METHOD AND DEVICE FOR TREATING A CARBON DIOXIDE-CONTAINING GAS STREAM

Abstract

The invention relates to a method and an apparatus for treating a carbon dioxide-containing gas stream. Precompressed raw gas stream (1) is partially liquefied in a cryogenic carbon dioxide purification stage (2, 3, 4). Part of the resultant liquid is used to obtain a gas stream having an elevated carbon dioxide content (7). From the non-liquefied raw gas, a gas stream having a reduced carbon dioxide content is obtained. This vent gas stream is expanded and the refrigeration generated is recovered for cooling the raw gas stream. The carbon dioxide gas stream is compressed (8) to a final pressure and fed to further utilization and/or storage. Another part of the liquid from the cryogenic carbon dioxide purification stage is fed in a liquid phase (9) to further utilization and/or storage (10).

Description

SUMMARY OF THE INVENTION

The invention relates to a method for treating a carbon dioxide-containing gas stream (raw gas stream), in particular a carbon dioxide-containing gas from a large-scale furnace plant. The precompressed raw gas stream is partially liquefied in a cryogenic carbon dioxide purification stage, and the liquid is separated off from which a gas stream having an elevated carbon dioxide content (carbon dioxide gas stream) is obtained by reevaporation. In addition, a gas stream having a reduced carbon dioxide content (vent gas stream) is obtained from the non-liquefied raw gas. This vent gas stream is expanded in at least one expansion turbine and the refrigeration generated in this process is recovered for cooling the raw gas stream. The carbon dioxide gas stream is compressed to a final pressure and fed to further utilization and/or storage. The invention also relates to an apparatus for carrying out the above-described method.

Carbon dioxide-containing gas streams are produced in all large-scale furnace plants which are operated with fossil fuels such as coal, oil or natural gas. These include, in particular, power plants, but also industrial furnaces, steam kettles and similar large-scale thermal plants for power and/or heat generation. In addition, carbon dioxide-containing gas streams are also formed in process plants of the chemical or petrochemical industry, such as cracking furnaces of olefin plants or steam reformers of synthesis gas plants. Owing to the harmful climatic effect of carbon dioxide gas, solutions are being sought in order to decrease the emissions of carbon dioxide-containing exhaust gases into the atmosphere.

Very recently, novel power plant concepts are proposed in which the fossil fuel, e.g. coal, is burnt with an oxygen-rich combustion gas, in particular with technically pure oxygen, or with oxygen-enriched air (oxygen combustion gas method). The oxygen fraction of this combustion gas is, e.g., 95 to 99.9% by volume. The resultant exhaust gas, which is also termed flue gas, contains principally carbon dioxide (CO₂) at a fraction of approximately 70 to 85% by volume. The purpose of these novel concepts is to inject the carbon dioxide formed in the combustion of the fossil fuels and present in concentrated form in the flue gas into suitable repositories, in particular into certain rock strata or salt water-bearing strata, and thereby to limit the emission of carbon dioxide into the atmosphere. The harmful climatic effect of greenhouse gases such as carbon dioxide is thereby reduced. Such power plants are termed in the specialist field “oxyfuel” power plants.

In the concepts known to date, a dedusting, denitrification and desulphurization of the flue gas proceed in sequential steps. Subsequently to this flue gas purification, the carbon dioxide-rich exhaust gas thus treated is compressed and fed to a carbon dioxide purification stage. There, typically, a gas substream having a reduced carbon dioxide content and another gas substream having an elevated carbon dioxide content are generated by way of a cryogenic separation method. The gas substream having an elevated carbon dioxide content is the desired carbon dioxide product stream, which is produced having a carbon dioxide content of, e.g., greater than 95% by volume and is provided for further use, in particular for transport to repositories. The gas substream having a reduced carbon dioxide content is produced as a subsidiary stream (what is termed vent gas) at 15 to 30 bar, preferably 18 to 25 bar, and contains predominantly the components not intended for the injection, in particular inert gases such as nitrogen (N₂) and argon (Ar) and oxygen (O₂). However, in this gas substream, fractions of carbon dioxide are also still present at a concentration of approximately 25 to 35% by volume. This vent gas is currently blown off into the atmosphere.

