WO2024124267A2

WO2024124267A2 - Coupled gas fermentation

Info

Publication number: WO2024124267A2
Application number: PCT/AT2023/060437
Authority: WO
Inventors: Oleksii OBOLENSKYI; Maximilian Lackner
Original assignee: Circe Biotechnologie GmbH
Current assignee: Circe Biotechnologie GmbH
Priority date: 2022-12-12
Filing date: 2023-12-12
Publication date: 2024-06-20
Anticipated expiration: 2025-06-12
Also published as: EP4634360A2; AT526749A3; WO2024124267A3; AT526749A2

Abstract

The present invention relates to a system and method for performing a coupled gas fermentation. The system comprises a first fermenter configured for operation in an aerobic mode and a second fermenter configured for operation in an anaerobic mode, wherein the first fermenter comprises a first inlet for introducing a first gaseous feedstock and the second fermenter comprises a second inlet for introducing a second gaseous feedstock, and wherein the first fermenter and the second fermenter are coupled with each other by a first connection element, wherein the first connection element is configured to feed at least a portion of a first tail gas from the first fermenter into the second fermenter for use as at least a portion of the second gaseous feedstock, characterized in that the first fermenter and the second fermenter are coupled with each other by a second connection element, wherein the second connection element is configured to feed at least a portion of a second tail gas from the second fermenter into the first fermenter for use as at least a portion of the first gaseous feedstock.

Description

Coupled gas fermentation

The present invention relates to a system for performing a coupled gas fermentation comprising a first fermenter configured for operation in an aerobic mode and a second fermenter configured for operation in an anaerobic mode, wherein the first fermenter comprises a first inlet for introducing a first gaseous feedstock and the second fermenter comprises a second inlet for introducing a second gaseous feedstock, and wherein the first fermenter and the second fermenter are coupled with each other by a first connection element, wherein the first connection element is configured to feed at least a portion of a first tail gas from the first fermenter into the second fermenter for use as at least a portion of the second gaseous feedstock. The present invention further relates to a method for performing a coupled gas fermentation.

During gas fermentation, a gaseous feedstock is fermented by microorganisms. The gaseous feedstock can be an industrial gas or a synthetic gas (synthesis gas, syngas) produced from the gasification of fossil fuels, biomass or (municipal) waste streams. Methanotrophic bacteria can convert methane (CH₄) into various products (e.g. single cell protein (SCP), polyhydroxyalkanoates (PHA) or other compounds of interest, such as ectoine) in the presence of oxygen (O2) via aerobic fermentation, hereby obtaining carbon dioxide (CO2) as by-product. Methanotrophs can use methane as their sole carbon and energy source. Bacteria, but also other microorganisms, such as archaea, can be methanotrophs. Methylotrophs are microorganisms that can feed on methanol. Some microorganisms can feed on both methane and methanol. Other anaerobic methanotrophs can also exist which are not considered here.

US 2021/0024961 Al describes a combination of a primary fermentation that converts gas to acetate with a secondary fermentation that converts acetate to a target. A portion of the first fermentation broth is transferred to a second bioreactor containing a culture of at least one second microorganism in a second liquid nutrient medium.

US 2022/0220433 Al describes a process for cultivating a microorganism capable of metabolising methane.

US 2019/0249315 Al describes the integration of a CO-consuming process, such as a gas fermentation process, with a CO2 electrolysis process.

US 2019/0390158 Al describes microorganisms and bioprocesses are provided that convert gaseous substrates, such as renewable H2 and waste CO2 producer gas, or syngas into high-protein biomass.

Acetogenic bacteria can feed on carbon monoxide (CO) or carbon dioxide (CO2) in the presence of hydrogen (H2) via anaerobic gas fermentation, e.g. to produce single cell protein, biopolymers or other compounds of interest. As such, anaerobic gas fermentation offers a route for carbon capturing and can make use of carbon dioxide obtained as by-product during aerobic fermentation.

Accordingly, combining anaerobic and aerobic gas fermentation allows for a reduction of the carbon footprint. Such a two-stage fermentation is known in the state of art, e.g. from Molitor et al., Current Opinion in Chemical Biology, 2017, 41, 84-92, and from Marcellin et al., Current Opinion in Chemical Biotechnology, 2022, 76, 102723. For example, in a first stage, acetate can be produced by syngas fermentation under anaerobic conditions, followed by a second stage to convert the produced acetate under aerobic conditions into single cell protein. Carbon dioxide, which is produced as byproduct in the aerobic fermentation can then be reintroduced into the first stage.

However, methane obtained as by-product (e.g. as non-consumed component from fed gas) during anaerobic fermentation negatively impacts the carbon footprint, as it needs to be either released into the atmosphere or captured in an additional step. It is an object of the present invention to further lower the carbon footprint resulting from a combined anaerobic and aerobic gas fermentation.

The present invention relates to a system for performing a coupled gas fermentation as outlined above, wherein the first fermenter and the second fermenter are coupled with each other by a second connection element, wherein the second connection element is configured to feed at least a portion of a second tail gas from the second fermenter into the first fermenter for use as at least a portion of the first gaseous feedstock.

The present invention further relates to a method for performing a coupled gas fermentation comprising the steps of

(a) providing a first fermenter and a second fermenter, each comprising a fermentation broth, wherein the first fermenter is operated in an aerobic mode and the second fermenter is operated in an anaerobic mode, and wherein the first and second fermenter are coupled through a gas phase,

(b) introducing a first gaseous feedstock comprising methane into the first fermenter and converting said first gaseous feedstock to a first product and a first tail gas, and introducing a second gaseous feedstock comprising carbon dioxide into the second fermenter and converting said second gaseous feedstock to a second product and a second tail gas, wherein at least a portion of the first tail gas is fed through a first connection element into the second fermenter for use as at least a portion of the second gaseous feedstock, and wherein at least a portion of the second tail gas is fed through a second connection element into the first fermenter for use as at least a portion of the first gaseous feedstock. The present invention allows to use at least a portion of the first tail gas to produce the second product and at least a portion of the second tail gas to produce the first product. Preferably, the first tail gas comprises carbon dioxide and the second tail gas comprises methane. As such, at least the portion of the first tail gas comprising carbon dioxide can be fed into the second fermenter, and at least the portion of the second tail gas comprising methane can be fed into the first fermenter. Methane thus does not have to be released into the atmosphere or be captured in an additional step. Particularly, it was discovered that the anaerobic fermentation in the second fermenter can be performed with no other carbon source apart from the carbon source comprised in the first tail gas, particularly carbon dioxide, which brings a considerable environmental benefit and lowers the carbon footprint. CO2 that is introduced into the aerobic, first fermenter can travel through it and can become enriched, as CH₄ is consumed, and CH₄ that is introduced into the acetogenic, second fermenter can likewise pass through and can become enriched, as CO2 is consumed. Since carbon-containing gases can thus be completely consumed in a coupled gas fermentation according to the present invention, the carbon footprint can be considerably improved.

When initiating the coupled gas fermentation, at least one of the first and second gaseous feedstocks consists of a gas externally introduced into the system. When the gaseous feedstock comprises carbon dioxide, the fermentation can be started by introducing said gaseous feedstock into the second fermenter, as carbon dioxide has a better solubility in the fermentation broth (more precisely, in water) than methane. The fermentation can also be started by introducing the first gaseous feedstock into the first fermenter, e.g. when a gas source comprising a high amount of methane is used, such as natural gas or biogas. Alternatively, the fermentation can be started by introducing gaseous feedstocks both in the first and second fermenter.

During fermentation, tail gases are formed which can be reused in the respective other fermenter. For example, in subsequent cycles of the fermentation (i.e. after having introduced a gaseous feedstock into the first and/or second fermenter), the first gaseous feedstock can comprise externally introduced gas (e.g. oxygen) and at least a portion of the second tail gas (e.g. methane), whilst the second gaseous feedstock can comprise externally introduced gas (e.g. hydrogen) and at least a portion of the first tail gas. The second gaseous feedstock can even consist of the first tail gas or, preferably, of a portion thereof after having removed at least oxygen comprised in the first tail gas. Preferably, tail gas comprising at least one of methane, hydrogen and carbon dioxide is exchanged between the first and the second fermenter. A composition of each of the first and second tail gases can be continuously measured for process control, e.g. by means of a gas sensor. Gas sensors can be arranged in the first and second connection elements. The fermentation broth comprised in the first and second fermenters can comprise microorganisms selected from bacteria, archaea, algae, yeasts or a mixture thereof. The fermentation broth comprised in the first fermenter can comprise methanothrops, whilst the fermentation broth comprised in the second fermenter can comprise acetogens. Any aerobic microorganism that yields a product of commercial interest can be grown in the first fermenter, and any anaerobic microorganism that yields a product of commercial interest can be grown in the second fermenter. Preferably, the method is carried out with pure or mixed cultures comprising at least one methanotrophic bacterium and one acetogenic bacterium. Further preferably, the method is carried out with pure or mixed cultures comprising at least one methylotrophic bacterium feeding on methanol in the first, aerobic fermenter.

According to the present application, methanothrops are understood to be aerobic methanotrophs, i.e. microorganisms that can metabolize CH₄ through oxidation with O2 to yield target product(s) and CO2 as by-product. Methylotrophic yeast can be e.g. Pichia pastoris.

