Producing synthesis gas

Abstract

According to the present invention, there is provided a process for producing a synthesis gas, by first carrying out a digestion of an organic biomass, and subsequently reacting the thus obtained biogas stream with a reagent mixture containing a carbonaceous material, to cause a reduction of the CO2 into carbon monoxide (CO), with simultaneous release of hydrogen, whereby synthesis gas is obtained.

Description

PRODUCING SYNTHESIS GASFIELD

[0001] The present invention relates to a process for producing synthesis gas by utilizing the methane and carbon dioxide obtained from the digestion of biomass.BACKGROUND

[0002] Biogas is a renewable energy source produced from natural raw materials, such as organic waste, typically by digestion. The gas composition is primarily methane (CH4) and carbon dioxide (CO2), but may also contain small amounts of other gaseous components, such as moisture. Due to its contents, its release in large amounts is harmful for the environment, whereby its recovery is typically attempted. When recovered, the biogas has mainly been utilized either in the production of methane, or as a fuel.

[0003] Since these previous alternatives leave the carbon dioxide as a waste or unutilized by-product, and since the use of the biogas as a fuel leaves the full potential of the components of the biogas unutilized, further alternatives for the utilization of these biogas streams would be needed.SUMMARY OF THE INVENTION

[0004] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0005] According to a first aspect of the present invention, there is provided a process that utilizes both the methane and the carbon dioxide of a biogas stream obtained from the digestion of an organic biomass.

[0006] According to a second aspect of the invention, there is provided a process that utilizes these components of a biogas stream to produce synthesis gas.

[0007] According to a further aspect, there is provided a process that utilizes biomass formed of waste materials as a raw material.

[0008] The present invention thus relates to a process for producing synthesis gas, by a) first carrying out a digestion of an organic biomass to obtain a biogas stream containing methane (CH4) and carbon dioxide (CO2), and b) then reacting the biogas stream with a reagent mixture containing a carbonaceous material, to cause a reduction of the CO2 into carbon monoxide (CO), with simultaneous release of hydrogen, whereby synthesis gas is obtained.

[0009] The process, and particularly step b), thus utilizes, among others, the following Reactions (1) or (2)

[0010] Several advantages are achieved using the present process. Among others, the invention provides means for utilizing the entire biogas stream obtained from the digestion of organic biomass, such as waste biomass. Conventionally, this stream has been used mainly for energy, or the methane therein has been recovered and used separately. This recovery, however, leaves the carbon dioxide as a by-product. The present invention utilizes also the carbon dioxide as a valued component.

[0011] Further, according to one option, even the digestate obtained from such digestions can be utilized.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIGURE 1 illustrates a process configuration encompassed by the present invention;

[0013] FIGURES 2 and 3 illustrate process configurations of different embodiments of the invention, which can be combined or utilized separately; and

[0014] FIGURE 4 is a graph showing the contents of a synthesis gas stream obtained in the present process.EMBODIMENTS

[0015] DEFINITIONSIn the present context, the term “biomass” encompasses a solid biological mass obtained from nature. Typically, such a biomass is obtained from agriculture, forestry or aquafarming. When formed of waste materials, it may, for example, be obtained as a by-product of agriculture, forestry or aquafarming, or it can be formed of sludge, manure, municipal waste, food waste or landfill material.The term “biogas”, in turn, encompasses gaseous streams generated from such biomass sources, e.g. by digestion. The term also covers landfill gas. Typically, these biogas streams contain mainly methane (CFU) and carbon dioxide (CO2), but may contain also trace amounts of moisture, nitrogen (N2), hydrogen (H2) and oxygen (O2), and possibly hydrogen sulphide (H2S), and volatile solids.Likewise, the term “synthesis gas” encompasses a mixture of gaseous and possibly liquid components including carbon monoxide (CO) and hydrogen (H2). Typically, it contains also carbon dioxide (CO2) and methane (CH4).When “gas streams” or “gas mixtures” are mentioned, it is typically referred to wet gas or humid gas.

