WO2012092982A1

WO2012092982A1 - Method and apparatus for co2 capture

Info

Publication number: WO2012092982A1
Application number: PCT/EP2011/050177
Authority: WO
Inventors: Torbjørn FIVELAND; Henrik Solgaard Andersen; Jens Bragdø SMITH
Original assignee: Statoil Petroleum As
Priority date: 2011-01-07
Filing date: 2011-01-07
Publication date: 2012-07-12

Abstract

Disclosed is a method for capturing C0₂ from a gas stream by introducing droplets of an absorption liquid into the gas stream mainly in the flow direction of the gas. C0₂ is captured from the gas stream during a capture phase by means of the absorption liquid droplets, where the absorption liquid droplets are airborne during the capture phase, absorption liquid droplets are introduced into the gas stream with a velocity high enough to ensure internal circulation inside the absorption liquid droplets, and the absorption liquid droplets are introduced into the gas stream with a Sauter mean diameter in the range of 50μm - 500μm. Heat is extracted from the gas stream before droplets of absorption liquid are introduced into the gas stream An apparatus suitable for conducting said method is also disclosed.

Description

Method and apparatus for C0₂ capture

The present invention relates to an apparatus for capturing C0₂ from an exhaust gas stream and a method therefore.

In the combustion of a fuel, such as coal, oil, gas, peat, waste, etc., in a combustion plant, such as those associated with boiler systems for providing steam to a power plant, a hot process gas (or flue gas) is generated. Such a flue gas will often contain, among other things, carbon dioxide (C0₂). The negative environmental effects of releasing carbon dioxide to the atmosphere have been widely recognised, and have resulted in the development of processes adapted for removing carbon dioxide from the hot process gas generated in the combustion of the above mentioned fuels.

The conventional method for removing C0₂ from exhaust gas would be by use of a standard absorption-desorption process illustrated in figure 1. In this process the exhaust gas has its pressure boosted by a blower either before or after an indirect or direct contact cooler. Then the exhaust gas is fed to an absorption tower where it is counter-currently brought into contact with an absorbent flowing downwards. In the top of the column a wash section is fitted to remove, essentially with water, remnants of absorbent following the exhaust gas from the C0₂ removal section. The absorbent rich in C0₂ from the absorber bottom is pumped to the top of the desorption column via a heat recovery heat exchanger rendering the rich absorbent pre-heated before entering the desorption tower. In the desorption tower the C0₂ is stripped by steam moving up the tower. Water and absorbent following C0₂ over the top is recovered in the condenser over the desorber top. Vapour is formed in the reboiler from where the absorbent lean in C0₂ is pumped via the heat recovery heat exchanger and a cooler to the top of the absorption column.

The known processes for removing C0₂ from exhaust gas involve equipment that causes a significant pressure drop in the exhaust gas. If such a pressure drop is allowed, it would cause a pressure build-up in the outlet of the power generating plant or other plant generating the exhaust gas. This is undesirable. In the case of a gas turbine it would lead to reduced efficiency in the power generating process. To counter this drawback a costly exhaust gas blower is needed. A further problem with existing technology is that the absorption tower and the preceding exhaust gas cooler are costly items.

The standard C0₂ capture plant also needs a significant area to build upon.

WO00/74816 discloses a system for C0₂ capture. The system may be arranged as a horizontal channel where the exhaust gas is brought in contact with two different absorption liquids in two adjacent sections. A screen is included to avoid liquid to be transported from one section into the next section. The liquids are being regenerated and recalculated. In the article "Critical flow atomizer in S0₂ spray scrubbing" by Bandyopadhyay et al (Chemical Engineering Journal 139, pp. 29-41, 2008), it is concluded S0₂ removal efficiency is increased with the increase in liquid flow rate, liquid-to-gas flow rate ratio, atomizing air pressure, droplet velocity. The same conclusion is reached by Srinivasan et al in the article "Mass transfer to droplets formed by the controlled breakup of a cylindrical jet - physical absorption" (Chemical Engineering Science, Vol. 43, No. 12, pp. 3141-3150, 1988)

The present invention relates to C0₂ capture from exhaust gas, and it is a so called post combustion technology. The present invention may be utilized in connection with gases coming from different kind of facilities. These facilities could be combined cycle gas fired power plants; coal fired power plants, boilers, cement factories, refineries, heating furnaces of endothermic processes such as steam reforming of natural gas or similar sources of flue gas containing C0₂.

