07.10.2008 | Autor / Editor: ESI Group / Marcel Dröttboom
Jaap P.J. Ruijgrok discusses a well proven storage concept for non-free flowing bulk materials like FGD gypsum.
Today we are technically capable of designing totally enclosed storage and handling systems for storing huge quantities of bulk solids, many of which are produced and/or stored on site at coal-fired power stations. The enclosed storage systems applied can be divided into three main categories:
This paper will mainly discuss the flat bottom storage silo or Eurosilo® system for storing a wide range of non-free flowing bulk solids in capacities ranging up to 100,000 m3. The heart of the system consists of sturdy screw conveyors and, if necessary, a central slotted column. The design allows easy access for maintenance and cleaning, achieved by a spacious design of transfer stations, and walkways, etc. The application of water repellent liners inside transfer chutes, on the screw flights, etc., avoids hang-ups thus providing a high percentage of equipment availability.
The structural design of most Eurosilo® walls consists of a steel structure with an inner and outer wall.
The inner wooden lining is supported by horizontal ring beams, to which the horizontal silo pressure is transmitted. The columns are part of silo structure. They enclose the inner part and are pin jointed to the foundation. The silo structure is externally shielded with standard wall cladding. For bulk materials such as coal, soybeans, limestone, and so on, a slip formed concrete wall structure may be applied.
2. The Eurosilo® System and Very Cohesive Materials
Very cohesive materials could not be stored in the original Eurosilos because material was arching and rat holing occurred. Therefore, a slotted column has been developed.
The material to be reclaimed is fed through the slots formed by horizontal flat rings. Subsequently the bulk material descends freely by gravity through the vertical channel as indicated. This reclaiming concept eliminates many of the complex influences of bulk material characteristics. Its fully automated mechanism fills and reclaims the bulk material in horizontal layers. In this way, wall loads will be kept minimal, symmetrical and predictable. Simultaneous filling and reclaiming is possible.
An important feature of the Eurosilo concept is the way the screw reclaim mechanism operates; linked with a slotted column the notorious „flow - no flow“ syndrome is eliminated, thus ensuring the controlled reclamation of even a difficult-to-handle bulk material such as FGD gypsum.
Before discussing the handling and storage of this material in more detail, we need to focus on its poor flow properties, which cause a number of serious problems. By way of an example, the „flowability“ of FGD gypsum with a free moisture content of 9 % is shown. The unconfined yield strength (σp) has been plotted against the major consolidation stress (σi). The characteristics lie completely in the worst flowability zone as defined by Dr. Jenike. These zones are separated from one another only by differently valued flow functions.
As additional reference, the curves of raw coal, soy meal and wet sand have been shown in the figure. Note how far the wet gypsum curves lie above those of soy meal, which in the food industry is traditionally known as one of the toughest materials to handle. The time consolidation after 7 days of storage already has an extremely negative effect on the flowability. Therefore, even experts wonder whether such a material can be handled at all and, even more important, whether it can be smoothly reclaimed and controlled after a certain time of storage. The answer is NO, unless a sound common sense principle is used, like the Eurosilo® system.
It is known that the free moisture content of the gypsum will concentrate in the lower regions of the silo, when it is being stored for a longer period. Therefore the FGD-Gypsum silos are equipped with a drainage system on the silo bottom.
The free moisture percolates downwards and is drained off via a membrane (geo textile) and a layer of granular material in which the drainage tubes are installed. An additional dewatering of at least 1% will be achieved by evaporation from the upper layer.
At long time storage, due to capillarity, the gypsum mass (which in some ways behave like fine sand with an average particle size of 50 micron) may be saturated with water up to a height of approx. 1 meter above the drainage layer. To avoid this layer of wet gypsum we have recently developed a system (patent pending), which creates an under pressure within the drainage system. This way, an additional dewatering of the stored gypsum can be achieved.
Today’s environmental requirements have increased the application of large capacity silos. The general advantages are:
Preventing possible contamination of surrounding land by runoff from outside stockpiles, or polluted drain water at the stockyard itself, are major arguments in favour of covered storage, and against open stockpiles. Covered storage also prevents deterioration of material due to weather conditions like frost and rain, thus maintaining quality standards, reducing maintenance cost and/or improving operations including energy saving if increased moisture need not to be removed.
The duration of material storage is important when loading and reclaiming criteria are determined. Usually the costs of components spiral with increasing bulk flows. Clearly, high loading and reclaiming capacities should not be contemplated for long time storage facilities. The optimal storage capacity and the required infeed and outfeed rates are being determined by the logistics of each location.
However, in case of FGD-Gypsum storage silos equipped with a bottom discharge system (plow feeder) the facility is restricted to a maximum storage period of approx. 72 hrs. The Eurosilo storage system can store the gypsum much longer than three months due to the fact that hang-ups of the material are eliminated.
The reliability of gravity flow storage systems highly depends on the accuracy of the design parameters, the constant bulk material quality as well as on various other conditions. Mayor deviations from the design parameters often lead to failing gravity flow. With the Eurosilo system, the stored material can be changed at any time as this does not affect the reclaim; this system offers true flexibility.
Fig. 6 shows the advantages of the Eurosilo concept if storage capacity and space requirement are compared.
In many cases, existing coal fired power stations have to be equipped with FGD-installations. Very often however, there is not much space left for the installation of the FGD gypsum dewatering system. In such cases, extended silo structures are provided; and the entire dewatering installation is placed on top of the Eurosilo system where the FGD-gypsum slurry is directly pumped (see Fig. 1), thus saving:
Of utmost importance in any new venture is the need for an accurate, up to date assessment of costs and savings of the new system. Initial investment costs give only half the picture. The total cost of the investment must be taken over the expected life span of the project in order to get a true approximation. Operating costs include not only the obvious costs such as interest, electrical power consumption, maintenance etc., but also hidden ones; such as the cost of product loss due to dusting, and quality loss due to moisture content.
The Eurosilo concept has been compared to both open and closed stockpile storage with respect to investment and return. Considering the advantages and drawbacks of the various systems, a comparable initial price has only been available in a few cases. But the economical choice is shifting to the Eurosilo system quickly, due to the increased need for environmental protection, homogenizing and drying functions, the requirement of transhipment without degradation, the avoidance of segregation, and the need for dust prevention. The aspect of land costs and space availability, especially in urban zones, can also have a great impact on the initial cost of a storage system. The biggest difference is the operating cost (see Fig. 7: Capital cost comparison).
The following data illustrate the value of the Eurosilo system. The first Eurosilo® for the storage of FGD-Gypsum came on line in 1982. Since then more than 50 systems have been delivered, featuring operating figures such as:
Noticeable examples of the Eurosilo system are the numerous FGD-gypsum silos at various power stations, working around the clock in Germany, The Netherlands, UK, Finland, Denmark, Japan, Poland, Singapore, Taiwan or China.
ESI Eurosilo BV
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