Patent Description:
Exhaust gas of different kinds is generated in a myriad of different situations, for example in connection with propulsion of vessels. Large ships are typically driven by engines operating on sulphur containing fuel, more particularly high sulphur heavy fuel oil, or low sulphur fuel like VLSFO, ULSFO or diesel. In the combustion of such fuel, exhaust gas containing nitrogen oxides (NOX), and possibly sulphur oxides (SOX) depending on the fuel sulphur level, is formed. The exhaust gas typically also contains particulate matter, such as soot, oil, heavy metals and black carbon (BC) primarily consisting of sub-micron elemental carbon particulates. In order to reduce the impact of the exhaust gas on the environment, the exhaust gas should be cleaned from these pollutants before it is released into the atmosphere. For example, the exhaust gas could be passed through a wet scrubber for removal of sulphur oxides and particulate matter, and/or treated in a SCR reactor for removal of nitrogen oxides.

The scrubber could be a so-called open loop scrubber, which uses the natural alkalinity of seawater to wash out the sulphur oxides from the exhaust gas. Seawater is then fed from the sea, through the scrubber for absorption of SOX and particulate matter from the exhaust gas, before it is discharged back to the sea.

Alternatively, the scrubber could be a so-called closed loop scrubber, which uses circulating freshwater or seawater, typically in combination with an alkaline agent like sodium hydroxide (NaOH) or sodium carbonate (Na<NUM>CO<NUM>), to wash out sulphur oxides and particulate matter from the exhaust gas. In such a scrubber, the amounts of aqueous sulphite, sulphate salts and particulate matter in the circulating freshwater or seawater are gradually increasing. Thus, to control the quality of the circulating freshwater or seawater, a small amount of it may occasionally or continuously be replaced by clean freshwater or seawater and either be stored on the ship or be discharged overboard after cleaning from particulate matter.

The scrubbers used for this purpose today are capable of removing most of the sulphur oxides and some, but less, of the particulate matter from the exhaust gas. SOX emissions are already regulated by the IMO worldwide and regulation of black carbon and particulate matter in general is expected in the future. In view thereof, there is a need for an exhaust gas cleaning technique enabling removal of more particulate matter from exhaust gas.

<CIT> discloses a soot filter comprising filter candles arranged parallel to one another in a ring.

<CIT> discloses an apparatus for filtering solid particles from a fluid.

An object of the present invention is to provide an improved exhaust gas cleaning system for cleaning exhaust gas, an improved method for cleaning exhaust gas and an improved use of an exhaust gas cleaning system for cleaning exhaust gas on-board a ship.

The basic concept of the invention is to provide cleaning of exhaust gas by means of a particle filter device which allows for increased particulate matter removal from the exhaust gas. The exhaust gas cleaning system, method and use according to the invention are defined in the appended claims and discussed below.

An exhaust gas cleaning system according to the invention is for cleaning exhaust gas, e.g. onboard a ship. The exhaust gas cleaning system comprises a particle filter device, which in turn comprises a casing and a plurality of hollow, ceramic filter rods arranged inside an exhaust gas passage of the casing and extending vertically and along each other when the exhaust gas system is installed. The filter rods are partly or completely arranged within the exhaust gas passage. The particle filter device further comprises an exhaust gas inlet arranged to allow exhaust gas to flow into the casing upstream the exhaust gas passage, and an exhaust gas outlet arranged to allow exhaust gas to flow out of the casing downstream the exhaust gas passage. The particle filter device is configured to guide exhaust gas from the exhaust gas inlet, through said exhaust gas passage and to the exhaust gas outlet. The particle filter device further comprises a perforated plate arranged downstream the exhaust gas inlet and upstream said exhaust gas passage. The plate extends at least partly along said filter rods and it partly blocks or closes an exhaust gas flow path from the exhaust gas inlet to said exhaust gas passage. The perforated plate defines openings arranged to allow exhaust gas to flow into said exhaust gas passage. The filter rods are gas permeable to allow exhaust gas to penetrate, during filtration, a respective wall of the filter rods and flow into said filter rods. Further, a respective open upper end of the filter rods are in communication with said exhaust gas outlet so as to allow exhaust gas to leave the casing.

The casing may be of any suitable shape and material, such as for example stainless steel and/or carbon steel, and the exhaust gas passage may have any suitable shape, such as the shape of a rectangular parallelepiped.

A plurality or all of the filter rods may be separated from each other and comprised in a filter set up of so-called candle type. As compared to e.g. a filter of so-called wall flow type, a candle type filter is: flexible since its configuration can easily be changed by addition or removal of filter rods, easy to clean since it enables efficient soot blowing, and cheap since the filter rods can easily be mass produced. Separation of the filter rods enables distribution of exhaust gas all the way around the filter rods, and it also enables use of much filter rod surface for exhaust gas filtration. Further, the filter rods may be arranged in a formation which promotes a more even distribution of the exhaust gas around the filter rods and an effective exhaust gas filtration. The filter rods may have any suitable cross section, such as a circular, oval or polygonal, and any wall thickness. For example, the filter rods may have a circular cross section and an outer diameter of <NUM>-<NUM>, a length of <NUM>,<NUM>-<NUM> and a wall thickness of <NUM>-<NUM>.

In that the filter rods extend vertically and not horizontally, they are less fragile and less prone to being damaged.

The filter rods each have an open upper end. By upper end is meant the end that is furthest from the ground or a floor of a space in which the exhaust gas system is arranged. The filter rods may each have a closed lower end. By lower end is meant the end that is closest to the ground or a floor of a space in which the exhaust gas system is arranged.

