Defining the two thermochemical pathways

Pyrolysis and gasification are both thermochemical processes that use heat to break down complex polymer chains in plastic waste into simpler molecules — but the presence or absence of an oxidising agent is the single most important factor that distinguishes them. Pyrolysis operates in a strictly oxygen-free (inert) atmosphere, while gasification deliberately introduces a controlled, sub-stoichiometric quantity of an oxidant such as oxygen, steam, or air.

This seemingly simple difference — the presence or absence of oxygen — cascades into entirely different reaction chemistries, reactor designs, operating temperatures, and end products. In pyrolysis, thermal energy alone drives the cracking of polymer chains through endothermic decomposition reactions. In gasification, partial oxidation reactions provide some of the process heat internally, while reforming reactions convert carbonaceous intermediates into the target syngas mixture of hydrogen (H₂) and carbon monoxide (CO).

A third thermochemical route, combustion, involves complete oxidation and produces only heat, carbon dioxide, and water — no recoverable chemical products. Pyrolysis and gasification are therefore distinguished from combustion by their ability to recover material value, not just energy value, from plastic waste streams. This distinction is central to their positioning within advanced recycling frameworks, as recognised by organisations including OECD in its analyses of plastic circularity.

How process conditions shape product outcomes

Temperature, heating rate, residence time, and reactor configuration are the four primary process variables that determine product distribution in both pyrolysis and gasification — and they operate differently in each pathway. In pyrolysis, these variables allow engineers to tune the process toward three distinct operating modes, each producing a different dominant product.

Pyrolysis operating modes

Slow pyrolysis (also called carbonisation) operates at lower temperatures, typically below 400 °C, with long residence times, and favours the production of solid char. Intermediate or conventional pyrolysis at 400–600 °C with moderate residence times produces a balanced mix of oil, gas, and char. Fast pyrolysis maximises liquid yield by using rapid heating rates and short vapour residence times at temperatures in the 400–600 °C range, quickly quenching the volatile products before secondary cracking reactions convert them to gas. Flash pyrolysis pushes heating rates even higher, approaching very short contact times, and can shift the product balance further toward gas-phase outputs.

Gasification temperature regimes

Gasification does not have equivalent operating mode sub-categories in the same sense, but temperature and oxidant choice are equally decisive. At the lower end of the gasification temperature window (700–900 °C), tar formation can be a significant problem — incomplete conversion of heavier hydrocarbons creates condensable tars that foul downstream equipment. Higher temperatures (above 1000 °C) favour more complete gasification and cleaner syngas, but require greater energy input and impose more demanding materials requirements on the reactor. The choice of oxidant also matters: steam gasification favours hydrogen-rich syngas, while air gasification dilutes the product with nitrogen, reducing syngas calorific value.

“The presence or absence of oxygen is the single chemical variable that separates pyrolysis from gasification — and it determines everything from reactor design to the market value of the end product.”

Product profiles: pyrolysis oil versus syngas

The most commercially significant distinction between pyrolysis and gasification for plastic waste lies in their primary output products — pyrolysis oil (also called pyrolytic naphtha or plastic-derived fuel oil) in the case of pyrolysis, and syngas in the case of gasification. These products have fundamentally different downstream markets, quality requirements, and value chains.

Pyrolysis oil: a liquid hydrocarbon product

Pyrolysis oil from plastic waste is a complex mixture of hydrocarbons — aliphatic and aromatic compounds — whose composition reflects the polymer feedstock. Polyolefins (polyethylene and polypropylene) tend to produce aliphatic-rich oils suitable as refinery feedstock or fuel blending components. Polystyrene pyrolysis yields oils with high aromatic content, including styrene monomer that can be recovered and repolymerised. The liquid product can in principle be used directly as a fuel, co-processed in conventional petroleum refineries, or further upgraded through hydrotreating to produce specification-grade transportation fuels or chemical feedstocks.

