DK201370637A1 - A combustion engine system - Google Patents

Abstract

The present invention relates to a combustion engine system. The combustion engine system comprises an internal combustion engine having engine data and being powered by a fuel having a sulphur content of at least 0.05% and generating an exhaust gas; a heat exchanger arranged downstream of the internal combustion engine; and a gas purification arrangement for purifying the exhaust gas from the internal combustion engine. The gas purification arrangement comprises a NOx reduction unit fluidly connected with the internal combustion engine for receiving the exhaust gas, the NOx reduction unit comprising one or more catalytic reactor(s) having a volume of at least 200 litres; a reducing agent supply unit fluidly connected to the NOx reduction unit; and a NOx sensor arranged downstream of the NOx reduction unit for measuring a NOx concentration in the exhaust gas. The reducing agent supply unit comprises a dosing unit for dosing an amount of reducing agent to the exhaust gas in or before entering the NOx reduction unit, wherein the amount of reducing agent is derived from the engine data. The gas purification arrangement further comprises a control unit adapted to reduce the amount of reducing agent supplied to the NOx reduction unit when the NOx concentration measured by the NOx sensor is below a pre-set value of NOx concentration in the exhaust gas. The present invention further relates to a NOx reduction method of reducing an ammonia bisulphate (ABS) formation in a heat exchanger in a combustion engine system.

Description

A COMBUSTION ENGINE SYSTEM

Field of the invention

The present invention relates to a combustion engine system and a NOx reduction method of reducing an ammonia bi-sulfate (ABS) formation in a heat exchanger in a combustion engine system.

Background art

Vessels having internal marine combustion engines have been subjected to restricted emissions of NOx by guidelines from the Internal Maritime Organization (IMO). The recent proposal for Tier III guidelines sets a goal of reducing the NOx emission to 3.4 g / kWh when the vessel is within a certain distance from the shore of an Emission Control Area.

Marine applications represent a challenge in terms of both dynamic operation and high sulfur levels in the fuel. Furthermore, marine vessels are often operated with varying fuel qualities depending on where the marine vessel bunkered fuel load, and the same vessel will often operate under wildly varying ambient conditions during a normal trip, i.e. tropical conditions in the Caribbean Sea and winter conditions in the North Sea. The varying fuel quality, operating conditions and varying ambient conditions will have a significant impact on the NOx emissions of the marine engine. The present systems of controlling the NOx emission of a marine combustion engine are therefore not sufficient to reach the goal of reducing the NQX emission to 3.4 g / kWh.

Summary of the invention

It is an object of the present invention to wholly or partly overcome the above disadvantages and drawbacks of the prior art. More specifically, It is an object to provide an improved internal combustion engine system which is capable of fulfilling the Tier III guidelines of having a reduced NOx concentration

Furthermore, it is an object to avoid ammonia slips in order to prevent ABS formation in the heat exchanger of an internal combustion engine system while still being able to fulfill the Tier III guidelines.

The above objects, together with numerous other objects, advantages, and features, which will become evident from the description below, are accomplished by a solution in accordance with the present invention by a combustion engine system comprising: - an internal combustion engine having engine data and being powered by a fuel having a sulfur content of at least 0.05% and generating an exhaust gas, - a heat exchanger arranged downstream of the internal combustion engine, and - a gas purification arrangement for purifying the exhaust gas from the internal combustion engine, the gas purification arrangement comprising: - an NGX reduction unit fluidly connected to the internal combustion engine for receiving the exhaust gas, the NOx reduction unit comprising one or more catalytic reactor (s) having a volume of at least 200 liters, - a reducing agent supply unit fluidly connected to the NOx reduction unit, the reducing agent supply unit comprising: a dosing unit for dosing an amount of r educating agent to the exhaust gas in or before entering the NOx reduction unit, the amount of reducing agent is derived from the engine data, and - a ΝΟχ sensor arranged downstream of the NOx reduction unit for measuring an NQX concentration in the exhaust gas, The gas purification arrangement further comprises a control unit adapted to reduce the amount of reducing agent supplied to the NOx reduction unit when the NOx concentration measured by the NOx sensor is below a pre-set value of NOx concentration in the exhaust gas.

