GB2333471A

GB2333471A - Improvements in catalyst systems

Info

Publication number: GB2333471A
Application number: GB9801450A
Authority: GB
Inventors: Anders Andreasson; Eive Tord Ronny Allansson; Nils Christian Myers; Ari Juhani Uusimaki; James Patrick Warren
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1998-01-24
Filing date: 1998-01-24
Publication date: 1999-07-28
Also published as: GB9801450D0

Abstract

A system for reducing NOx in diesel engine exhausts comprises a continuously regenerating trap ("CRT") with a lean-NOx catalyst mounted upstream of the CRT. An injector for diesel fuel is mounted upstream of the lean-NOx catalyst and doses diesel fuel into the exhaust gas according to engine-out NOx. All regulated emissions are reduced, and NOx can be reduced by 40-50%.

Description

IMPROVEMENTS IN CATALYST SYSTEMS This invention concerns improvements in Catalyst Systems, more especially it concerns improvements in systems for removing pollutants from diesel engine exhausts.

One of the most innovative advances in treating diesel engine exhausts is Johnson Matthey's Continuously Regenerating Trap ("CRT"), described in principle in US Patent 4,902,487. This equipment removes particulates from the exhaust by passing the exhaust gas over an oxidation catalyst to convert NO to NO2, trapping the soot particulates downstream of the catalyst and reacting or combusting the soot with the NO2. Although this is very effective to remove soot particulates, it is not at all effective to convert NOx to N2.

Our tests indicate that at low temperatures, below about 250"C, there is a few percent reduction of NOx, but at higher temperatures there is no significant conversion.

As emission regulations tighten, it has become necessary to increase the removal of NOx from lean-burn engine exhausts such as from diesel engines. We tried to improve the NOx results from a CRT unit by fitting a conventional lean-NOx catalyst downstream of the CRT. A slight increase in NOx removal was observed, but this still remained below 5%. A further attempt to improve NOx conversion was made by fitting the lean-NOx catalyst upstream of the CRT, but although there was a slight improvement in NOx conversion, it remained unacceptably low and peaked at about 6%. It is clear that there remains a need for improving NOx performance whilst still retaining the excellent particulate-removing performance of the CRT.

The present invention provides an emission control system for diesel engine exhausts, comprising a CRT and mounted upstream of said trap a lean-NOx catalyst and mounted upstream of said catalyst, a diesel fuel injection source.

The invention further provides a method of reducing emissions in the exhaust gases from diesel engines, comprising in a first step dosing the exhaust gas with diesel fuel, passing the enriched exhaust gas over a lean-NOx catalyst to convert a portion of the NOx to N2 and to oxidise hydrocarbons and carbon monoxide, and thereafter passing the treated exhaust gas through a CRT to remove particulates therefrom.

The CRT used in the present invention is desirably as described in USP 4,902,487.

In the present invention, however, it has been found advantageous to use as the filter or trap, a refractory ceramic honeycomb monolith, preferably of higher cell density than conventional in this use, eg 100 to 200 cells/sq inch (= 15 to 31 cells/sq cm) and of correspondingly shorter length, whereby the lean-NOx catalyst can be fitted without overall increase in volume in the CRT housing. Such a filter or trap may be of SiC or cordierite, although other materials may be used bearing in mind the need to avoid excessive backpressure and blockage of cells. The oxidation catalyst used in the CRT is desirably a highlyloaded platinum catalyst. The catalyst may contain other components.

The actual lean-NOx catalyst used is not critical to the invention, and the skilled person has knowledge of suitable compositions and suitable constructions. However, it is presently preferred to use a copper-zeolite (ZSM-5) catalyst available from Johnson Matthey under the designation DF-37.

The skilled person can readily design a diesel fuel injector using known technology and the on-board diesel fuel pumps and lines. Control of the volume or weight of the diesel fuel actually injected is suitably dependent upon engine speed and load. Suitable diesel fuel injection equipment is described in SAE paper 952489, "Development of Test Methods for Lean-NOx Catalyst Evaluation". Preferably, the amount of diesel fuel injected into the exhaust gas is dependent upon engine-out NOx; it is not possible to measure NOx directly so control of quantities of diesel fuel may be effected by sensing quantities of NO and/or NO2 or by a pre-calibration.

