Method and system for separating flue gas from natural gas combustion

Abstract

The present application relates to a kind of natural gas combustion flue gas separation utilization method and its system, comprising the following steps: S1, the flue gas to be treated is catalytically oxidized, so that residual oxygen in flue gas is fully reacted and eliminated;S2, flue gas enters preheater, heat exchange is carried out in preheater, so that the heat of flue gas is reduced;S3, compressor is used to compress flue gas, the temperature of flue gas is increased after being compressed by compressor, high temperature flue gas enters heat exchanger and is cooled by cooling water, wherein the water vapor contained is formed into liquid after cooling and discharged etc..The present application has the advantages that: flue gas generated by natural gas combustion is compressed and two-stage expansion refrigeration is used, liquid carbon dioxide and liquid nitrogen are separated in turn. The separation and utilization of natural gas combustion flue gas are realized, and the carbon dioxide discharged to the atmosphere is significantly reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to a natural gas combustion flue gas separation utilization method and system thereof, which is used for the separation of combustion exhaust gas of natural gas, obtains liquid carbon dioxide and liquid nitrogen, turns waste into treasure, and greatly reduces carbon dioxide emissions. BACKGROUND

[0002] Natural gas is a very important energy form, and its use range and share are increasing year by year due to its low carbon and low pollution characteristics. In the field of industrial boilers, civil heating and the like, the main application way with the largest proportion of natural gas is direct combustion to generate heat and exhaust flue gas containing carbon dioxide and water vapor. Under normal circumstances, the flue gas is directly discharged to the atmosphere without any utilization value. SUMMARY

[0003] In order to overcome the defects of the prior art, the present application separates the flue gas generated by natural gas combustion to obtain liquid carbon dioxide and liquid nitrogen, which can be widely used in industrial and civil fields as products; and the amount of finally emitted carbon dioxide is greatly reduced. The present application provides a natural gas combustion flue gas separation utilization method and system thereof, and the technical scheme of the present application is as follows:

[0004] A natural gas combustion flue gas separation utilization method, comprising the following steps:

[0005] S1, catalytically oxidizing the flue gas to be treated to fully react and eliminate the residual oxygen in the flue gas;

[0006] S2, the flue gas enters a preheater to exchange heat and reduce the heat of the flue gas; S3, a compressor is used to compress the flue gas, the temperature of the flue gas is increased after being compressed by the compressor, the high-temperature flue gas enters a heat exchanger to be cooled by cooling water, and the water vapor contained in the flue gas is formed into liquid and discharged after being cooled;

[0007] S4, the flue gas enters a pre-cooler to be frozen, and the water contained in the flue gas is further discharged in the freezing process;

[0008] S5, the frozen gas is divided into two paths, the first path of the gas enters a first expander to drive the first expander to work, and then the pressure and temperature are reduced to continuously generate cold energy; the second path of the gas does not expand to keep the pressure, and the first path of the gas after expansion cooling is cooled with the high-pressure second path of the gas in a first condenser to cool the second path of the gas, wherein the carbon dioxide in the flue gas is liquefied to realize separation and enter a first liquid tank;

[0009] S6, the gas after the carbon dioxide is separated is divided into two paths again, which are the third path gas and the fourth path gas, wherein the third path gas is expanded by the second expander to reduce pressure and temperature, the fourth path gas keeps the pressure and exchanges cold with the third path gas in the second condenser to make the fourth path gas condense in the high pressure and low temperature state, and the nitrogen in the fourth path gas becomes liquid and is stored in the second liquid tank.

[0010] In the steps S5 and S6, the first path gas and the third path gas used as the refrigeration working medium after the completion of the cold exchange and the condensation of the residual gas are collectively referred to as the residual gas, which is introduced into the pre-cooler to be used as the cold source of the pre-cooling link and is increased in temperature in the pre-cooler.

[0011] In the step S4, the residual gas after the preheating is sent to the preheater.

