Comparing carbon-saving potential of the pyrolysis of non-recycled municipal plastic waste: Influences of system scales and end products

Pyrolysis of non-recycled municipal plastic waste has the potential to mitigate the plastic waste crisis and to produce transport fuels (diesel or hydrogen), as compared with the conventional methods such as incineration or landfilling that have potential adverse environmental consequences. Transport fuel production from non-recycled municipal plastic waste contributes to the transition to more sustainable waste management. However, little is known about the influences of system scales and end product selection on the carbon footprints of the plastic waste treatment. Here, we applied the approach of life cycle assessment to compare the carbon-saving potential of centralized, large-scale and decentralized, small-scale pyrolysis systems producing diesel and hydrogen in the United Kingdom. It is shown that centralized systems had a lower global warming potential results than decentralized systems despite their greater transportation distances. The global warming potentials of diesel production for centralized and decentralized systems were 801 and 1345 kg CO 2 -eq. per tonne of non-recycled municipal plastic waste, respectively, whilst hydrogen production had much higher global warming potentials which are 7110 and 7990 kg CO 2 -eq per tonne of non-recycled municipal plastic waste, respectively. In hydrogen-producing systems, crude carbon nanotubes are also produced, and the purification process of carbon nanotubes requires significant material resource (hydrochloric acid and deionized water) inputs, resulting in high Scope 2 and 3 emissions. Also, it is worth noting that the global warming potential related to the Scope 1 emissions for the diesel-producing systems exceeds 80%, whereas the hydrogen-producing systems yield opposite results. 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. Decentralized, small-scale hydrogen production from plastic waste had the highest global warming potential among all as the conversion process of municipal plastic waste to hydrogen and carbon nanotubes for small-scale systems is more energy and resource intensive. Sensitivity analysis showed that various factors, such as feedstock composition and the heating energy consumption in the conversion process, had less influence on the global warming potential of centralized systems as compared to decentralized systems.


Two birds with one stone: sustainable post-use of non-recycled MPW and decarbonization of the transport sector
One of the biggest global environmental challenges humankind faces is plastic pollution.To promote the transition from a linear economy to a circular economy and minimize plastic pollution, municipal plastic waste (MPW) needs to be recycled (Biakhmetov et al., 2022;Praveenkumar et al., 2024).Various initiatives and programs have been developed worldwide to improve plastics circularity.For example, 1000 organizations producing more than 20% of global packaging plastic worked together to boost recycling rate (Biakhmetov et al., 2023;Ellen MacArthur Foundation, 2020, 2023).EU has an ambition to achieve 55% plastic packaging recycling by 2030, while significant changes in MPW collection and sorting practices, plastic product design, and market-level interventions are necessary to achieve this goal (Antonopoulos et al., 2021).
Mechanical recycling dominates in MPW management in the EU (Lase et al., 2023).However, mechanical recycling is not applicable to all kinds of MPW and there are still significant amounts of MPW that cannot be mechanically recycled or, indeed, do not even undergo recycling at all.For example, in the UK, Municipal Solid Waste (MSW) is typically transported to Material Recovery Facilities to be sorted by waste fraction types.PET and HDPE bottles are easily separated and mechanically recycled.However, other MPW containing recyclable plastics is mainly sorted to make plastic bales, as it cannot be further separated to the required purity for mechanical recycling (Burgess et al., 2021).Non-recycled plastic waste is usually dumped in landfill or incinerated, both of which generate a high carbon footprint (Eriksson and Finnveden, 2009).Hence, we need to explore the use of alternative technologies to lower the carbon footprint of non-recycled MPW management.
One of the waste management methods that is considered more sustainable than landfill or incineration is the production of diesel from non-recycled MPW.To explore alternative pathways for non-recycled MPW management, it is important to adopt a case study approach to assess its real-world relevance and conduct in-depth analysis.For example, it allows to frame the context to scope 1-3 emissions pathways to obtain more accurate and inclusive results.For this study, Glasgow, one of the largest cities in UK and the largest city in Scotland, is chosen as the case study city.
Diesel shortages and soaring prices are amongst the major challenges experienced by many countries due to unstable geopolitical situations worldwide (Millard, 2022).In 2019, 3,216,360 tonnes of oil equivalent energy were consumed by the transport sector (Department for Business, 2021), with around 60% in the form of diesel in Scotland (Haig et al., 2018).INEOS in Grangemouth is the only crude oil refinery plant in Scotland, and it only serves a quarter of Scotland's road transport fuel demands (Haig et al., 2018); with the majority of fuel used in the transport sector being imported.It is worth noting that the majority of north-western Europe's oil refineries are configured to produce petrol rather than diesel, which further negatively affects Scotland's transport fuel security.Diesel production from non-recycled MPW could have a positive influence on Scotland's overall energy security and relieve its dependence on the import of diesel, whilst at the same time allowing a shift towards more sustainable MPW management.
Hydrogen can also be recovered from non-recycled MPW, which has the potential to contribute to Scotland's ambitious plan for a future hydrogen economy.The transport sector was the biggest emitter of greenhouse gases (GHG) in Scotland, with a net emission of 9.5 MtCO 2eq in 2018 (Scottish Government, 2022).As an overall strategy to reach the zero-emission target in the transport sector, the UK and Scottish Governments have decided to ban fossil fuel car sales by 2030 and 2032, respectively (Agency, 2019;UK, 2020).This means that non-fossil fuel vehicles working on electricity and hydrogen will play a greater role (Haugen et al., 2022;Manigandan et al., 2023).While electric vehicles are poised to continue dominating the market over hydrogen fuel cell vehicles, it is beneficial to broaden consumer choice in the proliferation of low GHG emission vehicles (Kim et al., 2020).Additionally, despite one of the main advantages of electric vehicles being their relatively higher energy efficiency in fuelling or a more established charging infrastructure, hydrogen fuel cell technologies applied in heavy-duty vehicles such as buses and trucks have the potential to compete with heavy-duty electric vehicles due to specific economic and mileage advantages.For instance, it was reported that a hydrogen fuel cell truck costed around $135,503-249,900, whereas an electric truck costed $164,641-585,000 (Cunanan et al., 2021).A full electric battery is enough for driving 62-500 miles, while 660-1104 miles can be fulfilled by a hydrogen fuel cell truck.Moreover, a hydrogen fuel cell truck has a lighter energy storage system than an electric truck, and thus potentially has a larger cargo weight.
Glasgow has already taken action to increase the number of low emissions vehicles, particularly those powered by hydrogen fuel cells.Twenty waste collection and transportation lorries fuelled by hydrogen are planned to be delivered to the Glasgow City Council (Glasgow City Council, 2021).Transport Scotland has provided funding of £805,000 to convert 23 winter gritters working on diesel to dual fuel hydrogen (Glasgow City Council, 2019).The Glasgow City Council has an ambitious plan to make all of its cars emission-free by the end of 2029 (Glasgow City Council, 2019).It is expected that the demand for hydrogen fuel in the transport sector in Glasgow, and Scotland and UK in general, will increase significantly, so that any shortage in hydrogen supply should be prevented so as not to affect the operational costs of hydrogen fuel cell vehicles.Hydrogen is typically produced from natural gas, which is characterised by a greater carbon footprint (Williams, 2020).The UK government has aimed to produce 10 GW of hydrogen annually from fossil fuel-free sources for the transportation and industrial sectors by 2030 (Department for Energy Security and Net Zero and Department for Business, 2022).The use of MPW to produce hydrogen could reduce the UK's and Scotland's dependence on fossil fuel resources and reduce the carbon footprint of hydrogen production and application.

