pyrolysis (thermal decomposition)

Pyrolysis (from Greek: pyro = fire, lysis = separation) is a thermochemical process in which organic materials are decomposed by heat in an inert (oxygen-free) atmosphere, breaking long polymer chains into shorter hydrocarbon molecules. The process operates at 350–700°C for plastic waste and 400–550°C for tyre/rubber waste, held under positive nitrogen or recycled gas blanket pressure of 0–5 bar. The three product streams are: pyrolysis oil (also called pyro-oil or liquid fuel, 35–75% of input by weight depending on feedstock); non-condensable gas (syngas, 5–20%, typically used to fire the pyrolysis reactor itself); and char/carbon black residue (10–40% of input, composition depending on feedstock ash and inorganic content).

Product yields by feedstock (approximate, operating at 450–500°C, well-designed reactor): PE (HDPE/LDPE): oil 65–75%, gas 10–20%, char 5–12%; PP: oil 70–80%, gas 10–18%, char 5–10%; PS: oil 55–70% (high aromatic content), gas 8–15%, char 10–15%; PET: oil 20–30% (low yield, high oxygen content, poor quality), char 30–40% — PET is a poor pyrolysis feedstock and should be routed to depolymerisation or mechanical recycling; End-of-life tyres (ELT): oil 40–60%, gas 5–15%, char 30–40% (recovered carbon black, rCB); MLP: oil 35–55%, gas 10–20%, char 15–30%. In India, the pyrolysis sector is governed by the Hazardous and Other Wastes (Management and Transboundary Movement) Rules 2016 (if the process generates hazardous residue) and the PWM Rules 2016 (for plastic feedstock).

Reactor types in commercial use in India: batch retort reactors (Rs 25–80 lakh per unit, 1–5 TPD, majority of small Indian operators); semi-continuous reactors (Rs 80 lakh–Rs 2 crore, 5–10 TPD, continuous feed with batch product withdrawal); continuous rotary kiln reactors (Rs 3–12 crore, 10–50 TPD, for industrial-scale operations). Batch reactors dominate at small scale due to lower capital cost but have higher labour cost, lower throughput, and more process variability than continuous systems. The key operational challenge in all reactor types is PVC contamination: PVC in feedstock above 0.5% generates HCl gas during pyrolysis, which corrodes the reactor internals and condenser trains (requiring Rs 5–15 lakh in annual maintenance versus Rs 50,000–1 lakh without PVC), and requires corrosion-resistant metallurgy or acid gas scrubbers.

For Indian entrepreneurs entering plastic or tyre pyrolysis, the four critical decisions are: (1) reactor type and scale — batch for <5 TPD first-time operators, continuous for >10 TPD with secured feedstock; (2) feedstock purity — build or access a pre-sorting line to eliminate PVC, PET, and high-chlorine MLP from the input; (3) oil offtake — secure a written offtake agreement with a fuel blender, marine bunker supplier, or industrial fuel user before investing, as pyrolysis oil is not a drop-in diesel replacement without upgrading; (4) regulatory approvals — CTE, CTO, CPCB/SPCB authorisation for plastic waste, and fire NOC are all required; pyrolysis plants with hydrocarbon storage are classified as hazardous manufacturing and require additional District Collector or fire department clearances under state factory rules.