Process and KILN for calcining mineral rocks with a FLUE gas treatment unit

Abstract

Kiln (1) and Process for calcining carbonate mineral rock with a treatment of a first portion of exhaust gas discharged from the kiln in a flue gas treatment unit (25) where before being treated, said first portion of said exhaust gas is heated in a heat exchanger (23) to collect heat from a gas stream being either heated cooling gas or hot combustion fumes

Description

[0001] PROCESS AND KILN FOR CALCINING MINERAL ROCKS WITH A FLUE GAS TREATMENT UNIT

[0002] The present invention relates to a process for calcining carbonate mineral rock in a carbonate mineral rock kiln comprising:

[0003] loading the carbonate mineral rock in the kiln,

[0004] firing said carbonate mineral rock, in a combustion zone, with the decarbonation thereof into calcined material with a release of hot combustion fumes, discharging exhaust gas containing at least the combustion fumes, dust and pollutants at a temperature comprised between 80 to 220°C, in particular between 80°C to 200°C, more particularly between 90°C and 180°C, more preferably between 100°C and 160°C.,

[0005] cooling the calcined material via cooling gas, in particular cooling air, said cooling gas being introduced cold (i.e. between about 30°C to about 100°C) at a first inlet through the calcined material and being heated upon contacting said calcined material in a cooling zone by heat exchange to form a heated cooling gas,

[0006] unloading the calcined material from the cooling zone of the kiln,

[0007] treating a first portion of said exhaust gas discharged from the kiln in a flue gas treatment unit to remove at least partly pollutants and forming a clean exhaust gas,

[0008] Nowadays, flue gas treatment is more and more an attention point for industrial players.

[0009] In the technical domain of calcination of carbonate mineral rocks, the exhaust gas contains the combustion fumes from the combustion of fuel, but also CO2 from the calcination of the carbonate mineral rock, which will encounter a decarbonation. Consequently, the exhaust gas from carbonate mineral rock calcination contains typically at least 15 %of CO2 on a dry basis for a process carrying out a conventional combustion of fuel and this CO2 content can go up to 60, 70, 80, 90, 95 and even more than 98 % CO2 for example in processes carrying out combustion of fuel in oxygen enriched conditions also called in oxyfuel operation.

[0010] By the terms “rocks, carbonate mineral rocks, limestone rocks”, it is meant according to the present invention pieces of raw carbonated material having a median particle size 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.

[0011] 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 or dolomite when the magnesium content is close to the one of calcium on a molar basis.

[0012] The calcined mineral according to the present patent application is typically a calcium-magnesium oxide, also known as quicklime when containing a low amount of magnesium and dolime when the magnesium content is close to the one of calcium on a molar basis.

[0013] CO2 capture from the exhaust gas stream is today a key challenge and for being successful, many parameters should be controlled and many challenges should be addressed. One of them is the composition of the exhaust gas stream and especially its content in VOC (Volatile Organic Compounds), TOC (total organic carbon) or (THC) total hydrocarbon, SOx (including SO2 and SO3), NOx, called pollutants in the meaning of the present patent application for common carbon capture.

[0014] Those pollutants should be removed in a purification step for ensuring an efficient carbon capture step on the exhaust gas and therefore ultimately integrate some costly removal steps. Further, the pollutants will end up in a water exhaust stream that will also need to be treated at high cost.

[0015] Accordingly, the reduction of the content of pollutants in the exhaust gas will consequently have a positive impact on the aforementioned.

[0016] The present invention relates to the reduction of the content of pollutant in the exhaust gas.

[0017] Efficient pollutants capture often involves a temperature challenge. In some kilns, the exhaust gas exits the kiln at a temperature comprised between 80 to 220°C

[0018] Some flue gas treatments require the flue gas to be reheated to a minimum temperature. For example, SCR (Selective Catalytic Reduction) of NOx requires a temperature between 200°C and 400 °C, with an optimum around 350°C. Also, the filtration through a bag filter requires the flue gas temperature to be sufficiently higher than the acid dew point in the gas and lower than the maximum temperature that these bags can sustain.

[0019] For CO2 capture, the problem of pollutant capture is even more important since many pollutants are problematic and will have to be reduced to extremely lowlevels. It may be more cost effective to reduce as much as possible some pollutants like NOx before entering the CO2 concentration and purification plant.

[0020] Some of these treatments may require flue gas reheating. Flue gas reheating can be done by a set of burners or electrical heaters (possibly combined with heat exchangers), which are costly to operate.

[0021] In this respect some solutions have been foreseen, such as for example in document WO2024 / 126781, which provide a process as mentioned in the beginning specially adapted to Parallel flow regenerative kiln (PFRK) having an SCR system situated downstream the dust removal system and being arranged to reduce the NOx in the exhaust gas, comprising a low temperature catalyst which is arranged to operate at a temperature of between 150°C and 160°C, and at least one injection lance which is placed in a duct which is situated between the dust removal system and the catalyst and which is arranged to inject reducing agent in the exhaust gas present in the duct. As it can be seen according to this document a special low temperature catalyst system has been foreseen for corresponding to the temperature of the exhaust gas discharged from the kiln.

[0022] However, while being a relevant solution, it is nevertheless a niche solution and it is clearly a challenge for an industrial player to change its capture device and replace it with a low temperature catalyst since it is the most expensive portion of the NOx capture unit and the efficiency at this low temperature may not be sufficient with afterwards CO2 capture requirements or may require additional purification upon entering the CO2 capture step.

[0023] Another solution is discussed in CN214389610. This system provides a downstream heat recovery system to collect heat from the clean exhaust gas and use this heat to preheat the incoming exhaust gas to be treated. However, while making a step forward, this system requires a mixture between the preheated exhaust gas to be treated and hot exhaust gas from the kiln to ensure that proper temperature is achieved.

[0024] The present invention intends to solve at least partially these drawbacks by providing a process adaptable to different kilns, efficient in terms of energy consumption and which is compatible with CO2 capture units.

[0025] To solve this problem, it is provided according to the present invention, a process for calcining carbonate mineral rocks as mentioned in the beginning, characterized in that before being treated in a flue gas treatment unit, said first portion of said exhaust gas discharged from the kiln is further heated in a heat exchanger byflowing into a first pathway in the heat exchanger by collecting heat from a gas stream flowing in a second pathway of the heat exchanger, said gas stream being either heated cooling gas or hot combustion fumes or both, said heated cooling gas being extracted outside of the kiln after having traversed at least partially said calcined materiel in said cooling zone through a heated cooling gas exit in fluid communication with said heat exchanger or said hot combustion fumes being extracted outside the kiln through a hot combustion gas outlet in fluid communication with said heat exchanger, said hot combustion fumes or said heated cooling gas being extracted at a temperature higher than the temperature of said exhaust gas discharged from the kiln.

