EP3523404A1

EP3523404A1 - Method for hydrothermal carbonization of sludge in chemical pulp mills

Info

Publication number: EP3523404A1
Application number: EP17858813.3A
Authority: EP
Inventors: Fredrik Lundqvist; Erik ODÉN; Fredrik ÖHMAN
Original assignee: C Green Technology AB
Current assignee: C Green Technology AB
Priority date: 2016-10-05
Filing date: 2017-09-26
Publication date: 2019-08-14
Also published as: BR112019006748A2; EP3523404A4; WO2018067055A1; SE540135C2; CL2019000893A1; SE1651305A1; JP2019537674A

Abstract

A method and system for the treatment of sludge in a chemical pulp mill including a step of hydrothermal carbonization (HTC) of a sludge stream, wherein said sludge stream is fed into a HTC arrangement and subjected to elevated temperature and pressure, producing at least one solids fraction, at least one liquid fraction, and at least one gaseous fraction; wherein said solids fraction is fed to a power boiler of said pulp mill and burned to generate steam; said liquid fraction is combined with weak black liquor from the pulp mill and fed to a black liquor evaporator of said pulp mill, and the resulting concentrated black liquor is burned in a recovery boiler of said pulp mill.

Description

Method for hydrothermal carbonization of sludge in chemical pulp mills

Technical field

[0001 ] The present disclosure relates generally to the treatment of sludge generated in chemical pulp mills, and in particular to a method and system involving a step of hydrothermal carbonization of said sludge, feeding the resulting solids and liquid fractions to the power boiler and recovery boiler respectively, and thus integrating the sludge handling in the steam generation for the pulp mill.

Background

[0002] Sludge, a slurry of solids and water, including dissolved material, is what remains after wastewater treatment in municipal and industrial wastewater treatment plants. Industrial wastewater treatment plants treat water effluents from different industrial processes, for example pulp and paper mills, industrial food production facilities etc. All wastewater treatment plants in use generate sludge that needs to be handled in one way or the other. The sludge is either recovered directly from the plant after dewatering (aerobic sludge) or first treated

anaerobically for biogas production where part of the sludge is digested and the remainder is recovered as anaerobic sludge, also called biosludge.

[0003] Large quantities of wastewater are generated in the production of pulp and paper. These must be properly handled in order to avoid severe pollution of the environment. Current wastewater treatment in pulp and paper mills combines sedimentation, chemical precipitation, flotation and biological treatment where the specific combination of techniques is determined by the local conditions.

Unavoidably, the wastewater treatment generates large volumes of sludge.

[0004] The handling of sludge in chemical pulp and paper mills, in particular surplus sludge from biological treatment of liquid effluent, also known as

biosludge, is a considerable problem and carries significant specific costs per ton sludge treated. There are today three main solutions in use, and all these are associated with specific disadvantages. [0005] The sludge can be incinerated in the power boiler of the pulp mill, contributing to the production of steam necessary for the pulp and paper making process. This has the advantage of avoiding that the inorganic content of the sludge is introduced into the closed pulp mill cycle. With suitable treatment of flue gases and ashes, harmful chemicals and heavy metals can be neutralized or even recovered. Sludge with high concentrations of organic and/or biological

components is however generally very difficult to dewater. The water content is frequently so high that the net heating value if incinerated in a power plant is very low or even negative. Combustion of sludge will therefore increase the load on the power boiler as well as require support fuel.

[0006] Further, the handling of wet sludge is problematic due to the exposure to pathogens, smell and the formation of poisonous hydrogen sulfide gas. Thermal drying of the sludge consumes about the same amount of heat as the latent heat in the sludge. Thus, this is not an economically attractive alternative. It is therefore fair to say that sludge is currently viewed as a biofuel of poor quality due to its high water content, and consequently some pulp mills treat it solely as a disposal problem.