It should be stressed that "communicating" and "communication", throughout the text, means "communicating directly or indirectly" and "direct or indirect communication", respectively. Similarly, "receiving", "feeding", "outputting" etc., throughout the text, means "receiving directly or indirectly", "feeding directly or indirectly", and "outputting directly or indirectly", respectively.

Further, it should be stressed that, throughout this text, the term "exhaust gas" is used for untreated exhaust gas as well as exhaust gas cleaned to different degrees.

Herein, by "upstream" is meant "before" as seen in an exhaust gas flow direction, and by "downstream" is meant "after" as seen in an exhaust gas flow direction.

As said above, the perforated plate, which may be of any suitable material, such as stainless steel, partly blocks or closes an exhaust gas flow path from the exhaust gas inlet to the exhaust gas passage. The exhaust gas flow path is only "partly" blocked since the openings of the plate still allows exhaust gas to pass the plate. The openings of the plate may have any suitable design, e.g. be circular, oval or polygonal, or any mix thereof. The open area of the perforated plate may be <NUM>-<NUM>%, and preferably <NUM>-<NUM>%. Further, the exhaust gas flow path may be only "partly" blocked due to the perforated plate being separated from an inside of the casing such that exhaust gas may flow, not only through, but also around, the perforated plate.

As said above, the filter rods are gas permeable so as to allow exhaust gas to penetrate the filter rod walls and flow into an interior of the filter rods. When the exhaust gas penetrates the filter rod walls, particulate matter such as soot, oil, heavy metals and black carbon is deposited on an outside surface of the filter rods while the rest of the exhaust gas flows into the filter rods. Thereby, the exhaust gas is filtered and cleaned from particulate matter before flowing upwards towards the open upper ends of the filter rods and the exhaust gas outlet.

In that the particle filter device comprises a perforated plate which the exhaust gas needs to pass before reaching the exhaust gas passage containing the filter rods, the exhaust gas may be distributed relatively even around the filter rods which is beneficial from a exhaust gas filtering efficiency point of view.

The perforated plate may comprise opposing first and second outer side sections, wherein at least a portion of the first outer side section, e.g. an upper portion, is bent, possibly angled, around a vertical axis in a direction towards the filter rods.

In that at least a portion of the first outer side section is bent in the way specified above, the perforated plate is "folded", and may direct the exhaust gas, towards the filter rods so as to improve the distribution of the exhaust gas around the filter rods.

Naturally, also at least a portion of the second outer side section, e.g. an upper portion, may be bent around a vertical axis in a direction towards the filter rods.

The perforated plate may further comprise an outer lower section, wherein at least a portion of the outer lower section is bent, possibly angled, around a horizontal axis in a direction towards the filter rods.

In that at least a portion of the outer lower section is bent in the way specified above, the perforated plate is "folded", and may direct the exhaust gas, towards the filter rods so as to improve the distribution of the exhaust gas around the filter rods.

The exhaust gas cleaning system may further comprise an elongate plate reinforcement projection extending from the perforated plate towards the filter rods. The plate reinforcement projection need not extend directly or straight, but could also extend obliquely, towards the filter rods. The plate reinforcement projection may be differently positioned on the plate. As an example, the plate reinforcement projection may extend from, and/or along at least a portion of, an outer edge of the perforated plate. Further, the plate reinforcement projection may, or may not, be integrally formed with the perforated plate. As revealed by the name, the plate reinforcement projection may strengthen the perforated plate such that uncontrolled vibrations, deformation and damages to the perforated plate can be avoided. Further, depending on its position on the perforated plate, the plate reinforcement projection may decrease the risk of a "standing vortex" or turbulence being generated by the outer edge of the perforated plate, which "standing vortex" could increase the risk of uncontrolled vibrations in the filter rods arranged closest to the perforated plate, and especially corners thereof.

The filter rods may be divided into a number n > <NUM> of groups. Further, a distance between adjacent ones of at least a majority of the filter rods within each of said groups may be < x, and a distance between adjacent ones of the filter rods of two adjacent ones of the groups may be > x so as to form n-<NUM> intermediate distribution channels. Each of these one or more intermediate distribution channels will extend between two adjacent ones of the groups and may promote a more even distribution of the exhaust gas around the filter rods.

At least one of said intermediate distribution channels may extend in a direction away from the perforated plate. Such a design may promote exhaust gas distribution around the filter rods arranged most distant to the perforated plate.

A set of outer filter rods of the filter rods may be arranged on a distance from the casing so as to form a first outer distribution channel extending between said set of outer filter rods and the casing. This first outer distribution channel may promote a more even distribution of the exhaust gas around the filter rods.

Naturally, another set of outer filter rods of the filter rods may be arranged on a distance from the casing so as to form a second outer distribution channel extending between said another set of outer filter rods and the casing, and possibly parallel to the first outer distribution channel. This second outer distribution channel may even further promote a more even distribution of the exhaust gas around the filter rods.

The first and/or the second outer distribution channel may extend in a direction away from the perforated plate. Such a design may promote exhaust gas distribution around the filter rods arranged most distant to the perforated plate.

Herein, when reference is made to 'distance between filter rods' or 'distance between filter rods and casing', the distance is measured from an outer surface of the filter rods.

