Method for preparing carbon aerogel microspheres and increasing yield of biogas by anaerobic digestion of kitchen waste

Abstract

The invention relates to the field of anaerobic digestion reinforcement and porous conductive carbon material preparation, and discloses a method for preparing carbon aerogel microspheres and increasing yield of biogas by anaerobic digestion of kitchen waste. Dispersing the charcoal powder and thermosetting phenolic resin in a sodium alginate solution to obtain slurry, and dropwise adding a calcium chloride solution for ionic crosslinking to form hydrogel microspheres; and after washing and drying, pyrolyzing and carbonizing in an inert atmosphere at 300-600 DEG C to obtain the spherical porous conductive carbon aerogel microspheres. 0.5-10 g / L of the microbial agent is added into a kitchen waste medium-temperature anaerobic digestion system at the temperature of 35-40 DEG C, the methane production delay can be shortened, and the biogas / methane yield can be increased. According to the embodiment of the invention, the accumulated biogas yield is 816.67-878.11 mL / g VS under the conditions that the temperature is 37 + / -1 DEG C and the concentration of 5g / L is added, and is improved by 31.94-41.87% compared with that of a group which is not added with the biogas.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of solid waste resource processing and anaerobic digestion intensification, and particularly relates to a carbon aerogel microsphere prepared by sodium alginate / calcium ion cross-linking molding and pyrolysis carbonization in an inert atmosphere and a method for increasing biogas production in anaerobic digestion of kitchen waste. BACKGROUND

[0002] Kitchen waste is rich in easily biodegradable organic components and belongs to one of the main organic solid wastes in municipal solid waste. Anaerobic digestion technology can convert kitchen waste into biogas / methane and achieve reduction and resource utilization, which is an important way for kitchen waste treatment.

[0003] However, in engineering operation, the anaerobic digestion process of kitchen waste often has problems such as rate mismatch between hydrolysis acidification and methanogenesis stage, accumulation of volatile fatty acids (VFA), acidification of the system, long lag phase of methanogenesis and low biogas / methane yield, which affects the stability and productivity of the system.

[0004] In order to alleviate the above problems, the existing technology often adds exogenous materials (such as biochar, activated carbon and other conductive carbon materials) to the anaerobic digestion system to provide microbial attachment carriers and promote interspecies electron transfer, thereby intensifying the methanogenesis process.

[0005] However, the existing powdery exogenous conductive materials are easy to lose and difficult to recover under liquid phase stirring / shearing conditions, and it is difficult to maintain the intensification effect for a long time. Some high-performance conductive materials have high preparation cost, complex process or insufficient mechanical strength, which limits the engineering application.

[0006] Carbon aerogels and other three-dimensional porous conductive carbon materials usually have high specific surface area and good electrical conductivity, which can provide support for microbial attachment / immobilization and system electron transfer, but traditional blocky or brittle aerogel materials are difficult to maintain structural integrity in the anaerobic digestion stirring environment, and are not easy to recover.

[0007] Therefore, it is necessary to provide an exogenous conductive carbon material suitable for the anaerobic digestion system of kitchen waste and its preparation and application scheme, so that it has high mechanical integrity under liquid phase stirring conditions, can be recycled and reused, and can improve the biogas / methane yield. SUMMARY

[0008] Technical problems to be solved In view of the problems of rate mismatching between hydrolysis acidification and methanogenesis stages, VFA accumulation, long methanogenesis lag phase and low biogas / methane yield in the anaerobic digestion of kitchen waste, and in view of the engineering limitations of existing powdered exogenous conductive carbon materials, such as easy loss, difficult recovery, difficult long-term stable maintenance of effect under liquid phase stirring / shearing conditions, and high preparation cost, complex process and insufficient mechanical strength of some high-performance conductive materials, the present application aims to provide a scalable, cost-controllable and high-mechanical-integrity conductive porous carbon microsphere and its application method in the anaerobic digestion of kitchen waste, so as to shorten the methanogenesis lag phase and improve the biogas / methane yield under mesophilic anaerobic digestion conditions.

