CN114736934B

CN114736934B - Method for preparing biogas by promoting anaerobic co-fermentation of livestock manure straw by adding biochar

Info

Publication number: CN114736934B
Application number: CN202210574258.7A
Authority: CN
Inventors: 徐建玲; 王汉席; 刘学军; 孙寄添; 王昕宇; 邹丽
Original assignee: Northeast Normal University
Current assignee: Northeast Normal University
Priority date: 2022-05-24
Filing date: 2022-05-24
Publication date: 2024-03-26
Anticipated expiration: 2042-05-24
Also published as: ZA202303147B; CN114736934A

Abstract

The invention discloses a method for preparing biogas by promoting anaerobic co-fermentation of livestock manure straw by adding biochar, which comprises the following steps: (1) Cleaning impurities in fresh livestock manure, air-drying straws and crushing the straws; (2) Soaking the straw in sodium hydroxide aqueous solution, then washing with water, and finally carrying out ultrasonic treatment; (3) Adding livestock manure, straw, biochar and water into a fermentation tank, sealing, heating at the same time, introducing biogas slurry after sealing, and performing anaerobic fermentation to obtain biogas. The method for preparing the biogas can effectively improve the total gas yield and the total methane yield and shorten the gas production stagnation time.

Description

Method for preparing biogas by promoting anaerobic co-fermentation of livestock manure straw by adding biochar

Technical Field

The invention belongs to the technical field of biogas preparation, and particularly relates to a method for preparing biogas by promoting anaerobic co-fermentation of livestock manure straws by adding biochar.

Background

Organic waste represents a significant proportion of the total amount of waste produced worldwide, such as organic fractions of municipal solid waste, wastewater treatment sludge, agricultural waste, forest residues, and the like. Agricultural wastes mainly comprise planting wastes, breeding wastes, farm and pasture product processing wastes and the like generated in the agricultural production process, and livestock and poultry feces are difficult to treat in the agricultural wastes, and the yield is huge. Livestock manure has a huge yield in the world.

China is also a large country of agricultural animal husbandry, 38 hundred million tons of livestock manure are produced each year, 1570.42 tens of thousands of pigs are placed in the end of 2018 years of Jilin province, 249.56 tens of thousands of cattle are placed in the end of 2018 years of Jilin province, 45062.26 tens of thousands of poultry are placed in the end, and 625.027 tens of thousands of tons of pig manure, 1135.58 tons of poultry manure and 1821.79 tons of pig manure are produced according to the pollution discharge coefficient calculation.

Because the feces contain more carbon and nitrogen elements and have higher economic value, and the stacking of the feces can cause the problems of greenhouse gas emission, diseases and the like, the livestock feces need to be treated, and the treatment method of the organic waste has various forms including anaerobic fermentation, aerobic composting, sanitary landfill and the like, and the organic waste treatment refers to the physical, chemical and biological treatment of the organic waste and pollutants thereof, so that the pollution to the environment is reduced and even the waste is turned into wealth.

Anaerobic fermentation (AD for short), also known as anaerobic digestion, is an efficient biochemical pathway for converting organic solid waste and wastewater into energy and valuable products. Anaerobic fermentation has been operated globally commercially for decades as a resource recovery technology supporting recycling economies. The anaerobic fermentation process generates methane, the methane can be converted into heat energy and power, the digested matters after anaerobic digestion are rich in nutrition and can be safely spread in the field as soil conditioner, so that the soil fertility is effectively improved, the physical and chemical properties of the soil are improved, and how to improve the total gas yield and the total methane yield and shorten the gas production stagnation time become the main research direction of anaerobic fermentation related researches.

Therefore, how to develop a method for preparing biogas by adding biochar to promote livestock manure straw anaerobic co-fermentation is a technical problem to be solved by the technicians in the field.

Disclosure of Invention

In view of the above, the invention provides a method for preparing biogas by promoting anaerobic co-fermentation of livestock manure straw by adding biochar.

