Determination of Aromaticity in Co-Pyrolysis Products: Analytical Methods, Standards, and Applicatio

Determination of aromaticity in co-pyrolysis products is a critical analytical task for evaluating the quality, stability, and downstream value of oils, tars, and chars produced from the co-pyrolysis of biomass, plastics, coal, and sludge. Accurate aromaticity measurement relies on techniques such as quantitative 13C NMR, 1H NMR, FTIR spectroscopy, XRD, and elemental analysis, many of which are covered by ASTM and IP standards. This article provides a comprehensive technical guide to sample types, testing items, analytical methods, quality control, and typical applications of co-pyrolysis product aromaticity determination in accredited third-party laboratories.

Technical Overview

Co-pyrolysis is the simultaneous thermal decomposition of two or more feedstocks—such as biomass with waste plastics, coal with agricultural residues, or municipal sludge with lignite—under an oxygen-free or oxygen-limited atmosphere. The interaction between feedstocks frequently produces synergistic effects that significantly alter the yield and chemical structure of the resulting oils, tars, and solid chars compared with the pyrolysis of each feedstock alone.

Aromaticity describes the fraction of carbon or hydrogen atoms located in aromatic ring structures within a sample. It is one of the most important structural descriptors for co-pyrolysis products because it directly correlates with heating value, viscosity, aging stability, coking propensity, soot formation tendency, and the suitability of a product as a precursor for carbon materials or as a feedstock for downstream refining. Determining aromaticity therefore allows researchers and process engineers to quantify synergy effects, optimize feedstock blending ratios and reactor temperatures, and assess product compliance with fuel and chemical specifications.

No single parameter fully defines aromaticity. Depending on the physical state of the product, laboratories report complementary indicators, including the total aromatic carbon fraction (fa) from 13C NMR, aromatic proton fraction (Ha) from 1H NMR, semi-quantitative infrared aromaticity indices, crystallite structural parameters for solid chars, and the concentrations of individual aromatic compounds such as BTX and polycyclic aromatic hydrocarbons (PAHs) obtained by chromatographic methods.

Testing Samples

Liquid Co-Pyrolysis Products

Liquid products include bio-oil and co-pyrolysis tar generated from fixed-bed, fluidized-bed, auger, or rotary reactors. Typical feedstock combinations submitted for aromaticity testing include biomass and polyethylene, biomass and polystyrene, coal and biomass, and sludge and coal blends. Liquid samples are homogenized, moisture-corrected, and dissolved in an appropriate deuterated solvent prior to spectroscopic analysis.

Solid Co-Pyrolysis Products

Solid products include semi-coke, char, and biochar produced during co-pyrolysis. These materials are dried, ground to a fine and uniform particle size, and sieved to ensure representative sampling. Solid-state spectroscopic and diffraction techniques are applied to determine the aromatic carbon fraction, the degree of aromatic condensation, and the size of aromatic crystallite domains.

Heavy and Intermediate Fractions

Waxes, pitches, heavy oil fractions, and asphaltene-rich residues separated from co-pyrolysis liquids are frequently analyzed individually, because aromaticity in these fractions strongly influences upgrading behavior and coke formation. Fractionation according to saturates, aromatics, resins, and asphaltenes may be performed before measurement to improve data interpretation.

Testing Items

Common testing items for co-pyrolysis product aromaticity determination include the following parameters:

• Total aromatic carbon fraction (fa) and aliphatic carbon fraction
• Aromatic proton fraction (Ha) and aliphatic hydrogen distribution (Hα, Hβ, Hγ)
• Aromatic substitution index and degree of ring condensation
• Bridgehead carbon and protonated aromatic carbon contents
• Aromatic C–O and carboxyl carbon contributions (phenolic and carboxylic functionalities)
• FTIR-based semi-quantitative aromaticity index
• XRD crystallite parameters of chars, including interlayer spacing (d002), stacking height (Lc), lateral size (La), and XRD-derived aromaticity
• Raman D-band to G-band intensity ratio (ID/IG) as an indicator of aromatic structural ordering
• Atomic carbon-to-hydrogen ratio and Brown–Ladner structural parameters
• Quantification of individual aromatic compounds, including benzene, toluene, ethylbenzene, xylenes, naphthalene, and priority PAHs

Testing Methods

Quantitative Carbon-13 Nuclear Magnetic Resonance

13C NMR is widely regarded as the most direct method for determining aromatic carbon fraction. Liquid products are analyzed by liquid-state 13C NMR with inverse-gated decoupling to suppress the nuclear Overhauser effect and enable quantitative integration, while chars are analyzed by solid-state cross-polarization with magic-angle spinning (CP/MAS) techniques, often combined with dipolar dephasing to isolate non-protonated aromatic carbons. The aromatic carbon fraction is calculated from the integrated signal area between approximately 90 and 165 ppm relative to the total carbon signal. Standard practices for high-resolution NMR spectroscopy and related ASTM methods, including ASTM D5292 for aromatic carbon contents of hydrocarbon oils, provide the basis for method validation.

