A method of treating and a system for treating a lignin residue

Abstract

The present technology relates to a method of treating a lignin residue. The method comprises the steps of pretreating a lignin residue whereby a lignin-rich liquid stream is provided, contacting the lignin-rich liquid stream with an oxidant in an oxidation step whereby at least one oxidation product is formed in a liquid comprising oxidation products, separating the at least one oxidation product from an oxidized stream, circulating the remaining stream comprising at least one non-recovered product to a digester, and digesting the non-recovered product to form biogas.

Description

A METHOD OF TREATING AND A SYSTEM FOR TREATING A LIGNIN RESIDUE TECHNICAL FIELD

[0001] The present invention relates to a method of treating a lignin residue. The present invention further relates to a system for treating a lignin residue?BACKGROUND

[0002] Lignin is a complex organic polymer found in the cell walls of plants, particularly in wood and bark. It provides structural support, rigidity, and resistance to decay, making it essential to the plant’s overall strength and durability. Lignin is second only to cellulose as the most abundant organic material on Earth. It plays a crucial role in the water transport system of vascular plants by reinforcing the cell walls and allowing them to withstand the pressure of water movement.

[0003] Lignin is found in varying proportions depending on the type of plant. In softwoods, lignin makes up about 24-35% of the oven-dry weight, while in hardwoods it constitutes around 17-25 %. This variation in lignin content affects the properties and uses of the wood. Despite its abundance, lignin has relatively few industrial applications beyond being used as a fuel. However, ongoing research aims to find new ways to utilize lignin in various industries, including the production of biofuels, chemicals, and materials.

[0004] Lignin residue is typically produced as a byproduct of various industrial processes that involve the extraction or processing of cellulose from plant materials. One common method is the kraft process used in paper manufacturing, where lignin is separated from cellulose fibres through chemical pulping. Another process is the sulphite pulping method, which also isolate lignin during the production of paper and other cellulose-based products. Additionally, lignin can be obtained from bioethanol production where lignocellulosic biomass is broken down to fermentable sugars, leaving lignin as a residue. These processes not only provide valuable cellulose but also generate significant amounts of lignin, which can be further utilised, treated, or processed for various applications. Further, in biogas production processes in which lignocellulosic materials are used as a raw material, lignin residue is obtainable as a byproduct.SUMMARY

[0005] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims. It is an aim of the present invention to overcome at least some of the problems described above and provide a method of treating and a system for treating a lignin residue. According to a first aspect of the invention, there is provided a method of treating a lignin residue. According to a second aspect of the invention, there is provided a system for treating a lignin residue.

[0006] By means of the invention it has surprisingly been found that the yield of oxidation products and the yield of biogas from lignin obtainable from a lignocellulosic material can be improved.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIGURE 1 is a flow chart illustrating a method of treating a lignin residue according to at least some embodiments

[0008] FIGURE 2 is a flow chart illustrating a method of treating a lignin residue according to at least some embodiments.

[0009] FIGURE 3 is a flow chart illustrating a method of treating a lignin residue according to at least some embodiments.

[0010] FIGURE 4 is a flow chart illustrating a method of treating a lignin residue according to at least some embodiments.

[0011] FIGURE 5 is a flow chart illustrating a method of treating a lignin residue according to at least some embodiments.

[0012] FIGURE 6 is a flow chart illustrating a method of treating a lignin residue according to at least some embodiments.

[0013] FIGURE 7 illustrates an oxidation process of a method of treating a lignin residue according to at least some embodiments.

[0014] FIGURE 8 illustrates an oxidation process of a method of treating a lignin residue according to at least some embodiments.

[0015] FIGURE 9 illustrates a separation process of a method of treating a lignin residue according to at least some embodiments.

[0016] FIGURE 10 illustrates a separation process of a method of treating a lignin residue according to at least some embodiments.

[0017] FIGURE 11 illustrates a separation process of a method of treating a lignin residue according to at least some embodiments.EMBODIMENTS DEFINITIONS

[0018] For the purposes of embodiments of the present invention the term, “lignin residue” shall be taken to mean side stream material or waste material from a plant that processes lignin-containingbiomass, such as lignocellulosic material. Lignin residue may for example come from biological processes such as the biological digestion of lignin-containing biomass, a digestate from biomass processing, or for example from cooking processes such as kraft pulping or sulphite pulping.

