Method for enhancing hydrolytic acidification performance of mixed raw material by biogas residue biochar prepared from anaerobic fermentation discharged material

Abstract

The invention relates to a method for enhancing hydrolytic acidification performance of a mixed raw material by biogas residue biochar prepared from anaerobic fermentation discharged materials, and belongs to the field of anaerobic digestion of organic solid wastes. According to the method, the kitchen waste and the corn straw are used as the mixed raw material, the biogas residue biochar is prepared from the wet anaerobic digestion discharged material added with the shell powder and the nano ferroferric oxide, and the biogas residue biochar is applied to the acid production stage of the mixed raw material to improve the hydrolytic acidification performance, so that the hydrogen production performance, the system stability and the biogas production performance of the mixed raw material are improved. Compared with a control group, the accumulated hydrogen yield of the mixed raw material is increased by 126.8%-133.0%, and the maximum hydrogen production peak is increased by 48.7%-67.9%. The maximum daily volatile fatty acid yield of an anaerobic system is improved by 33.7%-51.4% compared with that of a control group, and the stability of the system is also obviously enhanced. In the biogas production stage, the biogas yield is improved by 42.0%-51.8% compared with that of a control group. The method can significantly improve the hydrolytic acidification and biogas production performance of the mixed raw material.

Description

Technical Field

[0001] The present invention belongs to the field of anaerobic digestion of organic solid waste, and specifically relates to a method for preparing biogas residue biochar from anaerobic fermentation output, adding the biogas residue biochar into a kitchen waste and corn stalk co-fermentation acid production system, and improving the biogas yield by enhancing the hydrolysis and acidification performance of the mixed raw materials. Background Art

[0002] The treatment and utilization of a large amount of biogas residue produced by wet anaerobic digestion has always been a matter of great concern. Biogas residue will form biogas residue biochar through pyrolysis, gasification, and hydrothermal carbonization, which is widely used in improving the anaerobic efficiency of organic matter and methane production. However, the effect of hydrolysis and acidification needs further study. The first step of anaerobic digestion is to hydrolyze and acidify the raw materials to convert macromolecular organic matter into H2 and volatile fatty acids (VFAs), providing sufficient substrate for the subsequent methane production stage. Therefore, how to use the biogas residue biochar prepared by the discharge of large-scale biogas projects, and quickly hydrolyze and acidify the raw materials to obtain clean gas in steps is a key issue. Guo et al. (GUO YD, ZHENG Y, WANG YK, et al. Bioresource Technology, 2022, 363: 127900.) added granular activated carbon (GAC) to the acid production stage of the anaerobic reactor and found that the system VFAs production and biogas production were significantly improved, but the hydrogen production performance was not obvious. Therefore, in order to solve the problems of low utilization of sludge and hydrolysis and acidification performance of raw materials, the present invention proposes a method in which the anaerobic fermentation output of alkaline substance shell powder and conductive material nano-ferroferric oxide (nFe3O4) is added to prepare sludge biochar, and add it to the acid production stage of mixed raw materials. This significantly improves the hydrolysis performance of the mixed raw materials and the stability of the system, and can enhance the subsequent biogas production efficiency. Summary of the invention

[0003] In view of the problems of poor utilization efficiency of biogas residue and low hydrolysis and acidification performance of raw materials in biogas projects, the present invention proposes a method for preparing biogas residue biochar from anaerobic fermentation output by adding alkaline substances and conductive materials, and applying the biogas residue biochar in the acid production stage of mixed raw materials to improve the hydrolysis and acidification performance. The method mainly includes four parts: raw material preparation, preparation of biogas residue biochar, testing of acid production performance of anaerobic fermentation of mixed raw materials and testing of biogas production performance of mixed raw materials.

