WO1992008541A1

WO1992008541A1 - METHOD AND APPARATUS FOR PURIFYING EXHAUST WITH RESPECT TO SO2, NOx AND DUST

Info

Publication number: WO1992008541A1
Application number: PCT/NO1991/000139
Authority: WO
Inventors: Erik Tollefsen
Original assignee: A/S Miljøutvikling
Priority date: 1990-11-12
Filing date: 1991-11-12
Publication date: 1992-05-29
Also published as: NO904892L; NO904892D0

Abstract

A method and an apparatus for purifying gases contaminated by SO2 and NOx particularly for purifying exhaust gas from a combustion engine (1) in a marine vehicle, the exhaust gas being cooled in a heat exchanger (2) and transported into a first scrubber (3), partly filled with sea water, together with small sea water droplets, beneath the sea water surface, resulting in absorption of SO2 in the bulk phase (19) in the scrubber (3). Then, the partly purified exhaust gas flows out of the SO2 scrubber (3) through the top of the same and into a second scrubber (4), the scrubber (4) being partly packed with a material (17) to effect intimate contact between gas and liquid flowing counter currently through the same. The gas flowing through the packing material (17) contacts the downwards flowing sea water absorbent supplied from the SO2 scrubber (3) added urea or ammonia, thus absorbing NOx in the absorbent. Then, the absorbent flows back to the SO2 scrubber (3), whereas the purified gas flows out of the NOx scrubber (4) to the atmosphere by means of a fan (6). Excessive contaminated sea water is withdrawn from the SO2 scrubber through an overflow pipe (8) to the environments.

Description

Method and apparatus for purifying exhaust with respect to SO₂, NO_x and dust.

The present invention relates to a method and an apparatus for purifying gase contaminated by sulphur dioxide, nitrous oxides, soot particles and equivalent, particularly t purifying exhaust gases from an internal combustion engine in marine engines.

Lately, an increasing attention has been paid to the environment, in which particular attention has been drawn to exhaust pollution from internal combustion engines and equivalent. This consciousness is being reflected in the authorities' demands for use of exhaust catalysts for automobiles. However, with respect to the total fuel consumption, the consumption for automobiles constitutes only a small part. In 1976, e.g. the norwegian shipping abroad constituted totally 74% of the total consumption of energy resources in the shape of oil within transportation.

However, in difference from automobiles, no suitable methods for purifying exhaust from marine engines exist today. Thus, looking at the contribution to the global pollution from marine engines, a purifying apparatus for such engines would therefore represent a substantial contri¬ bution to the global environment, particularly with respect to compounds containing sulphur and nitrous gases.

Several purifying methods for plants onshore, such as oil based power plants, exist today. For example, in DE Patent no. 2.304.784, a purification process for treating sulphur dioxide is described, and to a certain extent nitrous oxides, comprising supply of hydrogen peroxide. This system is based upon a constant exhaust volume rate, use of a concentration vessel, two condensers and a vaporizer, and would rise problems for implementation on a ship, in which storage of the peroxide would represent a safety problem.

DE Patent no. 2.555.220 describes a method for absorbing nitrous gases by means of powdered lime and hydrogen peroxide, in which the powdered lime and the peroxide being sprayed into the gas in a downstream direction. Then, the gas is filtrated for removing the powder containing absorbed impurities.

In DE printed publication no. 2.436.363 B2, a method for oxidizing NO by supplying hydrogen peroxide in the gas phase is described, whereupon the gas is treated with nitric acid. The object of this method is identical with the object of the present invention, but the chemical process is substantially different from the present invention. However, the apparatus according to the abovementioned printed publication is not illustrated. Like the abovementioned references, these raw materials also would represent a safety problem.

DE printed publication no. 2.331.156B2 describes a method for dry purification of sulphur dioxide, based upon a powdered reagent. The reagent, e.g. powdered lime, is supplied to the exhaust gas in a downstream direction, in which the exhaust previously have been supplied with steam in order to achieve enhanced bonding. Then, the gas/lime mixture is filtrated in a dry filter. This process is essentially different from the present invention.

