Modified biomass fuel and preparation method thereof

Abstract

The invention relates to the field of biomass fuel preparation, in particular to modified biomass fuel and a preparation method thereof. The biomass fuel provided by the invention is prepared by carrying out in-situ modification on a biomass base material through a specific composite additive; the composite additive is composed of small organic molecules containing phosphate groups and carboxylic acid groups and Ca / Mg / Fe alkaline earth metal salt, the small organic molecules and the Ca / Mg / Fe alkaline earth metal salt are subjected to an in-situ esterification reaction on the surface of biomass, a stable organic metal complex is generated, and a coating layer is formed. In the combustion process, the organic metal complex participates in ash conversion in a molecular state, potassium, sodium, silicon and other elements are inhibited from forming a low-melting-point molten phase through a solid-gas ash migration mechanism, and the technical problems that biomass briquette combustion is large in ash content and serious in slagging are solved from the source. The preparation process of the modified biomass fuel is simple and controllable, is suitable for industrial production, and can be widely applied to combustion equipment such as industrial boilers and civil stoves.

Description

Technical Field

[0001] This invention relates to the field of biomass fuel preparation, specifically to a modified biomass fuel and its preparation method. Background Technology

[0002] Biomass fuel, as an important component of renewable energy, boasts advantages such as low carbon emissions and abundant resources. However, minerals in natural biomass, such as potassium, sodium, and silicon, easily form ash during combustion. Among these, low-melting-point compounds like potassium silicate and sodium sodium silicate, formed by the reaction of potassium oxide and sodium oxide with silicon dioxide, can lead to slagging and ash accumulation on the furnace heating surfaces. In severe cases, this can cause pipe blockage, furnace corrosion, reduced combustion efficiency, and threats to equipment safety. Current technologies primarily address the slagging problem in biomass fuels through post-fuel molding intervention or passive treatment during the combustion stage. For example, low-ash raw materials can be screened, or inorganic ash removers such as kaolin, alumina, and limestone can be added to increase the ash melting point through physical dilution or chemical reaction. Alternatively, single-functional group additives can be used for modification. However, screening low-ash raw materials is limited by resources and has poor applicability. Adding ash removers will reduce the calorific value of the fuel, and the reaction is not targeted, resulting in unstable anti-slagging effect. Although adding single-functional group modifications can partially chelate metal ions, it cannot form a stable cross-linked structure. During combustion, metal ions are easily desorbed and will still combine with silicon to form a molten phase.

[0003] Therefore, this invention provides a modified biomass fuel and its preparation method, which not only improves the anti-slagging performance of biomass fuel, but also has high calorific value retention, strong complex layer stability, wide raw material adaptability and environmental friendliness, making it a biomass fuel with great research and application prospects. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention aims to develop a modified biomass fuel and its preparation method. The biomass fuel provided by this invention is obtained by in-situ modification of biomass substrate using a specific composite additive. The composite additive consists of small organic molecules containing phosphate and carboxylic acid groups and Ca / Mg / Fe alkaline earth metal salts. These react in-situ on the biomass surface to form a stable organometallic complex and a coating layer. This solves the technical problems of high ash content and severe slagging during biomass briquette combustion from the source. It can be widely used in industrial boilers, domestic stoves, and other combustion equipment.

[0005] This invention discloses a method for preparing modified biomass fuel, comprising the following steps: S1 substrate pretreatment: After crushing the biomass raw materials, pass them through an 80-100 mesh sieve, place them in a forced-air drying oven to dry, and after cooling, measure the biomass substrate with a moisture content of 9-11% for later use; Preparation of S2 composite complexing additive: Weigh out components A and B in molar ratio and add them to deionized water. Stir until completely dissolved to prepare a composite complexing additive solution. Then adjust the pH value to 3.5~4.5 before use. S3 In-situ esterification-complexation reaction: The pretreated biomass substrate from step S1 and the composite complex additive solution prepared in step S2 are added to the reactor and stirred at 80°C for 2 hours. During this period, the pH value is monitored every 20 minutes and maintained within the range of 3.5 to 4.5. The reactants are obtained after the reaction is completed. S4 Drying: Place the reactants obtained in step S3 into a drying oven, set the drying temperature of the drying oven to 100℃ and dry for 1 hour to obtain dried biomass; S5 Molding: The dried biomass obtained in step S4 is fed into a molding machine. The temperature of the molding machine is set to 160℃, the pressure to 25MPa, and the pressure is held for 60s to obtain modified biomass fuel.

