Technical Field

The present invention relates to a method of utilising black liquor lignin, according to the preamble of Claim 1 ,

In this method, at least part of the lignin contained in the black liquor is carbonated after a possible separation to carbon.

The present invention also relates to the use according to Claim 21.

Background

A method of separating lignin from black liquor by precipitation, and use of the lignin product thus generated as fuel, is known. In this respect, state of the art is described in the patent application publications WO2006/031 175 and WO2006/038 863.

In the known solution, the precipitation of lignin is carried out by acidifying black liquor in a two-stage process, in which case carbon dioxide is first fed in in order to at least partly precipitate the lignin, after which sulphuric acid is used for washing the lignin.

One of the problems associated with the use of sulphuric acid is that sulphur compounds are transferred along with the sulphuric acid into the washing solutions of lignin, and if these spent liquors are recycled to the recovery unit, the sulphur compounds end up in the ash of the soda recovery unit. This, in turn, affects the sulphur balance of the pulp mill, in which case correction of the balance requires an increased feed of fresh chemicals.

Use of lignin as fuel is, in turn, associated with poor heat resistance and relatively low carbon content.

Carbonisation of alkali lignin to fine carbon is described in GB publication No. 372 336. There is no mention of the use of this carbon. CN 103131497 describes, in turn, a method of producing carbonated carbon pieces, in which case black liquor is mixed with finely ground logging waste, and the thus obtained mixture is compressed and carbonised in a compression mould, Summary of the Invention

The problem to be solved it is an aim of the present invention to eliminate at least some of the problems described above and to provide a completely new solution for utilising the lignin contained in black liquor.

Characteristic features of the Invention The present invention is based on the idea that the lignin of the spent cooking liquor is subjected to heat treatment in order to convert the lignin into carbon that is essentially free from organic material. The heat treatment is most suitably carried out by carbonisation, for example with wet pyrolvsis (by the hydrothermal carbonisation process). From the product thus generated, in the following also referred to as "lignin carbon", at least a part is separated and subjected to burning wherein the energy comprised in this energy carrier is released.

The lignin carbon can be more preferably used as fuel in a lime rebuming kiln, in which case it replaces partly or totally the fuel conventionally used.

Burning of lignin carbon to finely divided ground material is more preferable. Use in powder form is particularly preferred.

More specifically, the method according to the present invention is mainly characterized by what is stated in the characterizing part of Claim 1. The use according to the present invention is, in turn, characterized by what is stated in Claim 21 ,

Advantages achieved with the Invention

Considerable advantages are ach eved with the present invention. By working according to the present invention, it is possible by carbonation to produce lignin carbon from black liquor, either for 1) only the needs of a Si me reburning kiln or a similar recovery boiler, or for 2) the needs of both a lime reburning kiln or a similar recovery boiler, and for use as a filler, for example for the rubber market. It is possible to run a charring plant continuously with good basic load for the needs of a lime reburning kiln, Market carbon is produced according to sales.

Generally, the calorific value of carbon is at, the level of 30 to 33 GJ/t, i.e. 8.3 to 9.2 MWh/t. This is significantly higher than the calorific value of mixed lignin, the calorific value of which is in the range of 25 to 26 GJ/t, i.e. approximately 7.1 MWh/t.

The lignin carbon used in the present technology is more affordable than the above described lignin which is separated from black liquor, which lignin is for example used after drying as a fuel. The latter is chemically equivaieni to the lignin in black liquor, i .e. it has the same chemical composition. Lignin carbon, in turn, has a different composition: in this case, the organic material is carbonised to carbon. The carbon content of the product is therefore higher than that of the black liquor lignin (typically of the order of 75 % by- weight, versus 60 % by weight for the lignin), and further the heat resistance of lignin carbon is better than the heat resistance of the black liquor lignin, Therefore, the present lignin carbon is even suitable, as such, as both fuel and as a filler or a pigment, it is not possible to use the lignin for the latter purposes, but its potential applications are glues.

In the following, preferred embodiments will be examined in more detail with the help of the accompanying drawing. Figure 1 shows a simplified process flowchart of one embodiment of the new technology. Embodiments

As will appear from the above, in the present solution, in order to utilise the Signin that is contained in the black liquor or a similar Hgnin-bearing solution or suspension, iignin is carbonated to carbon.

