Thermogravimetric Analysis of Dechlorination During Co-Pyrolysis of Sawdust and PVC: Experimental Pr

Thermogravimetric analysis (TGA) is the core experimental technique for investigating dechlorination behavior during the co-pyrolysis of sawdust and polyvinyl chloride (PVC), providing quantitative insight into mass loss stages, reaction kinetics, and chlorine release characteristics. This technical article presents a comprehensive overview of the sawdust–PVC pyrolysis dechlorination TGA experiment, covering sample preparation, testing items, instrument protocols, kinetic evaluation methods, and industrial applications. Engineers, researchers, and laboratory professionals can use this guide to design reproducible TGA experiments and interpret dechlorination performance data for biomass–plastic co-processing systems.

Technical Overview

Co-pyrolysis of sawdust and PVC is a widely studied thermochemical conversion route that combines a renewable lignocellulosic biomass with a chlorine-rich plastic waste stream. When heated under an inert atmosphere, PVC decomposes in two distinct stages. The first stage, typically occurring between 200 °C and 360 °C, is the dehydrochlorination step, in which nearly all of the bound chlorine is liberated as hydrogen chloride (HCl) gas while a polyene char skeleton remains. The second stage, generally between 400 °C and 550 °C, involves scission and aromatization of the polyene chains to yield aromatic oils and carbonaceous residue.

Sawdust pyrolysis overlaps significantly with the PVC dehydrochlorination window. Hemicellulose decomposes between approximately 220 °C and 315 °C, cellulose between 315 °C and 400 °C, and lignin degrades progressively across a broad range from roughly 160 °C to 900 °C. This thermal overlap creates strong interactions between the two materials: the released HCl can catalyze dehydration and char formation in cellulose, while alkali and alkaline earth metals (AAEMs) naturally present in biomass, such as potassium, calcium, and magnesium, can act as in-situ chlorine scavengers that retain part of the chlorine as stable inorganic chlorides in the solid char. In addition, hydrogen-rich radicals from biomass can stabilize PVC-derived radicals, altering product distribution.

Thermogravimetric analysis measures the mass of a milligram-scale sample continuously as a function of temperature or time under a precisely controlled atmosphere and heating program. The first derivative of the mass loss curve (DTG) resolves overlapping reaction events into discrete peaks, enabling quantitative determination of characteristic temperatures, stage-resolved mass losses, residual char yields, and apparent kinetic parameters. When TGA is coupled with evolved gas analysis techniques or followed by chlorine quantification of the solid residue, the degree of dechlorination achieved during co-pyrolysis can be directly evaluated, making the sawdust–PVC dechlorination TGA experiment a fundamental tool for process design and mechanism research.

Testing Samples

Sawdust (Biomass Feedstock)

Commercial or laboratory-prepared sawdust from species such as pine, poplar, fir, or birch is dried to below 5 wt% moisture, milled, and sieved to a narrow particle size fraction, typically below 0.25–0.5 mm, to minimize heat and mass transfer gradients within the TGA sample bed. Prior to testing, the sawdust is characterized for moisture, volatile matter, fixed carbon, and ash content following biomass proximate analysis standards such as ISO 18122 and ISO 18123, and for elemental composition including carbon, hydrogen, and nitrogen according to ISO 16948. Baseline chlorine and sulfur contents should also be established, for example by reference to ISO 16994, so that chlorine mass balances account for the native halogen content of the biomass.

PVC Reference Material

The PVC sample may be a pure suspension-grade resin powder or a washed and shredded waste PVC material. The resin is dried in an oven at approximately 105 °C for 12–24 hours and sieved to a particle size comparable to the sawdust fraction. Documenting the chlorine content of the PVC (theoretically about 56.7 wt% for pure PVC) is essential because it defines the total chlorine inventory of the blend and the maximum HCl release potential.

