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NOx (Nitrogen Oxides): Pollutant Profile

Direct answer: NOx is the collective term for nitric oxide (NO) and nitrogen dioxide (NO₂) — acid-forming, oxidizing combustion gases. NO dominates at the flame, NO₂ forms downstream by oxidation. NOx is removed either by reduction to N₂ (SCR / SNCR, the standard industrial routes) or by absorption routes, and its treatment is dominated by temperature window, sulfur content and dust load.

Definition

  • NO — nitric oxide, colorless, poorly water-soluble, the main species at combustion temperature.
  • NO₂ — nitrogen dioxide, reddish-brown, water-reactive (forms nitric acid), toxic; the species most emission limits are expressed in terms of.
  • Emission regulations generally state limits as NOx, calculated as NO₂ equivalents.

Industrial sources

  • Coal-, oil- and gas-fired power boilers and industrial boilers
  • Cement kilns and lime kilns
  • Glass melting furnaces
  • Steel: sintering machines, coke ovens, reheating furnaces
  • Waste incineration and biomass combustion
  • Nitric acid production and nitration process vents

Relevant properties (Literature Value)

Property NO NO₂
Molar mass 30.01 g/mol 46.01 g/mol
Boiling point −151.8 °C 21.2 °C
Water solubility Low Reactive with water (HNO₃ formation)
Corrosivity Moderate High in humid gas (acid dew point)

NO₂ in humid flue gas condenses as nitric acid, which sets the acid-dew-point constraint for low-temperature equipment downstream of any NOx device.

Treatment challenges

  1. No single pollutant — the NO/NO₂ ratio changes with temperature, oxygen and residence time; test and design for both.
  2. Temperature window — SCR catalysts work in a defined window (typical 200–420 °C for conventional V-Mo-Ti); below it ammonium bisulfate deposits, above it NH₃ oxidizes back to NOx.
  3. Catalyst poisons — SO₂ (forming sulfates and ammonium bisulfate), alkali and alkaline-earth metals, arsenic, phosphorus; each fuel carries its own poison profile.
  4. Ammonia management — the reducing agent itself is regulated; slip must stay controlled.
  5. Dust and erosion — high-dust gas wears catalyst channels and plugs pitch.

Suitable materials

  • V-Mo-Ti SCR catalysts (plate or honeycomb) — the industrial standard; see the SCR DeNOx Complete Guide.
  • Low-temperature SCR formulations for 150–200 °C duties — see Low-Temperature SCR.
  • Zeolites and activated carbon serve NOx roles only in specialized niches (low-temperature adsorption, combined systems), not as primary NOx destruction materials.

Suitable technologies

  • SCR — selective catalytic reduction with NH₃ (or urea): the default for large flows and high removal (design 80–95%); SCR vs SNCR.
  • SNCR — reagent injection without catalyst, for moderate removal at lower capital cost.
  • Absorption/scrubbing — for NO₂-rich streams (e.g. nitric acid plants).

Operating conditions that matter

  • NOx inlet concentration and NO/NO₂ split
  • Gas temperature at the catalyst (and its stability over load changes)
  • SO₂ / SO₃ concentration and humidity (bisulfate and acid-dew-point limits)
  • Dust concentration and particle size (erosion, plugging)
  • NH₃/NOx molar ratio (typical operating range 0.8–1.05 — beyond 1, slip rises)

Data type: the parameter ranges above are Typical Value engineering references; product-specific windows come from manufacturer specification. See Data Classification.

Testing

  • Catalyst activity is verified by simulation testing on actual gas — SCR Activity Testing.
  • Inlet/outlet NOx by continuous emission monitoring (CEMS) or extractive sampling — CEMS & Monitoring.

Limitations

  • SCR below ~200 °C (conventional catalysts): poor activity and bisulfate risk.
  • High SO₂ fuels shorten conventional catalyst life and raise SO₂→SO₃ oxidation.
  • NO alone resists scrubbing; wet routes only suit NO₂-rich streams.
  • Ammonia supply and slip compliance add system complexity.