System and process for producing biomass pellets

Abstract

A system for producing biomass pellets comprising a pan mill for comminuting a biomass material, a treatment means in which the comminuted biomass material is treated with an oxidizing reactant and a pellet press for pressing pellets from the treated biomass material. The invention further relates to an associated process. The intention is to initiate during pellet production oxidation reactions which can otherwise result in autoignition of the pellets in pellet heaps.

Claims

1. A system for producing biomass pellets comprising a pan mill (15, 28) for comminuting a biomass material, a treatment apparatus (25, 30) in which the comminuted biomass material is treated with an oxidizing reactant, the treatment apparatus comprising a reactant supply, a reactant channel extending from the reactant supply and defining an orifice, wherein the treatment apparatus delivers the reactant from the supply to the orifice so that the reactant exits from the orifice and treats the comminuted biomass material and a pellet press (29) for pressing pellets from the treated biomass material.

2. The system as claimed in claim 1, wherein the pan mill (15, 18) comprises a perforated die (20) in the form of a flat die.

3. The system as claimed in claim 1, wherein the treatment apparatus (25) comprises a spraying apparatus (26) for contacting the comminuted biomass material with the oxidizing reactant in liquid form.

4. The system as claimed in claim 1, wherein the oxidizing reactant is hydrogen peroxide.

5. The system as claimed in claim 1, wherein the treatment apparatus (30) comprises an opening (32) for contacting the biomass material with the oxidizing reactant in a gaseous state.

6. The system as claimed in claim 5, wherein the oxidizing reactant is ozone.

7. The system as claimed in claim 5, wherein the treatment apparatus (30) is configured as a pneumatic conveyor.

8. The system as claimed in claim 1, wherein upon comminution by the pan mill (15) the biomass material has a moisture content of at least 20% by weight.

9. The system as claimed in claim 1, wherein a dryer (24) is arranged between the pan mill (15) and the treatment apparatus (25, 30).

10. The system as claimed in claim 1, wherein an intermediate storage means (27) is arranged between the treatment apparatus (25) and the pellet press (29).

11. A process for producing biomass pellets comprising the steps of: comminuting biomass material with a pan mill (15, 28); treating the comminuted biomass material with an oxidizing reactant to produce a treated biomass material; and pressing the treated biomass material into pellets.

12. The process for producing biomass pellets of claim 11, comprising: storing the treated biomass material for a predetermined period of time before said step of pressing.

13. The process for producing biomass pellets of claim 11, comprising: drying the comminuted biomass material before said step of treating.

14. The process for producing biomass pellets of claim 11, wherein said step of treating comprises: spraying said comminuted biomass material with an oxidizing reactant in liquid form.

15. The process for producing biomass pellets of claim 14, wherein said oxidizing reactant in liquid form comprises hydrogen peroxide diluted with water.

16. The process for producing biomass pellets of claim 11, wherein said step of treating comprises: exposing said comminuted biomass material to an oxidizing reactant in gaseous form.

17. The process for producing biomass pellets of claim 16, wherein said oxidizing reactant is ozone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is elucidated hereinbelow by way of advantageous embodiments making reference to the accompanying figures. In the figures:

(2) FIG. 1 shows a schematic illustration of a system according to the invention,

(3) FIG. 2 shows a pan mill of a system according to the invention,

(4) FIG. 3 shows an alternative embodiment of a system according to the invention.

DETAILED DESCRIPTION

(5) Supplied to the system for producing biomass pellets shown in FIG. 1 is a biomass starting material in a coarsely pre-comminuted state in the form of chips. The chips may be predominantly or entirely generated from a wood material.

(6) The feeding of the biomass starting material is carried out via a feed hopper 14 of a first pan mill 15. As shown in the enlarged view of the pan mill 15 in FIG. 2 the material falls onto the top side of a perforated die 20 and is there comminuted by compressive and shear forces exerted by the pan rollers 19. The biomass material is converted into the form of predominantly fibrous shavings which pass through the holes 21 in the perforated die 20 as soon as they are sufficiently small. The biomass material can exit the first pan mill 15 via an outlet opening 23.

(7) The construction of the pan mill 15 is as follows: a floorstanding machine housing 12 carries a driveshaft 16 configured as a vertical shaft which is rotatably mounted in the machine housing 14. Via a drivetrain 17 the driveshaft 16 may be set into rotation with a drive motor (not shown). Connected to the driveshaft 16 are horizontal stub axles 18. A pan roller 19 is rotatably mounted on each stub axle 18.

