Definition
NOx, SOx and CO emissions
NOx, SOx and CO are key regulated gaseous emissions from combustion plants. Cleaning affects them indirectly by preserving heat transfer and abatement performance.
- Subject
- KPIs and measurements
- Also known as
- NOx, SOx, SO2, CO emissions, nitrogen oxides, sulphur oxides, carbon monoxide
NOx, SOx and CO are key regulated gaseous emissions from combustion plants. NOx means nitrogen oxides, mainly NO and NO2. SOx means sulphur oxides, mainly SO2 with smaller amounts of SO3. CO is carbon monoxide, a marker of incomplete combustion.
They are distinct from particulate matter and opacity, but they interact with particulate-control and boiler-cleaning systems through combustion stability, catalyst performance, acid formation and deposit chemistry.
Sources and controls
| Pollutant | Formation | Typical controls |
|---|---|---|
| NOx | Thermal NOx from high flame temperature and fuel NOx from fuel-bound nitrogen | Low-NOx burners, over-fire air, FGR, SNCR and SCR |
| SO2 | Oxidation of sulphur in fuel | Fuel selection, wet FGD, dry sorbent injection and circulating dry scrubbers |
| SO3 | Further oxidation of SO2, sometimes promoted by SCR catalyst | Catalyst formulation, temperature control, sorbents and acid dew point management |
| CO | Incomplete combustion from poor mixing, low oxygen, low temperature or short residence time | Combustion tuning, air distribution and stable fuel feed |
Measurement and compliance
Large combustion plants typically use CEMS for NOx, SO2, CO and oxygen. Permit limits may be expressed on dry gas, corrected oxygen and averaging periods ranging from minutes to months. Engineers must compare data on the same basis before drawing conclusions.
How cleaning affects gaseous emissions
Sonic horns do not capture gaseous pollutants directly. Their effect is indirect:
- Clean SCR catalyst maintains NOx reduction and reduces ammonia over-injection.
- Cleaner heat-transfer surfaces preserve furnace and gas-temperature profiles.
- Clean air heaters reduce cold-end fouling and ammonium-bisulphate accumulation.
- Stable hopper and dust handling reduces particulate carryover that can disturb downstream equipment.
- Avoided fouling helps maintain stable combustion air and draft control, which can affect CO.
If a plant has high NOx because of poor burner settings, high SO2 because of fuel sulphur, or high CO because of insufficient air mixing, acoustic cleaning is not the primary fix. It supports the equipment that controls those pollutants.
Operating and diagnostic detail
NOx, SOx and CO respond to different parts of the plant. NOx is driven by fuel nitrogen, flame temperature, oxygen distribution and DeNOx systems. SOx follows fuel sulphur and sulphur-capture equipment. CO reflects combustion completeness, mixing, residence time and oxygen availability. A single upset can move several of them at once, but the corrective action is not the same for each pollutant.
Operators use continuous monitors, oxygen readings, boiler load, burner settings, fuel analysis, reagent flow, FGD performance and opacity trends to diagnose changes. High CO with low oxygen may point to air distribution or burner tuning. High NOx after an SCR may point to catalyst, ammonia distribution or temperature. High SO2 generally points to fuel sulphur or scrubbing performance rather than boiler cleanliness.
Acoustic cleaning supports this system indirectly. Clean heat-transfer surfaces help stable steam temperature and draft. Clean air heaters preserve combustion-air temperature and fan margin. Clear SCR faces protect NOx reduction. Clear hoppers and ESP internals reduce particulate carryover that can interfere with downstream equipment. It is part of stability management, not a direct chemical control for NOx, SOx or CO.
Measurement context
These gases are usually reported with oxygen correction, moisture basis and averaging period defined by the permit or test method. Comparing numbers without those details can mislead operators. A NOx reading at 6 percent O2 is not directly comparable with one at 11 percent O2, and a short CO spike during load change has a different operational meaning from a sustained high average.
Cleaning systems should therefore be assessed against stable, normalised trends. If acoustic cleaning improves draft or air-heater performance, the pollutant benefit may appear indirectly through steadier combustion and abatement operation rather than an immediate step change in a gas analyser.
Related terms
Explore the subject
Related terms
4 terms
- Selective Catalytic ReductionSCR reduces NOx by injecting ammonia upstream of a catalyst. Temperature, mixing, catalyst condition and ash control determine performance.
- Continuous Emissions Monitoring SystemCEMS instruments measure stack emissions in real time - opacity, PM, NOx, SOx, CO, O2, moisture - providing the data on which environmental compliance is judged.
- Opacity (stack)Opacity is the percentage of light obscured by particulate in stack gas. It is monitored by transmissometer and often tracks ESP or baghouse upset.
- Industrial Emissions DirectiveThe IED is the EU framework for integrated industrial pollution control. It links permits to BAT conclusions, emission limits, monitoring and inspection duties.
References