Microalgae recycling method based on microwave-assisted pyrolysis

Abstract

The invention provides a microalgae recycling method based on microwave-assisted pyrolysis. The method comprises the following steps: pretreating microalgae, mixing the pretreated microalgae with a microwave absorption material, carrying out microwave-assisted heating for pyrolysis, purging volatile components by carrier gas to pass through a condensation absorption device so as to obtain bio-oil which can be condensed and absorbed and biogas which cannot be condensed and absorbed, and taking the residual solid residue in the device as biochar. The bio-oil not only can be used as fuel, but also can be used for extracting and preparing high-value chemicals; the biogas mainly comprises hydrogen, methane, carbon monoxide and carbon dioxide, can be used as fuel gas, and can also be further processed into synthesis gas for chemical synthesis; the biochar is rich in nutrient elements such as nitrogen, phosphorus and potassium, so that the biochar is widely applied to soil improvement and water pollution remediation. In addition, different product yields and properties can be obtained by regulating and controlling pyrolysis conditions. According to the method, microalgae biomass energy can be fully utilized, and high economic and environmental effects are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of biomass energy utilization, and particularly relates to a microalgae resource utilization method based on microwave-assisted pyrolysis. Background Art

[0002] With the rapid growth of the world's population and the development of industrialization, human demand for energy is increasing. From 2000 to 2017, global energy demand increased from 439EJ to 627EJ at an average annual growth rate of 2.1%, and multiple reports predict that energy demand will reach 800-900EJ by 2040. However, greenhouse gases emitted by the large-scale use of fossil energy are considered to be the main factor causing climate change. Studies have shown that the global average temperature is about 1°C higher than before industrialization. If climate change is not well controlled, humans will face a series of disasters caused by climate change in the future, such as increased extreme weather, rising sea levels, and species extinction. In order to cope with these problems, the development of renewable energy has become a research hotspot. Among them, biomass energy has received widespread attention in my country and even in other countries around the world due to its advantages such as carbon neutrality and abundant raw materials.

[0003] Among the many biomass energy raw materials, microalgae have been selected as the raw material for the third generation of biofuels due to their unique advantages, replacing the second generation of biofuels derived from non-food raw materials such as agricultural waste. Compared with terrestrial crops, microalgae have higher photosynthesis efficiency, faster growth rate and stronger carbon dioxide fixation ability. In addition, microalgae can also be cultivated with carbon dioxide in exhaust gas and nutrients in wastewater from factories such as steel mills, thereby achieving multiple purposes such as reducing pollution, recycling nutrients, and not occupying arable land.

[0004] The utilization of biomass resources by pyrolysis technology is one of the most studied technologies in the past 50 years. Pyrolysis is defined as the thermal degradation of biomass by heat in the absence of oxygen to produce three-phase products, namely bio-oil, biochar and biogas. However, in the traditional electric heating pyrolysis process, heat is mainly transferred to the biomass by conduction, convection and radiation, and the biomass is heated from the outside to the inside, which usually leads to disadvantages such as low pyrolysis efficiency and low product quality. Summary of the invention

[0005] The purpose of the present invention is to provide a method for recycling microalgae resources based on microwave-assisted pyrolysis, which can alleviate energy crisis, greenhouse effect and environmental pollution.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for recycling microalgae resources based on microwave-assisted pyrolysis comprises the following steps:

[0008] Step S1: Microalgae pretreatment:

[0009] Pre-treating the microalgae to improve its subsequent pyrolysis efficiency;

[0010] Step S2: Microwave assisted pyrolysis:

[0011] The pretreated microalgae are mixed with a microwave absorbing material and subjected to microwave-assisted pyrolysis, and the volatiles generated during the pyrolysis are blown out of the pyrolysis chamber by a carrier gas;

[0012] S3: Condensation and separation of volatiles:

[0013] The volatile matter is processed by a condensation absorption device, the condensable part is collected as bio-oil, and the non-condensable part is separated, dried and collected as biogas;

[0014] S4: Biochar Collection:

[0015] After the microwave-assisted pyrolysis is completed, the solid residue left by the microalgae is biochar.

