MARINE VESSEL MACHINERY AND A METHOD OF CONTROLLING EXHAUST GAS EMISSIONS IN A MARINE VESSEL

Technical field

The invention relates to sulphur exhaust controlling of exhaust gases in marine vessels. The invention particularly relates to a marine vessel machinery, comprising at least two fuel combustion units, such as main engines in a diesel- mechanical or diesel-electric propulsion configuration, a fuel feeding system for feeding fuel to the at least two combustion units, exhaust gas systems for lead- ing away the exhaust gases generated during the combustion of the fuel, separate first fuel tank for first fuel and a second fuel tank for second fuel, at least one exhaust gas sulphur emission control device in connection with the first combustion unit of the at least two combustion units, according to preamble of claim 1. The invention relates also to method according to the preamble of claim 5.

Background art

Environmental issues of various types of marine vessel are presently under increasing interest. Particularly, it is important that the sulphur emissions in ex- haust gases are at low level. As a result, the demand for marine fuel with low sulphur content is increasing, and so is the price compared to conventional marine heavy fuel oil (HFO) used in diesel engines and oil-fired boilers on ships.

Cleaning of exhaust gases in respect of the reduction of sulphur oxides by scrubbing is gaining an increasing amount of interest. For example in publica- tion WO 9944722 there is shown a wet scrubber for reducing contaminants in exhaust of diesel engines on ships. Installing scrubbers in a ship means a considerable investment particularly in a large vessel with several engines. It is also technically difficult to find room for a scrubber in connection with each engine. The size of a scrubber is considerably large since back pressure of the engine

exhaust system must be kept at relatively low level. Furthermore, operating scrubbers involve also some operating expenses in the form of pumping costs and/or costs for scrubber chemicals.

On the other hand, the sulphur oxide emissions may be kept low obviously by operating on fuels with low sulphur content. This is naturally possible, but such a fuel is typically more expensive than regular one, particular when distillate fuels (Light Fuel Oil, LFO) with a typical sulphur content of 0.1 - 0.2 % is in question.

In publication GB 2058211 there is shown a diesel engine fuel mixing system for mixing higher-grade and lower-grade fuel oils and feeding the mixture to a diesel engine.

However, blending of LFO (Light Fuel Oil) and HFO (Heavy Fuel Oil) in proportions which would be required for substantial reduction of sulphur emission would lead to such proportions that induce a risk of incompatibility, thus jeop- ardizing the safety of the vessel .

When two different fuels are mixed with such proportions which would result in proper sulphur exhaust reductions, there is a risk of incompatibility, which may cause clogging of fuel filters and separators and sticking of fuel injection pumps (asphaltene deposits). Compatibility problems are related to the fuel's stability reserve. HFOs are rather aromatic and contain considerable amounts of asphal- tenes. If the stability reserve of such a HFO is low, it cannot stand mixing more paraffinic LFO into it, and as a consequence asphaltenes will precipitate out of the blend. The risk is generally not very high, but incompatibility may occur e.g. if two fuels originate from different crude oils or have gone through different re- fining processes. Incompatibility problems may occur during transients when switching fuel, but also during steady-state, e.g. when blending fuels with certain ratio.

An objective of the invention is to provide marine vessel machinery for controlling exhaust gas emissions.

A specific object of the invention is to provide marine vessel machinery and method of controlling exhaust gas emissions in a marine vessel with which the installation of numerous space consuming exhaust gas scrubbers is avoided with simultaneously providing the total exhaust gas sulphur emission to be at acceptable level.

Disclosure of the invention

Objectives of the invention are met substantially as is disclosed in claims 1 and 5. The other claims present more details of different embodiments of the inven- tion.

The basic idea of the invention is that one of the used means for reducing sulphur emissions provides so efficient reduction that the other may be less effective still resulting in total sulphur emissions in exhaust gases with adequate level. This way all the fuel combustion units do not need to be provided with ex- pensive and space consuming means for reducing sulphur emissions.