Pyrolysis to generate diesel and hydrogen
There are various technologies that can be applied to treat MPW, namely pyrolysis, gasification, hydrogenation, etc. Pyrolysis has various advantages over the other technologies, one of the most important of which is its higher technology readiness level (Lee et al., 2021;Pires Costa et al., 2022;Spreafico et al., 2021;Thiounn and Smith, 2020;Vollmer et al., 2020).In a report by the United Nations Environmental Programme (2009), pyrolysis was stated to be one of the commercially available technologies enabling the conversion of plastic to energy.Moreover, catalytic pyrolysis is more tolerant to higher levels of MPW contamination than mechanical recycling (Ragaert et al., 2017).It is challenging for mechanical recycling to process mixed MPW which is less of an issue for the technology (Burgess et al., 2021).However, MPW typically contains PET and PVC, which could lead to corrosion issues and negatively impact the thermochemical conversion process, product quality, and even lead to secondary contamination if present at high levels.PET can be easily separated from other MPW fractions and then mechanically recycled, while MPW with a high PVC content can be directed to a dechlorination reactor operating at a low temperature (around 300 • C) to remove chlorine (Biakhmetov et al., 2023).MPW pyrolysis produces oil that can act as a substitute for crude oil in the production of polymers and diesel (Biakhmetov et al., 2023;Haig et al., 2018).Further, MPW can be treated by two-stage pyrolysis-catalytic reforming process at higher temperatures (500-800 o C) in the presence of metal-based catalysts to produce hydrogen-rich gas with carbon nanotubes (Acomb et al., 2016;Cai et al., 2021;Li et al., 2023).Consequently, hydrogen can be separated and used as a fuel for fuel cell electric vehicles.
In general, MPW management systems can be deployed in either centralized or decentralized modes.The main difference between the two is regarding the scale and efficiency, which affect their environmental and economic performance.Recently, the number of comparative LCA (Life Cycle Assessment) studies of centralized and decentralized systems have been increased to define their relative environmental footprints (Gupta et al., 2022;Quinteiro et al., 2020).In many cases, centralized systems are preferable due to their better economic benefits, but they can nevertheless have a worse or better environmental performance than decentralized systems depending on several factors.To the best of our knowledge, there have been limited studies on the influences of system scales and end product selection on the carbon footprints of diesel and hydrogen production from pyrolysis-based MPW treatment.The existing LCA of MPW pyrolysis and other waste management technologies focuses on large or centralized MPW.However, Pires Costa et al. (2022) mentioned in their study that large-scale centralized pyrolysis facilities should be compared with small-scale pyrolysis facilities built locally as there was still an argument as to how GHG emissions from transportation could affect the overall results.A large-scale waste pyrolysis facility is usually located some distance away from the city, and the density of plastic waste is low.In this case, the transportation of waste, pyrolysis products, and by-products for large-scale facilities may have a significant carbon footprint, as compared to small-scale facilities that can be located nearer to waste collection points.
The majority of existing studies have not considered the environmental burden of the end-of-life use of products resulting from plastic waste pyrolysis (Bora et al., 2020;Khoo, 2019).It is important to assess the quantity of products derived from plastic waste and the extent of the environmental burden/benefit that comes from using such products.Moreover, Pires Costa et al. (2022) highlighted that there were plenty of LCA studies where plastic waste pyrolysis was compared with incineration and landfill.It is well known that plastic waste pyrolysis can achieve better environmental performance than these waste management methods (Krüger et al., 2020).For instance, in the study by Haig et al. (2018), GWP of incineration of one tonne of MPW to produce electricity is around 2 tonnes of CO 2 , whereas GWP of diesel production via MPW pyrolysis is less than 0.3 tonne CO 2 per tonne of MPW.However, there is a knowledge gap in comparing the various types of plastic waste pyrolysis technologies with each other (Pires Costa et al., 2022).
To address the above-mentioned knowledge gaps, the global warming potential (GWP) of centralized and decentralized MPW pyrolysis plants for producing diesel and hydrogen was compared based on their development in Glasgow, UK using the approach of LCA.In this study, the scenarios for hydrogen and diesel production from MPW were illustrated in Section 2.1.Section 2.2 primarily describes the main phases of the LCA study, following which the results are provided in Section 3.1.The results are compared with those of existing studies in Section 3.2.Finally, a sensitivity analysis was conducted to assess the influences of various factors (feedstock composition, contamination, Plastic-to-Energy (PtE) conversion factor, transportation distance, pyrolysis, ratios of pyrolysis diesel in blended fuel, etc.) on GWP in Section 3.3.

