Title of Invention

AN APPARATUS FOR REDUCING THE CONTENT OF NITROGEN OXIDES (NOX) IN THE EXHAUST GAS OF A LEAN BURN ENGINE AND METHOD THEREOF

Abstract

An apparatus for reducing the content of nitrogen oxides (NOx) in the exhaust gas of a lean burn engine, which apparatus comprises: (a) an exhaust capable of allowing exhaust gases to flow therethrough; (b) a selective catalytic reduction catalyst located in the flow-path of the exhaust gas and being capable of (i) catalysing the reduction of the NOx by ammonia to nitrogen and (ii) adsorbing and desorbing ammonia during the engine cycle; (c) means for supplying ammonia from an ammonia source to the catalyst; and (d) switching means for intermittently supplying ammonia during an engine cycle thereby enabling (i) the catalyst to adsorb ammonia when ammonia supply is switched on and (ii) the adsorbed ammonia to react with NOx when ammonia supply is switched off, characterised in that the catayst comprises a zeolite and the-switching off means is triggered on and at pre-set temperature levels of the catalyst:

Full Text

FORM 2 THE PATENTS ACT 1970 [39 OF 1970] COMPLETE SPECIFICATION [See Section 10] "PROCESS AND APPARATUS FOR REDUCING THE NITROGEN OXIDE CONTENT IN EMXHAUST GASED BY THE CONTROLLED ADDITION OF -NII3" JOHNSON MATTHEY PUBLIC LIMITED COMPANY, a British company of 2-4 Cockspur Street, Trafalgar Square, London SWIY 5BQ, United Kingdom The following specification particularly describes and ascertain the nature of the invention and the manner in which it is to be performed :- WO 99/55446 PCT7CB99/0I285 PROCESS AND APPARATUS FOR REDUCING THE NITROGEN OXIDE CONTENT IN EXHAUST GASES BY THE CONTROLLED ADDITION OF NH3 This invention concerns combatting air pollution from the exhaust gas of a lean bum engine. In particular, it concerns apparatus for. and a method of, reducing the content ot 5 nitrogen oxides (NOx) in such gas. Lean burn engines (which have an air-fuel ratio greater than 14.7, generally in the range 19-50) exhibit higher fuel economy and lower hydrocarbon emissions than do stoichiometrically operated engines and are increasing in number. Emissions from diesel 10 engines are now being regulated by legislation, and whilst it is not too difficult to meet; regulations on hydrocarbon or CO emissions, it is difficult to meet regulations on NOx emissions. Since exhaust gas from lean burn engines such as diesel engines is high in oxygen content throughout the engine cycle, it is more difficult to reduce NOx to nitrogen than in the case of stoichiometrically operated engines. The difficulty is compounded by the 15 lower gas temperature. Various approaches are being considered to reduce NOx under the oxidising conditions. One approach is that of selective catalytic reduction (SCR) with hydrocarbon, but a catalyst of sufficient activity and durability to achieve the required conversion has not been found. Another approach is to adsorb the NOx by an adsorbent when the exhaust gas is lean (ie when there is a stoichiometric excess of oxygen) and release 20 and reduce the adsorbed NOx when the exhaust gas is rich, the exhaust gas being made rich periodically. During the lean operation. NO is oxidised to NO2, which can then react readily with the adsorbent surface to form nitrate. This approach, though, is constrained at low temperature by restricted ability to form N02 and by adsorbent regeneration and at high temperature by sulphur poisoning. Most adsorbents operate in a certain temperature window 25 and are deactivated by sulphate formation. The approach of the present invention is that of SCR of NOx by NH3. This approach has been applied to static diesel engines using a V205-Ti02 type catalyst. The application of NH3 SCR technology to the control of NOx emission from lean 30 bum vehicles, however, requires a suitable NH3 supply strategy, especially at low temperature, for various reasons. The engine-out NOx varies with temperature, so the amount of NH3 supplied must be well controlled as a function of the temperature to maintain I 2 4 * • * the appropriate stoichiometry for the reaction; an insufficient supply of NH3 results in inadequate NOx reduction, whilst an excess may cause NH3 to slip past the catalyst. Whilst at sufficiently high temperature, the catalyst can selectively oxidise that excess NH3 to N2, at low temperature, the unreacted NH3 will be emitted as such. Even