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

Dechlorination performance evaluation of wood sawdust–PVC co-pyrolysis quantifies how effectively chlorine is released, captured, and redistributed when PVC is thermally treated together with lignocellulosic biomass. This technical article describes the sample preparation, key performance indicators, analytical methods, and typical results used to assess HCl evolution, chlorine partitioning among char, oil, and gas, and overall product quality in sawdust–PVC pyrolysis systems.

Technical Overview

Polyvinyl chloride (PVC) contains approximately 56–57 wt% chlorine and is one of the most widely consumed thermoplastics, generating large volumes of difficult-to-treat waste. Thermal pyrolysis offers a route to convert PVC into oil, gas, and char, but the release of hydrogen chloride (HCl) and the formation of chlorinated organic compounds cause severe equipment corrosion, catalyst poisoning, and degradation of liquid product quality. Evaluating dechlorination performance is therefore a prerequisite for designing any thermal conversion process that accepts PVC-containing feeds.

PVC pyrolysis proceeds in two characteristic stages: a dehydrochlorination stage at approximately 200–360 °C, in which conjugated polyene structures form and HCl is released, and a subsequent chain-scission stage at approximately 360–550 °C, which generates aromatic oils and non-condensable gases. When wood sawdust is co-pyrolyzed with PVC, several synergistic effects influence chlorine behavior: alkali and alkaline earth metals (K, Ca, Mg, Na) present in biomass ash react with evolved HCl to form stable inorganic chlorides retained in the char, while hydrogen-donor volatiles and free radicals released from cellulose, hemicellulose, and lignin interact with PVC decomposition intermediates, shifting HCl release toward lower temperatures and suppressing the conversion of chlorine into chlorinated aromatics in the oil.

A systematic performance evaluation quantifies these effects across controllable variables, including pyrolysis temperature, residence time, heating rate, and the PVC-to-sawdust blending ratio. The core objectives are summarized below:

• Determine the temperature window and kinetics of the dehydrochlorination stage for the blend.
• Quantify total dechlorination efficiency of the solid phase and the partitioning of chlorine among gas, tar/oil, and char.
• Assess how much chlorine is chemically fixed in the char as inorganic chlorides rather than emitted or dissolved into the oil.
• Evaluate product quality, including chlorine content in bio-oil, char heating value, and gas composition.

Testing Samples

Virgin and Waste PVC Samples

Both virgin PVC resin powder and post-consumer PVC materials (pipes, window profiles, films, and cable insulation) can be evaluated. Waste PVC samples are washed to remove surface contamination, dried at 60–80 °C to constant mass, and milled and sieved to a particle size below 0.25 mm. The baseline chlorine content of each PVC sample is determined prior to testing, since plasticizers, fillers, and stabilizers in waste PVC can significantly alter pyrolysis behavior.

Wood Sawdust Samples

Sawdust from common softwood and hardwood species (for example pine, fir, and poplar) is air-dried, further dried at 105 °C for 12–24 hours, and sieved to a specified fraction, typically 0.18–0.42 mm for laboratory-scale studies. The ash composition, particularly the K, Ca, Na, and Mg contents, is characterized because these elements govern the chlorine-fixation capacity of the biomass. Moisture content is controlled to below 10 wt% to ensure reproducible thermal behavior.

Sawdust–PVC Blend Samples

Blends are prepared at defined PVC-to-sawdust mass ratios, commonly 3:1, 1:1, and 1:3, by mechanical homogenization or pelletization. Reference samples of pure PVC and pure sawdust are tested in parallel to isolate synergistic effects. Typical specimen masses are 5–10 mg for thermogravimetric analysis and 5–20 g for fixed-bed pyrolysis experiments, with each condition tested in triplicate to establish statistical reliability.

Testing Items

The evaluation program covers thermal behavior, chlorine release and redistribution, and end-product quality. Key testing items include:

• Thermogravimetric characteristics: onset decomposition temperature, maximum mass-loss rate temperature, overlapping or separation of decomposition stages, and residual char yield.
• Dehydrochlorination behavior: initial and final dehydrochlorination temperatures, HCl evolution rate profile, and cumulative HCl release.
• Dechlorination efficiency: the percentage of feed chlorine removed from the solid phase at each condition.
• Chlorine distribution: the mass fraction of feed chlorine partitioned to the gas phase (predominantly HCl), the pyrolysis oil, and the char.
• Chlorine fixation in char: identification of inorganic chloride species (such as KCl and CaCl2) formed through ash–HCl reactions.
• Oil quality: total chlorine content of the condensable fraction and concentration of chlorinated organic compounds.
• Gas quality: composition of non-condensable gases, including HCl, H2, CH4, CO, CO2, and light hydrocarbons.
• Kinetic parameters: apparent activation energy and reaction order of the dehydrochlorination stage.

Testing Methods

Coupled Thermal Analysis

Thermogravimetric analysis (TGA) and TGA coupled with Fourier-transform infrared spectroscopy (TGA-FTIR) are performed under high-purity nitrogen purge at 50–100 mL/min, heating rates of 10, 20, and 30 °C/min, and a final temperature of 800–900 °C. Evolved gases are monitored in real time, with HCl identified through its characteristic infrared absorption band. This configuration reveals how sawdust addition shifts the dehydrochlorination stage and alters volatilization behavior.

