Application Solution of High-Temperature Oxygen Sensors in Flue Gas Measurement

Boilers are the most common thermal equipment in industrial production, and their combustion efficiency directly affects energy costs, pollutant emissions, and operational safety. Maintaining a reasonable excess air coefficient is the core to achieving economic combustion, and the oxygen content in flue gas is the most direct and reliable indicator of combustion status. Real-time monitoring of flue gas oxygen allows dynamic adjustment of the air-fuel ratio, avoiding heat loss caused by excess air or incomplete combustion due to oxygen deficiency.

In practice, flue gas oxygen is usually controlled between 4%–4.5% for optimal performance. When boiler load is below 80% of rated load, oxygen content should be slightly increased to prevent oxygen-deficient flashover risks during sudden load increases. Accurate oxygen measurement is not only the basis for combustion optimization but also essential for calculating boiler efficiency, assessing air leakage, and diagnosing burner performance. Therefore, fast-response, high-stability, high-temperature oxygen sensors have become standard in modern boiler control systems.

Importance and Challenges of Flue Gas Oxygen Monitoring

Boiler flue gas contains complex components at high temperatures (usually 200℃–600℃), dust, water vapor, and corrosive gases, posing high requirements for sensor durability, selectivity, and anti-interference ability. Traditional electrochemical oxygen sensors are prone to poisoning, severe drift, and frequent maintenance in high-temperature flue gas. Paramagnetic oxygen analyzers, though accurate, are bulky, slow to respond, and expensive, unsuitable for most small and medium boilers.

Modern combustion control systems require oxygen analyzers to meet:

• Short response time (seconds) to follow load changes;
• Minimal long-term zero and span drift, reducing frequent calibration;
• High-temperature and corrosion resistance to withstand harsh flue gas conditions;
• Standard output signals for easy integration with PLC, DCS, or combustion controllers;
• Compact size and flexible installation for retrofit projects.

Only sensors meeting these criteria can reliably support closed-loop oxygen control in boilers.

Core Advantages of Nexisense SGA-400/700 Smart Oxygen Modules

The Nexisense SGA-400/700 series smart oxygen sensor modules are high-performance solutions specifically developed for industrial flue gas oxygen measurement. They use imported oxygen sensor cores with signal amplification, intelligent computation, full-range three-point temperature and humidity compensation, and high-precision standard gas calibration to output processed standard signals.

Technical highlights include:

• Compact modular design, easy to embed in sampling systems or insert directly into flue ducts;
• Calibration-free, plug-and-play, reducing on-site commissioning and maintenance costs;
• Flexible output: 0–5V analog (customizable) + TTL digital signal, compatible with PLC, DCS, DDC, and data acquisition systems;
• Full-range temperature and humidity compensation, eliminating environmental influences;
• High stability with minimal zero and span drift over long periods;
• Suitable for various industrial applications, including boilers, kilns, incinerators, and cogeneration systems.

Compared to traditional split oxygen analyzers, Nexisense modules maintain high accuracy while significantly simplifying system integration, making advanced closed-loop oxygen control accessible even for small to medium boiler rooms.

Typical Flue Gas Measurement Applications and Installation

Common deployment methods:

• Direct insertion: Sensor installed via flange or thread into straight flue sections, suitable for flue gas temperatures ≤600℃;
• Extractive: Used with heated sampling lines, filters, and cooling/drying units for higher temperature or dusty flue gas;
• Multi-point monitoring: Multiple points across different boiler loads or burners to monitor combustion uniformity.

Recommended installation is after the economizer and before the air preheater (flue temperature ~180℃–350℃), representing typical conditions and facilitating maintenance. Installation notes:

• Sample points away from elbows and reducers to avoid turbulence;
• Sensor probe facing downward or horizontal to prevent condensation drops;
• Heated lines above dew point to prevent clogging and corrosion;
• Signal cables separated from high-voltage lines to reduce EMI.

Combustion Optimization and Energy Savings

Industrial boiler users adopting Nexisense smart oxygen modules achieved:

• Flue gas oxygen fluctuations reduced from ±1.5% to ±0.3%, stabilizing combustion;
• Flue gas temperature reduced by 5–12℃, heat loss decreased by ~0.5%–1.2%;
• Fuel consumption decreased by 2%–6% depending on boiler type and prior operation;
• Significant reduction in CO and NOx emissions, easing environmental compliance;
• Less frequent manual inspections and calibrations, lowering operational costs.

Closed-loop oxygen control enables automatic adjustment of air and fuel based on load and fuel quality, achieving precise "air follows fuel, oxygen prioritized" combustion management, delivering dual benefits of energy savings and emissions reduction.

System Integration and Long-Term Maintenance

Nexisense modules with 0–5V and TTL outputs integrate easily into existing control systems. Typical integration:

• Module output → analog input module → PLC → control valves/frequency converters → forced draft/fan;
• Or via RS485 interface to DCS for centralized monitoring and trend analysis.

Maintenance recommendations:

• Inspect and clean probe every 6–12 months, use compressed air if needed;
• Periodically verify output against standard gas;
• Monitor flue draft and flow effects on measurement;
• Align with boiler overhaul cycles for full inspection or sensor replacement.

Proper maintenance ensures 3–5 years of high-precision operation.

FAQ

1. Optimal flue gas oxygen content? 4%–4.5% at rated load; increase to 5%–6% at low load for safety and efficiency.
2. Why temperature and humidity compensation? Fluctuations affect oxygen partial pressure measurement; compensation ensures accurate data under all conditions.
3. Does Nexisense module require field calibration? Pre-calibrated with high-precision three-point standard; plug-and-play.
4. Supported output signals? 0–5V analog (adjustable) and TTL digital signals, compatible with various control systems.
5. Maximum flue gas temperature? Direct insertion ≤600℃; higher temperatures require extractive heated sampling.
6. When to replace sensor? If drift exceeds limits, response slows, or probe heavily coated/corroded.
7. Can it be used in chain grate or circulating fluidized bed boilers? Yes, modules have strong adaptability.
8. Benefits after oxygen monitoring adoption? Improved combustion efficiency, reduced fuel consumption, lower emissions, enhanced boiler stability and safety.

Conclusion

Flue gas oxygen monitoring is key to economic combustion, energy savings, and emission compliance. Nexisense SGA-400/700 smart oxygen sensor modules provide compact, reliable, calibration-free, and easily integrated solutions for industrial boilers. They enable precise combustion monitoring, air-fuel optimization, energy and emission reductions, lower operational costs, and improved equipment management. With increasing industrial energy efficiency and environmental requirements, intelligent combustion control with reliable oxygen sensors will become standard in future boiler rooms, ensuring green and efficient operations.