[0012] In the step S1, the catalytic oxidation is performed on the mesh catalytic bed made of the rare metal, the reaction temperature is above 600℃, the catalyst on the catalytic bed is the palladium-carbon, specifically, the palladium powder is attached to the active carbon skeleton to form the granular material with the particle size of 5-10mm, the amount of the natural gas added for the catalytic combustion is determined according to the residual oxygen content in the flue gas, so that the residual oxygen content in the flue gas is below 0.5%, the flue gas oxygen content monitor is used for online monitoring, when the residual oxygen content in the flue gas exceeds the standard, the amount of the natural gas added is increased, otherwise, the amount of the natural gas added is reduced, and the residence time of the catalytic combustion is 3-10s.

[0013] In the step S2, the temperature of the flue gas is reduced to 40-50℃ after the flue gas passes through the preheater, in the step S3, the pressure of the gas compressed by the compressor is 10-15MPa, and in the step S4, the temperature of the frozen flue gas is 1-3℃.

[0014] In the step S5, the temperature of the second path gas after the temperature reduction is -35--30℃, in the step S6, the pressure of the gas after the carbon dioxide is separated is 6MPa, and the temperature is -30--25℃, and the temperature of the third path gas after the expansion, pressure reduction and temperature reduction by the second expander is below -176℃.

[0015] The temperature of the residual gas is increased to -4-0℃ in the pre-cooler, and then the residual gas is sent to the preheater and heated to above 160℃.

[0016] In the step S1, the flue gas to be treated is introduced into the catalytic oxidation space by the fan.

[0017] The present invention also relates to a flue gas separation and utilization system for natural gas combustion, comprising a catalytic oxidation chamber 1, a preheater 2, a compressor 3, a heat exchanger 4, and a precooler 5 connected in sequence by pipelines. The catalytic oxidation chamber 1 is provided with a natural gas inlet. A first pipeline and a second pipeline are connected in parallel at the outlet of the precooler 5. A first expander 6 and a first condenser 7 are connected in series on the first pipeline. The inlet of the first condenser 7 is connected to the second pipeline. The outlet of the first condenser 7 is connected to the inlet of a first liquid tank 8 (for storing liquid carbon dioxide). A third pipeline and a fourth pipeline are connected in parallel at the outlet of the first liquid tank 8. A second expander 9 and a second condenser 10 are connected in series on the third pipeline. The inlet of the second condenser 10 is connected to the fourth pipeline. The outlet of the second condenser 10 is connected to a second liquid tank 11 (for storing liquid nitrogen).

[0018] A first reflux pipe is installed between the first condenser 7 and the second condenser 10 and the precooler 5; a second reflux pipe is installed between the precooler 5 and the preheater 2; and a third reflux pipe is installed between the second liquid tank 11 and the precooler 5.

[0019] A blower 12 is installed at the entrance of the catalytic oxidation chamber 1.

[0020] The advantages of this invention are: it uses compression and two-stage expansion refrigeration to sequentially separate liquid carbon dioxide and liquid nitrogen from the flue gas generated by natural gas combustion. This achieves the separation and utilization of natural gas combustion flue gas and significantly reduces carbon dioxide emissions into the atmosphere. The produced carbon dioxide can be used as dry ice, as a raw material for oilfield injection, urea production, and methanol production; the nitrogen can be used for cryogenic refrigeration, as a raw material for ammonia synthesis, and as a protective gas in chemical production, among other applications; the process is simple and has low energy consumption. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the main structure of the system of the present invention. Detailed Implementation

[0022] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer as a result. However, these embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope of the present invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0023] See Figure 1 A method for separating and utilizing flue gas from natural gas combustion includes the following steps:

[0024] S1. Catalytically oxidize the flue gas to be treated so that the residual oxygen in the flue gas can be fully reacted and eliminated;

[0025] S2. The flue gas enters the preheater and undergoes heat exchange, which reduces the heat of the flue gas.

[0026] S3. The flue gas is compressed by a compressor. After the flue gas is compressed by the compressor, the temperature rises. The high-temperature flue gas enters the heat exchanger and is cooled by cooling water. The water vapor contained in it turns into liquid after cooling and is discharged.

[0027] S4. The flue gas enters the precooler to freeze it. During the freezing process, the moisture contained in the flue gas is further discharged.

