Decarbonation process of mineral carbonates in a plurality of kilns

Abstract

The present invention relates to a process for calcining mineral carbonate, in particular limestone and / or dolostone, in a multiple parallel flow regenerative kiln system comprising N parallel flow regenerative kilns, where N is an integer greater than or equal to 2, each parallel flow regenerative kiln, comprising at least two shafts: a first (10) and a second (20) shaft interconnected by a gas transfer channel (30), said process comprising the steps of, controlling the firing mode and the non-firing mode of the N parallel flow regenerative kilns in such a manner that all the N parallel flow regenerative kilns substantially have the same number of cycles over a predetermined time period and storing the common exhaust gas stream in a variable volume reservoir (300).

Description

DECARBONATION PROCESS OF MINERAL CARBONATES IN A PLURALITY OF KILNS Technical Field

[0001] The present invention relates to a process for calcinating mineral carbonates and a multiple parallel flow regenerative kiln system for carrying said process. Background Art

[0002] This invention relates to the optimal integration of a carbon capture unit in a lime plant. Typically, a carbon capture (CC) installation, includes the following modules: a flue gas cleaning unit, a flue gas cooling unit, a CO2separation unit for separating CO2from other gases and CO2purification unit. A carbon capture installation requires a substantially steady flow in order to operate efficiently.

[0003] In the specific case of a parallel flow regenerative lime kiln, the flue gas (a.k.a exhaust gas) production is not steady and follows a specific pattern (e.g. the flue gas flowrate and composition change during a cycle). This pattern is linked to the inherent cyclicity of the parallel flow regenerative kiln operation, with different sequences being carried out one after another. This translates into undesirable variations that are not compatible with the smooth flow requirements of the carbon capture installation.

[0004] The application of a carbon capture installation to a plurality of lime kilns, in particular parallel flow regenerative kilns therefore implies many challenges. Aims of the Invention

[0005] The invention aims to provide a solution to overcome at least one drawback of the prior art.

[0006] More specifically, the invention aims to provide a process and a system for allowing a smooth production of exhaust gas that is suitable for sequestration or use. Furthermore, the measures of the invention allow to obtain a smooth exhaust gas flow, preferably a substantially constant flow. Summary of the Invention

[0007] For the above purpose, the invention is directed to a process for calcining mineral carbonate, in particular limestone and / or dolostone, in a multiple parallel flow regenerative kiln system comprising N parallel flow regenerative kilns, where N is an integer greater than or equal to 2, each parallel flow regenerative kiln comprising at least two shafts: a first and a second shaft interconnected by a gas transfer channel, said process comprising the steps of, for each parallel flow regenerative kiln:- a loading of mineral carbonate at the top of the shafts, - a cooling of calcinated mineral carbonate in a lower end portion of the shafts with a cooling gas, - an unloading of calcined mineral carbonate at the bottom of the shafts, - each shaft operating alternately in a firing mode and in a preheating mode, with one shaft being in the firing mode for a predetermined firing time period F while another shaft is in the preheating mode, and inversely, -the fuel supply being temporally interrupted during a firing suspension mode for a predetermined firing suspension time period NF, while the shafts are swapped from the firing mode to the preheating mode, and inversely, preferably the firing suspension mode successively comprising a burnt-out phase, a reversal phase and a pressurizing phase; -with the firing mode comprising, in the presence of said mineral carbonate, a combustion of fuel in the presence of an oxygen-containing gas so as to obtain a calcination of said mineral carbonate to give calcined mineral carbonate, an emission of exhaust gas generated by the combustion of the fuel and the decarbonation of the mineral carbonate, and a passage of the exhaust gas from the shaft in the firing mode to the other shaft in the preheating mode by means of said gas transfer channel, - the preheating mode comprising a heat exchange between said mineral carbonate and said combustion gas transferred via said gas transfer channel, - the cooling of calcined mineral carbonate comprises a heat exchange between said calcined mineral carbonate and said cooling gas, - said process further comprising the steps of: - optionally combining exhaust gas streams exiting from each parallel flow regenerative kiln to form a common exhaust gas stream, - optionally storing the common exhaust gas stream in a variable volume reservoir, in particular an inflatable gas holder or a membrane gas holder. - controlling the firing mode and the firing suspension mode of the N parallel flow regenerative kilns in such a manner that: - optionally the N parallel flow regenerative kilns have the same or substantially the same number of cycles over a predetermined time period, preferably from 6 to 24 h, in particular a 24-hour period, wherein the ratio between the highest or higher number of cycles over the predetermined time period in any one of the N parallelflow regenerative kilns and the lowest or lower number of cycles over the predetermined time period in any other one of the N parallel flow regenerative kilns is comprised in the range from 1.0 to 1.05, preferably from 1.0 to 1.03, more preferably from 1.0 to 1.01 or is equal to 1.0; and / or - optionally the ratio between the highest or higher number of cycles over the predetermined time period in any one of the N parallel flow regenerative kilns and the lowest or lower number of cycles over the predetermined time period in any other one of the N parallel flow regenerative kilns is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1.0 to 1.05, even more preferably from 1.0 to 1.03, in particular from 1.0 to 1.01, or is equal to 1.0; and / or - optionally the ratio between the longest cycle duration in any one of the N parallel flow regenerative kilns and the shortest cycle duration in the one or any other one of the N parallel flow regenerative kilns, is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1 to 1.05, in particular is equal to 1.0, over the predetermined time period, wherein the cycle duration C represents a time window covering the firing mode and the subsequent firing suspension mode; and / or - optionally the N parallel flow regenerative kilns have the same cycle duration C or substantially the same cycle duration C over the predetermined time period, wherein the cycle duration C represents a time window covering the firing mode and the subsequent firing suspension mode, wherein the ratio between the longest cycle duration in any one of the N parallel flow regenerative kilns and the shortest cycle duration in the one or any other one of the N parallel flow regenerative kilns, is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1 to 1.05, in particular is equal to 1.0, over the predetermined time period; and / or -optionally the average of the number of cycles per day over the predetermined time period for each of the N parallel flow regenerative kilns is comprised in a range from 80 to 130 cycles per day, in particular from 85 to 125 cycles per day, in particular from 90 to 120 cycles per day; and / or - optionally at least two of the N parallel flow regenerative kilns are operated in a predefined desynchronized manner from one another.

[0008] According to specific embodiments of the invention, the process for calcining mineral carbonate comprises one or more of the following step(s) / feature(s): Controlling at least one of -a rotation speed and a pitch of a blade or vane of a fluid displacement device, inparticular a fan device, preferably arranged downstream from the variable volume reservoir, and / or -an opening of a valve, preferably arranged downstream from the variable volume reservoir, so that a variation in the CO2 concentration in the common exhaust gas stream and / or a variation in the flow rate of the common exhaust gas stream volume are(is) minimized, in particular the variable volume reservoir not being emptied and / or the volume of the variable volume reservoir not exceeding a preset threshold, preferably during the predetermined time period; the maximal volume of the variable volume reservoir is at least equal tofactor) = 2 in case N is equal to 2 or 3, or CF = N-1 in case N is higher than 3,preferably lower than 1.2, wherein represents the total exhaust gas volume displaced during the firing time period F of a reference kiln selected among the N parallel flow regenerative kilns and represents the total exhaust gas volume displaced during the firing suspension time period NF of the reference kiln, wherein the reference kiln is one of the N parallel flow regenerative kilns with either the largest total exhaust gas volume displaced, the larger total exhaust gas volume displaced, one of the largest total volumes displaced, one of the larger total volumes displaced or one of the identical total volumes displaced among the N parallel flow regenerative kilns during the firing time period F and the firing suspension time period NF, in particular on average over the predetermined time period; the firing mode and the subsequent firing suspension mode of the N parallel flow regenerative kilns are controlled in such a manner that at least one offset, in particular each offset, between two firing modes directly adjacent to one another, is equal to or is comprised in the range from 0.9 to 1.1 preferably from1.05, more preferably when N; the firing mode and the subsequent firing suspension mode of the N parallel flow regenerative kilns are controlled in such a manner that at least one offset, in particular each offset, between two firing modes directly adjacent to one another, is equal to or is comprised in the range from 0.9 to 1.1 preferably from 0.95 to 1.05; filtering each exhaust gas stream in a filter unit;cooling the common exhaust gas stream in at least one cooling unit, and optionally removing H2O from the common exhaust gas stream in said unit; further comprising removing N2 and 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 from the common exhaust gas stream in one or more CO2separation units selected from the group comprising a membrane separation unit, a cryogenic separation unit, a desublimation separation unit, a low temperature distillation unit, a pressure swing absorption unit, a potassium salt wash unit and an amine wash unit.