A long exhaust channel will be needed in almost all cases of C0₂ capture from exhaust gas for transporting the gas from the plant generating the gas to the plant for capturing C0₂. Putting it good use does not involve extra cost for the exhaust channel as such.

The channel is expected to be essentially horizontal, but it could have an angle between 0° and 60°. The direction of the slope can go either way, and the direction of the slope may change along the path of the channel. The channel may also change direction one or several times, from 1 to 360 degrees.

It is an aim of the present invention is to provide a method and apparatus for removing C0₂ from an exhaust gas stream, where the method and apparatus provides a reduced pressure loss, does not depend on the use of exhaust gas blowers and preferably requires less energy than the traditional method. Furthermore, it is an aim to provide a solution which has a considerably smaller footprint and weight, thereby also enabling use of a C0₂ capture plant of the kind described herein on a weight and space sensitive location, such as on a platform, a vessel or an onshore location. Examples of onshore locations that are space sensitive may be existing power plants where a C0₂ capture system is to retrofitted, and where space is limited and/or additional real estate extremely expensive. It is also a goal to provide a solution which can be integrated with a new efficient desorption method and apparatus.

Another example of a weight and space sensitive location in need of a C0₂ capture plant may be an offshore platform or vessel. In order to provide energy and heat to the various processes onboard an offshore platform or vessel, it is customary to arrange a power plant onboard the offshore platform or vessel. The power plant burns a small fraction of the produced

hydrocarbons in order to drive a power generator. The power may be used to energize various kinds of pumps, compressors and other users onboard the offshore platform or vessel, e.g.

booster pumps, injection pumps, various kinds of power driven tools and cranes, light and heating of living quarters etc. A drawback with such hydrocarbon burning power plants, is that it produces unwanted large amounts of C0₂. In order to reduce the emissions of C0₂, it is an option to capture C0₂ from the exhaust gas produced by the hydrocarbon burning power plant, or possibly electrify the power supply on the offshore platform or vessel. Conventional systems for capturing C0₂ are considered to be too heavy and space demanding for use on an offshore platform or vessel. There exist a need for a more compact and light weight C0₂ capturing system that may be fitted, or even retro-fitted, onboard an offshore platform or vessel.

It is also an aim for the present invention to provide heat for use either in the C0₂ capture plant process itself, or for other uses or users related to or in the vicinity of the C0₂ capture plant.

A further aim is to provide a system and a method that can be effectively combined to a plant utilizing recycling of exhaust gas. It is also an aim to provide a system which allows for combination with pre-treatment systems for removing other unwanted compounds within the gas stream. The abovementioned aims are reached by means of a system and method according to the enclosed independent claims. Further advantageous features and embodiments are mentioned in the dependent claims. According to one aspect of the present invention, heat is extracted from the exhaust gas before the exhaust gas enters into a C0₂ capture section of the C0₂ capture plant.

According to another aspect of the present invention, the heat extracted from the exhaust gas before the exhaust gas enters into a C0₂ capture section of the C0₂ capture plant, is utilized in a subsequent absorption fluid stripping stage.

According to yet another aspect of the present invention, the removal of heat from the exhaust gas before the exhaust gas enters into a C0₂ capture section of the C0₂ capture plant enables the use of lighter materials and more heat sensitive materials in the subsequent C0₂ capture section of the C0₂ capture plant, thereby lowering the overall weight and size of the C0₂ capture plant.

The present invention reduces both capital cost and saves energy.

These and other objectives are reached by the method according to claim 1 and an apparatus according to claim 6. Other benefits and advantageous embodiments are set out in the dependent claims.

