JP2019537674A - A method for hydrothermal carbonization of sludge in a chemical pulp mill. - Google Patents

Abstract

A method and system for the treatment of sludge in a chemical pulp mill, comprising a step of hydrothermal carbonization HTC of the sludge stream, wherein the sludge stream is fed into an HTC facility, subjected to a temperature rise and pressure, Producing at least one solid fraction, at least one liquid fraction, and at least one gas fraction, wherein the solid fraction is supplied to a power boiler of the pulp mill and burned to generate steam; A method wherein the liquid fraction is combined with light black liquor from a pulp mill and fed to a black liquor evaporator of the pulp mill, and the resulting concentrated black liquor is burned in a recovery boiler of the pulp mill. system.

Description

  The present disclosure relates generally to the treatment of sludge generated in a chemical pulp mill, specifically involving the step of hydrothermal carbonization of the sludge and transferring the resulting solid and liquid fractions to a power generation boiler and a recovery boiler, respectively. A method and system for providing and thus integrating sludge handling in steam generation for a pulp mill.

  Sludge, which is a solid and water slurry containing dissolved substances, remains after treating wastewater in municipal and industrial wastewater treatment plants. Industrial wastewater treatment plants treat water waste from different industrial processes, such as, for example, pulp and paper mills, industrial food production facilities, and the like. All wastewater treatment plants in use generate sludge that needs to be handled in some way. Sludge is either recovered directly from the plant after dewatering (aerobic sludge) or is first treated anaerobically for biogas production, where some of the sludge is It is digested and the rest is recovered as anaerobic sludge, also called biological sludge.

  In the production of pulp and paper, large amounts of wastewater are generated. These must be properly handled to avoid serious environmental pollution. Current wastewater treatment in pulp and paper mills combines sedimentation, chemical sedimentation, flotation, and biological treatment, where the particular combination of technologies is determined by local conditions. Inevitably, wastewater treatment generates large amounts of sludge.

  The handling of sludge in chemical paper pulp mills, especially excess sludge from the biological treatment of liquid effluents, also called biological sludge, is a major problem, and the processing of sludge per ton is very costly. Today, there are three main solutions used, all of which are associated with certain disadvantages.

  Sludge can be incinerated in the power boiler of a pulp mill, contributing to the generation of steam required for the paper pulp making process. This has the advantage of avoiding the inorganic content of the sludge being taken up in the closed pulp mill cycle. With suitable treatment of flue gas and ash, harmful chemicals and heavy metals can be neutralized or even recovered. However, sludge having a high concentration of organic and / or biological components is generally very difficult to dewater. Due to the often very high water content, the net heating value when incinerated in a power plant is very low or even negative. Therefore, the combustion of sludge not only increases the load on the power generation boiler, but also requires auxiliary fuel.

  Furthermore, handling of wet sludge is problematic due to exposure to pathogens, odors, and the formation of toxic hydrogen sulfide gas. Thermal drying of sludge consumes approximately the same amount of heat as the latent heat in the sludge. Therefore, this is not an economically attractive alternative. Thus, at present, it may be said that sludge is regarded as a low quality biofuel due to its high water content, and as a result, some pulp mills treat sludge simply as a disposal problem.

  Another alternative is combustion in a recovery boiler. The sludge is first hydrolyzed with heat and alkali and then mixed with black liquor, after which the mixture is fed to an evaporator to increase its dry solids content before combustion in a recovery boiler . The advantage of this alternative is that a multiple effect evaporator plant can be used to dewater the sludge prior to combustion in the recovery boiler, and the energy content of the sludge is utilized. However, this often requires a hydrolysis treatment before the alkali is filled. Ash from sludge contains inert components such as phosphorus and silica, which accumulate in the lime cycle and contribute to increased demand for lime making. In addition, the amount of used lime to be handled increases. Thus, ultimately, this alternative is associated with enormous fixed handling costs of treating sludge per ton.

  In addition, sludge can produce huge amounts of aluminum, calcium, iron, potassium, and chlorides that can cause problems in pulp mills, such as corrosion, contamination, green liquor and the poorer dewatering properties of lime mud. It contains other non-process elements (NPEs: Non-Process Elements). Each of these inevitably adds significantly to the operating costs and maintenance requirements of the pulp mill.

