A method for co-preparing nitrogen-doped porous carbon to capture CO2 from flue gas using microalgae to fix CO2 from flue gas.

Abstract

This invention provides a method for co-preparing nitrogen-doped porous carbon to capture CO2 from flue gas using microalgae fixation of flue gas. The method includes: feeding a first stream of flue gas into a vertical microalgae reactor to cultivate microalgae using the CO2 in the flue gas, obtaining a microalgae feed solution, which is then dried to obtain algae powder; sequentially subjecting the algae powder to pore formation, nitrogen doping, activation, and washing to obtain nitrogen-doped porous carbon; and feeding a second stream of flue gas into an adsorption tower to capture CO2 from the flue gas using an adsorbent comprising nitrogen-doped porous carbon. This method utilizes a vertical microalgae reactor for microalgae cultivation, significantly reducing the required floor space. Simultaneously, using the cultivated microalgae as a biomass raw material for activated carbon adsorbents increases the efficiency of biological CO2 utilization while significantly reducing the preparation cost of adsorbents, increasing adsorption volume, and integrating microalgae carbon fixation and adsorption-based carbon capture processes, thereby reducing the operating cost of the carbon capture system.

Description

Technical Field

[0001] This invention relates to the field of carbon capture technology, and more specifically, to a method for co-preparing nitrogen-doped porous carbon to capture CO2 from flue gas by fixing CO2 in microalgae. Background Technology

[0002] It is estimated that CO2 accounts for 68% of all greenhouse gas emissions, making it one of the main contributors to global warming. Currently, efforts are needed to peak CO2 emissions before 2030 and strive to achieve carbon neutrality before 2060. CO2 capture methods can be broadly categorized into physical methods, absorption methods, adsorption methods, and biological methods. Chemical absorption is a relatively safe and suitable method for long-term CO2 fixation. The key to CO2 emission reduction lies in CCUS (carbon capture, utilization, and storage).

[0003] Adsorption is widely considered a promising technology for capturing low-concentration CO2 due to its advantages such as low regeneration energy consumption, a wide variety of adsorbents, long service life, easy regeneration, no byproducts, and low operating costs. Adsorption is further divided into physical adsorption and chemical adsorption. Physical adsorption is simpler to operate, has better adsorbent regeneration performance, lower energy consumption, and is more widely used. However, the core of adsorption lies in the selection of the adsorbent. High-efficiency and high-quality adsorbents need to possess advantages such as large gas adsorption capacity, good selectivity, fast adsorption-desorption rates, and low preparation and regeneration costs. Research and development of adsorbents has always been a key focus in CO2 capture using adsorption methods. Currently, commonly used adsorbents include zeolite molecular sieves, MOFs, carbon nanotubes, graphene, activated carbon, and activated carbon fibers. Taking activated carbon as an example, some traditional materials used in its production are mainly wood and coal, which are relatively expensive and have high costs.

[0004] Compared to absorption and adsorption CO2 capture technologies, microalgae CO2 fixation is also a promising carbon reduction pathway. Current reports indicate that microalgae in the exponential growth phase of an aerated photosynthetic reactor can achieve an energy conversion efficiency of 5%–7%, realizing a sustainable cycle from solar energy to fuel and the resource utilization of CO2. Microalgae have a short lifespan (3–7 days), strong environmental adaptability, and can be further processed into food, feed, cosmetics, nutritional products, pharmaceuticals, and biofuels, offering significant economic benefits. Currently, there is a wide variety of microalgae in nature, with many promising candidate species for large-scale cultivation to reduce CO2 emissions from flue gas. Furthermore, the carbon tolerance of wild-type microalgae strains can be improved through mutagenesis and domestication, providing feasibility for fixing CO2 from coal-fired power plants. Microalgae capture and fixation of CO2 is a highly promising carbon reduction pathway. However, the bottleneck of microalgae CO2 fixation technology from coal-fired power plants is the large footprint and low CO2 utilization efficiency of traditional racetrack-type microalgae reactors, which cannot meet the CO2 capture requirements of large-scale coal-fired power plants. Summary of the Invention

[0005] The main objective of this invention is to provide a method for co-preparing nitrogen-doped porous carbon to capture CO2 from flue gas by fixing CO2 in microalgae, in order to solve the problems of high cost of adsorbent preparation and small adsorption capacity in the adsorption method for capturing CO2 in flue gas in the prior art; and the large footprint and low CO2 utilization efficiency of traditional racetrack-type microalgae reactors when fixing CO2 in microalgae.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for co-preparing nitrogen-doped porous carbon to capture CO2 from flue gas using microalgae fixation of flue gas is provided. The flue gas is divided into two parts, namely a first flue gas and a second flue gas. The method includes the following steps: Step S1, the first flue gas is fed into a vertical microalgae reactor to cultivate microalgae using CO2 in the flue gas to obtain microalgae feed liquid, which is then dried to obtain algae powder; Step S2, the algae powder is sequentially subjected to pore formation, nitrogen doping, activation, and washing to obtain nitrogen-doped porous carbon; Step S3, the second flue gas is fed into an adsorption tower to capture CO2 in the flue gas using an adsorbent, the adsorbent including nitrogen-doped porous carbon.

