Microalgae carbon sequestration system and method

Abstract

The invention discloses a microalgae carbon sequestration system and method, and belongs to the field of carbon dioxide capture and utilization. The system comprises a natural pond, a gas supply unit, a photobioreactor, an MBR separation tank and the like, and stable operation is guaranteed through PLC automatic control and pipeline electric heat tracing. According to the method, wild type marine nannochloropsis oculata is taken as a material, CO2 gradient domestication is carried out, variable light intensity, bacteria-algae synergism and precise nutrition addition culture are combined, and then separation, concentration, dehydration and algal fat extraction are carried out to prepare the biodiesel. According to the method, the cost is reduced and the water body is purified by utilizing a natural pond water source, the microalgae carbon sequestration efficiency and biomass are remarkably improved, CO2 in a steel mill is converted into high-value biodiesel, and a low-cost and sustainable scheme is provided for carbon neutrality.

Description

Technical Field

[0001] This invention relates to the field of carbon dioxide replenishment and utilization technology, specifically to a microalgae carbon fixation system and method. Background Technology

[0002] CO2 produced by the combustion of traditional energy sources is a major contributor to the global greenhouse effect, and how to efficiently capture and utilize CO2 is a pressing global problem. Existing CO2 fixation methods are mainly divided into three categories:

[0003] Physical sealing method: has high requirements for geological conditions and space, high energy consumption and cost, and has the risk of leakage, and is not suitable for long-term storage;

[0004] Chemical adsorption: The reaction rate is fast, but it requires high temperature and high pressure conditions (which indirectly increases CO2 emissions), and it depends on expensive catalysts. The products are prone to causing secondary pollution.

[0005] Biological carbon fixation and transformation (CCUS) is a bio-carbon fixation technology centered on microalgae. It has the advantages of fast growth rate, strong adaptability to extreme environments, high carbon fixation efficiency and low cost. Moreover, the lipids and proteins synthesized by microalgae can be used in food, feed and biofuel fields, making it a research hotspot of CCUS technology.

[0006] Existing patents related to microalgae carbon fixation have shortcomings:

[0007] The patent application No. 201810933261.7, entitled "A method to improve the oil production efficiency and carbon fixation rate of microalgae", only controls the carbon fixation efficiency by controlling basic conditions such as culture medium nutrition, CO2 concentration and temperature, and the culture strategy is singular.

[0008] The patent application No. 202211344280.9, entitled "A microalgae carbon fixation device and its usage method", promotes CO2 dissolution through aeration pipes and injection pipes, but the device structure is significantly different from that of the present invention and does not involve algae domestication and high-value conversion.

[0009] The patent application No. 202310430727.2, entitled "A Microalgae Carbon Fixation Device for Flue Gas Waste Heat Recovery System", only combines waste heat recovery to achieve CO2 emission reduction, without optimizing the microalgae carbon fixation performance and product utilization pathway.

[0010] Therefore, it is necessary to develop a microalgae carbon fixation system and method that integrates efficient algae domestication, precise environmental control, synergistic effects between bacteria and algae, and high-value conversion, so as to further improve the carbon fixation efficiency and resource value of CO2 in steel plants. Summary of the Invention

[0011] The purpose of this invention is to provide a microalgae carbon fixation system and method to solve the problems mentioned in the background art.

[0012] To achieve the above objectives, the present invention provides the following technical solution: a microalgae carbon fixation system, characterized in that it includes a natural pond, an aeration unit, a nutrient tank, a photobioreactor, an MBR separation tank, a concentration tank, a centrifuge, and an electrically heated dryer;

[0013] The natural pond and photobioreactor are connected by a booster pump and pipelines. The gas supply unit and photobioreactor are connected by an inlet pipe, valve, gas flow meter, and outlet pipe. The nutrient tank and photobioreactor are connected by a pump and nutrient solution pipe. The photobioreactor and MBR separation tank are connected by a submersible pump, outlet pipe, suction pump, and return pipe. The MBR separation tank and concentration tank are connected by a pump, outlet pipe, and return pipe. The concentrate tank and centrifuge are connected by a pump and pipelines. The centrifuge and MBR separation tank are connected by a pump and return pipe. The centrifuge and electric heating dryer are connected by pipelines.

[0014] The entire system is automated through PLC equipment, and all pipelines are equipped with electric heat tracing devices.

[0015] Preferably, the photobioreactor is equipped with a submersible mixer, a light compensation device, an aerator, an ultrasonic level gauge, a pH meter, and a turbidity meter. The submersible mixer is used to realize the circulation of algal solution, the light compensation device is used to provide a light source for microalgae growth, the aerator is used to disperse CO2 gas into small bubbles and mix it thoroughly with the algal suspension, the ultrasonic level gauge is used to control the liquid level in the photobioreactor, the pH meter is used to monitor the acidity and alkalinity of the algal solution, and the turbidity meter is used to monitor the concentration of the algal solution.

