Determination of Chlorine Content in Solid Residue from Sawdust-PVC Co-Pyrolysis

Determining the chlorine content in solid residues from the co-pyrolysis of sawdust and PVC is essential for evaluating char quality, controlling corrosion and HCl emissions, and ensuring environmental compliance. This article explains the sample types, testing items, analytical methods, and recognized standards—such as ASTM D6721, ASTM D4208, ISO 587, and ISO 16994—commonly applied by accredited third-party laboratories for quantifying chlorine in pyrolysis chars. Reliable chlorine data also supports chlorine partitioning studies, dechlorination optimization, and safe utilization of pyrolysis solid residues.

Technical Overview

Co-pyrolysis of lignocellulosic biomass such as wood sawdust with polyvinyl chloride (PVC) has become an important route for valorizing waste wood-plastic composites (WPCs), flooring offcuts, profiles, and mixed biomass-plastic streams. During thermal treatment, PVC decomposes in two stages: an initial dehydrochlorination step at approximately 200–360 °C releases hydrogen chloride, followed by cracking of the residual polyene structure at higher temperatures. Although most chlorine leaves the solid phase as HCl, a measurable fraction is retained in the final solid residue, mainly because alkali and alkaline earth metals in biomass ash (K, Na, Ca, Mg) capture chlorine as stable inorganic chlorides such as KCl, NaCl, and CaCl2.

Residual chlorine in the pyrolysis char directly affects downstream value and compliance. Elevated chlorine accelerates high-temperature corrosion of boilers and furnaces, promotes ash slagging and fouling, generates HCl emissions during char combustion, and may disqualify the char from use as a solid fuel or soil amendment. Consequently, accurate determination of total chlorine and water-soluble chlorine in the solid residue is fundamental for chlorine mass balance studies, dechlorination process optimization, residue classification, and regulatory reporting. Depending on blend ratio, peak temperature, residence time, and ash composition, chlorine levels in sawdust-PVC pyrolysis chars typically range from several hundred mg/kg up to several weight percent on a dry basis.

Testing Samples

Char from Laboratory Fixed-Bed or Tubular Reactors

Solid residues obtained from slow pyrolysis of sawdust-PVC blends at 300–700 °C under an inert nitrogen atmosphere. Samples are typically quenched under inert gas, ground to a particle size below 0.2–0.5 mm, homogenized by cone-and-quarter or riffling, and dried at 105 °C to constant mass before analysis.

Residue from Continuous or Pilot-Scale Pyrolysis Units

Char, semi-coke, or bottom residues discharged from rotary kilns, screw reactors, or fluidized-bed systems processing waste wood-plastic composites and PVC-containing biomass blends. These heterogeneous industrial residues require careful sub-sampling to ensure representativeness.

Post-Treated and Washed Residues

Water-washed, acid-leached, or additive-treated (for example CaO- or KOH-modified) chars used to evaluate residual insoluble chlorine after dechlorination steps, as well as finished biochar or char-based fuel products intended for commercial use.

Testing Items

The core analytical scope for chlorine characterization of sawdust-PVC pyrolysis solid residues includes:

• Total chlorine content, reported in mg/kg or wt% on a dry basis
• Water-soluble chlorine, extracted with boiling water or ultrasonic-assisted water leaching
• Water-insoluble chlorine, including organically bound chlorine and stable inorganic chlorides, calculated by difference
• Supporting proximate parameters such as moisture, ash, and volatile matter, which define the reporting basis and help interpret ash-related chlorine capture
• Optional elemental screening of Cl, S, K, Na, and Ca to assess chlorine capture mechanisms in the char matrix

Testing Methods

Oxygen Bomb Combustion Followed by Ion Chromatography or Titration

The sample is combusted in a sealed oxygen bomb containing an absorbing solution, converting all chlorine species to chloride. Chloride in the absorbate is then quantified by ion chromatography in accordance with ISO 10304-1 or by potentiometric titration with silver nitrate. This approach follows the principles of ASTM D4208 and ASTM D6721 and is well suited for chlorine levels from mg/kg to low percentage concentrations.

