GB2301865A

GB2301865A - Treatment of recirculated exhaust gases

Info

Publication number: GB2301865A
Application number: GB9611989A
Authority: GB
Inventors: David Christopher Walto Morley
Original assignee: CDSS Ltd
Current assignee: CDSS Ltd
Priority date: 1995-06-08
Filing date: 1996-06-07
Publication date: 1996-12-18
Also published as: GB9611989D0; GB9511611D0

Abstract

Exhaust gas recirculated back to a combustion chamber, e.g. in a closed cycle diesel engine 1, is treated with an aqueous ammonia solution in a unit 9 to remove carbon dioxide. Crystals of ammonium bicarbonate may be removed from ammonia solution circulated from the unit 9 to a tank 5. Metered quantities of liquid or gaseous ammonia from a tank 6 may be added to sea water from the exterior of a vessel wall 23 (Fig. 3) which is supplied to the unit 9.

Description

Title: "Recirculatory System" Description of Invention This invention relates to a recirculatory system, hereinafter referred to as being of the kind specified, comprising a combustion chamber, means to cause fuel to combust in the combustion chamber, means to feed exhaust gas from the combustion chamber, a circuit through which the exhaust gas is fed from the combustion chamber and some of the exhaust gas is returned to the combustion chamber and the circuit including means to treat at least some of the exhaust gas to remove a part therefrom. An example of such a system is disclosed in EP-A-118284.

The invention has been particularly, but not exclusively, developed for use with an internal combustion engine such as a compression ignition engine, spark ignition engine or gas turbine, or an external combustion engine such as a Stirling engine or steam engine. The invention is, however, also applicable to other combustion processes where it is desired to recirculate at least part of the exhaust gas back to a combustion chamber.

In EP-A-118284 the means to treat at least some of the exhaust gas to remove part therefrom comprises means to treat the exhaust gas with water so that carbon dioxide is dissolved in the water according to Henry's Law. A relatively high flow rate of water is required, for example 0.15-0.25 litres of water per second per kilowatt of engine output power. This causes a relatively large power loss due to the need to pump water into treating relationship with the exhaust gas.

In another approach it has been suggested to treat the exhaust gas with potassium hydroxide so that the carbon dioxide is converted to a non-volatile storable potassium bicarbonate. However, this system requires a substantial quantity of potassium bicarbonate to be stored, typically 6kgs of potassium hydroxide solution per kilowatt hour of running.

Especially for relatively small installations, for example submarines with power units of less than 100 kw power output, it is often difficult to make one of the existing systems sufficiently compact.

It is accordingly an object of the present invention to provide a recirculatory system of the kind specified which overcomes or reduces the disadvantages described hereinbefore.

According to the present invention we provide a recirculatory system of the kind specified wherein the means to treat at least some of the exhaust gas to remove part therefrom comprises means to treat the exhaust gas with an aqueous solution of ammonia.

As a result of treating the exhaust gas with an aqueous solution of ammonia a considerably greater amount of carbon dioxide can be removed from the exhaust gas than in the system of EP-A-118284 where the exhaust gas is treated with water alone. For example, treatment of the exhaust gas with water at one bar absolute results in the removal of up to about 2gms of carbon dioxide per lkg of water, whilst in the present example up to 70gms of carbon dioxide may be removed from the exhaust gas per lkg of aqueous solution of ammonium.

The system may be provided in a vessel or other enclosure.

Ammonia supply means may be provided to supply ammonia to treat water supplied from a water storage means.

Where the system is provided in a vessel or other enclosure, the water storage means may be provided within the vessel or other enclosure or externally and carried thereby.

Alternatively an ammonia supply means may be provided to supply arnmonia to treat water supplied from an external source of water.

Where the system is provided in a vessel or other enclosure the external source may be a body of water in which the vessel or other container is, or is at least partly, immersed.

The means to treat the exhaust gas with the aqueous solution of ammonia may comprise an exhaust gas contacting device in which the exhaust gas is contacted with the aqueous solution of ammonia.

Where the water is supplied from an external source of water the water may be fed from the external source to a treatment means where the water is treated with ammonia to form an aqueous solution of ammonia and means may be provided to feed the aqueous solution of ammonia to said contacting means.

Alternatively, where water is supplied from a water storage means, means are provided to recirculate the ammonia solution through the contacting device and means are provided to supply ammonia from said supply means to maintain the aqueous solution of ammonia at a desired predetermined concentration.

Since no water enters or leaves the system the carbon dioxide may be removed from the exhaust until the ammonia is exhausted.

The aqueous solution of ammonia may be of such concentration that some of the carbon dioxide dissolves in the water of the solution and some of the carbon dioxide reacts with the ammonia to form ammonium bicarbonate in solution.

