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 extracting heated cooling gas coming from a collection tunnel outside of the kiln and / or at least a part of combustion fumes from a connecting channel to form extracted gas, wherein during said extracting step, an extraction device is arranged to perform a pressure reduction and a flow rate control of the extracted gas.

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 mean particle size dso comprised between 20 mm to 20 cm, preferably higher than 25 mm, preferably lower than 18 cm, more preferably lower than 16 cm, and typically between 3 and 15 cm.

[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.

[0006] 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 theexception of the port of the periphery already occupied by the crossover channel.

[0007] 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.

[0008] 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 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 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 works 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.

[0009] The classical method for calcining carbonate mineral stones in a parallel flow regenerative kiln having at least two shaftsinterconnected by a connecting channel, comprises, in standard operation,

[0010] - 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,

[0011] - cooling the calcined material with cooling air in a cooling zone, with formation of a heated cooling air, by heat exchange,

[0012] - discharging the calcined material from the bottom of the shafts,

[0013] - exhausting an exhaust effluent from the kiln,

[0014] - 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,

[0015] - the calcining mode comprising :

[0016] said loading step of carbonate mineral stones at the top of a kiln shaft,

[0017] 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,

[0018] - said preheating mode comprising :

[0019] 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

[0020] 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 air at the bottom of each of said shafts or only of the shaft working in the calcining mode.

[0021] 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.

[0022] According to the invention, standard operation means that the kiln produces the calcined material in a continuous operation, or quasi-continuous manner (semi-batch production). This operation does not concern the phases of starting, stopping or maintenance of the kiln.

[0023] 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.

[0024] The calcination reaction of limestone into quicklime is :

[0025] CaCCh (solid) + heat

[0026]

[0027] 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 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 fuelcombustion contributes also to an important release of CO2. Globally the current calcination methods actively participate in increasing the greenhouse effect.

[0028] 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 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 dinitrogen from the air used.

[0029] To capture this CO2, it may be considered to use an “end-of-pipe” method of CO2 concentration and 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.

[0030] More recently, to be able to capture the CO2 emitted in a PFRK furnace, operating PFRK 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.

[0031] For the same purpose, it has also been proposed to replace all the air from the method, combustion air carrying the solid fuel 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 CaCOs. 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.

[0032] 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.

[0033] Standard operation means that the furnace is in normal service during which it produces calcined material in semi-continuous manner. This mode therefore does not apply to the start-up and shutdown phases of the furnace or to maintenance in the event of a malfunction.

[0034] 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.

[0035] 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 leads to CO generation. The generated CO is typically reburned in contact with the cooling air that comes into 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.

[0036] In PFRK kiln working in oxy-com bustion, the exhaust 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 effluent such as disclosed in CN 10500081 and in JP2002060254. Other proposals are to isolate the cooling air from the exhaust effluent (see WO2022 / 238384, W 02022 / 238385, WO2022 / 229120, WO2024 / 141577, WO2024 / 141578, LU 103028, WO2024 / 141398).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 effluent concentrated in CO2 and the cooling air.

[0037] 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.

[0038] The prior art indicates that heated cooling air extraction in the cooling zone is important to avoid mixing between the exhaust 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.

[0039] The heated cooling air and / or exhaust effluents present in the furnace, PFRK, are under pressurization conditions, typically with a gauge pressure between 150 to 400 mbar (pressure between 150 to 400 mbar higher than the ambient atmospheric pressure). In order to be able to reuse and / or treat the heated cooling air and / or the exhaust effluents of the furnace, in practice there is still a need to provide a method for calcining carbonate mineral stones to be carried out in PFRK allowing to finely control the extraction of the heated cooling air and / or exhaust effluents without changing the cyclical operation thereof and with few or no changes to the structure thereof. This method should be able to regulate rapidly and with high precision, in a manner that is robust overtime and simple to implement, the extraction rate of the heated cooling air and / or exhaust effluents present in the furnace under a high pressure drop condition, wherein the pressure condition can pass during extraction from an absolute pressure comprise between 1,15 and 1,40 bars to 1 bar (atmospheric conditions).

[0040] 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,

[0041] - 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,

[0042] - cooling the calcined material with cooling gas, preferably cooling air, in a cooling zone, with formation of heated cooling gas by heat exchange,

[0043] - discharging the calcined material from the bottom of the shafts,

[0044] - exhausting an exhaust effluent from the kiln,

[0045] - 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 inversion means,

[0046] - the calcining mode comprising :

[0047] • 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

[0048] • through said connecting channel, a passage of said combustion fumes toward the at least one shaft working in a preheating mode,- said preheating mode comprising :

[0049] • sgid prehegting step of the looded corbonote mineral stones by heot exchonge with soid combustion fumes coming from the connecting channel, which is ascending and flows in counter-current through the loaded carbonate mineral stones, and

[0050] • 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

[0051] extracting (a) at least a part of said heated cooling gas coming from a collection tunnel outside of the kiln and / or (b) at least a part of said combustion fumes from the connecting channel,

[0052] characterized in that during said extracting step of said at least a part of said heated cooling gas outside of said kiln and / or of at least a part of said combustion fumes from the connecting channel, said heated cooling gas or said combustion fumes is extracted through at least one extraction device, forming an extracted gas said extracting device being provided to control the flow rate and the pressure of said extracted gas, wherein (i) said extracted gas enters into an extraction channel of said extraction device through a gas input located in an upstream position at a gauge pressure comprised between 150 and 500 mbar,

[0053] (ii) said extracted gas encounters at least two regulations comprising - a regulation A being either a pressure reduction to a gauge pressure setpoint comprised between -50 mbar and 100 mbar, through at least one means of pressure reduction, or a flow rate control A through at least a first flow rate valve, and

[0054] - a regulation B being another flow rate control B through at least a second flow rate valve to give a controlled flow rate extracted gas, wherein said extracted gas encounters first regulation A then regulation B or first regulation B then regulation A,

[0055] and wherein the flow rate value either of extracted gas, or of said controlled flow rate extracted gas is measured by at least one flow meter,and transmitted to at least one flow rate controller, said at least flow rate controller being provided to automatically adjust said at least a first and / or a second flow rate valve to adapt the flow rate of said controlled flow rate extracted gas to a flow rate setpoint, said controlled flow rate extracted gas further exiting the extraction device through a gas output located in a downstream position,

[0056] and wherein, in the presence of said flow rate control A through at least a first flow rate valve, said extracted gas having a first flow rate passing through said first flow rate valve and a second flow rate passing through said second flow rate valve wherein said first flow rate is comprised between 10% and 40% of a total flow rate wherein said total flow rate is the sum of said first flow rate and said second flow rate.

