GB2350804A - Removing particulate matter from gas by thermophoresis and combustion - Google Patents

Abstract

Particulate matter in hot gas streams, eg very small sooty particles in petrol engine exhaust gas, is caused to deposit and agglomerate on a cooled surface, and then caused to combust by either heating eg using microwaves, or by passing the gas through a non-thermal plasma to generate oxidising species. The cooled surface may be a metal or ceramic eg alumina wall of an exhaust pipe or a though-flow monolith or filter in the exhaust pipe or box. It may be cooled by water, air or a heat pump. The combustion is intermittent and controlled by a timer, or in response to engine operating conditions or the detection of a significant deposit. If microwave heating is used with a ceramic cooled surface, the particles are heated to a temperature at which combustion takes place in the exhaust gas environment, without any heating of the cooled surface. Alternatively combustion is induced by a plasma generator which acts on the NOx, oxygen, ozone or water vapour in the exhaust gas. The combustion may be aided by catalytic surfaces. The apparatus may be used for diesel engine exhausts, and/or combined with exhaust gas recirculation.

Description

1 2350804 IMPROVEMENTS IN POLLUTION CONTROL This invention concerns

improvements in pollution control, and more particularly concerns a method and apparatus for removing small sooty particulates from gas streams.

It is very well known that gas-bome particulates can cause health problems if inhaled. It is also well known that diesel engines emit particulates, especially upon start-up and under heavy load, and this can be observed. Concern about particulates has led to the introduction of various national or European regulations to control the quantity of particulates emitted from diesel engines. We have patented and commercially introduced a device called the "CRT TM, trap for heavy duty diesel engines (USP 4,902, 487) which has made available a self-regenerating diesel filter device.

However, it is not generally appreciated that many modem gasoline engines emit large quantities of very small (<O.Ipm generally 50 to 100 nm) sooty particulates, which because of their small size are not observed by the naked eye. Nonetheless, the size of these particulates (hereinafter called "PM" (= Particulate Matter)) is such that they can be inhaled into the deepest recesses of the lungs and may even be more hazardous than the PM from diesel engines. We believe that this PM problem is found with the latest types of lean-bum gasoline engines, such as gasoline direct injection, in which uneven combustion characteristics can lead to PM formation. The fuel efficiency of these engines will result in their increasing use in private cars and small utility vehicles. Also, recent research indicates that there is considerable PM generation from "stoichiometric" (air/fuel ratios of approximately unity) gasoline engines operated at high speed, such as during motorway driving. Such PM may be generated in equivalent amounts to PM from diesel engines, but because of their smaller size, this equates to larger numbers of particles.

We do not believe that there currently exists any operable device for removing such small PM from gasoline engine exhausts, but consider that there will be an increasing need when the complete emissions characteristics of gasoline engines are better appreciated and that eventually regulations in respect of PM will be introduced.

The present invention is believed to have particular usefulness in the control of PM in gasoline engine exhaust gases, but may be applied to many similar exhaust or other 2 gases containing such sooty PM. It should be understood that exhausts from diesel engines are to be included. For ease of description, however, this specification concentrates on gasoline exhaust gases.

It has been appreciated in the above-mentioned USP 4,902,487 that the oxidation of trapped PM using oxygen/air requires rather a high temperature (which may not be met with regularly in diesel engine exhausts) and that the use of N02 offers a process of combustion that can be carried out at appreciably lower temperatures. This technology, however, is not easily adapted to gasoline engines.

The present invention provides a method of removing PM from hot gas streams, comprising the steps of a) causing deposition and agglomeration of the PM by passing the gas stream over a is cooled surface, and b) causing the combustion of the agglomerated and deposited PM.

The invention ftirther provides an apparatus for the removal of PM from hot gas streams, comprising a surface for deposition and agglomeration of said PM, cooling means for said surface to cause such deposition and agglomeration, and means to cause combustion of deposited and agglomerated PM.

