Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad, is a soft brown fuel with characteristics that put it somewhere between coal and peat. It is considered the lowest rank of coal; it is mined in Germany, Russia, the United States, Australia and many European countries, and it is used almost exclusively as a fuel for steam-electric power generation.
The application of vibrating mills for grinding of lignite shall be presented by an industrial example of RWE Power, the former Vipeak Company. The coal refining plant Fortuna-Nord in Bergheim is one of three remaining plants of RWE Power.
During the process design phase in the 1970s it became evident that the fine fraction needed to be removed in orderto optimize the economic efficiency. For coking two hearth type furnaces are installed in which the dry lignite is heated on a special plate. In this process the lignite loses its residual water and its gaseous components. This process is run under oxygen deficiency in order to avoid lignite combustion. The volatile components are used for steam generation and the hot carbon is cooled down by water and air for further transportation. With both hearth-type furnaces lignite as well as activated coke can be treated.
The most profitable solution is to remove all fines < 1 mm. The fraction < 0.3 mm can be sold as filter dust whereas the coarser fraction has to be reduced in size first. In order to achieve the fineness of the filter dust RWE Power installed three Palla-type vibrating mills.
Before installing the vibrating mills, grinding tests were executed. The test results showed that the largest throughput could be reached with a frequency of 1.000 min-1 and when using rods as grinding media. After installation of the vibrating mills the guaranteed throughput was considerably exceeded.
Due to its low volume and its low air throughput in the grinding cylinders the risk of fire is very low. The up- and downstream aggregates are connected to the vibrating mill in an airtight way, so that the vibrating mill does not cause any dust emissions.
Vibrating mills have hitherto proved their worth as individual machines. Vibration grinding represents an economic and reliable solution since the required product grain sizes can be achieved in a simple continuous grinding process without air classifying and thus with a very low air flow rate, whichmeans a very low risk of explosion.
This paper covers how vibration grinding works and the design and construction as well as the operating modes of the vibrating mill. In addition, the paper comments on the most important adaptation features. It quotes particulars of the correct type of grinding media and their size. Diagrams and figures explain and define the limits of application.
Moreover, the plants installed at RWE Power, former Vipeak company, with vibrating mills in Fortuna-Nord are described and operational data round out the conception that a grinding plant with a group of vibrating mills can be operated economically.
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