Method for preparing fermentable sugar from forest biomass

Abstract

The invention discloses a method for preparing fermentable sugar from forest biomass, which comprises the following steps: 1) cleaning, drying and crushing forest biomass to obtain forest biomass powder; 2) dispersing the forest biomass powder in an aldehyde acid aqueous solution for hydrothermal pretreatment, and then carrying out solid-liquid separation to obtain a solid containing glucan and lignin and a hydrolysate containing xylose; and 3) washing, drying and ball-milling the solid containing glucan and lignin, adding cellulase and a surfactant, carrying out enzymolysis, and carrying out solid-liquid separation to obtain a solid containing lignin and an enzymatic hydrolysate containing fermentable sugar. Directional removal of hemicellulose in the forest biomass and efficient enzymolysis conversion of cellulose into fermentable sugar are achieved through a chemical-physical synergistic mechanism, and the method has the advantages of being good in raw material adaptability, easy and convenient to operate, single in reagent, environmentally friendly and the like and has wide industrial application prospects in the field of biomass refining.

Description

Technical Field

[0001] This invention relates to the field of biomass utilization technology, specifically to a method for preparing fermentable sugars using forest biomass. Background Technology

[0002] Lignocellulosic biomass (e.g., agricultural residues, forestry residues, etc.) is the most abundant renewable organic resource on Earth. It can be converted into high-value products (e.g., cellulose and hemicellulose can be used to prepare biofuels and platform compounds; lignin can be used to prepare aromatic chemicals), showing great application potential. However, due to the dense anti-degradation barrier formed by carbohydrates and lignin through ether and ester bonds in lignocellulosic biomass, the separation and conversion of components are severely hindered. This results in existing lignocellulosic biomass utilization methods generally suffering from problems such as cumbersome operation, difficulty in component separation, low conversion efficiency, high energy consumption, and environmental unfriendliness, making it difficult to meet practical application needs.

[0003] Therefore, it is of great significance to develop a method for the high-value utilization of lignocellulose biomass that has advantages such as good raw material adaptability, simple operation, single reagent, and environmental friendliness. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing fermentable sugars using forest biomass.

[0005] The technical solution adopted in this invention is: A method for preparing fermentable sugars using forest biomass includes the following steps: 1) The forest biomass is washed, dried and crushed to obtain forest biomass powder; 2) The forest biomass powder is dispersed in an aqueous aldehyde acid solution for hydrothermal pretreatment, followed by solid-liquid separation to obtain a solid containing dextran and lignin, as well as a hydrolysate containing xylose; 3) The solid containing dextran and lignin is washed with water, dried and ball-milled, then cellulase and surfactant are added for enzymatic hydrolysis, followed by solid-liquid separation to obtain a solid containing lignin and an enzymatic hydrolysate containing fermentable sugars.

[0006] Preferably, the forest biomass in step 1) is at least one of eucalyptus, poplar, acacia, wheat straw, cotton stalks, hemp stalks, and corn cobs.

[0007] Preferably, the particle size of the forest biomass powder in step 1) is 150μm to 830μm.

[0008] Preferably, in step 2), the ratio of forest biomass powder to aldehyde acid aqueous solution is 1g:10mL to 20mL.

[0009] Preferably, the aqueous aldehyde solution in step 2) is at least one of glyoxylic acid aqueous solution and propionic acid aqueous solution.

[0010] Preferably, the concentration of the aldehyde acid aqueous solution in step 2) is 0.1 mol / L to 1.0 mol / L.

[0011] Preferably, the hydrothermal pretreatment in step 2) is carried out at a temperature of 120℃ to 170℃ for a treatment time of 10 min to 60 min.

[0012] Preferably, the solid-liquid separation method in step 2) is vacuum filtration.

[0013] Preferably, the ball milling in step 3) is carried out at a ball mill speed of 400 rpm to 500 rpm, and the ball milling time is 5 min to 30 min.

[0014] Preferably, the amount of cellulase used in step 3) is 10 FPU / g to 30 FPU / g.

