CA3170251A1

CA3170251A1 - Hydrogen biomethane process and apparatus

Info

Publication number: CA3170251A1
Application number: CA3170251A
Authority: CA
Inventors: Andrew Benedek; Juan Josse; Aaron BENEDEK
Original assignee: Anaergia Inc
Current assignee: Anaergia Inc
Priority date: 2021-06-18
Filing date: 2022-06-20
Publication date: 2022-12-18
Also published as: WO2022261790A1

Abstract

The specification describes systems and processes for producing methane gas from organic waste and ways to reduce GHG emissions from waste-to-energy facilities by biologically upgrading biogas. The system uses an anaerobic digester for receiving organic waste and producing a digester biogas, a biological upgrading reactor for receiving biogas from an anaerobic digester and increasing the methane content of the biogas; and a membrane system immersed in the upgrading reactor for diffusing an external source of hydrogen to the upgrading reactor to favor the conversion of carbon dioxide into methane by micro-organisms in the upgrading reactor. The specification also describes synergies between a hydrogen biomethanation (HBM) process and a supercritical CO2 power generation process (sCO2).

Description

HYDROGEN BIOMETHANE PROCESS AND APPARATUS
TECHNICAL FIELD
[001] The technical field generally relates to waste-to-energy facilities and more specifically to renewable energy production using a biogas upgrading reactor with a hydrogen diffusing membrane.
BACKGROUND

[002] Producing energy such as biogas and/or renewable natural gas from waste organic feedstock is becoming more common due, for example, to stricter regulations on the landfilling of organic waste. Current approaches to produce renewable natural gas from organic waste can comprise anaerobic digesters where micro-organisms convert a feedstock organic material in the waste into biogas comprising mainly methane gas and carbon dioxide but also other components.
Upgrading the biogas produced comprises separating the non-methane components such as carbon dioxide (CO2), hydrogen sulfide (H2S), water (H20), ammonia (NH3) in order to provide the highest concentration of methane.

[003] There are various challenges and shortcomings related to producing biogas and renewable natural gas in waste-to-energy facilities. For example, although the composition of biogas typically comprises 35-45% CO2, this greenhouse gas is often released to the atmosphere.
There is therefore a need for novel technologies and approaches that address some of the challenges and shortcomings by further reducing the amount of carbon dioxide released to the atmosphere from waste-to-energy facilities.

[004] In addition, another challenge or shortcoming of producing and upgrading biogas at large scale is the production of electricity for use is certain steps of the process. There is a need for a system and method of producing electricity at a biogas plant that has synergies with the production and upgrading of biogas.

[005] Emerging technologies have achieved electrical generation with turbines using supercritical CO2 (sCO2) as the high energy motor fluid to drive turbines, instead of steam.
Supercritical CO2 can be used in Brayton Cycle and in Allam Cycle turbines to generate electricity.
These technologies rely on a combustor that operates with natural gas and oxygen instead of air, to combust the gaseous fuel. The use of oxygen instead of air avoids the formation of NOx as a combustion flue gas by-product. Oxygen is produced from air by separating Nitrogen, oxygen, argon and other impurities by a pressure swing adsorption (PSA) or vacuum pressure swing absorption (VPSA) system. Methane combustion with oxygen forms CO2 and water.
The combustion products reach higher temperatures than combustion with air and is cleaner. The high temperature CO2 produced in the combustor can be compressed to generate supercritical CO2 which is used as the fluid to move the turbine and generate electricity. Water contained in low pressure CO2 after the turbine is removed in a water separator, and dry low p[pressure CO2 is compressed to recirculate into the system but a portion must be removed from the system and be either sold to industry to replace other sources of CO2, injected in underground deep formation for sequestration, or injected into oil wells for enhanced oil recovery. These methods of CO2 sequestration are costly and require CO2 transportation to the point of use or sequestration, which add to the carbon footprint of such facilities and operations. More sustainable closed loop uses for the CO2 that must be removed (bled) from the supercritical CO2 turbine power generation plants is described herein.
SUMMARY

[006] It is therefore an object of the present invention to provide a process for producing methane from organic waste comprising feeding organic waste (24) to an anaerobic digester (1) to produce a digester biogas (9) comprising methane and carbon dioxide;
transferring the digester biogas (9) from the anaerobic digester (1) to an upgrading reactor (3) comprising micro-organisms in a mixed liquor; and providing an external source of hydrogen (21) to the upgrading reactor (3) to favor the conversion of carbon dioxide present in the digester biogas (9) diffused into the upgrading reactor (3) and hydrogen into methane (and water) by micro-organisms. This process is hereinafter called a hydrogen biomethane (HBM) process.

[007] In some embodiments, hydrogen (21) is diffused across a submerged membrane system (4) located inside the upgrading reactor (3) because the membrane system (4) provides a high surface area for a diffusion transfer of the hydrogen to the mixed liquor.

