GB2229117A

GB2229117A - "System for electrostatic removal of particulates from exhaust and flue gases"

Info

Publication number: GB2229117A
Application number: GB9002692A
Authority: GB
Inventors: Angelo Colletta; Gian Paolo Giromella; Medardo Pinti
Original assignee: SVILUPPO MATERIALI SpA; Centro Sviluppo Materiali SpA
Current assignee: SVILUPPO MATERIALI SpA; Centro Sviluppo Materiali SpA
Priority date: 1989-02-10
Filing date: 1990-02-07
Publication date: 1990-09-19
Anticipated expiration: 2010-02-07
Also published as: SE9000484L; DD291934A5; FR2642989B1; KR930009718B1; GB2229117B; HU900704D0; SE9000484D0; HUT52717A; FR2642989A1; BR9000571A; DE9001339U1; ES2020120A6; GB9002692D0; JPH02241559A; IT1230462B; DE4003565A1; KR900012683A; IT8947638A0; RU1808096C

Description

1 91System for removal of iparticulates from exhaust and flue gases" This

invention concerns a system for removal of particulates from exhaust and flue gases produced by burning fossil fuels. More precisely it concerns a system for eliminating or at least greatly decreasing the emission of particulates from diesel engine exhausts or from flue gases resulting from all those applications involving the burning of gas oil or heavier oils, even in mixes with aromatic fractions (heating plants, portable power generators, large power stations, etc.) or coal.

Although, for simplicity, reference is made principally to diesel engines in the remainder of the description, it is obvious that the points put forward and the conclusions reached refer to and hold good equally for other applications.

Combustion of hydrocarbons, starting from molecules with about ten carbon atoms or simple aromatic molecules, gives rise not only to the usual products of combustion such as CO, C02 and H20, but also to products such as unburnt organic compounds, soot and nitrogen oxides.

The composition of exhaust gas can vary considerably, depending on the quantity of air used for combustion. However, for a good process yield (i. e. to obtain sufficient power) the air/fuel ratio is fixed at around certain values which make it difficult to avoid the emission of particulates.

The phenomenon is particularly evident to the observer in the case of automotive diesel engines, which are thus accused of causing heavy pollution and of 5 seriously harming public health.

Generally speaking, these accusations are groundless. In fact, compared with gasoline engines, diesels emit from four to six times less carbon monoxide (which is a powerful poison, since it blocks the blood- oxygen exchange), about half the amount of unburnt hydrocarbons (which are highly carcinogenic), and around half the nitrogen oxides (which are among those responsible for acid rain).

However, diesel engines emit some forty times more carbon particles (which in the long term can cause bronchitis and other chest complaints), as well as sulphur dioxide (also responsible for acid rain, but which could be eliminated if the diesel fuel were desulphurised, as is done with gasoline). 20 It should also be observed that soot adsorbs unburnt hydrocarbons and hence acts as a carrier for these carcinogenic agents. To sum up. there can be no doubt that the soot produced by the combustion of gas oil, fuel oil and the like is extremely unpleasant and that such emissions should be eliminated or at least greatly reduced.

Some decrease in the particulate quantity caused by the combustion of hydrocarbons can be achieved 1 by adjusting the combustion by electronic means. However, this adjustment will only reduce the grade of smoke by about 20%, which is insufficient for many applications. Many efforts have thus been made to 5 resolve this problem.

Numerous patents (e.g. JP 63-232817, EP 283420 and 114696, US 4622810, 4604868 and 4571938, etc.) use ceramic filters of various descriptions which mechanically trap the particulates and are periodically regenerated, for instance by combustion utilising hot gases produced by a special burner or by a stream of preheated air. The filter is often also impregnated with a catalyst,, platinum for instance, to facilitate combustion.

In this manner the particulate quantity is greatly reduced. However, the complexity of the solutions adopted to date, the cost and fragility of the ceramic filter, and the cost and deterioration of the catalysts (e.g. due to poisoning) have so far rendered the use of such filters uneconomic, especially for vehicles.

