A method for producing a thermally stabilized lignin

Abstract

The present invention relates to a method for producing a thermally stabilized lignin, the method comprises the steps of providing lignin and subjecting the 5 lignin to heat treatment in a fluidized bed reactor, wherein the fluidized bed reactor comprises a fluidizing media and the velocity of the fluidizing media is between 0,1-1,0 m / s and the final temperature during heat treatment is above 200°C. The invention also relates a stabilized lignin, a carbon enriched material and the use of the carbon enriched material. 10

Description

[0001] A METHOD FOR PRODUCING A THERMALLY STABILIZED LIGNIN

[0002] Field of the invention

[0003] The present invention relates to a method for obtaining a thermally stabilized lignin, a stabilized lignin produced according to the method and a carbon enriched material produced from the stabilized lignin.

[0004] Background

[0005] Lignin is an aromatic polymer, which is a major constituent in e.g. wood and one of the most abundant carbon sources on earth. In recent years, with the development and commercialization of technologies to extract lignin in a highly purified, solid and particularized form from the pulp-making process, it has attracted significant attention as a possible renewable substitute to primarily aromatic chemical precursors currently sourced from the petrochemical industry.

[0006] Today, the most commercially relevant source of lignin is kraft lignin, obtained from hardwood or softwood through the kraft process. The lignin can be separated from alkaline black liquor using for example membrane- or ultrafiltration. One common separation process is described in W02006031175 A1. In this process lignin is precipitated from alkaline black liquor by addition of acid and then filtered off. The lignin filter cake is in the next step re-slurried under acidic conditions and washed. The lignin filter cake may then be dried and / or pulverized.

[0007] Lignin can be used as a starting material to obtain a carbon enriched material. During such a process, the lignin is converted to carbon by heating. The process normally comprises several different heat treatment steps, one pre-heating step which will make the lignin more suitable for the following second heat treatment step which will convert the lignin to carbon. The preheating step is preferably carried out in an oxidative atmosphere. Pre-heating may also be referred to as thermal stabilization and is carried out at a temperature lower than the temperature required for conversion of the lignin to a carbon enriched material. Examples of carbon enriched materials are charcoal, activated carbon and amorphous carbon such as hard carbon.

[0008] One problem, with heat treatment of lignin is that lignin undergoes plastic deformation / melting, aggressive swelling and foaming during heating. There is therefore important to control the temperature during the heating steps to avoid problems related to the thermoplastic behavior of lignin.

[0009] Thus, there is still a need for an improved process of obtaining a thermally stabilized lignin, that is suitable for conversion to a carbon enriched material, and where the method is more efficient than prior art methods.

[0010] Summary of the invention

[0011] It is an object of the present invention to provide a method for producing a thermally stabilized lignin in a controlled and efficient way.

[0012] It is a further object of the present invention to provide an improved method for producing a stabilized lignin more suitable for conversion to a carbon enriched material.

[0013] It is a further object of the present invention to provide a stabilized lignin suitable for conversion to a carbon enriched material as well as a carbon enriched material.

[0014] The above mentioned objects, as well as other objects as will be realized by the skilled person in light of the present disclosure, are achieved by the various aspects of the present disclosure.

[0015] According to a first aspect, the present invention relates a method for producing a thermally stabilized lignin, the method comprises the steps of providing lignin and subjecting the lignin to heat treatment in a fluidized bed reactor, wherein the fluidized bed reactor comprises a fluidizing media and the velocity of the fluidizing media is between 0,1 -1,0 m / s and the final temperature during heat treatment is above 200°C.

[0016] Thermal stabilization of lignin is carried out at a temperature lower than the temperature required for conversion of the lignin to a carbon enriched material. The term “thermally stabilized lignin” as used herein refers to a lignin that has been subjected to a heat treatment step (or thermal stabilization step) to improve the stability of the lignin and in particular its ability to be subjected to further heat treatment without melting / swelling.

[0017] It has surprisingly been found that it is possible to produce a thermally stabilized lignin by heat treatment of the lignin in a fluidized bed reactor in a more efficient way. The heat treatment in the fluidized bed reactor can be controlled in a better way making the heat treatment both more efficient but also safer. The stabilization reaction of lignin releases energy at certain temperature ranges. With a fluidized bed reactor, it has been found possible to control the temperature within the reactor and keep the temperature at desired level. In this way it is possible to control the process time at certain temperature ranges in a very exact and controlled manner. Thereby, thermal runaway of the process can more easily be prevented.