[0016] Preferably, in step S1, the microalgae include natural algae species, mutant algae species, and transgenic algae species.

[0017] Preferably, in step S1, the pretreatment method includes drying, grinding and screening.

[0018] Preferably, in step S2, the carrier gas includes argon, nitrogen, and carbon dioxide, and the flow rate of the carrier gas is 50-2000 mL / min.

[0019] Preferably, in step S2, the microwave absorbing material includes silicon nitride, silicon carbide, activated carbon, biochar, and iron oxide.

[0020] Preferably, in step S2, the mixing method of the pretreated microalgae and the microwave absorbing material includes: placing the microalgae and microwave absorbing material powder in a quartz crucible for physical stirring and mixing, and placing the microalgae in a crucible made of microwave absorbing material. Can the underlined part be changed to: or placing the microalgae in a crucible made of microwave absorbing material for pyrolysis treatment.

[0021] Preferably, in step S2, the frequency of the microwave-assisted pyrolysis is 915-3000 MHz, the microwave power is 200-1400 W, the pyrolysis temperature is 200-1100° C., and the pyrolysis time is 20-120 min.

[0022] Preferably, in step S3, an organic solvent is added to the condensation absorption device as an absorbent to enhance the collection of bio-oil.

[0023] Preferably, the absorbent comprises dichloromethane.

[0024] Preferably, in step S2, the biogas contains carbon monoxide, methane, carbon dioxide and hydrogen.

[0025] Research has shown that during microwave-assisted pyrolysis, microwaves can penetrate biomass particles and convert microwave energy into heat energy inside the particles. Heat energy continuously accumulates inside the biomass particles and transfers outward, forming rapid and uniform heating. Therefore, microwave-assisted pyrolysis can become a key technology to promote the comprehensive and efficient utilization of microalgae biomass energy, and it is necessary to construct an effective method.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The present invention converts microalgae into bio-oil, biogas and biochar through microwave-assisted pyrolysis technology, thereby realizing high-value utilization of waste biomass. The present invention fully utilizes the nutrient elements in sewage (such as biogas slurry) to cultivate microalgae, reduce eutrophication of water bodies, improve the level of sewage resource utilization, and promote resource recycling.

[0028] (1) The bio-oil produced by the present invention can be used as fuel oil or chemical raw material, and the biogas (such as methane, hydrogen, carbon monoxide) can be used for combustion for heating or power generation, partially replacing fossil energy. The product distribution is optimized through microwave-assisted pyrolysis, the energy utilization efficiency is improved, and the development of clean energy is promoted; the development of renewable energy is realized, and the dependence on fossil fuels is reduced.

[0029] (2) The present invention uses CO2 emitted by industries such as steel mills 2 As a carbon source for the growth of microalgae, it promotes the fixation of carbon dioxide by microalgae and reduces greenhouse gas emissions. Combined with microalgae carbon capture technology, it promotes the construction of a low-carbon industrial system. It reduces industrial carbon emissions and realizes carbon capture and utilization.

[0030] (3) The biochar of the present invention can be used for water pollution control (removing heavy metals and organic pollutants in water) and soil improvement (improving soil fertility and enhancing carbon sink capacity). Combined with sewage treatment and soil remediation applications, the dual benefits of pollutant reduction and resource recycling can be achieved. The optimization of environmental governance and the improvement of ecological restoration capabilities are achieved.

[0031] (4) The present invention adopts microwave-assisted pyrolysis, which has the advantages of fast heating rate, low energy consumption, uniform heating, etc. compared with the traditional pyrolysis method, shortens the pyrolysis time, and improves the conversion efficiency. The present invention can accurately control the yield of bio-oil, biogas and biochar by adjusting parameters such as microwave power, pyrolysis temperature, and microwave absorbing materials to meet different application requirements. It achieves the improvement of pyrolysis efficiency and optimization of product distribution.

[0032] (5) The present invention improves the level of microalgae resource utilization through green technology and achieves the coordinated development of energy and environmental protection. This technology has broad application prospects in multiple fields such as renewable energy, industrial carbon emission reduction, and environmental governance, and can provide important support for the global sustainable development strategy. It can promote sustainable development and alleviate energy crises and environmental pollution.

