WO2008104070A1

WO2008104070A1 - Emission reduction system using wet scrubbing

Info

Publication number: WO2008104070A1
Application number: PCT/CA2008/000377
Authority: WO
Inventors: Erik Paul Johannes
Original assignee: Nxtgen Emission Controls Inc.
Priority date: 2007-02-27
Filing date: 2008-02-26
Publication date: 2008-09-04

Abstract

A wet scrubbing subsystem reduces atmospheric emissions of nitrogen oxides (NOx) and sulfur oxides (SOx) from an engine exhaust stream. The exhaust stream is first directed through a catalytic oxidation device that converts NO to NO2. In marine applications, the wet scrubbing subsystem can employ seawater scrubbing. The wet scrubbing subsystem can involve a one-stage or multi-stage scrubbing process. The oxidation catalysts in the catalytic oxidation device tend to become poisoned by sulfur species in the exhaust stream. If more than one catalytic oxidation device or catalyst bed is used, one device or bed can be regenerated while another continues to oxidize NO in the exhaust stream. A syngas generator can be employed to produce a syngas stream for use in regenerating the oxidation catalyst. An exhaust stream flow diverter and syngas flow distribution device can be employed to direct the various streams to enable simultaneous catalyst regeneration and exhaust after-treatment.

Description

EMISSION REDUCTION SYSTEM USING WET

SCRUBBING

Field of the Invention

5 [0001] The present invention relates to the after-treatment of combustion engine exhaust streams to reduce the amount of deleterious constituents in the exhaust streams. More particularly, the present invention relates to a combustion engine exhaust after- treatment system and method that incorporates catalytic oxidation, o wet scrubbing, and the production of syngas for catalyst regeneration.

Background of the Invention

[0002] Governments have legislated increasingly strict emissions regulations to reduce the exhaust gas emissions from5 combustion sources, including emissions of nitrogen oxides (NO_x) and sulfur oxides (SO_x). Meeting such regulations poses significant challenges, especially for marine vessels which typically use lean burn combustion engines, lower grades of fuel, and propulsion systems that are customized for a particular vessel o type and application. For marine applications, size, weight, durability and maintenance are some of the considerations when selecting an on-board emissions reduction system.

[0003] In lean burn combustion engines, NO_x, SO_x and particulate matter are typically formed during the combustion 5 process. Typically the NO_x comprises 70% - 95% nitric oxide (NO) and a portion of nitrogen dioxide (NO₂). The sulfur content of the fuel (for example, sulfur in diesel or natural gas) results in the formation of SO_x.

[0004] There is a large potential for improvements in emissions reduction with advances in exhaust after-treatment technologies. Several exhaust after-treatment technologies include selective catalytic reduction (SCR), lean NO_x traps (LNT), also known as NO_x adsorbers, and seawater scrubbers or, more generally, wet gas scrubbers.

[0005] Selective catalytic reduction (SCR) exhaust after- treatment systems have been used for NO_x reduction in large stationary applications where the engines are operated in a relatively steady state condition. Ammonia (NH₃) or aqueous urea, (NH₂)₂CO, is commonly used as the SCR reducing agent. Use of urea, as a precursor to ammonia, is preferred over direct use of ammonia as urea is non-toxic and safe to handle. Urea is sprayed into the SCR catalytic bed in an amount that essentially matches the amount OfNO_x to be reduced. The urea breaks down into ammonia components and then chemically reduces the NO_x which is converted primarily into N₂, O₂ and H₂O.

[0006] For mobile applications, engines can be operated under transient duty cycles resulting in highly variable exhaust mass flows, exhaust temperatures and NO_x concentrations. The shortcoming of using of urea as a reducing agent in mobile SCR exhaust after-treatment systems include:

[0007] (a) It is effective for reducing NO_x emissions only.

[0008] (b) There is a requirement for an additional liquid (aqueous urea) to be stored and maintained onboard the vessel.

[0009] (c) Additional space is occupied by the SCR system and urea storage.

[0010] (d) The additional weight of the SCR system and urea o storage reduces the usable load capacity of the vessel.