DE Patent no. 2.607.587 describes a process, in which hydrogen peroxide (or other peroxides) is supplied in two steps to the exhaust in a downstream direction, whereupon the exhaust gas is directed in to a demister. From the demister, the reagent is withdrawn and either removed from the process or recycled to the gas supply pipe. Then, the gas is guided from the demister to a scrubber, carrying out a final washing using the abovementioned reagents (hydrogen peroxide etc.). This system is substantially different from the present invention, both with respect to construction and chemistry.

The object of the present invention is providing a process for removing impurities from exhaust gases, particularly exhaust gases from marine engines, in which the impurities substantially comprising nitrous oxides and sulphur dioxide, with a minimum use of problematic chemicals/reagents .

The invention is expressed in the characterizing part of claim 1 and 4. Further features appears from the independent claims 2-3 and 5-7. The main principle for the present invention is that the exhaust initially is purified with respect to sulphur dioxide in a first scrubber (the SO₂ scrubber), in which the absorbent preferably comprising water only. The gaseous sulphur dioxide is dissolved and partly dissociated in the sea water. The partly purified exhaust is then brought to a second scrubber (the NO_x scrubber), in which the gas is washed counter-currently with recycled sea water from the SO₂ scrubber, the sea water intermixed with ammonia or urea. The ammonia itself, or ammonia from urea, results in the formation of ammonium salts, so that the content of nitrous gases in the exhaust gas being considerably reduced. Then, the absorbent leaves the second scrubber and drips/flows down into the first scrubber.

The advantage with this principle is that the consumption of chemicals is low compared with similar systems. If urea is being used in addition as purifying reagent for nitrous gases, a considerable gain is achieved with respect to storage. Pure urea is provided in the shape of powder or granules, and the storage conditions are substantially satisfied with dryness. On the other hand, ammonia requires storing in pressurized containers, which always will represent a certain element of risk with respect to leakage. In the following, the invention is described in further details with reference to Figure 1, illustrating a simple flow sheet of the combined purification process.

In Figure 1, a simple process flow sheet is illustrated, in which 1 designates the internal combustion engine. The exhaust gas from this engine is brought through a heat exchanger, alternatively together with exhaust from auxiliary engine from pipe 11, in which the exhaust being heat exchanged with fresh air, preferably down to a temperature less than 120 °C. Then, the cooled exhaust is brought through the pipe 18, to which seawater is being sprayed through e.g. nozzles in order to disperse the sea water in the gas phase. Then, the exhaust, containing small dispersed sea water droplets, is transported into the SO₂ scrubber 3 through a diffuser 7. The purpose of the diffuser, is to spread the gas, for example through a sieve tray, before dispersing the gas in the liquid phase 19 in the SO₂ scrubber 3. Sulphur dioxide is, because of its high water solubility, dissolved and partly dissociated, so that the sulphur dioxide content in the exhaust gas bubbling up to the liquid surface in the SO₂ scrubber 3, being considerably reduced. The liquid level in the SO₂ scrubber 3 can be controlled as desired by means of an overflow pipe 8, which is vertically adjustable. By adjusting the elevation of the overflow pipe 8, the temperature in the bulk phase 19 and in the exhaust is adjusted accordingly, since the liquid being withdrawn through this pipe having a higher temperature than the rest of the liquid phase. The sea water containing sulfuric acid/sulphurous acid may be drained directly into the sea, if the circumstances allow such draining. However, if the ship is located in a port or in sensitive areas, an equivalent amount of lime may be supplied to the SO₂ scrubber 3, in order to cause precipitation of muddy potassium sulphate. This mud is withdrawn in suitable amounts to a mud container 13, whereupon the mud, when desired, is pumped out of the container 13 by means of a pump 14, either to the sea or to another suitable recipient.

