LARGE SUPERCHARGED DIESEL ENGINE WITH SCR REACTOR

The present invention relates to a large supercharged diesel engine, such as a main engine of a ship, fitted with a SCR (Selective Catalytic Reduction) reactor for purifying exhaust gases from NO _x .

BACKGROUND ART

General awareness of environmental issues is increasing rapidly. Within the IMO (International Maritime Organisation) there are now discussions of emission limitations in the form of air pollution at sea. Authorities in various parts of the world are taking similar steps. An example is the proposed EPA (US

Environmental Protection Agency) rules currently under discussion.

NO _x in the exhaust gas can be reduced with primary and/or secondary reduction methods. Primary methods are methods that affect the engine combustion process directly. The actual degree of reduction depends on engine type and reduction method, but varies from 10% to more than 50%.

Secondary methods are means of reducing the emission level without changing the engine performance from its fuel optimized setting, using equipment that does not form part of the engine itself. The most successful secondary method so far is the SCR (Selective Catalytic

Reduction) method of removing NO _x . This method makes it possible to reduce the NO _x level by more than 95% by adding ammonia or urea to the exhaust gas before it enters a catalytic converter.

The SCR reactor contains several layers of catalyst . The catalyst volume and, consequently, the size of the reactor depend on the activity of the catalyst, the desired degree of NO _x reduction required. The catalyst has typically a monolithic structure, which means that it consists of blocks of catalyst with a large number of parallel channels, the walls of which are catalytically active.

The exhaust gases must have a temperature of at least 280-350 ⁰ C, depending on fuel sulphur content, i.e. high sulphur content requires high temperatures and low temperatures requires low temperatures, at the inlet of the SCR reactor for an effective conversion of NO _x into N ₂ and H ₂ 0.

The exhaust gases at the high pressure side of the turbine of the turbocharger have a temperature of approximately 350-450 ⁰ C, whilst the exhaust gases at the low pressure side of the turbine of the turbocharger typically have a temperature of approximately 250-300 ⁰ C.

Consequently, known large two-stroke diesel engines, operating on HFO, have been fitted with a SCR reactor at the high pressure side of the turbine of the turbocharger. However, there have been a lot of complications associated with the construction of SCR reactors at the high pressure side of the turbine due to the fact that these reactors include very large pipes and containers that have to be able to resist a pressure of approximately 4 bar and are exposed to temperature

changes between approximately 20 and 400 ⁰ C. Heat expansion and fixation have caused great design problems.

In order to avoid these problems, there have been suggestions to move the SCR reactor to the low pressure side of the turbine of turbocharger .

State of the art engines system with a high total fuel efficiency prepared for combined cycle operation, so- called "hot" engines operate with an exhaust gas temperature at the low pressure side of the turbine of the turbocharger with the temperature of approximately 290-300 ⁰ C as opposed to 250 ⁰ C in conventional engines. The increase in exhaust gas temperature in the "hot" engines is achieved by changing the timing of the opening of the exhaust valves and matching of the turbochargers . This change causes the efficiency of the engine itself to fall from approximately 50% to approximately 48.7%. In order to compensate for the engine efficiency drop in it is known to apply both an exhaust gas heated steam boiler driving a steam turbine downstream of the turbocharger turbine or downstream of the SCR reactor for recovering some of the energy in the exhaust gases. The amount of energy that is produced by a steam generator driven with steam provided by the exhaust gas boiler is approximately 7.7% of the engine output at the crankshaft. Further the turbine of the turbocharger receives significantly more energy from the hotter exhaust gases. However, the turbocharger does not require additional energy. The additional energy of the exhaust gases at the high pressure side is in the "hot" engine concept also put to use. This is done by connecting the shaft of the

turbocharger via a transmission to an electric generator or by branching off a portion of the exhaust gases at the high pressure side of the turbocharger turbine and using the branched off portion of the exhaust gases to drive a power turbine (gas turbine) connected to an electric generator. The amount of energy that is produced by the generator driven by the power turbine is approximately 4.4% of the engine output at the crankshaft. Thus, the overall fuel efficiency of the "hot" engine is

48.7 + ((7.7 + 4.4)*0.487) = 54.6%.

