Method and kiln for calcining carbonate mineral stones in a parallel flow regenerative kiln (PFRK) with rectangular shaft

Abstract

Method and kiln for calcining carbonate mineral stones in a rectangular shaft parallel flow regenerative kiln (PFRK) in common conditions of combustion, with improved extraction of cooling gas.

Description

[0001]« Method and kiln for calcining carbonate mineral stones in a parallel flow regenerative kiln (PFRK) with rectangular shaft» The present invention relates to a method for calcining carbonate mineral stones in a parallel flow regenerative kiln (PFRK). Such a kiln comprises at least two shafts interconnected by means of a crossover channel. In each shaft the stones are introduced in a top portion and follow a downward gravity displacement during which the stones are successively preheated, calcined and thereafter cooled in order to be collected in a low portion of each shaft. By the terms “stones, carbonate mineral stones, limestone stones”, it is meant according to the present invention pieces of raw carbonated material having a median particle size d50comprised between 20 mm to 12 cm, preferably higher than 25 mm, preferably lower than 10 cm, more preferably lower than 8 cm, and typically between 3 and 5 cm. Carbonate mineral according to the present patent application is typically a calcium-magnesium carbonate, also known as limestone, when containing low amount of magnesium, and dolostone, when the magnesium content is close to the one of calcium on a molar basis. The calcined mineral according to the present patent application is typically a calcium-magnesium oxide, also known as quicklime, when containing a low amount of magnesium and dolime when the magnesium content is close to the one of calcium on a molar basis. A Parallel Flow Regenerative Kiln of rectangular section usually has 2 shafts, which do not work in a continuous way. In standard operation, in every period, usually of 12 to 20 minutes, fuel is injected inside a calcining zone of one shaft by means of lances and is burned in presence of combustion air. Thereafter the descending calcined product is cooled in a cooling zone by heat exchange with a cooling gas introduced at the bottom of the shaft. The flue gas comprises or consists of the combustion fumes, the gas of decarbonation and the heated cooling gas. This flue gas is drawn into another shaft through the crossover channel and goes thereafter through the stones present in this shaft and thereafter outward the kiln. So, in this “preheating” shaft, the stones are preheated by the exiting flue gas. Consequently, during this period the shaft wherein the combustion takes place works according to a calcining mode (calcining shaft) and the shaft wherein the flue gas is drawn through the stones works according to a preheating mode (preheating shaft). Thereafter, there is a period, usually between 30 seconds and 2 minutes, called inversion period, which is provided, notably for reverting the air and fuel circuits. And the shaft having worked in a calcining mode works now in a preheating mode and the shaft having worked in a preheating mode works now in a calcining mode. The classical method for calcining carbonate mineral stones in a parallel flow regenerative kiln having at least two shafts interconnected by a crossover channel, comprises, in standard operation, - loading carbonate mineral stones at the top of each shaft, - preheating these loaded stones in a preheating zone, - calcining these preheated stones in a calcination zone with production of a decarbonated calcined material, - cooling the calcined material with cooling gas in a cooling zone, with formation of a heated cooling gas, by heat exchange, - discharging the calcined material from the bottom of the shafts, - exhausting an exhaust effluent from the kiln, - each shaft alternately working in a calcining mode and in a preheating mode, one shaft working in a calcining mode during a predetermined time period during which at least another shaft works in a preheating mode, and inversely, - the calcining mode comprising : said calcining step by means of an increase of temperature inside said carbonate mineral stones having been preheated, with production of said decarbonated calcined material and release of a gas stream which flows in co-current with the calcined material, and through said crossover channel, a passage of said gas stream toward the at least one shaft working in a preheating mode, - said preheating mode comprising : said preheating step of the loaded carbonate mineral stones by heat exchange with said gas stream coming from the crossover channel, which is ascending and flows in counter-current through the loaded carbonate mineral stones, and said exhausting step of said gas stream as exhaust effluent at the top of said at least one shaft in preheating mode, said cooling step comprising a supply of cooling gas at the bottom of each of said shafts or only of the shaft working in the calcining mode. In the calcining zone of a classical kiln, it is required in calcining mode to inject and burn a fuel into the mass of the stones to be calcined under the preheated stones in order to benefit from the heat of the flue gas that was transferred to the stone in the preheating zone. In preheating mode, the stones introduced into the kiln are at ambient temperature and the flue gas drawn outside the kiln has a temperature typically comprised between 80 and 250°C, preferably between 100 and 200°C and generally at about 150°C, limiting the energy losses. According to the invention, standard operation means that the kiln produces the calcined material in a continuous manner. This operation does not concern the phases of starting, stopping or maintenance of the kiln. According to the invention, carbonate mineral stones particularly mean calcareous stones (limestones), dolomitic stones (dolostones or unburnt dolomites) and / or magnesite stones which are calcined into quicklime, (quick) dolime and / or magnesia, respectively. The calcination reaction of limestone into quicklime is : CaCO3 (solid) + heat CaO (solid)+ CO2 (gas) This is a reversible endothermic reaction and the lime recombines with the CO2 at the first opportunity below 900°C, with an equilibrium and more or less fast kinetics depending on the temperature and the ambient concentration of CO2. Below 850 to 900°C lime and CO2 can easily recombine. But from a temperature of the order of 900°C the starting stones give off a significative volume of CO2during their decarbonation. In order to obtain such a decarbonation, the temperature must consequently be significatively increased in the calcining zone. Today this increase is mainly obtained by combustion of a fuel, frequently fossil, in presence of an oxidizer such as air. In turn this fuel combustion contributes also to an important release of CO2. Globally the current calcination methods actively participate in increasing the greenhouse effect. This common calcination method also has the disadvantage that the fuel is burnt with air and the calcined product is cooled by air. This results in an exhaust effluent being released at the top of the kiln having a high level of diatomic nitrogen and a comparatively low level of CO2 (volume concentration of about 20% to 27% on dry gas) which is expensive to capture because of the large presence of dinitrogen from the air used. To capture this CO2, it may be considered to use an “end-of- pipe” method of CO2concentration and abatement, notably cryogenic or by chemical solvent called "amines", which is the most widespread technique applied to the kiln fumes at the end of the line, after the dust collection filter However, for carrying out end-of-pipe methods as aforementioned, constrains are existing, notably in terms of concentration of CO2, but also in terms of compliancy of the fumes, which may require intermediate devices, the price and the use of hazardous solvents. Alternatively, one may consider to increase the CO2 concentration in the exhaust effluent. In this respect, to be able to capture the CO2 emitted in a PFRK kiln, operating PFRK in oxy-combustion seems to be a promising solution, since the exhaust effluent will have a higher CO2 concentration. Standard mode means that the kiln is in normal service during which it continuously produces calcined material. This mode therefore does not apply to the start-up and shut-down phases of the kiln or to maintenance in the event of a malfunction. In PFRK kiln working in oxy-combustion, the exhaust effluent should be as concentrated in CO2as possible. Accordingly, there has been some proposals to produce concentrated exhaust gas. One proposal that was discussed above is to replace the cooling air by exhaust effluent such as disclosed in CN10500081 and in JP2002060254. Other proposals are to isolate the cooling air from the exhaust effluent (see WO2022 / 002869, WO2022 / 238385, WO2022 / 238384 or WO2022 / 229120). In WO2022 / 002869, it was proposed to extract the cooling air in a ring collector, through a collector tunnel or through a central device, located below the connecting channel to avoid as much as possible mixing between the exhaust effluent concentrated in CO2and the cooling air. However, today, while the operation of PFRK in oxy- combustion seems to be a very promising solution, there remains uncertainties about feasibility and even no pilot kiln has been tested yet. therefore, the oxy-combustion operation while theoretically known and understood has also not yet been tested with respect to impact on lime quality and this technology appears still quite expensive for producing commodities products, despite the little required modifications to the kiln structure. In view of the aforementioned, it is to be expected that PFRK operated in common combustion will remain the gold standard for a while. However, as aforementioned, being able to concentrate the CO2 in the exhaust effluent can yield to a better efficiency in CO2 recovery in the fumes and reduce the CO2 emission in the atmosphere. There is therefore a need to provide a method to be carried out in PFRK under common combustion operation without changing the cyclical operation thereof and with few or no changes to the structure thereof while making it possible to concentrate and improve the capture the CO2present in the exhaust effluents emitted by the kiln at a level that can be industrially valorized while not operating in oxy-combustion. To solve these problems, the present invention provides a method for calcining carbonate mineral stones in a parallel flow regenerative kiln (such as operating in common combustion) having at least a first and a second shafts interconnected by a crossover channel, comprising, in standard operation, - loading carbonate mineral stones at the top of each shaft, - preheating these loaded stones in a preheating zone, - calcining these preheated stones in a calcination zone with production of a decarbonated calcined material, - cooling the calcined material with cooling gas in a cooling zone, with formation of heated cooling gas by heat exchange, - discharging the calcined material from the bottom of the shafts, - exhausting an exhaust effluent from the kiln, - each shaft alternately working in a calcining mode and in a preheating mode, one shaft working in a calcining mode during a predetermined time period during which at least another shaft works in a preheating mode, and inversely after activation of the inversion means, - the calcining mode comprising : • Upon said preheated carbonate mineral stones descending into said shaft, decarbonation of the preheated carbonate mineral stones with the release of combustion fumes descending co-currently in the shaft in calcining mode, and • through said crossover channel, a passage of said combustion fumes toward the at least one shaft working in a preheating mode, - said preheating mode comprising : • said preheating step of the loaded carbonate mineral stones by heat exchange with said combustion fumes coming from the crossover channel, which is ascending and flows in counter-current through the loaded carbonate mineral stones, and • said exhausting step of said combustion fumes as exhaust effluent at the top of said at least one shaft in preheating mode, said method further comprises a step of withdrawal outside of the kiln of an extracted heated cooling gas formed by from 20 to 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft from - (i) the shaft in calcining mode or, - (ii) from the shaft in preheating mode or, - (iii) from both the shaft