TECHNICAL FIELD

The invention relates to the field of methods for processing biomass material comprising washing and thermally treating the biomass material followed by steam explosion discharge. The invention also relates to a corresponding system.

BACKGROUND

There is a growing demand for biomass-based fuels, such as pellets or briquettes, to replace or supplement coal or other fossil fuels in power plants. Biomass-based pellets (and briquettes) may be produced with or without thermal treatment of the biomass. Pellets produced without thermal treatment are usually referred to as white pellets. Pellets produced with thermal treatment are usually referred to as black pellets due to their darker color. One advantageous method for thermal treatment is steam explosion. Steam explosion refers to a process step where the material undergoes a rapid/instantaneous pressure decrease. Thus, the hot and softened biomass from the thermal treatment is released or blown from the reactor through a blow valve or orifice, while the pressure drops to an environment with substantially lower pressure, such as below 5 bar, or preferably to substantially atmospheric pressure. The structure of the biomass breaks, partly due to the expanding steam, and partly by the shear forces and impact during the blow through the orifice or valve. Steam explosion treatment simplifies subsequent pelletizing or briquetting due to the (still) soft material being easy to densify. Steam explosion treatment improves the strength of the resulting pellet due to various substances such as lignin and sugars being released during the steam explosion.

Biomass is a limited resource, however. The demand for sustainable fuels increases the price of high- quality fuels, such as wood pellets. Consequently, there is pressure to be able to utilize lower grade feedstocks for instance residual biomass from agriculture, like straw, husks, empty fruit bunch (EFB) etc. One problem with using such lower grade feedstocks is that the resulting fuel/pellets may be of inadequate quality, mainly due to their high content of inorganic material (ash).

Several techniques for improving the fuel quality are used and proposed, such as steam explosion (as mentioned above), washing, hydrothermal carbonization and addition of combustion additives. All these techniques have limitations and presently cannot, using low-grade feedstocks as raw material, provide fuels of sufficiently high quality to fulfill the highest requirements, for instance in high temperature combustion. Thus, there is a need for an improved method for processing biomass. SUMMARY

An object of the invention is to provide a method and a corresponding system which solves or at least improves on the problems mentioned above.

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 processing biomass material is provided. The method comprises washing the biomass material in one or more washing stages, feeding at least a portion of the washed biomass material to at least one pressurized reactor, thermally treating the biomass material at elevated pressure and temperature by means of adding steam to the at least one reactor, discharging the biomass material and blow steam from the at least one reactor by means of steam explosion discharge, separating the blow steam from the discharged thermally treated biomass material, condensing at least part of the separated blow steam to obtain an acidic condensate, and recirculating at least a portion of said acidic condensate to at least one of said washing stages such as to reduce pH of the biomass material during washing.

In other words, the method comprises washing the biomass material in one or more washing stages, thermally treating at least part of the washed biomass material at elevated pressure and temperature in at least one pressurized reactor by means of adding steam thereto, discharging the thermally treated biomass material along with blow steam (steam and vapors released from the biomass material) by means of steam explosion discharge. The discharged material is separated into solid phase and gas phase, i.e., the blow steam is separated from the thermally treated biomass material. At least part, or all, of the separated blow steam is condensed to obtain an acidic condensate. At least part, or all, of the acidic condensate is recirculated to the process upstream of said thermally treatment where it is used (as washing liquid) for washing of biomass raw material.

It is understood that blow steam in this context refers to a mixture of (water) steam and volatile organic compounds released from the biomass material during thermal treatment / steam explosion discharge of the biomass material. More specifically, the blow steam comprises beside steam, the main component, gas-phase acidic compounds like formic and acetic acid, and non-acidic compounds like furfural and methanol derived from hemicelluloses during thermal treatment of the biomass. The acids produced is typically around 3 - 4 % of the mass of the biomass. In the following is the mixture of steam and vapor denoted "blow steam". It is furthermore understood that washing stage refers to a process stage in which a washing liquid is added to the biomass material and spent washing liquid is dewatered from the washed pulp. Adding of washing liquid and dewatering may be carried out by a washing apparatus such as a dewatering screw. It is furthermore understood that hydrolysis reactions will take place in the pressurized reactor during the thermally treating. The thermally treating step may also be referred to as hydrothermal treatment or pre-hydrolysis. Pre-hydrolysis may be carried out to hydrolyze the hemicellulose content of the biomass and is carried out at milder conditions compared to a hydrolysis process in which cellulose is hydrolyzed to sugar.

The biomass material may be of the low-grade type described in the background section (straw, husks, or empty fruit bunches for example), but the invention can equally well be used with higher-grade types of biomasses.