Scenario description
Glasgow is the fourth largest in the UK with a population of 614,520 in 2021 (Population UK, 2023).It is also the largest city in terms of waste generation: 258,941 tonnes of MSW or 408 kg/capita/year were generated in 2021 (Scottish Environment Protection Agency, 2023).Overall, 8% of all MSW generated in Scotland is plastics, 5672 tonnes of which were recycled in Glasgow in 2021 (Scottish Environment Protection Agency, 2023).MPW that is not suitable for mechanical recycling usually ends up in landfill or is sent to other countries, amounting around 15,000 tonnes.In this study, plastics that are non-recycled or rejected by recycling processes are defined as a feedstock for pyrolysis systems.
Four scenarios are defined as illustrated in Table 1: C1, D1, C2, and D2.Scenarios C1 and C2 denote centralized plants, whilst scenarios D1 and D2 denote decentralized plants.The annual capacity of the centralized pyrolysis plant is ~15,500 tonnes of MPW, that is all nonrecycled MPW generated in Glasgow is transported to this site for treatment.The capacity of the decentralized plants is set to be 3300 tonnes per year, which corresponds to the capacity of typical small commercial-scale pyrolysis plants, as reported by (Haig et al., 2018).In scenarios C1 and D1, MPW transported to the plant is treated by pyrolysis to produce crude oil substitute, which is then distilled to produce oil containing diesel-range hydrocarbons.The main products in the C2 and D2 scenarios are hydrogen, carbon nanotubes and diesel, which are recovered by two-step pyrolysis-catalytic reforming in the presence of metal-based catalysts.Diesel and hydrogen produced from all scenarios are used as fuel for buses in Glasgow.

Goal and scope definition
LCA was conducted based on the ISO 14040 and 14044 standards, and proceeds in four sequential stages: goal and scope definition; inventory analysis; impact assessment; and interpretation (ISO, 2006a;ISO, 2006b).The goal of the LCA study is to ascertain and compare the environmental footprints of large-scale centralized and small-scale decentralized MPW pyrolysis systems.The scope of this study is illustrated in the Graphical Abstract, and it considers six stages, namely (a) In this LCA study, two different functional units (FU) were considered, namely, the treatment of 1 tonne of non-recycled MPW from a waste management perspective, and 100 km travelled by bus.

Life cycle inventory analysis
Life cycle inventory analysis defines the inputs and outputs (materials, energy, products, by-products, and emissions) of each stage involved in the systems, and a summary of them can be seen in Table 2. Data for foreground processes such as MPW sorting, pyrolysis, product purification, etc., were obtained from research papers and documents, whilst data for background processes, such as electricity and diesel generation and distribution, were obtained from GaBi software databases.The system boundary and mass and energy flows were simulated in the GaBi software and their balance for all four scenarios is shown in Fig. 1.

MPW collection, transportation and sorting systems.
MSW waste is collected every eight days (Glasgow City Council, 2023).It was assumed that a lorry working with diesel collects 10-15 tonnes of MSW per trip and the maximum mass capacity of such lorries is 14-16 tonnes or 30.6 m 3 (PE International, 2015).The density of MSW is 408.5 kg/m 3 , which is within the range of MSW densities stated in other studies related to waste collection and transportation (Jaunich et al., 2016).Overall, 25% and 75% of the diesel used for MPW collection is consumed in the idling and driving of automated side loader trucks, respectively (Jaunich et al., 2016).Approximately 17.7 million MSW bins are collected per year in Glasgow (Glasgow City Council, 2015).It can thus be calculated that the average weight of MSW collected per stop is around 10.57 kg, 8.3% of which is plastics.
The influence of distance between collection points, as per the study by Liljenström and Finnveden (2015) has a significant impact on scope 1 emissions.This data was not automatically collated for the Glasgow vehicles.Instead, we estimated the average distance between waste collection points by using the Distance tool in the ArcGIS Pro program.First, four different datasets for Glasgow were taken from the Digimap, namely neighbourhood areal classification, topography with road network data for Glasgow, OS Open UPRN that contains property and address information, and UK buildings (EDINA Society Digimap Service, 2011;2022a;2022b;2022c). Second, all four datasets were integrated into ArcGIS Pro, as illustrated in Fig. 2. The map resulting from the integration of the four datasets contains all dwellings and road networks for Glasgow.There are no data regarding waste collection points for Glasgow, and it was assumed that waste collection points for each dwelling are located along the closest road.The 'Generate Near Table (Analysis)' tool was used to locate the waste collection points on the closest road, and then 1000 collection points were randomly selected.The road distances between the randomly selected point and the closest collection point were received to calculate the mean value of these distances, which is 17.3 m.It is worth noting that selecting more than 1000 collection points does not significantly impact the mean value result.Also, there are some issues with property locations as some of them are located outside the building footprints in the ArcGIS pro map.Collection points selected for the analysis were visually assessed to address these issues.
PtE plants in the UK are usually located beyond the outskirts of cities (on average around 100 km away) (PE International, 2015); it is considered that the pyrolysis plant for the C scenarios is located 100 km away from Glasgow.For the C scenarios, prior to transportation to the pyrolysis facility, the MPW is bailed, with each bale's weight being around 400 kg (Haig et al., 2018).Overall, 48 MPW bales are fit into a lorry for transportation.
First, MSW collected is transported to the sorting facility, where MPW is separated from other waste fractions.The MPW composition was defined based on the dataset obtained from Zero Waste Scotland (2010) and Foster (2008), and is shown in Table 1A in the Appendix.Sorted MPW mainly contains PE (28.5 wt%), PP (22.2 wt%), and PS (4 wt%), which is favourable for the pyrolysis process; MSW also contains impurities (16.5 wt%) and undesirable plastics such as PET (15.3 wt%) and PVC (3.5 wt%) for the pyrolysis process (Krüger et al., 2020).The MPW composition, featured by the particularly high content of PE and PP plastics, medium content of PET, and low content of PS and PVC, aligns with other studies defining MPW composition for developed countries (Bodzay and Bánhegyi, 2016;Edjabou et al., 2021;López et al., 2010).Extra sorting is applied to separate polyolefin MPW from them.The efficiency of the sorting process and the composition of sorted non-recycled MPW were assumed based on the data for the Meilo sorting plant in Germany, as it is one of the widely applied sorting setups (Krüger et al., 2020).Sorted non-recycled MPW mainly contains PE, PS, PP, and a small amount of PVC (<0.5 wt%) and PET (<3 wt%).The pyrolysis technology vendors reported that pyrolysis is tolerant of MPW containing up to 10% wt.PVC and 15% wt.PET (Haig et al., 2018).In this study, the PVC and PET content in MPW is much lower than the recommended limits for pyrolysis.
The presence of unwanted materials in plastic feedstock can affect the quality of resultant products and the pyrolysis process itself (Borsodi et al., 2011;Jeswani et al., 2021).The impurities received after the extra sorting have lower calorific value compared to non-recycled MPW sent