Further, according to the present application, acetogens are understood to be microorganisms that take their carbon from CO2 and/or CO and their energy from H2. They can form acetate in their metabolism. The acetogens can be tolerant to CH₄, but do not produce it in significant amounts (as would methanogenic bacteria in a biogas plant do, which is also an anaerobic process).

SCP can be made from methane using one or more of the following strains: Pseudomonas sp., Methylomonas methanica, Methylococcus capsulatus, Methylococcus vinelandii, Methylomonas carbonatophila, Methylococcus capsulatus strain Bath, Methylosinus sporium, Methylomonas rubrum, Methylococcus ucrainicus, Methylosinus trichosporium, Methylomonas rosaceus, Methylococcus fulvus, Methylosinus trichosporium TG, Methylomonas agile, Methylococcus thermophilus, Methylocystis parvus, Methylomonas albus, Methylococcus albus, Methylobacterium organophilum, Methylomonas streptobacterium, Methylococcus minimus, Mycobacterium methanicum, Methylomonas methanooxidans, Methylococcus luteus, Mycobacterium cuneatum, Methylomonas methanitrificans, Methylococcus bovis, Nocardia rhodochrous, Pseudomonas strain L-8, Methylococcus chroococcus, Nocardia ucrainica, Pseudomonas strain L-47, Methylococcus whittenburii, Rhodopseudomonas gelati nos, Hansenula polymorpha, Pichia pastoris, Pseudomonas sp. PHB can be made from methane using one or more of the following strains: Methylocystis sp. WRRC1, Methylosinus trichosporium, Methylobacterium organophilum, Methylocystis hirsute. Ectoine can be made from methane using Methylomicrobium alcaliphilum and/or Methylomicrobium buryatense.

The microorganisms can be selected from naturally occurring and/or genetically modified microorganisms. For example, each fermentation broth can comprise from 0.1 to 10 wt% microorganisms and 90 to 99.9 wt% water (including nutrients, not considering the gas fraction, such as gas bubbles). The typical gas fraction in the fermenter is 5 to 20 vol%. When starting the fermentation with an inoculum, the cell density can be lower, e.g. below 0.1 wt% of microorganisms. During growth, a density of 0.1 to 10 wt% can be reached for methanotrophs; typical values during operation of the fermenter can range from 1 to 30 g / 1 of dry weight, i.e. 0.1 to 3 wt%, and water from 99.9 to 97 wt%. For acetogens, values can be in the same range.

The first product that can be obtained via aerobic fermentation can comprise single cell protein (e.g. for feed and/or food applications), acids (e.g. amino acids, nucleic acids), alcohols, monomers to produce biopolymers (e.g. lactic acid, lactate), and biopolymers (e.g. polyhydroxyalkanoates, such as polyhydroxybutyrate (PHB); polylactic acid (PLA); polyhydroxy-butyrate-co-valerate (PHBV); EPS (extracellular polymeric substance).

The second product that can be obtained via anaerobic fermentation can comprise single cell protein (e.g. for feed and/or food applications), alcohols, acids (e.g. amino acids; nucleic acids; volatile fatty acids comprising Cz-Cg carboxylic acids), monomers to produce biopolymers (e.g. lactic acid, lactate), and biopolymers (e.g. polyhydroxyalkanoates (PHA), such as PHB, PLA, PHBV).

Preferably, the first and second product comprise single cell protein (SCP) for feed or food applications, building blocks for polymers such as lactic acid (LA), acetic acid (acetate), biopolymers such as polyhydroxyalkanoates (PHA), particularly polyhydroxybutyrate (PHB), or small molecules such as ectoine.

The fermentation broth in the first fermenter can comprise hydrogen-oxidizing bacteria (knallgas bacteria) and/or photosynthetic organisms (microalgae). Also, ammonia can directly be added to the fermentation broth of the first fermenter (e.g. in salt form, as nitrogen source) instead of introducing it as part of the first gaseous feedstock. For example, with the strains Methylococcus capsulatus and/or Methylophilus methylotrophus, methane can be converted to single cell protein (SCP), whilst with the strains Methylocystis parvus OBBP, Methylobacterium extorquens and/or Methylocystis sp. GB25, biopolymers (e.g. polyhydroxybutyrate (PHB)) can be produced. Accordingly, the fermentation broth in the first fermenter can comprise methanothrops capable of producing SCP and/or PHB and other PHA. Pure cultures and mixed cultures can be used. A mixed culture comprising the strains Methylococcus capsulatus, Alca ligenses acidovorans, Bacillus brevis and Bacillus firmus is advantageous to produce SCP from methane in the first fermenter, particularly when the mixed culture comprises 70 to 90% (by number) of Methylococcus capsulatus.

The fermentation broth of the first fermenter can further comprise methylotrophic bacteria, which allows for a conversion into methanol. This yields a higher biomass density and productivity, due to a better solubility of methanol in water than methane. As methanol-consuming strain to produce SCP, the presence of Methylophilus methylotrophus in the fermentation broth of the first fermenter has proven to be advantageous. To produce PHB, a fermentation broth comprising Methylobacterium extorquens is preferred. Synthetic methylotrophy and methanotrophy can also be achieved, for example with a fermentation broth comprising Saccharomyces cerevisiae (Kelso et al., ACS Synth. Biol. 2022, 11, 2548-2563).

The fermentation broth in the second fermenter can comprise anaerobic archaea. For example, Clostridium ljungdahlii or Clostridium carboxidivorans can be used for conversion to ethanol, and Acetobacerium woodii can be used for conversion to acetate. In a certain embodiment, microalgae (e.g. Synechocystis sp. strain PCC 6714) can be used for conversion to products of interest such as PHB or lactate. Accordingly, the fermentation broth in the second fermenter can comprise acetogens capable of producing lactate, ethanol, acetate, PHB and/or other products of interest.

Co-culturing of different microorganisms in the fermenters allows to efficiently use different feedstocks. For example, co-culturing with hydrogen oxidizing bacteria (HOB), microalgae and/or aerobic archaea in the first fermenter can yield a favorable composition of the first tail gas, of which at least a portion is fed into the second fermenter. Likewise, co-culturing with anaerobic archaea in the second fermenter can yield a favorable composition of the second tail gas, of which at least a portion is fed into the first fermenter.

The first gaseous feedstock can comprise, in addition to methane, also carbon monoxide, carbon dioxide, hydrogen, nitrogen, ammonia, or a mixture thereof. Preferably, the first gaseous feedstock comprises oxygen (O2) to allow for an operation in aerobic mode. The first feedstock can further comprise nitrogen, which can be used for stripping at least a portion of the first tail gas, particularly, carbon dioxide, from the first fermenter. The gaseous feedstocks can be supplied at different locations in the fermenters.

For example, the first gaseous feedstock can comprise 10 to 95 vol% methane and 5 to 60 vol% oxygen. Preferably, the first gaseous feedstock comprises methane and oxygen, such as 40 to 90 vol% methane and 10 to 60 vol% oxygen. It was found that when selecting such a composition, a high growth rate of cells comprising the first product, and a high yield of the first product can be obtained. The exact composition can depend on the composition of the fermentation broth (i.e. the used microorganisms) and the desired product.

The first fermenter can be operated with an excess of methane. The risk of explosion can then be reduced or avoided. Unconsumed methane can be comprised in the first tail gas and be fed into the second fermenter, where it can be purified from carbon dioxide and subsequently be sent back to the first fermenter where it can again be used for the aerobic fermentation. The unconsumed methane can thus circulate in the system.

To further reduce the risk of explosion, the first gaseous feedstock can comprise an inert gas, particularly nitrogen; and/or the first fermenter can be purged with an inert gas. A lower or upper explosive limit (LEL, UEL) in a headspace area of the first fermenter can be measured, and the inert gas can be introduced in case a certain value, e.g. 30% of the LEL or UEL, is reached.

The second gaseous feedstock can comprise, in addition to carbon dioxide, also carbon monoxide, methane, hydrogen, or a mixture thereof. As the second fermenter is an anaerobic fermenter, carbon dioxide (and optionally carbon monoxide) is consumed (i.e. reduced) during fermentation. Accordingly, a reducing agent can be comprised in the second gaseous feedstock. Preferably, the second gaseous feedstock comprises hydrogen (H2).

Further, the second fermenter being operated in an anaerobic mode typically implies that substantially no oxygen is present. However, a small amount of oxygen may be tolerated depending on the types of microorganisms comprised in the fermentation broth, e.g. by aerotolerant anaerobes and microaerophiles. Preferably, the second gaseous feedstock comprises less than 1 vol% oxygen.

The second gaseous feedstock can comprise carbon dioxide and hydrogen, e.g. 20-40 vol% carbon dioxide and 60-80 vol% hydrogen, particularly a ratio of carbon dioxide to hydrogen of 1:2. Preferably, the second gaseous feedstock comprises carbon dioxide, methane and hydrogen. As source for methane and carbon dioxide, the first tail gas can be used. Hydrogen can either be comprised in the first tail gas or introduced using an external gas source. Preferably, the second gaseous feedstock comprises 5-70 vol% methane or carbon monoxide, 10-70 vol% carbon dioxide and 10-70 vol% hydrogen; more preferably, 10-55 vol% methane or carbon monoxide, 15-30 vol% carbon dioxide and 30-60 vol% hydrogen. It was found that when selecting such a composition, a high growth rate of cells comprising the second product, and a high yield of the second product can be obtained. The ranges of 5-70 vol% methane or carbon monoxide and 10-55 vol% methane or carbon monoxide are preferably 5-70 vol% methane and 10-55 vol% methane, respectively.

