Exhaust gas recycling system

Abstract

To provide an exhaust gas recycling system which efficiently recovers carbon dioxide, reduces energy consumption in an air separation device, and increases efficiency of recycling of exhaust gas.SOLUTION: An exhaust gas recycling system 1 includes: a carbon dioxide solidification device 30 for solidifying and separating carbon dioxide in combustion exhaust gas in fossil fuel by heat exchange between the combustion exhaust gas and a coolant; an exhaust gas blower 10 for sending the combustion exhaust gas to the carbon dioxide solidification device 30; exhaust gas supply paths 62 and 63 for connecting the exhaust gas blower 10 and the carbon dioxide solidification device 30; a chimney 70 for diffusing excess exhaust gas which is not solidified in the carbon dioxide solidification device 30; an excess exhaust gas discharge path 64 for connecting the carbon dioxide solidification device 30 and the chimney 70; an air separation device 40 for producing nitrogen and oxygen by cryogenic separation; and an excess exhaust gas branch path 65 for supplying excess exhaust gas which is branched from the excess exhaust gas discharge path 64 and not solidified to an air compressor 41 of the air separation device 40.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an exhaust gas recycling system that separates and recovers carbon dioxide from exhaust gas, and in particular to an exhaust gas recycling system that separates and recovers carbon dioxide by solidifying it through heat exchange with a refrigerant, and further relates to an exhaust gas recycling system in which the remaining gas after the separation and recovery is recycled using an air separation unit. [Background technology]

[0002] Generally, most of the carbon dioxide released into the atmosphere is contained in exhaust gas from the combustion of fossil fuels. Because carbon dioxide has an impact on global warming, there are studies being conducted on ways to reduce carbon dioxide emissions or utilize it. However, the concentration of carbon dioxide contained in exhaust gas is low, ranging from a few percent to several tens of percent, and separation of carbon dioxide from the exhaust gas is essential for utilization.

[0003] Technologies for separating carbon dioxide from gases that contain carbon dioxide have long been used industrially, and well-known methods include those using hot potassium carbonate or amine-based absorption liquids, those using gas separation membranes, and the pressure swing adsorption (PSA) and temperature swing adsorption (TSA) methods that use adsorbents.

[0004] However, these separation technologies have only been used in cases where removing the carbon dioxide increases the value of the remaining gas or where there are benefits to using carbon dioxide, such as in enhanced oil recovery (EOR), and all of these methods consume a large amount of energy to separate the carbon dioxide. If fossil fuels are used to generate that energy, the effect of reducing carbon dioxide emissions will be small, and the amount of energy obtained from each fossil fuel will also decrease, making them economically unacceptable. Therefore, there is a demand for new energy-saving carbon dioxide separation technology that consumes as little energy as possible when separating carbon dioxide as a measure against global warming.

[0005] On the other hand, from the perspective of energy conservation, issues have also been pointed out regarding gas production facilities that use cryogenic separation. Specifically, cryogenic separation is listed as one of the industries that are being targeted for expanding the benchmarking of gas production facilities under the Energy Conservation Act, and there is a strong demand for energy conservation in this type of facility. In particular, the separation process accounts for 79% of the energy used in the entire facility, and since the feed air compressor that compresses the feed air consumes a significant proportion of energy, reducing the energy consumption of the feed air compressor is considered an issue.

[0006] In this regard, Patent Document 1 discloses a method of cooling the feed air by heat exchange between surplus product nitrogen (waste nitrogen) and the feed air, thereby reducing the power consumption of the feed air compressor. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2008-224186 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, as described in Patent Document 1, even if waste nitrogen at about 10° C. is used, the cooling effect of the feed air is extremely limited, and is ineffective in winter when the feed air is at a low temperature. In addition, since waste nitrogen has no effect of increasing the pressure of the feed air, the pressure difference of the feed air compressor cannot be reduced, and the effect of reducing the power consumption of the feed air compressor is also limited.