[0015] Preferably, the cellulase in step 3) comprises endoglucanase, exoglucanase, β-glucosidase, and xylanase.

[0016] Preferably, the amount of surfactant used in step 3) is 5 mg / g to 20 mg / g.

[0017] Preferably, the surfactant in step 3) is at least one of Tween 80, rhamnolipin, and polyethylene glycol (PEG).

[0018] Preferably, the number-average molecular weight of the polyethylene glycol is 4000 g / mol to 8000 g / mol.

[0019] Preferably, the enzymatic hydrolysis in step 3) is carried out at a temperature of 45℃ to 55℃ for a time of 3h to 72h.

[0020] Preferably, the enzymatic hydrolysis in step 3) is carried out in a shaker at a speed of 150 rpm to 250 rpm.

[0021] Preferably, the solid-liquid separation method in step 3) is vacuum filtration.

[0022] The principle of this invention: This invention utilizes aldehyde acid (containing bifunctional groups of carboxyl and aldehyde groups) to achieve efficient removal of xylan, while using aldehyde groups to stabilize the β-O-4 bond structure of lignin. In response to the shielding effect of lignin on cellulase, ball milling is used to increase the specific surface area of ​​the fiber, and surfactants are used to reduce the hydrophobicity of lignin. A three-level synergistic system of "chemical pretreatment-physical activation-interface regulation" is constructed, which improves the accessibility of cellulase and ultimately enables the efficient and high-value utilization of forest biomass resources.

[0023] The beneficial effects of this invention are: this invention achieves the targeted removal of hemicellulose and the efficient enzymatic hydrolysis of cellulose into fermentable sugars in forest biomass through a chemical-physical synergistic mechanism. This method has advantages such as good raw material adaptability, simple operation, single reagent, and environmental friendliness, and has broad prospects for industrial application in the field of biomass refining.

[0024] Specifically: 1) The aldehyde acid used in the hydrothermal pretreatment stage of this invention has dual functionality. It can not only efficiently and selectively remove xylan components, but also stabilize the β-O-4 bond structure of lignin through aldehyde groups, thereby improving the potential utilization value of multiple components. 2) The aldehyde acid used in the hydrothermal pretreatment stage of this invention is a simple organic acid, which is inexpensive and milder than inorganic acids, and has less corrosiveness to production equipment. 3) The ball mill used in this invention has the advantages of low energy consumption and high residue fragmentation efficiency, which can significantly increase the specific surface area of ​​the fiber, thereby improving the saccharification efficiency and making it highly feasible for industrial application. Attached Figure Description

[0025] Figure 1 The graph shows the test results of xylan content, lignin content and dextran content of the solid products from the hydrothermal pretreatment in step 3) of Examples 1, Comparative Examples 1 and 2.

[0026] Figure 2 The graph shows the test results of xylan removal rate, lignin removal rate and dextran retention rate of the solid products from the hydrothermal pretreatment in step 3) of Examples 1, Comparative Examples 1 and 2.

[0027] Figure 3 The graph shows the glucose yield test results corresponding to different enzymatic hydrolysis times in Examples 1-2 and Comparative Examples 1-4. Detailed Implementation

[0028] The present invention will be further explained and described below with reference to specific embodiments.

[0029] Example 1: A method for preparing fermentable sugars using forest biomass, comprising the following steps: 1) After washing the eucalyptus powder with water and air drying, it is then mechanically ground and sieved through a 40-mesh sieve and a 60-mesh sieve to obtain forest biomass powder (particle size 150μm~830μm). 2) Add 2.0g of forest biomass powder to the reaction vessel, then add 20mL of 0.43mol / L glyoxylic acid aqueous solution, seal the reaction vessel and keep it at 150℃ for 30min. After naturally cooling to room temperature, filter to obtain a solid containing dextran and lignin and a hydrolysate containing xylose. 3) The solid containing dextran and lignin was washed with deionized water until neutral, then freeze-dried to obtain a hydrothermally pretreated solid product. This product was then transferred to a ball mill and milled at 500 rpm for 20 minutes. 0.2 g of the milled hydrothermally pretreated solid product was added to an enzymatic hydrolysis device pre-filled with 10 mL of sodium citrate buffer solution (pH 4.8). 20 FPU / g of cellulase (Novozymes' Cellic) was then added. ® The enzyme was prepared by adding CTec2 and 5 mg / g Tween 80, then placing the enzyme hydrolysis device on a shaker and controlling the shaker speed at 180 rpm at 50°C for 72 h. After filtration, a solid containing lignin and an enzyme hydrolysate containing fermentable sugars were obtained.

