METHOD FOR CALCINING MINERAL ROCK IN A REGENERATIVE PARALLEL-FLOW VERTICAL FURNACE AND FURNACE FOR THIS PURPOSE

Description

[0001] The present invention relates to a method for calcining carbonate mineral rock in a regenerative parallel flow vertical straight furnace and to the furnace used.

[0002] The vertical parallel flow regenerative kiln or regenerative lime kiln known as "PFRK" (Parallel Flow Regenerative Kiln) has an energy efficiency of 85% to 90%; this is the highest in the lime sector, or even in the entire energy-intensive industry sector: cement, steel, glass, etc. 60% of lime in Europe is produced in this type of kiln. This proportion is set to increase in Europe and worldwide, taking into account the roadmaps for the energy and ecological transition.

[0003] The "PFRK" kiln is a straight kiln with double vertical shafts where the fuel is injected alternately into one shaft and then into another for approximately 10 to 15 minutes with a rest period between cycles of approximately 1 minute to 2 minutes to reverse the air and fuel circuits. This is the "inversion" period. The two shafts are connected by a connecting flue. When one shaft is in combustion (firing mode), the hot combustion fumes pass through the connecting flue (gas transfer path) and give up part of their heat to the mineral rock to be calcined to preheat it in the other shaft, called in regeneration or preheating mode. The shafts of the PFRK kiln are either cylindrical or rectangular. In some cases, there are three shafts, two in preheating and one in firing. The problems and solutions listed below are valid for all PFRK kiln geometries.

[0004] The process implemented in these known furnaces includes, in production mode, a loading of carbonate mineral rock at the top of the furnace, a preheating of this rock, a firing of this rock with its decarbonation into calcined material, a cooling of the calcined material by cooling air, and an unloading of the calcined material at the bottom of the tanks, each tank operating alternately in firing mode and in preheating mode, one tank being in firing mode for a predetermined period of time while at least one other tank is in preheating mode, and vice versa, the firing mode comprising: said loading of carbonate mineral rock at the top of the tank in firing mode, in the presence of said preheated carbonate mineral rock being lowered into this tank,a combustion of fuel in the presence of oxygen so as to obtain said cooking of this rock and its decarbonation into calcined material with release of combustion fumes in the form of a gaseous current descending in co-current in the tank in cooking mode, and a passage of said gaseous current containing these combustion fumes from the tank in cooking mode to said at least one tank in preheating mode via said gas transfer path, the preheating mode comprising: said preheating of the charged carbonated mineral rock by heat exchange with the gaseous current containing the combustion fumes, coming from the gas transfer path, which is ascending in said at least one tank in preheating mode, in counter-current of said charged carbonated mineral rock, and an evacuation from the furnace of a gaseous effluent based on the gaseous current containing the combustion fumes, at the top of said at least one tank in preheating mode.

[0005] For the purposes of the present invention, carbonate mineral rock is understood to mean in particular limestone, dolomite and / or magnesite which are calcined into quicklime, quickdolomite and / or magnesia respectively. The calcination equation for limestone into lime is as follows: CaCO 3 (solid) + heat ←→ CaO (solid) + CO 2 (gas)

[0006] It is a reversible endothermic reaction and lime recombines with CO2 at the first opportunity below 900°C, with equilibrium and kinetics more or less rapid depending on the temperature and the ambient concentration of CO2.

[0007] During this process, the initial limestone or dolomite rock 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.

[0008] This very common calcination 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 effluent with a high level of diatomic nitrogen, and a comparatively low level of CO 2 (volume concentration of the order of 20% to 27% on dry gas), which is expensive to capture due to the high presence of nitrogen from the air used.

[0009] To capture this CO2, we could consider using the chemical solvent abatement process called "amines", which is the most widespread technique and applied to furnace fumes at the end of the line, after the dust removal filter. But the cyclical nature of the PFRK furnace, with a furnace shutdown of 1 to 2 minutes every 10 to 15 minutes, is incompatible with this technique, which is also very expensive and uses a solvent that is not sustainable in relation to environmental legislation.

[0010] To capture the CO 2 emitted in a PFRK kiln, it has already been proposed to replace, in the process implemented, all the process air, combustion air carrying the solid fuel and cooling air, with recycled combustion fumes and to introduce pure oxygen into the tank in firing mode (see CN105000811). For any person skilled in the art, it is clear that this process is impractical, because the lime will recarbonate during cooling. As seen above, the CO 2 cannot be recirculated to cool the lime, because it will immediately recombine with this CO 2 to reform CaCO 3 . On the other hand, the use of pure oxygen at the kiln head would pose serious problems in terms of material compatibility and this supply will not constitute a sufficient mass flow to effectively recover the heat accumulated in the regeneration zone.The disadvantages and feasibility problems of this process have already been discussed in patent application US2020 / 0048146.

