Chemical Recycling Pyrolysis Yield Loss Conversion Factors for Mass Balance Accounting Ledgers

Yield

Mass Deficits at the Pyrolysis Boundary

In feedstocks processed through thermal depolymerization, one hundred metric tons of post-consumer mixed polyolefins yield approximately sixty-five to seventy-five tons of liquid oil, ten to twenty tons of non-condensable pyrogas, and five to fifteen tons of carbonaceous char. Sourcing teams reviewing Sustainability Declarations under the International Sustainability and Carbon Certification (ISCC PLUS) or REDcert2 systems encounter this shrinkage immediately upon reconciling input delivery tickets against certified output allocations. The discrepancy between physical mass arriving at the cracking unit and the credit balance declared in mass balance software represents the first friction point in chain-of-custody audits.

Mass balances operate across administrative boundaries that treat physical transformation as a quantitative conversion factor. A conversion factor equals the quotient of total eligible product mass divided by total raw material input mass over a fixed accounting period, which standard certification schemes limit to a maximum of three months. A pyrolysis plant feeding 1,000 metric tons of sorted post-consumer plastic waste to produce 720 metric tons of condensed pyrolysis oil establishes an operational baseline conversion factor of 0.72.

The remaining 280 tons exit the accounting envelope as heavy residues, light process gases, or moisture losses.

Auditors penalize facilities that assign recycled content credits across this non-condensable and solid fraction to commercial resins. When an operator records 1,000 tons of waste plastic feedstock and assigns all 1,000 credit tons to outbound pyrolysis oil shipped to a steam cracker, the mass balance ledger overstates certified recycled volume by 38.8 percent. Regulatory inspection under the European Union Packaging and Packaging Waste Regulation (PPWR) drafts and national plastics taxation bodies treats this allocation method as fraud.

The physical reactor output alone determines the mass eligible for credit issuance.

A raw conversion factor calculated without deducted pyrolysis char and non-condensable fuel gas credits creates an immediate over-allocation vulnerability during third-party mass balance audits.

Liquid yields fluctuate with polymer composition, bulk contamination, and reactor operating parameters. High-density polyethylene and polypropylene streams deliver liquid yields approaching eighty percent under fluidized bed pyrolyzers operated at 450 to 520 degrees Celsius. Conversely, mixed municipal sorting fractions containing polyethylene terephthalate, polyvinyl chloride, polyamides, and residual paper fiber drop liquid condensates below fifty-five percent while generating corrosive hydrogen chloride and carbon oxides.

These process gases combust within internal thermal oxidizers to provide furnace heat. Under strict attribution rules, fuel consumed to drive internal heating cannot generate recycled resin credits.

The auditor traces this separation through the mass ledger before clearing downstream credit transfers. If the facility fails to meter fuel gas, flare volumes, and bottom ash, certification bodies invalidate the entire mass balance balance sheet for that quarter. The consequence is immediate decertification of the credit batch, stranding the buyer with uncredentialed virgin polymers subject to primary plastics packaging taxes.

Tar

Contaminant Partitioning and Slag Losses

Pyrolysis input streams rarely match the analytical purity of virgin petrochemical feedstocks. Moisture content in washed post-consumer bales averages five to eight percent, while inorganic ash, titanium dioxide pigment, calcium carbonate fillers, and residual biomass account for four to twelve percent of total dry intake mass. During thermal cracking in rotary kilns or auger reactors, these substances concentrate into the heavy bottoms, solid residues, and high-boiling wax fractions generally categorized as char and tar.

Heteroatom distribution creates further material losses through clean-up processes. Post-consumer waste bales contain organic chlorine from PVC and PVDC packaging films, organic nitrogen from polyurethane adhesives and multi-layer polyamide barriers, and silicones from personal care packaging gaskets. Chlorinated compounds degrade into hydrochloric acid inside the condensation train, demanding alkaline wash steps or upstream dehalogenation stages that produce neutralized salt slurries.

These slurries remove mass from the hydrocarbon product stream.

Pre-treatment plants designed to strip chlorine and oxygen compounds from raw pyrolysis oil impose a second conversion penalty. Hydroprocessing units operating at 40 to 100 bar hydrogen pressure and 300 to 380 degrees Celsius convert chlorinated hydrocarbons to ethane, propane, and hydrogen chloride, while hydrodeoxygenation eliminates oxygen atoms from organic esters as water. A plant processing 1,000 kilograms of raw pyrolysis oil with 3,000 parts per million chlorine and 1.5 percent oxygen by weight loses mass to water and gas formation during hydrotreatment, reducing net clean pyrolysis naphtha output.

