Microalgae culture, harvesting and energy utilization process and system

Abstract

The invention relates to the field of microalgae culture and energy utilization. More specifically, the invention relates to a microalgae culture, harvesting and energy utilization process and system, cultured microalgae are harvested in a flocculation mode, ferric nitrate is used as a flocculant, ammonia water is added to adjust pH, generated ammonium nitrate can be recycled as a nitrogen source, and iron ions are changed into ferric hydroxide precipitates to be removed; the harvested microalgae is subjected to pyrolysis to prepare high-calorific-value bio-oil and biogas, iron-containing pyrolysis residues are further treated to obtain Fe3O4-BC serving as a magnetic flocculant, allelochemicals in an algae solution can be adsorbed and removed while microalgae flocculation is enhanced, and the inhibition effect of the culture solution on the microalgae during cyclic utilization is reduced. According to the invention, the culture, harvesting and energy utilization of microalgae can be synchronously realized, the problems of large water consumption in microalgae culture, difficult harvesting, incapability of effectively recycling a culture solution and the like are effectively solved, and a technical basis is provided for industrial application of energy microalgae.

Description

Technical Field

[0001] The present invention relates to the field of microalgae cultivation and energy utilization. More specifically, the present invention relates to a process and system for cultivating, harvesting and energy utilization of microalgae. Background Art

[0002] Fossil fuel shortages and severe environmental pollution caused by fossil energy consumption have become two major challenges hindering global sustainable development. Biomass energy, with its abundant reserves, has gradually become a hot topic in new energy research both domestically and internationally. Among various biomass sources, energy microalgae, as an emerging biofuel feedstock, possess broad application value and research prospects due to their high oil content and rapid growth rate. However, the industrialization of energy microalgae applications remains constrained by numerous challenges, the most significant of which are the efficient harvesting of microalgae and the recycling of culture media.

[0003] Currently, the main methods for harvesting microalgae include centrifugation, sedimentation, filtration, flotation, and flocculation. Centrifugation uses external centrifugal force to rapidly sediment algae cells in a solution, achieving algae-water separation. Sedimentation utilizes long-term sedimentation to achieve natural harvesting of microalgae. Filtration separates algae from water by passing microalgae through a filter membrane with a certain pore size. Flotation uses highly dispersed microbubbles in water as carriers to adhere to algae cells, causing them to float to the surface for collection. Flocculation involves adding flocculants to the algae solution to create bridges or a net-catching and sweeping effect between the particles, causing the suspended algae particles in the solution to accumulate and grow larger, thereby accelerating the aggregation of the algae and achieving the purpose of algae-water separation. The centrifugation method consumes too much energy and is limited by the size of the equipment, making it unsuitable for large-scale microalgae harvesting. The sedimentation method has problems such as long harvesting time and low harvesting efficiency. The filter screen or membrane of the filtration method is easily clogged, and the harvesting effect on algae with smaller cells is poor. The flotation method consumes a lot of energy and has a low harvesting efficiency. The flocculation harvesting method has the advantages of simple operation, wide application range, mature technical means, and low energy consumption. It can effectively reduce the harvesting cost and is considered to be a microalgae harvesting technology with the most application potential and development prospects.

[0004] Microalgae flocculation and harvesting mainly include chemical flocculation, biological flocculation, and physical flocculation. Chemical flocculation, which involves adding chemical flocculants to the algae solution for harvesting, is currently the most widely used flocculation method. Biological flocculation uses viscous substances produced by the organisms themselves or their metabolism to aggregate algae cells through netting or bridging. Physical flocculation mainly includes electroflocculation and magnetic flocculation. Electroflocculation uses metal ions released by electrolysis to flocculate microalgae through adsorption and electrical neutralization. Simultaneously, gases (such as H2 and O2) generated at the cathode cause the microalgae flocs to float, enabling the microalgae to be harvested. Magnetic flocculation utilizes the electrostatic interaction between magnetic nanoparticles and microalgae cells to aggregate the microalgae in the culture medium. An external magnetic field is then applied to drive the magnetic nanoparticles and separate them from the culture medium, enabling rapid harvesting of the microalgae. Magnetic flocculation alone is not very effective and is often used in combination with chemical flocculation. Furthermore, the low-cost preparation and efficient recovery of magnetic materials are major bottlenecks restricting the application of this technology in microalgae flocculation.