In order to achieve the above purpose, the present invention adopts the following technical scheme:

a method for preparing biogas by promoting anaerobic co-fermentation of livestock manure straw by adding biochar comprises the following steps:

(1) Cleaning impurities in fresh livestock manure, air-drying straws and crushing the straws;

(2) Soaking the straw in sodium hydroxide aqueous solution, then washing with water, and finally carrying out ultrasonic treatment;

(3) Adding livestock manure, straw, biochar and water into a fermentation tank, sealing, heating at the same time, introducing biogas slurry after sealing, and performing anaerobic fermentation to obtain biogas.

The invention has the beneficial effects that: the method for preparing the biogas can effectively improve the total gas yield and the total methane yield and shorten the gas production stagnation time.

Further, the livestock manure is pig manure.

Further, the straw is corn straw.

Further, in the step (1), the straws are air-dried until the water content is 0-20%, and the straws are crushed into 16-30 meshes.

Further, in the step (1), the biochar is prepared by anoxic burning of corn stalks for 2 hours at the temperature of 450 ℃ and the heating rate is 10 ℃/min.

In the step (2), the mass-volume ratio of the straw to the sodium hydroxide aqueous solution is 0.112mg/mL, and the concentration of the sodium hydroxide aqueous solution is more than or equal to 0.5mol/L.

Further, in the step (2), the soaking time is 24 hours.

Further, in the above step (2), the washing solution is washed with water until the pH of the washing solution becomes 7.

Further, in the step (2), the frequency of the ultrasonic treatment is 40KHz, and the time of the ultrasonic treatment is 4-6min.

Further, in the step (3), the carbon nitrogen ratio of the livestock manure and straw mixture is 28, the addition amount of the biochar is 2-8% of the total solid content of the livestock manure and the straw, and the addition amount of the water is 7-9% of the total solid content of the livestock manure, the straw, the biochar and the water.

In the step (3), the addition amount of the biogas slurry is 9-11% of the volume of the raw material in the fermentation tank after the sealing is finished.

In the step (3), the heating temperature is 35+/-1 ℃, the sealing time is 1-4 days, and biogas slurry is introduced after sealing is finished for anaerobic fermentation.

Further, in the step (3), the anaerobic fermentation temperature is 33-37 ℃ and the anaerobic fermentation time is 30-40 days.

Drawings

FIG. 1 is a diagram of an anaerobic fermentation reaction apparatus, wherein: the device comprises a 1-constant-temperature water bath, a 2-anaerobic fermentation reactor, a 3-biogas slurry sampling port, a 4-biogas transmission device, a 5-check valve, a 6-biogas sampling port, a 7-gas collecting bottle, an 8-drainage weighing device and a 9-drainage pipe;

FIG. 2 is a graph of daily biogas yield of pig manure corn straw anaerobic co-fermentation with biochar addition;

FIG. 3 is a diagram of total biogas yield of pig manure corn straw anaerobic co-fermentation with biochar added;

FIG. 4 is a graph showing the percentage change of methane produced by pig manure and corn stalks anaerobic co-fermentation with biochar added per day;

FIG. 5 is a graph of daily methane yield change in the anaerobic co-fermentation of biochar-added porcine corn stover;

FIG. 6 is a graph of the change of the total methane yield of the anaerobic co-fermentation of the pig corn stalks added with the biochar;

FIG. 7 is a diagram of the change of the physicochemical properties of the anaerobic co-fermentation of pig manure and corn stalks added with biochar;

FIG. 8 is an enlarged view of the change of the anaerobic co-fermentation physicochemical properties of pig manure and corn stalks added with biochar;

fig. 9 is a PCA analysis chart of pig manure biochar.

