Kinetic Evaluation of Dechlorination Reactions in the Co-Pyrolysis of Wood Sawdust and PVC

This article presents a systematic kinetic evaluation of dechlorination reactions during the co-pyrolysis of wood sawdust and polyvinyl chloride (PVC), combining multi-heating-rate thermogravimetric analysis with evolved gas detection and chlorine mass balance determination. By applying isoconversional and model-fitting methods, the apparent activation energy, pre-exponential factor, and most probable reaction mechanism of the dechlorination stage are quantified. The resulting kinetic data support co-pyrolysis reactor design, chlorine migration control, corrosion mitigation, and the production of low-chlorine fuels and chemicals.

Technical Overview

The thermal decomposition of polyvinyl chloride (PVC) proceeds through a distinct dechlorination stage in which labile allylic chlorine substituents eliminate as hydrogen chloride (HCl), typically between approximately 200 °C and 360 °C, followed by polyene chain scission and aromatic char formation between approximately 400 °C and 550 °C. When PVC is co-pyrolyzed with lignocellulosic biomass such as wood sawdust, the hemicellulose (220–315 °C) and cellulose (315–400 °C) decomposition windows overlap the dechlorination window, producing measurable interaction effects on mass loss, chlorine release, and char chemistry.

A rigorous kinetic evaluation of the dechlorination reaction quantifies the apparent activation energy (Ea), the pre-exponential factor (A), the most probable mechanism function f(α), and the associated thermodynamic quantities (ΔH‡, ΔG‡, ΔS‡). These parameters determine how rapidly chlorine is liberated at any given temperature and residence time, how the sawdust matrix promotes or inhibits HCl evolution, and whether kinetic synergy occurs in the blend relative to the weighted behavior of the individual components.

The evaluation is performed by accredited third-party laboratories using multi-heating-rate thermogravimetric analysis coupled with evolved gas analysis, supported by chlorine mass balance determination across the char, condensable oil, and permanent gas fractions. Results are directly applicable to reactor design, chlorine migration control, corrosion mitigation, and the production of low-chlorine bio-oil and syngas.

Testing Samples

PVC Materials

Virgin suspension-grade PVC resin with a defined degree of polymerization (e.g., K-value 66–68), rigid PVC granules, and recycled PVC from profiles, films, or cable insulation. Plasticizer and thermal stabilizer contents are documented because these additives can shift the dechlorination onset temperature by 10–40 °C.

Woody Biomass

Pine, fir, poplar, or mixed hardwood sawdust sieved to below 0.5 mm and oven-dried at 105 °C to constant mass. Ash composition, particularly alkali and alkaline earth metals (K, Ca, Mg), is recorded because these species can catalyze dechlorination and promote in-situ chlorine fixation as KCl and CaCl2.

Sawdust–PVC Blends

Binary mixtures prepared at sawdust-to-PVC mass ratios of 3:1, 1:1, and 1:3, homogenized by grinding and mixing to eliminate concentration gradients. The blend series brackets the interaction range and enables deviation analysis against the additive non-interacting baseline.

Reference and Spike Samples

Certified reference materials and laboratory-prepared standards with known chlorine content, used for calibration verification, method blank correction, and recovery validation.

Testing Items

The evaluation scope covers sample characterization, dechlorination conversion measurement, kinetic parameter determination, and chlorine speciation across product phases:

• Proximate analysis (moisture, volatile matter, fixed carbon, ash) by thermogravimetry per ASTM E1131
• Ultimate analysis (C, H, N, S) and calorific value of each component and blend
• Total chlorine, water-soluble chlorine, and organic chlorine by oxygen-bomb combustion followed by ion chromatographic or ion-selective electrode detection (ASTM D4208 / ISO 587 equivalents)
• Mass loss (TG) and derivative (DTG) profiles under inert atmosphere at multiple heating rates
• Dechlorination conversion degree α(T) and dechlorination rate dα/dT over the 150–400 °C window
• Apparent activation energy Ea as a function of conversion by isoconversional analysis
• Pre-exponential factor A, reaction order n, and most probable mechanism function f(α)
• Thermodynamic activation parameters: change in enthalpy ΔH‡, Gibbs free energy ΔG‡, and entropy ΔS‡
• HCl evolution onset temperature, peak temperature, completion temperature, and instantaneous release rate
• Chlorine distribution and mass balance among pyrolysis char, tar/bio-oil, and gas at selected final temperatures
• Synergy indices comparing experimental blend behavior with linearly weighted individual-component behavior

Testing Methods

Multi-Heating-Rate Thermogravimetry

Samples of 5–10 mg are heated from ambient temperature to 900 °C under high-purity nitrogen (flow 60–100 mL/min) at heating rates of 5, 10, 20, and 40 K/min, following the general conventions of ASTM E1131 and ISO 11358. Small sample masses and shallow crucibles minimize heat and mass transfer artifacts so that the measured rates reflect intrinsic chemical kinetics.

Isoconversional Model-Free Kinetic Analysis

Apparent activation energies are extracted at conversion steps from α = 0.05 to 0.95 using the Friedman differential method and the integral Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), and Starink methods. Agreement among the methods within approximately 5–10 kJ/mol confirms data consistency and distinguishes the dechlorination-controlled region from biomass-devolatilization-controlled and char-forming regions.

