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How to Take Temperature Measurements Inside a Cement Kiln

The inside of a cement kiln is an extremely challenging environment for making any kind of temperature measurement. Kiln rotation, high temperatures, and the movement of the sintering material along the kiln, make thermocouples and other contact-based temperature sensors impractical.

Radiation thermometers, or pyrometers, allow a non-contact temperature measurement, but their effectiveness can be limited by the high-dust conditions within the burning zone and small target point within a large, dynamic kiln area.

Nevertheless, the temperature at the burning zone, the hottest part of the process, is an important measurement, because it shows whether or not there has been a complete transformation from DiCalcium Silicate (C2S) to TriCalcium Silicate (C3S), a process which occurs at around 1300 to 1450 °C (2372 to 2642 °F).

Ratio pyrometers are often used to measure temperatures in the burning zone, as they are much less susceptible to errors caused by obscuration and dust. Combined with a peak-picker algorithm, a ratio thermometer can give an accurate temperature measurement even when there is 95% obscuration in the field of view.

This provides an effective measurement for general monitoring but does not supply much information about the kiln operation. Only a single measurement value from a small part of the kiln is provided, and so the readings can be deceptive if the target area is not optimised.

How to Take Temperature Measurements Inside a Cement KilnAdvantages of Using A Thermal Imager for Kiln Measurements

To obtain more detailed process information about the kiln, a more modern approach is to use a short-wavelength thermal imager, with a near infrared sensor and borescope lens.

This provides much more information about conditions within the burning zone. Borescopes have been used for many years for inspection and diagnostic purposes, but in recent years infrared versions, such as AMETEK Land’s NIR-B borescope camera models have been developed for thermal imaging applications.

The NIR-B borescope models are only 61 mm (2.4 in.) in diameter, including the water-cooled jacket, so can be inserted through a small viewing port without any major disruption to the process.

Because the kiln rotates continuously, the clinker collects on one side. This allows the borescope to be installed under the burner towards the opposite side, providing sighting onto the clinker and flame.

How to Take Temperature Measurements Inside a Cement KilnTo minimise exposure to process conditions, the tip of the borescope is usually slightly recessed into the wall. This does not affect its function, and the large 95° x 71° (NIR-B-2K model) field of view enables it to form an image of all the relevant parts of the process.

The NIR-B produces a detailed, live radiometric image, with accurate temperature information that allows the operator to measure any subset of over 300,000 live data points with the NIR-B or nearly 3 million live data points with the NIR-B-2K.

The wide-angle image, with high spatial resolution, allows accurate temperature measurements and important information about the process conditions within the burning zone.

The image processing software provided with the NIR-B models can be configured to measure multiple regions of interest and, as with a ratio pyrometer, defining up to 100 ROI’s (region of interest) in the thermal field of view, the process minimum, average and maximum temperature readings can be monitored at different areas within in the image to control the process, whilst having a contentious view into the process 24/7.

AMETEK Land NIR-BThe thermal image provides important data about kiln condition and qualitative information such as flame propagation and formation of ash rings within the kiln.

By using Near Infrared (NIR) wavelengths, the resultant thermal image suffers less from the scattering effect, caused by high energy particles within the field of view than a conventional video camera or borescope operating at visible wavelengths.

This makes it particularly valuable, as it can accurately measure temperature and provide high-quality process imaging simultaneously.

Read more about cement kiln interior monitoring in our Critical Measurements in Cement Manufacture Application Note.
 

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