Process for calcining lime or dolomite and annular shaft kiln used therefor

Description

[0001] The present invention relates to a method for calcining lime or dolomite in an annular straight kiln having an outer cylinder and an inner cylinder forming an annular space between them. A method of this type is known which comprises a supply of calcined material, limestone or dolomitic, at the top of the annular space into which it descends, an introduction of first combustion fumes into said annular space, at a first level of the furnace, an introduction of second combustion fumes into said annular space, at a second level of the furnace lower than the aforementioned first level, preheating and calcining of the calcined material during descent into the annular space, in contact with the first combustion fumes and second combustion fumes which undergo an upward draft, in counter-current to the calcined material, and are evacuated at the top of the furnace in the form of an upper gaseous current, cooling of the calcined material by supplying product cooling air to the bottom of the annular space, suction of this cooling air inside the internal cylinder, at a third level of the furnace lower than the aforementioned second level,by forming a lower gas stream containing the product cooling air, and a discharge of calcined lime or dolomite at the bottom of the furnace (see international patent application WO2018 / 002151).

[0002] During this process, the starting limestone or dolomite material releases a significant volume of CO 2 during its calcination into lime or dolomite. In addition, to achieve this calcination, high temperatures must be reached and therefore fuels must be burned, which, in turn, causes a significant release of CO 2 . Overall, calcination processes have the disadvantage of actively contributing to the increase in the greenhouse effect.

[0003] This very common process also has the disadvantage of providing for combustion of fuel with air and cooling of the calcined product by air. This results in the release at the top of the furnace of a gaseous stream having a high level of diatomic nitrogen, and a comparatively low level of CO 2 (volume concentration of the order of 25% to 35% on dry gas), which is expensive to capture due to the high presence of nitrogen from the air used.

[0004] Attempts have already been made to avoid these drawbacks.

[0005] A calcination furnace is already known where CO 2 recycling is planned (US2020 / 0048146). In this furnace, the CO 2 from calcination is partly heated and recycled into the furnace to cause the calcination of the raw material and partly released from the apparatus in the form of concentrated CO 2 effluent. The recycled gas must, however, be heated to the calcination temperature. For this, the combustion of a fuel is required and is carried out in air, releasing a gaseous effluent where the CO 2 is in diluted form.

[0006] A fluidized bed boiler is also known in which limestone is introduced in order to capture, through the lime formed, the sulfur from the fuel used and thus to evacuate the sulfur in the ashes in the form of calcium sulfate. After several heat exchanges, in particular to produce steam, a portion of the flue gases is, after dust removal, reheated by heat exchange with these same flue gases leaving the boiler and it is then mixed with dioxygen to form an oxidizer suitable for carrying out combustion with the fuel in the boiler (see US2010 / 0077947).

[0007] The present invention aims, for the purposes of use or sequestration, to allow partial or total capture of the CO 2 released during the calcination process in an annular straight furnace, not only during the actual calcination, but also during the combustion of the fuel necessary to reach the calcination temperature, and this without modification, or without major modification, of the annular straight furnace and the process implemented therein. The main objective of annular straight calcination furnaces must obviously be maintained, that is to say the production of calcined material of high quality and purity.

[0008] To solve this problem it is expected that the method indicated above will include a withdrawal of a portion of the upper gas stream, discharged from the furnace, a formation of a first oxidizing mixture by mixing this portion of the upper gas stream with pure oxygen, a combustion of a fuel in this first oxidizing mixture, to form said second combustion fumes, and an extraction of the lower gas stream containing the product cooling air from the internal cylinder, followed by its discharge from the furnace

[0009] This combustion in oxygen, giving rise to the second combustion fumes, has the effect of producing in them mainly CO2 alongside some impurities, present in the form of traces in the fuel and in the material to be calcined, and a little oxygen not consumed by the combustion of the fuel. This obviously results in a drastic increase in the CO2 content of the gas stream released at the top of the furnace.

[0010] Combustion of the fuel in pure oxygen would give rise to flame temperatures that would be too high for the usual furnace equipment. Therefore, according to the invention, provision is made to take a portion of the upper gas stream rich in CO 2 and mix it with the oxygen. Instead of a usual oxidant formed from the O 2 + N 2 mixture in air, an O 2 + CO 2 mixture at an appropriate flame temperature is thus obtained, by producing at the top of the furnace a gas stream increasingly concentrated in CO 2 . This CO 2 thus becomes usable or sequestrable under favorable conditions, which makes it possible to radically reduce the contribution to the greenhouse effect of the furnace.

[0011] Finally, following this process, the product cooling air introduced at the bottom of the furnace is completely channeled so as to in no way dilute the gaseous effluent escaping at the top of the furnace.

[0012] The implementation of this process does not necessarily require any particular arrangement of the furnace itself. The only modifications to be made to it may simply be external to the furnace and consist of changing the circuits of the fumes leaving the furnace, in particular by providing an outlet for the lower gas stream separate from that of the upper gas stream, as well as providing at least one source of pure dioxygen.

