Method for preparing high-yield low-nitrogen biomass oil based on organic component regulation and control of algal mud co-hydrothermal liquefaction

Abstract

The invention belongs to the technical field of preparation of biomass energy from high-nitrogen-content organic solid waste, and particularly relates to a method for preparing high-yield low-nitrogen biomass oil based on organic component regulation and control of algal mud co-hydrothermal liquefaction, and the method comprises the following steps: S1) carrying out alkali thermal hydrolysis pretreatment on waste algal mud, and then carrying out solid-liquid separation to obtain a low-nitrogen algal mud solid; and S2) taking an Fe-Co loaded modified molecular sieve as a catalyst, enabling the low-nitrogen algae mud solid to be in contact with the lipid-rich waste biomass, and performing co-hydrothermal liquefaction reaction to obtain the low-nitrogen biomass oil. Through the organic combination of the technical scheme, the key problem in the oil preparation process by algae mud hot liquefaction is solved synergistically, the source nitrogen reduction is obvious, the biomass oil yield is improved, the nitrogen migration path is optimized, and the high-quality low-nitrogen biomass oil product is obtained.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of biomass energy, and particularly relates to a method for preparing high-yield low-nitrogen biomass oil based on organic component regulation of algal sludge co-hydrothermal liquefaction. BACKGROUND

[0002] Lake eutrophication has become a global environmental problem, causing frequent outbreaks of blue-green algae blooms. After large-scale aggregation or death of blue-green algae, large-scale salvage is usually required to reduce the risk of water pollution. The algal liquid salvaged by the algal water separation station produces a large amount of waste algal sludge biomass. Although this algal sludge has a high organic matter content and is a potential biomass resource, its extremely high water content (usually up to 85%-90%), easy spoilage and potential release of toxins pose great challenges to subsequent treatment and disposal.

[0003] At present, common treatment methods for waste algal sludge include composting, landfilling and drying incineration. However, composting is difficult to control due to fast spoilage, heavy odor and unbalanced nutrient composition of algal sludge. Landfilling and incineration require further dewatering or drying of algal sludge, which increases disposal costs. Landfilling requires occupation of landfill capacity and poses risks of landfill instability and leachate pollution. Drying incineration produces flue gas containing NOx and dioxin, which easily causes secondary environmental pollution risk.

[0004] Hydrothermal liquefaction technology is considered as one of the promising core technologies for biomass resource utilization. It avoids the high-energy consumption of drying pretreatment. Under the conditions of 250-350℃ and 5-15Mpa, the organic matter in biomass is rapidly converted into high-energy-density biomass oil fuel through a series of complex reactions such as pyrolysis, hydrolysis, decarboxylation and polymerization, realizing high-value utilization of high-moisture organic solid waste. However, high-nitrogen biomass (N content usually >8wt%) such as algal sludge will have its nitrogen-containing components such as proteins and amino acids migrate to the biomass oil product through deamination and Maillard reactions during hydrothermal conversion, resulting in 20%-40% of nitrogen accumulating in the biomass oil in a form difficult to degrade, which reduces the oil heat value and combustion stability and causes subsequent NOx emission problems. Therefore, the high nitrogen content in biomass oil is one of the key bottlenecks for large-scale commercial application of hydrothermal liquefaction technology of nitrogen-containing biomass (especially protein-rich algae), significantly increasing the cost, technical difficulty and environmental pressure of subsequent upgrading.

[0005] In order to break through the bottleneck of high nitrogen in algae sludge hydrothermal liquefaction biomass oil, improve the economy and practicability of the technology, how to effectively reduce the nitrogen content in biomass oil while ensuring the yield and quality of biomass oil is a core scientific problem and technical difficulty to be solved, which has important significance for promoting the practical application of hydrothermal liquefaction technology, promoting the conversion of waste biomass energy and alleviating environmental pollution. SUMMARY

[0006] Therefore, the technical problem to be solved by the present application is to provide a method for preparing high-yield low-nitrogen biomass oil based on organic component regulation of algae sludge co-hydrothermal liquefaction.

[0007] The present application provides a comprehensive method integrating raw material pretreatment, organic component regulation and catalytic process optimization, which can effectively reduce the nitrogen content in algae sludge hydrothermal liquefaction bio-oil, while improving the yield of biomass oil, ultimately obtaining high-yield low-nitrogen biomass oil, and improving the economy and feasibility of algae sludge resource utilization.