By having a control unit adapted to reduce the amount of reducing agent supplied to the NOx reduction unit, the amount of reducing agent supplied to the NOx reduction unit can be easily regulated without adjusting the entire set-up of the dosing unit. Furthermore, the NOx sensor does not measure the NOx concentration in the gas every time the dosing unit doses an amount of reducing agent. NOx sensors adapted to measure NOx concentration are very costly, and one NOx sensor is only capable of making a certain number of measurements before being replaced. Therefore, by the present combustion engine system, the NOx sensor only measures the NOx concentration at certain intervals independent of the dosages, e, g. once every 10 doses, resulting in a 10 times longer life time of the NOx sensor.

In one embodiment, the reducing agent may comprise ammonia (NH3).

Thus, the reducing agent may comprise an aqueous urea solution, an aqueous ammonia solution or an anhydrous ammonia (gaseous ammonia) solution.

In marine applications, careful control of the amount of reducing agent is of vital importance for firstly achieving the required NOx emission limits and, on the other hand, minimizing consumption of reducing agent and avoiding undesired ammonia slip. Ammonia slip occurring in a marine engine system together with marine fuel sulfur will have the adverse effect of ABS deposition in the vessel downstream of the heat-exchanging equipment. Such deposits can impair the engine operation and hamper the operation of the vessel.

When the NOx sensor detects that the NOx concentration in the gas is below a certain value, and thus that the NGX concentration has been reduced sufficiently, the NH3 starts to build up inside the catalytic reactor. After a while, the catalytic reactor cannot accumulate any more NH3 (ammonia), and the ammonia is emitted from the catalytic reactor along with the gas and further into the heat exchanger, also called an ammonia slip. When the ammonia-containing gas cools down in the heat exchanger, Ammonia-Bi-Sulphate (ABS) precipitates on the inner surface of the heat exchanger because the exhaust gas also comprises sulfur originating from the engine fuel. Such ABS precipitation is therefore mainly a problem for diesel engines running on sulfur-containing fuel and not for automotive engines typically running on ultra-low sulfur diesel fuel (ULSD). The ABS precipitation limits the function of the heat exchanger, and it is furthermore very difficult to remove the ABS precipitation from the heat exchanger again, which often leads to a replacement of the heat exchanger.

In another embodiment, the catalytic reactor may be configured to retain at least 100 grams of NH3 during operation, preferably at least 150 grams of NH3 during operation, and more preferably at least 180 grams of NH3 during operation.

Furthermore, the reducing agent supply unit may comprise a container comprising reducing agent.

In addition, the amount of reducing agent dosed from the container by the dosing unit may be a pre-set amount of reducing agent derived from the engine data.

During shop test and commissioning test of an interna! combustion engine, within the marine industry, engine data are derived for the basic control of the gas purification arrangement.

In another embodiment, the catalytic reactor of the gas purification arrangement may be arranged on a high-pressure side of the internal combustion engine.

Moreover, the combustion engine system may further comprise a turbocharger arranged downstream of the internal combustion engine.

So, the turbocharger may comprise a turbine and a compressor.

In addition, the NOx sensor may be arranged between the catalytic reactor and the turbocharger or between the turbocharger and the heat exchanger.

Furthermore, the high-pressure side of the internal combustion engine may be downstream of the internal! combustion engine and upstream of the turbocharger.

The combustion engine system may further comprise a temperature sensor arranged upstream and / or downstream of the catalytic reactor for measuring the temperature of the gas.

Furthermore, the combustion engine system may comprise a flow sensor for measuring a volume flow of the exhaust gas downstream of the internal combustion engine.

In addition, the combustion engine system may comprise a timer adapted to control an activation of the NOx sensor.

In one embodiment, the timer may activate the NOx sensor at certain intervals.

The intervals may vary depending on the measurement made by the NOx sensor.

The gas purification arrangement may further comprise a disconnecting means for disconnecting the NOx sensor for maintenance or replacement.

Additionally, the gas purification arrangement may comprise a flushing unit for flushing a supply line fluidly connecting the container and the catalytic reactor.

Also, the gas purification arrangement may comprise a gas sampling unit arranged upstream and / or downstream of the catalytic reactor for sampling gas.

In an embodiment, the pre-set value may be 3.4 g NOx / kWh emitted from the internal combustion engine, preferably less than 3.4 g NΟχ / kWh emitted from the internal combustion engine.

