SE1250441A1 - Exhaust after-treatment system, and method in connection with such a system - Google Patents

Abstract

The invention relates to an exhaust post-treatment system (2) for a combustion engine (4) which produces an exhaust flow (6), which system (2) comprises at least one diesel oxidation catalyst (DOC) and/or at least one diesel particle filter (DPF), and at least one selective reduction catalyst (SCR catalyst). The system further comprises a reducing agent device (8) adapted to supplying a reducing agent (10) to the exhaust flow (6) upstream of said SCR catalyst, a first NOx sensor (12) situated upstream of said DOC and/or DPF and adapted to measuring the content of nitrogen oxide compounds (NOx) in the exhaust flow and to producing on the basis thereof a first NOx output signal (NOX1), a second NOx sensor (14) situated downstream of said SCR catalyst and adapted to measuring the content of nitrogen oxide compounds (NOx) in the exhaust flow and to producing on the basis thereof a second NOx output signal (NOX2), and at least one temperature sensor (16) adapted to monitoring the temperature of the exhaust flow and to producing on the basis thereof at least a first temperature signal (T1). The exhaust post- treatment system (2) comprises a calculation unit (18) to which said first and second NOx output signals (NOXl, NOX2) and said first temperature signal (T1) are arranged to be conveyed, said first and second NOx sensors (12, 14) are adapted to measuring the content of nitrogen oxide compounds NOX1 upstream of said DOC and/or DPF and to measuring at substantially the same time the content of nitrogen oxide compounds NOX2 downstream of the SCR catalyst when no reducing agent (10) is being supplied to the exhaust flow (6) from the reducing agent device (8), and said temperature sensor (16) is adapted to measuring the temperature T when the measurements of NOXl and NOX2 are taking place, which calculation unit (18) is adapted to comparing NOX2 with a detection criterion, which preferably comprises comparing with a predetermined threshold value NOXtr related to the measured temperature, and to generating on the basis of the comparison an indicating signal (20).

Description

Title Field of the Invention Field of the Invention The present invention relates to an exhaust aftertreatment system, and a method associated with such a system, according to the preambles of the independent claims.

In particular, the method and system are adapted to detect sulfur poisoning of a diesel oxidation catalyst (DOC) in the exhaust aftertreatment system.

An internal combustion engine burns an air and fuel mixture to generate a driving torque. The combustion process generates exhaust gases that are emitted from the engine to the atmosphere. The exhaust gases include nitrogen oxides (NOx), carbon dioxide (CO2), carbon monoxide (CO) and particles. NO is a collective name to describe the exhaust gases which primarily consist of nitrogen oxide (NO) and nitrogen dioxide (NO2). An exhaust after-treatment system treats the exhaust emissions to reduce the emissions before they are released to the atmosphere. In an exemplary exhaust aftertreatment system, a metering system injects a reducing agent into the exhaust gases upstream of a selective catalytic reduction catalyst (SCR catalyst). The exhaust gas and reducing agent mixture reacts in the SCR catalyst, thereby reducing the amounts of NOx released into the atmosphere.

An example of a reducing agent is liquid urea, commercially available as AdBlue®. This liquid is a non-toxic urea solution in water used to chemically reduce emissions of nitrogen oxides, especially for heavy diesel vehicles.

The reducing agent reacts with NO in the SCR catalyst to effect the NO reduction. More specifically, the reducing agent is broken down to form ammonia (NH3) which in turn reacts with NO to form water and nitrogen gas (N2).

To achieve the described NO reduction, NH 3 must be stored in the SCR catalyst. For the SCR catalyst to work efficiently, the storage level must be at an adequate level. In more detail, the NO reduction, or conversion efficiency, depends on the storage level. In order to maintain a high conversion efficiency under different operating conditions, the NH3 layer must be maintained. However, as the temperature of the SCR catalyst increases, the NH3 level must be reduced to avoid NH3 emissions (ie excess NH3 is released from the SCR catalyst), which may reduce the conversion efficiency of the catalyst.

In summary, in order to meet stricter environmental requirements, more and more vehicle manufacturers are using SCR catalyst systems to purify diesel exhaust gases from nitrogen oxides (N0x). This is done by injecting ammonia solution into an SCR catalyst which helps to convert NO particles into nitrogen and water. The exhaust gas purification strategy should take into account that enough NO is converted at the same time as you do not want to inject too much ammonia, both for the economy and the million.

