Method for calcining carbonate mineral stones in a parallel flow regenerative kiln and implemented kiln

Abstract

Method for calcining carbonate mineral stones in a parallel flow regenerative kiln wherein the method comprises an extraction step of the heated cooling gas through a series of extracting port located in an effective cooling zone within a cooling zone below a connecting channel, said heating cooling gas enters into an implementation port orifice and passes into a port duct to reach an evacuation element at an average extraction velocity comprised between 0.5 and 5 m / s.

Description

[0001] Method for calcining carbonate mineral stones in a parallel flow regenerative kiln and implemented kiln.

[0002] 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 connecting 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.

[0003] 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 d₅₀ comprised 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.

[0004] By “connecting channel”, it is meant according to the present invention, all the ducting and spaces void of stones that allow the combustion gases to flow from the shaft in calcining mode to the shaft(s) in preheating mode. This connecting channel comprises one or several crossover channels and possibly peripheral channels.

[0005] By “crossover channel”, it is meant according to the present invention, the straight part (as seen from top) of the connecting channel located between two shafts. If peripheral channels are existing, the peripheral channels of two shafts will be connected by a crossover channel. By “peripheral channel”, it is meant according to the present invention, the part of the connecting channel located at the periphery, or around a shaft, particularly in the case of a circular shaft, at the exception of the part of the periphery already occupied by the crossover channel.

[0006] 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.

[0007] A Parallel Flow Regenerative Kiln usually has 2 to 3 shafts, of circular or rectangular section, which do not work in a continuous way. In standard operation, in every period, usually of 10 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 air 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 air. This flue gas is drawn into another shaft (or the 2 other shafts) through the connecting channel and goes thereafter through the stones present in this shaft (or those 2 other shafts) and thereafter outward the kiln. So, in this “preheating” shaft(s), 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 and the shaft(s) wherein the flue gas is drawn through the stones work(s) according to a preheating mode. 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 (or one of the 2 other shafts) 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 connecting channel, comprises, in standard operation,

[0008] - 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,

[0009] - cooling the calcined material with cooling air in a cooling zone, with formation of a heated cooling air, by heat exchange,

[0010] - discharging the calcined material from the bottom of the shafts,

[0011] - exhausting a gaseous effluent from the kiln,

[0012] - each shaft alternately working in a calcining mode and in a preheating mode, one shaft working in a calcination mode during a predetermined time period during which at least another shaft works in a preheating mode, and inversely,

[0013] - the calcining mode comprising:

[0014] said loading step of carbonate mineral stones at the top of a kiln shaft,

[0015] 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 connecting channel, a passage of said gas stream toward the at least one shaft working in a preheating mode,

[0016] - said preheating mode comprising:

[0017] said preheating step of the loaded carbonate mineral stones by heat exchange with said gas stream coming from the connecting channel, which is ascending and flows in counter-current through the loaded carbonate mineral stones, and said exhausting step of said gas stream as gaseous effluent at the top of said at least one shaft in preheating mode,

[0018] said cooling step comprising a supply of cooling air at the bottom of each of said shafts or only of the shaft working in the calcining mode.

[0019] 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 between 100°C and 150°C, limiting the energy losses.

[0020] According to the invention, standard operation means that the kiln produces the calcined material in a continuous operation. This operation does not concern the phases of starting, stopping or maintenance of the kiln.

[0021] 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.

[0022] The calcination reaction of limestone into quicklime is:

[0023] CaCO₃ (solid) + heat

[0024]

[0025] 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°C lime and CO2 can easily recombine. But from a temperature of the order of 900°C and above the stones give off a significative volume of CO2 during 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.

[0026] 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 a gaseous effluent being released at the top of the furnace 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 costly to capture because of the large presence of diatomic nitrogen from the air used.

[0027] To capture this CO2, it may be considered to use an “end-of-pipe” method of abatement, notably cryogenic or by chemical solvent called "amines", which is the most widespread technique applied to the furnace 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.

[0028] More recently, to be able to capture the CO2 emitted in a PFRK furnace, operating PFKR in oxy-combustion has been proposed (see for example JP2002060254). However, the concentration of CO2 in the exhaust effluent is still below 50 % vol on a dry basis.

[0029] For the same purpose, it has also been proposed to replace all the air from the method, combustion air and cooling air, with recycled combustion fumes and introducing pure oxygen into the shaft in calcining mode (see CN 105000811). For any person skilled in the art, it is clear that this process is unfeasible, since the lime will recarbonate during cooling. As seen above, the CO2 cannot be recirculated to cool the lime, since the lime will immediately recombine with this CO2 to form again a carbonate, notably CaCO₃. On the other hand, using pure oxygen at the top of the furnace poses serious problems in terms of material compatibility (notably due to excessive temperature) and this input will not be a sufficient mass flow to effectively recover the heat accumulated in the regeneration area. The disadvantages and feasibility problems of this method have also been discussed in the patent application US2020 / 0048146.

[0030] It should also be noted that the cooling air in the PFRK, in contrast to the rotary kiln, for example, does not play a significant role on the combustion and the calcination process in the shaft in calcining mode.

[0031] Standard mode means that the furnace 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 furnace or to maintenance in the event of a malfunction.

[0032] Variation of the common calcination process have been proposed in order to improve capture of CO2, such as for example in W02022 / 002869 or WO2022 / 229120.

[0033] In PFRK kilns operation with a regular combustion of fuel in presence of combustion air, the mixing between fuel and comburant gas is relatively poor in the combustion shaft. This potentially leads to CO generation. The generated CO is typically reburned in contact with the cooling air that comes in contact with the combustion fumes in the connecting channel or a bit earlier. Even if the mixing of these two streams is relatively poor, the vast excess of cooling air allows to reburn the CO at the level of the connecting channel.

[0034] In PFRK kiln working in oxy-com bustion, the exhaust gaseous effluent should be as concentrated in CO2 as possible. Accordingly, there has been some proposal to produce concentrated exhaust gas. One proposal that was discussed above is to replace the cooling air by exhaust gaseous effluent such as disclosed in CN 10500081 and in JP2002060254. Other proposals are to isolate the cooling air from the exhaust gaseous effluent (see W02022 / 238385, WO2022 / 238384 or WO2022 / 229120). In W02022 / 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 gaseous effluent concentrated in CO2 and the cooling air.

