Abstract:
The present disclosure refers to a method and apparatus for recirculating an exhaust gas flow of a large internal combustion engine having an air inlet and an exhaust gas outlet. The method may comprise the steps of diverting a first partial exhaust gas flow at the exhaust gas outlet of the large internal combustion engine; cooling the exhaust gas of the first partial exhaust gas flow; compressing the cooled first partial exhaust gas flow; cooling the compressed exhaust gas of the first partial exhaust gas flow; and supplying the cooled and compressed first partial exhaust gas flow to the air inlet of the large internal combustion engine.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure refers to an exhaust gas recirculation system configured to be used in internal combustion engines, e.g., large internal combustion engines having an air inlet and an exhaust gas outlet, operated with fuels such as e.g. heavy fuel oils, and the disclosure refers to a large internal combustion engine comprising an air inlet, an exhaust gas outlet, and an exhaust gas recirculation system. Furthermore, the present disclosure refers to a method for recirculating an exhaust gas flow of an internal combustion engine having an air inlet and an exhaust gas outlet. 
       BACKGROUND 
       [0002]    Generally, the terminology “large internal combustion engine” used herein may refer to internal combustion engines which may be used as main or auxiliary engines of ships/vessels such as cruiser liners, cargo ships, container ships, and tankers, or in power plants for production of heat and/or electricity. In particular, large internal combustion engines may be configured to burn at least one fuel selected from the group consisting of diesel and heavy fuel oil (HFO). 
         [0003]    Heavy fuel oil may contain “asphaltenes”. Asphaltenes may be defined as molecular substances that are found in crude oil, along with resins, aromatic hydrocarbon, and alkane (i.e., saturated hydrocarbons). Asphaltenes may consist primarily of carbon, hydrogen, nitrogen, oxygen, and sulphur, as well as trace amounts of vanadium and nickel. The C:H ratio may be approximately 1:1.2, depending on the asphaltene source. 
         [0004]    Asphaltenes may also be defined operationally as the n-heptane (C 7 H 16 ) insoluble, toluene (C 6 H 5 CH3) soluble component of a carbaonaceous material such as crude oil, bitumen or coal. Asphaltenes have been shown to have a distribution of molecular mass mlecular mass in the range of 400 atomic mass unit to 1500 u with a maximum around 750 u. All techniques are now roughly in accord, including many different mass spectral methods (ESI FT-ICR MS, APPI, APCI FIMS, LDI) and many different diffusion techniques (time-resolved fluorescence depolarization (TRFD), fluorescence correlation spectroscopy (FCS), Taylor dispersion). Aggregation of asphaltenes at very low concentrations (in toluene) led to aggregate weights being misinterpreted as molecular weights with techniques such as VPO or GPC. The chemical structure is difficult to ascertain, due to the complex nature of the asphaltenes, but has been studied by all available techniques including X-ray, elemental, and pyrolysis GC-FID-GC-MS. However, it is undisputed that the asphaltenes are composed mainly of polyaromatic carbon i.e. polycondensed aromatic benzene units with oxygen, nitrogen, and sulphur, (NSO-compounds) combined with minor amounts of a series of heavy metals, particularly vanadium and nickel which occur in porphyrin structures. Furthermore, asphaltene rotational diffusion measurements show that small PAH chromophores (blue fluorescing) are in small asphaltene molecules while big PAH chromophores (red fluorescing) are in big molecules. This implies that there is only one fused Polycyclic aromatic hydrocarbon (PAH) ring system per molecule. Very recent fragmentation studies by FT ICR-MS strongly support this ‘island’ molecular architecture refuting the ‘archipelago’ molecular architecture. Asphaltenes may be today widely recognized as soluble, chemically altered fragments of Kerogen which migrated out of the source rock for the oil, during oil catagenesis. Asphaltenes had been thought to be held in solution in oil by resins (similar structure and chemistry, but smaller) but recent data shows this is incorrect. Indeed it has recently been suggested that asphaltenes are nanocolloidally suspended in crude oil and in toluene solutions of sufficient concentrations. In any event for low surface tension liquids such as alkanes and toluene, surfactants are not necessary to maintain nanocolloidal suspensions of asphaltenes. 