  Finally, sludge may be designated for external use, for example, as landfill or landfill covering, as fertilizer, or as a soil substitute. However, this does not imply a costly and environmentally attractive treatment since it involves the transport of large amounts of high moisture content materials. In addition, when the sludge rots, there is a high emission of nitrous oxide and methane to the atmosphere. Methane is a greenhouse gas and is not as present in the atmosphere as carbon dioxide, but is initially much more destructive to climate because of how effectively it absorbs heat. In short, methane emissions must be minimized.

  Hydrothermal carbonization (HTC), also known as wet roasting, is a thermochemical process used in the manufacture of carbonized materials that are similar in composition to coal. It involves exposing a wet carbohydrate feedstock, such as sludge, to elevated temperatures (180 ° C.-350 ° C.) and pressures (up to 2-5 MPa) in a closed system. The resulting product is often referred to as biochar, a product that is friable, hydrophobic, and has a significantly increased energy density compared to the starting material.

  WO 2015/025076 (Valmet Technologies Oy) relates to the application of HTC processes in pulp mills. According to this application, lignin is separated from a lignin-containing liquid medium, such as pulp mill black liquor, for further processing. The method comprises at least the following steps: a) a precipitation stage in which a pH-lowering agent is added to the lignin-containing slurry to precipitate the lignin; b) the lignin cake from the remaining liquid phase of the lignin-containing slurry C) the lignin cake is suspended to obtain a lignin suspension, a suspension stage, and d) the lignin suspension is processed to obtain a slurry of carbon-containing material. A) a hydrothermal carbonization stage, and e) a second separation stage in which the carbon-containing material is separated from the slurry.

WO2015 / 025076

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

  It has been found that hydrothermal carbonization of chemical pulp mill sludge, especially biological sludge, can be effectively integrated into a chemical pulp and paper mill given that some non-intuitive adaptation of the process is performed. I have. The resulting system is energy efficient, favorably contributes to the generation of steam for the paper pulp making process, and eliminates a major problem in that the sludge volume is significantly reduced or even eliminated.

A first aspect relates to a method for the treatment of sludge in a chemical pulp mill, comprising the step of hydrothermal carbonization (HTC) of the sludge stream,
The sludge stream is fed into an HTC facility comprising an HTC reactor and subjected to elevated temperature and pressure to produce at least one solids fraction, at least one liquid fraction, and at least one gas fraction;
The at least one solids fraction is fed to a power boiler of the pulp mill and burned to generate steam;
The at least one liquid fraction is combined with light black liquor from a pulp mill and supplied to a black liquor evaporator of the pulp mill;
-The resulting concentrated black liquor is burned in the pulp mill recovery boiler.

  According to an embodiment of the first aspect, the at least one gas fraction from the HTC facility is combined with air and the resulting gas mixture is fed to a power boiler.

  According to another embodiment of the first aspect, the liquid fraction undergoes oxidation before being combined with the black liquor, freely combinable with the above.

  According to another embodiment of the first aspect, a portion of the steam generated by the power boiler can be combined freely with the above, with a portion of the steam generated by the power boiler being sent to the HTC facility, the HTC reactor, or to the reactor and / or facility. Used to heat one or more of the sludge influents.

  In the above embodiment, the temperature increase in the HTC step is a temperature in a section of 180 to 350 ° C, preferably 180 to 300 ° C, and most preferably 180 to 230 ° C.

  According to the examples, the pH in the hydrothermal carbonization reaction is controlled, most preferably to a pH above 5, in order to ensure or optimize the enrichment of the NPE in the solid product.

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

A second aspect relates to a system for treatment of sludge from a chemical pulp mill comprising a hydrothermal carbonization (HTC) facility comprising an HTC reactor, wherein the system comprises:
-Equipment for taking the sludge into the reactor;
-A heater for heating the sludge;
-At least one separation facility in or downstream of the HTC reactor for producing at least one solids fraction, at least one liquid fraction, and at least one gas fraction;
A supply facility for supplying the at least one solids fraction to a power boiler of the pulp mill;
A supply facility for supplying the at least one liquid fraction to the black liquor evaporator of the pulp mill.

