Characteristics Analysis of the Dechlorination Stage in Co-Pyrolysis of Sawdust and PVC

Understanding the dechlorination stage during co-pyrolysis of sawdust and polyvinyl chloride (PVC) is essential for controlling chlorine migration, suppressing hydrogen chloride emissions, and upgrading pyrolysis oil quality in biomass–plastic waste valorization. This article presents a systematic characteristics analysis of the dechlorination stage, covering thermogravimetric behavior, evolved gas profiles, chlorine retention in solid residues, and reaction kinetics evaluated according to internationally recognized methods in accredited third-party laboratories. The results provide practical guidance for blend ratio selection, temperature staging, and clean co-pyrolysis process design.

Technical Overview

Thermal degradation of PVC proceeds through two well-separated mass-loss stages. The first stage, occurring approximately between 230 °C and 360 °C, is the dechlorination stage. Weakly bonded chlorine at structural defects such as allylic chlorides, head-to-head configurations, and tertiary chlorine sites initiates a zip-like elimination reaction that strips hydrogen chloride (HCl) from the polymer backbone, converting PVC into conjugated polyene sequences. In this stage, more than 90–95% of the total organically bound chlorine is released, and the liberated HCl further autocatalyzes dehydrochlorination. The second stage, typically above 400 °C, involves scission and cyclization of the polyene chains into aromatic hydrocarbons.

Sawdust, composed mainly of hemicellulose, cellulose, and lignin, decomposes in the range of approximately 200–400 °C, which strongly overlaps the PVC dechlorination window. This overlap creates substantial interaction between the two feedstocks. Hydrogen-rich radicals and volatile fragments released from the biomass matrix can stabilize chlorine-terminated chain ends, while inherent alkali and alkaline earth metals (K, Ca, Mg, Na) in the sawdust ash react with evolved HCl to form stable inorganic chlorides such as KCl and CaCl2, retaining chlorine in the solid residue. Simultaneously, HCl from PVC catalyzes dehydration and char-forming reactions of cellulose, slightly lowering its decomposition temperature and increasing solid yield. The net effect observed in most blends is an earlier dechlorination onset, a broadened and attenuated HCl release peak, and a shift of chlorine distribution from the gas and oil phases toward the char phase.

Characterizing the dechlorination stage therefore requires quantifying temperature boundaries, mass-loss behavior, HCl evolution dynamics, chlorine partitioning among char, oil, and gas, and the apparent kinetic parameters of the dehydrochlorination reaction in both pure and blended systems.

Testing Samples

Pure Component Reference Samples

Virgin PVC resin powder (defined particle size, e.g., below 0.2 mm) and air-dried sawdust of a documented species, such as pine or fir, sieved to a narrow particle size fraction were prepared as baseline references. Proximate analysis (moisture, volatile matter, fixed carbon, ash) and ultimate analysis (C, H, O, N, S, Cl) were performed on each feedstock so that all subsequent interactions could be attributed to co-pyrolysis chemistry rather than compositional drift.

Sawdust–PVC Blend Samples

Binary blends were prepared at mass ratios of 3:1, 1:1, and 1:3 (sawdust to PVC) by mechanical mixing to homogeneity. Pre-drying at 105 °C ensured that moisture evolution did not interfere with the dechlorination signal. Each blend ratio corresponds to a distinct chlorine load and a distinct biomass-derived hydrogen and mineral supply, allowing the promoting and inhibiting effects to be separated as a function of blend composition.

Pretreated and Control Samples

Acid-washed (demineralized) sawdust was included as a mechanistic control. By removing alkali and alkaline earth metals before pyrolysis, any reduction in chlorine retention confirms the role of mineral capture, while any residual synergistic effect is attributed to radical hydrogen transfer. All samples were stored in desiccators prior to analysis.

Testing Items

The characterization program for the dechlorination stage included the following test items:

• Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves under inert atmosphere for all samples
• Onset, peak, and endset temperatures of the dechlorination stage and associated mass-loss fractions
• Temperature-resolved HCl evolution profiles quantified from evolved gas analysis
• Chlorine retention ratio in solid residues (char) after staged heating at target dechlorination temperatures
• Chlorine content in condensable pyrolysis oil and non-condensable gas for chlorine mass balance
• Identification of chlorinated organic species (e.g., chlorobenzenes, chlorophenols) in the oil phase
• Apparent activation energy and pre-exponential factor of the dehydrochlorination reaction
• Morphology and chlorine distribution mapping of chars by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS)
• Functional group evolution of solid residues by Fourier-transform infrared spectroscopy (FTIR)

Testing Methods

Thermogravimetric Analysis

Simultaneous TG/DTG experiments were conducted in high-purity nitrogen at flow rates of 60–100 mL/min, using sample masses of 5–10 mg and heating rates of 5, 10, 20, and 40 °C/min from ambient temperature to 600 °C or above. Testing followed the general principles of ISO 11358-1 for polymer thermogravimetry and ASTM E1131 for compositional analysis by thermogravimetry. Multiple heating rates enable iso-conversional kinetic evaluation of the dechlorination stage.

