SE538378C2 - Method for detecting sulfur poisoning in an exhaust after-treatment system - Google Patents

Abstract

Summary The invention relates to an exhaust gas aftertreatment system (2) for an internal combustion engine (4) which emits an exhaust gas (6), the exhaust aftertreatment system (2) comprises at least one diesel oxidation catalyst (DOC), at least one selective catalytic reduction catalyst (SCR catalyst). The system further comprises a reducing agent device (8) adapted to supply a reducing agent (10) to the exhaust gas stream (6) upstream of said SCR catalyst, a first NOx sensor (12) arranged upstream of said DOC and adapted to feed the content of nitrogen oxide compounds (NOx) in the exhaust stream and emitting a first NOx output signal (NOX1) in dependent & 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) in depending thereon, and at least one temperature sensor (16) arranged to sense the temperature of the exhaust stream and to emit at least one first temperature signal (Ti) thereon. The exhaust after-treatment system (2) comprises a calculating unit (18) and that said first and second NOx output signals (NOX1, NOX2) and said first temperature signal (Ti) are adapted to be called 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 the DOC and substantially simultaneously feed the content of nitric oxide compounds NOX2 downstream of the SCR catalyst when no reducing agent (10) is supplied to the exhaust stream (6) from the reducing agent device (8), and said temperature sensor (16) to math. the temperature T dA the feeds of NOX1 and NOX2 take place, the calculation unit (18) being adapted to compare NOX2 with a detection criterion, which preferably comprises comparing with a predetermined threshold value NOXtr, related to the measured temperature, and generating an indication signal (20) in depending on the comparison.

Description

Title Method for detecting sulfur sequestration in an exhaust gas aftertreatment system Field of invention The present invention relates to an exhaust gas treatment system, and a method in connection with such a system, according to the preambles of the independent claims. In particular, the method and system are adapted to detect detoxification by in the exhaust gas treatment system.

An internal combustion engine pre-embers a lu fl and fuel mixture to generate a driving torque. The combustion process generates exhaust gases emitted from the engine to the atmosphere. The exhaust gases include nitrogen oxides (NOX), carbon dioxide (CO2), carbon monoxide (CO) and particles. NOX is a collection standard for describing the exhaust gases which primarily consist of nitric oxide (NO) and nitrogen dioxide (NO2). An exhaust gas after-treatment system treats exhaust emissions to reduce emissions before being released into the atmosphere. In an exemplary exhaust gas treatment system, a dosing system injects a reducing agent into the exhaust gases upstream of a selective catalytic reduction catalyst (SCR catalyst). The exhaust 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 fl surface 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 NOX in the SCR catalyst to effect the NOX reduction. More specifically, the reducing agent is broken down to form ammonia (NH3) which in turn reacts with NOX to form water and nitrogen (Ng).

To achieve the described NOX reduction, NH3 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 NOX reduction, or conversion efficiency, 2 depends on the storage level. In order to maintain a high conversion efficiency during different still states, the NH 3 layer must be maintained. However, as the temperature of the SCR catalyst increases, the NH 3 level must be reduced to avoid NH 2 emissions (ie excess NH 3 is emitted from the SCR catalyst), which may reduce the conversion efficiency of the catalyst.

In summary, in order to meet stricter environmental requirements, all your vehicle manufacturers use SCR catalyst systems to purify diesel exhaust gases from nitrogen oxides (NOX). This is done by injecting ammonia solution into an SCR catalyst that helps convert NOX particles into nitrogen and water. The exhaust gas purification strategy should take into account that sufficient NOX is converted while not wanting to inject too much ammonia, for reasons of both the driving economy and the environment.

Exhaust after-treatment systems also use at least one diesel oxidation catalyst (DOC), and also one or more diesel particulate matter (DPF) which is often coated with a single-catalytic coating. The purpose of the coating is, among other things, to generate a sufficient amount of NO 2 to effect passive oxidation of soot captured by DPF. This occurs among armates according to the reaction C + NO 2 -> CO + NO.

