Elimination of nitrogen oxides during a cold start of a lean-burn engine

Abstract

A method for the selective removal of NOx from the exhaust stream of an internal combustion engine, particularly during a "cold start" of the engine, comprising: the exhaust stream being successively passed through a first lean NOx trap (LNT) and a second lean NOx trap (LNT), wherein the lean NOx traps comprise a catalyst capable of catalyzing the reduction of NOx to N2 and a compound capable of storing NOx as nitrates; NOx being stored as nitrates in the first LNT at a temperature below the catalytic temperature at which the catalyst in the first LNT catalyzes the reduction of NOx to N2; NOx being stored as nitrates in the second LNT under fuel-poor conditions when the first LNT reaches a temperature at which it releases the stored NOx; and the stored NOx in the first LNT being released under stoichiometric or fuel-rich engine operating conditions. N2 is reduced,when the temperature of the first LNT reaches the catalytic temperature.

Description

FIELD OF THE INVENTIONThe present invention relates generally to the catalytic reduction of nitrogen oxides in engine exhaust gas, and more particularly to the reduction of NOx emissions from an exhaust gas stream during a cold start of a lean burn engine.BACKGROUND OF THE INVENTIONCertain compounds in the exhaust gas stream of a combustion process, such as in the exhaust gas stream of an internal combustion engine, are undesirable because their release to the environment contributes to poorer air quality. As a result, they must be controlled to protect the environment and meet or exceed governmental emissions regulations. Among such undesirable compounds are oxides of nitrogen, referred to as NOx. There are a wide variety of combustion processes that produce NOx, such as coal or oil fired furnaces, piston engines (including gasoline and diesel engines), and gas turbine engines. In each of these combustion processes, control actions are needed to prevent or reduce NOx emissions to the atmosphere to improve air quality and meet governmental regulations.In order to remove contaminants (e.g., NOx, CO, and hydrocarbons) from an exhaust gas flow of an engine, catalytic converters including a three-way catalyst have been incorporated into automobiles. In most low load vehicles in the United States, the vehicle engines operate in a stoichiometric mode, i.e., stoichiometric amounts of fuel and air are fed to the engine, and after combustion, the exhaust stream contains roughly equal amounts of residual oxygen and reducing agents (e.g., hydrocarbons, CO and H 2). In this exhaust flow, the three-way catalysts may reduce NOxto N 2 and oxidize CO, H 2 and hydrocarbons to CO 2 and H 2 O. However, in a fuel-rich exhaust gas, CO and hydrocarbons cannot be completely oxidized to CO 2 and H 2 O because of an insufficient amount of oxygen present in the exhaust gas. On the other hand, in a fuel-lean exhaust gas, NOx cannot be completely reduced to N 2 due to an insufficient amount of reducing agents and an excess of oxygen in the exhaust gas.Lean burn engines typically operate in a low fuel mode, i.e., an amount of air exceeding the stoichiometric amount is injected with fuel into the engine cylinders. Lean burn engines provide better fuel economy as compared to stoichiometric engines. A typical example of a lean burn engine is a diesel engine. However, the low fuel exhaust flow from the lean burn engines makes the three-way catalysts unsuitable for converting NOx into N2 and for reducing NOx emissions at the tail pipe due to the above-mentioned reason. Lean NOx traps or storage catalysts, sometimes called LNT, are considered one of the leading technologies for removing NOx from the exhaust flow of lean-burn engines. They contain catalysts which reduce NOx to N 2 and compounds (such as metal nitrates) which can store NOx as nitrates. However, most LNTs operate most effectively in a temperature range of about 250-550° C. and undesirable NOx emissions to the atmosphere are more likely to occur at temperatures outside this range. At temperatures below about 250° C., such as during a cold start of an engine, the LNT does not effectively convert NOx to nitrates (for storage) or NOx released to a fuel rich mode to N 2 in a low fuel mode. If the LNT temperature is above about 550° C., the metal nitrates are not stable enough to store the