A method for graded conversion and high-value utilization of lignite

Abstract

The present invention discloses a method for the graded conversion and high-value utilization of lignite, belonging to the fields of lignite resource utilization and fine chemical engineering. This method involves oxidizing lignite in an alkaline aqueous solution to produce an alkali-soluble depolymerization product, which is then electrolyzed in an alkaline system to obtain water-soluble fulvic acid and small-molecule carboxylic acids, while also producing hydrogen. This method uses oxygen to oxidize and depolymerize lignite in an alkaline aqueous solution, achieving mild reaction conditions, high lignite conversion, and low alkali usage and CO2 generation. The oxidative depolymerization product undergoes solid-liquid separation, and the resulting alkali-soluble depolymerization product can be directly used for electrolysis without the need for acid-base neutralization and refining. Using transition metal-loaded nickel as the anode and constant potential electrolysis at 60-80°C, a conversion product primarily composed of fulvic acid is obtained, while simultaneously producing high-purity hydrogen at the cathode. Compared to traditional chemical oxidation methods, this method offers significant technical advantages, including high lignite utilization, safety, cleanliness, and low carbon emissions.

Description

Technical Field

[0001] The present invention belongs to the fields of comprehensive energy utilization, lignite resource utilization, and fine chemical technology. Specifically, it relates to a method for the hierarchical conversion and high-value utilization of lignite, which involves mild oxidative depolymerization of lignite in an alkaline system to produce humic acid, followed by electrolysis of the humic acid to obtain fulvic acid and small-molecule carboxylic acids, and the co-production of hydrogen. Background Art

[0002] Lignite is a cheap, low-metamorphic coal resource with high oxygen content and moisture content, low calorific value, and easy to spontaneously combust. my country is rich in lignite resources, which are mainly used in traditional energy production methods such as combustion for power generation, with low utilization efficiency and large carbon emissions. However, lignite has a high H / C ratio and is relatively chemically active. Selective oxidation and depolymerization of lignite under mild conditions to prepare oxygen-containing chemicals has significant structural advantages. In particular, the oxidation of lignite in aqueous media can avoid high-energy consumption processes such as dehydration and deoxygenation, and obtain oxygen-containing chemical raw materials such as coal-based humic acid, fulvic acid, aromatic carboxylic acids and small molecule fatty carboxylic acids, which are widely used in the preparation of fine chemicals and advanced carbon materials, ecological restoration and environmental governance, as well as agriculture and medicine and other related fields. Achieve clean conversion and high-value utilization of lignite. Therefore, for CC((CH2) n ) and CO (ether bond) and other cross-linked aromatic ring macromolecules and the structural characteristics of lignite rich in oxygen-containing functional groups such as phenolic hydroxyl and carboxyl groups. Efficient oxidative depolymerization under mild conditions to prepare oxygen-containing chemicals can achieve low-carbon conversion and high-value utilization of lignite, make up for the shortage of petroleum resources, and have important development and utilization prospects.

[0003] Selective oxidation of lignite in aqueous media has always been one of the important research areas of coal conversion. The preparation of coal-based humic acid by nitric acid oxidation is one of the main production processes of commercial humic acid. However, the high cost and NO xProblems such as pollution, process safety, and nitration side reactions have remained unresolved. Hydrogen peroxide is a clean, efficient oxidant, and its use in the oxidation of lignite to produce oxygenated chemicals has also attracted considerable attention. While lignite oxidation with hydrogen peroxide can yield chemicals such as phenylcarboxylic acids and small fatty acids, its poor safety profile has limited its large-scale production and application. While the alkali-oxygen oxidation of coal using O₂ or air as the oxidant is cost-effective, it requires concentrated alkaline solutions, resulting in high alkali consumption. In particular, the production of phenylcarboxylic acids and water-soluble organic acids requires deep oxidative cracking of the coal's macromolecular structure at high temperatures, resulting in high CO₂ production and low yields of the target products. For example, at 200-300°C and an alkali / coal weight ratio of 2.5, using an initial O₂ pressure of 5-6 MPa, the mass yield of water-soluble acids can reach as high as 54.9% (Journal of Fuel Chemistry and Technology, 1997, 25:373). However, in the conventional alkali-oxygen oxidation of lignite to produce water-soluble organic carboxylic acids, more than 50% of the carbon is oxidized to CO2 (Fuel 2020, 280: 118652; Energy Fuels 1997, 11 (1): 227), resulting in low lignite utilization efficiency. Based on the study of lignite structure and oxidative reactivity, the applicant proposed a new method for preparing coal-based humic acid by mild oxidative depolymerization of lignite (Fuel 2022, 308: 122043; 315: 123277). Through the selective oxidative cracking of cross-linked structures such as CC and CO, a coal-based humic acid yield of more than 80% was obtained at below 100°C in a dilute alkaline solution (alkali-coal mass ratio 1 / 3), and no CO2 generation was detected, thus providing a new conversion technology for low-carbon and efficient utilization of lignite.