The diesel fuel used is low-sulphur, that is, less than 50ppm S, preferably less than I Oppm S.

It is believed that the invention has particular application to heavy duty diesel engines, especially vehicle engines, but the invention should not be regarded as limited thereto, and may be applied to stationary engines and light duty engines.

The present invention is illustrated by the results of experimental tests, which are plotted on the accompanying figures.

TEST 1 (Comparative) A Volvo D1OA 6-cylinder, 10 litre, turbocharged engine mounted on a test bed was used for all tests described herein. The engine is a direct injection, water-cooled engine which meets European Stage II emissions regulations. The exhaust was fitted with a CRT unit consisting of a housing containing a 10.5 inch (26.67cm) diameter, 6 inch (15.24cm) length ceramic honeycomb monolith of 400 cells/sq in (62 cells/sq cm), carrying 75g/ft3 Pt (2.65g/litre Pt) and an identical volume filter manufactured in 200 cells inch (31 cells/sq cm) SiC. The CRT unit was conditioned by operating the engine for 30 minutes so that the temperature immediately before the CRT unit was 530"C, and at constant engine speed and load.

The results of conversion of various gases in the exhaust gas were determined by analysis before and after treatment with the CRT unit, and the results for various operating temperatures are plotted on Figure 1. It can readily be seen that there is insignificant conversion of NOx at all temperatures. Conversion of NO is to NO2, and thus total NOx remains largely unchanged. The Pt catalyst in the CRT is effective to reduce some NOx at about 200"C.

TEST 2 (Comparative) The same engine and CRT unit as in Test 1 was used, but with the insertion of a 10.5 inch diameter (26.67cm), 6 inch length (15.24cm), commercial Cu-ZSM-5 lean-NOx catalyst supplied by Johnson Matthey as DF37 in the CRT housing downstream of the filter.

The identical tests as in Test 1 were carried out and the results are plotted in Figure 2. A very slight improvement in NOx conversion was observed, but this still remains below about 5%.

TEST 3 (Comparative) The same engine and the modified CRT unit of Test 2 were used, but with a further modification involving the moving of the lean-NOx catalyst upstream of the CRT. The identical tests were performed as in Test 1, and the results are plotted in Figure 3. The NOx conversion results remain inadequate.

TEST 4 The modified unit described in Test 3 was further modified by mounting a diesel fuel injector upstream of the lean-NOx catalyst and injecting at a fuel/NOx weight ratio of 2.5. Repeating the tests gave results that are plotted in Figure 4. It can be seen that NOx conversions are close to 50% in the region of 400 C.

Particulate emissions were below < 0.01 (g/kWh) according to the ECE R49 test procedure, and were identical to the results from the engine using a CRT alone, as in Test 1.

Claims

CLAIMS 1. An emission control system for diesel engine exhausts, comprising a continuously regenerating trap and mounted upstream of said trap a lean-NOx catalyst, and mounted upstream of said catalyst, a diesel fuel injection source.
2. A system according to claim 1, wherein the lean-NOx catalyst is a copper on zeolite catalyst.
3. A system according to claim 1 or 2, mounted in a single housing.
4. A system according to claim 3, wherein the trap comprises a refractory ceramic honeycomb.
5. A system according to claim 4, wherein the honeycomb has about 100 to 200 cells/sq in (15-31 cells/sq cm).
6. A system according to any one of the preceding claims, comprising a controller for the injection of diesel fuel which controller acts to dose diesel fuel in an amount of about 2.5 times the mass of NOx.
7. A method of reducing emissions, especially NOx, in the exhaust gases from diesel engines, comprising in a first step dosing the exhaust gas with diesel fuel, to form an enriched exhaust gas, passing the enriched exhaust gas over a lean-NOx catalyst to convert a portion ofthe NOx to N2 and to oxidise hydrocarbons and carbon monoxide, and thereafter passing the treated exhaust gas through a continuously regenerating trap to remove particulates therefrom.
8. A method according to claim 7, comprising dosing the diesel fuel according to engine-out NOx emissions.