High-activity red mud-biomass ash composite oxygen carrier as well as multistage construction method and application thereof

Abstract

The invention discloses a high-activity red mud-biomass ash composite oxygen carrier and a multistage construction method and application thereof, and belongs to the technical field of solid waste resource utilization, pretreated red mud powder and biomass ash are compounded, and the biomass ash composite red mud oxygen carrier is prepared after drying and roasting; through modification of K and Ca components in the biomass ash, a high-activity phase is directionally generated in the red mud, an alkaline site is introduced, the oxygen vacancy concentration is increased, and the surface property is optimized; when the oxygen carrier is applied to a chemical looping gasification process, the release activation energy of lattice oxygen can be reduced, the reaction rate and the carbon conversion rate can be improved, the generation of CO2 can be inhibited, the selectivity and the yield of synthesis gas can be improved, and the oxygen carrier has excellent anti-sintering capability and cycling stability; a large amount of solid waste red mud and biomass ash are used as raw materials, the oxygen carrier preparation process has the characteristics of low cost, simple process and environmental friendliness, and a new way is provided for high-value utilization of the red mud and the biomass ash and industrial application of a chemical looping technology.

Description

Technical Field

[0001] This invention relates to the field of solid waste resource utilization and chemical chaining technology, specifically to a method and application for preparing a highly active and stable chemical chaining oxygen carrier using red mud and biomass ash, two major solid wastes, as raw materials. Background Technology

[0002] Red mud is a major solid waste generated during the alumina industrial production process, producing approximately 1.0-1.8 tons of red mud for every ton of alumina produced. Global annual red mud emissions are enormous, with accumulated stockpiles exceeding 4 billion tons, making the red mud stockpiling problem particularly severe. Red mud is typically highly alkaline and contains various heavy metals. Long-term open-air stockpiling not only occupies vast amounts of land resources but also poses a risk of dam failure. Its leachate can lead to serious environmental problems such as soil alkalization and groundwater pollution, urgently requiring the search for large-scale, high-value-added resource utilization methods. Furthermore, the large amounts of biomass ash (such as rice husk ash and straw ash) generated during biomass energy utilization also face disposal challenges. Biomass ash is rich in alkali metals and alkaline earth metals such as potassium and calcium; improper disposal can easily cause soil and water pollution due to the soluble alkali metals within it.

[0003] Red mud's main chemical components are Fe2O3, Al2O3, and SiO2, with a high iron content. In recent years, utilizing the abundant iron oxides in red mud as an oxygen carrier in chemical looping technology has become a promising high-value utilization pathway. Chemical looping gasification technology provides oxygen for the partial oxidation of fuel through lattice oxygen in the oxygen carrier, producing high-quality syngas in one step, offering advantages such as internal CO2 separation, low energy consumption, and high efficiency. Replacing traditional artificially synthesized oxygen carriers with red mud can achieve "waste treatment," reducing the raw material cost of oxygen carriers and achieving both environmental and economic benefits. However, the direct application of raw red mud as an oxygen carrier still faces many technical bottlenecks. First, its reactivity is low, and the release rate and depth of lattice oxygen are slow, resulting in a slow gasification reaction rate, low carbon conversion rate, and unsatisfactory syngas yield. Secondly, the inert aluminum, silicon, calcium, and sodium components in red mud readily react with the active iron phase at high temperatures to form low-melting-point inactive phases such as nepheline and calcite, which cover the active sites, leading to sintering and agglomeration of the oxygen carrier and severely impairing its cycle stability and mechanical strength. Furthermore, red mud oxygen carriers have weak control over the gasification reaction pathway, are prone to over-oxidation, generating CO2 and reducing the selectivity and quality of syngas.