The present invention will be described in more detail with reference to the enclosed figures; wherein:

Figure 1 illustrates a conventional absorption-desorption process;

Figure 2 illustrates a flow sheet of an embodiment of the present invention;

Figure 3 illustrates an embodiment where the channel includes direct contact cooling and a washing section;

Figure 4 shows the operating and equilibrium lines for the C0₂ absorption process shown in figure 3;

Figure 5 illustrates an embodiment with an integrated pre-treatment section;

Figure 6 illustrates the embodiment with exhaust gas recycling; and

Figure 7 shows a cross-section showing the relative velocity of the internal circulation pattern developed in a liquid drop moving in gas. Figure 1 shows a conventional method for removing C0₂ from exhaust gas using a standard absorption-desorption process. In this process the exhaust gas P10 has its pressure boosted by a blower P21 either before (as illustrated) or after an indirect or direct contact cooler P20. Then the exhaust gas is fed to an absorption tower P22 where it is contacted counter-currently with an absorbent P40 flowing downwards. In the top of the column a wash section is fitted to remove, essentially with water, remnants of absorbent following the exhaust gas from the C0₂ removal section. Washing liquid P41 is entered at the top and redrawn further down as P42. The C0₂ depleted exhaust gas is removed over the top as P12. The absorbent rich in C0₂ P32 from the absorber bottom is pumped to the top of the desorption column P30 via a heat recovery heat exchanger P28 rendering the rich absorbent P36 pre-heated before entering the desorption tower P30. In the desorption tower the C0₂ is stripped by steam moving up the tower. Water and absorbent following C0₂ over the top is recovered in the condenser P33 over the desorber top. Vapour is formed in the reboiler P31 from where the absorbent lean in C0₂ P38 is pumped via the heat recovery heat exchanger P28 and a cooler P29 to the top of the absorption column P22. Steam is supplied to the reboiler as stream P61. The isolated C0₂ leaves as stream P14.

Figure 2 illustrates the main fluid flows of a C0₂ capture section of a C0₂ capture plant where the present invention may find it's utility. Exhaust gas 10 enters the channel 1 at one end.

Absorption liquid comprising a C0₂ absorbent and a diluent is sprayed into the channel from a nozzle arrangement 15. The absorption liquid is sprayed mainly in the flow direction of the exhaust gas and with a speed high enough to at least compensate for the pressure loss in the first part of the channel. The droplets of absorption liquid moves through the exhaust gas stream and absorbs C0₂ there from. The C0₂ rich absorption liquid is collected upstream at collection point 23 at the lower part of the channel. The droplets are collected by the use of a demister/droplet catcher. The C0₂ rich absorption liquid 19 is pumped via pump 34 into conduit 32 connected to a desorption plant. The desorption plant may be a traditional desorption plant as illustrated in figure 1 or it can be any other system for desorbing C0₂ from an absorbent liquid. In the embodiment illustrated on figure 2 the exhaust gas continues downstream in the channel and a second absorption liquid is sprayed into the gas from a nozzle arrangement 17. The absorption liquid is sprayed mainly in the flow direction of the exhaust gas and with a speed high enough to at least compensate for the pressure loss in this second part of the channel. The droplets of absorption liquid move through the gas stream and absorbs C0₂ there from. The C0₂ rich absorption liquid is collected upstream at collection point 24 at the bottom of the channel. The C0₂ rich absorption liquid collected at point 24 is pumped via pump 16 up to the nozzle arrangement 15. The exhaust gas continues downstream in the channel and lean absorption liquid 40 is sprayed into the gas from a nozzle arrangement. The absorption liquid is sprayed mainly in the flow direction of the exhaust gas and with a speed high enough to at least compensate for the pressure los in this third part of the channel. The droplets of absorption liquid move through the exhaust gas stream and absorb C0₂ there from. The C0₂ rich absorption liquid is collected upstream at collection point 25 at the lower part of the channel. The CO₂ rich absorption liquid collected at point 25 is pumped via pump 18 up to the nozzle arrangement 17. The CO₂ depleted exhaust gas leaves the channel at the other end as stream 12.

The channel may be horizontal or have an angle of up to 60 degrees. The channel may further include one or more demisters or similar arrangement to collect the droplets of absorption liquid. The droplets will then be introduced at a speed large enough to push the gas stream forward through the demisters.

Figure 2 illustrates the basic configuration of cross-flow treatment in the exhaust gas channel. The nozzles in this figure are pointing downwards. This is, however, only for convenience of drawing. The intention is to point the nozzles mainly in the direction of the gas flow, but other configurations may also be feasible, e.g. an array or cluster of nozzles pointing in various directions. More examples could be given.