[0016] Preferably, the gas supply unit includes a CO2 storage tank, an air compressor, a pressure reducing valve, a gas flow meter, and a gas mixing chamber; the CO2 storage tank is used to store high-concentration CO2 flue gas from the steel plant, and the air compressor sends air into the gas mixing chamber through the pressure reducing valve and the gas flow meter, where it is mixed and diluted with the high-concentration CO2 output from the CO2 storage tank in a proportional manner, and then the gas supply rate is adjusted by the flow meter before being introduced into the photobioreactor.

[0017] Preferably, the MBR separation tank is equipped with an ultrafiltration hollow fiber membrane for algae-water separation.

[0018] This invention also provides a carbon fixation method for the above-mentioned microalgae carbon fixation system, specifically including the following steps:

[0019] Step 1: Domestication of High-Quality Algal Strains: Using wild-type marine microalgae as material, a gradient of increasing CO2 concentration was adopted for domestication. The domestication stages were as follows: 1) air; 2) 3% CO2 / 97% N2; 3) 6% CO2 / 94% N2; 4) 10% CO2 / 90% N2; 5) 12% CO2 / 88% N2. Each stage contained 3 rounds of cultivation, with each round lasting 8 days, so that the microalgae gradually acquired the ability to adapt to CO2 in a collective manner.

[0020] Step 2: Microalgae cultivation: Natural pond water is injected into the photobioreactor via a booster pump. The gas supply unit introduces CO2 gas at a concentration of 12%–15% into the photobioreactor. The microalgae acclimatized in Step 1 are then cultivated in the reactor for an 8-day cultivation period: the light intensity is 650–750 μmol·m⁻² s⁻¹ for the first 2 days, then 1100–1200 μmol·m⁻² s⁻¹ for days 3–4, and then 900–1000 μmol·m⁻² s⁻¹ for days 5–8. The light-dark cycle is 16 h: 8 h, and the pH of the algal solution is controlled at 7.5–8.5.

[0021] Step 3: Microbial Inoculation and Nutrient Supplementation: Simultaneously with the addition of microalgae in Step 2, microbial inoculation is added to the photobioreactor. The inoculation mainly consists of five genera: Aeromonas, Shigella, Clostridium, Orthomonas, and Klebsiella. The microbial-to-algae ratio is controlled at 3:1 to 15:1. At the same time, inorganic carbon sources, organic carbon sources, nitrogen sources, and phosphorus sources are added to the algal solution in the nutrient tank: the inorganic carbon source is sodium bicarbonate, with the carbon number controlled to be the same as that of glucose with a COD of 100–500 mg / L; the organic carbon source is sodium acetate or glucose, with the addition amount controlled at a COD of 100–500 mg / L; the nitrogen source is sodium nitrate, with the addition amount controlled at 100–350 mg / L; and the phosphorus source is potassium dihydrogen phosphate, with the addition amount controlled at 4–24 mg / L.

[0022] Step 4: Algal liquid separation: When the turbidity of the algal liquid is >4000 NTU, algal liquid separation is carried out. The algal water is filtered through the ultrafiltration hollow fiber membrane in the MBR separation tank by a submersible pump to separate high-concentration algal slurry. The clean water obtained by the suction pump is returned to the photobioreactor through the return water pipe.

[0023] Step 5: Algae slurry concentration: The high-concentration algae slurry separated is pumped into a concentration tank for further concentration to obtain algae mud with a water content of <95%. The separated clear water overflows through the return water pipe and returns to the MBR separation tank for circulation separation.

[0024] Step 6: Dewatering of algae sludge: The algae sludge is sent to a centrifuge for dewatering until the moisture content is <90%, and then dried in an electric heating dryer until the moisture content is <10%. The centrifugal wastewater is returned to the MBR separation tank.

[0025] Step 7: Algal lipid extraction: The dried microalgae powder is ultrasonically crushed at a power density of 0.5-3 W / ml, with a 30-second interval after 30 seconds of crushing, and the ultrasonic treatment time is 2-12 minutes; after ultrasonication, algal lipids are extracted using a dichloromethane-methanol solution at an extraction temperature of 30-50℃ for 4-8 hours. After extraction, the mixture is filtered, and the solvent is evaporated using a rotary evaporator.

[0026] Step 8: Algal lipid decolorization: Mix the extracted algal lipid with activated carbon for decolorization. The decolorization temperature is 50-70℃, the decolorization time is 20-40 min, and the activated carbon dosage is 5-15%. After decolorization, filter and then wash and dry with acetone-water mixture.