[0030] Composition analysis: The xylose-containing hydrolysate from step 2) of this embodiment was diluted with ultrapure water and then detected by high performance liquid chromatography (HPLC). The results showed that the xylose concentration of the xylose-containing hydrolysate was 9.76 g / L and the concentration of the derivative inhibitor furfural was 0.46 g / L.

[0031] Example 2: (The dosage of Tween 80 was adjusted to 20 mg / g, and the rest was the same as in Example 1) A method for preparing fermentable sugars using forest biomass, comprising the following steps: 1) After washing the eucalyptus powder with water and air drying, it is then mechanically ground and sieved through a 40-mesh sieve and a 60-mesh sieve to obtain forest biomass powder (particle size 150μm~830μm). 2) Add 2.0g of forest biomass powder to the reaction vessel, then add 20mL of 0.43mol / L glyoxylic acid aqueous solution, seal the reaction vessel and keep it at 150℃ for 30min. After naturally cooling to room temperature, filter to obtain a solid containing dextran and lignin and a hydrolysate containing xylose. 3) The solid containing dextran and lignin was washed with deionized water until neutral, and then freeze-dried to obtain a hydrothermal pretreated solid product. The product was then transferred to a ball mill and milled at 500 rpm for 20 min. 0.2 g of the milled hydrothermal pretreated solid product was added to an enzymatic hydrolysis device pre-filled with 10 mL of sodium citrate buffer solution at pH 4.8. 20 FPU / g of cellulase (same as in Example 1) and 20 mg / g of Tween 80 were then added. The enzymatic hydrolysis device was placed on a shaker and the shaker was rotated at 180 rpm at 50 °C for 72 h. The mixture was then filtered to obtain a solid containing lignin and an enzymatic hydrolysate containing fermentable sugars.

[0032] Comparative Example 1: (Acetic acid was used, ball milling was not performed, and Tween 80 was not added; otherwise, it was the same as Example 1) A method for preparing fermentable sugars using forest biomass, comprising the following steps: 1) After washing the eucalyptus powder with water and air drying, it is then mechanically ground and sieved through a 40-mesh sieve and a 60-mesh sieve to obtain forest biomass powder (particle size 150μm~830μm). 2) Add 2.0g of forest biomass powder to the reactor, then add 20mL of acetic acid aqueous solution with a concentration of 0.43mol / L. After sealing the reactor, keep it at 150℃ for 30min, cool it naturally to room temperature, filter it to obtain a solid containing dextran and lignin and a hydrolysate containing xylose. 3) The solid containing dextran and lignin was washed with deionized water until neutral, and then freeze-dried to obtain a hydrothermal pretreated solid product. 0.2g of the hydrothermal pretreated solid product was added to an enzymatic hydrolysis device pre-filled with 10mL of sodium citrate buffer solution at pH=4.8, and then 20FPU / g of cellulase (same as in Example 1) was added. The enzymatic hydrolysis device was then placed on a shaker, and the shaker speed was controlled at 180rpm at a temperature of 50℃ for 72h. After filtration, a solid containing lignin and an enzymatic hydrolysate containing fermentable sugars were obtained.

