Method of calcining mineral rock in a regenerative parallel flow shaft furnace and furnace used

Abstract

The invention relates to a method for calcining mineral rocks in a regenerative parallel flow vertical furnace, comprising collecting a portion of gaseous effluent discharged from the shaft in a preheating mode (2) into a circulation loop (18), forming an oxidation mixture by mixing said collected portion from the gaseous effluent with high concentration molecular oxygen from a source (20) of molecular oxygen, and introducing said oxidation mixture into the top of the shaft in a firing mode (1) to ensure combustion of fuel in the presence of oxygen, said gaseous effluent discharged from the furnace containing a high concentration of CO2.

Description

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

[0002] The energy efficiency of a regenerative parallel flow shaft furnace or parallel flow regenerative kiln (PFRK) can reach 85% to 90%; this efficiency is the highest not only in the lime industry, but even in the whole energy-intensive cement, steel and glass industries. In Europe, 60% of lime is produced from this type of furnace. This proportion will increase in Europe and worldwide, taking into account the roadmap for the energy and ecological transition.

[0003] The "PFRK" furnace is a vertical double shaft furnace in which fuel is injected alternately into the two shafts for about 10 to 15 minutes, with a stop time of about 1 to 2 minutes between cycles to reverse the air and fuel circuits. This is the "reversal" period. The two shafts are connected to each other by a connecting channel. When one of the shafts is burning (firing mode), the hot combustion fumes pass through the connecting channel (gas transfer channel) and provide part of their heat to the mineral rocks to be calcined, in order to preheat them in the other shaft in the regeneration or preheating mode. The shafts of the PFRK furnace are cylindrical or rectangular. In some cases there are three shafts, two of which are in preheating mode and one in firing mode. The problems and solutions outlined below apply to PFRK furnaces of all geometries.

[0004] The method used in these known furnaces comprises, in the production mode:

[0005] - loading of carbonate mineral rocks at the top of the shaft,

[0006] - preheating of the rocks,

[0007] - firing of the rocks, decarbonating them into calcined material,

[0008] - cooling of the calcined material using cooling air, and

[0009] - unloading of the calcined material at the bottom of the shaft,

[0010] - each shaft is alternately operated in firing mode and in preheating mode, one shaft being in firing mode for a predetermined period of time while at least the other shaft is in preheating mode, and vice versa,

[0011] - the firing mode comprises:

[0012] - loading of carbonate mineral rocks at the top of the shaft in firing mode,

[0013] combusting fuel in the presence of said preheated carbonate mineral rock descending into the shaft to obtain said firing of the rock and decarbonating the rock into calcined material, combustion fumes being released in the form of a gas stream descending co-currently in the shaft in firing mode, and

[0014] comprising said gas stream of combustion fumes from the shaft in firing mode moving to said at least one shaft in preheating mode using said gas transfer channel.

[0015] - the preheating mode comprises:

[0016] said preheating of the loaded carbonate mineral rock by heat exchange with a gas stream comprising combustion fumes from the gas transfer channel, combustion fumes ascending counter-currently in said at least one shaft in preheating mode to said loaded carbonate mineral rock, and

[0017] said at least one shaft in preheating mode top discharging a gaseous effluent based on a gas stream comprising combustion fumes from the furnace.

[0018] For the purposes of the present invention, carbonate mineral rock means in particular limestone rock, dolomite rock and / or magnesite, which are respectively calcined into quicklime, calcined dolomite and / or magnesia. The equation for the calcination of limestone into quicklime is as follows:

[0019] CaC03(solid) + heat <-> CaO (solid) + CO2(gas)

[0020] This is a reversible endothermic reaction, quicklime recombines with CO2 at a first time below 900°C, with an equilibrium and more or less fast kinetics depending on the temperature and the environmental concentration of CO2.

[0021] Thus, in this process, the original limestone or dolomite rock releases a large amount of CO2 during its calcination into quicklime or dolomite. In addition, high temperatures must be reached to carry out this calcination, and therefore fuel must be burned, which in turn leads to the release of a large amount of CO2. Overall, the calcination process has the drawback of actively contributing to the greenhouse effect.

[0022] This common calcination process also has the drawback that fuel is burned with air and that the calcination product is cooled by air. This results in a gaseous effluent being discharged at the top of the furnace containing a high concentration of diatomic nitrogen and a relatively low concentration of CO2 (volume concentration of dry gas of about 20% to 27%), the cost of capturing CO2 being high due to the large amount of diatomic nitrogen contained in the air used.

[0023] To capture this CO2, an abatement method using a chemical solvent called "amines" can be considered, which is the most widespread technique applied to the flue gases at the end of the production line, after the dust filter. However, the cyclic nature of the PFRK furnace, i.e. 1 to 2 minutes of shutdown every 10 to 15 minutes, is incompatible with this technique, which is also very costly and involves solvents that are not sustainable from an environmental legislation point of view.

