Temperature Measurements in Cement Manufacture
December 14, 2018
Portland cement is an essential ingredient for the construction industry, used to make concrete and mortar. There are more than 2,000 active cement plants around the world producing over 3.5 billion tonnes per year. They produce cement by heating crushed limestone and other ingredients in a rotary kiln to produce clinker, which is milled to a fine powder to produce the finished product.
Temperature monitoring in and around the kiln is essential to an efficient process and to produce cement of consistent quality. It also enables early detection of kiln refractory issues or insufficiently quenched clinker, which could lead to production stoppages if left unchecked.
The Cement Manufacturing Process
Cement is created by heating crushed limestone with clay, slate, blast furnace slag and other materials, then cooling and grinding the resultant clinker to create a fine powder.
Most modern cement plants use a dry process which is more thermally efficient than the older wet process. The process consists of the following stages:
- Quarried limestone is crushed along with clay, slate, blast furnace slag and other components, to a size of 75 mm or less.
- The raw materials are heated in the multi-stage pre-heater and then fed into a rotary kiln which raises their temperature to approximately 1,370° C.
- As they progress along the rotary kiln towards the firing zone, the raw materials lose moisture and other components and form a rock-like substance called clinker, with mineral lumps roughly 25 mm in diameter.
- Upon leaving the kiln, the hot clinker is cooled and the heat is recycled to the pre-heater. This improves efficiency, saves money and reduces the environmental impact of the process.
- The cool clinker is milled to form a fine powder and any additional cement components – for example, calcium sulphate to control the setting time – are added.
- The finished cement is bagged and shipped.
Temperature Measurement Points
Temperature monitoring at key stages is critical for the close control of the manufacturing process, ensuring process efficiency, consistent product quality and prolonging the life of the kiln.
The important points for these measurements are in and around the rotary kiln, measuring:
- The burning zone
- The kiln shell
- The clinker cooler
The Rotary Kiln
The rotary kiln is the key component in a cement plant. It is a long steel cylinder, lined with refractory brickwork to protect the outer shell from the high temperatures of the interior.
The kiln is very large, up to 4 m diameter and up to 50 m long. It typically rotates three to five times per minute. As it rotates, the cement material works its way down towards the burning zone, heating up as it does so. This arrangement means that temperatures may vary considerably within the kiln, making monitoring more difficult.
The Burning Zone
Also known as the firing zone, the burning zone is at the lower end of the rotary kiln, where the cement components are heated to about 1,300° C to 1,450° C, forming clinker.
Accurate temperature measurements are critical for product quality, environmental performance and kiln lifetime.
Temperature Solutions in the Burning Zone
How it Works: Firing Zone Temperature Measurement
The inside of a cement kiln is an extremely challenging environment for making any kind of measurement. Temperatures are high, there is a lot of dust and tumbling clinker can damage in situ measuring instruments.
Traditionally, single-point measurements have been made here using a ratio pyrometer with a peak picker algorithm, such as the Spot R100. This measurement technique allows the device to measure the hottest material visible through the dust, even with up to 95% obscuration. It provides an effective measurement for general monitoring but does not supply much information about the kiln operation.
A superior method, providing more detailed process information about the kiln, uses Ametek Land’s near-infrared borescope (NIR-B) thermal imager. Because the kiln rotates continuously, the clinker collects on one side. This means that, for best effect, the NIR-B can be installed under the burner towards the other side, sighting onto the clinker.
The NIR-B produces a detailed, live radiometric image, with accurate temperature information that allows the operator to measure any subset of 324,000 live data points. This enables measurement of the flame temperature in the burner zone, clinker temperature and refractory wall temperature. The live image provides the operator with a lot of information that can’t be obtained from a single spot measurement, such as identifying ash rings inside the kiln. It is also superior to a standard video camera as it is less prone to being affected by heavy dust levels in the kiln.
NIR-B imagers provide not only temperature measurement in the burning zone but also a live video view of the conditions within the kiln.
Recommended Product: NIR-B
A short-wavelength, radiometric infrared borescope camera, the NIR-B is specifically designed to measure temperature profiles in furnace interiors.
Producing a detailed, live 656 x 494 thermal image, the NIR-B allows the operator to select from thousands of live data points for highly specific optimization. In the high particulate environment of the rotary kiln, it provides a clear image of the kiln interior.
With a choice of 90° or 44° viewing angles, the NIR-B requires only a narrow hole through the firing hood. High-performance water cooling allows it to operate in hot environments, while an auto-retraction system can remove the camera from the kiln if the cooling fails.
