METHOD OF CALCINING MINERAL ROCK IN A REGENERATIVE PARALLEL-FLOW VERTICAL SHAFT FURNACE, AND FURNACE USED

Abstract

A method for calcining mineral rock in a regenerative parallel-flow vertical shaft furnace including the steps of collecting a portion of the gaseous effluent discharged, in preheating mode, from the furnace shaft in a recirculating circuit, forming an oxidizing mixture by mixing the portion collected from the gaseous effluent with concentrated dioxygen from a dioxygen source, and inserting the oxidizing mixture into the top of the shaft in firing mode so as to ensure the combustion of fuel in the presence of oxygen. The gaseous effluent discharged from the furnace having a high concentration of CO.sub.2.

Claims

1. Method for calcining mineral rock in a regenerative parallel-flow vertical shaft furnace, wherein at least two shafts are interconnected via a gas transfer channel, the method comprising, in production mode, loading the carbonate mineral rock at a top of the furnace, preheating said rock, firing said rock with the decarbonation thereof into calcined material, cooling the calcined material via cooling air, and unloading the calcined material at a bottom of the shafts, each shaft operating alternately in firing mode and in preheating mode, one shaft being in firing mode for a predetermined time period 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 descending into said shaft, combusting fuel in the presence of oxygen so as to obtain said firing of said rock and the decarbonation thereof into calcined material with the release of combustion fumes in the form of a gaseous stream descending co-currently in the shaft in firing mode, and said gaseous stream containing these combustion fumes moving from the shaft in firing mode to said at least one shaft in preheating mode using said gas transfer channel, the preheating mode comprising: said preheating of the loaded carbonate mineral rock via heat exchange with the gaseous stream containing the combustion fumes from the gas transfer channel, which is ascending in said at least one shaft in preheating mode, counter-currently to said loaded carbonate mineral rock, and discharging from the furnace a gaseous effluent based on the gaseous stream containing the combustion fumes, at the top of said at least one shaft in preheating mode, the preheating mode further comprises collecting a portion of the gaseous effluent discharged from the furnace, forming an oxidizing mixture by mixing said collected portion of the gaseous effluent discharged from the furnace with concentrated dioxygen, and introducing this oxidizing mixture at the top of the shaft in firing mode so as to ensure said fuel combustion in the presence of oxygen, the gaseous effluent discharged from the furnace being concentrated in CO.sub.2.

2. Method according to claim 1, wherein said cooling of the calcined material comprises, at the bottom of each of the shafts, supplying cooling air which flows counter-currently through the descending calcined material and is heated on contact with it, in that the heated cooling air mixes with the gaseous stream containing the combustion fumes in the shaft in firing mode before moving through the gas transfer channel and, after moving, with this gaseous stream in said at least one shaft in preheating mode, and in that the gaseous effluent concentrated in CO.sub.2 discharged from the furnace contains the combustion fumes and the cooling air.

3. Method according to claim 1, wherein said cooling of the calcined material comprises, at the bottom of the only shaft in firing mode, supplying cooling air which flows counter-currently through the descending calcined material and is heated on contact with it, in that the heated cooling air mixes with the gaseous stream containing the combustion fumes before moving through the gas transfer channel, and in that the gaseous effluent concentrated in CO.sub.2 discharged from the furnace contains the combustion fumes and the cooling air.

4. Method according to claim 2, wherein the cooling air is supplied to the furnace in a total volume equal 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, wherein the total volume of cooling air supplied to the furnace is about 40 to 60% of said thermodynamic minimum.

6. Method according to claim 1, wherein said cooling of the calcined material comprises, at the bottom of each of the shafts or at the bottom of the only shaft in firing mode, supplying cooling air which flows counter-currently through the descending calcined material and is heated on contact with it, in that the method further comprises removing the heated cooling air from the furnace and in that the gaseous effluent discharged from the furnace contains a CO.sub.2 content of at least 90% by volume on dry gas.