In exhaust aftertreatment systems, at least one diesel oxidation catalyst (DOC) is also used, and also one or more diesel particulate filters (DPF) which are often coated with a catalytic coating. The purpose of the coating is, among other things, to generate a sufficient amount of NO2 to achieve passive oxidation of soot captured by DPF. This occurs, among other things, according to the reaction C + NO2 -> CO + NO.

The formation of NO2 in the DOC will depend, among other things, on the mass flow of the exhaust gases and the temperature in the DOC. In addition to river and temperature dependence, DOC and / or the catalytic coating store in DPF, sulfur (S), which may be present in the exhaust gases, at lower temperatures and emit sulfur at temperatures typically higher than 400 ° C. If the chorus ratio causes the DOC to absorb a lot of sulfur, the DOC will be poisoned, ie. the formation of NO2 is hampered. The NO2 content after DPF will also depend on the state of DPF with respect to sulfur poisoning. Sulfur is thus the main reason why the formation of NO2 decreases in DOC and in the catalytic coating for DPF. The actual temperatures for sulfur absorption and sulfur release depend on the specific catalyst mixture and the specific chorus ratio.

As low-sulfur diesel fuel (below 10 ppm) is used, which is now widely available in Europe and the United States, it will take several hours or days of engine operation without exceeding 400 ° C exhaust temperatures to achieve a marked reduction in NO2 formation in DOC and / or in the DPF coated with catalytic material. Driving heavy vehicles in such a way is unusual but can occur. However, sulfur poisoning of DOC and / or of the coated DPF can occur after shorter times if the driver is driving with fuel that has a higher sulfur content, e.g. when coming to 'ander without legal sulfur fuel, or if you accidentally shave refueling fuel with a high sulfur content.

It is important to detect such poisoning and remove the sulfur from the DOC. Sulfur is removed from the DOC and / or the coated DPF by heating the catalysts to Over 400 ° C for a long time e.g. more than five minutes, which can be done by injecting fuel into the exhaust gases or by activating a burner.

The temperature used in the desulfurization does not affect the SCR catalyst and during the desulfurization it reaches a temperature where it works very efficiently and the effect of the ratio between NO 2 and NO is minimal.

The sensors used to feed the content of nitrogen oxides in the exhaust gases are often very expensive components. NOx sensors are often made of ceramic metal oxides, usually yttrium-stabilized zirconium (YSZ). YSZ is compressed into a solid ceramic that conducts oxygen ions at high temperatures, from approx. 400 ° C. To obtain a feed signal, a pair of sub-metal electrodes are placed on the surface and the voltage or current variations of an electrical signal can then be fed as a function of the NOx concentration.

High demands are placed on the sensor to achieve the sensibility and robustness required when feeding the exhaust stream. Therefore, the cost of NOx sensors is high.

The NOx sensor emits an output signal that represents the sum of the content of NO and NO2.

Upstream DOC (see Figure 1) the exhaust gases contain about 90% (+/- 5%) NO and the rest NO2. This ratio can be estimated from theoretical models.

It is possible that the NOx sensor has different sensitivity for NO and for NO2, respectively, where the sensitivity for NO is higher than for NO2. The output signal SNox from the NOx sensor can then be expressed as: SNox = Ax [NO] + Bx [NO2], ddr A> B.

WO-2010/068147 describes how a sulfur poisoning diagnosis is carried out in a system with a NOx sensor before ASC (the last part of certain SCR catalysts with special coating) and one after.

The diagnosis then uses the NOx levels to detect and regulate the SCR catalyst. WO-2010/068147 discloses a method for after-treatment of exhaust gases, where the system comprises DOC and DPF. The method describes that one can detect sulfur poisoning in DOC and DPF by math. DOC and DPFs are able to form NO2. The ability to form NO2 can i.a. mdtas by NOx sensors and then jdmfOras with estimated values.

US-2008/216466, US-2003/032188 and US-2005/109022 are examples of patent documents showing different methods for removing unwanted substances (eg sulfur) from a catalyst. For example, the degree of sulfur poisoning is calculated by feeding the catalyst's ability to repel NOx gases. This is achieved by feeding the content of NOx gases before and after the catalyst.

Sulfur poisoning of DOC and of DPF with catalytic coatings is a common problem that can be caused, among other things, by the use of fuel with too high a sulfur content. There are currently no reliable methods for obtaining an indication that sulfur poisoning exists and the purpose of the present invention is to provide such a method.