[0007] Further, step S2 includes: step S21, performing a first pyrolysis on algae powder under a nitrogen atmosphere to create pores and obtain porous carbon; step S22, performing a first mixing of porous carbon with a nitrogen source and water, drying and then performing a second pyrolysis under an air atmosphere to dope nitrogen, washing with water and drying to obtain nitrogen-doped material; step S23, performing a second mixing of nitrogen-doped material with solid alkali and water, drying and then performing a third pyrolysis under a nitrogen atmosphere to activate material, obtaining activated material; step S24, performing acid washing and water washing sequentially to wash until the washing liquid is neutral, drying to obtain nitrogen-doped porous carbon.

[0008] Furthermore, in step S21, the nitrogen flow rate for the first pyrolysis is 5–10 L / min, the temperature is 800–900 °C, and the time is 30–60 min.

[0009] Further, in step S22, the nitrogen source includes one or more of urea, molasses, and peptone; the weight ratio of porous carbon to nitrogen source is 1:(0.8-1.2), the solid-liquid ratio of porous carbon to water is (25-50 kg):(250-500 L); and / or the first mixing time is 2-5 h; and / or the second pyrolysis temperature is 300-400 °C, and the time is 80-100 min.

[0010] Further, in step S23, the solid alkali includes KOH and / or NaOH; the weight ratio of nitrogen dopant to solid alkali is 1:(1.8-2.2), the solid-liquid ratio of nitrogen dopant to water is (25-50 kg):(500-1000 L); and / or the second mixing time is 100-140 min; and / or the nitrogen flow rate for the third pyrolysis is 5-10 L / min, the temperature is 550-650 °C, and the time is 1-2 h.

[0011] Further, in step S24, acid washing is performed using one or more of hydrochloric acid, sulfuric acid, and phosphoric acid at a concentration of 0.8–1.2 mol / L.

[0012] Furthermore, the nitrogen-doped porous carbon has a pore size of 2–250 nm, a porosity of 60–95%, and a specific surface area of ​​200–5000 m². 2 / g, nitrogen doping amount is 2-20wt.%.

[0013] Further, in step S1, the microalgae include one or more of Chlorella, Microcystis, and Arthrospira; and / or the flow rate of the first flue gas is 2–10 Nm³. 3 / h; and / or the operating temperature of the vertical microalgae reactor is 25-35℃, the light intensity is 2200-17500 lux, and the photoperiod is 8-12h; and / or the nutrient solution of the vertical microalgae reactor includes 3mL 35% w / v HCl, 6.0g / L NaHCO3, 7.5g / L Na2CO3, 3.5g / L NaNO3, 1g / L K2HPO4, 0.1g / L MgSO4·7H2O, 0.5g / L K2SO4, 0.01g / L FeSO4·7H2O, 0.08g / L Na2EDTA, and 0.04g / L CaCl2.

[0014] Furthermore, in step S3, the flow rate of the second flue gas is 200–1000 Nm³. 3 / h; and / or the loading of nitrogen-doped porous carbon is 100-500 kg, and the operating temperature of the adsorption tower is 0-35℃.

[0015] Furthermore, the volume percentage of CO2 in the flue gas is 3-15%, and the volume percentage of H2O is 5-20%; preferably, the flue gas includes one or more of coal-fired flue gas, gas-fired flue gas, biomass boiler flue gas, steel plant flue gas, and building materials plant flue gas.

[0016] The technical solution of this invention first utilizes microalgae, which are highly tolerant to coal-fired flue gas, to fix a portion of the CO2 in the flue gas. Simultaneously, the CO2 in the flue gas is used to cultivate the microalgae on a large scale. Then, the resulting microalgae are used as biomass raw material for producing activated carbon adsorbents, preparing nitrogen-doped porous carbon with high CO2 adsorption capacity. This carbon is then provided as an adsorbent in the adsorption-based flue gas CO2 capture process. This method allows for the use of vertical microalgae reactors for microalgae cultivation, significantly reducing the floor space required. Using the cultivated microalgae as biomass raw material for activated carbon adsorbents increases the efficiency of biological CO2 utilization while greatly reducing the preparation cost of the adsorbent. Furthermore, the nitrogen-doped porous carbon prepared through pore formation, nitrogen doping, activation, and washing exhibits a significantly increased adsorption volume and thus increased adsorption capacity. This method integrates microalgae carbon fixation and adsorption-based carbon capture processes. By cultivating microalgae in flue gas and then preparing the adsorbent, the preparation cost of the adsorbent in carbon capture is reduced, greatly improving the economics of carbon capture and lowering the operating cost of the carbon capture system. Attached Figure Description

[0017] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0018] Figure 1 A schematic diagram of a carbon capture system according to an embodiment of the present invention is shown.