[0019] “Lignin-rich liquid stream” shall be taken to mean a liquid stream in which the concentration of lignin is greater than in the lignin residue. The lignin-rich liquid stream has a dry solids content in the range of 1-30 wt%?

[0020] The present invention relates to a method of treating a lignin residue. The lignin residue is treated to provide a lignin-rich liquid stream which is in turn contacted with an oxidant to form a liquid comprising oxidation products. At least one oxidation product is separated from the liquid comprising oxidation products to provide a remaining oxidised stream. The remaining oxidised stream is circulated to a digester, where at least one component of the stream is digested to form biogas.

[0021] FIGURE 1 illustrates a method and system according to at least some embodiments. A lignin residue 10 from biomass processing is directed to a pretreatment unit 100 into which a washing solution 20 is provided to wash the lignin residue 10, whereby non lignin solids 30 are removed from an outlet of the pretreatment unit 100 providing a lignin-rich liquid 300 which is removed from the pretreatment unit 100 via an outlet and introduced into an oxidation unit 101 through an inlet adapted for feeding the lignin rich liquid 300 into the oxidation unit 101. The oxidation unit 101 has a further inlet adapted to feed oxidant 40 into the oxidation unit 101, where it is contacted with the lignin-rich liquid. The lignin rich liquid 300 is typically contacted with oxidant under alkaline conditions. An alkali 50 is fed into the oxidation unit 101 through an inlet adapted for feeding alkali 50 into the oxidation unit 101, where the alkali 50 is contacted with the lignin-rich liquid 300 and the oxidant 50, whereby a gas byproduct is produced. The gas 60 is vented through an outlet and optionally recovered for further processing. The liquid comprising oxidation products 310 is then fed from an outlet of the oxidation unit through an inlet into a separation unit 102. In the separation unit impurities 80 are separated and removed through an outlet, methanol 90 is separated and recovered through an outlet. Liquid 70 is removed from an outlet of the separation unit and the remaining composition 320 is directed through an outlet from the oxidation unit to a digestion unit 103 in which the composition 320 is contacted with anaerobic micro-organisms which digest at least a part of the composition 320 to form biogas 330 which is recovered through an outlet. Remaining undigested residue 200 is recovered and discarded or optionally directed to further processing, e.g. the undigested residue 200 may be processed into fertiliser or soil improver or compost. In a further embodiment the remaining undigested residue 200 is directed to the pretreatment unit 100. Optionally the pretreatment unit 100 comprises a filtration unit 104 (as shown in Figure 2). In one embodiment the pretreatment unit comprises a feed tank 105 as shown in figures 3, 4, and 5. In an embodiment the pretreatment unit 100 comprises a mechanical pretreatment unit 106 as shown in figures 4, 5 and 6. Suitably, the separation unit 102 comprises one or more separators as shown in Figure 9.

[0022] FIGURE 2 illustrates a method and system according to at least some embodiments. A lignin residue from biomass processing 10 is directed for pretreatment in a pretreatment unit comprising afiltration unit 104 in which the lignin residue 10 is contacted with a washing liquid 20. Solids 30 are filtered from the lignin residue 10, whereby a lignin rich liquid 300 is obtained, which is in then directed to further processing in an oxidation unit 101 (not shown).

[0023] FIGURE 3 illustrates a method and system according to at least some embodiments. A lignin residue from biomass processing 10 is directed for pretreatment in a pretreatment unit comprising a feed tank 105. In the feed tank 105 the lignin residue 10 is contacted with an alkaline chemical solution 110, whereby lignin in the lignin residue 10 is dissolved to provide a solution comprising dissolved lignin 120. The solution comprising dissolved lignin 120 is then directed to a filtration unit 104, where solids 30 are filtered from the solution comprising dissolved lignin 120 and removed from the filtration unit 104 and from the pretreatment unit whereby a lignin-rich residue 300 is obtained and directed to an oxidation unit 101 (not shown).

