A method for preparing biochar from fly ash under alkaline treatment

Abstract

The application provides a method for preparing biochar from fly ash under alkali treatment, which comprises the following steps: adding fly ash and corn straw into an alkaline solution, and fully reacting to obtain a mixture of fly ash and corn straw after alkali treatment; and calcining the mixture to obtain biochar. In the process of converting biomass into biochar, the aromatic ring structure is greatly increased, a dense structure is formed on the surface of the biochar, the influence of external conditions on the biochar itself is reduced and prevented, thereby inhibiting the oxidation, decomposition and release of carbon molecules, and the thermal stability and chemical stability of the biochar are improved.

Description

Technical Field

[0001] This invention belongs to the field of coal-based solid waste resource utilization and biochar technology, specifically relating to a method for preparing highly stable biochar from fly ash under alkali treatment. Background Technology

[0002] Biochar is a highly aromatic, carbon-rich solid material produced by carbonizing biomass under protective gas (anaerobic) conditions. Its important role in global climate change and environmental protection is increasingly evident. Biochar is a good heavy metal stabilizer; its stability mainly depends on its physicochemical properties, such as surface functional groups, porous structure, specific surface area, pore structure, and mineral composition. Previous studies have investigated the stability of biochar using optical, thermal, and chemical methods. In recent years, modification with mineral materials has improved the functional properties of biochar, such as enhancing its adsorption capacity for heavy metal ions. However, further improvements are needed in the thermal and oxidative stability of biochar. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides a method for preparing biochar from fly ash under alkali treatment. The method involves adding fly ash and corn stalks to an alkaline solution and allowing them to react fully to obtain an alkali-treated mixture of fly ash and corn stalks. This mixture is then calcined to obtain biochar. During the alkali treatment and calcination process, the molecular structure of the biomass gradually transforms into a stable aromatic ring structure, forming a protective layer on the biochar surface. This inhibits the oxidation, decomposition, and release of carbon molecules, thereby enhancing the thermal and chemical stability of the biochar.

[0004] The purpose of this invention is to provide a method for preparing biochar from fly ash under alkali treatment. The method involves adding fly ash and corn stalks to an alkaline solution, allowing them to react fully to obtain an alkaline-treated mixture of fly ash and corn stalks, which is then calcined to obtain biochar.

[0005] The mass ratio of fly ash to corn stalks is 1:(1-9).

[0006] The alkaline solution is an inorganic alkaline solution.

[0007] The calcination is carried out under a protective atmosphere at 200-800℃ for 1.5-3.5 hours.

[0008] The present invention also aims to provide a biochar prepared by the method for preparing biochar from fly ash under alkali treatment.

[0009] The present invention has the following beneficial effects:

[0010] (1) This invention utilizes fly ash to prepare biochar, which significantly improves the thermal and chemical stability of biochar.

[0011] (2) In this invention, fly ash and corn stalks are treated with alkali at the same time and then roasted. Under the effect of increased preparation temperature and fly ash doping, the macromolecules such as hemicellulose, cellulose, and polysaccharides in biomass are reduced and gradually transformed into stable aromatic ring structures, which greatly promotes the environmental resistance and stability of biochar.

[0012] (3) Fly ash and corn stalks form a dense structure on the surface of biochar during alkali treatment and roasting, which reduces and prevents changes in external conditions from affecting the biochar itself, thereby inhibiting the oxidation, decomposition and release of carbon molecules and improving the thermal and chemical stability of biochar. Attached Figure Description

[0013] Figure 1 This invention AB is shown. 300-1:2 AB 500-1:2 AB 700-1:2 AB 500-1:5 BC 500 Scanning electron microscope image, Figure 1 (a) is BC 500 SEM image, Figure 1 (b) is AB 500-1:2 SEM image, Figure 1 (c) is AB 500-1:5 SEM image, Figure 1 (d) is AB 300-1:2 SEM image, Figure 1 (e) is AB 700-1:2 SEM image;

[0014] Figure 2 This invention BC is shown. 500 AB 500-1:2 AB 500-1:5 AB 300-1:2 AB 700-1:2 XPS full spectrum, Figure 2 (a) is BC 500 , Figure 2 (b) is AB 500-1:2 , Figure 2 (c) is AB 500-1:5 , Figure 2 (d) is AB 300-1:2 , Figure 2 (e) is AB 700-1:2 ;

[0015] Figure 3 This invention BC is shown. 500 C1S and Si2p XPS spectra;

