Field of the invention

The present invention relates to a method and apparatus for torrefaction of biomass material at low temperature so as to obtain a product having high carbon content with possible hydrophobic characteristics.

Background of the invention

Torrefaction of biomass material is known since long and the so formed product has high carbon content. The torrefied product may be used to replace coal in many applications and has many advantages regarding handling properties. It is for instance hydrophobic and will thus have low moisture content. Moreover, it will not be affected by weather and wind and demands no particular storage conditions. The torrefaction of biomass material is a mainly exothermic reaction starting at about 260°C and performed in an inert atmosphere, thus it is a gasification of the biomass material at low temperature. Since the reaction is in most cases exothermic the reaction velocity must be controlled. Normally this is done by cooling the jacket of the reactor. However, when performing torrefaction a pyrolytic gas is formed. This pyrolytic gas when cooled forms a sticky tar which will thus adhere to the inner surfaces of the reactor and interfere with the torrefied product so formed. However, the Applicant has noted that when torrefaction of biomass material of a certain type it may be necessary to, instead of cooling, heat said material. Summary of the invention

The object with the invention is to provide a method and an apparatus by which the formation of sticky tar on the inner surfaces of the reactor is eliminated. According to the invention this is achieved by a method for torrefaction of biomass material at low temperature,

comprising the steps of:

- providing a finely divided biomass material,

- feeding said finely divided biomass material to at least one heater for drying and heating said biomass material,

- feeding said dried biomass material to at least one rotating torrefaction reactor, said reactor being provided with a heated jacket and in the circumferential direction regularly spaced internal cooling tubes having a distance in the radial direction to the jacket, whereby said cooling tubes are so arranged that at least some of them, during rotation of said at least one reactor, will be in contact with and cool the biomass material being torrefied.

According to the invention this is also achieved by an

apparatus for torrefaction of biomass material at low

temperature comprising means for feeding a finely divided biomass material to at least one heater for drying and heating said biomass material, at least one torrefaction reactor connected to said at least one heater, and means for

collecting, cooling and transporting the so formed torrefied product to a suitable storage bunker, wherein the reactor comprises a rotating reactor having an external stationary casing and an internal jacket and provided with in the circumferential direction regularly spaced cooling tubes having a distance in the radial direction relative to the jacket, and wherein the internal jacket of reactor is adapted to be heated and the cooling tubes are adapted be cooled by feeding a cooling fluid to the same so as to cool the biomass material being torrefied.

Brief description of the drawings The present invention is described below with reference to the accompanying drawings, on which

- Fig. 1 is a schematic view of an apparatus for torrefaction of biomass material according to the invention,

- Fig. 2 is a schematic sectional side view taken at line II- II in Fig. 3 of a reactor used in the apparatus in Fig. 1, and

- Fig. 3 is a schematic sectional end view taken at line III- III in Fig. 2 of the reactor in Fig. 2.

Description of preferred embodiments

In the description below by biomass material is meant any materials having a suitable organic compound content, such as plant material, waste material, etc.

As seen in Fig. 1-3 an apparatus 1 for torrefaction of biomass material at low temperature comprises means 2 for feeding a finely divided biomass material to an inlet end 3 of at least one heater 4, preferably at least one tubular heater, and most preferably at least one rotating tubular heater, for drying and heating said biomass material. The heater 4 is connected at its outlet end 5 to an inlet end 11 of at least one

torrefaction reactor 10. At an outlet end 12 of the reactor 10 suitable means (not shown) are arranged for on one hand collecting, cooling and transporting the so formed torrefied product to a suitable storage bunker (not shown) and on the other hand collecting possible formed process gas.

The reactor 10 is preferably a rotating reactor 10 having an external stationary casing 13 and internal rotating jacket 14 provided internally in the circumferential direction with regularly spaced cooling tubes 15 having a distance in the radial direction relative to the jacket (Fig. 2 and 3) . When torrefying the biomass material in the reactor 10 the internal jacket 14 of the reactor 10 is adapted to be heated by a suitable heating fluid so as to torrefy the biomass material in an inert atmosphere, i.e. without any inlet of air or oxygen to the reactor. The treatment temperature of the reactor 10 lies within the interval of 250 to 320°C,

preferably within the interval of 270 to 300°C.

As schematically seen in Fig. 2 and 3 a heating fluid is fed into the reactor preferably at its bottom at A and is released preferably at its top at B. The heating fluid is preferably heated gas. When so doing an exothermic reaction is started within the biomass material. To control the reaction velocity the biomass material has to be cooled. According to the invention this is done by the in the circumferential direction regularly spaced cooling tubes 15 which are adapted be cooled by suitable cooling fluid which is fed into the tubes at C of the reactor and released from the tubes at D of the reactor.

More precisely, the in the circumferential direction regularly spaced cooling tubes 15 are so placed in the reactor 10 that they extend between respective ends 15,16 of the reactor and are located close to the inner surface of the internal jacket 14, so when the reactor with biomass material is rotated the biomass material will progressively cover the lowest cooling tubes in the reactor and will thereby be cooled. The torrefied product is preferably cooled in counter-current flow.

However, as mentioned above a pyrolytic gas is formed when torrefying the biomass material under these conditions.

Therefore, the temperature of the inner surface of the

internal rotating jacket 14 should be kept at a temperature above the liquefying temperature of the pyrolytic gas. Since the cooling tubes are cool pyrolytic gas will liquefy on the tubes and form a sticky tar. The torrefied product will thus adhere to the cooling tubes 15 but will be worn off as the cooling tubes pass through the torrefied product at the bottom of the rotating reactor.

Regarding the cooling of the cooling tubes 15 the cooling fluid is preferably feed only to the tubes partly or wholly in contact with the torrefied product, thus to the cooling tubes at the bottom of the reactor.