The formation of NO2 in the DOC will, among other things, depend on the mass fate of the exhaust gases and the temperature in the DOC. In addition to the flow and temperature dependence, the DOC and / or the catalytic coating stores in DPF, sulfur (S), which can be found in the exhaust gases, at camp temperatures and emits sulfur at temperatures typically higher than 400 ° C. Driving conditions mean that the DOC absorbs a lot of sulfur, the DOC will be released, ie the formation of NO2 will be inhibited. The NO2 content after DPF will also depend on the state of the DPF with respect to sulfur deposition. Sulfur is thus the main reason for the formation of NO2 decreases in DOC and in the catalytic coating fed DPF. Defective temperatures for sulfur absorption and sulfur release depend on the specific catalyst mixture and the specific driving conditions.

When diesel fuel with a low sulfur content (below 10 ppm) is used, which is now widely available in Europe and the USA, it will take several hours or days of engine operation 3 without exceeding exhaust temperatures of 400 ° C to achieve a noticeable 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, can sulfur poisoning by DOC and / or by the coated DPF occurs after shorter times the driver runs on fuel that has a higher sulfur content, e.g. when driving in countries without low-sulfur fuel, or if you accidentally refuel with a high sulfur content.

Therefore, 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 catalyst medium above 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 desulphurisation does not affect the SCR catalyst and the sub-desulphurisation has a temperature where it works very efficiently and the effect on the ratio of NO2 to NO is minimal.

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

High demands are placed on the sensor in order to achieve the sensitivity and robustness required when measuring exhaust emissions. Therefore, the cost of NOX sensors is high.

The NOX sensor emits an output signal that represents the sum of the contents of NO and NO2.

Upstream of DOC (see Figure 1), the exhaust gases contain about 90% (+/- 5%) NO and the remainder N02. This ratio can be estimated from theoretical models. It is known that the NOX sensor has different sensitivities to NO and to NO2, respectively, where the sensitivity to NO is higher than to NO2. The output signal SNOX from the NOX sensor can then be expressed as: SNOX = A> <[NO] + B> <[NO2], where A> B.

W0-2010 / 068147 describes how a sulfur poisoning diagnosis is carried out in a system with NOX sensor before ASC (the last part of some SCR catalysts with special coating) and an e fi er.

The diagnosis then uses the NOX levels to detect and regulate the SCR catalyst. WO-2010/068147 shows a method for after-treatment of exhaust gases, where the system includes DOC and DPF. The method describes that sulfur deposition in DOC and DPF can be detected by measuring the ability of DOC and DPF to form NO2. The ability to form N02 can i.a. measured by NOX sensors and then ironed with estimated values.

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

Sulfur poisoning of DOC and of DPF with catalytic coatings is a known problem which among the arm can be caused by if fuel with too high a sulfur content is used. At present, there are no reliable methods for obtaining an indication that sulfur poisoning exists and the purpose of the present invention is to state such a method.

In the current situation, it can happen that a warning light is incorrectly lit which indicates high emission levels, which has been indirectly caused by a sulfur poisoning. Another point of view is to get an indication earlier that sulfur poisoning may be present and then be able to initiate countermeasures.

SUMMARY OF THE INVENTION The above-mentioned view is achieved with the invention defined by the independent claims.

Preferred embodiments are claimed by the dependent claims.

A NOX sensor is adapted to emit an output signal representing the sum of the contents of the NO and NO2 contents and according to the present invention the fact is used that the NOX sensors used have different sensitivities to NO and NO2 which is used to diagnose the sulfur deposition. The ability of DOCs and DPFs to convert NO to NO2 is a good indicator of sulfur poisoning.

Since the proportion of NO2 increases in relation to the proportion of NO due to the oxidation that normally takes place in DOC / DPF, as this is not poisoned, this means that the signal strength of the pre-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 fate sperm but reaches a maximum before falling again. However, the level never falls below the 150 ° C level.

