1. Best Feedstocks for Pyrolysis: PE, PP and PS

Polyethylene (PE), polypropylene (PP) and polystyrene (PS) are generally among the most suitable plastics for pyrolysis. They consist mainly of carbon and hydrogen, making them good candidates for producing liquid hydrocarbons.

Pyrolysis oils from polyolefins (LDPE, HDPE and PP) and PS typically have calorific values of approximately 39.5–46.67 MJ/kg and densities of 0.71–0.92 g/cm³.

Liquid yield varies with feedstock composition and operating conditions. The general trend is:

PS > PP ≈ LDPE > HDPE > PET > PVC

PS can achieve particularly high liquid yields because its aromatic structure favours the production of styrene and other aromatic compounds. Some studies report liquid yields of 90–91.38% under optimised conditions.

PP can achieve liquid yields of approximately 65–86% under suitable conditions, with products primarily consisting of hydrocarbons.

PE behaves differently. Without catalysts, it can produce substantial amounts of wax rather than lighter liquid products. However, appropriate temperature control and catalytic processing can improve liquid production. Under certain conditions, LDPE has achieved liquid oil yields of around 45% at 450°C.

For pyrolysis projects, PE, PP and PS should generally be prioritised during feedstock selection and pre-treatment.

2. Mixed Waste Plastics: Particle Size and Consistency Matter

In commercial applications, feedstock rarely consists of a single polymer. It often contains a mixture of plastic films, bags, trays, containers and other packaging materials, including plastic-rich fractions recovered from municipal solid waste (MSW).

Mixed plastics can be processed through pyrolysis, but variations in composition and particle size make feedstock preparation particularly important.

One comparative study of PE, mixed plastics and PS reported a maximum liquid yield of approximately 45% for mixed plastics at 500°C, with a residence time of 120 minutes and a particle size of 3 cm. Under optimised conditions, liquid yields reached 75.39% for PE and 91.38% for PS when particle sizes were adjusted to 2–2.75 cm.

These results illustrate the importance of feedstock characteristics. Particle size affects heat transfer and mass transfer during pyrolysis. When mixed feedstock has inconsistent particle sizes, variations in heating behaviour can make reactor operation more difficult to control.

How Harden Machinery's Shredder Helps Prepare Mixed Plastic Feedstock

Harden Machinery's fine shredding technology helps control particle size when processing mixed waste plastics.

Plastic films, bags, trays and rigid containers have different shapes, thicknesses and mechanical properties. A suitable shredding process must handle these materials efficiently while maintaining a consistent output size.

The Harden Machinery SG3000RF fine shredder features an adjustable cutting gap and an integrated screening structure. This configuration helps achieve the target particle size in a single shredding stage, reducing recirculation, energy consumption and equipment wear.

Consistent particle sizing also helps reduce excessive fines that may be carried into downstream equipment with pyrolysis vapours, potentially contributing to pipe blockages and other operational problems.

By improving feedstock uniformity, the SG3000RF helps reduce variations in particle size that can affect downstream processing. This supports more stable operation and helps manage the risks associated with inconsistent mixed-plastic feedstock.

Effective pre-treatment starts with controlled particle size and consistent feedstock preparation.

3. Plastics That Require Special Attention: PVC and PET

Not all plastics are equally suitable for pyrolysis. PVC and PET present specific challenges and should be carefully identified during feedstock sorting.

PVC: A Major Source of Chlorine Contamination

Polyvinyl chloride (PVC) contains approximately 56–56.8% chlorine by weight. During the initial stages of thermal decomposition, typically around 250–320°C, PVC can release significant quantities of hydrogen chloride (HCl).

HCl can corrode processing equipment and introduce chlorine compounds into the pyrolysis products. Chlorine contamination may also deactivate catalysts used in downstream upgrading processes.

Some pyrolysis processes require chlorine concentrations in the plastic feedstock to be below 0.2% by weight, while certain chemical feedstock specifications impose much stricter limits, potentially at parts-per-million (ppm) levels.

For this reason, PVC should be identified and removed where required by the process and product specifications.

PET: High Oxygen Content and Different Recycling Requirements

Polyethylene terephthalate (PET) presents a different challenge. Its molecular structure contains oxygen, which affects the composition and properties of its pyrolysis products.

PET pyrolysis liquids typically have lower calorific values, around 21.1–28.2 MJ/kg, compared with products derived from PE, PP and PS. They may also contain more oxygenated compounds, which can affect product stability and downstream processing requirements.

Mixing PET with polyolefin-rich feedstock can therefore change the quality and composition of the resulting oil.

PET is often better suited to dedicated recycling processes that recover its constituent monomers, depending on the material quality and available technology.

The key is not simply to shred all plastics together, but to identify and separate materials according to the requirements of the intended recycling process.

4. How to Select and Prepare Plastic Feedstock for Pyrolysis

Suitable pyrolysis feedstock generally contains a high proportion of hydrocarbon-based plastics, particularly PE, PP and PS, with controlled levels of chlorine, oxygen-containing polymers and other contaminants.

For practical feedstock preparation, the main priorities are:

• Identify suitable polymers: Prioritise PE, PP and PS where appropriate for the target process.

• Remove problematic materials: Separate PVC and manage PET according to feedstock specifications and the intended recycling route.

• Control particle size: Use shredding and screening to achieve a consistent feedstock size.

• Reduce impurities: Remove unwanted materials that could affect product quality, equipment reliability or downstream processing.

• Maintain feedstock consistency: Minimise variations in composition and particle size to support stable reactor operation.

Feedstock selection determines what enters the pyrolysis system. Pre-treatment determines how consistently that material can be processed.

After sorting, Harden Machinery's fine shredding technology plays an important role in balancing particle size control, processing efficiency and equipment wear. By preparing a more uniform plastic feedstock, the SG3000RF supports the front-end requirements of waste plastic chemical recycling and helps address the challenges of processing mixed plastic streams.