Rotary kiln

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Rotary kiln of the Rohrdorf cement works
Cross-section of a rotary kiln in the German Cement Museum in Hemmoor

A rotary kiln ( DRO ) is a cylindrical furnace for continuous processes in process engineering that rotates continuously around its own axis during normal operation. Combined with a slight inclination of the axis of rotation , the rotary movement ensures product or fuel transport.

Types

Rotary kilns can be heated both directly and indirectly. With direct heating, the heat is supplied from within the furnace, for example by a burner . Any product in the rotary kiln is in direct contact with the resulting flue gas. With indirect heating, the heat is transferred from outside into the reaction chamber. This can also be done, for example, via the exhaust gas from the process.

Construction of directly heated rotary kilns

An early rotary kiln (cylinder furnace) for the large-scale implementation of the Leblanc process in the second half of the 19th century

A directly heated rotary kiln usually consists of the following components:

  • an inlet housing to feed or fill the continuously treated materials
  • the rotary kiln
  • a discharge housing or furnace head
  • a burner equipment that sits on the furnace head.
  • the lining made of refractory materials
  • the storage of two or more supports, wheels and axles
  • the longitudinal positioning control with an axial roller
  • the drive by means of a gearbox and ring gear or by means of friction drive on driven rollers
  • the seals on the furnace inlet and outlet to seal the rotating barrel against the stationary furnace head parts

In the case of directly heated rotary kilns, a distinction is made between cocurrent and countercurrent firing, depending on whether the exhaust gas and the material to be fired are moving in the same or opposite direction.

Construction of indirectly heated rotary kilns

An indirectly heated rotary kiln usually consists of the following components:

  • an inlet casing, usually only one feed screw with a shield
  • the rotary kiln
  • the discharge housing
  • the sealing systems, which seal off the product room atmosphere from the air, often with the use of sealing gas
  • the bearings , traditionally with a raceway and rollers, modern with large ball bearings or plain bearings
  • the drive by means of a compact gearbox and gear ring
  • the externally insulated heating muffle, which provides the required amount of heat, usually through flue gas or heating rods

Furthermore, built-in components can be present in rotary kilns which mainly affect the transport of solids. Lifting blades, for example, ensure that the feed material trickles down through the hot gas atmosphere, thereby improving the heat transfer. Damming rings at the discharge increase the degree of filling in the oven. Chain fixtures are particularly suitable for wet input material to avoid or remove caking. Specifically introduced geometric bodies, such as steel balls, improve the mixing and increase the heat transfer.

Process engineering peculiarities

The product is transported inside the rotary kiln by the rotary movement and the inclination of the kiln axis.

Due to their length and inner diameter (several meters each possible), rotary kilns are also suitable for treating inhomogeneous materials. The raw materials can have very different consistencies and lumps. For example, solids, sludges and barrels can be introduced.

A rotary kiln is a high-quality investment with costs of several million euros. As a rule, rotary kilns are operated continuously. The mostly 50-week operating time span ("continuous operation") from lighting the stove to turning it off for maintenance and repair work is called travel time or stove travel .

Because of the mostly difficult thermal conditions and the brick lining, which reacts sensitively to rapid temperature fluctuations, the continuous operation of a rotary kiln must not be suddenly interrupted. In the event of a power failure or damage to the drive , emergency devices must be in place (auxiliary drive or emergency running device) that allow the furnace to continue turning until it is emptied or the temperature is reduced to a safe area. Rotary kilns that suddenly come to a standstill when fully loaded with hot materials can bend due to the unilateral effects of heat and weight from the material left standing or the brickwork can be damaged, which can lead to the furnace being destroyed.

In the cement industry, wastewater is fed to the rotary kilns in some systems for flame cooling and thus to reduce nitrogen oxide emissions .

Applications of rotary kilns

Directly heated DRO

Indirectly heated DRO

literature

  • VDI 3460 part 1 : 2014-02 emission reduction; Thermal waste treatment; Basics (Emission control; Thermal waste treatment; Fundamentals). Beuth Verlag, Berlin. Pp. 53-57.

