Sawdust-PVC Co-Pyrolysis Dechlorination Testing: Chlorine Release, Distribution, and Removal Efficie

Sawdust-PVC co-pyrolysis dechlorination testing quantifies hydrochloric acid release, chlorine distribution among solid, liquid, and gaseous products, and overall dechlorination efficiency when biomass and polyvinyl chloride are thermally decomposed together. Accredited third-party laboratories perform this testing using thermogravimetric analysis, controlled-temperature pyrolysis, and ion chromatography in accordance with recognized ASTM, ISO, and GB standards. The resulting data supports process optimization, corrosion control, pyrolysis oil quality assurance, and compliance with chlorine specifications for fuels and derived chemicals.

Technical Overview

Polyvinyl chloride (PVC) is one of the most chlorine-rich commodity plastics, containing approximately 56.8 wt% organically bound chlorine. When PVC is heated under pyrolysis conditions, it decomposes in two characteristic stages: an initial dehydrochlorination step between approximately 200 °C and 380 °C, in which hydrogen chloride (HCl) gas is released and conjugated polyene structures form, followed by a secondary cracking step between approximately 380 °C and 550 °C that generates aromatic hydrocarbons. The liberated HCl is highly corrosive to reactors, heat exchangers, and downstream piping, and residual organochlorine compounds contaminate the pyrolysis oil, limiting its use as a fuel or chemical feedstock.

Co-pyrolysis of PVC with lignocellulosic biomass such as sawdust has been widely studied as a dechlorination strategy. Sawdust contributes hydrogen and hydroxyl radicals that promote the conversion of organically bound chlorine into gaseous HCl, while naturally occurring alkali and alkaline earth metals (potassium, calcium, magnesium) in the biomass ash can partially capture chlorine in the solid char. The interaction between the two materials also shifts degradation temperatures, alters product yields, and changes the chlorine partition ratio among the gas, liquid, and solid phases.

Sawdust-PVC pyrolysis dechlorination testing provides quantitative data on these phenomena. Typical parameters include total chlorine in feedstock and products, HCl evolution onset temperature, chlorine mass balance closure, and dechlorination efficiency as a function of blend ratio, heating rate, and final pyrolysis temperature. Testing is conducted by accredited third-party laboratories following recognized methodologies such as ASTM E1131 for compositional analysis by thermogravimetry, ASTM D7359 for total chlorine by oxidative pyrohydrolytic combustion with ion chromatography detection, ISO 587 and GB/T 3558 for chlorine in solid fuels using the Eschka method, and ASTM D4929 for organic chloride determination in hydrocarbon liquids.

Testing Samples

Feedstock Materials

Representative samples of the raw materials are tested first to establish the chlorine baseline. These typically include sawdust from softwood or hardwood species (pine, spruce, poplar, eucalyptus), virgin or waste PVC in powder, pellet, granulate, or film form, and homogenized sawdust-PVC blends prepared at defined mass ratios, commonly ranging from 1:9 to 9:1. All feedstock samples are dried, milled to a consistent particle size (typically below 0.5 mm), and characterized for proximate and ultimate composition before pyrolysis testing.

Pyrolysis Solid Residues (Char)

The solid char recovered after pyrolysis is analyzed for residual chlorine to determine how much chlorine was retained in the solid phase through alkali metal capture or carbon-bound organochlorine formation. Char chlorine content directly affects disposal classification, soil amendment suitability, and activated carbon applications.

Pyrolysis Liquids (Bio-Oil)

The condensed liquid product, often separated into an organic oil phase and an aqueous phase, is analyzed for both water-soluble chloride and organically bound chlorine. Chlorine in pyrolysis oil is a critical quality parameter, as most fuel specifications and refinery acceptance criteria require total chlorine below 10–50 mg/kg.

Gas Absorption Solutions

Gaseous HCl evolved during pyrolysis is captured in alkaline absorption solutions (typically sodium hydroxide or hydrogen peroxide solutions arranged in serial impingers) placed downstream of the reactor. These scrubber solutions are subsequently analyzed to quantify chlorine transferred to the gas phase, enabling closure of the chlorine mass balance.

Testing Items

Dechlorination testing programs for sawdust-PVC co-pyrolysis typically include the following measurable parameters:

• Total chlorine content in sawdust, PVC, and blended feedstock
• Thermogravimetric (TG) and derivative thermogravimetric (DTG) degradation profiles under inert atmosphere
• HCl evolution onset temperature and peak release temperature
• Evolved gas composition analysis, including HCl, H2O, CO, CO2, and light hydrocarbons
• Chlorine distribution among gas, liquid (oil and aqueous phase), and solid char products
• Dechlorination efficiency at defined final pyrolysis temperatures (e.g., 300 °C, 400 °C, 500 °C, 600 °C)
• Water-soluble chloride and organically bound chlorine in pyrolysis oil
• Residual chlorine in char and its leaching behavior
• Chlorine mass balance closure across the pyrolysis system
• Product yield distribution (char, oil, gas) as a function of blend ratio and temperature
• Identification of organochlorine species in condensates, such as chlorobenzenes and chlorophenols

Testing Methods

Thermogravimetric Analysis (TGA) and TGA-FTIR

Thermogravimetric analysis following ASTM E1131 principles is used to characterize the thermal degradation sequence of sawdust, PVC, and their blends. Samples of 5–20 mg are heated under nitrogen at controlled rates (typically 10 °C/min) from ambient temperature to 800–900 °C. When coupled with Fourier-transform infrared spectroscopy (TGA-FTIR), the evolved gas stream is monitored in real time, allowing the onset temperature, peak intensity, and cumulative quantity of HCl release to be determined precisely, and revealing interaction effects between the biomass and PVC degradation stages.