[0009] The present invention has been made to solve these problems, and has an object to provide an exhaust gas recycling system that efficiently recovers carbon dioxide and reduces energy consumption in an air separation unit, thereby improving the efficiency of exhaust gas recycling. [Means for solving the problem]

[0010] (1) The exhaust gas recycling system according to the present invention comprises: a carbon dioxide solidifier that solidifies and separates carbon dioxide in the combustion exhaust gas by heat exchange between the combustion exhaust gas of a fossil fuel and a refrigerant; an exhaust gas blower that delivers the combustion exhaust gas to the carbon dioxide solidifier; an exhaust gas supply line connecting the exhaust gas blower and the carbon dioxide solidifier; a chimney for dissipating excess exhaust gas that has not been solidified in the carbon dioxide solidifier; an excess exhaust gas exhaust passage connecting the carbon dioxide solidifier and the chimney; an air separation unit for producing nitrogen and oxygen by cryogenic separation; and an excess exhaust gas branch passage which branches off from the excess exhaust gas discharge passage and supplies the unsolidified excess exhaust gas to an air compressor of the air separation plant.

[0011] (2) The exhaust gas recycling system according to the present invention further comprises: a carbon dioxide solidifier that solidifies and separates carbon dioxide in the combustion exhaust gas by heat exchange between the combustion exhaust gas of a fossil fuel and a refrigerant; an exhaust gas blower that delivers the combustion exhaust gas to the carbon dioxide solidifier; an exhaust gas supply line connecting the exhaust gas blower and the carbon dioxide solidifier; a moisture solidifier that is disposed in the exhaust gas supply passage and solidifies and separates moisture by heat exchange with the refrigerant; a chimney for dissipating excess exhaust gas that has not been solidified in the carbon dioxide solidifier; an air separation unit for producing nitrogen and oxygen by cryogenic separation; and a low-temperature exhaust gas branch line that branches off from the exhaust gas supply line between the moisture solidifier and the carbon dioxide solidifier and supplies the low-temperature exhaust gas from which moisture has been separated by the moisture solidifier to an air compressor of the air separation unit. Effect of the Invention

[0012] The present invention can significantly reduce the energy consumption of the feed air compressor by introducing into the feed air compressor of the cryogenic separation equipment exhaust gas that has been cooled and dehumidified by a refrigerant to solidify the carbon dioxide in the exhaust gas, and that has been pressurized for return to the chimney. [Brief description of the drawings]

[0013] [Figure 1] 1 is a schematic diagram showing an exhaust gas recycling system according to a first embodiment of the present invention. [Diagram 2] FIG. 5 is a schematic diagram showing an exhaust gas recycling system according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] [Embodiment 1] First, the configuration and functions of the first embodiment of the present invention will be described with reference to FIG. The exhaust gas recycling system 1 according to the first embodiment of the present invention includes a first exhaust gas supply pipe 61, an exhaust gas blower 10, a second exhaust gas supply pipe 62, a moisture solidifier 20, a third exhaust gas supply pipe 63, a carbon dioxide solidifier 30, an excess exhaust gas discharge pipe 64, a chimney 70, an excess exhaust gas branch pipe 65, a cryogenic air separation unit 40, an air compressor 41, and an exhaust gas heat exchanger 50.

[0015] The first exhaust gas supply pipe 61 branches off from the flue 66 and is connected to the exhaust gas blower 10 , and introduces a part of the exhaust gas in the flue 65 into the exhaust gas blower 10 . The exhaust gas blower 10 is a typical centrifugal blower, which increases the pressure of the exhaust gas in the flue 66 by an amount equivalent to the pressure loss caused when the exhaust gas is circulated through a series of equipment and returned to the flue.

[0016] The second exhaust gas supply pipe 62 is connected to the exhaust gas blower 10 and the moisture solidification device 20 , and introduces the exhaust gas pressurized by the exhaust gas blower 10 into the moisture solidification device 20 via the exhaust gas heat exchanger 50 .

[0017] The moisture solidifier 20 is a scraping type heat exchanger that cools the exhaust gas by heat exchange with LNG (or its low-temperature evaporated gas) that has been heated through the carbon dioxide solidifier 30, and removes the moisture in the exhaust gas by solidifying it. Ice formed on the surface of the heat transfer tube is scraped off by a mechanical mechanism (not shown) and discharged to the outside.

