SE542710C2

SE542710C2 - Hydrothermal carbonization and wet oxidation of sludge

Info

Publication number: SE542710C2
Application number: SE1851671A
Authority: SE
Inventors: Fredrik Öhman; Fredrik Lundqvist; Staffan Jakobsson; Ted Johansson
Original assignee: C Green Tech Ab
Priority date: 2018-12-27
Filing date: 2018-12-27
Publication date: 2020-06-30
Also published as: SE1851671A1

Abstract

There is provided a method of hydrothermal carbonization of a sludge, comprising the steps of:a) pre-heating the sludge by mixing it with an oxidizing steam fraction to obtain a preheated sludge;b) heating the pre-heated sludge with at least one further steam fraction to obtain a heated sludge;c) subjecting the heated sludge to hydrothermal carbonization (HTC) in a reactor (107.507) to obtain a HTC-treated slurry; andd) subjecting the HTC-treated slurry to flashing in at least one step to obtain the at least one further steam fraction and a pre-cooled slurry;e) subjecting the pre-cooled slurry flashing to obtain a low-pressure steam fraction having a pressure of 1.00-2.00 bar, preferably 1.00-1.30 bar, more preferably 1.00-1.05 bar; andf) adding a pressurized oxidizing gas to the low-pressure steam fraction to obtain the oxidizing steam fraction, which has a higher pressure than the low-pressure steam fraction. A corresponding system (100, 200, 300, 400, 500, 600) is also provided.

Description

HYDROTHERMAL CARBONIZATION AND WET OXIDATION OF SLUDGE TECHNICAL FIELD

[0001] The present disclosure relates to a method of hydrothermal carbonization of sludge (HTC), in particular municipal or industrial sludge from a wastewater treatment plants.

BACKGROUND

[0002] Sludge is typically what remains after wastewater treatment in municipal or industrial wastewater treatment plants. Municipal wastewater treatment plants treat wastewater from cities while industrial wastewater treatment plants treat water effluents from different industrial processes, for example pulp and paper mills, industrial food production facilities etc. Animal farming is also a considerable source of wastewater and sludge, for example large-scale pig farming. Embodiments of the present disclosure will be useful in all these areas.

[0003] The technologies for wastewater treatment are similar on a general level, but include specific solutions depending on the character of the waste streams to be treated, basic design, local requirements and environmental concerns. In larger plants in Sweden, the wastewater treatment process often comprises mechanical and chemical pretreatment followed by biological treatment steps. In some cases different forms of e.g. chemical treatment is also applied to remove remaining problematic substances, for example drug residues, toxic organic substances etc., in the treated water. In smaller plants some of these stages may often be omitted.

[0004] Almost all wastewater treatment plants in use generate sludge that needs to be handled. 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 after dewatering.

[0005] Wastewater treatment plants world wide produce several hundred millions metric tonnes of sludge every year and the amount is rapidly growing. In Sweden, the total sludge volume in tons of dry solids per year (tDS/y) was reported to be 250 000 in 2010 and the current figure is estimated to be higher. Sludge handling is thus an enormous challenge for society, and present solutions are associated with high cost and frequently also a negative environmental impact. [ooo6] Starting from 1986, the European Union has adopted several directives regulating the treatment and disposal of waste water sludge, addressing different aspects such as the use of sludge as landfill, the recovery of phosphorus, incineration of sludge etc. The various directives are reflected in national legislation in the individual member states, and for example in Sweden, the disposal of sludge in landfill has been prohibited since 2005.

[0007] Today, the main uses for wastewater sludge are fertilization in agriculture and forestry/silviculture, mixing into plant soil for ground construction projects and the coverage and restoration of landfills, incineration with energy recovery, recovery of chemicals and the production of fertilizers, and finally landfill, however provided that the sludge has undergone specific pretreatment, such as composting.

[0008] Incineration of the sludge, with energy recovery and suitable treatment of flue gases and ashes to destroy harmful chemicals and safely handle heavy metals, remains an attractive alternative. The exact composition of the sludge however depends on the composition of the incoming wastewater and the type of wastewater treatment plant. Sludge with high concentrations of organic and/or biological components is generally 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 and the addition of support fuels, often fossil fuel, may be necessary. Further, today’s sludge handling is normally associated with emission of foul odour requiring certain preconditions regarding e.g. storage and handling.

[0009] C-Green Technology AB has developed a process for treatment of sludge involving a step of hydrothermal carbonization (HTC).

SUMMARY

[0010] The operation of many hydrothermal carbonization (HTC) systems normally requires external energy for process heating purposes, supplied in the form of e.g. electricity, natural gas, steam or high temperature waste heat. The present inventors have found that the need for continuous supply of external energy for heating purposes in HTC treatment of sludge can be eliminated or at least significantly reduced by wetoxidizing the sludge. Further, the present inventors have found that the supply of oxidizing gas for wet-oxidizing the sludge can be used to drive the recirculation of lowpressure steam recovered from the process. Oxidizing gas can thus be added to obtain two different energy-saving effects at the same time.