[0011] It should also be noted that the cooling air in the PFRK kiln, unlike the rotary kiln for example, has no direct impact on the combustion and calcination process in the tank during firing mode. It has no expected effect on product quality.

[0012] Production mode means that the furnace is in its normal operation during which it continuously produces calcined material. This mode therefore does not concern the phases of starting the furnace, stopping it or maintenance in the event of a malfunction.

[0013] The present invention aims to at least partially remedy the problem of significant CO 2 emissions into the atmosphere from PFRK type furnaces, without modifying their cyclic operation and by making little or no modifications to their structure. It therefore also aims to enable capture of the CO 2 present in the gaseous effluents emitted by the furnace.

[0014] To solve this problem, according to the invention, a method of calcining mineral rock in a vertical straight furnace with regenerative parallel flows as indicated at the beginning has been provided, this method further comprising a sampling of a portion of the gaseous effluent discharged from the furnace, a formation of an oxidizing mixture by mixing this portion taken from the gaseous effluent discharged from the furnace with concentrated oxygen, and an introduction of this oxidizing mixture at the top of the tank in cooking mode so as to ensure said combustion of fuel in the presence of oxygen,

[0015] the gaseous effluent discharged from the furnace being concentrated in CO2.

[0016] Combustion of the fuel in concentrated oxygen would result in flame temperatures that are too high for conventional furnace equipment. Therefore, the invention provides for the removal of part of the CO2-rich gaseous effluent and mixing it with oxygen. Instead of a conventional oxidant consisting of the O2 + N2 mixture from air, an O2 + CO2 mixture with an appropriate flame temperature is thus obtained.

[0017] The combustion of the fuel in oxygen gives rise to the gas stream containing the combustion fumes and to the calcination of the carbonate rock. This has the effect of producing mainly CO 2 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. These combustion fumes naturally also include the CO 2 brought into the combustion mixture. This obviously results, compared to the conventional process, in a drastic increase in the CO 2 content of the gaseous effluent released at the top of the furnace. By gaseous effluent concentrated in CO 2 , it is meant according to the invention that it has a CO 2 content of at least 60%, in particular at least 80%, and particularly advantageously at least 90% by volume on dry gas.This CO2 thus becomes usable or sequestrable under favorable conditions, which makes it possible to radically reduce the contribution to the greenhouse effect of the furnace.

[0018] Implementing this process does not necessarily require any special adjustments to the furnace itself. The only modifications that need to be made to it may simply be external to the furnace and consist of changing the effluent circuits leaving the furnace and providing at least one source of concentrated oxygen.

[0019] Concentrated oxygen (also referred to as oxygen hereinafter) is understood to mean, according to the invention, a gas whose oxygen content exceeds 50% by volume. It will preferably be equal to or greater than 90%, in particular 93%, advantageously 98 to 100% by volume. The source of concentrated oxygen may, for example, be an air separation unit which separates the air into oxygen and nitrogen and which operates in parallel with the furnace, or a tank of oxygen installed next to the furnace. Advantageously, the combustion of fuel takes place in the presence of an excess of oxygen, preferably of the order of 5 to 50%, in particular 10 to 30%, advantageously 15 to 25% by volume relative to the stoichiometric requirements of the combustion.

[0020] 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. Advantageously, in the case of a solid fuel, its introduction into the tank in cooking mode is carried out in a granular or powdered form using a portion of said part taken from the gaseous effluent discharged from the furnace, as a carrier gas. CO 2 from any other source may also be provided as a carrier gas.

[0021] According to one embodiment of the invention, the cooling of the calcined material comprises, at the bottom of each of the tanks, a cooling air supply which crosses in counter-current the descending calcined material and is heated upon contact with it, the heated cooling air mixing, in the tank in cooking mode, with the gaseous stream containing the combustion fumes, before its passage through the gas transfer path, and, in said at least one tank in preheating mode, with this gaseous stream after this passage, the gaseous effluent concentrated in CO 2 which is evacuated from the furnace containing the combustion fumes and the cooling air. In this case, only the combustion air of the conventional process is replaced by the oxidizing mixture based on the gaseous effluent concentrated in CO 2 , evacuated from the furnace, and oxygen.Such a process makes it possible to increase the CO 2 content of the gaseous effluent discharged from the conventional PFRK furnace from 20 to 27% by volume on dry gas to a value of at least 35%, advantageously at least 45%, and even up to 65% by volume on dry gas in a furnace according to the invention. For illustration, a PFRK furnace implementing this process can advantageously replace the coke oven currently used in soda makers, to provide fumes with 40% by volume of CO 2 . Furthermore, the PFRK is a “sustainable” furnace, energy-efficient, and above all it eliminates all the environmental problems linked to coke ovens, including significant emissions of pollutants (CO, NH 3 , H 2 S, etc.).