[0005] Microalgae cultivation consumes significant amounts of water resources, and a certain amount of metal ions remains in the wastewater after harvesting. Direct discharge without treatment can pollute the environment. Recycling the supernatant left after harvesting the algae can save cultivation costs and simplify subsequent supernatant processing, which has significant economic significance. However, recycling the flocculated supernatant remains a challenge. Residual flocculants in the flocculated culture fluid may affect the normal growth of microalgae. Some studies have shown that some secondary metabolites released by microalgae during cultivation can inhibit their growth, resulting in lower algae yields when circulating the supernatant for microalgae cultivation. Flocculants remaining after harvesting can also affect the growth of microalgae. Summary of the Invention

[0006] The purpose of the present invention is to provide a microalgae cultivation, harvesting and energy utilization process and system, which can simultaneously realize the cultivation, harvesting and energy utilization of microalgae, effectively solve the problems of high water consumption in microalgae cultivation, difficult harvesting, and inability to effectively recycle culture fluid, and provide a technical basis for the industrial application of energy microalgae.

[0007] In order to achieve these purposes and other advantages according to the present invention, a method for flocculating and harvesting microalgae is provided, comprising the following steps:

[0008] Adding ferric nitrate to the microalgae culture solution until the ferric nitrate concentration in the microalgae culture solution is 0.05-0.10 g / L;

[0009] A mixture of Fe3O4-BC and Fe-BC was added to the microalgae culture solution at a dosage of 15-20 mg / L, and after stirring evenly, ammonia water was added to adjust the pH of the microalgae culture solution to 8.5-9.0. After slowly stirring evenly, the solution was allowed to stand, and the flocculated sediment at the bottom of the flocculation tank was collected to obtain an algae slurry mixture.

[0010] Furthermore, in the microalgae flocculation and harvesting method, the mass ratio of Fe3O4-BC to Fe-BC in the mixture added to the microalgae culture solution is 10:1.

[0011] Furthermore, in the microalgae flocculation and harvesting method, the microalgae culture solution is placed in a magnetic field with a magnetic field strength of 50-100 mT.

[0012] The present invention also provides a process for cultivating, harvesting and converting microalgae into energy, comprising the following steps:

[0013] S1. When the chlorophyll a content in the microalgae culture solution exceeds 50 μg / L, the microalgae in the microalgae culture solution are harvested using the above-mentioned microalgae flocculation harvesting method to obtain an algae slurry mixture and a supernatant;

[0014] S2. Nitric acid is added to the supernatant obtained in S1 to adjust its pH to 7.0-7.5, and then 0.01-0.02% of the mass of the supernatant is added with Fe3O4-BC to cause precipitation. After supplementing nitrogen, phosphorus and trace elements, the supernatant is recycled as a microalgae culture solution;

[0015] S3, subjecting the algae slurry mixture obtained in S1 to ultrasonic treatment, centrifugation, and then drying to obtain a microalgae biomass product;

[0016] S4, sending the microalgae biomass product obtained in S3 into a biomass pyrolysis furnace for high-temperature pyrolysis to obtain bio-oil and biogas, as well as residual Fe-BC. The tail gas of the biomass pyrolysis furnace is used to dry the algae pulp mixture in S3;

[0017] S5. The residual Fe-BC obtained in S4 is introduced into water vapor at high temperature to prepare Fe3O4-BC.

[0018] Furthermore, in the microalgae cultivation, harvesting and energy utilization process, the algae slurry mixture in S3 is ultrasonically treated for 30-60 minutes at an ultrasonic intensity of 0.5-1.0 W / mL.

[0019] Furthermore, in the microalgae cultivation, harvesting and energy utilization process, the centrifugal separation in S2 specifically includes:

[0020] S3.1. Centrifuge the ultrasonically treated algae slurry mixture at 3000 g for 15-30 minutes to obtain ferric hydroxide colloid and a primary high-concentration algae slurry;

[0021] S3.2. Centrifuge the high-concentration algae pulp obtained in S3.1 at a centrifugal force of 5000g for 20-30 minutes to obtain a secondary high-concentration algae pulp. The secondary high-concentration algae pulp is dried to obtain the microalgae biomass product. The liquid obtained by centrifugation is returned to the microalgae culture medium for recycling.

[0022] Furthermore, in the microalgae cultivation, harvesting and energy utilization process, after the microalgae culture solution has been recycled 10-15 times, or when the ammonia nitrogen concentration in the microalgae culture solution exceeds 1000 mg / L, the microalgae culture solution is filtered through an activated carbon filter column, wherein the activated carbon filter column uses the Fe3O4-BC obtained in S5.