[0004] Coal-based humic acid is the primary source of commercial humic acid and has been widely used in industry, agriculture, medicine, and environmental protection. In recent years, with its promising application potential in the production of new carbon-based materials and bulk chemicals, humic acid has attracted increasing global attention as a potential organic resource being developed and utilized. Currently, coal-based humic acid is primarily produced from lignite and weathered coal via alkali extraction or nitric acid oxidation. These methods result in low humic acid yields, high acid and alkali consumption, and high costs. While oxidative depolymerization under mild conditions can significantly reduce alkali consumption, increase humic acid yields, and enable its use in the production of bulk fine chemicals such as high-efficiency concrete water reducers and coal-water slurry dispersants (ZL 202010259004.7; 202011523968.4; 202110767465.X), the humic acid obtained through mild oxidation is primarily alkali-soluble black humic acid, limiting its application. In order to obtain fulvic acid with higher utilization value, or even water-soluble organic acids such as benzene polycarboxylic acids, it is necessary to increase the oxidation reaction temperature and the amount of alkali, which will lead to the generation of a large amount of CO2 (Green Chem 2020, 22:7233).

[0005] In recent years, water electrolysis, utilizing renewable energy sources such as wind and solar power, has been considered the most promising "green hydrogen" production technology for development and application, potentially addressing the storage and transportation challenges of distributed energy. Among these, the use of biomass oxidation coupled with water electrolysis to produce hydrogen simultaneously yields value-added chemicals, making it a research hotspot in the fields of hydrogen energy and biomass conversion (AdvEnergy Mater 2021, 11: 2102-292). Its technical advantage lies in the use of biomass selective oxidation to produce value-added chemicals, replacing the complete oxidation product, CO2, with low-overpotential electrolysis, improving current efficiency, and achieving product value. Compared to biomass, coal has a higher energy density, making water-assisted hydrogen electrolysis a promising option for large-scale, low-cost hydrogen production (J Power Sources 2021, 483: 2291-75). However, conventional coal-assisted water electrolysis primarily produces H2 and CO2, which do not meet the requirements of "green hydrogen." Furthermore, the poor solubility of solid coal severely constrains the electrolysis kinetics. Consequently, water-coal slurry electrolysis to produce hydrogen has not been widely adopted in industry. Summary of the Invention

[0006] Given the energy conversion advantages and value-added benefits of biomass-assisted water electrolysis for hydrogen production in alkaline media, this paper proposes a technical approach to directly assist in hydrogen production by electrolysis of water by depolymerizing lignite under mild conditions to produce alkali-soluble products. Using electrochemical oxidation, the chemical energy and electrical energy generated during the lignite oxidation process are coupled to convert them into clean hydrogen energy. Simultaneously, oxygenated chemicals such as fulvic acid and small-molecule water-soluble organic acids are produced, achieving low-carbon conversion and high-value utilization of lignite.

[0007] The present invention is achieved through the following technical solutions.

[0008] The present invention provides a method for graded conversion and high-value utilization of lignite. The method comprises the following steps: mild oxidative depolymerization of lignite in an alkaline aqueous solution to obtain an alkali-soluble depolymerized product, followed by electrolysis in an alkaline system to obtain water-soluble fulvic acid and small molecule carboxylic acids.