[0022] According to a particular embodiment of the invention, the cooling air is brought to the furnace in a total volume equivalent to or less than a thermodynamic minimum necessary to cool the calcined material to a reference temperature of 100°C. Advantageously, the total volume of cooling air brought to the furnace may be of the order of 40 to 60% of said thermodynamic minimum, preferably equal to 50% thereof. In this case, the discharged product will have a temperature higher than normal operation. It will then be necessary to adapt the unloading equipment to materials resistant to this temperature.

[0023] It is also advantageous to provide that said cooling of the calcined material comprises, at the bottom of the single tank in firing mode, a cooling air supply which crosses in counter-current the descending calcined material and is heated in contact with it, the heated cooling air mixing with the gaseous stream containing the combustion fumes, before its passage through the gas transfer path, and the CO2-concentrated gaseous effluent which is discharged from the furnace containing the combustion fumes and the cooling air. In this case too, the cooling air can be supplied to the furnace in a total volume less than a thermodynamic minimum necessary to cool the calcined material to a reference temperature of 100°C. Advantageously, the total volume of cooling air supplied to the furnace can thus be of the order of 40 to 60% of said thermodynamic minimum, preferably equal to 50% thereof.

[0024] According to a particularly advantageous embodiment of the invention, said cooling of the calcined material comprises, at the bottom of each of the tanks or at the bottom of the single tank in firing mode, a supply of cooling air which crosses in counter-current the descending calcined material and is heated in contact with it, the method further comprising an extraction from the furnace of the heated cooling air, the gaseous effluent discharged from the furnace containing a CO 2 content of at least 90% by volume on dry gas, preferably at least 95%. In this case the gaseous effluent discharged from the furnace is formed almost exclusively from combustion fumes. It becomes possible to use such a gas in specialized industries or to sequester it.

[0025] According to a particular embodiment of the invention, the method further comprises a heat exchange between the heated cooling air, extracted from the furnace, and said portion taken from the gaseous effluent discharged from the furnace, before or after its mixing with concentrated oxygen. This allows heat recovery in the combustion mixture to be introduced into the tank in cooking mode.

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

[0027] The present invention also relates to a regenerative parallel flow vertical straight furnace, of the PFRK type.

[0028] Such an oven includes at least two tanks interconnected by a gas transfer path, each of said tanks comprising, in the in-service or out-of-service position, at least one fuel supply device, at least one oxidant supply opening containing oxygen for the combustion of the fuel, an inlet for loading a carbonate mineral rock, at the top of the tanks, an outlet for unloading calcined material produced, at the bottom of the tanks, a gaseous effluent discharge conduit at the top of the tanks, which is connected to a chimney, and a cooling air supply for cooling the calcined material produced, the furnace comprising a system for reversing the operation of the tanks, arranged so that each tank operates, in production mode, alternately in cooking mode and in preheating mode, one tank being in cooking mode for a predetermined period of time while at least one other tank is in preheating mode, and vice versa,this inversion system controlling for this purpose the said positions in service and out of service.

[0029] According to the invention, the oven further comprises a recirculation circuit which is arranged between the aforementioned gaseous effluent discharge conduit of the tanks and said oxidant supply openings of the tanks and in which said inversion system controls a sampling of at least a portion of gaseous effluent from a tank in preheating mode, and a source of concentrated oxygen which communicates with the recirculation circuit to supply it with concentrated oxygen and thus form an oxidant mixture, said oxidant supply opening of the tank in cooking mode being brought into the operating position by said inversion system to ensure combustion of the fuel.

[0030] As explained above, the PFRK furnace has a cyclic operation, with each tank operating for a predetermined period of time in firing mode, then, after a 1-2 minute reversal time, in preheating mode, and so on. During the reversal time, the reversal system synchronously controls all the changes necessary to switch from one mode to the other, for example, by opening the lances of the fuel supply device in the tank when it is in firing mode and closing them when it switches to preheating mode. The reversal system therefore controls not only numerous valves and dampers, but also the operation of the loading and unloading equipment or that of various suction, pumping or injection elements.

[0031] As can be seen, the furnace according to the invention only has a few structural modifications made to the exterior of the furnace. Existing furnaces can therefore be easily adapted to implement a calcination process according to the invention.