The first and/or second gaseous feedstocks can originate from various fossil and/or renewable gas sources. For example, synthetic gas (syngas; i.e. a gas comprising hydrogen and carbon monoxide), gasification gas, biogas, (synthetic) natural gas, air, waste gas (e.g. from a landfill, a coal mine or an industrial process, such as combustion, incineration, steel or cement production), or a mixture thereof can be used. Preferably, a solid feedstock (such as biomass, waste or coal) is converted into methane and carbon dioxide to be used in the first and second fermenters, respectively.

A source for methane can be selected e.g. from natural gas (which can comprise 85 to 95% methane), biogas (which can comprise 50 to 75% methane; and can also comprise carbon dioxide), landfill gas (e.g. 50% methane, 50% carbon dioxide), biomass gasification (e.g. obtained from dry biomass through gasification), and the Sabatier process, where CO2 can be reduced to CH₄. A source for oxygen can be selected e.g. from air or electrolysis of water. Hydrogen can be produced by different routes, e.g. electrolysis (driven e.g. by solar energy, wind energy, nuclear energy), coal gasification and steam reforming. Preferably, hydrogen is produced from renewable energy. Further preferably, hydrogen is produced by electrolysis of water. Carbon dioxide can e.g. be obtained from a cement plant or a waste incineration plant. For carbon dioxide, such point sources are preferred over direct air capture. Carbon monoxide for use in the second gaseous feedstock can be generated by means of carbon dioxide electrolysis, either by using carbon dioxide from an external source, or by using carbon dioxide comprised in the first tail gas.

The first and second gaseous feedstocks can both be from the same gas source. This simplifies the set-up of the system, as the same gas stream can be introduced into both fermenters. This is an advantage over a catalytic process wherein impurities can considerably shorten lifetime, thus causing the need of frequent stops. When using the same gas source for both gaseous feedstocks, oxygen can be introduced separately into the first fermenter to perform the aerobic fermentation, e.g. via an injection nozzle or sparger in one or more locations.

The fermenters can be operated as per available gaseous feedstock (and also microorganisms for the fermentation broths). Also, a fluctuating gaseous feedstock (with a gas composition varying over time) can be used, e.g. directly from industry, due a changing composition of the tail gases with proceeding fermentation time. Accordingly, costly gas storage tanks can be avoided. The decision on which gaseous feedstocks and which microorganisms to use can be made by artificial intelligence, taking into account supply and demand, making the invention suitable for use in a biorefinery.

In an embodiment, the first gaseous feedstock comprises oxygen (O2) and/or the second gaseous feedstock comprises hydrogen (H2). This allows for an efficient aerobic fermentation in the first fermenter in the presence of oxygen, and an efficient anaerobic fermentation in the second fermenter in the presence of hydrogen.

The first and/or second gaseous feedstock can be pre-treated in a gas pre-treatment unit before being introduced into the first and/or second fermenter. Pre-treating the gaseous feedstocks can comprise any of converting carbon dioxide and hydrogen by means of the Sabatier process; applying direct air capture (e.g. to obtain carbon dioxide and/or methane directly from the atmosphere); filtering a specific gas; converting synthetic gas to methane (SNG); converting methane to synthesis gas; splitting water to oxygen and hydrogen; and conversion by thermochemical, catalytic and/or biotechnological means. When splitting off undesired gas (e.g. higher hydrocarbons) during pretreatment, said gas can be burned to generate thermal energy, which can then be used to operate the system, e.g. the first fermenter and the second fermenter, e.g. by heating or cooling. By means of the Sabatier process, carbon dioxide and hydrogen can be converted to methane and water. To accelerate the reaction, a catalyst can be used, such as a nickel catalyst as per the state-of- the-art. The water produced in the Sabatier process can subsequently be split into oxygen and hydrogen via electrolysis in an electrolyzer, followed by introducing at least a portion of the oxygen into the first fermenter and at least a portion of the hydrogen into the second fermenter.

The first and second connection elements are configured to feed at least a portion of the first tail gas to the second fermenter and at least a portion of the second tail gas to the first fermenter. The connection elements can comprise connection pipes. For example, the first connection element can comprise a first connection pipe, and the second connection element can comprise a second connection pipe. Each connection pipe can comprise two or more segments.

An opening of the first connection element (e.g. one end of a first connection pipe) can enclose a first opening of the first fermenter, such that at least a portion of the first tail gas can exit the first fermenter through said first opening and be guided through the first connection element. To subsequently feed said portion of the first tail gas into the second fermenter, another opening of the first connection element (e.g. the other end of the first connection pipe) can enclose a first opening of the second fermenter (which can be arranged in a mantle or a cover of the second fermenter), through which the portion of the first tail gas can enter the second fermenter. Similarly, an opening of the second connection element (e.g. one end of a second connection pipe) can enclose a second opening of the second fermenter, such that at least a portion of the second tail gas can exit the second fermenter through said second opening and be guided through the second connection element. To subsequently feed said portion of the second tail gas into the first fermenter, another opening of the second connection element (e.g. the other end of the second connection pipe) can enclose a second opening of the first fermenter, through which the portion of the second tail gas can enter the first fermenter.

To allow for a gas flow into the first and second openings of the respective fermenters, said openings can be arranged above a fermentation volume (i.e. the volume fraction of the respective fermenter filled with fermentation broth). For example, the first and second openings can each be arranged in a mantle or a lid of the respective fermenters. The first and second openings can further be arranged opposite to each other, such that gas flows through the respective openings cannot interfere with each other.

Each of said first and second openings in the respective fermenters can be equipped with a closing element, e.g. a lid or a valve. For example, this allows to close the openings of the first fermenter when introducing the first gaseous feedstock, such that an undesired passage of the first gaseous feedstock towards the second fermenter through the first and/or second opening can be avoided. Also, when guiding a portion of the first tail gas through the first opening of the first fermenter into the first connection element and towards the second fermenter, the second opening of the first fermenter can be closed, such that an undesired passage of a portion of the first tail gas into the second connection element can be avoided. Typically, the fermenters are operated continuously, but batch processing is also feasible.

The system can further comprise a first feeding element connected with the first inlet and a second feeding element connected with the second inlet, wherein the first connection element is coupled with the second feeding element, and the second connection element is coupled with the first feeding element. This allows to combine at least a portion of the first tail gas fed through the first connection element with externally introduced gas to form the second gaseous feedstock already before introducing said second gaseous feedstock into the second fermenter. As a consequence, the gas composition can be homogenized, and the process stability can be increased. Likewise, at least a portion of the second tail gas fed through the second connection element can be combined with externally introduced gas to form the first gaseous feedstock already before introducing said first gaseous feedstock into the first fermenter. The first feeding element can comprise a first pipe and the second feeding element can comprise a second pipe. Each feeding pipe can comprise two or more segments.

The system can further comprise a gas processing unit configured to alter a composition of the first tail gas before feeding at least a portion of the first tail gas into the second fermenter or to alter a composition of the second tail gas before feeding at least a portion of the second tail gas into the first fermenter. Accordingly, at least one component/fraction of the first tail gas and the second tail gas can be guided through a gas processing unit to alter a composition of the first tail gas or the second tail gas before feeding at least a portion of the first tail gas or the second tail gas to the second fermenter or the first fermenter.

The gas processing unit can be arranged in the first connection element or in the second connection element. For example, when the connection elements comprise connection pipes, with each connection pipe comprising two segments, the gas processing unit can be arranged between two segments of the first or second connection pipe.

The gas processing unit can comprise a filter (e.g. a sieve, a membrane), an adsorber (e.g. a scrubber), a catalyst, or a combination thereof. Accordingly, treatment of the first tail gas or second tail gas can comprise filtering, adsorption, absorption, or a combination thereof.

The gas processing unit can comprise an oxygen removal unit configured to remove at least a portion of oxygen comprised in the first tail gas before feeding a portion of the first tail gas into the second fermenter. Accordingly, oxygen can be removed at least partially, preferably completely, from the first tail gas before feeding a portion of the first tail gas into the second fermenter. The oxygen removal unit can comprise an oxygen scrubber, an oxygen scavenger, and/or a filter. Alternatively, treatment of the first tail gas can comprise dilution with another gas to reduce the oxygen content, in order to not impede the anaerobic fermentation in the second fermenter. It is noted that no limitations were found regarding maximum contents of residual methane comprised in the first tail gas and fed into the second fermenter, and residual carbon dioxide comprised in the second tail gas and fed into the first fermenter.

The gas processing unit can also comprise a sieve to filter gases with respect to their molecule size. The sieve can be configured to be flush with an inner wall of the first or second connection element, such that the gas inevitably passes the sieve. A gas that cannot pass through the sieve can be sucked off (e.g. via vacuum suction), be removed from the first or second connection element, and optionally be subjected to a further treatment, whilst a gas that can pass through the sieve can be fed through the respective connection element to the respective fermenter. Alternatively, a gas that cannot pass through the filter can be fed through the respective connection element to the respective fermenter, whilst a gas that can pass through the sieve can be removed and optionally subjected to a further treatment.

