THE CAPTURE AND REMOVAL OF GASES FROM OTHER GASES IN A GAS

STREAM

Area of the Invention

The present invention isdirected to the field of Thermo-acoustic Stirling Hybrid Engines and Refrigerators (TASHER) they being Stirling hybrid devices that achieve cryogenic temperatures without moving parts. In particular the invention relates to a new separation method and apparatus for liquefying and removing wanted or unwanted gases from a gas stream, selectively capturing and storing liquefied gases, then allowing the release of desirable gases back into the atmosphere conserving the energy contained in the cooling accompanying their release.

Background to the Invention

The liquefaction of gases to enable their storage, or selective removal, has heretofore been generally achieved at costs which limit the usage of the technology, these costs, for instance, being particularly critical to the removal of greenhouse gases from the atmosphere which has importance in the field of climate change.

The conventional process for liquefaction of natural gas, or methane, is currently compressor based refrigeration.

Because of the temperatures required for this process it is usual to employ multi-stage compressors with the appropriate inter-cooling. The draw backs to this process are that it is not very efficient, multi-stage and it has a high maintenance penalty.

The other method of providing refrigeration is via thermo-acoustic refrigeration, which can potentially be made very efficient and because it has no moving parts and requires almost no maintenance although doubt has previously been has expressed as to

whether this method can economically liquify gases so as to remove "greenhouse gases" from exhaust gases resulting from combustion.

One known problem is that current thermo-acoustic refrigeration systems use conventional burners and produce levels of NOx that are way above the maximum levels permitted in many countries. Additionally, they make difficultthe development of "knife edge" heat sources as are most desirable forthe production of the most efficient acoustic wave in the TASHER.

Using pulse combustion burners however increases the thermal efficiency over current systems. The exhaust gases from the pulse combustion system can be heat exchanged with the incoming combustion air and all combustion gases cooled together to delete the greenhouse gases.

Combined we believe that a combination of pulse combustion and thermo-acoustic cooling could be used to make an economic and effective answer to the capture and storage for commercial use of wanted gases and eventual sequestration and storage of the unwanted gases.

It is known that pulse combustion can release 96-98% of the available heat from a fuel with virtually no release of oxides of nitrogen or sulphur and is economical to apply while conventional burners or the use of an electric heat source are generally more expensive and less efficient to apply.

We believe that the application of the technology mentioned above can overcome the drawbacks of existing TASHER units, thereby providing a more economic means of liquefaction of gases, or of cooling generally. In addition the size of units is scalable which is an essential feature in dealing with power station exhaust gases of substantial volume.

Sequestration of greenhouse gases is most desirable, but is difficult and expensive if underground sequestration is to be practiced. This methodology fights against the laws

of nature so it requires considerable force (pressure), hence a lot of energy, to pump the greenhouse gases underground, followed by never ending monitoring to ensure safety. If the underground caverns contain resources which are, or may be, valuable, then they too are sequestrated, from economic usage.

A better solution would be to deposit the liquefied greenhouse gases in the sea at a depth and temperature which will keep them liquid and, for greater security, under a blanket of silts which will protect them from movement arising from the most extreme mechanical or geological perturbations.

Even greater security could be achieved if the gases were encased in a plastic film which will also allow their re-use if so desired.

The undersea methods would appear to have costs of some 25% of the costs applicable to geosequestration in terrestrial cavities.

Therefore, there is still a significant unfilled need for a new method and apparatus for the liquefaction of gases and selective treatment of all or some of them and the undesired gases need to be sequestrated at an economic cost which . remains undemonstrated.

Outline of the Invention

It is an object of the invention to provide improvements over the prior art to enable the economic liquefaction of greenhouse gases, together with more economic liquefaction of desirable gases such as methane. It is a further object of the invention to provide a means whereby, once liquefied, the gases can be separated and the undesirable gases sequestrated. It is a further object of the invention that these objects be achieved in a highly efficient manner and answer the existing needs.

It is also an object of the invention to determine how unwanted recovered gases and vapours may be collected and permanently disposed of for geological time scales.

