EXHAUST GAS TREATMENT SYSTEM AND EXHAUST GAS PURIFICATION METHOD

Abstract

The present invention is directed to solve problems in conventional HC-SCR systems and provide cost-effective exhaust gas treatment systems with high NOx removal rates especially at low temperatures. A hydrocarbon selective catalytic reduction (HC-SCR) system in which H.sub.2 is added to a diesel oxidation catalyst (DOC) along with hydrocarbon. In other words, it can be said as an exchange gas purification method including removing NOx from an exhaust gas by adding H.sub.2 to a diesel oxidation catalyst (DOC) along with hydrocarbon in a hydrocarbon selective catalytic reduction (HC-SCR) system.

Claims

1. An exhaust gas treatment system wherein H.sub.2 is added to a diesel oxidation catalyst along with hydrocarbon in a hydrocarbon selective catalytic reduction (HC-SCR) system.

2. An exhaust gas treatment system wherein in a hydrocarbon selective catalytic reduction (HC-SCR) system comprising, in order of inflow of an exhaust gas, an upstream diesel oxidation catalyst removing NOx from the exhaust gas using hydrocarbon, a diesel particulate filter, and a downstream diesel oxidation catalyst removing an excess of the hydrocarbon by oxidation, H.sub.2 is added to the upstream diesel oxidation catalyst along with hydrocarbon.

3. An exhaust gas purification method comprising: removing NOx from an exhaust gas by adding H.sub.2 to a diesel oxidation catalyst along with hydrocarbon in a hydrocarbon selective catalytic reduction (HC-SCR) system.

4. An exhaust gas treatment system comprising, in order of inflow of an exhaust gas, a diesel oxidation catalyst removing NOx from the exhaust gas using hydrocarbon, a diesel particulate filter, and a urea SCR catalyst removing NOx, H.sub.2 is added to the diesel oxidation catalyst along with hydrocarbon.

5. The exhaust gas treatment system according to claim 4, further comprising, downstream of the urea SCR catalyst, a catalyst removing excess ammonia that is a degradation product of the urea.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a diagrammatic representation of a system according to a first embodiment;

[0012] FIG. 2 is a graph showing the relation between NOx removal rates and temperatures;

[0013] FIG. 3 is a graph showing the relation between NOx removal rates and concentrations of added H.sub.2 at a temperature between 100 C. and 200 C.;

[0014] FIG. 4A is a graph showing the relation between muffler inlet temperatures and engine operating times in the 1199 mode of FTP (US regulation);

[0015] FIG. 4B is a graph showing the average values at each time interval in FIG. 4A;

[0016] FIG. 5 is a graph showing the relation between HC removal rates and temperatures;

[0017] FIG. 6 is a graph showing the relation between NOx removal rates and temperatures;

[0018] FIG. 7 is a graph showing the relation between NOx removal rates and HC concentrations at 170 C.;

[0019] FIG. 8 is a diagrammatic representation of a system according to a second embodiment;

[0020] FIG. 9 is a graph showing the relation between NOx removal rates and temperatures;

[0021] FIG. 10 is a graph showing the relation between NOx removal rates and concentrations of added H.sub.2 at a temperature between 100 C. and 200 C.;

[0022] FIG. 11A is a graph showing the relation between muffler inlet temperatures and engine operating times in the 1199 mode of FTP (US regulation);

[0023] FIG. 11B is a graph showing the average values at each time interval in FIG. 11A;

[0024] FIG. 12 is a graph showing the relation between HC removal rates and temperatures;

[0025] FIG. 13 is a graph showing the relation between NOx removal rates and temperatures;

[0026] FIG. 14 is a graph showing the relation between NOx removal rates and HC concentrations at 170 C.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Embodiments of the present invention are described below, but the scope of the present invention is not limited to the description including Examples.

First Embodiment

[0028] HC-SCR systems (exhaust gas treatment systems) convert harmful components (e.g., NOx) in exhaust gas from automobile engines into harmless components before the exhaust gas is emitted into the atmosphere, and these systems are usually disposed at the bottom of the automobiles.