The invention is based on the insight to combine two pretreatment methods (steam explosion and washing) in an innovative way, and more specifically on the insight that (formic and acetic) acid released from the biomass hemicellulose during thermal treatment and steam explosion, may be condensed to form an acidic condensate, which advantageously can be used upstream of the thermal treatment as washing liquid. As explained above, steam explosion simplifies production of the pellets and improve the quality of the pellets. Washing is known to be able to decrease the total amount ash, dissolving harmful ash components, for example sodium (Na), potassium (K) and chloride (Cl). Na and K typically cause slagging and fouling and Cl corrosion problems in a boiler. It is however known that washing with neutral (pH = 6-8) water is not very efficient. Typically, the reduction of alkaline compounds (Na + K) is not fulfilling the highest requirement of fuel (less than 2000 ppm). This can be enhanced by countercurrent stepwise washing, where fresh water is introduced to the biomass in last washing step. Typically, water washing of wheat straw for instance leaves 5000 ppm ash in the washed material. The inventor realized that the acidic condensate is advantageously used as a washing agent/liquid in the washing stage prior to thermal treatment to enhance the washing effect and reduce the ash content. Consequently, a higher-quality fuel can be obtained. Furthermore, the amount of washing water may be reduced. Furthermore, use of combustion additives (for reducing of ash stickiness in a boiler) may be reduced or even eliminated.

Furthermore, the addition of acid upstream of the thermal treatment (during washing) will improve acid catalyzed hydrolyzation in the reactor. The reaction rates of hydrolysis is increased in the thermal treatment step which results in higher processing capacity of a certain reactor volume, and thus results in a more efficient thermal treatment process and typically also a cleaner biomass. Furthermore, the wastewater treatment may be enhanced as the acidic compounds react with inorganic material forming separable salts. The invention is further based in the insight that various, non-acidic compounds are released during thermal treatment. Furfural is such a compound, and it may be recovered from the acidic condensate. In embodiments, the method further comprises collecting vapors from reliefs on the at least one pressurized reactor, wherein the condensing further comprises condensing at least part of the collected vapors to obtain said acidic condensate. Thus, a first stream comprises at least part of the blow steam separated from thermally treated biomass at discharging of the pressurized reactor and a second stream comprises at least part of vapors collected from reliefs on the at least one pressurized reactor. The condensing comprises condensing the first stream (of blow steam) and the second stream (of vapors). In other words, steam/vapors subjected to condensation may originate both from the reactor discharging step and the from reliefs on the pressurized reactor during thermal treatment.

In embodiments, at least a part of the acidic condensate may be subjected to a furfural separation step such as to provide an acidic condensate substantially free from furfural. Processes for furfural separation are known in the art and will not be described in further detail here. Separating furfural from the acidic condensate may be advantageous since a high amount of furfural can affect the process negatively and it is a high value platform chemical which can be sold.

In embodiments, the acidic condensate may be added at a rate such as to reduce the pH of the biomass material to less than 5, preferably less than 4 and most preferably less than 3.

In embodiments, the acidic condensate added to at least one of said washing stages (as washing liquid) at least partly comprises said acidic condensate substantially free from furfural. In other words, the acidic condensate added to at least one of said washing stages may be acidic condensate substantially free from furfural or a mixture of acidic condensate (from the condensing step) and acidic condensate substantially free from furfural (from the furfural separation step).

The washing may be conducted in several steps, i.e., using at least two washing stages arranged in series, and the acidic condensate is preferably added to the last washing stage in said series to obtain counter-current washing of biomass material.

In embodiments, the method further comprises separation of non-acidic chemicals from the acidic condensate prior to using it for washing.

In embodiments, the method further comprises, prior to said feeding, reducing the moisture content of the washed biomass material. The moisture content may be reduced by mechanically dewatering the washed biomass material to obtain a moisture content in the range 35-55 %. Optionally, the washed and dewatered biomass material may be thermally dried to obtain a moisture content in the range 8-25 %.

In embodiments, the method further comprises, prior to feeding the biomass material to the at least one pressurized reactor, separating the biomass material into a first biomass stream and a second biomass stream, which first biomass stream is fed to said at least one reactor and which second stream is added to the discharged thermally treated first stream. In other words, only part of the biomass material is steam exploded. This may be advantageous since the amount of required heating energy is reduced. The share of the biomass that is steam exploded and/ or the severity of the steam explosion process is controlled to receive a suitable flow of acidic condensate for washing of biomass feedstock such that the quality requirements (ash content and durability) of pellets or briquettes are satisfied. Advantageously, this embodiment is combined with the above-mentioned embodiment in such a manner that the biomass material is dewatered and thermally dried before separating.

