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 tech- niques 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. 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 combus- tion process intake air. Hence, a solution should be found for reducing the proportion of nitrogen in the combustion air in order to make the high investment costs of carbon dioxide removal equipment economically justifiable. However, using oxygen in lieu of air (in a so-called oxygen combustion process) allows the concentration of carbon dioxide (C02) in exhaust gas to be increased higher thus also making its capture substantially simpler and cheaper. As the proportion of carbon dioxide increases and simultaneously the overall volume of gas flowing through the combustion system decreases due to the reduced volume of nitrogen, the carbon dioxide removal equipment can be dimensioned for a smaller flow rate of gas.

In oxygen combustion, the fuel is combusted with the help of oxygen separated from the air. Oxygen combustion has been developed for use in gas turbine technology.

Herein, however, it has been necessary to make substantial changes in gas turbines and their compressors designed for large gas volumes to adapt them for different flow rates and modified combustion process. Control of combustion temperature is a particularly grave problem when a gas turbine is run using oxygen combustion.

Therefore, oxygen combustion in gas turbines becomes an extremely costly process.

As separation of oxygen from ambient air has become a relatively cost-efficient possibility by means of concurrent technology, any chance of making advantageous energy-production methods based on oxygen combustion could make oxygen combustion a viable technique for reduction of carbon dioxide emissions in energy generation using separation of carbon dioxide from the flue gases.

It is an object of the invention to provide a method capable of energy generation by means of oxygen combustion technology and, simultaneously, increasing the propor- tion of carbon dioxide in flue gases so that separation of carbon dioxide from flue gases can be carried out in a manner of higher cost-efficiency than in the prior art.

The goal of the invention is achieved by way of replacing the combustion air of an internal combustion engine with a mixture of oxygen and recirculated exhaust gas.

More specifically, the method according to the invention for operating an internal com- bustion engine is characterized by what is stated in the characterizing part of claim 1.

The invention offers significant benefits.

By virtue of the invention, the carbon dioxide concentration of exhaust gas dis- charged from an internal combustion engine can be elevated substantially thus making the separation treatment of carbon dioxide easier and more cost-efficient. As the proportion of carbon dioxide in the overall volume of exhaust gas to be treated increases, the operation of the separation equipment becomes more efficient.

Conversion of an internal combustion engine, such as a diesel engine, for oxygen combustion is simple and can be carried out without major changes in the engine design. Because the combustion takes place in a closed combustion space of the engine cylinder, the combustion process is easy to control by adjusting the propor- tions of fuel, oxygen and recirculated exhaust gas, complemented with water or steam injection. As compared with a gas turbine, the operation of an internal com- bustion engine is not so critical in regard to an exactly set volume rate of the intake gas mixture, thus allowing the use of the recirculated exhaust gas for controlling the engine temperature and combining oxygen with the fuel being used into a gas mixture that is capable of efficient burning in the combustion space but yet is free from the risk of spontaneous ignition elsewhere in the system. The amount of infeed oxygen may be controlled to a stoichiometric ratio with the fuel being fed without the need for using excess oxygen. Therefore, the proportion of oxygen in the com- bustible gas mixture to be fed into the combustion space may differ from the oxygen concentration of ambient air by being, e. g. , lower than in the ambient air. Diesel engines in particular are capable of using many different fuels already in existing installations, whereby oxygen combustion gives an extra possibility of controlling the combustion process by adjusting the oxygen concentration of the combustible gas mixture. Resultingly, the engine may be run using biofuels such vegetable oils. Due to the efficient capture of carbon dioxide, the carbon dioxide balance sheet may indicate negative net emission. In conventional combustion processes the introduc- tion of a sufficient amount of oxygen is accomplished by means of pressurizing the

intake air with the help of a compressor. In contrast, the amount of oxygen fed in oxygen combustion is controlled by adjusting the injected amount of oxygen. When recirculated by a charger, the pressure of the recirculated exhaust gas may be lower than the pressure of combustion air conventionally used. Hence, a charger of smaller size can be used and its energy consumption is reduced. Simultaneously, the temperature of exhaust gas leaving the charger becomes higher thus expanding their utilization possibilities in steam generation, for instance. In a comprehensive aspect, the application of oxygen combustion into internal combustion engine technology invokes smaller need for modifications as compared with gas turbine technology.

