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

Abstract

Kiln 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 where before being treated, said first portion of said exhaust gas is preheated in a RGGH.

Description

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

[0002] The present invention relates to a method for calcining carbonate mineral rocks 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 rocks, limestone stones”, 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.

[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 the exception of the part of the periphery already occupied by the crossover channel.

[0007] A Parallel Flow Regenerative Kiln usually has 2 to 3 shafts, of circular or rectangular section, which do not work in a continuous way. In standard operation, in every period, usually of 12 to 20 minutes, fuel is injected inside a calcining zone of one shaft by means of lances and is burned in presence of combustion air.Thereafter the descending calcined product is cooled in a cooling zone by heat exchange with a cooling gas introduced at the bottom of the shaft. The flue gas comprises or consists of the combustion fumes, the gas of decarbonation and the heated cooling gas. This flue gas is drawn into another shaft (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 outwards 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 (calcining shaft) and the shaft(s) wherein the flue gas is drawn through the stones works according to a preheating mode (preheating shaft). Thereafter, there is a period, usually between 30 seconds and 2 minutes, called reversion 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.

[0008] The classical method for calcining carbonate mineral rocks in a parallel flow regenerative kiln having at least two shafts interconnected by a connecting channel, comprises, in standard operation,

[0009] - loading carbonate mineral rocks at the top of each shaft, - preheating these loaded stones in a preheating zone,

[0010] - calcining these preheated stones in a calcination zone with production of a decarbonated calcined material,

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

[0012] - discharging the calcined material from the bottom of the shafts, - exhausting an exhaust effluent from the kiln,

[0013] - 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,

[0014] - the calcining mode comprising:

[0015] said calcining step by means of an increase of temperature inside said carbonate mineral rocks having been preheated, with production of saiddecarbonated calcined material and release of a gas stream which flows in cocurrent with the calcined material, and

[0016] through said connecting channel, a passage of said gas stream toward the at least one shaft working in a preheating mode,

[0017] - said preheating mode comprising:

[0018] said preheating step of the loaded carbonate mineral rocks 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 rocks, and

[0019] said exhausting step of said gas stream as exhaust effluent at the top of said at least one shaft in preheating mode,

[0020] said cooling step comprising a supply of cooling gas at the bottom of each of said shafts or only of the shaft working in the calcining mode.

[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.Jn 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 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, limiting the energy losses.

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

[0023] According to the invention, carbonate mineral rocks 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] CaCOs (solid) + heat -> CaO (solid)+ CO2 (gas)

[0026] 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 lessfast 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.

[0027] Today this increase is mainly obtained by combustion of a fuel, frequently fossil, in presence of an oxidizer such as air. In turn this fuel combustion contributes also to an important release of CO2. Globally the current calcination methods actively participate in increasing the greenhouse effect.

[0028] 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, TOC (total organic carbon), THC (total hydrocarbon), SO2, NOx, called pollutants in the meaning of the present patent application for common carbon capture. Those pollutants could prevent an efficient carbon capture step on the exhaust gas.

[0029] The present invention relates therefore to the reduction of the content of pollutant in the exhaust gas.

[0030] Efficient pollutants capture often involves a temperature challenge. In some kilns, the exhaust gas exits the kiln at a temperature comprised between 80°C and 220°C, depending on the type of kilns. It is also to be noted that the temperature of the flue gas exiting a PFRK is not constant but varies during the cycle. Indeed, during the burning of fuel, the flue gas exiting the kiln can reach high temperatures, such as 240°C and even sometimes up to 250°C at some point. However, at other periods, the flue gas temperature of the kiln will be much lower, especially during or immediately after the reversion period.

[0031] Some flue gas treatments require the flue gas to be reheated to a minimum temperature. For example, SCR (Selective catalytic reduction) requires a temperature between 200 and 400 °C, with an optimum around 350°C, catalytic oxidation of organics requires temperatures over 350°C. Even filtration through a bag filter requires to be sufficiently higher than the dew point in the gas.Stricter regulation and limitations of carbon capture technologies will make it necessary to reach very low levels of NOx in the exhaust gas of lime kilns. Current technologies to reduce NOx are SNCR and SCR.

[0032] SNCR (Selective non-catalytic reduction) consists of injecting a reagent (usually ammonia or urea) which reacts with NOx at high temperature (between 900 and 1050 °C). This technology is applicable on rotary kilns but tests on shaft kilns indicated low or no NOx reduction notably due to too short residence time of the reagent inside the kiln.