The cooled surface, onto which the sooty PM is attracted to deposit and agglomerate, may suitably be the walls of an exhaust pipe in the engine exhaust system and although this surface may be metal, it is preferably a ceramic, for reasons described below. Other cooled structures including a through-flow monolith or a filter system may be placed in the exhaust pipe or in a vessel or "box" through which the exhaust gases flow. Cooling may be achieved by water- cooling the appropriate section, by forced-air cooling or by using a heat- pump. The actual method of cooling is not critical to the present invention, and the size of the surface may be determined by experiment which relates the temperatures available and the characteristics (temperature, flow rate, PM content etc.) of the particular exhaust gas.

A presently-preferred method in a first embodiment of the invention of causing the combustion of the agglomerated PM is to use microwave heating of the agglomerated and deposited PM. In such a system a ceramic, e.g. alumina, section of exhaust pipe can be 3 actively cooled, but is not itself heated by the microwaves. The agglomerated PM can be heated to a temperature at which combustion takes place in the exhaust gas environment.

In a second embodiment of the invention, plasma-induced combustion of the agglomerated and deposited PM is used. A non-thermal plasma is believed to be particularly effective to cause combustion of sooty PM, and commercially- available plasma generators may be adapted and used in the invention. This embodiment has the advantage that there is no need to heat up the agglomerated PM or the cooled surface, and the active gas species in the treated exhaust gases are themselves effective oxidants. These species may include species comprising or derived from NO, N023 02. Ozone and, significantly, water vapour. The species found to be particularly effective may, if desired, be increased in concentration either continuously or intermittently by engine management and/or by adding such gases or vapours upstream of the apparatus of the invention. It is to be noted that the exhaust gases from stoichiometric gasoline engines generally contain inadequate levels of oxygen for combustion of carbon, and therefore other oxidising agents are necessary. Under these conditions, the exhaust gases do, however, contain significant levels of water vapour, and we have found that plasma treatment of water vapour generates useful oxidants.

It should be understood that the combustion process may be aided by the presence of one or more catalytic surfaces which may be the cooled surface or may be upstream of the cooled surface to condition the PM andlor the exhaust gases to promote the capturing and combustion processes of the invention. The usefulness of such catalytic surfaces, and the particular catalyst, may be established by routine experiment.

Although it is envisaged that the invention will be operated throughout each engine operating cycle, it is also contemplated that especially the combustion may be intermittent and controlled either by a simple timing system or actuated in response to the detection of significant agglomeration or in response to engine operating conditions which have been pre-determined to correspond to significant PM generation. Such intermittent operation may be more energy-efficient, and may be adequate to meet the regulations when introduced.

The invention may, of course, be combined with more conventional pollution control measures. Exhaust gas recirculation CWX) is particularly contemplated, and the 4 apparatus and method of the invention may be incorporated in the main exhaust gas stream and/or in the EGR stream. If the invention is incorporated in the EGR stream, -it is particularly beneficial to reduce PM to avoid degradation of engine components, lubricants and the EGR valve, and preferably, to permit cooling of the EGR stream without unnecessary accumulation of PM in the cooler.

Claims (8)

1. A method of removing PM from hot gas streams, comprising the steps of.

   a) causing deposition and agglomeration of the PM by passing the gas stream over a cooled surface, and b) causing the combustion of the agglomerated and deposited PM.
2. 2. A method according to claim 1, wherein the PM is a sub-micron PM in the exhaust gases from a gasoline engine.
3. 3. A method according to claim 1 or 2, wherein combustion is caused by heating the agglomerated and deposited PM using microwaves.
4. 4. A method according to claim 1 or 2, wherein combustion is caused by passing the gas stream through a plasma.
5. 5. An apparatus for the removal of PM from a hot gas stream, comprising a surface for deposition and agglomeration of said PM, cooling means for said surface to cause zo such deposition and agglomeration, and means to cause combustion of deposited and agglomerated PM.
6. 6. An apparatus according to claim 5, wherein the means to cause combustion comprises microwave means to cause heating of the deposited and agglomerated PM.
7. 7. An apparatus according to claim 5, wherein the means to cause combustion comprises plasma generating means to treat said hot gas stream.
8. 8. An apparatus according to claims 5,6 or 7, fitted in the exhaust system and/or an EGR system in a gasoline engine.