[0028] S5. The refrigerated gas is divided into two streams. The first stream of gas enters the first expander, which drives the first expander to do work. Subsequently, the pressure and temperature drop, continuously generating cooling capacity. The second stream of gas maintains its pressure and does not expand. The first stream of gas, after expansion and refrigeration, exchanges heat with the high-pressure second stream of gas in the first condenser to cool the second stream of gas. In this process, the carbon dioxide in the flue gas is liquefied and separated, and enters the first liquid tank.

[0029] S6. After the carbon dioxide is separated, the gas is further divided into two streams, namely the third stream gas and the fourth stream gas. The third stream gas is expanded, depressurized and cooled by the second expander, while the fourth stream gas maintains the pressure and exchanges heat with the third stream gas in the second condenser. This allows the fourth stream gas to condense under high pressure and low temperature, and the nitrogen in it becomes liquid and enters the second liquid tank for storage.

[0030] In steps S5 and S6, the first and third gases, which are used as refrigerants through expansion refrigeration, after completing the heat exchange, and the residual gas after condensation, are collectively referred to as residual gas. This residual gas enters the precooler and is used as the cold source for the precooling process, where its temperature is increased.

[0031] In step S4, the preheated residual gas is sent to the preheater.

[0032] In step S1, the catalytic oxidation is carried out on a mesh catalytic bed made of rare metals at a reaction temperature above 600°C. The catalyst on the catalytic bed is palladium on carbon, specifically palladium powder attached to an activated carbon framework to form particulate matter with a particle size of 5-10 mm. The amount of natural gas added for catalytic combustion is based on the residual oxygen content in the flue gas, ensuring that the volume percentage of residual oxygen in the flue gas is below 0.5%. Online monitoring is performed using a flue gas oxygen content monitoring instrument; when the residual oxygen content in the flue gas exceeds this standard, the amount of supplementary natural gas is increased, and vice versa. The residence time for catalytic combustion is between 3 and 10 seconds.

[0033] In step S2, the flue gas temperature is reduced to 40-50°C after passing through the preheater; in step S3, the compressor compresses the gas at a pressure of 10-15 MPa; in step S4, the temperature of the cooled flue gas is 1-3°C.

[0034] In step S5, the second gas is cooled to a temperature of -35 to -30°C; in step S6, the gas pressure after carbon dioxide separation is 6 MPa and the temperature is -30 to -25°C; the third gas is cooled to below -176°C after expansion, decompression and cooling by the second expander.

[0035] The residual gas is heated to -4 to 0°C in a precooler; it is then sent to a preheater and heated to above 160°C.

[0036] In step S1, the flue gas to be treated is introduced into the catalytic oxidation space by a blower.

[0037] A flue gas separation and utilization system for natural gas combustion, characterized in that it includes a catalytic oxidation chamber, a preheater, a compressor, a heat exchanger, and a precooler connected sequentially by pipelines. A first pipeline and a second pipeline are connected in parallel at the outlet of the precooler. A first expander and a first condenser are connected in series on the first pipeline. The inlet of the first condenser is connected to the second pipeline. The outlet of the first condenser is connected to the inlet of a first liquid tank. A third pipeline and a fourth pipeline are connected in parallel at the outlet of the first liquid tank. A second expander and a second condenser are connected in series on the third pipeline. The inlet of the second condenser is connected to the fourth pipeline. The outlet of the second condenser is connected to a second liquid tank.

[0038] A first reflux pipe is installed between the first condenser and the second condenser and the precooler; a second reflux pipe is installed between the precooler and the preheater; and a third reflux pipe is installed between the second liquid tank and the precooler.

[0039] A blower is installed at the entrance of the catalytic oxidation chamber.

[0040] The working principle of this invention is as follows: The main advantage of this method is that it can simultaneously collect liquid carbon dioxide and liquid nitrogen. The flue gas is collected from a widely used natural gas boiler (the flue gas produced by the combustion of natural gas in the boiler, whose main components include: nitrogen, carbon dioxide, water vapor, residual oxygen, argon, trace amounts of sulfur dioxide, and other trace amounts of inert gases, etc.), wherein the volume ratio of each component is approximately: nitrogen 71%, water vapor 17%, carbon dioxide 8%, oxygen 2%, and other gases 2%, and is sent into this system by a fan.

