Exhaust scrubbing

Abstract

An apparatus 10 for scrubbing exhaust gases from an engine comprising a quenching chamber 21 of a high temperature resistant material, the quenching chamber containing nozzles 25 for spraying aqueous

Description

The present invention relates to an apparatus and a process for scrubbing exhaust gases from an engine, in particular from a marine engine. Engines such as diesel engines are widely used on marine vessels, either for propulsion or as a source of auxiliary power. The fuel for such an engine is typically derived from fossil fuels, so its use generates carbon dioxide which, if released into the atmosphere, will contribute to global warming. It would be desirable to be able to prevent such release of carbon dioxide. Exhaust gases typically contain other contaminants such as sulphur dioxide, and scrubbing has been proposed as a way of preventing the release of such contaminants. It is important that any such scrubbing process does not impose a large pressure drop on the exhaust gases, as this would have a detrimental impacton the performance of the engine. For example GB 2 501633 (Oceanox) describes a system for treating exhaust gases in which the exhaust gases are first quenched, to lower their temperature, and are then scrubbed; the quenching section includes nozzles to spray water, and the scrubbing duct also includes nozzles to spray water. According to the present invention there is provided an apparatus for scrubbing exhaust gases from an engine, the apparatus comprising: - a quenching chamber of a high temperature resistant material, the quenching chamber containing nozzles for spraying aqueous droplets into the exhaust gases; - a scrubbing duct of sufficient length and cross-sectional area that the residence time for exhaust gases in the scrubbing duct is greater than 2 seconds, the scrubbing duct comprising nozzles for spraying droplets of scrubbing liquid so that the droplets fall through the flowing exhaust gases; and also comprising: - a processing plant to supply the scrubbing liquid, the scrubbing liquid having a pH between pH8 and pH12; wherein the scrubbing duct comprises a plurality of scrubbing chambers, and wherein the quenching chamber and the scrubbing chambers are linked in series by flow ducts, each flow duct being of cross-sectional area no more than 1 / 10 the cross-sectional area of the chambers to which it is connected. The ratio of the cross-sectional area of the chamber to that of the flow ducts may be more than 10, but typically less than 100 for example 20, 30, 35, 40 or 50. Consequently the mean velocity of the gas flow rate through the chambers is less than that through the flow ducts. The reduction in flow velocity on entry into each scrubbing chamber has been found to be beneficial to the scrubbing process. The processing plant may comprise a unit to treat sea water, if the engine is on a sea-going vessel, so as to produce the scrubbing liquid of the required pH. For example this unit may add chemicals to environmentally-obtained water, for example sodium hydroxide, calcium hydroxide, and / or calcium chloride. The scrubbing liquid may be between pHll and pH12 for example. There may also be a demister unit at the downstream end of the scrubbing duct to remove droplets from the flowing exhaust gases. Neither the quenching chamber nor the scrubbing duct contain a packed region, so there is little pressure drop imposed on the exhaust gases. The total pressure drop is preferably less than 10 kPa (76mm Hg). After passing through the demister unit, the treated exhaust gases, which consist primarily of nitrogen and water vapour, may be discharged to the environment. Preferably the residence time for exhaust gases in the scrubbing duct is greater than 3 seconds, and more preferably greater than 4 seconds. The dimensions of the scrubbing duct may be between 2 m and 10 m, for example between 4 m and 6 m, and the cross-sectional area is selected in accordance with the maximum exhaust gases flow to achieve the required residence time. The scrubbing apparatus may be connectable into an exhaust-carrying duct by means of a diversion valve, such that when the exhaust gases scrubbing apparatus is not operating the diversion valve may be actuated so that the exhaust gases are not passed through the scrubbing apparatus. If there is a problem in the water supply system, the exhaust gases can be discharged directly into the atmosphere until the problem is rectified. Consequently the scrubbing duct can be made of a comparatively lightweight material such as a plastic or glass-reinforced plastic, as it is not exposed to the high temperature of the unquenched exhaust gases. It is nevertheless feasible to make the scrubbing duct of a denser material such as steel, and indeed it may be made of the same material as the quenching chamber. The quenching chamber and the scrubbing duct are separate items, linked by a flow duct to carry cooled exhaust gases from the outlet of the quenching chamber to the inlet of the