[0016] The present invention thus relates to a process for producing synthesis gas, by a) carrying out a digestion of an organic biomass to obtain a biogas stream containing methane (CH4) and carbon dioxide (CO2), and b) reacting the biogas stream with a reagent mixture containing a carbonaceous material, to cause a reduction of the CO2 into carbon monoxide (CO), with simultaneous release of hydrogen, whereby synthesis gas is obtained.

[0017] A typical process configuration is shown in Fig. 1, and embodiments in Figs.2 and 3.

[0018] The organic biomass is selected from biomass formed of waste materials, i.e. particularly formed of biowaste materials, the organic biomass preferably being selected from waste-based biomass containing landfill materials, crop residues, municipal solid waste, sludge from waste water treatment, or livestock manure, or mixtures of these, most suitably consisting of these waste materials.

[0019] When carrying out a digestion on such a biomass, it results in a solid / liquid digestate and a gas stream composed of biogas. Preferably the digestion is an anaerobic digestion.

[0020] The digestion can be a digestion taking place naturally in such biomass, optionally with adjusted conditions, such as adjusted temperature and / or atmospheric content, or alternatively the digestion can be carried out by addition of microorganisms, particularly methane-generating microorganisms (methanogenic microorganisms), e.g. of the order methanopyrales, methanococcales, methanobacteriales, methanosarcinales, methanomicrobiales, methanocellales, methanomassiliicoccales, halobacteriales, themoplasmatales, or archaeoglobales.

[0021] In the past, the biogas streams produced by digestion of biomass have mainly been used as energy sources, and for the production of biomethane (CPU). The present invention, however, finds value also in the carbon dioxide (CO2) of the biogas. Therefore, there is no need for separate measures for increasing the CPU-content of the biogas.Instead, a wide range of contents of components in the biogas are acceptable. For example, the biogas may have a CH4 / CO2 molar ratio at a level of 0.3 / 3 - 3 / 0.3, preferably 0.8 / 1.2 - 3 / 0.3, and is either naturally obtained or separately adjusted to said level, e.g. by gas separations or addition of desired components.

[0022] The biogas stream can be used as such in step b), in the obtained contents, together with the reagent mixture, without separation to remove undesired components of the stream. Alternatively, the CH4 and / or CO2 content(s) can be increased, e.g. by purification or removal of trace elements.

[0023] Any gas separations mentioned herein may typically take place by using sorbents or solvents (e.g. scrubbing), by membrane separation, or by distillation or condensation (e.g. using cryogenics).

[0024] In an embodiment, the carbon dioxide (CO2) content of the biogas obtained from step a), and used in step b), is increased by addition of additional CO2, preferably wherein the additional CO2 to be added to the process is obtained from a calcination, a pyrolysis or a waste incineration, or alternatively from ethanol production, a brewery or biomethane purification.

[0025] In one preferred embodiment, a calcination of a carbonate-containing rawmaterial is carried out to produce the additional CO2. Any known calcination equipment can be used in the process, such as a fluidized bed reactor, shaft furnace, flash calciner or rotary kiln. This equipment can either be placed on the same site as the equipment for carrying out the main process of the invention, or the produced CO2 can be separately transported to this site.

[0026] The temperature used in this optional calcination step is typically >750 °C, preferably 750 - 1100 °C, more preferably 900 - 1100 °C, and, generally, the calcination is operated at a pressure that is close to atmospheric pressure, typically at a slight overpressure level of 0.0001 to 0.5 bar. Electric calcination can also be operated at a slight vacuum, which further facilitates the calcination reaction. In practice, however, the calcination reaction occurs within the range of -0.5 to 2 bar pressure, preferably 0.0001 - 0.5 bar. As already implied, it is an option to carry out the calcination at least partly, or entirely, by electrical heating.

