WO2008143432A1

WO2008143432A1 - Purifying device for exhaust gas of internal combustion engine

Info

Publication number: WO2008143432A1
Application number: PCT/KR2008/002733
Authority: WO
Inventors: Dowoan Kim; Yunguen Cho; Yongwoo Kim; Hongseok Jung; Woojin Lee
Original assignee: Sk Energy Co., Ltd.
Priority date: 2007-05-18
Filing date: 2008-05-16
Publication date: 2008-11-27
Also published as: KR20080101451A

Abstract

The present invention relates to an exhaust gas purifying device for a diesel engine, which efficiently purifies nitrogen oxides ('NOx') and particulate matter in an internal combustion engine. More specifically, the present invention provides an exhaust gas purifying device having catalyst section for selective reduction of nitrogen oxides (SCR), equipped with a diesel injection unit (or called as a reductant injection unit) which can inject light oil (if necessary, hydrocarbon such as gasoline may be used) at the fore-end of the catalyst section for selective reduction of nitrogen oxides, wherein light oil as reductant flowing from a reductant injection unit into the exhaust gas purifying device via injection is treated by a plasma modifier, to be subjected to partial oxidation, so that modified reductant is injected to the fore-end of the SCR in the exhaust gas purifying device.

Description

[DESCRIPTION]

[invention Title]

PURIFYING DEVICE FOR EXHAUST GAS OF INTERNAL COMBUSTION ENGINE [Technical Field]

The present invention relates to a purifying device for exhaust gas of a diesel engine, which efficiently purifies nitrogen oxides (NO_x) and particulate matter (PM) contained in the exhaust from a diesel engine. [Background Art]

On the basis of excellent reliability with low fuel efficiency, diesel engines are applied in various industrial fields including automobile, ship and general industry, and the demand continually increases due to its ability for high power and high- load operation. In the 3L Car Program or Super Car Project, which is proceeded for the purpose of low fuel efficiency vehicles, employing a diesel engine becomes a settled matter. Thus, increase of vehicles having diesel engines is anticipated. However, in developed countries, diesel vehicles are recognized as principal cause of air pollution, constituting up to 40% of entire air pollution.

In order to deal with this problem, individual countries intensify restrictions of exhaust gas of diesel engines. Since the air pollution from a diesel vehicle essentially comes from nitrogen oxides (NO_x) , improvements and modifications in combustion performance of diesel engines have been intensively- developed to reduce the concentration of nitrogen oxide, as primary subject material for restrictions in exhaust gas of diesel vehicles.

As a conventional solution to overcome air pollution by nitrogen oxides (NO_x) generated from internal combustion engines causing serious environmental problems in urban areas, techniques for removing NO_x generated from engines (in particular, internal combustion engines) by means of selective catalytic reduction (SCR) using a variety of hydrocarbons have been known.

In order to substantially utilize selective catalytic reduction, researches for developing a catalyst for reducing NO_x which can be stably operable in an appropriate temperature range must be accomplished. For example, catalysts such as zeolite carrying metal or metal oxides, or a conventional technique disclosed by Korean Patent Application No. 10-2006- 63597 (filed by the present Applicant) can be referred. Conventional reductants for selective catalytic reduction include ammonia, urea and hydrocarbons. Among them, hydrocarbons are preferable since ammonia and urea are disadvantageous in that they require an infrastructure for separate supplying. When an oxygenated hydrocarbon substance such as aldehydes and ethanol is used, performance of removing NO_x is excellent, but an additional device and a storage vessel are required for a vehicle to supply said reductant. This results in a bulky equipment with complicated design and increased cost, so that commercial practice of substantial application cannot be achieved. When a small amount of hydrogen is additionally supplied as a reductant, superior performance in reducing NO_x can be secured to the case without hydrogen, but an additional vessel for hydrogen storage is required to result in difficulties of practical application.

When light oil (diesel fuel comprising hydrocarbons) is used as reductant for an exhaust gas purifying device, on the other hand, the effect of reducing nitrogen compounds is meek as compared to the case using oxygenated hydrocarbons (aldehydes, alcohols) or ammonia. Thus there exist needs to develop devices which do not require separate equipment or a storage vessel for reductant, but enable reduction of nitrogen oxides as using reductant such as oxygenated hydrocarbons with high reactivity. Accordingly, urgently required is development of simple and efficient exhaust gas purifying devices having excellent ability to remove nitrogen oxides in wide temperature range including substantially low temperature, without requiring additional use of heterogeneous reductant. [Disclosure] [Technical Problem]