[008] In some embodiments the micro-organisms comprise hydrogenotrophic methanogens.
In other embodiments the hydrogen (21) is renewable hydrogen and/or the electricity used to produce the hydrogen is renewable electricity.

[009] In other embodiments, digester biogas (9) is fed to the upgrading reactor (3) from underneath the submerged membrane system (4) using a biogas diffuser (5), wherein diffused biogas scours the membrane system (4) as it raises through the mixed liquor to prevent biofilm formation and other contamination of membrane surfaces and provide mixing to maintain the micro-organisms in suspension.

[0010] In some embodiments the micro-organisms are suspended in the mixed liquor inside the upgrading reactor (3) and operating the anaerobic digester is done in thermophilic conditions, in other embodiments, the upgrading reactor (3) can be operated in mesophilic conditions. .

[0011] In other embodiments, a biologically upgraded biogas (16) is conditioned in a biogas conditioner (29) to remove contaminants comprising one or more of H2S, moisture, VOC, siloxane and ammonia, wherein the conditioning step uses one or any combination of membranes, water wash, amine scrubbers, PSA, cryogenic gas separation, or other method to remove the contaminants.

[0012] In yet other embodiments a nutrient-rich filtrate (11) from liquid digestate (10) is provided to the upgrading reactor (3) as a nutrient and carbon source as co-substrate for growth.

[0013] In some embodiments, waste mixed liquor from the upgrading reactor (3) is recirculated back to the anaerobic digester (1) for it to be dewatered together with the residue of the digester (1). In other embodiments a recirculated gas (15) is extracted from a headspace of the upgrading reactor (3) and recirculated back to the biogas diffuser (5) to avoid losing carbon dioxide and dissolving it in the mixed liquor water column of the upgrading reactor (3) to facilitate further conversion of this carbon dioxide to methane with the hydrogen introduced through the membranes (4).

[0014] In other embodiments, a CO2-rich tail gas (17) of the polishing upgrader (8), which contains mostly CO2, is returned to the biogas diffuser (5). This avoids losing CO2 and enhances further conversion to methane in the reactor (3) with the hydrogen (21) introduced through the membrane (4). The biogas conditioner (29) is used as pre-treatment of the biologically upgraded biogas (16) before feeding it to a polishing upgrader (8). The polishing upgrader (8) is used for polishing the conditioned biogas (31) to meet high quality biomethane specifications required for the final biomethane product (18) to be injected into a pipeline.

[0015] It is also an object of the present invention to provide a system for converting organic waste into methane comprising an anaerobic digester (1) for receiving the organic waste (24) and producing a digester biogas (9); an upgrading reactor (3) for receiving the digester biogas (9) from the anaerobic digester (1) and increasing the methane content of the digester biogas (9); and a membrane system (4) immersed in the upgrading reactor (3) for diffusing an external source of hydrogen (21) into the upgrading reactor (3).

[0016] In some embodiments, a biogas diffuser (5) is located under the membrane system (4) for feeding digester biogas (9) into the upgrading reactor (3). In other embodiments, the upgrading reactor (3) is an anaerobic suspended growth mixed liquor reactor.

[0017] In some embodiments, a biologically upgraded biogas conditioner (29) removes contaminants comprising one or more of H2S, moisture, VOC, siloxane and ammonia, wherein, in some embodiments, the biogas conditioner (29) uses one or any combination of membranes, water wash, amine scrubbers, PSA, cryogenic, or other method to remove the contaminants from the biologically upgraded biogas (16).

[0018] In some embodiments, a recirculation loop returns waste mixed liquor from the upgrading reactor (3) to the anaerobic digester (1). In other embodiments, a recirculation loop returns a recirculated gas (15) from a headspace of the upgrading reactor (3) to the biogas diffuser (5).

[0019] In yet other embodiments, a recirculation loop returns a CO2-rich tail gas (17) from the polishing upgrader (8) back to the biogas diffuser (5).

[0020] It is also an object of the present invention to provide an upgrading reactor (3) for biologically upgrading biogas comprising a reactor tank for growing micro-organisms in suspension; a membrane system (4) inside the reactor tank for receiving a source of external hydrogen and diffusing the hydrogen into the reactor tank using a high surface area of the membrane system; a biogas diffusing strip located underneath the membrane system (4) for feeding biogas into the reactor (3) and scouring the membranes to maintain them free of excessive biofilm accumulation.

[0021] In some embodiments, the HBM process described herein is combined with a supercritical CO2 power generation system (SCO2) due to the synergies between these two systems (HBM-SCO2). In the combined system, at least a portion of the methane (18) (i.e.
renewable natural gas ¨ RNG) produced in the HBM process is combusted with pure oxygen in a methane oxy-combustor (RNG-02 Combustor - 43). In the combined system, at least a portion of the oxygen (40) is produced by a water electrolyzer (6) that is used to generate the hydrogen gas (21) required for the HBM process. In the combined system, at least a portion of the electricity (41) produced in the sCO2 process is used to power a water electrolyzer (6).
In the combined system, a water separator (44) can separate the working fluid into water (45) and CO2 (47) where at least a portion of the separated CO2 (47) is recirculated to the upgrading reactor (3). In the combined system, at least a portion of the water (45) separated in the water separator (44) can be provided to the water electrolyzer (6).

