Electrical production of gaseous fuel for combustion

Abstract

The present invention provides a process for modifying a carbon dioxide (CO2) -containing flue gas stream using electrical heating, and utilizing the obtained modified gas stream in a subsequent combustion, in steps comprising - obtaining a first gas stream containing carbon dioxide (CO2) as an industrial flue gas stream, - reacting the obtained first gas stream, optionally combined with one or more further CO2-containing gases or gas mixtures, at an elevated temperature achieved by electrical heating, with a carbonaceous reactant to form a reduction mixture and to cause a reduction reaction, where the carbon dioxide (CO2) reacts into carbon monoxide (CO), thereby producing a second gas stream containing carbon monoxide (CO), and - utilizing the obtained second gas stream having an elevated temperature as a gaseous fuel in a combustion.

Description

ELECTRICAL PRODUCTION OF GASEOUS FUEL FOR COMBUSTIONFIELD

[0001] The present invention relates to a process for modifying a carbon dioxide (CO2) -containing flue gas stream using electrical heating, and further utilizing the modified gas stream in a combustion, thus integrating the electrical processing of a flue gas with a subsequent combustion, while providing also said combustion with an increased potential for electrification.BACKGROUND

[0002] Conventional combustion processes are based on the burning of traditional fuels, including fossil fuels. These are efficient, but cause significant emissions, among others, of carbon dioxide in the flue gases. Due to increased requirements on the use of renewable energy, there is an existing need for sustainable development of these combustion processes, to achieve reduced emissions and a more efficient use of energy.

[0003] Utilizing the carbon dioxide produced as a by-product in various industrial processes has been the goal of many publications in the past (see e.g. WO 2015015161 Al), but there is still a need for solutions that could be integrated with combustion processes, while reducing the need for burning fossil fuels, and providing increased possibilities for powering the combustion via electrical heating.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 the carbon dioxide of a flue gas stream, e.g. in the form of an industrial flue gas stream.

[0006] According to a second aspect of the invention, there is provided a process that utilizes this carbon dioxide in a process that has the potential to be heated fully by electrical means.

[0007] According to a further aspect, there is provided a process for reacting the carbon dioxide of a flue gas into a more valuable product, and integrating this process with a subsequent combustion.

[0008] The present invention thus relates to a process for modifying a carbon dioxide (CO2) -containing flue gas stream using electrical heating, and utilizing the obtained modified gas stream, typically still a hot modified gas stream, in a subsequent combustion, in steps comprisingobtaining a first gas stream containing carbon dioxide (CO2) as an industrial flue gas stream,reacting the obtained first gas stream, optionally combined with one or more further CCh-containing gases or gas mixtures, at an elevated temperature achieved by electrical heating, with a carbonaceous reactant to form a reduction mixture and to cause a reduction reaction, where the carbon dioxide (CO2) reacts into carbon monoxide (CO), thereby producing a second gas stream containing carbon monoxide (CO), andutilizing the obtained second gas stream having an elevated temperature as a gaseous fuel in a combustion.

[0009] While many high-temperature processes require the use of fuels for achieving the required temperatures, the present process is based on a different principle. The reduction step requires heating, but this heating is achieved with electrical heating means. Further, the product obtained in the reduction step is utilized further in a subsequent combustion while it still maintains a high temperature. Thus, the effects of the electrical heating are utilized also in the subsequent combustion.

[0010] Several advantages are achieved using the present process. Among others, an industrial flue gas can be utilized in a fully electrical process that extends the benefits of the electrical energy also to a subsequent combustion, and results in a more renewable combustion process. Likewise, the present process is capable of increasing the amount of renewable fuels that can be utilized in combustions.

[0011] Further, an embodiment of the invention provides means for utilizing the carbon dioxide of a flue gas stream obtained from a preceding first combustion process, and for processing it into a form that can be circulated back to the same combustion, or to a second combustion. Conventionally, such flue gas streams from combustions have been used mainly as direct sources of energy, although the energy value of these streams is quite low. The present invention provides a means for increasing this energy value.