scrubbing duct. The direction of flow of the exhaust gases through the quenching chamber and through each scrubbing chamber may be generally horizontal, so that the droplets are falling in a direction substantially transverse to the gas flow direction, and in that case the droplets and the exhaust gases are substantially in a crossflow relationship. For example, the direction of flow may be no more than 45° from the horizontal, more preferably not more than 30° from horizontal, and may indeed be less than 10° from the horizontal. Each scrubbing chamber preferably has a longitudinal axis, the gas flow direction being substantially parallel to the longitudinal axis; and such a horizontal flow arrangement may be such that in use the longitudinal axis is not more than 30° from the horizontal. Although such a horizontal flow arrangement is often beneficial, and may enable the pressure drop to be minimised, there may also be situations where the flow direction is arranged to be significantly different, and may even be generally vertical. The system is primarily intended for use on a ship, so that in practice, as the ship proceeds through waves, the inclination of the direction of flow from the horizontal will in practice vary. The quenching chamber and the scrubbing chambers may be aligned, that is to say having their longitudinal axes aligned, but this is not essential. To fit the quenching chamber and the scrubbing duct into the space available it may be preferable to arrange the quenching chamber and the scrubbing chambers alongside each other, or one above another, connected by flow ducts that follow U-shaped paths. The number of scrubbing chambers must be at least two but may be more: this multi chamber approach can be sized appropriately for the size of engine in a low footprint way which can be adjusted for applicability to a variety of vessels and indeed the chambers may not need to be collocated allowing them to be positioned where space allows. The size of and number of scrubbing chambers can be adjusted to suit the engine output and make installation as simple as possible. The flow ducts that interconnect the quenching chamber and the scrubbing chamber lead to an alternating increase in flow area and decrease in velocity / decrease in flow area and increase in velocity of the gas a plurality of times, and this assists in bringing about the reaction with the low pH water, whilst not increasing back pressure. The process is specifically designed to remove CO?, but by nature of the reactions will also remove up to 85% of SOx. It is therefore a combined process. The aqueous liquid supplied to the nozzles in the quenching chamber may be seawater, preferably filtered to remove any particulate material; or it may be the scrubbing liquid. The apparatus comprises an outlet for treated exhaust gases from the scrubbing duct, and an outlet for scrubbing liquid that has passed through the exhaust gases in the scrubbing duct. The apparatus may also comprise a filter to treat the scrubbing liquid that has passed through the scrubbing duct, for example a hydro-cyclone, to remove any particulate material, so the remaining waste liquid can be discharged back to the sea. The particulates emerge with some water as a sludge from the top of the hydro-cyclone (as they consist of carbon and unburnt fuel and are less dense than water), and this sludge can be stored for safe subsequent disposal. The waste liquid, that is the scrubbing liquid that has passed through the scrubbing duct, and from which the particulate material has been removed (if a filter or hydro-cyclone is provided), contains the bulk of the gaseous contaminants from the exhaust gases, in particular the carbon dioxide, sulphur dioxide and nitrogen oxides, and may be diluted with additional seawater before being discharged into the sea. The waste liquid, in this example, is not recirculated. It will be appreciated that the contaminants such as sulphur dioxide and NOx are not detrimental to the environment when introduced in dilute form into the sea. The invention, in a second aspect, provides a process for scrubbing exhaust gases from an engine, comprising: passing the exhaust gas through a quenching chamber of a high temperature resistant material, and spraying aqueous droplets from nozzles into the exhaust gases to lower their temperature and so produce cooler exhaust gases; then passing the cooler exhaust gases through a scrubbing duct of sufficient length and cross-sectional area that the residence time for the exhaust gases in the scrubbing duct is greater than 2 seconds, the scrubbing duct comprising nozzles; spraying droplets of a scrubbing liquid from the nozzles of the scrubbing duct so that the droplets fall through the flowing exhaust gases; wherein the scrubbing liquid is supplied from a processing plant, the scrubbing liquid having a pH between pH8 and pH12; and wherein the scrubbing duct comprises a plurality of scrubbing chambers, and the quenching chamber and the scrubbing chambers are linked in