[0032] According to the invention, the tanks are of circular section, said gas transfer path is a connecting flue which connects peripheral channels arranged around each tank so as to allow a gas transfer and, below the connecting flue, the tanks are provided with a collector ring communicating with a draw-off element so as to allow extraction from the heated cooling air furnace, the circular tanks further advantageously comprise, at the bottom of the tank, a central collector element communicating with a draw-off element so as to allow extraction from the heated cooling air furnace, below the connecting flue;or the tanks are of rectangular section, a first side of one tank facing a first side of the other neighboring tank and each tank having a second side which is opposite those which face each other, the transfer path for gas being a connecting flue which directly connects one tank to the other by their first sides, and, below the connecting flue, said first sides and said second sides of the tanks are each provided with a collecting tunnel communicating with a draw-off element so as to allow extraction from the furnace of heated cooling air.;

[0033] According to one embodiment of the invention, the furnace comprises, as a source of oxygen for the recirculation circuit, an air separation unit capable of separating air into oxygen and nitrogen. An oxygen tank can also be provided. Advantageously, a heat exchanger supplied with heated cooling air, extracted from the furnace, is mounted on the recirculation circuit to heat the aforementioned combustion mixture before it is supplied to the tank in cooking mode.

[0034] Other details and features of the oven according to the invention are indicated in the appended claims.

[0035] Other features of the invention will also emerge from the description given below, without limitation and with reference to the attached drawings. There figure 1 schematically represents a classic PFRK oven. The Figures 2a and 2brepresent a numerical modeling of the mass% concentration of oxygen in the gas streams in a conventional circular cross-section PFRK furnace and in a conventional rectangular cross-section PFRK furnace. The figures 3 And 4 schematically illustrate several embodiments of a circular section furnace according to the invention. The Figure 5 is a broken representation of an embodiment of a rectangular section furnace according to the invention.

[0036] In the figures, identical or similar elements bear the same references. Conventionally, the tank illustrated on the left is in cooking mode and the tank illustrated on the right in preheating mode. To avoid cluttering the drawings with standard elements, such as loading or unloading equipment, they are not shown or are shown very schematically.

[0037] As can be seen in the figure 1, the PFRK kiln illustrated is a straight kiln with double shafts 1, 2 where the fuel is injected alternately into one shaft 1 then into another 2 for about 12' with a stopping period between cycles of 1' to 2' to reverse the circuits. This is the "inversion" period. The two shafts have a circular section and are provided with peripheral channels 13 which are interconnected by a connecting flue 3. The shafts are divided in height into three zones, the preheating zone A where the carbonate rock is preheated before calcination, the combustion zone B where the carbonate rock is fired and the cooling zone C where the calcined material is cooled.

[0038] When a tank is in cooking mode, here tank 1, a fuel supply device in the form of lances 4 injects a fuel 9 into the tank, which, in the example illustrated, is natural gas. The carbonate rock, loaded at the top of the tank through an inlet 5 in the open position, gradually descends into it. Combustion air is introduced at the top of the tank through an inlet opening 6, which allows combustion of the fuel at the outlet of the lances 4 and decarbonation of the carbonate rock into calcined material 10. The gas stream 11 formed by the combustion and decarbonation descends in co-current with the calcined material and, via the peripheral channel 13, passes into the connecting flue 3. Cooling air is, through a supply duct 7, introduced at the bottom of the tank, in counter-current with the calcined material, to cool it.The heated cooling air 12 mixes with the gas stream containing the combustion fumes 11 to pass into the connecting flue 3. The calcined material is discharged through the outlet 8 into an unloading equipment 24.

[0039] When a tank is in preheating mode, here tank 2, the fuel supply device is closed and the lances 4 are therefore out of service. The same applies to the inlet 5 for the carbonate rock and the opening 6 for the supply of combustion air. On the other hand, the supply duct 7 for the cooling air and the outlet 8 for the calcined material remain in the open position. After heat exchange with the descending calcined material 10, the heated cooling air mixes with the gas stream 11 which, coming from the connecting flue 3, reaches the tank via the peripheral channel 13. This gas stream 11 progresses to the top of the tank where it is evacuated from the furnace by an evacuation duct 14 and transferred to a chimney 15. In the tank in firing mode 1, this evacuation duct 14 is closed.

[0040] The furnace also includes a reversing system 16 shown schematically. It controls the operation of the tanks in a synchronized manner, during the time of reversal of the tanks, and this directly or remotely. It controls the activation or deactivation of all the elements of the furnace so that, in production mode, each tank operates alternately in cooking mode and in preheating mode.

[0041] In some cases there are three tanks, two for preheating and one for combustion.

[0042] There figure 1 shows a kiln designed for the production of 430 tonnes of lime per day. All gas flow rates mentioned below are expressed in Nm 3 / t of lime produced.

[0043] In order to react with the gas injected as fuel into tank 1, 1120 Nm 3 < / t of combustion air are used so as to obtain an excess of air of 19% by weight compared to the stoichiometric requirements, and so as to form a volume of 100 Nm 3 < / t of CO 2 during combustion. The mass concentration of oxygen in the incoming gas is 23%, since it is air. The temperature reached is then well above 900°C, which leads to decarbonation of the limestone rock with a release of 380 Nm 3 < / t of CO 2 . To cool the lime produced to a temperature of around 100°C, 290 Nm 3 < / t of cooling air are introduced through the bottom of the two tanks, which makes a total of 580 Nm 3 < / t. At the chimney we obtain 2250 Nm 3 < / t of gaseous effluent which contains 480 Nm 3 < / t of CO 2 , that is to say that this gaseous effluent has a CO 2 content of 23% on dry gas.At this low content, CO2 is difficult to exploit or sequester and the gaseous effluent is therefore completely released into the atmosphere.

