Converting carbon dioxide formed during heat treatments into suitable components for use as fuel

Abstract

According to the present invention, there is provided a process for utilizing a gas stream containing carbon dioxide (CO2), formed during the heat treatment of a raw material containing carbonates, in reducing the carbon dioxide of said stream to carbon monoxide, to produce a gas mixture containing carbon monoxide (CO), and subsequently utilizing at least a fraction of the CO- containing gas mixture as fuel to one or more heat treatment steps.

Description

CONVERTING CARBON DIOXIDE FORMED DURING HEAT TREATMENTS INTOSUITABLE COMPONENTS FOR USE AS FUELFIELD

[0001] The present invention relates to a process for converting a gas stream containing carbon dioxide, formed by heat treating a carbonate-containing raw material, into suitable components for supplying to a high-temperature heat treatment to be further used, among others as fuel.BACKGROUND

[0002] Conventional calcinations and other similar heat treatments typically rely on the heat produced by the burning of fossil fuels. Such processes result in significant carbon emissions. For example, cement and lime production processes have been considered to be among the world largest CO2 emitters.

[0003] There are numerous development projects taking place worldwide aimed at electrifying such processes.

[0004] Similarly, there are numerous development projects taking place worldwide aimed at carbon capture and the utilization of the carbon dioxide formed in high-temperature processes, such as described in WO 2015015161 Al. In other projects, a lot of focus is being put on developing replacements to fossil fuels.

[0005] There is, however, still a need for further development in providing more environmental solutions for high-temperature processes and for their off gases.SUMMARY OF THE INVENTION

[0006] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0007] According to a first aspect of the present invention, there is provided a process that provides a new route for the utilization of the gas stream containing carbon dioxide (CO2) that is formed during calcination and / or other high-temperature treatments.

[0008] According to a second aspect of the present invention, there is provided a process that utilizes at least some of the carbon of the compounds formed during calcination and / or other high-temperature treatments in the same or other high-temperature process steps.

[0009] The present invention thus relates to a process for utilizing a gas stream containing carbon dioxide (CO2), formed during the heat treatment of a raw material containing carbonates, in reducing the carbon dioxide of said stream to carbon monoxide, among others, in Reactions (1) and (2)to produce a gas mixture containing carbon monoxide (CO), and subsequently utilizing at least a fraction of the CO-containing gas mixture as fuel in the calcination and / or a further heat treatment.

[0010] Several advantages are achieved using the present process. Among others, carbon dioxide (CO2) emissions generated from calcinations and / or other high-temperature treatments can be transformed into carbon monoxide (CO) and possibly further into synthesis gas, serving as an efficient fuel source for combustion processes. The produced CO has a high energy content, with almost twice the energy content compared to utilized carbonaceous material. Thus, the new process decreases the consumption of conventional fuels, and it cuts down the carbon emissions of fuel combustion.

[0011] Further, the optional electric calcination offers a cost-effective solution of capturing CO2 by releasing this unavoidable CO2 from the carbonates of the raw material in high concentration and simultaneously decreasing the consumption of energy from burning fossil fuels. Further, the electric calcination eliminates emissions of e.g. carbon and carbon dioxide from thermal energy production.

[0012] Particularly for cement production, the new process enhances the process sustainability by decreasing the need for external fuel supply and decreasing the emissions.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIGURES 1 - 4 illustrate process configurations encompassed by the present invention; Figs. 1 and 2 illustrating the alternative of combining the reduction with a heat treatment, Figs. 3 and 4 illustrating the alternative of carrying out a separate reduction of the CO2-containing gas stream obtained from a heat treatment step, further with Figs. 1 and 3 illustrating the alternative of carrying out only one heat treatment step, and Figs. 2 and 4 illustrating the alternative of carrying out two (or more) heat treatment steps.

[0014] FIGURE 5 illustrates a process configuration incorporating some preferred embodiments of the below description, showing for example the optional addition of further reagents (O2, H2, steam) to either the heat treatment step(s) or the reduction step, or both, as well as using dotted lines to show the optional recovery of the CCh-containing gas stream and / or the CO-containing gas stream, and to show the optional use of the CO-containing gas stream as dilution gas.