MPW-to-energy production.
For all scenarios, the impurities obtained after the extra sorting are incinerated to produce electricity fed into the grid systems and heating energy used for local businesses or district heating.Incinerating one tonne of impurities produces 4.1 MJ of electricity and 12.15 MJ of heating energy (Jeswani et al., 2021).It is worth noting that heating energy and electricity generated from the impurities have carbon saving by displacing the heating energy generation from natural gas and electricity in the UK grids.Before the plastics are fed into the pyrolysis reactor, they need to be dried as a high content of moisture could cause various issues such as temperature reduction in the reactor, prolonging the pyrolysis process, and increasing the production of unwanted gas and oil (C.Chen et al., 2014).The heating energy consumed by the large (C1 and C2 scenarios) and small dryers (D1 and D2 scenarios) is 0.45 and 1.36 MJ/kg feedstock, respectively, with 5.3% of the feedstock mass being evaporated as moisture (Haig et al., 2018).
Dried MPW is fed into the pyrolysis reactor, where the feedstock in the pyrolysis reactor is broken down into lighter molecules in the absence of air (D.D. Chen et al., 2014).The main difference between scenarios C1/D1 (diesel production) and C2/D2 (hydrogen production) is that for the former scenarios, an oil distillation process is applied after pyrolysis, while for the latter, a catalytic reforming process is applied.For scenarios C1 and D1, dried MPW is degraded into less complex hydrocarbon chains in the form of vapor in the pyrolysis reactor.The condensable hydrocarbon molecules are then condensed and separated from gas and solid products.The liquid obtained after the condensation process is considered as crude oil substitute, which is subsequently pumped into the oil distillation system to produce diesel-like oil that can be mixed with diesel.The oil obtained after the distillation process is called diesel-like pyrolysis oil, as it contains hydrocarbon molecules within the conventional diesel range (C 5 -C 18 ) (Haig et al., 2018).The yields of diesel-like oil, gas, and solid products and residues are 71.81wt %, 10.03 wt%, and 18.16 wt%, respectively.Overall, 5.7 MJ of heating energy is consumed per kg of dried feedstock for the pyrolysis process in scenario C1, while for scenario D1, three times more heating energy is consumed (Haig et al., 2018).Also, the gas and char produced from the pyrolysis process and residue from the oil distillation process are combusted to produce the necessary heat for the pyrolysis plants, and thus B. Biakhmetov et al. have a carbon footprint.In scenarios C1 and D1, approximately 90 kg of pyrolysis char are produced per FU, 1 tonne of non-recycled MPW, and their combustion emits approximately 300 kg CO 2 -eq (Ahamed et al., 2020).It is worth noting that the pyrolysis process consumes more than 90% of the heating energy used in the plants, and the rest is consumed by the processes such as dryer, oil distillation, etc.
Two-stage pyrolysis-catalytic reforming of MPW in the presence of a metal-based catalyst is applied to produce hydrogen-rich gas and carbon nanotubes (Acomb et al., 2016;Prabu and Chiang, 2022).In the first stage, in the pyrolysis reactor, plastic feedstock is broken down into mixture of shorter hydrocarbon volatiles at 500 o C. In the second stage, hydrocarbon molecules volatiles are pumped into another catalytic reforming reactor, where they are decomposed and diffused on the nickel-based catalyst in an oxygen-free environment at a temperature of 700-800 o C (Biakhmetov et al., 2023;Li et al., 2023;Yao and Wang, 2020).Overall, around 2.5 tonnes of Ni-based catalyst is used per FU (1 tonne of non-recycled MPW) in the C2 and D2 scenarios, and GWP associated with it was assumed based on the study by Ahamed et al. (2020).The GWP of producing 1 kg of nickel-based catalyst is 1.54 kg CO 2 -eq, with half of them associated with direct emissions occurring during catalyst preparation.The main products of catalytic reforming process are hydrogen-rich gas (42 wt%) and carbon nanotubes (38 wt %).Additionally, a small amount of liquid product containing hydrocarbon molecules C 6 -C 22 is produced which is then distilled to achieve diesel-like oil quality (Cai et al., 2021).
The carbon nanotubes obtained from the process can be used in various ways, from serving as a fuel for heating and electricity generation, attributed to their high carbon content, to becoming value-added materials due to their unique characteristics such as extraordinary strength and stiffness, relatively high electrical and thermal conductivity, and chemical stability (Inshakova et al., 2020).To comply with circular economy practices and maximize resource recovery (Biakhmetov et al., 2022), carbon nanotubes are used as value-added materials instead of being employed as fuel for heating energy and electricity generation in this study.A number of studies indicate that simple carbon nanotubes produced from waste can be utilized as reinforcement to LDPE, resulting in improved tensile and Charpy impact properties (Borsodi et al., 2016), in the automotive and construction B. Biakhmetov et al. industries, as well as in the production of lithium-ion batteries (Dagle et al., 2017).However, carbon nanotubes obtained from MPW termed crude carbon nanotubes as they contain undesired amorphous carbons or can be polluted with metal-based catalysts (Guo et al., 2007).Thus, they can be purified and then modified to acquire properties close to required property depending on the application purpose (Wang et al., 2022).
Carbon nanotubes obtained from the catalytic reforming process are deposited on the nickel-based catalysts and need to be separated.An acid washing process is applied for this purpose, using hydrochloric acid (HCl) and deionized water as input materials (Griffiths et al., 2013).The main waste output of this process, wastewater, is sent to the wastewater treatment plant where Ni metals are easily recovered by adding calcium oxide (Ahamed et al., 2020).The acid washing process does not have any direct GHG emissions, but it is highly a material intensive process due to the HCl and deionized water inputs.Consequently, there are indirect GHG emissions associated with preparation of these input materials.Also, it is believed that the combustion of carbon nanotubes is less environmentally and economically friendly, because of significant resource inputs into the acid washing process for their production.If carbon nanotubes are used as value-added materials, there will be carbon saving by displacing fossil fuel-based carbon nanotubes production, positively affecting the overall GWP of non-recycled MPW management systems producing hydrogen and carbon nanotubes.
There is lack of data about the amount of energy used in two-stage pyrolysis-catalytic reforming process.This study considered data from other thermo-chemical technologies that are similar to the two-step pyrolysis-catalytic reforming considered in this study.Hydrogen-rich gas is the main product of two-stage pyrolysis-catalytic reforming, containing more than 30 wt% hydrogen, 20 wt% methane, and other hydrocarbon gases (C 2 -C 4 ).Hydrogen-rich gas is pumped to be treated by the polybed pressure swing adsorption (PSA) technology, where hydrogen is separated from other gases, which is used due to the following advantages: (a) it is the most commercially available technology to produce ultrapure hydrogen (99.99+%) (Luberti and Ahn, 2022;Meyers, 2016;Ruthven and Pressure, 1994;Voss, 2005;Yang, 1997), and (b) it has relatively high hydrogen recoveries (60-90%) (Ronald Long, 2011).Since not all hydrogen is recovered after the PSA process, the remaining gas still contains a proportion of hydrogen and methane, which is combusted to recover heating energy for auxiliary demands of the systems.Overall, 322.17 MJ of electricity is consumed by the PSA process to process 357.97 kg of hydrogen-rich gas produced from the pyrolysis-catalytic reforming process for scenarios C2 and D2 (Lui et al., 2022;Valente et al., 2019).The crude oil substitute produced from the two-stage pyrolysis-catalytic reforming process is distilled to produce pyrolysis diesel.