[0011] As a first aspect of the present disclosure, there is provided a method of hydrothermal carbonization (HTC) of a sludge, comprising the steps of: a) pre-heating the sludge by mixing it with an oxidizing steam fraction to obtain a preheated sludge; b) heating the pre-heated sludge with at least one further steam fraction to obtain a heated sludge; c) subjecting the heated sludge to HTC in a reactor (HTC reactor) to obtain a HTC-treated slurry; and d) subjecting the HTC-treated slurry to flashing in at least one step to obtain the at least one further steam fraction and a pre-cooled slurry; e) subjecting the pre-cooled slurry flashing to obtain a low-pressure steam fraction having a pressure of 1.00-2.00 bar; and f) adding a pressurized oxidizing gas to the low-pressure steam fraction to obtain the oxidizing steam fraction, which has a higher pressure than the low-pressure steam fraction.

[0012] Step f) thus facilitates the recovery of the heat of the low-pressure steam fraction while introducing the oxidizing gas that results in the exothermic wet oxidation reactions.

[0013] Preferably, an ejector is used for adding the pressurized oxidizing gas to the low-pressure steam fraction in step f). The ejector is preferably designed and operated such that the motive/suction mass flow ratio enables the dew point of the discharge gas to be 75-100 °C, preferably > 90 °C.

[0014] The pressurized oxidizing gas is preferably pressurized oxygen gas. In the context of the present disclosure, oxygen gas refers to a gas comprising at least 80 % oxygen by volume, preferably at least 95 % oxygen by volume. A benefit of using oxygen gas instead of air is that less inert gas is introduced and that a higher dew point of the discharge gas is easier obtained.

[0015] However, the pressurized oxidizing gas may, in one embodiment, be pressurized air. In such an embodiment nitrogen gas may be purged during step b) (preferably during the later stages of step b)), between steps b) and c) and/or during step c). Gas may for example be purged through gas outlets arranged on pump tanks used in step b) and through an gas outlet arranged on the HTC reactor used for step c) (see the figures). A benefit of using air instead of oxygen gas is that the cost for providing oxygen gas is avoided.

[0016] The lower the pressure of the low-pressure steam fraction, the higher the degree of heat recovery. Hence, the pressure of the low-pressure steam fraction is preferably 1.00-1.30 bar and more preferably 1.00-1.10 bar, such as 1.00-1.05 bar.

[0017] The pressure of the pressurized oxidizing gas is preferably at least 5.0 bar, more preferably at least 7.0 bar, such as at least 12.0 bar. For economic efficiency, a typical upper limit may be 30 bar.

[0018] The oxidizing steam fraction (which is the result of adding the pressurized oxidizing gas to the low-pressure steam fraction) preferably has a pressure of at least 1.24 bar, more preferably at least 1.5 bar. For economic efficiency, at typical upper limit may be 5 bar. In a particularly preferred embodiment, the pressure of the oxidizing steam fraction is 2.0-3.0 bar, such as 2.0-2.5 bar.

[0019] Step b) preferably comprises heating the pre-heated sludge with at least two further steam fractions, preferably by adding the at least two further steam fractions to the sludge in steam mixers, e.g. venturi mixers. Such heating is preferably carried out stepwise, wherein the further steam fraction having the highest temperature is added last. The at least two further steam fractions are preferably obtained by, in step d), subjecting the HTC-treated slurry from step c) to flashing in at least two steps. Such a procedure results in efficient heat recovery.

[0020] To boost the temperature, pressurized oxidizing gas may also be added to the heated sludge prior to step c). The pressure of the heated sludge is relatively high.

Consequently, the pressurized oxidizing gas added at this point typically has a pressure of at least 20 bar, such as 20-30 bar. Optionally, the heated sludge is retained in a reactor for a period of time after the addition of the pressurized gas, but before being fed to the HTC reactor. The average retention time in such a reactor may be 5-50 % of the average retention time in the HTC reactor.

[0021] An alternative or complementary way of boosting the temperature is to add pressurized oxidizing gas to the HTC-treated slurry prior to step d). Thereby, the temperature of the HTC-treated slurry will be increased, which means that steam of higher temperature will be obtained from step d). The pressure of the HTC-treated slurry is relatively high. Consequently, the pressurized oxidizing gas added at this point typically has a pressure of at least 20 bar, such as 20-30 bar. Optionally, the HTC-treated slurry is retained in a reactor for a period of time after the addition of the pressurized gas, but before step d). The average retention time in such a reactor may be 5-50 % of the average retention time in the HTC reactor.

[0022] Another alternative or complementary way of boosting the temperature is to: - separate a fraction from the HTC-treated slurry prior to step d); - add a pressurized oxidizing gas to the separated fraction to obtain a wet-oxidized fraction; and - add the wet-oxidized fraction to the heated sludge.

For the same reason as above, the pressurized oxidizing gas typically has a pressure of at least 20 bar, such as 20-30 bar. The wet-oxidized fraction may be added to the heated sludge upstream the HTC reactor or in the HTC reactor. Optionally, the separated fraction is retained in a reactor for a period of time after the addition of the pressurized gas, but before it is added to the heated sludge. The average retention time in such a reactor may be 5-50 % of the average retention time in the HTC reactor.