[0011] (e) There is a limited distribution infrastructure for urea.

[0012] (f) The durability of catalyst is low due to sulfur 5 poisoning at high sulfur levels.

[0013] (g) Precise control of the urea injection rate is required. Insufficient injection can result in low NO_x conversions, while excess injection can result in the undesirable release of ammonia to the o atmosphere. - A -

[0014] (h) There can be formation of undesirable byproducts, for example, explosive ammonium nitrate (NH₄NO₃) if the NH₃/NO_X molar ratio is >1 and the reaction temperatures fall below 200⁰C.

[0015] (i) There is potential for regulatory non-compliance if the urea supply is not properly maintained. The exhaust after-treatment system is ineffective without a proper supply of urea. Depending on the safeguards that are incorporated into the system, operators can inadvertently or, in some circumstances deliberately, continue to operate the engine after the urea supply has been consumed.

[0016] Lean NO_x traps (LNT) operate by adsorbing nitrogen oxides (NO_x) from the engine exhaust during lean (excess oxygen) exhaust conditions and desorbing and catalytically converting NO_x into nitrogen (N₂) during rich (excess fuel) exhaust conditions. The rich exhaust conditions required for regeneration of the LNT can be created by introducing a reducing agent into the engine exhaust stream. This can be achieved through several methods, for example, injecting excess fuel into the combustion chamber during or after the engine combustion process, injecting fuel directly into the engine exhaust stream, or using an on-board fuel reformer to create a gas mixture comprising hydrogen (H₂) and carbon monoxide (CO) (referred to as syngas) and introducing it into the engine exhaust stream to regenerate the LNT. [0017] Lean NO_x traps typically contain an oxidation catalyst, for example platinum (Pt); an adsorbent, for example barium oxide (BaO); and a reduction catalyst, for example rhodium (Rh). The exhaust stream of a lean burn combustion engine can contain concentrations of sulfur species, originating from the engine fuel and oil. As the engine exhaust flows through the LNT, the sulfur species are preferentially adsorbed over NO_x, occupying available adsorbent sites and "poisoning" the catalyst. Methods to effectively desulfate the LNT are being improved, but sulfur o poisoning is still generally an issue when the fuel contains high levels of sulfur.

[0018] Seawater scrubbing is a prior approach to reducing atmospheric emissions of sulfur oxides (SO_x) from thermal power stations, metal smelters, oil refineries and marine propulsion5 systems. During this process, seawater is typically pumped and introduced into the combustion exhaust stream. The acidic SO_x gas is absorbed and neutralized by the natural alkalinity of the seawater to produce sulfates. Sulfates occur naturally in seawater and the additional levels from the scrubbing process have no o environmental impact. The seawater is separated or knocked out of the exhaust stream and returned back to the sea. Nitrogen oxides (NO_x) are also absorbed to some extent by the seawater scrubbing process, so seawater scrubbing offers the potential advantage of reducing the atmospheric emissions of both NO_x and5 SO_x. [0019] The primary shortcoming in using seawater scrubbing for NO_x reduction is that the seawater can readily absorb nitrogen dioxide (NO₂), but not nitric oxide (NO). Since the majority of NO_x from a diesel engine is NO and not NO₂, the NO needs to be converted into NO₂ before it can be effectively absorbed by the seawater and converted to nitrates.

Summary of the Invention

[0020] In the present approach, a wet scrubbing subsystem is used to reduce the atmospheric emissions of both nitrogen oxides (NO_x) and sulfur oxides (SO_x) from an engine exhaust stream. To improve the removal OfNO_x, the engine exhaust stream is first directed through a catalytic oxidation device that converts NO to NO₂. In marine applications the wet scrubbing subsystem can conveniently employ seawater scrubbing. The wet scrubbing subsystem can involve a one-stage or multi-stage scrubbing process. The oxidation catalysts in the catalytic oxidation device tend to become poisoned by sulfur species in the engine exhaust stream that originate from the fuel source, so it is desirable to periodically regenerate the device. If more than one catalytic oxidation device or catalyst bed is used, it is possible to regenerate one device or bed while another continues to perform oxidation of NO in the engine exhaust stream. A syngas generator can be employed to produce a syngas stream that can be used to regenerate the oxidation catalyst. An exhaust stream flow diverter and syngas flow distribution device can employed to direct the various streams to enable simultaneous catalyst regeneration and exhaust after-treatment. A control module can be employed to control the various devices within the exhaust after-treatment system.

