GB2332379A

GB2332379A - Gas purification

Info

Publication number: GB2332379A
Application number: GB9726714A
Authority: GB
Inventors: David Leslie Segal; Anthony Robert Martin; James Timothy Shawcross
Original assignee: AEA Technology PLC
Current assignee: Ricardo AEA Ltd
Priority date: 1997-12-19
Filing date: 1997-12-19
Publication date: 1999-06-23
Also published as: GB9726714D0; GB2332379A9

Abstract

A reactor for the plasma assisted processing of gaseous media, including a bed of gas permeable material consisting of a mixture of a ferroelectric and non-ferroelectric dielectric material.

Description

1 - Gas Purification 2332379 The present invention relates to the

plasma-assisted processing of gaseous media and in particular to the reduction of the emission of carbonaceous and nitrogenous oxide combustion products from the exhausts of internal combustion engines.

One of the ma3or problems associated with the development and use of internal combustion engines is the noxious exhaust emissions from such engines. Two of the most deleterious materials, particularly in the case of diesel engines, are particulate matter (primarily carbon) and oxides of nitrogen (NOJ. Increasingly severe emission control regulations are forcing internal combustion engine and vehicle manufacturers to find more efficient ways of removing these materials in particular from internal combustion engine exhaust emissions. Unfortunately, in practice, it is found that techniques which improve the situation in relation to one of the above components of internal combustion engine exhaust emissions tend to worsen the situation in relation to the other. Even so, a variety of systems for trapping particulate emissions from internal combustion engine exhausts have been investigated, particularly in relation to making such particulate emission traps capable of being regenerated when they have become saturated with particulate material.

Examples of such diesel exhaust particulate filters are to be found in European patent applications EP 0 010 384; US patents 4,505,107; 4,485, 622; 4,427,418; and 4,276,066; EP 0 244 061; EP 0 112 634 and EP 0 132 166.

In all the above cases, the particulate matter is removed from diesel exhaust gases by a simple, physical trapping of particulate matter in the interstices of a porous, usually ceramic, filter body, which is then regenerated by heating the filter body to a temperature at which the trapped diesel exhaust particulates are burnt off. In most cases the filter body is monolithic, although EP 0 010 384 does mention the use of ceramic beads, wire meshes or metal screens as well. US patent 4,427,418 discloses the use of ceramic coated wire or ceramic fibre.

GB patent 2,274,412 discloses a method and apparatus for removing particulate and other pollutants from internal combustion engine exhaust gases, in which the exhaust gases are passed through a bed of charged pellets of material, preferably ferroelectric, having high dielectric constant (that is of the order of at least 1,000). In addition to removing particulates by oxidation, especially electrical discharge assisted oxidation, there is disclosed the reduction of NO, gases to nitrogen, by the use of pellets adapted to catalyse the NOx reduction as exemplified by the use of barium titanate as the ferroelectric material for the pellets.

Also, US patents 3 983 021, 5 147 516 and 5 284 556 disclose the catalytic reduction of nitrogen oxides. However, US 3 983 021 is solely concerned with the reduction of NO to N in a silent glow discharge, the temperature of which is kept below a value of at which the oxidation of N or NO to higher oxides of nitrogen does not occur. There is no mention of any simultaneous removal of hydrocarbons.

1 i 1 1 - 3 Although, so-called contact bodies are used in the process of US 3 983 021, and some of those disclosed may have some catalytic properties, catalysis does not appear to be a necessary feature of the process of US 3 983 021.

Other surface properties, such as adsorption on large surface area materials, are the basis of the process o US 3 983 021.

US patent 5 147 516 does refer to the use of catalysts to remove NO... but the catalytic materials involved are defined very specifically as being sulphur tolerant and deriving their catalytic activity from their form rather than their surface properties.

Also, the operating conditions are very tightly defined. There is no specific mention of the type, if any, of electric discharge involved. All that is disclosed is that the NO,., removal depends upon electronmolecule interactions, facilitated by the structure of the 'coronacatalytic, materials, not the inter-molecular interactions involved in the present invention. There is no mention of the simultaneous removal of hydrocarbons from the gas streams being treated by the invention of US 5 147 516.

US patent 5 284 556 does disclose the removal of hydrocarbons from internal combustion engine exhaust emissions. However, the process involved is purely one of dissociation in an electrical discharge of the so- called silent, type, that is to say, a discharge which occurs between two electrodes at least one of which is insulated. The device described is an open discharge chamber, not a packed bed device. Mention is made of the possible deposition of a NO reducing catalyst on one of the electrodes.

4 In a broader context, the precipitation of charged particulate matter by electrostatic forces also is known. However, in this case, precipitation usually takes place upon large planar electrodes or metal screens.

It is an object of the present invention to provide an improved method for the removal of particulates and nitrogenous oxides from the exhaust gases emitted by 10 internal combustion engines.

According to the present invention there is provided a method of reducing exhaust emissions from internal combustion engines, comprising the operation of passing exhaust gases from an internal combustion engine through a bed of active material and applying an electric potential across the active material sufficient to create an electrical discharge in the interstices of the active material wherein the active material is a gas permeable bed including a combination of a metal oxide containing ferroelectric material and a non-ferroelectric metal oxide containing dielectric material.

According to the present invention in a particular aspect there is provided a reactor for reducing exhaust emissions from an internal combustion engine, comprising a reactor chamber adapted to form part of an internal combustion engine exhaust system, the reactor chamber including at least two electrodes between which there is disposed active material through which, in use, exhaust gases are constrained to pass, wherein the active material is a gas permeable bed including a combination of a metal oxide containing ferroelectric material and a non-ferroelectric metal oxide containing dielectric material.