[0044] There Figure 2a represents a numerical model of the PFRK furnace with a circular section showing the path of the gases as a function of their oxygen content. It only shows the combustion zone B, from the end of the lances, and the cooling zone C, and therefore the top of the tanks is not represented.

[0045] Zones a: in the tank in cooking mode, cooling air (at the bottom) and combustion air (at the top, just above the end of the lances) with an O 2 content of 23% by weight.

[0046] Zones b: jets of combustion fumes emitted by the lances, in which there is almost no oxygen left and between which we can still find a little unreacted O2.

[0047] Zone c: the fumes penetrate deep into the cooling zone C, gradually mixing with the cooling air. They push the gas mixture peripherally into the peripheral channel 13, then the connecting flue 3.

[0048] Zone d: in the tank in preheating mode, cooling air.

[0049] Zone e: mixing between the gas stream from the peripheral channel 13 and the cooling air. The further one progresses towards the centre of the tank, the more the residual O2 content increases.

[0050] There Figure 2b represents such a numerical model on a PFRK furnace whose tanks have a rectangular section. Here the distribution of gas flows is no longer symmetrical as in the case of circular tanks.

[0051] There figure 3is a view of a furnace according to the present invention. In this case, there is no modification of the structure of the furnace. On the outside, a separation member 17 is provided on the discharge duct 14, capable of taking a portion of the gaseous effluent discharged from the furnace and introducing it into a recirculation circuit 18. In this circuit, the portion of gaseous effluent taken is advantageously treated in a treatment unit 19, where it can, for example, be filtered and / or dried. An air separation unit 20 separates air supplied by the duct 21 into N 2 discharged by the duct 22 and into O 2 brought to the recirculation circuit 18 by the supply duct 23. This circuit 18 then conducts the oxidizing mixture formed from the recirculated fraction of gaseous effluent and O 2 concentrated at the top of each of the tanks to the supply opening 6.

[0052] The operation of the oven of the figure 3is similar to that of a PFRK oven. The separation member 17 is in continuous operation, as are the treatment unit 19 and the air separation unit 20. As already seen, the reversing system 16 closes the exhaust duct 14 at the top of the tank in cooking mode. On the other hand, it opens, at the top of this tank, the supply opening 6 to allow the introduction of the combustion mixture, while it is closed at the top of the tank in preheating mode.

[0053] The same quantity of carbonate rock and the same flow rates of fuel and cooling air are used as in the conventional furnace described above. 830 Nm 3 < / t of gaseous effluent discharged from the furnace, rich in CO 2 , are taken through the recirculation conduit 18. 160 Nm 3 < / t of O 2 are mixed with this recirculated effluent, so as to maintain the same mass concentration of 23% of O 2 in the combustion mixture thus formed and to obtain during combustion the same excess of oxygen of 19% by weight compared to the stoichiometric requirements. The nitrogen N 2 in the combustion air is thus replaced by its mass equivalent of CO 2 . As it is heavier than nitrogen (specific weight of 1.977 compared to 1.25 g / Nm 3< ), the total volume of fumes decreases in the furnace, which causes a reduction in the pressure drop of 13% compared to the conventional furnace.At the chimney there is a release of 1240 Nm 3 < / t of gaseous effluent which now contains 43% by volume on dry gas of CO 2 . At this content, as explained above, industrial exploitation becomes possible, for example in soda makers.

[0054] As a variant in such a furnace according to the invention, it is possible, to further reduce the air supply in the process, to reduce the cooling air flow rate. For example, this supply can be reduced by 50%, which makes 290 Nm 3 < / t of cooling air. This reduced volume can be introduced through the supply duct 7 of the single tank in firing mode or by using the supply ducts 7 of both tanks. This measure has the effect of reducing the dilution of the fumes by 50%. This results in less cooling of the calcined material which is evacuated through the outlets 8. It then becomes necessary to provide unloading equipment which can withstand a temperature higher than 100°C, for example a refractory steel unloading table and steel drag chains.As the lime comes out hotter, there was therefore less heat recovery by the cooling air, which is compensated by a slight increase in the fuel supply at a flow rate such that it causes the formation of 120 Nm 3 < / t of CO 2 during combustion. This increase in turn requires, in the gaseous effluent discharged from the kiln of 1730 Nm 3 < / t, an adaptation of its withdrawal in the recirculation circuit to 865 Nm 3 < / t and a mixing of this withdrawn effluent with 200 Nm 3 < / t of O 2 so as to maintain in the combustion mixture thus formed the same mass concentration of 23% of O 2 and to obtain during combustion the same excess of oxygen of 19% by weight compared to the stoichiometric requirements. At the chimney, only 865 Nm 3 < / t of gaseous effluent is obtained with a high CO 2 content of 63% by volume on dry gas.