A remarkable benefit of the invention is the elimination of nitrogen oxide emissions or at least a substantial reduction thereof, because nitrogen is eliminated from the combustion process or at least reduced to a minimum amount.

Next, the invention will be examined with the help of exemplary embodiments by making reference to the appended drawings in which: FIG. 1 shows a schematic block diagram of an arrangement according to the invention; FIG. 2 shows a first embodiment of a detail in the diagram of FIG. 1; FIG. 3 shows a second embodiment of a detail in the diagram of FIG. 1; FIG. 4 shows a third embodiment of a detail in the diagram of FIG. 1; FIG. 5 shows a fourth embodiment of a detail in the diagram of FIG. 1; FIG. 6 shows a schematic block diagram of another arrangement according to the invention; FIG. 7 shows a first embodiment of a detail in the diagram of FIG. 6;

FIG. 8 shows a second embodiment of a detail in the diagram of FIG. 6; FIG. 9 shows a third embodiment of a detail in the diagram of FIG. 6; FIG. 10 shows a fourth embodiment of a detail in the diagram of FIG. 6; FIG. 11 shows a fifth embodiment of a detail in the diagram of FIG. 6; FIG. 12 shows a sixth embodiment of a detail in the diagram of FIG. 6; and FIG. 13 shows a seventh embodiment of a detail in the diagram of FIG. 6.

In the context of the following description, the term"internal combustion engine" must be understood to refer to a spark-ignited engine, diesel engine or any like engine based on combusting the fuel in a closed combustion space. The engine may have a two-stroke or four-stroke design. The most advantageous embodiment of the invention runs the process in 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 configurations of FIGS. 1 and 6 do not illustrate the actual combustion engine section that is shown in FIGS. 2-5 and 7-10.

As shown in the diagrams, the internal combustion engine discharges its exhaust gases, or flue gases, along line 1 to turbine wheel 2 of the outlet side of a charger, whereby the kinetic energy imparted by the flowing gases drives the charger turbine wheel at the inlet side (FIGS. 2-5,7-10). 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. 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 terminating in the exhaust gas stack 8 whereto the exhaust gases, which are rich with nitrogen compounds during engine startup, are passed. As the combustion 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 inlet-side turbine section of the engine's charger. 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.

The cooled, dried and cleaned exhaust gases are passed via the inlet-side turbine sec- tion of the charger wherefrom they exit at a higher pressure. After leaving the charger, a portion of the carbon-dioxide-containing exhaust gases is branched aside and taken to a carbon dioxide capture process. As ambient air is used during the engine startup, the exhaust gases leaving the first heat exchanger 3 are first passed to the exhaust gas stack 8. As soon as the C02 concentration of exhaust gases after switching to oxygen combustion reaches a sufficiently high level, the C02 capture process can be begun.

In FIG. 2 is shown the inlet side of the engine in a schematic block diagram.

The treated exhaust gases are taken along line 6 shown in FIG. 1 to the inlet-side turbine section 9 of the charger, where the gases are supercharged above the ambient pressure by 1 bar, for instance. In the construction according to the invention, the supercharging pressure produced by the charger can be set to a level conventionally used in turbocharged engines. The supercharged exhaust gas is passed to lines 10 and 11, of which line 11 exits in the combustion space of engine cylinder 22. Line 10 leads to the carbon dioxide capture equipment, whereby this line passes that portion of exhaust gases that is not needed as the recirculated exhaust gas of the engine. As the carbon dioxide concentration of the recirculated exhaust gas is high, separation of carbon dioxide from the exhaust gas can be made at a high efficiency. The excess portion of exhaust gas is used as both an intermediate gas of the combustion process and flushing/cooling gas of the combustion space and exhaust valves. The exhaust gas is passed to the combustion space along line 11 via a dedicated valve. In practice, the valve may be the inlet valve of the engine. Into the combustion space is also injected oxygen along line 12 and fuel along line 14. Using conventional fuel injection techniques, the fuel may be fed directly into the combustion space, whereby also oxygen can be introduced in the same fashion. When necessary, water or steam can be injected along lines 15 into the exhaust gas flow just prior to the cylinder 22 or, alternatively, directly into the cylinder. With the help of water mist or steam, the engine temperature and the combustion process can be controlled, whereby the detrimental emissions of the engine may be reduced. The engine may have a single- volume combustion space or equipped with a precombustion chamber. The combustion gas mixture introduced into the precombustion chamber may comprise oxygen alone or, e. g. , a mixture of oxygen, exhaust gas and steam. Therefore, the gas mixture composition in the precombustion chamber is different from that filling the actual combustion chamber. The precombustion chamber may also have a separate infeed nozzle or nozzles for injecting a cooling gas (exhaust gas).

