NO329851B1 - Methods and facilities for cleaning exhaust from diesel engines - Google Patents

Abstract

A method and plant for cleaning exhaust gas from a marine diesel engine, comprising a diesel engine and a compressor for compression of air for introduction into the engine, wherein the compressor is driven by a turbocharger driven by exhaust gas from the diesel engine, wherein the exhaust gas leaving the diesel engine is purified in a NOX removal unit and a SOX removal unit before the exhaust gas is expanded in the turbocharger is described.

Description

The field of invention

The present invention relates to a method and a plant for cleaning exhaust gas from a diesel engine. More particularly, the present invention relates to a method and a plant for the removal or significant reduction of particles, SOx, NOx and possibly CO2 from the exhaust gas from diesel engines, such as marine diesel engines.

Background for the invention

Marine diesel engines are major emission sources for SOx, NOx, particles and CO2. Emissions of SOx and NOx are of great concern to the industry.

SOx (mainly SO2, some SO3) originates from the combustion of sulfur-containing compounds in the fuel oil. Typically, heavy fuel oils are preferred due to lower costs. Heavy fuel oils contain the largest amounts of sulfur compounds (typically 4.5% by weight or more) and therefore cause the largest emissions of SOx. New regulations may require the use of fuel oil with a lower sulfur content, typically below 1% by weight. Such fuel oils will cost around 10 to 15% more. Even with this improved fuel oil, the exhaust gas will contain several hundred ppm SOx on a volume basis.

In comparison, an exhaust gas from coal burning with a suitable sorbent added to the incinerator will contain well below 20 ppm SOx.

The requirements to use low-sulphur fuel oil mean that there will be an increased need for this fuel oil, and a corresponding reduction in the need for fuel oil with a high sulfur content. Fuel oil with a high sulfur content can therefore become relatively less expensive and the economic benefits of using such oil can increase over time.

The use of oil with a low sulfur content will further require modifications in the use of additives to reduce wear in the cylinder liner.

As an alternative to low-sulphur fuel oils, ships can have exhaust gas cleaning systems that trap SOx.

Environmental effects of SOx include reduced visibility due to fine particles that scatter and absorb light. SOx accelerates metal corrosion and causes damage to building materials. The gas also causes acid reflux and damage to vegetation. Although difficult to measure, there are indications that SOx can cause temporary respiratory distress and reduced lung function in humans. All required reductions in SOx emissions are driven by such environmental considerations.

NOx (mainly NO, some NO2) originates from the fuel (fuel NOx) or from reactions between nitrogen and oxygen in the combustion process (thermal NOx). Marine diesel engines typically emit around 8 g/kWh NOx. This corresponds to several hundred ppm NOx in the fuel gas on a volume basis. Some environmentally restricted areas may require greatly reduced NOx emissions, perhaps 2 to 3 g/kWh. This corresponds to exhaust 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 have been seen for some areas for gas turbines.

NOx reacts with oxygen in the presence of volatile organic compounds and sunlight to form ozone at ground level. NOx also accelerates damage to materials and can absorb visible light and thus reduce visibility. The gas affects vegetation. In coastal areas, it can give rise to competition between plant species, lead to explosive algal growth and oxygen reduction. Fish kills in fish farms due to NOx have been reported. On land, NOx can reduce some crops. Possible health effects include changes in lung function in people with respiratory diseases.

Although today of less concern than SOx of NOx, particulate matter from marine diesel engines may be responsible for reduced visibility. It can cause damage to materials. It can also settle on leaf surfaces and reduce photosynthesis.

EP 1 795 721 A1 describes a particle trap for removing particulate matter arranged between the exhaust manifold and the turbine of the turbo of a diesel engine. US 4,335,684 describes an ash removal unit arranged between the exhaust manifold and the turbine of a turbo on a diesel engine. None of the publications describe the removal of SOx or NOx.

US 2008/0022667 and US 6,058,700 both describe devices for cleaning exhaust gas from engines by removing or reducing SOx and NOx. The main focus in both publications is control systems for monitoring emissions of the aforementioned gases and the control systems to reduce emissions.

