Apparatus and method for removal of by-products from exhaust gases of a combustion engine

Technical field

The present invention relates to a method and an apparatus for removal of by-products from exhaust gases from a combustion engine equipped with a NO_x reduction catalyst. This is achieved by a combination of arranging a by product reducing catalyst in the exhaust gases and by means of engine control for the control of the right temperature range and air fuel ratio range for the desired catalytic processes to take place.

Background of the invention

When attempting to reduce the emissions of nitrogen oxides (NO_x) from internal combustion engines, a lot of efforts have been made to modify the combustion conditions in order to reduce the NO_x-emissions, while still maintaining the combustion efficiency at a satisfactory level.

Amongst the traditional techniques for the reduction of NO_x-emissions may, inter alia, the technique of Exhaust Gas Recirculation (EGR) be mentioned, as well as special designs of fuel injectors and combustion chambers. Other important parameters are compression, fuel injection time and fuel injection pressure. Techniques involving water injection, the use of fuel/water emulsions and so-called Selective Catalytic Reduction (SCR) by ammonia, have also been employed. Thereby, it has been found that a one-sided optimization of the combustion efficiency often results in increased NO_x-emissions.

Today, it is required that the NO_x-emissions are reduced and decreased fuel consumption is expected by both customers and by international agreements like the Kyoto protocol. There are also strong demands on reduced emissions of other compounds which are potentially hazardous to the environment, e.g. hydrocarbons.

Accordingly, there is an increased need for catalytic converters which also are able to treat exhaust gases from diesel and lean burn engines. Therefore, a number of different catalytic converters have been developed and are well-known from commercial applications in e.g. vehicles.

Typically, conventional catalytic converters comprise one or several substrates, or monolith bricks as they sometimes are called. Such bricks or monoliths are often in the form of a ceramic honeycomb substrate or metallic substrates, with through passages or cells, and can be furnished with a porous surface coating.

Particles of a suitable catalyst are coated on the surface of the substrate, and the design of the coating has been optimized in order to maximise the surface area over which catalytic reactions take place. Common groups of catalysts for lean NO_x reduction are: <ul id="ul0001" list-style="none" compact="compact"><li>1, SCR catalysts like vanadium on titanium or a base metal that is ion exchanged in zeolites. A reducing agent like ammonia or Urea is added to the exhaust in front of the catalyst.</li><li>2, NO_x storage reduction type catalysts that often contain Ba-carbonates in close connection to noble metals like Pt and Rh dispersed on a support like Ceria. The stored NO_x is periodically regenerated by rich exhaust conditions.</li><li>3, HC-SCR catalysts like Cu-zeolites or silver or aluminium or noble metals, where hydrocarbons are dosed to the exhaust gas in front of the catalyst.</li></ul> There are also a number of other catalytic systems which may be applied.

It is also previously known to use crystalline aluminium silicates, so-called zeolites, loaded with a suitable catalyst. The use of zeolites in connection with the catalytic conversion of exhaust gases is disclosed in e.g. EP 0 499 931 Al and EP 0 445 408 A.

Furthermore, it is also previously known to combine several different catalytic substrates, or to arrange a so-called after-burner in the catalytic conversion process. Such arrangements are disclosed in e.g. U.S. Patent No. 5 465 574.

It is also previously known to use a honeycomb monolith of corrugated metal foil, having a suitable catalyst carried or supported on its surface.

It has also been suggested, e.g. in EP 0 483 708 A1, to combine a conventional ceramic catalytic converter with an electrically heatable catalytic converter in order to ensure that the optimum temperature for catalytic conversion is maintained.

Thus, a number of different catalyst materials, devices, and arrangements for the catalytic conversion of exhaust gases have been described in the art.         A)     NO_x + H_xC_y → N₂ + H₂O + CO₂ + CO and         B)     O₂ + H_xC_y →H₂O + CO₂

Acidic catalysts (H+) and acidic zeolites, doped with Ag or other suitable catalysts, have been found to be selective in the sense that NO₂ will readily be converted, whereas NO will not. This can be a great disadvantage since NO is predominant in "lean" exhaust gases from e.g. diesel and lean burn engines.