[0006] Preferably, in step S1, the biomass raw material is one or two of pine sawdust, corn stalks, wheat stalks, and cotton stalks.

[0007] Preferably, in step S2, component A is a monoethyl phosphate-itaconic acid copolymer, sodium dihydrogen phosphate-maleic anhydride-acrylic acid terpolymer, or tributyl phosphate-fumaric acid copolymer.

[0008] Preferably, the molecular weight of the ethyl phosphate-itaconic acid copolymer is 600-900, and the molar ratio of phosphate groups to carboxylic acid groups is 2:1.

[0009] Preferably, the molecular weight of the sodium dihydrogen phosphate-maleic anhydride-acrylic acid terpolymer is 800-1200, and the molar ratio of phosphate groups to carboxylic acid groups is 1.5:1.

[0010] Preferably, the molecular weight of the tributyl phosphate-fumaric acid copolymer is 500-800, and the molar ratio of phosphate groups to carboxylic acid groups is 2.5:1.

[0011] Preferably, in step S2, component B is a complex salt of calcium chloride and magnesium sulfate, a complex salt of calcium acetate and ferric nitrate, or a complex salt of magnesium nitrate and calcium carbonate.

[0012] Preferably, the molar ratio of calcium chloride to magnesium sulfate in the calcium chloride-magnesium sulfate composite salt is 1:1; the molar ratio of calcium acetate to ferric nitrate in the calcium acetate-ferric nitrate composite salt is 2:1; and the molar ratio of magnesium nitrate to calcium carbonate in the magnesium nitrate-calcium carbonate composite salt is 1.5:1.

[0013] Preferably, in step S3, the solution used to adjust the pH value is a sodium hydroxide solution with a concentration of 0.5 mol / L or a hydrochloric acid solution with a concentration of 0.5 mol / L.

[0014] A modified biomass fuel, prepared by any of the above-mentioned methods for preparing modified biomass fuel.

[0015] Compared with the prior art, the beneficial effects of the present invention are: This invention provides a modified biomass fuel and its preparation method, which has the following characteristics: (1) The modified biomass fuel prepared by the preparation method provided by the present invention has a cross-linked organometallic complex layer, which can efficiently lock potassium and sodium ions, with a slagging index ≤0.55 and a furnace deposition rate ≤4.8%, which is more than 50% lower than the traditional technology, and greatly improves the safety and stability of furnace operation.

[0016] (2) The amount of the composite complex additive added in this invention is only 1~3%, without the introduction of high ash inert components, the high calorific value of the fuel is ≥18.0MJ / kg, and the retention rate is ≥98%, which effectively solves the problem of calorific value dilution caused by traditional ash removal agents.

[0017] (3) FT-IR and TGA tests show that the thermal decomposition rate of the complex layer is 10-14% at 300-900℃, and it can maintain structural stability within the combustion temperature range to ensure that metal ions do not desorb.

[0018] (4) Modified biomass fuel can be prepared using the preparation method provided by the present invention without screening low ash raw materials. It is applicable to common biomass such as pine sawdust, straw, and cotton stalks, and can be adapted to composite biomass base materials, thus broadening the sources of raw materials.

[0019] (5) The preparation method provided by the present invention does not require any new special equipment in the preparation process, the reaction conditions are mild, it is compatible with existing biomass briquetting production lines, and the industrial promotion cost is low.