If DOC / DPF is poisoned, it will not decrease as much. If it is completely poisoned, 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 signal signal that is far too high, ie. too high a NO / NO2 content.

By comparing actual NOX signals measured downstream of the SCR catalyst with expected signals and possibly relating these to previously measured signals as well as completed refueling, according to the invention, a deteriorated NO to NO2 conversion, and thus sulfur poisoning, can be detected.

An advantage of the invention is that reliable sensors can be used to detect a process that would otherwise require a separate sensor or a workshop visit. Thereby, at an earlier stage, one can get an indication of sulfur forgiveness and earlier put in countermeasures.

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 nitric oxide levels in the exhaust gases of the first and second NOX sensors.

Figure 3 is a schematic fate diagram 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 after-treatment system according to the present invention.

Figure 1 shows an exhaust gas aftertreatment system 2 for an internal combustion engine 4 emitting a single exhaust gas 6. The exhaust aftertreatment 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 catalytic reduction. also a diesel particulate filter (DPF) arranged downstream of the DOC. The diesel particulate filter may be uncoated or coated with a catalytic coating. Soot and ash are collected here and there is also a certain conversion of NO to N02.

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 6 upstream SCR catalyst where NO and NO 2 (NOX) are then converted to Ng. The amount of reducing agent that is supplied is controlled by a control means (not shown) among the arm depending on the sensed content of nitrogen oxides and the temperature of the exhaust gases.

A first NOX sensor 12 is arranged upstream of the DOC and which is adapted to measure the content of nitrogen oxide compounds (NOX) in the exhaust gas and to emit a first NOX output signal (NOX1) depending thereon. A second NOX sensor 14 is provided downstream of the SCR catalyst and adapted to measure the content of nitric oxide compounds (NOX) in the exhaust gas and to emit a second NOX output signal (NOX2) accordingly, and at least one temperature sensor 16 is provided to sense the temperature. and outputting at least a first temperature signal (T1) depending thereon. The clock shows four temperature sensors 16 for measuring the temperature of the exhaust gas at different points of the exhaust after-treatment system. These are located before and after the DOC and before and after the SCR catalyst and emit the temperature signals T1, T2, T3 and T4, respectively.

The exhaust gas negotiation system 2 comprises, according to the invention, a calculation unit 18 and the first and second NOX output signals (N OX1, NOX2) and said first temperature signal (T1), or one or more of the temperature signals, are adapted to be applied to the calculation unit 18. The first and second NOX sensors 12, 14 are adapted to measure the content of nitric oxide compounds NOX1 upstream of the DOC and substantially simultaneously measure the content of nitric oxide compounds NOX2 downstream of the SCR catalyst when no reducing agent 10 is supplied to the gas 6 from the reducing agent device 8. Further, the temperature sensor (s) of the temperature sensor NOX2 happens.

Then, in the calculation unit 18, NOX2, or a value related to NOX2, is compared with a detection criterion related to the measured 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 is generated if NOX2 is greater than NOXtr. In this embodiment, for example, NOXtr can be selected to be in a predetermined relationship with NOX1.

Figure 2 is a graph showing the nitric oxide levels in the exhaust gases of the first and second NOX sensors. It should be emphasized that the graph is primarily intended to illustrate the aspects that are important for illustrating the invention and is therefore simplified.

As can be seen from the fi gure, the content of nitric oxide NOX1 measured by the first NOX sensor 12 is constant regardless of the temperature. In the measurement made with the second NOX sensor 14 when no reducing agent is applied, the amplitude of the output signal will decrease if the DOC (and where applicable DPF) functions as intended, ie. if no sulfur deposition has taken place. Then the proportion of NO2 in NOX will increase at the expense of the proportion of NO and due to the sensor's different sensitivity to NO and NO2 the amplitude of NOX2 will then decrease. This is illustrated in the clock by NOX2 decreasing with increasing temperatures.