[0012] In addition to providing the means for utilizing the carbon dioxide of flue gases, the process can be fully integrated with a combustion, and can recirculate the produced flue gas stream containing carbon dioxide in the combustion. In an alternative to the fully integrated process with recirculation, the invention can be used to feed renewable energy from one combustion to a second parallel combustion.

[0013] Finally, with a suitable choice of carbonaceous materials, the reduction reaction described herein can be operated in an ecological manner, using biomaterials. Also the amount of fossil fuels used in the combustion can be reduced, which will also have an impact on the amount of harmful emissions.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIGURE 1 illustrates a process configuration encompassed by the present invention;

[0015] FIGURE 2 illustrates a process configuration of an embodiment of the invention.EMBODIMENTS

[0016] DEFINITIONSIn the present context, the term “flue gas” encompasses gaseous streams generated from industrial processes, typically in high-temperature processes. In the present context, the flue gases are rich in carbon dioxide (CO2), but also typically contain one or more of methane (CH4), moisture (H2O), nitrogen (N2),hydrogen (H2) and oxygen (O2), and possibly hydrogen sulphide (H2S), and volatile solids.Further, 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, and is typically rich in carbonaceous materials.When “gas streams” or “gas mixtures” are mentioned, it is typically referred to wet gas or humid gas. When “separating” or “purifying” such gas streams, these steps 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).

[0017] The present invention thus relates to a process for modifying a carbon dioxide (CO2) -containing flue gas stream with electrical heating, and utilizing the modified gas stream (typically a hot modified gas stream) in a combustion, in steps comprising- obtaining a first gas stream containing carbon dioxide (CO2) as a flue gas stream,- reacting the obtained first gas stream, optionally combined with one or more further CCh-containing gases or gas mixtures, at an elevated temperature achieved by electrical heating, with a carbonaceous reactant to form a reduction mixture and to cause a reduction reaction, where the carbon dioxide (CO2) reacts into carbon monoxide (CO), thereby producing a second gas stream containing carbon monoxide (CO), and - utilizing the obtained second gas stream gas stream having an elevated temperature as a gaseous fuel in a combustion.

[0018] A typical process configuration is shown in Fig. 1, which illustrates the combination of a carbonaceous material and a flue gas into a reduction mixture that is carried to a reduction reaction, from which a second gas stream is separated and led to a combustion.

[0019] The first gas stream used in the above process is preferably a flue gas from an industrial process, which can be a combustion, and it is preferably rich in CO2, more preferably having a CO2 content of 10-40 vol-%, or 10-70 vol-%, or even 10-90 vol-%, and most suitably said gas stream contains also nitrogen gas (N2).

[0020] Fig. 2 illustrates the process configuration of some preferred embodiments, showing further integration options, with also the first gas stream being obtained as a flue gas from a combustion, which in one embodiment shown in Fig. 2 is the “main” combustion, thus allowing a recirculation of a gas stream in the process, whereas in another embodiment (as shown by the dotted lines of Fig. 2) the first gas stream is obtained from a “separate” combustion, different from the main combustion where the second gas stream is subsequently utilized. In yet a further embodiment, the first gas stream can be a combination of flue gases obtained from these two combustions. When the main combustion is intended in the disclosure below, it generally refers only to a “combustion” (as shown in Fig. 1), whereas the separate combustion (as shown in Fig. 2) is named “separate”.

[0021] In the past, the flue gas streams of combustions have mainly been used as energy sources, and for the production of biomethane (CH4). However, the heat value of these flue gases is not very high and further alternatives for its utilization would be highly sought after. The present invention finds further value in this flue gas stream obtained from a combustion, as shown in the embodiments of Fig. 2.

[0022] In the reduction reaction, the first gas stream can either be used as the only gaseous reactant, or it can be used as a combination of the first gas stream and a further CO2-containing gas or gas mixture.

[0023] Thus, the reduction mixture is formed of a gaseous phase, combined with the carbonaceous reactant, wherein the gaseous phase is formed of the first gas stream, or of a mixture of the first gas stream and a further CCh-containing gas or gas mixture. The components of the final reduction mixture, formed when combining the carbonaceous reactant with said gaseous phase, are then reacted in a reduction reaction by heating said mixture. The heating is achieved using electricity, such as by using a heating resistor, a plasma heater or an induction heater, whereby the reduction is an electrical reduction step.