[0004] To improve the performance of red mud oxygen carriers, existing research often employs metal oxide modification. Common strategies include impregnation, doping, or co-precipitation treatment of red mud with metals such as Cu, Ni, Mn, and Ce, or their oxides. While these methods can enhance reactivity to some extent, they still have significant limitations: precious metals (such as Ni and Cu) are expensive and prone to carbon deposition and poisoning; some metals (such as Mn and Cu) are easily volatilized at high temperatures or form more complex low-melting-point compounds with the red mud matrix, accelerating oxygen carrier deactivation; most modification methods focus on improving initial activity, with limited improvement on cycle stability, and it is difficult to simultaneously optimize gasification efficiency and subsequent residue resource recovery performance. In particular, there are no reports in the current technology of using biomass ash, a major solid waste, for synergistic modification of red mud oxygen carriers to achieve a synergistic improvement in cost, activity, and stability. Therefore, developing a low-cost, effective, and green modification method that can simultaneously enhance the reactivity, selectivity, and stability of red mud oxygen carriers is of great significance for promoting the high-value utilization of red mud and biomass ash and the industrial application of chemical chaining technology. To date, no published literature or patents have reported methods for modifying red mud oxygen carriers with biomass ash and successfully enhancing their reactivity, selectivity, and stability through the synergistic effect of its multiple components. Summary of the Invention

[0005] Technical problems addressed: Existing technologies for red mud oxygen carriers suffer from low reactivity, poor lattice oxygen release capacity, easy sintering and deactivation, poor syngas selectivity, and difficulty in efficiently recovering gasification residues. Furthermore, existing modification methods are costly and may introduce secondary pollution. This invention provides a highly active red mud-biomass ash composite oxygen carrier and its multi-stage construction method and application. It achieves an efficient, economical, and environmentally friendly method for modifying red mud oxygen carriers. Through biomass ash composite modification, the activation energy for lattice oxygen release from the red mud oxygen carrier is significantly reduced, increasing its reaction rate with fuel and carbon conversion efficiency. The gasification reaction pathway is controlled, effectively inhibiting excessive CO oxidation to CO2, improving syngas selectivity and total yield. The oxygen carrier's anti-sintering ability and recyclability are improved, ensuring high reactivity even after multiple oxidation-reduction cycles. Modification guides the directional phase transformation of iron in the gasification residue, enriching iron into magnetic Fe3O4, laying the phase foundation for subsequent iron separation and recovery, and realizing an integrated "gasification-resource recovery" process.

[0006] To achieve the above objectives, this application provides a highly active red mud-biomass ash composite oxygen carrier and its multi-level construction method and application. The core of this method lies in the deep activation of red mud raw materials, composite with biomass ash, structural directional control, and surface engineering through a series of precisely designed physicochemical steps, ultimately constructing a composite oxygen carrier with high activity, high selectivity, high stability, and an ideal microstructure. This is achieved through the following technical solutions: A multi-stage construction method for a highly active red mud-biomass ash composite oxygen carrier, comprising the following steps: The first step involves sequentially subjecting the red mud raw material to mechanical activation, dilute acid washing, and thermal activation: the mechanical activation employs high-energy ball milling, controlling the milling intensity and time to increase the specific surface area of ​​the red mud to 3-8 times its original value; the dilute acid washing uses a 0.1-1.0 mol / L dilute nitric acid or dilute hydrochloric acid solution, with a liquid-to-solid ratio of 3:1-8:1, a temperature of 60-90℃, and a washing time of 1-3 hours; the thermal activation involves calcining the red mud at 500-700℃ in air for 1-2 hours to obtain a dry red mud base. The second step involves physically mixing the red mud dry base obtained in the first step with biomass ash powder at a mass ratio of 100:5-100:30 to obtain a composite substrate; preparing a solution containing a structure-directing agent, wherein the structure-directing agent is a decomposable organic macromolecule or surfactant, and its amount is 1%-5% of the total mass of the composite substrate; and loading the solution containing the structure-directing agent onto the composite substrate using an equal-volume or over-volume impregnation method to obtain the impregnated and aged material. The third step involves freeze-drying or spray-drying the impregnated and aged material, followed by calcination under programmed temperature control, inert atmosphere, or oxidizing atmosphere. The calcination process is divided into three stages: the first stage involves heating at 2-5℃ / min to 300-500℃ and holding for 0.5-1 hour; the second stage involves heating at 5-10℃ / min to 800-950℃ and holding for 1-2 hours; and the third stage involves rapidly heating to 950-1100℃ under an oxidizing atmosphere and holding for 1-3 hours, ultimately forming a composite oxygen carrier with iron oxide as the main active phase, rich in alkaline active sites introduced by biomass ash, and possessing a multi-level porous structure. The fourth step involves subjecting the calcined composite oxygen carrier to short-term steam treatment at a temperature of 400-600℃ for 0.5-2 hours to further regulate its surface acidity / alkalinity and oxygen species properties, thereby obtaining a highly active red mud-biomass ash composite oxygen carrier.