in calcining mode and the shaft in preheating mode with a predetermined amount x of heated cooling gas being extracted from the shaft in calcining mode and a predetermined amount y of heated cooling gas being extracted from the shaft in preheating mode with x+y being equal to the amount of extracted heated cooling gas, i.e. to the amount comprised between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft. As it can be seen the method according to the present invention comprises in standard operation, the steps of conducting the kiln where the carbonate mineral stones are loaded at the top of each shaft, then preheated in a preheating zone, calcined in a calcination zone with the production of a decarbonated calcined material and combustion fumes before being cooled with cooling gas in a cooling zone. The cooling step is accompanied by the formation of heated cooling gas by heat exchange, i.e. the cooling gas capture the heat from the calcined stones. After being cooled, the calcined material is discharged from the bottom of the shafts. During the method according to the present invention, an exhaust effluent containing the combustion fumes is exhausted from the kiln and each shaft alternately works in a calcining mode and in a preheating mode, one shaft working in a calcining mode during a predetermined time period during which the other shaft works in a preheating mode, and inversely after activation of the inversion means. In the method according to the present invention, the calcining mode comprises a decarbonation of the preheated carbonate mineral stones upon descending into said shaft, with the release of combustion fumes descending co-currently in the shaft in calcining mode passing then through said connecting channel, toward the at least one shaft working in a preheating mode. The preheating mode comprises the preheating step of the loaded carbonate mineral stones by heat exchange with said fumes coming from the connecting channel, which is ascending and flows in counter-current through the loaded carbonate mineral stones, and the exhausting step of said combustion fumes as exhaust effluent at the top of the shaft in preheating mode. The method according to the present invention contemplates a step of withdrawal outside of the kiln of an extracted heated cooling gas formed by from 20 to 90 vol% of said heated cooling gas to avoid dilution of CO2 in the exhaust effluent by the cooling gas, which allow to exit an exhaust effluent having a higher concentration in CO2. This CO2can then be used or sequestered under favorable conditions, drastically decreasing the contribution of the kiln to the greenhouse effect. The withdrawal outside of the kiln also allow to reduce the kiln pressure drop, which is most often the limiting factor for the production rate of a PFRK. Accordingly and surprisingly, a production increase can be reached with the step of withdrawal of the cooling gas. The amount of cooling gas that is extracted is comprised between 20 and 90% with respect to the introduced amount of cooling gas from any of the shafts and should be controlled to keep the kiln flue gas temperature above the dew point of about 60°C and is preferably set by security above 80°C. The withdrawal step can be performed from any of the shaft (i.e. from the shaft in calcining mode or from the shaft in preheating mode) but can also be performed from both the shaft in calcining mode and the shaft in preheating mode with a predetermined amount x of heated cooling gas being extracted from the shaft in calcining mode and a predetermined amount y of heated cooling gas being extracted from the shaft in preheating mode with x+y being equal to the amount of extracted heated cooling gas, i.e. to the amount comprised between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft. As it can be seen, the amount of extracted cooling gas should not necessary be the same in the shaft in calcining mode and in the shaft in preheating mode. Further, for simplification, one may prefer to extract the heated cooling gas from only one shaft. In a preferred embodiment, the step of withdrawal outside of the kiln of an extracted heated cooling gas formed by from 20 to 90 vol% of said heated cooling gas is a step of withdrawal outside of the kiln of an extracted heated cooling gas formed by from 30 to 80 vol%, preferably from 35 to 75 vol%, more preferably from 40 to 70 vol%, even more preferably from 45 to 65 vol% of said heated cooling gas, so the amount of extracted heated cooling gas is comprised between 30 and 80 vol%, preferably between 35 and 75 vol%, more preferably between 40 and 70 vol%, even more preferably between 45 and 65 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft, said step of withdrawal of heated cooling gas is preferably controlled by the temperature of the exhaust effluent exhausted at the top of the shaft in preheating mode in such a way that said amount of extracted heated cooling gas is continuously controlled, preferably continuously modulated to maintain the temperature of the exhaust effluent above a threshold value of at least 60°C, preferably above 70°C, more preferably above 80°C. In a preferred embodiment of the present invention, when said extracted heated cooling gas is extracted from the shaft in calcining mode, said extracted heated cooling gas is extracted : - from 50 to 100% by volume on dry gas basis relative to the introduced cooling gas in the same shaft by exit means located in a first part (36) of a first shaft (1) or by exit means located in a first part (37) of the second shaft (2), and - from 0 to 50% by volume on dry gas basis relative to the introduced cooling gas in the same shaft by exit means located in a second part (35) of said first shaft (1) or by exit means located in a second part (38) of said second shaft (2), and when said extracted heated cooling gas is extracted from the shaft in preheating mode, said extracted heated cooling gas is extracted : - from 50 to 100% by volume on dry gas basis relative to the introduced cooling gas in the same shaft by exit means located in said second part (35) of said first shaft (1) or by exit means located in said second part (38) of said second shaft (2), and - from 0 to 50% by volume on dry gas basis relative to the introduced cooling gas in the same shaft by exit means located in said first part (36) of said first shaft (1) or by exit means located in said first part (37) of said second shaft (2). According to the present invention, the kiln possesses external wall, wherein the first shaft has an external wall comprising 4 sides and the second shaft has an external wall comprising 4 sides. According to the present invention, the first part of the first shaft and second shaft comprises an interior side, being the side which faces respectively the second shaft and the first shaft, and a proximal zone of the two adjacent sides, in proximity of said interior side. According to the present invention, the second part of the first shaft and second shaft comprises an exterior side, being the side opposed to the interior side and a distal zone of the two adjacent sides, in proximity of said exterior side. Indeed, it has been identified according to the present invention that the heated cooling gas should be extracted at different location depending to the role of the shaft and the period in the cycle of calcining / preheating to maximize the recovery of CO2and keep as low as possible cooling gas leakage. In the shaft in calcining mode the combustion fumes and the CO2 exhausted from the decarbonation of the carbonate stone form the exhaust effluent. This effluent pushes the cooling gas towards the first part of the first shaft when the first shaft is in calcining mode or towards the first part of the second shaft when the second shaft is in calcining mode. In the preheating shaft, the exhaust effluent arrives from the calcining shaft and is mainly spread towards the inside walls of the cooling zone. Due to its momentum, the exhaust effluent pushes the heated cooling gas to the second part of the first shaft when the first shaft is in preheating mode or towards the second part of the second shaft when the second shaft is in preheating mode. The process according to the present invention foresees an extraction step of the heated cooling gas in the calcining shaft in majority (from 50 to 100%) by volume on dry gas basis, relative to the introduced cooling gas in the same shaft, by exit means located in a first part of a first shaft or by exit means located in a first part of the second shaft, while a minority portion of the heated cooling gas, from 0 to 50% by volume on dry gas basis relative to the introduced cooling gas in the same shaft may be at the same time extracted from the shaft in calcining mode by exit means located in a second part of said first shaft 1 or by exit means located in a second part of said second shaft. Thereby, the fact that in the calcining shaft the cooling gas is extracted in majority in the first part of the shaft, below the crossover channel and that in the preheating shaft the cooling gas is extracted in majority through exit means located in the second part of the shaft allows to minimize the CO2dilution of the exhaust effluent by minimizing leakage of cooling gas in the exhaust effluent, but also reduce the leakage of CO2in the cooling gas where it end ultimately lost. Within the meaning of the present invention, the momentum of a fluid flow is defined as the mass flow of this fluid (in kg / s) multiplied by its average velocity (in m / s). It is expressed in Newtons (kg*m / s²). Advantageously, in the method according to the present invention: when said extracted heated cooling gas is extracted from the shaft in calcining mode, said extracted heated cooling gas is extracted : - from 75 to 99%, more preferably from 80 to 98% by volume on dry gas basis relative to the introduced cooling gas in the same shaft through said exit means located in a first part (36) of a first shaft (1) or by exit means located in a first part (37) of the second shaft (2) and - from 0 to 30%, preferably from 1 to 25%, more preferably from 2 to 20% by volume on dry gas basis relative to the introduced cooling gas in the same shaft through said exit means located in a second part (35) of said first shaft (1) or by exit means located in a second part (38) of said second shaft (2) and when said extracted heated cooling gas is extracted from the shaft in preheating mode, said extracted heated cooling gas is extracted : - from 0 to 30%, preferably from 1 to 25%, more preferably from 2 to 20% by volume on dry gas basis relative to the introduced cooling gas in the same shaft through said exit means located in said first part (36) of said first shaft (1) or by exit means located in said first part (37) of said second shaft (2) and - from 70 to 100%, preferably from 75 to 99%, more preferably from 80 to 98% by volume on dry gas basis relative to the introduced cooling gas in the same shaft through said exit means located in said second part (35) of said first shaft (1) or by exit means located in said second part (38) of said second shaft (2). Preferably, the loading of carbonate mineral stones occurs during the inversion period at the top of the shaft that will work in a preheating mode after said inversion period. Alternatively, or in addition, the loading of carbonate mineral stones occurs during the cycle at the top of the shaft that works in a preheating mode or at the top of the shaft that works in a calcining mode. Preferably, the loading of carbonate mineral stones occurs during the cycle at the top of the shaft that works in a preheating mode, but can optionally also be done during reversal. In a preferred embodiment according to the present invention, the method further comprises injection of combustion air and fuel in the calcining shaft, preferably in the calcination zone, to perform the decarbonation of preheated carbonate mineral stones through a combustion of fuel, the combustion fumes comprising CO2from the decarbonation of carbonate mineral stones and from combustion of fuel. Preferably, the combustion air is injected in excess with respect to the stoichiometric conditions for fuel combustion, for example with an excess factor of at least 1.05, preferably 1.1. As it can be seen, the method according to the present invention is carried out in a kiln working in common operation, meaning with combustion of fuel in presence of combustion air. Under common operation, it has appeared that even if the (released) fumes can’t be