[0009] Technical solution To achieve the above-mentioned purpose, the present application provides a preparation method of carbon aerogel microspheres and a method for improving the biogas production of kitchen waste anaerobic digestion. The preparation method of carbon aerogel microspheres comprises the following steps: S1, preparation of precursor hydrogel microspheres: preparing a sodium alginate aqueous solution; adding biochar powder and thermosetting phenolic resin into the sodium alginate aqueous solution and mixing and dispersing to obtain a precursor slurry; adding the precursor slurry into a calcium chloride aqueous solution by dripping to form and solidify into hydrogel microspheres under ionic crosslinking action. 2+ S2, drying and pyrolysis carbonization to prepare carbon aerogel microspheres: washing and drying the hydrogel microspheres, and then pyrolyzing and carbonizing under inert atmosphere to obtain carbon aerogel microspheres.

[0010] S2, drying and pyrolysis carbonization to prepare carbon aerogel microspheres: washing and drying the hydrogel microspheres, and then pyrolyzing and carbonizing under inert atmosphere to obtain carbon aerogel microspheres.

[0011] In the above preparation process, one or more of the following conditions are preferably met: The dripping can adopt a peristaltic pump, a syringe or a titration device to drip the precursor slurry into the calcium chloride aqueous solution through a nozzle; the particle size of the microspheres can be adjusted by selecting the inner diameter of the nozzle, controlling the dripping flow rate and the dropping height. For example, when the inner diameter of the nozzle is 3.0-5.0 mm and the dropping height is 5-30 cm, the wet-state diameter of the obtained hydrogel microspheres is 3-4 mm; and the diameter of the obtained carbon aerogel microspheres after drying and pyrolysis carbonization is 2-3 mm.

[0012] The biochar powder is ground and passed through a 80-200 mesh sieve, preferably a 100 mesh sieve, to facilitate dispersion and dripping.

[0013] The mass fraction of the sodium alginate aqueous solution is 2%-6%, preferably 4%.

[0014] The mass ratio of the sodium alginate aqueous solution, the biochar powder and the thermosetting phenolic resin is 80:(2-6):1; the precursor slurry can be stirred for 1-2 min first, and then ultrasonically dispersed for 10-20 min to improve uniformity.

[0015] The concentration of the calcium chloride aqueous solution is 0.1-1.0 mol / L, preferably 0.5 mol / L; the curing time is 6-24 h, preferably ≥12 h; after curing, the product is washed with clean water and immersed in distilled water for more than 8 h to remove the surface Ca 2+ .

[0016] The drying can be normal-temperature drying to constant weight, or drying to constant weight at 60-80 ℃; more preferably, drying at 60 ℃±1 ℃ for more than 48 h.

[0017] The pyrolysis carbonization is carried out in a rotary furnace or a tubular furnace, and inert gas (such as nitrogen or argon) is introduced for 10-30 min before pyrolysis; the heating rate is 5-20 ℃ / min; the pyrolysis temperature is 300-600 ℃; and the residence time is 30-180 min.

[0018] The application also provides an application method in anaerobic digestion of kitchen waste: the carbon aerogel microspheres are added as an exogenous additive to an anaerobic digestion system of kitchen waste to shorten the methanogenic lag phase and improve the biogas / methane yield.

[0019] The anaerobic digestion temperature is 35-40 ℃, more preferably 37±1 ℃; and the initial pH is 6.8-7.8, more preferably 7.4.

[0020] The carbon aerogel microspheres are added in an amount of 0.5-10 g / L, more preferably 5 g / L.

[0021] The carbon aerogel microspheres can be added at one time in the start-up stage of anaerobic digestion, or added in batches during the operation; when added in batches, the addition can be carried out in the hydrolysis-acidification stage and / or the methanogenesis stage.

[0022] After the anaerobic digestion is completed, the carbon aerogel microspheres can be recovered by screening, sedimentation or filtration; after recovery, the carbon aerogel microspheres can be washed with clean water and dried for reuse. The steps of recovery-washing-drying-reuse can be repeated for 2-10 times, preferably at least 3 times, to reduce the loss of exogenous materials and improve the material utilization efficiency.

[0023] The anaerobic digestion system of kitchen waste can be a batch reactor, a sequencing batch reactor or a continuous reactor; the continuous reactor is preferably a continuous stirred tank reactor (CSTR).