[0024] To be able to capture the CO2 emitted in a PFRK furnace, in the method used it has been proposed to replace all the air in the process, the combustion air that carries the solid fuel and the cooling air with recirculated combustion flue gases and to introduce pure oxygen into the shaft in the firing mode (see CN105000811). It is obvious to the person skilled in the art that this process is not feasible because the quicklime would re-carbonate during the cooling. As mentioned above, the CO2 cannot be recirculated to cool the quicklime because it would immediately recombine with the CO2, re- generating CaCO3. On the other hand, the use of pure oxygen at the top of the furnace would raise serious material compatibility problems and this input would not provide a sufficient mass flow to effectively recover the heat accumulated in the regeneration zone. The drawbacks and feasibility problems of this method have been discussed in patent application US2020 / 0048146.

[0025] It should also be noted that the cooling air in the PFRK furnace has no direct influence on the combustion and calcination processes in the shaft in the firing mode, compared to a converter, for example. It has no expected influence on the quality of the product.

[0026] The production mode means that the furnace is in normal working conditions, during which the calcined material is continuously produced. This mode is therefore not applicable to the start-up and shut-down phases of the furnace, nor to the maintenance in case of malfunction.

[0027] The present invention aims to at least partially solve the problem of the emission of large amounts of CO2 from a PFRK furnace, without changing its cyclic operation and almost without changing its structure. The present invention also aims to make it possible to capture the CO2 present in the gaseous effluent emitted by the furnace.

[0028] To solve this problem, the present invention provides a method for calcining mineral rocks in a regenerative parallel flow shaft furnace as described in the opening paragraph, which method further comprises:

[0029] - collecting a portion of the gaseous effluent emitted from the furnace,

[0030] - forming an oxidation mixture by mixing the portion of the gaseous effluent emitted from the furnace collected with a high concentration of molecular oxygen, and

[0031] - introducing the oxidation mixture into the top of the shaft in the firing mode to ensure the combustion of the fuel in the presence of oxygen,

[0032] The gaseous effluent exiting the furnace contains a high concentration of CO2.

[0033] The combustion of the fuel in a high concentration of molecular oxygen leads to a flame temperature which is too high for the usual furnace equipment. The application also provides for collecting a part of the gaseous effluent rich in CO2 and mixing it with the molecular oxygen. An O2+N2 mixture is thus obtained at a suitable flame temperature, instead of the oxidizing agent usually formed by the air mixture of O2+N2.

[0034] The combustion of the fuel in molecular oxygen leads to a gaseous stream containing the combustion fumes and the calcination of carbonate rocks. This mainly produces CO2 with some impurities, which are present in trace amounts in the fuel and the material to be calcined, and some oxygen which has not been consumed by the combustion of the fuel. These combustion fumes also naturally contain the CO2 provided to the oxidizing mixture. This obviously leads to a significant increase in the CO2 content of the gaseous effluent emitted from the top of the furnace compared to the conventional method. According to the application, the gaseous effluent rich in CO2 means that its CO2 content is at least 35% by volume of dry gas, advantageously at least 45%, preferably at least 60%, in particular at least 80%, particularly advantageously at least 90%. This CO2 can then be used or sequestered under advantageous conditions, thus greatly reducing the contribution of the furnace to the greenhouse effect.

[0035] The use of this method does not necessarily require any particular design of the furnace itself. The only change made to the furnace can be outside the furnace, including the modification of the effluent circuit leaving the furnace and the provision of at least one source of high concentration of molecular oxygen.

[0036] According to the application, high concentration of molecular oxygen (hereinafter simply oxygen), means a gas whose oxygen content exceeds 50% by volume. It is preferably equal to or greater than 90%, in particular 93%, advantageously 98% to 100% by volume. The source of high concentration of molecular oxygen can be, for example, an air separation unit which separates air into molecular oxygen and molecular nitrogen and works in parallel with the furnace, or a tank of molecular oxygen installed next to the furnace. Advantageously, the combustion of the fuel is carried out in the presence of an excess of oxygen relative to the stoichiometric combustion requirement, preferably from about 5% to 50%, in particular from 10% to 30%, advantageously from 15% to 25% by volume.

[0037] According to the application, the fuel means any solid, liquid or gaseous fuel, such as natural gas, hydrogen, biogas, fuel oil, oil, coal or coke powder, solid biomass (such as sawdust), solid recycled fuel (such as plastics, paper, cardboard, etc.). Advantageously, in the case of solid fuel, a part of the collected portion of the gaseous effluent exiting the furnace is used as a carrier gas, which is introduced in the form of granules or powder into the furnace shaft in the firing mode. CO2 from any other source can also be provided as a carrier gas.