[0057] Indeed, a method according to the present invention having the above-mentioned features and centered on the fact that the extracting step of cooling gas and / or of at least a part of the combustion fumes (forming the extracted gas) is performed through an extraction device wherein the extracted gas encounters a regulation A being either a pressure reduction to a gauge pressure setpoint comprised between -50 mbarand 100 mbar, through at least one means of pressure reduction, or a flow rate control A through at least a first flow rate valve, and a regulation B being another flow rate control through at least a second flow rate valve to give a controlled flow rate extracted gas, permits to finely control the flow rate and the pressure drop of the gas. In addition, the fact that a flow rate controller allows adapting automatically the flow rate of the gas flow permits having a rapid and very precise control of the gas flow rate at the output of the extraction device. This extraction device as described allows having an extracted gas with a very stable flow rate over time while being in the presence of a high pressure drop conditions. Indeed, a single regulation, for example a single valve, would not work properly because a large valve would be needed in order to create the large pressure drop or a first control of the flow rate, the opening would have to be very little and therefore the valve would not be in the openingrange for fine flow confrol. On the other hand, if the valve would be carefully designed small so that it would create the large pressure drop while being significantly open, it would be incapable to adapt to varying pressure conditions inside the kiln. Such extraction device is particularly advantageous for post-treatments and / or reuse of the combustion fumes and / or heated cooling gas after being extracted from the kiln. In addition, the extraction device coupled with a PFRK is quite simple having few components which renders this extraction device very stable and robust over time.

[0058] Preferably, the method according to the present invention further comprises recirculating a fraction of the exhaust effluent exhausted from the top of said at least one shaft in preheating mode, and injecting the exhaust 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 a step of oxy-com busting the fuel.

[0059] 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.

[0060] According to the present invention, oxy-com busting fuel in the presence of oxygen preferably means a combustion of fuel in a comburant mixture containing significantly less dinitrogen than ambient air, in particular a mix of an O2-rich stream and recycled flue gas.

[0061] Preferably, said extracted gas enters into said extraction channel of said extraction device through a gas input located in an upstream position at a gauge pressure comprised between 150 and 450 mbar, preferably between 200 and 400 mbar.Preferably, said pressure reduction is performed to a gauge pressure setpoint comprised between -40 mbar and 80 mbar, more preferably between -20 mbar and 70 mbar.

[0062] 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.

[0063] 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.

[0064] Preferably, by the terms C>2-rich gas or stream, it is meant according to the present invention, a gas 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 gas 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.

[0065] According to the present invention, in a preferred embodiment, oxygen is introduced in the kiln, preferably in the combustion area of the kiln in the shaft in calcination mode, 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 is 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.

[0066] In other words, the total amount of oxygen introduced in the kiln, preferably in the combustion area of the kiln in the shaft in calcination mode, is equal to the amount of oxygen needed for a stoichiometriccombustion for the oxy-combustion of fuel in presence of oxygen multiplied by on 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.

[0067] 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.

[0068] Preferably, the gas temperature in the combustion zone is comprised between 1100°C and 1500 °C, more preferably between 1200°C and 1400°C.

[0069] 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.

[0070] 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.

[0071] 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 or any mixture thereof and preferably air.

[0072] 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 connecting channel during said extracting step through said extracting device.Further, in a particular embodiment, said heated cooling gas, preferably said heated air is extracted outside of the kiln during said extracting step through said extracting device.

[0073] Preferably, the temperature of the heated cooling gas, preferably of the heated air forming the extracted gas, extracted at a level below the connecting channel during said extracting step is comprised between 500°C and 1000°C, more preferably between 650°C and 950°C.

[0074] More preferably, the method according to the present invention comprises at least one heat exchange between the heated cooling gas, preferably of the heated air, which has been extracted outside the kiln, and said recirculated fraction of exhaust effluent before injection to the shaft in calcining mode.

[0075] For example, when the combustion is carried out in presence of a mixture of exhaust 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 gas, removed from the furnace, and said collected portion of exhaust effluent discharged from the furnace occurs then before or after it is mixed with concentrated dioxygen.

[0076] Preferably, the O2-rich gas 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 dioxygen.

[0077] 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 oxy-combustion (oxyfuel operation), the comburant supplied to the combustion shaft is preferably also high purity oxygen mixed with recycled exhaust effluent. The overallquantity of oxygen supplied to the kiln will be calculated by a 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 O2- rich stream) and oxygen sent to the connecting channel (second O2-rich stream).

[0078] Concerning the quantity of oxygen possibly supplied to the connecting channel, it is advantageous to heat it up to high temperature, such as for example above 500°C, preferably above 650°C before injection. This can advantageously be done by said aforementioned heat exchange or by burning a small quantity of fuel (preferably gaseous or liquid, but low-ash solids could also be possible) in the oxygen stream before injection. Another advantage is that high temperature oxygen is a very strong oxidant and would therefore more readily react with the unburnt, leading to more efficient reduction.

[0079] Preferably, during said extraction step, said extracted gas encounter said pressure reduction before said flow rate control.

[0080] Preferably, within the method according to the present invention the pressure of said extracted gas is measured by a pressure sensor and wherein a difference between said pressure and said pressure setpoint is determined by a pressure controller, said means of pressure reduction, being actuated, preferably automatically actuated, by said pressure controller, and triggered by said difference in order to reduce said difference.

[0081] Preferably, within the method according to the present invention said automatic adjustment of said at least first and / or second flow rate valve by said at least one flow rate controller has a response time shorter than said actuation of said means of pressure reduction by said pressure controller.Preferably, within the method according to the present invention said extraction step, further comprises a passage of said heated cooling gas or said combustion fumes through a heat exchanger and / or through a filter provided to dedust said extracted gas, said passage through said heat exchanger and / or through said filter being upstream and / or downstream of the passage of said extracted gas through said at least one means of pressure reduction and / or through said at least first and / or second flow rate valve.

[0082] Preferably, the 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 connecting channel.

[0083] Preferably, the heated cooling gas is heated cooling air. 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 a 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 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 exhaust effluent discharged from the top of the furnace, compared to the conventional method.

