Docket No.: [P2022-1200WO01/ PG22505PC00] 1 THERMOCHEMICAL CATALYST KICK-START TECHNICAL FIELD The disclosure relates generally to exhaust aftertreatment systems. In particular aspects the disclosure relates to thermochemical catalytic kick-starters. The disclosure can be applied in heavy-duty vehicles such as trucks, buses, marine vessels and construction equipment, and also in some types of gensets. Although the disclosure may be described with respect to a particular vehicle or system, the disclosure is not restricted to any particular vehicle or system. BACKGROUND Modern engines are equipped with catalytic exhaust aftertreatment systems that require temperatures within a specified range to ensure adequate catalytic activity, for periodic regeneration and for the prevention of excessive physical and/or chemical fouling. To maximize catalytic activity, the exhaust catalysts should reach the required minimum activation temperature as quickly as possible after a cold start and remain above a minimum threshold throughout the entire time that the engine is operational. The measures required to achieve these objectives depend on engine type, duty cycle and the level of exhaust emission control required. The minimum temperatures requirements for urea SCR systems and DOCs are examples of what is required before some aftertreatment system components can function effectively. For engines with urea SCR catalysts, exhaust temperatures above 185-200°C are typically required before urea dosing is enabled. Dosing at lower temperatures can lead to the formation of urea-based deposits and poor NOx conversion. For engines that use a DOC to generate additional heat for downstream catalysts, the DOC catalyst temperature should exceed 200-250°C before the catalyst will oxidize hydrocarbons and produce the required amount of heat. Some developments that are forcing further refinement of exhaust thermal management include the Euro Stage VI standards that aim to further reduce NOx emissions from off-road heavy-duty engines and vehicles. Meeting these requirements will require significant reductions in cold start NOx emissions through more rapid SCR catalyst temperature rise, earlier supply of ammonia to the SCR catalyst and the prevention of SCR catalyst cooling during idle and low load operation. One object of the inventive concept disclosed herein is therefore to provide an emission reduction system wherein exhaust catalysts in the exhaust aftertreatment device are able to rapidly reach the required minimum activation temperature after a cold start of the combustion engine and thereafter Docket No.: [P2022-1200WO01/ PG22505PC00] 2 remain above a minimum threshold throughout the entire time that the combustion engine is operational. SUMMARY According to a first aspect there is provided an emission reduction system for reducing the amount of one or more undesirable substances in a flow of emissions discharged from a combustion engine of a vehicle, the emission reduction system comprising - an exhaust aftertreatment device capable of reducing a quantity of the one or more undesirable substances from the flow of emissions, the undesirable substances including undesirable chemicals, undesirable particles, or both; and - a thermochemical reactor comprising one or more reactants capable of undergoing thermochemical reactions in cycles of endothermic and exothermic chemical reactions; and - one or more valves for controlling the flow of emissions in the emission reduction system. The first aspect of the disclosure may seek to provide an emission reduction system wherein exhaust catalysts in the exhaust aftertreatment device are able to reach the required minimum activation temperature quickly after a cold start and remain above a minimum threshold throughout the entire time that the combustion engine is operational. A technical benefit may include that the energy required to reach a threshold temperature for activation of the catalysts in the exhaust aftertreatment device at a cold start is generated within the emission reduction system itself without addition of external energy from the outside. Optionally in some examples, including in at least one preferred example, the emission reduction system is configured to be operated in an aftertreatment device warm-up mode actuated in relation to a cold start of the combustion engine wherein at least one of ambient air and the flow of emissions discharged from the combustion engine passes through the thermochemical reactor absorbing thermal energy therefrom, prior to passing through the exhaust aftertreatment device releasing thermal energy thereto. A technical benefit may include that in relation to a cold start of the combustion engine, ambient air and/or the flow of emissions are heated in the thermochemical reactor and thereafter passed through the exhaust aftertreatment device, warming it to a temperature above or at least close to the threshold temperature required to activate the catalysts in the exhaust aftertreatment device. A technical benefit of heating ambient air and passing it through the exhaust aftertreatment device may include that the temperature of the exhaust aftertreatment device is increased without use of the flow of emissions from the combustion engine or by use of the ambient air and the flow of emissions from the combustion engine. This may result in faster heating of the exhaust aftertreatment device. Ambient air as used herein means air, or gas, from an environment external to the emission reduction system Docket No.: [P2022-1200WO01/ PG22505PC00] 3 and/or the vehicle, e.g., gas from the atmosphere of Earth or from a pressurized gas tank. The ambient air is different from the flow of emissions from the combustion engine. Optionally in some examples, including in at least one preferred example, in the aftertreatment device warm-up mode, the emission reduction system is configured to be operated in a first phase wherein ambient air heated by the thermochemical reactor passes through the exhaust aftertreatment device before the combustion engine is started. A technical benefit may include that the exhaust aftertreatment device is heated before the combustion engine is started, for example to the threshold temperature required to activate the catalysts in the exhaust aftertreatment device, or to a temperature closer to the threshold temperature required to activate the catalysts in the exhaust aftertreatment device. This may result in a reduced amount of untreated emissions exiting the exhaust aftertreatment device. Optionally in some examples, including in at least one preferred example, the emission reduction system is configured to be operated in the first phase, and optionally prohibit the combustion engine from being started, until the exhaust aftertreatment device fulfils an aftertreatment device temperature acceptance criterion. A technical benefit may include that a more controlled heating of the exhaust aftertreatment device is achieved, allowing the exhaust aftertreatment device to be heated before the combustion engine is started. Prohibiting the combustion engine from being started may imply a reduced risk of untreated emissions exiting the exhaust aftertreatment device. By way of example, the aftertreatment device temperature acceptance criterion may correspond to at least one threshold, such as reaching a threshold temperature of the exhaust aftertreatment device and/or exceeding a threshold time period. As other non-limiting examples, the aftertreatment device temperature acceptance criterion may correspond to reaching a range of acceptable operation values associated with the exhaust aftertreatment device and/or fulfilling an acceptance test of the exhaust aftertreatment device. Optionally in some examples, including in at least one preferred example, the emission reduction system is configured to initiate the first phase in relation to the cold start of the combustion engine when a first criterion is fulfilled. A technical benefit may include that the first phase is initiated when required, thereby, e.g., avoiding unnecessary or unwanted use of the first phase. The first criterion may correspond to at least one temperature being below a threshold, such as when the combustion engine is below a threshold temperature, when the exhaust aftertreatment device is below a threshold temperature, and/or when ambient air is below a threshold temperature. The threshold temperatures of the combustion engine, the exhaust aftertreatment device and/or the ambient air may be the same or different. Other tests