Influence of Mixing Ratio on Dechlorination in Sawdust-PVC Co-Pyrolysis: A Professional Testing Guid

This technical article examines how the sawdust-to-PVC mixing ratio influences dechlorination behavior during co-pyrolysis, covering synergistic reaction mechanisms, sample design, analytical items, and internationally recognized test methods. It explains how chlorine is distributed among the char, oil, and gas phases and why blend proportion is the key variable controlling HCl release, oil quality, and corrosion risk. Engineers, researchers, and waste-to-energy developers can use this framework to optimize biomass-plastic co-processing at laboratory or pilot scale.

Technical Overview

Co-pyrolysis of polyvinyl chloride (PVC) with lignocellulosic biomass such as sawdust has become a practical route for converting chlorinated plastic waste into liquid products and carbonaceous char while controlling chlorine-related problems. When heated, PVC decomposes in two well-defined stages: an initial dehydrochlorination step between approximately 250 °C and 360 °C that releases the majority of bound chlorine as hydrogen chloride (HCl) gas, followed by cracking, cyclization, and condensation of the residual polyene chain above 400 °C.

Sawdust contributes cellulose, hemicellulose, and lignin that decompose within a broadly overlapping 250–450 °C window, releasing hydrogen-rich volatiles and leaving behind a char enriched in alkali and alkaline-earth metals (AAEMs), particularly potassium, calcium, and magnesium. During co-pyrolysis these two feedstocks interact synergistically: hydrogen-donor volatiles stabilize chlorine-containing radical fragments and suppress the formation of chlorinated aromatics, while potassium and calcium species chemically capture gaseous HCl as stable inorganic chlorides that remain fixed in the solid char.

Because these interactions are stoichiometrically limited, the relative proportion of sawdust to PVC becomes the decisive process variable. The mixing ratio governs the extent and temperature of dehydrochlorination, the distribution of chlorine among the gas, oil, and char phases, the acidity and chlorine content of the pyrolysis oil, and downstream corrosion risk. A systematic mixing-ratio test program quantifies this behavior and identifies the blend that maximizes chlorine migration into the gas or char phase while maintaining acceptable oil yield and quality.

• Quantify the chlorine distribution across char, oil, and gas as a function of blend proportion
• Identify the mixing ratio at which dechlorination synergy is maximized
• Evaluate whether AAEM-captured chlorine is permanently fixed in the char matrix
• Characterize pyrolysis oil quality, acidity, and chlorinated organic content at each ratio
• Provide kinetic and product-yield data for reactor design and scale-up

Testing Samples

Reference PVC Sample

A well-characterized PVC feedstock, typically virgin suspension-grade resin or a washed and sorted PVC waste stream, is ground and sieved to a particle size below 0.5 mm to ensure homogeneous heat and mass transfer. The reference sample establishes the baseline dehydrochlorination profile, total chlorine content (theoretically approximately 56.7 wt% for pure PVC), and product distribution in the absence of biomass interaction.

Biomass Sawdust Sample

Sawdust from a single, documented wood species such as pine, fir, or beech is dried to below 5 wt% moisture, milled, and sieved to match the PVC particle size fraction. Proximate analysis, ultimate analysis, and intrinsic AAEM content are determined, because the potassium and calcium concentration in the ash directly controls the chlorine-capture capacity of the char and therefore the interpretation of mixing-ratio effects.

Binary Blend Series at Graded Mixing Ratios

Sawdust and PVC are mechanically premixed on a dry mass basis at graded ratios, for example 0:100, 10:90, 20:80, 30:70, 50:50, 70:30, 90:10, and 100:0 (sawdust to PVC, weight percent). Ratios of 10–40 wt% PVC receive particular attention, since this region typically exhibits the strongest interaction between biomass volatiles, AAEM species, and HCl release. Each blend is prepared in triplicate to support statistical evaluation of deviations from additive behavior.

Optional Control Samples

Acid-washed sawdust, in which AAEM species have been removed by dilute acid pretreatment, may be blended with PVC to isolate the contribution of inorganic capture from that of hydrogen-donor volatiles. Similarly, demineralized char or model compounds can be used to verify whether the observed chlorine fixation is chemical fixation, physical adsorption, or simple mechanical entrainment.

