Thermal Stability Testing of Co-Pyrolysis Products: Methods, Standards, and Applications

Thermal stability testing of co-pyrolysis products is essential for evaluating the safety, storage performance, and downstream processability of oils, chars, and waxes generated from the co-pyrolysis of biomass, plastics, coal, and waste materials. Using techniques such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and accelerated aging protocols, accredited third-party laboratories quantify how these products resist degradation under thermal stress. This article explains the key testing samples, measurable parameters, standardized methods, and industrial applications of thermal stability evaluation for co-pyrolysis products.

Technical Overview

Co-pyrolysis is the simultaneous thermal decomposition of two or more feedstocks—typically biomass blended with waste plastics, coal, scrap tires, or municipal sludge—in an oxygen-free or oxygen-limited environment at temperatures generally ranging from 350 °C to 700 °C. The synergy between feedstocks can increase oil yield, reduce oxygenated compounds, and improve the calorific value of products compared with the pyrolysis of individual materials. However, the resulting liquid oils, solid chars, waxes, and condensable fractions are chemically complex and often thermally unstable.

During storage, transportation, and downstream processing, exposure to heat, oxygen, and light can trigger polymerization, oxidation, esterification, phase separation, coke formation, and progressive viscosity growth. Thermal stability testing quantifies this degradation behavior through controlled laboratory heating experiments, thermogravimetric profiling, oxidation induction measurements, and before-and-after characterization of critical quality attributes. The resulting data support blend formulation, shelf-life definition, safety classification, and commercial release decisions.

• Quantifies degradation onset temperature and mass-loss behavior under programmed heating
• Assesses oxidation resistance via onset temperature (OOT) and oxidation induction time (OIT)
• Tracks aging-induced changes in viscosity, acid number, water content, and phase behavior
• Supports quality control, regulatory compliance, and R&D optimization of co-pyrolysis processes

Testing Samples

Co-Pyrolysis Oil (Liquid Products)

Liquid fractions produced from feedstock combinations such as biomass with polyethylene, biomass with polypropylene, lignocellulosic residue with waste tires, or algae with plastics. These oils are typically heterogeneous, contain both aqueous and organic phases, and are among the most stability-sensitive co-pyrolysis products.

Co-Pyrolysis Char and Biochar

Solid carbonaceous residues recovered from the co-pyrolysis reactor. Testing evaluates char thermal resilience, residual volatile matter, fixed carbon stability, and ash behavior relevant to combustion, gasification, or soil amendment applications.

Wax and Semi-Solid Fractions

Higher-molecular-weight paraffinic and oxygenated waxes, particularly from plastic-rich feedstock blends. These fractions are assessed for melting behavior, thermal cracking tendency, and oxidation susceptibility during heated storage and handling.

Feedstock Blends (Pre-Pyrolysis Screening)

TGA-based co-pyrolysis studies are also performed on the raw blend before reaction to characterize decomposition synergy, reaction temperature windows, and predicted product distribution, guiding reactor design and blend ratio optimization.

Upgraded and Catalytically Treated Products

Hydrotreated, esterified, emulsified, or catalytically upgraded co-pyrolysis oils that require verification of improved stability relative to the crude product.

Testing Items

Thermal stability evaluation of co-pyrolysis products typically covers the following measurable parameters:

• Thermogravimetric (TG/DTG) profile: mass-loss curve, derivative peaks, and devolatilization rate
• Thermal degradation onset temperature and maximum-rate decomposition temperature
• Oxidation onset temperature (OOT) under oxygen or air atmosphere
• Oxidation induction time (OIT) at isothermal conditions, including pressure DSC variants
• Residual mass, char yield, fixed carbon, and ash content after controlled heating
• Accelerated aging stability: change in kinematic viscosity and acid number after thermal exposure
• Water content and moisture stability before and after aging
• Phase separation tendency and sediment or coke formation during heated storage
• Flash point and fire-safety screening of liquid products
• Boiling point distribution and light-end loss during simulated thermal stressing
• Heating value (higher and lower calorific value) stability across aging cycles

Testing Methods

Thermogravimetric Analysis (TGA)

Small samples (typically 5–10 mg) are heated at a controlled rate under nitrogen, air, or oxygen atmospheres while mass is continuously recorded. Relevant standards include ISO 11358 for thermogravimetry of polymers, ASTM E1131 for compositional analysis by TGA, and ASTM E2550 for thermal stability determination by TGA under inert atmosphere. TGA defines the degradation onset, rate maxima, and residue, and enables kinetic modeling of decomposition.

