WO2016063136A1

WO2016063136A1 - System and method for efficient cryogenic carbon dioxide capture from flue gas

Info

Publication number: WO2016063136A1
Application number: PCT/IB2015/002355
Authority: WO
Inventors: Hisayuki HANDA
Original assignee: Handa Hisayuki
Priority date: 2014-10-21
Filing date: 2015-10-20
Publication date: 2016-04-28

Abstract

A system and method are provided to cryogenically remove carbon dioxide contained in a flue gas stream produced by a fossil-fuel-fired electric power generation plant. At least one heat exchanger cools the flue gas stream down to a temperature close to, yet above, a saturation temperature, which depends on the partial pressure of the carbon dioxide contained in the flue gas. A deposition-sublimation chamber connected to the heat exchanger contacts the cooled flue gas stream with at least one cold surface for deposition of the carbon dioxide as dry ice and utilizes the latent heat released by deposition to sublimate dry ice previously formed on at least one other surface to pure carbon dioxide gas utilizing the latent heat. A carbon dioxide reservoir connected to the deposition-sublimation chamber stores the pure carbon dioxide gas. A first scavenging subsystem scavenges flue gas remaining in the deposition-sublimation chamber. A second scavenging subsystem scavenges carbon dioxide gas remaining in the deposition-sublimation chamber. The cost of installation and operation of the system and method substantially reduces costs of carbon dioxide removal from flue gas. The system and method reduce emissions of carbon dioxide and in turn decrease the amount of greenhouse gases contributing to global warming to mitigate damage to the environment.

Description

Title

SYSTEM AND METHOD FOR EFFICIENT CRYOGENIC CARBON DIOXIDE CAPTURE FROM FLUE GAS

Field

The present invention relates generally to extraction of carbon compounds, for example, the removal of carbon dioxide from a gaseous feed stream, and, more particularly, to cryogenic capture of carbon dioxide from, for example, flue gas generated by an electric power generation plant to reduce emissions of carbon dioxide into the atmosphere and in turn decrease the amount of greenhouse gases contributing to global warming to mitigate damage to the environment.

Background

Globally, the generation of electric power relies primarily on the combustion of fossil fuels, mainly coal, which provides the heat source for steam-driven turbine electric power generators. The emissions from electric power generation plants pollute the environment. The emissions are also said to be a contributing cause of global warming.

Various techniques are known to reduce carbon dioxide emissions from flue gas emitted by electric power generation plants. One known technique is the Mono Ethanol Amine (MEA) system already in commercial use. However, the cost of an MEA-based system is reported to cost US$100M to install and consumes approximately 40% of the electricity produced by the electric power generation plant (4.0 GJ/ton-CO₂) during operation. Another known technique is the membrane system currently known to be the most energy-efficient process (0.7 GJ/ton-CO₂). Consequently, there is a need for a carbon dioxide removal system and method that are considerably less expensive to install and impose a smaller penalty on the electricity that is generated by the electric power generation plant to operate. Summary

The various examples in accordance with the present invention remove carbon dioxide contained in the flue gas of fossil-fuel-fired electric power generation plants. In accordance with the present invention, the various examples of a system and method provide efficient cryogenic capture of carbon dioxide present in the flue gas produced by a fossil fuel burning electric power generation plant. The principle underlying the various examples of the system and method in accordance with the present invention is that when carbon dioxide is deposited as dry ice on a solid surface, latent heat is released. In accordance with the various examples of the system and method of the present invention, that latent heat is conducted to the other side of the surface on which dry ice has previously been deposited, . where the latent heat may be utilized for sublimation of previously formed dry ice on that other side of the surface. The various examples of the system and method in accordance with the present invention transfer the latent heat of deposition and utilize the latent heat for sublimation. Utilizing the latent heat requires less input energy and results in an energy efficient carbon dioxide capture system and method.

By way of one non-limiting example, a system and method in accordance with the present invention are provided to functionally remove carbon dioxide from flue gas. First, an incoming flue gas stream is cooled down to a suitable temperature close to, yet above, a saturation temperature, which depends on the partial pressure of the carbon dioxide contained in the flue gas. Second, the cooled flue gas stream is then brought in contact with cold surfaces for dry ice deposition. Third, the dry ice is sublimated to pure carbon dioxide gas utilizing the latent heat of deposition, and the carbon dioxide gas is captured.

By way of a non-limiting example in accordance with the present invention, a three- stage cryogenic system is provided for removal of carbon dioxide from a flue gas stream. The system comprises a preparatory stage consisting of a plurality of heat exchangers. In accordance with one preferred example, there are three heat exchangers. The plurality of heat exchangers is employed to lower the temperature of the flue gas stream to approximately a deposition temperature of the carbon dioxide present in the flue gas, wherein the deposition temperature depends on the partial pressure of the carbon dioxide. In accordance with a preferred example, two flow streams are fed to the heat exchangers to provide cooling to lower the temperature of the flue gas stream. One flow stream is the flue gas stream after depositing carbon dioxide in a deposition-sublimation chamber, which is utilized for cooling in a first heat exchanger and in turn in a third heat exchanger. Another flow stream is the captured carbon dioxide gas stream exiting the deposition-sublimation chamber, which is utilized for cooling in a second heat exchanger.

The system further comprises a separation stage consisting of the deposition- sublimation chamber. The deposition-sublimation chamber removes carbon dioxide from the flue gas stream. To implement the deposition-sublimation chamber, a deposition-sublimation tower is preferably provided. The deposition-sublimation tower comprises a heat exchanger with two sets of flow passages or channels. Although the system may be implemented with one such deposition-sublimation tower, a plurality of such towers is preferable for continuous operation to process a continuous stream of flue gas. In accordance with one non-limiting example, carbon dioxide deposits on pre-cooled walls of a first set of channels comprising the deposition-sublimation tower. This phenomenon releases both the sensible heat and the latent heat of deposition of the carbon dioxide, which is transferred to the other side of the walls to a second set of channels and is utilized for sublimation of dry ice previously deposited on the walls of the second set of channels.

The system further comprises a plurality of scavenging subsystems. When the flue gas stream to the deposition-sublimation tower of the preferred implementation of the deposition-sublimation chamber is stopped temporarily after a first cycle of operation, there remains flue gas with the deposited dry ice on the walls of the first set of channels and carbon dioxide gas in the second set of channels.

A first scavenging subsystem is provided to scavenge flue gas remaining in the deposition-sublimation chamber. In the preferred implementation, at the beginning of a cycle of operation, a carbon dioxide gas stream from a scavenging carbon dioxide reservoir with appropriate pressure and temperature conditions scavenges the remaining flue gas in a first set of channels of the deposition-sublimation tower, which is routed to and stored in an N2+O₂+CO₂ reservoir and later utilized to cool a carbon dioxide gas stream fed to the scavenging carbon dioxide reservoir. Some of the scavenging carbon dioxide gas is expected to deposit and release latent heat, but that latent heat is utilized to sublimate newly deposited dry ice in a second set of channels. Thus, a heat balance is preserved.