[0033] SCR is the same reaction over a catalyst bed that reduces the temperature needed to 200-450°C, with a maximum efficiency around 350°C. As the exit temperature of PFRKs is generally too low (can be <130°C), the gas needs to be reheated.

[0034] When flue gas treatment is applied on exhaust gas, Flue gas has to be reheated and the reheating can be done by a set of burners or electrical heaters (possibly combined with heat exchangers) to a temperature compatible with the flue gas treatment. However, those solutions are costly to operate.

[0035] In this respect some solutions have been foreseen, such as for example in document WC2024 / 126781 which provides a process as mentioned in the beginning specially adapted to Parallel flow regenerative kiln having an SCR system situated downstream the dust treatment 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 treatment 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.

[0036] 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.

[0037] Indeed, the operation according to WO 2024 / 126781 using a low temperature catalyst operating at ~155°C limits the reheating need, but lowtemperature catalysts will be more expensive, with lower efficiency, and will require a larger bed (the most expensive part). The risk of unreacted reagent, also known as “ammonia slip” is also higher.

[0038] Further, this document does not take into account the cyclic operation of the PFR kiln which creates irregularities in the exhaust gas discharge in terms of flow rate, pressure and pollutant concentration.

[0039] WO 2024 / 002933 proposes to reheat the gas by a fixed bed regenerative exchanger (also called fixed bed gas-to-gas heater of FBGGH). The ‘cold gas’ from the kiln will pass through the FBGGH to be heated by the hot gas exiting the SCR. This allows to limit the heat losses to the stack and, thanks to the high efficiency of FBGGH, only a small burner or electrical heater is needed to compensate for these losses.

[0040] The FBGGH operates in a cyclic manner similar to a PFRK and the flow needs to be periodically reversed. Accordingly, a complex set of valves is necessary and WO 2024 / 002933 gives numerous examples of valves arrangement for this. FBGGH also have the problem that during their reversal, some gas will go directly to the stack without being treated. WO 2024 / 002933 Al proposes to synchronize the cycles of the FBGGH to the cycles of the PFRK to minimize this impact.

[0041] As it can be seen, while some solutions for efficient pollutant capture have been recently developed, there is still a need to find efficient solution in terms of energy costs, efficiency of pollutant capture, adaptable to the cyclic nature of PFR kilns.

[0042] The present invention encounters to solve at least a part of these drawbacks by providing a process efficient pollutant capture, reasonable energy costs and adaptable to the cyclic nature of PFR kiln for the calcination of carbonate mineral rocks.

[0043] To solve this problem, it is provided according to the present invention, a process for calcining carbonate mineral rocks in a parallel flow regenerative kiln having at least two shafts interconnected by a connecting channel, comprising, in standard operation,

[0044] - loading carbonate mineral rocks 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,

[0045] - cooling the calcined material with cooling gas in a cooling zone, with formation of heated cooling gas by heat exchange,

[0046] - discharging the calcined material from the bottom of the shafts, - discharging an exhaust gas from the kiln at a discharging 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,

[0047] - each shaft alternately working in a calcining mode and in a preheating mode, one shaft working in a calcining mode during a predetermined time period during which at least another shaft works in a preheating mode, and inversely after activation of the reversion means,

[0048] - the calcining mode comprising:

[0049] • Upon soid preheoted corbonote mineral rocks descending into soid shaft, decarbonation of the preheated carbonate mineral rocks with the release of combustion fumes descending co- currently in the shaft in calcination mode, and

[0050] • through said connecting channel, a passage of said combustion fumes toward the at least one shaft working in a preheating mode,

[0051] - said preheating mode comprising:

[0052] • sgid prehegfing step of the looded corbonote mineral rocks by heot exchonge with soid combustion fumes coming from the connecting channel, which is ascending and flows in countercurrent through the loaded carbonate mineral rocks, and

[0053] • said exhausting step of said combustion fumes as exhaust gas at the top of said at least one shaft in preheating mode

[0054] - treating a first portion of said exhaust gas discharged from the kiln in a flue gas treatment unit to remove at least partly pollutants forming a clean exhaust gas, said first portion being reheated to a controlled flue gas treatment temperature before entering the flue gas treatment unit by reheating means.The process for calcining carbonate mineral rocks according to the present invention is further characterized in that it comprises a step of preheating said first portion of said exhaust gas discharged from the kiln before being re-heated, said preheating step being performed in a rotary gas-to-gas heater (or RGGH) having a cold gas entry of a first pathway and a hot gas entry of a second pathway and rotative heat accumulation elements for communicating the heat from the hot gas flowing in the second pathway, accumulated in the rotative heat accumulation elements to the cold gas flowing in the first pathway, said first portion of exhaust gas entering the RGGH by said cold gas entry and being preheated by said accumulated heat to exit the RGGH with a temperature above said discharging temperature, said hot gas entry being fed by hot gas chosen amongst clean exhaust gas, combustion fumes and heated cooling gas, such as heated cooling air.