The present invention is to overcome the problems of conventional techniques as described above. The object of the present invention is to provide a novel exhaust gas purifying device having excellent performance in reducing NO_x, regardless of driving condition of engine, which induces difference of performance in reducing NO_x depending on the temperature of exhaust gas . The present invention provides an exhaust gas purifying device having catalyst section for selective reduction of nitrogen oxides (SCR) , equipped with a diesel injection unit (or called as a reductant injection unit) which can inject light oil (if necessary, hydrocarbon such as gasoline may be used) at the fore-end of the catalyst section for selective reduction of nitrogen oxides, wherein light oil as reductant flowing from a reductant injection unit into the exhaust gas purifying device via injection is treated by a plasma modifier after the injection, to be subjected to partial oxidation, so that modified reductant is injected to the fore-end of the SCR in the exhaust gas purifying device.

[Technical Solution]

In order to overcome the disadvantages as described above, the present inventors have exerted themselves to intensive studies for a long time, and found that reduction of nitrogen oxides can be promoted if light oil is used as reductant to decrease nitrogen oxides in an exhaust gas purifying device for an internal combustion engine, and it is injected to the fore-end of catalyst for reducing nitrogen oxides (SCR) in the exhaust gas purifying device to make the reductant contact with SCR, and the reductant after injection is treated in a plasma modifier to cause partial oxidation; and completed the present invention.

Specifically, The present invention provides an exhaust gas purifying device having catalyst section for selective reduction of nitrogen oxides (SCR) , equipped with a diesel injection unit (or called as reductant injection unit) which can inject light oil (if necessary, hydrocarbon such as gasoline may be used) at the fore-end of the catalyst section for selective reduction of nitrogen oxides, wherein light oil as reductant flowing from a reductant injection unit into the exhaust gas purifying device via injection is treated by a plasma modifier after the injection, to be subjected to partial oxidation, so that modified reductant is injected to the fore-end of the SCR in the exhaust gas purifying device.

The exhaust gas purifying device according to the invention is not restricted to any structural arrangement or shape of the device, or specific SCR catalyst, as long as light oil is partially oxidized in a plasma modifier and then injected to the fore-end of SCR of the exhaust gas purifying device to give contact. According to a specific embodiment of the invention, provided is an exhaust gas purifying device, wherein selective catalyst (SCR) section for selective reduction of nitrogen oxides and a filter section for removing particulate matter are provided in the order from the upstream of the exhaust gas flow, and a reductant injection unit and a plasma modifier are sequentially equipped at the fore-end of the catalyst section for selective reduction of nitrogen oxides, to give the flow into an exhaust gas inlet section or a SCR fore-end of the exhaust gas purifying device (see Fig. 1) . Thus, the present inventors have found that the efficiency of decreasing NO_x is surprisingly enhanced when reductant such as diesel or gasoline is modified by passing through a plasma modifier and then contacted with the catalyst section for reduction of nitrogen, and completed the invention. This phenomenon occurs because hydrocarbon such as light oil is partially oxidized in a plasma modifier to aldehydes or alcohols as oxygenated hydrocarbon, which plays a role to reduce nitrogen oxides at low temperature by means of the plasma modifier, without requiring external supply of any substance. Diesel or the like is partially oxidized in the plasma modifier to generate some hydrogen, which activates the reaction of nitrogen oxides to improve the reduction performance of nitrogen oxides. Fig. 2 shows that the reduction (%) of NO_x increases when silver (Ag) catalyst carried on alumina is contacted, in case of using ethanol as a low molecular weight oxygenated hydrocarbon, and diesel fuel, as reductant, respectively. As can be seen from the graph, substantial decrease (%) in NO_x occurs at about 300^°C when ethanol (product of partial oxidation of diesel fuel) is used as reductant, while reductive activity of NO_x increases above 500 ^°C when using only diesel fuel.

Fig. 3 shows improvement of decrease in NO_x by using an embodiment wherein a small amount of hydrogen is added to diesel as reductant. As confirmed by the present inventors, the temperature of treating NO_x largely increases at low temperature level of 300^°C, in case that low molecular weight hydrogen is added as reductant. Thus, a process using oxygenated hydrocarbon and hydrogen may afford decrease of NO_x at low temperature, but the process additionally requires a reductant vessel and a hydrogen storage vessel, with difficulties to ensure stability. Therefore, the aforementioned process is restricted to be used for diesel engines .