[0021] So, embodiments of a method of operating an engine system comprising an engine, an exhaust after-treatment system and at least one wet scrubbing device, comprise:

[0022] (a) directing a fuel stream from a fuel supply to the engine, directing an air stream to the engine, and o operating the engine to produce an engine exhaust stream;

[0023] (b) directing at least a portion of the engine exhaust stream to at least one catalytic oxidation device in the exhaust after-treatment system, thereby 5 converting nitric oxide (NO) into nitrogen dioxide

(NO₂);

[0024] (c) directing at least a portion of the engine exhaust stream from the catalytic oxidation device to the at least one wet scrubbing device in the exhaust after- o treatment system, for reducing the NO_x and SO_x emissions from the engine system.

[0025] The wet scrubbing device can comprise a seawater scrubber. In some embodiments there is a first and second seawater scrubber, and the method comprises directing the engine exhaust gas from the catalytic oxidation device through the first seawater scrubber and then through the second seawater scrubber.

[0026] The method can further involve operating a syngas generator to produce a syngas stream. At least a portion of the syngas stream can be at least periodically directed to the catalytic oxidation device to regenerate the device. If the engine system comprises more than one catalytic oxidation device (for example, a first and second catalytic oxidation device) the method can comprise selectively directing syngas from the syngas generator to the first catalytic oxidation device while at the same time directing the at least at portion of the engine exhaust stream to the second catalytic oxidation device and vice versa so that it is possible to regenerate one device while another continues to perform oxidation of NO in the engine exhaust stream.

[0027] The syngas generator is supplied with a fuel and an oxidant stream, and in some embodiments a water-containing stream. The fuel can conveniently be supplied from the same fuel supply that is used to supply the engine. The oxidant stream can comprises air and/or at least a portion of the engine exhaust stream.

[0028] Embodiments of a combustion engine and exhaust after- treatment system comprise: [0029] (a) an engine and a fuel tank and fuel line for directing fuel to the engine;

[0030] (b) an engine exhaust stream line connected to receive an exhaust stream from the engine;

[0031] (c) at least one catalytic oxidation device fluidly connected to at least periodically receive engine exhaust from the engine via the engine exhaust stream line, to produce a treated engine exhaust stream;

[0032] (d) at least one wet scrubbing device located downstream of the at least one catalytic oxidation device and fluidly connected to receive the treated engine exhaust from the at least one catalytic oxidation device.

[0033] The at least one wet scrubbing device can comprise a seawater scrubber, and in some embodiments comprises at least a first and second seawater scrubber, wherein the first seawater scrubber is connected to receive the treated engine exhaust from the at least one catalytic oxidation device, and the second seawater scrubber is connected to receive the treated engine exhaust from the first seawater scrubber.

[0034] The combustion engine and exhaust after-treatment system can further comprise a syngas generator fluidly connected to at least periodically supply syngas to each the catalytic oxidation device. The system can comprise at least a first and second catalytic oxidation device, and optionally at least one exhaust stream flow diverter for selectively directing the exhaust 5 stream to the first or second catalytic oxidation device. The system can comprise at least one syngas flow distribution device for selectively directing syngas to the first or second catalytic oxidation device.

Brief Description of the Drawing(s) o [0035] FIG. 1 is a schematic diagram of an embodiment of a diesel engine system having an exhaust after-treatment system that employs oxidation catalysts, a syngas generator and seawater scrubbers.

