Method and device for converting carbon dioxide in flue gas into natural gas

Abstract

A device for converting carbon dioxide in flue gas into natural gas using dump energy. The device includes a transformer and rectifier device, an electrolytic cell, a turbine, a carbon dioxide heater, a primary fixed bed reactor, a secondary fixed bed reactor, a natural gas condenser, and a process water line. An outlet of the transformer and rectifier device is connected to a power interface of the electrolytic cell, a gas-liquid outlet of a cathode of the electrolytic cell is connected to a gas-liquid inlet of a hydrogen separator, and a liquid outlet of the hydrogen separator is connected to a liquid reflux port of the cathode of the electrolytic cell.

Claims

1. A device for converting carbon dioxide in flue gas into natural gas using dump energy, the device comprising: a) a transformer and rectifier device; b) an electrolytic cell; c) a turbine; d) a carbon dioxide heater; e) a primary fixed bed reactor; f) a secondary fixed bed reactor; g) a natural gas condenser; and h) a process water line; wherein an outlet of the transformer and rectifier device is connected to a power interface of the electrolytic cell, a gas-liquid outlet of a cathode of the electrolytic cell is connected to a gas-liquid inlet of a hydrogen separator, a liquid outlet of the hydrogen separator is connected to a liquid reflux port of the cathode of the electrolytic cell, a H.sub.2 outlet of the hydrogen separator is connected to an inlet of a hydrogen cooler, both an outlet of the hydrogen cooler and an outlet of the carbon dioxide heater are connected to an inlet of the primary fixed bed reactor; an outlet of the primary fixed bed reactor is connected to an inlet of the secondary fixed bed reactor successively through a superheater and a mixed gas line of a primary heat exchanger, and an outlet of the secondary fixed bed reactor is connected to an inlet of the natural gas condenser successively through a secondary heat exchanger and a mixed gas line of a preheater; and the process water line is connected to an aqueous medium inlet of the preheater, an aqueous medium outlet of the preheater is connected to a steam inlet of the superheater through a steam pocket, a steam outlet of the superheater is connected to a steam inlet of the turbine, and an electric outlet of the turbine is connected to an inlet of the transformer and rectifier device.

2. The device of claim 1, wherein a mixed gas outlet of the primary heat exchanger is provided with a bypass connected to a heat medium inlet of a circulating heat exchanger, a heat medium outlet of the circulating heat exchanger is connected to an inlet of a circulating compressor through a circulating cooler, an outlet of the circulating compressor is connected to a heated medium inlet of the circulating heat exchanger, and a heated medium outlet of the circulating heat exchanger is connected to the inlet of the primary fixed bed reactor.

3. The device of claim 2, wherein an intermediate fixed bed reactor is provided between the primary fixed bed reactor and the secondary fixed bed reactor; an inlet of the intermediate fixed bed reactor is connected to the mixed gas outlet of the primary heat exchanger, and an outlet of the intermediate fixed bed reactor is connected to the inlet of the secondary fixed bed reactor through an intermediate heat exchanger.

4. The device of claim 2, wherein a steam exhaust outlet of the turbine is connected to the process water line through a steam exhaust condenser.

5. The device of claim 2, wherein the process water line is connected to the gas-liquid inlet of the hydrogen separator.

6. The device of claim 2, wherein a condensed water outlet of the natural gas condenser is connected to the aqueous medium inlet of the preheater.

7. The device of claim 1, wherein an intermediate fixed bed reactor is provided between the primary fixed bed reactor and the secondary fixed bed reactor; an inlet of the intermediate fixed bed reactor is connected to a mixed gas outlet of the primary heat exchanger, and an outlet of the intermediate fixed bed reactor is connected to the inlet of the secondary fixed bed reactor through an intermediate heat exchanger.

8. The device of claim 1, wherein a steam exhaust outlet of the turbine is connected to the process water line through a steam exhaust condenser.

9. The device of claim 1, wherein the process water line is connected to the gas-liquid inlet of the hydrogen separator.

10. The device of claim 1, wherein a condensed water outlet of the natural gas condenser is connected to the aqueous medium inlet of the preheater.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a structural diagram of a device for converting carbon dioxide in flue gas into natural gas by dump energy; and

(2) FIG. 2 is a structural diagram of another device for converting carbon dioxide in flue gas into natural gas by dump energy.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(3) The method and device in the invention are further illustrated in detail in the light of the drawings and specific embodiments as follows:

Example 1

(4) A device for converting carbon dioxide into natural gas by dump energy, as shown in FIG. 1, comprises a transformer and rectifier device 1, an electrolytic cell 2, a turbine 4, a carbon dioxide heater 21, a primary fixed bed reactor 13, a secondary fixed bed reactor 11, a natural gas condenser 8 and a process water line 3. The outlet of the transformer and rectifier device 1 is connected to the power interface of the electrolytic cell 2. The gas-liquid outlet of the anode of the electrolytic cell 2 is connected to the gas-liquid inlet of the oxygen separator 20, liquid outlet of the oxygen separator 20 is connected to the liquid reflux port of the anode of the electrolytic cell 2, O.sub.2 outlet of the oxygen separator 20 is connected to the inlet of the oxygen cooler 19, and outlet of the oxygen cooler 19 is connected to a pressurized tank car or a filling device of O.sub.2 (not shown in the figure) for other industrial use. The gas-liquid outlet of the cathode of the electrolytic cell 2 is connected to the gas-liquid inlet of the hydrogen separator 18, and the gas-liquid inlet of the hydrogen separator 18 is also connected to the process water line 3 to supplement water losses. The liquid outlet of the hydrogen separator 18 is connected to the liquid reflux port of the cathode of the electrolytic cell 2, H.sub.2 outlet of the hydrogen separator 18 is connected to the inlet of the hydrogen cooler 17, outlet of the hydrogen cooler 17 is connected to the outlet of the carbon dioxide heater 21 and also connected to the inlet of the primary fixed bed reactor 13, so as to transport fresh H.sub.2 and CO.sub.2 to the primary fixed bed reactor 13.

(5) Outlet of the primary fixed bed reactor 13 is connected to the inlet of the secondary fixed bed reactor 11 successively through a superheater 6 and mixed gas line of a primary heat exchanger 7, the mixed gas outlet of the primary heat exchanger 7 is still provided with a bypass connected to the heat medium inlet of a circulating heat exchanger 16, the heat medium outlet of the circulating heat exchanger 16 is connected to the inlet of a circulating compressor 14 through a circulating cooler 15, the outlet of the circulating compressor 14 is connected to the heated medium inlet of the circulating heat exchanger 16, and the heated medium outlet of the circulating heat exchanger 16 is connected to the inlet of the primary fixed bed reactor 13.

(6) Outlet of the secondary fixed bed reactor 11 is successively connected to the inlet of a natural gas condenser 8 through a secondary heat exchanger 10 and mixed gas line of a preheater 9. The process water line 3 is connected to the aqueous medium inlet of the preheater 9, aqueous medium outlet of the preheater 9 is connected to the steam inlet of the superheater 6 through a steam pocket 12, the steam outlet of the superheater 6 is connected to the steam inlet of a turbine 4, and the steam exhaust outlet of the turbine 4 is connected to the process water line 3 through a steam exhaust condenser 5, and the electric outlet of the turbine 4 is connected to the inlet of the transformer and rectifier device 1 to provide electric energy for water electrolysis. In addition, the condensed water outlet of the natural gas condenser 8 may also be connected to the aqueous medium inlet of the preheater 9 (not shown in the figure) to send the condensed water back to the system for recycling.

(7) The process flow of the above device for converting carbon dioxide in flue gas into natural gas by dump energy is as follows:

(8) Dump energy arising from renewable energy generation, such as solar energy, hydroenergy or wind energy etc., is converted to required current through the transformer and rectifier device 1 to provide working power supply for the electrolytic cell 2. Potassium hydroxide solution with the density of 1.2-1.4 kg/m.sup.3 is used as the electrolyte solution within the electrolytic cell 2, and the reaction temperature is controlled at 90±2° C. Here, anode and cathode of the electrolytic cell 2 respectively generate O.sub.2 and H.sub.2 carrying the electrolyte solution. The electrolyte solution is removed from O.sub.2 generated therein with an oxygen separator 20, and is transported back to the electrolytic cell 2 to further participate in the reaction. Afterwards, O.sub.2 is cooled in an oxygen cooler 19 to 45° C. or so for water removal, and then delivered to a pressurized tank car or a filling device for industrial use. The electrolyte solution is removed from H.sub.2 generated therein with a hydrogen separator 18, and is transported back to the electrolytic cell 2 to further participate in the reaction. Afterwards, H.sub.2 is cooled in a hydrogen cooler 17 to 45° C. or so for water removal, and then enters the reaction in the next step. Water losses in electrolysis is introduced into the hydrogen separator 18 through the process water line 3, is then supplemented to the electrolytic cell 2, and is also used to cool the heat generated in the water electrolysis process.

