WO2020253959A1

WO2020253959A1 - A method of and assembly for purifying exhaust gas of an internal combustion piston engine

Info

Publication number: WO2020253959A1
Application number: PCT/EP2019/066254
Authority: WO
Inventors: Jan Torrkulla; Nahil SERHAN
Original assignee: Wärtsilä Finland Oy
Priority date: 2019-06-19
Filing date: 2019-06-19
Publication date: 2020-12-24

Abstract

Invention relates to a method of and apparatus for purifying exhaust gas of an internal combustion piston engine (10). The method comprising steps of leading exhaust gas generated by combustion of fuel in the engine (10) through a particulate filter (22) and collecting particulate material into the particulate filter (22), arranging circumstances in the particulate filter (22) which brings about a regeneration of the particulate filter (22) by oxidation of the particulate material by NO₂ based regeneration, and feeding reactant gas from a source of NH₃ containing gas (24) through a catalysing process where the NH₃ is oxidized forming NO₂ into the reactant gas, introducing the NO₂ containing reactant gas into the exhaust gas of the engine (10) at location upstream the particulate filter (22) and regenerating the particulate filter (22) by inducing oxidation reaction between the NO₂ containing reactant gas and carbon containing particulate material.

Description

A method of and assembly for purifying exhaust gas of an internal combustion piston engine

Technical field

[001 ] The present invention relates to a method of purifying exhaust gas of an internal combustion piston engine, comprising steps of leading exhaust gas gen erated by combustion of fuel in the engine through a particulate filter and collect ing particulate material into the particulate filter, arranging circumstances in the particulate filter which brings about a regeneration of the particulate filter by oxi dation of the particulate material by NO2 based regeneration according to the preamble of claim 1. Invention relates also to and assembly as recited in the preamble of the independent apparatus claim.

Background art

[002] Internal combustion piston engines, when combusting fuel while operat ing the engine, produce various gaseous emissions but also particulate matter which both are harmful for the environment. It is known as such to provide an internal combustion piston engine with a particulate filter. A diesel particulate filter (DPF in the following) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine. DPF removes such particulate mat ter very efficiently, virtually all, but the removed particles accumulate in the filter. Therefore, in order to avoid blockage of the filter or excessive back pressure to the engine the particulate matter needs to be removed from the DPF from time to time. Removal of the particulate material is often called as regeneration. Re generation is the process of removing the accumulated particulate matter from the filter. This is typically done either passively using the engine's exhaust heat in normal operation or by adding a catalyst to the filter, or actively introducing heat into filter by operating the engine such that unburned fuel will escape with the exhaust gas and and/or adding fuel directly to the exhaust gas which, when combusts, increases the temperature in the DPF high enough to oxidize the par ticulate matter into gaseous products. To facilitate oxidation of the surplus fuel in the exhaust gas the exhaust system may be provided with a Diesel Oxidation Catalyst (DOC in the following). The fuel and exhaust gas mixture passes through the DOC creating temperatures high enough to burn off the accumulated partic ulate matter. Once the pressure drop across the DPF lowers to a desired value, the process ends.

[003] Depending on the DPF system design and on the engine operating con ditions (exhaust gas temperature), the oxidation of carbon in soot can occur via reactions with oxygen or nitrogen dioxide. Oxygen is present in diesel exhaust at sufficient concentrations at nearly all operating conditions. However, relatively high temperatures are necessary to achieve appreciable regeneration rates with O2. Oxygen based regeneration is mostly seen in active DPF systems that do not utilize catalysts. Nitrogen dioxide based regeneration can be conducted at lower temperatures than oxygen regeneration using DOC. NO2 is an effective soot ox idizer that can ensure the passive regeneration of the DPF also at the normal exhaust temperatures. However, the NO2 concentrations must be increased, which is typically realized via catalytic oxidation of NO i.e. by increasing the NO2 : NO ratio in NO_x. Oxidation of soot by NO2 is the dominant regeneration mechanism in most catalytic (passive and active) DPF systems. In passive re generation using DOC, nitrogen monoxide NO is oxidised to nitrogen dioxide NO2 on a catalytically coated surface. This additionally generated NO2 reacts with the soot particles from temperatures as low as approx. 220°C. The soot is oxidised and the NO2 is reduced back to nitrogen monoxide NO.