if the proper 5 stoichiometry of NH3 is provided, the catalyst may not be sufficiently active at low temperature to react all the NH3 with the NOx. For example, Figure 1 shows the reaction of NH3 with NOx over a non-metallised zeolite as a function of temperature at a stoichiometry of 1:1 at an inlet concentration of 200ppm. It can be seen that at temperatures below 300°C the reduction does not proceed to any significant extent Furthermore, it has 10 been reported that the presence of excess NH3 at low temperature could lead to the formation of NH4N03 and (NH4 )2So4 There is also evidence that the presence of excess gas phase NH3 can inhibit the NH3 SCR reaction over some catalysts at low temperature. Urea is usually the preferred form of storing NH3 on a vehicle. Urea is readily available and is stable in water solution. However, it only hydrolyses readily to NH3 at temperatures greater than 15 150°C, and may not be a suitable source of NH3 at low temperature. Exhaust gas temperatures, though, vary over an engine cycle and for the average light duty diesel car a significant fraction of that cycle is at low temperature. Thus, the control of NOx at low temperature is a problem. 20 25 Methods have been suggested to mitigate this problem. For instance, US-A-5,785,937, JP-A-07136465 and US-A-4,963,332 all suggest the use of ammonia as a reductant to convert NOx to nitrogen over a catalyst EP-A-0773354 also describes the use of ammonia to reduce NOx to nitrogen. However, ammonia is synthesis in situ over a three-way catalyst during the rich burning phase of the engine and the supply of ammonia is triggered as a function of the stoichiometry of the fuel in terms of the fuel to air ratio not as a function of temperature. The present invention provides an improved apparatus and method for reducing the content of NOx. 30 Accordingly, the invention provides an apparatus for reducing the content of nitrogen oxides (NO J in the exhaust gas of a lean burn engine, which apparatus comprises: (a) an exhaust capable of allowing exhaust gases to flow therethrough; (b) a selective catalytic reduction catalyst located in the flow-path of the exhaust gas and being capable of (i) catalysing the reduction of the NO, by ammonia to nitrogen and (ii) adsorbing and desorbing ammonia during the engine cycle; (c) means for supplying ammonia from an ammonia source to the catalyst; and (d) switching means for intermittently supplying ammonia during an engine cycle thereby enabling (i) the catalyst to adsorb ammonia when ammonia supply is switched on and (ii) the adsorbed ammonia to react with NOx when ammonia supply is switched off, characterised in that the catalyst comprises a zeolite and the switching means is triggered on and off at pre-set temperature levels of the catalyst. -4- .AMEHPCD SHEE1 T • ••«• •• •• • ••••• The invention provides also a method of reducing the content of nitrogen oxides (NOx) in the exhaust gas of a lean bum engine, which method comprises passing the exhaust gas over a selective catalytic reduction catalyst which catalyses the reduction of the NOx by 5 ammonia to nitrogen and which adsorbs and desorbs ammonia during the engine cycle, ammonia being supplied intermittently to the catalyst during the engine cycle, the catalyst adsorbing ammonia during its supply and the ammonia which has been adsorbed reacting with the NOx when the ammonia is not supplied. 10 We have discovered that ammonia can be adsorbed on a SCR catalyst and thereafter used in the NOx reduction when ammonia is not being supplied. It is an advantage to be able to achieve the NOx reduction while supplying the ammonia intermittently. In particular, the ammonia supply can be halted and yet NOx reduction occur when the temperature of the catalyst is low and supply would have the problems referred to above. 