Arrangement for feeding wood particles into impregnating

Abstract

An arrangement for feeding wood particles into an impregnating stage of a wood treatment process comprises a feed silo (301, 701), at least one impregnating vessel (305, 601, 602, 603, 703) for receiving wood particles into said impregnat- ing stage, and two or more conveyors (302, 303, 304, 501, 502, 503, 702) between said feed silo (301, 701) and said at least one impregnating vessel (305, 601, 602, 603, 703), for transferring wood particles from said feed silo (301, 701) to said at least one impregnating vessel (305, 601, 602, 603, 703). Each of said con- veyors (302, 303, 304, 501, 502, 503, 702) is a compressing con- veyor for applying pressure to the wood particles on their way through the respective conveyor.

Description

ARRANGEMENT FOR FEEDING WOOD PARTICLES INTO IMPREGNAT- ING FIELD OF THE INVENTION The invention concerns the general technical field of converting biomass into chemical bioproducts in industrial scale. In particular the invention con- cerns the technology of pretreating wood particles and moving them between various pretreating stages. BACKGROUND OF THE INVENTION The production of biomass-based chemicals may use for example wood particles as the main raw materi- al. In a biomass-to-sugar process the wood particles may be subjected to various kinds of pretreatment such as washing, impregnating with water and / or other liq- uids, and heating, in order to prepare them for the later stages of the process. A pretreatment process may involve soaking the wood particles in steam or hot water, then soaking them in dilute acid, and subsequently taking the acid- impregnated wood particles into a hemihydrolysis reac- tor where a steam explosion reaction breaks the parti- cles into reaction products such as cellulose, hemi- cellulose (so-called C5 sugar), and lignin. Mechanical conveyors such as screw feeders transfer the impreg- nated wood particles between the stages of the pre- treatment process. A screw feeder is an example of a compressing conveyor that applies a squeezing force to the trans- ferred material. This is a useful property when the liquid content of the material needs to be influenced upon. As the material passes through the screw feeder it becomes compressed, removing some previously ab- sorbed liquid and causing some - at least partially 2 elastic - compressing deformation. Releasing the com- pressed material from the screw feeder makes it readi- ly absorb a new impregnating liquid if one is availa- ble, as the elastic deformation relaxes. Not only the actual pretreatment stages but also the conveyor solutions between them that move the wood particles forward in the process have an effect on how the process works, so they must be designed and dimensioned accordingly. SUMMARY According to a first aspect there is provided an arrangement for feeding wood particles into an im- pregnating stage of a wood treatment process. The ar- rangement comprises a feed silo, at least one impreg- nating vessel for receiving wood particles into said impregnating stage, and two or more conveyors between said feed silo and said at least one impregnating ves- sel. The conveyors are used for transferring wood par- ticles from said feed silo to said at least one im- pregnating vessel. Each of said conveyors is a com- pressing conveyor for applying pressure to the wood particles on their way through the respective convey- or. According to a second aspect there is provid- ed a method for feeding wood particles from a feed si- lo into an impregnating stage of a wood treatment pro- cess. The method comprises transferring the wood par- ticles into at least one impregnating vessel for said impregnating stage using at least two compressing con- veyors that apply pressure to the wood particles on their way through said at least two conveyors. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the described 3 embodiments and constitute a part of this specifica- tion, illustrate various advantageous features and ex- amples of their combinations. In the drawings: Figure 1 illustrates a chemical refining pro- cess on a general level, Figure 2 illustrates an example of process stages in pretreatment, Figure 3 illustrates an example of process stages and equipment, Figure 4 illustrates another example of pro- cess stages and equipment, Figure 5 illustrates another example of pro- cess stages and equipment, and Figure 6 illustrates another example of pro- cess stages and equipment. DETAILED DESCRIPTION Numerical attributes such as first, second, third, and so on are used in this description and the appended claims for the purpose of giving unambiguous names to concepts. They do not refer to any particular order, unless otherwise explicitly stated. In the context of this description the term wood particles refers to a material that consists mainly of pieces of wood formed by cutting or chipping