Testing Items

The core objective is to trace chlorine through the entire pyrolysis system while characterizing the decomposition behavior and product quality at every mixing ratio. A comprehensive test program typically includes the following items.

• Feedstock characterization: moisture, ash, volatile matter, fixed carbon, elemental composition (C, H, N, S, Cl), higher heating value, and bulk density
• Thermogravimetric behavior: mass-loss (TG) and derivative (DTG) curves, dehydrochlorination onset temperature, peak degradation rate, overlap of decomposition windows, and residual char yield
• Product distribution: char yield, oil yield, reaction water, and gas yield at target pyrolysis temperatures such as 500 °C or 600 °C
• Chlorine speciation: total chlorine in char, oil, and gas; water-soluble inorganic chlorine versus organically bound chlorine in the oil; HCl concentration in the syngas
• Chlorine mass balance and derived indices: dechlorination efficiency, chlorine fixation rate in char, and closure of the chlorine balance across all phases
• Pyrolysis oil quality: water content, pH, density, heating value, and GC-MS speciation including benzene derivatives, phenols, and chlorinated aromatics such as chlorobenzene and chlorophenols
• Char characterization: surface area, functional groups by FTIR, morphology and chlorine mapping by SEM-EDS, and crystalline chloride phases such as KCl or CaCl2 by XRD
• Pyrolysis kinetics: apparent activation energy and reaction order for each blend using iso-conversional and model-fitting methods

Testing Methods

Feedstock Characterization Methods

Moisture, ash, and volatile matter in the sawdust are determined according to ISO 18134, ISO 11722, and ISO 18123 or the equivalent EN 14774, EN 14775, and EN 15148 procedures, with ASTM E870 and ASTM D1102 applied to wood fuels. Carbon, hydrogen, and nitrogen are measured per ISO 16948 or ASTM D5373. Total chlorine and sulfur in the solid feedstocks are determined per ISO 16994 or EN 15289, with GB/T 3558 referenced where coal-based method equivalences are required. Higher heating values of feedstocks and products are measured by adiabatic bomb calorimetry per ISO 18125 or ASTM D240.

Thermogravimetric and Evolved-Gas Analysis

Small-sample screening is performed on a thermogravimetric analyzer operated under high-purity nitrogen at 50–100 mL/min, heating from ambient temperature to 800 °C at 10 °C/min, following the principles of ASTM E1131 for compositional analysis by thermogravimetry. Coupled TGA-FTIR optionally provides qualitative, time-resolved detection of evolved HCl, benzene, and light hydrocarbons. Measured mass-loss curves of each blend are compared against the mass-weighted average of the pure components to compute a synergy index, and replicates establish the significance of deviations.

Bench-Scale Fixed-Bed Pyrolysis Trials

Quantitative product and chlorine distribution data are generated in a quartz or stainless-steel fixed-bed reactor loaded with 10–50 g of sample per run. The reactor is purged with nitrogen, heated at 10 °C/min to the target temperature, and held isothermally for 20–30 minutes. Condensable vapors are collected in a staged condenser train with the permanent gas metered downstream, while acidic gases are trapped in alkaline impinger bottles. Each mixing ratio is run in triplicate, and overall mass balance closure of 95% or better is required for a valid run.

Chlorine Determination in Solid and Liquid Products

Total chlorine in feedstock and char is determined by oxygen-bomb combustion followed by ion chromatographic quantification of the absorbed chloride, consistent with ASTM D4208 and ISO 16994 practice. Water-soluble inorganic chlorine is measured by aqueous extraction and ion chromatography per ISO 10304-1. Total chlorine in the pyrolysis oil is measured by oxidative combustion and microcoulometry following the approach of ASTM D5808 and ASTM D4929, with the organic chlorine fraction obtained by difference after water washing. X-ray fluorescence screening and SEM-EDS mapping provide complementary spatial and semi-quantitative chlorine data on the char.

Gas-Phase HCl Analysis

Hydrogen chloride in the product gas is captured in impingers containing dilute sodium hydroxide solution, and the absorbed chloride is quantified by ion chromatography per ISO 10304-1. On-line FTIR monitoring of the reactor off-gas may be added to resolve the temperature window and kinetics of HCl release in real time, which is particularly valuable for comparing dehydrochlorination shifts across mixing ratios.