[0023] Yet another alternative or complementary way of boosting the temperature is to: - separate a fraction from the HTC-treated slurry prior to step d); - add a pressurized oxidizing gas to the separated fraction to obtain a wet-oxidized fraction; and - subject the wet-oxidized fraction to flashing to obtain a high -temperature steam fraction that is used to further heat the heated sludge prior to step c).

For the same reason as above, the pressurized oxidizing gas typically has a pressure of at least 20 bar, such as 20-30 bar. Optionally, the separated fraction is retained in a reactor for a period of time after the addition of the pressurized gas, but before it is subjected to flashing. The average retention time in such a reactor may be 5-50 % of the average retention time in the HTC reactor.

[0024] The total suspended solids (TSS) content of the separated fraction may be lower than the TSS content of the HTC-treated slurry. This can be achieved by a particle separation device arranged downstream the HTC reactor or by a HTC reactor designed to provide a particle-lean fraction and a particle-rich fraction (see patent application SE 1750284 A1).

[0025] The TSS content of the separated fraction may for example be lower than 50 g/1, preferably lower than 30 g/1, more preferably lower than 20 g/1 and most preferably in an interval of 0-10 g/1.

[0026] The volumetric flow rate of the separated fraction may for example be 10-50 % of the volumetric flow rate of the heated sludge directly after step b).

[0027] The sludge of the present disclosure is preferably a municipal or industrial sludge from a wastewater treatment plant.

[0028] The dry solids content (also referred to as “Total Solids”) of the sludge depends on its source of production but is typically 1-35 %, such as 2-35 %, such as 3-32 %. If the sludge is anaerobic sludge, the dry solids content is normally 13-32 %. If the sludge is aerobic sludge, the dry solids content is typically 5-17 %. The ash content is typically 5-65 %, such as 10-50 %, such as 35-45 %, of the dry weight of the sludge. The higher heating value (HHV) of the sludge is typically 10-26 MJ/kg DS, such as 15-22 MJ/kg (dry weight).

[0029] The wet oxidation(s) of the present disclosure typically does not use the whole energy content of the slurry. 50-95 %, such as 75-95 %, such as 84-91 % of the heat content of the untreated sludge typically remains after the method of the present disclosure.

[0030] The present disclosure facilitates the separation of phosphorus (P).

Accordingly, the sludge of the present disclosure may comprise phosphorus, e.g. in an amount of 0.5-9 % of the dry weight of the sludge, such as 1-9 % of the dry weight of the sludge, such as 1.5-9 % of the dry weight of the sludge.

[0031] The sludge of the present disclosure preferably comprises carbon (C), e.g. in an amount of 9-50 % of the dry weight of the sludge, such as 20-50 % of the dry weight of the sludge.

[0032] When the oxidizing agent is oxygen gas, it may be added in a total amount of 60-300 kg per tonne of dry sludge processed by the method, preferably 150-290 kg per tonne of dry sludge processed by the method, more preferably 200-260 kg per tonne of dry sludge processed by the method.

[0033] The temperature of the preheated sludge (i.e. the temperature after step a)) is typically 70-95 °C and preferably 80-90 °C. The temperature of the HTC-treated slurry, which may be considered to be the “reaction temperature” of the HTC reaction, is typically 180-250 °C , preferably 180-230 °C and more preferably 190-225 °C.

[0034] The average retention time in the HTC reactor is typically 0.25-8 h, preferably 0.5-2 h, such as 1-2 h.

[0035] Otherwise, the embodiments of the second aspect discussed below apply mutatis mutandis to the first aspect.

[0036] As a second aspect of the present disclosure, there is provided a system for hydrothermal carbonization (HTC) of a sludge, comprising: - a HTC reactor for subjecting sludge to a HTC such that HTC-treated slurry is obtained; - a sludge routing arrangement for routing sludge to the HTC reactor, which sludge routing arrangement comprises a preheating arrangement and a further heating arrangement, wherein the further heating arrangement is arranged downstream the preheating arrangement; and - a slurry routing arrangement for routing HTC-treated slurry from the HTC reactor, which slurry routing arrangement comprises a first and a second flashing arrangement, which second flashing arrangement is arranged downstream the first flashing arrangement; - a first steam routing arrangement for routing steam from the first flashing arrangement to the further heating arrangement; - a second steam routing arrangement for routing steam from the second flashing arrangement to the preheating arrangement, which second steam routing arrangement comprises an ejector for adding a pressurized oxidizing gas to the steam.

[0037] As understood by the skilled person, the system is adapted for a continuous process and the HTC reactor is a continuous reactor.

[0038] In an embodiment of the second aspect, the system further comprises a pressure vessel, which is connected to an inlet of the ejector. In operation, the pressure vessel typically contains oxidizing agent, such as oxygen, having a pressure of about 14 bar. Optionally, a compressor is arranged between the pressure vessel and the inlet of the ejector. In such case, the pressure may be increased to about 20-25 bar. Oxygen may be generated on-site or off-site. For on-site generation, an oxygen separation apparatus, such as a cryogenic separation apparatus, may be connected to the pressure vessel. In case of off-site generation, the pressurized oxidizing gas may be supplied to the pressure vessel by a tanker truck.

