WO2025228775A1

WO2025228775A1 - Decarbonation process of carbonated materials in a multi-shaft vertical kiln

Info

Publication number: WO2025228775A1
Application number: PCT/EP2025/061183
Authority: WO
Inventors: Alex AUBERT; Marie-Laure VAN DE BERG; Bernard MAITER
Original assignee: Carmeuse Technologies Sa
Current assignee: Carmeuse Technologies Sa
Priority date: 2024-04-29
Filing date: 2025-04-24
Publication date: 2025-11-06
Anticipated expiration: 2026-10-29

Abstract

Multi-shaft vertical kiln (MSVK) for decarbonating carbonated materials (10), preferably carbonated mineral, in particular limestone and/or dolomitic limestone, said kiln (MSVK) comprising a first (100), a second (200), and optionally a third shaft (300) with preheating zones (110, 210, 310), heating zones (120, 220, 320) and cooling zones (130, 230, 330) and a transfer (412, 423, 432) channel between each shaft (100, 200, 300), said kiln (MSVK) being arranged for being cooled with one or more cooling streams (90), said kiln (MSVK) comprising a regenerative heat exchangers (4000) in fluid communication with an opening formed in a wall portion of the or each transfer channel (412, 423, 432) and a process to operate said kiln (MSVK).

Description

DECARBONATION PROCESS OF CARBONATED MATERIALS IN A MULTI-SHAFT

VERTICAL KILN

Technical Field

[0001] The present invention relates to a decarbonation process of carbonated materials and to a multi-shaft vertical kiln for carrying said process and a method for retrofitting a parallel-flow regenerative kiln.

Background Art

[0002] The increasing concentration of carbon dioxide in the atmosphere is recognized as one of the causes of global warming, which is one of the greatest concerns of present days. This increase is largely owed to human actions and particularly to the combustion of carbon-containing fossil fuel, for instance for transportation, household heating, power generation, etc., and in energy-intensive industries such as steel, cement, and lime manufacturing.

[0003] Within the lime-production process, natural limestone (mainly composed of calcium carbonate) is heated to a temperature above 900°C, in particular above 910°C in order to cause its calcination into quicklime (calcium oxide) and carbon dioxide according to the following reversible reaction :

CaCCh CaO + CO2 AH = 178 kJ/mol : Equation 1

[0004] Calcium oxide is considered as one of the most important raw materials and is used in a multitude of applications such as steel manufacturing, construction, agriculture, flue gas and water treatment as well as in glass, paper, and food industry. The global annual production is estimated to be above 250 million tons.

[0005] As indicated in Equation 1 , CO2 is a co-product of the lime-production process meaning that approximately 760 to 790 kg of CO2 is unavoidably generated when producing 1 ton of lime. Moreover, the heat required for heating limestone and for conducting the reaction is usually provided by the combustion of a carbonaceous fuel, which results in additional production of CO2 (ranging between 200 and more than 700 kg per ton of lime depending on the nature of the fuel and efficiency of the kiln).

[0006] The use of vertical shaft kiln prevails in the lime industry as they are particularly suitable for the production of lumpy quicklime compared to other types of furnaces, such as rotary kiln, and because they have the advantage of lower specific energy input. [0007] In a single-shaft vertical kiln, limestone or dolomitic limestone is fed through the top of the shaft and the produced lime is discharged at its bottom. In the pre-heating zone, the limestone is heated by hot gases flowing upward from the combustion zone. In the combustion zone, heat is produced through the direct firing of a fuel to reach a temperature above 900°C, preferably 910°C and consequently causing the decomposition of the limestone into quicklime and CO2. The lime then enters the cooling zone where it is cooled by air fed from the bottom of the shaft. The produced lime is finally discharged, ground, and sieved into the desired particle size. Flue gas leaves the shaft at the top of the pre-heating zone and is fed to a filter system before it is vented to the atmosphere. Specific energy consumption for such single-shaft vertical kilns ranges between 4 and 5 GJ per ton of lime.

[0008] Parallel-flow regenerative kilns (PFRK) are a variant of vertical shafts that are considered as the best-available technology for lime production with design capacity up to 800 tons per day. They consist in several vertical shafts (usually 2 or 3) connected by a transfer channel. Each shaft operates alternately according to a defined sequence. Initially, fuel is burnt in one of the shafts (“in combustion”) with combustion air flowing downwards (“parallel flow” with the limestone). Hot gases are then transferred to the other shafts (“in regeneration”) through the transfer channel in order to pre-heat limestone in said other shafts. A reversal between combustion and regeneration shafts typically occurs every 10 to 16 minutes.

[0009] This operational mode enables optimal recovery of the heat contained in product and hot gases bringing the specific energy consumption down to about 3.6 GJ per ton of lime. The combustion of the fuels required to bring this heat results in the production of approximately 200 kg of CO2 per ton of lime when natural gas is used.

[0010] The lime industry is making efforts for reducing its CO2 emissions by improving energy efficiency (including investment in more efficient kilns), using lower-carbon energy sources (e.g. replacing coal by natural gas or biomass) or supplying lime plants with renewable electricity. The CO2 related to energy can thus be reduced to some extent. Nevertheless, none of these actions impact the CO2 which is inherently produced during decarbonation of limestone.

[0011] A route for further reducing emission consists in capturing CO2 from the lime kiln flue gas for permanent sequestration (typically in underground geological formation) or recycling for further usage (e.g. for the production of synthetic fuels). Those processes are known under the generic term CCLIS (Carbon Capture, Utilization and Storage).

[0012] Combustion air and cooling air used in conventional PFRK lime kilns contains approximately 79 vol% nitrogen resulting in CO2 concentration in flue gas not higher than 15-25 vol%. Additional measures are thus required to obtain a CO2 stream that is sufficiently concentrated to be compatible with transportation, sequestration and/or .utilization.

[0013] Several technologies have been investigated for concentrating the CO2 stream.

[0014] In particular WO2022238385 discloses a multiple shaft vertical kiln, in which the combustion and cooling sequences have been dissociated to minimize the mixing of CO2 and air.

Aims of the Invention

[0015] The invention aims to provide a solution to overcome at least one drawback of the teaching provided by the prior art.

[0016] More specifically, the invention aims to provide a process and a device for simultaneously allowing to maintain a very similar energy consumption as a standard PFR kiln, preferably to recover the maximum amount of CO2, more preferably to allow a decarbonation with a high production throughput of a product (e.g. quicklime, dolime) with a CC>2-enriched exhaust stream, in particular a CCh-rich stream that is suitable for sequestration or use.