[0055] In fact, a tailor-made CO2 concentration between 40% and 65% by volume of CO2 can be established at the stack by adjusting the amount of cooling air between 100% and 50% of the minimum thermodynamic volume required to cool the calcined material to a reference temperature of 100°C. A higher CO2 concentration could be achieved by reducing the cooling air supply below 50%, within the limit of compatibility of the lime temperature with the high-temperature unloading and transport system installed for this.

[0056] There figure 4 is a view of an advantageous furnace according to the present invention. As can be seen, this embodiment includes the features of the embodiment according to the figure 3 , but, in addition, it involves a slight transformation at the level of the external structure of the oven.

[0057] The aim here is to extract the cooling air heated by contact with the calcined material, by installing an extraction system. The tanks 1 and 2 are each provided, below the connecting flue 3 and the peripheral channels 13, with a collector ring 25 which communicates with a extraction element 26 so as to allow extraction from the furnace of the heated cooling air. In this way, part or all of the combustion air can be extracted, if necessary also extracting a small proportion of the combustion fumes. Indeed, as shown in Figure 2a, because the descending gases penetrate deep into the cooling zone C, the cooling air is pushed towards the outer walls of the furnace where the collector ring is arranged. The tanks may further comprise, at the bottom, a central collector element 27 communicating with the draw-off element 26 so as to also allow a central extraction of heated cooling air, below the connecting flue 3.

[0058] In the case of rectangular furnaces, it is also possible to extract the cooling air without a collector ring, by lateral intakes. As can be seen from Fig. 5, each tank has 4 sides. One side 28 of a tank faces a side 29 of the neighboring tank and each tank has a second side 30 and 31 respectively which is opposite those which face each other. The gas transfer path is a connecting flue 3 which directly connects one tank to the other by their sides 28 and 29. Below the connecting flue, the sides 28 to 31 are each provided with a collecting tunnel 32 to 35 respectively communicating with a withdrawal element 26 so as to allow extraction from the furnace of heated cooling air.

[0059] As the distribution of gas currents in a rectangular shaft furnace is not symmetrical (see Figure 2b), the cooling air is only pushed by the hot fumes to one side. Also, in the furnace shown where tank 1 is in baking mode and tank 2 in preheating mode, the reversing system 16 only opens the collecting tunnels 32 and 34. During the following cycle, only the collecting tunnels 33 and 35 will be open.

[0060] In the oven shown on the figure 4, the same quantity of carbonate rock and the same cooling air flow rates are used as in the conventional furnace described above. The heated cooling air is extracted from the furnace by the draw-off element 26. In the tank 1, a fuel introduction is carried out such that 105 Nm 3 < / t of CO 2 is formed during combustion. 1330 Nm 3 < / t of gaseous effluent is evacuated at the top of the tank 2. 730 Nm 3 < / t of this evacuated gaseous effluent, rich in CO 2 , is withdrawn via the recirculation conduit 18. 220 Nm 3 < / t of O 2 are mixed with this recirculated effluent, so as to maintain the same mass concentration of 23% of O 2 in the combustion mixture thus formed and to obtain during combustion the same excess of oxygen of 19% by weight compared to the stoichiometric requirements. Thus, only 600 Nm 3 < / t of gaseous effluent with a content of 96% on dry gas of CO 2 is obtained at the chimney.

[0061] In the oven shown on the figure 4 , to recover part of the energy from the hot air extracted by the extraction element 26, it is possible to provide a heat exchange with the part of gaseous effluent recirculated by means of a heat exchanger 36, before or after its mixing with concentrated oxygen.

[0062] It is also possible to provide, in addition, in the connecting flue 3 and the peripheral channels 13, an injection of a fraction of said part taken from the gaseous effluent discharged from the furnace via an injection conduit 37. Optionally, beforehand, a heat exchange between the heated cooling air, extracted from the furnace, and this aforementioned fraction to be injected can take place using a heat exchanger, for example the heat exchanger 36. In the absence of the latter, a heat exchanger not shown can be provided on the injection conduit 37.

[0063] According to yet another variant, the temperature in the connecting flue can be mitigated by injecting water at selected locations in the flue and / or the peripheral ring. This added water has no diluting effect on the CO2 concentration in dry gas.

[0064] Such arrangements for recovering heat from the heated cooling air extracted from the furnace using a heat exchanger, as well as such devices for injecting CO2 or water into the connecting flue, can of course also be provided with rectangular shaft furnaces.

