Boiler Flue Gas NOx Online Monitoring Solution: Reliable Guarantee for Ultra-Low Emissions

Against the backdrop of China's "Carbon Peak and Carbon Neutrality" goals and continuous air pollution prevention efforts, boilers are key energy equipment in industrial and heating sectors, and NOx emissions in their flue gas have become a primary regulatory focus. NOx not only contributes to photochemical smog and fine particulate matter formation, but also directly leads to acid rain and ozone exceedances. According to GB 13271-2014 and GB 13223-2011 standards, many regions have tightened NOx emission limits to 30–50 mg/m³, with some key areas requiring below 20 mg/m³ for ultra-low emissions.

Achieving precise denitrification, compliant emissions, and regulatory management cannot rely solely on periodic manual sampling. Continuous, automatic, and reliable online NOx monitoring has become a rigid demand for boiler operators, thermal power plants, and environmental regulators. Nexisense SGA-900 series NOx online monitoring system is developed specifically for high-temperature, high-dust, and high-humidity boiler flue gas conditions.

Hazards of Boiler Flue Gas NOx and Monitoring Necessity

• NOx primarily exists as NO and NO₂, with NO over 90%, which rapidly oxidizes to NO₂ in the atmosphere.

• Environmental hazards include: contributing to photochemical smog and ozone exceedance, forming secondary PM2.5 with SO₂ and NH₃, causing soil acidification and eutrophication, and long-term exposure increases respiratory and cardiovascular risks.

• Monitoring challenges: flue gas temperatures 200–800℃, high dust concentration, high moisture content, complex composition, traditional sensors easily fail.

National standards HJ 397-2017 and HJ 943-2018 mandate CEMS (Continuous Emission Monitoring System) for automatic online monitoring with direct data transmission to environmental platforms.

Core Components and Working Principle of SGA-900

• High-temperature sampling probe (up to 800℃)

• High-temperature filter + efficient dust removal

• Condensation dehumidifier + desiccant

• Vacuum sampling pump + flow meter

• Imported high-precision NOx analyzer (electrochemical or UV differential absorption optional)

• Industrial-grade control unit + data acquisition & transmission module

• Explosion-proof/corrosion-resistant enclosure (IP65+)

Typical workflow:

1. Probe inserted into flue gas duct to directly extract hot flue gas

2. Gas passes through high-temperature filter to remove large particulates

3. Condensation dehumidification removes moisture, preventing condensation interference

4. Clean, dry gas enters the analyzer chamber

5. Sensor converts NOx concentration into electrical signal, displaying real-time mg/m³ values

6. Data transmitted via 4–20mA, RS485 Modbus RTU, or HJ212 protocol to control room/environmental platform

System supports automatic calibration, fault self-diagnosis, and data retransmission, ensuring continuity and traceability.

High-Temperature Probe: Key Technology for Boilers

• Special alloy and multi-stage insulation design enable stable sampling at 800℃

• Customizable probe length (0.5–3 meters)

• Built-in ceramic or alloy filter core to prevent clogging

• Optional heating/cooling to prevent overheating or condensation

• Quick assembly for easy maintenance and replacement

Integration and Compliance Application

• Seamless connection to DCS, PLC, or third-party CEMS platforms

• Real-time display and historical trend curves

• Automatic over-limit alarms (audible, visual, SMS)

• Linkage with SCR/SNCR systems for automatic ammonia/urea injection adjustment

• Emission control: flue gas released only when within standards

• Direct data transmission to regulators, generating daily/monthly reports

Installation and Maintenance Guidelines

• Probe placement: straight section, 4–6 duct diameters downstream of bends

• Probe insertion depth: 1/3–1/2 of duct radius

• Pre-treatment cabinet installed on platform for easy maintenance

• Ensure heating lines and cooling water circuits are unobstructed

• Weekly filter and condensate checks

• Monthly zero/span calibration with standard gas

• Quarterly probe and pre-treatment cleaning

• Sensor lifespan: 2–4 years, modular replacement available

• Average annual system availability >95% with proper maintenance

  
  

FAQ

1. High flue gas temperatures (600–800℃) prevent direct use of ordinary NOx analyzers. SGA-900 uses high-temperature probe + pre-treatment to reduce sample gas to ambient temperature while removing dust and moisture.

2. High dust and humidity are managed by high-temperature ceramic filters, auto-backflush, dual-stage dehumidifiers, and desiccants. Regular weekly inspections prevent clogging and sensor poisoning.

3. Supports electrochemical and UV differential absorption (UV-DOAS) NOx detection. Electrochemical is cost-effective for small/medium boilers; UV is precise and interference-resistant, recommended for large coal boilers and ultra-low emission upgrades.

4. Data meets environmental platform requirements via built-in HJ212-2017 protocol, 4G/5G/Ethernet transmission, breakpoint resume, data recovery, and fault alerts.

5. Automatic linkage with SCR/SNCR systems through 4–20mA or digital outputs for ammonia dosing control and optional shut-down or load reduction when over-limit.

6. Probe maintenance: monthly filter check, quarterly internal cleaning, ceramic filter replacement every 3–6 months, alloy probe lasts 5+ years, modular design enables quick replacement.

7. One system usually monitors one boiler/duct. Multi-boiler setups require multiple probes or parallel systems for accurate source tracking.

8. Monitoring data helps optimize boiler operation, NOx reduction, and ammonia/urea consumption, lowering operating costs and secondary pollution.

Conclusion

Boiler flue gas NOx control is key to improving air quality. Nexisense SGA-900, with high-temperature sampling, reliable pre-treatment, precise analysis, and stable data transmission, provides a full-chain monitoring and control solution for coal and gas boilers. Investing in a professional, reliable NOx online monitoring system ensures compliance, supports green transformation, reduces operating costs, and enhances competitiveness.