[004] EP 2853707 B1 discloses an exhaust gas purification system for remov ing particulate matters and nitrogen oxides in the exhaust gas of an internal com- bustion piston engine. The engine is provided with a DPF device and an oxidation catalyst device upstream the DPF and an ammonia-based solution feeder be tween the oxidation catalyst and the DPF and a NOx selective catalytic reduction device in the exhaust system. According to the document with the suitable ar rangement of the exhaust gas purification units, the NH₃ production rate is im- proved to improve the NO_x removal rate while the temperature of the DPF device is kept high to increase the time and frequency of continuous regeneration, thus decreasing the frequency of forced regeneration of the DPF. [005] EP0758713 A1 discloses a method in which carbon particles collected by a DPF can be easily burned by NO2, thereby being removed from the DPF with out increasing the amount of NO released to the atmosphere. Nitrogen monoxide in the exhaust gas of a diesel engine is first oxidized to NO2 by an oxidizing cat- alyst. The exhaust gas containing NO2 formed by oxidation of nitrogen monoxide is fed to the DPF, and NO2 in the exhaust gas reacts with the carbon particles trapped in the DPF. When the NO2 reacts with carbon particles, carbon particles are oxidized (burned) by NO2 and removed from DPF, and at the same time, NO2 is reduced to NO by the carbon particles. The exhaust gas containing NO formed by the reaction between the carbon particles and NO2 is fed to an NOx absorbent in which NO is removed from the exhaust gas.

[006] EP 2670958 A1 discloses a method of regeneration of a particle filter in tended to treat an exhaust flow arising from combustion in a combustion engine, regeneration involves fuel being supplied to post-treatment system, and the method comprises raising a temperature of said particle filter by fuel being sup plied to said exhaust flow for oxidation in said post-treatment system, discontin uing said supply of fuel to said exhaust flow when said particle filter temperature rises to a first level, and resuming supply of fuel for oxidation to said post-treat ment system when said particle filter temperature drops to a second level which is lower than said first level. The stage of discontinuing fuel supply to the exhaust flow makes it possible for passive, NC>2-based regeneration to take place at high temperature while at the same time a large conversion of NO_x to NO2 can take place, with a resulting high regeneration rate when fuel supply is discontinued.

[007] However, in some applications high the sulphur content in the fuel is a limiting factor for use of an oxidizing catalyst. In such cases SO₂ is contained in the exhaust gas and therefore, if an oxidizing catalyst is used for oxidizing NO to NO₂, SO₂ in the exhaust gas is also oxidized by the oxidizing catalyst and forms sulfate, SO₃. Sulfate in the exhaust gas is not collected by the DPF but is released to the atmosphere. Further, since sulfate is detected as particulate matter, the amount of the diesel particulates released to the atmosphere increases due to sulfate formed by the oxidizing catalyst if the method. [008] An object of the invention is to provide a method of and an assembly for purifying exhaust gas of an internal combustion piston engine in which the per formance is considerably improved compared to the prior art solutions.

Disclosure of the Invention

[009] Objects of the invention can be met substantially as is disclosed in the independent claims and in the other claims describing more details of different embodiments of the invention.

[0010] According to an embodiment of the invention a method of purifying ex haust gas of an internal combustion piston engine comprises steps of leading exhaust gas generated by combustion of fuel in the engine through a particulate filter and collecting particulate material into the particulate filter, and arranging circumstances in the particulate filter which brings about a regeneration of the particulate filter by oxidation of the particulate material by NO2 based regenera tion. And further the method comprises feeding reactant gas from a source of NH3 containing gas through a catalysing process where the NH₃ is oxidized form ing NO₂ into the reactant gas, introducing the NO₂ containing reactant gas into the exhaust gas of the engine at location upstream the particulate filter and re generating the particulate filter by inducing oxidation reaction between the NO₂ containing reactant gas and carbon containing particulate material.