[0035] In W 02022 / 229118, it was proposed that the cooling gas heated in the cooling zone is discharged from the cooling zone of the shaft via a cooling gas discharge device, and that the amount of cooling gas discharged from the cooling zone of the shaft via the cooling gas discharge device can be regulated. In particular, it is described that a gas analysis device is connected to a control device for transmitting the determined oxygen and / or CO2 content of the cooling gas and / or exhaust effluent. Based on the amount of CO2 and / or oxygen in the cooling gas and / or exhaust effluent, a control element can adapt the amount of oxidizing agent to be introduced in the shaft.

[0036] The prior art indicates that heated cooling air extraction in the cooling zone is important to avoid mixing between the exhaust gaseous effluent concentrated in CO2 and the cooling air, and that regulation of cooling air extraction is advantageous to optimize a recirculation circuit arranged between the effluent discharge duct of the shafts and the oxidant supply openings of the shafts.

[0037] In practice, the extraction of the heated cooling air / gas present in the furnace may entrain a lot of fines and combustion waste present in the furnace. In order to be able to reuse and / or treat the heated cooling air / gas of the furnace, there is still a need to provide a method for calcining carbonate mineral stones to be carried out in PFRK allowing an extraction of the heated cooling air / gas without changing the cyclical operation thereof and with few or no changes to the structure thereof while carrying a minimum of fines / dust with the extracted heated cooling air / gas so as not to obstruct the heated cooling air / gas treatment device(s) located downstream of the furnace. In addition, the extraction of the heated cooling gas should be performed with an adequate flow rate in order to ensure an optimal operational mode of the furnace while allowing to recover a maximum concentration of CO2 in the exhaust effluent. Moreover, the temperature of the extracted heated cooling gas should be sufficiently high so as to be able to reuse the heated cooling gas in post-treatment operations. This method should be able to promote at the same time a high extraction rate of the heated cooling gas at a sufficiently high temperature while avoiding to entrain fines and combustion waste during the extraction of the heated cooling air / gas.

[0038] To solve these problems, the present invention provides a method for calcining carbonate mineral stones in a parallel flow regenerative kiln having at least two shafts interconnected by a connecting channel, comprising, in standard operation,

[0039] - 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,

[0040] - cooling the calcined material with cooling gas in a cooling zone, said cooling zone presenting a cooling zone height, with formation of heated cooling gas by heat exchange,

[0041] - discharging the calcined material from the bottom of the shafts,

[0042] - exhausting a gaseous effluent from the kiln,

[0043] - each shaft alternately working in a calcining mode and in a preheating mode, one shaft working in a calcination 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,

[0044] - the calcining mode comprising:

[0045] • 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 calcination mode, and through said connecting channel, a passage of said combustion fumes toward the at least one shaft working in a preheating mode

[0046] - said preheating mode comprising:

[0047] • said preheating step of the loaded carbonate mineral stones by heat exchange with said combustion fumes coming from the connecting channel, which is ascending and flows in counter-current through the loaded carbonate mineral stones, and

[0048] • said exhausting step of said fumes as exhaust effluent at the top of said at least one shaft in preheating mode, said method further comprising

[0049] extracting said heated cooling gas from said shaft working in a calcining mode and / or from said shaft working in a preheating mode through a cooling gas extraction system comprising a series of extracting ports and an evacuation element,

[0050] wherein each extracting port of the series of extracting ports comprises - an implementation port orifice in a peripheral wall of said shaft working in a calcining mode and / or from said shaft working in a preheating mode and / or into a wall of a central collector element, said implementation port orifice, facing the interior volume of the shafts, is located into an effective cooling zone within said cooling zone below the connecting channel, and

[0051] - a port duct crossing said peripheral wall of said shaft working in a calcining mode and / or of said shaft working in a preheating mode and / or crossing said wall of said central collector element, said port duct being arranged to fluidly connect the interior volume of each shaft to said evacuation element,

[0052] wherein said heating cooling gas enters into said implementation port orifice and passes into said port duct to reach said evacuation element at an average extraction velocity comprised between 0.5 and 5 m / s, preferably between 0.5 and 3.5 m / s,

[0053] wherein said effective cooling zone presents an effective cooling zone height, said effective cooling zone height having a height comprised between 25% and 75% of the cooling zone height starting from the base of the shaft (effective cooling zone height / cooling zone height).

[0054] Indeed, a method according to the present invention having the above-mentioned features and centered on the fact that the extracting step of heated cooling gas is performed through a cooling gas extraction system comprising a series of extracting ports and an evacuation element, wherein each extracting port having an implementation port orifice being located into the effective cooling zone below the connecting channel, and wherein the heating cooling gas enters into the implementation port orifice of each extracting port and passes into the port duct to reach the evacuation element at an average extraction velocity comprised between 0.5 and 5 m / s, preferably between 0.5 and 3.5 m / s, is able to extract a heated cooling gas at a sufficiently high temperature to be reuse in post-treatment operations, while being at an adequate extraction rate to ensure an optimal operational mode of the furnace, and avoiding to entrain fines and combustion waste that can obstruct devices used in the post-treatment operations of the extracted heated cooling air.

[0055] Within the meaning of the present invention, the cooling zone height is preferably the vertical height between the base of the shaft, i.e. the discharge table, and the base of the connecting channel, i.e. the bottom end of the burning zone.

[0056] Within the meaning of the present invention, the average extraction velocity is preferably the mean of the extraction velocity measured at least in a bottom point of the port duct, for example in a point of the port duct located in the bottom face of the port duct, and in an upper point of the port duct, for example in a point of the port duct located in the upper face of the port duct. Alternatively, or in addition, the average extraction velocity is preferably the sum of the extraction velocity measured at N given points of the port duct divided by N, for example (i) a first point located in a bottom point of the port duct, for example in a point of the port duct located in the bottom face of the port duct, and (ii) a second point located in an upper point of the port duct, for example in a point of the port duct located in the upper face of the port duct, divided by two.

[0057] Preferably, the method according to the present invention further comprises recirculating a fraction of the gaseous effluent exhausted from the top of said at least one shaft in preheating mode, and injecting the gaseous effluent, exhausted from the top of said at least one shaft in preheating mode, to the shaft in calcining mode, either under the form of a comburant mixture or with an injection of a combustion stream containing a comburant, for said step of oxy-com busting the fuel.