[0041] After the catalytic oxidation stage, an appropriate amount of natural gas is added to carry out catalytic oxidation. The structure is a mesh catalytic bed made of rare metals, where the oxidation reaction occurs at a surface temperature above 600℃, allowing residual oxygen in the flue gas to fully react and be eliminated.

[0042] After the catalytic oxidation reaction, the gas contains virtually no oxygen, and the carbon dioxide content increases to about 10%. At this point, the flue gas enters the preheater, serving as a heat source for preheating the subsequent gas and transferring heat to it. After heat transfer, the flue gas temperature drops to 40-50°C, a lower temperature that is beneficial for the operation of the subsequent compressor.

[0043] The gas is compressed using a compressor to a pressure of 10–15 MPa. After compression, the gas temperature increases, and it enters a heat exchanger where it is cooled by cooling water. Most of the water vapor contained in the gas condenses into liquid and is discharged after cooling.

[0044] The gas enters the precooler, where it is cooled using lower-temperature gas from the later stage as a cold source. The cooled gas has a temperature of 1–3°C, and any moisture it contains is further removed. After this process, the gas contains approximately 86% nitrogen, 11% carbon dioxide, and about 3% other gases.

[0045] The gas is divided into two streams. The first stream enters the first expander, driving it to do work. Subsequently, the pressure and temperature decrease, and due to the refrigeration effect of adiabatic expansion, it continuously generates cooling. The second stream maintains its pressure and does not expand. After expansion and cooling, the first stream gas exchanges heat with the high-pressure second stream gas in the first condenser, cooling the second stream gas to approximately -30°C. The carbon dioxide in the second stream liquefies and separates, entering the first liquid tank. The gas after carbon dioxide separation contains over 95% nitrogen, less than 1% carbon dioxide, and approximately 4% other gases. (The purpose of using the expander is to recover the kinetic energy of the gas, generating kinetic energy to compensate for the power consumption of the upstream compressor. The expander is preferably coaxially operated with the compressor.)

[0046] After the carbon dioxide is separated, the gas has a pressure of approximately 6 MPa and a temperature of approximately -30°C. It is then split into two streams, referred to as the third and fourth streams. The third stream gas undergoes expansion, decompression, and cooling in a second expander, reaching a temperature below -176°C. The fourth stream gas maintains its pressure and exchanges heat with the third stream gas in a second condenser, causing it to condense under high pressure and low temperature. The nitrogen in the fourth stream becomes liquid and enters a second liquid tank for storage. The purpose of using the expander here is also to recover kinetic energy.

[0047] The residual gases, after the first and third gases, which are used as refrigerants in expansion refrigeration, have completed their heat exchange, as well as the gases remaining after condensation, are collectively referred to as residual gases. These residual gases enter precoolers, where they are used as the cold source for the precooling process, and their temperature is raised to -4 to 0°C. They are then sent to preheaters, where they are heated to above 160°C.

[0048] The preheated residual gas is sent to the upstream section, where it contains less than 50% of the carbon dioxide found in ordinary flue gas. Approximately 80% of this gas is returned to the system for reprocessing, while the remaining 20% ​​is emitted through the chimney. This emission is to prevent the gradual accumulation of inert gases and harmful substances. Ultimately, the total amount of carbon dioxide emitted from the chimney is reduced by more than 90% compared to the original state.

[0049] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for the utilization of flue gas from natural gas combustion, characterized in that, The method comprises the following steps: S1, the flue gas to be treated is subjected to catalytic oxidation, so that the residual oxygen in the flue gas is fully reacted and eliminated; S2, the flue gas enters a preheater, and heat exchange is performed in the preheater, so that the heat of the flue gas is reduced; S3, a compressor is used to compress the flue gas, the temperature of the flue gas is increased after being compressed by the compressor, the high-temperature flue gas enters a heat exchanger, and is cooled by cooling water, and water vapor contained in the flue gas is formed into liquid and discharged after being cooled; S4, the flue gas enters a pre-cooler, and the flue gas is frozen, and the water contained in the flue gas is further discharged in the freezing process; S5, the frozen gas is divided into two paths, the first path of the gas enters a first expander, and drives the first expander to work, and then the pressure and temperature of the first path of the gas are reduced, and cold energy is continuously generated; the second path of the gas is not expanded, the first path of the gas after expansion refrigeration and the second path of the gas at high pressure are subjected to heat exchange in a first condenser, the second path of the gas is cooled, carbon dioxide in the flue gas is liquefied, and separation is realized, and the carbon dioxide enters a first liquid tank; S6, the gas after separation of carbon dioxide is divided into two paths again, which are a third path of the gas and a fourth path of the gas, the third path of the gas is subjected to expansion refrigeration by a second expander, the fourth path of the gas is kept at a constant pressure, and heat exchange is realized between the third path of the gas and the fourth path of the gas in a second condenser, so that the fourth path of the gas is condensed at high pressure and low temperature, nitrogen in the fourth path of the gas is liquefied, and the nitrogen enters a second liquid tank for storage.