During the day, it can happen that a warning light is illuminated incorrectly, which indicates too high emission levels, which has been indirectly caused by a sulfur poisoning. Another object of the invention is to previously obtain an indication that sulfur poisoning may be present and then be able to initiate counter-guards.

SUMMARY OF THE INVENTION The above objects are achieved by the invention defined by the independent claims.

Preferred embodiments are defined by the dependent claims.

A NOx sensor is adapted to emit an output signal representing the sum of the content of the NO and NO2 contents and according to the present invention the fact is used that the NOx sensors used have different probabilities for NO and NO2 which is used to make a diagnosis of the sulfur refinement. The ability of DOCs and DPFs to convert NO to NO2 is a good indicator of sulfur refining.

Since the proportion of NO2 increases in relation to the proportion of NO due to the oxidation that normally takes place in DOC / DPF, when this is not poisoned, this means that the signal strength of the output signal from the NOx sensor downstream of the SCR catalyst (NOX2) (see Figure 1) will decrease at temperatures Above 150 ° C. However, the decrease is not linear over the temperature and river range but reaches a maximum before it falls again. However, the level never goes below the level of 150 ° C.

If the DOC / DPF is poisoned, it will not decrease that much. Since it is completely forged, there will be essentially no change in the signal. This means, for example, that a calculated virtual NOx signal after DOC / DPF is incorrectly corrected and may therefore indicate a far too high signal strength, ie. one for high content NO / NO2.

By comparing real NOx signals fed downstream of the SCR catalyst with pre-mixed signals and possibly relating these to previously fed signals and through refueling, according to the invention, a condensed NO to NO2 conversion, and damned sulfur deposition, can be detected.

An advantage of the invention is that existing sensors can be used to detect a fault that would otherwise require a separate sensor or a workshop visit. In this way, at an earlier stage, an indication of sulfur poisoning and antidote can be used earlier.

Brief Description of the Drawings Figure 1 is a schematic illustration of an exhaust aftertreatment system according to the present invention.

Figure 2 is a graph showing the nitrogen oxide levels in the exhaust gases for the first and second NOR sensors.

Figure 3 is a schematic flow chart illustrating the present invention.

Detailed Description of Preferred Embodiments of the Invention Preferred embodiments of the invention will now be described with reference to the accompanying drawings.

Figure 1 is a schematic illustration of an exhaust aftertreatment system according to the present invention.

Figure 1 shows an exhaust after-treatment system 2 for an internal combustion engine 4 which emits an exhaust gas flow 6. The exhaust after-treatment system 2 comprises at least one diesel oxidation catalyst (DOC), the purpose of which is, inter alia, to convert NO to NO 2, and at least one selective catalytic reduction catalyst (SCR). . Preferably, a diesel particulate filter (DPF) is also arranged downstream of the DOC. The diesel particulate filter may be uncoated or coated with a catalytic coating. Has soot and ash accumulate and it also happens has a certain conversion of NO to NO2.

The system further comprises a reducing agent device 8 adapted to supply a reducing agent 10, for example urea or ammonia, to the exhaust gas stream 6 upstream of said SCR catalyst since NO and NO 2 (NO x) are converted to N 2. The amount of reducing agent that is supplied is controlled by a control means (not shown), among other things depending on the content of nitrogen oxides and the temperature of the exhaust gases.

A first NOx sensor 12 is arranged upstream of the said DOC and which is adapted to supply the content of nitrogen oxide compounds (NOx) in the exhaust gas flow and to emit a first NOx output signal (NOX1) in dependence thereon. A second NOx sensor 14 is provided downstream of said SCR catalyst and adapted to feed the content of nitric oxide compounds (NOx) in the exhaust stream and to emit a second NOx output signal (NOX2) in dependent & ray, and at least one temperature sensor 16 is provided to sense the temperature. for the exhaust gas flow and to emit at least one first temperature signal (Ti) depending thereon. The figure shows four temperature sensors 16 for measuring the temperature of the exhaust flow at different points of the exhaust after-treatment system. These are located before and after DOC and before and after the SCR catalyst and emit the temperature signals Ti, T2, T3 and T4 respectively.

The exhaust after-treatment system 2 comprises, according to the invention, a calculation unit 18 and the first and second NOx output signals (NOX1, NOX2) and said first temperature signal (Ti), or one or more of the temperature signals, are adapted to be applied to the calculation unit 18. The first and second NOx the sensors 12, 14 are adapted to math. the content of nitric oxide compounds NOX1 upstream of the said DOC and substantially simultaneously feed the content of nitric oxide compounds NOX2 downstream of the SCR catalyst (no reducing agent 10 is supplied to the exhaust stream 6 from the reducing agent device 8. Furthermore, the temperature sensor or temperature sensors 16 are fed and the temperature) NOX2 happens.