NOx.Therefore, despite efforts to develop an apparatus or system effective to reduce NOxto nitrogen in a lean burn engine (such as a diesel engine), the need for adequate conversion efficiency has been unsatisfered. Moreover, there is a continuing need for improved effectiveness in treating NOx emissions from any combustion process, but particularly during a cold start of an engine.SUMMARY OF THE INVENTIONA method is provided for selectively removing NOx from the exhaust flow of an internal combustion engine, in particular during a cold start of the engine, according to the claims.The method for selectively removing NOx from the exhaust flow of an internal combustion engine, particularly during a cold start of the engine, includes passing the exhaust through a first (close coupled) LNT and a second (main) LNT in sequence. The tightly coupled LNT and the main LNT contain catalysts that catalyze reduction of NOxto N 2 and compounds that can store NOxas nitrates. During a cold start, NOx contained in the exhaust gas exiting the engine is initially stored at the close-coupled LNT. As the engine warms up, it heats the close coupled and main LNTs. Once the close coupled LNT has been warmed above ambient temperature by the hot exhaust gas, it reaches its catalytic temperature (typically 250°) and becomes catalytically active to convert NOxto N 2 as excess reductants leave the engine (e.g., hydrocarbons, CO, or H 2) are available.When the temperature of the close coupled LNT is higher than about 350° C., the stored NOx may be released under rich as well as lean modes. In a fuel rich mode, the released NOxis converted to N2by the catalysts at the close coupled LNT. In a low fuel mode, the close coupled LNT will also release the stored NOx. However, the released NOx cannot be converted to N2 at the close-coupled LNT due to the lack of available reducing agents. Because the main LNT is disposed farther away from the exhaust manifold of the engine than the close-coupled LNT, the temperature of the main LNT is generally lower than that of the close-coupled LNT. NOx released from the close coupled LNT is stored at the main LNT where it may be reduced to N 2 during high fuel modes.The method of claims is also intended to reduce the total NOx released to the environment during operation of a vehicle including a lean burn engine. The cold start operation and the engine operation are carried out as above. After the engine is turned off, the close coupled LNT may be regenerated, such as by heating and by air provided by a heater or airflow element. The heater on the close-coupled LNT and an air flow element, e.g., an air pump, may be turned on for a short period of time (e.g., 30 seconds) to heat the close-coupled LNT to a temperature at which the close-coupled LNT will release its absorbed NOx (e.g., about 600° C.). The NOx released during this regeneration of the close-coupled LNT may be absorbed by the main LNT, thereby releasing little or no NOx into the atmosphere. The tightly coupled LNT may then cool in the air.BRIEF DESCRIPTION OF THE DRAWINGSThe advantages of this invention will become apparent upon consideration of the following detailed disclosure of the invention, particularly taken in conjunction with the accompanying drawings, in which: FIG. 1 is a schematic illustration of an apparatus for removing NOx for performing a method according to an exemplary embodiment of the present invention; FIG. 2 is a graph of NOx concentration at the output of the main lean NOx trap (main LNT) with a lean exhaust gas supply containing 250 ppm NO during a NOx storage capacity measurement at 150° C; FIG. 3 is a graph of engine output emission and temperature data during cycle 1 of a cold start test procedure FTP (U.S. Federal Test Procedure); and FIG. 4 is a graph of NOx concentration at the exit of a main lean NOx trap (main LNT) with a lean exhaust gas supply containing 250 ppm NO during a NOx storage capacity measurement at 150° C., a temperature rise up to about 600° C., and a subsequent cooling down to 150° C.DETAILED DESCRIPTION OF THE INVENTIONIt is known that internal combustion engines may operate in fuel rich (rich), stoichiometric or fuel lean (lean) modes with respect to fuel / oxygen ratio. For example, when an engine is operating in a fuel rich mode, the exhaust flow after combustion in the engine cylinders contains excess reductant (e.g., fuel, CO, and H2) and hardly