Tuesday, September 2, 2014

Oxygen Analyzers in Industrial Combustion

zirconia sensor
Diagram of zirconia
sensor for oxygen
measurement.
(Courtesy of Wikipedia)
Oxygen analyzers are used to measure the quantity of oxygen in air or in other mixtures of gases, such as exhaust flue gas from a process. Oxygen, along with nitrogen and hydrogen, is a major component of air making up 20.9% of the air we breath. Oxygen is also used in the industrial combustion process, and when fuel is burned in presence of oxygen, CO, CO2, and H2O are by-products of that combustion. By measuring the oxygen in combustion process the proper air-to-fuel ratio can be maintained for optimized process efficiency. 

Oxygen analyzers are used to determine exhaust gas concentration of oxygen in many applications from internal combustion engines to combustion applications in industrial plants. Many industries such as powergen, refineries, chemical plants, cement plants, asphalt production, and agrochemicals all require oxygen measurement.

Among the many ways to measure oxygen, the leading technologies are the zirconia cell, electrochemical, infrared, ultrasonic and laser methods. Each method has its own advantages and disadvantages. Most oxygen analyzers, though, are based on the zirconia cell.

Zirconia based oxygen measurement is accomplished by relating conductivity of a zirconia ceramic cell to oxygen concentration. Ceramic zirconia cells allow oxygen ions to pass through at elevated temperatures (around around 800°C). In the zirconia cell, the sample gas containing the oxygen which needs to be measured is supplied at one side of the sensor. On other side of the sensor, a reference gas is supplied whose oxygen concentration is fixed and known. Since zirconia cells have the ability to pass oxygen ions, oxygen ions move from the high concentration side (i.e. reference gas in case of combustion process) to the low concentration side (sample gas). The movement of these ions generate an electromotive force (EMF or voltage). By measuring this EMF, the oxygen content can be determined. The EMF generated depends on two properties; temperature, which is elevated and maintained; and oxygen concentration which is determined by using air as a reference gas.

It is very important to keep the temperature elevated and constant in order to prevent condensation. Moisture deposits on the zirconia cell can cause damage. The air (reference gas) should also be kept flowing to keep the oxygen analyzer in proper working condition.

To calibrate an oxygen analyzer, a gas with a known oxygen concentration is required. There are two types of calibration, two point calibration and single point calibration. When calibrating using two reference gases, a low oxygen concentration gas is used to set the zero point, while a high oxygen concentration is used to set the span. Single point calibration uses only air as the high concentration gas to set the span. 

To receive more information about any industrial combustion application contact CTi Combustion at 925-208-4250.