Pellet
A pellet ( English pellet , balls', 'spheres' from Old French pelote , games Ball' and Latin pila , play ball ',' ball ',' pile '- hence linguistic relationship with the pill and the platoon ) is a small body of compacted material in spherical or cylindrical shape . The term is mostly used in the plural, as pellets are not used individually but as bulk goods .
The process for the production of pellets, in the process technology of agglomeration associated is referred to as pelletizing designated. Pelleting is used in many different areas, such as the processing of fuels (e.g. wood pellets ), the conditioning of raw materials and waste and others. The purpose is to improve the handling of the pelletized material, e.g. B. by increasing the density, avoiding dust and / or improving the dosability.
If the pressed material is relatively large (over 4 cm in diameter) or in an angular shape, it is called briquette . As an intermediate stage, so-called maxipellets with a cross-section of 16-20 mm are also produced today. This production technology has been developed primarily for the compression of moist biomass, but also wood waste, with the advantage of less energy consumption.
purpose
Pelleting a material can offer several advantages:
- Increase in bulk density: By pressing the pellets, the bulk density can be significantly increased compared to the starting material. Wood pellets , for example, have a bulk density of around 650 kg / m³ that is several times higher than sawdust of around 200 kg / m³. This means that storage and transport costs are significantly lower and the transportability is increased.
- Improvement of the handling of the material: By converting the small-structured material (e.g. dust / powder, sawdust) into pellets, dust formation during storage, transport or use is avoided. In the case of toxic material, contamination is avoided and in the case of flammable materials dust explosions are prevented.
- Standardization of the material size: By converting the starting material into pellets of a certain size, the size-related properties are evened out. So z. B. conveyor systems ( screw conveyors, etc.), as they u. a. are used in wood pellet heating systems can be operated reliably. Dosability can also be improved.
- Transportability: material that has to be transported in large quantities, such as B. firewood or animal feed, can quickly and efficiently in pellet form by blowing with an air stream z. B. be transported by a truck into a storage silo .
- Avoiding segregation: If the starting material is a heterogeneous mixture of different substances (e.g. animal feed), pelleting can prevent segregation.
- Reduction of dust explosions because more compact particles are transported or blown in
species
Pelleting is used in many areas:
- For biogenic fuels :
- Wood pellets made from sawdust and other wood waste, but also from forest wood, are becoming increasingly important. They are usually used as fuel in pellet heating systems . In Germany and Austria, wood pellets for pellet boilers are standardized in accordance with DIN 51731 and ÖNORM M 7135. Wood pellets of inferior quality are used as industrial pellets as fuel in power plants. Are less common
- Peat pellets ,
- Straw pellets or
- Residual value pellets , e.g. B. from waste from flour mills
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Supplementary feed (concentrate) is pressed into pellets for easier handling.
- Sugar beet pulp pellets; in sugar production from sugar beet which squeezed beet residues are processed into pellets which animals (cattle) can be given as food.
- Hop pellets make it easier to dose in beer production.
- In the industry:
- Pre-product in pharmacy for compression into tablets or for filling into capsules
- Iron ore pellets are the primary products of iron production .
- With pellets of uranium dioxide to fuel rods for nuclear reactors in a fuel element factory filled ( assembly ).
- Catalyst pellets are used as a carrier for the actual catalyst in heterogeneously catalyzed chemical reactions
production method
A raw material is crushed and passed through a die, e.g. B. a flat die press , pressed. This compacts the material evenly and strongly. Pellets are always produced dry. Binders may be used for better consistency . If the material to be compacted is a sticky, adhesive substance, agglomeration into granules can also be achieved with a pelletizing plate , an inclined rotating disk or drum . In this way, z. B. expanded clay , cat litter and. Ä. Shaped.
Concentrate feed
Feed pellets are produced to reduce the dusting of the feed and to improve after-drying.
Wood pellets
The production of wood pellets can be divided into different steps:
- Provision of raw materials: The raw materials have to meet certain properties so that, on the one hand, the production of pellets is technically possible. On the other hand, they also influence the quality of the end product. Is z. B. If wood that has not been debarked is used, the ash content can rise above the values permitted for certain quality classes according to various standards.
- Processing of raw materials: If round wood is used, debarking may be necessary. By crushing, e.g. B. with a hammer mill , chips are generated. The smaller the chips, the better pelleting is possible. The binding properties are optimized by setting a water content of 10 to 15%.
- Addition of auxiliary agents : In addition to the lignin which is naturally present in the wood , other binding agents such as B. starch or molasses may be necessary. Other auxiliaries can be added, the z. B. optimize the ash softening point .
- Pelleting (pressing): With a muller press the starting material through a flat or annular die pressed (flat or annular die) with holes according to the desired pellet diameter. The heat generated during pressing (up to 130 ° C) activates the binding agent and ensures that the individual chips bond. A knife cuts the pellet strands to the desired length after they exit the die.
- Delivery: The pellets are delivered loose in silo vehicles with a blowing device or packed in sacks or big bags .
Different pellet standards are used to classify them in different quality classes.
Ore pellets
Ore pellets typically have a diameter of 10 to 15 mm and, in addition to iron ore, also contain aggregates , so that they can be used as mortars for the extraction of pig iron without further processing . For pelletizing, the ore powder is mixed with water on rotating plates or in rotary kiln drums. The moist ore globules that form are called green pellets . They are then dried and fired between 1000 and 1200 ° C before the actual ore reduction takes place.
Foam pellets
Foam pellets are used in medicine and dentistry for the pressure-free and economical application of liquid or pasty drugs . In dentistry, for example, they serve as carriers of refrigerants for testing the sensitivity of teeth. Furthermore, they are in the jewelry and cosmetics industry used and are used in the packaging industry , in the photo - and pressure range and furniture - and decoration area.
literature
- Martin Kaltschmitt, Hans Hartmann, Hermann Hofbauer (Hrsg.): Energy from biomass. Basics, techniques and procedures. Springer Verlag, Berlin / Heidelberg 2009, ISBN 978-3-540-85094-6 .
Web links
Footnotes
- ^ A b Martin Kaltschmitt, Hans Hartmann, Hermann Hofbauer (eds.): Energy from biomass. Basics, techniques and procedures. Springer Verlag, Berlin / Heidelberg 2009, ISBN 978-3-540-85094-6 .
- ↑ Raw material uranium: origin of energy , website of the Gundremmingen nuclear power plant, accessed on January 6, 2010.
- ↑ Bertram Philipp, Peter Stevens: Fundamentals of industrial chemistry. VCH Verlagsgesellschaft, 1987, ISBN 3-527-25991-0 , p. 208.