Exhaust produced by diesel engines has a relatively high content of polluting particles composed of carbon, unburned fuel, and partially burned fuel. Filters are conventionally disposed in engine exhaust systems to remove the particles from the exhaust of automobile or truck diesel engines, a need dictated to a large extent by increasingly stringent governmental regulations in the United States and European countries regarding maximum allowable particulates in automotive emission gases. Generally, particulates trapped by the filtering devices are then periodically combusted in the filter so as to regenerate the filtering surfaces, the combustion being initiated, for example, by electrical means or fuel burner devices associated with the overall trap design, or by variable operation of the engine itself or other means to provide to the filter an exhaust stream sufficiently hot, or chemically active, to initiate the combustion process. This is an expensive process that can even become non-effective for high concentration of particles in the exhaust gases.

In another flue gas clean-up process, particulates are removed by wet-scrubbing.

In order to wash the exhaust gas generated in a combustion chamber and to separate (scrub) at least partially remaining ashes or other solid materials entrained in the hot gas and resulting from the combustion, in the method known from European patent 374 323-B1 the gas is guided through a liquid bath, commonly a water bath. The removal pipe is an immersion tube that is immersed into the liquid bath and the introduction and distribution of the hot gas is achieved with the zig-zag-shaped free edge of the immersion tube. The gas introduced into the liquid bath is likely to cause the formation of gas bubbles. These bubbles can incorporate particulates. When the bubbles reach the surface of the liquid, any

particles remaining within the bubbles will be released into the gas phase. One way to improve this situation is to ensure that any bubbles which do form are as small as possible. The invention described here is designed to achieve this objective.

Another method is known from US 5,397, 381. A hot gas is supplied with a removal pipe into a scrubber. The gas is introduced into at least one liquid bath of the scrubber, forcibly dispersed to form small bubbles over at least a portion of a flow path of the hot gas through the liquid bath, so that the contact surface area is significantly increased.

In still another method described in US 4,820, 391, the flue gas is additionally stripped of sulphur and nitrogen oxides by passing through an aqueous slurry of lime or limestone for reaction with S02 to form insoluble calcium sulfate and/or calcium sulfite. In a variation, known as the dual alkaline method, the flue gas is passed through a scrubbing solution containing a sulfur dioxide absorbent such as sodium carbonate which reacts with S02 to form sodium bisulfite. These processes used alone were not effective in removing fine partilculates along with oxides of sulfur and nitrogen.

Therefore, there is a need for improving the removal of fine particulates, especially of sizes in the sub-micron range. An approach developed by the inventors of the present patent application in an earlier effort (PCT/GB99/03930) includes the use of an apparatus including a container for liquid in which is submerged a pipe transporting a particulate-containing gas. The pipe permits passage of the gas through the liquid so as to wet the particulates, thereby retaining them in the liquid while the gas passes to the exhaust. Additionally, the gas stream is cooled.

Although this process proved to be effective in the simultaneous removal of particulates along with a certain degree of removal of nitrogen and sulphur oxides from the exhaust or combustion gases, various problems have arisen. The surface area at the boundary between the liquid phase and gas phase, was still insufficient to cope with the large amount of fine particulates and provide the degree of removal required by the latest air pollution laws.

A further problem arises with the liquid droplets suspended in the gas stream, thereby increasing the moisture content of the exhaust stream, causing condensation in the pipes and reducing the productivity of the particulate removal device. Moreover, this may cause a more severe corrosion of the pipes and fittings and reduce the lifetime of the apparatus.

Furthermore, the liquid in the apparatus for removing particulates is continuously contaminated with fine particulates and chemical substances originally presented in the exhaust gas. The impurities in the liquid decrease the efficiency of the dispersion of the liquid and removal of the particulates.

Still another problem with conventional diesel engines is the noise generated during their operation. Vehicle exhaust silencers are well known to minimise noise by removing the undesirable acoustic shock waves emerging from the engine.

One conventional method of doing this is to have baffles installed within the exhaust system. These baffles are designed to behave as tuned resonant cavities, which cause destructive interference and thereby cancel out the undesirable audible effects.