[0085] According to the invention, an exhaust 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 be further concentrated, purified and captured, 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 preferably using a portion of said collected portion of exhaust 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 02-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 further concentration and purification for producing a substantially pure CO2 gas, before or after the step of collecting the exhaust effluent in a buffer, preferably after.

[0089] In a preferred embodiment, the exhaust effluent discharged from the shaft in preheating mode has a temperature of 60°C to 160° C. preferably 100° C.

[0090] In a preferred embodiment according to the present invention, during the step of recirculating said fraction of the exhaust effluent and before the step of injecting the exhaust effluent to the shaft in calcining mode, the exhaust effluent is cooled into a heat exchanger in which water is condensed and discarded forming a cooled and dried exhaust 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.

[0091] Within the meaning of the present invention, by the wording “dried exhaust effluent”, it is meant a gaseous effluent substantiallydepleted in water vapor. Obviously, in the dried exhaust effluent, some % of water vapor can remain.

[0092] In yet a preferred embodiment, a portion of the cooled and dried exhaust effluent is further introduced at the top of the shaft in calcining mode at a temperature below 200°C, preferably below 100°C, more preferably between 30 and 50°C, to keep the benefit from the regeneration, with a slightly higher pressure.

[0093] Other embodiments of the method according to the present invention are mentioned in the appended claims.

[0094] The present invention also relates to a parallel-flow regenerative kiln for implementing the method according to the present invention, comprising

[0095] - at least two shafts, interconnected by a connecting channel,

[0096] - each of said shafts comprising, in the on or off position, - at least one fuel supply device,

[0097] - at least one supply opening for oxygen-containing oxidant, - an inlet, for loading carbonate mineral stones, at the top of the shafts,

[0098] - an outlet for unloading the calcined material produced, at the bottom of the shafts,

[0099] - an exhaust effluent discharge duct at the top of the shafts, which is connected to a chimney, and

[0100] - a supply of cooling gas to cool the calcined material produced,

[0101] the kiln comprising a system for reversing the operation of the shafts, arranged so that each shaft, in standard operation, 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,

[0102] 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

[0103] - an extraction device connected to a heated cooling gas output or to the connecting channel and provided to control the extraction of heated cooling gas and / or of the combustion fumes forming an extracted gas, wherein said extraction device comprises:

[0104] o a gas input located in an upstream position and arranged to receive said extracted gas,

[0105] o a gas output located in a downstream position and arranged to evacuate said extracted gas, o at least a means for a regulation A through at least one means of pressure reduction or through at least a first flow rate valve and a means for a regulation B through at least a second flow rate valve, o an extraction channel arranged to allow a fluid communication between said gas input and said gas output wherein said extraction channel comprises said means for a regulation A and said means for a regulation B, wherein alternatively said extraction channel comprises a main extraction channel and a secondary extraction channel in fluid communication with the main extraction channel, said main extraction channel comprises said means for a regulation B while said secondary extraction channel comprises said means for a regulation A, said means for a regulation A and B being arranged to allow a passage of at least 60% of the flow of said extracted gas into the main extraction channel (flow of the extracted gas in the main extraction channel / sum of the flow of the extracted gas in themain extraction channel and the flow of the extraction gas in the secondary extraction channel), o a flow meter arranged to measure the flow rate of said extracted gas, said flow meter being in operational connection with said means for a regulation B,

[0106] o at least one flow rate controller in operational connection with said means for a regulation B and with said flow meter,

[0107] wherein

[0108] said extraction device comprises

[0109] (i) a configuration in series wherein said means for a regulation A and B are arranged in series and are both located in and in fluid communication with said main extraction channel, wherein said means for a regulation A is a means of pressure reduction arranged to reduce the pressure of said extracted gas and said means for a regulation B is said second flow rate valve preferably an automated flow rate valve, arranged to control the flow rate of said extracted gas, or

[0110] (ii) a configuration in parallel wherein said means for a regulation A is a first flow rate valve located in and in fluid communication with the secondary extraction channel and said means for a regulation B is said second flow rate valve located in and in fluid communication with said main extraction channel.

[0111] Preferably, said at least one means of pressure reduction is located in an upstream said at least (second) flow rate valve.

[0112] Preferably, said extraction device comprises said configuration in series and said configuration in parallel. Preferably, said extraction device comprises at least 2 of said configuration in parallel, preferably at least 3, 4, 5, 6, 7, or 8 of said configuration in parallel. Preferably, said extraction device comprises at least 2 of saidconfiguration in series, preferably at least 3, 4, 5, 6, 7, or 8 of said configuration in series. Alternatively, or preferably, said extraction device comprises a combination of at least 2 of said configuration in parallel and at least 2 of said configuration in series.

[0113] Preferably, the parallel-flow regenerative kiln according to the present invention comprises one extraction device per shaft.

[0114] Preferably, said extraction device further comprises a means for a regulation C through at least one means of pressure reduction and / or through at least a flow rate valve.

[0115] 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.

[0116] Preferably or optionally, said at least one means of pressure reduction comprises

[0117] (i) at least one static restriction means arrange to reduce the diameter of the extraction channel on at least a portion of the extraction channel in order to reduce the pressure of the extracted gas passing through said static restriction means, and / or

[0118] (ii) at least one manual valve of pressure reduction, and / or (iii) at least one automated valve of pressure reduction.

[0119] Preferably, said extraction device further comprises a pressure sensor in fluid communication with said extraction channel, wherein said pressure sensor is preferably located downstream said at least one means of pressure reduction.

[0120] Preferably, said extraction device further comprises a pressure controller in operational connection with said pressure sensor and with said at least one automated valve of pressure reduction.

[0121] Preferably, said at least (first and / or second) flow rate valve is an automated flow rate valve having an open position able to let said extracted gas passing through without a flow rate change and at least aclose position able to let said extracted gas passing through with a flow rate change.

[0122] Preferably, said pressure controller is in operational connection with said pressure sensor and with said at least one automated valve of pressure reduction, and wherein said flow rate controller is in operational connection with said flow meter and with said automated flow rate valve.

[0123] Preferably, said extraction device further comprises a second controller in operational connection with said flow meter and with said at least one flow rate valve.

[0124] Preferably, said flow rate controller has a shorter response time than said pressure controller.

