Iron-based oxygen carrier based on directional regulation of pore structure of fly ash and preparation method thereof

Abstract

The application belongs to the technical field of industrial solid waste resource utilization, and particularly relates to an iron-based oxygen carrier based on directional regulation of pore structure of fly ash and a preparation method thereof. The preparation method comprises the following steps: preparing a mixed metal salt solution containing an iron source and an aluminum source; calcining fly ash at high temperature to pre-activate the fly ash as a pore structure directional regulation agent, mixing the mixed metal salt solution, adding a template agent, stirring uniformly, and obtaining a precursor mixed solution; adding a precipitant dropwise into the precursor mixed solution to perform a co-precipitation reaction, obtaining a precursor precipitate; separating and washing and drying the precursor precipitate, and then calcining at high temperature to obtain the iron-based oxygen carrier. The iron-based oxygen carrier effectively inhibits high-temperature grain sintering while improving the specific surface area and mesopore ratio, provides an oxygen carrier material with low cost and high stability for chemical chain hydrogen production, and opens up a new way for high-value utilization of industrial solid waste fly ash.

Description

Technical Field

[0001] This invention belongs to the field of industrial solid waste resource utilization technology, specifically relating to an iron-based oxygen carrier based on the directional controlled pore structure of fly ash and its preparation method. Background Technology

[0002] Chemical looping hydrogen production technology, which involves the cyclic oxidation-reduction of an oxygen carrier between a fuel reactor, a steam reactor, and an air reactor, achieves fuel conversion, in-situ carbon dioxide separation, and pure hydrogen generation. It is considered a highly promising low-carbon hydrogen production pathway. Iron-based oxygen carriers, due to their abundant raw materials, low cost, environmental friendliness, and excellent resistance to carbon buildup, have become one of the most promising candidate materials for industrial application in this system.

[0003] However, iron-based oxygen supports still face three major technical bottlenecks in practical applications: First, the specific surface area of ​​iron-based oxygen supports prepared by traditional methods is generally low (usually less than 15m²). 2 The density of active sites is insufficient, which limits the gas-solid reaction rate. Secondly, the pore size distribution is uneven. The proportion of macropores is too high, which leads to a decrease in mechanical strength. The proportion of micropores is too high, which hinders gas diffusion. There is a lack of precise control over the mesoporous structure, and the proportion of mesopores (2-50nm) is low. Thirdly, under high-temperature cycling conditions of 800~1000℃, α-Fe2O3 grains are prone to abnormal growth and sintering agglomeration, which leads to a significant decrease in specific surface area, pore collapse, and a significant deterioration in cycling stability.

[0004] Existing improvement strategies mainly include: adding inert supports (such as Al2O3 and MgAl2O4) to inhibit sintering, doping with transition metals (such as Cu, Ce, and Ni) to enhance activity, and using template methods to construct mesoporous structures. However, these methods generally suffer from high costs, complex processes, and uneven dispersion of active components. In particular, although industrial solid waste fly ash is rich in SiO2, Al2O3, and alkaline components (CaO, K2O, and Na2O), possessing the potential to act as a pore structure regulator and inert framework, the introduction of fly ash in existing technologies is mostly through simple mechanical mixing, failing to fully utilize its synergistic effects in pore construction, grain inhibition, and interface nucleation. The hydrophobic surface, microbubble adhesion, and lightweight buoyancy of fly ash particles make it difficult to disperse uniformly in aqueous systems, resulting in the inability of metal hydroxides to nucleate in situ on their surface, easy peeling off of active components, and unstable pore structure regulation effects.

[0005] Therefore, there is an urgent need to develop a method that can directionally regulate the pore structure of iron-based oxygen carriers using fly ash, thereby increasing the specific surface area and the proportion of mesopores while suppressing high-temperature grain growth, achieving the dual goals of solid waste resource utilization and material performance optimization. Summary of the Invention

[0006] To address the technical problems of low specific surface area, uneven pore size distribution, easy sintering at high temperatures, and difficulty in uniformly dispersing fly ash in the existing technologies, this invention provides an iron-based oxygen carrier based on the directional control of fly ash pore structure and its preparation method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing an iron-based oxygen carrier based on the directional controlled pore structure of fly ash includes the following steps: S1. Prepare a mixed metal salt solution containing iron and aluminum sources; S2. Fly ash is pre-activated by high-temperature calcination and used as a pore structure orientation regulator. It is then incorporated into the mixed metal salt solution described in step S1, and a template agent is added. The mixture is stirred evenly to obtain a precursor mixture. S3. Add a precipitant to the precursor mixture described in step S2 to carry out a co-precipitation reaction and obtain the precursor precipitate. S4. Separate the precursor precipitate from step S3, wash and dry it, and then calcine it at high temperature to obtain the iron-based oxygen carrier.