Conventionally, in practice, noise reduction and purification of exhaust gases are arranged so that the exhaust gases are led in succession through a catalytic unit and a separate silencer. This leads to an expensive and space-requiring construction, causing problems especially when a gas cleaning arrangement is to be installed afterwards in an engine located in a restricted space.

To some degree attempts have been made to combine these functions. An example of this is the solution disclosed in the publication WO 93/24744 in which two stationary silencer units are built in a uniform housing and between them is arranged a separate element providing catalytic cleaning of the exhaust gas. This solution is rather complicated in its construction and is more suitable for stationary applications, so it cannot be easily modified for different applications.

According to US 5,832, 720, a device representing a combination of an exhaust catalytic element and sound silencer is disclosed. Elements included in the system include on the one hand sound silencing elements having no essential effect on the composition of the exhaust gas and on the other hand elements

applicable for catalytic purification of the exhaust gas. The combined device can be further modified as to either increase the catalytic efficiency, or sound silencing properties, depending on the current demands.

Thus, it is an object of the present invention to provide a method with which the efficiency for removal of particulates is increased and the scrubbing of possibly entrained solid particles is improved.

It is a further object to provide a process in which particular material can be removed along with the oxides of sulfur and the oxides of nitrogen.

It is a further object to provide separation of suspended water droplets from the exit gas stream. It is a further object to provide a process in which particulate material entrapped by the scrubbing liquid can be removed along with the oxides of sulfur and oxides of nitrogen before the liquid is re-circulated to the process.

It is also an object to provide a simple filtration and regeneration method for a scrubbing liquid.

It is still another object to provide such a new particulate removal device that can be easily applied or modified in a simple way to effect the sound silencing. A further aim is to provide a solution with an uncomplicated construction and with the further advantages of low cost and no additional space requirements.

SUMMARY OF THE INVENTION In accordance with the present invention, an apparatus for cleaning an exhaust gas containing particulates, and, optionally, sulphur and nitrogen oxides, includes a means for injecting a fluid into the exhaust gas stream. The interaction of the high-velocity gas stream and the fluid causes the fluid to break up into a mist of droplets. This results in a large surface area interface between the particulate- laden gas and the liquid, which maximises the probability of wetting of the particles entrained in the gas stream. The injection of liquid also serves to cool the exhaust gas stream, reducing it to a temperature below its dewpoint. This causes condensation of water vapour generated by the combustion process in the engine, which is assisted by the nucleation centres provided by the carbonaceous

particulates. By these means the proportion of the particulate matter entrained in the liquid increases.

According to a simplest embodiment of the invention, the apparatus for removing particulate material from an exhaust gas stream comprises an inlet gas pipe for feeding the exhaust gas, provided with a means for injecting liquid into the gas stream so as to form a highly dispersed gas/liquid mixture; a gas/liquid mixing unit having a lower chamber for receiving the gas/liquid mixture and an upper chamber for separation of the mixture, the upper chamber being provided with a scrubber cone packed with filling and a splash plate mounted over the scrubber cone; and a means for circulating liquid into the mixing unit, provided with means for cooling liquid before re-circulating it into the apparatus. According to a second embodiment, an apparatus for cleaning exhaust gas containing particulates, sulphur and nitrogen oxides further includes a means for cyclonic droplet separation, comprising a cyclone cone mounted over the splash plate. This cone functions to increase the angular velocity of the gas as it travels down the cone, thus tending to throw suspended water droplets outwards towards the surface of the cone, from where they run downwards under gravity into a water reservoir disposed on the splash plate. The water thus collected is returned to the external reservoir by a drain pipe.

According to a third embodiment of the invention, an apparatus for cleaning exhaust gas containing particulates, sulphur and nitrogen oxides further includes a liquid filtration system for cleaning up the scrubbing liquid before re-circulating it into the process. The liquid filtration system consists of one or more filters which are in circuit with the liquid flowing from the diesel particulate removal apparatus.

