NL2024258B1

NL2024258B1 - System and method for nox removal

Info

Publication number: NL2024258B1
Application number: NL2024258A
Authority: NL
Inventors: Pot Eric-Jan; In 't Groen Dennis; Wiersma Leo; De Waard Eltjo
Original assignee: Visser & Smit Bv
Priority date: 2019-11-18
Filing date: 2019-11-18
Publication date: 2021-07-29
Also published as: NL2025852B1; NL2025852A

Abstract

-19- ABSTRACT A system and method for lowering NOX concentration in a gas ﬂow comprising a ﬁrst gas inlet unit adapted for introducing a primary gas ﬂow and a second gas inlet unit adapted for introducing a secondary gas ﬂow; wherein the secondary ﬂow comprises ambient air; a ﬁrst 5 catalytic chamber, conﬁgured to receive a combined gas ﬂow comprising the primary and secondary gas ﬂow; comprising a reduction catalyst; a control means adapted to control the amount of a reductant that is introduced to the ﬁrst catalytic chamber for exposure to the reduction catalyst; at least one heating chamber upstream of the ﬁrst catalytic chamber for heating at least a portion of the combined gas ﬂow; a ﬂow inducing means adapted to draw the 10 gas ﬂow through the system; wherein the system is adapted to control rotational speed of the ﬂow inducing means to control towards a predetermined gas ﬂow rate.

Description

-1-

SYSTEM AND METHOD FOR NOX REMOVAL Field of the Invention

[0001] The invention relates to a system and method for lowering NOx concentration in a gas flow, and in particular in a gas flow comprising exhaust gas from one or more internal combustion engines. Description of the Background Art

[0002] A system comprising a reduction catalyst for lowering the NOx concentration in gas flows from diesel engines is known in the art.

[0003] US 10161281 B2 discloses an exhaust purification system including an engine, a lean NOx trap (LNT) mounted on an exhaust pipe and enabled to absorb nitrogen oxide contained in an exhaust gas at a lean air/fuel ratio or to release the adsorbed NOx at a rich air/fuel ratio and a selective catalytic reduction (SCR) catalyst to reduce the NOx. Oxygen sensors detect the concentration of oxygen in the exhaust gas and additional air is injected upstream of the SCR when the air/fuel ratio is rich.

[0004] US 2002/0148220 Al discloses a diesel powered vehicle provided with an SCR system which uses an external reducing reagent to convert NOx emissions on the reducing catalyst, whereby actual NOx emissions produced by the engine are filtered using a variable NOx time constant in turn correlated to the catalyst temperature.

[0005] WO 2015/109072 Al discloses an apparatus for a gas turbine power plant that is allowed to operate continuously from 100% load down to approximately 40% load, wherein cumulative emissions are reduced through the addition of a series of CO oxidation catalysts and, if necessary, a high NO: reduction SCR catalyst.

[0006] US 2002/0104309 Al discloses a deNOx apparatus with a NOx adsorber catalyst for adsorbing and releasing NOx, and an exhaust recirculating circuit for mixing an exhaust gas into intake air and an exhaust recirculating amount control means for recirculating a predetermined amount of the exhaust gas for reducing NO, when the adsorbed NOx accumulation amount is a predetermined value or less, and recirculating the aforementioned predetermined amount or more of exhaust gas to bring an air fuel ratio into a rich state when the adsorbed NOx accumulation amount exceeds a predetermined value and is to be released.

[0007] US 2012/0222406 Al describes an exhaust purification system for an internal combustion engine capable of lean-burn driving, comprising an non-selective reduction (NSR)

2- catalyst, an SCR disposed downstream of the NSR catalyst, a NOx sensor disposed downstream of the SCR and a rich-spike means for causing a rich-spike.

[0008] The systems described in the prior art are all systems that are designed for direct connection to a specific engine and optimized for that certain type of engine and for a certain 5S gas flow rate, certain NOx concentration, certain size, etc. A disadvantage of such systems is that a system designed for one type of vehicle with a certain type of engine would operate inefficiently if used in a different vehicle. This is especially a problem for construction vehicles or non-road vehicles and machines, where each individually optimized system is built into the vehicle, taking up space and contributing to an increased weight of the vehicle. Additionally, any required maintenance of such a system would require that the vehicle or machine is taken out of service.

[0009] Another disadvantage of some prior art systems which use fuel injections, or other engine modifications to reduce NOx concentrations in exhaust gas, is that they compromise the performance of the engine.

[0010] Another disadvantage of some of the prior art systems is that the reduction catalysts only operate properly under essentially constant gas flow and within a certain temperature range. Engines that work under different loads emit exhaust gas at different flow rates and with different temperatures, depending on the engine efficiency. As a result, during a significant amount of time that the engine runs, the reduction catalyst does not optimally lower the NOx concentration in the exhaust gas due to fluctuations in gas flow and temperature. Moreover, some of the prior art systems are controlled such that when the engine is idling, the reduction catalysts are not operating, resulting 1n significant NOx emissions.

