Calculation and Evaluation of Dechlorination Rate in Co-Pyrolysis of Wood Sawdust and PVC

The dechlorination rate is the key performance metric for quantifying how effectively pyrolysis converts organically bound chlorine in PVC into removable hydrogen chloride when wood sawdust is co-pyrolyzed with plastic waste. Accurate evaluation depends on rigorous chlorine mass balancing across the char, oil, and gas phases, supported by ion chromatography and combustion-based chlorine assays. This article explains the calculation methodology, laboratory testing workflow, influencing factors, and practical applications of dechlorination rate assessment for sawdust-PVC pyrolysis systems.

Technical Overview

Polyvinyl chloride (PVC) contains approximately 56.7 to 57.3 wt% organically bound chlorine, making it the most chlorine-rich commodity plastic. During thermal treatment, PVC degrades in two well-defined stages: an initial dehydrochlorination step between roughly 200 and 360 degrees Celsius, in which hydrogen chloride (HCl) is eliminated and conjugated polyene structures form, followed by chain scission and condensation of the polyene backbone between 400 and 550 degrees Celsius. If not controlled, the released HCl corrodes reactors and heat exchangers, poisons downstream catalysts, contaminates pyrolysis oil with organochlorine compounds, and complicates compliance with acid gas emission limits.

Co-pyrolyzing wood sawdust with PVC is a widely studied strategy for mitigating these problems. Sawdust acts as a hydrogen and hydroxyl radical donor, accelerates the thermal decomposition of PVC, and contributes alkali and alkaline earth metals, principally potassium, calcium, and magnesium, that capture a portion of the evolved chlorine as stable inorganic chlorides retained in the char. The net effect is faster chlorine release from the polymer structure, reduced organochlorine formation in the oil, and partial in-situ chlorine fixation.

The dechlorination rate is the quantitative measure of this process. Two calculation conventions are commonly used. The solid-basis dechlorination rate expresses the percentage of feedstock chlorine that has left the solid residue after pyrolysis, while the gas-basis rate expresses the percentage of feedstock chlorine actually recovered as HCl in the gas absorption train. The difference between the two values reflects chlorine transferred into the liquid product or recaptured by mineral matter in the char. A complete evaluation therefore closes the chlorine mass balance across all three product phases.

Typical results reported in the literature show that dehydrochlorination of PVC in sawdust blends exceeds 90 percent at final temperatures of 350 degrees Celsius and above, and can reach 97 to 99 percent at 450 to 500 degrees Celsius with sufficient residence time. Adding chlorine sorbents such as CaO, Ca(OH)2, or KOH further suppresses gaseous HCl without altering the degree of polymer dechlorination.

Testing Samples

Wood Sawdust (Biomass Feedstock)

Sawdust from common species such as pine, fir, or poplar is air-dried, ground, and sieved to a defined particle size fraction, typically 0.25 to 0.5 mm for laboratory fixed-bed reactors. The sample is oven-dried at 105 degrees Celsius to constant mass and stored in sealed containers. Moisture, ash, and volatile matter should be documented, for example according to ASTM E871, ASTM E872, and ASTM D1102 procedures for wood analysis.

PVC Material

Either suspension-grade PVC resin or washed and milled waste PVC may be used. The chlorine content of the as-received material must be determined experimentally, since additives, plasticizers, and fillers in waste PVC can shift the chlorine fraction away from the theoretical value. Particle size should be matched to the sawdust fraction to ensure homogeneous blending and uniform heat transfer.

Sawdust-PVC Blends

Blends are prepared at defined dry mass ratios, commonly 3:1, 1:1, and 1:3 sawdust to PVC, by tumble or planetary mixing for at least 30 minutes. Each blend is prepared in triplicate, and the blend chlorine content is either calculated from component analyses or verified by direct combustion analysis of the mixed sample.

Pyrolysis Products and Process Streams

Collected samples for chlorine partitioning include the solid residue (char or semi-coke), the condensed liquid product (pyrolysis oil and aqueous condensate), gas absorption solutions from impinger trains, and any particulate retained on filters or in transfer lines, which must be rinsed and included in the balance to avoid systematic losses.

Testing Items

The evaluation program typically comprises the following measurable items:

• Total chlorine content of the sawdust, PVC, and each blend (wt%, dry basis)
• Chlorine retained in the solid pyrolysis residue (mg/g residue and wt%)
• Chlorine transferred into the pyrolysis oil and aqueous condensate
• HCl concentration in the pyrolysis gas, expressed as mg Cl per gram of feed or mg per cubic meter of gas
• Solid-basis dechlorination rate (percent of feed chlorine removed from the solid)
• Gas-basis dechlorination rate (percent of feed chlorine recovered as HCl)
• Chlorine distribution among char, oil, and gas phases (percent of total chlorine)
• Chlorine mass balance closure (percent recovery)
• Product yields of char, oil, and gas for correlation with dechlorination behavior
• Optional speciation of organochlorine compounds in oil by pyrolysis-GC/MS and chlorine mapping in char by SEM-EDS or XPS

Testing Methods

Chlorine Determination in Solids

Total chlorine in feedstocks, blends, and chars is determined by oxygen bomb combustion followed by ion chromatographic quantification of the absorbed chloride, following the principles of ASTM D4208, or by oxidative hydrolysis microcoulometry per ASTM D6721. The classical Eschka method described in ISO 587 and GB/T 3558 provides a gravimetric-territory alternative suitable for high-throughput screening. Each determination is performed in duplicate with reagent blanks, and calibration standards must bracket the expected chloride concentration.