Based on this, the process according to the drawing comprises treatment of a cooking liquor used in the pulping in such a way that it is possible to produce Iignin carbon (reference number 10 of the figure) from the Iignin that is dissolved or dispersed in the cooking liquor, at least part of which Iignin carbon can be used to produce energy, especially thermal energy 14, 15. Another part can be used for example as a filler in the rubber industry 16. The reference number 10 means in general those operations which are related to treatment of the initial material, for example by precipitation of Iignin, by HTC treatment and by drying.

As described in the drawing, it is possible to use as feed of the process, for example, spent cooking liquor, sulphate black liquor, of the sulphate cooking. The dry matter percentage of the black liquor may be 25 to 45 %. However, these are only examples of the limits of the dry-matter content,

Most suitably, the Iignin of the black liquor is first precipitated from the black liquor, after which the generated Iignin is carbonated to carbon 10.

In one embodiment 1 to 50 % by weight, typically 5 to 30 % by weight, especially approximately 10 to 20 % by weight, of the Iignin in the black liquor is carbonated after a possible precipitation.

The carbonisation is carried out in unit 10 preferably using the hydrothermal carbonisation process, which will be described in more detail below. in the first embodiment, the initial material used in the carbonisation process is black liquor or a similar Hgnin-bearing dispersion, as such. in another embodiment, the initial material used in the carbonisation process is separated lignin, especially lignin that is separated from black liquor or a similar lignm-bearing dispersion, more preferably a separated lignin that is precipitated from black liquor.

An example of such a lignin is a lignin that is produced by acid precipitation. This type of lignin can be generated by using the LignoBoost™ method.

The precipitation can be a single or multistage process.

Material generated after the first or the only stage of the acid precipitation can be fed to the carbonisation 10.

Typically, the treatment, the purpose of which is to separate the lignin from the black liquor or a similar lignm-bearing dispersion, comprises at least two stages. First, the lignin is precipitated with a gaseous substance, such as carbon dioxide, in the first stage of the precipitation, and then with a mineral acid, such as sulphuric acid, in the second stage of the precipitation. For these process stages, reference is made to what is described in the published application publications WO2006/031 175 and WO2006/038 863, the contents of which are herewith incorporated by reference,

In the present solution, where the lignin is carbonated, it is possible, in the two-stage acid precipitation, to use significantly less acid in the second stage of the precipitation than in prior methods, where the lignin is recovered and then used as such after the separation. Experimentally it has been established that in the present solution, the amount of acid may be even half of what is normally required for precipitation-based lignin production.

It is preferable to feed into the carbonisation 10, precipitated lignin which has been treated with a mineral acid, such as sulphuric acid. This solution is particularly advantageous if it is desired to separate the metals in the lignin, especially alkali metal ions such as sodium, which are detrimental to the use of lignin carbon as fuel.

In a third embodiment, black liquor or a similar lignm-bearing dispersion, and separated lignin which is, for example, precipitated with the methods described above, are used alternating!)'. It is of course possible to use a raw materia! that comprises both black liquor (or a similar lignin containing dispersion) and separated lignin. In particular, mixtures of black liquor and lignin that is precipitated with acids, are used in this case. In these mixtures, the ratio between the lignin contained in the black liquor and the lignin precipitated with acids (based on weight) are usually 1 : 100 ... 300: 1 , especially approximately 1 :50 ... 50: 1 .

A particularly interesting method of carbonating lignin carbon is liquid pyroiysis

(hydrothermal carbonisation, HTC), This process is also called wet pyroiysis. In the hydrothermal carbonisation process, the material to be pyrolysed is treated in an acidic environment, and at an elevated temperature and pressure. In the HTC process, the lignin is chemically transformed. A small amount of hydrogen and oxygen are removed and the chemical bonds break down at the same time.

In one embodiment, the HTC process is carried out in such a way that the pH value of the liquid or the dispersion to be pyrolysed is reduced to below 7, preferably the pH value is reduced to clearly acidic (for example to the pH value of 1 to 6, or even less). In another embodiment, which can be combined with the previous one, the temperature is increased to approximately 150 to 300 °C, especially approximately 180 to 250 °C. In the previous embodiments, the pressure is higher than normal atmospheric pressure, typically approximately 1.5 to 50 bar (a), preferably approximately 2 to 30 bar (a), especially approximately 3 to 25 bar (a), for example approximately 18 to 22 bar (a). In the described conditions, the organic material decomposes and at least approximately 50 % by weight of it, typically at least approximately 60 % by weight, especially at least approximately 70 % by weight, most suitably even 80 % by weight of it is converted into carbon.

