Technical Field

[0001] The present invention relates to a decarbonation system for carbonated materials, namely a parallel flow regenerative kiln (PFRK) with a NOx purification device (i.e. Selective catalytic reduction).

Background Art

[0002] Selective catalytic reduction (SCR) for NOx abatement is a well-known technique. It consists in reducing NOx over a catalytic bed with ammonia or ammonia precursor injections.

[0003] Current selective catalytic reduction devices have an effective temperature window (typically 220°C-350°C). However, this window is higher than the typical fume temperatures exiting a parallel flow regenerative kiln that are typically below 150°C, even below 100°C in some applications. Therefore, in order to apply this technology to a parallel flow regenerative kiln PFRK with a significant NOx abatement rate, the fumes, namely the exhaust gas need to be reheated. The energy requirement for reheating makes it at a first glance economically unsound.

[0004] In order to reduce the costs associated with the NOx abatement, it has been proposed to recover the thermal energy used to raise the temperature of the exhaust gas. Typical energy recovery on selective catalyst reduction (SCR) systems relies on indirect plate heat exchangers, which have several drawbacks such a high pressure drop and high sensitivity to dust clogging.

[0005] Typical SCR systems are used on continuous processes (boilers, cement kilns, engines). The temperature and reagent addition control are based on reactive control thanks to a retroaction loop between NOx measured emissions and the aforementioned parameters (kind of PID control). However, this type of control reactiveness is too slow to capture typical PFRK emission dynamic pattern which results from the inherent cyclicity of the process.

[0006] Alternatively, to address abatement of NOx in parallel flow regenerative kiln, it has been proposed in WO18220520, unlike the known SNCR (Selective Non-catalytic Reduction) and typical honeycomb SCR (Selective Catalytic Reaction), catalytic bags constituted by a catalytic fabric preferably with a base of vanadium oxide . This solution activates the NOx reduction reaction on the catalytic materials (i.e. vanadium oxide) using an ammonia-based reagent. Even though such a solution requires a lower exhaust temperature of 180°C to 220 °C, it is complex to implement. Furthermore, this solution fails to recover the heat energy necessary to reheat the kiln exhaust gas to the operating temperature window, thereby reducing its effectiveness and economic viability The use of filtering materials integrated with the catalytic materials renders this solution sensitive to clogging.

Aims of the Invention

[0007] The invention aims to provide a solution to overcome at least one drawback of the teaching provided by the prior art.

[0008] More specifically, the invention aims to provide a device for improving the NOx abatement while remaining economically viable.

Summary of the Invention

[0009] For the above purpose, the invention is directed to a decarbonation system for carbonate materials, in particular limestone and/or dolomite, comprising: a parallel flow regenerative kiln comprising at least a first and a second shaft, and optionally a third shaft with preheating, combustion and cooling zones and a cross-over channel between each shaft, said kiln being arranged to heat alternately the carbonated materials in the first or second shaft, by a combustion of fuel, up to a temperature range in which carbon dioxide of the carbonated materials is released, the combustion of fuel and the decarbonatation generating exhaust gas comprising NOx and CO2; a NOx selective catalyst reduction device, arranged downstream from said kiln for removing nitrogen oxides in exhaust gas generated by said kiln in cyclic manner, said device comprising: at least one heat compensation unit adapted to heat the exhaust gas up to a temperature range suitable for the reduction of nitrogen oxides present in the exhaust gas; at least one catalyst bed for the selective reduction of NOx, in particular a catalyst bed or a first and second catalyst beds for the selective reduction of NOx, the or each catalyst bed being fluidly connected to the at least one heat compensation unit; a first and a second regenerators comprising a heat exchange surface in contact with the exhaust gas in use and adapted to intermittently store the thermal energy of the exhaust gas, each regenerator being respectively fluidly connected to the at least one catalyst bed; at least one injection module comprising an injector feeding the exhaust gas with ammonia and/or an ammonia precursor such as urea; a control valve assembly fluidly connected to the NOx selective catalyst reduction device, said valve being adapted, in a first position, to guide the exhaust gas through the first regenerator, the at least one catalyst bed the second regenerator, and in a second position, to guide the exhaust gas through the second regenerator, the at least one catalyst bed, the first regenerator.