[0027] Step 9: Biodiesel Preparation: The decolorized algal lipids are mixed with methanol and catalyst and stirred to react. The resulting mixture is allowed to stand and separate into layers. The upper layer of crude biodiesel is separated, washed with hot water, and then evaporated in a water bath to remove excess methanol. Finally, it is vacuum dried to remove moisture, yielding microalgae biodiesel. The alcohol-to-oil molar ratio is 20:1–30:1. The catalyst is LVK-H900 lipase or Novozym lipase, with a catalyst dosage of 8%–12%. The reaction temperature is 50–70℃, and the reaction time is 3–5 hours.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] 1. The microalgae carbon fixation system uses natural ponds as the water source for microalgae cultivation, which reduces the amount of nutrient solution added for microalgae cultivation and at the same time plays a role in purifying natural water bodies.

[0030] 2. By employing multiple methods such as incremental CO2 concentration gradient acclimatization, variable light intensity strategy, and synergistic effects between bacteria and algae, algae with high carbon fixation performance and tolerance to high CO2 concentrations are screened and cultivated to maximize microalgal biomass and oil production.

[0031] 3. Extract microalgae oil for the preparation of high-value biodiesel products, realizing the high-value utilization of CO2 from steel plants after conversion into biomass. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Example 1

[0034] This embodiment of a microalgae carbon fixation system includes a natural pond, an aeration unit, a nutrient tank, a photobioreactor, an MBR separation tank, a concentration tank, a centrifuge, and an electrically heated dryer. The natural pond and the photobioreactor are connected via a booster pump and pipelines; the aeration unit and the photobioreactor are connected via an inlet pipe, valves, a gas flow meter, and an outlet pipe; the nutrient tank and the photobioreactor are connected via a pump and nutrient solution pipes; the photobioreactor and the MBR separation tank are connected via a submersible pump, an outlet pipe, a suction pump, and a return pipe; the MBR separation tank and the concentration tank are connected via a pump, an outlet pipe, and a return pipe; the concentration tank and the centrifuge are connected via a pump and pipelines; the centrifuge and the MBR separation tank are connected via a pump and a return pipe; and the centrifuge and the electrically heated dryer are connected via pipelines.

[0035] Furthermore, the photobioreactor is equipped with a submersible mixer, a light compensation device, an aerator, an ultrasonic level gauge, a pH meter, and a turbidity meter. The submersible mixer enables the circulation of the algal solution, the light compensation device provides a light source for microalgae growth, the aerator disperses CO2 gas into small bubbles and mixes them thoroughly with the algal suspension, the ultrasonic level gauge controls the liquid level in the photobioreactor, the pH meter monitors the pH of the algal solution, and the turbidity meter monitors the concentration of the algal solution.

[0036] Furthermore, the gas supply unit includes a CO2 storage tank, an air compressor, a pressure reducing valve, a gas flow meter, and a gas mixing chamber. The CO2 storage tank is used to store flue gas containing high concentrations of CO2 from the steel plant. The air compressor delivers air through the pressure reducing valve and the gas flow meter into the gas mixing chamber, where it is mixed and diluted with the high concentration of CO2 supplied by the CO2 storage tank in a certain proportion. The mixed gas is then supplied at different gas supply rates by adjusting the flow meter, thereby providing a carbon source for the microalgae cultivated in the photobioreactor.

[0037] Furthermore, the MBR separation tank is equipped with an ultrafiltration hollow fiber membrane for algae-water separation;

[0038] Furthermore, the entire system uses PLC equipment to automate the control of various instruments, reducing the need for manual labor;

[0039] Furthermore, the entire system's piping is equipped with an electric heat tracing device to prevent the pipes from freezing when the temperature is low.

[0040] A method for carbon fixation by microalgae, characterized by comprising the following steps:

[0041] Step 1: Domestication of high-quality algae: Using wild-type marine microalgae as material, a gradient of increasing CO2 concentration was adopted for domestication, including five domestication stages: 1) air; 2) 3% CO2 / 97% N2; 3) 6% CO2 / 94% N2; 4) 10% CO2 / 90% N2; 5) 12% CO2 / 88% N2. Each stage included three culture cycles, and each culture cycle lasted for eight days, so that the microalgae gradually acquired the ability to adapt to CO2 in a collective manner.

[0042] Step 2: Microalgae cultivation: Natural pond water is injected into the photobioreactor via a booster pump, and CO2 gas with a concentration of 12% is introduced into the reactor by the gas supply unit. The microalgae acclimatized in Step 1 are cultivated in the reactor for an 8-day cultivation period: the light intensity is 650 μmol·m⁻²s⁻¹ for the first 2 days, 1100 μmol·m⁻²s⁻¹ for days 3-4, and 900 μmol·m⁻²s⁻¹ for days 5-8. The light-dark cycle is 16h:8h, and the pH of the algal solution is controlled at 7.5.