US Patent 4741746 suggests the use of an electrostatic precipitator with corona effect to precipitate the carbon particles from diesel exhaust gases.

US Patent 4587808, also concerning diesel engines, provides for the use of a molecular dissociator which, with a charge of up to 150 KV, causes dissociation of the CO, C02 and NOX molecules and unburnt hydrocarbons into the constituent chemical elements, and the subsequent removal of the carbon particles thus produced, as well as of those already present in the exhaust gas, by means of an electrostatic precipitator and a cyclone.

At the 1987 Paris Meeting on Air Pollution Caused by Transport, it was reported, however, that when industrially-derived systems are installed in cars, their efficiency is dubious (as in the case of cyclones) or their size is excessive and their complexity prohibitive (electrostatic separators), (cf. Pollution Atmospherique, Special Number, December 1987, pp 268285).

It is evident from the f oregoing that in the case of applications of limited size (such as automotive diesel engines, etc. or domestic heating plants) the technical and economic problems bound up with reduction in the grade of smoke of gases resulting from the 0 combustion of hydrocarbons are far from being resolved.

The objective of this invention is to eliminate these drawbacks by providing a simple, inexpensive, compact system ensuring a marked reduction of particulate emissions in gases resulting from the combustion of hydrocarbons, as well as simple means, operating together with said system, for eliminating the soot separated.

According to.;he invention there is provided a 2 1 system for reduction of particulates in exhaust or flue gases produced by the combustion of fossil fuels, comprising a high voltage generator, means for electrostatic separation of the particulates f rom the gases comprising at least one pair of conductors between which, in use of the system, the gases are passed, each of said conductors being electrically connected to one of the terminals of said voltage generator, and means for eliminating the particles separated from the gases.

Said conductors of one pair of conductors may comprise two spaced facing surfaces, electrically insulated from one another, in a conduit through which said exhaust or flue gases pass, in use.

Alternatively, one conductor of a pair of said conductors may have at least one point directed towards the other conductor, which comprises a surface extending in front of said one conductor from which it is electrically insulated.

Said one conductor of a pair of said conductors may comprise the inner surf ace of a conduit through which said exhaust or flue gases passf in use, the other conductor comprising an elongate element, electrically insulated from said surface, extending along and within said conduit. Said elongate element preferably extends substantially axially along said conduit.

Said means for eliminating the particulates may consist of means for raising the temperature of the particulate matter separated f rom the exhaust or f lue gases and burning it.

Said means for raising the temperature of the collected particulates can take the form of a small auxiliary burner which delivers hot gases rich in oxidizing agents onto the particulates, or an electric heating device which heats the particulates in a stream of exhaust gas or air or a mixture thereof. The heating device may consist of at least one spark- or glow-plug.

Said means for eliminating the particulate matter separated from the exhaust or flue gases may consist of removable interchangeable means that can be dismantled from the system to be cleaned apart and replaced by other similar means already clean.

The invention includes within its scope a method of reducing particulates in exhaust or flue gases produced by the combustion of fossil fuels, comprising passing the gases between two spaced conductors each of which is connected to one of the terminals of a voltage generator, whereby the particulates are electrostatically separated from the gases, and eliminating the particulates so separated.

In applications where the exhaust or flue gas line runs vertically, as in the case of chimneys, smoke stacks and the exhausts of some commercial vehicles, the means for eliminating the separated particulates can consist of a collecting trap installed in the lowest part of the part icul ate -stripping system and the point where the gases enter the vertical run. Collection of particulates in the trap is facilitated by use of an appropriate system, such as a vibrator which is operated during a break in the combustion process or gas flow.

Yet another embodiment provides for use of a suitably-sized particulateremoval system which also acts as a soot container that can be separated from the exhaust/flue line, so that it can be removed and cleaned 10 every so often, and then replaced and reused.