The exhaust gas inlet may be arranged at a top portion of the exhaust gas passage, i.e. above the lower ends of the filter rods. More particularly, the exhaust gas inlet may be arranged between or within two imaginary separated horizontal planes which define an extension of an upper half of the perforated plate. Thereby, an upwards directed exhaust gas flow around the filter rods may be minimized which, in turn, may minimize an upwards flow of particulate matter originating from the exhaust gas. Consequently, collection of the particulate matter may be facilitated.

The particle filter device may further comprise a soot blowing arrangement arranged to blow gas into the open upper ends of the filter rods to loosen particles or particulate matter deposited by the exhaust gas on an outside surface of the filter rods. Thereby, a possibility of cleaning the filter rods so as to maintain their filtering capability is offered.

The soot blowing arrangement may be arranged to blow gas into the open upper ends of a subset of the filter rods at a time to loosen particles or particulate matter deposited by the exhaust gas on an outside surface of said subset of the filter rods. By blowing gas into only a subset, instead of into all, of the filter rods at a time, the exhaust gas cleaning system may still be operated, and may not have to be shut down, during the filter rod cleaning. For example, a subset of filter rods may be the filter rods arranged along one and the same straight line.

The casing may define a dirt collection space for collecting the particles or particulate matter loosened from the outside surface of said subset of the filter rods. The dirt collection space may be arranged underneath said exhaust gas passage. Thereby, gravity may aid in collecting loose particles or particulate matter from the exhaust gas passage in said dirt collection space. Further, the dirt collection space may be tapered in a direction downwards. Thereby, gravity may aid in collecting loose particles or particulate matter in a bottom of the dirt collection space. Furthermore, the particle filter device may comprise a mechanism for opening and closing the bottom of the dirt collection space for discharge of the particulate matter. This mechanism may comprise an automatic or manual gastight outlet valve.

The filter rods may be coated or impregnated with a substance comprising at least a first catalyst. The complete filter rods, or only parts thereof, may be coated or impregnated. The first catalyst could be a reduction catalyst for promoting reduction of nitrogen oxides contained in the exhaust gas, or an oxidation catalyst for promoting oxidation of hydrocarbons contained in the exhaust gas. The substance could also comprise a second catalyst in the form of a reduction catalyst or an oxidation catalyst.

The exhaust gas cleaning system may further comprise a scrubber arranged downstream the particle filter device for cleaning the filtered exhaust gas from sulphur oxides. Additionally/alternatively, the exhaust gas cleaning system may comprise a boiler for exhaust gas heat recovery arranged downstream the particle filter device. By having the boiler arranged after the exhaust gas cleaning system, filtered exhaust gas is fed to through boiler which may result in less fouling of the surfaces of the boiler as compared to if unfiltered exhaust gas instead was fed through the boiler.

The exhaust gas cleaning system may further comprise means for occasionally introducing hot gas, instead of the exhaust gas to be cleaned by the exhaust gas cleaning system, into the exhaust gas passage for regeneration of the filter rods. These means may comprise the exhaust gas inlet, wherein the exhaust gas inlet may be arranged to allow either exhaust gas or the hot gas intended for filter rod regeneration to flow into the casing upstream said exhaust gas passage.

The method according to the invention is for cleaning exhaust gas by means of a particle filter device. The particle filter device comprises a casing and a plurality of hollow, ceramic, gas permeable filter rods arranged at least partly inside an exhaust gas passage of the casing and extending essentially vertically and along each other. The particle filter device further comprises an exhaust gas inlet arranged to allow exhaust gas to flow into the casing upstream the exhaust gas passage, an exhaust gas outlet arranged to allow exhaust gas to flow out of the casing downstream the exhaust gas passage, and a perforated plate arranged downstream the exhaust gas inlet and upstream said exhaust gas passage, which plate extends at least partly along said filter rods and partly blocks an exhaust gas flow path from the exhaust gas inlet to said exhaust gas passage. The method comprises the steps of feeding exhaust gas into the casing and feeding exhaust gas through openings of the perforated plate into the exhaust gas passage. The method further comprises the steps of filtering exhaust gas by allowing it to penetrate a respective wall of said filter rods and flow into said filter rods, releasing exhaust gas through a respective open upper end of the filter rods, and feeding exhaust gas out of the casing.

The method may comprise the step of guiding exhaust gas inside the casing by means of opposing first and second outer side sections of the perforated plate. At least a portion of the first outer side section may be bent around a vertical axis in a direction towards the filter rods.

The method may comprise the step of guiding exhaust gas inside the casing by means of an outer lower section of the perforated plate. At least a portion of the outer lower section may be bent around a horizontal axis in a direction towards the filter rods.

The method may comprise the step of feeding exhaust gas in n-<NUM> intermediate distribution channels formed inside the exhaust gas passage. The filter rods may be divided into a number n > <NUM> of groups, and a distance between adjacent ones of at least a majority, possibly all, of the filter rods within each of said groups may be < x. Further, a distance between adjacent ones of the filter rods of two adjacent ones of the groups may be > x so as to form said n-<NUM> intermediate distribution channels. Each one of said intermediate distribution channels may extend between two adjacent ones of the groups.

At least one of the intermediate distribution channels may extend in a direction away from the perforated plate.

The method may comprise the step of feeding exhaust gas in a first outer distribution channel formed inside the exhaust gas passage. A set of outer filter rods of the filter rods may be arranged on a distance from the casing so as to form said first outer distribution channel extending between said set of outer filter rods and the casing.

The first outer distribution channel may extend in a direction away from the perforated plate.

The method may comprise the step of feeding exhaust gas into the casing between two separated horizontal planes which define an extension of an upper half of the perforated plate.