Detailed Description of Preferred Embodiment(s) 5 [0036] In the present system and method, a combustion engine system employs an after-treatment system that results in reduced atmospheric emissions of nitrogen oxides (NO_x) and sulfur oxides (SO_x). The present system and method comprise:

[0037] (a) a combustion engine with a fuel intake and air o intake, the engine producing an exhaust gas stream comprising NO_x and SO_x; [0038] (b) one or more catalytic oxidation devices for oxidizing the nitric oxide (NO) in the engine exhaust stream to nitrogen dioxide (NO₂); and

[0039] (c) a wet scrubbing subsystem that employs one or more wet scrubbers (such as seawater scrubbers) for introducing water into the engine exhaust stream downstream of the catalytic oxidation device. The wet scrubbing subsystem supplies, meters, and introduces the water into the engine exhaust stream at one or more locations. The wet scrubber absorbs NO₂ and SO_x from the engine exhaust stream, and then releases the water.

[0040] In addition, preferred embodiments of the present system and method can further comprise some or all of the following:

[0041] (d) a syngas generator that produces and supplies a syngas stream, as a regenerating agent, to the one or more catalytic oxidation devices for regeneration of the catalyst;

[0042] (e) at least one flow distribution device to direct a regenerating agent to the one or more catalytic oxidation devices at the appropriate time;

[0043] (f) an exhaust stream flow diverter to direct the engine exhaust gas stream to the appropriate exhaust after-treatment device at the appropriate time;

[0044] (g) a control module that employs one or more controllers and software for controlling various components of the exhaust after-treatment system, such as the syngas generator system, exhaust gas flow diverter system, wet scrubbing subsystem, water supply and water exhaust system.

[0045] FIG. 1 is a schematic illustration of a marine diesel engine system 10 comprising an exhaust after-treatment system that employs two catalytic oxidation devices and a pair of seawater scrubbers to remediate the engine exhaust stream, and a syngas generator to produce syngas to regenerate the catalyst beds in the catalytic oxidation devices. In engine system 10 of FIG. 1, fuel tank 11 supplies diesel fuel via fuel conduit 12 and fuel conduit 13 to combustion engine 14. Combustion engine 14 can be, for example, a reciprocating piston engine which comprises one or more cylinders, a Wankel rotary engine comprising one or more rotors, a gas turbine, or another type of engine. Ambient air is drawn through an air intake subsystem (not shown in FIG. 1) and is introduced into engine 14. The fuel and air mixture combusts in engine 14, creating mechanical and heat energy. The engine exhaust stream is then expelled from engine 14 via exhaust manifold 15, to optional turbo-compressor 16, and then via exhaust conduit 17 to flow diverter 18. [0046] In FIG. 1, engine exhaust gas flow diverter 18 receives control signals from control module 42 (control lines not shown in FIG. 1) and typically operates in one of three positions at a given time. In the first position, diverter 18 directs the engine exhaust stream via conduit 20 to catalyst bed 22, while limiting or preventing the exhaust stream from flowing via conduit 19 to catalyst bed 21. In catalyst bed 22, NO in the exhaust stream is oxidized to NO₂. At the same time catalyst bed 21 can be regenerated. The treated exhaust stream then flows from catalyst bed 22 via conduit 23 into primary scrubber 24. In the second diverter position, which is employed for a limited transitional period, flow diverter 18 allows the engine exhaust stream to flow via conduit 19 to catalyst bed 21, and via conduit 20 to catalyst bed 22. The treated exhaust stream then flows from catalyst bed 21 and catalyst bed 22 via conduit 23 into primary scrubber 24. In the third diverter position, diverter 18 directs the engine exhaust stream via conduit 19 to catalyst bed 21, while limiting or preventing the exhaust stream from flowing via conduit 20 to catalyst bed 22. In catalyst bed 21, NO in the exhaust stream is oxidized to NO₂. At the same time catalyst bed 22 can be regenerated. The treated exhaust stream flows from catalyst bed 21 via conduit 23 into primary scrubber 24. An optional NO_x sensor (not shown in FIG. 1) can be located in exhaust conduit 23, to aid in the control of flow diverter 18. [0047] As the treated engine exhaust stream flows through primary scrubber 24, seawater is introduced and sprayed into the exhaust stream via first seawater conduit 39. The cold seawater mixes with and cools the exhaust stream, adsorbing the NO_x (which has largely been catalytically oxidized to NO₂ in bed 21 and/or 22) and SO_x from the stream. Primary scrubber 24 then separates the seawater from the exhaust stream, draining the seawater back into the sea through seawater drain conduit 40. The exhaust stream exits primary scrubber 24 and flows via conduit 25 into main scrubber 26 for the secondary scrubbing process. Seawater is introduced and sprayed into the exhaust stream via second seawater conduit 38. The seawater mixes with and further cools the exhaust stream, further adsorbing the NO₂ and SO_x from the stream. Particulate matter is also captured in the seawater in scrubbers 24 and 26. Main scrubber 26 then separates the seawater from the exhaust stream, draining the seawater back into the sea through seawater drain conduit 41. The treated and scrubbed exhaust stream is then released to the surrounding environment via exhaust conduit 27.

