The invention relates to a method according to the preamble of claim 1 for replacing the flushing air used in large internal combustion engines so that the carbon dioxide content of exhaust gases may be elevated. As known, a higher carbon dioxide content in the exhaust gases of an engine allows the capture of carbon dioxide to take place in a more profitable fashion. Even a partial capture of carbon dioxide improves the carbon balance sheet of energy generation.

The most important one of"greenhouse gases"whose concentrations in the atmos- phere can be affected by man is carbon dioxide. Depending on the computational method used, its contribution to the greenhouse effect is 72 to 80 %. In Finland, for instance, 39 % of the total emissions of carbon dioxide are traceable to energy generation and electric energy transmission.

Plural methods are available for reducing the level of carbon dioxide emissions.

Multiple techniques of relatively low cost ("least regrets"approach) can be used for cutting down carbon dioxide emissions, such as efficiency improvements in energy generation/utilization, change of fuel from coal to natural gas, improved forestation and utilization of advantageously renewable energy sources. However, these techniques are hampered by the limited scope of their contribution. It is a generally accepted opinion that the actions listed above are not sufficient in the medium-long and long run.

In energy generation with fossil fuels, the goal of reduced emissions over a time span of a medium-long and long scale requires capture, useful recycling and storage of carbon dioxide. To this end, methods have been developed for removal of carbon dioxide from flue gases. However, when installed in conventional power plants em- ployed today, both the operating and investment costs of these techniques become very high due to the low concentration of carbon dioxide in the flue gases. Inasmuch

as air consists chiefly of nitrogen, the concentration of carbon dioxide in flue gases cannot be increased substantially if air is used as the sole source of oxygen in the combustion process intake air. Hence, a solution should be found capable of reducing the proportion of nitrogen in the combustion air in order to make the high investment costs of carbon dioxide removal equipment economically justifiable.

In internal combustion engine types, the actual combustion process takes places almost stoichiometrically, whereby the carbon dioxide concentration of exhaust gases is limited by nitrogen stemming from the combustion air. For instance, in large diesel engines, the combustion space of the cylinders is flushed with air after the working <BR> <BR> stroke in order to, e. g. , prevent excessive heating of the engine. While flushing air does not contribute to the combustion process, it significantly increases the overall fuel-air ratio. Hence, flushing cylinders with air raises the total air ratio even above two. Obviously, since merely the nitrogen entering with the combustion air causes substantial dilution of the carbon dioxide concentration of exhaust gases, doubling the engine intake air volume makes the exhaust gas carbon dioxide concentration so low that removal of carbon dioxide becomes extremely expensive and technically difficult inasmuch as the capture process must be designed for treating large volumes of a gas mixture, wherein the concentration of the gas component to be removed is low.

It is an object of the invention to provide a method capable of increasing the propor- tion of carbon dioxide in exhaust gases of internal combustion engines.

The goal of the invention is achieved by way of using recirculated exhaust gas of the engine as the flushing gas.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

The invention offers significant benefits.

By virtue of the using exhaust gas for flushing the combustion spaces of an engine, the carbon dioxide concentration of exhaust gas discharged from an internal combus- tion engine can be elevated thus making the carbon dioxide capture process easier.

The method is particularly suited for use in engines equipped with a cylinder flushing system and may be applied thereto without major changes in engine construction or operation. Furthermore, the invention makes it possible to reduce the environmental impact of energy generation thus posing an enticing route toward generating plants or, e. g. , ship engines equipped with this novel solution.

Next, the invention will be examined with the help of exemplary embodiments by making reference to the appended drawing in which: FIG. 1 shows a schematic block diagram of an arrangement according to the invention.

In the context of the following description, the term"internal combustion engine" must be understood to refer to any engine based on combusting the fuel in a closed combustion space. The engine 12 may have a two-stroke or four-stroke design. In accordance with the most advantageous embodiment of the invention, the engine is a turbocharged diesel engine. The engine may be fueled by heavy or light fuel oil, natural gas, pyrolysis oil, biogas collected from a dump, biofuel oil, gasification oil, blast furnace gas or other fuel capable of being introduced in a reliable fashion into the combustion space of such an internal combustion engine. Typical applications of the invention are found in diesel engine power plants and ship engines having an output power rating in excess of 0.5 MW.

The internal combustion engine 12 discharges its exhaust gases, or flue gases, along line 1 to turbine wheel 2 of the outlet side of a turbocharger, whereby the kinetic energy imparted by the flowing gases drives the turbocharger turbine wheel 9 at the inlet side. Next, the hot exhaust gases are passed to a first heat exchanger 3 for recovery of their heat content that may be utilized for preheating or as process heat, for instance. Inasmuch as the temperature of the exhaust gas in the heat exchanger

used for heat recovery cannot be allowed to drop very low in order to keep the temperature of the heat-transferring medium reasonably high, the exhaust gases must be taken to a second heat exchanger 4, wherein the gases are cooled approximately down to the ambient temperature. This kind of cooling is mandatory, since one of the functions of the recirculated exhaust gas is to cool the engine. The connection between the first heat exchanger 3 and the second heat exchanger 4 is branched by a line 8 terminating in the carbon dioxide capture process and the exhaust gas stack whereto also all the exhaust gases are passed during engine startup. As the combus- tion of a fuel forms water, the cooled exhaust gases are dried in a dryer 5. From dryer 5 the exhaust gases are taken along a line 6 to the flushing valves of the engine and, thus, into the combustion spaces of the engine. Inasmuch as a turbocharged engine runs at all times at an elevated pressure, whereby there is ever a vacuum in the cylinders even during the flushing cycle, the flushing gas must be pressurized by means of a fan or compressor 10 mounted on line 6 prior to its introduction into the engine combustion space.

Depending on the type of fuel used, the exhaust gases leaving the engine may contain various obnoxious components such as sulfur, ash, other solids, alkali metal or heavy metal elements. If released to the environment as such, these are detrimental and may also cause damage to the engine and its accessories even very rapidly. Consequently, exhaust gases must generally be subjected to cleaning. The cleaning equipment 7 may be located, e. g., between the outlet-side turbine section and the first heat exchanger, between the first and the second heat exchanger or between the second heat exchanger and the dryer. The type of cleaning system required must be selected according to the fuel being combusted, whereby some fuels rich with obnoxious components, such as certain heavy fuel oils, may necessitate a series connection of different cleaning equipment. In contrast, natural gas may be combusted without any cleaning equipment at all.

A turbocharger feeds combustion air to engine 12. Line 11 passes fresh air to the turbocharger compressor section which is connected to the inlet manifold of the engine and thus feeds the pressurized combustion air to the engine. Line 2 feeds fuel

to the engine. In this exemplary embodiment, engine 12 drives a generator 13, for instance. When necessary, also water or steam injection may be used for engine temperature and emission level control. The air exiting in a pressurized state from the turbocharger may be cooled by means of an intercooler.

All the above discussion concerns a single cylinder only. Obviously, the connections in a multicylinder engine must be replicated for each cylinder. Inasmuch as one important function of the recirculated exhaust gas is to cool down the engine, the risk of engine overheating dictates that the amount of recirculated exhaust gas cannot be made too small. For the same purpose, the flushing gas must be cooled down to a sufficiently low temperature, however, generally only close to the ambient temperature.