Data Processing and Evaluation Indices

Chlorine distribution is reported as the mass percentage of feed chlorine recovered in each phase. Dechlorination efficiency is calculated as the total feed chlorine minus the chlorine retained in char and oil, divided by total feed chlorine, expressed as a percentage. The chlorine fixation rate in char is defined as char chlorine divided by total feed chlorine. Synergy is assessed by comparing measured yields and chlorine partitioning of each blend against linear additive predictions from the pure components, and apparent kinetic parameters are extracted using Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, or Coats-Redfern methods.

Applications

Testing of mixing-ratio effects on dechlorination supports a wide range of industrial and research applications in thermochemical waste conversion.

• Blend optimization for co-pyrolysis and co-gasification of biomass with PVC-rich plastic waste or refuse-derived fuel streams
• Corrosion mitigation design for condensers, heat exchangers, and piping through data-driven chlorine partitioning predictions
• Protection of downstream upgrading catalysts by controlling organic chlorine and chlorinated aromatic precursors in pyrolysis oil
• Production of chlorine-fixed biochar, with verified speciation, for use as adsorbent or soil amendment
• Process licensing, regulatory compliance dossiers, and quality documentation prepared with data from accredited third-party laboratories
• Academic research on biomass-plastic interactions, AAEM-mediated chlorine capture, and pyrolysis reaction kinetics
• Pilot-plant and commercial reactor design supported by verified product yields and chlorine balances

FAQ

What mixing ratio typically gives the best dechlorination performance?

In most reported systems, blends containing roughly 10–40 wt% PVC exhibit the strongest synergy, because biomass volatiles and AAEM species are present in sufficient excess relative to the released HCl. Above this range, the chlorine-capture capacity of the sawdust ash saturates, and excess HCl escapes to the gas or condenses into the oil. The optimum must nevertheless be verified experimentally for each specific feedstock pair, since wood species, ash chemistry, and PVC grade all shift the result.

At what temperature does PVC release chlorine during co-pyrolysis?

Pure PVC begins dehydrochlorination at approximately 250–280 °C and reaches its maximum HCl release rate near 300–320 °C. In the presence of sawdust, catalytic effects from AAEM species and heat-transfer interactions commonly shift the onset to a slightly lower temperature and broaden the release window, which is one of the key diagnostic observations of a mixing-ratio test.

Why does sawdust promote dechlorination of PVC?

Sawdust acts through two complementary mechanisms. First, its hydrogen-rich volatiles stabilize chlorine-containing radicals and reduce the incorporation of chlorine into aromatic oil compounds. Second, potassium, calcium, and magnesium in the biomass ash react with gaseous HCl to form thermally stable inorganic chlorides that remain locked in the char, effectively transferring chlorine from the problematic gas and oil phases into the manageable solid phase.

How is chlorine in the pyrolysis oil measured?

Total chlorine in the oil is measured by oxidative combustion followed by microcoulometric detection, following the methodology of ASTM D5808 and ASTM D4929. Water-soluble chloride is quantified separately by extraction and ion chromatography per ISO 10304-1, and organic chlorine is obtained by difference. Chlorinated aromatic species are additionally profiled by GC-MS.

What sample quantity and turnaround are typical for such a test program?

A complete ratio series with triplicate runs generally requires several kilograms of blended feedstock in total, with individual thermogravimetric tests using only milligram-scale samples and fixed-bed runs using 10–50 g charges. Depending on the scope of characterization, a full program including chemical analysis of all phases typically requires three to six weeks, with kinetics and advanced char characterization extending the schedule modestly.

Which standards govern this type of testing?

No single integrated standard covers biomass-PVC co-pyrolysis; instead, the program is assembled from recognized methods published by ISO, ASTM, EN, and GB organizations, including ISO 16994 and EN 15289 for chlorine in solid fuels, ASTM D4208 for bomb-combustion chlorine, ASTM E1131 for thermogravimetric analysis, ASTM D5808 and ASTM D4929 for organic chlorine in liquids, and ISO 10304 for ion-chromatographic anion determination. Accredited third-party laboratories routinely combine these methods into a coherent, traceable test protocol.