[0023] Furthermore, in the microalgae cultivation, harvesting and energy utilization process, after the high-temperature pyrolysis in S4 is completed, air is introduced into the biomass pyrolysis furnace to burn the residual Fe-BC, and the gas generated by the combustion is absorbed by concentrated ammonia water to obtain ammonium bicarbonate, which serves as a carbon source and nitrogen source for microalgae cultivation.

[0024] The present invention also provides a microalgae cultivation, harvesting and energy utilization system, which uses the above-mentioned microalgae cultivation, harvesting and energy utilization process, including:

[0025] microalgae cultivation tank;

[0026] a flocculation tank, the water inlet of which is connected to the water outlet of the microalgae cultivation tank through an algae liquid discharge pipe;

[0027] A regulating tank, whose water inlet is connected to the water outlet of the flocculation tank, and whose water outlet is connected to the flocculation liquid return inlet of the microalgae cultivation tank through an algae liquid discharge pipe, and a plurality of sets of electromagnetic coils are arranged in the conical groove at the bottom of the flocculation tank;

[0028] An activated carbon filtration device, wherein the liquid inlet and the liquid outlet are connected to the flocculation supernatant return pipe through a first pipe and a second pipe respectively, a first valve is provided on the portion of the flocculation supernatant return pipe located between the first pipe and the second pipe, and a second valve is provided on both the first pipe and the second pipe;

[0029] a first dosing device, which is used to add chemicals to the regulating tank;

[0030] a second dosing device, which is used to add chemicals to the flocculation tank;

[0031] a first water quality online monitoring device, which is used to detect the water quality in the regulating tank;

[0032] A second water quality online monitoring device, which is used to detect the water quality in the flocculation tank;

[0033] a third water quality online monitoring device, which is used to detect the water quality in the microalgae cultivation pool;

[0034] A paddle wheel is arranged in the microalgae cultivation pond.

[0035] Furthermore, in the microalgae cultivation, harvesting and energy utilization system, a U-shaped first baffle is vertically provided in the microalgae cultivation pool, a second baffle is vertically provided in the first baffle, one end of the second baffle extends out of the first baffle and is connected to the inner wall of the microalgae cultivation pool, and there is a gap between the other end and the arc portion of the first baffle.

[0036] The beneficial effects of the present invention are:

[0037] The present invention provides a process and system for cultivating, harvesting, and utilizing microalgae for energy. Cultivated microalgae are harvested via flocculation, using ferric nitrate as a flocculant. When the culture solution pH is adjusted to 8.5-9.0 with ammonia, a 99.8% recovery rate can be achieved. The resulting ammonium nitrate can be recycled as a nitrogen source, and the iron ions are precipitated and removed as ferric hydroxide, eliminating the introduction of other ions into the culture solution and the resulting high osmotic pressure. At a pH of 8.5, over 99.5% of the iron ions are removed, and the remaining iron ions serve as trace elements necessary for microalgae growth. The harvested microalgae are then pyrolyzed to produce high-calorific-value bio-oil and biogas. The iron-containing pyrolysis residue is further processed to produce nano-ferroferric oxide-loaded biochar, or Fe₃O₄-BC. This biochar can be returned to the microalgae flocculation tank as a magnetic flocculant, enhancing the growth of the microalgae while simultaneously adsorbing and removing allelochemicals from the algae solution, reducing the inhibitory effects of the culture solution on the microalgae during recycling. The waste heat from the pyrolysis process is used to dry the microalgae, and the resulting water vapor is used as a raw material for producing Fe₃O₄-BC. The process and system provided by this invention can simultaneously cultivate, harvest, and utilize microalgae for energy, effectively addressing issues such as high water consumption in microalgae cultivation, difficulty in harvesting, and the inability to effectively recycle the culture medium. This provides a technical foundation for the industrial application of energy-rich microalgae.

[0038] Other advantages, objectives and features of the present invention will be reflected in part from the following description and will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a flow chart of the microalgae cultivation, harvesting and energy utilization process of the present invention;

[0040] Figure 2 This is a schematic diagram of the structure of the microalgae cultivation, harvesting and energy utilization system of the present invention.

[0041] Wherein, the accompanying drawings are marked as follows:

[0042] 1-Microalgae cultivation tank; 2-Flocculation supernatant return pipe; 3-Flocculation liquid return inlet; 4-Second valve; 5-Activated carbon filter device; 6-First valve; 7-Second valve; 8-Regulating tank; 9-First water quality online monitoring device; 10-First dosing device; 11-Second water quality online monitoring device; 12-Flocculation tank; 13-Dosing pipe; 14-Second dosing device; 15-Algae liquid discharge pipe; 16-Third water quality online monitoring device; 17-Paddle wheel. DETAILED DESCRIPTION

[0043] The present invention is further described in detail below with reference to the embodiments so that those skilled in the art can implement the invention with reference to the description.