[0033] The present invention will convert microalgae into bio-oil, biogas and biochar through microwave-assisted pyrolysis, thereby turning waste into treasure. A large amount of sewage such as biogas slurry, which contains nutrients, can be used to cultivate microalgae. In addition, the tail gas containing carbon dioxide emitted by factories such as steel mills can also be used as a carbon source for the growth of microalgae. The bio-oil and biogas generated by microwave-assisted pyrolysis of microalgae can replace part of the fossil fuels, and the biochar can be used for environmental applications such as water pollution control and soil improvement. In summary, the microalgae resource technology based on microwave-assisted pyrolysis has great potential to alleviate the energy crisis, greenhouse effect and environmental pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A schematic diagram of a process flow of a method for recycling microalgae resources based on microwave-assisted pyrolysis provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0036] like Figure 1 As shown, the microalgae resource utilization method based on microwave-assisted pyrolysis of the present invention comprises the following steps:

[0037] Step S1: Microalgae pretreatment:

[0038] The microalgae is dried to a moisture content of less than 15%, then ground and finally sieved through a sieve to improve its subsequent pyrolysis efficiency.

[0039] In this embodiment, the microalgae include natural algae species, mutant algae species, and transgenic algae species, specifically including but not limited to Chlorella, Spirulina, and Scenedesmus.

[0040] Step S2: Microwave assisted pyrolysis:

[0041] The pretreated microalgae are mixed with a microwave absorbing material and subjected to microwave-assisted pyrolysis. The volatiles generated during the pyrolysis process are blown out of the pyrolysis chamber by a carrier gas selected from an inert gas (such as argon) or a reactive gas (such as carbon dioxide) passing through the pyrolysis chamber at a flow rate of 50-2000 mL / min. The inert gas is used to exclude air and blow the volatiles generated during the pyrolysis of the microalgae out of the pyrolysis chamber to collect biogas and bio-oil, while the reactive gas, in addition to the above functions, also has the function of changing the yield distribution and properties of the pyrolysis products.

[0042] In this embodiment, the pretreatment method includes drying, grinding, and screening. Further, the mixing method of the pretreated microalgae and the microwave absorbing material includes: placing the microalgae and microwave absorbing material powder in a quartz crucible for physical stirring and mixing, and placing the microalgae in a crucible made of microwave absorbing material. Can the underlined part be changed to: or placing the microalgae in a crucible made of microwave absorbing material for pyrolysis treatment.

[0043] The mixing method of the pretreated microalgae and the microwave absorbent includes: placing the microalgae and microwave absorbent material powder in a quartz crucible for physical stirring and mixing, or placing the microalgae in a crucible made of microwave absorbent material.

[0044] In this embodiment, the microalgae itself has a weak ability to absorb microwaves. In order to improve the pyrolysis efficiency, microwave absorbing materials are added. The microwave absorbing materials include silicon nitride, silicon carbide, activated carbon, biochar, iron oxide and other materials with strong microwave absorbing ability. It is worth noting that some microwave absorbing materials also have a catalytic effect.

[0045] S3: Condensation and separation of volatiles:

[0046] The volatiles are processed by a condensation absorption device, the condensable part is collected as bio-oil, and the non-condensable part is collected as biogas after separation and drying; the absorbent in the condensation absorption device includes dichloromethane. Biogas contains carbon monoxide, methane, carbon dioxide and hydrogen.

[0047] In this embodiment, an organic solvent such as dichloromethane can be added to the condensation absorption device as an absorbent to enhance the collection of bio-oil.

[0048] The organic solvent can be removed by evaporation to obtain bio-oil, which can be used as a fuel or to extract and prepare high-value chemicals, such as nitriles, which are widely used in medicine and agrochemicals.

