Nitrogen Dioxide (NO2)

Nitrogen dioxide (NO₂) is a reddish-brown, sharp-smelling toxic gas, the most regulated member of the nitrogen oxides (NOₓ) family. It forms when nitrogen and oxygen in combustion air combine at high flame temperatures — so any hot combustion source produces it, regardless of fuel sulphur. In the NAAQS, NO₂ has a 24-hour ambient limit of 80 µg/m³ and an annual limit of 40 µg/m³ (30 µg/m³ in ecologically sensitive areas).

NO₂ damages health and the environment on three fronts. It is a respiratory toxin that inflames airways and reduces lung function, with children and asthmatics most affected. In sunlight it reacts with VOCs to form ground-level ozone and photochemical smog. And it contributes to acid rain as nitric acid, alongside SO₂. NO₂ is also a precursor to secondary nitrate PM2.5.

Because NO₂ is a product of combustion temperature rather than fuel composition, it appears at every recycling-related combustion source — DG sets, boilers, dryers, kilns, and the burners on pyrolysis reactors. Higher flame temperatures and excess air both raise NOₓ formation, so the same equipment can emit more or less NO₂ depending on how it is tuned and loaded.

Control is about combustion management rather than fuel switching. Low-NOₓ burners, flue-gas recirculation, staged combustion and good air-fuel control reduce thermal NOₓ at source; for large sources, selective catalytic or non-catalytic reduction (SCR/SNCR) with ammonia or urea cuts it in the flue gas. NO₂/NOₓ is a standard stack-test parameter and an OCEMS-monitored value for larger plants; it is reported in the consent as both stack concentration and ambient impact.