[0038] According to an embodiment of the application, the cooling of the calcined material comprises the supply of cooling air at the bottom of each shaft, which counter-currently passes through the descending calcined material and is heated upon contact therewith, the heated cooling air mixing with the gas stream containing combustion fumes in the shafts in firing mode before moving through the gas transfer channel and, after moving, mixing with the gas stream in the at least one shaft in preheating mode, the CO2-enriched gaseous effluent discharged from the grate containing combustion fumes and cooling air. In this case, only the combustion air in the conventional process is replaced by an oxidizing mixture based on CO2-enriched gaseous effluent and oxygen discharged from the grate. Such a process makes it possible to increase the CO2 content of the gaseous effluent discharged from a conventional PFRK grate from 20 to 27% by dry gas volume to at least 35%, advantageously at least 45%, and even up to 65% by dry gas volume, in the furnace of the application. For example, a PFRK furnace using this process can effectively replace the coke furnace currently used in soda plants to provide a fume containing 40% CO2 by volume. Moreover, the PFRK is a "sustainable" energy-saving furnace, and above all, it eliminates all the environmental problems associated with coke furnaces, including the massive emission of pollutants (CO, NH3, H2S, etc.).

[0039] According to a particular embodiment of the application, the cooling air is supplied to the furnace in a total volume equal to or less than the thermodynamic minimum required to cool the calcined material to a reference temperature of 100°C. Advantageously, the total volume of cooling air supplied to the furnace can be between about 40% and 60% of said thermodynamic minimum, preferably equal to 50%. In this case, the temperature of the unloaded product will be higher than normal operation. It is therefore necessary to adapt the unloading equipment to materials that can withstand this temperature.

[0040] It can also be advantageously provided that the cooling of the calcined material comprises the supply of cooling air at the bottom of the only shaft in firing mode, which counter-currently passes through the descending calcined material and is heated upon contact therewith, the heated cooling air mixing with the gas stream containing combustion fumes before moving through the gas transfer channel, the CO2-enriched gaseous effluent discharged from the grate containing combustion fumes and cooling air. In this case, the total volume of cooling air supplied to the furnace can also be less than the thermodynamic minimum required to cool the calcined material to a reference temperature of 100°C. Advantageously, therefore, the total volume of cooling air supplied to the furnace can be between about 40% and 60% of said thermodynamic minimum, preferably equal to 50%.

[0041] According to a particularly advantageous embodiment of the application, the cooling of the calcined material comprises the supply of cooling air at the bottom of each shaft or at the bottom of the only shaft in firing mode, which cooling air flows countercurrently through the descending calcined material and is heated on contact therewith, the method further comprising the removal of the heated cooling air from the furnace, the CO2 content in the gaseous effluent discharged from the furnace being at least 90% by dry gas volume, preferably at least 95%. In this case, the gaseous effluent discharged from the furnace is formed almost entirely of combustion fumes. The use of this gas or its sequestration is made possible in the specialized industry.

[0042] According to a particular embodiment of the application, the method further comprises heat exchange between the heated cooling air removed from the furnace and the said collected portion of gaseous effluent discharged from the furnace, before or after mixing with the high-concentration molecular oxygen. This makes it possible to introduce heat recovery in the oxidation mixture into the shafts in firing mode.

[0043] Other details and features of the said method of the application are mentioned in the attached claims.

[0044] The application also relates to a PFRK type regenerative parallel flow vertical furnace.

[0045] Such a furnace comprises:

[0046] - at least two shafts connected to one another by a gas transfer channel,

[0047] Each of the said shafts comprises, in the open or closed position:

[0048] - at least one fuel supply device,

[0049] - at least one oxygen-containing oxidizer supply opening for the combustion of the fuel,

[0050] - an inlet at the top of the shaft for loading of the carbonate mineral rock,

[0051] - an outlet at the bottom of the shaft for unloading of the produced calcined material,

[0052] - a gas discharge duct connected to a chimney at the top of the shaft, and

[0053] - a cooling air supply device for cooling of the produced calcined material,

[0054] The said furnace comprises a system for the reversing operation of the shafts, which system is arranged to cause each shaft to operate alternately in firing mode and in preheating mode in the production mode, one shaft being in firing mode for a predetermined period of time while at least another shaft is in preheating mode, and vice versa, the reversing system thus controlling the said open and closed positions.

[0055] The furnace according to the invention further comprises:

[0056] - a recirculation circuit arranged between the gas discharge duct of the above-mentioned shaft and the oxidizing agent supply opening of the shaft, and wherein the reversing system controls the collection of at least part of the gaseous effluent from the shaft in preheating mode, and

[0057] - a source of high-concentration molecular oxygen connected to the recirculation circuit to supply it with high-concentration molecular oxygen to form an oxidizing mixture, the oxidizing agent supply opening of the shaft in firing mode being supplied in the open position by the reversing system to ensure the combustion of the fuel.