[0044] It should be noted that the experimental methods described in the following embodiments are conventional methods unless otherwise specified, and the reagents and materials are commercially available unless otherwise specified; in the description of the present invention, the terms "horizontal", "longitudinal", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention.

[0045] like Figure 1 As shown, an embodiment of the present invention provides a process for cultivating, harvesting and utilizing microalgae for energy, comprising the following steps:

[0046] S1. Prepare a microalgae culture medium and inoculate Chlorella vulgaris for cultivation. The main nutritional indicators of the culture medium are: TN mass concentration of 500 mg / L and TP mass concentration of 50 mg / L. During the microalgae cultivation process, turn on ultraviolet light every 12 hours for 30 minutes to promote the production of a certain amount of reactive oxygen (hydrogen peroxide) in the microalgae.

[0047] When the chlorophyll a content in the microalgae culture solution exceeds 50 μg / L, the microalgae in the microalgae culture solution are harvested to obtain an algae pulp mixture and a supernatant;

[0048] The microalgae flocculation and harvesting method comprises the following steps:

[0049] Adding ferric nitrate to the microalgae culture solution until the ferric nitrate concentration in the microalgae culture solution is 0.05-0.10 g / L;

[0050] A mixture of powdered Fe₃O₄-BC and Fe-BC is added to a microalgae culture at a dosage of 15 mg / L, with a mass ratio of 10:1. After uniform stirring, 25-30% ammonia water is added to adjust the pH of the microalgae culture to 8.5-9.0. Addition of ammonia water is stopped, and the culture is slowly stirred for 2 minutes before being allowed to stand for 10 minutes. The flocculated sediment at the bottom of the flocculation tank is collected to obtain an algae slurry mixture. The microalgae culture is then placed in a magnetic field with a strength of 50-100 mT. The magnetic field accelerates the sinking of the Fe₃O₄-BC-attached flocs, shortening sedimentation time by over 90%.

[0051] Ferric nitrate is used as a flocculant, and the pH is adjusted with ammonia water to generate ferric hydroxide colloid for flocculating microalgae. More than 99.5% of the iron ions are removed by precipitation. Ammonia nitrogen and nitric nitrogen can provide a nitrogen source for the growth of microalgae. No anions other than nitrate are introduced during the flocculation and precipitation process, which will not cause the osmotic pressure of the culture medium to be too high, which is not conducive to the growth of microalgae.

[0052] Ferric nitrate reacts with Fe in Fe-BC to produce Fe 2+ , Fe 2+ It can produce a Fenton-like reaction with hydrogen peroxide in microalgae, generating hydroxyl free radicals with strong oxidizing ability, which promotes the decomposition of microalgae allelochemicals. Related reaction formula:

[0053] Fe 2+ +H2O2→Fe 3+ +OH — + OH

[0054] When Fe₃O₄-BC is mixed with an acidic ferric nitrate solution, it releases adsorbed phosphorus for use in microalgae cultivation. The strong complexation between ferric nitrate and microalgae cell wall polysaccharides and proteins forms a three-dimensional cross-linked network, which is directly converted into a highly stable iron-carbon composite skeleton during pyrolysis. Endogenous volatiles (such as CO₂ and H₂O) generated by microalgae pyrolysis act as in-situ activators, simultaneously creating pores and magnetic loading.

[0055] S2. Add nitric acid to the supernatant obtained in S1 to adjust its pH to 7.0-7.5, then add 0.01-0.02% of Fe3O4-BC by weight of the supernatant, stir evenly and allow to precipitate, take the supernatant, and supplement nitrogen, phosphorus and trace elements to the supernatant before recycling it as a microalgae culture solution;

[0056] S3, subjecting the algae slurry mixture obtained in S1 to ultrasonic treatment, centrifugation, and then drying to obtain a microalgae biomass product;

[0057] S4. The microalgae biomass product obtained in S3 is sent to a biomass pyrolysis furnace for high-temperature pyrolysis to obtain bio-oil and biogas, as well as residual Fe-BC. The exhaust gas from the biomass pyrolysis furnace is used to dry the algae slurry mixture in S3. The reaction conditions of the biomass pyrolysis furnace are: heating to 450-500°C at a heating rate of 20-30°C / min under air-tight conditions, and maintaining at this temperature for 2 hours.

[0058] S5. The residual Fe-BC obtained in S4 is introduced into water vapor under high temperature conditions to prepare Fe3O4-BC, i.e. magnetic biochar.