[0058] Preferably, during said extracting step of the heated cooling gas outside of the kiln from said shaft working in a calcining mode and / or from said shaft working in a preheating mode, the heated cooling gas is extracted from 50 to 100% by volume on dry gas basis relative to the introduced cooling air in the same shaft from the shaft in calcining mode through said implementation port orifice in a peripheral wall of said shaft working in a calcining mode, and from 0 to 50% by volume on dry gas basis relative to the introduced cooling air in the same shaft from the shaft in calcining mode through said implementation port orifice into the wall of said central collector element and the heated cooling gas is extracted from 0 to 50% by volume on dry gas basis relative to the introduced cooling air in the same shaft from the shaft in preheating mode through said implementation port orifice in a peripheral wall of said shaft working in a preheating mode and from 50 to 100% by volume on dry gas basis relative to the introduced cooling air in the same shaft from the shaft in preheating mode through said implementation port orifice into the wall of said central collector element of said shaft working in a preheating mode. Preferably, during said extracting step of the heated cooling gas outside of the kiln from said shaft working in a calcining mode and / or from said shaft working in a preheating mode, the heated cooling gas is extracted 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 air in the same shaft from the shaft in calcining mode through said implementation port orifice in a peripheral wall of said shaft working in a calcining mode 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 air in the same shaft from the shaft in calcining mode through said implementation port orifice into the wall of said central collector element and the 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 air in the same shaft from the shaft in preheating mode through said implementation port orifice in a peripheral wall of said shaft working in a preheating mode 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 air in the same shaft from the shaft in preheating mode through said implementation port orifice into the wall of said central collector element of said shaft working in a preheating mode.

[0059] Preferably, the decarbonation of the preheated carbonate mineral stones in the calcining mode further comprises oxy-com busting fuel in the presence of oxygen so as to obtain said calcination of said stones in said combustion zone.

[0060] According to another particular embodiment of the present invention, said combustion stream containing a comburant is chosen amongst C>2-rich gas mixture, in particular pure oxygen, a steam-based gas mixture containing oxygen or their mixture.

[0061] Preferably, by the terms C>2-rich gas mixture, it is meant according to the present invention, a mixing gas stream containing more than 70 vol% on dry basis of di-oxygen with respect to the volume of combustion stream containing a comburant, more preferably a mixing gas stream containing more than 80 vol%, even more than 90 vol%, more particularly more than 93 vol% on dry basis di-oxygen, with respect to the volume of combustion stream containing a comburant, such as for example oxygen generated by pressure swing adsorption method which has generally a O₂ concentration of 93 vol% on dry basis di-oxygen.

[0062] Preferably, when the combustion stream is a mixture of steam (water vapor) and O₂-rich gas mixture, the volume ratio between steam and the O₂-rich gas mixture is of at least 10-90, at least 20-80, at least 30-70, at least 40-60, at least 50-50, at least 60-40, at least 70-30, at least 80-20, at least 90-10.

[0063] According to the present invention, in a preferred embodiment, oxygen is introduced in the kiln at one or more locations to provide a total amount of oxygen introduced in the kiln higher than the amount required for a stoichiometric combustion for the oxy-combustion of fuel in presence of oxygen in excess, and is preferably introduced at an excess from 2 to 30%, preferably from 3 to 20 %, in particular from 4 to 17%, advantageously from 5 to 15 % in volume with respect to the stoichiometric need of the combustion reaction.

[0064] In other words, the total amount of oxygen introduced in the kiln is equal to the amount of oxygen needed for a stoichiometric combustion for the oxy-combustion of fuel in presence of oxygen multiplied by an excess factor from 1.02 to 1.30, preferably 1.03 to 1.2, in particular from 1.04 to 1.17, advantageously from 1.05 to 1.15.

[0065] More particularly, in the method according to the present invention, the oxy-com busting step of fuel in the presence of oxygen is carried out in the combustion zone fed by the exhaust effluent and by the combustion stream containing a comburant, simultaneously or separately, or by a mixture of said exhaust effluent and said combustion stream containing a comburant. According to the present invention, in a preferred embodiment, the average extraction velocity of the heating cooling gas is comprised between 0.75 and 3 m / s, preferably between 0.8 and 2.8 m / s.

[0066] According to the present invention, in a preferred embodiment, the cooling gas is introduced at a flow rate comprised between 0.6 Nm³ / kg and 1.1 Nm³ / kg of lime produced, preferably between 0.7 Nm³ / kg and 1 Nm³ / kg, more preferably between 0.8 Nm³ / kg and 1 Nm³ / kg, even more preferably between 0.85 Nm³ / kg and 0.95 Nm³ / kg.

[0067] According to the present invention, in a preferred embodiment, the heated cooling gas has a mean temperature, at said implementation port orifice, comprised between 500 and 1000 °C, preferably between 500 to 900 °C.

[0068] Within the meaning of the present invention, the mean temperature of the heated cooling gas at said implementation port orifice is the mean of the temperature measured at least in a bottom point of the port duct, for example in a point of the port duct located in the bottom face of the port duct, and in an upper point of the port duct, for example in a point of the port duct located in the upper face of the port duct.

[0069] According to the present invention, in a preferred embodiment, the 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.

[0070] According to a particular embodiment, the exhaust effluent is partially or fully collected in at least one buffer after said exhausting step to absorb fluctuation due to the operation of the kiln, especially during the inversion phase. The presence of the buffer ensures a regular feeding of exhaust effluent to the kiln or to any downstream device, such as purification and / or concentration units, filters,...

[0071] By the terms “at least one buffer”, it is meant according to the present invention at least one device of any kind of gas storage allowing to store a gas flow for a predetermined period of time. In a further particular embodiment of the present invention, 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.

[0072] In a variant of the particular embodiment, according to the present invention, said cooling step comprises a supply of cooling gas at the bottom of the shaft having worked in the preheating mode and before the activation of the inversion means, in order to have each shaft encountering sequentially said preheating mode, a cooling step and then a calcining mode.

[0073] In yet a preferred embodiment of the present invention, said cooling gas is air, nitrogen (such as nitrogen from the air separation unit when present) or steam and preferably air.

[0074] In a particular embodiment of the present invention, said cooling gas, preferably said air, supplied at the bottom of each shaft or of the shaft under calcining mode is ascending, flowing in counter-current through the calcined mineral stones forming a heated cooling gas, preferably a heated air, said heated cooling gas, preferably said heated air being extracted at a level below the cross-over channel.

[0075] Further, in a particular embodiment, said heated cooling gas, preferably said heated air is extracted outside of the kiln through said cooling gas extraction system.

[0076] Preferably, the temperature of the heated cooling gas, preferably of the heated air, extracted at a level below the cross-over channel is comprised between 500°C and 1000°C, more preferably between 700°C and 950°C.

[0077] More preferably, the method according to the present invention comprises at least one heat exchange between the heated cooling air, which has been extracted outside the kiln, and said recirculated fraction of gaseous effluent before injection to the shaft in calcining mode.

