IntroductionThe challenge for cement producers today is to keep the operational costs low, which is not easy. Energy prices have reached levels never seen before, are continuing to rise, and customers expect the best quality cement for the lowest price. Additionally, as if this is not enough, environmental legislation requires a very strict energy policy and, even in countries like China, cement plants risk being shut down if they do not achieve the mandated amount of energy savings.

Siemens has a long tradition in finding innovative ways to reduce energy consumption and offers a large range of proven solutions.

Sometimes, large investments are made, like the installation of waste heat recovery, or the installation of a wind turbine farm to generate electricity.

Other, less drastic changes are now becoming general practice. For example, by replacing inefficient motors with high efficiency ones, up to 40% of energy can be saved, or by replacing fixed speed drives with variable ones, up to 70% can be saved. On top of this, Siemens has also developed a fuel management system for secondary fuels and a very advanced power management system that is part of the Totally Integrated Power concept.

Saving EnergyMark Yseboodt, Siemens NV, Belgium, explains how

comprehensive instrumentation helps in saving energy and costs.

Reprinted from WoRld CemenT June 2008www.worldcement.com

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These are all very effective ways to improve fuel consumption but, to get to the point where the plant operates at minimum cost, one can go a step further by automating the process in the most efficient way. To achieve this, every part of the process has to run without interruptions and as smoothly as possible, preferably without sudden changes. For this purpose, Siemens has developed the automation package CEMAT, based on the reliable Simatic PCS 7 control system. However, even the best performing automation system can only be efficient when the correct information from the field is communicated quickly and without interruptions. Over the past few years, Siemens has continually invested in order to be able to offer the most comprehensive instrumentation solution for the cement industry. These sensors work in combination with CEMAT to provide a highly stable and optimised production process with the maximum availability.

Limestone quarryCrushers are the biggest energy consumers in a quarry and it is important that they run with the highest possible efficiency and availability. It is very important that a crusher never runs empty because this is a waste of energy. Choke feeding is the most common way of ensuring that a minimum level of rock will be maintained in the crusher bowl. The crusher never runs empty, but – even more importantly – the efficiency of the crusher is maximised. When the level is very low, too many rocks are not crushed to the right size on the first pass. These then need to be recycled to the correct size, resulting in double the energy consumption.

The level in the limestone bin is important to guarantee a constant feed of limestone into the crusher. If the level is too low, it will demand more limestone. This could be a signal for a truck driver telling him to dump more limestone into the bin, or it can control a conveyer transporting rock into the limestone bin.

A level measurement is the best solution to maintain a minimum level of limestone in the crusher bowl. This will control the feed into the crusher. For example, it can be used to start or stop a vibrating feeder depending on the level in the crusher. For primary crushers, the level of rock box underneath the crusher is measured frequently to guarantee the crusher does not jam or run empty. If the box runs empty, the crushed stone will hit the conveyer with greater impact and the falling stones will damage the conveyer belt, resulting in increased wear and reduced lifetime.

Measuring the level in this harsh environment is not easy and it is important that the right technology is used. Non-contact technology is definitely the best choice, while ultrasonic technology is the most suitable because a crusher will expose the sensor to excessive mechanical stress. Vibrations and rocks, which might hit the sensor, are too demanding for compact measuring devices like radar or laser that contain electronics. Siemens offers fully potted Echomax transducers that resist the mechanical stress. The Sitrans LU transmitter is mounted separately. This proven solution will ensure a flawless measurement under all conditions.

For inventory purposes, the feed rate of the limestone conveyer to the crusher, and sometimes the feed rate after the crusher, is monitored with belt scales. The Siemens Milltronics MSI is a heavy-duty single idler belt scale that was designed specifically for this kind of application. It uses two triple beam parallel loadcells.

The special design eliminates the horizontal forces of the moving belt and provides an accuracy of 0.5% with a single idler scale. The design allows the scale to be very compact and easy to install. The idler is mounted directly on the scale, avoiding any additional moving parts and pivots that can wear out or need maintenance. The idler is the same as the other idlers

Figure 3. It is important to have a belt scale that provides accurate measurement and has

low maintenance requirements.

Figure 2. Ultrasonic transducers resist the mechanical stress in a crusher

better than anything else.

Figure 1. The control system uses reliable information from the field to control the crusher.

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from the conveyer, so there is no need for expensive scale quality idlers and this is an additional benefit for the management of spare part inventory.

Clinker productionIn this part of the process, much energy is used to maintain the correct temperature in the kiln. The most efficient combustion can be achieved at the ideal combustion point where the amount of CO and O2 determine the amount of fuel that is needed. Inefficient combustion in the kiln will have an enormous effect on the fuel bills. Many cement plants are now using alternative fuels and burn many different materials. The characteristics of these fuels can vary a lot and their combustion is optimally controlled by measuring CO, O2 and NOx in the kiln exhaust gases. CEMAT uses this information to optimise the fuel management.