[0039] In another embodiment, the system further comprises a compressor for compressing an oxidizing gas to obtain the pressurized oxidizing gas, which compressor is connected to an inlet of the ejector. Further, a pressure vessel may be connected to an inlet of the compressor. In operation, the pressure vessel typically contains oxidizing gas, such as oxygen gas, having a pressure of about 5 bar. When the oxidizing gas is oxygen gas, a pressure swing adsorption (PSA) apparatus may be connected to the pressure vessel for on-site generation.

[0040] The preheating arrangement preferably comprises a steam mixer, such as a venturi mixer. The steam mixer typically has a steam inlet connected to the second steam routing arrangement, another inlet for receiving sludge and an outlet. The outlet is preferably connected to a pump tank.

[0041] The further heating arrangement preferably comprises at least two steam mixers, such as at least two venturi mixers, arranged in series, each having an steam inlet connected to the first steam routing arrangement, another inlet for receiving sludge and an outlet, which is preferably connected to a pump tank.

[0042] The first flashing arrangement preferably comprises at least two flashing vessels arranged in series, each having an inlet for receiving slurry, a steam outlet connected to the first steam routing arrangement and a slurry outlet. The first steam routing arrangement preferably connects the steam outlet of the first flashing vessel of the first flashing arrangement to the steam inlet of last steam mixer of the further heating arrangement. Thereby, the first steam routing arrangement can route steam from the first flashing vessel of the first flashing arrangement exclusively to the last steam mixer of the further heating arrangement.

[0043] The second flashing arrangement preferably comprises a flashing vessel having an inlet for receiving slurry, a steam outlet connected to the second steam routing arrangement and a slurry outlet. Accordingly, the second steam routing arrangement preferably connects the steam outlet of the flashing vessel of the second flashing arrangement to the steam inlet of the steam mixer of the preheating arrangement. Thereby, the second steam routing arrangement can route steam from the second flashing arrangement exclusively to the preheating arrangement.

[0044] In one embodiment, the sludge routing arrangement comprises a gas mixer arranged downstream the further heating arrangement. A gas inlet of the gas mixer may be connected to a pressure vessel or a compressor. Thereby, a pressurized oxidizing gas, such as pressurized oxygen gas can be added to the heated sludge. Further, the sludge routing arrangement may comprise a reactor arranged downstream the gas mixer. The volume of such a reactor may be 5-50% of the volume of the HTC reactor.

[0045] In another embodiment, the slurry routing arrangement comprises a gas mixer arranged upstream the first flashing arrangement. A gas inlet of the gas mixer may be connected to a pressure vessel or a compressor. Thereby, a pressurized oxidizing gas, such as pressurized oxygen gas can be added to the slurry. Further, the slurry routing arrangement may comprise a reactor arranged downstream the gas mixer, but upstream the first flashing arrangement. The volume of such a reactor may be 5-50% of the volume of the HTC reactor.

[0046] In another embodiment, the system comprises a slurry fraction routing arrangement for routing a fraction of the HTC-treated slurry to a point in which it is mixed with heated sludge from the further heating arrangement. Hence, the slurry fraction routing arrangement connects a position on the slurry routing arrangement that is located upstream the first flashing arrangement to a position on the sludge routing arrangement that is located downstream the further heating arrangement. In such an embodiment, the slurry fraction routing arrangement comprises a gas mixer. A gas inlet of the gas mixer may be connected to a pressure vessel or a compressor.

Thereby, a pressurized oxidizing gas, such as pressurized oxygen gas can be added to the slurry. Further, the slurry fraction routing arrangement may comprise a reactor arranged downstream the gas mixer. The volume of such a reactor may be 5-50% of the volume of the HTC reactor.

[0047] In another embodiment, the system comprises a particle-lean fraction routing arrangement for routing a particle-lean fraction of the HTC-treated slurry to a point in which it is mixed with heated sludge from the further heating arrangement. The particle-lean fraction routing arrangement connects an upper outlet of the HTC reactor to a position on the sludge routing arrangement that is located downstream the further heating arrangement. The particle-lean fraction routing arrangement comprises a gas mixer. A gas inlet of the gas mixer may be connected to a pressure vessel or a compressor. Thereby, a pressurized oxidizing gas, such as pressurized oxygen gas can be added to the particle-lean fraction. Further, the particle-lean fraction routing arrangement may comprise a reactor arranged downstream the gas mixer. The volume of such a reactor may be 5-50% of the volume of the HTC reactor. The upper outlet of the HTC reactor is arranged above a lower outlet of the HTC reactor, which is the outlet connected to the slurry routing arrangement.

