Straw hydrothermal pretreatment method and application

Abstract

The invention provides a straw hydrothermal pretreatment method and application. The straw hydrothermal pretreatment method comprises the following steps: cutting straws into pieces; the method comprises the following steps: dispersing straws in a mixed solution of hydrogen peroxide and organic acid, stirring and reacting for a period of time; the method comprises the following steps: putting straws and coffee residues into a hydrothermal reaction kettle, and adding a hydrothermal solution for hydrothermal reaction to obtain pretreated straws; according to the method, the hydrogen peroxide and the organic acid are adopted to pre-treat the straw, the oxidation effect of the hydrogen peroxide and the hydrolysis promotion effect of the organic acid are combined, cellulose, hemicellulose and lignin structures in the straw can be effectively destroyed, the hydrolysis efficiency is improved, and generation of by-products is reduced; then the pretreated straw and coffee residues are subjected to a hydrothermal reaction together, caffeine rich in the coffee residues can inhibit an excessive oxidation reaction in the hydrothermal reaction and can also protect reaction intermediate products, then generation of furan derivatives is reduced, and the lignin degradation rate and the methane yield are increased.

Description

Technical Field

[0001] The invention relates to the technical field of straw anaerobic fermentation, and in particular to a straw hydrothermal pretreatment method and application. Background Art

[0002] As an important component of biomass resources, straw comes from agricultural waste and has a huge output. It mainly covers the stems and leaves of crops such as wheat, rice, corn and potatoes. Its structural characteristics are complex and rich in crude cellulose, lignin and other components, showing a wide range of application potential. Historically, the treatment of straw has evolved from simple incineration to more sophisticated crushing and returning to the field, feed utilization and wood processing and conversion. At present, straw can not only be directly converted into bio-oil through technical means, but also used as a fermentation raw material to prepare feed. Reasonable disposal and comprehensive utilization of straw resources can not only significantly reduce carbon emissions, but also help to achieve high added value utilization of straw resources, which is of great significance to promoting the sustainable development of agriculture.

[0003] Anaerobic fermentation technology can convert various organic matter into renewable energy biogas. The main components of biogas are methane, carbon dioxide, hydrogen sulfide, etc. Biogas can be directly burned for use, or purified as a source of biogas, which is in line with the construction direction of ecological civilization in the new era and has great development potential. The main raw materials of anaerobic fermentation technology are sludge, kitchen waste, livestock and poultry manure and lignocellulosic biomass, among which lignocellulosic biomass is widely used in anaerobic digestion technology due to its wide source and renewable characteristics.

[0004] Hydrothermal pretreatment is an effective method for converting lignocellulosic biomass (such as straw, olive branches, bamboo and rapeseed) into biofuels or high-value products. It is usually carried out in a closed system under conditions below the critical point of water, which can effectively destroy its stubborn structure and accelerate the anaerobic digestion process. Studies have shown that hydrothermal pretreatment can change the chemical structure of lignin and promote the anaerobic digestion rate. However, furan derivatives produced during the hydrolysis of straw will polymerize at high temperatures, inhibit the activity of methanogenic bacteria, and affect the bioconversion rate.

[0005] Therefore, reducing the furan derivatives produced during hydrothermal pretreatment of straw, thereby increasing the lignin degradation rate and methane production, is an urgent problem to be solved in the field of anaerobic fermentation of straw. Summary of the invention

[0006] In view of the above-mentioned shortcomings of the prior art, the purpose of the present invention is to provide a straw hydrothermal pretreatment method to solve the problem that furan derivatives are easily produced during the hydrothermal pretreatment of straw in the prior art; in addition, the present invention will also provide a straw anaerobic fermentation method.

[0007] To achieve the above objectives and other related objectives, the present invention provides the following technical methods: A first aspect of the present invention provides a method for hydrothermal pretreatment of straw, comprising the following steps: S1, cutting the straw into pieces, washing and drying them for later use; S2, dispersing the straw obtained in step S1 in a mixed solution of hydrogen peroxide and an organic acid, stirring and reacting for a period of time, filtering out the straw after the reaction is completed, washing, drying and setting aside for later use; S3. Put the straw and coffee residue obtained in step S2 into a hydrothermal reaction kettle, and add hydrothermal liquid to carry out hydrothermal reaction. After the reaction is completed, separate the solid phase product and the liquid phase product, and dry the solid phase product to constant weight to obtain the pretreated straw.