[0043] Step 3: Microbial Inoculation: Simultaneously with the addition of microalgae in Step 2, microbial inoculation is added to the photobioreactor. The inoculation primarily consists of five genera: Aeromonas, Shigella, Clostridium, Orthomonas, and Klebsiella, with a microbial-to-algae ratio controlled at 3:1. At the same time, inorganic carbon source, organic carbon source, nitrogen source, and phosphorus source are added to the algae solution in the nutrient tank: the inorganic carbon source is sodium bicarbonate, with the carbon number controlled to be the same as that of glucose with a COD of 100 mg / L; the organic carbon source is sodium acetate, with the addition amount controlled at a COD of 100 mg / L; the nitrogen source is sodium nitrate, with the addition amount controlled at 100 mg / L; and the phosphorus source is potassium dihydrogen phosphate, with the addition amount controlled at 4 mg / L.

[0044] Step 4: Algal liquid separation: When the turbidity of the algal liquid is >4000 NTU, algal liquid separation is carried out. The algal water is filtered through the ultrafiltration hollow fiber membrane in the MBR separation tank by a submersible pump to separate high-concentration algal slurry. The clean water obtained by the suction pump is returned to the photobioreactor through the return water pipe.

[0045] Step 5: Algae slurry concentration: The high-concentration algae slurry separated is pumped into a concentration tank for further concentration to obtain algae mud with a water content of <95%. The separated clear water overflows through the return water pipe and returns to the MBR separation tank for circulation separation.

[0046] Step 6: Algae sludge dewatering: The algae sludge is pumped into a centrifuge for further dewatering until the moisture content is <90%, and then enters an electric heating dryer to dry until the moisture content is <10%. The wastewater after centrifugation is pumped back to the MBR separation tank for circulation separation.

[0047] Step 7: Algal lipid extraction: The dried microalgae powder was ultrasonically crushed at a power density of 0.5 W / ml, with a 30-second interval after crushing for 30 seconds, and a total ultrasonic treatment time of 2 minutes. After ultrasonic treatment, algal lipids were extracted using a dichloromethane-methanol solution at a temperature of 30℃ for 4 hours. After extraction, the mixture was filtered, and the solvent was evaporated using a rotary evaporator.

[0048] Step 8: Algal lipid decolorization: Mix the extracted algal lipid with activated carbon for decolorization at a temperature of 50℃ for 20 minutes. Use 5% activated carbon. After decolorization, filter the mixture and wash and dry it with an acetone-water mixture.

[0049] Step 9: Biodiesel Preparation: The decolorized algal lipids, methanol, and catalyst are mixed and stirred for reaction. The resulting mixture is allowed to stand and separate into layers. The upper layer of crude biodiesel is separated, washed with hot water, and then evaporated in a water bath to remove excess methanol. Finally, it is vacuum dried to remove moisture, yielding microalgae biodiesel. The alcohol-to-oil molar ratio is 20:1, the catalyst is Novozym lipase (8% catalyst dosage), the reaction temperature is 50℃, and the reaction time is 3 hours.

[0050] After implementing the above steps, the microalgae fixed CO2 at a rate of 1.7 kg / kg, the microalgae biomass at a rate of 1.5 g / L, the algal lipid dry weight content at a rate of 28.34%, the oil yield at a rate of 20.93%, and the biodiesel yield at a rate of 65.43%.

[0051] Example 2

[0052] This embodiment of a microalgae carbon fixation system includes a natural pond, an aeration unit, a nutrient tank, a photobioreactor, an MBR separation tank, a concentration tank, a centrifuge, and an electrically heated dryer. The natural pond and the photobioreactor are connected via a booster pump and pipelines; the aeration unit and the photobioreactor are connected via an inlet pipe, valves, a gas flow meter, and an outlet pipe; the nutrient tank and the photobioreactor are connected via a pump and nutrient solution pipes; the photobioreactor and the MBR separation tank are connected via a submersible pump, an outlet pipe, a suction pump, and a return pipe; the MBR separation tank and the concentration tank are connected via a pump, an outlet pipe, and a return pipe; the concentration tank and the centrifuge are connected via a pump and pipelines; the centrifuge and the MBR separation tank are connected via a pump and a return pipe; and the centrifuge and the electrically heated dryer are connected via pipelines.

[0053] Furthermore, the photobioreactor is equipped with a submersible mixer, a light compensation device, an aerator, an ultrasonic level gauge, a pH meter, and a turbidity meter. The submersible mixer enables the circulation of the algal solution, the light compensation device provides a light source for microalgae growth, the aerator disperses CO2 gas into small bubbles and mixes them thoroughly with the algal suspension, the ultrasonic level gauge controls the liquid level in the photobioreactor, the pH meter monitors the pH of the algal solution, and the turbidity meter monitors the concentration of the algal solution.