[0049] The main components of biogas are carbon monoxide, methane, carbon dioxide and hydrogen. The relative content of carbon monoxide, methane and hydrogen can be increased by adjusting the reaction conditions (such as pyrolysis temperature). Biogas is considered to be a promising fuel source or raw material for producing chemicals, especially for the direct synthesis of methanol and ethylene glycol based on hydrogen and carbon monoxide.

[0050] S4: Biochar Collection:

[0051] After the microwave-assisted pyrolysis is completed, the microalgae in the crucible are cooled to room temperature under the protection of carrier gas, and the solid residue remaining in the crucible is biochar.

[0052] In this embodiment, the frequency of microwave-assisted pyrolysis is 915-3000 MHz, the microwave power is 200-1400 W, preferably 400-1000 W; the pyrolysis temperature is 200-1100° C., preferably 400-800° C.; the pyrolysis time is 20-120 min, preferably 30-90 min.

[0053] Microwave-assisted pyrolysis has a higher pyrolysis efficiency than traditional external heating pyrolysis. This is because, during the microwave-assisted pyrolysis process, electromagnetic waves can penetrate the raw materials and induce volume heating from the inside out. This not only overcomes the problem of uneven heating, but also reduces the overall energy consumption. In addition, microwave-assisted pyrolysis can also provide higher heating rates and product selectivity.

[0054] Microalgae have high protein content, and the biochar produced by microwave-assisted pyrolysis contains nutrients such as nitrogen, phosphorus, and potassium. It can be applied to the soil as a slow-release fertilizer to increase crop yields. The application of biochar in soil improvement can not only improve soil fertility, but also improve soil structure, repair soil pollution, and increase soil carbon sequestration. In addition, the high nitrogen content of microalgae biochar makes it widely used as an adsorbent and catalyst for sewage treatment.

[0055] The present invention will be further described below with reference to the embodiments.

[0056] Embodiment 1:

[0057] A method for recycling pyrolytic Chlorella vulgaris based on microwave-assisted pyrolysis, the steps are as follows:

[0058] Step S1: drying the Chlorella vulgaris and sieving it with a 200-mesh sieve;

[0059] Step S2: placing in a silicon nitride crucible for microwave-assisted pyrolysis, the microwave frequency is 2450 MHz, the power is 1000 W, the pyrolysis time is 66 min, and the volatiles generated during the pyrolysis process are blown out of the pyrolysis chamber by 80 mL / min of high-purity argon gas;

[0060] Step S3: passing the volatile matter generated in step S2 through a condensation absorption device filled with dichloromethane, the condensable absorption part is bio-oil, and the non-condensable absorption part is separated, dried and collected with a gas collection bag to obtain biogas whose main components are hydrogen, carbon monoxide, methane and carbon dioxide;

[0061] Step S4: After the microwave-assisted pyrolysis process is completed, the solid residue of Chlorella pyrenoidosa in the silicon nitride crucible is cooled to room temperature under the protection of argon gas, and the solid residue is biochar.

[0062] In order to verify the yield and properties of the three-phase products under different conditions, the pyrolysis temperature in step (1) was set to 400°C, 600°C and 800°C while keeping other conditions unchanged.

[0063] After microwave-assisted pyrolysis, the products were immediately analyzed, and the yield distribution and properties of the three-phase products were shown in Tables 1, 2, 3 and 4.

[0064] Table 1 Yield distribution of three-phase products at different pyrolysis temperatures

[0065] 400℃ 600℃ 800℃ Biochar (wt.%) 33.18 26.96 24.68 Bio-oil (wt.%) 38.89 39.58 37.35 Biogas (wt.%) 27.92 33.46 37.97

[0066] Table 2 Distribution of biogas components at different pyrolysis temperatures

[0067] 400℃ 600℃ 800℃ hydrogen(%) 3.84 38.28 41.06 Carbon monoxide (%) 10.43 14.60 12.20 Methane (%) 4.84 21.26 13.87 carbon dioxide(%) 80.89 33.27 32.87

[0068] Table 3 Element content and calorific value of biochar at different pyrolysis temperatures