[0024] The carbon aerogel microspheres referred to in the application are spherical porous conductive carbon microspheres obtained by drying and inert-atmosphere pyrolysis carbonization using a sodium alginate and calcium ion crosslinked hydrogel microsphere as a forming template; the conductive framework is composed of a porous network formed by carbonization of sodium alginate and reinforcing carbon formed by carbonization of thermosetting phenolic resin, and biochar particles are dispersed and embedded therein, so that the mechanical integrity of the microspheres is improved while maintaining the three-dimensional interconnected pore structure.

[0025] It should be noted that although it can be speculated that carbon aerogel microspheres can strengthen the anaerobic digestion process by providing an electrically conductive carbon skeleton with a three-dimensional interconnected pore structure, enhancing microbial attachment and promoting interspecies direct electron transfer, etc., the present application is not limited to this mechanism of action.

[0026] Advantages Compared with the prior art, the present application has at least the following advantages: Improved mechanical integrity, suitable for liquid phase stirring conditions and easy to recover: by introducing thermosetting phenolic resin as a reinforcing component, the microsphere skeleton is more stable after carbonization, and the shape retention rate is about 89%-92.8% under simulated anaerobic digestion stirring abrasion conditions, higher than the comparative sample without reinforcing agent, and the microsphere morphology is easy to sieve and recycle and reuse.

[0027] Pore structure and leaching characteristics: mercury intrusion method showed that the porosity of carbon aerogel microspheres was 38.67%-51.66%, the median pore size (volume distribution) was 331.55-1576 nm, and the pore size distribution covered sub-micron to micron level (see Table 2, Figures 5-7 ); The leaching liquid pH was 8.76-9.65, and the conductivity was shown in Table 2.

[0028] Improved biogas and methane yield: under batch anaerobic digestion conditions, compared with Comparative Example 1 without adding external materials, the cumulative biogas yield of Examples 1-3 was increased by 31.94%-41.87%, and the cumulative methane yield was also increased (see Table 1, Figure 2 ).

[0029] Shortened lag time / maintenance time of methanogenesis process: cumulative methane production rate curves show that the starting time of the system with added carbon aerogel microspheres into the rapid methanogenesis stage is advanced, and the methanogenesis process shows a more gentle change trend (see Figure 3 ).

[0030] Relative simplification of process chain, with the basis for engineering scale-up: the preparation process uses common raw material system and drop crosslinking molding process, which can be prepared in an inert atmosphere furnace at a relatively mild pyrolysis temperature range, and has the feasibility of scale-up.

[0031] Recyclable and reusable, suitable for engineering operation: the microsphere morphology is easy to recover by sieving / settling, etc. and can be used again, which can be recycled and reused in multiple operation cycles, thereby reducing the amount of external material supplement and improving the engineering economy. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The shape retention rate of carbon aerogel microspheres under simulated anaerobic digestion operating conditions is compared.

[0033] Figure 2 Cumulative biogas and methane production rates of Examples 1-3 and Comparative Examples 1-2 are compared in the following figures, where the left figure is the comparison of cumulative biogas production rates, and the right figure is the comparison of cumulative methane production rates.

[0034] Figure 3 Cumulative methane production rates under different conditions of adding materials are shown in the following figures, where the left figure is the comparison of cumulative methane production rates under the conditions of not adding and adding different carbon aerogel microspheres, and the right figure is the comparison of cumulative methane production rates under the conditions of not adding, adding sludge biochar and adding sludge carbon aerogel microspheres.

[0035] Figure 4 Scanning electron microscope images of different materials are shown in the following figures, where the left column is 1 kX and the right column is 10 kX; (a) is sludge biochar, (b) is cyanobacterial carbon aerogel microspheres, (c) is sludge residue carbon aerogel microspheres, and (d) is sludge carbon aerogel microspheres.

[0036] Figure 5 Pore size distribution of carbon aerogel microspheres from different sources is shown in the following figure.

[0037] Figure 6 Comparison of pore size distribution of sludge-based carbon aerogel microspheres and sludge biochar is shown in the following figure.

[0038] Figure 7 Comparison of pore volume of sludge-based carbon aerogel microspheres and sludge biochar in different pore size ranges is shown in the following figure. DETAILED DESCRIPTION

[0039] The technical solutions of the present application will be further described below in combination with the drawings and examples. It should be understood that the examples are only used to explain the present application, but not to limit the protection scope of the present application.