[0020] (6) The modified biomass fuel prepared by the preparation method provided by the present invention has reduced ash emissions after combustion, and the combustion products of the complex layer are non-toxic inorganic compounds such as phosphates and carbonates, with no secondary pollution, which meets the requirements of green and low-carbon development. Detailed Implementation

[0021] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0022] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0023] Example 1: A method for preparing modified biomass fuel, comprising the following steps: S1 base material pretreatment: After crushing the pine wood chips, pass them through an 80-100 mesh sieve, place them in a forced-air drying oven and dry them at 110℃. Then, when the moisture content is measured to be 9%, stop drying and take them out for use.

[0024] Preparation of S2 composite complexing additive: Component A and component B are weighed at a molar ratio of 1:1.2 and added to deionized water. The mixture is stirred until completely dissolved to prepare a composite complexing additive solution. The pH value is then adjusted to 3.5-4.5 for later use. Component A is a phosphate monoethyl ester-itaconic acid copolymer with a molecular weight of 600-900 and a molar ratio of phosphate to carboxylic acid of 2:1. Component B is a composite salt composed of calcium chloride and magnesium sulfate at a molar ratio of 1:1.

[0025] S3 In-situ esterification-complexation reaction: The pretreated biomass substrate from step S1 and the composite complex additive solution obtained in step S2 are added to the reactor at a mass ratio of 1:0.03. The mixture is stirred at 80°C for 2 hours, during which the pH value is monitored every 20 minutes and maintained within the range of 3.5 to 4.5. The reactants are obtained after the reaction is completed. The solution used is a 0.5 mol / L sodium hydroxide solution or a 0.5 mol / L hydrochloric acid solution.

[0026] S4 Drying: Place the reactants obtained in step S3 into a drying oven, set the drying temperature of the drying oven to 100℃ and dry for 1 hour to obtain dried biomass.

[0027] S5 Molding: The dried biomass obtained in step S4 is fed into a molding machine. The temperature of the molding machine is set to 160℃, the pressure to 25MPa, and the pressure is held for 60s to obtain modified biomass fuel.

[0028] Example 2: In step S2, component A was replaced with a sodium dihydrogen phosphate-maleic anhydride-acrylic acid terpolymer with a phosphate group to carboxylic acid group molar ratio of 1.5:1 and a molecular weight of 800~1200. The remaining steps and conditions were the same as in Example 1.

[0029] Example 3: In step S2, component A was replaced with a tributyl phosphate-fumaric acid copolymer with a phosphate group to carboxylic acid group molar ratio of 2.5:1 and a molecular weight of 500-800. The remaining steps and conditions were the same as in Example 1.

[0030] Example 4: In step S2, component B was replaced with a composite salt composed of calcium acetate and ferric nitrate in a molar ratio of 2:1. All other steps and conditions were the same as in Example 1.

[0031] Example 5: In step S2, component B was replaced with a composite salt composed of magnesium nitrate and calcium carbonate in a molar ratio of 1.5:1. All other steps and conditions were the same as in Example 1.

[0032] The modified biomass fuels obtained in Examples 1-5 were subjected to performance testing, and the testing methods are as follows: (1) Ash content: determined according to GB / T28731-2012 "Method for determination of ash content in solid biomass fuels"; (2) Slagging Index (RSI): According to DL / T5240-2010 standard, the ash deformation temperature (DT), softening temperature (ST), and flow temperature (FT) are measured using an ash melting point tester. RSI = (ST-1000) / 250 (RSI≤0.6 is considered qualified for anti-slagging). (3) Higher heating value: determined by oxygen bomb calorimeter in accordance with GB / T30727-2014 "Method for Determination of Heating Value of Solid Biomass Fuel"; (4) Furnace deposition rate: Simulate the working conditions of a biomass power generation boiler (combustion temperature 850-920℃, excess air coefficient 1.2, combustion time 10h), and weigh the ratio of the mass of ash deposited in the furnace to the mass of total ash. (5) Stability of complex layer: The characteristic peaks of ester bond (1730-1740 cm-1) and complex bond (1060-1180 cm-1) were characterized by FT-IR, and the thermal decomposition rate of complex layer at 300-900℃ was analyzed by TGA.