7. Method according to claim 6, wherein it further comprises a heat exchange between the heated cooling air, removed from the furnace, and said collected portion of gaseous effluent discharged from the furnace, before or after it is mixed with concentrated dioxygen.

8. Method according to claim 6, wherein it further comprises, in the gas transfer channel, injecting a fraction of said collected portion of gaseous effluent discharged from the furnace and, optionally before this injection, a heat exchange between the heated cooling air, removed from the furnace, and the above-mentioned fraction to be injected.

9. Method according to claim 1, wherein it further comprises, in the gas transfer channel, injecting water.

10. Method according to claim 1, wherein said fuel combustion comprises introducing a gaseous, liquid or solid fuel into the shaft in firing mode and in that, in the case of a solid fuel, said introduction is carried out using a portion of said collected portion of gaseous effluent discharged from the furnace, or using another source of CO.sub.2 as a carrier gas.

11. Method according to claim 1, wherein said fuel combustion occurs in the presence of an excess of oxygen relative to stoichiometric requirements.

12. Regenerative parallel-flow vertical shaft furnace for implementing the method according to claim 1, comprising at least two shafts, interconnected by a gas transfer channel, each of said shafts comprising, in the on or off position, at least one fuel supply device, at least one supply opening for oxygen-containing oxidant, an inlet, for loading carbonate mineral rock, at the top of the shafts, an outlet for unloading the calcined material produced, at the bottom of the shafts, a gaseous effluent discharge duct at the top of the shafts, which is connected to a chimney, and a supply of cooling air to cool the calcined material produced, the furnace comprising a system for reversing the operation of the shafts, arranged so that each shaft, in production mode, operates alternately in firing mode and in preheating mode, a shaft being in firing mode for a predetermined time period while at least one other shaft is in preheating mode, and vice-versa, this reversing system therefore controlling said on and off positions, wherein it further comprises a recirculation circuit which is arranged between the above-mentioned gaseous effluent discharge duct of the shafts and said oxidant supply openings of the shafts, a separating member, capable of collecting a portion of gaseous effluent discharged from the furnace via the duct and introducing it into the recirculation circuit, and a source of concentrated dioxygen that is connected with the recirculation circuit in order to supply it with concentrated dioxygen and thereby form an oxidizing mixture, said oxidant supply opening of the shaft in firing mode being supplied in the on position via said reversing system to ensure fuel combustion.

13. Furnace according to claim 12, wherein the shafts have a circular cross-section, in that said gas transfer channel is a connecting flue that connects the peripheral channels arranged around each shaft so as to allow a transfer of gas and in that, below the connecting flue, the shafts are provided with a collector ring connecting with an evacuation element so as to allow heated cooling air to be removed from the furnace.

14. Furnace according to claim 13, wherein the circular shafts further comprise, at the bottom, a central collector element connecting with an evacuation element so as to allow heated cooling air to be removed from the furnace, below the connecting flue.

15. Furnace according to claim 12, wherein the shafts have a rectangular cross-section, in that a first side of a shaft faces a first side of a neighbouring shaft and each shaft comprises a second side that is opposite those facing each other and in that the gas transfer channel is a connecting flue which directly connects one shaft to the other via their first sides, and in that, below the connecting flue, said first sides and said second sides of the shafts are provided with a collection tunnel connecting with an evacuation element so as to allow heated cooling air to be removed from the furnace.

16. Furnace according to claim 12, wherein the furnace comprises, as a dioxygen source for the recirculation circuit, a unit for separating air into dioxygen and dinitrogen.

17. Furnace according to wherein a heat exchanger supplied with heated cooling air removed from the furnace, is mounted on the recirculation circuit.

18. Furnace according to claim 12, wherein it comprises equipment for unloading calcined material that is resistant to temperatures greater than 100° C.

Description

[0061] Other features of the invention will also be apparent from the description below, which is non-limiting and refers to the appended drawings.

[0062] FIG. 1 schematically shows a conventional PFRK furnace.