Summary of the Invention

[0017] For the above purpose, the invention is directed to a decarbonation process of carbonated materials, preferably carbonated mineral, in particular limestone and/or dolomitic limestone, in a multi-shaft vertical kiln, said kiln comprising a first, a second, and optionally a third shaft with preheating zones, heating zones and cooling zones and a transfer channel between each shaft, alternately heating carbonated materials by a combustion of at least one fuel with at least one comburent up to a temperature range in which carbon dioxide of the carbonated materials is released, the combustion of the fuel and the decarbonation generating an exhaust gas, the decarbonated materials being cooled in the cooling zones with one or more cooling streams, wherein a mixing between the exhaust gas and the one or more cooling streams is minimized or avoided by operating said kiln in a mode in which a cooling cycle interposed between two subsequent alternating heating cycles between the first and the second or the third shaft, is such that the decarbonated materials in at least the first, the second and/or the third shaft are cooled with the one or more cooling streams while a supply of the fuel in each shaft is stopped, said process further comprising the steps of :

- transferring thermal energy from the one or more cooling streams extracted from said kiln during the cooling cycle to a portion of a recirculated exhaust gas extracted from said kiln during the latter of the two subsequent alternating heating cycles or another heating cycle subsequent to the cooling cycle and

- supplying said kiln with the portion of a recirculated exhaust gas, as heated, during the latter of the two subsequent alternating heating cycles or the other heating cycle subsequent to the cooling cycle.

[0018] According to specific embodiments of the invention, the decarbonation process of carbonated materials comprises one or more of the following steps/features: transferring the thermal energy of at least some of the one or more cooling streams to a heat storage medium of a regenerative heat exchanger, said streams exiting from at least one transfer channel, in particular the transfer channel between the first and second shaft, said heat storage medium being adapted to intermittently stored the thermal energy of said cooling streams, during the cooling cycle; feeding the portion of a recirculated exhaust gas, as heated into the at least one transfer channel, in particular the transfer channel between the first and second shaft, during the latter of the two subsequent alternating heating cycles or the other heating cycle subsequent to the cooling cycle, via the regenerative heat exchanger. controlling the flow rate of the portion of a recirculated exhaust gas, in particular supplying said portion intermittently and/or at different flow rates, during the latter of the two subsequent alternating heating cycles or the other heating cycle subsequent to the cooling cycle; extracting the recirculated exhaust gas from an upper portion of the shaft(s) in regeneration; supplying the shaft in combustion with another portion of the recirculated exhaust gas during the latter of the two subsequent alternating heating cycles or the other heating cycle; each shaft of the multi-shaft vertical kiln comprises supply pipes adapted to supply the at least one fuel, in particular fuel lances, said process further comprising, during the latter of the two subsequent alternating heating cycles or the other heating cycle, the step of supplying one or more of the supply pipes of the shaft(s) in regeneration with a further portion of the recirculated exhaust gas so as to cool said supply pipes; the feeding of the one or more cooling streams in at least one of the first, the second or third shaft is stopped, during the two subsequent alternating heating cycles or the other heating cycle subsequent to the cooling cycle, or wherein an amount of the one or more cooling streams to be supplied in at least one the first, the second or third, during the two subsequent alternating heating cycles or the other heating cycle subsequent to the cooling cycle, is controlled in such a manner that the cooling stream amount to be supplied in the at least one the first, the second or third shaft, does not exceed 18%, preferably 10%, in volume of the amount of the one or more cooling streams to be supplied during the cooling cycle; between the two subsequent alternating heating cycles in the first and the second or the third shaft, depressurizing the first and the second and optionally the third shaft for a predetermined time period before the cooling cycle, preferably the first and the second and optionally the third shaft are depressurized down to reach a level comprised in the range of 1 to 600 mbars, preferably 500 mbars, under the atmospheric pressure.

[0019] The invention is also directed to a multi-shaft vertical kiln for decarbonating carbonated materials, preferably carbonated mineral, in particular limestone and/or dolomitic limestone, said kiln (MSVK) comprising a first, a second, and optionally a third shaft with preheating zones, heating zones and cooling zones and a transfer channel between each shaft, said kiln being arranged for being cooled with one or more cooling streams, preferably said kiln being adapted for carrying out the process according the invention, said kiln comprising a regenerative heat exchangers in fluid communication with an opening formed in a wall portion of the or each transfer channel.

[0020] According to specific embodiments of the invention, the multi-shaft vertical kiln comprises one or more of the following features: the regenerative heat exchanger comprises or consists in a pebble heater. the regenerative heat exchanger is a single bed regenerator; the regenerative heat exchanger further comprises a cleaning circuit adapted to circulate and clean a heat storage medium of the regenerative heat exchanger, in particular pebbles of the pebble heater; the regenerative heat exchanger is positioned between the first and the second shafts; the regenerative heat exchanger is positioned between the first, the second and the third shafts; the multi-shaft vertical kiln has a specific energy consumption of 3.2 to 4.2 GJ per ton, preferably 3.5 to 3.7 GJ per ton of decarbonated materials and/or the multi-shaft vertical kiln has output rate of 30 to 800 tons, preferably 30 to 330 tons of carbonated materials per day.

[0021] The invention is also directed to method of retrofitting a parallel-flow regenerative kiln comprising a first, a second, and optionally a third shaft with preheating zones, heating zones and cooling zones and a transfer channel between each shaft, said kiln being arranged for being cooled with one or more cooling streams, into the multi-shaft vertical kiln according to the invention, further comprising the steps of:

- performing an opening in the transfer channel arranged between the first and second shaft;

-providing a regenerative heat exchanger, preferably comprising or consisting in a pebble heater;

- fluidly connecting an opening of the regenerative heat exchanger to the opening of the transfer channel arranged between the first and second shaft ;

- integrating a recycle exhaust gas passage arrangement, said arrangement being adapted to recirculate the exhaust gas exiting one of the shafts, while said shaft being in regeneration in use, into another opening of the regenerative heat exchanger and optionally an opening in another shaft, while said shaft being in combustion in use;

-loading a memory medium of a control unit of said kiln with a computer program comprising instructions which, when the program is executed by the control unit, cause said unit to implement the steps of the process according to the invention.

[0022] The CO2 produced by the decarbonation of the limestone pebbles and optionally by the combustion of the fuel is recovered up to 95%, compared to “classical” multi-shaft vertical kiln operation with an “intermitted flush”, which typically reach between 60 and 70% of CO2 recovery, that do not by pass the cooling stream in the shaft in regeneration.

