Field of the invention

The present invention relates to a method and a plant for purification of exhaust gas from a diesel engine. More specifically, the invention relates to a method and a plant for removing or substantially reducing particles, SO _x , NO _x and optionally, CO ₂ from the exhaust gas from diesel engines, such as marine diesel engines.

Background of the invention

Marine diesel engines are major emitters of SOx, NOx, particulate matter and

CO2. Currently, the emissions of SOx and NOx are of great concern to the industry.

SOx (mainly SO2, some SO3) originates from the combustion of sulphurous compounds in the fuel oil. Typically heavy fuel oil is preferred because it carries the lowest cost. Heavy fuel oil also contains the largest amounts of sulphurous compounds (typically 4.5% or more by weight) and therefore causes the largest emission of SOx. New regulations may require the use of fuel oil with lower sulphur content, typically below 1.5% by weight. Such fuel oil cost about 10 to 15% more. Even with this improved fuel oil, the flue gas will contain several hundred ppm SOx on a volume basis.

In comparison, flue gas from coal firing with an appropriate sorbent added to the combustor can contain well below 20 ppm SOx.

The requirement to use low sulphur fuel oil means that there will be an increase in demand for this fuel oil, and a corresponding decreased demand for high sulphur fuel oil. High sulphur fuel oil may therefore become relatively less expensive, and the economic benefit of using such fuel oil may increase with time. The use of low sulphur oil will additionally require modifications in the use of additives in order to reduce wear and tear in the cylinder liner.

As an alternative to low sulphur fuel oil, ships may have flue gas purification systems which capture SOx.

Environmental effects of SOx include impaired visibility because fine particles scatter and absorb light. SOx accelerates metal corrosion and causes damage to building materials. The gas also causes acid rain and damage to vegetation, Although difficult to measure, there are indications that SOx may cause temporary breathing impairment and reduced lung function in humans. Any required reductions in SOx emissions are driven by such environmental concerns.

NOx (mainly NO, some NO ₂ ) originates from the fuel (fuel NOx) and from reactions between nitrogen and oxygen in the combustion process (thermal NOx). Marine diesel engines typically emit about 8 g/kWh NOx. This corresponds to several hundred ppm NOx in the flue gas on a volume basis. Some environmentally restricted areas may require much reduced NOx emissions, perhaps 2 to 3 g/kWh. This still corresponds to flue gas NOx concentrations in the hundred ppm range on a volume basis.

In comparison, gas turbines typically emit less than 25 ppm NOx on a volume basis. Requirements down to 5 ppm are seen in some areas for gas turbines.

NOx reacts with oxygen in the presence of volatile organic compounds and sunlight to form ground level ozone. In some areas, ground level ozone is one of the most intractable air pollution problems. NOx also accelerates damage to materials, and may absorb visible light, causing reduced visibility. The gas affects vegetation. In coastal waters, it may lead to changes in competition between plant species, leading to explosive algae growth and oxygen depletion. Death of fishes in fish farming facilities due to NO _x has also been reported. On land, NO _x may reduce yield of some crops. Possible health effects include undesirable changes in lung function in people with respiratory illnesses. Although currently of less concern than SOx and NOx, particulate matter from marine diesel engines may be responsible for reduction in visibility. It may cause damage to materials. It may also settle on leaf surfaces, reducing photosynthesis.

EP 1 795 721 A1 describes a particulate trap to remove particular matter arranged between the exhaust manifold and the turbine of the turbo of a diesel engine. US 4.335.684 describes a ash removal unit provided between the exhaust manifold and the turbine of the turbo of a diesel motor. Neither of the publications describes removal of SOx or NOx.

US 2008/0022667 and US 6.058.700 both describe devices for purifying exhaust gas from engines by removal or reduction of NOx and SOx. The main focus of both publications is control systems for monitoring the emission of said gases and to control the system to reduce the emission.