[0013] By pure oxygen (also called oxygen hereinafter) is meant according to the invention a gas whose oxygen content exceeds 50% by volume. It will preferably be equal to or greater than 95%, advantageously from 98 to 100% by volume. The source of pure oxygen can, for example, be an air separation unit which separates the air into oxygen and nitrogen (also called nitrogen hereinafter) and which operates in parallel with the furnace, or a tank of oxygen installed next to the furnace.

[0014] According to the invention, fuel means any solid, liquid or gaseous fuel, for example natural gas, hydrogen, biogas, fuel oil, oils, coal or powdered coke, solid biomass, such as sawdust, solid recovered fuel, such as plastics, paper, cardboard, etc.

[0015] According to one embodiment of the invention, said upward draft and said suction in said inner cylinder are adjusted so as to balance in the annular space in such a way that a first part of the second combustion fumes is brought, by said upward draft of the annular space, in counter-current to the material to be calcined, and that a second part of the second combustion fumes is brought to the bottom of the annular space, in co-current to the material to be calcined, then inside the inner cylinder, by said suction of the product cooling air, forming a mixture of product cooling air and second combustion fumes, as aforesaid lower gas stream extracted from the inner cylinder and discharged from the furnace.

[0016] According to this embodiment, the furnace has not undergone any internal arrangement, that is to say that it provides, as in conventional annular straight furnaces, a calcination zone in counter-current to the material to be calcined and a calcination zone in co-current with the latter. In the upper gas stream, the CO 2 concentration on dry gas is equal to or greater than 80% by volume on dry gas, in particular greater than 85% by volume on dry gas, and preferably greater than 90% or even 95% by volume on dry gas, and is therefore perfectly captureable. In this embodiment, the lower gas stream, formed from a mixture of product cooling air and CO 2 with a lower CO2 concentration (approximately 40-60% on dry gas by volume), is evacuated separately, for example via a chimney, or reconcentrated into CO2 with a suitable process or even used as such in other industrial processes.In fact, at this lower concentration, which is however higher than that of the gas stream leaving a usual annular straight furnace, the lower gas stream can be used industrially.

[0017] According to another embodiment of the invention, said upward draft and said suction in said inner cylinder are adjusted so as to balance in the annular space in such a way that all of the second combustion fumes introduced into the annular space undergo the above-mentioned upward draft and are incorporated into said upper gas stream, the lower gas stream extracted from the inner cylinder and discharged from the furnace being formed solely by the product cooling air. Such a method is made possible for example by a modification of the mutual location between the return vents allowing suction of the product cooling air inside the inner cylinder and the lower combustion chambers where the lower combustion fumes are produced.According to this embodiment, the lower gas stream is formed solely of product cooling air, the temperature of which has increased significantly upon contact with the calcined material in the cooling zone and all the CO2 produced by the calcination and by the combustion giving rise to the lower combustion fumes is captured in the upper gas stream discharged from the furnace, after its cooling upon contact with the material to be calcined, during the preheating step.

[0018] According to a particular embodiment of the invention, upstream of said combustion giving rise to the second combustion fumes, the method comprises a heat exchange between, on the one hand, said first oxidizing mixture or said portion taken from the upper gas stream and, on the other hand, said lower gas stream, before its evacuation from the furnace. As indicated above, depending on the case, the lower gas stream is formed from a mixture of product cooling air and second combustion fumes or simply product cooling air. This lower gas stream has been heated in contact with the calcined material and advantageously makes it possible, by heat exchange, to regulate to the appropriate temperature the portion of the upper gas stream which will be or is mixed with the oxygen. Indeed, on leaving the furnace, the upper gas stream has been largely cooled in contact with the material to be calcined.The lower its temperature, the more efficient the furnace is, because this means that the preheating of the material to be calcined has been very effective. To allow the combustion of fuel giving rise to the second combustion fumes, it is indeed preferable for the oxidant to reach an appropriate temperature. By this heat recovery, the lower gas stream is also favorably cooled before its evacuation, whether by elimination into the atmosphere, or its reuse for various purposes.

[0019] According to the invention, the dioxygen is mixed with said portion taken from the upper gas stream discharged from the top of the furnace, upstream or downstream of said heat exchange.

[0020] According to another embodiment of the method according to the present invention, this comprises, to form said first combustion fumes, a combustion of a fuel in a second oxidizing mixture of carbon dioxide and pure dioxygen. The usual air-based oxidizer is here also replaced, at the combustion chambers where the first combustion fumes are produced, by a mixture of dioxygen and a gas concentrated in CO 2 . The pure dioxygen can come from an air separation unit or an oxygen tank. These can be the same as those used to supply the lower combustion chambers or be different devices. The gas concentrated in CO 2 , mixed with dioxygen, can come from any source of CO 2 external to the furnace or specific to it. The second oxidizing mixture can also be a gaseous mixture O 2 + CO 2 used previously to cool the casing of the internal cylinder.