The present inventors endeavored to contrive a solution to use conventional diesel fuel as reductant to carry out partial oxidation into alcohols or aldehydes to a certain level, and to provide thereby the effect of using oxygenated hydrocarbon and hydrogen. As the result, the inventors provides a device for removing NO_x by decreasing the reduction percentage of NO_x in wide temperature range including low temperature, wherein conventional diesel fuel reductant is passed through a plasma modifier to carry out partial oxidation, and then contacted with catalyst section for selective reduction of nitrogen oxides.

When the reductant is partially oxidized by means of a plasma modifier as in the present invention, external supply of reductant such as hydrogen, alcohols and aldehydes (reductant having activities at low temperature) is not required, nor additional device for storage thereof. Thus, a system available for practical use can be developed without enlarging the bulk of the exhaust gas purifying system. As described above, the exhaust gas purifying device according to the invention is not restricted to any structural arrangement or shape of the device, as long as diesel is partially oxidized in a plasma modifier at the fore-end of the catalyst section for selective reduction of nitrogen oxides and then injected. For instance, the plasma modifier may be positioned at the fore-end of the catalyst section for selective reduction inside the exhaust gas reducing unit to be canned together, or externally equipped.

F)

[Advantageous Effects]

The exhaust gas purifying device for diesel engine according to the invention exhibits excellent performance of decreasing nitrogen oxides at practical exhaust gas

10 temperature, a temperature range from 250 to 600 ^°C (the temperature range of exhaust gas of the diesel vehicle used in the present invention) , with high efficiency of removing particulate matter. Diesel fuel modified by a plasma modifier is used as reductant to remove nitrogen oxides via selective

If) reduction, while simultaneously regenerating the filter for removing particulate matter. Thus, the present invention provides novel exhaust gas purifying devices with excellent performance of decreasing NO_x, regardless the driving condition of engines, which may induce difference of performance of 0 decreasing NO_x depending on the temperature of exhaust gas.

[Brief Description of Drawings]

Fig. 1 shows a schematic diagram of an exhaust gas purifying device according to the present invention. Fig. 2 shows comparison of performance of NO_x reduction by diesel and ethanol, respectively, as reductant.

Fig. 3 shows comparison of performance of decreasing NO_x by using diesel reductant, depending on whether hydrogen is added or not .

description of symbols of significant parts of the drawings>

1: fuel tank 2 : diesel engine

3: reductant injection unit

4 : plasma modifier

5 : catalyst section for selective reduction of nitrogen oxides 6: filter section for removing particulate matter

7 : exhaust inlet pipe

8 : exhaust pipe

10: exhaust gas purifying device

[Best Mode]

Now the present invention is described in more detail by referring to specific embodiments to achieve the object of the present invention, that is an exhaust gas purifying system for a diesel engine wherein the catalyst section for selective reduction of nitrogen oxides and a filter section for removing particulate matter are sequentially provided from the upstream of the exhaust gas flow, and a reductant injection unit and a plasma modifier are sequentially equipped at the fore-end of the catalyst section for selective reduction of nitrogen oxides .

Fig. 1 shows an embodiment of an exhaust gas purifying system equipped with a plasma modifier according to the invention. First, exhaust gas from engine flows into the fore- end of an exhaust gas purifying device (10) via exhaust inlet pipe (7) , and the exhaust gas flows from a fuel tank (1) to a plasma modifier (4) through a reductant injection unit (3) . The reductant partially oxidized thereby is mixed with the exhaust gas in a step prior to a catalyst section for selective reduction of nitrogen oxides in the exhaust gas purifying device to contact the catalyst section for selective reduction (5) at the fore-end of the exhaust gas purifying system, and decreases nitrogen oxides by reduction. When a filter section for removing particulate matter (6) is equipped, the exhaust gas treated in the catalyst section (5) for selective reduction is discharged outward thereby. In the device, the reductant injection unit gives injection adjacent to the fore-end of the exhaust gas purifying device, and the plasma modifier is provided inside the exhaust gas purifying device. Since the filter section for removing particulate matter is well described by prior arts including Korean Patent Application No. 10-2006-0063597, specific description of the same is omitted in the present invention.

5 Now the SCR catalyst section according to the invention is described in more detail. The catalyst section is not restricted as long as any SCR catalyst section described in the afore-mentioned Korean Patent Application, or any other from prior arts is selected. For example, catalysts carrying

10 silver (Ag) components, copper (Cu) components or a mixture thereof may be used. A substance for carrying silver (Ag) component can be selected from silver (Ag) in a reduced state, silver oxide (Ag₂O) , silver chloride (AgCl) , silver nitrate (AgNO₃), silver sulfate (Ag₂SO₄) and mixtures thereof. A

If) substance for carrying copper can be selected from copper (Cu) in a reduced state, copper (cuprous or cupric) oxide, copper acetate, copper nitrate, copper sulfate or mixtures thereof. As the carrier for the catalyst, a substance selected from alumina (Al₂O₃) , silica (SiO₂) , titania (TiO₂) , ceria (CeO₂) , 0 zirconia (ZrO₂) or zeolite, or a mixture thereof may be used. Among them, alumina is preferably used in terms of performance of decreasing nitrogen oxides.