  According to an embodiment of the second aspect, the system comprises equipment for combining the at least one gas fraction with air and supplying a resulting gas mixture into the power boiler.

  According to another embodiment of the second aspect, in any combination with the above, the system comprises a heater arranged to receive steam from the power boiler. Preferably, the heater is adapted to heat the HTC reactor or to heat sludge influent to the reactor.

  According to another embodiment of the second aspect, the system provides a reactor for oxidizing a liquid fraction, and an oxidant in the reactor, freely combinable with other embodiments. And an apparatus for recovering heat generated in the oxidation.

  According to another embodiment of the second aspect, the chemical pulp mill operates according to the Kraft process (sulfate process), freely combinable with the above embodiment. Preferably, the sludge in question is biological sludge from the treatment of wastewater from the pulp mill. The sludge can also be a combination of sludge from a pulp mill and sludge from other sources, for example, from municipal wastewater treatment.

  According to yet another embodiment, the system is housed in a mobile unit, preferably in a shipping container, which can be freely combined with the above embodiments.

  Embodiments of the present invention are described in further detail below in the description, examples, and appended claims, and in the drawings.

FIG. 4 schematically illustrates how an HTC process can be integrated into a chemical pulp mill. FIG. 1 schematically illustrates the components of an HTC facility, an HTC reactor, and auxiliary equipment.

  Before describing the present invention, the terms used in the present specification are used for the purpose of describing particular embodiments only, and the scope of the present invention shall be limited only by the appended claims and their equivalents. It should be understood that the limitations are not intended to be limiting.

  It is noted that as used herein and in the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Should.

  As described in the Background section, the production of paper pulp generates large amounts of wastewater. Inevitably, wastewater treatment generates large amounts of sludge.

  Sludge, and especially biological sludge, contains relatively high concentrations of inorganic compounds (non-process elements, NPE) that are undesirable in the process. Even if the biological sludge is a small stream compared to, for example, the wood influx into the pulp mill, the high concentration of NPE results in a significant total influx into the pulp mill. In a report published in 2003, Johan Dahlbomb discusses the effects of NPEs on chemical recovery systems at kraft pulp mills (Dahlbomb, J., Effekter av PFG vid indus naming absoni s bon ab s a n a n a n a n g o n a b a n a n a n s a n a b a n s a n t e s English translation: Effects of Non Process Elements in the chemical recovery system of a Kraft pulp mill from the institutional org. sk Service AB, S2-226, 2003).

  Dahlbom investigated the relative contribution of NPE in biological sludge to the total influx in Swedish pulp mills. The results are shown in Table 1.

  It is clear that the contribution from biological sludge is significant for all NPEs investigated. The present inventors have successfully addressed this and other related problems with the methods and systems disclosed herein. Hydrothermal carbonization (HTC) has been applied to treat sludge from chemical pulp mills to provide high dry solids, high energy solid products containing most of the undesirable inorganic compounds such as P and Si. It is a surprising finding that it can be produced and this product can be efficiently burned in a factory power boiler to produce steam and factory power. Some of the generated steam may be used to fuel a hydrothermal carbonization process. If necessary, HTC off-gas can be taken into the power boiler along with the combustion air to destroy possible organic residues and minimize the cost of gas treatment in the HTC plant.

  Due to integration possibilities, the system disclosed herein is preferably constructed in close proximity to a power boiler. When retrofitting an existing pulp mill or planning and building a new mill, the integration of the system can be optimized by those skilled in the art based on the advice provided herein. In the case of existing pulp mills, it is recalled that the system as disclosed herein is provided in a mobile unit that is located near and connected to the power generation burner.

A first aspect relates to a method for treating sludge in a chemical pulp mill, comprising a step of hydrothermal carbonization (HTC) of the sludge stream,
The sludge stream is fed into an HTC facility comprising an HTC reactor and subjected to elevated temperature and pressure to produce at least one solids fraction, at least one liquid fraction, and at least one gas fraction;
The at least one solids fraction is fed to a power boiler of the pulp mill and burned to generate steam;
The at least one liquid fraction is combined with a light black liquor from a pulp mill and supplied to a black liquor evaporator of the pulp mill;
-The resulting concentrated black liquor is burned in the recovery boiler of the pulp mill.

