Catalytic reduction of NOx

Abstract

A system for NOx reduction in combustion gases, especially from diesel engines, incorporates an oxidation catalyst to convert at least a portion of NO to NO2, a particulate filter, a source of reductant such as NH3 and an SCR catalyst. Considerable improvements in NOx conversion are observed.

Description

The present invention concerns improvements in selective catalytic reduction of NO_x in waste gas streams such as diesel engine exhausts or other lean exhaust gases such as from gasoline direct injection (GDI).

The technique named SCR (Selective Catalytic Reduction) is well established for industrial plant combustion gases, and may be broadly described as passing a hot exhaust gas over a catalyst in the presence of a nitrogenous reductant, especially ammonia or urea. This is effective to reduce the NO_x content of the exhaust gases by about 20-25% at about 250° C., or possibly rather higher using a platinum catalyst, although platinum catalysts tend to oxidise NH₃ to NO_x during higher temperature operation. We believe that SCR systems have been proposed for NO_x reduction for vehicle engine exhausts, especially large or heavy duty diesel engines, but this does require on-board storage of such reductants, and is not believed to have met with commercial acceptability at this time.

We believe that if there could be a significant improvement in performance of SCR systems, they would find wider usage and may be introduced into vehicular applications. It is an aim of the present invention to improve significantly the conversion of NO_x in a SCR system, and to improve the control of other pollutants using a SCR system.

Accordingly, the present invention provides an improved SCR catalyst system, comprising in combination and in order, an oxidation catalyst effective to convert NO to NO₂, a particulate filter, a source of reductant fluid and downstream of said source, an SCR catalyst.

The invention further provides an improved method of reducing NO_x in gas streams containing NO and particulates comprising passing such gas stream over an oxidation catalyst under conditions effective to convert at least a portion of NO in the gas stream to NO₂, removing at least a portion of said particulates, adding reductant fluid to the gas stream containing enhanced NO₂ to form a gas mixture, and passing the gas mixture over an SCR catalyst.

Although the present invention provides, at least in its preferred embodiments, the opportunity to reduce very significantly the NO_x emissions from the lean (high in oxygen) exhaust gases from diesel and similar engines, it is to be noted that the invention also permits very good reductions in the levels of other regulated pollutants, especially hydrocarbons and particulates.

The invention is believed to have particular application to the exhausts from heavy duty diesel engines, especially vehicle engines, e.g. truck or bus engines, but is not to be regarded as being limited thereto. Other applications might be LDD (light duty diesel), GDI, CNG (compressed natural gas) engines, ships or stationary sources. For simplicity, however, the majority of this description concerns such vehicle engines.

We have surprisingly found that a “pre-oxidising” step, which is not generally considered necessary because of the low content of CO and unburnt fuel in diesel exhausts, is particularly effective in increasing the conversion of NO_x to N₂ by the SCR system. We also believe that minimising the levels of hydrocarbons in the gases may assist in the conversion of NO to NO₂. This may be achieved catalytically and/or by engine design or management. Desirably, the NO₂/NO ratio is adjusted according to the present invention to the most beneficial such ratio for the particular SCR catalyst and CO and hydrocarbons are oxidized prior to the SCR catalyst. Thus, our preliminary results indicate that for a transition metal/zeolite SCR catalyst it is desirable to convert all NO to NO₂, whereas for a rare earth-based SCR catalyst, a high ratio is desirable providing there is some NO, and for other transition metal-based catalysts gas mixtures are notably better than either substantially only NO or NO₂. Even more surprisingly, the incorporation of a particulate filter permits still higher conversions of NO_x.

The oxidation catalyst may be any suitable catalyst, and is generally available to those skilled in art. For example, a Pt catalyst deposited upon a ceramic or metal through-flow honeycomb support is particularly suitable. Suitable catalysts are e.g. Pt/A12O3 catalysts, containing 1-150 g Pt/ft³ (0.035-5.3 g Pt/liter) catalyst volume depending on the NO₂/NO ratio required. Such catalysts may contain other components providing there is a beneficial effect or at least no significant adverse effect.