[0026] As it can be seen, the process according to the present invention uses heat from hot gas extracted from the kiln process at a point hotter than the kiln flue gas exit such as for example higher than 350°C, preferably higher than 400°C, and even more preferably higher than 500°C, to reheat the exhaust gas or to stabilize the temperature of the exhaust gas, by the mean of a heat exchanger between this extracted hot gas and the flue gas exiting the kiln installation; thus reducing or suppressing the need for additional burners or electrical heaters.

[0027] Indeed, depending on the type of kiln, sometimes, the temperature of the exhaust gas discharged from the kiln is too low for an efficient flue gas treatment and sometimes, the exhaust gas discharged from the kiln can have a sufficient temperature on a predefined period, but can present highly fluctuating temperature during other periods. The heat exchange thanks to the extracted hot gas can be used according to the present invention either to reheat the exhaust gas or to stabilize its temperature at a temperature allowing efficient flue gas treatment, while reducing or suppressing the need for additional burners or electrical heaters.

[0028] The hot gas can be hot combustion fumes or heated cooling gas. In those cases, either the costs or the total CO2 balance can be advantageous since the fuel used for the combustion in the carbonate mineral rock kiln can possibly be a solid fuel cheaper or emitting less CO2 than the natural gas normally used in reheater burners.

[0029] When the hot gas is heated cooling gas, for example cooling air, having been heated by heat exchange through the calcined mineral rock, significant heat will therefore be available. Cooling gas or cooling air extraction can potentially be applied to parallel flow regenerative kiln operating in conventional or oxyfuel conditions and has the added benefit of rising the concentration of CO2 in the flue gas, thus making the capture easier.Advantageously, in the process according to the present invention, the kiln is a regenerative parallel-flow vertical shaft kiln (PFRK), wherein at least two shafts are interconnected via a connecting channel, wherein each shaft operates alternately in firing mode and in preheating mode, one shaft being in firing mode for a predetermined time period while at least one other shaft is in preheating mode, and vice-versa, the process comprising an inverting step during a predefined inversion period during which a shaft in preheating mode is shifted to the firing mode and the shaft in firing mode is shifted to the preheating mode;

[0030] the firing mode comprising:

[0031] o combusting fuel in the presence of oxygen so as to obtain said firing of said carbonate mineral rock and the decarbonation thereof into calcined material with the release of combustion fumes in the form of combustion fumes descending co-currently in the shaft in firing mode, and o said combustion fumes moving from the shaft in firing mode to said at least one shaft in preheating mode using said connecting channel and forming at least partially said exhaust gas,

[0032] the preheating mode comprising:

[0033] o preheating of the loaded carbonate mineral rock via heat exchange with the exhaust gas from the connecting channel, which is ascending in said at least one shaft in preheating mode, counter-currently to said loaded carbonate mineral rock, and

[0034] o said step of discharging exhaust gas.

[0035] Indeed the cyclic nature of the kiln and the inversion period occurring during the process according to the present invention represents a double challenge: on one side, the exhaust gas due to the preheating step exiting the preheating shaft have a lower temperature compared to other kiln type and on the other side, the inversion period and the cyclic operation creates fluctuation in the exhaust gas features that needed to be stabilized before entering a gas treatment unit for being highly efficient. Therefore, the present invention is particularly suitable for PFR kilns by allowing both to reheat the exhaust gas before entering the gas treatment unit and stabilize its temperature with recovered heat from the process for calcining carbonate.

[0036] Preferably, in the process of calcining carbonate mineral rocks according to the present invention, the step of treating the exhaust gas discharged from the kiln comprises a NOx reduction step in a SCR system.Nitrogen oxides (NOx), including nitric oxide (NO) and nitrogen dioxide (NO2), are the most significant gaseous pollutant, apart from CO2 emitted by a carbonate mineral rocks kiln. The NOx emissions are mainly dependent on the design of the kiln and, for a particular kiln, on the fuel nitrogen content and combustion temperature.

[0037] Two well deployed technologies exist and consist of Selective Non-Catalytic Reduction (SNCR) system to reduce NOx and Selective Catalytic Reduction (SCR). The SNCR system injects a reducing agent such as an ammonia solution, ammonia precursor compounds or a urea solution into the flue gas produced by the carbonate mineral rock kilns to reduce the NOx into N2 and water vapor. However, while being quite efficient, SNCR operates in a narrow range of (relatively high) temperature, requires a contact time that is quite high, with large excess of reducing agent and present a reduced level of efficiency when the gas flow and NOx emission level is not constant.

[0038] SCR has been already described for carbonate mineral rocks kiln, such as in PFRK (see WO 2018 / 220520 or ON 114184053). However, the efficiency of SCR does not yet reach the level needed for CO2 capture (about 65% of NOx capture).

[0039] According to the present invention, the use of SCR is made possible and efficient through the integration of the heat exchanger recovering heat from the main calcining process allowing to reach much higher temperature such as around 350°C, making the NOx capture much more efficient, while staying within a reasonable cost window. Further, the improved level of NOx reduction is made now compatible with the standard of a carbon capture treatment.

[0040] Advantageously, in the process of calcining carbonate mineral rocks according to the present invention, the step of treating the exhaust gas discharged from the kiln comprises a SOx reduction step.

[0041] In a preferred embodiment, in the process of calcining carbonate mineral rocks according to the present invention, the step of treating the exhaust gas discharged from the kiln comprises a VOC and / or total organic carbon (TOC) or total hydrocarbon (THC) reduction step.

[0042] Preferably, the process according to the present invention comprises a step of CO2 capture from said clean exhaust gas.

[0043] In a preferred embodiment according to the present invention, wherein a second portion of the exhaust gas discharged from the kiln is derived through a derivation system, preferably located upstream of the flue gas treatment unit, saidsecond portion of said exhaust gas being collected and mixed with oxygen to form an oxidizing mixture and re-introduced in the kiln for providing an oxidizing atmosphere in the combustion zone.