Distribution and end-of-life use of products.
In Scotland, a large share of fuel (petrol and diesel) for transport is sourced through INEOS's oil refinery located on the Firth of Forth in Grangemouth, Scotland, and it is assumed for the purpose of this study that distilled pyrolysis diesel, with a density of 850 kg/m 3 , is transported to this plant (Haig et al., 2018).The shortest and most optimal route to this oil refinery from Glasgow is via the M80 motorway, the distance for which is around 45 km.Mixed diesel from oil refinery plant is transported back to the fuel station in Glasgow, where buses are fuelled.
The pyrolysis oil obtained after the distillation process has properties very close to those of conventional diesel (Faisal et al., 2023).The diesel engine does not require considerable modification to use a mixture of conventional diesel and pyrolysis diesel produced from MPW.The most Fig. 2. Glasgow city neighbourhoods' territory with road networks, dwellings and waste collection points.
important factor is the effect of pyrolysis diesel on engine performance and emissions characteristics (Biakhmetov et al., 2023;Erdogan et al., 2019;Ramalingam et al., 2018;Sekar et al., 2022).The negative effects of pyrolysis diesel, such as poor combustion, knocking, or combustion noise, can be mitigated by modifying the blending ratio of conventional and MPW diesel.It should be stated that pyrolysis diesel produced from a mixture of plastics cannot fully substitute conventional diesel or be blended with conventional diesel at a high ratio (Biakhmetov et al., 2023).Also, the pyrolysis diesel obtained and then transported to the oil refinery plant could still contain some impurities and undesired hydrocarbon chains.However, this issue can be sorted out, as the oil refinery plant has the capability to adjust the composition of pyrolysis diesel or remove all undesired components through the purification or other processes it has (Haig et al., 2018).
The blending ratio of pyrolysis diesel in a blended fuel mainly depends on the composition of pyrolysis diesel.Pyrolysis diesel contains a higher amount of unsaturated hydrocarbons than conventional diesel (Biakhmetov et al., 2023).Combustion of diesel containing more double-bond hydrocarbons results in a higher heat release rate due to the higher dissociation energy required for breaking bonds (Mangesh et al., 2020).Thus, it could cause issues like lowering brake thermal efficiency, incomplete combustion, etc.The applicable mixing ratio of pyrolysis diesel and conventional diesel depends on the contents of saturated and unsaturated hydrocarbons, and blended diesel must meet the requirements for automotive diesel fuel standards, for example, in the EU, EN 590:2014+A1:201 (Biakhmetov et al., 2023).There are existing studies assessing the optimal ratio for applicable mixed diesel.Some found that pyrolysis diesel could substitute 40-60 wt% of conventional diesel (Januszewicz et al., 2023;Kalargaris et al., 2017).However, most existing studies recommended blending pyrolysis diesel with conventional diesel in a ratio of up to 15-20 wt% which does not significantly affect combustion characteristics in diesel engines (Das et al., 2020;Januszewicz et al., 2023;Mangesh et al., 2020;Rajak et al., 2022).
The hydrogen produced in scenarios C2 and D2 is used to support local hydrogen fuel cell electric buses in Glasgow.Additional energy is required to store and transfer hydrogen.Overall, 11.34 MJ of electricity was required per kilogram of hydrogen, with 4.14 MJ for compression and the remaining energy used for storage (Lui et al., 2022).The hydrogen produced, usually at around 10-20 bar pressure, is compressed to 200 bar for storing and transportation purposes (Lozanovski et al., 2011).The pyrolysis diesel produced in scenarios C2 and D2 is transported to the oil refinery located outside of Glasgow which is the same as scenarios C1 and D1.The fuel consumption part of the comparison study by Ally and Pryor (2016) is used to assess the carbon footprint of bus operations powered by fuel from MPW pyrolysis.The main by-product in all scenarios is ash received from the combustion process to produce heating energy.Ultimate and proximate analyses of MPW in many studies show that MPW contains a small amount of ash (Aboulkas and El Bouadili, 2010;Park et al., 2012;Rajendran et al., 2020;Sharuddin et al., 2017), which can typically deposit with carbon and other inert solid products at the bottom of the reactor during the pyrolysis process, which is considered a solid product.In this study, the overall solid product was combusted due to the high carbon content, and the ash was left as a by-product.Based on data released by the Scottish Environment Protection Agency (2015), the ash produced is defined as a non-hazardous inert material, and transported to landfill without any pretreatment (50 km road transport).