[0008] To achieve the above-mentioned purpose, the present application provides a method for preparing high-yield low-nitrogen biomass oil based on organic component regulation of algae sludge co-hydrothermal liquefaction, comprising the following steps:

[0009] S1) Alkaline hydrolysis pretreatment of waste algae sludge, followed by solid-liquid separation to obtain low-nitrogen algae sludge solid;

[0010] S2) Using Fe-Co loaded modified molecular sieve as catalyst, contacting the above-mentioned low-nitrogen algae sludge solid with lipid-rich waste biomass and carrying out co-hydrothermal liquefaction reaction to obtain low-nitrogen biomass oil.

[0011] Figure 1 The present application provides a method for preparing high-yield low-nitrogen biomass oil based on organic component regulation of algae sludge co-hydrothermal liquefaction.

[0012] The present application does not specially limit the water content and N content of the waste algae sludge, and general waste algae sludge is suitable for the method provided by the present application. In some specific embodiments, the water content of the waste algae sludge is 84.3% to 97.5%, and the N content is 5.5wt% to 10.1wt%.

[0013] The present application first carries out alkaline hydrolysis pretreatment on waste algae sludge. Preferably, the alkaline hydrolysis pretreatment is carried out using an aqueous solution of alkali metal hydroxide, more preferably using an aqueous solution of potassium hydroxide or sodium hydroxide, and more preferably using an aqueous solution of potassium hydroxide.

[0014] The present application carries out alkali thermal hydrolysis pretreatment on waste algae mud, strengthens the deamination reaction of protein components in the algae mud, promotes the transfer of nitrogen elements in the algae mud raw material to the liquid phase (aqueous phase) and mainly exists in the form of ammonia / ammonium salt and is removed. Through this step, low-nitrogen algae mud solids are obtained as the main raw material for subsequent hydrothermal liquefaction, which significantly reduces the initial nitrogen content in the hydrothermal liquefaction reaction system, achieving the purpose of "source nitrogen reduction".

[0015] The test results show that the alkali thermal hydrolysis pretreatment can improve the biomass oil yield, strengthen the removal of nitrogen source, reduce the N content, and remove 30%-60% of the nitrogen in the algae mud.

[0016] The temperature of the alkali thermal hydrolysis pretreatment is preferably 90-160°C, and in some specific embodiments, the temperature of the alkali thermal hydrolysis pretreatment is 150°C.

[0017] The time of the alkali thermal hydrolysis pretreatment is preferably 15-60 min, and in some specific embodiments, the time of the alkali thermal hydrolysis pretreatment is 60 min.

[0018] The pH value of the alkali thermal hydrolysis pretreatment is 10-13, i.e. the pH value of the solution used in the alkali thermal hydrolysis pretreatment is 10-13, and in some specific embodiments, the pH value of the alkali thermal hydrolysis pretreatment is 12.

[0019] In some specific embodiments, the alkali thermal hydrolysis pretreatment is carried out in a hydrothermal kettle.

[0020] After the reaction is completed, preferably, solid-liquid separation is carried out by suction filtration to obtain a nitrogen-rich liquid phase and a low-nitrogen solid phase, the separated solid is collected and dried, i.e. low-nitrogen algae mud solids are obtained.

[0021] The temperature of the drying is preferably 50-70°C, and in some specific embodiments, the temperature of the drying is 60°C.

[0022] Then the low-nitrogen algae mud solids obtained above are used as the main raw material for subsequent co-hydrothermal liquefaction for co-hydrothermal liquefaction reaction.

[0023] The present application introduces Fe-Co supported modified molecular sieves as catalysts in co-hydrothermal liquefaction reaction, which greatly improves the biomass oil yield and reduces the N content. The molar ratio of Fe to Co is preferably (1:1) to (1:4).

[0024] The source of the Fe-Co supported modified molecular sieves is not particularly limited in the present application and can be purchased or prepared by oneself, and preferably prepared by the following method:

[0025] Mixing water-soluble Fe-containing compounds and water-soluble Co-containing compounds into a paste-like solution in an aqueous solution;

[0026] The above paste solution was contacted with a molecular sieve and calcined to obtain a Fe-Co supported and modified molecular sieve.