Methods for removing NOx and SOx are well known in the art, see, for example, the following review articles by Joseph A. MacDonald in Energy Tech Magazine: "Power Plant Emissions Controls: SO2removal technologies & systems", http://www.energy-tech. com/index.cfm?PagelD=108&c2e=236&e2e=0&rs=0&artid=543, "Power Plant Emissions Controls NOxreduction technologies & systems", http://www.energy-tech.com/index.cfm?PagelD=108&c2e=236&e2e =0&rs=0&artid=466, and "Combined SO2/NOXremoval processes yield lower costs, no waste, salable by-products", http://www.energypubs.com/index.cfm?PagelD=108&c2e=236&e2e=0&rs=0&ar time=569. C02 from marine diesel engines is one of the main contributors to today's increase in atmospheric C02 and thus possibly to global warming. A cost-effective reduction of CO2 emissions from marine diesel engines would therefore be a valuable contribution to the required reduction in CO2 emissions. Captured CO2 can be condensed into liquid and stored on board. Typical storage conditions are around -50 °C and 7 barg in a container with a diameter of up to 10 metres. CO2 has increasing value for increased oil extraction.

Summary of the invention

According to a first aspect, the present invention relates to a method for cleaning exhaust gas from a marine diesel engine comprising a diesel engine and a compressor for compressing air for introduction into the engine, where the compressor is driven by a turbo expander driven by exhaust gas from the diesel engine, in which the exhaust gas leaving the diesel engine is cleaned in a unit for the removal of NOx and a unit for the removal of SOx before the exhaust gas is expanded in the turboexpander.

According to another aspect, the present invention relates to a marine diesel installation comprising a diesel engine comprising a diesel engine and a compressor for compressing air for introduction into the engine, the compressor being driven by a turboexpander powered by exhaust gas from the diesel engine, wherein the installation further comprises a unit for removal of SOx and a unit for removal of NOx between the exhaust manifold of the diesel engine and the turboexpander.

Brief description of the figures

Figure 1 is a simplified representation of a marine diesel installation according to known technology, Figure 2 is a simplified 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 present invention

Figure 1 is a schematic representation of a marine diesel engine installation on board a ship. The installation comprises a diesel engine 1, which receives air for combustion through an air inlet 2, and a liquid fuel through a primary fuel line 3. The air is compressed in an air compressor 4, typically to a pressure of 2 to 3 bar. The compressed air is normally cooled by means of an intercooler 5 before the air is introduced into the diesel engine 1.

The exhaust gas is extracted from the diesel engine, typically at a pressure of 3 to 5 bar, through an exhaust manifold 6 and is introduced into a turboexpander 8 where

the exhaust gas is expanded to a pressure close to 1 bar, and is drawn out through an exhaust line 10 and released to the atmosphere. The air compressor 4 and the turboexpander 8 are preferably arranged on a common shaft so that the air compressor is driven by the turboexpander.

In addition, a main shaft 11 has also been provided for transferring the mechanical energy from the engine, and cooling lines 12,12' for the introduction and withdrawal of cooling water, respectively.

Figure 2 illustrates a first embodiment of the present invention where means for cleaning the exhaust gas are provided between the exhaust manifold 6 and the turboexpander 8.

According to the embodiment in Figure 2, the exhaust gas leaving the manifold 6 is introduced into a filter 20 to remove or significantly reduce the amount of particles in the exhaust gas. The filter 20 can be of a high-temperature metal cassette type with automatic online pulsing for the removal of particles.

The filtered air from the filter 20 is introduced into a NOx reduction unit 21. The NOx reduction unit 21 illustrated is a selective catalytic reduction unit (SCR), into which NO3 is introduced through an NH3 line 22. NOx and NH3 react with water vapor and oxygen present in the exhaust gas according to the following equations:

to give water and nitrogen gas.

SCR is well known and is widely used to remove NOx from combustion gases. Depending on the catalyst, the SCR operates in the temperature range from 300 to 400 °C. However, the use of a pressurized SCR is not common even though it offers significant advantages both with respect to volume and efficiency requirements. As an alternative to SCR, the NO content in NOx can be oxidized to NO2, which is more water-soluble than NO. N02 can then be removed in downstream scrubbing equipment.