Additionally, a second scavenging subsystem is provided to scavenge carbon dioxide gas remaining in the deposition-sublimation chamber. In the preferred implementation, at the beginning of the next cycle of operation, the incoming flue gas stream scavenges carbon dioxide gas remaining in the second set of channels of the deposition-sublimation tower, which is routed to and stored in a scavenging carbon dioxide reservoir. The pressure of the carbon dioxide gas is raised by a compressor to a level needed for the restoration of the original scavenging carbon dioxide reservoir pressure and then the temperature of the carbon dioxide gas is restored to its initial level by a heat exchanger utilizing the scavenged flue gas stream from the N2+O₂+CO₂ reservoir.

By way of an additional non-limiting example, the deposition -sublimation tower to preferably implement the deposition-sublimation chamber comprises a core section. The core section of the deposition-sublimation tower comprises two sets of channels located such that a plurality of walls separates the two sets of channels. The deposition-sublimation tower also comprises an inlet section configured such that one set of channels can receive flow independent of the other set of channels. The deposition-sublimation tower further comprises an outlet section having a configuration similar to the inlet section such that the outgoing flows can be separately routed.

By way of a further non-limiting example, the walls between the two sets of channels of the deposition-sublimation tower function to transfer heat across the walls. The walls are structured such that latent heat released by deposition of carbon dioxide on one side of the walls does not warm the wall itself but is instead utilized to sublimate dry ice formed on the other side of the walls. Accordingly, the walls are preferably constructed of a material with a low heat conductivity suitable for heat insulation and having inserted heat conductors extending through the walls to transfer heat from one side of the walls to the other side of the walls.

In accordance with the preferred examples of the present invention, the processed flue gas exiting the deposition- sublimation chamber consists mostly of nitrogen and oxygen and is estimated to have 0 ~ 1.5% residual carbon dioxide depending on operating conditions and is utilized to cool down the incoming flue gas stream. Also, the separated carbon dioxide is utilized to cool down the incoming flue gas stream. Consequently, the system and method in accordance with present invention require a minimum energy input except for the initial conditioning of the deposition-sublimation chamber and for operation of compressors comprising the system.

The energy requirement for the system in accordance with an example of the present invention is estimated at 0.357 ~ 0.386 GJ/ton-CO₂ avoided, which is less than 10% of the energy requirement of the Mono Ethanol Amine (MEA) systems already in commercial use and about one half of the energy requirement of the membrane systems currently known to be the most energy-efficient process (0.7 GJ/ton-CO₂). The energy cost of carbon dioxide capture, exclusive of transportation and sequestration costs, based on the average electricity price for industrial usage in the U.S.A. of 6.6 cents per kWH in 2013, is estimated at $6.55 ~ $7.08 per ton-CO₂ avoided for one example of the system in accordance with the present invention.

Advantageously, the system in accordance with the various examples of the present invention is expected to be considerably less expensive to install and would impose a much smaller penalty on the electricity that is generated by the electric power generation plant to operate. The cost of installation and operation of the system and method in accordance with the various examples of the present invention may be utilized to substantially reduce costs of carbon dioxide removal from flue gas. Additionally, the system and method in accordance with the various examples of the present invention efficiently reduce emissions of carbon dioxide and in turn decrease the amount of greenhouse gases contributing to global warming to mitigate damage to the environment.

Brief Description of Drawings

The various examples in accordance with the present invention will be described in conjunction with the accompanying figures of the drawing to facilitate an understanding of various examples of the present invention. In the drawing:

Figure 1 is a block diagram of a system in accordance with an example of the present invention.

Figure 2 is schematic diagram of a deposition-sublimation tower in accordance with an example implementation of the deposition-sublimation chamber of the system shown in Figure 1.

Figure 3 is a schematic diagram of the inlets of the deposition-sublimation tower shown in Figure 2, the outlets being a mirror image of the inlets.

Figure 4 is a schematic diagram of a preferred implementation of the deposition- sublimation tower shown in Figure 2 illustrating two independent inlet passages leading to two sets of neighboring channels in the deposition-sublimation tower.

Figure 5 illustrates an insulating wall having metal conductors comprising the deposition-sublimation tower shown in Figure 2 for utilization of latent heat for sublimation and to promote carbon dioxide deposition by stirring of flue gas.

Figure 6 is a schematic diagram of a scavenging system shown in Figure 1 in accordance with an example of the present invention. Figure 7 is a schematic diagram of an example subsystem for maintaining constant pressure of carbon dioxide in a carbon dioxide reservoir in accordance with the present invention.

Figure 8 illustrates in Figure 8A pre-conditioning of the deposition-sublimation tower shown in Figure 2 in accordance with an example method of the present invention followed in Figure 8B by normal cyclic operation of the deposition-sublimation tower shown in Figure 2 in accordance with an example method of the present invention.

Figure 9 is a timing diagram for operation of the scavenging system shown in Figure 6 in accordance with an example method of the present invention.

Figure 10 is a diagram to illustrate the concept of CO₂ avoided-

Figure 11 shows power requirements for three carbon dioxide recovery cases and associated costs per ton-CO₂ recovered in accordance with an example of the present invention.

Description of Embodiments

A basic physical property of carbon dioxide (CO₂) is that CO₂ transforms from the gaseous state directly to the solid state (i.e., dry ice) for pressure below what is known as the "triple-point pressure." The transformation of CO₂ to dry ice is known as "deposition." Conversely, the transformation of dry ice to the gaseous state in the form of pure CO₂ gas is known as "sublimation."

The present invention leverages the physical property of CO₂ of direct transformation from gas to solid phase and vice versa (deposition and sublimation) for a particular combination of pressure and temperature. Accordingly, in accordance with one non-limiting example of the present invention, a flue gas stream is cooled down to a temperature close to a deposition temperature for a given partial pressure of CO₂ contained in the flue gas stream prior to entering a deposition-sublimation chamber (hereafter referred to as the "D-S chamber") preferably comprising a deposition-sublimation tower having two sets of channels in which deposition and sublimation alternately take place. By preparing the walls separating the channels at a suitably low temperature, CO₂ contained in the flue gas stream will deposit on one side of the walls of one set of channels during a deposition cycle, and the phase change releases the latent heat of deposition. The latent heat is transferred to the other side of the walls separating the one set of channels from a second set of channels, where the latent heat is utilized to sublimate dry ice previously formed on the walls of the second set of channels. The resulting deposition and sublimation maintains the temperature of the walls relatively constant.

Furthermore, after CO₂ is deposited, the residual flue gas consisting of nitrogen (N₂), oxygen (O₂), and residual CO₂, if any, is routed from the deposition-sublimation tower to a N₂+O₂+CO₂ reservoir. Also, the sublimated CO₂ gas is routed to a CO₂ reservoir. Gases from these reservoirs are then preferably utilized to cool the flue gas stream in order to establish appropriate conditions for deposition to take place in the deposition-sublimation tower.

