ANAEROBIC DIGESTION SYSTEM FOR PRODUCTION OF BIOGAS WITH A REDUCED HYDROGEN SULPHIDE CONTENT AND METHOD FOR PRODUCTION OF BIOGAS WITH A REDUCED HYDROGEN SULPHIDE CONTENT IN AN ANAEROBIC DIGESTION SYSTEM

Abstract

The application relates to an anaerobic digestion system for production of biogas with a reduced hydrogen sulphide content. The system comprises an airtight reactor vessel comprising: sludge comprising a surface layer and a volume underneath the surface layer comprising bio-matter and micro-organisms that are able to anaerobically digest the bio-matter, thereby producing biogas comprising hydrogen sulphide, a headspace above the surface layer of the sludge into which biogas migrates out of the sludge, a spraying system for spraying an aqueous nitrate containing solution in the headspace using a carrier gas stream to form a nitrate containing interphase layer between the surface layer of the sludge and the headspace, with the proviso that the reactor vessel does not comprise a membrane or fabric positioned in the headspace.

Claims

1-20. (canceled)

21. Anaerobic digestion system for production of biogas with a reduced hydrogen sulphide content, comprising an airtight reactor vessel (10) comprising a sludge (11) comprising a surface layer (11a); and a volume (11b) underneath the surface layer comprising bio-matter and micro-organisms that are able to anaerobically digest the bio-matter into biogas, the biogas comprising hydrogen sulphide; a headspace (12) above the surface layer (11a) of the sludge into which purified biogas migrates out of the sludge (11); a spraying system for spraying an aqueous nitrate containing solution in the headspace (12) using a carrier gas stream to form a nitrate containing interphase layer (11c) between the surface layer (11a) of the sludge and the headspace (12), wherein the interphase layer (11c) comprises the same species of micro-organisms as the ones present in the sludge (11), and wherein at least part of the nitrate of the interphase layer (11c) is used by the micro-organisms present in the interphase layer (11c) to oxidize at least part of the hydrogen sulphide present in the produced biogas to sulphate or sulphur, resulting in the headspace (12) being filled with biogas with a reduced hydrogen sulphide content; with the proviso that the reactor vessel (10) does not comprise a membrane or fabric positioned in the headspace (12).

22. Anaerobic digestion system according to claim 21, wherein the gas in the carrier gas stream is recirculated biogas.

23. Anaerobic digestion system according to claim 21, wherein the gas in the carrier gas stream is compressed gas from a source outside the reactor vessel (10), more specifically compressed nitrogen gas.

24. Anaerobic digestion system according to claim 22, wherein the anaerobic digestion system comprises a pipeline (18) leading the purified biogas out of the headspace (12), and wherein the anaerobic digestion system comprises a gas tight vacuum pump (14) to suck biogas out of the headspace (12) of the reactor vessel (10) or the pipeline (18).

25. Anaerobic digestion system according to claim 21, wherein the spraying system comprises one or more venturi nozzles (13), arranged to produce aqueous nitrate containing solution droplets using the carrier gas stream.

26. Anaerobic digestion system according to claim 25, wherein a gas tight vacuum pump (14), optionally connected to a control box (20), is connected to the one or more venturi nozzles (13) to press the recirculated biogas serving as the carrier gas stream through the one or more venturi nozzles (13).

27. Anaerobic digestion system according to claim 25, wherein the anaerobic digestion system comprises a low speed and low pressure dosage pump (15), optionally connected to a control box (20), to apply the aqueous nitrate containing solution to the one or more venturi nozzles (13).

28. Anaerobic digestion system according to claim 21, wherein the aqueous nitrate containing solution includes an aqueous sodium nitrate solution, an aqueous potassium nitrate solution, an aqueous calcium nitrate solution, an aqueous magnesium nitrate solution or an aqueous nitric acid solution and/or wherein the aqueous nitrate containing solution has a concentration of between 10 to 30 weight % of nitrate.

29. Anaerobic digestion system according to claim 21, further comprising a hydrogen sulphide gas sensor (21) to measure the concentration of hydrogen sulphide in the biogas leaving the headspace (12).

30. Anaerobic digestion system according to claim 21, further comprising a storage tank (16) for storing the aqueous nitrate containing solution, the storage tank optionally comprising a temperature sensor (22), optionally connected to a control box (20), and/or a level sensor (17), optionally connected to the control box (20).

