SE2051477A1 - Method and system for producing solid combustible material from biomass - Google Patents

Abstract

Method for producing solid combustible material from biomass material is provided. The method comprises providing (1) biomass material, continuously introducing (2) the biomass material into a pressurized reactor at a first volume rate V1 per minute, subjecting (3) the biomass material to thermal treatment in said pressurized reactor at elevated pressure and temperature by means of adding steam, continuously discharging (5) the biomass material from the pressurized reactor, subjecting the discharged biomass to steam explosion (6), and collecting (7) a solid portion of the steam exploded biomass, which solid portion forms at least part of the solid combustible material, further comprising, in connection with said continuously discharging (5), compressing (4) the biomass material such that it is discharged at a second volume rate V2 per minute being lower than V1. A corresponding system is also provided.

Description

METHOD AND SYSTEM FOR PRODUCING SOLID COMBUSTIBLE MATERIAL FROM BIOMASS TECHNICAL FIELD The invention relates to the field of production of solid combustible material such as pellets or briquettes from biomass material.

BACKGROUND There is a growing demand for biomass based solid combustible fuels (such as pellets or briquettes)to replace or supplement coal or other fossil fuels in power plants. Biomass-based pellets (andbriquettes) may be produced with or without thermal treatment of the biomass. Pellets producedwithout thermal treatment are usually referred to as white pellets. Pellets produced with thermaltreatment are usually referred to as black pellets due to their darker color. One advantageousmethod for thermal treatment is steam explosion. Steam explosion refers to a process step wherethe material undergoes a rapid/instantaneous pressure decrease. Thus, the hot and softenedbiomass from the thermal treatment is released or blown from the reactor through a blow valve ororifice, while the pressure drops to an environment with substantially lower pressure, such as below5 bar, or preferably to substantially atmospheric pressure. The structure of the biomass breaks,partly due to the expanding steam, and also by the shear forces and impact during the blow throughthe orifice or valve. Steam explosion may simplify subsequent pelletizing or briquetting due to the(still) soft su bstrate being easier to densify. Steam explosion may furthermore improve the strengthof the resulting pellet due to various substances such as lignin and sugars being released during the steam explosion.

The term solid refers to a state that is solid (i.e. neither gas nor liquid) at ambient pressure and roomtemperature. The solid combustible material does not necessarily need to have a certain size andmay be granular or in the form of a powder. Thus, although the present application mainly refers to pellets, it is understood that the solid combustible fuel may take on any size or shape.

A process for producing combustible material from wood chips is disclosed in WO2020089187A1which comprises continuous introduction of wood fragments at a predetermined volume per minuteinto a pressurized reactor and continuous steam explosion discharge from the reactor at the samepredetermined volume of wood fragments per minute. A problem with this method is that pellets formed from the resulting combustible material may be of greatly varying quality.

SUMMARYAn object of the invention is to solve or improve on at least the problem mentioned above in the background section.

These and other objects are achieved by the present invention by means of a method and a system according to the independent claims.