[0048] In another embodiment, the system comprises a particle-lean fraction routing arrangement for routing a particle-lean fraction of the HTC-treated slurry to a flashing vessel that is not receiving slurry from the HTC reactor (i.e. that is not part of the first flashing arrangement). The particle-lean fraction routing arrangement connects an upper outlet of the HTC reactor or an outlet of a separation device arranged on the slurry routing arrangement upstream the first flashing arrangement to the flashing vessel. A steam outlet of the flashing vessel is connected to the last steam mixer of the further heating device. A liquid outlet of the flashing vessel is advantageously connected to the slurry routing arrangement, preferably to a position located upstream the first flashing arrangement. The particle-lean fraction routing arrangement comprises a gas mixer. A gas inlet of the gas mixer may be connected to a pressure vessel or a compressor. Thereby, a pressurized oxidizing gas, such as pressurized oxygen gas can be added to the particle-lean fraction. Further, the particle-lean fraction routing arrangement may comprise a reactor arranged downstream the gas mixer. The volume of such a reactor may be 5-50% of the volume of the HTC reactor. If an upper outlet of the HTC reactor is employed, it is arranged above a lower outlet of the HTC reactor that is connected to the slurry routing arrangement.

[0049] Otherwise, the embodiments of the first aspect apply mutatis mutandis to the second aspect.

[0050] As an alternative configuration, there is provided a method of hydrothermal carbonization (HTC) of a sludge, comprising the steps of: a) pre-heating the sludge with a first steam fraction to obtain a preheated sludge; b) heating the pre-heated sludge with at least one further steam fraction to obtain a heated sludge; c) subjecting the heated sludge to HTC in a reactor to obtain a HTC-treated slurry; d) subjecting the HTC-treated slurry to flashing in at least one step to obtain said at least one further steam fraction and a pre-cooled slurry; e) adding a pressurized oxidizing gas to at least one of said at least one further steam fraction before it is/they are used for heating in step b); and f) subjecting the pre-cooled slurry flashing to obtain said first steam fraction.

[0051] The pressurized oxidizing gas, which is preferably pressurized oxygen gas, increases the pressure of the fraction(s) to which it is added. One or more ejector(s) is/are preferably used for the addition(s) of step e).

[0052] The fraction(s) to which the pressurized oxidizing gas is added in step e) may for example have a pressure of 3.0-8. o bar, such as 4.5-6.0 bar.

[0053] Otherwise, the embodiments of the aspects discussed above apply mutatis mutandis to the alternative configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figures 1-6 illustrate different exemplary embodiments of a system according to the present disclosure.

DETAILED DESCRIPTION

[0055] A first exemplary embodiment of a system too according to the present disclosure is schematically illustrated in Fig. 1. For the process in the system, a sludge is received from a source, which can be a municipal wastewater treatment plant, an industrial process, or an installation in agriculture or animal farming. The sludge may be received directly from the plant or from a storage tank that forms part of the system too. The sludge typically has an initial temperature of about 30°C and a dry matter content of about 30 % when it is a municipal sludge and about 15 % when it is a sludge from a pulp and paper mill. After optional initial heating (not shown), e.g. by a liquid stream from the same process/system, the sludge is preheated in a preheating arrangement 101 comprising a steam mixer 102. A pump tank I02t and a pump I02p are preferably arranged downstream the steam mixer 102 in that order. The pump tank i02t preferably comprises a gas outlet 102g comprising a valve 102v.

[0056] In a further heating arrangement 103, the preheated sludge from the preheating arrangement 101 is heated in by stepwise additions of steam, e.g. in a first 104, a second 105 and a third 106 steam mixer arranged in series. Downstream each steam mixer 104, 105, 106, a pump tank i04t, tost, io6t and a pump 104P, 105p, 106p are preferably arranged. Each pump tank 104t, 105t, 106t of the further heating arrangement preferably comprises a gas outlet 104g, 105g, 106g comprising a valve 104V, 105V, io6v. The last pump io6p brings the pressure up to the pressure of the HTC reaction.

[0057] The heated sludge from the further heating arrangement 103 is subjected to HTC in a reactor 107. Thereby, HTC-treated slurry is obtained. The temperature of the HTC-treated slurry is typically 200-215 °C when it is withdrawn from the reactor 107. The reactor preferably comprises a gas outlet 107g comprising a valve 107V.

[0058] The HTC-treated slurry from the reactor 107 is subjected to flashing in a first flashing arrangement 108, which produces at least one steam fraction that is used to heat the sludge in the further heating arrangement 103. Preferably, the first flashing arrangement 108 comprises several flashing vessels arranged in series to produce steam fractions of different temperatures. For example, the flashing arrangement 108 may comprise: a first flashing vessel 109 that produces a steam fraction of relatively high temperature that is routed to the third steam mixer 106 of the further heating arrangement 103; a second flashing vessel 110 that produces a steam fraction of medium temperature that is routed to the second steam mixer 105 of the further heating arrangement 103; and a third flashing vessel 111 that produces a steam fraction of relatively low temperature that is routed to the first steam mixer 104 of the further heating arrangement 103.

[0059] The pre-cooled slurry obtained from first flashing arrangement 108 is subjected to flashing in a flashing vessel, which constitutes a second flashing arrangement 112. Thereby, a low-pressure steam fraction and a cooled slurry is obtained. The low-pressure steam fraction advantageously has a pressure of only 1.00-1.05 bar (but the pressure may be up to 2.00 bar).