The carrier is coated on a support, and used in a preferable amount of 0.5 to 4 g/in³ based on total catalyst. If the amount of carrier is less than 0.5 g/in³, absolute quantity of the catalyst carried is insufficient to cause decrease of the catalytic performance. If it is more than 4 g/in³, catalytic performance does not increase any more, only causing difficulties in the preparation.

The support for SCR section is a support of flow-through type, comprised of heat-resistant ceramic or metal substance. Specific example is a cordierite honeycomb structural body.

The silver or copper component or a mixture thereof is preferably used in an amount of 0.1 to 10% by weight based on the carrier. If the content is less than 0.1% by weight, the absolute quantity of catalytic component is insufficient to cause decrease in catalytic performance. If it is more than 10% by weight, the component exists in metal state (disadvantageous for reaction) to cause noticeable deterioration of catalytic performance.

The SCR section comprising DeNO_x catalyst according to the present invention additionally carries platinum group component to further enhance the performance of decreasing nitrogen oxides at the actual exhaust gas temperature, from 250 to 600 ^°C . The catalyst for decreasing nitrogen oxides according to the present invention exhibits excellent reduction performance of nitrogen oxide at actual exhaust gas temperature, from 250 to 600^°C. More excellent performance can be obtained at a temperature range from 350 to 450^°C when platinum group catalyst is additionally carried.

Usable metal to be carried on DeNO_x catalytic section may be one or more metal (s) selected from Pt, Pd, Ru, Ir and Rh. As starting material for palladium (Pd) , used may be palladium nitrate, palladium chloride, tetraminepalladiumdichloride, or the like. As starting material for platinum (Pt), used may be chloroplatinic acid, diaminenitritoplatinum, diaminetetrachloroplatinum or the like. As starting material for rhodium (Rh), used may be rhodium chloride, rhodium nitrate, triaminerhodium hexachloride, or the like. The content of the platinum group metal is from 0.0001 to 0.5% by weight, preferably from 0.0005 to 0.2% by weight, more preferably from 0.001 to 0.1% by weight. If the content is less than 0.0001% by weight, the metal is of no use to enhance the performance. If the content is more than 0.5% by weight, the performance abruptly decreases.

The filter section (DPF) for removing particulate matter (PM) according to the invention is a filter carrying platinum group catalyst. The element carried as platinum group catalyst is one or more element (s) selected from a group consisting of Pt, Pd, Ir and Rh, but Pt or Pd is more preferably used among them. The content of platinum group catalyst carried on DPF is from 0.01% by weight to 5% by weight based on the support coated with carrier. If the content is less than 0.01% by weight, the effect of removing particulate matter does not occur. If the content is increased over 5% by weight, the performance is not improved any more, to provide disadvantageous economy.

DPF may additionally carry co-catalyst component for enhancing oxidation ability in addition to platinum group catalyst. The oxidative co-catalyst component plays a role to inhibit oxidation of sulfurous acid gas or increase surface activity of platinum group catalyst. Platinum group catalyst usually shows high activity in oxidation of soluble organic fraction (SOF) , but causes undesirable result in that sulfurous acid gas (SO₂) (combustion product of sulfur contained in light oil) is oxidized to form sulfate salt, thereby rather increasing the particulate matter. In order to overcome the problem, one or more catalyst component (s) selected from V, W and Mo may be further used in addition to the platinum group catalyst. One or more catalyst component (s) selected from V, W and Mo, as described above is preferably contained in an amount from 0.01% by weight to 2% by weight on the basis of support coated with carrier. When the content is less than 0.01% by weight, effect of inhibiting oxidation of sulfurous acid gas does not occur. If the content is more than 2% by weight, there is no advantage of further addition but only providing economic burden. Since alkaline metal component such as K and Mg has flowability at low temperature, it plays a role to enhance the rate of catalytic reaction by increasing surface activity of platinum group catalyst and facilitating contact of the catalyst with particulate matter. Alkaline metal is preferably used in an amount from 0.01% by weight to 1% by weight based on the support coated with carrier. If the content is less than 0.01% by weight, the effect of increasing the reaction rate of platinum group catalyst does not occur. If the content is more than 1% by weight, there is no advantage of further addition but only providing economic burden.