[0063] FIGS. 2a and 2b show a digital modelling of the oxygen mass % concentration of the gaseous streams in a conventional PFRK furnace with a circular cross-section and in a conventional PFRK furnace with a rectangular cross-section.

[0064] FIGS. 3 and 4 schematically show several embodiments of the furnace with a circular cross-section according to the invention.

[0065] FIG. 5 is a fragmented representation of an embodiment of the furnace with a rectangular cross-section according to the invention.

[0066] In the figures, identical or similar parts use the same references. Conventionally, the shaft shown on the left is in firing mode and the shaft shown on the right is in preheating mode. Standard parts, such as loading or unloading equipment, are not shown or they are shown very schematically, in order to not overload the drawings.

[0067] As can be seen in FIG. 1, the PFRK furnace shown is a vertical double-shaft furnace 1, 2, where the fuel is injected alternately in one shaft 1 then in another 2 for approximately 12 minutes with a stop period between cycles of 1 to 2 minutes to reverse the circuits. This is the “reversing” period. Both shafts have a circular cross-section and are provided with peripheral channels 13 which are interconnected by a connecting flue 3. The shafts are divided vertically into three areas, the preheating area A where the carbonate rock is preheated before calcination, the combustion area B where the firing of the carbonate rock occurs and the cooling area C where the cooling of the calcined material occurs.

[0068] When a shaft is in firing mode, here the shaft 1, a fuel supply device in the form of nozzles 4 injects a fuel 9 into the shaft, which, in the example shown, is natural gas. The carbonate rock, loaded at the top of the shaft via an inlet 5 in the open position, progressively descends in the shaft. Combustion air is introduced at the top of the shaft via a supply opening 6, which allows for fuel combustion at the outlet of the nozzles 4 and a decarbonation of the carbonate rock to calcined material 10. The gaseous stream 11 formed by the combustion and decarbonation descends co-currently to the calcined material and, using the peripheral channel 13, moves into the connecting flue 3. Cooling air is introduced via a supply duct 7 at the bottom of the shaft, counter-currently to the calcined material, to cool it. The heated cooling air 12 mixes with the gaseous stream containing the combustion fumes 11 in order to move into the connecting flue 3. The calcined material is unloaded via the outlet 8 into a piece of unloading equipment 24.

[0069] When a shaft is in preheating mode, here the shaft 2, the fuel supply device is closed and the nozzles 4 are therefore off. The same applies to the inlet 5 for the carbonate rock and to the opening 6 for supplying combustion air. However, 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 gaseous stream 11 which, from the connecting flue 3, enters the shaft via the peripheral channel 13. This gaseous stream 11 progresses until it reaches the top of the shaft where it is discharged from the furnace via a discharge duct 14 and transferred to a chimney 15. In the shaft in firing mode 1, this discharge duct 14 is closed.

[0070] The furnace also comprises a reversing system 16, shown schematically. It controls, in a synchronised manner, the operation of the shafts during the reversing time of the shafts, either directly or remotely. It controls the switching on and off of all elements of the furnace in such a way that, in production mode, each shaft operates alternately in firing mode and in preheating mode.

[0071] In some cases, there are three shafts, two in preheating mode and one in combustion.

[0072] FIG. 1 shows a furnace designed for producing 430 tonnes of lime per day. All of the gas flows mentioned in the following are expressed in Nm.sup.3/t of lime produced.

[0073] In order to react with the gas injected as a fuel into the shaft 1, 1120 Nm.sup.3/t of combustion air are used to obtain an excess of air of 19% by weight relative to stoichiometric requirements, and in order to form a volume of 100 Nm.sup.3/t of CO.sub.2 at combustion. The mass concentration of oxygen in the entering gas is 23%, since it is air. The temperature reached is then far above 900° C., causing a decarbonation of the limestone rock with a release of 380 Nm.sup.3/t of CO.sub.2. In order to cool the lime produced to a temperature of about 100° C., 290 Nm.sup.3/t of cooling air are introduced via the bottom of both shafts, which makes a total of 580 Nm.sup.3/t. At the chimney, 2250 Nm.sup.3/t of gaseous effluent are obtained which contains 480 Nm.sup.3/t of CO.sub.2, i.e., this gaseous effluent has a CO.sub.2 content of 23% on dry gas. The CO.sub.2 is difficult to use or sequester at this low content and the gaseous effluent is therefore totally released into the atmosphere.