[0011] Thus, there is a particular need for a system or method to reduce NOx levels in exhaust gas from engines that does not have, or has to a lesser extent, one or more of the above- mentioned disadvantages of the prior art. In particular, there is a need to significantly reduce the NOx emission of non-road diesel engines that are used at e.g. construction sites that currently significantly suffer from one or more of these disadvantages.

Brief Summary of the Invention

[0012] The present invention relates to a system for lowering NOx concentration in a gas flow comprising: - a first gas inlet unit adapted for introducing a primary gas flow; - a second gas inlet unit adapted for introducing a secondary gas flow, wherein the secondary flow comprises ambient air;

-3- - a first catalytic chamber, configured to receive a combined gas flow comprising the primary and secondary gas flow, wherein the first catalytic chamber comprises a comprising a reduction catalyst and is adapted for contacting the combined gas flow with the reduction catalyst; - a means adapted to introduce a controlled amount of a reductant onto the reduction catalyst or into the combined gas flow for contact with the reduction catalyst; - at least one heating means upstream of the first catalytic chamber adapted to provide heating of at least a portion of at least one of the primary gas flow, the secondary gas flow, and/or the combined gas flow to achieve a temperature of the combined gas flow of at least 200 °C, preferably at least 250 °C, more preferably at least 300 °C; and - a flow inducing means adapted to draw the combined gas flow through at least the first catalytic chamber, wherein the system 1s adapted to control rotational speed of the flow inducing means to control towards a predetermined flow rate of the combined gas flow.

[0013] The first gas inlet unit is preferably an intake for exhaust gas from internal combustion engines and is therefore preferably connected or arranged in a flow communication to one or more exhaust pipes deriving from diesel engines. The connection is preferably done via flexible coupling, preferably via a chute.

[0014] The primary gas flow is introduced to the system via the first gas inlet unit and preferably comprises exhaust gas from one or more diesel engines, preferably from one or more diesel non-road engines. Non-road engines are engines that are used for other purposes than a motor vehicle that is used primarily on a public roadway. The primary gas flow most preferably comprises exhaust gas from non-road construction vehicles or construction equipment. The first gas inlet unit may be connected or in flow communication via a flexible coupling with the engine, and may be connected or arranged in flow communication with a flow inducing means that is adapted to draw primary gas into the system.

[0015] The second gas inlet unit is a gas intake unit for ambient air, and is preferably also connected or arranged in flow communication with the flow inducing means that is adapted to draw ambient air into the system.

[0016] In a preferred embodiment, the primary and secondary gas flows are introduced to the system via the same flexible coupling and are in flow communication with the flow inducing means. The flow inducing means can draw a gas flow into the system via a rotational movement. The flow inducing means is preferably a ventilation fan that is adapted to vary the rotational speed and as such control the gas flow that is drawn into the system. Ambient air starts flowing in as a secondary gas flow when the flow of the primary gas flow is lower than

-4- the flow induced by the rotational speed of the ventilation fan. As such, the flow of the combined gas flow in the system may be controlled by varying the rotational speed of the ventilation fan.

[0017] Preferably, the reduction catalyst is a selective catalytic reduction (SCR) catalyst, because the SCR catalyst results in higher NOx conversion yields compared to other reduction catalysts and allows for a more simple configuration of the first catalytic chamber.

[0018] The reductant used for the catalytic reduction of NOx in the first catalytic chamber is preferably in the form of an aqueous urea solution. The injected urea breaks down in the hot gas stream. The water evaporates and becomes steam. The urea becomes carbon dioxide (COz), water (H20) and ammonia (NH3). The ammonia attaches itself to the surface of the SCR catalyst and reacts with the NOx and forms nitrogen (Nz) and water (H20). A urea solution is safer to store than ammonia and is therefore the preferred reductant. The reductant is preferably injected directly into the combined gas flow in the first catalytic chamber before the combined gas flow reaches the catalyst as to enable mixing with the NOx in the combined gas flow for improved catalytic performance.

[0019] Preferably, the system further comprises NO sensors adapted to measure the concentration of NOx in the combined gas flow. As such, the system is adapted to control the amount of reductant that is introduced based on the measured NOx concentration in the combined gas flow.