The source of reductant fluid conveniently uses existing technology to inject fluid into the gas stream. For example, in the tests for the present invention, a mass controller was used to control supply of compressed NH₃, which was injected through an annular injector ring mounted in the exhaust pipe. The injector ring had a plurality of injection ports arranged around its periphery. A conventional diesel fuel injection system including pump and injector nozzle has been used to inject urea by the present applicants. A stream of compressed air was also injected around the nozzle; this provided good mixing and cooling.

The reductant fluid is suitably NH₃, but other reductant fluids including urea, ammonium carbamate and hydrocarbons including diesel fuel may also be considered. Diesel fuel is, of course, carried on board a diesel-powered vehicle, but diesel fuel itself is a less selective reductant than NH₃ and is presently not preferred.

Suitable SCR catalysts are available in the art and include Cu-based and vanadia-based catalysts. A preferred catalyst at present is a V₂O₅/WO₃/TiO₂ catalyst, supported on a honeycomb through-flow support. Although such a catalyst has shown good performance in the tests described hereafter and is commercially available, we have found that sustained high temperature operation can cause catalyst deactivation. Heavy duty diesel engines, which are almost exclusively turbocharged, can produce exhaust gases at greater than 500° C. under conditions of high load and/or high speed, and such temperatures are sufficient to cause catalyst deactivation. In one embodiment of the invention, therefore, cooling means is provided upstream of the SCR catalyst. Cooling means may suitably be activated by sensing high catalyst temperatures or by other, less direct, means, such as determining conditions likely to lead to high catalyst temperatures. Suitable cooling means include water injection upstream of the SCR catalyst, or air injection, for example utilising the engine turbocharger to provide a stream of fresh intake air by-passing the engine. We have observed a loss of activity of the catalyst, however, using water injection, and air injection by modifying the turbocharger leads to higher space velocity over the catalyst which tends to reduce NO_x conversion. Preferably, the preferred SCR catalyst is maintained at a temperature from 160° C. to 450° C.

We believe that in its presently preferred embodiments, the present invention may depend upon an incomplete conversion of NO to NO₂. Desirably, therefore, the oxidation catalyst, or the oxidation catalyst together with the particulate trap if used, yields a gas stream entering the SCR catalyst having a ratio of NO to NO₂ of from about 4:1 to about 1:3 by vol, for the commercial vanadia-type catalyst. As mentioned above, other SCR catalysts perform better with different NO/NO₂ ratios. We do not believe that it has previously been suggested to adjust the NO/NO₂ ratio in order to improve NO_x reduction.

The present invention incorporates a particulate trap downstream of the oxidation catalyst. We discovered that soot-type particulates may be removed from a particulate trap by “combustion” at relatively low temperatures in the presence of NO₂. In effect, the incorporation of such a particulate trap serves to clean the exhaust gas of particulates without causing accumulation, with resultant blockage or back-pressure problems, whilst simultaneously reducing a proportion of the NO_x. Suitable particulate traps are generally available, and are desirably of the type known as wall-flow filters, generally manufactured from a ceramic, but other designs of particulate trap, including woven knitted or non-woven heat-resistant fabrics, may be used.

It may be desirable to incorporate a clean-up catalyst downstream of the SCR catalyst, to remove any NH₃ or derivatives thereof which could pass through unreacted or as by-products. Suitable clean-up catalysts are available to the skilled person.

A particularly interesting possibility arising from the present invention has especial application to light duty diesel engines (car and utility vehicles) and permits a significant reduction in volume and weight of the exhaust gas after-treatment system, in a suitable engineered system.

Several tests have been carried out in making the present invention. These are described below, and are supported by results shown in graphical form in the attached drawings.

A commercial 10 liter turbocharged heavy duty diesel engine on a test-bed was used for all the tests described herein.

Test 1—(Comparative)

A conventional SCR system using a commercial V₂O₅/WO₃/TiO₂ catalyst, was adapted and fitted to the exhaust system of the engine. NH₃ was injected upstream of the SCR catalyst at varying ratios. The NH₃ was supplied from a cylinder of compressed gas and a conventional mass flow controller used to control the flow of NH₃ gas to an experimental injection ring. The injection ring was a 10 cm diameter annular ring provided with 20 small injection ports arranged to inject gas in the direction of the exhaust gas flow. NO_x conversions were determined by fitting a NO_x analyser before and after the SCR catalyst and are plotted against exhaust gas temperature in FIG. 1. Temperatures were altered by maintaining the engine speed constant and altering the torque applied.