The combustion engine system may further comprise an exhaust gas receiver.

Moreover, the combustion engine system may comprise a by-pass channel for by-pass part of the exhaust gas led from the internal combustion engine bypassing the ΝΟχ reduction unit to the turbine of the turbocharger.

The present invention furthermore relates to a NOx reduction method of reducing an ammonia bi-sulphate (ABS) formation in a heat exchanger in a combustion engine system as described above, the method comprising the steps of: - determining engine data of the internal combustion engine , - calculating an amount of reducing agent to be dosed to the exhaust gas in or before entering the NOx reduction unit based on the engine data and operation conditions of the internal combustion engine in order to reduce a NOx concentration in the exhaust gas to a pre -set value of NOx concentration, - dosing the amount of reducing agent to the NOx reduction unit, - measuring the NC3X concentration of the gas downstream of the NQX reduction unit and upstream of the boiler by means of the NOx sensor, - comparing the measured NOx concentration with the pre-set value of NOx concentration, and - reducing the amount of reducing agent by means of the control unit if the measured NOx concentration is below the pre-set value.

In an embodiment, the step of reducing the amount of reducing agent may be repeated until the measured NQX concentration is above the pre-set value in order to use accumulated reducing agent in the NOx reduction unit.

In addition, the reducing agent may comprise ammonia (NH3).

Furthermore, the step of reducing the amount of reducing agent may be performed when at least two subsequent measurements of the NOx concentration are below the pre-set value.

In another embodiment, the NOx reduction method may further comprise the step of activating the NOx sensor at a predetermined interval by means of a timer.

Moreover, the NOx reduction method may comprise the step of adjusting the predetermined interval at which the timer activates the NGX sensor.

The step of reducing the amount of reducing agent may be performed when a second of the two subsequent measurements of MOx concentration is below a first of the two measurements of NOx concentration.

Finally, the NOx reduction method may further comprise the step of activating the NQX sensor more frequently when a second of the two subsequent measurements of NOx concentration is below a first of the two measurements of NQX concentration.

Brief description of the drawings

The invention and its many advantages will be described in more detail below with reference to the accompanying schematic drawings, which for the purpose of illustration show some non-limiting embodiments and in which

FIG. 1 shows a diagram of a combustion engine system having a gas purification arrangement,

FIG. 2 shows a diagram of a combustion engine system having a gas purification arrangement on a high-pressure side,

FIG. 3 shows a diagram of another embodiment of the combustion engine system having a NOx sensor arranged downstream of the turbine,

FIG. 4 shows a diagram of another embodiment of the combustion engine system having a NOx sensor arranged upstream of the turbine,

FIG. 5 shows a diagram of a combustion engine system having a gas purification arrangement at an iow pressure side,

FIG. 6 shows a diagram of another combustion engine system having several sensors,

FIG. 7 shows a diagram of another combustion engine system having a gas purification arrangement with two catalytic reactors, and

FIG. 8 shows a graph of NGX measurements before and after the catalytic reactor is filled with ammonia.

All the figures are highly schematic and not necessarily to scale, and they show only those parts which are necessary in order to elucidate the invention, other parts being omitted or merely suggested.

Detailed description of the invention

FIG. 1 shows a combustion engine system 1 comprising an internal combustion engine 2 and a gas purification arrangement 4 for purifying exhaust gas from the internal combustion engine 2. The interna! combustion engine 2 is powered by a fuel, such as heavy fuel, with a sulfur content of at least 0.05% and generates exhaust gas which is purified in the gas purification arrangement 4 to reduce the NOx concentration in the gas. The gas purification arrangement 4 comprises a ΝΟχ reduction unit 5 fluidly connected to the engine 2 for receiving the exhaust gas and a catalytic reactor 6 in which the reduction process takes place. The emission of NQX is reduced by dosing an amount of reducing agent to the exhaust gas in or before entering the catalytic reactor 6 of the NOx reduction unit 5. Thus, the gas purification arrangement 4 comprises a reducing agent supply unit 7 and a dosing unit. 9 for dosing the predetermined amount of reducing agent. The reducing agent supply unit 7 is fluidly connected to the NQX reduction unit 5 so that, the dosing unit. 9 is capable of ejecting the reducing agent into the exhaust gas. The amount of reducing agent 18 is derived from engine data collected when implementing the internal combustion engine 2 and subsequently testing the internal combustion engine 2, Furthermore, the amount of reducing agent 18 is adjusted for variations in ambient temperature, humidity and pressure. The reducing agent 18 may be any solution suitable for reducing the NOx concentration in the exhaust gas. The reducing agent 18 may thus comprise ammonia (NH3). The reducing agent 18 may comprise an aqueous urea solution, an aqueous ammonia solution or an anhydrous ammonia (gaseous ammonia) solution, as such solutions are very efficient for the reducing process.