[0065] It is obvious that one can also design an oven similar to the one shown in the figure 4 where the cooling air is injected at the bottom of only one of the two tanks.

[0066] Table 1 below shows the flow rates in a conventional furnace and in different furnace variants according to the invention, and Table 2 shows the quantities of the different gaseous elements at the furnace inlet.

[0067] In the examples column, 1 means a classic PFRK oven, 2 and 3 are ovens according to the figure 3 with variable cooling air flow rates and 4 and 5 are ovens according to the figure 4 with and without heat exchanger. Table 1 Combustion air Nm 3< / t Cooling air Nm 3< / t Injection of O 2 Nm 3 < / t Recycled effluent Nm 3< / t Upper effluent from the furnace Nm 3< / t Chimney effluent Nm 3< / t DP* 1 1120 580 0 - 2250 2250 DP 0 2 0 580 160 830 2070 1240 -13% 3 0 290 200 865 1730 865 -26% 4 0 580 220 730 1330 600 -49% 5 0 580 250 820 1440 620 -39% *DP = pressure loss Table 2 Injection of O 2 Nm 3 < / t O 2 kg / t N 2 kg / t CO 2 kg / t O 2% by mass Stoichiometric O 2 kg / t Excess O 2% 1 0 333 1090 0,89 23 281 19 2 160 331 448 646 23 278 19 3 200 367 277 915 23 303 18 4 220 346 5 1143 23 294 18 5 250 394 6 1284 23 327 20

[0068] It is 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.

[0069] For example, it would be advantageous to replace the air-cooled fuel injection lances with thermally insulated lances.

Claims

1. Method for calcining mineral rock in a parallel-flow regenerative vertical shaft kiln, wherein at least two shafts are interconnected by a gas transfer channel, this method comprising, in production mode, - loading carbonate mineral rock at the top of the kiln, - preheating this rock, - firing this rock with its decarbonation into calcined material, - cooling the calcined material by cooling air, and - unloading the calcined material at the bottom of the shafts, - each shaft operating alternately in firing mode and in preheating mode, one shaft being in firing mode for a predetermined period of time while at least one other shaft is in preheating mode, and vice versa, - the firing mode comprising: said loading of carbonate mineral rock at the top of the shaft in firing mode, in the presence of said preheated carbonate mineral rock, during descending into this shaft, a combustion of fuel in the presence of oxygen so as to obtain said firing of this rock and its decarbonation into calcined material with the release of combustion fumes in the form of a gaseous current descending in a co-current manner in the shaft in firing mode, and passage of said gaseous current containing these combustion fumes from the shaft in firing mode to said at least one shaft in preheating mode via said gas transfer channel, - the preheating mode comprising: said preheating of the loaded carbonate mineral rock by heat exchange with the gas stream containing the combustion fumes, coming from the gas transfer channel, which is ascending in said at least one shaft in preheating mode, in a counter-current manner of said loaded carbonate mineral rock, and discharging from the kiln of a gaseous effluent originating from the gas stream containing the combustion fumes, at the top of said at least one shaft in preheating mode, characterised in that it further comprises - collecting a part of the gaseous effluent discharged from the kiln, - formation of an oxidising mixture by mixing said part collected from the gaseous effluent discharged from the kiln with concentrated dioxygen, and - an introduction of this oxidising mixture at the top of the shaft in firing mode so as to ensure said combustion of fuel in the presence of oxygen, the gaseous effluent discharged from the kiln being concentrated in CO2 and having a CO2 content of at least 60%, in particular at least 80%, and particularly advantageously at least 90% by volume for dry gas.

2. Method according to claim 1, characterised in that said cooling of the calcined material comprises, at the bottom of each of the shafts, a cooling air supply which passes in a counter-current manner through the descending calcined material and is heated upon contact therewith, in that the heated cooling air mixes, in the shaft in firing mode, with the gas stream containing the combustion fumes, before it passes through the gas transfer channel, and, in said at least one shaft in preheating mode, with this gas stream after this passing-through and in that the gaseous effluent concentrated in CO2 which is discharged from the kiln contains the combustion fumes and the cooling air.

3. Method according to claim 1, characterised in that said cooling of the calcined material comprises, at the bottom of the single shaft in firing mode, a supply of cooling air which passes in a counter-current manner through the descending calcined material and is heated on contact therewith, in that the heated cooling air mixes with the gas stream containing the combustion fumes, before it passes through the gas transfer channel, and in that the gaseous effluent concentrated in CO2 which is discharged from the kiln contains the combustion fumes and the cooling air.

4. Method according to either of claims 2 and 3, characterised in that the cooling air is supplied to the kiln in a total volume equivalent to or less than a thermodynamic minimum necessary to cool the calcined material to a reference temperature of 100°C.

5. Method according to claim 4, characterised in that the total volume of cooling air supplied to the kiln is of the order of 40 to 60% of said thermodynamic minimum, preferably equal to 50% thereof.