[0024] FIGURE 4 illustrates a method and system according to at least some embodiments. A lignin residue from biomass processing 10 is directed for pretreatment in a pretreatment unit 100 comprising a mechanical pretreatment unit 106 where the residue undergoes a mechanical treatment e.g. steam explosion or grinding. Mechanically-treated lignin residue 130 is then directed from the mechanical pretreatment unit 106 to a feed tank 105 where it is contacted with an alkaline chemical solution 110 whereby lignin in the mechanically-treated lignin residue 130 is dissolved to provide a mechanically and chemically-treated residue comprising dissolved lignin 140. The residue comprising dissolved lignin 140 is then directed to a filtration unit 104, where solids 30 are filtered from the solution comprising dissolved lignin 140 and removed from the filtration unit 104 and from the pretreatment unit whereby a lignin-rich residue 300 is obtained and directed to an oxidation unit 101 (not shown).

[0025] FIGURE 5 illustrates a method and system according to at least some embodiments. A lignin residue from biomass processing 10 is directed for pretreatment in a pretreatment unit comprising a feed tank 105 in which the lignin residue 10 is contacted with an alkaline chemical solution 110, whereby lignin in the lignin residue is dissolved to provide a solution comprising dissolved lignin 120. The solution comprising dissolved lignin 120 is then directed to a mechanical pretreatment unit 106 for mechanical treatment e.g. steam explosion or grinding. Chemically and mechanically treated lignin residue 150 is then directed from the mechanical pretreatment unit 106 to a filtration unit 104, where solids 30 are filtered from the solution comprising dissolved lignin 140 and removed from the filtration unit 104 and from the pretreatment unit whereby a lignin-rich residue 300 is obtained and directed to an oxidation unit 101 (not shown).

[0026] FIGURE 6 illustrates a method and system according to at least some embodiments. A lignin residue from biomass processing 10 is directed for pretreatment in a pretreatment unit comprising a mechanical pretreatment unit 106 for mechanical pretreatment e.g. steam explosion or grinding. Mechanically-treated lignin residue 130 is then directed from the mechanical pretreatment unit to a filtration unit 104 in which it is washed with a washing solution 20 and filtered to remove solids 30which are removed from the filtration unit 104 and the pretreatment unit 100 to provide a lignin-rich liquid 300, which is directed to an oxidation unit 101 (not shown).

[0027] FIGURE 7 illustrates a method and system according to at least some embodiments. A lignin-rich liquid 300 obtainable from a pretreatment unit 100 (not shown) is directed to an oxidation unit 101 in which it is contacted with an oxidant 40 fed into the oxidation unit 101 through a first inlet and with an alkali 50 fed into the oxidation unit through a second inlet. The oxidation unit 101 is equipped with a vent for venting gas 60 and a further outlet for removal of a liquid comprising oxidation products 310 for delivery to further processing, such as a separation unit 102 (not shown).

[0028] FIGURE 8 illustrates a method and system according to at least some embodiments. A lignin-rich liquid 300 obtainable from a pretreatment unit 100 (not shown) is directed to an oxidation unit 101 in which it is contacted with an oxidant 40 fed into the oxidation unit 101 through an inlet. The oxidation unit 101 is equipped with a vent for venting gas 60 and a further outlet for removal of a liquid comprising oxidation products 310 for delivery to further processing, such as a separation unit 102 (not shown).

[0029] FIGURE 9 illustrates a method and system according to at least some embodiments. A liquid comprising oxidation products 310 obtainable from an oxidation unit 101 (not shown) is directed into a first separator 107 of a separation unit comprising a first separator 107 and a second separator 108. In the first separator 107, e.g. a distillation column, non-methanolic components 160 are removed from the liquid comprising oxidation products 310 to provide a first methanolic fraction 170. The first methanolic fraction 170 is directed from the first separator 107 to the second separator 108 from which purified methanol 180 is obtained and directed to further processing, leaving a second fraction comprising nonrecovered oxidation products 320, which products 320 are directed to further processing.

[0030] FIGURE 10 illustrates a method and system according to at least some embodiments. A liquid comprising oxidation products 310 obtainable from an oxidation unit 101 (not shown) is directed into a separator 107, e.g. a distillation column of a separation unit. In the separator 107 methanol 190 is recovered from the liquid comprising oxidation products 310 and directed to further processing. A stream comprising non-recovered oxidation products 320 is obtained and directed to a further process.