[0021] For example, the method may include a withdrawal of an additional portion of the upper gas stream, discharged from the furnace, a mixing of this additional portion of the upper gas stream with pure oxygen to form said second oxidizing mixture, and a combustion of a fuel in this second oxidizing mixture to form said first combustion fumes.

[0022] In this case, no air is used to form the lower combustion fumes or the upper combustion fumes. As a result, the gas stream discharged from the blast furnace is particularly concentrated and therefore perfectly usable or sequestrable.

[0023] According to an advantageous embodiment, the method further comprises a recovery outside the furnace of a gaseous fraction formed from the first combustion fumes and at least part of the second combustion fumes drawn towards the top of the furnace and, upstream of said combustion giving rise to the first combustion fumes, a heat exchange between, on the one hand, this recovered gaseous fraction and, on the other hand, the aforementioned second combustion mixture.

[0024] According to a particular embodiment of the invention, the method comprises cooling the internal cylinder with a gaseous mixture of carbon dioxide and oxygen. After this cooling, this gaseous mixture can serve as an additional oxidant mixture for said combustion of the fuel giving rise to the second combustion fumes. It can also serve as a second oxidant mixture or as an additional oxidant mixture for said combustion of the fuel giving rise to the first combustion fumes.

[0025] According to the method according to the invention, the above-mentioned upper gas stream has a CO2 concentration on dry gas equal to or greater than 80% by volume, in particular greater than 85% by volume, and preferably greater than 90% or even 95% by volume on dry gas.

[0026] Other details and features of the method according to the invention are indicated in the appended claims.

[0027] The invention also relates to a straight annular kiln for calcining lime or dolomite.

[0028] A typical annular straight furnace comprises an outer cylinder, an inner cylinder forming an annular space with the outer cylinder, at the top of the furnace, a feed inlet for introducing into the annular space a calcined limestone or dolomite material, several upper combustion chambers, which are arranged at a first level of the furnace, and in which combustion of a fuel takes place, so as to introduce first combustion fumes into the annular space, several lower combustion chambers, which are arranged at a second level of the furnace lower than said first level and in which combustion of a fuel takes place, so as to introduce second combustion fumes into the annular space, a product cooling air inlet at the bottom of the annular space, a discharge for collecting the calcined lime or dolomite at the bottom of the furnace, upward draft means which discharge out of the top of the furnace, through an outlet duct,an upper gas stream formed from the first combustion fumes and second combustion fumes, return vents provided in the inner cylinder at a third level lower than the aforementioned second level, and suction means which, through said return vents, suck the product cooling air from the annular space into the inner cylinder, forming therein a lower gas stream containing the product cooling air.

[0029] As explained previously, such a furnace has the disadvantage of releasing into the atmosphere large volumes of flue gas containing a high nitrogen content and a low concentration of CO2.

[0030] An annular straight furnace according to the invention further comprises a recirculation circuit which is arranged between the outlet duct of the above-mentioned upper gas stream and said lower combustion chambers and from which a part of the above-mentioned upper gas stream discharged at the top of the furnace is taken, a source of pure dioxygen which communicates with the recirculation circuit and supplies dioxygen to the part of the above-mentioned upper gas stream passing into the recirculation circuit, thereby forming a first oxidizing mixture for said combustion of fuel in said lower combustion chambers, and at least one extraction duct through which said lower gas stream containing the product cooling air is extracted from the internal cylinder and then discharged from the furnace.

[0031] According to one embodiment of the invention, a heat exchanger is arranged in the recirculation circuit so as to allow an exchange of heat between, on the one hand, said first combustion mixture or said portion taken from the upper gas stream passing through this recirculation circuit and, on the other hand, the lower gas stream extracted from the internal cylinder by said extraction conduit.

[0032] The gaseous stream of combustion fumes which leaves at the top of the furnace is cooled during the preheating stage of the material to be calcined. By passing through the aforementioned heat exchanger, the withdrawn and recirculated part of this gaseous stream as well as the dioxygen can be heated to a temperature suitable for combustion in the lower combustion chambers.

[0033] According to a particularly advantageous embodiment of the furnace according to the invention, it further comprises a conduit for supplying a combustion mixture which supplies the upper combustion chambers and which is connected to a source of pure oxygen and to a source of CO 2 and / or to a source of a gaseous mixture O 2 + CO 2 .

[0034] According to a particular embodiment of the invention, the furnace comprises, as a source of CO 2 , a recycling circuit which is arranged between the outlet conduit of the above-mentioned upper gas stream and the conduit for supplying the combustion mixture to the upper combustion chambers and in which an additional portion taken from the above-mentioned upper gas stream passes.

[0035] According to another embodiment, the source of a gas mixture O 2 + CO 2 , said above, is connected, by a supply conduit, to a cooling system of the internal cylinder of the furnace, and then to the conduit for supplying the combustion mixture.