[0078] For example, when the combustion is carried out in presence of a mixture of gaseous effluent and concentrated dioxygen, the mixture is preferably performed in a mixing chamber in fluid communication with the recirculation of exhaust effluent and one dioxygen source and with the combustion zone of the shaft of the kiln in calcining mode. The heat exchange between the heated cooling air, removed from the furnace, and said collected portion of gaseous effluent discharged from the furnace occurs then before or after it is mixed with concentrated dioxygen.

[0079] Alternatively or in addition to, in the method according to the present invention, at least one heat exchange between the heated cooling air which has been extracted outside the kiln and said mixing gas stream, before passing through the series of mixing gas entry means is comprised to provide the increase of temperature to the mixing gas stream.

[0080] In a particular embodiment of the present invention, said combustion stream comprising a comburant is C>2-rich gas and said mixing gas stream is C>2-rich mixing gas stream and wherein the total amount of oxygen is calculated by a control system according to the stoichiometric requirement for combustion multiplied by an excess factor supplied by the operator, said amount of oxygen being spread between a first O2- rich stream and a second 02-rich stream, said first O₂-rich stream being supplied to the combustion zone of the shaft in calcining mode and said second O₂-rich stream being supplied to the connecting channel.

[0081] Preferably, the O₂-rich gas and / or the O₂-rich mixing gas stream has an oxygen content comprised between 70 and 100 vol% on dry basis di-oxygen, preferably of at least 80 vol% on dry basis di-oxygen, More particularly of at least 90 vol% on dry basis di-oxygen and most preferably around 93 vol% on dry basis di-oxygen.

[0082] It is important to keep the overall oxygen excess for combustion low to keep the final CO2 in the exhaust effluent sufficiently high (i.e. to not dilute with additional oxygen). In oxyfuel operation, the comburant supplied to the combustion shaft is preferably also high purity oxygen mixed with recycled exhaust effluent. The overall quantity of oxygen supplied to the kiln will be calculated by the control system according to the stoichiometric requirement for combustion, multiplied by an excess factor supplied by the operator (typically between 1.02 and 1.30, preferably between 1.03 and 1.2, in particular from 1.04 to 1.17, advantageously from 1.05 to 1.15). This global quantity will then, if relevant, be split between oxygen sent to the shaft in calcining mode, at the level of or in the combustion zone (first C>2-rich stream) and oxygen sent to the connecting channel (second C>2-rich stream).

[0083] The use of this oxy-combustion method does not necessarily require any particular design of the furnace itself. The only changes to be made to the furnace may be simply external to the furnace and consist of changing the effluent circuits leaving the furnace and providing at least one source of concentrated dioxygen. As it can be seen, the method according to the present invention carries out fuel combustion in dioxygen which results in the mixing gas stream containing the combustion fumes and in the calcination of the carbonate stones. This produces mainly CO2 and steam with some impurities, present as traces in the fuel and in the material to be calcined, and some oxygen not used up by the fuel combustion.

[0084] Naturally, these combustion fumes also contain the CO2 supplied to the oxidizing mixture. This evidently results in a significant increase in the CO2 content of the gaseous effluent discharged from the top of the furnace, compared to the conventional method.

[0085] According to the invention, a gaseous effluent concentrated in CO2 means that it has a CO2 content of at least 60%, more preferably of at least 70%, more particularly of at least 75%, especially at least 80% and particularly advantageously at least 90% by volume on dry gas. This CO2 can then be used or sequestered under favorable conditions, drastically decreasing the contribution of the furnace to the greenhouse effect.

[0086] Advantageously according to the present invention, said fuel combustion 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 a portion of said collected portion of gaseous effluent discharged from the furnace, or using another source of CO2 as a carrier gas.

[0087] More particularly according to the present invention, the O₂-rich gas or the O₂-rich stream is produced in one air separation unit, said separation unit producing from an air entry, a stream of oxygen and a stream of nitrogen.

[0088] In a 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 for producing a substantially pure CO2 gas, before or after the step of collecting the exhaust effluent in said buffer, preferably after.

[0089] In a preferred embodiment, the method according to the present invention comprises a step of reuse of the produced CO2 gas and / or a step of revalorization of the produced CO2 gas, for example use of CO2 gas to cool down the lances and / or use of CO2 gas to dedust filters, use of CO2 gas as a carrier gas as indicated above and / or use of CO2 gas as an air substitute.

[0090] In a preferred embodiment, the exhaust effluent discharged from the shaft in preheating mode has a temperature of 60°C to 200° C. preferably 100° C.

[0091] In a variant according to the present invention, said mixing gas stream is exhaust effluent or substantially pure CO2 gas, optionally fed from the buffer or storage unit. In this embodiment, all the comburant, preferably all the oxygen for combustion was already supplied to the shaft in calcining mode as a comburant mix.

[0092] In a preferred embodiment according to the present invention, during the step of recirculating said fraction of the gaseous effluent and before the step of injecting the gaseous effluent to the shaft in calcining mode, the gaseous effluent is cooled into a heat exchanger in which water is condensed and discarded forming a cooled and dried gaseous effluent. This is also especially advantageous when the mixing gas stream is steam. The extra steam injected will be removed by the gas cooling step and would therefore not affect the CO2 concentration.

[0093] In yet a preferred embodiment, a portion of the cooled and dried gaseous effluent is further introduced at the top of the shaft in calcining mode at a temperature below 300°C, preferably below 200°C, more preferably below 100°C, even more preferably between 30 and 50°C, to keep the benefit from the regeneration, with a slightly higher pressure.

[0094] Other embodiments of the method according to the present invention are mentioned in the appended claims.

[0095] The present invention also relates to a parallel-flow regenerative kiln for implementing the method according to the present invention, comprising

[0096] - at least two shafts, interconnected by a connecting channel,

[0097] - each of said shafts comprising, in the on or off position, - at least one fuel supply device,

[0098] - at least one supply opening for oxygen-containing oxidant,

[0099] - an inlet, for loading carbonate mineral stones, at the top of the shafts,

[0100] - an outlet for unloading the calcined material produced, at the bottom of the shafts,

[0101] - a exhaust effluent discharge duct at the top of the shafts, which is connected to a chimney, and

[0102] - a supply of cooling gas to cool the calcined material produced,

[0103] the furnace 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 it further comprises

[0104] - said connecting channel being provided to transfer combustion fumes from the shaft in calcining mode to at least one shaft in preheating mode,