[0033] The test results obtained using the above testing methods are shown in the table below:

[0034] As shown in the table above, the modified biomass fuel prepared in Example 1 exhibits the best performance and the most stable cross-linked organometallic complex layer structure. This is because the 2:1 ratio of phosphate groups to carboxylic acid groups achieves dual chelation of potassium and sodium ions. The copolymer chain length of 600-900 molecular weight is moderate, which can fully coat the biomass surface and form a dense cross-linked network, resulting in the highest efficiency in locking metal ions. Therefore, the slagging index and furnace deposition rate are the lowest. The 1:1 composite salt of calcium chloride and magnesium sulfate optimizes the pore structure of the complex layer through complementary ionic radii, reducing ash residue, thus resulting in the lowest ash content. The stable complex layer has a thermal decomposition rate of only 10.2% within the combustion temperature range (300-900℃), avoiding calorific value loss due to metal ion desorption, thus resulting in the highest heating value. Example 2: The reduced phosphate group ratio weakened the chelating ability for metal ions, resulting in a slightly higher slagging index and furnace deposition rate compared to Example 1. Example 3: The increased phosphate group ratio, however, reduced the molecular weight. The shorter copolymer chains led to insufficient density of the cross-linked composite layer, resulting in a higher thermal decomposition rate than Example 1 and slightly poorer overall performance. The composite salts of Examples 4 and 5 lacked Ca. 2+ With Mg 2+ The synergistic effect of Fe in ferric nitrate leads to a decrease in the stability of the complex layer. 3+ The ionic radius is too small, resulting in insufficient complexation bond strength; the CO3 in calcium carbonate 2- It readily reacts with organic acids to generate bubbles, damaging the integrity of the complex layer. Therefore, the ash content, slagging index, and furnace deposition rate of both embodiments are higher than those of Embodiment 1, while the high heating value is slightly lower.

[0035] Example 6: The molar ratio of component A to component B in step S2 was screened (1:0.6, 1:0.8, 1:1, 1:1.2, 1:1.4, 1:1.6), and the remaining steps and conditions were the same as in Example 1.

[0036]

[0037] As shown in the table above, when component B is insufficient, it cannot fully undergo esterification-complexation with all the phosphate and carboxylic acid groups in component A, resulting in an incomplete cross-linked organometallic complex layer. Some potassium and sodium ions are not effectively locked, leading to a higher slagging index and furnace deposition rate. At the same time, the unreacted organic functional groups will slightly reduce the fuel calorific value, and the insufficient density of the complex layer will increase the thermal decomposition rate. When component B is excessive, unreacted calcium chloride and magnesium sulfate will remain as inorganic impurities, leading to an increase in ash content. Excessive metal ions will destroy the cross-linked structure of the complex layer, resulting in a decrease in its density and an increase in the thermal decomposition rate, which in turn increases the slagging index and furnace deposition rate. At the same time, excessive inorganic components will dilute the fuel calorific value.

[0038] Example 7: The biomass raw materials in step S1 were screened (pine sawdust, corn stalks, wheat stalks, pine sawdust to corn stalks mass ratio 1:1, pine sawdust to wheat stalks mass ratio 1:1, wheat stalks to corn stalks mass ratio 1:1), and the remaining steps and conditions were the same as in Example 1.

[0039]

[0040] As can be seen from the data in the table above, whether it is a single common biomass such as pine sawdust, corn stalks, or wheat stalks, or a composite base of any two, it can be efficiently modified by the preparation method provided by this invention. There is no need to screen low-ash raw materials, which greatly broadens the source of raw materials for biomass fuel.

[0041] Example 8: The mass ratio of biomass base material to composite complex additive solution in step S3 was screened (1:0.01, 1:0.02, 1:0.03, 1:0.04, 1:0.05), and the remaining steps and conditions were the same as in Example 1.