Web links

Commons : Rotary Kilns  - Collection of Images, Videos, and Audio Files

Individual evidence

  1. ↑ a b VDI 3460 sheet 1 : 2014-02 emission reduction; Thermal waste treatment; Basics (Emission control; Thermal waste treatment; Fundamentals). Beuth Verlag, Berlin. Pp. 63-64.
  2. ^ H. Ost: Textbook of Technical Chemistry , published by Robert Oppenheim, Berlin, 1890, p. 78.
  3. ↑ Separation, recycling and disposal of waste. In: Hazardous substances - cleanliness. Air . 62, No. 10, 2002, ISSN  0949-8036 , pp. 430-431.
  4. ↑ VDI 2094: 2003-03 Emission Reduction; Cement plants (emission control; cement plants). Beuth Verlag, Berlin, p. 19.
  5. ↑ a b DIN EN 19694-3: 2016-10 emissions from stationary sources; Determination of greenhouse gases (GHG) from energy-intensive industries; Part 3: Cement Industry; German version EN 19694-3: 2016. Beuth Verlag, Berlin, p. 46.
  6. ↑ VDI 2094: 2003-03 Emission Reduction; Cement plants (emission control; cement plants). Beuth Verlag, Berlin, p. 7.
  7. ↑ DIN EN 19694-5: 2016-10 emissions from stationary sources; Determination of greenhouse gases (GHG) from energy-intensive industries; Part 5: Lime Industry; German version EN 19694-5: 2016. Beuth Verlag, Berlin, p. 7.
  8. ↑ VDI 2301: 1993-01 Reduction of emissions; Incineration of waste from hospitals and other public health facilities (Emission control; incineration of solid wastes from hospitals and other public health facilities). Beuth Verlag, Berlin, p. 9.
  9. ↑ VDI 3460 sheet 1: 2014-02 emission reduction; Thermal waste treatment; Basics (Emission control; Thermal waste treatment; Fundamentals). Beuth Verlag, Berlin. Pp. 53-54.
  10. ↑ a b VDI 2578: 2017-03 emission reduction; Glassworks (emission control; glassworks). Beuth Verlag, Berlin, pp. 3-4.
  11. ↑ DIN EN 12915-2 Products for the treatment of water for human consumption; Granulated activated carbon; Part 2: reactivated granulated activated carbon; German version EN 12915-2: 2009. Beuth Verlag, Berlin, p. 10.
  12. ↑ VDI 3897: 2007-12 emission reduction; Systems for soil air extraction and groundwater stripping systems (Emission control; Soil vapor extraction and groundwater stripping systems). Beuth Verlag, Berlin, p. 32.
  13. ↑ DIN EN 19694-2 emissions from stationary sources; Determination of greenhouse gases (GHG) from energy-intensive industries; Part 2: Steel and iron industry; German version EN 19694-2: 2016. Beuth Verlag, Berlin, p. 50.
  14. ↑ DIN 17022-3: 1989-04 heat treatment of ferrous materials; Method of heat treatment; Case hardening. Beuth Verlag, Berlin, p. 15.
  15. ↑ VDI 3898: 2013-01 Emission Reduction; Dry mechanical, physical-chemical, thermal and biological soil treatment systems (emission control; Plants for dry mechanical, physio-chemical, thermal and biological soil treatment). Beuth Verlag, Berlin, p. 25.
  16. ↑ VDI 3674: 2013-04 Exhaust gas cleaning through adsorption; Process gas and waste gas cleaning (Waste gas cleaning by adsorption; Process gas and waste gas cleaning). Beuth Verlag, Berlin, p. 21.
  17. ↑ VDI 2102 sheet 2: 2013-04 emission reduction; Copper and copper alloy melting plants (Emission control; Copper and copper alloy melting plants). Beuth Verlag, Berlin, p. 14.