Detailed Description

The following description of the technical solutions in the embodiments of the present invention will be clear and complete, and it is obvious that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

Example 1

The method for preparing the biogas by promoting the anaerobic co-fermentation of the livestock manure straw by adding the biochar comprises the following steps of:

(1) Performing anoxic combustion on corn stalks for 2 hours at 450 ℃ to obtain biochar, cleaning impurities such as stones and grass in fresh pig manure at a heating rate of 10 ℃/min, air-drying the corn stalks until the water content is 2%, and crushing the corn stalks to 25 meshes;

(2) Soaking the straws in a sodium hydroxide aqueous solution for 24 hours, wherein the mass volume ratio of the straws to the sodium hydroxide aqueous solution is 0.112mg/mL, the concentration of the sodium hydroxide aqueous solution is 0.5mol/L, then washing with water until the pH of a washing solution is 7, and finally carrying out ultrasonic treatment with the frequency of 40KHz and the ultrasonic treatment time of 5min;

(3) Adding pig manure, straw, biochar and water into a fermentation tank, wherein the carbon nitrogen ratio of the pig manure and straw mixture is 28, the addition amount of the biochar is 2% of the total solid content of the livestock manure and the straw, the addition amount of the water is 8% of the total solid content of the pig manure, the straw, the biochar and the water, sealing and heating simultaneously, the heating temperature is 35+/-1 ℃, the sealing time is 3 days, introducing biogas slurry for anaerobic fermentation after sealing, the anaerobic fermentation temperature is 35+/-1 ℃, the anaerobic fermentation time is 33 days, and the addition amount of the biogas slurry is 10% of the volume of the raw materials in the fermentation tank after sealing, so as to obtain the biogas.

Example 2

(1) Performing anoxic combustion on corn stalks for 2 hours at 450 ℃ to obtain biochar, cleaning impurities such as stones and grass in fresh pig manure at a heating rate of 10 ℃/min, air-drying the corn stalks until the water content is 10%, and crushing the corn stalks to 16 meshes;

(2) Soaking the straws in a sodium hydroxide aqueous solution for 24 hours, wherein the mass volume ratio of the straws to the sodium hydroxide aqueous solution is 0.112mg/mL, the concentration of the sodium hydroxide aqueous solution is 0.6mol/L, then washing with water until the pH of a washing solution is 7, and finally carrying out ultrasonic treatment with the frequency of 40KHz and the ultrasonic treatment time of 4min;

(3) Adding pig manure, straw, biochar and water into a fermentation tank, wherein the carbon nitrogen ratio of the pig manure and straw mixture is 28, the addition amount of the biochar is 4% of the total solid content of the pig manure and the straw, the addition amount of the water is 7% of the total solid content of the pig manure, the straw, the biochar and the water, sealing and heating simultaneously, the heating temperature is 35+/-1 ℃, the sealing time is1 day, biogas slurry is introduced after sealing is finished for anaerobic fermentation, the anaerobic fermentation temperature is 34+/-1 ℃, the anaerobic fermentation time is 39 days, and the addition amount of the biogas slurry is 9% of the volume of the raw materials in the fermentation tank after sealing is finished, so that biogas is obtained.

Example 3

(1) Performing anoxic combustion on corn stalks for 2 hours at 450 ℃ to obtain biochar, cleaning impurities such as stones and grass in fresh pig manure at a heating rate of 10 ℃/min, air-drying the corn stalks until the water content is 20%, and crushing the corn stalks to 30 meshes;

(2) Soaking the straws in a sodium hydroxide aqueous solution for 24 hours, wherein the mass volume ratio of the straws to the sodium hydroxide aqueous solution is 0.112mg/mL, the concentration of the sodium hydroxide aqueous solution is 0.7mol/L, then washing with water until the pH of a washing solution is 7, and finally carrying out ultrasonic treatment with the frequency of 40KHz and the ultrasonic treatment time of 6min;