[0010] According to specific embodiments of the invention, the decarbonation system comprises one or more of the following features: controller operably coupled to the control valve assembly for controlling at least one control valve assembly switching either from the first to the second position or vice versa to synchronize the NOx selective catalyst reduction device with the emission cycles of the parallel flow regenerative kiln; the timing of the at least one control valve assembly switching is based on at least one operating parameter of the parallel flow regenerative kiln; the least one operating parameter of the parallel flow regenerative kiln comprises at least one time parameter, in particular one or more values selected from the group comprising: a value of the point in time of the beginning a combustion cycle and a value of the duration of said cycle; the controller is configured to control the at least one control valve assembly switching such as the cumulative duration of at least two directly subsequent cycles of the NOx selective catalyst reduction device, preferably between 2 to 20 cycles , in particular 3 to 10 cycles, more preferably 5 to 7 cycles is equal to or substantially equal to the duration of a reference combustion, wherein the duration of a given cycle of the NOx selective catalyst reduction device is defined as the duration between a given control valve assembly switching and a directly subsequent control valve assembly switching; the controller is configured to control the at least one control valve assembly switching such as the duration of a given cycle of the NOx selective catalyst reduction device is equal to or substantially equal to the duration of a reference combustion cycle, wherein the duration of the given cycle of the NOx selective catalyst reduction device is defined as the duration between a given control valve assembly switching and a directly subsequent control valve assembly switching; point in time of the first of a series of valve assembly switchings of the at least two directly subsequent cycles or the given cycle is synchronized with the beginning of a given combustion cycle, with either non phase shift or a constant phase shift offset; the parallel flow regenerative kiln being configured to actuate the control valve assembly, upon detection or expectation of a kiln reversal phase characterized by a swapping of the burning and non-burning shaft, in a third position in which the exhaust gas is prevented to flow between the parallel flow regenerative kiln and the NOx selective catalyst reduction device; the controller is further operatively coupled to :

-at least one injector valve so as to control the ammonia and/or ammonia precursor injection rate and/or timing, and/or

-at least one heat source, in particular a fuel burner, of the heat compensation unit so as to control the heat release; the controller is configured to perform a predictive control of at least one of the control valve assembly, the at least one injector valve and/or the at least one heat source of the heat compensation unit; a bypass valve is arranged in parallel with the control valve assembly, said valve being configured so that at least a portion of an exhaust gas flow generated by the parallel flow regenerative kiln bypasses the NOx selective catalyst reduction device, preferably the exhaust gas flow bypassed representing at least 10% of the entire flow generated by the parallel flow regenerative kiln, preferably at least 50%, in particular at least 80%; the control valve assembly comprises either a four-way distributor with at least three positions, namely the first, the second and a third position, or a plurality of two-way valves, in particular four or six valves; the at least one injection module comprises a first and a second injection module, wherein the first injection module is fluidly positioned between the first regenerator and the at least one catalyst bed for the selective reduction of NOx, and the second injection module is fluidly positioned between the second regenerator and the at least one catalyst bed for the selective reduction of NOx; the at least one injection module comprises a first and a second injection module, wherein the first injection module is fluidly positioned upstream relative to the first regenerator when the control valve assembly is in the first position, and the second injection module is fluidly positioned upstream relative to the second regenerator when the control valve assembly is in the second position; a first and a second catalyst beds for the oxidation of CO and/or VOC are positioned between the at least one catalyst bed for the selective reduction of NOx and first and the second regenerators, respectively; the at least one heat compensation unit is fluidly positioned between the first and the second catalyst bed for the selective reduction of NOx; the at least one heat compensation unit comprises a first and a second compensation unit and the catalyst bed for the selective reduction of NOx is fluidly positioned between the first and the second compensation unit; the NOx selective catalyst reduction device is free of a catalyst bed for the oxidation of CO and/or VOC; the active catalytic elements of the first and the second catalyst beds for the selective reduction of NOx are selected from the group comprising either oxides of base metals, such as vanadium, molybdenum and tungsten, zeolites, or one or more precious metals, or any combination thereof.