[0055] As it can be seen, the process according to the present invention use a rotary regenerative heat exchanger (also called rotary gas-to-gas heater or RGGH) to preheat the flue gas, limiting the need of re-heating the flue gas before flue gas treatment and allowing to recover the heat from another hot gas, such as clean exhaust gas, combustion fumes or heated cooling gas in an easy way. Indeed, RGGH operates continuously and therefore avoids a complex set of vanes to adapt to the cyclic nature of calcination in a parallel flow regenerative kiln, especially the reversal period. Further, RGGH can be designed with a low height so that automatic cleaning system with high pressure gas or water can be integrated, which makes their usage also possible before filters.

[0056] In a preferred embodiment according to the present invention, the process further comprises during the preheating of the said first portion of said exhaust gas an injection of a pressurized sealing gas chosen amongst clean exhaust gas, steam, air, nitrogen, clean CO2 and their combination at sealing means through which the rotative heat accumulation elements pass through, between the first pathway and the second pathway.

[0057] Typically, pressurized sealing gas has a pressure above the gas in the first and the second pathway, such as for example between 5 and 100 mbar above the pressure of the gas of the first and the second pathway.Using in the process according to the present invention a RGGH in fluid communication with the exhaust gas discharging port with sealing gas injected, such as clean exhaust gas or clean CO2 (the latter avoiding the bypass of untreated exhaust gas to the stack), allows to have extremely low (even indistinguishable from zero) leakage between the first and the second pathway. This is further advantageous with respect to a fixed bed gas-to-gas heater, in which, beside the need of complex valves and operations, especially during reversal as explained above, leakage will always occur.

[0058] Accordingly, the process according to the present invention reduces the environmental impact of the operation of carbonate mineral rocks Advantageously, in the process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to the present invention, said reheating step is performed by one or more burners, one or more oxyfuels burners, one or more electrical heaters, one or more plasma torches, one or more heat exchangers.

[0059] More particularly, in the process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according the present invention, said re-heating step is performed in said heat exchanger, where said first portion of said exhaust gas discharged from the kiln flows into a first pathway in the heat exchanger and is heated by collecting heat from a gas stream flowing in a second pathway of the heat exchanger, said gas stream being either heated cooling air or hot combustion fumes or even their combination, said heated cooling air 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 or their combination.

[0060] In a preferred embodiment of the process for calcining carbonate mineral rocks according to the present invention, said hot combustion fumes are extracted outside the kiln through a hot combustion gas outlet located in the connecting channel, preferably in the cross-over channel.

[0061] In a further preferred embodiment of the process for calcining carbonate mineral rocks according to the present invention, at least a part of saidheated cooling air is extracted outside of the kiln from a central collecting element located in the bottom of the cooling zone and / or from the cooling zone below the connecting channel, preferably from the top of the cooling zone, through extraction peripheral orifices or through a peripheral circular channel located at the top of the cooling zone, below the connecting channel.

[0062] In yet a preferred embodiment according to the present invention, at least a part of said heated cooling air is extracted outside of the kiln from a central collecting element located in the bottom of the cooling zone from the shaft in preheating mode.

[0063] In another preferred embodiment according to the present invention, at least a part of said heated cooling air is extracted outside of the kiln from a peripheral circular channel located at the top of the cooling zone, below the connecting channel from the shaft in firing mode.

[0064] Preferably according to the present invention, the step of treating the exhaust gas discharged from the kiln comprises a NOXreduction step in a SCR system, a SOXreduction step, and / or a VOC or TOC (total organic carbon) or THC (total hydrocarbon) reduction step.

[0065] Preferably, according to the present invention, the process for calcining carbonate mineral rocks in a parallel flow regenerative kiln comprises a step of CO2 capture from said clean exhaust gas.

[0066] In a particular embodiment of the process according to the present invention, a second portion of the exhaust gas discharged from the kiln is derived through a derivation system, located upstream of the flue gas treatment unit, said second 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, preferably at the top of the shaft in firing mode or at the top of the combustion zone of the shaft in firing mode so as to ensure said fuel combustion in the presence of oxygen; the combustion is accordingly performed in oxyfuel.