The method may comprise the step of blowing gas into the open upper ends of a subset of the filter rods at a time to loosen particles deposited by the exhaust gas on an outside surface of the subset of the filter rods.

The method may comprise the step of collecting the particles loosened from the outside surface of the subset of the filter rods in a dirt collection space arranged underneath the exhaust gas passage.

The method may comprise the step of providing the filter rods with a coating or an impregnation of a substance comprising at least a first catalyst. The first catalyst may be a reduction catalyst for promoting reduction of nitrogen oxides contained in the exhaust gas, or an oxidation catalyst for promoting oxidation of hydrocarbons contained in the exhaust gas.

The method may comprise the step of feeding the filtered exhaust gas through a scrubber for cleaning it from sulphur oxides. Additionally/alternatively, the method may comprise the step of feeding the filtered exhaust gas through a boiler for recovering heat from it.

The method may comprise the step of occasionally introducing hot gas, instead of the exhaust gas to be cleaned by the exhaust gas cleaning system, into the exhaust gas passage for regeneration of the filter rods.

A use of an exhaust gas cleaning system according to the invention is for cleaning exhaust gas onboard a ship.

The above discussed advantages of the different embodiments of the exhaust gas cleaning system according to the invention are also present for the corresponding different embodiments of the method for cleaning exhaust gas and the use according to the present invention.

The invention will now be described in more detail with reference to the appended schematic drawings, in which.

<FIG> illustrates an exhaust gas cleaning system <NUM> used for cleaning exhaust gas from an engine <NUM> installed onboard a ship (not illustrated) from nitrogen oxides, sulphur oxides, hydrocarbons and particulate matter, such as black carbon. The exhaust gas cleaning system <NUM> comprises a urea supply <NUM>, a hot gas supply <NUM>, a particle filter device <NUM>, a boiler <NUM> and a scrubber <NUM>. In a first operation mode of the exhaust gas cleaning system <NUM>, the hot gas supply <NUM> is inactive while exhaust gas discharged from the engine <NUM> is fed in turn through the particle filter device <NUM>, the boiler <NUM> and the scrubber <NUM> before it is released into the atmosphere as illustrated by arrow A1. Urea from the urea supply <NUM> is injected into the exhaust gas before it is fed to the particle filter device <NUM>. When injected into the exhaust gas which is hot, the urea quickly decomposes into ammonia. The exhaust gas discharged from the particle filter device <NUM> is fed through the boiler <NUM> to recover heat from the exhaust gas. The heat recovered may, for example, be used to heat water and produce steam needed onboard the ship. The design and operation of exhaust gas heat recovery boilers are well-known and will not be described herein. The exhaust gas discharged from the boiler <NUM> is fed through the scrubber <NUM> for further removal of particulate matter but especially to clean the exhaust gas from sulphur oxides. The scrubber may be a wet scrubber of open loop type or a closed loop type or a hybrid thereof. The design and operation of exhaust gas scrubbers are well-known and will not be described herein. The exhaust gas fed through the particle filter device <NUM> is filtered to be cleaned from particulate matter. The rest of this description will be focused on the particle filter device <NUM> and the method performed by means of it.

With reference to <FIG> and <FIG>, the particle filter device <NUM> comprises casing <NUM> defining an exhaust gas reception space <NUM>, an exhaust gas passage <NUM>, an exhaust gas discharge space <NUM> and a dirt collection space <NUM>. The part of the casing <NUM> defining the dirt collection space <NUM> is made of stainless steel while the rest of the casing <NUM> is made of carbon steel. The exhaust gas discharge space <NUM> is arranged above the exhaust gas passage <NUM>, and the dirt collection space <NUM> is arranged below the exhaust gas passage <NUM>. Further, the particle filter device <NUM> comprises an exhaust gas inlet <NUM> communicating with the exhaust gas reception space <NUM>, an exhaust gas outlet <NUM> communicating with the exhaust gas discharge space <NUM>, a perforated plate <NUM>, a hole plate <NUM> and a plurality of separated elongate gas permeable hollow ceramic filter rods or pipes <NUM>.

The dirt collection space <NUM> is funnel shaped and provided with a mechanism <NUM> for opening and closing its bottom in connection with emptying of the dirt collection space <NUM>. The material, here loosened deposits, emptied from the dirt collection space <NUM> is stored in a stainless steel container <NUM> arranged underneath the dirt collection space <NUM>. To facilitate collection of the loosened deposits at the bottom of the dirt collection space <NUM> and then in the container <NUM>, the particle filter device further comprises a hammer or vibrator <NUM> arranged on the outside of the dirt collection space <NUM>.

The exhaust gas inlet <NUM> extends into a short side portion <NUM> of the casing <NUM>, at the height of a respective upper portion of the filter rods <NUM>, i.e. between two imaginary separated horizontal planes h1 and h2 defining an extension of an upper half <NUM> (<FIG>) of the perforated plate <NUM>.

The exhaust gas outlet <NUM> extends out of a short side portion <NUM> of the casing <NUM>, which side portion <NUM> is opposite to the side portion <NUM> of the casing <NUM>, above the upper imaginary horizontal plane h2.