[0029] FIG. 1 illustrates a diagrammatic representation of an HC-SCR system according to this embodiment. The HC-SCR system comprises, in order of exhaust gas inflow, an upstream diesel oxidation catalyst (upstream DOC; denoted as 1st DOC in FIG. 1), a diesel particulate filter (DPF), and a downstream diesel oxidation catalyst (downstream DOC; denoted as 2nd DOC in FIG. 1).

[0030] <Diesel Oxidation Catalyst (DOC)>

[0031] DOC converts, on itself, HC, CO, and NOx in exhaust gas into harmless components. The DOC in the present embodiment has two stages, an upstream DOC and a downstream DOC, in order of exhaust gas inflow. The downstream DOC is not an essential component.

[0032] Examples of upstream DOC compositions include noble metals such as Pt and Pd, and alumina, but any composition can be used if it shows oxidation activity. In addition, two or more noble metals can be used in a form similar to that of an alloy. Cocatalysts such as CeO.sub.2 and ZrO.sub.2 can also be used.

[0033] Examples of substrates for supporting the upstream DOC include alumina (Al.sub.2O.sub.3), lanthanum (La), and silica (SiO.sub.2), but are not limited thereto.

[0034] The upstream DOC has the role of converting HC and NOx, which are harmful components in the exhaust gas emitted from engines, into harmless components.

[0035] In the HC-SCR system according to this embodiment, a light oil component is added upstream of the upstream DOC. Because the amount of HC in the exhaust gas is trace, the amount of HC in the reaction system is intentionally increased by the HC contained in the light oil component. Thus, purification is performed by promoting the reduction reaction between HC and NOx in the exhaust gas. However, sufficient NOx removal efficiency cannot be obtained only by adding HC.

[0036] The HC-SCR system according to this embodiment enhances NOx removal performance by adding H.sub.2 along with HC to the upstream DOC. It can be anticipated that this occurs because the reaction intermediate of NOx can be efficiently decomposed by reducing the surface of a catalyst such as Pt with the addition of H.sub.2.

[0037] Furthermore, by adding H.sub.2, the present invention also has the advantage that NOx can be removed even at such a low temperature that urea does not react (in an environment where the urea SCR system does not function).

[0038] The downstream DOC is typically provided downstream of the DPF and has the role of removing excess HC by oxidation. In the HC-SCR system, light oil is intentionally added in the aforementioned manner, and the light oil may be added more than the usual amount to remove NOx in some cases. Many HCs that cannot be consumed or removed by the upstream DOC or the DPF remain. The downstream DOC is provided to remove such HCs.

[0039] Examples of downstream DOC compositions include noble metals such as Pt and Pd, and alumina, similar to the upstream DOC, but are not limited thereto. Moreover, an alloy and a cocatalyst can be used similar to the upstream DOC. Furthermore, the same examples of substrates for the upstream DOC can also be used in this case.

[0040] <Diesel Particulate Filter (DPF)>

[0041] DPF is a device that captures particulate matter (PM) contained in the exhaust gas. There is no limit to the types of DPF, and any known types can be used.

[0042] The heat of the exhaust gas alone is insufficiently to raise the temperature and the PM cannot be completely burned off and tends to clog the DPF.

[0043] Therefore, the DPF makes good use of the reaction heat generated by intentionally adding light oil components to the upstream DOC, thereby to remove the PM by burning it off.

[0044] <Other Structures>

[0045] A urea SCR catalyst that removes NOx with urea can be provided downstream of the DPF. At low temperatures, the upstream DOC can play the role of removing NOx by adding light oil and H.sub.2 to the upstream DOC; at high temperatures, the urea SCR catalyst can play the role of removing NOx by adding urea to the urea SCR catalyst. Accordingly, it is possible to enhance the NOx removal performance using the hybrid effect.

