Verification Experiment on the Dechlorination Effect of Co-Pyrolyzing Sawdust with PVC

The co-pyrolysis of sawdust with polyvinyl chloride (PVC) is a proven strategy for suppressing hydrochloric acid (HCl) emissions and transferring chlorine into stable inorganic chlorides retained in solid char. This article presents a complete verification experiment covering sample preparation, fixed-bed pyrolysis protocols, chlorine speciation analysis in accordance with ISO, ASTM, EN, and GB standards, and the calculation of dechlorination efficiency. The procedures described are suitable for in-house research laboratories as well as accredited third-party laboratories seeking to validate chlorine mass balance and dechlorination performance.

Technical Overview

Polyvinyl chloride contains approximately 56–58 wt% chlorine by mass, and its thermal degradation proceeds through a well-documented dehydrochlorination stage between roughly 250 °C and 380 °C, during which large quantities of HCl are released. In pyrolysis, gasification, and waste-to-energy systems, this released chlorine causes severe corrosion of reactors and heat exchangers, poisons catalysts, and contaminates pyrolysis oil with organochlorine compounds that prevent the oil from meeting fuel specifications.

Co-feeding lignocellulosic biomass such as sawdust (wood flour) with PVC has been widely studied as an in-situ dechlorination strategy. Sawdust contributes hydrogen-rich volatiles from cellulose, hemicellulose, and lignin decomposition, which overlap temporally with the PVC dehydrochlorination window. In addition, the alkali and alkaline earth metals naturally present in biomass ash, principally potassium, calcium, and magnesium, act as chlorine scavengers that convert gaseous HCl into thermally stable inorganic chlorides such as KCl and CaCl2 immobilized in the solid char. Synergistic radical interactions between biomass volatiles and PVC degradation intermediates further suppress the formation of chlorinated aromatics in the liquid product.

The objective of this verification experiment is to quantitatively confirm the dechlorination effect of sawdust during co-pyrolysis with PVC by measuring the chlorine distribution among the char, condensable oil, and gas phases, calculating dechlorination efficiency and HCl emission reduction against a PVC-only baseline, and verifying chlorine mass balance closure in accordance with ISO, ASTM, EN, and GB standard methods. The experimental design is fully transferable to accredited third-party laboratories for independent confirmation.

Testing Samples

Sawdust (Wood Flour) Feedstock

The biomass sample typically consists of air-dried sawdust from softwood or hardwood species such as pine or poplar, milled and sieved to a particle size below 0.5 mm to ensure homogeneous mixing and reproducible heat transfer. Characterization should be performed before testing and includes the following items:

• Proximate analysis (moisture, volatile matter, ash, fixed carbon) per ISO 18122, ISO 18123, ISO 18134, and GB/T 28731
• Ultimate analysis (carbon, hydrogen, nitrogen, sulfur) per ISO 16948 and GB/T 30733
• Total chlorine and sulfur content per ISO 16994
• Higher heating value per ISO 18125, EN 14918, or GB/T 30727
• Ash composition, particularly potassium, sodium, calcium, and magnesium, determined by X-ray fluorescence or inductively coupled plasma techniques after ashing

PVC Reference Material

The PVC sample may be either analytical-grade suspension PVC resin powder or washed and shredded waste PVC. The chlorine content must be verified experimentally, and it should fall within the expected range of 56–58 wt%. Thermal stability, filler content, and additive package should be documented by thermogravimetric analysis, and the chlorine content should be confirmed by combustion ion chromatography per ASTM D7359 or by the Eschka method per ISO 587.

Blended Test Specimens

Blended specimens are prepared gravimetrically at defined PVC-to-sawdust mass ratios, commonly 3:1, 1:1, and 1:3, to bracket the expected synergistic range. Each blend is mixed in a tumble mixer for at least 30 minutes to ensure homogeneity. Control specimens of PVC-only and sawdust-only are prepared in parallel to establish baseline chlorine behavior, and all tests are performed in triplicate to support statistical evaluation.