[0054] Furthermore, the gas supply unit includes a CO2 storage tank, an air compressor, a pressure reducing valve, a gas flow meter, and a gas mixing chamber. The CO2 storage tank is used to store flue gas containing high concentrations of CO2 from the steel plant. The air compressor delivers air through the pressure reducing valve and the gas flow meter into the gas mixing chamber, where it is mixed and diluted with the high concentration of CO2 supplied by the CO2 storage tank in a certain proportion. The mixed gas is then supplied at different gas supply rates by adjusting the flow meter, thereby providing a carbon source for the microalgae cultivated in the photobioreactor.

[0055] Furthermore, the MBR separation tank is equipped with an ultrafiltration hollow fiber membrane for algae-water separation;

[0056] Furthermore, the entire system uses PLC equipment to automate the control of various instruments, reducing the need for manual labor;

[0057] Furthermore, the entire system's piping is equipped with an electric heat tracing device to prevent the pipes from freezing when the temperature is low.

[0058] A method for carbon fixation by microalgae, characterized by comprising the following steps:

[0059] Step 1: Domestication of high-quality algae: Using wild-type marine microalgae as material, a gradient of increasing CO2 concentration was adopted for domestication, including five domestication stages: 1) air; 2) 3% CO2 / 97% N2; 3) 6% CO2 / 94% N2; 4) 10% CO2 / 90% N2; 5) 12% CO2 / 88% N2. Each stage included three culture cycles, and each culture cycle lasted for eight days, so that the microalgae gradually acquired the ability to adapt to CO2 in a collective manner.

[0060] Step 2: Microalgae cultivation: Natural pond water is injected into the photobioreactor via a booster pump, and CO2 gas with a concentration of 14% is introduced into the reactor by the gas supply unit. The microalgae acclimatized in Step 1 are cultivated in the reactor for an 8-day cultivation period: the light intensity is 700 μmol·m⁻²s⁻¹ for the first 2 days, 1150 μmol·m⁻²s⁻¹ for days 3-4, and 950 μmol·m⁻²s⁻¹ for days 5-8. The light-dark cycle is 16h:8h, and the pH of the algal solution is controlled at 8.0.

[0061] Step 3: Microbial Inoculation: Simultaneously with the addition of microalgae in Step 2, microbial inoculation is added to the photobioreactor. The inoculation primarily consists of five genera: Aeromonas, Shigella, Clostridium, Orthomonas, and Klebsiella, with a microbial-to-algae ratio controlled at 15:1. At the same time, inorganic carbon source, organic carbon source, nitrogen source, and phosphorus source are added to the algae solution in the nutrient tank: the inorganic carbon source is sodium bicarbonate, with the carbon number controlled to be the same as that of glucose with a COD of 500 mg / L; the organic carbon source is glucose, with the addition amount controlled at a COD of 500 mg / L; the nitrogen source is sodium nitrate, with the addition amount controlled at 200 mg / L; and the phosphorus source is potassium dihydrogen phosphate, with the addition amount controlled at 10 mg / L.

[0062] Step 4: Algal liquid separation: When the turbidity of the algal liquid is >4000 NTU, algal liquid separation is carried out. The algal water is filtered through the ultrafiltration hollow fiber membrane in the MBR separation tank by a submersible pump to separate high-concentration algal slurry. The clean water obtained by the suction pump is returned to the photobioreactor through the return water pipe.

[0063] Step 5: Algae slurry concentration: The high-concentration algae slurry separated is pumped into a concentration tank for further concentration to obtain algae mud with a water content of <95%. The separated clear water overflows through the return water pipe and returns to the MBR separation tank for circulation separation.

[0064] Step 6: Algae sludge dewatering: The algae sludge is pumped into a centrifuge for further dewatering until the moisture content is <90%, and then enters an electric heating dryer to dry until the moisture content is <10%. The wastewater after centrifugation is pumped back to the MBR separation tank for circulation separation.

[0065] Step 7: Algal lipid extraction: The dried microalgae powder was ultrasonically crushed at a power density of 2W / ml, with a 30-second interval after crushing, and a total ultrasonic treatment time of 10 minutes. After ultrasonic treatment, algal lipids were extracted using a dichloromethane-methanol solution at a temperature of 40℃ for 6 hours. After extraction, the mixture was filtered, and the solvent was evaporated using a rotary evaporator.

[0066] Step 8: Algal lipid decolorization: Mix the extracted algal lipid with activated carbon for decolorization at a temperature of 60℃ for 30 minutes. Use 10% activated carbon. After decolorization, filter the mixture and wash and dry it with an acetone-water mixture.