[0009] The invention is also related to a multiple parallel flow regenerative kiln system, preferably adapted to carry out the process according to the invention, said system comprising N parallel flow regenerative kilns with N being greater or equal to 2, for calcinating mineral carbonate, in particular limestone and / or dolostone, each parallel flow regenerative kiln comprising: - at least two shafts interconnected by a gas transfer channel, with each one of said shafts comprising: - at least one device for supplying fuel, - at least one supply of an oxygen-containing gas for the combustion of the fuel, - an inlet for the loading of mineral carbonate, - an outlet for the unloading of said calcined mineral carbonate produced, and - a exhaust passage for removal of the exhaust gas; wherein said system further comprises: - a common exhaust passage arranged downstream from the exhaust passage of the N parallel flow regenerative kilns; - a gas holder, preferably a variable volume reservoir, in particular an inflatable gas holder or a membrane gas holder, arranged downstream from the common exhaust passage.

[0010] According to specific embodiments of the invention, the multiple parallel flow regenerative kiln system comprises one or more of the following feature(s): the maximal volume of the variable volume reservoir is at least equal to.. and lower thancase N is equal to 2 or 3, or CF = N-1 in case N is higher than 3, preferably lower than 1.2wherein represents the total exhaust gas volume displaced during the firing time period F of a reference kiln selected among the Nparallel flow regenerative kilns and represents the total exhaust gas volume displaced during the firing suspension time period NF of the reference kiln, wherein the reference kiln is one of the N parallel flow regenerative kilns with either the largest total exhaust gas volume displaced, the larger total exhaust gas volume displaced, one of the largest total volumes displaced, one of the larger total volumes displaced or one of the identical total volumes displaced among the N parallel flow regenerative kilns during the firing time period F and the firing suspension time period NF, in particular on average over the predetermined time period; the predetermined time period being from 6 to 24 h; the predetermined time period being a 24-hour period; at least one of a fluid displacement device, in particular a fan device and / or an opening of a valve, preferably arranged downstream from the variable volume reservoir, to control an exhaust gas flow rate downstream form the variable volume reservoir; a filter unit disposed in each exhaust passage; at least one cooling unit, in particular a heat exchanger, more preferably a condenser, disposed in the common exhaust passage; the one or more CO2separation units being selected from the group comprising a membrane separation unit, a cryogenic separation unit, a desublimation separation unit, a low temperature distillation unit, a pressure swing absorption unit, a potassium salt wash unit and an amine wash unit; the variable volume reservoir being arranged upstream from the one or more CO2 separation units; the multiple parallel flow regenerative kiln system is configured such as the N parallel flow regenerative kilns have the same or substantially the same number of cycles over the predetermined time period, wherein the ratio between the highest or higher number of cycles over the predetermined time period in any one of the N parallel flow regenerative kilns and the lowest or lower number of cycles over the predetermined time period in any other one of the N parallel flow regenerative kilns is comprised in the range from 1.0 to 1.05, preferably from 1.0 to 1.03, more preferably from 1.0 to 1.01, or is equal to 1.0; the ratio between the highest or higher number of cycles over the predeterminedtime period in any one of the N parallel flow regenerative kilns and the lowest or lower number of cycles over the predetermined time period in any other one of the N parallel flow regenerative kilns is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1.0 to 1.05, even more preferably from 1.0 to 1.03, in particular from 1.0 to 1.01, or is equal to 1.0; the multiple parallel flow regenerative kiln system is configured such as the N parallel flow regenerative kilns have the same cycle duration C or substantially the same cycle duration C over the predetermined time period, wherein the cycle duration C represents a time window covering the firing mode and the subsequent firing suspension mode, wherein the ratio between the longest cycle duration in any one of the N parallel flow regenerative kilns and the shortest cycle duration in the one or any other one of the N parallel flow regenerative kilns, is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1 to 1.05, in particular is equal to 1.0, over the predetermined time period; the multiple parallel flow regenerative kiln system is configured such as the average of the number of cycles per day over the predetermined time period for each of the N parallel flow regenerative kilns is comprised in a range from 80 to 130 cycles per day, in particular from 85 to 125 cycles per day, in particular from 90 to 120 cycles per day; the multiple parallel flow regenerative kiln system is configured such as at least two of the N parallel flow regenerative kilns are operated in a predefined desynchronized manner from one another.

[0011] The measures of the present invention allow that a carbon capture installation benefits from a scaling effect (lower specific CAPEX and OPEX) as it is deployed on multiple kilns (a.k.a. kiln battery). In another words, an unique installation is preferred compared to a set of several carbone capture installations dedicated for each of the kilns. Moreover, the measures of the invention permit a lower variability in the exhaust gas flow, thereby allowing to work more often close to the design capacity. Said measures lower the capital cost per unit of CO2treated. In another words, it is not necessary anymore to select an unduly large installation, that would have been otherwise needed to cope with larger peak flow. Brief Description of Drawings

[0012] Aspects of the invention will now be described in more details with reference to the appended drawings, wherein same reference numerals illustrate same features.

[0013] Figure 1 illustrates kiln operation phases for 90 and 120 cycles per day.

[0014] Figure 2 illustrates flow variations over several cycles and the volume requirement for a variable volume reservoir for a single kiln in operation.

[0015] Figures 3A and 3B illustrate flow variations over several cycles and the optimization of the buffer volume for a group of two kilns in operation with a first phasing and a second phasing.

[0016] Figure 4 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of three kilns in operation.

[0017] Figure 5 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of four kilns in operation.

[0018] Figure 6 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of five kilns in operation.

[0019] Figure 7 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of six kilns in operation.

[0020] Figure 8 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of seven kilns in operation.

[0021] Figure 9 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of eight kilns in operation.

[0022] Figure 10 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of six kilns in operation with a constant offset.

[0023] Figure 11 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of eight kilns in operation with a variable offset.

[0024] Figure 12 illustrates buffer volume variations as well as input flow and output flow.

[0025] Figure 13 shows a multiple parallel flow regenerative kiln system according to an embodiment of the invention.

[0026] List of reference symbols K#1,K#2, K#3, K#4 Parallel flow regenerative kiln(s) K#5, K#6, K#7,K#8 CPU CO2purification unit 10,20 1st, 2ndshafts 30 Transfer channel50 Exhaust passages 90 Common exhaust passage 100 Filter(s) 200 Cooling unit, Heat exchanger(s) 300 Variable volume reservoir 400 Fluid displacement device, in particular a fan device 500 CO2Separation unit C Cycle time / duration is the time taken for a (full) cycle, wherein the (full) cycle comprises a pressurising time, a firing time, a burn-out time and a reversal time P Pressurising time is time needed to re-install the (full) air flow inside the kiln after the reversal time B Burn-out time is the time without fuel (but with air flow) between the firing time and the reversal time R Reversal time is the time for the reversal sequence, without any flow F Firing time (period) is the time when fuel is injected in the shaft in combustion NF Firing suspension mode comprises the burn-out time, the reversal time and the pressurising time S Start of a cycle is the time 0 of the cycle Detailed description

[0027] The present invention will now be described in details with reference to the accompanying drawings and their reference numbers, in which illustrative and non- limitative embodiments of the invention are shown.

[0028] Traditionally, the production settings of a battery of parallel flow regenerative kilns are based on the production rate determination for each individual kiln. It appears that the production rate per day (a.k.a. over the 24-hour period) for each parallel flow regenerative kiln varies as the kilns do not have the same size. Also, the production is tailored to cope with the overall demand that fluctuates, depending on the client demand and / or seasonal changes. The number of cycles per day for each parallel flow regenerative kiln are then determined on the basis of the production rates per day. As a consequence, the number of cycles per day for each parallel flow regenerative kiln generally varies significantly from one kiln to another. It has been discovered that to ensure a steady exhaust stream compatible with an optimized CAPEX, the number of cycles per day for each parallel flow regenerative kiln should be first set and then the production rate per day for each parallel flow regenerative kiln is then defined. Thanks to this new approach the number of cycles per day for each parallel flow regenerative kiln can be set such that it is substantially identical for all kilns and that the number of cycles per day is set in an optimal range of 80 to 130, preferably 85 to 125, in particular 90 to120. If the number of cycles per day is higher than around 120, the operation of the kiln is deteriorated as the incompressible dead production period, required to ensure the non- firing time NF (Firing suspension mode) should not exceed 25% kiln utilization. On the other hand, if the number of cycles per day is lower than around 90, the operation of the kiln is deteriorated as the cycle duration (a.k.a. cycle time) becomes too long between 2 shafts, some hysteresis pattern can be observed causing large variation in the exhaust stream not only in terms of flow rate but also of gas composition and temperature. When the number of cycles during for instance a 24-hour period for each parallel flow regenerative kiln is set to be identical or substantially identical, the exhaust steam generated by the kilns can be balanced if they are operated with an adapted offset pattern. While a skilled person would aim to achieve kilns with identical or substantially identical cycles for said period with predefined offsets, it is important to allow for a margin, as one of the kilns may need to be temporarily operated with a different control pattern. To accommodate such circumstances, the ratio between the kiln with the highest number of cycles for the 24-hour period and the kiln with the lowest number of cycles for the 24-hour period is acceptable if it falls within the range [1-1.25], preferably [1-1.1], more preferably [1-1.05], even more preferably [1.0-1.03], in particular [1-1.03], notably [1-1.01]. Even if a preferable duration for the monitoring of the kiln is a 24-hour period, the parameters of the kilns can be defined as equivalent day, and the duration of the monitoring (the predetermined time period) can lengthen or shorten (e.g. 6 -24h).