[0027] No further additives are required in the calcination, but according to one option, it may be carried out in the presence of steam, preferably in the form of superheated steam and / or oxygen and / or hydrogen.

[0028] Regardless of the choice of process used for producing the additional CO2, it is typically obtained as a by-product stream, and is purified to increase its CO2 content and / or to remove undesired components, such as oxygen, nitrogen or sulphur dioxide, before using it in the addition of additional CO2. This purification can take place e.g. bywashing, scrubbing, cooling or drying the gas stream, or by a combination of two or more such techniques.

[0029] A further option is to increase the methane content in the biogas before step b) by addition of additional methane. The optional added methane is typically selected from synthetic methane, biomethane and natural gas.

[0030] The carbonaceous material used in step b) is typically a solid material, or a mixture of different solid materials. In an alternative, the carbonaceous material is mainly formed of solid material, particularly to 80w-% or more, or 90w-% or more.

[0031] Further, the carbonaceous material used in step b) is typically carbon, preferably in the form of char, charcoal, coke, petroleum coke, or biochar, or spent activated carbon, or a hydrocarbon, preferably in the form of methanol (CH3OH), methane (CH4), ethylene (C2H4), propylene (ChHe), butenes (C4H8), or formic acid, more preferably in the form of methane (CH4), ethylene (C2H4), propylene (C3H6), or butenes (C4H8), which are advantageous in that they lack oxygen in their structures, or a mixture of hydrocarbons. Alternatively, a mixture of carbon and hydrocarbon(s) can be used in the reduction reaction, or a woody biomass, such as wood chips, or a combination of any of these. The optional biomass is preferably formed of wood chips or other crude or dried biomass, thus preferably excluding refined carbon products.

[0032] The term “biochar” is intended to cover all carbon materials obtained from biomaterials, i.e. conventional biochar, as well as biocoke and biocharcoal, and torrefied biomass. The potential sources of the include both fresh biomass and waste materials, with waste materials being a preferred option particularly when aiming for an ecological improvement.

[0033] In one embodiment, the carbonaceous material is obtained from a digestate, in turn obtained from a digestion of an organic biomass, preferably from a digestion of a biowaste material, most suitably from an anaerobic digestion. Thus, in one alternative, the carbonaceous material may be obtained from the same or a similar digestion as the digestion of step a) of the present process. In a second alternative, it may be obtained from a different digestion process. In a third alternative, it may be obtained from an entirely different production step. A digestate would, however, preferably be subjected to one ormore treatment steps, e.g. moisture reduction, such as mechanical press drying, and / or thermal drying, and optionally pyrolysis, before the carbonaceous material can be recovered therefrom, and this recovery could take place e.g. by decanting or filtering.

[0034] Thus, in a preferred embodiment, the process of the invention includes the steps of a’) digesting an organic biomass to produce a carbonaceous digestate and a biogas stream containing methane (CH4) and carbon dioxide (CO2), al) recovering the carbonaceous digestate comprising a solid digestate and a liquid digestate, a2) separating the solid digestate and liquid digestate, a3) treating the solid digestate thermally to obtain a dried and optionally pyrolyzed material, a4) re-combining the thus treated and recovered carbonaceous material with the biogas stream, and b’) reacting the biogas stream with the thus treated carbonaceous material to cause a reduction of the CO2 into carbon monoxide (CO), with simultaneous release of hydrogen, whereby synthesis gas is obtained.

[0035] In the above-described optional steps al-a4, step a2 includes a mechanical drying, while the thermal step a3 can be either a thermal drying step or a high-temperature treatment step, such as a pyrolysis, or two separate thermal steps, typically with a thermal drying being carried out before a high-temperature treatment step.

[0036] The reaction to produce synthesis gas takes place in step b), and is preferably carried out by utilizing proportioning of the components of the biogas stream, reagent mixture and optional added reagents, in order to obtain the desired proportions of gas components in the synthesis gas. The proportioning is, again, typically carried out by gas separations or addition of desired components into the mixtures.