The perforated plate <NUM> and opposing imaginary extensions e of the same, which are illustrated with dash-dot-dot lines in <FIG>, separate, or define the border between, the exhaust gas reception space <NUM> and the exhaust gas passage <NUM>. As is clear from <FIG>, the perforated plate <NUM> does not transversally extend all the way through, i.e. between two opposing vertical long side portions <NUM> of, the casing <NUM> to allow for exhaust gas to flow between the perforated plate <NUM> and the two long side portions <NUM> of the casing <NUM>. However, with reference to <FIG>, the perforated plate <NUM> longitudinally extends all the way through, i.e. between top and bottom portions <NUM> and <NUM>, respectively, of, the casing <NUM> to prevent exhaust gas to flow between the perforated plate <NUM> and the top and bottom portions <NUM> and <NUM> of the casing <NUM>. With reference to <FIG> the perforated plate <NUM> comprises a plurality of circular openings <NUM> arranged to allow for exhaust gas to flow through the perforated plate <NUM> along an exhaust gas flow path P (<FIG>). The perforated plate <NUM> is of stainless steel and comprises opposing downwards extending longitudinal first and second outer side sections <NUM> and <NUM> and a transverse outer lower section <NUM>. The outer lower section <NUM> has the shape of a trapezoid and it is folded around a horizontal axis H1 in a direction towards the filter rods <NUM>. A respective rectangular upper portion 43u and 45u of the first and second outer side sections <NUM> and <NUM> are folded towards the filter rods <NUM> and each other around a respective vertical axis V1 and V2, while a respective lower portion <NUM> and <NUM> of the first and second outer side sections <NUM> and <NUM> are folded towards each other around a respective inclined axis I1 and I2. Thereby, the perforated plate <NUM> has a trough shape which, as will be further discussed below, will promote a more uniform exhaust gas distribution in the exhaust gas passage <NUM>.

With reference to <FIG>, the perforated plate <NUM> is provided with two outer plate reinforcement projections 46a and four inner plate reinforcement projections 46b in the form of elongate flanges. The outer and inner plate reinforcement projections 46a and 46b are welded onto a surface <NUM> of the perforated plate <NUM> arranged to face the filter rods <NUM>, and they extend essentially horizontally in a direction towards the filter rods <NUM>. The outer plate reinforcement projections 46a extend along a respective long side <NUM> of the perforated plate <NUM> and project from an outer edge <NUM> thereof. The inner plate reinforcement projections 46b form a cross between the outer plate reinforcement projection 46a.

With reference to <FIG>, the hole plate <NUM> separates the exhaust gas passage <NUM> from the exhaust gas discharge space <NUM>. The hole plate <NUM> defines a plurality of circular holes <NUM> which are larger than the openings <NUM> of the perforated plate <NUM>. The filter rods <NUM> extend vertically and along each other. Each of the filter rods <NUM> has a circular cross section, an open upper end <NUM> and a closed lower end <NUM>. At and around the upper end <NUM> of each of the filter rods <NUM>, a thickness of a filter rod wall <NUM> is locally increased so as to form a flange <NUM> with an outer diameter exceeding a diameter of the holes <NUM>. Each of the filter rods <NUM> extends through a respective one of the holes <NUM> of the hole plate <NUM> such that a major part of the filter rod extends within the exhaust gas passage <NUM> and the flange <NUM> of the filter rod is arranged within the exhaust gas discharge space <NUM>. Thus, the filter rods <NUM> discharge into the exhaust gas discharge space <NUM>. The filter rods <NUM> are impregnated with a substance containing an oxidation catalyst as well as an reduction catalyst. Here, the oxidation catalyst is based on a noble metal such as palladium or platinum but any suitable oxidation catalyst is conceivable. Similarly, here the reduction catalyst is based on vanadium pentoxide in combination with titanium dioxide but any suitable reduction catalyst is conceivable.

With reference to <FIG> the filter rods <NUM> are arranged in a specific pattern. More particularly, they are divided into a first and a second group <NUM> and <NUM> of filter rods arranged on opposite sides of a horizontal center axis C of the exhaust gas passage <NUM>. In <FIG>, a total of eight rows of filter rods <NUM>, four rows on each side of the horizontal center axis C, each row containing nine filter rods <NUM>, are illustrated. However, the number of filter rod rows, and the number of filter rods in each row, can be varied endlessly. The filter rods <NUM> within one and the same group of the first and second groups <NUM> and <NUM> are arranged closer than one of the filter rods <NUM> within the first group <NUM> and one of the filter rods <NUM> within the second group <NUM>. Thereby, an intermediate distribution channel <NUM> is formed between the first and second groups <NUM> and <NUM> of filter rods <NUM>, which intermediate distribution channel <NUM> extends from a center of the perforated plate <NUM> in a direction from the perforated plate <NUM> and the exhaust gas inlet <NUM>. Further, the filter rods <NUM> within one and the same group of the first and second groups <NUM> and <NUM> are arranged closer than outer filter rods 33a of the filter rods <NUM> within the first group <NUM>, or outer filter rods 33b of the filter rods <NUM> within the second group <NUM>, and the casing <NUM>. Thereby, a first outer distribution channel <NUM> is formed between the first group <NUM> of filter rods <NUM> and the casing <NUM>, while a second outer distribution channel <NUM> is formed between the second group <NUM> of filter rods <NUM> and the casing <NUM>. The first and second outer distribution channels <NUM> and <NUM> extend in a direction from the perforated plate <NUM> and the exhaust gas inlet <NUM>, more particularly essentially parallel to the intermediate distribution channel <NUM> and the horizontal center axis C of the exhaust gas passage <NUM>.