[0029] In order to reduce to the exhaust flow variation, the exhaust passage of a set of kilns are grouped to form a common exhaust passage connected to a variable volume reservoir, thereby reaching a balancing and a buffering of the common exhaust flow supplied to the exhaust gas treatment unit.

[0030] A further measure to reduce the exhaust flow variation is to ensure that at least two kilns are not synchronized from one another (e.g. all kilns present cycles that are offset from one another).

[0031] Operating all kilns with a substantially identical cycle time combined with an asynchronization of the kiln cycle permits to generate a regular common exhaust flow that balance out the unevenness of the individuals exhaust flow. Moreover, the features of the invention allow for instance to group 4 kilns with 120 cycles per day without having an unfavorable situation where two kilns are in phase or partially in phase. Likewise, the measures of the invention allow to group 5 kilns with 90 cycles per day without having an unfavorable situation where two kilns are in phase or partially in phase.

[0032] Advantageously, a fluid displacement device, in particular a fan device isprovided downstream from the variable volume reservoir. Ideally, the fluid displacement device, in particular a fan device should expand or compress the exhaust gas flow in order to smooth it, preferably to render it substantially constant.

[0033] Preferably, the fluid displacement device, in particular a fan device 400 is activated in order to prevent that the inflatable or membrane gas holder is emptied or does not exceed a reference value. The reference value may ensure a margin with regards to the maximal allowable volume of the variable volume reservoir. In case, the inflatable or membrane gas holder reaches this reference value, the flow rate downstream from the fluid displacement device, in particular a fan device is increased so as to preemptively lower the amount of fluid stored in the tank (e.g. moving away from the nominal or full capacity thereof) and ensure that any common exhaust gas peak can be absorbed by the inflatable or membrane gas holder, without causing some exhaust gas disruption in gas treatment unit 500 arranged downstream. Likewise, the fluid displacement device, in particular a fan device can be controlled through a frequency drive variation speed system so that the variable volume reservoir is not emptied.

[0034] Complementary to or alternatively, a throttle valve provided downstream from the variable volume reservoir can control the exhaust gas flow.

[0035] Figure 1 illustrates kiln operation phases for 90 and 120 cycles per day.

[0036] Figure 2 illustrates flow variations over several cycles and the volume requirement for a variable volume reservoir for a single kiln in operation. The parallel flow regenerative kiln K#1 is operated on a basis of 120 cycles per day, implying a cycle C of 12 minutes. In this cycle of 12 minutes, the firing mode F lasts 9 minutes and the non- firing mode NF lasts 3 minutes. In the non-firing mode NF, there are 3 sub-modes: - pressurizing mode P, with a flow rate exiting the kiln stemming from the combustion comburent flow rate (e.g. combustion air) and cooling air flow rate injected at the top and the bottom of the kiln and CO2generated by heated stones. The flow rated exiting the kiln during the pressurizing mode is estimated to represent around 75% of the flow rate when the kiln is in the firing mode. During the pressurizing mode, no fuel is injected. - burn-out mode, with a flow rate exiting the kiln stemming from the combustion comburent flow rate (e.g. combustion air) and cooling air flow rate injected at the top and the bottom of the kiln and CO2generated by heated stones. The flow rated exiting the kiln during the burn-out mode is estimated to represent 75% of the flow rate when the kiln is in the firing mode. During the burn-out mode, no fuel is injected. - reversal mode, with no flow is present as the combustion comburent flow (e.g. combustion air) and cooling air flow are stopped as well as the fuel, the kiln being set toatmospheric pressure, the CO2generated by heated stones is not collected in the exhaust passage. Therefore, the flow rate exiting the kiln during the reversal mode is estimated to be equal to zero.

[0037] To deliver a balanced and substantially constant flow, a buffer volume (also known as a variable volume reservoir, in particular a inflatable or membrane gas holder) of 10.7% relative to the total exhaust gas volume displaced over a full cycle time C by the kiln is preferred. The buffer volume refers to the maximal volume of a variable volume reservoir. In case several kilns are operated independently with individual CO2 purification units (no common exhaust gas stream), an optimized buffer size for each parallel flow regenerative kiln would also correspond to 10.7% of the total exhaust gas volume displaced over a full cycle time C for each parallel flow regenerative kiln. If all kilns are connected to a common buffer, preferably via a common exhaust passage, one would expect a buffer size of N x 10.7% relative to the cumulative exhaust flow rate, implying a significantly large buffer and significant CAPEX and OPEX. Thanks to the measure of the invention, it is possible to reduce the size of the buffer while operating the kilns and the exhaust gas treatment under optimized conditions.

[0038] Figure 3A illustrates flow variations over several cycles and the optimization of the buffer volume for a group of two kilns in operation. As for the kiln in Figure 2, each parallel flow regenerative kiln K#1, K#2 in Figure 3A is operated on a basis of 120 cycles per day, implying a cycle C of 12 minutes. In this cycle of 12 minutes, the firing mode F lasts 9 minutes and the non-firing mode NF lasts 3 minutes. As for the kiln shown in Figure 2, the non-firing mode (a.k.a. firing suspension mode) NF comprises 3 sub-modes, wherein each parallel flow regenerative kiln is operated with the same air and fuel flow rate hypothesis as in Figure 2. The offset between the firing phases of the first and second kilns is of 7 minutes. The buffer volume recommended in this case, amounts to 4.8% of the total exhaust gas volume displaced over a cycle time C by both kilns. This illustrated that the buffer volume is significantly reduced compared to a situation where two kilns are connected to a separate buffer with a volume of 10.7% each, namely 10.7% of the sum of the total exhaust gas volume displaced by both kilns as described previously.

[0039] It is observed that the starting time S of the cycle of the second kiln should be advantageously positioned 6 minutes after the starting time S of the cycle of the first kiln, to allow to fill enough the buffer (meaning at half of the cycle), as illustrated in Figure 3B. The buffer volume recommended in this case, amounts to 3.6% of the total exhaust gas volume displaced over a cycle time C by both kilns, K#1, K#2, as illustrated in Figure 3B. The required volume for the buffer in Figure 3B is reduced compared to that of Figure3A.

[0040] In case one of the two kilns is stopped for a prolonged period of time, for instance following a client order drop, it is advisable as a precautionary measure to have a buffer dimensioned as if one kiln is operated has a buffer optimized for its operation as for Figure 2. Moreover, the buffer size should be dimensioned according to the flow rate specifications of the one of the two kilns with the bigger capacity.

[0041] It is also observed that phasing the second kiln too late (after half of the cycle) as disclosed in Figure 3A, leads to the need for an optimized buffer with a larger size compared to a situation with regular offset as described previously in Figure 3B. Moreover, starting the second kiln too early, leads to a lack of flow, as the buffer will not be filled enough causing flow disruption.

[0042] Therefore, a balanced filling of the buffer may be advantageously achieved with an even distribution of the offsets and without any partial overlapping of the fire- suspension phases of at least two kilns. However, this control strategy is preferably applicable to a grouping of a limited number of kilns (e.g. the maximal allowable number of kilns is lower than or equal to C / NF, namely the ratio of the cycle time C and the fire- suspension mode duration NF). For instance, with a daily cycle number of 120, consequently a cycle time of 12 minutes and a fire-suspension duration of 3 minutes, a maximum of 4 kilns can be operated with an even distribution of the offsets without any partial overlapping of the fire-suspension phases of at least two kilns.

[0043] Figure 4 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of three kilns in operation. Each parallel flow regenerative kiln K#1, K#2, K#3 is operated on a basis of 120 cycles per day, implying a cycle C of 12 minutes. As for any of the kilns shown in Figure 2, 3A and 3B, the non-firing mode NF comprises 3 sub-modes, wherein each parallel flow regenerative kiln is operated with the same air and fuel flow rate hypothesis as in Figure 2, 3A and 3B. A shift of 4 minutes is set between each consecutive kiln, thereby fulfilling the preferable phasing range criterium. This duration amounts to 1 / 3 of the cycle time C, to avoid that at least two kilns are simultaneous in phase. In this three-kiln configuration, it is observed that the buffer should have a volume of 2.0% of the total exhaust gas volume displaced over the cycle time C by the three kilns, leading to a substantial gain in size.

[0044] Figure 5 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of four kilns in operation. Each parallel flow regenerative kiln K#1, K#2, K#3, K#4 is operated on a basis of 120 cycles per day, implying a cycle Cof 12 minutes. As for any of the kilns shown in any of Figures 2, 3A, 3B and 4, the non- firing mode NF comprises 3 sub-modes, wherein each parallel flow regenerative kiln is operated with the same air and fuel flow rate hypothesis as in any of Figures 2, 3A, 3B and 4. A shift of 3 minutes is set between each consecutive kiln, thereby fulfilling the preferable phasing range criterium. This duration amounts to ¼ of the cycle time C, to avoid that at least two kilns are simultaneous in phase. In this four-kiln configuration, it is observed that the buffer should have a volume of 1.2% of the total exhaust gas volume displaced over the cycle time C by the four kilns, leading to a substantial gain in size.