[0004] (1) Raw material preparation

[0005] The raw materials include corn stalks, restaurant kitchen waste and inoculants. Corn stalks were taken from the suburbs of Tianjin, air-dried under natural conditions, cut into 3-5 cm small segments with a guillotine, and then crushed with a grinder and passed through a 20-mesh sieve, and placed in a dry and cool place for use. The total solid content (TS) of corn stalks is 91.98%-93.01%, the volatile solid content (VS) is 85.24%-85.81%, and the carbon-nitrogen ratio (C / N) is 49.06-52.07. Restaurant kitchen waste was taken from the canteen, mainly food residues, with high moisture content, high oil and salt content, and easy to ferment and stink. The TS of restaurant kitchen waste is 22.69%-28.25%, VS is 21.55%-28.25%, C / N is 17.25-20.84, and the crude fat content is 24.78-27.52%. Before crushing, pick out the non-degradable debris and crush it with a food grinder, then put the crushed food waste into a ziplock bag for later use. The inoculum is taken from the biogas station, which is the output of anaerobic digestion of cow dung that has been running for more than 60 days. Let the inoculum stand for more than 24 hours, remove a small amount of supernatant, and control the TS of the inoculum to more than 12%. If the TS of the inoculum is lower than 12%, after skimming a small amount of supernatant, it needs to be placed in a constant temperature incubator at 35±1℃ to evaporate the water in the inoculum, thereby increasing its TS.

[0006] (2) Preparation of biochar from sludge

[0007] 1). Obtaining biogas residue

[0008] Food waste and inoculum were placed in 8 anaerobic digestion reactors, with food waste: inoculum = 3:1 (based on TS ratio). Then, shell powder of 4%, 6%, 8% and 10% based on food waste TS and nFe3O4 of 0.05, 0.1, 0.2 and 0.4 g / gVS based on food waste VS were added to 8 different reactors. Shell powder was taken from Tianjin Binhai New Area, naturally air-dried and crushed to less than 70 mesh. Shell powder mainly contains 80%-95% CaCO3 and Ca3(PO4)2, and the particle size of nFe3O4 is 10-30nm. The anaerobic device was operated until no more gas was produced, the residual VFAs concentration in the system was below 200 mg / L, and the anaerobic digestion test was terminated when the system pH was above 7.8. By accumulating methane production, it was determined that the optimal dosage of shell powder and nFe3O4 was 8% and 0.1 g / gVS respectively. The residual materials after the anaerobic treatment of the two optimal dosage reactors were separated by solid-liquid separation with a screen, and the separated solid part was digested into biogas residue. The digestion reactor was configured again, and food waste and inoculum were added, with food waste: inoculum = 3:1 (based on TS ratio), and 8% shell powder based on food waste TS and 0.1g / gVS nFe3O4 based on food waste VS were added at the same time. After running until no more gas was produced, the biogas liquid was separated to obtain digestion residue again.

[0009] 2). Preparation of biochar from sludge

[0010] The digested biogas residue was placed in a crucible and then dried in an oven at 105°C until the moisture content was 0%. It was ground in a mortar and then placed in a porcelain boat, sealed and wrapped with tin foil, and pyrolyzed in a closed tubular furnace at a temperature of 550°C for two hours to obtain biogas residue biochar. The heating rate of the tubular furnace was set to 5°C / min, and nitrogen (0.5L / min) was continuously filled during the heating process to ensure anaerobic conditions. After the tubular furnace cooled down, the fired biogas residue biochar was taken out and further ground in a mortar through a 100-mesh sieve, and then sealed and stored in a sub-bag for later use. The digested biogas residue containing shell powder, nFe3O4, shell powder and nFe3O4 was prepared into biogas residue biochar through the above process and named BC-OS, BC-NFO and BC-NFO-OS respectively. The prepared BC-OS contained 15%-18% calcium, and the BC-NFO contained 21%-24% iron. In addition, the functional group analysis of the three biochars was performed by infrared spectroscopy. The vibration of the CH, CO, and Si-O groups of the characteristic peaks of BC-OS and BC-NFO-OS was significantly stronger than that of BC-NFO. Calcium and iron were loaded on the biochar to form Ca-O and Fe-O groups. The molar ratio of iron to calcium in BC-NFO-OS was calculated to be 1.59-1.89 (iron / calcium).