[0036] Advantageously, in the annular straight furnace according to the invention, the combustion mixture supply conduit is arranged so as to supply not only the upper combustion chambers, but also the lower combustion chambers.

[0037] According to an advantageous embodiment of the invention, the furnace comprises, at its top, means for separating a gaseous fraction formed from the first combustion fumes and at least part of the second combustion fumes drawn towards the top of the furnace, a conduit for recovering this separated gaseous fraction and a heat exchanger mounted in the recycling circuit so as to allow a heat exchange between, on the one hand, the separated gaseous fraction passing through the recovery conduit and, on the other hand, the second oxidizing mixture.

[0038] Other details and particularities of the invention will emerge from the description given below, by way of illustration, of exemplary embodiments according to the invention. There figure 1 represents an axial sectional view of a conventional annular straight calcination furnace. The figure 2 schematically represents a usual annular straight furnace similar to that of the figure1 , so as to schematically illustrate its operation. figures 3 à 5 schematically represent several embodiments of annular straight furnaces which have been arranged according to the invention.

[0039] As is clear from the figure 1 , which represents a furnace as described and illustrated in international patent application WO2018 / 002151, a conventional annular straight furnace for calcining limestone or dolomitic rock comprises an outer cylinder 1 and an inner cylinder 2 forming between them an annular space 3 into which the material to be calcined descends. The raw material is introduced from the top of the furnace at the inlet 4 and the cooked product is discharged from the bottom through the discharge 8. The fuel is injected at two levels by means of the burners of several upper 5 and lower 6 combustion chambers (from 4 to 6 chambers depending on the capacity of the furnace). Generally, 1 / 3 of the fuel is injected into chambers 5 and 2 / 3 into chambers 6. All of the fumes from the upper chambers 5 and part of the fumes from the lower chambers 6 are drawn upwards by a draft fan 9, therefore counter to the movement of the material charge, and evacuated through the outlet duct 12.In zone A of the furnace, the material to be calcined is preheated and in zone B, counter-current calcination takes place. The other part of the flue gases from the lower combustion chambers 6 is drawn downwards by a depression created at the level of the return vents 10 provided in the internal cylinder 2, lower than the combustion chambers 6. This is zone C of co-current calcination.

[0040] Product cooling air is introduced through the inlet 7 at the bottom of the annular space 3, to cool the calcined material in the cooling zone D. At the level of the gills 10 the flue gases from the co-current calcination zone C then mix with the product cooling air introduced at the bottom of the furnace at 7. This mixture forms a lower gas stream which, via several recycling ducts 41, is extracted from the inner cylinder 2 by suction means, such as several recirculation eductors 11 (one per lower combustion chamber), and is recirculated in the lower combustion chambers 6 by recirculation ducts 13, and this to serve as combustion gas (see also the schematic illustration of this recirculation on the figure 2 ).

[0041] This furnace further comprises an upper cylinder 23 which serves to separate a fraction of the combustion fumes which rise in the furnace and which directs this gaseous fraction through a recovery duct 26 into a heat recuperator 24. In this, a heat exchange takes place between the recovered gaseous fraction and air supplied by a motor air supply duct 31. This motor air, thus preheated, is brought through a transfer duct 32 to the recirculation eductors 11 to enable them to operate.

[0042] Cooling air is also, through a supply duct 33, introduced into a cooling system of the internal cylinder 2 to cool the latter. Exemple 1 (according to the prior art).

[0043] There figure 2 represents a conventional annular straight furnace schematically.

[0044] For example, this kiln can produce 170 tonnes of lime per day. Limestone is introduced into the kiln through inlet 4 and is calcined to form a product in the form of quicklime powder, which is discharged at inlet 8.

[0045] The fuel introduced into the combustion chambers 5 and 6 may be a solid fuel, such as coke and powdered coal, a biomass, or a fluid fuel, such as natural gas, hydrogen, among others. It is burned in these chambers in the presence of an oxidant. It is brought to the burners of the chambers 5 by a fuel supply conduit14 and to the burners of the chambers 6 by a fuel supply conduit15.

[0046] In the upper combustion chambers 5, the oxidant is air, which, as is known, contains on average 79% by volume of N 2 and 21% by volume of O 2 . This air is, in this exemplary embodiment, the cooling air of the internal cylinder 2 supplied by the supply duct 33, heated during its passage through the cooling system of the internal cylinder 2 and recycled in the form of combustion air to the burners of the combustion chambers 5 and 6 by recycling ducts 34 and 35 respectively.

[0047] The upward draft by conventional draft means 9 draws upwards the combustion fumes from the upper combustion chambers 5 and a portion of the combustion fumes from the lower combustion chambers 6. By suction means, such as the recirculation eductors 11, only one of which is shown, another portion of the combustion fumes from the lower combustion chambers 6 is drawn downwards, then drawn inside the internal cylinder 2 through the return vents 10. The balance between the upward draft and the downward suction in the annular space 3 is represented by a broken line 30. The descending portion of the fumes from the lower combustion chambers 6 typically represents 20% to 40% by volume of these fumes.