[0058] As mentioned above, the PFRK furnace has a cyclic operation, each shaft being operated in firing mode for a predetermined period of time, then in preheating mode for a reversing time of 1 to 2 minutes, and so on. During the reversal, the reversing system controls in a synchronized manner all the changes required to pass from one mode to the other, for example the opening of the nozzles of the fuel supply device when the shaft is in firing mode, and the closing of these nozzles when it passes to preheating mode. Thus, the reversing system controls not only numerous valves and gates, but also the operation of loading and unloading equipment or various suction, pumping or injection elements.

[0059] It can be seen that the furnace according to the invention has only a few changes in the external structure of the furnace. Thus, existing furnaces can easily be arranged to implement the calcination method according to the invention.

[0060] According to an embodiment of the invention, the shafts have a circular cross-section, the gas transport channels being connecting flues which connect the peripheral channels arranged around each shaft for gas transport, and below the connecting flues, the shafts are provided with a collector ring connected to an evacuation element for the evacuation of the heated cooling air from the interior of the furnace. Advantageously, the circular shafts further comprise, at the bottom of the shaft, a central collector element connected to an evacuation element for the evacuation of the heated cooling air from the interior of the furnace below the connecting flues.

[0061] According to another embodiment of the furnace according to the invention, the shafts have a rectangular cross-section, the first side of a shaft facing the first side of another adjacent shaft, and each shaft comprising those second sides which face each other, the gas transport channels being connecting flues which directly connect one shaft to another through their first sides, and below the connecting flues, the first and second sides of the shafts are respectively provided with a collection channel connected to an evacuation element for the evacuation of the heated cooling air from the interior of the furnace.

[0062] According to an embodiment of the application, the furnace comprises an air separation unit as a source of molecular oxygen of the recirculation loop for separating air into molecular oxygen and molecular nitrogen. An oxygen tank can also be provided. Advantageously, a heat exchanger supplied with heated cooling air evacuated from the furnace is installed on the recirculation loop in order to heat the above-mentioned oxidation mixture before it is supplied to the shaft in the firing mode.

[0063] Other details and features of the furnace of the application are mentioned in the attached claims.

[0064] Other features of the application will also become apparent from the following non-limiting drawing description.

[0065] Figure 1 A conventional PFRK furnace is schematically shown.

[0066] Figure 2a and 2b Numerical models of the oxygen mass percentage concentration of the gas flow in a conventional PFRK furnace with a circular cross-section and in a conventional PFRK furnace with a rectangular cross-section are shown.

[0067] Figure 3 and Figure 4 Several embodiments of the furnace of the application with a circular cross-section are schematically shown.

[0068] Figure 5 is a partial view of an embodiment of the furnace of the application with a rectangular cross-section.

[0069] In the figures, identical or similar parts use the same reference numbers. Generally, the shaft shown on the left is in the firing mode and the shaft shown on the right is in the preheating mode. Standard components, such as loading or unloading devices, are not shown or are very schematically shown in order to avoid overloading the drawings.

[0070] It can be seen from Figure 1 that the PFRK furnace shown is a vertical double-shaft furnace 1, 2 in which the fuel is injected alternately into the shaft 1 and the shaft 2 for about 12 minutes, with a stop time of 1 to 2 minutes between cycles to reverse the circuit. This is the "reversal" period. Both shafts have a circular cross-section and are provided with peripheral channels 13 which are connected to each other by a connecting flue 3. The shafts are vertically divided into three zones: a preheating zone A (preheating of the carbonate before calcination), a combustion zone B (firing of the carbonate) and a cooling zone C (cooling of the calcined material).

[0071] When the shaft is in the burning mode, here shaft 1, a fuel supply in the form of a nozzle 4 injects fuel 9 into the shaft, which in the shown embodiment is natural gas. Carbonate rock, which is loaded at the top of the shaft via an inlet 5 in the open position, is gradually decarbonated in the shaft. Combustion air is introduced at the top of the shaft through a supply opening 6, which causes the fuel to burn at the outlet of the nozzle 4 and the carbonate rock to be decarbonated into calcined material 10. A gas stream 11 formed by the burning and decarbonation flows down alongside the calcined material and moves into the connecting flue 3 using the peripheral channel 13. Cooling air is introduced through a supply pipe 7 at the bottom of the shaft, countercurrent to the calcined material to cool it. The heated cooling air 12 mixes with the gas stream containing the combustion fumes 11 in order to move into the connecting flue 3. The calcined material is unloaded via an outlet 8 into an unloading device 24.

[0072] When the shaft is in the preheating mode, here shaft 2, the fuel supply is closed and the nozzle 4 is therefore closed. The same applies to the inlet 5 for carbonate rock and the opening 6 for the supply of combustion air. However, the supply pipe 7 for 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 enters the shaft from the connecting flue 3 via the peripheral channel 13. This gas stream 11 proceeds until it reaches the top of the shaft, where it is discharged from the furnace through a discharge pipe 14 and diverted to a chimney 15. In the furnace in the burning mode 1, this discharge pipe 14 is closed.

