A prior-art fuel pellet of this type is made of saw- dust with a small addition of binding agent in the form of lignin. A major disadvantage of this pellet is that it easily disintegrates, which makes transport as well as handling more difficult. It may disintegrate to such an extent as to form dust, which can cause allergic persons trouble and, moreover, result in dust explosions. This prior-art pellet easily absorb moisture and, thus, has a relatively low thermal value. Besides, moist pellets may cause problems of mould in connection with storing.

Another prior-art fuel pellet of this type is made of plastic-coated cardboard, such as the plastic-coated cardboard which is used in milk cartons and the like.

This pellet is somewhat more moisture-repellent and has a higher thermal value than the pellet which is made of sawdust, nor does it disintegrate like the pellet of saw- dust. A great disadvantage of the pellet made of plastic- coated cardboard is, however, that in combustion it yields a large amount of ash (up to about 10% by weight), which has to be taken care of in one way or another.

The object of the present invention is to provide a fuel pellet which eliminates the disadvantages of the pellet made of sawdust without the amount of ash obtained in connection with combustion being much increased, that is a fuel pellet which does not disintegrate, is mois- ture-repellent, has a high thermal value and in combus- tion yields a small amount of ash.

According to the invention, this object is achieved by a fuel pellet which consists of 30-50% by weight of plastic, 25-35% by weight of cardboard and 25-35% by weight of sawdust.

In a preferred embodiment, the fuel pellet consists of 30% by weight of plastic, 35% by weight of cardboard and 35% by weight of sawdust.

Five different types of pellets have been made of recycled material, which has been ground, torn and/or chopped in order to form material particles which then, in a prior-art manner, have been compressed to pellets.

In this case, use has been made of inflammable plastic from recovery plants intended for plastic materials (how- ever not PVC and Teflon), such plastic-coated cardboard as is used in milk cartons and the like, and dry sawdust from carpentry shops, especially manufacturers of parquet floors.

These five different pellets, which are here desig- nated sample 1,2,3,4 and 5, were composed as indicated in Table 1.

Table 1 Sample Sample Sample Sample Sample 1 2 3 4 5 Sawdust (% by weight) 90 0 40 30 35 Plastic (% by weight) 10 0 10 15 30 Cardboard (% by weight) 0 100 50 55 35 As shown, sample 1 contained 90% by weight of saw- dust and 10% by weight of plastic. Sample 1 thus essen- tially corresponds to the known sawdust pellet as de- scribed by way of introduction. Sample 2 corresponds to the known cardboard pellet as described by way of intro- duction.

As is further shown in Table 1, sample 3 contained 40% by weight of sawdust, 10% by weight of plastic and 50% by weight of cardboard and sample 4 contained 30% by

weight of sawdust, 15% by weight of plastic and 55% by weight of cardboard. As is also evident, sample 5, which corresponds to a preferred embodiment of a pellet accord- ing to the present invention, contained 35% by weight of sawdust, 30% by weight of plastic and 35% by weight of cardboard.

The characteristics of the five samples as regards thermal value, ash, disintegration and moisture- repellency have been examined. The obtained results are shown in Table 2.

Table 2 Sample Sample Sample Sample Sample 1 2 3 4 5 Sawdust (% by weight) 90 0 40 30 35 Plastic (% by weight) 10 0 10 15 30 Cardboard (% by weight) 0 100 50 55 35 Thermal value (MJ/kg) 18.43 18.88 17.97 17.96 19.08 Ash (% by weight) 0.50 9.20 4.80 4.70 2.80 Disintegration (1-5) 2 5 3 4 5 Moisture- repellency (1-5) 1 2 3 3 5 The samples have as regards thermal value and ash been examined by the Swedish National Testing and Re- search Institute (SP) in Boras, method SP 0492 (= ISO 1928) and SP 0502 (mod. SS 18 71 71), respectively, being used. The thermal value indicated in Table 2 constitutes the effective thermal value at a constant pressure. Dis- integration and moisture-repellency have only been esti-

mated subjectively and visually and are given in the range 1-5, where 5 corresponds to good values, that is a low degree of disintegration and a high degree of mois- ture-repellency and 1 corresponds to poor values, that is a high degree of disintegration and a low degree of mois- ture-repellency.

The characteristics of the prior-art pellets (sam- ples 1 and 2) as described by way of introduction are evident from Table 2.

Furthermore, Table 2 clearly shows that sample 5 corresponds to a pellet which does not disintegrate, is moisture-repellent, has a high thermal value and yields a small amount of ash.

If, on the basis of the composition of sample 5 (35% by weight of sawdust, 30% by weight of plastic and 35% by weight of cardboard), the sawdust content is increased, the characteristics as to disintegration and moisture- repellency deteriorate, but the amount of ash decreases.

If the sawdust content is decreased, the opposite result is obtained, whereas the thermal value increases if the sawdust is replaced with plastic but decreases if the sawdust is replaced with cardboard.

If the plastic content is decreased from 30% by weight, only a negative change of the characteristics of the pellet seems to be obtained by the disintegration and moisture-repellency deteriorating, the ash content in- creasing and the thermal value decreasing. If the plastic content is increased, the opposite changes of the charac- teristics are obtained. However, it should be noted that the thermal value in connection with an increase of the plastic content may be so high that the ovens which are available cannot handle the generation of heat without being damaged.

If the cardboard content is increased from 35% by weight and this occurs at the expense of the plastic con- tent, all of the above-mentioned four characteristics de- teriorate, while a decrease in favour of the plastic con-

tent results in the opposite effect. If, on the contrary, the cardboard content is increased at the expense of the sawdust content, the characteristics are improved as re- gards disintegration, while the amount of ash increases and the thermal value decreases. A decrease of the card- board content in favour of the sawdust content results in the opposite effect.

In view of that stated above, it has been found that the pellet conveniently contains 25-35% by weight of saw- dust, 30-50% by weight of plastic and 25-35% by weight of cardboard.

The SP has also analysed the five samples as regards the content of other substances. The analyses are summed up regarding some of these substances in Table 3.

Table 3 Substance Sample Sample Sample Sample Sample 1. 2 3 4 5 Sulphur (% by weight) 0.01 0.06 0.05 0.06 0.06 Chlorine (% by weight) 0.01 0.06 0.05 0.04 0.02 Cadmium (mg/kg) 0.11 <0.05 0.18 0.34 0.09 Manganese (mg/kg) 84.50 22.50 65.50 59.00 39.50 Zinc (mg/kg) 9.90 24.00 37.00 34.00 20.50 It should be noted that when determining the above- indicated ranges of components comprised in the pellet, naturally, also the influence of the components on the content of sulphur, chlorine, cadmium, manganese and zinc, as shown in Table 3, has been taken into considera- tion.