[0067] Advantageously, the oxidizing mixture is performed with said second portion of the discharged exhaust gas and concentrated dioxygen in a mixing chamber, said oxidizing mixture being fed to the kiln, preferably in the combustion zone.In another 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.

[0068] According to a preferred embodiment of the present invention, the loading of carbonate mineral rocks occurs during the reversion period at the top of the shaft that will work in a preheating mode after said reversion period.

[0069] 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 or at the top of the shaft that works in a calcination mode. Preferably, the loading of carbonate mineral stones occurs during the cycle at the top of the shaft that works in a preheating mode, but can optionally also be done during reversal.

[0070] Preferably, 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 treatment system.

[0071] Other embodiments of the process according to the present invention are mentioned in the appended claims.

[0072] The present invention relates to a parallel flow regenerative kiln for calcining carbonate mineral rocks comprising

[0073] - at least two shafts, interconnected by a connecting channel, having each a preheating zone, a combustion zone and a cooling zone,

[0074] - each of said shafts comprising, in the on or off position,

[0075] - at least one fuel supply device,

[0076] - an inlet, for loading carbonate mineral rocks, at the top of the shafts, - an outlet for unloading the calcined material produced, at the bottom of the shafts,

[0077] - 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 at the top of the shafts, and

[0078] - a supply of cooling gas to cool the calcined material produced,- the kiln comprising a system for reversing the operation of the shafts, arranged so that each shaft, in standard mode, operates alternately in calcining mode and in preheating mode, a shaft being in calcining mode for a predetermined time period while at least one other shaft is in preheating mode, and vice-versa, this reversing system therefore controlling said on and off positions,

[0079] - 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 forming a clean exhaust gas, said flue gas treatment unit having a clean exhaust gas outlet,

[0080] - a reheater, provided upstream the flue gas treatment unit in fluid communication with the gas discharging port and with the flue gas treatment unit to heat the exhaust gas at a controlled flue gas treatment temperature.

[0081] The parallel flow regenerative kiln according to the present invention is further characterized in that it comprises a rotary gas to gas heater RGGH having a cold gas entry of a first pathway and a hot gas entry of a second pathway and rotative heat accumulation elements for communicating the heat from the hot gas flowing in the second pathway, accumulated in the rotative heat accumulation elements to the cold gas flowing in the first pathway for preheating the discharged exhaust gas upstream the entry in the flue gas treatment unit, said first portion of exhaust gas entering the RGGH by said cold gas entry and being preheated by said accumulated heat to exit the RGGH with a temperature above said discharging temperature and below said flue gas treatment temperature, said hot gas entry being fed by hot gas chosen amongst clean exhaust gas, combustion fumes and heated cooling gas, such as heated cooling air.

[0082] Preferably, in one embodiment according to the present invention, the rotary gas to gas heater RGGH further comprises sealing means through which the rotative heat accumulation elements pass through, between the first pathway and the second pathway and injection means provided for injecting a pressurized sealing gas chosen amongst clean exhaust gas, preferably cold clean exhaust gas and clean CO2.According to the present invention, said reheater comprises one or more burners, one or more oxyfuels burners, one or more electrical heaters, one or more plasma torches, one or more heat exchangers.

[0083] Alternatively, or in addition, according to the present invention, said reheater comprises said 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 a heated cooling gas outlet or with a hot combustion fumes outlet in fluid communication with the combustion zone or their combination.

[0084] In a preferred embodiment of the kiln according to the present invention, the flue gas treatment unit comprises a SCR system, a SOx reduction system and / or a VOC or TOC (total organic carbon) or THC (total hydrocarbon) reduction system.

[0085] In addition, according to one embodiment of the present invention, the kiln further comprises a CO2 capture unit, downstream of said flue gas treatment unit.

[0086] In a preferred embodiment, the kiln according to the present invention 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, located upstream of the flue gas treatment unit and with an injection system provided in the kiln, more preferably in the kiln and also communicating with the combustion zone.

[0087] More preferably, in the kiln according to the present invention, 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 chamber, upstream the oxidizing inlet, said injection system being 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, with combustion performed in oxyfuel.

[0088] In a preferred embodiment, the kiln according to the present invention comprises loading activation means, synchronized with the reversion system, provided to load the carbonate mineral rocks during the reversion period at the top of the shaft that will work in a preheating mode after said reversion period.