In marine applications within internal combustion engines, careful control of the amount of reducing agent is of vital importance for firstly achieving the required NOx emission limits of the Tier III and, on the other hand, minimizing consumption of reducing agent and avoiding undesired ammonia. slip. As shown in FIG. 1, the combustion engine system 1 further comprises a heat exchanger 3, such as a boiler, arranged downstream of the internal combustion engine 2 and the gas purification arrangement 4. Even though the catalytic reactor 6 has a total volume of at least 200 liters, excess NH3 (ammonia) may build up and fill up the catalytic reactor 6 if the dosed amount of ammonia is not controlled. Once the catalytic reactor 6 is filled, ammonia may slip and enter the heat exchanger 3 where the ammonia and sulfur will deposit as ABS (Ammonia Bi-Sulfate) because the exhaust gas also comprises sulfur from the heavy fuel. Once formed, the ABS is not easily removed, which reduces the function of the heat exchanger 3. Thus, such deposits can impair the engine operation and hamper the operation of the vessel, such as a container ship.

In order to avoid ABS precipitation, the gas purification arrangement 4 further comprises a control unit 16 adapted to reduce the amount of reducing agent 18 supplied to the NOx reduction unit 5 when the NQX concentration in the exhaust gas measured by a NOx sensor 10 is below a pre-set value. The control unit 16 cooperates with the NOx sensor 10 arranged between the NOx reduction unit 5 and the heat exchanger 3 on measuring the NOx concentration in the exhaust gas. The NOx sensor 10 measures the NOx concentration downstream of the catalytic reactor 6, and if the NOx concentration of the gas has been reduced to a certain level, such as 3.4 g / kVVh, the control unit 16 limits the amount of ammonia dosed to the exhaust gas in or upstream of the catalytic reactor 6.

By being able to reduce the amount of reducing agent 18 supplied to the NOx reduction unit 5, the amount of reducing agent supplied to the NOx reduction unit 5 can be easily regulated without having to adjust the entire set-up of the interna! combustion engine 2. Furthermore, the NOx sensor 10 does not have to measure the NOx concentration in the gas every time the dosing unit 9 doses an amount of reducing agent 18, as this amount is pre-set in the dosing unit 9 if no control signal from the control unit to dose a different amount of reducing agent 18 is received. NOx sensors 10 adapted to measure NOx concentration are very costly, and one NOx sensor 10 is only capable of making a certain number of measurements before having to be replaced. In the present combustion engine system 1, the NOx sensor 10 only needs to measure the NOx concentration at certain intervals independent of the dosages, e.g. once every 10 doses, resulting in a 10 times longer life time of the NOx sensor 10.

When the NOx sensor 10 detects that the NOx concentration in the gas is below a certain value, and thus the NGX concentration has been reduced more than needed, the NH3 starts to build up inside the catalytic reactor 6. In order to avoid that after a while, an accumulation of ammonia in the cataiytic reactor 6 results in an ammonia slip, the control unit 16 reduces the amount of reducing agent 18 emitted to the gas.

The cataiytic reactor 6 is configured to retain or accumulate at least 100 grams of NH3 during operation. Depending on the temperature and pressure, the catalytic reactor 6 can accumulate at least 150 grams of NH3 during operation, and even at least 180 grams of NH3 during operation.

The amount of reducing agent 18 comprising NH3 dosed by the dosing unit 9 may be a pre-set amount of reducing agent 18 which is determined from the engine data. When setting up the internal combustion engine 2, the constant, pre-set amount of reducing agent 18 is found during extensive testing of the interna! combustion engine 2 and is based on calculations during development of the internal combustion engine 2. In this embodiment, the dose amount is thus constant and can only be regulated by the control unit 16 which receives information regarding the NOx measurements from the NOx sensor 10.