2. A natural gas fired flue gas separation utilization method according to claim 1, characterized in that, In the steps S5 and S6, the first path of the gas and the third path of the gas used as refrigeration working medium after completion of cold energy exchange and the residual gas after condensation are collectively referred to as residual gas, the residual gas enters the pre-cooler and is used as a cold source of the pre-cooling link, and the temperature of the residual gas is increased in the pre-cooler.

3. A natural gas fired flue gas separation utilization method according to claim 1, characterized in that, In the step S4, the preheated residual gas is sent to the preheater.

4. A natural gas fired flue gas separation utilization method according to claim 2 or 3, characterized in that, In the step S1, the catalytic oxidation is performed on a mesh-shaped catalytic bed made of a rare metal, the reaction temperature is higher than 600 DEG C, the catalyst on the catalytic bed is palladium-carbon, specifically, the palladium-carbon is a granular material formed by attaching palladium powder to an activated carbon skeleton, the particle size is 5-10 mm, the amount of natural gas added for catalytic combustion is determined according to the residual oxygen content in the flue gas, the residual oxygen content in the flue gas is less than 0.5%, an oxygen content monitoring instrument is used for online monitoring, when the residual oxygen content in the flue gas is greater than 0.5%, the amount of natural gas added is increased, and when the residual oxygen content in the flue gas is less than 0.5%, the amount of natural gas added is reduced, and the residence time of catalytic combustion is 3-10 s.

5. A natural gas fired flue gas separation utilization method according to claim 2 or 3, characterized in that, In the step S2, the temperature of the flue gas after passing through the preheater is reduced to 40-50 DEG C, in the step S3, the pressure of the gas compressed by the compressor is 10-15 MPa, and in the step S4, the temperature of the flue gas after being frozen is 1-3 DEG C.

6. A natural gas fired flue gas separation utilization method according to claim 2, characterized by, The temperature of the residual gas in the pre-cooler is increased to-4-0 DEG C, and then the residual gas is sent to the preheater and heated to higher than 160 DEG C.

7. The method of claim 1, wherein the natural gas is combusted in a furnace. In the step S1, the flue gas to be treated is introduced into the space for catalytic oxidation by a fan.

8. A natural gas fired flue gas separation utilization system for carrying out the method of any one of claims 1 to 7, characterized by The application relates to a catalytic oxidation device, which comprises a catalytic oxidation chamber, a preheater, a compressor, a heat exchanger and a pre-cooler connected in sequence through pipes, a first pipeline and a second pipeline are formed in parallel at the outlet of the pre-cooler, a first expander and a first condenser are connected in series on the first pipeline, the inlet of the first condenser is connected to the second pipeline, the outlet of the first condenser is connected to the inlet of a first liquid tank, a third pipeline and a fourth pipeline are formed in parallel at the outlet of the first liquid tank, a second expander and a second condenser are connected in series on the third pipeline, the inlet of the second condenser is connected to the fourth pipeline, and the outlet of the second condenser is connected to a second liquid tank.

9. A natural gas fired flue gas separation utilization system according to claim 8, wherein, A first backflow pipe is arranged between the first condenser and the second condenser and the pre-cooler; a second backflow pipe is arranged between the pre-cooler and the preheater; and a third backflow pipe is arranged between the second liquid tank and the pre-cooler. A fan is arranged at the inlet of the catalytic oxidation chamber.

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