VOC (VOC)

Volatile Organic Compounds (VOCs) are carbon-based chemicals with high vapour pressures at ambient temperature, meaning they readily evaporate into air. They span a wide range of molecules — benzene, toluene, ethylbenzene, xylene (collectively BTEX), formaldehyde, methanol, and most light hydrocarbons from C₁ to C₁₂. In waste-processing sectors, VOCs are released during plastic pyrolysis, tyre pyrolysis, solvent-based plastic dissolution, and from the storage tanks of pyrolysis oil and recovered solvents.

VOCs matter for three distinct reasons. First, they are regulated air pollutants. Under India's Air (Prevention and Control of Pollution) Act, 1981 and CPCB emission standards for pyrolysis units, total non-methane VOC concentration in stack gases is typically capped at 20 mg/Nm³, with individual species like benzene capped at 5 mg/Nm³ because of its known carcinogenicity. State Pollution Control Boards include VOC monitoring as a Consent to Operate condition for any plant handling hydrocarbon streams. Second, VOCs are tropospheric ozone precursors — they react with NOₓ in sunlight to form ground-level ozone and photochemical smog, which is why metropolitan airsheds in Delhi and Mumbai treat them as priority pollutants. Third, many VOCs are occupational health hazards, with workplace exposure limits enforced under the Factories Act, 1948 (e.g., benzene TWA of 1 ppm).

Control technologies depend on concentration and flow rate. Low-concentration, high-volume streams (under 1,000 ppm) are typically treated with regenerative thermal oxidisers or biofilters. High-concentration streams are routed to flares, activated carbon adsorption beds, or condensed and recovered as product. In a tyre pyrolysis plant, the non-condensable gas stream is usually burned in the reactor furnace itself, both as fuel and as VOC destruction. Trade-offs centre on capex versus thermal recovery: oxidation destroys product value, while adsorption preserves it but adds spent-carbon disposal cost.