Testing Items

The verification experiment is designed around a closed chlorine mass balance, which requires the following testing items:

• Total chlorine content of the feed materials and the solid pyrolysis char
• Speciation of chlorine in the char into water-soluble inorganic chloride and residual organic chlorine
• HCl concentration and total gaseous chlorine in the pyrolysis off-gas
• Organic chlorine content of the condensable pyrolysis oil or tar
• Chlorine mass balance closure across the char, oil, and gas phases
• Proximate and ultimate analysis of all solid streams
• Heating value of feed blends and derived chars
• Thermogravimetric analysis coupled with FTIR evolved gas analysis to map the HCl release temperature window
• Scanning electron microscopy with energy-dispersive spectroscopy, X-ray fluorescence, or X-ray diffraction identification of KCl and CaCl2 phases in the char

Testing Methods

Sample Preparation and Characterization

All samples are dried to constant mass at 105 °C in accordance with ISO 18134, milled, and sieved. Moisture is re-measured immediately before each pyrolysis run so that chlorine calculations can be expressed on a dry basis. Feed chlorine is determined in duplicate by combustion ion chromatography per ASTM D7359, cross-checked where applicable by the oxygen bomb method per ASTM D4208 or the Eschka method per ISO 587.

Fixed-Bed Pyrolysis Protocol

Pyrolysis is carried out in an electrically heated horizontal quartz tube reactor under a nitrogen purge of 100–200 mL/min. A nominal charge of 5–10 g of blend is heated at approximately 10 °C/min to a target temperature within the 250–600 °C verification range, with a recommended two-stage profile holding near 320 °C to complete PVC dehydrochlorination and near 500 °C to complete biomass devolatilization. An isothermal residence time of 20–30 minutes is applied at each stage. Volatiles pass through an ice-cooled condensation train for oil collection, followed by gas washing bottles charged with 0.1 mol/L sodium hydroxide solution for quantitative HCl absorption. The solid char is recovered, weighed, and homogenized for analysis.

Chlorine Determination and Speciation

Total chlorine in the char and condensed oil is determined by combustion ion chromatography per ASTM D7359. Inorganic chloride in the char is extracted with deionized water at 60 °C for two hours and quantified by ion chromatography per ISO 10304-1 or by silver nitrate titration; organic chlorine in the char is obtained by difference. Chloride absorbed in the alkaline impinger solution is quantified by ion chromatography, with a titrimetric confirmation per the principles of ISO 587 where required. This analytical scheme yields the chlorine distribution among the three product phases required for mass balance evaluation.

Instrumental Verification of Chloride Fixation

To confirm the mechanistic pathway, char residues are examined by SEM-EDS to locate chlorine-rich domains, by XRF to quantify chlorine and alkali metal co-enrichment, and by XRD to identify crystalline KCl and CaCl2 phases. TGA-FTIR analysis of the parent blend is used to verify that the HCl evolution peak is attenuated and shifted relative to the PVC-only control, providing an independent qualitative confirmation of in-situ chlorine capture.

Data Reduction and Acceptance Criteria

The following performance indicators are calculated from the measured chlorine partition:

• Chlorine fixation rate equals chlorine retained in char divided by chlorine in the feed, expressed as a percentage
• Dechlorination efficiency equals one minus the chlorine in oil plus chlorine in gas divided by chlorine in the feed, expressed as a percentage
• HCl emission reduction equals the HCl yield of the PVC-only baseline minus the HCl yield of the blend, divided by the PVC-only baseline yield, expressed as a percentage

Acceptance criteria for a valid verification include a chlorine mass balance closure between 95% and 105%, a relative standard deviation of no more than 5% among triplicate runs, and spike recoveries for chloride determination between 90% and 110%. Based on commonly reported literature ranges for biomass-PVC systems, well-designed runs at 1:1 to 1:3 PVC-to-sawdust ratios between 300 °C and 450 °C are expected to raise char chlorine fixation substantially above the PVC-only baseline and to reduce both gaseous HCl and oil chlorine content; the experiment verifies the actual values for the specific feedstocks under test.