[0059] After centrifugation, the microalgae flocs still contain a certain amount of iron ions. During the high-temperature pyrolysis process, the microalgae produce reducing gases such as CO and H₂, which reduce the iron oxide to zero-valent iron. Heating the zero-valent iron-loaded biochar (Fe-BC) at 800°C for 2 hours under steam can produce Fe₃O₄-BC. The reaction is shown in the following equation:

[0060] Fe(OH)3→Fe2O3+H2O↑

[0061] Fe2O3+CO→Fe+CO2

[0062] Fe2O3+H2→Fe+H2O

[0063] Fe+H2O→Fe3O4+H2

[0064] The algae slurry mixture in S3 is ultrasonically treated at an ultrasonic intensity of 0.5-1.0 W / mL for 30-60 min.

[0065] The role of ultrasonic treatment is to fully desorb nutrients such as nitrogen and phosphorus adsorbed on microalgae flocs.

[0066] Among them, the centrifugal separation in S2 specifically includes:

[0067] S3.1. Centrifuge the ultrasonically treated algae slurry mixture at 3000 g for 15-30 minutes to obtain ferric hydroxide colloid and a primary high-concentration algae slurry;

[0068] S3.2. Centrifuge the high-concentration algae pulp obtained in S3.1 at a centrifugal force of 5000g for 20-30 minutes to obtain a secondary high-concentration algae pulp. The secondary high-concentration algae pulp is dried to obtain the microalgae biomass product. The liquid obtained by centrifugation is returned to the microalgae culture medium for recycling.

[0069] A two-step centrifugation process separates the iron hydroxide colloid from the microalgae in the flocculated sediment, recovering the iron while also preventing the negative impact of excessive iron content on the microalgae's energy utilization. A certain amount of iron remains in the microalgae after separation, catalyzing the production of bio-oil and biogas during pyrolysis and serving as a source of magnetite for the subsequent Fe3O4-BC reaction.

[0070] After the microalgae culture solution has been recycled 10-15 times, or when the ammonia nitrogen concentration in the microalgae culture solution exceeds 1000 mg / L, the microalgae culture solution is filtered through an activated carbon filter column using the Fe3O4-BC obtained in S5.

[0071] To prevent the culture medium from being affected by excessive total nitrogen concentration after multiple cycles, which may affect the growth of microalgae, the culture medium needs to pass through a carbon filter column after 10-15 cycles or when the ammonia nitrogen concentration in the culture medium exceeds 1000 mg / L. This high concentration of ammonia nitrogen in the culture medium will inhibit the growth of microalgae and needs to be passed through a carbon filter column. At this time, the excessive concentration of ammonium nitrate in the culture medium will be adsorbed and removed. The carbon filter column is made of organic glass, and the culture medium stays in it for 100-200 seconds. The filter carbon is Fe3O4-BC obtained in S5, and commercial activated carbon can also be used. The adsorption saturated carbon column is regenerated by heating desorption. During heating desorption, nitrogen-containing substances such as ammonium nitrate are decomposed into gaseous substances and escape.

[0072] Among them, after the high-temperature pyrolysis in S4 is completed, air is introduced into the biomass pyrolysis furnace to burn the residual Fe-BC, and the gas produced by the combustion is absorbed by concentrated ammonia water to obtain ammonium bicarbonate, which serves as the carbon source and nitrogen source for microalgae cultivation.

[0073] When the system reaches a certain stage of operation, due to the continuous accumulation of biochar, residual biochar will be generated. The method for treating this part of biochar is as follows: after the microalgae are pyrolyzed at high temperature in the biomass pyrolysis furnace, nitrogen is introduced to expel the biogas in the furnace and then air is immediately introduced. The residual biochar is burned to CO2, and the generated gas is absorbed by concentrated ammonia water to generate ammonium bicarbonate, which can be used as a carbon and nitrogen source in the microalgae cultivation process. The relevant chemical reaction formula is:

[0074] C(biochar)+O2→CO2

[0075] NH3·H2O+CO2→(NH4)2CO3+H2O

[0076] (NH4)2CO3+CO2+H2O→NH4HCO3

[0077] like Figure 2As shown, an embodiment of the present invention provides a microalgae cultivation, harvesting and energy utilization system, which adopts the above-mentioned microalgae cultivation, harvesting and energy utilization process, including:

[0078] microalgae cultivation tank;

[0079] a flocculation tank, the water inlet of which is connected to the water outlet of the microalgae cultivation tank through an algae liquid discharge pipe;