[0007] Another alternative is combustion in the recovery boiler. Sludge is first hydrolysed with heat and alkali and then mixed with black liquor before feeding the mixture to the evaporators to increase its dry solids content before combustion in the recovery boiler. An advantage of this alternative is that a multi-effect evaporation plant can be used to dewater the sludge before combustion in the recovery boiler, and the energy content of the sludge is utilized. This however often requires a pre-hydrolysis treatment where alkali is charged. The ash from the sludge contains inert components such as phosphorus and silica which will accumulate in the lime cycle and give rise to an increased make-up lime demand. Additionally, the amount of spent lime that has to be handled will increase. In the end, this alternative is therefore associated with significant specific handling costs per ton sludge treated. [0008] In addition, the sludge contains significant amounts of other non-process elements (NPEs) such as aluminum, calcium, iron, potassium and chlorides which can cause problems in the pulp mill, such as corrosion, fouling, worse dewatering properties of green liquor dregs and lime mud etc. Each of these is bound to significantly increase the operating costs and maintenance requirements of the pulp mill.

[0009] Finally, the sludge can be designated for external use, for example as landfill or landfill cover, as fertilizer or as soil replacement products. This however represents a costly and environmentally less attractive treatment, as it includes the transport of large volumes of material with high moisture content. Additionally, there will be high emissions of nitrous oxide and methane to the atmosphere as the sludge decomposes. Methane is a greenhouse gas, and although it doesn't linger as long in the atmosphere as carbon dioxide, it is initially far more

devastating to the climate because of how effectively it absorbs heat. It follows that methane emissions must be minimized.

[0010] Hydrothermal carbonization (HTC) also known as wet torrefaction is a thermochemical process used in the production of charred matter similar in composition to coal. It involves subjecting a wet, carbohydrate feedstock, such as sludge, to an elevated temperature (180 °C - 350 °C) and pressure (up to 2.- 5 MPa) in a closed system. The resulting product is frequently referred to as bio char, a product which is friable, hydrophobic, and has a significantly increased energy density compared to the starting material.

[001 1 ] WO 2015/025076 (Valmet Technologies Oy) relates to the application of a HTC process in a pulp mill. According to the application, lignin is separated from a lignin containing liquid medium, such as pulp mill black liquor, and subjected to further treatment. The method comprises at least the following steps: a) a precipitation stage, wherein a pH lowering agent is added to the lignin containing slurry for precipitating lignin, b) followed by a first separation stage, wherein the precipitated lignin is separated as a lignin cake from the remaining liquid phase of the lignin containing slurry, c) a suspending stage, wherein the lignin cake is suspended for obtaining a lignin suspension, d) a hydrothermal carbonization stage, wherein the lignin suspension is treated for obtaining a slurry of carbon containing material, and e) a second separation stage, wherein the carbon containing material is separated from the slurry.

Summary

[0012] It has been found that hydrothermal carbonization of chemical pulp and paper mill sludge, in particular biosludge, can be effectively integrated in the chemical pulp and paper mill provided that some non-intuitive adaptations of the process are performed. The resulting system is energy efficient, contributes favorably to the generation of steam for the pulp and paper making process, and eliminates a considerable problem in that the sludge volumes are significantly reduced or even eliminated.

[0013] A first aspect relates to a method for treatment of sludge in a chemical pulp mill including a step of hydrothermal carbonization (HTC) of a sludge stream, wherein

- said sludge stream is fed into a HTC arrangement comprising a HTC reactor and subjected to elevated temperature and pressure, producing at least one solids fraction, at least one liquid fraction, and at least one gaseous fraction;

- said at least one solids fraction is fed to a power boiler of said pulp mill and burned to generate steam;

- said at least one liquid fraction is combined with weak black liquor from the pulp mill and fed to a black liquor evaporator of said pulp mill, and

- the resulting concentrated black liquor is burned in a recovery boiler of said pulp mill.

[0014] According to an embodiment of said first aspect, said at least one gaseous fraction from said HTC arrangement is combined with air and the resulting mixed gas fed into the power boiler. [0015] According to another embodiment of said first aspect, freely combinable with the above, the liquid fraction is subjected to oxidation before it is combined with black liquor.