         [0005]    Heavy oils, and biodegraded oils (as bacteria can not assimilate asphalten[e]s, but readily consume saturated hydrocarbons and certain aromatic hydrocarbon isomers—enzymatically controlled) may contain much higher proportions of asphaltenes than do medium API gravity oils or light crude oils. Condensates are virtually devoid of asphaltenes. 
         [0006]    The water content of heavy fuel oils may vary widely. Water may come from several different sources, it can be either fresh or saline. It can also originate from e.g. condensation in the installation&#39;s storage tanks. If water is sweet and well emulsified in heavy fuel oil, the effective energy content of the fuel decreases with increasing water content, leading to an increase in fuel consumption. If heavy fuel oil is contaminated with seawater, the chlorine in the salt may cause corrosion of the fuel handling system, including the fuel injection equipment. 
         [0007]    Sulphur in fuel may cause cold corrosion and corrosive wear, especially in low load operation. Sulphur may also contribute to deposit formation in the exhaust system, normally together with vanadium and/or sodium in the form of sulphates. The deposits can also cause high-temperature corrosion. Sulphur is a fuel property that has been the subject of much discussion recently. For example the International Maritime Organization (IMO) is proposing, and the European Union (EU) has set limits on fuel sulphur content to reduce emissions of sulphur oxides. 
         [0008]    High ash content may be detrimental in several ways. Different ash constituents, like vanadium, nickel, sodium, aluminium and silicon can cause different kinds of problems. Aluminium and silicon oxides originate from the refining process, and can cause severe abrasive wear mainly to the injection pumps and nozzles, but also to the cylinder liner and piston rings. Efficient fuel separation is a must for minimizing component wear. 
         [0009]    Oxides of vanadium and sodium, mainly sodium vanadyl vanadates, are formed during combustion, and will mix or react with oxides and vanadates of other ash constituents, e.g. nickel, calcium, silicon and sulphur. The sticking temperature of the mixture may cause a deposit to be formed on an exhaust valve, in the exhaust gas system, or in the turbocharger. This deposit may be highly corrosive in molten salt. It may destroy the protective oxide layer, e.g. on an exhaust valve, and leads to hot corrosion and a burned valve. Deposits and hot corrosion in the turbocharger, especially on the nozzle ring and turbine blades, may reduce turbocharger efficiency. 
         [0010]    The gas exchange may also be disturbed, less air flows through the engine, and thus the thermal load on the engine may increase. Deposit formation may increase at increased temperatures and engine outputs. Several measures are necessary to avoid these problems when running on high-ash heavy fuel oils. It is important, for example, to have an efficient fuel separation system, to clean the turbocharger regularly, and to ensure strict quality control of the bunkered fuel, i.e. to see that the amounts of ash and dangerous ash constituents stay low. It is also essential to ensure clean air filters and charge air coolers by regular cleaning based on pressure drop monitoring. 
         [0011]    A high carbon residue content may lead to deposit formation in the combustion chamber and in the exhaust system, especially at low engine loads. 
         [0012]    A high asphaltene content may contribute to deposit formation in the combustion chamber and in the exhaust system, especially at low loads. Asphaltenes are complex, highly aromatic compounds with a high molecular weight that usually contain sulphur, nitrogen and oxygen, as well as metals like vanadium, nickel and iron. A high asphaltene content indicates that a fuel may be difficult to ignite and will burn slowly. If heavy fuel oil is unstable, the asphaltenes will precipitate from the fuel and block filters and/or cause deposits in the fuel system, as well as lead to excessive sludge formation in the fuel separator. Moreover, when operating on heavy fuel oils with high asphaltene content, good performance of the lubricating oil should be emphasized. It is important that the lubricating oil is able to bind the combustion residues containing asphaltenes from blow-by gases entering the crankcase and thus prevent deposit formation on the engine component surfaces. 
         [0013]    All heavy fuel oils contain a certain amount of sediment. The sediment can be both organic and inorganic. Total sediment content (TSP analysis) describes the fuel&#39;s cleanliness (presence of sand, rust, dirt, catalyst fines and other solid/inorganic contaminants), stability (resistance to breakdown and precipitate asphaltenes), and compatibility with another fuel quality. 