[0024] Other useful electrical heating means include electric arc heaters and infrared heaters. However, a preferred alternative for electrical heating in the herein described process is heating by heating resistors. Particularly compared to plasma heaters, resistors are highly energy efficient. Induction heaters, in turn, are not suitable for heating gases, while the further mentioned electrical heating means are less efficient, in general, than the other alternatives.

[0025] Resistors require only a moderate temperature to provide the desired heating levels, they are easily adjusted to the desired temperature, and the provided energy can be efficiently transferred to the desired process step or unit or reaction. Thus, for example, the heat created by the resistors can be efficiently utilized in the reduction step described herein, and even in heating the gases used as reagents in one or more of the steps of this process.

[0026] Compared to resistors, for example plasma heaters have the disadvantage of creating heat loss, as cooling of the plasma is required. Further, the plasma can create a point temperature that is significantly higher than the temperature required for the reaction being carried out. This high point temperature can be harmful e.g. for the activated carbon optionally produced as a solid by-product in the reduction reaction, which activated carbon might achieve a lower grade, or a weaker carbon structure, or the high point temperature of the plasma might even cause partial melting of the activated carbon.

[0027] A further option for achieving heating in the reduction step, without requiring fossil fuels, is to pre-heat the CCh-containing first gas stream by electrical heating before carrying out the reduction reaction. Typically, the above-mentioned heating means are suitable also for this purpose. As indicated above, the resistors are particularly preferred for use also in heating this CCh-containing first gas stream due to above-mentioned reasons, and also because plasma heaters require controlled and clean gas streams than flue gases applied in this invention.

[0028] Thus, the reduction step may be carried out for example in the presence of added CO2, mixed into a gaseous phase of the reduction mixture.

[0029] The above-described gaseous phase of the reduction mixture, formed of either the first gas stream or a mixture of the first gas stream and a further CCh-containing gas or gas mixture, may have a CO2 content as low as 10-30 vol-%. However, in another alternative its CO2 content is 40-90 vol-%, preferably 60-70 vol-%.

[0030] Thus, there is no requirement to purify or concentrate the first gas stream led to the reduction step, as the first gas stream can be utilized as such, even without added CO2.

[0031] Further, in one alternative the first gas stream is carried to the reduction step without a preceding concentrating step or purifying step. This is a particularly advantageous option, since concentrating the CO2 in such a gas stream can be challenging. However, in another alternative, the CO2 content of the first gas stream is increased before the reduction step is carried out, by concentrating the CO2, either by adding further CO2 to the first gas stream, or by separating one or more gaseous components, other than CO2, from the first gas stream or by adding further CO2 to the gas stream.

[0032] Typically, in this alternative, the CO2 content of the first gas stream can be increased by a purification to remove undesired components, such as oxygen, nitrogen or sulphur dioxide. This purification can take place e.g. by washing, scrubbing, cooling or drying the gas stream, or by a combination of two or more such techniques.

[0033] The reactants of the reduction mixture and their concentrations are preferably selected to favour the reaction of the carbon dioxide with the carbonaceous reactant into carbon monoxide in a reduction reaction following Reaction (1)

[0034] Particularly, high amounts of hydrogen are avoided in the reaction, in order to avoid Reaction (2), as this reaction would not give an equal yield of CO productCO2+ H2-> CO + H2O (2)

[0035] In an embodiment of this process, the carbonaceous reactant used in the reduction step contains carbon, preferably in the form of char, charcoal, coke, or petroleumcoke, or biochar or activated carbon, and / or it contains a hydrocarbon, preferably in the form of methanol (CH3OH), methane (CH4), ethylene (C2H4), propylene (CaHe), butenes (C4H8), or formic acid, more preferably in the form of methane (CH4), ethylene (C2H4), propylene (CaHe), or butenes (C4H8), which are advantageous in that they lack oxygen in their structures, or a mixture of hydrocarbons, or it contains a biomass 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. Alternatively, the carbonaceous reactant is formed of one or more carbons or one or more hydrocarbons, or it is formed of a mixture of carbon(s) and hydrocarbon(s), or it is a combination of any of these, more preferably being formed of carbon(s) or a mixture of carbon(s) and hydrocarbon(s), the carbon(s) particularly selected from char and / or biochar. The same carbonaceous reactant can optionally be utilized as a fuel component also in the combustion.

