Hazardous Substances Testing of Pyrolysis Products from Recycled Plastics

Hazardous substances testing of pyrolysis products from recycled plastics is essential for verifying the safety, compliance, and market acceptance of pyrolysis oil, gas, and char generated through chemical recycling. Contaminants such as halogens, heavy metals, PAHs, PCBs, and dioxins can be carried over from mixed plastic waste streams and must be quantified using standardized analytical methods. This article outlines the key sample types, testing items, international methods and standards, and typical applications for accredited third-party laboratories serving the chemical recycling industry.

Technical Overview

Pyrolysis is a core technology in chemical recycling, thermally decomposing mixed waste plastics, typically polyethylene, polypropylene, and polystyrene, in an oxygen-free reactor at approximately 400 to 600 °C to yield pyrolysis oil, non-condensable gas, and solid char. Because recycled plastic feedstocks frequently contain PVC residues, electronic-grade plastics bearing brominated flame retardants, inks, adhesives, labels, pigments, and metal contaminants, the resulting products can carry hazardous substances that must be identified and quantified before commercial use.

The hazard profile of pyrolysis products is shaped by the chemistry of the feedstock. Chlorine from PVC converts to hydrogen chloride and organochlorinated compounds, causing corrosion of downstream equipment and poisoning of refinery catalysts. Brominated flame retardants can release hydrogen bromide and persist as regulated compounds such as PBDEs. Heavy-metal-based pigments and stabilizers, including cadmium, lead, chromium, and antimony, tend to concentrate in the solid char. Polycyclic aromatic hydrocarbons are inherently formed during thermal degradation, while dioxins and furans may arise in halogen-rich streams under unfavorable conditions. Volatile organic compounds such as benzene, toluene, ethylbenzene, xylenes, and styrene are also present in the liquid and gaseous fractions.

Systematic hazardous substances testing therefore serves four objectives: process control at the recycling plant, acceptance of pyrolysis products by refineries and compounders, correct regulatory classification of outputs as products rather than waste, and protection of human health and the environment. Accredited third-party laboratories operating under ISO/IEC 17025 quality systems provide the analytical data required to meet these objectives.

Testing Samples

Pyrolysis Oil and Wax

The liquid condensate, which may range from a light oil to a waxy heavy fraction, is the most commonly tested product. It is analyzed for total halogens, heavy metals, PAHs, PCBs, sulfur, and nitrogen. Careful homogenization and temperature-controlled sampling are required, since waxy fractions can segregate during storage and bias results.

Solid Char and Recovered Carbon Black

The carbonaceous solid residue concentrates inorganic contaminants from the feedstock. Testing covers total heavy metals, PAHs, PCBs, ash content, and leaching behavior, which determine whether the char can be upgraded and reused as recovered carbon black or must be managed as a residue.

Pyrolysis Gas (Non-Condensable Fraction)

The gaseous product is screened for acidic components such as hydrogen chloride and hydrogen bromide, hydrogen sulfide, ammonia, and volatile organic compounds, together with calorific value, because these parameters govern corrosion risk, emission compliance, and suitability for use as process fuel.

Process Wastewater and Scrubber Liquor

Condensate water and gas-cleaning liquors accumulate chlorides, bromides, phenols, and dissolved metals. Their characterization supports wastewater treatment design and discharge compliance.

Recycled Plastic Feedstock (Pre-Screening)

Incoming flakes, pellets, or regrind can be screened before pyrolysis using rapid techniques such as X-ray fluorescence and near-infrared identification. Early detection of halogen-rich or flame-retarded streams significantly reduces the risk of contaminating the entire product slate.

Testing Items

The scope of testing is tailored to the feedstock composition and the intended end use of each product stream. Typical testing items include:

• Total halogens: chlorine, bromine, and fluorine, including organic-bound chlorine, for which refinery and steam cracker feedstock specifications commonly demand levels below 10 mg/kg
• Heavy metals: lead, cadmium, mercury, arsenic, chromium including chromium(VI), nickel, zinc, copper, antimony, and tin
• Polycyclic aromatic hydrocarbons: the 16 priority PAHs, with particular attention to benzo[a]pyrene and naphthalene
• Polychlorinated biphenyls (PCBs) originating from legacy plastic waste
• Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF)
• Volatile and semi-volatile organic compounds: benzene, toluene, ethylbenzene, xylenes, styrene, phenols, and aldehydes
• Brominated flame retardants: PBDEs, HBCD, and related substances
• Phthalate plasticizers carried over from flexible PVC and other resins
• Sulfur and nitrogen compounds relevant to corrosion, odor, and emissions
• Basic quality parameters: moisture, ash, density, viscosity, flash point, calorific value, and elemental CHN composition

Testing Methods

Reliable results depend on representative sampling and validated sample preparation, including homogenization of waxy oils, microwave-assisted acid digestion of chars, and gas-tight containment of volatile fractions. The principal analytical techniques are summarized below.