may also be used to verify if the first criterion is fulfilled, such as an emission test of the combustion engine exhaust or a time since the combustion engine was last use. Additionally, or alternatively, the first criterion being fulfilled may correspond to when a user, such as a vehicle driver, accepts a delayed start of the combustion engine. As such, according to an example, the emission reduction system may be configured to use data which is indicative of user input relating Docket No.: [P2022-1200WO01/ PG22505PC00] 4 to if the user accepts a delayed start of the combustion engine or not. As another example, the first criterion being fulfilled may correspond to a planned upcoming start of the combustion engine. For example, the first phase may be initiated a time period before the planned upcoming start. The time period before the planned upcoming start may be predetermined, and/or it may vary depending on current conditions, such as ambient temperature, combustion engine temperature and/or exhaust aftertreatment device temperature. Optionally in some examples, including in at least one preferred example, the emission reduction system is configured to refrain from initiating the first phase in relation to the cold start of the combustion engine when a second criterion is fulfilled. A technical benefit may include that the first phase is initiated when required, thereby, e.g., avoiding unnecessary or unwanted use of the first phase. The second criterion may correspond to at least one threshold, such as when the combustion engine is above a threshold temperature, when the exhaust aftertreatment device is above a threshold temperature, and/or when ambient air is above a threshold temperature. Additionally, or alternatively, the second criterion being fulfilled may correspond to when a user, such as a vehicle driver, denies a delayed start of the combustion engine. As such, according to an example, the emission reduction system may be configured to use data which is indicative of user input relating to if the user accepts a delayed start of the combustion engine or not. Optionally in some examples, including in at least one preferred example, in the aftertreatment device warm-up mode, the emission reduction system is configured to be operated in a second phase wherein a mix of ambient air and the flow of emissions discharged from the combustion engine passes through the thermochemical reactor absorbing thermal energy therefrom, prior to passing through the exhaust aftertreatment device releasing thermal energy thereto. A technical benefit may include faster heating of the exhaust aftertreatment device, using both heated ambient air and the heated flow of emissions. Optionally in some examples, including in at least one preferred example, the second phase is subsequent to the first phase. A technical benefit may include that a step-wise process is achieved wherein the exhaust aftertreatment device is heated by heated ambient air followed by heating the exhaust aftertreatment device by the mix of heated ambient air and the heated flow of emissions from the combustion engine. Optionally in some examples, including in at least one preferred example, in the aftertreatment device warm-up mode, the emission reduction system is configured to be operated in a third phase wherein the flow of emissions discharged from the combustion engine passes through the thermochemical reactor absorbing thermal energy therefrom, prior to passing through the exhaust aftertreatment device releasing thermal energy thereto. A technical benefit may include that the heated flow of emissions is only used for heating the exhaust aftertreatment device. Docket No.: [P2022-1200WO01/ PG22505PC00] 5 Optionally in some examples, including in at least one preferred example, the third phase is subsequent to the first phase and/or the second phase. A technical benefit may include that a step-wise process is achieved. For example, the exhaust aftertreatment device may be heated by heated ambient air followed by a mix of heated ambient air and the heated flow of emissions and thereafter by heating the exhaust aftertreatment device only by the heated flow of emissions. As another example, the exhaust aftertreatment device may be heated by heated ambient air followed by heating the exhaust after- treatment device only by the heated flow of emissions. As yet another example, the exhaust aftertreatment device may be heated by heated ambient air followed by a mix of heated ambient air and the heated flow of emissions. Optionally in some examples, including in at least one preferred example, the emission reduction system is configured to select to initiate either the first, second or third phase of the aftertreatment device warm-up mode in dependence on a current condition of at least one of the combustion engine and the exhaust aftertreatment device, and/or in dependence on a user input. A technical benefit may include that the most suitable phase for the current circumstances is used for heating the exhaust aftertreatment device in relation to starting the combustion engine. Optionally in some examples, including in at least one preferred example, the emission reduction system further comprises a flow source, such as a fan or compressor, configured to aid movement of the heated ambient air and/or the heated flow of emissions to the exhaust aftertreatment device, wherein optionally the emission reduction system is at least configured to activate the flow source during the first phase of the aftertreatment device warm-up mode. A technical benefit may include faster heating of the exhaust aftertreatment device, allowing a higher flow rate of heated ambient air and/or heated flow of emissions passing through the exhaust aftertreatment device. The flow source may be located downstream of the thermochemical reactor and upstream of the exhaust aftertreatment device. Alternatively, the flow source may be located upstream of the thermochemical reactor and upstream or downstream of the exhaust aftertreatment device. By way of example, the emission reduction system may be configured to activate the flow source during the second phase of the aftertreatment device warm-up mode, and/or during the third phase of the aftertreatment device warm- up mode. As yet another example, the emission reduction system may be configured to vary a fan or compressor speed during the aftertreatment device warm-up mode in dependence on a condition of the combustion engine, the exhaust aftertreatment device and/or the thermochemical reactor, such as in dependence on a temperature of the combustion engine and/or the exhaust aftertreatment device, and/or in dependence on a current warming capacity of the thermochemical reactor. For example, the fan or compressor speed may be gradually increased in relation to a gradual increase of the warming capacity of the thermochemical reactor during the aftertreatment device warm-up mode. A technical benefit may include a more energy efficient warming procedure of the exhaust aftertreatment device. Docket No.: [P2022-1200WO01/ PG22505PC00] 6 Optionally in some examples, including in at least one preferred example, the emission reduction system is configured to use ambient air from a pressurized tank in order to aid movement of the ambient air in the emission reduction system. A technical benefit may include that heated ambient air may efficiently flow through the exhaust aftertreatment device, e.g., without need of a fan or compressor. Optionally in some examples, including in at least one preferred example, the emission reduction system is configured so that the heated ambient air and the flow of emissions from the combustion engine enter the exhaust aftertreatment device at a common inlet of the exhaust aftertreatment device. A technical benefit may include that the same inlet, and not separate inlets, is used for the ambient air and the flow of emissions. This may result in a more cost-effective configuration. Optionally in some examples, including in at least one preferred example, the one or more valves in the emission reduction system are positioned with respect to the combustion engine, the exhaust aftertreatment device and the thermochemical reactor so that the emission reduction system is capable of operating in at least two modes including: (A) a heat-charging mode active during operation of the combustion engine, wherein the flow of emissions discharged from a hot combustion engine passes the exhaust aftertreatment device prior to passing through the thermochemical reactor releasing