CN112725383A - Treatment method for improving methane production performance of straw anaerobic fermentation and application thereof - Google Patents
Treatment method for improving methane production performance of straw anaerobic fermentation and application thereof Download PDFInfo
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Abstract
The invention belongs to the technical field of biogas preparation by anaerobic fermentation and conversion of organic solid wastes, and particularly relates to a treatment method for improving the methane production performance by straw anaerobic fermentation and application thereof. Aiming at the problems that the crop straws are difficult to degrade in the anaerobic fermentation process and the pretreatment cost is high, the invention provides an environment-friendly, simple, convenient and efficient method for pretreating agricultural and forestry waste. According to the method, acid liquor generated in the anaerobic fermentation acidification process of crop straws is used for replacing a traditional acidic chemical reagent to dip and swell the raw material, and then the pretreatment is performed on the biomass raw material by combining with a mechanical disc mill with low energy consumption, so that the complex structure of the lignocellulose biomass can be effectively destroyed, the cellulose crystallinity is reduced, the porosity of the raw material is improved, and the gas production efficiency of the biomass is improved. The invention combines the anaerobic fermentation process and the biomass pretreatment process, provides a new way for preparing the biogas by the high-efficiency conversion of biomass resources, and has important significance for the development of biogas engineering.
Description
Technical Field
The invention belongs to the technical field of biogas preparation by anaerobic fermentation and conversion of organic solid wastes, and particularly relates to a treatment method for improving the methane production performance by straw anaerobic fermentation and application thereof.
Background
Lignocellulosic biomass has great potential in the production of sustainable energy as the largest renewable organic resource on earth. The lignocellulose resources are fully utilized, so that the dependence of the human society on petrochemical resources can be reduced, and the current severe resource and environmental crisis can be relieved. According to the data of the statistical bureau, the crop straw yield in 2019 is about 8.63 hundred million tons, and the corn straw yield is 2.84 hundred million tons. The straws are usually directly discarded in the field or burnt on the spot, which causes huge environmental pollution and waste of biological resources. Therefore, the treatment of the straws in a recycling, reducing and harmless way is very important.
The lignocellulose part of the straw can be used for producing clean energy, namely methane through anaerobic fermentation, so that the emission of greenhouse gases is reduced, and the ecological environment in rural areas is improved. At present, for the technology of producing biogas by straw anaerobic fermentation, the main technical challenge is that the structure of lignocellulose in the straw is complex and compact, so that the carbon conversion rate of the straw in the anaerobic fermentation process is low. Thus, pretreatment of the straw prior to anaerobic fermentation becomes particularly important.
Currently, the pretreatment methods for lignocellulosic raw materials mainly include physical pretreatment, chemical pretreatment, biological pretreatment, combined pretreatment, and the like. Physical pretreatment can effectively destroy the compact structure of lignocellulose, but they generally require high energy consumption and specialized equipment. Chemical pretreatment has a good effect on improving the gas yield of anaerobic fermentation, but the large-scale application of the chemical pretreatment is hindered by the problems of the cost of adding chemical reagents and secondary pollution. In addition, chemical pretreatment is also prone to produce various fermentation inhibitors (e.g., 5-hydroxymethylfurfural and furfural). The advantages of biological pretreatment are mild reaction conditions, low energy consumption, easy operation, long pretreatment time and small gas production promotion. The ideal pretreatment process for producing biogas should have the characteristics of high efficiency, low cost, simple operation and easy industrial scale. The combined pretreatment refers to the pretreatment of the biomass resources such as straws and the like by two or more single pretreatment methods, and the advantages of each single pretreatment can be fully exerted to make up for the respective disadvantages. The combined pretreatment technology has proven to be an effective and promising method for biomass pretreatment, and is also a hot spot of current biomass pretreatment research.
Mechanochemical pretreatment of lignocellulosic biomass is a combined pretreatment method combining physical and chemical actions, which can efficiently destroy the complex structure of lignocellulose, thereby improving the contact area of enzymes and the conversion effect. If the problems of high chemical reagent cost and adverse effect on the environment can be well solved, and the energy consumption of mechanical treatment is reduced, the mechanochemical pretreatment can be quickly applied to the biogas industry. Therefore, there is an urgent need to improve the pretreatment process and develop an efficient, mild, green, and low-cost mechanochemical pretreatment method.