Features
- High measurement accuracy
- Short-wavelength sensor
- High-performance water-cooling
- Integrated air purge
Benefits
- Optimum process control
- Simple installation and ease of use
- 24/7 monitoring
- Two-year warranty
Recommended Product: Spot R100
Should a single-spot measurement be preferred, the Spot R100 pyrometer is ideal. This is a ratio pyrometer with peak picking, allowing measurements in dusty environments with up to 95% obscuration.
Designed for easy, one-person installation, the Spot range provides an accurate standalone solution, with no separate signal processor required. Ethernet, Modbus TCP, 4 to 20 mA analog outputs, alarm outputs and image streaming are all available.
With models using a variety of operating wavelengths and temperature ranges for different process requirements, the Spot range includes remotely accessible motorized focusing. This enables it to deliver accurate stable measurements quickly, reducing maintenance time and enabling faster process adjustments.
Features
- Self-contained, single-sensor solution
- Range of digital and analog communications
- Local and remote motorized focus control
- Easy plug-and-play installation
Benefits
- Peak picker mode measures in high dust
- No separate processor required
- Scratch-resistant sapphire protection window
- Faster, more accurate measurements
Kiln Shell Monitoring
Monitoring the exterior kiln shell temperature is critical. The steel kiln shell is protected from the extremely high process temperature by one or two layers of refractory brickwork. If this brickwork fails, either by erosion or physical loss of bricks, the metal shell can be severely damaged, putting the kiln out of action.
Detecting hot spots indicative of refractory damage helps to avoid costly maintenance or unplanned shutdowns. Continuous monitoring along the length of the kiln provides early detection of problems, allowing for repairs to be made before serious damage occurs.
How it Works: Temperature Solutions in the Kiln Shell
Traditionally, thermocouples have been extensively used to take temperature measurements along the moving kiln. However, these give poor coverage and are unreliable. A better measurement is provided by non-contact technologies, and a common method is to use a handheld, portable pyrometer to repeatedly measure along the rotating kiln.
However, this method is labour intensive and is not very effective, as it does not provide full coverage of the kiln shell. The measurement also depends on the operator’s skill, so repeatability is poor.
A much more effective technique is to use a linescanner to map the temperature of the outer shell of the kiln. Ametek Land’s solution for this application is the LSP-HD 62, a compact, high-accuracy linescanner specifically designed to produce thermal images of moving processes.
Installed to view the rotating kiln with an 80° scan angle, it samples 1,000 points along a single line, with an industry-leading scan speed of 150 Hz.
This helps build up a picture of the complete shell temperature, identifying any aberrations. For long kilns, multiple scanners can be used – Ametek Land’s proprietary Landscan Windows Control and Analyse (WCA) software will unify the data from these scanners to form one overall picture of the kiln temperature.
Tire Slip Monitor
Optional position sensors allow the WCA server to measure the rotation speed of each kiln tire and display it alongside the shell temperature. Accurate detection of kiln tire slip further reduces the likelihood of refractory damage.
Recommended Product: LSP-HD 62
Designed to operate in harsh environments, the LSP-HD uses high-quality optics to produce high-definition thermal images at unrivalled scan speeds, for outstanding process control.
All LSP-HD scanners feature rugged sapphire protection windows that resist scratches, acids and solvents. They also have a robust scanner assembly with only one moving part, and a die-cast housing water cooling and air purging.
The LSP-HD 62 is optimized for cement manufacturing applications, with a temperature range from 100 to 600° C. A plug-and-play industrial Ethernet connection provides real-time processed data, enabling refractory problems to be identified and analyzed quickly.
Features
- High-resolution optical system
- Operates in hot, dusty industrial environments
- Easy installation with single Ethernet cable
- Range of data output formats
Benefits
- Real-time thermal displays for accurate results
- Industry-leading scan speeds
- Detects even the smallest temperature differences
- Installation costs significantly reduced
Using a Thermowell
It is also possible to measure the temperature inside the kiln, away from the burning zone, using a Spot M160 pyrometer and a thermowell. A thermowell is a closed-ended tube that goes through the wall of the kiln and the refractory. The Spot pyrometer is set up to sight onto the end of the tube. Each time the kiln rotates, the Spot M160 can take a measurement of the temperature at the end of the thermowell.
Because the thermowell is a closed-ended tube, it acts in a similar way to a theoretically ideal cavity radiation source. This means that emissivity is not an issue for the thermowell material and provides an accurate determination of the temperature inside the kiln at that point.