         [0014]    An internal combustion engine may include one or more turbochargers for compressing a fluid which is supplied to one or more combustion chambers within corresponding combustion cylinders of an internal combustion engine. The fluid compressed by the compressor may be intake air or fresh air. Each turbocharger may typically include a turbine driven by an exhaust gas flow of the engine and a compressor which is driven by the turbine. The compressor may receive the fluid to be compressed and may supply the compressed fluid to the combustion chambers. 
         [0015]    An exhaust gas recirculation system (EGR system) may be used for controlling the generation of undesirable pollutant gases and particulate matter in the operation of internal combustion engines. Such systems have proven particularly useful in internal combustion engines used in e.g. ships and generator sets. EGR systems primarily recirculate a part of the exhaust gas into the intake air supply of the internal combustion engine. The exhaust gas which is reintroduced to the engine&#39;s cylinder may reduce the concentration of oxygen, which intern lowers the maximum combustion temperature within a cylinder and slows the chemical reaction of the combustion process, decreasing the formulation of nitro oxide (NOx). Furthermore, the exhaust gases may typically contain unburned hydrocarbons which are burned on reintroduction into the engine cylinder, which may further reduce the emission of exhaust gas by-products which would be emitted as undesirable pollutants from the internal combustion engine. 
         [0016]    An EGR system as described above, may be used to control the amount of exhaust gas which is mixed with the combustion air before introduction into an intake manifold. 
         [0017]    WO 96/18030 discloses an arrangement for recirculation of exhaust gases in turbocharged or supercharged engines with turbines in series. This known arrangement includes an EGR turbine driven by the exhaust gases discharged at the exhaust gas outlet of the internal combustion engine. The EGR turbines drive an EGR compressor which compresses a partial exhaust gas flow. An exhaust gas cooler is arranged downstream of the EGR compressor between the EGR compressor and an inlet manifold of the internal combustion engine. The exhaust gas cooler may serve to compensate an increased temperature imported to the recirculated exhaust gases by the EGR compressor. Alternatively, the exhaust gas cooler may be placed upstream of the EGR compressor. 
         [0018]    EP 0 707 142 A1 (corresponding to U.S. Pat. No. 5,564,275 A) discloses a method and apparatus for high-pressure and exhaust gas recirculation on a supercharged internal combustion engine. Here, a third exhaust gas flow of the internal combustion engine may expand separately from a first exhaust gas flow and may provide power to compress the second exhaust gas flow. A turbine is arranged as a drive for the exhaust gas compressor. The exhaust gas duct system has a third exhaust gas duct connected to the turbine. An exhaust gas cooler is arranged in the second exhaust gas duct downstream of the exhaust gas compressor. 
         [0019]    A similar exhaust gas recirculation system for an internal combustion engine for passenger cars or trucks is shown in WO 2008/062254 A1. Here, an EGR turbine is located upstream of an EGR cooler. Thus, EGR gas flowing from an exhaust manifold goes through the EGR turbine prior to entering the EGR cooler. Due to the pressure reduction occurring the turbine, the EGR gas temperature may be lowered at the intake manifold inlets, and less cooling power from the engine cooling system may be required to cool down the EGR gas in the EGR cooler. 
         [0020]    A problem may arrive within the duct or line system of the exhaust gas recirculation system if burning fuels commonly used for operating in large combustion engines may produce sulphur acid or the like, contained within the recirculated exhaust gases. The sulphur acid and/or other components (e.g. sulphur as such, sulphur together with vanadium and/or sodium in the form of sulphates, ash constituents like vanadium, nickel, sodium, aluminum and silicon) of the exhaust gas (e.g. H 2 SO 3  or H 2 SO 4 ) may have a negative effect on the exhaust gas line system. Corrosion may be one of the negative effects generated by one or more of the components of the exhaust gas flow, for example, sulphur acid etc. may result in strong corrosion of the exhaust gas line systems. 
         [0021]    The present disclosure is directed, at least in part, to improving or overcoming one or more aspects of prior systems. 