Figure 3 shows that exhaust gas enters the exhaust gas channel that would normally be void of process equipment for the 150-250 meters leading to the conventional CO₂ capture plant. At a convenient point shortly after entry the exhaust gas is here, in section C, sprayed with cooling water to form a direct contact cooler. The cooling water is recycled except a possible purge. The recycle is via pump and cooler to a point where this stream is mixed with compressed gas in the spray nozzles (atomizing nozzles). Droplets created in this section are collected in the downstream droplet catchers. In another embodiment, the pressure of the cooling water is increased to 5-100 bars, preferably in the range 5-10 bar, with a pump before it exits through spray nozzles. The absorbent liquid may also be introduced to the channel in the same way. The gas for nozzle spraying is compressed in a compressor common for all nozzle batteries that uses atomizing nozzles. In one embodiment, the suction gas is exhaust gas conveniently extracted from the channel downstream of the DCC section droplet catchers. The cooled exhaust gas now enters C0₂ absorption section Al where is contacted concurrently and cross-currently with the C0₂ richest absorbent solution passing through the absorption process. The liquid is again sprayed into the channel via nozzles. The liquid droplets are captured in the downstream droplet catchers. The rich absorbent liquid collected is pumped from the Al section to the desorption process not further described here. The liquid absorbent sprayed into section Al is pumped from section A2 where there is less C0₂ in the exhaust gas and the outlet liquid is thus less rich in C0₂ than that coming out of the Al section. The operating and equilibrium lines for the C0₂ removal process are shown in figure 4. Also the A2 section has gas liquid contact following the same pattern as in section Al. The liquid to section A2 comes from section A3 where the C0₂ levels are the lowest in both the exhaust gas and the liquid. The absorbent liquid sprayed into section A3 is the lean absorbent coming back from the desorption process in a regenerated condition. The droplet catchers downstream of section A3 would favourably be designed to do a more rigid droplet capture than the other sections since any slippage of absorbent will put a higher demand on the absorbent recovery section W. The function of section W is to wash essentially all absorbent carried with the gas from section A3 out. This is achieved by circulating essentially water over the section via a pump and a cooler. A bleed to recycle caught absorbent and a make-up water stream would be applied as convenient to the recycle stream. The potential for removing absorbent from the exhaust gas is determined by the concentration of free absorbent in the wash liquid, and its temperature. There may a need for more than one such wash section, and that may be easily added.

It has been found that the droplet sprays are pushing the gas along the channel to the extent that no exhaust gas blower is needed. The number of stages needed for C0₂ absorption is a trade-off against absorbent flow. In principle one stage would be enough if sufficient liquid was circulated, but this would imply a lot of liquid. Two stages or more are conceivable. In the standard counter-current absorption column it may be shown that 2 to 3 equilibrium stages would suffice. According to one embodiment, the C0₂ capture plant may be combined with a pre-treatment section and a recycling of exhaust gas. These features are described in more detail in figure 5 and 6. In figure 5, one embodiment ofthe C0₂ capture plant is shown extended with exhaust gas pre- treatment. This is relevant for coal fired power stations and a variety of industrial settings where C0₂ recovery is needed. The pre-treatment could have one or more duties. It could e.g. be a sea water wash where the buffering propertied of sea water is exploited to absorb S0₂ from the exhaust gas. If this was not done, S0₂ would react irreversibly with the alkaline absorbent used to catch C0₂ thus leading to a greater consumption. Such a process could also scrub the exhaust gas for particles. Both these functions would typically be required downstream of coal burning. From an aluminium melter the exhaust gas might contain HF, and more examples could be given. The fluid regeneration in the pre-treatment section could e.g. be a filter to contain particles. In the case of S0₂ absorption into sea water the best course of action is to have a bleed where S0₂ is piped with sea water as sulphite that would in turn be oxidised to sulphate in the sea water, a substance that is already in sea water in abundance.

The pre-treatment section could use the same technologies for nozzles and droplet catchers as the other sections.

In figure 6, one embodiment of the C0₂ capture plant is shown integrated with a pre-treatment section and combined with an exhaust gas recycle (EGR). The advantage of using an EGR is that the volumetric exhaust gas flow is significantly reduced thus enabling a reduction in the cross- sectional area in the gas flow sections and the higher C0₂ content in the exhaust gas which reduces the capital cost of treatment.