[0037] The reaction can be carried out with no added reagents other than those mentioned above (biogas stream and carbonaceous material), but according to another alternative it can be carried out with additives. Typically, it is carried out with added water or steam, preferably superheated steam, but also added oxygen, e.g. added in the form of oxygen gas (O2), and / or added hydrogen, e.g. added as hydrogen gas (H2) can be used. Inaddition to providing the desired proportions, e.g. the optional added oxygen makes it possible to increase the amount of carbonaceous material in the reaction, which would be beneficial for the energy efficiency of the process.

[0038] The additional oxygen and / or hydrogen can for example be produced by electrolysis from water using Reaction (3),The oxygen and / or hydrogen can thus optionally be produced at the same site of the reaction of step b).

[0039] The reaction in step b) is typically carried out with heating, preferably at least partly achieved by electrical heating, which can be achieved e.g. by using resistive heaters.

[0040] Thus, a temperature of >800 °C, preferably 900 - 1200 °C, more preferably 1000 - 1200 °C, is advantageously used in the reaction. The pressure, in turn, is preferably close to atmospheric pressure, or slightly adjusted pressure, more typically a pressure in the range of -1 - 10 bar, preferably 0 - 2 bar.

[0041] The herein described process thus results in synthesis gas, which typically contains carbon monoxide (CO), hydrogen (H2), unreacted carbon dioxide (CO2), and methane (CFU), and traces of other unreacted components from the CO2-containing gas mixture.

[0042] The synthesis gas produced according to the method of the present invention typically contains a higher proportion of hydrogen and a lower proportion of carbon monoxide (and generally also of carbon dioxide) compared to a synthesis gas produced with similar reaction conditions but without adding methane to the reaction mixture.

[0043] For example, the synthesis gas produced according to the present method may contain < 50 vol-%, preferably 30 - 50 vol-%, carbon monoxide, and > 20 vol-%, preferably 30 - 50 vol-% hydrogen (H2), whereas a synthesis gas produced as described, but without methane addition, typically contains >70 vol-% carbon monoxide.

[0044] To increase the value of the reaction in step b), the proportions of the reagents fed to the reaction may be adjusted to 10 - 50 mol-% CO2, 10 - 50 mol-% carbon, 10 - 50 mol-% water or steam and 10 - 50 mol-% methane. This is, again, for example achieved by gas separations or by adding desired components to the reagent mixture. The adjustment of proportions will result in a H2 / CO molar ratio in the synthesis gas of 0.5 - 2.5.

[0045] Further, this adjustment of the molar ratios in the synthesis gas can be done, as indicated, by adjusting the proportions of reagents. Further adjustment can, however, be done, e.g. by varying the reaction conditions, such as temperature and pressure (see above). An additional option is to contact the biogas stream with one or more catalysts, such as nickel, calcium oxide, magnesium oxide, zinc oxide, or aluminium oxide, to facilitate providing a desired proportion of gas components in the synthesis gas, and optionally to increase the reaction rate and / or to lower the temperature required for the desired reactions to take place.

[0046] In an embodiment, the proportions of reagents in step b) are adjusted to about 40 mol% CO2, 20 mol % carbon, 20 mol % water or steam and 20 mol% methane, to obtain a Fischer Tropsch ratio in the synthesis gas.

[0047] In another embodiment, the proportions of reagents in step b) are adjusted to about 20 mol% CO2, 40 mol% carbon, and either 40 mol % water or steam or 40 mol% methane to obtain a synthesis gas particularly suitable for lower alkyl alcohol production.

[0048] The synthesis gas can be further used as a fuel in energy production or in other high-temperature processes, or as a hydrogen or carbon source in such processes, or in the manufacture of synthetic fuels, chemicals or plastics. Further, it can be used as a reducing agent to convert iron ore into sponge iron. The iron ore for this optional conversion is typically in the form of hematite (Fe2O3) or magnetite (Fe3O4).