(8) The pan rollers 19 run on the top side of the perforated die 20 which has been provided with a plurality of holes 21. The holes 21 extend from an inlet end facing the pan rollers 19 through the perforated die 20 to an outlet end on the underside of the perforated die 20. When the driveshaft 16 is rotating the pan rollers 19 move along a circular track on the surface of the die 20.

(9) The biomass starting material supplied to the first pan mill 15 may have a moisture content of for example 40% to 50% by weight. The comminution operation in the first pan mill 15 does not substantially alter the moisture content and the moisture content of the predominantly fibrous shavings exiting the first pan mill 15 is therefore of the same order of magnitude.

(10) Arranged downstream of the first pan mill 15 is a dryer 24 with which the moisture content of the biomass material is reduced to 10% to 15% by weight for example. Arranged downstream of the dryer 24 is a treatment means 25 in which the biomass material is treated with an oxidizing reactant.

(11) The treatment means 25 may be configured as an intermediate container in which the biomass material is arranged over a large surface area so that the surface of the biomass material is readily accessible. It is also possible for the means 25 to be a transport means on which the biomass material is for example conveyed on a conveyor belt. The biomass material may be arranged on the conveyor belt such that the surface of the biomass material is readily accessible.

(12) The storage/transport means 25 comprises a reactant supply 33, a reactant channel 34, a spraying apparatus 26 defining orifices from which the biomass material may be sprayed with an oxidizing reactant in liquid form. The oxidizing reactant may for example be hydrogen peroxide diluted with water so that the proportion of hydrogen peroxide and the solution is for example 30%. The amount of solution distributed may be 1% by weight for example, based on the weight of the biomass material. Due to the arrangement of the biomass material over a large surface area the oxidizing reactant can bring about oxidation reactions in the biomass material to substantial extents. These oxidation reactions which already take place during production of the pellets are no longer available to bring about heating in a later pellet heap.

(13) Following the treatment with the oxidizing reactant in the storage/transport means 25 is a rest phase of several hours during which the oxidation reactions initiated by the reactant can take place. In FIG. 1 this is indicated schematically via an intermediate storage means 27.

(14) The biomass material treated in this way is comminuted further in a second pan mill 28. The second pan mill 28 differs from the first pan mill 15 in that the holes 21 in the perforated die 20 have a smaller diameter. The biomass material is broken down into smaller predominantly fibrous constituents before it can pass through the holes 21 of the second pan mill 28.

(15) After the second comminution operation the biomass material may be pressed to afford pellets in a pellet press 29. This may be done using customary pellet presses in which the biomass material is for example pressed through a ring die or a flat die. There are pellet presses having a certain similarity to the pan mills 15, 28 in terms of their construction. These pellet presses differ from the pan mills 15, 28 in that the pass-through channels in the flat die have a markedly greater length than the holes 21 in the perforated die 20. After sufficient comminution the biomass material can pass through the holes 21 in the perforated die 20 without being compressed further. In the pellet press the friction against the channel walls is sufficiently great for the material to be compressed to afford pellets.

(16) In the alternative embodiment according to FIG. 3 the second pan mill 28 is arranged immediately downstream of the dryer 24. The biomass material is discharged from the second pan mill 28 in further comminuted form and is transported to the pellet press 29 with a pneumatic conveying means 30. A reactant supply 33 includes a reactant channel communicating with an orifice 32. In this embodiment, the reactant supply 33 is a source of ionized air, for example in the form of ozone. The pneumatic conveying means 30 comprises a blower 31 which generates an air flow of sufficient speed to entrain the constituents of the biomass material.

(17) A substream of ionized air, for example in the form of ozone, is added to this airstream via an opening 32. The ozone acts as an oxidizing reactant upon the predominantly fibrous constituents of the biomass material. Since the predominantly fibrous constituents of the biomass material are small and the pneumatic conveying brings about intensive contact between the ozone and the predominantly fibrous constituents a treatment duration shorter than in the embodiment according to FIG. 1 is sufficient to achieve the desired effect. Taking place in the biomass material are oxidation reactions which are then no longer available to bring about heating in a later pellet heap.