(3) Adding pig manure, straw, biochar and water into a fermentation tank, wherein the carbon nitrogen ratio of the pig manure and straw mixture is 28, the addition amount of the biochar is 6% of the total solid content of the pig manure and the straw, the addition amount of the water is 9% of the total solid content of the pig manure, the straw, the biochar and the water, sealing and heating simultaneously, the heating temperature is 35+/-1 ℃, the sealing time is 4 days, introducing biogas slurry for anaerobic fermentation after sealing, the anaerobic fermentation temperature of the anaerobic fermentation is 36+/-1 ℃, the anaerobic fermentation time is 30 days, and the addition amount of the biogas slurry is 11% of the volume of the raw materials in the fermentation tank after sealing, so as to obtain biogas.

Example 4

(3) Adding pig manure, straw, biochar and water into a fermentation tank, wherein the carbon nitrogen ratio of the pig manure and straw mixture is 28, the addition amount of the biochar is 8% of the total solid content of the pig manure and the straw, the addition amount of the water is 8% of the total solid content of the pig manure, the straw, the biochar and the water, sealing and heating simultaneously, the heating temperature is 35+/-1 ℃, the sealing time is 3 days, introducing biogas slurry for anaerobic fermentation after sealing, the anaerobic fermentation temperature is 35+/-1 ℃, the anaerobic fermentation time is 39 days, and the addition amount of the biogas slurry is 10% of the volume of the raw materials in the fermentation tank after sealing, so as to obtain the biogas.

Effect experiment

1. Experimental method

(2) Soaking the straws in a sodium hydroxide aqueous solution for 24 hours, wherein the mass volume ratio of the straws to the sodium hydroxide aqueous solution is 0.112mg/mL, the concentration of the sodium hydroxide aqueous solution is 0.5mol/L, then washing the straws with water to pH 7, and finally carrying out ultrasonic treatment, wherein the frequency of the ultrasonic treatment is 40KHz, and the ultrasonic treatment time is 5min;

(3) A500 mL conical flask is used as a reactor (effective volume is 610 mL), a rubber plug and glass cement are used for sealing, a constant temperature water bath kettle is used for heating, a 1L gas collecting bottle is used for collecting gas, the content of biogas is calculated by a drainage method, the glass cement and medical vaseline are used for sealing the joint of the device to prevent air leakage, pig manure, straw, biochar and water are added into the reactor shown in the figure 1, the carbon nitrogen ratio of the pig manure and straw mixture is 28, the addition amount of the biochar is 0-8% of the total solid content of the pig manure and the straw, the addition amount of the water is 8% of the total solid content of the pig manure, the straw, the biochar and the water, the sealing is carried out in the constant temperature water bath kettle, the heating temperature is 35+/-1 ℃, the sealing time is 3 days, biogas slurry is introduced after the sealing is finished, the anaerobic fermentation temperature is 35+/-1 ℃, and the addition amount of the biogas slurry is 10% of the volume of the raw materials in the fermentation tank after the sealing is finished, and the biogas is prepared.

The time when biogas slurry was added was defined as the start time. Followed by 12 per day: 00 detecting gas production, taking biogas slurry every 3 days, and detecting ammonia nitrogen, VFAs, pH and conductivity. And collecting biogas every three days to determine the methane content in the biogas and determining the methane yield. Methane production was fitted using a modified Gompertz model to investigate the gas production performance of anaerobic fermentation. And (3) carrying out principal component analysis on physical and chemical indexes and methane yield, and researching the mechanism of the additive affecting anaerobic fermentation gas production performance through principal component analysis results.

Table 1 groups of experimental groups and nomenclature

2. Measurement method

The volume of liquid in the measuring cylinder and the ambient temperature were recorded at 12:00 pm each day, the reactor was shaken for one minute each morning, midnight and evening each day, and the gas was collected with a gas collection bag and the methane content was measured with a portable methane detector and recorded. Sampling every 3 days in the anaerobic digestion process, taking 10mL of biogas slurry through a three-way valve each time, adding a proper amount of distilled water according to the solid content of the biogas slurry, measuring pH and conductivity, centrifuging, and taking supernatant to measure ammonia nitrogen and VFAs. Anaerobic fermentation is considered to be over when the gas production is below 10% of the maximum gas production.

