Determination of Chlorine Content in Pyrolysis Tar from Wood Sawdust-PVC Co-Pyrolysis

Determining the chlorine content of pyrolysis tar produced from the co-pyrolysis of wood sawdust and PVC is critical for assessing oil quality, corrosion risk, and downstream upgrading feasibility. Because PVC contains approximately 57 wt% chlorine, a significant fraction of chlorine can migrate into the tar during pyrolysis, where it exists as dissolved hydrogen chloride, inorganic chlorides, and organochlorine compounds. This article provides a comprehensive technical guide covering sample handling, testing items, analytical methods based on recognized standards, and practical applications for chlorine determination in sawdust-PVC pyrolysis tar.

Technical Overview

Co-pyrolysis of lignocellulosic biomass, such as wood sawdust, with waste polyvinyl chloride (PVC) has attracted growing interest as a route to convert mixed biomass and plastic waste into liquid fuels and chemicals. During pyrolysis, typically performed at 400-700 °C under an oxygen-free atmosphere, PVC undergoes a well-documented two-stage degradation. The first stage, dehydrochlorination, occurs at approximately 200-360 °C and releases a large portion of the polymer's chlorine content, roughly 57 wt% of the PVC mass, as hydrogen chloride gas. The second stage involves scission of the remaining polyene structure to form aromatic and olefinic products.

The chlorine released during dehydrochlorization partitions among three product streams: the permanent gas phase as HCl and chlorinated light hydrocarbons, the solid char where alkali and alkaline-earth metals in the biomass ash (mainly potassium and calcium) capture chlorine as inorganic salts, and the condensable tar where chlorine is present as dissolved HCl, water-soluble inorganic chloride, and organic chlorine compounds such as methyl chloride, chlorophenols, and chlorobenzenes. The distribution among these phases depends strongly on the PVC blending ratio, pyrolysis temperature, heating rate, vapor residence time, and the composition of the biomass ash.

Chlorine levels in the tar can span a wide range. Tar derived from clean sawdust alone typically contains less than 50-100 mg/kg of chlorine, while tar from blends containing 10-30 wt% PVC can carry several hundred to several thousand mg/kg, and tar from PVC-rich feeds may exceed 1 wt%. Even at trace levels, chlorine creates serious technical problems: HCl formation causes low-temperature and high-temperature corrosion of reactors, condensers, and downstream piping; chloride species poison hydrotreating, fluid catalytic cracking, and reforming catalysts; and combustion of chlorine-bearing tar can generate HCl emissions and serve as a precursor environment for chlorinated aromatic byproducts.

Accurate chlorine determination is therefore an essential quality-control and research measurement. It supports feedstock blending optimization, evaluation of in-situ dechlorination strategies, qualification of pyrolysis oil as a fuel or refinery co-feed, and complete chlorine mass balances across the pyrolysis process. Accredited third-party laboratories routinely perform this analysis using combustion-based standard methods originally developed for petroleum products and solid fuels, adapted to the specific matrix characteristics of biomass-plastic pyrolysis tars.

Testing Samples

Whole Pyrolysis Tar from Reactor Condenser Trains

The primary sample type is the combined organic condensate collected from the condensation system of a fixed-bed, fluidized-bed, auger, or rotary pyrolysis reactor. Best practice is to collect the tar from all condenser stages, typically a first condenser at 0-5 °C and a second trap at -20 °C or colder, so that light chlorinated volatiles are retained. Because pyrolysis tar is viscous, heterogeneous, and prone to phase separation, representative subsampling requires warming the bulk sample to 40-50 °C, mechanical agitation or rolling for at least 15-30 minutes, and immediate subdivision into analysis aliquots. Samples should be stored in amber glass bottles with minimal headspace, preferably under nitrogen, and refrigerated at approximately 4 °C to limit aging, polymerization, and loss of volatile chlorinated species.