[0009] (1) Oxidative depolymerization of lignite: lignite: alkali: water = 1:0.3 to 1:30 by mass are added to a high-pressure reactor. The temperature is raised to 60-100°C in a water bath with stirring. O₂ is introduced to 0-3 MPa and the reaction is carried out at a constant temperature for 0.5-5 hours. After the reaction is completed, the solid residual coal is separated by centrifugation to obtain a soluble depolymerized product alkali solution.

[0010] (2) Electrolyte preparation: KOH and deionized water were added to the soluble depolymerized alkaline solution obtained in (1), and the pH was adjusted to 13-14 and the humic acid content was 5-15 g / L to prepare the electrolyte.

[0011] (3) Electrolysis: Add the electrolyte prepared in step (2) to the anode chamber of an H-type electrolytic cell. Simultaneously, add an equal volume of an alkaline solution of equal concentration to the cathode. Select transition metal-loaded nickel foam as the anode, platinum sheet as the cathode, and an anion exchange membrane as the diaphragm. Perform constant potential electrolysis at a temperature of 60-80°C and an anode potential of 0.3-1V (vs Ag / AgCl), and collect the gases generated at the cathode and cathode, respectively. Electrolyze for 1-5 hours, terminate the reaction, and cool to room temperature.

[0012] (4) Product separation: The product solution obtained in (3) was acidified to pH = 1 using 5 mol / L sulfuric acid, allowed to settle for 12 h, and insoluble humic acid was separated by centrifugation. After washing with deionized water, it was dried and weighed to obtain black humic acid + brown humic acid. The liquid product obtained after acid precipitation was extracted with butanone to obtain the product fulvic acid. The butanone extract was neutralized with alkali to neutrality and then used for TOC analysis to determine the total organic carbon content of small molecular products such as soluble carboxylic acids.

[0013] As an optimization, the lignite in step (1) is a low-metamorphosis lignite with a dry ash-free carbon content of 50-70%, preferably one of Xilin Gol lignite, Xiaolongtan lignite and Zhaotong lignite; the alkali is KOH or NaOH, preferably KOH; the oxidative depolymerization conditions are: lignite: alkali: water = 1:0.3-1:30 (mass ratio), 60-100°C, and O2 pressure 0-3MPa.

[0014] In the step (2), the base is KOH or NaOH, preferably KOH; and the pH is preferably 13.7.

[0015] In step (3), the anode is a transition metal-loaded nickel foam, preferably Co, Ni, Cu, Cr, or a combination thereof, prepared by electrochemical deposition. The electrolysis conditions are a temperature of 60-80° C., an anode potential of 0.3-1 V (vs Ag / AgCl), and constant potential electrolysis.

[0016] The present invention provides a method for obtaining humic acid by mild oxidative depolymerization of lignite in an alkaline aqueous solution, followed by electrolysis in an alkaline system to obtain water-soluble fulvic acid and small molecule carboxylic acids. The method has the following innovative features:

[0017] (1) Based on the weak covalent cross-linking structure of lignite, such as methylene and ether bonds, which are easily oxidized, selective depolymerization of lignite can be achieved through oxidation and alkali dissolution under mild conditions, resulting in alkali-soluble depolymerization products. At the same time, the production of CO2 by overoxidation at high temperatures is avoided, and the amount of alkali used is significantly reduced.

[0018] (2) Using the alkaline depolymerization products of lignite instead of insoluble lignite can significantly improve the anodic oxidation kinetics and increase the efficiency of anode-assisted cathode hydrogen evolution. At the same time, electrochemical oxidation replaces chemical oxidation to improve energy efficiency.

[0019] (3) Using transition metal-loaded nickel foam to replace traditional precious metal electrodes such as Pt can improve the anode electrocatalytic oxidation activity, reduce the anode overpotential, inhibit anode oxygen evolution and overoxidation, and achieve anode product value-added.

[0020] Compared with the prior art, the present invention has the following technical effects:

[0021] 1. This invention achieves the selective oxidation of lignite to produce chemical raw materials such as fulvic acid and small molecule carboxylic acids, while also producing hydrogen. The reaction conditions are mild and CO2 emissions are low. Compared to traditional chemical oxidation methods, this invention offers significant advantages, including high lignite utilization, safety, cleanliness, and low carbon emissions.