1 - Preferably the ferroelectric and conventional dielectric materials are in the form of pellets and the bed of active material is in the form of a mixture of pellets of the ferroelectric and dielectric materials.

A suitable ferroelectric material is barium titanate and suitable non ferroelectric dielectric materials are titania, preferably in the anatase phase, or zirconia.

The invention will now be described, by way of example, with reference to the accompanying drawing which is a longitudinal section of a schematic reactor for treating the exhaust gases from internal combustion engines.

Referring to the drawing, a reactor 1 for treating the exhaust gases from an internal combustion engine to reduce the emissions of pollutants such as carbonaceous and nitrogenous oxide combustion products therefrom consists of a stainless steel chamber 2 which has an inlet stub 3 and an outlet stub 4 by means of which it can be connected into the exhaust system of an internal combustion engine (not shown in the drawing). The chamber 2 is arranged in use to be connected to an earthing point 5. Within the chamber 2 are a cylindrical inner electrode 6 made of perforated stainless steel and an earthed outer electrode 14 co- axial with the inner electrode 6 and also made of stainless steel. The electrodes 6 and 14 are held in position within the chamber 2 by insulating supports 7 and 8. The space 11 between the electrodes 6 and 14 and insulating supports 7 and 8 is filled with a mixture of pellets illustrated highly diagramatically at 12, some of which are made of a ferroelectric metal oxide and the remainder of which are made of a dielectric, but not ferroelectric, metal oxide. The insulating support 7 has a number of axially oriented - 6 holes 13 disposed around its periphery so that exhaust gases entering the reactor 1 are directed into the space 15 between the outer casing 2 of the reactor 1, the insulating supports 7 and 8 and the outer electrode 14.

The exhaust gases then are constrained to pass radially through the bed of pellets 12 before passing through the inner electrode 6 and leaving the reactor 1. A high voltage feed-through 10 connects output from a source 9 of pulsed or continous potential to the inner electrode 6. The potential is thus applied across the bed of pellets 12 and is sufficient to excite a plasma in the exhaust gases in the interstices between the pellets 12. A convenient Dotential for this purpose is a potential of about 10 kV to 30 kV which may be a pulsed direct potential or a continuously varying alternating potential, or may be an interrupted continuous direct potential. Typically we employ a potential of 20 kV per 30 mm of bed depth. The pellets 12 can be in any convenient form such as chips, spheres, or extrudates and may include binders such as gamma-alumina.

In use, the ferroelectric material acts to oxidise carbonaceous combustion products to carbon dioxide, whereas the dielectric material acts to limit the production of nitrogenous oxides by the ferroelectric material.

An example of a suitable material for use for the treatment of internal combustion engine exhaust gases is a mixture comprising 3 parts by volume of barium chips and one part by volume of zirconia pellets. suitable form of barium titanate is that marketed chips by Cerac and a suitable form of zirconia is manufactured and marketed by the Norton Chemical Products Corporation under their designation XZ This material does not contain a ceramic binder.

i j j titanate A as that Process 16052. An - 7 alternative form of zirconia which can be used is nonporous zirconia beads which have a diameter of 2 mm and are obtainable from Zirconia Sales UK Ltd.

A second mixture suitable for use for the treatment of internal combustion engine exhaust gases is a mixture of equal parts by volume of pellets of barium titanate and titania. Preferably the titania is in the anatase phase such as that manufactured and marketed by Norton Chemical Process Products Corporation under their designation XT 25376.

claims 1. A method of reducing exhaust emissions from internal combustion engines, comprising the operation of passing exhaust gases from an internal combustion engine through a bed of active material and applying an electric potential across the active material sufficient to create an electrical discharge in the interstices of the active material wherein the active material is a gas permeable bed including a combination of a metal oxide containing ferroelectric material and a non-ferroelectric metal oxide containing dielectric material.

2. A reactor for reducing exhaust emissions from an internal combus7-ion engine, comprising a reactor chamber adapted to form part of an internal combustion engine exhaust system, the reactor chamber including at least two electrodes between which there is disposed active material through which, in use, exhaust gases are constrained to pass, wherein the active material is a gas permeable bed including a combination of a metal oxide containing ferroelectric material and a non-ferroelectric metal oxide containing dielectric material.

Claims

3. A reactor according to Claim 2, wherein the ferroelectric material is

barium titanate.

4. A reactor according to Claim 2 or Claim 3 wherein the nonferroelectric dielectric material is titania or zirconia.

5. A reactor according to Claim 4 wherein the nonferroelectric dielectric material is zirconia.

i 1 1 1 6. A reactor according to Claim 5 wherein the zirconia is present in the mixture in the proportion of one part or zirconia to 3 of barium titanate.

7. A reactor according to Claim 4 wherein the nonferroelectric material is titania.

8. A reactor according to Claim 7 wherein the titania is present in the mixture in the proportion of one part of titania to one part of barium titanate.

9. A reactor according to Claim 7 or Claim 8 wherein the titania is in the anatase phase.

10. A reactor according to any of Claims 2 to 9 wherein the gas permeable bed is formed by a mixture of discrete pellets of the ferroelectric and non-ferroelectric dielectric materials.

11. A method of reducing exhaust emissions from an internal combustion engine substantially as hereinbefore described and with reference to the accompanying drawing.

12. A reactor for reducing exhaust emissions from an internal combustion engine substantially as hereinbefore described and with reference to the accompanying drawing.