thermal energy therein; and (B) an aftertreatment device warm-up mode actuated at a cold start of the combustion engine, wherein at least one of ambient air and the flow of emissions discharged from the cold combustion engine passes through the thermochemical reactor absorbing thermal energy therefrom prior to passing through the exhaust aftertreatment device and releasing thermal energy thereto. Technical benefits may include that thermal energy generated by the exhaust aftertreatment device, which during normal operation is discharged with the flow of emissions to the exterior of the vehicle, is instead transferred by means of the flow of emissions to a thermochemical reactor. Thereafter, at a cold start of the combustion engine, instead of discharging the untreated flow of emissions exiting from the cold combustion engine directly to the environment, it passes to the thermochemical reactor and absorbs thermal energy therefrom. Thereafter the heated flow of emissions passes to the cold exhaust aftertreatment device releasing thermal energy thereto, warming it to the threshold temperature required to activate the catalysts in the exhaust aftertreatment device. Optionally in some examples, including in at least one preferred example, the one or more valves are diverter valves having at least a first inlet and at least two outlets, wherein - at least a first diverter valve is positioned upstream of the thermochemical reactor and downstream of the exhaust aftertreatment device; and/or - at least a second diverter valve is positioned upstream of the exhaust aftertreatment device and downstream of the thermochemical reactor. A technical benefit may include that the diverter valves may be capable of changing the path in which the flow of emissions flows through components or Docket No.: [P2022-1200WO01/ PG22505PC00] 7 exits the emission reduction system. For example, at least a first valve may be capable of changing the path of the flow of emissions from exiting the emission reduction system at an exit point, partly passing through to the thermochemical reactor and partly to an exit point, or alternatively, completely passing through to the thermochemical reactor. In another example, at least a second valve may be capable of diverting the flow of emissions to be discharged at an exit point of the emission reduction system or alternatively to pass it on to the exhaust aftertreatment device. Optionally in some examples, including in at least one preferred example, the heat-charging mode includes - passing the flow of emissions discharged from a hot combustion engine through the exhaust aftertreatment device absorbing thermal energy therefrom, whereafter at least some of the flow of emissions passes through a first valve to the thermochemical reactor releasing thermal energy present in the flow of emissions to the thermochemical reactor, thereby initiating an endothermic chemical reaction in the one or more reactants present in the thermochemical reactor. After releasing thermal energy to the thermochemical reactor, the flow of emissions may pass through the second valve and exit the emission reduction system at an exit point. A technical benefit may include that in the heat charging mode, at least some of the thermal energy generated by the exhaust aftertreatment device during operation of the combustion engine is used to initiate an endothermic chemical reaction in the one or more reactants present in the thermochemical reactor. Optionally in some examples, including in at least one preferred example, the aftertreatment device warm-up mode includes - an actuator configured to initiate an exothermic chemical reaction in the one or more reactants present in the thermochemical reactor; and - passing the ambient air and/or the flow of emissions discharged from the cold combustion engine to the thermochemical reactor, wherein the ambient air and/or the flow of emissions absorbs thermal energy released during the exothermic reaction in the thermochemical reactor, and whereafter the heated ambient air and/or the flow of emissions is passed through the second valve to the exhaust aftertreatment device releasing absorbed thermal energy thereto. A technical benefit may include that energy loaded into the thermochemical reactor during the heat charging mode, may at a cold start of the combustion engine be released by means of an actuator and initiate an exothermic chemical reaction in the thermochemical reactor. Thermal energy released during the exothermic chemical reaction is absorbed by the ambient air and/or the flow of emissions passing through the thermochemical reactor to the exhaust aftertreatment device wherein the absorbed thermal energy is released thereby raising the activation energy of the catalysts in the exhaust aftertreatment device to the required threshold temperature. By way of example, the actuator may be configured to initiate the exothermic chemical reaction in the one or more reactants present in the thermochemical reactor in response to one of the first, second or Docket No.: [P2022-1200WO01/ PG22505PC00] 8 third phases being initiated. As an example, the actuator may be configured to initiate the exothermic chemical reaction in response to a user input indicative of starting the combustion engine. For example, the user input may correspond to at least one of the user opening a vehicle door (e.g. cab door), an electronic key is detected close to the combustion engine, combustion ignition is activated, a combustion engine start sequence is initiated, etc. As another example, the actuator may be configured to initiate the exothermic chemical reaction in response to a battery status becoming low in a genset which indicates that the generator is about to start. Optionally in some examples, including in at least one preferred example, the flow of emissions discharged from the cold combustion engine are passed through the exhaust aftertreatment device, and the first valve prior to the thermochemical reactor. A technical benefit may include that as soon as the threshold temperature for activation of the catalysts in the exhaust aftertreatment device has been reached the flow of emissions is treated since it continuously flows through the exhaust aftertreatment device. Optionally in some examples, including in at least one preferred example, the thermochemical reactor comprises - a gas storage; and - a heat reaction chamber comprising one or more reactants capable of generating a gas upon addition of thermal energy. A technical benefit may include that during the heat-charging mode, thermal energy released from the flow of emissions to the thermochemical reactor is used to initiate an endothermic chemical reaction in the one or more reactants present in the heat reaction chamber to generate a gas. Thus, thermal energy provided by means of a heated flow of emissions passing by the thermochemical reactor is converted to chemical energy in the form of a gas and the gas may be stored in the gas storage. Optionally in some examples, including in at least one preferred example, the thermochemical reactor is further provided with - a compressor arranged downstream of the heat reaction chamber and upstream of the gas storage, the compressor being adapted to increase the pressure of the gas flowing into the gas storage from the heat reaction chamber; and - a pressure relief valve arranged downstream of the gas storage and upstream of the heat reaction chamber, the pressure relief valve being adapted to control the pressure of the gas flowing into the reaction chamber from the gas storage. A technical benefit may include that gas evolved in the heat reaction chamber during the endothermic chemical reaction can be pressurized by means of the compressor into a small volume upon transfer to the gas storage. Pressurized gas stored in the gas storage may be transferred back to the heat reaction chamber by means of a pressure release valve which releases the pressurized gas in a controlled manner. Docket No.: [P2022-1200WO01/ PG22505PC00] 9 Optionally in some examples, including in at least one preferred example, during the heat-charging mode, thermal energy released from the flow of emissions to the thermochemical reactor initiates an endothermic reaction in the one or more reactants provided in the heat reaction chamber and generates a gas, whereafter the compressor increases the pressure of the gas upon transfer of the gas to the gas storage. A technical benefit may include that upon compression of the gas, the temperature of the gas decreases and the energy