Downstream steam crackers impose rigorous specification limits on pyrolysis oil feeds to prevent rapid furnace tube coking and catalyst deactivation in selective hydrogenation units. Pyrolysis oil specification limits for steam crackers demand total chlorine content below 1 to 5 parts per million, total nitrogen below 10 to 50 parts per million, silicon below 1 to 2 parts per million, and sodium, calcium, and iron levels below 1 part per million each. Meeting these thresholds requires distillation passes where ten to twenty percent of the heaviest hydrocarbon fraction separates as an asphalt-like pitch.

If the conversion factor applied to the ledger fails to deduct this pitch, the auditor identifies an unsustainable mass inflation across the supply tier.

Suppliers defend these gaps by claiming that process bottoms constitute secondary raw materials for asphalt or circular carbon black, but their exclusion from plastics mass balance ledgers is absolute.

Splitter

Steam Cracker Mass Allocation Mechanics

Hydroprocessed pyrolysis oil entering a steam cracker blends with conventional fossil inputs like fossil naphtha, liquefied petroleum gas, or ethane. Steam cracking breaks these long-chain paraffinic and naphthenic molecules into light olefins, primarily ethylene and propylene, alongside a spectrum of secondary co-products. Ethylene yield from liquid naphtha ranges between twenty-five and thirty-four percent, while propylene yield spans thirteen to eighteen percent, depending upon furnace coil outlet temperature and cracking severity.

The steam cracker does not segregate pyrolysis-derived molecules from fossil molecules. Traceability depends entirely upon accounting ledgers that divide outbound chemicals using mathematically defined conversion factors. The essential commercial dispute centers on whether mass balance credits follow the stoichiometric reality of petrochemical cracking or follow arbitrary administrative assignments across high-value product lines.

A mass balance ledger attributing cracker yields to ethylene without proportional allocation to pyrolysis fuel oil and methane gas breaches ISO 22095 chain of custody rules.

Under the fuel-exempt allocation model, credits derived from pyrolysis oil processing are assigned strictly to high-value chemicals and polymers. The portions of cracked output that enter fuel fractions, including methane-rich fuel gas, cracked gasoline, and pyrolysis fuel oil (carbon black feedstock), carry no recycled credits. In contrast, under the proportional allocation model, every chemical product exiting the cracker receives a uniform share of the recycled attribute corresponding strictly to its mass yield fraction.

Consider an operational cracking campaign where a petrochemical facility feeds 10,000 metric tons of mixed feed containing 1,000 metric tons of certified pyrolysis oil and 9,000 metric tons of fossil naphtha. High severity cracking produces thirty-two percent ethylene, sixteen percent propylene, eight percent butadiene, six percent benzene, fourteen percent pyrolysis gasoline, eighteen percent fuel gas (methane and hydrogen), and six percent heavy pyrolysis fuel oil.

Under proportional attribution, the 1,000 tons of pyrolysis oil input yield 320 tons of recycled ethylene credit, 160 tons of recycled propylene credit, 80 tons of recycled butadiene credit, 60 tons of recycled benzene credit, 140 tons of recycled pyrolysis gasoline, 180 tons of recycled methane fuel gas, and 60 tons of recycled heavy residue. The facility sells the ethylene to high-density polyethylene producers and the propylene to polypropylene producers, enabling a maximum of 480 tons of polyolefin packaging credit from the initial 1,000 tons of pyrolysis oil.

Under the contested polymer-only attribution model, the operator allocates the entire 1,000 tons of recycled content to the ethylene and propylene streams, zeroing out allocations to the pyrolysis gasoline and fuel gas fractions. This accounting manoeuvre inflates the effective conversion factor of the steam cracker from 0.48 to 1.00 for polymer products. The legal validity of this practice remains a primary battleground in European chemical packaging policy.

Credit

Ledger Balancing Rules and Attribution Limits

Mass balance ledgers resemble double-entry bank accounts where credits represent kilogram-equivalent units of circular polymer attributes. Transactions follow international standards such as ISO 22095 for chain of custody, supported by sector systems including ISCC PLUS, REDcert2, and RSB (Roundtable on Sustainable Biomaterials). These programs mandate balance settlement within designated credit validity windows to prevent unbounded speculative trading.

Credit ledgers operate within three distinct physical boundaries: site-specific, contiguous logistics cluster, and national corporate entity. Under site-specific mass balance, transfers occur exclusively among process units linked directly by pipe or localized transfer within a single industrial perimeter. Contiguous logistics clusters allow balancing between plants connected by dedicated pipelines, such as ethylene grids running through northwest Europe.