[0016] According to another embodiment of said first aspect, freely combinable with the above, part of the steam generated by the power boiler is used to heat one or more of the HTC arrangement, the HTC reactor or the inflow of sludge to said reactor and/or arrangement.

[0017] In the above embodiments, the elevated temperature in said HTC step is a temperature in the interval of 180 - 350 °C, preferably 180 - 300 °C, and most preferably 180 to 230°C.

[0018] According to an embodiment, in order to ensure and optimize the enrichment of NPEs in the solid product, the pH in the hydrothermal carbonization reaction is controlled, most preferably to a pH above 5.

[0019] According to another embodiment, freely combinable with the above embodiments, the chemical pulp mill operates according to the kraft process (the sulfate process). Preferably the sludge in question is the biosludge from the treatment of wastewater from said pulp mill. The sludge can also be a combination of sludge from the pulp mill and sludge from other sources, for example from municipal waste water treatment.

[0020] A second aspect relates to a system for treatment of sludge from a chemical pulp mill, comprising a hydrothermal carbonization (HTC) arrangement, comprising a HTC reactor, wherein said system further comprises

- an arrangement for introducing said sludge into said reactor;

- a heater for heating said sludge;

- at least one separation arrangement in or downstream of said HTC reactor for producing at least one solids fraction, at least one liquid fraction, and at least one gaseous fraction; - a feeding arrangement for feeding said at least one solids fraction to a power boiler of said pulp mill; and

- a feeding arrangement for feeding said at least one liquid fraction to a black liquor evaporator of said pulp mill.

[0021 ] According to an embodiment of said second aspect, the system comprises an arrangement for combining said at least one gaseous fraction with air and feeding the resulting mixed gas into said power boiler.

[0022] According to another embodiment of said second aspect, freely combinable with the above, the system comprises a heater arranged to receive steam from said power boiler. Preferably said heater is adapted to heat said HTC reactor or to heat the inflow of sludge to said reactor.

[0023] According to another embodiment of said second aspect, freely combinable with the other embodiments, the system comprises a reactor for oxidation of the liquid fraction, as well as auxiliary equipment for feeding an oxidizing agent into said reactor, and equipment for recovering heat generated in said oxidation.

[0024] According to another embodiment of said second aspect, freely combinable with the above embodiments, the chemical pulp mill operates according to the kraft process (the sulfate process). Preferably the sludge in question is biosludge from the treatment of wastewater from said pulp mill. The sludge can also be a combination of sludge from the pulp mill and sludge from other sources, for example from municipal waste water treatment.

[0025] According to yet another embodiment, freely combinable with the above embodiments, the system is housed in a mobile unit, preferably a shipping container. Brief description of the drawings

[0026] Embodiments of the invention will be described in further detail below, in the description, examples and appended claims, and in the drawings in which

Fig. 1 schematically illustrates how the HTC process can be integrated in a chemical pulp mill; and

Fig. 2 schematically shows components of the HTC arrangement, the HTC reactor and auxiliary equipment.

Description of embodiments

[0027] Before the present invention is described, it is to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims and equivalents thereof.

[0028] It must be noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0029] As described in the background section, the production of pulp and paper generates large quantities of wastewater. Unavoidably, the wastewater treatment generates large volumes of sludge.

[0030] Sludge and in particular biosludge contains comparatively high concentrations of inorganic compounds that are unwanted in the process (non- process elements, NPEs). Even though the biosludge is a small stream compared to e.g. the wood intake to the pulp mill, the high concentration of NPEs leads to a significant total intake to the pulp mill. In a report published 2003, Johan Dahlbom discusses the effects of NPEs in the chemical recovery system of a kraft pulp mill [Dahlbom, J., Effekter av PFG vid indunstning och forbranning av bioslam i ett massabruks sodapanna (English translation: Effects of Non Process Elements in the chemical recovery system of a kraft pulp mill from the incineration in the recovery boiler of biological sludge), Rapport 798, Varmeforsk Service AB, S2- 226, 2003].

