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Measuring Gas Temperatures In Combustion Processes By Infrared Pyrometry

Infrared thermometry is used in a wide range of industrial processes to measure the temperature of objects as diverse as a blast furnace shell, glass in a melt tank or the clinker in a cement kiln. 

In each of these cases, the thermometer works by measuring the intensity of radiation emitted by the object at a specific wavelength. Typical applications like these measure the surface temperature of a solid or liquid, but it is also possible to measure the temperature of a hot gas using infrared pyrometry.

Gas temperatures are important in many furnaces. The furnace exit gas temperature (FEGT) is an important parameter in the operation of a steam boiler. Its 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.

Municipal waste incinerators require a minimum temperature and residence time to ensure complete destruction of the waste materials. This is especially important when plastics are being incinerated because incomplete destruction can lead to emissions of highly toxic chemicals such as dioxins.

Measuring Gas Temperatures - Derek Stuart
Figure 1: A hot band is a molecular transition between two excited states

There are some specific problems in measuring gas temperatures that do not apply to measurements of solid or liquid objects with a well-defined surface, where we need to look through the gases in the air to see the surface in question.

For these applications, it is important to choose a wavelength where the main gases in the air are transparent, otherwise we will not be able to detect the infrared radiation emitted by the object. For gas measurements, we need a wavelength where the gas emits infrared radiation or, to put it technically, where it has high emissivity. However we still need to see through the cool gases in the foreground, so that we do not inadvertently measure their temperature. Fortunately, molecular spectroscopy provides a way to solve both of these problems. A “hot band” in the infrared spectrum of a gas is a spectral feature which only appears when the gas is hot.

Most common infrared spectra are related to transitions between the ground state and an excited state. Hot bands are transitions between two excited states, and only appear when the gas has enough thermal energy to populate the lower state, shown as “Excited 1” in Figure 1. Because the cold gases don’t absorb or emit radiation in the hot band, we are able to look through the cool gases outside the furnace and measure the gases inside the furnace. Fortunately, carbon dioxide has a hot band at a convenient wavelength, and it is produced in most combustion processes. so we can measure the gas temperature using an infrared thermometer with an appropriate spectral filter.

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 CDB is emitted by the gas. Otherwise the pyrometer will detect radiation from both the gas and the far wall of the furnace. This is mainly an issue for small furnaces running with a lot of excess air. Figure 2 shows the measurement error which results from different CO2 concentrations and pathlengths. 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 - Derek Stuart
Figure 2: Measurement uncertainty at 1475 °C (2690 °F) for different CO2 concentrations.


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