This article presents a systematic experimental analysis of the dechlorination mechanism during co-pyrolysis of wood sawdust and polyvinyl chloride (PVC), integrating thermogravimetric analysis, evolved gas monitoring, and chlorine mass balance testing. It explains how alkali and alkaline-earth metals in biomass char capture hydrogen chloride, how radical interactions accelerate PVC dehydrochlorination, and how chlorine is redistributed among the gas, oil, and char phases. The findings support the design of cleaner co-pyrolysis processes for chlorine-rich plastic waste and biomass valorization.
Technical Overview
Polyvinyl chloride (PVC) contains approximately 56 to 57 percent chlorine by mass, and its thermal degradation proceeds in two characteristic stages. The first stage, occurring between roughly 200 and 360 degrees Celsius, is dehydrochlorination: hydrogen chloride (HCl) is eliminated from the polymer backbone, generating conjugated polyene sequences. The second stage, between approximately 400 and 520 degrees Celsius, involves cyclization, aromatization, and chain scission of the polyene structure, releasing benzene, toluene, naphthalene, and heavier aromatics. Uncontrolled HCl release causes severe reactor corrosion, poisons downstream catalysts, and drives the formation of chlorinated organic contaminants in pyrolysis oil, which is a major barrier to energy recovery from chlorine-containing plastic waste.
Wood sawdust is an abundant lignocellulosic biomass composed mainly of cellulose, hemicellulose, and lignin, together with inherent alkali and alkaline-earth metallic (AAEM) species such as potassium, calcium, magnesium, and sodium. When sawdust and PVC are co-pyrolyzed, the AAEM species in the nascent biomass char act as natural in-situ sorbents for HCl, while radical species generated from biomass decomposition interact with the degrading PVC chain. This produces a measurable synergistic dechlorination effect that can be quantified experimentally. The principal mechanisms investigated in this experimental program are:
- Radical-assisted dehydrochlorination, in which hydrogen, hydroxyl, methoxy, and phenoxy radicals derived from biomass abstract hydrogen and chlorine-bearing sites on the PVC chain, promoting earlier and more complete HCl elimination.
- AAEM neutralization, in which potassium and calcium species in the sawdust char chemically capture HCl to form stable inorganic chlorides such as KCl and CaCl2.
- Char adsorption, in which the porous, freshly formed biomass char physically and chemically adsorbs HCl and organic chlorine intermediates.
- Chlorine redistribution, in which chlorine migrates from the volatile oil phase into the solid char and the aqueous condensate, sharply reducing organic chlorine in the pyrolysis oil.
- Cross-phase catalysis, in which released HCl in turn promotes dehydration and char-forming reactions in cellulose, altering the overall degradation profile of the blend.
The experimental program is designed to verify these mechanisms through coupled thermal analysis, controlled fixed-bed pyrolysis, chlorine mass balance quantification, and solid residue characterization, with control experiments using demineralized sawdust to isolate the contribution of AAEM species.
Testing Samples
Virgin PVC Resin Powder
Suspension-grade PVC resin with a known chlorine content of approximately 57 percent by mass is used as the reference plastic feedstock. The resin is ground and sieved to a particle size below 0.25 millimeters and dried at 105 degrees Celsius for 24 hours to eliminate moisture interference in thermogravimetric measurements.
Wood Sawdust Feedstock
Sawdust from softwood species such as pine or spruce is dried, milled, and sieved to below 0.5 millimeters. The feedstock is characterized for moisture, volatile matter, ash, fixed carbon, ultimate composition (carbon, hydrogen, nitrogen, sulfur), calorific value, and AAEM content, with water-soluble chloride and metals determined according to the EN ISO 16995 procedure for solid biofuels.
Sawdust-PVC Blend Mixtures
Binary blends are prepared at sawdust-to-PVC mass ratios of 1:2, 1:1, 2:1, and 4:1, along with the pure components as baselines. Each blend is homogenized mechanically for at least 30 minutes to ensure compositional uniformity and reproducible interaction between the two materials during heating.