[0048] In the lower combustion chambers 6, the oxidant consists of the mixture of combustion fumes drawn downwards and the product cooling air, a mixture which is, using the eductors 11, extracted from the internal cylinder 2 through the recycling conduits 41 and recirculated in the chambers 6 by the recirculation conduits 13. This gaseous mixture contains a low concentration of CO 2 and O 2 , for example approximately 9% by volume of CO 2 and 14% by volume of O 2 , the remainder comprising nitrogen and usual impurities, present in trace form in the fuel.

[0049] Each recirculation eductor 11 operates using the engine air which is supplied by the supply duct 31, preheated in the heat recovery unit 24 and then brought to the eductor 11 by the transfer duct 32.

[0050] The recirculated gaseous combustion mixture in the lower combustion chambers 6 has an approximate temperature of 850°C, which is adequate to ensure proper combustion of the fuel. The flow rate of this recirculated combustion mixture is variable and depends on the furnace setting.

[0051] In order to maintain reasonable temperatures, the upper combustion chambers 5 operate with insufficient air (approximately 50% by volume) and the lower combustion chambers 6 with excess air (approximately 150%).

[0052] In this example, product cooling air is provided at a rate of 800 Nm 3 < / t of lime produced, air which when introduced into the kiln is at ambient temperature. The gas mixture, which is recirculated to the eductors at a temperature of approximately 850°C, represents approximately 1600 Nm 3 < / t of lime and the upper gas stream, which is discharged at the top of the kiln through the outlet duct 12 at a temperature of approximately 250°C, subjected to filtration in a dust filter 16 and released into the atmosphere through the chimney 17, represents 2000 Nm 3 < / t of lime.

[0053] 367 Nm 3< of CO 2 / t of lime are released during the transformation of limestone into lime. 215 Nm 3< of CO 2 / t of lime are produced by the combustion of the fuel, at a rate of 71 Nm 3< in the upper combustion chambers 5 and 144 Nm 3< in the lower combustion chambers 6. In total, this process therefore produces 582 Nm 3< of CO 2 / t of lime which are evacuated in the gas stream passing through the chimney 17. This gas stream contains approximately 30% by volume of CO 2 on dry gas, this CO 2 is therefore difficult to directly sequester or exploit. Exemple 2 (in accordance with the invention)

[0054] There figure 3 represents a usual straight annular furnace, of the type shown in the figure 2 , but arranged according to the invention.

[0055] The same material to be calcined as in Example 1 is introduced into the furnace. The same fuels are used. The combustion chambers are identical. However, the lower combustion chambers 6 have been lowered to a level just above the return vents 10. This minor modification in the interior of the furnace facilitates the operation of the furnace completely in counter-current, with an equilibrium line 30 also lowered. In this furnace there is no longer a co-current calcination zone and all the combustion fumes leaving the lower combustion chambers 6 are drawn upwards like those leaving the upper combustion chambers 5 to together form the upper gas stream.

[0056] A portion of the upper gas stream, discharged at the top of the furnace through the outlet duct 12 at a temperature of approximately 250°C, is, using a suction means 38, taken from a recirculation circuit 18 which leads to the lower combustion chambers 6.

[0057] In the illustrated example, to form a first combustion mixture, this portion of the upper gas stream is mixed with pure dioxygen. This comes from a dioxygen source, such as an air separation unit 19. This CO2 + O2 gas mixture is an equivalent mass mixture of recycled CO2 and O2 coming from the dioxygen source.

[0058] The lower gas stream, drawn into the inner cylinder 2, is formed of only the product cooling air, introduced at ambient temperature through the inlet 7, and passed into the inner cylinder 2 through the return vents 10. This product cooling air is extracted from the inner cylinder 2 using a suction means, such as a suction fan 36, at a temperature of approximately 850°C, then eliminated by an exhaust duct 37 to a chimney 20, passing through a dust filter 21. There are therefore no more recirculation eductors 11, nor any need for motor air for their operation, nor recirculation of the lower gas stream. The cooling air follows, between its entry and its exit from the furnace, a clean circuit separate from the fumes forming the upper gas stream.

[0059] Part of the heat of this lower gas stream is, via a heat exchanger 22, recovered from the CO2 + O2 mixture passing through the recirculation circuit 18. The lower gas stream is then removed to the chimney 2 at a temperature of 200°C, while the combustion mixture has a temperature of 650°C at the outlet of the heat exchanger.

[0060] The upper combustion chambers 5 are, in the example illustrated, supplied with a second combustion mixture similar to the aforementioned first combustion mixture. This can be done by taking it from the recirculation circuit 18 or for greater flexibility by a mixture supplied to measure from an independent system.