Quality Assurance and Interlaboratory Verification

Quality control measures include reagent blanks, duplicate digestions, multi-point calibration with certified chloride standards, and analysis of certified reference materials for solid fuel chlorine where available. For regulatory submissions or commercial disputes, parallel specimens should be dispatched to accredited third-party laboratories for confirmation testing using the same ISO, ASTM, EN, and GB methods, with results compared against the acceptance criteria defined above.

Applications

The verified dechlorination protocol supports a range of engineering and commercial applications in thermal conversion and waste valorization:

• Design of co-pyrolysis processes that upgrade waste PVC together with forestry and furniture-industry sawdust into chlorine-reduced fuel products
• Quality control in the production of refuse-derived fuel, where chlorine limits directly govern boiler corrosion risk and stack compliance
• Protection of downstream equipment by quantifying HCl loadings for scrubber and corrosion allowance design in gasifiers and pyrolysis plants
• Optimization of blend ratios and temperature profiles to minimize organochlorine contamination of bio-oil prior to upgrading
• Academic and industrial research on synergistic effects between biomass alkali metals and chlorine-containing polymers
• Preparation of defensible technical documentation for permitting authorities, supported where necessary by accredited third-party laboratories

FAQ

Why does sawdust enhance PVC dechlorination during pyrolysis?

Sawdust contributes hydrogen-rich volatiles and alkali and alkaline earth metal species. The volatiles react with chlorine-containing intermediates, while metals such as potassium and calcium chemically fix gaseous HCl as stable KCl and CaCl2 in the char, shifting chlorine from the corrosive gas and oil phases into the manageable solid phase.

What temperature window is most effective for verification?

The most informative window is 300–450 °C, because this range overlaps the PVC dehydrochlorination stage and the main biomass devolatilization stage, maximizing the interaction between the two decomposition processes. A staged profile holding near 320 °C and then 500 °C captures both the dechlorination and char-forming regimes.

How is dechlorination efficiency calculated?

Dechlorination efficiency is calculated as one minus the sum of chlorine reporting to the oil and gas phases divided by the total chlorine in the feed, multiplied by 100. A complementary indicator, the chlorine fixation rate, expresses the chlorine retained in char as a percentage of feed chlorine.

What blend ratio should be tested first?

A PVC-to-sawdust ratio of 1:1 is a practical starting point, bracketed by 3:1 and 1:3 specimens. Ratios rich in sawdust generally provide more alkali metal scavenging capacity, while PVC-rich ratios stress the system toward its dechlorination limit, so the full series identifies the optimal operating point.

Which standards govern the analytical work?

Key methods include ASTM D7359 and ISO 587 for chlorine determination, ISO 10304-1 for chloride by ion chromatography, ISO 16994 for chlorine in solid biofuels, the ISO 18123 and ISO 18122 series and GB/T 28731 for proximate analysis, ISO 16948 and GB/T 30733 for ultimate analysis, and ISO 18125 or EN 14918 for calorific value.

Can the results be independently confirmed?

Yes. The entire protocol, including specimen preparation and the chlorine mass balance calculation, is designed so that parallel specimens can be submitted to accredited third-party laboratories for confirmation using identical ISO, ASTM, EN, and GB methods, ensuring that reported dechlorination efficiencies are reproducible and defensible.

What residual chlorine issues remain after verification?

Even with strong sawdust synergism, a fraction of chlorine typically remains in the pyrolysis oil as organic chlorine, and the chlorine-laden char requires controlled disposal or further stabilization. The verification data quantify these residual streams so that downstream polishing steps and char management strategies can be sized appropriately.