[0042]

[0043] As shown in the table above, when the content of composite complexing additives is insufficient, it cannot cover all active sites on the surface of biomass, and can only form a discontinuous, poorly dense cross-linked organometallic complex layer. Some potassium, sodium, and other easily slagging metal ions are not effectively locked and will still form a low-melting-point molten phase with silicon during combustion, resulting in a high slagging index and furnace deposition rate. At the same time, the incomplete complex layer has weak thermal stability, and the thermal decomposition rate will also increase. Unreacted biomass active sites are prone to side reactions, leading to a decrease in fuel calorific value. When the content of composite complexing additives is excessive, the composite complexing additives exceeding the reaction requirements cannot fully participate in the esterification-complexation reaction and will remain in a free state on the biomass surface, leading to an increase in ash content. Excessive additives will also destroy the cross-linked structure of the complex layer, reduce its density, increase the thermal decomposition rate, and thus lead to a reversal in the slagging index and furnace deposition rate. At the same time, free inorganic metal salts will dilute the fuel calorific value.

[0044] Comparative Example 1: A method for preparing biomass fuel, comprising the following steps: S1 base material pretreatment: After crushing the pine wood chips, pass them through an 80-100 mesh sieve, place them in a forced-air drying oven and dry them at 110℃. Then, when the moisture content is measured to be 9%, stop drying and take them out for use.

[0045] S2 Molding: The dried biomass base material obtained in step S1 is fed into the molding machine. The temperature of the molding machine is set to 160℃, the pressure to 25MPa, and the pressure is held for 60s to obtain biomass fuel.

[0046] Comparative Example 2: A method for preparing modified biomass fuel, comprising the following steps: S1 base material pretreatment: After crushing the pine wood chips, pass them through an 80-100 mesh sieve, place them in a forced-air drying oven and dry them at 110℃. Then, when the moisture content is measured to be 9%, stop drying and take them out for use.

[0047] S2 Modification: The pretreated biomass substrate from step S1 and alumina were added to the reactor at a mass ratio of 1:0.03. The mixture was stirred at 80°C for 2 hours. During the reaction, the pH value was monitored every 20 minutes and maintained within the range of 3.5 to 4.5. The reactants were obtained after the reaction was completed. The solution used was either a 0.5 mol / L sodium hydroxide solution or a 0.5 mol / L hydrochloric acid solution.

[0048] S3 Drying: Place the reactants obtained in step S2 into a drying oven, set the drying temperature of the drying oven to 100℃ and dry for 1 hour to obtain dried biomass.

[0049] S4 Molding: The dried biomass obtained in step S3 is fed into a molding machine. The temperature of the molding machine is set to 160℃, the pressure to 25MPa, and the pressure is held for 60s to obtain modified biomass fuel.

[0050] Comparative Example 3: A method for preparing modified biomass fuel, comprising the following steps: S1 base material pretreatment: After crushing the pine wood chips, pass them through an 80-100 mesh sieve, place them in a forced-air drying oven and dry them at 110℃. Then, when the moisture content is measured to be 9%, stop drying and take them out for use.

[0051] S2 Modification: The pretreated biomass substrate from step S1 and kaolin were added to the reactor at a mass ratio of 1:0.03. The mixture was stirred at 80°C for 2 hours. During this period, the pH value was monitored every 20 minutes and maintained within the range of 3.5 to 4.5. The reactants were obtained after the reaction was completed. The solution used was either a 0.5 mol / L sodium hydroxide solution or a 0.5 mol / L hydrochloric acid solution.

[0052] S3 Drying: Place the reactants obtained in step S2 into a drying oven, set the drying temperature of the drying oven to 100℃ and dry for 1 hour to obtain dried biomass.

[0053] S4 Molding: The dried biomass obtained in step S3 is fed into a molding machine. The temperature of the molding machine is set to 160℃, the pressure to 25MPa, and the pressure is held for 60s to obtain modified biomass fuel.

[0054] Comparative Example 4: A method for preparing modified biomass fuel, comprising the following steps: S1 base material pretreatment: After crushing the pine wood chips, pass them through an 80-100 mesh sieve, place them in a forced-air drying oven and dry them at 110℃. Then, when the moisture content is measured to be 9%, stop drying and take them out for use.

