Evaluation of Solid-Phase Dechlorination Performance in Sawdust-PVC Co-Pyrolysis: Testing Protocol,

Solid-phase dechlorination evaluation quantifies how effectively chlorine released from PVC during co-pyrolysis with sawdust is captured and immobilized in the char residue instead of escaping as corrosive hydrogen chloride gas. This article presents the underlying reaction chemistry, sample preparation requirements, key testing items, standardized analytical methods, and industrial applications for assessing dechlorination performance in sawdust-PVC pyrolysis systems. Reliable evaluation of chlorine fixation efficiency supports corrosion control, fuel quality assurance, and compliant waste-to-energy process design.

Technical Overview

Polyvinyl chloride (PVC) contains approximately 56.7 wt% chlorine chemically bound to the polymer backbone. During thermal treatment, PVC undergoes a two-stage decomposition: an initial dehydrochlorination step between approximately 200 °C and 360 °C that releases hydrogen chloride (HCl) and forms polyene sequences, followed by chain scission, cyclization, and aromatization reactions between approximately 360 °C and 550 °C. The liberated HCl causes severe corrosion of reactors and heat-exchange surfaces, contaminates pyrolysis oil with organochlorine compounds, and acts as a precursor for chlorinated organic pollutant formation in downstream oxidative processes.

Co-pyrolysis with lignocellulosic biomass such as sawdust offers a pathway to intercept this chlorine within the solid residue. Sawdust contains alkali and alkaline earth metals (AAEM), principally potassium, calcium, magnesium, and sodium, distributed in the wood matrix and its ash fraction. As HCl evolves from the degrading PVC, these metal species react with the gas to form thermally stable inorganic chlorides, such as KCl, CaCl₂, and MgCl₂, that remain trapped in the developing char structure. Hydroxyl and carboxyl groups on the biomass surface, together with the porous char skeleton, further promote adsorption and chemisorption of chlorine species. The interaction between PVC-derived polyene chains and biomass radicals also affects char yield and cross-linking behavior.

Solid-phase dechlorination performance is evaluated through a chlorine mass balance across the three product phases (solid char, liquid condensate, and gas). The core metric is the chlorine retention efficiency, defined as:

Dechlorination Efficiency (%) = (Mass of chlorine retained in char ÷ Total mass of chlorine in the feedstock) × 100

Key process variables influencing the result include peak pyrolysis temperature, heating rate, residence time, PVC-to-sawdust blending ratio, particle size, and the intrinsic AAEM content of the biomass. A well-designed evaluation program isolates these variables to identify the operating window that maximizes chlorine fixation in the solid phase while maintaining acceptable char yield and fuel properties.

Testing Samples

PVC Feedstock Samples

Both virgin PVC resin and real-world waste PVC (e.g., cable insulation, packaging film, flooring) can be evaluated. Samples are cryogenically or mechanically milled to a particle size below 0.5 mm, dried at 105 °C to constant mass, and stored in desiccators. For waste PVC, the presence of plasticizers, stabilizers, and fillers is documented because calcium carbonate or metal stabilizers in commercial formulations can themselves contribute to chlorine capture and must be accounted for in the mass balance.

Sawdust Samples

Sawdust from documented wood species (pine, poplar, fir, or mixed forestry residues) is sieved to a controlled fraction, typically 40–80 mesh, dried to below 5 wt% moisture, and characterized for ash content and ash composition. The concentrations of potassium, calcium, magnesium, and sodium are determined because AAEM loading directly governs the theoretical chlorine capture capacity of the blend.

Sawdust-PVC Blend Samples

Binary mixtures are prepared at defined mass ratios, commonly PVC:sawdust of 1:3, 1:1, and 3:1, along with pure PVC and pure sawdust as reference materials. Each blend is homogenized by tumble mixing for at least 30 minutes to ensure uniform contact between the two solid phases, which is critical for reproducible chlorine capture during the solid-state reaction.

Pyrolysis Solid Residues (Chars)

Char samples are generated under controlled pyrolysis conditions and recovered quantitatively from the reactor. Each char is weighed, homogenized, ground, and analyzed for residual chlorine, ash, and elemental composition. Parallel char samples from pure PVC and pure sawdust pyrolysis at identical conditions serve as controls for distinguishing genuine synergistic capture from simple mixing behavior.

Testing Items

The evaluation program typically encompasses the following testing items:

• Total chlorine content of the feedstocks and the pyrolysis chars
• Chlorine retention (solid-phase dechlorination) efficiency and chlorine distribution among solid, liquid, and gas phases
• Proximate analysis: moisture, volatile matter, ash, and fixed carbon
• Ultimate analysis: carbon, hydrogen, nitrogen, sulfur, and oxygen by difference
• Thermogravimetric parameters: onset decomposition temperature, maximum mass-loss rate temperature (DTG peaks), and residual mass
• AAEM composition of biomass and char ashes (K, Na, Ca, Mg) by inductively coupled plasma or X-ray fluorescence methods
• Crystalline phase identification of the char by X-ray diffraction, confirming the formation of inorganic chlorides such as KCl and CaCl₂
• Elemental mapping of chlorine distribution on char surfaces by scanning electron microscopy with energy-dispersive spectroscopy
• Char yield and carbon retention as supporting indicators of process efficiency

Testing Methods

Sample Preparation and Pyrolysis Procedure

All samples are conditioned in accordance with recognized solid-fuel preparation practice prior to testing. Pyrolysis is conducted in a fixed-bed or horizontal tubular quartz reactor under a high-purity nitrogen purge (typically 100–300 mL/min) to exclude oxygen. A standard temperature program heats the sample at approximately 10 °C/min to target temperatures between 250 °C and 600 °C, followed by an isothermal hold of 20–60 minutes. The volatile products pass through a condensation train for tar and aqueous phase recovery, and the permanent gas stream, including HCl, is bubbled through dilute sodium hydroxide absorber solutions so that the gas-phase chlorine fraction can be quantified by ion chromatography of the absorbing solution.