[0023] The CO2 concentration level in the exhaust gas will reach a level of 30 to 45 % (volumetric) dry basis without Flue Gas Recirculation and without O2 (using air as comburant) and >85% (volumetric) dry basis using Flue Gas Recirculation and O2.

[0024] Thanks to the measures of the invention, the energy consumption and/or the production rate are similar to usual parallel flow regenerative kilns, for the same concentration level of CO2 in the exhaust gas, in the range 30 to >85% (volumetric) dry basis. This result is achieved through higher heat transfer efficiency achieved with a regenerative heat exchanger such as a pebble heater. Indeed, the pebble heater technology produces very low exergy losses due to its high heat exchange surface and the low temperature difference between the gases exiting and subsequently entering on each side of the pebble bed (typically 70°C), resulting in an exergy efficiency of >92%, ideally > 95%. In many applications, this temperature difference is even less than 50 K, with the recorded minimum being 15 K, leading to an exergy efficiency of above 98%.

[0025] The combination of a regenerative heat exchanger and a multi-shaft vertical kiln operating with an intermitted flush pattern generates synergies as one pebble heater module can be selected. Indeed, energy transfer from a cooling cycle to a subsequent heating cycle, allows operation with a single pebble heater module compared to a steady flow conditions, where at least two pebble heater modules alternately operated are required.

[0026] As the pebble heater could receive cooling flow from the kiln with a certain amount of dust, the single pebble heater module can be designed with the possibility to circulate the pebble heater load and to remove the dust out of it.

[0027] During the kiln cooling cycle, the cooling flow going to the pebble heater is set to a high flow rate during a limited time (e.g. maximum 3 minutes). During the kiln heating cycle (e.g. minimum 8 minutes), which is longer than the kiln cooling cycle, the flow that is heated into the pebble heater can be set to either a high rate during a short time (e.g. 3 minutes), or a low rate during a long time (e.g. 8 minutes). The final goal is to recover a maximum of the energy stored during the kiln cooling cycle.

[0028] The measures of the invention can be implemented in many types of existing parallel-flow-regenerative kiln without undue capital expenses. Few modifications may comprise for instance the additional opening(s) in the channel area, the provision of additional valve(s), new software and a regenerative heat exchanger. Especially, a single pebble heater can be positioned between two or three shafts or in the proximity of the kiln.

Brief Description of Drawings

[0029] Aspects of the invention will now be described in more details with reference to the appended drawings, wherein same reference numerals illustrate same features.

[0030] Figures 1 A and 1 B show a first embodiment according to the invention.

[0031] Figure 2 shows a second embodiment according to the invention.

[0032] Figures 3A and 3B show a third embodiment according to the invention. [0033] Figures 4A and 4B show a fourth embodiment according to the invention.

[0034] Figure 5 shows a fifth embodiment according to the invention.

[0035] Figure 6 shows a sixth embodiment according to the invention.

[0036] Figure 7 shows a pebble heater as used in the first embodiment according to the invention.

[0037] Figure 8 shows a cleaning circuit to recirculate and clean pebbles of a pebble heater.

[0038] List of reference symbols

MSVK multi-shaft vertical kiln CPU CO2 purification unit ASU Air separation Unit 10 carbonated materials 20 Fuel 30, 31 , 32 Comburent 40 exhaust gas (from fuel + decarbonation) 41 recirculated gas 50 decarbonated materials 90 cooling streams: 100,200,300 1st, 2nd, 3rd shafts 110,210,310 preheating zones 111 ,211 upper end of preheating zones 120,220,320 heating zones 130,230,330 cooling zones 132,232,332 lower end of cooling zone 412,423,431 transfer channels 3000 membrane separation unit 4000 regenerative heat exchanger 4100 pebble bed 4500 cleaning circuit to clean and circulate a heat storage medium 4510 valve 4520 tank 4530 pneumatic transport conduit 4540 cyclone 4550 pneumatic pump Detailed description

[0039] The present invention will now be described in detail with reference to the accompanying drawings and their reference numbers, in which illustrative and non- limitative embodiments of the invention are shown. [0040] Figure 1A shows a multi-shaft vertical kiln (MSVK) leading to a CO2 enriched exhaust gas. For instance, the control of the opening or closing of the valves (e.g. louvers) as well as the activation of the blowers are set up so that the contact of combustion flows and cooling flows are minimized or not existent. Indeed, between two subsequent, alternating heating cycles between the first 100 and the second 200 shafts, the decarbonated materials 50 in at least the first 100 and/or the second 200 shaft are cooled with a cooling stream 90, in particular air, while a supply of the fuel 20 and optionally the at least one comburent 30, 31 , 32 in each shaft 100, 200 is stopped. This way of operating the MSVK in which the cooling steams 90 and the exhaust gas stream are separated in the “time” allows to generate exhaust gas with a high CO2 content. This way of operating the MSVK is termed as “intermittent flush”. The control of the MVSK can comprise the following sequential cycles:

[0041] Cycle 1 comprises feeding the first shaft 100 with fuel 20, at least one comburent 30, 31 , 32 (e.g. air, CCh-enriched air, oxygen-enriched air or substantially pure oxygen) and the recirculated exhaust gas 41 from the second shaft 200, while transferring the generated exhaust gas 40 to the second shaft 200 via the transfer channel 412. During Cycle 1 , the heat accumulated in a heat accumulation media of a regenerative heat exchanger 4000 is recovered through heating at least a portion of the recirculated exhaust gas 41 that is extracted from an upper portion of the shaft 200 in regeneration. The recirculated exhaust gas 41 exiting the regenerative heat exchanger 4000 is then injected in the transfer channel 412. Typically, the regenerative heat exchanger is a pebble heater.

[0042] Cycle 2 comprises feeding the first 100 and the second 200 shaft with cooling streams 90 at the lower portions, in particular the lower ends 132 and 232 of their cooling zones. The heated cooling stream 90 is then extracted from the transfer channel 412. The heated cooling stream 90 typically lies in the temperature range from 700° to 1100°C ideally from 900 to 1000°C and contains high quantity of sensible thermal energy. At least some, preferably a majority, in particular most of this thermal energy (e.g. up to around 95%) is then absorbed and stored in the heat accumulation media of the regenerative heat exchanger 4000 once the heated cooling stream 90 is fed in the regenerative heat exchanger 4000. The cooled cooling steam 90 extracted from the regenerative heat exchanger 4000 is then discharged preferably to a stack through a filter (the existing filter of the kiln).