Since the aforementioned phase changes do not occur simultaneously, they are timed to take place in alternate channels. For this purpose, in accordance with a non-limiting example of the present invention, multiple D-S chambers each preferably implemented by a deposition-sublimation tower are preferably provided in order to render the whole process continuous for a continuously flowing flue gas stream. However, to simplify the following description, a system having one D-S chamber preferably implemented by a deposition- sublimation tower will be described.

By way of a non-limiting example in accordance with the present invention, a three- stage cryogenic system 10 for removal of CO₂ from a flue gas stream 12 is shown in Figure 1. The system 10 comprises a preparatory subsystem 14 consisting of a plurality of heat exchangers. In accordance with one preferred example, there are three heat exchangers 16, 18, and 20. The heat exchangers 16, 18, and 20 are employed to lower the temperature of the flue gas stream 12 to approximately a deposition temperature of the CO₂ present in the flue gas stream, wherein the deposition temperature depends on the partial pressure of the CO₂.

In accordance with a preferred example, two flow streams are fed to the heat exchangers 16, 18, and 20 to provide cooling to lower the temperature of the flue gas stream 12. One flow stream 22 is the CO₂ captured following sublimation in a (D-S) chamber 24, which is utilized for cooling by the heat exchanger 18. Another flow stream 26 is a residual flue gas stream which is used for cooling by the heat exchanger 20 and in turn by the heat exchanger 16 and is then released to the atmosphere through a compressor 27.

Considered in more detail, insofar as preparation of the flue gas stream 12 is concerned, it is assumed that the flue gas stream is dehydrated prior to reaching the heat exchanger 16 shown in Figure 1. The temperature of the incoming flue gas stream 12 is lowered through the three heat exchangers 16, 18, and 20 prior to entering the D-S chamber 24 such that deposition in the D-S chamber will occur readily. The flue gas stream 12 first passes through the heat exchanger 16, exchanging heat with the N₂+O₂+CO₂ stream 26 exiting the heat exchanger 20. The partially cooled flue gas stream 12 then enters the heat exchanger 18 in which the temperature of the flue gas stream is further lowered by heat transfer to CO₂ gas received from a CO₂ reservoir 28. Lastly, the partially cooled flue gas stream 12 passes through the heat exchanger 20 where the flue gas stream exchanges heat with the N2+O₂+CO₂ stream 26 exiting from a turbine 30, which receives a flow from an N2+O₂+CO₂ reservoir 32. The CO₂ reservoir 28 and the N2+O₂+CO₂ reservoir 32 are replenished with respective streams exiting the D-S chamber 24 as will be described in detail below.

As shown in Figure 1, the system 10 further comprises a separation subsystem consisting of the D-S chamber 24. The D-S chamber 24 removes CO₂ from the flue gas stream 12.

By way of a non-limiting example in accordance with the present invention, the D-S chamber 24 preferably consists of a deposition-sublimation tower 24A comprising a heat exchanger with two independent sets of flow passages. Although the system 10 may be implemented with one such deposition-sublimation tower 24A, a plurality of such towers is preferable for continuous operation to process a continuous flue gas stream 12. In accordance with one preferred implementation of the deposition-sublimation tower 24A, CO₂ in the flue gas stream 12 deposits on one side of pre-cooled walls 24AB of a first set of channels comprising the deposition-sublimation tower. This phenomenon releases both the sensible heat and the latent heat of deposition of the CO₂, which is transferred to the other side of the walls 24AB separating the first set of channels from a second set of channels and is utilized for sublimation of dry ice previously deposited on the other side of the walls of the second set of channels.

Considered in more detail, the deposition-sublimation tower 24A is a heat exchanger as shown in Figure 2. As shown in Figure 2, the deposition-sublimation tower 24A comprises a core section 200. The core section 200 of the deposition-sublimation tower 24A comprises two sets of channels 202A and 202B structured such that the channels of the first set of channels 202A alternate with the channels of the second set of channels 202B and are separated by the walls 204AB such that each of the two flow passages next to each other share a wall which serves as the heat transfer medium required for the aforementioned deposition and sublimation. In one preferred geometry, an even number of channels 202A and 202B is provided, and the channels are arranged in a circular configuration. Also, as shown in Figures 2 and 3 the deposition-sublimation tower 24A comprises an inlet section 206 having a first inlet 206A connected to the first set of channels 202A and a second inlet 206B connected to the second set of channels 202B configured such that each of the first set of channels 202 A and the second set of channels 202B can receive flow independent of the other as shown in Figure 3. Although not shown in Figure 3, the deposition-sublimation tower 24A comprises an outlet section that is a mirror image of the inlet section 206.

The deposition-sublimation tower 24A also comprises two concentric vertical ducts 208A and 208B in the center of the deposition-sublimation tower for distribution of the incoming partially cooled flue gas stream 12 to the respective sets of channels 202A and 202B, as shown in Figure 4. At the inlets 206A and 206B to the respective ducts 208A and 208B, the inner duct 208A is connected to receive the partially cooled flue gas stream 12 exiting the heat exchanger 20 through one outlet of a valve 34, whereas the outer duct 208B is connected to another outlet of the valve 34. Hence, the valve 34 controls which duct,208A or 208B is fed with the incoming partially cooled flue gas stream 12. A similar duct arrangement is employed using valves 36 and 38 for the outgoing flows from the two sets of channels 202A and 202B of the deposition-sublimation tower 24A as will be described in more detail below.

As shown in Figure 4, the two independent ducts 208A and 208B connect to the two sets of channels 202A and 202B, respectively. One incoming flow stream passes through the inner duct 208A and enters the first set of channels 202A, and the other incoming flow passes through the outer duct 208B and enters the second set of channels 202B. Each pair of neighboring channels 202A and 202B provide flow passages separated by one wall 204AB between them as shown in Figure 4. Consequently, the deposition-sublimation tower 24A guides the incoming flows to two separate yet adjoining flow passages as shown in Figure 4. While the first set of channels 202A is fed with the partially cooled flue gas stream 12 for CO₂ deposition, the second set of channels 202B with dry ice previously deposited on the wall 204AB of the second set of channels receives heat for sublimation through the wall between the channels 202A and 202B. The valve 34 operates to alternate the incoming flow stream between the channels 202A and 202B from one cycle to the next.

In the channels 202A and 202B of the deposition-sublimation tower 24A in which CO₂ sublimation occurs, the pressure has to be at a value lower than the pressure of the flue gas stream 12 exiting the heat exchanger 20, so that the flue gas stream can enter the channels 202A and 202B. As shown in Figure 1, the CO₂ reservoir 28 is connected to the deposition- sublimation tower 24 A by the valve 36 and will have a pressure appearing at the outlet of the deposition-sublimation tower 24A. As CO₂ gas in the CO₂ reservoir 28 will be sucked out by a compressor 40, the pressure of CO₂ gas in the deposition-sublimation tower 24A and the CO₂ reservoir 28 will be maintained as sublimation progresses.