According to a first aspect of the invention, a method for producing solid combustible material frombiomass material is provided. The method comprises providing biomass material, continuouslyintroducing the biomass material into a pressurized reactor at a first volume rate V1 per minute,subjecting the biomass material to thermal treatment in said pressurized reactor at elevatedpressure and temperature by means of adding steam, continuously discharging the biomass materialfrom the pressurized reactor, subjecting the discharged biomass to steam explosion, and collecting orseparating a solid portion of the steam exploded biomass, which solid portion forms at least part ofthe solid combustible material. The method further comprises, in connection with the continuouslydischarging, compressing the biomass material such that it is discharged at a second volume rate V2per minute being lower than V1. The method may furthermore comprise forming fuel pellets or briquettes from the solid combustible material. ln other words, the biomass material is discharged at a volume rate V2 which is lower than thevolume rate V1 at which the biomass material is introduced to the pressurized reactor. lt is notedthat the introduction of biomass material into the reactor may involve a compression of the material(due to the use of for instance a plug screw feeder) followed by decompression of the biomassmaterial upon entry into the reactor. The volume rate V1 is defined with respect to the biomassmaterial prior to feeding to the reactor, which normally is stored at ambient pressure. Afterdischarge, the solid portion is separated in a known manner from the gas phase (for example bymeans of a cyclone) and is collected. The solid combustible material is formed at least partly from thecollected solid portion in the sense that it can be mixed with another source of combustible material and/or that additives such as glue or lignin may be added. lt is important to note that the compressing step takes place in connection with the discharging stepin the sense that the compression takes place near or at the outlet of the pressurized reactor, i.e.either outside/immediately outside the pressurized reactor or in a bottom or end portion of thepressurized reactor from which the biomass is discharged. Put differently, the compressing step may be considered part of the discharging step. The compressing is thus not to be confused with packing biomass into a pressurized reactor as disclosed in US7303707B2, where the biomass in the whole reactor is compressed. lt is also important to note that the compression is quite low, typically within a range from 1.1 to 1.6,preferably from 1.2 to 1.5, and most preferably from 1.3 to 1.5. The compression is thus not to be interpreted as an extrusion (which typically involves higher compression rates).

The invention is based on the insight that improved quality of the solid combustible material can beachieved by compressing the biomass in connection with discharge from the reactor such that thebiomass discharges at a lower volume rate per minute than the volume rate with which the biomassmaterial is introduced into the reactor. The compression should however not be as high as inextrusion. This insight is based on the realization that such relatively mild compression may result ina higher concentration of acetic acid as well as side products like furfural formed in the wood duringthe thermal treatment which in turn may lead to faster kinetics and condensation of lignin andfurfural, which may result in higher energy content and stronger pellets. The compression at discharge also means that the flow out of the reactor can be more accurately controlled. ln embodiments, the compressing is achieved at least partly by means of at least one dischargescrew. The discharge screw may provide a compression ratio within a range from 1.1 to 1.6,preferably from 1.2 to 1.5, and most preferably from 1.3 to 1.5. Such discharge screws are known inthe art and will not be described in further detail herein. The discharge screw may be arranged at anoutlet of the pressurized reactor in fluid communication therewith. The discharge screw may bearranged outside the reactor and in connection with the outlet or may be arranged inside the reactor at a bottom or end portion thereof in connection with the outlet.

According to a second aspect of the invention, a system for producing solid combustible materialfrom biomass material is provided. The system comprises a pressurized reactor, a feeding deviceconfigured to continuously introduce biomass material into said pressurized reactor at a first volumerate V1 per minute, means for addition of steam to provide continuous thermal treatment of thebiomass at elevated pressure and temperature, a discharging arrangement configured tocontinuously discharge the biomass material from the pressurized reactor, a steam explosion deviceconnected to the discharging arrangement and being configured to subject the thermally treatedbiomass to steam explosion, collection means configured to collect a solid portion of steam explodedbiomass, which solid portion forms at least part of said solid combustible material. The system further comprises compression means for compressing the biomass material at a compression ratio within a range from 1.1 to 1.6, preferably from 1.2 to 1.5, and most preferably from 1.3 to 1.5. Thecompression means is arranged in connection with the discharging arrangement such that the biomass material is discharged at a second volume rate V2 per minute being lower than V1.

In embodiments, the pressurized reactor is a substantially horizontally arranged reactor, but mayalternatively be a substantially vertically arranged reactor. The steam explosion device may be in theform of a blow valve or orifice. The system furthermore preferably comprises separating means, suchas a cyclone, configured to separate steam from the steam exploded biomass prior to said collection meanS.