[0089] In a preferred embodiment, the kiln according to the present invention comprises a dust treatment unit upstream the flue gas treatment unit.Other embodiments of the parallel flow regenerative kiln for calcining carbonate mineral rocks according to the present invention are mentioned in the appended claims

[0090] Other characteristics and advantages of the present invention will be derived from the non-limitative following description, and by making reference to the drawings.

[0091] In the drawings, figure 1 represents a parallel flow regenerative kiln according to the present invention carrying out the process according to the present invention, where the combustion is performed with combustion air and where the RGGH is followed by a reheater.

[0092] Figure 2 represents a parallel flow regenerative kiln according to the present invention carrying out the process according to the present invention, where the combustion is performed in oxyfuel and where the RGGH is followed by a reheater.

[0093] Figure 3 represents a parallel flow regenerative kiln according to the present invention carrying out the process according to the present invention, where the combustion is performed with combustion air and where the RGGH is followed by a heat exchanger using combustion fumes as hot gas.

[0094] Figure 4 represents a parallel flow regenerative kiln according to the present invention carrying out the process according to the present invention, where the combustion is performed with combustion air and where the RGGH is followed by a heat exchanger using heated cooling air as hot gas

[0095] Figure 5 represents a parallel flow regenerative kiln according to the present invention carrying out the process according to the present invention, where the combustion is performed in oxyfuel and where the RGGH is followed by a heat exchanger using combustion fumes as hot gas.

[0096] Figure 6 represents a parallel flow regenerative kiln according to the present invention carrying out the process according to the present invention, where the combustion is performed in oxyfuel and where the RGGH is followed by a heat exchanger using heated cooling air as hot gas.

[0097] In the drawings, the same reference numbers have been allocated to the same or analog elementAs it can be seen, Figure 1 represents a parallel flow regenerative kiln according to the present invention.

[0098] 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.

[0099] As can be seen in figure 1, the parallel-flow regenerative kiln 1 is a vertical double-shaft kiln 1, 2 where the fuel is injected alternately into 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 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.

[0100] When a shaft is in calcination mode, here the shaft 2, 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.

[0101] When a shaft is in preheating mode, here the shaft 2, the fuel supply device is closed and the lances 4 do not receive any fuel, but still can be supplied with cooling gas. The inlet 5 of the shaft is supplied by the carbonate stones. Theopening 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. The discharged exhaust gas contains 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 and is transferred to a chimney 15 or a CO2 capture unit 21. At least a first portion of the exhaust gas discharged from the duct 14, after being filtered in a dedusting device such as a filter 22, is treated in a flue gas treatment unit 25.

[0102] The first portion of the exhaust gas can be a portion from 10 to 100 % of the total exhaust gas on a volume basis.

[0103] Before entering the flue gas treatment unit 25, the dedusted exhaust gas from the filter 22 enters a reheater 23 to reach the flue gas treatment temperature. The flue gas treatment unit can be a NOXreduction step in a SCR system, a SOXreduction step, a VOC or TOC (total organic carbon) or THC (total hydrocarbon) reduction step and is in the illustrated embodiment, a SCR system 25 in which the reheated exhaust gas is fed via duct 39, in which a solution of ammonia / urea is sprayed through the lance 35.

[0104] As explained earlier in this patent application, the present invention further comprises a step of preheating said first portion of said exhaust gas discharged from the kiln before being re-heated in the reheater 23. The preheating step is performed in a rotary gas to gas heater RGGH 36 having a cold gas entry 41 of a first pathway which extends between the cold gas entry 41 and the hot gas exit 43. The rotary gas to gas heater RGGH 36 has also a hot gas entry 42 of a second pathway which extends between the hot gas entry 42 and a cold gas exit 44. In the RGGH, rotative heat accumulation elements are included for communicating the heat from the hot gas flowing in the second pathway, accumulated in the rotative heat accumulation elements to the cold gas flowing in the first pathway. Therefore, the first portion of exhaust gas entering the RGGH by said cold gas entry 41 ispreheated by said accumulated heat to exit the RGGH through the hot gas exit 43 with a temperature above said discharging temperature, said hot gas entry being fed by clean exhaust gas 38. The clean exhaust gas exits the RGGH through the cold gas exit 44, which is in fluid communication with the chimney 15 or a CO2 capture unit 21.

[0105] 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.

[0106] In some cases, there are three shafts, two in preheating mode and one in combustion.

[0107] In the shaft in calcination mode 1 , this discharge duct 14 is closed. Figure 2 represents a variation according to the present invention where the combustion is performed in oxyfuel.