Methods and plants for removal of NO _x and SO _x are also known from several publications, such as e.g. the following review articles by Joseph A. MacDonald in Energy Tech Magazine: "Power Plant Emissions Controls: SO ₂ removal technologies & systems", http://www.energy- tech.com/index.cfm?PagelD=108&c2e=236&e2e=0&rs=0 &artid=543, "Power Plant Emissions Controls NO _x reduction technologies & systems", http://www.energy- tech.com/index.cfm?PagelD=108&c2e=236&e2e=0&rs=0 &artid=466, and

"Combined Sθ ₂ /NO _x removal processes yield lower costs, no waste, salable byproducts", http://www.energypubs.com/index.cfm?PagelD=108&c2e=236&a mp;e2e=0&rs=0&arti d=569

CO ₂ from marine diesel engines is one of a few major contributors to the current increase in atmospheric CO ₂ and hence possibly to global warming. A cost effective reduction of the emission of CO ₂ from marine diesel engines could therefore be a valuable contribution to the required reduction in CO ₂ emission. Captured CO2 may be liquefied and stored on board. Typical storage condition is about -50 deg C and 7 barg, container diameter up to 10 meters. CO2 has increasing value for enhanced oil recovery purposes.

Summary of the invention

According to a first aspect, the present invention relates to a method for purification of exhaust gas from a marine diesel engine comprising a diesel motor and a compressor for compression of air for introduction into the engine, where the compressor is driven by a turbo expander driven by exhaust gas from the diesel motor, wherein the exhaust gas leaving the diesel motor is purified in a unit for removal of NO _x and a unit for removal of SO _x before the exhaust gas is expanded in the turbo expander.

According to a second aspect, the present invention relates to a marine diesel installation, comprising a diesel engine comprising a diesel motor and a compressor for compression of air for introduction into the engine, where the compressor is driven by a turbo expander driven by exhaust gas from the diesel motor, wherein the installation further comprises a unit for removal of SO _x and a unit for removal of NO _x between an exhaust manifold of the diesel motor and the turbo expander.

Short description of the figures

Figure 1 is a simplified representation of a marine diesel installation according to the prior art,

Figure 2 is a simpliefied representation of a first embodiment of the present invention,

Figure 3 is a simplified representation of a second embodiment of the present invention, Figure 4 is a simplified representation of a third embodiment of the present invention,

Figure 5 is a simplified representation of a fourth embodiment of the present invention, and Figure 6 is a simplified representation of a fifth embodiment of the present invention.

Detailed description of the invention Figure 1 is a schematic view of a maritime diesel engine installation onboard a ship. The installation comprises a diesel engine 1, receiving air for combustion through an air inlet line 2, and liquid fuel through a primary fuel line 3. The air is compressed in an air compressor 4, typically to a pressure from 2 to 3 bar. The compressed air is normally cooled by means of an intercooler 5 before the air is introduced into the diesel motor 1.

The exhaust gas is withdrawn from the diesel engine, typically at a pressure from

3 to 5 bar through an exhaust manifold 6 and is introduced into a turbo expander 8 where the exhaust is expanded to a pressure close to 1 bar, and is withdrawn trough an exhaust line 10 and released into the atmosphere. The air compressor

4 and the turbo expander 8 are preferably arranged on a common axle 9 so that the air compressor is driven by the turbo expander.

Additionally, a main axle 11 for transfer of the mechanical energy from the engine, and cooling lines 12, 12' for introduction and withdrawal, respectively, of cooling water, are also provided.

Figure 2 illustrates a first embodiment of the present invention, where means for purification of the exhaust gas are provided between the exhaust manifold 6 and the turbo expander 8.

According to the embodiment of figure 2, exhaust gas leaving the manifold 6 is introduced into a filter 20 to remove or substantially reduce the amount of particles in the exhaust gas. The filter 20 may be a high temperature metal cartridge type, with automatic on line pulsing for particle removal.

The filtered air form the filter 20, is introduced into a NO _x reduction unit 21. The NO _x reduction unit 21 illustrated is a Selective Catalytic Reduction (SCR) unit, into which NH ₃ is introduced through a NH ₃ line 22. NO _x and NH ₃ react with water vapour and oxygen present in the exhaust gas according to the following equations:

4NO + 4NH ₃ + O ₂ = 4N ₂ + 6H ₂ O 2NO ₂ + 4NH ₃ + 02 =3N ₂ + 6H ₂ O 6NO + 4NH ₃ = 5N ₂ + 6H ₂ O 6NO ₂ + 8 NH ₃ = 7N ₂ + 12H ₂ O

to give water and nitrogen gas.