[0125] Preferably, the parallel-flow regenerative kiln according to the present invention further comprises a recirculation circuit which is arranged between the above-mentioned exhaust effluent discharge duct of the shafts and said oxidant supply openings of the shafts, and a separating member, capable of collecting a portion of exhaust effluent discharged from the furnace via the duct and introducing it into the recirculation circuit, and a source of concentrated dioxygen, connected with the recirculation circuit in order to supply it with concentrated dioxygen and thereby form an oxidizing mixture, said oxidant supply opening of the shaft in calcining mode being supplied in the on position via said reversing system to ensure fuel combustion.

[0126] Preferably, a heat exchanger supplied with heated cooling gas removed from the furnace, is mounted on the recirculation circuit and wherein said at least one means of pressure reduction of said extraction device is located upstream said heat exchanger and wherein said at least (second) flow rate valve of said extraction device is located downstream said heat exchanger.

[0127] Preferably, the recirculation circuit is connected to at least one buffer unit.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 at least the quantity corresponding to 10 seconds of the gas flow in the connected pipe.

[0128] Preferably, said recirculation circuit is connected to storage unit provided to store a CC>2-rich exhaust effluent, optionally before or after a buffer unit.

[0129] Preferably, the shafts have a circular cross-section, wherein said connecting channel comprises a crossover channel and the peripheral channels, the crossover 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 an evacuation element so as to allow heated cooling gas to be removed from the furnace, said evacuation element having an heated cooling gas output connected to said extraction device.

[0130] Preferably, the circular shafts further comprise, at the bottom, a central collector element connecting with said evacuation element so as to allow heated cooling gas to be removed from the furnace, below the connecting channel.

[0131] 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 connecting channel is a crossover 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 an evacuation element wherein said evacuation element having an heated cooling gas output connected to said extraction device so as to allow heated cooling gas to be removed from the furnace.According to on 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 dinitrogen. An oxygen tank may also be provided. Advantageously, a heat exchanger supplied with heated cooling gas 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.

[0132] Preferably, the heated cooling gas is heated cooling air. In still a preferred embodiment, the kiln according to the present invention comprises, downstream or upstream pressurization means, heating means to heat said mixing gas stream before injection in the connecting channel, said heating means being preferably chosen amongst a heat exchanger, a combustion chamber, electrical heater.

[0133] Preferably, said recirculation circuit is connected to at least one buffer unit.

[0134] 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 exhaust effluent, optionally before or after a buffer unit.

[0135] Preferably, the extraction device comprises at least one valve of pressure reduction and one valve of flow rate control located in series, wherein a position controller controls said at least one valve of pressure reduction and a flow rate controller controls said valve of flow rate control, wherein the position of said valve of flow rate is transmitted through a position transmitter to said position controller and wherein the position of said at least one valve of pressure reduction is transmitted by a position transmitter to said position controller.

[0136] Preferably, the extraction device comprises a first and a second valves of pressure reduction and one valve of flow rate control located in series, wherein a first and a second position controller respectively control said first and said second valves of pressure reduction and a flow rate controller controls said flow rate valve, wherein said flowrate valve controls through a position transmitter said second position controller and wherein the position of said second valve of pressure reduction is transmitted by a position transmitter to said first position controller.

[0137] Other embodiments of the parallel-flow regenerative kiln according to the present invention are mentioned in the appended claims.

[0138] Other features and details of the kiln according to the invention are indicated in the appended claims.

[0139] Other particularities of the invention will also result from the non-limiting description given below, with reference to the Figures illustrating the present invention.

[0140] FIG. 1 schematically shows a conventional PFRK furnace of circular cross-section.

[0141] FIGS. 2, 3, 4, 5, 6, 13, and 14 schematically show several embodiments of the furnace according to the invention comprising different embodiments of the extraction device.

[0142] FIG. 7 schematically shows a PFRK furnace comprising a recirculation circuit for oxyfuel combustion (prior art).

[0143] FIGS. 8, 9, 10, 11, 12, 15, and 16 schematically show several embodiments of the PFRK furnace comprising a recirculation circuit for oxyfuel combustion according to the present invention comprising different embodiments of the extraction device.

[0144] 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.

[0145] 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 carbonate stones occurs and the cooling area C where the cooling of the calcined material occurs.

[0146] 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.

[0147] 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 inlet 5 for the carbonate stones and to the opening 6 for supplying combustion air. However, the supply duct 7 for the cooling air and the outlet 8 for the calcined material remain in the open position. After heat exchange with the descending calcined material 10, the heated cooling air mixes with the combustion fumes 11 which, from the crossover channel 3, enters the shaft via the peripheral channel 13. The combustion fumes 11 mixed with the heated cooling air (including the one coming from the bottom of the preheating shaft) progresses until reachingT1

[0148] 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.

[0149] 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.

[0150] In some cases, there are three shafts, two in preheating mode and one in combustion.

[0151] FIG. 2 is a view of an advantageous furnace according to the present invention. As can be seen, this embodiment comprises an extraction device 1.1, 1.2 connected to the collector ring 25 on each shaft 1 , 2 and provided to control the extraction of heated cooling gas forming an extracted gas, wherein said extraction device 1.1 comprises:

[0152] o a gas input 28 located in an upstream position and arranged to receive said extracted gas,

[0153] o a gas output 29 located in a downstream position and arranged to evacuate said extracted gas, o at least a means for a regulation A through at least one means of pressure reduction 26 arranged to reduce the pressure of said extracted gas and a means for a regulation B through at least a (second) flow rate valve 27 (flow rate valve 27), preferably an automatic valve, arranged to control the flow rate of said extracted gas,

[0154] o an extraction channel 30 arranged to allow a fluid communication between said gas input 28 and said gas output 29, wherein said extraction channel 30comprises said means for a regulation A and said means for a regulation B

[0155] o a flow meter 31 arranged to measure the flow rate of said extracted gas, said flow meter 31 being in operational connection with said at least one flow rate valve 27,

[0156] o a flow rate controller 32 in operational connection with said flow rate valve 27 and with said flow meter 31,

[0157] o a pressure sensor 34 in fluid communication with the extraction channel 30.

[0158] The means of pressure reduction 26 and the flow rate valve 27 are in series and are both in fluid communication with said extraction channel 30. In addition, the pressure of said extracted gas is measured by the pressure sensor 34 in fluid communication with the extraction channel 30 and wherein a difference between said pressure and said pressure setpoint is determined by a pressure controller 33, said means of pressure reduction 26, being actuated, preferably automatically actuated, by said pressure controller 33, and triggered by said difference in order to reduce said difference.