By way of one non-limiting example in accordance with the present invention, the walls 204AB comprising the deposition-sublimation tower 24A are structured so that heat released by CO₂ deposition on one side of the walls between the channels 202A and 202B does not warm the walls but is instead utilized to sublimate dry ice on the other side of the walls. When CO₂ deposits on the walls 204AB, heat is released as a result of two phenomena: a) sensible heat when the temperature of CO₂ as well as the other components of the partially cooled flue gas stream 12 are lowered to a saturation temperature; and b) latent heat of deposition resulting from the phase change of CO₂ gas into dry ice. It is reasonable to assume that the thermal energy originating from these two sources of heat will be absorbed by the walls 204AB, rather than the flue gas. Since the original temperature of the walls 204AB is made uniform as will be described in detail below, heat conduction on one side of the walls will fan out in a near 180° hemispherical direction, rather than taking the shortest path toward the other side of the walls. To address this situation, the walls 204AB are preferably constructed of a low heat conduction material for the walls proper and a high heat conduction material connecting the two sides of the walls. Structural integrity and strength of the walls 204AB also have to be taken into account as well as the thermal characteristics and ease of production.

Accordingly, as shown in Figure 5, the walls 204AB are constructed of a material 500 with a low heat conductivity suitable for heat insulation. In this regard, glass reinforced plastic is known to be a good insulator for cryogenic applications and is preferably employed to construct the walls 204AB. Additionally, the walls have inserted heat conductors 502, for example, constructed of copper, to provide the function of heat transfer between the channels 202A and 202B.

The heat conductors 502 of the walls 204AB may project from the walls into the flow region in order to stir the boundary layer flow and the main flow for promoting contact with the walls, since it is known that the deposition rate of CO₂ is controlled by how well the flow region containing CO₂ gas comes in contact with the walls. With the wall configuration shown in Figure 5, the partially cooled flue gas stream 12 after depositing a portion of the CO₂ content on the walls 204AB is forced to mix rather than relying on a natural diffusion to even out the CO₂ concentration, so that the remaining flue gas next in contact with the walls will have a good amount of CO₂ content.

In order to maintain the CO₂ deposition process continuous, the following criteria should be noted. The temperature of the walls 204AB of the channels 202A and 202B is set low enough for the deposition process to continue until a target percentage of CO₂ is removed. The length and area of the channels 202A and 202B should be optimized to achieve the target percentage of CO₂ removal. Also, sublimation heat needs to be in balance with the heat released by deposition. A small adjustment of heat released is possible by changing the duration of the incoming flue gas stream 12.

The system 10 shown in Figure 1 further comprises a scavenging subsystem 600 shown in Figure 6. On the one hand, the scavenging subsystem 600 scavenges remaining flue gas after passing into the D-S chamber 24. In the preferred implementation in which the D-S chamber 24 consists of the deposition-sublimation tower 24A, when the partially cooled flue gas stream 12 to the deposition-sublimation tower is stopped temporarily, residual flue gas remains in the deposition-sublimation tower with dry ice deposited on the walls 204AB of one set of channels 202A or 202B and CO₂ gas in the other set of channels. By way of one non-limiting example, scavenging subsystem 600 utilizes a CO₂ gas stream from a scavenging CO₂ reservoir 42 with appropriate pressure and temperature conditions to alternately scavenge remaining flue gas in channels 202A and 202B via the valve 34. The scavenged flue gas is routed by the valve 36 to and stored in an N₂+O₂+CO₂ reservoir 44 from scavenging and later used to cool the CO₂ gas stream returning to the scavenging CO₂ reservoir 42. Some of the scavenging CO₂ gas is expected to deposit and release some heat, but this heat is utilized to sublimate newly deposited dry ice on the walls 204AB of the set of channels on which CO₂ has been deposited prior to scavenging. Thus, a heat balance is preserved.

Additionally, the scavenging subsystem 600 preferably alternately scavenges remaining CO₂ gas in the channels 202A or 202B of the deposition-sublimation chamber tower 24A in which CO₂ was deposited in a previous cycle. At the beginning of the next cycle, the incoming partially cooled flue gas stream 12 alternately scavenges the CO₂ gas remaining in the channels 202A or 202B of the deposition-sublimation chamber tower 24A, which is routed via the valve 38 to the scavenging CO₂ reservoir 42. In this process, the pressure of the CO₂ gas stream is raised by a compressor 46 to a level necessary for the restoration of the original reservoir pressure and then the temperature of the CO₂ gas stream is restored to its initial level by a heat exchanger 48 with cooling supplied by the scavenged flue gas stream from the N2+O₂+CO₂ reservoir 44 from scavenging, after which the scavenged flue gas stream is expelled to the atmosphere through a compressor 50.

The above-described cyclic operation is then repeated. Since the flue gas stream 12 is continuous, multiple systems 10 shown in Figure 1 may be used to provide spare time for scavenging of the channels 202A and 202B of the deposition-sublimation chamber tower 24A. A configuration comprising a plurality of systems 10 wiil also maintain the electric power generation plant operation in continuous operation in the case of a failure of any one of the components of the CO₂ removal system.

The outgoing flue gas consisting mostly of nitrogen and oxygen routed to and stored in the N₂+O₂+CO₂ reservoir 32 from the D-S chamber 24 is estimated to have 0 ~ 1.5% residual CO₂ depending on design conditions, and is utilized to cool the incoming flue gas stream 12 as described above. Also, th^'e separated CO₂ stream routed to and stored in the CO₂ reservoir 28 is utilized to cool down the incoming flue gas stream 12. If the system 10 is thermodynamically designed properly, there is a minimum energy input necessary for extended operation after the initial conditioning of the D-S chamber 24 and for operation of the compressors 27, 40, 46, and 50.

When choosing thermal conditions in the scavenging CO₂ reservoir 42, there are two requirements: a) pressure has to be higher than the average pressure in the channels 202A and 202B of the deposition-sublimation tower 24A after CO₂ deposition in those channels, so that the CO₂ stream can enter and perform scavenging; and b) average pressure after scavenging must be less than the pressure of the incoming flue gas stream 12.