Then, in the calculation unit 18, NOX2, or a value related to NOX2, is compared with a detection criterion related to the supplied temperature, and an indication signal 20 is generated depending on the comparison.

According to one embodiment, the detection criterion is a predetermined threshold value NOXtr related to the measured temperature, and the indication signal 20 is generated if NOX2 is stone an NOXtr. In this embodiment, NOXtr can, for example, be selected so that it is in a predetermined relationship with NOX1.

Figure 2 is a graph showing the nitrogen oxide levels in the exhaust gases for the first and second NOR sensors. It should be emphasized that the graph is primarily intended to illustrate the aspects which are important in illustrating the invention and are therefore simplified.

As can be seen from the figure, the content of nitrogen oxide NOX1 measured by the first NOR sensor 12 is constant regardless of the temperature. When feeding with the second NOx sensor 14 when no reducing agent is supplied, the amplitude of the output signal will decrease if the DOC (and, where applicable, the DPF) functions as intended, ie. if no sulfur poisoning has occurred. Then the proportion of NO2 in NO will increase at the expense of the proportion of NO and due to the sensor's different probability for NO and NO2 the amplitude for NOX2 will then decrease. This is illustrated in the figure by NOX2 decreasing with increasing temperatures.

By, at the same temperature (in the figure 350 ° C), comparing NOX1 and NOX2 with a threshold value NOXtr as Oiler for this temperature (in this case 350 ° C) one can get an indication of whether NOX2 shows an expected decrease. In the figure, the sensor signal NOX2 at 350 ° C is lower than NOXtr, which indicates that no sulfur poisoning is present.

The figure also shows a curve NOX2 '(dotted line) which shows output signals picked up by the second NON sensor 14 in another case. The NOx sensor output signal is above NOXtr at 500 ° C, which may indicate sulfur poisoning by DOC (and / or DPF).

According to another embodiment, the calculation unit 18 is adapted to perform the comparison by determining ANON = INOX1-NOX21, comparing ANON with the predetermined threshold value NOXtr ', which constitutes the detection criterion, and generating the indication signal 20 if ANOX is less than NOXtr'. In this embodiment, the difference between NOX1 and NOX2 is thus compared with a threshold value NOXtr '.

The threshold values are preferably chosen so that a certain deviation from the NOX2 requirement causes an indication to occur. In the embodiment shown in Figure 2, this may mean that NOXtr must be, for example, 10% higher than the corresponding "normal" value of NOX2. And in the second embodiment, when the difference between NOX1 and NOX2 is compared with NOXtr ', NOXtr' is chosen, for example, 10% less than the "normal" difference.

For both embodiments it is preferred that there is preferably a table, or a simple database, arranged, in e.g. the calculation unit 18, which contains the associated value of the temperatures for the exhaust gas flow and the said predetermined threshold value NOXtr or NOXtr '. To determine NOXtr, one starts from NOX1 and lets the threshold value form a predetermined part of NOX1 which is in relation to the temperature. Of course, it is also possible to directly calculate these threshold values based on the relationships between temperature and NOx content.

According to one embodiment, the indication signal 20 is adapted to indicate that DOC, and / or DPF, is sulfur poisoned. The indication signal 20 may be in the form of an alarm signal to the driver that the vehicle should go to a workshop. It can also mean that an antidote is directly salted in to remove sulfur from DOC and / or DPF, for example by raising the temperature of the exhaust gases in a controlled manner.

Since it is true that the output signal from one NOx sensor is differently sensitive to NO and NO2, the output signal from the other NOx sensor is adjusted with an appropriate adjustment value in order to thereby obtain a "true" NOx value which can then be set in relation to the output signal. from the first NOx sensor. In practice, this meant adding an adjustment value to NOX2, which thereby gives a higher value. If DOC and / or DPF are poisoned, then NOX2 will increase, and if an adjustment value is then added, an excessively high, and thus incorrect, NOx content will be indicated.

The exhaust after-treatment system is, according to a further embodiment, adapted to calibrate the output signal NOX2 from the second NOx sensor 14 in view of the output signal NOX1 from the first NOx sensor 12. This is done by letting exhaust gases pass through the system at a low temperature, preferably less than 150 ° C, dA substantially no oxidation takes place in DOC, and dA no reducing agent is supplied to the exhaust gas stream from the reducing agent device 8.