The features of the embodiments described above are combinable in any practically realizable way toform embodiments having combinations of these features. Further, all features and advantages ofembodiments described above with reference to the first aspect of the invention may be applied in corresponding embodiments of the second aspect of the invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGSAbove discussed and other aspects of the present invention will now be described in more detailusing the appended drawings, which show presently preferred embodiments of the invention,wherein: fig. 1 shows a flowchart of an embodiment of the method according to the first aspectof the invention, fig. 2 is a schematic side view illustration of an embodiment of the system according tothe second aspect of the invention, and fig.3 is a schematic side view illustration of another embodiment of the system according to the second aspect of the invention.

DETAILED DESCRIPTION Fig. 1 shows a flowchart of an embodiment of the method according to the first aspect of theinvention. The method comprises providing 1 biomass material, which may comprise providingprepared disintegrated hardwood material from a storage bin, for instance in the form of wood chipsof suitable dimension, typically within a range of from 50 to 100 mm at their largest dimension, andof suitable moisture content, for instance within a range from 5 to 10%. In an alternativeembodiment, the step of providing biomass material 1 may comprise at least one of providing wholewood logs, de-barking, reducing the size of the biomass to wood chips, and drying the size-reduced biomass to obtain wood chips having the above-mentioned dimensions and moisture content. The biomass material is thereafter continuously introduced 2 into a pressurized reactor at a first volumerate V1 per minute and subjected to thermal treatment 3 in the pressurized reactor at elevatedpressure and temperature by means of adding steam. The thermal treatment at elevated pressureand temperature is performed at a pressure of 11.5-25 bar (g), at a temperature of 190-225°C and fora duration of 7-11 minutes. The steam is added at a rate of 0,25 to 0,8 ton per ton biomass material.The thermally treated biomass is compressed 4 at a bottom/end portion of the reactor and isthereafter continuously discharged 5 from the reactor. Due to the compressing, the biomass materialis discharged at a second volume rate V2 per minute being lower than V1, where the compressing isperformed such that V2/V1 is within a range from 1.3 to 1.5. After compression and discharge, thebiomass material is subjected to steam explosion 6, where the pressure is reduced to atmosphericpressure. The solid and gas phases of the steam-exploded biomass material is separated 7,whereafter the solid portion is collected, which forms at least part of the solid combustible material.The method further optionally comprises forming 9 fuel pellets or briquettes from the solidcombustible material. Step 9 may be preceded by an optional drying step 8. Providing wood chipswith a low moisture content such as 5 to 10% prior to thermal treatment is advantageous, sincedrying 8 of the solid combustible material prior to said forming pellets or briquettes may not benecessary. Processes and devices for pelletizing and forming briquettes are known in the art, for example from WO 2017089648A1 and will not be described in further detail here. ln other embodiments, the compressing step is performed immediately outside the pressurizedreactor. ln such embodiments, the compressing step may be considered part of the discharge stepsince the biomass material supplied to the steam explosion step is supplied at rate V2/min in the same manner as in the embodiment in fig. 1.

Fig. 2 is a schematic side view illustration of an embodiment of the system according to the secondaspect of the invention. The system comprises an elongated reactor vessel 11 arranged substantiallyhorizontally, which vessel has a circular cross section, i.e. has a cylindrical shape. The vesselcomprises an inlet 12c for receiving the biomass material arranged at an uppermost portion of thevessel. The inlet is arranged at a first longitudinal end of the vessel (to the left in the figure). Thevessel furthermore comprises an outlet 14a at a second (opposite) longitudinal end of the vessel atthe lowermost portion of the vessel. A feeding device 12 comprising a vertically arranged pipe 12b isconnected to the inlet 12c, and a plug screw feeder 12a is connected to an upper portion of the pipe12b for feeding pressurized biomass material thereto. The plug screw feeder is provided with uncompressed biomass material at a volume flow V1 per minute. A conveying screw 18 is arranged inside the reactor vessel to convey the material from the inlet towards the outlet (only the protruding axle portions are visible in the figure).