initially provided as thermal energy to the thermochemical reactor may now be stored in the gas storage as chemical energy in a cold state until needed. Optionally in some examples, including in at least one preferred example, the emission reduction system is configured to increase the pressure of the gas in the gas storage by means of the compressor to a level in the range of 20 to 80 bars. A technical benefit may include that a large amount of chemical energy is stored in the gas storage, wherein the chemical energy can be converted to thermal energy when needed. Optionally in some examples, including in at least one preferred example, during the aftertreatment device warm-up mode, the actuator actuates the pressure relief valve to allow gas stored in the gas storage to flow into the reaction chamber, thereby initiating an exothermic reaction in the one or more reactants provided in the reaction chamber releasing thermal energy therein. A technical benefit may include that when the gas stored in the gas storage is released back into the heat reaction chamber, it reacts with the one or more reactants present in the heat reaction chamber. During the reaction the chemical energy stored as gas is instantly converted into thermal energy which heats the thermochemical reactor. When passing through the thermochemical reactor the ambient air and/or flow of emissions discharged from the combustion engine during a cold start absorbs thermal energy generated during the exothermic chemical reaction and thereafter releases it to the exhaust aftertreatment device raising the temperature of the exhaust aftertreatment device to the threshold temperature for activating the catalysts. Optionally in some examples, including in at least one preferred example, the chemical reactant is a metal carbonate or a metal hydride. The thermochemical reactor comprises a hot side and a cold side. The hot side is the heat reaction chamber in which thermal energy released from the hot flow of emissions is converted to stable and storable energy, or where storable energy is converted to thermal energy, and the cold side is a gas storage in which the energy is stored in the form of a gas. At the hot side, the temperature of the thermochemical reactor may be at least 300 °C and up to 700 °C or above. When thermal energy is supplied to the hot side of the thermochemical rector a chemical reaction occurs which produces a gas, such as either a carbon dioxide (CO2) gas or a hydrogen (H2) gas. The gas is transferred from the hot side to the cold side where the gas is stored. The energy in the form of a gas may be stored at ambient temperatures for long time periods (years) with minimal loss of energy. Docket No.: [P2022-1200WO01/ PG22505PC00] 10 When the gas stored in the cold side of the thermochemical rector is transferred back to the hot side, the energy of the gas will be released in the reaction chamber, and the temperature of the hot side may increase to a temperature of at least 500 °C and up to 700 °C or above. It may be important that the hot side, i.e., the heat reaction chamber is insulated to minimize thermal energy loss during energy recovery. A technical benefit may include that the thermal energy supplied by the flow of emissions may be converted to chemical energy which may be stored in a cold state, with a minimal energy lost during storage. When needed the stored chemical energy may be converted back into thermal energy and used to warm up the exhaust aftertreatment device. Optionally in some examples, including in at least one preferred example, the emission reduction system is capable of operating in at least three modes, including a by-pass mode (C) wherein the flow of emissions by-passes the thermochemical reactor and is discharged from the emission reduction system at one or more exit points. A technical benefit may include that when the combustion engine is operating at a steady-state and the gas storage is fully charged with chemical energy, there is no need for providing further thermal energy to the thermochemical reactor and the by-pass mode is activated to prevent the thermochemical reactor from becoming overheated. According to a second aspect there is provided a vehicle comprising the emission reduction system disclosed herein. The second aspect of the disclosure may seek to provide a vehicle wherein exhaust catalysts in the exhaust aftertreatment device are able to reach the required minimum activation temperature quickly after a cold start and remain above a minimum threshold throughout the entire time that the combustion engine of the vehicle is operational. A technical benefit may include that the energy required to reach the activation temperature of the catalysts in an exhaust aftertreatment device at a cold start of the vehicle is generated within the emission reduction system itself without addition of external energy from the outside. According to a third aspect of the disclosure there is provided a method for reducing the amount of one or more undesirable substances in a flow of emissions discharged from a combustion engine of a vehicle, the method comprising using the emission reduction system disclosed herein and alternating between a heat-charging mode (A) during operation of the combustion engine and an aftertreatment device warm-up mode (B), actuated at cold start of the combustion engine, wherein during the thermochemical reactor heat-charging mode (A), A-i) the flow of emissions discharged from the combustion engine is passed through an exhaust aftertreatment device absorbing thermal energy therefrom; and A-ii) at least some of the heated flow of emissions exiting the exhaust aftertreatment device is passed to a thermochemical reactor releasing thermal energy present in the heated flow of emissions to the thermochemical reactor, thereby initiating an endothermic chemical reaction in the one or more reactants present in the thermochemical reactor; and during the aftertreatment device warm-up mode (B), Docket No.: [P2022-1200WO01/ PG22505PC00] 11 B-i) initiating an exothermic chemical reaction in the one or more reactants present in the thermochemical reactor by means of an actuator; and B-ii) passing ambient air and/or the flow of emissions discharged from a cold combustion engine through the thermochemical reactor, whereby the ambient air and/or the flow of emissions absorb thermal energy released during the exothermic reaction in the thermochemical reactor; and B-iii) passing the ambient air and/or the flow of emissions to the exhaust aftertreatment device transferring absorbed thermal energy thereto, wherein B-ii) and B-iii) are repeated until the exhaust aftertreatment device has reached a threshold temperature Tth, whereafter the thermochemical reactor heat-charging mode is initiated. The third aspect of the disclosure may seek to provide a method wherein exhaust catalysts in an exhaust aftertreatment device are able to reach the required minimum activation temperature quickly after a cold start of a vehicle, and thereafter remain above a minimum threshold temperature throughout the entire time that the combustion engine of the vehicle is operational. A technical benefit may include that the energy required to reach the activation temperature of the catalysts in an exhaust aftertreatment device at a cold start of the vehicle is generated within the emission reduction system itself without addition of external energy from the outside. The above aspects, accompanying claims, and/or examples disclosed herein above and later below may be suitably combined with each other as would be apparent to anyone of ordinary skill in the arts. Additional features and advantages are disclosed in the following description, claims, and drawings, and in part will be readily apparent therefrom to those skilled in the art or recognized by practicing the disclosure as described herein. There are also disclosed herein control units, computer readable media, and computer program products associated with the above discussed technical benefits. BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a more detailed description of aspects of the disclosure cited as examples. Fig.1 is an exemplary emission reduction system according to one example. Fig.2 is an exemplary view of the thermochemical reactor of the emission reduction system according to one example. Fig.3 is an exemplary view of a vehicle comprising the emission reduction system according to one example. Fig.4 is an exemplary flowchart of a method according to one example. Docket No.: [P2022-1200WO01/ PG22505PC00] 