[0105] - a heated cooling gas extraction system arranged to withdrawn said heated cooling gas from the kiln, said heated cooling gas extraction system comprising (i) a series of cooling gas extracting ports located in a peripheral wall of said shaft and / or into a wall of a central collector element, and (ii) an evacuation element, wherein each cooling gas extraction port of said series of cooling gas extracting ports comprises an implementation port orifice being in fluid communication with a port duct, said port duct crossing said peripheral wall of said shaft and / or said wall of said central collector element and fluidly connecting said implementation port orifice to said evacuation element, said implementation port orifice being located into an effective cooling zone inside the cooling zone below said connecting channel in an inner side of a peripheral wall of said shaft and / or into an inner side of a wall of a central collector element, wherein said inner side of a peripheral wall of said shaft and / or said inner side of a wall of a central collector element face the interior volume of said shaft, said effective cooling zone presenting an effective cooling zone height, said effective cooling zone height having a height comprised between 25% and 75% of the cooling zone height starting from the base of the shaft (effective cooling zone height / cooling zone height),

[0106] wherein a. each implementation port orifice of each cooling gas extraction port has a cross-section with a height H and a width W wherein the ratio H / W is equal to or higher than 1, and wherein said cross-section has a surface comprised between 0.1 m2and 0.75 m2,

[0107] b. each port duct has a longitudinal axis forming a positive angle, preferably of positive angle comprised between 0 degrees and 75 degrees, with an horizontal axis of the base of the shaft (angle from the longitudinal axis of the base of the shaft to the longitudinal axis of the port duct taken in a counterclockwise direction), wherein each port duct passes from said inner side of said peripheral wall of said shaft and / or from said inner side of said wall of a central collector element to an outer side of said peripheral wall and / or through an outer side of said wall of the central collector element, wherein said outer side of said peripheral wall and / or said outer side of said wall of the central collector element is in fluid connection with said evacuation element.

[0108] The kiln according to the invention only has a few structural changes to the exterior of the furnace. Therefore, existing parallel-flow regenerative kilns may be easily arranged to implement a calcining method according to the invention.

[0109] In a preferred embodiment according to the present invention, the kiln further comprises

[0110] - a recirculation circuit which is arranged between the above-mentioned gaseous effluent discharge duct of the shafts and said oxidant supply openings of the shafts, and - a separating member, capable of collecting a portion of gaseous effluent discharged from the furnace via the duct and introducing it into the recirculation circuit. In a preferred embodiment according to the present invention, the series of cooling gas extracting ports comprises between 6 and 14 cooling gas extraction ports located in a peripheral wall of said shaft and / or between 1 and 6 cooling gas extraction ports located into a wall of a central collector element.

[0111] In a preferred embodiment according to the present invention, each implementation port orifice is located at a height comprised between 1 m and 6 m (height from the base of the shaft).

[0112] In a preferred embodiment according to the present invention, all cooling gas extraction ports of said series of cooling gas extracting ports located in a peripheral wall of said shaft are located substantially at the same height (as compared to the base of each shaft).

[0113] In a preferred embodiment according to the present invention, all cooling gas extraction ports of said series of cooling gas extracting ports located in a peripheral wall are distributed evenly around the perimeter of each shaft.

[0114] In a preferred embodiment according to the present invention, each cooling gas extraction port has a height comprised between 50 cm and 150 cm, preferably, between 70 cm and 90 cm.

[0115] In a preferred embodiment according to the present invention, each cooling gas extraction port has a width comprised between 20 cm and 80 cm, preferably between 30 cm and 50 cm.

[0116] In a preferred embodiment according to the present invention, said heated cooling gas extraction system further comprises in a downstream position of said port duct:

[0117] - an extraction channel,

[0118] - at least one means of pressure reduction arranged to reduce the pressure of said extracted gas, - at least one flow rate valve, preferably an automatic valve, arranged to control the flow rate of said extracted gas, - a flow meter arranged to measure the flow rate of said extracted gas, said flow meter being in operational connection with said at least one flow rate valve,

[0119] - a flow rate controller in operational connection with said at least one flow rate valve and with said flow meter, wherein said at least one means of pressure reduction and said at least one flow rate valve are in series and are both in fluid communication with said extraction channel.

[0120] In a preferred embodiment according to the present invention, the recirculation circuit is connected to at least one buffer unit.

[0121] In a preferred embodiment according to the present invention, the shafts have a circular cross-section, wherein said connecting channel comprises a crossover channel and the peripheral channels, the cross over channel connecting the peripheral channels arranged around each shaft so as to allow a transfer of gas and wherein, below the connecting channel, the shafts are provided with a collector ring connecting with said evacuation element of said heated cooling gas extraction system and facing said outer side of said peripheral wall so as to allow heated cooling air to be removed from the furnace.

[0122] In a preferred embodiment according to the present invention, a heat exchanger supplied with heated cooling air removed from the furnace, is mounted on the recirculation circuit.

[0123] Advantageously, the circular shafts further comprise, at the bottom, a central collector element connecting with an evacuation element so as to allow heated cooling air to be removed from the furnace, below the connecting channel.

[0124] According to another embodiment of the furnace according to the invention, the shafts have a rectangular cross-section, in that a first side of a shaft faces a first side of a neighboring shaft and each shaft comprises a second side that is opposite those facing each other and in that the crossover channel is a connecting channel which directly connects one shaft to the other via their first sides, and in that, below the connecting channel, said first sides and said second sides of the shafts are provided with a collection tunnel connecting with said evacuation element of said heated cooling gas extraction system so as to allow heated cooling air to be removed from the furnace.

[0125] According to an embodiment of the invention, the furnace comprises, as a dioxygen source for the recirculation circuit, an air separation unit for separating air into dioxygen and diatomic nitrogen. An oxygen tank may also be provided. Advantageously, a heat exchanger supplied with heated cooling air removed from the furnace is mounted on the recirculation circuit to heat the above-mentioned oxidizing mixture before it is supplied to the shaft in calcination mode.

[0126] According to an embodiment of the invention, each port duct of each extracting ports of the series of extracting ports is connected to a collector peripheral ring, said collector peripheral ring being connected to a first extracting element / evacuation element so as to allow heated cooling gas to be removed from the kiln. Preferably, said first extracting element is controlled by a control means. Preferably, at the bottom the shafts comprise a central collector element connected with a second extracting element / evacuation element so as to allow heated cooling gas to be removed from the kiln, said second extracting element being preferably controlled by a second control means, said first control means and said second control means being synchronized with said reversing system.

[0127] Preferably, said recirculation circuit is connected to at least one buffer unit.

[0128] In a particular embodiment of the present invention, in the kiln of the present invention, said recirculation circuit is connected to storage unit provided to store a CC>2-rich gaseous effluent, optionally before or after a buffer unit.