The system further comprises means for injecting steam in the form of a set of steam injectionorifices 13a-d (schematically illustrated as arrows) arranged to supply steam to the biomass. The setof steam injection orifices comprises a plurality of orifices 13a-d mounted in through holes in thebottom of the reactor vessel along a straight line along the extension thereof. ln other embodiments,steam injection orifices may be arranged at different positions, for example (partly) around thecircumference of the reactor, as disclosed in a WO2019088906. The outlet 14a is connected tocompression means in the form of a discharge screw 14b providing a compression ratio within arange from 1.3 to 1.5 such that the discharged volume flow V2 per minute from the reactor is lowerthan the volume flow V1 per minute into the reactor. A schematically illustrated steam explosiondevice in the form of a blow valve 15 is connected to the outlet of the discharge screw 14b. Thepressure decreases rapidly over the blow valve 15 to substantially ambient pressure. The blow valve15 is connected to a cyclone separator 17 which separates the thermally treated and steam-explodedbiomass material into a gas phase and a solid phase. The solid phase is collected in collection means(storage bin) 16. The storage bin is connected to a pelletizing device 19 to form pellets from the solidphase of the biomass collected in the storage bin. Optionally, additives may be added to the pelletizing device, as is known in the art.

Fig.3 is a schematic side view illustration of another embodiment of the system according to thesecond aspect of the invention. The system comprises an elongated reactor vessel 21 arrangedsubstantially vertically, which vessel has a substantially conical shape. The vessel comprises an inlet22c for receiving the biomass material arranged at an uppermost portion of the vessel. The inlet isarranged at the upper end of the vessel. The vessel furthermore comprises an outlet 24a at thebottom right portion of the vessel. A feeding device 22 comprising a vertically arranged pipe 22b isconnected to the inlet 22c, and a plug screw feeder 22a is connected to an upper portion of the pipe22b for feeding pressurized biomass material thereto. The plug screw feeder is provided with uncompressed biomass material at a volume flow V1 per minute.

The system further comprises means for injecting steam, which may be in the form of a set of steaminjection orifices (not shown) arranged around the circumference of the reactor at a lower part of thevessel. Compression means are provided in the form of a rotary scraper 24c rotating around asubstantially vertical axis. The rotary scraper 24c force feeds the material to a discharge screw 24b arranged in the reactor vessel at the bottom thereof. The discharge screw discharges the biomass material through outlet 24a. The compression provided by the rotary scraper (alone or together withoptional compression provided by the discharge screw) results in that the discharged volume flow V2per minute from the reactor is lower than the volume flow V1 per minute into the reactor, whereV1/V2 is within a range from 1.3 to 1.5. A schematically illustrated steam explosion device in theform of a blow valve 25 is connected to the outlet 24a. The description above regarding blow valve15, cyclone separator 17, collection means 16 and pelletizing device 19 with reference to fig. 2 applies to the corresponding devices ref. 25, 27, 26 and 29 in fig. 3.

The description above and the appended drawings are to be considered as non-limiting examples ofthe invention. The person skilled in the art realizes that several changes and modifications may bemade within the scope of the invention. For example, the compressing may take place inside thereactor in a bottom/end portion thereof rather than outside the reactor. Furthermore, steam may beadded at different positions than shown in fig. 2. Further, the feeding device may take on any knownform and does not necessarily need to comprise a plug screw feeder. Furthermore, the suppliedbiomass does not necessarily need to be hardwood, but may be only partly hardwood, or may be another type of biomass altogether.

Claims (19)

1. Method for producing solid combustible material from biomass material, comprising- providing (1) biomass material; continuously introducing (2) the biomass material into a pressurized reactor at a firstvolume rate V1 per minute; subjecting (3) the biomass material to thermal treatment in said pressurized reactor atelevated pressure and temperature by means of adding steam; continuously discharging (5) the biomass material from the pressurized reactor;subjecting the discharged biomass to steam explosion (6), and collecting (7) a solid portion of the steam exploded biomass, which solid portion forms atleast part of said solid combustible material, further comprising, in connection with said continuously discharging (5), compressing (4) the biomass material such that it is discharged at a second volume rate V2 per minute being lower than V1.