[0080] A regulating tank, whose water inlet is connected to the water outlet of the flocculation tank, and whose water outlet is connected to the flocculation liquid return inlet of the microalgae cultivation tank through an algae liquid discharge pipe, and a plurality of sets of electromagnetic coils are arranged in the conical groove at the bottom of the flocculation tank;

[0081] An activated carbon filtration device, wherein the liquid inlet and the liquid outlet are connected to the flocculation supernatant return pipe through a first pipe and a second pipe respectively, a first valve is provided on the portion of the flocculation supernatant return pipe located between the first pipe and the second pipe, and a second valve is provided on both the first pipe and the second pipe;

[0082] a first dosing device, which is used to add chemicals to the regulating tank;

[0083] a second dosing device, which is used to add chemicals to the flocculation tank;

[0084] a first water quality online monitoring device, which is used to detect the water quality in the regulating tank;

[0085] A second water quality online monitoring device, which is used to detect the water quality in the flocculation tank;

[0086] a third online water quality monitoring device, which is used to detect the water quality in the microalgae cultivation pond; the first online water quality monitoring device, the second online water quality monitoring device, and the third online water quality monitoring device can all monitor online the real-time indicators of ammonia nitrogen, nitrate nitrogen, total nitrogen, total phosphorus, pH, dissolved oxygen, and chlorophyll;

[0087] A paddle wheel is arranged in the microalgae cultivation pond.

[0088] In the above embodiment, when the turbine is working, it drives the microalgae culture liquid in the microalgae culture tank to flow through the flocculation tank and the regulating tank in sequence, and finally returns to the microalgae culture tank.

[0089] When the activated carbon filter device is not in use, the first valve is opened and the second valve is closed. The microalgae culture liquid in the microalgae culture pool enters the flocculation pool through the algae liquid discharge pipe for flocculation. After flocculation is completed, the supernatant in the flocculation pool is discharged into the regulating pool for water quality adjustment. After adjustment, it is sent to the microalgae culture pool through the flocculation supernatant return pipe.

[0090] When the activated carbon filter device is in use, the first valve is closed and the second valve is opened. The supernatant in the flocculation tank is discharged into the regulating tank for water quality adjustment, flows through the activated carbon filter device, filters the supernatant through the activated carbon filter device, and finally is sent to the microalgae culture tank through the flocculation supernatant return pipe.

[0091] Preferably, in the microalgae cultivation, harvesting and energy utilization system, a U-shaped first baffle is vertically provided in the microalgae cultivation pool, a second baffle is vertically provided in the first baffle, one end of the second baffle extends out of the first baffle and is connected to the inner wall of the microalgae cultivation pool, and a gap is formed between the other end thereof and the arc portion of the first baffle.

[0092] In the above embodiment, if Figure 2 As shown, the microalgae cultivation ponds of the present invention are racetrack-shaped, with individual pools measuring 20-30 meters long, 3-5 meters wide, and 1.5-2.0 meters deep. Two sets of propellers are installed in each racetrack pool to power the water flow. The propeller blades are designed with a porous structure, which increases the CO2 dissolution efficiency during agitation and provides a carbon source for the growth of microalgae.

[0093] The runway-shaped pools are 20-30m long, 3-5m wide, and 1.5-2.0m deep. Two sets of propellers are installed in each runway pool to power the water flow. The propeller blades are designed with a porous structure, which increases the CO2 dissolution efficiency when agitated, providing a carbon source for the growth of microalgae.

[0094] The embodiment of the present invention provides a microalgae cultivation, harvesting and energy utilization process, using the above-mentioned microalgae cultivation, harvesting and energy utilization system, setting up three sets of parallel raceway culture tanks, each tank size is 25m×4m×1.8m, equipped with 6 sets of multi-hole propellers (pore diameter 2mm, porosity 40%), with an adjustable speed range of 5-15rpm

[0095] Online monitoring system configuration: pH / DO composite electrode (measurement accuracy ±0.1), chlorophyll fluorescence probe (detection limit 1μg / L), and spectral nutrient salt analysis module.

[0096] Harvesting unit: Flocculation tank volume 20m 3 The bottom of the machine is equipped with a 30° conical mud collecting trough and a built-in 16-set electromagnetic coil array (maximum field strength 120mT). It is equipped with an automatic ferric nitrate dosing system (metering accuracy ±1%) and an ammonia pH adjustment unit (flow control ±0.5L / min).

[0097] Microalgae energy system: The biomass pyrolysis furnace adopts a double-chamber design, with the main reaction chamber volume of 4m 3 (Working temperature 500±10℃), regeneration chamber volume 1m 3(working temperature 800±15℃), set up waste heat recovery system, tubular heat exchanger area 20m 2 , steam generation capacity 50kg / h.