Chromatographic and Mass Spectrometric Analysis

Headspace and purge-and-trap GC-MS are used for VOCs, while GC-MS/MS provides the sensitivity required for PAHs and phthalates in complex hydrocarbon matrices. PCBs are determined by GC-ECD or GC-MS, and dioxins and furans are confirmed by high-resolution GC-MS at trace levels consistent with the EN 1948 methodology for related thermal processes.

Elemental and Inorganic Analysis

Heavy metals are quantified by ICP-OES or ICP-MS following acid digestion, with cold-vapor AAS employed for mercury. Rapid X-ray fluorescence screening, following the analytical approach of the IEC 62321 series, is widely used for incoming plastic feedstock and for spot checks of bromine and metal hot spots.

Combustion-Based Halogen and Sulfur Determination

Combustion ion chromatography in accordance with EN 14582 and ASTM D7359 delivers total fluorine, chlorine, and bromine in liquids and solids. Microcoulometric titration per ASTM D4929 targets organic-bound chloride in hydrocarbon liquids. Sulfur is measured by energy-dispersive X-ray fluorescence (ASTM D4294, ISO 20884) or ultraviolet fluorescence (ASTM D5453).

Fuel and Physical-Chemical Characterization

Product quality is profiled using ASTM D5291 for carbon, hydrogen, and nitrogen, ASTM D482 for ash, ASTM D1298 for density, ASTM D445 for viscosity, ASTM D93 for flash point, ASTM D6304 for water by Karl Fischer titration, and ASTM D240 or ISO 1928 for calorific value. These data allow benchmarking against established fuel specifications such as ASTM D396 and EN 590 after upgrading.

Leaching Assessment for Solid Residues

Char destined for reuse is evaluated with leaching tests such as the EN 12457 series or the leaching toxicity criteria of the GB 5085.3 standard, confirming that immobilized metals do not migrate under environmental exposure conditions.

Applications

Hazardous substances testing of pyrolysis products underpins decision-making across the entire chemical recycling value chain:

• Plant quality control: incoming feedstock screening and batch release testing at pyrolysis facilities to prevent off-specification production
• Refinery and steam cracker acceptance: demonstrating compliance with chlorine, sulfur, and metal specifications that protect downstream catalysts during co-processing of pyrolysis oil
• Recovered carbon black qualification: verifying PAH, heavy metal, and leaching compliance before substituting virgin carbon black in rubber and plastic compounds
• Regulatory classification: supporting end-of-waste and product status determinations under hazardous waste identification frameworks such as the GB 5085 series
• Emission and environmental compliance: validating that acid gas and dioxin levels in process off-gas meet stringent limits comparable to GB 18484 requirements
• Chemical regulatory submissions: generating compositional data for EU REACH-type registrations of pyrolysis-derived substances
• Sustainability claims: substantiating recycled content declarations under schemes such as UL 2809 and supporting mass-balance documentation
• Research and development: comparing catalyst performance, residence times, and decontamination efficiencies during process optimization

FAQ

Why is chlorine the most critical contaminant in pyrolysis oil?

Chlorine originates mainly from PVC contamination and converts to hydrogen chloride and chlorinated organics during pyrolysis. These species corrode reactors and piping, deactivate downstream refining catalysts, and complicate co-processing. Most cracker and refinery specifications therefore impose total chlorine limits on the order of 10 mg/kg or lower, making combustion ion chromatography one of the most frequently requested tests.

Can dioxins actually form during plastic pyrolysis?

The oxygen-limited pyrolysis atmosphere generally suppresses dioxin formation, but the risk cannot be ignored when halogen-rich feedstocks are combined with catalytic metals such as copper or iron at certain temperature windows. High-resolution GC-MS confirmation on both product and off-gas streams provides the evidence needed for regulatory confidence.

Is testing required before pyrolysis oil can be used as cracker feedstock?

Yes. Refineries and steam cracking operators require a certified contaminant profile covering halogens, sulfur, nitrogen, metals, and volatile aromatics before accepting pyrolysis oil, either neat or in co-feeding. Batch certificates from accredited third-party laboratories are the standard basis for such acceptance.

How is pyrolysis char evaluated for safe reuse?

Char is assessed for total heavy metals, PAHs, PCBs, and ash, followed by leaching tests such as EN 12457 or the GB 5085.3 leaching toxicity procedure. Only char that meets both total-content and leaching criteria is suitable for upgrading into recovered carbon black for polymer and rubber applications.

How frequently should hazardous substances testing be performed?

For variable mixed-plastic feedstocks, rapid screening per lot combined with periodic full characterization is recommended. Stable, well-sorted feedstocks permit reduced frequency, while any feedstock change, process upset, or new end-use application should trigger a complete re-evaluation.

Does feedstock pre-screening reduce downstream testing burdens?

Substantially. X-ray fluorescence screening for bromine and metals, near-infrared resin identification, and halogen spot tests at the intake stage prevent contaminated batches from entering the reactor, stabilizing product quality and lowering the volume of confirmatory laboratory testing required downstream.