The gas processing unit can also comprise a membrane, such as a hollow fiber membrane, for gas separation. The membrane can be flush with an inner wall of the first or second connection element, such that the gas inevitably passes the membrane.

The composition of the first or second tail gas cannot only be altered by means of separation of different gases from each other (e.g. via filtering), but also via chemical treatment, which can be performed either as alternative or in addition to gas separation. When the second tail gas comprises carbon dioxide, it can be guided through the gas processing unit to convert the carbon dioxide into methane. The formed methane can be used for the first gaseous feedstock, which allows for an even more significant reduction of the carbon footprint.

The composition of the first or second tail gas can be altered in presence of an external gas introduced into the gas processing unit. For example, biogas, natural gas, landfill gas or air can be used, which can optionally be subjected to pre-treatment in a gas pre-treatment unit before being introduced into the gas processing unit.

The system can also comprise two gas processing units, wherein one gas processing unit is configured to treat the first tail gas and the other gas processing unit is configured to treat the second tail gas. The system can further comprise an electrolyzer configured to split water into oxygen and hydrogen, wherein the electrolyzer can be connected with the first inlet and the second inlet to feed at least a portion of the oxygen into the first fermenter and at least a portion of the hydrogen into the second fermenter. Accordingly, water can be split into oxygen and hydrogen in the electrolyzer connected with the first fermenter and the second fermenter, wherein at least a portion of the oxygen is fed into the first fermenter and at least a portion of the hydrogen is fed into the second fermenter. The electrolyzer can be comprised in the gas pre-treatment unit to treat externally introduced water. For example, ground water or water from a river, a stream or a lake can be used, which can optionally be pre-treated, e.g. subjected to filtering, deionization and/or distillation. Alternatively or additionally, water which can be formed as by-product in the first fermenter during aerobic fermentation can be subjected to electrolysis. For example, water comprised as water vapor in the first tail gas can be separated by means of a gas processing unit and be guided to the electrolyzer to produce hydrogen and oxygen. This allows for an improved circulation of gases/vapors already present in the system.

The system can further comprise an electrolyzer configured to produce carbon monoxide from carbon dioxide, wherein the electrolyzer can be connected with the second inlet to feed the carbon monoxide into the second fermenter as part of the second gaseous feedstock. Accordingly, carbon dioxide can be converted to carbon monoxide into the electrolyzer. The carbon dioxide used for the conversion in the electrolyzer can be from an external source, and/or it can be the carbon dioxide comprised in the first tail gas, or a portion thereof.

The system can further comprise a stock vessel and/or a discharge vessel connectable to the first fermenter and/or the second fermenter. Accordingly, the first fermenter or the second fermenter can be connected to a discharge vessel to empty the first fermenter or the second fermenter; and/or the first fermenter or the second fermenter can be connected to a stock vessel comprising a fermentation broth to refill the first fermenter or the second fermenter. This arrangement can shorten undesirable breaks when the fermentation broth of the first or second fermenter is dormant (i.e., consumed) and needs to be renewed, hereby improving the operating efficiency. Both the stock vessel and/or the discharge vessel can be constructed similarly to the fermenters. The filling volume of each vessel can correspond to the fermentation volume of the fermenters, or at least to the fermentation volume of the larger of the fermenters (in case the first and second fermenters do not have the same size). This ensures that the total content of any of the fermenters can be discharged into the discharge vessel, and that any of the fermenters can be filled up to maximum level using the stock vessel. The fermentation broth of the stock vessel can have a composition similar or identical to that of the composition of the fermentation broth of the first or second fermenter. This ensures a stable processing, and the processing parameters (e.g. temperature, pressure) do not need to be adapted to another composition.

In the stock vessel, the fermentation broth can be stirred continuously. Using a rotation speed of from 1 to 50 rpm in the stock vessel was found to be beneficial. Optionally, a minimum gas supply can be used to maintain the cultures in the stock vessels.

The system can also comprise two or more stock vessels connectable to the first fermenter or the second fermenter; and/or the system can comprise two or more discharge vessels connectable to the first fermenter or the second fermenter. One of the stock vessels can comprise a fermentation broth for aerobic fermentation, and the other can comprise a fermentation broth for anaerobic fermentation. This further improves the operating efficiency, as for example, both the first and second fermenter can be emptied or refilled simultaneously.

The first fermenter and the second fermenter can both be operable in aerobic mode and anaerobic mode. That is, when refilling the first and second fermenter, the respective fermentation broths and gaseous feedstocks can be chosen such that the first fermenter, which was previously operated in an aerobic mode, can then be operated in an anaerobic mode, and the second fermenter, which was previously operated in an anaerobic mode, can then be operated in an aerobic mode. Accordingly, the first fermenter and the second fermenter can be alternately operated in an aerobic mode and in an anaerobic mode. Reversing the aerobic/anaerobic nature of the fermenters can reduce the risk of contamination. For example, when employing two discharge vessels and two stock vessels, the aerobic/anaerobic nature of the fermenters can even be reversed within some hours instead of within some days. This allows for an improved flexibility to adapt to different feedstocks, making the present invention suitable for use in a biorefinery.

The first and second fermenters can each be a loop reactor, a bubble column or an air lift reactor. They can each have a cylindrical shape. When using cylindric fermenters, the diameter-height ratio can vary from 1:3 to 1:50. A plug flow reactor (PFR) (type of loop reactor) can result in a higher feedstock conversion than e.g. a continuously stirred thank reactor (CSTR).

At industrial scale, the first fermenter and the second fermenter can have a fermentation volume from 20 to 2,000 m³, preferably 50 to 500 m³. The fermentation volume intends to refer to the volume fraction of the fermenter filled with fermentation broth, which can be 65 to 90% of the total volume of the fermenter. The gas hold-up ratio can range from 5 to 30 vol%. The gas hold-up ratio intends to refer to the fraction of the fermentation broth (excl. headspace for degassing/defoaming) that comprises gas bubbles. It can be in the same range for both fermenters. In the aerobic fermenter, for instance, 30% can be approached when using air as oxidizer instead of oxygen.

The design of the first and second fermenter can be adapted to one another to allow for an optimum conversion. For example, the fermenters can have a similar or identical size and geometry, particularly a similar or identical fermentation volume. Alternatively, the filling degree can be varied, e.g. by adapting the working volume.

A gassing rate of the first fermenter and the second fermenter can be from 0.05 to 2.5 vessel volumes per minute (vvm), preferably from 0.2 to 0.6 vvm. This was found to yield optimum processing conditions under cultivation conditions. The gassing rate signifies the amount of gaseous feedstock introduced into the first or second fermenter per time. Introducing an additional inert gas (e.g. nitrogen) leads to an increase of the gassing rate. The lower the gassing rate, the less energy can be consumed and the higher the gas conversion rate can be, particularly in a plug flow type system such as a loop reactor. On the other hand, a higher gassing rate will increase mixing and mass transfer, so a balance is needed. The term "vvm" stands for "vessel volumes per minute", which means that e.g. for a fermenter of 100 m³ volume, preferably 20 to 60 m³ of gas are fed. The vvm are calculated in operational m³, not standard m³ (which would be calculated at 1.01325 bar and 273.15 K). Typical operating conditions of the fermenters are 20 to 40 °C and an ambient pressure of up to a few bar. Depending on the position of a gas bubble or volume element in the fermenter, a different hydrostatic pressure can occur, e.g. + 1 bar (gauge) at 10 m water column and + 2 bar (gauge) at 20 m water column, which can compress the gas bubbles at lower sections of the fermenters accordingly.

A gas phase coupling rate between the first fermenter and the second fermenter can be from 0.05 to 2.5 vessel volumes per minute (vvm), preferably from 0.1 to 0.4 vvm. This signifies that a gas stream between the two fermenters (with the gas stream being the sum of streams flowing through first and second connection elements) is typically 10 to 40% of the volume of the second fermenter (i.e., volume of the entire fermenter, not only fermentation volume).

The first fermenter and the second fermenter can each comprise a loop fermenter. This was found to yield the best fermentation results (high yield, high growth rate). For example, the first and second fermenter can be airlift reactors having an external loop. The loop fermenters can be operated with a pump (e.g. an axial flow pump or a centrifugal pump). The first and second fermenters can also each comprise a bubble column or a stirred tank. A reactor length of each loop fermenter can range from 10 to 60 m at industrial scale for each leg (i.e. in x-, y- and z-direction). For example, a folded design as is common with loop reactors used for polyolefin polymerization can be used. The inner diameter of each loop fermenter can be in the range of 0.1 to 2.5 m. The inlet for introducing the first or second gaseous feedstock can be arranged on a side of the loop fermenter opposite to a side on which an outlet is arranged that allows the first or second tail gas to exit the respective fermenter. As such, the gas flows of the introduced gaseous feedstock and the tail gas exiting the fermenter cannot interfere with each other. A gas flow rate of from 0.05 to 2.5 vvm, preferably from 0.2 to 0.6 vvm in each loop fermenter was found to yield optimum processing conditions.