Normally, the following main reactions will take place over a conventional Pt-catalyst:         C)     NO + 1/2 O₂ → NO₂         D)     O₂ + H_xC_y → H₂O+ CO₂         E)     2 NO + H_xC_y → N₂O + H₂O + CO₂

In a NO_x storage reduction type catalyst as described e. g. in US 5 740 669 a NO_x absorbent is used for removing the NO_x in the exhaust gas (a so called NO_x trap). The NO_x absorbent absorbs NO_x in the exhaust gas when the air-fuel ratio of the exhaust gas is lean and releases the absorbed NO_x and reduces it to nitrogen when the air-fuel ratio of the exhaust gas is rich or stoichiometric. To prevent the NO_x absorbent from being saturated with the absorbed NO_x, the NO_x absorbent must be regenerated periodically by causing the NO_x in the absorbent to be released and reduced.

In the described methods for the removal of NO_x in the exhaust gas, there is under certain circumstance an undesired residue of NO_x in the catalytically converted exhasust gases. Many attempts have been made to solve this problem with different types of catalysts, by means of combining different catalysts, and by means of adding an additional amount of hydrocarbon to the exhaust gases in order to deacrease NO_x with a sufficient amount of hydrocarbon. When the engine runs rich secondary air supply can enhance the byproduct conversion further.

However, many of the previous solutions have been associated with the problem of undesired oxidation of hydrocarbons (H_xC_y) over at least some surfaces of the oxidation catalyst, which preferably only should catalyse oxidation of nitrogen monoxide (NO) into nitrogen dioxide (NO₂), according to reaction C).

Another disadvantage associated with many previously known catalysts is that, during certain conditions, they will catalyse reaction E) which produces dinitrogen oxide (N₂O).

This reaction is undesired and it is preferred that the nitrogen oxides (NO_x) in the exhaust gases are converted into nitrogen (N₂) to the highest possible degree, and not into dinitrogen oxide (N₂O).

During control of an engine it is further desired to operate the engine for short periods with a deficiency of oxygen, so called rich operation of the engine, in order to regenerate the NO_x trap. A drawback occurring at rich operation of the engine is that during these periods of rich operation hydrocarbons, HC, are present to some extent in the exhaust gases. Further, nitrous oxide, N₂O, is formed over a catalyst, in particular over a Pt containing catalyst.

One document connected to N₂O treatment is EP 01027919 A3 (D1). In said document it is disclosed a method and a system for treating exhaust gases generated by a diesel engine by locating two catalyst components in the engine exhaust gas passage. The first catalyst component which is exposed to oxidizing diesel exhaust is located nearest the engine and is a nitrogen oxide absorbent. The second component is a catalyst such as a lean-NO_x catalyst which is capable of converting the exhaust gas passing over it including reducing oxides desorbed from the first component into nitrogen (N₂) or nitrous oxide (N₂O). Materials like hydrocarbons or ammonia or urea may be injected into the vicinity of the second catalyst to aid in the reaction.

JP 2001182530 (D2) describes an oxidation catalyst capable of oxidizing at least methane and a trap catalyst trapping NO_x jointly used. A combustion air-fuel ratio for an engine is controlled to a rich side of a theoretical air-fuel ratio necessary.

Document D1 does not mention how to reduce nitrous oxide from the exhaust gas and D2 does not mention treatment of N₂O at all. Little has been described in the art about actions to decrease by-product, such as methane and nitrous oxide formation, in NO_x reduction emission control systems.