[0043] Cycle 3 comprises feeding the second shaft 200 with the fuel 20, the at least one comburent 30, 31 , 32 (e.g. air, CCh-enriched air, oxygen-enriched air or substantially pure oxygen) and the recirculated exhaust gas 41 from the first shaft 100, while transferring the generated exhaust gas 40 to the first shaft 100 via the transfer channel 412. During Cycle 3, the heat accumulated in the heat accumulation media of the regenerative heat exchanger 4000 is recovered through heating at least a portion of the recirculated exhaust gas 41 that is extracted from an upper portion the shaft 100 in regeneration. The recirculated exhaust gas 41 exiting the regenerative heat exchanger 4000 is then injected in the transfer channel 412.

[0044] Cycle 4 comprises feeding at least the first 100 and second shaft 200 with cooling streams 90 at the lower portion 132 and 232 of their cooling zones. The heated cooling stream 90 is then extracted from the transfer channel 412. At least some, preferably a majority, in particular most of this thermal energy (e.g. up to around 95%) is then absorbed and stored in the heat accumulation media of the regenerative heat exchanger 4000 once the heated cooling stream 90 is fed in the regenerative heat exchanger 4000. The cooled cooling steam 90 extracted from the regenerative heat exchanger 4000 is then discharged to the stack through the filter (the existing filter of the kiln).

[0045] Thanks to these measures, the multi-shaft vertical kiln MSVK using air as an oxidizer reaches CO2 concentration in the exhaust gas in the range of 30 to 45%(volumetric) dry basis and a CO2 recovery rate >95%, providing the cooling of the fuel lances in the shaft in regeneration with recirculated exhaust gas 41 (a fuel lances cooling is not illustrated in Figure 1A, but can be implemented to increase the CO2 content in the exhaust gas 40). The multi-shaft vertical kiln MSVK using N2 depleted comburent as an oxidizer reaches CO2 concentration in the exhaust gas >85% (dry basis and volumetric) and a CO2 recovery rate >95%, providing the cooling of the fuel lances in the shaft in regeneration with recirculated exhaust gas 41 (a fuel lances cooling is not illustrated in Figure 1A, but can be implemented in increase the CO2 content in the exhaust gas 40).

[0046] Alternatively, only one shaft is flushed per cooling cycle, instead of both shafts as illustrated in Figure 1A.

[0047] In Figure 1 A is illustrated a transfer channel 412 with a (substantially) straight shape. An alternative design for the transfer channel 412 can be considered such a ring shaped transfer channel.

[0048] Figure 1A shows that the cooling stream 90 is stopped during the heating cycles 1 and 3. Alternatively, during a heating cycle a reduced cooling stream (compared to the amount of air supplied during a cooling cycle) is supplied in the shaft in combustion to prevent the intrusion of exhaust gas 40 into the cooling zone of the shaft in combustion, thereby minimizing the possibility that some decarbonated materials 50 in the upper portion of the cooling zone of the shaft in combustion undergo a recarbonation.

[0049] In relation with Figure 1A, an example on how to carry out the invention is detailed. This example is just an example of one configuration. Different other configurations are also possible. A standard parallel flow regenerative kiln is generally operated with 120 cycles per day, meaning a cycle time of 12 minutes. For a MSVK operating with an intermittent flush, the same number of cycles and the same cycle time of 12 minutes is selected for the example. A MSVK operating with an intermittent flush mode has have 3 different phases: (i) the combustion mode (with O2 and/or air, and/or exhaust gas recirculation) during for instance (around) 8 minutes, also known as combustion or heating such as cycle 1 , 3, (ii) the cooling mode during for instance (around) 3 minutes, also known as cooling cycle, such as cycle 2 , 4 and (iii) the reversal time during for instance (around) 1 minute, for a total cycle time of for instance (around) 12 minutes. The reversal time is phase principally dedicated for loading the kiln with carbonated material and switching the flow control valves to set up the next heating phase. Such a reversal time is not illustrated in Figure 1A. During the reversal time, no cooling air or comburent flow is supplied to MSVK kiln and the pebble heater.

[0050] During for instance the (around) 8 minutes of the combustion mode (cycle 1 , 3), a high concentrated CO2 (>85%) fumes coming out of the kiln will be split into 4 different flows: i . A portion of the recirculated gas flow is directed into the pebble heater 4000 to be heated using the energy stored in the pebble heater during the cooling cycle. It is then directed to the transfer channel 412 at a high temperature, representing approximately 21 % of the total exhaust gas volume exiting the top of the shaft during regeneration; ii . Another portion of the recirculated exhaust gas flow is fed to the top of the kiln MSVK, mixed with O2, and sent to the shaft for combustion. This portion represents approximately 45% of the total exhaust gas volume exiting the top of the shaft during regeneration; iii . A further portion of the recirculated exhaust gas flow is used for lance cooling, representing approximately 3% of the total exhaust gas volume exiting the top of the shaft during regeneration; iv. A portion of the exhaust gas exiting the top of the shaft is either discharged to the stack/exit or fed into a gas holder. This portion represents approximately 31% of the total exhaust gas volume exiting the top of the shaft during regeneration.

[0051] During for instance the (around) 3 minutes of the cooling mode, cooling air is supplied at the bottom of each shaft. Then the cooling air is extracted out of the transfer channel 412 and directed to the pebble heater 4000. The pebble heater absorbs thermal energy that will be released in the subsequent combustion mode. The flow of the cooling air is preferably in the range of (around) 0,65 Nm³/kg of lime produced by the kiln, but dispatched into the kiln during the limited time of the cooling mode, meaning a high instantaneous flow, to cover the full requested flow in a limited time.

[0052] The respective flows of fumes and cooling air passing through the pebble heater are adjusted in order to balance and recover all the energy stored and extracted from the pebble heater.

[0053] The invention is not limited to the above-mentioned sequence and can follow various patterns that can be adjusted depending on the circumstances.

[0054] We understand by the at least one comburent, an oxidizing agent such as either air (e.g. ambient air) , oxygen-enriched air or substantially pure oxygen, alone or in combination with the exhaust gas or substantially pure CO2 used during heating cycles. Preferably, the comburent is an oxygen-enriched air or substantially pure oxygen. One or more comburents are foreseen, in particular:

- a comburent 30, or

- a first 31 and a second comburent 32, preferably injected via the fuel lances

[0055] Figure 1 B schematically shows a multi-shaft vertical shaft MSVK of Figure 1 A in cycle 1. Three separate supply passages opening in a shaft are shown in Figure 1A and 1 B:

- a first passage arranged at an upper portion of the multi-shaft vertical kiln (e.g. PFRK) traditionally supplying a (first) comburent 30, 31 (e.g. primary air supply). Even if Figure 1 B shows one first supply passage, the multi-shaft vertical kiln MSVK may comprise more than one first supply passage per shaft 100, 200. The one or more first passage outlet openings are arranged in the corresponding shaft 100, 200. In the present disclosure, the comburent 30 or the first comburent 31 is preferably oxygen- enriched air or substantially pure oxygen.