[0027] The water-soluble Fe-containing compound is preferably an Fe-containing nitrate. In some specific embodiments, the water-soluble Fe-containing compound is Fe(NO3)3·9H2O.

[0028] The water-soluble Co-containing compound is preferably a Co-containing nitrate. In some specific embodiments, the water-soluble Co-containing compound is Fe(NO3)2·9H2O.

[0029] The molecular sieve is preferably HZSM-5.

[0030] In the Fe-Co-modified molecular sieve, the ratio of the total mass of Fe and Co to the mass of the molecular sieve is preferably 1:(5-15), more preferably 1:10.

[0031] The preferred method for contact is to drop the above-mentioned paste-like solution onto the surface of the molecular sieve and allow it to make full contact.

[0032] The method of ensuring sufficient contact can be any method known to those skilled in the art, including but not limited to vibration on a shaker to ensure sufficient contact. The vibration time is preferably 20-50 minutes, more preferably 30 minutes.

[0033] The calcination temperature is preferably 500-600℃, more preferably 550℃; the calcination time is preferably 3-8h, more preferably 5h.

[0034] Preferably, the material is dried before calcination. The drying temperature is preferably 100-110℃, more preferably 105℃; the drying time is preferably 8-12h, more preferably 10h.

[0035] In some specific embodiments, the catalyst is Fe2-Co8@HSZM-5 molecular sieve.

[0036] Preferably, the Fe2-Co8@HSZM-5 molecular sieve is prepared according to the following method:

[0037] Fe(NO3)3·9H2O and Co(NO3)2·6H2O (Fe:Co mass ratio 2:8) and a small amount of ultrapure water were prepared into a paste solution. HZSM-5 powder was weighed and placed in a beaker (metal mass to HZSM-5 mass ratio 1:10). The prepared paste solution was evenly dropped onto the surface of the HZSM-5 molecular sieve using a dropper. The beaker was then placed on a shaker and shaken (preferably for 30 minutes). After shaking, it was placed in an oven for continuous drying at a constant temperature. The dried powder removed from the oven was then calcined in a muffle furnace. After calcination, it was ground into powder to obtain the Fe2-Co8@HSZM-5 catalyst.

[0038] The Fe-Co-supported modified molecular sieve, acting as a catalyst, can regulate the nitrogen migration pathway during hydrothermal liquefaction, inhibiting nitrogen from entering the oil phase and thus reducing the nitrogen content of biomass oil. Its mechanism of action includes: catalytic cracking of intermediate nitrogen-containing compounds, inhibiting Maillard reactions between nitrogen-containing intermediates (such as amino compounds) and carbohydrate degradation products, and inhibiting amidation reactions between nitrogen-containing components (such as amines / ammonia) and carboxylic acids to generate oil-soluble nitrogen-containing organic compounds (such as amides). Based on this, the catalyst guides nitrogen to migrate and transform into the aqueous phase (mainly in the form of NH4+) or the solid phase (in the form of pyridine nitrogen and quaternary nitrogen), maximally suppressing nitrogen from entering the final biomass oil product.

[0039] Preferably, the amount of catalyst added is 5% to 10% of the dry weight of the low-nitrogen algae mud solids.

[0040] The above-mentioned hydrothermal liquefaction process involves the addition of lipid-rich waste biomass to further improve biomass oil yield and reduce nitrogen content. The low-nitrogen algal sludge solids and the lipid-rich waste biomass exhibit a synergistic effect under hydrothermal conditions. This synergistic effect includes, but is not limited to, the active participation of lipid components in promoting the formation of high-yield biomass oil and contributing to improved biomass oil quality (such as H / C ratio, calorific value, etc.).

[0041] This invention does not specifically limit the type or source of the lipid-rich waste biomass, including but not limited to kitchen waste or its treatment products. The dry basis lipid content of the lipid-rich waste biomass is preferably ≥18 wt%.

[0042] In some specific implementations, the lipid-rich waste biomass is wet kitchen waste, wherein the water content is 72.6%, the nitrogen content is 2.03 wt%, and the lipid content is 20.32 wt%.