A filter aid, such as keiselguhr, bentonite or similar natural or artificial material, is added to the liquid, along with a substance such as an alkali metal salt or nitrate, which enhances subsequent combustion of the carbonaceous particles. Once the filter has filled up with particulates, it is isolated from the water flow (at which point the water flow is diverted to another filter if available) and heated to dry and burn off the carbonaceous matter. The filter is then backflushed to return the filter aid to the liquid. If more than one filter is provided, then one can be in circuit at any

given time when another is being regenerated. In other aspects of the invention, a method of cleaning up the exhaust gas using the claimed apparatus is provided.

Further, the invention relates to the use of a diesel exhaust cleaning device as a silencer, in a case, where, in addition to particulate removal, there appears a need for sound silencing. Then, in accordance with the invention, the traditional sound silencer can be optionally detached and removed, while the particulate removal device would be connected to the exhaust pipe directly to effect the sound silencing.

It is believed that the injection of water into the device serves to absorb the acoustic shock waves which are the principal source of noise generated from an internal combustion engine exhaust and serve therefore as an effective silencer.

The precise means by which the silencing effect is achieved has still to be elucidated, but without being bound by a particular theory, it is believed that the aerosol of water droplets generated within the device acts as a viscous damper and absorbs the shock wave energy, thereby behaving as a silencer.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustrated in the accompanying drawings wherein: FIG. 1 is a schematic diagram of a first embodiment of the invention, showing an apparatus for removal of particulate material from an exhaust gas with the continuous re-circulation of the liquid.

FIG. 2 is a schematic diagram showing in greater detail a mixing unit of the apparatus according to the invention.

FIG. 3 is a schematic diagram of a second embodiment of the apparatus for removal of particulate material from an exhaust gas. It incorporates a cyclone cone for droplet separation.

FIG. 4 is the enlarged schematic diagram of the upper chamber of the second embodiment of the apparatus for removal particulate material.

FIG. 5 illustrates the process of separation of water droplets from the exhaust gas in the upper chamber of the second embodiment of the apparatus.

FIG. 6 shows a supplemental system for cleaning the scrubber liquid used in the apparatus for removal of particulates.

DETAILED DESCRIPTION OF THE INVENTION The first embodiment of the invention is described with reference to FIGS. 1-2 of the drawings.

In FIG. 1, an apparatus is presented comprising a gas/water mixing unit 1 having a lower chamber 3 for contacting dispersed water with gas entering the chamber through an exhaust gas inlet pipe 2. A cone 4 filled with wire mesh is disposed within the gas/water mixing unit 1 to further facilitate removal of particulates from the gas/water mixture. A splash plate 5 is provided to prevent liquid droplets from exiting the mixing unit 1. The splash plate also includes a series of curved guide vanes 26 as shown in Fig 2. For collecting liquid, an upper chamber water gallery 6 is located below the splash plate 5, from where it is constantly drained to the external reservoir 10 via drain pipe 8. The mixing unit 1 is further provided with exit cone 16 and exhaust gas exit pipe 7. A lower drain pipe 9 serves to re- circulate a portion of fluid to external fluid reservoir 10 provided with pump 12 connected to fluid return pipe 11. A cooler 13 is provided to cool the recirculated liquid before re-entering the system.

The apparatus operates as follows. An exhaust gas such as a combustion gas or other flue gas enters the apparatus through an inlet pipe 2 and is contacted with a fluid, such as water, which is injected by a fluid injection nozzle 14 and is further fed into a lower chamber 3 wherein the particulates and various acid gases such as sulphur oxides and oxides of nitrogen are removed.

Fluid recirculated from fluid reservoir 10 is injected into the exhaust gas stream through injection nozzle 14 to cause the interaction of the high-velocity gas stream and the fluid. The gas stream, now laden with a mist of water droplets, passes into the lower chamber 3 at a tangent to the wall of the chamber (see Fig. 2), where it starts to rotate around the vertical axis of the vessel. This circular motion causes a proportion of the water droplets, primarily the larger heavier ones, to be thrown to the walls of the chamber and run down to join the bulk of the liquid. The circular motion also causes energy transfer to the bulk of the liquid, causing it to

rotate. Greater gas flows will cause a more turbulent motion in the liquid, increasing its surface area and hence the potential for gas/liquid interaction, therefore improving the efficiency of the device at high engine speeds and loads.