[0022] 2. The present invention provides a method for producing humic acid by mild oxidative depolymerization of lignite in an alkaline aqueous solution. The method achieves mild reaction conditions, high humic acid yield, and low CO2 production. When the reaction conditions are 80°C and the lignite:KOH:water mass ratio is 1:1:30, the humic acid yield reaches 61.15%, with no detectable CO2 production.

[0023] 3. The present invention provides a method for preparing fulvic acid by electrochemically oxidizing and upgrading humic acid. This method, which requires no external oxidant, exhibits mild reaction conditions, high humic acid utilization, simple operation, and the ability to co-produce hydrogen, among other technical advantages. At an electrolysis temperature of 80°C, a pH of 13.7, 10 g / L of humic acid, and an anode potential of 0.7 V (vs Ag / AgCl), the humic acid conversion rate reached 89.11% after 2 hours of electrolysis, with anode CO2 generation of less than 0.5%, and a cathode product H2 (purity >99.9%) exhibiting a Faradaic efficiency of 98.8%. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention provides a technical route for a method of graded conversion and high-value utilization of lignite.

[0025] Figure 2 1. The ESI FT-ICR MS spectrum of the electrolysis product fulvic acid in Example 1 of the present invention and its analysis results.

[0026] Figure 3 This is the MALD-TOF / MS spectrum of the electrolysis products humic acid and humic acid in Example 1 of the present invention. DETAILED DESCRIPTION

[0027] The present invention is described in detail below with reference to the accompanying drawings and embodiments, but the present invention is not limited to the following embodiments.

[0028] By the attached Figure 1It can be seen that the technical route of the present invention includes: (1) converting solid coal into alkali-soluble products through oxidative depolymerization in an alkaline medium, and then using it for electrolysis under alkaline conditions, converting the solid (coal particles)-liquid (electrolyte)-solid (electrode catalyst) multiphase reaction of the traditional water-coal slurry into a liquid (electrolyte + alkali-soluble)-solid (electrode catalyst) two-phase reaction, significantly improving the electrolysis kinetics process and increasing the electrolysis efficiency. (2) Using transition metal catalysts, the selective oxidation of the alkali-soluble products of lignite oxidative depolymerization is achieved under optimized electrolysis conditions, obtaining high-value products such as small molecular carboxylic acids and fulvic acid, which has a significant value-added effect compared to the CO2 anode product of traditional water-coal slurry electrolysis. (3) Through electrolysis, distributed renewable electricity is coupled with the chemical energy of lignite oxidation to produce coal-based chemicals, realizing hydrogen energy production, with high energy conversion efficiency and clean process.

[0029] Example 1

[0030] (1) Lignite Oxidative Depolymerization: Zhaotong lignite: KOH: water = 1:1:30 by mass were added to a high-pressure reactor, heated to 80°C with stirring, and reacted at normal pressure for 5 hours. After the reaction, the solid residue was removed by centrifugation to obtain a soluble depolymerized alkaline solution.

[0031] (2) Electrolyte preparation: KOH and deionized water were added to the soluble depolymerized alkaline solution obtained in (1), and the pH was adjusted to 14 and humic acid was 10 g / L to prepare the electrolyte.

[0032] (3) Electrolysis: Add the electrolyte prepared in step (2) to the anode chamber of an H-type electrolytic cell. Simultaneously, add an equal volume of a KOH solution with a pH of 14 to the cathode. Select Co loaded on nickel foam as the anode, platinum sheet as the cathode, and a Fumasep FAB-PK-130 anion exchange membrane as the separator. Perform constant potential electrolysis at a temperature of 80°C and an anode potential of 0.5 V (vs Ag / AgCl), and collect the gases generated at the cathode and cathode, respectively. Electrolyze for 2 h, terminate the reaction, and cool to room temperature.

[0033] (4) Product separation: The product solution obtained in (3) was acidified to pH = 1 using 5 mol / L sulfuric acid, allowed to settle for 12 h, and insoluble humic acid was separated by centrifugation. After washing with deionized water, it was dried and weighed to obtain fulvic acid + fulvic acid. The liquid product obtained after acid precipitation was extracted with butanone to obtain the fulvic acid product. The butanone extract was neutralized with alkali to neutrality and then used for TOC analysis to determine the total organic carbon content of small molecular products such as soluble carboxylic acids.