[0016] Figure 4 This invention AB is shown. 500-1:2XPS spectra of C1S, Si2p, and Al2p;

[0017] Figure 5 This invention AB is shown. 500-1:5 XPS spectra of C1S, Si2p, and Al2p;

[0018] Figure 6 This invention AB is shown. 300-1:2 XPS spectra of C1S, Si2p, and Al2p;

[0019] Figure 7 This invention AB is shown. 700-1:2 XPS spectra of C1S, Si2p, and Al2p;

[0020] Figure 8 a shows the invention BC 500 AB 500-1:2 AB 500-1:5 AB 300-1:2 AB 700-1:2 Van-Krevelen plot; Figure 8 b shows the BC of the present invention 500 AB 500-1:2 AB 500-1:5 AB 300-1:2 AB 700-1:2 Thermogravimetric analysis diagram; Figure 8 c shows the BC of the present invention 500 AB 500-1:2 AB 500-1:5 AB 300-1:2 AB 700-1:2 Thermogravimetric test chart for micro-businesses; Figure 8 d shows the BC of the present invention 500 AB 500-1:2 AB 500-1:5 AB 300-1:2 AB 700-1:2 A comparison chart of carbon loss rates. Detailed Implementation

[0021] The present invention will now be described in detail through specific embodiments, and the features and advantages of the present invention will become clearer and more explicit with these descriptions.

[0022] This invention provides a method for preparing biochar from fly ash under alkali treatment. The method involves adding fly ash and corn stalks to an alkaline solution, allowing them to react fully to obtain an alkali-treated mixture of fly ash and corn stalks, which is then calcined to obtain biochar.

[0023] The fly ash is a product of the aerobic combustion of coal, and its components include SiO2, Al2O3, Fe2O3, CaO, MgO, TiO2, K2O, etc.

[0024] With my country placing increasing emphasis on ecological and environmental protection, the resource-based treatment of solid waste from industrial production processes has become particularly important. Fly ash is a major solid waste discharged from coal-fired power plants, with a huge output and low utilization rate, primarily used in the construction industry. Mineral additives are one of the effective means to improve the stability of biochar. Different mineral types and their forms will cause different pyrolysis processes in biochar, leading to different physicochemical properties and changes in biochar stability, thus affecting its environmental application. As a silicate-based waste, fly ash contains various inorganic mineral components such as SiO2 and Al2O3. Through chemical bonding and physical coating, biochar oxidation can be reduced, enhancing the stability of biochar.

[0025] In this invention, fly ash is added to treat biomass raw materials. By reacting with components such as SiO2 and Al2O3 in the fly ash, the physicochemical properties and microstructure of biochar are improved, achieving the goal of regulating biochar stability using fly ash. The stability of biochar is assessed using the carbon elemental rating (R), which is determined by the H / C and O / C ratios, mass loss, and carbon loss rate. The method in this invention optimizes the preparation process of modified corn stalk biochar materials, improves their stability, and simultaneously achieves resource utilization of fly ash and agricultural solid waste, effectively reducing environmental risks and enhancing agricultural and environmental benefits.

[0026] The fly ash has a particle size of 12-30 mesh, preferably 14-25 mesh, and more preferably 16-20 mesh. The corn stalk has a particle size of 12-30 mesh, preferably 14-25 mesh, and more preferably 16-20 mesh. Fly ash and corn stalk within the above particle size range are easily dispersed in alkaline solution, which is beneficial for sufficient contact and improves the thermal and oxidative stability of biochar.

[0027] The mass ratio of fly ash to corn stalks is 1:(1-9), preferably 1:(1.5-7), and more preferably 1:(2-5). Within the above range, appropriately increasing the amount of fly ash results in a higher silicon content in the biochar material, a larger surface mineral coverage area, and a lower thermal decomposition mass loss rate. The addition of fly ash can protect the biochar from oxidation and improve its chemical stability. A lower H / C molar ratio also improves the aromaticity of the biochar.

[0028] The alkaline solution is an inorganic alkaline solution, preferably an aqueous solution of an alkali metal hydroxide, more preferably an aqueous solution of sodium hydroxide and / or an aqueous solution of potassium hydroxide, such as an aqueous solution of sodium hydroxide. The concentration of the alkaline solution is 2-10 wt%, preferably 3-8 wt%, more preferably 4-6 wt%.

[0029] When the fly ash and corn stalks are added to the alkaline solution, the solid-liquid mass ratio is 1:(7-16), preferably 1:(8-14), and more preferably 1:(9-12).