The first fermenter and the second fermenter can each be operated at an elevated temperature, such as up to 65 °C, preferably of from 20 to 55 °C, more preferably from 30 to 45 °C, particularly preferably from 25 to 40 °C. Using a temperature in the specified range was found to yield a good balance between processing efficiency (time, cost) and yield. Thermophilic strains are economically advantageous because hot feed gasses can be used in the anaerobic fermenter (e.g. from a gasification process) and less cooling is required in the aerobic fermenter. The cost savings for cooling are counteracted by a lower gas solubility at elevated temperatures, so a balance needs to be established, which depends on the microorganism(s) and was found to be typically between 25 and 40 °C. The second fermenter can be operated at a higher temperature than the first fermenter, for example at 10 to 40 °C higher, or at 20 to 30 °C higher, especially if the fermentation broth of the second fermenter comprises thermophilic acetogens. The first tail gas can have a high specific heat capacity, such that a part of the thermal energy required in the second fermenter can be introduced via said first tail gas, hereby achieving a reduction of the overall energy consumption. Temperature control is important to allow for a smooth fermentation. The selected temperature preferably remains within a range of +/- 3 °C, more preferably +/- 1 °C.

Further, the first fermenter and the second fermenter can each be operated at a pressure of from 0.1 to 10 bar (gauge), preferably from 1 to 5 bar (gauge). A pressure in this range was found to yield a good balance between investment costs, processing efficiency (time, cost) and yield. In general, a higher temperature was found to decrease the solubility of gas in water, whilst the volumetric mass transfer coefficient (k_La value) was found to increase with increasing energy intake and increasing pressure.

For an optimized processing, both the first fermenter and the second fermenter can be operated at a temperature of from 20 to 65 °C and a pressure of from 0.1 to 10 bar (gauge), i.e. 1.1 to 11 bar (atmospheric).

Continuous operation is preferred over batch mode, as a higher time-space yield can be achieved. It was found that continuous fermentation can be maintained over more than 30 days. Particularly, the fermentation can be performed for 15 to 60 days, which can be followed by purging, cleaning and restarting the fermentation. This can prevent random mutations of the strains to manifest themselves.

The pH value of the fermentation broth comprised in the first fermenter can range from 3.0 to 7.5, preferably 5.5 to 6.5, whilst the pH value of the fermentation broth comprised in the second fermenter can range from 3 to 7.5. The pH value can be adjusted with acids or bases. The pH value can be increased with ammonia (NH3), its ammonium salts or NaOH (the dissolved CO2 will automatically lower the pH). A pH reduction can be achieved for example with hydrochloric acid (HCI). Control of the pH value is particularly important when introducing carbon dioxide into the fermentation broth (the solubility of CO2(_g) in water is 1.45 g/l at 25 °C and 1 bar).

Preferably, the coupled gas fermentation is performed in a continuous mode at a temperature of 20 to 65 °C, a gassing rate of 0.05 to 2.5 vvm, a pH value of 3.5 to 7.5, a pressure of 0.1 to 10 bar (gauge; corresponds to 1.1 to 11 bar atmospheric pressure), for each fermenter.

The gas utilization rate can be from 80 to 99.9 vol%, preferably 95 to 99 vol%, both for methane in the aerobic fermentation and for carbon dioxide in the anaerobic fermentation. The values in vol% refer to the fraction of gaseous feedstock which is introduced into the respective fermenter to the fraction of tail gas which was not consumed during fermentation.

The specific energy intake in the coupled gas fermentation can be from 0.1 to 10 kW/m³, preferably 0.5 to 5 kW/m³. In general, in a continuous stirred-tank reactor (CSTR), energy input can be provided by a stirrer, whereas in a bubble column reactor, energy input can come from a gas phase. In a loop reactor, the energy input into the system can be provided by an external pump and a gas phase. Energy input is needed to achieve a good mixing and hence a high k_La value.

In general, the mass flow (mol/h) of educt gas that is supplied from an external gas source to a fermenter (comprising methane for the first fermenter, and carbon dioxide for the second fermenter) is higher than the mass flow (mol/h) of tail gas from the respective other fermenter. Ratios of 100:1 up to 3:1 were tested and found to work. When a high mass flow of unconsumed methane exits in the first fermenter, gets cleaned from carbon dioxide in the second fermenter and is fed back into the first fermenter, said first fermenter can be operated with an excess of methane, which can be beneficial in terms of safety (operation outside the upper explosive limit) and productivity (as the fermenter is then not limited by methane supply). Likewise, the second, anaerobic fermenter can be operated with an excess of hydrogen and/or an excess of carbon dioxide, which can be fed back from the first fermenter.

The first product can comprise single cell protein (SCP) formed by methanotrophs. Such SCP can be used for feed (animals) or food (humans) applications, due to the high protein (amino acid) content. For example, a chemical equation for the production of dry cells comprising 70 wt% SCP (CH1.8O0.5N02) in the first fermenter with a fermentation broth comprising Methylococcus capsulatus can be drawn as follows:

CH₄ + 1.52 O2 + 0.09 NH3 -> 0.456 CH1.8O0.5N02 + 0.544 CO2 + 1.72 H2O (molar ratios); in this case, the first gaseous feedstock can comprise methane and oxygen, whilst ammonia can be directly added to the first fermentation broth to provide the necessary nitrogen. Note that the composition of the SCP biomass can vary. CH1.8O0.5N02 was found to be a typical composition (not taking into account other elements, such as sulfur). The ratio of C:H:O:N is l:1.8:0.5:0.2 but also compositions of 1.0:1.5-2.1:0.4- 0.6:0.15-0.25 were found.

Taking into account the molar mass of constituents, the formula above can be recalculated on a mass basis:

16 CH₄ + 48.64 O₂ + 1.62 NH₃ ->11.218 CH1.8O0.5N02 + 23.94 CO₂ + 30.96 H₂O (weight ratios); this means that 16 g methane and 48.64 g oxygen yield 23.94 g carbon dioxide. In terms of volumes, this signifies that 22.35 I methane and 43.01 1 oxygen yield 11.97 I carbon dioxide.

This formula can be based on the target product (SCP) as follows:

1.43 CH₄ + 4.34 O2 + 0.14 NH3 ->1 CH1.8O0.5N0.2 + 2.13 CO2 + 2.76 H2O (weight ratios); this means that to produce 1 t SCP, theoretically (optimum case i.e. without metabolic losses), 1.43 t methane is required and 2.13 t carbon dioxide are formed as by-product. However, as methanotrophs breathe during aerobic fermentation, these theoretical numbers are reduced in reality, such that a higher amount of methane is converted to carbon dioxide. Accordingly, a faster growth rate in the first fermenter can correspond to a higher SCP yield.

It can be estimated that in reality, to produce 1 1 SCP, an input of about 21 methane is required (i.e. about 40% more than theoretically needed). This ratio can be improved by process optimization, e.g. when using a loop fermenter with a plug flow profile, wherein methane can be fully consumed at the end of each cycle. With optimized processing parameters, a productivity of from 0.2 to 20 g biomass (dry cell weight (dew)) per liter and per hour can be obtained, particularly 1 to 10 g biomass per liter and hour, or typically 3 to 7 g biomass per liter and per hour.

When using methane from natural gas, the following calculation can be made: Methane has a density of about 660 g/m³. Accordingly, 1 t methane corresponds to 1,520 m³ methane, thus 2 t methane correspond to 3,040 m³ methane. Natural gas can comprise 85 to 95 wt% methane, such that about 3,200 to 3,600 m³ natural gas is needed to produce 1 1 SCP (3,040/0.95 = 3,200 m³; 3,040/0.85 = 3,600 m³).

To avoid a risk of explosion, the amount of methane comprised in the first gaseous feedstock can be doubled:

2 CH₄ + 1.52 O₂ + 0.09 NH₃ -> 0.456 CH1.8O0.5N02 + 0.707 CO₂ + 1.72 H₂O + 0.7 CH₄ (molar ratios); the first gaseous feedstock thus comprises 57 vol% methane and 43 vol% oxygen. The first tail gas comprises unconsumed methane, which can be fed to the second fermenter and subsequently circulated back to the first fermenter for further participation in the aerobic reaction.

The first product can also comprise SCP and polyhydroxybutyrate (PHB). For example, a chemical equation for the production of dry cells comprising 50 wt% PHB and 30 wt% SCP (C2.23H3.4O1.25N0.1) in the first fermenter can be drawn as follows:

1.24 CH₄ + 3.8 O₂ + 0.045 NH₃ -> 0.456 C2.23H3.4O1.25N0.1+ 0.22 CO₂ + 1.77 H₂O (molar ratios)

0.8 CH4 + 5.2 O2 + 0.2 NH3 -> 1 C2.23H3.4O1.25N0.1 + 0.4 CO2 + 1.4 H2O (weight ratios)

1.68 CH₄ + 10.34 O₂ + 0.46 NH₃ -> 1 PHB + 0.82 CO₂ + 2.71 H₂O + 0.6 SCP (weight ratios)

2.81 CH₄ + 17.24 O₂ + 0.77 NH₃ -> 1.67 PHB + 1.37 CO₂ + 4.52 H₂O + 1 SCP (weight ratios)

This means that to produce 1 1 dry cells, theoretically, 0.8 t methane is required and 0.4 t carbon dioxide is formed as by-product. Again, in reality, a higher amount of methane is converted to carbon dioxide, and the yield of SCP and PHB is lower. It can be estimated that instead of 1.68 t methane to produce 1 1 PHB and 0.6 1 SCP, about 2 to 2.3 t methane are required, which corresponds to 3,030 to 3,480 m³ methane, or, based on a methane content of 85 to 95 wt% in natural gas, about 3,100 to 4,100 m³ natural gas.