When the C⁽¾ gas in the scavenging CO₂ reservoir 42 is sucked out, pressure in that reservoir will tend to decrease, but the pressure needs to be maintained at the original level. By way of a non-limiting example in accordance with the present invention, Figure 7 shows a subsystem 700 to maintain the pressure in the scavenging CO₂ reservoir 42 substantially constant. The subsystem 700 comprises a piston 702 and a driving mechanism 704 including an electric motor 706. The shaft of the motor 706 is connected to a one-way clutch 708, on the opposite side of which is a shaft carrying a pinion gear 710. The pinion gear 710 is engaged with a rack 712 mounted on the rod of the piston 702, The piston 702 is structured such that the weight of the piston is supported by the initial pressure of the CO₂ gas in the scavenging CO₂ reservoir 42 and is located at its top dead center position. As CO₂ gas is depleted in the scavenging CO₂ reservoir 42, the piston 702 moves down by its own weight due to gravity, keeping the CO₂ gas pressure in the scavenging CO₂ reservoir 42 constant. When the depletion of the CO₂ gas in the scavenging CO₂ reservoir 42 stops, the piston 702 is located at its bottom dead center position. The motor 706 will then start to lift the piston 702 back to its top dead center position. Then, a new supply of CO₂ scavenged from the channels 202A and 202B of the deposition-sublimation tower 24A enters in to refill the scavenging CO₂ reservoir 42.

Referring again to Figure 1 , the CO₂ stream originating from the scavenging CO₂ reservoir 42 is utilized to scavenge flue gas remaining in the channels 202A and 202B of the deposition-sublimation tower 24A. The residual flue gas from scavenging will be stored in the N₂+O₂+CO₂ reservoir 44. When the flue gas stream 12 scavenges CO₂ gas remaining in the channels 202A and 202B of the deposition-sublimation tower 24A, the CO₂ has to be conditioned for returning to the scavenging CO₂ reservoir 42. Compensation for pressure losses and temperature changes in the system is provided in order for the system 10 to cycle. Here, the compressor 46 is employed to compensate for pressure losses, and the heat exchanger 48 is employed to compensate for temperature changes. For the heat exchanger 48, the scavenged flue gas stored in the N₂+O₂+CO₂ reservoir 44 is utilized.

By way of a non-limiting example, a method in accordance with the present invention for deposition-sublimation is schematically depicted in Figure 1. It is assumed that the flue gas stream 12 has been dehydrated prior to entering the system 10. A stepwise description of the method is as follows.

Φ The flue gas stream 12 entering at the right top corner of Figure 1 is cooled by the outgoing stream of N₂+O₂+CO₂ from the N₂+O₂+CO₂ reservoir 32 and the CO₂ stream from the CO₂ reservoir 28 in the three heat exchangers 16, 18, and 20 before entering the deposition-sublimation tower 24 A preferably comprising the D-S chamber 24.

(D By the time the flue gas stream 12 reaches the inlets 206A and 206B of the deposition-sublimation tower 24A, the flue gas stream is cooled to an appropriate temperature which is chosen to be close enough, yet above, the deposition temperature of CO₂ for a given partial pressure (e.g., -98°C for CO₂ partial pressure of 0.168 bar).

(D The flue gas stream 12 feeds into one set of channels 202A and 202B of the deposition-sublimation tower 24A. The walls 204AB of the channels 202A and 202B of the deposition-sublimation tower 24A are cooled in advance to a sufficiently low temperature such that CO₂ deposition can take place on the walls 204 AB as the flue gas stream 12 passes through the deposition-sublimation tower 24A to the N₂+O₂+CO₂ reservoir 32 through the valve 38. (3) When dry ice forms on the surfaces of the walls 204 AB of one set of channels 202A and 202B of the deposition-sublimation tower 24A, the latent heat of deposition is released. The latent heat is transferred through the walls 204 AB of the one set of channels 202A and 202B of the deposition-sublimation tower 24A in which deposition of CO₂ occurs. Dry ice previously formed on the heat receiving side of the walls 204AB of the other set of channels 202A and 202B of the deposition-sublimation tower 24A during the previous cycle will sublimate. The resulting CO₂ gas is routed out of the set of channels 202A and 202B of the deposition-sublimation tower 24A in which sublimation occurs to the CO₂ reservoir 28 through the valve 36, thus replenishing the CO₂ gas previously used to cool the incoming flue gas stream 12,

© Having removed most, if not all, of the CO₂₎ the N₂+O₂+CO₂ stream from the N₂+O₂+CO₂ reservoir 32 is exhausted to the atmosphere after the heat exchanger 16 via the compressor 27. The separated CO₂ is compressed by the compressor 40 for transportation, enhanced oil recovery (EOR), sequestration, or other handling.

The thermodynamic considerations are as follows. Continuous separation of CO₂ from the flue gas stream 12 takes place over a certain temperature range as the deposition phenomenon itself lowers the partial pressure of CO₂. This in turn requires a continuous decrease of saturation temperature. To illustrate, the conditions of the flue gas stream 12 starting at 1 bar pressure containing 14% CO₂ (i.e., a CO₂ partial pressure of 0.14 bar), and saturation temperature of -100°C undergoing a continuous separation of CO₂ are shown in Table 1. CO₂ deposition releases 585.2 kJ/kg of latent heat at -100°C. Hence, any CO₂ separation process requires a way to dispose of the latent heat, as well as reducing the temperature of the flue gas stream 12 sufficiently for the succeeding deposition to take place.

TABLE 1.

CO2 SATURATION TEMPERATURES AT VARIOUS CONCENTRATIONS

* This table was prepared for 1 kg of flue gas at 1 bar with the initial C02 partial pressure of 14% ** Saturation temperatures were estimated with the equation based on a model by W. F. Giauque and C, J. Egan, 1937

Temperature (° K)

Figure 2. CO₂ Sublimation Curve

In accordance with a non-limiting example of the present invention, a method for initial pre-conditioning of the temperature of the walls 204AB of the channels 202A and 202B of the deposition-sublimation tower 24A is provided to enable continuous CO₂ deposition. Thereafter, heat conduction through the walls 204 AB to enable sublimation of dry ice plays a role in heat disposal. Hence, each cycle of operation may be expected to maintain the temperature of the walls 204AB relatively constant. By setting the initial wall temperature at -120°C, 90% of the CO₂ is deposited out of the flue gas stream 12, By lowering the temperature further to -159°C, 100% removal of CO₂ may be achieved.

A method for initial pre-conditioning of the temperature of the walls 204AB of the deposition-sublimation tower 24A in accordance with an example of the present invention is shown in Figure 8. A first step of the pre-conditioning method 800 is priming which occurs at time 1 in Figure 8, as indicated by the numeral 802. Figure 8 illustrates the preconditioning of the two sets of channels 202A and 202B before the initial flue gas stream 12 feeds into the deposition-sublimation tower 24A. During the step 802, the two sets of channels 202A and 202B are cooled down to a desired temperature by flowing N₂ gas into both channels.

During a second step of the method which occurs at time 2 in Figure 8, the flue gas stream 12 feeds into one of the two sets of channels 202 A and 202B of the deposition- sublimation tower 24A, as indicated by the numeral 804. Since deposition of CO₂ releases latent heat, flowing N₂ flow into the other set of channels 202A and 202B is continued to keep the temperature of the walls 204AB at a desired level.

