The process of treating waste in biogas installations to produce methane gas is previously known. In such a process water and humus are also obtained. The methane gas can be burned to produce energy; the water obtained in the process is preferably recycled to the process; and the humus obtained in the digestion process used e.g. as substrate or filling soil in green areas. The biogas reactor, where the digestion takes place, forms the central part of the biogas installation. Digestion refers to a process where an organic substance is allowed to decompose in an anaerobic environment. The purpose of this digestion process is to decompose the organic substance, so that all the inherent energy can be utilized. During such a decomposition process methane gas is produced.

Biogas reactors of the prior art generally comprise large vertical vessels, with a height of a few dozen meters and a diameter of about 10 m. The volume of the reactor vessel is dimensioned so that it can receive several dozen of daily portions of biomass to be digested. When new biomass is fed into the biogas reactor, the already digested material must be removed from the vessel.

New matter is usually fed through a feeding pipe into the reactor, where it is received in a so- called antechamber. An antechamber is used to prevent

the new denser material from immediately sinking to the bottom of the reactor. The digestion process starts in the antechamber, from where the matter after a short time is conducted to the so-called main chamber of the reactor.

The use of such antechambers has, however, entailed some problems. When new waste is led into the antechamber, the digestion process starts, which causes the new waste quickly to rise up to the surface of the waste in the main chamber. This new waste soon dries in the upper parts of the main chamber, forming there a thick and impermeable surface.

Usually the main chamber comprises different kinds of scraping devices or impellers for mixing the matter that rises up to the surface during the digestion process. However, the waste dries quickly and forms thick layers around these scraping devices and impel- lers. Thus either a void forms around the stirrers, hindering them from mixing the waste effectively, or dried up waste accumulates on them, hampering their functioning. New waste continuously rises up to the surface of the reactor and the accumulated thick crust continues to increase.

The problems associated with known solutions are avoided with the present invention. An effective mixing of the new waste with the already digested sludge in the main chamber can be achieved and crust formation in the upper parts of the reactor thus altogether avoided.

The solution to the problems mentioned above has been achieved by means of the characteristics of the present invention set forth in the claims.

It is characteristic of the present invention that the reactor comprises at least one screw device, rotatingly arranged to the upper part of the reactor,

for mixing inflowing organic matter with organic matter already in the reactor, the screw device extending in a substantially vertical direction, so that the greater part of its length is immersed in the waste.

Waste is understood to refer to the matter that is being digested in a biogas reactor; this will here- after be called digested sludge. Waste also refers to new matter that is fed into the reactor to join the digestion process and that will hereafter be called biomass.

Many considerable advantages over the prior art are achieved by the present invention. New and old waste, i.e. digested sludge and new inflowing biomass are effectively mixed in the biogas reactor and thus crust formation in the reactor is avoided. When crust formation is avoided, it saves the maintenance staff a lot of work, since such crust formations have previously been dissolved manually or mechanically. Moreover, it is not necessary to have scraping or mixing tools in the gas space of the reactor, which simplifies the construc- tion of the reactor and further facilitates maintenance.

The mixing of the waste materials also renders digestion of all the matter led into the reactor effective and more reliable.

In the following the invention will be described in greater detail with reference to the drawings, in which Fig. 1 shows a cross-section of a reactor of the present invention, and Fig. 2 shows a sectional view of the reactor in Fig. 1, taken along line I-I.

A biogas reactor 1 of the invention is a main component of a biogas installation. The functioning of such a biogas installation can briefly be described so that organic waste is supplied to a mixing tank by means

of a conveyor. Water and/or slurry from a separate slurry tank is also supplied to the mixing tank. These waste components are mixed to form biomass in the mixing tank. The biomass is then conveyed further to a biogas reactor 1. The reactor comprises a main chamber 2, where the waste is subjected to digestion in an anaerobic environment. The digestion goes on for about two weeks, so that methane gas is produced, whereafter the gas is led from the top of the reactor to a storage tank for gas. Digested sludge can be discharged from the reactor and compressed to obtain humus and water. The water is recycled to the digestion process and the humus transported away to be used.