The Clinker Exit and Beyond
Clinker is the term given to the solid lumps of cement that leave the rotary kiln. Once out of the kiln, the hot clinker is transferred to a grate cooler, where it is cooled by air blowers.
The cooled clinker is then taken away by conveyor belt, either for immediate milling into powder, or for storage until it is milled later. In either case, it is essential to monitor the clinker for any rogue hot inclusions that could damage the conveyor belt.
How it Works: Temperature Solutions for Clinker Monitoring
Even very small clinker fragments can cause serious problems if they are too hot. They can damage the conveyor belt and shut down the entire manufacturing process. Single-spot pyrometers won’t detect these small hot spots, as they average the temperature over the whole field of view.
Instead, a high-resolution solution is required that can detect a small hot spot with a rapid response. Ametek Land’s HotSpotIR scans the belt at 100 scans per second, detecting 1,000 small temperature spots each time.
This detects any uncooled clinker fragments that pass by, activating a high-speed alarm to alert the operator. Preventive action can then be taken, safeguarding the equipment and avoiding a lengthy shutdown and costly loss of production.
A single-spot pyrometer will average out the whole area it sees, potentially missing small hot clinker fragments.
A HotSpotIR scans the belt at 100 scans a second and in each scan, it detects 1,000 small temperature spots.
Recommended Product: HotSpotIR
A compact, fixed-focus, high-speed scanning system, the HotSpotIR is specifically developed to detect hot inclusions on a moving conveyor.
Designed for industrial environments, it rapidly identifies hot particles, preventing damage and avoiding costly shutdowns.
With high-resolution monitoring across 1,000 temperature spots, user adjustable scanning speed up to 100 Hz and repeatability of ±0.5° C, the HotSpotIR can detect hot spots as small as 25 mm.
Easy to install, it uses non-contact infrared scanning to measure a range from 20° C to 250° C.
The HotSpotIR connects to a dedicated processor and measures the entire belt surface, activating an alarm that can be set to trigger a fire suppression system or divert the material to a safe location.
The continuous monitoring means hot spots can be detected and the alarm triggered in one-hundredth of a second, allowing the operator to respond quickly.
Features
- Wide scan angle of 80°
- Built-in laser targeting system
- Fast, 100 Hz scanning speed
- Withstands high ambient temperatures
Benefits
- Increases confidence in safety
- Rapid-response alarm system
- Helps reduce insurance costs
- Prevents damage and downtime
Other Solutions for Cement Manufacturing
Ametek Land also supplies industry-leading solutions for other applications in the cement manufacturing process.
- Emissions monitoring throughout the production process.
- Material build-up monitoring at the feed chute and spray tower.
- Hot spot and CO detection during the storage, conveying and milling of the coal used for heating.
- Flue gas analysis throughout the plant.
(Download the cement and lime brochure: www.ametek-land.com)
Ametek Land Combustion and Emissions Monitoring
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Ametek Land Non-Contact Measurement Solutions
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| A high-resolution, fast thermal imager that provides detailed temperature information in harsh industrial environments. |
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Ametek Process Instruments Analysers
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Ametek Land Solutions for Temperature Measurements in Cement Manufacture
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| A short-wavelength borescope thermal imager that provides high-resolution images with a wide 90° view, in a through-the wall design. |
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| An Ethernet-controlled compact infrared linescanner to produce advanced thermal imaging in moving processes. |
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| These full-featured, high-performance pyrometers for fixed, non-contact, infrared spot temperature measurements and a range of process requirements. |
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| Continuous infrared thermal line scanning detects small, hot inclusions on the conveyor, with alarms set to operate an inerting or diverting system to prevent expensive belt or downstream fires. |
Services
Ametek Land’s in-house service centers provide after-sales services to ensure the best performance from each system. This includes technical support, certification, calibration, commissioning, repairs, servicing, preventive maintenance and training. Their highly trained technicians can also visit sites to cover planned maintenance schedules and repair emergency breakdowns.
Summary
Temperature measurements are critical to the cement manufacturing process to ensure consistent cement quality and to prolong the lifespan of the equipment. Monitoring is focused on the rotary kiln and there are several measurement techniques that are applicable.
While single-spot measurements from a ratio pyrometer are effective, the most comprehensive information is obtained from thermal imaging using an infrared borescope. This provides an overall picture of kiln activity at the burner end, allowing the operator to see a live view of operations in the firing zone and beyond. A linescanner can help safeguard the kiln shell against damage by detecting compromised refractory brickwork at an early stage.
In addition, there are many other points where analysis is effective, including hot spot detection and emissions monitoring.
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