Pneumatic conveying systems in the cement and cement processing industry.
The correct storage of bulk material and the selection of a suitable transport system is becoming increasingly important for plant owners nowadays. This is particularly of great significance wherever a high service life, few to no interruptions, high throughputs, and the lowest possible power consumption are in demand. In cement plants with outputs from 1000 t/d to 3000 t/d and in the cement-processing industry, all of these requirements on the plant parts have to be fulfilled.
In addition to mechanical bulk material handling (such as belt conveyor transport), pneumatic conveying systems are used as a transport method in all the various process stages of cement manufacturing. The pneumatic conveying methods comprise the dense phase conveying with pressure vessels, lean phase conveying with rotary-valves, or even conveying using air slides. The use of these technologies is accordingly determined by the task and requirements. All three conveying technologies have the advantages of a closed conveying system in common, which is equivalent with a minimisation of environmental pollution and dust emission.
Pneumatic conveying systems are used, for example, for the loading of coal dust into the rotary furnace and calciner. Since this fine dust is extremely abrasive, the conveying system and pipeline must be accordingly protected against wear, for example, using ceramics-lined pipelines, pipe bends, and/or special pipe elbows. Protection against wear, however, should also be taken into consideration for dense phase conveying systems even if the wear effect is lower in the case of these systems due to a lower conveying speed (5 – 25 m/s) in comparison with the lean phase conveying systems (20 – 35 m/s). On the one hand, the wear affect depends on the particle properties (hardness, grain shape, and grain size) and to a greater degree on the conveying speed (rule: wear = K * vn; K= constant, v= speed, n = 2-6) and operating conditions (product temperature, amount of bulk material) on the other.
In cement plants, secondary fuels like sewage sludge have been increasingly used instead of coal dust in the last few years. In these processes, the dried sewage sludge is conveyed to the desired points using pneumatic dense or lean phase conveying methods.
Cement clinker is grinded in a ball mill and then often transported to the silos using a dense phase conveying system. In this case, the grinded product falls into a pressure vessel and is then blown away through the pipeline at a pressure of about 3 bar. To increase output and guarantee continuous operation, two alternating vessels can also be used. Whilst one vessel is conveying, the other is being filled.
An important point in the case of pneumatic conveying systems is that the air is accordingly conditioned. Since humid cement has a tendency to create lumps, the generated compressed air should be dehumidified using an appropriate dryer.
Another application for using a dense phase conveying system is the bulk material transport of fly ash. The combination of a pressure vessel with a special pipeline construction (internal bypass pipes) makes it possible to convey the abrasive products at a low speed. In addition to the advantage of the wear minimization, this system is best-suited for safe transport without pipeline blockages.
Pneumatic conveying systems are also often used for cement loading or discharging, for example, when loading or unloading ships or bulk silo train cars. In the summer of 2009, Gericke was thus able to commission a pneuwork® pneumatic cement unloading system, designed as a dense phase conveying system, at the company Transportbeton GmbH & Co. KG in Vienna, Austria. The task was to unload the tank cars loaded with cement and convey the cement from the unloading point about 200 metres to two existing silos at an output of 50-55 t/h.
The five silo cars in total are parked by the railway company at a fixed unloading point at the customer site and unloaded using a common conveying pipeline. The silo cars consist of four pressure containers with a content of 13 m3 each and are designed for a maximum unloading pressure of 2.0 bar. The containers are equipped with an aereated bottom and have a lateral discharge with DN80. For each silo car with four containers, two dockings points are present in the pipeline installed next to the rails, which are designed in DN125/150. The docking points have a shut off valve and are connected to the lateral discharge spout of the railway car pressure vessel using a flexible hose line. The air supply line by means of which the compressed air of five connecting points in total are conducted to the respective silo car is also laid parallel to the conveying pipeline. The conveying air is provided by a screw compressor that achieves a maximum end pressure 3 bar installed next to the track. Depending on the container, the unloading pressure lies between 1 and 2 bar and the energy consumption is at about 1.9 kW per ton of conveyed material. To distribute the cement over the two silos, a diverter valve with two slide gate valves was also provided. Furthermore a special silo inlet station was delivered to divert the cement gently and with low wear into the respective silo.
In addition to silo car unloading systems, Gericke provides under the brand name
pneuwork® conveying system especially designed for abrasive products, bulk materials at high temperature level up to 300 °C or throughputs up to 200 t/hr.
The construction and design of the pressure vessels and the pertinent equipment is designed for robustness, reliability, the product requirements, and the industrial market environment. The pressure vessels are equipped with a fluidising bottom or cone, a special abrasion-resistant inlet cone that seals off the pressure vessel. The pneuwork® system is completed by ancillary equipment such as diverter valves and abrasion resistant elbows. The individual components are adapted according to the necessary requirements.
pneuwork® is a technology and brand of the Gericke Group.