[0040] In the following examples, the kitchen waste slurry can be taken from the buffer tank of the kitchen waste pretreatment system; the concentration is 65,000-95,000 mg / L in terms of total solids (TS), and the mass ratio of volatile solids (VS) to TS is 85%-95%. The methanogenic inoculum sludge can be taken from the continuous stirred tank reactor (CSTR) of the kitchen waste anaerobic digestion system; the concentration is 15,000-17,000 mg / L in terms of TS, and the mass ratio of VS to TS is 50%-60%. Unless otherwise specified, the inoculation ratio of the batch anaerobic digestion test is 10% (volume ratio), the initial pH is adjusted to 7.4, and the carbon aerogel microsphere dosage is 5 g / L; the gas production and gas composition are collected and measured every two days.

[0041] The determination of TS and VS can be carried out according to the commonly used methods for municipal wastewater / sludge and organic solid waste, wherein the VS is the weight loss of the sample after burning at 600°C for 2 hours. The biogas volume can be determined by the syringe method or the drainage gas collection method after standing for 30 minutes at room temperature (25±3°C); the gas components (CH4, CO2, etc.) can be determined by a gas chromatograph equipped with a thermal conductivity detector, and the external standard quantitative curve is established by using standard gas and is calibrated periodically.

[0042] To evaluate the anti-breaking ability of the microspheres, an oscillation abrasion test simulating the stirring condition of anaerobic digestion can be used to evaluate the shape retention rate: the microspheres with an initial mass of m0 are placed in a 25 mL density bottle filled with deionized water, and after oscillation at 37±1°C and 130 rpm for 72 hours, the microspheres that remain intact are recovered by sieving and dried (60°C) and weighed as m1, and the shape retention rate (%) is calculated as m1 / m0x100%. The comparative sample is the microspheres prepared under the same conditions but without the addition of thermosetting phenolic resin, which breaks under the evaluation conditions; the shape retention rate of the microspheres with the reinforcing agent is about 89%-92.8% (see Figure 1 ).

[0043] To characterize the leaching properties and pore structure of the microspheres, the following determinations are carried out: pH value and conductivity determination: the biochar or carbon aerogel microspheres are added to ultrapure water at 1% (w / w), and after oscillation in a 37°C constant temperature shaker (130 rpm) for 24 hours, the pH value and conductivity of the suspension are determined, and the same sample is determined in parallel.

[0044] Pore size and pore volume analysis: the mercury intrusion method is used for determination, and the sample is first dried overnight in a 105°C oven to remove water, and then placed in an automatic mercury porosimeter (MicroActive AutoPore V 9600) for determination.

[0045] Implementation and evaluation method of recycling and reuse To verify the recyclability and reusability of the carbon aerogel microspheres in the anaerobic digestion system, the recycling-reloading test can be carried out as follows: after completing an anaerobic digestion cycle, the microspheres are recovered by sieving, sedimentation or filtration; sieving is preferred to achieve rapid separation of the microspheres from the digestion residue / digestion liquid, with a screen aperture smaller than the diameter of the microspheres. The recovered microspheres can be washed with clean water or deionized water, and if necessary, can be lightly ultrasonicated to remove surface attachments; then dried at room temperature to constant weight or dried at 60-80°C to constant weight to obtain microspheres that can be reloaded.

[0046] The recycling-washing-drying-reloading steps can be continuously repeated at least 2 times, preferably at least 3 times. The microsphere recovery rate and the shape retention rate of each cycle can be recorded respectively in the cycle test, and the cumulative biogas / methane yield of each cycle is counted to evaluate the stability of the material in multi-cycle operation. Illustratively, the microsphere recovery rate can be calculated according to formula (I): the mass of the microspheres after recovery and drying m r , i.e. the recovery rate (%) = m r / m0x 100%; the microsphere shape retention rate can be calculated according to formula (II): the mass of the microspheres retaining the original shape m i , i.e. the shape retention rate (%) = m i / m0x 100%.

[0047] Control test settings and effect evaluation This section is used to explain the setting of the control material and the comparison test scheme, and the related results are only used for effect evaluation and explanation, and do not constitute a limitation on the scope of protection of the claims.