  A general example of the integration of an HTC process in a chemical pulp mill is illustrated in FIG. 1, where a sludge stream (A) is fed to an HTC facility (HTC) from which the resulting liquid fraction The minute (B) is fed to a pulp mill black liquor evaporator and mixed with a light black liquor, sometimes also called a light black liquor. The resulting solids fraction is then burned in a recovery boiler. The resulting steam can be used to heat the evaporator, and even for other energy needs in the pulp mill. The resulting solids fraction (C) is fed to the pulp mill power boiler, preferably together with the HTC offgas (D). The steam fraction (E) produced by the power boiler can be used to heat HTC equipment, HTC reactors, or incoming sludge.

  The HTC facility preferably comprises several components as schematically illustrated in FIG. Here, the dashed line delimits the HTC equipment, which is simply shown in FIG. 1 as "HTC box". The HTC reactor (10) is shown as a central component of the HTC facility. Sludge (A) is fed into an apparatus (50) for receiving and diluting sludge, for example, HTC equipment in a tank with a mixer. The sludge may then be preheated in a preheater (60), which may be any suitable type of heat exchanger, such as a double tube or shell and tube heat exchanger. The preheater (60) is preferably heated by steam (E) from one or more flash recovery steps (30). The final heater (70) is preferably provided immediately before the HTC reactor (10). The final heater (70) is preferably heated by steam (F) from a power boiler. Selection of the type, configuration, and dimensions of the preheater and final heater can be made by those skilled in the art.

  According to one embodiment, the solids and liquid fractions are taken directly from the HTC reactor (not shown). In the embodiment shown in FIG. 2, a single liquid HTC product is taken from the HTC reactor (10) and undergoes separation in one or more steps. Here, the product first undergoes a step of flash evaporation (30), also referred to as flash vapor recovery, followed by another step of solid / liquid separation (40). The flash evaporation step produces steam (E) that can be used to preheat incoming sludge. The second step of solid / liquid separation (40) may be selected from a settling step, a filtration step, a filter press, a solid / liquid separator screw or compressor, or the like. In one embodiment, for example, when using a filter press, the solids fraction will be concentrated to a fraction of 60-75% DS and the liquid fraction will contain less than 10% DS.

  The HTC reactor (10) also produces an off-gas which is preferably fed to a condenser (20) provided with a coolant (G), which may be water. The condensate is recycled to the sludge receiving and diluting step (50), and the off-gas (D) is preferably fed to a power boiler for combustion. Optionally, the off-gas can be mixed with air, preferably compressed air, before supplying to the power boiler.

  The HTC facility shown within the dashed line in FIG. 2 is only an example of how the HTC step is integrated into a pulp mill.

  In the above examples, the sludge stream referred to is a sludge stream generated in a chemical pulp mill or a combination of different sludge streams generated at least in part in a chemical pulp mill. Thus, the sludge can be a combination of sludge from a pulp mill and sludge from other sources, for example, from municipal wastewater treatment. In the present method and system, the sludge from the chemical pulp mill is efficiently burned in the power boiler of the mill, producing high energy solids with a high dry solids content that can be used to generate steam and mill power. Used to produce things. Interestingly, most of the inert components such as P and Si are contained in the solid product.

  According to the examples, in order to ensure or optimize the enrichment of NPE in the solid product, the pH in the hydrothermal carbonization reaction is controlled by a suitable alkaline compound (eg black liquor, spent lime mud). , Most preferably to a pH greater than 5. Elements already at pH 5, such as Ca and P, begin to dissolve to a considerable extent in process water. Further pH adjustments, for example increasing the pH to neutral or alkaline pH, are preferably performed to influence the distribution of NPE between the HTC liquid fraction and the HTC solids fraction.

  According to an embodiment of the first aspect, the liquid from hydrothermal carbonization, usually containing about 15-30% of organic matter, is sent to a pulp mill evaporation plant where it is evaporated together with black liquor. It will be burned. The advantage is that its energy content is likely to generate at least enough steam to drive evaporation, and also to generate excess for the pulp process. In the hydrothermal carbonization process, the solution will be heated to a maximum of approximately 90 ° C., and thus requires little residual heat when it is taken into the evaporator train.