[0020] The heating means may be placed in a separate heating chamber that is configured to receive at least a portion of at least one of the primary gas flow, the secondary gas flow and/or the combined gas flow, and to heat this gas flow. The heating means is placed upstream of the first catalytic chamber so as to heat the gas flow prior to exposure to the catalyst. Heating the gas flow via the heating means may be necessary when ambient air is introduced to the system, asa combined gas flow comprising ambient air typically has a temperature below the optimum working temperature of the reduction catalyst. A minimum temperature of the heated gas flow of 200 °C is preferred, more preferably 250 °C, still more preferably 300 °C, in order to enable efficient functioning of the NOx reduction catalyst in the first catalytic chamber. In addition, an advantage of a heating means is that it allows the system to work independently from the activity and temperature of the (diesel) engine(s) that are connected to the system. As such, even when the engine(s) are running on low load, at low activity, or at a low temperature or when the engine(s) are idling, and the resulting temperature of the exhaust gas is too low, or when heat loss occurs in e.g. the gas inlet units, the heating means heats the gas flow to the required temperature for efficient NOx reduction in the gas flow.

-5.

[0021] The system preferably also comprises a temperature controller adapted to control the temperature of the gas flow at the heating means or in the heating chamber. The temperature controller allows for first measuring the temperature of the gas flow prior to heating and compares this to the desired temperature setting. Advantageously, the temperature controller provides feedback as to whether the desired temperature has been reached. Another advantage of using a temperature controller is that energy for heating is used more efficiently. Furthermore, in the case that the gas flow already has a temperature above 200 °C, preferably above 250 °C, more preferably above 300 °C, the temperature controller can shut the heating off, as the desired temperature has already been reached.

[0022] Preferably, the system also comprises NOx sensors adapted to measure the NOx concentration in the primary and/or in the secondary gas flow before they are combined. The measured concentrations are then registered in a central control unit. Preferably, the rotational speed of the flow inducing means is adapted to be controlled based on the information provided by the NOx sensors towards achieving a predetermined NOx conversion rate in the combined gas flow.

[0023] Preferably, the system further comprises a NOx adsorbent, preferably placed downstream of the first catalytic chamber comprising the reduction catalyst, that is adapted for adsorbing NOx from the combined gas flow. The advantage of using a NOx adsorbent is to achieve a higher NOx removal rate from the combined gas flow. In particular, placing the NOx adsorbent downstream of the first catalytic chamber results in adsorbing the last residues of NO, that were not yet converted by the reduction catalyst. When such adsorbents are placed upstream of the reduction catalyst, the adsorbents are exposed to higher concentrations of NOx, resulting in a fast saturation of the adsorbents. Good results may be obtained using activated carbon as NOx adsorbent.

[0024] Itis preferred that the system comprises a second catalytic chamber upstream of the first catalytic chamber, configured to receive at least a portion of at least one of the primary gas flow, the secondary gas flow and/or the combined gas flow, wherein the second catalytic chamber comprises an oxidation catalyst. The operation of an SCR catalyst is more effective when the exhaust gases from the diesel engine contain a high concentration of NO: (NO: breaks down more easily than NO). Typically, however, the NOx in the exhaust gas from diesel engines consists of 20% NO»: and 80% NO. An oxidation catalyst can oxidize the NO to NO: to obtain a molar NO/NO: ratio in the combined gas flow to a molar ratio between 10:90 — 50:50, preferably between 20:80 — 40:60, more preferably 30:70. Good results for the SCR

-6- catalyst may be obtained for gas flows having a NO/NO2 molar ratio of approximately 70% NO: and 30% NO.

[0025] Any oxidation catalyst known to the skilled worker may be used, but preferably a Diesel Oxidation Catalyst (DOC) is used, as this catalyst is designed for treating exhaust gas 5S from diesel engines. The DOC is also capable of converting undesired CO and hydrocarbons in the exhaust gas into CO: and H:O.

[0026] It is preferred that the oxidation catalyst is placed downstream of the heating means, as the oxidation catalyst functions best at a minimum temperature of 200 °C, preferably at least 250 °C, more preferably at least 300 °C.

[0027] The system may further also comprise a diesel particulate filter (DPF), which is a self- generating soot filter configured to receive at least a portion of at least one of the primary gas flow, secondary gas flow and/or the combined gas flow. Preferably, the DPF 1s placed upstream of the first catalytic chamber. In this way, soot particles are caught before they reach the reduction catalyst and block the reduction catalyst. The DPF is preferably placed downstream of the heating means, because the filters work better when the gas flow is heated to a temperature of at least 200 °C, preferably at least 250 °C, most preferably at least 300 °C.

[0028] An advantage of the system according to the invention is that it is configured to be placed in a single housing unit that can be adapted to be placed on a trailer and/or on a motorized vehicle and/or adapted to be arranged in flow communication with at least one internal combustion engine. As such, the system can be used on e.g. construction sites, where the system is connected to multiple machines or vehicles. When the system is placed on a trailer it can be towed to follow the machines or vehicles while lowering the NOx concentration in the exhaust gasses of these machines and vehicles. The system according to the invention may also be configured to be directly connected on or to the vehicle or machine or equipment.

[0029] Preferably, the system is adapted to operate in a plurality of predetermined modes, wherein the system is adapted to adjust the rotational speed of the flow inducing means, such as a ventilation fan, to set a different NOx removal rate at each mode.