series by flow ducts, each flow duct being of cross-sectional area no more than 1 / 10 the cross-sectional area of the chambers to which it is connected. The second aspect of the invention may utilise one or more of the features described and summarised above. The quenching chamber preferably is arranged to cool the exhaust gases to below 100°C, for example to below 70°C or to below 50°C. As mentioned above, cooling the exhaust gases to such as extent makes it possible to make the scrubbing duct of a comparatively lightweight material such as a plastic or glass-reinforced plastic. The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a flow diagram of an exhaust gases scrubbing apparatus; and Figure 2 shows a perspective view, showing more details of the scrubbing apparatus of Figure 1. Referring to figure 1, there is shown part of an exhaust duct 12 from a marine engine, along with a scrubbing system 10. The exhaust duct 12 is modified by installing a side duct 14 of substantially the same cross-sectional area as the exhaust duct 12, and a diverter valve 16 (represented schematically). In the position as shown the exhaust gases from the engines are all diverted into the side duct 14, as indicated by the arrows, but if the scrubbing system 10 is not functioning, then the diverter valve 16 can be moved into an alternative position 16a (shown in broken lines) in which the exhaust gases do not flow through the side duct 14. The side duct 14, as indicated schematically, supplies the exhaust gases to a treatment unit 20, and the treated exhaust gases emerge as a clean gas stream R which may be discharged directly to the environment as shown, or alternatively may be fed back into the exhaust duct 12 above the diverter valve 16 for discharge into the environment. The treatment unit 20 consists of a quenching chamber 21 constructed of heat-resistant steel such as Inconel (trade mark), and a scrubbing duct 22 constructed of glass reinforced vinyl ester resin (shown schematically as a single vessel); there may also be a demisting section 26 downstream of the scrubbing duct 22, also of glass-reinforced vinyl ester resin and enclosing a polypropylene fibre pad, but in some embodiments this demisting section 26 may be omitted. Seawater S is supplied by a pump 23 and through a duplex filter 24 to injection nozzles 25 within the quenching chamber 21. Some of the seawater S that has passed through the filter 24 flows through a treatment unit 27 and a pH regulator 28 to injection nozzles 30 within the scrubbing duct 22. The injection nozzles 25 may also be also supplied with compressed air C, so they produce a fine mist of droplets. The droplets within the quenching chamber 21 evaporate and cool the exhaust gases, typically from above 400°C down to below 50°C, for example to about 30°C, and ensure that the gas stream is saturated with water vapour as it enters the scrubbing duct 22. The treatment unit 27 and the pH regulator 28 produce alkaline water at between pH8 and pH12, for example at pHll. As indicated by an arrow labelled NaOH, the alkaline water may be produced by adding a chemical (such as NaOH, Ca(OH)2 or CaCI2) to the water from the filter 24. This alkaline liquid is supplied to the nozzles 30, so that as the cooled exhaust gases pass through the scrubbing duct 22 they are exposed to sprays of alkaline droplets from the nozzles 30, the arrangement being substantially a crossflow arrangement with the droplets primarily travelling in a direction transverse to the longitudinal axis of the duct 22 and so to the flow direction of the exhaust gases. The droplets remove particulate material and water-soluble contaminant gases such as SO2 and NOX, and because they are alkaline they also remove CO2, to form a waste water stream 34. The waste water stream 34 flows down under gravity into a hydro-cyclone 35 which separates off a sludge stream P containing particulate material such as carbon and any unburnt hydrocarbons from the fuel (these materials being less dense than the water). The quantity of material in the sludge stream P is not large, typically being less than 20 g per tonne of fuel that has been used, more typically about 10 g / tonne. This may be stored in a container for subsequent safe disposal; it may be dosed with a neutralising agent; and in some cases it may be stored in a ship's organic sludge disposal system. The remainder of the waste water stream emerges through an outlet duct 36 which is connected to an eductor (or jet pump) 37 fed with a stream of seawater S by a pump 38. The eductor 37 hence produces a dilute waste water stream Q which may be discharged back into the sea. The eductor 37 creates suction, which enhances the flow through the hydro-cyclone 35. The hydrocyclone 35, the eductor 37 and the pump 38 together constitute a waste water treatment unit 33. Referring now to figure 2 there is shown a perspective view of the treatment unit 20, which includes the quenching chamber 21, and the scrubbing duct 22. The quenching chamber 21 is generally