[001 1 ] Thus, the process where the NH₃ is oxidized forming NO₂ containing re actant gas comprises oxidation facilitated by a catalyst reactor through which the NH₃ containing gas is arranged to flow.

[0012] According to an embodiment of the invention the source of NH3 contain ing gas comprises a process of providing gas other than the exhaust gas of the engine having temperature of 200 - 500 Celsius degrees, injecting urea into the hot gas and evaporating the urea and producing NH3 containing gas from the urea.

[0013] According to an embodiment of the invention the reactant gas led through a first oxidation catalyst, and heat exchanger decreasing the temperature of the reactant gas and further through a second oxidation catalyst for producing NO₂ containing reactant gas.

[0014] Advantageously the temperature of the reactant gas to be less than or equal to 300 Celsius degrees when introduced into the second oxidation catalyst. [0015] According to an embodiment of the invention the reactant gas led through a first oxidation catalyst, and heat exchanger decreasing the temperature of the reactant gas and dry air at temperature of less than 50 Celsius degrees is mixed with the reactant gas and transforming NO in the NO2 containing reactant gas into NO2. [0016] According to an embodiment of the invention NH3 is oxidized into NO2 using several catalyst reactors though which the reactant gas is fed.

[0017] According to an embodiment of the invention the process includes com busting fuel having sulphur as its component wherein the exhaust gas contains sulphur oxide and the method comprises purifying sulphur oxide containing ex- haust gas, and wherein the reactant gas containing NO2 is based on gas other than the exhaust gas of the engine, advantageously on ambient air.

[0018] An assembly for purifying exhaust gas of an internal combustion piston engine comprises a particulate filter arranged in connection with an exhaust gas conduit of the engine, configured to collect particulate material from exhaust gas generated by combustion of fuel in the engine. The assembly comprises further a source of NH3 containing gas and a conduit connecting the source of NH3 con taining gas with the exhaust gas conduit of the engine at location upstream to the particulate filter, and an oxidation catalyst arranged to the conduit, configured to oxidize NH3 thus forming NO2 containing reactant gas, the oxidation catalyst ar- ranged to the conduit between the source of NH3 containing gas and the exhaust gas conduit of the engine, further comprising an outlet in the conduit for introduc ing the NO2 containing reactant gas into the exhaust gas of the engine for regen erating the particulate filter by inducing oxidation reaction between the NO2 con taining reactant gas and carbon containing particulate material in the particulate filter. [0019] According to an embodiment of the invention the assembly further com prises a source of NH3 containing gas configured to practise a process of provid ing gas other than the exhaust gas of the engine having temperature of 200 - 500 Celsius degrees and means for injecting urea into hot gas and evaporating the urea and producing NH3 containing gas from the urea.

[0020] According to an embodiment of the invention the assembly further com prises a first oxidation catalyst, and a heat exchanger and a second oxidation catalyst in the conduit for producing NO2 containing reactant gas.

[0021] According to an embodiment of the invention the assembly comprises several catalyst reactors though which the reactant gas is fed for producing NO2 containing reactant gas.

[0022] According to an embodiment of the invention the conduit connecting the source of NH3 containing gas with the exhaust gas conduit of the engine opens into the surrounding air which is used as base of the reactant gas. [0023] Invention has several benefits, of which some are mentioned in the fol lowing: there is no need for a separate burner for increasing the temperature for regenerating the DPF, possible sulphur in the fuel does not hamper the regener ation, high concentration of NO2 in the exhaust gas before the DPF can be effi ciently obtained, possible NH₃ slip can be utilized or eliminated in selective cata- lytic reduction downstream the DPF.