The exhaust gas, along with a proportion of the suspended water droplets (the smaller, lighter fraction) and any remaining untrapped particulates, now passes up through cone 4. The cone is filled with wire mesh which serves to provide a surface, on to which the water droplets can impinge and be captured. As the water-laden gas passes up through the cone its velocity decreases, in response to the increasing cross-sectional area of the cone. For a given gas velocity there will be an equilibrium point at which the weight of the water droplets is balanced (on average) by the force from the upward-moving gas. This ensures that there is a significant quantity of water suspended within the mesh through which the exhaust gas must flow. This provides another opportunity for gas/liquid interaction and the further removal of particulate matter from the gas stream.

As the liquid is being constantly supplied from below, so it is thrown out of the top of the cone, where it impinges on the splash plate 5. This causes the water to fall to the upper chamber water gallery 6, from where it is constantly drained to the external reservoir 10 via drain pipe 8. The splash plate also includes a series of curved guide vanes 26 (Fig 2), which encourage the gas to move in a circular manner. This again encourages the suspended water droplets to impinge on the walls of the upper chamber. The gas in the upper chamber is already in circular motion due to the action of the guide vanes 26 (Fig 2) on the underside of the splash plate 5. It has been demonstrated that this provides improved separation of aerosol water droplets from the exiting gas stream. This also improves the overall particulate capture efficiency of the device as the suspended water droplets contain entrained particulate matter. The exhaust gas, which is now substantially free of suspended water droplets and cleaned of carbonaceous particulates, exits through exit cone 16 and pipe 7.

The working fluid is recirculated by draining (under gravity) the upper chamber through pipe 8, as stated above. A portion of the fluid from the lower chamber is also recirculated via drain pipe 9. The force for this draining process comes from the circular motion of the water in the lower part of the chamber; the exit pipe is

also tangential to the vessel (see figure 2 for clarification). The liquid from the pipes drains into the external reservoir 10, from where it is removed by pump 12 via pipe 11, and through a cooler 13. The cooler consists of a copper coil, through which the working fluid flows, immersed in a bath of cold water. In a mobile device it would probably take the form of a radiator, identical to that in a vehicle cooling system. The cooled water is then returned to the system through injection nozzle 14 and the cycle repeats. Drain tap 15 is used to completely drain the chamber for the purpose of fluid renewal.

In FIG. 3, a second embodiment of an apparatus according to the invention is presented, the apparatus comprising a gas/water mixing unit 1 having a lower chamber 3 for contacting dispersed water with gas entering the chamber through an exhaust gas inlet pipe 2. A cone 4 filled with wire mesh is disposed within the gas/water mixing unit 1 to further facilitate removal of particulates from the gas/water mixture. A splash plate 5 is provided to prevent liquid droplets from exiting the mixing unit 1. The splash plate also includes a series of curved guide vanes 26 as shown in Fig 2. For collecting liquid, an upper chamber water gallery 6 is located below the splash plate 5, from where it is constantly drained to the external reservoir 10 via drain pipe 8.

The improved embodiment of Fig. 3 comprises a mixing unit 1 provided with cyclone cone 16 mounted over the splash plate, a water collection reservoir 17 located under the cyclone cone 16, and additionally, a cyclone fluid drain pipe 18.

An exhaust gas exit pipe 7 is provided for discharging gas from the apparatus. A lower drain pipe 9 serves to re-circulate a portion of the fluid to an external fluid reservoir 10 provided with pump 12 connected to fluid return pipe 11. A cooler 13 is provided to cool the regenerated liquid before re-entering the system. The cone 16 functions to increase the angular velocity of the gas as it travels down the cone, thus tending to throw suspended water droplets outwards towards the surface of the cone 16, from where they run downwards under gravity into a water reservoir 17 disposed on the splash plate 5. The water thus collected is returned to the external reservoir 10 by drain pipe 18.

The operation of the apparatus according to a second embodiment of the invention will be described with reference to Figs. 3,4 and 5.