[0048] Seawater is drawn from the sea through seawater intake conduit 36 and seawater pump 37. The seawater stream is directed to the primary and main scrubbers 24 and 26 through seawater conduit 39 and seawater conduit 38, respectively. Additional optional flow control, distribution devices and sensors are not shown in FIG. 1. Such flow control devices can receive control signals from control module 42 to meter the flow of seawater to each of the scrubbers 24 and 26.

[0049] In FIG. 1, fuel tank 11 also supplies diesel fuel to syngas generator 31 via fuel conduit 12, metering pump 28 and fuel conduit 29. Metering pump 28 receives control signals from control module 42. The engine exhaust stream exits combustion engine 14 into exhaust manifold 15. At least a portion of the engine exhaust stream can be directed to syngas generator 31 via exhaust conduit 30. Syngas generator 31 mixes and converts the fuel and engine exhaust stream into product syngas. Product syngas exits syngas generator 31 and flows via syngas conduit 32 to syngas flow distribution valve 33. Syngas flow distribution valve 33 receives control signals from control module 42, which then directs the flow of syngas to either syngas conduit 35 and catalyst bed 21 , or syngas conduit 34 and catalyst bed 22, to regenerate the catalyst bed.

[0050] Details of some of the subsystems illustrated in FIG. 1 as well as other possible embodiments are now described in more detail below.

[0051] Turning first to the catalytic oxidation devices, in the present system and method, at least one oxidizing catalyst bed is employed to convert the nitric oxide (NO) in the engine exhaust gas stream into nitrogen dioxide (NO₂). The increase in the amount OfNO₂ increases the NO_x absorption capacity of the downstream seawater scrubbing process, thereby reducing the level of atmospheric NO_x emissions. The oxidation catalysts can, for example, comprise a platinum (Pt) catalyst and an alumina (AI₂O₃) carrier on a monolith substrate. Alternatively, a catalyst formulation comprising one or more platinum group metals such as platinum (Pt) or rhodium (Rh) can be employed. The catalyst composition, catalyst particle size, catalyst dispersion, substrate geometry, and other catalyst bed parameters can be optimized for NO oxidation, catalyst regeneration and low pressure loss across the catalyst bed. In some embodiments the temperature of the catalyst bed is maintained within a desired range, for example 350° ± 50⁰C, during the exhaust stream oxidation process and normal engine duty.

[0052] Preferably at least two oxidizing catalyst beds are employed, so that one can be actively oxidizing NO to NO₂ while the other is being regenerated. In preferred embodiments an exhaust gas flow diverter directs the engine exhaust stream through one catalyst bed while limiting or stopping the flow to the other. Under some operating conditions, the engine exhaust stream can be directed through more than one catalyst bed simultaneously. The flow diverter can be controlled by a control module which can switch the catalyst beds between operating to oxidize NO and being regenerated, for example, based on preprogrammed algorithms, or in response to monitored parameters in the system. [0053] Various configurations can be used for the two or more catalyst beds. For example, multiple catalyst beds can be contained in a single housing or enclosure, with internally separated conduits. The enclosure can be rotated, exposing at least one catalyst bed to the engine exhaust stream allowing for NO oxidation, while one or more other catalyst beds are simultaneously regenerated. Alternatively a rotating gas distribution manifold can be used in conjunction with a stationary single enclosure configuration containing multiple catalyst beds. The rotating gas distribution manifold can switch the catalyst beds between the NO oxidation process and regeneration process.