Geographic book-and-claim approaches that detach credit allocations from physical freight networks violate baseline standards for plastic food-contact declarations.

The accounting life of a mass balance credit spans a rolling twelve-month window under ISCC PLUS. Credits generated in January expire if not linked to finished polymer sales by December thirty-first of the same calendar year. Negative balances are restricted: a site cannot sell mass balance claims against projected future production runs except under short-term overdraft dispensations capped at three months and contingent upon verifiable maintenance shutdown schedules.

Credits drawn from an expired twelve-month certification reconciliation period lose legal validity upon customs inspection.

Regulatory frameworks diverge sharply over allowable attribution mechanisms:

• Chemical attribution matches the molecular yield distribution of the cracking and synthesis equipment, assigning losses to low-value coproducts and fuel streams.
• Polymer-only attribution redirects mass credits from fuels and aromatics to the primary polymer resin phases, enabling higher declared recycled content per ton of oil cracked.
• Controlled blending attribution mandates physical inclusion of recycled molecules inside every container dispatched, precluding purely administrative credit swaps across separate production lines.
• Energy-subtracted attribution removes raw waste equivalent consumed as process energy at any stage in the recycling chain from outbound product credit eligibility.

The selection of attribution method shifts the calculated landed cost of recycled resin substantially. When chemical attribution dictates ledger accounting, a converter must purchase 2.1 to 2.8 tons of certified pyrolysis oil equivalents to claim one single ton of circular polyethylene film. When a polymer-only assignment passes audit, that ratio drops to 1.3 to 1.5 tons of pyrolysis oil per ton of finished film.

Sourcing directors negotiating fixed-price supply agreements run substantial cost risks if their resin purchase contract omits the specific conversion factor algorithm applied to the batch.

Supply contracts specifying mass balance terms without designating an attribution clause leave the buyer exposed to retroactive volume re-rating. If a resin producer prices resin on a polymer-only basis and national regulators subsequently mandate chemical attribution under PPWR Article 7 secondary legislation, the available credit volume drops by forty to fifty percent overnight, inflating unit packaging taxes on uncredentialed plastic.

International audits under ISCC PLUS standard 205 impose non-conformity notices when processors cross-credit between chemically incompatible sites. Pyrolysis oil produced from post-consumer agricultural film cannot support bio-based polybutylene succinate claims, nor can a plant balance pyrolysis oil receipts against mechanical PET bottle flake inventories. The mass ledger must maintain distinct balance sheets for separate material categories: bio-based, bio-circular, and circular.

Purity

What Contaminants Survive Thermal Pyrolysis into Food Packaging?

A persistent marketing assertion surrounding chemical recycling suggests that thermal cracking obliterates all prior contamination, automatically resetting polymer safety to match virgin petrochemical standards. Gas chromatography with mass spectrometry (GC-MS) screenings of actual pyrolysis oil fractions, however, expose an array of synthetic additives, degraded process aids, and newly formed synthesis products. These non-intentionally added substances (NIAS) survive cracking or synthesize inside quenching units, challenging compliance under European Commission Regulation (EU) No 10/2011.

Halogenated substances demonstrate stubborn persistence across conversion steps. Brominated flame retardants present in electronic waste streams (such as polybrominated diphenyl ethers, tetrabromobisphenol A, and hexabromocyclododecane) crack into low-molecular-weight bromo-alkanes and bromo-aromatics that co-distill with naphtha fractions. In steam crackers, these brominated molecules behave like volatile organochlorines, presenting catalyst poison hazards and migrating unhindered into polyethylene packaging films.

Polycyclic aromatic hydrocarbons (PAHs) generate copiously during high-temperature thermal pyrolysis through Diels-Alder cyclization reactions of conjugated dienes. Benzo(a)pyrene, chrysene, and phenanthrene partition into the high-boiling pyrolysis oil fraction. While subsequent catalytic hydrotreatment hydrogenates these aromatic rings into saturated cycloparaffins, hydrotreating efficiency degrades as catalyst beds age, allowing PAH slippage into outbound monomer synthesis feeds.

Migration testing of finished articles blown from mass-balanced polyethylene resins must follow strict conditions outlined in Annex III and Annex V of Regulation (EU) 10/2011. Testing an all-polyolefin stand-up pouch intended for fatty dairy contact demands food simulant D2 (vegetable oil or alternative 95 percent ethanol depending upon fat extraction characteristics) applied for ten days at forty degrees Celsius for ambient storage, or two hours at one hundred degrees Celsius for hot-fill processes.