- a second passage (e.g. fuel lance) traditionally supplying fuel 20 (e.g. natural gas, oil) and optionally the second comburent 32 (e.g. air). Even if Figure 1 B shows only one second supply passage, the multi-shaft vertical kiln comprises one or more second supply passages per shaft 100, 200 generally under the form of fuel/air lances. For instance, a mixture of fuel 20 and the second comburent 32 (e.g. coke with the conveying second comburent such as air) can be supplied through at least a part of the lances. Alternatively, a group of lances supplies the second comburent 32 (e.g. air), while another group of lances supplies the fuel 20 (natural gas or oil). In the present disclosure, the second comburent 32 is preferably oxygen-enriched air or substantially pure oxygen.

- a third passage is shown in Figure 1 B and dedicated to the supply of the recirculated exhaust gas 41. In Figure 1 B, the third passage comprise a portion in common with the exhaust passage opening the corresponding shaft. Alternately, the third passage portion opening in the corresponding shaft is separated from the exhaust portion opening the shaft. The present disclosure is not limited to a single third passage. Indeed, it can be foreseen that one or more third passages are in fluid connection with the corresponding shaft 100, 200.

In an alternative preferred form (shown schematically in a “window” arranged above the MSVK in Figure 1 B), a downstream end of the third passage is connected to the first passage. The present disclosure is not limited to a single third passage connected to a single first passage. Indeed, it can be foreseen that one or more downstream ends of the third passage(s) are connected to one or more first passages. The one or more first passages can feed the corresponding shaft 100, 200 with:

- a gas mixture comprising the recirculated exhaust gas 41 and the first comburent 30 (e.g. CC>2-enriched air, oxygen-enriched air or substantially pure oxygen) according to a first preferred alternative, or

- the recirculated exhaust gas 41 according to a second preferred alternative.

In the above-mentioned first preferred alternative, the fuel 20 (e.g. natural gas or oil, dihydrogen) is supplied via the one or more second passages.

In the above-mentioned second preferred alternative, the one or more second passages supply both the second comburent 32 (e.g. CCh-enriched air, oxygen-enriched air or substantially pure oxygen) and the fuel 20 (e.g. natural gas, oil, coke or dihydrogen). For instance, a group of lances supply the second comburent 32 (e.g. oxygen-enriched air or substantially pure oxygen) while another group supplies the fuel 20 (e.g. natural gas, oil or dihydrogen).

The first, second and third passages can be found in other embodiments of the present invention. [0056] Figure 2 illustrates a second embodiment of the present invention that differs from the previous embodiment in that an ASU is provided to increase the concentration of O2 in the comburent, thereby enriching in CO2 the exhaust gas.

[0057] Figure 3A and 3B show a third embodiment of the present invention. It differs from the multi-shaft vertical MSVK of the preceding embodiment in that it comprises a third shaft 300. This embodiment is a generalization of the “intermittent flush” use to a three-shaft kiln. As for any one of the preceding embodiments, the operation sequences, in the third embodiment, follow the “intermittent flush” use in which an intermittent cooling cycle is interposed between two heating cycles. Figure 3A shows a top view of a MSVK kiln according to the third embodiment. Figure 3B presents a lateral view of an “unrolled” MSVK kiln, where all the shafts are arranged in a plane. Figure 3B also schematically shows the fluid flows during a given operating cycle, namely Cycle 1 .

[0058] A typical sequence for a three-shaft vertical kiln according to the third embodiment is described as follow:

[0059] Cycle 1 (shown in Fig. 3B) comprises heating the carbonated materials in the heating zone 120 of the first shaft 100 while transferring the generated exhaust gas 40 to the second shaft 200 and the third shaft 300, via the corresponding transfer channels 412 and 431. During Cycle 1 , the heat accumulated in a heat accumulation media of regenerative heat exchangers 4000 is recovered through heating at least a portion of the recirculated exhaust gas 41 that is extracted from an upper portion the shafts 200, 300 in regeneration. The recirculated exhaust gas 41 exiting the regenerative heat exchangers 4000 is then injected in the transfer channels 412, 423, 431.

[0060] Cycle 2 comprises cooling the decarbonated materials 50 in the first 100, the second 200 and the third 300 shafts, while the fuel 20 supply is stopped. The heated cooling stream 90 is then extracted from the transfer channels 412, 423, 431 . The heated cooling stream 90 typically lies in the temperature range from 700° to 1100°C, ideally from 900 to 1000°C, and contains high quantity of sensible thermal energy. At least some, preferably a majority, in particular most of this thermal energy (e.g. up to around 95%) is then absorbed and stored in the heat accumulation media of the regenerative heat exchangers 4000 once the heated cooling stream 90 is fed in the regenerative heat exchangers 4000. The cooled cooling steam 90 extracted from the regenerative heat exchangers 4000 is then discharged to a stack, through a filter (existing kiln filter).

[0061] Cycle 3 comprises heating the carbonated materials 10 in the heating zone 220 of the second shaft 200 while transferring the generated exhaust gas 40 to the first 100 and the third 300 shafts, via the corresponding transfer channel 412, 423. During Cycle 3, the heat accumulated in the heat accumulation media of regenerative heat exchangers 4000 is recovered through heating at least a portion of the recirculated exhaust gas 41 that is extracted from an upper portion the shafts 100, 300 in regeneration. The exhaust gas 40 exiting the regenerative heat exchangers 4000 is then injected in the transfer channels 412, 423, 431.

[0062] Cycle 4 comprises cooling the decarbonated materials 50 in the first 100, the second 200 and the third 300 shafts, while the fuel supply 20 is stopped. The heated cooling stream 90 is then extracted from the transfer channels 412, 423, 431. At least some, preferably a majority, in particular most of this thermal energy is then absorbed and stored in the heat accumulation media of the regenerative heat exchangers 4000 once the heated cooling stream 90 is fed in the regenerative heat exchangers 4000. The cooled cooling steam 90 extracted from the regenerative heat exchangers 4000 is then discharged to the stack, through the filter (existing kiln filter).