[0034] By the attached Figure 2 It can be seen that the main component of the electrolysis product fulvic acid is organic polycarboxylic acid, mainly including aromatic polycarboxylic acid and fatty polycarboxylic acid containing aromatic nucleus, which has the structural characteristics of fulvic acid.

[0035] By the attached Figure 3 It can be seen that the molecular weight of the electrolysis products humic acid + humic acid is mainly concentrated in the range of 500-2000 amu, which is significantly lower than the alkali-soluble products obtained by oxidative depolymerization of lignite.

[0036] Example 2

[0037] (1) Lignite Oxidative Depolymerization: Zhaotong lignite: KOH: water = 1:0.3:30 by mass were added to a high-pressure reactor, heated to 80°C with stirring, and oxygen was introduced to 1.0 MPa. The reaction was then continued at this constant temperature for 3 h. After the reaction, the solid residue was removed by centrifugation to obtain a soluble depolymerized alkaline solution.

[0038] (2) Electrolyte preparation: KOH and deionized water were added to the soluble depolymerized alkaline solution obtained in (1), and the pH was adjusted to 13.7 and humic acid was added to 10 g / L to prepare the electrolyte.

[0039] (3) Electrolysis: Add the electrolyte prepared in step (2) to the anode chamber of an H-type electrolytic cell. Simultaneously, add an equal volume of a KOH solution with a pH of 13.7 to the cathode. Select CoNi loaded on nickel foam as the anode, platinum sheet as the cathode, and Fumasep FAB-PK-130 anion exchange membrane as the diaphragm. Perform constant potential electrolysis at a temperature of 80°C and an anode potential of 0.3V (vs Ag / AgCl), and collect the gases generated at the cathode and cathode, respectively. Electrolyze for 2 hours, terminate the reaction, and cool to room temperature.

[0040] (4) Product separation: Same as Example 1.

[0041] Example 3

[0042] (1) Oxidative depolymerization of lignite: Zhaotong lignite: KOH: water = 1:1:30 by mass were added to a high-pressure reactor, heated to 60°C with stirring, and oxygen was introduced to 0.1 MPa. The reaction was continued at this constant temperature for 5 h. After the reaction was completed, the solid residue was removed by centrifugation to obtain a soluble depolymerized alkaline solution.

[0043] (2) Electrolyte preparation: KOH and deionized water were added to the soluble depolymerized alkaline solution obtained in (1), and the pH was adjusted to 13 and humic acid was 10 g / L to prepare the electrolyte.

[0044] (3) Electrolysis: Add the electrolyte prepared in step (2) to the anode chamber of an H-type electrolytic cell. Simultaneously, add an equal volume of a KOH solution with a pH of 13 to the cathode. Select CoNi loaded on nickel foam as the anode, platinum sheet as the cathode, and Fumasep FAB-PK-130 anion exchange membrane as the diaphragm. Perform constant potential electrolysis at a temperature of 80°C and an anode potential of 1.0 V (vs Ag / AgCl), and collect the gases generated at the cathode and cathode, respectively. Electrolyze for 1 hour, terminate the reaction, and cool to room temperature.

[0045] (4) Product separation: Same as Example 1.

[0046] Example 4

[0047] (1) Lignite Oxidative Depolymerization: Zhaotong lignite: KOH: water = 1:0.5:30 by mass was added to a high-pressure reactor. The temperature was raised to 100°C with stirring, and O₂ was introduced to 1.0 MPa. The reaction was carried out at a constant temperature for 0.5 h. After the reaction was completed, the solid residue was removed by centrifugation to obtain a soluble depolymerized alkaline solution.

[0048] (2) Electrolyte preparation: KOH and deionized water were added to the soluble depolymerized alkaline solution obtained in (1), and the pH was adjusted to 14 and the humic acid was 15 g / L to prepare the electrolyte.

[0049] (3) Electrolysis: Add the electrolyte prepared in step (2) to the anode chamber of an H-type electrolytic cell. Simultaneously, add an equal volume of a KOH solution with a pH of 14 to the cathode. Select CoNi loaded on nickel foam as the anode, platinum sheet as the cathode, and Fumasep FAB-PK-130 anion exchange membrane as the diaphragm. Perform constant potential electrolysis at a temperature of 60°C and an anode potential of 0.3V (vs Ag / AgCl), and collect the gases generated at the cathode and cathode, respectively. Electrolyze for 5 hours, terminate the reaction, and cool to room temperature.