[0043] The preferred mass ratio of the low-nitrogen algae mud solid to the lipid-rich waste biomass is (5:5) to (7:3). In some specific embodiments, the mass ratio of the low-nitrogen algae mud solid to the lipid-rich waste biomass is 5:5 or 7:3.

[0044] The preferred temperature for the hydrothermal liquefaction reaction is 275-350℃, more preferably 325℃.

[0045] The preferred time for the co-hydrothermal liquefaction reaction is 10-60 min, more preferably 30 min.

[0046] The hydrothermal liquefaction reaction is preferably carried out in an inert atmosphere, preferably argon.

[0047] Preferably, the co-hydrothermal liquefaction reaction is carried out in a high-temperature and high-pressure reactor.

[0048] The hydrothermal liquefaction reaction is preferably carried out under stirring conditions.

[0049] Preferably, after the hydrothermal liquefaction reaction is completed, the process further includes:

[0050] The reaction products were collected and separated by extraction with dichloromethane. After extraction, the system was separated into an aqueous phase, a low-nitrogen biomass oil phase, and a solid phase.

[0051] In some specific implementation schemes, after the reaction is completed, the reaction system is cooled to room temperature; the reaction vessel is opened, the reaction products are collected and extracted and separated with dichloromethane to obtain aqueous phase, solid phase and oil phase products, the oil phase product being biomass oil.

[0052] The method provided by this invention can achieve a biomass oil yield of 38.7wt%-39.5wt%, and reduce the N content in the biomass oil to 1.07wt%-1.38wt%, which significantly reduces the subsequent quality improvement cost and solves the bottleneck of high-nitrogen biomass oil production.

[0053] This invention, through the organic combination of the above technical solutions, synergistically solves the key problems in the hydrothermal liquefaction of algae mud for oil production, and has the following significant beneficial effects:

[0054] 1) Significant nitrogen reduction at the source: Through alkaline hot water hydrolysis pretreatment, protein nitrogen in algae mud raw materials is efficiently removed (the solid nitrogen content is reduced by 40%-60% after pretreatment), reducing the initial nitrogen load entering the hydrothermal reaction system.

[0055] 2) Improved biomass oil yield: By introducing lipid-rich waste biomass and pretreated low-nitrogen algal mud for co-hydrothermal liquefaction, the synergistic effect of lipid components is fully utilized, significantly improving the final biomass oil yield.

[0056] 3) Optimization of nitrogen migration pathway: By using a specific Fe-Co supported modified molecular sieve catalyst, the hydrothermal reaction pathway is precisely controlled, effectively suppressing key side reactions that lead to nitrogen entering the oil phase, and promoting the transfer of nitrogen to the aqueous and solid phases.

[0057] 4) High-quality, low-nitrogen biomass oil products: This technology achieves both high yield and low nitrogen content in biomass oil, significantly increases the calorific value of biomass oil, and significantly reduces the difficulty and cost of subsequent biomass oil upgrading and processing, thereby enhancing the overall economic efficiency and competitiveness of the technology. Attached Figure Description

[0058] Figure 1 The process flow diagram provided by this invention is for the preparation of high-yield, low-nitrogen biomass oil by hydrothermal liquefaction of algae mud based on organic component regulation. Detailed Implementation

[0059] To further illustrate the present invention, a detailed description is provided below with reference to embodiments. However, it should be understood that these descriptions are merely for further illustrating the features and advantages of the invention and are not intended to limit the scope of the claims. Unless otherwise specified, the experimental methods described in the following embodiments are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0060] Example 1:

[0061] (1) Weigh 500g of wet algae mud (moisture content of 90.5% and nitrogen content of 8.83wt%) into a 1L hydrothermal reactor, and add 250mL of potassium hydroxide aqueous solution with pH value of 12. Then tighten and seal the hydrothermal reactor. Heat the reactor to 150℃ and maintain for 60min to enhance hydrolysis. After the reaction is completed, use the vacuum filtration method to separate the solid and liquid. Collect the separated solid and place it in a vacuum drying oven to dry to constant weight at 60℃. Take a part of the solid sample for elemental analysis and find that the nitrogen content is 3.71wt%. The obtained low nitrogen algae mud solid is used as the main raw material for subsequent co-hydrothermal liquefaction.

