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Measuring Gas Temperatures in a Boiler with the CDA Pyrometer

Accurate temperature measurements within industrial boilers are important for safe, efficient, and reliable operation. However, direct measurements using thermocouples are difficult because the elevated temperatures, corrosive gases and high concentrations of particulate matter mean that probe lifetimes are very short.

Non-contact measurements using an infrared pyrometer allow the measuring instrument to be located outside the boiler, so it is never exposed to the aggressive furnace environment (Figure 1).

Measuring Gas Temperatures in a Boiler with the CDA Pyrometer

Figure 1: CDA views the furnace gases through a sighting tube on the furnace wall

A key design parameter for any large industrial boiler is the rate of heat transfer from the burning fuel through the water wall tubes to the steam side. The temperature of the combustion gases as they move from the radiant section of the boiler to the convective section is a key operating parameter for the boiler operator. Known as furnace exit gas temperature or FEGT, this value must be held within a closely defined range. Excessive temperature can damage the boiler tubes, while a low value reduces efficiency, increases fuel costs, and creates additional carbon dioxide emissions. A severe problem occurs when the FEGT rises above the ash fusion temperature, at which point the ash particles liquefy and rapidly deposit on the heat transfer surfaces.

Most infrared pyrometers are designed to measure hot objects within an oven or furnace, so they operate at wavelengths to see through the hot furnace gases. This is desirable in many applications, for example, where the user needs to measure the temperature of the boiler tubes, but it means they cannot be used to measure gas temperature. The AMETEK Land CDA pyrometer uses an infrared wavelength that is emitted by hot carbon dioxide (CO2). This allows it to see through the cold gases outside the furnace and measures the average temperature along its line of sight within the region of interest.

The CDA pyrometer can also measure gas temperatures at the boiler exit, where it passes to the air preheater, and in municipal waste incinerators where sufficiently high furnace temperature and long enough residence time are required to ensure complete burnup of the waste materials.

An important application consideration is that there must be enough CO2 molecules in the line of sight to ensure all of the radiation detected by the CDA is emitted by the gas. If this is not the case, the gas is somewhat transparent, so the measurement is influenced by both the gas temperature and the temperature of the boiler wall. Figure 2 shows the measurement error which results from different CO2 concentrations and pathlengths. The uncertainties are largest for small boilers with large amounts of excess air, where the CO2 concentration is low. In most large industrial boilers, the CO2 concentration is above 8%, and the pathlength is greater than 3 m (10 ft), so the measurement uncertainties are small. 
Measuring Gas Temperatures in a Boiler with the CDA Pyrometer

Figure 2: Measurement uncertainty at 1475 °C (2690 °F) for different CO2 concentrations.

Find out more about the CDA pyrometer here. 


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