[0074] FIG. 2a shows a digital modelling of the PFRK furnace with a circular cross-section, showing the routes of the gases according to their oxygen content. It only shows the combustion area B, from the end of the nozzles, and the cooling area C, and therefore the top of the shafts is not shown.

[0075] Areas a: in the shaft in firing mode, cooling air (at the bottom) and combustion air (at the top, just above the end of the nozzles) with an O.sub.2 content of 23% by weight.

[0076] Areas b: combustion fume jets emitted by the nozzles, in which there is hardly any oxygen left and between which some unreacted O.sub.2 can still be found.

[0077] Area c: the fumes penetrate deeply into the cooling area C by mixing gradually with the cooling air. They push the gaseous mixture peripherally into the peripheral channel 13, then the connecting flue 3.

[0078] Area d: in the shaft in preheating mode, cooling air.

[0079] Area e: mixture of the gaseous stream from the peripheral channel 13 and the cooling air. The closer to the centre of the shaft, the more the residual O.sub.2 content increases.

[0080] FIG. 2b shows such a digital modelling on a PFRK furnace whose shafts have a rectangular cross-section. Here, the distribution of gas flows is no longer symmetrical like it is in the case of circular shafts.

[0081] FIG. 3 is a view of a furnace according to the present invention. In this case, there are no changes to the structure of the furnace. A separating member 17, capable of collecting a portion of gaseous effluent discharged from the furnace and introducing it into the recirculation circuit 18 has been provided on the exterior, on the discharge duct 14. In this circuit, the collected portion of gaseous effluent is advantageously treated in a treatment unit 19, where it may, for example, be filtered and/or dried. An air separation unit 20 separates air supplied by the duct 21 into N.sub.2 discharged via the duct 22 and O.sub.2 supplied to the recirculation circuit 18 via the supply duct 23. This circuit 18 then brings the oxidising mixture formed from the recirculated portion of gaseous effluent and concentrated O.sub.2 to the top of each of the shafts at the supply opening 6.

[0082] The operation of the furnace in FIG. 3 is similar to a PFRK furnace. The separating member 17 is continuously in service, the same as the treatment unit 19 and the air separation unit 20. As has already been seen, the reversing system 16 closes the discharge duct 14 at the top of the shaft in firing mode. However, at the top of this shaft, it opens the supply opening 6 to allow the oxidising mixture to be introduced, while it is closed at the top of the shaft in preheating mode.

[0083] The same amount of carbonate rock and the same flows of fuel and cooling air are used as in the conventional furnace described above. 830 Nm.sup.3/t of gaseous effluent discharged from the furnace, rich in CO.sub.2, is collected via the recirculation duct 18. This recirculated effluent is mixed with 160 Nm.sup.3/t of O.sub.2, to maintain the same mass concentration of 23% of O.sub.2 in the oxidising mixture thus formed and to obtain the same excess of oxygen of 19% by weight relative to stoichiometric requirements during combustion. The nitrogen N.sub.2 of the combustion air is thus replaced by the mass equivalent thereof of CO.sub.2. Since this is heavier than dinitrogen (specific weight of 1.977 relative to 1.25 g/Nm.sup.3), the total volume of the fumes decreases in the furnace, which causes a decrease in the pressure drop of 13% relative to the conventional furnace. At the chimney, 1240 Nm.sup.3/t of gaseous effluent is released which, at present, contains 43% by volume on dry gas of CO.sub.2. As explained above, industrial use becomes possible at this content, for example in soda ash plants.