oxygen. On the other hand, when the engine is operating in a low fuel mode, the exhaust flow contains excess oxygen and few reductants. When the engine is operating in a stoichiometric mode, which is the conventional operating mode for gasoline engines, the exhaust stream contains approximately an equal amount of residual oxygen and reductants. For lean burn engines, three way catalysts cannot effectively remove NOx emissions from the low fuel exhaust. Therefore, novel catalyst technologies such as the lean NOx traps or trap catalysts have been developed to address this problem. In order for the LNTs to be able to effectively remove NOx emissions, the engines mostly operate in a low fuel mode with excursions of operation to a fuel rich mode. During the low fuel mode, the NOx in the exhaust flow is absorbed by the LNTs to form metal nitrates. During the fuel rich mode, the nitrates decompose and release NOx. The released NOx is reduced to N2 in the rich exhaust gas by excess reducing agents. Thus, during low fuel and rich modes, very little NOx passes through the LNT and into the emissions at the tailpipe. The operating temperature window for LNTs is typically between 250 and 550° C.Before an engine is started, the LNT catalyst is at ambient temperature. Once the engine has been started, the catalyst is slowly warmed by the hot exhaust gas from the engine. In the first few minutes, the catalyst temperature is substantially below 250°C, e.g., 50-150°C. This period of time when an engine is started but the catalyst is at a temperature below its operating temperature is called "cold start".Referring now to FIG. 1, there is shown a schematic illustration of an apparatus for eliminating NOxduring such a cold start of an engine for carrying out a method according to the present invention. Specifically, the apparatus combines a close-coupled cold start LNT (lean NOx trap) 11 with a main LNT (lean NOx trap) 12. As shown in FIG. 1, the close-coupled LNT 11 may be disposed under the hood of a motor vehicle, and the main LNT 12 may be disposed under a floor of the motor vehicle, e.g., under the floor of the passenger compartment. Thus, the main LNT 12 may be disposed at a location farther from the exhaust manifold of the engine than the close-coupled LNT 11. The close-coupled LNT 11 may be connected to the main LNT 12 through a hollow pipe 16 that allows passage of an exhaust gas flow. During a cold start of an engine, e.g., when the catalyst temperature is below 250° C., exhaust gas is directed from the engine 13 to the cold start LNT 11. The NOx from the cold exhaust gas is stored in the close-coupled LNT 11 before the close-coupled LNT 11 and the main LNT 12 reach their respective operating temperatures, about 250-450° C. and about 250-500° C., respectively. The stored NOx is not released from the close-coupled LNT 11 in a low fuel mode until the close-coupled LNT 11 reaches about 350° C. Thus, when the temperature of the close-coupled LNT 11 is between 250 and 350° C., the stored NOx is released only in a fuel-rich mode when excess reductants (e.g., hydrocarbons, CO, or H2) are available. Since the close-coupled LNT 11 is catalytically active at these temperatures, the close-coupled LNT 11 reduces the released NOx to N2.If the temperature of the close coupled LNT is higher than 350° C., the stored NOx may be released in both fuel rich and fuel lean modes. In a fuel rich mode, the released NOxis converted to N2by the catalysts at the close coupled LNT. In a low fuel mode, the close coupled LNT will also release the stored NOx. However, the released NOx is not converted to N2 at the close-coupled LNT due to the lack of available reducing agents. Since the main LNT is located farther from the exhaust manifold of the engine than the close-coupled LNT, the temperature of the main LNT will be lower than that of the close-coupled LNT, and as a result, the released NOx is stored at the main LNT and reduced to N 2 during rich modes.Once the close-coupled LNT 11 reaches an operating temperature corresponding to an effective catalytic temperature (e.g., about 250-450° C.), the engine 13 may operate in a low fuel mode with periodic operation in a fuel-rich mode to convert the NOx stored at the close-coupled LNT 11 to N2. For example, engine 13 may operate in a mode that cycles between lean and rich, such as, but is not limited to, 30 seconds lean / 2 seconds rich. During