[0019] The above figures include the following reference numerals:

[0020] 11. Coal-fired power generating unit; 12. Pre-washing tower; 21. Vertical microalgae reactor 1; 22. Vertical microalgae reactor 2; 23. Vertical microalgae reactor 3; 24. High-temperature reactor; 31. Absorption tower 1; 32. Absorption tower 2; 33. Absorption tower 3; 34. Regenerated gas cooler; 35. Regenerated gas separator; 36. Wastewater collection tank; 37. Desuperheating and depressurization device; A. Flue gas; A1. First flue gas; A2. Second flue gas; B. Dried algae powder; C. Nitrogen-doped porous carbon; D. Demineralized water from the power plant; E. Steam from the low-pressure cylinder of the power plant. Detailed Implementation

[0021] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0022] As described in the background section of this invention, existing technologies suffer from problems such as high adsorbent preparation costs and low adsorption capacity when using adsorption to capture CO2 from flue gas; and large footprint and low CO2 utilization efficiency when using microalgae to fix CO2 from flue gas. To address these issues, in a typical embodiment of this invention, a method for co-preparing nitrogen-doped porous carbon to capture CO2 from flue gas using microalgae fixation of flue gas is provided. The flue gas is divided into two parts, a first flue gas and a second flue gas. The method includes the following steps: Step S1, the first flue gas is fed into a vertical microalgae reactor to cultivate microalgae using CO2 in the flue gas, obtaining a microalgae feed solution, which is then dried to obtain algae powder; Step S2, the algae powder is sequentially subjected to pore formation, nitrogen doping, activation, and washing to obtain nitrogen-doped porous carbon; Step S3, the second flue gas is fed into an adsorption tower to capture CO2 from the flue gas using an adsorbent, which includes nitrogen-doped porous carbon.

[0023] The inventors unexpectedly discovered during their research that microalgae, due to their rapid growth rate and high carbon content, possess the potential for large-scale production of bio-activated carbon through high-temperature pyrolysis and carbonization. This invention divides flue gas into two parts. First, the first stream of flue gas is fed into a vertical microalgae reactor. Microalgae, highly tolerant of coal-fired flue gas, immobilize a portion of the CO2 in the flue gas, while simultaneously using the CO2 in the flue gas to cultivate the microalgae on a large scale, obtaining a microalgae slurry. After drying, algae powder is obtained, which serves as a biomass raw material for producing activated carbon adsorbents. Then, the algae powder undergoes sequential pore-forming, nitrogen-doping, activation, and washing to obtain nitrogen-doped porous carbon with high CO2 adsorption capacity, which is provided as an adsorbent in the adsorption-based flue gas CO2 capture process. Finally, the second stream of flue gas is fed into an adsorption tower to efficiently capture CO2 from the flue gas using the nitrogen-doped porous carbon.

[0024] The method of this invention utilizes a vertical microalgae reactor for microalgae cultivation, significantly reducing the required floor space. It uses CO2 from flue gas as the feed gas for microalgae growth, effectively fixing CO2 from coal-fired flue gas while simultaneously cultivating microalgae on a large scale. The resulting microalgae and other biomass products are then used as raw materials to synthesize activated carbon, which is used as an adsorbent in adsorption-based CO2 capture processes. This method increases the efficiency of biological CO2 utilization while significantly reducing the preparation cost of the adsorbent. Furthermore, the nitrogen-doped porous carbon prepared through pore formation, nitrogen doping, activation, and washing exhibits a significantly increased adsorption volume and thus an increased adsorption capacity. This invention integrates microalgae carbon fixation and adsorption-based carbon capture processes. By cultivating microalgae in flue gas and then preparing the adsorbent, the preparation cost of the adsorbent in carbon capture is reduced, greatly improving the economics of carbon capture and lowering the operating cost of the carbon capture system. The number of vertical microalgae reactors and adsorption towers can be adjusted as needed; for example, 1 to 3 vertical microalgae reactors and 1 to 3 adsorption towers can be used.

[0025] In a preferred embodiment, step S2 includes: step S21, performing a first pyrolysis on algae powder under a nitrogen atmosphere to create pores and obtain porous carbon; step S22, performing a first mixture of porous carbon, nitrogen source, and water, drying, and then performing a second pyrolysis under an air atmosphere to dope nitrogen, washing with water, and drying to obtain nitrogen-doped material; step S23, performing a second mixture of nitrogen-doped material, solid alkali, and water, drying, and then performing a third pyrolysis under a nitrogen atmosphere to activate material, obtaining activated material; step S24, performing acid washing and water washing sequentially to wash until the washing liquid is neutral, drying to obtain nitrogen-doped porous carbon.

[0026] Specifically, the algal powder is first pyrolyzed under a nitrogen atmosphere to create pores, resulting in porous carbon. Then, the porous carbon is mixed with a nitrogen source and water, dried, and then pyrolyzed again in air to increase the nitrogen doping degree of the porous carbon. After washing and drying, residual nitrogen source is removed, yielding nitrogen-doped material. Next, the nitrogen-doped material is mixed with solid alkali and water, dried, and then pyrolyzed again under a nitrogen atmosphere. Heating activation further increases the porosity of the porous carbon, yielding activated material. Finally, the activated material is sequentially acid-washed and water-washed until the washing liquid is neutral, and then dried to obtain nitrogen-doped porous carbon with high CO2 adsorption capacity. This preparation method is simple and easy to operate. It can convert microalgae cultivated in flue gas into porous carbon for CO2 capture in flue gas, further improving CO2 utilization and reducing the preparation cost of the carbon capture adsorbent, making it more suitable for industrial operation.