cylindrical, with an inlet duct 43 for the exhaust gas at the inlet end. At the other end the quenching chamber 21 communicates with a linking duct 44. The exhaust gas is fed into the quenching chamber 21 through the inlet duct 43, and there are several nozzles 25 (indicated schematically) in the wall of the quenching chamber 21, so the exhaust gases are sprayed with a mist of water droplets from several different directions. The scrubbing duct 22 comprises two generally cylindrical scrubbing chambers 40 arranged in series, each having multiple nozzles 30 distributed over the upper wall of the chamber 40, so alkaline water at between pHll and pH12 is sprayed in multiple directions into the flowing gas stream, the droplets then falling down to the bottom of the treatment vessel 40. The base of each vessel 40 slopes down to form a sump region (as indicated in figure 1), from which the waste water 34 flows out through an outflow pipe (not shown). The inlet duct 43 and the linking duct 44 are cylindrical and are aligned with the longitudinal axis of the quenching chamber 21. The linking duct 44 extends along a generally U-shaped path to feed the cooled exhaust gases into one end of the first scrubbing chamber 40, and a linking duct 45 that communicates with the opposite end of the first scrubbing chamber 40 extends along a generally U-shaped path to feed the gases into one end of the second scrubbing chamber 40. An outlet duct 46 from the opposite end of the second scrubbing chamber 40 carries the treated gas stream R. The treatment unit 20 in this example is arranged so the longitudinal axes of the quenching chamber 21 and of each of the scrubbing chambers 40 are close to horizontal, although if the treatment unit 20 is on a ship the angle from the horizontal will be affected by any waves. Consequently droplets falling under gravity move transverse to the gas flow direction in the quenching chamber 21 and in both the scrubbing chambers 40. It will be appreciated that the inlet duct 43 and the linking ducts 44 and 45 are each of significantly smaller cross-sectional area than the quenching chamber 21 or scrubbing chamber 40 with which they communicate. By way of example the dimensions of a test plant are shown in the Table: Table Quench Inlet Quench Linkl Scrubber 1 Link 2 Scrubber 2 Volume / m3 0.000393 0.2 0.006 0.2 0.006 0.2 Diameter / mm 50 572 100 572 100 572 Length / mm 400 851 800 851 800 851 Cross Section / mm2 1962.5 256839 7850 256839 7850 256839 In this example the outlet duct from the second scrubber has the same dimensions as the two linking ducts (Link 1 and Link 2 in the table). It will be observed that the ratio of the cross-sectional areas for gas flow are as follows: On entering the quenching chamber 21: ratio 130 On leaving the quenching chamber 21: ratio 33 On entering and leaving each scrubbing chamber 40: ratio 33 The total volume of the scrubbing duct 22 is sufficiently large that the residence time for the exhaust gases in it is just more than 4 seconds. The quantities of water and of alkaline solution that are supplied to the treatment unit 20 depend on the power of the engine. For each MW of engine power, 3 m3 of seawater may be supplied per hour to the quenching chamber 21, and about 20 m3 of alkaline seawater at between pHll and pH12 may be supplied per hour to each scrubbing chamber 40. The bulk of the scrubbing apparatus 10, in particular the treatment unit 20, would typically fit within the dimensions of a standard 40-foot container. The scrubbing apparatus 10 can therefore readily be retrofitted to a ship. Because the scrubbing duct 22 may be made of a relatively low-density material, the scrubbing apparatus 10 does not affect the stability of the ship, as it does not require the installation of heavy items high up in the ship. As the quenching chamber 21 and scrubbing duct 22 do not contain any packing material to disrupt gas flow, the scrubbing apparatus 10 does not impose a large back pressure on the exhaust duct 12. The components of the waste water treatment 33 may be installed below the treatment unit 20, for example on a lower deck of the ship, for example within the ship's engine room, as may the water supply pumps 23 and 38. It will be appreciated that the apparatus 10 described in relation to the figures is by way of example only. A scrubbing apparatus and its mode of operation may differ in various ways while remaining within the scope of the present invention, for example the quenching chamber and the scrubbing chambers may be of a different cross-sectional shape to that shown. It will be appreciated that the apparatus 10 can be installed as a module. As described above, the volume of the treatment unit 20 can be selected in accordance with the gas flow to be treated, which corresponds to the power of the engine. However, if a single module does not provide sufficient capacity it will be appreciated that, as an alternative, two or more such treatment units 20 may be 5 used in parallel, so providing greater capacity.