A method for cleaning exhaust gas from the engine <NUM> is performed by means of the exhaust gas cleaning system <NUM>. As said above, with reference to <FIG>, in a first operation mode of the exhaust gas cleaning system <NUM>, exhaust gas discharged from the engine <NUM> is fed in turn through the particle filter device <NUM>, the boiler <NUM> and the scrubber <NUM> before it is released into the atmosphere. With reference to the particle filter device <NUM> and <FIG>, exhaust gas from the engine <NUM> is fed into the casing <NUM>, more particularly into the exhaust gas reception space <NUM>, via the exhaust gas inlet <NUM>, i.e. at the height of a respective upper portion of the filter rods <NUM>. Then, the exhaust gas is fed past the perforated plate <NUM> through the openings <NUM> thereof, and through the passages between the perforated plate <NUM> and the two long side portions <NUM> of the casing <NUM> (<FIG>), into the exhaust gas passage <NUM>, and especially into the intermediate and the first and second outer distributions channels <NUM>, <NUM> and <NUM> defined therein by the filter rods <NUM>. The exhaust gas conveyed through the channels <NUM>, <NUM> and <NUM> is eventually forced into the groups <NUM> and <NUM> of filter rods <NUM> because of the lower exhaust gas pressure within the filter rod groups <NUM> and <NUM>. The presence and design of the perforated plate <NUM> promote a more even distribution of the exhaust gas inside the exhaust gas passage <NUM>. Particularly, the folded first and second outer side sections <NUM> and <NUM>, respectively, enables guiding of the exhaust gas towards the filter rods <NUM>.

Inside the exhaust gas passage <NUM> the exhaust gas spreads around the filter rods <NUM>. The intermediate and first and second outer distribution channels <NUM>, <NUM> and <NUM> aid in conveying the exhaust gas away from the perforated plate <NUM> and towards the filter rods <NUM> arranged most distant from the exhaust gas inlet <NUM>. Since the exhaust gas inlet <NUM> is arranged at the same height as a respective upper portion of the filter rods <NUM>, the exhaust gas density inside the exhaust gas passage <NUM> will be higher closer to the hole plate <NUM> than more distant therefrom, and an upwards directed exhaust gas flow inside the exhaust gas passage <NUM> will be minimized.

Inside the exhaust gas passage <NUM> the exhaust gas is filtered by penetrating the walls <NUM> of the filter rods <NUM> whereby filtered exhaust gas is received inside the filter rods <NUM> and soot and particulate matter is deposited on an outside surface <NUM> of the filter rods <NUM>. The deposits on the outside surface <NUM> of the filter rods <NUM> are gradually increasing and since the exhaust gas density is higher closer to the hole plate <NUM>, so is also the amount of deposits. Thereby, the exhaust gas inside the exhaust gas passage <NUM> is gradually forced downwards for penetration of the filter rod walls <NUM>. As said above, the filter rods <NUM> are impregnated with a substance containing an oxidation catalyst as well as an reduction catalyst. Therefore, when the exhaust gas contacts the filter rods <NUM>, the nitrogen oxides contained in the exhaust gas reacts, in the presence of the reduction catalyst, with ammonia, also contained in the exhaust gas and resulting from the previously discussed decomposition of urea, which results in a degradation of the nitrogen oxides into nitrogen and water. Further, the hydrocarbons contained in the exhaust gas are burned in the presence of the oxidation catalyst for reduction of the overall soot mass and regeneration of the filter rods <NUM>.

Filtered exhaust gas cleaned from nitrogen oxides and hydrocarbons is conveyed upwards inside the filter rods <NUM> and is discharged into the exhaust gas discharge space <NUM> via the open upper ends <NUM> of the filter rods <NUM>. Thereafter, it leaves the particle filter device <NUM> via the exhaust gas outlet <NUM>.

Thus, during operation of the exhaust gas cleaning system <NUM> in the first operation mode, there is a gradual build-up of soot and particulate matter deposits on the outside surface <NUM> of the filter rods <NUM>. These deposits may eventually cause malfunctioning of the particle filter device <NUM> and they should therefore be removed before getting to thick. In view thereof, with reference to <FIG>, the particle filter device <NUM> further comprises a soot blowing arrangement <NUM> arranged inside the exhaust gas discharge space <NUM>, i.e. above the exhaust gas passage <NUM> and the filter rods <NUM>. The soot blowing arrangement <NUM> comprises one gas pipe <NUM> for each one of the eight rows of filter rods <NUM>, which gas pipe <NUM> extends parallel to the respective row of filter rods <NUM>. Further, each of the gas pipes <NUM> is provided with one nozzle <NUM> for each one of the nine filter rods <NUM> in each row of filter rods <NUM>. More particularly, each of the nozzles <NUM> is aligned with a respective one of the filter rods <NUM> and arranged to blow gas, which is fed through the respective gas pipe <NUM>, into the open upper end <NUM> of the respective filter rod <NUM>. Gas is fed to the gas pipes <NUM> by means of an arrangement not illustrated in the figures and not further described herein.