31. Anaerobic digestion system according claim 30, wherein the storage tank (16) comprises a temperature sensor (22) connected to a control box (20) and a heat source (23) connected to the control box (20).

32. Method for producing biogas with a reduced hydrogen sulphide content in an anaerobic digestion system, comprising the steps of: providing an airtight reactor vessel; providing a sludge in the airtight reactor vessel having a surface layer and a volume underneath the surface layer comprising bio-matter and micro-organisms; anaerobically digesting the bio-matter, thereby producing biogas comprising hydrogen sulphide, the biogas migrating out of the sludge and rising up into the headspace of the reaction vessel; spraying an aqueous nitrate containing solution in a headspace of the reactor vessel above the surface layer using a carrier gas stream; forming a nitrate containing interphase layer between the surface layer of the sludge and the headspace, the interphase layer comprising the same species of micro-organisms as the ones present in the sludge; purifying the biogas, that is produced in the sludge and that passes through the interphase layer when migrating out of the sludge to rise up into the headspace, by oxidizing hydrogen sulphide present in the biogas to sulphate or sulphur by the micro-organisms present in the interphase layer using nitrate of the interphase layer, whereby the purified biogas enters the headspace; with the proviso that no membrane or fabric is positioned in the headspace of the reactor vessel.

33. Method according to claim 32, wherein the method comprises the step of continuously measuring the residual hydrogen sulphide concentration in the purified biogas.

34. Method according to claim 31, wherein a storage tank is provided for storing the aqueous nitrate containing solution, and wherein the method includes the step of continuously measuring the temperature of the aqueous nitrate containing solution in the storage tank and/or, wherein the method comprises the step of continuously measuring the level of the aqueous nitrate containing solution in the storage tank.

35. Method according to claim 34, wherein the storage tank comprises a heat source to heat the aqueous nitrate containing solution in the storage tank, wherein the method comprises the step of heating the aqueous nitrate containing solution in the storage tank if the temperature of the aqueous nitrate containing solution in the storage tank is below a predetermined temperature, in particular below 5 C.

36. Method according to claim 32, wherein the anaerobic digestion system comprises one or more of the following: a low speed and low pressure dosage pump to apply the aqueous nitrate containing solution to the one or more venturi nozzles and wherein the method comprises the step of continuously controlling the start and stop of the dosage pump; and/or a gas tight vacuum pump to suck biogas out of the headspace of the reactor vessel, and wherein the method comprises the step of controlling the start and stop of the vacuum pump.

37. Method according to claim 32, wherein the method comprises the step of continuously measuring the dosage of the aqueous nitrate containing solution in the headspace of the reactor vessel.

38. Computer program product comprising program code instructions for implementing a method according claim 32, wherein the anaerobic digestion system comprises a dosage pump (15) for applying the aqueous nitrate containing solution to one or more venturi nozzles (13) to the headspace of the reactor vessel, a control box (20) and a hydrogen sulphide gas sensor (21) to measure the concentration of the hydrogen sulphide in the biogas leaving the headspace; and wherein the computer program product comprises program code instructions for controlling the start and stop of the dosage pump, wherein a control box receives data from a hydrogen sulphide gas sensor and, based on a threshold value, activates the dosage pump (15).

39. Anaerobic digestion system according to claim 21 configured for executing a method for producing biogas with a reduced hydrogen sulphide content in an anaerobic digestion system, comprising the steps of: providing an airtight reactor vessel; providing a sludge in the airtight reactor vessel having a surface layer and a volume underneath the surface layer comprising bio-matter and micro-organisms; anaerobically digesting the bio-matter, thereby producing biogas comprising hydrogen sulphide, the biogas migrating out of the sludge and rising up into the headspace of the reaction vessel; spraying an aqueous nitrate containing solution in a headspace of the reactor vessel above the surface layer using a carrier gas stream; forming a nitrate containing interphase layer between the surface layer of the sludge and the headspace, the interphase layer comprising the same species of micro-organisms as the ones present in the sludge; purifying the biogas, that is produced in the sludge and that passes through the interphase layer when migrating out of the sludge to rise up into the headspace, by oxidizing hydrogen sulphide present in the biogas to sulphate or sulphur by the micro-organisms present in the interphase layer using nitrate of the interphase layer, whereby the purified biogas enters the headspace; with the proviso that no membrane or fabric is positioned in the headspace of the reactor vessel.