[0018] The third exhaust gas supply pipe 63 is connected to the moisture solidifier 20 and the carbon dioxide solidifier 30 , and introduces the low-temperature exhaust gas from which the moisture has been cooled and removed in the moisture solidifier 20 into the carbon dioxide solidifier 30 .

[0019] The carbon dioxide solidifier 30 is a scraping-type heat exchanger that further cools the low-temperature exhaust gas that has been cooled and dehydrated in the moisture solidifier 20 through heat exchange with LNG to generate dry ice on the surface of the heat transfer tube, and the generated dry ice is scraped off by a mechanical mechanism not shown.

[0020] The surplus exhaust gas discharge pipe 64 connects the carbon dioxide solidifier 30 to a flue 66 , and introduces the surplus exhaust gas containing nitrogen, oxygen, etc. that was not solidified in the carbon dioxide solidifier 30 into the flue 66 .

[0021] The chimney 70 is a typical steel chimney that dissipates the combustion exhaust gas from a high altitude. The chimney 70 also dissipates the excess exhaust gas introduced through the excess exhaust gas exhaust pipe 64.

[0022] The excess exhaust gas branch pipe 65 branches off from the excess exhaust gas discharge pipe 64 and is connected to the air compressor 41 of the air separation unit 40 , and introduces a portion of the excess exhaust gas flowing inside the excess exhaust gas discharge pipe 64 into the air compressor 41 .

[0023] The air separation unit 40 is a typical cryogenic separation type air separation unit, which produces oxygen, nitrogen, etc. from air pressurized by an air compressor 41 . The air separation unit 40 of the present invention may be one that produces at least oxygen and nitrogen, and may also produce, for example, argon in addition to oxygen and nitrogen.

[0024] The exhaust gas heat exchanger 50 is a typical shell-and-tube type heat exchanger, and exchanges heat between the excess exhaust gas flowing in the excess exhaust gas discharge pipe 64 and the exhaust gas flowing in the second exhaust gas supply pipe 62 .

[0025] Next, the operation of the exhaust gas recycling system 1 according to this embodiment will be described. Exhaust gas discharged from a combustion facility such as a power generation facility (not shown) is released into the atmosphere from a chimney 70, flows through a first exhaust gas supply pipe 61 branched off from the flue 66, and is introduced into the exhaust gas blower 10.

[0026] The exhaust gas introduced into the exhaust gas blower 10 is pressurized by the pressure loss in the flow path, flows through the second exhaust gas supply pipe 62, and is cooled and dehumidified by heat exchange with excess exhaust gas in the exhaust gas heat exchanger 50.

[0027] The exhaust gas cooled and dehumidified in the exhaust gas heat exchanger 50 is introduced into the moisture solidifier 20. The exhaust gas introduced into the moisture solidifier 20 is further cooled by heat exchange with LNG, which is a refrigerant, or NG, which is its evaporated gas. At this time, the exhaust gas is cooled to a low temperature (e.g., -90°C) at which carbon dioxide contained in the exhaust gas does not solidify due to its partial pressure, so that moisture solidifies on the surface of the heat transfer tube. The solidified moisture is scraped off by a mechanical mechanism (not shown) and discharged to the outside.

[0028] The low-temperature flue gas from which moisture has been removed passes through the third flue gas supply pipe 63 and is introduced into the carbon dioxide solidifier 30.

[0029] The low-temperature exhaust gas introduced into the carbon dioxide solidifier 30 is further cooled to a temperature at which carbon dioxide solidifies (for example, -130°C) by heat exchange with the LNG. As a result, the carbon dioxide in the exhaust gas solidifies on the surface of the heat transfer tube. The solidified carbon dioxide is scraped off by a mechanical mechanism (not shown) and discharged to the outside.