The amount of reducing agent 18 dosed by the dosing unit 9 is always set so that the NQX concentration is reduced sufficiently, and thus, during operation, the exhaust, gas will vary In NC3X concentration upstream of the gas purification arrangement 4, which may cause ammonia to accumulate in the catalytic reactor 6 during periods with lower NO * concentration in the exhaust gas. Thus, the dosing unit 9 is periodically set to somewhat overdose to ensure that the NOx concentration is reduced sufficiently to comply with the IMG regulations, and accumulated ammonia in the catalytic reactor 6 will not always be used later on. In this way, ammonia in combustion engine systems without a control unit 16 will accumulate ammonia, and ammonia slips will occur, causing ABS to be formed in the heat exchanger 3.

In FIG. 2, the reducing agent supply unit 7 has a container 8 with reducing agent 18 being in fluid communication with the dosing unit 9 for dosing the predetermined amount of reducing agent. The catalytic reactor 6 of the gas purification arrangement 4 is arranged on a high-pressure side of the internal combustion engine 2 which is downstream of the internal combustion engine 2 and upstream of a turbocharger 12. The turbocharger 12 comprises a turbine 13 which is driven by the exhaust gas and connected to a compressor 14 by means of a shaft 15 for rotating the compressor 14 in order to compress new gas to enter the internal combustion engine 2. The NQX sensor 10 is arranged between the catalytic reactor 6 and the turbine 13 of the turbocharger 12, and in FIG. 3, the NOx sensor 10 is arranged between the turbocharger and the heat exchanger 3 on the low-pressure side of the turbocharger.

In Figs. 1-3, he control unit 16 controls the dosing unit 9 based on the measurements performed by the NOx sensor 10, illustrated by dotted communication lines 20. In FIG. 3, the control unit 16 also communicates with the dosing unit 9 in order to adjust the amount of reducing agent 18 dosed by the dosing unit 9, e.g. If the dosing unit 9 has dosed too much ammonia over a longer period of time, the amount can be adjusted so that the accumulated amount of ammonia in the catalytic reactor 6 is used instead of dosing further ammonia. In this way, the catalytic reactor 6 is emptied of ammonia.

As shown in FIG. 3, the control unit 16 comprises a timer 17 adapted to control activation of the NOx sensor 10. The NOx sensor 10 does not measure the NOx concentration every time the dosing unit 9 is to dose ammonia, but measures the ΝΟχ concentration at certain intervals, and In order to control the NOx sensor, the timer is used to activate the NOx sensor 10 at these certain intervals. The control unit 16 is adapted to adjust, the timer 17 to activate the NGX sensor more frequently, e, g. when two subsequent measurements of NQX concentration are below the pre-set value, i.e. when the NOx concentration has been reduced to below 3.4 g / kWh, in order to activate the control unit 16 to reduce the dose of ammonia even further so that ammonia does not accumulate in the catalytic reactor 6. Thus, the pre-set value is approximately 3.4 g NOx / kWb in the gas emitted from the internal combustion engine 2. The pre-set value corresponds to the value when running a certification test on the internal combustion engine. The pre-set value may therefore also be an amount dosed as a function of the engine load, a percentage of a reduced amount of NOx as a function of the engine load, or a NOx emission as a function of the engine load. Ail these values thus correspond to e.g. a pre-set value of approximately 3.4 g / kWh when running a certification test.

The control unit 16 further receives information about the condition of the internal combustion engine 2, as indicated by the dotted line 11 in FIG. 4, to be able to adjust to dose amount, e.g. when the internal combustion engine 2 changes temperature, etc. As shown in Fig. 6, the combustion engine system 1 may further comprise a temperature sensor 27 arranged upstream of the catalytic reactor 6 for measuring a temperature of the gas. The temperature sensor 27 may also be arranged downstream of the catalytic reactor 6 to measure a gas temperature. The combustion engine system 1 further comprises a flow sensor 28 for measuring a volume flow of the exhaust gas downstream of the internal combustion engine 2. The combustion engine system 1 may also comprise other sensors, such as pressure sensors, etc. The control unit 16 is thus part of a control system of the internal combustion engine 2.