[0067] Step 9: Biodiesel Preparation: The decolorized algal lipids, methanol, and catalyst are mixed and stirred for reaction. The resulting mixture is allowed to stand and separate into layers. The upper layer of crude biodiesel is separated, washed with hot water, and then evaporated in a water bath to remove excess methanol. Finally, it is vacuum dried to remove moisture, yielding microalgae biodiesel. The alcohol-to-oil molar ratio is 25:1, the catalyst is LVK-H900 lipase (10% catalyst dosage), the reaction temperature is 60℃, and the reaction time is 4 hours.

[0068] After implementing the above steps, the microalgae fixed CO2 at a rate of 1.9 kg / kg, the microalgae biomass at 1.7 g / L, the algal lipid dry weight content at 30.15%, the oil yield at 23.5%, and the biodiesel yield at 70.46%.

[0069] Example 3

[0070] This embodiment of a microalgae carbon fixation system includes a natural pond, an aeration unit, a nutrient tank, a photobioreactor, an MBR separation tank, a concentration tank, a centrifuge, and an electrically heated dryer. The natural pond and the photobioreactor are connected via a booster pump and pipelines; the aeration unit and the photobioreactor are connected via an inlet pipe, valves, a gas flow meter, and an outlet pipe; the nutrient tank and the photobioreactor are connected via a pump and nutrient solution pipes; the photobioreactor and the MBR separation tank are connected via a submersible pump, an outlet pipe, a suction pump, and a return pipe; the MBR separation tank and the concentration tank are connected via a pump, an outlet pipe, and a return pipe; the concentration tank and the centrifuge are connected via a pump and pipelines; the centrifuge and the MBR separation tank are connected via a pump and a return pipe; and the centrifuge and the electrically heated dryer are connected via pipelines.

[0071] Furthermore, the photobioreactor is equipped with a submersible mixer, a light compensation device, an aerator, an ultrasonic level gauge, a pH meter, and a turbidity meter. The submersible mixer enables the circulation of the algal solution, the light compensation device provides a light source for microalgae growth, the aerator disperses CO2 gas into small bubbles and mixes them thoroughly with the algal suspension, the ultrasonic level gauge controls the liquid level in the photobioreactor, the pH meter monitors the pH of the algal solution, and the turbidity meter monitors the concentration of the algal solution.

[0072] Furthermore, the gas supply unit includes a CO2 storage tank, an air compressor, a pressure reducing valve, a gas flow meter, and a gas mixing chamber. The CO2 storage tank is used to store flue gas containing high concentrations of CO2 from the steel plant. The air compressor delivers air through the pressure reducing valve and the gas flow meter into the gas mixing chamber, where it is mixed and diluted with the high concentration of CO2 supplied by the CO2 storage tank in a certain proportion. The mixed gas is then supplied at different gas supply rates by adjusting the flow meter, thereby providing a carbon source for the microalgae cultivated in the photobioreactor.

[0073] Furthermore, the MBR separation tank is equipped with an ultrafiltration hollow fiber membrane for algae-water separation;

[0074] Furthermore, the entire system uses PLC equipment to automate the control of various instruments, reducing the need for manual labor;

[0075] Furthermore, the entire system's piping is equipped with an electric heat tracing device to prevent the pipes from freezing when the temperature is low.

[0076] A method for carbon fixation by microalgae, characterized by comprising the following steps:

[0077] Step 1: Domestication of high-quality algae: Using wild-type marine microalgae as material, a gradient of increasing CO2 concentration was adopted for domestication, including five domestication stages: 1) air; 2) 3% CO2 / 97% N2; 3) 6% CO2 / 94% N2; 4) 10% CO2 / 90% N2; 5) 12% CO2 / 88% N2. Each stage included three culture cycles, and each culture cycle lasted for eight days, so that the microalgae gradually acquired the ability to adapt to CO2 in a collective manner.

[0078] Step 2: Microalgae cultivation: Natural pond water is injected into the photobioreactor via a booster pump, and CO2 gas with a concentration of 15% is introduced into the reactor by the gas supply unit. The microalgae acclimatized in Step 1 are cultivated in the reactor for an 8-day cultivation period: the light intensity is 750 μmol·m⁻²s⁻¹ for the first 2 days, 1200 μmol·m⁻²s⁻¹ for days 3-4, and 1000 μmol·m⁻²s⁻¹ for days 5-8. The light-dark cycle is 16h:8h, and the pH of the algal solution is controlled at 8.5.

[0079] Step 3: Microbial Inoculation: Simultaneously with the addition of microalgae in Step 2, microbial inoculation is added to the photobioreactor. The inoculation primarily consists of five genera: Aeromonas, Shigella, Clostridium, Orthomonas, and Klebsiella, with a microbial-to-algae ratio controlled at 10:1. At the same time, inorganic carbon source, organic carbon source, nitrogen source, and phosphorus source are added to the algae solution in the nutrient tank: the inorganic carbon source is sodium bicarbonate, with the carbon number controlled to be the same as that of glucose with a COD of 300 mg / L; the organic carbon source is sodium acetate, with the dosage controlled at a COD of 300 mg / L; the nitrogen source is sodium nitrate, with the dosage controlled at 350 mg / L; and the phosphorus source is potassium dihydrogen phosphate, with the dosage controlled at 24 mg / L.

