1.  Field of the Invention
The invention is directed to an exhaust gas aftertreatment system for an internal combustion engine, particularly for a ship's diesel engine operated with heavy oil and to a method for exhaust gas aftertreatment of exhaust gas exiting an internal combustion engine, particularly a ship's diesel engine operated with heavy oil. 
2.  Description of the Related Art
It is a characteristic of internal combustion engines operated with heavy oil that the fuel that is used, i.e., the heavy oil, has a high sulfur content.  Sulfur oxides can react with other constituents of the exhaust gas and lead to deposits which impair the efficiency of the exhaust gas cleaning.  This is disadvantageous.  There is a need for an exhaust gas aftertreatment system for an internal combustion engine which also allows an efficient cleaning of exhaust gas in internal combustion engines operated with heavy oil. 
An internal combustion engine with an exhaust gas turbocharger and an exhaust gas cleaner is known from DE 10 2004 027 593 A1.  The exhaust gas turbocharger is constructed either as a one-stage exhaust gas turbocharger or as a two-stage exhaust gas turbocharger.  The exhaust gas cleaner comprises a SCR catalyst, which is positioned either downstream of the turbine of the exhaust gas turbocharger or upstream of the turbine of the exhaust gas turbocharger in the one-stage exhaust gas turbocharger.  For a two-stage exhaust gas turbocharger with a high-pressure exhaust gas turbocharger and a low-pressure exhaust gas turbocharger, the SCR catalyst is positioned between the high-pressure turbine of the high-pressure exhaust gas turbocharger and the low-pressure turbine of the low-pressure exhaust gas turbocharger. 
A SCR catalyst of an exhaust gas aftertreatment system uses ammonia as reductant.  In known exhaust gas aftertreatment systems, an aqueous urea solution is injected into the exhaust gas upstream of the SCR catalyst, this aqueous urea solution being decomposed and evaporated in the exhaust gas flow to form water vapor, carbon dioxide and ammonia.  For this purpose, a relatively long process line with an evaporator and a hydrolysis catalyst is required between a nozzle, which injects the aqueous urea solution into the exhaust gas and which is positioned in an exhaust gas line running between the internal combustion engine and the SCR catalyst, and the SCR catalyst which uses ammonia as a reductant.  This results in a relatively long construction of the exhaust gas aftertreatment system.  An exhaust gas aftertreatment system of this type is known from EP 0 487 886 B1. 
Further, it is known from EP 0 487 866 B1 to position the process line with the evaporator and hydrolysis catalyst in a separate line rather than in the exhaust gas line between the internal combustion engine and SCR catalyst.  However, this embodiment also requires a relatively long process line with an evaporator and a hydrolysis catalyst, which again results in a relatively long construction of the exhaust gas aftertreatment system.  Moreover, in such a configuration, a gas mixer is required in the exhaust gas line.