[0024] The exemplary embodiments of the invention presented in this patent ap plication are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as charac teristic of the invention are set forth in particular in the appended claims. Brief Description of Drawings

[0025] In the following, the invention will be described with reference to the ac companying exemplary, schematic drawings, in which

Figure 1 illustrates an assembly for purifying exhaust gas of an internal combus- tion piston engine according to an embodiment of the invention,

Figure 2 illustrates an assembly for purifying exhaust gas of an internal combus tion piston engine according to another embodiment of the invention,

Figure 3 illustrates a NO2 forming unit of an assembly for purifying exhaust gas of an internal combustion piston engine according to another embodiment of the invention,

Figure 4 illustrates a NO2 forming unit of an assembly for purifying exhaust gas of an internal combustion piston engine according to another embodiment of the invention,

Figure 5 illustrates a NO2 forming unit of an assembly for purifying exhaust gas of an internal combustion piston engine according to another embodiment of the invention,

Figure 6 illustrates a source of NH₃ containing gas of an assembly for purifying exhaust gas of an internal combustion piston engine according to another em bodiment of the invention, Figure 7 illustrates a source of NH₃ containing gas of an assembly for purifying exhaust gas of an internal combustion piston engine according to another em bodiment of the invention,

Figure 8 illustrates a source of NH₃ containing gas of an assembly for purifying exhaust gas of an internal combustion piston engine according to still another embodiment of the invention, and

Figure 9 illustrates an assembly for purifying exhaust gas of an internal combus tion piston engine according to still another embodiment of the invention. Detailed Description of Drawings

[0026] Figure 1 depicts schematically an internal combustion piston engine 10 which is of a type known as such, and is denoted as engine in the following. The engine 10 is provided with an exhaust gas conduit 12 which is configured to safely lead the exhaust gas generated by combustion of fuel in the engine to the surrounding air. The engine may be installed in a facility, such as a land based stationary power plant or in a vehicle, or in a marine vessel or other floating con struction, and the exhaust gas conduit is arranged to extend from an engine com partment through the structures of its installation facility to the outside 14 of the facility. The engine 10 and the exhaust conduit 12 include various necessary de vices in order to fulfil is intended operation, which devices are not shown here for clarity reasons.

[0027] The engine 10 is provided with an assembly 20 for purifying exhaust gas of an internal combustion piston engine. The assembly 20 comprises a particu- late filter 22, which may be referred to as a Diesel Particulate Filter DPF particu larly when used in connection with a diesel engine. Even if not shown here the assembly 20 for purifying exhaust gas usually includes also other means for pu rifying the gas. The particulate filter 22 is configured to collect particulate material from exhaust gas generated by combustion of fuel in the engine. The particulate material is mainly carbon containing material which may be referred to as soot. As is known as such in the art, particulate filter 22 will eventually fill up, which increases back pressure of the engine. The back pressure, in turn, if increases to too high level, is harmful to the engine, or at least to its performance. Therefore the particulate filter 22 needs to be regenerated to restore its acceptable opera- tion.

[0028] In the particulate filter 22, the oxidation of carbon or the soot accumulated in the particulate filter 22 occurs by NO2 based regeneration, via reactions with nitrogen dioxide added to the exhaust gas from an outside source. NO2 is an effective soot oxidizer that can ensure the passive regeneration of the particle filter also at the normal exhaust temperatures. Oxygen required for the regener ation is present in exhaust gas at sufficient concentrations at nearly all operating conditions of the engine. In nitrogen dioxide based regeneration NO2 reacts with the soot (carbon) particles from temperatures as low as approx. 220°C. The car bon is oxidised into carbon monoxide and the NO₂ is reduced back to nitrogen monoxide NO.

[0029] NO₂ concentration in the exhaust gas is increased by making use of an external source of NO₂ which is generally referred to by a numeral 21 in the fig ure 1. In this connection the term external source of NO₂ stands for that the NO₂ is not originating from the combustion process. In other words, in respect to pos sible NO₂ generated by combustion of fuel in the exhaust gas, additional NO₂ is fed to the exhaust gas from of nitrogen dioxide 21 outside the engine. In the embodiment of the figure 1 the nitrogen oxide is obtained via reactions from NH₃. The external source of nitrogen dioxide 21 comprises a source of NH₃ containing gas 24. There is a conduit 26 connecting the source of NH₃ containing gas 24 with the exhaust gas conduit 12 of the engine 10 at location upstream to the particulate filter 22. According to the invention at least a considerable portion, advantageously all, of NH₃ present in the NH₃ containing gas is converted into

NO₂ prior to feeding the gas into the exhaust gas of the engine 10.