[0046] Examples of urea SCR catalyst compositions include those containing metals such as Fe, Cu, and V, and include Fe-zeolite, Cu-zeolite, and V.sub.2O.sub.5 but are not limited thereto.

[0047] An ammonia slip catalyst (ASC) for removing excess ammonia can be provided downstream of the urea SCR catalyst. Examples of ASC compositions include combinations of a noble metal such as Pt or Pd and an SCR catalyst such as the Fe-zeolite or Cu-zeolite. The ASC works via the mechanism of converting NOx from ammonia oxidation with a noble metal catalyst into harmless components by catalytic reduction on the ASC catalyst with in-flow ammonia.

[0048] Next, the present invention is described by way of Examples, but the scope of the present invention is not limited to these Examples. It should be noted that % means % by volume.

Example 1

[0049] Changes in NOx gas removal characteristics were examined with H.sub.2 concentrations increased stepwise.

[0050] Composition of the Catalyst

[0051] The catalyst that is used in Example 1 corresponds to the upstream DOC. A specific composition of the catalyst is Pt 6.0 g/L and dimensions are 1.0 inch50 mm. The same applies to the second to fourth Examples.

[0052] Composition of the Simulated Gas

[0053] C.sub.3H.sub.6: 1300 ppmC, CO: 200 ppm, NO: 200 ppm, CO.sub.2: 5%, O.sub.2: 10%, H.sub.2O: 5%, SO.sub.2: 2 ppm, H.sub.2: (see the graph in FIG. 2), and N.sub.2: the balance. Note that ppmC is a unit of emission concentration; it is a product of concentration in ppm and the number of carbons.

[0054] (Evaluation Conditions) [0055] Catalytic heat treatment: 600 C., 50 hours [0056] Gas flow rate: 24 L/min (SV: 60000/h) [0057] Temperature: Measured while raising from room temperature to 500 C. and then decreasing at a rate of 10 C./min.

[0058] The results of the Example mentioned above are shown in FIGS. 2 and 3. FIG. 2 is a graph showing the relation between the NOx removal rates and temperatures. FIG. 3 is a graph showing the relation between the NOx removal rates and H.sub.2 concentrations at a temperature between 100 C. and 200 C. in which urea is not activated.

[0059] From the graph in FIG. 2, it can be understood that the larger the amount of the added H.sub.2 (concentration of the added H.sub.2) is, the more the peaks of the removal rate are shifted to lower temperatures. Among them, when the concentration of the added H.sub.2 is 16000 ppm, it is estimated that the peak of the removal rate is at 100 C. or less, and it is estimated that the removal reaction is actively occurring even at 100 C. or less.

[0060] Also, from the graphs in FIGS. 2 and 3, the maximum NOx removal rates get higher with the increase in H.sub.2 concentrations up to a certain point, but when the H.sub.2 concentration is 16000 ppm, both of the maximum NOx removal rate and the removal rate at a temperature between 100 C. and 200 C. decrease. It is assumed that this is because the added H.sub.2 activates the reaction between H.sub.2 and NOx, and NOx removal is occurring from a lower temperature. The NOx removal rate at each temperature varies with the change in amount of the added H.sub.2 as in the indicated experimental results; thus, this means that, by adapting the H.sub.2 concentrations to different engines, the required performance such as the required removal rate and engine temperature can be achieved.

[0061] FIG. 4 shows the relation between muffler inlet temperatures and engine operating times in the 1199 mode of the US Environmental Protection Agency (EPA) Federal Test Procedure, which is a method that ought to be evaluated for meeting the US regulatory compliance. Specifically, FIG. 4A shows automobile muffler inlet temperatures at each temperature, and FIG. 4B shows the average values at each time interval in FIG. 4A.

[0062] It can be understood from FIGS. 4A and 4B that the operating temperatures of the engine scarcely fall below 100 C. except when the engine is starting up. That is, it can be understood that, since most engine operating temperatures in a temperature range around and below 200 C. in which urea is not activated come between 100 C. and 200 C., it is preferable in the HC-SCR system that the amount of hydrogen addition is regulated in such a manner that high removal performance can be obtained in the range between 100 C. and 200 C.

