METHOD AND SYSTEM FOR HYDROTHERMAL CARBONIZATION AND WET OXIDATION OF SLUDGE

Abstract

There is provided a method of hydrothermal carbonization of a sludge from a wastewater treatment plant, comprising the steps of: preheating the sludge with at least one first steam fraction to obtain a preheated sludge; further heating the preheated sludge with a second steam fraction to obtain a heated sludge; subjecting the heated sludge to hydrothermal carbonization (HTC) in a reactor to obtain a HTC-treated slurry; mixing the HTC-treated slurry with an oxidizing agent, such as oxygen gas, to obtain a wet-oxidized slurry; subjecting the wet-oxidized slurry to flashing to obtain the second steam fraction and a pre-cooled slurry; subjecting the pre-cooled slurry to flashing in at least one step to obtain the at least one first steam fraction and a cooled slurry; separating the cooled slurry into a liquid fraction and a solids fraction; and routing the liquid fraction to the wastewater treatment plant for further treatment, wherein the second steam fraction is used for heating preheated sludge to the temperature of the HTC reaction. A corresponding system is also provided.

Claims

1. A method of hydrothermal carbonization of a sludge from a wastewater treatment plant, comprising the steps of: preheating the sludge with at least one first steam fraction to obtain a preheated sludge; further heating the preheated sludge with a second steam fraction to obtain a heated sludge; subjecting the heated sludge to hydrothermal carbonization (HTC) in a reactor to obtain a HTC-treated slurry; mixing the HTC-treated slurry with an oxidizing agent to obtain a wet-oxidized slurry; subjecting the wet-oxidized slurry to flashing to obtain the second steam fraction and a pre-cooled slurry; subjecting the pre-cooled slurry to flashing in at least one step to obtain the at least one first steam fraction and a cooled slurry; separating the cooled slurry into a liquid fraction and a solids fraction; and routing the liquid fraction to the wastewater treatment plant for further treatment, wherein the second steam fraction is used for heating preheated sludge to the temperature of the HTC reaction.

2. The method according to claim 1, wherein the temperature of the HTC-treated slurry is 180-250° C.

3. The method according to claim 1, wherein the temperature of the wet-oxidized slurry is 220-260° C.

4. The method according to claim 1, wherein the temperature of the second steam fraction is 190-240° C.

5. The method according to claim 1, wherein the step of further heating the preheated sludge results in a temperature increase of at least 10° C.

6. The method according to claim 1, wherein the temperature of the second steam fraction is 9-40° C. higher than the temperature of the heated sludge.

7. The method according to claim 1, wherein the temperature of the second steam fraction is 25-75° C. higher than the temperature of the preheated sludge.

8. The method according to claim 1, wherein the sludge is a municipal or industrial sludge.

9. The method according to claim 1, wherein the pre-cooled slurry is subjected to flashing in at least two steps to obtain at least two first steam fractions of different temperatures, which are used for sequential heating of the sludge in the preheating step.

10. The method according to claim 1, wherein the residence time between the step of mixing the HTC-treated slurry with the oxidizing agent and the step of subjecting the wet-oxidized slurry to flashing is 5-75 min.

11. A system for hydrothermal carbonization (HTC) of a sludge from a wastewater treatment plant, comprising: a reactor for subjecting sludge to a HTC such that HTC-treated slurry is obtained (HTC reactor); a separating arrangement for separating wet-oxidized and cooled slurry into a liquid fraction and a solids fraction; 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 to the separating arrangement, which slurry routing arrangement comprises a mixer for mixing the HTC-treated slurry with an oxidizing agent and a first and a second flashing arrangement, wherein the mixer is arranged upstream the first flashing arrangement and the 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; and a liquid fraction routing arrangement for routing the liquid fraction from the separating arrangement to the wastewater treatment plant.

12. The system according to claim 11, wherein the slurry routing arrangement comprises a second reactor arranged between the mixer and the first flashing arrangement.

13. The system according to claim 11, wherein the mixer is part of a second reactor.

14. The system according to claim 12, wherein the volume of the second reactor is 5-30% of the volume of the HTC reactor.