[0007] Furthermore, in the first step, mechanical activation involves high-energy ball milling with a ball-to-material ratio of 8:1-15:1, a rotation speed of 300-500 rpm, and a milling time of 30-120 minutes. This refines the particles, creates lattice defects, and significantly increases the specific surface area to 3-8 times that of the original sample, thereby exposing more reactive sites. The optimal acid concentration for the dilute acid washing is 0.3-0.6 mol / L to selectively remove some soluble alkali metals such as Na and amphoteric metal oxides such as Al2O3, reducing the risk of sintering caused by soluble alkali metals and amphoteric metal oxides at high temperatures, while further increasing porosity. The thermal activation stabilizes the phase, eliminates residual volatile components, and forms a stable initial pore structure.

[0008] Furthermore, in the second step, the biomass ash is one or more of rice husk ash, straw ash, and sawdust ash, with a K2O content of not less than 5%. The introduction of biomass ash aims to utilize its rich K and Ca elements to enhance the catalytic activity of red mud and regulate the reaction pathway. The structure-directing agent is selected from one or more of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and hexadecyltrimethylammonium bromide, which can create abundant hierarchical channels during subsequent roasting. The impregnation and aging process specifically involves thorough stirring followed by static aging for 12-24 hours to ensure that the structure-directing agent is evenly distributed and penetrates deep into the pores.

[0009] Furthermore, in the third step, freeze drying or spray drying is used to avoid component migration and pore collapse caused by conventional drying, thus better preserving the precursor structure. In the first stage of calcination, low-temperature decomposition causes the structure-directing agent to decompose and create pores. In the second stage, the precursor is formed by promoting the preliminary solid-phase reaction between the active components in biomass ash and the iron components in red mud in an inert or weakly reducing atmosphere to form the precursor. The inert atmosphere for calcination is nitrogen, argon, or a weakly reducing atmosphere. The weakly reducing atmosphere is a mixture of hydrogen and nitrogen with a volume fraction of 5%-10%. This atmosphere is conducive to ion migration and solid-phase reaction, forming a non-stoichiometric active phase that is more likely to release lattice oxygen, laying the foundation for the final high-performance oxygen carrier. In the third stage, the crystal phase is reconstructed and activated. The heating rate for calcination is 10-20℃ / min, and the temperature is raised to 980-1050℃. In this stage, the precursor phase is finally transformed into a stable and highly active composite metal oxide under an oxidizing environment, and the crystal growth and stabilization are completed.

[0010] Furthermore, in the fourth step, the short-term steam treatment, with a steam partial pressure of 10%-30%, can gently hydroxylate the surface, adjust the surface acidity and alkalinity, and increase the concentration of surface active oxygen species, thereby optimizing its reaction pathway with fuel gas.

[0011] This application also discloses a highly active red mud-biomass ash composite oxygen carrier prepared by any of the above-mentioned multi-level construction methods. The phase composition of the highly active red mud-biomass ash composite oxygen carrier includes iron oxides and composite oxides or silicate phases of potassium and calcium introduced from the biomass ash. The highly active red mud-biomass ash composite oxygen carrier has a bimodal multi-level porous structure, wherein mesopores with a pore size of 2-50 nm account for 60%-85% of the total pore volume, and the BET specific surface area is 15-200 m². 2 / g.

[0012] Furthermore, the highly active red mud-biomass ash composite oxygen carrier, when characterized by H2-TPR, shows that the area of ​​its low-temperature reduction peak (<600℃) accounts for no less than 40% of the total reduction peak area; and when characterized by Raman spectroscopy, the intensity of its oxygen vacancy-related defect peak is more than 50% higher than that of the unmodified red mud oxygen carrier.