[0030] Through the above process, moisture and carbon dioxide are removed from the exhaust gas, and the exhaust gas is cooled to the solidification temperature of carbon dioxide. The exhaust gas in this state flows through the excess exhaust gas exhaust pipe 64, cools the exhaust gas at the exhaust gas blower 10 outlet in the exhaust gas heat exchanger 50, and is heated itself. It is then introduced into the flue 66 and released from the chimney 70.

[0031] While having pressure to flow through excess exhaust gas discharge piping 64, the excess exhaust gas branches off and flows into excess exhaust gas branch piping 65, and is introduced into air compressor 41 in air separation unit 40. The flow rate of the excess exhaust gas introduced into air compressor 41 is determined by the operating status of air separation unit 40.

[0032] As a result, the air compressor 41 pressurizes the surplus exhaust gas that has been cooled to the carbon dioxide solidification temperature and has a pressure required for dissipation treatment from the chimney 70 . Table 1 shows a comparison of the power of the air compressor 41 between the conventional case of pressurizing the atmosphere and the present embodiment of pressurizing the surplus exhaust gas.

[0033] [Table 1]

[0034] As shown in Table 1, the power of a conventional air compressor is 82 kW per ton / h, whereas the power of air compressor 41 according to this embodiment is 28 kW, leading to a reduction of 66%.

[0035] According to the exhaust gas recycling system 1 of this embodiment, after the carbon dioxide is solidified and separated, the excess exhaust gas having pressure for dissipation in the chimney 70 is supplied to the air compressor 41, so that the compression power can be significantly reduced by reducing the pressure rise of the air compressor 41 and improving the compression efficiency.

[0036] Furthermore, since nitrogen, oxygen, etc. can be individually produced as products from the surplus exhaust gas composed of nitrogen, oxygen, etc. by the air separation unit 40, the exhaust gas can be efficiently recycled.

[0037] Furthermore, in this embodiment, in order to prevent moisture from being entrained in the solid carbon dioxide and being carried over to the subsequent carbon dioxide recycling process, moisture is removed by the moisture solidifier 20 before the carbon dioxide is solidified by the carbon dioxide solidification device 30. Although a mechanism for removing moisture is also provided in conventional air separation units, in this embodiment, a considerable amount of moisture is removed by solidifying carbon dioxide, so that the equipment load for removing moisture in the air separation unit 40 can be reduced to zero or significantly reduced.

[0038] [Embodiment 2] Next, the configuration and functions of the second embodiment will be described with reference to FIG. The same numbers are used for components having the same configurations and functions as those in the first embodiment. In the exhaust gas recycling system 100 according to the second embodiment, a low-temperature exhaust gas branch pipe 165 branches off from the third exhaust gas supply pipe 63, and introduces the low-temperature exhaust gas coming out of the moisture solidification device 20 into the air compressor 41 of the air separation unit 40.

[0039] Next, the operation of the exhaust gas recycling system 100 according to this embodiment will be described. Explanations of the same parts as those in the first embodiment will be omitted.

[0040] In the moisture solidification device 20, the exhaust gas is cooled to a low temperature (for example, -90°C) at which carbon dioxide does not solidify due to its partial pressure, and moisture is solidified and removed. Then, the exhaust gas is introduced into the carbon dioxide solidification device 30 through the third exhaust gas supply pipe 63, and a part of the exhaust gas flows through the low-temperature exhaust gas branch pipe 165 branched off from the third exhaust gas supply pipe 63 and is introduced into the air compressor 41.

[0041] Table 2 shows the results of a comparison of the power required for the air compressor between a conventional method of pressurizing the atmosphere and a method of pressurizing the surplus exhaust gas according to the present embodiment. Table 2 also shows the results for the first embodiment.

[0042] [Table 2]

[0043] As shown in Table 2, the power of the conventional air compressor 41 is 82 kW per ton / h, whereas the power of the air compressor 41 according to the present embodiment is 28 kW, resulting in a reduction of 66%.