[0045] Figure 6 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of five kilns in operation. Each parallel flow regenerative kiln K#1, K#2, K#3, K#4, K#5 is operated on a basis of 120 cycles per day, implying a cycle C of 12 minutes. As for any of the kilns shown in any of Figures 2, 3A, 3B, 4 and 5 the non-firing mode NF comprises 3 sub-modes, wherein each parallel flow regenerative kiln is operated with the same air and fuel flow rate hypothesis as in any of Figures 2, 3A, 3B, 4 and 5. Because the number of kilns becomes too large, the preferable phasing range criterium is not applicable. A shift of 3 minutes is set between each consecutive first four kilns forming a group of four kilns K#1, K#2, K#3, K#4. This duration amounts to ¼ of the cycle time C and allows to avoid that these four kilns are not simultaneous in phase with one another. The fifth kiln K#5 is synchronized with the first kiln K#1. In this five-kiln configuration, it is observed that the buffer should have a volume of 2.6% of the total exhaust gas volume displaced over the cycle time C by the five kilns, leading to a substantial gain in size even if a pair of kilns are synchronized with one another.

[0046] Figure 7 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of six kilns in operation. Each parallel flow regenerative kiln K#1, K#2, K#3, K#4, K#5, K#6 is operated on a basis of 120 cycles per day, implying a cycle C of 12 minutes. As for any of the kilns shown in any of Figures 2, 3A, 3B, 4, 5 and 6 the non-firing mode NF comprises 3 sub-modes, wherein each parallel flow regenerative kiln is operated with the same air and fuel flow rate hypothesis as in any of Figures 2, 3A, 3B, 4, 5 and 6. A shift of 3 minutes is set between each consecutive first four kilns forming a group of four kilns K#1, K#2, K#3, K#4. This duration amounts to ¼ of the cycle time C and allows to avoid that these four kilns are not simultaneous in phase with one another. The fifth kiln K#5 is synchronized with the first kiln K#1 and the sixth kiln K#6 is synchronized with the third kiln K#3. In this sixth-kiln configuration, it is observed that the buffer should have a volume of 2.0% of the total exhaust gas volume displaced over the cycle time C by the six kilns, leading to a substantial gain in size even if two pairsof kilns are synchronized with one another.

[0047] Figure 8 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of seven kilns in operation. Each parallel flow regenerative kiln K#1, K#2, K#3, K#4, K#5, K#6, K#7 is operated on a basis of 120 cycles per day, implying a cycle C of 12 minutes. As for any of the kilns shown in any of Figures 2, 3A, 3B, 4, 5, 6 and 7, the non-firing mode NF comprises 3 sub-modes, wherein each parallel flow regenerative kiln is operated with the same air and fuel flow rate hypothesis as in any of Figures 2, 3A, 3B, 4, 5, 6 and 7. A shift of 3 minutes is set between each consecutive first four kilns forming a group of four kilns K#1, K#2, K#3, K#4. This duration amounts to ¼ of the cycle time C and allows to avoid that these four kilns are not simultaneous in phase with one another. The fifth kiln K#5 is synchronized with the first kiln K#1, the sixth kiln K#6 is synchronized with the third kiln K#3 and the seventh kiln K#7 is synchronized with the fourth kiln K#4. In this seven-kiln configuration, it is observed that the buffer should have a volume of 2.5% of the total exhaust gas volume displaced over the cycle C time by the seven kilns, leading to a substantial gain in size even if three pairs of kilns are synchronized with one another.

[0048] Figure 9 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of eight kilns in operation. Each parallel flow regenerative kiln K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8 is operated on a basis of 120 cycles per day, implying a cycle C of 12 minutes. As for any of the kilns shown in any of Figures 2, 3A, 3B, 4, 5, 6, 7 and 8, the non-firing mode NF comprises 3 sub-modes, wherein each parallel flow regenerative kiln is operated with the same air and fuel flow rate hypothesis as in any of Figures 2, 3A, 3B, 4, 5, 6, 7 and 8. A shift of 3 minutes is set between each consecutive first four kilns forming a group of four kilns K#1, K#2, K#3, K#4. This duration amounts to ¼ of the cycle time C and allows to avoid that these four kilns are not simultaneous in phase with one another. The fifth kiln K#5 is synchronized with the first K#1 kiln, the sixth kiln K#6 is synchronized with the third kiln K#3, the seventh kiln K#7 is synchronized with the fourth kiln K#4 and the eighth kiln K#8 is synchronized with the second kiln K#2. In this eight-kiln configuration, it is observed that the buffer should have a volume of 1.2% of the total exhaust gas volume displaced over the cycle time C by the eight kilns, leading to a substantial gain in size even if four pairs of kilns are synchronized with one another.

[0049] The embodiments according to Figures 2, 3A, 3B, 4, 5, 6, 7, 8 and 9 show that an increase in the number of kilns allows to reduce the flow variations providing that at least one phase shift is set between at least two kilns. When the amplitude of the flowvariations is minimized, a smaller buffer volume is needed compared to a situation with no offset. Furthermore, an even cycle spacing of the kilns is recommended. However when the number of kilns becomes too large, corresponding to 5 kilns with a daily cycle number of 120 or 6 kilns with a daily cycle number of 90, a complete offset of the fire- suspension phases cannot be ensured. In this case, three scenarios can be envisaged for a large number of kilns when N > C / NF.

[0050] In a first scenario, at least two kilns can be synchronized while at least a group of 4 or 5 kilns are successively offset with a substantially constant shift (as illustrated in Figures 6 to 9).

[0051] In a second scenario, all kilns are controlled in such a manner that the firing phases are successively offset with a constant or a substantially constant shift. Figure 10 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of six kilns in operation, where all kilns are successively offset with a constant or a substantially constant shift. In this configuration, it is observed that the buffer should have a volume of 0.4% of the total exhaust gas volume displaced over a cycle time C by the six kilns. An even offset of the kilns is particularly advantageous, especially when the number of kilns exceeds five with a daily cycle number of 120. Indeed, flow variations are dampened by the number of kilns and advantageous phasing compensations. For example, a reduced flow (e.g., a burn-out phase in one kiln) is combined with another reduced flow from another kiln (e.g., a pressurization phase).

[0052] In a third scenario, referred to as the intermediate scenario and encompassing other phasing configurations not covered in the first and second scenarios, all kilns are successively offset with a variable shift, without having at least two kilns with concomitant firing phases (synchronized). For instance, Figure 11 illustrates flow variations over several cycles and the optimization of the buffer volume for a group of eight kilns in operation, where all kilns are successively offset with a non-constant shift. In this configuration, it is observed that the buffer should have a volume of 0.6% of the total exhaust gas volume displaced over a cycle time C by the eight kilns.

[0053] Surprisingly, it has been observed that another way to optimize the volume of a buffer is to determine the required volume of the buffer as if it were solely supplied by the largest or larger kiln of the battery in terms of volume of exhaust gas over a predefined period. This scenario applies when the kilns have different capacities. If all kilns connected to the buffer generate the same or substantially the same total exhaust gas volume displaced over the predefined period, the required buffer volume is determined based on this equal volume displaced. If at least two kilns have the largest or larger capacity overthis period, the buffer volume is determined based on one of these with largest or larger capacities. Such an approach still allows for substantial gains in terms of buffer volume but also ensures efficient operation if one or more kilns are shut down.

[0054] As illustrated in Figure 12, a recommended maximal volume for a buffer (a.k.a. variable volume reservoir) can be determined using the following formula that defines a minimal suitable maximal volume for said buffer:If the selected maximal volume is below this minimal threshold, the buffer will operate less efficiently to smooth the flow. If the selected maximal volume is well above this minimal threshold, the exhaust will be indeed smoothed without using the full potential of the buffer, causing undue capital investment. In this formula, represents the total exhaust gas volume displaced during the firing time period of the largest or larger parallel flow regenerative kiln K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8 of the battery if said kiln was operated alone. , NF, and F represent the total exhaust gas volume generated during the firing suspension time period and going to the buffer, the duration of the firing suspension, and the duration of the firing period, respectively, of the largest or larger one of the parallel flow regenerative kilns K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8 of the battery.

[0055] The upper graph in Figure 12 illustrates the evolution of the variable volume of the buffer. The variable volume follows a repetitive pattern with an ascending ramp during the firing phase and a descending ramp during the fire-suspension phase. The minimum volume required for the buffer corresponds to the height of the ramps.

[0056] The minimal required buffer volume is the volume needed to ensure that during the firing suspension period, the buffer has enough capacity to maintain the feeding of the downstream treatment unit at an ideal constant rate . The ideal constant rate is based on the weighted average of the total volumes displaced during the firing period and the non-firing period, as expressed by the formula:, where and represent the average flow rates during the firing phase and the non- firing phase, respectively.