Compared with other mechanical grinding methods, the disc grinder has the advantages of energy conservation, rapid processing and easy enlargement. Disc mills have long been widely used in the commercial production of pulp and paper. The disc mill can reduce the grain size of crop straws and increase the specific surface area of the crop straws, thereby improving the conversion rate of biomass. However, the disc mill has a limited effect when used alone.
Disclosure of Invention
Aiming at the problems that the crop straws are difficult to degrade in the anaerobic fermentation process and the pretreatment cost is high, the invention provides an environment-friendly, simple, convenient and efficient method for pretreating agricultural and forestry waste. The method utilizes acid liquor generated in the anaerobic fermentation acidification process of crop straws to replace the traditional acidic chemical reagent to dip and swell the raw material, and then combines a mechanical disc mill with lower energy consumption to pretreat the biomass raw material, so that the complex structure of the lignocellulose biomass can be effectively destroyed, the crystallinity of cellulose is reduced, the porosity of the raw material is improved, and the gas production efficiency of the biomass is improved. The method has the advantages of low cost, high efficiency, green and mild process, easy amplification and important practical application value. The invention combines the anaerobic fermentation process and the biomass pretreatment process, provides a new way for preparing the biogas by the high-efficiency conversion of biomass resources, and has important significance for the development of biogas engineering.
The invention is realized in such a way that a treatment method for improving the methane production performance of straw anaerobic fermentation comprises the following steps:
crushing straws, and soaking the straws in acid liquor; after filtering, grinding the solid by a disc mill to obtain a pretreated product; mixing the pretreated product with inoculated sludge, and performing anaerobic fermentation under the condition of medium temperature; the acid liquor comprises any one of straw, fermentation acid liquor in an acid production stage in the fermentation process of inoculated sludge and organic acid liquor; the inoculated sludge is a product obtained by performing continuous biogas fermentation on straws and animal wastes.
The liquid obtained by filtering can be continuously used as steeping liquor, and the obtained crop straws are subjected to disc grinding treatment to fully destroy the structure of the crop straws.
And (3) measuring the microstructure of the crop straws before and after pretreatment and the change of the methane yield in the anaerobic fermentation process. All data are used for evaluating the influence of pretreatment of acid soaking and disc grinding on anaerobic fermentation of crop straws.
Further, the solid-to-liquid ratio of the straw to the acid liquor is 1: 10.
Further, the soaking time in the acid liquor is 5 hours.
Further, the TS ratio of the pretreatment product to the inoculated sludge is 1: and 2, adjusting the TS to 6 percent and the pH value to be neutral.
Further, the medium temperature condition refers to 35 ± 1 ℃.
Further, the anaerobic fermentation time is 50 days, and the fermentation process is shaken for 1 to 5 times every day.
Further, the fermentation acid liquid is fermentation liquid obtained by fermenting corn straws and inoculated sludge for 7 days.
Further, the organic acid solution comprises any one of acetic acid, propionic acid and butyric acid, and the concentration is 0.5% -4%.
The TS (Total solids) referred to herein means the amount of solids remaining after evaporation of all of the water from the material, and VS (volatile solids) means the amount of volatile solids in the material.
The treatment method for improving the methane production performance of straw anaerobic fermentation is applied to the preparation of methane by using straw.
The components in the liquid are different in different stages in the anaerobic fermentation process, and the acidogenic stage is an essential path for the anaerobic fermentation conversion of biomass, wherein Volatile Fatty Acids (VFAs) are important intermediates for producing methane. These VFAs include various organic acids, (e.g., acetic acid, propionic acid and butyric acid). Therefore, the present invention uses fermented acid liquor instead of pure organic acid to impregnate biomass, and combines with disc mill to perform pretreatment of crop straw, thereby increasing the yield of methane.
In summary, the advantages and positive effects of the invention are:
(1) the methane yield is obviously improved. The accumulated methane yield of the straws pretreated by the fermentation acid liquor dipping and disc grinding is 198.41mL/g TS, and is 22.34% higher than that of untreated straws, wherein the pretreatment effect of the straws pretreated by the fermentation acid liquor dipping and disc grinding combined with 2% acetic acid is best, the total methane yield reaches 238.61mL/g TS, and the methane yield is 47.13% higher than that of the untreated straws.