[0008] Preferably, the iron source in step S1 is ferric nitrate nonahydrate, and the aluminum source is aluminum nitrate nonahydrate.

[0009] Preferably, the fly ash in step S2 is activated by high-temperature calcination, which includes: grinding and sieving the fly ash, heating it to 500-600°C in air at a heating rate of 5-20°C / min and holding it at that temperature for 1-3 hours.

[0010] Preferably, the fly ash composition is as follows (by weight percentage): SiO2 40-55%, Al2O3 20-35%, CaO 5-10%, Fe2O3 2-6%, K2O+Na2O 2-5%, and loss on ignition 5-15%.

[0011] Preferably, the molar ratio of Fe in the iron source to Al in the aluminum source in step S1 is 1-1.5:1, and the mass of the fly ash after high-temperature calcination and pre-activation in step S2 is 2.4-7.2% of the mass of Fe in the iron source.

[0012] Preferably, the template agent in step S2 is a polyvinyl alcohol solution, the mass concentration of the polyvinyl alcohol solution is 2-10%, and the mass of the polyvinyl alcohol solution is 2-10% of the mass of the mixed metal salt solution in step S1.

[0013] Preferably, the coprecipitation reaction temperature in step S3 is 50-90°C, and the precipitant is added dropwise until the pH is 8.5-9.5.

[0014] Preferably, the high-temperature calcination in step S4 is a gradient high-temperature calcination, which involves heating to 500-600°C at a heating rate of 5-20°C / min in an air atmosphere and holding for 2-3 hours, followed by heating to 800-950°C at a heating rate of 5-20°C / min and holding for 2-4 hours.

[0015] An iron-based oxygen carrier based on the directional controlled pore structure of fly ash, prepared by the above method.

[0016] Application of an iron-based oxygen carrier based on the directional controlled pore structure of fly ash, prepared by the above method, in a chemical looping hydrogen production process.

[0017] Preferably, the application includes: the iron-based oxygen carrier circulating lattice oxygen between the fuel reactor, the steam reactor and the air reactor to achieve fuel conversion and hydrogen generation.

[0018] The positive and beneficial effects of this invention are: 1. This invention overcomes the problem of aqueous dispersion of fly ash. By pre-activating fly ash through high-temperature calcination, residual carbon is removed, the glassy phase is destroyed, and alkaline components are converted into soluble oxides. Combined with a pre-mixed slurry wetting strategy, mechanical shearing and capillary force are used to remove microbubbles on the particle surface, which completely solves the industry problem of lightweight fly ash floating and agglomerating in the liquid phase. It guides it to form a uniform suspended skeleton in the liquid network. In the dynamic co-precipitation process, metal hydroxides achieve local in-situ nucleation and tight coating on the fly ash surface, which significantly improves the dispersion of active components and achieves uniform suspension and in-situ nucleation.

[0019] 2. The BET specific surface area of ​​the iron-based oxygen carrier prepared by this invention is 18.31-27.45 m². 2 / g, total pore volume is 0.1552-0.2183cm³ 3 / g, with a mesoporous content of 75.1-86.5%, and the specific surface area of ​​the iron-based oxygen carrier BET increased from 12.56m² in traditional methods. 2 / g increased to 27.45m 2 / g, total pore volume is 0.0896cm³ 3 / g increased to 0.2183cm 3 / g, the proportion of mesopores (2-50nm) increased from 62.3% to 86.5%. This invention precisely controls the pore structure, achieving a dual improvement in specific surface area and mesopore proportion. SiO2 and Al2O3 in fly ash interact with the iron-aluminum matrix at high temperatures to form stable aluminosilicate crystals, constructing a rigid inorganic framework and providing anti-sintering support. Simultaneously, residual carbon and volatiles in fly ash are oxidized and released during high-temperature calcination pre-activation, producing an in-situ etching effect on the matrix, forming a hierarchical interconnected network of micropores, mesopores, and macropores. The pore connectivity is significantly improved, and the dissolution of alkaline components further promotes local pore formation and defect formation. Polyvinyl alcohol occupies a certain space during the co-precipitation of iron hydroxide and aluminum hydroxide, and is converted into a gaseous state during subsequent calcination, thus generating a dense pore structure.

