Patent Description:
Ammonia-fuelled ships are increasingly being developed and built, or engines adapted to be driven by ammonia-fuel, following a <NUM> International Maritime Organisation commitment to cut International shipping's greenhouse gas emissions. Meanwhile, on ships and vessels with engines running on low-flashpoint fuels, regulations require the engine(s), and piping between fuel valve train(s), to be purged, typically with nitrogen, whenever the engine stops, either intentionally or from trips. During the purging process, some of the fuel will evaporate into the nitrogen purging stream, and so be vented from the vessel along with the nitrogen.

<CIT> discloses that the purge gas from a marine vessel engine contains ammonia and other contaminants and it is treated in a gas-liquid separation device before being vented.

<CIT>, <CIT>, <CIT>, <CIT> and <CIT> are also useful in understanding the background technology.

When the fuel is ammonia, it is desirable to minimise emissions of ammonia gas because ammonia is toxic. This is particularly for crew safety, and where an engine stops at unplanned times, for example due to an engine trip.

The present invention seeks to help overcome this problem.

According to one aspect of the present invention, there is provided a method of treating an ammonia-containing purge gas stream from an engine on a marine or seagoing vessel comprising at least the steps defined in Claim <NUM>.

Steps (a) and (b) of the method of treating are optionally a first stage of a batch process, as well as a first stage of a more general or overall process for treating a vent gas stream and regenerating the treatment process on a vessel.

The method further comprises the steps of:.

Such steps provide a second stage of an overall process, or a first stage of a regenerating process.

This provides a third stage of an overall method of treating, or a second stage of a regenerating process.

Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying drawings in which:.

The use of ammonia as a fuel in engines has been known for some time. It is now possible to provide what is termed "green ammonia", by reacting nitrogen separated from air with hydrogen made by wind or solar-powered water electrolysis. Green ammonia is potentially seen as environmentally friendlier than fossil fuels by having a no-carbon emissions. Green ammonia is also a cheaper fuel for the shipping industry than hydrogen, as it is easier to store and can be burned in standard internal combustion engines.

However, ammonia is also considered to be relatively toxic. Thus, the regulations that require a ship's or vessel's engine(s), and piping between fuel valve train(s), to be purged with nitrogen whenever the engine stops, either intentionally or from trips, may lead some of the ammonia fuel to be evaporated into the purging nitrogen stream, and thus be vented along with the nitrogen. Venting of ammonia is undesirable due to its toxicity.

An example of the present invention provides a method of treating an ammonia-containing purge gas stream from an engine on a marine or seagoing vessel comprising as a first stage, the steps of:.

The vessel may be any marine or seagoing vessel, including ships, carriers etc..

The nature of the engine may be an existing or standard internal combustion engine, or an engine that has been further developed to use ammonia fuel. Some engines are also able to use a mixture of ammonia and an additional fuel, and the present invention extends to all such engines. Purging gas streams are known in the art, and generally comprise one or more inert gases, including inert to the nature of the engine fuel. A purging gas stream is passed through an engine, line, tank, piping etc., to purge these of gases or liquids for various reasons, including a change of fuel in the engine, or cleanliness or to prevent undesirable build up. The commonest gas used for a purge gas stream is nitrogen, although other gases may be used or mixed.

The method of the present invention passes a purge gas stream on the vessel through an engine on the vessel, (i.e. a vessel engine), optionally also associated pipework and the piping between fuel valve train(s), to be purged, to provide a resultant ammonia-containing purge gas stream, i.e. including ammonia that has evaporated and needs to be purged away from the engine(s), etc..

In the present invention, such a stream is passed into a first tank containing water to provide a reduced-ammonia vent gas stream. Water is known to dissolve ammonia, such that the resultant gas passing through and out of the water will have a reduced-ammonia content, herein termed a reduced-ammonia vent gas stream.

Optionally, the first tank contains water comprising no or a de minimis amount of ammonia, such as a 'trace amount'.

Optionally, the ammonia-containing purge gas stream is passed through the water of the first tank with or under some form of excitation so as to increase the surface area between the ammonia-containing purge gas stream and the water. Various means and apparatus are known to achieve excitation of a gas stream through a liquid stream, including various dispersement means. One common means of achieving excitation in this way is sparging, which uses a sparging device, means or `head' intended to create bubbles of the vent gas stream, and therefore a high or higher surface area for ammonia absorption.

It is known that ammonia can dissolve in ammonia-free water even at ambient temperature and pressure. The absorption can be assisted with enhanced conditions such as above ambient pressure. The absorption provides a reduced-ammonia vent gas stream that may be at least <NUM>% free of its ammonia content, possibly ><NUM>% ammonia free, and even ><NUM>% ammonia free.

It is possible that where the reduced-ammonia gas stream is ><NUM>% ammonia free, the reduced-ammonia vent gas stream can be directly vented to atmosphere. Thus, the method of the present invention can further provide the step of:
venting the reduced-ammonia vent gas stream to atmosphere.