[0050] (4) Product separation: Same as Example 1.

[0051] Example 5

[0052] (1) Lignite Oxidative Depolymerization: Zhaotong lignite: NaOH: water (mass ratio = 1:0.6:30) were added to a high-pressure reactor, heated to 80°C with stirring, and oxygen (O₂) was introduced to 3.0 MPa. The reaction was continued at this constant temperature for 3 h. After the reaction, the solid residue was removed by centrifugation to obtain a soluble depolymerized alkaline solution.

[0053] (2) Electrolyte preparation: KOH and deionized water were added to the soluble depolymerized alkaline solution obtained in (1), and the pH was adjusted to 13.5 and humic acid to 5 g / L to prepare the electrolyte.

[0054] (3) Electrolysis: Add the electrolyte prepared in step (2) to the anode chamber of an H-type electrolytic cell. Simultaneously, add an equal volume of a NaOH solution with a pH of 13.5 to the cathode. Select Co loaded on nickel foam as the anode, platinum sheet as the cathode, and Fumasep FAB-PK-130 anion exchange membrane as the diaphragm. Perform constant potential electrolysis at a temperature of 60°C and an anode potential of 0.7 V (vs Ag / AgCl), and collect the gases generated at the cathode and cathode, respectively. Electrolyze for 2 h, terminate the reaction, and cool to room temperature.

[0055] (4) Product separation: Same as Example 1.

[0056] Example 6

[0057] (1) Oxidative depolymerization of lignite: Xiaolongtan lignite: KOH: water = 1:1:30 by mass was added to a high-pressure reactor, heated to 80°C with stirring, and oxygen was introduced to 1.0 MPa. The reaction was continued at this constant temperature for 2 h. After the reaction was completed, the solid residue was removed by centrifugation to obtain a soluble depolymerized alkaline solution.

[0058] (2) Electrolyte preparation: KOH and deionized water were added to the soluble depolymerized alkaline solution obtained in (1), and the pH was adjusted to 14 and humic acid was 10 g / L to prepare the electrolyte.

[0059] (3) Electrolysis: Add the electrolyte prepared in step (2) to the anode chamber of an H-type electrolytic cell. Simultaneously, add an equal volume of a KOH solution with a pH of 14 to the cathode. Select CoCu loaded on nickel foam as the anode, platinum sheet as the cathode, and Fumasep FAB-PK-130 anion exchange membrane as the diaphragm. Perform constant potential electrolysis at a temperature of 80°C and an anode potential of 0.3V (vs Ag / AgCl), and collect the gases generated at the cathode and cathode, respectively. Electrolyze for 2 hours, terminate the reaction, and cool to room temperature.

[0060] (4) Product separation: Same as Example 1.

[0061] Example 7

[0062] (1) Oxidative depolymerization of lignite: Xiaolongtan lignite: KOH: water = 1:0.6:30 by mass was added to a high-pressure reactor, heated to 100°C with stirring, and oxygen was introduced to 3.0 MPa. The reaction was carried out at this constant temperature for 2 h. After the reaction was completed, the solid residue was removed by centrifugation to obtain a soluble depolymerized alkaline solution.

[0063] (2) Electrolyte preparation: KOH and deionized water were added to the soluble depolymerized alkaline solution obtained in (1), and the pH was adjusted to 14 and humic acid was 10 g / L to prepare the electrolyte.

[0064] (3) Electrolysis: Add the electrolyte prepared in step (2) to the anode chamber of an H-type electrolytic cell. Simultaneously, add an equal volume of a KOH solution with a pH of 14 to the cathode. Select CuCr loaded on nickel foam as the anode, platinum sheet as the cathode, and Fumasep FAB-PK-130 anion exchange membrane as the diaphragm. Perform constant potential electrolysis at a temperature of 80°C and an anode potential of 0.5V (vs Ag / AgCl), and collect the gases generated at the cathode and cathode, respectively. Electrolyze for 3 hours, terminate the reaction, and cool to room temperature.