[0061] In the example illustrated on the figure 3 , a gaseous mixture CO 2 + O 2 is introduced into the furnace through the conduit 33 to cool the cylinder 2. It then passes into the heat recovery unit 24 before being directed to the burners of the upper and lower combustion chambers through the supply conduit 25, then the recycling conduits 39 and 40 respectively, and thus serves as a second combustion mixture for the upper combustion chambers 5 and as an additional combustion mixture for the lower combustion chambers 6. A heat exchange takes place, as in the usual furnace, with a fraction of the fumes from the upper gas stream separated by an upper cylinder 23.

[0062] If the fuel fed to the burners is a solid fuel, a CO2 + O2 mixture or pure CO2 can also be used as a carrier gas.

[0063] It should be noted that, since the density of CO 2 is higher than that of nitrogen, the volume equivalent CO 2 + O 2 is less than the volume of air replaced by about 30%. This results in a decrease of the same order of magnitude in the volume of recycled CO 2. This reduces the pressure drop of the furnace (about 20%) and compensates for the increase in pressure drop due to the downward movement of the lower combustion chambers 6 (about 15%).

[0064] As in the furnace of Example 1, in this example, 800 Nm 3 < / t of lime of product cooling air is provided at the inlet 7. Also as in Example 1, 367 Nm 3 < of CO 2 / t of lime are released during the transformation of limestone into lime. The same quantities of fuel as in Example 1 are applied to the combustion chambers, 71 Nm 3 < / t of lime in the upper combustion chambers 5 and 144 Nm 3 < / t of lime in the lower combustion chambers 6. And thus the total CO 2 production balance is the same as in the furnace of Example 1, namely 582 Nm 3 < of CO 2 / t of lime.

[0065] However, in the present example 2, the product cooling air introduced into the furnace 7 forms, in its entirety, the effluent of the chimney 20, without having been mixed with process fumes.

[0066] On the other hand, the combustion of the fuel in the upper combustion chambers no longer takes place in air, and the combustion in the lower combustion chambers no longer takes place in a mixture of air and combustion fumes. These combustions now take place in an oxidizing mixture of CO 2 + O 2 . As a result, in the fumes released, the CO 2 is no longer diluted in a large volume of nitrogen.

[0067] In the present example, the upper gas stream, at its outlet from the top of the furnace, represents 1550 Nm 3< / t of lime, 900 Nm 3< / t of lime are taken from it in the recirculation circuit 18 and we therefore obtain 650 Nm 3< / t of lime from a concentrated effluent, where the CO 2 content is greater than 85% by volume on dry gas.

[0068] The gaseous fraction taken from the upper gaseous stream therefore has the same CO2 content and is mixed with 200 Nm3 / t of dioxygen lime, which gives, to the lower combustion chambers 6, a supply of oxidant mixture of 1100 Nm3 / t of lime.

[0069] Under these conditions of concentrated effluent at the outlet of the top of the furnace, 100% capture of the CO2 present becomes possible for sequestration or exploitation. Exemple 3 (in accordance with the invention)

[0070] There figure 4 represents a straight annular furnace similar to a conventional furnace, as shown in the figure 2 , and arranged as in example 2.

[0071] However, in this example 3, instead of moving the lower combustion chambers 6 downwards, the recovery vents 10 have been relocated upwards, just below the level of these chambers. This adaptation of the usual furnace is simpler and therefore less expensive.

[0072] The rest of the process is unchanged compared to the furnace of Example 2. There is no longer a co-current calcination zone; in this furnace, this becomes a cooling zone. As in the furnace of Example 2, the combustion of the fuel takes place in the presence of first and second CO 2 + O 2 oxidizing mixtures, and the effluent obtained at the top of the furnace is very concentrated in CO 2 , at a value greater than 85% by volume on dry gas. Exemple 4 (in accordance with the invention)

[0073] There figure 5 represents a straight annular furnace similar to a conventional furnace, as shown in the figure 2. Here, only the arrangement of the elements external to the furnace is modified in accordance with what was provided in the furnaces according to the invention of examples 2 and 3. On the other hand, the furnace itself is not modified, the lower combustion chambers 6 and the return vents 10 are not relocated and there remains here a co-current calcination zone.

[0074] Therefore, part of the combustion fumes from the lower combustion chambers 6 is mixed with the product cooling air introduced into the kiln at inlet 7. This gas mixture extracted from the inner cylinder 2 and discharged through the chimney 20 is therefore a diluted effluent, because in addition to part of the combustion CO 2 and part of the calcination CO 2, it contains all the lime cooling air. However, thanks to this channeling of the cooling air, it can no longer in any way dilute the gaseous effluent leaving the top of the kiln. The CO 2 content of the lower gas stream, however, remains higher than in the effluents leaving the top of traditional annular straight kilns, approximately 40-60% on dry gas instead of 30%, but it is lower than that obtained in the gas stream discharged at the top of the kiln. This is not diluted in air and contains a CO2 content greater than 85% on dry gas.