However, even in the "hot" engine the temperature of the exhaust gases is insufficient to place the SCR reactor at the low pressure side of the turbine. In order to be able to place the SCR reactor at the low pressure side of the turbine of the turbocharger the temperature of the exhaust gases leaving the turbine must be increased from 290-300 ⁰ C to approximately 330 ⁰ C. This could be done by a combustion unit, such as a burner. However, a 40 ⁰ C temperature raise by means of a burner will lead to a

4.6% increase in overall fuel consumption of the engine

(an additional 4.6 % fuel is burned in the combustion unit) . Some of this additional energy can be recuperated with an efficiency of approximately 25% in the exhaust gas heated boiler and steam turbine downstreams of the SCR reactor. Due to the increase exhaust gas temperature at the low pressure side of the turbocharger turbine the output of the steam turbine will rise from 7.7% to 10.8% of the engine output (with the efficiency of the steam turbine being 27.9%). The overall efficiency of the system with the downstream SCR reactor is :

( 48 . 7 + ( ( 10 . 8 + 4 . 4 ) * 0 . 4 87 ) ) / 1 . 04 6 = 53 . 6 % .

Thus, the overall fuel efficiency will fall from 54.5% to 53.5% when compared with the SCR reactor at the high pressure side of the turbocharger turbine . Such a reduction in fuel efficiency is highly undesirable and will annihilate much of the progress in fuel efficiency in recent years .

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide a large two-stroke diesel engine that has a SCR reactor at the low pressure side of the turbine of the turbocharger with a high fuel efficiency. This object is achieved in accordance with claim 1 by a large supercharged diesel engine comprising a turbocharger having an exhaust gas-driven turbine and a compressor driven by the turbine for supplying charging air to the engine cylinders, a first exhaust conduit leading the exhaust gas from the cylinders to the inlet of the turbine, a SCR converter requiring a minimum temperature for the exhaust gas entering the SCR converter in order to effectively reduce NO _x in the exhaust gas to N ₂ and H ₂ O, a second exhaust conduit leading the exhaust gas from the outlet of the turbine to the inlet of the SCR converter, a third exhaust conduit leading the exhaust gas from the outlet of the SCR converter further on its way to the atmosphere, a heating unit that heats up the exhaust gas upstream of the turbine in order obtain at least said minimum temperature of the exhaust gas at the

inlet of the SCR converter, and a power turbine driven by exhaust gas that is branched off from the first exhaust conduit at a point downstream of the heating unit but upstream of the turbine, or a mechanical power takeoff from the shaft (8) of the turbocharger .

When placing the heating unit at the high pressure side of the turbine of the turbocharger, a 5.91- fuel consumption increase is required to reach the required exhaust gas temperature at the inlet of the SCR reactor, as opposed to fuel consumption increase of only 4.6% when the heating unit is placed at the low pressure side of the turbine of the turbocharger. However, the inventors arrived at the insight that by placing the heating unit upstream of the turbine as opposed to the more instinctively logical downstreams position close to the inlet of the SCR converter, the overall fuel efficiency can increased -despite the increased amount of fuel required by the combustion unit - since the energy used to increase the temperature of the exhaust gases can be recuperated with 100% efficiency in a power turbine driven by exhaust gas that is branched off from the exhaust conduit at the high pressure side of turbine.

Preferably, the engine comprises an exhaust gas boiler placed in the exhaust conduit downstreams of the SCR reactor, and the engine may further comprise a steam turbine driven by the steam produced by said exhaust gas boiler, thereby further increasing the overall fuel efficiency.

Preferably, the power turbine or the mechanical power takeoff is used to drive an electric generator.

The engine may further comprise an electric generator driven by the power turbine or by the power takeoff from the shaft of the turbocharger .

The heating unit can be a burner.

Approximately 20% of the potential expansion energy of the exhaust gas at the high pressure side of the turbine may be diverted or drawn from the turbine .

Preferably, the activation and/or intensity of the burner is controlled by a controller in response to a temperature sensor at - or upstreams from - the inlet of the SCR Reactor.