[0073] The furnace also comprises a reversing system 16, which is shown schematically. It controls the operation of the shafts during the reversing of the shafts in a synchronized manner, directly or remotely. It controls the switching of all elements of the furnace in such a way that in the production mode each shaft alternately operates in the burning mode and the preheating mode.

[0074] In some cases there are three shafts, of which two are in the preheating mode and one in the burning mode.

[0075] Figure 1 A furnace designed for the production of 430 tons of lime per day is shown. All gas flows mentioned below are expressed in Nm 3 / t of lime production.

[0076] In order to react with the gas injected into the shaft 1 as fuel, 1120 Nm 3 / t of combustion air is used to obtain an excess of air of 19% by weight relative to the stoichiometric requirement, and in order to form 100 Nm 3 / t of CO2 when burning. The mass concentration of oxygen in the incoming gas is 23% since it is air. The temperature reached is far above 900°C, resulting in the decarbonation of the limestone, releasing 380 Nm 3CO2. To cool the produced lime to a temperature of approximately 100°C, 290 Nm 3 / t of cooling air, for a total of 580 Nm 3 / t. At the chimney, 2250 Nm 3 / t of gaseous effluent, of which 480 Nm 3 / t of CO2, i.e. the CO2 content of the gaseous effluent is 23% of the dry gas. CO2 is difficult to use or sequester at such a low content, so the gaseous effluent is released completely into the atmosphere.

[0077] Figure 2a A numerical model of a PFRK furnace with a circular cross-section is shown, showing the route of the gases according to their oxygen content. It shows only the combustion zone B (starting from the end of the nozzle) and the cooling zone C, so the top of the shaft is not shown.

[0078] a zone: in the shaft in firing mode, cooling air (at the bottom) and combustion air (at the top, just above the end of the nozzle), with an O2 content of 23% (by weight).

[0079] b zone: combustion fumes emitted by the nozzle, with almost no oxygen left, with some unreacted O2 still in the middle of these fumes.

[0080] c zone: the fumes gradually mix with the cooling air, penetrating deeply into the cooling zone C. They push the gaseous mixture from the periphery into the peripheral channel 13 and then into the connecting flue 3.

[0081] d zone: cooling air in the shaft in preheating mode.

[0082] e zone: mixture of the gas flow from the peripheral channel 13 with the cooling air. The closer to the center of the shaft, the more the residual O2 content increases.

[0083] Figure 2b A numerical model of a PFRK furnace is shown, with a shaft with a rectangular cross-section. Here, the distribution of the gas flow is no longer symmetrical as with the circular shaft.

[0084] Figure 3is a view of a furnace according to the application. In this case, the structure of the furnace is not changed. The outside of the discharge pipe 14 is provided with a separation component 17, which is able to collect a portion of the gaseous effluent exiting from the furnace and introduce it into a recirculation circuit 18. In this circuit, the collected portion of gaseous effluent is advantageously treated in a treatment device 19, which can be filtered and / or dried, for example. An air separation device 20 separates the air supplied by the pipe 21 into N2, which is discharged via the pipe 22, and O2, which is supplied to the recirculation circuit 18 via the supply pipe 23. This circuit 18 then takes the oxidizing mixture formed by the recirculated portion of gaseous effluent and the concentrated O2 to the top of each shaft at the supply opening 6.

[0085] Figure 3 The operation of the shaft furnace is similar to that of the PFRK furnace. The separation component 17 is in continuous operation, like the treatment device 19 and the air separation device 20. As has been seen, the reversing system 16 closes the discharge pipe 14 at the top of the shaft in the firing mode. However, it opens the supply opening 6 of this shaft top in order to introduce the oxidizing mixture, while the supply opening 6 of the shaft top in the preheating mode is closed.

[0086] The same amount of carbonate rock is used as in the conventional furnace described above, as well as the same flow rates of fuel and cooling air. The gaseous effluent rich in CO2, 830 Nm 3 / t, exiting from the furnace is collected by the recirculation circuit 18. This recirculated effluent is mixed with 160 Nm 3 / t of O2 in order to maintain the O2 mass concentration unchanged at 23% in the oxidizing mixture thus formed and to obtain the same 19% excess oxygen by weight with respect to the stoichiometric requirement during combustion. Therefore, the nitrogen N2 in the combustion air is replaced by CO2 in a mass equivalent amount. Since it is heavier than nitrogen (specific weight of 1.977 with respect to 1.25 g / Nm 3 of nitrogen), the total volume of the flue gas in the furnace is reduced, resulting in a 13% reduction in pressure drop with respect to the conventional furnace. At the chimney, 1240 Nm 3 / t of gaseous effluent is released, which now contains 43% of CO2 by dry gas volume. As mentioned above, at this content, industrial uses become possible, for example in soda plants.