       SUMMARY OF THE DISCLOSURE 
       [0022]    According to a first aspect of the present disclosure an exhaust gas recirculation system configured to be used in, e.g., a large internal combustion engine having an air inlet and an exhaust gas outlet may comprise an exhaust gas recirculation line configured to recirculate a first partial exhaust gas flow to the air inlet of the internal combustion engine. In addition, an exhaust gas discharge line may be fluidly connected to the exhaust gas recirculation line and configured to discharge at least a second partial exhaust gas flow. An exhaust gas turbine may be provided and configured to be driven by the second partial exhaust gas flow. An exhaust gas compressor may be provided and configured to be driven by the exhaust gas turbine and to compress the exhaust gas of the first partial exhaust gas flow. Furthermore, a first exhaust gas cooler may be arranged upstream of the exhaust gas compressor and configured to cool the exhaust gas of the first partial exhaust gas flow to a first temperature. A second exhaust gas cooler may be arranged downstream of the exhaust gas compressor and configured to cool the compressed exhaust gas of the first partial exhaust gas flow to a second temperature. The first and second temperatures may be equal or different. 
         [0023]    In another aspect of the present disclosure an internal combustion engine such as a large internal combustion engine may comprise engine an air inlet, an exhaust gas outlet, and an exhaust gas recirculation system described herein. 
         [0024]    According to a further aspect of the present disclosure a method for recirculating an exhaust gas flow of a large internal combustion engine having air inlet and exhaust gas outlet may be provided. The method may comprise the steps of diverting a first partial exhaust gas flow, cooling the exhaust gas of the first partial exhaust gas flow and compressing the cooled first partial exhaust gas flow. Furthermore, the compressed exhaust gas of the first partial exhaust gas flow may be cooled again and may subsequently be supplied to the air inlet of the large internal combustion engine. 
         [0025]    Other features and aspects of this disclosure will be apparent from the following description, the accompanying drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic block diagram of an exemplary embodiment of the present disclosure, and 
           [0027]      FIG. 2  is a schematic internal combustion engine cooling circuit diagram of a further exemplary embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    A first exemplary embodiment of an exhaust gas recirculation system  5  configured to be used in a large internal combustion engine  10  having an air inlet  15  and an exhaust gas outlet  20  will be described in the following with reference to  FIG. 1 . 
         [0029]    Internal combustion engine  10  may comprise one or more cylinders and associated combustion chambers  25 . Combustion chambers  25  may be connected to both air inlet  15  (e.g. an inlet air manifold) and exhaust gas outlet  20 . Exhaust gas outlet  20  may be connected to an exhaust gas line  130 . Exhaust gas line  130  may include a first valve  135  and/or second valve  136 . Both valves  135  and  136  may be connected to a control unit  181 . Upstream of second valve  136  and downstream of first valve  135  an exhaust gas recirculation line  140  may branch off. An exhaust gas turbine  155  may be located downstream of second valve  136 . Exhaust gas turbine  155  may be a component of an exhaust gas supercharger system  150 . An exhaust gas compressor  160  may be driven via a shaft  165  by exhaust gas turbine  155 . Exhaust gas recirculation line  140  may be connected to a first exhaust gas cooler  170 . An outlet of exhaust gas cooler  170  may be connected to a further exhaust gas recirculation line  175 , which may supply the cooled exhaust gas to exhaust gas compressor  160 . 
         [0030]    Another part of exhaust gas recirculation line  175  may connect exhaust gas compressor  160  and a second exhaust gas cooler  180  with each other. A third valve  185  may be arranged downstream of second exhaust gas cooler  180 . First, second and third valve  185  each may consist of a simple throttle allowing blocking or unblocking the exhaust gas flow and the recirculation of exhaust gases via lines  140 ,  175 . An end part of the exhaust gas recirculation line may open downstream of valve  185  into manifold  15 . 
         [0031]    In the following, a multiple supercharger system arrangement of internal combustion engine  10  is described. A first supercharger system  30  may comprise a first turbine  35  and a first compressor  40  which may be driven by first turbine  35  via a shaft  45 . A second supercharger system  90  may comprise a second turbine  95  and a second compressor  100  which may be driven via a shaft  105  by second turbine  95 . A first intake air cooler  50  may be arranged downstream of first intake air compressor  40  and second intake air compressor  100 . Intake air lines  55  and  60  may interconnect first intake air cooler  50  with first intake air compressor  40  and second air compressor  100 . Second intake air compressor  100  may be connected to second air cooler  115 . Intake air cooler  115  in turn may connect to intake manifold  15  of internal combustion engine  10 . 