[0057] During the firing phase, the buffer is filled with the exhaust gas that occupies a volume corresponding to the integral of the difference between the exhaust gas streamentering the buffer and the constant flow rate exiting the buffer over the firing period F as illustrated in Figure 12, lower graph. The volume of the exhaust gas accumulated in the buffer corresponds to the area AF. During the non-firing phase, the buffer is emptied. The volume occupied by the exhaust gas discharged corresponds to the integral of the difference between the constant flow rate exiting the buffer and the exhaust gas stream entering the buffer over the non-firing period FN. The volume of the exhaust gas emptied from the buffer corresponds to the area ANF. Even if the above mentioned formulas have been established in a specific context with a periodic constant gas flow rate under a 0D and incompressible flow hypothesis in the buffer, they can be generalized to a more complex time dependent flow pattern (e.g. including periodic fluctuations) within the same hypothesis.

[0058] It should be noted that the total exhaust gas volume displaced produced during the non-firing phase can be established based on the contributions of the burn-out period, the reversal period, and the pressurization period as defined by the following formula: . . . , , where B, R, and P represent the respective durations of the burn-out time, the reversal time, and the pressurization time. Similarly, , , and represent the average flow rates during the burn-out time, the reversal time, and the pressurization time.

[0059] The flow rate is expressed in terms of volumetric flow rate. The teaching of the invention can be also described using mass flow rate.

[0060] Figure 13 shows a multiple parallel flow regenerative kiln system according to an embodiment of the invention. The parallel flow regenerative kiln system comprises two parallel flow regenerative kilns K#1, K#2. Each parallel flow regenerative kiln K#1, K#2 comprises two shafts 10, 20 interconnected by a gas transfer channel 30, wherein each shaft 10, 20 comprises: at least one device for supplying fuel (not illustrated in Figure 13), at least one device (not illustrated in Figure 13) for supply an oxygen-containing gas for the combustion of the fuel, an inlet (not illustrated in Figure 13) for the loading of carbonated materials such as mineral carbonate, and an outlet (not illustrated in Figure 13) for the unloading of decarbonated material such as calcined mineral carbonate produced, an exhaust passage 50 for removal of the exhaust gas (a.k.a. flue gas). The parallel flow regenerative kiln system further comprises a common exhaust passage 90 arranged downstream from the exhaust passages 50 of the two parallel flow regenerative kilns K#1, K#2.

[0061] In Figure 13, the parallel flow regenerative kiln system comprises at least one filter unit 100 adapted to remove the dusts contained in the exhaust gas exiting the parallel flow regenerative kilns K#1, K#2. Typically, the filter unit 100 comprises or consists in a bag filter and is foreseen for each parallel flow regenerative kiln K#1, K#2. Alternatively, a common filter 100 is foreseen for both kilns K#1, K#2. This alternative solution is not illustrated in Figure 13.

[0062] Furthermore, the exhaust gas is then cooled in a cooling unit 200, in particular a common heat exchanger. Alternatively, a cooling unit 200 is foreseen for each parallel flow regenerative kiln K#1, K#2. This alternative solution is not illustrated in Figure 13. Typically, a cooling unit 200 comprises or consists in one or more elements selected from the group: cooling tower, direct contact cooler and condenser. The use of a colling unit allows to remove the water present in the common exhaust gas stream, as a decrease of the temperature exhaust stream leads to the condensation the water vapor of the exhaust stream.

[0063] The exhaust gas stream is then fed to a variable volume reservoir 300, in particular an inflatable gas holder or a membrane gas holder in which the exhaust gas is stored when the exhaust gas stream flow rate is above it nominal value.

[0064] A fluid displacement device, in particular a fan device 400 is provided downstream from the variable volume reservoir 300. Preferably, the fluid displacement device, in particular a fan device 400 is activated in order to maintain the fumes flow as constant as possible, to prevent that the inflatable or membrane gas holder 400 does not exceed a reference value, for instance 90% of the maximal allowable volume, in particular 100% of the maximal allowable volume. In case, the inflatable or membrane gas holder reaches its maximal allowable volume, flow rate downstream from the fluid displacement device, in particular a fan device increase, in the way to minimize the disruption in gas treatment unit 500 arranged downstream.

[0065] The exhaust gas stream is then fed in a CO2separation unit 500. Typically, the CO2separation unit 500 is selected from the group comprising a membrane separation unit, a cryogenic separation unit, a desublimation separation unit, a low temperature distillation unit, a pressure swing absorption unit, a potassium salt wash unit and an amine wash unit. The CO2separation unit 500 comprises an exhaust gas inlet in fluid communication with the common exhaust passage 90, a first outlet for conveying a CO2enriched gas stream and a second outlet for conveying a CO2depleted gas stream. Although there are different technologies that can be used for the CO2separation, a CO2membrane separation unit 500 is illustrated in the following paragraph.

[0066] The membrane structure of a CO2membrane separation unit 500 preferably comprises a plurality of inner tubes with porous walls allowing molecules such as CO2to pass through the porous walls with a higher probability than N2 molecules (Figure 13). In other words, CO2 molecules have a higher penetration rate than N2 molecules. The gas molecule penetration rate depends on their size and diffusivity. Membrane separation technology requires a partial pressure difference, that can be achieved by increasing the pressure of the gas to be separated and / or decreasing the pressure of the gas separated. For this purpose, a compression element, comprising at least one compressor can be foreseen in the common exhaust passage 90 or can be integrated in the membrane separation unit 500.

[0067] The embodiment in Figure 13 shows one separation unit 500. Alternatively, a plurality of separation units 500 arranged in series or parallel can be implemented depending on the circumstances.

[0068] Typically, a given separation unit 500 is adapted to separated N2and 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.

[0069] The parallel flow regenerative kiln system in Figure 13 further comprises a CO2 purification unit (CPU) that is configured to remove from the concentrated CO2 stream produced by CO2 separation unit (500) 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. Preferably, the CO2 purification unit (CPU) is adapted to adjust the composition of the CO2 purified stream 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). Typically, the CPU relies on the liquefaction of the CO2enriched gas stream and the distillation of impurities. It should be noted that certain CO2separation technology such as desublimation separation technology, can perform the step of separation and purification all in one.

[0070] The control of a parallel flow regenerative kiln is dependent on not only the data regarding its own sequence timings but also on the data of the other(s) kiln(s), in particular timing sequence data.

[0071] The oxygen-containing gas is selected from the group comprising air, a substantially pure oxygen, or a mixture of them.

[0072] The meaning of ^substantially pure oxygen^ in the present disclosure is an oxygen gas comprising at least 90 % (dry volume) dioxygen (i.e. O2), preferably at least 95% (dry volume) dioxygen (i.e. O2).

[0073] The meaning of ^CO2 enriched^ in the present disclosure is a gas containing at least 19% CO2 dry volume, preferably at least 35 % CO2 dry volume, more preferably at least 50 % CO2 dry volume, most preferably at least 60 % CO2 dry volume, in particular at least 70% dry volume, notably at least 80% dry volume.

[0074] By the expression ^firing suspension mode^ is meant a phase that comprise a reversal phase, a burnout phase prior to said reversal phase and a pressurizing phase after the said reversal phase. The burnout phase starts with the fuel supply interruption and allows the unburned fuel to react with the comburent (e.g. air). During the reversal phase, a series of open / closing comburent and fuel valves is performed so that the comburant / fuel supply is shifted from one shaft to the other and the kiln pressure is back to atmospheric pressure. During the pressurizing phase, the kiln is closed again and the combustion and cooling air are injected into the kiln to pressurize it and re-install the correct air flow to be able to re-start the fuel injection again. Generally, the feeding with the rocks is performed during the reversal phase.

[0075] By ^firing time period^ is meant the programmed or regulated duration of a firing during a given cycle. This duration can be sorted in a storing media and retrieved by the electronic computer unit controlling of one or more of the parallel flow regenerative kilns. This duration can vary depending on predefined parameters such as the output rate and / or CO2 content extracted.

[0076] Equally, the duration of the fire-suspension mode can be predetermined. This duration can be sorted in a storing media and retrieved by the electronic computer unit controlling of one or more of the parallel flow regenerative kilns. This duration can vary depending on predefined parameters such as the output rate and / or CO2content extracted.

[0077] The embodiment of the disclosure presents a system with two parallel flow regenerative kilns. The present teaching also applies to a system with three, four and more kilns, wherein N, the number of parallel flow regenerative kilns is lower than 20, preferably lower that 16, in particular lower than 8.

[0078] The present embodiment of the disclosure shows parallel flow regenerative kilns with two shafts. The present teaching also applies to multi-shaft vertical kilns with three shafts.

[0079] By mineral carbonate is meant a material selected from the list comprising limestone, dolostone, carbonated hydrated lime, carbonated lime and any combination thereof.

[0080] In the present disclosure, the term ^cycle duration^ or ^cycle time^ relates a complete cycle covering the firing mode and the subsequent firing suspension mode, within a predetermined time period.

[0081] By a predetermined time period is meant a time period from 6 to 24 h, in particular a 24-hour period.

[0082] In the present disclosure, by the term ^number of cycles per day^ is meant the number of cycles starting S in the predetermined time period, multiplied by the ratio between 24 hours and the duration of the predetermined time period.