Control and Product Samples
Control samples include water-washed and dilute-acid-washed sawdust from which AAEM species have been removed, used to distinguish chemical capture from purely thermal effects. Product samples comprise solid chars collected at target temperatures between 250 and 600 degrees Celsius, condensed pyrolysis oils, and aqueous condensate fractions recovered from staged quench systems.
Testing Items
The experimental program covers the following measurement categories, each of which feeds into the overall dechlorination mechanism analysis.
Thermal Behavior and Degradation Kinetics
- Initial dehydrochlorination temperature and total mass loss of PVC, sawdust, and their blends.
- DTG peak temperatures and reaction intervals for each degradation stage.
- Apparent activation energies for the dehydrochlorination step, determined by iso-conversional analysis at multiple heating rates.
Chlorine Partition and Dechlorination Efficiency
- HCl evolution onset temperature, peak release rate, and cumulative HCl yield as a function of temperature.
- Chlorine distribution among the gas, liquid oil, aqueous phase, and solid char.
- Overall dechlorination efficiency of the pyrolysis oil, defined as the relative reduction of chlorine in the oil compared with the chlorine input in the feed blend.
- Concentration of organic chlorine species, such as chlorobenzenes and chlorophenols, in the oil.
Char and Product Characterization
- Crystalline inorganic chloride phases, particularly KCl and CaCl2, identified in the char.
- Elemental mapping of chlorine, potassium, and calcium on char surfaces.
- Ultimate analysis and calorific value of chars and oils.
- Gas composition, including HCl, CO, CO2, CH4, H2, and light hydrocarbons.
Testing Methods
Thermogravimetric Analysis Coupled with FTIR (TGA-FTIR)
Thermogravimetric runs are performed in high-purity nitrogen following the general framework of ISO 11358 and ASTM E1131, using sample masses of 5 to 10 milligrams, heating rates of 10 and 20 degrees Celsius per minute, and a final temperature of 800 degrees Celsius. Evolved gases are transferred through a heated line to an FTIR spectrometer, and gas-phase concentrations including HCl are monitored according to the principles of ASTM D6348, enabling real-time correlation between mass loss stages and HCl evolution peaks.
Fixed-Bed Temperature-Programmed Pyrolysis with HCl Trapping
A horizontal quartz tube furnace is used for gram-scale pyrolysis at target temperatures of 250, 300, 350, 400, 450, 500, and 600 degrees Celsius, with holding times of 15 to 30 minutes under continuous nitrogen flow. Downstream impinger bottles containing dilute sodium hydroxide solution trap evolved HCl, and the absorbed chloride is quantified by ion chromatography in accordance with ISO 10304 or ASTM D4327. This staged approach maps the temperature window of maximum chlorine release for each blend ratio.
Chlorine Mass Balance Protocols
Total chlorine in solid feedstocks and chars is determined using the Eschka combustion method described in ISO 587. Chlorine in pyrolysis oil is measured by oxygen-bomb combustion followed by ion chromatographic detection of the absorbed chloride, consistent with the approach of ASTM D808 for organic matrices. Chlorine in the aqueous condensate is analyzed directly by ion chromatography. The three-phase chlorine accounting closes the mass balance to within 95 percent or better and quantifies how co-pyrolysis shifts chlorine from the oil into the char and water phases.
Pyrolysis-GC/MS and Gas Chromatographic Analysis
Flash pyrolysis at 500 to 600 degrees Celsius coupled with gas chromatography-mass spectrometry is used to speciate volatile products, including PVC-derived aromatics, biomass-derived phenols, acids, and ketones, and residual chloro-organic compounds. Permanent gases and light hydrocarbons are analyzed by gas chromatography with thermal conductivity and flame ionization detection. Comparing speciation between pure PVC, pure sawdust, and their blends provides molecular-level evidence of radical interactions and chlorine scavenging.