Additionally or alternatively, the reduced-ammonia vent gas stream is further treated by one or more further treating processes or steps. One known further treatment step is use of scrubbing, in particular as an acid-scrubbing polishing step, intended to further capture or recover ammonia in the vent gas stream. Thus, the method of the present invention can further provide the step of:
further treating the reduced ammonia vent gas stream by acid scrubbing.

The steps outlined and described above provide a first stage of a process. Such steps may be carried out for a required time or duration, i.e. as a batch process, for treating an ammonia-containing vent gas stream.

A resultant ammonia-rich water remains in the first tank.

The present invention further provides as a second stage the steps of:.

Caustic solutions are well known in the art, and typically involve one or more alkalis. Concentrated forms of caustic solution are preferred. Desired concentration of caustic solutions for ammonia-release are well known in the art and not further described herein.

By providing a caustic solution in a second tank, and passing the caustic solution from the second tank into the first tank to mix with the ammonia-rich water therein, the caustic solution provides the release of the ammonia dissolved in the first tank in a gaseous form, to provide a gaseous ammonia stream, and a resultant solution, generally being a less concentrated caustic solution.

The passing of solutions between the first tank and the second tank and the second tank and the first tank may be provided by any suitable means, including one or more pumps etc. known in the art. Optionally, one pump is able to provide the means to pump liquids between both tanks, using appropriate valving and piping.

The gaseous ammonia stream can be directed either back to a suitable storage tank such as an ammonia fuel tank, or to an ammonia reliquefaction unit, or to both as desired or required. In this way, the ammonia that would previously have been vented with the vent gas stream has now been recovered.

The recovery of the ammonia provides a second stage of the method of treating an ammonia-containing vent gas stream, alternatively or additionally forming a first stage of a regenerating process of the system described herein.

The present invention further provides as a third stage the steps of:.

The heating of the resultant solution in the second tank may be provided by any suitable means, including electric, steam and other heating means known in the art.

The condensing of a water vapour stream may be provided by any suitable condenser, generally having a condensing stream running therethrough intended to reduce the temperature of the water vapour stream to provide a liquid water stream.

The liquid water stream is therefore now free of ammonia and ready for reuse back in the first tank, whilst the caustic solution remaining in the second tank has also been recovered and is ready for reuse when next required.

The recovery of the ammonia-free water and caustic solution provides a third stage of the method of treating an ammonia-containing vent gas stream, alternatively or additionally forming a second stage of a regenerating process of the system described herein.

Thus, overall, the present invention shows a method for treating and regenerating an ammonia-containing purge gas stream from an engine on a marine or seagoing vessel comprising at least the steps of:.

The use of consecutive lettering for the steps described herein is not limiting to that order where one or more of the steps could be carried out in an alternative sequence.

The skilled person can see that the steps (a) to (k) of the system can be repeated, such that the system provides a cycle.

Referring to the drawings, <FIG> shows, there is shown a method of treating an ammonia-containing purge gas stream <NUM> on a vessel (not shown) comprising at least the steps of providing the ammonia-containing purge gas stream <NUM> from a vessel engine being purged (not shown); and passing the ammonia-containing purge gas stream <NUM> into a first tank <NUM> containing water <NUM> to provide a reduced-ammonia vent gas stream.

The purging of engine(s) on a vessel by a purging gas stream such as nitrogen is well known in the art to remove gases away from the engine when it stops, either intentionally or not. Where the removed gas or gases are not harmful or toxic, they may be vented directly to atmosphere.

However, ammonia is relatively toxic. Thus, a purging gas stream for an ammonia-using engine should not be vented directly to atmosphere.

In the example of the present invention, the ammonia-containing purge gas stream <NUM> comprises the purging gas nitrogen, and ammonia. This stream <NUM> passes through an ammonia-free water <NUM> in the first tank <NUM>, so that the water <NUM> is able to absorb the ammonia content. The ammonia-containing purge gas stream <NUM> can be sparged through a suitable sparging device or head <NUM> at or near the bottom of the first tank <NUM>, to disperse the passage of the ammonia-containing purge gas stream <NUM> through the water <NUM> in a manner known in the art, such that the ammonia is better dissolved into the water <NUM>, and the vent gas is 'cleaned' of its ammonia content.

The resultant reduced-ammonia vent gas stream from the water <NUM> passes up through the first tank <NUM> into a suitable header pipe <NUM>. Following the opening of a vent valve <NUM>, the resultant reduced-ammonia or even ammonia-free vent gas stream can be either vented to atmosphere, or if required further treated by a subsequent acid-scrubbing polishing step or the like, along pipeline <NUM> in a manner known in the art, and not further described herein.

The provision of the ammonia-containing purge gas stream <NUM> needing to be cleaned can be provided as a first stage. This stage provides venting of a wholly or substantially ammonia-free vent gas to atmosphere, or where required, to a subsequent acid-scrubbing polishing step.

Once treatment of the ammonia-containing purge gas stream <NUM> has stopped or otherwise been completed, the vent valve <NUM> can be closed.

Over time, the water <NUM> in the first tank <NUM> becomes ammonia-rich and needs changing or cleaning to be able to absorb more ammonia. This is typically termed 'regenerating', and can either be a second stage of the method of treating, or a first stage of a regenerating process for regenerating the water in the tank <NUM>.