[0022] In some embodiments, the upgrading reactor (3) is dissociated from the anaerobic digester (1) whereby the residual CO2 from the sCO2 power generation system and the hydrogen from the water electrolyzer are provided to the upgrading reactor (3). In some embodiments, the micro-organisms in the upgrading reactor produce biogas that is upgraded to methane for use in the sCO2 process. In some embodiments, no external biogas is provided to the upgrading reactor.
In other embodiments, a portion of the recirculated biogas (15) is recirculated to the upgrading reactor (3). In some embodiments, nutrients (11) are provided to the upgrading reactor (3) to favor the growth of micro-organisms that convert hydrogen and CO2 into methane.

[0023] In some embodiments, there is provided a process for producing methane from organic waste comprising feeding organic waste (24) to an anaerobic digester (1) to produce a digester biogas (9) comprising methane and carbon dioxide; transferring the digester biogas (9) from the anaerobic digester (1) to an upgrading reactor (3) comprising micro-organisms in a mixed liquor;
and providing an external source of hydrogen (21) to the upgrading reactor (3) to favor the conversion of carbon dioxide and hydrogen into methane and water by the micro-organisms.

[0024] In some embodiments, there is provided a system for converting organic waste into methane comprising: an anaerobic digester (1) for receiving the organic waste (24) and producing a biogas (9); an upgrading reactor (3) for receiving the biogas (9) from the anaerobic digester (1) and increasing a methane content of the biogas (9), and producing a biologically upgraded biogas (16); and a membrane system (4) immersed in a mixed liquor within the upgrading reactor (3) for providing an external source of hydrogen (21) to the upgrading reactor (3).
The system can also include a biogas diffuser (5) located proximate the membrane system (4) for feeding the biogas into the upgrading reactor (3).

[0025] In some embodiments, there is provided a process for producing methane from organic waste comprising: feeding organic waste (24) to an anaerobic digester (1) to produce a digester biogas (9) comprising methane and carbon dioxide; transferring the digester biogas (9) from the anaerobic digester (1) to an upgrading reactor (3) comprising micro-organisms in a mixed liquor;
providing an external source of hydrogen (21) to the upgrading reactor (3) to favor the conversion of carbon dioxide and hydrogen into methane and water by the micro-organisms;
combusting at least a portion of the methane (18) obtained from the upgrading reactor in a methane combustor (43) to produce electricity (41); powering a water electrolyzer (6) with at least a portion of the electricity, the water electrolyzer (6) producing hydrogen gas (21) and oxygen gas (40); and providing at least a portion of the hydrogen gas (21) as at least part of the external source of hydrogen (21) fed to the upgrading reactor (3).

[0026] In some embodiments, there is provided an upgrading reactor for biologically upgrading biogas, comprising: a reactor tank for growing micro-organisms in suspension, the reactor tank having an inlet for receiving nutrients (11) and an outlet for wasting micro-organisms (14); a membrane system (4) inside the reactor tank for receiving a source of hydrogen (21) and diffusing the hydrogen (21) into the reactor tank; and a diffuser (5) located underneath the membrane system (4) for feeding a CO2-containing biogas into the reactor tank.

[0027] In some embodiments, there is provided a process for producing methane, comprising:
providing biogas (9) comprising methane and carbon dioxide to an upgrading reactor (3) comprising micro-organisms in a mixed liquor; providing an external source of hydrogen (21) to the upgrading reactor (3) to favor the conversion of carbon dioxide and hydrogen into methane and water by the micro-organisms; producing electricity from methane using a supercritical CO2 turbine/generator (42); using at least a portion of the electricity for water electrolysis to produce hydrogen gas (21); providing at least a portion of the hydrogen gas (21) as at least part of the external source of hydrogen (21) fed to the upgrading reactor (3).

[0028] In some embodiments, there is provided the use of a supercritical CO2 power generation system to provide electricity for an electrolyzer to produce hydrogen that is used in one or more of the processes for producing methane described herein.

[0029] It is also noted that the systems and processes described above and herein can be combined with one or more additional features described and/or illustrated herein.
BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a schematic block diagram of a process and system for converting organic waste into methane.

[0031] Figure 2 is a schematic block diagram of a process and system for converting organic waste into methane using synergies with a supercritical CO2 power generation process/system.

[0032] Figure 3 is a schematic block diagram of a process and system for producing methane using a combination of a HBM process/system and supercritical CO2 power generation process/system.
DETAILED DESCRIPTION

[0033] Techniques described herein relate to systems and processes that enhance performance of waste-to-energy facilities by recovering additional carbon dioxide using a hydrogen diffusing membrane system inside an anerobic bioreactor comprising hydrogenotrophic methanogens.