[0020] 3. The extremely small particle size of fly ash provides aggregation points during the co-precipitation process of aluminum hydroxide and ferric hydroxide. These scattered, fine aggregates ensure the formation of a high specific surface area. Simultaneously, fly ash contains inert components such as Al₂O₃, which readily form covalent bonds with ferric hydroxide during calcination to produce ferric oxide, creating a robust framework. Furthermore, fly ash contains a certain amount of alkali metals such as Na and K, which generate active vacancies in the crystal structure of ferric oxide formation, enhancing its reactivity.

[0021] The inert framework of fly ash exerts a spatial confinement effect under high-temperature calcination and cyclic conditions of 800-950℃, resulting in an α-Fe2O3 grain size of 28.7-35.2 nm, decreasing from 42.6 nm in the unmodified form to 28.7 nm. XRD and SEM characterization confirmed that fly ash did not alter the main active α-Fe2O3 crystalline phase, but rather inhibited abnormal grain growth through physical barrier and interface anchoring, delaying the specific surface area decay and pore collapse during high-temperature cycling, effectively suppressing high-temperature grain sintering, and significantly improving cycling stability.

[0022] When the fly ash content exceeds 4.8%, excessive fly ash particles adhere to each other, clogging pores and encapsulating active components, leading to a decline in performance. The preferred fly ash content in this invention is 2.4-7.2%. This invention, through precise control of fly ash content and synergistic optimization of the gradient calcination process, successfully achieves directional regulation of the pore structure of the iron-based oxygen carrier. While increasing the specific surface area and mesoporous proportion, it effectively inhibits high-temperature grain sintering, providing a low-cost, highly stable oxygen carrier material for chemical looping hydrogen production, and opening up new avenues for the high-value utilization of industrial solid waste fly ash.

[0023] 4. This invention uses fly ash, a byproduct of industrial coal combustion, as a pore structure regulator to replace or partially replace chemically synthesized carrier materials. While improving the performance of oxygen carriers, it significantly reduces raw material costs, which aligns with the green chemical industry concept of "treating waste with waste and turning waste into treasure". It provides an economically feasible material solution for the large-scale application of chemical chain hydrogen production technology, realizes the high-value utilization of industrial solid waste, and reduces material costs. Attached Figure Description

[0024] Figure 1 The XRD diffraction patterns of the iron-based oxygen carrier under different fly ash contents in the embodiments of the present invention are shown. Where: a - no fly ash added, b - 4.8% fly ash added.

[0025] Figure 2 These are SEM microstructure images of iron-based oxygen carriers with different fly ash content in the embodiments of the present invention. Where: a - no fly ash added, b - 2.4% fly ash added, c - 4.8% fly ash added, d - 7.2% fly ash added. Detailed Implementation

[0026] The present invention will be further described below with reference to some specific embodiments.

[0027] The composition of the fly ash used in this embodiment of the invention is as follows (by weight percentage): SiO2 48.6%, Al2O3 27.3%, CaO 6.8%, Fe2O3 4.2%, K2O+Na2O 3.5%, and loss on ignition 9.6%.