(2) The physical structure of the pretreated straw is obviously changed. The surface of the straw is changed from smooth to uneven, so that the contact area of the enzyme is increased.
(3) The pretreatment time is significantly reduced. The whole process of the acid soaking and disc grinding combined pretreatment needs less than 7 hours, and compared with the traditional microbial pretreatment method, the time is obviously shortened.
(4) The raw material cost is low, and the impregnation liquid can be recycled. The steeping liquor is fermentation acid liquor which is an intermediate produced in the anaerobic fermentation process, the cost is not consumed, and the steeped liquor can be reused after being filtered.
(5) The invention has simple operation and the used equipment is a disc grinder. The disc mill has the advantages of energy conservation, quick processing and easy enlargement, and is more suitable for industrial production.
Drawings
FIG. 1 is an experimental flow chart of the pretreatment of crop straw by acid impregnation combined disc grinding according to the present invention;
FIG. 2 shows the cumulative methane yield over time after the straw has been pretreated with fermentation acid solution + disc mill, self-acid preparation solution + disc mill, and biogas slurry + disc mill;
FIG. 3 shows the cumulative methane yield over time of straw pretreated with different types of acid solution and disc milling according to an embodiment of the present invention;
FIG. 4 is a graph of cumulative methane production over time after pretreatment of straw with different concentrations of acetic acid + disc mill in accordance with an embodiment of the present invention;
FIG. 5 is an SEM image of straw provided by an embodiment of the invention: (a) untreated; (b) pretreating with fermented acid liquor and disc grinding; (c) 2% acetic acid + disc mill pretreatment.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to examples, and the equipment and reagents used in the examples and test examples are commercially available without specific reference. The specific embodiments described herein are merely illustrative of the invention and are not intended to be limiting.
Various modifications to the precise description of the invention will be readily apparent to those skilled in the art from the information contained herein without departing from the spirit and scope of the appended claims. It is to be understood that the scope of the invention is not limited to the procedures, properties, or components defined, as these embodiments, as well as others described, are intended to be merely illustrative of particular aspects of the invention. Indeed, various modifications of the embodiments of the invention which are obvious to those skilled in the art or related fields are intended to be covered by the scope of the appended claims.
For a better understanding of the invention, and not as a limitation on the scope thereof, all numbers expressing quantities, percentages, and other numerical values used in this application are to be understood as being modified in all instances by the term "about". Accordingly, unless expressly indicated otherwise, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained. At the very least, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. In the present invention, "about" means within 10%, preferably within 5% of a given value or range.
The invention discloses a treatment method for improving the methane production performance of straw anaerobic fermentation and application thereof, and a flow chart is shown in figure 1. In all of the following experiments, the crop straw used was air-dried corn straw obtained from the pilot plant of Shanghai university of China. The inoculation sludge used in the research is taken from a 35 ℃ continuous biogas fermentation tank which takes corn straws and pig manure as raw materials in Hebei 28095, China (the corn straws and the pig manure, fermentation materials and the inoculation sludge are uniformly mixed according to a ratio of 1:1 (based on VS), tap water is used for diluting until TS is 5%, the feed solid content per day is gradually increased after a reactor is sealed, the reactor is backfilled for discharging, when the feed content in the reactor reaches a preset volume and the solid content reaches 10%, the solid content and organic load of the added materials are fixed), and the fermentation tank stably operates for more than 1 year before the inoculation sludge is taken. Before use, the inoculated sludge was activated by adding glucose (load: 0.5g/L) for 14 days at a temperature of 35 ℃ (in a controlled temperature chamber). The measurement results of the basic indexes of the raw material and the inoculated sludge are shown in Table 1. The anaerobic fermentation experimental device is a blue-cap bottle with the volume of 500 mL.
TABLE 1 Properties of the feedstock and the inoculated sludge
The TS (Total solids) referred to herein means the amount of solids remaining after evaporation of all of the water from the material, and VS (volatile solids) means the amount of volatile solids in the material.