[0027] In a preferred embodiment, in step S21, the nitrogen flow rate for the first pyrolysis is 5–10 L / min, the temperature is 800–900 °C, and the time is 30–60 min. Adjusting these parameters can optimize the pyrolysis and carbonization of carbon-based functional groups within the microalgae, thereby improving the pore structure and adsorption performance of the activated carbon and further optimizing CO2 capture efficiency.

[0028] To further optimize the nitrogen content and pore structure of activated carbon prepared from microalgae, in a preferred embodiment, in step S22, the nitrogen source includes one or more of urea, molasses, and peptone; the weight ratio of porous carbon to nitrogen source is 1:(0.8-1.2), the solid-liquid ratio of porous carbon to water is (25-50 kg):(250-500 L); and / or the first mixing time is 2-5 h; and / or the second pyrolysis temperature is 300-400 °C, and the time is 80-100 min. Within the above parameter range, the nitrogen doping content in the porous carbon can be further improved by adjusting the mass ratio of porous carbon to nitrogen source and the pyrolysis time.

[0029] To more quickly neutralize the adsorbent, in a preferred embodiment, in step S23, the solid alkali includes KOH and / or NaOH; the weight ratio of nitrogen-doped material to solid alkali is 1:(1.8–2.2), and the solid-liquid ratio of nitrogen-doped material to water is (25–50 kg):(500–1000 L); and / or the second mixing time is 100–140 min; and / or the nitrogen flow rate for the third pyrolysis is 5–10 L / min, the temperature is 550–650 °C, and the time is 1–2 h. Within the above parameter range, the pH of the adsorbent can be adjusted to improve the surface active sites of the adsorbent, thereby making it more conducive to the selective adsorption of CO2.

[0030] In a preferred embodiment, in step S24, acid washing is performed using one or more of hydrochloric acid, sulfuric acid, and phosphoric acid at a concentration of 0.8–1.2 mol / L, thereby further improving the purity and performance of the material.

[0031] In the preparation of nitrogen-doped porous carbon, the drying temperature for each step can be 90–100℃ and the time can be 12–24 h, in order to adjust the moisture content and storage stability of the material.

[0032] As described above, nitrogen-doped porous carbon with high CO2 adsorption capacity can be obtained by using microalgae as raw material through pore formation, nitrogen doping, activation, and washing. In a preferred embodiment, the pore size of the nitrogen-doped porous carbon is 2–250 nm, the porosity is 60–95%, and the specific surface area is 200–5000 m². 2 With a nitrogen doping content of 2–20 wt.%, it exhibits significantly improved CO2 adsorption performance and significantly reduced preparation cost.

[0033] In a preferred embodiment, in step S1, the microalgae include one or more of Chlorella, Microcystis, and Arthrospira; compared to other species, the above-mentioned microalgae have a faster growth rate, higher carbon content, and stronger tolerance to flue gas; and / or the flow rate of the first flue gas is 2-10 Nm³. 3 / h; and / or the operating temperature of the vertical microalgae reactor is 25–35℃, the light intensity is 2200–17500 lux, and the photoperiod is 8–12h; and / or the nutrient solution of the vertical microalgae reactor includes 3 mL 35% w / v HCl, 6.0 g / L NaHCO3, 7.5 g / L Na2CO3, 3.5 g / L NaNO3, 1 g / L K2HPO4, 0.1 g / L MgSO4·7H2O, 0.5 g / L K2SO4, 0.01 g / L FeSO4·7H2O, 0.08 g / L Na2EDTA, and 0.04 g / L CaCl2. Under the above conditions, it is beneficial to further cultivate microalgae with stronger tolerance to flue gas, so as to fully fix CO2 in flue gas and optimize CO2 fixation efficiency and activated carbon yield.

[0034] To fully utilize the adsorption properties of nitrogen-doped porous carbon and further improve carbon capture efficiency, in a preferred embodiment, the flow rate of the second flue gas in step S3 is 200–1000 Nm³. 3 / h; and / or the loading of nitrogen-doped porous carbon is 100-500 kg, and the operating temperature of the adsorption tower is 0-35℃.

[0035] In a preferred embodiment, the volume percentage of CO2 in the flue gas is 3-15%, and the volume percentage of H2O is 5-20%. Preferably, the flue gas includes one or more of coal-fired flue gas, gas-fired flue gas, biomass boiler flue gas, steel plant flue gas, and building materials plant flue gas, or other industrial waste gas rich in CO2, to increase the application scope of the present invention.

[0036] Typically, but not limitingly, in step S1, the flow rate of the first flue gas is 2 Nm³. 3 / h、3Nm 3 / h、4Nm 3 / h、5Nm 3 / h、6Nm 3 / h、7Nm 3 / h、8Nm 3 / h、9Nm 3 / h, 10Nm 3 / h or a range of values ​​consisting of any two of its values.