[0108] The operation of the kiln according to the present invention is the same as explained for figure 1 except that a derivation system 17, capable of collecting a portion of exhaust gas discharged from the kiln and introducing it into the recirculation circuit 18 has been provided on the exterior, on the discharge duct 14. The derivation system 17 can be located before or after the filter 22 and in the illustrated embodiment, it is located before the filter 22. 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 is advantageously treated in a treatment unit 19, inside which it may, for example, be filtered and / or dried. An air separation unit 20 separates air supplied by the duct 40 into N2 (discharged via the duct 45 and O2 supplied to the recirculation circuit 18 via the supply duct 46). This circuit 18 then brings the oxidising 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. The operation of the furnace in figure 2 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 supplyopening 6 to allow the oxidising mixture to be introduced, while it is closed at the top of the shaft in preheating mode.

[0109] Figure 3 represents a variation of the embodiment of figure 1 where the reheater is a heat exchanger 23.

[0110] The first portion of the exhaust gas from the discharging duct 14 enters then the filter 22, is preheated via the RGGH 36 and enters a first pathway 29 of a heat exchanger 23 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 cross over channel 3, through a combustion fumes exit 37, while it can be also extracted from the peripheral channel 13 (not illustrated) The gas from the second pathway of the heat exchanger 23 is further filtered in a filter 28 before reaching a chimney 15 or another gas collector.

[0111] Figure 4 represents the same embodiment as the one in figure 3, 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 peripheral 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.

[0112] In the present patent application, for simplicity of description and understanding, while due to the cyclic operation of PFR kiln, all the connections are present 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 has been described on the burning shaft.

[0113] The same applies for cooling air which may be extracted from the burning shaft, from the preheating shaft or both.

[0114] For the same reasons, gas control devices such as fans, blowers, control valves, isolation valves are omitted on the figures.

[0115] In a variation, the heated cooling air 12 can be extracted through a central collector.

[0116] In both cases (central or peripheral extraction of heated cooling air), 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 partonly being used in the heat exchanger 23, the remaining part being extracted to concentrate in CO2 the exhaust gas.

[0117] 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.

[0118] Figure 5 represents a variation of the embodiment of figure 3 where the combustion is performed in oxyfuel and where the reheater is a heat exchanger 23.

[0119] The first portion of the exhaust gas from the discharging duct 14 enters then the filter 22, is preheated via the RGGH 36 and enters a first pathway 29 of a heat exchanger 23 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 cross over channel 3 through a combustion fumes exit 37. The gas from the second pathway of the heat exchanger 23 is further filtered in a filter 28 before reaching a chimney 15 or another gas collector.

[0120] Figure 6 represents the same embodiment as the one in figure 5, 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 peripheral 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.

[0121] 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, the extracted hot gas to feed the second pathway of the heat exchanger 23 can also be heated cooling air 12 extracted from a heated cooling gas output 26 of a central extractor or even located in the cross over channel, where it can be extracted in the bottom part of this latter due to the segregation of the layers of gas inside the cross over channel if the kiln is not operating in oxyfuel conditions.

[0122] Similarly, the second pathway 30 of the heat exchanger can be fed by combustion gas extracted from the peripheral channel 17.

Claims

CLAIMS1. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln having at least two shafts interconnected by a connecting channel, comprising, in standard operation,- loading carbonate mineral rocks at the top of each shaft, - preheating these loaded stones in a preheating zone,- calcining these preheated stones in a calcination zone with production of a decarbonated calcined material,- cooling the calcined material with cooling gas in a cooling zone, with formation of heated cooling gas by heat exchange,- discharging the calcined material from the bottom of the shafts, - discharging an exhaust gas from the kiln at a discharging 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,- each shaft alternately working in a calcining mode and in a preheating mode, one shaft working in a calcining mode during a predetermined time period during which at least another shaft works in a preheating mode, and inversely after activation of the reversion means,- the calcining mode comprising:• Upon soid preheoted corbonote mineral rocks descending into soid shaft, decarbonation of the preheated carbonate mineral rocks with the release of combustion fumes descending co- currently in the shaft in calcination mode, and• through said connecting channel, a passage of said combustion fumes toward the at least one shaft working in a preheating mode,- said preheating mode comprising:• sgid prehegting step of the looded corbonote mineral rocks by heot exchonge with soid combustion fumes coming from the connecting channel, which is ascending and flows in countercurrent through the loaded carbonate mineral rocks, andsaid exhausting step of said combustion fumes as exhaust gas at the top of said at least one shaft in preheating mode treating a first portion of said exhaust gas discharged from the kiln in a flue gas treatment unit to remove at least partly pollutants forming a clean exhaust gas, said first portion being reheated to a controlled flue gas treatment temperature before entering the flue gas treatment unit by re-heating means, characterized in that said process further comprises a step of preheating said first portion of said exhaust gas discharged from the kiln before being re-heated, said preheating step being performed in a rotary gas to gas heater RGGH having a cold gas entry of a first pathway and a hot gas entry of a second pathway and rotative heat accumulation elements for communicating the heat from the hot gas flowing in the second pathway, accumulated in the rotative heat accumulation elements to the cold gas flowing in the first pathway, said first portion of exhaust gas entering the RGGH by said cold gas entry and being preheated by said accumulated heat to exit the RGGH with a temperature above said discharging temperature, said hot gas entry being fed by hot gas chosen amongst clean exhaust gas, combustion fumes and heated cooling gas, such as heated cooling air.

2. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to claim 1, further comprising during the preheating of the said first portion of said exhaust gas an injection of a pressurized sealing gas chosen amongst clean exhaust gas, steam, air, nitrogen, clean CO2 and their combination at sealing means through which the rotative heat accumulation elements pass through, between the first pathway and the second pathway.

3. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to claim 1 or claim 2, wherein said re-heating step is performed by one or more burners, one or more oxyfuels burners, one or more electrical heaters, one or more plasma torches, one or more heat exchanger.

4. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to claim 3, wherein said re-heating step is performed in said heat exchanger, where said first portion of said exhaust gas discharged from the kiln flows into a first pathway in the heat exchanger and is heated by collecting heat from a gas stream flowing in a second pathway of the heat exchanger, saidgas stream being either heated cooling air or hot combustion fumes or even their combination, said heated cooling air 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 or their combination.

5. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to claim 4, wherein said hot combustion fumes are extracted outside the kiln through a hot combustion gas outlet located in the connecting channel, preferably in the cross-over channel.

6. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to claim 4, wherein at least a part of said heated cooling air is extracted outside of the kiln from a central collecting element located in the bottom of the cooling zone and / or from the cooling zone below the connecting channel, preferably from the top of the cooling zone, through extraction peripheral orifices or through a peripheral circular channel located at the top of the cooling zone, below the connecting channel.

7. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to claim 6, wherein at least a part of said heated cooling air is extracted outside of the kiln from a central collecting element located in the bottom of the cooling zone from the shaft in preheating mode.

8. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to claim 6 or claim 7, wherein at least a part of said heated cooling air is extracted outside of the kiln from a peripheral circular channel located at the top of the cooling zone, below the connecting channel from the shaft in firing mode.

9. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to any of the previous claims, the step of treating the exhaust gas discharged from the kiln comprises a NOXreduction step in a SCR system.

10. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to any of the previous claims, wherein the step of treating the exhaust gas discharged from the kiln comprises a SOXreduction step.

11. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to any of the previous claims, wherein the step of treating the exhaust gas discharged from the kiln comprises a VOC or TOC (total organic carbon) orTHC (total hydrocarbon) reduction step.

12. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to any of the claims 1 to 11 , comprising a step of CO2 capture from said clean exhaust gas.

13. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to any of the claims 1 to 12, wherein a second portion of the exhaust gas discharged from the kiln is derived through a derivation system, located upstream of the flue gas treatment unit, said second portion of said exhaust gas being collected and mixed with oxygen to form an oxidizing mixture and reintroduced in the kiln for providing an oxidizing atmosphere in the combustion zone, preferably at the top of the shaft in firing mode or at the top of the combustion zone of the shaft in firing mode so as to ensure said fuel combustion in the presence of oxygen.

14. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to claim 13, wherein oxidizing mixture is performed with said second portion of the discharged exhaust gas and concentrated dioxygen in a mixing chamber, said oxidizing mixture being fed to the kiln, preferably in the combustion zone.

15. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln 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 in fluid communication with the second pathway of the heat exchanger.

16. Process for calcining carbonate mineral rocks in a parallel flow regenerative kiln according to any of the previous claims, wherein the loading of carbonate mineral rocks occurs during the reversion period at the top of the shaft that will work in a preheating mode after said reversion period.

17. Process for calcining carbonate mineral rocks in a carbonate mineral rock kiln according to any of the claims 1 to 16, wherein step of treating the first portion of said exhaust gas also comprises a step of flowing the exhaust gas through a dust treatment system.