[0087] As a variant of this furnace according to the application, in order to further reduce the air input in the process, the flow rate of the cooling air can be reduced. For example, this input can be reduced to 50%, i.e. 290 Nm 3of the cooling air. This reduced volume can be introduced through the supply pipe 7 of the shaft alone in the firing mode or through the supply pipes 7 of both shafts. This measurement method reduces the flue gas dilution by 50%. This results in less cooling of the calcined material exiting via the outlet 8. It is therefore necessary to provide unloading equipment that can withstand temperatures in excess of 100°C, such as refractory steel discharge platforms and steel drag chains. Due to the higher temperature at which the quicklime leaves, there is less heat recovery of the cooling air, which is compensated for by a small increase in the fuel input flow, resulting in the formation of 120 Nm 3 / t of CO2. In turn, this increase requires the collection of 1730 Nm 3 / t of gaseous effluent from the grate in the recirculation circuit to be changed to 865 Nm 3 / t, and mixing this collected effluent with 200 Nm 3 / t of O2 to maintain the 23% O2 mass concentration in the oxidation mixture thus formed unchanged and to obtain the same 19% excess oxygen by weight during combustion. Thus, at the chimney, only 865 Nm 3 / t of gaseous effluent is obtained, with a CO2 content of up to 63% by dry gas volume.

[0088] In fact, by adjusting the amount of cooling air to between 100% and 50% of the thermodynamic minimum required to cool the calcined material to a reference temperature of 100°C, it is possible to establish a custom CO2 concentration of between 40% and 65% by volume at the chimney. Higher concentrations of CO2 can be obtained by reducing the input of cooling air to below 50% within the temperature compatibility limits of the quicklime and the high-temperature unloading and transport systems installed for this purpose.

[0089] Figure 4 is a view of an advantageous furnace according to the present application. It can be seen that this embodiment comprises the features according to Figure 3 the embodiment, but in addition it comprises small modifications to the furnace external structure.

[0090] In this case, by installing a removal system, the heated cooling air is extracted by contact with the calcined material. Both the shaft 1 and the shaft 2 are provided below the connection flue 3 and the peripheral channel 13 with a collector ring 25, which is connected with an extraction element 26, in order to extract the heated cooling air from the furnace. In this way, it is possible to extract part or all of the combustion air by extracting a small part of the combustion flue gas, as required. In fact, as shown in Figure 2a the heated cooling air is pushed towards the outer wall of the furnace where the collector ring is arranged, due to the deep penetration of the descending gases into the cooling zone C. The shafts can further optionally comprise, at the bottom, a central collector element 27 connected with an extraction element 26, in order to concentrate the removal of the heated cooling air below the connection flue 3.

[0091] In the case of a rectangular furnace, cooling air can also be extracted using the side recovery zone even without a collector ring. Figure 5 As can be seen, each furnace body includes four sides. Side 28 of one furnace body faces side 29 of the adjacent furnace body, and each furnace body includes second sides 30 and 31, respectively opposite those sides that face each other. The gas transmission channel is the connecting flue 3, which directly connects one furnace body to another through its sides 28 and 29. Below the connecting flue, sides 28 to 31 are respectively provided with collection channels 32 to 35 connected to the evacuation element 26 to discharge heated cooling air from the furnace.

[0092] Due to the asymmetrical distribution of gas flow in a rectangular blast furnace (see...) Figure 2b The cooling air is pushed to one side only by the hot flue gas. Furthermore, in the furnace shown, furnace body 1 is in firing mode, furnace body 2 is in preheating mode, and the reversing system 16 only opens collection channels 32 and 34. In the following cycle, only collection channels 33 and 35 will be opened.

[0093] exist Figure 4 In the furnace shown, the same amount of carbonate rock and the same flow rate of cooling air as in the conventional furnace described above are used. The heated cooling air is exhausted from the furnace through the evacuation element 26. Fuel is introduced into the furnace body 1 to obtain 105 Nm³ during combustion. 3 / t of CO2 is formed. At the top of furnace body 2, 1330 Nm³ is discharged. 3 / t of gaseous effluent. 730 Nm³ is collected through recirculation loop 18. 3 / t of CO2-rich gaseous effluent discharged. This recirculated effluent contains 220 Nm³. 3 / t of O2 is mixed to maintain a constant O2 mass concentration of 23% in the resulting oxidizing mixture, and to obtain an excess of 19% by weight of oxygen relative to the stoichiometric requirements during combustion. Therefore, only 600 Nm³ of oxygen is obtained at the chimney. 3 / t of gaseous effluent, of which CO2 content is 96% by dry gas volume.

[0094] exist Figure 4 In the furnace shown, in order to recover some energy from the hot air removed by the evacuation element 26, heat exchange can be performed with a portion of the recirculated gas effluent using the heat exchanger 36 before or after mixing with high-concentration molecular oxygen.