Example 1
(1) Raw material treatment
Cutting the air-dried corn straws into 3-4 cm, crushing by a crusher, sieving by a 20-mesh sieve, and drying and storing for later use.
The biogas slurry is obtained from the liquid part after 40 days of medium temperature anaerobic fermentation of corn straws and cow dung (the TS ratio of the corn straws to the cow dung is 1:1, the TS ratio of the fermented material to the inoculated sludge is 1:1, and the total TS of the fermentation tank is 8%).
The preparation method of the fermented acid liquid comprises the following steps: carrying out medium-temperature anaerobic fermentation on the corn straws and the inoculated sludge which pass through a 20-mesh screen in a mode that the TS ratio is 3:2, wherein the total TS of a fermentation tank is 16%, monitoring the concentration of VFAs once a day to select the peak time of acid production, and taking out the fermentation liquor on the 7 th day.
The measurement results of the basic indexes of the biogas slurry and the fermentation acid are shown in table 2. The biogas slurry and the fermented acid solution are centrifuged at 5000r/min for 5 minutes, and the supernatant is stored in a refrigerator at 4 ℃ before use. After determining the content of VFAs in the fermented acid liquid by high performance liquid chromatography, the self-prepared acid liquid with the same concentration is prepared by pure acetic acid, propionic acid and butyric acid.
TABLE 2 Properties of biogas slurry and fermentation broth
(2) Pretreatment method
Soaking the crushed corn straws in biogas slurry, fermentation acid liquor and self-prepared acid liquor for 5 hours at room temperature (25 ℃), wherein the solid-to-liquid ratio is 1: 10. The steeped corn stover is filtered and the resulting liquor is used as steeping liquor. The solid was disk-milled 8 times with a disk mill at 2800 r/min. Direct anaerobic fermentation of ground corn stover alone (untreated) was used as a control.
(3) Anaerobic fermentation
The solids and inoculated sludge obtained from the pretreatment were mixed in a ratio of 1: a proportion of 2 (based on TS) was charged to the fermenters and distilled water was added to each fermenter to adjust the total TS to 6%. The initial pH of each fermentor was adjusted to 7.0. + -. 0.1 with NaOH or HCl solution. Introducing nitrogen into the fermentation tank for 2 min to form anaerobic condition, sealing with rubber stopper, and performing mesophilic anaerobic fermentation in a thermostatic chamber at 35 + -1 deg.C for 50 days while shaking 1-5 times daily.
(4) Results
The yield of methane in the corn straw pretreated according to the method of the invention is obviously changed compared with that of the untreated straw. FIG. 2 shows that the methane yield obtained by biogas slurry and disc mill pretreatment is 133.31mL/g TS, which is reduced by 17.80% compared with untreated straws; the yield of methane obtained by fermentation acid liquid and disc grinding pretreatment is 198.41mL/g TS, which is increased by 22.34% compared with untreated straws; the yield of methane obtained by the acid blending liquid and the disc grinding pretreatment is 189.53mL/g TS, which is increased by 16.86% compared with the untreated straw. Optimum methane production table is displayed by fermentation acid liquor and disc grinding pretreatmentThis is because the fermentation acid contains NH in comparison to the self-souring acid4 +-N and a microorganism. The methane accumulated yield of biogas slurry and disc mill pretreatment is the lowest, even lower than that of untreated straws. This is because biogas slurry is a weakly alkaline solution and contains a large amount of secondary metabolites, which makes it hardly effective for the pretreatment of biomass but has a strong inhibitory effect on methanogenic microorganisms in the anaerobic fermentation process.
FIG. 5 shows that untreated corn stover is smooth and continuous with no air holes, hindering the enzyme from contacting the fermentable portions. After pretreatment of the corn stalk with fermentation acid liquor and disc grinding, the smooth surface of the corn stalk disappears, and the fiber is seriously damaged. It can be shown that during the pretreatment process, a portion of the components are removed and the outer surface of the straw is fiberized, increasing the surface area accessible to the enzyme, thereby enhancing the anaerobic fermentation potential of the corn straw.
Example 2
(1) Raw material treatment
Cutting the air-dried corn straws into 3-4 cm, crushing by a crusher, sieving by a 20-mesh sieve, and drying and storing for later use.