[0034] An anaerobic digester (1) produces biogas by digesting any organic feedstock (24) including sewage sludge. The digester can operate on mesophilic or thermophilic conditions. The digester produces a digester biogas (9) which is a mixture of methane (CH4), carbon dioxide (CO2), moisture and gaseous contaminants such as hydrogen sulfide (H2S), siloxanes, ammonia (NH3), non-methane VOCs and others. CO2 typically accounts for 35% to 45% of the biogas volume.

[0035] The digester biogas (9) pressure is increased with a blower (not shown) and is directed into a fine bubble or coarse bubble biogas diffuser (5) placed under a gas porous membrane system (4), in an anaerobic biological upgrading reactor (3) designed to biologically upgrade digester biogas (9) by reacting CO2 in the biogas and injected renewable hydrogen (21) mediated by hydrogenotrophic micro-organisms to convert the hydrogen and the CO2 into methane and water. This increases the concentration of methane in the biologically upgraded biogas (16) out of the biological upgrading reactor (3). Digester biogas (9) is injected into the upgrading reactor (3) by bubbling it through a biogas diffuser (5). The biogas raises in the tank (3) and a portion of the CO2, which has high solubility (1 g/kg at 40 deg C), dissolves in the water column. The biogas injected this way into the reactor, beside serving as the source of CO2, also serves to physically scour the membrane surface to avoid biofilm accumulation. The biogas diffuser (5) is used for mixing the mixed liquor in the tank. Recirculated gas (15) can be extracted from the headspace of the upgrading reactor (3) and recirculated back to the biogas diffuser (5) to help replenish the content of CO2 solubilized in the liquid and make more CO2 available to react with the membrane-injected renewable hydrogen (21).

[0036] Renewable hydrogen (21) is injected into the biogas upgrading reactor (3) using a membrane system (4) such as submerged flat sheet membranes or hollow fiber membranes that are porous and allow gas permeation through the membrane surface and diffusion into the anaerobic mixed liquor. Hydrogen diffusion across a membrane system (4) is used to avoid the creation of bubbles that would reduce the ability for the low-solubility hydrogen to be dissolved and remain for longer time in the water column because membrane systems (4) provide a large surface area to allow the hydrogen to diffuse (30) into the mixed liquor.
Hydrogen gas has a very low solubility in water (0.0014 g/kg at 40 deg C). The anaerobic mixed liquor in the reactor (3) is a suspension of micro-organisms, mostly thermophilic or mesophilic hydrogenotrophic methanogens. These micro-organisms use CO2 as carbon source for growth. The membrane system (4) can be arranged as a cassette with a single unit or several stacked cassettes one on top of the other or placed side-by-side. Digester biogas (9) is diffused through fine or coarse bottom biogas diffuser (5) placed under the membrane system (4) and raises through the submerged membrane system (4).

[0037] Renewable hydrogen (21) is fed to the upgrading reactor (3) into the lumen of the membranes in one cassette or multiple cassettes making up the membrane system (4) and diffuses (30) into the mixed liquor and reacts with the CO2 dissolved in the water column mediated by the methanogenic micro-organisms in the upgrading reactor (3). This reaction produces methane and consumes CO2, which increases the methane content in the biogas that reaches the head space of the reactor. Methane has low solubility in water (0.014 g/kg at 40 C) and therefore raises to the reactor headspace. The pressure of a portion of higher methane content gas called recirculated gas (15) can be increased (using known methods) and recirculated to blend with the raw digester biogas (9) entering the biogas diffuser (5) in the reactor (3), which further increases methane content in the biologically upgraded biogas as there is more recirculated gas (15) than the amount of biologically upgraded biogas (16).
Although the biologically upgraded biogas (16) has been biologically upgraded, it still contains some minor amounts of CO2 and other gaseous contaminants. The pressure of biologically upgraded biogas (16) can be increased using known methods before going to a biogas conditioner (29) and further upgraded in a conventional biogas upgrading system called the polishing upgrader (8). The conditioning consists of H2S, moisture, VOC, siloxane and ammonia removal and the polishing consist of further separation of CO2 using membranes, water wash, amine scrubbers, PSA, cryogenic, or other method. The CO2-rich tail gas (17) separated in the polishing upgrader (8) is returned to the digester biogas (9) line that feeds the diffuser (5) to ensure no CO2 is lost and this CO2 is available to produce more methane in the biological upgrading reactor (3). The biologically upgraded biogas (16) undergoes polishing and further upgrading in the polishing upgrader (8), where it becomes pure methane with CO2 content below 5%, with quality dictated by the requirements of the receiving natural gas pipeline, and in the case of other uses like vehicle fuel, the polishing upgrader (8) can be designed to meet that specification and produce a final biomethane product (18) capable of meeting any particular purity specification.