Solid Residue and Microscopic Analysis
Chars are examined by X-ray diffraction to confirm the formation of crystalline KCl and CaCl2, by scanning electron microscopy with energy-dispersive X-ray spectroscopy to map the spatial correlation between chlorine and alkali metals on char surfaces, and by X-ray fluorescence for bulk elemental composition. Ash content is determined according to ASTM D3174 or EN 14775, and ultimate analysis follows ASTM D5373 or EN 15104, with calorific values measured under ISO 18125.
Control Experiments on the AAEM Mechanism
Parallel runs using demineralized sawdust blends isolate the contribution of inherent potassium and calcium. A significantly higher chlorine fraction in the oil and gas from demineralized blends, together with the disappearance of KCl and CaCl2 diffraction peaks in the resulting chars, directly confirms the AAEM neutralization pathway. All instrumental procedures can be executed in accredited third-party laboratories to ensure traceable and defensible data quality.
Applications
- Co-processing of waste PVC with forestry and agricultural residues to produce chlorine-reduced pyrolysis oil suitable for fuel or refining applications.
- Valorization of the organic fraction of municipal solid waste, where PVC contamination routinely degrades pyrolysis product quality.
- In-situ HCl scavenging design, in which biomass char or AAEM-rich sorbents replace synthetic additives, lowering operating cost and secondary waste.
- Corrosion mitigation and catalyst protection strategies for pyrolysis and gasification plant operators handling chlorine-bearing feedstocks.
- Feedstock blending guidelines and temperature window optimization for commercial waste-to-energy and chemical recycling facilities.
- Academic and industrial research into biomass-plastic synergies, radical chemistry, and solid-phase chlorine fixation mechanisms.
FAQ
At what temperature does PVC release hydrogen chloride during pyrolysis?
Dehydrochlorination of PVC begins at approximately 200 degrees Celsius, peaks between 280 and 320 degrees Celsius, and is largely complete by 360 degrees Celsius. In blends with sawdust, the onset temperature typically shifts slightly lower because biomass-derived radicals accelerate the elimination reaction.
Why does adding sawdust reduce the chlorine content of pyrolysis oil?
Three coupled effects are responsible. First, potassium and calcium species in the sawdust char react with HCl to form stable KCl and CaCl2, fixing chlorine in the solid phase. Second, the porous nascent char adsorbs HCl and chlorinated intermediates. Third, hydrogen-rich radicals from biomass decomposition promote more complete dehydrochlorination early in the process, before chlorine can be incorporated into condensable aromatic molecules.
Where does the chlorine ultimately end up after co-pyrolysis?
In optimized blends with a sawdust-to-PVC ratio of 2:1 or higher, typically 60 to 80 percent of the feed chlorine reports to the char as inorganic chlorides, with most of the remainder captured in the aqueous condensate. The chlorine remaining in the organic oil can be reduced by one order of magnitude or more compared with PVC pyrolyzed alone.
How is dechlorination efficiency calculated?
Dechlorination efficiency is defined as the percentage reduction of chlorine measured in the pyrolysis oil relative to the chlorine contained in the feed blend entering the reactor. It is obtained by closing a three-phase chlorine mass balance across the gas, liquid, and solid products.
Which analytical techniques are essential for this mechanism study?
TGA-FTIR defines degradation stages and HCl evolution windows, fixed-bed pyrolysis with impinger trapping quantifies HCl release, ion chromatography quantifies chloride in all phases, XRD and SEM-EDS confirm KCl and CaCl2 fixation in char, and Py-GC/MS reveals changes in chloro-organic speciation in the volatiles.
Do the experiments follow recognized standards?
Yes. Thermal analysis follows ISO 11358 and ASTM E1131, gas-phase FTIR follows ASTM D6348, chloride determination follows ISO 10304, ASTM D4327, ASTM D808, and ISO 587, and biofuel characterization follows EN ISO 16995, EN 14775, EN 15104, and ISO 18125, ensuring that results are comparable and traceable across laboratories.