The oxygen needed by the engine is produced by means of an oxygen separation

apparatus 13 connected to the oxygen feed line. As the present invention is particularly intended for use in conjunction with large engines used in ships or power plants, the high volume of oxygen need must be satisfied with the help of a dedicated system adapted to operate in conjunction with the power-generating engine. To line 12 between the oxygen-producing equipment 13 and the cylinder 22 is connected an air feed line 16 serving to feed compressed air along the oxygen feed line 12 to the combustion space of the engine at the instant of engine startup.

In the embodiment of FIG. 3, into the exhaust gas recirculation flow prior to cylinder 22 is mixed oxygen separated from air, whereupon the mixture is passed into the combustion space of the engine. The fuel is introduced into the mixture of exhaust gas and oxygen in the combustion space. When necessary, water mist or steam can be sprayed into the exhaust gas flow either prior to the cylinder or directly into the cylinder 22. In the illustrated embodiment the flushing/cooling of the combustion space and the exhaust valves of the cylinder is accomplished by means of recirculat- ed and cooled exhaust gas that is passed along a separate line 17 to the engine. The portion of exhaust gases used for flushing is directed to the combustion space via a separate valve adapted to the engine cylinder head. Compressed air used for starting the engine is passed along the oxygen feed line 12.

In the embodiment of FIG. 4, the fuel is mixed with the recirculated exhaust gas introduced into the oxygen flow before the exhaust gas enters the combustion space of cylinder 22. Thus, the recirculated exhaust gas forms a mixture with fuel and oxygen separated from the air, whereupon the mixture is passed to the combustion space of engine cylinder 22. When necessary, water mist or steam can be sprayed from lines 15 into the exhaust gas flow either prior to the cylinder or directly into the cylinder. The flushing/cooling of the combustion space and the exhaust valves of the cylinder is accomplished by means of recirculated exhaust gas. The portion of exhaust gases used for flushing is directed to the combustion space via a separate valve adapted to the engine cylinder head. Compressed air used for starting the engine is passed along the oxygen feed line 12 and recirculated exhaust gas duct.

In the embodiment of FIG. 5, the fuel is mixed with the recirculated exhaust gas, while oxygen is introduced along line 12 directly into the combustion space of cylinder 22. Fuel is introduced into the recirculated exhaust gas flow, whereupon the mixture is passed to the combustion space of engine cylinder. Also oxygen is injected into the combustion space. When necessary, water mist or steam can be sprayed into the exhaust gas flow either prior to the cylinder or directly into the cylinder. The flushing/cooling of the combustion space and the exhaust valves of the cylinder is accomplished by means of recirculated exhaust gas. The portion of exhaust gases used for flushing is directed to the combustion space via a separate valve adapted to the engine cylinder head. Compressed air used for starting the engine is passed along the oxygen feed line 12.

In all the above-described embodiments, the exhaust gas taken to carbon dioxide capture is treated in a pressurized state. As such pressurization is advantageous in the carbon dioxide capture process, the capacity of the engine charger can be advantageously utilized for pressurizing the exhaust gas. In the embodiment of FIG. 6j the recirculated portion of exhaust gas is passed to line 19, while the exhaust gas portion to be taken to the carbon dioxide capture process is passed to line 18 prior to passing the recirculated exhaust gas to the inlet-side turbine section of charger 9. One of the advantages of this embodiment is that the temperature of the exhaust gas portion taken to the carbon dioxide capture process is not elevated due to supercharging and that the volume rate of exhaust gas passed to the charger is reduced thus allowing the use of a smaller and less expensive charger.

Also this embodiment can utilize the same configurations as those used in the earlier described arrangements. The only difference is that, if mixing any additional compo- nent with the recirculated exhaust gas prior to its entry into the cylinder is considered necessary, the required amount of oxygen, fuel and/or water with the exhaust gas flow can be made either prior to or after the charger (FIGS. 7-10). When necessary, pressurization of the exhaust gas portion used for flushing in the configurations of FIGS. 11-13 may be carried out.