This invention will now be described in greater detail by reference to the accompanying drawings which illustrate it purely by way of example, while in no way limiting its objectives and scope. In the 15 drawings:

Figure 1 is a sectional view of one embodiment of the invention, and Figure 2 is a sectional view 'of the particulate-stripping part which can be removed from the 20 rest of the exhaust flue-gas system.

With reference to Figure 1, the steel (preferably stainless) exhaust or flue-gas pipe 1 is also one of the conductors of the part iculate-removal system, and is connected to the positive terminal of a 25 high-voltage generator (not shown).

The exhaust gases enter the pipe 1 through a conduit 9. The other conductor 2, connected to the other terminal of the generator, enters the pipe 1 through an insulator 8 and extends along the axis of said pipe for some distance, being partly insulated by means of an insulator 3. The free end of the conductor 2 is supported by a suitable insulator 4 in the form of small, spaced bars set between the pipe 1 and the conductor 2. The pipe 1 also has an external heater 5, a conduit 6 and spark or glow-plugs 10.

In operation, exhaust gases are delivered through the conduit 9 into the pipe 1, traversing it from right to lef t as see in Figure 1. In the space between the conductor 2 and the pipe 1, both of which are connected to the terminals of a voltage generator, the carbon particles in the exhaust gases are electrically charged and are attracted towards the inner wall of the pipe 1 where they are discharged and are deposited to form a deposit 7.

When this deposit has attained a certain thickness, or when the build-up of carbon within the whole of the pipe 1 has reached a stage where the conductors 1 and 2 cannot be perfectly insulated from one another, the voltage between the conductors decreases, very considerably in some cases.

At this point the heater 5, which can take the form of an electric resistance, is brought into action, and/or the glow or spark-plugs 10 are switched on. Alternatively or additionally very hot gas from an auxiliary burner, or air, is blown through the pipe 1. These operations, separately or in any combination, are designed to raise the temperature of the carbon deposit 7 to generally between 400 and 5501C, at which carbon burns.

To facilitate this operation, two or more of said pipes 1 are set in parallel, the soot being stripped f rom the exhaust gas in at least one of them, while the previously deposited particles are eliminated in at least one of the others. In this way, each of the pipes acts alternatively as a soot separator and an eliminator.

Selection of the desired functions and operation of valves to divert the exhaust gases from one pipe to the other can be advantageously controlled automatically by a timer or a microprocessor which receives its information from special probes.

With reference to Figure 2, where the same reference numerals indicate the same parts as in Figure 1, it can be seen that the pipe 1 f orms' the end part of an exhaust pipe 11 to which it is connected by a joint 12, which can be in the form of a bayonet fitting, for example.

The way the apparatus of Figure 2 operates as regards soot separation is just the same as that described in relation to the Figure 1 embodiment. Here, too, it is possible to position at least two pipes in parallel, the exhaust gas being made to pass alternatively through only one of said pipes or, if there are more than two, through only some of these, for the soot-stripping operation.

With the Figure 2 arrangement, once the layer of deposited soot reaches a predetermined level, the pipe 1 is disconnected f rom the main pipe 11, to be cleaned separately and replaced by another clean pipe.

Trials demonstrate that the particulate content of exhaust- or flue-gases can easily be reduced by at least 50%.

As an alternative embodiment, it has been seen that hot air can be sent into the pipe 1 even without interrupting the flow of exhaust gases. In this way it is possible to avoid the complications involved in positioning more than one conduit in parallel and to avoid the build-up of soot that could be ejected as a block from the exhaust pipe owning to vibrations, shocks or similar events.

The present invention has been tested on the exhaust system of a dieselelectric set with a 3860 cc engine and a generator rated at 40 KW at 1500 rpm. The engine was run at between 1300 and 1480 rpm during the tests.

The soot-stripping system consisted of one or two 1000 mm long, 150 mm internal diameter steel cylinders (in parallel and operating either together or alternatively) with a 15 mm diameter steel rod set axially.

A voltage of 30 KV at 0.5 nA was applied between the outer cylinder and the axial rod. Each test was run for an hour, a continuous sample of 5000 litres being taken from the gases which passed through the device. The sample was passed through a porous f ilter and the particulates collected in this manner were weighed. The results are given in the Table.

