Compaction of Saw Dust and Wood Shavings from Wood Waste

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For more than 60 years the company AMANDUS KAHL has built pelleting presses, which proved to be successful due to their robust construction, their economic efficiency, and their smooth running performance. Our machines are extremely suitable for compacting a large variety of products.

As for the pelleting of dried beet pulp in sugar beet factories, Amandus Kahl has been market leader in Western Europe for decades. A solid construction and a high availability in case of uninterrupted, continuous operation are indispensable prerequisites for processing these products. 

For many years KAHL pelleting presses have also been used to the complete satisfaction of our customers for the production of wood pellets, for example in Austria, Belgium, Bosnia + Herzegovina, Canada, the Croatia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Great Britain, Ireland, Italy, Japan, Latvia, Netherlands, Norway, Poland, Russia, Slovakia, Slovenia, Spain, Sweden, Switzerland, and the United States. The pellets produced are mainly used as combustible or animal litter.

What are pellets and how do they differ from briquettes or granulate?

Different processes are used for compaction, i.e. for pressing powdery, pasty or lumpy products into uniform bodies. Depending on the process applied and the shape, the final product is called granulate, pellet, or briquette. A clear definition of terms is not available, but the following could be determined:

Product: Granulate / Pellet / Briquette

Size: 0.5 - 3 mm / 2 - 30 mm / 30 - 60 mm 

Shape: undefined, but not powdery / cylindrical / geometrically defined

                            
In general, one of the following two methods is applied for compacting:

1.)   Agglomeration by agitation

In this case the adhesion of the particles is not reached by mechanical pressure acting from the outside, but by moving the product adding moisture and by subsequent drying.

This process is not appropriate for the compaction of saw dust and wood shavings.

2.)   Agglomeration by compression

A process, where the adhesion of the particles is promoted by forces acting from the outside. By means of suitable pelleting elements the product is compacted to such an extent that agglomerates of sufficient solidity are produced.

Within the processes of agglomeration by compression, "pelleting" always proves to be the best if larger product quantities have to be processed under economic aspects. This applies particularly to products which do not have strong natural adhesive powers and do not allow the addition of binders.

KAHL pelleting presses are built in a basic design:

As a flat die press, where the pan grinder rollers rotate on a horizontally arranged die and the product is pressed downwards through the die holes (Figure 1). 

Pelleting process and pelleting elements

The pan grinder rollers and the dies are the most important elements in the pelleting process.

Compaction takes place in the open effective 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 effective bores, thereby pushing the product plug in the bores slightly forward. To achieve this, the frictional force within the effective bores must not exceed the effective 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 (Figure 2).

A small layer of product is forced into the effective bores which is united to a pellet by means of the pressure and the adhesive powers of the product. 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 execution of the die. Kahl presses are used with rollers of up to 450 mm in diameter and 192 mm in width. The profiles of the running surfaces are available in grooved, perforated, and corrugated design.

The pan grinder head rotates at a speed of approximately 60 rpm. For this reason the running speed and thus the noise level of the flat die press are very low. Nevertheless the number of rollers ensure a high roll-over frequency of the product which is necessary for achieving a high throughput.

The respective design of the die is very important. The ratio between the diameter of the bores and the length of the effective bores is called "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 results concerning pellet quality and throughput of the machine. 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 together with the drive power.

Hydraulic system of the press

Today a hydraulic system for the exact adjustment of the rollers - also during operation of the press is "state-of-the-art".

By means of the product layer on the die a gap is produced between rollers and die which has an important influence on compaction. For this reason it is desirable to be able to modify this gap also during production and to check the operating state of the press by means of the product pressure against the pan grinder rollers.

Our hydraulic system is a proven accessory which offers these possibilities to the press operator. The process can be monitored and optimized during operation. On top of it, the hydraulic system is provided with an integrated pressure relief valve as safety device if foreign particles get into the press (Figure 3).

Product feeding and shearing effect

The product to be compacted is fed into the press by a suitable feeding screw, depending on the load. Due to the large press interior there is sufficient space for voluminous products, too. Forced feeding, which might cause blockages, is not required.

Individually adjusted scrapers serve for a uniform product distribution on the die.

In case of products with a high content of crude fibres such as wood the flat die press is equipped with cylindrical rollers which cause a shearing effect when running on the circular path of the die. This desired effect produces a twisting and primary crushing of the product on the die, so that it can be pelleted more easily.

Particularities of pelleting wood

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, i.e. the press interior, must be as large as possible. Further requirements made by the wood industry are a robust machine construction and a high availability of the plant in order to meet the required operational safety.

A sufficiently high throughput with the lowest possible specific energy consumption and a good pellet quality are further requirements, along with the aim to reach a long service life of the wear parts.

In comparison with other products wood does not have strong adhesive powers like starch-containing feed, for example. The antifriction properties are not very distinct, either. As a result a high frictional force is quickly produced in the effective bores, which also is necessary for compacting this product with its low own binding forces.

It is noteworthy that there are differences concerning the processing of different kinds of wood which depend to a large extent on the resin content. In case of hard wood, such as beech or oak, pelleting presses for shaping wood pellets require higher pressure forces than for soft wood and conifers. The pressure forces lead to the "specific energy consumption", which is between 40 and 60 kWh/t in case of wood processing.

Due to the high frictional resistance and the low adhesive powers of some kinds of wood preconditioning, i.e. pretreatment of the product before the actual pelleting process, is very important.

The product moisture, temperature, and crushing degree play an important part. In order to increase the moisture content of the product, water should be added partly in form of steam and the moistened wood should be given sufficient retention time for letting the moisture penetrate into the product.

For this reason an ideal plant is provided with a crushing system, e.g. a suitable hammer mill, a continuous mixer for the addition of steam and water, and a long-term conditioner for the retention time before pelleting (Figure 4).

Particularly for the pelletization of wood the company Amandus Kahl has developed pelleting presses with reinforced bearings and further construction details which meet the requirements made by the wood industry.

Pan grinder mill

AMANDUS KAHL has gone back to the well-known pan grinding tradition. The established KAHL flat die pelleting press, which is based on this principle, has been further developed to a "dieplate pan grinder mill". On the occasion of the trade fair "BioEnergy Europe 2008" in Hanover the pan grinder mill won the Innovation Award 2009 of the Deutscher Landwirtschaftsverlag DLV (German agricultural publishing house).

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 a few 100 kg/h up to 40 t/h and can replace the hammer mill, depending on the raw material.

Advantages over the hammer mill are the quiet operation even at full load, the low space requirement and the minimum energy consumption. Besides, it can be used without dedusting systems, cyclones, filters and exhaust air plants and free from ATEX requirements.

Co-firing in power plants is the newest application of defibered wet biomass. In a two-stage process, the biomass is defibered to particle sizes < 2 mm and then directly injected with the coal dust into the combustion zone. The fresh, defibered wood chips are not dried or pelleted, so that the energy consumption can be reduced drastically.