[0033] Comparative Example 2: (Glycolic acid was used, but ball milling was not performed, and Tween 80 was not added; otherwise, it was the same as Example 1) A method for preparing fermentable sugars using forest biomass, comprising the following steps: 1) After washing the eucalyptus powder with water and air drying, it is then mechanically ground and sieved through a 40-mesh sieve and a 60-mesh sieve to obtain forest biomass powder (particle size 150μm~830μm). 2) Add 2.0g of forest biomass powder to the reaction vessel, then add 20mL of 0.43mol / L glycolic acid aqueous solution, seal the reaction vessel and keep it at 150℃ for 30min. After naturally cooling to room temperature, filter to obtain a solid containing dextran and lignin and a hydrolysate containing xylose. 3) The solid containing dextran and lignin was washed with deionized water until neutral, and then freeze-dried to obtain a hydrothermal pretreated solid product. 0.2g of the hydrothermal pretreated solid product was added to an enzymatic hydrolysis device pre-filled with 10mL of sodium citrate buffer solution at pH=4.8, and then 20FPU / g of cellulase (same as in Example 1) was added. The enzymatic hydrolysis device was then placed on a shaker, and the shaker speed was controlled at 180rpm at a temperature of 50℃ for 72h. After filtration, a solid containing lignin and an enzymatic hydrolysate containing fermentable sugars were obtained.

[0034] Comparative Example 3: (No ball milling was performed, and Tween 80 was not added; otherwise, it was the same as Example 1) A method for preparing fermentable sugars using forest biomass, comprising the following steps: 1) After washing the eucalyptus powder with water and air drying, it is then mechanically ground and sieved through a 40-mesh sieve and a 60-mesh sieve to obtain forest biomass powder (particle size 150μm~830μm). 2) Add 2.0g of forest biomass powder to the reaction vessel, then add 20mL of 0.43mol / L glyoxylic acid aqueous solution, seal the reaction vessel and keep it at 150℃ for 30min. After naturally cooling to room temperature, filter to obtain a solid containing dextran and lignin and a hydrolysate containing xylose. 3) The solid containing dextran and lignin was washed with deionized water until neutral, and then freeze-dried to obtain a hydrothermal pretreated solid product. 0.2g of the hydrothermal pretreated solid product was added to an enzymatic hydrolysis device pre-filled with 10mL of sodium citrate buffer solution at pH=4.8, and then 20FPU / g of cellulase (same as in Example 1) was added. The enzymatic hydrolysis device was then placed on a shaker, and the shaker speed was controlled at 180rpm at a temperature of 50℃ for 72h. After filtration, a solid containing lignin and an enzymatic hydrolysate containing fermentable sugars were obtained.

[0035] Comparative Example 4: (Tween 80 was not added; otherwise, it was the same as Example 1) A method for preparing fermentable sugars using forest biomass, comprising the following steps: 1) After washing the eucalyptus powder with water and air drying, it is then mechanically ground and sieved through a 40-mesh sieve and a 60-mesh sieve to obtain forest biomass powder (particle size 150μm~830μm). 2) Add 2.0g of forest biomass powder to the reaction vessel, then add 20mL of 0.43mol / L glyoxylic acid aqueous solution, seal the reaction vessel and keep it at 150℃ for 30min. After naturally cooling to room temperature, filter to obtain a solid containing dextran and lignin and a hydrolysate containing xylose. 3) The solid containing dextran and lignin was washed with deionized water until neutral, and then freeze-dried to obtain a hydrothermal pretreated solid product. The product was then transferred to a ball mill and milled at 500 rpm for 20 min. 0.2 g of the milled hydrothermal pretreated solid product was added to an enzymatic hydrolysis device pre-filled with 10 mL of sodium citrate buffer solution at pH 4.8, and 20 FPU / g of cellulase (same as in Example 1) was added. The enzymatic hydrolysis device was then placed on a shaker and the shaker was rotated at 180 rpm at 50 °C for 72 h. The product was then filtered to obtain a solid containing lignin and an enzymatic hydrolysate containing fermentable sugars.