[0036] In a preferred embodiment, the carbonaceous reactant is selected from one or more carbons, such as the carbons of the above list.

[0037] In another preferred embodiment, the carbonaceous reactant is selected from a mixture of one or more carbons and one or more hydrocarbons, each preferably selected from the above lists, more preferably with the content of carbons in the carbonaceous reactant being 30-100 w-%, more preferably 50-90 w-%.

[0038] 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.

[0039] In one embodiment, the carbonaceous reactant is selected from a biomass or a waste material 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, the digestate further having been subjected to one or more treatment steps, e.g. moisture reduction, such as mechanical press drying, and / or thermal drying, and optionally pyrolysis, before the carbonaceous reactant can be recovered therefrom, and this recovery could take place e.g. by decanting or filtering.

[0040] In an embodiment, the carbonaceous reactant used in the reduction step is provided in the form of a carbonaceous bed through which the first gas stream or a more complex gas phase of the reduction mixture can be fed.

[0041] Thus, as indicated above, the reduction step may be carried out in the presence of added CO2. However, also other gaseous components may be added to the reduction mixture. For example, the reduction step may be carried out in the presence of steam, preferably in the form of superheated steam. Alternatively, the reduction step may be carried out in the presence of oxygen, e.g. as oxygen gas (O2).

[0042] The optionally used additional oxygen can for example be produced by electrolysis from water using Reaction (3),The oxygen can thus be produced at the same site of the reduction step.

[0043] The heating of the reduction mixture may take place before or during the reduction step, preferably by continuous heating, more preferably by heating both the reduction mixture during the reduction and the first gas stream, the carbonaceous reactant and / or a gaseous phase of the reduction mixture before combining the gases with the carbonaceous reactant.

[0044] The reduction reaction proceeds at an elevated temperature, preferably at a temperature of >800 °C, preferably 900 - 1200 °C, more preferably 900 - 1100 °C. An elevated temperature is important to achieve in the reduction step, since the flue gas stream obtained from a typical combustion generally only has a temperature of < 500 °C, which is not sufficient for the reduction reaction. However, heating the reduction mixture using typical heating means, using fossil fuels, would cause a less environmental overall process. Therefore, electrical heating is utilized to increase the temperature of the reduction mixture or the first gas stream from said <500 °C to the required >800 °C.

[0045] In an embodiment, the reactants of the reduction mixture and their concentrations are selected to ensure the reaction of the carbon dioxide with the carbonaceous reactant in a reduction step resulting in a second gas stream containing 20 -60 vol-% CO. Preferably, the second gas stream is thus enriched in CO, while typically alsocontaining unreacted nitrogen gas (N2) carried to the gas streams of the process with the CCh-containing flue gas, and unreacted CO2, more preferably containing 20 - 78 vol-% N2 and 2 - 30 vol-% CO2.

[0046] Further, the reduction reaction typically produces a solid residue, which contains activated carbon, which can be recovered, by separating it from the second gas stream, and can optionally be utilized as a reagent. For example, the activated carbon produced herein can be returned to the reduction reaction for use as the carbonaceous reactant.

[0047] The second gas stream, or at least a fraction thereof, is preferably used as such as a gaseous fuel in the combustion, without preceding separations or concentrating steps. Thus, it can be fed directly to the combustion. Optionally, the second gas stream can be used in concentrated form, i.e. further enriched in CO, or in purified form, or after a fraction of the unreacted CO2 remaining therein has been separated therefrom.

[0048] In a further embodiment, at least a fraction of the second gas stream, containing unreacted CO2, or a fraction of unreacted CO2 separated from the second gas stream, is returned to the reduction step to be added into the reduction mixture.