[0030] Preferably all measured data in the system are captured in a control system that is also accessible online.

[0031] The system typically requires electricity to operate, which can be supplied either by a generator set or via a direct connection to the grid.

[0032] The invention further relates to a method for lowering NO, concentration in a gas flow comprising the steps of: - introducing a primary gas flow;

-7- - introducing a secondary gas flow comprising ambient air; - combining the primary and secondary gas flows to form a combined gas flow; - contacting the combined gas flow with a reduction catalyst in a first catalytic chamber; - introducing a controlled amount of a reductant onto the reduction catalyst or into the combined gas flow for contact with the reduction catalyst; - heating at least a portion of at least one of the primary gas flow, the secondary gas flow and/or the combined gas flow, at a point upstream of the first catalytic chamber to achieve a temperature of the combined gas flow of at least 200 °C, preferably at least 250 °C, more preferably at least 300 °C; - drawing the combined gas flow through at least the first catalytic chamber via a flow inducing means; and - controlling rotational speed of the flow inducing means to control flow rate of the combined gas flow towards a predetermined flow rate.

[0033] Preferably, the primary gas flow comprises exhaust gas from one or more diesel engines. In this way the method can simultaneously lower the NOx concentration in the exhaust gas from multiple diesel engines. Preferably these engines are non-road engines found in construction vehicles and machines.

[0034] Preferably, the reduction of NOx is performed with a selective catalytic reduction (SCR) catalyst. The reductant is preferably in the form of an urea solution, as this a safe reductant to store. The reductant is preferably introduced to the reduction catalyst by injecting it into the combined gas flow (e.g. by spraying the urea solution from a nozzle) to ensure proper mixing with the NOx in the combined gas flow, but other methods may also be used.

[0035] Preferably, the amount of reductant that is injected in the combined gas flow is based on a measured concentration of NOx in the combined gas flow. This is advantageous because too much reductant would lead to the undesired presence of ammonia in the gas flow after catalytic reduction of NOx and if there is not enough reductant present on the catalyst, the NOx removal rate is decreased.

[0036] Preferably in the method according to the invention, the NOx concentration in the primary gas flow and/or the secondary gas flow is measured. On the basis of these measured concentrations, the rotational speed of the flow inducing means is adjusted as to realize a predetermined NOx removal rate from the combined gas flow.

[0037] The method according to the invention preferably also comprises contacting the combined gas flow with a NOx adsorbent, preferably an activated carbon, at some point after contacting the combined gas flow with the reduction catalyst. An advantage is that the

-8- concentration of NOx in the gas flow is further lowered. NOx residues that were not converted in the reduction catalyst can be filtered out of the gas flow by adsorbing on the activated carbon.

Good results are obtained with activated carbon as NOx adsorbent.

[0038] The method according to the invention preferably also comprises contacting at least a portion of at least one of the primary gas flow, the secondary gas flow, and/or the combined gas flow with an oxidation catalyst at some point before contacting the combined gas flow with a reduction catalyst in the first catalytic chamber. This oxidation catalyst is preferably a Diesel Oxidation Catalyst (DOC), because this catalyst is specifically designed to treat exhaust gases from diesel engines. Since a reduction catalyst, in particular the SCR catalyst, performs more optimally reducing NO: in comparison with NO, it is advantageous to oxidize at least a part of the NO in the NOx to NOa. Preferably, the oxidation catalyst oxidizes the NOx as to achieve a molar ratio for NO/NO: between 10:90 — 50:50, preferably between 20:80 — 40:60, more preferably 30:70. Good results with the SCR catalysts are obtained when the molar ratio is 30:70.

[0039] The method according to the invention may preferably also comprise contacting at least a portion of at least one of the primary gas flow, the secondary gas flow and/or the combined gas flow with a particulate filter at some point before contacting the combine gas flow with a reduction catalyst, to filter out soot particles in the gas flow as to prevent these particulates to clog the SCR catalyst. The particulate filter is preferably a Diesel Particulate Filter, because these filters are designed for filtering soot particles from exhaust gases from diesel engines.

[0040] Preferably the flow inducing means is a ventilation fan. Brief Description of the Drawings

[0041] The features and advantages of the invention will be appreciated upon reference to the following drawings, in which:

[0042] FIG. 1 is a simplified schematic diagram of one embodiment of the invention showing a system for lowering NOx concentration in a gas flow according to the invention; and

[0043] FIG. 21s a simplified diagram of one embodiment of the invention, showing a method for lowering NOx concentration in a gas flow according to the invention.

[0044] The drawings are intended for illustrative purposes only, and do not serve as restriction of the scope or the protection as specified in the claims.

-9- Detailed Description of the Invention

[0045] The following is a description of certain embodiments of the invention, given by way of example only and with reference to the drawings.

