INDUSTRY NEWS


PELLET PRESSES Compaction of sawdust and wood waste F


or more than 60 years, pellet presses built by Amandus Kahl GmbH & Co Kg of Reinbek, Germany, have enjoyed success due to their robust construction, cost-efficiency and reliable performance (Fig. 1). In a flat die press, the pan grinder rollers and the dies are the most important elements in the pelletising process. The pan grinder rollers rotate on a horizontally- arranged die and the product is pressed downwards through the die holes (Fig. 2).


Compaction takes place in the open bores of the die. The product is fed to the press vertically from above and is uniformly distributed into the pelleting chamber. A product layer is formed on the surface of the die. The rollers run over this layer and compact it. The pressure is continuously increasing whilst the product is being rolled towards the bores, thereby pushing the product plug in the bores slightly forward. To achieve this, the frictional force within the bores must not exceed the pressure created by the rollers. On the other hand, the frictional force must be high enough to cause a sufficient compaction of the product to a solid agglomerate. The individual layers of product form endless strands in the bores, which are cut at the lower side of the die to the desired pellet length by means of rotating knives. As for the pan grinder rollers, a large outer diameter and a solid


bearing are required. The profile of the running surface can be varied, but this does not have such an influence as the performance of the die. Kahl presses employ rollers of up to 450 mm in diameter and 192 mm in width. The running surfaces are available with either grooved, perforated or corrugated profiles. The design of the die is very important. The ratio between the diameter of the bores and the length of the bores is called the pelleting ratio. Together with the product properties, it determines the frictional force produced, so the pelleting ratio must be exactly adapted to the product in order to reach optimum pellet quality and maintain effective rates of production. It must be taken into account that the ratio of pellet surface to volume changes with decreasing pellet diameter. For this reason, dies which are manufactured for pelleting compound feed, dried beet pulp or other products cannot be used for compacting wood.


Another important parameter of the dies is the number of holes and thus the ‘open perforated surface’ available, which has a direct effect on the throughput of the press. Prerequisites for optimum operation are continuous product feeding into the press and a homogeneous, sufficiently pre- crushed product. The moisture should be kept very constant at a range of 12-15%.


Due to the large volume reduction when compacting wood and wood shavings, particularly sawdust from about 100 to 600 kg/m³, the capacity of the machine (the press interior) must be as large as possible.


Fig. 1. Kahl pellet press.


A sufficiently high throughput with the lowest possible specific energy consumption and a good pellet quality are further requirements, together with the aim of achieving a long service life. In comparison with other products, wood does not have strong adhesive powers such as starch-containing feed, for


Fig. 2. Pan grinder rollers on a horizontally-arranged die.


example. It is worth noting that there are differences involved in processing different kinds of wood, which depend to a large extent on the resin content. In the case of hardwood, such as beech or oak, pelleting presses for shaping wood pellets require higher pressure forces than for softwood and conifers. Pressure forces lead to the ‘specific energy consumption’, which is between 40 and 60 kWh/t in wood processing. Due to the high frictional resistance and the low adhesive powers of some kinds of wood, preconditioning (pre-treatment of the product before the actual pelleting process) is very important. The product‘s moisture, temperature and crushing degree play an important part. In order to increase the moisture content of the product, water should be added, partly as steam, and the moistened wood should be given sufficient retention time to allow the moisture to penetrate into the product.


Amandus Kahl has developed pellet presses with reinforced bearings and other changes specifically designed for pelletising wood that meet the requirements of the forest bioenergy industry. The company’s well-established flat


Prerequisites for optimum operation are continuous product feeding into the press and a homogeneous, sufficiently pre-crushed product. The moisture should be kept very constant at a range of 12-15%


die pelleting press has been further developed into a ‘dieplate pan grinder mill’.


Wet and dry biomass or wood chips can be crushed without problems and the capacity of the pan grinder mill even increases with the moisture of the raw material. Very wet wood does not cause any difficulties. The pan grinder mill reaches capacities of several 100 kg/h up to 40 t/h and can replace the hammer mill, depending on the raw material.


Advantages include quiet operation – even at full load, small space requirement and minimum energy consumption. More information from www.akahl.de


November 2013 9 9


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