as concentrated in CO2as under oxy-combustion operation, the valorisation of the CO2 in the fumes is nevertheless higher than under common condition facilitating therefore the recovery of this latter. Further as the production rate of the kiln is improved due to the reduced pressure drop, the yield of the method is enough to provide the profitability for the CO2recovery. According to said preferred embodiment, preferably said combustion of fuel comprises introducing a gaseous, liquid or solid fuel into the shaft in calcining mode and in that, in the case of a solid fuel, said introduction may be carried out using either air or a collected portion of exhaust effluent discharged from the kiln, or using another source of CO2 as a carrier gas or using another source of gas as a carrier gas, such as combustion air. According to said preferred embodiment, preferably, a portion of the exhaust effluent discharged from the kiln may be introduced in lances for fuel injection or in an outer envelope enclosing lances for fuel injection for cooling said lances in the shaft in preheating mode. According to said preferred embodiment, preferably, the exhaust effluent is partially or fully collected in at least one buffer after said exhausting step or in a storage tank or first in one buffer and then in a storage tank, the buffer and the storage tank being located in series or first in a storage tank and then in a buffer, the storage tank and the buffer being located in series. In one embodiment, the method according to the present invention further comprises a CO reduction step in the connecting channel by injecting a mixing gas through a series of mixing gas entry means at high velocity in such a way that a ratio of momentum J between the each gas stream and the combustion fumes is higher than 1. Within the meaning of the present invention, the momentum of a fluid flow is defined as the mass flow of this fluid (in kg / s) multiplied by its average velocity (in m / s). It is expressed in Newtons (kg*m / s²). The momentum ratio between a mixing gas and the combustion fumes flow is defined as the ratio of the momentum of the mixing gas and the momentum of the combustion fumes. The momentum of the mixing gas is measured at the mixing gas outlet. The momentum of the combustion fumes is measured at the cross-section perpendicular to its flow direction and intersecting the center of the mixing gas entry means. In order to complete the combustion of the CO, an intense mixing of the combustion fumes with comburant gas must be ensured. This is obtained by jet mixing, i.e. the injection of a mixing gas through entry means at high velocity in the connecting channel. The mixing gas can be steam, O2-rich stream or CO2-rich stream such as the exhaust effluent. Advantageously, said ratio of momentum J is higher or equal to 2, more preferably higher or equal to 4, more preferably higher or equal to 5, more preferably higher or equal to 6, in particular higher than or equal to 7, more particularly higher than or equal to 8, even higher than or equal to 9 or higher than or equal to 10 or higher than or equal to 11 or higher than or equal to 12 or higher than or equal to 13 or higher than or equal to 14 or higher than or equal to 15 or higher than or equal to 16 or higher than or equal to 17 or higher than or equal to 18 or higher than or equal to 19 or higher than or equal to 20 or higher than or equal to 21 or higher than or equal to 22 or higher than or equal to 23 or higher than or equal to 24 or higher than or equal to 25 or higher than or equal to 26 or higher than or equal to 27 or higher than or equal to 28 or higher than or equal to 29 or higher than or equal to 30 to ensure a good mixing between the mixing gas and the exhaust effluent. It must be noted that the mixing gas does not require to be continuous or constant. It can be advantageous to enhance mixing to supply the mixing gas in the form of jets incorporating periodic oscillations. Such periodic oscillations can notably take the form of intermittent pulsed jets, but also include sinusoidal oscillations in flow, pressure or velocity, or other shapes of oscillation waves. In the case of jets with periodic oscillations, including pulsed jets, the momentum shall be calculated using the average properties of the jet over the duration of several oscillations or pulsations. In one advantageous embodiment, said cooling step comprises a supply of cooling gas at the bottom of each of said shafts with the same rate of supply or a different rate of supply between the calcining shaft and the preheating shaft, or only of the shaft working in the calcining mode, or only in the shaft working in the preheating mode. Preferably, said cooling step comprises a supply of cooling gas at the bottom of each of said shafts. In a preferred embodiment, said cooling gas is air. In another preferred embodiment, the method according to the present invention comprises a step of collecting a portion of the exhaust effluent in a storage unit, preferably after purification and / or concentration for producing a substantially pure CO2gas, before or after the step of collecting the exhaust effluent in said buffer, preferably after. Other embodiments of the method according to the present invention are mentioned in the appended claims. The present invention also relates In a preferred embodiment, said cooling gas is air. In another preferred embodiment, the method according to the present invention comprises a step of collecting a portion of the exhaust effluent in a storage unit, preferably after purification and / or concentration for producing a substantially pure CO2 gas, before or after the step of collecting the exhaust effluent in said buffer, preferably after. Other embodiments of the method according to the present invention are mentioned in the appended claims. The present invention also relates to a parallel-flow regenerative kiln for implementing the method according to anyone of the preceding claims, comprising - at least a first and a second shafts having a rectangular cross-section, interconnected by a crossover channel, - each of said shafts comprising, in the on or off position, - at least one fuel supply device, - at least one supply opening of combustion air, - an inlet, for loading carbonate mineral stones, at the top of the shafts, - an outlet for unloading the calcined material produced, at the bottom of the shafts, an exhaust effluent discharge duct at the top of the shafts, which is connected to a chimney, and - a supply of cooling gas to cool the calcined material produced, the kiln comprising a system for reversing the operation of the shafts, arranged so that each shaft, in standard mode, operates alternately in calcining mode and in preheating mode, a shaft being in calcining 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 said crossover channel being provided to transfer combustion fumes from the shaft in calcining mode to at least one shaft in preheating mode, wherein the shafts have a rectangular cross-section wherein a first part (36) of said first shaft (1) faces a first part (37) of said second shaft (2), a second part (35) of said first shaft (1) is opposite to said first part (36) of said first shaft (1) and a second part (38) of said second shaft (2) is opposite to said first part (37) of said second shaft (2), said kiln being characterized in that the kiln is further provided with a cooling gas extraction device connected to and controlling a heated cooling gas extraction system provided to withdraw cooling gas, said cooling gas extraction device having an inlet to receive a signal from a temperature probe located in the exhaust effluent discharge duct and being provided to control said heated cooling gas extraction system to output an amount of extracted heated cooling gas by the heated cooling gas extraction system between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft over at least one cycle of the kiln, i.e. a sequence of at least one calcining mode, inversion and preheating mode for each shaft and to maintain the temperature of the exhaust effluent above a threshold value of at least 60°C, preferably above 70°C, more preferably above 80°C. In a preferred embodiment of the kiln according to the present invention, the heated cooling gas extraction device is connected to a modulating valve located on said heated cooling gas extraction system and modulate its opening degree to withdraw by the heated cooling gas extraction system an amount of heated cooling gas comprised between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft continuously over said at least one cycle of the kiln. In one alternative embodiment of the kiln according to the present invention, said heated cooling gas extraction system has an “open and off” position means provided to shift the heated cooling gas extraction system from an open-position to an off-position and vice versa, said open-position and off-position being actioned by said cooling gas extraction device upon a signal from a temperature probe located in the exhaust effluent discharge in such a way that the temperature of the exhaust effluent will remain in a limited range above said threshold, said cooling gas extraction device being further provided to control the flow rate of the extraction of heated cooling gas in the open-position of the heated cooling gas extraction system in such a way the amount of extracted heated cooling gas is comprised between 20 to 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft over said at least one cycle. According to the present invention, the amount of extracted heated cooling gas is for example comprised between 30 and 80 vol%, preferably between 35 and 75 vol%, more preferably between 40 and 70 vol%, even more preferably between 45 and 65 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft and said threshold value is of at least 60°C, preferably above 70°C, more preferably above 80°C. Preferably, in the kiln according to the present invention, the heated cooling gas extraction system is at least in fluid communication with the cooling zone of at least one shaft and is located on - (i) the shaft in calcining mode or, - (ii) the shaft in preheating mode or, - (iii) has a first and second heated cooling gas extraction system, respectively in fluid communication with the shaft in calcining mode and the shaft in preheating mode, both first and second heated cooling gas extraction system being controlled by the cooling gas extraction device to output a predetermined amount x of heated cooling gas being extracted from the shaft in calcining mode and a predetermined amount y of heated cooling gas being extracted from the shaft in preheating mode with x+y being equal to the amount of extracted heated cooling gas, i.e. to the amount comprised between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft. More preferably, in the kiln according to the present invention, the heated cooling gas extraction system is either, (i) Exit means located on the first part of the first and second shaft, below the cross-over channel, connected to a first extracting element being a part of the cooling gas extraction device so as to allow heated cooling gas to be removed from the kiln, said first extracting element being controlled by a first control means of the cooling gas extraction device or (ii) Exit means located in the second part of the first and second shaft connected with a second extracting element being a part of the cooling gas extraction device so as to allow heated cooling gas to be removed from the kiln, said second extracting element being controlled by a second control means of the cooling gas extraction device, said first control means and said second control means being preferably synchronized with said reversing system, or (iii) Exit means located on the first part of the first and second shaft, below the cross-over channel, connected to a first extracting element being a part of the cooling gas extraction device so as to allow heated cooling gas to be removed from the kiln, said first extracting element being controlled by a first control means of the cooling gas extraction device and exit means located in the second part of the first and second shaft connected with a second extracting element being a part of the cooling gas extraction device so as to allow heated cooling gas to be removed from the kiln, said second extracting element being controlled by a second control means of the cooling gas extraction device, said first control means and said second control means