[0030] For that purpose, there is a NO2 forming unit 28 arranged to the conduit 26 between the source of NH3 containing gas 24 and the exhaust gas conduit 12 of the engine 10 which produces NO2 containing reactant gas from the NH3 containing gas.

[0031] As is depicted in the figure 3 the NO2 forming unit 28 according to an embodiment of the invention comprises an oxidation catalyst 28.1 arranged to the conduit 26 between the source of NH3 containing gas 24 and the exhaust gas conduit 12 of the engine 10 which is configured to oxidize NH3, forming NO2 con- taining reactant gas for use in the particulate filter 22 to oxidize accumulated soot and regenerate the particulate filter 22 for reuse. The oxidation catalyst 28.1 ad vantageously comprise precious group metals, mainly platinum and or palladium. Other precious/non precious metals, including but limited to, rhodium can be also present. The catalysed reaction(s) consumes oxygen forming NO, NO2 and H2O from NH₃. [0032] Turning back to the figure 1 the conduit 26 is provided with an outlet 29 coupled to and opening into the exhaust gas conduit 12. This way the NO2 con taining reactant gas is introduced to the exhaust gas and made carried by the exhaust gas into the particulate filter 22 for regenerating the particulate filter by inducing oxidation reaction between the NO2 containing reactant gas and carbon containing particulate material.

[0033] The assembly 20 for purifying exhaust gas is operated to implement the method according to the invention in the following manner. A method of purifying exhaust gas of an internal combustion piston engine 10 comprise steps of leading exhaust gas generated by combustion of fuel in the enginelO through a particu late filter 22 in which particulate material is collected. When it is desired to regen erate the particulate filter 22 the circumstances in the particulate filter are con trolled to be such that a regeneration of the particulate filter by oxidation of the particulate material by NO₂ based regeneration will take place. The regeneration can be continuous or intermittent. The assembly 20 for purifying exhaust gas is operated such that reactant gas containing NH₃ is fed from the source of NH₃ containing gas 24 through the at least one oxidation catalyst 28.1 , to induce cat alysing process where the NH₃ is oxidized forming NO₂ containing reactant gas. The NO₂ containing reactant gas is then introduced into the exhaust gas of the engine 10 at location upstream the particulate filter 22. NO₂ containing reactant gas is conveyed along with the exhaust gas of the engine 10 to the particulate filter 22 so as to regenerate the particulate filter 22 by inducing oxidation reaction between the NO₂ containing reactant gas and carbon containing particulate ma terial accumulated in the particulate filter 22. The NO₂ containing reactant gas contains advantageous substantially clean gas, particularly sulphur oxide (SO_x) - free gas, advantageously air, in which NO₂ concentration is increased to result in NO₂ containing reactant gas suitable for regenerating the particulate filter 22.

[0034] Figure 4 shows another embodiment of the NO₂ forming unit according to the invention. As is depicted in the figure 4 the NO₂ forming unit 28 comprises a first oxidation catalyst 28.1 arranged to the conduit 26 downstream the source of NH₃ containing gas 24. Temperature of the gas entering the first oxidation cata lyst 28.1 is 200 - 500 Celsius degrees. The first oxidation catalyst 28.1 causes oxidation of NH₃ to NO and possibly including small amount of NO₂ in the reactant gas. The exact catalytic formula of the first oxidation catalyst is not critical for the application of the invention, and can be determined be simple experiments by a skilled person in the art. Next, the NO2 forming unit 28 comprises a heat ex changer 28.2 which is configured to control the temperature of the reactant gas to be suitable for a second oxidation catalyst 28.3 arranged to the conduit 26 downstream the heat exchanger 28.2. The second oxidation catalyst 28.3 causes oxidation of NO in the gas such that NO2 containing reactant gas is produced. The temperature of the reactant gas entering to the second oxidation catalyst