[0098] Specific harvesting process:

[0099] When the chlorophyll a concentration reaches 50μg / L, the algae liquid discharge valve will be automatically opened and discharged at a rate of 15m 3 / h flow rate to the flocculation tank to deliver the culture solution. The turbine is switched to low speed mode (8rpm) to maintain the continuous operation of the culture system. During the flocculation operation, 0.06g / L ferric nitrate solution (Fe 3+ Purity ≥99%), stirred for 5 minutes; added a mixture of Fe3O4-BC and Fe-BC (15 mg / L), stirred, and slowly added 28% concentrated ammonia water until the pH reached 8.8; after magnetic stirring (200 rpm) for 2 minutes, the magnetic field (80 mT) was activated, achieving 99.8% algae sedimentation within 15 minutes, a 92% reduction in sedimentation time compared to traditional gravity sedimentation.

[0100] The flocculated algae slurry was treated with a 20kHz probe ultrasonic instrument (0.8W / mL) for 45 minutes, increasing the release rate of nitrogen and phosphorus adsorbed on the algae to 80%. Centrifugation was then performed in a 3000g decanter centrifuge. The first centrifugation step (3000g, 20 minutes) recovered the iron hydroxide colloid in the lower layer, leaving a highly concentrated algae slurry on the upper layer. The second centrifugation step (5000g, 25 minutes) reduced the slurry's moisture content to 20%. The centrifuge was then returned to the flocculation tank, and the slurry was conveyed to the dryer via a screw conveyor. The drying system utilized the residual heat from pyrolysis (250°C flue gas) to further reduce the slurry's moisture content to 8%.

[0101] The pyrolysis process was heated to 480℃ at 25℃ / min under nitrogen protection, producing bio-oil (yield 32%), combustible gas (calorific value 18MJ / m 3 ), the reducing gas produced by pyrolysis (CO accounts for 15%, H2 accounts for 8%) 3+ Reduced to zero-valent iron (conversion rate 96%), and then introduced 110℃ superheated steam (flow rate 1.0m 3 / h), and the iron oxidation reaction was completed at 800 ° C to obtain magnetically enhanced Fe3O4-BC (the specific surface area increased by 40% to 650m 2 / g).

[0102] Treatment of supernatant: add nitric acid (adjust pH to 7.2) and Fe3O4-BC (0.015%) to the regulating tank, supplement nitrogen and phosphorus after precipitation (mainly supplement TP to a mass concentration of 40-50 mg / L), and return it to the culture tank.

[0103] Preparation of Fe3O4-BC: The residual biochar was pyrolyzed and steam was introduced at 800℃ for 2h to generate Fe3O4-BC (specific surface area 650m 2 / g), used as subsequent flocculant or adsorbent.

[0104] Treatment of residual carbon: Excess biochar is introduced into air and burned, and the tail gas is absorbed by concentrated ammonia water to generate ammonium bicarbonate (recycled as carbon and nitrogen sources).

[0105] After the culture fluid circulated 12 times, the online monitoring device detected that the ammonia nitrogen concentration reached 1000 mg / L, reaching the set threshold. At this point, the culture fluid circulation system closed the first valve and opened the two second valves simultaneously. The circulating culture fluid from the regulating tank no longer entered the culture tank directly, but was first filtered through the carbon column before being delivered to the culture tank. After the culture fluid remained in the carbon column for 150 seconds, the ammonia nitrogen concentration dropped to 500 mg / L.

[0106] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the description and implementation methods. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to the specific details and embodiments shown and described herein.

Claims

1. A method for harvesting microalgae by flocculation, characterized in that: The following steps are involved: Adding ferric nitrate to the microalgae culture solution until the ferric nitrate concentration in the microalgae culture solution is 0.05-0.10 g / L; A mixture of Fe3O4-BC and Fe-BC was added to the microalgae culture solution at a dosage of 15-20 mg / L, and after stirring evenly, ammonia water was added to adjust the pH of the microalgae culture solution to 8.5-9.

0. After slowly stirring evenly, the solution was allowed to stand, and the flocculated sediment at the bottom of the flocculation tank was collected to obtain an algae slurry mixture.

2. The microalgae flocculation and harvesting method according to claim 1, characterized in that: In the mixture added to the microalgae culture medium, the mass ratio of Fe3O4-BC to Fe-BC was 10:

1.

3. The microalgae flocculation and harvesting method according to claim 1, wherein: The microalgae culture solution is placed in a magnetic field with a magnetic field strength of 50-100 mT.