[0065] (4) Product separation: Same as Example 1.

[0066] Example 8

[0067] (1) Lignite Oxidative Depolymerization: Xilin Gol lignite: NaOH: water = 1:1:30 by mass were added to a high-pressure reactor, heated to 100°C with stirring, and oxygen was introduced to 3.0 MPa. The reaction was then continued at this constant temperature for 3 h. After the reaction, the solid residue was removed by centrifugation to obtain a soluble depolymerized alkaline solution.

[0068] (2) Electrolyte preparation: NaOH and deionized water were added to the soluble depolymerized alkaline solution obtained in (1), and the pH was adjusted to 14 and humic acid was 10 g / L to prepare the electrolyte.

[0069] (3) Electrolysis: Add the electrolyte prepared in step (2) to the anode chamber of an H-type electrolytic cell. Simultaneously, add an equal volume of a NaOH solution with a pH of 14 to the cathode. Select CoNi loaded on nickel foam as the anode, platinum sheet as the cathode, and Fumasep FAB-PK-130 anion exchange membrane as the diaphragm. Perform constant potential electrolysis at a temperature of 80°C and an anode potential of 0.3V (vs Ag / AgCl), and collect the gases generated at the cathode and cathode, respectively. Electrolyze for 2 hours, terminate the reaction, and cool to room temperature.

[0070] (4) Product separation: Same as Example 1.

[0071] The results of Examples 1 to 8 are shown in Tables 1 to 3.

[0072] Table 1 Results of oxidative depolymerization of lignite in the present invention

[0073]

[0074]

[0075] Table 2 Electrolysis results of alkali-soluble depolymers of the present invention

[0076]

[0077] Table 3 Composition distribution of lignite conversion products of the present invention

[0078]

[0079] *Indicates the carbon mass yield of soluble small molecular organic matter determined by TOC.

Claims

1. A method for graded conversion and high-value utilization of lignite, characterized in that The steps include: (1) Oxidative depolymerization of lignite: lignite: alkali: water = 1:0.3~1:30 by mass ratio is added to a high-pressure reactor in sequence, heated to 60~100℃ in a water bath under stirring, and oxygen is introduced to 0~3 MPa, and the reaction is carried out at a constant temperature for 0.5~5 h. After the reaction is completed, the solid residual coal is separated by centrifugation to obtain a soluble depolymerized product alkali solution; (2) Electrolyte preparation: KOH and deionized water are added to the soluble depolymerized alkaline solution obtained in step (1), and the pH is adjusted to 13-14 and the humic acid content is 5-15 g / L to prepare the electrolyte; (3) Electrolysis: Add the electrolyte prepared in step (2) to the anode chamber of an H-type electrolytic cell; simultaneously, add an equal volume and concentration of alkaline solution to the cathode; select transition metal-loaded nickel foam as the anode, platinum sheet as the cathode, and an anion exchange membrane as the diaphragm; perform constant potential electrolysis at a temperature of 60-80°C and an anode potential of 0.3-1 V, and collect the gases generated at the cathode and cathode respectively; perform electrolysis for 1-5 h, terminate the reaction, and cool to room temperature; The transition metal in the transition metal-loaded nickel foam is one or more of Co, Ni, Cu, and Cr; (4) Product separation: The product solution obtained in step (3) was acidified to pH = 1 using 5 mol / L sulfuric acid, allowed to settle for 12 h, and insoluble humic acid was separated by centrifugation. After washing with deionized water, it was dried and weighed to obtain black humic acid + brown humic acid. The liquid phase product obtained after acid precipitation was extracted with butanone to obtain the product fulvic acid.

2. The method for graded conversion and high-value utilization of lignite according to claim 1, characterized in that: The lignite in step (1) is low-metamorphosis lignite with a dry ash-free carbon content of 50-70% and a particle size of 80-200 mesh.

3. The method for graded conversion and high-value utilization of lignite according to claim 1, characterized in that: The base in steps (1) to (3) is KOH or NaOH.

4. The method for graded conversion and high-value utilization of lignite according to claim 3, characterized in that: The base in steps (1) to (3) is KOH.

5. The method for graded conversion and high-value utilization of lignite according to claim 1, characterized in that: In the step (2), pH=13.7.

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