the fuel-rich mode, the exhaust gas from the engine 13 has a low oxygen content and a high content of reducing agents such as CO and H2. These reducing agents may reduce the NOx stored at the close-coupled LNT 11 to nitrogen (N2). In fuel-rich conditions, any NOx released from the close-coupled LNT 11 and not converted by the reductants is absorbed to the main LNT 12 because the main LNT 12 is at a lower temperature than the close-coupled LNT 11. As a result, little, if any, NOx is released from the main LNT 12 through the tailpipe 17 and into the environment.Prior to a cold start of an engine, the close coupled LNT 11 has a high NOx storage capacity. However, after the engine 13 is turned on and the close-coupled LNT 11 is used to store NOx and convert the stored NOx into N 2 the close-coupled LNT 11 has a reduced storage capacity for NOx. Therefore, the close-coupled LNT 11 can be regenerated after each use. In one embodiment, the close coupled LNT, such as that shown in FIG. 1, may be regenerated by heating above a temperature at which the compounds in the LNT release NOx. For example, in FIG. 1, to regenerate the close-coupled LNT 11, the motor 13 is turned off, and a heater 15 (e.g., an electric heater) on the close-coupled LNT 11 and an air flow member 14 (e.g., an air pump) are turned on for a short period of time, e.g., 30 seconds. Since the NOx are stored at the LNTs 11 and 12 as nitrates, they decompose at elevated temperatures and release NOx. Thus, when the close-coupled LNT 11 is heated by the heater 15, the close-coupled LNT 11 releases its stored NOx. The released NOx is then absorbed by the main LNT 12. The absorbed NOx is then reduced to N2during the next vehicle operation. Because the NOx released by the close-coupled LNT 11 is absorbed by the main LNT 12, during this regeneration of the NOx storage capacity for the close-coupled LNT 11 there will be minimal, if any, release of NOx by the main LNT 12. Additionally, the close-coupled LNT 11 will have sufficient storage capacity for the next cold start of the engine.Each LNT catalyst has a fixed NOx storage capacity corresponding to the amount of NOx storage components (e.g., alkali metal, alkaline earth metals, and / or rare earth metals) in the catalyst. When the catalyst is saturated, e.g., when all NOx storage components have been converted to nitrates by NOx, the LNT may no longer store NOx. Before this full storage capacity is reached, the exhaust leaving the LNT should ideally contain no NOx, i.e., zero NOx breakthrough. However, even if the catalyst of the close-coupled LNT has a storage capacity of 50%, the NOx concentration at the outlet, i.e., the NOx breakthrough, as shown in FIG. 2, approaches 60% of the NOx concentration at the inlet at 150° C. This NOx breakthrough indicates that the LNT can only store about 40% of the incoming NOx even if it still has about 50% NOx storage capacity.Thus, in order for a close coupled LNT to have low NOx breakthrough and efficiently reduce NOx emissions during cold starts of an engine, the close coupled LNT should retain most of its NOx storage capacity. As a result, in a preferred embodiment of the present invention, the close-coupled LNT 11 is regenerated before each cold start of an engine. The regeneration process converts the stored metal nitrates to metal oxides, carbonates, or hydroxides and may be performed in both high fuel and low fuel exhaust gases. Regeneration in a fuel-rich exhaust may be performed at lower temperatures. However, at 150° C., the close-coupled LNT 11 recovered only about 50% of its capacity after one minute regeneration in a fuel-rich exhaust (see FIG. 4 ). In a low fuel exhaust, the close coupled LNT recovered its full storage capacity after a 30-second regeneration at 600° C. (see FIG. 4 ). To ensure that the regenerated, close coupled LNT retains its full storage capacity, it should be cooled in a NOx-free exhaust gas.Both the close coupled LNT 11 and the main LNT 12 may be made of a honeycomb substrate, the inner walls being coated with at least one catalyst. The catalysts typically contain noble metals (e.g., Pt, Pd, and / or Rh), and the NOx storage materials include, but are not limited to, oxides, hydroxide, and / or carbonates of alkali metals (e.g., Li, Na, or K), alkaline earth metals (e.g., Ca, Sr, or Ba), or lanthanum group metals (e.g., La, Ce, etc.), compounds containing elements that form