being preferably synchronized with said reversing system, wherein exit means comprises a series of orifices comprising X columns of N orifices and Y rows of T orifices, wherein X, N, Y and T are integer comprised between 0 and 20 in which X, N, Y and T cannot be all equal to 0 for each first parts and each second parts. In a preferred kiln according to the present invention, the first and the second extracting element both include at least one control element, able to extract, based on a signal respectively from the first or second control means, from 0 to 100% of the heated cooling gas. Preferably, in the kiln according to the present invention, each columns of N orifices and / or each rows of T orifices are connected to a collector tunnel (25) communicating with the first extracting element (26) and / or with the second extracting element (28) so as to allow extraction of heated cooling gas from the kiln. In a preferred kiln according to the present invention, the exit means comprises a series of orifices comprising X columns of N orifices and Y rows of T orifices wherein the columns and rows are staggered to provide staggered exit means. Preferably, the number of orifices N is different from one column to another column. Preferably, the number of orifices T is different from one row to another row. Preferably, the proximal zone of the two adjacent sides, in proximity of said interior side of the first shaft and the second shaft is in the shape of a polyhedron whose vertical section plane is triangular wherein - the base of the polyhedron is the base of the shaft, - the interior side is one face of the polyhedron , - any vertical line intersecting the upper edge of the polyhedron does not pass through the base of the polyhedron. Preferably, the proximal zone has a height which is comprised between 20% and 40% of the total height of the first shaft 1 or of the second shaft 2. Preferably, the distal zone of the two adjacent sides, in proximity of said exterior side is in a band shape having a height and a width wherein - the exterior side is one face of the band shape, - the height of the band shape is approximatively the height of the shafts , and - the width of the band shape in the upper part of the band shape is comprised between 5% and 20% in width starting from the exterior side as compared to the width of the base of the shaft, wherein the upper part of the band shape represents at least 60% in height towards the height of the band shape, - the width of the band shape in the bottom part of the band shape becomes larger when the height of the band shape decreases, wherein the width of the band shape is equal to the width of the base of the shaft at the bottom of the shafts. According to the present invention, by the term “width” it is preferably meant the horizontal dimension that is perpendicular to the plane of symmetry between the first and second shaft. According to the present invention, by the term “depth” it is preferably meant the horizontal dimension that is parallel to the plane of symmetry between the first and second shaft. According to the present invention, by the term “height” it is preferably meant the vertical dimension that is perpendicular to width and to the depth. Preferably, the exit means of the first shaft and second shaft are located : (i) inside the first part: either inside the proximal zone and / or inside said interior side, and (ii) inside the second part: either inside the distal zone and / or inside the exterior side. Preferably, the exit means of the first shaft and second shaft located inside the first part are located inside an upper part of the first part, such as an upper part of the polyhedron shape wherein said upper part starts at 25% in height, preferably 40% above the base of the polyhedron (% on the vertical line intersecting the upper edge of the polyhedron). Preferably, the exit means of the first part of the first shaft and the exit means of the second part of said second shaft is connected, preferably through a collector tunnel , to a first extracting element being controlled by a first control means, and wherein the exit means of the first inside of the second shaft and the exit means of the second part of the first shaft is connected, preferably through a collector tunnel , to a second extracting element being controlled by a second control means so as to allow heated cooling gas to be removed from the kiln, said first control means and said second control means being synchronized with said reversing system. Preferably, X is comprised between 1 and 50, more preferably between 1 and 10. Preferably, X is comprised between 1 and 50, more preferably between 1 and 10. Preferably, N is comprised between 1 and 100. Preferably, T is comprised between 1 and 100. In a preferred embodiment, in the kiln according to the present invention, the first and the second extracting element both include at least one control element, able to extract, based on a signal respectively from the first or second control means, from 0 to 100% of the heated cooling gas. Preferably, each column of N orifices and / or each row of T orifices are connected to a collector tunnel communicating with the first extracting element and / or with the second extracting element so as to allow extraction of heated cooling gas from the kiln. Preferably, each column of N orifices and each row of T orifices of the exit means are shifted between each other. Preferably, the exit means of each side of said first parts and said second parts are located in the bottom part of each side and / or in the middle part of each side and / or in the upper part of each side, preferably the exit means of said second parts are located at the top of said second parts . Preferably, the collector tunnel 25 comprises a series of openings means being able to provide (i) a fluid communication between the exit means and the first extracting element , or alternatively (ii) a fluid communication between the exit means and the second extracting element . Preferably, the exit means of the first shaft and / or the second shaft is a single opening covering respectively most of the depth and / or the width of the first shaft or of the second shaft. Preferably, the collector tunnel is fluidly connected to the first part of the first shaft and to the second part of the second shaft, and the collector tunnel’ is fluidly connected to the first part of the first shaft and to the second part of the first shaft , each collector tunnels comprising preferably an upper horizontal plane wherein the exit means , in the form of a single opening or a plurality of openings, are located. In yet a further particular embodiment, the kiln according to the present invention comprises equipment for unloading calcined material that is resistant to temperatures greater than 200°C, preferably greater than 300°C, such as equipment in refractory steel. In a preferred embodiment of the kiln according to the present invention, the exit of exhaust effluent is connected to at least one device chosen in the group comprising a filter, a gas cooler, a heat exchanger, a buffer, a storage vessel, a purification unit to purify CO2contained in the exhaust effluent, a concentration device to concentrate the CO2contained in the exhaust effluent, a pressurization device to pressurize the exhaust effluent, such as for producing liquid CO2, a gas heater, a scrubber, a separating element to divide exhaust gas effluent into at least a flow connected to the chimney and a flow connected to another device for treating the exhaust effluent and their combination. In a further preferred embodiment, said connecting channel comprises a series of mixing gas entry means, provided to feed a mixing gas from outside of the kiln into the lumen of the connecting channel, said series of mixing gas entry means being operatively connected to a pressurization device so as to be able to inject a mixing gas at high velocity in said connecting channel throughout said series of mixing gas entry means in such a way that a ratio of momentum J between the gas stream and the combustion fumes is higher than 1. Advantageously, said ratio of momentum J is higher or equal to 2, more preferably higher or equal to 4, more preferably higher or equal to 5, more preferably higher or equal to 6, in particular higher than or equal to 7, more particularly higher than or equal to 8, even higher than or equal to 9 or higher than or equal to 10 or higher than or equal to 11 or higher than or equal to 12 or higher than or equal to 13 or higher than or equal to 14 or higher than or equal to 15 or higher than or equal to 16 or higher than or equal to 17 or higher than or equal to 18 or higher than or equal to 19 or higher than or equal to 20 or higher than or equal to 21 or higher than or equal to 22 or higher than or equal to 23 or higher than or equal to 24 or higher than or equal to 25 or higher than or equal to 26 or higher than or equal to 27 or higher than or equal to 28 or higher than or equal to 29 or higher than or equal to 30 to ensure a good mixing between the mixing gas and the exhaust effluent. In yet a further preferred embodiment, said connecting channel comprises a series of obstacles arranged to increase the mixing between said mixing gas and said exhaust effluent. Advantageously, according to the present invention, the parallel-flow regenerative kiln comprises, downstream or upstream the pressurization means, heating means to heat said mixing gas before injection in the connecting channel, said heating means being preferably chosen amongst a heat exchanger, a combustion chamber, electrical heater. Other embodiments of the parallel-flow regenerative kiln according to the present invention are mentioned in the appended claims Other characteristics and advantages of the present invention will be derived from the non-limitative following description, and by referring to the drawings and the examples. In the drawings, figure 1 schematically shows a PFRK kiln of rectangular cross-section operating in common conditions of combustion. Figure 2 schematically shows one embodiment of the kiln with a rectangular cross-section according to the invention carrying out one embodiment of the process according to the present invention. Figure 3 schematically shows a further embodiment of the kiln with a rectangular cross-section according to the invention carrying out one embodiment of the process according to the present invention. Figure 4 schematically shows a further embodiment of the kiln with a rectangular cross-section according to the invention carrying out one embodiment of the process according to the present invention. Figure 5 schematically shows a further embodiment of the kiln with a rectangular cross-section according to the invention carrying out one embodiment of the process according to the present invention. Figure 6 schematically shows a further embodiment of the kiln with a rectangular cross-section according to the invention carrying out one embodiment of the process according to the present invention. Figure 7 schematically shows a further embodiment of the kiln with a rectangular cross-section according to the invention carrying out one embodiment of the process according to the present invention. Figure 8 schematically shows a further embodiment of the kiln with a rectangular cross-section according to the invention carrying out one embodiment of the process according to the present invention. Figure 9 schematically shows a further embodiment of the kiln with a rectangular cross-section according to the invention carrying out one embodiment of the process according to the present invention. Figure 10 represents a numerical modelling of the oxygen concentration in weight% of the several flows in the PFRK according to the present invention. Figure 11 illustrates different embodiments of the exit means of the kiln with a rectangular cross-section according to the invention. In the drawings, the same reference numbers have been allocated to the same or analog element. Conventionally, the shaft shown on the left is in calcining 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. As can be seen in figure 1, the parallel-flow regenerative kiln shown is a vertical double-shaft kiln 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 rectangular cross-section which are interconnected by a crossover channel comprising a crossover channel 3. The shafts are divided vertically into three areas, the preheating area A where the carbonate stones is preheated before calcination, the combustion