[0036] Effect test: 1) The xylan, lignin, and dextran contents of the hydrothermal pretreatment solid products from Step 3) of Examples 1, 1, and 2 were determined using the NREL standard method. The xylan removal rate, lignin removal rate, and dextran retention rate were calculated. The test results for the xylan, lignin, and dextran contents of the hydrothermal pretreatment solid products are as follows: Figure 1 (The "eucalyptus" in the figure represents the raw material "eucalyptus powder"), as shown, the test results of xylan removal rate, lignin removal rate, and dextran retention rate of the hydrothermal pretreatment solid product are as follows. Figure 2 As shown.

[0037] Depend on Figure 1 It can be known that: a) The glucan, xylan, and lignin contents of eucalyptus powder were 44.75%, 15.58%, and 28.13%, respectively, while the glucan, xylan, and lignin contents of the hydrothermal pretreated solid product (Example 1) obtained after glyoxylic acid pretreatment were 42.85%, 2.15%, and 23.66%, respectively. This indicates that glyoxylic acid has an excellent selective removal effect on xylan in eucalyptus powder. b) The glucan, xylan, and lignin contents of the hydrothermal pretreated solid product obtained after acetic acid pretreatment (Comparative Example 1) were 44.36%, 7.36%, and 24.24%, respectively. The glucan, xylan, and lignin contents of the hydrothermal pretreated solid product obtained after glycolic acid pretreatment (Comparative Example 2) were 43.66%, 4.04%, and 23.94%, respectively. This indicates that glyoxylic acid has a significantly better selective removal effect on xylan in eucalyptus powder than acetic acid and glycolic acid.

[0038] Depend on Figure 2 It can be known that: a) The xylan removal rate of the hydrothermal pretreated solid product (Example 1) obtained by glyoxylic acid pretreatment was as high as 86.20%, the lignin removal rate was only 15.89%, and the dextran retention rate was as high as 95.75%, indicating that glyoxylic acid has an excellent selective removal effect on xylan in eucalyptus powder. b) The xylan removal rate and lignin removal rate of the hydrothermal pretreated solid product obtained after acetic acid pretreatment (Comparative Example 1) were 52.76% and 13.83%, respectively, and the dextran retention rate was 99.13%. The xylan removal rate and lignin removal rate of the hydrothermal pretreated solid product obtained after glycolic acid pretreatment (Comparative Example 2) were 74.07% and 14.90%, respectively, and the dextran retention rate was 97.56%. This indicates that glyoxylic acid has a significantly better selective removal effect on xylan in eucalyptus powder than acetic acid and glycolic acid.

[0039] 2) The enzymatic hydrolysates containing fermentable sugars from Examples 1-2 and Comparative Examples 1-4 were subjected to HPLC analysis, and the glucose yield was calculated based on the results. The glucose yield test results corresponding to different hydrolysis times (3h, 6h, 12h, 36h, 48h, 60h, and 72h) are shown below. Figure 3 As shown.

[0040] Depend on Figure 3 It can be known that: a) The glucose yields corresponding to 72h of enzymatic hydrolysis in Comparative Examples 1 and 2 were 33.84% and 41.32%, respectively, which were much lower than 53.04% in Comparative Example 3. This indicates that the removal rate of xylan and the enzymatic hydrolysis efficiency of glucan in lignocellulose are somewhat correlated, and at the same time, it shows the advantages of glyoxylic acid hydrothermal pretreatment. b) The glucose yields corresponding to 72 hours of enzymatic hydrolysis in Comparative Examples 3 and 4 were 53.04% and 65.52%, respectively, which were much lower than 75.70% in Example 1 and 81.47% in Example 2 (which met the production requirements of industrial sugar fermentation), indicating that ball milling and surfactants can effectively improve the yield of fermentable sugars.