Fixed-Bed and Tube Furnace Pyrolysis

Combustion Ion Chromatography (CIC)

Total chlorine in feedstock, char, and oil samples is determined by oxidative pyrohydrolytic combustion followed by ion chromatography detection in accordance with ASTM D7359. This method offers low detection limits (sub-mg/kg level) and is suitable for both inorganic and organic chlorine species across all sample matrices.

Eschka Method

For solid fuels and chars, chlorine determination by the Eschka method in accordance with ISO 587 and GB/T 3558 provides a classical gravimetric or potentiometric endpoint approach. The sample is ignited with an Eschka mixture (magnesium oxide and sodium carbonate), converting chlorine to water-soluble chloride, which is then quantified by titration or ion-selective electrode.

Oxygen Bomb Combustion with Ion Chromatography

Chlorine in liquid products and oil phases can be determined by oxygen bomb combustion, in which the sample is fully oxidized in a sealed vessel and the resulting chloride in the absorbent solution is measured by ion chromatography. ASTM D4929 and related microcoulometric techniques are applied to quantify organic chloride in hydrocarbon fractions.

Pyrolysis-GC/MS

Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) is employed to identify and semi-quantify specific organochlorine compounds in the condensable products, including chlorobenzene, chlorophenols, and polychlorinated aromatics. This information is essential for assessing toxicity risk and downstream upgrading requirements.

Supporting Characterization

X-ray fluorescence (XRF) spectroscopy is used as a rapid screening technique for total chlorine in solids, while elemental analysis establishes carbon, hydrogen, nitrogen, and sulfur contents needed for stoichiometric mass balance calculations. Proximate analysis (moisture, volatile matter, ash, fixed carbon) supports interpretation of degradation behavior.

Applications

Dechlorination testing for sawdust-PVC co-pyrolysis serves a broad range of industrial and research applications:

• Co-pyrolysis process design, including optimization of blend ratio, heating rate, peak temperature, and residence time to maximize chlorine removal into the capturable gas phase
• Quality control of pyrolysis oil intended for boiler fuel, diesel blends, or refinery feedstock, where chlorine specifications are typically below 10–50 mg/kg
• Corrosion risk assessment for reactors, condensers, and gas cleaning systems exposed to HCl-containing product streams
• Qualification of refuse-derived fuel (RDF) and mixed plastic-biomass waste streams prior to thermal conversion
• Evaluation of dechlorination additives, catalysts, alkaline sorbents, and hydrodeoxygenation pretreatments
• Environmental compliance support, including characterization of char for disposal, land application, or use as activated carbon precursor
• Academic and industrial research into biomass-plastic synergies, radical interactions, and chlorine migration mechanisms
• Closed-loop chlorine mass balance validation for pilot plants and commercial pyrolysis facilities

FAQ

How is dechlorination efficiency calculated?

Dechlorination efficiency is typically expressed as the percentage of total chlorine in the feedstock that is removed from the solid residue and transferred to the gas phase as HCl, or captured overall. It is calculated as: Dechlorination efficiency (%) = (chlorine in feedstock − chlorine in solid residue) ÷ chlorine in feedstock × 100. An alternative formulation uses captured HCl chlorine in the absorption solution divided by total feedstock chlorine. Both calculations require accurate total chlorine analysis of all streams to close the mass balance.

At what temperature does PVC release chlorine during pyrolysis?

Dehydrochlorination of PVC begins at approximately 200 °C and reaches its maximum rate between 280 °C and 340 °C, with the majority of chlorine released as HCl by about 380 °C. The exact onset and peak temperatures shift when PVC is blended with sawdust due to hydrogen transfer and radical interactions between the two materials.

Why is sawdust added to PVC during pyrolysis?

Sawdust acts as a hydrogen donor that promotes conversion of organochlorine structures into gaseous HCl, dilutes the chlorine load per unit feedstock, and contributes alkali metals that capture a portion of chlorine in the char. Co-pyrolysis also improves liquid product yield and reduces the concentration of organochlorine contaminants in the oil compared with PVC pyrolysis alone.

What chlorine limits apply to pyrolysis oil?

Although limits vary by application, most industrial fuel oil and refinery feedstock specifications require total chlorine below approximately 10 mg/kg, with some applications permitting up to 50 mg/kg. Chlorine above these levels causes corrosion, catalyst poisoning, and emissions of chlorinated pollutants during combustion.

How is the evolved HCl gas quantified?

HCl released during pyrolysis is swept by an inert carrier gas into serial impingers containing alkaline absorption solution. The captured chloride is then quantified by ion chromatography or potentiometric titration, and the result is converted to HCl mass to determine gas-phase chlorine distribution.

How much sample is required for a complete test program?

A typical program covering feedstock characterization, multi-temperature pyrolysis runs, and product chlorine analysis requires approximately 200–500 g of homogenized sample per blend ratio. TGA-level tests require only milligram quantities, while fixed-bed reactor runs generally consume 5–50 g per condition. Exact requirements depend on the number of temperatures and replicate runs specified in the test plan.