Usually, the raw gas stream is precompressed to a desired pressure in upstream plant parts and dried, e.g., in adsorber stations. This means that the vent gas also is first still present in the compressed state. At present this pressure level is reduced by expansion valves.

In EP 1952874 A1 and EP 1953486 A1 (Air Products) it has already been proposed, after warming the vent gas and further heating by means of waste heat from the compression, to carry out a turbine expansion of the vent gas stream. However, utilization of the energy liberated in the turbine expansion, in particular of the refrigeration capacity produced in the expansion process, is not envisaged in this case.

In DE 102009039898 A1 (Linde), for improving the energy efficiency, it is proposed that the vent gas stream is expanded in at least one expansion turbine and both the resultant kinetic energy and also the refrigeration generated are utilized for energy recovery. For utilizing the kinetic energy, the expansion turbine can be coupled to a compressor (booster) which compresses the raw gas stream and/or the carbon dioxide gas stream. For utilizing the refrigeration generated in the expansion, the at least partially expanded vent gas stream can be brought into heat exchange with process streams that are to be cooled, e.g. the raw gas stream and/or the carbon dioxide gas stream.

The carbon dioxide gas stream is usually compressed by way of a final compressor to the required final pressure of above 80 bar (preferably 120 to 150 bar) for transport and subsequent sequestration.

Alternatively, for the carbon dioxide gas stream compression, liquefaction of the separated-off carbon dioxide-rich gas with subsequent pressure elevation by way of pumps is also possible. Here, however, the use of refrigerant is necessary. When external refrigeration from a refrigeration plant is used, a liquid carbon dioxide pure product is already present after the cryogenic separation, which liquid carbon dioxide pure product can be brought to the necessary final pressure by way of a pump. However, the use of a refrigeration plant (external refrigeration) increases the necessary energy consumption.

An object of the present invention is to design a method of the type mentioned at the outset and also an apparatus for carrying out the method in such a manner that the energy efficiency can be further improved.

Upon further study of the specification and appended claims, other objects and advantages of the invention will become apparent.

These objects are achieved in terms of the method in that, from some of the liquid that is separated off in the cryogenic carbon dioxide purification stage, a liquid stream having an elevated carbon dioxide content (carbon dioxide liquid stream) is obtained which is fed in a liquid phase to further utilization and/or storage.

Using the invention, an energy-sparing operation of the carbon dioxide purification stage is made possible without using external refrigeration from an external refrigeration plant. The refrigeration necessary for cooling and partial condensation of the raw gas stream can be provided via heat exchange with the evaporating liquid forming the carbon dioxide gas stream and also by heat exchange with the vent gas stream that is cooled by expansion in the expansion turbine. By branching off a carbon dioxide liquid stream, the final compression of the carbon dioxide gas stream can be relieved, whereby, in total, an improvement of the energy efficiency is achieved. Furthermore, in this manner an additional liquid carbon dioxide product can be provided without further energy consumption.

It has proved in this case that operation of the carbon dioxide purification stage without an external refrigeration plant is expedient when the carbon dioxide liquid stream amounts to 5 to 25%, preferably 10 to 15%, of the total liquid that is separated off in the cryogenic carbon dioxide purification stage.

According to a particularly advantageous embodiment of the invention, the carbon dioxide liquid stream is fed to the carbon dioxide gas stream after compression thereof to the final pressure. In this case the carbon dioxide liquid stream is expediently compressed to the final pressure by way of a liquid pump before it is fed to the carbon dioxide gas stream.

Another variant of the invention provides that the carbon dioxide liquid stream is temporarily stored in a liquid gas tank for further use, in particular in the food industry. Customers' wants can thereby be met by an additional liquid carbon dioxide product without additional energy expenditure and without the use of an external refrigeration plant.

A further possibility of use is in the use of the carbon dioxide liquid stream, after evaporation, as transport medium for the pneumatic transport (i.e., entrainment of solid particles, such as coal dust, by a gas stream) or as a lock gas of feedstocks, in particular coal dust, in large-scale furnace plants. For example, during preparation of combustible material (coal/lignite) for coal fired power plants, contact with oxygen has to be minimized. Carbon dioxide is possible gas to use for displacing air, thereby acting as a lock gas.