  According to another embodiment, the liquid from hydrothermal carbonization undergoes oxidation before being combined with the black liquor, freely combinable with other aspects and embodiments. The oxidation step is carried out at a temperature in the interval from 180 to 300C, preferably from 230 to 300C, by injecting an oxidizing agent into the first fraction. The oxidizing agent is preferably selected from oxygen, hydrogen peroxide, percarbonate, and percarbonate, and preferably oxygen containing gases such as air, most preferably compressed air.

  According to another embodiment of the first aspect, the at least one gas cut from the HTC reactor is combined with air and fed to a power boiler. This has the advantage of adding fuel to the boiler, ensuring the destruction of possible organic residues in the HTC gas, and minimizing the costs for gas treatment in the HTC plant.

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

  In the above embodiment, the temperature increase in the HTC step is a temperature in a section of 180 to 350 ° C, preferably 180 to 300 ° C, and most preferably 180 to 230 ° C.

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

A second aspect relates to a system for treating sludge from a chemical pulp mill, comprising a hydrothermal carbonization (HTC) reactor, wherein the system comprises:
-Equipment for taking the sludge into the reactor;
-A heater for heating the sludge;
A separation facility downstream of said HTC reactor for producing at least one solids fraction, at least one liquid fraction and at least one gas fraction;
A supply facility for supplying the at least one solids fraction to a power boiler of the pulp mill;
A supply facility for supplying the at least one liquid fraction to the black liquor evaporator of the pulp mill.

  According to an embodiment of the second aspect, the system comprises equipment for combining the at least one gas fraction with air and supplying a resulting gas mixture to the power boiler. As mentioned above, this has the advantage that potential organic residues are destroyed, thereby minimizing the requirements and costs for gas treatment in HTC plants.

  According to another embodiment of the second aspect, the system provides a reactor for oxidizing a liquid fraction, and an oxidant in the reactor, freely combinable with other embodiments. And an apparatus for recovering heat generated in the oxidation. Such a device may include a compressor for injecting compressed air into the liquid fraction. A gas-liquid separator or a flushing facility for cooling the oxidant may also be included.

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

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

  The system is preferably integrated within the pulp plant, for example, built in a new plant and fully integrated, or integrated as an add-on to an existing plant. According to yet another embodiment, the system is housed in a mobile unit, preferably in a shipping container, which can be freely combined with the above embodiments. It is preferably located in close proximity to the power boiler.

  At present, in kraft pulp mills as studied by Dahlbomb, where the sludge is mixed with black liquor, evaporated and burned in a recovery boiler, the introduction of this process is based on the NPE inflow to the factory closed cycle. And have a very positive effect. By recycling only the HTC solution, rather than the entire sludge stream, the total inflows of Al, Si, P, Ca, Cl, and Fe into the closed cycle of the plant were reduced to 23, 17, 11, 6, 6, and respectively. Reduce by 10%.

  This reduces both the need for fresh lime production and the discharge lime handling by 0.4-1 t / t DS sludge. Chloride reduction has a positive effect on corrosion and contamination of the recovery boiler heat transfer surface. The reduction of Al, Si and Ca not only has a positive effect on the contamination of the black liquor evaporator surface, but also improves the filtering properties of green liquor and lime mud.

  In a method or system as disclosed herein, the HTC product can be burned in a factory power boiler with a high dry solids content to be used for steam to drive the HTC process, as well as in the factory. Generates excess steam. The HTC liquor is evaporated together with the black liquor, and the dissolved organics are burned in a recovery boiler to generate sufficient steam to drive the evaporation. In this way, the energy content of the sludge is used efficiently while avoiding the problems associated with NPE inflow to the closed cycle of a chemical pulp mill.

  The inclusion of the step of oxidizing the liquid fraction allows heat to be extracted from the liquid fraction in the form of vapor, which contributes to the heating requirements of the pulp mill or of the HTC process. The oxidation step also allows the liquid volume to be reduced before mixing with the black liquor. In addition, the oxidation step causes chemical reactions within the liquid fraction, for example, the decomposition of various chemicals.