[0011] 3). Determination of the amount of biochar added

[0012] Food waste and inoculum were placed in the anaerobic digestion reactor, with food waste: inoculum = 3:1 (based on TS ratio), and the effective working volume of the reactor was 300 mL. Then 4-12 g / L (based on the effective working volume of the reactor) of biochar was added, and the time taken to reach 80% of the cumulative methane production (T 80 )Determine the optimal amount of sludge biochar added.

[0013] (3) Acid production performance of mixed raw materials by anaerobic fermentation

[0014] According to the ratio of kitchen waste: corn stalks = 5:1 (based on TS ratio), put them into a reaction bottle with an effective working volume of 300mL, and then add the inoculum at a ratio of mixed raw materials: inoculum = 3:1 (based on TS ratio). Finally, three different biochars of sludge were added to each reactor, and stirred with a stirring rod until all materials were fully and evenly distributed in the bottle. After sealing, it was placed in a constant temperature water tank at a temperature of 35±1℃ for hydrolysis and acidification test, and the test cycle was set to 7 days. During the test, the reaction bottle was manually shaken for 2-3 minutes every morning, noon and evening, and the shaking was stopped after the material was evenly distributed in the reaction bottle. The reaction bottle was divided into two types: gas testing bottle and property testing bottle. The gas testing bottle measured the hydrogen production of the mixed material every day, and the property testing bottle measured the pH value and VFAs value every day.

[0015] (4) Biogas production performance of mixed raw materials anaerobic fermentation

[0016] According to the parameters of anaerobic acid production performance, the reactor was reconfigured for hydrolysis and acidification. On the days with the highest VFAs production, the sealed reactor was opened, and the inoculum was added again according to the ratio of mixed raw materials: inoculum = 1:2 (based on TS ratio) and stirred with a stirring rod until the raw materials were evenly distributed. Nitrogen was introduced for 60 seconds to expel the air, and then the anaerobic digestion device was connected and placed in a constant temperature water tank. During the test, the reaction bottle was manually shaken for 2-3 minutes every morning, noon and evening. The shaking was stopped after the material was evenly distributed in the reaction bottle. The biogas production test period was set to 20 days.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] (1) Significantly improved hydrogen production performance

[0019] This experiment broke through the problem of the difficulty in utilizing biogas residues in large-scale biogas projects. The biogas residues from anaerobic fermentation were prepared as biogas residue biochar and added to the mixed raw material hydrolysis and acid production stage. The cumulative hydrogen production of different test groups was 27.53-28.29mL / gVS, which was 126.8%-133.0% higher than that of the control group. In addition, the maximum hydrogen production peak of different test groups was 13.49-15.23mL / gVS, which was 48.7%-67.9% higher than that of the control group. In the two-stage anaerobic digestion reaction in the literature, the cumulative hydrogen increase rate in 24 hours was about 10%.

[0020] (2) VFAs production and system stability are significantly improved

[0021] The addition of biochar from sludge significantly promoted the production of VFAs, and the maximum daily VFAs production increased by 33.7%-51.4% compared with the control group, which laid the foundation for the direct use of these components by methanogens in the next stage. After the addition of biochar from sludge to strengthen the hydrolysis and acidification of the mixed raw materials, the stability of the hydrolysis and acidification performance was significantly improved. The pH values ​​of the different test groups after the acid production ended were in the range of 4.32-4.96, while the pH of the control group continued to drop to 3.77.

[0022] (3) Biogas production increased significantly

[0023] The cumulative biogas production of different test groups was 654.09-688.84mL / gVS, which was 13.1%-19.1% higher than that of the control group. In addition, the biogas production of different test groups reached 80% of the cumulative biogas production in 12 days, while the control group took nearly 16 days. In the two-stage anaerobic digestion reaction in the literature, the maximum increase rate of cumulative methane production was about 15%.