[0095] Moreover, in the connecting flue 3 and the peripheral channel 13, it is also possible to inject, using the injection pipe 37, a portion of said collected portion of gaseous effluent coming out from the grate. Optionally, it is possible to use, in advance, a heat exchanger, for example the heat exchanger 36, for the exchange of heat between the heated cooling air removed from the furnace and the portion to be injected as described above. In the absence of a heat exchanger, it is possible to provide, on the injection pipe 37, a heat exchanger not shown.

[0096] According to another variant, it is possible to reduce the temperature in the connecting flue by injecting water at selected positions of the flue and / or of the peripheral ring. This added water has no dilution effect on the concentration of CO2 in the dry gas.

[0097] A rectangular furnace can naturally also be provided with this configuration for recovering heat from the heated cooling air coming out from the grate, using a heat exchanger and injecting CO2 or water in the connecting flue.

[0098] It is clear that it is possible to design a furnace similar to that shown in Figure 4 , in which the cooling air is injected only from the bottom of one of the two shafts.

[0099] Table 1 below includes the flows in the conventional furnace and in the variants of the different furnaces according to the present application, and Table 2 includes the amounts of the various gaseous elements at the inlet of the furnace.

[0100] In the example column, 1 indicates a conventional PFRK furnace, 2 and 3 are furnaces with variable cooling air flow according to Figure 3 , 4 and 5 are furnaces with and without heat exchanger according to Figure 4 .

[0101] Table 1

[0102]

[0103] * DP = pressure loss

[0104] Table 2

[0105]

[0106] It is understood that the present application is in no way limited to the above-described embodiments and that changes can be made without departing from the scope of the appended claims.

[0107] For example, it can be advantageous to provide for the substitution of the fuel injection nozzles cooled by air with thermally insulated nozzles.

Claims

1. A method for calcining mineral rocks in a regenerative parallel flow shaft furnace, in which at least two shafts are connected to each other by means of a gas transfer channel, said method comprising, in a production mode: - loading of carbonate mineral rocks at the top of the shafts, - preheating of said rocks, - firing of said rocks, decarbonating them into calcined material, - cooling of said calcined material by means of cooling air, and - unloading of said calcined material at the bottom of the shafts, - each shaft being alternately operated in a firing mode and in a preheating mode, one shaft being in the firing mode for a predetermined period of time while at least one other shaft is in the preheating mode, and vice versa, - said firing mode comprising: - loading of carbonate mineral rocks at the top of the shaft in the firing mode, - combustion of fuel in the presence of said preheated carbonate mineral rocks descending into said shaft, in the presence of oxygen, to obtain said firing of said rocks and decarbonate them into calcined material, combustion fumes being released in the form of a gas flow descending in parallel in said shaft in the firing mode, and - said gas flow comprising these combustion fumes moved from the shaft in the firing mode to said at least one shaft in the preheating mode using said gas transfer channel; - said preheating mode comprising: - said preheating of the loaded carbonate mineral rocks by heat exchange with a gas flow comprising combustion fumes from the gas transfer channel, combustion fumes ascending in counterflow in said at least one shaft in the preheating mode to said loaded carbonate mineral rocks, and - discharge of gaseous effluents based on a gas flow comprising combustion fumes from the furnace at the top of said at least one shaft in the preheating mode, - wherein said method further comprises: - collecting a portion of the gaseous effluents discharged from the furnace, - forming an oxidation mixture by mixing the collected portion of gaseous effluents discharged from the furnace with high concentration molecular oxygen, and - introducing said oxidation mixture into the top of the shaft in the firing mode to ensure the combustion of said fuel in the presence of oxygen, - said gaseous effluents discharged from the furnace containing a high concentration of CO2; the CO2 content of said gaseous effluents being at least 60% by dry gas volume.

2. The method according to claim 1, wherein the CO2 content of said gaseous effluents is at least 80% by dry gas volume.

3. The method according to claim 1, wherein the CO2 content of said gaseous effluents is at least 90% by dry gas volume.

4. The method according to claim 1, wherein cooling the calcined material comprises, at the bottom of each shaft, supplying cooling air which passes in counterflow through the descending calcined material and is heated on contact therewith, the heated cooling air being mixed, before moving through the gas transfer channel, with the gas flow containing combustion fumes in the shaft in the firing mode, and, after moving, with the gas flow in said at least one shaft in the preheating mode, the CO2-rich gaseous effluents discharged from the furnace containing combustion fumes and cooling air.

5. The method of claim 1, wherein cooling the calcined material comprises supplying cooling air at the bottom of the furnace body, which is the only part in the firing mode, the cooling air flowing countercurrently through the descending calcined material and being heated upon contact with it, the heated cooling air being mixed with a gas stream containing combustion flue gas before moving through a gas transmission channel, and a CO2-rich gaseous effluent discharged from the furnace containing combustion flue gas and cooling air.