TABLE

Engine No. pipes Voltage Particulates Percentage revs/ used in collected stripped min parallel Yes/No mg/cu.m (A-B/A) A 1300 1 No 13.46 B 1300 1 Yes 6.4 52.4 A 1350 2 No 15.0 B 1350 2 Yes 4.0 67.3 A 1380 2 No 16.4 B 1380 2 Yes 7.32 55.4 A 1410 1 No 17.5 B 1410 1 Yes 7.2 58.9 A 1430 1 No 21.3 B 1430 1 Yes 9.83 53.8 A 1430 2 No 22.0 B 1430 2 Yes 7.33 66.7 A 1480 1 No 24.8 B 1480 1 Yes 13.9 43.9 The deposited soot was rapidly burnt-off when a stream of air at 4900C was passed through the separation conduit.

If the exhaust gas was enriched with air, it was burnt during the separation process, the percentage stripped figure then being improved by 12 to 13 points.

1

Claims

A system for reduction of particulates in exhaust or flue gases produced by the combustion of fossil fuels, comprising a high voltage generator, means f or electrostatic separation of the particulates f rom the gases comprising at least one pair of conductors between which, in use of the system, the gases are passed, each of said conductors being electrically connected to one of the terminals of said voltage generator, and means for eliminating the particles separated from the gases.
2. A system according to Claim 1, wherein said conductors of one pair of conductors comprise two spaced facing surfaces, electrically insulated from one another, in a conduit through which said exhaust or flue gases pass, in use.
3. A system according to Claim 1,wherein one conductor of a pair of said conductors has at least one point directed towards the other conductor, which comprises a surface extending in front of said one conductor from which it is electrically insulated.
4. A system according to Claim 1,wherein one conductor of a pair of said conductors comprises the inner surface of a conduit through which said exhaust or flue gases pass, in use, and the other conductor comprises an elongate element, electrically insulated from said surface, extending along and within said conduit.

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5. A system according to Claim 4, wherein said elongate element extends substantially axiallyalong said conduit.
6. A system according to any of Claims 1 to 5, 5 wherein said means for eliminating the particulates consist of means for raising the temperature of the particulate matter separated from the exhaust or flue gases and burning it.
7. A system according to Claim 6, wherein said 10 means for raising the temperature of the particulate matter consist of an auxiliary burner which, in use, directs hot gases rich in oxidizing substances onto the said particulate matter.
8. A system according to Claim 6, wherein said 15 means for raising the temperature of the particulate matter consist of an electric heating device that heats said particulate matter in the presence of a stream of exhaust gas or air or a mixture of air and exhaust gas.
9. A system according to Claim 8, wherein said 20 heating device consists of at least one spark- or glowplug.
10. A system according to any of Claims 1 to 9, including means for enriching the exhaust gases with air during particulate precipitation to cause the combustion of at least part of the particulates.
11. A system according to any of Claims 1 to 5, wherein said means for eliminating the particulate matter separated from the exhaust or flue gases consist t A of removable interchangeable means that can be dismantled from the system to be cleaned apart and replaced by other similar means already clean.
12. A system for reduction of particulates in exhaust or flue gases, substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.
13. A method of reducing particulates in exhaust or flue gases produced by the combustion of fossil fuels, comprising passing the gases between two spaced conductors each of which is connected to one of the terminals of a voltage generator, whereby the particulates are electrostatically separated from the gases, and eliminating the particulates so separated.
14. A method according to Claim 13, wherein the separated particulates are eliminated by burning.
15. A method according to Claim 13, wherein the separated particulates are eliminated by collection and removal.

Publithed 1990 at The Patent Office, State He. 68 '11 High Holborn. London WC 1 R 4TP. Further copies maybe obtained from The Patent OfficeSalc& Branch, St Mary Cray. Orpington, Kent BR5 3RD.Printed by MulUplex t@cbnlques It& st MLry Cray, Kent, Con. V87