IBAU Hamburg has equipped more cement silos with central cone extraction systems than any other supplier and has always set the standards for this technology. Tried and tested solutions were the starting point for the company’s engineers to develop even better solutions for their customer’s requirements. In the last two years large-scale tests have been carried out at the silo plants of several clients to identify how the energy requirements of silo extraction can be reduced. The results are better than expected. Not only can up to 40 % of energy consumption be reduced, but wear in the downstream equipment can be minimized and bulk loading times significantly improved. It is also possible to reduce silo wall loads and improve silo safety. The new concept is available for all new IBAU central cone silos and existing silos can be modernized with a return on investment being achieved very quickly.
Energy efficiency has become a major focus of the cement industry and IBAU Hamburg has been looking at ways to reduce the energy demand of large central cone cement silos, which use compressed air for fluidising the cement material within the discharge process. Typically the energy requirement for such silos including material extraction and transport is in the range of 0.06 to 0.12 kWh/t of extracted cement, depending on the extraction rate. This energy requirement is relatively low, when compared to other material transport and especially grinding systems.
About 70 to 75 % of the power consumption is for the material extraction from the silo, while 25 to 30 % is for material transport from the silo to bulk loading stations, where the biggest energy consumers are the vibrating screens for sieving foreign bodies and the bulk loading winches. Normally silo aeration uses about 40 to 50 % of the power consumption for material extraction from the silo and the rest is used by filter fans, venting systems and control air for the devices.
However, besides the power requirement, there are many other important aspects to limiting excess compressed air for silo aeration. One such aspect is the wear of the flow-control gate, another is the noise from excess air being blown off via an overflow valve and another is the longer loading time due to excess air blocking the system. Additionally, during discharge in large central cone silos material flow channels can be formed, which can cause damage to the silo walls. The less aeration air is used, the smaller are the flow channels.
The central cone discharge technology
The central cone silo was introduced to the market in 1975 and is used for silos with diameters from 10 to 30 m and with storage capacities of up to 40 000 t. Such largecapacity silos for the storage of cement require an efficient and trouble-free discharge system.
The central cone has a displacement function for the material in the silo, which allows the material to flow freely during discharge. The central cone forms a ring area on the silo bottom, which is divided into individual aeration sections that are inclined slightly downwards towards the discharge openings in the cone. The silo bottom is equipped with open fluidslides (aeration pad type) that have an air-permeable fabric on the upper side. The aeration air is blown under the fabric in order to fluidise the cement on the fabric. Each aeration section has its own discharge outlet with a flow-control gate that provides a controlled discharge from the silo to the downstream collecting bin.
For silo discharge, one silo section is active at a time. This means that only the fluidslides of one section are aerated and the relevant flow-control gate is opened for discharge. The silo bottom is aerated section by section, so that all sections are aerated in a complete cycle. Each outlet has a right-hand and a left-hand aeration sector that can be actuated individually, one after the other, to generate small discharge flow funnels as required by the aeration scheme. So, for discharge only the bulk material above the activated section is in motion and with the uniformity of this procedure, large uncontrolled material movements and load peaks on the silo walls are avoided. The result is a controlled silo discharge with a controlled mass flow in the silos according to the “Safety First” principle.
The advantages of the IBAU Discharge system are:
- 100 % safe operation
- Complete usage of the storage volume
- No uncontrolled material movements within the silo
- Almost uniform discharge during an aeration cycle
- No interruption during silo operation
The quantities of material that can be discharged with such a system vary between 50 and 1 000 t/h, depending on the size of the silo and loading requirements. Only very small quantities of air are needed for the material discharge, and the aeration air is removed along with the discharged cement. Typically the energy requirement for such silos with extraction rates of 250 t/h is in the range of 0.09 to 0.11 kWh/t of extracted cement, of which only about 40 to 50 % is used for the silo aeration system.
The Gdischarge concept
The idea behind the Gdischarge (i.e. Gravity discharge) concept was to optimise the energy requirement for large cement silos with an advanced discharge control system using the latest rotary lobe blowers. With a controlled limitation of the differential pressure for the silo bottom aeration, the power used for the generation of the compressed air can be significantly reduced. Furthermore, due to less pressure loss and other energy saving operations, the energy consumption and energy costs are significantly reduced.