Sawdust–PVC Blends

Binary mixtures are prepared gravimetrically at defined mass ratios, commonly spanning 10 wt% to 50 wt% PVC in sawdust (for example, 90:10, 75:25, 50:50, and 25:75 sawdust-to-PVC ratios). The two powders are homogenized thoroughly, preferably by gentle mechanical mixing or grinding together, to ensure representative sub-samples at the milligram scale. Each blend ratio is tested alongside the pure components so that experimental mass loss curves can be compared with calculated linear-additive curves to identify synergistic or inhibitory interactions.

Testing Items

The sawdust–PVC dechlorination TGA experiment delivers a structured set of quantitative results:

• TG (mass loss) and DTG (derivative mass loss) curves for pure sawdust, pure PVC, and each blend under inert atmosphere.
• Characteristic temperature parameters: onset temperature of decomposition, peak temperature of each DTG event, and final temperature of each reaction stage.
• Stage-resolved mass losses: moisture evolution, the PVC dehydrochlorination stage, biomass devolatilization, secondary cracking of the polyene structure, and char consolidation.
• Solid residue (char) yield at the final temperature, typically 600–900 °C.
• Degree of dechlorination, calculated from the chlorine mass balance between the feed blend and the collected solid residue.
• Chlorine retention ratio in the char, indicating the in-situ chlorine capture capacity of biomass mineral matter.
• Synergy index, defined as the deviation between the experimental TG/DTG curves and the weighted additive curves of the individual components.
• Apparent kinetic parameters: activation energy, pre-exponential factor, and reaction order, obtained by iso-conversional and model-fitting methods.
• Thermodynamic parameters derived from kinetics, including enthalpy change, Gibbs free energy change, and entropy change of the decomposition reactions.
• Optional evolved gas profiles of HCl, H2O, CO, CO2, and light hydrocarbons obtained by TGA coupled with Fourier-transform infrared spectroscopy (TGA-FTIR) or mass spectrometry (TGA-MS).

Testing Methods

Thermogravimetric Measurement Protocol

Tests are performed on a calibrated thermogravimetric analyzer in accordance with the general principles of ISO 11358 and, where compositional breakdown is required, ASTM E1131. A typical protocol uses 5–10 mg of sample in an open alumina or platinum pan, a high-purity nitrogen purge flow of 50–100 mL/min, and heating from ambient temperature to 800–900 °C. To support model-free kinetic analysis, identical runs are repeated at several linear heating rates, most commonly 5, 10, 20, and 40 K/min. Temperature calibration and buoyancy baseline correction are conducted with empty-pan runs and certified melting point standards, consistent with recognized thermal analysis practice such as ASTM E2550 for thermal stability determination. Each condition is tested in duplicate or triplicate to confirm repeatability, with acceptable deviation typically kept within ±2–3% on mass loss.

Dechlorination Evaluation and Chlorine Quantification

The degree of dechlorination is determined by comparing the chlorine content of the feed blend with the chlorine remaining in the pyrolysis solid residue. Residues from parallel tube-furnace experiments, or material recovered from TGA runs where feasible, are combusted or digested using oxygen flask or bomb combustion techniques, and the resulting chloride is quantified by ion chromatography following ISO 10304 or ASTM D4327, or alternatively by potentiometric titration. The dechlorination efficiency is then calculated as the percentage of feed chlorine converted to volatile species, while the chlorine retention ratio expresses the fraction captured in the char. When TGA-FTIR or TGA-MS is available, the HCl evolution profile is integrated over the dehydrochlorination window to cross-validate the chlorine mass balance.

Kinetic and Interaction Analysis

Apparent activation energies are computed from multi-heating-rate data using model-free iso-conversional methods such as Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), and the Starink method, which yield activation energy as a function of conversion and reveal multi-step behavior. Model-fitting with the Coats–Redfern approach is then applied to estimate pre-exponential factors and reaction orders for each identified stage. Interaction effects are quantified by comparing the experimental TG curve of each blend against the calculated additive curve, with differences in mass loss or DTG peak intensity at given temperatures attributed to chemical and physical synergy, such as HCl-catalyzed biomass charring or AAEM-driven chlorine fixation.