[0038] The system utilizes renewable hydrogen (21), which is produced on-site by a water electrolyzer (6), powered by renewable electricity. Alternatively, renewable hydrogen generated elsewhere (27), can be transported in tanker trucks to fill on-site storage tank (28) and then be fed to the membrane system (4), thereby eliminating the need for an on-site electrolyzer (6). If an on-site electrolyzer (6) is used, an alternative to generating renewable electricity at the site (19) to power the electrolyzer (6), is to purchase renewable energy generated elsewhere and transmitted to the site through the electrical grid (26). As indicated, renewable electricity (19) can be produced locally or remotely using solar panels (7) or wind turbines (23).
A portion of the renewable electricity (19) generated on site or remotely can be dedicated to other uses (20). The renewable electricity can be produced elsewhere and transported via the electrical grid (26) and used for the electrolyzer (6) installed at the biogas plant site.
Alternatively, renewable hydrogen (21) produced elsewhere (27), can be purchased, transported to the site in tanker trucks and stored (28) for use in the process. If hydrogen is produced locally by an electrolyzer (6), a side product is oxygen (22), which can be further purified and sold for various uses as an industrial gas.

[0039] Hydrogenotrophic methanogenic micro-organisms grow in suspension in the reactor (3).
These micro-organisms must be wasted periodically to maintain an optimal solids retention time.
The micro-organisms are wasted (14) as required by pumping anaerobic mixed liquor from the gas upgrading reactor (3) to the anaerobic digester (1). The waste micro-organisms blend with the anaerobic digester (1) contents and is sent as liquid digestate (10) to a solids separation step (2), that can be a centrifuge, screw press, belt press, or other. The solids separation unit (2) produces a dewatered solid digestate cake (13) that can be used as a soil amendment or fertilizer and also can be dried or dried and pyrolyzed to produce biochar. The liquid fraction of the digestate is called filtrate or centrate. This liquid contains nutrients and carbon. A portion of the filtrate or centrate (11) can be used as nutrient source for the micro-organisms and carbon as one of the substrates for micro-organism growth in the gas upgrading reactor (3).
Other micronutrients (25) may be required that are added directly to the reactor (3). The balance of the filtrate or centrate (12) is treated for discharge or land applied, depending on the type of organic feedstock (24) the process is using and the local requirements for use or disposal of the liquid.

[0040] Synergies between the hydrogen biomethanation (HBM) process and supercritical CO2-based power generation.

[0041] Plants that use supercritical CO2 for power generation can use biomethane (renewable natural gas or RNG) derived from biogas instead of natural gas to avoid using fossil fuels. RNG
(18) can be produced from organics (24) that otherwise would end up in landfills and contribute to greenhouse gas formation through fugitive methane emissions from landfill gas or can be produced from energy crops which capture CO2 from the atmosphere to grow and produce biomass. Methane produced with the anaerobic digestion of such biomass produces a carbon negative methane for use in the combustor to produce supercritical CO2. RNG
(18) can be burned with oxygen (40) in the combustor (43) to produce supercritical CO2 for the sCO2 turbine (42).
The dry low-pressure CO2 (47) after the turbine and water separation is used for the production of biogas in the upgrading reactor (3) and hydrogen biomethanation (HBM) process. If the amount of CO2 required for the HBM process is less than the amount of CO2 produced in the sCO2 process, the bled CO2 (57) can be removed from the system and used for other purposes. These biological reactors with hydrogenotrophic archaea population are fed CO2 and hydrogen and they produce methane using the well-studied and understood hydrogen biomethanation (HBM) anaerobic process. Hydrogen (21) needed for HBM is produced from water using an electrolyzer (6). The electrical power (41) required for the electrolyzer is produced by the supercritical CO2 power plant and during the day can be supplemented by photovoltaic solar power (19). The biogas produced in the HBM process is upgraded to separate methane from CO2 and produce methane-rich RNG. The dry low-pressure CO2 (17) separated into the tail gas of the biogas upgrading step is returned to the upgrading reactor (3) to continue producing methane with hydrogen (21) introduced from the electrolyzer (6). The by-product oxygen (40) produced in the electrolyzer (6) when separating hydrogen (21) and oxygen (40) in water in the electrolyzer (6) is used in the methane combustor (43) to supplement oxygen produced from air (49) by PSA or VPSA systems (48). At least a portion of the water (45) used in the electrolyzer (6), is combustion water formed in the combustor by burning methane with oxygen to generate CO2 and H20. The H2O is removed from the CO2 after the sCO2 turbine (42) with a water separator (44) and can be a source of water (45) to the electrolyzer (6). Additional water (55) may be provided to the electrolyzer, as needed if water from the water separator (44) is not sufficient or required in other processes.

[0042] By reutilizing the CO2 by-product to produce more methane as carbon source fuel for the process and replacing a portion of the oxygen needed for the combustor with oxygen generated from water in the electrolyzer, the concept presented herein puts forward a closed loop with lower carbon footprint than current electric power generation technologies using supercritical CO2. The synergistic integration of hydrogen biomethanation (HBM) and electrolysis results in a self-generating alternative to CO2 sequestration.