[0044] In this case, the concentration of CO2 in the exhaust gas is increased together with the number of cycles of the exhaust gas. The oxygen can originate from combustion air or from concentrated dioxygen or both. When oxygen is originating from concentrated dioxygen, the CO2 concentration of the exhaust gas can reach a level from 60% to 98 % (especially with cooling gas extraction, preferably with cooling air extraction), on a volume basis. Increased CO2 concentration in the exhaust gas improves further the efficiency of CO2 capture

[0045] Preferably, in the process according to the present invention, the oxidizing mixture is performed with said second portion of the discharged exhaust gas and concentrated dioxygen.

[0046] Preferably, the oxidizing mixture is performed with said second portion of the discharged exhaust gas and concentrated dioxygen in a mixing device such as a static mixing device (for example a mixing chamber), a countercurrent injection mixing device and the like, said oxidizing mixture being fed to the kiln, preferably in the combustion zone.

[0047] In a further preferred embodiment according to the present invention, the heated cooling gas is extracted outside of the kiln and a portion of the heated cooling gas is collected through a duct connected to the heated cooling gas exit in fluid communication with the second pathway of the heat exchanger.

[0048] The operation in oxyfuel condition, combined with the extraction of heated cooling gas from the kiln reduces the level of dilution of the CO2 in the exhaust gas while providing more than enough heat to the second pathway of the heat exchanger, since the heated cooling gas can present a temperature comprised between 600°C and 800°C. This preferred embodiment allows a very promising CO2 capture, efficient and affordable. Indeed, on one side the exhaust gas has a high CO2 content and on the other side, the reduction of pollutant can be done at the ideal temperature at reasonable energetic costs.

[0049] Preferably, the heated cooling gas is heated cooling air.

[0050] When the kiln for carrying out the present invention is a parallel flow regenerative kiln, in yet a preferred embodiment according to the present invention, the oxidizing mixture is introduced at the top of the shaft in firing mode or at the top ofthe combustion zone of the shaft in firing mode so as to ensure said fuel combustion in the presence of oxygen.

[0051] In a further preferred embodiment of the process according to the present invention, the loading of carbonate mineral stones occurs during the inversion period at the top of any of the shafts, preferably of the shaft that will work in a preheating mode after said inversion period.

[0052] In other preferred embodiments according to the present invention, alternatively, or in addition, the loading of carbonate mineral stones occurs during the cycle at the top of the shaft that works in a preheating mode.

[0053] In one preferred embodiment according to the present invention, the step of treating the first portion of said exhaust gas also comprises a step of flowing the exhaust gas through a dust removal system, preferably as first flue gas treatment, i.e. before, when present NOx capture, SOx capture, VOC (volatile organic carbon) capture, TOC (total organic carbon) or total hydrocarbon (THC) capture, and the like.

[0054] In a further preferred embodiment according to the present invention, the step of treating the first portion of exhaust gas comprises a step of flowing the exhaust gas through a dust removal system thereby forming a dedusted first portion of exhaust gas, flowing of the dedusted first portion of exhaust gas to an SCR system, in order to reduce the amount of NOx in the exhaust gas.

[0055] In a variant according to the present invention, the process of calcining carbonate mineral rock further comprises a heat exchange step between the first portion of the exhaust gas discharged from the kiln and the clean exhaust gas, in particular through a rotary gas-to-gas heater, preferably after dedusting the exhaust gas and before NOx capture.

[0056] Other embodiments of the process according to the present invention are mentioned in the appended claims.

[0057] The present invention also relates to a carbonate mineral rock kiln for calcining carbonate mineral rock comprising

[0058] a combustion zone provided with fuel lances or burners in which carbonate mineral rocks are loaded and fired for the decarbonation thereof into calcined material with a release of hot combustion fumes ,

[0059] a gas discharging port for discharging exhaust gas containing at least the combustion fumes, dust and pollutants at a temperature comprised between 80 to 220°C, in particular between 80°C to 200°C, more particularly between 90°C and 180°C, more preferably between 100°C and 160°C a cooling zonedownstream of the combustion zone following a flow of calcined material, comprising :

[0060] o a cooling gas first inlet for cooling the calcined material via cooling gas, said cooling gas being introduced cold at a first inlet through the calcined material and being heated upon contacting said calcined material in a cooling zone by heat exchange to form a heated cooling gas, o a rock discharging port for unloading the calcined material from the cooling zone of the kiln,

[0061] a hot gas outlet chosen amongst a heated cooling gas outlet or a hot combustion fumes outlet or their combination,

[0062] a flue gas treatment unit having an inlet in fluid communication with said gas discharging port for treating a first portion of said exhaust gas discharged from the kiln to remove at least partly pollutants and forming a clean exhaust gas, said flue gas treatment unit having a clean exhaust gas outlet.

[0063] The kiln according to the present invention is characterized in that it further comprises a heat exchanger with a first pathway and a second pathway, said first pathway being in fluid communication with the gas discharging port and said second pathway being in fluid communication with either said heated cooling gas outlet orwith said combustion fumes outlet in fluid communication with the combustion zone, said first pathway being provided for heating said first portion of said exhaust gas by collecting heat from a gas stream flowing in said second pathway of the heat exchanger.

[0064] In a preferred embodiment, the carbonate mineral rock kiln is a regenerative parallel-flow (vertical) shaft kiln (PFRK) comprising at least two shafts interconnected via a connecting channel, wherein each shaft operates alternately in firing mode and in preheating mode, one shaft being in firing mode for a predetermined time period while at least one other shaft is in preheating mode, and vice-versa, the carbonate mineral rock kiln further comprising an inversion system provided to operate during a predefined inversion period during which one shaft in preheating mode is shifted to the firing mode and the shaft in firing mode is shifted to the preheating mode, wherein each shaft in firing mode comprises sequentially from top to bottom, a preheating zone (A) for preheating said carbonate mineral rocks, preferably fed by combustion gas, said combustion zone (B) and said cooling zone (C), .

[0065] 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 fromthe shaft in calcining mode to the shaft(s) in preheating mode. This connecting channel comprises one or several crossover channels and possibly peripheral channels.

[0066] 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.

[0067] 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.

[0068] In the carbonate mineral rock according to the present invention, the flue gas treatment unit comprises a SCR system.

[0069] In an advantageous embodiment of the kiln according to the present invention, the flue gas treatment unit comprises a SOx reduction system and / or a VOC and / or TOC orTHC reduction system.