[0015] FIGURE 6 illustrates the high CO2 concentrations, which can be achieved by electrically heated calcination reactor, as a function of the pressure.

[0016] FIGURE 7 is a graph showing the contents of a typical CO-containing gas stream obtained in a reduction reaction carried out in accordance with the invention by using three different carbonaceous materials having different reactivities.EMBODIMENTS

[0017] DEFINITIONSIn the present context, the term “heat treatment” encompasses high- temperature process steps, particularly steps that generate carbon dioxide as aby-product. Examples of high-temperature process steps include calcination, sintering and clinkering.When “gas streams” or “gas mixtures” are mentioned, it is typically referred to wet gas or humid gas saturated with liquid vapor.

[0018] The present invention thus relates to a process for utilizing a gas stream containing carbon dioxide (CO2), formed during the heat treatment of a raw material containing carbonates in steps comprising heat treating the raw material in one or more steps, at least one of which produces CO2, obtaining a gas stream containing carbon dioxide (CO2) from one or more heat treatment step(s), optionally, separating the CO2 from the remaining components of the gas stream to provide a concentrated CO2 gas stream, in reacting the obtained gas stream containing CO2 with a carbonaceous material to cause the reduction of the carbon dioxide to carbon monoxide, to produce a gas mixture containing carbon monoxide (CO), at least a fraction of the gas mixture being utilized as fuel in one or more of said heat treatment steps.

[0019] Typical process configurations are shown in Figs. 1 - 5.

[0020] The raw material is typically a carbon-containing material, preferably a material containing carbonates, more preferably containing the carbonates in the form of calcium carbonate (CaCOs), calcium magnesium carbonate (CaMgfCChh), magnesium carbonate (MgCCh), lithium carbonate (Li2CO3), potassium carbonate (K2CO3), or sodium carbonate (Na2CCh), or a mixture of two or more of these, most suitably being a mineral raw material, such as limestone or dolomite.

[0021] One or more heat treatment steps are used, preferably one or two. In case of using only one heat treatment step (see Figs. 1 and 3), it is preferably carried out as a calcination to produce a calcined material. Further, in case of using more than one heat treatment steps (see Figs. 2 and 4), it is preferred to carry out at least one heat treatment step, preferably a first heat treatment step, as a calcination. A second or a final heat treatment step might, in turn, be operated as e.g. a sintering step or a clinkering step.

[0022] The first or only heat treatment step may, for example, be carried out in a fluidized bed reactor, shaft furnace, flash calciner or rotary kiln. A rotary kiln is particularly preferred, and is typically in the form of a hollow cylindrical device with two ends. The hollow cylindrical device is configured to be rotated around an axis of rotation. The axis of rotation is typically orientated horizontally or substantially horizontally. The term ’’axis orientated horizontally” means an axis that is orientated perpendicular to a gravity vector or perpendicular to the normal on the surface of the Earth. Similarly, the term ’’axis orientated substantially horizontally” means an axis that is tilted a few degrees, for example less than 10 degrees, from the axis that is orientated perpendicular to a gravity vector or perpendicular to the normal on the surface of the Earth. In other words, when the rotary kiln is positioned substantially horizontally, a feed end is fixed at a higher level than a discharge end. Thus, the solid material carried through the kiln will gradually move from the feed end to the discharge end when the device is rotated. Additionally, the solid material is distributed over an inner surface of the cylindrical device by the rotation. The rotational speed of the hollow cylindrical device may be, for example, in the range between 0.1 and 10 revolutions per minute (rpm). The rotational speed may be adjustable. When the rotary kiln is positioned substantially horizontally, the feed inlet is positioned at the feed end of the device that is fixed at a higher level than the discharge end at which the outlet for the solid product is positioned. The heat treatment step carried out in said rotary kiln will thus take place during a time interval that is determined by the time it takes for the material to move from the feed end of the hollow cylindrical device to the discharge end. The heating can be applied using electrical heating equipment or a fuel-powered burner or by supplying a heated raw material to the kiln, or a combination of these, the optional heating equipment preferably positioned by the discharge end of the substantially horizontal rotary kiln. The heating equipment may be capable of adjusting the temperature within the cavity, for example in a range between 150 °C and 1600 °C.