[0043] In terms of experimentation, it was tested and shown that a bioreactor fed with CO2 and H2 produced CH4 and that H2 can be diffused through a membrane to saturation conditions.

[0044] Referring to Figure 1, it is noted that the digester biogas (9) can be subjected to treatments prior to being fed into the upgrading reactor (3). For example, the digester biogas (9) could be sent directly without processing or purified or enriched in terms of CO2. Since CH4 does not substantially further react in the upgrading reactor (3) enrichment in reactant CO2 could be performed in some scenarios.

[0045] The hydrogen that is fed into the upgrading reactor (3) can be provided in various ways and can take various forms in terms of the stream supplied to the reactor. For example, the hydrogen could be fed into the upgrading reactor via a fine or coarse bubble diffuser, using a mixing impeller with eductor, and/or through pressurizing the reactor to increase the solubility of the H2 in the liquid. The use of the membrane can be preferred as energy consumption can be reduced since no sparger energy is wasted, no pressure is required in the reactor to increase the partial pressure of H2 (although pressurization can be implemented), and the membrane provides a biological zone for specialized biomass to thrive. In addition, the stream fed to the upgrading reactor (3) can be pure hydrogen or a mixed hydrogen-containing stream.

[0046] As mentioned above, the biogas diffuser (5) can be configured and operated to scour the membrane system (4). The biogas is buoyant compared to the mixed liquor and thus the bubbles tend to rise and can scour the membrane. The injection rate and spacing of the biogas diffuser relative to the membrane system can be provided to facilitate different degrees of scouring. The biogas diffuser can inject the biogas continuously at the same or variable injection rates, or can inject the biogas cyclically where intermittent injection is provided based on process control parameters for example. The diffuser (5) is preferably located below the membrane system (4), but it could alternatively be side mounted to the membrane. Another alternative is to provide no diffuser and use an eductor and pump to transfer CO2 into the mixed liquor, for example.

[0047] In addition, a CO2-rich tail gas (e.g., from the polishing unit or another unit used to remove CO2 from the gas output of the upgrading reactor or another unit) can be fed into the upgrading reactor. The tail gas can be fed directly via a dedicated feed inlet that can include diffusers or blended with the biogas as illustrated in Figure 1.

Claims

1. A process for producing methane from organic waste comprising feeding organic waste (24) to an anaerobic digester (1) to produce a digester biogas (9) comprising methane and carbon dioxide;
transferring the digester biogas (9) from the anaerobic digester (1) to an upgrading reactor (3) comprising micro-organisms in a mixed liquor; and providing an external source of hydrogen (21) to the upgrading reactor (3) to favor the conversion of carbon dioxide and hydrogen into methane and water by the micro-organisms.

2. The process of claim 1, further comprising diffusing the hydrogen (21) across a membrane system (4) located inside the upgrading reactor (3) and into the mixed liquor.

3. The process of claim 2, wherein the membrane system (4) provides a surface area for enhanced transfer and dissolution of the hydrogen into the mixed liquor.

4. The process of claim 2 or 3, further comprising feeding the digester biogas (9) to the upgrading reactor (3) using a biogas diffuser (5) to dissolve CO2 in the mixed liquor of upgrading reactor (3).

5. The process of claim 4, wherein biogas from the biogas diffuser (5) scours the membrane system (4) to prevent biofilm accumulation.

6. The process of claim 4, wherein biogas from the biogas diffuser (5) scours the membrane system (4) to prevent contamination of membrane surfaces.

7. The process of any one of claims 4 to 6, wherein the biogas diffuser (5) is located underneath the membrane system (4).

8. The process of any one of claims 4 to 7, wherein biogas from the biogas diffuser (5) mixes the mixed liquor as the biogas moves toward a headspace of the upgrading reactor (3).

9. The process of any one of claims 1 to 8, wherein the external source of hydrogen (21) is a pure hydrogen stream that is supplied into the upgrading reactor (3).

10. The process of any one of claims 1 to 9, further comprising operating the anaerobic digester (1) in one of thermophilic conditions.

11. The process of any one of claims 1 to 9, further comprising operating the anaerobic digester (1) in conditions.

12. The process of any one of claims 1 to 11, wherein the upgrading reactor (3) produces a biologically upgraded biogas (16) and the process further comprises conditioning the biologically upgraded biogas (16) to remove contaminants comprising one or more of H2S, moisture, VOC, siloxane and ammonia to produce a conditioned biogas (31).

13. The process of claim 12, further comprising polishing the conditioned biogas (31) using a polishing upgrader (8) to remove CO2 therefrom to produce a polished biogas (18).

14. The process of claim 12 or 13, wherein the conditioning is performed using a biogas conditioner (29) that uses one or any combination of membranes, water wash, amine scrubbers, PSA, and cryogenic methods.

15. The process of any one of claims 1 to 11, further comprising polishing the biologically upgraded biogas (16) using a polishing upgrader (8) to remove CO2 therefrom to produce a polished biogas (18).