28.3 is cooled down, if necessary, to 200-300 Celsius degrees. The formulation of the second catalyst can be based on noble metals and metal oxides, multi metal oxide, perovskite type and carbon-based catalysts. However, the exact catalytic formula is not critical for the application of the invention, and can be determined be simple experiments by a skilled person in the art. Still, it is pre ferred to use a catalyst material formula that can yield over 90% NO-NO2 con- version in the temperature region 200-300 Celsius degrees.

[0035] Figure 5 shows still another embodiment of the NO2 forming unit 28 ac cording to the invention. As is depicted in the figure 5 the NO2 forming unit 28 comprises an oxidation catalyst 28.1 arranged downstream the source of NH3 containing gas 24. The oxidation catalyst 28.1 causes oxidation of NH3 to NO and NO2 in the reactant gas. Temperature of the gas entering the oxidation cat alyst 28.1 is 200 - 500 Celsius degrees. The oxidation catalyst comprise precious group metals, mainly platinum and or palladium. Other precious/non precious metals, including but limited to, rhodium can be also present in the catalyst. How ever, the exact catalytic formula is not critical for the application of the invention, and can be determined be simple experiments by a skilled person in the art. Next, the NO2 forming unit 28 comprises a heat exchanger 28.2 which is configured to control the temperature of the reactant gas by decreasing the temperature to about 150 Celsius degrees. Next, in the direction of the reactant gas flow the NO2 forming unit 28 comprise an oxidation tower 28.4 arranged to the conduit 26. The oxidation tower 28.4 causes oxidation of NO in the gas such that NO2 containing reactant gas is produced. The oxidation tower is coupled with a source of air 28.5 by means of which cold (temperature being 20-30 Celsius degrees) and dry (moisture content being 2- 4 gi kg_dryair) air is injected into the oxidation tower

28.4 into which the reactant gas transforming the NO into NO2 in the reactant gas. Temperature of the reactant gas after the oxidation tower 28.4 is about 50 Celsius degrees. The oxidation tower is considered as a container, such as an open tank in which the NO/air mixture will have enough time to mix and react.

[0036] Turning back to figure 1 , the source of NH3 containing gas 24 comprises a process of providing gas, separate from the exhaust gas of the engine, having temperature of 200-500 Celsius degrees, and injecting urea into the gas, evapo rating the urea and producing NH₃ containing reactant gas from the urea. The gas is advantageously air. NH₃ containing gas is fed through a catalysing process where the NH₃ is oxidized forming NO₂ into the reactant gas. This way the poi- soning of oxidation catalyst for producing NO₂ containing reactant gas can be avoided even if the engine is running with sulphur containing fuel.

[0037] Figure 6 shows in more detailed manner an embodiment of the source of NH3 containing gas 24 which comprises a urea gasification unit 24.1. The urea gasification unit 24.1 is configured to implement at least the steps of injection of the urea in the hot gas being at the temperature of 200 - 500 Celsius degrees, and mixing and evaporating the urea into the gas. The source of NH3 containing gas 24 comprises an optional hydrolysis catalyst 24.2 (preferred but not limited to titanium oxide T1O2) which can be introduced to further assist in hydrolysing the urea mixture into ammonia. [0038] Figure 7 shows in more detailed manner another embodiment of the source of NH3 containing gas 24 which comprises a storage of anhydrous liquid NH3 24.3. The conduit 26 is coupled with the storage of anhydrous liquid NH3 24.3 by means of ammonia is injected into the gas flow in the conduit 26 after compressed to suitable pressure. [0039] Figure 8 shows in more detailed manner a still another embodiment of the source of NH3 containing gas 24 which comprises a storage of NH₃ in solid state 24.4. The conduit 26 is coupled with the storage of NH₃ in solid state 24.4 by means of ammonia is injected into the gas flow in the conduit 26. In this em bodiment ammonia is stored in solid state in the form of metal ammine salts and/or ammine metal borohydrides. Storage chamber is heated by e.g. exhaust waste heat of the engine 10 assisted with electric heating to reach temperature in the region of 200-500 Celsius degrees, depending on the salts type storing the ammonia, and ammonia is released to the reactant gas flowing in the conduit 26.