In practise this base on providing fuel combustion units with exhaust gas emission reduction device and providing some other fuel combustion units with fuel blending units. The invention relates to vessels with a minimum of two fuel combustion units, like main engines, a number of which are equipped with exhaust gas emission reduction units, the at least some of the rest with fuel blending units.

Particularly, the invention relates to marine vessel machinery, comprising at least two fuel combustion units, such as main engines in a diesel-mechanical or diesel-electhc propulsion configuration, a fuel feeding system for feeding fuel to the at least two combustion units, exhaust gas systems for leading away the exhaust gases generated during the combustion of the fuel, separate first fuel tank for first fuel and a second fuel tank for second fuel and at least one exhaust gas sulphur emission control device in connection with the first combustion unit of the at least two combustion units. It is characteristic to the invention

that the fuel feeding system is provided with a fuel blending unit connected to the first and the second fuel tanks, and the first combustion unit of the at least two combustion units is connected to the first exhaust gas system provided with the at least one exhaust gas sulphur emission control device and to the first fuel tank upstream the fuel blending unit so that the first combustion unit utilizes only the fuel in the first fuel tank, and that the second combustion unit of the at least two combustion units is connected to the second exhaust gas system and to the fuel blending unit so that the second combustion unit utilizes the fuel coming from the fuel mixing unit.

This way the total sulphur content of the vessel's exhaust gases is low enough without a need of installing a scrubber for each engine and without a need of performing the fuel blending at ratios which would pose a risk of incompatibility.

The fuel blending unit comprises a mixer for enhancing mixing the fuels from the first and the second fuel tanks. The mixer may be a static mixer, which makes the arrangement quite simple.

Preferably the exhaust gas sulphur emission control device is a scrubber, in which water based solution is caused be in direct contact with the exhaust gas for efficient sulphur oxide removal.

According to method of controlling exhaust gas emissions in a marine vessel comprising at least two fuel combustion units, such as main engines in a diesel- mechanical or diesel-electric propulsion configuration, a fuel feeding system for feeding fuel to the at least two combustion units, exhaust gas systems for each combustion units for leading away the exhaust gases generated during the combustion of the fuel, separate first fuel tank for first fuel and a second fuel tank for second fuel, at least one exhaust gas sulphur emission control device in connection with the first exhaust system of the exhaust systems, the fuel from the first and the second fuel tanks having first and second sulphur content respectively are mixed to produce a fuel mixture, the fuel mixture is introduced into the second combustion unit wherein the fuel mixture is combusted produc- ing exhaust gases, the exhaust gases resulting from the second combustion unit having a fourth sulphur content is led to the atmosphere, fuel from the first

fuel tank is introduced in the first combustion unit wherein the fuel is combusted producing exhaust gases, the exhaust gases resulting from the first combustion unit is led to an exhaust gas sulphur emission control device wherein the sulphur content of the exhaust gases is reduced having a third sulphur content, and the ratio of fuel from the first and the second fuel tank in the fuel mixture is set so that the total combined sulphur content of the exhaust gases resulting from the second combustion unit and the exhaust gases resulting from the first combustion after leading through a exhaust gas sulphur emission control device, is less than a predetermined set value.

With applying the method according to the invention it is possible to avoid installing a space consuming and expensive scrubber to the exhaust ducting of each engine and still use fuel which has considerably high sulphur content without increasing the sulphur (oxide) emissions.

The total sulphur content of the exhaust gases corresponds to exhaust gas emission of combustion of fuel having sulphur content less than 1 ,5%. In practise the actual total sulphur content of the exhaust gases is depending on the blending ratio of the first and the second fuel as well as their actual sulphur content, thus in the future it is most likely that even lover total sulphur content may be obtained with the machinery and method according to the invention.

The total sulphur content of the exhaust gases is a weighted average of sulphur contents of individual exhaust gas mass flows.

The exhaust gas sulphur emission control device is a scrubber wherein the gas is subjected to direct contact with water-based solution.