This is done through math. NOX1 and NOX2, determine NOX2k = NOX1-NOX2, and to determine a calibrated value for NOX2 as NOX2 '= NOX2 + NOX2k. The comparison carried out by the calculating unit 18 takes place dA with the calibrated value NOX2 'instead of NOX2.

The present invention also relates to a method of an exhaust aftertreatment system for an internal combustion engine which emits an exhaust stream. The method is illustrated by the schematic flow chart in Figure 3.

The integral parts of the exhaust aftertreatment system and its functions have been described in detail above and reference is made herein to that description.

The method according to the invention comprises the steps of: A - measuring the content of nitric oxide compounds NOX1 upstream of DOC and essentially simultaneously feeding the content of nitric oxide compounds NOX2 downstream of the SCR catalyst as no reducing agent is supplied to the exhaust stream from the reducing agent device, B - feeding the temperature T1 NOX2, or a value related to NOX2, with a detection criterion related to the measured temperature, D - generate an indication signal depending on the comparison.

According to one embodiment, the detection criterion in step C is a predetermined threshold value NOXtr related to the measured temperature, and that in step D the said indication signal is generated if NOX2 is stone an NOXtr.

According to another embodiment, step A further comprises determining ANON = I NOX1-NOX2 I, and that said detection criterion in step C consists of a predetermined threshold value NOXtr 'which is compared with ANON, and that in step D said indication signal is generated if ANON is less and NOXtr '.

The predetermined threshold values are stored in, for example, a table or a database which contains the associated values of temperatures for the exhaust gas flow and the said predetermined threshold values (NOXtr and NOXtr ', respectively). As mentioned above, they can also be berdknas.

According to one embodiment, the indication signal is adapted to indicate that the DOC, and / or DPF, is sulfur poisoned. The indication signal can be in the form of an alarm signal to the driver that the vehicle should go to a workshop. It can also mean that an antidote is injected directly to remove sulfur from DOC and / or DPF, for example by raising the temperature of the exhaust gases in a controlled manner.

In a further embodiment of the present invention, the method comprises the steps of calibrating NOX2 at a low temperature, preferably less than 150 ° C, since essentially no oxidation takes place in DOC, and since no reducing agent is supplied to the exhaust gas stream from the reducing agent device. This is done through math. NOX1 and NOX2, determine a calibration value NOX2k = NOX1-NOX2, and determine a calibrated value for NOX2 as NOX2 '= NOX2 + NOX2k, and that the method steps described above are performed with the calibrated value NOX2' for NOX2.

The present invention is not limited to the above-described preferred embodiments.

Various alternatives, modifications and equivalents can be used. The above embodiments are, therefore, not to be construed as limiting the scope of the invention as defined by the appended claims.

Claims (13)