A first cycle of normal operation of the system 10 shown in Figure 1 begins at time 3 in Figure 8, as indicated by the numeral 806. The flue gas stream 12 feeds into the set of channels 202A and 202B of the deposition-sublimation tower 24A into which the flue gas was flowed at time 2 for CO₂ deposition, and the latent heat of deposition is used to sublimate dry ice deposited on the walls 204AB of the other set of channels 202 A and 202B.

During the next cycle of operation of the system 10 shown in Figure 1 at time 4 in Figure 8, as indicated by the numeral 808, the flue gas stream 12 again feeds into the set of channels 202A and 202B of the deposition-sublimation tower 24A into which the flue gas stream initially fed at time 2 in Figure 8, and the latent heat of deposition of CO₂ is used to sublimate dry ice on the walls 204AB of the other set of channels 202A and 202B deposited at time 3 in Figure 8.

After deposition, remaining N₂, O_2} and CO₂ are routed from the set of channels 202A and 202B of the deposition-sublimation tower 24A in which deposition occurs through the valve 38 to the N₂+O₂+CO₂ reservoir 32, and CO₂ gas produced by sublimation of dry ice in the other set of channels 202A and 202B is routed through the valve 36 to the CO₂ reservoir 28.

By way of a further non-limiting example in accordance with the present invention, a scavenging method is provided. A time sequence of steps for scavenging is shown in Figure 9, Two adjoining channels 202A and 202B of the deposition-sublimation tower 24A labeled X and Y, respectively, are shown, and events cyclically taking place in those channels are as follows.

As shown in Figure 9, at time 1 the flue gas stream 12 feeds through the valve 34 into channel X, as indicated by the numeral 902. The N₂+O₂+CO₂ gas minus the CO₂ deposition in channel X is routed through the valve 38 to the N₂+O₂+CO₂ reservoir 32.

Next, at time 2 after CO₂ deposition in channel X, the flow of the flue gas stream 12 into the deposition-sublimation tower 24A is temporarily stopped by the valve 34, as indicated by the numeral 904. Residual N₂+O₂+CO₂ gas remains in channel X.

Then, CO₂ gas from the scavenging CO₂ reservoir 42 is fed into channel X through the valve 34 at time 3 in order to scavenge the residual N2+O₂+CO₂ gas in channel X, which is routed to the N₂+O₂+CO₂ reservoir 44 through the valve 36, as indicated by the numeral 906.

At time 4 shown in Figure 9, the flue gas stream 12 feeds through the valve 34 into channel Y for CO₂ deposition in channel Y and release of latent heat which is transferred through the wall 204AB to sublimate dry ice in channel X, as indicated by the numeral 908. In the initial period of this step, CO₂ gas in channel Y is scavenged and routed through the valve 38, compressor 46, and heat exchanger 48 to the scavenging CO₂ reservoir 42. In the subsequent period of this step, N₂+O₂+CO₂ gas minus the CO₂ deposition in channel Y is routed through the valve 38 to the N₂+O₂+CO₂ reservoir 32.

At time 4 in Figure 9, when the flue gas stream 12 enters the channel Y and scavenges the remaining CO₂, the CO₂ stream is routed back to the scavenging CO₂ reservoir 42 through the compressor 46 and the heat exchanger 48, by which the conditions of the CO₂ stream are adjusted to the initial temperature and pressure conditions in the scavenging CO₂ reservoir. To achieve the temperature condition, the cold side of the heat exchanger 48 is fed with a

N2+O₂+CO₂ stream from the N2+O₂+CO₂ reservoir 44. Calculation shows that the amount of N₂+O₂+CC>2 used for cooling the CO₂ stream going back to the CO₂ reservoir 42 is about 10% of the scavenged amount, so it will become necessary to evacuate with the compressor 50 before the beginning of the next cycle.

Next, at time 5 shown in Figure 9, CO₂ gas in channel X, including gas sublimated by transfer of latent heat from channel Y resulting from CO₂ deposition, is routed through the valve 36 to replenish the CO₂ reservoir 28 until the pressure in channel X becomes equal to the initial pressure in the CO₂ reservoir 28, as indicated by the numeral 910.

The above steps are then repeated with the channels X and Y switched and so forth. To recapitulate, the flue gas stream 12 is fed into one set of channels 202A and 202B of the deposition-sublimation tower 24A for a certain duration before the flue gas stream is directed to the other set of channels 202A and 202B of the deposition-sublimation tower 24A. An amount of flue gas processed in the deposition-sublimation tower 24A in one cycle can be estimated as follows for 90% recovery.

The density of dry ice is known to range from 1 ,400 to 1 ,600 kg/m³, so using the larger value, 65 kg of CO₂ deposited per second will occupy roughly 0.0406 m³. The flue gas flow rate is 470.92 kg/sec and its density at -98°C is 2.9166 kg/m³. Hence, the minimum volume of the channels 202A and 202B of the deposition-sublimation tower 24A should be 161.46 m³. These numbers yield flow duration for filling the channels 202A and 202B with dry ice as 3,976 sec. If one-tenth of the accumulation is picked for a typical operation, one cycle will take approximately 400 sec. This means that the scavenging process of the flue gas remaining in the channels 202A and 202B takes place once every 400 seconds.

As described earlier, when CO₂ deposition occurs on the walls 204AB of the channels 202A and 202B of the deposition-sublimation tower 24A, heat is created due to two phenomena: a) sensible heat when the temperature of CO₂ as well as the other components of the flue gas stream 12 is lowered to a saturation temperature and b) latent heat of deposition for the phase change of CO₂ gas into dry ice, which may suggest a possible imbalance of heat on the two sides of the walls 204AB as the latent heat of sublimation varies with temperature as shown in Table 2.

TABLE 2.

Temperature dependence of latent heat of sublimation (Ref. "Properties of Carbon Dioxide" by UNION ENGINEERING,

Snaremonsevej 27, 700 Fredericia, Denmark) The CO₂ stream from the scavenging CO₂ reservoir 42 may be used to adjust the heat requirement for sublimation. However, the following approach may eliminate the concern regarding a potential heat imbalance.

One way of scavenging the flow remaining in the channels 202A and 202B is to feed CO₂ gas at a sufficient pressure to drive out the remaining flue gas. This may be

implemented as follows.

As shown in Figure 1, CO₂ gas from the scavenging CO₂ reservoir 42 with a pressure of 0.799 bar may be fed into the set of channels 202A and 202B in which CO₂ deposition occurs. Since the leftover flue gas is at an average pressure of 0.779 bar or less, the CO₂ stream should force the remaining flue gas out of the set of channels 202A and 202B in which CO₂ deposition is occurring. It is possible that the new CO₂ stream deposits additional dry ice with a release of additional heat, but the additional heat will be absorbed by dry ice already on the walls 204AB of the set of channels 202A and 202B in which the deposition is occurring. Then, the flue gas stream 12 fed to the other set of channels 202 A and 202B producing CO₂ deposition will release heat to sublimate all the dry ice, and the resulting CO₂ gas will be routed to the CO₂ reservoir 28, which is at a pressure lower than 0.779 bar because of evacuation of its contents by the compressor 40.

