GB2228072A - Device for removal of particulates from exhausts and flue gases - Google Patents

Description

"Device for removal of Darticulates from exhaust and flue gases" This

invention concerns a device for removal of particulates from exhaust and flue gases. 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 the 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 same holds good for the combustion of coal, at least as far as soot is concerned.

The composition of exhaust gas can vary considerably, depending on the quality of air used for combustion. However, for a good process yield (e.g. 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 oxides (also responsible for acid rain, but which could be eliminatedif the diesel fuel were desulphurised, as is done with gasoline).

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 emitted by the combustion of gas oil, fuel oil and the like is extremely unpleasant and that such emissions should thus be eliminated or at least greatly reduced.

Some decrease in the particulate quantity caused by the combustion of hydrocarbons can be achieved i 1 i 1 i 1 i i i by adjusting the combustion by electronic means. However, this adjustment will only reduce the particulates by about 20%, which is not sufficient for a wide variety of applications. Many efforts have thus 5 been made to resolve this problem.

Numerous patents (e.g. J63-232817, EP 283240 and 114696, U.S. 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, usually platinum, 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 filters, and the cost and deterioration of the catalysts (due to poisoning, for instance) have so far rendered the use of such filters uneconomic, especially for vehicles.

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

U.S. Patent 4587808, also concerning diesel engines, provides for the use of a molecular dissociator which, with a charge of up to 150 kV, causes is.

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 separator 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 f rom the foregoing 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 of the particulate content of gases resulting from the combustion of hydrocarbons are far from being resolved.

The objective of this invention is to eliminate the drawbacks inherent in existing particulate stripping systems by providing a simple, inexpensive, compact device for the reduction of particulate emissions in gases resulting from the combustion of hydrocarbons.

According to this invention there is provided a device for reduction of exhaust gas particulates in an 1 1 i i i exhaust pipe, comprising a two-pole high-voltage generator, a plurality of conductors alternately connected to one or other of said poles, means for introducing air into said pipe upstream of said conductors, and means for regulating said means for introducing air.

In one embodiment according to the invention said conductors each consist of a conductive metal grid arranged inside said pipe perpendicular to the general direction of flow of said exhaust gases.

The ratio between the cross-sectional area of each grid and that of the pipe cross-section is preferably less than 50% and more than 10%, and preferably less than 35%.

These values are dictated by the need f or a good balance between the necessity of not unduly obstructing the cross- section of the pipe, while having an adequate area covered by the electric discharge.

Alternatively. said conductors can consist of grids and/or plates, and/or wires. for example arranged to form flats or tubes, set parallel to the general direction of flow of said exhaust gases.

In this case the ratio between the total cross-sectional area of said conductors and that of the cross-section of said pipe is preferably between 35 and 40%, while the conductors extend parallel to the axis of said pipe for a length up to ten pipe diameters.

In any of the arrangements according to the 1 i i invention each of said conductors may have a great number of protruding points, f or example protruding at right angles from their surface.

The voltage supply to the conductors is preferably such as to provide an electric field between 50 and 98% of that which would cause a discharge in the gas at the operating temperature and under the relevant working conditions.

Said means for introducing air into said pipe may consist of a valved conduit or line and a pipe.

Said means for controlling the means for introducing air into said pipe may consist of a probe which analyses the exhaust gas and transmits the results to a microprocessor which controls a pump and a valve f or introducing the desired quantity of air into the pipe upstream of said conductors.

The present 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 the aims and scope of the invention. In the drawings:

Figure 1 is a general schematic layout of an embodiment of the invention, Figure 2 is a sectional view of an embodiment in which the conductors are set parallel to the general direction of gas flow, and Figure 3 is a sectional perspective view of an embodiment in which the conductors are set perpendicular 1 i 1 1 1 i i 1 1 1 i 1 1 i i i i i to the general direction of gas flow.

With reference to Figure 1, a conduit 2 is introduced into a pipe 1 carrying exhaust gas. Said conduit is complete with conductors (not shown) connected via cables 4 and 5 to a voltage generator 3. A microprocessor 9 processes signals concerning the composition of the gas analysed by a probe 6, and controls a valve 8 in a line 7 connected to the conduit 2, to admit into the latter the desired quantity of air, which is fed via a pump 10.

In operation, the probe 6 analyses the gas, especially as regards its CO, C02 and 02 content. The signals from the probe are sent to the microprocessor 9 which - on the basis of pre-established programs and other information regarding specific fuel consumption- establishes the quantity of unburnt material and soot in the gas, and hence the amount of air needed f or their combustion and, consequently, controls the pump 10 and valve 8.

At the same time, a high voltage is established between the conductors, through the voltage generator 3.