The method according to the first aspect of the invention may be used for different purposes. For example, the method may further comprise forming fuel pellets and/or briquettes from the discharged thermally treated biomass material (optionally mixed with the above-mentioned second biomass stream). Furthermore, a combustion additive may be added to the at least one pressurized reactor and/or to the discharged thermally treated biomass (optionally mixed with the above-mentioned second biomass stream). Mixing with a combustion additive is described for example in WO2017089648, which is hereby incorporated by reference.

According to a second aspect of the invention, a system for processing biomass material is provided. The system comprises at least one washer arranged to receive the biomass material and to wash biomass material, at least one pressurized reactor arranged to receive the washed biomass material, the at least one reactor being provided with means for adding steam into said at least one reactor for thermal treatment of the biomass material at elevated pressure and temperature. At least one steam explosion discharge device is arranged to discharge the biomass material and blow steam from the at least one reactor. Separating means is connected to the at least one steam explosion discharge device to separate said blow steam from discharged thermally treated biomass material, and at least one condensing device is arranged to receive said blow steam from said separating means and to condense said vapor to obtain an acidic condensate, and a feeding arrangement is arranged to recirculate at least a portion of said acidic condensate to upstream of said at least one reactor. More specifically, the feeding arrangement may be arranged to recirculate at least a portion of said acidic condensate to one or more the at least one washer (for use as washing liquid/agent). The system may comprise at least two washers arranged in series, wherein the feeding arrangement is arranged to recirculate at least a portion of the acidic condensate to the last washer in said series to obtain counter-current washing of biomass material.

In embodiments, the system further comprises means for adding washing water to at least one of the washers, preferably to the last washer in said series. The system may furthermore comprise means for providing catalyzing mineral acid to at least one of the washers, preferably to the last washer in said series.

In embodiments of the system, the system further comprises a system for separation of furfural from acidic condensate to provide an acidic condensate substantially free from furfural for washing of biomass material. In other words, the feeding arrangement may be arranged to receive the acidic condensate substantially free from furfural from the system for separation to provide acidic condensate substantially free from furfural as washing liquid to one or more of the at least one washer. In such embodiments, the feeding arrangement may be arranged to provide solely acidic condensate substantially free from furfural as washing liquid to one or more of the at least one washer, or a mixture of acidic condensate (from the separating device) and acidic condensate substantially free from furfural (from the system for separation).

In embodiments, the system further comprises a drying arrangement comprising at least one mechanical dewatering apparatus and/or at least one thermal drying apparatus, said drying arrangement being arranged to receive washed biomass material from the at least one washer, said at least one pressurized reactor being arranged to receive washed and dried biomass material from the drying arrangement.

In embodiments, the system further comprises a separating arrangement arranged upstream of the at least one pressurized reactor to separate the biomass material into a first biomass stream and a second biomass stream, which first stream is fed to said at least one reactor, the system further comprising a mixing arrangement arranged to mix said second biomass stream from the separating arrangement with discharged thermally treated first biomass stream from the at least one steam explosion discharge device. Advantageously, this embodiment is combined with the above-mentioned embodiment in such a manner that the biomass material is dewatered and thermally dried before separating. In embodiments, the system further comprises at least one pelleting device arranged to receive thermally treated biomass material from the at least one pressurized reactor or mixed biomass from the mixing arrangement to produce fuel pellets.

In embodiments, the system further comprises means for collecting vapors from reliefs on the at least one reactor, said means for collecting being connected to said condensing device to provide collected vapors thereto, said at least one condensing device being arranged to condense said blow steam and said vapors to obtain said acidic condensate.

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

BRIEF DESCRIPTION OF THE DRAWINGS

Above discussed and other aspects of the present invention will now be described in more detail using the appended drawings, which show presently preferred embodiments of the invention, wherein: fig. 1 shows a flow chart illustrating an embodiment of the method according to the first aspect of the invention; and fig. 2 shows a schematic illustration of an embodiment of the system according to the second aspect of the invention.

DETAILED DESCRIPTION

Fig. 1 shows a flow chart illustrating an embodiment of the method according to the first aspect of the invention. The method comprises washing 1 the biomass material using one or more washing stages. The washed biomass is then dewatered mechanically 15, for example with a screw-press before it is thermally dried 16, for example with a low temperature belt dryer. Washed and dried biomass material is separated 2 into a first biomass stream and a second biomass stream, which first stream of washed and dried biomass material is fed 3 to at least one pressurized reactor in which the biomass is thermally treated 4 at elevated pressure and temperature by means of adding steam to the at least one reactor. A combustion additive may be added to the at least one pressurized reactor as part of step 4. The biomass material and steam are discharged 6 from the at least one reactor by means of steam explosion discharge. Blow steam (steam and vapor released from the biomass material) are thereafter separated 7 from the discharged thermally treated biomass material. The method further comprises collecting 5 vapors released from reliefs on the at least one pressurized reactor.