The exhaust gas, now being primarily free from sulphur dioxide, and having a temperature less than 60°C, leaves the SO₂ scrubber 3 through the top of said scrubber and in to the lower part of the NO_x scrubber 4. The NO_x scrubber 4 is partly packed with a material allowing intimate contact between gas and liquid flowing counter-currently. Preferably, the packing comprise porous spheres of expanded clay such as "Leca" . The washing medium is brought into the vessel 4 at the top of said vessel, and is dispersed into small droplets by means of nozzles 20 or similarly. Before the seawater enters the nozzles 20, an aqueous solution of urea or ammonia is added. When this liquid mixture is moving downwards through the packing material 17 and contacts the upwards rising exhaust gas, salt formation from ammonia and the nitrous gases in the exhaust will occur. Then, the washing medium leaves the NO_x scrubber 4 through the bottom, and drip/flow down to the SO₂ scrubber 3. The exhaust, now being partly or substantially free from nitrous gases and sulphur dioxide/dust, leaves the NO_x scrubber 4 by means of an outlet fan 6, to which heated fresh air from the heat exchanger 2 may be supplied in order to heat up the exhaust gas to a temperature above the dew point temperature of the exhaust. This heating of the exhaust prevents water condensation, and for example icing in the smokestack in winter may be avoided.

If the exhaust supply from the main engine 1 or from the auxiliary engine is small, fresh air is supplied via the throttle valve 15. In this way, a constant gas charge to the scrubbers 3 and 4 is maintained. The consumption of ammonia or urea is among other things dependent on the fuel consumption in the engine, the sulphur content in the fuel and the pH value in the bulk phase in the SO₂ scrubber. Generally, the required amount of ammonia equivalents is 2-13 grams per kilogram fuel consumed in the engine. When regulating the process, a natural choice will be to monitor the pH value in the bulk phase 19 in the SO₂ scrubber. For example, for diesel fuels, 2 grams NH_j/kilogram fuel is required ifpH = 8, while 4-6 grams NH₃/kilogram fuel is required if pH = 6. This can naturally be adjusted by means of the quantity of fresh sea water entering the exhaust duct 18.

With respect to the degree of purification for this process, the reduction of nitrous gases is ca. 70% and sulphur dioxide is ca. 99%, and the degree of purification with respect to particles and dust is greater than 99%. In Table 1 below, a typical example of purification efficiency in exhaust gas from a diesel engine, utilizing the process according to the present invention. In Table 2, purification efficiency in percent for diesel and so-called marine fuel including consumption of ammonia/urea is illustrated. As appears from Table 2, the consumption of ammonia/urea is substantially higher for marine fuel than for diesel, because of the relatively high content of sulphur in marine fuel (2.5%) compared with diesel (0.7%).

Compound

SO,

NO,

Particles/dust

TABLE 2 Degree of purification (%)

The apparatus according to the present invention is operated in a corrosive environment, and accordingly, the choice of construction materials are critical for both the viability of the plant and the operating regularity. Thus, all parts contacting the exhaust or the absorbent should preferably be constructed of plastics.

Claims

Claims:

1. A method for purifying gases contaminated by SO₂, NO_x, soot particles and equivalent, particularly for purifying exhaust gases from a combustion engine (1) in a marine engine, c h a r a c t e r i z e d in: cooling the exhaust gas in a heat exchanger (2) to a temperature below 120 °C; transporting the exhaust gas into a duct (18) together with sea water being sprayed through nozzles into the duct from a pump (9), resulting in the formation of small droplets uniformly distributed in the exhaust gas; transporting the exhaust gas comprising dispersed sea water into a container (3) (SO₂ scrubber) partly filled with sea water, through a diffuser (7) beneath the liquid surface, thus absorbing particles and dust in the bulk phase (19) in the SO₂ scrubber (3); transporting the partly purified exhaust gas, preferably having a temperature of less than