Figs. 1 and 2 illustrate the reactor in greater detail. The biogas reactor 1 comprises a cylindrical reactor vessel that preferably tapers conically at the bottom. The conical tapering directs the waste towards the discharge opening 3, which is provided at the bottom and equipped with a valve 4. The digested sludge is dis- charged through this discharge opening.

The organic matter to be digested, i.e. the biomass, is fed into the biogas reactor through a feeding pipe 5. The feeding pipe directs the matter into a receiving tank 6 provided in the reactor. This so- called antechamber 6 is placed in the lower part of the casing 7 of the reactor vessel to receive the biomass, instead of the biomass being immediately mixed with the digested sludge and thus directly introduced into the digestion process in the main chamber. The volume of the antechamber is preferably so dimensioned that it can receive several daily portions of biomass collected at waste disposal.

When new biomass has been supplied to the antechamber 6, the digestion process accelerates. The gas formed during digestion continues to rise upwards

and is -led, via a gas pipe 9, provided at the top 8 of the main chamber, further to a gas container for stor- ing. The new biomass rises up to the upper parts of the main tank 2 and gradually reaches the surface of the digested sludge 10. When the biomass comes into contact with the gas space 11 in the upper parts of the main chamber, it quickly dries. This quickly drying biomass easily forms a disadvantageous, thick and hard crust that surrounds ordinary stirrers in the main chamber.

To avoid crust formation the biogas reactor 1 of the present invention has at least one screw device 13 that rotates around its longitudinal axis 12, and has a relatively large diameter. The screw device is mounted in a substantially vertical position in the upper part of the reactor, preferably so that its actuator 14 is placed outside the main chamber. The screw device 13 is attached to the actuator from its upper end 15, whereas the substantially rigid shaft extends into the main chamber and further down into the waste.

The screw device 13 is not in any way anchored to the surrounding reactor constructions in the main chamber 2, but its other end 16 freely hangs down into the waste. Thus disadvantageous maintenance work in the main chamber can be avoided. To prevent strain in the screw device, caused by more or less solid accumulations of waste hitting the screw device, the lower, free end of the screw device has a substantially tapering conical form.

The screw device 13 extends considerably below the surface 10 of the waste in the main chamber 2. Thus the screw device, by its rotating movement, intercepts the new biomass that rises up from the antechamber 6 and pushes it back down into the digested sludge, simultaneously mixing the new biomass with the already partly digested matter.

The length of the screw device is preferably greater than a third of the height of the reactor 1. In its present embodiment the screw device of the invention is arranged so as to extend from the upper parts of the antechamber 6 to a position above the surface of the waste matter 10. Thus the new biomass rising upwards can be quickly intercepted by the screw device and effec- tively mixed with the old, already digested matter.

The mixing process that results when such screw devices are used advantageously evens out the density of the biomass, simultaneously further releasing gases produced in the new biomass, after which they rise upwards and are led out of the reactor. Since the vis- cosity both in the biomass and the digested sludge varies, it is essential that the rotation speed and direction can be freely chosen for the screw devices during the different steps of the process.

Tests have proved that the best mixing results are achieved by employing one screw device 13 in the reactor 1. The screw device is then placed substantially along the central axis of the biogas reactor.

Furthermore a very effective mixing is achieved if the diameter of the screw device corresponds to about one fourth of the diameter of the reactor vessel. In a present installation there are three screw devices placed in a substantially symmetrical manner in the upper part of the reactor. By this arrangement a sufficient mixing of biomass and digested sludge is achieved, and thus crust formation in the upper part of the main vessel can also be effectively prevented.

By employing the screw devices of the invention, the need for special scraping means or propeller means in the reactor can be avoided.

Since the digestion process continuously proceeds in the biomass that has risen out of the

antechamber, gas formation is gradually reduced and the biomass becomes denser with time. Owing to this increase in density of the waste matter, the waste will again start sinking towards the bottom of the reactor. When the waste has reached the bottom of the vessel, its organic matter has practically vanished and the material has been converted into energy poor matter that can be led out of the reactor through the discharge opening at the bottom of the vessel.

The invention has been described above by means of one embodiment only, and therefore it is pointed out that the details of the invention can be modified in many ways within the scope of the appended claims. The shape, length and position of the screw devices can differ from those illustrated. It is also possible to use one or more screw devices.