[0011] The present invention also relates to a method for operating the decarbonation system, said method comprising the steps of : heating carbonated materials in the first shaft by combustion of fuel, up to a temperature range in which carbon dioxide of the carbonated materials is released, during a first combustion cycle, the combustion of fuel and the decarbonatation generating exhaust gas comprising NOx and CO2, heating carbonated materials in the second shaft by combustion of fuel, up to a temperature range in which carbon dioxide of the carbonated materials is released, during a second combustion cycle, the combustion of fuel and the decarbonatation generating exhaust gas comprising NOx and CO2; preferably repeating the two preceding steps.

[0012] According to specific embodiments of the invention, the method comprises one or more of the following features: switching the at least one control valve assembly from the first position to the second position or vice versa, preferably at the beginning of, during or at the end of the first or second combustion cycle; controlling the heat released by the at least one heat compensation unit so that the exhaust gas temperature entering the at least one catalyst bed for the selective reduction of NOx is comprised in the range: 150-400°C, in particular 180-350°C, preferably 220-350°C; controlling the at least one control valve assembly switching such as the cumulative duration of a given cycle of the NOx selective catalyst reduction device is equal to or substantially equal to the duration of a reference combustion cycle, in particular the first of second combustion cycle, wherein the duration of the given cycle of the NOx selective catalyst reduction device is defined as the duration between the given control valve assembly switching and a directly subsequent control valve assembly switching; controlling the at least one control valve assembly switching such as the cumulative duration of at least two directly subsequent cycles of the NOx selective catalyst reduction device, preferably in the range from 2 to 20 cycles , in particular in the range from 3 to 10 cycles, more preferably in the range from 5 to 7 cycles is equal to or substantially equal to the duration of a reference combustion cycle, in particular the first of second combustion cycle, wherein the duration of a given cycle of the NOx selective catalyst reduction device is defined as the duration between a given control valve assembly switching and a directly subsequent control valve assembly switching; controlling the switching of the at least one control valve assembly so that the point in time of the first of a series of valve assembly switchings of the at least two directly subsequent cycles or the given cycle is synchronized with the beginning of a given combustion cycle, in particular the first or the second combustion cycle, with either non phase shift or a constant phase shift offset..

Brief Description of Drawings

[0013] Aspects of the invention will now be described in more detail with reference to the appended drawings, wherein same reference numerals illustrate same features.

[0014] Figures 1A, 1 B show an embodiment with two catalyst beds for the selective reduction of NOx according to the invention according to the invention.

[0015] Figure 2 shows the nitrogen oxide concentration cyclic evolution in the exhaust gas generated by a parallel flow regenerative kiln.

[0016] Figures 3A, 3B show a further embodiment according to the invention with a specific control valve assembly, namely a combination of two three-way valves.

[0017] Figures 4A, 4B show a further embodiment according to the invention suitable for a parallel flow regenerative kiln generating exhaust gas with low volatile organic and carbon monoxide .

[0018] Figures 5A, 5B show a further embodiment according to the invention with a specific control valve assembly, namely a combination of six two-way valves.

[0019] Figures 6A, 6B show a further embodiment according to the invention with another specific control valve assembly, namely a combination of four two-way valves.

[0020] Figures 7A, 7B show a further embodiment with one catalyst bed for the selective reduction of NOx according to the invention. [0021] Figures 8A, 8B show an alternative embodiment with one catalyst bed for the selective reduction of NOx according to the invention.

[0022] Figures 9A, 9B show a further embodiment with an alternative arrangement of injection units according to the invention. [0023] Figures 10A, 10B show a further embodiment with the alternative arrangement of injection units and one catalyst bed for the selective reduction of NOx according to the invention.

[0024] List of reference symbols Detailed description

[0025] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may however be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness.