[0054] As the engine exhaust stream flows through the catalyst bed, sulfur originating from the engine fuel and oil can be converted into sulfur compounds such as, for example, sulfur trioxide (SO₃) and/or sulfuric acid (H₂SO-O, which are generally preferentially adsorbed by the oxidation catalyst. In preferred embodiments, syngas is periodically introduced into the catalyst bed to regenerate the catalyst, restoring its desired performance. Preferably the syngas is introduced within a specific range for example, 0.95 to 1.0 stoichiometry, or under a slight oxygen deficient condition. If the syngas introduction rate is below a threshold value, the regeneration rate can be reduced. If the syngas introduction rate and stoichiometry exceeds a threshold value, hydrogen sulfide (H₂S) that can poison the catalyst can form. Generally it is advantageous if the temperature of the catalyst bed is increased during the regeneration process, for example to within the range 450° to 500⁰C, to increase the regeneration rate. The temperature of the catalyst bed can be increased and controlled by the metering the amounts of syngas and engine exhaust gas supplied to bed.

[0055] Although the use of syngas is preferred, the catalyst beds can be regenerated using other regenerating methods, agents and/or combinations thereof. For example, thermal regeneration without the use of a regenerating agent can be used. Hydrogen (H₂) or carbon monoxide (CO) alone, or various other gaseous or liquid fuels or hydrocarbons can be used as the regenerating agent.

[0056] For mobile applications the supply of syngas (or other regenerating agent) is preferably generated on-board, due to challenges related to on-board storage, maintenance and the current absence of a re-fueling infrastructure for syngas or other suitable regenerating agents. This can be accomplished with the use of a fuel processor, such as a syngas generator.

[0057] In preferred embodiments of the present system and method, a syngas generator (SGG) converts a fuel and a portion of the engine exhaust stream into syngas. The engine exhaust stream typically contains oxygen (O₂), water (H₂O), carbon dioxide (CO₂), nitrogen (N2) and sensible heat, which can be useful for the production of syngas. Alternatively, an air stream can be supplied to the SGG in combination with or in lieu of the engine exhaust stream. The air stream can be supplied from an air compressor which supplies inlet air to the engine, for example turbo- compressor or supercharger, or from a separate air supply subsystem with an air compressor (not shown in FIG. 1). The fuel used by the SGG can conveniently be the same fuel that is used in the combustion engine. A different fuel can be used, although this would generally require a separate secondary fuel source and supply system specifically for the SGG. The SGG design is preferably non-catalytic, and the SGG is preferably operated continuously while the combustion engine is operating to reduce thermal cycling of the SGG and to reliably provide syngas when needed. The syngas generator can alternatively be a catalytic type of reformer such as a partial oxidation or auto-thermal reformer. Seawater can be distilled to supply water to an auto-thermal type reformer in marine applications.

[0058] In preferred embodiments a syngas flow distribution device, such as a valve, is employed to distribute the product syngas from the SGG to the various catalyst beds (and optionally to other components of the system) at the appropriate time. The SGG and syngas flow distribution device can be controlled by a control module.

[0059] Turning now to the wet scrubbing subsystem. A one- stage scrubbing process involving a single seawater scrubber can be employed, however in preferred embodiments of the present system and method, a two-stage seawater scrubbing process is employed to reduce the amount OfNO_x and SO_x in the engine exhaust stream. The exhaust stream is directed through a primary seawater scrubbing process before proceeding onto a secondary or main seawater scrubbing process. Seawater is pumped from the sea, optionally metered, and sprayed into the engine exhaust stream to increase the contact area between the seawater and exhaust gas, thereby increasing the heat transfer and gas absorption rates. The seawater can be metered and introduced at a rate which cools the exhaust gas stream to within a temperature range that enhances the NO_x and SO_x absorption rate while reducing the formation of visible fog released into the atmosphere. Reducing the exhaust gas temperature using the seawater offers several other advantages. For example, it can reduce the exhaust system volume, reduce the temperature of the exhaust conduits lessening the potential of accidents and injuries, reduce the tendency for corrosion in the exhaust gas system, and reduce the exhaust gas infrared signature. The seawater scrubber also functions to separate the introduced seawater from the exhaust stream, allowing the seawater to be returned to the sea. A seawater scrubber can also function as a silencer, providing noise attenuation. A control module can be employed to control the seawater introduction rate into the engine exhaust stream in each scrubber. A metering pump can be used to meter the amount of seawater introduced into the exhaust gas stream in each scrubber. [0060] In many applications seawater supply is abundant and convenient, and the natural alkalinity of seawater assists in the absorption and conversion of SO_x and NO₂. However, other wet scrubbing devices and techniques can be used instead of seawater scrubbers.