In this context the sulphur content means in addition to sulphur as such, also sulphur-containing substances possibly present in the fuels and exhaust gases, like SO ₂ , H ₂ SO ₄ , etc.

Brief Description of Drawings

In the following the invention will be described with the reference to the accompanying schematic drawing, in which

Figure 1 illustrates an embodiment of machinery for controlling exhaust gas emission is a marine vessel, Figure 2 illustrates an embodiment of the invention.

Detailed Description of Drawings

In figure 1 there is shown a marine vessel 1 , which is a ship, which is provided with machinery including two combustion engines 2, 2' acting as fuel combustion units. This should be understood so that the engines 2, 2' may also represent for two groups of several engines. In this illustrative example the ship is provided with electric propulsion system with the engines 2, 2' being provided with respective generators (not shown) arranged to provide electricity through a network 7 to the propulsion motor 3. The engines 2, 2' are provided with exhaust system 10 for leading the exhaust gases away, eventually to the atmosphere.

The machinery comprises further at least two separate fuel tanks 4, 5 for two different types of fuel or fuels having different sulphur contents like HFO and LFO. The tanks 4, 5 are shown here only schematically and represent e.g. so called day tanks of the ship. The tanks 4, 5 are in connection with the engines 2 through a fuel feeding system 6.

The fuel feeding system comprises a first fuel supply line 5.1 which is in connection with the first fuel tank 5 and the first engine 2. The first fuel tank 5 is ar- ranged for the first fuel having a first sulphur content, like heavy fuel oil (HFO). Typically the first sulphur content is at level below 4,5%, typically about 3%. The fuel feeding system comprises also a second fuel supply line 4.1 , 4.2, having a first part 4.1 and a second part 4.2, the first part 4.1 connecting the second fuel tank 4 to a blending unit 11 and the second part connecting the blending unit 11 with the second engine 2'. The blending unit 11 is also connected to the first

fuel supply line 5.1 with a connection line 5.2. The second fuel tank 4 is arranged for a second fuel having a second sulphur content, like light fuel oil (MDO). Typically the second sulphur content is below 0,2%.

Now, in order to keep the total sulphur content in the exhaust gases of the ves- sel having at least two separate combustion units, like the combustion engines, at acceptable (certain predetermined set value) level, the engines (or generally two sets of engines) are provided with separate means for reducing sulphur emissions of the vessel's exhaust gas. The basic idea is that one of the means for reducing sulphur emissions provides so efficient reduction that the other may be less effective still resulting in total emissions with adequate level. This way all the engines do not need to be provided with expensive and space consuming means for reducing sulphur emissions.

The fist engine 2 is in connection with a first exhaust gas system which is provided with at least one exhaust gas sulphur emission control device 9. The ex- haust gas sulphur emission control device is a scrubber, in which water-based solution is arranged to contact with the exhaust gas thus cleaning the gas also chemically, bounding the sulphur oxides from the gas to the scrubbing liquid.

It is preferable to utilize sea water as bases for the solution used in the scrubbers in marine vessels. The actual construction and process used in a scrub- bers must be considered case by case, which may vary accordingly.

The reduction of sulphur oxides in a scrubber is typically over 80%. Additionally, the engines 2 connected to such exhaust system which are provided with a scrubber are also connected to the first fuel tank 5 containing fuel having substantially high sulphur content (HFO). This way the exhaust gas sulphur emis- sions resulted in combusting heavy fuel oil will become lower than the total sulphur content in the exhaust gases of the whole vessel (certain predetermined set value).

In the machinery according to the invention the second engine 2' (or set of second engines) in turn is connected to a second exhaust system 10' which is not provided with an exhaust gas sulphur emission control device. In stead, the sec-

ond engine 2' is connected to a blending unit 11 by means of a second part 4.2 of the second fuel supply line. The blending unit 11 is connected to both the second fuel tank 4 and to the first fuel tank 5, in this case actually to the first particularly to the first fuel supply line 5.1.