larger pieces of wood such as trees, branches, logging residues, stumps, roots, and wood waste. The size of the wood particles may vary in a wide range from a few millimetres to a few centimetres, so the wood parti- cles meant here are typically larger than those meant with the term sawdust. The wood used to make wood par- ticles may be debarked or it main contain bark. For a wood-to-sugar process the preferred raw material is broadleaf wood due to its relatively high inherent sugar content, but the use of other kinds of wood is not excluded. The terms wood chips, wooden chips, or just chips can be used to mean the same thing as wood 4 particles. The term chips is used in the appended drawing because it is short. Fig. 1 illustrates schematically how in a method and arrangement for treating wood particles the wood particles may go to pretreatment, schematically illustrated as 101. The purpose of the pretreatment 101 is to prepare the incoming wood particles for ef- ficient use in the process, by removing some unwanted impurities, by compensating for some of the natural fluctuations in the characteristics of the material, and by breaking down the natural structure of the wood material. Hemicellulose (C5 sugars) can be collected from the pretreatment 101, and cellulose (or lignocel- lulosic material) can be taken further to a hydrolysis 102 to produce carbohydrates of desired kind. Fig. 2 illustrates an example of a product flow through various stages that all belong to the pretreatment 101 of fig. 1. Washing 201 is done with water, removing some mainly inorganic impurities such as sand. Washed wood particles are taken to steam treatment 202 for the purpose of removing air from in- side the wood particles and to preheat them to an ele- vated temperature. Steam-treated wood particles are taken to dilute acid treatment 203 for impregnating them with a dilute acid solution. The aim of the di- lute acid treatment 203 is to make the dilute acid so- lution penetrate into the wood particles as evenly as possible. The acid-impregnated wood particles are taken to hemihydrolysis (not separately shown in fig. 2) where they are under elevated pressure and tempera- ture. At the output of the hemihydrolysis the wood particles undergo a steam explosion 204 that breaks their structure. The output stream from the steam ex- plosion 204 goes through steam separation (not sepa- rately shown) to mixing 205 where water is added and the resulting mass is homogenized mechanically to 5 break up agglomerates. Solids and liquids may then be separated at 206 for feeding into later process stag- es. It has been found that in a pretreatment pro- cess of a commercially viable scale the use of a known plug screw feeder to feed wood particles into an im- pregnating stage may result in less than optimal im- pregnating. This is an important finding, because spa- tially even absorption of the dilute acid solution in the wood particle would be highly desirable: it af- fects the quality of the product later in the process. The relatively large flow rates of a commercially via- ble wood-to-sugar process mean that it is not feasible to assume that a wood particle would spend more than some minutes in the impregnating vessel 401. Aiming at longer times would mean that the impregnating vessel 401 would need to be larger than is practical to build. However, after only some minutes under the in- fluence of the acidic solution the wood particle may not have been fully penetrated, or at least the spa- tial distribution of acid inside the wood particle is not completely even. This problem is made even worse if the wood particle was not in optimal condition (shape, size, amount of bark in unbarked feedstock, not compressed enough) for absorbing the dilute acid solution when it came to the impregnating stage. The compression ratios of commercially avail- able plug screw feeders are between 1.5 and 6, and the compression ratio tends to become lower when one goes towards larger equipment and larger material flows. This is an underlying cause of the problem that in commercially viable, industrial scale processes the work done on the wood particle in the compressing con- veyor before an impregnating stage may not be suffi- cient for effective impregnation. It has been found that more efficient com- pressing, and consequently a better initial condition of the wood particles for impregnating, can be achieved by using two or more conveyors between the pre-steaming silo and the impregnating vessel(s). This way the compressing work done per wood particle by an individual conveyor becomes larger. Fig. 3 illustrates schematically a part of an arrangement for pretreating wood particles in a wood treatment process. As a part thereof there is an ar- rangement for feeding the wood particles into an im- pregnating stage of the wood treatment process. The arrangement comprises a feed silo, which in the embod- iment of