[0023] In a specific embodiment, possible to use for example as a section of a cement manufacturing process, it is preferred to use two heat treatment steps, whereby the first one is operated as a calcination and the second one is operated as a clinkering.

[0024] The heating of the heat treatment step(s) is preferably at least partly achieved via electrical heating, and optionally by combining the electrical heating with heating achieved by combustion, i.e. by burning fuel. It may, however, also be carried out entirelyby electrical heating. The electrical heating is particularly suitable for first heat treatment step(s), especially when carried out at the below mentioned lower temperature range of 750- 1000 °C, and most suitably when the heat treatment is carried out as the above- mentioned optional calcination. Likewise, the combustion is particularly suitable for subsequent heat treatment steps.

[0025] Thus, this electrical heating is particularly suitable for use in the calcination step when applied to the above-mentioned cement manufacturing process.

[0026] The temperature in the heat treatment step(s) is typically >750, preferably 750 - 1600 °C, more preferably 900 - 1450 °C, even more preferably 900 - 1200 °C, and most suitably 950-1050 °C, typically determined by measuring the temperature of the gas surrounding the material being heated. Generally, these heat treatments are carried out at a pressure that is close to atmospheric pressure, typically at a slight overpressure level of 0.1 to 0.5 bar, but electric calcination can also be operated at a slight vacuum, which facilitates the calcination reaction. In practice, the calcination reaction occurs within the range of -0.5 to 2 bar pressure, preferably 0.1 - 0.5 bar, while the pressures for other heat treatments have larger variation within the range of -1 - 10 bar. In case more than one heat treatment steps are carried out, their temperatures can naturally be different from the temperatures of the other heat treatment steps, such as achieved by operating each heat treatment step at a higher temperature than the previous one. Thus, a preferred alternative is to operate the first heat treatment step at a temperature of 750 - 1100 °C, more preferably 900 - 1000 °C, and the final heat treatment step at a temperature of 1000 - 1600 °C, more preferably 1200- 1500 °C, and most suitably 1300 - 1450 °C. An optional intermediate step could thus be carried out at a temperature of 850 - 1200 °C. The duration of each of these heat treatment steps may range from 2 seconds to 10 days, partly based on the temperature and on the rate of energy transfer, which are significantly influenced by the type of equipment and process. Further, the selected raw material has an influence. Typically, the residence time in a rotary kiln could be 1 - 30 h, or 6 - 24 h, while the residence time in a fluid bed reactor could be as low as seconds, and the residence time in a shaft kiln could be as long as 12 - 50 h, or 16 - 48 h.

[0027] The scope of this description also includes the alternative, wherein a preheating step is carried out, e.g. to pre-dry the raw material before the above-described heat treatment(s), which pre-heating step can be carried out at a lower temperature than the heattreatment step(s), such as a temperature of 150 - 800 °C, preferably 200 - 750 °C, and more preferably 500 - 750 °C.

[0028] No further additives are required in the heat treatments, but in an embodiment, one or more heat treatment steps are carried out in the presence of steam (see further reagents of Fig. 5), preferably in the form of superheated steam. This steam (H2O) will, among others, have the effect of reducing the temperature required for the heat treatments, which will further increase the durability of the process equipment and the wear resistance of the materials of the equipment, as well as improve the structural strength of the equipment structures. Further, it may improve both the energy efficiency of the process and the quality of the products. Further, the steam and other added components can also be used to optimize the specific contents of the gas stream containing CO2 or the gas mixture containing carbon monoxide (CO), the steam (as H2O), or added hydrogen or oxygen, increasing the hydrogen content of the formed gas mixture, or increasing the CO2 content, respectively.

[0029] In another embodiment, one or more heat treatment steps are carried out in the presence of oxygen, e.g. in the form of oxygen gas (O2) (see the further reagents of Fig. 5), thus resulting in a higher quality for the CO2 in the resulting gas stream, and a more efficient burning, particularly in high-temperature heat-treatments, such as clinkering steps, also allowing an adjustment of the temperatures used in such steps, thus providing further variety in the choice of fuel as the requirements for the energy content are reduced. The oxygen can for example be oxygen produced by electrolysis from water using Reaction (4),Thus, the oxygen can optionally be produced at the same site of the present process, and is preferably used as a mixture of oxygen gas and steam.