[0083] By ^multiple parallel flow regenerative kiln system comprising N parallel flow regenerative kilns^ is meant a system comprising N parallel flow regenerative kilns in operation and grouped to supply a given (variable volume) reservoir. The plant where said kilns are based may comprise another or other parallel flow regenerative kilns that are either grouped to supply another variable volume reservoir, one or more stacks, or one or more exhaust treatment systems, or operated individually.

[0084] A variable volume reservoir according to the present disclosure preferably consists of or comprises an inflatable gas holder or a membrane gas holder expanding or contracting depending on the amount of exhaust gas stored therein. Alternatively, the variable volume reservoir may consist of or comprise a bellows, a bladder reservoir or equivalent. Also a variable volume reservoir may comprise a series of fixed volume reservoirs interconnected by valves, wherein an adapted control of the valves allow a step-wise filing of the fixed volume reservoirs. The variable volume reservoir (e.g. inflatable or membrane gas holder) may be exposed to the atmospheric pressure. This solution is preferred when the exhaust gas pressure inside is close to the atmospheric pressure. Alternatively, the variable volume reservoir can take place within a gas storing chamber in which the variable volume reservoir is disposed. This solution is preferred when the exhaust gas pressure is pressurized compared to the atmospheric pressure. A variable volume reservoir can be passively controlled to the extent that the variable volume is a function of the elastic properties of a gas containing element selected from the list comprising: flexible wall(s), a membrane, bladder, bellows, one or more resilient means acting on at least one slidable wall, or any combination thereof. In case of a gas storing chamber, the pressure exerted by the gas present in the gas storing chamber alsoinfluences the displacement of the gas containing element. But, in a passive control, the pressure, such as atmospheric pressure would not be controlled (e.g. inflatable or membrane gas holder exposed to air). Alternatively, the control of the gas pressure within the gas storing chamber can be actively controlled to the extent that at least one actuator influences the displacement of at least a portion of the gas containing element (e.g. sliding wall(s)). Equally the pressure exerted on the gas containing element can be controlled. Alternatively, a hydraulic storing chamber can be used instead of the gas storing chamber, with the gas being replaced by hydraulic fluid. A solution with a variable volume reservoir is preferred for the present invention as it allows to smooth efficiently the common exhaust gas stream downstream the reservoir. Alternatively, a fixed volume reservoir can be foreseen. However such a solution may require a more complex design and / or control of the fan and / or valve(s) that should be adapted to control larger flow and / or pressure variation. Even additional valve(s) and / or fan(s) may be foreseen upstream from the reservoir to compensate for flow variations.

[0085] The variable volume reservoir can comprise one chamber or a series of chambers fluidly interconnected (e.g. in series and / or in parallel).

[0086] By ^the maximal volume of the variable volume reservoir^ is meant the volume for storage of the variable volume reservoir when said reservoir (e.g. inflatable and / or membrane gas holder) is in its maximal deployment capacity in operation and / or the reservoir (e.g. inflatable and / or membrane gas holder) is restricted by the volume defined by its gas containing element (e.g. reaching its maximal allowable volume (ex: stroke for a sliding element), for instance as defined by a manufacturer.

[0087] The ^total exhaust gas volume displaced^ is determined at the inlet of the variable volume reservoir, ensuring that the pressure and temperature are the same or substantially the same at the inlet of the variable volume reservoir and inside the cavity delimited by the variable volume reservoir, assuming e.g. adiabatic conditions (no heat transfer) and that the kinetic energy of the exhaust gas flow at the inlet of the variable volume reservoir is negligible compared to its enthalpy. The total exhaust gas volume displaced can be directly measured or estimated at the inlet of the variable volume reservoir with for instance a volume flow rate measurement, or indirectly estimated based on the exhaust gas flow exiting the N parallel flow regenerative kilns, the evolution of the exhaust gas composition (e.g. condensation cause water removal), as well as the temperatures and pressures determined at the exit of the kilns and at the inlet of the variable volume reservoir, to account for the pressure and temperature variations in the exhaust gas streams or the common exhaust stream in the exhaust passages andcommon exhaust passage. The ^total exhaust gas volume displaced^ of given kiln can be determined at the inlet of the variable volume reservoir assuming that the other or all other kilns connected to said reservoir are(is) not in operation.