In the following analyses, all properties of gas mixtures are calculated as a sum of individual values weighted with their respective mass fraction. For example, the ratio of specific heat capacities y for the N₂+O₂+CO₂ stream at a specified temperature is estimated as follows:

The analysis for the heat exchanger 16 is as follows. The flue gas stream 12 is first fed to the heat exchanger 16 after dehydration. Hence, the flow stream can be assumed to be free of water. Analyses here apply heat balance equations known to persons skilled in the art, e.g., the equations found in The Fundamentals of Heat Exchangers by Dean A. Bartlett. However, one point to be considered is the estimation of fluid properties for two streams. For example,

Mass fractions for the incoming flue gas: N₂ - 0.813

O₂ - 0.033

CO₂ - 0.154 Mass fraction for the N₂+O₂+CO₂ stream after 90% deposition: N₂ _ 0.952

O₂ - 0.039

CO₂ - 0.009

Where,

KM = ε (m_c*Cp_c / m_h*Cp_h) and "_c & _h" indicate cold and hot side, respectively

ε: Heat exchanger effectiveness

TFGO: Incoming flue gas temperature = 0°C assuming dehydration at this temperature

TFGK Flue gas temperature exiting the heat exchanger 16

TN₂+O2+CO24: Temperature of gas exiting the heat exchanger 20

TN2+O2+CO2 S: Temperature of the outgoing N₂+O₂+CO₂ stream m _c: Mass flow rate of the N₂+O₂+CO₂ stream 26 m _n: Mass flow rate of the flue gas stream 12

T₄ is initially unknown, so it is set at an arbitrary value, which is corrected by iteration.

The analysis for the heat exchanger 18 is as follows. The heat exchanger 18 receives flow streams exiting the heat exchanger 16 and the CO₂ reservoir 28, so inlet conditions are known:

m _c: Mass flow rate of the CO₂ stream from the CO₂ reservoir 28, equal to the amount of CO₂ sublimated and exiting the D-S chamber 24 m _h: Mass flow rate of the flue gas stream 12

T _h : TFGI from Equation 2, above

T _c: Ti, which is the temperature of CO₂ exiting the CO₂ reservoir 28

From the heat exchanger relationship,

The analysis for the heat exchanger 20 is as follows. The heat exchanger 20 receives flows from the heat exchanger 18 and from the turbine 30. The heat exchanger 20 passes the cooled flue gas stream 12 to the D-S chamber 24 for deposition. Hence, a suitable temperature of the flue gas stream 12 exiting the heat exchanger 20 should be selected depending on the initial partial pressure of CO₂ in the flue gas stream.

m_c: Mass flow rate of the N2+O₂+CO₂ stream 26 from the turbine 30.

This is calculated as a difference between the flue gas flow rate and the

CO₂ recovery rate.

m_n: Mass flow rate of the flue gas stream 12

Equations (2) through (7) are iteratively solved until the T4 value of the heat exchanger 16 converges.

The analysis for the turbine 30 is as follows. Since the outlet temperature from the D- S chamber 24 may be estimated as will be described below, the temperature T_3A above can be calculated as well as an expansion ratio of the turbine 30:

Where,

Polytropic efficiency of the turbine 30

Average specific heat ratio of N₂+O₂+CO₂ across the turbine 30

Turbine power is calculated according to the following relationship:

The analysis for the compressor 40 is as follows. The compression ratio of the compressor 40 is estimated from the pressure in the CO₂ reservoir 28 adjusted for a pressure loss in the heat exchanger 18 and the atmospheric pressure.

Where,

Compressor power is calculated according to the following relationship:

The analysis of the flue gas expansion in the D-S chamber 24 is as follows. The flue gas stream 12 enters the D-S chamber 24 preferably consisting of the deposition-sublimation tower 24A comprising the channels 202A and 202B and deposits dry ice. With this process of shedding CO₂ and losing the CO₂ partial pressure, the flue gas stream 12 undergoes adiabatic expansion, if a simplified assumption of no heat transfer to the stream is assumed.

The mass fraction of each component gas is known at the inlets of the deposition- sublimation tower 24A and is revised at the exit in accordance with the loss of CO₂ due to deposition. Partial pressure of each component of the flue gas stream 12 at the exit of the channels 202A and 202B of the deposition-sublimation tower 24A may be estimated together with an assumption of 5% aerodynamic pressure loss through the channels. Then, the pressure of the flue gas at the exit of the deposition-sublimation tower 24A can be calculated.

An exit temperature is evaluated using the adiabatic expansion relationship:

Where,

T_DS2: Temperature of flue gas at the exit of the channels 202A and 202B of the deposition-sublimation tower 24A

T_DS1: Temperature of flue gas stream 12 at the inlets 206A and 206B to the respective channels 202A and 202B of the deposition-sublimation tower 24A p_DS1 and p_DS2: Pressures at the inlets 206A and 206B and outlets, respectively, of the deposition-sublimation tower 24A

Gamma: Average value of the ratio of specific heats at the inlets 206A and 206B and outlets of the channels 202A and 202B of the deposition- sublimation tower 24A for the flue gas; its estimation requires iteration.

Thermodynamics gives a minimum energy required for CO₂ separation estimated from the free energy of mixing for ideal gases given by the following expression:

Where,

R is the CO₂ ideal gas constant (0.1889 kJ/kg K)

T is the working temperature

X_i is the final partial pressure fraction Using Equation (13) for X_t = 0.015 (i.e., 1.5% at the end of the capture process) and the working temperature of 154.25 K,

E _{free energy} = 0.12 GJ/ton CO₂

This is the thermodynamic minimum energy requirement for CO₂ removal from the flue gas stream 12 of the fossil-fuel-fired electric power generation plant. The above formula ignores the change of free energy of the air (flue gas) when CO₂ is extracted. This is expected to require up to 1% correction. See "Capturing CO₂ from the atmosphere: rationale and process design considerations" by David W. Keith, et al, Geo-Engineering Climate Change: Environmental Necessity or Pandora's Box, eds. Brian Launder and J. Michael T. Thompson, Cambridge University Press, 2010. The results of the present analysis can, therefore, be judged as a realistic value compared to this thermodynamic minimum limit.

Since a CO₂ capturing technology requires electrical energy, it has been suggested and generally adopted to use what is called "CO₂ avoided" rather than "CO₂ recovered " for analysis. This concept is schematically illustrated in Figure 10 (adopted from "An

Introduction to CO₂ Separation and Capture Technologies" by Howard Herzog, MIT Energy Laboratory, August, 1999).