[0037] Typically, but not limitingly, in step S21, the nitrogen flow rate for the first pyrolysis is 5 L / min, 6 L / min, 7 L / min, 8 L / min, 9 L / min, 10 L / min, or any two of these values; the temperature is 800℃, 810℃, 820℃, 830℃, 840℃, 850℃, 860℃, 870℃, 880℃, 890℃, 900℃, or any two of these values; and the time is 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or any two of these values.

[0038] Typically, but not limitingly, in step S22, the weight ratio of porous carbon to nitrogen source is 1:0.8, 1:0.9, 1:1.0, 1:1.1, 1:1.2, or any two of these values; the temperature of the second pyrolysis is 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, or any two of these values; and the time is 80 min, 85 min, 90 min, 95 min, 100 min, or any two of these values.

[0039] Typically, but not limitingly, in step S23, the weight ratio of nitrogen-doped material to solid alkali is 1:1.8, 1:1.9, 1:2.0, 1:2.1, 1:2.2, or any two of these values; the nitrogen flow rate for the third pyrolysis is 5 L / min, 6 L / min, 7 L / min, 8 L / min, 9 L / min, 10 L / min, or any two of these values; the temperature is 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, or any two of these values; and the time is 1h, 1.2h, 1.5h, 1.8h, 2h, or any two of these values.

[0040] Typically, but not limitingly, in step S3, the flow rate of the second flue gas is 200 Nm³. 3 / h, 300Nm 3 / h, 400Nm 3 / h, 500Nm 3 / h, 600Nm 3 / h, 700Nm 3 / h, 800Nm 3 / h, 900Nm 3 / h, 1000Nm 3 / h or any two of these values; the loading of nitrogen-doped porous carbon is 100kg, 200kg, 300kg, 400kg, 500kg or any two of these values; and the operating temperature of the adsorption tower is 0℃, 10℃, 15℃, 20℃, 25℃, 30℃, 35℃ or any two of these values.

[0041] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0042] Example 1

[0043] A schematic diagram of a carbon capture system is shown below. Figure 1As shown, flue gas A from coal-fired power generation unit 11 is separated into two streams after being purified by pre-washing tower 12. The first stream, flue gas A1, serves as the carbon source for vertical microalgae reactors 1 21, 2 22, and 3 23. The vertical microalgae reactors are used as devices for capturing CO2 from the flue gas, and the CO2 in the flue gas is used to cultivate microalgae. The microalgae cultivated based on the CO2 in the flue gas are dried to obtain dried algae powder B, which is then sent to a high-temperature reactor 24 for carbonization (where the high-temperature reactor 24 is a schematic diagram of multiple devices). Specifically, the process involves pore formation, nitrogen doping, activation, and washing to obtain nitrogen-doped porous carbon C, which serves as a CO2 adsorbent. Another stream of flue gas from the coal-fired unit (second flue gas A2) is sent to absorption towers 1-31, 2-32, and 3-33. The adsorbent used is nitrogen-doped porous carbon prepared based on microalgae, which captures CO2 in the flue gas. Steam E from the low-pressure cylinder of the power plant is de-cooled and de-pressurized by desuperheating and demineralization device 37, and then mixed with demineralized water D from the power plant before being sent to each absorption tower. The absorbed liquid in the tower, along with the demineralized water D, is sent to the regeneration gas cooler 34 for cooling, and then to the regeneration gas separator 35. Excess demineralized water D from the power plant is sent to the wastewater collection pond 36 for collection and treatment. (The number of vertical microalgae reactors and adsorption towers can be adjusted according to actual conditions, which is understandable to those skilled in the art and will not be elaborated further here.)

[0044] The flue gas from the coal-fired power plant (with a volume percentage of 11.13% CO2 and 10.65% H2O) was divided into two parts: the first flue gas and the second flue gas.

[0045] Step S1, the first stream of flue gas (flow rate of 6 Nm³) 3 The microalgae (including Chlorella, Microspiral, and Arthrospira) are fed into a vertical microalgae reactor (operating temperature 30℃, light intensity 10000 lux, photoperiod 10h, nutrient solution including 3mL 35% w / v HCl, 6.0g / L NaHCO3, 7.5g / L Na2CO3, 3.5g / L NaNO3, 1g / L K2HPO4, 0.1g / L MgSO4·7H2O, 0.5g / L K2SO4, 0.01g / L FeSO4·7H2O, 0.08g / L Na2EDTA, 0.04g / L CaCl2) to utilize CO2 in the flue gas for microalgae cultivation (microalgae include Chlorella, Microspiral, and Arthrospira) to obtain microalgae feed solution, which is then dried to obtain algae powder.

[0046] Step S2 involves sequentially performing pore-forming, nitrogen-doping, activation, and washing of the algae powder to obtain nitrogen-doped porous carbon, as detailed below:

[0047] Step S21: Place the algae powder into a quartz tube furnace and perform the first pyrolysis under a nitrogen atmosphere (nitrogen flow rate of 8L / min, temperature of 850℃, time of 45min) to create pores and obtain porous carbon.