[0008] The present invention uses hydrogen peroxide in conjunction with organic acid to pretreat the straw in step S2, combining the oxidizing effect of hydrogen peroxide and the hydrolysis promoting effect of organic acid, thereby effectively destroying the cellulose, hemicellulose and lignin structures in the straw, improving the hydrolysis efficiency and reducing the generation of by-products. In addition, the organic acid can not only adjust the pH value and enhance the activity of the hydrolysis reaction, but also be added as an additional carbon source to the subsequent hydrothermal reaction, effectively reducing the temperature and time required for the subsequent hydrothermal reaction, and improving the energy efficiency of the hydrothermal reaction.

[0009] In step S3, the present invention hydrothermally reacts the pretreated corn stalks with the coffee residues. In addition to hydrolyzing the stalks, the complex organic matter in the coffee residues can also be hydrolyzed to release fermentable sugars or organic matter, which are then converted into methane or other useful products through anaerobic fermentation. This process can not only effectively utilize the two wastes of straw and coffee residues, but also generate renewable energy, with the dual advantages of environmental friendliness and resource recycling. In addition, coffee residues are rich in caffeine, which is a natural antioxidant that can inhibit excessive oxidation reactions in hydrothermal reactions, reduce the generation of free radicals, and thus reduce the generation of furan derivatives, and antioxidants help protect the intermediates of the reaction to prevent them from undergoing further cracking reactions. At the same time, phenolic substances (such as caffeic acid, chlorogenic acid, etc.) in coffee residues can be further degraded under hydrothermal conditions to generate different organic acids (such as acetic acid, lactic acid, etc.) or other intermediates, which help decompose more organic matter and provide raw materials for subsequent anaerobic fermentation processes.

[0010] Therefore, the combined hydrolysis of straw and coffee residues will produce a synergistic effect, improve the conversion efficiency and reduce the generation of harmful by-products.

[0011] Preferably, in step S1, the straw is cut to 30 mm to 40 mm.

[0012] Preferably, in step S1, the chopped straw is washed with clean water and then placed in a 40° C. oven for drying.

[0013] Preferably, in step S2, the organic acid includes at least one of acetic acid, citric acid and oxalic acid.

[0014] Preferably, in step S2, the concentration of hydrogen peroxide is between 3% and 5% (v / v), and the concentration of the organic acid is between 1% and 3% (v / v).

[0015] Preferably, in step S2, the mass volume ratio of the straw to the mixed solution is 1:(4-10), more preferably 1:6.

[0016] Preferably, in step S2, the stirring reaction time is 40 min to 180 min; and the stirring rate is 100 rpm to 200 rpm.

[0017] Preferably, in step S3, the mass ratio of the straw to the coffee residue is (1-5): 1. More preferably, the mass ratio of the straw to the coffee residue is 3:1.

[0018] Preferably, in step S3, the solid-liquid ratio in the hydrothermal reaction is 1:(4-10), that is, the ratio of the total mass of the straw and the coffee residue to the mass of the hydrothermal liquid is 1:(4-10). More preferably, the solid-liquid ratio in the hydrothermal reaction is 1:6.

[0019] Preferably, in step S3, the hydrothermal fluid includes at least one of water and biogas slurry.

[0020] More preferably, the hydrothermal liquid is a combination of water and biogas slurry. By using water and biogas slurry together for hydrothermal reaction, the nitrogen source in the biogas slurry can be doped with straw, which not only helps to optimize the hydrolysis process and improve the hydrolysis rate of cellulose and hemicellulose, but also the organic acid and nitrogen source in the biogas slurry can adjust the reaction environment, promote the conversion of by-products, and effectively reduce the generation of furan derivatives.

[0021] More preferably, the mass ratio of the total mass of the straw and the coffee residue to the biogas slurry is 1:(1-2). A suitable biogas slurry ratio can prevent the accumulation of by-products while ensuring sufficient hydrolysis of the straw.

[0022] Preferably, in step S3, the temperature of the hydrothermal reaction is 120° C. to 200° C.; and the time of the hydrothermal reaction is 30 min to 180 min.

[0023] Preferably, in step S3, the solid phase product and the liquid phase product are separated by glass fiber filter paper.

[0024] Preferably, in step S3, the obtained solid phase product is dried in a constant temperature forced air drying oven at 40° C. to constant weight.

[0025] A second aspect of the present invention provides a method for anaerobic fermentation of straw, comprising the following steps: T1. Mix the pretreated straw and the active anaerobic sludge evenly, add water to dilute to a TS concentration of 4% to 10%, and obtain a mixed material to be fermented; T2. subjecting the mixed material to be fermented to anaerobic fermentation.