Example 2

[0063] Changes in HC gas removal characteristics were examined with H.sub.2 concentrations increased stepwise. The evaluation conditions are the same as those in Example 1.

[0064] Composition of the Simulated Gas

[0065] C.sub.3H.sub.6: 1300 ppmC, CO: 200 ppm, NO: 200 ppm, CO.sub.2: 5%, O.sub.2: 10%, H.sub.2O: 5%, SO.sub.2: 2 ppm, H.sub.2: (see the graph in FIG. 5), and N.sub.2: the balance.

[0066] The results of the Example mentioned above are shown in FIG. 5. FIG. 5 is a graph showing the relation between temperatures and HC removal rates.

[0067] From the graph in FIG. 5, it can be understood that, in a temperature range around and below 200 C. in which urea it not activated, the HC removal (reaction) rate increases and the peaks of the removal rate are shifted to low temperatures as in Example 1 with the increase in amount of the added H.sub.2 (concentration of the added H.sub.2). Above all, when the concentration of the added H.sub.2 is 16000 ppm, the removal rate is almost 100% at 100 C., and the removal reaction occurs even at 100 C. and less in which urea cannot be activated. It can be understood that the estimation made in Example 1 is correct.

[0068] In summary, it can be anticipated that, from the results of Examples 1 and 2, in the HC-SCR system of the present invention, HC is used for NOx removal at least in the temperature range in which urea is not activated, and the enhancement of the HC activity is related to the addition of H.sub.2 (concentration of the added H.sub.2).

Example 3

[0069] The relation among the presence/absence of H.sub.2, the presence/absence of HCs, and the NOx removal rates was examined. The evaluation conditions are the same as those in Example 1.

[0070] Composition of the Simulated Gas

[0071] C.sub.3H.sub.6: 0 or 1300 ppmC, CO: 200 ppm, NO: 200 ppm, CO.sub.2: 5%, O.sub.2: 10%, H.sub.2O: 5%, SO.sub.2: 2 ppm, H.sub.2: 0 or 2000 ppm, and N.sub.2: the balance.

[0072] The results of the Example mentioned above are shown in FIG. 6. FIG. 6 is a graph showing the relation between the NOx removal rates and temperatures.

[0073] From the graph in FIG. 6, in the temperature range around and below 200 C., the NOx removal rate (with H.sub.2 and without HC) is exceptionally high, followed by (without H.sub.2, with HC) and (with H.sub.2 and HC) in this order. On the other hand, it can be understood that (without H.sub.2 and HC) shows almost no sign of removal.

[0074] That is, H.sub.2 alone does not exhibit superior NOx removal performance, and the removal rate gets high when H.sub.2 is combined with HC. Therefore, it is understood that H.sub.2 can exhibits its removal performance subject to be used in the HC-SCR system. In addition, the removal rate is higher with the addition of H.sub.2 than with HC alone that represents the conventional art. Thus, it is estimated that H.sub.2 promotes the HC-SCR reaction.

Example 4

[0075] The relation among H.sub.2 concentrations and HC concentrations at 170 C. and the NOx removal rates was examined. Note that 170 C. is the temperature corresponded to the highest removal rate (with H.sub.2 and HC) in Example 3. The evaluation conditions are the same as those in Example 1.

[0076] Composition of the Simulated Gas

[0077] C.sub.3H.sub.6: see the graph, CO: 200 ppm, NO: 200 ppm, CO.sub.2: 5%, O.sub.2: 10%, H.sub.2O: 5%, SO.sub.2: 2 ppm, H.sub.2: (see the graph in FIG. 7), and N.sub.2: the balance.

[0078] The results of the Example mentioned above are shown in FIG. 7. FIG. 7 is a graph showing the relation between the NOx removal rates and HC concentrations at 170 C.