[0159] The gas output 29 is fluidly connected to a downstream reuse unit and / or to a downstream device 35. The extraction device 1.1, 1.2 is arranged to reduce the pression of the extracted gas that passes through the means of pressure reduction 26 from a gauge pressure comprised between 150 mbar and 500 mbar in a upstream position at the gas input 28 to a gauge pressure setpoint comprised between -50 mbar and 100 mbar, in a downstream position, after the passage through the means of pressure reduction 26. In addition, the flow rate controller 32 in operational connection with said flow rate valve 27 and with said flow meter 31 allows to finely control the flow rate of the extracted gas to the flow rate setpoint. Indeed, said controlled flow rate extracted gas is measured by a flow meter 31, and transmitted to a flow rate controller 32, said flow ratecontroller 32 being provided to automatically adjust said at least one flow rate valve 27 to adapt the flow rate of said controlled flow rate extracted gas to this flow rate setpoint, said controlled flow rate extracted gas further exiting the extraction device 1.1 through a gas output 29 located in a downstream position. In this embodiment, the extracted gas encounter the pressure reduction before the flow rate control. A schematic extraction device 1.2 is connected to the shaft 1. The extraction device 1.2 is identical to the extraction device 1.1 and includes the same components of the extraction device 1.1 (not shown).

[0160] FIG. 3 illustrates another advantageous furnace according to the present invention. As can be seen, in this embodiment the extraction device 1.1, 1.2 comprises a gas input 28 located upstream to two means of pressure reduction 26.1 , 26.2 (valves of pressure reduction) arranged in series. These valves of pressure reduction 26.1 , 26.2 are located upstream to one flow rate valve 27, preferably an automatic flow rate valve, arranged to control the flow rate of said extracted gas. Each valve of pressure reduction 26.1, 26.2 is an automated valve of pressure reduction operatively connected to (i) a pressure sensor 34 located downstream to the respective automated valve of pressure reduction 26.1, 26.2, and to (ii) a pressure controller 33.1, 33.2. The flow rate valve 27 is operatively connected to a flow meter 31 located downstream to the flow rate valve 27, and to a flow rate controller 32. Each pressure controller 33.1 , 33.2 sets a predetermined pressure setpoint to be reached after each automated valve of pressure reduction 26.1, 26.2. In addition, and optionally, the extraction device 1.1, 1.2 also comprises a static restriction means 36 located upstream to the two automated valves of pressure reduction 26.1 , 26.2. In FIG. 3, the schematic extraction device 1.2 is connected to the shaft 1. The extraction device 1.2 is identical to the extraction device 1.1 and includes the same components of the extraction device 1.1 (not shown).

[0161] FIG. 4 illustrates another advantageous furnace according to the present invention. As can be seen, in this embodiment the extractiondevice comprises a first 26.2 and a second 26.1 valves of pressure reduction in series located upstream to one valve 27 of flow rate control. A first static restriction means 36.1 is located between the two valves of pressure reduction 26.2, 26.1, while a second static restriction means 36.2 is located downstream the first static restriction means 36.1 between the second valves of pressure reduction 26.1 and the flow rate valve 27. The first automated valve of pressure reduction 26.2 is optional in this embodiment, and is controlled by a first position controller 33.1 while the second automated valve of pressure reduction 26.1 is controlled by a second position controller 33.2. The flow rate valve 27 is controlled by a flow rate controller 32. In this embodiment, the position of the flow rate valve 27 is transmitted to the second position controller 33.2 which acts on the second automated valve of pressure reduction 26.1. The position of the second automated valve of pressure reduction 26.1 is in turn transmitted by a position transmitter to the first position controller 33.1 which acts on the first valve of pressure reduction 26.2. In this embodiment, the first 33.1 and second 33.2 position controller act so that the flow rate valve 27, and also the second automated valve of pressure reduction 26.1, returns to an average position, preferably a predefined average position. In other words, if the flow rate valve 27 closes too much, the second automated valve of pressure reduction 26.1 will react by closing a little bit in turn, so that the flow rate valve 27 returns to its average position. The first automated valve of pressure reduction 26.2 will then react in the same way.

[0162] In this embodiment, the flow rate controller 32 has a shorter response time than the second position controller 33.2, while the second position controller 33.2 has a shorter response time than the first position controller 33.1.

[0163] In the context of the present invention, the term “response time” is preferably defined as the time interval between the application of change of input (e.g., setpoint change) and the point at which thecontroller output reaches a predetermined percentage (usually 63.2%) of the range between the initial output value to the final output value.

[0164] In FIG. 4, the schematic extraction device 1.2 is connected to the shaft 1. The extraction device 1.2 is identical to the extraction device 1.1 and includes the same components of the extraction device 1.1 (not shown).

[0165] FIG. 5 illustrates another advantageous furnace according to the present invention. As can be seen, in this embodiment the extraction device 1.1 comprises three valves of pressure reduction 26.1 , 26.2, 37 and one flow rate valve 27 located in series but in a different order as compared to the other embodiments. In this embodiment, a first manual valve of pressure reduction 37 located upstream to a second automated valve of pressure reduction 26.1 and a third automated valve of pressure reduction 26.2. A flow rate valve 27 is located between the second 26.1 and the third automated valve of pressure reduction 26.2. A flowmeter 31 , located upstream the first manual valve of pressure reduction 37, transmits the flow rate value to the flow rate controller 32. The position of the flow rate valve 27 is transmitted, through a position transmitter, to the second pressure controller 33.2, which in turn regulates the third automated valve of pressure reduction 26.2. A pressure sensor 34 located between the flow rate valve 27 and the third automated valve of pressure reduction 26.2 regulated the first pressure controller 33.1 that in turn acts on the second automated valve of pressure reduction 26.1. This embodiment of the furnace according to the present invention illustrates the fact that that many configurations are possible for the sensors and valves within the extraction device 1.1, 1.2.

[0166] In FIG. 5, the schematic extraction device 1.2 is connected to the shaft 1. The extraction device 1.2 is identical to the extraction device 1.1 and includes the same components of the extraction device 1.1 (not shown).