For utilization of the kinetic energy, the expansion turbine can also be coupled to at least one compressor (booster), in such a manner that the expansion turbine compresses the raw gas stream and/or the carbon dioxide product stream during the at least partial expansion of the vent gas stream. For utilization of the refrigeration generated in the expansion, the at least partially expanded vent gas stream is preferably brought into heat exchange with process streams that are to be cooled, e.g. the raw gas stream and/or the carbon dioxide product stream. By expansion of the vent gas, process-internal refrigeration output can be provided and external refrigeration can thereby be spared.

The invention further relates to an apparatus for treating a carbon dioxide-containing gas stream (raw gas stream), in particular from a large-scale furnace plant, having a carbon dioxide purification appliance that is charged with the precompressed raw gas stream. The carbon dioxide purification appliance comprises an outlet line for a gas stream having an elevated carbon dioxide content (carbon dioxide gas stream) and an outlet line for a gas stream having a reduced carbon dioxide content (vent gas stream). The outlet line for the carbon dioxide gas stream is connected via a final compressor to a utilization appliance and/or repository, whereas the outlet line for the vent gas stream is connected to at least one expansion turbine which comprises an outlet line for the at least partially expanded vent gas stream. The outlet line for the at least partially expanded vent gas stream is connected to a heat-exchange appliance which is chargeable with the precompressed raw gas stream, the carbon dioxide gas stream and the vent gas stream.

The objects are achieved in terms of the apparatus in that the carbon dioxide purification appliance additionally comprises an outlet line for a liquid stream having an elevated carbon dioxide content (carbon dioxide liquid stream). This outlet line bypasses the heat-exchange appliance and the final compressor and is connected directly to a utilization appliance and/or storage appliance for liquid having an elevated carbon dioxide content.

Preferably, the outlet line for the carbon dioxide liquid stream comprises a liquid pump and, downstream of the final compressor, the carbon dioxide liquid stream is introduced into the outlet line for the carbon dioxide gas stream.

The invention is suitable for all conceivable large-scale furnace plants in which carbon dioxide-containing gas streams are produced. These include, e.g., fossil-fuel-fired power plants, industrial furnaces, steam kettles and similar large-scale thermal plants for power and/or heat generation. The invention can be used particularly advantageously in large-scale furnace plants which are supplied with technically pure oxygen or oxygen-enriched air as combustion gas and in which, accordingly, exhaust gas streams having high carbon dioxide concentrations are produced. In particular, the invention is suitable for what are termed low-CO₂ coal power plants which are operated with oxygen as combustion gas (“oxyfuel” power plants) and in which the carbon dioxide that is present in the exhaust gas in high concentration is separated off and injected below ground (“CO₂ capture technology”).

The invention is associated with a large number of advantages.

The final compressor for the carbon dioxide gas stream is relieved, which leads to energy savings and also to a reduction of capital costs. The demands on the operation of the final compressor are reduced so, for example, a smaller compressor can be used thereby reducing capital costs. In addition, the energy balance at the heat-exchange appliance is optimally utilized. In some circumstances, the intake temperature at the final compressor can be adjusted in such a manner that simpler materials (no high-alloy steels) can be used. Furthermore, a prepurified liquid carbon dioxide product can be provided from the system which can be utilized, for example, for treatment to give a food-specific carbon dioxide product (external utilization) or else also as liquid store in a tank system.

The carbon dioxide liquid stream can also be used for other applications (e.g. treatment to give purified seal gas for the oxyfuel process, use as transport medium for the pneumatic transport of coal dust in the oxyfuel process, storage of liquid carbon dioxide for use as start-up gas or charge gas after evaporation).