[0046] Figure 1 shows an embodiment of a system 1 for lowering NOx concentration in a gas flow according to the invention. The system 1 comprises a first gas inlet unit 5 adapted for introducing a primary gas flow 4. In this embodiment, the first gas inlet unit 5 is an exhaust gas intake and is connected to the exhaust pipe 3 of a diesel engine 2. The first gas inlet unit 5 may also be connected to or arranged in a flow communication with more than one exhaust pipe deriving from different diesel engines. The connection is preferably done via flexible coupling 12, preferably via a chute.

[0047] The primary gas flow 4 is introduced to the system via the first gas inlet unit 5 comprises exhaust gas from the diesel engine 2. The diesel engine may originate from vehicles or may be non-road engines originating from construction vehicles, machines, or equipment. The preferred NOx sensor 22 measures the NOx concentration in the primary gas flow 4 and provides this information to the main control cabinet 50.

[0048] The second gas inlet unit 7 is a gas intake for ambient air. The secondary gas flow 6 therefore mainly comprises air, which is introduced to the system via a ventilation fan 25. The fan 25 draws a gas flow into the system via a rotational movement that is controlled by the controller 26. The rotational speed of the fan 25 can be varied by the controller 26 so as to vary the gas flow in the system. The preferred NOx sensor 23 measures the NOx concentration in the secondary gas flow 6 and provides this information to the main control cabinet 50.

[0049] In the embodiment according to the invention shown in Figure 1, the primary 4 and secondary 6 gas flows are introduced to the system 1 via the same flexible coupling 12 and are in flow communication with the fan 25. Ambient air starts flowing in as a secondary gas flow 6 when the flow of the primary gas flow 4 is lower than the flow induced by the rotational speed of the fan 25. This can be the situation when the rotational speed of the fan is high and/or when the engine 2 works at low load with a low amount of exhaust gas. In this particular embodiment, the primary 4 and secondary 6 gas flows are combined to form combined gas flow 9. It is also possible to combine the two gas flows at a later stage in the system, as long as the two gas flows are combined prior to entering the first catalytic chamber 16.

[0050] The concentration of NOx in the combined gas flow 9 is measured by a NOx sensor 42 and the flow rate is measured by air flow measurer 8. The concentration of NOx in the ambient air is measured from a sample of ambient air 40 by a NOx sensor 41. The information obtained from these measurements is sent to the main control cabinet 50.

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[0051] The combined gas flow 9 enters the heating chamber 10 comprising heating means 11, which is controlled by a temperature controller 13. The combined gas flow 9 is heated to a temperature of at least 200 °C, more preferably at least 250 °C, most preferably at least 300 °C to form the heated combined gas flow 19. The combined gas flow requires this minimum 5S temperature as to ensure optimized catalytic activity in the first 16 and/or second 14 catalytic chamber and/or in the Diesel Particulate Filter 19. The temperature controller allows for first measuring the temperature of the gas flow 9 prior to heating and compares this to the desired temperature setting. An advantage of using a temperature controller 13 is that energy for heating is used more efficiently. The temperature controller 13 is controlled by the main control cabinet 50.

[0052] The heated combined gas flow 9 passes through the Diesel Particulate Filter 19 to filter out soot particles and subsequently enters the second catalytic chamber 14 comprising a catalyst bed comprising the Diesel Oxidation Catalyst (DOC) 15. In the second catalyst chamber 14, the DOC 15 converts at least partially the NO into N02. Preferably, the DOC oxidizes the NOx as to achieve a molar ratio for NO/NO: between 10:90 — 50:50, preferably between 20:80 — 40:60, more preferably 30:70. A gas flow comprising more NO» than NO is more easily reduced by the SCR catalyst 17 in the first catalytic chamber 16. Good results with the SCR catalysts 17 are obtained when the molar ratio is 30:70.

[0053] The first catalytic chamber 16 is receives combined gas flow 9. Preferably, the combined gas flow 9 has been heated in the heating chamber 10, and/or has passed the particulate filter 19 and/or and has been treated by the DOC 15 in the second catalytic chamber

14. The first catalytic chamber 16 comprises the selective catalytic reduction (SCR) catalyst in the form of one or more catalyst beds 17. The invention is not limited to the SCR catalyst. Other suitable NOx reduction catalysts known to the skilled worker may be used, such as NOx storage-reduction NSR catalysts, zeolite-based catalysts, or any other suitable reduction catalyst. The reduction catalyst in the first catalytic chamber 16 is preferably a selective catalytic SCR reduction catalyst 17, because the SCR catalyst results in higher NOx conversion yields compared to other catalysts and allows for a more simple configuration of the catalytic chamber 16. The chamber may comprise the catalyst in the form of one or more catalyst beds

17. Preferably, the chamber is configured as fixed-bed reactors, as indicated in Figure 1. In this way, the gas flows through a catalyst bed, making the catalyst work more efficient.

