Oxidation and Subsequent Hydrothermal Carbonization of Sludge

Abstract

There is provided a method of hydrothermal carbonization of a sludge, comprising the steps of: a) preheating the sludge to obtain a preheated sludge; b) adding an oxidizing agent, such as oxygen gas, to the preheated sludge; and c) subjecting the sludge from step b) to hydrothermal carbonization (HTC) in a reactor to obtain a HTC-treated sludge.

Claims

1. A method of hydrothermal carbonization of a sludge, comprising the steps of: a) preheating the sludge to obtain a preheated sludge; b) adding an oxidizing agent to the preheated sludge; and c) subjecting the sludge from step b) to hydrothermal carbonization (HTC) in a reactor to obtain a HTC-treated sludge.

2. The method according to claim 1, further comprising the step: d) subjecting the HTC-treated sludge from step c) to flashing to obtain at least one steam fraction and a cooled fraction, wherein the at least one steam fraction is used in the preheating of step a).

3. The method according to claim 1, wherein the sludge passes through a reactor for wet oxidation between step b) and step c).

4. The method of claim 3, wherein the volume of the reactor for wet oxidation is smaller than the volume of the reactor of step c).

5. The method of claim 4, wherein the volume of the reactor for wet oxidation is 10-50% of the volume of the reactor of step c).

6. The method of claim 5, wherein the volume of the reactor for wet oxidation is 20-40% of the volume of the reactor of step c).

7. The method according to claim 1, wherein the sludge is a municipal or industrial sludge from a wastewater treatment plant.

8. The method according to claim 1, wherein the temperature of the HTC-treated sludge in step c) is 180-250 C.

9. The method according to claim 1, wherein the temperature of the HTC-treated sludge in step c) is at least 20 C. higher than the temperature of the preheated sludge to which the oxidizing agent is added in step b).

10. The method according to claim 1, wherein the temperature of the preheated sludge to which the oxidizing agent is added in step b) is 145-195 C.

11. The method according to claim 1, wherein the average retention time in the reactor of step c) is 0.25-8 h.

12. The method according to claim 1, wherein the amount of oxidizing agent added in step b) is such that wet oxidation reactions reduces the higher heating value (HHV) of the sludge by 5-49%.

13. The method according to claim 1, wherein the oxidizing agent is oxygen gas.

14. The method according to claim 1, wherein the temperature of the HTC-treated sludge in step c) is 190-225 C.

15. The method according to claim 1, wherein the temperature of the HTC-treated sludge in step c) is at least 30 C. higher than the temperature of the preheated sludge to which the oxidizing agent is added in step b).

16. The method according to claim 1, wherein the temperature of the preheated sludge to which the oxidizing agent is added in step b) is 165-190 C.

17. The method according to claim 1, wherein the average retention time in the reactor of step c) is 0.5-2 h.

18. The method according to claim 1, wherein the amount of oxidizing agent added in step b) is such that wet oxidation reactions reduces the higher heating value (HHV) of the sludge by 6-15%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIGS. 1 and 2 illustrate different exemplary embodiments of a method according to the present disclosure.

DETAILED DESCRIPTION

[0035] A first exemplary embodiment of a method according to the present disclosure is schematically illustrated in FIG. 1. A sludge is received (A) 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. 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), e.g. by a liquid stream from the same process/system, the sludge is preheated in a preheating arrangement 101. The preheating is preferably carried out by stepwise additions of steam, e.g. in a first 102, a second 103 and a third 104 steam mixer arranged in series. Downstream each steam mixer 102, 103, 104, a pump 102p, 103p, 104p is arranged. After the preheating arrangement 101, a preheated sludge having a temperature of about 175 C. is obtained.

[0036] Oxygen gas is added to the preheated sludge in an oxygen gas mixer 105 to achieve partial wet oxidation of the preheated sludge. The oxygen gas mixer is connected to an oxygen tank (not shown). The amount of oxygen gas may be about 130 kg per tonne of dry sludge processed in the system. The wet oxidation is not instantaneous. Rather, it will be ongoing when the sludge flows downstream the oxygen gas mixer 105.

[0037] After the addition of oxygen, the sludge is routed to a reactor 106 for hydrothermal carbonization (HTC) of the sludge. In an upstream section of the reactor 106, the wet oxidation reactions may be ongoing, which means that the temperature is typically lower in the upstream section of the reactor 106 than in a downstream section of the reactor 106. In the downstream section of the reactor, the temperature is typically within the range of 205-215 C.