[0049] The particular suitability of the product in fuel applications and in high- temperature processes is achieved with the help of the advantageous chemistry of the product.

[0050] The invention, however, also relates to the further conversion of the synthesis gas into hydrocarbon products (see Fig. 3), wherein the synthesis gas or the remaining unreacted CO-containing gas mixture with the hydrogen-containing gas mixture is reacted further in conditions that produce hydrocarbons, such as methanol (CH3OH), methane (CH4), ethylene (C2H4), propylene (CsHe), butenes (C4H8), or formic acid (HCOOH), or higher alkanes, higher olefins, synthetic gasoline, diesel fuel, kerosene or waxes and lubricants. For example, for methane production, suitable conditions include using a pressure of 1 - 10 bar and a temperature of 300 - 400 °C, and for methanol production, suitable conditions include a pressure of 50 - 80 bar and a temperature of 200 - 300 °C.

[0051] Said higher alkanes and higher olefins may include for example nonane, nonene, decane, decene, or further alkanes or olefins having > 10 carbon atoms. The waxes may, in turn, be polyethylene waxes, or paraffin waxes, or other commonly used waxes, while the lubricants may be polyolefin-based lubricants or similarly other commonly used lubricants.

[0052] In a preferred embodiment, the further conversion of the synthesis gas into hydrocarbons is optimized to obtain a product rich in methane and / or methanol. This is preferably achieved by utilizing conditions of methanation (4)3H2+ CO - CH4+ H2O (4) and / or methanol formation (5)respectively. The methanation reaction of CO utilizes a H2 / CO molecular ratio of 3:1 and is typically catalyzed by a nickel catalyst, while the methanol synthesis utilizes a H2 / CO ratio of 2: 1 and is typically catalyzed by a copper catalyst.

[0053] The conditions that produce hydrocarbons may, for example, involve varying the temperature or the pressure of the conversion reaction, or using one or more catalysts, such as iron, cobalt, nickel, and ruthenium-based catalysts, such as Ru / TiCh or Ni / AhO;, or alternatively biocatalysts.

[0054] In an embodiment of the invention, a further reaction can be carried out on the reaction mixture of step b) or on the synthesis gas obtained from step b), the further reaction carried out by subjecting the reaction mixture to a temperature of 150 - 300 °C, and a pressure of about 20-40 bar abs, to obtain a Fischer Tropsch gas mixture.

[0055] In another embodiment of the invention, a further reaction can be carried out on the reaction mixture of step b) or on the synthesis gas obtained from step b), the further reaction carried out by subjecting the reaction mixture to a temperature of 150 - 500 °C, preferably 200 - 300 °C, and a pressure of about 50-150 bar abs, to obtain a lower alkyl alcohol product. Particularly, the conditions of the optional further reaction are adjusted to produce methanol (CH3OH) or ethanol (C2H5OH), most suitably to produce methanol.

[0056] Also in the further reaction of the two previous embodiments, a catalyst can be used, preferably selected from iron, cobalt, nickel, and ruthenium-based catalysts, such as Ru / TiCh or Ni / AfrCfi, or alternatively a biocatalyst. For biocatalysts it is, however, preferred, and even sufficient, to use lower temperatures, typically within the range of 45- 80°C, or 60-80°C.

[0057] In a further embodiment, the synthesis gas, or at least a fraction thereof, containing unreacted CO2, or unreacted CO2 separated from this fraction, can be recycled to the reaction step b) for producing synthesis gas.

[0058] In a further option of the invention, the synthesis gas obtained in step b) can be used e.g. as a reducing agent to convert iron ore into sponge iron, or as a fuel in energy production or in other high-temperature processes.

[0059] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0060] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the presentinvention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

[0061] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0062] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0063] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0064] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.EXAMPLE - Synthesis gas products formed using a biogas stream

[0065] A digestate and a biogas stream containing mainly carbon dioxide (CO2) and methane (CH4) were recovered from a waste water sludge as a by-product of waste water treatment. The biogas stream was recovered, and carried to a separate tubular reaction unit.