For the PHB production, the typical composition of biomass (dew) was found to be C2.23H3.4O1.25N0.1, i.e. ratio of C:H:O:N of 1:2.23:3.4:1.25:0.1. The actual composition can depend on strain and PHB yield and compositions of C:H:O:N of 2.18-2.18:3.3-3.5:1.2-1.3:0.05-0.15 were found. In case of other target products, such as ectoine, the calculations can be done accordingly.

With regard to the production of methane with the Sabatier process, 1 1 methane can be produced using 2.75 1 carbon dioxide, as shown below:

1 CO2 + 4 H2 -> 1 CH₄ + 2 H2O (molar ratios)

2.75 CO₂ + 0.5 H₂ -> 1 CH₄ + 3.5 H₂O (weight ratios)

In sum, for a coupled gas fermentation according to the present invention, the following chemical equations can be drawn, with (a) being the aerobic fermentation, (b) the anaerobic fermentation, (c) electrolysis in an electrolyzer, (d) air splitting in a gas pre-treatment unit, and (e)-(f) the net process. In this example, the first fermentation broth comprises Methylococcus capsulatus to produce single cell protein in the first fermenter, and the second fermentation broth comprises Acetobacterium woodii to produce lactate. However, also other strains and/or mixed cultures can be used. Lactate is of interest because it can be converted into polylactic acid (PLA), a popular biopolymer.

(a) 2 CH₄ + 1.52 O₂ + 0.09 NH₃ -> 0.456 CH1.8O0.5N02 + 0.707 CO₂ + 1.72 H₂O + 0.7 CH₄ (molar ratios)

(b) 1.42 H2 + 0.71 CO2 -> 0.24 lactate + 0.7 CH₄ (molar ratios)

(c) 0.71 H₂O -> 1.42 H₂ + 0.71 O₂ (molar ratios) (d) 4.05 air -> 3.24 N2 + 0.81 O2 (molar ratios)

(e) 1.3 CH₄ + 1.52 O₂ + 0.09 NH₃ + 1.421 H₂ -> 0.456 CH1.8O0.5N02 + 0.24 lactate (molar ratios)

(f) 20.8 CH₄ + 48.64 O₂ + 1.53 NH₃ + 2.841 H₂ -> 11.218 CH1.8O0.5N0.2+ 21.595 lactate (weight ratios)

(g) 1 CH₄+ 2.34 O₂ + 0.07 NH₃ + 0.137 H₂ -> 0.539 CH1.8O0.5N02 + 1.038 lactate (weight ratios)

Accordingly, 11 methane can yield 0.5391 SCP and 1.0381 lactate without any emissions of carbonbased gases.

To obtain the first product and the second product, for example, the fermentation broths can be subjected to concentration and drying, using unit operations as known in the state of the art, e.g. in case of SCP, centrifugation and subsequent spray-drying. To save energy, flocculation, decanting and drum drying can also be applied, or vacuum drying. In case of an intracellular compound, such as PHB, cell lysis, and separation/purification can be performed, e.g. using solvents as known in the art. With an extracellular compound, downstream processing can be facilitated.

Single cell protein comprised in the first product can then be used for food applications, whereas lactate comprised in the second product can be used to produce polylactic acid (PLA), which can e.g. be used for the manufacture of polymer films, fibers, bottles and bio-degradable medical devices or other articles.

All of the embodiments of this invention as disclosed in the present application are interrelated, and each embodiment and/or disclosed characteristic feature may be combined with each other and also as any combination of two or more embodiments/characteristic features.

Short description of the figures

Each of Fig. 1-6 shows a schematic visualization of a system for performing a coupled gas fermentation as disclosed herein. For clarity reasons, each system comprises different components (e.g. electrolyzer, gas pre-treatment unit, stock vessel, discharge vessel), however, it is readily evident that any of the components can be used in combination.

Fig. 7 shows a schematic visualization of system for performing a coupled gas fermentation comprising several components and gas sources.

Fig. 8 and 9 show ternary diagrams for an aerobic fermentation, whilst Fig. 10 and 11 show ternary diagrams for an anaerobic fermentation. Fig. la shows a schematic system la for performing a coupled gas fermentation comprising a first fermenter 2a operated in an aerobic mode and a second fermenter 2b operated in an anaerobic mode. A first gaseous feedstock comprising methane and oxygen is introduced into the first fermenter 2a, whilst a second gaseous feedstock comprising hydrogen and optionally carbon dioxide is introduced into the second fermenter 2b. As can further be seen from Fig. 1, at least a portion of a first tail gas comprising carbon dioxide is fed into the second fermenter 2b, whilst at least a portion of a second tail gas comprising methane is fed into the first fermenter 2a. Methane can optionally also be comprised in the second gaseous feedstock and introduced into the second fermenter 2b. As methane cannot be consumed in the second fermenter 2b, it becomes part of the second tail gas and is fed into the first fermenter 2a. Also, carbon dioxide can optionally be comprised in the first gaseous feedstock and introduced into the first fermenter 2a. As carbon dioxide cannot be consumed in the first fermenter 2a, it becomes part of the first tail gas and is introduced into the second fermenter 2b.

Alternatively, as can be seen from Fig. lb, the system la is operated using only a first gaseous feedstock that comprises methane, hydrogen and oxygen from an external source. As such, the second gaseous feedstock consists of the first tail gas; or the second gaseous feedstocks consists of at least of a portion of the first tail gas, when the first tail gas is subjected to a first gas processing unit (not shown), e.g. to remove any oxygen not consumed in the first fermenter.

Fig. 2 displays another schematic system lb for performing a coupled gas fermentation. In addition to the system la (Fig. la-b), the system lb comprises a first gas processing unit 3a configured to alter a composition of a first tail gas before feeding at least a portion of the first tail gas comprising carbon dioxide into the second fermenter 2b. The system lb further comprises a second gas processing unit 3b configured to alter a composition of a second tail gas before feeding at least a portion of the second tail gas comprising methane into the first fermenter 2a.

A further schematic system lc for performing a coupled gas fermentation is shown in Fig. 3. In addition to the system la (Fig. la-b), the system lc comprises a gas pre-treatment unit 4. At least a portion of externally introduced gas is introduced into said gas pre-treatment unit 4 to produce a first gaseous feedstock comprising methane and oxygen to be introduced into the first fermenter 2a, and a second gaseous feedstock comprising hydrogen and optionally carbon dioxide to be introduced into the second fermenter 2b. The dashed arrows signify that in addition to gas being subjected to pretreatment, gas from an external source can also be directly introduced into the first fermenter 2a or the second fermenter 2b to form part of the first/second gaseous feedstock. In Fig. 4, another schematic system Id for performing a coupled gas fermentation is visualized. In contrast to the system lc of Fig. 3, the system Id comprises a first gas pre-treatment unit 4a to produce a first gaseous feedstock comprising methane and oxygen, and a second gas pre-treatment unit 4b to produce a second gaseous feedstock comprising hydrogen and optionally carbon dioxide. Also, gas from the first gas pre-treatment unit 4a can be introduced into the second fermenter 2b, and/or gas from the second gas pre-treatment unit 4b can be introduced into the first fermenter 2a, as indicated with the dotted arrows.

Fig. 5 shows a further schematic system le for performing a coupled gas fermentation. In addition to the system la (Fig. la-b), the system le comprises an electrolyzer 5 configured to split water into oxygen and hydrogen. At least a portion of the oxygen is fed into the first fermenter 2a, and at least a portion of the hydrogen is fed into the second fermenter 2b. The water subjected to electrolysis is introduced externally. In addition thereto or alternatively, water generated as by-product in the first fermenter 2a can be subjected to electrolysis, as indicated with the dashed arrow.

Another schematic system If similar to the system la (Fig. la-b) is shown in Fig. 6. The system If comprises two discharge vessels 6a, 6b and two stock vessels 7a, 7b. Each of said vessels is connectable to the first fermenter 2a or the second fermenter 2b. Accordingly, the first fermenter 2a or the second fermenter 2b can each be emptied by connecting it with one of the discharge vessels 6a, 6b. Further, the first fermenter 2a or the second fermenter 2b can each be refilled by connecting it with one of the stock vessels 7a, 7b. One of the stock vessels 7a, 7b can comprise an anaerobic fermentation broth, whilst the other one can comprise an aerobic fermentation broth. Refilling allows to either maintain the aerobic/anaerobic nature of the respective fermenter, or to reverse it, which can be beneficial to avoid contaminations.

Fig. 7 shows a more complex, schematic system lg for performing a coupled gas fermentation. Said system lg comprises a first fermenter 2a and a second fermenter 2b. Gas from an external gas source, such as natural gas or biogas, or SNG from biomass or coal, is pre-treated in a first gas pre-treatment unit 4a to separate methane from other hydrocarbons. Methane is then introduced into the first fermenter 2a as part of a first gaseous feedstock, whilst the other hydrocarbons can be burned to generate energy to operate the fermenters. As can further be seen from Fig. 7, air is pre-treated in a first gas pre-treatment unit 4a in order to separate oxygen for use in the first gaseous feedstock; optionally, nitrogen can optionally also be used for the first gaseous feedstock.

