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Rotary kilns never stand idle

Published by
World Cement,

Author: Thorsten Sienk, Specialised Journalist, Bodenwerder.

Rotary kilns never stand idle

Pneumatic conveying technology rules the flow of materials in the cement industry. Blowers convey the fine material in piping throughout the production process. The availability requirements of the packaged units are high, as production would cease without blower air. Dyckerhoff has used AERZEN solutions for decades in its Lengerich cement plant. Core values and an innovative approach to air compression are the things which inspire the customer from the region known as Münsterland.

In Lengerich, they specialise in deep holes. The more difficult the operating conditions are, the more Dyckerhoff’s competences are called for. The cement plant in the Münsterland region is a unique location in Germany for so-called deep well cement, suitable for situations where material has to be put into the bore holes of gas fields and crude oil fields all over the world to seal and stabilise the outer walls at depths of 6000 to 8000 m. The prevailing temperatures and pressures pose a real challenge for deep well cement, which is why the parent rock, and additional aggregates, must be of very special compositions. Within Germany, these materials are available only in Lengerich.

Sintering creates new connections in the rotary kiln

Whether it’s for simple Portland cement, special types of cement for civil engineering, blast furnace slag cement, or mixtures with nanocrystalline structures, Dyckhoff’s basis is always limestone, found right on their doorstep, and a composition of aggregates. These materials are mixed, well ground and then heated in a rotary kiln. The industry calls the output clinker. To obtain this material, the mixture is exposed to a sintering process. The sintering zone in rotary kiln 8, with a length of 58 m, reaches temperatues of about 1500°C. The kiln is one of two rotary kilns in Lengerich that Dyckerhoff commissioned in 2001, and which is still one of the most modern kilns in Europe due to its high-energy efficiency. “The minerals almost liquefy in the sintering zone,” explains Heinz Hülsmeier. Hülsmeier is an institution at Dyckerhoff, having worked, until his retirement, in the maintenance area, and now guides factory tours.

Hot process air is one of the most important energy sources, as heat ultimately determines the production process. Dyckerhoff uses fuel oil for heating up, and then switches to pulverised lignite that is blown into the rotating kiln by means of a burner lance. The lignite burns immediately. Dyckerhoff can also burn derived fuels in these kilns; this fibrous and highly-calorific mass is called fluff, which is accrued during the reclamation process for recycling sacks and mainly consists of plastic.

Blower technology for fuel supply

Dyckerhoff also uses the principle of pneumatic conveying technology to supply the burner. Positive Displacement Blowers (GM25, 304 to 1452 m³/h, 55 kW max) of the series Delta Blower are used here. The packaged unit supplies volume flow of 16 m3 per minute at a motor connected load of 36 kW for the transport of the pulverised lignite and the fluff. With the new Generation 5 of Delta Blower, AERZEN has a series in the market that is extraordinary in terms of its effectiveness.

Both units at Dyckerhoff ensure that the fuel from the silos reaches the burner nozzle under optimal pressure and in sufficient quantity. The biggest cost drivers in cement production are increased energy costs. As a result, the company has responded by increasingly using derived fuels. Burning fluff brings a lot of advantages in ecological terms, as primary energy sources are conserved. One tonne of fluff provides almost as much energy as one tonne of lignite.

Dust is generated during the production process, which consists of raw material, is separated by electrical gas cleaning (EGR) and collected in silos. Afterwards, the EGR dust is returned in the process - this is achieved by means of a conveying system where AERZEN Delta Screw packages (VML 18, 380 to 1.190 m³/h, 75 kW max) provide the necessary volume flow of 18 m3 per minute in the piping. Today, electrical filters are an established technology for removing dust from exhaust air and for ensuring that the amount of pure dust is significantly below 10mg/m³. Dyckerhoff applies smaller positive displacement blowers (GM4 S, 46 to 342 m³/h, 15 kW max) from AERZEN, which blow air into the silo via a pneumatic ground, and which regularly mix the material with an air volume of six cubic metres per minute. This procedure avoids agglutination of fine material in the silo.

The unit is located in the area where the raw preparation of the untreated meal takes place. A vertical mill crushes the limestone into a fine pre-dried meal which is blown through transport piping from a storage silo into the heat exchanger which is more than 100 m high. Here, the raw meal is brought to temperature and flows through zones which are progressively hotter and hotter, until finally the furnace intake of the rotary kiln is reached. The mineral mixture has now reached a temperature of about 850°C. In order to achieve maximum energy efficiency, Dyckerhoff feeds the heat exchanger with the hot air from the clinker cooler at the end of the rotary kiln. Thus, it is possible to supply less energy to the heat exchanger.

Delta Screw packages type VML 18, with a motor rating of 45 kW, a pressure of 2.25 bar and a volume flow of 1080 m3 per hour each, are those which keep the transport network running from the storage silo to the kiln. AERZEN offers single-stage, oil-free screw compressors in different sizes, with volume flows of between 950 and 15 000 m3 per hour. The units are designed as universal tools, dimensioned for maximum energy efficiency, offering the possibility of combining any compressors and accessory components on a modular basis. The compressors can thus be adapted to suit the relevant application.

Close co-operation in service as well

Rotary kilns are designed for continuous operation because of the high-energy consumption during firing alone. Furthermore, these units are very sensitive to temperature changes - the lining of the interior would suffer damage because of alternating hot and cool phases. “This would lead to expansion gaps, and the whole lining would start crumbling and falling off in large pieces”, explains Heinz Hülsmeier. If this were to happen, repairs would be necessary, and these would cause a week of standstill - a long period of production downtime for Dyckerhoff. Therefore, the group exclusively applies technology that ensures maximum availability - supported by services. “With a daily capacity of up to 3700 tonnes, a week at standstill means failure of about 20 000 tonnes”, says Hülsmeier. The co-operation with AERZEN on the service level is very close, and the engineering always seeks to standardise as much as possible to limit the spare parts stockage. “We work hand-in-hand on preventive maintenance,” adds Hülsmeier. As soon as the service technician from AERZEN sets off, the maintenance team in Lengerich begins the preparation work. “Our engineers are well trained and experienced in blower technology. Therefore, we ask experts to visit only to do fine-tuning jobs,” Hülsmeier notes.

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