[0048] In order to more comprehensively evaluate the comprehensive strengthening effect of carbon aerogel microspheres in the anaerobic digestion of kitchen waste, in addition to the blank control (without adding exogenous materials) and the biochar control (adding powdered biochar), other common exogenous material control groups can be set up, such as activated carbon, magnetite (Fe3O4) particles, and conductive carbon black; each control group preferably uses the same or similar loading amount range (for example, 0.5-10 g / L) and is operated under the same reactor, temperature, inoculation ratio, and substrate conditions to reduce the influence of non-target factors on the comparison results. The particle size of the control material can be adapted according to the material form; for example, the particle size range of powdered materials can be controlled by sieving, and the particle size grade of granular materials can be selected to be comparable to that of the microspheres, or the particle size distribution thereof is explicitly stated in the specification for comparison.

[0049] The comparison indicators can include but are not limited to: start-up lag phase, peak VFA and its fall-back time, cumulative biogas / methane yield per unit VS, methane volume fraction, pH / alkalinity stability, and recovery convenience and recycling performance of the exogenous material, etc.

[0050] Structure and conductivity characterization method This section is used to explain the characterization method of the pore structure and conductivity of carbon aerogel microspheres, and to explain the reason why the three-dimensional interconnected pores (including micron-scale channels) can provide a channel for microorganisms to attach to / enter the interior of the material; the related mechanistic explanation is only for illustration and does not constitute a limitation on the scope of protection of the present application.

[0051] The pore structure and conductivity of carbon aerogel microspheres can be characterized by the following test means: Pore structure and specific surface area: nitrogen adsorption-desorption (BET) test can be used to obtain specific surface area and pore volume information, and BJH or DFT model can be used to analyze mesopore size distribution; if necessary, mercury intrusion test can be used to test macropore size distribution. Thus, the mass transfer and attachment interface of the microspheres can be characterized from the perspective of hierarchical pore structure of micropores / mesopores / macropores.

[0052] Morphology and pore size: scanning electron microscopy (SEM) can be used to observe the surface and cross-sectional morphology of the microspheres; the cross-section of the microspheres can be sampled to identify the connected pores and pore wall structure, and statistical analysis of the pore size can be performed. Combined with the morphology shown in FIG. 1, in some embodiments, in addition to nanoscale pores, the microspheres also have sub-micron to micron scale connected pores, which can provide space conditions for microorganisms to enter the pores and attach inside the material. Figure 4

[0053] Conductivity: four-probe or two-probe method can be used to measure the resistivity / conductivity of the microsphere material. Exemplarily, the microspheres can be ground or pressed into a certain size of test sample, and the volume resistivity is measured under constant pressure; the resistance of a single particle or a particle packed bed can also be tested to characterize the conductivity of the microspheres in wet state / running state.

[0054] Mechanism verification and process monitoring The process monitoring and mechanism verification described in this section are optional, which are used to further characterize the influence of adding carbon aerogel microspheres on the anaerobic digestion process of kitchen waste, and to explain the reasons for the formation of technical effects; the above content should not be understood as a limitation on the protection scope of the present application. The following tests can be carried out as needed (the listed methods are examples): Process parameter monitoring: digestion liquid samples can be collected at a preset sampling period, and pH, alkalinity, and total volatile fatty acid (VFA) and components (such as acetic acid, propionic acid, butyric acid, etc.) can be measured to evaluate the matching state of hydrolysis acidification and methanogenesis stages and the acidification accumulation / recovery trend; VFA can be measured by gas chromatography or liquid chromatography, and alkalinity can be measured by titration.

[0055] Gas production process monitoring: cumulative biogas production, unit VS gas production, and gas production rate (or daily gas production) can be monitored, and biogas components (such as CH4, CO2, etc.) can be measured to calculate the cumulative methane production rate; gas components can be measured by gas chromatography, and volume conversion and calibration can be performed as needed.

[0056] ​Microbial and interface characterization (optional): surface and cross-section observation (e.g. SEM / CLSM) can be performed on the recovered microspheres to characterize microbial adhesion and pore distribution; EPS extraction and component analysis (e.g. protein / polysaccharide) can be performed on the digested sludge or microsphere-attached biofilm to evaluate adhesion and flocculation properties; 16S rRNA sequencing and / or qPCR (e.g. mcrA) quantification can be performed to characterize community structure and abundance changes of methanogenic functional bacteria; if necessary, ORP monitoring or electrochemical characterization can be performed to assist in illustrating the trend of changes in the electron transfer environment. The related mechanism explanation is only used to explain the source of technical effects, and does not constitute a limitation on the protection scope of the present application.