[0031] FIGURE 11 illustrates a method and system according to at least some embodiments. A liquid comprising oxidation products 310 obtainable from an oxidation unit 101 (not shown) is directed to a pre-evaporation unit 109 of a separation unit comprising a pre-evaporation unit, a first separator 107 and a second separator 108. In the pre-evaporation unit 109 pre-evaporated methanol 210 is recovered from the liquid comprising oxidation products 310. The remaining liquid 220 is directed to a first separator 107, e.g. a distillation column, where non-methanolic components 160 are removed providing a first methanolic fraction 170 which is directed to a second separator 108. In the second separator 108, e.g. a distillation column, purified methanol 180 is recovered. A further fraction comprising non-recovered oxidation products 320 is directed to a further process.

[0032] As described above, the present technology relates to a method of treating a lignin residue. In an embodiment the method comprises the steps of pretreating a lignin residue, whereby a lignin-rich liquid stream is provided, contacting the lignin-rich liquid stream with an oxidant in an oxidation step whereby a liquid comprising oxidation products is formed, separating at least one oxidation product from the liquid comprising oxidation products to provide a remaining stream, circulating the remaining stream comprising at least one non-recovered product to a digester, and digesting the non-recovered product to form biogas. The pretreating comprises separating fibrous material and / or non lignin solids from the lignin residue.

[0033]

[0034] There are various sources of lignin as described above. In an embodiment the lignin residue is a side stream material or a waste material from a plant that processes lignin-containing biomass, such as lignocellulosic material. In a further embodiment the lignin residue comes from biological processes such as the biological digestion of lignin-containing biomass, a digestate from biomass processing. In one embodiment the lignin residue comes from cooking processes such as kraft pulping or sulphite pulping. In embodiments, any source of lignin is suitable for providing a lignin residue. In one embodiment the lignin residue is obtainable from one or more lignocellulosic materials selected from the group consisting of hardwood, softwood, com stover, sugarcane bagasse, wheat straw, rice husks, switchgrass, miscanthus, olive pits, coconut shells, and palm kernel shells. In an embodiment the ligninrich liquid stream has a dry solids content in the range of up to 30 wt %, by weight of the stream, for example 1 to 30 wt %, preferably of 2 to 30 wt %, suitably 5 to 28 wt %, by weight of the stream. While it is possible to oxidise lignin in lignin-rich liquid streams with dry solids contents of more than 30 wt %, oxidation becomes more challenging as the solids content increases and the oxidant to lignin ratio increases unnecessarily.

[0035] In a further embodiment the dry solids content has a lignin content in the range of 15 to 40 wt %, by weight of the dry solids, preferably 18 to 35 wt %, suitably 20 to 30 wt % by weight of the dry solids, for example 21, 22, 23, 24, 25, 26, 27, 28 or 29 wt % by weight of the dry solids

[0036] In one embodiment the pretreatment step is chemical followed by washing and / or mechanical followed by washing. In one embodiment washing is carried out after a chemical pretreatment and after a mechanical pretreatment, i.e. there are two washings. In a further embodiment washing is carried only after the chemical and / or mechanical steps are completed, i.e. there is one washing only. In embodiments, washing displaces lignin bound to fibres into the liquid phase making the lignin more accessible to oxidation. In an embodiment washing is carried out a neutral pH for example at a pH of about 7, meaning a pH in the range of 6.5 to 7.5, suitably in the range of 6.8 to 7.2. The washing water including the lignin is then found in the lignin-rich stream which goes to oxidation and then further after removal of at least one oxidation product into the digester. In an embodiment washing is carried out in filtration equipment, for example in a filter press or other such suitable filtration equipment. In one embodiment the washing water has an adjusted pH, preferably the pH of the washingwater is more than 9.0, typically in the range of 9.1 to 13.0, suitably 10. O to 12.0, for example 11.0. In cases in which the pH is too low, the pH is typically adjusted with one or more of the alkaline chemicals mentioned herein, preferably NaOH.

[0037] In a further embodiment the pretreatment step comprises mechanical steps only. In such an embodiment the lignin residue is subject to a mechanical treatment including one or more steps selected from drying, steam explosion and grinding, before water is added and the mechanically pretreated lignin residue is directed to oxidation in an oxidation unit.