The vapors from the collecting step 5 and blow steam from the separating step 7 are condensed 8 to obtain an acidic condensate. The acidic condensate is optionally subjected to a furfural separation step 9 to provide an acidic condensate substantially free from furfural, which is recirculated to upstream of the thermally treatment by adding the acidic condensate to at least one of the washing stages (as washing liquid) such as to reduce pH of the biomass material during washing. The acidic condensate is added at a rate such as to reduce the pH of the biomass material to less than 3. In this embodiment, the recirculated acidic condensate is substantially free from furfural, if the optional step 9 is used. In other embodiments, a mixture of acidic condensate from steps 8 and 9 can be recirculated. The second stream of washed and dried biomass from the separation step 2 is mixed 11 with (added to) the discharged thermally treated biomass material, which thereafter is pelletized 14.

Fig. 2 shows a schematic illustration of an embodiment of the system according to the second aspect of the invention. The system comprises three washers lOla-c arranged in series, wherein biomass feedstock is fed to the first washer 101a wherein washing liquid is added to the last washer 101c in the series to obtain counter-current washing of biomass material. The washed biomass from the last washer 101c is dewatered mechanically to 35-55 % moisture with for example a screw-press 116, prior to drying it to 8-25 % moisture for example with a low temperature belt dryer 117. The washed and dry product from the dryer 117 is conducted to a divider 115 to separate the washed biomass material into a first biomass stream and a second biomass stream, which first stream of washed and dried biomass material is fed to the pressurized reactor 104, which reactor is provided with means 104a for adding steam (schematically shown as a steam injector provided with an adjustable valve) into the at least one reactor for thermal treatment of the biomass material at elevated pressure and temperature. The reactor 104 is furthermore (optionally) provided with an injector 104b for adding a combustion additive. At the bottom of the pressurized reactor, a discharge screw is provided which feeds the biomass material towards a steam explosion discharge device 106 schematically shown as a valve. Separating means in the form of a cyclone 107 is connected to the at least one steam explosion discharge device 106 to separate blow steam released with the discharged biomass material. Blow steam from the cyclone 107 is conducted to a condensing device 108.

The system further comprises means for collecting vapors from reliefs on the at least one reactor, shown schematically as ref. 118, to the condensing device 108. The condensing device condenses vapor to obtain an acidic condensate which is conducted to a system 109 (optional) for separation of furfural to provide an acidic condensate substantially free from furfural. A feeding arrangement 114 is arranged to receive the acidic condensate. The feeding arrangement 114 is connected to the last washer 101c in the series to provide acidic condensate thereto. The acidic condensate is added (as washing liquid) to the washer 101c together with fresh washing water to obtain an acidic washing. Optionally, it may be possible to further acidulate the washing process by adding a catalyzing mineral acid to the washers. Such an option is of interest with heavily contaminated feedstocks (high ash content) if the acidic condensate flow from steam explosion is too low to obtain good washing.

The cyclone 107 is furthermore connected to a mixing device 116 to provide the solid portion (the thermally treated biomass) thereto. In the mixing device 116, the second stream of washed and dried (but not thermally treated) biomass from divider 115 is mixed with the thermally treated biomass to provide a biomass mixture which is provided to a pelleting device 114 which outputs fuel pellets.

Typical operating conditions of the embodiments described above with reference to figures 1-2 are as follows:

Temperature in reactor: 140-225 °C

Pressure: corresponding pressure 2 - 30 bar(g)

Residence time: 1 min - 3 hours, preferably 3-20 minutes.

Although not explicitly shown or described above, it is understood that the system in figure 2 comprises feeding devices such as feeding screws and/or pumps where appropriate to convey the biomass material and acidic condensate between the shown devices.

It is further noted that the devices forming part of the described embodiments of the system, and used in the embodiments of the method, (such as washers, screw-presses, dryers, pressurized reactors, condensing arrangements, systems for furfural separation, pelleting devices, cyclones, steam explosion discharge devices and dividers for separating streams of biomass slurries) are all well known in the art and will therefore not be described in further detail herein.

The description above and the appended drawings are to be considered as non-limiting examples of the invention. The person skilled in the art realizes that several changes and modifications may be made within the scope of the invention. Further, the furfural separation system and/or the means for addition of combustion additive may be omitted. Further, the number of washers/washing stages may vary if the acidic condensate is added to the last washer in the series. Further, the reactor does not need to be vertical as shown in fig. 2 but may be horizontal instead.