[0026] The decomposition effect of the lignocellulose structure of the pretreated straw is better, which can improve the gas production and fermentation efficiency of the anaerobic fermentation of the straw. In addition, the coffee residue contained in the pretreated straw will produce organic acids such as caffeic acid after hydrothermal treatment, which can adjust the pH of the mixed material to be fermented to a certain extent, so there is no need to add additional acidic medium for adjustment.

[0027] Preferably, in step T1, the TS concentration is 6% to 8%.

[0028] Preferably, in step T1, the concentration of the activated anaerobic sludge is 25% to 35%.

[0029] Preferably, in step T2, the temperature of anaerobic fermentation is 34° C. to 38° C., and the time of anaerobic fermentation is 20 to 30 days.

[0030] As described above, the straw hydrothermal pretreatment method and application of the present invention have the following beneficial effects: The present invention firstly adopts hydrogen peroxide in coordination with organic acid to pretreat the straw, combines the oxidizing effect of hydrogen peroxide with the hydrolysis promoting effect of organic acid, can effectively destroy the cellulose, hemicellulose and lignin structures in the straw, improves the hydrolysis efficiency and reduces the generation of by-products; then the pretreated straw is subjected to hydrothermal reaction with coffee residues, and the caffeine rich in the coffee residues can inhibit excessive oxidation reaction in the hydrothermal reaction, and can also protect the reaction intermediates, thereby reducing the generation of furan derivatives, which is helpful to improve the lignin degradation rate and methane yield.

[0031] The present invention combines hydrothermal pretreatment with anaerobic fermentation for the treatment of straw and coffee residues, which can not only effectively increase the available organic matter in the straw and reduce the generation of furan derivatives, thereby improving the methane conversion efficiency in anaerobic fermentation; the present invention provides an efficient and environmentally friendly treatment method for agricultural waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Process flow diagram for hydrothermal pretreatment of corn stover and anaerobic digestion to produce methane.

[0033] Figure 2These are SEM images of corn stalks, where (a) is untreated corn stalks, and (b) and (c) are corn stalks after hydrothermal pretreatment.

[0034] Figure 3 It is the cumulative methane production of Examples 1-3 and Comparative Examples 1-2. DETAILED DESCRIPTION

[0035] The following is a description of the implementation of the present invention by means of specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification.

[0036] Example 1 A pretreated straw preparation method comprises the following steps: S1. Cut the corn stalks into pieces of about 40 mm in length using a chopper, wash with water, and then put them into a 40° C. oven for drying before use; S2, prepare 6L of a mixed aqueous solution of hydrogen peroxide and citric acid, wherein the concentration of hydrogen peroxide is 4% and the concentration of citric acid is 2%, take 1kg of the straw obtained in step S1 and put it into the mixed solution, stir at 120rpm for 2 hours, filter out the straw after the reaction, wash and dry it for later use; S3. Take 300 g of the straw obtained in step S2 and 100 g of coffee residue, put them into a hydrothermal reactor, add 600 g of biogas slurry, and supplement water to a solid-liquid ratio of 1:6. Send the high-temperature steam generated by the boiler to the hydrothermal reactor to heat it. The heating temperature is 165° C. and the hydrothermal reaction time is 150 min. After the reaction is completed, the solid phase product and the liquid phase product are separated by glass fiber filter paper. After the solid phase product is dried to constant weight, the pretreated straw is obtained.

[0037] The pretreated straw obtained in this example was photographed with a SEM image. Figure 2 As shown in (b) and (c). The SEM image of the untreated corn stalk is shown in (a), and its surface is relatively smooth and flat, showing a compact structural organization. The dense structure hinders the degradation of the corn stalk by microorganisms, which will reduce the gas production of anaerobic fermentation. After the hydrothermal pretreatment of the present invention, the surface of the corn stalk becomes rough, with holes and cracks, and the wax layer structure on the surface of the corn stalk is damaged to a certain extent. At the same time, the dense reticular skeleton structure is destroyed, the specific surface area is increased, and the accessibility of microorganisms to the corn stalk is increased, indicating that the pretreatment can promote the decomposition of the lignocellulose structure in the corn stalk to a certain extent, thereby improving the gas production of the anaerobic fermentation of the corn stalk.