[0038] In one embodiment the pretreatment step comprises contacting the lignin residue with an alkali, preferably selected from the group consisting of NaOH, KOH, an alkaline sidestream from the plant and a mixture thereof. For example, sidestreams from sulphite pulp mills containing alkali, typically NaOH are readily available. In one embodiment the pretreatment step comprises separating fibrous material from the lignin residue. The separating can be carried out by various methods. In an embodiment the fibrous material is separated from the lignin residue by filtration. In an embodiment the lignin residue is contacted with a washing solution. The washing can take place after a mechanical pretreatment step, after a chemical pretreatment step or after both a mechanical pretreatment step and after a chemical pretreatment step. After washing solid fibrous particles are removed by filtration in embodiments. Suitable filtration methods include pressure filtration (such as filter press), vacuum filtration, membrane separation (micro-, ultra- and nanofiltration). Typically, filtration is carried out by screw or belt press; since screw and belt presses are often found in biogas plants.

[0039] The pretreatment method according to embodiments is selected based on qualities of the lignin residue to be treated. As the lignin content of the residue to be pretreated increases, the need for effective chemical pretreatments and washings decreases.

[0040] As described above, after the pretreatment step, the lignin-rich stream undergoes an oxidation step. In an embodiment, the oxidant is selected from the group consisting of air, oxygen, ozone, peroxide, hydrogen peroxide and a combination thereof. Preferably, the oxidant is selected from the group of air and oxygen gas. The mild oxidative properties of air and oxygen gas reduce the risk that methanol present in the lignin-rich stream is oxidized to methanoic acid. In a further embodiment the oxidation step further comprises contacting the lignin-rich stream with an alkali, preferably selected from the group consisting of NaOH, KOH, an alkaline sidestream from the plant and a combination thereof. In one embodiment the alkali is waste from the pretreatment step.

[0041] After the oxidation step, at least one oxidation product is separated and recovered from the oxidized sidestream, which is a liquid comprising oxidation products. In one embodiment the separating comprises distilling, scrubbing, stripping and / or filtering for recovering the at least one oxidation product from the oxidized stream. Oxidation products formed in the oxidation step are typically alcohols and carboxylic acids, for example in an embodiment at least one oxidation product is methanol.

[0042] After recovering the at least one oxidation product from the oxidized stream, the remainder of the stream is directed to a digester. In the digester at least one non-recovered product in the remainder of the oxidized stream is contacted with anaerobic bacteria.

[0043] In an embodiment the at least one non-recovered product is contacted with anaerobic microorganisms such as anaerobic bacteria and / or fungi and / or archaea in the digester. In a further embodiment the anaerobic microorganisms feed on the at least one non-recovered product producing biogas and optionally other products. The biogas is typically recovered from the digester and directed to further processes. In an embodiment, the anaerobic microorganisms are selected from the group consisting of hydrolytic bacteria, acidogenic bacteria, acetogenic bacteria, methanogenic bacteria and a mixture thereof. Hydrolytic bacteria are useful in converting organic macromolecules such as carbohydrates, proteins and lipids into sugars, amino acids and fatty acids, which in turn may be converted into organic acids such as acetic acid, propionic acid, formic acid, lactic acid, butyric acid, and succinic acid, alcohols and ketones such as ethanol, methanol, glycerol and acetone, and acetate, carbon dioxide and hydrogen by acidogenic bacteria. Fatty acids and alcohols are readily converted into acetate, carbon dioxide, and hydrogen by acetogenic bacteria, and acetate, carbon dioxide, and hydrogen are converted by methanogenic bacteria into methane and carbon dioxide.

[0044] As hereinbefore described, the lignin residue is obtainable from a divers range of sources. In one embodiment, at least a part of the lignin residue is obtained from the anaerobic digester.

[0045] The present technology further relates to a system for treating a lignin residue. In an embodiment the system comprises a pretreatment unit equipped with an inlet for receiving a lignin residue from a plant, and an outlet for a lignin-rich liquid stream, an oxidation unit equipped with an inlet for receiving a lignin-rich liquid stream from the outlet of the pretreatment unit, an inlet for an oxidant, and an outlet for an oxidized stream, a separation unit equipped with an inlet for receiving an oxidized stream from the oxidation unit, at least one separation device, an outlet for recovered methanol, and an outlet for non-recovered oxidation products.