Screening tests using head-space gas chromatography coupled to flame ionization detectors and quadrupole time-of-flight mass spectrometry (HS-GC-QTOF) regularly surface oligomeric series. These linear, branched, and cyclic hydrocarbon structures (POSH – Polyolefin Oligomeric Saturated Hydrocarbons, and MOAH – Mineral Oil Aromatic Hydrocarbons) generate during thermal cracking. If hydroprocessing fails to eliminate the aromatic double bonds, MOAH fractions containing one to three aromatic rings carry over into the ethylene feedstock stream.

Under draft European recommendations for MOAH thresholds in food packaging, levels above 0.5 milligrams per kilogram in dry foods or 1.0 milligram per kilogram in fatty foods prompt regulatory interventions and product withdrawals. Because mass balance accounting distributes the attribute of circularity rather than physically segregated pristine molecules, finished mass-balanced resins physically incorporate conventional fossil molecules alongside the chemical recycling product. This dilution effect frequently depresses physical NIAS concentrations in final packaging resins below standard analytical limits of quantification (typically 0.01 milligrams per kilogram), creating a compliance paradox where an analytically pure lot sits upon an undocumented chemical synthesis history.

The auditor demands extraction test reports verifying that non-target screening revealed no toxicologically concerning signals under EFSA guidance on tiered risk assessment for packaging materials.

Audit

Reconciling the Declarations Dossier

A mass balance transaction falls apart inside the documentation binder. Sourcing teams receiving shipments of circular low-density polyethylene or polypropylene film resins cannot establish compliance based upon a supplier commercial invoice alone. Defending the recycled content claim against customs authorities, fiscal tax inspectors, and national packaging registries requires an unbroken chain of custody file.

The dossier starts with the sustainability certificate of the polymer producer, verified against the official registry of the approved certification scheme. An active ISCC PLUS certificate lists the production address, the operational scope (such as converter, compounding site, or polymerisation unit), and the certified material categories. The certificate must remain valid on the exact day the polymer batch cross-ships through the factory gates.

Accompanying every shipment is the Sustainability Declaration (SD), also referred to as a Proof of Sustainability (PoS). This document links directly to the commercial invoice and bill of lading by unique batch numbers and gross delivery quantities. The Sustainability Declaration details: the certified material claim (e.g.

Circular LDPE), the scheme certificate registration code, the net mass of physical resin delivered, the absolute quantity of sustainable or circular credits allocated to that physical delivery, the greenhouse gas emissions per ton of product if a carbon claim accompanies the shipment, and the explicit mass balance attribution model used (proportional or polymer-only).

A common compliance failure emerges when the sustainability declaration covers a gross dry weight of 24 metric tons, while the attached mass balance allocation credit specifies only 7.2 metric tons. This spread indicates a thirty percent mass-balanced product. The remaining 16.8 metric tons represent standard fossil resin serving as the physical diluent.

If the purchasing brand markets the finished containers as hundred-percent chemically recycled plastic, customs authorities and trading standards units flag the consumer packaging as misleading under consumer protection rules. The buyer must verify the declaration’s credit ratio matches the customer-facing label claims.

The technical dossier must integrate the Declaration of Compliance (DoC) for food contact alongside the Sustainability Declaration. Regulation (EU) 10/2011 Annex IV demands that the DoC list all substances subject to specific migration limits (SML), dual-use additives, and confirmation that the article complies with overall migration limits (OML) of 10 milligrams per square decimeter or 60 milligrams per kilogram of simulant under standard test exposures.

A DoC covering circular resin cannot rely upon generic raw-material assertions. It requires supporting laboratory test reports with clear analytical identification: accredited test laboratory ISO/IEC 17025 certification numbers, exposure times and temperatures (e.g. ten days at forty degrees Celsius in three percent acetic acid and vegetable oil), measured specific migration quantities with their specific measurement uncertainties, and toxicological assessments of any NIAS identified above 0.01 milligrams per kilogram food equivalent.

The gap between the sample sent for testing and the commercial lot dispatched poses a serious audit liability. A resin producer testing an optimized pilot run of pyrolysis oil produces an immaculate migration report. Subsequent full-scale production campaigns incorporating mixed commercial wastes with elevated post-consumer PVC or adhesive residues encounter variable cracking profiles.

If the plant alters its waste feedstock specification without revalidating migration profiles, the DoC becomes obsolete. Dossier auditors scrutinize the time delta between the laboratory test date and the batch release date on the commercial delivery ticket.

Customs authorities executing border checks under national plastic packaging taxes demand cross-verification between physical dispatch weights and electronic registry records. When an importer declares an exemption from a tax based on circular plastic claims, customs interrogates the mass balance registry ledger to verify that the corresponding credits were permanently retired rather than banked or resold.