Description

DESCRIPTION OF THE FIGURES

[0047] FIG. 1 shows a schematic representation of an anaerobic digestion system for biogas production according to the prior art using a fabric installed in the headspace of the reactor vessel;

[0048] FIG. 2 shows a schematic representation of an anaerobic digestion system for biogas production according to an embodiment of the application.

DETAILED DESCRIPTION OF THE INVENTION

[0049] The anaerobic digestion system according to the present application, as shown in FIG. 2, comprises an airtight covered reactor vessel (10). On the one hand, the reactor vessel (10) comprises an anaerobic (fermentation) sludge (also called effluent) (11) having a surface layer (11a) and a volume underneath the surface layer (11a) comprising organic matter and micro-organisms that are able to anaerobically digest the organic matter producing biogas. The biogas consists primarily of methane, carbon dioxide, and (undesired) hydrogen sulphide. On the other hand, the reactor vessel (10) comprises a headspace (12) which is filled with biogas that migrates out of the fermenting sludge (11) and rises up into the headspace (12). The reactor vessel (10) does not comprise a membrane or fabric positioned in the headspace (12).

[0050] In order to reduce the hydrogen sulphide content in the biogas rising up into the headspace (12) of the reactor vessel (10), a spraying system is installed in the headspace (12) that is arranged to spray an aqueous nitrate containing solution in the headspace (12) using a carrier gas stream. The spraying system may comprise one or more spraying nozzles (13) using the carrier gas stream to spray the aqueous nitrate containing solution in the headspace (12). The combination of the spraying nozzles (13) and the carrier gas stream takes care that the aqueous nitrate containing solution that is injected as droplets are ripped apart by the carrier gas stream and sprayed out into the headspace (12). From there, they are evenly distributed in the headspace (12) forming an aerosol or mist of aqueous nitrate containing solution in the headspace (12) that finally settles on the surface layer (11a) of the sludge (11). In this way, an interphase layer (11c) of aqueous nitrate containing solution is formed on the surface layer (11a) of the sludge (11). This interphase layer (11c) is thus situated between the surface layer (11b) of the sludge (11) and the headspace (12). The average thickness of the interphase layer (11c) as a result of aerosol precipitation ranges from about 0.01 mm to about 1 mm, preferably from about 0.05 mm to about 0.5 mm, more preferably is about 0.02 mm. More in particular, venturi nozzles (13) are applied as spraying nozzles since these have the best atomizing effect for highly viscous liquids including aqueous nitrate containing solutions.

[0051] The interphase layer (11c) is inhabited by the same species of micro-organisms as the anaerobic sludge (11). However, due to the lower redox potential in the interphase layer (11c) than in the sludge (11), the metabolism of the microorganisms in the sludge (11) is different from the metabolism of the microorganisms in the interphase layer (11c). This higher redox potential in the sludge (11) leads to hydrogen sulphide production. The biogas with the hydrogen sulphide passes through the interphase layer (11c), in which the hydrogen sulphide is oxidized to sulphates by the microorganisms present in the interphase layer (11c) using nitrate present therein. The biogas that enters the headspace (12) consequently has a reduced hydrogen sulphide content. Accordingly, hydrogen sulphide is effectively removed from the produced biogas even before it reaches the headspace (12).

[0052] The nitrate concentration in the interphase layer (11c) must be high enough to stimulate the sulphide oxidation by the present micro-organisms. The aqueous nitrate containing solution typically has a concentration of between 10 to 30 weight % of nitrate. The aqueous nitrate containing solution may include an aqueous sodium nitrate solution, an aqueous potassium nitrate solution, an aqueous calcium nitrate solution, an aqueous magnesium nitrate solution or an aqueous nitric acid solution.

[0053] More in particular, an aqueous calcium nitrate solution is sprayed into the headspace (12), through which a calcium nitrate rich interphase layer is formed between the surface layer (11a) of the sludge (11) and the headspace (12). The hydrogen sulphide that is formed and that passes through the interphase layer (11c), will be oxidized to sulphates that react with calcium, and consequently precipitate as calcium sulphate (gypsum). The precipitated gypsum is removed from the reactor vessel (10) together with the digested solid sludge fraction. This digested solid sludge fraction is more in particular continuously pumped out of the reactor vessel (10).