[0048] Step S22: The porous carbon, urea, and water are first mixed (the weight ratio of porous carbon to urea is 1:1, the solid-liquid ratio of porous carbon to water is 35kg:350L, and the mixing time is 3.5h). After drying, the mixture is placed in a quartz tube furnace and subjected to a second pyrolysis in an air atmosphere (temperature is 350℃, time is 90min) to perform nitrogen doping. The mixture is then washed with water and dried to obtain the nitrogen-doped material.

[0049] Step S23: The nitrogen-doped material, solid KOH, and water are mixed for the second time (the weight ratio of nitrogen-doped material to solid KOH is 1:2, the solid-liquid ratio of nitrogen-doped material to water is 35kg:700L, and the mixing time is 120min). After drying, the mixture is placed in a quartz tube furnace and subjected to a third pyrolysis under a nitrogen atmosphere (nitrogen flow rate is 8L / min, temperature is 600℃, and time is 1.5h) to activate the material and obtain the activated material.

[0050] Step S24: The activated material is sequentially acid-washed (with 1 mol / L hydrochloric acid) and washed with water until the washing solution is neutral. After drying, nitrogen-doped porous carbon (pore size 130 nm, porosity 80%, specific surface area 2600 m²) is obtained. 2 / g, nitrogen doping content is 11wt.%).

[0051] Step S3, the second flue gas (flow rate of 600 Nm) 3 The CO2 in the flue gas is fed into an adsorption tower (with a nitrogen-doped porous carbon loading of 300 kg and an adsorption tower operating temperature of 20 °C) to capture CO2 in the flue gas using the adsorbent, which includes nitrogen-doped porous carbon.

[0052] The temperature for each drying step is 95℃, and the time is 16 hours.

[0053] Example 2

[0054] A schematic diagram of a carbon capture system is shown below. Figure 1 As shown, the flue gas from a coal-fired power plant (with a volume percentage of 11.13% CO2 and 10.65% H2O) is divided into two parts: the first flue gas and the second flue gas.

[0055] Step S1, the first stream of flue gas (flow rate of 2 Nm³ / h) is... 3The nutrient solution (containing 3 mL 35% w / v HCl, 6.0 g / L NaHCO3, 7.5 g / L Na2CO3, 3.5 g / L NaNO3, 1 g / L K2HPO4, 0.1 g / L MgSO4·7H2O, 0.5 g / L K2SO4, 0.01 g / L FeSO4·7H2O, 0.08 g / L Na2EDTA, and 0.04 g / L CaCl2) is fed into a vertical microalgae reactor (operating temperature 25℃, light intensity 17500 lux, photoperiod 8h, nutrient solution including 3 mL 35% w / v HCl, 6.0 g / L NaHCO3, 7.5 g / L Na2CO3, 3.5 g / L NaNO3, 1 g / L K2HPO4, 0.1 g / L MgSO4·7H2O, 0.5 g / L K2SO4, 0.01 g / L FeSO4·7H2O, 0.08 g / L Na2EDTA, and 0.04 g / L CaCl2) to utilize CO2 in the flue gas for the cultivation of microalgae (including Chlorella, Microspirochetus, and Arthrospira) to obtain microalgae feed solution, which is then dried to obtain algae powder.

[0056] Step S2 involves sequentially performing pore-forming, nitrogen-doping, activation, and washing of the algae powder to obtain nitrogen-doped porous carbon, as detailed below:

[0057] Step S21: Place the algae powder into a quartz tube furnace and perform the first pyrolysis under a nitrogen atmosphere (nitrogen flow rate of 5 L / min, temperature of 800℃, time of 60 min) to create pores and obtain porous carbon.

[0058] Step S22: The porous carbon, urea, and water are first mixed (the weight ratio of porous carbon to urea is 1:0.8, the solid-liquid ratio of porous carbon to water is 25kg:250L, and the mixing time is 2h). After drying, the mixture is placed in a quartz tube furnace and subjected to a second pyrolysis in an air atmosphere (temperature is 300℃, time is 100min) to perform nitrogen doping. The mixture is then washed with water and dried to obtain the nitrogen-doped material.

[0059] Step S23: The nitrogen-doped material, solid NaOH, and water are mixed for the second time (the weight ratio of nitrogen-doped material to solid NaOH is 1:1.8, the solid-liquid ratio of nitrogen-doped material to water is 25kg:500L, and the mixing time is 100min). After drying, the mixture is placed in a quartz tube furnace and subjected to a third pyrolysis under a nitrogen atmosphere (nitrogen flow rate is 5L / min, temperature is 550℃, and time is 2h) to activate the material and obtain the activated material.

[0060] Step S24: The activated material is sequentially acid-washed (with 0.8 mol / L hydrochloric acid) and then washed with water until the washing solution is neutral. After drying, nitrogen-doped porous carbon (pore size 250 nm, porosity 60%, specific surface area 200 m²) is obtained. 2 / g, nitrogen doping content is 2wt.%).