[0167] FIG. 6 illustrates another advantageous furnace according to the present invention. As can be seen, in this embodiment the extractiondevice 1.1 comprises a automated valve of pressure reduction 26 operatively connected to a pressure sensor 34 and to a pressure controller 33, and wherein the automated valve of pressure reduction 26 is located upstream to a flow rate valve 27. The flow rate valve 27 is operatively connected to a flow meter 31 and a flow rate controller 32. Between the automated valve of pressure reduction 26 and the flow rate valve 27, the extraction channel 30 is in fluid communication with a heat exchanger 38 and a filter 39. It is advantageous that the flow rate valve 27 treats less hot and dust-free gas, for example dust-free air, which represents gentler conditions for a valve which must act with precision. Alternatively, the automated valve of pressure reduction 26 could also be located after the heat exchanger 38 to be subject to less high temperature. Preferably, the automated valve of pressure reduction 26 is located before, upstream, the filter 39, so as to maintain the filter 39 at reduced pressure. This also adds the advantage that the pressure drop of the heat exchanger 38 and the filter 39 contribute to the overall pressure reduction.

[0168] The different embodiments described in Figures 2 to 6 have been illustrated for cooling gas extraction, such as the heated cooling gas extracted from the collector ring 25. Of course, similar embodiments can be performed for the extraction of cooling gas from a central collecting element 43 (see Figures 7 to 12 and 15 and 16) .

[0169] When the PFRK has rectangular shafts, the extraction of the cooling gas is performed laterally through exit means located either on a first side, being the side of the shaft facing the other shaft or on a second side, opposed to the first side of each shaft. The extracted cooling gas exit the kiln through the exit means in the first side or in the second side or both.

[0170] Typically from the calcining shaft, the cooling gas is air and is concentrated around the first side of the calcining shaft and it is advantageous to extract the cooling gas mainly through the exit means located on the first side mainly and possibly to a lower extend through the exit means located on the second side of the calcining shaft. From the preheating shaft, the cooling air is concentrated around the second sideof the preheating shaft and it is advantageous to extract the cooling gas mainly through the exit means located on the second side and possibly to a lower extend through the exit means located in the first side.

[0171] The extraction device 1.1, 1.2 explained here above for the cooling gas in circular shaft is also applicable in rectangular shaft and can be therefore located after the exit means of the first side and of the second side of the calcining shaft or preheating shaft.

[0172] In FIG. 6, the schematic extraction device 1.2 is connected to the shaft 1. The extraction device 1.2 is identical to the extraction device 1.1 and includes the same components of the extraction device 1.1 (not shown).

[0173] FIG. 7 is a view of a PFRK furnace comprising a recirculation circuit for oxyfuel combustion. As can be seen, this embodiment comprises separating member 17, capable of collecting a portion of exhaust 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 exhaust 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 02 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 exhaust effluent and concentrated 02 to the top of each of the shafts at the supply opening 6.

[0174] 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.

[0175] In addition, the heated cooling gas is extracted after contact with the calcined material, by installing a removal system. Indeed, theshafts 1 and 2 are each provided with the collector ring 25, below the crossover channel 3, which connects with an evacuation element 40 so as to allow heated cooling gas to be removed from the furnace. In this way, a portion or all of the combustion air may be extracted, as required, by also extracting a small proportion of combustion fumes. The shafts may further optionally comprise, at the bottom, a central collector element 43 connecting with the evacuation element 40 as to also allow a central removal of the heated cooling gas, below the crossover channel 3. Moreover, in order to recover a portion of the energy from the hot air removed by the evacuation element 40, a heat exchange may be provided with the portion of recirculated exhaust effluent using a heat exchanger 41, before or after the mixing thereof with concentrated dioxygen.

[0176] In the connecting channel or crossover channel 3, an injection of a fraction of said collected portion of exhaust effluent discharged from the furnace using an injection duct 42 may also be provided. Optionally beforehand, a heat exchange between the heated cooling gas removed from the furnace, and this above-mentioned fraction to be injected may occur using a heat exchanger, for example the heat exchanger 41. In the absence thereof, another heater not shown may be provided on the injection duct 42.

[0177] Preferably, in the connecting channel or crossover channel 3, the injection of a fraction of said collected portion of exhaust effluent discharged from the furnace using the injection duct 42 also comprises an addition of oxygen by a fluid connection of the injection duct 42 with the supply duct 23.

[0178] Figures 8 to 12 describe the embodiments described respectively in Figures 2 to 6 wherein the conventional PFRK furnace of circular cross-section has been replaced by a PFRK furnace comprising a recirculation circuit for oxyfuel combustion. In each embodiments described above, the extraction device 1.1, 1.2 can be located downstream to heated cooling gas output wherein the gas input 28 of theextraction device 1.1, 1.2 is fluidly connected to the central collecting element 43, and / or to the collector ring 25, and / or through the connecting channel 3, 13.

[0179] In FIGS. 8 to 12, the schematic extraction device 1.2 is connected to the shaft 1. The extraction device 1.2 is identical to the extraction device 1.1 and includes the same components of the extraction device 1.1 (not shown). The extraction device 1.2 represented in FIGS. 8 to 12 is identical to the respective extraction device 1.1 represented in the embodiments of FIGS. 8 to 12.

[0180] The extraction devices 1.1 and 1.2 are connected to the collector ring 25 and to the central collecting element 43. Isolation valves 1.11 are arranged to permit (open position) or prevent (close position) the extraction of the heated cooling gas from the central collecting element 43 while isolation valves 1.12 are arranged to permit (open position) or prevent (close position) the extraction of the heated cooling gas from the collector ring 25.

[0181] FIG. 13 shows another advantageous embodiment of the furnace according to the present invention which is identical to the embodiments described in Figures 2 to 6 except that the extraction device 1.1 and 1.2 comprises a main extraction channel 30.1 and a secondary extraction channel 30.2 in fluid communication with the main extraction channel 30.1 wherein a first flow rate valve 27.1 is located into the main extraction channel in operational connection with a first flow meter 31.2 located into the main channel and with a flow rate controller 32.1 located in a downstream position in the extraction channel. The extraction device 1.1 and 1.2 also comprises a second flow rate valve 27.2 located into the secondary extraction channel 30.2 wherein said second flow rate valve 27.2 is in operational connection with a second flow meter 31.1 and with a flow rate controller 32.2. The first and second flow rate valve 27.1 , 27.2 being arranged to allow a passage of at least 60% of the flow of said extracted gas in the main extraction channel 30.1 (flow of the extracted gas in the main extraction channel 30.1 / sum of the flow of theextracted gas in the main extraction channel 30.1 and the flow of the extraction gas in the secondary extraction channel 30.2).