fig. 3 is a pre-steaming silo 301 for treat- ing the wood particles with steam. The arrangement comprises an impregnating vessel 305 for receiving wood particles in the impregnating stage that is dis- cussed here. The impregnating vessel 305 comprises an input for a dilute solution of an acid, such as sul- phuric acid, for soaking the received wood particles in said dilute acid solution in order to prepare them for hemihydrolysis and steam explosion in a reactor 307 further downstream in the process. Other acids that could be used for said impregnation comprise - but are not limited to - nitric acid, phosphoric acid, lactic acid, acetic acid, formic acid and carbonic ac- id. The arrangement comprises two or more convey- ors 302, 303, and 304 between the pre-steaming silo 301 and the impregnating vessel 305 for transferring wood particles from the pre-steaming silo 301 to the impregnating vessel 305. Each of these conveyors is a compressing conveyor for applying pressure to the wood particles on their way to through the respective con- veyor. Concerning the feed silo, which in fig. 3 is the pre-steaming silo 301, it is not a necessary re- quirement that it is used for pre-steaming. As was de- scribed above with reference to fig. 2, advantageous effects of pre-steaming include removing air from in- side the wood particle and preheating. These or corre- sponding effects can be achieved otherwise than by ap- plying steam; for example various vacuum and microwave related techniques can be used. If the silo is a pre- steaming silo, the steam used therein may be pure steam of water or it may contain additives, such as some acid, to enhance the desired effect. The fact that the conveyors 302 to 304 are fed from a silo has an advantageous effect, because stable operation and constant effect on the transferred material of plug screw feeders of force feed screws (which are examples of compressing conveyors) are easy to achieve by using a silo for feeding and by maintaining the surface lev- el of the material inside the silo sufficiently high. Taken the relatively large flow rates of a commercially viable wood-to-sugar process it is not feasible to assume that a wood particle would spend more than some minutes in the impregnating vessel 305. Aiming at longer times would mean that the impregnat- ing vessel 305 would need to be larger than is practi- cal to build. However, after only some minutes under the influence of the acidic solution the acid may not have fully penetrated the wood particle, or at least the spatial distribution of acid inside the wood par- ticle may not be completely even. It has been found advantageous to provide a soaking silo as a temporary storage in which the spatial distribution of acid in- side the wood particle has time to even out. In the schematic presentation of fig. 3 the soaking silo would appear in block 306. The residence time in the soaking silo 502 may be in the orders of some tens of minutes. In an embodiment the residence time in the soaking silo 502 is not more than 60 minutes. In an- other, preferred embodiment the residence time in the soaking silo 502 is not more than 30 minutes. In the arrangement of fig. 3 the two or more conveyors operate in parallel. That is, each of the two or more conveyors 302 to 304 comprises a respec- tive conveyor input and a respective conveyor output, and the conveyor inputs are coupled to the pre- steaming silo 301 in parallel for receiving a respec- tive component stream of wood particles from the pre- steaming silo 301. There is only one, common impreg- nating vessel 305, which is coupled to at least two (here: all) of the conveyor outputs for receiving at least two (here: all) of the component streams. Taken that there is a certain gross material flow through the process, each of the N parallel com- pressing conveyors (N = 2, 3, ...) conveys an <semantics>1 / N<annotation encoding="application / x-tex">1 / N< / annotation>< / semantics> frac- tion of the gross material flow. In total, N times the work of an individual compressing conveyor is per- formed on the material flow. This ensures much more thoroughly conditioned wood particles at the beginning of the impregnating stage in the impregnating vessel 305 than if only one compressing conveyor would be used for transferring. Fig. 4 illustrates an embodiment that is oth- erwise similar to that of fig. 3 but there is provided a divider 401 between the feed silo (the pre-steaming silo 301) and the conveyor inputs for dividing wood particles from the feed silo into the component streams. In fig. 3 it was assumed that the conveyor inputs were simply installed at the bottom of the pre- steaming silo 301. The divider 401 may be a static, mechanical divider like a wedge-shaped barrier that mechanically guides wood particles from the pre- steaming silo 301 to the conveyor inputs. Additionally or alternatively it may comprise conveyors of its own, like a