[0070] In another preferred embodiment, the kiln according to the present invention comprises a CO2 capture unit, downstream of said flue gas treatment unit.

[0071] In a preferred embodiment, the carbonate mineral rock kiln further comprises a recirculation circuit in fluid communication with the gas discharging port provided to derive a second portion of the exhaust gas from the kiln, preferably located upstream of the flue gas treatment unit and with an injection system provided either in the kiln, or in the kiln and communicating with the combustion zone.

[0072] Preferably, the injection system comprises an oxidizing inlet to inject the exhaust gas recirculated in the recirculation circuit and an oxidizing gas, and optionally a mixing device such as a static mixing device (for example a mixing chamber), a countercurrent injection mixing device and the like, upstream the oxidizing inlet.

[0073] When a mixing chamber is present, the exhaust gas and the oxidizing gas are injected together as a mixture.

[0074] Preferably, according to the present invention, the injection system is provided to inject the exhaust gas recirculated in the recirculation circuit and an oxidizing gas in the combustion zone.

[0075] More specifically, when the kiln according to the present invention is a parallel flow regenerative kiln, the injection system is provided to inject the exhaust gas recirculated in the recirculation circuit and an oxidizing gas in the combustion zone of the shaft in firing mode.More preferably, when the kiln according to the present invention is a parallel flow regenerative kiln comprising loading activation means, synchronized with the inversion system, provided to load the carbonate mineral stones during the inversion period at the top of any of the shaft, preferably the shaft that will work in a preheating mode after said inversion period.

[0076] Alternatively, or in addition, the loading of carbonate mineral stones occurs during the cycle at the top of the shaft that works in a preheating mode.

[0077] In a preferred embodiment, the carbonate mineral rock kiln further comprises a dust treatment unit upstream the flue gas treatment unit or downstream the flue gas treatment unit.

[0078] In a further preferred embodiment, the carbonate mineral rock kiln further comprises a rotary gas-to-gas heater upstream the flue gas treatment unit having a cold gas entry in fluid communication with the exhaust gas discharged and a hot gas entry, in fluid communication with the clean exhaust gas outlet from the SCR, bringing the advantage of its compactness and efficiency to replace two traditional heat exchangers (one to reheat the flue gas and a second to cool it back down.

[0079] Preferably, the heated cooling gas is heated cooling air.

[0080] Other embodiments of the kiln according to the present invention are mentioned in the appended claims

[0081] Other characteristics and advantages of the present invention will be derived from the non-limitative following description, and by making reference to the drawings.

[0082] In the drawings, figure 1 represents one embodiment of the carbonate mineral rock kiln according to the present invention.

[0083] Figure 2 represents a variation of the embodiment of figure 1.

[0084] Figure 3 represents a variation of the embodiment of figure 1.

[0085] Figure 4 represents another embodiment of the carbonate mineral rock kiln according to the present invention.

[0086] Figure 5 represents a variation of the embodiment of figure 4.

[0087] Figure 6 represents another embodiment of the carbonate mineral rock kiln according to the present invention, where the gas passing through the second pathway of the heat exchanger is used to transfer heat in a second pathway of a second heat exchanger, downstream the flue gas treatment unit.

[0088] Figure 7 represents a variation of the embodiment of figure 6.Figure 8 represents another embodiment of the carbonate mineral rock kiln according to the present invention where heat recovery is improved.

[0089] Figure 9 represents a variation of the embodiment of figure 8.

[0090] Figure 10 represents another embodiment of the carbonate mineral rock kiln according to the present invention, further comprising a rotary gas to gas heater.

[0091] Figure 11 represents another embodiment of the carbonate mineral rock kiln according to the present invention where the kiln is a parallel flow regenerative kiln.

[0092] Figure 12 represents yet another embodiment of the carbonate mineral rock kiln according to the present invention where the kiln is a parallel flow regenerative kiln

[0093] Figure 13 represents another embodiment of the carbonate mineral rock kiln according to the present invention where the kiln is a parallel flow regenerative kiln operating in oxyfuel conditions.

[0094] In the drawings, the same reference numbers have been allocated to the same or analog element.

[0095] As it can be seen on figure 1, the present invention relates to a process for calcining carbonate mineral rock in a carbonate mineral rock kiln 1. The kiln 1 according to the present invention comprises a combustion zone B provided with fuel lances or at least one burner 4 in which carbonate mineral rocks are loaded from the carbonate mineral rock inlet 5. The loaded carbonate mineral rocks are fired for the decarbonation thereof into calcined material 10 with a release of hot combustion fumes 11.

[0096] The kiln 1 comprises a gas discharging port 14 for discharging exhaust gas containing at least the combustion fumes, dust and pollutants at a temperature comprised between 80 to 220°C, in particular between 80°C to 200°C, more particularly between 90°C and 180°C, more preferably between 100°C and 160°C .

[0097] The kiln 1 also comprises a cooling zone C downstream the combustion zone, following the flow of calcined material (from the combustion zone to the exit of the kiln), and which is in proximity of, preferably below, the combustion zone B, which comprises a cooling gas first inlet 7 for cooling the calcined material 10 via cooling gas, said cooling gas being introduced cold at a first inlet 7 through the calcined material and is heated upon contacting said calcined material in the cooling zone C by heat exchange to form a heated cooling gas 12. The cooling zone C further comprises a rock discharging port 8 for unloading the calcined material from the cooling zone C of the kiln, optionally via a discharging device (not shown on figure 1).The kiln further comprises a hot gas outlet chosen amongst a heated cooling gas outlet 26 or a combustion fumes outlet 37 or a combination thereof.

[0098] In the kiln according to the present invention, the kiln further comprises a flue gas treatment unit 25 having an inlet in fluid communication with said gas discharging port 14 for treating a first portion of said exhaust gas discharged from the kiln to remove at least partly pollutants and forming a clean exhaust gas , said flue gas treatment unit having a clean exhaust gas outlet for exiting the clean exhaust gas 38. The first portion of the exhaust gas can be a portion from 10 to 100 % of the total exhaust gas on a volume basis.

[0099] In the illustrated embodiment on figure 1, a filter 22 is present downstream the flue gas treatment unit 25. The dedusting through the filter as a dust removal system 22 and the flue gas treatment unit 25 form together, within the meaning of the present patent application the step of treating the first portion of said exhaust gas.