[0049] The CO-containing second gas stream is typically fed directly from the reduction step to the first or second combustion to be utilized at fuel therein. Thus, said gas stream can maintain its elevated temperature and allow the heat of the reduction step to be utilized also in said combustion(s).

[0050] In a preferred embodiment, the second gas stream is fed to the combustion in a manner that allows it to at least essentially maintain its elevated temperature. Optionally, it may be further heated before leading it to said combustion(s), preferably by electrical heating. The above listed electrical heating means can be used also for this purpose.

[0051] In an embodiment, the temperature of the second gas stream led into the combustion is > 750 °C, preferably 900 - 1200 °C, more preferably 1000 - 1200 °C.

[0052] The CO content of the second gas stream is typically monitored, and if the CO content in said second gas stream is >70 vol-%, the combustion can be operated using the second gas stream as the only fuel, whereas in case of a lower CO content, the second gas stream is typically mixed with one or more further fuel components to increase the heat value of the combined fuels, the further fuels preferably also being gaseous fuel components.

[0053] The combustion can use any solid, liquid or gaseous fuel, or a combination of two or more different fuels. Even fossil fuels can be used, although the aim with the invention is to decrease the need for such fossil fuels, and the present process can be operated fully without fossil fuels.

[0054] As stated above, the present invention provides the option of integrating a combustion with a process for modifying a CCh-containing gas stream into a different stream having a higher heat value. In this integrated process, both the prepared gaseous fuel (i.e. the second gas stream) and the heat used in the reduction reaction are utilized in the subsequent combustion.

[0055] In a further alternative, also the first gas stream can be obtained from said combustion, thus providing further integration. Thus, the flue gases formed in the combustion can be recovered, modified, and recirculated back to the same combustion.

[0056] As mentioned above, however, it is also possible to obtain the first gas stream from a separate combustion, or as another type of industrial flue gas.

[0057] Similarly to the reduction step, also the combustion, whereto the second gas stream is fed, may be operated by utilizing electrical heating, preferably at least partly achieved by electrical heating of the CO-containing second gas stream fed as a gaseous fuel to said combustion. In order to be able to utilize the heat of the reduction step also in the combustion, these process steps are most suitably carried out on the same site.

[0058] 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.

[0059] 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 present invention. 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.EXAMPLESExample 1 - Reduction of carbon dioxide into carbon monoxide to obtain a product suitable for combustion or co-firing

[0064] In the experiment set-up, nitrogen and carbon dioxide were fed into the electrically heated reactor together with biochar. Nitrogen was fed as purge gas corresponding to 40 vol-% of the gas feed. Consequently, 60 vol-% of the gas feed was CO2. The reactor temperature (achieved by electrical heating) was about 900 °C. As a result, the gas composition presented in the following Table 1 was obtained, and the shown results obtained by measuring as an average value from stable operation.Table 1. Average gas composition during stable operation of experiments.

[0065] The results show how CO2 is converted to CO in the reactor kept at 900 °C.Also a small amount of hydrogen is formed from the moisture present in the biochar. Based on the heating values of these combustible compounds of the produced gas, a heating value of 5.6 MJ / kg is estimated for the produced gas mixture. If the reactor temperature would be increased, the CO concentration of the produced gas mixture would also increase, whereas the CO2 concentration would be lowered, enabling an even higher heating value of the produced gas mixture.INDUSTRIAL APPLICABILITY

[0066] The present invention can utilize the carbon dioxide of a flue gas stream obtained from a combustion. Further, the process has the potential to be fully powered with electricity.

[0067] Likewise, the process of the invention has the potential to be fully integrated with a combustion, and can modify an industrial flue gas stream containing carbon dioxide, and can lead it back to the combustion as a gas stream with a higher energy value.

