Cities are desperate for ways to stop plastics piling up in landfills, sewers and the sea. A growing body of research — paired with recent pilot projects and hot local debates — shows pyrolysis (a thermal process that turns mixed plastics into oil, gas and solids) can play a limited but useful role if planners model it carefully, set strict rules, and pair small facilities with better sorting, reuse and reduction policies. This article explains what the models show, what real pilots reveal on the ground, and how cities can judge whether a local, small-scale pyrolysis unit belongs in a responsible waste plan. According to the United Nations Environment Programme, municipal waste is rising fast, and cities must choose technologies that fit wider circular-economy goals.
What the Modelling Actually Says — and What it Doesn’t
Modellers who examine local waste flows and pyrolysis chemistry are clear about two things: results depend heavily on the feedstock, and small units can only ever process a fraction of a city’s plastic if the city aims to manage most waste sustainably. A report by the U.S. National Renewable Energy Laboratory (NREL) and related technical papers found that pyrolysis tolerates contaminated, mixed plastics better than mechanical recycling, but only a very small share of the plastic entering a pyrolysis plant typically returns as product that can be used again as plastic — often because much of the output becomes fuels or other non-circular streams. According to NREL, only about 0.1–6% of the plastic entering a typical pyrolysis process may end up as recycled plastic suitable to re-enter the consumer chain, depending on feed type and downstream processing.
Academic modelling at both laboratory and pilot scale supports this: researchers simulating mixed-plastic feedstocks show that yields and oil quality vary significantly with the share of PVC, PET, food-contaminated films, and multilayer packaging. These variations affect both the economics and the emissions profile. Small modular units—such as shipping-container-style reactors often proposed for urban deployment—have different energy balances compared with large integrated plants because they cannot capture economies of scale or share advanced downstream refining.
Studies emphasise that effective local models must connect three layers: waste collection and sorting performance, reactor yield curves by feed composition, and the city’s intended use of the pyrolysis oil (whether refining it into virgin polymer, burning it for energy, or exporting it). These system-level links ultimately determine whether a local unit reduces landfill burden and carbon emissions, or merely shifts pollution elsewhere. According to recent peer-reviewed studies, detailed mixture-to-product modelling is essential for accurate local planning.
Real-World Pilots Under the Microscope
Models meet politics in neighbourhoods. Small pilots are springing up around the world; some are praised as innovations, while others face community pushback. A vivid recent example is a small modular plant in Rohnert Park, California, where the local startup Resynergi proposed a shipping-container-style pyrolysis unit to process local film and bag waste. Neighbours staged protests at the farmers’ market and at City Hall, worried about emissions near a school. City and air-quality agencies carried out technical reviews and public consultations.
One nearby teacher told reporters, “My neighbours are going crazy — they’re protesting at the farmers’ market,” illustrating how easily local trust can erode when residents feel left out of planning. The same reporting noted both company explanations about engineered emission controls and regulators’ draft findings that limits would be met — a reminder that community acceptance is as crucial as technical compliance. According to the San Francisco Chronicle, residents, parents, and environmental groups openly challenged the project in public meetings.
Contrast that local controversy with another, larger-scale example: Mura Technology’s commercial plant in the UK uses a supercritical water process (different from conventional pyrolysis) and aims for higher conversion efficiencies. Company spokespeople claim conversion rates in the high 80s to low 90s, and lifecycle analyses show lower warming potential than some pyrolysis routes. The lesson for cities is practical: different thermal technologies produce very different outputs, and claims of high circularity require independent verification before a city relies on them to meet targets. According to Reuters reporting on Mura’s technology, lifecycle studies by the European Commission’s Joint Research Centre found materially lower global warming potential than standard pyrolysis for that specific process.