[0061] In preferred embodiments of the present system and method, a control module with various sensors and preprogrammed control logic is employed to control the exhaust after- treatment system and devices. The control module can control various functions, for example, the operation of the syngas generator, the rate of the syngas generation, distribution of product syngas, distribution of the engine exhaust stream, regeneration of catalyst beds, the seawater scrubbing water introduction rate, and seawater release. The control module can employ various sensors such as, temperature, pressure, flow rate, location, speed, nitric oxide (NO) and other gas sensors. Alternatively, there can be more than one control module employed to control various aspects of the exhaust after-treatment system. For example separate and/or individual control modules could be used to control the seawater supply and flow control, syngas generator, syngas flow distribution device and engine exhaust gas diverter.

[0062] In preferred embodiments, at least some of the exhaust after-treatment components are made from titanium or a titanium alloy, offering an increase in resistance to corrosion. [0063] The combustion engine system can comprise a combustion engine or burner, for example an internal combustion engine or a gas turbine. The combustion engine system may or may not employ a turbo-compressor, supercharger or exhaust gas recirculation (EGR).

[0064] The combustion engine can be fueled by diesel, gas oil, marine gas oil, intermediate fuel oil, fuel oil, residual fuel oil, gasoline, kerosene, natural gas, liquid propane gas (LPG), jet fuel, coal or other suitable fuels, depending on the application.

[0065] Additional exhaust after-treatment devices can be employed in the system, such as a diesel oxidation catalyst device (DOC) for reducing the level of CO and unburned hydrocarbon, and/or a diesel particulate filter (DPF) for reducing particulate matter in the exhaust stream. Syngas can optionally be directed to the DOC and/or DPF to enhance the effectiveness of the devices at various times, for example, by elevating the temperature of the device to the desired operating temperature.

[0066] The present system and method is particularly suited for use in marine propulsion and/or power generation applications (as used herein, reference to "marine applications" includes freshwater applications as well sea- or ocean-based applications). However, embodiments of the present system and method can be used in other industries, for example stationary power generation, metal smelting and oil refining processes and in other motive and stationary applications for example power generation, off shore oil, gas drilling or production platforms.

[0067] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.

Claims

What is claimed is:

1. A method of operating an engine system comprising an engine, an exhaust after-treatment system and at least one wet scrubbing device, the method comprising: (a) directing a fuel stream from a fuel supply to said engine, directing an air stream to said engine, and operating said engine to produce an engine exhaust stream;

(b) directing at least a portion of said engine exhaust stream to at least one catalytic oxidation device in said exhaust after-treatment system, thereby converting nitric oxide (NO) into nitrogen dioxide (NO₂);

(c) directing at least a portion of said engine exhaust5 stream from said catalytic oxidation device to said at least one wet scrubbing device in said exhaust after- treatment system, for reducing the NO_x and SO_x emissions from said engine system.

2. The method of claim 1 , wherein said at least one wet o scrubbing device comprises a seawater scrubber.

3. The method of claim 1 , wherein said at least one wet scrubbing device comprises at least a first and second seawater scrubber, said method comprising in step (c) directing said engine exhaust gas from said catalytic oxidation device through said first seawater scrubber and then through said second seawater scrubber.

4. The method of claim 1 , wherein said exhaust after- treatment system further comprises a syngas generator that is operated to produce a syngas stream.