[0054] The system 1 further comprises an urea storage tank 30 in which the reductant 34 for the SCR catalyst 17 is stored in the form of urea in demineralized water (32% urea following DIN 70070 ~ Adblue). The urea solution 34 is injected into the combined gas flow 9 at injection

-11- unit 33 making use of dosing pump 31 and compressor 32 controlled by the main control cabinet 50 on the basis of the measured NOx concentration in combined gas flow 9 by NOx sensor 42. The urea storage tank 30 and/or dosing pump 31 and/or the compressor 32 may be present inside the first catalytic chamber 16 or may be located outside the first catalytic 5S chamber 16, as long as they are present in the system 1 and as long as the reductant 34 is introduced for exposure to the same catalyst bed 17 as the combined flow 9 is exposed to, such that the catalyst 17 can reduce the NOx in the combined gas flow making use of the reductant

34.

[0055] The injected urea 34 breaks down in the hot gas stream 9. The water evaporates and becomes steam. The urea becomes carbon dioxide (COz), water (H20) and ammonia (NHz). The ammonia attaches itself to the surface of the SCR catalyst and reacts with the NOx and forms nitrogen (N2) and water (H20). The reductant 34 is preferably injected into the heated combined gas flow 29 prior to contacting with the first SCR catalyst bed 17. This enables proper conversion of urea into ammonia and enables mixing of the reductant 34 with the NOx in the gas flow 9 for improved catalytic performance of the SCR catalyst 17. The reductant 34 that is introduced to the SCR catalyst may be in the form of an aqueous urea solution, but may also be anhydrous ammonia or aqueous ammonia or any other suitable reductant. Preferably, the reductant is in the form of an aqueous urea solution, because an urea solution is safer to store than ammonia.

[0056] The first catalytic chamber 16, second catalytic chamber 14, and heating chamber 10 may be individual single spaces but may also comprise several spaces or be divided up in several chambers that may be connected or be linked in flow communication with each other. The first catalytic chamber 16 and/or the second catalytic chamber 14 and/or the particulate filter 19 and/or the heating chamber 10 may also all be combined in one big chamber comprising individual compartments.

[0057] After leaving the first catalytic chamber 16, the temperature of the combined gas flow 9 and the NOx concentration are measured with a temperature sensor 36 and a NOx sensor 37. The measured parameters are sent to the main control cabinet 50 and serve as feedback signals for the performance of the SCR catalyst 17 in the first catalytic chamber 16. A too low temperature or a high concentration of NOx may indicate a malfunctioning in the system 1. A too low temperature may indicate that the SCR catalyst 17 has deactivated. When this is the case, the main control cabinet 50 gives a signal to stop the injection of reductant 34. In this way, a maintenance requirement signal may be provided by the system to the operator.

-12-

[0058] The combined gas flow 9 with a lowered NOx concentration enters the NOx adsorbent 20 comprising activated coal, that adsorbs NOx that has not been reduced in the first catalytic chamber 16 from the combined gas flow 9. The advantage of using a NO, adsorbent 20 1s to achieve a higher NOx removal rate from the gas flow yielding a gas flow with a lowered NOx concentration 9. In particular, placing the NO, adsorbent downstream of the first catalytic chamber 16 results in adsorbing the last residues of NOx that were not yet converted by the reduction catalyst 17. When such adsorbents are placed upstream of the reduction catalyst, the adsorbents are exposed to higher concentrations of NOx, resulting in a fast saturation of the adsorbents. Preferably, the NOx adsorbent is an activated carbon adsorbent.

[0059] The ventilation fan 25 can draw the primary, secondary, and/or combined gas flow through the system. The ventilation fan 25 is configured to vary the rotational speed allowing a higher or lower amount of volume of gas to flow through the system per minute. The system 1 comprises a main control cabinet 50 that can control the rotational speed of the ventilation fan to set a predetermined gas flow rate through the system 1. Preferably, the rotational speed of the flow inducing means is controlled based on the information provided by the NOx sensors 22, 23, 37, 41, 42, and/or 43 towards achieving a predetermined NOx conversion rate.

[0060] The embodiment of the system 1 according to the invention as shown in Figure 1 is designed to capture the NOx from a 400 kW stage 1 diesel engine (7.5 kg NOx / hour) and can work in three modes corresponding to three different NOx conversion rates: LOW: 1.2 KG NOx / hr MEDIUM: 3.6 KG NOx / hr HIGH: 7.5 KG NOx / hr The required reduction level/mode can be selected in the controller by a switch on the main control cabinet 50.

[0061] The system | according the embodiment of Figure 1 may be placed in a single housing unit that can be placed on a trailer and/or on a motorized vehicle and/or can be arranged in flow communication with at least one diesel engine and/or can be directly placed on or connected to the equipment/machine.