[0018] The fluidized bed reactor, operates by suspending lignin within a flow of a fluidizing media, creating a mixture that behaves similarly to a fluid

[0019] The lignin is introduced at the top or from the side of the fluidized bed reactor. The added lignin forms, together with the fluidizing media, a bed inside the fluidized bed reactor.

[0020] The fluidizing media is introduced from the bottom of the reactor at a sufficient velocity to lift and suspend the lignin within the reactor. As the fluidizing media flows through the bed of lignin, it causes the lignin to become suspended and mix freely. The upward force of the fluidizing media makes the lignin to behave as a fluid and the bed to expand. The bed expansion may be between 15-50%. This fluid-like state allows for excellent mixing. Within the reactor, the suspended lignin provides a large surface area, enabling efficient reactions. The fluidizing media flows through a gas distribution plate that distributes the gas evenly over the entire cross section of the fluidized bed reactor. The gas distribution plate is also designed to prevent the lignin to pass through the plate to ensure that the lignin remains in the reactor. It is preferred that the reactor is in the form of a cylinder.

[0021] The heating of the lignin in the fluidized bed reactor is done by the feeding of heated fluidizing media to the fluidized bed reactor which transfers the heat to the lignin inside the bed. In addition, the reactor walls can also be heated. Due to the efficient mixing of the bed and the fluidizing media, the heat transfer is very efficient. A fluidized bed reactor can have one or multiple heating zones, variating geometry, and be operated in either continuous or batch mode.

[0022] The main temperature control of the bed is done with regulation of the temperature of the fluidizing media. If the temperature of the reactor increases too fast, the temperature of the fluidizing media is decreased to slow down the process. In this way it is possible to control the reaction temperature and time at each temperature range in a very controlled way. The temperature within the reactor can be controlled at a range of ±5 °C, preferably ± 2°C and even more preferred at a range of ±1 °C.

[0023] The fluidized bed reactor preferably comprises a fluidizing media that comprises oxygen, thus the heat treatment is performed in an oxidative atmosphere. It is preferred that the fluidizing media is a gas, preferably an oxygen containing gas such as oxygen gas or air. Oxidative species that can react to cross-link lignin are preferably present in the fluidizing media. The heat treatment may beside oxygen also be carried out for example in the presence of iodine, ozone, nitrogen dioxide, nitrobenzene, hydrogen peroxide and peracetic acid. Alternatively, any suitable oxidative species may be supplied in a nitrogen atmosphere. The lignin may be cross-linked by heat treatment in the oxidative atmosphere. This will increase the thermal stability of the lignin during eventual subsequent heat treatment. In particular, the processability of the lignin in terms of avoiding melting / swelling and retaining shape and dimension during any subsequent heat treatment is improved by the heat treatment step.

[0024] The velocity of the fluidizing media is between 0,1 -1,0 m / s, preferably between 0,3-0, 8 m / s and even more preferred 0,4-0, 6 m / s. It has been found that by using a sufficiently high velocity of the fluidizing media in the fluidized bed reactor it is possible to maintain a stable temperature at higher temperatures without the problem of exothermic reactions causing a thermal runaway. The needed velocity also depends on the density and the size and shape of the lignin in order for the fluidizing media to be able to create the desired bed expansion. With velocity of the fluidizing media is meant the superficial velocity of the fluidizing media. The superficial velocity is calculated by dividing the volumetric gas flow rate of the fluidizing media by the total cross-sectional area of the bed reactor. The cross-sectional area of the bed is measured at the fluidized zone of the reactor.

[0025] The increased control of the reactions that occurs during heat treatment of the lignin in the fluidized bed reactor also makes it possible to increase the final temperature of the heat treatment. It has been found that it is possible to increase the final temperature during the heat treatment to at least above 200°C, even more preferred above 250°C, preferably above 280°C, more preferably above 300°C, even more preferably above 320°C or more preferred above 350°C by the use of a fluidized bed reactor. The final temperature of the heat treatment is preferably between 200-400°C, more preferred between 250-400°C, more preferred between 280-400°C, more preferred between 300-390°C and even more preferred between 320-380°C. With final temperature is meant the average temperature used during the last 5 minutes of the heat treatment. The heat treatment of the lignin can be carried out continuously or in batch mode. The ability to use higher temperatures in the heat treatment and be able to maintain it for a desired retention time, increases the desired reactions between oxygen and lignin.