[0100] The clean exhaust gas can be sent to a CO2 capture unit 21, i.e. a unit able to recover CO2 from the exhaust gas for further valorization, such as a cryogenic unit or a concentration unit, notably working with amines or any other suitable device. Alternatively, the clean exhaust gas can be sent to a chimney, for example if the CO2 is not recovered.

[0101] The kiln 1 according to the present invention further comprises a heat exchanger 23 with a first pathway 29 and a second pathway 30. The first pathway 29 is in fluid communication with the gas discharging port 14 and the second pathway 30 is in fluid communication with either said heated cooling gas outlet 26 or with the combustion fumes outlet 37 in fluid communication with the combustion zone B. The first pathway is provided for heating said first portion of said exhaust gas by collecting heat from a gas stream flowing in said second pathway 30 of the heat exchanger 23, i.e. hot combustion fumes or heated cooling gas extracted at a temperature higher than the temperature of the discharged exhaust gas. The process and the kiln according to the present invention accordingly propose to use heat from hot gas extracted from the kiln process at a point hotter than the kiln exhaust gas exit to reheat the flue gas, by the mean of a heat exchanger between this extracted hot gas and the exhaust gas exiting the kiln installation; thus reducing or suppressing the need for additional burners or electrical heaters for such reheating of the flue gas.

[0102] The flue gas treatment unit can be a NOx reduction step in a SCR system, possibly with a SOx reduction step, a VOC and / or TOC or THC reduction step. In theillustrated embodiment, the kiln 1 according to the present invention also comprises a preheating zone A for preheating the carbonate mineral rocks before the calcination.

[0103] While not illustrated in figure 1, a second portion of the exhaust gas discharged from the kiln by the exhaust gas discharging port 14 can be derived through a derivation system, preferably located upstream of the flue gas treatment unit. In this case, the second portion of said exhaust gas can be collected and mixed with oxygen to form an oxidizing mixture and re-introduced in the kiln for providing an oxidizing atmosphere in the combustion zone.

[0104] The oxidizing mixture can be performed by rejoining two feeding duct or through a mixing device such as a static mixing device (such as a mixing chamber), a countercurrent injection mixing device and the like. If a mixing device, for example a mixing chamber is present, the oxidizing mixture is performed with said second portion of the discharged exhaust gas and concentrated dioxygen and is fed to the kiln, preferably to the combustion zone.

[0105] Also, additional dust removal system or filter can be located, such as before the chimney 15.

[0106] Figure 2 represents another embodiment of figure 1, where a gas cooler 43 is foreseen before the filter 22. The dedusting through the filter such as a dust removal system 22, the cooler 43 and the flue gas treatment unit 25 form together, within the meaning of the present patent application the step of treating the first portion of said exhaust gas. Also in this case, additional dust removal system or filter can be located, such as before the chimney 15.

[0107] Figure 3 represent another embodiment of figure 1, where the dust removal system or filter 22 is located upstream the flue gas treatment unit. It goes without being said that the filter or dust removal system 22 can be located in several locations without departing from the scope of the present invention. Further the position of the filter or dust removing system can vary depending on the type of kiln and temperature of the exhaust gas discharged from the kiln because some filters or dust removal system are not necessarily compatible with all exhaust gas temperatures.

[0108] Also, the gas cooler has been illustrated in figure 2 but can also be present on the embodiment of the other figures.

[0109] Accordingly, within the meaning of the present invention, the step of treating the first portion of said exhaust gas contains at least a passage through a gas treatment unit and a filter, such as a dust removal system, but can contain additionalpassage through several devices, such as a gas cooler, a condenser, other filters of dust removal system, and the like.

[0110] As previously explained, Figure 4 represents another embodiment of the carbonate mineral rock kiln according to the present invention. The embodiment of figure 4 is the same as the one on figure 3, but further comprising a heater 31, downstream the heat exchanger, to provide additional heat to the discharged exhaust gas exiting the first pathway of the heat exchanger 23, for example, when the temperature at the exit of the heat exchanger is not enough or not stable enough for the flue gas treatment unit operation. The heater 31 can be one or more burners, one or more oxyfuels burners, one or more electrical heaters, one or more plasma torches. The kiln according to figure 4 therefore combines a partial reheating of the discharged exhaust gas through the aforementioned heat exchanger 23 with a heater 31.

[0111] As explained previously, the filter or dust removal system 22 has been illustrated upstream the gas treatment unit 25, but in other embodiments, the filter or dust removal system 22 can also be located downstream the gas treatment unit 25, optionally with a gas cooler or a condenser (see Figure 5).

[0112] Figure 6 represents another embodiment of the carbonate mineral rock kiln according to the present invention. The embodiment of figure 6 is the same as the one on figure 3, but the hot gas 11,12 from outlet 37 or 26 passing through the second pathway 30 of the heat exchanger 23 is further used to transfer heat in a second pathway 30 of a second heat exchanger 32, downstream the flue gas treatment unit. This can be done for cooling down the clean exhaust gas and recover for example a part of its heat.

[0113] If the second heat exchanger is used to cool down the clean exhaust gas, a hot gas stream 33 is collected at the exit of the second pathway of the second heat exchanger, which can be further used in any application requiring hot gas. Especially when the hot gas is heated cooling air, this hot gas stream 33 can be used for drying , such as for drying sludges, limestone, biomass,

[0114] Figure 7 represents a variation of Figure 6, where the dust removal system or filter 22 is located downstream the second heat exchanger 32, possibly after a gas cooler (not illustrated).

[0115] Figure 8 represents another embodiment of the carbonate mineral rock kiln according to the present invention where heat recovery is improved. The embodiment of figure 8 is the same as the one on figure 3, but the hot gas 11,12 fromoutlet 37 or 26 passing through the second pathway 30 of the heat exchanger 23 is filtered through a filter 28, and used to feed a boiler or economizer 34 before reaching the chimney, especially when it is heated cooling air. The steam generated by the boiler 34 can be used to feed a waste heat power generation system 44. Further, the clean exhaust gas 38 can also be used to feed a second gas exchanger 32 or a boiler before reaching for example a CO2 capture unit. The boiler can then generate steam and feed a waste heat power generation 44.

[0116] In a variant according to the present invention, the generated steam can be used in an amine capture process CO2 recovery and valorization or other CO2 capture process that would require steam for regeneration.