Evolved Gas Analysis by TG-FTIR

A heated transfer line (approximately 200 °C) coupled the thermobalance to an FTIR spectrometer so that evolved gases were monitored in real time. HCl was identified and quantified through its characteristic absorption in the 2600–3000 cm-1 stretching region using external calibration, generating temperature-resolved release curves. CO, CO2, CH4, light hydrocarbons, and oxygenated compounds were tracked simultaneously to reveal interaction pathways between HCl and biomass volatiles.

Flash Pyrolysis-GC/MS

Pyrolysis-gas chromatography/mass spectrometry was performed at temperatures bracketing the dechlorination stage (e.g., 300, 350, and 400 °C) to identify chlorinated aromatics and hydrocarbon products at quasi-steady stage conditions. This complements TG-FTIR by resolving individual organochlorine species that would otherwise contaminate downstream oil fractions.

Chlorine Quantification and Mass Balance

Total chlorine in feedstocks, chars, and oils was determined by oxygen-flask or bomb combustion followed by ion chromatography or potentiometric titration, consistent with the approach of ISO 587 for chlorine in solid fuels. X-ray fluorescence (XRF) spectroscopy provided rapid semi-quantitative screening of chlorine and alkali metals in solids. Chlorine retention ratios were calculated as the fraction of feedstock chlorine fixed in the char after defined heat treatments, and a full chlorine mass balance across char, oil, and gas phases closed within acceptable analytical uncertainty.

Kinetic Analysis

Model-free iso-conversional methods, including Kissinger-Akahira-Sunose (KAS), Ozawa-Flynn-Wall (OFW), and Starink approaches, were applied to multi-heating-rate data to obtain apparent activation energy as a function of conversion. Model-fitting with the Coats-Redfern method was used to infer probable reaction mechanisms. Kinetic testing practice aligned with ASTM E2890 for the determination of kinetic parameters by thermogravimetry. In typical pure PVC, the dehydrochlorination stage exhibits an apparent activation energy of roughly 80–130 kJ/mol, while blends show composition-dependent shifts reflecting interaction effects.

Applications

Dechlorination stage characterization directly supports several engineering applications:

• Two-stage co-pyrolysis process design, in which a low-temperature stage (280–350 °C) preferentially strips chlorine as HCl before the oil-forming, high-temperature cracking stage, producing low-chlorine pyrolysis oil
• Blend ratio optimization for municipal solid waste and refuse-derived fuel processing, where PVC is the dominant chlorine carrier and sawdust or other biomass acts as both a hydrogen donor and a chlorine scavenger
• Corrosion mitigation in boilers, gasifiers, and downstream condensers by predicting HCl release peaks and verifying chlorine fixation in char
• Compliance with fuel and oil quality specifications that impose strict limits on organic and inorganic chlorine content
• Production of chlorine-functionalized or metal-chloride-loaded chars for adsorbent and catalytic applications
• Reactor scale-up, where kinetic parameters feed residence time and heat-transfer models for rotary kilns, fluidized beds, and screw reactors

FAQ

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

The dechlorination stage spans approximately 230–360 °C, with a maximum HCl release rate near 290–320 °C at moderate heating rates. In blends with sawdust, the onset temperature typically shifts 10–25 °C lower and the release peak broadens due to heat-transfer and radical interaction effects.

How does sawdust affect PVC dechlorination?

Sawdust influences dechlorination through two mechanisms. First, hydrogen-rich radicals from decomposing cellulose, hemicellulose, and lignin help saturize chlorine-bearing chain ends and can accelerate HCl elimination. Second, inherent potassium, calcium, and magnesium in the biomass ash capture HCl as stable inorganic chlorides, retaining a significant portion of chlorine in the char and reducing both gas-phase HCl and organochlorines in the oil.

Does a higher sawdust proportion always improve dechlorination performance?

No. Although chlorine capture generally improves with biomass share, excessive sawdust lowers the effective energy density of the feed, increases char yield, and dilutes hydrocarbon recovery from PVC. An optimum blend ratio, often near equimass or moderately biomass-rich conditions depending on the target product, balances chlorine fixation against oil yield and quality, and should be established experimentally for each feedstock pair.

Why is dechlorination control important for pyrolysis oil quality?

Chlorine in pyrolysis oil causes corrosion of storage and processing equipment, deactivates upgrading catalysts, and typically exceeds the chlorine limits of commercial fuel specifications. Removing chlorine during a dedicated low-temperature stage before the main oil-forming reactions is the most economical route to compliant, upgradable pyrolysis liquids.

What experimental evidence confirms the alkali metal capture mechanism?

Comparative tests with acid-washed (demineralized) sawdust demonstrate sharply reduced chlorine retention in char relative to raw sawdust blends, while SEM-EDS mapping shows chlorine co-located with potassium and calcium in char particles. Together with diminished KCl and CaCl2 signatures in XRF spectra, these results confirm mineral capture as a principal chlorine sink.

Which standards govern the testing of the dechlorination stage?

Thermogravimetry follows ISO 11358-1 and ASTM E1131, kinetic analysis follows ASTM E2890, and chlorine determination in solid fuels follows ISO 587. All analyses described here can be commissioned at accredited third-party laboratories operating under ISO/IEC 17025-accredited quality systems, ensuring traceable and defensible results.