Differential Scanning Calorimetry (DSC) and Oxidation Testing

DSC measures heat flow associated with melting, crystallization, glass transition, and oxidative exotherms. OOT is determined per ASTM E2009, general DSC characterization follows ISO 11357 and ASTM E537, and OIT (including pressurized DSC per ASTM D5483) quantifies the time-to-oxidation under isothermal high-temperature conditions, which is a direct indicator of oxidative storage stability.

Accelerated Thermal Aging

Bulk liquid samples are stored at elevated temperatures (commonly 40 °C, 60 °C, or 80 °C) for defined periods under inert and air atmospheres. Before-and-after measurements of kinematic viscosity (ASTM D445), acid number (ASTM D664), water content (ASTM E203 or Karl Fischer), and visual phase stability allow calculation of aging rates and projected shelf life.

Flash Point and Fire-Safety Screening

Liquid co-pyrolysis products are classified for transport and handling safety using Pensky-Martens closed cup testing (ASTM D93) or rapidly equilibrated small-scale methods, with thermal history sensitivity noted through repeat testing after aging.

Char and Solid Residue Testing

Solid products are characterized using ASTM D1762 for charcoal analysis, ISO 18122 for ash content, ISO 18123 for bulk density and related properties, and EN 15148 for volatile matter, establishing the thermal robustness of chars for combustion, gasification, or soil amendment uses.

Applications

Thermal stability testing of co-pyrolysis products serves a wide range of industrial and research purposes:

• Biomass–plastic co-pyrolysis fuel development, including drop-in fuel candidate qualification
• Waste tire and mixed plastic valorization projects requiring stable pyrolysis oil supply
• Coal–biomass co-processing research and co-firing feedstock qualification
• Biochar certification for agricultural soil amendment and carbon sequestration programs
• Blending and refinery co-processing decisions where instability can foul equipment and catalysts
• Shelf-life determination and storage/transport condition definition for commercial products
• Academic and industrial R&D, kinetic modeling, and publication-grade data generation
• Quality control for continuous co-pyrolysis plants and batch process validation

FAQ

Why is thermal stability critical for co-pyrolysis oil?

Co-pyrolysis oil contains reactive oxygenated compounds, olefins, and unsaturated species that continue reacting during storage. Instability leads to viscosity growth, acidity increase, sediment formation, and phase separation, all of which impair pumpability, combustion performance, and compatibility with refinery or boiler systems.

How much sample is required for testing?

TGA and DSC methods require only 5–20 mg per run, while accelerated aging studies and post-aging property testing typically require 100–500 mL of liquid product or 200–500 g of solid char, depending on the test matrix.

How long does thermal stability testing take?

TGA, DSC, OOT, and OIT measurements are typically completed within one to three working days. Accelerated aging studies may run from several days to several weeks depending on the selected temperature and exposure duration.

Which standards govern the testing?

Commonly applied frameworks include ISO 11358, ISO 11357, ISO 18122, ISO 18123, ASTM E1131, ASTM E2550, ASTM E2009, ASTM E537, ASTM D5483, ASTM D445, ASTM D664, ASTM D93, ASTM E203, ASTM D1762, and EN 15148, selected according to product state and project objectives.

Can test results support fuel blending decisions?

Yes. By correlating oxidation induction behavior, viscosity drift, and sediment formation with blending ratios, laboratories can identify stable blend windows and define handling limits for co-processing co-pyrolysis oil with conventional diesel, heavy fuel oil, or refinery intermediates.