Interpretation and sensitivity analysis
This LCA phase includes a description of the final results and checks the completeness of the study as a whole.Also, breakdowns of GWP results based on the stages involved in the production of hydrogen and diesel, as well as scope 1-3 emissions, are described to provide a more inclusive and accurate results.Finally, sensitivity analysis is conducted to understand how some of the uncertainties affect the final results.Various uncertainties could affect the final GWP results, and some such that have profound impact were chosen.These are described below.-The distance from the transfer station to the MPW pyrolysis plant for centralized scenarios was assumed based on the distance reported in other studies (PE International, 2015).In the study by Haig et al. (2018), a few options for the locations of centralized MPW pyrolysis plants in Scotland were suggested, one of which was on the outskirts of Glasgow.This means there is no need for the transfer station, as any MPW collected can be transported directly to the pyrolysis facility.In another option, the MPW pyrolysis facility was located 230 km away from Glasgow.-The distance between MPW collection points.During the data collection about MPW collection and transportation, it was found that the average distances between collection points could vary.-MPW feedstock composition.Contamination of feedstocks with other materials such as glue, paint, dirt, food, and other inert materials reduces the proportion of useable plastic, which is the main source of any oil or gas produced.Inert materials are usually deposited onto the char produced (Williams et al., 2023).Also, polymer composition (some polymers such as PET or PVC) has negative effects on the conversion efficiencies of systems from feedstock to diesel or hydrogen fuel, whilst rigid plastics can be more suitable for the pyrolysis process than film plastics as the latter lead to the production of increased amounts of residue (Haig et al., 2018).In the sensitivity analysis, the upper and lower bounds for contamination and the content of undesired polymers (PET and PVC) for all scenarios are ±20%.This variation impacts the useable MPW content for the thermochemical conversion process, as well as the energy usage for removing contamination and the content of undesired polymers.-The efficiency of hydrogen recovery from the gas produced.In this study, the maximum hydrogen recovery from PSA (90%) was considered.However, the efficiency of hydrogen recovery from PSA can vary from 60% to 90% (Ronald Long, 2011), and affects the overall GWP.-Heating energy used for the conversion process.There are many factors affecting the amount of heating energy required for the conversion process such as the scale of reactors, conversion efficiency, etc.In Haig et al. (2018), it is noted that the heating energy can vary depending on the scale of the facility.The variations of the input heating energy are ±10%.
After defining the upper and lower bounds of the various uncertainties, Monte Carlo simulations were used to assess their impacts on the GWP results.Triangular distributions were used for the analysis (Doubilet et al., 1985).Overall, 1000 iterations were run for each uncertainty factor.Thereafter, the percentage change in GWP for each uncertainty factor was calculated for all four scenarios using Eq.(1).
where GWP main is the main result of the scenarios, and GWP mont.car. is the maximum or minimum values of the results from the Monte Carlo simulation for each uncertainty factor.Despite pyrolysis diesel having properties close to fossil fuel-based conventional diesel, it cannot fully substitute conventional diesel (Biakhmetov et al., 2023).However, it can be blended with conventional diesel in ratios of up to 20 wt% without compromising the combustion efficiency, as described in the previous subsection.In the sensitivity analysis, the impact of the blending ratio of pyrolysis diesel produced from the C1 and D1 scenarios on the GWP of producing 1 tonne of applicable diesel fuel was assessed.

Environmental impacts of four scenarios
The results of GWP calculations for all scenarios are illustrated in Fig. 3a (FU =  The GWP of MPW collection, transportation, and sorting systems for the D scenarios is 22.51 kg CO 2 -eq.per tonne of MPW, whilst the GWP for the centralized systems is 1.5 times higher than for the decentralized systems.For scenario C1 and C2 systems, a higher carbon footprint can be explained by the fact that the MPW feedstock is transported to MPW pyrolysis facilities that are located 100 km away from the city.Also, MPW sorted from other waste fractions needs to be bailed before transportation.Fig. 4 (a) shows that the carbon footprint associated with MPW collection, transportation, and sorting systems is 4.13% overall for scenario C1 and around 1.67% for scenarios D1 and followed by 0.46% and 0.28% for scenarios C2 and D2, respectively.In Fig. 4b, scope 1 emissions for centralized MPW collection, transportation, and sorting systems are 33.22%,while they are 16.41% for decentralized MPW collection, transportation, and sorting systems.This difference can be explained by direct GHG emissions from driving truck from the transfer station to the pyrolysis facility.
The MPW-to-energy production stage in Fig. 3 includes incineration of impurities from the extra sorting process, feedstock pretreatment, pyrolysis, and oil distillation for scenarios C1 and D1, whilst for scenarios C2 and D2 it includes incineration of impurities, feedstock pretreatment, two-stage pyrolysis-catalytic reforming, oil distillation, PSA and acid washing.Hydrogen production is more energy intensive than diesel production and, consequently, its environmental footprint is much higher.The pie charts displayed in Fig. 4 (a) indicate the share of each stage involved to produce hydrogen and diesel with respect to the overall carbon footprints.In Fig. 4 (a), the GWP proportion of MPW-toenergy production is also much higher than other parts, namely (a) MPW collection, transportation, and sorting, and (b) product and byproduct distribution.It is worth noting that GHG emissions from MPW-to-energy production are ordered from low to high as C1, D1, C2, and D2.The share of scope 1 emissions from MPW-to-energy production is over 90% for the C1 and D1 scenarios, while the C2 and D2 scenarios have much lower scope 1 emissions, with indirect GHG emissions being dominant.The plastic-to-hydrogen and carbon nanotubes conversion process is not only highly energy-intensive but also demands other resources (catalyst, deionized water, HCl, etc.) in large quantities, resulting in high indirect GHG emissions.
The GWP of the distribution of products stage is less than that of the MPW-to-energy production stage and higher than that of the MPW collection, transportation, and sorting stage.Also, it is worth noting that the GWP of distribution of products and byproducts for scenarios C2 and D2 is almost 7 times higher than scenarios C1 and D1 as storage and transportation of hydrogen is more energy and resource intensive as compared to diesel.Moreover, wastewater sent to the wastewater treatment plant is processed by adding quicklime to recover metals, resulting in high shares of scope 1 and 2 emissions for the C2 and D2 scenarios.
The end-of-life-use and displacement stages are significant as they change the overall results of this study.The end-of-life-use of hydrogen and diesel produced for all scenarios are fuels for public transport -  B. Biakhmetov et al. buses.Scenarios C1 and D1 produce diesel to drive 1107.71km each of them and their GWP is 1915.6 kg CO 2 -eq.per tonne of MPW.Diesel and hydrogen produced in scenarios C2 and D2 are enough to drive 1094 km, their GWP is 479.92 kg CO 2 -eq.per tonne of MPW.It is worth noting that the results are slightly changed (by 1.2%) when the FU of 100 km driven by bus is applied.These results highlight the fact that it is meaningful to assess the environmental footprint based on the consideration of different types of FUs for these kinds of studies.
The carbon saving associated with the displacement of fossil fuelbased diesel, hydrogen, carbon nanotubes, and energy production further improve the environmental performance of the systems, as detailed in Table 3.The GaBi database for the UK was used to calculate the carbon saving for diesel, electricity, and heating energy displacement.However, since there was a lack of specific data on the hydrogen production from fossil fuels for the UK, the hydrogen production from natural gas was simulated in the GaBi software.The carbon saving by the production of carbon nanotubes to displace fossil fuel-based carbon nanotubes was calculated based on the study by Temizel-Sekeryan et al. (2021).The carbon saving obtained from the heating energy displacement for the C1 scenario is the highest compared to other scenarios.Overall, in the C1 scenario, 7329 MJ of heating energy is produced per FU (1 tonne of non-recycled MPW) from the waste product on the pyrolysis site, which is much higher than the consumed heating energy for the thermochemical conversion process (5660 MJ).In other scenarios (D1, C2 and D2), the combustion of waste products (pyrolysis char, pyrolysis gas, and oil distillation residue) does not produce enough heating energy for the whole systems, and as a result, natural gas is combusted to fill the deficiency.For all the scenarios, there is carbon saving from displacing grid heat with the heating energy obtained from the incineration of sorting process residues as shown in Table 3.The carbon savings by displacements for C2 and D2 scenarios are 10 times of those for C1 and D1 scenarios, mainly attributed to significant emissions from the production of hydrogen and carbon nanotubes from fossil fuels that are displaced.
Overall, centralized systems show better environmental performance than decentralized systems.Notably, D2 shows the worst net environmental performance (3376 kg CO 2 -eq.per tonne of MPW), whilst C1 indicates the best net environmental performance (2114 kg CO 2 -eq.per tonne of MPW).Despite that the hydrogen and diesel production stage for scenarios C2 and D2 has higher GWP than the diesel production stage for scenarios C1 and D1, the former scenarios outperformed the latter scenarios in terms of carbon saving by displacements overall.It can be explained that diesel buses emit GHG while hydrogen fuel cell buses do not.Also, the carbon saving by the displacement of fossil fuel-based hydrogen using the hydrogen from the pyrolysis is greater than that from the GWP reduction by the displacement of fossil fuel-based diesel using the diesel from the pyrolysis, and the carbon saving from carbon nanotubes significantly improve the environmental performance of C2 and D2 scenarios.
The GWP results for all scenarios are positive, but they could turn to negative values if changes are applied.For example, centralized systems can be located as close as possible to the transfer station, which reduced the GHG emission associated with the non-recycled MPW transportation.Alternatively, the carbon dioxide emitted from thermochemical processes can be captured and stored in geologic formations, which can further reduce the overall GWP.These possibilities could be addressed by future studies.This LCA study exclusively focuses on comparing the GWP of hydrogen and diesel production from MPW as pyrolysis-catalytic reforming is an energy-intensive technology that can have a profound impact on GWP.However, GWP abatement is not the only environmental impacts that are relevant to the deployment of the technology.It is recommended future research can explore other impact categories, such as acidification, eutrophication, PM2.5, water depletion, etc., based on similar system boundaries defined in this study.The GWP was 265.8 kg CO2-eq.per tonne MPW which is higher than that reported in this study: where the difference can be explained by transportation and feedstock properties.The MPW collected was transported 900 km to the pyrolysis plant; polymer feedstock contained more contamination and undesired polymers such as PET and PVC, which affected the transportation and conversion efficiency of feedstock into fuel.Li et al. (2022) Non-recycled pharmaceutic surgical plastic masks Non-catalytic pyrolysis 798 kg pyrolysis oil, 168 kg gas and 27 kg char The experimental data regarding pyrolysis process was used.The transportation of feedstock, products, and by-products were excluded, and natural gas was used to produce the heating energy for pyrolysis conversion process.There were no processes run to purify the oil produced by pyrolysis.Khoo (2019) 30,000 tonnes of MPW (40%