12 DETAILED DESCRIPTION Aspects set forth below represent the necessary information to enable those skilled in the art to practice the disclosure. With reference to Fig.1 a combustion engine 11 in a vehicle 12 emits a flow of emissions to an emission reduction system 10 as disclosed herein. The emission reduction system 10 comprises at least one exhaust aftertreatment device 20 capable of reducing a quantity of the one or more undesirable substances from a flow of emissions, at least one thermochemical reactor 30 comprising one or more reactants capable of undergoing thermochemical reactions in cycles of endothermic and exothermic reactions, and one or more valves 40a, 40b, 40c that enable fluid communication of the flow of emissions discharged from the combustion engine 11 of the vehicle 12 through the emission reduction system 10 to one or more exit points 50a, 50b from the system 10. The flow of emissions may contain one or more undesirable substances resulting from a combustion reaction taking place in a combustion engine 11 of a vehicle 12 during operation of the vehicle 12. The one or more undesirable substances in the flow of emissions may include one or more chemicals that are provided to the combustion engine 11 (e.g., one or more reactants), one or more reaction products, or any combination thereof. The combustion engine 11 may react one or more hydrocarbon reactants to produce energy, preferably by a combustion reaction with oxygen. Any hydrocarbon reactant may be employed. The hydrocarbon reactant may be a fossil fuel, such as oil, natural gas, coal, gasoline, diesel fuel, a bio-fuel derived from one or more biological organisms, such as from plants, algae, animals, or any combination thereof; or a synthetic fuels derived from organic or inorganic reactants and the like. The undesirable substances may also contain particles, such as soot particles. The exhaust aftertreatment device 20 described herein is capable of reducing a quantity of the one or more undesirable substances from the flow of emissions that flows through the exhaust aftertreatment device 20. Preferably, the exhaust aftertreatment device 20 is capable of reducing the amount of nitrogen oxides, hydrocarbons, carbon monoxide, particulate matter, or any combination thereof flowing through the exhaust aftertreatment device 20. Advantageously, the exhaust aftertreatment device 20 is capable of catalytically reacting nitrogen oxides, such as to form nitrogen gas, oxygen gas, or preferably both. Advantageously, the exhaust aftertreatment device 20 is also capable of catalytically reacting a hydrocarbon to produce, at least carbon dioxide and water, and catalytically react carbon monoxide to produce carbon dioxide. Preferably, the exhaust aftertreatment device 20 comprises at least a diesel oxidation catalyst (DOC) unit 21 wherein hydrocarbons (HC) and carbon monoxides (CO) emission levels are reduced. Furthermore, the exhaust aftertreatment device 20 advantageously comprises at least a diesel particulate filter (DPF) 22 which will trap and convert soot and other particles to ash. The exhaust aftertreatment device 20 may further comprise a selective catalytic reduction (SCR) unit 23, either Docket No.: [P2022-1200WO01/ PG22505PC00] 13 included in the DPF 22 or as a stand-alone unit. The SCR unit 23 reduces NOx emissions by injecting urea into the hot flow of emissions which will create a homogenous mixture of NOx and ammonia flowing through the SCR unit 23 where it is converted to harmless nitrogen gas (N2) and water vapor (H2O). The thermochemical reactor 30 described herein (see Fig.2), comprises one or more reactants capable of undergoing thermochemical reactions in cycles of endothermic and exothermic reactions. Endothermic reactions are chemical reactions in which the reactants absorb heat energy from the surroundings to form products. An exothermic reaction is a chemical reaction in which less energy is needed to break bonds in the reactants than is released when new bonds form in the products. During an exothermic reaction, energy is constantly given off, often in the form of heat. Examples of exothermic reactions are combustion reactions. The thermochemical reactor 30 may as shown in fig.2 comprise a gas storage 31 and a heat reaction chamber 32. The one or more reagents present in the heat reaction chamber 32 is advantageously a metal carbonate or a metal hydride which will release gas when heated. The gas storage 31 is configured to store gas. As shown, a compressor 33 may be arranged downstream of the heat reaction chamber 32 and upstream the gas storage 31. The compressor 33 is configured to move the gas generated during the endothermic reaction when the heat reaction chamber 32 is heated, while at the same time increasing the pressure of the gas upon transfer of the gas to the gas storage 31 from the heat reaction chamber 32 (see arrow G _A in Fig.2). The gas is released from the metal carbonate/metal hydride with a relatively low pressure when the reaction chamber 32 is heated. The pressure in the reaction chamber 32 may be between one to a few bars when the reaction chamber 32 is heated to 500-600 °C. However, to be able to store the gas within the volume of the emission reduction system 10, the gas must be stored under high pressure in the gas storage 31. As an example, when the temperature is 20°C and a pressure of 65 bar carbon dioxide liquifies and can be stored in a relatively small storage tank. For example, the emission reduction system 10 may be configured to increase the pressure of the gas in the gas storage 31 by means of the compressor 33 to a level in the range of 20 to 80 bars. Thereby, a large amount of energy can be stored in the gas storage 31. As further shown, a pressure relief valve 34 may be arranged downstream of the gas storage 31 and upstream of the heat reaction chamber 32. The pressure relief valve 34 is configured to control the pressure of the gas flowing from the gas storage 31 to the heat reaction chamber 32. The valve 34 is completely closed when gas is stored in the gas storage 31. An actuator 61 (see Fig.1) may be configured to initiate the pressure release valve 34 to release gas from the gas storage 31 into the heat reaction chamber 32 (see arrow GB in Fig.2). Upon re-entrance of the gas to the reaction chamber 32, heat is evolved in an exothermic reaction when the gas reacts with the metal oxide in the heat reaction chamber 32. The thermochemical reactor 30 is adapted to convert heat to storable energy, and to convert the storable energy back to heat. The thermochemical reactor 30 has one hot side and one cold side which Docket No.: [P2022-1200WO01/ PG22505PC00] 14 are thermally insulated from each other. The hot side comprises a heat reaction chamber 32 capable of being heated by a heated flow of emissions and comprises one or more reactants capable of undergoing endothermic and exothermic chemical reactions in cycles. For example, when a reaction chamber 32 comprising a metal carbonate is heated by passing a heated flow of emissions therethrough, a chemical reaction occurs wherein carbon dioxide (CO2) gas is released from the metal carbonate. The gas is thereafter transferred from the hot side to the cold side by means of the compressor 33, where it is stored as a gas or a liquid. Examples of suitable metals that may be used are e.g., sodium, magnesium, titanium, calcium, aluminum, iron, strontium or barium. The valves 40a, 40b, 40c in the emission reduction system 10 described herein may be positioned with respect to the combustion engine 11, the exhaust aftertreatment device 20 and the thermochemical reactor 30 so that the system 10 is capable of operating in at least two modes including: (A) a heat-charging mode during operation of the combustion engine 11, wherein the flow of emissions discharged from the combustion engine 11 passes the exhaust aftertreatment device 20 prior to passing through the thermochemical reactor 30, absorbing thermal energy in the exhaust aftertreatment device 20 and releasing it to the thermochemical reactor 30 to initiate an endothermic chemical reaction therein; and (B) an aftertreatment device warm-up mode actuated at a cold start of the combustion engine 11, wherein at least one of ambient air and the flow of emissions discharged from the combustion engine 11 passes through the thermochemical reactor 30 prior to passing through the exhaust aftertreatment device 20, absorbing thermal energy generated by an exothermic reaction in the thermochemical reactor 30 and releasing it to the exhaust