Fluid recirculated from fluid reservoir 10 is injected into the exhaust gas stream through injection nozzle 14 to cause the interaction of the high-velocity gas stream and the fluid. The gas stream, now laden with a mist of water droplets, passes into the lower chamber 3 at a tangent to the wall of the chamber (see Figs. 4 and 5), where it starts to rotate around the vertical axis of the vessel. This circular motion causes a proportion of the water droplets, primarily the larger heavier ones, to be thrown to the walls of the chamber and run down to join the bulk of the liquid. The circular motion also causes energy transfer to the bulk of the liquid, causing it to rotate. Greater gas flows will cause a more turbulent motion in the liquid, increasing its surface area and hence the potential for gas/liquid interaction, therefore improving the efficiency of the device at high engine speeds and loads.

The exhaust gas, along with a proportion of the suspended water droplets (the smaller, lighter fraction) and any remaining untrapped particulates, now passes up through cone 4. The cone is filled with wire mesh which serves to provide a surface, onto which the water droplets can impinge and be captured. As the water-laden gas passes up through the cone its velocity decreases, in response to the increasing cross-sectional area of the cone. For a given gas velocity there will be an equilibrium point at which the weight of the water droplets is balanced (on average) by the force from the upward-moving gas. This ensures that there is a significant quantity of water suspended within the mesh through which the exhaust gas must flow. This provides another opportunity for gas/liquid interaction and the further removal of particulate matter from the gas stream.

The operation of the cyclone is explained in more detail with reference to Fig. 5, where A-water laden gas emerges from lower chamber and is directed into circular motion by guide vanes on splash plate ; B-Rotational motion of gas causes some liquid to be thrown out to walls of container; C-gas enters cyclone and starts to rotate faster as cone diameter decreases; D-gas is rotating very quickly and throws out more water droplets to walls of cyclone ;

E-Water enters collection vessel and gas undergoes flow reversal and proceeds up exit pipe; F-gas exits through pipe; G-water is drained to external reservoir.

As the liquid is being constantly supplied from below, so it is thrown out of the top of the cone, where it impinges on the splash plate 5. This causes the water to fall to the upper chamber water gallery 6, from where it is constantly drained to the external reservoir 10 via drain pipe 8. The splash plate 5 also includes a series of curved guide vanes 26, which encourage the gas to move in a circular manner.

This again encourages the suspended water droplets to impinge on the walls of the upper chamber. The gas in the upper chamber is already in circular motion due to the action of the guide vanes 26 on the underside of splash plate 5, and the cyclone 16 acts to accelerate this motion. It has been demonstrated that this provides improved separation of aerosol water droplets from the exiting gas stream. This also improves the overall particulate capture efficiency of the device as the suspended water droplets contain entrained particulate matter. The exhaust gas, which is now substantially free of suspended water droplets and cleaned of carbonaceous particulates, exits through exit cone 16 and pipe 7.

The working fluid is recirculated by draining (under gravity) the upper chamber through pipe 8, as stated above. A portion of the fluid from the lower chamber is also recirculated via drain pipe 9. The force for this draining process comes from the circular motion of the water in the lower part of the chamber; the exit pipe is also tangential to the vessel (see Fig. 3 for clarification). The liquid from the pipes drains into the external reservoir 10, from where it is removed by pump 12 via pipe 11, and through a cooler 13. The cooler consists of a copper coil through which the working fluid flows, immersed in a bath of cold water. In a mobile device it would probably take the form of a radiator, identical to that in a vehicle cooling system. The cooled water is then returned to the system through injection nozzle 14 and the cycle repeats. Drain tap 15 is used to completely drain the chamber for the purpose of fluid renewal.

An additional drain pipe 18 extends to reach almost to the bottom of the cyclone cone 16. Gas in the cyclone has a tendency to undergo flow reversal at the bottom of the cone, where it flows up the inside of the vortex, and hence in this case, emerges from the exit pipe 7 substantially free of suspended water droplets and cleaned of carbonaceous particulates. Thus, the efficiency of removal of water droplets is significantly increased. A supplemental system for fluid filtering and regeneration according to a third embodiment of the invention is shown in Fig.