[0030] The heat treatment step(s) produce both a solid product, e.g. containing calcium oxide (CaO) in case of a calcination, as well as the gas stream containing CO2 that is utilized in the following reduction step. In case more than one heat treatment step is carried out, said gas stream can be obtained from one or more of these heat treatment steps, preferably from the first heat treatment step, or from both the first and from one or more further heat treatment steps. Particularly calcinations result in a gas stream containing CO2.This gas stream obtained as an intermediate product is rich in CO2, typically containing >50 vol-% CO2, or more than 70 vol-%, or even more than 90 vol-% (see Fig. 6), while it preferably contains 85 - 95 vol-%. Further, it may contain oxygen (O2), carbon monoxide (CO), hydrogen (H2), nitrogen (N2), and / or steam (H2O), and traces of other gases.

[0031] The gas stream containing carbon dioxide (CO2), obtained from the heat treatment step(s), can be recovered before reacting it with the carbonaceous material in a separate process step, i.e. a separate reduction step (see Figs. 3, 4 and 5). This recovery may take place e.g. by separating the gaseous fraction from the solid product via a gas outlet in the heat treatment unit. If it is recovered, it is preferably fed to the reduction step directly, or it may be purified or concentrated before taking part in the reduction step, to increase the CO2 content of this gas stream, and / or to remove undesired components, such as excess 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, and preferably results in a CO2 concentration of >85 vol-%. However, even if the reduction may be carried out in a reduction unit that is separate from the heat treatment unit(s), these units form an integrated sequence of units.

[0032] The temperature of the stream containing CO2 may even be sufficient to provide an optimal reduction step without further heating, particularly if no separate step for concentrating the CO2 of the stream has been carried out. As an alternative, the stream containing CO2 can be heated separately, preferably to a temperature of >900 °C, more preferably 900 - 1500 °C, or 920-1500 °C, or 940-1500 °C, or 950-1500 °C, before taking part in any further reactions, such as the reduction step. This alternative may also be applied when a particularly low temperature has been applied in the heat treatment step(s), such as a temperature below 900 °C, or a below 800 °C. Further, the reduction step can be carried out at a pressure that is close to atmospheric pressure, or at slightly reduced pressure, more typically at a pressure in the range of -1 - 10 bar, preferably 0 - 2 bar. Further, the gas stream can be contacted with one or more catalysts, such as nickel, calcium oxide, magnesium oxide, zinc oxide, or aluminium oxide, preferably in catalytic amounts, before further reactions.

[0033] In another alternative, the gas stream containing carbon dioxide (CO2), obtained from the heat treatment step(s), can be used as such in the heat treatment stepwithout preceding separation of the gas stream (see Figs. 1 and 2). Thus, the heating of the heat treatment steps can be utilized also for this reduction reaction, whereby also the heating of the reduction reaction may be at least partly achieved by electrical heating, e.g. combined with fuel burning, or alternatively entirely by electrical heating. Further, also in said reduction step, a temperature of 900 - 1500 °C may, again, be used, more preferably 900 - 1100 °C, or 920-1100 °C, or 940-1100 °C, and most suitably 950-1050 °C. This alternative is particularly suitable for embodiments, where the heating takes place by electric heating. Preferably, the reduction reaction will begin immediately in the feed end of the heat treatment unit, or in the area of the heat treatment unit that is close to the feed end / area. Depending on the efficiency of the process and the selected heat treatment unit, the reduction reaction might be complete or essentially complete by the time the solid material reaches the discharge end, or the reaction might be completed or essentially completed at the discharge end, where the heating possibly is most efficient.

[0034] The carbonaceous material used in the reduction reaction is typically carbon, preferably in the form of char, charcoal, coke, petroleum coke or biochar, or a hydrocarbon or an acid or alcohol thereof, preferably in the form of methane (CH4), ethylene (C2H4), propylene (ChHe), butenes (C4H8), or formic acid, or methanol (CH3OH), or a mixture of hydrocarbons, or a mixture of carbon and hydrocarbon(s), or a 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.