The basic properties of the fermentation raw materials were measured for water content, TOC, TN, TS and VS. The basic properties of the raw materials are shown in Table 2. The specific method for measurement is as follows:

(1) pH value. And mixing the biogas slurry uniformly, and measuring the temperature and the pH value by using a pH meter.

(2) Total Organic Carbon (TOC) content. The potassium dichromate volumetric method is adopted. The organic carbon in the air-dried sample is oxidized by using a quantitative potassium dichromate-sulfuric acid solution under the heating condition, and the redundant potassium dichromate is titrated by using a ferrous sulfate standard solution and simultaneously uses silicon dioxide as a blank test.

(3) Total Nitrogen (TN). Nitrogen in the dried sample is digested with sulfuric acid-hydrogen peroxide and converted into ammonium nitrogen. Ammonia distilled off after alkalization was absorbed with boric acid solution, titrated with standard acid solution, and the total nitrogen content in the sample was calculated.

(4) Volatile Fatty Acids (VFAs). Colorimetric method comprises centrifuging biogas slurry at 5000 rpm for 15min, collecting supernatant, adding ethylene glycol, sulfuric acid, heating in boiling water bath for 3min, cooling with cold water, adding hydroxylamine sulfate 0.5mL10%, sodium hydroxide 2mL4.5mol, and acid ferric chloride 10mL, fixing volume to 25mL, developing, and measuring at 500nm with spectrophotometer.

(5) Ammonia nitrogen. 10mL of the supernatant was centrifuged at 5000 rpm for 15min and the supernatant was measured by a phenol nitrobenzene-sodium dichloroisocyanurate color development method.

(6) And (5) gas production. Biogas production was measured by drainage.

(7) Total solids content (TS). Measured by a dry weight loss method, and dried at 105 ℃ for 24 hours.

(8) Volatile solids content (VS). The measurement was carried out by a muffle furnace firing method, and the measurement was carried out by firing at 550℃for 2 hours.

(9) Methane content. Methane content the methane concentration was measured with a portable methane detector.

TABLE 2 basic Properties of fermentation raw materials

	TOC(g/kg·TS)	TN(g/kg·TS)	C/N	TS(％)	VS(％TS)
						Pig manure	395.62	16.76	23.61	23.61	76.31
Straw	559.07	4.83	115.74	93.62	97.93

3. Data processing

Three parallel samples are taken during sampling in the anaerobic fermentation system, and the final measurement result is averaged. Data statistics were performed using microsoft excel2016, modeling total methane production by a modified Gompertz model, shown in equation (1), using origin 9.0. The effect of physicochemical properties on gas production performance was studied by Principal Component Analysis (PCA) using Canaco 5.

Wherein: p is the cumulative methane yield in mL/g (in terms of VS); p (P) ₀ For final methane potential, units are mL/g (in terms of VS); rm is the maximum methanogenic rate in mL/(g.d) (in VS); lambda is the residence time in d; e is a constant 2.718282.

4. Results and analysis

4.1.1 influence of biochar addition on gas production

The daily gas production of pig manure fermentation with added biochar is shown in figure 2, the maximum gas production per day is P6C experimental group, 21.91mL/g VS is produced, and the peak of gas production is reached at the highest speed, and the addition of biochar can improve the gas production and make the gas production more stable. The effect on the gas production cycle is not obvious, the blank group reaction is ended at 35 days, the fermentation cycle is 30-39 days after adding the biochar, the reaction is fastest after adding 6% of the biochar, and the reaction is ended at 30 days.