SCR as such is well known and widely used to remove NO _x from combustion gases. Depending on the catalyst, the SCR operates in the temperature range from 300 to 400 ^c C. However, the use of a pressurized SCR is not common although it gives significant advantages both with respect to space requirement and efficiency. As an alternative to SCR, the NO content in the NOx may be oxidized to NO2 which is more water soluble than NO. The N02 may be removed in downstream scrubbing equipment.

From the SCR unit 21 , the combustion gas being reduced in NO _x is cooled in a heat exchanger 23 and subsequently purified in a scrubber 24 where the combustion gas is scrubbed by countercurrent flow to a SO _x solvent, such as water in a contact section 25 of the scrubber. The solvent is collected in the bottom of the scrubber and is withdrawn through a line 26. The solvent in line 26 is split between a solvent removal line 27 where the solvent is removed for deposition, or regeneration, and recirculation via a pump 28. The re-circulated solvent is cooled in a cooler 30 and introduced into the scrubber through a line 31. Makeup solvent to replace the volume removed through line 27, is introduced through a line 29.

The scrubbed combustion gas is withdrawn from the top of the scrubber through a purified combustion gas line 33. A demister 32 is preferably arranged in the top of the scrubber to remove or at least reduce the amount of solvent droplets in the purified gas withdrawn in line 33. The gas in line 33 is reheated against gas from the SCR unit 21 , in the heat exchanger 23 before the gas is expanded over the turbo expander 8 and released through exhaust line 10.

This first embodiment is compact and efficient. By arranging the scrubber 24 downstream for the SCR unit 21 , a unwanted slip of NH ₃ into the atmosphere is avoided. Some ammonium sulphate may, however, be formed as fine particles in the heat exchanger 23.

Figure 3 illustrates an alternative embodiment, where SO _x is removed from the exhaust gas before removal of NO _x . Only the features that separate this embodiment from the embodiment of figure 2 are described in detail.

In this embodiment the gas leaving the filter 20 is cooled in the heat exchanger 23 and introduced into the scrubber 25. The scrubbed gas is then reheated in the heat exchanger before introduction into the SCR unit 21.

The solvent used in the scrubber is preferably water or an alkaline aqueous solution, such as slurries of CaCO ₃ or CaO, and solutions of ammonia, sodium hydroxide, sodium carbonate, potassium hydroxide or magnesium hydroxide, or sea water. If environmentally accepted, the used absorbent may be released into the sea. If not, precipitated sulphur compounds is collected and deposited, and the remaining absorbent may be re-circulated after make up with the absorbing agent to make up for the amount that is removed due to the reaction with sulphur.

This second embodiment is also compact and efficient. By arranging the scrubber 24 upstream for the SCR unit 21 , there may be a slip of NH ₃ into the atmosphere. This solution gives very clean working conditions for the SCR and the formation of ammonium sulphate is thus avoided.

The use of sea water as a solvent for the SO _x scrubbing may be preferred on a ship if it is allowed to release the used solvent into the ocean. The use of se water may however, require an additional washing step to remove any salt present in the gas leaving the scrubber. Figure 4 illustrates one alternative for implementing such a washing step in a plant according to figure 2.

Figure 4 illustrates a water scrubber 40 arranged at the top of the scrubber 24. The gas leaving the scrubber 24 is introduced into the water scrubber 40 through a line 41. The gas entering the water scrubber is washed by countercurrent flow to water in a contact section 44. Water is introduced through a water line 42. Water leaving the contact section is collected in the bottom of the water washer and is removed through line 43 and is re-circulated back into the water washer again by means of a pump 45. A predetermined amount of the water in line 43 is withdrawn through a used water line 47 and is replaced by make up water in a make up line 46. The cleaned exhaust gas is withdrawn from the contact section 44 in line 33, as described with reference to figure 2. A demister 48 is preferably arranged at the top of the water washer to reduce or remove water droplets in the gas leaving the contact section 44 before it is withdrawn through line 33.