[0026] Figure 1A shows an embodiment of a decarbonation system for carbonated materials. The decarbonation system comprises a parallel flow regenerative kiln 10 generating exhaust gas. The exhaust gas contains harmfull substances such as nitrogen oxides and needs to be purified to a certain extent to meet emission requirements. For this purpose, a filter 20 where patricules in suspenesion in the exhaust gas are removed is arranged in an exhaust line. Once filtered, the exhaust gas is then routed to a NOx selective catalyst reduction device 30 via a control valve assembly 40 before being release in the atmosphere through a fume stack 60.

[0027] A parallel flow regenerative kiln 10 generally comprises at least a first and a second shaft, and optionally a third shaft and a cross-over channel between each shaft. The kiln 10 is arranged to heat alternately by a combustion reaction the carbonated materials in the first or second shaft (and eventually the thrid shaft) up to a temperature range in which carbon dioxide of the carbonated materials is released during at least one combustion cycle. A combustion cycle can be identified by the pressurization of the shafts and/or the presence of fuel injections within one shaft over a predefined period.

[0028] A traditional parallel-flow regenerative 10 kiln comprises a first shaft and a second shaft with preheating zones heating (i.e. combustion) zones and cooling zones, as well as a cross-over channel arranged between the first and second shafts. In use, the carbonated materials are introduced at an upper portion of each shaft. The carbonated materials slowly move to the bottom. In the preheating zones, the carbonated materials are essentially preheated with the alternating regenerative exhaust gas. In the combustion zones, the carbonated materials are alternately heated by a combustion of fuel with at least one comburent, in particular air and or substantially pure oxygen, up to a temperature range in which carbon dioxide of the carbonated materials is released. Both the combustion of the fuel with the at least one comburent and the decarbonatation generate the exhaust gas. The decarbonated materials formed after the release of the CO2 from the carbonated materials are directly cooled in the cooling zones by an air stream that is burned with the fuel. Tipically, the carbonated material can be limestone and dolomitic limestone.

[0029] In a parallel-flow regenerative 10 kiln, the main NOx source is generally the fuel NOx, However thermal NOx are also present resulting from the exposurre of oxygen to nitrogen present in air at high temperatures during the combustion. In Figure 1A, the nitrogen oxides are treated in the NOx selective catalyst reduction device 30 that comprises the following elements: a heat compensation unit 32, two catalyst beds for the selective reduction of NOx 36A, 36B, two regenerators 34A, 34B, two catalyst beds 35A, 35B, fo the oxidation of CO and/or VOC, two injection modules 38A, 38B and two opposed flow openings 39A, 39B.

[0030] The two catalyst beds for the selective reduction of NOx 36A, 36B (also known as NOx selective reduction catalyst) of the NOx selective catalyst reduction device 30 can be made from various porous ceramic materials used as a support, such as titanium oxide or alumina, and active catalytic components usually either oxides of base metals (such as vanadium, molybdenum and tungsten), zeolites, or various precious metals. The reduction requires a minimal temperature and typically operates in the range: 220-350°C. It should be noted that the temperature of the exhaust gas generated by the parallel flow regenerative kiln 10 is below 150°C and is, therefore, outside the NOx selective reduction catalyst operating window, in which an effective abatement is expected. Operation below 220°C would lead to a lower abatement rate.

[0031] In order to increase the temperature of the exhaust gas, an heat compensation unit 32 is foreseen. One or more heat sources 32.1 , in particular a fuel burner are used to heat the exhaust gas up to a temperature range in which the reduction of the nitrogen oxides present in the exhaust gas takes place. As an alternative or in combination with the one or more fuel burners, an electric heat source can be used.