As shown in figure 3, the total biogas yield of pig manure fermentation with added biochar is slightly lower than that of a control group, the total biogas yield is 112.33mL/g VS, the biogas yield can be improved by adding other amounts of biochar, the biogas yield with added 6% of biochar is 188.74mL/g VS, and the total biogas yield is 121.96mL/g VS higher than that of the control group.

4.1.2 influence of biochar addition on biogas Components

The results of the experimental group gas production of pig manure corn straw anaerobic co-fermentation with biochar are shown in table 3. The addition of biochar significantly promotes methane yield, and the final methane yield is improved by 18.25% -150.15%, and the influence on the reaction period is not obvious, wherein the final methane percentage of the added 6% is the highest, and the total methane yield accounts for 60% of the total methane yield.

TABLE 3 influence of charcoal addition on anaerobic fermentation of pig manure to produce gas

The change of methane percentage in the pig manure experimental group is shown in fig. 4, the methane percentage is slightly higher than that of the control group after the biochar is added in the first 9 days, the addition of 6% of the biochar can enable the system to rapidly start producing methane, but the addition of 8% of the biochar can slightly prolong the time for reaching the peak of gas production. Overall, the addition of biochar increases the methane percentage.

The daily methane yield of the pig manure anaerobic co-fermentation after addition of biochar is shown in figure 5. As can be seen from the figure, the experimental group with 6% biochar added rapidly produced methane, reaching the peak of gas production for the first time on day six, producing methane 14.31mL/g VS. The control group has less methane production, the balance is destroyed after the peak of methane production is reached, the methane production is rapidly reduced, the methane production is stable after the biochar is added, and the continuous methane production time is long after the peak of methane production is reached.

Daily methane production for the pig manure biochar group is shown in figure 6. As can be seen from the graph, after the biochar is added, the total methane yield of all experimental groups is higher than that of a control group, the P6C experimental group rapidly produces methane, the total methane yield is always higher than that of the control group, the P8C experimental group with the most methane production produces 119.74mL/g VS methane, and the P6C experimental group with the most gas production produces 111.25mL/g VS methane, which is far higher than that of the control group 47.86mL/g VS methane.

4.1.3 influence of biochar addition on physicochemical index

The effect of adding biochar on anaerobic co-fermentation of pig manure and corn straw is shown in figure 7, respectively.

The pH of the experimental group added with 6% of biochar in the pig manure experimental group quickly rises to be neutral and starts to produce gas, and the pH of the other experimental groups firstly drops and then rises, which indicates that acidification is not completely finished and the most suitable neutral condition of methanogen is not gradually recovered until the ninth day.

In the pig manure experimental group, ammonia nitrogen is very high on the 0 th day when the reaction starts to introduce biogas slurry, and the phenomenon of ammonia nitrogen inhibition possibly exists due to the high nitrogen content of the pig manure.

The conductivity shows a decreasing trend in the pig manure, and the conductivity in the pig manure is about 3.0. The conductivity reaches a peak around the 5 th day, and the biochar group is saturated in the wetland system and has a dynamic process of adsorption release, so that the conductivity floats up and down and is not obvious.

Because biochar stimulates the anaerobic fermentation process, VFAs can be decomposed by methanogenic bacteria after being accumulated to a certain extent, and the concentration of the VFAs is reduced after 12 days of reaction due to the utilization of the VFAs in the methanogenic process, the concentration of the VFAs in the middle and later stages of reaction is reduced by adding the biochar, and the problem of anaerobic fermentation system damage caused by the accumulation of the VFAs is solved.

4.2.1 investigation of gas production Properties by biochar

The gas production performance of anaerobic fermentation can be analyzed by a model, and the methane production result of the modified Gompertz model fitting is shown in Table 4, and the final methane production potential can be improved by adding biochar into pig manure, and the final methane production potential is improved from 47.43mL/g VS to 122.24mL/g VS of the P2C experimental group.

