A comparative study of aromatic content in pyrolysis oils from waste plastics and tires: Assessing common refinery methods

Significance of the Topic
Pyrolysis oil produced from waste plastics and tires via thermal decomposition presents a promising route for chemical recycling. Effective utilization of these oils as petrochemical feedstocks or fuels depends critically on accurate quantification of aromatic hydrocarbons. Aromatics influence fuel properties (heat value, combustion emissions) and steam‐cracking behavior (coke formation). However, pyrolysis oils differ markedly from conventional petroleum fractions in high olefin and heteroatom content, complicating standard aromatic measurement techniques.

Study Objectives and Overview
This research systematically compares three widely accessible refinery methods for aromatic analysis—FIA (ASTM D1319), HPLC‐RI (EN 12916 / ASTM D6379 / D6591), and GC×GC‐FID (ASTM D8396)—across crude and hydrotreated oils from plastic and tire pyrolysis, and their distilled kerosene (150–250 °C) and gas oil (250–360 °C) fractions. Model compounds representing common olefins, dienes and aromatics were used to elucidate method biases.

Analytical Methods and Instrumentation
• Fluorescence Indicator Adsorption (FIA, ASTM D1319): Silica gel displacement column with fluorescent dye, visual ultraviolet‐band measurement. Applicable to boiling point <315 °C.
• HPLC with Refractive Index Detection (HPLC‐RI, EN 12916 / ASTM D6379 / D6591): Polar NH2/CN column, n-heptane mobile phase, calibrated with o-xylene (mono), fluorene (di), phenanthrene (tri).
• Comprehensive GC×GC‐FID (ASTM D8396): Two‐dimensional separation (DB-17 ms / DB-1 HT), helium carrier, flame‐ionization detection, quantitative carbon‐number response correction.
• Reference saturate isolation: Ag–SiO2 adsorption to isolate saturates; aromatics = 100 − saturates.

Validation with Model Compounds
• FIA misclassifies common pyrolysis‐oil dienes (α,ω-dienes, limonene, α-terpinene) as aromatics, causing overestimation.
• HPLC‐RI exhibits non‐uniform refractive‐index responses: cycloaromatics and aromatic olefins show higher signals than alkylbenzenes. Diasromatic response varies widely (76–100 % vs. fluorene standard). Triaromatic response often exceeds phenanthrene calibration. Compounds eluting outside integration windows are lost.
• GC×GC‐FID provides uniform FID response across structures and resolves co-elutions, enabling accurate group quantification.

Main Results and Discussion
Kerosenes (150–250 °C fractions):
• Crude oils: FIA reports ~40 % higher aromatics due to dienes/heteroatom detection. HPLC‐RI overestimates monoaromatics by >5 % vs. GC×GC; diaromatics vary with calibration choice (fluorene vs. 1-methylnaphthalene).
• Deep hydrotreatment (360 °C/10 MPa): All methods align within 5 %; HPLC‐RI still ~10 % higher.
• Aviation‐grade requirement (max 25 vol % aromatics): All methods reliably assess compliance.
Gas Oils (250–360 °C fractions):
• Trends mirror kerosenes: steep total aromatics drop at severe hydrotreatment; initial ring hydrogenation yields monoaromatics.
• HPLC‐RI overreports monoaromatics (cycloaromatic‐rich samples) and underreports polyaromatics by 30–100 %.
• GC×GC vs. saturate reference: GC×GC aromatics within ±5 % of measured 100 − saturates for polyolefin oils; slight overestimate in deep‐treated tire oils.
Neat Pyrolysis Oils:
• FIA unsuitable for boiling >315 °C and high heteroatom/olefin content; only valid on fully treated light pyrolysis oils.
• HPLC‐RI yields 10 % lower aromatics vs. GC×GC; response variability drives underestimation.

Practical Benefits and Applications
GC×GC‐FID emerges as the most reliable approach for aromatic analysis of pyrolysis oils and fractions, especially when tracking hydrotreatment progress or meeting distillate specifications. While FIA and HPLC‐RI offer faster throughput, they suffer significant biases in complex pyrolysis matrices. Accurate aromatic quantification via GC×GC supports process optimization, quality control, and end‐use compliance (fuels, steam cracking feedstocks).

Future Trends and Potential Uses
• Wider adoption of GC×GC‐FID and method standardization (e.g., ASTM D8396 interlaboratory validation) for non‐petroleum feedstocks.
• Improved calibration for HPLC‐RI: structure‐specific factors or compound‐class mixes to reduce response bias.
• Expanded use of alternative detectors (GC‐VUV, SFC‐FID) and data‐analysis algorithms to automate group‐type assignment.
• Integration of high‐resolution MS in 2D GC workflows for compositional insights and deconvolution of heteroatoms/olefins.
• Feedback into hydrotreatment catalyst and process design based on accurate aromatic group quantification.

Conclusions
In pyrolysis‐oil matrices rich in olefins and heteroatoms, conventional FIA and HPLC‐RI methods misestimate aromatic content due to co‐elution, non‐uniform detector response, and calibration limitations. GC×GC‐FID provides superior separation and quantitative accuracy, aligning closely with independent saturate‐isolation data. For robust monitoring of aromatic content in waste plastics and tire pyrolysis oils and their hydrotreatment products, GC×GC‐FID is recommended despite longer analysis times. HPLC-RI may be retained for routine middle‐distillate screening if calibrated with representative standards; FIA is limited to light, fully treated cuts.

References
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