[0026] The bed temperature is increased during the heat treatment, such as a stepwise increase of the temperature or using a temperature gradient. Lignin can be fed at ambient temperature, and it may be possible to start the lignin stabilization at a fluidizing media temperature between 100-200°C and then increase the temperature to achieve desired temperature curve and the desired final bed temperature.

[0027] It has also been found that by being able to increase the temperature of the lignin stabilization step, it is possible to increase the overall thermal yield of the process to obtain a carbon enriched material. A higher overall thermal yield indicates a more efficient process, with less energy wasted and more effective utilization of heat to achieve the desired treatment results. This concept is important in optimizing heat treatment processes to minimize energy consumption and maximize productivity.

[0028] The heat treatment is carried out for at least 30 minutes, i.e. the residence time of the lignin inside the fluidized bed reactor is at least 30 minutes. In one embodiment, the heat treatment is carried out for at least 1 hour, or at least 1.5 hours. Preferably, the heat treatment is carried out for less than 12 hours, or less than 6 hours. It is preferred that the heat treatment is carried out for a period between 30 minutes and 6 hours, preferably between 1 hour and 3 hours. The heat treatment may be carried out at the same temperature throughout the entire heating stage or may be carried out at varying temperatures, such as a stepwise increase of the temperature or using a temperature gradient.

[0029] Another way to control the heat treatment process and the exothermic reactions, to avoid thermal runaway, is by diluting or removing the volatiles formed during the heat treatment. By heat treating lignin different kind of volatiles, such as carbon dioxide, methanol, methyl formate, dimethyl disulfide and / or methoxyphenols, are released due to the thermal degradation of the lignin. The volatiles can be removed from the fluidized bed with the fluidizing media passing through the bed. It is intended throughout the present disclosure that the term "lignin" refers to any kind of lignin.

[0030] Examples of said lignin are, but are not limited to, lignin obtained from vegetable raw material such as wood, e.g. softwood lignin, hardwood lignin, and lignin from annular plants. Also, the lignin can be chemically modified.

[0031] Preferably, the lignin has been purified or isolated before being used in the method according to the present invention. The lignin may be isolated from black liquor and optionally be further purified before being used in the method according to the present invention. The purification is typically such that the purity of the lignin is at least 90%, preferably at least 95%, more preferably at least 98%, based on the dry weight of the lignin material. Thus, the lignin used in the method of the present invention preferably contains less than 10%, preferably less than 5%, more preferably less than 2% impurities, such as cellulose, carbohydrates and inorganic compounds, based on the dry weight of the lignin.

[0032] The lignin used in method according to the present invention may be obtained through different extraction methods such as an organosolv process or a kraft process. The lignin may also be obtained from processes such as steam explosion or acidic pre-treatment followed by enzymatic hydrolysis.

[0033] Preferably, the lignin used in the method according to the present invention is kraft lignin, i.e. lignin obtained through the kraft process. The kraft lignin may be obtained from hardwood or softwood. The lignin may be obtained by the process disclosed in W02006031175 A1 commonly referred to as the LignoBoost process. Typically, this process involves the steps of precipitation of lignin from alkaline black liquor by acidification; separation of the precipitated lignin; and re-slurrying the lignin under acidic conditions at least once. After washing, the precipitated lignin is obtained as a moist filter cake comprising loosely aggregated solid lignin particles. The obtained lignin may be dried and pulverized and thus provided as solid particles. Thus, the lignin may be in form of a powder. The drying of the lignin powder is carried out by methods and equipment known in the art. The dried lignin in powder form may have a moisture content of less than 45 wt%. Preferably, the moisture content of the lignin is less than 25 wt%, preferably less than 10 wt%, more preferably less than 8 wt%. The moisture content of the lignin may be at least 1 wt%, such as at least 5 wt%. The temperature during the drying is preferably in the range of from 80 to 160°C, more preferably in the range of from 100 to 120°C.