The soot blowing arrangement <NUM> is operated in response to a change in backpressure inside the particle filter device <NUM>. More particularly, the particle filter device <NUM> comprises a first pressure sensor (not illustrated) arranged inside the exhaust gas passage <NUM> and a second pressure sensor (not illustrated) arranged inside the exhaust gas discharge space <NUM>. When the difference between the pressures measured by means of the first and second pressure sensors exceeds a predetermined threshold value, this indicates that the soot and particulate matter deposits on the outside surface <NUM> of the filter rods <NUM> are starting to get too thick and that the soot blowing arrangement <NUM> should be operated. Then, short bursts of pressurized gas, e.g. air, is fed through the gas pipes <NUM>, one of the gas pipes <NUM> at a time. The pressurized gas is ejected from the nozzles <NUM> into the corresponding filter rods <NUM> to create a shock wave that causes loosening of the deposits from the outside surface <NUM> of the filter rods <NUM>. Since the soot blowing operation is made for one row of, i.e. only a subset of the, filter rods <NUM> at a time, it does not require a shut-down of the particle filter device <NUM> which may be operated normally in the meantime. The loosened deposits fall downwards by gravity, which is possible due to the minimization of the upwards directed exhaust gas flow inside the exhaust gas passage <NUM>. Eventually, the loosened deposits end up in the dirt collection space <NUM>. Since the part of the casing <NUM> defining the dirt collection space <NUM> is made of stainless steel which has relatively good "sliding" properties, and collection of the deposits at the bottom of the dirt collection space <NUM> is facilitated. Also, the provision of the hammer or vibrator <NUM> on the outside of the dirt collection space <NUM> helps to collect the deposits at the dirt collection space bottom. When the dirt collection space <NUM> needs to be emptied, the mechanism <NUM> is operated and the deposits are discharged to the container <NUM>.

Thus, in the first operation mode of the exhaust gas cleaning system <NUM>, exhaust gas from the engine <NUM> is fed through the particle filter device <NUM>, the boiler <NUM> and the scrubber <NUM> for removal of soot and particular matter, together with nitrogen oxides and hydrocarbons, and eventually also sulphur oxides, from the exhaust gas. As discussed above, during operation of the exhaust gas system <NUM> in the first operation mode, soot and particulate matter from the exhaust gas adhere to the outside surface <NUM> of the filter rods <NUM>. When the backpressure in the particle filter device <NUM> becomes too high, the soot blowing arrangement <NUM> is operated and gas is injected into the filter rods <NUM>, one filter rod row after the other. Thereby, soot and particulate matter are peeled off from the filter rods <NUM>. However, additional deeper cleaning of the particle filter device <NUM> may be necessary once in a while, to assure proper operation of the exhaust gas system <NUM>.

In view of the above, the exhaust gas system <NUM> is also arranged for operation in a second operation mode. In the second operation mode, with reference to <FIG>, the exhaust gas flow from the engine <NUM> to the particle filter device <NUM> is shut off, just like the urea flow from the urea supply <NUM>. Instead, the hot gas supply <NUM> is activated and blows hot gas through the particle filter device <NUM>. The hot gas supply <NUM> may comprise an electrical heater for heating gas. The hot gas may originate from a burner or the engine itself, possibly from before a turbocharger. The hot gas follows the same, above specified, way as the exhaust gas through the particle filter device <NUM>. After discharge from the particle filter device <NUM>, the hot gas is discharged from the exhaust gas system <NUM> as illustrated by arrow A2, or it is fed through the boiler <NUM> and possibly also the scrubber <NUM>. Inside the exhaust gas passage <NUM> the hot gas boosts the regeneration of the filter rods <NUM> by intensifying the burning of hydrocarbons remains on and in the wall <NUM> of the filter rods <NUM>. After the intensified thermal regeneration, the exhaust gas system <NUM> is again ready for the first operation mode. Switching between the first and second operation modes can be done automatically or manually.

<FIG> illustrates a particle filter device <NUM> of an exhaust gas cleaning system according to another embodiment of the invention. This particle filter device <NUM> is very similar to the particle filter device <NUM> described above with reference to <FIG>, and hereinafter, primarily the differing features will be discussed.

The filter rods <NUM> are arranged in a specific pattern. More particularly, they are divided into first, second and third groups <NUM>, <NUM> and <NUM> of filter rods. The third group <NUM> is centrally arranged inside the exhaust gas passage <NUM>, while the first and second groups <NUM> and <NUM> are arranged on opposite sides of the third group <NUM>. In <FIG>, a total of twelve rows of filter rods <NUM>, four rows in each of the groups, each row containing nine filter rods <NUM>, are illustrated. The filter rods <NUM> within one and the same group of the first, second and third groups <NUM>, <NUM> and <NUM> are arranged closer than two filter rods <NUM> within different ones of the groups. Thereby, an intermediate distribution channel 61a is formed between the first and third groups <NUM> and <NUM> of filter rods <NUM>, and an intermediate distribution channel 61b is formed between the second and third groups <NUM> and <NUM> of filter rods <NUM>, which intermediate distribution channels 61a and 61b extend in a direction from the perforated plate <NUM>. Like in particle filter device <NUM> illustrated in <FIG>, the filter rods <NUM> and the casing <NUM> of the particle filter device <NUM> illustrated in <FIG> define first and second outer distribution channels <NUM> and <NUM>.