When multistage compression is used to bring the pressure of the carbon dioxide gas stream to the required final pressure, the carbon dioxide liquid stream, after a supercritical compression of the carbon dioxide gas stream (>72 bar), can be fed (in the supercritical state) upstream of the suction side of the next-following compressor stage (or pump). The temperature falls and the density increases thereby. Owing to the higher density, the energy requirement of the subsequent compressor stages/pumps for achieving the required final pressure falls.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further embodiments of the invention are described in more detail hereinafter with reference to working examples shown schematically in the figures, in comparison with the previous prior art. Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein:

FIG. 1 shows a block diagram of a carbon dioxide treatment plant with expansion of the vent gas via an expansion turbine as per the prior art according to EP 1952874 A1;

FIG. 2 shows a block diagram of a carbon dioxide treatment plant with expansion of the vent gas via expansion turbines with energy recovery as per the prior art according to DE 102009039898 A1;

FIG. 3 shows a block diagram of a carbon dioxide treatment plant with removal of a separate carbon dioxide liquid stream with subsequent compression by way of a liquid pump to the required final pressure for pipeline transport; and

FIG. 4 shows a block diagram of a carbon dioxide treatment plant with removal of a separate carbon dioxide liquid stream with subsequent temporary storage for external use.

FIG. 1 shows a conventional treatment of a carbon dioxide-containing raw gas stream from a coal power plant as per the prior art according to EP 1952874 A1 (Air Products). The raw gas stream, after compression in the raw gas compressor 1, is fed via a heat-exchange unit 2 to a primary separator 3 for separating off carbon dioxide. The vent gas from the primary separator 3 is introduced into the heat-exchange unit 2 and then fed to a secondary separator 4. The carbon dioxide product stream is withdrawn, respectively, from the bottoms of primary carbon dioxide separator 3 and secondary separator 4, introduced into the central heat-exchange unit 2 (and in the case of the vent gas from the primary separator 3 additionally via a CO₂ product compressor 7), and then subjected to a final compression 8 in order finally to be fed via a pipeline (carbon dioxide pipeline) 9, e.g. to an injection below ground. The vent gas of the secondary separator 4 is withdrawn from the top of the secondary separator 4, likewise introduced into the central heat-exchange unit 2 and finally, downstream of a further warming in the heat exchanger 5, expanded via a turbine 6 in order to be delivered to the atmosphere.

In contrast to the method shown in FIG. 1 for carbon dioxide treatment, the method according to DE 102009039898 A1 (Linde), shown in FIG. 2, offers the advantage of energy recovery in the expansion of the vent gas. In this method, as in the method shown in FIG. 1, two carbon dioxide separators 3 and 4 and also a central heat-exchange unit 2 are provided. In contrast to the method as per FIG. 1, however, simple expansion of the vent gas via a single turbine does not occur, but instead a stepwise expansion via two expansion turbines 5 and 6 is performed. By way of the stepwise expansion of the vent gas stream, the formation of solid carbon dioxide in the vent gas can be prevented. Downstream of the expansion in the first expansion turbine 5, the vent gas stream is warmed in the central heat-exchange unit 2 and then expanded further to close to atmospheric pressure in the second expansion turbine 6 and again warmed in the central heat-exchange unit 2. The available pressure level of the vent gas can thereby be completely exploited. The vent gas that is cold after the expansion is warmed in the central heat-exchange unit 2 against the process streams that are to be cooled. The vent gas thereby provides some of the refrigeration capacity necessary in the process.

FIG. 3 shows a carbon dioxide treatment according to the invention. The process procedure differs from that shown in FIG. 2 in that some of the liquid carbon dioxide separated off in the primary separator 3 is branched off and fed via a carbon dioxide liquid pump 9 downstream of the final compressor 8 to the CO₂ pipeline 10. In this procedure the carbon dioxide liquid stream is brought to the required final pressure for the pipeline transport by way of the carbon dioxide liquid pump 9. Since this carbon dioxide liquid stream by passes the final compressor 8, the operational demands on the final compression 8 are reduced thereby increasing energy efficiency and reducing capital costs. The energy balance at the heat-exchange unit 12 can be used optimally. The intake temperature at the final compressor 8 can be adjusted in such a manner that simpler materials (e.g. no high-alloy steels) can be used.