Comparison with existing studies
A number of LCA studies that considered similar systems and configurations are chosen for comparison, as shown in Fig. 5, with the main findings of this comparison summarized in Table 4.It is worth noting that, despite similarities, the selected studies have different system boundaries, data sources, etc. in general.An attempt is made to identify the factors responsible for the differences between the various results and findings that would be interesting for future studies.
The GWP results for scenarios C1 and D1 are compared to similar existing studies that consider where plastic feedstock goes through the pyrolysis process to produce pyrolysis diesel or oil that can be blended with conventional diesel.The existing studies' results are adapted for comparison with this study: the FU applied for the purposes of the comparison is kg CO 2 -eq.per tonne plastics.For scenarios C2 and D2, a two-stage pyrolysis-catalytic reforming system is considered, and its main products were hydrogen, carbon nanotubes and diesel.To the best of our knowledge, there are no existing LCA studies that consider the systems and configurations similar to scenarios C2 and D2.There is only one study assessing the environmental footprint of two-stage pyrolysiscatalytic reforming of flexible packaging plastic waste, where the main products are oil, hydrogen-rich gas, and multi-walled carbon nanotubes (Ahamed et al., 2020).In this study, oil is produced, and oil purification processes are not included; the hydrogen-rich gas is considered as a fuel for heat recovery, and hydrogen separation and cleaning processes are excluded as well.Hence, the results of C2 and D2 are not comparable with that of Ahamed et al. (2020), and they are not included in the comparison.
Overall, most of the other studies did not explore the influences of pyrolysis configuration and parameters towards GWP.Also, the scale of pyrolysis plants has a profound impact on the environmental performance and conversion efficiency of plastic feedstock to products, and most of existing studies described in this sub-section did not have information about pyrolysis plants' scales.Only a few studies accounted for feedstock collection and transportation stages.This study shows that it is important to include MPW collection and transportation in the assessment as it can account for up to 4-5% of overall GWP for largescale pyrolysis systems producing diesel.