[0061] Step S3, the second stream of flue gas (flow rate of 200 Nm) 3The gas is fed into an adsorption tower (with a nitrogen-doped porous carbon loading of 100 kg and an adsorption tower operating temperature of 0 °C) to capture CO2 in the flue gas using an adsorbent, which includes nitrogen-doped porous carbon.

[0062] The temperature for each drying step is 90℃, and the time is 24 hours.

[0063] Example 3

[0064] A schematic diagram of a carbon capture system is shown below. Figure 1 As shown, the flue gas from a coal-fired power plant (with a volume percentage of 11.13% CO2 and 10.65% H2O) is divided into two parts: the first flue gas and the second flue gas.

[0065] Step S1, the first stream of flue gas (flow rate of 10 Nm³) 3 The microalgae (including Chlorella, Microspiral, and Arthrospira) are fed into a vertical microalgae reactor (operating temperature 35℃, light intensity 2200 lux, photoperiod 12h, nutrient solution including 3mL 35% w / v HCl, 6.0g / L NaHCO3, 7.5g / L Na2CO3, 3.5g / L NaNO3, 1g / L K2HPO4, 0.1g / L MgSO4·7H2O, 0.5g / L K2SO4, 0.01g / L FeSO4·7H2O, 0.08g / L Na2EDTA, 0.04g / L CaCl2) to utilize CO2 in the flue gas for microalgae cultivation (microalgae include Chlorella, Microspiral, and Arthrospira) to obtain microalgae feed solution, which is then dried to obtain algae powder.

[0066] Step S2 involves sequentially performing pore-forming, nitrogen-doping, activation, and washing of the algae powder to obtain nitrogen-doped porous carbon, as detailed below:

[0067] Step S21: Place the algae powder into a quartz tube furnace and perform the first pyrolysis under a nitrogen atmosphere (nitrogen flow rate of 10 L / min, temperature of 900℃, time of 30 min) to create pores and obtain porous carbon.

[0068] Step S22: The porous carbon, urea, and water are first mixed (the weight ratio of porous carbon to urea is 1:1.2, the solid-liquid ratio of porous carbon to water is 50kg:500L, and the mixing time is 5h). After drying, the mixture is placed in a quartz tube furnace and subjected to a second pyrolysis in an air atmosphere (temperature is 400℃, time is 80min) to perform nitrogen doping. The mixture is then washed with water and dried to obtain the nitrogen-doped material.

[0069] Step S23: The nitrogen-doped material, solid KOH, and water are mixed for the second time (the weight ratio of nitrogen-doped material to solid KOH is 1:2.2, the solid-liquid ratio of nitrogen-doped material to water is 50kg:1000L, and the mixing time is 140min). After drying, the mixture is placed in a quartz tube furnace and subjected to a third pyrolysis under a nitrogen atmosphere (nitrogen flow rate is 10L / min, temperature is 650℃, and time is 1h) to activate the material and obtain the activated material.

[0070] Step S24: The activated material is sequentially acid-washed (with 1.2 mol / L hydrochloric acid) and then washed with water until the washing solution is neutral. After drying, nitrogen-doped porous carbon (pore size 2 nm, porosity 95%, specific surface area 5000 m²) is obtained. 2 / g, nitrogen doping content is 20wt.%).

[0071] Step S3, the second flue gas (flow rate of 1000 Nm) 3 The CO2 in the flue gas is fed into an adsorption tower (with a nitrogen-doped porous carbon loading of 500 kg and an adsorption tower operating temperature of 35°C) to capture CO2 in the flue gas using the adsorbent, which includes nitrogen-doped porous carbon.

[0072] The temperature for each drying step is 100℃ and the time is 12 hours.

[0073] Comparative Example 1

[0074] The difference from Example 1 is that only a microalgae reactor is used for carbon capture, and the microalgae reactor is a racetrack-type reactor.

[0075] Comparative Example 2

[0076] The difference from Example 1 is that only an adsorption tower is used for carbon capture, and the adsorbent in the adsorption tower is coal-based activated carbon.

[0077] The performance test results of the above embodiments and comparative examples are shown in Table 1:

[0078] Test method:

[0079] CO2 fixation rate of microalgae: Reference DOI: 10.1111 / 1751-7915.13497;

[0080] CO2 adsorption capacity: Reference DOI: 10.1021 / acs.energyfuels.9b01638.

[0081] Table 1

[0082]

[0083] As can be seen from the above, compared with the comparative examples, the embodiments of the present invention first utilize microalgae with strong tolerance to coal-fired flue gas to fix part of the CO2 in the flue gas, and simultaneously use the CO2 in the flue gas to cultivate the microalgae on a large scale. Then, the obtained microalgae are used as biomass raw materials for producing activated carbon adsorbents to prepare nitrogen-doped porous carbon with high CO2 adsorption capacity, which is then provided as an adsorbent in the adsorption-based flue gas CO2 capture process. The method of the present invention can use a vertical microalgae reactor for microalgae cultivation, which greatly reduces the floor space required. At the same time, using the cultivated microalgae as biomass raw materials for activated carbon adsorbents can increase the efficiency of biological CO2 utilization while greatly reducing the preparation cost of adsorbents. Furthermore, the nitrogen-doped porous carbon prepared through pore formation, nitrogen doping, activation, and washing has a significantly increased adsorption volume and an increased adsorption capacity. The method of the present invention integrates microalgae carbon fixation and adsorption-based carbon capture processes. By cultivating microalgae in flue gas and then preparing adsorbents, the preparation cost of adsorbents in carbon capture is reduced, greatly improving the economics of carbon capture and reducing the operating cost of the carbon capture system.