[0024] In order to solve the problem that biogas residue biochar has incomplete hydrolysis and acidification performance on organic solid waste, the effluent from wet anaerobic digestion with the addition of shell powder and nFe3O4 was prepared into biogas residue biochar, which was used in the acid production stage of mixed raw materials to improve the hydrolysis and acidification performance, while enhancing the subsequent biogas production performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the biogas production performance effect diagram of the present invention DETAILED DESCRIPTION

[0026] (1) Raw material preparation

[0027] The raw materials include restaurant waste, corn stalks and inoculants. Restaurant waste is taken from the canteen. Bones and plastic bags are removed manually before grinding. Then, it is ground into a pulp using a food grinder and placed in a ziplock bag for later use. After the corn stalks are air-dried under natural conditions, they are cut into 3-5cm small segments with a guillotine, and then crushed with a grinder and passed through a 20-mesh sieve. The inoculants are taken from a biogas station, which is the output of anaerobic digestion of cow dung that has been running for more than 60 days.

[0028] (2) Preparation of biochar from sludge

[0029] 1). Obtaining biogas residue

[0030] 15gTS food waste and 5gTS inoculum were placed in the anaerobic digestion reactor, and then 0.6, 0.9, 1.2 and 1.5g of shell powder and 0.7, 1.4, 2.8 and 5.6g of nFe3O4 were added to 8 different reactors respectively. Then, 25gTS inoculum was added on the 5th day when the VFAs production was the highest to start the biogas production test. The anaerobic device was operated until no more gas was produced, the residual VFAs concentration in the system was below 200mg / L, and the system pH was above 7.8 to end the anaerobic digestion test. The optimal dosage of shell powder and nFe3O4 was determined to be 1.2g (based on 8% of food waste TS) and 1.4g (based on 0.1g / gVS of food waste VS) by the cumulative methane production of food waste. The remaining materials after the anaerobic end of the two reactors with the optimal dosage were separated into solid and liquid using a sieve, and the separated solid part was the digested biogas residue. The digestion reactor was reconfigured, and food waste and inoculum were put in, with food waste: inoculum = 3:1 (based on TS ratio). At the same time, 1.2 g of shell powder and 1.4 g of nFe3O4 were added. The reactor was operated until no gas was produced, and the biogas slurry was separated to obtain the digestion residue again.

[0031] Table 1 Cumulative methane production under different exogenous additives

[0032]

[0033] 2). Preparation of biochar from sludge

[0034] The digested biogas residue was dried in an oven at 105°C, ground and placed in a porcelain boat. The temperature and heating flow rate of the tubular furnace were set to 550°C and 5°C / min, respectively. The porcelain boat was placed in the tubular furnace and sealed for pyrolysis for 2 hours to obtain biogas residue biochar. After the tubular furnace cooled down, the fired biogas residue biochar was taken out and further ground and sieved (100 mesh), and then sealed and stored in sub-bags. The anaerobic digested biogas residue containing shell powder, nFe3O4, shell powder and nFe3O4 was prepared into biogas residue biochar through the above process and named BC-OS, BC-NFO and BC-NFO-OS.

[0035] 3). Determination of the amount of biochar added

[0036] In an anaerobic digestion reactor with an effective working volume of 300 mL, 15 g TS of food waste and 5 g TS of inoculum were placed, and then 1.2, 1.8, 2.4, 3, and 3.6 g of biochar were added, respectively. 80 The optimal addition amount of sludge biochar was determined to be 10g / L.

[0037] Table 2 The time taken to reach 80% of the cumulative methane production at different dosages

[0038]

[0039] (3) Acid production performance of mixed raw materials by anaerobic fermentation

[0040] The organic load of the test was set at 50 g / L. 12.5 g TS of food waste and 2.5 g TS of corn stalks were placed in a reaction bottle with a working volume of 300 mL, and then 5 g TS of inoculum was added (mixed raw materials: inoculum = 3:1). Finally, 3 g of different biochar residues were added to each reactor, the materials in the reactor were stirred evenly with a stirring rod, and the remaining materials of the stirring rod were flushed into the reaction bottle with a small amount of pure water, and nitrogen was filled for 60 seconds to expel the air. The sealed reactor was placed in a constant temperature water tank at 35 ± 1 ° C for hydrolysis and acid production test, and the test cycle was 7 days. During the test, the reaction bottle was manually shaken for 2-3 minutes every morning, noon and evening, and the shaking was stopped after the materials were evenly distributed in the reaction bottle. The reaction bottle is divided into two types: gas test bottle and property test bottle. The gas test bottle determines the hydrogen production of the mixed material by drainage gas collection method every day, and the property test bottle is opened every day to take a certain amount of samples to determine the pH value and VFAs value. After the test bottle was sampled, nitrogen was introduced for 60 seconds to expel the air and then placed in a constant temperature water tank. The cumulative hydrogen production results of the test are shown in Table 3.