Figure 7 is a cross-section showing the relative velocity of the internal circulation pattern developed in a liquid drop moving in gas. The gas motion is in the horizontal direction and results in a doughnut shaped, toroid flow known as a Hill's vortex. The cause of the internal circulation is the shear force at the surface of the liquid drop, created by the gas moving along the surface. It is known that a liquid drop moving through a viscous fluid, e.g. gas stream comprising C0₂, will tend to circulate internally due to the shear stress applied at its interface by the ambient fluid. Heat and mass transfer are greatly augmented by a reduction of the boundary layer thickness. Compared to a so-called rigid drop (i.e. a liquid drop with no, or very little, internal circulation), the transfer coefficients for a liquid drop with internal circulation is at least 2-4 times higher.

According to an advantageous embodiment, an absorption liquid, e.g. amine, is introduced or sprayed into a channel 1 by the use of atomizing nozzles 15, 17, 40. A flue gas 10 comprising a gas stream comprising C0₂ moves through the channel 1 with a velocity of 5-15 m/s. The diameter of the flue gas channel 1 may depend on the amount of flue gas produced by the power plant, cement factory or similar, but it will in most cases be between 3 and 10 meters. The flow conditions in the flue gas channel will thus be highly turbulent with a Reynolds number » 100 000.

The absorption liquid leaves the nozzle or nozzles 15, 17, 40 as small droplets with a velocity of 30-120 m/s. It is expected that the droplets will be turbulent for a short while after they leave the nozzle, 1-2 seconds. The relative velocity difference between the absorption liquid doplets and the flue gas causes high shear stress on the droplets which will help sustain an internal circulation inside the droplets and possibly sustain turbulent conditions inside the droplets. The mass transfer in the region adjacent to the nozzles will thus be extremely high.

A major drawback of packed bed absorber is the ability to mass transfer of C0₂(g) to C0₂(aq). The mass transfer rate depends on the gas film thickness and a corresponding diffusion. These again depend on flow rates. In packed bed absorbers, laminar flow will occur, which results in significantly lower mass transfer of C0₂(g) to C0₂(aq) compared to turbulent flow conditions. The high turbulence in the channel 1 and the turbulence/internal circulation in the droplets results in significantly reduced resistance to mass transfer. As opposed to conventional methods for absorbing C0₂ from a flue gas 10, the transport of C0₂ from the flue gas 10 into the absorption liquid droplets will be much higher due to reduced film thickness and the transport of C0₂(aq) is not dependent on diffusion, but by convection. The reaction with absorbent will thus be a lot faster. Absorption liquid droplet size can be varied by changing pressure on the absorption liquid before the nozzle or nozzles, or by the absorption liquid flow rate through the nozzle or nozzles. The size and shape of the nozzle or nozzles will also have an effect on the absorption liquid droplet size. The relative difference in velocity between the mean gas stream and the mean absorption liquid droplet velocity will also affect the droplet size. If the velocity ratio between the mean gas stream velocity and the mean absorption liquid droplet velocity is greater than approximately 3 when the absorption liquid leaves the absorption liquid introduction means, preferably in the range of 6-10, this will help ensure internal circulation in the absorption liquid droplets introduced in the C0₂ gas stream, and that the Sauter mean diameter of the absorption liquid droplets is kept relatively small, preferably on the order of 50μιη - 500μιη.

The residence or flight time of the absorption liquid droplets through the channel 1 is also important. As the absorption liquid droplets moves through the flue gas channel, the initial collision between the droplets and the flue gas will contribute towards further atomization of the droplets. Simultaneously, the shear forces/stress on the droplets will help sustain an internal circulation inside the droplets. In this initial phase of the absorption liquid droplet flight, the mass transfer of C0₂ from the flue gas and into the absorption liquid droplets reach a peak. As the absorption liquid droplets move along the channel 1, their velocity decreases due to multiple collisions and drag forces (the kinetic energy is transferred from droplet to the flue gas).

Furthermore, the absorption liquid droplets may also increase in size due to coalescence, further decreasing their velocity and a reduction of the active liquid surface area. The absorption liquid droplets also start to saturate due to reaction with C0₂(aq). In effect, the mass transfer of C0₂ from the flue gas and into the absorption liquid droplets starts to decrease. This period between the introduction of the absorption liquid droplets into the channel 1 and a very diminished mass transfer of C0₂ from the flue gas, defines the desired residence or flight time of the absorption liquid droplets in the gas stream, and thereby also helps determine a preferable length of the channel 1 before the absorption liquid is collected, e.g. by droplet catchers. In light of this, it can be understood that any obstacles in the channel, e.g. packing material of a packed bed absorber etc., will only shorten the residence or flight time, and thus be of detriment for the mass transfer of C0₂ from the flue gas and into the absorption liquid droplets. Also, any obstacles in the channel, e.g. packing material etc., may increase pressure loss along the channel, which preferably should be avoided.