In the blending unit the first fuel having substantially high (first) sulphur content (e.g. HFO) from the first fuel tank 5 is blended with the second fuel having substantially low (second) sulphur content (e.g. LFO) from the second fuel tank 4 in proportions with minimum risk of incompatibility. The blending is performed so that the percentage of the first fuel in second fuel is less than 30%, preferably less than 20%.

Even if the sulphur content of the blended fuel is relatively high, which is also the case of the exhaust gas sulphur content level (a fourth sulphur content), the exhaust gas the total sulphur content in the exhaust gases of the whole vessel, that is combined exhaust of the first and the second engine, is still below the certain predetermined set value.

This way the total sulphur content of the vessel's exhaust gases is low enough without a need of installing a scrubber for each engine and without a need of performing the fuel blending at ratios which would pose a risk of incompatibility.

In practise the blending ratio is preferably determined based on the sulphur con- tents of the both fuels when the ship is refuelled. Another, more sophisticated manner would be arranging proper measurement devices to each exhaust duct, that is, mass flow meter and sulphur content meter. By defining a weighted mean value of exhaust sulphur content the blending ratio of the fuel may be controlled in real time.

For example, if the sulphur content of the first fuel is about 3,0%, and 0,1 % for the second fuel, with blending ratio of about 10% it is possible to reach the 1.5% level for the total ship exhaust emission.

In figure 2 there is shown another embodiment of the invention, which has the basic elements corresponding to those of figure 1. In this embodiment there is a

set of first engines 2 comprising two engines and a set of second engines 2' comprising also two engines. The first fuel supply line 5.1 for delivering the first fuel to the engines 2 has a section with dotted line. This stands for the fact that the line may comprise fuel treatment devices not relevant to the invention like pumps, filters, fuel heaters etc.

The first part of the second fuel supply line 4.1 is arranged between the second fuel tank 4 and the fuel blending unit 11. In figure 2 the fuel blending unit comprises a static mixer 11.4 in which the fist fuel and the second fuel are efficiently mixed. The first part 4.1 of the second fuel supply line comprises a coarse filter 11.3, a pump 11.2 with adjustable capacity and the first flow meter 11.3. There is also a second flow meter 11.5 provided in the line after the mixer 11.4. The flow meters and adjustable pump 11.2 makes it possible to accomplish proper ratio of the fist and second fuel after the blending unit 11 by adjusting the capacity of the pump 11.2 so that desired ratio is achieved based on the meas- urements of the flow meters 11.3, 11.5. The pump 11.2 is preferably operated by frequency controlled electric motor so that continuous speed control is achieved.

The connection line 5.2 is arranged to connect the mixer 11.4 and the first fuel supply line 5.1 so that fuel from the first tank 5 may be fed to the mixer and blended with the fuel coming from the second tank 4. The connection line 5.2 comprises a pump 5.3 and a filter 5.4 upstream the mixer 11.4.

The mixer 11.4 is connected to a fuel mixing tank 4.3, which ensures that the fuel is as homogeneous as possible. The fuel in the mixing tank 4.2 is led to the second set of engines 2' to be combusted there in. The second part of the sec- ond fuel supply ling 4.2 is provided with a pump 4.4, a fuel heater 4.5 and a vis- cosimeter 4.6 based on the information of which the fuel heater 4.5 is controlled. In figure 2 there is also shown a fuel return line 4.7 leading from the engine back to the mixing tank. The mixer may be also of other type instead of being a static one. Additionally, even if not shown in the figures there may be addi- tional heater (s) provided upstream the mixer or integrated in the mixer itself.

It is clear that the invention is not limited to the examples mentioned above but can be implemented in many other different embodiments within the scope of the inventive idea. The fuel combustion unit may be instead of or in addition to the main engines, mentioned above, auxiliary engines or even oil-fired boilers. Instead of mixing HFO and LFO, the two fuels with different sulphur contents may be two different HFOs or two different LFOs. Instead of one first fuel tank 5 and one second fuel tank 4 there may be more than one tank for each type of fuel.