[0057] Pore structure, porosity and leachate pH / conductivity This section is optional. In some embodiments, to balance mass transfer and electron transfer, the carbon aerogel microspheres can have one or more of the following pore structure and leaching characteristics parameters: Porosity: the porosity measured by mercury intrusion method is, for example, 38.67% to 51.66%.

[0058] Pore size: the median pore size (volume distribution) measured by mercury intrusion method is, for example, 331.55 to 1576 nm, and the pore size distribution can cover sub-micron to micron scale (see Figures 5-7 ).

[0059] Leaching pH and conductivity: a suspension is prepared according to 1% (w / w) and ultrapure water, and the pH is measured to be, for example, 8.76 to 9.65, and the conductivity is measured to be, for example, 908.5 to 1492.5 (units see Table 2) after 24 h of shaking at 37°C and 130 rpm.

[0060] The above parameters are example data of the embodiments, which are only used to illustrate the material characteristics and do not constitute a limitation on the protection scope of the present application.

[0061] Example 1: Preparation of cyanobacteria-based carbon aerogel microspheres and their application in anaerobic digestion Cyanobacteria-based biochar preparation: after drying the cyanobacteria raw material to a constant weight, it is placed in an inert atmosphere furnace, and nitrogen is introduced for 30 min to replace the air; then it is heated to 400°C at a rate of 10°C / min and held at this temperature for 1.5 h, and then cooled to obtain cyanobacteria-based biochar; the biochar is ground and sieved through a 100 mesh sieve, and then dried for use.

[0062] Microsphere preparation: sodium alginate was dissolved in water to prepare a 4% (mass fraction) sodium alginate aqueous solution; 80:6:1 of the mass ratio of the sodium alginate aqueous solution, biochar and thermosetting phenolic resin was added and stirred for 1-2 min, and then ultrasonic dispersion was performed for 15 min to obtain a uniform slurry; the slurry was dropped into a 0.5 mol / L CaCl2 aqueous solution through a peristaltic pump to form hydrogel microspheres, and the microspheres were solidified in the CaCl2 solution for 12 h; after being taken out, the microspheres were rinsed with water several times and immersed in distilled water for more than 8 h; then the microspheres were dried at 60°C until the weight was constant (preferably for more than 48 h); the dried microspheres were placed in a rotary furnace, nitrogen was introduced for 20 min, the temperature was raised to 400°C at a rate of 10°C / min and maintained for 90 min, and then cooled to obtain cyanobacteria-based carbon aerogel microspheres.

[0063] Anaerobic digestion: 500 mL serum bottles were taken as batch reactors (working volume 400 mL), and methanogenic inoculum sludge and kitchen waste slurry were added according to an inoculation ratio of 10% (volume ratio), the initial pH was adjusted to 7.4, and 5 g / L of cyanobacteria-based carbon aerogel microspheres were added; then nitrogen was introduced for 20 min to establish an anaerobic environment and the serum bottles were sealed, and the reaction was carried out in a constant-temperature shaker (37±1°C, 130 rpm). The gas production and gas composition were measured every two days, and the cumulative biogas / methane yield was calculated according to the consumed VS (initial-final). The cumulative biogas yield of Example 1 was 816.67 mL / g VS, of which the cumulative methane yield was 618.28 mL / g VS. Each test group was carried out at least twice in parallel, and the average values of the parallel tests are listed in Table 1.

[0064] Example 2: Preparation of biogas residue-based carbon aerogel microspheres and their application in anaerobic digestion Biogas residue-based biochar preparation: after the dewatered biogas residue was dried to a constant weight, it was placed in an inert atmosphere furnace, nitrogen was introduced for 30 min to replace the air; then the temperature was raised to 400°C at a rate of 10°C / min and maintained for 1.5 h, and then cooled to obtain biogas residue-based biochar; the biochar was ground and sieved through a 100-mesh sieve, and then dried for use.