Further objects, features, advantages and properties of the large turbocharged diesel engine will become apparent from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present description, the invention will be explained in more detail with reference to the exemplary embodiments shown in the drawings, in which:

Fig. 1 illustrates a diagram of the intake and exhaust systems of an internal combustion engine according to a first embodiment of the invention, and

Fig. 2 illustrates a diagram of the intake and exhaust systems of an internal combustion engine according to a second embodiment of the invention.

DETAILED DESCRIPTION

In the following detailed description, the invention will be described by the preferred embodiments. Fig. 1 shows a large turbocharged two-stroke diesel engine of the crosshead type 1 with its intake and exhaust systems. The engine 1 has a charging air receiver 2 and an exhaust gas receiver 3. The exhaust valves belonging to the combustion chambers are indicated by 4. The engine 1 may e.g. be used as the main engine in an ocean going vessel or as a stationary engine for operating a generator in a power station. The total output of the engine may, for example, range from 5,000 to 110,000 kW, but the invention may also be used in four-stroke diesel engines with an output of, for example, 1,000 kW.

The charging air is passed from the charging air receiver 2 to the scavenging air ports (not shown) of the individual cylinders. When the exhaust valve 4 is opened, the exhaust gas flows through a first exhaust conduit into the exhaust receiver 3 and onwards through a first exhaust conduit 5 to a turbine 6 of a turbocharger, from which the exhaust gas flows away through a second exhaust conduit 7. Through a shaft 8, the turbine 6 drives a compressor 9 supplied via an air inlet 10. The compressor 9 delivers pressurized charging air to a charging air conduit 11 leading to the charging air receiver 2.

The intake air in the conduit 11 passes through an intercooler 12 for cooling the charging air - that leaves the compressor at approximately 200 °C - to a temperature between 36 and 80 ⁰ C.

The cooled charging air passes via an auxiliary blower 16 driven by an electric motor 17 that pressurizes the charging air flow in low or partial load conditions to the charging air receiver 2. At higher loads the turbocharger compressor 9 delivers sufficient compressed scavenging air and then the auxiliary blower 16 is bypassed via a non-return valve 15.

A heating unit 19, preferably in the form of a combustion unit such as a burner, is disposed in the first exhaust conduit 5, i.e. upstream of the turbine 6 for increasing the temperature of the exhaust gas in the first exhaust conduit 5. The exhaust gases in the first exhaust gas conduit 5 have to be heated up to an extent that results in the exhaust gases leaving the turbine 6 of the turbocharger having a temperature of at least 33O ⁰ C.

For a "hot" large two-stoke diesel engine in which the exhaust gases when leaving the turbine 6 are approximately 290-300 ⁰ C the temperature increase to be applied to the exhaust gases in the first exhaust conduit

5 is approximately 50 ⁰ C. The amount of additional fuel used by the heating unit 19 for heating up the exhaust gases is in a large two-stroke diesel engine at the high pressure side of the turbocharger turbine is about 5.8% of the fuel consumption of the engine itself.

For conventional large two-stroke diesel engines, in which the exhaust gas temperature when leaving the turbine of the turbocharger is about 250 ⁰ C, the temperature increase of the gases in the first exhaust conduit 5 has to be approximately 100 ⁰ C.

A conduit 30 branches off from the exhaust conduit 5 downstream of the combustion unit 19 but upstream of the turbine 6. The conduit 30 leads a portion (approximately 20% in a large two-stroke diesel engine) of the exhaust gases to an additional power turbine 31. The additional power turbine 31 drives an electric generator 32. The power turbine 31 has an output approximately equivalent to 7.0% of the output of the large two-stroke diesel engine 1.

The surplus of energy in the exhaust gas stream is thus converted to electric power, i.e. energy with a high exergy. The amount of exhaust gas that is branched off to the power turbine 31 can be regulated by a variable flow regulator (not shown) in conduit 30. The exhaust gases that leave the power turbine 31 are reintroduced into the main exhaust gas flow at the low pressure side of the turbine 6 upstreams the SCR reactor.

The second exhaust conduit 7 leads the exhaust gas from the outlet of the turbine 6 to the inlet of a SCR reactor 20. If the temperature of the exhaust gas at the inlet of the SCR reactor 20 is sufficiently high, i.e. typically above approximately 330 ⁰ C, the NO _x in the exhaust gas is converted to N ₂ and H ₂ O .