Claims

Claims1. A process for modifying a carbon dioxide (CO2) -containing flue gas stream using electrical heating, and utilizing the modified gas stream in a combustion, the process being characterized by steps comprisingobtaining a first gas stream containing carbon dioxide (CO2) having a temperature of < 500 °C as a flue gas stream, and using electrical heating to increase its temperature to a level of >800 °C,reacting the obtained first gas stream, optionally combined with one or more further CCh-containing gases or gas mixtures, at a temperature of 900 - 1200 °C achieved by electrical heating using a heating resistor, with a carbonaceous reactant to form a reduction mixture and to cause a reduction reaction, where the carbon dioxide (CO2) reacts into carbon monoxide (CO), thereby producing a second gas stream containing carbon monoxide (CO), andutilizing the obtained second gas stream having an elevated temperature as a gaseous fuel in a combustion.

2. The process of claim 1, wherein the CO2 content of the first gas stream obtained from the combustion is 10-40 vol-%, or 10-70 vol-%, or even 10-90 vol-% and the first gas stream typically contains also nitrogen gas (N2).

3. The process of claim 1 or 2,wherein the CO2 content in the first gas stream is increased before the reduction step is carried out, either by adding further CO2 to the first gas stream, or by separating one or more gaseous components, other than CO2, from the first gas stream.

4. The process of any preceding claim, wherein a gaseous phase of the reduction mixture containing the first gas stream, or a mixture of the first gas stream and a further CO2-containing gas, before the reduction reaction, has a CO2 content of 40-90 vol-%, preferably 60-70 vol-%.

5. The process of any preceding claim, wherein the reduction reaction is carried out by heating to a temperature of 900 - 1100 °C.

6. The process of any preceding claim, wherein the elevated temperature during the reduction reaction is achieved by electrical heating carried out by heating the first gas stream, the carbonaceous reactant and / or a gaseous phase of the reduction mixture, preferably by continuous heating.

7. The process of any preceding claim, wherein the carbonaceous reactant used in the reduction reaction contains carbon, preferably in the form of char, charcoal, coke, or petroleum coke, or biochar or spent activated carbon, and / or it contains a hydrocarbon or a mixture of hydrocarbons, or it contains a biomass, such as wood chips, or it is formed of one or more carbons or one or more hydrocarbons, or it is formed of a mixture of carbon(s) and hydrocarbon(s), or it is a combination of any of these, more preferably being formed of carbon(s) or a mixture of carbon(s) and hydrocarbon(s), the carbon(s) particularly selected from char and / or biochar.

8. The process of claim 7, wherein the carbonaceous reactant is selected from one or more carbons, preferably from char, charcoal, coke, or petroleum coke, or biochar or spent activated carbon.

9. The process of claim 7, wherein the carbonaceous reactant is selected from a mixture of one or more carbons and one or more hydrocarbons, the content of carbons preferably being 30-100 w-% of the entire carbonaceous reactant, more preferably 50-90 w-%.

10. The process of any of claims 1-2 and 4-9, wherein the first gas stream is carried to the reduction step without a preceding concentrating step or purifying step.

11. The process of any of claims 1 to 9, wherein the reduction step is carried out in the presence of added CO2, mixed with the first gas stream into a gaseous phase of the reduction mixture.

12. The process of any preceding claim, wherein the reduction step is carried out in the presence of steam, which preferably is in the form of superheated steam, mixed into a gaseous phase of the reduction mixture.

13. The process of any preceding claim, wherein the second gas stream is further heated before leading it to the combustion, preferably by electrical heating.

14. The process of any preceding claim, wherein the second gas stream, or a fraction thereof, is used as such as a gaseous fuel in the combustion, optionally in concentrated or purified form, or after a fraction of the unreacted CO2 remaining therein has been separated therefrom.

15. The process of any preceding claim, wherein at least a fraction of the second gas stream, containing unreacted CO2, or a fraction of unreacted CO2 separated from the second gas stream, is returned to the reduction step.

16. The process of any preceding claim, wherein the CO content of the second gas stream is monitored, and if the CO content in the second gas stream is >70 vol-%, the combustion is operated using said second gas stream as the only fuel, whereas in case of a lower CO content, the second gas stream is mixed with one or more further gaseous fuel components.

17. The process of any preceding claim, wherein the temperature of the second gas stream led into the combustion is > 750 °C, preferably 900 - 1200 °C, more preferably 1000 - 1200 °C.

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