         [0032]    Exhaust gas outlet  20  of internal combustion engine  10  may be connected to second turbine  95  of second supercharger system  90 . Second turbine  95  of supercharger system  90  in turn may be connected to first turbine  35  of first supercharger system  30 . An exhaust gas line  85  may branch off from exhaust gas outlet  20  to guide a partial exhaust gas flow to second turbine  95 . Downstream of exhaust gas outlet  20  and upstream of second turbine  95  a branch line  80  may be provided and connected to a valve  75 . Valve  75  in turn may be connected to a exhaust gas line  70  opening into connecting line  65  located between first turbine  35  and second turbine  95 . 
         [0033]      FIG. 2  shows a schematic cooling circuit for an internal combustion engine as, e.g., shown in  FIG. 1 . The cooling circuit of the internal combustion engine  10  may comprise a low temperature (LT) cooling circuit  400 , a high temperature (HT) cooling circuit  500  and a super high temperature (SHT) cooling circuit  600 . 
         [0034]    LT cooling circuit  400  may comprise a LT cooler  410  supplied with, e.g., sea water by means of a sea water pump  405 . The temperature of the cooling medium, e.g. water (separate from sea water) circulating in LT cooling circuit  400  pumped by LT pump  425  may be, e.g. 32° C. The portion of the cooling medium circulating in LT cooling circuit  400  not flowing through cooler  410  may be adjusted by valve  420  and dependant on the temperature detected by temperature sensor  440 . Accordingly, the temperature of the cooling medium circulating may be adjusted to the desired value, e.g. 32° C. Accordingly, temperature sensor  440  and valve  420  may be connected to control unit  181 . 
         [0035]    The cooling medium of LT cooling circuit  400  may flow after passing LT cooler  410  through second exhaust gas cooler  180  and parallel to first intake cooler  50 . A temperature display  440  may be arranged downstream of intake air cooler  50 . A further temperature display  445  may be arranged downstream of exhaust gas cooler  180 . The temperature of the cooling medium leaving the second exhaust gas cooler  180  may be about 47.6° C. The temperature of the cooling medium leaving intake air cooler  50  may be about 44.90° C. 
         [0036]    The high temperature (HT) cooling circuit  500  may comprise a HT cooler  505 , a HT pump  510 , the second intake air cooler  115  and internal combustion engine  10 . A temperature sensor  530  may be located downstream of engine  10 . A temperature display  515  may be located upstream of HT pump  510  and another temperature display may be located upstream of engine  10 . The cooling medium of HT cooling circuit  500  may be pumped by HT pump  510  through second intake air cooler  115  and the engine  10 . The temperature displayed on display  515  may be about 69° C. The temperature of the cooling medium leaving intake air cooler  115  may be about 80° C., which may be displayed by display  520 . After the cooling medium, e.g. water, is recirculated through internal combustion engine  10  its temperature may be about 90° C. The temperature may be determined by sensor  530 . Again, via a valve  540  the part of the cooling medium recirculated within the HT cooling circuit  500  not passing through HT cooler  505  may be adjusted. 
         [0037]    Finally, super high temperature (SHT) cooling circuit  600  is explained. SHT cooling circuit  600  may comprise a SHT cooler  605  connected to HT cooler  505 . Via a pump  615  the cooling medium of the SHT cooling circuit  600  is pumped through first exhaust gas cooler  170 . The temperature of the cooling medium may be around 150° C., which may be determined by sensor  610 . The cooling medium temperature downstream of first exhaust gas cooler  170  may be around 162° C. This temperature may be displayed on temperature display  620 . Again, via a valve  630  the part of the cooing medium recirculated within circuit  600  not passing through SHT cooler  605  may be adjusted. The advantage to supply first exhaust gas cooler  170  with very high temperature water may be that it can be guaranteed that at least the surfaces of first exhaust gas cooler  170  do not have a temperature below a dew point of a specific component, e.g. sulphur acid etc., of the exhaust gas to be cooled by first exhaust gas cooler  170 . Thus, disadvantage corrosion may within first exhaust gas cooler  170  may be reduced or even prevented. In addition, it might be possible to reduce or even avoid corrosion in exhaust gas compressor  160  and at the entry of low temperature cooler  180 . 