The invention is a method for the selective capture and removal of gases and vapors from a gas stream using thermo-acoustic means including the steps of

cooling the gas stream using at least one heat exchanger;

passing the stream to a thermo-acoustic refrigeration process;

removing the gases in a cascade process.

In order that the invention may be more readily understood we will describe by way of non limiting example a specific embodiment of the invention with reference to the accompanying drawings.

Brief Description of the Drawing Figures

Fig 1 Shows the general arrangement of the capture process;

Fig. 2 Shows a means of coupling thermo-acoustic refrigerators;

Fig. 3 Shows disposal methods for carbon dioxide;

Description of an embodiment of the invention

In the following embodiment of the invention the example of a process for the permanent removal of carbon dioxide is described however the invention is not restricted to the processing of carbon dioxide only and can equally be applied to other gases. For convenience sake however it will be described here in terms of its application to the treatment of carbon dioxide.

Carbon Dioxide Removal

In this embodiment of the invention the gas to be captured is carbon dioxide (CO ₂₎ and

this is cooled to a liquid state under pressure or solid state. This CO ₂ can be passed to a repository.

If the gas stream contains methane, this can be collected for use.

The oxygen and nitrogen can be passed to. atmosphere but before being so sent, can act as a heat exchange medium for the incoming gas stream.

Gases containing CO ₂ are invariably the end products of a combustion process or the natural constituents of gases from gas and oil wells. In the case of the former, these gases are normally hot and can be in the vicinity of 900 ⁰ C. It is to be noted however that the invention is applicable whether or not the gas stream may be hot.

Figure 1 shows the general arrangement of the capture process. Heat exchangers are shown, the first of which 1 is used to partially cool down the incoming hot gas stream containing the CO ₂ . The second heat exchanger 2 is used to further cool down the now warm incoming hot gas stream containing the CO ₂ using some readily available coolant such as ambient air or cold water.

The second heat exchanger 2 is used to remove the bulk of the water from the incoming hot gas stream prior to the refrigeration step. This heat exchanger utilises a coolant such as water or ambient air 25. Both these heat exchangers may have pulsating flows, whereby the size of the heat exchangers required are considerably reduced and their thermal efficiency is boosted.

While the arrangement of heat exchangers shown is not the only arrangement that can be employed it is preferred in this embodiment of the invention that a third heat exchanger 3 be used to further cool the incoming gas stream with the coldest stream of nitrogen and remnant oxygen from the refrigeration system.

The refrigeration process employs a thermo-acoustic refrigerator system 10. The energy to drive each thermo-acoustic refrigerator 30 is provided by an external pulse combustion system 15. The use of pulse combustion enables the thermal efficiency to

be markedly increased over current systems used to add heat to a Thermo-Acoustic Driver, (TAD) 41 , without incurring the penalty of increased emissions of environmentally damaging gases such as the various oxides of nitrogen.

The exhaust gases from the pulse combustion system are heat exchanged with the incoming combustion air which enables the temperature at the hot end of the TAD to be maintained at the highest possible temperature, commensurate with the materials of construction.

In the invention one or morethermo-acoustic refrigerators 30 can be used when linked together as shown in Figure 2. This coupling method is applicable to both the thermo- acoustic driver (TAD) and thermo-acoustic Stirling Hybrid engine (TASHE) of orifice pulse tube refrigerators.

This practice is desirable as a TASHER can have the drawback of being very high. However it is possible to bend the TASHER into a "U" shape and further benefits can arise from two TASHER units being combined in a "U" shape, preferably with the join being at the cold end and the heat engine at the top end. The TASHER can then be tuned to reduce noise and to mutually assist another TASHER with which it is joined. The tuning can be achieved by conventional loud speakers placed along the TASHER.

The basis of this method is to form a U tube 35 with two TAD or TASHER units with the join 36 being at the coldest end of the refrigerator part where the orifice sits. There being a common orifice 38 between two of the TAD or TASHER units.

By this means each TAD or TASHER unit drives the other unit. Both units will automatically go into 180° out of phase resonance when started. Should this not occur the phasing can be achieved by placing suitably tuned closed ended tubes to each TAD or TASHER unit as shown in Figure 2. Within each side tube resides a conventional loud speaker 40 which is driven at the resonant frequency of the main TAD or TASHER unit shells but with the voltage applied at 180° out of phase to each of the loud speakers.