[0054] Since the carrier gas stream may not influence the biogas quality, the carrier gas stream more in particular can be recirculated biogas (21). Therefore, a gas tight vacuum pump (14) may be provided to suck the biogas out of the headspace (12) or a pipeline (18) leading the purified biogas out of the headspace (12), and press it through the one or more venturi nozzles (13). The carrier gas can also be a compressed gas from a source outside the reactor vessel (10) such as compressed nitrogen gas.

[0055] The aqueous nitrate containing solution is applied to the one or more venturi nozzles (13) by means of a dosage pump (15), more in particular a low pressure and low speed dosage pump. It is possible to use such a pump since the atomization of the aqueous nitrate containing solution is done via the venturi-effect of the venturi-nozzles (13) and does not need to be done by means of the pump itself. The dosage pump (15) sucks aqueous nitrate containing solution out of a covered storage tank (16).

[0056] The biogas that migrates out of the interphase layer (11c) into the headspace (12) and that is practically free from hydrogen sulphide then moves out of the headspace (12) to the pipeline (18) to either be used further as energy source or to move to a recirculation pipeline (19) to use the recirculated biogas as carrier gas for the spraying of the aqueous nitrate containing solution.

[0057] The dosage pump (15) can be controlled to prevent over dosage and to stop dosage whenever the storage tank (16) reaches a lowest level. It is important that the dosage lines are always filled with aqueous nitrate containing solution as maintenance inside the reactor vessel (10) is difficult. In addition, there is a risk that biogas could be released through empty dosage lines. The control of the dosage pump (15) can be performed by means of a control box (20). The level of the storage tank (16) can be measured using a level sensor (17) provided in the storage tank (16). If the level sensor (17) detects a lowest level in the storage tank (16), the level sensor (17) will send a signal to the control box (20) that in its turn will send a signal to the dosage pump (15) to stop dosage of aqueous nitrate containing solution to the nozzles (13).

[0058] The storage tank (16) can be arranged with a temperature sensor (not shown on the figure) to measure the temperature of the aqueous nitrate containing solution. This is especially important when the storage tank (16) is located outside. An aqueous calcium nitrate solution with 25 weight % of calcium nitrate for instance has a freezing point of about 8 C. The measurement of the temperature in the aqueous nitrate containing solution is more in particular done continuously. The storage tank (16) can furthermore be provided with a heat source (not shown on the figure) to heat the storage tank (16) when the temperature of the aqueous nitrate containing solution is below a predetermined temperature, for instance 5 C. The heat source can for instance be a heater or a loop from re-cooling units of a power station. This control can be performed by a control box (20). If the temperature sensor measures a temperature of the aqueous nitrate containing solution which is less than the predetermined temperature, a signal can be send to the control box (20) that in its turn sends a signal to the heat source to start heating the aqueous nitrate containing solution in the storage tank (16).

[0059] A hydrogen sulphide gas sensor (21) can be provided to measure the concentration of hydrogen sulphide in the biogas present in the pipeline (18) leading the purified biogas out of the headspace (12). The measurement of the concentration of hydrogen sulphide in the biogas leaving the headspace (12) is used to adjust the dosage of aqueous nitrate containing solution to the nozzles (13). This adjustment can be controlled by means of a control box (20).

[0060] Also the start and stop of the dosage pump (15) and the start and stop of the gas tight vacuum pump (14) can be controlled by a control box (20).

[0061] The anaerobic digestion system can comprise one or more control boxes (20) to perform the control over the devices and signals as mentioned in the paragraphs before.

[0062] The method according to the application involves the steps of first of all providing an airtight reactor vessel (10) having a sludge (11) with a surface layer (11a) and a volume (11b) underneath the surface layer (11a) comprising bio-matter and micro-organisms. The method further comprises the step of anaerobic digestion of the bio-matter by the micro-organisms present in the sludge (11) to produce biogas. This biogas will comprise unwanted hydrogen sulphide. The method also comprises the step of spraying an aqueous nitrate containing solution in the headspace (12) above the surface layer (11a) using a carrier gas stream. This step is followed by the formation of a nitrate containing interphase layer (11c) between the surface layer (11a) of the sludge (11) and the headspace (12). This interphase layer (11c) comprises the same species of micro-organisms as the ones present in the sludge (11). A following step is the purification of the biogas formed by the fermentation sludge (11) through oxidation of hydrogen sulphide present in the biogas to sulphate or sulphur by the micro-organisms present in the interphase layer (11c) using nitrate of the interphase layer (11c). Thereafter, the purified biogas migrates into the headspace (12). No membrane or fabric is provided in the headspace (12).