[0117] Figure 10 represents a carbonate mineral rock kiln according to the present invention. In the illustrated embodiment, the flue gas treatment unit 25 comprises a SCR in which the reheated exhaust gas from the first pathway 29 is fed via duct 39, in which an ammonia or urea solution is sprayed through the lance (or lances) 35. The clean exhaust gas 38 from the SCR unit is fed to a rotary gas to gas heater (RGGH) 36 to collect the heat of the clean exhaust gas before being sent to a chimney 15 or to a CO2 capture unit 21. The exhaust gas discharged from the kiln via the discharging port 14 is introduced into the RGGH 36 to be preheated by the clean exhaust gas 38 before reheating through the first pathway of the heat exchanger 23.

[0118] The flue gas treatment unit 25 may also comprises filters and other pollutants removal unit.

[0119] Figure 11 represents another embodiment of the carbonate mineral rock kiln according to the present invention, based on the embodiment of figure 3, where the kiln is a parallel flow regenerative kiln.

[0120] Conventionally, the shaft shown on the left is in calcining mode and the shaft shown on the right is in preheating mode. Standard parts, such as loading or unloading equipment, are not shown or they are shown very schematically, in order to not overload the drawings.

[0121] As can be seen in figure 11 , the parallel-flow regenerative kiln 1 is a vertical double-shaft kiln 2a, 2b where the fuel is injected alternately in one shaft 2a then in another 2b 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 zones, the preheating zone A where the carbonate mineral rocks is preheated before calcination,the combustion zone B where the calcination of the carbonate mineral rocks occurs and the cooling zone C where the cooling of the calcined material occurs.

[0122] When a shaft is in calcination mode, here the shaft 2a, 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 mineral rocks, loaded at the top of the shaft via an inlet 5, progressively descend 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 combustion fumes 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 gas is introduced via a supply duct 7 at the bottom of the shaft, counter-currently to the calcined material 10, to cool it. The heated cooling gas 12 introduced in the calcination shaft mixes with the combustion fumes 11 in order to move into the crossover channel 3 and forms exhaust gas. The calcined material is unloaded via the outlet 8 into a piece of unloading equipment 24.

[0123] In the present patent application, for simplicity of description and understanding, while due to the cyclic operation of PER kiln, all the connections are presents on both shafts 2a, 2b, they have been described and illustrated only on the shaft having the related role in the description. For example the connection related to combustion have been described on the burning shaft.

[0124] The same applies for cooling air which may be extract from the burning shaft, from the preheating shaft or both.

[0125] For the same reasons, gas control devices such as fans, blowers, control valves, isolation valves are omitted on the figures.

[0126] When a shaft is in preheating mode, here the shaft 2b, the fuel supply device is closed and the lances 4 do not receive any fuel, but still can be supplied with cooling gas. The carbonate stones can be supplied by the inlet 5 of the shaft in preheating mode. The opening 6 for supplying combustion air is closed. However, the supply duct 7 for the cooling gas 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 gas mixes with the combustion fumes 11 which, from the crossover channel 3, enters the shaft via the peripheral channel 13. The exhaust gas containing the combustion fumes 11 progresses until reaching the top of the shaft where it is discharged from the kiln via a discharge duct 14 and transferred to a CO2 capture unit 21, after being filtered in a dedusting device such as a filter 22 and a flue gas treatmentunit 25. Before entering the flue gas treatment unit 25, the dedusted exhaust gas from the filter 22 enters a first pathway 29 of a heat exchanger to be reheated to the predefined temperature for the optimal flue gas treatment. The second pathway 30 of the heat exchanger is fed by combustion gas extracted from the connecting channel 13, such as for example from the peripheral channel or from the cross over channel 3. The gas from the second pathway of the heat exchanger 23 is further filtered in a filter 28 before reaching a chimney or another gas collector for further gas treatment.

[0127] In the shaft in calcination mode 2a, this discharge duct 14 is closed.

[0128] The kiln also comprises a reversing system 16, shown schematically. It controls, in a synchronized manner, the operation of the shafts during the reversing time of the shafts, either directly or remotely. It controls the on and off switching of all elements of the kiln in such a way that, in production mode, each shaft operates alternately in calcination mode and in preheating mode.

[0129] In some cases, there are three shafts, two in preheating mode and one in combustion.

[0130] Figure 12 represents the same embodiment as the one in figure 11 , except that the extracted hot gas to feed the second pathway of the heat exchanger 23 is heated cooling air 12 extracted from a heated cooling gas output 26, located below the connecting channel 13. The heated cooling gas output is preferably located in a peripheral collector ring in fluid communication with the cooling zone of the kiln.

[0131] In a variation, the heated cooling air 12 can be extracted through the central collector 27. In both cases, extracting the heated cooling air is further an advantage to reduce dilution of CO2 in the exhaust gas by the cooling air. The heated cooling air can be partially extracted for being used in the heat exchanger 23 or entirely extracted, one part only being used in the heat exchanger 23, the remaining part being extracted to concentrate in CO2 the exhaust gas.

[0132] As a variation, the heated cooling air can be extracted from one or several shafts, from both the peripheral channel and the central extractor, with same ratio or different ratio, at the same time of not. For example, it is contemplated the possibility to extract the heated cooling air from one shaft during a certain phase through the peripheral channel while it can be extracted from another shaft at the same time or not from the central extractor.

[0133] Figure 1 represents another embodiment of the carbonate mineral rock kiln according to the present invention where the kiln is a parallel flow regenerative kiln operating in oxyfuel conditions. The operation of the kiln according to the presentinvention is the same as explained for figure 6 and figure 7 except that a derivation system 17 , capable of collecting a portion of exhaust gas discharged from the furnace and introducing it into the recirculation circuit 18 has been provided on the exterior, on the discharge duct 14. The derivation system 17 divides the exhaust gas into a first portion and a second portion. The second portion of the exhaust gas is transferred in the recirculation circuit 18 where it is advantageously treated in a treatment unit 1 , where it may, for example, be filtered, cooled and / or dried. An air separation unit 20 separates air supplied by the duct 40 into N2 discharged via the duct 41 and O2 supplied to the recirculation circuit 18 via the supply duct 42 . 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 .

[0134] In a variation, the derivation system 17 is located downstream the filter 22. In this case, the second portion of the exhaust gas transferred to the recirculation circuit is treated in the treatment unit for cooling and / or drying but does not need to be dedusted.