Alternatively the aqueous solution of ammonia may be of such concentration that the carbon dioxide reacts with the ammonia solution to form a precipitate of ammonium bicarbonate and means being provided to remove the precipitate.

Said means to remove the precipitate may be a scrolling centrifuge, a filter or other means to separate a precipitate from the solution.

The ammonia supply means may comprise a vessel containing liquid ammonium, or a concentrated solution of ammonia, or, alternatively the ammonia supply means may comprise supply means for a precursor constituent or constituents to produce ammonia by reaction between a plurality of precursors or by the action of heat on at least one precursor.

In a more specific aspect of the invention the recirculatory system may comprise an internal combustion engine such as a diesel engine comprising a combustion chamber, delivery means to deliver insert carrier gas, combustion supporting gas and fuel into the combustion chamber, means to cause the fuel to burn in the combustion chamber, means to exhaust exhaust gas from the combustion chamber, a circuit through which some at least of the exhaust gas is ducted from the combustion chamber and returned to the combustion chamber to provide the carrier gas, the circuit including said means in which the exhaust gas is treated to remove carbon dioxide from the exhaust gas, and supply means to supply a monatomic inert gas to the combustion chamber and means are provided to control the amount of monatomic gas supplied by the supply means, such that the gamma value of the mixture of gases in the combustion chamber has a value lying in a predetermined range.

In this specification the term "aqueous solution of ammonia" is used to refer to the liquid which results when ammonia has been contacted with water irrespective of the precise chemical composition of the liquid.

When ammonia passes into water it forms an aqueous solution of ammonia, in reality, however, it is understood that the ammonia is generally converted into ammonium ions and forms a solution of ammonium hydroxide NH4.OH.

Embodiments of the invention will now be described with reference to the accompanying drawings wherein: FIGURE 1 is a schematic illustration of a first embodiment of the invention, as applied to a diesel engine, FIGURE 2 is a schematic illustration similar to Figure 1 showing an alternative embodiment of the invention, and FIGURE 3 is a schematic illustration similar to Figure 1 showing another alternative embodiment of the invention.

Referring now to Figure 1, the internal combustion engine which is the first embodiment of this invention has been devised for operating both under conditions of normal aspiration at atmospheric pressure and in environments where communication with free-atmosphere is undesirable, or under water, in which latter context the engine will hereinafter be described. The engine comprises a piston/cylinder unit 1 designed to operate on diesel cycle and hence having a combustion chamber and an inlet valve or valves through which a mixture of gases containing oxygen are admitted to the cylinder, and an exhaust valve or valves through which exhaust gas is ducted from the cylinder. The engine also comprises means 50 to inject fuel into the cylinder, and a circuit 51 through which some at least of the exhaust gas is ducted from the combustion chamber 1 and returned thereto.The circuit 51 includes a heat exchange unit 2, and a spray cooling unit 3, and a duct 4 connected to a treatment unit 9. If desired either or both of the heat exchange unit and spray cooling unit may be omitted if not required.

In the operation of the engine in a non-atmospheric or closed cycle, oxygen from a store 10 thereof is delivered to a manifold 12 at a working pressure of between 1.5 and 10 Bara, and is ducted into the engine cylinder together with carrier gas. The gas charge is compressed within the engine cylinder, causing the temperature of the gas to increase, and fuel is injected, producing combustion of the fuel. Exhaust gas is ducted from the combustion chamber through the duct 4 to the spray cooler 3, in which the temperature of the exhaust gases is reduced, and through the first cooling unit 2 to the treatment unit 9, in which the exhaust gases are treated to remove carbon dioxide as hereinafter to be described in more detail.

The thus treated exhaust gas is passed to a sensor unit 15 in which the oxygen content of the gas is measured, and back into the manifold 12 in which argon and oxygen are added to the treated exhaust gas from reservoirs 10, 11 respectively.

A pressure sensor unit 53 in one of the combustion chambers is arranged to provide a control signal to a metering valve 13 which meters the amount of argon fed to the manifold 12 from the reservoir 11 to ensure that the proportion of argon in the inducted gas provides the desired ratio of specific heats, gamma. In the present example, the sensor 53 gives a measure of the pressure rise inside the cylinder from which a measure of gamma may be deduced.

During an initial stage in the operation of the engine on closed cycle, the proportions of carbon dioxide and argon in the circulating gas will increase until an equilibrium condition is established (dependent upon the characteristics of the absorption process) in which the rate at which carbon dioxide is removed by absorption in the aqueous solution is equal to the rate at which carbon dioxide is added during the combustion process.