area B where the calcination of the carbonate stones occurs and the cooling area C where the cooling of the calcined material occurs. When a shaft is in calcining mode, here the shaft 1, a fuel supply device in the form of lances 4 injects a fuel 9 into the shaft, which, in the example shown, is natural gas. The carbonate stones, loaded at the top of the shaft via an inlet 5, 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 lances 4 and a decarbonation of the carbonate stones to calcined material 10. The exhaust gas 11 formed by the combustion and decarbonation descends co-currently to the calcined material and, moves into the crossover channel 3. Cooling gas 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 gas 12 introduced in the calcination shaft mixes with the combustion fumes 11 in order to move into the crossover channel 3. The calcined material is unloaded via the outlet 8 into a piece of unloading equipment 24. When a shaft is in preheating mode, here the shaft 2, the fuel supply device is closed and the lances 4 do not receive any fuel, but still can be supplied with cooling gas. The inlet 5 of the shaft in preheating mode can be supplied by the carbonate stones one or more times. The opening 6 for supplying combustion air is closed. However, the supply duct 7 for the cooling gas remains in the open position. The unloading equipment 24 continues to discharge lime from the kiln through outlet 8.. After heat exchange with the descending calcined material 10, the heated cooling gas mixes with the combustion fumes 11 which, from the crossover channel 3, enters the shaft. The combustion fumes 11 progresses until reaching the top of the shaft where it is discharged from the kiln via a discharge duct 14 and transferred to a chimney 15, possibly after treatment in equipment’s such as filters. In the shaft in calcining mode 1, this discharge duct 14 is closed. The kiln also comprises a reversing system 16, shown schematically. It controls, in a synchronized manner, the operation of the shafts during the reversing time of the shafts, either directly or remotely. It controls the on and off switching of all elements of the kiln in such a way that, in production mode, each shaft operates alternately in calcining mode and in preheating mode. Figure 2 is a view of preferred embodiment of the kiln according to the present invention where the cooling gas is extracted below the cross-over channel 3 by exit means 39 in fluid connection with a collector tunnel 25, 25’ connected with a first extracting element 26 being a part of the cooling gas extraction device so as to allow heated cooling gas to be removed from the kiln, said first extracting element 26 being controlled by a first control means of the cooling gas extraction device. The exit means 39 and the collector tunnel 25, 25’ are located on the first part 36 of the first shaft 1 and of the first part 37 of the second shaft 2. The first part 36 of the first shaft 1 and the first part 37 of the second shaft are described more in details below and are facing each other. The collector tunnel 25, 25’ has been shown on the two shafts of the illustrated kiln, but it can be only present on one shaft. The collector tunnel can be also present on the two shafts but only working when the shaft is in calcining mode. The collector tunnel 25 can alternatively be present on the two shafts but only working when the shaft is in preheating mode. In the two latter cases, the first extracting element 26 will be activated by the inversion means 16. The heated cooling gas extraction system on figure 2 is accordingly the exit means 39 and the collector tunnel 25 connected to and controlled by the first extraction means 26 of the cooling gas extraction device. The cooling gas extraction device has an input to receive a signal from a temperature probe located in the exhaust effluent discharge duct and being provided to control said heated cooling gas extraction system to output an amount of extracted heated cooling gas by the heated cooling gas extraction system between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft over at least one cycle of the kiln, i.e. a sequence of at least one calcining mode, inversion and preheating mode for each shaft and to maintain the temperature of the exhaust effluent above a threshold value of at least 60°C, preferably above 70°C, more preferably above 80°C. Further, In one embodiment, the heated cooling gas extraction device (first extraction means 26) is connected to a modulating valve located on said heated cooling gas extraction system and modulate its opening degree to withdraw by the heated cooling gas extraction system an amount of heated cooling gas comprised between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft continuously over said at least one cycle of the kiln In another embodiment, the heated cooling gas extraction system has an “open and off” position means provided to shift the heated cooling gas extraction system from an open-position to an off-position and vice versa, said open-position and off-position being actioned by said cooling gas extraction device upon a signal from a temperature probe located in the exhaust effluent discharge in such a way that the temperature of the exhaust effluent will remain in a limited range above said threshold. The cooling gas extraction device (first extraction means 26) is further provided to control the flow rate of the extraction of heated cooling gas in the “open” position of the heated cooling gas extraction system in such a way the amount of extracted heated cooling gas is comprised between 20 to 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft. Preferably, the amount of extracted heated cooling gas is for example comprised between 30 and 80 vol%, preferably between 35 and 75 vol%, more preferably between 40 and 70 vol%, even more preferably between 45 and 65 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft and said threshold value is of at least 60°C, preferably above 70°C, more preferably above 80°C. The kiln according to the present invention can optionally also comprise a heat exchanger 17, fed by the extracted heated cooling gas, for example to preheat the combustion air or the fuel 9. Figure 3 shows an alternative embodiment of the present invention where the where the cooling gas is extracted below the cross- over channel 3, by exit means 39 in fluid connection with a collector tunnel 25, 25’ in the second part 35 of the first shaft 1 and in the second part 38 of the second shaft 2. Below the cross-over channel, the heated cooling gas is hot enough to allow heat recovery through a heat exchanger for example heat exchanger 17. As it will be more apparent from the following, the exit means 39 and the collector tunnel 25 can be located at any height of the second part of the shaft, i.e. well above the cross- over channel 3. The collector tunnels 25, 25’are connected with a second extracting element 28 being a part of the cooling gas extraction device so as to allow heated cooling gas to be removed from the kiln, said second extracting element 28 being controlled by a second control means of the cooling gas extraction device. Collector tunnels 25, 25’ have been shown on the second part of the two shafts of the illustrated kiln, but it can be only present on the second part of one shaft. The collector tunnel 25, 25’ can be also present on the two shafts but only working when the shaft is in calcining mode. The collector tunnels 25, 25’ can alternatively be present on the two shafts but only working when the shaft is in preheating mode. In the two latter cases, the second extracting element 28 will be activated by the inversion means 16. The heated cooling gas extraction system on figure 3 is accordingly the exit means 38 and collector tunnel 25, 25’connected to and controlled by the second extraction means 28 of the cooling gas extraction device. The heated cooling extraction device has an input to receive a signal from a temperature probe located in the exhaust effluent discharge duct and being provided to control said heated cooling gas extraction system to output an amount of extracted heated cooling gas by the heated cooling gas extraction system between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft over at least one cycle of the kiln, i.e. a sequence of at least one calcining mode, inversion and preheating mode for each shaft and to maintain the temperature of the exhaust effluent above a threshold value of at least 60°C, preferably above 70°C, more preferably above 80°C. Further, In one embodiment, the heated cooling gas extraction device (second extraction means 28) is connected to a modulating valve located on said heated cooling gas extraction system and modulate its opening degree to withdraw by the heated cooling gas extraction system an amount of heated cooling gas comprised between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft continuously over said at least one cycle of the kiln In another embodiment, the heated cooling gas extraction system has an “open and off” position means provided to shift the heated cooling gas extraction system from an open-position to a off-position and vice versa, said open-position and off-position being actioned by said cooling gas extraction device upon a signal from a temperature probe located in the exhaust effluent discharge in such a way that the temperature of the exhaust effluent will remain in a limited range above said threshold. Preferably, the amount of extracted heated cooling gas is for example comprised between 30 and 80 vol%, preferably between 35 and 75 vol%, more preferably between 40 and 70 vol%, even more preferably between 45 and 65 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft and said threshold value is of at least 60°C, preferably above 70°C, more preferably above 80°C. The kiln according to the present invention can optionally also comprise a heat exchanger 17, fed by the extracted heated cooling gas, for example to preheat the combustion air or the fuel 9. Figure 4 shows another alternative embodiment of the present invention where the shafts are both provided with exit means 39 and collector tunnel 25, 25’on the first part of each shaft respectively, connected to a first extracting element 26 and exit means 39 and collector tunnel 25, 25’on the second part of each shaft respectively, connected with a second extracting element 28 to extract the cooling gas which has been heated by heat exchange with the descending stones. The heated cooling gas extraction system comprises accordingly the exit means 39 and collector tunnel 25, 25’ of the first part 36, 37 of the first shaft 1 and second shaft 2 and the exit means of the second part 35, 38 of the first shaft 1 and second shaft 2 while the cooling gas extraction device comprises the first extracting element 26 and the second extracting element 28. The first extracting element 26 and the second extracting element 28 have each an input to receive a signal from a temperature probe located in the exhaust effluent discharge duct 14 and being provided to control respectively the exit means 39 and collector tunnel 25, 25’ of the first part 36, 37 of the first shaft 1 and second shaft 2 and the exit means 39 of the second part 35, 38 of the first shaft 1 and second shaft 2 to output an amount of extracted heated cooling gas by the heated cooling gas extraction system between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft over at least one cycle of the kiln, i.e. a sequence of at least one calcining mode, inversion and preheating mode for each shaft and to maintain the temperature of the exhaust effluent above a threshold value of at least 60°C, preferably above 70°C, more preferably above 80°C. Further, In one embodiment, the first extraction means 26 and second extraction means 28 are each connected to a modulating valve located on the exit means 39 and collector tunnel 25, 25’ of the first part 36, 37 of the first shaft 1 and second shaft 2 and the exit means of the second part 35, 38 of the first shaft 1 and second shaft 2 and modulate its opening degree to withdraw by them an amount of heated cooling gas comprised between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft continuously over said at least one cycle of the kiln. In another embodiment, the cooling gas extraction device (first extraction means 26 and second extraction means 28) is provided to control an “on and off” position of said heated cooling gas extraction system (exit means 39 and collector tunnel 25, 25’ of the first part 