WO1999030807A1

WO1999030807A1 - Plasma reactor for processing of gases and ferroelectric particles for use therein

Info

Publication number: WO1999030807A1
Application number: PCT/GB1998/003664
Authority: WO
Inventors: David Leslie Segal
Original assignee: Aea Technology Plc
Priority date: 1997-12-12
Filing date: 1998-12-08
Publication date: 1999-06-24
Also published as: AU1495499A; GB9726188D0

Abstract

A reactor for the plasma assisted processing of gaseous media, in which there is included a bed of pellets made from sintered barium titanate or other ferroelectric powder formed into spheres or pellets using a binder preferably comprising a material containing silicon, titanium, aluminium or mixtures of these elements.

Description

PLASMA REACTOR FOR PROCESSING OF GASES AND FERROELECTRIC PARTICLES FOR USE THEREIN

The present invention relates to the processing of gaseous media by means of plasmas established in the 5 gaseous media, and more specifically to the removal of carbonaceous particles and other pollutants from the exhaust emissions of internal combustion engines.

Our earlier patent number GB 2 274 412 discloses a 0 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 particles of a ferro-electric material having a high dielectric constant to which a voltage of some tens 5 of ilovolts is applied. This voltage is sufficient to excite plasma discharges in the exhaust gases which remove carbonaceous particles from the exhaust gases by oxidative processes. Among the materials disclosed- for use in the bed of charged particles is barium titanate, 0 which can be adapted to cause, in addition, the removal of NO,, from the exhaust gases by reduction to N₂. The use of barium titanate for the plasma assisted processing of gaseous media also is disclosed in A. Mizu o IEEE Transactions on Industry Applications, Vol 28 pp 535-540 5 May/June 1992 and R. Zhang e al IEEE Transactions on Industry Applications 1996, Vol 32 pp 113-117.

However in each of the above cases, the barium titanate is in the form of spheres or pellets, or other 0 shapes which may be extrudates. Barium titanate is a material which is difficult to fabricate into a particulate form as relatively high temperatures, of the order of 1200°C to 1500°C are required to sinter it effectively. This is also true for other ferroelectric 5 materials. These high temperatures mean also that the resultant material is dense with a low porosity; the

-3 density of tetragonal barium titanate is 6.017 g cm and

-3 the density of hexagonal barium titanate is 5.806 g cm .

The low porosity is an advantage when the material is to be used for the plasma discharge processing of gaseous media in general, because the permittivity of the material is not decreased by the presence of pores.

According to the invention there is provided a reactor for the plasma assisted processing of gaseous media comprising a chamber including an inlet and outlet for a gaseous medium to be processed, a gas permeable bed of particulate ferroelectric material, means for directing the gaseous medium through the bed of particulate ferroelectric material and means for applying a potential across the bed of particulate ferroelectric material sufficient to excite an electrical discharge in the said gaseous medium, wherein the particulate ferroelectric material is made by mixing the ferroelectric material in powder form with a binder, forming the mixture into particles of the desired size and shape, and consolidating the particles by heat treatmen .

The binder conveniently comprises silicon- containing, aluminium-containing or titanium-containing material, or combinations of these materials which can be prepared by sol gel techniques.

Preferably the binder comprises a silica-titania gel (which may be referred to as a silico-titanate gel).

Preferably the ferroelectric material comprises barium titanate or calcium titanate. Also according to the invention there is provided a process for producing a material for use in the plasma assisted processing of gaseous materials, characterised in that there is included the operations of mixing ferroelectric material in powder form with a binder, forming the mixture into particles of a desired size and shape and consolidating the particles by heating them to a temperature approaching but less than that required to form the ferroelectric material.

The said heat treatment is carried out at a temperature which is significantly lower than the sintering temperature (1200°C to 1500°C) required for forming the ferroelectric material powder. The heat treatment temperature is chosen so as to achieve desired strength and porosity (density) in the product particles, these being to some extent conflicting in that heating to higher temperatures tends to increase strength but reduce porosity.

Also, these characteristics are affected by the proportion of binder present, but, in accordance with the present invention, this is desirably kept low (e.g. less than 10% by weight, preferably less than 5% by weight) to avoid or minimise degradation of the ferroelectric properties or dielectric constant of the ferroelectric material .

Thus typically, the binder is present at around 3 weight per cent and is mixed with sintered barium titanate powder having, for example, a permittivity of 1410 at a temperature of 25°C and frequency of 1 kHz, formed into particles of the desired size and shape (e.g. spheres or pellets) and heat treated at a temperature of the order of 200°C to produce the particulate barium titanate material. The barium titanate so formed is

-3 porous and typically has a packing density of 2 g cm , properties which make it particularly suitable for use in connection with the treatment of vehicle exhaust gases. However, the barium titanate so formed is dusty. The use of a higher heat treatment temperature, but less than the temperature required to form the barium titanate powders, improves the strength and hardness of the material as well as reducing its dustiness.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing which is a schematic longitudinal section of a reactor for the treatment of internal combustion engine exhaust gases, to remove noxious components therefrom.