  Other advantages will be apparent to one skilled in the art from a study of the present disclosure, including examples, claims, and drawings.

"Example 1"
HTC Treatment of Biological Sludge from Swedish Pulp Mill Biological sludge from a Swedish pulp mill was treated by hydrothermal carbonization at a temperature of 200 ° C. in a stirred 0.5 l batch reactor. Biological sludge was charged into the reactor and heated to the carbonization temperature. After carbonization, the reactor was quickly cooled and the resulting slurry was filtered. Separate analyzes were performed on the solid and liquid products to determine the distribution of NPE in each phase.

  Referring to Table 2, the results indicated a significant enrichment of key NPEs within the solid HTC product. Thus, the recycling of HTC liquor to the black liquor evaporator can have a very small effect on the amount of NPE in a closed pulp mill cycle.

  The dewatering properties were tested by pouring a sample of the slurry into a Buchner funnel before and after carbonization. Filtration was measured as kg DS / m2 filter surface x h. The HTC treated slurry exhibited significantly improved (> 300%) dewatering properties compared to the original biological sludge. This enables efficient mechanical dewatering up to a dry solids content of 65-75%. This also allows for an efficient use of the energy content in the sludge by combustion in the power boiler of the factory, where the main content of NPE is found in the ash.

  Without further elaboration, it is believed that one skilled in the art can, using the description including examples, utilize the present invention to its fullest extent. Also, while the present invention has been disclosed herein in terms of its preferred embodiments, which constitute the best mode currently known to the inventors, various modifications and variations will be apparent to those skilled in the art. It is to be understood that modifications may be made without departing from the scope of the invention as set forth in the claims below.

  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 examples disclosed herein are for illustrative purposes and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims (10)

A method for treating sludge in a chemical pulp mill, comprising a step of hydrothermal carbonization (HTC) of the sludge stream, comprising:
The sludge stream is fed into an HTC facility comprising an HTC reactor and subjected to elevated temperature and pressure to produce at least one solids fraction, at least one liquid fraction, and at least one gas fraction;
The at least one solids fraction is fed to a pulp mill power boiler and burned to generate steam;
The at least one liquid fraction is combined with light black liquor from the pulp mill and supplied to a black liquor evaporator of the pulp mill;
The process wherein the resulting concentrated black liquor is burned in a recovery boiler of the pulp mill.   The method of claim 1, wherein the at least one gas cut from the HTC facility is combined with air and a resulting gas mixture is provided to the power boiler.   The method of claim 1 or 2, wherein a portion of the steam generated by the power boiler is used to heat the HTC reactor or sludge influent to the reactor.   The method according to any one of claims 1 to 3, wherein the temperature increase in the HTC step is a temperature in a section of 180 to 350C, preferably 180 to 300C, most preferably 180 to 230C.   5. The chemical pulp mill according to any one of the preceding claims, wherein the chemical pulp mill operates according to a kraft process (sulfate process) and the sludge comprises biological sludge from the treatment of wastewater from the pulp mill. Method. A system for treating sludge from a chemical pulp mill, comprising a hydrothermal carbonization (HTC) reactor, said system comprising:
Equipment for taking the sludge into the HTC reactor;
A heater for heating the sludge,
A separation facility within the HTC reactor or downstream of the HTC reactor for producing at least one solid fraction, at least one liquid fraction, and at least one gas fraction;
A supply facility for supplying the at least one solid fraction to a power boiler of the pulp mill;
A supply facility for supplying the at least one liquid fraction to the pulp mill black liquor evaporator.   The system according to claim 6, wherein the system comprises facilities for combining the at least one gas fraction with air and supplying a resulting gas mixture into the power boiler.   The system comprises a heater arranged to receive steam from the power boiler, wherein the heater heats the HTC reactor or heats sludge influent to the reactor. 8. The system according to claim 6 or 7, wherein the system is adapted to:   9. The chemical pulp mill according to any one of claims 6 to 8, wherein the chemical pulp mill operates according to a kraft method (sulphate method) and the sludge comprises biological sludge from the treatment of wastewater from the pulp mill. system.   10. The system according to any one of claims 6 to 9, wherein the system or parts of the system are housed in a mobile unit, preferably in a shipping container.