18. Parallel-flow regenerative kiln for calcining carbonate mineral rocks, comprising- at least two shafts, interconnected by a connecting channel, having each a preheating zone, a combustion zone and a cooling zone,- each of said shafts comprising, in the on or off position,- at least one fuel supply device,- an inlet, for loading carbonate mineral rocks, at the top of the shafts, - an outlet for unloading the calcined material produced, at the bottom of the shafts,- 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°Cat the top of the shafts, and- a supply of cooling gas to cool the calcined material produced, - the kiln comprising a system for reversing the operation of the shafts, arranged so that each shaft, in standard mode, operates alternately in calcining mode and in preheating mode, a shaft being in calcining mode for a predetermined time period while at least one other shaft is in preheating mode, and vice-versa, this reversing system therefore controlling said on and off positions,- 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 forming a clean exhaust gas, said flue gas treatment unit having a clean exhaust gas outlet,- a reheater, provided upstream the flue gas treatment unit in fluid communication with the gas discharging port and with the flue gas treatment unit to heat the exhaust gas at a controlled flue gas treatment temperature,characterized in that it further comprises a rotary gas to gas heater RGGH having a cold gas entry of a first pathway and a hot gas entry of a second pathway and rotative heat accumulation elements for communicating the heat from the hot gas flowing in the second pathway, accumulated in the rotative heat accumulation elements to the cold gas flowing in the first pathway for preheating the discharged exhaust gas upstream the entry in the flue gas treatment unit, said first portion of exhaust gas entering the RGGH by said cold gas entry and being preheated by said accumulated heat to exit the RGGH with a temperature above said discharging temperature and below said flue gas treatment temperature, said hot gas entry being fed by hot gas chosen amongst clean exhaust gas, combustion fumes and heated cooling gas, such as heated cooling air.

19. Parallel-flow regenerative kiln for calcining carbonate mineral rocks according to claim 18, wherein the rotary gas to gas heater RGGH further comprises sealing means through which the rotative heat accumulation elements pass through, between the first pathway and the second pathway and injection means provided for injecting a pressurized sealing gas chosen amongst clean exhaust gas, steam, air, nitrogen, clean CO2 and their combination.

20. Parallel-flow regenerative kiln for calcining carbonate mineral rocks according to claim 18 or claim 1 , wherein said reheater comprises one or more burners, one or more oxyfuels burners, one or more electrical heaters, one or more plasma torches, one or more heat exchangers.

21. Parallel-flow regenerative kiln for calcining carbonate mineral rocks according to claim 18 or 19, wherein said reheater comprises said 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 a heated cooling gas outlet or with a hot combustion fumes outlet in fluid communication with the combustion zone or their combination.

22. Parallel-flow regenerative kiln for calcining carbonate mineral rocks according to any of the claims 18 to 21, wherein the flue gas treatment unit comprises a SCR system.

23. Parallel-flow regenerative kiln for calcining carbonate mineral rocks according to any of the claims 18 to 22, wherein the flue gas treatment unit comprises a SOx reduction system.

24. Parallel-flow regenerative kiln for calcining carbonate mineral rocks according to any of the claims 18 to 23, wherein the flue gas treatment unit comprises a VOC or TOC (total organic carbon) or THC (total hydrocarbon) reduction system.

25. Parallel-flow regenerative kiln for calcining carbonate mineral rocks according to any of the claims 18 to 24, comprising a CO2 capture unit, downstream of said flue gas treatment unit.

26. Parallel-flow regenerative kiln for calcining carbonate mineral rocks according to any of the claims 18 to 25, further comprising a recirculation circuit in fluid communication with the gas discharging port provided to derive a second portion of the exhaust gas from the kiln, located upstream of the flue gas treatment unit and with an injection system provided in the kiln, more preferably in the kiln and also communicating with the combustion zone.

27. Parallel-flow regenerative kiln for calcining carbonate mineral rocks according to claim 26, wherein 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 chamber, upstream the oxidizing inlet, said injection system being 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.

28. Parallel-flow regenerative kiln for calcining carbonate mineral rocks according to any of the claims 18 to 27 comprising loading activation means, synchronized with the reversion system, provided to load the carbonate mineral rocks during the reversion period at the top of the shaft that will work in a preheating mode after said reversion period.

29. Parallel-flow regenerative kiln for calcining carbonate mineral rocks according to any of the claims 18 to 28, comprising a dust treatment unit upstream the flue gas treatment unit.

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