The absorption of C0₂ takes place while the absorption liquid droplets are airborne, i.e.

suspended in the gas stream containing C0₂. This is also referred to as the capture phase. The capture phase takes place in the capture zone. The capture zone can be defined as the area or volume between the absorption liquid introduction means and a collection point of the absorption liquid downstream of the absorption liquid introduction means. It is preferred that no obstacles, e.g. packing materials or other surfaces, which may result in that absorption liquid collects in or on the obstacles, are present in this capture zone or during the capture phase. The main benefit of the present invention is obtained by providing a transfer of C0₂ from the gas stream and into the absorption liquid while the absorption liquid is airborne or suspended in the gas stream.

However, it is conceivable that a further C0₂ capturing stage comprising a packed bed absorber or some other capture means is provided after the capture zone according to the present invention. For example, collection means 23 for collecting C0₂ saturated absorption liquid droplets downstream of the absorption liquid introduction means 15, 17, 40 may in part comprise a packed bed absorber or some other capture means. The temperature of the absorption liquid introduced into the gas stream is in the range of 20° to 80° C, preferably in the range of 20° to 50° C. However, this depends on the kind of absorption liquid used, and it is conceivable that other absorption liquids with other temperature ranges may be utilized. It is understood that the benefits of the presently described C0₂ capture plant can be obtained even when varying the various parameters of the process. Parameters that have an effect on the mass transfer of C0₂ from the flue gas and into the absorption liquid droplets are:

-channel diameter

-channel shape

-channel length

-residence or flight time of absorption liquid droplets

-channel surface

-number of nozzles

-placement of nozzles

-shape and design of nozzles

-pressure of absorption liquid droplets before exiting nozzles

-flow rate of absorption liquid droplets through nozzles

-velocity of flue gas

-velocity of absorption liquid droplets

-velocity ratio between the flue gas and the absorption liquid droplets

-temperature of absorption liquid droplets

-temperature of flue gas

-concentration of C0₂ in flue gas

-flow rate of flue gas -concentration of absorption liquid

-viscosity of absorption liquid

etc. According to the present invention, heat extraction means are provided in the exhaust channel before the C0₂ capture section, whereby the temperature of the exhaust gas is drawn down from e.g. 600°C to 200°C. The heat extractions means may comprise of one or more heat exchangers comprising a heat transfer medium or refrigerant such as water, ammonia, carbon dioxide and non-halogenated hydrocarbons, or any other viable heat transfer medium.

The heat extraction means may comprise of any viable heat exchanger, such as shell and tube heat exchangers, plate heat exchangers, plate fin heat exchangers etc. The heat transfer medium may be arranged with a concurrent flow, counter current flow, or spiral flow/cross flow configuration. A spiral flow/cross flow configuration may be preferred because of its low pressure loss characteristics.

According to one embodiment of the present invention, the heat removed by the heat extraction means is conducted to, and utilized in, the stripper unit of the desorption plant. As mentioned above, the desorbtion plant may be a traditional desorption plant as illustrated in figure 1 or it can be any other system for desorbing C0₂ from an absorbent fluid. The stipping unit of a desorption plant requires a large amount of heat since the desorption of C0₂ from the absorbent fluid is an endothermic reaction, and according to the present invention, this heat may be, at least in part, extracted from the exhaust gas from a power plant before the exhaust gas enters into the C0₂ capture section of the exhaust channel. Although, according to one embodiment of the present invention, heat extracted from the exhaust gas from a power plant before the exhaust gas enters into the C0₂ capture section of the exhaust channel may be used in a stripping unit, the amount of heat extracted may not be sufficient to carry out the full stripping process. Additional heat may have to be provided from some other heat source, e.g condensing of steam or some other medium able to carry and transfer enough heat.