[0041] Table 3 Hydrogen production performance of mixed raw materials in the acid production stage under different additions of biochar from sludge

[0042]

[0043]

[0044] This experiment solved the problem of insignificant improvement of biogas residue in biogas projects. Biogas residue with exogenous additives was prepared into biogas residue biochar and added to the mixed raw material hydrolysis and acid production stage. The cumulative hydrogen production of different experimental groups was 27.53-28.29mL / gVS, which was 126.8%-133.0% higher than that of the control group. In addition, the 12-hour hydrogen production peak of different experimental groups was 13.49-15.23mL / gVS, which was 48.7%-67.9% higher than that of the control group.

[0045] At the same time, the addition of sludge biochar significantly increased the production of VFAs and system stability.

[0046] Table 4 VFAs production and system stability of mixed raw materials under different additions of biochar from sludge

[0047]

[0048] VFAs are important intermediates in anaerobic digestion. Complex organic matter is decomposed into VFAs under the action of hydrolytic acidifying bacteria, and VFAs further produce hydrogen and biogas under the action of microorganisms. The significant increase in its yield indicates the effective decomposition of anaerobic digestion raw materials. The addition of biochar from sludge significantly increased the yield of VFAs. The maximum daily VFAs yields of BC-NFO, BC-OS and BC-NFO-OS were 11103.45, 12573.11 and 12119.41 mg / L, respectively, which were 33.7%-51.4% higher than those of the control group. In addition, the addition of biochar from sludge promoted the production of acetic acid and butyric acid. The proportion of acetic acid and butyric acid in each experimental group was 76.05%-81.19%, while that in the control group was only 63.55%, which laid the foundation for the direct use of these components by methanogens in the next stage.

[0049] In addition, the addition of biochar increased the pH of each test group, which is a parameter for measuring system stability. At the end of the acid production phase, the pH of each test group rebounded to between 4.32 and 4.95, while the pH of the control group continued to drop to 3.77 and showed no upward trend.

[0050] (4) Anaerobic fermentation of mixed raw materials to produce biogas

[0051] The hydrolysis and acidification reactor was reconfigured, and 25 g TS inoculum was added to the biogas production test on the day with the highest VFAs production (day 4). The cumulative biogas production results are shown in Table 5. In the biogas production stage, the cumulative biogas production of the experimental group was 519.32-554.98 mL / gVS for 12 days, which was 42.0%-51.8% higher than that of the control group. This experiment fully demonstrated that the preparation of biogas residue biochar from biogas residues of biogas projects with exogenous substances can promote the hydrolysis and acidification performance of food waste and corn straw, and can also produce methane by in situ anaerobic digestion. This not only provides ideas for the utilization of biogas residues in biogas projects, but also shortens the time cost of the hydrolysis and acidification recovery process, and increases the hydrogen production in the acid production stage and the subsequent biogas production. Hydrogen and biogas are obtained separately in different time periods, thereby creating higher ecological and environmental value and economic benefits.

[0052] Table 5 Cumulative biogas production of mixed raw materials under different additions of biochar

[0053]