36, 37 of the first shaft 1and second shaft2 and of the second part 35, 38 of the first shaft 1 and second shaft 2), said open-position and off-position being actioned by said cooling gas extraction device upon a signal from a temperature probe located in the exhaust effluent discharge in such a way that the temperature of the exhaust effluent will remain in a limited range above said threshold. Further, the cooling gas extraction device (first extraction means 26 and second extraction means 28) is further provided to control the flow rate of the extraction of heated cooling gas in the “on” position of the heated cooling gas extraction system in such a way the amount of extracted heated cooling gas is comprised between 20 to 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft. In the illustrated embodiment, the first extracting element 26 and the second extracting element 28 both include at least one control element, able to extract, based on a signal respectively from the first or second control means, from 0 to 100% of the heated cooling gas. The control element of the first extracting element 26 connected to exit means 39 and collector tunnel 25 of the first part 36 of the shaft in calcining mode allows for example to extract from 50 to 100%, preferably from 70 to 100%, preferably from 75 to 99%, more preferably from 80 to 98% by volume on dry gas basis of heated cooling gas relative to the introduced cooling gas in the same shaft from the shaft in calcining mode through said collector tunnel 25’, based on a signal from the first control means. The control element of the second extracting element 28 connected to the exit means 39 and collector tunnel 25’ on the second part 35 of the shaft in calcining mode allows to extract from 0 to 50%, preferably from 0 to 30%, preferably from 1 to 25%, more preferably from 2 to 20% by volume on dry gas basis of heated cooling gas relative to the introduced cooling gas in the same shaft from the shaft in calcining mode through said collector tunnel 25’, based on a signal from the second control means. The control element of the first extracting element 26 connected to the exit means 39 and collector tunnel 25’ of the first part 37 of the shaft in preheating mode allows for example to extract from 0 to 50%, preferably from 0 to 30%, preferably from 1 to 25%, more preferably from 2 to 20% by volume on dry gas basis of heated cooling gas relative to the introduced cooling gas in the same shaft from the shaft in preheating mode through said collector tunnel 25’, based on a signal from the first control means. The control element of the second extracting element 28 connected to the exit means 39 and collector tunnel 25 of the second part 38 of the shaft in preheating mode allows to extract from 50 to 100%, preferably from 70 to 100%, preferably from 75 to 99%, more preferably from 80 to 98% by volume on dry gas basis of heated cooling gas relative to the introduced cooling gas in the same shaft from the shaft in preheating mode through the collector tunnel 25, based on a signal from the second control means. Moreover, in order to recover a portion of the energy from the hot air removed by the extraction element 26, (but also possibly 28) a heat exchange may be provided with the combustion air or the fuel 9, through the heat exchanger 17 Figure 5 illustrates an embodiment of the kiln with a rectangular cross-section according to the invention with a focus on the first parts 36, 37 and second parts 35, 38 of the shafts, together with a schematic representation of the proximal zone of the first shaft 36’ and the distal zone 38’ of the second shaft. The proximal zone of the second shaft and the distal zone of the first shaft are not represented. Figure 6 further illustrates an embodiment of the kiln with a rectangular cross-section according to the invention with a focus on the connection of the collector tunnel 25 to the first extracting element 26 and the connection of the collector tunnel 25’ with the second extracting element 28. In more details, the shafts are provided, below the crossover channel, with a collector tunnel 25, 25’. The collector tunnel 25 connects the exit means 39 of the first part 36 of the first shaft 1 and the exit means 39 of the second part 38 of the neighboring shaft 2 with a first extracting element 26 so as to allow heated cooling gas to be removed from the kiln, said first extracting element 26 being controlled by a first control means (not shown), and the collector tunnel 25’ connects the exit means 39 of the second part 35 of the first shaft 1 and the exit means 39 of the first part 37 of the second shaft 2 with the second extracting element 28 so as to allow heated cooling gas to be removed from the kiln, said second extracting element 28 being controlled by a second control means (not shown), said first control means and said second control means being synchronized with said reversing system. In the embodiment of Fig. 6, the collector tunnels 25 and 25’ connecting exit means 39 are all located in the bottom of the shaft, below the crossover channel. As illustrated, the collector tunnel(s) 25 connected to exit means 39 are located inside the first part of the first shaft 1 and inside the second part of the second shaft 2. The collector tunnel(s) 25’ connected to exit means 39 are located inside the second part of the first shaft and inside the first part of the second shaft. Alternatively to the embodiment of Fig. 6, Figure 7 illustrates an embodiment of the kiln with a rectangular cross-section according to the invention with a focus on the connection of the collector tunnel(s) 25 to the first extracting element 26 and the connection of the collector tunnel(s) 25’ with the second extracting element 28 wherein (i) the collector tunnels 25’ each connected to exit means 39 are located inside an upper part of the second part of the first shaft and inside an upper part of the first part of the second shaft , and (ii) the collector tunnels 25 each connected to exit means 39 are located inside an upper part of the second part of the second shaft and inside an upper part of the first part of the first shaft. In addition, Figure 8 illustrates an embodiment of the kiln with a rectangular cross-section according to the invention combining the embodiment of Fig.6 and the embodiment of Fig.7 wherein (i) the collector tunnels 25’ each connected to exit means 39 are located inside an upper part and a bottom part of the second part of the first shaft and inside an upper part of the first part of the second shaft, and (ii) the collector tunnels 25 each connected to exit means 39 are located inside an upper part and a bottom part of the second part of the second shaft and inside an upper part of the first part of the first shaft. In the illustrated embodiment, the first extracting element 26 and the second extracting element 28 both include at least one control element, able to extract, based on a signal respectively from the first or second control means, from 0 to 100% of the heated cooling gas. The control element of the first extracting element 26 connected to the collector tunnel 25 of the shaft in calcining mode allows for example to extract from 50 to 100%, preferably from 70 to 100%, preferably from 75 to 99%, more preferably from 80 to 98% by volume on dry gas basis of heated cooling gas from the shaft in calcining mode, relative to the introduced cooling gas in the same shaft, through said collector tunnel 25, based on a signal from the first control means. The control element of the second extracting element 28 connected to the collector tunnel 25’ of the shaft in calcining mode allows for example to extract from 0 to 50%, preferably from 0 to 40%, preferably from 0 to 30%, more preferably from 0 to 25% by volume on dry gas basis of heated cooling gas from the shaft in calcining mode, relative to the introduced cooling gas in the same shaft, through said collector tunnel 25’, based on a signal from the second control means. The control element of the first extracting element 26 connected to the collector tunnel 25 of the shaft in preheating mode allows to extract from 50 to 100%, preferably from 70 to 100%, preferably from 75 to 99%, more preferably from 80 to 98% by volume on dry gas basis of heated cooling gas from the shaft in preheating mode, relative to the introduced cooling gas in the same shaft, through said collector tunnel 25, based on a signal from the first control means. The control element of the second extracting element 28 connected to the collector tunnel 25’ of the shaft in preheating mode allows for example to extract from 0 to 50%, preferably from 0 to 30%, preferably from 1 to 25%, more preferably from 2 to 20% by volume on dry gas basis of heated cooling gas from the shaft in preheating mode, relative to the introduced cooling gas in the same shaft, through said collector tunnel 25’, based on a signal from the first control means. Alternatively, extracting element can have a closed position and an open position, wherein the first extracting element connected to the collector tunnel 25 of the shaft in calcining mode is in open position to extract nearly 100% by volume of extracted (or to be extracted) heated cooling gas on dry gas basis and the second extracting element connected to the collector tunnel 25’ of the shaft in calcining mode is in closed position to not extract cooling gas or only extract a very few % of the extracted (or to be extracted) heated cooling gas due to some unavoidable leakage. The first extracting element connected to the collector tunnel 25 of the shaft in preheating mode is in open position to extract nearly 100% of heated cooling gas by volume on dry gas basis and the second extracting element connected to the collector tunnel 25’ of the shaft in preheating mode is in closed position to not extract cooling gas or almost extract a very few % of the heated cooling gas due to some unavoidable leakage. Alternatively, Figure 7A illustrates an embodiment of the kiln with a rectangular cross-section according to the invention wherein each collector tunnel 25, 25’ of the first shaft 1 and the second shaft 2 comprises an upper horizontal plane 25’’, wherein each collector tunnel 25, 25’ is connected to the exit means 39 and wherein the exit means 39 is a single opening located in the upper horizontal plane 25’’, wherein the single opening covering respectively most of the depth and / or the width of the shaft 1 and / or of the shaft 2. Figure 7b illustrates an enlargement of one collector tunnel 25 having the upper horizontal plane 25’’ and wherein the collector tunnel 25 is connected to the exit means 39 wherein the exit means 39 consists of a single opening. Figure 8 represents a numerical modeling of the Parallel-flow regenerative kiln with a rectangular 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 shafts is not represented. Zones a: in the shaft in calcining mode, combustion air mixed with fuel. Zones b: jets of combustion fumes emitted by the lances, in which there is almost no more oxygen and between which we can still find a little O2 from combustion air which has not reacted. Zone c’: interface between the fumes and the cooling gas / air in the shaft in calcining mode. Zone d: in the shaft in preheating mode, cooling gas / air. Zone e: mixture mainly composed of cooling air introduced in the shaft of preheating mode and a low portion of gas stream coming from the crossover channel. Zone f: the fumes penetrate deep into cooling zone C, below the level of the crossover channel in the shaft in calcining mode, mixing little with the cooling gas, such as cooling air. They push the gas mixture laterally to the crossover channel 3. Zone g: cooling gas / air in the shaft in calcining mode,. Figure 11 illustrates different embodiments of the exit means 39 wherein Fig. 11A illustrates a series of exit means comprising X columns of N aligned orifices. Fig. 11B illustrates a series of exit means where the columns and rows are staggered providing an arrangement of staggered exit means. The exit means can be in the form of a single slit shape (Fig. 11C) or in the form of a plurality of slit shape (Fig.11D). The present invention was illustrated with some embodiments but of course other combinations are possible such as for example a combination of the embodiment of figure 2 and the embodiment of figure 3. It should be understood that the present invention is not limited to the described embodiments and that variations can be applied without going outside of the scope of the appended claims.