[0084] As a variant in such a furnace according to the invention, in order to further decrease the input of air in the method, the flow of cooling air may be reduced. For example, this input may be reduced to 50%, which is 290 Nm.sup.3/t of cooling air. This reduced volume may be introduced via the supply duct 7 of the only shaft in firing mode or by making use of the supply ducts 7 of both shafts. This measurement reduces the dilution of the fumes by 50%. This results in less cooling of the calcined material that is discharged via the outlets 8. Thus, it becomes necessary to provide unloading equipment that is resistant to a temperature higher than 100° C., for example, a refractory steel unloading table and steel drag chains. Since the lime exits hotter, there has been less heat recovery by the cooling air, which is compensated by a small increase in the input of fuel to a flow such that it causes 120 Nm.sup.3/t of CO.sub.2 to be formed at combustion. In turn, this increase requires changing the collection of 1730 Nm.sup.3/t of gaseous effluent discharged from the furnace in the recirculation circuit to 865 Nm.sup.3/t and mixing this collected effluent with 200 Nm.sup.3/t of O.sub.2 so as to maintain the same mass concentration of 23% of O.sub.2 in the oxidising mixture thus formed and to obtain the same excess of oxygen of 19% by weight relative to stoichiometric requirements during combustion. At the chimney, only 865 Nm.sup.3/t of gaseous effluent with a high CO.sub.2 content of 63% by volume on dry gas is thus obtained.

[0085] In fact, a custom CO.sub.2 concentration between 40% and 65% by volume of CO.sub.2 can be established at the chimney by adjusting the amount of cooling air to between 100% and 50% of the thermodynamic minimum volume necessary to cool the calcined material to a reference temperature of 100° C. A higher concentration of CO.sub.2 may be able to be obtained by reducing the input of cooling air to below 50%, within the temperature compatibility limit of the lime with the high-temperature unloading and transport system installed for this purpose.

[0086] FIG. 4 is a view of an advantageous furnace according to the present invention. As can be seen, this embodiment comprises the features of the embodiment according to FIG. 3, but, in addition, it comprises a small transformation of the external structure of the furnace.

[0087] In this case, the heated cooling air is extracted by contact with the calcined material, by installing a removal system. The shafts 1 and 2 are each provided with a collector ring 25, below the connecting flue 3 and the peripheral channels 13, which connects with an evacuation element 26 so as to allow heated cooling air to be removed from the furnace. In this way, a portion or all of the combustion air may be extracted, as required, by also extracting a small proportion of combustion fumes. Indeed, as FIG. 2a shows, because the descending gases penetrate deeply into the cooling area C, the cooling air is pushed towards the external walls of the furnace where the collector ring is arranged. The shafts may further optionally comprise, at the bottom, a central collector element 27 connecting with the evacuation element 26 as to also allow a central removal of the heated cooling air, below the connecting flue 3.

[0088] In the case of rectangular furnaces, it is also possible to extract the cooling air without a collector ring, using side recovery areas. As can be seen in FIG. 5, each shaft includes 4 sides. A side 28 of one shaft faces a side 29 of the neighbouring shaft and each shaft includes a second side 30 and 31, respectively, which is opposite to those facing each other. The gas transfer channel is a connecting flue 3 which directly connects one shaft to the other via the sides 28 and 29 thereof. Below the connecting flue, the sides 28 to 31 are each provided with a collection tunnel 32 to 35, respectively, connecting with an evacuation element 26 so as to allow heated cooling air to be removed from the furnace.

[0089] Since the spread of gaseous streams in a rectangular-shaft furnace is not symmetrical (see FIG. 2b), the cooling air is only pushed by the hot fumes to one side. Also, in the furnace shown where the shaft 1 is in firing mode and the shaft 2 in preheating mode, the reversing system 16 only opens the collection tunnels 32 and 34. During the following cycle, only the collection tunnels 33 and 35 will be open.