[0055] S2 Modification: The pretreated biomass substrate from step S1 and component A (component A is a phosphate monoethyl ester-itaconic acid copolymer with a molecular weight of 600-900 and a molar ratio of phosphate to carboxylic acid of 2:1) are added to a reactor at a mass ratio of 1:0.03. The mixture is stirred at 80°C for 2 hours, and the pH value is monitored every 20 minutes to maintain it within the range of 3.5-4.5. The reactants are obtained after the reaction is completed. The solution used is a 0.5 mol / L sodium hydroxide solution or a 0.5 mol / L hydrochloric acid solution.

[0056] S3 Drying: Place the reactants obtained in step S2 into a drying oven, set the drying temperature of the drying oven to 100℃ and dry for 1 hour to obtain dried biomass.

[0057] S4 Molding: The dried biomass obtained in step S3 is fed into a molding machine. The temperature of the molding machine is set to 160℃, the pressure to 25MPa, and the pressure is held for 60s to obtain modified biomass fuel.

[0058] Comparative Example 5: A method for preparing modified biomass fuel, comprising the following steps: S1 base material pretreatment: After crushing the pine wood chips, pass them through an 80-100 mesh sieve, place them in a forced-air drying oven and dry them at 110℃. Then, when the moisture content is measured to be 9%, stop drying and take them out for use.

[0059] S2 Modification: The pretreated biomass substrate from step S1 and component B (component B is a composite salt composed of calcium chloride and magnesium sulfate in a molar ratio of 1:1) are added to the reactor at a mass ratio of 1:0.03. The mixture is stirred at 80°C for 2 hours, during which the pH value is monitored every 20 minutes and maintained within the range of 3.5 to 4.5. The reactants are obtained after the reaction is completed. The solution used is either a 0.5 mol / L sodium hydroxide solution or a 0.5 mol / L hydrochloric acid solution.

[0060] S3 Drying: Place the reactants obtained in step S2 into a drying oven, set the drying temperature of the drying oven to 100℃ and dry for 1 hour to obtain dried biomass.

[0061] S4 Molding: The dried biomass obtained in step S3 is fed into a molding machine. The temperature of the molding machine is set to 160℃, the pressure to 25MPa, and the pressure is held for 60s to obtain modified biomass fuel.

[0062] The biomass fuels prepared in Comparative Examples 1-5 were subjected to performance testing, and the test results are shown in the table below:

[0063] As shown in the table above, Comparative Example 1, without any modification treatment, only produced fuel through raw material pretreatment and molding. The data shows that its performance in all aspects is at the worst level. The ash content is 67.2% higher than that of Example 1, the slagging index is 3.5 times that of Example 1, and the furnace deposition rate is 5.6 times that of Example 1. Furthermore, it lacks a cross-linked organometallic complex layer, making it unable to lock in easily slagging metal ions such as potassium and sodium. During combustion, the metal ions and silicon form a large amount of low-melting-point molten phase, resulting in severe slagging and deposition.

[0064] Comparative Examples 2 and 3 used industrially common alumina and kaolin as ash-reducing agents. Although these could reduce the slagging index to some extent, they had significant drawbacks. The ash content increased significantly, by 112.1-125.9% compared to Example 1. This is because the inorganic ash-reducing agent itself is a high-ash inert component, and it can only intervene in ash melting through physical dilution, failing to reduce the total ash content. The higher heating value decreased significantly, by 10.9-16.1% compared to Example 1, and the calorific value retention rate was only 89.1-90.7%. The slagging index was still much higher than that of Example 1, indicating limited anti-slagging effect, and no complex layer was formed, making it impossible to lock in metal ions at the source.