[0062] (2) Preparation of Fe2-Co8@HSZM-5 catalyst: 0.579 g of Fe(NO3)3·9H2O and 1.580 g of Co(NO3)2·6H2O were weighed and a small amount of ultrapure water was added to prepare a paste solution. 4 g of HZSM-5 powder was weighed and placed in a beaker (the mass ratio of metal to HZSM-5 was 1:10). The prepared paste solution was evenly dropped onto the surface of the molecular sieve HZSM-5 using a dropper. The beaker was then placed on a shaker and shaken for 30 min. After shaking, it was placed in an oven at 105℃ and dried continuously for 10 h. The dried powder taken out of the oven was then placed in a muffle furnace and calcined at 550℃ for 5 h. After calcination, it was taken out and ground into powder to obtain the Fe2-Co8@HSZM-5 catalyst.

[0063] (3) Accurately weigh 2.5g of the dry low-nitrogen algae mud solid from step (1) into a 100mL high-temperature and high-pressure reactor. At the same time, weigh 9.12g of wet kitchen waste (moisture content 72.6%, N content 2.03wt%, lipid content 20.32wt%) and 41.0mL of deionized water and add them to the reactor. The dry basis mixing ratio of algae mud and kitchen waste in the resulting mixed raw materials is 5:5. Continue to add 0.25g of Fe2-Co8@HSZM-5 catalyst powder prepared in step (2) to the reactor, mix evenly, and continuously introduce argon gas for 5min to remove air from the reactor. Then seal the reactor. Set the stirring speed to 150rpm and heat the reactor to 325℃ and maintain it for 30min. After the reaction is completed, use a fan to cool it down to room temperature. Open the reactor, collect the reaction products and extract and separate them with dichloromethane to obtain aqueous phase, solid phase and oil phase products. The oil phase product is biomass oil. The biomass oil yield was 39.5%, and the nitrogen content was measured to be 1.07 wt%.

[0064] Comparative Example 1:

[0065] 52.6 g of wet algal mud (90.5% water content, 8.83 wt% nitrogen content) was weighed and added to a 100 mL high-temperature and high-pressure reactor. Argon gas was continuously introduced for 5 min to remove air from the reactor, and then the reactor was sealed. The stirring speed was set to 150 rpm, and the reactor was heated to 325 °C and maintained for 30 min. After the reaction was completed, the reactor was cooled to room temperature using a fan. The reactor was opened, the reaction products were collected, and extracted and separated with dichloromethane to obtain the aqueous phase, solid phase, and biomass oil product. The biomass oil yield was 24.2%, and the nitrogen content was measured to be 6.82 wt%.

[0066] It can be seen from Example 1 and Comparative Example 1 that the biomass oil yield prepared from the untreated raw algal mud under the same hydrothermal liquefaction reaction conditions is low, and the nitrogen content in the biomass oil is high.

[0067] Comparative Example 2:

[0068] 26.3g of wet algae mud (moisture content 90.5%, nitrogen content 8.83wt%) and 9.12g of wet kitchen waste (moisture content 72.6%, nitrogen content 2.03wt%, lipid content 20.32wt%) were weighed and added to a 100mL high-temperature and high-pressure reactor. Then, 17.2mL of deionized water was added to adjust the total moisture content to 90.5%, resulting in a dry basis mixing ratio of 5:5 for the algae mud and kitchen waste in the mixed raw materials. Argon gas was continuously introduced for 5min to remove air from the reactor, and then the reactor was sealed. The stirring speed was set to 150rpm, and the reactor was heated to 325℃ and maintained for 30min. After the reaction was completed, the reactor was cooled to room temperature using a fan. The reactor was opened, the reaction products were collected, and extracted and separated with dichloromethane to obtain the aqueous phase, solid phase, and biomass oil product. The biomass oil yield was 33.6%, and the nitrogen content was measured to be 6.03wt%.

[0069] As can be seen from Example 1 and Comparative Example 2, the biomass oil prepared by co-hydrothermal liquefaction of untreated raw algal mud and kitchen waste under the same hydrothermal parameters has an increased yield and a decreased nitrogen content compared with Comparative Example 1. This indicates that co-hydrothermal liquefaction can promote the increase of biomass oil yield, but the effect of nitrogen reduction is limited without pretreatment and catalyst conditions.