[0062] The combined gas flow with a lowered NOx concentration 9 is blown out of the system 1 via air outlet unit 28 into the ambient air. In the air outlet there is another NOx sensor 42 and an NH:-sensor 18. The measured parameters are sent to the main control cabinet 50 and serve as feedback signals for the performance of the SCR catalyst 17 and the NOx adsorbent

20. A high concentration of NOx may indicate saturation of the NOx adsorbent and a high concentration of NH; may indicate a malfunctioning of the dosing pump 31 or deactivation of

-13- the SCR catalyst 17. In this way, a maintenance requirement signal may be provided by the system to the operator. In this embodiment, in case the NOx level is higher than the measured NOx level in the ambient air (as measured by NOx sensor 41), the system 1 operates insufficiently and shall provide a red signal 49. For this embodiment, in the case that the NH: concentration is too high, not only a red signal 49 is provided, but the system is also stopped, because a high NH: concentration is dangerous for the environment.

[0063] Figure 2 shows an embodiment of a method for lowering NO, concentration in a gas flow according to the invention comprising steps S01-S06.

[0064] Step SO1 comprises introducing a primary gas flow 4 comprising exhaust gas. The exhaust gas may originate from one or more internal combustion engines, such as diesel engines.

[0065] Step S02 comprises introducing a secondary gas flow 6 comprising ambient air.

[0066] Step S03 comprises providing a ventilation fan 25 that draws the primary 4 and secondary 6 gas flow and forms a combined gas flow 9.

[0067] Step S04 comprises heating the combined gas flow 9 to a temperature of at least 300 °C.

[0068] Step S05 comprises contacting the heated combined gas flow 9 to a diesel oxidation catalyst DOC to decrease the NO/NO: molar ratio in the gas flow.

[0069] Step S06 comprises contacting the combined gas flow 9 after being contacted with the DOC with a selective catalytic reduction SCR catalyst in the presence of a reductant 34 as to reduce the NO, in the gas flow.

[0070] Step SO7 comprises contacting the combined gas flow 9 after being contacted with the SCR catalyst with an activated coal NOx adsorbent 20 as to adsorb the NOx in the gas flow that had not been reduced by the SCR catalyst and thereby lower the NOx concentration in the gas flow further.

[0071] The description above is intended to be illustrative, not limiting. It will be apparent to the person skilled in the art that various alternative and equivalent embodiments are possible for implementing the invention without departing from the scope of the claims set out below.

Claims

Conclusions l. A system (1) for reducing NOx concentration in a gas stream (9), the system comprising: a first gas inlet unit (5) arranged for introducing a primary gas stream (4), a second gas inlet unit (7) arranged for introducing a secondary gas stream (6), the secondary gas stream (6) comprising ambient air; a first catalytic chamber (16) configured to receive a combined gas stream (9) comprising the primary and secondary gas streams (4,6), the first catalytic chamber (16) comprising a reduction catalyst (17) and arranged to contacting the combined gas stream (9) with the reduction catalyst (17); means (31) arranged to introduce a controlled amount of reducing agents (34) onto the reducing catalyst (17) or into the combined gas stream (9) for contact with the reducing catalyst (17), at least one heating means (11) upstream of the first catalytic converter chamber (16) arranged to provide heating of at least a portion of at least one of the primary gas stream (4), the secondary gas stream (6) and/or the combined gas stream (9) to maintain a temperature of the combined gas stream ( 9) of at least 200°C, preferably at least 250°C, more preferably to reach at least 300°C; and a flow inducing means (25) arranged to draw the combined gas stream (9) through at least the first catalytic chamber (16), the system (1) being adapted to control the rotational speed of the flow inducing means (25) to a predetermined flow rate of the combined gas stream (9).

System (1) according to claim 1, wherein the primary gas stream (4) comprises exhaust gas from one or more diesel engines (2).

System (1) according to any one of the preceding claims, wherein the reduction catalyst (17) is a selective catalytic reduction catalyst.

A system (1) according to any one of the preceding claims, wherein the reducing agent (34) is in the form of an aqueous urea solution.

A system (1) according to any one of the preceding claims, further comprising a NOx sensor (42) arranged to measure the NOx concentration in the combined gas stream (9), the system (1) being arranged to measure the amount of reducing agents introduced by the means (31) based on the measured NOx concentration.

A system (1) according to any one of the preceding claims, further comprising a temperature controller (13) arranged to control the temperature of the gas flow (4, 6 and/or 9) at the heating means (11).

System (1) according to any one of the preceding claims, further comprising NOx-S sensors (22, 23) arranged to measure the NOx concentration in the primary and in the secondary gas streams (4,6).

System (1) according to claim 7, wherein the system (1) is arranged to control the rotational speed of the flow inducing means (25) on the basis of the measured NOx concentration in the primary and secondary gas flows (4,6).