[0080] Step 4: Algal liquid separation: When the turbidity of the algal liquid is >4000 NTU, algal liquid separation is carried out. The algal water is filtered through the ultrafiltration hollow fiber membrane in the MBR separation tank by a submersible pump to separate high-concentration algal slurry. The clean water obtained by the suction pump is returned to the photobioreactor through the return water pipe.

[0081] Step 5: Algae slurry concentration: The high-concentration algae slurry separated is pumped into a concentration tank for further concentration to obtain algae mud with a water content of <95%. The separated clear water overflows through the return water pipe and returns to the MBR separation tank for circulation separation.

[0082] Step 6: Algae sludge dewatering: The algae sludge is pumped into a centrifuge for further dewatering until the moisture content is <90%, and then enters an electric heating dryer to dry until the moisture content is <10%. The wastewater after centrifugation is pumped back to the MBR separation tank for circulation separation.

[0083] Step 7: Algal lipid extraction: The dried microalgae powder was ultrasonically crushed at a power density of 3W / ml, with a 30-second interval after crushing, and a total ultrasonic treatment time of 12 minutes. After ultrasonic treatment, algal lipids were extracted using a dichloromethane-methanol solution at a temperature of 50℃ for 8 hours. After extraction, the mixture was filtered, and the solvent was evaporated using a rotary evaporator.

[0084] Step 8: Algal lipid decolorization: Mix the extracted algal lipid with activated carbon for decolorization at a temperature of 70℃ for 40 minutes. Use 15% activated carbon. After decolorization, filter the mixture and wash and dry it with an acetone-water mixture.

[0085] Step 9: Biodiesel Preparation: The decolorized algal lipids, methanol, and catalyst are mixed and stirred for reaction. The resulting mixture is allowed to stand and separate into layers. The upper layer of crude biodiesel is separated, washed with hot water, and then evaporated in a water bath to remove excess methanol. Finally, it is vacuum dried to remove moisture, yielding microalgae biodiesel. The alcohol-to-oil molar ratio is 30:1, the catalyst is Novozym lipase (12% catalyst dosage), the reaction temperature is 70℃, and the reaction time is 5 hours.

[0086] After implementing the above steps, the microalgae fixed CO2 at a rate of 1.8 kg / kg, the microalgae biomass at a rate of 1.6 g / L, the algal lipid dry weight content at a rate of 29.81%, the oil yield at a rate of 22.22%, and the biodiesel yield at a rate of 68.74%.

[0087] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A microalgal carbon sequestration system, characterized by: The natural pond, the air supply unit, the nutrient pool, the photobioreactor, the MBR separation tank, the concentration tank, the centrifuge and the electric heating dryer are connected through the lifting pump and the pipeline. The natural pond and the photobioreactor are connected through the lifting pump and the pipeline, the air supply unit and the photobioreactor are connected through the air inlet pipe, the valve, the gas flow meter and the air outlet pipe, the nutrient pool and the photobioreactor are connected through the pump and the nutrient liquid pipe, the photobioreactor and the MBR separation tank are connected through the submersible pump, the water outlet pipe, the suction pump and the water return pipe, the MBR separation tank and the concentration tank are connected through the pump, the water outlet pipe and the water return pipe, the thick water tank and the centrifuge are connected through the pump and the pipeline, the centrifuge and the MBR separation tank are connected through the pump and the water return pipe, and the centrifuge and the electric heating dryer are connected through the pipeline. The whole system is automatically controlled through the PLC device, and all the pipelines are provided with the electric heating device.

2. The microalgal carbon sequestration system of claim 1, wherein: The photobioreactor is provided with the submersible agitator, the light compensation device, the aerator, the ultrasonic liquid level meter, the pH meter and the turbidimeter, the submersible agitator is used for realizing the circulation flow of the algal liquid, the light compensation device is used for providing the microalgae growth light source, the aerator is used for dispersing the CO2 gas into small bubbles and fully mixing the small bubbles with the algal suspension, the ultrasonic liquid level meter is used for controlling the liquid level in the photobioreactor, the pH meter is used for monitoring the acidity and alkalinity of the algal liquid, and the turbidimeter is used for monitoring the concentration of the algal liquid.

3. The microalgal carbon sequestration system of claim 2, wherein: The air supply unit comprises the CO2 storage tank, the air compressor, the pressure reducing valve, the gas flow meter and the gas mixing chamber, the CO2 storage tank is used for storing the steel plant flue gas containing high-concentration CO2, the air compressor sends the air into the gas mixing chamber through the pressure reducing valve and the gas flow meter, the high-concentration CO2 output from the CO2 storage tank is mixed and diluted in proportion, and then the air supply rate is adjusted through the flow meter to enter the photobioreactor.