Measuring these components in the extreme environment of the kiln is very demanding for the sample system, as temperatures reach 1400 °C, there are very high dust loads, a lot of mechanical stress, and aggressive alkalis, sulfates and chlorides. For environments such as this, Siemens has developed the FLK sampling probe, which is installed in the back of the kiln and is equipped with a thermal cooling liquid to enable the sample gas to be cooled to 180 °C. The advantage of a cooling circuit at this high temperature – a water-cooled system is normally cooled to 80 °C – is that condensation in the sample tube is avoided and the subsequent risk of dust clogging is reduced to a minimum. Automatic back purging of the sample line at regular intervals will increase the availability of the measurement and the process will run more smoothly. This is an area where, due to the increased accuracy and availability of the measured value, significant cost savings can be realised, considering today’s fuel prices. The return on investment for such an installation is around 12 - 18 months.

To further improve the process, measuring points are installed in the calcinator and in the preheater.

To control the clinker cooler, the thickness of the clinker bed has to be measured. Traditionally, differential pressure transmitters are used to give an indication of the clinker level. Pressure transmitters provide very accurate pressure indication, but when they are used for level measurement they can only provide an indication of the level and not an accurate measurement. Pressure measurements do not take into consideration the density of the clinker. When the consistency of the density varies, there will be variations in the pressure, even when the level does not vary in the same way. Pressure sensors sometimes detect peaks, caused by the clinker having a kind of explosion but, in fact, the level itself would not change much. This makes

it difficult to control the speed of the cooler and very often a camera has to be mounted into the cooler to allow the operator to manually adjust the speed. The only way to have an accurate level reading is by using a level measurement device that is capable of taking measurements in a dusty environment with temperatures up to 100 °C. A proven solution for many coolers is to install a radar level measurement. Radar technology

Figure 4. The energy consumption can be substantially reduced when the clinker

production area is well automated.

Figure 5. The FLK kiln sampling probe is developed to reduce the total cost of ownership.

Figure 6. The Sitrans LR 260 solid radar accurately measures the clinker bed depth in the clinker cooler.

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is non-contact and is unaffected by dust and temperature. The Siemens Sitrans LR260 is the perfect instrument for this application. It is a high frequency radar device, developed to measure solids of up to 30 m, even in extremely dusty conditions. When installed on a cooler just after the kiln outlet, it gives an accurate reading of the clinker level. This provides perfect automation of the cooler without any need for intervention from the operator. The advantage is not only that the cooler runs better, but it also reflects on the stability of the kiln. This is because when the speed of the cooler varies it will have a direct effect on the kiln control.

By optimising the kiln combustion and controlling the cooler speed, the plant will have a more consistent clinker quality with less energy consumption overall.

The cement millGrinding cement consumes a lot of energy. To achieve the best quality cement, the right dosing of clinker, gypsum and other additives is very important. Accurate information about the inventory of these products allows for uninterrupted production.

Clinker silos typically have large diameters, with 30 m as the norm. They are also very tall. The Sitrans LR 460 radar with a measuring range of 100 m is specifically developed for this kind of application and the same radar is also suitable for the additive bins, providing a very accurate level measurement.

Accurate rationing of these materials is typically done with weigh feeders. Siemens manufactures the weigh feeders according to the customer specification, meaning that every feeder is specifially designed for its job. The recipes for the different types of cement can be easily adjusted in the control system CEMAT.

To improve the operation of the mill, grinding aids are added to the process. These optimise the overall performance and efficiency of the finishing mill. In addition, these liquid additives provide finer granulation and improved flow properties, thus lowering the handling costs and improving the quality of the cement. Accurate dosing of the additives is essential for the process: too little and the mill does not run at optimum performance; too much and the material passes through the mill without being ground sufficiently. To accurately measure the grinding aids, Siemens utilises the Sitrans FC coriolis flow meter.

The advantage of the coriolis principle is that it measures both very low flow rates and non-conductive materials. Typically, grinding agents are non-conductive, rendering traditional electromagnetic type flow meters ineffective. The addition of the liquid additives allows the operator to dose less clinker and more inexpensive substitutes such as slag or fillers. Accurate and reliable dosing of these additives increases the grinding output of the mill, improves productivity and lowers overall energy costs.

When the mix is ground, the quality is checked and the rejects are returned to the mill. These rejects are measured with an impact flow meter. The Siemens Milltronics impact flow meter is ideal for this application as it can be installed in-line, requires very little headroom and can be easily fitted into an existing application. The flow rate of rejects is a good indication of the efficiency of the finishing mill, allowing the control system to optimise the feed rates and dosing of grinding agents.

For weigh feeders, impact flowmeters and belt scales, Siemens has developed a module called Siwarex FTC that can be installed into a Simatic S7 PLC. The units are connected directly to the control system and CEMAT has all function blocks available to interface with the devices and control the process. A similar module is available for the coriolis flowmeters, the Siflow FC 070.

The advantage of such an approach is that the instruments are connected directly to the control system without the need of a separate transmitter. This saves a lot of engineering, installation and commissioning time and the

complete process automation is carried out inside the control system.

Meeting needsAdvanced automation can save costs, but only when suitable, accurate and reliable instrumentation and gas analysers are used. It will optimise the operation of the plant and improve the efficiency and product quality.

With many years of experience in the cement industry, Siemens has developed an extensive range of solutions, including the most comprehensive instrumentation product range.__________________________________________________l

Figure 7. Accurate ratio control of clinker, additives and grinding aids

increases the efficiency of the mill and reduces the energy consumption.

Figure 8. Coriolis mass flowmeters are the most accurate

way of dosing grinding aids.

Figure 9. Siwarex modules allow belt scales, weigh feeders and flowmeters to be integrated directly into PCS 7.

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