[0090] In the furnace shown in FIG. 4, the same amount of carbonate rock and the same flows of cooling air are used as in the conventional furnace described above. The heated cooling air is removed from the furnace via the evacuation element 26. In the shaft 1, a fuel introduction is performed such that the formation of 105 Nm.sup.3/t of CO.sub.2 is obtained at combustion. At the top of the shaft 2, 1330 Nm.sup.3/t of gaseous effluent is discharged. 730 Nm.sup.3/t of this discharged gaseous effluent rich in CO.sub.2 is collected via the recirculation circuit 18. This recirculated effluent is mixed with 220 Nm.sup.3/t of O.sub.2 so as to maintain the same mass concentration of 23% of O.sub.2 in the oxidising mixture thus formed and to obtain the same excess of oxygen of 19% by weight relative to stoichiometric requirements during combustion. At the chimney, only 600 Nm.sup.3/t of gaseous effluent with a content of 96% on dry gas of CO.sub.2 is thus obtained.

[0091] In the furnace shown in FIG. 4, in order to recover a portion of the energy from the hot air removed by the evacuation element 26, a heat exchange may be provided with the portion of recirculated gaseous effluent using a heat exchanger 36, before or after the mixing thereof with concentrated dioxygen.

[0092] Furthermore, in the connecting flue 3 and the peripheral channels 13, an injection of a fraction of said collected portion of gaseous effluent discharged from the furnace using an injection duct 37 may also be provided. Optionally beforehand, a heat exchange between the heated cooling air removed from the furnace, and this above-mentioned fraction to be injected may occur using a heat exchanger, for example the heat exchanger 36. In the absence thereof, a heat exchanger not shown may be provided on the injection duct 37.

[0093] According to yet another variant, the temperature in the connecting flue may be mitigated by injecting water at selected locations of the flue and/or peripheral ring. This added water has no dilution effect on the concentration of CO.sub.2 on dry gas.

[0094] Such arrangements to recover the heat from the heated cooling air removed from the furnace, using a heat exchanger as well as such CO.sub.2 or water injecting devices in the connecting flue may also naturally be provided with rectangular-shaft furnaces.

[0095] It is clear that a furnace similar to that shown in FIG. 4 may be designed, where the cooling air is injected at the bottom of only one of the two shafts.

[0096] Table 1 below includes the flows in a conventional furnace and in different furnace variants according to the invention and Table 2 includes the amounts of the various gaseous elements at the inlet of the furnaces.

[0097] In the examples column, 1 indicates a conventional PFRK furnace, 2 and 3 are furnaces according to FIG. 3 with variable flows of cooling air and 4 and 5 are furnaces according to FIG. 4 with and without a heat exchanger.

TABLE-US-00001 TABLE 1 Effluent at O.sub.2 Recycled the top of Chimney Combustion Cooling air injection effluent the furnace effluent air Nm.sup.3/t Nm.sup.3/t Nm.sup.3/t Nm.sup.3/t Nm.sup.3/t Nm.sup.3/t DP* 1 1120 0 2250 2250 DP 2 0 160 1240 % 3 0 200 865 % 4 0 220 1330 % 5 0 250 1440 −39% *DP = pressure loss indicates data missing or illegible when filed

TABLE-US-00002 TABLE 2 O.sub.2 Stoichio- Excess injection O.sub.2 N.sub.2 CO.sub.2 Mass % metric of Nm.sup.3/t of O.sub.2 O.sub.2 kg/t O.sub.2% 1 0 333 0.89 23 281 19 2 160 331 23 278 19 3 200 367 23 18 4 220 1143 23 18 5 250 23 20 indicates data missing or illegible when filed

[0098] It is understood that the present invention is in no way limited to the embodiments described above and that changes can be made without departing from the scope of the appended claims.

[0099] For example, replacing the fuel injection nozzles, cooled by air, with thermally insulated nozzles may be advantageously provided.