Fixed-Bed Pyrolysis

Laboratory-scale pyrolysis is conducted in a tubular quartz reactor loaded with 5–20 g of sample under nitrogen flow of 100–300 mL/min. The furnace is heated to target temperatures between 250 and 600 °C at 10–20 °C/min and held isothermally for 15–60 minutes. HCl in the product gas is captured in serial impinger bottles containing an alkaline absorption solution, condensable tar is collected in chilled condensers, and the char is recovered and weighed for subsequent chlorine analysis. Flash pyrolysis coupled with gas chromatography–mass spectrometry (Py-GC/MS) may be used to profile chlorinated organics in the volatiles.

Chlorine Determination

Total chlorine in feed materials, chars, and oils is measured by oxygen-bomb combustion followed by ion chromatography in accordance with ASTM D4208, or by the Eschka fusion method in accordance with ISO 587. Chlorine in solid biomass matrices is determined per ISO 16994. Chloride in the impinger absorption solutions is quantified by ion chromatography. The mineral form of retained chlorine is identified by X-ray diffraction, X-ray fluorescence, and scanning electron microscopy with energy-dispersive spectroscopy, confirming whether chlorine is fixed as KCl, CaCl2, or other stable species.

Data Processing and Performance Indicators

Solid-phase dechlorination efficiency is calculated as D = (m_Cl,in − m_Cl,char) / m_Cl,in × 100%, where m_Cl,in is the chlorine mass in the feed and m_Cl,char is the chlorine mass remaining in the char. Complementary indicators include the HCl selectivity (chlorine evolved as HCl relative to total chlorine evolved), the chlorine fixation ratio in char, and the chlorine mass balance closure, which should fall within 90–110% for a valid test series. Reported results include mean values with measurement uncertainty, typically within ±3% for chlorine balances.

Applications

The evaluation results directly support engineering decisions and regulatory compliance in waste-to-energy and resource recovery projects:

• Co-processing of waste PVC with forestry and woodworking residues to produce cleaner pyrolysis products.
• Formulation of low-chlorine waste-derived and biomass-derived solid fuels with controlled corrosion risk.
• Design of two-stage pyrolysis systems in which a dedicated low-temperature dehydrochlorination step precedes main-chain cracking.
• Specification of flue-gas cleaning and HCl abatement capacity for pyrolysis and gasification plants.
• Quality control of bio-oil and upgraded char destined for combustion, gasification, or soil amendment, subject to washing of soluble chlorides.
• Research and development support for kinetic modeling, reactor scale-up, and life-cycle assessment of co-pyrolysis pathways.
• Generation of compliance data for waste fuel specifications, with confirmatory testing available through accredited third-party laboratories.

FAQ

Why does adding sawdust improve PVC dechlorination?

Sawdust contributes alkali and alkaline earth metals that chemically capture HCl as stable inorganic chlorides in the char, while biomass-derived hydrogen-rich volatiles and radicals promote early HCl release and suppress chlorine transfer into aromatic oil compounds. The net effect is a lower chlorine burden in the liquid and gaseous products compared with PVC pyrolysis alone.

What temperature range is most effective for dechlorination?

The dehydrochlorination stage of PVC occurs mainly between 200 and 360 °C, and most evaluations target isothermal holding between 300 and 400 °C for 30 minutes to maximize chlorine removal before extensive chain scission begins. Higher temperatures of 400–500 °C drive more complete devolatilization but can increase the formation of chlorinated organics in the oil.

What dechlorination efficiency can typically be achieved?

With blending ratios near 1:1 and appropriate temperature control, solid-phase dechlorination efficiencies above 90% are commonly reported, with roughly 15–40% of the feed chlorine fixed in the char as inorganic chlorides depending on the ash composition of the sawdust. Actual values are feed-specific and must be confirmed experimentally.

How is HCl in the product gas measured?

Product gas is passed through serial impingers containing an alkaline absorption solution, and the collected chloride is quantified by ion chromatography. This wet-chemistry approach provides reliable cumulative HCl yields and supports closure of the overall chlorine mass balance.

Which standards govern the test methods?

Chlorine determination references ASTM D4208 for oxygen-bomb combustion, ISO 587 for the Eschka method, and ISO 16994 for chlorine in solid biofuels. Thermal analysis procedures follow generally accepted laboratory practice, and confirmatory testing may be conducted by accredited third-party laboratories.

Does a higher temperature always improve performance?

No. Although raising the temperature accelerates chlorine release, excessive temperatures promote chlorine incorporation into heavy aromatic oils, increase energy consumption, and may cause ash slagging. A balanced condition identified through systematic evaluation generally outperforms simply maximizing temperature.

Can the resulting char be utilized?

If chlorine is predominantly fixed as water-soluble inorganic chlorides, the char can be washed and considered for use as a solid fuel, sorbent, or soil amendment after verification of residual chloride levels. Characterization of the chlorine species in the char is an essential part of the evaluation.