16. The process of claim 13 or 15, wherein the polishing upgrader (8) uses one or any combination of membranes, water wash, amine scrubbers, PSA, and cryogenic methods.

17. The process of any one of claims 1 to 16, wherein processing the biologically upgraded biogas (16) generates a CO2-rich tail gas (17), and at least a portion of the CO2-rich tail gas (17) is recycled back to the upgrading reactor (3).

18. The process of claim 13, 15 or 16, further comprising returning a CO2-rich tail gas (17) produced by the polishing upgrader (8) back to the upgrading reactor (3).

19. The process of any one of claims 1 to 18, further comprising providing a nutrient and soluble carbon-rich filtrate (11) from liquid digestate (10) to the upgrading reactor (3).

20. The process of any one of claims 1 to 19, further comprising recirculating waste mixed liquor from the upgrading reactor (3) back to the anaerobic digester (1).

21. The process of any one of claims 4 to 8, further comprising extracting a recirculated gas (15) from a headspace of the upgrading reactor (3) and recirculating back to the biogas diffuser (5)-

22. The process of any one of claims 1 to 20, further comprising extracting a headspace gas from a headspace of the upgrading reactor (3) and recirculating at least a portion thereof back into the mixed liquor of the upgrading reactor (3).

23. The process of any one of claims 1 to 22, further comprising subjecting the digester biogas (9) to purification or enrichment to produce a pretreated biogas which is then fed into the upgrading reactor (3).

24. The process of any one of claims 1 to 22, wherein the digester biogas (9) is fed directly from the anaerobic digester (1) to the upgrading reactor (3).

25. The process of any one of claims 1 to 24, wherein the micro-organisms comprise hydrogenotrophic methanogens.

26. The process of any one of claims 1 to 25, wherein the hydrogen (21) is renewable hydrogen.

27. The process of any one of claims 1 to 26, wherein the electricity used to produce the hydrogen is renewable electricity.

28. The process of any one of claims 1 to 27, wherein the external source of hydrogen is provided via storage tanks.

29. The process of any one of claims 1 to 27, wherein at least a portion of the external source of hydrogen is provided from an electrolyzer (6).

30. The process of any one of claims 1 to 29, further comprising combusting methane (18) in a methane combustor (43) to produce electricity (41) using a supercritical CO2 turbine/generator (42), and using the electricity to power water electrolysis that produces hydrogen gas (21) that is used as at least a portion of the external source of hydrogen (21).

31. A system for converting organic waste into methane comprising:
an anaerobic digester (1) for receiving the organic waste (24) and producing a biogas (9);

an upgrading reactor (3) for receiving the biogas (9) from the anaerobic digester (1) and increasing a methane content of the biogas (9), and producing a biologically upgraded biogas (16); and a membrane system (4) immersed in a mixed liquor within the upgrading reactor (3) for providing an external source of hydrogen (21) to the upgrading reactor (3).

32. The system of claim 30, further comprising a biogas diffuser (5) located proximate the membrane system (4) for feeding the biogas into the upgrading reactor (3).

33. The system of claim 31, wherein the biogas diffuser (5) is located underneath the membrane system (4).

34. The system of claim 31 or 32, wherein the biogas diffuser (5) is configured to provide mixing in the upgrading reactor (3) and scouring of the membrane system (4).

35. The system of any one of claims 31 to 33, wherein the biogas diffuser (5) is configured to keep the micro-organisms in suspension in the upgrading reactor (3) by upward mixing effect created by the digester biogas (9) diffused by the biogas diffuser (5) at a bottom of the reactor (3).

36. The system of any one of claims 30 to 34, wherein the micro-organisms comprise hydrogenotrophic methanogens.

37. The system of any one of claims 30 to 35, further comprising a biogas conditioner (29) to receive the biologically upgraded biogas (16) and remove contaminants comprising one or more of H2S, moisture, VOC, siloxane and ammonia to produce a conditioned biogas (31).

38. The system of claim 36, further comprising a polishing upgrader (8) to remove CO2 from the conditioned biogas (31), the polishing upgrader (8) using one or any combination of membranes, water wash, amine scrubbers, PSA, and cryogenic methods.

39. The system of any one of claims 30 to 37, further comprising a waste mixed liquor line (14) in fluid communication with the upgrading reactor (3) and configured to recirculate waste mixed liquor from the upgrading reactor (3) to the anaerobic digester (1).

40. The system of any one of claims 30 to 38, further comprising a recirculation loop in fluid communication with a headspace of the upgrading reactor (3) for returning a recirculated gas (15) from the headspace to a lower region of the upgrading reactor (3).

41. The system of any one of claims 31 to 34, further comprising a recirculation loop in fluid communication with a headspace of the upgrading reactor (3) for returning a recirculated gas (15) from the headspace to the biogas diffuser (5).

42. The system of any one of claims 30 to 38, further comprising a recirculation loop for returning CO2-rich tail gas (17) removed from the biologically upgraded biogas (16) back into the upgrading reactor (3).