[0135] The operation of the furnace in figure 8 is similar to a PFRK furnace. The separating member 17 is continuously in service, the same as the treatment unit 19 and the air separation unit 20 . As has already been seen, the reversing system 16 closes the discharge duct 14 at the top of the shaft in firing mode. However, at the top of this shaft, it opens the supply opening 6 to allow the oxidizing mixture to be introduced, while it is closed at the top of the shaft in preheating mode.

[0136] 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. For example, the present invention has been disclosed with reheating the exhaust gas with hot gas from the same kiln. It is also contemplated within the meaning of the present invention that the hot gas can be extracted from a kiln to reheat the flue gas of another kiln (for example, using hot gas extracted from a rotary kiln, or flue gas from a rotary kiln to reheat flue gas from a PFRK kiln). Further, different embodiments have been specifically disclosed and it is of course, within the scope of the present invention also foreseen to combine different aspects, such as the embodiment of figure 4 or 6 with the one of figure 11, 12 or 13.

Claims

1. CLAIMS1. Process for calcining carbonate mineral rock in a carbonate mineral rock kiln ( 1 ) comprising:loading the carbonate mineral rock in the kiln ( 1 ),firing said carbonate mineral rock, in a combustion zone (B), with the decarbonation thereof into calcined material (10) with a release of hot combustion fumes (11),discharging exhaust gas containing at least the combustion fumes, dust and pollutants at a temperature comprised between 80 to 220°C, in particular between 80°C to 200°C, more particularly between 90°C and 180°C, more preferably between 100°C and 160°C,cooling the calcined material (10) via cooling gas, (7) said cooling gas (7) being introduced cold at a first inlet through the calcined material (10) and being heated upon contacting said calcined material (10) in a cooling zone (C) by heat exchange to form a heated cooling gas,unloading the calcined material (10) from the cooling zone (C) of the kiln, treating a first portion of said exhaust gas discharged from the kiln in a flue gas treatment unit (25) to remove at least partly pollutants and forming a clean exhaust gas (38),characterized in that before being treated in a flue gas treatment unit (25), said first portion of said exhaust gas discharged from the kiln is further heated in a heat exchanger (23) by flowing into a first pathway (29) in the heat exchanger (23) by collecting heat from a gas stream flowing in a second pathway (30) of the heat exchanger (23), said gas stream being either heated cooling gas (26) or hot combustion fumes (37) or both, said heated cooling gas being extracted outside of the kiln (1 ) after having contacted at least partially said calcined materiel (10) in said cooling zone (C) through a heated cooling gas exit (26) in fluid communication with said heat exchanger (23) or said hot combustion fumes being extracted outside the kiln through a hot combustion gas outlet (37) in fluid communication with said heat exchanger (23), said hot combustion fumes or said heated cooling gas being extracted at a temperature higher than the temperature of the discharged exhaust gas.

2. Process for calcining carbonate mineral rock in a carbonate mineral rock kiln (1) according to claim 1, wherein the kiln (1) is a regenerative parallel-flow vertical shaft kiln (PFRK), wherein at least two shafts (2a, 2b) are interconnected via a connecting channel ( 13), wherein each shaft operates alternately in firing mode and in preheating mode, one shaft being in firing mode for a predetermined time period while at least one other shaft is in preheating mode, and vice-versa, the process comprising an inverting step during a predefined inversion period during which a shaft in preheating mode is shifted to the firing mode and the shaft in firing mode is shifted to the preheating mode;the firing mode comprising:o combusting fuel in the presence of oxygen so as to obtain said firing of said carbonate mineral rock and the decarbonation thereof into calcined material (10) with the release of combustion fumes (11) in the form of combustion fumes descending co-currently in the shaft in firing mode (2a), ando said combustion fumes moving from the shaft in firing mode (2a) to said at least one shaft in preheating mode (2b) using said connecting channel (13) and forming at least partially said exhaust gas,the preheating mode comprising:o preheating of the loaded carbonate mineral rock via heat exchange with the exhaust gas from the connecting channel, which is ascending in said at least one shaft in preheating mode (2b), counter-currently to said loaded carbonate mineral rock, ando said step of discharging exhaust gas3. Process for calcining carbonate mineral rock in a carbonate mineral rock kiln (1) according to claim 1 or claim 2, wherein the step of treating the exhaust gas discharged from the kiln comprises a NOx reduction step in a SCR system.

4. Process for calcining carbonate mineral rock in a carbonate mineral rock kiln (1) according to any of the claims 1 to 3, wherein the step of treating the exhaust gas discharged from the kiln comprises a SOx reduction step.

5. Process for calcining carbonate mineral rock in a carbonate mineral rock kiln ( 1 ) according any of the claims 1 to 4, wherein the step of treating the exhaust gas discharged from the kiln comprises a VOC and / or TOC or THC reduction step.

6. Process for calcining carbonate mineral rock in a carbonate mineral rock kiln (1) according to any of the claims 1 to 5, comprising a step of CO2 capture from said clean exhaust gas.

7. Process for calcining carbonate mineral rock in a carbonate mineral rock kiln (1) according to any of the claims 1 to 6, wherein a second portion of the exhaust gas discharged from the kiln is derived through a derivation system (17), preferably located upstream of the flue gas treatment unit (25), said second portion of said exhaust gas being collected and mixed with oxygen to form an oxidizing mixture and re-introduced in the kiln ( 1 ) for providing an oxidizing atmosphere in the combustion zone (B).

8. Process for calcining carbonate mineral rocks in a carbonate mineral rock kiln (1) according to claim 7, wherein oxidizing mixture is performed with said second portion of the discharged exhaust gas and concentrated dioxygen in a mixing device such as a static mixing device, a countercurrent injection mixing device and the like, said oxidizing mixture being fed to the kiln ( 1 ), preferably in the combustion zone (B).

9. Process for calcining carbonate mineral rock in a carbonate mineral rock kiln ( 1 ) according to any of the previous claim, wherein the heated cooling gas is extracted outside of the kiln and wherein a portion of the heated cooling gas is collected through a duct connected to the heated cooling gas exit (26) in fluid communication with the second pathway (30) of the heat exchanger (23).

10. Process for calcining carbonate mineral rock in a carbonate mineral rock kiln ( 1 ) according to any of the previous claim, wherein the hot combustion fumes is extracted outside of the kiln and wherein a portion of the hot combustion fumes is collected through a duct connected to the hot combustion fumes exit (37) in fluid communication with the second pathway (30) of the heat exchanger (23).