[0035] In another alternative, the carbonaceous reactant used in the reduction reaction contains carbon, preferably in the form of char, charcoal, coke, or petroleum coke, or biochar or activated carbon, and / or it contains a hydrocarbon, preferably in the form of methane (CH4), ethylene (C2H4), propylene (C3H6), butenes (C4H8), or formic acid, or methanol (CH3OH), 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, or it contains a biomass or a woody biomass, such as wood chips, or a combination of any of these. In a further alternative, 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) andhydrocarbon(s), the carbon(s) particularly selected from char and / or biochar. In a preferred alternative, 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-%.

[0036] 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.

[0037] The gas stream containing CO2 can be used in the reduction step without further additives, but can, in an embodiment, also be reacted in the presence of steam (see the further reagents of Fig. 5), preferably in the form of superheated steam, thus providing a further means for heating, while also resulting in a gas stream containing hydrogen.

[0038] In another embodiment, the gas stream containing CO2 is reacted in the presence of oxygen gas (O2) (see the further reagents of Fig. 5), for example oxygen produced by electrolysis, e.g., at the same site of the present process. The oxygen will facilitate the heating of the reduction step, but will also take part in the reactions to produce carbon monoxide, via Reaction (4)

[0039] A further alternative is again to use a mixture of oxygen gas and steam.

[0040] In a further embodiment, the gas stream containing CO2 is reacted in the presence of a hydrogen-containing gas, which can be either separately added hydrogen (see the further reagents of Fig. 5), or hydrogen carried to the reaction with the gas stream or with another added stream, such as the above-mentioned steam.

[0041] In yet a further embodiment, the gas stream containing CO2 is reacted with the carbonaceous material in the presence of added CO2, e.g. used to initiate the reduction reaction.

[0042] To further facilitate the reaction, it is possible to contact the CCh-containing gas stream with one or more catalysts, such as nickel, calcium oxide, magnesium oxide, zinc oxide, or aluminium oxide, to the reduction reaction. The catalyst is typically added as a solid material, e.g. a fixed bed or a coating, in the reduction unit, or it may be added to the carbonaceous material or the stream containing CO2 before reacting into a CO- containing product gas mixture.

[0043] The above-described optional reagents can either be added separately to the reduction mixture in the reduction unit, or they can be premixed with the carbonaceous material or the CCh-containing gas stream before taking part in the reduction reaction. Preferably any solid reagents and materials are premixed with the carbonaceous material before the reduction reaction takes place, while any gaseous reagents and materials are premixed with the CCh-containing gas stream.

[0044] Thus, the process described herein results in the production of a CO- containing gas mixture which can be utilized further in the process, preferably having a CO content of more than 60 vol-%, more preferably more than 70 vol-%, or even more than 80 vol-% (see Fig. 7). This gas mixture may, in addition to carbon monoxide, contain e.g. unreacted carbon dioxide, as well as hydrogen (H2) and other common gaseous components, such as oxygen (O2), nitrogen (N2), ethylene (C2H4), methane (CH4), ethane (C2H6) and / or sulphur dioxide (SO2), typically in trace amounts, such as in a combined content of the O2, N2, C2H4, CH4, C2H6 and SO2 of less than 10 vol-% (see “others” of Fig. 7). When a biomass has been used as a carbon source in the reduction reaction, the CO- containing gas mixture formed as a product will typically contain a more complex mixture of components, such as the mentioned ethylene, methane and ethane. When an excess of carbon is used in the reduction, and the reduction is carried out in a unit that is separate from the heat treatment unit, also activated carbon can be generated.

[0045] The CO-containing gas mixture is thus recovered (see Fig. 5), and utilized as a fuel in one or more of said heat treatment steps, preferably in a final heat treatment step, for example in a sintering or clinkering step. Further, a fraction thereof may also be utilized as a carbon source, or alternatively as a diluting gas for a calcination, to adjust the contents of materials during the calcination, e.g. to reduce recarbonization, to facilitate calcination at lower temperatures, and to adjust the quality of the calcined product.According to one option, the carbon monoxide can be recovered from the mixture, or its concentration increased, before its use as fuel or carbon source.