double screw feeder that receives a flow of wood particles from the silo in the middle and that exhibits helixes of opposite handedness towards its two ends. 9 Fig. 5 illustrates an embodiment in which the compressing conveyors operate in series. That is, each of the two or more conveyors 501 to 503 comprises a respective conveyor input and a respective conveyor output, of which only the conveyor input of a first conveyor 501 is coupled to the feed silo (the pre- steaming silo 301) for receiving wood particles from the feed silo. The conveyor output of the first con- veyor 501 is coupled to the conveyor input of the sec- ond conveyor 502 for transferring the wood particles through said first 501 and second 502 conveyors in se- quence on their way from the feed silo to the impreg- nating stage. Fig. 5 illustrates schematically the possibility of having more than two compressing con- veyors in series (see block 503 in fig. 5). In an embodiment where two or more compress- ing converters operate in series, in order to fully achieve the advantageous effect of double compressing work on the wood particle the compression stages should follow each other directly enough, with minimum delay in between so that the mechanical deformation achieved in a preceding compression does not relax too much before the next compression. Therefore the con- veyor output of the preceding conveyor is most advan- tageously directly coupled to the conveyor input of the next conveyor, with few or no intermittent pieces of apparatus therebetween. Fig. 6 illustrates an embodiment that is oth- erwise similar to that of fig. 3 or fig. 4 (the divid- er 401 being optional) but there are separate impreg- nating vessels coupled to at least two of the com- pressing conveyors 302 to 304. That is, a first im- pregnating vessel 601 is coupled to the conveyor out- put of a first conveyor 302 to receive a first compo- nent stream, and a second impregnating vessel 602 is coupled to the conveyor output of a second conveyor 303 to receive a second component stream. The arrange- ment comprises a combiner stage after said first 601 and second 602 impregnating vessels for combining out- puts of the first 601 and second 602 impregnating ves- sels to a later stage of the wood treatment process. In the schematic presentation of fig. 6 the combiner stage is included in block 306. An additional advantageous effect of using at least two compressing conveyors before the impregnat- ing stage has been found when some further stages of the process are considered in more detail. Fig. 7 il- lustrates schematically an apparatus for pretreating wood particles. A pre-steaming silo 701 is provided for treating the wood particles with steam, i.e. im- plementing at least part of the stage shown as stage 202 in fig. 2. At least two compressing conveyors 702 are provided for transferring steam-treated wood par- ticles from the pre-steaming silo 701 to one or more impregnating vessels 703 in which the dilute acid treatment stage 203 of fig. 2 is implemented. Down- stream from the one or more impregnating vessels 703 there may be provided one or more soaking silos 704 for giving the dilute acid solution more time to pene- trate into the inner parts of the wood particles. One or more further compressing conveyors 705 may be used to transfer the acid-impregnated wood particles into a reactor 706 in which the hemihydrolysis reaction takes place and at the output of which the steam explosion takes place. The dry matter content and acid content of the material that goes into the reactor 706 are im- portant process parameters, because they have an ef- fect on how the desired reactions proceed in the reac- tor 706 and downstream from it in the process. The re- actor 706 is pressurized, and the compressing capabil- ity of the immediately preceding compressing conveyor 705 may be used to aid the feeding of the material flow into the reactor 706. 11 Now when two or more compressing conveyors 702 have been used before the impregnating vessel(s) 703, the penetration of dilute acid solution into the wood particle may be more complete than if only one compressing conveyor had been used at stage 702; the material may be "wetter" of dilute acid. The extent to which this occurs can be controlled by controlling, among others, the operation of the compressing convey- ors 702. In order to achieve the desired dry matter content and acid content at the input of the reactor 706, the operation of the later compressing conveyor 705 can be controlled. There is a wider control win- dow, i.e. more accurate controlling possibilities, for the last-mentioned due to the fact that two or more compressing conveyors were used at stage 702. It is obvious to a person skilled in the art that with the advancement of technology, the ideas ex- plained above may be implemented in various ways. The claimed scope is thus not limited to the examples de- scribed above.