Combustion Ion Chromatography (CIC)

The sample undergoes oxidative pyrohydrolytic combustion at approximately 950–1100 °C in a humidified oxygen stream, converting chlorine to HCl, which is trapped in an absorption solution and analyzed online by ion chromatography, consistent with ASTM D7359 principles. CIC offers low detection limits, automated operation, and excellent tolerance to heterogeneous carbonaceous matrices, making it a preferred confirmatory technique for pyrolysis chars.

Eschka Mixture Fusion Method

The sample is mixed with an Eschka blend of magnesium oxide and sodium carbonate and ignited at elevated temperature, fixing chlorine as alkali chloride. The residue is leached with water or dilute acid, and chloride is determined by argentometric or potentiometric titration, following procedures analogous to ISO 587 and GB/T 30729 for solid fuels and solid biofuels. The method is robust and inexpensive, though it is more labor-intensive at very low chlorine levels.

High-Temperature Pyrohydrolysis

Chlorine is volatilized as HCl in a moist air or oxygen flow over a heated quartz or platinum catalyst and trapped in an absorbing solution for subsequent ion chromatographic or spectrophotometric determination. This technique is effective for tightly bound chlorine in mineral-rich chars.

X-Ray Fluorescence Screening

Energy-dispersive XRF provides rapid, semi-quantitative screening of total chlorine in ground and pelletized char samples, typically with practical quantification limits around 100–200 mg/kg. It is widely used for preliminary screening before confirmatory combustion-based analysis.

Quality assurance measures applied by accredited third-party laboratories include certified reference materials, procedural blanks, duplicate analyses, matrix spike recoveries of 90–110%, and reporting of limits of quantification, moisture basis, and measurement uncertainty with each result.

Applications

• Chlorine mass balance and partitioning studies across char, tar, and gas phases in sawdust-PVC co-pyrolysis research
• Fuel quality assessment of pyrolysis char and semi-coke for combustion applications, including corrosion and HCl emission control
• Optimization of dechlorination strategies, such as peak temperature selection, extended residence time, alkaline additive dosing, and post-pyrolysis water washing
• Environmental compliance screening for char disposal, co-processing, or utilization as biochar or adsorbent material
• Recycling process development for waste wood-plastic composites within circular economy and industrial symbiosis programs
• Academic and industrial R&D on chlorine migration mechanisms and ash-chlorine interactions during thermochemical conversion

FAQ

Why does chlorine remain in the solid residue if PVC fully releases HCl during pyrolysis?

Although PVC dehydrochlorination is essentially complete above 400 °C, alkali and alkaline earth metals in the sawdust ash react with evolved HCl to form thermally stable chlorides such as KCl and CaCl2. A portion of this captured chlorine, along with some char-bound organic chlorine, remains in the solid residue, with retention generally increasing with ash content and moderate peak temperatures, and decreasing at very high temperatures where alkali chlorides begin to volatilize.

What chlorine levels are typically found in sawdust-PVC pyrolysis chars?

Raw sawdust usually contains less than 500 mg/kg of chlorine, while PVC contains approximately 57 wt% chlorine. Depending on the blend ratio, pyrolysis temperature, residence time, and ash chemistry, char chlorine content commonly falls between 0.05 and 3 wt% on a dry basis, with washed chars often reduced to below 0.1 wt%.

Which analytical method is most suitable for this matrix?

Combustion ion chromatography is generally preferred for confirmatory analysis because it quantifies all chlorine species with low detection limits and minimal matrix interference. Oxygen bomb combustion with ion chromatography or titration and the Eschka fusion method remain reliable alternatives, while XRF serves as a fast screening tool.

How should samples be prepared before testing?

Residues should be ground to below 0.2–0.5 mm, thoroughly homogenized, and dried at 105 °C to constant weight. Because chars are heterogeneous and hygroscopic, airtight storage and representative sub-sampling are critical for reproducible results.

How can residual chlorine in the char be reduced?

Effective strategies include raising the final pyrolysis temperature and holding time to promote chloride volatilization, co-feeding alkaline sorbents such as CaO to steer chlorine into recoverable gas-phase or filterable salts, and applying post-pyrolysis hot-water washing to remove soluble chlorides before fuel or soil applications.