The supply of air for the initial charge for closed cycle operation may be achieved by supplementary supply bottles, or air may be ducted from available free-space in the engine compartments, of course one closed cycle operation has started, further air is not required but only a continual supply of oxygen. If engine is shut down from a closed cycle system in balanced operation, the initial charge is suitable for another start-up without additions.

Alternatively operation of the engine is initiated in the atmosphere on "open cycle", in which the exhaust gases are vented to atmosphere via two-way valves MV, and air is drawn into the manifold from atmosphere via two-way valves MV, and then switched over to closed cycle, in which an increasing proportion of exhaust gas is ducted to the absorber by progressively closing valves to atmosphere, causing, in consequence, an increasing proportion of treated exhaust gases to be returned to the manifold with a corresponding closing of valves to atmosphere. When change over to closed cycle has been completed, the engine may be submerged to its intended location.

The engine which is the first embodiment of this invention is designed to operate in air, or a charge which simulates air, that is with a mixture of charge gases having a gamma ratio of approximately 1.4.

In this embodiment the system is operated with a fixed quantity of fresh water carried within a pressure wall 23 of the container or vessel within which the system is provided. Water is not taken into or passed out of the interior of the wall.

A tank 5 for containing a quantity of water or of ammonia solution is provided. Ammonia liquid is stored in a reservoir 6 and is fed by a metering pump 7 and pressure relief valve 18 to a contacting device 19 in a circuit 52 through which water/ammonia solution is circulated by a pump 8 between the tank 5 and the treatment unit 9.

In this embodiment ammonia is fed into the water in the contacting device 19 so as to bring the ammonia level above that needed to cause precipitation of bi-carbonates when carbon dioxide is contacted with the water in the treatment unit 9.

Accordingly there is more than the 2.50 wt.% of ammonia in the water and it is convenient in practice to bring the water near to being saturated with ammonia.

The treatment unit 9 may be of any suitable type but preferably comprises a rotor provided with wire mesh or other material having a high surface area to volume ratio through which the water is thrown radially outwardly by centrifugal force whilst the exhaust gas is caused to pass therethrough in counterfiow. This treatment unit achieves rapid contact with the sea water and is compact. The treatment unit 9 is provided with a level control 20 to ensure that the treatment unit is not flooded or run below a predetermined water level.

When the carbon dioxide of the exhaust gas comes into contact with the ammonia solution a fine slurry of bicarbonate crystals is formed. This slurry is passed to a device suitable for removing crystals from the water. A preferred device is a scrolling centrifuge such as is provided as shown at 16 diagrammatically, but other devices may be employed such as filters or white wheels, commonly used in the art of preparing clays.

The crystals of bicarbonate are drawn off to a holding vessel 17 and the water and any residual ammonia is passed back to the tank 5 and recirculated to the contacting device 19 to be re-treated with ammonia.

In a second embodiment, shown in Figure 2, in which the same reference numerals have been used to refer to corresponding parts as were used in Figure 1, it may be convenient only to pass part of the recycle gases through the absorber. In this way the required amount of carbon dioxide may still be removed from the recycle gases but the ratio of gas flow to water flow in the contacting device may be brought into a ratio more suitable for efficient mass transfer.

In order to achieve this a gas bypass duct 54 is provided and a pump 21. The pump 21 is arranged to pass a minority fraction of the gas to the treatment unit 9 after raising the pressure through a ratio which is high enough to overcome any pressure drops in the contacting device 9 but not so high as to unnecessarily waste power. A ratio of 1.2:1 is suitable in some cases, for example.

In a third embodiment, shown in Figure 3, in which the same reference numerals have been used to refer to corresponding parts as were used in Figure 1, in the treatment unit 9 the exhaust gases are treated with sea water which has itself been treated with ammonia to form an aqueous solution of ammonia. In the present example the water is sea water taken in at an inlet 22 through a vessel pressure wall 23. The water taken in at the inlet 22 is fed via a conduit 55 to a contactor 19 where the sea water is contacted with ammonia by means of a metering pump 7 working towards a pressure relief valve 18 which is set at a predetermined pressure, in the present example 15 bar. The pump 7 feeds the ammonia from a reservoir 6 which provides a store of liquid ammonia. The metering pump arrangement 7, 18 ensures that the ammonia is kept liquid so that the pump 7 will work properly and not be troubled with vapour formation.

The amount of ammonia added to the water is controlled by controlling the delivery provided by the pump 7 to provide a solution of a desired concentration.

The sea water is subjected to a change of pressure of between one and ten Bara (bars absolute) depending upon the pressure at which the diesel engine is operating. The change of pressure may be effected by any suitable means but a preferred method is to link the changes of pressure from sea pressure to diesel engine operating pressure and the subsequent diesel engine operating pressure to sea pressure by means of a water management system 59 such as is disclosed and claimed in our patent no. EP-A-0142362B.