[0084] Furthermore, it can be seen that the overall effect is better when all process parameters are within the preferred range of the present invention.

[0085] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for co-preparing nitrogen-doped porous carbon to capture CO2 from flue gas using microalgae to fix CO2 from flue gas, characterized in that, The method of dividing the flue gas into two parts, namely a first stream of flue gas and a second stream of flue gas, includes the following steps: Step S1: The first stream of flue gas is sent into a vertical microalgae reactor to use CO2 in the flue gas to cultivate microalgae, obtain microalgae feed liquid, and then dry it to obtain algae powder. Step S2 involves sequentially performing pore-forming, nitrogen-doping, activation, and washing on the algae powder to obtain nitrogen-doped porous carbon; including: Step S21: The algae powder is subjected to a first pyrolysis under a nitrogen atmosphere to create pores and obtain porous carbon. Step S22: The porous carbon, nitrogen source, and water are first mixed, dried, and then subjected to a second pyrolysis in an air atmosphere to perform nitrogen doping. The mixture is then washed with water and dried to obtain the nitrogen-doped material. Step S23: The nitrogen-doped material is mixed with solid alkali and water for a second time, dried, and then subjected to a third pyrolysis under a nitrogen atmosphere to activate the material and obtain activated material. Step S24: The activated material is sequentially acid-washed and water-washed to perform the washing process until the washing solution is neutral, and then dried to obtain the nitrogen-doped porous carbon. Step S3: The second stream of flue gas is fed into an adsorption tower to capture CO2 in the flue gas using an adsorbent, wherein the adsorbent is the nitrogen-doped porous carbon prepared in step S2. In step S21, the nitrogen flow rate of the first pyrolysis is 5~10 L / min, the temperature is 800~900℃, and the time is 30~60 min. In step S22, the nitrogen source includes one or more of urea, molasses, and peptone; the weight ratio of the porous carbon to the nitrogen source is 1:(0.8~1.2); the temperature of the second pyrolysis is 300~400℃, and the time is 80~100min; In step S23, the solid alkali includes KOH and / or NaOH; the weight ratio of the nitrogen-doped material to the solid alkali is 1:(1.8~2.2); the nitrogen flow rate of the third pyrolysis is 5~10 L / min, the temperature is 550~650℃, and the time is 1~2 h; The nitrogen-doped porous carbon has a pore size of 2-250 nm, a porosity of 60-95%, and a specific surface area of ​​200-5000 m². 2 / g, with nitrogen doping amount of 2~20wt.%.

2. The method according to claim 1, characterized in that, In step S22, the solid-liquid ratio of the porous carbon to water is (25~50kg):(250~500L); and / or the first mixing time is 2~5h.

3. The method according to claim 1, characterized in that, In step S23, the solid-liquid ratio of the nitrogen-doped material to water is (25~50kg):(500~1000L); and / or the second mixing time is 100~140min.

4. The method according to claim 1, characterized in that, In step S24, the acid washing is performed using one or more of hydrochloric acid, sulfuric acid, and phosphoric acid at a concentration of 0.8~1.2 mol / L.

5. The method according to any one of claims 1 to 4, characterized in that, In step S1 The microalgae include one or more of Chlorella, Microcystis, and Arthrospira; and / or The flow rate of the first flue gas is 2~10 Nm³. 3 / h; and / or The vertical microalgae reactor operates at a temperature of 25-35℃, a light intensity of 2200-17500 lux, and a photoperiod of 8-12 hours; and / or The nutrient solution of the vertical microalgae reactor includes 3 mL of 35% w / v HCl, 6.0 g / L NaHCO3, 7.5 g / L Na2CO3, 3.5 g / L NaNO3, 1 g / L K2HPO4, 0.1 g / L MgSO4·7H2O, 0.5 g / L K2SO4, 0.01 g / L FeSO4·7H2O, 0.08 g / L Na2EDTA, and 0.04 g / L CaCl2.

6. The method according to any one of claims 1 to 4, characterized in that, In step S3 The flow rate of the second flue gas is 200~1000 Nm³. 3 / h; and / or The nitrogen-doped porous carbon has a loading of 100~500 kg, and the adsorption tower operates at a temperature of 0~35℃.

7. The method according to any one of claims 1 to 4, characterized in that, The volume percentage of CO2 in the flue gas is 3-15%, and the volume percentage of H2O is 5-20%; the flue gas includes one or more of the following: coal-fired flue gas, gas-fired flue gas, biomass boiler flue gas, steel plant flue gas, and building materials plant flue gas.

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