Analytical Instrumentation

Lambda transmitter principle

Lambda transmitter principle 1

Which instrument is used to monitor O2 and CO in flue gas?

  • The by-products of combustion (CO/H2) have been used as indications of the quality of the combustion process is a new idea for binary burner regulation that is based on a modified zirconium dioxide probe to provide an alternative to current technology for O2 trim. 
  • This research’s ultimate goal was to produce a dynamic, self-improving regulatory technique that would help industrial combustion systems further cut back on exhaust gas losses.
  • In order to conserve energy and prevent harm to the environment, property, and public health, combustion processes must be monitored and managed. 
  • It is not possible to determine whether combustion is complete by just measuring the oxygen content of exhaust gases. 
  • Therefore, it is crucial to identify and reduce the amounts of unburned components in the exhaust stream. 
  • These unburned substances include hydrogen (H2) and carbon monoxide (CO). 
  • Hydrogen and carbon monoxide emissions usually occur in conjunction in the exhaust gas when incomplete combustion occurs.
  • For the first time, unburned components in exhaust gases of gaseous fuels may now be quickly and maintenance-free measured in location, allowing for the optimization and regulation of combustion. 
  • The dual sensor monitors and reports the oxygen level simultaneously for safe limit value shut-down.

What is the working principle of lambda?

Measurement principle

Lambda transmitter principle2

Sensor technology principle for the O2 electrode

The Combination Probe is built around a heated zirconium dioxide ceramic (ZrO2) electrochemical measurement cell.

It has three electrodes.

  1.  O2 electrode (platinum)
  2. COe electrode (platinum/noble metal)
  3. Reference electrode (platinum)
  • A ceramic tube made of zirconium dioxide that is sealed on one side serves as the probe. 
  • It extends into the emissions channel of the combustion system, separating the measurement gas compartment from the reference gas compartment to prevent any gas from escaping. 
  • In the reference gas compartment, the zirconium dioxide ceramic’s inner side contains the reference electrode.
  • In the measuring gas compartment, the two measuring electrodes for O2 and CO/H2 are situated on the ceramic’s exterior side.
  • The probe is heated to a temperature of about 650 °C by an inbuilt heater, which also regulates this temperature.
  • At this temperature, the zirconium dioxide ceramic conducts oxygen ions, resulting in the generation of the two sensor signal voltages that can be measured: UO2 (between the reference and O2 electrodes) and UCOe (between the reference and COe electrodes).
  • The sensor voltage UO2 [mV] corresponds to the known Nernst voltage, which is dependent on the sensor temperature T [K] and the logarithm for the O2 partial pressure ratio between the reference and measuring chambers, with the constants k = 0.21543 [mV/K] and the sensor-specific offset voltage U0[mV]. following the formula:

UO2 = U0+kTln(pO2,ref/pO2,meas).

The probe’s U0 is established by calibrating it with ambient air: 

The last component of the equation becomes zero 

when pO2,ref = pO2,

Meas = 0.21, 

and the offset voltage is measured U0 = UO2 at 21 vol.% O2

“Nernst sensor characteristic Us = f (O2)” depicts a typical Nernst O2 characteristic (UO2) at a typical sensor temperature T = 923° [K] with a typical offset voltage U0 = -5 [mV].

 Lambda transmitter principle 3

Principle of sensor technology for the COe electrode

The COe electrode is identical to the O2 electrode with the exception of the electro-chemical and catalytic capabilities in the signal materials, which allow the detection of combustible components like CO and H2.

The COe electrode also develops the Nernst voltage UO2 during “clean” combustion, and the characteristics of the two electrodes are identical. 

A non-Nernst voltage UCOe also occurs on the COe electrode in the case of incomplete combustion and in the presence of combustible materials, and the characteristics for both electrodes shift apart (Refer the “Typical signal characteristics for the two  sensor voltages”).

Lambda transmitter principle 4
Lambda transmitter principle 5

These two voltages add up to form the complete sensor signal UCO/H2 on the COe electrode: UCO/H2 = UO2 + UCOe. 

The concentration of combustible components COe in ppm can be calculated using the formula UCOe = UCO/H2 – UO2 by subtracting the oxygen content, as determined by the O2 electrode, from the overall sensor signal. 

The normal route for COe concentrations as O2 content steadily decreases is shown by the dashed line on the “Typical signal characteristics” graph for the two sensor voltages.

When going into the “deficient air” range, the COe concentration at the so-called “emission edge” goes up a lot because of incomplete combustion caused by not enough air for combustion. 

Additionally depicted are the sensors’ resulting signal characteristics UO2 (continuous line) and UCO/H2 (dotted dashed line).

According to the Nernst principle, the two sensor signals UO2 and UCO/H2 in the surplus air range with clean COe-free combustion are similar to one another and display the present oxygen concentration in the exhaust gas channel.

Due to the additional non-Nernst COe signal present close to the emission edge, the sensor signal for the COe electrode UCO/H2 grows at an abnormally rapid pace.

La mbda transmitter principle 6

The two sensor voltages, UO2 and UCO/H2, and their usual signal properties in respect to the O2 concentration in the emissions channel. 

Also demonstrated is the typical property of flammable elements COe.

In addition to the absolute sensor signals UCO/H2 and UO2, the relative change in the sensor signals over time dUO2/dt and dUCO/H2/dt, as well as, in particular, the signal dynamic range for the COe electrode, can be utilized to calculate the emission edge.(see “Dynamic range of the COe electrode signal UCO/H2 in the incomplete combustion range”).

What is the purpose of the lambda sensor?

Advantages of Lambda transmitter

  • Lambda transmitters continuously monitor the oxygen content in flue gases, allowing for precise control of air-to-fuel ratio. This optimization improves combustion efficiency, reducing fuel consumption and lowering operating costs.
  • By maintaining the ideal air-to-fuel ratio, lambda transmitters help minimize the production of harmful pollutants such as nitrogen oxides (NOx) and carbon monoxide (CO), resulting in compliance with environmental regulations.
  • Lambda transmitters provide real-time data on flue gas conditions, enabling immediate detection of potential combustion issues, such as incomplete combustion, which can lead to hazardous situations.
  • The precise control of air-to-fuel ratio ensures consistent combustion, leading to stable and reliable burner performance, reducing downtime and maintenance requirements.
  •  With better combustion efficiency, less fuel is wasted, resulting in lower energy consumption and decreased greenhouse gas emissions.
  • By reducing fluctuations in combustion conditions, lambda transmitters help protect burners and related components from excessive wear and tear, prolonging their operational life.
  • Many lambda transmitters offer remote access and control capabilities, allowing operators to monitor and adjust combustion parameters from a central location, enhancing convenience and flexibility.
  • Lambda transmitters can promptly identify deviations in combustion conditions, enabling rapid troubleshooting and preventing potential damages to the system.
  • Lambda transmitters can be integrated into various burner types and sizes, making them suitable for a wide range of industrial applications.
  • Using lambda transmitters assists industries in adhering to strict emission regulations, avoiding penalties and reputational damage associated with non-compliance.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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