[0038] The HTC-treated sludge from the reactor 106 is subjected to flashing in a flashing arrangement 107, which produces at least one steam fraction that is used to preheat the sludge in the preheating arrangement 101. Preferably, the flashing arrangement 107 comprises several flashing vessels arranged in series to produce steam fractions of different temperatures. For example, the flashing arrangement 107 may comprise: a first flashing vessel 108 that produces a steam fraction of relatively high temperature that is routed to the third steam mixer 104 of the preheating arrangement 101; a second flashing vessel 109 that produces a steam fraction of medium temperature that is routed to the second steam mixer 103 of the preheating arrangement 101; and a third flashing vessel 110 that produces a steam fraction of relatively low temperature that is routed to the first steam mixer 102 of the preheating arrangement 101.

[0039] The cooled slurry obtained downstream the flashing arrangement 107 is dewatered (not shown) such that at least one liquid stream (that can be used for initial heating and/or dilution of incoming sludge) and a thick fraction comprising HTC coal are obtained.

[0040] The system 100 may comprise a heater 111 using external heat, such as an electrical heater, for cold starting the process. The heater 111 is preferably arranged downstream the oxygen gas mixer, but upstream the reactor 106.

[0041] A second exemplary embodiment of a system according to the present disclosure is schematically illustrated in FIG. 2. A sludge is received (A) 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. 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), e.g. by a liquid stream from the same process/system, the sludge is preheated in a preheating arrangement 101. The preheating is preferably carried out by stepwise additions of steam, e.g. in a first 102, a second 103 and a third 104 steam mixer arranged in series. Downstream each steam mixer 102, 103, 104, a pump 102p, 103p, 104p is arranged. After the preheating arrangement 101, a preheated sludge having a temperature of about 175 C. is obtained.

[0042] Oxygen gas is added to the preheated sludge in an oxygen gas mixer 105 to achieve partial wet oxidation of the preheated sludge. The oxygen gas mixer is connected to an oxygen tank (not shown). The amount of oxygen gas may be about 130 kg per tonne of dry sludge processed in the system. The wet oxidation is not instantaneous. Therefore, a reactor 112 for wet oxidation is arranged downstream the oxygen gas mixer 105. The retention time of the sludge in such a reactor 112 may be about 30 min, which is considered to be sufficient for the wet oxidation reactions.

[0043] The temperature of the wet-oxidized sludge from the reactor 112 for wet oxidation is typically 200-215 C. This wet-oxidized sludge is routed to a reactor 106 for hydrothermal carbonization (HTC) of the sludge. The temperature may vary slightly between different positions in the HTC reactor 106 because of exothermic reactions (e.g. the HTC reactions and possibly oxidations by remaining oxygen) and heat losses. The retention time of the sludge in the HTC reactor 106 may be about 1.5 h, i.e. about three times longer than the retention time in the reactor 112 for wet oxidation. Accordingly, the volume of the HTC reactor may be three times greater than the volume of the reactor 106 for wet oxidation.

[0044] The HTC-treated sludge from the reactor 106 is subjected to flashing in a flashing arrangement 107, which produces at least one steam fraction that is used to preheat the sludge in the preheating arrangement 101. Preferably, the flashing arrangement 107 comprises several flashing vessels arranged in series to produce steam fractions of different temperatures. For example, the flashing arrangement 107 may comprise: a first flashing vessel 108 that produces a steam fraction of relatively high temperature that is routed to the third steam mixer 104 of the preheating arrangement 101; a second flashing vessel 109 that produces a steam fraction of medium temperature that is routed to the second steam mixer 103 of the preheating arrangement 101; and a third flashing vessel 110 that produces a steam fraction of relatively low temperature that is routed to the first steam mixer 102 of the preheating arrangement 101.

[0045] The cooled slurry obtained downstream the flashing arrangement 107 is dewatered (not shown) such that at least one liquid stream (that can be used for initial heating and/or dilution of incoming sludge) and a thick fraction comprising HTC coal are obtained.

[0046] The system 100 may comprise a heater 111 using external heat, such as an electrical heater, for cold starting the process. The heater 111 is preferably arranged downstream the oxygen gas mixer, but upstream the reactor 106.