[0041] Example 3: (Using propionic acid, otherwise the same as Example 1) A method for preparing fermentable sugars using forest biomass, comprising the following steps: 1) After washing the eucalyptus powder with water and air drying, it is then mechanically ground and sieved through a 40-mesh sieve and a 60-mesh sieve to obtain forest biomass powder (particle size 150μm~830μm). 2) Add 2.0g of forest biomass powder to the reactor, then add 20mL of 0.43mol / L propionic acid aqueous solution, seal the reactor and keep it at 150℃ for 30min. After cooling to room temperature, filter to obtain a solid containing dextran and lignin and a hydrolysate containing xylose. 3) The solid containing dextran and lignin was washed with deionized water until neutral, and then freeze-dried to obtain a hydrothermal pretreated solid product. The product was then transferred to a ball mill and milled at 500 rpm for 20 min. 0.2 g of the milled hydrothermal pretreated solid product was added to an enzymatic hydrolysis device pre-filled with 10 mL of sodium citrate buffer solution at pH 4.8. 20 FPU / g of cellulase (same as in Example 1) and 5 mg / g of Tween 80 were then added. The enzymatic hydrolysis device was placed on a shaker and the shaker was rotated at 180 rpm at 50 °C for 72 h. The product was then filtered to obtain a solid containing lignin and an enzymatic hydrolysate containing fermentable sugars.

[0042] According to the test (test method is the same as in Example 1), the content of dextran, xylan and lignin in the solid product of hydrothermal pretreatment in step 3) of this example is 43.06%, 2.56% and 25.03% respectively. The xylan removal rate and lignin removal rate are 83.57% and 11.02% respectively, and the dextran retention rate is 96.22%. This shows that propoxyl has excellent selective removal effect on xylan in eucalyptus powder, but the effect is slightly weaker than that of glyoxyl.

[0043] Example 4: (Using wheat straw powder, otherwise the same as Example 1) A method for preparing fermentable sugars using forest biomass, comprising the following steps: 1) The wheat straw powder (containing 40.18% glucan, 27.58% xylan and 20.96% lignin) was washed with water, air-dried, mechanically ground, and then sieved through a 40-mesh sieve and a 60-mesh sieve to obtain forest biomass powder (particle size 150μm~830μm). 2) Add 2.0g of forest biomass powder to the reaction vessel, then add 20mL of 0.43mol / L glyoxylic acid aqueous solution, seal the reaction vessel and keep it at 150℃ for 30min. After naturally cooling to room temperature, filter to obtain a solid containing dextran and lignin and a hydrolysate containing xylose. 3) The solid containing dextran and lignin was washed with deionized water until neutral, and then freeze-dried to obtain a hydrothermal pretreated solid product. The product was then transferred to a ball mill and milled at 500 rpm for 20 min. 0.2 g of the milled hydrothermal pretreated solid product was added to an enzymatic hydrolysis device pre-filled with 10 mL of sodium citrate buffer solution at pH 4.8. 20 FPU / g of cellulase (same as in Example 1) and 5 mg / g of Tween 80 were then added. The enzymatic hydrolysis device was placed on a shaker and the shaker was rotated at 180 rpm at 50 °C for 72 h. The product was then filtered to obtain a solid containing lignin and an enzymatic hydrolysate containing fermentable sugars.

[0044] According to the test (test method is the same as in Example 1), the content of dextran, xylan and lignin in the solid product of hydrothermal pretreatment in step 3) of this example is 38.65%, 2.24% and 18.44% respectively. The xylan removal rate and lignin removal rate are 91.88% and 12.02% respectively, and the dextran retention rate is 96.19%, indicating that glyoxylic acid has an excellent selective removal effect on xylan in wheat straw powder.