stable nitrates (e.g., alkali metals, alkaline earth metals, and / or rare earth metals), or a combination of such elements.In a low fuel mode, NOx in the exhaust gas is oxidized and stored as nitrates at the LNT catalyst. The stored NOx may be released under both fuel rich and fuel lean conditions depending on the catalyst temperature and catalyst formulation. For example, barium is a component commonly used in NOx trap catalysts. Barium nitrate is stable in a low fuel exhaust gas at temperatures up to about 600°C. However, barium nitrate will decompose and release NOx at temperatures as low as 250° C. in a fuel-rich exhaust gas. Therefore, under normal operating conditions for lean-burn engines in a lean-burn mode, an LNT will not release NOx, as the exhaust temperature is typically below 600° C. The released NOx will remain in a fuel-lean exhaust gas or in a fuel-rich exhaust gas NOx before the LNT reaches its operating temperature range, e.g., 250-550° C. Moreover, the released NOx is converted from the LNT in a rich exhaust gas to N2 when the LNT reaches its operating temperature.Having generally described this invention, a further understanding can be obtained by reference to certain specific examples set forth below which are provided for purposes of illustration only and are not intended to be exhaustive or limiting unless otherwise indicated.Table 1 below shows the experimental conditions for the following examples. The total flow rate was 6 liters per minute. Table 1 Table 1O 210 %0CO03,6 %NO250 ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm0 or 60 ppmH 201,2 %HC0200 ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppmH 2 O5%5%CO 25%5%Example 1In FIG. 2, the NOx concentration profile at the output of a main LNT with a low fuel air supply containing 250 ppm NO during a storage capacity measurement at 150° C. is seen. At time 0, a gas flow was switched from a fuel rich feed containing no NO to a fuel lean feed containing about 250 ppm NO. The difference between the NOx concentration at the outlet and the NOx concentration at the inlet (250 ppm) measured with a chemiluminescence NOx analyzer corresponds to the amount of NOx absorbed by the catalyst. A blank cordierite sample was used to correct the flow dynamics. Integrating the difference between the NOx concentrations at the outlet after the empty cordierite and the catalyst over time yields the total amount of NO absorbed by the catalyst, i.e., yields the storage capacity.After about 200 seconds, the outlet NOx concentration approached but did not reach the inlet NOx concentration, e.g., the outlet NOx reached 250 ppm. This result suggests that although most of the storage of NOxwas completed in the first 200 seconds of measurement, there was still an additional, though much slower, storage process of NOxthat continued to occur after the first 200 seconds had elapsed. Although not wishing to be bound by theory, this additional storage process may be due to NOx absorption on the alumina support. However, because this is a much slower process, it plays a relatively minor role in the ability of the LNT to remove NOx from an exhaust flow of an engine during a cold start of the engine.Using an estimate of the amount of NOx absorbed excluding the contribution from the slower NOx absorption, the storage capacity was calculated to be about 0.6 grams of NO / L catalyst. On the other hand, during an FTP (U.S. Federal Test Procedure) drive cycle, cumulative NOx emission out of the engine during a cold start (e.g., less than about 200° C.) for a 4.9 L engine was determined to be about 0.1-0.2 grams (see FIG. 3 ). As shown in Figure 3, substantial NOx breakthrough began to occur after only the first 50 seconds of measurement when about 50% of the storage capacity of the LNT had been consumed. This demonstrates that a reasonably sized LNT (e.g., greater than 1 liter) may have enough storage capacity to remove NOx emissions from an engine exhaust during a cold start. However, it is preferred that the LNT maintain greater than 50% of the original storage capacity to avoid substantial NOx breakthrough.Example 2In FIG. 4, the NOx concentration at the outlet during a storage capacity measurement is 150° C. and a subsequent temperature rise to 600° C. in a fuel-lean feed. The composition of the low fuel exhaust gas supply is given in Table 1 above. After the close-coupled LNT was saturated with NOx at 