[0028] Example 1

[0029] The specific steps of the preparation method of an iron-based oxygen carrier based on the directional controlled pore structure of fly ash are as follows: S1. High-temperature calcination pre-activation of fly ash: Place fly ash in a ball mill and grind for 30 min. Pass it through a 200-mesh sieve (pore size 75 μm). Place the sieved powder into a muffle furnace and heat it to 500℃ at a heating rate of 5℃ / min in air atmosphere. Keep it at the same temperature for 2 h, and then let it cool naturally to room temperature. After this pre-activation treatment, residual carbon and volatiles in fly ash are removed, the glass phase structure is destroyed, the alkaline components are converted into soluble oxides, and the color changes from gray-black to gray-white. S2. Preparation of mixed metal salt solution: Weigh 30.3g of ferric nitrate nonahydrate Fe(NO3)3·9H2O and 23.5g of aluminum nitrate nonahydrate Al(NO3)3·9H2O. The molar ratio of Fe in the iron source to Al in the aluminum source is 1.2:1. Dissolve the two in 200mL of deionized water and stir for 30min to obtain a uniform brownish-red mixed metal salt solution. S3. Fly ash pre-mixing and dispersion: Take 0.2g of the pre-activated fly ash from step S1 (the mass of fly ash is 4.8% of the mass of Fe in the iron source), add a trace amount of deionized water, and mix it into a homogeneous high-concentration slurry without bubbles by mechanical extrusion and grinding. Add the slurry to the mixed metal salt solution from step S2 and stir vigorously at 800 rpm for 60 min to break up the agglomeration between fly ash particles and form a stable suspended skeleton. S4. Template agent addition and co-precipitation: Add 12.5g of a 5% polyvinyl alcohol (PVA) solution to the mixture described in step S3, stir evenly to form a stable suspension precursor mixture; S5. Add 25%-28% ammonia solution at a rate of 1 mL / min to the precursor mixture described in step S4, and continue stirring at 60°C until the pH of the system rises to 9.0. Stop adding the solution and continue stirring for 1 hour to generate a brown gel-like precipitate. S6. Washing and Separation: Separate the supernatant (mainly ammonium nitrate and water) remaining after co-precipitation in step S5 using decantation. Add 300-500 mL of warm water (40-50℃) to the bottom precipitate and stir to wash. Repeat the settling and decantation process 3-4 times until Cl is undetectable in the washing solution. - (No white precipitate was observed in the silver nitrate test). After the final wash, the precipitate was collected by vacuum filtration. S7. Gradient High-Temperature Calcination: The precipitate washed in step S6 is dried in a vacuum drying oven at 105℃ for 12 hours to obtain a dried precursor. The precursor is ground and placed in a muffle furnace. The temperature is increased to 500℃ at 5℃ / min under air atmosphere and held for 2 hours to completely pyrolyze the PVA template agent and organic matter. Then, the temperature is increased to 900℃ at the same rate and held for 3 hours to complete the crystal transformation and pore structure fixation. Finally, after natural cooling to room temperature, the iron-based oxygen carrier product with a particle size of 0.2-0.3mm is obtained by sieving and is designated as Fe-2 sample.

[0030] Example 2

[0031] The preparation method of the iron-based oxygen carrier based on the directional control of the pore structure of fly ash in this embodiment adopts the same preparation process as in Example 1, only changing the amount of fly ash: the amount of fly ash added is set to 0% (Fe-0 sample, no fly ash added), 2.4% (Fe-1 sample, 0.1g fly ash) and 7.2% (Fe-3 sample, 0.3g fly ash) of the Fe mass in the iron source, respectively, while the amount of other raw materials and process parameters remain unchanged.

[0032] Performance characterization and results The iron-based oxygen support samples obtained in Examples 1 and 2 were systematically characterized: 1. BET specific surface area and pore structure test The results were obtained using a low-temperature nitrogen adsorption-desorption method: Fe-0 sample: BET specific surface area: 12.56 m² 2 / g, Total pore volume: 0.0896cm³ 3 / g, average pore size: 28.54nm, mesopores (2-50nm) percentage: 62.3%; Fe-1 sample: BET specific surface area: 18.31 m² 2 / g, Total pore volume: 0.1552cm³ 3 / g, average pore size: 31.11nm, mesopore content: 75.1%; Fe-2 sample: BET specific surface area: 27.45 m² 2 / g, total pore volume: 0.2183cm³ 3 / g, average pore size: 27.40nm, mesoporous content: 86.5%; Fe-3 sample: BET specific surface area: 22.68 m² 2 / g, Total pore volume: 0.1875cm³ 3 / g, average pore size: 29.36nm, mesopore ratio: 79.4%.

[0033] 2. XRD phase and grain size analysis Depend on Figure 1 (Fe-0 and Fe-2 samples) It can be seen that the main crystalline phase of all samples is α-Fe2O3, with no other impurity phases generated. The grain size of the iron-based oxygen support in each embodiment was calculated using the Scherrer formula: Fe-0: 42.6 nm; Fe-1: 35.2 nm; Fe-2: 28.7 nm; Fe-3: 31.5 nm; The results showed that the α-Fe2O3 grain size was significantly reduced after the addition of fly ash, with the inhibition effect being most significant at the 4.8% dosage in Example 1, confirming that the spatial confinement effect of the inert skeleton of fly ash effectively inhibited the abnormal growth of high-temperature grains.