[0046] In an embodiment the pretreatment unit comprises a feed tank. The feed tank preferably comprises an inlet for a lignin residue, an inlet for a chemical solution, and an outlet for a solution comprising dissolved lignin. Optionally the feed tank is equipped with an agitator.

[0047] In an embodiment the pretreatment unit comprises a filtration unit. The filtration unit is adapted to receive a lignin residue or to receive a feed of a solution comprising dissolved lignin from an outlet of feed tank, or to receive mechanically and chemically treated residue from a mechanical pretreatment unit. The filtration unit comprises an inlet for washing water and an outlet for solids washed from the chemically and mechanically treated residue. Typically the filtration unit comprises a filtration system such as filter press that includes a washing system, and a filtrate tank for the lignin-rich liquid, The filtration unit comprises an outlet for a lignin-rich liquid.

[0048] In a further embodiment the pretreatment unit comprises a mechanical pretreatment unit. In one embodiment the mechanical pretreatment unit is adapted to receive a lignin residue and mechanically treat for example grind solids in the lignin residue or subject the lignin residue to steam explosion to provide a mechanically treated lignin residue via an outlet to an inlet of a feed tank or to an inlet of a fdtration unit. In one embodiment the mechanical treatment unit is adapted to receive a composition comprising dissolved lignin from a feed tank, and mechanically treat the composition comprising dissolved lignin by mechanically grinding the composition or subjecting the composition to steam explosion to provide a chemically and mechanically treated residue to a filtration unit.

[0049] In some systems oxidation is carried out with the addition of an alkali. Thus, in an embodiment the oxidation unit comprises an inlet for a lignin rich liquid, an inlet for oxidant, a gas vent for venting gas such as oxygen or gas and an outlet for a composition comprising oxidation products. In one embodiment the oxidation unit comprises an inlet for a lignin-rich liquid, an inlet for an oxidant and in inlet for an alkali, and an outlet for a composition comprising oxidized products. In an embodiment The oxidation unit further comprises a gas vent for venting gas such as oxygen gas or air. In a further embodiment the oxidation unit is equipped with an agitator (not shown) for agitating the content of the oxidation unit and a heat exchanger (not shown) for adjusting the temperature inside the oxidation unit. Typically, the heat exchanger is configured to lower the temperature in the oxidation unit. Heat removed from the oxidation unit by the heat exchanger is preferably transferred for use in further processes.

[0050] The separation unit comprises at least one separator, or one or more separators or more than one separator. In one embodiment the separators are selected from distillation column, stripping column and scrubber. In one embodiment the separators are distillation columns, in one embodiment the separators are stripping columns. In one embodiment the separators are scrubbers. In a further embodiment the first separator is different to a subsequent separator, e.g. a first separator may be a distillation column and a second separator may be a scrubber or a stripping column. In a further embodiment one or more of the separators are equipped with a heat exchanger for heating or cooling the separators as required. In one embodiment heat is provided to the separators from heat recovered from the oxidation unit. In a further embodiment, heat is provided to one separator to heat recovered from a further separator. In embodiments the separation unit further comprises a pre -evaporation unit. The preevaporation unit is configured to receive a composition comprising oxidation products from an oxidation unit. The pre -evaporation unit has an outlet for pre-evaporated methanol recovered from the composition comprising oxidation products and an outlet for the remaining liquid connected to an inlet of a separator. In embodiments a first separator is configured to remove impurities from the remaining liquid to provide methanol with reduced impurities, which is directed to a further separator. In the second separator, purified methanol is recovered through a first outlet. Non-recovered oxidation products are directed through a second outlet to further processing.

[0051] In one embodiment the system further comprises a digester for receiving non-recovered oxidation products from the separation unit. The digester has an outlet for biogas and an outlet forundigested residues. In an embodiment the outlet for undigested residues is connected to an inlet to a pretreatment unit configured for receiving a lignin residue. In an embodiment the digester is selected from the group consisting of batch digester, continuous stirred tank reactor (CSTR), upflow anaerobic sludge blanket reactor (UASB), and hybrid digester.