Sensitivity analysis
The impacts of various factors/parameters on GWP for all four scenarios are assessed as shown in Fig. 6.It is shown that the uncertainty factors have a greater influence on decentralized systems rather than centralized ones.For example, the upper and lower bounds from the influence of heating energy for the C1 scenario are around ±4% whilst they are around ±10% for D1 scenario.Also, it is worth noting that uncertainty factors related to the transportation have a greater impact on C1 and D1 scenarios compared to C2 and D2 scenarios.
The influence of the ratio between pyrolysis diesel and conventional diesel in the blended fuel towards GWP was further assessed, and its results are illustrated in Fig. 7.The GWP of producing 1 tonne of diesel from fossil fuel is 645.2 kg CO 2 -eq., while the GWP of producing 1 tonne of pyrolysis diesel for C1 and D1 scenarios are 431.6 and 1471.8 kg CO 2eq., respectively.Thus, the GWP of the blended fuel that contains pyrolysis diesel from the large-scale system is lower than the GWP of fossil diesel, while the fuel containing pyrolysis diesel from the D1 scenario shows the opposite.Specifically, the GWP of producing 1 tonne of the fuel that contains 20% pyrolysis diesel from the C1 system and 80% conventional diesel is 602.4 kg CO 2 -eq., which is about 75% of the GWP of producing 1 tonne of fuel containing 20% pyrolysis fuel from the D1 system.It means that the blended fuel containing the pyrolysis diesel produced from the large-scale system has a lower carbon footprint than the fuel containing the pyrolysis diesel from the small-scale system.

Limitations and future studies
A number of gaps in the current knowledge are identified that shall be addressed by future research.
-There is a lack of data about the hydrogen production from industrial scale pyrolysis-catalytic reforming of MPW (Lui et al., 2022).Most of the studies are lab-scale, and there are only a few pilot-scale studies (Sharma and Batra, 2020).Accordingly, lab-scale data was used for the C2 and D2 scenarios in this study due to the lack of pilot and industrial-scale data.Pilot-scale studies of hydrogen production from MPW pyrolysis-catalytic reforming are needed for more accurate and comprehensive environmental assessment of the development.-There are a number of LCA studies on diesel production from MPW that used data provided by the commercial and industrial waste management sector (Haig et al., 2018;Krüger et al., 2020).To the best of our knowledge, there were no LCA studies of two-stage pyrolysis of MPW producing hydrogen.A widely used MPW pyrolysis configuration, i.e. two-stage pyrolysis-catalytic reforming in the presence of a nickel-based catalyst) was considered in this study.
There are many other MPW pyrolysis configurations or designs that can be used to produce hydrogen.For example, steam can be introduced at the second stage of the process (Yao et al., 2018;Zhang et al., 2017), and there are alternative catalysts can be used to increase gas yield and its hydrogen content (Yao et al., 2021).Indeed, there is little information about the environmental footprints of the other pyrolysis designs.Further LCA studies are necessary to explore the influences of pyrolysis configurations on hydrogen production and ultimate environmental impacts.-This study considered the average available volume and feedstock composition (non-recycled MPW).However, these parameters can vary on a daily, and even seasonal basis (Lui et al., 2022).More studies are needed to define feedstock volume and composition variations and their impacts on the environmental footprints.For example, MPW feedstock could contain multilayered and composite plastics, and it is difficult to identify and separate them (Bassey et al., 2023).Composite plastics usually have residues, paper, metals, and halogens, which can negatively affect the efficiency of the pyrolysis process and its products (Kusenberg et al., 2022).-This LCA study compared the GWP of centralized and decentralized hydrogen and diesel production from non-recycled MPW.The transportation part has a relatively small GWP impact compared to thermochemical conversion or product upgrading processes.However, there is little information about the techno-economic feasibility of centralized and decentralized systems.Optimisation can be done for both centralized and decentralized systems that achieve a balance between environmental and economic benefits.In such optimisation studies, social impacts could be considered, contributing to a more comprehensive understanding of the socio-economic and environmental impacts of the development.

Conclusions
This study investigated the GWP of centralized and decentralized pyrolysis systems that convert non-recycled MPW to diesel and/or hydrogen.Centralized, large-scale systems producing hydrogen and diesel show better environmental performance than decentralized, small-scale systems.The GWP of hydrogen production is much higher than that for diesel production but the compensation from the end-of-life use of fuels, and the displacement of their production from fossil fuels, as well as heating energy, electricity, and carbon nanotubes, significantly affect the overall GWP results.Decentralized hydrogen production shows the highest GHG emissions (7989.6 kg CO 2 -eq.per tonne of feedstock).Sensitivity analysis shows that centralized systems are less influenced by uncertainty factors compared to the decentralized ones.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Table 2A
Various inputs and outputs for all four scenarios considered in detail.
No. (corresponding to the processes illustrated in Fig. 1 to the PtE facilities.However, they contain a relatively high carbon content and can thus be sent to an incineration plant to recover electricity and heating energy.For D scenarios, it was assumed that the extra sorting facilities are near to waste incineration facilities around Glasgow (hence, transportation is minimum and not considered), whilst the impurities obtained from the extra sorting are transported to the incineration plant located 50 km away from the extra sorting facilities for C scenarios.The outputs and inputs of incineration are described in detail in the next subsection.The GWP burden allocation of the MSW sorting process was performed with regard to the mass of sorted waste streams(Civancik-Uslu et al., 2021).

Fig. 3 .
Fig. 3. a) GWP for each scenario, where the FU is 1 tonne of MPW; b) GWP for each scenario, where the FU is 100 km travelled by bus.

Fig. 4 .
Fig. 4. The GWP breakdown of the four scenarios based on (a) the stages involved in the production of hydrogen and diesel, and (b) scope 1-3 emissions.
1 tonne of feedstock) and Fig. 3b (FU = 100 km travelled by bus).Each scenario includes four parts: (a) MPW collection, transportation, and sorting systems, (b) MPW-to-energy production, (c) distribution and end-of-life-use of products and byproducts; and (d) displacement of fossil fuel-based diesel, hydrogen, heating energy and electricity.

Fig. 5 .
Fig. 5. Comparison of GWP results with those of other existing studies (FU is kg CO 2 -eq.per tonne feedstock).

Fig. 6 .
Fig. 6.Sensitivity map illustrating the impact of factors on GWP ((a) FU is 1 tonne of MPW; (b) FU is 100 km travelled by bus).

Fig. 7 .
Fig. 7. Impact of the blending ratio of pyrolysis diesel produced from C1 and D1 scenarios on GWP of producing 1 tonne of blended fuel.

Table 1
Scenario description in terms of waste transportation, PtE technologies, product distribution and annual capacity.

Table 2
Summary for various inputs and outputs based on FU, 1 metric tonne of MPW for all four scenarios considered.

Table 3
The carbon (GWP) saving by the displacements of heating energy, electricity, diesel, hydrogen and carbon nanotubes per FU (1 tonne of non-recycled MPW) for all scenarios.

Table 4
Comparison between the results of this study and those of other existing ones. )