[0129] Other features and details of the kiln according to the invention are indicated in the appended claims. Other particularities of the invention will also result from the non-limiting description given below, with reference to the Figures illustrating the present invention.

[0130] FIG. 1 schematically shows a conventional PFRK furnace of circular cross-section.

[0131] FIG. 2 schematically shows an embodiment of PFRK furnace of circular cross-section comprising a collector peripheral ring.

[0132] FIG. 3 schematically shows a PFRK furnace comprising a recirculation circuit for oxy-fuel combustion (prior art).

[0133] FIG. 4 illustrates a view of a PFRK furnace comprising a collector peripheral ring (see Figure 2) with the cutting plans A-A, B-B and C-C.

[0134] FIG. 5 schematically shows a cooling gas extracting port comprising an implementation port orifice and a port duct.

[0135] FIG. 6 schematically shows a view from above of the PFRK furnace according to the present invention comprising the series of cooling gas extracting ports.

[0136] In the figures, identical or similar parts use the same references. Conventionally, the shaft shown on the left is in calcination 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.

[0137] As can be seen in FIG. 1, the PFRK furnace shown is a vertical double-shaft furnace 1, 2, where the fuel is injected alternately in one shaft 1 then in another 2 for approximately 12 minutes with a stop period between cycles of 1 to 2 minutes to reverse the circuits. This is the “reversing” period. Both shafts have a circular cross-section and are provided with peripheral channels 13 which are interconnected by a 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 16

[0138] carbonate stones occurs and the cooling area C where the cooling of the calcined material occurs.

[0139] When a shaft is in calcination 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 in the open position, progressively descends in the shaft. Combustion air is introduced at the top of the shaft via a supply opening 6, which allows for fuel combustion at the outlet of the lances 4 and a decarbonation of the carbonate stones to calcined material 10. The mixing gas stream 11 formed by the combustion and decarbonation descends co-currently to the calcined material and, using the peripheral channel 13, moves into the crossover channel 3. Cooling air is introduced via a supply duct 7 at the bottom of the shaft, counter-currently to the calcined material, to cool it. The heated cooling air 12 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.

[0140] When a shaft is in preheating mode, here the shaft 2, the fuel supply device is closed and the lances 4 are therefore off. The same applies to the opening 6 for supplying combustion air. However, the supply duct 7 for the cooling air and the outlet 8 for the calcined material remain in the open position. During cycle, the inlet 5 for the carbonate stones is open during the loading carbonate mineral stones. After heat exchange with the descending calcined material 10, the heated cooling air mixes with the combustion fumes 11 which, from the crossover channel 3, enters the shaft via the peripheral channel 13. The combustion fumes 11 progresses until reaching the top of the shaft where it is discharged from the furnace via a discharge duct 14 and transferred to a chimney 15, possibly after treatment in equipment’s such as filters. In the shaft in calcination mode 1, this discharge duct 14 is closed.

[0141] The furnace 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 furnace in such a way that, in production mode, each shaft operates alternately in calcination mode and in preheating mode.

[0142] In some cases, there are three shafts, two in preheating mode and one in combustion.

[0143] FIG. 2 is a view of a PFRK furnace comprising a collector peripheral ring (25).

[0144] FIG. 3 is a view of a PFRK furnace comprising a recirculation circuit for oxy-fuel combustion. As can be seen, this embodiment comprises separating member 17, capable of collecting a portion of gaseous effluent discharged from the furnace and introducing it into the recirculation circuit 18, and which has been provided on the exterior, on the discharge duct 14. In this circuit, the collected portion of gaseous effluent is advantageously treated in a treatment unit 19, where it may, for example, be filtered and / or dried. An air separation unit 20 separates air supplied by the duct 21 into N2 discharged via the duct 22 and O2 supplied to the recirculation circuit 18 via the supply duct 23. This circuit 18 then brings the oxidizing mixture formed from the recirculated portion of gaseous effluent and concentrated O2 to the top of each of the shafts at the supply opening 6.

[0145] The separating member 17 is continuously in service during combustion, the same as the treatment unit 19 and the air separation unit 20. As has already been seen, the reversing system 16 closes the discharge duct 14 at the top of the shaft in calcination mode. However, at the top of this shaft, it opens the supply opening 6 to allow the oxidizing mixture to be introduced, while it is closed at the top of the shaft in preheating mode.

[0146] In addition, the heated cooling air is extracted after contact with the calcined material, by installing a removal system. Indeed, the shafts 1 and 2 are each provided with a collector ring 25, below the crossover channel 3, which connects with an evacuation element 26 so as to allow heated cooling air to be removed from the furnace. The shafts may further optionally comprise, at the bottom, a central collector element 27 connecting with the evacuation element 26 as to also allow a central removal of the heated cooling air, below the crossover channel 3. Moreover, in order to recover a portion of the energy from the hot air removed by the evacuation element 26, a heat exchange may be provided with the portion of recirculated gaseous effluent using a heat exchanger 36, before or after the mixing thereof with concentrated dioxygen.

[0147] In the connecting channel or crossover channel 3, an injection of a fraction of said collected portion of gaseous effluent discharged from the furnace using an injection duct 37 may also be provided. Optionally beforehand, a heat exchange between the heated cooling air removed from the furnace, and this above-mentioned fraction to be injected may occur using a heat exchanger, for example the heat exchanger 36. In the absence thereof, another heater not shown may be provided on the injection duct 37.

[0148] Preferably, in the connecting channel or crossover channel 3, the injection of a fraction of said collected portion of gaseous effluent discharged from the furnace using the injection duct 37 also comprises an addition of oxygen by a fluid connection of the injection duct 37 with the supply duct 23.

[0149] Figure 4 illustrates a view of a PFRK furnace comprising a collector peripheral ring (25) (see Figure 2) with the cutting plans A-A, B-B and C-C.

[0150] Figure 5A schematically shows a cooling gas extracting port 38 comprising an implementation port orifice 39 and a port duct 40. The port duct 40 having a longitudinal axis LI forming a positive angle alpha, preferably of positive angle alpha comprised between 0 degrees and 75 degrees, preferably between 40 degrees and 75 degrees with the horizontal axis L2 of the base of the shaft (angle from the longitudinal axis of the base of the shaft L2 to the longitudinal axis of the port duct LI taken in a counterclockwise direction). Figure 5B illustrates different embodiments of the shape of the implementation port orifice 39. Preferably, each implementation port orifice 39 of each cooling gas extracting port 38 has a vertical rectangular shape or a vertical oval shape or a square shape or a circular shape.