[0032] The reduction of the nitrogen oxides requires the presence of a reductant (i.e. ammonia or equivalent) injected via two injection modules 38A, 38B. Each module 38A, 38B comprises one or more injectors feeding the exhaust gas with ammonia and/or an ammonia precursor. An ammonia precursor, such as urea can be used as an alternative to ammonia. Advantageously, urea decomposes into ammonia in a temperature region comparable to that for the reduction of the nitrogen oxides. The embodiment in Figure 1A presents only two injection modules 38A, 38B. The decarbonatation system can comprise more than two injection modules 38A, 38B. For instance two complementary injection modules can be respectively arranged adjacent to the catalyst beds for the selective reduction of NOx 36A, 36B. Furthermore, a (single) injection module can be arranged upstream from the control valve assembly 40.

[0033] In Figure 1A, the NOx selective catalyst reduction device 30 comprises a central node, to which two branches are connected, each branch comprising successively one NOx selective reduction catalyst 36A, 36B, one oxidation catalyst 35A, 35B, one regenerator 34A, 34B, one injection module 38A, 38B, and one flow opening 39A, 39B. In Figure 1A, the at least one heat compensation unit 32 comprises a single unit disposed at the central node. Alternatively, two heat compensation units 32 can be foreseen arranged adjacent to the respective NOx selective reduction catalyst 36A, 36B. Furthermore, the embodiment in Figure 1 comprises two branches. However, other configurations are possible such as with three branches. Preferably, the elements of the NOx selective catalyst reduction device 30 are arranged in a stack (e.g. horizontal or vertical stack) so has to reduce the heat losses. Alternatively, said elements can be arranged in different units separated from one another.

[0034] In Figure 1A, the configuration of the NOx selective catalyst reduction device 30 based on a central node with extending branches in combination with a control valve assembly 40, in particular a four way valve with at least two position allows an efficient use of the heat generated in the heat compensation unit 32. Indeed, in the first position of the control valve assembly 40 as shown in Figure 1A, the exhaust gas is heated successively by the regenerator 34A, the oxidation catalyst 35A substrate, the NOx selective reduction catalyst 36A substrate and heat source(s) 32.1 that are activated, if needed, in order to reach the activation temperature of the downstream NOx selective reduction catalyst 36B. The heat sources 32.1 are symbolically represented by heating resistors. This measure allows to reach the temperature threshold required by the NOx selective reduction catalyst 36B to ensure an efficient reduction of the nitrogen oxides in presence of ammonia. The exhaust gas is marginally further heated in the NOx selective reduction catalyst 36B thanks to the exothermic nitrogen oxide reduction reaction.

[0035] The exhaust can be further heated in the oxidation catalyst 35B thanks to the exothermic oxidation of the volatile organic compounds (VOC) and carbon monoxide (CO) present in the exhaust gas resulting from a partial combustion of the fuel in the kiln 10 and optionally the one or more heat sources 32.1 using a fuel burner. However, the amount of compounds such as VOC and carbon monoxide (CO) is relatively too low in kiln operation in usual operation conditions, to the extent that the heat resulting from the unburnt fuel remains in general marginal. A portion of the heat present in the exhaust gas is absorbed in the NOx selective reduction catalyst 36B substrate, in the oxidation catalyst 35B substrate and finally in the regenerator 34B, by solid/gas heat transfer. A regenerator 34A, 34B (also known as regenerative heat exchanger) is a type of heat exchanger where heat from a hot fluid (i.e. the above exhaust gas heated) is intermittently stored in a thermal storage medium before it is transferred to a cold fluid (exhaust gas described below entering the regenerator 34B in Figure 1 B). To accomplish this, the hot fluid is brought into contact with the heat storage medium, then the fluid is displaced with the cold fluid (thanks to the control valve assembly), which absorbs the heat in the next cycle when the control valve assembly is in a second position.

[0036] In Figure 1 B, the exhaust gas entering NOx selective catalyst reduction device 30 through the first flow opening 39B is supplied with the ammonia or ammonia precursor in the injection module 38B. In Figure 1 B , the injection module 38B is positioned upstream (with regard to the exhaust gas flow direction during injection) of the regenerator 34B so as to allow a residence time for the vaporization of the ammonia and/or ammonia precursor. Furthermore, the heat released by the regenerator 34B promotes the endothermic dissociation reaction of an ammonia precursor such as urea into ammonia. Moreover, in the presence of the regenerator 34B, the relatively cold exhaust gas stream is heated by the regenerator 34B, that releases the energy absorbed in the previous cycle. Thanks to this measure, the heat generated by the heating compensation unit 32 is recycled ensuring a efficient consumption of the energy supplied to heat up the exhaust gas exiting the kiln 10.