A purpose of the present invention is to provide an apparatus and a method for the reduction of hydrocarbons and nitrous oxide in the exhaust gases during periods of operation of a combustion engine, particularly during rich operation of the said engine. EP 1 256 700 (D3) describes an apparatus for purifying exhaust gas from a combustion engine. The apparatus includes means for parameter tuning of the engine and a NOx reduction catalyst including means for NOx reduction. The apparatus is further provided with a catalyst containing a noble metal catalyst. Said apparatus is not aimed at reducing hydrocarbons or nitrous oxide in the exhaust gas of the combustion engine. Thus, the parameter tuning does not, specifically, involve control of the engine for the reduction of the amount of hydrocarbons and/or nitrous oxide in the exhaust gas. DE 196 41 644 (D4) describes an apparatus used in process industry for making noble gases cleaner when decomposing air to produce noble gases. The apparatus is thus provided with a catalyst for reducing methane and nitrous oxide from a gas mixture during the process. The catalyst used for this purpose is Palladium. Both the amount of methane and nitrous oxide are reduced from the gas mixture by use of Palladium as the catalyst during said process of stationary process conditions. This process is not verified to be applicable during non-stationary conditions of the kind occurring in a combustion engine. Further, an optimization of the catalysts for the specific reduction of the amount of each of hydrocarbons and nitrous oxide is not possible with the use of one single catalyst for both substances. It is further not known how the chemical reactions are affected by a parameter tuning of the engine when using a single specific catalyst.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is presented an apparatus for the removal of methane, CH₄, and nitrous oxide, N₂O, from the exhaust gases of a combustion engine during operation of the engine, particularly during rich operation of the engine, wherein the apparatus further comprises: <ul id="ul0002" list-style="dash" compact="compact"><li>means for parameter tuning of the engine,</li><li>a NO_x reduction catalyst including means for NO_x reduction and</li><li>catalysts for reducing methane and/or nitrous oxide in the exhaust gases of the engine.</li></ul>

The actions to be taken for arriving at the first aspect of the invention are to: <ul id="ul0003" list-style="dash" compact="compact"><li>make arrangements for keeping the exhaust gases within the right temperature range, sufficient for methane oxidation and above N₂O equilibrium. N₂O is in equilibrium with NO and NO₂ between 200°C and 350°C (depending on partial pressures of oxygen and NO_x),</li><li>make arrangements for remaining in the right air-fuel ratio range. This is achieved by controlling the engine and the combustion in such a way that rich operation is accomplished during short periods, whereby ammonia generation is avoided. Further, avoiding too rich operation keeps the generation of ammonia at a low level. The engine is controlled such that the combustion conditions are close to an air-fuel ratio, where the generation of nitrous gas, N₂O, can be kept at a low level. To stay in the right air-fuel ratio is further important to avoid hysteresis effects on the reduction of methane, and</li><li>control the engine for reduction of emissions of methane out of the engine. This step is achieved by means of modifying engine parameters such as: EGR (Exhaust Gas Recirculation), injection timing, inlet gas (air+egr) temperature, boost pressure, valve timing and injection pressure.</li></ul>

NO_x reduction is achieved by use of at least one of: means for fuel injection timing, means for control of fuel injection length or pressure, means for valve timing, means for throttling, means for controlling engine air compressor, means for control of engine inlet gas temperature and injector for reducing agent in the exhaust pipe upstream the catalyst.

The parameter tuning of the engine is achieved by controlling the engine by use of at least one of the parameters, engine torque, engine speed, value of content of NO_x in exhaust gases, lambda probe signal level.

Said means listed for arriving at NO_x reduction and said means for achieving parameter tuning are well known in the art and will not be described in more details herein.

For the reduction of methane Paladium, Pd, can be used as a catalyst. A particle filter may be used for hosting the catalyst. The catalyst is further, at least in a preferred embodiment, an NSR (Nitrogen Storing Reduction) type catalyst, which may have Ceria or Zirconia components as oxygen storing components (OSC).

For the reduction of nitrous oxide Fe-zeolite may be used as the catalytic agent.

According to a second aspect of the present invention there is presented a method for the removal of methane, CH₄, and nitrous oxide, N₂O, from the exhaust gases of a combustion engine during operation of the engine, particularly during rich operation of the engine, as stated in the independent method claim. Further aspects of the method are listed in the dependent method claims.

Still other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWING

The enclosed figure is a schematical illustration of a system description of a device for the removal of by-products from exhaust gases from an engine.

EMBODIMENTS OF THE INVENTION

In the following a number of embodiments of the present invention will be described with support by the enclosed drawing.