CO₂ avoided IS the amount of CO₂ recovered which comprises the original emission prior to the installation of the CO₂ capturing system minus the amount of emission due to the capturing system. Three cases of CO₂ recovery were analyzed, 90%, 95%, and 100% recovery.

Results are tabulated in Figure 1 1. For each case, the energy requirement is shown in the last column. Values are 0.386, 0.372, and 0.357 GJ/ton-CO₂ avoided for the respective cases that were analyzed. The scavenging energy requirement for the scavenging subsystem in accordance with the example of the present invention was divided by 400, as the scavenging subsystem operates once every 400 seconds as described above.

These efficiency values are superior to values for other CO₂ capturing systems reported in the literature. For a comparison, the membrane method presently considered to be the most efficient method of CO₂ recovery is known to have an energy requirement of 0.7 GJ/ton-CO₂.

Based on an average US industrial rate of electricity at 6.6 cents per kWH, the above results yield an energy cost of US$6.55 ~ $7.08 per ton-CO₂ avoided-

The examples in accordance with the present invention described and analyzed above efficiently remove CO₂ from the flue gas produced by fossil-fuel-fired electric power generation plants. The above estimates indicate a reasonable and attractive energy efficiency for the system and method in accordance with the examples of the present invention.

All equipment except the described deposition-sublimation tower 24A is commercially available industrial equipment, indicating relatively low costs compared to Mono Ethanol Amine (MEA) based facilities currently in operation, which are reported to cost US$1 OOM to install and consume approximately 40% of the electricity produced (4.0 GJ/ton-CO₂) for operation. The system and method in accordance with the examples of the present invention would be considerably less expensive to install and operate and would impose a much smaller energy penalty to operate.

The ultimate destination of CO₂ captured by the system and method in accordance with the examples of the present invention was not considered here beyond pointing out that the captured CO₂ has many uses, for example, as a building material produced from CO₂ using a mineral solution, carbonation of beverages, enhanced oil recovery (EOR), etc.

Presently, EOR is known to be the most promising market, while oil prices remain above a certain level. Moreover, recent media news reported a global shortage of CO₂ for industrial applications, which provides an added incentive for the development of an efficient CO₂ recovery system, in addition to the dire need for reduction of CO₂ emissions and a decrease of current atmospheric CO₂ concentration.

While the foregoing description has been with reference to particular examples of the present invention, it will be appreciated by persons skilled in the art that changes in these examples may be made without departing from the principles and spirit of the invention. Accordingly, the scope of the present invention can only be ascertained with reference to the appended claims.

Claims

WHAT IS CLAIMED IS: Claims

1. A system to cryogenically remove carbon dioxide contained in a flue gas stream produced by a fossil-fuel-fired electric power generation plant, comprising:

at least one heat exchanger to cool the flue gas stream down to a temperature close to, yet above, a saturation temperature, which depends on the partial pressure of the carbon dioxide contained in the flue gas;

a deposition-sublimation chamber connected to the at least one heat exchanger to contact the cooled flue gas stream with at least one cold surface for deposition of the carbon dioxide as dry ice and to utilize the latent heat released by deposition to sublimate dry ice previously formed on at least one other surface to pure carbon dioxide gas utilizing the latent heat;

a carbon dioxide reservoir connected to the deposition-sublimation chamber to store the pure carbon dioxide gas; and

a plurality of scavenging subsystems, comprising:

a first scavenging subsystem connected to the deposition-sublimation chamber to scavenge flue gas remaining in the deposition-sublimation chamber; and

a second scavenging subsystem connected to the deposition-sublimation chamber to scavenge carbon dioxide gas remaining in the deposition-sublimation chamber.

2. A system as recited in claim 1 wherein three heat exchangers are connected in series to the deposition-sublimation chamber to lower the temperature of the flue gas stream to approximately a deposition temperature of the carbon dioxide present in the flue gas stream, wherein the deposition temperature depends on the partial pressure of the carbon dioxide.

3. A system as recited in claim 2 wherein two flow streams are fed to the heat exchangers to provide cooling to lower the temperature of the flue gas stream, and wherein a first flow stream is the flue gas stream after depositing carbon dioxide in the deposition- sublimation chamber, which is utilized for cooling in a first heat exchanger and in turn in a third heat exchanger, and a second flow stream is a CO₂ gas stream exiting the deposition- sublimation chamber, which is utilized for cooling in a second heat exchanger,

4. A system as recited in claim 1 wherein the deposition-sublimation chamber is a deposition-sublimation tower comprising a heat exchanger having two sets of channels, and wherein carbon dioxide deposits on pre-cooled surfaces of a first set of channels and releases latent heat of deposition of the carbon dioxide, which is transferred to surfaces of a second set of channels and is utilized for sublimation of dry ice previously deposited on the surfaces of the second set of channels.

5. A system as recited in claim 4 wherein the system comprises a plurality of deposition-sublimation towers for continuous operation to process a continuous flue gas stream.

6. A system as recited in claim 4 wherein, when the flue gas stream to the deposition-sublimation tower is stopped temporarily after a first cycle of operation, there remains flue gas with the deposited dry ice on the surfaces of the first set of channels and carbon dioxide gas in the second set of channels, and wherein at the beginning of a next cycle of operation, a carbon dioxide gas stream from a scavenging carbon dioxide reservoir with appropriate pressure and temperature conditions scavenges the remaining flue gas in the first set of channels, which is routed to and stored in an N2+Q2+CO₂ reservoir and later utilized to cool a carbon dioxide gas stream fed to a scavenging carbon dioxide reservoir, and at the beginning of the next cycle of operation, the incoming flue gas stream scavenges carbon dioxide gas remaining in the second set of channels, which is routed to and stored in the scavenging carbon dioxide reservoir.

7. A system as recited in claim 6 wherein the pressure of the carbon dioxide gas is raised by a compressor to a level needed for the restoration of the original scavenging carbon dioxide reservoir pressure and then the temperature of the carbon dioxide gas is restored to its initial level by a heat exchanger utilizing the scavenged flue gas stream from the N2+O₂+CO₂ reservoir.

8. A system as recited in claim 4 wherein the deposition-sublimation tower comprises a core section having the two sets of channels located such that a plurality of walls separates the two sets of channels, an inlet section configured such that one set of channels can receive flow independent of the other set of channels, and an outlet section having a configuration similar to the inlet section such that the outgoing flows can be separately routed.

9. A system as recited in claim 8 wherein the walls between the two sets of channels of the deposition-sublimation tower function to transfer heat across the walls, and wherein the walls are structured such that latent heat released by deposition of carbon dioxide on one side of the walls does not warm the wall itself but is instead utilized to sublimate dry ice formed on the other side of the walls.

10. A system as recited in claim 9 wherein the walls are constructed of a material with a low heat conductivity suitable for heat insulation and having inserted heat conductors extending through the walls to transfer heat from one side of the walls to the other side of the walls.