43. The system of any one of claims 31 to 34 or 40, further comprising a recirculation loop for returning CO2-rich tail gas (17) removed from the biologically upgraded biogas (16) back to the biogas diffuser (5).

44. The system of claim 37, further comprising a recirculation loop for returning CO2-rich tail gas (17) removed from the polishing upgrader (8) back into the upgrading reactor (3).

45. The system of any one of claims 30 to 44, further comprising a controller for receiving and processing data from the system in order to optimize operation of the system.

46. The system of any one of claims 30 to 45, further comprising a methane combustor (43) configured to receive a portion of methane generated by the upgrading reactor (3) and a supercritical CO2 turbine/generator (42) coupled to the methane combustor and configured to produce electricity (41), and an electrolyzer powered at least in part by the electricity (41) and configured to produce hydrogen gas (21) that is used as at least a portion of the external source of hydrogen (21) in the upgrading reactor (3).

47. A process for producing methane from organic waste comprising feeding organic waste (24) to an anaerobic digester (1) to produce a digester biogas (9) comprising methane and carbon dioxide;
transferring the digester biogas (9) from the anaerobic digester (1) to an upgrading reactor (3) comprising micro-organisms in a mixed liquor;

providing an external source of hydrogen (21) to the upgrading reactor (3) to favor the conversion of carbon dioxide and hydrogen into methane and water by the micro-organisms;
combusting at least a portion of the methane (18) obtained from the upgrading reactor in a methane combustor (43) to produce electricity (41);
powering a water electrolyzer (6) with at least a portion of the electricity, the water electrolyzer (6) producing hydrogen gas (21) and oxygen gas (40); and providing at least a portion of the hydrogen gas (21) as at least part of the external source of hydrogen (21) fed to the upgrading reactor (3).

48. The process of claim 47, wherein producing the electricity (41) is performed using a supercritical CO2 turbine/generator (42).

49. The process of claim 47 or 48, wherein the water electrolyzer (6) produces oxygen gas (40) and the process further comprises using the oxygen gas (41) from the water electrolyzer (6) as a source of oxygen in the methane combustor (43).

50. The process of 48, further comprising using a water separator (44) to separate a stream from the supercritical CO2turbine/generator (42) into CO2(47) and H20 (45).

51. The process of claim 50, further comprising recirculating the separated CO2 (47) from the supercritical CO2turbine/generator (42) to the upgrading reactor (3).

52. The process of claim 50 or 51, further comprising recirculating the separated H20 (45) from the water separator (44) to the water electrolyzer (6).

53. An upgrading reactor for biologically upgrading biogas, comprising:
a reactor tank for growing micro-organisms in suspension, the reactor tank having an inlet for receiving nutrients (11) and an outlet for wasting micro-organisms (14);
a membrane system (4) inside the reactor tank for receiving a source of hydrogen (21) and diffusing the hydrogen (21) into the reactor tank; and a diffuser (5) located underneath the membrane system (4) for feeding a CO2-containing biogas into the reactor tank.

54. The upgrading reactor of claim 53, wherein the diffuser (5) is located relative to the membrane system (4) to scour a surface of the membrane system (4) with the CO2-containing biogas.

55. The upgrading reactor of claim 53 or 54, wherein the diffuser (5) is configured to provide the CO2-containing biogas that is at least partly obtained from an anaerobic digester.

56. The upgrading reactor of any one of claims 53 to 55, further comprising a recycle line in fluid communication with the diffuser (5) and a polishing upgrader (8) that receives biologically upgraded biogas generated by the upgrading reactor and produced a polished biogas and CO2tail gas, the recycle line being configured to feed the CO2tail gas back into the diffuser (5).

57. The upgrading reactor of any one of claims 53 to 56, further comprising a headspace recirculation line and wherein the diffuser (5) is in fluid communication with the headspace recirculation line to receive recirculated biogas (15) therefrom.

58. A process for producing methane comprising combining a hydrogen biomethane process as described herein with a supercritical CO2power generation system comprising a methane oxy-combustor (43) and a supercritical turbine/generator (42).

59. A process for producing methane, comprising:
providing biogas (9) comprising methane and carbon dioxide to an upgrading reactor (3) comprising micro-organisms in a mixed liquor;
providing an external source of hydrogen (21) to the upgrading reactor (3) to favor the conversion of carbon dioxide and hydrogen into methane and water by the micro-organisms;
producing electricity from methane using a supercritical turbine/generator (42);
using at least a portion of the electricity for water electrolysis to produce hydrogen gas (21);
providing at least a portion of the hydrogen gas (21) as at least part of the external source of hydrogen (21) fed to the upgrading reactor (3).

60. The process of claim 59, further comprising one or more features of any one of claims 1 to 58 and/or as described and/or illustrated herein.

61. Use of a supercritical CO2 power generation system to provide electricity for an electrolyzer to produce hydrogen that is used in a process, system or reactor as defined in any one of claims 1 to 57.