Abstract
Waste plastics and end-of-life tyres are long-chain hydrocarbons. Pyrolysis — heating in the absence of oxygen — breaks those chains into smaller molecules that condense to a liquid, leave as gas, or remain as solid carbon. Because the liquid spans a broad range of molecular sizes, it is not a single fuel. Green Fuel Tech upgrades the liquid and separates it by boiling point into defined cuts, each matched to the market its molecules suit best.
01Polymer feedstocks
The molecular output of pyrolysis begins with the polymer that goes in. Polyolefins (PE and PP) are made only of carbon and hydrogen and are the preferred feed. Other polymers bring aromatics, chlorine or oxygen.
Polyethylene (HDPE / LDPE)
–(CH₂–CH₂)ₙ–
Linear paraffins and α-olefins across a wide carbon range; waxy heavy ends at lower severity.
Polypropylene (PP)
–(CH₂–CH(CH₃))ₙ–
Branched paraffins and olefins; tertiary carbons crack readily, favouring lighter, mid-range molecules.
Polystyrene (PS)
–(CH₂–CH(C₆H₅))ₙ–
Largely depolymerises to styrene monomer and aromatics (toluene, ethylbenzene).
PVC
–(CH₂–CHCl)ₙ–
Releases HCl on heating and forms organochlorides — corrosive and a catalyst poison.
PET
–(O–CO–C₆H₄–CO–O–CH₂CH₂)ₙ–
Oxygenated products (benzoic / terephthalic acids) that can plug equipment and raise acidity.
02Pyrolysis chemistry
Polyolefin pyrolysis proceeds by a free-radical chain mechanism, typically at roughly 400–550 °C. Bonds in the polymer backbone break, and the radical fragments react further until stable molecules form.
Initiation — random scission
R–CH₂–CH₂–R′ → R–CH₂• + •CH₂–R′
Heat breaks a C–C bond at a random point along the chain, creating two radicals.
Propagation — β-scission
R–CH₂–CH₂–CH•–R′ → R–CH₂• + CH₂=CH–R′
The bond one position away from the radical breaks, releasing a molecule with a double bond (an olefin) and a new, shorter radical.
Hydrogen transfer
R• + R′–CH₂–R″ → R–H + R′–CH•–R″
A radical takes a hydrogen atom from another chain, becoming a stable paraffin and moving the radical site elsewhere.
Termination
R• + R′• → R–R′
Two radicals combine, ending the chain.
The balance of these steps sets the product: β-scission produces olefins (molecules with a C=C double bond) and hydrogen transfer produces paraffins (fully saturated chains). Higher temperature and longer residence time crack molecules further, shifting output from wax and heavy oil toward lighter liquids and gas. Catalysts can steer selectivity toward specific carbon ranges or aromatics.
Lower severity
More wax & heavy oil
Moderate severity
Maximum liquid — broad C5–C40
Higher severity
More gas & aromatics
03Tyre pyrolysis
A tyre is a composite: natural and synthetic rubber (polyisoprene, styrene-butadiene), carbon black filler, steel wire, textile and additives such as sulphur and zinc oxide. Pyrolysis cracks the rubber into oil and gas. Polyisoprene yields limonene and other terpenes alongside aromatics, so tyre oil is more aromatic and higher in sulphur than polyolefin oil. The carbon black and steel do not react — they leave as recovered carbon black and clean steel wire.
04Heteroatoms & contaminants
Atoms other than carbon and hydrogen — chlorine, nitrogen, sulphur, oxygen and metals — matter more than their small concentrations suggest. Chlorine forms HCl and corrodes equipment; nitrogen and sulphur poison refinery hydrotreating catalysts; oxygen reduces stability. Feedstock control is the first defence, followed by vapour-phase treatment and guard beds.
05Upgrading
Raw pyrolysis oil is reactive: olefins can link together and form gums over time. Upgrading removes water and solids, reduces contaminants and stabilises the oil before separation. Downstream, refiners use hydrotreatment to saturate olefins and remove sulphur and nitrogen:
CH₂=CH–R + H₂ → CH₃–CH₂–R
Hydrogen adds across the double bond, turning an olefin into a stable paraffin.
06Fractionation
A hydrocarbon's boiling point rises with its size. In a distillation column the oil is heated at the base; vapour rises through a series of trays, cooling as it goes. Heavy molecules condense low in the column, light ones climb higher. Drawing liquid from different heights separates the oil into boiling-range cuts.
The choice of cut points is a trade-off. A narrow cut gives a cleaner, more consistent product but a lower yield; a broad cut recovers more volume but carries more light and heavy molecules into the product. Distillation separates molecules by size — it does not remove chlorine, saturate olefins or change aromatic content. That is the role of upgrading.
07Molecular basis of each product
Each product is defined by the size and type of the molecules in it, and that decides where it is useful.
Circular Naphtha
Light paraffins, olefins and some aromatics. The molecular size used as steam-cracker feed to make ethylene and propylene — closing the loop back to new plastic.
Jet-RCF
Kerosene-range paraffins, iso-paraffins, naphthenes and aromatics. A refinery feedstock; hydrotreatment and qualification are needed before any aviation use.
Diesel-Range Circular Fuel
Middle-distillate paraffins with good ignition quality; long linear chains from polyethylene give high cetane character but can affect cold flow.
Marine-Fuel-Range Product
Heavier paraffins, waxes and polyaromatics. Suited to industrial and marine fuel ranges, or recycled to the reactor for further cracking.
Recovered Carbon Black
From tyres: the original carbon black filler plus char and ash (silica, zinc oxide). Structure and surface area decide its reuse as a filler or pigment.
Recovered Steel
Tyre bead and belt wire is unchanged by pyrolysis and leaves the reactor clean of rubber, ready for steel recycling.
08Limitations & quality assurance
Product composition varies with feedstock mix, reactor conditions and cut points. For this reason Green Fuel Tech does not claim universal compliance with any fuel standard. Each batch is characterised — typically by simulated distillation, density, flash point, GC-MS composition and elemental analysis — and supplied against an agreed specification with a Certificate of Analysis. Jet-RCF in particular is a refinery feedstock, not finished SAF or certified Jet A-1.
Glossary
- Paraffin
- A saturated hydrocarbon — every carbon bond is single.
- Olefin
- A hydrocarbon with at least one C=C double bond; reactive.
- Aromatic
- A hydrocarbon containing a benzene-type ring.
- Naphthene
- A saturated ring-shaped hydrocarbon.
- Carbon number (Cn)
- The number of carbon atoms in a molecule.
- Heteroatom
- Any atom other than carbon or hydrogen, e.g. Cl, N, S, O.
- COA
- Certificate of Analysis — measured properties of a specific batch.