[0060] A steam routing arrangement 113 connects the second flashing arrangement 112 to the steam mixer 102 of the preheating arrangement 101. The steam routing arrangement 113 comprises an ejector 114 for adding a pressurized oxidizing gas to the low-pressure steam fraction from the second flashing arrangement 112 such that an oxidizing steam fraction is obtained. By the addition of the pressurized oxidizing gas, which typically has a pressure of more than 5 bar, the oxidizing steam fraction obtains a higher pressure than the low-pressure steam fraction. Typically, the pressure of the oxidizing steam fraction is at least 1.24 bar, preferably at least 1.50 bar, such as 2.003-00 bar. This increase in pressure allows the oxidizing steam fraction to be added to the sludge in the steam mixer 102 of the preheating arrangement 101. In other words, the addition of the pressurized oxidizing gas facilitates efficient heat recovery in the system too.

[0061] At the same time, the addition of the pressurized oxidizing gas introduces an oxidizing agent that will react with components of the sludge downstream the preheating arrangement 101. In particular, such wet oxidation reactions will take place when the temperature of the sludge has been increased further, i.e. in and after the further heating arrangement 103. The wet oxidation reactions taking place downstream the further heating arrangement will heat the sludge beyond the temperature that can be reached by steam-based heat recovery.

[0062] In one embodiment, the pressurized oxidizing gas is pressurized oxygen gas, i.e. gas comprising at least 85% O2. Benefits of such an embodiment is that oxygen gas is highly reactive, no or only a small amount of inert gas dilutes the sludge and no or only a limited amount of energy is wasted on compressing an inert gas.

[0063] In another embodiment, the pressurized oxidizing gas is pressurized air, i.e. a gas comprising about 21% O2. A benefit of such an embodiment is that no generation of oxygen gas is necessary, which saves energy. Inert gas introduced with the air may be purged through the gas outlets 102g, 104g, 105g, 106g, 107g by opening the valves 102v, 104V, 105V, io6v, 107V, in particular the downstream valves 105V, io6v, 107V. It is less preferred to purge a substantial amount of gas from the upstream valves 102v, 104V because at these points, a significant portion of the oxygen of the added air has typically not reacted yet.

[0064] The system too may comprise a heater using external heat (not shown), such as an electrical heater, for cold starting the process. Such a heater is preferably arranged downstream the further heating arrangement 103, but upstream the reactor 107.

[0065] A second exemplary embodiment of a system 200 according to the present disclosure is schematically illustrated in Fig. 2. The system 200 is the same as the system discussed above with reference to Fig. 1 except for an extra addition pressurized oxidizing gas, such as pressurized oxygen gas, which is made to the heated sludge for an extra temperature boost. For this extra addition, a gas mixer 215, such as an oxygen gas mixer, is arranged downstream the further heating arrangement 103, but upstream the reactor 107. Optionally, a reactor 216 for the wet oxidation reactions is arranged between the gas mixer 215 and the reactor 107. An alternative to the wet oxidation reactor 216 is to allow the wet oxidation reactions to be completed in the HTC reactor 107. If the amount of pressurized oxidizing agent that is added in the gas mixer 215 is relatively small, it may also be the case that the wet oxidation reactions are more or less completed already in pipe leading to the HTC reactor 107.

[0066] In the embodiment of Fig. 2, the amount of oxidizing gas added in the ejector 114 may be lower than in the embodiment of Fig. 1. One option is to add pressurized air in the ejector 114 and pressurized oxygen gas in the gas mixer 215. At the point of the gas mixer 215, the pressure of the heated sludge is relatively high, which means that the pressure of the pressurized oxygen gas added in the gas mixer 215 typically has to be above 20 bar.

[0067] A third exemplary embodiment of a system 300 according to the present disclosure is schematically illustrated in Fig. 3. The system 300 is the same as the system discussed above with reference to Fig. 1 except for an extra addition pressurized oxidizing gas, such as pressurized oxygen gas, which is made to the HTC-treated slurry to facilitate the generation of a steam fraction of particularly high temperature in the first flashing arrangement 108. For this extra addition, a gas mixer 315, such as an oxygen gas mixer, is arranged downstream the HTC reactor 107, but upstream the first flashing arrangement 108. Optionally, a reactor 316 for the wet oxidation reactions is arranged between the gas mixer 315 and the first flashing arrangement 108. If such a reactor is omitted and all the oxidizing gas added in the gas mixer 315 has not reacted by the time the slurry reaches the first flashing arrangement 108, the remaining oxidizing gas will be flashed off with the steam and recirculated to the sludge in the further heating arrangement 103. Consequently, the unreacted oxidizing gas will typically not be wasted.

[0068] In the embodiment of Fig. 3, the amount of oxidizing gas added in the ejector 114 may be lower than in the embodiment of Fig. 1. One option is to add pressurized air in the ejector 114 and pressurized oxygen gas in the gas mixer 315. The pressure of the HTC-treated slurry is relatively high, which means that the pressure of the pressurized oxygen gas added in the gas mixer 315 typically has to be above 20 bar.