No compromises have been made to the IBAU central cone concept which is characterised by a number of separate discharge outlets. All system components such as fans and metering devices are designed for maximum discharge capacity and optimised by the Gdischarge system for the required discharge situation. This means that during operation the air volume flow of the blower is automatically adjusted via a controller. In the control procedure alternative discharge requirements for downstream conveying to varying numbers of trucks, railcars, mixers and packing plants can be integrated.
A frequency converter for the blower and a pressure sensor at the blower form the control unit, which is linked to an intelligent controller. This controller regulates the motor speed of the blower to adjust the volume flow depending on the measured back pressure in the aeration pipe.
Target and actual pressure in the flow line regulate the air quantity of the blower. The blowers that are used by IBAU allow a very wide control range from 25 to 100 %. It goes without saying that these blowers are robust and durable, very easy to service and maintain and provide complete oil-free aeration air. In another option, control of the under pressure in the silo and venting air can be incorporated. Measuring the material flow from the silo discharge is best when the collecting bin for the silo discharge is equipped with a weighing system.
Test results and Gdischarge advantages
The new silo discharge system has been tested by IBAU Hamburg at different plants in Western Europe under real operating conditions. The results of the Gdischarge system are very impressive. In the tests, first the power consumption, loading capacities and the loading time of the existing silo systems were analysed and attempts were made to optimise them. The main process parameter variations in the tests were the back pressure of the flow line and the loading capacities. After the initial tests the silos were modified and the Gdischarge system was installed.
In most of the tests at the different silo facilities the existing roots blowers were not exchanged and only frequency converters for the blowers were installed. At one location in Germany the blowers were also exchanged and newly equipped with the latest technology including integrated frequency controllers. In addition to the frequency converter a pressure control system comprising of a pressure sensor, pressure measuring line and a controller were installed.
Table 1 summarizes the latest results at a Lafarge-Holcim cement plant in Germany. The results of 12 truck loadings without the new Gdischarge system are compared with the results of a 15 truck loadings test with Gdischarge. At the beginning of the tests the optimal target pressure in the flow line was measured. All the data in Table 1 are normalized, and given per ton of loading capacity or as an average figure.
The loading capacity (flow rate) could be increased from 100.7 t/h without pressure control to 140.6 t/h with pressure control, which corcorresponds to an increase of 39.6 %. Concurrently the power consumption was reduced from 0.033 kWh/t without pressure control to 0.019 kWh/t with pressure control which equates to a power reduction of 41.6 %. Finally, and not surprisingly to the experts, the loading time was improved by almost 40 %.
Furthermore, the wear is reduced within the system of flowcontrol gates and valves due to a reduction in the air quantities and velocities used, resulting in a reduction of the maintenance costs of the system. The filter loads are also reduced due to the lower air quantities in the system and smaller filter systems can be designed leading to significant cost advantages.
Another very positive effect is that faster loading operations for trucks and railcars can be achieved due to less aeration air in the system needing to be removed from the vessels during the loading operation. The reduction in loading times is about 30 to 40 %, which means that loading procedures can be significantly improved. In some cases where an additional loading lane is planned, this investment can be avoided with the installation of the efficient Gdischarge system. Cement producers should consider all the benefits of the Gdischarge system when considering the costs of upgrading existing systems.
Last but not least, a very positive effect is achieved on the formation of flow funnels in the silo. Because of the reduced silo aeration pressure and aeration air quantities, the flow funnels in the silo are smaller in diameter and the core funnels do not touch the silo walls so that the horizontal pressures on the silo walls are more homogenous within the silo and peak loads are reduced further.
The new Gdischarge system has been tested under real operational conditions by several cement producers. The test results from different silos and different extraction rates are achievable at any site and show a number of advantages when compared to conventional discharge systems. Therefore, IBAU Hamburg has decided to use the Gdischarge system as its new standard for all new cement silos and self-discharging cement carriers.
The Gdischarge system is already successful in operation on multiple self-discharging cement carriers and in a reference silo in Malaysia. Several new IBAU silos which are equipped with Gdischarge will be commissioned this year worldwide.
Because of its modular design, existing silos can also be modified and equipped with the system. In a next step, measurements by 3D-laser scanning are envisaged to illustrate the “Safety concept” and the reduced formation of flow funnels in the silos.