[0065] Comparative Example 4 was modified with only component A (monoethyl phosphate-itaconic acid copolymer), and Comparative Example 5 was modified with only component B (calcium chloride and magnesium sulfate composite salt). Neither achieved the modification effect of Example 1. In Comparative Example 4 (component A only), the single functional group could only partially chelate metal ions and could not form a cross-linked complex layer. The stability of the complex structure was extremely poor, with a thermal decomposition rate as high as 36.2%. During combustion, metal ions were easily desorbed, resulting in a still high slagging index and furnace deposition rate. In Comparative Example 5 (component B only), without the synergistic effect of functional groups of small organic molecules, a complex layer could not be formed. Slagging could only be slightly improved through the physical adsorption of metal ions. The slagging index and furnace deposition rate were similar to those of Comparative Example 1, with limited performance improvement.

[0066] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing modified biomass fuel, characterized in that, Includes the following steps: S1 substrate pretreatment: After crushing the biomass raw materials, pass them through an 80-100 mesh sieve, place them in a forced-air drying oven to dry, and after cooling, measure the biomass substrate with a moisture content of 9-11% for later use; Preparation of S2 composite complexing additive: Weigh out components A and B in molar ratio and add them to deionized water. Stir until completely dissolved to prepare a composite complexing additive solution. Then adjust the pH value to 3.5~4.5 before use. S3 In-situ esterification-complexation reaction: The pretreated biomass substrate from step S1 and the composite complex additive solution prepared in step S2 are added to the reactor and stirred at 80°C for 2 hours. During this period, the pH value is monitored every 20 minutes and maintained within the range of 3.5 to 4.

5. The reactants are obtained after the reaction is completed. S4 Drying: Place the reactants obtained in step S3 into a drying oven, set the drying temperature of the drying oven to 100℃ and dry for 1 hour to obtain dried biomass; S5 Molding: The dried biomass obtained in step S4 is fed into a molding machine. The temperature of the molding machine is set to 160℃, the pressure to 25MPa, and the pressure is held for 60s to obtain modified biomass fuel.

2. The method for preparing modified biomass fuel according to claim 1, characterized in that, In step S1, the biomass raw material is one or two of pine sawdust, corn stalks, wheat stalks, and cotton stalks.

3. The method for preparing modified biomass fuel according to claim 1, characterized in that, In step S2, component A is a monoethyl phosphate-itaconic acid copolymer, sodium dihydrogen phosphate-maleic anhydride-acrylic acid terpolymer, or tributyl phosphate-fumaric acid copolymer.

4. The method for preparing modified biomass fuel according to claim 3, characterized in that, The molecular weight of the monoethyl phosphate-itaconic acid copolymer is 600-900, and the molar ratio of phosphate groups to carboxylic acid groups is 2:

1.

5. The method for preparing modified biomass fuel according to claim 3, characterized in that, The molecular weight of the sodium dihydrogen phosphate-maleic anhydride-acrylic acid terpolymer is 800~1200, and the molar ratio of phosphate groups to carboxylic acid groups is 1.5:

1.

6. The method for preparing a modified biomass fuel according to claim 3, characterized in that, The tributyl phosphate-fumaric acid copolymer has a molecular weight of 500-800 and a molar ratio of phosphate groups to carboxylic acid groups of 2.5:

1.

7. The method for preparing modified biomass fuel according to claim 1, characterized in that, In step S2, component B is a complex salt of calcium chloride and magnesium sulfate, a complex salt of calcium acetate and ferric nitrate, or a complex salt of magnesium nitrate and calcium carbonate.

8. The method for preparing modified biomass fuel according to claim 7, characterized in that, The molar ratio of calcium chloride to magnesium sulfate in the calcium chloride-magnesium sulfate composite salt is 1:1; the molar ratio of calcium acetate to ferric nitrate in the calcium acetate-ferric nitrate composite salt is 2:1; and the molar ratio of magnesium nitrate to calcium carbonate in the magnesium nitrate-calcium carbonate composite salt is 1.5:

1.

9. The method for preparing a modified biomass fuel according to claim 1, characterized in that, In step S3, the solution used to adjust the pH value is either a sodium hydroxide solution with a concentration of 0.5 mol / L or a hydrochloric acid solution with a concentration of 0.5 mol / L.

10. Modified biomass fuel prepared by any one of the modified biomass fuel preparation methods according to claims 1 to 9.

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