According to the present invention, one of the main advantages of extracting heat from the exhaust gas before the exhaust gas enters into the C0₂ capture section of the exhaust channel, is that the temperature of the exhaust channel is reduced considerably, thereby permitting the use of exhaust gas channel materials that are significantly lighter than otherwise would have been possible. According to one embodiment of the present invention, the exhaust gas channel itself and parts associated with and in contact with the exhaust gas channel, may be made of lightweight materials such as plastic materials, i.e. polymers/elastomers. Other lightweight materials may comprise composite materials, i.e. fibreglass composite, carbon fibre composite, glass-ceramic matrix composite etc.

When choosing polymers/elastomers suitable for high temperature applications, there exists a number of viable options. The polymer/elastomer material has to be chosen in considereation of the maximum service temperature versus strength. Viable polymer/elastomer materials may be PTFE (260°C), PEEK (260°C), PFA (260°C), FEP (200°C), PEI (180°C), and/or PET/PBT (170°C). The maximum service temperatures of these materials are indicated in brackets.

Reinforced polymer/elastomer materials, e.g. carbon fibres, ceramic fibres etc. may also be considered, especially if the material requires certain structural load bearing characteristics.

By using lightweight materials such as polymers/elastomers, the weight of the exhaust gas channel itself and parts associated with and in contact with the exhaust gas channel, may be drastically reduced. The reduction in weight also has a significant and positive effect on the need for support and load bearing structures, thereby reducing weight and space requirements even further.

By bringing down the temperature of the exhaust gas before the exhaust gas enters into the C0₂ capture section of the exhaust channel according to the present invention, using the extracted heat in the stripper unit, and reducing the overall weight and size of the exhaust channel and C0₂ capture section, the present invention provides a method and apparatus that may be used in a weight and space sensitive location, such as on a platform, a vessel or an onshore location, and it may also be suitable for retrofitting a C0₂ capture system in such a location.

Although it has been mentioned that a reduced pressure loss through the exhaust channels is preferred in order to avoid the need for exhaust gas blowers, and it is understood that the extraction of heat from the exhaust gas by means of heat extraction means before the exhaust gas enters into the C0₂ capture section of the exhaust channel, will cause a pressure drop of the exhaust gas in the exhaust channel, this pressure drop is not of great concern since it takes place before the C0₂ capture section. Avoiding a pressure drop is of greatest importance in the C0₂ capture section itself, and the benefits of the C0₂ capture section described herein are not adversely effected by the present invention.

Claims

C l a i m s

1. Method for capturing C0₂ from a gas stream comprising the steps of

- introducing droplets of an absorption liquid into the gas stream mainly in the flow direction of the gas;

-capturing C0₂ from the gas stream during a capture phase by means of the absorption liquid droplets, where the absorption liquid droplets are airborne during the capture phase;

c h a r a c t e r i z e d i n t h a t the method further comprises the step of:

-extracting heat from the gas stream before droplets of absorption liquid are introduced into the gas stream.

2. Method according to claim 1, wherein the heat extracted from the gas stream before droplets of absorption liquid are introduced into the gas stream is utilized in a subsequent absorption liquid stripping stage.

3. Method according to claim 1 or 2, wherein the steps of capturing C0₂ from a gas stream are conducted in a structure comprising lightweight materials such as polymer/elastomer materials and/or composite materials, said structure comprising a gas stream channel and means for supporting said gas stream channel.

4. Apparatus for capturing C0₂ from a gas stream comprising:

-absorption liquid introduction means (15, 17, 40) for introducing droplets of an absorption liquid mainly in the flow direction of the C0₂ gas stream (10), and

-a capture zone wherein the absorption liquid droplets capture C0₂ from the gas stream (10), where the absorption liquid droplets are airborne throughout the capture zone, said capture zone being defined by a gas stream channel,

c h a r a c t e r i z e d i n that the apparatus, inside the gas stream channel and before the capture zone, comprises means for extracting heat from the gas stream, where the heat extracted by said heat extracting means is arranged to be used in a C0₂ desorbing stripping unit for desorption of C0₂ from the absorption liquid used to absorb C0₂ from said gas stream (10), and wherein said gas stream channel and means for supporting said gas stream channel comprises lightweight materials such as polymer/elastomer materials and/or composite materials.

5. Apparatus according to claim 4, wherein said polymer/elastomer material comprises PTFE, PEEK, PFA, FEP, PEI, and/or PET/PBT.

6. Apparatus according to claim 4, wherein said composite material comprises fibreglass composite, carbon fibre composite, and/or glass-ceramic matrix composite.