5. The method of claim 4, wherein at least a portion of said syngas stream is at least periodically directed to said catalytic oxidation device to regenerate said device.

6. The method of claim 5, wherein the temperature of said catalytic oxidation device is increased to 450° - 500⁰C when syngas is directed to said device.

7. The method of claim 5, wherein said engine system comprises at least a first and second catalytic oxidation device, said method comprising selectively directing syngas from said syngas generator to said first catalytic oxidation device while at the same time directing said at least at portion of said engine exhaust stream to said second catalytic oxidation device.

8. The method of claim 4, wherein at least a portion of said engine exhaust stream is directed to said syngas generator.

9. The method of claim 4, wherein an air stream is supplied to said syngas generator.

10. The method of claim 4, wherein fuel from said fuel supply is directed to said syngas generator.

11. The method of claim 4, wherein a water-containing stream is supplied to said syngas generator.

12. The method of claim 7, wherein said exhaust after- treatment system further comprises at least one exhaust stream flow diverter.

13. The method of claim 12, wherein said exhaust stream flow diverter device is operated to selectively direct at least ao portion of said engine exhaust stream to said first and second catalytic oxidation devices.

14. The method of claim 7, wherein said exhaust after- treatment system further comprises at least one syngas flow distribution device. 5

15. The method of claim 14, wherein said syngas flow distribution device is operated to direct at least a portion of said syngas stream to said first and second catalytic oxidation catalyst devices.

16. The method in claim 1 , wherein said exhaust after- o treatment system further comprises at least one control module device.

17. The method in claim 16 wherein said control module is operated to sense and control said exhaust after-treatment system and devices.

18. The method of claim 1 , wherein said engine system is on-board a marine vessel.

19. The method of claim 18, wherein said engine system is used for propulsion of said vessel.

20. A combustion engine and exhaust after-treatment system comprising: (a) an engine and a fuel tank and a fuel line for directing fuel to said engine;

(b) an engine exhaust stream line connected to receive an exhaust stream from said engine;

(c) at least one catalytic oxidation device fluidly connected to at least periodically receive engine exhaust from said engine via said engine exhaust stream line, to produce a treated engine exhaust stream;

(d) at least one wet scrubbing device located downstream of said at least one catalytic oxidation device and fluidly connected to receive said treated engine exhaust from said at least one catalytic oxidation device.

21. The combustion engine and exhaust after-treatment system of claim 20, wherein said at least one wet scrubbing device comprises a seawater scrubber.

22. The combustion engine and exhaust after-treatment

5 system of claim 20, wherein said at least one wet scrubbing device comprises at least a first and second seawater scrubber, wherein said first seawater scrubber is connected to receive said treated engine exhaust from said at least one catalytic oxidation device, and said second seawater scrubber is connected to receive said o treated engine exhaust from said first seawater scrubber.

23. The combustion engine and exhaust after-treatment system of claim 20, wherein said exhaust after-treatment system further comprises a syngas generator fluidly connected to at least periodically supply syngas to each said catalytic oxidation device. 5

24. The combustion engine and exhaust after-treatment system of claim 23, wherein said system comprises at least a first and second catalytic oxidation device.

25. The combustion engine and exhaust after-treatment system of claim 24, wherein said exhaust after-treatment system o further comprises at least one exhaust stream flow diverter for selectively directing said exhaust stream to said first or second catalytic oxidation device.

26. The combustion engine and exhaust after-treatment system of claim 25, wherein said exhaust after-treatment system further comprises at least one syngas flow distribution device for selectively directing syngas to said first or second catalytic

5 oxidation device.

27. The combustion engine and exhaust after-treatment system of claim 20, wherein said exhaust after-treatment system further comprises at least one control module.

28. The combustion engine and exhaust after-treatment i o system of claim 20, wherein said exhaust after-treatment system comprises at least one component made from titanium or a titanium alloy.

29. A marine vessel comprising the combustion engine and exhaust after-treatment system of claim 20.

15 30. The marine vessel of claim 29, wherein said combustion engine is configured to propel said vessel.