Claims

1. A method for preparing biochar from anaerobic fermentation to enhance the hydrolysis and acidification performance of mixed raw materials, characterized in that: It includes four parts: raw material preparation, preparation of biochar from sludge, acid production performance of anaerobic fermentation of mixed raw materials, and biogas production performance test of mixed raw materials; (1) Raw material preparation The raw materials include corn stalks, restaurant kitchen waste and inoculants; after the corn stalks are air-dried under natural conditions, they are cut into small segments of 3-5 cm with a guillotine, and then crushed with a grinder, passed through a 20-mesh screen, and placed in a dry and cool place for use; the total solid content TS of the corn stalks is 91.98%-93.01%, the volatile solid content VS is 85.34%-85.81%, and the carbon-nitrogen ratio C / N is 49.06-52.07; before crushing, the restaurant kitchen waste is firstly selected to remove non-degradable debris and then crushed with a food grinder, and then the crushed restaurant kitchen waste is put into a ziplock bag for use; the inoculants are taken from a biogas station and are the output of anaerobic digestion of cow dung that has been running for more than 60 days; the inoculants are kept still for more than 24 hours, a small amount of supernatant is removed, and the TS of the inoculants is controlled to be above 12%; if the TS of the inoculants is lower than 12%, the supernatant is skimmed off and then placed in a constant temperature incubator at 35±1°C to evaporate the water in the inoculants, thereby increasing the TS; (2) Preparation of biochar from sludge 1). Obtaining biogas residue Put food waste and inoculum into an anaerobic digestion reactor, the mass ratio of food waste to inoculum total solid content TS=3:1; then add 4%-10% shell powder based on food waste TS or 0.05-0.4g / gVS of nFe3O4 based on food waste VS; the shell powder is naturally air-dried and crushed to less than 70 mesh, and the particle size of nFe3O4 is 10-30nm; the anaerobic device is operated until no gas is produced, the residual VFAs concentration in the system is below 200mg / L, and the system pH is above 7.8, and the anaerobic digestion test is terminated; the remaining material after the anaerobic end is separated into solid and liquid with a sieve, and the separated solid part is digested biogas residue; or shell powder and nano-iron tetroxide nFe3O4 are added at the same time; 2). Preparation of biochar from sludge The digested biogas residue was placed in a crucible, and then placed in an oven at 105°C to dry until the moisture content was 0%; the digested biogas residue was ground in a mortar and then placed in a porcelain boat, which was sealed and wrapped with tin foil, and pyrolyzed in a closed tubular furnace at a temperature of 550°C for two hours to obtain biogas residue biochar; the heating rate of the tubular furnace was set to 5°C / min, and nitrogen was continuously filled during the heating process to ensure anaerobic conditions; after the tubular furnace was cooled down, the fired biogas residue biochar was taken out and further ground in a mortar through a 100-mesh sieve, and then sealed and stored in a sub-bag for later use; the digested biogas residue containing shell powder, nFe3O4, shell powder and nFe3O4 was prepared into biogas residue biochar through the above process and named BC-OS, BC-NFO and BC-NFO-OS respectively; the molar ratio of iron to calcium in BC-NFO-OS was: 1.59-1.89; (3) Anaerobic fermentation of mixed raw materials to produce acid Kitchen waste and corn stalks were put into the reaction bottle in a ratio of 5:1 in terms of total solid content TS, and then the inoculum was added in a ratio of 3:1 in terms of total solid content TS of the mixed raw material. Finally, 4-12 g / L of biochar was added to each reactor, and stirred with a stirring rod until all the materials were fully and evenly distributed in the bottle. After sealing, it was placed in a constant temperature water tank at a temperature of 35±1°C for hydrolysis and acidification test, and the test period was set to 7 days. During the test, the reaction bottle was manually shaken for 2-3 minutes every morning, noon and evening, and the shaking was stopped after the materials were evenly distributed in the reaction bottle. The pH value and volatile fatty acid VFAs value were measured every day.

2. The method according to claim 1, characterized in that Also includes: (4) Biogas production performance of mixed raw materials anaerobic fermentation On the days with the highest production of volatile fatty acids (VFAs), open the sealed reaction bottle, add the inoculum again in a ratio of mixed raw materials: total solid content TS of the inoculum = 1:2, and stir with a stirring rod until the raw materials are evenly distributed. After nitrogen is passed for 60 seconds to expel the air, connect the anaerobic digestion device and put it into a constant temperature water tank. During the test, manually shake the reaction bottle for 2-3 minutes every morning, noon and evening. Stop shaking after the material is evenly distributed in the reaction bottle. The biogas production test period is set to 20 days.

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