aftertreatment device 20. During normal operation (i.e., at steady state), the combustion engine 11 generates heat which is transferred to the flow of emissions exiting the combustion engine 11. A warm, fully operational diesel engine may discharge a flow of emissions having a temperature of about 200°C to 700°C depending on the speed and load of the engine. However, most heavy-duty diesel engines produce exhaust temperatures in the range of 300-450°C. During the heat-charging mode (A) the combustion engine 11 is operating at a steady state and the flow of emissions leaving the combustion engine 11 has a relatively high temperature of about 200-700°C, or normally about 300-450°C when it enters the exhaust aftertreatment device 20. When passing through the exhaust aftertreatment device 20 comprising e.g., the above mentioned DOC unit 21, the DPF 22 and the SCR unit 23, the number of undesirable substances is reduced from the flow of emissions in a manner known to the skilled person. The temperature of the flow of emissions exiting the exhaust aftertreatment device 20 may be about 550-650°C. In the emission reduction system 10 described herein, at least some of the hot flow of emissions exiting the exhaust aftertreatment device 20 passes through a first valve 40a and is thereafter passed Docket No.: [P2022-1200WO01/ PG22505PC00] 15 onwards to the thermochemical reactor 30. The first valve 40a is advantageously a diverter valve which has the capability of diverting the flow of emissions in two or more directions. The first valve 40a may divert at least some or the entire flow of emissions either to the thermochemical reactor 30 or to a first exit point 50a through which the hot flow of emissions exits the emission reduction system 10 to the surroundings. Inside the thermochemical reactor 30 the hot flow of emissions enters a heating coil (not shown) arranged in the heat reaction chamber 32 and thermal energy from the flow of emissions is transferred to the heat reaction chamber 32 (see arrow (A) in Fig.2). The supplied thermal energy initiates an endothermic reaction in the reagents disposed inside the reaction chamber 32 and a gas is generated. The compressor 33 arranged downstream of the heat reaction chamber 32 and upstream of the gas storage 31, pressurizes the gas upon transfer of the gas to the gas storage 31 (see arrow G _A in Fig.2). Compression of the gas reduces the temperature of the gas in the gas storage 31 compared to the temperature of the gas in the reaction chamber 32. After the gas has been transferred to the gas storage 31, it can be stored for years with only minimal loss of energy. After releasing thermal energy to the heat reaction chamber 32, the flow of emission may pass from the thermochemical reactor 30 through a second valve 40b to a second exit point 50b and exits the emission reduction system 10 to the surroundings. At cold start of the combustion engine 11, the combustion engine 11, as well as all components of the emission reduction system 10 is at ambient temperature. By way of example, the cold-starting of a combustion engine 11 in a vehicle 12 may occur when the temperature of the combustion engine 11 is about 50° C or less, about 30° C or less, about 0° C or less, or about −20° C or less. Cold starting may occur after the combustion engine 11 has been off for about 5 minutes or more, about 20 minutes or more, about 1 hour or more, or about 3 hours or more. It will be appreciated that the time for the combustion engine 11 and/or the exhaust aftertreatment device 20 to cool (e.g., below the lower limit operating temperature of the exhaust aftertreatment device 20) may depend on the ambient temperature, the thermal mass, the initial temperatures, and the rate at which the thermal energy is removed. At cold start, the flow of emissions exiting the combustion engine 11 has a temperature which is relatively low compared to the temperature when the combustion engine 11 is operating normally at a steady state. Also, the exhaust aftertreatment device 20 is at a temperature which is far below the threshold temperature of about 175-225°C required to activate the catalysts in the exhaust aftertreatment device 20. Any flow of emissions entering the exhaust aftertreatment device 20 before the threshold temperature has been reached will not be properly treated. Thus, it is of uttermost importance during the aftertreatment device warm-up mode that as little as possible of the cold flow of emissions exits to the surroundings before the exhaust aftertreatment device 20 is fully activated. Docket No.: [P2022-1200WO01/ PG22505PC00] 16 At cold start of the combustion engine 11, an actuator 61 is simultaneously, or at least substantially simultaneously, actuated to initiate an exothermic chemical reaction in the thermochemical reactor 30. The exothermic chemical reaction is initiated when the pressure relief valve 34 arranged downstream of the gas storage 31 and upstream of the heat reaction chamber 32, releases the stored gas from the gas storage 31 allowing it to flow to the heat reaction chamber 32 (see arrow GB in Fig.2). When the released gas reacts with the one or more reactants in the heat reaction chamber 32, thermal energy is generated. The actuator 61 may e.g., be a battery powered coil configured to actuate the pressure relief valve 34 to release the stored gas back into the heat reaction chamber 32. The entire flow of emissions exiting the cold combustion engine 11 may pass untreated through the exhaust aftertreatment device 20, through the first valve 40a to the thermochemical reactor 30. Reaching the thermochemical reactor 30, the cold flow of emissions flows through the heating coil in the heat reaction chamber 32 and absorbs thermal energy generated in the exothermal reaction when the stored gas is released into the heat reaction chamber 32 (see arrow (B) in Fig.2). After absorbing heat from the thermochemical reactor 30, the heated flow of emissions may pass through the second valve 40b and on to the exhaust aftertreatment device 20 releasing absorbed thermal energy thereto. Advantageously, also the second valve 40b is a diverter valve having one inlet and at least two outlets. The second valve 40b may pass the flow of emissions from the thermochemical reactor 30 to either the exhaust aftertreatment device 20, or to a second exit point 50b, of the emission reduction system 10 releasing the flow of emissions to the surroundings. A fan 65, or any other flow source, such as a compressor, may be located downstream of the second valve 40b and upstream of the exhaust aftertreatment device 20 to aid movement of the heated flow of emissions to the exhaust aftertreatment device 20. Alternatively, as depicted by a dashed arrow in Fig.1, the cold flow of emissions discharged from the cold combustion engine 11 may by-pass the exhaust aftertreatment device 20 and instead pass directly to the first valve 40a and the thermochemical reactor 30 absorbing thermal energy generated by the exothermic reaction. In this case a third diverter valve 40c may be located downstream of the combustion engine 11 and upstream of the exhaust aftertreatment device 20 for generating this flow. The absorption of thermal energy generated in the exothermic reaction followed by its release to the exhaust aftertreatment device 20 may continue until the exhaust aftertreatment device 20 has reached the required threshold temperature for activating the catalysts in the exhaust aftertreatment device 20. The threshold temperature for activating the catalysts in the exhaust aftertreatment device 20 may be about 175-225°C. The aftertreatment device warm-up mode (B) may not necessarily only use the flow of emissions from the combustion engine 11 for heating the exhaust aftertreatment device 20, but may additionally or Docket No.: [P2022-1200WO01/ PG22505PC00] 17 alternatively use ambient air, see e.g., arrow AA in FIG.1, which is heated in the thermochemical reactor 30 and then passed to the exhaust aftertreatment device 20. For example, in the aftertreatment device warm-up mode, the emission reduction system 10 may be configured to be operated in a first phase wherein ambient air heated by the thermochemical reactor 30 passes through the exhaust aftertreatment device 20 before the combustion engine 11 is started. The exhaust aftertreatment device 20 may thereby be heated before the combustion engine 11 is started, for example to the threshold temperature required to activate the catalysts in the exhaust aftertreatment device 20, or to a temperature closer to the threshold