Figure 5 illustrates a fourth embodiment of the present invention. This embodiment is a further development of the embodiment of figure 3, where a washing step is introduced for the gas leaving the SCR unit 21 in a line 50. The gas in line 50 is cooled in a heat exchanger 51 before it is introduced into a washing section 53, where the gas is scrubbed by counter-current flow against water in a contact section 53. Water is introduced at the top of the contact section 53 through a water line 54. The water is collected in the bottom of the washing unit and is withdrawn through a line 55, and re-circulated into line 54 by a pump 56. A cooler 57 is preferably arranged in line 54 to cool the water. A part of the water in line 55 is withdrawn through a waste water line 58, and is replaced by water that is introduced through a make up water line 59. The washed gas is withdrawn from the washing unit through a line 60 and is reheated in heat exchanger 51 before being introduced into turbo expander 8. A demister 61 is preferably arranged at the top of the washing unit to remove water droplets from the gas. The operating conditions for the SCR is very good as the gas is cleaned both of dust and SO _x , as in the embodiment according to figure 3. Accordingly there is no formation of ammonium sulphate. Additionally, the additional washing step removes, or substantially reduces, the slip of NH ₃ to the atmosphere.

Figure 6 is a schematic view of a combined plant 100 for NOx/SOx removal, and CO2 capture. Exhaust gas withdrawn from the diesel engine 1 in the manifold 6 is filtered in a filter 20 as above. The filtered exhaust gas is then cooled in a cooler 101 before being introduced into a compressor 102 and compressed to a pressure of 10 to 25 bar. Air is introduced into an air compressor 104 through an air inlet 103, and is also compressed to a pressure corresponding to the pressure of the exhaust gas. Both compressions are preferably multistage processes with cooling, as indicated in the figures between the steps.

The compressed air and exhaust gas is combined in line 105 and is introduced into a combustion chamber 106 as an oxygen containing gas for combustion of pressurized diesel fuel that is introduced into the combustion chamber 106 through a additional fuel line 107. In the combustion chamber 106, the fuel is combusted under a pressure corresponding to the pressure of the incoming oxygen containing gas, i.e. 10 to 25 bar, under generation of steam in steam coils 108 arranged in the combustion chamber.

The steam generated in the steam coils is withdrawn through a steam line 109 and is expanded in a turbine 110 to produce electrical power in a generator 111. A steam line 112 may also be provided to deliver steam to other steam consuming processes in the plant to give energy integration of the total process.

After expansion in the turbine 110, the expanded steam is condensed in a condenser 113 before the condensate is returned to the steam coils 108 via line 113 and high pressure pump 114.

The combustion gas formed by the combustion in the combustion chamber 106 is withdrawn from the combustion chamber and filtered through a filter 115 before being introduced into a SCR unit 116 to remove NOx by reaction with NH ₃ introduced through an ammonia line 117, as described above. The exhaust gas leaving the SCR unit 116 is cooled in a heat exchanger 118 before being introduced into a venturi scrubber 119, where the exhaust gas is scrubbed by a solvent to remove SOx. The solvent is preferably one of the solvents as described with reference to the SCR unit 21 above, such as calcium carbonate.

The exhaust gas leaving he venturi scrubber is again cooled by introduction into a lower chamber 120' of a direct contact heat exchanger 120 where the gas is cooled by direct contact with water in a contact section. The cooled gas from the direct contact heat exchanger 120 is introduced into the bottom of an absorber 121 , where CO ₂ is absorbed by a liquid absorbent by countercurrent flow through a contact zone in the absorber. The not absorbed exhaust gas is withdrawn from the absorber 121 and introduced into an upper section 120" of the direct contact heat exchanger 120 where the gas is heated against circulating water.

Water is circulated in the direct contact heat exchanger by means of a pump 123 pumping water through a line 124. The water in line 124 is introduced at the top of the upper chamber 120" and flows counter-current to the gas flowing upwards in a contact zone in the upper chamber. The water from the contact zone is collected at the bottom of the upper chamber and transferred to the top of the lower chamber 120' through a line 125. In the lower chamber the gas from the venturi scrubber is cooled by the water by countercurrent flow in a contact zone therein. The water leaving the contact zone is collected at the bottom of the lower chamber and is introduced into the pump 123 to be re-circulated.