15 The stored ammonia can be used as reductant for NOx over the same catalyst without the presence of gas phase NH3. The ammonia can be supplied without the exhaust gas so that the catalyst adsorbs the ammonia and then the exhaust gas passed over the catalyst for the NOx reduction to occur 20 Preferably, however, the exhaust gas is passed continuously over the catalyst. The invention uses adsorption and desorption characteristics of the required catalyst A higher amount of NH3 will be adsorbed, and hence be available for subsequent reaction, if adsorption is at a lower temperature than temperatures at which the catalyst adsorbs less 25 NH3. Preferably NH3 is adsorbed at a temperature at which a large amount is adsorbed; the temperature is preferably below that of maximum desorption. The temperature, however, is preferably above that at which any significant formation of ammonium salts occurs. Figure 2 shows the desorption profile from zeolite ZSM5 (non-metallised) of NH3 which had been pre-adsorbed at 100°C. It can be seen that at say 300DC more NH3 is retained, WO 99/55446 PCT/GB99/01205 10 15 20 adsorbed, than at say 400°C, and that the temperature of maximum desorption is about 370°C. Bearing in mind that the desorption of NH3 is endothermic, it can also be seen that if NH3 were adsorbed at say 300°C and then heated, NH3 would be desorbed in accordance with the graph so that less would be available for subsequent reaction, while if NH3 were adsorbed at the same temperature, 300°C, and cooled, NH3 would not be desorbed so the adsorbed NH3 would be available for subsequent reaction. NH3 stored on the ZSM5 catalyst at 250°C can effectively be used to reduce NOx at a temperature as low as 150°C under exhaust conditions simulating those of a light duty diesel car. Figure 3 shows the NH3 uptake of ZSM5 catalyst (non-metallised) from a gas mixture containing 4.5% C TABLE 1 NH3 Adsorption as a Function of NH3 Concentration 25 30 6 10 WO 99/55446 PCT/GB99/0I205 5 The means to make the supply of ammonia intermittent during the engine cycle in the present apparatus can be a switch which switches the ammonia supply on and off dependent on the level of NOx conversion occurring over the SCR catalyst. Preferably, however, the means to make the supply of ammonia intermittent comprises a switch to switch on the means to supply the ammonia when the temperature of the catalyst rises above a set level (i) during the engine cycle, and to switch off the means to supply the ammonia when the temperature of the catalyst falls below a set level (ii). The set level (i) is preferably in the range 25(M00°C, especially in the range 250-350°C. The set level (ii) is preferably in the range 200-250°C. The ammonia can be supplied for instance 1 -30 times per minute. The source of ammonia and means to supply it from the source to the catalyst can be conventional. Compounds of ammonia as a solid or a solution in water are preferred. The 15 compounds are preferably urea or ammonium carbamate. The means to supply the ammonia from the source to the catalyst can be a pipe through which it is injected into the exhaust gas up-stream of the catalyst. Thus, the present invention can be employed to provide a method of promoting the conversion of NOx under oxidising conditions in an exhaust fitted with a means of injecting NH3 and a catalyst which adsorbs NH3 during parts of the engine cycle 20 in which the exhaust gas is sufficiently wanned for the hydrolysis of NH3 precursor and injection of ammonia and ammonia is adsorbed by the catalyst for use as reductant for NOx during parts of the engine cycle in which the exhaust gas is cooler, without the need for the continuous injection of NH3 into the exhaust gas. 25 It can be seen that the invention provides an exhaust system for an engine operating generally under lean conditions, which exhibits a higher exhaust gas temperature and a lower exhaust gas temperature, the lower exhaust gas temperature being inadequate for the effective hydrolysis of NH3 precursor and injection of NH3 (generally a temperature below 200°C), and anNH3 SCR catalyst arranged and constructed so that during the higher exhaust 30 gas temperature parts of the engine cycle the catalyst adsorbs NH3 and during the lower 7 WO 99/55446 PCT/GB99/0I205 6 exhaust gas temperature parts of the engine cycle the adsorbed NH3 is used as reductant for NOx. The catalyst can be any which has the required characteristics of the present catalyst. The same material can both selectively catalyse the reduction and also adsorb and desorb the ammonia, and this is preferred. However, different materials in the catalyst can perform the two functions, one material catalysing and one material adsorbing and desorbing. When different materials are employed, they can be physically separate or, preferably, in admixture one with another. A zeolite can perform both functions or a zeolite can be employed which performs one function