[0069] A fourth exemplary embodiment of a system 400 according to the present disclosure is schematically illustrated in Fig. 4. The system 400 is the same as the system discussed above with reference to Fig. 1 except for an extra addition of pressurized oxidizing gas, such as pressurized oxygen gas, which is made to a fraction of the HTC-treated slurry separated prior to the first flashing arrangement 108. Thereby, a wet-oxidized fraction is obtained. This wet-oxidized fraction is added to the heated sludge at a point 417 downstream the further heating arrangement 103, but upstream the HTC reactor 107. For this addition, a t-type connection or a more advanced device for mixing two flows may be used. For the extra addition of pressurized oxidizing gas, a gas mixer 415, such as an oxygen gas mixer, is used. Optionally, a reactor 416 for the wet oxidation reactions is arranged downstream the gas mixer 415, but upstream the HTC reactor 107. An alternative to the wet oxidation reactor 416 is to allow the wet oxidation reactions to be completed in the HTC reactor 107. If the amount of pressurized oxidizing agent that is added in the gas mixer 415 is relatively small, it may also be the case that the wet oxidation reactions are more or less completed already in piping leading to the HTC reactor 107.

[0070] In the embodiment of Fig. 4, the amount of oxidizing gas added in the ejector 114 may be lower than in the embodiment of Fig. 1. One option is to add pressurized air in the ejector 114 and pressurized oxygen gas in the gas mixer 415. The pressure of the HTC-treated slurry is relatively high, which means that the pressure of the pressurized oxygen gas added in the gas mixer 415 typically has to be above 20 bar.

[0071] A fifth exemplary embodiment of a system 500 according to the present disclosure is schematically illustrated in Fig. 5. The system 500 is basically the same as the system discussed above with reference to Fig. 1, but there are some differences. In the system 500, an extra addition pressurized oxidizing gas, such as pressurized oxygen gas, is made to a particle-lean fraction of the HTC-treated slurry to boost the temperature.

[0072] In detail, the heated sludge from the further heating arrangement 103 is merged with a wet-oxidized fraction in a T-type connection 517 (or a more advanced device for mixing two flows), to form a reaction mixture, which is fed to a vertical reactor 507 (having a special design), in which the reaction mixture is subjected to HTC and the HTC-treated slurry is separated into a particle-lean fraction and a particle-rich fraction. The vertical reactor 507 comprises: a reactor inlet 508 arranged at the top of the vertical reactor 507; a first channel 509 extending downwardly from the reactor inlet 508 for guiding the reaction mixture from the inlet 508 to a bottom section of the reactor 508; a second channel 510 extending upwardly from the bottom section to a recirculation outlet 511 for withdrawing the particle-lean fraction; and a lower outlet 512 for withdrawing the particle-rich fraction. The design of the vertical reactor 507 enables fluidization that facilitates the separation into the particle-lean fraction and the particlerich fraction (see patent application SE 1750284 A1). The particle-rich fraction is subjected to flashing in the same was as the HTC-treated slurry in the system 100 discussed with reference to figure 1.

[0073] Part of the particle-lean stream may be recirculated to a bottom inlet of the vertical reactor 507 (not shown). A flow through such a bottom inlet aids the fluidization.

[0074] Pressurized oxidizing gas, such as pressurized oxygen gas, is added to the other part of the particle-lean fraction or to all of it in case the recirculation to the bottom inlet is omitted. By the addition of pressurized oxidizing gas, the wet-oxidized fraction is obtained.

[0075] A gas mixer 515, such as an oxygen gas mixer, is used for the addition of the pressurized oxidizing gas. Optionally, a reactor 516 for the wet oxidation reactions is arranged downstream the gas mixer 515, but upstream the vertical HTC reactor 507. An alternative to the wet oxidation reactor 516 is to allow the wet oxidation reactions to be completed in the vertical HTC reactor 507. If the amount of pressurized oxidizing agent that is added in the gas mixer 515 is relatively small, it may also be the case that the wet oxidation reactions are more or less completed already in piping leading to the vertical HTC reactor 507.

[0076] The volumetric flow rate of the wet-oxidized fraction is typically 10-50% of the volumetric flow rate of the heated sludge.

[0077] In the embodiment of Fig. 5, the amount of oxidizing gas added in the ejector 114 may be lower than in the embodiment of Fig. 1. One option is to add pressurized air in the ejector 114 and pressurized oxygen gas in the gas mixer 515. The pressure of the particle-lean fraction is relatively high, which means that the pressure of the pressurized oxygen gas added in the gas mixer 515 typically has to be above 20 bar.

[0078] A sixth exemplary embodiment of a system 600 according to the present disclosure is schematically illustrated in Fig. 6. The system 600 is similar to the system 500 discussed above with reference to Fig. 5, but there are some differences. In the system 600, the wet-oxidized fraction is not added to the heated sludge. Instead, it is subjected to flashing in a flashing vessel 650 to generate a steam fraction of particularly high temperature, which is the last steam fraction to be added in the further heating arrangement 103, and a liquid fraction that is merged with the particle-rich fraction prior to the first flashing arrangement 108.