[0079] From the graph in FIG. 7, it can be understood that, under the condition with no H.sub.2 added, the NOx removal rates do not increase even with increased HC concentrations. On the other hand, under the condition with no HC added, almost no difference was observed in removal rates regardless of the presence or absence of H.sub.2. Furthermore, under the condition with HC, the higher the H.sub.2 concentration was, the higher the NOx removal rate was.

Second Embodiment

[0080] The invention described in this embodiment is, in an exhaust gas treatment system comprising, in order of exhaust gas inflow, a diesel oxidation catalyst removing NOx from the exhaust gas using hydrocarbon, a diesel particulate filter, and a urea SCR catalyst removing NOx, H.sub.2 is added to the diesel oxidation catalyst along with hydrocarbon.

[0081] Furthermore, the invention described in this embodiment is the exhaust gas treatment system including, downstream of the urea SCR catalyst, a catalyst removing excess ammonia that is a degradation product of the urea.

[0082] The invention described in the embodiment resulted in the enhancement in NOx removal performance compared with conventional HC-SCR systems. In particular, the NOx removal performance was enhanced in a wide temperature range including lower temperatures in which urea is not activated.

[0083] The exhaust gas treatment system according to this embodiment promotes HC-SCR reactions between light oil and NOx and enhances NOx removal performance by adding hydrogen (H.sub.2) along with hydrocarbon when it is added to the diesel oxidation catalyst. In addition, the exhaust gas system according to the embodiment is a hybrid system that can ensure high NOx removal performance over a wide temperature range by using the urea SCR system in a high temperature range in which urea is activated and using the HC-SCR system utilizing H.sub.2 in a low temperature range in which urea is not activated. Specifically, it is possible to achieve hybrid systems that can ensure high NOx removal performance at almost all temperatures in the engine's operating temperature range by using, in the lower temperature range, the HC-SCR system in which HC and H.sub.2 are both present and using the urea SCR system in the high temperature range. Details are described below.

[0084] FIG. 8 shows a diagrammatic representation of an exhaust gas treatment system according to this embodiment. Exhaust gas treatment systems convert harmful components (e.g., NOx) in exhaust gas from automobile engines into harmless components before the exhaust gas is emitted into the atmosphere. These systems are disposed at the bottom of the automobiles. Exhaust gas treatment systems comprise, in order of exhaust gas inflow, a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF), a urea SCR catalyst (urea SCR), and an ammonia slip catalyst (ASC).

[0085] <Diesel Oxidation Catalyst (DOC)>

[0086] DOC converts, over itself, HC, CO, and NOx in exhaust gas to harmless components.

[0087] Examples of upstream DOC compositions include noble metals such as Pt and Pd, and alumina, but any composition can be used if it shows oxidation activity. In addition, two or more noble metals can be used in a form similar to that of an alloy. Cocatalysts such as CeO.sub.2 and ZrO.sub.2 can also be used.

[0088] Examples of substrates for supporting the upstream DOC include alumina (Al.sub.2O.sub.3), lanthanum (La), and silica (SiO.sub.2), but are not limited thereto.

[0089] In the exhaust gas treatment system, a light oil component is added upstream of the upstream DOC. Because the amount of HC in the exhaust gas is trace, the amount of HC in the reaction system is intentionally increased by the HC contained in the light oil component. Thus, purification is performed by promoting the reduction reaction between HC and NOx in the exhaust gas. However, sufficient NOx removal efficiency cannot be obtained only by adding HC.

[0090] The exhaust gas treatment system enhances NOx removal performance by adding H.sub.2 along with HC to the DOC. It can be anticipated that this occurs because the reaction intermediate of NOx can be efficiently decomposed by reducing the surface of a catalyst with the addition of H.sub.2.

[0091] Furthermore, by adding H.sub.2, the present invention also has the advantage that NOx removal performance is enhanced in such a low temperature range that urea is not activated (in an environment where it cannot function as the urea SCR system).

[0092] <Diesel Particulate Filter (DPF)>

[0093] DPF is a device that captures particulate matter (PM) contained in the exhaust gas. There is no limit to the types of DPF, and any known types can be used.