[0044] According to the flue gas recycling system 100 of this embodiment, the flue gas flowing through the third flue gas supply pipe 63, which has been cooled to about -90°C and has had moisture removed, and which has a higher pressure than in the first embodiment by the amount of the pressure loss in the carbon dioxide solidification device 30, can be compressed by the air compressor 41. Therefore, compared to the flue gas recycling system 1 of the first embodiment, the compression efficiency is reduced due to the higher inlet temperature of the air compressor 41, but the increase in pressure is reduced due to the higher inlet pressure, and as a result, an energy saving effect equivalent to that of the first embodiment is obtained.

[0045] In addition, the air separation unit 40 is also equipped with carbon dioxide removal equipment as in the conventional case, and in the event that the carbon dioxide solidification capacity of the carbon dioxide solidifier 30 is insufficient due to a lack of LNG cold energy or the like, the air separation unit 40 can share the load.

[0046] Although the carbon dioxide solidifier 30 has been described as a scraping-type heat exchanger, the type of carbon dioxide solidifier of the present invention is not limited to a scraping-type heat exchanger, and may be any type that can selectively solidify carbon dioxide by heat exchange with a refrigerant, such as dropping dry ice with an air blower or knocker, or a type in which carbon dioxide is solidified and then pressurized to liquefy and discharged. Similarly, the moisture solidifier 20 is not limited to a scraping-type heat exchanger, and may be any type that can selectively solidify and remove moisture, such as dropping dry ice with an air blower or knocker.

[0047] In addition, although the above description uses LNG as a refrigerant, any refrigerant that is in a liquid or gaseous state depending on the pressure and at a temperature that can solidify the carbon dioxide in the exhaust gas can be used, and single-component fluids such as nitrogen or hydrogen, or multi-component fluids based on fluorocarbons can also be selected as appropriate. [Industrial Applicability]

[0048] The present invention can be used as an exhaust gas recycling system in which carbon dioxide is solidified by heat exchange with a refrigerant, and then separated and recovered, and the exhaust gas is recycled by an air separation unit. [Explanation of symbols]

[0049] 10 Exhaust gas blower 20 Moisture solidifier 30 Carbon dioxide solidifier 40 Air Separation Unit 50 Exhaust gas heat exchanger 61 First exhaust gas supply pipe 62 Second exhaust gas supply pipe 63 Third exhaust gas supply pipe 64 Excess exhaust gas exhaust piping 65 Excess exhaust gas branch piping 66 Flue 70 Chimney 165 Low temperature exhaust gas branch pipe

Claims

1. a carbon dioxide solidifier that solidifies and separates carbon dioxide in the combustion exhaust gas by heat exchange between the combustion exhaust gas of a fossil fuel and a refrigerant; an exhaust gas blower that delivers the combustion exhaust gas to the carbon dioxide solidifier; an exhaust gas supply line connecting the exhaust gas blower and the carbon dioxide solidifier; a chimney for dissipating excess exhaust gas that has not been solidified in the carbon dioxide solidifier; an excess exhaust gas exhaust passage connecting the carbon dioxide solidifier and the chimney; an air separation unit for producing nitrogen and oxygen by cryogenic separation; an excess exhaust gas branching passage that branches off from the excess exhaust gas discharge passage and supplies the unsolidified excess exhaust gas to an air compressor of the air separation plant.

2. a carbon dioxide solidifier that solidifies and separates carbon dioxide in the combustion exhaust gas by heat exchange between the combustion exhaust gas of a fossil fuel and a refrigerant; an exhaust gas blower that delivers the combustion exhaust gas to the carbon dioxide solidifier; an exhaust gas supply line connecting the exhaust gas blower and the carbon dioxide solidifier; a moisture solidifier that is disposed in the exhaust gas supply passage and solidifies and separates moisture by heat exchange with the refrigerant; a chimney for dissipating excess exhaust gas that has not been solidified in the carbon dioxide solidifier; an air separation unit for producing nitrogen and oxygen by cryogenic separation; a low-temperature exhaust gas branch line that branches off from the exhaust gas supply line between the moisture solidifier and the carbon dioxide solidifier, and supplies low-temperature exhaust gas from which moisture has been separated by the moisture solidifier to an air compressor of the air separation unit.

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