[0088] Embodiments as discussed above are defined by the following numbered clauses: 1. A process for calcining mineral carbonate, in particular limestone and / or dolostone, in a multiple parallel flow regenerative kiln system comprising N parallel flow regenerative kilns, where N is an integer greater than or equal to 2, each parallel flow regenerative kiln (K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8), comprising at least two shafts: a first (10) and a second (20) shaft interconnected by a gas transfer channel (30), said process comprising the steps of, for each parallel flow regenerative kiln (K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8): - a loading of mineral carbonate at the top of the shafts (10, 20), - a cooling of calcinated mineral carbonate in a lower end portion of the shafts with a cooling gas ; - an unloading of calcined mineral carbonate at the bottom of the shafts (10, 20), - each shaft operating alternately in a firing mode and in a preheating mode, with one shaft (10, 20) being in the firing mode for a predetermined firing time period F while another shaft (10, 20) is in the preheating mode, and inversely, -the fuel supply being temporally interrupted during a firing suspension mode for a predetermined firing suspension time period NF, while the shafts (10, 20) are swapped from the firing mode to the preheating mode, and inversely, preferably the firing suspension mode successively comprising a burnt out phase, a reversal phase and a pressurizing phase; -with the firing mode comprising, in the presence of said mineral carbonate, a combustion of fuel in the presence of an oxygen-containing gas so as to obtain a calcination of said mineral carbonate to give calcined mineral carbonate, an emission of exhaust gas generated by the combustion of the fuel and the decarbonation of the mineral carbonate, and a passage of the exhaust gas from the shaft in the firing mode to the other shaft (10, 20) in the preheating mode by means of said gas transfer channel (30), - the preheating mode comprising a heat exchange between said mineral carbonate and said combustion gas transferred via said gas transfer channel (30), - the cooling of calcined mineral carbonate comprises a heat exchange between said calcined mineral carbonate and said cooling gas;- said process further comprising the steps of: - optionally combining exhaust gas streams exiting from each parallel flow regenerative kiln (K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8) to form a common exhaust gas stream; - optionally storing the common exhaust gas stream in a gas holder, preferably a variable volume reservoir (300), in particular an inflatable gas holder or a membrane gas holder; - controlling the firing mode and the firing suspension mode of the N parallel flow regenerative kilns in such a manner that -optionally the N parallel flow regenerative kilns have the same or substantially the same number of cycles over a predetermined time period, preferably from 6 to 24 h, in particular a 24-hour period, wherein the ratio between the highest or higher number of cycles over the predetermined time period in any one of the N parallel flow regenerative kilns and the lowest or lower number of cycles over the predetermined time period in any other one of the N parallel flow regenerative kilns is comprised in the range from 1.0 to 1.05, preferably from 1.0 to 1.03, more preferably from 1.0 to 1.01, or is equal to 1.0; -optionally the ratio between the highest or higher number of cycles over the predetermined time period in any one of the N parallel flow regenerative kilns and the lowest or lower number of cycles over the predetermined time period in any other one of the N parallel flow regenerative kilns is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1.0 to 1.05, even more preferably from 1.0 to 1.03, in particular from 1.0 to 1.01, or is equal to 1.0; - optionally the N parallel flow regenerative kilns have the same cycle duration C or substantially the same cycle duration C over the predetermined time period, wherein the cycle duration C represents a time window covering the firing mode and the subsequent firing suspension mode, wherein the ratio between the longest cycle duration in any one of the N parallel flow regenerative kilns and the shortest cycle duration in the one or any other one of the N parallel flow regenerative kilns, is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1.0 to 1.05, in particular is equal to 1.0, over the predetermined time period; - optionally the ratio between the longest cycle duration in any one of the Nparallel flow regenerative kilns and the shortest cycle duration in the one or any other one of the N parallel flow regenerative kilns, is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1.0 to 1.05, in particular is equal to 1.0, over the predetermined time period, wherein the cycle duration C represents a time window covering the firing mode and the subsequent firing suspension mode; - optionally the average of the number of cycles per day over the predetermined time period for each of the N parallel flow regenerative kilns is comprised in a range from 80 to 130 cycles per day, in particular from 85 to 125 cycles per day, in particular from 90 to 120 cycles per day; and / or - optionally at least two of the N parallel flow regenerative kilns are operated in a predefined desynchronized manner from one another. 2. The process according to clause 1, further comprising controlling: - at least one of a rotation speed and a pitch of a blade or vane of a fluid displacement device, in particular a fan device (400), preferably arranged downstream from the variable volume reservoir (300) and / or -an opening of a valve, preferably arranged downstream from the variable volume reservoir (300), so that a variation in the CO2 concentration in the common exhaust gas stream and / or a variation in the flow rate of the common exhaust gas stream volume are(is) minimized, in particular, the variable volume reservoir not being emptied and / or the volume of the variable volume reservoir not exceeding a preset threshold, preferably during the predetermined time period. 3. The process according to any one of the preceding clauses, wherein the maximal volume of the variable volume reservoir (300) is at least equal tolower than , where CF (correction factor) = 2 in case N is equal to 2 or 3, or CF = N-1 in case N is higher than 3, preferably lowerthan 1.2, wherein represents the total exhaust gas volume displaced during the firing time period F of a reference kiln selected among the N parallel flow regenerative kilns (K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8) and represents the total exhaust gas volume displaced during the firing suspension time period NF of the reference kiln, wherein the reference kiln is one of the N parallel flow regenerative kilns (K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8) with either the largest total exhaust gas volume displaced, the larger total exhaust gas volume displaced, one of the largest total volumesdisplaced, one of the larger total volumes displaced or one of the identical total volumes displaced among the N parallel flow regenerative kilns (K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8) during the firing time period F and the firing suspension time period NF, in particular on average over the predetermined time period. 4. The process according to any one of the preceding clauses, wherein the firing mode and the subsequent firing suspension mode of the N parallel flow regenerative kilns are controlled in such a manner that at least one offset, in particular each offset, between two firing modes directly adjacent to one another either: is equal to or is comprised in the range from 0.9 to 1.1 preferably from 0.95 to 1.05, more preferably when N , or is equal to or is comprised in the range from 0.9 to 1.1 preferably from 0.95 to 1.05; 5. The process according to any one of the preceding clauses, further comprising filtering each exhaust gas stream in a filter unit (100). 6. The process according to any one of the preceding clauses, further comprising cooling the common exhaust gas stream in at least one cooling unit (200), and optionally removing H2O from the common exhaust gas stream in said unit (200). 7. The process according to any one of the preceding clauses, further comprising removing N2and 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 from the common exhaust gas stream in one or more CO2 separation units (500) selected from the group comprising a membrane separation unit, a cryogenic separation unit, a desublimation separation unit, a low temperature distillation unit, a pressure swing absorption unit, a potassium salt wash unit and an amine wash unit. 8. A multiple parallel flow regenerative kiln system, preferably adapted to carry out the process according to any of the preceding clauses, said system comprising N parallel flow regenerative kilns (K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8) with N being greater or equal to 2, for calcinating mineral carbonate, in particular limestone and / or dolostone, each parallel flow regenerative kiln comprising: - at least two shafts (10, 20) interconnected by a gas transfer channel (30), with each one of said shafts (10, 20) comprising:- at least one device for supplying fuel, - at least one supply of an oxygen-containing gas for the combustion of the fuel, - an inlet for the loading of mineral carbonate, - an outlet for the unloading of said calcined mineral carbonate produced, and - a exhaust passage (50) for removal of the exhaust gas; wherein said system further comprises: - a common exhaust passage (90) arranged downstream from the exhaust passage (50) of the N parallel flow regenerative kilns (K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8); - a gas holder, preferably a variable volume reservoir (300), in particular an inflatable gas holder or a membrane gas holder, arranged downstream from the common exhaust passage (90). 9. The system according to clause 8, wherein the maximal volume of the . . variable volume reservoir (300) is at least equal to and lower thancase N is equal to 2 or 3,or CF = N-1 in case N is higher than 3 preferably lower than 1.2, wherein represents the total exhaust gas volume displaced during the firing time period F of a reference kiln selected among the N parallel flow regenerative kilns (K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8) and represents the total exhaust gas volume displaced during the firing suspension time period NF of the reference kiln, wherein the reference kiln is one of the N parallel flow regenerative kilns (K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8) with either the largest total exhaust gas volume displaced, the larger total exhaust gas volume displaced, one of the largest total volumes displaced, one of the larger total volumes displaced or one of the identical total volumes displaced among the N parallel flow regenerative kilns (K#1, K#2, K#3, K#4, K#5, K#6, K#7, K#8) during the firing time period F and the firing suspension time period NF , in particular on average over the predetermined time period. 10. The system according to any one of clauses 8 to 9, further comprising at least one of a fluid displacement device, in particular a fan device (400) and / or an opening of a valve, preferably arranged downstream from the variable volume reservoir (300), to control an exhaust gas flow rate downstream form the variable volume reservoir (300). 11. The system according to any one of clauses 8 to 10, comprising a filter (100) unit disposed in each exhaust passage (50). 12. The system according to any one of clauses 8 to 11, comprising at least one cooling unit (200), in particular a heat exchanger, more preferably a condenser,disposed in the common exhaust passage (90). 13. The system according to any one of clauses 8 to 12, comprising one or more CO2separation units (500) selected from the group comprising a membrane separation unit, a cryogenic separation unit, a desublimation separation unit, a low temperature distillation unit, a pressure swing absorption unit, a potassium salt wash unit and an amine wash unit, preferably the variable volume reservoir (300) being arranged upstream from the one or more CO2 separation units (500). 14. The system according to any one of clauses 8 to 13, wherein the multiple parallel flow regenerative kiln system is configured such as the N parallel flow regenerative kilns have the same or substantially the same number of cycles over the predetermined time period, wherein the ratio between the highest or higher number of cycles over the predetermined time period in any one of the N parallel flow regenerative kilns and the lowest or lower number of cycles over the predetermined time period in any other one of the N parallel flow regenerative kilns is comprised in the range from 1.0 to 1.05, preferably from 1.0 to 1.03, more preferably from 1.0 to 1.01, in particular is equal to 1.0. 15. The system according to any one of clauses 8 to 14, wherein the multiple parallel flow regenerative kiln system is configured such as the ratio between the highest or higher number of cycles over the predetermined time period in any one of the N parallel flow regenerative kilns and the lowest or lower number of cycles over the predetermined time period in any other one of the N parallel flow regenerative kilns is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1.0 to 1.05, even more preferably from 1.0 to 1.03, in particular from 1.0 to 1.01, or is equal to 1.0. 16. The system according to any one of clauses 8 to 15, wherein the multiple parallel flow regenerative kiln system is configured such as the N parallel flow regenerative kilns have the same cycle duration C or substantially the same cycle duration C over the predetermined time period, wherein the cycle duration C represents a time window covering the firing mode and the subsequent firing suspension mode, wherein the ratio between the longest cycle duration in any one of the N parallel flow regenerative kilns and the shortest cycle duration in the one or any other one of the N parallel flow regenerative kilns, is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1 to 1.05, in particular is equal to 1.0, over the predetermined time period. 17. The system according to any one of clauses 8 to 16, wherein the multiple parallel flow regenerative kiln system is configured such as the ratio between the longest cycle duration in any one of the N parallel flow regenerative kilns and the shortest cycle duration in the one or any other one of the N parallel flow regenerative kilns, is comprisedin the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1 to 1.05, in particular is equal to 1.0, over the predetermined time period, wherein the cycle duration C represents a time window covering the firing mode and the subsequent firing suspension mode. 18. The system according to any one of clauses 8 to 17, wherein the multiple parallel flow regenerative kiln system is configured such as the average of the number of cycles C per day over the predetermined time period for each of the N parallel flow regenerative kilns is comprised in a range from 80 to 130 cycles per day, in particular from 85 to 125 cycles per day, in particular from 90 to 120 cycles per day; 19. The system according to any one of clauses 8 to 18, wherein the multiple parallel flow regenerative kiln system is configured such as at least two of the N parallel flow regenerative kilns are operated in a predefined desynchronized manner from one another.

[0089] 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

CLAIMS1. A process for calcining mineral carbonate, in particular limestone and / ordolostone, in a multiple parallel flow regenerative kiln system comprising N parallel flowregenerative kilns, where N is an integer greater than or equal to 2, each parallel flowregenerative kiln, comprising at least two shafts: a first (10) and a second (20) shaftinterconnected by a gas transfer channel (30), said process comprising the steps of, foreach parallel flow regenerative kiln: -a loading of mineral carbonate at the top of the shafts (10, 20),- a cooling of calcinated mineral carbonate in a lower end portion of the shafts with acooling gas ; -an unloading of calcined mineral carbonate at the bottom of the shafts (10, 20),- each shaft operating alternately in a firing mode and in a preheating mode, with oneshaft (10, 20) being in the firing mode for a predetermined firing time period F whileanother shaft (10, 20) is in the preheating mode, and inversely,-the fuel supply being temporally interrupted during a firing suspension mode for apredetermined firing suspension time period NF, while the shafts (10, 20) are swappedfrom the firing mode to the preheating mode, and inversely, preferably the firingsuspension mode successively comprising a burnt out phase, a reversal phase and a pressurizing phase; -with the firing mode comprising, in the presence of said mineral carbonate, a combustion of fuel in the presence of an oxygen-containing gas so as to obtain acalcination of said mineral carbonate to give calcined mineral carbonate, an emissionof exhaust gas generated by the combustion of the fuel and the decarbonation of the mineral carbonate, and a passage of the exhaust gas from the shaft in the firing modeto the other shaft (10, 20) in the preheating mode by means of said gas transfer channel(30), -the preheating mode comprising a heat exchange between said mineral carbonateand said combustion gas transferred via said gas transfer channel (30),- the cooling of calcined mineral carbonate comprises a heat exchange between saidcalcined mineral carbonate and said cooling gas;- said process further comprising the steps of:- combining exhaust gas streams exiting from each parallel flow regenerative kiln toform a common exhaust gas stream; -storing the common exhaust gas stream in a variable volume reservoir (300), inparticular an inflatable gas holder or a membrane gas holder.- controlling the firing mode and the firing suspension mode of the N parallel flowregenerative kilns in such a manner-that the ratio between the highest or higher number of cycles over apredetermined time period, preferably from 6 to 24 h, in particular a 24-hourperiod in any one of the N parallel flow regenerative kilns and the lowest or lowernumber of cycles over the predetermined time period in any other one of the Nparallel flow regenerative kilns is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1.0 to 1.05, even morepreferably from 1.0 to 1.03, in particular from 1.0 to 1.01,or is equal to 1.0,-that the average of the number of cycles per day over the predetermined timeperiod for each of the N parallel flow regenerative kilns is comprised in a rangefrom 80 to 130 cycles per day, in particular from 85 to 125 cycles per day, inparticular from 90 to 120 cycles per day, and-that at least two of the N parallel flow regenerative kilns are operated in a predefined desynchronized manner from one another.