[0038] Example 2 A pretreated straw preparation method comprises the following steps: S1. Cut the corn stalks into pieces of about 40 mm in length using a chopper, wash with water, and then put them into a 40° C. oven for drying before use; S2, prepare 6L of a mixed aqueous solution of hydrogen peroxide and citric acid, wherein the concentration of hydrogen peroxide is 4% and the concentration of citric acid is 2%, take 1kg of the straw obtained in step S1 and put it into the mixed solution, stir at 120rpm for 2 hours, filter out the straw after the reaction, wash and dry it for later use; S3. Take 300 g of the straw obtained in step S2 and 100 g of coffee residue, put them into a hydrothermal reactor, add 600 g of biogas slurry, and supplement water to a solid-liquid ratio of 1:6. Send the high-temperature steam generated by the boiler to the hydrothermal reactor to heat it. The heating temperature is 135° C. and the hydrothermal reaction time is 120 min. After the reaction is completed, the solid phase product and the liquid phase product are separated by glass fiber filter paper. After the solid phase product is dried to constant weight, the pretreated straw is obtained.

[0039] Example 3 A pretreated straw preparation method comprises the following steps: S1. Cut the corn stalks into pieces of about 40 mm in length using a chopper, wash with water, and then put them into a 40° C. oven for drying before use; S2, prepare 6L of a mixed aqueous solution of hydrogen peroxide and citric acid, wherein the concentration of hydrogen peroxide is 4% and the concentration of citric acid is 2%, take 1kg of the straw obtained in step S1 and put it into the mixed solution, stir at 120rpm for 2 hours, filter out the straw after the reaction, wash and dry it for later use; S3. Take 300 g of the straw obtained in step S2 and 100 g of coffee residue, put them into a hydrothermal reactor, add 600 g of biogas slurry, and supplement water to a solid-liquid ratio of 1:6. Send the high-temperature steam generated by the boiler to the hydrothermal reactor to heat it. The heating temperature is 197° C. and the hydrothermal reaction time is 40 min. After the reaction is completed, the solid phase product and the liquid phase product are separated by glass fiber filter paper. After the solid phase product is dried to constant weight, the pretreated straw is obtained.

[0040] Comparative Example 1 A pretreated straw preparation method comprises the following steps: S1. Cut the corn stalks into pieces of about 40 mm in length using a chopper, wash with water, and then put them into a 40° C. oven for drying before use; S2. Take 400 g of the straw obtained in step S1 and put it into a hydrothermal reactor, add 600 g of biogas slurry, and supplement water to a solid-liquid ratio of 1:6. Send the high-temperature steam generated by the boiler to the hydrothermal reactor to heat it. The heating temperature is 165° C. and the hydrothermal reaction time is 150 min. After the reaction is completed, the solid phase product and the liquid phase product are separated by glass fiber filter paper. After the solid phase product is dried to constant weight, the pretreated straw is obtained.

[0041] Comparative Example 2 A pretreated straw preparation method comprises the following steps: S1. Cut the corn stalks into pieces of about 40 mm in length using a chopper, wash with water, and then put them into a 40° C. oven for drying before use; S2, prepare 6L of a mixed aqueous solution of hydrogen peroxide and citric acid, wherein the concentration of hydrogen peroxide is 4% and the concentration of citric acid is 2%, take 1kg of the straw obtained in step S1 and put it into the mixed solution, stir at 120rpm for 2 hours, filter out the straw after the reaction, wash and dry it for later use; S3. Take 400 g of the straw obtained in step S2 and put it into a hydrothermal reactor, add 600 g of biogas slurry, and supplement water to a solid-liquid ratio of 1:6. Send the high-temperature steam generated by the boiler to the hydrothermal reactor to heat it. The heating temperature is 165° C. and the hydrothermal reaction time is 150 min. After the reaction is completed, the solid phase product and the liquid phase product are separated by glass fiber filter paper. After the solid phase product is dried to constant weight, the pretreated straw is obtained.

[0042] Experimental data The pretreated straw prepared in Examples 1 to 3 and Comparative Examples 1 to 2 is subjected to anaerobic fermentation, comprising the following steps: T1, respectively taking 24g of the pretreated straw prepared in Examples 1-3 and Comparative Examples 1-2 and mixing them evenly with 120mL of active anaerobic sludge, and diluting them to 400mL with water to obtain a mixed material to be fermented; T2, the mixed material to be fermented was subjected to anaerobic fermentation at a fermentation temperature of 35°C, and the cumulative methane production within 30 days was recorded. The results are shown in Tables 1 and Figure 3 As shown. 90 The time required for anaerobic fermentation of straw to produce 90% of the total gas production.