[0063] Cycle 5 comprises heating the carbonated materials 10 in the heating zone 320 of the third shaft 300 while transferring the exhaust gas 40 generated to the first 100 and second 200 shafts, via the corresponding transfer channel 431 , 423. During Cycle 5, the heat accumulated in the heat accumulation media of regenerative heat exchangers 4000 is recovered through heating at least a portion of the recirculated exhaust gas 41 that is extracted from an upper portion the shafts 100, 200 in regeneration. The exhaust gas 40 exiting the regenerative heat exchangers 4000 is then injected in the transfer channels 412, 423, 431.

[0064] Cycle 6 comprises cooling the decarbonated materials 50 in the first 100, the second 200 and the third 300 shafts while the fuel 20 supply is stopped. The heated cooling stream 90 is then extracted from the transfer channels 412, 423, 431. At least some, preferably a majority, in particular most of this thermal energy is then absorbed and stored in the heat accumulation media of the regenerative heat exchangers 4000 once the heated cooling stream 90 is fed in the regenerative heat exchangers 4000. The cooled cooling steam 90 extracted from the regenerative heat exchangers 4000 is then discharged to the stack, through the filter (existing kiln filter).

[0065] Thanks to these measures, the multi-shaft vertical kiln MSVK using air as an oxidizer reaches CO2 concentration in the exhaust gas in the range of 30 to 45% (volumetric) dry basis, providing the cooling of the fuel lances in the regenerative shaft with recirculated exhaust gas 41 (a fuel lances cooling is not illustrated in Figure 3B, but can be implemented in increase the CO2 content in the exhaust gas 40). The multi-shaft vertical kiln MSVK using N2 depleted comburent as an oxidizer reaches CO2 concentration in the exhaust gas >85% (dry basis and volumetric) dry basis, providing the cooling of the fuel lances in the shaft in regeneration with recirculated exhaust gas 41 (a fuel lances cooling is not illustrated in Figure 1 B, but can be implemented in increase the CO2 content in the exhaust gas 40)

[0066] The invention is not limited to the above-mentioned sequence and can follow various patterns that can be adjusted depending on the circumstances.

[0067] Figures 4A and 4B illustrate a fourth embodiment of the present invention that differs from the previous embodiment in that a single regenerative heat exchanger preferably disposed in the center of the three shafts is provided instead of three regenerative heat exchangers 4000 preferably disposed between each pair of shafts.

[0068] Figure 5 shows a MSVK according to the fifth embodiment and differs from the first embodiment in that a membrane separation unit 3000 is provided in the exhaust line so as to enrich the exhaust gas in CO2 before being treated in the CPU. The membrane separation unit arranged in an exhaust line would separate N2 molecules from O2 and CO2 molecules. As the exhaust gas 40 comprises water resulting from the combustion of hydrogen containing fuel, a water separation unit (e.g. condenser) is preferably provided upstream from the membrane separation unit. Such a water separation unit is not illustrated in Figure 5.

[0069] Figure 6 shows a sixth embodiment according to the invention in which a kiln MSVK is depressurized (e.g. cycle vacuum 1 and cycle vacuum 2) between a given heating cycle (e.g. cycle 1 and cycle 3) and its subsequent cooling cycle (e.g. cycle 2 and cycle 4). The effect of this measure is that the equilibrium of Equation 1 is shifted to the right according to Le Chatelier's principle, thereby enhancing the release of CO2 from heated carbonated materials 10, in particular in the heating zones 120, 220 of the first shaft 100 and second 200 shaft. Furthermore, this measure allows to pump the CO2 already present in the kiln MSVK, for instance in the space between the pebbles of the carbonated 10 and decarbonated 50 materials at the start of the depressurization. Consequently, during the subsequent cooling phase, the extraction of CO2 present in combustion zones 120, 220 by the cooling stream 90 is minimized, because a large amount of the extractable CO2 had been already extracted during the vacuum phase (e.g. cycle vacuum 1 and cycle vacuum 2). Thus, the concentration of CO2 contained in the heated cooling streams 91 can be reduced. Preferably, the shafts 100, 200 are depressurized down to reach a level comprised in the range 1 to 600 mbars, in particular 500 mbars, under the atmospheric pressure. In particular, the predetermined time period amounts to at least 1 minutes, in particular at least 2 minutes, notably less than 10 minutes. The sixth embodiment also differs than the first embodiment in that only one shaft is cooled during the cooling cycle 2 and 4, Alternatively, both shafts can be cooled.

[0070] In Figure 6, the depressurization is ensured by a reversible (bidirectional) pump that is also used for recycling the exhaust gas 40 into the shafts 100, 200 during their combustion. In an alternative embodiment (not shown), a dedicated pump can be foreseen for the depressurization of the kiln MSVK besides a pump dedicated to the recycling of the exhaust gas 40. Such a depressurization pump can be arranged in parallel to the exhaust recycling pump or in a separate passage connected to the kiln MSVK.

[0071] Figure 7 represents a schematic cross section of a regenerative heat exchanger, in particular a pebble heater. In operation, the heat absorbing medium, in particular the pebble bed 4100 absorbs the thermal energy of the heated cooling stream 90 exiting the transfer channel 412 of a two-shaft kiln during its cooling mode. During the heating mode of the two-shaft kiln, a part of the recycled exhaust gas 41 is heated in contact with the pebble bed 4100. Typically, the pebble bed is made of pebbles with a diameter ranging from 2 to 10 mm, preferably made of aluminum oxide. Even if a cylindrical regenerative heat exchanger with a radial flow is presented in figure 7, other lay-out can be considered as well as an alternative. For a two shaft kiln, the heat absorbing medium is selected such that the pebble bed mass of the pebble heater time its specific heat capacity is able to recover the heat available in the cooling stream 90. This measure ensures that the thermal energy contained the heated cooling streams 90 exiting the cooling zones that normally heats directly the materials in the heating zone of the shaft in regeneration, is not wasted. Indeed, this thermal energy is first absorbed in the pebble bed 4100 during the cooling cycle and then redistributed during the heating cycle via the recirculated exhaust gas 41 . Alternatively, instead of pebble heater, a regenerative heat exchanger such as honey comb or matrix regenerator can be selected. Advantageously, a multi-shaft kiln with “intermittent flush” allows to use of a single regenerator/ single bed regenerator without the need of distributing valves, simplifying the system and its adaptation. A dual-bed regenerator can also be selected, although this solution is more complex to implement as it requires valves to alternate the flows between the regenerator modules or beds. Even if the figure 7 is illustrated with a two-shaft kiln, this solution can be extended to a 3-shaft kiln as illustrated in Figure 3A and 3B.