Under the United Kingdom Plastic Packaging Tax, which charges 217.85 pounds per metric ton on plastic packaging containing less than thirty percent recycled content, administrative failures in tracking these conversion rates trigger retro-assessed levies across multi-ton imports. The financial penalty attaches directly to the importer of record.

Ledger

Worked Conversion Calculation from Bale to Blown Film

To ground the compounding arithmetic of mass balance yields, we walk through an explicit, production-scale conversion scenario. The material starts as 100 metric tons of post-consumer mixed polyolefin packaging waste at a sorting facility and finishes as blown low-density polyethylene film manufactured for secondary pouch packaging.

Assume the input feedstock comprises post-consumer bales sampled at six percent moisture, five percent inorganic contamination (mineral ash, fillers, residual dirt), three percent non-polyolefin polymers (PET, polyamide, PVC), and eighty-six percent mixed polyethylene and polypropylene. The target output is circular LDPE film certified under proportional chemical attribution rules.

Phase one processes the feedstock through shredding, mechanical dry cleaning, and pyrolysis cracking. The 100 tons of wet raw bales drop to 89 tons of dry mixed polymer feed following moisture loss and preliminary particulate screening. This dry feed enters a continuous pyrolysis reactor operating at 480 degrees Celsius.

Reactor cracking yields seventy-four percent condensed liquid pyrolysis oil, eighteen percent non-condensable pyrogas (diverted immediately to fire the pyrolysis furnace tubes), and eight percent solid carbon char bottoms. The reactor converts the 89 tons of clean dry feedstock into 65.86 tons of raw pyrolysis oil, 16.02 tons of internal fuel gas, and 7.12 tons of solid char. The unadjusted physical conversion factor of the pyrolysis stage equals 0.6586 relative to raw bale input.

Phase two subjects the 65.86 tons of raw pyrolysis oil to distillation, dechlorination, and hydrotreatment to meet steam cracker feed specifications. Heavy distillation waxes and asphaltic pitch discard 12 percent of the mass as refinery bottoms. Hydrotreatment consumes hydrogen while stripping 1.2 percent of mass as water and light gases.

This purification drops the volume of cracker-ready circular naphtha from 65.86 tons to 57.17 tons. The cumulative conversion factor from the initial post-consumer waste bale now stands at 0.5717.

Phase three transports the 57.17 tons of clean pyrolysis naphtha to a petrochemical steam cracker. The steam cracker operates under strict proportional attribution, yielding the chemical product slate detailed in the following reconciliation schedule.

The cracker yields 18.01 tons of circular ethylene. Polyethylene synthesis into LDPE resin exhibits a high polymerization efficiency of ninety-eight percent, with two percent loss to purge gases, low-molecular-weight waxes, and fine polymer dusts. The 18.01 tons of ethylene produce 17.65 tons of mass-balanced circular LDPE pellets.

Phase four extrudes the LDPE pellets into blown packaging film. Film converting lines generate startup scrap, edge-trim scrap, and roll-change losses totaling four percent of resin throughput. This film manufacturing loss reduces the net output of finished circular film to 16.94 metric tons.

The final, true conversion factor for this complete manufacturing chain—from post-consumer mixed waste bale to customer-delivered circular film under proportional attribution—is 0.1694, or roughly seventeen percent. To deliver one single metric ton of certified circular packaging film, the complete industrial network consumes 5.90 metric tons of raw post-consumer plastic bales.

Under polymer-only attribution, the steam cracker operator assigns the entire 57.17 tons of input naphtha mass to the polymerizable olefin streams. That approach yields approximately 37.1 tons of circular film from the same starting bales, generating an inflated conversion factor of 0.371. The 2.19-fold difference between these two conversion models demonstrates how choice of attribution methodology alters material demand and unit economics.

When an auditor reviews the converter’s mass balance ledger, reconciliation requires verified tracking across each stage: 100 metric tons of baled waste yielding 65.86 tons of pyrolysis liquid, 57.17 tons of hydrotreated naphtha, 18.01 tons of cracker ethylene, 17.65 tons of virgin-grade LDPE polymer, and 16.94 tons of finished blown film. If the converter declares thirty metric tons of circular film output from this initial 100-ton intake using proportional allocation accounting, the audit team flags a volume discrepancy of 13.06 metric tons. The discrepancy invalidates the mass balance declaration for the entire production lot.

Whether national tax enforcement authorities will accept polymer-only allocation adjustments across future packaging cycles remains subject to ongoing legislative negotiation across key international jurisdictions.