[0046] In an embodiment, at least a fraction of the CO-containing gas mixture, containing unreacted CO2, is returned to a heat treatment step, such as a calcination step, or a sintering or clinkering step, into the used reactor (e.g. calcinator) as a dilution gas (see Fig. 5). A further option is to return the CO-containing gas mixture or a fraction thereof, containing unreacted CO2, or unreacted CO2 separated from this fraction, to the reduction step.

[0047] Due to the suitable reaction scheme, the process described herein may be used as a section of a cement manufacturing process. In such a case, as mentioned above, it is preferred to use two heat treatment steps, whereby the first one is operated as a calcination and the second one is operated as a clinkering. Likewise, it is advantageous in this cement application to use electrical heating in the calcination step. Thus, by using an electrical calcination, as well as electrical heating in the reduction step, and by recovering and utilizing the carbon of the calcination off gas, an essentially carbon neutral cement production process can be achieved.

[0048] Other application areas include lime and pulp processes, or other similar combustion processes.

[0049] 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.

[0050] 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.

[0051] As used herein, a plurality of items, structural elements, compositionalelements, 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.

[0052] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following 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.

[0053] 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.

[0054] 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 - Gas products formed by calcination followed by reduction

[0055] A calcination of a calcium carbonate (CaCCh) material (limestone, containing 97 % CaCCh) was carried out at 1000 °C, using electric heating, and a solid product of calcium oxide (CaO) was obtained, as well as a gaseous product containing mainly carbondioxide (CO2), obtained from the carbonate and released as its own gas stream. Fig. 6 illustrates the CO2 content in a calcination reactor operated as described herein, as a function of the pressure.

[0056] The solid CaO product was transferred to a second heat treatment step for clinkering. This second step requires higher temperature, and this further heat production was achieved through combustion by utilizing CO-containing gas mixtures (similar to the herein produced) as fuel.

[0057] The gaseous product was collected, purified and carried to a reduction unit.

[0058] In the reduction unit, the carbon dioxide in the gaseous product is reduced with the help of a carbonaceous material fed into the reaction. The reaction requires energy, which can be transferred to the reaction with the heat of the gaseous product fed into it, but in the present example, the reduction unit was further heated by resistors. The CCh-containing gaseous stream was combined with various carbon materials, providing different test results. The contents of gaseous products were continually measured. Three different carbon materials were used, being char coal produced from different natural raw materials, char coal1 being obtained by pyrolysis from wood chips and having the highest reactivity, char coal2 being obtained by pyrolysis from bark waste, and char coal 3 being obtained by pyrolysis from a digestate. Thus, different reduction products were obtained (CO-containing gas mixture), having different gas contents (contents determined by FTIR). The results are shown in Fig. 7, providing the contents of carbon monoxide (CO), carbon dioxide (CO2), water (or steam, i.e. H2O), and other components, which generally may include oxygen (O2), nitrogen (N2), hydrogen (H2), ethylene (C2H4), methane (CH4), ethane (C2H6) and / or sulphur dioxide (SO2), and in the present example the specific content of this fraction was disregarded due to the small size of the fraction.

[0059] As the results show, high CO contents were achieved for the CO-containing gas mixtures, thus providing gas mixtures having a high energy content. Such gas mixtures are therefore highly suitable for use as such as fuel in the burners of e.g. clinker kilns or other similar kilns.INDUSTRIAL APPLICABILITY

[0060] The present invention provides a further alternative for utilizing the gas stream containing carbon dioxide (CO2) that is formed during calcinations and other high- temperature treatments in industrial processes.

[0061] Due to the suitable reaction scheme, the process steps of the invention are most suitably used as a section of a cement manufacturing process. One preferred option is the so-called retrofit technology, where existing process units are converted into suitable units for this new use, e.g. by providing electric heating to existing heat treatment units. CITATION LISTPatent LiteratureWO 2015015161 Al

Claims

CLAIMS:

1. Process for utilizing a gas stream containing carbon dioxide (CO2), formed during the heat treatment of a raw material containing carbonates in steps comprising heat treating the raw material in one or more steps, at least one of which produces CO2, obtaining a gas stream containing carbon dioxide (CO2) from one or more heat treatment step(s), optionally, separating the CO2 from the remaining components of the gas stream to provide a concentrated CO2 gas stream, in reacting the obtained gas stream containing CO2 with a carbonaceous material to cause the reduction of the carbon dioxide to carbon monoxide to produce a gas mixture containing carbon monoxide (CO), the process being characterized by carrying out the reduction of the carbon dioxide to carbon monoxide with heating, and by utilizing at least a fraction of the CO-containing gas mixture as fuel in one or more of said heat treatment steps.