[0094] The heat of the exhaust gas alone is insufficiently to raise the temperature and the PM cannot be completely burned off and tends to clog the DPF.

[0095] Therefore, the DPF makes good use of the reaction heat generated by intentionally adding light oil components to the DOC, thereby to remove the PM by burning it off.

[0096] <Urea SCR Catalyst (Urea SCR)>

[0097] A urea SCR is a catalyst for removing NOx with urea and is provided downstream of the DPF. In a low temperature range in which urea is not activated, the DOC can play the role of removing NOx by adding light oil and H.sub.2 to the DOC; in a high temperature range, the urea SCR can play the role of removing NOx by adding urea to the urea SCR. Accordingly, it is possible to enhance the NOx removal performance using the hybrid effect over a wide temperature range.

[0098] Examples of urea SCR compositions include those containing metals such as Fe, Cu, and V, and include Fe-zeolite, Cu-zeolite, and V.sub.2O.sub.5 but are not limited thereto.

[0099] <Ammonia Slip Catalyst (ASC)>

[0100] ASC is a catalyst for removing excess ammonia that did not participate in the reaction in the urea SCR and is provided downstream of the urea SCR.

[0101] Examples of ASC compositions include combinations of a noble metal catalyst such as Pt or Pd and a urea SCR catalyst such as the Fe-zeolite or Cu-zeolite.

[0102] In the ASC, ammonia is oxidized into NOx on a noble metal catalyst and that NOx is reacted with ammonia newly flowing from the urea SCR on the urea SCR catalyst to convert the ammonia into nitrogen and water, thereby converting both ammonia and NOx into harmless components. Note that ASC is not an essential component.

[0103] <Other Structures>

[0104] Besides, a DOC (hereinafter, upstream DOC) can be provided and one DOC (not shown; hereinafter, downstream DOC) that removes excess HC by oxidation can be provided downstream of the DPF. In the exhaust gas treatment system according to this embodiment, light oil may be added more than the usual amount to remove NOx in some cases. In such cases, many HCs that cannot be consumed or removed by the upstream DOC remain. Specifically, unlike DOCs, urea SCRs usually contain no platinum group metal. Thus, excess HCs that could not be removed are accumulated on a urea SCR or reach the ASC through the urea SCR in some cases. The downstream DOC is provided to remove such excess HCs.

[0105] Examples of downstream DOC compositions include noble metals such as Pt and Pd, and alumina, similar to the upstream DOC, but are not limited thereto. Moreover, an alloy and a cocatalyst can be used similar to the upstream DOC. Furthermore, the same examples of substrates for the upstream DOC can also be used in this case.

[0106] The downstream DOC can be provided between the DPF and the urea SCR, between the urea SCR and the ASC, or downstream of the ASC.

[0107] Next, the present invention is described by way of Examples, but the scope of the present invention is not limited to these Examples. It should be noted that % means % by volume.

Example 5

[0108] Changes in NOx removal characteristics were examined with H.sub.2 concentrations increased stepwise.

[0109] Composition of the Catalyst

[0110] The catalyst that is used in Example 5 corresponds to the DOC. A specific composition of the catalyst is Pt 6.0 g/L and dimensions are 1.0 inch50 mm. The same applies to the sixth to eighth Examples.

[0111] Composition of the Simulated Gas

[0112] C.sub.3H.sub.6: 1300 ppmC, CO: 200 ppm, NO: 200 ppm, CO.sub.2: 5%, O.sub.2: 10%, H.sub.2O: 5%, SO.sub.2: 2 ppm, H.sub.2: (see the graph in FIG. 9), and N.sub.2: the balance. Note that ppmC is a unit of emission concentration; it is a product of concentration in ppm and the number of carbons.

[0113] (Evaluation Conditions) [0114] Catalytic heat treatment: 600 C., 50 hours [0115] Gas flow rate: 24 L/min (SV: 60000/h) [0116] Temperature: Measured while raising from room temperature to 500 C. and then decreasing at a rate of 10 C./min.