6. For designating elements, similar to the elements shown in Fig. 1, the same reference numbers are used.

According to Fig. 6, the scrubber fluid returned from the upper and lower chambers of the mixing unit flows along pipes 8,9 and, optionally 18, to the external reservoir 10. It is pumped from the reservoir via return pipe 11 by a pump 12.

When the filter is in operation (i. e. removing particles from the fluid), the liquid passes up through crossover valve 36 which is set to allow liquid to pass into the filter 39 but not into the bypass pipe 37. Cleaned fluid exiting the filter then passes through the second crossover valve 38 which is set to allow liquid to pass through non-return valve 40 then into cooler feed pipe 41 but not into backflush pipe 35.

From here the liquid flows through cooler 13 into the injection nozzle 14.

When the filter requires regeneration, crossover valve 36 is switched to allow fluid to pass into bypass pipe 37 and then on to cooler 13. Backflow of liquid into the filter outlet is prevented by non-return valve 40. Crossover valve 38 is set as before, such that backflush pipe 35 is closed. Regeneration of the filter is then achieved by means of an internal electrical heating element (not shown) or by other means such as utilising heat from the engine exhaust.

After regeneration, the filter must be backflushed to wash out the trapped filter aid and return it to the working fluid. This is achieved by setting crossover valve 38 such that liquid can flow from backflush pipe 35 back into the outlet of the filter 39.

Crossover valve 36 is then set to a third position, which allows liquid to flow from the filter inlet into bypass pipe 37. In this way, fluid flow through the filter is reversed. Once the filter aid has been flushed from the filter, the valves are set to allow flow of fluid through the filter in the normal direction as detailed above.

Multiple filters may be used such that there is always at least one in operation during the regeneration cycle of the other filter (s). This is achieved by duplication of those components in Figure 4 which are surrounded by the dotted line and placing them in the fluid circuit in parallel with the existing filter system.

It is seen that the present invention provides a process for the simultaneous removal of particulates, optionally, along with dissolved substances, such a S02 and NOX, from exhaust and flue gas streams. The liquid can be re-circulated and filtered, while the regeneration of the filter is achieved by means of an internal electrical heating or by other means such as utilising heat from the engine exhaust. The liquid may also contain additives, to encourage incorporation of the polluants into the liquid. These additives may include surfactant wetting agents and sodium carbonate. The latter can assist in the removal of nitrogen oxides and sulphur oxides from the gas stream.

Further, observation has been made that the particulate removal apparatus for cleaning exhaust gases according to the present invention has a substantial sound silencing property, whereby, when silencing needs appear, conventional sound silencing elements can easily be replaced by exhaust cleaning elements without increased need for space or weakened sound silencing or particulate removal performance.

The sound silencing efficiency had been estimated in the course of preliminary experiments conducted on a 1.9 litre 4-cylinder Diesel engine. These experiments confirm that the Diesel particulate removal device acts as an effective silencer. At this stage our observations have been confined to perceived noise levels, as observed by the engine operator.

It was observed that, without the particulate capture device connected to the exhaust of the engine, there was significant noise being emitted due to the pulsating nature of the issuing high velocity gas. Once the particulate capture device was connected to the gas stream, it was further observed that the gas stream issuing from the device was of a steady state, i. e. the pulsating effect previously observed had been abated and the perceived noise level had been greatly reduced. It has been concluded that the device could be utilised as an effective replacement for silencers (mufflers) currently used for noise abatement

on internal combustion engines. More quantitative data will be obtained in the future.

At increased requirements for sound silencing, several particulate removal elements can be installed in series. Alternatively, the conventional silencer can be connected downstream of the particulate removal device of the present invention.

The invention provides a significant reduction of diesel engine weight, when used to replace both a traditional exhaust cleaning device and silencer, or can be used in a case in which there is an outstanding need for sound silencing, for example if a conventional sound silencer is damaged or unavailable for some other reasons.

Then in accordance with the invention, the traditional sound silencer can be detached and removed, while the particulates removal device would be connected to the exhaust pipe directly to effect the sound silencing function.

Although the present invention is described in terms of specific materials, embodiments and process steps, it will be clear to one skilled in the art that various modifications can be made within the scope of the invention as defined in the accompanying claims.