In any of the previously described embodiments the ammonia, instead of being drawn from a pressurised tank containing a liquid ammonia, may be drawn from a reservoir of gaseous ammonia or may be made by the reaction of at least two precursors or by the thermal decomposition of at least one precursor.

Other details of the recirculating system including possible modifications of the recirculating system are described in our patent EP-B0118284.

Whilst in principle it is possible to dissolve 30 times the amount of carbon dioxide into an aqueous solution of ammonia than is possible with water according to Henry's Law, in practice such an extent of solution may not be achieved, since the extent of solution depends on other factors, such as the flow of gases and liquid and the effectiveness of the treatment unit 9.

The features disclosed in the foregoing description, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, or a class or group of substances or compositions, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A recirculatory system of the kind specified wherein the means to treat at least some of the exhaust gas to remove part therefrom comprises means to treat the exhaust gas with an aqueous solution of ammonia.

2. A system according to Claim 1 wherein the system is provided in a vessel or other enclosure.

3. A system according to Claim 1 or Claim 2 wherein an ammonia supply means is provided to supply ammonia to treat water supplied from a water storage means.

4. A system according to Claim 3 when dependent on Claim 2 wherein the water storage means is provided within the vessel or other enclosure.

5. A system according to Claim 3 when dependent on Claim 2 wherein the water storage means is provided externally of the vessel or other enclosure and carried thereby.

6. A system according to Claim 1 or Claim 2 wherein an ammonia supply means is provided to supply ammonia to treat water supplied from an external source of water.

7. A system according to Claim 6 when dependent on Claim 2 wherein the external source is a body of water in which the vessel or other container is, or is at least partly, immersed, in use.

8. A system according to any one of the preceding claims wherein the means to treat the exhaust gas with the aqueous solution of ammonia comprises an exhaust gas contacting device in which the exhaust gas is contacted with the aqueous solution of ammonia.

9. A system according to Claim 8 when dependent on Claim 6 or Claim 7 wherein the water is fed from the external source to a treatment means where the water is treated with ammonia to form an aqueous solution of ammonia and means are provided to feed the aqueous solution of ammonia to said contacting device.

10. A system according to Claim 8 when dependent on any one of Claims 3 to 5 wherein means are provided to recirculate the ammonia solution through the contacting device and means are provided to supply ammonia from said supply means to maintain the aqueous solution of ammonia at a desired predetermined concentration.

11. A system according to any one of the preceding claims wherein no water enters or leaves the system and the carbon dioxide is removed from the exhaust until the ammonia is exhausted.

12. A system according to any one of the preceding claims wherein the aqueous solution of ammonia is of such concentration that some of the carbon dioxide dissolves in the water of the solution and some of the carbon dioxide reacts with the ammonia to form ammonium bicarbonate in solution.

13. A system according to any one of Claims 1 to 11 wherein the aqueous solution of ammonia is of such concentration that the carbon dioxide reacts with the ammonia solution to form a precipitate of ammonium bicarbonate and means are provided to remove the precipitate.

14. A system according to Claim 13 wherein said means to remove the precipitate is a scrolling centrifuge, a filter or other means to separate a precipitate from the solution.

15. A system according to Claim 3 or any one of Claims 4 to 14 when dependent on Claim 3 wherein the ammonia supply means comprises a vessel containing liquid ammonia, or a concentrated solution of ammonia.

16. A system according to Claim 3 or any one of Claims 4 to 14 when dependent on Claim 3 wherein the ammonia supply means comprises supply means for a precursor constituent or constituents to produce ammonia by reaction between a plurality of precursors or by the action of heat on at least one precursor.

17. A system substantially as hereinbefore described with reference to Figure 1, Figure 2, or Figure 3 of the accompanying drawings.

18. A recirculatory system according to any one of the preceding claims comprising an internal combustion engine comprising a combustion chamber, delivery means to deliver insert carrier gas, combustion supporting gas and fuel into the combustion chamber, means to cause the fuel to burn in the combustion chamber, means to exhaust exhaust gas from the combustion chamber, a circuit through which some at least of the exhaust gas is ducted from the combustion chamber and returned to the combustion chamber to provide the carrier gas, the circuit including said means in which the exhaust gas is treated to remove carbon dioxide from the exhaust gas, and supply means to supply a monatomic inert gas to the combustion chamber and means are provided to control the amount of monatomic gas supplied by the supply means, such that the gamma value of the mixture of gases in the combustion chamber has a value lying in a predetermined range.