[0031 ] Dahlbom investigated the relative contribution of NPEs in biosludge to the overall intake in a Swedish pulp mill. His results are shown in Table 1.

Table 1 . Relative contribution of different NPEs in biosludge

[0032] It is clear that the contribution from biosludge is significant for all the investigated NPEs. The present inventors have successfully addressed this problem and other, related problems, by the methods and systems disclosed herein. It is a surprising finding that hydrothermal carbonization (HTC) can be applied to treat sludge from chemical pulp mills to produce an energy-rich solid product at high dry solids content, containing most of the unwanted inorganic compounds such as P and Si, which can be fired efficiently in the mill power boiler to generate steam and power for the mill. Part of the generated steam can be used to fuel the hydrothermal carbonization process. If needed, the HTC off-gas can be introduced together with the combustion air in the power boiler to destruct possible organic residues and minimize costs for gas treatment in the HTC plant.

[0033] Due to the integration possibilities, a system as disclosed herein is preferably built in close proximity to the power boiler. When refurbishing an existing pulp mill, or when projecting and building a new mill, the integration of the system can be optimized by a person skilled in the art, based on the guidance obtained herein. For existing pulp mills, it is conceived that a system as disclosed herein is supplied in a mobile unit, to be placed in vicinity of the power burner and connected thereto. [0034] A first aspect relates to a method for treatment of sludge in a chemical pulp mill including a step of hydrothermal carbonization (HTC) of a sludge stream, wherein

- said sludge stream is fed into a HTC arrangement comprising a HTC

reactor, and subjected to elevated temperature and pressure, producing at least one solids fraction, at least one liquid fraction, and at least one gaseous fraction;

[0035] A general embodiment of the integration of a HTC process in a chemical pulp mill is illustrated in Fig. 1 where a sludge stream (A) is fed to a HTC arrangement (HTC) from which the resulting liquid fraction (B) is fed to the black liquid evaporators of the pulp mill, and mixed with thin black liquor, sometimes also referred to as weak black liquor. The resulting solids fraction is then combusted in the recovery boiler. The resulting steam can be used to heat the evaporators and also for other energy needs in the pulp mill. The resulting solids fraction (C) is fed to the power boiler of the pulp mill, preferably together with the HTC off-gas (D). A fraction (E) of the steam generated by the power boiler can be used for heating the HTC arrangement, the HTC reactor or the incoming sludge.

[0036] The HTC arrangement preferably comprises several components as schematically illustrated in Fig. 2. Here, the dashed line delimits the HTC

arrangement shown simply as a "HTC-box" in Fig. 1. A HTC reactor (10) is shown as the central component of the HTC arrangement. Sludge (A) is fed into the HTC arrangement in to equipment (50) for receiving and diluting the sludge, for example a tank with a mixer. The sludge can then be pre-heated in a pre-heater (60) which can be any type of suitable heat exchanger, such as a double-tube or a tube-and-shell type heat exchanger. The pre-heater (60) is preferably heated by steam (E) from one or more flash recovery steps (30). A final heater (70) is preferably provided immediately before the HTC reactor (10). The final heater (70) is preferably heated by steam (F) from the power boiler. The choice of type, configuration and dimension of the pre-heater and final heater can be made by a skilled person.

[0037] According to one embodiment, a solids fraction and a liquid fraction are withdrawn directly from the HTC reactor (not shown). In the embodiment shown in Fig. 2, a single liquid HTC product is withdrawn from the HTC reactor (10) and subjected to separation in one or more steps. Here, it is first subjected to a step of flash evaporation (30), also referred to as flash steam recovery, followed by another step of solids / liquid separation (40). The flash evaporation step produces steam (E) which can be used to pre-heat the incoming sludge. The second step of solids / liquid separation (40) can be chosen from a sedimentation step, a filtering step, a filter press, a solids / liquid separator screw or compactor, or the like. In one embodiment, for example when using a filter press, the solids fraction is concentrated to an interval of 60 - 75 % DS and the liquid fraction will contain less than 10 % DS.

