NMHC (NMHC)

Non-Methane Hydrocarbons (NMHC) is the regulated category of volatile organic carbon compounds excluding methane that are released to atmosphere from industrial processes, fuel combustion, and evaporative losses. Methane is excluded because, while it is a potent greenhouse gas, it is largely unreactive in the lower atmosphere on the relevant time scales for local air quality. NMHC species — alkanes, alkenes, aromatics (BTEX), oxygenates, halogenated hydrocarbons — are reactive and participate in photochemical reactions that form ground-level ozone and secondary organic aerosols (PM2.5), making them targets of urban air quality regulation in India.

NMHC species relevant to waste-processing sectors and typical industrial sources include:

• BTEX (benzene, toluene, ethylbenzene, xylene): from tyre pyrolysis off-gas, plastic pyrolysis distillates, solvent recovery operations.
• Aliphatic alkanes (C2-C8): from CNG/CBG handling, engine slip, fuel storage breathing.
• Aldehydes and ketones: from incomplete combustion and biogas engine exhaust.
• Chlorinated hydrocarbons (PVC pyrolysis, e-waste plastic processing): dichloromethane, vinyl chloride.

Indian regulatory limits on NMHC emissions:

• BS-VI vehicle exhaust: 68 mg/km NMHC for CNG passenger cars.
• CPCB stack limits for incinerators: TOC (total organic carbon) 10 mg/Nm3 daily average.
• Workplace air (BTEX): benzene 1 ppm 8-hour TWA (carcinogen), toluene 50 ppm.
• Vapour recovery systems on fuel terminals mandated by CPCB since 2016.

For tyre pyrolysis, plastic pyrolysis, and waste oil processing — three of the eight sectors covered here — NMHC emissions are the dominant air quality concern. Pyrolysis condensate handling, distillation vents, and storage tank breathing release substantial BTEX unless captured by carbon adsorption beds, thermal oxidisers, or vapour recovery units. The trade-off is capex versus environmental compliance: a regenerative thermal oxidiser (RTO) for a 10 TPD tyre pyrolysis plant costs 40-90 lakh INR but achieves 99% VOC destruction and unlocks CPCB Consent to Operate. Activated carbon scrubbers are cheaper (10-25 lakh INR) but require periodic carbon change-out (3-12 months) and produce hazardous spent carbon needing licensed disposal.

NMHC reduction also has co-benefits: ozone formation potential drops, neighbourhood odour complaints diminish, and BTEX exposure to workers (some of which are recognised carcinogens) falls below permissible exposure limits. CPCB Consent to Operate increasingly requires continuous TOC monitoring on combustion stacks of pyrolysis plants above 5 TPD, making robust NMHC management a precondition for operating the unit at all.