[0079] The steam fraction of particularly high temperature is advantageously routed to a third steam mixer 106 of the further heating arrangement 103, which means that the first flashing arrangement 108 may comprise a second flashing vessel 110 that produces a steam fraction that is routed to a second steam mixer 105 of the further heating arrangement 103 and a third flashing vessel 111 that produces a steam fraction that is routed to a first steam mixer 104 of the further heating arrangement 103.

[0080] If a reactor 516 is not arranged downstream the gas mixer 515 and all the oxidizing gas added in the gas mixer 515 has not reacted by the time the wet-oxidized fraction reaches the first flashing vessel 650, the remaining oxidizing gas will be flashed off with the steam and recirculated to the sludge in the further heating arrangement 103. Consequently, the unreacted oxidizing gas will typically not be wasted.

Claims

1. A method of hydrothermal carbonization (HTC) of a sludge, characterized in that the method comprises the steps of: a) pre-heating the sludge by mixing it with an oxidizing steam fraction to obtain a preheated sludge; b) heating the pre-heated sludge with at least one further steam fraction to obtain a heated sludge; c) subjecting the heated sludge to HTC in a reactor (107, 507) to obtain a HTC-treated slurry; d) subjecting the HTC-treated slurry to flashing in at least one step to obtain the at least one further steam fraction and a pre-cooled slurry; e) subjecting the pre-cooled slurry to flashing to obtain a low-pressure steam fraction having a pressure of 1.00-2.00 bar, preferably 1.00-1.30 bar, more preferably 1.00-1.05 bar; and f) adding a pressurized oxidizing gas to the low-pressure steam fraction to obtain the oxidizing steam fraction, which has a higher pressure than the low-pressure steam fraction.

2. The method according to claim 1, wherein step b) comprises heating the preheated sludge with at least two further steam fractions and step d) comprises subjecting the HTC-treated slurry from step c) to flashing in at least two steps to obtain the at least two further steam fractions.

3. The method according to claim 1 or 2, wherein the pressurized oxidizing gas is added to the low-pressure steam fraction in an ejector (114).

4. The method according to any one of the preceding claims, wherein further pressurized oxidizing gas is added to the heated sludge prior to step c) or to the HTC-treated slurry prior to step d).

5. The method according to any one of the preceding claims, further comprising: - separating a fraction from the HTC-treated slurry prior to step d); - adding a pressurized oxidizing gas to the separated fraction to obtain a wet-oxidized fraction; and - adding the wet-oxidized fraction to the heated sludge.

6. The method according to any one of claims 1-4, further comprising: - separating a fraction from the HTC-treated slurry prior to step d); - adding a pressurized oxidizing gas to the separated fraction to obtain a wet-oxidized fraction; and - subjecting the wet-oxidized fraction to flashing to obtain a high-temperature steam fraction that is used to further heat the heated sludge prior to step c).

7. The method according to any one of the preceding claims, wherein the sludge is a municipal or industrial sludge from a wastewater treatment plant.

8. The method according to any one of the preceding claims, wherein the temperature of the HTC-treated slurry is 180-250 °C, preferably 180-230 °C and more preferably 190-225 °C.

9. The method according to any one of the preceding claims, wherein the pressure of the oxidizing steam fraction is at least 1.24 bar, preferably at least 1.60 bar, such as 2.00-3.00 bar.

10. The method according to any one of the preceding claims, wherein the temperature of the preheated sludge is 70-95 °C, preferably 80-90 °C.

11. A system (100, 200, 300, 400, 500, 600) for hydrothermal carbonization (HTC) of a sludge, comprising: - a HTC reactor (107, 507) for subjecting sludge to HTC such that HTC-treated slurry is obtained; - a sludge routing arrangement for routing sludge to the HTC reactor, which sludge routing arrangement comprises a preheating arrangement (101) and a further heating arrangement (103), wherein the further heating arrangement is arranged downstream the preheating arrangement; and - a slurry routing arrangement for routing HTC-treated slurry from the HTC reactor, which slurry routing arrangement comprises a first (108) and a second flashing arrangement (112), which second flashing arrangement is arranged downstream the first flashing arrangement; - a first steam routing arrangement for routing steam from the first flashing arrangement to the further heating arrangement; - a second steam routing arrangement (113) for routing steam from the second flashing arrangement to the preheating arrangement, characterized in that the second steam routing arrangement comprises an ejector (114) for adding a pressurized oxidizing gas to the steam.

12. The system of claim 11, further comprising a pressure vessel, which is connected to an inlet of the ejector.

13. The system of claim 12, further comprising an oxygen separation apparatus, such as a cryogenic separation apparatus, which is connected to the pressure vessel.

14. The system of claim 11, further comprising a compressor for compressing an oxidizing gas to obtain the pressurized oxidizing gas, which compressor is connected to an inlet of the ejector, and optionally a pressure vessel, which is connected to an inlet of the compressor.

15. The system of claim 14, further comprising an oxygen separation apparatus, such as a pressure swing adsorption (PSA) apparatus, which is connected to the pressure vessel.