[0182] Advantageously, such parallel configuration wherein two flow rate valves in a parallel configuration: the first flow rate valve located in the main extraction channel and the second flow rate located in the secondary extraction channel, wherein the secondary extraction channel is parallel to the main extraction channel allows a first regulation of the flow rate by the first flow rate valve in operational connection with the first flow meter 31.2 and with the flow rate controller 32.1, while the second flow rate valve 27.2 in operational connection with a second flow meter 31.1 and with a flow rate controller 32.2 allows a complementary, finetuned, regulation of the flow rate. Preferably, the flow rate controller 32.1 has a longer response time than the response time of the flow rate controller 32.2. The extraction device 1.2 represented in FIG. 13 is identical to the extraction device 1.1 represented in FIG. 13.

[0183] FIG. 14 shows another advantageous embodiment of the furnace according to the present invention which is identical to the embodiment described in Figure 13 except that within the extraction device 1.1, 1.2 the first flow meter 31.2 has been removed and the position of the second flow rate valve 27.2 is transmitted to a position controller 32.3 which acts on the first flow rate valve 27.1. The position controller 32.3 acts to bring back the position of the second flow rate valve 27.2 to an average predefined position. In this embodiment, only one flow meter is needed. The extraction device 1.2 represented in FIG. 14 is identical to the extraction device 1.1 represented in FIG. 14.

[0184] FIG. 15 is the same embodiment of the embodiment described in FIG. 13 except that the furnace is a PFRK furnace comprising a recirculation circuit for oxyfuel combustion as described in FIG. 7. The extraction device 1.2 represented in FIG. 15 is identical to the extraction device 1.1 represented in FIG. 15.

[0185] FIG. 16 is the same embodiment of the embodiment described in FIG. 14 except that the furnace is a PFRK furnace comprisinga recirculation circuit for oxyfuel combustion as described in FIG. 7. The extraction device 1.2 represented in FIG. 16 is identical to the extraction device 1.1 represented in FIG. 16

[0186] Obviously, the present invention is not limited to the disclosed embodiment and several modifications may be provided without being outside the scope of the appended claims.

Claims

38CLAIMS1. 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), 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 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 is39ascending 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 (a) at least a part of said heated cooling gas coming from a collection tunnel outside of the kiln and / or (b) at least a part of said combustion fumes from the connecting channel,characterized in that during said extracting step of said at least a part of said cooling gas outside of said kiln and / or of at least a part of said combustion fumes from the connecting channel, said heated cooling gas and / or said combustion fumes is extracted through at least one extraction device, forming an extracted gas, said extracting device being provided to control the flow rate and the pressure of said extracted gas, wherein (i) said extracted gas enters into an extraction channel of said extraction device through a gas input located in an upstream position at a gauge pressure comprised between 150 mbar and 500 mbar,(ii) said extracted gas encounters at least two regulations comprising - a regulation A being either a pressure reduction to a gauge pressure setpoint comprised between -50 mbar and 100 mbar, through at least one means of pressure reduction, or a flow rate control A through at least a first flow rate valve, and- a regulation B being a flow rate control B through at least a second flow rate valve to give a controlled flow rate extracted gas,wherein said extracted gas encounters first regulation A then regulation B or first regulation B then regulation A,and wherein the flow rate value either of extracted gas, or of said controlled flow rate extracted gas is measured by at least one flow meter, and transmitted to at least one flow rate controller, said at least one flow rate controller being provided to automatically adjust said at least a first and / or a second flow rate valve to adapt the flow rate of said controlled flow rate extracted gas to a flow rate setpoint, said controlled flow rate40extracted gas further exiting the extraction device through a gas output located in a downstream position,and wherein, in the presence of said flow rate control A through at least said first flow rate valve, said extracted gas having a first flow rate passing through said first flow rate valve and a second flow rate passing through said second flow rate valve wherein said first flow rate is comprised between 10% and 40% of a total flow rate wherein said total flow rate is the sum of said first flow rate and said second flow rate.

2. Method according to claim 1, further comprising a regulation C being either a pressure reduction through at least one means of pressure reduction and / or a flow rate control through at least a flow rate valve.

3. Method according to claim 1 or claim 2, further comprising recirculating a fraction of the exhaust effluent exhausted from the top of said at least one shaft in preheating mode, and injecting the exhaust 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.

4. Method according to any of the preceding claims, wherein said 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 (B) .

5. Method according to any of the preceding claims, wherein the pressure of said extracted gas is measured by a pressure sensor and wherein a difference between said pressure and said pressure setpoint is determined by a pressure controller, said means of pressure reduction, being actuated, preferably automatically actuated, by said pressure controller, and triggered by said difference in order to reduce said difference.

6. Method according to claim 5, wherein said automatic adjustment of said at least first and / or second flow rate valve by said at least one flow rate controller has a response time shorter than said actuation of said means of pressure reduction by said pressure controller.

7. Method according to any of the preceding claims, wherein said extraction step, further comprises a passage of said heated cooling gas or said combustion fumes through a heat exchanger and / or through a filter provided to dedust said extracted gas, said passage through said heat exchanger and / or through said filter being upstream and / or downstream of the passage of said extracted gas through said at least one means of pressure reduction and / or through said at least first and / or second flow rate valve.

8. Method according to any of the preceding claims, wherein the position of the flow rate valve (27) is transmitted to a second position controller (33.2) which acts on a second automated valve of pressure reduction (26.1), wherein the position of the second automated valve of pressure reduction (26.1) is in turn transmitted by a position transmitter to a first position controller (33.1) which acts on a first pressure reduction valve (26.2), wherein the first (33.1) and second (33.2) position controller act so that the flow rate valve (27), and also the second automated valve of pressure reduction (26.1), return to an average position, preferably a predefined average position.