11. Process for calcining carbonate mineral rocks in a carbonate mineral rock kiln (1) according to any of the claims 7, 8, 9 or 10, when depending on claim 2, wherein the oxidizing mixture is introduced at the top of the shaft in firing mode (2a) or at the top of the combustion zone (B) of the shaft in firing mode (2a) so as to ensure said fuel combustion in the presence of oxygen.

12. Process for calcining carbonate mineral rocks in a carbonate mineral rock kiln (1) according to claim 11, wherein the loading of carbonate mineral stones occurs during the cycle at the top of the shaft in preheating mode or during the inversion period at the top of any of the shafts, preferably of the shaft that will work in a preheating mode after said inversion period.

13. Process for calcining carbonate mineral rocks in a carbonate mineral rock kiln according to any of the claims 1 to 12, wherein step of treating the firstportion of said exhaust gas also comprises a step of flowing the exhaust gas through a dust removal system (22).

14. Process for calcining carbonate rocks in a carbonate mineral rock kiln (1) according to any of the previous claims, wherein the step of treating the first portion of exhaust gas comprises a step of flowing the exhaust gas through a dust removal system (22) thereby forming a dedusted first portion of exhaust gas, flowing of the dedusted first portion of exhaust gas to an SCR- system, in order to reduce the amount of NOx in the exhaust gas.

15. Process for calcining carbonate mineral rocks in a carbonate mineral rock kiln according to any of the previous claims, further comprising a heat exchange step between the first portion of the exhaust gas discharged from the kiln and the clean exhaust gas, in particular through a rotary gas-to-gas heater (36).

16. Mineral carbonate rock kiln (1) for calcining carbonate mineral rock comprisinga combustion zone (B) provided with fuel lances (4) or burners in which carbonate mineral rocks are loaded and fired for the decarbonation thereof into calcined material ( 10) with a release of hot combustion fumes (11),a gas discharging port (14) for discharging exhaust gas containing at least the combustion fumes (11), dust and pollutants at a temperature comprised between 80 to 220°C, in particular between 80°C to 200°C, more particularly between 90°C and 180°C, more preferably between 100°C and 160°C , a cooling zone (C) downstream of the combustion zone (B) following a flow of calcined material, comprising :o a cooling gas first inlet (7) for cooling the calcined material (10) via cooling gas (12), said cooling gas (12) being introduced cold at a first inlet (7) through the calcined material and being heated upon contacting said calcined material (10) in a cooling zone (C) by heat exchange to form a heated cooling gas,o a rock discharging port (24) for unloading the calcined material from the cooling zone of the kiln,a hot gas outlet chosen amongst a heated cooling gas outlet (26) and a hot combustion fumes outlet (37) or their combination,a flue gas treatment unit (25) having an inlet in fluid communication with said gas discharging port (14) for treating a first portion of said exhaust gas dischargedfrom the kiln (1) to remove at least partly pollutants and forming a clean exhaust gas, said flue gas treatment unit having a clean exhaust gas outlet (38), characterized in that it further comprises a heat exchanger (23) with a first pathway (29) and a second pathway (30), said first pathway (29) being in fluid communication with the gas discharging port (14) and said second pathway (30) being in fluid communication with either said heated cooling gas outlet (26) or with said hot combustion fumes outlet (37) in fluid communication with the combustion zone (B), said first pathway (29) being provided for heating said first portion of said exhaust gas by collecting heat from a gas stream flowing in said second pathway (30) of the heat exchanger,17. Carbonate mineral rock kiln according to claim 16, wherein the kiln is a regenerative parallel-flow vertical shaft kiln ( PFRK) comprising at least two shafts (2a, 2b) interconnected via a connecting channel (13), wherein each shaft operating alternately in firing mode and in preheating mode, one shaft being in firing mode for a predetermined time period while at least one other shaft is in preheating mode, and vice-versa, the carbonate mineral rock kiln further comprising an inversion system (16) provided to operate during a predefined inversion period during which a shaft in preheating mode(2b) is shifted to the firing mode and the shaft in firing mode (2a) is shifted to the preheating mode and wherein each shaft in firing mode comprises sequentially from top to bottom, a preheating zone (A) for preheating said carbonate mineral rocks, said combustion zone (B) and said cooling zone (C).

18. Carbonate mineral rock kiln according to claim 16 or claim 17, wherein the flue gas treatment unit (25) comprises a SCR system.

19. Carbonate mineral rock kiln according to any of the claims 16 to 18, wherein the flue gas treatment unit (25) comprises a SOx reduction system.

20. Carbonate mineral rock kiln according any of the claims 16 to 19, wherein the flue gas treatment unit comprises a VOC orTotal Organic Carbon (TOC) or Total Hydrocarbon (THC) reduction system.

21. Carbonate mineral rock kiln according to any of the claims 16 to 20, comprising a CO2 capture unit (21 ), downstream of said flue gas treatment unit (25).

22. Carbonate mineral rock kiln according to any of the claims 16 to 21, further comprising a recirculation circuit (18) in fluid communication with the gas discharging port (14) provided to derive a second portion of the exhaust gas from the kiln, preferably located upstream of the flue gas treatment unit (25) and with aninjection system provided either in the kiln, or in the kiln and communicating with the combustion zone.

23. Carbonate mineral rock kiln (1) according to claim 22, wherein the injection system comprises an oxidizing inlet (42) to inject the exhaust gas recirculated in the recirculation circuit and an oxidizing gas, and optionally a mixing device (20), such as a static mixing device, a countercurrent injection mixing device and the like, upstream the oxidizing inlet.

24. Carbonate mineral rock kiln (1) according to claims 22 and 23, wherein the injection system is provided to inject the exhaust gas recirculated in the recirculation circuit (18) and an oxidizing gas in the combustion zone (B).

25. Carbonate mineral rock kiln (1) according to claim 23 or 24, when depending on claim 17, comprising loading activation means, synchronized with the inversion system ( 16), provided to load the carbonate mineral stones during the inversion period at the top of the shaft that will work in a preheating mode after said inversion period.

26. Carbonate mineral rock kiln according to any of the claims 16 to 25, comprising a dust treatment unit (22) upstream the flue gas treatment unit (25).

27. Carbonate mineral rock kiln according to any of the claims 16 to 26, comprising a rotary gas-to-gas heater (36) upstream the flue gas treatment unit (25) having a cold gas entry in fluid communication with the exhaust gas discharged and a hot gas entry, in fluid communication with the clean exhaust gas outlet.

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