This invention relates to a method for drying a fibrous or granular material, preferably peat, in a rotary drier, primary drying air being supplied to the driver axially and being substantially moisture satur- ated at its passage through the drier due to absorp¬ tion of water from the material to be dried. The in¬ vention also comprises a device for drying a fibrous or granular material.

A method and a device for drying wood chips and peat are previously known, the material being dried in a first space surrounding the space in which the final drying takes place. This two-step drying requires a relatively complicated device, the handling of the peat being relatively circumstantial as the peat must be transferred to the internal space for final drying after pre-drying in the external space.

A device for continuous dewatering of peat is also known, where peat is supplied to a stationary, perforated drum, electric current being conducted through the peat, which will then give off water vapour, Air is fed into the drum, absorbs the water vapour and is pressed out through the perforations in the drum.

Thus, the known technique for drying of peat is relatively complicated and slow. MoreOver, it is not especially efficient as too low air temperatures are used.

It is the object of this invention to provide a method and a device for drying preferably peat where drying air is supplied both axially and radially to a rotary drier. Besides, very high air temperatures, about 140 C, are used which means that the drying, is carried out rapidly and efficiently. As the drying air is substantially moisture saturated during the whole

drying process the risk of so-called dust explosion is in principle eliminated.

The object of the invention is realized by means of a method and a device given the characteristic features set out in the appended claims.

An illustrative example of a device according to the invention will be described below with reference to the enclosed drawing, where in Fig 1 is shown schematically a longitudinal section of a drying unit according to the invention and Fig 2 shows a section taken on the line II-II in Fig 1.

The drying unit 1 shown in Figs. 1 and 2 comprises a rotary drier 2, which on its inside, see Fig 2, is provided with flanges 3, ensuring good stirring of the material placed in the drum 2. The drum 2 is prefer¬ ably cylindrical and has a great number of holes 4 on its mantle surface. These holes have preferably a dia etre of about 3 mm.

As is apparent from Fig 1 the rotary drier 2 has ^aj ι inlet opening 5 and a discharge opening.

A great number of finned pipes 7 surround the drum 2 and are arranged at some distance from this. In operation of the drying unit 1 hot water or some other suitable medium is circulating in these finned pipes 7. The drum 2 is supported by two bearings 8, which have the shape of rings running about the drum 2. The drum 2 is rotatable within these bearings 8. This rotation is created by a drive source, e g an electric motor, and suitable transmitting means. An insulating mantle 9 is also included in the drying unit 1, which mantle extends along the major portion of the drum 2, the inlet and outlet openings 5 and 6 however not being surrounded by the mantle 9. As is apparent from Figs. 1 and 2 the mantle 9 has an up- wardly tapering inlet 10 for drying air.

The drying unit described above works in the following way.

The drying principle of the unit 1 is built on the fact that raw peat is dried in the rotary drier 2 by 5 evaporation of the water bound in the peat.

The primary drying air supplied to the drum 2 - axially at the inlet opening has a relatively high temperature, about 150 C, when entering the drum 2. The supply of the primary drying air is preferably 10 carried out by means of a fan not shown in the figures. As the temperature of the primary drying air is so high a continuous disintegration of the particles takes place, the water in the particles being evaporated and taken up by the supplied drying air. * 15 When the drying air supplied at the inlet opening 5 absorbs water vapour its temperature will sink to the order of 70°C when the air leaves the drum 2 at the outlet opening 6. At this stage the primary drying air is almost moisture saturated. 20 After the drying air has left the drum 2 via the outlet opening 6 it is dehydrated, e g in a heat ex¬ changer or condenser not shown in the figures. After this the secondary drying air is blown via fans or the like (not shown) into the mantle 9 through the inlet 25 opening 10. When the secondary drying air is supplied to the mantle 9 after the de-moistening, its tempera¬ ture is down at about 50 C. As mentioned above hot water or the like is circulating in the finned pipes 7, the temperature of the hot water being of the order of 30 200 C. The hot water in the finned pipes 7 can e g consist of cooling water from some plant, waste heat thus being utilized for heating the drying air.

When the dehydrated drying air having a temperature of about 50°C is supplied, to the drum 2 transversely ^' 35 via the holes 4, said drying air will pass past the

hot water pipes 7, the drying air being heated at its passage of the pipes 7..The temperature of the so-called cross-flow air is raised from the above- mentioned about 50°C to about 140°C. The fact that the drum 2 is rotating and that the drying air meets the peat both in a. countercurrent and cross-flow direction contributes to an extraordinarily efficient heat transfer. The result of said heat transfer, in turn, is that the peat balls successively burst and are converted into peat granulate having a diametre of 4-6 mm. By the bursting the moisture trans¬ mitting surface is increased with a simultaneous in¬ crease of the dry content in the peat. Despite the in¬ creasing dry content in the peat the drying effect is constant during the entire drying process due to the continuous bursting of the particles.

The risk of so-called dust explosion in the drum 2 is very slight, when using the process of the present invention as the drying air during the whole drying process has a high moisture content due to the fact that the water released at bursting is absorbed by the drying air in the form of water vapour. As there is a possibility within the scope of the invention of using exhaust gases as drying air this is also positive considering the risk of explosion as exhaust gases always contain water vapour in principle.

The peat granulate obtained as the result of peat drying is extraordinarily well suited for automated solid fuel systems. The drying air used with the drying process according to the claimed invention can be obtained in many ways. A so-called hot air boiler is possible, which is e g fired with oil. This boiler has then preferably a water-carried cooling system. The heated air is used as primary drying air, but it can be mixed

with the exhaust gases formed in the oil combustion. Cooling water from the boiler can be circulated in the finned pipes 7 for heating the secondary drying air. It is also possible within the scope of the in- vention that the secondary drying air is preheated in another way than through passage of the cam pipes. The temperature of the secondary drying air can e g be raised in such a way that it passes through a hot air boiler. The invention is of course by no means restricted to the illustrative example described above and considering the generation of the primary drying air in particular the variations are numerous. Thus, the invention can be freely varied within the scope of the accompanying drawings.