In the variant of the invention shown in FIG. 4, the liquid carbon dioxide that is separated off from the primary separator 3 is first temporarily stored in a liquid carbon dioxide tank 11. The liquid carbon dioxide can be fed, as required, from tank 11 via the liquid pump 9 to the CO₂ pipeline 10 and/or loaded into a transport vehicle 12 for further external utilization (e.g. in the food industry) and/or evaporated in a CO₂ evaporator 13 and provided for internal utilization (e.g. as start-up or feed gas).

The entire disclosure[s] of all applications, patents and publications, cited herein and of corresponding German Application No. 10 2011 014 678.4, filed Mar. 22, 2011, are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

Claims

1. A method for treating a carbon dioxide-containing gas stream comprising:

partially liquefying precompressed raw carbon dioxide-containing gas stream in a cryogenic carbon dioxide purification stage,

removing from said cryogenic carbon dioxide purification stage a liquid stream having an elevated carbon dioxide content and a gas stream having a reduced carbon dioxide content,

reevaporating a first portion of the liquid stream having an elevated carbon dioxide content to obtain a gas stream having an elevated carbon dioxide content,

expanding the gas stream having a reduced carbon dioxide content in at least one expansion turbine and recovering the resultant refrigeration generated in this expansion for cooling the precompressed raw carbon dioxide-containing gas stream,

compressing the gas stream having an elevated carbon dioxide content to a final pressure, and

feeding the resultant compressed gas stream having an elevated carbon dioxide content to further utilization and/or storage,

wherein a second portion of the liquid stream having an elevated carbon dioxide content removed from said cryogenic carbon dioxide purification stage is fed in a liquid phase to further utilization and/or storage.

2. The method according to claim 1, wherein said precompressed raw carbon dioxide-containing gas stream is a raw gas stream form a large-scale furnace plant.

3. The method according to claim 1, wherein the second portion of the liquid stream having an elevated carbon dioxide content is 5 to 25% of the total liquid removed from the cryogenic carbon dioxide purification stage.

4. The method according to claim 3, wherein the second portion of the liquid stream having an elevated carbon dioxide content is 10 to 15% of the total liquid removed from the cryogenic carbon dioxide purification stage.

5. The method according to claim 1, wherein the second portion of the liquid stream having an elevated carbon dioxide content is introduced into the gas stream having an elevated carbon dioxide content after compression of the second portion of the liquid stream having an elevated carbon dioxide content to the final pressure.

6. The method according to claim 5, wherein the second portion of the liquid stream is compressed to the final pressure by way of a liquid pump before it is introduced into the gas stream having an elevated carbon dioxide content.

7. The method according to claim 1, wherein said second portion of the liquid stream having an elevated carbon dioxide content is temporarily stored in a liquid gas tank for further use.

8. The method according to claim 3, said second portion of the liquid stream having an elevated carbon dioxide content is used as transport medium for the pneumatic transport of feedstocks.

9. An apparatus for treating a carbon dioxide-containing gas stream, said apparatus comprising:

a carbon dioxide purification appliance comprising an inlet for introducing a precompressed raw carbon dioxide-containing gas stream, an outlet line for removal of a gas stream having an elevated carbon dioxide content and another outlet line for removal of a gas stream having a reduced carbon dioxide content,

said outlet line for removal of a gas stream having an elevated carbon dioxide content being connected via a final compressor to a utilization appliance and/or repository,

said another outlet line for removal of a gas stream having a reduced carbon dioxide content being connected to at least one expansion turbine comprising an outlet line for removal of at least partially expanded gas stream having a reduced carbon dioxide content which is connected to an inlet of a heat-exchange appliance, and said heat exchange appliance further having inlets for introduction of said precompressed raw gas stream and said gas stream having an elevated carbon dioxide content,

wherein said carbon dioxide purification appliance additionally comprises an outlet line for a liquid stream having an elevated carbon dioxide content, which bypasses said heat-exchange appliance and said final compressor, and is connected directly to a utilization appliance and/or storage appliance for liquid having an elevated carbon dioxide content.

10. The apparatus according to claim 9, wherein said outlet line for a liquid stream having an elevated carbon dioxide content comprises a liquid pump and, downstream of said final compressor, outlet line for a liquid stream having an elevated carbon dioxide content communicates with said the outlet line for removal of a gas stream having an elevated carbon dioxide content being.