Model Fitting and Mechanism Discrimination

The Coats–Redfern method is applied against a library of reaction models, including first-order, nucleation (Avrami–Erofeev), one-, two-, and three-dimensional diffusion (including Ginstling–Brounshtein), and contracting geometry models. The optimal mechanism is confirmed by master-plot analysis, and the complete kinetic triplet (Ea, A, f(α)) is reported with 95% confidence intervals derived from regression statistics.

Evolved Gas Analysis

Online TG–FTIR monitors the HCl absorption band near 2880 cm⁻¹, while TG–MS tracks the m/z 36 and m/z 38 isotopologue signals, providing time-resolved HCl release profiles that are cross-correlated with DTG mass loss to isolate the dechlorination contribution. Transfer lines are maintained above 180 °C to prevent HCl adsorption and condensation losses.

Chlorine Mass Balance and Speciation

Chars and condensates collected at defined final temperatures (300, 400, and 550 °C) are combusted in an oxygen bomb or extracted with hot water, and chloride is quantified by ion chromatography following ASTM D4327-type methodology. Water-insoluble organic chlorine in tar is determined after Eschka-type fusion, enabling a closed chlorine mass balance across the three product phases.

Data Reduction and Quality Assurance

Peak deconvolution of overlapping DTG bands separates the hemicellulose, cellulose, PVC dechlorination, and PVC polyene-cracking contributions. Each condition is run in triplicate, blank and reference-material runs bracket each batch, and reported uncertainties combine instrumental repeatability with regression error.

Applications

• Design and scale-up of co-pyrolysis reactors for waste PVC and biomass, including determination of optimal temperature ramps and residence times for maximal chlorine release ahead of oil-forming reactions
• Formulation of refuse-derived fuel (RDF) and engineered feedstock blends with predictable HCl emission profiles for gasification and combustion systems
• Screening of chlorine-fixing additives such as CaO, Ca(OH)2, KOH, and zeolites by quantifying their effect on dechlorination activation energy and residual organic chlorine in char
• Corrosion mitigation in boilers, syngas coolers, and downstream equipment through kinetic prediction of HCl release timing and concentration
• Production of low-chlorine bio-oil and chlorine-lean syngas, supporting fuel-quality compliance and downstream catalyst protection
• Environmental compliance documentation for chlorinated waste co-processing, including management of chlorinated aromatic precursor formation via kinetic control of the 250–450 °C window
• Academic and industrial R&D support, providing reproducible kinetic triplets and thermodynamic parameters for model validation and publication

FAQ

At what temperature does PVC dechlorination occur during co-pyrolysis with sawdust?

In nitrogen, PVC releases the majority of its chlorine as HCl between approximately 220 °C and 360 °C. In sawdust blends, the onset typically shifts slightly lower because biomass-derived radicals and hydrogen donors accelerate allylic chlorine elimination, while alkali metals in the wood ash can simultaneously capture part of the released chlorine as stable inorganic chlorides.

Why are multiple heating rates required for the kinetic evaluation?

Single-heating-rate experiments cannot separate the temperature dependence of the rate constant from the conversion dependence of the mechanism function, which often produces compensating, non-unique kinetic triplets. Isoconversional analysis at four heating rates resolves Ea(α) without assuming a reaction model, revealing whether the dechlorination mechanism changes across the conversion range.

How is the degree of dechlorination calculated?

The dechlorination conversion is computed as α = (m0 − mt)/(m0 − mf) restricted to the dechlorination DTG band, or directly from cumulative HCl evolution as the fraction of total chlorine released up to temperature T. Both definitions are reported, with the HCl-based value preferred whenever evolved gas data are available.

How is synergy between sawdust and PVC assessed?

Experimental mass loss, HCl evolution, and kinetic parameters of the blend are compared with the mass-weighted average of the neat components measured under identical conditions. Statistically significant deviations, typically expressed as synergy indices, indicate that interactions genuinely alter the dechlorination pathway rather than reflecting simple additive decomposition.

Which conversion range should be used for isoconversional analysis?

The reliable window is generally α = 0.05–0.95, with data below 0.05 and above 0.95 excluded because baseline drift and tailing effects inflate regression error. For dechlorination specifically, the analysis is confined to the isolated DTG band after deconvolution so that Ea reflects the C–Cl bond scission and HCl elimination step rather than overlapping cellulose decomposition.

What typical activation energies are expected?

Reported apparent activation energies for PVC dechlorination generally fall between 80 and 150 kJ/mol depending on PVC grade, stabilizer content, and the kinetic method applied. Sawdust blends commonly show moderate reductions of 5–20 kJ/mol in the dechlorination band, consistent with hydrogen-transfer assistance from biomass volatiles; exact values are sample-specific and must be established experimentally.

Which standards govern the testing?

Thermogravimetric procedures follow ASTM E1131 and ISO 11358 conventions, thermal stability screening references ASTM E2550, and chlorine determination references ASTM D4208, ISO 587, and related combustion-ion chromatography methodologies. All work is performed by accredited third-party laboratories under documented quality management systems.