The CO ₂ depleted and partly heated gas is withdrawn from the top section 120" through a line 126 and is heated in the heat exchanger 118 before being expanded over a turbine 127 driving a generator 128 to produce electricity power. The expanded gas from the turbine 127, having a pressure close to the pressure of the exhaust gas originally leaving the diesel engine 1 , is heated again in the heat exchanger 101 , before the gas is introduced as driving gas into the turbo expander 8 and is released as a purified exhaust gas through line 10. The absorbent collected at the bottom of the absorber 121 , being rich in CO ₂ is withdrawn from the absorber in a line 130 and is flashed in a flash vessel 131 , to a pressure below the absorber 121 operating pressure, to flash off and reduce the level of oxygen in the absorbent and thus finally in the CO ₂ fraction isolated later. The gas stripped off the rich absorbent comprises mainly oxygen and a minor amount of H2O and CO ₂ , is vented to the atmosphere through a vent line 132. The liquid phase from the flash vessel is introduced into a regenerator column 133 where the absorbent is stripped by counter-current flow of steam in a contact zone. The steam for stripping of the absorbent is generated in a reboiler 134 by heating of a part of stripped absorbent that is collected at the bottom of the regenerator column 133. Stripped, or lean absorbent is re-circulated from the bottom of the regeneration column 133 and is introduced at the top of the absorber 121.

The absorbent used in the absorber / regenerator circuit for capturing CO ₂ is preferably an aqueous absorbent. At high pressure even water may be used as the absorbent but the most preferred absorbent is an aqueous carbonate solution. Amines that are commonly used for capturing CO ₂ may not be used due to a too high partial pressure of oxygen in the gas to be treated, causing degradation of the amines.

The CO ₂ that is stripped of the absorbent in the regenerating column is withdrawn trough a line 135, partly condensed in a cooler 136 and flashed in a flash tank 137 to remove water before the gas phase from the flash tank is compressed by means of a compressor 139 driven by a motor 140, to give CO ₂ that is withdrawn through a CO ₂ line 141 for further treatment and deposition. The liquid phase formed in the flash tank 137 is returned into the regenerator column through a line 138.

The plant described with reference to figure 6 has a fuel consumption that is about 15 % higher then the diesel engine alone giving the same power output. The price reduction by allowing the engine to be run on high sulphur fuel, is however, about 15 %. The benefits are thus mainly connected to the total reduction of pollution into the surroundings and to any fees and taxes that have to be paid to the authorities for emission of NO _x , SO _x and CO ₂ .

The above described processes for removal of SO _x and NO _x, are introduced between the exhaust manifold and the turbo expander. Accordingly, the SCR and the scrubber operates at an elevated pressure of typically 3.5 to 5 bar, dependent on the characteristics of the diesel engine and the adjustments thereof. The high pressure makes it possible to reduce the size of the SCR unit and the scrubber, and thus the space requirement and the cost thereof, making the purification method and plant thereof practically and economically possible to implement onboard a ship.

The purification as described above results in a pressure drop of the exhaust gas from the manifold to the turbo expander of 0.5 to 1 bar. This pressure drop may be compensated at least partly, by the turbo expander being more efficient over time due to particles being removed from the exhaust gas. Particles may reduce the efficiency of the turbo expander over time. Alternatively, the engine valves may be adjusted to give a slightly higher pressure of the exhaust gas leaving the diesel engine. Such an adjustment may result in a minor reduction of the output of the engine.

The tables below are simulation examples.

Table 1 is a comparison of a marine diesel engine according to the prior art as described with reference to figure 1 (example 1 ) with the embodiment of figure 6 under different operation conditions (examples 2 to 4). Table 1

* After 5% parasitic power deducted from diesel engine shaft output - service (not testbed) conditions

Table 2 is a comparison of a marine diesel engine according to the prior art (Example 5) with reference to figure 1 , whereas examples 6 to 8, are simulations on an embodiment according to figure 2. The values given for examples 6 to 8 will however, be valid also for the embodiment of figure 3 as the sequence of the purification steps therein is irrelevant for the simulation.

Table 2

^* After 5% parasitic power deducted from diesel engine shaft output service (not testbed) conditions