Claims

1. An apparatus for scrubbing exhaust gases from an engine, the apparatus comprising:- a quenching chamber of a high temperature resistant material, the quenching chamber containing nozzles for spraying aqueous droplets into the exhaust gases;- a scrubbing duct of sufficient length and cross-sectional area that the residence time for exhaust gases in the scrubbing duct is greater than 2 seconds, the scrubbing duct comprising nozzles for spraying droplets of scrubbing liquid so that the droplets fall through the flowing exhaust gases; and also comprising:- a processing plant to supply the scrubbing liquid, the scrubbing liquid having a pH between pH8 and pH12, wherein the scrubbing duct comprises a plurality of scrubbing chambers, and wherein the quenching chamber and the scrubbing chambers are linked in series by flow ducts, each flow duct being of cross-sectional area no more than 1 / 10 the cross-sectional area of the chambers to which it is connected.

2. An apparatus as claimed in claim 1 wherein the ratio of the cross-sectional area of the chamber to that of the flow ducts is more than 10, but less than 200 for example 20, 30, 35,40 or 50.

3. An apparatus as claimed in claim 1 or claim 2 wherein the processing plant is arranged to add chemicals to environmentally-obtained water, for example sodium hydroxide, calcium hydroxide, and / or calcium chloride.

4. An apparatus as claimed in any one of the preceding claims wherein the length of the scrubbing duct is between 2 m and 10 m, more preferably between 4 m and 6 m, and the cross-sectional area is selected in accordance with the maximum exhaust gases flow to achieve a required residence time.

5. An apparatus as claimed in any one of the preceding claims that is connectable into an exhaust-carrying duct by means of a diversion valve, such that when the exhaust gases scrubbing apparatus is not operating the diversion valve may be actuated so that the exhaust gases are not passed through the scrubbing apparatus.

6. An apparatus as claimed in any one of the preceding claims wherein the scrubbing duct is made of a plastic or fibre-reinforced plastic.

7. An apparatus as claimed in any one of the preceding claims wherein the quenching chamber and the scrubbing chambers are arranged alongside each other, or one above another, and connected by flow ducts that follow U-shaped paths.

8. An apparatus as claimed in any one of the preceding claims that comprises an outlet for treated exhaust gases from the scrubbing duct, and an outlet for scrubbing liquid that has passed through the exhaust gases in the scrubbing duct, and the apparatus comprises means to treat the scrubbing liquid that has passed through the scrubbing duct, to remove particulate material, so the remaining waste liquid can be discharged back to the environment.

9. An apparatus as claimed in claim 8 wherein the means to remove particulate material from the scrubbing liquid is a hydro-cyclone.

10. A process for scrubbing exhaust gases from an engine, comprising:- passing the exhaust gas through a quenching chamber of a high temperature resistant material, and spraying aqueous droplets from nozzles into the exhaust gases to lower their temperature and so produce cooler exhaust gases;- then passing the cooler exhaust gases through a scrubbing duct of sufficient length and cross-sectional area that the residence time for the exhaust gases in the scrubbing duct is greater than 2 seconds, the scrubbing duct comprising nozzles; and- spraying droplets of a scrubbing liquid from the nozzles of the scrubbing duct so that the droplets fall through the flowing exhaust gases;- wherein the scrubbing liquid is supplied from a processing plant, the scrubbing liquid having a pH between pH8 and pH12, wherein the scrubbing duct comprises a plurality of scrubbing chambers, and wherein the quenching chamber and the scrubbing chambers are linked in series by flow ducts, each flow duct being of cross-sectional area no more than 1 / 10 the cross-sectional area of the chambers to which it is connected.

11. A process as claimed in claim 10 wherein the ratio of the cross-sectional area of the chamber to that of the flow ducts is more than 10, but less than 200 for example 20, 30, 35, 40 or 50.

12. A process as claimed in claim 10 or claim 11 wherein the quenching chamber is arranged to cool the exhaust gases to below 100°C, for example to below 70°C or to below 50°C.

13. A process as claimed in claim 10, claim 11 or claim 12 wherein the total pressure drop, when the exhaust gases flow through the quenching chamber and the scrubbing duct, is less than 10 kPa (76 mm Hg).

14. A process as claimed in any one of claims 10 to 13 wherein the residence time for exhaust gases in the scrubbing duct is greater than 3 seconds, and more preferably greater than 4 seconds.Application No: GB2317491.5Examiner: Joe MahoneyClaims searched: 1-14Date of search: 9 April 2024Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X 1-14 US2014 / 0338532 Al (SOGAARD) - Please see figures 8A, 8B and paragraphs [0024], [0033], [0035], [0101] in particular. X 1-14 WO2012 / 117233 Al (HOLNESS et.al) - Please see whole document. X 1-4,6-14 US3420508 A (HURST &DARLINGER) - Please see figures and column 1 lines 52-62, column 2 lines 48-65, column 4 lines 36-44. A - KR101630074 Bl (KIM et.al) - Please see abstract and figures, taking note of expansion unit 112 for quenching.Categories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if P Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:International Classification:Subclass Subgroup Valid From F01N 0003 / 04 01 / 01 / 2006 BOID 0047 / 06 01 / 01 / 2006 BOID 0053 / 48 01 / 01 / 2006 BOID 0053 / 92 01 / 01 / 2006 F01N 0013 / 00 01 / 01 / 2010

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