4. A process for cultivating, harvesting and converting microalgae into energy, characterized in that: The following steps are involved: S1. When the chlorophyll a content in the microalgae culture solution exceeds 50 μg / L, harvesting the microalgae from the microalgae culture solution using the microalgae flocculation harvesting method according to any one of claims 1 to 3 to obtain an algae slurry mixture and a supernatant; S2. Nitric acid is added to the supernatant obtained in S1 to adjust its pH to 7.0-7.5, and then 0.01-0.02% of the mass of the supernatant is added with Fe3O4-BC to cause precipitation. After supplementing nitrogen, phosphorus and trace elements, the supernatant is recycled as a microalgae culture solution; S3, subjecting the algae slurry mixture obtained in S1 to ultrasonic treatment, centrifugation, and then drying to obtain a microalgae biomass product; S4, sending the microalgae biomass product obtained in S3 into a biomass pyrolysis furnace for high-temperature pyrolysis to obtain bio-oil and biogas, as well as residual Fe-BC. The tail gas of the biomass pyrolysis furnace is used to dry the algae pulp mixture in S3; S5. The Fe-BC obtained in S4 is introduced into water vapor at high temperature to prepare Fe3O4-BC.

5. The microalgae cultivation, harvesting and energy utilization process according to claim 4, characterized in that: The algae slurry mixture in S3 was ultrasonically treated at an ultrasonic intensity of 0.5-1.0 W / mL for 30-60 min.

6. The microalgae cultivation, harvesting and energy utilization process according to claim 4, characterized in that: The centrifugal separation in S2 specifically includes: S3.

1. Centrifuge the ultrasonically treated algae slurry mixture at 3000 g for 15-30 minutes to obtain ferric hydroxide colloid and a primary high-concentration algae slurry; S3.

2. Centrifuge the high-concentration algae pulp obtained in S3.1 at a centrifugal force of 5000g for 20-30 minutes to obtain a secondary high-concentration algae pulp. The secondary high-concentration algae pulp is dried to obtain the microalgae biomass product. The liquid obtained by centrifugation is returned to the microalgae culture medium for recycling.

7. The microalgae cultivation, harvesting and energy utilization process according to claim 4, characterized in that: After the microalgae culture solution has been recycled 10-15 times, or when the ammonia nitrogen concentration in the microalgae culture solution exceeds 1000 mg / L, the microalgae culture solution is filtered through an activated carbon filter column using the Fe3O4-BC obtained in S5.

8. The microalgae cultivation, harvesting and energy utilization process according to claim 4, characterized in that: After the high-temperature pyrolysis in S4 is completed, air is introduced into the biomass pyrolysis furnace to burn the residual Fe-BC, and the gas produced by the combustion is absorbed by concentrated ammonia water to obtain ammonium bicarbonate, which serves as the carbon and nitrogen source for microalgae cultivation.

9. A microalgae cultivation, harvesting and energy utilization system, using the microalgae cultivation, harvesting and energy utilization process according to any one of claims 4 to 8, characterized in that: include: microalgae cultivation tank; a flocculation tank, the water inlet of which is connected to the water outlet of the microalgae cultivation tank through an algae liquid discharge pipe; A regulating tank, whose water inlet is connected to the water outlet of the flocculation tank, and whose water outlet is connected to the flocculation liquid return inlet of the microalgae cultivation tank through an algae liquid discharge pipe, and a plurality of sets of electromagnetic coils are arranged in the conical groove at the bottom of the flocculation tank; An activated carbon filtration device, wherein the liquid inlet and the liquid outlet are connected to the flocculation supernatant return pipe through a first pipe and a second pipe respectively, a first valve is provided on the portion of the flocculation supernatant return pipe located between the first pipe and the second pipe, and a second valve is provided on both the first pipe and the second pipe; a first dosing device, which is used to add chemicals to the regulating tank; a second dosing device, which is used to add chemicals to the flocculation tank; a first water quality online monitoring device, which is used to detect the water quality in the regulating tank; A second water quality online monitoring device, which is used to detect the water quality in the flocculation tank; a third water quality online monitoring device, which is used to detect the water quality in the microalgae cultivation pool; A paddle wheel is arranged in the microalgae cultivation pond.

10. The microalgae cultivation, harvesting and energy utilization system according to claim 9, characterized in that: A U-shaped first baffle is vertically provided in the microalgae cultivation pool, and a second baffle is vertically provided in the first baffle. One end of the second baffle extends out of the first baffle and is connected to the inner wall of the microalgae cultivation pool, and there is a gap between the other end and the arc portion of the first baffle.

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