In addition, to the organic, pyrolysabJe material being converted into the described percentage of carbon, minor amounts of gas (0.1 to 5 % by weight of the organic material), and the remainder, typically 5 to 20 % by weight, of liquid components, are generated in the hydrothermal carbonisation process. The particular advantages of the hydrothermal carbonisation include the fact that it can be used to treat moist feed without time- consuming and energy-consuming drying. The black liquor can be brought to the carbonisation as such. The separated lignin, if it can be used as solid material, is typically elutriated in water or in acid. The lignin percentage of the dispersion or solution to be treated is most suitably approximately 0. 1 to 50 % by weight, especially approximately 1 to 30 % by weight.

Based on the above, in one embodiment of the present technology:

- lignin carbon is produced either directly from black liquor (or from a similar

solution or suspension that comprises dissolved lignin), or from the lignin that is separated from it,

- the black liquor (or a similar solution or suspension) or the separated lignin is fed to the hydrothermal carbonisation process or a similar carbonisation process where, if necessary, a possible solid material is elutriated or dissolved,

- in the carbonisation process, the pH value of the liquid phase that comprises lignin is reduced, if necessary, by adding acid, and the temperature is increased, either immediately or gradually, to approximately 180 to 250 °C,

- the pressure is simultaneously increased either immediately or gradually to

approximately 20 bar (absolute pressure), and

- the temperature and the pressure are maintained until the desired portion of the lignin has been converted into carbon, after which

- the generated lignin carbon is recovered or brought further for processing.

After the processing, the lignin carbon is dried particularly in such a way that its moisture content is less than 20 % by weight, especially its moisture content is less than

approximately 10 % by weight.

Regardless of the type of the initial material that is fed into the carbonisation, the generated product is typically lignin carbon, the carbon percentage of which is more than 50 % by weight, especially it is approximately 60 to 95 % by weight, for example approximately 65 to 85 % by weight.

The generated lignin carbon is similar to carbon black. Most suitably, the carbon is finely divided. In one embodiment, the lignin is carbonated to finely divided carbon.

In one embodiment, the particle size of the product is smaller than or equal to 50 mesh.

The product can be granulated or used as such, either as a fuel or in filler or pigment applications, in the drawing, the reference number 11 represents an ungranulated carbon storage, the reference number 12 represents a carbon granulation unit, and the reference number 13 a granulated carbon storage.

In one embodiment, lignin carbon that is used as a powdery fuel comprises, in addition to the particles described above, the size of which is smaller than or equal to 50 mesh, also particles, the size of which i s smaller than approximately 80 mesh, for example smaller than approximately 200 mesh, for example smaller than 325 mesh, for example smaller than 400 mesh.

The concept "particle size" described above means the average particle size, which is often marked "ø".. The calorific value of the product generated in the carbonisation is approximately 28 to 35 GJ/t, especiaily approximately 30 to 33 GJ/t. so it is suitable to be used as a fuel. Based on the particle size, it is also possible, after possible additional processing, to use the lignin carbon as a filler or a pigment. The heat resistance of lignin carbon (soot/carbon black) is better than that of the black liquor-lignin - the product does not melt at temperatures of 200 °C or lower.

Lignin carbon is easy to carry in piping also at high temperatures Lignin carbon cannot agglomerate near the burning site, nor during the burning itself, but pulverised fuel firing possible.

The reference number 14 refers to the burning of lignin carbon. Local burning can be carried out in a lime sludge reburning kiln or a power boiler. In one embodiment, the fuel, which is a powdery solid, is transferred from the

ungranulated carbon storage 11 with the conveyors 17, preferably pneumatically, from the storage to burning, According to a preferred embodiment, the burning 14 is carried out as pulverised fuel firing.

The reference number 15 refers to a burning unit, which is located separately from the place of production of the carbon. For example, it is possible to transfer the product from the granulated carbon storage 13 to the burning site in question 15 using bulk transport (as loose material) or in sacks 20. The burning 15 can be carried out in a lime sludge reburning kiln or a power boiler.

In one embodiment, the burning is carried out as pulverised fuel firing. In another embodiment, the burning is carried out in a bubbling fluidised bed or a circulating fluidised bed.

This processing 15 may also case-specifically include storage of the fuel before burning, as well as grinding of it.