[0075] In this embodiment, variants for the supply of CO 2 and O 2 to the combustion mixture supply duct 25 are also shown in broken lines. The oxygen can come from an air separation unit 28, from the air separation unit 19 or from any other source of O 2 . The CO 2 can come from an independent CO 2 source 27. It can also come from the upper gas stream passing through the outlet duct 12, an additional part of which is taken from a recycling circuit 29. Here, it is also possible to provide, as in examples 2 and 3, a CO 2 + O 2 gas mixture to cool the internal cylinder, supply the duct 25, then the combustion chambers 5 and 6.

[0076] It should be understood that the present invention is in no way limited to the embodiments described above and that many modifications may be made thereto without departing from the scope of the appended claims.

[0077] In all embodiments, both that of example 1 of the prior art, and examples 2 to 4 according to the invention, it is possible to envisage a variant where the upper cylinder 23 is absent, which prevents heat recovery in a recuperator 24.

[0078] According to the invention, it can also be provided that the supply of oxygen into the recirculation circuit 18 takes place downstream of the heat exchanger 22.

[0079] Likewise, according to the invention, it is possible to arrange the suction means 38, which serves to take a portion of the upper gas stream in the recirculation circuit 18, in the section of this circuit which is located downstream of the heat exchanger 22 and upstream of the lower combustion chambers 6.

Claims

1. Method of calcining lime or dolomite in a annular shaft kiln having an outer cylinder and an inner cylinder forming an annular space between them, comprising - a supply of limestone or dolomite material to be calcined at the top of the annular space into which it descends, - introducing first combustion fumes into said annular space, at a first level of the kiln, - introducing second combustion fumes into said annular space, at a second level of the kiln lower than the aforementioned first level, - preheating and calcining of the material to be calcined during descent into the annular space, by contact with the first combustion fumes and second combustion fumes which undergo an upward draught, in counter-current to the material to be calcined, and are removed at the top of the kiln in the form of an upper gas stream, - cooling of the calcined material by supplying product cooling air at the bottom of the annular space, - suctioning this product cooling air inside the inner cylinder, at a third level of the kiln lower than the aforementioned second level, forming therein a lower gas stream containing the product cooling air, and - discharging calcined lime or dolomite at the bottom of the kiln, characterised in that it comprises - withdrawing a portion of the upper gas stream, discharged from the kiln, - a formation of a first oxidising mixture by mixing this sampled portion of the upper gas stream with pure dioxygen, - a combustion of a fuel in this first oxidising mixture, to form said second combustion fumes, and - an extraction of the lower gas stream containing the product cooling air from the inner cylinder, followed by its removal from the kiln.

2. Method according to claim 1, characterised in that said upward draught and said suction in said inner cylinder are adjusted so as to be balanced in the annular space in such a way that a first portion of the second combustion fumes is brought, by said upward draught of the annular space, in counter-current to the material to be calcined, and that a second portion of the second combustion fumes is brought down from the annular space, in co-current to the material to be calcined, and then inside the inner cylinder, by said suction of the product cooling air, forming a mixture of product cooling air and second combustion fumes, as aforementioned lower gas stream extracted from the inner cylinder and discharged from the kiln.

3. Method according to claim 1, characterised in that said upward draught and said suction in said inner cylinder are adjusted so as to be balanced in the annular space in such a way that all of the second combustion fumes introduced into the annular space undergo the aforementioned upward draught and are incorporated into said upper gas stream, with the lower gas stream extracted from the inner cylinder and discharged from the kiln being formed solely of product cooling air.

4. Method according to claim 2, characterised in that, upstream of said combustion giving rise to the second combustion fumes, it comprises a heat exchange between, on the one hand, said first oxidising mixture or said portion taken from the upper gas stream and, on the other hand, said lower gas stream formed from the mixture of product cooling air and second combustion fumes, before its removal from the kiln.

5. Method according to claim 3, characterised in that, upstream of said combustion giving rise to the second combustion fumes, it comprises a heat exchange between, on the one hand, said first oxidising mixture or said portion taken from the upper gas stream and, on the other hand, the product cooling air forming said lower gas stream, before its removal from the kiln.

6. Method according to one of claims 1 to 5, characterised in that the upper gas stream and the lower gas stream containing the cooling air undergo filtration.

7. Method according to any one of claims 1 to 6, characterised in that it comprises, to form said first combustion fumes, a combustion of a fuel in a second oxidising mixture of carbon dioxide and pure dioxygen.

8. Method according to claim 7, characterised in that it comprises - withdrawing an additional portion of the upper gas stream, removed from the kiln, - mixing this additional portion of the upper gas stream with pure dioxygen to form said second oxidising mixture, and - a combustion of a fuel in this second oxidising mixture to form said first combustion fumes.