[0066] In this reaction unit, the biogas stream, and a separately added carbonaceous material (biochar, obtained by pyrolysis from the above digestate) were allowed to react under heating to a temperature of 1000 °C using resistors. The contents of the synthesis gas thus obtained were measured (the hydrogen content measured with gas chromatograph and the other contents obtained using Fourier Transform Infrared Spectrometer) and the results are shown in Fig. 4. For reference samples, the same reaction was carried out also using only CO2 (instead of the biogas stream; shown as the first column for each component of Fig. 4).

[0067] As the results show, the present invention allows the formation of particularly high H2 contents, while the reaction proceeds without requiring high amounts of added hydrogen. In other words, no electrolyzers are required to boost the hydrogen content in the production of the synthesis gas.INDUSTRIAL APPLICABILITY

[0068] The present invention is useful for producing synthesis gas, which can be further converted into hydrocarbon products.

[0069] Further, the invention is useful in utilizing the biogas stream formed e.g. in digestions of biomass, also including naturally occurring digestions.

Claims

CLAIMS:

1. Process for producing synthesis gas, characterized by a) carrying out a digestion of an organic biomass to obtain a biogas stream containing methane (CH4) and carbon dioxide (CO2), and b) reacting the biogas stream with a reagent mixture containing a carbonaceous material, to cause a reduction of the CO2 into carbon monoxide (CO), with simultaneous release of hydrogen, whereby synthesis gas is obtained.

2. The process of claim 1, wherein the organic biomass is selected from biomass formed of waste materials, particularly formed of biowaste materials, the organic biomass preferably being selected from waste-based biomass containing landfill materials, crop residues, municipal solid waste, sludge from waste water treatment, or livestock manure, or mixtures of these, most suitably consisting of these waste materials.

3. The process of claim 1 or 2, wherein the digestion is an anaerobic digestion.

4. The process of any preceding claim, wherein the biogas stream has a CH4 / CO2 molar ratio that is adjusted to a level of 0.3 / 3 - 3 / 0.3, preferably 0.8 / 1.2 - 3 / 0.3.

5. The process of any preceding claim, wherein the biogas stream is used as such together with the reagent mixture in step b), without separation of components of the stream.

6. The process of any preceding claim, wherein the biogas stream is purified before reaction in step b).

7. The process of any preceding claim, wherein the carbon dioxide (CO2) content of the biogas obtained from step a) is increased by addition of additional CO2, preferably wherein the additional CO2 to be added to the process is obtained from a calcination, a pyrolysis or a waste incineration, or alternatively from ethanol production, a brewery or biomethane purification.

8. The process of claim 7, wherein the additional CO2 is obtained as a by-product stream, and is purified to increase its CO2 content and / or to remove undesired components, such as oxygen, nitrogen or sulphur dioxide, before using it in the addition of additional CO2.

9. The process of any preceding claim, wherein the carbonaceous material is carbon, preferably in the form of char, charcoal, coke, petroleum coke, or biochar, or spent activated carbon, or a woody biomass, such as wood chips, or a hydrocarbon, preferably in the form of methanol (CH3OH), methane (CH4), ethylene (C2H4), propylene (C3H6), butenes (C4H8), or formic acid, more preferably in the form of methane (CH4), ethylene (C2H4), propylene (C3H6), or butenes (C4H8), or a combination of any of these.

10. The process of any preceding claim, wherein the carbonaceous material is obtained from a digestate, in turn obtained from a digestion of an organic biomass, preferably from a digestion of a biowaste material, most suitably from an anaerobic digestion, and the carbonaceous material is preferably recovered from the digestate before use in step b).