A number of tests were run at different quantities of NH₃ injection, from 60% to 100% of theoretical, calculated at 1:1 NH₃/NO and 4:3 NH₃/NO₂. It can readily be seen that at low temperatures, corresponding to light load, conversions are about 25%, and the highest conversions require stoichiometric (100%) addition of NH₃ at catalyst temperatures of from 325 to 400° C., and reach about 90%. However, we have determined that at greater than about 70% of stoichiometric NH₃ injection, NH₃ slips through the SCR catalyst unreacted, and can cause further pollution problems.

Test 2 (Comparative)

The test rig was modified by inserting into the exhaust pipe upstream of the NH₃ injection, a commercial platinum oxidation catalyst of 10.5 inch diameter and 6 inch length (26.67 cm diameter and 15.24 cm length) containing log Pt/ft³ (=0.35 g/liter) of catalyst volume. Identical tests were run, and it was observed from the results plotted in FIG. 2, that even at 225° C., the conversion of NO_x has increased from 25% to >60%. The greatest conversions were in excess of 95%. No slippage of NH₃ was observed in this test nor in the following test.

Test 3

The test rig was modified further, by inserting a particulate trap before the NH₃ injection point, and the tests run again under the same conditions at 100% NH₃ injection and a space velocity in the range 40,000 to 70,000 hr⁻¹ over the SCR catalyst. The results are plotted and shown in FIG. 3. Surprisingly, there is a dramatic improvement in NO_x conversion, to above 90% at 225° C., and reaching 100% at 350° C. Additionally, of course, the particulates which are the most visible pollutant from diesel engines, are also controlled.

Test 4

An R49 test with 80% NH₃ injection was carried out over a V₂O₅/WO₃/TiO₂ SCR catalyst. This gave 67% particulate, 89% HC and 87% NO_x conversion; the results are plotted in FIG. 4.

Additionally tests have been carried out with a different diesel engine, and the excellent results illustrated in Test 3 and 4 above have been confirmed.

The results have been confirmed also for a non-vanadium SCR catalyst.

Claims

1. A method of reducing pollutants, including particulates and NOx, in a gas stream, comprising passing said gas stream over an oxidation catalyst under conditions effective to convert a portion of NO in the gas stream to NO2 to produce a gas mixture comprising a mixture of NO and NO2, wherein the gas mixture has an enhanced NO2 content relative to the gas stream entering the oxidation catalyst, removing at least a portion of said particulates in a particulate trap located downstream of the oxidation catalyst, reacting trapped particulate with NO2, adding reductant fluid to the gas stream to form a gas mixture downstream of said trap, and passing the gas mixture over an SCR catalyst under NOx reduction conditions.

2. A method according to claim 1, wherein said gas stream is the exhaust from a diesel, GDI or DNG engine.

3. A method according to claim 1, wherein the gas stream is cooled before reaching the SCR catalyst.

4. A method according to claim 1, wherein the NO to NO2 ratio of the gas mixture is adjusted to a level pre-determined to be optimum for the SCR catalyst, by oxidation of NO over said oxidation catalyst.

5. A method according to claim 1, wherein the SCR catalyst is maintained at a temperature from 160° C. to 450° C.

6. A method according to claim 1, wherein the SCR catalyst includes a component selected from the group consisting of a transition metal and a rare-earth metal.

7. A method according to claim 1, wherein the SCR catalyst includes a component selected from the group consisting of copper and vanadium.

8. A method according to claim 1, wherein the SCR catalyst is V2O5/WO3/TiO2.

9. A method according to claim 4, wherein the ratio of NO:NO2 leaving the oxidation catalyst is adjusted to about 4:3.

10. A method according to claim 1, wherein the reductant fluid is a hydrocarbon.

11. A method according to claim 1, wherein the reductant fluid is selected from the group consisting of ammonia, ammonium carbamate and urea.

12. A method according to claim 10, further comprising the step of contacting the gas mixture leaving the SCR catalyst with a clean-up catalyst to remove NH3 or derivatives thereof.

13. A method according to claim 1, wherein the space velocity of the exhaust gas over the SCR catalyst is in the range 40,000 to 70,000 h−1.