Syngas: a gaseous platform chemical

Syngas from plastic gasification is a mixture primarily of hydrogen (H₂) and carbon monoxide (CO), with smaller amounts of carbon dioxide, methane, and other light hydrocarbons. The H₂:CO ratio is a critical quality parameter and can be adjusted through the water-gas shift reaction. Syngas has several high-value downstream applications: direct combustion in gas turbines or engines for power generation; catalytic conversion to methanol; Fischer-Tropsch synthesis to produce synthetic fuels; and hydrogen separation for use as a clean fuel or chemical feedstock. As noted by IEA in its hydrogen and clean energy analyses, syngas-derived hydrogen from waste feedstocks is an area of active interest for decarbonisation pathways.

The char produced as a co-product of pyrolysis deserves separate consideration. Depending on feedstock purity and process conditions, pyrolysis char may contain significant inorganic content (from plastic additives and fillers) that limits its use as a carbon black substitute or soil amendment. In gasification, the equivalent residue is a mineral ash, typically with very low carbon content due to the more complete conversion at higher temperatures.

Feedstock suitability and contamination tolerance

Pyrolysis is generally more tolerant of mixed and contaminated plastic feedstocks than gasification, which is one reason it has attracted significant commercial development for post-consumer plastic waste streams. Gasification, by contrast, can handle a broader range of carbonaceous materials — including mixed municipal solid waste, biomass, and heavily contaminated plastics — because the higher operating temperatures and partial oxidation environment are less sensitive to feedstock variability in terms of product quality.

Chlorine-containing plastics — principally PVC — present a specific challenge for both pathways. In pyrolysis, chlorine is released as hydrogen chloride (HCl) in the vapour phase, where it can cause corrosion and contaminate the liquid product. In gasification, chlorine similarly reports to the syngas as HCl and must be removed before downstream catalytic processes. Both pathways therefore require pre-sorting or pre-treatment to reduce PVC content, or downstream gas cleaning systems to manage chlorine. Standards bodies including ISO have begun developing quality specifications for chemically recycled plastic outputs that address these contaminant concerns.

Nitrogen-containing polymers (polyurethanes, polyamides) present analogous challenges: pyrolysis generates nitrogen-containing organic compounds in the oil, while gasification produces ammonia and hydrogen cyanide in the syngas that require removal. Moisture content is a greater concern for pyrolysis than gasification — high moisture in the feedstock reduces pyrolysis oil yield and quality, whereas in gasification, steam can serve as both a reactant and a moderating agent.

Role in the circular economy for plastics

Both pyrolysis and gasification are classified as chemical recycling or advanced recycling routes within the circular economy framework for plastics — a classification that distinguishes them from mechanical recycling (which preserves polymer structure) and from energy recovery through incineration (which destroys material value entirely). Their role is to handle the fraction of plastic waste that is not amenable to mechanical recycling due to contamination, degradation, or mixed polymer composition.

The Ellen MacArthur Foundation, whose work on circular economy principles for plastics is widely referenced in policy and industry contexts, has noted that chemical recycling routes including pyrolysis and gasification are necessary complements to mechanical recycling for achieving high overall plastic circularity — but should not substitute for mechanical recycling where that route is viable. This positioning shapes how both technologies are regulated, incentivised, and reported under emerging extended producer responsibility (EPR) frameworks in the European Union and elsewhere.

From an IP strategy perspective, both pathways have generated substantial patent activity across reactor design, catalyst development, feedstock pre-treatment, product upgrading, and process integration. R&D teams at PatSnap have observed that the patent landscape in plastic waste thermochemical conversion reflects a competitive and rapidly evolving technology space, with activity from petrochemical majors, dedicated advanced recycling companies, and academic institutions. Understanding which claims are already staked — and where white space exists — is a prerequisite for effective R&D investment decisions in this field. The PatSnap IP intelligence platform provides the tools to map this landscape systematically.