[0065] Microsphere preparation and anaerobic digestion steps are the same as in Example 1, except that the biochar is replaced by biogas residue-based biochar. The cumulative biogas yield of Example 2 was 864.08 mL / g VS, of which the cumulative methane yield was 642.14 mL / g VS.

[0066] Example 3: Preparation of sludge-based carbon aerogel microspheres and their application in anaerobic digestion Sludge-based biochar preparation: after the dewatered sludge was dried to a constant weight, it was placed in an inert atmosphere furnace, nitrogen was introduced for 30 min to replace the air; then the temperature was raised to 400°C at a rate of 10°C / min and maintained for 1.5 h, and then cooled to obtain sludge-based biochar; the biochar was ground and sieved through a 100-mesh sieve, and then dried for use.

[0067] The microspheres were prepared and the anaerobic digestion step was the same as Example 1, except that the biochar was replaced by sludge-based biochar. The cumulative biogas yield of Example 3 was 878.11 mL / g VS, of which the cumulative methane yield was 662.33 mL / g VS.

[0068] Comparative Example 1: No addition of exogenous material The seed sludge and kitchen waste slurry were added in a serum bottle with a working volume of 400 mL at an inoculation ratio of 10%, and no exogenous material was added; after nitrogen filling for 20 min, the serum bottle was sealed and shaken at 37±1 ℃ and 130 rpm. The cumulative biogas yield of Comparative Example 1 was 618.95 mL / g VS, of which the cumulative methane yield was 468.40 mL / g VS.

[0069] Comparative Example 2: Addition of sludge-based biochar The sludge-based biochar was ground and dried after passing through a 100-mesh sieve for standby use; 5 g / L of sludge-based biochar was added in the anaerobic digestion system, and the rest of the conditions were the same as those of Comparative Example 1. The cumulative biogas yield of Comparative Example 2 was 800.14 mL / g VS, of which the cumulative methane yield was 589.31 mL / g VS.

[0070] Table 1 Comparison of effects of Examples 1-3 and Comparative Examples 1-2 Note: The data in the table are the average values of parallel tests As shown in Table 1 and Figure 2 compared with the group without addition (Comparative Example 1), the addition of carbon aerogel microspheres can increase the cumulative biogas yield to 816.67-878.11 mL / g VS, with an increase of 31.94%-41.87%; compared with the addition of sludge-based biochar (Comparative Example 2), the carbon aerogel microspheres still show higher methane yield at the same addition amount.

[0071] Table 2 Pore structure, porosity and leaching pH and conductivity of different materials Note: The pore structure parameters were measured by mercury injection method; the pH and conductivity were measured after preparing a suspension of 1% (w / w) with ultrapure water and shaking at 37 ℃ and 130 rpm for 24 h. The mercury injection results of the powdered biochar may include inter-particle pores, and the related data are for reference only.

[0072] The pore size distribution results are shown in Figures 5-7As shown in FIG. 2, the pore size distribution of the carbon aerogel microspheres of different sources mainly ranges from hundreds of nanometers to several microns; compared with the powdered sludge biochar, the pore size distribution of the sludge-based carbon aerogel microspheres shifts to a larger pore size range (see FIG. 2), which may help to improve the accessibility of microorganisms into the pores and increase the available interface inside the carrier. Figure 6 、 Figure 7 ,

[0073] The scanning electron microscopy results are shown in FIG. 3, and the carbon aerogel microspheres exhibit a more developed pore structure and continuous skeleton morphology compared with the biochar, which can provide a larger interface for microbial attachment and material / electron transfer. Figure 4 As can be seen from the mercury intrusion porosimetry (FIG. 4), the pore size distribution of the microspheres covers the sub-micron to micron level, which provides spatial conditions for cell-scale microorganisms to enter the connected pores and attach inside the material, thereby possibly enhancing the microbial-carrier interface interaction and improving the mass transfer and electron transfer environment of the methanogenesis process. The above mechanistic explanation is only used to explain the beneficial effects and does not constitute a limitation on the protection scope of the present application. Figures 5-7

[0074] In the present application, unless otherwise stated, the "dosing amount" is based on the working volume of the anaerobic digestion system; the cumulative biogas yield and the cumulative methane yield are based on the consumed VS (initial-end). The biogas volume is measured at room temperature (25±3℃); if conversion to standard condition volume is required, conversion can be performed according to the ideal gas state equation, and the present application is not limited by the volume conversion method.