[0040] Depending on the practical case anyone of the above described NO2 forming unit 28 can be used in connection with anyone of the above described source of NH3 containing gas 24. However it has been found that the most ad vantageous combinations of the embodiments are based on the source of NH3 containing gas 24 according to figure 6 combined with NO2 forming unit 28 ac cording to figure 4 or 5.

[0041] Figure 2 discloses schematically an internal combustion piston engine 10 which is called simply as an engine in the following. The engine is connected to a source of fuel 30, which may contain some amounts of sulphur. The engine 10 is provided with an exhaust gas conduit 12 which is configured to lead the ex haust gas generated by combustion of fuel in the engine to the surrounding air. Also in this embodiment the engine 10 is provided with an assembly 20 for puri- fying exhaust gas of an internal combustion piston engine 10. The assembly 20 comprises a particulate filter 22, which may be referred to as a Diesel Particulate Filter DPF particularly when used in connection with a diesel engine. The partic ulate filter 22 is configured to collect particulate material from exhaust gas gen erated by combustion of fuel in the engine. [0042] Regeneration of the particulate filter 22 is accomplished be the oxidation of carbon of the soot accumulated in the particulate filter 22, which occurs by NO2 based regeneration, via reactions with nitrogen dioxide added to the exhaust gas from an outside source. Oxygen required for the regeneration is present in ex haust gas at sufficient concentrations at nearly all operating conditions of the engine. NO2 is an effective soot oxidizer that can ensure the passive regeneration of the particle filter also at the normal exhaust temperatures. NO2 concentration in the exhaust gas is increased by making use of an external source of NO2 which is generally referred to by a numeral 21 in the figure 2 also. Additionally to what is described in the connection with the embodiment of the figure 1 the conduit 26 which transports gas is provided with one or more heat exchangers 25 configured to transfer heat from coolant of the engine 10 and/or from the exhaust gas flowing in the exhaust gas conduit 12 to the reactant gas. The heat required to heat up the gas to the desired temperature is thus at least partially recovered from engine but the conduit 26 may also be provided with additional heat exchangers, like electric heaters as well (not shown). The reactant gas is in the conduit 26 is not originating from the combustion process and therefore it does not contain sulphur oxides, even if the engine may use fuel 30 containing sulphur. However the re- actant gas is heated to suitable temperature by making use of heat produced by the engine 10. The assembly 20 for purifying exhaust gas is in other respects similar to that shown in the figure 1 and it is also operated in similar manner.

[0043] Figure 9 discloses an embodiment which corresponds to that shown in the figure 2 but being further provided with a selective catalytic reduction (SCR) unit 23 in the exhaust gas conduit 12 for NOx removal from the exhaust gas. Now, as an additional benefit of the invention the source of NH₃ containing gas 24 may be used for supplying NH₃ to the SCR unit 23. The assembly 20 for puri fying exhaust gas of an internal combustion piston engine 10 comprises a branch conduit 26’ which runs from the conduit 26, starting between the source of NH₃ containing gas 24 and N02 forming unit 28 and ending to the exhaust gas conduit 12 upstream the SCR unit 23. The SCR unit 23 may be located, contrary what is shown in the figure 9, upstream the particulate filter 23 in the direction of the exhaust gas flow, but when located downstream the particulate filter 23 in the direction of the exhaust gas flow, as is shown in the figure, the assembly 20 pro- vides an additional benefit of consuming possible NH₃ slip through the particulate filter 23 in the SCR unit 23.

[0044] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred em bodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the in vention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodi ment when such combination is technically feasible.