         [0038]    It may also be possible to separate the various cooling circuits  400 ,  500 , and  600  from each other. This alternative, exemplary embodiment of a cooling system including three separate cooling circuits, namely LT cooling circuit  400 , HT cooling circuit  500 , and SHT cooling circuit  600 , is shown  FIG. 2  in dashed lines. In this case each cooling circuit  400 ,  500 , and  600  may have its own sea water pump  405 ′ and its own return line. Hence, in this exemplary embodiment of cooling system  400 ,  500 ,  600  there is no fluid connection between LT cooler  410  and HT  505 , or between HT cooler  505  and SHT cooler  605 . 
       INDUSTRIAL APPLICABILITY 
       [0039]    In the following, the basic operation of the above exemplary embodiment of exhaust gas recirculation system  5  is explained with reference to  FIGS. 1 and 2 . During normal operation of the internal combustion engine  10  a continuous flow of exhaust gas leaves the internal combustion engine  10  at the exhaust gas outlet  20 . A partial exhaust gas flow enters via line  85  second turbine  95  of supercharger system  90 . Via valve  75  the part of the exhaust gas flowing within line  85  by-passing the second turbine  95  may be adjusted. The exhaust gas entering second turbine  95  may cause rotation of second turbine  95  and, consequently, second compressor  100  may be driven. Exhaust gas discharged by second turbine  95  may enter into first turbine  35  of first supercharger system  30 . Again, first turbine  35  may be caused to rotate due to exhaust gas entering. After leaving first turbine  35  exhaust gas flow  210  may be guided into the atmosphere or anywhere else. 
         [0040]    The rotation of first turbine  35  may cause via shaft  45  rotation of compressor  40 . Due to the rotation of first compressor  40  intake air  200  may be compressed and guided through first intake air cooler  50  to second intake air compressor  100 . Accordingly, intake air  200  may be compressed, cooled, and again compressed. There may be only one supercharger system or more than two supercharger systems, contrary to the exemplary embodiment shown in  FIG. 1 . The intake air compressed at least once may flow in intake air line  110  to second intake air cooler  115 . There, the compressed intake air may be cooled again and then may flow via intake air line  120  into manifold  15 , where the intake air compressed and cooled may mix with recirculated exhaust gas flow  190 . 
         [0041]    As shown in  FIG. 1 , an exhaust gas  190  flow to be recirculated may enter into first exhaust gas cooler  170  and may be then compressed by exhaust gas compressor  160 . The exhaust gas flow  190  leaving compressor  160  may have, due to the compression step, a higher temperature as desired and, therefore, may be cooled within second exhaust gas cooler  180  before the cooled and compressed exhaust gas flow  190  may enter into manifold  15 . If a specific portion of the exhaust gas may be guided through line  130  and the rest of the exhaust gases may be recirculated through lines  140  and  175 , valves  135  and  185  receive both a control signal from control unit  181  causing that both valves  135  and  185  unblock the flowing path. Simultaneously, valve  136  may receive an appropriate control signal causing that the passage of exhaust gases is only partly blocked so that a specific portion of exhaust gas may flow through turbine  155  and the rest of the exhaust gas may be recirculated. 
         [0042]    In an exemplary embodiment of the present disclosure first exhaust gas cooler  170  may be arranged as far as possible downstream of exhaust gas compressor  160 . In case that first exhaust gas cooler  170  is controlled such that the exhaust gas leaving exhaust gas cooler  170  has a temperature below the dew point of a component of the exhaust gas flow, e.g. sulphur or sulphured acid, corrosion or other negative effects within the piping system up to exhaust gas compressor  160  or even up to second exhaust gas cooler  180  may be reduced or prohibited. Second exhaust gas cooler  180  may be controlled by control unit  181  such that the temperature of the exhaust gas leaving exhaust gas cooler  180  is relatively low, e.g. below the dew point of the component of the exhaust gas mentioned above. 
         [0043]    An advantage of cooling of the exhaust gas may be that the efficiency of the combustion of the mixture of cooled and compressed exhaust gas  190  and cooled and compressed intake air  120  at air inlet  15  of internal combustion engine  10  may be improved. 
         [0044]    Although the preferred embodiments of this invention have been described herein, improvements and modifications may be incorporated without departing from the scope of the following claims.