[0038] Said HTC reactor (10) also produces off-gas which preferably is fed to a condenser (20) supplied with a cooling liquid (G) which can be water. The condensate is recirculated to the sludge receiving and dilution step (50) and the off-gas (D) is preferably fed to the power boiler and combusted. Optionally the off- gas can be mixed with air, preferably compressed air, before feeding to the power boiler.

[0039] The HTC arrangement shown within the dashed line in Fig. 2 is only an example of how the HTC step can be integrated in a pulp mill.

[0040] In the above embodiments, the sludge stream referred to is a sludge stream generated in the chemical pulp mill, or a combination of different sludge streams, at least some of which are generated in the chemical pulp mill. The sludge can thus be a combination of sludge from the pulp mill and sludge from other sources, for example from municipal waste water treatment. In the present method and system, sludge from the chemical pulp mill is used to produce an energy-rich solid product with high dry solids content which can be fired efficiently in the bower boiler of the mill and used to generate steam and power for the mill. Interestingly, most of the inert components, such as P and Si are contained in the solid product.

[0041 ] According to an embodiment, in order to ensure and optimize the enrichment of NPEs in the solid product, the pH in the hydrothermal carbonization reaction is controlled, most preferably to a pH above 5, by the addition of suitable alkaline compounds (e.g. black liquor, spent lime mud). Already at pH 5, elements such as Ca and P will start to dissolve to a greater degree in the process water. Further pH adjustments, for example increasing the pH to neutral or alkaline pH, are preferably performed to influence the distribution of NPEs between the HTC liquid and HTC solids fraction.

[0042] According to an embodiment of the first aspect, the liquid from the hydrothermal carbonization, which normally contains about 15-30% of the organic material, is sent to the pulp mill evaporation plant, where it will be evaporated and combusted together with the black liquor. An advantage is that its energy content will generate at least enough steam to drive the evaporation and very likely also generate a surplus for the pulp process. In the hydrothermal carbonization process, the liquor will be heated up to around 90 °C, thus requiring very little preheating when it is introduced into the evaporator train.

[0043] According to another embodiment, freely combinable with the other aspects and embodiments, the liquid from the hydrothermal carbonization is subjected to oxidation before being combined with the black liquor. The oxidation step is performed at a temperature in the interval of 180 - 300 °C, preferably 230 to 300°C, by the injection of an oxidizing agent into said first fraction. Said oxidizing agent is preferably chosen from oxygen, hydrogen peroxide, percarbonate, and percarbonic acids, and preferably an oxygen containing gas, such as air, most preferably compressed air.

[0044] According to another embodiment of said first aspect, said at least one gaseous fraction from the HTC reactor is combined with air and fed to the power boiler. This has the advantages that it adds fuel to the boiler; it ensures destruction of possible organic residues in the HTC gas; and minimizes costs for gas treatment in the HTC plant.

[0045] According to another embodiment, freely combinable with the above, part of the steam generated by the power boiler is used to heat the HTC reactor or the inflow of sludge to said reactor.

[0046] In the above embodiments, the elevated temperature in said HTC step is a temperature in the interval of 180 - 350 °C, preferably 180 - 300 °C, most preferably 180 - 230°C.

[0047] According to another embodiment, freely combinable with the above embodiments, the chemical pulp mill operates according to the kraft process (the sulfate process). Preferably said sludge is biosludge from the treatment of wastewater from said pulp mill. The sludge can also be a combination of sludge from the pulp mill and sludge from other sources, for example from municipal waste water treatment.