9. Method according to one of claims 7 and 8, characterised in that it further comprises a recovery outside the kiln of a gaseous fraction formed from the first combustion fumes and at least a portion of the second combustion fumes drawn towards the top of the kiln and, upstream of said combustion giving rise to the first combustion fumes, a heat exchange between, on the one hand, this recovered gaseous fraction and, on the other hand, the second aforementioned oxidising mixture.

10. Method according to any one of claims 1 to 6, characterised in that it comprises cooling the inner cylinder by a gaseous mixture of carbon dioxide and pure dioxygen which, after this cooling, serves as an additional oxidising mixture for said combustion of the fuel giving rise to the second combustion fumes.

11. Method according to any one of claims 7 to 10, characterised in that it comprises cooling the inner cylinder by a gaseous mixture of carbon dioxide and pure dioxygen which, after this cooling, serves as a second oxidising mixture or an additional oxidising mixture for said combustion of the fuel giving rise to the first combustion fumes.

12. Method according to any one of the preceding claims, characterised in that the aforementioned upper gas stream has a CO2 concentration in dry gas equal to or greater than 80% by volume, in particular 85% by volume, advantageously 90% by volume, preferably 95% by volume.

13. Method according to one of claims 2 and 4, characterised in that the aforementioned lower gas stream has a CO2 concentration in dry gas equal to or greater than 40% by volume.

14. Annular shaft kiln for calcining lime or dolomite, comprising - an outer cylinder (1), - an inner cylinder (2) forming an annular space (3) with the outer cylinder, - at the top of the kiln, a supply inlet (4) for introducing into the annular space a limestone or dolomite material to be calcined, - several upper combustion chambers (5), which are arranged at a first level of the kiln, and in which a combustion of a fuel takes place so as to introduce first combustion fumes into the annular space, - several lower combustion chambers (6), which are arranged at a second level of the kiln lower than said first level and in which a combustion of a fuel takes place, so as to introduce second combustion fumes into the annular space, - a product cooling air inlet (7) at the bottom of the annular space, - an unloading device (8) for collecting the calcined lime or dolomite at the bottom of the kiln, - upward draught means (9) which discharge from the top of the kiln, through an outlet duct (12), an upper gas stream formed of the first combustion fumes and second combustion fumes, - return vents (10) provided in the inner cylinder (2) at a third level lower than the aforementioned second level, and - suction means which, through said return vents (10), draw the cooling air from the annular space into the inner cylinder (2) forming therein a lower gas stream containing the product cooling air, characterised in that it further comprises - a recirculation circuit (18) which is arranged between the outlet duct (12) of the aforementioned upper gas stream and said lower combustion chambers (6) and from which a portion of the aforementioned upper gas stream removed at the top of the kiln is taken, - a source of pure dioxygen (19) which communicates with the recirculation circuit and supplies dioxygen to the portion of the aforementioned upper gas stream passing through the recirculation circuit (18), thereby forming a first oxidising mixture for said fuel combustion in said lower combustion chambers (6), and - at least one extraction duct (37) from which said lower gas stream containing product cooling air is extracted from the inner cylinder (2) and then removed from the kiln.

15. Annular shaft kiln according to claim 14, characterised in that a heat exchanger (22) is arranged in the recirculation circuit (18) so as to allow a heat exchange between, on the one hand, said first combustion mixture passing through this recirculation circuit (18) or said portion taken from the upper gas stream and, on the other hand, the lower gas stream extracted from the inner cylinder (2) by said extraction duct (37).

16. Annular shaft kiln according to one of claims 14 and 15, characterised in that it further comprises a combustion mixture supply duct (25) which supplies the upper combustion chambers (5) and which is connected to a source of pure dioxygen (27) and to a source of CO2 (28, 29) and / or to a source of a gas mixture O2 + CO2.

17. Annular shaft kiln according to claim 16, characterised in that it comprises, as a source of CO2, a recycling circuit (29) which is arranged between the outlet duct (12) of the aforementioned upper gas stream and the duct for supplying the combustion mixture (25) to the upper combustion chambers (5) and in which an additional portion taken from the aforementioned upper gas stream passes.

18. Annular shaft kiln according to claim 16, characterised in that the aforementioned source of a gas mixture O2 + CO2 is connected, by a supply duct (33), to a cooling system of the inner cylinder (2) of the kiln, and then to the duct for supplying the combustion mixture (25).

19. Annular shaft kiln according to one of claims 16 to 18, characterised in that the duct for supplying the combustion mixture (25) is arranged so as to also supply the lower combustion chambers (6).

20. Annular shaft kiln according to one of claims 16 to 19, characterised in that it comprises at the top of the kiln separation means (23) for a gaseous fraction formed of the first combustion fumes and at least part of the second combustion fumes drawn towards the top of the kiln, a recovery duct (26) for this separated gaseous fraction and a heat exchanger (24) mounted in this recovery duct so as to allow a heat exchange between, on the one hand, the separated gaseous fraction passing through the recovery duct (26) and, on the other hand, the oxidising gas mixture passing through the oxidising mixture supply duct (25).