4. The microalgal carbon sequestration system of claim 2, wherein: The MBR separation tank is provided with the ultrafiltration hollow fiber membrane for separating the algal water.

5. A method for carbon sequestration by microalgae based on the system according to any one of claims 1 to 4, characterized in that: The method comprises the following steps: Step 1: high-quality algae domestication: taking the wild-type marine microalgae as the material, adopting the incremental CO2 concentration gradient domestication, and sequentially performing the following domestication stages: 1) air; 2) 3% CO2 / 97% N2; 3) 6% CO2 / 94% N2; 4) 10% CO2 / 90% N2; 5) 12% CO2 / 88% N2; each stage contains 3 cycles of culture periods, and each cycle of period is 8 days, so that the microalgae gradually obtain the population CO2 adaptation ability; Step 2: microalgae culture: the photobioreactor injects the natural pond water source through the lifting pump, and the air supply unit enters the CO2 gas with the concentration of 12% to 15% into the photobioreactor, the microalgae domesticated through step 1 are cultured in the reactor, the culture period is 8 days: the light intensity is 650 to 750 μmol·m-2s-1 in the first 2 days, is converted into 1100 to 1200 μmol·m-2s-1 in the 3rd to 4th days, is converted into 900 to 1000 μmol·m-2s-1 in the 5th to 8th days, the light and dark cycle is 16h:8h, and the pH of the algal liquid is controlled to be 7.5 to 8.

5. Step 3: Microbial Inoculation and Nutrient Supplementation: Simultaneously with the addition of microalgae in Step 2, microbial inoculation is added to the photobioreactor. The inoculation mainly consists of five genera: Aeromonas, Shigella, Clostridium, Orthomonas, and Klebsiella. The microbial-to-algae ratio is controlled at 3:1 to 15:

1. At the same time, inorganic carbon sources, organic carbon sources, nitrogen sources, and phosphorus sources are added to the algal solution in the nutrient tank: the inorganic carbon source is sodium bicarbonate, with the carbon number controlled to be the same as that of glucose with a COD of 100–500 mg / L; the organic carbon source is sodium acetate or glucose, with the addition amount controlled at a COD of 100–500 mg / L; the nitrogen source is sodium nitrate, with the addition amount controlled at 100–350 mg / L; and the phosphorus source is potassium dihydrogen phosphate, with the addition amount controlled at 4–24 mg / L. Step 4: Algal liquid separation: When the turbidity of the algal liquid is >4000 NTU, algal liquid separation is carried out. The algal water is filtered through the ultrafiltration hollow fiber membrane in the MBR separation tank by a submersible pump to separate high-concentration algal slurry. The clean water obtained by the suction pump is returned to the photobioreactor through the return water pipe. Step 5: Algae slurry concentration: The high-concentration algae slurry separated is pumped into a concentration tank for further concentration to obtain algae mud with a water content of <95%. The separated clear water overflows through the return water pipe and returns to the MBR separation tank for circulation separation. Step 6: Dewatering of algae sludge: The algae sludge is sent to a centrifuge for dewatering until the moisture content is <90%, and then dried in an electric heating dryer until the moisture content is <10%. The centrifugal wastewater is returned to the MBR separation tank. Step 7: Algal lipid extraction: The dried microalgae powder is ultrasonically crushed at a power density of 0.5-3 W / ml, with a 30-second interval after 30 seconds of crushing, and the ultrasonic treatment time is 2-12 minutes; after ultrasonication, algal lipids are extracted using a dichloromethane-methanol solution at an extraction temperature of 30-50℃ for 4-8 hours. After extraction, the mixture is filtered, and the solvent is evaporated using a rotary evaporator. Step 8: Algal lipid decolorization: Mix the extracted algal lipid with activated carbon for decolorization. The decolorization temperature is 50-70℃, the decolorization time is 20-40 min, and the activated carbon dosage is 5-15%. After decolorization, filter and then wash and dry with acetone-water mixture. Step 9: Biodiesel Preparation: The decolorized algal lipids are mixed with methanol and catalyst and stirred to react. The resulting mixture is allowed to stand and separate into layers. The upper layer of crude biodiesel is separated, washed with hot water, and then evaporated in a water bath to remove excess methanol. Finally, it is vacuum dried to remove moisture, yielding microalgae biodiesel. The alcohol-to-oil molar ratio is 20:1–30:

1. The catalyst is LVK-H900 lipase or Novozym lipase, with a catalyst dosage of 8%–12%. The reaction temperature is 50–70℃, and the reaction time is 3–5 hours.