Data Reporting and Quality Control

All results are reported with explicit test conditions: sample mass, particle size, blend ratio, purge gas flow, heating rate, temperature program, and final temperature. Calibration records, replicate statistics, and instrument detection limits accompany the dataset. Where third-party verification is required, testing may be arranged through accredited third-party laboratories operating under ISO/IEC 17025 management systems, while the analytical methodologies themselves remain anchored to published ISO and ASTM standards.

Applications

The sawdust–PVC dechlorination TGA experiment supports a broad range of engineering and research objectives:

• Co-pyrolysis process design: selecting blend ratios and temperature programs that maximize volatile conversion while promoting chlorine release or capture at controlled stages.
• Corrosion mitigation: predicting HCl release profiles to protect incinerators, gasifiers, and pyrolysis reactors from high-temperature chlorine corrosion and slagging.
• Pyrolysis oil quality improvement: reducing organic chlorine contamination in bio-oil and plastic-derived oil by steering chlorine into the gas or char phase.
• Waste-to-energy feedstock formulation: blending biomass with PVC-containing refuse-derived fuels while respecting chlorine limits for combustion equipment.
• Feedstock recycling of PVC waste: establishing thermal windows in which dehydrochlorination is complete before main chain cracking, enabling chlorine-lean polyene intermediates.
• Biochar engineering: producing mineral-rich chars in which potassium and calcium chlorides are stabilized, relevant to nutrient recycling and sorbent development.
• Fundamental research: elucidating interaction mechanisms between hydrogen chloride, lignocellulosic polymers, and AAEM species during mixed feedstock pyrolysis.
• Regulatory and academic reporting: supplying standardized kinetic and compositional data for environmental permitting, lifecycle assessment, and peer-reviewed publication.

FAQ

At what temperature does PVC dechlorination occur during co-pyrolysis with sawdust? Dehydrochlorination of PVC typically begins near 200–250 °C and peaks around 280–320 °C, with most bound chlorine released as HCl below 360 °C. This window overlaps strongly with hemicellulose and cellulose decomposition in sawdust, which is the basis of the observed interactions.

How does sawdust influence PVC dechlorination? Sawdust affects dechlorination in two opposing ways. HCl released from PVC can acid-catalyze biomass dehydration and increase char yield, while AAEM species in the biomass ash can capture a portion of the chlorine as thermally stable inorganic chlorides retained in the char. The net effect depends on blend ratio, ash composition, and heating rate, and is quantified by the synergy index in the TGA experiment.

Why are multiple heating rates used in the experiment? Multiple heating rates (for example 5, 10, 20, and 40 K/min) are required by iso-conversional methods such as KAS and FWO to compute activation energy without assuming a reaction model beforehand, providing more reliable kinetics for the complex, multi-stage sawdust–PVC decomposition.

What atmosphere is used, and can the experiment be extended? High-purity nitrogen is the standard inert atmosphere. The same framework can be extended to CO2 or steam atmospheres to study co-gasification behavior, or coupled online with FTIR or MS analyzers to track HCl and other gases in real time.

How is dechlorination efficiency calculated? Dechlorination efficiency equals one minus the ratio of chlorine remaining in the solid residue to the chlorine in the original feed, expressed as a percentage. Chlorine is measured in both the feed and residue by ion chromatography per ISO 10304 or ASTM D4327, or by potentiometric titration after combustion digestion.

Which standards govern this testing? Thermogravimetric procedures reference ISO 11358 for polymer thermogravimetry, ASTM E1131 for compositional analysis by TG, and ASTM E2550 for thermal stability. Biomass characterization references ISO 18122, ISO 18123, ISO 16948, and ISO 16994, and chloride quantification references ISO 10304 and ASTM D4327.

What sample size and preparation are critical for reliable results? Small, uniform samples of 5–10 mg with particle sizes below roughly 0.5 mm minimize thermal gradients and ensure that measured mass loss reflects intrinsic reaction kinetics rather than heat or mass transfer artifacts. Thorough homogenization of blends is equally important because sub-milligram sampling errors can distort interaction analysis.