6. The method of claim 4, wherein the cooling air is supplied to the furnace in a total volume equal to or less than the thermodynamic minimum required to cool the calcined material to a reference temperature of 100°C.

7. The method of claim 6, wherein the total volume of the cooling air supplied to the furnace is 40% to 60% of the thermodynamic minimum.

8. The method of claim 7, wherein the total volume of the cooling air supplied to the furnace is 50% of the thermodynamic minimum.

9. The method of claim 1, wherein cooling the calcined material comprises supplying cooling air at the bottom of each furnace body or at the bottom of the only furnace body in firing mode, the cooling air flowing countercurrently through the descending calcined material and being heated upon contact with it, the method further comprising removing the heated cooling air from the furnace, the CO2 content in the gaseous effluent discharged from the furnace being at least 90% by dry gas volume.

10. The method of claim 9, wherein the method further comprises, before or after mixing with high concentrations of molecular oxygen, performing heat exchange between heated cooling air removed from the furnace and the collection portion of the gaseous effluent discharged from the furnace.

11. The method of any one of claims 9 and 10, wherein the method further comprises injecting a portion of the collection portion of the gaseous effluent discharged from the furnace into the gas transmission channel, and optionally performing a heat exchange between heated cooling air removed from the furnace and the portion to be injected prior to injection.

12. The method of any one of claims 1 to 10, wherein the method further comprises injecting water into the gas transmission channel.

13. The method of any one of claims 1 to 10, wherein the fuel combustion comprises introducing gaseous, liquid, or solid fuel into the furnace body in a firing mode, and in the case of solid fuel, the introduction is carried out using a portion of the collection portion of the gaseous effluent discharged from the furnace, or using CO2 from another source as a carrier gas.

14. The method of any one of claims 1 to 10, wherein the fuel combustion is carried out in the presence of excess oxygen relative to stoichiometric requirements.

15. A regenerative parallel-flow vertical blast furnace for carrying out the method as described in any one of claims 1 to 13, comprising: -At least two furnace bodies (1, 2) are interconnected via gas transmission channels. -Each of the furnace bodies includes the following in the open or closed position: -At least one fuel supply device (4), -At least one oxygen-containing oxidant supply opening (6), -The inlet (5) located at the top of the furnace body for loading carbonate mineral rocks, - an outlet (8) at the bottom of the shaft for unloading the calcined material produced, - a gas discharge duct (14) at the top of the shaft connected to a chimney (15), and - cooling air supply means (7) for cooling the calcined material produced, the furnace comprising a system (16) for shaft reversing operation, arranged to alternate the operation of each shaft in production mode with firing mode and preheating mode, one shaft being in firing mode for a predetermined period of time while at least another shaft is in preheating mode, and vice versa, the reversing system (16) thus controlling the opening and closing positions, wherein the method further comprises: - a recirculation circuit (18) arranged between the gas discharge duct (14) of the above-mentioned shaft and the oxidizing agent supply opening (6) of the shaft, - a separation component (17) able to collect a portion of the gaseous effluent exiting from the furnace through the duct (14) and introduce it into the recirculation circuit (18), and - a high concentration molecular oxygen source (20) connected to the recirculation circuit (18) to supply it with high concentration molecular oxygen, thus forming an oxidizing mixture, the oxidizing agent supply opening (6) of the shaft in firing mode being supplied in the opening position by the reversing system (16) to ensure fuel combustion; - wherein the shafts have a circular section, the gas transmission channels are connection flues (3) which connect the peripheral channels (13) arranged around each shaft for gas transmission, and below the connection flues (3) the shafts are provided with collector rings (25) connected to evacuation elements (26) for the discharge of heated cooling air from the furnace; - or wherein the shafts have a rectangular section, a first side (28) of a shaft (1) facing a first side (29) of another adjacent shaft (2), and each shaft comprising second sides (30, 31) opposite the first sides (28, 29) facing each other, the gas transmission channels are connection flues (3) which directly connect one shaft to another through their first sides (28, 29), and below the connection flues the first and second sides of the shafts are respectively provided with collection channels (32-35) connected to evacuation elements (26) for the discharge of heated cooling air from the furnace.

16. The furnace of claim 15, wherein the circular shafts further comprise, at the bottom, a central collector element (27) connected to an evacuation element (26) for the discharge of heated cooling air from the furnace below the connection flues (3).

17. The furnace of claim 15 or 16, wherein the furnace comprises a device (37) as a source of molecular oxygen of the recirculation circuit for separating air into molecular oxygen and molecular nitrogen.

18. The furnace of claim 15 or 16, wherein a heat exchanger (36) supplied with heated cooling air discharged from the furnace is installed on the recirculation circuit (18).

19. The furnace of claim 15 or 16, wherein the furnace comprises a device (24) for unloading calcined material that can withstand high temperatures in excess of 100°C.

Images