In the enclosed figure a combustion engine is symbolized by a block denoted with the reference numeral 1. Exhaust gases 2 are lead through an exhaust pipe 3 to a catalyst device 4. In the disclosed embodiment the catalyst device is divided into two parts, a primary catalyst 4a and a secondary catalyst 4b. The primary catalyst 4a is, in the described example, an NSR type catalyst, known in the art as described above, with NO_x storing and reduction properties. The secondary catalyst 4b is a clean-up catalyst taking care of remaining undesired by-products in the exhaust gases let out from the primary catalyst 4a. Exhaust gases released to the atmosphere are denoted by reference numeral 5. The arrows denoted by 6 indicates signals for the control of the engine by means of parameters and control signals as described in paragraphs 28, 29 and 30.

The secondary catalyst 4b contains catalysts for reducing, according to a preferred embodiment of the invention, the content of methane or nitrous oxide or both methane and nitrous oxide in combination, from the exhaust gases. This is accomplished in combination with engine control, where measures are performed to control the engine by means of parameters which tune the engine for minimizing the formation of methane and/or nitrous gas from the engine.

Further, the engine is arranged to remain in a correct air-fuel ratio by a control of the engine and the combustion in such a way that rich operation is accomplished during short periods. Measures for such operation of the engine are known in the art, whereby the provisions of details for such control are not described herein. Still further, a reading of at least one of the values: engine torque, engine speed, exhaust mass flow, value of content of NO_x in exhaust gases and lambda probe signal for modifying engine parameters like fuel injection timing, fuel injection length, fuel injection pressure, valve timing, throttling, air compressor control, inlet gas temperature, to control the engine to stay in a predetermined air fuel ratio, is each a measure known in the art and is also not described herein.

Housed in the secondary catalyst a first catalytic agent for the reduction of nitrous oxide is present. As an example, without being restricted to such use, the first catalytic agent of the disclosed embodiment is Fe zeolite.

Also housed in the secondary catalyst a second catalytic agent is provided for the reduction of methane present in the exhaust gas downstream the primary catalyst. Without being restricted to such use, a Pd-selective catalyst is arranged for use as the second catalytic agent for said methane reduction.

In one embodiment of the invention the first catalytic agent is provided in a first layer of the catalyst, wherein the second catalytic agent is provided in a second layer of the catalyst.

In another embodiment of the invention the methane reducing catalyst and/or the nitrous oxide reducing catalyst is arranged in a separate monolith

In still another embodiment of the invention the methane reducing catalyst and/or the nitrous oxide reducing catalyst is arranged in a powder penetrated by the exhaust gases.

In still a further embodiment of the invention the catalyst compounds are arranged in the same housing, wherein the methane reducing catalyst and/or the nitrous oxide reducing catalyst is arranged in a catalyst powder mixed with the NO_x reduction catalyst.

Below a short description of the process according to the invention follows. Engine gases let out from the engine during rich or lean operation are substantially: CO₂, NO, O₂, H_xC_y, (CH₄), CO, H₂O. Hydrocarbons are indicated by H_xC_y, wherein methane is indicated as the substance which according to the invention is removed by means of a processing in the secondary catalyst.

In a regeneration step processed in the primary catalyst nitrogen oxide, NO, and nitrogen dioxide, NO₂, are over the first catalyst reduced to nitrogen, N₂, according to the schematic reaction:         NO/NO₂ + reducing agent → N₂ (H₂O and or CO₂) By-products from the reaction are water and/or carbon dioxide.

As discussed above another undesired reaction will take place in the primary catalyst, wherein nitrogen dioxide over the reducing agent will be reduced to nitrous oxide, N₂O, according to the simplified formula:         NO₂ + reducing agent → N₂O (H₂O and or CO₂)

In the secondary catalyst methane is oxidized into carbon dioxide and water over a Pd-selective catalyst (that is a catalyst selective for the reaction between methane and oxygen), according to the formula:         CH₄ + O₂ → CO₂ + H₂O

Nitrous oxide generated in the primary catalyst will be processed in the secondary catalyst according to the formula:         N₂O + reducing agent → N₂ (H₂O + CO₂), wherein nitrous oxide is converted into nitrogen, N₂, be means of the first catalytic agent. By-products at the reaction are water and carbon dioxide.

As obvious from the reactions taking place in the secondary catalyst the gas components released to the atmosphere are nitrogen, water and carbon dioxide.

As a further measure to improve the efficiency of the reduction process during rich running of the engine, as described, secondary air may be added to the exhaust gases by means of an air pump providing air upstream of the catalyst.