(2) Pretreatment method
The crushed corn stalks are immersed in 1 percent of sulfuric acid, 1 percent of phosphoric acid, 1 percent of acetic acid, 1 percent of propionic acid and 1 percent of butyric acid (volume ratio) for 5 hours at room temperature (25 ℃), and the solid-liquid ratio is 1: 10. Filtering the soaked corn straw, continuously using the obtained liquid as soaking liquid, and grinding the solid for 8 times by a disc grinder at the rotation speed of 2800 r/min.
(3) Anaerobic fermentation
The solids and inoculated sludge obtained from the pretreatment were mixed in a ratio of 1: a proportion of 2 (based on TS) was charged to the fermenters and distilled water was added to each fermenter to adjust the total TS to 6%. The initial pH of each fermentor was adjusted to 7.0. + -. 0.1 with NaOH solution. Introducing nitrogen into the fermentation tank for 2 min to form anaerobic condition, sealing with rubber stopper, and performing mesophilic anaerobic fermentation in a thermostatic chamber at 35 + -1 deg.C for 50 days while shaking 1-5 times daily.
(4) Results
The fermentation acid liquid is proved to be an ideal steeping liquid for corn stalk acid steeping and disc grinding pretreatment, and contains various organic acids. Therefore, it is necessary to study which acid in the fermentation acid liquor plays a dominant role in the pretreatment process of the acid impregnation combined with disc milling. As can be seen from table 2, the fermentation acid solution mainly contains acetic acid, butyric acid and propionic acid, of which acetic acid predominates. The yield of methane in the corn straw pretreated according to the method of the invention is obviously changed compared with that of the untreated straw. Figure 3 shows that all three organic acids effectively enhanced methane production and that butyric and acetic acids produced better gas than propionic acid. This result may be due to the strong swelling action of organic acids on lignocellulosic biomass, which, under the action of the disc mill, can promote the degradation of the biomass, thus completely destroying the recalcitrant structure of the lignocellulose. The pretreatment process can generate hair-shaped microfibrils on the surface of the biomass, increase the specific surface area of lignocellulose and improve the performance of anaerobic fermentation gas generation. Wherein, the yield of methane obtained by 1% acetic acid and disc mill pretreatment is 197.15mL/g TS, which is increased by 21.56% compared with untreated straws; the yield of methane obtained by 1% propionic acid and disc mill pretreatment is 181.82mL/g TS, which is increased by 12.11% compared with untreated straws; the yield of methane obtained by 1% butyric acid plus disc mill pretreatment is 207.75mL/g TS, which is 28.10% higher than that of untreated straws. In addition, for better comparison, sulfuric acid and phosphoric acid (as common mineral acids) were also used for acid impregnation in combination with disk mill pretreatment of corn stover. However, the results are not satisfactory because mineral acids (such as sulfuric acid) tend to produce fermentation-inhibiting byproducts such as furfural and 5-hydroxymethylfurfural during biomass pretreatment.
Example 3
(1) Raw material treatment
Cutting the air-dried corn straws into 3-4 cm, crushing by a crusher, sieving by a 20-mesh sieve, and drying and storing for later use.
(2) Pretreatment method
The crushed corn straws are soaked in 0.5 percent acetic acid, 2 percent acetic acid and 4 percent acetic acid for 5 hours at room temperature (25 ℃), and the solid-liquid ratio is 1: 10. Filtering the soaked corn straw, continuously using the obtained liquid as soaking liquid, and grinding the solid for 8 times by a disc grinder at the rotation speed of 2800 r/min. A control group with only a millstone and only 1% acetic acid impregnation was set.
(3) Anaerobic fermentation
The solids and inoculated sludge obtained from the pretreatment were mixed in a ratio of 1: a proportion of 2 (based on TS) was charged to the fermenters and distilled water was added to each fermenter to adjust the total TS to 6%. The initial pH of each fermentor was adjusted to 7.0. + -. 0.1 with NaOH solution. Introducing nitrogen into the fermentation tank for 2 min to form anaerobic condition, sealing with rubber stopper, and performing mesophilic anaerobic fermentation in a thermostatic chamber at 35 + -1 deg.C for 50 days while shaking 1-5 times daily.