11. The process of any preceding claim, including the steps of: a’) digesting an organic biomass to produce a carbonaceous digestate and a biogas stream containing methane (CH4) and carbon dioxide (CO2), al) recovering the carbonaceous digestate, a2) separating the solid digestate from the liquid digestate, a3) treating the solid digestate thermally to obtain a dried and optionally pyrolyzed material, a4) re-combining the thus treated carbonaceous material with the biogas stream, and b’) reacting the biogas stream with the thus treated carbonaceous material to cause a reduction of the CO2 into carbon monoxide (CO), with simultaneous release of hydrogen, whereby synthesis gas is obtained.

12. The process of any preceding claim, wherein the methane content in the biogas is increased before step b) by addition of additional methane.

13. The process of claim 12, wherein the additional methane is selected from synthetic methane, biomethane and natural gas.

14. The process of any preceding claim, wherein the reaction into synthesis gas is carried out by utilizing proportioning of the components of the biogas stream, reagent mixture and optional added reagents, in order to obtain the desired proportions of gas components in the synthesis gas.

15. The process of any preceding claim, wherein the reaction into synthesis gas is carried out in the presence of water or steam, preferably superheated steam, and / or in the presence of oxygen, e.g. as oxygen gas (O2), and / or in the presence of added hydrogen, e.g. added as hydrogen gas (H2).

16. The process of any preceding claim, wherein the reaction into synthesis gas is carried out with added methane to provide an excess of methane in the reaction.

17. The process of any preceding claim, wherein the reaction into synthesis gas is carried out with heating, preferably at least partly achieved by electrical heating, more preferably achieved by using resistive heaters.

18. The process of any preceding claim, wherein the reaction into synthesis gas is carried out by heating the gas stream and / or the reaction mixture to a temperature of >800 °C, preferably 900 - 1200 °C, more preferably 1000 - 1200 °C.

19. The process of any preceding claim, wherein the biogas stream is contacted with one or more catalysts, such as nickel, calcium oxide, magnesium oxide, zinc oxide, or aluminium oxide, to provide a desired proportion of gas components in the synthesis gas.

20. The process of any preceding claim, wherein the proportions of the components of the biogas stream and reagent mixture fed to the reaction in step b) are adjusted to 10 - 50 mol-% CO2, 10 - 50 mol-% carbon, 10 - 50 mol-% water or steam and 10 - 50 mol-% methane in order to achieve a H2 / CO molar ratio in the synthesis gas of 0.5 - 2.5.

21. The process of any of claims 1 to 20, wherein the proportions of gas components in the synthesis gas are adjusted to about 40 mol% CO2, 20 mol % carbon, 20 mol % water or steam and 20 mol% methane, to obtain a Fischer Tropsch ratio in the synthesis gas.

22. The process of any of claims 1 to 20, wherein the proportions of gas components in the synthesis gas are adjusted to about 20 mol% CO2, 40 mol% carbon, and either 40 mol % water or steam or 40 mol% methane to obtain a synthesis gas particularly suitable for lower alkyl alcohol production.

23. The process of any of claims 1 to 21, wherein the synthesis gas is carried to a further reaction, and subjected to a temperature of 150 - 300 °C, and a pressure of about 20-40 bar abs, to obtain a Fischer Tropsch gas mixture.

24. The process of any of claims 1 to 20 and 22, wherein the synthesis gas is carried to a further reaction, and subjected to a temperature of 150 -500 °C, preferably 200 - 300 °C, and a pressure of about 50-150 bar abs, to obtain a lower alkyl alcohol product.

25. The process of claim 24, wherein the conditions of the further reaction are adjusted to produce methanol (CH3OH) or ethanol (C2H5OH).

26. The process of claim 24 or 25, wherein a catalyst is used in the reaction, preferably selected from iron, cobalt, nickel, and ruthenium-based catalysts, such as Ru / TiCh or Ni / AhCh, or alternatively a biocatalyst.

27. The process of any preceding claim, wherein at least a fraction of the synthesis gas mixture, containing unreacted CO2, or unreacted CO2 separated from this fraction, is recycled to the reaction step for producing synthesis gas.

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