together with a different material, which may or may not be a zeolite, which performs the other function. The catalyst preferably comprises a zeolite. The zeolite can be metallised or non-metallised, and can have various silica-to-alumina ratios. Examples are metallised or non-metallised ZSM5, mordenite, y zeolite and P zeolite. Preferred is ZSM5 or ion-exchanged or metal impregnated ZSM5 such as Cu/ZSM5. It may be desirable that the zeolite contains metal, especially Cu, Ce, Fe or Pt; this can improve the low temperature SCR activity. The zeolite can contain for instance 1-10% of metal by weight. The catalyst should have an appropriate structure, for instance in terms of pore size or surface acid sites, to trap and release NH3. The catalyst is preferably carried out on a support substrate, in particular a honeycomb monolith of the flow-through type. The monolith can be metal or ceramic. The substrate can be conventional. Nitrogen oxide (NO) is usually the most abundant nitrogen oxide in an engine exhaust 25 stream, but at lower temperatures the reaction of the adsorbed NH3 on a zeolite catalyst occurs more readily with N02 than with NO. Accordingly it is often desirably to oxidise NO to N02 up-stream of the SCR catalyst, particularly at low temperature. 10 15 20 The present engine has diesel or petrol (gasoline)engine. The diesel engine can 30 be a light dity or heavy duty diesel engine. The engine is preferably that of a vehicle. 8 WO 99/55446 PCT/GB99/01205 7 The invention is illustrated by the accompanying drawings, which are graphs in which: Figure 1 shows NOx and NH3 concentrations in simulated exhaust gas against temperature after treatment by zeolite ZSM5, the NH3 being supplied continuously; 5 Figure 2 shows the temperature programmed desorption (TPD) of NH3 from ZSM5 which had been pre-adsorbed at 100°C, the graph showing, in arbitrary units, the concentration of ammonia in the gas against temperature; Figure 3 shows the NH3 concentration in a full simulated exhaust gas mixture containing 4.5% C02, 12% 02?4.5% H2Q. 200ppm C07 lOOppm C3IIh. ZOpprr. SO_ 10 and 200ppm NH3 with the balance N2 after passage over ZSM5 at 250°C against time, and hence shows the NH3 uptake by the zeolite; Figure 4 shows the NOx concentration remaining in simulated exhaust gas after passage over the zeolite containing adsorbed NH3 resulting from the adsorption shown in Figure 3 against time; 15 Figure 5 shows the NOx concentration remaining in simulated exhaust gas containing 200ppm NO, 200ppm CO, 12% 02 and 14% C02 with the balance N2 after passage over ZSM5 with and without pre-adsorption of NH3 against temperature; Figure 6 shows the corresponding effect to that shown in Figure 5 of successive cycles of the NH3 pre-adsorption followed by subjection to the simulated exhaust gas: 20 Figure 7 corresponds to Figure 5 but with the simulated exhaust gas containing also hydrocarbon; Figure 8 corresponds to Figure 7 but with the simulated exhaust gas containing also H20 and S02; Figure 9 corresponds to Figure 5 but with the simulated exhaust gas containing N02 25 instead of NO; Figure 10 corresponds to Figure 9 but with the simulated exhaust gas containing also hydrocarbon; Figure 11 corresponds to Figure 10 but with the simulated exhaust gas containing also H20 and S02; 30 Figure 12 shows NOx concentration and temperature against time during part of an engine cycle; WO 99/55446 PCT/GB99/01295 8 Figure 13 corresponds to Figure 12 but shows the effect of intermittent supply of NH3; Figure 14 shows the NOx concentration remaining in simulated exhaust gas after passage over Cu/ZSM5 with and without pre-adsorption of NH3 against temperature; 5 and Figure 15 shows the NOx concentration remaining in simulated exhaust gas which is that used in relation to Figure 14 but containing also hydrocarbon, H20 and S02, after passage over Cu/ZSM5 with pre-adsorption of NH3 against temperature. Figures 1-4 arc discussed further hereinbefore, and Figures 5-15 hereinafter 10 The invention is illustrated also by the following Examples. EXAMPLE 1 Reaction of NO With Pre-adsorhed NH3 Over Nnn-metallised ZSM5 15 This Example shows the effect of pre-adsorbing NH3 at 250°C on the conversion of NOx over a non-metallised zeolite in a simple gas mixture containing NOx, CO, C02 and 02 during a light-off test from room temperature to 400°C. The gas stream containing NO (200ppm), CO (200ppm), 0, (12%). C02 f 14%) with the balance N2 at a flow rate of 2 litres 20 per minute was first passed over the non-metallised zeolite (0.4g) from room temperature to 400°C at a heating rate of 50°C per minute and the NOx at the outlet measured. In a subsequent experiment, the catalyst temperature was first raised to 250°C and 200ppm NH3 was added to the gas stream, the zeolite was exposed to that stream for 5 minutes and then the NH3 switched off, and the catalyst was cooled to room temperature and the rapid light- 25 off repeated. Figure 5 shows the outlet NOx concentration for these experiments. It can be seen mat in the case where NH3 was not pre-adsorbed over the catalyst, some of the NOx is adsorbed on the zeolite at low temperature and is then subsequently released between 150°C and 350°C, but that when NH3 was pre-adsorbed on the zeolite, the zeolite did not adsorb a significant amount of NOx at low temperature. Furthermore, it can be seen that a 30 decrease in the outlet NOx concentration occurs from 150°C to 450°C due to the reaction of the NOx with the pre-adsorbed NH3. This effect of reacting the adsorbed NH3 with the /O WO 99/55446 PCT/GB99/012G5 9 NOx can be repeated over successive cycles with NH3 injection at 250°C between each cycle, as is shown in Figure 6. We have also shown that even in the presence of other gaseous components such as 5 hydrocarbon, H20 and S02, the adsorption of NH3 will readily occur on the zeolite and can be used to reduce NOx. For example, Figure 7 shows the effect of adding 200ppm C3H6 to the gas mixture in similar tests to those described above and Figure 8 shows the effect of further addition of H20 (10%) and S02 (20ppm). It can be seen that in both cases NOx was reduced by the adsorbed NH3. 10 EXAMPLE 2 Reaction of NO2 with Pre-adsorbed NH3 Over Non-metallised ZSMS The selective catalytic reduction of NOx by NH3 under oxidising conditions proceeds 15 more rapidly at low temperature if N02 instead of NO is present. The present Example shows that NH3 pre-adsorbed on a zeolite catalyst can be used to reduce N02 even at a temperature as low as 100°C. This was demonstrated by rapid light-off tests analogous to that described above in Example 1. In the first experiment, a simple gas mixture containing N02 (200ppm), CO (200ppm), 02 (12%), C02 (14%) with the balance N2 at a flow rate of 20 2 litres per minute was passed over the non-metallised zeolite (0.4g) from room temperature to 400°C at a heating rate of 50°C per minute. In a subsequent experiment, the catalyst temperature was first raised to 250°C and 200ppm NH3 was added to the gas stream, the zeolite was exposed to that stream for 5 minutes and then the NH3 was switched off, and the catalyst was cooled to room temperature and the rapid light-off repeated. Figure 9 shows 25 the outlet NOx concentration from these experiments. It can be seen that in the absence of pre-adsorbed NH3, N02 is adsorbed at low temperature over the zeolite and is released between 100°C and 300°C, but when NH3 was pre-adsorbed on the catalyst, significant NOx reduction is shown over the entire temperature window up to 400°C. 30 We have also shown that even in the presence of hydrocarbon, H20 and SO, adsorbed NH3 will readily react with N02. Figure 10 shows the effect of adding C3H6 on the reaction WO 99/55446 PCT/GB99/0I295 10 of pre-adsorbed NH3 with NOx, and Figure 11 demonstrates the effect with addition of H20 and S02. EXAMPLE 3 5 Reaction of NO; With Pre-adsorbed NH3 Over Non-metallised ZSM5 in Cvcle Test In most cases, exhaust gas temperature varies during an engine cycle and for a significant fraction of that time the temperature can be low. We have shown that by injecting NH3 over a set temperature during the cvcle. the adsorbed NH* can subseauentlv 10 be utilised in reducing NOx at both low and high temperature. In the experiment, exhaust gas containing C02 (14%), 02 (12%), H,0 (10%), CO (200ppm). C3H6 (200ppm), SO, (20ppm) and NO, (200ppm) was cycled between 150°C and 350°C with a dwell of approximately 5 minutes at 250°C during the cooling-down part of the cycle. The NH3 injection was switched on when the temperature was at 350°C and switched off when the 15 temperature fell to 250°C. Figure 12 shows the outlet NOx concentration and the temperature against time without any NH3 injection, and Figure 13 shows the effect of the cycling with the intermittent injection of NH3. In both Figures, the ordinate scale gives the degrees C for the temperature graph and the parts per million (ppm) for the NOx graph. 