Monitoring cement kilns: thermal imaging gives early warning of lining failure

In cement production, the furnaces or kilns that are used for blending the raw materials are a critical asset, which are often at risk of overheating, which can cause serious damage to the kiln shell. It is therefore important to have an effective way of monitoring the heating process to prevent damage to these critical assets.

Cement production is a complex process in which one of the steps consists of blending limestone - cement’s main ingredient - with other components in large rotary furnaces. These furnaces or kilns are a critical asset of a cement production plant, heating their contents to temperatures up to 1500°C. There is, however, a risk of overheating, which can cause serious damage to the kiln shell.

How cement is made

To understand the importance of the rotary kiln in the cement production process and the use of thermal cameras for this process, let’s first take a look at the cement production process.

Figure 1: A typical cement kiln.

Cement plants are usually located closely either to hot spots in their downstream market or to areas with sufficient quantities of raw materials. Basic constituents for cement (limestone and clay) are taken from quarries in these areas. The production of cement requires two main steps: first, clinker is produced from raw materials, and then cement is produced from cement clinker.

The raw materials are delivered in bulk, crushed and homogenised into a mixture which is fed into a rotary kiln. This is an enormous rotating pipe 60 to 90 m long and up to 5 m in diameter. This huge kiln is heated by a flame inside the structure to a temperature of 1500°C. The kiln is slightly inclined to allow for the materials to slowly reach the other end, where it is quickly cooled to 100-200°C. Four basic oxides in the correct proportions make cement clinker: calcium oxide (65%), silicon oxide (20%), alumina oxide (10%) and iron oxide (5%). These elements mixed homogeneously when heated to over 1450°C will combine to form the product of this phase, called ‘clinker’. The resulting solid grains are then stored in silos.

The second phase is handled in a cement grinding mill, which may be located in a different place to the clinker plant. Gypsum (calcium sulphates) and possibly additional cementitious or inert materials (limestone) are added to the clinker. All constituents are ground, leading to the fine and homogenous powder that we know of as cement.

Figure 2: Schematic representation of a rotary kiln.

The rotary kiln

Inside the rotary kiln, there is a refractory lining which insulates the steel shell from the high temperatures inside the kiln and protects it from the corrosive properties of the process material. This lining consists of refractory bricks or cast refractory concrete and needs to be replaced on a regular basis whenever the lining gets worn. The lifetime of the refractory lining can be prolonged by maintaining a coating of the processed cement material on the refractory surface. The thickness of the lining is generally in the range 80 to 300 mm. A typical refractory layer will be capable of maintaining a temperature drop of 1000°C or more between its hot and cold faces. The shell temperature needs to be maintained below around 350°C in order to protect the outer steel from damage.

This is where thermal imaging comes in. Using thermal imaging cameras, the kiln shell can be continuously monitored and when needed, early warnings of ‘hotspots’ indicative of refractory failure can be given.

Thermal imaging

Thermal imaging is the use of cameras constructed with specialty sensors that ‘see’ thermal energy emitted from an object. Thermal, or infrared energy, is light that is not visible to the human eye because its wavelength is too long to be detected. It’s the part of the electromagnetic spectrum that we perceive as heat. Infrared allows us to see what our eyes cannot. Thermal imaging cameras produce images of invisible infrared or heat radiation. Based on temperature differences between objects, thermal imaging produces a clear image. It is an excellent tool for predictive maintenance, building inspections, research and development, and automation applications. It can see in total darkness, in the darkest of nights, through fog, in the far distance and through smoke.

Protecting the kiln shell

The shell is critical for the operational performance of the kiln. Thermal imaging cameras can at least detect two different problems regarding this shell.

Firstly, during operation, a ring of cement coating is piling up inside the shell on the refractory brick surface. On the one hand, this is beneficial, because it lowers the shell temperature, reducing heat losses and protecting the refractory material. On the other hand, furnace operators need to be aware that this coating doesn’t get too thick, because this will reduce the internal diameter and, as a result, reduce the furnace’s production performance. By detecting low temperatures on the kiln shell, thermal imaging cameras can make operators aware of this problem.

Secondly, unstable cement coating or sudden detachment of coating material easily leads to problems with the refractory material and can cause refractory bricks to fall off. As the protecting layer is then damaged and its thickness reduced, hot spots are formed inside the shell, which results in loss of energy and disturbed kiln operation. To protect the steel shell from damage, its temperature should remain below 350°C. This can, of course, easily be monitored with thermal imaging cameras.