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

The även clock also shows a NOX2 ”curve (dotted) showing output signals picked up by the second NOX sensor 14 at another time. Here, the output signal of the NOX sensor is above NOXtr at 500 ° C, which may indicate sulfur deposition of DOC (and / or DPF).

According to another embodiment, the calculation unit 18 is adapted to perform the comparison by determining ANOX = I NOX1-NOX2 | , to compare ANOX with the predetermined threshold value NOXtr ', which constitutes the detection exciter, and that the generating 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 of NOX2 is required for a single indication to take place. In the embodiment shown in Figure 2, this may mean that NOXtr shall be, for example, 10% higher than the corresponding “normal” value for NOX2. And in the second embodiment, when the difference between NOX1 and NOX2 is compared with NOXtr ", NOXtr" is selected, for example, 10% less than the "normal" difference.

For both embodiments it applies that there is preferably a table, or a single database, arranged, in e.g. the calculation unit 18, which contains the associated values of exhaust fume temperatures and the said predetermined threshold values NOXtr or NOXtr '. To determine NOXtr, one starts from NOX1 and lets the threshold value constitute 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 that exist between temperature and the NOX content. According to one embodiment, the indication signal 20 is adapted to indicate that the 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 direct is used to remove sulfur from DOC and / or DPF, for example by raising the temperature of the exhaust gases in a controlled manner.

As it is known that the output signal from one NOX sensor is differently sensitive to NO and NO, the output signal from the other NOX sensor is adjusted with a suitable adjustment value 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 means that an adjustment value is added to NOX2, which thereby has a higher value. If DOC and / or DPF are poisoned, NOX2 will increase, and if an adjustment value is then added, an excessively high, and thus incorrect, NOX content will be indicated.

The exhaust gas treatment system is, according to a further embodiment, adapted to calibrate the output signal NOX2 from the second NOX sensor 14 with regard to the output signal NOX1 from the first NOX sensor l2. This is done by allowing exhaust gases to pass through the system at a low temperature, preferably lower than 150 ° C, when substantially no oxidation takes place in the DOC, and when no reducing agent 10 is supplied to the exhaust gas discharge device 8.

This is done by measuring NOX1 and NOX2, determining NOX2k = NOX1-NOX2, and determining a calibrated value for NOX2 as NOX2 ”= NOX2 + NOX2k. The comparison performed by the calculation unit 18 then takes place with the calibrated value NOX2 ”instead of NOX2.

The present invention also relates to a method for an exhaust gas treatment system for a single-combustion engine which emits an exhaust fate. The method is illustrated by the schematic fate diagram in Figure 3.

The components of the exhaust aftertreatment system and its functions have been described in detail above and reference is made here 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 substantially simultaneously measuring the content of nitric oxide compounds NOX2 downstream of the SCR catalyst when no reducing agent is added to the gas from the reducing agent, B - measuring the NO2 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 consists of a predetermined threshold value NOXtr related to the measured temperature, and that in step D said indication signal is generated if NOX2 is greater than NOXtr.

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

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

According to one embodiment, the indication signal is adapted to indicate that DOC, and / or DPF, is sulfur-deposited. The indication signal may be in the form of an alarm signal to the driver the night the vehicle is to go to a workshop. It can also mean that an antidote is directly introduced 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 lower than 150 ° C, when substantially no oxidation takes place in DOC, and when no reducing agent is supplied, the exhaust gas from the reducing agent device. This is done by measuring NOX1 and NOX2, determining a ll calibration value NOX2k = NOX1-NOX2, and determining 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 forms of embodiment are therefore not to be construed as limiting the scope of the invention, as defined by the appended claims.