[0030] Fly ash and corn stalks are added to an alkaline solution. The temperature for a complete reaction is 20-40℃, preferably 22-35℃, more preferably 24-30℃, and the reaction time is 6-10 hours, preferably 7-9 hours. Preferably, after adding the fly ash and corn stalks to the alkaline solution, the mixture is shaken at a frequency of 120-180 r / min. After thorough mixing, the mixture is allowed to stand, filtered, and dried to obtain an alkaline-treated mixture of fly ash and corn stalks.

[0031] The calcination is carried out under a protective atmosphere at 200-800℃ for 1.5-3.5 hours. Preferably, it is carried out at 300-700℃ for 2-3 hours. Appropriately increasing the calcination temperature gradually reduces the H / C molar ratio of the biochar, which can promote the transformation of aliphatic compounds in the biochar into aromatic compounds, thereby improving the thermal and chemical stability of the biochar.

[0032] This invention also provides biochar prepared using the aforementioned method for preparing biochar from fly ash under alkali treatment. Its specific surface area can reach 19 m². 2 / g or higher, even reaching 60m 2 The carbon content is above 0.55 g / g. At 900℃, its thermal decomposition mass loss rate is less than 22 wt%; the carbon loss after oxidation with a 5 wt% H₂O₂ aqueous solution is less than 30 wt%; the H / C molar ratio is less than 0.55. Its carbon elemental rating (R) is less than 1, and can even reach below 0.9. Carbon elemental rating (R) = H / C + mass loss rate + carbon loss rate.

[0033] Example

[0034] Example 1

[0035] Fly ash from a coal-fired power plant in Shanxi Province and corn stalks from farmland were collected. The fly ash was dried at 105℃ and pulverized to a particle size of less than 18 mesh. The main oxide components and their mass percentages of the fly ash are shown in Table 1. The corn stalks were dried at 85℃ and similarly pulverized to a particle size of less than 18 mesh.

[0036] Table 1

[0037]

[0038] Fly ash and corn stalks were mixed at a mass ratio of 1:2, and then a 5 mol / L NaOH aqueous solution was added to achieve a solid-liquid mass ratio of 1:10. The mixture was then placed in a shaker at 150 r / min for 8 hours at 25°C to ensure thorough mixing of the fly ash and corn stalk biomass. After static filtration, the mixture was dried in a forced-air drying oven. The mixture was then ground in a mortar and pestle, and the solid mixture was placed in a nickel box and calcined in a tube furnace at 300, 500, and 700°C in a N2 atmosphere for 2 hours, respectively. After cooling to room temperature, the mixture was washed with deionized water at least three times until the pH of the deionized water no longer changed, yielding three types of alkali-impregnated fly ash-modified corn stalk biochar, namely A, B, and C. 300-1:2 AB 500-1:2 AB 700-1:2 Based on corn stalks, AB 300-1:2 AB 500-1:2 AB 700-1:2 The yields were 92.14%, 83.25%, and 78.05%, respectively.

[0039] Example 2

[0040] Biochar AB was prepared according to Example 1. 500-1:2 AB was prepared by the method 500-1:5 The only difference is that fly ash and corn stalks are mixed at a mass ratio of 1:5. Based on corn stalks, AB... 500-1:5 The yield was 79.59%.

[0041] Comparative Example

[0042] Comparative Example 1

[0043] Biochar AB was prepared according to Example 1. 500-1:2 Methods for preparing biochar BC 500 The only difference is that fly ash is not added. Based on corn stalks, BC 500 The yield was 29.1%.

[0044] Comparative Example 2

[0045] The fly ash from Example 1 was dried at 105°C and pulverized to a particle size of less than 18 mesh. The corn stalks were dried at 85°C and pulverized to a particle size of less than 18 mesh.

[0046] Mix 10g of fly ash and 15g of NaOH evenly, place them in a nickel crucible, and heat-treat them in a muffle furnace at 600℃ for half an hour. After cooling, remove them from the muffle furnace to obtain alkali-fused fly ash, which is then crushed to a particle size of less than 18 mesh.

[0047] Alkali-fused fly ash and corn stalks were mixed at a mass ratio of 1:2 and added to deionized water to make a solid-liquid mass ratio of 1:10. The mixture was then reacted in a shaker at 25°C and 150 rpm for 8 hours. After standing and filtering, the mixture was dried in a forced-air drying oven and crushed to obtain a pretreated mixture.