(9) Meanwhile, CO.sub.2 trapped from flue gas is purified, introduced into the carbon dioxide heater 21, heated, and mixed with H.sub.2 purified through water removal at the volume ratio of H.sub.2:CO.sub.2=4:1 to fresh gas, which is transported to the primary fixed bed reactor 13 for strong exothermic reaction (methanation). In order to control the reaction heat of methanation of H.sub.2 and CO.sub.2, certain amount of CH.sub.4 may be added into the CO.sub.2 heater 21 generally at the volume ratio of H.sub.2:CO.sub.2:CH.sub.4=4:1:0.5. Addition of CH.sub.4 can be stopped after the reaction is stable. The primary fixed bed reactor 13 is kept at the inlet temperature of 250-300° C., reaction pressure of 3-4 MPa, and outlet temperature of 600-700° C. In the presence of a nickel-based catalyst, most H.sub.2 reacts with CO.sub.2 to generate high-temperature mixed gas of CH.sub.4 and water vapor. The high-temperature mixed gas is cooled to 250-300° C. successively through the superheater 6 and primary heat exchanger 7, and then divided into two parts. Where, a part of high-temperature mixed gas enters a circulating cooler 15 through the heat medium line of the circulating heat exchanger 16, cooled to 30-40° C. after heat exchange, pressurized to 3-4 MPa and heated to 180-200° C. with a circulating compressor 14, finally further heated to 250-300° C. through the heated medium line of the circulating heat exchanger 16, and mixed with fresh H.sub.2 and CO.sub.2 at such a ratio that the volume content of CO.sub.2 in the mixed gas is 6-8%. The mixed gas is transported to the primary fixed bed reactor 13, and the cycle is repeated. Preheating fresh H.sub.2 and CO.sub.2 in above circulation can greatly reduce energy consumption and control the outlet temperature of the primary fixed bed reactor 13. Another part of high-temperature mixed gas is introduced into the secondary fixed bed reactor 11, which is kept at the inlet temperature of 250-300° C., reaction pressure of 3-4 MPa, and outlet temperature of 350-500° C., so that the unreacted H.sub.2 and CO.sub.2 therein continue to complete the strong exothermic reaction (methanation), until complete reaction of all raw materials.

(10) The high-temperature mixed gas of CH.sub.4 and water vapor from the secondary fixed bed reactor 11 is cooled successively through a secondary heat exchanger 10 and a preheater 9, further cooled through a natural gas condenser 8, where gas CH.sub.4 is cooled to 45-50° C., and flows out from the gas output of the natural gas condenser 8. CH.sub.4 with the purity of more than 94% is pressurized to SNG/LNG (natural gas/liquefied natural gas), and is transported through pipeline to the existing pipe network/tank car for storage and use; while the condensed water therein flows out from the condensed water output of the natural gas condenser 8, and is transported to the aqueous medium inlet of the preheater 9 for recycling.

(11) In the above strong exothermic reaction process of methanation, the process water is introduced into the preheater 9 through the process water line 3, and is heated to superheated water through heat exchange therein. Superheated water is transported to a steam pocket 12 through pipeline to evaporate into water vapor therein. Water vapor is transported to the superheater 6 through pipeline to convert to superheated water vapor under given pressure by further heating. The superheated vapor enters the turbine 4 through pipeline, the high-speed superheated water vapor drives the blades of the turbine 4 to rotate for power generation, the generated energy returns to the transformer and rectifier device 1 for voltage transformation, rectification, and further use for water electrolysis, so as to make full use of the waste heat in the strong exothermic reaction of methanation. The steam exhaust generated after the turbine is driven for power generation is transported to a steam exhaust condenser 5, and is condensed to water, which is transported back to the process water line 3 for recycling.

Example 2

(12) Another device for converting carbon dioxide into natural gas by dump energy, as shown in FIG. 2, has the structure and process flow basically the same as that in Example 1, except that an intermediate fixed bed reactor 22 is provided between the primary fixed bed reactor 13 and the secondary fixed bed reactor 11. The inlet of the intermediate fixed bed reactor 22 is connected to the mixed gas outlet of the primary heat exchanger 7, and the outlet of the intermediate fixed bed reactor 22 is connected to the inlet of the secondary fixed bed reactor 11 through an intermediate heat exchanger 23. In this way, three stage fixed bed reactors are provided, so as to distribute the methanation reaction rate of H.sub.2 and CO.sub.2 in three stages, and ensure complete reaction of the raw materials. At the same time, inlet and outlet temperature of the three stage fixed bed reactors can be reduced successively, so as to obtain corresponding quality of steam (temperature, pressure), and meet the needs of the turbine 4.