The resulting U tube thermo-acoustic driver UTAD or UTASHER units require less energy to drive themselves than they would in total on their own. It should be noted however that the position of the side arm closed ended tubes with the loud speakers , is not critical and may be placed at any suitable location.

The refrigeration process removes the various gases such as CO ₂ (26), SOx (27) and NOx (28) from the incoming hot gas stream in a cascade process except for the nitrogen and remnant oxygen from the main combustion process or, in the case of methane sources such as gas wells, coal mine ventilation exit shafts or bio-processes that produce methane, the methane itself which is valuable.

The remnant cold stream of nitrogen and oxygen gases is now used to cool the incoming hot gas stream in the first heat exchanger, while itself being heated up to be put 20 into the stack.

The methane recovery process is dictated by whether the methane is required as a gas or is itself to be liquefied. If just methane gas is required, the now cool methane is used in the first heat exchanger to cool down the incoming raw methane steam containing water vapour, CO ₂ and other minor quantities of different gases which are to be separated from the methane.

Carbon Dioxide Storage

The CO ₂ (26) is now in a liquid state at high ^' pressure or in a solid state.

The long term removal of CO ₂ can be achieved in a variety of ways and is based on the fact that CO ₂ remains in a liquid state provided the repository temperature is below 30 ⁰ C and the pressure is above 715QkPa. The repository temperature has to be below -45 ⁰ C and the pressure has to be above 715OkPa, if the CO ₂ is to be deposited in the solid state for it to remain solid, The lower the available pressure in the repository, the lower the temperature has to be to keep the CO ₂ in the desired state.

The disposal methods (shown in Figure 3) all involve depositing the CO ₂ using pumping means 50 below the ocean surface 70 into a deep water based environment such as the ocean or an aquifer.

The first method 51 involves piping liquid CO ₂ at pressure to a point in the ocean where the depth is sufficient to keep the CO ₂ in its liquid form and the density differences between the CO ₂ and the sea water cause the CO ₂ to sink to the bottom of the ocean floor which can be well away from the point of discharge.

The second method 52 is an extension of the first method, whereby the CO ₂ is kept in a pipe 55 until it reaches the maximum depth of the ocean floor. The pipe work from the point of discharge in the first method can be made of a flexible high density film allowing the CO ₂ to take the pipe down to the maximum depth of the ocean floor.

The third method 53 involves encapsulating the liquid CO ₂ in a suitable material such as high density plastic to form a "sausage" 56 like structure or a package, to which heavy solid material may be added to increase the density well above that of ocean or saline aquifer so that any currents present do not carry the "sausage" or package away from the intended drop zone.

The "sausages" or packages can be pumped along a pipe in a similar fashion to "pigs" for oil and chemical pipelines. The "sausage" like structure or package is forced along the pipe to the point of discharge as in method 51 at which point density differences take over and the "sausage" or package travels down to the ocean floor. The liquid CO ₂ can be used as a lubricant in the pipe for the sausage like structure.

Methods 52 and 53 can be combined in which the flexible plastic pipe becomes a very foπg "pig" or'"sausage" up to several kilometers long. Once filled the pipe is sealed off and dropped to the ocean floor and a new flexible plastic pipe is placed on the pipe to recommence the filling.

These methods of encapsulating the CO ₂ and keeping it contained stop any interaction with the surrounding marine life and also make it easy to recover should it be needed at a future time.

The last method 54 involves using a drag plough 57 on a chain which contains the opening of a pipe 58 which is connected to the ship 60 at the surface which is pumping the carbon dioxide down. The drag plough is pulled through silts on the sea floor such that CO ₂ is deposited underneath 59 where it can remain undisturbed.

A mixture of solid and liquid CO ₂ slush can be used in the above disposal methods.

This invention described here provides an improved method of removing gases selectively from a gas stream and while we have described here one specific embodiment of the invention it is to be understood that variations and modifications in this can be made without departing from the spirit and scope of the invention.