From the SCR unit 21, the combustion gas with reduced NOx is cooled in a heat exchanger 23 and then cleaned in a scrubber 24 where the combustion gas is washed by countercurrent to an SOx solvent, such as water in a contact section 25 in the scrubber. The solvent is collected at the bottom of the scrubber and is drawn out through a line 26. The solvent in line 26 is split between a solvent outlet line 27 where the solvent is removed for deposition, or regeneration, and recycling via a pump 28. The recycled 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 drawn out from the top of the scrubber through a line 33 for purified combustion gas.

The gas in line 33 is again heated against gas from the SCR unit 21, in the heat exchanger 23 before the gas is expanded over the turboexpander 8 and released through the exhaust line 10 Aug.

The first embodiment is compact and efficient. By arranging the scrubber 24 downstream of the SCR unit 21, an unwanted release of NH3 to 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 SOx is removed from the exhaust gas before removal of NOx. Only the features which distinguish this embodiment from the embodiment in 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 heated again 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, potassium hydroxide, or magnesium hydroxide, or sea water. If it is environmentally acceptable, the used absorbent can be released into the sea. If not, precipitated sulfur compounds are collected and deposited, and the remaining absorbent can be recycled after adding absorbent to replace the amount removed due to reaction with sulfur.

This second embodiment is also compact and efficient. By arranging the scrubber 24 upstream of the SCR unit 21, NH3 can be released into the atmosphere. This solution provides very clean working conditions for SCR and the formation of ammonium sulphate is thus avoided.

The use of seawater as a solvent for SOx scrubbing may be preferred on board a ship if it is permitted to release the used solvent into the sea. However, the use of seawater requires an additional washing step to remove any salt present in the gas leaving the scrubber. Figure 4 illustrates an alternative for implementing such a washing step in a plant according to Figure 2. Figure 4 illustrates a water scrubber 40 arranged on top of the scrubber 24. The gas that leaves the scrubber 24 is introduced into the water scrubber 40 through a line 41. The gas that comes into the water scrubber is washed by counter current flow to water in a contact section 44. Water is introduced through a water line 42. Water leaving the contact section is collected at the bottom of the water scrubber and is removed through line 43 and is recycled back inside the water scrubber 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 purified exhaust gas is withdrawn from the contact section 44 in line 33, as described by reference to figure 2. A demister 48 is preferably arranged on top of the water sink to reduce or remove water droplets in the gas which fo rlates the contact section before it is pulled out through line 33.

Figure 5 illustrates a fourth embodiment of the present invention. This embodiment is a further development of the embodiment in 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 unit 52 where the gas is scrubbed by countercurrent 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 at the bottom of the washing unit and is drawn out through a line 55 , and recirculated into line 54 by a pump 56. A cooler 57 is preferably arranged in line 54 to cool the water. A portion of the water in line 55 is withdrawn through a waste water line 58 and is replaced by water introduced through a makeup water line 59. The washed gas is withdrawn from the washing unit through a line 60 and is again heated in the heat exchanger 51 before being introduced into the turboexpander 8. A demister 61 is preferably arranged on top of the washing unit to remove droplets from the gas.

The operating conditions for SCR are very good as the gas is cleaned both before dust and SOx, as in the embodiment according to figure 3. Consequently, there is no formation of ammonium sulphate. In addition, the further step removes or significantly reduces the release of NH3 to the atmosphere.

Figure 6 is a schematic sketch of a combined plant 100 for NOx/SOx removal and C02 capture. Exhaust gas extracted 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 it is 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 multi-stage processes with cooling, as indicated in the figure between the stages.

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

The steam generated in the steam coils is drawn out 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 arranged to supply steam to other steam consuming processes into the plant to provide energy for integration of the overall process.