[0034] The lignin may also be agglomerated. The term “agglomerated lignin” as used herein refers to macroscopic particles in turn comprising clustered smaller particles of lignin. By providing the lignin in agglomerated form, a more compact and hard material is achieved. Hard agglomerates are advantageous during subsequent processing as they can resist physical impact during processing. In addition, the tendency for dusting is reduced when lignin is provided in agglomerated form. The agglomerated lignin of the present invention is prepared by a method comprising a step of compacting lignin. This means that the agglomerated lignin is not in the form of spontaneously aggregated secondary lignin particles formed during e.g. precipitation of lignin. The agglomerated lignin preferably has a bulk density in the range of from 0.3 to 1.0 g / cm3, preferably between 0.4 to 0.8 g / cm3, or even more preferred between 0.5 to 0.7 g / cm3. Due to the production process to produce agglomerated lignin which often involves both compacting, mixing and / or kneading, the agglomerated lignin is compacted and thus has a higher bulk density than a lignin material that has been loosely aggregated. The agglomerated lignin preferably has a moisture content of less than 30 wt%, preferably less than 20wt%, more preferred less than 10wt% and even more preferred less than 5wt%. By the present invention it is possible to also treat lignin with high moisture contents.

[0035] The lignin preferably has an average particle size between 0,1-5 mm, preferably between 0,5-2 mm. The average particle size of lignin is important to make the heat treatment in the fluidized bed as efficient as possible. It is preferred that the lignin particles have a uniform particle size.

[0036] In the present application, the average particle size is defined as the volume average particle size (Dvso). This value refers to the maximum particle size below which 50% of the volume of the sample exists. The particle size is in the context of the present invention taken to be the diameter of the particle. The average particle size may be determined using for example laser diffraction. In the context of the present invention, the diameter of a particle is the equivalent spherical diameter of the particle, if the particle is not spherical. The equivalent spherical diameter is the diameter of a sphere of equivalent volume.

[0037] Depending on the use of the obtained thermally stabilized lignin, the material may be pulverized to a smaller particle size. The method according to the first aspect may therefore comprise an additional step of pulverizing the stabilized lignin. Pulverization is preferably carried out after drying.

[0038] Pulverization may be carried out by any method as known in the art. Some examples include a cutting mill, blade mixer, ball-mill, impact mill, hammer mill and / or jet-mill. Optionally, fine / coarse particle selection by classification and / or sieving may be performed subsequent to the pulverization. After pulverization, the average particle size of the obtained lignin powder may for example be in the range of from 1 to 100 pm, or from 1 to 50 pm, or from 1 to 20 pm.

[0039] The method further comprises the step of measuring the temperature of the bed of the fluidized bed reactor. This can be done by placing one or more thermocouples inside the bed or by any other temperature measurement method to measure the temperature of the bed during the reaction in the fluidized bed reactor. Fluidized bed temperature can also be estimated by measuring the temperature of the fluidizing media after it has passed through the bed. The temperature of the fluidized bed is controlled by the temperature and / or flow of the fluidizing media. In it thus possible to control and easily adjust the bed temperature to follow e.g. a pre-defined heating curve or to have a desired retention time at a certain temperature. The temperature of the bed is preferably uniform. With uniform is meant that the temperature of the bed is within ±10 °C, preferably within ±5 °C. The uniform mixing of lignin and the fluidizing media makes it possible to achieve a uniform temperature within the bed.

[0040] The method may further comprise the steps of subjecting the thermally stabilized lignin to a heat treatment at one or more temperatures in the range of from 300 to 1500°C, wherein the heat treatment is carried out for a total time in the range of from 30 minutes to 10 hours, so as to obtain a carbon enriched material. The carbon enriched material of the present invention is suitable for use in secondary batteries. The carbon enriched material, preferably in powder form, of the present invention is preferably used as an active material in a negative electrode of a non-aqueous secondary battery, such as a lithium-ion battery or sodium-ion battery. When used for producing such a negative electrode, any suitable method to form such a negative electrode may be utilized. In the formation of the negative electrode, the carbon enriched material may be processed together with further components. Such further components may include, for example, one or more binders to form the carbon enriched material into an electrode, conductive materials, such as carbon black, carbon nanotubes or metal powders, and / or further Li storage materials, such as graphite or lithium. For example, the binders may be selected from, but are not limited to, poly(vinylidene fluoride), poly(tetrafluoroethylene), carboxymethylcellulose, natural butadiene rubber, synthetic butadiene rubber, polyacrylate, poly(acrylic acid), alginate, etc., or from combinations thereof. Optionally, a solvent such as e.g. 1-methyl-2-pyrrolidone, 1-ethyl-2-pyrrolidone, water, or acetone is utilized during the processing.