[0048] Take the pretreated mixture, calcine it at 500°C for 2 hours under a nitrogen atmosphere, and then remove the sample after cooling to room temperature. Wash the obtained product with deionized water until the pH value of the deionized water no longer changes, and then dry it to obtain alkali-fused AB. 500-1:2 .

[0049] Comparative Example 3

[0050] Alkali-molten AB was prepared according to the method in Comparative Example 2. 500-1:5 The only difference is that the ratio of alkali-fused fly ash to corn stalks is 1:5 by mass.

[0051] Experimental Example

[0052] Experimental Example 1

[0053] Test AB 300-1:2 AB 500-1:2 AB 700-1:2 AB 500-1:5 BC 500 The scanning electron microscope image is as follows: Figure 1 As shown, Figure 1 (a) is BC 500 SEM image, Figure 1 (b) is AB 500-1:2 SEM image, Figure 1 (c) is AB 500-1:5 SEM image, Figure 1 (d) is AB 300-1:2 SEM image, Figure 1 (e) is AB 700-1:2 SEM image.

[0054] Experiment Example 2

[0055] Test AB 300-1:2 AB 500-1:2 AB 700-1:2 AB 500-1:5 BC 500 Ash content, volatile matter and fixed carbon content.

[0056] A sample of biochar, dried at 85℃, was placed in a nickel crucible (total mass: mg). The nickel crucible was calcined and then stored in a desiccator before use. Under air atmosphere, the sample was heated to 815℃ in a muffle furnace at a heating rate of 10℃ / min and calcined for 1 hour. After cooling to room temperature in the furnace, the sample was weighed (m2). Ash content (%) = (m - m2) / m1 × 100%.

[0057] A sample of biochar, dried at 85℃, was placed in a nickel crucible (total mass: mg). The nickel crucible was calcined and then stored in a desiccator before use. Under air atmosphere, the sample was heated to 915℃ in a muffle furnace at a heating rate of 10℃ / min for 7 min. After treatment, the sample was removed and cooled to room temperature in a desiccator, and weighed as m2. Volatile matter content (%) = (m - m2) / m1 × 100%.

[0058] Fixed carbon content (%) = 100% - ash content (%) - volatile matter content (%).

[0059] Table 2

[0060]

[0061] BC 500 AB 500-1:2 AB 500-1:5 The ash content increased with increasing fly ash content, while the volatile matter decreased slightly. This is because fly ash catalyzes the biochar pyrolysis process, promoting the conversion of organic matter into a more stable form. The ash content increased with increasing roasting temperature, indicating that raising the roasting temperature is beneficial for promoting the conversion of biological organic matter into biochar.

[0062] Experimental Example 3

[0063] AB was measured using the MicroActive 5.01 instrument manufactured by Micro Instruments, Inc., USA. 300-1:2 AB 500-1:2 AB 700-1:2 AB 500-1:5 BC 500 Specific surface area (BET surface area), average pore size (nm), and pore volume (cm³) 3 / g), where the adsorbate is N2.

[0064] The pH value of the sample was determined according to GB / T 12496.7-1999 "Test Methods for Determination of pH Value of Wood-based Activated Carbon". 1g of sample was placed in a 100mL Erlenmeyer flask and a suspension was prepared by mixing fly ash and distilled water at a mass ratio of 1:20. The mixture was slowly boiled in a water bath for 5 minutes. Water was added to evaporate the mixture and it was filtered. The first 5mL of the filtrate was discarded, and the remaining filtrate was cooled to room temperature before the pH value was determined.

[0065] The content of C and H elements in the material was determined using an elemental analyzer (Elementar UNICUBE), and the molar ratio of H to C, H / C, was calculated.

[0066] Table 3

[0067]

[0068] Compared to BC 500 AB 300-1:2 AB 500-1:2 AB 700-1:2 AB 500-1:5 The specific surface area and pore volume increase, and the structure of biochar gradually changes from layered to a structure with obvious depressions. Under high temperature conditions, the increased pore size of biochar leads to the collapse of biochar pores and carbon layers. Figure 1 Consistent.

[0069] Compared to AB 500-1:2 AB 500-1:5 alkali fusion AB 500-1:2 AB and alkali fusion 500-1:5 The yield is significantly reduced, the pH value decreases, and the alkalinity weakens. In the preparation method of this invention, the alkaline oxides in fly ash promote the increase of biochar alkalinity. The higher the preparation temperature, the higher the alkalinity of the pyrolytic biochar. High temperature conditions promote the formation of alkaline functional groups.

[0070] AB 500-1:2 and AB 500-1:5 It has a lower H / C ratio, higher aromaticity, and stronger stability.