A system (1) according to any one of the preceding claims, further comprising a NOx adsorbent (20) downstream of the first catalytic chamber (16) adapted to adsorb NOx from the combined gas stream (9), said NOx adsorbent (20) is preferably an activated carbon adsorbent.

A system (1) according to any preceding claim, further comprising a second catalytic chamber (14) upstream of the first catalytic chamber (16) configured to receive at least a portion of at least one of the primary gas streams (4 ), the secondary gas stream (6) and/or the combined gas stream (9), wherein the second catalytic chamber (14) comprises an oxidation catalyst (15), preferably a diesel oxidation catalyst.

A system (1) according to claim 10, wherein the second catalytic chamber (14) is arranged to control the NO/NO:molar ratio in the combined gas stream (9) to a molar ratio between 10:90 - 50:50. , preferably between 20:80-40:60, more preferably before 30:70.

A system (1) according to any preceding claim, further comprising a particulate filter (19) upstream of the first catalytic chamber (16), configured to receive at least a portion of at least one of the primary gas streams (4) , the secondary gas stream (6) and/or the combined gas stream (9), wherein the particulate filter (19) is preferably a diesel particulate filter.

A system (1) according to any one of the preceding claims, wherein the current inducing means (25) is a ventilation fan.

A system (1) according to any one of the preceding claims, wherein the system (1) is placed in a single housing unit which is adapted to be placed on a trailer and/or on a motorized vehicle and/or arranged to be set up in power connection with at least one diesel engine.

A system (1) according to any one of the preceding claims, wherein the system (1) is arranged to operate in a plurality of predetermined modes, wherein the system (1) is arranged to vary the rotational speed of the current inducing means (25). to set a different NOx take-off rate at each mode 1n.

A method of reducing the NOx concentration in a gas stream (9) comprising the steps of: providing a primary gas stream (4); providing a secondary gas stream (6) comprising ambient air; combining the primary and secondary gas streams (4, 6) to form a combined gas stream (9); contacting the combined gas stream (9) with a reduction catalyst (17) in a first catalytic chamber (16); introducing a controlled amount of reducing agents (34) onto the reducing catalyst (17) or into the combined gas stream (9) before contacting the reducing catalyst (17); heating at least a portion of at least one of the primary gas stream (4), the secondary gas stream (6) and/or the combined gas stream (9), at a point upstream of the first catalytic chamber (16), to form a to reach a temperature of the combined gas stream (9) of at least 200°C, preferably at least 250°C, more preferably at least 300°C; drawing the combined gas stream (9) through at least the first catalytic chamber (16) via a flow inducing means (25); and controlling the rotational speed of the flow inducing means (25) to control the flow rate of the combined gas flow (9) to a predetermined flow rate.

A method according to claim 16, wherein the primary gas stream (4) comprises exhaust gas from one or more diesel engines (2).

A process according to any one of claims 16-17, wherein the reduction catalyst (17) is a selective catalytic reduction catalyst.

A method according to any one of claims 16-18, wherein the reducing agent (34) is in the form of an aqueous urea solution.

The method of any one of claims 16-19, further comprising measuring the NOx concentration in the combined gas stream (9) prior to contact with the reducing catalyst (17) and controlling the amount of reducing agents (34) used. introduced based on the measured NOx concentration.

A method according to any one of claims 16-20, further comprising measuring the NOx concentration in the primary and secondary gas streams (4, 6) and controlling the rotational speed of the flow inducing means (25) based on the measured NOx concentration.

The method of any one of claims 16 to 21, further comprising contacting the combined gas stream (9) with a NOx adsorbent (2) at some point after contacting the combined gas stream (9) with the reduction catalyst (17), wherein the NOx adsorbent (20) is preferably an activated carbon.

A method according to any one of claims 16-22, further comprising contacting at least a portion of at least one of the primary gas stream (4), the secondary gas stream (6) and/or the combined gas stream (9) with an oxidation catalyst (15) in a second catalytic chamber (14) at some point before contacting the combined gas stream (9) with a reduction catalyst (17) in the first catalytic chamber (16), the oxidation catalyst (15) at preferably a diesel oxidation catalyst.

Process according to claim 23, wherein the oxidation catalyst (15) is arranged to control NO/NO 2 molar ratio in the combined gas stream (9) to a molar ratio between 10:90 - 50:50, preferably between 20: 80 - 40:60, more preferably 30:70.

A method according to any one of claims 16-24, further comprising contacting at least a portion of at least one of the primary gas stream (4), the secondary gas stream (6) and/or the combined gas stream (9) with a particulate filter (19) at some point before contacting the combined gas stream (9) with a reduction catalyst (17), the particulate filter (19) preferably being a diesel particulate filter.

A method according to any one of claims 16-25, wherein the current inducing means (25) is a ventilation fan.