[0041] The second aspect of the present invention relates to a stabilized lignin obtainable by the method according to the first aspect. The reactions between oxygen and lignin in the heating step improves the processability of the lignin in the following process steps forming a harder carbonized layer on the surface of the lignin. Furthermore, the glass transition temperature (Tg) of lignin increases after the heating step, so that the Tg of the thermally stabilized lignin is higher than the lignin prior to heating. In this case the Tg of the lignin is very high since the process is done in an efficient process often at very high temperatures.

[0042] It is preferred that the stabilized lignin preferably has a bulk density in the range of from 0.4 to 0.8 g / cm3, or from 0.5 to 0.7 g / cm3. The heat treatment might lead to a slight increase or decrease in bulk density compared to the agglomerated lignin prior to heat treatment. The bulk density of the thermally stabilized lignin will however preferably remain within the same range as prior to the heating step.

[0043] The third aspect of the present invention relates to a negative electrode for a non-aqueous secondary battery comprising the carbon enriched material obtainable by the method according to the first aspect. The use of stabilized lignin according to the method makes it possible to produce a negative electrode material with improved properties.

[0044] The fourth aspect of the present invention relates to the use of the carbon enriched material obtainable by the method according to the first aspect as an active material in a negative electrode of a non-aqueous secondary battery.

[0045] In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

Claims

Claims1. A method for producing a thermally stabilized lignin, the method comprises the steps of providing lignin and subjecting the lignin to heat treatment in a fluidized bed reactor, wherein the fluidized bed reactor comprises a fluidizing media and the velocity of the fluidizing media is between 0,1-1 ,0 m / s and the final temperature during heat treatment is above 200°C.

2. The method according to claim 1 wherein the fluidizing media comprises oxygen.

3. The method according to any of the preceding claims wherein the velocity of the fluidizing media is between 0,3-0, 8 m / s, preferably between 0,4-0, 6 m / s.

4. The method according to any of the preceding claims, wherein the final temperature of the heat treatment is above 250°C, preferably between 250-400°C, more preferred between 300-400°C and even more preferred between 320-380°C.

5. The method according to any of the preceding claims wherein the heat treatment in the fluidized bed reactor is carried out for a period of at least 30 minutes.

6. The method according to any of the preceding claims wherein at least some of the volatiles formed during heat treatment are diluted and / or removed from the fluidized bed reactor.

7. The method according to any of the preceding claims wherein the lignin is in form of a powder.

8. The method according to any of the preceding claims wherein the lignin is agglomerated lignin.

9. The method according to any of the preceding claims wherein the lignin has an average particle size between 0,1-5 mm, preferably between 0,5-2 mm.

10. The method according to any one of the preceding claims, wherein the lignin is kraft lignin.11.The method according to any of the preceding claims wherein the lignin and the fluidizing media forms a bed inside the fluidized bed reactor and the temperature of the bed is measured.

12. The method according to claim 11 wherein the temperature of the bed is controlled by adjusting the flow and / or the temperature of the fluidizing media.

13. The method according to any of the preceding claims wherein the lignin and the fluidizing media forms a bed in the fluidized bed reactor and the temperature of the bed is uniform.

14. The method according to any of the preceding claims wherein the method further comprises the steps of subjecting the thermally stabilized lignin to a heat treatment at one or more temperatures in the range of from 300 to 1500°C, wherein the heat treatment is carried out for a total time in the range of from 30 minutes to 10 hours, so as to obtain a carbon enriched material.

15. A stabilized lignin produced according to any of the claims 1-13.

16. A negative electrode for a non-aqueous secondary battery comprising the carbon enriched material obtainable by the method according to any one of claims 14 as an active material.

7. Use of the carbon enriched material obtainable by the method according to any one of claims 14 as an active material in a negative electrode of a non-aqueous secondary battery.