Proton Nuclear Magnetic Resonance

1H NMR classifies hydrogen atoms into aromatic and aliphatic environments. The aromatic proton fraction is obtained by integrating the 6.0–9.0 ppm region, while alpha, beta, and gamma aliphatic hydrogen fractions are integrated across the remaining spectral regions. When combined with elemental analysis, 1H NMR data support the calculation of Brown–Ladner structural parameters, which estimate aromaticity, substitution degree, and ring condensation. IP 468 and ASTM D5292 describe the application of high-resolution NMR to the determination of aromatic hydrogen and aromatic carbon in petroleum-derived and related hydrocarbon streams.

Fourier Transform Infrared Spectroscopy

FTIR provides a rapid, low-cost estimate of aromaticity. The aromatic C–H stretching band near 3030 cm-1 is compared with aliphatic C–H stretching bands between 2800 and 3000 cm-1 to calculate a semi-quantitative aromaticity factor. Additional aromatic ring stretching bands near 1600 and 1500 cm-1 and out-of-plane C–H bending bands between 700 and 900 cm-1 are curve-fitted to evaluate substitution patterns of aromatic rings. FTIR is particularly valuable for tracking relative aromaticity changes across process conditions in large sample sets.

X-Ray Diffraction and Raman Spectroscopy for Solid Chars

For semi-coke and char samples, XRD evaluates the stacking structure of aromatic layers. The aromaticity is estimated from the ratio of the (002) aromatic band area to the combined area of the (002) band and the gamma band associated with aliphatic side chains, while Bragg and Scherrer equations yield interlayer spacing, crystallite height, and lateral aromatic layer size. Raman spectroscopy complements XRD by resolving the disorder-related D band and graphitic G band; the ID/IG ratio serves as a sensitive indicator of aromatic ordering and graphitization degree during co-pyrolysis and subsequent heat treatment.

Elemental Analysis and Correlation Methods

Carbon, hydrogen, nitrogen, and sulfur contents determined according to ASTM D5373 and ASTM D3176 provide the atomic H/C ratio, a classical first-order indicator of aromaticity. Plotting results on a van Krevelen diagram allows visualization of aromatization and deoxygenation trends among different feedstock blends and process severities. Elemental data are also essential inputs for NMR-based structural parameter calculations.

Chromatographic and Spectrophotometric Quantification of Aromatics

Gas chromatography–mass spectrometry (GC-MS) identifies and quantifies volatile aromatic species, including BTX and light PAHs, with reference to methods such as ASTM D5769 for aromatic hydrocarbons in spark-ignition engine fuels. High-performance liquid chromatography methods, including those consistent with ASTM D6379 and ASTM D6591, determine mono-, di-, and polynuclear aromatic hydrocarbon type contents in middle-distillate-range products. Ultraviolet fluorescence and ultraviolet-visible absorption spectroscopy, including synchronous scanning techniques, characterize the distribution of aromatic ring sizes in heavy fractions that are not directly amenable to chromatographic analysis.

Applications

Aromaticity determination for co-pyrolysis products supports a wide range of research, industrial, and regulatory activities:

• Quantifying synergistic effects between biomass, plastics, coal, and sludge blends and optimizing blend ratios and pyrolysis temperatures
• Assessing the stability, aging behavior, and heating value of co-pyrolysis oils for renewable fuel development
• Evaluating semi-coke and char as precursors for activated carbon, needle coke, carbon fibers, and graphitic materials
• Screening catalysts and upgrading processes, such as hydrodeoxygenation and catalytic cracking, by tracking aromaticity reduction or aromatics enrichment
• Monitoring PAH formation to control toxic and carcinogenic compound levels in products and process streams
• Supporting academic research, pilot-plant scale-up studies, and quality documentation required for commercialization

FAQ

Q: Why is aromaticity a key quality indicator for co-pyrolysis products?

A: Aromaticity governs heating value, viscosity, thermal stability, and coking tendency in liquids, and reflects the degree of carbonization in solid chars. It also indicates how strongly the co-pyrolysis process has transformed the feedstock structure, making it essential for comparing products and quantifying synergy.

Q: Which method provides the most reliable aromaticity value?

A: Quantitative 13C NMR is generally considered the most direct and defensible technique for aromatic carbon fraction. However, best practice combines NMR with 1H NMR, FTIR, elemental analysis, and, for solids, XRD and Raman spectroscopy to obtain a complete structural picture.

Q: How do synergistic effects in co-pyrolysis influence aromaticity?

A: Hydrogen-rich plastics can donate hydrogen to biomass-derived radicals, suppressing repolymerization and reducing char aromaticity while enriching light aromatics in the oil. Conversely, interactions between certain feedstocks can promote condensation and increase the aromaticity of heavy fractions and chars.

Q: What sample preparation is required?

A: Liquid samples are homogenized, moisture-adjusted, and dissolved in deuterated solvents; solid samples are dried, ground, and sieved to a uniform particle size. Heavy fractions may undergo column-based separation to isolate aromatic-rich components before analysis.

Q: How is data quality assured?

A: Accredited third-party laboratories apply calibrated instrumentation, internal standards, replicate measurements with relative standard deviation controls, and reference material checks. Reports typically include raw spectra, integration regions, calculated aromaticity parameters, and measurement uncertainty.

Q: What is a typical turnaround time?

A: Routine FTIR or elemental analysis can often be completed within a few working days, whereas quantitative NMR and comprehensive multi-technique characterization packages may require one to two weeks depending on sample throughput and the number of requested parameters.