[0037] Figure 2 shows a nitrogen oxide concentration profile over successive combustion cycles of a parallel flow regenerative kiln 10. The profile measured by a NOx sensor positioned in an exhaust channel fluidly arranged downstream from the first and second shaft of the parallel flow regenerative kiln 10, is essentially cyclic. The NOx measures are performed according to the reference standards such as JRC Reference Report on Monitoring of Emissions to Air and Water from IED Installations. Within each cycle, the nitrogen oxide concentration varies significantly rendering the control of the timing and amounts of ammonia and/or a precursor thereof challenging. It has been discovered that the control can be optimized by synchronizing the cycles of the parallel flow regenerative kiln 10 and the cycles of the NOx selective catalyst reduction device 30. For instance, the points in time of the switching of the control valve assembly is dependent on at least one time parameter, in particular one or more values selected from the group comprising: a value of the point in time of the beginning of a combustion cycle and a value of the duration of the combustion cycle.

[0038] By a combustion cycle is meant a phase during which the first or the second or optionally the third shaft is heated up so as to ensure the decarbonation of carbonated materials. On a two shaft kiln, the combustion cycles alternate between the first and the second shafts. Typically, a given combustion cycle is interposed between two directly adjacent transfers cycles used for loading the kiln with carbonated materials and discharge of decarbonated materials. Typically, a combustion cycle beginning can be defined as the point in time when the fuel for the combustion ensuring the temperature increase to reach the decarbonation conditions starts or the point in time when one or more valves for the material feeding or discharging in the kiln are closed. In a similar way, a combustion cycle end can be defined as the point in time when the fuel for the combustion ensuring the decarbonation conditions stops or the point in time when one or more of the valves for the material feeding or discharging in the kiln are open.

[0039] In particular, it is advantageous to control the at least one control valve assembly 30 switching such as the cumulative duration of one cycle or at least two directly subsequent cycles of the of NOx selective catalyst reduction device 30 is equal to the duration of a reference combustion cycle of the parallel flow regenerative kiln 10. The reference combustion cycle can be a preceding cycle, a target cycle, an averaged cycle based on the previous cycles. This measure ensure that the NOx selective catalyst reduction device 30 will essentially retrieves the same initial condition after each cycle of the kiln 10. This approach can be advantageously combined with a predictive control using receding horizons matching one or more cycles of the kiln 10. Preferably, the cycles of the of NOx selective catalyst reduction device 30 substantially present the same duration.

[0040] It has been further discovered that a specific number (positive integer) of cycles, in particular 2 to 20 cycles, preferably 3 to 10 cycles, more preferably 5 to 7 cycles should match the duration of the at least one combustion cycle of the parallel flow regenerative kiln 10. The duration of a given cycle of the of NOx selective catalyst reduction device 30 is defined as the duration between a given control valve assembly switching and a directly subsequent control valve assembly switching. The preferred number hinges on a compromise between two antagonist effects. A higher switching frequency will either increase the heat recovery efficacy for the same installation or decrease its size for the same efficiency. However, a higher switching frequency creates “dead times” (i.e. time during which the flue gas isn’t treated) during which the exhaust gas bypasses the catalyst.

[0041] A controller of the decarbonation system controls the switching timings of the control valve assembly 40, by means of a programmable computing unit and (a) power interface(s). The programmable computing unit and optionally the power interface(s) can be operatively coupled to :

-at least one injector valve so as to control the ammonia and/or ammonia precursor injection rate and timing, and/or

-at least one heat source 32.1 , in particular a fuel burner, of the heat compensation unit 32 so as to control the heat release.