In the embodiment of FIG. 7, fuel, oxygen and recirculated exhaust gas are fed each separately into the combustion space of cylinder 22. Water mist or steam is mixed with the recirculated exhaust gas flow or is injected directly into the cylinder. In the embodiment of FIG. 8 is used flushing gas that is passed into the combustion space of cylinder 22 along line 17 and via a separate flushing valve. Oxygen is mixed with the pressurized flow of recirculated exhaust gas before the entry of gas into cylinder 22. In the embodiment of FIG. 9, fuel and oxygen are mixed with the pressurized flow of recirculated exhaust gas before the entry of the gas into the combustion space of cylinder 22, while in the embodiment of FIG. 10 fuel is mixed with the pressur- ized flow of recirculated exhaust gas and oxygen is fed directly into cylinder 22. In the embodiment of FIG. 11, the flushing gas of cylinder 22 is taken from the exhaust gas discharge line 18 to line 20, wherefrom the flushing gas is passed to the combus- tion space of cylinder 22 via a flushing valve. As the flushing gas must be pressurized prior to its introduction into the engine combustion space, flushing gas line 20 is equipped with a fan or compressor 21. Fuel is mixed with the recirculated exhaust gas flow prior to the inlet-side turbine section of charger 9 connected to the engine. The embodiment of FIG. 12 is similar to the embodiment of FIG. 11 with the exception that oxygen in this configuration is mixed with the recirculated exhaust gas flow prior to the inlet-side turbine section of charger 9, while fuel is mixed after charger 9 but before the entry of the recirculated exhaust gas into cylinder 22.

Further, FIG. 13 illustrates a configuration wherein both oxygen and fuel are mixed with the recirculated exhaust gas flow prior to the charger.

All the above discussion concerns a single cylinder only. Obviously, the connections in a multicylinder engine must be replicated for each cylinder.

The recirculated portion of exhaust gas serves to form together with fuel and oxygen a mixture suitable for use in an internal combustion engine. In the simplest fashion this takes place by cooling the exhaust gas down to the ambient temperature and then using the ambient air as the oxygen source in the combustible mixture. Herein, the carbon dioxide of the exhaust gas replaces the nitrogen of ambient air. However, as pointed out in the foregoing, more advantageous combustion efficiency can be at-

tained by varying particularly the oxygen content of the mixture according to its fuel content. Then an important function of the recirculated exhaust gas is to cool the engine As the risk of overheating the engine sets a lower limit to the proportion of exhaust gas in the combustible mixture. In the case that the proportion of recirculated exhaust gas in regard to the amount of fuel being combusted is desired to be reduced, internal combustion engines currently used must be provided with more efficient cooling. The amount of inlet gas fed to an engine must also be kept quite large to generate a sufficiently high end pressure of the compression cycle in the cylinder.

Therefore, the partial gas flows of an engine must be adjusted close to those of an engine that uses ambient air as the oxidizing-gas source unless major modifications are made in the engine construction. Herein, the recirculated exhaust gas must be conditioned as to its temperature and oxygen content equivalent to the ambient air.

As a rule, oxygen combustion in an internal combustion engine gives a higher increase in combustion efficiency than in a gas turbine system.

In addition to those described in the foregoing, an engine may need and, in fact, needs plural different accessories. For instance, an intercooler is conventionally used after the charger inasmuch supercharging heats up the exiting gas mixture thus deteriorating its cooling effect. While distillation could be used for obtaining oxygen, recently developed techniques based on molecular sieve ion polymer membranes are becoming economically more attractive. Also carbon dioxide can be captured using a number of already existing methods. However, the present invention is not limited as to any specific oxygen generation or separation method or carbon dioxide capture technique. It only requires that a sufficiently abundant source of oxygen is available and that at least a portion of the carbon dioxide contained in the exhaust gases is captured. The oxidizing gas used in the combustion process need not be very pure as to its oxygen content. Hence, the oxygen-rich gas mixture may also contain other gases including nitrogen. Nevertheless, the oxygen concentration of the oxidizing gas must be higher than in ambient air in order to attain the benefits of improved carbon dioxide removal offered by the invention. Obviously, the higher the oxygen concentration in the oxidizing gas mixture and the lower the proportion of nitrogen therein, the higher the gain from the invention.