<FIG> shows a second tank <NUM> containing a caustic solution, optionally a concentrated caustic solution. When required, the caustic solution <NUM> can be pumped via a bottom pipeline <NUM>, pump valve <NUM>, pump <NUM>, next pump valve <NUM> and pipeline <NUM>, into the first tank <NUM>. The caustic solution <NUM> then reacts with the ammonia-rich water <NUM> in the first tank <NUM>, to release the absorbed ammonia, forming ammonia gas in a manner known in the art. By opening of return valve <NUM>, the gaseous ammonia stream thus created in the first tank <NUM> can be released through the header pipe <NUM>, return valve <NUM>, and then either be returned to a storage or fuel tank <NUM>, and/or sent to reliquefaction through pipeline <NUM>, by the use of appropriate valves after the initial return valve <NUM>. In this way, the ammonia provided by the ammonia-containing purge gas stream <NUM> and absorbed by the water <NUM>, is wholly or substantially recovered.

In either a third stage of the method of the present invention as described herein, or a second stage of a regenerating process, the resultant solution in the first tank <NUM> can be passed back into the second tank <NUM> via the transfer pump <NUM> via closure of the pump valves <NUM> and <NUM> and opening of the return valves <NUM> and <NUM> in by-pass pipelines <NUM> and <NUM>.

Thus, the resultant solution from the first tank <NUM> passes into the second tank <NUM>. The resultant solution can now be heated by a steam stream <NUM> through a control valve <NUM>, (and resulting in a steam condensate stream <NUM> out of the second tank <NUM>). The heating of the resultant solution in the second tank <NUM> provides a water vapour stream that passes upwardly and through a header pipe <NUM>, through a control valve <NUM>, into a suitable water condenser <NUM> to be condensed. The so-formed condensed liquid water stream <NUM> that is recovered in this way is ammonia- and caustic-free, can be provided back into the first tank <NUM> to provide the water for step (b) as described above, i.e. the clean water able to absorb the ammonia in the ammonia-containing purge gas stream <NUM> to provide a reduced-ammonia vent gas stream through piping <NUM> and <NUM>.

Following the regenerating process, there is provided recovery and return of the ammonia-free water in the first tank <NUM>, and of the caustic solution <NUM> in the second tank <NUM>, ready for further treatment of ammonia-containing vent gas in another cycle as described above.

Thus, the process is complete, and the cycle may begin again.

Depending upon the required purge, and selected water volumes, <NUM>% ammonia recovery can be approached, with greater than <NUM>% ammonia recover being expected. If, instead of directly venting the reduced-ammonia vent gas stream to atmosphere, subsequent acid-scrub polishing is used, the acid consumption decreases proportionately.

The skilled person can see that the methods and steps described above show an overall method for treating and regenerating an ammonia-containing purge gas stream on a vessel comprising at least the steps of:.

The skilled person can see that the steps (a) to (k) of the method can be repeated, such that the method provides a cycle.

<FIG> also shows an apparatus an apparatus for reducing ammonia content of a vessel engine vent gas stream comprising:.

<FIG> is a graph of molar composition of ammonia in a reduced-ammonia vent gas against time. This is an example illustrating the reduction in vented ammonia content during the first stage of processing described hereinabove. <FIG> can be interpreted in relation to <FIG>, with the following parameters. The ammonia-containing purge gas stream <NUM> flows at a rate of <NUM>/h with a composition containing <NUM> mol% ammonia, (with the balance being nitrogen), into the first tank <NUM>. The first tank <NUM> contains <NUM><NUM> of water <NUM>. The ambient temperature, which is assumed to be the prevailing temperature in the system itself, is <NUM>. The pressure in the first tank <NUM> is maintained at <NUM> bar(g) using the control valve <NUM>. <FIG> shows the ammonia content of the reduced-ammonia vent gas in pipeline <NUM> (and ultimately pipeline <NUM>) as a function of time.

Claim 1:
A method of treating an ammonia-containing purge gas stream (<NUM>) from an engine on a marine or seagoing vessel comprising at least the steps of:
(a) in a first stage, providing the ammonia-containing purge gas stream (<NUM>) from the vessel engine;
(b) passing the ammonia-containing purge gas stream through a first tank (<NUM>) containing water (<NUM>) to provide a reduced-ammonia vent gas stream;
(c) in a second stage, providing a second tank (<NUM>) containing a caustic solution (<NUM>);
(d) passing the caustic solution from the second tank into the first tank to provide a resultant solution and a gaseous ammonia stream; and
(e) passing the gaseous ammonia stream to a storage tank (<NUM>) or a reliquefaction unit;
(f) in a third stage, passing the resultant solution from the first tank (<NUM>) into the second tank (<NUM>);
(g) heating the resultant solution in the second tank to provide a water vapour stream and a recharged caustic solution;
(h) condensing the water vapour stream to provide a liquid water stream (<NUM>); and
(i) passing the liquid water stream into the first tank (<NUM>) to provide the water of step (b).