60°C,out of the SO₂ scrubber (3) through the upper part of the same and further into a second container (4) (NO_x scrubber), the NO_x scrubber being partly filled with a material (17) effecting intimate contact between gas and liquid flowing counter-currently through the same; the gas, flowing in an upwards direction through the packing material (17), contacts a downward flowing absorbent (sea water), the absorbent being removed from the SO₂ scrubber (3) by means of a pump (12) followed by addition of urea or ammonia from a supply line (22), the amount of ammonia or urea being added comprising 2-13 grams ammonia equivalents per kg fuel consumed in the engine (1); supplying the resulting mixture of sea water and ammonia or urea into the upper part of the NO_x scrubber (4) through nozzles (20) or equivalent, and allowing the same to flow downwards counter currently with respect to the upwards flowing gas from the SO₂ scrubber (3); transporting the gas out of the NO_x scrubber (4) into the atmosphere by means of a fan (6), the gas being partly or substantially completely purified with respect to nitrous gases and SO₂/dust particles, respectively, and removing contaminated sea water in excess from the SO₂ scrubber (3) via an overflow pipe (8) to the environments.

2. A method in accordance with claim 1, c h a r a c t e r i z e d in adding heated fresh air from the heat exchanger (2) to the purified exhaust gas upstream of the fan (6) to elevate the temperature of the exhaust gas above the dew point temperature, thus avoiding water condensing in the fan or exhaust pipe.

3. A method in accordance with claim 1 or 2, c h a r a c t e r i z e d in adding an aqueous CaO solution to the exhaust gas in the duct (18) to effect precipitation of absorbed SO₂ as CaSO₄ in the SO₂ scrubber (3), the calcium sulphate being withdrawn from the SO₂ scrubber (3) when required.

4. An apparatus for purifying gases contaminated by SO₂ NO_x, soot particles and equivalent, particularly for purifying exhaust gases from a combustion engine (1) in a marine engine, c h a r a c t e r i z e d in: a heat exchanger (2) for cooling the exhaust gas against fresh air, the heat exchanger (2) being connected with a duct (18) downstream, the duct (18) being connected with a supply line (21) for addition of sea water from a pump (9) through e.g. nozzles (not illustrated); the duct (18) being extended into a container (3) (SO₂ scrubber) partly filled with sea water, the downstream section of the duct (18) being provided with a diffuser (7), the diffuser being arranged to be submerged beneath the liquid surface in the SO₂ scrubber (3), so that cooled gas comprising dispersed sea water droplets contacts the bulk phase (19) in the SO₂ scrubber beneath the liquid surface to effect the absorption of SO₂, particles and dust in the bulk phase

(19); the upper part of the SO₂ scrubber being provided with a duct connection to transport purified gas to a second container (4) (NO_x scrubber), the NO_x scrubber (4) being partly packed with a material (17) effecting intimate contact between gas and liquid flowing counter-currently through the same, and further in its upper section provided with a pipe connection from the bottom part of the SO₂ scrubber again connected with further a pipe connection (22) for supply of ammonia or urea, to transport sea water from the SO₂ scrubber to the NO_x scrubber by means of a pump (12) through nozzles or equivalent (20) into the upper section of the NO_x scrubber, thus spraying sea water, withdrawn from the bulk phase (19) in the SO₂ scrubber, and added ammonia or urea from supply pipe (22), over the packing material (17) in the NO_x scrubber, said sea water flowing downwards through the packing material (17) and contacting upwards flowing partly purified gas from the SO₂ scrubber; the NO_x scrubber (4) is provided with a vent pipe (10) for the removal of purified gas to the atmosphere by a fan (6), the SO₂ scrubber (3) being provided with an overflow pipe (8) effecting removal of sea water in excess from the bulk phase (19) in the SO₂ scrubber to the environments.

5. An apparatus in accordance with claim 4, c h a r a c t e r i z e d in that the packing material (17) in the container (2) comprising porous spheres of expanded clay (Leca™) or equivalent having an average diameter of 50 mm.

6. An apparatus in accordance with claim 4 or 5, c h a r a c t e r i z e d in that the overflow pipe (8) is arranged to be vertically adjustable with respect to the SO₂ scrubber (3) to adjust the liquid level in the SO₂ scrubber to control temperature and space velocity.

7. An apparatus in accordance with claim 4-6, c h a r a c t e r i z e d in that all components of the apparatus contacting corrosive media are constructed of plastic.