20 ' EXAMPLE 4 Reaction of NO With Pre-adsorhed NH3 Over Cu/ZSM5 This Example shows the effect of pre-adsorbing NH3 at 250°C on the conversion of NOx over a Cu-impregnated ZSM5 (containing 5% copper by weight) in a simple gas 25 mixture containing NOx, CO, C02 and 02 during a light-off test from room temperature to 400°C. The gas stream containing NO (200ppm), CO (200ppm), 02 (12%), C02 (14%) with the balance N2 at a flow rate of 2 litres per minute was first passed over the Cu/ZSM5 (0.4g) from room temperature to 400°C at a heating rate of 50°C per minute and the NOx at the outlet measured. In a subsequent experiment, the catalyst temperature was first raised 30 to 250°C and 200ppm NH3 was added to the gas stream, the Cu/ZSM5 was exposed to that stream for 5 minutes and then the NH3 was switched off, the catalyst was cooled to room - 12- WO»/55446 PCT/GB99/01205 11 temperature rapidly and the light-off repeated. Figure 14 shows the outlet NOx concentration for these experiments. It can be seen that in the case where NH3 was not pre-adsorbed on the catalyst, some of the NOx is adsorbed on the zeolite at low temperature, and is then subsequently released at higher temperature, but the pre-adsorption of NH3 at 250°C 5 suppresses the amount of NOx adsorbed at low temperature, with significant NOx reduction by the pre-adsorbed NH3 at temperatures greater than 125°C. Similarly, even in the presence of other gaseous components such as hydrocarbon, H-,0 and S02 the adsorption of NH3 u Ml occur readilv over the Cu/ZSM5 and can he used 10 to reduce NOx. For example, Figure 15 shows the effect of pre-adsorbing NH3 on the Cu/ZSM5 at 250°C from a gas mixture containing NO, H20, C02, CO, C3H6, S02 and 02 and the reduction of NOx by the adsorbed NH3 during a light-off test. -13- WE CLAIM: 1. An apparatus for reducing the content of nitrogen oxides (NOx) in the exhaust gas of a lean burn engine, which apparatus comprises: (a) an exhaust capable of allowing exhaust gases to flow therethrough; (b) a selective catalytic reduction catalyst located in the flow-path of the exhaust gas and being capable of (i) catalysing the reduction of the NOx by ammonia to nitrogen and (ii) adsorbing and desorbing ammonia during the engine cycle; (c) means for supplying ammonia from an ammonia source to the catalyst; and (d) switching means for intermittently supplying ammonia during an engine cycle thereby enabling (i) the catalyst to adsorb ammonia when ammonia supply is switched on and (ii) the adsorbed ammonia to react with NOx when ammonia supply is switched off, characterised in that the catayst comprises a zeolite and the-switching off means is triggered on and at pre-set temperature levels of the catalyst: 2. An apparatus as claimed in any one of the preceding claims wherein the catalyst is capable of selectively catalysing the reduction of NOx and also adsorbing and desorbing ammonia. 3. Apparatus as claimed in any one of the preceding claims wherein the zeolite catalyst is ZSM-5. 4. An apparatus as claimed in any one of the preceding Claims wherein the zeolite catalyst is non-metallised. 5. Apparatus as claimed in any one of the claims 1-4 wherein the zeolite contains a metal selected from copper, iron, cerium and platinum. 6. A method of reducing the content of nitrogen oxides (NOx) in the exhaust gas of a lean burn engine, which method comprises: passing the exhaust gases over a selective catalytic reduction „ comprising zeolite catalyst located in the flow-path of the exhaust gas which catalyses the reduction of the NOx by ammonia and which catalyst adsorbs and desorbs ammonia during an engine cycle intermittently triggered by the catalyst temperature during the engine cycle whereby the supply of ammonia is switched on when the catalyst temperature is above a pre-set level i.e. in the range from 250-400 °C and switched off when it is below said set level i.e. in the range from 200-250 °C such that (i) the catalyst adsorbs ammonia the supply is switched on and (ii) the adsorbed ammonia reacts with NOx when ammonia supply is switched off. Dated this 25th day of October 2000. (SANJA^P^^R) OF REMFR^SAGAR ATTORNEY FOR THE'APPLICANTS

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