Claims (14)

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 produce a catalyst, a first NOx sensor (12) arranged upstream of said DOC and adapted to feed the content of nitrogen oxide compounds (NON) in the exhaust stream and to emit a first NOx output signal (NOX1) in dependent drive, a second NOx sensor (14) arranged downstream of said SCR catalyst and adapted to man. the content of nitrogen oxide compounds (NON) in the exhaust stream and to emit a second NOx output signal (NOX2) in dependent drive, and at least one temperature sensor (16) arranged to supply the temperature of the exhaust stream and to emit at least one first temperature signal (Ti) depending thereon, The method comprises the steps of: A - feeding the content of nitrogen oxide compounds NOX2 downstream of the said SCR catalyst as no reducing agent Of-Ors the exhaust stream from the reducing agent device, B - feeding the temperature T when the NOX2 is fed, C - comparing NOX2, or a value related to NOX2, with a predetermined detection criterion related to the fed temperature, D - generating an indication signal depending on the comparison, said indication signal being adapted to indicate that said DOC is sulfur poisoned. The method of claim 1, wherein step A further comprises feeding the content of nitric oxide compounds NOX1 upstream of said DOC substantially while feeding the content of nitric oxide compounds NOX2 downstream of said SCR catalyst. The method according to claim 1 or 2, 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 2, wherein step A further comprises determining ANON = I NOX1-NOX2 I, that step C comprises comparing ANON with a predetermined 12 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 the oceans 3 or 4, wherein a table is provided which contains the associated values of temperatures for the exhaust gas flow and one of the said predetermined threshold values (NOXtr, NOXtr '). The method according to any of the preceding claims, wherein the exhaust aftertreatment system comprises at least one diesel particulate filter (DPF) arranged downstream of said DOC, and wherein said indication signal is adapted to also indicate that said diesel particulate filter (DPF) 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 substantially no oxidation takes place in DOC, and then no reducing agent is supplied to the exhaust stream from the reducing agent device, by feeding 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' father NOX2. An exhaust aftertreatment system (2) for a combustion engine (4) emitting an exhaust stream (6), the exhaust aftertreatment system (2) comprises at least one diesel oxidation catalyst (DOC), at least one selective catalytic reduction catalyst (SCR catalyst), a reducing agent (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 (NON) in the exhaust stream and emitting a first NOx output signal ( NOX1) in dependence thereon, a second NOx sensor (14) arranged downstream of said SCR catalyst and adapted to feed the content of nitrogen oxide emissions (NON) in the exhaust stream and to emit a second NOx output signal (NOX2) in dependence thereon, and at least one temperature sensor (16) arranged to supply the temperature for the exhaust gas flow and to emit at least one first temperature signal (T1) depending thereon, 13 characterized by that exhaust after-treatment system et (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 be papered said calculating unit (18), said first and other 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 said SCR catalyst as no reducing agent (10) is supplied to the exhaust stream (6) from the reducing agent device (8), and said temperature sensor (16) adapted to feed the temperature T when the feeds of NOX1 and NOX2 take place, the bending unit (18) being adapted to compare NOX2, or a value related to NOX2, with a detection criterion related to the raised temperature, and to generate an indication signal (20) in dependence of the iron release, said indication signal (20) being adapted to indicate that said DOC is sulfur poisoned. The exhaust aftertreatment system according to claim 8, 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 8, wherein the counting unit (18) is adapted to comprise determining ANON = I NOX1-NOX2 I, comparing ANON with a predetermined threshold value NOXtr 'constituting said detection criterion, and generating the indication signal (20) if ANON is less than NOXtr '. The exhaust aftertreatment system according to any one of claims 9 and 10, wherein a table is provided which contains the associated value of temperatures before the exhaust flow and said predetermined threshold value NOXtr or NOXtr '. The exhaust aftertreatment system according to any one of claims 8-11, wherein the system is adapted to calibrate NOX2 at a low temperature, preferably lower than 150 ° C, since essentially 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 14 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 8-12, wherein the system comprises at least one diesel particulate filter (DPF) arranged downstream of said DOC. The exhaust aftertreatment system of claim 13, wherein said indication signal (20) is adapted to awn indicate that said diesel particulate filter (DPF) is sulfur poisoned. 1/2