A third exhaust conduit 22 leads the charging air from the outlet of the SCR reactor 20 to the inlet of a boiler 25. A fourth exhaust conduit 27 leads the exhaust gas from the outlet of the boiler 25 to the inlet of a silencer 28. A fifth exhaust conduit 29 leads the exhaust gas from the outlet of the silencer 28 to the atmosphere.

The boiler 25 uses the heat in the exhaust gas stream to produce (superheated) steam under pressure. A conduit 34 leads the steam produced by the boiler 25 to a steam turbine 37. The steam turbine 37 drives an electric generator 35. The steam turbine has an output power equivalent to approximately 10.8% of the output of the large two-stroke diesel engine.

Figure 2 shows an alternative embodiment of the invention. This embodiment corresponds substantially to the first embodiment, except that the power turbine is replaced with a power takeoff from the turbocharger . Hereto, a transmission 36 connects the shaft 8 of the turbocharger with an electric generator 33.

The fuel efficiency of the large two-stroke diesel engine 1 is 48.7%. The overall fuel efficiency in both embodiments is:

(48.7 + ((10.8 + 7.0)*0.487)) / 1.058 = 54.2%.

The engine according to the invention with the heating unit 19 on the high pressure side of the turbine 6 is with 54.2% significantly more fuel efficient than the

engine described in the background art with the heating unit on the low pressure side of the turbine 6 and a fuel efficiency of 53.6%.

Examples :

1. Hot engine with SCR at high pressure side (prior art)

2. Burner on low pressure side of turbocharger turbine

3. Burner on high pressure side of turbocharger turbine

Thus, the constructional advantage of a SCR reactor on the low pressure side of the turbocharger turbine 6 can be obtained with only a small decrease in fuel efficiency when compared to the "hot engine" with the SCR reactor on the high pressure side of the turbocharger turbine that has many practical constructional problems.

In both embodiments of the invention, the steam produced by the boiler 25 could be used for other purposes than driving a steam turbine, such as for heating purposes.

Each of the embodiments can be provided with a temperature sensor (not shown) placed near the inlet of the SCR reactor 20 for measuring the temperature of the

exhaust gases in the second exhaust conduit 7. The signal of the temperature sensor is communicated to a controller (not shown). The controller controls heating unit 19. The controller increases the activity of the heating unit 19 when the temperature of the exhaust gases in the second exhaust gas conduit 7 is not sufficiently high and reduces the activity of the combustion unit 19 when the temperature of the exhaust gases in the second exhaust gas conduit 7 is above the minimum temperature for an effective operation of the SCR reactor.

Both embodiments can be devised as a so-call humid air engine (not shown), e.g. an engine that is operated with charging/scavenging air with a very high absolute water (vapour) content . The charging air is in this variation of the invention approximately 60 to 90 ⁰ C (as opposed to 37 ⁰ C in a conventional engine) and the absolute humidity is about 40 to 80 g/kg i.e. approximately 4 to 8 times the water (vapour) content of a "non-humid air" motor. The humidification (obtained by injecting relatively warm water in a "scrubber" (not shown) ) causes the energy content of the charging/scavenging air and thus of the exhaust gases to increase substantially. The additional energy in the charging air is obtained in two ways:

by reducing the amount of energy withdrawn from the charging/scavenging air by the intercooler, i.e. the amount of "waste" energy created by the intercooler is reduced, and by injecting water warmed up with hot water from the cooling system of the engine, i.e. injecting water containing "waste energy" .

The additional energy in the exhaust gases can be relatively efficiently recuperated in the power turbine, and thus an even higher overall fuel efficiency than indicated in the examples above can be obtained.

The term "comprising" as used in the claims does not exclude other elements or steps. The term "a" or "an" as used in the claims does not exclude a plurality. The reference signs used in the claims shall not be construed as limiting the scope.

Thus, while the preferred embodiments of the devices and methods have been described with reference to the environment in which they were developed, they are merely illustrative of the principles of the inventions. Other embodiments and configurations may be devised without departing from the spirit of the invention and the scope of the appended claims.