[0063] The method includes the step of continuously measuring the residual hydrogen sulphide concentration in the purified biogas, this by means of the hydrogen sulphide gas sensor (21) present in the pipeline (19) transporting the biogas out of the headspace (12).

[0064] The method can further include the step of continuously measuring the temperature of the aqueous nitrate containing solution present in the storage tank (16). The method can also comprise the step of heating the aqueous nitrate containing solution in the storage tank (16) using a heat source (not shown on the figure) when the temperature of the aqueous nitrate containing solution in the storage tank is below a predetermined temperature, in particular when it is below 5 C.

[0065] The method further possibly comprises the step of continuously measuring the level of the aqueous nitrate containing solution in the storage tank (16).

[0066] The method according to the application can also comprise the step of continuously controlling the start and stop of the low speed and low pressure dosage pump to apply the aqueous nitrate containing solution to the one or more spraying nozzles, this allowing to stop the dosage pump to prevent overdose or to stop dosage in case the level of the aqueous nitrate containing solution in the storage tank reaches its lowest level.

[0067] The method also possibly comprises the step of controlling the start and stop of the vacuum pump sucking the biogas out of the headspace of the reactor vessel.

[0068] The method according to the application may also comprise the step of continuously measuring the dosage of the aqueous nitrate containing solution in the headspace (12) of the reaction vessel (10).

[0069] The method according to the application can be computer-implemented. This allows efficient execution of the method as described above. Further can be provided a computer program product comprising program code instructions for implementing a method according to the application as described above. Accordingly, system can be efficiently programmed to execute the abovementioned method according to the application.

[0070] In particular embodiments, the computer program product for executing the abovementioned method according to the present application utilizes data received by a control box (20) from one or more sensors, such as a hydrogen sulphide gas sensor (21) for measuring the concentration of hydrogen sulphide in the biogas leaving the headspace (12), a level sensor and/or temperature sensor in the storage tank (16). In certain embodiments, the computer program product comprises program code instructions for controlling the start and stop of a dosage pump (15, 14) wherein a control box receives data from a hydrogen sulphide gas sensor and, based on a threshold value, activates the dosage pump (15, 14).

[0071] Also a system according to the application as described above that is configured for executing a method according to the application as described above is provided.

Example

[0072] In a biogas plant having an annual biogas production of 750.000 m.sup.3/yr, the H.sub.2S concentration in the biogas is in average 2.500 ppm, what equals to 3.750 mg/m.sup.3. The approximate annual production of H.sub.2S in the anaerobic digester used to produce the biogas is about 2.800 kg/yr. To reduce the content of H.sub.2S in the biogas produced by the (fermentation) sludge, annually, 7.500 kg/yr of calcium nitrate is used. The dosage is in average 5.1 liter per hour when applied as a 20 wt. % aqueous calcium nitrate solution. The 20 wt. % aqueous calcium nitrate solution is made from a 45 wt. % aqueous calcium nitrate solution by dilution prior to injection via a venturi nozzle spray injection system resulting in an aerosol that precipitates on the surface layer (11a) of the sludge (11). The sludge (11) in the anaerobic digester has a surface of 314 m.sup.2. The average thickness of the interphase layer (11c) as a result of aerosol precipitation is about 0.02 mm. The advantage of adding the aqueous calcium nitrate solution using the venturi nozzle spray injection system is that the addition of calcium nitrate in the headspace (12) is restricted through which, once the aerosol (or mist) settles on the surface layer (11a) of the sludge (11), the concentration of calcium nitrate in the interphase layer (11c) will also be restricted. Consequently, the calcium nitrate will be consumed by the microorganisms close to the surface layer (11a) and will not sink deeper into the sludge (11). Furthermore, despite of the fact that the sludge (11) typically is mixed, the calcium nitrate is consumed before it can be mixed along with the sludge (11) since the mixing speed is considerably low.

[0073] The H.sub.2S-concentration in the biogas present in the headspace of the reactor vessel is reduced to the desired level of 200 ppm.