Referring to the drawing, a reactor 1 for removing pollutants, such as NO... or carbonaceous combustion products, from the exhaust from an internal combustion engine consists of a cylindrical 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. The chamber 2 is arranged, in use, to be connected to an earthing point 5. Perforated cylindrical stainless steel inner and outer electrodes 6 and 14 are positioned co-axially within the chamber 2 by means of two electrically insulating supports 7 and 8. The upstream end of the inner electrode 6 is closed off and is arranged to be connected via an insulating feed through 10 to a source 9 of an electrical potential of the order of tens of kilσvolts which may be a regularly pulsed direct or continuously varying alternating potential, or an interrupted continuous potential. Typically we employ a potential of 20 kV per 30 mm of bed depth. The space 11 bounded by the electrodes 6 and 14 and the supports 7 and 8 is filled, in this example, with a bed illustrated highly diagrammatically at 12 of particulate barium titanate in the form of spheres, although other shapes, such as solid or hollow cylinders, or even irregular pellets, can be used.

The support 7 nearer the inlet stub 3 has a number of axial holes 13 disposed regularly around its periphery so that incoming exhaust gases are constrained to pass into the space 15 between the outer electrode 14 and the chamber 2 of the reactor 1 and thence radially through the bed 12 of barium titanate spheres before passing through the inner electrode 6 and leaving the chamber 2 via the exhaust stub 4.

If desired, the support 7 can be made to be gas permeable in the annular region between where the electrodes 6 and 14 join it, so that gases can pass into the bed 12 of barium titanate spheres axially as well as radially.

In another arrangement, which is not illustrated, the electrodes 6 and 14 are impermeable and the supports 7 and 8 have an annulus of axial holes such that gases are directed to pass through the bed 12 of barium titanate axially.

The barium titanate spheres in the bed 12 incorporate a binder comprising a silica-titania gel. The binder is present at a proportion of about 3 weight per cent of the barium titanate. This proportion of binder enables the spheres in the bed 12 to be fabricated by heat treatment at about 200°C. The resultant spheres

-3 are somewhat porous and have a density of about 2 gm cm .

Also the presence of such a relatively small proportion of the binder material does not degrade significantly the ferro-electric properties or the dielectric constant of the barium titanate. The binder also has hydrophobic properties with respect to water vapour contained in the exhaust gases, which also is an advantage compared with particles of pure barium titanate.

However, spheres of barium titanate so formed are dusty. A significant improvement in the strength and hardness, and a reduction in the porosity and dustiness of the product material is achieved when the heat treatment is carried out at higher temperatures. For example, barium titanate spheres having diameters between 3 and 4 mm prepared with a silica titania gel binder at a

-3 temperature of 1150° had a density packing of 2.6 gm cm .

Although at such a temperature the binder may react with the barium titanate to form a crystalline phase, for example Ba₂TiSi₂0_H, this does not appear to have a detrimental effect on the performance of the spheres.

Claims

1. A reactor for the plasma assisted processing of gaseous media comprising a chamber (2) including an inlet (3) and outlet (4) for a gaseous medium to be processed, a gas permeable bed (12) of particulate ferroelectric material, means (6, 7, 8, 13, 14) for directing the gaseous medium through the bed (12) of particulate ferroelectric material and means (9, 10) for applying a potential across the bed (12) of particulate ferroelectric material sufficient to excite an electrical discharge in the said gaseous medium, characterised in that the particulate ferroelectric material is made by mixing the ferroelectric material in powder form with a binder, forming the mixture into particles of the desired size and shape, and consolidating the particles by heat treatment .

2. A reactor according to claim 1 characterised in that the binder comprises a silicon-containing material a titanium-containing material, an aluminium-containing material or combinations of these materials, each of the materials being such as to be capable of being prepared in a gel form..

3. A reactor according to claim 1 or claim 2, characterised in that the binder comprises a silica- . titania gel.

4. A reactor according to any of claims 1 to 3 characterised in that the particulate ferroelectric material includes less than five per cent by weight of binder.

5. A reactor according to any of the preceding claims, characterised in that the ferroelectric material comprises barium titanate.

6. A reactor according to any of claims 1 to 4, characterised in that the ferroelectric material comprises calcium titanate.

7. A process for producing a material for use in a reactor for the plasma assisted processing of gaseous material, characterised in that there is included the operations of mixing ferroelectric material in powder form with a binder, forming the mixture into particles of a desired size and shape and consolidating the particles by heating them to a temperature approaching but less than that required to form the ferroelectric material.

8. A process according to claim 7 characterised in that the binder comprises a sol gel derived material containing silicon, titanium, aluminium or a combination of these materials.

9. A process according to claim 7 or claim 8 characterised in that the ferroelectric material contains less than ten per cent by weight of binder.

10. A process according to any of claims 7 to 9 characterised in that there is included the operations of mixing barium titanate powder with approximately 3 per cent by weight of a binder comprising a silica titania gel, forming the mixture into particles having a size in the range 3 to 4 mm and subjecting the particles to a temperature in the region of 1000┬░C.

11. A process according to claim 10 characterised in that the particles are spheres and the temperature is 1150┬░C, the product material having a density packing of

2.6 gm cm .