9. Parallel-flow regenerative kiln for implementing the method according to any one 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 (6), 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 kiln 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 (3, 13) being provided to transfer combustion fumes from the shaft in calcining mode to at least one shaft in preheating mode- an extraction device connected to a heated cooling gas output and / or to the connecting channel and provided to control the extraction of heated cooling gas and / or of the combustion forming an extracted gas, wherein said extraction device comprises:o a gas input located in an upstream position and arranged to receive said extracted gas,o a gas output located in a downstream position and arranged to evacuate said extracted gas, o at least a means for a regulation A through at least one means of pressure reduction or through at least a first flow rate valve and a means for a regulation B through at least a second flow rate valve,o an extraction channel arranged to allow a fluid communication between said gas input and said gas output wherein said extraction channel43comprises said means for a regulation A and said means for a regulation B, wherein alternatively said extraction channel comprises a main extraction channel and a secondary extraction channel in fluid communication with the main extraction channel, said main extraction channel comprising said means for a regulation B while said secondary extraction channel comprising said means for a regulation A, said means fora regulation A and said means for a regulation B being arranged to allow a passage of at least 60% of the flow of said extracted gas into the main extraction channel (flow of the extracted gas in the main extraction channel I sum of the flow of the extracted gas in the main extraction channel and the flow of the extraction gas in the secondary extraction channel),o a flow meter arranged to measure the flow rate of said extracted gas, said flow meter being in operational connection with said means for a regulation B,o at least one flow rate controller in operational connection with said means for a regulation B and with said flow meter,whereinsaid extraction device comprises(iii) a configuration in series wherein said means for a regulation A and B are arranged in series and are both located in and in fluid communication with said main extraction channel, wherein said means for a regulation A is a means of pressure reduction arranged to reduce the pressure of said extracted gas and said means for a regulation B is said second flow rate valve,44preferably an automated flow rate valve, arranged to control the flow rate of said extracted gas, or(iv) a configuration in parallel wherein said means for a regulation A is a first flow rate valve located in and in fluid communication with the secondary extraction channel and said means for a regulation B is said second flow rate valve, preferably an automated flow rate valve, located in and in fluid communication with said main extraction channel.

10. Parallel-flow regenerative kiln according to claim 9, wherein said extraction device comprises a configuration in series and a configuration in parallel.

11. Parallel-flow regenerative kiln according to claim9 or claim 10, wherein said extraction device further comprises a means for a regulation C through at least one means of pressure reduction and / or through at least a first flow rate valve.

12. Parallel-flow regenerative kiln according to any of the claims 9 to 11 , comprising one extraction device per shaft (1, 2).

13. Parallel-flow regenerative kiln according to any of the claims 9 to 12, wherein said at least one means of pressure reduction comprises(i) at least one static restriction means arranged to reduce the diameter of the extraction channel on at least a portion of the extraction channel in order to reduce the pressure of the extracted gas passing through said static restriction means, and / or(ii) at least one manual valve of pressure reduction, and / or (iii) at least one automated valve of pressure reduction.

14. Parallel-flow regenerative kiln according to any of the claims 9 to 13, wherein said extraction device further comprises a pressure sensor in fluid communication with said extraction channel, wherein said pressure sensor is preferably located downstream said at least one means of pressure reduction.4515. Parallel-flow regenerative kiln according to claim 14, wherein said extraction device further comprises a pressure controller in operational connection with said pressure sensor and with said at least one automated valve of pressure reduction.

16. Parallel-flow regenerative kiln according to claim 14 or 15, wherein said pressure controller is in operational connection with said pressure sensor and with said at least one automated valve of pressure reduction, and wherein said flow rate controller is in operational connection with said flow meter and with said automated flow rate valve.

17. Parallel-flow regenerative kiln according to any of the claims 9 to 16, wherein said extraction device comprises at least one valve of pressure reduction and one valve of flow rate control located in series, wherein a position controller controls said at least one valve of pressure reduction and a flow rate controller controls said valve of flow rate control, wherein the position of said valve of flow rate is transmitted through a position transmitter to said position controller and wherein the position of said at least one valve of pressure reduction is transmitted by a position transmitter to said first position controller.

18. Parallel-flow regenerative kiln according to any of the claims 9 to 17, further comprising- a recirculation circuit (18) which is arranged between the above-mentioned exhaust 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 exhaust effluent discharged from the furnace via the duct and introducing it into the recirculation circuit (18), - a source of concentrated dioxygen (20), connected with the recirculation circuit (18) in order to supply it with concentrated dioxygen and thereby form an oxidizing mixture, said oxidant supply opening of the shaft in46calcining mode being supplied in the on position via said reversing system (16) to ensure fuel combustion.

19. Parallel-flow regenerative kiln according to any one of claims 9 to 18, wherein a heat exchanger (41) supplied with heated cooling gas removed from the furnace, is mounted on the recirculation circuit (18) and wherein said extraction device is located downstream and / or upstream said heat exchanger (41 ).

20. Parallel-flow regenerative kiln according to any of the claims 9 to 19, wherein said recirculation circuit (18) is connected to at least one buffer unit.

21. Parallel-flow regenerative kiln according to any one of claims 9 to 20, 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 allow a transfer of gas and wherein, below the connecting channel (3, 13), the shafts (1, 2) are provided with a collector ring (25) connecting with an evacuation element (40) so as to allow heated cooling gas to be removed from the furnace, said evacuation element (40) having an heated cooling gas output connected to said extraction device.

22. Parallel-flow regenerative kiln according to claim 21, wherein the circular shafts further comprise, at the bottom, a central collector element (43) connecting with said evacuation element (40) so as to allow heated cooling gas to be removed from the furnace, below the connecting channel.

23. 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 connecting channel is a crossover channel (3) which directly connects one shaft to the other via their first sides, and in that, below the connecting channel (3), said first sides and said second sides of the shafts47are provided with a collection tunnel (25) connecting with an evacuation element (40) so as to allow heated cooling gas to be removed from the furnace, said evacuation element (40) having an heated cooling gas output connected to said extraction device.

24. Parallel-flow regenerative kiln according to any one of claims 9 to 23, wherein the extraction device (1.1, 1.2) comprises said main extraction channel (30.1 ) and said secondary extraction channel (30.2) in fluid communication with said main extraction channel (30.1 ) wherein said first flow rate valve (27.1) is located into said main extraction channel and is in operational connection with a first flow meter (31.2) located into said main channel and with a flow rate controller (32.1) located in a downstream position in said extraction channel, wherein said extraction device (1.1, 1.2) also comprises a second flow rate valve (27.2) located into said secondary extraction channel (30.2) wherein said second flow rate valve (27.2) is in operational connection with a second flow meter (31.1) and with a flow rate controller (32.2), wherein the first and second flow rate valve (27.1 , 27.2) being arranged to allow a passage of at least 60% of the flow of said extracted gas in the main extraction channel (30.1 ) (flow of the extracted gas in the main extraction channel 30.1 I sum of the flow of the extracted gas in the main extraction channel 30.1 and the flow of the extraction gas in the secondary extraction channel 30.2).

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