(4) Results
In the anaerobic fermentation process, complex organic matters are decomposed into volatile fatty acids and alcohols by zymogenic bacteria, and the volatile fatty acids except acetic acid are converted into acetic acid by the bacteria to be utilized by methanogens, so that the method is of great importance to the research of the acetic acid.
Acetic acid is the major VFAs for the anaerobic fermentation conversion of corn stover to methane, and therefore, it is necessary to study the effect of acetic acid concentration on acid impregnation in combination with disc mill pretreatment. The yield of methane in the corn straw pretreated according to the method of the invention is obviously changed compared with that of the untreated straw. FIG. 4 shows that 0.5% acetic acid + disc mill pretreatment gave a methane yield of 190.24mL/g TS, a 17.30% increase over untreated straw; the yield of methane obtained by 2% acetic acid and disc mill pretreatment is 238.61mL/g TS, which is increased by 47.13% compared with untreated straws; the yield of methane obtained by the pretreatment of 4% acetic acid and disc mill is 200.54mL/g TS, which is increased by 23.65% compared with untreated straws; pretreatment of corn stover with either pan milling or 1% acetic acid impregnation alone can increase the production of biomethane (9.96% and 7.92% increase, respectively).
FIG. 5 shows that after 2% acetic acid + disc mill pretreatment, the corn stover surface was rough, with streaky dents, increasing the surface area accessible to the enzyme. This shows that the surface structure of lignocellulose can be well destroyed by the acid impregnation combined with disc grinding pretreatment, thereby increasing the methane yield.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.
Claims (9)
1. A treatment method for improving the methane production performance of straw anaerobic fermentation is characterized by comprising the following steps:
crushing straws, and soaking the straws in acid liquor; after filtering, grinding the solid by a disc mill to obtain a pretreated product; mixing the pretreated product with inoculated sludge, and performing anaerobic fermentation under the condition of medium temperature; the acid liquor comprises any one of straw, fermentation acid liquor in an acid production stage in the fermentation process of inoculated sludge and organic acid liquor; the inoculated sludge is a product obtained by performing continuous biogas fermentation on straws and animal wastes.
2. The treatment method for improving the methane production performance of straw anaerobic fermentation according to claim 1, characterized in that: the solid-liquid ratio of the straw to the acid liquor is 1: 10.
3. The treatment method for improving the methane production performance of straw anaerobic fermentation according to claim 1, characterized in that: soaking in acid solution for 5 hr.
4. The treatment method for improving the methane production performance of straw anaerobic fermentation according to claim 1, characterized in that: the TS ratio of the pretreatment product to the inoculated sludge is 1: 2, adjusting the total TS to 6 percent and the pH value to be neutral.
5. The treatment method for improving the methane production performance of straw anaerobic fermentation according to claim 1, characterized in that: the medium temperature condition refers to 35 +/-1 ℃.
6. The treatment method for improving the methane production performance of straw anaerobic fermentation according to claim 1, characterized in that: the anaerobic fermentation time is 50 days, and the fermentation process is shaken for 1 to 5 times every day.
7. The treatment method for improving the methane production performance of straw anaerobic fermentation according to claim 1, characterized in that: the fermentation acid liquid is fermentation liquid obtained by fermenting corn straws and inoculated sludge for 7 days.
8. The treatment method for improving the methane production performance of straw anaerobic fermentation according to claim 1, characterized in that: the organic acid solution comprises any one of acetic acid, propionic acid and butyric acid, and the concentration is 0.5-4%.
9. The use of a treatment process as claimed in any one of claims 1 to 8 to improve the methane production performance of anaerobic straw fermentation in the production of methane from straw.
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| CN114181977A (en) * | 2022-01-24 | 2022-03-15 | 中国农业大学 | Pretreatment method for enhancing methane production performance of straws by using anaerobic fermentation self-product and application |
| CN115029386A (en) * | 2022-05-07 | 2022-09-09 | 中国林业科学研究院林产化学工业研究所 | Method for improving yield and productivity of methane prepared by solid-state anaerobic fermentation of agricultural and forestry residues |
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