temperature required to activate the catalysts in the exhaust aftertreatment device 20. The emission reduction system 10 may be configured to be operated in the first phase, and optionally prohibit the combustion engine 11 from being started, until the exhaust aftertreatment device 20 fulfils an aftertreatment device temperature acceptance criterion. The aftertreatment device temperature acceptance criterion may correspond to at least one threshold, such as reaching a threshold temperature of the exhaust aftertreatment device 20 and/or exceeding a threshold time period. As another example, the aftertreatment device temperature acceptance criterion may correspond to reaching a range of acceptable operation values associated with the exhaust aftertreatment device 20, such as a range of values comprising temperature of at least one of the DOC unit 21, the DPF 22 and the SCR unit 23. Additionally or alternatively, the aftertreatment device temperature acceptance criterion may correspond to fulfilling an acceptance test of the exhaust aftertreatment device 20. The emission reduction system 10 may be configured to initiate the first phase in relation to the cold start of the combustion engine 11 when a first criterion is fulfilled. The first criterion may correspond to at least one temperature being below a threshold, such as when the combustion engine 11 is below a threshold temperature, when the exhaust aftertreatment device 20 is below a threshold temperature, and/or when ambient air is below a threshold temperature. Additionally, or alternatively, the first criterion being fulfilled may correspond to when a user, such as a vehicle driver, accepts a delayed start of the combustion engine 11. As such, according to an example, the emission reduction system 10 may be configured to use data which is indicative of user input relating to if the user accepts a delayed start of the combustion engine 11 or not. As another example, the first criterion being fulfilled may correspond to a planned upcoming start of the combustion engine 11. For example, the first phase may be initiated a time period before the planned upcoming start. The time period before the planned upcoming start may be predetermined, and/or it may vary depending on current conditions, such as ambient temperature, combustion engine temperature and/or exhaust aftertreatment device temperature. The planned upcoming start of the combustion engine 11 may be planned by a user and/or it may be retrieved from a predetermined driving schedule for the vehicle 12. For example, the first phase may be initiated 5-30 minutes before the planned upcoming start of the combustion engine 11. Docket No.: [P2022-1200WO01/ PG22505PC00] 18 The emission reduction system 10 may be configured to refrain from initiating the first phase in relation to the cold start of the combustion engine 11 when a second criterion is fulfilled. The second criterion may correspond to at least one threshold, such as when the combustion engine 11 is above a threshold temperature, when the exhaust aftertreatment device 20 is above a threshold temperature, and/or when ambient air is above a threshold temperature. For example, if it is determined that the flow of emissions heated in the thermochemical reactor 30 can be used for reaching the threshold temperature required to activate the catalysts in a relatively short time, the emission reduction system 10 may be configured to refrain from initiating the first phase. Additionally, or alternatively, the second criterion being fulfilled may correspond to when a user, such as a vehicle driver, denies a delayed start of the combustion engine 11. A late start may for example be denied if the user needs to immediately start driving the vehicle 12. The emission reduction system 10 may additionally or alternatively be configured to start the combustion engine 11 once the first phase is completed or configured to accept starting of the combustion engine 11 once the first phase is completed, i.e., it does no longer prohibit a start. The emission reduction system 10 may additionally or alternatively be configured to be operated in a second phase wherein a mix of ambient air and the flow of emissions discharged from the combustion engine 11 passes through the thermochemical reactor 30 absorbing thermal energy therefrom, prior to passing through the exhaust aftertreatment device 20 releasing thermal energy thereto. The second phase may be subsequent to the first phase. The emission reduction system 10 may additionally or alternatively be configured to be operated in a third phase wherein the flow of emissions discharged from the combustion engine 11 passes through the thermochemical reactor 30 absorbing thermal energy therefrom, prior to passing through the exhaust aftertreatment device 20 releasing thermal energy thereto. The third phase may be subsequent to the first phase and/or the second phase. When at least two of the three phases are used subsequently, the emission reduction system 10 may be configured to trigger to change from one of the phases to another one of the phases based on at least one of data indicative of a temperature of the exhaust aftertreatment device 20 and time data. The time data may be predetermined, such as set so that one of the phases is not performed for too long time. The time data may for example be determined based on practical tests or computer simulation relating to how long time it normally takes for the exhaust aftertreatment device 20 to be heated to a temperature, or close to a temperature, where the catalysts in the exhaust aftertreatment device 20 are activated. The emission reduction system 10 may be configured to select to initiate either the first, second or third phase of the aftertreatment device warm-up mode in dependence on a current condition of at least Docket No.: [P2022-1200WO01/ PG22505PC00] 19 one of the combustion engine 11 and the exhaust aftertreatment device 20, and/or in dependence on a user input. If only two of the aforementioned phases are available, the emission reduction system 10 may be configured to select to initiate one of the two phases in dependence on a current condition of at least one of the combustion engine 11 and the exhaust aftertreatment device 20, and/or in dependence on a user input. The current condition may for example relate to a temperature of at least one of the combustion engine 11 and the exhaust aftertreatment device 20. Additionally or alternatively, the current condition may relate to if one of the phases has been performed before selecting to initiate another one of the phases. The emission reduction system 10 may at least be configured to activate the above-mentioned fan 65 during the first phase of the aftertreatment device warm-up mode. As shown, the flow source 65 may be located downstream of the thermochemical reactor 30 and upstream of the exhaust aftertreatment device 20. Alternatively, the flow source may be located upstream of the thermochemical reactor and upstream or downstream of the exhaust aftertreatment device. The emission reduction system 10 may additionally or alternatively be configured to activate the flow source 65 during the second phase of the aftertreatment device warm-up mode, and/or during the third phase of the aftertreatment device warm-up mode. The ambient air may be stored in a pressurized tank (not shown), such as a tank used for driving pneumatic actuators of a vehicle. As such, if the ambient air is at least partly stored in a pressurized tank, a flow of air through the thermochemical reactor 30 and the exhaust aftertreatment device 20 may be generated when the pressurized ambient air is released from the tank. The emission reduction system 10 may be configured to vary a fan or compressor speed during the aftertreatment device warm-up mode in dependence on a condition of the combustion engine 11, the exhaust aftertreatment device 20 and/or the thermochemical reactor 30, such as in dependence on a temperature of the combustion engine 11 and/or the exhaust aftertreatment device 20, and/or in dependence on a current warming capacity of the thermochemical reactor 30. For example, the fan or compressor speed may be gradually increased in relation to a gradual increase of the warming capacity of the thermochemical reactor 30 during the aftertreatment device warm-up mode. In a similar manner, the release of air from the above-mentioned pressurized tank may be varied in dependence on a condition of the combustion engine 11, the exhaust aftertreatment device 20 and/or the thermochemical reactor 