FIG. 5 shows a diagram of the combustion engine system 1 having a gas purification arrangement 4 at a low pressure side of the turbocharger 12. The gas purification arrangement 4 is thus arranged fluidly downstream of the turbine 13 of the turbocharger 12 so that the IOx reduction unit 5 is fluidly connected to the interna! combustion engine 2 in order to receive the gas after the gas has passed the turbine 13. The NOx sensor 10 is arranged downstream of the NOx reduction unit 5 upstream of the heat exchanger 3.

The control unit 16 is adapted to reduce the amount of ammonia dosed to the exhaust gas just before the exhaust gas enters the catalytic reactor 6.

In Fig. 6, the gas purification arrangement further comprises a disconnecting means 19 for disconnecting the NOx sensor 10 during maintenance, calibration or replacement. Since the dosing unit 9 is not dependent on measurements from the ΝΟχ sensor 10, maintenance work can be performed between two measurements by disconnecting the NGX sensor 10 by means of the disconnecting means 19. Thus, during maintenance or displacement of the NOx sensor 10, the combustion engine system 1 is capable of running and performing NOx reduction. As shown in FIG. 6, the dosing unit 9 may dose the reducing agent 18 directly into the catalytic reactor 6.

The gas purification arrangement further comprises a flushing unit 21 for cleaning the ΝΟχ sensor 10. Another flushing unit (not shown) could be arranged for flushing a supply line 22 fluidly connecting the container 8 and the catalytic reactor 6. The gas purification arrangement further comprises a gas sampling unit 23 arranged in connection with the NOx sensor 10 for sampling gas for control purposes. In FIG. 6, the combustion engine system 1 further comprises an exhaust gas receiver 24. Furthermore, the combustion engine system 1 comprises a by-pass channel 25 for by-passing part of the exhaust gas led from the internal combustion engine 2 by-passing the NGX reduction unit 5 to the turbine of the turbocharger 12.

In FIG. 7, the combustion engine system 1 comprises an NQX reduction unit 5 having two catalytic reactors 6. The dosing unit 9 doses an amount of reducing agent 18 into the flow of exhaust gas upstream of the catalytic reactors 6, and the flow is then fed to the catalytic reactors 6. In another combustion engine system, the reducing agent supply unit 7 comprises two dosing units 9, both controlled by the control unit 16 so that each dosing unit 9 doses reducing agent to a catalytic reactor 6.

FIG. 8 shows a graph of NOx measurements before and after the catalytic reactor is filled with ammonia, and an ammonia slip occurs at point "B". Once the NOx concentration is reduced too much, overdosing occurs and starts at point "A". When the overdosing has occurred for some time, approximately 1 hour, the catalytic reactor is filled with ammonia, and the ammonia slip occurs at point "B" and thereafter. The ammonia slip can be greatly reduced or completely removed by using the invention. Shortly after time A, the control unit can use the NOx sensor signal to limit the dosing to get the NOx concentration above the pre set value. In this case, excess ammonia disturbance in the catalytic reactor will be avoided, and no or very little ammonia slip will occur at time B.

Ammonia bi-sulphate (ABS) formation in a heat exchanger 3 in a combustion engine system 1 is reduced, which limits the dosed amount of reducing agent 18 if the measured NOx concentration is below the pre-set value. First, engine data of the internal combustion engine 2 is determined, and an amount of reducing agent 18 is calculated based on the engine data and operation conditions of the internal combustion engine 2, which amount of reducing agent 18 is dosed to the gas in or before entering the NOx reduction unit 5. Then, the amount of reducing agent 18 is dosed to the NOx reduction unit 5 by the dosing unit 9, and the NOx concentration of the gas downstream of the NOx reduction unit 5 and upstream of the heat exchanger 3 is measured by means of the NOx sensor 10. The measured NOx concentration is then compared with the pre-set value of NOx concentration, and the dosed amount of reducing agent 18 is reduced by means of the control unit 16 if the measured NGX concentration is below the pre-set value. The step of reducing the amount of reducing agent 18 is repeated until the measured NOx concentration is above the pre-set value in order to use accumulated reducing agent 18 in the NOx reduction unit 5.

The step of reducing the amount of reducing agent 18 is not performed in one embodiment until at least two subsequent measurements of the NOx concentration are below the pre-set value, e.g. below 3.4 g NOx / kWh. This is to ensure that the measurement is not just a peak in the line of measurements. Furthermore, the step of reducing the amount of reducing agent may not be performed until a second of the two subsequent measurements of NQX concentration is below a first of the two measurements of NQX concentration so as to show that the NOx concentration is decreasing before limiting the dosed amount further by the control unit 16.