6. Method according to claim 1, characterised in that said cooling of the calcined material comprises, at the bottom of each of the shafts or at the bottom of the single shaft in firing mode, a supply of cooling air which passes in a counter-current manner through the descending calcined material and is heated upon contact therewith, in that the method further comprises an extraction of the heated cooling air from the kiln and in that the gaseous effluent discharged from the kiln contains a CO2 content of at least 90% by volume on dry gas.

7. Method according to claim 6, characterised in that it further comprises a heat exchange between the heated cooling air, extracted from the kiln, and said portion collected from the gaseous effluent discharged from the kiln, before or after its mixing with concentrated dioxygen.

8. Method according to either of claims 6 and 7, characterised in that it further comprises, in the gas transfer channel, an injection of a fraction of said portion collected from the gaseous effluent discharged from the kiln and, optionally before this injection, a heat exchange between the heated cooling air, extracted from the kiln, and the aforementioned fraction to be injected.

9. Method according to any one of claims 1 to 8, characterised in that it further comprises, in the gas transfer channel, an injection of water.

10. Method according to any one of claims 1 to 9, characterised in that said combustion of fuel comprises an introduction into the shaft in firing mode of a gaseous, liquid or solid fuel and in that, in the case of a solid fuel, said introduction is carried out using a portion of said part collected from the gaseous effluent discharged from the kiln, or another source of CO2, as a carrier gas.

11. Method according to any one of claims 1 to 10, characterised in that said combustion of fuel takes place in the presence of an excess of oxygen relative to the stoichiometric requirements.

12. Parallel-flow regenerative vertical shaft kiln for implementing the method according to any one of claims 1 to 11, comprising - at least two shafts (1, 2) interconnected by a gas transfer channel, - each of said shafts comprising, in the in-operation or not-in-operation position, - at least one fuel supply device (4), - at least one oxygen-containing oxidant supply opening (6), - an inlet (5) for loading a carbonate mineral rock, at the top of the shafts, - an outlet (8) for unloading calcined material produced, at the bottom of the shafts, - a gas effluent discharge duct (14) at the top of the shafts, which is connected to a chimney (15), and - a cooling air supply (7) for cooling the calcined material produced, the kiln comprising a system (16) for switching the operation of the shafts, arranged so that each shaft operates, in production mode, alternately in firing and in preheating mode, one shaft being in firing mode for a predetermined period of time while at least one other shaft is in preheating mode, and vice versa, this switching system (16) controlling for this purpose said in-operation or not-in-operation positions, characterised in that it further comprises - a recirculating circuit (18) which is arranged between the aforementioned gaseous effluent discharge duct (14) of the shafts and said oxidant supply openings (6) of the shafts, - a separating body (17), capable of taking a portion of the gaseous effluent discharged from the kiln via the duct (14) and introducing it into the recirculating circuit (18), and - a source of concentrated dioxygen (20) which communicates with the recirculating circuit (18) so as to supply it with concentrated dioxygen and therefore form an oxidant mixture, said oxidant supply opening (6) of the shaft in firing mode (1) being brought into the operating position by said switching system (16) to ensure combustion of the fuel and, (i) characterised in that the shafts are of circular cross-section, in that said gas transfer channel is a connecting channel (3) which connects peripheral channels (13) arranged around each shaft so as to allow a transfer of gas and in that, below the connecting channel (3), the shafts are provided with a collector ring (25) communicating with a extraction element (26) so as to allow an extraction from the kiln of heated cooling air, or (ii) characterised in that the shafts are of rectangular cross-section, in that a first side (28) of a shaft (1) faces a first side (29) of an adjacent shaft (2) and each shaft has a second side (30, 31) which is opposite those which face each other (28, 29) and in that the transfer path for gas is a connecting channel (3) which directly connects one shaft to the other by their first sides (28, 29), and in that, below the connecting channel, said first sides and said second sides of the shafts are provided with a collector tunnel (32 to 35) communicating with an extraction element (26) so as to allow extraction from the kiln of heated cooling air.

13. Kiln according to claim 12, characterised in that the circular shafts further comprise, at the bottom, a central collector element (27) communicating with an extraction element (26) so as to allow extraction from the kiln of heated cooling air, below the connecting channel (3).

14. Kiln according to any one of claims 12 to 13, characterised in that the kiln comprises, as a source of dioxygen for the recirculating circuit, a unit for separating air (20) into dioxygen and dinitrogen.

15. Kiln according to any one of claims 12 to 14, characterised in that a heat exchanger (36) supplied with heated cooling air, extracted from the kiln, is fitted on the recirculating circuit (18).

16. Kiln according to any one of claims 12 to 15, characterised in that it comprises equipment for unloading calcined material (24) which can resist temperatures above 100°C.