[0151] Figure 6 schematically shows a view from above of the PFRK furnace according to the present invention comprising the series of cooling gas extracting ports 38 in the cutting plan A-A. In this embodiment, a series of 6 cooling gas extracting ports is present in the peripheral wall of the shaft 1 and in the peripheral wall of the shaft 2. Each cooling gas extracting ports 38 comprises an implementation port orifice passing the peripheral wall and forming a fluid connection between the interior volume of the shafts 1, 2 and the collector ring 25. The cutting plans B-B and C-C shows a view from above of the PFRK furnace according to the present invention respectively above and below the series of cooling gas extracting ports (see also the position of the cutting plans A-A, B-B and C-C in Figure 4).

[0152] Examples. - A parallel flow regenerative kiln according to the present invention having a capacity of 300 tpd (Example 1 ), 600 tpd (Example 2), and 800 tpd (Example 3) of calcined mineral stones, where fuel is burned in oxy-fuel conditions has been numerically modelized. The kiln comprises a central collecting element and a peripheral circular collecting channel.

[0153] In order to illustrate the present invention, notably to ensure a sufficient flow rate of cooling gas to promote an optimal range of working conditions of the furnace allowing an extraction of the heated cooling gas at a sufficient temperature to be able to use the extracted heated cooling gas in post-treatment conditions, a parallel flow regenerative kiln according to the present invention, where fuel (lignite) is burned, has been numerically modelized.

[0154] From the hypothesis that the flow rate of cooling air can fluctuate in a range of 700 to 900 Nm3 / h per ton of lime and that the average speed of the extracted heated cooling gas in the port duct is between 1 m / s and 3 m / s in order not to bring too much dust in pipes, the cooling gas extraction port may advantageously have the characteristics of Table 1.

[0155] Table 1: Characteristics of the kiln operational conditions and the cooling gas extraction port

[0156]

[0157] 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

CLAIMS1. Method for calcining carbonate mineral stones in a parallel flow regenerative kiln having at least two shafts (1, 2) interconnected by a connecting channel (13, 3), comprising, in standard operation,- loading carbonate mineral stones at the top of each shaft, - preheating these loaded stones in a preheating zone (A), - calcining these preheated stones in a calcination zone (B) with production of a decarbonated calcined material,- cooling the calcined material with cooling gas in a cooling zone (C), said cooling zone (C) presenting a cooling zone height, with formation of heated cooling gas by heat exchange,- discharging the calcined material from the bottom of the shafts,- exhausting a gaseous effluent from the kiln,- each shaft alternately working in a calcining mode and in a preheating mode, one shaft working in a calcination 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 calcination mode, and• through said connecting channel (3, 13), 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 connecting channel, which is ascending and flows in counter-current through the loaded carbonate mineral stones, and• said exhausting step of said fumes as exhaust effluent at the top of said at least one shaft in preheating mode, said method further comprisingextracting said heated cooling gas from said shaft working in a calcining mode and / or from said shaft working in a preheating mode through a cooling gas extraction system comprising a series of extracting ports and an evacuation element,wherein each extracting port of the series of extracting ports comprises - an implementation port orifice in a peripheral wall of said shaft working in a calcining mode and / or from said shaft working in a preheating mode and / or into a wall of a central collector element, said implementation port orifice, facing the interior volume of the shafts, is located into an effective cooling zone within said cooling zone below the connecting channel, and- a port duct crossing said peripheral wall of said shaft working in a calcining mode and / or of said shaft working in a preheating mode and / or crossing said wall of said central collector element, said port duct being arranged to fluidly connect the interior volume of each shaft to said evacuation element,wherein said heating cooling gas enters into said implementation port orifice and passes into said port duct to reach said evacuation element at an average extraction velocity comprised between 0.5 and 5 m / s, preferably between 0.5 and 3.5 m / s, andwherein said effective cooling zone presents an effective cooling zone height, said effective cooling zone height having a height comprised between 25% and 75% of the cooling zone height starting from the base of the shaft (effective cooling zone height / cooling zone height).

2. Method according to claim 1, wherein said average extraction velocity of said heating cooling gas is comprised between 0.75 and 3 m / s, preferably between 0.8 and 2.8 m / s.

3. Method according to claim 1 or claim 2, wherein said cooling gas is introduced at a flow rate comprised between 0.6 Nm³ / kg and 1.1 Nm³ / kg of lime produced, preferably between 0.7 Nm³ / kg and 1 Nm³ / kg, more preferably between 0.8 Nm³ / kg and 1 Nm³ / kg, even more preferably between 0.85 Nm³ / kg and 0.95 Nm³ / kg.

4. Method according to any of the preceding claims, wherein said heated cooling gas has a mean temperature, at said implementation port orifice, comprised between 500 °C and 900 °C.

5. Method 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.

6. Method according to any of the preceding claims, wherein said cooling gas is air, nitrogen (such as nitrogen from the air separation unit when present) or steam and preferably air.

7. Method according to any of the preceding claims, wherein said extracting step of the heated cooling gas outside of the kiln from said shaft working in a calcining mode and / or from said shaft working in a preheating mode, the heated cooling gas is extracted from 50 to 100% by volume on dry gas basis relative to the introduced cooling air in the same shaft from the shaft in calcining mode through said implementation port orifice in a peripheral wall of said shaft working in a calcining mode, and from 0 to 50% by volume on dry gas basis relative to the introduced cooling air in the same shaft from the shaft in calcining mode through said implementation port orifice into the wall of said central collector element and the heated cooling gas is extracted from 0 to 50% by volume on dry gas basis relative to the introduced cooling air in the same shaft from the shaft in preheating mode through said implementation port orifice in a peripheral wall of said shaft working in a preheating mode and from 50 to100% by volume on dry gas basis relative to the introduced cooling air in the same shaft from the shaft in preheating mode through said implementation port orifice into the wall of said central collector element of said shaft working in a preheating mode.

8. Method according to any of the preceding claims, wherein during said extracting step of the heated cooling gas outside of the kiln from said shaft working in a calcining mode and / or from said shaft working in a preheating mode, the heated cooling gas is extracted 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 air in the same shaft from the shaft in calcining mode through said implementation port orifice in a peripheral wall of said shaft working in a calcining mode 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 air in the same shaft from the shaft in calcining mode through said implementation port orifice into the wall of said central collector element and the 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 air in the same shaft from the shaft in preheating mode through said implementation port orifice in a peripheral wall of said shaft working in a preheating mode 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 air in the same shaft from the shaft in preheating mode through said implementation port orifice into the wall of said central collector element of said shaft working in a preheating mode.