[0072] Figure 8 depicts how pebbles can be effectively cleaned. For this purpose, a cleaning circuit 4500, is adapted to recirculate and clean the pebbles. One or more valves 4510 (e.g. a first valve controlling the amount of pebbles to recirculate and a second valve to ensuring air tightness of the system) , control the cleaning process, which can be performed at regular intervals. Once the one or more valve 4510 is open, the pebbles fall by gravity into a tank 4520 where they are entrained by an airflow supplied by a pneumatic pump 4550. The pebbles carried by the airflow are then transported to a separator, specifically a cyclone 4540, through a conduit 4530. In the separator 4540, the pebbles are separated and then guided back to fill the top of the pebble bed. The pebbles extracted from their filter bed 4100 undergo cleaning through shocks and air flush. The contaminated air exiting the separator 4540 is preferably filtered before being released into the atmosphere. This filtration process is not illustrated in Figure 8.

[0073] In Figures 1 A, 1 B, 2, 3Ab 3B, 4A, 4B, the regenerative heat exchanger(s) 4000 is(are) positioned with a volume defined by the shafts. These examples allow to optimize the space available as well as the length of the connecting pipes. Alternatively, the regenerative heat exchanger(4000) can also be positioned separately from the shafts, depending on the circumstances.

[0074] Preferably, the CO2 purification unit (CPU) is configured to remove at least one of the following elements: acid gases, O2, Ar, CO, H2O, NOx, sulfur compounds, heavy metals, in particular Hg, Cd, and/or organic compounds, in particular CH4, benzene, hydrocarbons. More preferably, the CO2 purification unit (CPU) is adapted to adjust the composition of the exhaust gas 40 to the specification required by a carbon capture and utilization or carbon capture and storage application, preferably with a CO2 content above 80% (dry volume) and more preferably above 95% (dry volume).

[0075] Advantageously, the fuel 20 used in a multi-shaft vertical MSVK kiln according to the invention, in particular in any of the previous embodiments is either carbon- containing fuel or dihydrogen-containing fuel or a mixture of them. A typical fuel can be either wood, coal, peat, dung, coke, charcoal, petroleum, diesel, gasoline, kerosene, LPG, coal tar, naphtha, ethanol, natural gas, hydrogen, propane, methane, coal gas, water gas, blast furnace gas, coke oven gas, CNG or any combination of them. Furthermore, the multi-shaft vertical MSVK can use, for instance, two sources of fuel with different compositions.

[0076] Advantageously, the decarbonated materials 50 produced in a multi-shaft vertical kiln according to the invention, in particular in any of the previous embodiments have a residual CO2 <5%, preferably <2%.

[0077] Advantageously, the combustion of at least one fuel 20 with the at least one comburent 30, 31 , 32 is under an oxygen-to-fuel equivalence ratio greater or equal to 0.9, preferably greater than 1 .02.

[0078] The meaning of “multi vertical-shaft kiln” in the present disclosure is a kiln comprising at least two shafts 100, 200. The shafts are not coaxial and are disposed side by side to the extent that any shaft of a group consisting of the first and second, and optimally the third shaft 100, 200, 300 is not encircled by the other or another shaft 100, 200, 300 of said group. In other words, the transfer channel(s) 412, 423, 431 are arranged outside the shafts 100, 200, 300. This definition excludes a annular-shaft kiln in case it were interpreted as being a multi vertical-shaft kiln. A parallel-flow regenerative kiln is a specific form of a multi vertical-shaft kiln in the present definition. The multi vertical-shaft kiln of the first to the sixth embodiment falls under the definition of a parallel-flow regenerative kiln (in German: “Gleich Gegenstrom Regernativ Oferi"). According to the invention, the term “vertical” in “multi vertical-shaft kiln” does not necessarily require that the longitudinal axes of the shafts 100, 200, 300 have an exact vertical orientation. Rather, an exact vertical directional component of the alignment should be sufficient, with regard to an advantageous gravity-related transport of the material in the shafts, an angle between the actual alignment and the exact vertical alignment amounts to at most 30°, preferably at most 15° and particularly preferably of 0° (exactly vertical alignment).

[0079] Each shaft 100, 200, 300 of the multi-shaft vertical kiln comprises a preheating zone 110, 210, 310, a heating zone 120, 220, 320 and a cooling zone 130, 230, 330. A transfer channel 412, 423, 431 is disposed between each shaft 100, 200, 300. According to the present disclosure, the junction between the heating zones 120, 220, 320 and the cooling zones 130, 230, 330 is preferably substantially aligned with the lower end of the transfer channel(s) 412, 423, 431.

[0080] By ’’lower (end) portion/zone” of an element is meant that the fluid is introduced at an elevation from the lower end of said element/zone not exceeding 50 percent of the total height of said element/zone, in particular the fluid being introduced at the lower end.

[0081] By “upper (end) portion/zone” of an element is meant that the fluid is introduced at an elevation from the upper end of said element/zone not exceeding 50 percent of the total height of said element/zone, in particular the fluid being introduced at the upper end.

[0082] By “extracted gas or stream” is meant that the “gas or stream" is transferred either by suction or expansion.

[0083] The present disclosure presents a multi-shaft vertical kiln with two or three shafts.

[0084] Although the present invention has been described and illustrated in detail, it is understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims

1. Decarbonation process of carbonated materials (10), preferably carbonated mineral, in particular limestone and/or dolomitic limestone, in a multi-shaft vertical kiln (MSVK), said kiln (MSVK) comprising a first (100), a second (200), and optionally a third (300) shaft with preheating zones (110, 210, 310), heating zones (120, 220, 320) and cooling zones (130, 230, 330) and a transfer channel (412, 423, 431) between each shaft, alternately heating carbonated materials by a combustion of at least one fuel (20) with at least one comburent (30, 31 , 32) up to a temperature range in which carbon dioxide of the carbonated materials (50) is released, the combustion of the fuel and the decarbonation generating an exhaust gas (40), the decarbonated materials being cooled in the cooling zones (130, 230, 330) with one or more cooling streams (90), wherein a mixing between the exhaust gas (40) and the one or more cooling streams (90) is minimized or avoided by operating said kiln (MSVK) in a mode in which a cooling cycle interposed between two subsequent alternating heating cycles between the first (100) and the second (200) or the third (300) shaft, is such that the decarbonated materials (50) in at least the first (100), the second (200) and/or the third (300) shaft are cooled with the one or more cooling streams (90) while a supply of the fuel (20) in each shaft is stopped, said process further comprising the steps of :

- transferring thermal energy from the one or more cooling streams (90) extracted from said kiln (MSVK) during the cooling cycle to a portion of a recirculated exhaust gas (41) extracted from said kiln (MSVK) during the latter of the two subsequent alternating heating cycles or another heating cycle subsequent to the cooling cycle and

- supplying said kiln (MSVK) with the portion of a recirculated exhaust gas (41), as heated, during the latter of the two subsequent alternating heating cycles or the other heating cycle subsequent to the cooling cycle.