TABLE 4 Gompertz model corrected pig manure fitting methanogenesis results

From the residence time point of view, the addition of 2% and 8% of biochar in the pig manure biochar group can prolong the gas production residence time, but the addition of 4% or 6% of biochar can significantly shorten the gas production residence time. Hydrolysis is an important speed limiting step of anaerobic fermentation, and the shortening of residence time also represents that biochar promotes the hydrolysis and acidification processes in anaerobic fermentation, so that methanogens are rapidly propagated after inoculums are introduced, and an anaerobic fermentation system is rapidly stabilized. On the other hand, the biochar can promote direct electron transfer among seeds, so that an anaerobic fermentation system can enter a methanogenesis stage more quickly, and the retention time is shortened.

From the aspect of fitting degree, the fitting degree in pig manure is better and is more than 0.985. Because the biogas slurry contains a small amount of volatile fatty acid, the system is easier to start, and because the biochar can promote anaerobic fermentation, the generated VFAs are quickly utilized, and the system is quickly started. The residence time of P6C in this study was the shortest, which was 80.66% shorter than that in the control group.

4.2.2 mechanism analysis of biochar affecting gas production Performance

From the point of view of the gas production results, the addition of biochar promotes the gas production effect of anaerobic fermentation, shortens the gas production period, and many studies have shown that hydrolysis is an important rate-limiting step, and that hydrolysis and methane production are performed simultaneously for an anaerobic fermentation system. The key to increasing the gas production rate is to accelerate the hydrolysis stage. At present, most students accelerate anaerobic fermentation through two-phase anaerobic fermentation, separate a hydrolysis stage from a methanogenesis stage, and obtain obvious research results. And a proper pretreatment method can also accelerate the hydrolysis rate. The invention provides a new idea, and the corn straw is subjected to NaOH alkaline pretreatment and added into an anaerobic fermentation system of livestock manure to form a co-fermentation system, so that researches show that the alkaline pretreatment can thin the cell wall, and the treated straw is more easily utilized by methanogens and hydrolytic bacteria. The biochar additive is added on the basis of the method, so that a growth space of methanogens is provided, and the methane production lag phase is further advanced. The pH value is from pH 7 before treatment to about 6 at the beginning of the reaction, and the hydrolysis and acidification stages have been started for some time.

Biochar is an example of an adsorbent made from agricultural residues as it is cheaper than adsorbents such as activated carbon, zeolite, etc., and its use is increasing. Although biochar can theoretically have a pH buffering effect, the addition of biochar does not have a significant pH buffering effect from the experimental results. This is probably due to the biochar's reduced late gas production lag phase while buffering the pH, improving efficiency, thus accelerating pH changes.

Ammonia nitrogen has been found by scholars to be toxic to anaerobic fermentation because it can cross the cell membrane, thereby affecting anaerobic fermentation bacteria. Researchers have shown that free ammonia is more toxic than ammonia nitrogen because it is able to penetrate cell membranes. Ammonia inhibition generally occurs in continuous feeding anaerobic fermentation and high solid content anaerobic dry fermentation, and ammonia nitrogen inhibition in sequencing batch anaerobic wet fermentation is reported, on one hand, because the solid content is low, generated ammonia nitrogen is diluted into biogas slurry in a vibrating mode to slow down the influence on anaerobic fermentation, on the other hand, because fermentation raw materials are added into a reactor before the reaction starts and are not fed in the middle, the total nitrogen content is low, a small amount of ammonia nitrogen also has a stimulation and excitation effect on anaerobic microorganisms, so that anaerobic fermentation is promoted, the change of the ammonia nitrogen in the anaerobic fermentation process is detected, and the whole anaerobic fermentation process is helped to be known. In the invention, the ammonia nitrogen concentration of the pig manure is higher, but the ammonia nitrogen concentration of the pig manure is far from 3390mg/L for similar experiment.