[0048] A second aspect relates to a system for treatment of sludge from a chemical pulp mill, comprising a hydrothermal carbonization (HTC) reactor, wherein said system further comprises

- an arrangement for introducing said sludge into said reactor;

- a heater for heating said sludge;

- a separation arrangement downstream of said HTC reactor for producing at least one solids fraction, at least one liquid fraction, and at least one gaseous fraction; - a feeding arrangement for feeding said at least one solids fraction to a power boiler of said pulp mill; and

[0049] According to an embodiment of said second aspect, the system comprises an arrangement for combining said at least one gaseous fraction with air and feeding the resulting mixed gas to said power boiler. As stated above, this has an advantage in that possible organic residues are destructed, which minimizes the requirements and costs for gas treatment in the HTC plant.

[0050] According to another embodiment of said second aspect, freely combinable with the other embodiments, the system comprises a reactor for oxidation of the liquid fraction, as well as auxiliary equipment for feeding an oxidizing agent into said reactor, and equipment for recovering heat generated in said oxidation. Such equipment can comprise a compressor for injecting compressed air into the liquid fraction. A vapor-liquid separator or flashing arrangement for cooling the oxidized liquid can also be included.

[0051 ] According to another embodiment, freely combinable with the above, the system comprises a heater arranged to receive steam from said power boiler. Preferably said heater is adapted to heat said HTC reactor or to heat the inflow of sludge to said reactor.

[0052] According to another embodiment, freely combinable with the above embodiments, the chemical pulp mill operates according to the kraft process (the sulfate process). Preferably said sludge is biosludge from the treatment of wastewater from said pulp mill. The sludge can also be a combination of sludge from the pulp mill and sludge from other sources, for example from municipal waste water treatment.

[0053] The system is preferably integrated in the pulp plant, for example built into and fully integrated in new plants, or as an add-on to existing plants.

According to yet another embodiment, freely combinable with the above embodiments, the system is housed in a mobile unit, preferably a shipping container. This is preferably placed in close proximity to the power boiler.

[0054] It is conceived that, in a kraft pulp mill such as that studied by

Dahlbom, where sludge currently is mixed with black liquor, evaporated and combusted in the recovery boiler, introduction of this process will have a very positive impact on the NPE intake to the closed cycle of the mill. Recycling of only the HTC liquid instead of the whole sludge stream would reduce the total intake of Al, Si, P, Ca, CI and Fe to the closed cycle of the mill by 23, 17, 1 1 , 6, 6 and 10% respectively.

[0055] This reduces both the need for fresh lime make-up and discharged lime handling by 0.4-1 t/t DS sludge. Reduction of chlorides will have a positive impact on corrosion and fouling of the recovery boiler heat transfer surfaces.

Reduction of Al, Si and Ca will positively affect fouling on black liquor evaporator surfaces as well as improve the filtration properties of green liquor dregs and lime mud.

[0056] In a method or in a system as disclosed herein, the HTC product is combusted in the mill power boiler at high dry solids content to generate steam to drive the HTC process as well as a steam surplus that can be used in the mill. The HTC liquid is evaporated together with the black liquor and the dissolved organics are combusted in the recovery boiler generating enough steam to drive the evaporation. Thus, the energy content of the sludge is efficiently used, while at the same time avoiding the problems related to NPE intake to the closed cycle of the chemical pulp mill.

[0057] Including a step of oxidation of the liquid fraction makes it possible to extract heat in the form of steam from the liquid fraction, contributing to the heating requirements of the pulp mill or to the heating requirements of the HTC process. The oxidation step also makes it possible to reduce the liquid volume before mixing with the black liquor. Additionally, the oxidation step will result in chemical reactions in the liquid fraction, for example decomposing various chemical substances. [0058] Other advantages will be evident to a skilled person upon study of the present disclosure, including the examples, claims and drawings.

Examples

Example 1 . HTC-treatment of biosludge from a Swedish pulp mill

[0059] Biosludge from a Swedish pulp mill was treated with hydrothermal carbonization at a temperature of 200 °C in a stirred 0.5 I batch reactor. Biosludge was loaded into the reactor and heated to the carbonization temperature. After carbonization, the reactor was rapidly cooled, and the resulting slurry filtered. Separate analyses were made of the solid and liquid product to determine the distribution of NPEs in the respective phase.