There is also a strong body of critical literature. Environmental groups and independent analyses have documented cases where pyrolysis facilities generated hazardous waste or shipped large quantities of residues for disposal; some facilities even closed after encountering problems. A 2025 briefing by NRDC found that several U.S. pyrolysis/chemical-recycling facilities produced large amounts of hazardous waste between 2021 and 2024 and warned that, in practice, pyrolysis often yields fuels and residues rather than materials that truly close the plastic loop. The report suggested the need to model end-use pathways for pyrolysis outputs, to demand transparency on hazardous residues, and noted that pyrolysis facilities in the U.S. have sent millions of pounds of hazardous waste off-site in recent years.NRDC
How a City Should Use Modelling to Decide
If a city asks whether a small, mixed-plastic pyrolysis unit “fits” its waste strategy, here are the practical, model-driven steps planners should insist on — not speculation, but concrete modelling and rules a city can enforce.
First, require a transparent material-flow model that starts at the kerb: how much mixed plastic will be reliably collected, what contamination rates are realistic, and what will be diverted to reuse and mechanical recycling before anything heats up. System-level analyses show that interventions that reduce and separate waste at source can dramatically change whether pyrolysis makes sense economically and environmentally.
Second, demand a feed-to-product mass-balance model from the operator that shows realistic yields across the actual local feed mix — not idealised laboratory plastics. The model should disclose expected shares of oil, gas, char, and hazardous residues, and show where each fraction will be sent (local refinery, export, incinerator, landfill). An independent technical review should be part of the permit. Peer-reviewed mixture modelling shows that yield curves and oil quality depend on small changes in feed composition; planners who treat pyrolysis as a monolithic technology will be surprised by the results. Researchers note that modelling oil composition from mixed feeds is complex but feasible and must be done to inform decisions.
Third, require a binding plan for end uses and emissions: will the oil be upgraded into polymer feedstock, or burnt as fuel? If the latter, then counting that output as “recycling” is misleading and will likely worsen the city’s greenhouse gases (GHGs). The NREL framework shows how relative climate outcomes change when pyrolysis oil displaces crude oil versus when it replaces mechanical recycling — modelling both scenarios is essential. Planners should require life-cycle and health risk assessments as part of the approval process. Analyses suggest that climate and circularity outcomes vary widely depending on how the pyrolysis oil is used.
Fourth, bind operators to strict transparency, monitoring, and community engagement: continuous emissions monitoring, third-party audits of hazardous residues, and clear transport plans for any off-site wastes are non-negotiable. The Rohnert Park example shows that technical permits alone do not substitute for local trust; community concerns can and should be addressed before operations begin. Local pushback centred on proximity to schools and a perceived lack of early consultation.
Finally, use pyrolysis only as one small tool in a broader strategy that prioritises reduction, reuse, and mechanical recycling where feasible. UNEP’s modelling of municipal waste futures shows that cities that commit to circular systems gain economically and reduce overall harm; thermal routes should be complementary and tightly governed, not the city’s main “fix” for plastic. Combining prevention and better management limits long-term costs and offers net gains compared with business-as-usual.
Conclusion — Practical Advice for Mayors and Planners
Model first, permit conditionally, and treat small pyrolysis units as laboratory-backed pilots rather than finished solutions. Require: (1) an open, peer-reviewed material-flow model tied to local collection rates; (2) a feed-composition–to–product-yield model with independent validation; (3) lifecycle and health-risk assessments; and (4) legally binding clauses on monitoring, residue management, and community engagement. Where cities have applied these steps, pilot projects have been easier to evaluate and controversial surprises have been rarer. Where the steps were skipped, projects often triggered community protests or regulatory re-examinations.
Balance honesty about limitations—most pyrolysis plants today produce fuels and residues, not large volumes of recycled plastic—with readiness to test verified innovations that genuinely increase material circularity. Recent studies and reporting make this clear: modelling shows potential, but only strict rules and public transparency can turn that potential into safe, useful municipal practice. According to work by NRDC, NREL, and UNEP between 2023 and 2025, the right approach is careful, model-led, and precautionary.