[0042] The NOx selective catalyst reduction device 30 can comprise at least a temperature sensor to monitor the exhaust temperature and/or a mass flow sensor (e.g. using at least one pressure sensor, hot-film mass meter, etc.), connected to the controller. The controller can also use a model for the temperature and/or mass flow, alternatively or complementary to the sensor measurements.

[0043] The parallel flow regenerative kiln 10 can advantageously be arranged to be simultaneous fed with the carbonated materials and discharged from the decarbonated materials, during a (periodic) transfer cycle (also known as kiln reversal with swapping of the burning and non-burning shaft), preferably directly subsequent one of the at least one combustion cycle (see Figure 2). During one of the transfer cycles, the controller preferably actuates the control valve assembly 40, upon detection or expectation of one transfer cycle, in a third position in which the exhaust gas is prevented to circulate through the first and second flow openings 39A, 39B.

[0044] Advantageously, the exhaust line comprises a bypass passage with a throttle valve 50 so that at least a portion of the exhaust gas bypasses the NOx selective catalyst reduction device 30 when the throttle valve is open. This measure is particularly useful when the decarbonatation system already meets the nitrogen oxide emissions with only a fraction of the exhaust steam purified. The throttle valve 50 adjusts the amount of exhaust gas to be bypassed, continuously from a close position to an open position.

[0045] In the embodiment according to Figures 3A, 3B, the control valve assembly 40 preferably comprises an arrangement of two three-way valves. An appropriated control of the two three-way valves reproduces the operation a four-way control valve assembly (i.e. distributor) with three position, namely parallel flows, switched flows, and blocked flows.

[0046] The embodiment according to Figures 4A, 4B differs from that of Figures 1A, 1 B in that the NOx selective catalyst reduction device 30 does not comprise any oxidation catalyst. This measure is preferred when the exhaust gas generated by the parallel flow regenerative kiln 10 presents a low content of VOC or CO, rendering an oxidation catalyst superfluous.

[0047] Figures 5A, 5B show a further embodiment according to the invention with a specific control valve assembly, namely a combination of six two-way valves.

[0048] Figures 6A, 6B show a further embodiment according to the invention with a specific control valve assembly, namely a combination of four two-way valves.

[0049] Figures 7A, 7B show a further embodiment with one catalyst bed for the selective reduction of NOx with heat source(s) 32.1 integrated in said catalyst .

[0050] Figures 8A, 8B show an alternative embodiment with one catalyst bed 36 for the selective reduction of NOx, wherein heat sources 32.1 are disposed on both sides of the catalyst bed selective reduction of NOx 36.

[0051] Figures 9A, 9B show a further embodiment based on that of Figure 1A, 1 B. The difference between the embodiment of Figures 9A, 9B and that of Figures 1A, 1 B is that each injector module 38A, 38B is positioned downstream from and adjacent to the corresponding regenerator 34A, 34B when said module 38A, 38B are activated (i.e. injecting ammonia/ammonia precursor) instead of being of being positioned upstream thereof as shown in Figure 1A, 1 B. In this configuration, the ammonia or ammonia precursor is injected in a hotter environment allowing a better dispersion of the ammonia or ammonia precursor.

[0052] In an embodiment not shown, injector modules 38A, 38B can be positioned not only downstream but also upstream from the regenerators 34A, 34B in order to optimize the vaporization.

[0053] Figures 10A, 10B show a further embodiment based on that of Figure 9A, 9B. The difference between the embodiment of Figures 10A, 10B and that of Figure 9A, 9B is the presence of single catalyst bed 36 for selective NOx reduction instead of two catalyst bed 36A, 36B as shown in Figure 9A, 9B.

[0054] The embodiments show one NOx selective catalyst reduction device 30 fluidly connected to one parallel flow regenerative kiln 10. Nevertheless a plurality of NOx selective catalyst reduction devices 30 can be provided downstream from and fluidly connected to a given parallel flow regenerative kiln 10 depending on the needs and size.

[0055] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

[0056] The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the invention may be practiced in many ways, and is therefore not limited to the embodiments disclosed. It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to include any specific characteristics of the features or aspects of the invention with which that terminology is associated.