Phase-Separated and Fractionated Tar Samples

Co-pyrolysis tars frequently separate into an organic phase and an aqueous phase, and the two phases exhibit very different chlorine behavior. The aqueous condensate often carries a large share of the water-soluble inorganic chloride, while the organic phase retains organically bound chlorine. Laboratories therefore analyze each phase separately when a chlorine partition assessment is required. Fractionated samples, such as light oil, middle distillate, and heavy residue obtained by vacuum distillation or solvent fractionation, may also be submitted to identify how chlorine is distributed across the boiling range, which is important for designing dechlorination or upgrading processes.

Feedstock, Char, and Gas Companion Samples

To construct a full chlorine mass balance, testing programs should include the feed blend (sawdust, PVC, and any dechlorination additives such as CaO or Na2CO3), the solid char, and absorber solutions from the gas line where HCl is trapped in alkaline solution for subsequent ion chromatographic analysis. These companion samples allow researchers to quantify chlorine migration to each product stream and to evaluate process strategies such as staged pyrolysis that exploits the low-temperature dehydrochlorination of PVC ahead of biomass decomposition.

Testing Items

The core testing items for chlorine determination in sawdust-PVC pyrolysis tar are listed below:

• Total chlorine content of the tar, reported in mg/kg (ppmw) or wt%, covering the full range from sub-10 mg/kg trace levels to several wt% for PVC-rich feeds.
• Inorganic (water-extractable) chloride, determined by aqueous extraction of the tar followed by ion chromatography or potentiometric titration, representing dissolved HCl and salts.
• Organic chlorine, calculated by difference between total chlorine and inorganic chloride, or measured directly by combustion-based techniques on the extracted organic phase.
• Chlorine content of companion streams: feedstock blend, char, aqueous condensate, and alkaline gas-absorption solution, for complete mass balance studies.
• Speciation screening of individual chlorinated organic compounds, such as chloromethane, chloroethene, chlorophenols, and chlorobenzenes, by gas chromatography-mass spectrometry or GC with electron capture detection.
• Complementary tar characterization required for data interpretation and moisture correction: water content by Karl Fischer titration (ASTM E203), ash content, density, viscosity, higher heating value, elemental composition (ASTM D5291), and acid number.

Moisture correction deserves particular attention because pyrolysis tars can contain 10-40 wt% water depending on feedstock and condensation conditions, and chlorine results should be reported on a consistent basis, either as-received or dry-oil basis, to allow valid comparison between samples and studies.

Testing Methods

Oxygen Bomb Combustion Followed by Ion Chromatography

Oxygen bomb combustion, following the principle of ASTM D808, is the classical approach for total chlorine in heavy petroleum products and is well suited to viscous, high-chlorine pyrolysis tars. A weighed portion of 0.2-1.0 g of tar, often mixed with a combustion aid such as paraffin oil or a gelatin capsule to ensure complete oxidation, is ignited in a sealed bomb charged with oxygen at approximately 3 MPa. All chlorine compounds are oxidized and converted to chloride, which is absorbed in an aqueous alkaline absorbing solution containing sodium carbonate or sodium hydroxide with hydrogen peroxide. The bomb is rinsed quantitatively, and the combined absorbate is analyzed for chloride by suppressed ion chromatography in accordance with the principles of ISO 10304 or ASTM D4327, or alternatively by potentiometric titration with standard silver nitrate solution. The method provides reliable results from approximately 10 mg/kg up to wt% levels.

Automated Combustion Ion Chromatography (CIC)

Combustion ion chromatography per ASTM D7359 is the preferred modern method for total halogen determination at trace levels. A small sample, typically 10-100 mg injected as a solution in isopropanol, xylene, or a hydrocarbon solvent, is introduced into a horizontal combustion furnace operating at 900-1050 °C under an argon-oxygen atmosphere. Combustion gases are routed into an absorption module where the hydrogen chloride formed is trapped in ultrapure water or dilute hydrogen peroxide, and the resulting solution is transferred automatically to the ion chromatograph. Quantification by peak area against external chloride calibration yields lower limits of quantification in the range of 0.5-1 mg/kg, making the technique ideal for evaluating tar after dechlorination treatment or for low-PVC blends. Method validation should address combustion completeness, carryover between high-chlorine samples, and matrix-matched calibration.