[0045] Example 5: (Using corn cob powder, otherwise the same as Example 1) A method for preparing fermentable sugars using forest biomass, comprising the following steps: 1) The corn cob powder (containing 42.50% glucan, 31.65% xylan and 15.69% lignin) was washed with water, air-dried, mechanically ground, and then sieved through a 40-mesh sieve and a 60-mesh sieve to obtain forest biomass powder (particle size 150μm~830μm). 2) Add 2.0g of forest biomass powder to the reaction vessel, then add 20mL of 0.43mol / L glyoxylic acid aqueous solution, seal the reaction vessel and keep it at 150℃ for 30min. After naturally cooling to room temperature, filter to obtain a solid containing dextran and lignin and a hydrolysate containing xylose. 3) The solid containing dextran and lignin was washed with deionized water until neutral, and then freeze-dried to obtain a hydrothermal pretreated solid product. The product was then transferred to a ball mill and milled at 500 rpm for 20 min. 0.2 g of the milled hydrothermal pretreated solid product was added to an enzymatic hydrolysis device pre-filled with 10 mL of sodium citrate buffer solution at pH 4.8. 20 FPU / g of cellulase (same as in Example 1) and 5 mg / g of Tween 80 were then added. The enzymatic hydrolysis device was placed on a shaker and the shaker was rotated at 180 rpm at 50°C for 72 h. The product was then filtered to obtain a solid containing lignin and an enzymatic hydrolysate containing fermentable sugars.

[0046] According to the test (test method is the same as in Example 1), the content of dextran, xylan and lignin in the solid product of hydrothermal pretreatment in step 3) of this example is 41.03%, 2.22% and 13.59% respectively. The xylan removal rate and lignin removal rate are 92.98% and 13.38% respectively, and the dextran retention rate is 96.54%, indicating that glyoxylic acid has an excellent selective removal effect on xylan in corn cob powder.

[0047] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing fermentable sugars using forest biomass, characterized in that, Includes the following steps: 1) The forest biomass is washed, dried and crushed to obtain forest biomass powder; 2) The forest biomass powder is dispersed in an aqueous aldehyde acid solution for hydrothermal pretreatment, followed by solid-liquid separation to obtain a solid containing dextran and lignin, as well as a hydrolysate containing xylose; 3) The solid containing dextran and lignin is washed with water, dried and ball-milled, then cellulase and surfactant are added for enzymatic hydrolysis, followed by solid-liquid separation to obtain a solid containing lignin and an enzymatic hydrolysate containing fermentable sugars.

2. The method for preparing fermentable sugars using forest biomass according to claim 1, characterized in that: Step 1) The forest biomass is at least one of eucalyptus, poplar, acacia, wheat straw, cotton stalks, hemp stalks, and corn cobs.

3. The method for preparing fermentable sugars from forest biomass according to claim 1 or 2, characterized in that: Step 1) The particle size of the forest biomass powder is 150μm to 830μm.

4. The method for preparing fermentable sugars using forest biomass according to claim 1, characterized in that: Step 2) The ratio of forest biomass powder to aldehyde acid aqueous solution is 1g:10mL~20mL.

5. The method for preparing fermentable sugars from forest biomass according to claim 1 or 4, characterized in that: Step 2) The aldehyde acid aqueous solution is at least one of glyoxylic acid aqueous solution and propionic acid aqueous solution.

6. The method for preparing fermentable sugars from forest biomass according to claim 1 or 4, characterized in that: Step 2) The concentration of the aldehyde acid aqueous solution is 0.1 mol / L to 1.0 mol / L.

7. The method for preparing fermentable sugars from forest biomass according to claim 1 or 4, characterized in that: Step 2) The hydrothermal pretreatment is carried out at a temperature of 120℃~170℃ for a time of 10min~60min.

8. The method for preparing fermentable sugars from forest biomass according to claim 1, characterized in that: Step 3) The amount of cellulase used is 10 FPU / g to 30 FPU / g; Step 3) The cellulase consists of endoglucanase, exoglucanase, β-glucosidase and xylanase.

9. The method for preparing fermentable sugars from forest biomass according to claim 1 or 8, characterized in that: The amount of surfactant used in step 3) is 5 mg / g to 20 mg / g; the surfactant in step 3) is at least one of Tween 80, rhamnolipid, and polyethylene glycol.

10. The method for preparing fermentable sugars from forest biomass according to claim 1 or 8, characterized in that: Step 3) The enzymatic hydrolysis is carried out at a temperature of 45℃~55℃ for 3h~72h.

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