[0075] Without deviating from the overall concept of the present application, those skilled in the art can reasonably adjust the precursor slurry composition, droplet parameters, pyrolysis conditions and dosing strategies according to the nature of the food waste, the type of reactor and the operating load, and any equivalent replacement or modification shall fall within the protection scope of the present application.

[0076] The above examples are only used to explain the technical solutions of the present application, and are not a limitation on the protection scope of the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application shall be included in the protection scope of the present application.​

Claims

1. A method for preparing carbon aerogel microspheres, characterized in that, The process includes the following steps: S1, preparing an aqueous solution of sodium alginate, and mixing and dispersing biochar powder and thermosetting phenolic resin in the sodium alginate aqueous solution to obtain a precursor slurry; S2, dropping the precursor slurry into an aqueous solution of calcium chloride, and then... 2+ The hydrogel microspheres are solidified under ionic crosslinking and then washed and dried. In step S3, the dried hydrogel microspheres are pyrolyzed and carbonized under an inert atmosphere to obtain carbon aerogel microspheres.

2. The preparation method according to claim 1, characterized in that, The biochar powder is ground and passed through an 80-200 mesh sieve; the biochar powder is selected from cyanobacteria-based biochar, biogas residue-based biochar and / or sludge-based biochar.

3. The preparation method according to claim 1, characterized in that, The sodium alginate aqueous solution has a mass fraction of 2% to 6%.

4. The preparation method according to claim 1, characterized in that, The mass ratio of sodium alginate aqueous solution, biochar powder and thermosetting phenolic resin is 80:(2-6):1; the precursor slurry is stirred for 1-2 min and then ultrasonically dispersed for 10-20 min.

5. The preparation method according to claim 1, characterized in that, The concentration of the calcium chloride aqueous solution is 0.1–1.0 mol / L; the curing time is 6–24 h; after curing, it is washed with clean water and then immersed in distilled water for more than 8 h to remove surface Ca. 2+ .

6. The preparation method according to claim 1, characterized in that, The drying process is carried out at room temperature to constant weight, or at 60–80 °C to constant weight; the pyrolysis carbonization is carried out in a rotary kiln or tube furnace, with an inert gas introduced for 10–30 min before pyrolysis; the heating rate is 5–20 °C / min; the pyrolysis temperature is 300–600 °C; and the residence time is 30–180 min.

7. A carbon aerogel microsphere, characterized in that, The carbon aerogel microspheres are spherical porous conductive carbon microspheres, prepared by the preparation method according to any one of claims 1 to 6, wherein the diameter of the carbon aerogel microspheres is 0.5 to 5 mm; the carbon aerogel microspheres have a three-dimensional interconnected pore structure, including a hierarchical pore structure of micropores, mesopores and macropores, and the pore structure includes through channels at the submicron to micron scale; the conductive framework of the carbon aerogel microspheres includes a porous network formed by carbonization of sodium alginate and reinforcing carbon formed by carbonization of thermosetting phenolic resin, and biochar particles are dispersed and embedded in the conductive framework.

8. A method for improving the biogas production from the anaerobic digestion of kitchen waste, characterized in that, The carbon aerogel microspheres described in claim 7 are added as an exogenous material to the anaerobic digestion system of food waste.

9. The method according to claim 8, characterized in that, The dosage of the carbon aerogel microspheres is 0.5–10 g / L; the anaerobic digestion is mesophilic anaerobic digestion, with a digestion temperature of 35–40 °C and an initial pH of 6.8–7.8; the carbon aerogel microspheres are added all at once during the anaerobic digestion initiation stage, or added in batches during the anaerobic digestion process; when adding in batches, the volatile fatty acid content and / or alkalinity are monitored, and the timing and / or dosage of the batch addition are adjusted according to the monitoring results.

10. The method according to claim 8, characterized in that, It also includes the steps of recovering carbon aerogel microspheres by sieving, sedimentation or filtration after anaerobic digestion, and washing, drying and re-adding the recovered carbon aerogel microspheres; wherein the sieve mesh size used for sieving is smaller than the diameter of the carbon aerogel microspheres; the recovery-washing-drying-re-addition step is repeated at least twice.

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