2. Process according to Claim 1 , further comprising transferring the thermal energy of at least some of the one or more cooling streams (90) to a heat storage medium of a regenerative heat exchanger (4000), said streams exiting from at least one transfer channel (412, 423, 431), in particular the transfer channel (412) between the first (100) and second shaft (200), said heat storage medium being adapted to intermittently stored the thermal energy of said cooling streams (90), during the cooling cycle.

3. Process according to the preceding claim, further comprising feeding the portion of a recirculated exhaust gas (41), as heated into the at least one transfer channel (412, 423, 431), in particular the transfer channel (412) between the first (100) and second shaft (200), during the latter of the two subsequent alternating heating cycles or the other heating cycle subsequent to the cooling cycle, via the regenerative heat exchanger (4000).

4. Process according to the preceding claim, further comprising controlling the flow rate of the portion of a recirculated exhaust gas, in particular supplying said portion intermittently and/or at different flow rates, during the latter of the two subsequent alternating heating cycles or the other heating cycle subsequent to the cooling cycle.

5. Process according to any of the preceding claims, further comprising extracting the recirculated exhaust gas (41) from an upper portion (111 , 211 , 311) of the shaft(s) (100, 200, 300) in regeneration.

6. Process according to any of the preceding claims, further comprising supplying the shaft (100, 200, 300) in combustion with another portion of the recirculated exhaust gas (41) during the latter of the two subsequent alternating heating cycles or the other heating cycle.

7. Process according to any of the preceding claims, wherein each shaft (100, 200, 300) of the (MSVK) multi-shaft vertical kiln comprises supply pipes adapted to supply the at least one fuel (20), in particular fuel lances, said process further comprising, during the latter of the two subsequent alternating heating cycles or the other heating cycle, the step of supplying one or more of the supply pipes of the shaft(s) (200, 100, 300) in regeneration with a further portion of the recirculated exhaust gas (41) so as to cool said supply pipes.

8. Process according to any of the preceding claims, wherein the feeding of the one or more cooling streams (90) in at least one the first (100), the second (200) or third (300) shaft is stopped, during the two subsequent alternating heating cycles or the other heating cycle subsequent to the cooling cycle, or wherein an amount of the one or more cooling streams (90) to be supplied in at least one the first (100), the second (200) or third (300), during the two subsequent alternating heating cycles or the other heating cycle subsequent to the cooling cycle, is controlled in such a manner that the cooling stream amount to be supplied in the at least one the first (100), the second (200) or third (300) shaft, does not exceed 18%, preferably 10%, in volume of the amount of the one or more cooling streams (90) to be supplied during the cooling cycle.

9. Process according to any of the preceding claims, further comprising between the two subsequent alternating heating cycles in the first (100) and the second (200) or the third (300) shaft, depressurizing the first (100) and the second (200) and optionally the third (300) shaft for a predetermined time period before the cooling cycle, preferably the first (100) and the second (200) and optionally the third (300) shaft are depressurized down to reach a level comprised in the range of 1 to 600 mbars, preferably 500 mbars, under the atmospheric pressure.

10. Multi-shaft vertical kiln (MSVK) for decarbonating carbonated materials (10), preferably carbonated mineral, in particular limestone and/or dolomitic limestone, said kiln (MSVK) comprising a first (100), a second (200), and optionally a third shaft (300) with preheating zones (110, 210, 310), heating zones (120, 220, 320) and cooling zones (130, 230, 330) and a transfer (412, 423, 432) channel between each shaft (100, 200, 300), said kiln (MSVK) being arranged for being cooled with one or more cooling streams (90), preferably said kiln (MSVK) being adapted for carrying out the process according to any of the preceding claims, said kiln (MSVK) comprising a regenerative heat exchangers (4000) in fluid communication with an opening formed in at least a wall portion of the or each transfer channel (412, 423, 432).

11. The kiln according to the preceding claim, wherein the regenerative heat exchanger (4000) comprises or consists in a pebble heater.

12. The kiln according to Claim 10, preferably in combination with Claim 11 , wherein the regenerative heat exchanger (4000) further comprises a cleaning circuit adapted to circulate and clean a heat storage medium of the regenerative heat exchanger (4000), in particular pebbles of the pebble heater.

13. The kiln according to any of Claims 10 to 12, wherein the regenerative heat exchanger (4000) is positioned between the first (100) and the second (200) shafts.

14. The kiln according to any of Claims 10 to 13, wherein the multi-shaft vertical kiln (MSVK) has a specific energy consumption of 3.2 to 4.2 GJ per ton, preferably 3.5 to 3.7 GJ per ton of decarbonated materials (50) and/or the multi-shaft vertical kiln (MSVK) has output rate of 30 to 800 tons, preferably 30 to 330 tons of carbonated materials (10) per day.

15. Method of retrofitting a parallel-flow regenerative kiln comprising a first (100), a second (200), and optionally a third shaft (300) with preheating zones (110, 210, 310), heating zones (120, 220, 320) and cooling zones (130, 230, 330) and a transfer (412, 423, 432) channel between each shaft (100, 200, 300), said kiln (MSVK) being arranged for being cooled with one or more cooling streams (90), into a multi-shaft vertical kiln (MSVK) according to any of Claims 10 to 14, further comprising the steps :

- performing an opening in the transfer channel arranged between the first and second shaft; - providing a regenerative heat exchanger (4000), preferably comprising or consisting in a pebble heater;

- fluidly connecting an opening of the regenerative heat exchanger (4000) to the opening in the transfer channel arranged between the first and second shaft ; - integrating a recycle exhaust gas passage arrangement, said arrangement being adapted to recirculate the exhaust gas exiting one of the shafts, while said shaft being in regeneration in use, into another opening of the regenerative heat exchanger (4000) and optionally an opening in another shaft in combustion, while said shaft being in combustion in use; -loading a memory medium of a control unit of said kiln with a computer program comprising instructions which, when the program is executed by the control unit, cause said unit to implement the steps of the process according to one of claims 1 to 9.