In all cases described above, the carbon obtained from the carbonisation can be mixed with another fuel, prior to feeding into the recovery boiler.

Besides that the lignin carbon can be burned as described above, part of it can also be used as a filler or a pigment. In Figure 1 , this use is indicated by reference number 16. It is possible to direct the carbon from the ungranulated carbon storage 11 or,

correspondingly, from the granulated carbon storage 13, for another use, by bulk transport 18, 19, for example pneumatically, Alternatively, the granulated carbon can be transferred in sacks 20 from the granulated carbon storage 13 for other uses 16. Easy handling of the lignin carbon and also burning of it for example as pulverised fuel firing represent important advantages. For filler or pigment purposes the iignin carbon is used, for example, in industrial rubber and other elastomer products, such as conveyor belts, or in vehicles or in heavy-duty rubber tyres, or in thermoplastic p!asiic products, in the example, carbon is supplied using the process according to the drawing 1) only for the needs of the lime sludge rebuming kiln, or 2) for the both the l ime sludge reburning kiln and the rubber market, i.e. the rubber industry needs. The latter case is suitable for situations where the goal is to enter the rubber market, but where the market, entry is slower than the production capacity of the plant. In this case, adequate dimensioning of the precipitation and treatment plant provides process technical support for the time of entering the market. It is possible to am the charring plant continuously with a good basic load for the needs of the lime sludge reburning kiln and to produce market carbon according to sales. In one embodiment, the procedure is that during the first period the Iignin is separated and carbonated only to be used as fuel for the recovery boiler, and during the second period the Iignin is separated and carbonated both to be used as fuel for the recovery boiler and to be used in the production of value added products. At the pulp mill, it is possible to separate the volume needed for the lime sludge reburning kiln (for example 200 GWh/a) and an equal volume for the rubber market, In this case, the burning chamber is still fully manageable regarding the superheating. Possible vapour deficit is easy to correct by increasing the production, if there is a need. Based on the above, in a preferred embodiment, at least as much Iignin is separated from the black liquor of the pulp mill as is necessary to fill, after the carbonisation, the fuel need of the lime sludge reburning kiln

- during continuous operation of the pulp mill, or

- during a pre-selected period of time, or

- during a steady- state operation.

In one embodiment, at least 5 % by weight of all the carbon produced is used as a fuel, especially at least 10 % by weight, for example at least 15 % by weight, Case-specifically, even all the lignin carbon is used as fuel, but usually at maximum 95 % by weight is used as fuel . Typically, approximately 10 to 99 % by weight, for example approximately 20 to 90 % by weight, such as 30 to 75 % by weight is used as fuel.

Industrial applicability

The present invention can be applied in processes which use solutions that comprise dissolved lignin. More preferably, the present invention is suitable for pulp mills and similar plants, where lignin is separated from lignocellulose-bearing raw material, especially fibrous raw material, by dissolving it into a (cooking) liquid. The cooking liquid may be alkaline, neutral or acidic. More preferably, the present invention is applied at sulphate mills, where cooking liquid is available, i.e. black liquor, which comprises approximately 10 to 100 g/i of dissolved lignin.

Part of the lignin carbon which is generated by carbonisation is used as fuel for a lime reburaing kiln or another recovery' boiler, such as a power boiler, for example a heat boiler. Part is also used as such, i.e. in applications where particles of lignin carbon or granules prepared from the carbonisation are utilised without burning, preferably as unmodified, for example to utilise their absorption capacity, for example as activated carbon or as a filler or a pigment, for example in polymer or rubber products.

One embodiment comprises the use of fine carbon, which is generated by wet pyroiysis from the lignin in black liquor, as fuel for a lime reburaing lain.

Reference number list

10 Unit where lignin precipitation, HTC treatment and possible drying

operations are carried out

1 1 Ungranuiated carbon storage

12 Carbon granulation

13 Granulated carbon storage

14 Local burning in a lime sludge returning kiln or a power boiler

15 Burning outside the carbon production site 16 Other industrial use of carbon than burning in a lime sludge rebuming kiln or a power boiler

17 Transfer of carbon to the burner with various conveyors, for example

pneumatically, typically as bulk transport

18 Transfer of carbon by bulk transport

19 Transfer of carbon in sacks or by bulk transport

20 Transfer of carbon by bulk transport (loose) or in sacks

Citation list

WO2006/031 175

WO2006/038863,.

GB 372336

CN 103131497