12 Patent claims A method of an exhaust aftertreatment system for an internal combustion engine emitting an exhaust stream, the exhaust aftertreatment system comprising at least one diesel oxidation catalyst (DOC), at least one selective reduction catalyst (SCR catalyst), a reducing agent device adapted to supply the scrubbing agent to the scrubber. a first NON sensor (NOX1) arranged upstream of said DOC and adapted to feed the content of nitric oxide compounds (NON) in the exhaust stream and to emit a first NOx output signal (NOX1) depending thereon, a second NOR sensor (NOX2) arranged downstream of said SCR -catalyst and adapted to math. the content of nitric oxide compounds (NON) in the exhaust stream and emitting a second NON output signal (NOX2) depending thereon, and at least one temperature sensor (16) arranged to supply the temperature of the exhaust gas stream and emitting at least one first temperature signal (Ti) thereon, k aline te ckn ad that the method comprises the steps of: A - measuring the content of nitric oxide compounds NOX1 upstream of DOC and essentially simultaneously feeding the content of nitric oxide compounds NOX2 downstream of SCR catalyst when no reducing agent is supplied to the exhaust stream from the reducing agent, B - feeding the NO2 T occurs, C - compares NOX2, or a value related to NOX2, with a predetermined detection criterion related to the measured temperature, D - generates an indication signal depending on the comparison. The method according to claim 1, wherein said detection criterion in step C is a predetermined threshold value NOXtr related to the measured temperature, and in step D generating said indication signal if NOX2 is stone an NOXtr. The method of claim 1, wherein step A further comprises determining ANON = I NOX1-NOX2 I, that step C comprises comparing ANON with a predetermined threshold value NOXtr 'constituting said detection criterion, and in step D generating said indication signal if ANON is smaller than NOXtr '. The method according to any one of claims 2 or 3, wherein a table is provided 13 which contains coherent values of temperatures for the exhaust gas flow and one of said predetermined threshold values (NOXtr, NOXtr '). The method of any preceding claim, wherein said indication signal is adapted to indicate that said DOC is sulfur poisoned. The method according to any of the preceding claims, wherein the method comprises the steps of calibrating NOX2 at a low temperature, preferably lower than 150 ° C, since essentially no oxidation takes place in DOC, and when no reducing agent is supplied to the exhaust stream from the reducing agent device, by math. NOX1 and NOX2, determine a calibration value NOX2k = NOX1-NOX2, and to determine a calibrated value for NOX2 as NOX2 '= NOX2 + NOX2k, the method steps according to any of the preceding requirements being performed with the calibrated value NOX2' for NOX2. An exhaust after-treatment system (2) for an internal combustion engine (4) which emits an exhaust gas (6), the exhaust after-treatment system (2) comprises at least one diesel oxidation catalyst (DOC), at least one selective catalytic reduction catalyst (SCR catalyst), a reducing agent device (8) supplying a reducing agent (10) to the exhaust stream (6) upstream of said SCR catalyst, a first NOx sensor (12) arranged upstream of said DOC and adapted to feed the content of nitric oxide compounds (NOx) in the exhaust stream and emitting a first NOx output signal ( NOX1) in dependent a: ray, a second NOx sensor (14) arranged downstream of said SCR catalyst and adapted to feed the content of nitrogen oxide compounds (NOx) in the exhaust stream and to emit a second NOx output signal (NOX2) depending thereon, and at least a temperature sensor (16) arranged to measure the temperature of the exhaust gas flow and to emit at least one first temperature signal (Ti) depending thereon, may be due to the exhaust gas aftertreatment system (2) comprises a calculating unit (18) and that said first and second NOx outputs (NOX1, NOX2) and said first temperature signal (Ti) are adapted to power said calculating unit (18), said first and second NOx sensors (12, 14) are adapted to feed the content of nitric oxide compounds NOX1 upstream of said DOC and substantially simultaneously feed the content of nitric oxide compounds NOX2 downstream of SCR catalyst when no reducing agent 14 (10) is supplied to the exhaust stream (6) from the reducing agent device (8), and said temperature sensor (16) to math. the temperature T when the feeds of NOX1 and NOX2 take place, the calculation unit (18) being adapted to compare NOX2, or a value related to NOX2, with a detection criterion related to the supplied temperature, and to generate an indication signal (20) depending on the comparison. The exhaust aftertreatment system of claim 7, wherein said detection criterion is a predetermined threshold value NOXtr, and generating said indication signal (20) if NOX2 is greater than NOXtr. The exhaust aftertreatment system according to claim 7, wherein the calculating unit (18) is adapted to comprise determining ANON = I NOX1-NOX2 L to compare ANON with a predetermined threshold value NOXtr 'constituting said detection criterion, and to generate the indication signal (20) if ANON is smaller and NOXtr '. The exhaust aftertreatment system according to any one of claims 8 and 9, wherein a table is provided which contains the associated values of temperatures for the exhaust flow and said predetermined threshold values NOXtr or NOXtr '. The exhaust aftertreatment system according to any one of claims 7-10, wherein said indication signal (20) is adapted to indicate that said DOC is sulfur poisoned. The exhaust aftertreatment system according to any one of claims 6-9, wherein the system is adapted to calibrate NOX2 at a low temperature, preferably lower than 150 ° C, where substantially no oxidation takes place in DOC, and when no reducing agent (10) is supplied to the exhaust stream from the reducing agent device ( 8), by math. NOX1 and NOX2, determine NOX2k = NOX1-NOX2, and to determine a calibrated value for NOX2 as NOX2 '= NOX2 + NOX2k, the comparison being carried out by the calculating unit (18) with the calibrated value NOX2' instead of NOX2. The exhaust aftertreatment system according to any one of claims 7-12, wherein the system comprises at least one diesel particulate filter (DPF) arranged downstream of said DOC. 1/2 2 16 6 12 T1,1, T2 8 SCR 16 DOC 166 DPF T3v T4v 16 14 18 NOX2 NOX1 ‘1,