Microcoulometric Titration

Oxidative microcoulometry, following the approach of ASTM D4929 Procedure A for organic chloride in crude oils and ASTM D5808 for trace chlorine in aromatic hydrocarbons, combines high-temperature combustion with sensitive coulometric detection of the chloride generated. The tar sample is diluted in a suitable solvent and injected into a combustion tube at approximately 900-1000 °C; the hydrogen chloride produced is titrated in situ with silver ions generated coulometrically, and the charge consumed is proportional to the chlorine mass. Detection limits down to approximately 1 mg/kg are achievable. Because the technique responds to combustible organic chlorides most efficiently, it is best applied to the organic phase after aqueous extraction when a clear organic-versus-inorganic chlorine distinction is required.

X-Ray Fluorescence Spectrometry

Wavelength-dispersive X-ray fluorescence, as described in ASTM D4929 Procedure B and ASTM D7220, offers rapid, non-destructive screening of chlorine in oils with minimal sample preparation. The tar is homogenized, diluted if necessary to reduce viscosity, and presented in a sealed cup with a polyester film window. Practical detection limits for chlorine, a low-atomic-number element, are on the order of 3-10 mg/kg depending on instrument configuration and counting time, and careful matrix-matched calibration using organochlorine standards in a similar hydrocarbon-oxygen matrix is essential because the high oxygen and water content of pyrolysis tar introduces absorption and enhancement effects. XRF is most valuable as a high-throughput screening tool for research programs analyzing large sample sets.

Eschka Combustion and Potentiometric Titration for Solid Companion Samples

For the feedstock blend and char, where chlorine occurs largely as inorganic salts trapped in the mineral matter, the Eschka method in accordance with ISO 587 for solid mineral fuels remains a robust option. The sample is ashed with an intimate mixture of magnesium oxide and sodium carbonate at approximately 850 °C, converting all chlorine to soluble chloride, which is then leached with hot water and determined by potentiometric titration with silver nitrate or by ion chromatography. Applying the same approach to heavy tar residues ensures consistency of the chlorine mass balance across solid and semi-solid streams.

Sample Preparation, Calibration, and Quality Control

Regardless of the instrumental technique, data quality in pyrolysis tar analysis depends on disciplined sample preparation and quality control. Recommended practices include: thorough homogenization of the bulk tar at 40-50 °C before subsampling; use of validated solvent dilution schemes with chlorine-free solvents; moisture determination on the same aliquot batch for result correction; method blanks covering all reagents and combustion aids; duplicate analyses with acceptance criteria of relative deviation below 5-10% depending on concentration; matrix spike recoveries in the range of 90-110%; and, where available, analysis of certified reference materials of chlorine-in-oil to verify calibration traceability. For very high-chlorine PVC-derived tars, dilution series should be prepared to place results within the calibrated range and prevent carryover contamination.

Applications

Chlorine determination data for sawdust-PVC pyrolysis tar serve a broad range of engineering and research purposes:

• Feedstock blending optimization: quantifying the relationship between PVC ratio in the sawdust blend and chlorine concentration in the tar, enabling selection of blend ratios that keep tar chlorine within fuel or co-processing targets.
• Evaluation of in-situ dechlorination strategies: comparing tar chlorine levels before and after addition of calcium oxide, calcium hydroxide, sodium carbonate, or other sorbents that capture HCl in the char phase.
• Assessment of staged or stepwise pyrolysis: verifying that a low-temperature stage at 250-350 °C, designed to release PVC chlorine before biomass devolatilization, effectively reduces chlorine transfer to the tar.
• Fuel and refinery co-feed qualification: demonstrating compliance with chlorine or organic chloride limits imposed for boiler firing, diesel-engine operation, fluid catalytic cracking co-processing, and hydrotreater feed protection, where catalyst suppliers typically require single-digit to low-tens mg/kg levels.
• Corrosion and materials engineering: providing input chlorine data for dew-point calculations of hydrochloric acid in condenser and flue-gas systems and for selection of construction materials.
• Environmental compliance support: underpinning HCl emission estimates for combustion of the pyrolysis oil and providing baseline data for waste-to-energy permitting and environmental impact assessments.
• Academic and pilot-scale research: enabling chlorine migration studies, kinetic modeling of dehydrochlorination, and closure of elemental chlorine mass balances across gas, tar, and char products.

FAQ

Why is chlorine in sawdust-PVC pyrolysis tar such a critical parameter?

Chlorine in the tar converts to hydrogen chloride during storage, heating, or combustion. HCl condensing below its dew point causes severe acid corrosion of condensers and pipelines, while at elevated temperatures it promotes high-temperature corrosion and metal chlorination. In catalytic upgrading, even trace chloride deactivates nickel, noble-metal, and zeolite catalysts through poisoning and sintering. In addition, chlorine-bearing fuels raise concerns about HCl stack emissions and the formation of chlorinated organic byproducts during combustion.

What chlorine concentrations are typically found in this tar?

Concentrations depend almost linearly on the PVC fraction in the feed over moderate ranges. Tar from clean wood sawdust generally contains below 50-100 mg/kg total chlorine. Blends containing 10-30 wt% PVC commonly produce tar with several hundred to several thousand mg/kg, and PVC-dominated feeds can yield tar exceeding 1-2 wt% chlorine. Temperature and vapor residence time also matter, with faster quenching generally increasing chlorine retention in the condensate.

Which analytical method is most appropriate?

For total chlorine across the full concentration range, oxygen bomb combustion with ion chromatographic finish is robust and economical. For trace-level work below 10 mg/kg, automated combustion ion chromatography per ASTM D7359 or microcoulometric titration per ASTM D4929 or ASTM D5808 provides the required sensitivity. X-ray fluorescence per ASTM D7220 is recommended for rapid screening of large sample sets. A well-designed program often combines a combustion-based reference method with XRF screening.

How should viscous, heterogeneous tar samples be handled before analysis?

The sample should be warmed to 40-50 °C to reduce viscosity, mixed thoroughly by rolling or mechanical agitation for at least 15-30 minutes, and subsampled immediately. For instrumental methods, the aliquot is dissolved in a chlorine-free solvent such as isopropanol or xylene at a validated dilution. Water content must be determined in parallel by Karl Fischer titration so that results can be reported on a consistent moisture basis.

How can inorganic and organic chlorine in the tar be distinguished?

The standard approach is a two-step scheme: the tar is first extracted with water under controlled conditions and the aqueous extract is analyzed for chloride by ion chromatography, giving the inorganic fraction; total chlorine is then measured on the washed organic phase or on a separate aliquot, and organic chlorine is obtained either directly or by difference. This distinction is valuable because inorganic chloride can be removed by washing, whereas organochlorines require thermal or catalytic dechlorination.

What process strategies effectively reduce chlorine in the tar?

The most widely reported approaches include staged pyrolysis that exploits the 250-350 °C dehydrochlorination of PVC in a separate low-temperature stage with gas evacuation; in-situ addition of alkali sorbents such as CaO, Ca(OH)2, or Na2CO3 that fix chlorine in the char; hot vapor filtration upstream of condensers; post-synthesis water washing or alkaline washing of the tar; and catalytic hydrodechlorination during upgrading. Chlorine analysis of the tar before and after each intervention is the key performance metric for all of these strategies.