When the gas flows between the conductors, the soot contained therein lowers the dielectric constant of the system, causing a strong spark to be discharged onto the carbon particles, rendering them incandescent, so that they are burnt up.

Enrichment of the gas with air and the high i 1 i i i 1 temperature attained permit most of the carbon particles to be burnt, while ensuring the production essentially of carbon dioxide. In this way, for example, the very low carbon monoxide content of diesel engine exhaust gases is maintained and, in some cases, even decreased.

Two embodiments of the invention are illustrated in Figures 2 and 3 respectively.

In Figure 2, the conductors, in the f orm of flat plates 12, 121 and 13 are set parallel to the general direction of flow of the gas, from left to right. Cables 4 and 5 respectively feed plates 12, 121 and 13, cable 5 passing through the wall of the conduit 2 via an electrically insulated section 11. The plates can have protruding points 14 to assist the formation of electric discharges between the carbon particles and the plates themselves.

In Figure 3, the conductors, in the f orm of circular grids 12, 1211 13 and 131. are set perpendicular to the general direction of gas flow, being fed via cables 4 and 5, while being kept electrically insulated f rom. the conduit 2 by insulators 11 which also permit the passage of the cables 4 and 5 through the wall of conduit 2.

of course, other shapes and arrangements of conductors 12 and 13 can be employed without departing from the scope of the invention.

The present invention has been tested on the exhaust system of a dieselelectric set with a 3860 cc Z i 1 1 i 1 i i i 1 1 1 1 i -g- engine and a generator rated at 40 kW at 1500 rpm. The engine was run at 1400 rpm during the tests.

The conductors, in the f orm of a grid, as in Figure 3, were set 3 cm apart. The diameter of the four pairs of conductors was 20% less than the internal diameter of the exhaust pipe. The voltage between the conductors of a pair of conductors ranged from 50 to 65 kV. The amount of space occupied trasversely by each grid, as defined above, amounted to 22% of the cross-sectional area of the pipe.

The engine was run f or one hour during each test and the particulate removal device was operated for f ixed periods. At the same time, the exhaust gas was sampled and the sample was passed through a weighed filter. After the passage of one cubic metre of gas the filter, with its particulate load, was reweighed.

The average quantity of particulates contained in the exhaust gas which was not treated as per this invention was 17.2 mg/m3, the variability being between -3 and +4 mg/m3.

Insertion of the pairs of grids one by one resulted in a decrease in the particulate content of the treated gas, varying from about 15% (in the case of one pair of grids with voltage of 52 kV) to about 70% (with four pairs of grids and voltage of 60 kV).

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Claims (11)

1. A device for reduction of exhaust gas particulates in an exhaust pipe, comprising: a two-pole, high-voltage generator, a plurality of conductors alternately connected to one or other of said poles, means for introducing air into said pipe upstream of said conductors, and means for regulating said means for introducing air.

2. A device according to Claim 1, wherein said 10 conductors each consists of a conductive metal grid arranged inside said pipe perpendicular to the general direction of flow of said exhaust gases.

3. A device according to Claim 2, wherein the ratio between the crosssectional area of each grid and that of the pipe cross-section is less than 50% and more than 10%.

4. A device according to Claim 3, wherein said ratio is less than 35%.

5. A device according to Claim 1, wherein said 20 conductors are in the form of grids and/or plates and/or wires set parallel to the general direction of flow of said exhaust gases.

6. A device according to Claim 5, wherein the ratio between the total cross-sectional area of said conductors and that of the cross-section of said pipe is between 35 and 4%, with said conductors extending parallel to the axis of said pipe for a length of up to ten pipe diameters.

i 1 i 1 1 1 l i i 1 i 1 i 1 i i 1 A wherein consists

7. A device according to any of Claims 1 to 6, wherein each of said conductors has a great number of protruding points.

8. A device according to any of Claims 1 to 7, 5 wherein said highvoltage generator feeds said conductors to form an electric field between 50 and 98% of that needed to cause a discharge between the conductors under the operating conditions.

9 A device according to any of Claims 1 to 8, said means for introducing air into said pipe of a valved conduit or line and a pipe.

10. A device according to any of Claims 1 to 9, wherein said means for controlling the means for introducing air into said pipe consists of a probe which analyses the exhaust gas and transmits the results to a micro processor which controls a pump and a valve for introducing the desired quantity of air into the pipe upstream of said conductors.

11. A device for reduction of exhaust gas particulates in an exhaust pipe substantially as hereinbefore described with reference to any of Figures 1 to 3 of the accompanying drawings.

Published 1990ct The Patent Office. State House. 66 71 High Holborn. London WC1R4TP.PUrther copies maybe obtained from The Patent Office. Sales Branch, St Mary Cray. Orpington. Kent BR5 3RD. Printed by Multiplex techniques ltd. St MarY Cray. Kent. Con 187