Furthermore, the NOx sensor may be activated more frequently when a second of the two subsequent measurements of NQX concentration is below a first of the two measurements of NOx concentration because an overdose is detected and ammonia may then accumulate in the catalytic reactor, and thus, the NOx concentration of the gas needs to be measured more frequently in order to prevent an ammonia slip.

Although the invention has been described in the above in connection with preferred embodiments of the invention, it. will be evident to a person skilled in the art that several modifications are conceivable without departing from the invention as defined by the following claims.

Claims (10)

1. A combustion engine system (1) comprising: - an interna! combustion engine (2) having engine data and being powered by a fuel having a sulphur content of at least 0.05% and generating an exhaust gas, - a heat exchanger (3) arranged downstream of the internal combustion engine, and - a gas purification arrangement (4) for purifying the exhaust gas from the internal combustion engine (2), the gas purification arrangement (4) comprising: - a NOx reduction unit (5) fluidly connected with the internal combustion engine (2) for receiving the exhaust gas, the NOx reduction unit (5) comprising one or more catalytic reactor(s) (6) having a volume of at least 200 litres, - a reducing agent supply unit (7) fluidly connected to the NGX reduction unit (5), the reducing agent supply unit (7) comprising: - a dosing unit (9) for dosing an amount of reducing agent (18) to the exhaust gas in or before entering the NOx reduction unit (5), wherein the amount of reducing agent (18) Is derived from the engine data, and - a NGX sensor (10) arranged downstream of the NOx reduction unit (5) for measuring a NOx concentration in the exhaust gas, wherein the gas purification arrangement (4) further comprises a control unit (16) adapted to reduce the amount of reducing agent (18) supplied to the NQX reduction unit (5) when the NGX concentration measured by the NGX sensor (10) Is below a pre-set value of NOx concentration in the exhaust gas.

2. A combustion engine system according to claim 1, wherein the reducing agent (18) comprises ammonia.

3. A combustion engine system according to claim 1 or 2, wherein the catalytic reactor (6) is configured to retain at least 100 grams of NH3 during operation, preferably at least 150 grams of NH3 during operation, and more preferably at least 180 grams of NH3 during operation.

4. A combustion engine system according to claim 1 or 2, further comprising a timer (17) adapted to control an activation of the NOx sensor (10).

5. A combustion engine system according to any of the preceding claims, wherein the gas purification arrangement (4) further comprises a disconnecting means (19) for disconnecting the NOx sensor (10) for maintenance or replacement.

6. A ΝΟχ reduction method of reducing an ammonia bi-sulphate (ABS) formation in a heat exchanger (3) in a combustion engine system (1) according to claims 1-5, the method comprising the steps of: - determining engine data of the internal combustion engine (2), - calculating an amount of reducing agent (18) to be dosed to the exhaust gas in or before entering the NOx reduction unit (5) based on the engine data and operation conditions of the internal combustion engine (2) in order to reduce a ΝΟχ concentration in the exhaust gas to a pre-set value of NQX concentration, - dosing the amount of reducing agent (18) to the NOx reduction unit (5), - measuring the NGX concentration of the gas downstream of the NGX reduction unit (5) and upstream of the heat exchanger (3) by means of the NQX sensor (10), - comparing the measured NGX concentration with the pre-set value of NQX concentration, and - reducing the amount of reducing agent (18) by means of the control unit (16) if the measured NQX concentration is below the pre-set value.

7. A NOx reduction method according to claim 6, wherein the step of reducing the amount of reducing agent (18) is repeated until the measured NQX concentration is above the pre-set value in order to use accumulated reducing agent (18) in the NOx reduction unit (5).

8. A NOx reduction method according to claim 6 or 7, wherein the step of reducing the amount of reducing agent (18) is performed when at least two subsequent measurements of the NGx concentration are below the pre-set value.

9. A NOx reduction method according to claim 8, further comprising the step of activating the NOx sensor (10) at a predetermined interval by means of a timer (17).

10. A NGX reduction method according to claim 9, further comprising the step of adjusting the predetermined intervai at. which the timer (17) activates the N0X sensor.