9. Parallel-flow regenerative kiln for implementing the method according to anyone of the preceding claims, comprising- at least two shafts (1, 2), interconnected by a connecting channel (3, 13),- each of said shafts (1, 2) comprising, in the on or off position, - at least one fuel supply device (4),- at least one supply opening (6) for oxygen-containing oxidant (23),- an inlet (5), for loading carbonate mineral stones, at the top of the shafts,- an outlet (8) for unloading the calcined material produced, at the bottom of the shafts,- a 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 furnace comprising a system (16) 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 (16) therefore controlling said on and off positions, wherein it further comprises - said connecting channel being provided to transfer combustion fumes from the shaft in calcining mode to at least one shaft in preheating mode,- a heated cooling gas extraction system arranged to withdrawn said heated cooling gas from the kiln, said heated cooling gas extraction system comprising (i) a series of cooling gas extracting ports located in a peripheral wall of said shaft and / or into a wall of a central collector element, and (ii) an evacuation element, wherein each cooling gas extraction port of said series of cooling gas extracting ports comprises an implementation port orifice being in fluid communication with a port duct, said port duct crossing said peripheral wall of said shaft and / or said wall of said central collector element and fluidly connecting said implementation port orifice to said evacuation element, said implementation port orificebeing located into an effective cooling zone inside the cooling zone below said connecting channel in an inner side of a peripheral wall of said shaft and / or into an inner side of a wall of a central collector element, wherein said inner side of a peripheral wall of said shaft and / or said inner side of a wall of a central collector element face the interior volume of said shaft, said effective cooling zone presenting an effective cooling zone height, said effective cooling zone height having a height comprised between 25% and 75% of the cooling zone height starting from the base of the shaft (effective cooling zone height / cooling zone height),whereina. each implementation port orifice of each cooling gas extraction port has a cross-section with a height H and a width W wherein the ratio H / W is equal to or higher than 1, and wherein said cross-section has a surface comprised between 0.1 m2and 0.75 m2,b. each port duct has a longitudinal axis forming a positive angle, preferably of positive angle comprised between 0 degrees and 75 degrees, with an horizontal axis of the base of the shaft (angle from the longitudinal axis of the base of the shaft to the longitudinal axis of the port duct taken in a counterclockwise direction), wherein each port duct passes from said inner side of said peripheral wall of said shaft and / or from said inner side of said wall of a central collector element to an outer side of said peripheral wall and / or through an outer side of said wall of the central collector element, wherein said outer side of said peripheral wall and / or said outer side of said wall of the central collectorelement is in fluid connection with said evacuation element.

10. Parallel-flow regenerative kiln according to claim 9, further comprising- a recirculation circuit (18) which is arranged between the above-mentioned gaseous effluent discharge duct of the shafts and said oxidant supply openings of the shafts (6), and- a separating member (17), capable of collecting a portion of gaseous effluent discharged from the furnace via the duct and introducing it into the recirculation circuit (18).

11. Parallel-flow regenerative kiln according to claim 9 or claim 10, wherein said series of cooling gas extracting ports comprises between 6 and 14 cooling gas extraction ports located in a peripheral wall of said shaft and / or between 1 and 6 cooling gas extraction ports located into a wall of a central collector element.

12. Parallel-flow regenerative kiln according to anyone of claims 9 to 11, wherein all cooling gas extraction ports of said series of cooling gas extracting ports located in a peripheral wall of said shaft are located substantially at the same height (as compared to the base of each shaft).

13. Parallel-flow regenerative kiln according to anyone of claims 9 to 12, wherein all cooling gas extraction ports of said series of cooling gas extracting ports located in a peripheral wall are distributed evenly around the perimeter of each shaft.

14. Parallel-flow regenerative kiln according to anyone of claims 9 to 13, wherein each cooling gas extraction port has a height comprised between 50 cm and 150 cm, preferably, between 70 cm and 90 cm.

15. Parallel-flow regenerative kiln according to anyone of claims 9 to 14, wherein each cooling gas extraction port has a widthcomprised between 20 cm and 80 cm, preferably between 30 cm and 50 cm.

16. Parallel-flow regenerative kiln according to anyone of claims 9 to 15, wherein each port duct of each extracting ports of the series of extracting ports is connected to a collector peripheral ring, said collector peripheral ring being connected to said evacuation element so as to allow heated cooling gas to be removed from the kiln.

17. Parallel-flow regenerative kiln according to anyone of claims 9 to 16, wherein said heated cooling gas extraction system further comprises in a downstream position of said port duct:- an extraction channel,- at least one means of pressure reduction arranged to reduce the pressure of said extracted gas, - at least one flow rate valve, preferably an automatic valve, arranged to control the flow rate of said extracted gas,- a flow meter arranged to measure the flow rate of said extracted gas, said flow meter being in operational connection with said at least one flow rate valve,- a flow rate controller in operational connection with said at least one flow rate valve and with said flow meter, wherein said at least one means of pressure reduction and said at least one flow rate valve are in series and are both in fluid communication with said extraction channel.

18. Parallel-flow regenerative kiln according to any of the claims 9 to 17, wherein said recirculation circuit (18) is connected to at least one buffer unit.

19. Parallel-flow regenerative kiln according to any one of claims 9 to 18, wherein the shafts have a circular cross-section, wherein said connecting channel comprises a crossover channel (3) and the peripheral channels (13), the cross over channel (3) connecting the peripheral channels (13) arranged around each shaft (1, 2) so as to allowa transferof gas and wherein, below the connecting channel (3, 13), the shafts (1, 2) are provided with a collector ring (25) connecting with said evacuation element (26) of said heated cooling gas extraction system and facing said outer side of said peripheral wall so as to allow heated cooling air to be removed from the furnace.

20. Parallel-flow regenerative kiln according to claim 19, wherein the circular shafts further comprise, at the bottom, said central collector element (27) connecting with said evacuation element (26) so as to allow heated cooling air to be removed from the furnace, below the connecting channel.

21. Parallel-flow regenerative kiln according to any one of claims 9 to 20, wherein the shafts have a rectangular cross-section, in that a first side of a shaft faces a first side of a neighboring shaft and each shaft comprises a second side that is opposite those facing each other and in that the crossover channel is a connecting channel which directly connects one shaft to the other via their first sides, and in that, below the connecting channel, said first sides and said second sides of the shafts are provided with a collection tunnel connecting with said evacuation element of said heated cooling gas extraction system so as to allow heated cooling air to be removed from the furnace.

22. Parallel-flow regenerative kiln according to any one of claims 9 to 21, wherein a heat exchanger supplied with heated cooling air removed from the furnace, is mounted on the recirculation circuit.

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