[0043] Table 1. Anaerobic fermentation results of pretreated straw in Examples 1 to 3 and Comparative Examples 1 to 2

[0044] By comparing Example 1 with Comparative Examples 1 and 2, it can be seen that the present invention first pre-treats corn stalks with hydrogen peroxide and organic acids, and then hydrothermally treats the corn stalks with coffee residues before anaerobic fermentation, which can effectively improve the hydrolysis efficiency, reduce the generation of by-products, and optimize the composition of the product, thereby significantly improving the methane production and fermentation efficiency. In Comparative Example 1, the stalks are directly hydrothermally treated before anaerobic fermentation. Since there are many furan derivatives produced by the hydrolysis of the stalks in the anaerobic fermentation, the activity of methanogens is inhibited, so not only the methane production is small, but also the methane production efficiency is greatly reduced. In Comparative Example 2, the method of the present invention is first used to pre-treat the stalks with hydrogen peroxide and organic acids, which destroys the dense structure of the stalks, and promotes the hydrolysis of the stalks to a certain extent compared with Comparative Example 1, so that the methane production efficiency is greatly improved, but since the problem of furan derivatives has not been solved, the methane production is not significantly improved.

[0045] In summary, the present invention first uses hydrogen peroxide in conjunction with organic acid to pretreat the straw, combines the oxidation effect of hydrogen peroxide and the hydrolysis promotion effect of organic acid, can effectively destroy the cellulose, hemicellulose and lignin structure in the straw, improve the hydrolysis efficiency and reduce the generation of by-products; then the pretreated straw is subjected to a hydrothermal reaction with coffee residues, and the caffeine rich in the coffee residues can inhibit excessive oxidation reaction in the hydrothermal reaction, and can also protect the reaction intermediates, thereby reducing the generation of furan derivatives, and helping to improve the lignin degradation rate and methane production. Therefore, the present invention effectively overcomes the various shortcomings in the prior art and has a high industrial utilization value.

[0046] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A method for hydrothermal pretreatment of straw, characterized in that: The following steps are involved: S1, cutting the straw into pieces, washing and drying them for later use; S2, dispersing the straw obtained in step S1 in a mixed solution of hydrogen peroxide and an organic acid, stirring and reacting for a period of time, filtering out the straw after the reaction is completed, washing, drying and setting aside for later use; S3. Put the straw and coffee residue obtained in step S2 into a hydrothermal reaction kettle, and add hydrothermal liquid to carry out hydrothermal reaction. After the reaction is completed, separate the solid phase product and the liquid phase product, and dry the solid phase product to constant weight to obtain the pretreated straw.

2. The straw hydrothermal pretreatment method according to claim 1, characterized in that: In step S1, the straw is cut to 30 mm to 40 mm.

3. The straw hydrothermal pretreatment method according to claim 1, characterized in that: In step S2, the organic acid includes at least one of acetic acid, citric acid, and oxalic acid.

4. The straw hydrothermal pretreatment method according to claim 1, characterized in that: In step S3, the mass ratio of straw to coffee residue is (1-5):

1.

5. The straw hydrothermal pretreatment method according to claim 1, characterized in that: In step S3, the solid-liquid ratio in the hydrothermal reaction is 1:(4~10).

6. The straw hydrothermal pretreatment method according to claim 1, characterized in that: In step S3, the hydrothermal fluid includes at least one of water and biogas slurry.

7. The straw hydrothermal pretreatment method according to claim 6, characterized in that: The hydrothermal liquid is a combination of water and biogas slurry, and the mass ratio of the total mass of the straw and coffee residue to the biogas slurry is 1: (1-2).

8. The straw hydrothermal pretreatment method according to claim 1, characterized in that: In step S3, the temperature of the hydrothermal reaction is 120° C. to 200° C.; the time of the hydrothermal reaction is 30 min to 180 min.

9. A method for anaerobic fermentation of straw, characterized in that: Anaerobic fermentation is carried out using the pretreated straw as described in any one of claims 1 to 8.

10. The method for anaerobic fermentation of straw according to claim 9, characterized in that: The specific steps include: T1. Mix the pretreated straw and the active anaerobic sludge evenly, add water to dilute to a TS concentration of 4% to 10%, and obtain a mixed material to be fermented; T2. subjecting the mixed material to be fermented to anaerobic fermentation.

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