[0034] SEM microstructure observation Figure 2 These are SEM microstructure images of the iron-based oxygen carrier under different fly ash content in the embodiments of the present invention. Figure 2 It can be known that: Fe-0 sample (no doping): The surface exhibits a dense porous structure, but the pore size is relatively small; Fe-1 sample (2.4% doping): The surface is relatively rough, with local sintered dense areas, and pores are initially formed but unevenly distributed, with some channels closed. Fe-2 sample (4.8% doping): loose and porous structure, fine and uniform particles, dense and interconnected mesoporous network visible on the surface, no large agglomeration or dense sintered area, and the best pore connectivity. Fe-3 sample (7.2% admixture): Fly ash agglomeration and flocculent areas appeared, some pores were blocked, a coating layer appeared on the particle surface, and the pore connectivity deteriorated.

[0035] In summary, the SEM results are in high agreement with the BET and XRD data, confirming that 4.8% is the optimal fly ash content.

[0036] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing an iron-based oxygen carrier based on the directional controlled pore structure of fly ash, characterized in that, The steps include the following: S1. Prepare a mixed metal salt solution containing iron and aluminum sources; S2. Fly ash is pre-activated by high-temperature calcination and used as a pore structure orientation regulator. It is then incorporated into the mixed metal salt solution described in step S1, and a template agent is added. The mixture is stirred evenly to obtain a precursor mixture. S3. Add a precipitant to the precursor mixture described in step S2 to carry out a co-precipitation reaction and obtain the precursor precipitate. S4. Separate the precursor precipitate from step S3, wash and dry it, and then calcine it at high temperature to obtain the iron-based oxygen carrier.

2. The preparation method of the iron-based oxygen carrier based on the directional controlled pore structure of fly ash according to claim 1, characterized in that, The iron source in step S1 is ferric nitrate nonahydrate, and the aluminum source is aluminum nitrate nonahydrate.

3. The method for preparing an iron-based oxygen carrier based on the directional controlled pore structure of fly ash according to claim 1, characterized in that, The activation of fly ash by high-temperature calcination in step S2 includes: grinding and sieving the fly ash, heating it to 500-600℃ in air at a heating rate of 5-20℃ / min and holding it at that temperature for 1-3 hours.

4. The preparation method of the iron-based oxygen carrier based on the directional controlled pore structure of fly ash according to claim 3, characterized in that, The composition of the fly ash is as follows (by weight percentage): SiO2 40-55%, Al2O3 20-35%, CaO 5-10%, Fe2O3 2-6%, K2O+Na2O 2-5%, and loss on ignition 5-15%.

5. The method for preparing an iron-based oxygen carrier based on the directional controlled pore structure of fly ash according to claim 1, characterized in that, The molar ratio of Fe in the iron source to Al in the aluminum source in step S1 is 1-1.5:1, and the mass of the fly ash after high-temperature calcination and pre-activation in step S2 is 2.4-7.2% of the mass of Fe in the iron source.

6. The method for preparing an iron-based oxygen carrier based on the directional controlled pore structure of fly ash according to claim 1, characterized in that, The template agent in step S2 is a polyvinyl alcohol solution with a mass concentration of 2-10%, and the mass of the polyvinyl alcohol solution is 2-10% of the mass of the mixed metal salt solution in step S1.

7. The method for preparing an iron-based oxygen carrier based on the directional controlled pore structure of fly ash according to claim 1, characterized in that, The coprecipitation reaction temperature in step S3 is 50-90℃, and the precipitant is added dropwise until the pH is 8.5-9.

5.

8. The method for preparing an iron-based oxygen carrier based on the directional controlled pore structure of fly ash according to claim 1, characterized in that, The high-temperature calcination in step S4 is a gradient high-temperature calcination, which involves heating to 500-600℃ at a heating rate of 5-20℃ / min in an air atmosphere and holding for 2-3 hours, followed by heating to 800-950℃ at a heating rate of 5-20℃ / min and holding for 2-4 hours.

9. An iron-based oxygen carrier based on the directional controlled pore structure of fly ash, prepared by the preparation method according to any one of claims 1-8.

10. The application of an iron-based oxygen carrier prepared by any one of claims 1-8 in a chemical looping hydrogen production process.

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