[0117] The results of the Example mentioned above are shown in FIGS. 9 and 10. FIG. 9 is a graph showing the relation between the NOx removal rates and temperatures. FIG. 10 is a graph showing the relation between the NOx removal rates and H.sub.2 concentrations at a temperature between 100 C. and 200 C. in which urea is not activated.

[0118] From the graph in FIG. 9, it can be understood that the larger the amount of the added H.sub.2 (concentration of the added H.sub.2) is, the more the maximum removal rates are shifted to lower temperatures. Among them, when the concentration of the added H.sub.2 is 16000 ppm, it is presumed that the maximum removal rate is at 100 C. or less, and it is presumed that the removal reaction is actively occurring even at 100 C. or less.

[0119] On the other hand, from the graph in FIG. 9, it is presumed that the maximum NOx removal rate increases with the increase in H.sub.2 concentration up to the H.sub.2 concentration of 8000 ppm but then decreases when the H.sub.2 concentration reaches 16000 ppm.

[0120] Furthermore, from the graph in FIG. 10, it is presumed that the NOx removal rate between 100 C. and 200 C. increases with the increase in H.sub.2 concentration up to the H.sub.2 concentration of 8000 ppm but then decreases when the H.sub.2 concentration reaches 16000 ppm.

[0121] It is assumed that this is because the added H.sub.2 activates the reaction between H.sub.2 and NOx, and NOx removal is occurring from a lower temperature. The NOx removal rate at each temperature varies with the change in amount of the added H.sub.2 as in the indicated experimental results; thus, this means that, by adapting the H.sub.2 concentrations to different engines, various kinds of required performance, such as temperature ranges where a specific NOx removal rate or a high NOx removal rate is required, can be met.

[0122] On the other hand, focusing on a temperature range of 200 C. and above in which urea is activated, as shown in the graph in FIG. 9, it can be understood that the NOx removal rate decreases with the increase in temperature even with the addition of H.sub.2. Accordingly, it can be understood that sufficient NOx removal performance cannot be expected in the temperature range of 200 C. and above in which urea is activated merely by adding H.sub.2.

[0123] FIG. 11 shows the relation between muffler inlet temperatures and engine operating times in the 1199 mode of the US Environmental Protection Agency (EPA) Federal Test Procedure, which is a method that ought to be evaluated for meeting the US regulatory compliance. Specifically, FIG. 11A shows automobile muffler inlet temperatures at each temperature, and FIG. 11B shows the average values at each time interval in FIG. 11A.

[0124] It can be understood from FIGS. 11A and 11B that the operating temperatures of the engine scarcely fall below 100 C. except when the engine is starting up. That is, it can be understood that, since most engine operating temperatures in a temperature range around and below 200 C. in which urea is not activated come between 100 C. and 200 C., it is preferable in the exhaust gas treatment system that the amount of hydrogen addition is regulated in such a manner that high NOx removal performance can be obtained in the range between 100 C. and 200 C.

Example 6

[0125] Changes in NOx removal characteristics with HC were examined with H.sub.2 concentrations increased stepwise. The evaluation conditions are the same as those in Example 1.

[0126] Composition of the Simulated Gas

[0127] C.sub.3H.sub.6: 1300 ppmC, CO: 200 ppm, NO: 200 ppm, CO.sub.2: 5%, O.sub.2: 10%, H.sub.2O: 5%, SO.sub.2: 2 ppm, H.sub.2: (see the graph in FIG. 12), and N.sub.2: the balance.

[0128] The results of the Example mentioned above are shown in FIG. 12. FIG. 12 is a graph showing the relation between temperatures and HC removal rates.

[0129] From the graph in FIG. 12, it can be understood that, in a temperature range around and below 200 C. in which urea it not activated, the removal (reaction) rate with HC increases and the peaks of the removal rate are shifted to low temperatures with the increase in amount of the added H.sub.2 (concentration of the added H.sub.2). Above all, when the concentration of the added H.sub.2 is 16000 ppm, the removal rate is almost 100% at 100 C., and it is estimated that NOx removal with HC occurs even at 100 C. and less in which urea is not activated.