[0070] Comparative Example 3:

[0071] (1) Weigh 500g of wet algae mud (moisture content of 90.5% and nitrogen content of 8.83wt%) into a 1L hydrothermal reactor, and add 250mL of potassium hydroxide aqueous solution with pH value of 12. Then tighten and seal the hydrothermal reactor. Heat the reactor to 150℃ and maintain for 60min to enhance hydrolysis. After the reaction is completed, use the vacuum filtration method to separate the solid and liquid. Collect the separated solid and place it in a vacuum drying oven to dry to constant weight at 60℃. Take a part of the solid sample for elemental analysis and find that the nitrogen content is 3.71wt%. The obtained low nitrogen algae mud solid is used as the main raw material for subsequent hydrothermal liquefaction.

[0072] (2) Accurately weigh 5.0 g of the dried low-nitrogen algal mud solid from step (1) into a 100 mL high-temperature and high-pressure reactor, and simultaneously add 47.6 mL of deionized water to the reactor to adjust the total water content to 90.5%; continue to add 0.25 g of Fe2-Co8@HSZM-5 catalyst powder, mix evenly, and continuously purge with argon gas for 5 min to remove air from the reactor, then seal the reactor; set the stirring speed to 150 rpm, heat the reactor to 325℃, and maintain for 30 min; after the reaction is complete, use a fan to cool it to room temperature; open the reactor, collect the reaction product, and extract and separate it with dichloromethane to obtain aqueous phase, solid phase, and oil phase products, the oil phase product being biomass oil. The biomass oil yield was 29.0%, and the measured N content was 2.79 wt%.

[0073] As can be seen from Example 1 and Comparative Examples 1 and 3, alkaline hot hydrolysis pretreatment can effectively reduce the initial nitrogen content entering the reaction system, and the addition of catalyst can effectively inhibit nitrogen from entering the oil phase product, but the biomass oil yield is slightly lower.

[0074] Comparative Example 4:

[0075] (1) Weigh 500g of wet algae mud (moisture content of 90.5% and nitrogen content of 8.83wt%) into a 1L hydrothermal reactor, and add 250mL of potassium hydroxide aqueous solution with pH value of 12. Then tighten and seal the hydrothermal reactor. Heat the reactor to 150℃ and maintain for 60min to enhance hydrolysis. After the reaction is completed, use the vacuum filtration method to separate the solid and liquid. Collect the separated solid and place it in a vacuum drying oven to dry to constant weight at 60℃. Take a part of the solid sample for elemental analysis and find that the nitrogen content is 3.71wt%. The obtained low nitrogen algae mud solid is used as the main raw material for subsequent co-hydrothermal liquefaction.

[0076] (2) Accurately weigh 2.5g of the dried low-nitrogen algae mud solid from step (1) into a 100mL high-temperature and high-pressure reactor. Simultaneously weigh 9.12g of wet kitchen waste (moisture content 72.6%, N content 2.03wt%, lipid content 20.32wt%) and 41.0mL of deionized water into the reactor to adjust the total moisture content to 90.5%. The dry basis mixing ratio of the algae mud and kitchen waste in the resulting mixed raw materials is 5:5. Mix evenly, continuously introduce argon gas for 5min to remove air from the reactor, and then seal the reactor. Set the stirring speed to 150rpm and heat the reactor to 325℃, maintaining it for 30min. After the reaction is complete, use a fan to cool it to room temperature. Open the reactor, collect the reaction products, and extract and separate them with dichloromethane to obtain aqueous, solid, and oil phase products. The oil phase product is biomass oil. The biomass oil yield is 34.2%, and the N content is measured to be 4.36wt%.

[0077] As can be seen from Example 1 and Comparative Example 4, the catalyst plays a crucial role in further significantly reducing the nitrogen content in biomass oil in the co-hydrothermal system.

[0078] Example 2:

[0079] (1) Weigh 500g of wet algae mud (moisture content of 90.5% and nitrogen content of 8.83wt%) into a 1L hydrothermal reactor, and add 250mL of potassium hydroxide aqueous solution with pH value of 12. Then tighten and seal the hydrothermal reactor. Heat the reactor to 150℃ and maintain for 60min to enhance hydrolysis. After the reaction is completed, use the vacuum filtration method to separate the solid and liquid. Collect the separated solid and place it in a vacuum drying oven to dry to constant weight at 60℃. Take a part of the solid sample for elemental analysis and find that the nitrogen content is 3.71wt%. The obtained low nitrogen algae mud solid is used as the main raw material for subsequent co-hydrothermal liquefaction.