[0052] In a further embodiment the pretreatment unit further comprises an inlet for addition of a chemical. As described above in relation to embodiments of the first aspect, a pretreatment step may involve a chemical step, for example contacting the lignin residue with an alkali. The inlet for the addition of a chemical allows the introduction to the pretreatment unit of a chemical for such a chemical step.

[0053] In one embodiment the pretreatment unit further comprises a mechanical separation device. In an embodiment the mechanical separation device is present to separate fibrous material from the lignin residue. Various mechanical separation devices can be adapted to this purpose, for example in an embodiment the separation device is selected from the group consisting of grinder, crusher, shredder, chipper, refiner and steam explosion equipment

[0054] In an embodiment the oxidation unit comprises a further inlet for addition of a chemical. Typically, the chemical is alkaline, preferably selected from the group consisting of NaOH, KOH, an alkaline sidestream from the plant, and a combination thereof.

[0055] As described above, the separation unit comprises a separation device.

[0056] The desired temperature inside the digester depends on the anaerobic microorganisms present therein for example mesophilic methanogenic bacteria thrive between 20°C and 45°C and digest at a temperature in the range of 30°C to 38°C, and thermophilic methanogenic bacteria digest at a temperature in the range of 49°C to 57°C. Other suitable bacteria mentioned above may thrive at different temperatures. Thus, in an embodiment a heat exchanger for adjusting the temperature in the digester is positioned between the separation unit and the digester. In an embodiment the heat exchanger is adapted to adjust the temperature in the digester to a temperature in the range of 20°C to 57°C, preferably 30°C to 49°C, suitably 38°C to 45°C, for example 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56 or 57°C.

[0057] The following examples illustrate embodiments and advantages thereof described above.Oxidation of digestate lignin

[0058] Straw-based digestate sample from a biogas plant was pretreatment with three different methods to produce liquors suitable for oxidation. The different methods can be seen in table 1. Mechanical pretreatment contained drying and grinding the digestate to smaller particles and dissolving to water. The water and alkali washes included dissolving the digestate into the washing medium andfiltrating the solution with a mesh bag to obtain a liquor. The wash pretreatment was done using deionised water and NaOH solution with two different concentrations. Warm bath was applied during two of the washing pretreatments before the filtration steps.

[0059] After the pretreatment the liquors were subjected to oxidation. To study the effect of alkali, oxidation was done with and without alkali addition. For the tests with alkali addition, IM NaOH was added to the sample to reach pH 13. The tests without alkali addition were done at pH 10. In both scenarios, the solution was oxidized with oxygen at 90 °C temperature and under 24 bar pressure for 120 minutes. The oxidation conditions of each sample are shown in the table below.Table 1. Oxidation conditions of digestate lignin.

[0060] The production of carboxylic acids was analysed with HPLC method and the production of methanol with GC-FID method. For carboxylic acids, five acids were chosen to be analysed: acetic acid, formic acid, oxalic acid, glycolic acid and lactic acid. The yields of produced carboxylic acids and methanol are shown in the tables below.Table 2. Acids yield of digestate lignin oxidation (kg / t lignin).Table 3. Methanol yield of digestate lignin oxidation.Oxidation of lignin residue from kraft pulp process

[0061] Oxidation of lignin residue from a kraft pulp process was studied. A sample of pure lignin was originally in a powdery form but was pretreated into finer powder. Around 80 g of the powder was then dissolved into 1 litre of water to make it suitable for oxidation.

[0062] The effect of alkali addition was also studied with kraft lignin. In samples with aadded alkali, IM NaOH was added. Both samples with and without additional alkali were oxidized under three different pressures: 12, 17 and 19 bar. Oxidation was carried out at a temperature of 90 °C for 120 minutes using oxygen. The oxidation conditions of the samples are shown in the table below.Table 4. Oxidation conditions of kraft lignin.

[0063] The production of carboxylic acids was analysed with HPLC method and the production of methanol with GC-FID method. Five carboxylic acids were analysed: acetic acid, formic acid, oxalic acid, glycolic acid and lactic acid. The yields of produced carboxylic acids and methanol are shown in the tables below.Table 5. Acids yield of kraft lignin oxidation (kg / t lignin).Table 6. Methanol yield of kraft lignin oxidation.Anaerobic digestion

[0064] The oxidized digestate-based liquors were subjected to biodegradability testing to study the potential of biogas production. The test was done with a standard inoculum and unadapted anaerobicbacteria. The pH of the samples was set around 7. The production of biogas was measured as volume and is shown in the tables below.Table 7. Biogas potential of oxidized digestate per t lignin.