Claims

« CLAIMS » 1. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln having at least a first (1) and a second shafts (2) interconnected by a crossover channel (3), comprising, in standard operation, - loading carbonate mineral stones at the top of each shaft (1, 2), - preheating these loaded stones in a preheating zone, - calcining these preheated stones in a calcination zone with production of a decarbonated calcined material (10), - cooling the calcined material (10) with cooling gas in a cooling zone, with formation of heated cooling gas by heat exchange, - discharging the calcined material (10) from the bottom of the shafts (1, 2), - exhausting an exhaust effluent from the kiln, - each shaft alternately working in a calcining mode and in a preheating mode, one shaft working in a calcining mode during a predetermined time period during which at least another shaft works in a preheating mode, and inversely after activation of the inversion means, - the calcining mode comprising : • Upon said preheated carbonate mineral stones descending into said shaft, decarbonation of the preheated carbonate mineral stones with the release of combustion fumes (11) descending co-currently in the shaft in calcining mode, and • through said crossover channel (3), a passage of said combustion fumes toward the at least one shaft working in a preheating mode, - said preheating mode comprising :• said preheating step of the loaded carbonate mineral stones by heat exchange with said combustion fumes (11) coming from the crossover channel (3), which is ascending and flows in counter-current through the loaded carbonate mineral stones, and • said exhausting step of said combustion fumes (11) as exhaust effluent at the top of said at least one shaft in preheating mode, characterized in that said method further comprises a step of withdrawal outside of the kiln of an extracted heated cooling gas formed by from 20 to 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft from - (i) the shaft in calcining mode or, - (ii) from the shaft in preheating mode or, - (iii) from both the shaft in calcining mode and the shaft in preheating mode with a predetermined amount x of heated cooling gas being extracted from the shaft in calcining mode and a predetermined amount y of heated cooling gas being extracted from the shaft in preheating mode with x+y being equal to the amount of extracted heated cooling gas, i.e. to the amount comprised between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft.

2. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln according to claim 1, wherein the step of withdrawal outside of the kiln of an extracted heated cooling gas formed by from 20 to 90 vol% of said heated cooling gas is a step of withdrawal outside of the kiln of an extracted heated cooling gas formed by from 30 to 80 vol%, preferably from 35 to 75 vol%, more preferably from 40 to 70 vol%, even more preferably from 45 to 65 vol% of said heated cooling gas,so the amount of extracted heated cooling gas is comprised between 30 and 80 vol%, preferably between 35 and 75 vol%, more preferably between 40 and 70 vol%, even more preferably between 45 and 65 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft, said step of withdrawal of heated cooling gas is preferably controlled by the temperature of the exhaust effluent exhausted at the top of the shaft in preheating mode in such a way that said amount of extracted heated cooling gas is continuously controlled, preferably continuously modulated to maintain the temperature of the exhaust effluent above a threshold value of at least 60°C, preferably above 70°C, more preferably above 80°C.

3. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln according to claim 1 or claim 2, wherein when said extracted heated cooling gas is extracted from the shaft in calcining mode, said extracted heated cooling gas is extracted - from 50 to 100% by volume on dry gas basis relative to the introduced cooling gas in the same shaft by exit means located in a first part (36) of a first shaft (1) or by exit means located in a first part (37) of the second shaft (2), and - from 0 to 50% by volume on dry gas basis relative to the introduced cooling gas in the same shaft by exit means located in a second part (35) of said first shaft (1) or by exit means located in a second part (38) of said second shaft (2), and when said extracted heated cooling gas is extracted from the shaft in preheating mode, said extracted heated cooling gas is extracted - from 50 to 100% by volume on dry gas basis relative to the introduced cooling gas in the same shaft by exit means located in said second part (35) of said first shaft (1) or by exit means located in said second part (38) of said second shaft (2), and- from 0 to 50% by volume on dry gas basis relative to the introduced cooling gas in the same shaft by exit means located in said first part (36) of said first shaft (1) or by exit means located in said first part (37) of said second shaft (2).

4. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln according to any of the claims 1 to 3, wherein when said extracted heated cooling gas is extracted from the shaft in calcining mode, said extracted heated cooling gas is extracted - from 75 to 99%, more preferably from 80 to 98% by volume on dry gas basis relative to the introduced cooling gas in the same shaft through said exit means located in a first part (36) of a first shaft (1) or by exit means located in a first part (37) of the second shaft (2) and - from 0 to 30%, preferably from 1 to 25%, more preferably from 2 to 20% by volume on dry gas basis relative to the introduced cooling gas in the same shaft through said exit means located in a second part (35) of said first shaft (1) or by exit means located in a second part (38) of said second shaft (2) and when said extracted heated cooling gas is extracted from the shaft in preheating mode, said extracted heated cooling gas is extracted - from 0 to 30%, preferably from 1 to 25%, more preferably from 2 to 20% by volume on dry gas basis relative to the introduced cooling gas in the same shaft through said exit means located in said first part (36) of said first shaft (1) or by exit means located in said first part (37) of said second shaft (2) and - from 70 to 100%, preferably from 75 to 99%, more preferably from 80 to 98% by volume on dry gas basis relative to the introduced cooling gas in the same shaft through said exit means located in said second part (35) of said first shaft (1) or by exit means located in said second part (38) of said second shaft (2).

5. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln according to any of the claims 1 to 4, further comprising injection of combustion air and fuel in the shaft in calcining mode, preferably in the calcination zone, to perform the decarbonation of preheated carbonate mineral stones through a combustion of fuel, the combustion fumes comprising CO2 from the decarbonation of carbonate mineral stones and from combustion of fuel.

6. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln according to claim 5, wherein said combustion of fuel comprises introducing a gaseous, liquid or solid fuel into the shaft in calcining mode and in that, in the case of a solid fuel, said introduction is carried out using air or a collected portion of exhaust effluent discharged from the kiln, or using another source of CO2as a carrier gas or using another source of gas as a carrier gas, such as combustion air.

7. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln according to claim 5 or claim 6, wherein a portion of the exhaust effluent discharged from the kiln is introduced in lances (4) for fuel injection or in an outer envelope enclosing lances (4) for fuel injection for cooling said lances (4) in the shaft in preheating mode.

8. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln according to any of the claims 1 to 7, wherein the exhaust effluent is partially or fully collected in at least one buffer after said exhausting step.

9. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln according to any of the preceding claims, wherein said cooling step comprises a supply of cooling gas at the bottom of each of said shafts or only of the shaft working in the calcining mode, or only in the shaft working in the preheating mode.

10. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln according to any of the preceding claims, wherein said cooling gas is air.

11. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln according to any of the preceding claims, further comprising a step of collecting a portion of the exhaust effluent in a storage unit, preferably after purification and / or concentration for producing a substantially pure CO2 gas, before or after the step of collecting the exhaust effluent in said buffer, preferably after.

12. Parallel-flow regenerative kiln for carrying out the method according to anyone of the preceding claims, comprising - at least a first (1) and a second shafts (2) having a rectangular cross-section, interconnected by a crossover channel (3), - each of said shafts (1, 2) comprising, in the on or off position, - at least one fuel supply device, - at least one supply opening of combustion air, - an inlet (5), for loading carbonate mineral stones, at the top of the shafts (1, 2), - an outlet (8) for unloading the calcined material produced, at the bottom of the shafts (1, 2), - an exhaust effluent discharge duct (14) at the top of the shafts (1, 2), which is connected to a chimney (15), and - a supply of cooling gas (7) to cool the calcined material produced, the kiln comprising a system (16) for reversing the operation of the shafts (1, 2), arranged so that each shaft, in standard mode, operates alternately in calcining mode and in preheating mode, a shaft being in calcining mode for a predetermined time period while at least one other shaft is in preheating mode, and vice-versa, this reversing system (16) therefore controlling said on and off positions,wherein said crossover channel (3) is provided to transfer combustion fumes (11) from the shaft in calcining mode to at least one shaft in preheating mode, wherein the shafts have a rectangular cross-section wherein a first part (36) of said first shaft (1) faces a first part (37) of said second shaft (2), a second part (35) of said first shaft (1) is opposite to said first part (36) of said first shaft (1) and a second part (38) of said second shaft (2) is opposite to said first part (37) of said second shaft (2), said kiln being characterized in that the kiln is further provided with a cooling gas extraction device connected to and controlling a heated cooling gas extraction system provided to withdraw cooling gas, said cooling gas extraction device having an input to receive a signal from a temperature probe located in the exhaust effluent discharge duct (14) and being provided to control said heated cooling gas extraction system to output an amount of extracted heated cooling gas by the heated cooling gas extraction system between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft over at least one cycle of the kiln, i.e. a sequence of at least one calcining mode, inversion and preheating mode for each shaft and to maintain the temperature of the exhaust effluent above a threshold value of at least 60°C, preferably above 70°C, more preferably above 80°C.

13. Parallel-flow regenerative kiln according to claim 12, wherein the heated cooling gas extraction device is connected to a modulating valve located on said heated cooling gas extraction system and modulate its opening degree to withdraw by the heated cooling gas extraction system an amount of heated cooling gas comprised between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft continuously over said at least one cycle of the kiln.

14. Parallel-flow regenerative kiln according to claim 12, wherein said heated cooling gas extraction system has an “open and off” position means provided to shift the heated cooling gas extraction system from an open-position to an off-position and vice versa, said open-position and off-position being actioned by said cooling gas extraction device upon a signal from a temperature probe located in the exhaust effluent discharge in such a way that the temperature of the exhaust effluent will remain in a limited range above said threshold, said cooling gas extraction device being further provided to control the flow rate of the extraction of heated cooling gas in the open-position of the heated cooling gas extraction system in such a way the amount of extracted heated cooling gas is comprised between 20 to 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft over said at least one cycle 15. Parallel-flow regenerative kiln according to any of the claims 12 to 14, wherein the heated cooling gas extraction system is at least in fluid communication with the cooling zone of at least one shaft and actionable from a on position to an off-position and vice versa, said heated cooling gas extraction system being located on (i) the shaft in calcining mode or, (ii) the shaft in preheating mode or, (iii) has a first and second heated cooling gas extraction system, respectively in fluid communication with the shaft in calcining mode and the shaft in preheating mode, both first and second heated cooling gas extraction system being controlled by the cooling gas extraction device to output a predetermined amount x of heated cooling gas being extracted from the shaft in calcining mode and a predetermined amount y of heated cooling gas being extracted from the shaft in preheating mode with x+y being equal to the amount of extracted heated coolinggas, i.e. to the amount comprised between 20 and 90 vol% of said heated cooling gas relative to the introduced cooling gas in the same shaft.

16. Parallel-flow regenerative kiln according to any of the claims 12 to 15, wherein the heated cooling gas extraction system is either, (i) Exit means (39) located on the first part (36, 37) of the first (1) and second shaft (2), below the cross-over channel (3), connected to a first extracting element (26) being a part of the cooling gas extraction device so as to allow heated cooling gas to be removed from the kiln, said first extracting element (26) being controlled by a first control means of the cooling gas extraction device or (ii) Exit means (39) located in the second part (35, 38) of the first (1) and second shaft (2) connected with a second extracting element (28) being a part of the cooling gas extraction device so as to allow heated cooling gas to be removed from the kiln, said second extracting element (28) being controlled by a second control means of the cooling gas extraction device, said first control means and said second control means being preferably synchronized with said reversing system (16), or (iii) Exit means (39) located on the first part (36, 37) of the first (1) and second shaft (2), below the cross-over channel (3), connected to a first extracting element (26) being a part of the cooling gas extraction device so as to allow heated cooling gas to be removed from the kiln, said first extracting element (26) being controlled by a first control means of the cooling gas extraction device and exit means (39) located in the second part (35, 38) of the first (1) and second shaft (2) connected with a second extracting element (28) being a part of the cooling gas extraction device so as to allow heated cooling gas to be removed from the kiln, said second extracting element (28)being controlled by a second control means of the cooling gas extraction device, said first control means and said second control means being preferably synchronized with said reversing system (16)wherein exit means (39) comprises a series of orifices comprising X columns of N orifices and Y rows of T orifices, wherein X, N, Y and T are integer comprised between 0 and 20 in which X, N, Y and T cannot be all equal to 0 for each first parts (36, 37) and each second parts (35, 38).

17. Parallel-flow regenerative kiln according to claim 16, wherein the first and the second extracting element both include at least one control element, able to extract, based on a signal respectively from the first or second control means, from 0 to 100% of the heated cooling gas.

18. Parallel-flow regenerative kiln according to claim 16 or claim 17, wherein each columns of N orifices and / or each rows of T orifices are connected to a collector tunnel (25) communicating with the first extracting element (26) and / or with the second extracting element (28) so as to allow extraction of heated cooling gas from the kiln.

19. Parallel-flow regenerative kiln according to any of the claims 16 to 18, wherein each columns of N orifices and each rows of T orifices of the exit means (39) are shifted between each other.

20. Parallel-flow regenerative kiln according to any of the claims 16 to 19, wherein the exit means (39) of each side of said first parts (36,37) and said second parts (35, 38) are located in the bottom part of each side and / or in the middle part of each side and / or in the upper part of each side, preferably the exit means (39) of said second parts (35, 38) are located at the top of said second parts (35, 38).

21. Parallel-flow regenerative kiln according to any of the claims 16 to 20, wherein said collector tunnel (25) comprises a series of openings means being able to provide (i) a fluid communication betweenthe exit means (39) and the first extracting element (26), or (ii) a fluid communication between the exit means (39) and the second extracting element (28).

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