After expansion in the turbine 110, the expanded steam is condensed in the 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 an SCR unit 116 to remove NOx by reaction with NH3 introduced through an ammonia line 117, as described above. The exhaust gas leaving the SCR unit 16 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 the venturi scrubber is again cooled by introduction into a lower chamber 120' in 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 CO2 is absorbed by a liquid absorbent by countercurrent flow through a contact zone in the absorber. The unabsorbed 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 into the direct contact heat exchanger by means of a pump 123 which pumps water through a line 124. The water in line 124 is introduced at the top of the upper chamber 120" and flows countercurrently to the gas flowing upward into a contact zone in the upper 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 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 introduced into the pump 123 to be recycled.

The CO2-depleted and partially 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 which drives a generator 128 to produce electricity. The expanded gas from the turbine 127 which has a pressure close to the pressure of the exhaust gas initially leaving the diesel engine 1, is heated again in the heat exchanger 101, before the gas is introduced as propellant gas into the turboexpander 8 and is released as purified exhaust gas through line 10.

The absorbent collected at the bottom of the absorber 121 which is rich in CO2 is withdrawn from the absorber in a line 130 and is flashed in a flash vessel 131, to a pressure lower than the operating pressure of the absorber 121, to flash off and reduce the level of oxygen in the absorber and thus finally in the C02 fraction which is isolated later. The gas stripped from the rich absorbent, comprising mainly oxygen and a minor amount of H 2 O and CO 2 , is vented to the atmosphere through a vent line 132. The liquid phase from the flash vessel is introduced to a regenerator column 133 where the absorbent is stripped by countercurrent flow of steam in a contact zone. The steam for stripping the absorbent is generated in a reboiler 134 by heating a portion of the stripped absorbent which is collected at the bottom of the regenerator column 133. The stripped and poor absorbent is recycled from the bottom of the regeneration column 133 and is introduced at the top of the absorber 121.

The absorbent that is used in the absorber / regenerator circuit for capturing CO2 is preferably an aqueous absorbent. At high pressure, even water can be used as the absorbent, but the most preferred absorbent is an aqueous carbonate solution. Amines that are commonly used for capturing CO2 cannot be used due to a too high partial pressure of oxygen in the gas to be treated, which leads to degradation of amines.

CO2 stripped of absorbent in the regeneration column is withdrawn through a line 135, partially 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 a compressor 139 driven by a motor 140, for to provide CO2 which is extracted through a CO2 line 141 for further treatment and disposal. The liquid phase formed in the flash tank 137 is returned into the regeneration column through a line 138.

The plant described with reference to figure 6 has a fuel consumption that is about 15% higher than the diesel engine alone to give the same power output. However, the cost reduction by allowing the engine to be run on fuel with a higher sulfur content is around 15%. The benefits are thus mainly associated with the total reduction in pollution of the surroundings and any fees and taxes that must be paid to the authorities for emissions of NOx, SOx and CO2.

The processes described above for the removal of SOx and NOx are introduced between the exhaust manifold and the turboexpander. Accordingly, the SCR and the scrubber are operated at an elevated pressure of typically 3.5 to 5 bar, depending on the characteristics of the diesel engine and its tuning. The high pressure makes it possible to reduce the size of the SCR unit and the scrubber and thus the spatial requirements and costs thereof, to make the cleaning method and the facility practically and economically possible to implement on board a ship.

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

The tables below are simulation examples

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

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

Claims (3)

1. A method of cleaning exhaust gas from a marine diesel engine, comprising a diesel engine and a compressor for compressing air for introduction into the engine, the compressor being driven by a turboexpander driven by exhaust gas from the diesel engine, wherein the exhaust gas leaving the diesel engine is cleaned in a unit for the removal of NOx and a unit for the removal of SOx before the exhaust gas is expanded in the turboexpander. 2. The method according to claim 1, where the particles are removed from the exhaust gas before the gas is introduced to the units for removing NOx and SOx. 3. A marine diesel installation, comprising a diesel engine (1) and a compressor (4) for compressing air for introduction into the engine (1), where the compressor (4) is driven by a turboexpander (8) driven by exhaust gas from the diesel engine (1), characterized in that the installation further comprises a unit (24) for the removal of SOx and a unit (21) for the removal of NOx between the exhaust manifold (6) of the diesel engine (1) and the turboexpander (8).