and NPVs, from which the best scenarios in terms of balance between environmental and economic performance was identified using the TOPSIS and LINMAP approaches. The TOPSIS approach defined a scenario that aligns perfectly with the ideal scenario, achieving the lowest GWP (-2570.42 kg CO₂-eq. per tonne of NMPW) and the highest NPV (£300,315.65 per tonne of NMPW). This demonstrates that the TOPSIS method effectively balances environmental and economic performance of NMPW management system. In contrast, the LINMAP approach obtained a less optimal scenario, with a moderate GWP reduction (-1025.28 kg CO₂-eq. per tonne of NMPW) and a negative NPV (-£1,402.92 per tonne of NMPW). This means that the TOPSIS approach is recommended for selecting optimal scenarios, as it provides the best balance between environmental and economic performance for NMPW management systems utilizing pyrolysis-based thermochemical conversion technologies.

Based on the LCA, CBA, and MOO studies, several recommendations were developed for practical applications, and a few of them are worth highlighting. Currently, among the scenarios considered in this study, only diesel production from NMPW in a large-scale plant is economically feasible and can achieve a negative GWP if a CCS unit is applied. Additionally, pyrolysis plants should be located close to feedstock collection sources to reduce transportation costs and minimize environmental impact. When selecting between large-scale NMPW systems and small-scale systems utilizing pyrolysis-based thermochemical conversion processes, the large-scale option is recommended due to its superior environmental and economic performance.