What Acid Gases and Odorants Emanate from High-Temperature Plastic Pyrolysis?
When waste plastics are heated above 300 °C in the absence of oxygen, thermal cracking breaks long hydrocarbon chains into smaller, volatile fragments. This pyrolysis process, increasingly used to produce fuel oil or syngas, also releases a complex mixture of acid gases and malodorous compounds that can be corrosive, toxic and environmentally harmful. Understanding their origin, composition and concentration is essential for designing effective abatement systems and ensuring occupational safety.
1. Hydrogen Chloride (HCl) – The Dominant Acid Gas
Polyvinyl chloride (PVC) contains 57 % chlorine by mass. Above 280 °C the C-Cl bond homolyzes, releasing HCl at rates exceeding 2 g Cl⁻ per kg PVC. US-EPA Method 26A field tests show that a 500 kg h⁻¹ mixed-plastic pyrolyser can emit 1,200 mg m⁻³ HCl-eighty times the OSHA 8-hour ceiling (7 mg m⁻³). The gas is highly corrosive, attacking carbon steel at 0.5 mm yr⁻¹ and forming secondary aerosol H₂SO₄ when SO₂ is present.
2. Sulfur Oxides (SO₂, SO₃) – From Flame Retardants
Halogenated flame retardants (HBCD, DBDPE) and rubber fillers decompose between 320 and 400 °C, releasing SO₂ and traces of SO₃. Concentrations typically 200–400 mg m⁻³ in stack gas, contributing to acid rain formation and respiratory irritation.
3. Nitrogen Oxides (NO, NO₂) – Thermal and Fuel NO
Nitrogen-containing polymers (PA, PU) and ammonia-based stabilisers form fuel-NO above 350 °C. NOx levels 150–300 mg m⁻³ have been measured, promoting photochemical smog and ozone formation.
4. Low-Molecular-Weight Organic Acids
Pyrolysis of PET and PA yields acetic, propionic and benzoic acids at 50–150 mg m⁻³. These compounds impart a rancid vinegar odour and lower the pH of condensate to <3, accelerating corrosion of ductwork and scrubbers.
5. Malodorous and Toxic Organics
Breakdown of PE/PP produces C₂–C₆ aldehydes (acrolein, acetaldehyde) and ketones at 80–200 mg m⁻³, responsible for sharp, sweet smells and eye irritation. Benzene, toluene and styrene from PS and ABS occur at 100–400 mg m⁻³; benzene is a Category 1A carcinogen with a unit risk of 6 × 10⁻⁶ per µg m⁻³. Skatole and indole from nitrogen-containing plastics impart fecal odours even at ppb levels.
Health & Environmental Impact
Short-term exposure to 10 mg m⁻³ HCl causes throat burning and bronchial constriction; 50 mg m⁻³ SO₂ induces asthma-like symptoms. Chronic inhalation of benzene at 1 mg m⁻³ increases leukaemia risk by 1 in 10,000. Acid gases lower rain pH, damaging crops and buildings. Odour compounds reduce neighbourhood amenity and can trigger nuisance complaints.
Control Technologies
a) Wet scrubbers with caustic soda (NaOH) achieve >99 % HCl and SO₂ removal.
b) Selective catalytic reduction (SCR) using V₂O₅/TiO₂ catalyst reduces NOx by 90 %.
c) Activated-carbon beds adsorb VOCs and mercaptans; catalytic oxidation converts them to CO₂ and H₂O at 350 °C.
d) Bio-filters using nitrifying bacteria remove NH₃ and amines at 95 % efficiency.
Real-Time Monitoring
FTIR multi-gas analysers now provide 1-second resolution for HCl, SO₂, NOx and benzene, triggering alarms at 50 % of regulatory limits and ensuring pyrolysis plants operate within occupational and environmental safety boundaries.