[0071] From an economic efficiency perspective, the alkaline impregnation treatment of fly ash and corn stalks in this invention only requires one pyrolysis, which reduces energy consumption during the preparation process. Alkaline melting pretreatment of fly ash requires two carbonization reactions to obtain biochar, which consumes more energy and has a lower yield.

[0072] Experiment Example 4

[0073] Measurement of BC 500 AB 500-1:2 AB 500-1:5 AB 300-1:2 AB 700-1:2The full XPS (X-ray photoelectron spectroscopy) spectrum, as shown in the figure below. Figure 2 (a)~ Figure 2 As shown in (e), peak positions of C, Al, and Si were fitted, specifically as follows: Figures 3-7 As shown, BC 500 for Figure 3 AB 500-1:2 for Figure 4 AB 500-1:5 for Figure 5 AB 300-1:2 for Figure 6 AB 700-1:2 for Figure 7 .

[0074] According to XPS analysis, compared with BC 500 In comparison, AB 300-1:2 AB 500-1:2 AB 700-1:2 AB 500-1:5 The O content increased, the C content decreased, and elements such as Al, Mg, and Ca appeared. Figure 2 (a)~ Figure 2 (e) Pyrolysis temperature has a significant effect on the functional groups of biochar. As the temperature increases, the O content in biochar decreases, the C content increases, and the CC / C=C / CH ratio gradually increases. Figure 4 , Figure 6 , Figure 7 This indicates that increasing temperature is beneficial for improving biochar stability. The proportions of oxygen-containing functional groups such as C-OH / COC and C=O / O=CC first increased and then decreased. It is speculated that below 500℃, the addition of fly ash plays a dominant role, and the abundant oxide components in fly ash promote the increase of oxygen-containing functional groups in biochar. Above 500℃, the pyrolysis temperature plays a dominant role, with an increase in aromatic functional groups and a decrease in aliphatic functional groups such as hydroxyl and carboxyl groups, resulting in more stable biochar.

[0075] Comparison of A and B 500-1:2 and AB 500-1:5 AB 500-1:2 The O content increases and the C content decreases. Figure 2 (a)~ Figure 2 (e) This is because fly ash contains various oxides but has very little or no carbon content. Analysis Figure 5 It was found that fly ash regulation can promote AB 500-1:5 The formation of C-OH and COC occurs because oxides such as SiO2 and Al2O3 in fly ash react with carbon in biochar under high temperature conditions, leading to an increase in oxygen-containing functional groups.

[0076] It is speculated that the formation of a Si-C coupling system in the biochar matrix affects the arrangement of C and functional groups, forming a dense structure on the surface of biochar. This reduces and prevents changes in external conditions from affecting the biochar itself, thereby inhibiting the oxidation, decomposition and release of carbon molecules and improving the thermal and chemical stability of biochar.

[0077] Increasing the preparation temperature and adding fly ash increases the proportion of aromatic carbon in biochar. It can be inferred that under the influence of increased preparation temperature and fly ash doping, the amount of macromolecules such as hemicellulose, cellulose, and polysaccharides in biomass decreases and gradually transforms into stable aromatic ring structures, which greatly promotes the environmental resistance and stability of biochar.

[0078] Experimental Example 5

[0079] TG-DSC, a simultaneous thermal analyzer manufactured by TA Instruments, USA, was used to determine AB. 300-1:2 AB 500-1:2 AB 700-1:2 AB 500-1:5 BC 500 The thermogravimetric curves of micro-businesses and their corresponding thermogravimetric curves are as follows: Figure 8 As shown in Table 4, the mass loss rate at 900℃ is the mass loss rate at 900℃.

[0080] AB 300-1:2 AB 500-1:2 AB 700-1:2 AB 500-1:5 BC 500 The chemical stability of biochar was analyzed by examining the changes in carbon loss rate of biochar under oxidation in 5wt% H2O2 solution. The specific carbon loss rate under H2O2 is shown in Table 4.