Figure 3: This view presents the kiln ‘pipe’ temperature on the surface as a thermal image. The kiln is rotating in real speed.

A kiln monitoring system

Two companies recently teamed up to develop the IRT KilnMonitor, a computer system that allows cement production operators to monitor, process and trace data from several kilns at once. The first company, INPROTEC IRT, is an official FLIR Systems distributor for Italy. Based in Milan (Italy), INPROTEC IRT has a wide expertise in high-tech equipment for industrial safety applications. The second company, Grayess (USA), specialises in the design, manufacture and marketing of special customised infrared thermal imaging solutions and software for a wide variety of applications.

The IRT KilnMonitor system includes FLIR A-Series cameras, which monitor the kiln temperature in real time. In addition, it includes a kiln visualisation module (2D and 3D) and a thermographic analysis module.

The system makes use of three A315 cameras each scanning one third of the 60 m long rotary kiln. These thermal video streams are distributed to a visualisation system inside the central control room and provide operators with a 24/7, real-time view of the kiln operation and performance. The kiln has a rotation time of around 30 seconds and the IRT KilnMonitor is synchronised to the rotation time to build up a thermal image.

Whenever the kiln shell reaches an undesired temperature, operators receive dedicated software alerts which allow them to take the appropriate remedial actions. For example, hot spots in the thermal image of the furnace can indicate that refractory bricks got detached from the refractory lining and that the protective kiln layer is getting less thick. This may require the furnace operators to reduce the temperature of the burner or even shut the system down in order to prevent severe damage and avoid excessive costs.

Figure 4: This viewing mode virtually cuts the kiln at some position and shows the kiln interior (bricks and coating).

Accurate thermal views

To give control room operators the best possible view of the situation, the IRT KilnMonitor generates several different viewing modes based on the information received from the thermal imaging cameras:

• A three-dimensional view of the whole kiln, rotating in real time, and showing the temperature distribution along the kiln.
• A three-dimensional view of the kiln interior at a cross-sectional ‘slice’, showing the refractory bricks and coating.
• A kiln end view in section.

Thermal imaging cameras versus scanners

Previous solutions for kiln monitoring have used thermal imaging scanners, rather than cameras.

When scanners are used, then theoretically one scanner unit can suffice to monitor an entire 60 m rotary kiln. However, when using a scanner, the unit needs to be placed at a certain distance so ‘see’ the whole kiln, and the view of the kiln should not be obscured. In practice, this is not always possible. Thermal scanners can be quite bulky and are not very flexible in terms of installation. In many cases, a rotary kiln is installed inside a dedicated production hall, so taking into account that a thermal imaging scanner has a maximum viewing angle of 120°, it is very often impossible to install a thermal scanner at sufficient distance from the rotary kiln and avoid obstacles that are blocking the view. For example, with many rotary kiln installations, there is a secondary air tube which directs hot air out of the rotary kiln to be used as an energy source. This secondary tube will often be an obstacle.

In contrast, although multiple thermal imaging cameras will be needed to replace a single thermal scanner, they are much smaller, much lighter and much more flexible in terms of placement and installation. They are therefore the preferred solution for installations where space is limited. In the KilnMonitor system design, a FLIR A315 camera with a 90° lens is used. In this case, three thermal imaging cameras cover the total pipe length of 60 metres, which is still lower in cost than one thermal imaging scanner.

In addition, when comparing thermal cameras with thermal imaging scanners, the thermal imaging cameras offer the end customer a less expensive solution.

High resolution

The FLIR A315 and A615 are compact and affordable thermal imaging cameras, fully controlled by a PC. With a thermal sensitivity of <50 mK, they can capture fine image details and temperature difference information. “We definitely need the high resolution,” said INPROTEC’s Roberto Ricca. “For an ideal installation, we often opt for a 90° lens, because then you only need to use two or three cameras to cover the entire kiln length. For a German customer, we integrated the FLIR A315 with 90° lens and it delivered on the promise: very high image quality and very accurate detail.”

Using the higher resolution of the FLIR A615 for future installations of the IRT Kilnmonitor system will provide even higher resolution. The FLIR A615 has a resolution of 640 x 480 pixels that allows more accuracy and shows more detail at a greater distance. Taking into account a rotary kiln of 60 metres length, it would provide an image where each pixel represents 10 cm of the pipe.