Water from a water source and/or from the first fermenter 2a (where water is formed as by-product during aerobic fermentation) is introduced into an electrolyzer 5 to split the water into hydrogen and oxygen. At least a portion of the oxygen is then introduced into the first fermenter 2a as part of the first gaseous feedstock. At least a portion of the hydrogen is introduced into the second fermenter 2b as part of a second gaseous feedstock, and into a first gas processing unit 3a. Carbon dioxide is also introduced into said first gas processing unit 3a, originating from the first fermenter 2a (where carbon dioxide is comprised in a first tail gas), and/or from an external carbon dioxide source after being subjected to pre-treatment by means of a second gas pre-treatment unit 4b. Pre-treated carbon dioxide obtained from an external carbon dioxide source can also be introduced into the second fermenter 2b to form part of the second gaseous feedstock.

In said first gas processing unit 3a as shown in Fig. 7, hydrogen and carbon dioxide react to form methane via the Sabatier or a related process. The formed methane is then fed into the first fermenter 2a, along with methane originating from the second fermenter 2b (where methane is comprised in a second tail gas), hereby using said methane as part of the first gaseous feedstock.

During the aerobic fermentation in the first fermenter 2a, a first product is formed which can comprise e.g. single cell protein (SCP) and/or polyhydroxybutyrate (PHB) and/or other products of interest. During the anaerobic fermentation in the second fermenter 2b, a second product is formed which can comprise e.g. lactate and/or acetate and/or other products of interest.

Fig. 8 shows a ternary diagram for an aerobic fermentation. Reaction A proceeds from point a to point a', using a first gaseous feedstock comprising 40 vol% methane and 60 vol% oxygen and yielding (theoretically) 100 vol% carbon dioxide, i.e. full conversion (all amounts in vol% are equal to amounts in mol% in a first approximation). Reaction B starts at point b, using a first gaseous feedstock comprising 46 vol% methane and 54 vol% oxygen, and proceeds to point b', which is identical to point a', i.e. full conversion into carbon dioxide is obtained (theoretically). Point b" denotes an intermediate point in time, or an end point of reaction B in case of incomplete conversion. Reaction C starts at point c, using a first gaseous feedstock comprising 57 vol% methane and 43 vol% oxygen, and proceeds to point c' (50 vol% CO2, 50 vol% CH₄).

As can be seen from Fig. 8, an area in the ternary diagram marked with X (shaded in grey) signifies the explosive limit of methane in oxygen; namely 5-60 vol% methane in pure oxygen (which corresponds to 5-15 vol% methane in 20 vol% oxygen, e.g. in air). Accordingly, reaction C is advantageous from a safety point of view, as said reaction C leaves the explosive limit as it proceeds. Note that this diagram is for atmospheric pressure. For instance, at ambient pressure and temperature, the lower and upper explosive limits (LEL and UEL) for methane in air are 5 to 15%, whereas at 300 bar, LEL and UEL are 3 to 60%, respectively. Also, with increasing temperature, LEL and UEL start moving into leaner and richer mixture regimes. Reactions A, B and C are three typical examples, and starting points a, b and c are also 3 typical examples. Fig. 9 shows the ternary diagram of Fig. 8 concerning an aerobic fermentation, but additionally includes an area marked with Y, covering compositions of the first tail gas comprising 0-20 vol% oxygen, 0-60 vol% methane and 40-100 vol% carbon dioxide (all amounts in vol% are equal to amounts in mol% in a first approximation). As oxygen hinders the anaerobic fermentation in the second fermenter, it should be tried to minimize the residual amount of oxygen in the first tail gas. Reaching a final reaction point in an area marked with Y" is preferred, which area covers compositions of the first tail gas comprising 0-20 vol% oxygen, 0-60 vol% methane and 40-100 vol% carbon dioxide. The area Y' is a part of Y, which is a preferred "landing field", containing 0-20 vol% O2, 0-60 vol% CH₄ and 40-100 vol% CO2. The area Y" is a part of Y and also a part of Y'. It ranges from 0-5 vol% O2, 10-60 vol% CH₄ and 40-90 vol% CO2.

Fig. 10 displays a ternary diagram for an anaerobic fermentation when using a second gaseous feedstock comprising hydrogen, methane and carbon dioxide. It is noted that this composition was chosen for simplification and to be able to visualize the anaerobic fermentation in a ternary diagram. If the second feedstock comprises further gases, e.g. carbon monoxide, the respective ratios have to be adapted accordingly, as is readily obvious to a person skilled in the art.

Point A of Fig. 10 signifies a composition of the second gaseous feedstock comprising a ratio of hydrogen to carbon dioxide of 2:1 vol%/vol%, which is suitable in case of lactic acid production.

Point c' signifies a composition comprising 50 vol% methane and 50 vol% carbon dioxide (all amounts in vol% are equal to amounts in mol% in a first approximation), which is a typical composition of a biogas as external gas source or the first tail gas. Point b' is pure carbon dioxide, whilst point D signifies pure hydrogen. When mixing the composition of point c' with that of point D, a composition according to point B is obtained, which comprises 50 vol% hydrogen, 25 vol% methane and 25 vol% oxygen and can be used as second gaseous feedstock. Upon anaerobic fermentation, point E will be reached, i.e. a second tail gas consisting of methane remains (when there is no nitrogen in the system and full conversion is achieved). The trajectory of this reaction is B -> E.

When the anaerobic fermentation is performed with an excess of hydrogen, the end point will be located on line D-E in areas of the diagram of Fig. 10 marked with X and Y. For instance, starting at point H, there is no methane in the system, and only hydrogen will remain in the end, so the end point is D. When, however, hydrogen is mixed with a composition according to point c', the starting mixture of the second fermenter corresponds to point C, and the trajectory of the reaction is C -> F.

With regard to the area marked with X, it comprises compositions of the second tail gas comprising not more than 10 vol% carbon dioxide), up to 50 vol% hydrogen and 50-100 vol% methane. The area marked with Y comprises compositions with 0-20 vol% hydrogen and 0-10 vol% carbon dioxide, with the remainder being methane. Accordingly, carbon dioxide can be (almost) completely consumed, such that an end point in the area marked with Y is preferred. For example, a second gaseous feedstock comprising a composition according to point H can be used (76 vol% hydrogen and 23 vol% carbon dioxide), and the reaction can proceed until point F is reached, i.e. a second tail gas comprising 33 vol% hydrogen and 67 vol% methane.

Any leftover hydrogen and carbon dioxide can be introduced into the first fermenter together with unconsumed methane (as second tail gas). The methane can be consumed in the first fermenter, and carbon dioxide can be generated. The first tail gas comprising hydrogen, carbon dioxide, and optionally unconsumed methane, can then be fed into the second fermenter. In the sense of the present invention, the unconsumed gas fraction is sent to the other fermenter for usage, so that the total carbon dioxide and methane emissions from the process can minimized and ideally completely avoided.

Fig. 11 shows the ternary diagram of Fig. 10 concerning an anaerobic fermentation, but additionally includes an area marked with Z (shaded), covering compositions of the second gaseous feedstock as supplied to the fermenter (possibly after mixing the "return" stream from the first fermenter with additional feedstock, e.g. syngas) comprising 0-50 vol% methane, 10-70 vol% carbon dioxide and 30- 90 vol% hydrogen, which composition can comprise hydrogen and at least a portion of the first tail gas. A typical composition of the second gaseous feedstock is shown in point B, comprising 25 vol% methane, 25 vol% carbon dioxide and 50 vol% hydrogen.

Examples

The following examples are supposed to further illustrate the invention as described within this application without any intention to limit the scope of the invention.

Example 1 - Growth rate constants in dependence on fermentation time and composition of gaseous feedstock

A coupled gas fermentation was performed using Methylocystis sp. GB25 as a methantropic producer of PHB, and Acetobacterium woodii as an acetogenic strain to produce acetate. The fermentation volume in each fermenter was 30 I (however, the process can also be carried out at industrial scale). The composition of the fermentation broths was as follows (in mg/l): KH2PO4 - 3400; K2HPO4 - 435O; MgSO₄ - 7 H₂O - 712; CuSO₄ - 5 H₂O - 7.85; MnSO₄ - H₂O - 8.12; FeSO₄ - 7 H₂O - 49.8; ZnSO₄ - 4.4; COSO4 • 7 H2O - 0.36; CaCh • 2 H₂O - 50.47; Na₂MoO • 2 H2O - 2.52; H3BO3 - 12.8. The content of the strains in each fermentation broth was 20 g/l fermentation broth.

Various compositions of the first and second gaseous feedstocks were employed. The fermentation was performed for 100 h. During fermentation, the first fermenter was operated at a temperature of 30 °C and an atmospheric pressure of 2 bar, whilst the second fermenter was operated at a temperature of 35 °C and an atmospheric pressure of 1.5 bar. The pH value of the fermentation broth in the first fermenter was adjusted to 6.5 with ammonia, whilst the pH value of the fermentation broth in the second fermenter was 6.3. Gassing rates were varied between 0.2 and 0.6 vvm.