Chlorine Determination

Total chlorine in solid samples is determined by oxygen bomb combustion followed by ion chromatography or coulometric detection of the absorbed chloride, consistent with the principle of ASTM D4208. As a cross-check, the classical Eschka mixture fusion method followed by potentiometric titration, consistent with ISO 587 and GB/T 3558, is applied to representative split samples. Agreement between the two methods within the repeatability limits validates the chlorine data used in the mass balance.

Thermal and Structural Characterization

Thermogravimetric analysis is performed under ASTM E1131 and ISO 11358 principles to record the degradation profile of each blend and to identify the temperature window in which PVC dehydrochlorination overlaps with biomass decomposition, which is where solid-phase chlorine capture is most pronounced. X-ray diffraction identifies crystalline chloride phases in the char, and SEM-EDS mapping verifies whether chlorine is uniformly dispersed across the char matrix or concentrated near AAEM-rich domains.

Mass Balance and Efficiency Calculation

The chlorine input equals the feed mass multiplied by its chlorine concentration; the chlorine outputs are the char-bound chlorine, the chloride measured in the liquid condensate, and the chloride recovered from the gas absorbers. Solid-phase dechlorination efficiency is then the char-bound chlorine divided by the chlorine input. A mass balance closure between 95% and 105% is generally required for the data set to be accepted as valid.

Quality Assurance

• Method blanks and reagent blanks are run with every analytical batch
• Duplicate preparations establish repeatability of pyrolysis results, with chlorine retention efficiency typically required to agree within 5 percentage points
• Certified reference materials of comparable matrices are analyzed alongside samples to verify analytical accuracy
• All work is performed by accredited third-party laboratories operating under ISO/IEC 17025-aligned quality management systems

Applications

Evaluation of solid-phase dechlorination in sawdust-PVC co-pyrolysis directly supports several engineering and environmental objectives:

• Co-processing of waste PVC with forestry and agricultural residues, converting two difficult waste streams into a manageable solid product
• Production of low-chlorine char fuels and solid recovered fuels, where chlorine content determines fuel classification under EN-type specifications and governs acceptance in cement kilns and industrial boilers
• Mitigation of high-temperature and low-temperature chlorine corrosion in combustion and gasification equipment, reducing maintenance cost and extending equipment life
• Improvement of pyrolysis oil quality by suppressing HCl and organochlorine transfer into the liquid product, which is essential for subsequent upgrading or use as a chemical feedstock
• Screening of dechlorination additives and optimization of blending ratios, temperatures, and residence times during process development
• Provision of defensible data for environmental permits, emissions inventories, and technology due diligence in waste-to-energy projects

FAQ

What level of solid-phase dechlorination efficiency is considered acceptable? There is no single universal threshold, as acceptance depends on the downstream use of the char. In research practice, blends processed in the 350–450 °C window commonly retain a substantially higher fraction of chlorine in the char than pure PVC alone, and comparative testing against pure-PVC controls is the primary basis for judging synergy. For fuel applications, the absolute chlorine content of the final char must be checked against the relevant fuel specification.

Which pyrolysis temperature maximizes chlorine fixation? Moderate temperatures of roughly 350–450 °C generally provide the best compromise. Below this range, PVC dehydrochlorination is incomplete, leaving organic chlorine in the residue; above it, fixed inorganic chlorides can partially volatilize or the char structure can sinter, reducing capture efficiency.

Why is sawdust specifically effective? Sawdust combines a favorable AAEM ash composition, a porous char-forming lignocellulosic matrix, and abundant oxygenated surface functional groups, all of which promote conversion of evolved HCl into stable inorganic chlorides held within the solid phase.

Which standards govern the chlorine measurements? Total chlorine is commonly determined per ASTM D4208 (oxygen bomb method) or per ISO 587 / GB/T 3558 (Eschka method). Thermogravimetric characterization follows ASTM E1131 and ISO 11358, while proximate and ultimate analyses follow the corresponding ASTM or ISO solid-fuel standards.

How much sample is needed and how long does evaluation take? A complete program typically requires a few hundred grams of each blend and can be completed by an accredited third-party laboratory within two to three weeks, including pyrolysis trials, chromatographic analysis, structural characterization, and mass balance reporting.

Can the gas-phase HCl emission also be quantified? Yes. HCl swept from the reactor is captured in alkaline absorbing solutions and quantified by ion chromatography, allowing the complete three-phase chlorine distribution to be reported alongside the solid-phase retention efficiency.