2. Method according to claim 1, wherein V1/V2 is within a range from 1.1 to 1.6, preferablyfrom 1.2 to 1.5, and most preferably from 1.3 to 1.5.

3. Method according to any of the preceding claims, wherein said compressing is achieved atleast partly by means of at least one discharge screw.

4. Method according to any of the preceding claims, wherein said compressing (4) takes placeimmediately outside the pressurized reactor as part of said discharging (5).

5. Method according to any of claims 1-3, wherein said compressing (4) takes place in a bottomor end portion of the pressurized reactor.

6. Method according to any of the preceding claims, further comprising forming fuel pellets orbriquettes from said solid combustible material.

7. Method according to claim 4, wherein said wood material has a moisture content within arange from 5 to 10%, and wherein no drying of the solid combustible material is performedprior to said forming pellets or briquettes.

8. Method according to any of the preceding claims, wherein said biomass material comprisesdisintegrated wood material.

9. Method according to claim 6, wherein said disintegrated wood material comprises woodchips and/or wood microchips and/or hammer milled wood and/or saw dust.

10. Method according to claim 7, wherein said wood chips have a size within a range of from 30mm to 125 mm at their largest dimension, preferably from 50 to 100 mm at their largest dimension.

11. Method according to claim 7, wherein said wood microchips have a size within a range from1 mm to 50mm at their largest dimension, preferably from 5 to 30 mm at their largestdimension.

12. Method according to any of claims 5-9, wherein said wood material is at least partlyhardwood material.

13. Method according to any of claims 5-10, wherein said wood material has a moisture contentwithin a range from 5 to 30%, preferable from 5 to 15%.

14. Method according to any of the preceding claims, wherein said thermal treatment isperformed at a pressure of 11.5-25 bar (g).

15. Method according to any of the preceding claims, wherein said thermal treatment isperformed for 4-15 minutes, preferably 5-12 minutes, and most preferably 7-11 minutes.Method according to any of the preceding claims, wherein said thermal treatment isperformed at a temperature of 190-225°C.

16. Method according to any of the preceding claims, wherein the thermal treatment isperformed for a duration and at a temperature such that the severity factor is within a rangefrom 3,5 to 4,9, preferably from 3,7 to 4,2, more preferably from 3,7 to 3,95 or from 3,95 to4,2.

17. Method according to any of the preceding claims as dependent on claim 4 or 5, wherein saidforming comprises forming pellets having a size in a direction perpendicular to itslongitudinal direction of from 5 mm to 15 mm.

18. Method according to any of the preceding claims as dependent on claim 4 or 5, wherein saidforming comprises forming pellets having a density of from 700 kg/m3 to 1200 kg/m3.Method according to any of the preceding claims, wherein said subjecting the biomassmaterial to thermal treatment comprises adding steam at a rate of 0,25 to 0,8 ton per tonbiomass material.

19. System for producing solid combustible material from biomass material, comprising - a pressurized reactor (11; 21); a feeding device (12a-c; 22a-c) configured to continuously introduce biomass material into said pressurized reactor; - means for addition of steam (13a-e) into the reactor (11; 21) to provide continuousthermal treatment of the biomass at elevated pressure and temperature; - a discharging arrangement (14; 24) configured to continuously discharge the biomassmaterial from the pressurized reactor, - a steam explosion device (15; 25) connected to the discharging arrangement and being configured to subject the thermally treated biomass to steam explosion; - collection means (16; 26) configured to collect a solid portion of steam explodedbiomass, which solid portion forms at least part of said solid combustible material,further comprising compression means (14b; 24b-c) for compressing the biomass material ata compression ratio within a range from 1.1 to 1.6, preferably from 1.2 to 1.5, and mostpreferably from 1.3 to 1.5, said compression means being arranged in connection with said discharging arrangement prior to said steam explosion device.