2. The process according to Claim 1, wherein the N parallel flow regenerativekilns have the same cycle duration C or substantially the same cycle duration C over thepredetermined time period, wherein the cycle duration C represents a time window covering the firing mode and the subsequent firing suspension mode, wherein the ratio between the longest cycle duration in any one of the N parallel flow regenerative kilns andthe shortest cycle duration in the one or any other one of the N parallel flow regenerativekilns, is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, morepreferably from 1.0 to 1.05, in particular is equal to 1.0, over the predetermined timeperiod.

3. The process according to Claim 1 or 2, further comprising controlling:- at least one of a rotation speed and a pitch of a blade or vane of a fluid displacementdevice, in particular a fan device (400), preferably arranged downstream from the variablevolume reservoir (300) and / or-an opening of a valve, preferably arranged downstream from the variable volumereservoir (300),so that a variation in the CO2 concentration in the common exhaust gas stream and / or avariation in the flow rate of the common exhaust gas stream volume are minimized, inparticular, the variable volume reservoir not being emptied and / or the volume of thevariable volume reservoir not exceeding a preset threshold, preferably during thepredetermined time period.

4. The process according to any one of the preceding claims, wherein themaximal volume of the variable volume reservoir (300) is at least equal to^^^ × ^^ ^ × ^^^^^^^ − ^^^^^ and lower than CF × ^^^ × ^^^^^^ −^ × ^^^^^^^^ , where CF (correctionfactor) = 2 in case N is equal to 2 or 3, or CF = N-1 in case N is higher than 3 , preferablylower than 1.2 ×−^ × ^^^^^^^ ), wherein ^^represents the total exhaust gas volumedisplaced during the firing time period F of a reference kiln selected among the N parallelflow regenerative kilns and ^^^ represents the total exhaust gas volume displaced duringthe firing suspension time period NF of the reference kiln, wherein the reference kiln isone of the N parallel flow regenerative kilns with either the largest total exhaust gasvolume displaced, the larger total exhaust gas volume displaced, one of the largest totalvolumes displaced, one of the larger total volumes displaced or one of the identical totalvolumes displaced among the N parallel flow regenerative kilns during the firing timeperiod F and the firing suspension time period NF, in particular on average over thepredetermined time period.

5. The process according to any one of the preceding claims, wherein thefiring mode and the subsequent firing suspension mode of the N parallel flow regenerativekilns are controlled in such a manner that at least one offset, in particular each offset,between two firing modes directly adjacent to one another either:^ is equal to^^^^ ^^^^ ^^ sed in the range from ^^× 0.9^^^^ or is comprito^^× 1.1^^^^ ^^^^ eferably from× 1.05, more pr^^^^ preferably when N ≤, or ^is equal to^^^^ ^^^^ ^^^^ ^ or is comprised in the range from ^× 0.9 to^× 1.1^^^^× 0.95 to^^^^ preferably from ^ ^× 1.05.

6. The process according to any one of the preceding claims, furthercomprising filtering each exhaust gas stream in a filter unit (100).

7. The process according to any one of the preceding claims, furthercomprising cooling the common exhaust gas stream in at least one cooling unit (200), andoptionally removing H2O from the common exhaust gas stream in said unit (200).

8. The process according to any one of the preceding claims, furthercomprising removing N2 and 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 from the common exhaust gas stream in one ormore CO2separation units (500) selected from the group comprising a membrane separation unit, a cryogenic separation unit, a desublimation separation unit, a low temperature distillation unit, a pressure swing absorption unit, a potassium salt wash unit and an amine wash unit.

9. A multiple parallel flow regenerative kiln system, preferably adapted tocarry out the process according to any of the preceding claims, said system comprising Nparallel flow regenerative kilns with N being greater or equal to 2, for calcinating mineralcarbonate, in particular limestone and / or dolostone, each parallel flow regenerative kilncomprising:- at least two shafts (10, 20) interconnected by a gas transfer channel (30), with each oneof said shafts (10, 20) comprising:- at least one device for supplying fuel,- at least one supply of an oxygen-containing gas for the combustion of the fuel,- an inlet for the loading of mineral carbonate,- an outlet for the unloading of said calcined mineral carbonate produced, and- a exhaust passage (50) for removal of the exhaust gas;wherein said system further comprises: -a common exhaust passage (90) arranged downstream from the exhaust passage (50)of the N parallel flow regenerative kilns;- a variable volume reservoir (300), in particular an inflatable gas holder or a membranegas holder, arranged downstream from the common exhaust passage (90).

10. The system according to Claim 9, wherein the maximal volume of thevariable volume reservoir (300) is at least equal to ^ . ^^^<img src='' class="img-anchor img-center" img-id="IMGF000036_0001" / >^^^^) and lowerthan ^^ × (^^ .^^ ^ . ^^^^^^^ − ^^^^) , where CF (correction factor) = 2 in case N is equal to 2 or3, or CF = N-1 in case N is higher than 3, preferably lower than 1.2 × ^ × ^^^− ^^^^ ), wherein ^^represents the total exhaust gas volume displaced during the firing time periodF of a reference kiln selected among the N parallel flow regenerative kilns and ^^^represents the total exhaust gas volume displaced during the firing suspension time periodNF of the reference kiln, wherein the reference kiln is one of the N parallel flowregenerative kilns with either the largest total exhaust gas volume displaced, the largertotal exhaust gas volume displaced, one of the largest total volumes displaced, one of thelarger total volumes displaced or one of the identical total volumes displaced among theN parallel flow regenerative kilns during the firing time period F and the firing suspensiontime period NF, in particular on average over the predetermined time period.

11. The system according to any one of Claims 9 to 10, further comprising atleast one of a fluid displacement device, in particular a fan device (400) and / or an openingof a valve, preferably arranged downstream from the variable volume reservoir (300), tocontrol an exhaust gas flow rate downstream form the variable volume reservoir (300).

12. The system according to any one of Claims 9 to 11, comprising a filter (100)unit disposed in each exhaust passage (50).

13. The system according to any one of Claims 9 to 12, comprising at least onecooling unit (200), in particular a heat exchanger, more preferably a condenser, disposedin the common exhaust passage (90).

14. The system according to any one of Claims 9 to 13, comprising one or moreCO2 separation units (500) selected from the group comprising a membrane separation unit, a cryogenic separation unit, a desublimation separation unit, a low temperature distillation unit, a pressure swing absorption unit, a potassium salt wash unit and an aminewash unit, preferably the variable volume reservoir (300) being arranged upstream fromthe one or more CO2 separation units (500).

15. The system according to any one of Claims 9 to 14, wherein the multipleparallel flow regenerative kiln system is configured such as at least one of the followingconditions is fulfilled: -the ratio between the highest or higher number of cycles over the predetermined timeperiod, in any one of the N parallel flow regenerative kilns and the lowest or lowernumber of cycles over the predetermined time period in any other one of the N parallelflow regenerative kilns is comprised in the range from 1.0 to 1.25, preferably from 1.0 to 1.10, more preferably from 1.0 to 1.05, even more preferably from 1.0 to 1.03, inparticular from 1.0 to 1.01, or is equal to 1.0;- the N parallel flow regenerative kilns have the same cycle duration C or substantiallythe same cycle duration C over the predetermined time period, wherein the cycleduration C represents a time window covering the firing mode and the subsequent firing suspension mode, wherein the ratio between the longest cycle duration in any one ofthe N parallel flow regenerative kilns and the shortest cycle duration in the one or anyother one of the N parallel flow regenerative kilns, is comprised in the range from 1.0to 1.25, preferably from 1.0 to 1.10, more preferably from 1 to 1.05, in particular is equal to 1.0, over the predetermined time period; -the average of the number of cycles C per day over the predetermined time periodfor each of the N parallel flow regenerative kilns is comprised in a range from 80 to 130cycles per day, in particular from 85 to 125 cycles per day, in particular from 90 to 120cycles per day; and / or-at least two of the N parallel flow regenerative kilns are operated in a predefined desynchronized manner from one another.

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