During a long time there have been requirements in the market for a biofuel with a high energy content which could be based on rest products, usual in the community, like fibre sludge and sludge from purifying plants and/or other raw materials like fibre- based materials.

This invention intends to provide for these requirements, which has been made possible by a new method for producing biofuel. The new method is characterized by the combination of the following steps : 1) The fibre- and/or sludge material is mixed with bio-oil in a mixing chamber ; 2) The mentioned mixture is heated up for 5-30 min at a temperature of 90 - 170°C, whereby water is driven away and the material is dried ; 3) The mentioned mixture is de-oiled in a pressing device, whereafter removed bio-oil is returned to the process and the de-oiled material is formed to pellets/briquettes ; 4) The formed, pressed products are homogenized. i. e. are given a supplementary, thermal treatment at a temperature of 50-200°C, a chemical reaction being started up, by which the products are crystallized, i. e. are hardened to a material with qualities being like those of carbon.

The steps 1-3 have certain similarities with what have been previously known by the Japanese patents Nos. 61-296 and 63-120792.

The great and important difference is that this patent application comprises a step 4, which means that the products after pressing and forming go through a hardening process, whereby the products are given very good qualities, which shall be described more closely below.

A principal sketch over a flowchart containing a plurality of stations for carrying out the new method is shown in the enclosed figure.

In the mentioned figure is shown a mixing chamber with the denotation 1. which chamber is supplied with a fibre- and/or sludge material respectively bio-oil, which is

indicated by the denotations 2,3.

The mixture formed in the mixing chamber 1 is subjected to heating for 5-30 min at a temperature of 90-170°C, whereby water is driven away from the fibre- and/or sludge material. This is indicated in the figure by means of the stations 4 and 5.

From the station 5 the de-watered mixture is taken to a pressing device 6, where de-oiling is made, bio-oil, pressed away, being returned to the process at the same time as the de-oiled material is formed to pellets/briquettes.

From the station 6 the formed, pressed products, i. e. the pellets/briquettes, are taken to the station 7, where these ones are subjected to a homogenization process, i. e. are given a supplementary, thermal treatment at a temperature of 50-200°C, a chemical reaction being started up, by which the products are crystallized, i. e. are hardened to a material with qualities like those of carbon.

Due to that fact a final product with very good qualities is obtained : 1) It contains 100% biofuel. In this respect the new product complies with the Kyoto-agreement, which was driven through by the United Nations, and according to which a world-wide reduction of the greenhouse gases are strived for.

2) It has a high energy content (exceeding 5, 5 MWh/ton.

3) It can be stored outdoors due to the fact that it partly is hydrophobic. partly does not emit any emissions to the environment ; 4) It is simple to divide into pieces, which is of value for dust firing.

The products are taken from the station 7 to a terminal station 8, which is in the form of a fuel storage.

In order to illustrate the invention further, an example of a method for carrying out the invention shall hereinafter be described, there being assumed that a plant being at hand which is dimensioned for treating 1 ton sludge per hour continuously.

1 ton sludge containing 800 kg water and 200 kg dry substance are fed continuously dring 1 h into a mixing chamber. where it is mixed with 200 kg new bio- oil and 1300 kg re-circulated bio-oil. The mixture (2500 kg), formed in that way, is heated up to 90°C (energy consumption = 100 kWh). whereafter it is pumped to a station for further heating. In this connection the pressure is raised to 4 bar and the temperature to 140°C (energy consumption = 410 kWh). In this case there is a so-called flush-tank

for driving away water. The hot sludge- and oilmixture are spurted onto the innerwalls of the tank. 720 kg water being driven away per hour. The remaining sludge- and oilmixture is thereafter pumped to a pressing and formation device, where 1310 kg biooil is separated from the mixture and is re-circulated to the mixing chamber.

Thereafter, the reduced sludge- and oilmixture (470 kg) is formed in this case to pellets. In order that the specific, final product shall be obtained, that is the result of this invention, the pellets have to go through the previously mentioned hardening process.

Thus, the pellets are taken to an oven, where the hardening goes on for 3 hours at a temperature of 140°C, the mentioned crystallization being made. The final pellets (465 kg/h) are taken out of the oven and are allowed to be cooled down in a fuel storage.

In the following method claim, all steps 1-4 have been inserted into the characterizing part of the claim. The reason why is that we will make clear that we apply for patent protection of the combination of steps, which at least to certain parts are previously known, and comprises the new, special step 4 which is that step that makes the production of the final product, described in the text, possible.

The invention, of course, can be modified within the scope of the following claims.