According to the invention, the addition of the biochar significantly promotes the methane yield in the pretreated pig manure, P8C and P6C in the experiment are two experimental groups with the highest methane yield in a single day, and the daily methane ratio of four experimental groups added with the biochar is obviously improved, because the biochar can promote the inter-species electron transfer, the methanogen activity is higher, and the methane yield is increased.

The conductivity of biochar fluctuates with time after adsorption saturation even in pure water and formulated municipal tail water. The straw biochar particles are light in weight, form suspended solids, and can increase EC in the liquid. Biochar also has stronger adsorptivity. In addition, the ash concentration of the straw biochar discharged into the wastewater is greatly changed, and the concentration of suspended matters is obviously increased along with the introduction of pollutants, so that the EC is changed. The straw biochar mainly contains C, H, N and O, wherein the content of C element is highest. Addition to anaerobic fermentation systems provides a living space for anaerobic bacteria and also causes fluctuations in conductivity.

The results of the PCA analysis of the pig manure experimental group are shown in FIG. 9. PCA analysis of pig manure fermentation with added biochar shows that Axis1 and Axis2 respectively account for 78.88% and 98.35% of methane yield variation. The addition of biochar appears to be significantly positively correlated with pH and significantly negatively correlated with the change in conductivity. Biochar may affect methane production by affecting changes in pH and conductivity, affecting anaerobic bacterial activity. Higher VFAs is a major factor in lower methane production and higher ammonia nitrogen is one of the factors responsible for lower methane production.

5. Conclusion(s)

(1) The addition of biochar in the pig manure experimental group can obviously improve the total gas yield and the total methane yield and shorten the gas production stagnation time. The total methane content is improved by 18.53-150.18%, and the gas production period change is not obvious. The addition of 6% of biochar is the optimal addition amount for the pig manure co-fermentation experiment.

(2) The addition of biochar influences the activity of anaerobic bacteria by influencing the change of pH and conductivity in a pig manure co-fermentation system, thereby influencing the methane yield.

The description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the present invention is not to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. The method for preparing the biogas by promoting the anaerobic co-fermentation of the livestock manure straw by adding the biochar is characterized by comprising the following steps of:

(3) Adding livestock manure, straw, biochar and water into a fermentation tank, sealing, heating at the same time, introducing biogas slurry after sealing, and performing anaerobic fermentation to obtain biogas;

the livestock manure is pig manure;

in the step (2), the frequency of ultrasonic treatment is 40KHz, and the time of ultrasonic treatment is 4-6min;

in the step (2), the mass volume ratio of the straw to the sodium hydroxide aqueous solution is 0.112mg/mL, and the concentration of the sodium hydroxide aqueous solution is more than or equal to 0.5mol/L;

in the step (3), the biochar is prepared by anoxic burning of corn stalks for 2 hours at the temperature of 450 ℃ and the heating rate is 10 ℃/min;

in the step (3), the carbon-nitrogen ratio of the livestock manure and straw mixture is 28, and the addition amount of the biochar is 6% of the total solid content of the livestock manure and the straw;

in the step (3), the anaerobic fermentation temperature is 33-37 ℃, and the anaerobic fermentation time is 30-40 days;

in the step (3), the addition amount of water is such that the total solid content of the livestock manure, the straw, the biochar and the water reaches 7-9%;

in the step (3), the addition amount of the biogas slurry is 9-11% of the volume of the raw materials in the fermentation tank after the sealing is finished;

in the step (3), the heating temperature is 35+/-1 ℃, the sealing time is 1-4 days, and biogas slurry is introduced for anaerobic fermentation after sealing.

2. The method for preparing biogas by adding biochar to promote anaerobic co-fermentation of livestock manure and straw according to claim 1, wherein in the step (1), the straw is air-dried to a water content of 0-20%, and the straw is crushed to 16-30 meshes.

3. The method for producing biogas by adding biochar to promote anaerobic co-fermentation of livestock manure straw according to claim 1, wherein in step (2), the biogas is washed with water until the pH of the washing solution is 7.