[0081] Semi-simulated aging experiments are considered an effective method for evaluating the oxidizing properties of biochar in soil and reflecting its stability. Chemical oxidation with hydrogen peroxide (H2O2) is an accelerated aging method, representing the stability of biochar in the natural environment. Chemical oxidation of biochar for 48 hours can simulate approximately 100 years of environmental aging under temperate conditions, demonstrating its carbon sequestration potential. 0.5 g of biochar was mixed with 35 mL of H2O2 (5 wt%) in a 50 mL test tube and stirred slowly for 1 min. The test tube was then covered and incubated in a 60℃ water bath for 48 h. After the heating process, all test tubes were placed in a drying oven and dried at 105℃ for 60 hours until the H2O2 evaporated and the biochar reached a constant weight. The carbon loss was expressed as the difference in total carbon before and after H2O2 oxidation, as shown below:

[0082] Carbon loss rate = (C1×M1 - C2×M2) / (C1×M1)×100%

[0083] Wherein, C_loss represents the carbon loss % after oxidation of biochar in a 5wt% H2O2 aqueous solution; C1 and C2 represent the carbon content of biochar before and after oxidation, respectively, in %; M1 and M2 represent the mass of biochar before and after oxidation, respectively, in g.

[0084] Carbon elemental rating (R) = H / C + mass loss rate + carbon loss rate. The higher the R value, the worse the stability of the biochar. Here, H / C is the molar ratio of H to C.

[0085] Table 4

[0086]

[0087] BC 500 Located in lignin or carboxyl-containing aliphatic positions, AB 300-1:2 AB 500-1:2 AB 700-1:2 AB 500-1:5 Located in the position of aromatic hydrocarbons ( Figure 8 a) This indicates that the addition of fly ash causes the transformation of aliphatic to aromatic compounds in biochar. From 300℃ to 700℃, the carbon loss rate of biochar significantly decreases. (AB) 700-1:2 The carbon loss rate is only 8.56% ( Figure 8 d). The greatest mass loss of biochar occurs when the temperature is raised to 900℃, reaching 75% of the original mass. Figure 8 b、 Figure 8 c). The higher the pyrolysis temperature, the higher the proportion of aromatic carbon in the biochar, and the better its thermal and chemical stability. AB 500-1:2 The carbon loss rate was 9.93%, slightly higher than AB. 700-1:2 ( Figure 8 d), but considering factors such as yield and energy consumption, biochar prepared at 500℃ has strong resistance to chemical oxidation and is more economical.

[0088] AB 500-1:5 There is a significant mass loss at around 500℃. Figure 8 b、 Figure 8 c), this is because AB 500-1:5 The composition contains a large proportion of corn straw biomass, which is due to the thermal degradation of hemicellulose and cellulose in the biomass. (Comparison with A and B) 500-1:2 and AB 500-1:5 The carbon loss rate of biochar decreases with increasing fly ash doping ratio. Figure 8 d).

[0089] The present invention has been described in detail above with reference to specific embodiments and / or exemplary examples, as well as the accompanying drawings. However, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing biochar from fly ash under alkali treatment, wherein fly ash and corn stalks are added to an alkaline solution and allowed to react fully to obtain an alkali-treated mixture of fly ash and corn stalks, which is then calcined to obtain biochar with a specific surface area of ​​19 m². 2 The biochar has a mass loss rate of less than 22 wt% after thermal decomposition at 900℃; the carbon loss after oxidation with a 5 wt% H2O2 aqueous solution is less than 30 wt%; and the H / C molar ratio is less than 0.

55. The fly ash has a particle size of 12-30 mesh, and the corn stalk has a particle size of 12-30 mesh. The mass ratio of fly ash to corn stalks is 1:(1-9). The alkaline solution is an aqueous solution of an alkali metal hydroxide, and the concentration of the alkaline solution is 2-10 wt%. When fly ash and corn stalks are added to an alkaline solution, the solid-liquid mass ratio is 1:(7-16). The temperature for complete reaction of fly ash and corn stalks in the alkaline solution is 20-40℃, and the reaction time is 6-10 hours. The calcination is carried out under a protective atmosphere at 200-800℃ for 1.5-3.5 hours.

2. The method according to claim 1, characterized in that, The mass ratio of fly ash to corn stalks is 1:(1.5-7).

3. The method according to claim 2, characterized in that, The mass ratio of fly ash to corn stalks is 1:(2-5).

4. The method according to claim 1, characterized in that, The alkaline solution is an aqueous solution of sodium hydroxide and / or an aqueous solution of potassium hydroxide.

5. The method according to claim 1, characterized in that, When the fly ash and corn stalks are added to the alkaline solution, the solid-liquid mass ratio is 1:(8-14).

6. The method according to claim 5, characterized in that, When the fly ash and corn stalks are added to the alkaline solution, the solid-liquid mass ratio is 1:(9-12).

7. The method according to claim 1, characterized in that, The roasting is carried out at 300-700℃ for 2-3 hours.

Images