The present invention relates to a multi-level drying system for drying animal manure, said system comprising at least one level provided with a conveyor belt, means for supplying air and means for exhausting air. The present invention furthermore relates to a method for drying animal manure by using such a multi-level drying system.

A multi-level drying system of this kind for drying animal manure is known from European Patent Application No. 0 740 900. According to said European patent application a conveyor belt is provided under chicken cages, which conveyor discharges the chicken manure to a lower level. Said lower level is built up of chicken cages, under which a conveyor belt is likewise disposed, whereby the manure from the higher level, which has fallen off the conveyor belt, as well as the manure from the cages of the present level is discharged to a drying section located under this level. Said drying section comprises a conveyor belt, on which the manure to be dried is dried by using air supply means provided above said conveyor belt. Movable air blades are longitudinally aligned above the unit for storage of the manure or droppings to be dried, in the transverse direction thereof, so as to provide an optimum aeration of the manure to be dried. An appropriate displacement of the air above the manure will be effected by moving such air blades to and from. A drawback of such a multi-level drying system is the fact that the underside of the manure, which lies on the conveyor belt, is not sufficiently dried. During the drying process a hard dry layer is formed circumferentially on the manure, which impedes the transport of moisture from the inside of the manure to the outside. In addition to that the movable air blades are liable to malfunction, which stands in the way of an optimum continuous operation.

The object of the present application is to eliminate the above drawbacks and in order to accomplish that objective the multi¬ level drying system for drying animal manure as referred to in the introduction is characterized in that each level comprises an air permeable conveyor belt, whereby said means for supplying air is located under said air permeable conveyor belt and said means for exhausting air is located

above said air permeable conveyor belt, whereby said levels are separated from each other by partition plates which are impermeable to air.

In this manner a multi-level drying system for drying animal manure is provided, wherein the drying process in the animal manure takes place in an advantageous manner as a result of air being supplied at the bottom side of the air permeable conveyor belt and said air subsequently being exhausted at the upper side of said conveyor belt. Since the levels are separated from each other by air impermeable partition plates, no mixing of the air flows being used in the levels will take place, thus providing an optimum drying action.

The term "animal manure" used herein relates to manure from poultry, chickens, cows and other animals, with poultry manure being preferred in connection with the dry matter content thereof.

In a preferred embodiment of the multi-level drying system according to the present invention said air permeable conveyor belt is made of woven natural or synthetic fibres, in particular polyester fibres or Aramide (trademark) fibres. Fibres of this type possess sufficient strength for being used as a conveyor belt, and in addition to that they are permeable to air. Furthermore it is also possible to use a conveyor belt provided with perforations. It is to be understood, however, that such perforations must be dimensioned in such a manner that the material to be dried will remain present on the conveyor belt, whilst the air flow being supplied at the bottom side of the conveyor belt can move through the perforations in the conveyor belt. The multi-level drying system according to the present invention is preferably designed in such a manner that said air impermeable partition plates are disposed at an angle with respect to the conveyor belt. Air baffles of this type are known from European Patent Application No. 0047 043, where they are used for directing a forced air flow over manure to be dried, which is present on a conveyor belt. In a construction of this type a forced air flow will only be provided at the first part of the conveyor belt, whilst the air impermeable partition plates according to the present invention extend along the entire length of the conveyor belt at each level, so that no mixing with air from the other levels will take place.

In the multi-level drying system according to the present invention the animal manure being discharged via the conveyor belt

of one level is preferably supplied to the conveyor belt of the other level located therebelow. The advantage of such a multi-level drying system is that the manure to be dried passes a number of levels in order to be dried in this manner. According to such a multi-level drying system the dry matter content of the animal manure will decrease from the uppermost level in the direction of the lowermost level. Since the levels are separated from each other by air impermeable partition plates, the drying process will take place at an accelerated rate in comparison with the multi-level drying systems according to prior art, since the manure to be dried is exposed to dry air at every level anew.

The present invention furthermore relates to a method for drying animal manure by using the above-described multi-level drying system. The method according to the present invention is characterized in that the animal manure to be dried is supplied to the conveyor belt of the uppermost level, whereby the air is supplied at the bottom side of said conveyor belt and exits from said level via the upper side of said conveyor belt, after which the animal manure being discharged from the uppermost level is supplied as manure to be dried to the conveyor belt of the level located therebelow, which level is aerated in the same manner as the level located thereabove, with no mixing of the air flows used in said two levels taking place.

Preferably the air coming from a stable is used as the air flow to be supplied to each level. In order to achieve an optimum drying action, however, it is preferred to use air having a temperature of at least 10 °C. It is also possible, however, to use outside air or a mixture of outside air and stable air.

The multi-level drying system according to the present invention will be explained in more detail in the accompanying Figures.

Figure 1 is a side view of the multi-level drying system according to the present invention.

Figure 2 is a side view of the multi-level drying system according to the present invention, wherein air from a stable is used as the air flow to be supplied to the respective levels.

In Figure 1 the manure from a stable is placed on the uppermost conveyor belt 6 of multi-level drying system 1 by means of a conveyor element 2 and a manure distributing element 3. The manure to be dried is moved from position 5 to position 7 by causing conveyor belt 6

to move in the direction indicated by the arrow. Conveyor belt 6 is driven by means of drive unit 4. The manure present on conveyor belt 6 is then transferred, at position 7, to conveyor belt 8 located therebelow, and subsequently moved in the direction indicated by the arrow to position 14. The manure to be dried moves off conveyor belt 8 and lands on conveyor belt 9 located therebelow. Then the manure to be dried is moved on conveyor belt 9 from position 15 to position 16, and subsequently discharged to conveyor belt 10 located therebelow. The manure subsequently passes conveyor belts 11 and 12 in a similar manner, and the dried manure is received in receptacle 13. The arrows in Figure 1 indicate the direction of transport of the animal manure to be dried on conveyor belts 6, 8, 9, 10, 11 and 12.

Figure 2 diagrammatically illustrates the multi-level drying system 1 according to the present invention in cross-sectional view. Air 26 from stable 25 is supplied to respective conveyor belts 6, 8, 9, 10, 11 and 12 via distributing element 24. The air 26 to be supplied at the bottom side of conveyor belt 6 impinges on air impermeable partition plate 18, and as a result is supplied at the bottom side of conveyor belt 6 and discharged from this level via air impermeable partition plate 17. It is also possible, however, to leave out partition plate 17, whereby the roof, illustrated in hatched lines, functions as an impermeable partition plate. The conveyor belt 8 located below conveyor belt 6 is dried with air 26 at its bottom side, and said air 26 exits conveyor belt 8 via air impermeable partition plate 18. The use of air impermeable partition plates 17 and 18 prevents the air flows 26 that are being supplied to conveyor belts 6, 8 from being mixed. The same principle applies to respective conveyor belts 9, 10, 11 and 12, under which air impermeable partition plates 19, 20, 21, 22 and 23 are disposed.

The present invention will be explained in more detail hereafter by means of an example.

Chicken manure having a dry matter content of 25% and a density of 800 kg/m ³ was supplied to the uppermost conveyor belt of a multi-level drying system comprising 9 levels, each level having a length of 11 m. The manure to be dried was placed on the uppermost conveyor belt in a 10 cm thick layer and dried to a final dry matter content of 85% and a density of 300 kg/m ³ , the total residence time being 48 hours. The air being used had a temperature of 25 °C and a humidity level of 55%.

Additional experiments have shown that a reduction of the drying period could be achieved by preheating, and possibly pre-drying, the air from the stable.