[0130] In summary, it can be understood that, from the results of Examples 5 and 6, NOx is efficiently removed with HC at least in the temperature range between 100 C. and 200 C. of the temperature range in which urea is not activated. In addition, the HC activity in this temperature range is enhanced depending on the concentration of the added H.sub.2.

Example 7

[0131] The relation among the presence/absence of H.sub.2, the presence/absence of HCs, and the NOx removal rates was examined. The evaluation conditions are the same as those in Example 5.

[0132] Composition of the Simulated Gas

[0133] C.sub.3H.sub.6: 0 or 1300 ppmC, CO: 200 ppm, NO: 200 ppm, CO.sub.2: 5%, O.sub.2: 10%, H.sub.2O: 5%, SO.sub.2: 2 ppm, H.sub.2: 0 or 2000 ppm, and N.sub.2: the balance.

[0134] The results of the Example mentioned above are shown in FIG. 13. FIG. 13 is a graph showing the relation between the NOx removal rates and temperatures.

[0135] From the graph in FIG. 13, it can be understood that for the H.sub.2 concentration of 2000 ppm, the HC removal rate exceeds 90% only after the temperature reaches 170 C. Then, in the case where the H.sub.2 concentration is 2000 ppm, it can be anticipated that the HC-SCR reaction proceeds well at around 170 C. Thus, focusing on the NOx removal rate at 170 C. in the graph in FIG. 13, it can be understood that (with H.sub.2 and HC) exhibits the highest rate.

[0136] That is, compared with the base condition (without H.sub.2 and with HC), H.sub.2 alone does not enhance the NOx removal performance. In contrast, it can be understood that, by using H.sub.2 in combination with HC, higher removal rates can be achieved than those obtained under the base condition. Furthermore, it can be understood that higher removal rates are obtained with the addition of H.sub.2 than HC alone. From this result, it is presumed that H.sub.2 promotes the HC-SCR reactions.

[0137] On the other hand, focusing on a temperature range of 200 C. and above in which urea is activated, it can be understood that the NOx removal rate in the case of (with H.sub.2 and HC) decreases with the increase in temperature similar to the case of Example 5.

Example 8

[0138] The relation among H.sub.2 concentrations and HC concentrations at 170 C. and the NOx removal rates was examined. Note that 170 C. is the temperature corresponded to the highest removal rate (with H.sub.2 and HC) in Example 7. The evaluation conditions are the same as those in Example 5.

[0139] Composition of the Simulated Gas

[0140] C.sub.3H.sub.6: see the graph, CO: 200 ppm, NO: 200 ppm, CO.sub.2: 5%, O.sub.2: 10%, H.sub.2O: 5%, SO.sub.2: 2 ppm, H.sub.2: (see the graph in FIG. 14), and N.sub.2: the balance.

[0141] The results of the Example mentioned above are shown in FIG. 14. FIG. 14 is a graph showing the relation between the NOx removal rates and HC concentrations at 170 C.

[0142] From the graph in FIG. 14, under the condition with no HC added, almost no difference was observed in removal rates regardless of the presence or absence of H.sub.2. Under the condition with HC, the higher the H.sub.2 concentration was, the higher the NOx removal rate was. Note that under conditions where on H.sub.2 was added, the NOx removal rates were kept low and changed little regardless of the concentration of HC.

[0143] This result revealed that NOx removal rates are dependent on H.sub.2 concentrations on the premise that HC is present together with H.sub.2.

[0144] From the above, it can be understood that NOx removal rates significantly increase by adding H.sub.2 to DOC along with hydrocarbon in a low temperature range around and below 200 C. in which urea is not activated. In contrast, in a high temperature range around and above 200 C. in which urea is activated, no increase in NOx removal rate by the addition of H.sub.2 can be found.