150° C., the temperature was gradually increased to 600° C. During heating, additional NOx storage occurred with a small absorption peak at about 250°C followed by a much larger absorption peak at about 330°C. Although not wishing to be bound by theory, the small absorption peak at 250° C. may be due to a small storage component in the catalyst, and the larger absorption peak at 330° C. may be due to the formation of barium nitrate. At 350° C., the LNT began releasing the stored NOx. As shown in FIG. 4, the peak release of NOx occurred at about 420° C. At about 600°C, the NOx concentration at the outlet returned to the level of the NOx concentration at the inlet, indicating that the LNT no longer stores or releases NOx at 600°C. The same inlet / outlet concentration of NOx at 600° C. indicates that the NOx storage capacity of the LNT in low fuel exhaust supplies may be thermally regenerated.In order to verify that the LNT restore its storage capacity after high temperature regeneration, the LNT in a low fuel exhaust gas supply containing about 250 ppm of NO was cooled to 150° C. following regeneration at 600° C. (see FIG. 4 ). It was found that although the LNT was completely regenerated, its storage capacity was completely consumed during the cooling process in the temperature range of 350 to 500° C. Therefore, very little, if any, storage capacity remained at 150°C. In order to maintain the NOx storage capacity of the LNT, the LNTs are preferably cooled in a NOx-free gas stream after high temperature thermal regeneration in a low fuel gas stream.The invention of this application has been described above both generically and with respect to specific embodiments. Although the invention has been set forth by what is considered to be the preferred embodiments, a wide variety of alternatives known to those skilled in the art may be chosen within the generic disclosure. The invention is not to be further limited except by the wording of the appended claims.

Claims

A method for selectively removing NOx from the exhaust flow of an internal combustion engine, particularly during a "cold start" of the engine, comprising: passing the exhaust flow sequentially through a first lean NOx trap (LNT) and a second lean NOx trap (LNT), wherein the lean NOx traps comprise a catalyst capable of catalyzing the reduction of a NOx to N 2 and a compound capable of storing NOx as nitrates, that NOx in the first LNT at a temperature below a catalytic temperature at which the catalyst in the first LNT catalyzes the reduction of NOx to N 2, as nitrates, NOx is stored in the second LNT as nitrates under low fuel conditions when the first LNT reaches a temperature at which it releases the stored NOx, and the stored NOx is reduced to N 2 in the first LNT under stoichiometric or fuel-rich engine operating conditions when the temperature of the first LNT reaches the catalytic temperature.The method of claim 1, wherein the second LNT is operated at a lower temperature than the first LNT.The method of claim 1 or 2, further comprising operating the engine with alternating cycles of rich and lean fuel.The method of any preceding claim, wherein the first LNT has been regenerated prior to the cold start to release stored NOx.The method of claim 4, wherein regenerating the first LNT comprises heating the first LNT to a temperature sufficient to release stored NOx from the first LNT.The method of claim 5, wherein the temperature sufficient to release stored NOx from the first LNT is about 600°C.The method of claim 5 or 6, wherein the first LNT is heated for a period of about 30 seconds.The method of any one of claims 5 to 7, wherein the first LNT is cooled in an environment that does not contain NOx.The method of any one of claims 5 to 8, wherein the heating step is carried out in the presence of a fuel-rich exhaust gas.The method of any one of claims 5 to 8, wherein the heating step is carried out in the presence of a low fuel exhaust gas.The method according to any one of claims 5 to 10, wherein the heating step is carried out after the motor has been turned off.The method of any preceding claim, wherein the first LNT is less than the second LNT.The method of any preceding claim, wherein the first lean NOx trap is disposed below the hood of a motor vehicle and the second lean NOx trap is disposed below a floor of the motor vehicle.

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