[0065] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0066] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0067] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0068] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the preceding description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding ofembodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0069] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0070] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.

[0071] As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two or more elements are joined by “and” or “or”, mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements.INDUSTRIAL APPLICABILITY

[0072] At least some embodiments of the present invention find industrial application in treating lignin residues, whereby not only traditional oxidation products are recovered for use as raw materials in the production of further chemicals and chemical products, but also biogas for use in fuels may be recovered. Lignin residues may come from various industries such as the wood processing industry, the pulp and paper industry and / or from agriculture. Agricultural residues such as wheat straw, rice husks, com stover, sugarcane bagasse, olive stones , broccoli stems, river cane etc. may be processed in embodiments of the invention.REFERENCE NUMBERS

Claims

CLAIMS1. A method of treating a lignin residue comprising the steps of:• pretreating a lignin residue, whereby a lignin-rich liquid stream is provided,• contacting the lignin-rich liquid stream with an oxidant in an oxidation step whereby a liquid comprising oxidation products is formed,• separating at least one oxidation product from the liquid comprising oxidation products to provide a remaining stream,• circulating the remaining stream comprising at least one non-recovered product to a digester, and• digesting the non-recovered product to form biogascharacterized in that the pretreating comprises separating fibrous material from the lignin residue.

2. The method according to claim 1, wherein the pretreatment step is chemical and / or mechanical followed by washing.

3. The method according to claim 1 or 2, wherein the oxidation step further comprises contacting the lignin-rich stream with an alkali, preferably selected from the group consisting of NaOH, KOH, an alkaline sidestream from the plant and a combination thereof.

4. The method according to any of the preceding claims, wherein the pretreatment step comprises contacting the lignin residue with an alkali, preferably selected from the group consisting of NaOH, KOH, an alkaline sidestream from the plant and a mixture thereof.

5. The method according to any of the preceding claims, wherein the oxidant is selected from the group consisting of air, oxygen, ozone, peroxide, hydrogen peroxide and a combination thereof.

6. The method according to any of the preceding claims, wherein the separating at least one oxidation product from the liquid comprising oxidation products to provide a remaining stream comprises distilling, scrubbing, stripping and / or filtering for recovering the at least one oxidation product from the liquid comprising oxidation products.

7. The method according to any of the preceding claims, wherein the at least one non-recovered product is contacted with anaerobic bacteria in the digester.

8. The method according to any of the preceding claims, wherein the lignin residue is obtained from the digester.

9. A system comprising:• a pretreatment unit equipped witho an inlet for receiving a lignin residue from a plant, ando an outlet for a lignin-rich liquid stream;• an oxidation unit equipped witho an inlet for receiving the lignin-rich liquid stream from the outlet of the pretreatment unit,o an inlet for an oxidant, ando an outlet for a liquid comprising oxidation products;• a separation unit equipped witho an inlet for receiving the liquid comprising oxidation products from the oxidation unit,o at least one separator,o an outlet for a recovered oxidation product, ando an outlet for a remaining stream; and• a digester equipped to receive the remaining stream and produce biogas.

10. The system according to claim 9 wherein the pretreatment unit further comprises an inlet for addition of a chemical.

11. The system according to claim 9 or 10, wherein the pretreatment unit further comprises a mechanical separation device.

12. The system according to any of claims 9 to 11, wherein the oxidation unit comprises a further inlet for addition of a chemical.

13. The system according to any of claims 9 to 12, wherein the separator is selected from the group consisting of distillation column, stripping column and scrubber.• The system according to any of claims 9 to 13, wherein the digester is equipped withan inlet for receiving the remaining stream from the separation unit,• an outlet for biogas, and• an outlet for undigested residues, optionally connected to an inlet of the pretreatment unit (100) for receiving lignin residue (10).

14. The system according to any of claims 9 to 14, wherein between the separation unit and the digester is positioned a heat exchanger for adjusting the temperature in the digester.

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