Application of Gas Sensors in Flue Gas Analyzers
3

Application of Gas Sensors in Flue Gas Analyzers

With the development of global industrialization, the massive consumption of fossil fuels has exerted considerable impacts on the environment. In the 12th Five-Year Plan for Energy Conservation and Emission Reduction issued by the State Council, the state has refined and quantified energy conservation and emission reduction measures and targets for key industries and key sectors. Against this background, the market for emission monitoring instruments and meters related to flue gas emission and combustion efficiency monitoring is poised for explosive growth. To meet the business demands and development trends of the Chinese market, UK-based DDS Company has launched the S+4OXLF, a long-life lead-free oxygen sensor applicable to both industrial applications and flue gas analysis.

I. Traditional Lead-Based Oxygen Sensors in Flue Gas Analyzers

Traditional oxygen sensors for emission gas detection can be regarded as two-electrode metal-air batteries, consisting of an air cathode, a lead anode, and an alkaline electrolyte. The cathode is a polytetrafluoroethylene (PTFE) film coated with an active catalyst, and the anode is a lead block, both enclosed in a hermetically sealed metal or plastic container. Air enters the sensor through a capillary tube on the top, and the cathode and anode are connected to two pins of the sensor via current-collecting wires.
When oxygen reaches the working electrode, a reduction reaction occurs, generating hydroxide ions that migrate to the lead electrode through the electrolyte. An oxidation reaction then takes place at the lead electrode, producing lead oxide. By converting the current into a voltage via an external known resistor, the oxygen concentration can be calculated.
Although lead-based oxygen sensors have a history of more than 30 years, they have inherent limitations:
  1. Service life affected by combustion products
    Combustion products generally contain acidic gases such as carbon dioxide (CO₂), nitrogen oxides (NOₓ), and sulfur dioxide (SO₂). Since the sensor interior uses an alkaline electrolyte, the pH value of the electrolyte decreases over prolonged use, accelerating the consumption of lead fuel and shortening the sensor lifespan.
  2. Failure to meet lead-free requirements
    Although lead-based sensors currently enjoy exemptions under RoHS and WEEE directives, lead-free sensors will gradually replace lead-based ones as an irreversible environmental trend.
  3. Large weight and size
    Extra lead mass is required to extend the service life, resulting in bulky and heavy sensors.

II. Technical Features of Long-Life Lead-Free Oxygen Sensors

In response to the drawbacks of lead-based oxygen sensors, the second-generation three-electrode long-life lead-free oxygen sensor S+4OXLF has been developed.
  1. Long service life
    During the entire reaction process, the highly active electrodes only act as catalysts for oxidation or reduction without any consumption, eliminating the issue of anode fuel depletion in traditional lead-oxygen sensors. The unique structural design of the S+4OXLF effectively controls the volatilization of the internal electrolyte, greatly extending its service life compared with conventional oxygen sensors.
    It can operate for more than 5 years within a temperature range of -40℃ to 60℃ and a relative humidity range of 15% to 90%.
  2. Acid gas resistance
    The S+4OXLF adopts an acidic electrolyte internally, which effectively resists interference from acidic gases produced by the combustion of various fuels such as natural gas, petroleum, wood, and biomass, ensuring its adaptability in diverse fuel application environments.
  3. Light weight
    Lead-based oxygen sensors for emission monitoring (e.g., S+5OX/5FO) weigh approximately 40g, while the S+4OXLF weighs only 5g, showing a significant weight advantage. This provides a necessary condition for developing compact and lightweight combustion efficiency meters.
  4. RoHS and WEEE compliant
    The S+4OXLF is lead-free, enabling instruments to obtain RoHS and WEEE certifications.
  5. Complete technical documentation
    We provide customers with comprehensive documentation including sensor characteristics, operating principles, and reference circuits to support their applications.

Market Applications

The sensor has been well received by end-users due to its compact size, resistance to acidic gas interference, and long service life. Its specific applications are as follows:
  1. Online flue gas analyzers
    At present, zirconia or partial-pressure lead-oxygen sensors are widely used for online oxygen monitoring in China. The theoretical service life of the latter is nearly 5 years, but in practice, it only lasts about one year due to fluctuations in intake temperature and humidity. Moreover, its weak resistance to high-concentration acidic gas interference limits its application scope.
    The S+4OXLF is immune to intake temperature and humidity changes and can operate stably for 5 years even under high-concentration acidic gas interference, significantly reducing maintenance costs (product replacement and travel expenses) caused by sensor failure within the warranty period.
  2. Combustion efficiency analyzers
    Currently, combustion efficiency analyzers are mainly used for industrial boiler efficiency analysis. With the increasing popularity of civil boilers in households, especially the successful application of condensing boilers, the household heating market will experience explosive growth, driving the expanding demand for combustion efficiency analyzers and improving monitoring mechanisms. Both boiler manufacturers and maintenance service providers will need to equip such analyzers.
  3. Portable flue gas analyzers
    Most domestic flue gas analyzers adopt a box-type structure, equipped with lead-oxygen sensors and bulky Series 3 toxic gas sensors. The S+4OXLF can serve as a replacement, helping instrument manufacturers develop lighter, thinner, shorter, and more compact products.