Pyrolysis Test Execution

Laboratory evaluation is carried out in a fixed-bed tubular reactor, a thermogravimetric analyzer for kinetic screening, or a micro-pyrolyzer coupled to GC/MS for product speciation. A typical program heats 5 to 10 g of sample at 10 degrees Celsius per minute under nitrogen purge of 50 to 100 mL/min to a final temperature between 350 and 600 degrees Celsius, holding isothermally for 20 to 30 minutes. Char and oil are weighed to establish product yields, and the gas stream is routed directly to the absorption train.

Gas-Phase HCl Capture and Quantification

The pyrolysis off-gas passes through a series of impingers charged with dilute sodium hydroxide solution, typically 0.1 mol/L, placed after the condensation system and kept in an ice bath. Chloride in each impinger solution is quantified by suppressed ion chromatography following ASTM D4327. The absorbed chloride mass, corrected for dilution and blank contributions, is converted to equivalent HCl and summed across all impingers to give the gas-phase chlorine yield. A breakthrough impinger is included to verify complete capture.

Liquid Product Chlorine Analysis

An aliquot of the pyrolysis oil is combusted in an oxygen bomb, and the absorber solution is analyzed by ion chromatography to determine total chlorine in the liquid product. Water-soluble chloride in the aqueous condensate is measured directly by ion chromatography after filtration.

Dechlorination Rate Calculation

The solid-basis dechlorination rate is calculated as DR(solid) = [(m0 x wCl,0) - (mr x wCl,r)] / (m0 x wCl,0) x 100, where m0 is the initial dry feed mass, wCl,0 is the chlorine mass fraction of the feed, mr is the dry mass of solid residue, and wCl,r is the chlorine mass fraction of the residue. The gas-basis dechlorination rate is calculated as DR(gas) = mCl,gas / (m0 x wCl,0) x 100, where mCl,gas is the total chloride mass recovered from the impinger train. Chlorine distribution percentages are obtained by dividing each phase mass by the feed chlorine input. Mass balance closure is computed as CB = (mCl,char + mCl,oil + mCl,gas) / (m0 x wCl,0) x 100.

Data Acceptance and Quality Control

Acceptable runs should close the chlorine balance within 95 to 105 percent. Results outside this window trigger investigation of leaks, condensation on cold transfer lines, impinger breakthrough, or incomplete combustion during solid analysis. Triplicate pyrolysis runs with a relative standard deviation below 5 percent on the dechlorination rate are recommended for publication-grade or client-facing data, and all work may be performed or verified by accredited third-party laboratories.

Applications

Dechlorination rate calculation and evaluation serve a broad range of engineering and research purposes:

• Design of pre-dechlorination stages for PVC-containing waste ahead of gasification or combustion units
• Optimization of co-pyrolysis conditions for producing chlorine-compliant bio-oil from biomass and plastic mixtures
• Sizing of HCl absorption systems and selection of corrosion-resistant materials for pilot and commercial pyrolysis plants
• Screening of chlorine sorbents and catalysts such as CaO, MgO, and biomass ash for in-situ chlorine fixation
• Quality control of solid recovered fuel (SRF/RDF) formulations containing mixed biomass and plastic streams
• Kinetic modeling of PVC dehydrochlorination and synergistic biomass interactions in academic research
• Generation of inventory data for life cycle assessment and regulatory demonstration of acid gas control performance

FAQ

Question: What dechlorination rate can realistically be achieved in sawdust-PVC co-pyrolysis? Answer: Dehydrochlorination of the PVC fraction typically exceeds 90 percent at a final temperature of 350 degrees Celsius and reaches 97 to 99 percent at 450 to 500 degrees Celsius with adequate hold time. Co-feeding sawdust generally accelerates chlorine release and promotes fixation of part of the chlorine as inorganic chloride in the char.

Question: Why do the solid-basis and gas-basis dechlorination rates differ? Answer: The solid-basis rate measures chlorine leaving the polymer and residue structure, while the gas-basis rate measures HCl actually captured downstream. Chlorine recaptured by potassium and calcium in the char, or transferred into the pyrolysis oil, causes the gas-basis value to be lower. The difference is a useful indicator of the in-situ chlorine fixation capacity of the biomass minerals.

Question: Which standards govern the analytical work? Answer: Chlorine in solids is determined per ASTM D4208 or ASTM D6721, with ISO 587 and GB/T 3558 as recognized alternative methods. Chloride in absorption solutions and condensates is quantified by ion chromatography per ASTM D4327. Accredited third-party laboratories can perform these analyses to support certification or dispute resolution.

Question: What constitutes a valid chlorine mass balance? Answer: Recovery between 95 and 105 percent of the feed chlorine across char, oil, and gas is generally considered valid. Balances outside this range usually indicate vapor losses in transfer lines, impinger breakthrough, or analytical drift and require re-testing.

Question: Can the calculated dechlorination rate exceed 100 percent? Answer: No. Values above 100 percent are physically impossible and signal measurement error, most commonly moisture interference in residue weighing, contamination of absorption solutions, or an underestimated feed chlorine content.

Question: How do heating rate and blend ratio affect the result? Answer: Higher heating rates and finer particles accelerate heat and mass transfer and shift dehydrochlorination to slightly higher temperatures. Increasing the sawdust share raises hydrogen availability and alkali metal content, which typically increases the overall dechlorination rate while reducing the fraction of chlorine that exits as free HCl gas.