Claims

1. A method of purifying exhaust gas of an internal combustion piston en gine (10) comprising steps of leading exhaust gas generated by combustion of fuel in the engine (10) through a particulate filter (22) and collecting particulate material into the particulate filter (22), arranging circumstances in the particulate filter (22) which brings about a regeneration of the particulate filter (22) by oxida tion of the particulate material by NO2 based regeneration, characterized by feeding reactant gas from a source of NH₃ containing gas (24) through a catalys- ing process where the NH₃ is oxidized forming NO₂ into the reactant gas, intro ducing the NO₂ containing reactant gas into the exhaust gas of the engine (10) at location upstream the particulate filter (22) and regenerating the particulate filter (22) by inducing oxidation reaction between the NO₂ containing reactant gas and carbon containing particulate material.

2. A method of purifying exhaust gas according to claim 1 , characterized in that the process where the NH₃ is oxidized forming NO₂ containing reactant gas comprises oxidation facilitated by a catalyst reactor (28.1 , 28.3)) through which the NH₃ containing gas (24) is arranged to flow.

3. A method of purifying exhaust gas according to claim 1 , characterized in that the source of NH3 containing gas (24) comprises a process of providing re actant gas other than the exhaust gas of the engine (10) having temperature of 200-500 Celsius degrees, injecting urea into the hot gas and evaporating the urea and producing NH3 containing gas (24) from the urea.

4. A method of purifying exhaust gas according to claim 3, characterized in that the reactant gas led through a first oxidation catalyst (28.1 ), and heat ex changer (28.2) decreasing the temperature of the reactant gas and further through a second oxidation catalyst (28.3) producing NO2 containing reactant gas.

5. A method of purifying exhaust gas according to claim 4, characterized in that the reactant gas led through a first oxidation catalyst (28.1 ), and heat ex changer (28.2) decreasing the temperature of the reactant gas and dry air at temperature of less than 50 Celsius degrees is mixed with the reactant gas and transforming NO in the NO2 containing reactant gas into NO2.

6. A method of purifying exhaust gas according to claim 2, characterized in that NH3 is oxidized into NO2 using several catalyst reactors though which the reactant gas is fed.

7. A method of purifying exhaust gas according to claim 1 , characterized by combusting fuel having sulphur as its component wherein the exhaust gas contains sulphur oxide and the method comprises purifying sulphur oxide con taining exhaust gas.

8. An assembly for purifying exhaust gas of an internal combustion piston engine (10) comprising a particulate filter (22) arranged in connection with an exhaust gas conduit of the engine (10), configured to collect particulate material from exhaust gas generated by combustion of fuel in the engine (10), characterized in the that the assembly further comprises a source of NH3 containing gas (24) and a conduit connecting the source of NH3 containing gas (24) with the exhaust gas conduit of the engine (10) at location upstream to the particulate filter (22), and an oxidation catalyst arranged to the conduit, configured to oxidize NH3_, forming NO2 containing reactant gas, and arranged to the conduit between the source of NH3 containing gas (24) and the exhaust gas conduit of the engine (10), outlet in the conduit for introducing the NO2 containing reactant gas into the exhaust gas of the engine (10) for regenerating the particulate filter (22) by in ducing oxidation reaction between the N02 containing reactant gas and carbon containing particulate material.

9. An assembly for purifying exhaust gas of an internal combustion piston engine (10) according to claim 8, characterized in that the assembly further com prises a source of NH₃ containing gas (24) configured to practise a process of providing gas other than the exhaust gas of the engine (10) having temperature of 200 - 500 Celsius degrees and means for injecting urea into hot gas, and means for evaporating the urea and producing NH₃ containing gas (24) from the urea.

10. An assembly for purifying exhaust gas of an internal combustion piston engine (10) according to claim 8, characterized in that the assembly further com prises a first oxidation catalyst, and a heat exchanger and a second oxidation catalyst in the conduit for producing NO2 containing reactant gas.

1 1. An assembly for purifying exhaust gas of an internal combustion piston engine (10) according to claim 8, characterized in that the conduit opens into the surrounding air which is used as base of the reactant gas.