[0069] 400℃ 600℃ 800℃ Carbon (wt.%) 61.30 60.68 67.27 Hydrogen (wt.%) 3.67 1.80 0.61 Nitrogen (wt.%) 10.10 8.28 7.97 Oxygen (wt.%) 8.16 8.62 4.10 Sulfur (wt.%) 0.33 0.20 0.13 Calorific value (MJ / kg) 24.41 21.87 23.26

[0070] Table 4 Element content and calorific value of bio-oil at different pyrolysis temperatures

[0071] 400℃ 600℃ 800℃ Carbon (wt.%) 64.10 66.83 66.64 Hydrogen (wt.%) 7.81 8.91 7.79 Nitrogen (wt.%) 9.87 9.35 9.92 Oxygen (wt.%) 17.63 14.59 15.23 Sulfur (wt.%) 0.60 0.32 0.39 Calorific value (MJ / kg) 29.66 32.21 30.76

[0072] As can be seen from the above table, by adjusting the pyrolysis temperature, the yield distribution of the three-phase products obtained in the microalgae resource method of this embodiment is different, wherein the yield of biochar decreases with increasing temperature, while the yield of biogas increases with increasing temperature. In addition, the properties of the three-phase products are also significantly affected by temperature. Among them, the hydrogen content in biogas increases with increasing temperature, while the content of carbon dioxide decreases with increasing temperature. The nitrogen content in biochar accounts for 7.97-10.10wt.%, showing the potential to replace nitrogen fertilizer. The calorific value of bio-oil is 29.66-32.21MJ / kg depending on the content of the main elements. In short, the yield and properties of the product can be controlled by regulating the pyrolysis conditions, and the method is flexible and controllable.

[0073] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" 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 referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0074] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0075] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for recycling microalgae resources based on microwave-assisted pyrolysis, characterized in that: The steps include: Step S1: Microalgae pretreatment: Pre-treating the microalgae to improve its subsequent pyrolysis efficiency; Step S2: Microwave assisted pyrolysis: The pretreated microalgae are mixed with a microwave absorbing material and subjected to microwave-assisted pyrolysis, and the volatiles generated during the pyrolysis are blown out of the pyrolysis chamber by a carrier gas; S3: Condensation and separation of volatiles: The volatile matter is processed by a condensation absorption device, the condensable part is collected as bio-oil, and the non-condensable part is separated, dried and collected as biogas; S4: Biochar Collection: After the microwave-assisted pyrolysis is completed, the solid residue left by the microalgae is biochar.

2. The method for recycling microalgae according to claim 1, characterized in that: In step S1, the microalgae include natural algae species, mutant algae species, and transgenic algae species.

3. The microalgae resource utilization method according to claim 1, characterized in that: In step S1, the pretreatment method includes drying, grinding and screening.

4. The method for recycling microalgae according to claim 1, characterized in that: In step S2, the carrier gas includes argon, nitrogen, and carbon dioxide, and the flow rate of the carrier gas is 50-2000 mL / min.

5. The method for recycling microalgae according to claim 1, characterized in that: In step S2, the microwave absorbing material includes silicon nitride, silicon carbide, activated carbon, biochar, and iron oxide.

6. The method for recycling microalgae according to claim 1, characterized in that: In step S2, the pretreated microalgae and the microwave absorbing material are mixed in a manner including: placing the microalgae and microwave absorbing material powder in a quartz crucible for physical stirring and mixing, or placing the microalgae in a crucible made of microwave absorbing material for pyrolysis treatment.

7. The method for recycling microalgae according to claim 1, characterized in that: In step S2, the frequency of the microwave-assisted pyrolysis is 915-3000 MHz, the microwave power is 200-1400 W, the pyrolysis temperature is 200-1100° C., and the pyrolysis time is 20-120 min.

8. The method for recycling microalgae according to claim 1, characterized in that: In step S3, an organic solvent is added to the condensation absorption device as an absorbent to enhance the collection of bio-oil.

9. The method for recycling microalgae according to claim 8, characterized in that: In step S3, the absorbent includes dichloromethane.

10. The method for recycling microalgae according to claim 1, characterized in that: In step S2, the biogas contains carbon monoxide, methane, carbon dioxide and hydrogen.

Images