[0080] (2) Accurately weigh 3.5g of the dry low-nitrogen algae mud solid from step (1) into a 100mL high-temperature and high-pressure reactor. At the same time, weigh 5.47g of wet kitchen waste (moisture content 72.6%, N content 2.03wt%, lipid content 20.32wt%) and 43.6mL of deionized water and add them to the reactor. The dry basis mixing ratio of the algae mud and kitchen waste in the resulting mixed raw materials is 7:3. Continue to add 0.25g of Fe2-Co8@HSZM-5 catalyst powder to the reactor, mix evenly, and continuously introduce argon gas for 5min to remove air from the reactor. Then seal the reactor. Set the stirring speed to 150rpm and heat the reactor to 325℃ and maintain it for 30min. After the reaction is completed, use a fan to cool it down to room temperature. Open the reactor, collect the reaction products and extract and separate them with dichloromethane to obtain aqueous phase, solid phase and oil phase products. The oil phase product is biomass oil. The biomass oil yield was 38.7%, and the nitrogen content was measured to be 1.38 wt%.

[0081] Examples 1 and 2 show that the mixing ratio of algal mud and kitchen waste in hydrothermal liquefaction affects the yield and nitrogen content of the prepared biomass oil. The higher the proportion of fat-rich waste biomass kitchen waste, the better it is for improving the yield and quality of biomass oil.

[0082] This invention employs a comprehensive strategy of alkaline hydrolysis pretreatment, addition of kitchen waste to regulate organic components, and selection of Fe-Co supported molecular sieves as a catalyst. This significantly improves biomass oil yield while substantially reducing the nitrogen content of the biomass oil to 1.07 wt%, far lower than the results of all control examples. This fully demonstrates the effectiveness and superiority of the three-step synergistic approach of this invention. In particular, this invention effectively addresses the balance between the difficulty of denitrification in high-nitrogen biomass hydrothermal liquefaction and ensuring oil yield.

[0083] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing high-yield low-nitrogen biomass oil by co-hydrothermal liquefaction of algal sludge based on organic components, comprising the following steps: S1) performing alkaline hydrothermal pretreatment on waste algal sludge, then performing solid-liquid separation to obtain low-nitrogen algal sludge solids; S2) using Fe-Co modified molecular sieve as catalyst, contacting the low-nitrogen algal sludge solids with lipid-rich waste biomass and performing co-hydrothermal liquefaction reaction to obtain low-nitrogen biomass oil.

2. The method of claim 1, wherein, The alkaline hydrothermal pretreatment is performed using an aqueous solution of alkali metal hydroxide.

3. The method of claim 1, wherein, The temperature of the alkaline hydrothermal pretreatment is 90-160℃. The time of the alkaline hydrothermal pretreatment is 15-60min. The pH value of the alkaline hydrothermal pretreatment is 10-13.

4. The method of claim 1, wherein, The molar ratio of Fe to Co in the Fe-Co modified molecular sieve is (1:1) ~ (1:4).

5. The method of claim 1, wherein, The Fe-Co modified molecular sieve is Fe2-Co8@HSZM-5 molecular sieve. The addition amount of the catalyst is 5%~10% of the dry weight of the low-nitrogen algal sludge solids.

6. The method of claim 1, wherein, The mass ratio of the low-nitrogen algal sludge solids to the lipid-rich waste biomass is (5:5) : (7:3).

7. The method of claim 1, wherein, The lipid-rich waste biomass is kitchen garbage or its processing product, and the dry basis lipid content is ≥18wt%.

8. The method of claim 1, wherein, The temperature of the co-hydrothermal liquefaction reaction is 275-350℃. The time of the co-hydrothermal liquefaction reaction is 10-60min.

9. The method of claim 1, wherein, The co-hydrothermal liquefaction reaction is performed in a high-temperature high-pressure reaction kettle.

10. The method of claim 1, wherein, After the co-hydrothermal liquefaction reaction, the following steps are further included: Collecting the reaction product and performing extraction separation with dichloromethane.

Images