30, such as in dependence on a temperature of the combustion engine 11 and/or the exhaust aftertreatment device 20, and/or in dependence on a current warming capacity of the thermochemical reactor 30. As shown in Fig.1, the emission reduction system 10 may be configured so that the heated ambient air and the flow of emissions from the combustion engine 11 enter the exhaust aftertreatment device 20 at a common inlet 24 of the exhaust aftertreatment device 20. Docket No.: [P2022-1200WO01/ PG22505PC00] 20 The ambient air may be entered into the emission reduction system 10 at any suitable entry point, such as from the exit point 50a via the first valve 40a. To avoid overheating of the thermochemical reactor 30, the emission reduction system 10 may operate in a third mode, a by-pass mode (C) which may be invoked when the combustion engine 11 is operating at a steady state and the gas storage 31 in the thermochemical reactor 30 is fully charged with gas. In the by-pass mode the hot flow of emissions exiting the exhaust aftertreatment device 20 may pass through the first valve 40a to a first exit point 50a and may be released into the surroundings instead of being passed on to the thermochemical reactor 30. The system of Fig.1 comprises a controller 60, or control unit, which in its simplest configuration may comprise a processing unit 62, a memory 63 and a communication interface 64. The controller 60 is arranged to control various functions of the previously described system 20, for example arranged to control the first, second and/or third phases as mentioned in the above. For example, the controller 60 may receive various signals corresponding to detected temperatures from temperature sensors present in e.g., the combustion engine 11, the exhaust aftertreatment device 20, and the thermochemical reactor 30 and based on the received temperature data, control the operation of described valves 40a, 40b, 40c, actuator 61 and/or flow source 65. Thus, the communication interface 64 may be configured to receive signals from the sensors and/or data relating to user input and/or temperature data and output control signals to control valve actuators (not shown), the actuator 61, the flow source 65, etc. in response to the received signals and/or data. The memory 63 in communication with the processing unit 62 may contain instructions and/or data. The instructions in the memory 63 may control the operations of the processing unit 62 when executed by the same. Consequently, the instructions may cause the processing unit 62 at least to: - during the heat-charging mode (A), output a first control signal allowing flow of emissions discharged from a hot combustion engine 11 to pass through an exhaust aftertreatment device 20 absorbing thermal energy therefrom; and output a second control signal allowing at least some of the heated flow of emissions exiting the exhaust aftertreatment device 20 to pass to a thermochemical reactor 30 releasing thermal energy present in the flow of emissions to the thermochemical reactor 30, thereby initiating an endothermic chemical reaction in the one or more reactants present in the thermochemical reactor 30; and during the aftertreatment device warm-up mode (B): 1) controlling the actuator 61 initiating an exothermic chemical reaction in the one or more reactants present in the thermochemical reactor 30; 2) output a third control signal allowing passage of ambient air and/or the flow of emissions discharged from a cold combustion engine 11 through the thermochemical reactor 30, whereby the ambient air and/or the flow of emissions absorbs thermal energy released during the exothermic chemical reaction in the thermochemical reactor 30; and 3) output a fourth control signal allowing passage of the ambient air and/or the flow of emissions to the Docket No.: [P2022-1200WO01/ PG22505PC00] 21 exhaust aftertreatment device 20 transferring absorbed thermal energy thereto. Steps 2) and 3) may be repeated until the exhaust aftertreatment device 20 has reached a threshold temperature Tth, whereafter the thermochemical reactor heat-charging mode may be initiated. Of course, the controller 60 can be realized as a single processor or part of a processor. Even though the controller 60 may receive various signals corresponding to detected temperatures from temperature sensors as mentioned in the above, the controller 60 may additionally or alternatively receive data indicative of the temperature of e.g., the combustion engine 11, the exhaust aftertreatment device 20, and the thermochemical reactor 30 by use of system models and/or correlations between the different components in the emission reduction system 10. The thresholds as mentioned herein, e.g., temperature thresholds, may be determined by practical experimentation of the emission reduction system 10 and/or by computer simulations. Fig.3 is an exemplary view of a vehicle 12 comprising the emission reduction system 10 and a combustion engine 11 according to one example. The vehicle 12 is in this example a truck. It shall however be noted that the disclosure is also applicable to any other type of vehicle comprising an emission reduction system and a combustion engine, such as a bus, a passenger car and a construction equipment, such as a wheel loader, an excavator, etc. The vehicle may for example also be a marine vessel. As another example not shown, the emission reduction system 10 may be part of a genset, such as a diesel generator or any other generator set comprising a combustion engine. Fig.4 depicts a flowchart of a method for reducing the amount of one or more undesirable substances in a flow of emissions discharged from a combustion engine 11 of a vehicle 12. The method comprises using the emission reduction system 10 as disclosed herein, alternating between a heat-charging mode (A) during operation of the combustion engine 11 and an aftertreatment device warm-up mode (B) actuated at cold start of the combustion engine 11, wherein: A-i) the flow of emissions discharged from a hot combustion engine 11 is passed through an exhaust aftertreatment device 20 absorbing thermal energy therefrom; and A-ii) at least some of the heated flow of emissions exiting the exhaust aftertreatment device 20 is passed to a thermochemical reactor 30 releasing thermal energy present in the flow of emissions to the thermochemical reactor 30, thereby initiating an endothermic chemical reaction in the one or more reactants present in the thermochemical reactor 30; and during the aftertreatment device warm-up mode (B): B-i) initiating an exothermic chemical reaction in the one or more reactants present in the thermochemical reactor 30 by means of an actuator 61; and B-ii) passing ambient air and/or the flow of emissions discharged from a cold combustion engine 11 through the thermochemical reactor 30, whereby the ambient air and/or the flow of emissions absorbs thermal energy released during the exothermic chemical reaction in the thermochemical reactor 30; Docket No.: [P2022-1200WO01/ PG22505PC00] 22 and B-iii) passing the ambient air and/or the flow of emissions to the exhaust aftertreatment device 20 transferring absorbed thermal energy thereto, wherein B-ii) and B-iii) are repeated until the exhaust aftertreatment device (20) has reached a threshold temperature Tth, whereafter the thermochemical reactor heat-charging mode (A) is initiated. The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms "comprises" "comprising" "includes," and/or "including" when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element without departing from the scope of the present disclosure. Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe a relationship of one element to another element as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. It will be understood that when an element is referred to as being connected or coupled to another element, it can be directly connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being directly connected or directly coupled to another element, there are no intervening elements present. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. It is to be understood that the present disclosure is not limited to the aspects described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the present disclosure and appended claims. In the Docket No.: [P2022-1200WO01/ PG22505PC00] 23 drawings and specification, there have been disclosed aspects for purposes of illustration only and not for purposes of limitation, the scope of the inventive concepts being set forth in the following claims.