Claims

1. An arrangement for processing wood particles, comprising: a feed silo; at least one impregnating vessel configured to subject wood particles received therein to an impregnating stage of a wood treatment process; for feeding said wood particles into said impregnating stage, two or more conveyors between said feed silo and said at least one impregnating vessel, for transferring said wood particles from said feed silo to said at least one impregnating vessel; and downstream from said impregnating stage in the process, a reactor configured to subject wood particles impregnated in said impregnating stage to hemihydrolysis and steam explosion; wherein each of said two or more conveyors is a compressing conveyor is configured to apply pressure to the wood particles on their way through the respective conveyor and comprises a respective conveyor input and a respective conveyor output, and wherein the conveyor inputs of said two or more conveyors are coupled to said feed silo in parallel for receiving a respective component stream of the wood particles from said feed silo for causing an elastic compressing deformation of the wood particles in said two or more conveyors and subsequently relaxing the elastic deformation when releasing the wood particles into the impregnating stage.

2. The arrangement according to claim 1, wherein said feed silo is a pre-steaming silo for treating said wood particles with steam.

3. The arrangement according to claim 1, wherein each of said two or more conveyors is a plug screw feeder or a force feed screw.

4. The arrangement according to claim 1, further comprising a divider between said feed silo and said conveyor inputs for dividing the wood particles from said feed silo into said component streams.

5. The arrangement according to claim 1, wherein one of said at least one impregnating vessels is coupled to at least two of said conveyor outputs for receiving at least two of said component streams.

6. The arrangement according to claim 1, wherein a first of said at least one impregnating vessel is coupled to the conveyor output of a first of said two or more conveyors, to receive a first component stream of said component streams, a second impregnating vessel is coupled to the conveyor output of a second of said two or more conveyors, to receive a second of said component streams, the arrangement further comprising a combiner stage after said first and second impregnating vessels for combining outputs of said first and second impregnating vessels. An arrangement for processing wood particles, comprising: a feed silo; at least one impregnating vessel configured to receive wood particles received therein to an impregnating stage of a wood treatment process; for feeding said wood particles into said impregnating stage, two or more conveyors between said feed silo and said at least one impregnating vessel, for transferring wood particles from said feed silo to said at least one impregnating vessel; and downstream from said impregnating stage, a reactor configured to subject wood particles impregnated in said impregnating stage to hemihydrolysis and steam explosion; wherein each of said two or more conveyors is a compressing conveyor for applying pressure to the wood particles on their way through the respective conveyor and comprises a respective conveyor input and a respective conveyor output, wherein each of said two or more conveyors is a plug screw feeder, and the conveyor input of a first conveyor of said two or more conveyors is coupled to said feed silo for receiving said wood particles from said feed silo, and the conveyor output of said first conveyor is coupled to the conveyor input of a second conveyor of said two or more conveyors for transferring said wood particles through said first and second conveyors in sequence on their way from said feed silo to said impregnating stage for causing an elastic compressing deformation of the wood particles in said two or more conveyors and subsequently relaxing the elastic deformation when releasing the wood particles into the impregnating stage.

8. The arrangement according to claim 7, wherein said feed silo is a pre-steaming silo for treating said wood particles with steam.

9. The arrangement according to claim 1, wherein said impregnating stage comprises soaking said wood particles in a dilute acid solution.

10. The arrangement according to claim 7, wherein a first of said at least one impregnating vessel is coupled to the conveyor output of a first of said two or more conveyors, to receive a first component stream of said component streams, a second impregnating vessel is coupled to the conveyor output of a second of said two or more conveyors, to receive a second of said component streams, the arrangement further comprising a combiner stage after said first and second impregnating vessels for combining outputs of said first and second impregnating vessels.

11. A method for processing wood particles, com- prising: - feeding wood particles from a feed silo into an im- pregnating stage of a wood treatment process, and - transferring outputs of at least one impregnating vessel used for said impregnating stage into a reac- tor, and - using said reactor to subject wood particles impreg- nated in said impregnating stage to hemihydrolysis and steam explosion; the method comprising: - transferring the wood particles into said at least one impregnating vessel for said impregnating stage using at least two compressing conveyors that apply pressure to the wood particles on their way through said at least two conveyors so that either a) said at least two conveyors are used in parallel to transfer at least two component streams of wood parti- cles from said feed silo to said impregnating stage or b) said at least two conveyors each are plug screw feeders used in series to transfer a stream of wood particles from said silo through both of said two con- veyors to said impregnating stage, by receiving said wood particles from said feed silo into a conveyor in- put of a first conveyor and coupling a conveyor output of said first conveyor to a conveyor input of a second conveyor; - during said transferring, causing an at least par- tially elastic compressing deformation of the wood particles in said at least two conveyors; and - subsequently relaxing the elastic deformation when releasing the wood particles into the impregnating stage.

12. The method according to claim 11, comprising treating said wood particles with steam in said feed silo.

13. The method according to claim 11, wherein in case a) at least two of said component streams are fed into a common impregnating vessel for said impregnat- ing stage.

14. The method according to claim 11, wherein in case a): - a first component stream of said component streams is fed into a first impregnating vessel for said im- pregnating stage, 17 - a second component stream of said component streams is fed into a second impregnating vessel for said im- pregnating stage, - outputs of said first and second impregnating ves- sels are combined into a common material stream.

15. The method according to any of claims 11 to 14, wherein said impregnating stage comprises soaking said wood particles in a dilute acid solution.