In line with the above discussions with reference to <FIG>, the intermediate and first and second outer distribution channels 61a, 61b, <NUM> and <NUM> are arranged to convey the exhaust gas before it is force into the groups <NUM>, <NUM> and <NUM> of filter rods <NUM> to promote a more uniform exhaust gas distribution inside the exhaust gas passage <NUM>. To guide the exhaust gas into all of the intermediate and first and second outer distribution channels 61a, 61b, <NUM> and <NUM>, the perforated plate <NUM> comprises a number of guide vanes <NUM> in the form of elongate plates or flanges welded onto the perforated plate <NUM>. The guide vanes <NUM> are angled in relation to a normal direction of the perforated plate <NUM> so as to guide exhaust gas especially into the intermediate distribution channels 61a and 61b. The guide vanes <NUM> may extend along the complete, or only part of the, longitudinal extension of the perforated plate <NUM> and their design, number and position may vary depending on the prevailing circumstances, such as the size of the perforated plate <NUM> and the number of intermediate distribution channels. The guide vanes <NUM> could be arranged on a side of the perforated plate <NUM> facing away from the filter rods <NUM>, as is illustrated in <FIG>, and/or on a side of the perforated plate <NUM> facing the filter rods <NUM>. As already said, the main purpose of the guide vanes <NUM> is to promote a more even distribution of the exhaust gas around the filter rods <NUM> to improve a performance and functionality of the exhaust gas filter device <NUM>. Another purpose of the guide vanes <NUM> is to increase the stiffness of the perforated plate.

The above described embodiments of the present invention should only be seen as examples. A person skilled in the art realizes that the embodiments discussed can be varied in a number of ways without deviating from the inventive conception.

As an example, the soot blowing arrangement need not be arranged to blow gas into only one row of filter rods at a time. According to an alternative embodiment, the soot blowing arrangement is instead arranged to blow gas into all filter rods at the same time. Such an embodiment may require cessation of the exhaust gas feed through the particle filter device, which in turn could require a valve at the exhaust gas inlet and/or at the exhaust gas outlet of the particle filter device. Further, in such an embodiment it may be suitable to have multiple particle filter devices of which one is always available for exhaust gas cleaning.

As another example, which is particularly relevant for filter rods with larger diameters, such as diameters of <NUM> and more, the open upper ends of the filter rods may be provided with venturi inlets. The venturi may draw extra gas into the filter rods during soot blowing and thereby create a more powerful shock wave inside the filter rods. At the same time a possibility to use less pressurized gas is offered.

As yet another example, the soot blowing arrangement need not comprise one gas pipe and one set of nozzles for each row of filter rods. In such an embodiment one or more gas pipes, with associated nozzles, could be movable and able to blow gas into the filter rods of more than one of the rows of filter rods.

Further, the soot blowing arrangement need not comprise any nozzles. Instead, the gas could be ejected directly from holes in the pipe/pipes.

Moreover, in the above described embodiment, the soot blowing arrangement is operated when the difference between the pressures measured by means of the first and second pressure sensors exceeds a predetermined threshold value. In an alternative embodiment, the soot blowing arrangement could instead be operated with predetermined time intervals. In yet another alternative embodiment, the soot blowing arrangement could be operated with predetermined time intervals unless said predetermined threshold value is exceeded, which would shorten the time between two successive operations.

Naturally, the boiler and/or the scrubber may be left out in an exhaust gas cleaning system according to the invention. As an example, if the engine is fueled by a low sulphur fuel, then it may be possible to omit the scrubber.

The exhaust gas system according to the invention could comprise a heating device, such as an electrical heater, for heating the exhaust gas before it is fed to the particle filter device to increase the conversion of nitrogen oxides into nitrogen and water and/or the oxidation of hydrocarbons, inside the exhaust gas passage of the particle filter device. This heating device could be used also for producing hot gas for the hot gas supply which is active in the second operation mode of the exhaust gas cleaning system.

The exhaust gas system according to the invention could comprise a draft fan for overcoming the backpressure caused by the particle filter device and drawing the exhaust gas through the particle filter device, which fan could be arranged either before or after the particle filter device.

As a final example, ammonia instead of urea could be injected into the exhaust gas before it is fed to the particle filter device.

It should be stressed that the attributes first, second, third, etc. is used herein just for distinguishing purposes and not to express any kind of specific order.

Claim 1:
Exhaust gas cleaning system (<NUM>) for cleaning exhaust gas, which exhaust gas cleaning system (<NUM>) comprises a particle filter device (<NUM>), in turn comprising
a casing (<NUM>),
a plurality of hollow ceramic filter rods (<NUM>) arranged at least partly inside an exhaust gas passage (<NUM>) of the casing (<NUM>) and extending vertically and along each other when the exhaust gas cleaning system (<NUM>) is installed,
an exhaust gas inlet (<NUM>) arranged to allow exhaust gas to flow into the casing (<NUM>) upstream said exhaust gas passage (<NUM>), and
an exhaust gas outlet (<NUM>) arranged to allow exhaust gas to flow out of the casing (<NUM>) downstream said exhaust gas passage (<NUM>),
wherein the particle filter device (<NUM>) is configured to guide exhaust gas from the exhaust gas inlet (<NUM>), through said exhaust gas passage (<NUM>) and to the exhaust gas outlet (<NUM>), the particle filter device (<NUM>) further comprising
a perforated plate (<NUM>) arranged downstream the exhaust gas inlet (<NUM>) and upstream said exhaust gas passage (<NUM>), which perforated plate (<NUM>) extends at least partly along said filter rods (<NUM>) and partly blocks an exhaust gas flow path (P) from the exhaust gas inlet (<NUM>) to said exhaust gas passage (<NUM>),
wherein the perforated plate (<NUM>) defines openings (<NUM>) arranged to allow exhaust gas to flow into said exhaust gas passage (<NUM>), said filter rods (<NUM>) are gas permeable to allow exhaust gas to penetrate, during filtration, a respective wall (<NUM>) of said filter rods (<NUM>) and flow into said filter rods (<NUM>), and a respective open upper end (<NUM>) of the filter rods (<NUM>) are in communication with said exhaust gas outlet (<NUM>) so as to allow exhaust gas to leave the casing (<NUM>).