[0060] The results showed a significant enrichment of important NPEs in the solid HTC product, see Table 2. Thus, recycling of the HTC liquid to the black liquor evaporators is possible with only a small impact on the NPE amounts in the closed pulp mill cycle.

Table 2. Distribution of NPEs in sludge, HTC solids and HTC liquid

[0061 ] Dewatering properties were tested by pouring samples of the slurry, before and after carbonization, onto a Buchner funnel. Filtering was measured as kg DS/m2 filter surface x h. The HTC treated slurry exhibited significantly improved (over 300%) dewatering properties compared to the original biosludge. This will allow efficient mechanical dewatering to up to 65-75% dry solids content. This in turn allows efficient utilization of the energy content in the sludge by combustion in the power boiler of the mill, where the main content of NPEs will be found in the ash.

[0062] Without further elaboration, it is believed that a person skilled in the art can, using the present description, including the examples, utilize the present invention to its fullest extent. Also, although the invention has been described herein with regard to its preferred embodiments, which constitute the best mode presently known to the inventors, it should be understood that various changes and modifications as would be obvious to one having the ordinary skill in this art may be made without departing from the scope of the invention which is set forth in the claims appended hereto.

[0063] Thus, while various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

1. A method for treatment of sludge in a chemical pulp mill including a step of hydrothermal carbonization (HTC) of a sludge stream, characterized in that said sludge stream is fed into a HTC arrangement comprising a HTC reactor, and subjected to elevated temperature and pressure, producing at least one solids fraction, at least one liquid fraction, and at least one gaseous fraction; said at least one solids fraction is fed to a power boiler of said pulp mill and burned to generate steam; said at least one liquid fraction is combined with weak black liquor from the pulp mill and fed to a black liquor evaporator of said pulp mill, and the resulting concentrated black liquor is burned in a recovery boiler of said pulp mill.

2. The method according to claim 1 , wherein said at least one gaseous fraction from the HTC arrangement is combined with air and the resulting mixed gas is fed to the power boiler.

3. The method according to claim 1 or 2, wherein part of the steam generated by the power boiler is used to heat the HTC reactor or the inflow of sludge to said reactor.

4. The method according to any one of claims 1 - 3, wherein the elevated temperature in said HTC step is a temperature in the interval of 180 - 350 °C, preferably 180 - 300 °C, most preferably 180 - 230°C.

5. The method according to any one of claims 1 - 4, wherein the chemical pulp mill operates according to the kraft process (sulfate process) and said sludge comprises biosludge from the treatment of wastewater from said pulp mill.

6. A system for treatment of sludge from a chemical pulp mill, comprising a hydrothermal carbonization (HTC) reactor, characterized in that said system further comprises an arrangement for introducing said sludge into said HTC reactor; a heater for heating said sludge; a separation arrangement in or downstream of said HTC reactor for producing at least one solids fraction, at least one liquid fraction, and at least one gaseous fraction; a feeding arrangement for feeding said at least one solids fraction to a power boiler of said pulp mill; and a feeding arrangement for feeding said at least one liquid fraction to a black liquor evaporator of said pulp mill.

7. The system according to claim 6, wherein the system comprises an arrangement for combining said at least one gaseous fraction with air and feeding the resulting mixed gas into said power boiler.

8. The system according to claim 6 or 7, wherein the system comprises a heater arranged to receive steam from said power boiler, said heater adapted to heat said HTC reactor or to heat the inflow of sludge to said reactor.

9. The system according to any one of claims 6 - 8, wherein the chemical pulp mill operates according to the kraft process (sulfate process) and said sludge comprises biosludge from the treatment of wastewater from said pulp mill.

10. The system according to any one of claims 6 - 9, wherein said system or parts thereof is/are housed in a mobile unit, preferably in a shipping container.