15. The system according to claim 11, wherein: the second flashing arrangement comprises at least two flashing vessels arranged in series to provide steam fractions of different temperatures; the preheating arrangement comprises at least two steam mixers arranged in series; and the second steam routing arrangement connects the at least two flashing vessels to the at least two steam mixers such that the sludge can be preheated stepwise.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] FIG. 1 illustrate an exemplary embodiment of a system for sludge treatment according to the present disclosure.

DETAILED DESCRIPTION

[0057] An exemplary embodiment of a system 100 according to the present disclosure is schematically illustrated in FIG. 1. A sludge is received from a wastewater treatment plant 101. The sludge may be received directly from the plant 101 or from a storage tank (not shown) that forms part of the system 100. The sludge typically has an initial temperature of about 30° C. and a dry matter content of about 30%. After optional initial heating (not shown), the sludge is preheated in a preheating arrangement 103. The preheating is preferably carried out by stepwise additions of steam, e.g. in a first 104 and a second 105 steam mixer arranged in series. Downstream each steam mixer 104, 105, a pump 104P, 105P is arranged. After the preheating arrangement 103, a preheated sludge having a temperature of about 175° C. is obtained. The preheated sludge is heated further in a further heating arrangement 106, which is typically a steam mixer and after which a pump 106p is arranged. The further heated sludge is fed to a reactor 108, in which the sludge undergoes hydrothermal carbonization (HTC). An HTC-treated slurry, which typically has a temperature of 200-215° C. is withdrawn from the reactor 108. The pressure of the HTC-treated slurry is slightly increased a pump (not shown). Oxygen gas is then added to the HTC-treated slurry in an oxygen gas mixer 110. The oxygen gas mixer is connected to a pressurized oxygen tank (not shown). The amount of oxygen gas may be about 250 kg per tonne of dry sludge processed in the system 100. The wet oxidation is not instantaneous. Rather, it will be ongoing when the fraction flows downstream the oxygen gas mixer 110. To allow time for the exothermic wet oxidation reactions, a second reactor 111 may therefore be arranged downstream the oxygen gas mixer 110. The wet-oxidized slurry obtained from the second reactor 111 typically has a temperature of about 240° C. and is led to a first flashing arrangement 112 producing a high-temperature steam fraction having a temperature of about 220° C. and a pre-cooled slurry. The high-temperature steam fraction is routed to the further heating arrangement 106 and is thus used to heat the preheated sludge.

[0058] The pre-cooled slurry is subjected to flashing in a second flashing arrangement 113, which produces at least one steam fraction that is used to preheat the sludge in the preheating arrangement 103. Preferably, the flashing arrangement 113 comprises several flashing vessels arranged in series to produce steam fractions of different temperatures. For example, the flashing arrangement 113 may comprise: a first flashing vessel 114 that produces a steam fraction of “medium” temperature that is routed to the second steam mixer 105 of the preheating arrangement 103; and a second flashing vessel 115 that produces a steam fraction of relatively low temperature that is routed to the first steam mixer 104 of the preheating arrangement 103.

[0059] The cooled slurry obtained downstream the second flashing arrangement 113 is separated by a separating arrangement 116 into a liquid stream that is routed back to the wastewater treatment plant 101 for further treatment and a solids fraction that has been enriched in ash components, such as phosphorous, by the upstream wet oxidation process.

[0060] The system 100 may comprise a heater 107 using external heat, such as an electrical heater, for cold-starting the process. The heater 107 is preferably arranged downstream the further heating arrangement 106, but upstream the reactor 108.

[0061] The preheating arrangement 103, the further heating arrangement 106, the associated pumps and the heater 107 forms part of a sludge routing arrangement 102 for routing sludge to the reactor 108.

[0062] The oxygen gas mixer 110, the second reactor 111, the first flashing arrangement 112 and the second flashing arrangement 113 forms part of a slurry routing arrangement 109 for routing HTC-treated slurry from the reactor 108 to the separating arrangement 116.