As a carrier for the catalyst component, employed may be alumina (Al₂O₃) , silica (SiO₂) , titania (TiO₂) , ceria (CeO₂) , zirconia (ZrO₂) or zeolite, or a mixture thereof.

DPF according to the invention can remove particulate matter trapped on the filter by using nitrogen dioxide generated from oxidation of nitrogen monoxide in exhaust gas, and also remove accumulated particulate matter by using temperature rise of exhaust gas with oxidation of diesel injected from the fore-end of catalyst for decreasing nitrogen oxides. In order to ensure safety in case of excessive rise of temperature of DPF, the support is preferably made of cordierite with high heat resistance, ceramic containing SiC, alloy material containing Ni alloy or FeCr alloy.

If such a material having high heat resistance is used as support, safe operation is secured against high temperature occurred upon regeneration of DPF, and heat generated from oxidation of particulate matter can be utilized to result in saving of diesel fuel.

The plasma modifier is not particularly restricted as long as it can generate plasma in a conventional manner. Depending on the energy and temperature of plasma, some portion of reductant may exist in liquid state, and the other in gas state, or all the reductant may exist in gas state. Since it may be injected to exhaust stream in liquid state by temperature control of the injection site, the appearance of the modified and partially oxidized reductant doesn't matter. Thus the present invention provides a device for reduction of NOx by converting diesel fuel (as reductant) to alcohols, aldehydes or the like due to partial oxidation, at a suitable temperature for an exhaust gas purifying system for diesel engine. According to the invention, more efficient NO_x reductant is provided by the device, by partially oxidizing (modifying) diesel fuel as conventional reductant by means of a plasma modifier.

In an embodiment of the present invention, provided is a device for decreasing NO_x from diesel engine, which performs the steps of providing an engine exhaust gas stream; injecting diesel as reductant to a plasma modifier; modifying diesel fuel as reductant by means of the plasma modifier; selectively mixing the exhaust gas stream with the reductant fuel modified as above; and reducing nitrogen oxides by introducing the reductant gas mixture into the catalyst section of selective reduction of nitrogen oxides.

The reductant injection unit according to the invention is equipped for an exhaust gas purifying device generally comprising an exhaust gas inlet, an exhaust gas exit, and at least one catalyst section for decreasing nitrogen oxides . The reductant injection unit is positioned at the fore-end of the catalyst section for reduction of nitrogen oxides, which promotes reduction of NO_x by virtue of reductant flowing into the catalyst. At least a part of the modified reductant reacts with the catalyst section, to cause temperature rise of exhaust gas .

[industrial Applicability]

The exhaust gas purifying device for diesel engine according to the invention exhibits excellent reduction performance of nitrogen oxides at practical exhaust gas temperature, a temperature range from 250 to 600^°C (the temperature range of exhaust gas of the diesel vehicle used in the present invention) , with high efficiency of removing particulate matter. Diesel fuel modified by a plasma modifier is used as reductant to remove nitrogen oxides via selective reduction, while simultaneously regenerating the filter for removing particulate matter. Thus, the present invention provides novel exhaust gas purifying devices with excellent performance of NO_x reduction, regardless the driving condition of engines, which may induce difference of performance of decreasing NO_x depending on the temperature of exhaust gas.

Claims

[CLAIMS]

[Claim l]

An exhaust gas purifying device for an internal combustion engine, which provides a catalyst section for selective reduction of nitrogen oxides; a reductant injection unit at the fore-end of said catalyst section for selective reduction; and a plasma modifier for plasma treatment of the reductant to result in partial oxidation of the reductant,.

[Claim 2]

An exhaust gas purifying device for a diesel engine according to claim 1, wherein the reductant is light oil.

[Claim 3] An exhaust gas purifying device for a diesel engine according to claim 2, wherein the catalyst section for selective reduction of nitrogen oxides and a filter section for removing particulate matter are sequentially provided from the upstream of the exhaust gas flow.

[Claim 4]

A process for decreasing nitrogen oxides by reduction of NO_x, which comprises the steps of providing an engine exhaust gas stream,- supplying reductarit to a plasma modifier; partially oxidizing the reductant by means of the plasma modifier; mixing the exhaust gas with the reductant modified by plasma; and reacting the mixture of the plasma modified reductant and exhaust gas .

[Claim 5] A process for decreasing nitrogen oxides by reduction of NO_x according to claim 4, wherein the reductant is diesel fuel.