2. The process of claim 1, wherein the raw material is a material containing carbonates, preferably containing the carbonates in the form of calcium carbonate (CaCO;), calcium magnesium carbonate (CaMg(CO3)2), magnesium carbonate (MgCCh), lithium carbonate (Li2CO3), potassium carbonate (K2CO3), or sodium carbonate (Na2CCh), more preferably being a mineral raw material, such as limestone or dolomite.

3. The process of claim 1 or 2, wherein the temperature in the heat treatment step(s) is >750, preferably 750 - 1600 °C, more preferably 900 - 1450 °C, and most suitably 900 - 1200 °C.

4. The process of any preceding claim, wherein at least one heat treatment step is carried out as a calcination step to produce a calcined material.

5. The process of claim 4, wherein the heating of this calcination step is at least partly achieved via electrical heating.

6. The process of claim 4 or 5, wherein this calcinating heat treatment is carried out at a temperature of 750 - 1100 °C, preferably 900 - 1000 °C.

7. The process of any preceding claim, wherein one or more heat treatment steps are carried out in the presence of steam, preferably in the form of superheated steam.

8. The process of any preceding claim, wherein one or more heat treatment steps are carried out in the presence of oxygen, e.g. as oxygen gas (O2).

9. The process of any preceding claim, wherein one or more heat treatment steps are carried out as a further heat treatment after a calcination of the raw material, for example to produce a clinkered material.

10. The process of claim 9, which further heat treatment step is carried out at a temperature of 1000 - 1600 °C, more preferably 1200 - 1500 °C, and most suitably 1300 - 1450 °C.

11. The process of any preceding claim, wherein the carbonaceous material is carbon, preferably in the form of char, charcoal, coke, or petroleum coke, or biochar, or it is a hydrocarbon or a mixture of hydrocarbons, a mixture of carbon and hydrocarbon(s), or a biomass, such as wood chips, or a combination of any of these.

12. The process of any preceding claim, wherein the gas stream containing CO2 is reacted with the carbonaceous material in the presence of added CO2.

13. The process of any of claims 1 to 12, wherein the gas stream containing carbon dioxide (CO2), obtained from the heat treatment step(s), is recovered before reacting it with the carbonaceous material in a separate reduction step, and optionally activated carbon is obtained from the gas stream.

14. The process of any preceding claim, wherein the gas stream containing CO2 is reacted in the presence of steam, preferably in the form of superheated steam15. The process of any preceding claim, wherein the gas stream containing CO2 is reacted in the presence of oxygen, e.g. as oxygen gas (O2).

16. The process of any preceding claim, wherein the gas stream containing CO2 is reacted in the presence of hydrogen-containing gas.

17. The process of any of claims 1 to 12, wherein the gas stream containing carbon dioxide (CO2), obtained from the heat treatment step(s), is reacted with the carbonaceous material without preceding separation of the gas stream.

18. The process of any preceding claim, wherein the reduction of the carbon dioxide to carbon monoxide is carried out with heating that is at least partly achieved by electrical heating.

19. The process of any preceding claim, wherein at least a fraction of the CO-containing gas mixture, containing unreacted CO2, is returned to one or more heat treatment step(s), e.g. for use as dilution gas.

20. The process of any preceding claim, wherein at least a fraction of the CO-containing gas mixture, containing unreacted CO2, or unreacted CO2 separated from this fraction, is returned to the reduction step.

21. The process of any preceding claim, wherein at least a fraction of the CO-containing gas mixture is utilized in a burner of a heat treatment unit, preferably being a burner of a calcination unit or a clinkering unit.

22. The process of any preceding claim, wherein one or more catalysts, such as nickel, calcium oxide, magnesium oxide, zinc oxide, or aluminium oxide, is contacted with the stream containing CO2 in the reduction reaction.

23. The process of any preceding claim, being a section of a cement manufacturing process.

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