Growth rate constants of cells comprising the final product in dependence on the fermentation time and the composition of the first and second gaseous feedstocks are listed in Tables 1 and 2, respectively. In general, during the growth phase, a bacterial culture mimics a first -order chemical reaction, i.e. the rate of increase of cells is proportional to the number of bacteria present at that time. The constant of proportionality p is an index of the growth rate and is called the growth rate constant. The value of p can be determined from the following equations:

In N_t - In No = p(t - to) p = [(logic N - logic No) * 2.303] / (t - to)

By measuring the increase in the number of cells during a certain time period, the growth rate constant can be calculated. Furthermore, the maximum growth rate constant p_max for each composition of the first and second gaseous feedstock was determined from the following equation:

Pmax — [I n (p(t=75h)) — I n (P(t=5h))] / (75— 5)

As can be seen from Table 1, the growth rate during the aerobic fermentation is hardly affected by the presence of 10-50 vol% carbon dioxide. With regard to the anaerobic fermentation, it can be seen from Table 2 that the presence of 10 or 25 vol% methane does not cause a significant reduction of the growth rate, whilst a higher methane content of 45 and 55 vol% leads to a reduced growth rate.

Table 1: Growth rate constants during aerobic fermentation in dependence on fermentation time and composition of the first gaseous feedstock

Table 2: Growth rate constants during anaerobic fermentation in dependence on fermentation time and composition of the second gaseous feedstock

Example 2 - Maximum growth rate constants in dependence on pH value

Maximum growth rate constants were determined in dependence on the pH value of the first fermentation broth comprising Methylocystis sp. GB25. The pH value was varied from 7.0 to 5.5 by adding ammonia. The composition of the fermentation broth and the processing parameters according to example 1 were used. The set pH value was maintained throughout the experiment for 100 hours. The first gaseous feed included methane and air in a ratio of 1:5 (vol%/vol%), which corresponds to approx. CH₄:O2 =1:1.

The maximum growth rate constants according to Table 3 were obtained, showing an increase of the maximum growth rate constant with increasing pH value.

Table 3: Growth rate constants during aerobic fermentation in dependence on pH value

Example 3 - Maximum growth rate constants in dependence on temperature

Maximum growth rate constants were determined in dependence on the fermentation temperature in the second fermenter, with the fermentation broth comprising Acetobacterium woodii. The temperature was varied from 25 to 40 °C. The composition of the fermentation broth and the processing parameters according to example 1 were used. The set temperature value was maintained throughout the experiment for 100 hours. The second gaseous feedstock comprised 20 vol% CH₄, 40 vol% CO2 and 40 vol% H2.

The maximum growth rate constants according to Table 4 were obtained, showing an increase of the maximum growth rate constant with increasing temperature. Up to 30 °C, no significant difference in pmax was observed.

Table 4: Growth rate constants during anaerobic fermentation in dependence on temperature

Example 4 - Influence of feedstock source Gaseous feedstocks from different sources, and thus having different compositions, were employed to assess the dependence of the feedstock composition on the yield of single cell protein (SCP) produced in the first fermenter, lactic acid produced in the second fermenter, the efficiency of the coupled gas fermentation. For comparability of the results displayed in Table 5, the coupled gas fermentations with different sources of the feedstock are all based on 1 1 methane being introduced into the first fermenter, and a corresponding amount of hydrogen added to consume all carbon dioxide in the system in the second fermenter. In all cases, the coupled gas fermentation was performed CCh-neutral. For instance, when using biogas as external gas source, a comparably high amount of hydrogen is needed to consume the carbon dioxide from both the first tail gas and the biogas.

Methane was obtained from natural gas, biogas or from a methanation process, e.g. of coal or biomass. Olive pomace was used as an example of a non-food waste organic material. SNG (synthetic natural gas) is accessible through syngas at a technology readiness level (TRL) of 9, or through direct methanation of coal/biomass, which are at a lower TRL. In general, TRLs are based on a scale from 1 to 9, with 9 being the most mature technology. Olive pomace gasification and methanation were demonstrated in the lab but have notyet been commercialized, neither has large scale SNG production from biomass been commercialized, in contrast to SNG from coal.

With the inventive method, a TRL of up to 9 can be achieved, which signifies that the method is well proven in operational environment, and there is a very broad feedstock flexibility, rendering the process CO2-neutral, both with renewable and with fossil resources.

At a given energy content of a feedstock (depending on its chemical composition), the actually obtained yield is lower than the theoretically obtainable yield, as some of the energy is consumed by metabolism (i.e., breathing of microorganisms of the fermentation broth). Accordingly, the efficiency is the yield obtained in reality in %, with respect to the theoretical yield. As can be seen from Table 5, the inventive method has an efficiency of up to 52.3% when using coal or biogas as gaseous feedstock.

Table 5 shows two columns for carbon dioxide. The first column signifies the amount of carbon dioxide supplied from an external gas source, e.g. biogas. The second column signifies the amount of carbon dioxide converted in the second fermenter. As it can be seen, all feedstock sources result in different amounts of target products, but there is no carbon dioxide emission, since the two fermentation processes are matched to each other so that all carbon dioxide can be consumed. Since the inventive method also aims at zero methane emission, through the feeding of unconsumed methane from the first fermenter into the second fermenter and then back into the first fermenter, zero carbon dioxide equivalents are targeted and can be realized with this method, which is an important achievement when making bulk commodities such as SCP, PHB and LA, and/or specialties, such as ectoine. Compared to traditional methods, the environmental footprint in terms of energy consumption, land use and water use can be strongly reduced, which makes the present invention a vital building block of a circular, carbon-reduced economy.

Table 5: Yield, efficiency and technology readiness level (TRL) in dependence on source of gaseous feedstock

Claims

1. System for performing a coupled gas fermentation comprising a first fermenter configured for operation in an aerobic mode and a second fermenter configured for operation in an anaerobic mode, wherein the first fermenter comprises a first inlet for introducing a first gaseous feedstock and the second fermenter comprises a second inlet for introducing a second gaseous feedstock, and wherein the first fermenter and the second fermenter are coupled with each other by a first connection element, wherein the first connection element is configured to feed at least a portion of a first tail gas from the first fermenter into the second fermenter for use as at least a portion of the second gaseous feedstock, characterized in that the first fermenter and the second fermenter are coupled with each other by a second connection element, wherein the second connection element is configured to feed at least a portion of a second tail gas from the second fermenter into the first fermenter for use as at least a portion of the first gaseous feedstock.

2. The system according to claim 1, characterized in that the system further comprises a first feeding element connected with the first inlet and a second feeding element connected with the second inlet, wherein the first connection element is coupled with the second feeding element, and the second connection element is coupled with the first feeding element.

3. The system according to claim 1 or 2, characterized in that the system further comprises a gas processing unit configured to alter a composition of the first tail gas before feeding at least a portion of the first tail gas into the second fermenter or to alter a composition of the second tail gas before feeding at least a portion of the second tail gas into the first fermenter.

4. The system according to any of claims 1 to 3, characterized in that the system further comprises an electrolyzer configured to split water into oxygen and hydrogen, wherein the electrolyzer is connected with the first inlet and the second inlet to feed at least a portion of the oxygen into the first fermenter and at least a portion of the hydrogen into the second fermenter.

5. The system according to any of claims 1 to 4, characterized in that the system further comprises a stock vessel and/or a discharge vessel connectable to the first fermenter and/or the second fermenter.

6. The system according to any of claims 1 to 5, characterized in that the first fermenter and the second fermenter each comprise a loop fermenter.

7. Method for performing a coupled gas fermentation comprising the steps of

(b) introducing a first gaseous feedstock comprising methane into the first fermenter and converting said first gaseous feedstock to a first product and a first tail gas, and introducing a second gaseous feedstock comprising carbon dioxide into the second fermenter and converting said second gaseous feedstock to a second product and a second tail gas, wherein at least a portion of the first tail gas is fed through a first connection element into the second fermenter for use as at least a portion of the second gaseous feedstock, characterized in that at least a portion of the second tail gas is fed through a second connection element into the first fermenter for use as at least a portion of the first gaseous feedstock.

8. The method according to claim 7, characterized in that the first tail gas comprises carbon dioxide and the second tail gas comprises methane.

9. The method according to claim 7 or 8, characterized in that at least one of the first tail gas and the second tail gas is guided through a gas processing unit to alter a composition of the first tail gas or the second tail gas before feeding at least a portion of the first tail gas or the second tail gas to the second fermenter or the first fermenter.

10. The method according to any of claims 7 to 9, characterized in that the first gaseous feedstock comprises 40 to 90 vol% methane and 10 to 60 vol% oxygen, and/or the second gaseous feedstock comprises 10 to 55 vol% methane or carbon monoxide, preferably 10 to 55 vol% methane; 15 to 30 vol% carbon dioxide and 30 to 60 vol% hydrogen.

11. The method for gas fermentation according to any of claims 7 to 10, characterized in that water is split into oxygen and hydrogen in an electrolyzer connected with the first fermenter and the second fermenter, wherein at least a portion of the oxygen is fed into the first fermenter and at least a portion of the hydrogen is fed into the second fermenter.

12. The method according to any of claims 7 to 11, characterized in that the first fermenter or the second fermenter is connected to a discharge vessel to empty the first fermenter or the second fermenter.

13. The method according to any of claims 7 to 12, characterized in that the first fermenter or the second fermenter is connected to a stock vessel comprising a fermentation broth to refill the first fermenter or the second fermenter.

14. Method for gas fermentation according to any of claims 7 to 13, characterized in that the first gaseous feedstock comprises oxygen and/or the second gaseous feedstock comprises hydrogen.

15. Method for gas fermentation according to any of claims 7 to 14, characterized in that the first fermenter and the second fermenter each comprise a loop fermenter.