[0013] The application of a highly active red mud-biomass ash composite oxygen carrier prepared by a multi-stage construction method in a chemical looping gasification process. The highly active red mud-biomass ash composite oxygen carrier is suitable for the gasification of carbonaceous fuels, which are one or more of coal, biomass, and organic solid waste. The gasification reaction temperature is 950-1150℃.

[0014] A chemical chain gasification system, wherein the chemical chain gasification system uses a highly active red mud-biomass ash composite oxygen carrier prepared by any of the above-described multi-stage construction methods as an oxygen carrier, and the system is equipped with an online activation or regeneration unit for the highly active red mud-biomass ash composite oxygen carrier, wherein the regeneration unit can introduce air containing 5%-15% water vapor by volume to periodically treat the deactivated oxygen carrier.

[0015] This application provides a highly active red mud-biomass ash composite oxygen carrier, its multi-level construction method, and its application, which has the following beneficial effects: 1. This invention uses bulk solid waste red mud and biomass ash as the main raw materials, realizing the upgrade of "waste treatment". The raw material cost is far lower than the traditional method of using pure chemicals (such as Sr, Ni, Cu, Mn, etc.), and the environmental and economic benefits are extremely significant.

[0016] 2. Biomass ash modification is not a simple physical mixing process. Its rich K, Ca and other components interact with the red mud components during roasting. On the one hand, it acts as an excellent gasification catalyst, significantly reducing the activation energy of carbonaceous fuel gasification. On the other hand, it may form new active phases or optimize the electronic structure of the original iron oxides, introducing a large number of oxygen vacancies as fast channels for lattice oxygen migration, thereby significantly reducing the activation energy barrier for lattice oxygen release and migration. The various gasification indicators of biomass ash-modified red mud oxygen carriers are better than those of unmodified red mud, and they exhibit cost-effectiveness comparable to or even better than some expensive artificial synthetic oxygen carriers. 3. This invention, by introducing components such as Si and Ca into biomass ash, can potentially form high-melting-point silicate or aluminate phases at high temperatures. These phases act as a "skeleton," effectively preventing the migration and aggregation of active components such as sodium and iron in red mud at high temperatures. This significantly inhibits the growth, sintering, and agglomeration of active iron oxide grains at high temperatures. After five high-temperature oxidation-reduction cycles, the unmodified red mud oxygen carrier exhibits severe melting and agglomeration, with an average grain size increase exceeding 50%. In contrast, the biomass ash-modified oxygen carrier maintains a clear porous structure and its original particle morphology, with an average grain size increase of less than 8%. This structural stability directly translates into excellent cyclic reaction performance. After ten consecutive gasification-regeneration cycles, the biomass ash-modified oxygen carrier prepared by this invention can still maintain a carbon conversion rate of over 85% and a syngas selectivity of over 82%, with a performance degradation rate far lower than that of the unmodified oxygen carrier (whose carbon conversion rate drops below 65% in the fifth cycle). 4. This invention uses two types of bulk solid waste, red mud and biomass ash, as the main raw materials to replace expensive pure chemicals. The modifier used is an inexpensive and non-toxic solid waste, which completely avoids the high cost and problems of easy poisoning and carbon deposition caused by using precious metals (Ni, Cu) or other chemicals. The entire preparation process is simple, compatible with the existing catalyst industrial production process, easy to scale up and implement industrially, and its raw material cost is highly competitive. At the same time, the method does not generate secondary pollutants throughout the process. It is a high-value utilization technology path of "treating waste with waste and turning two kinds of waste into one treasure", which has significant economic and environmental benefits. Detailed Implementation

[0017] The technical solution of the present invention will now be described in detail with reference to preferred embodiments.

[0018] Example 1: A multi-stage construction method for a highly active red mud-biomass ash composite oxygen carrier, the specific steps of which are as follows: Preparation method: Red mud raw material was mechanically activated at a ball-to-material ratio of 10:1 and a rotation speed of 400 rpm for 60 minutes to increase the specific surface area from the original 15 m² / g to 60 m² / g. It was then washed for 2 hours with 0.5 mol / L dilute nitric acid at a liquid-to-solid ratio of 5:1 and a temperature of 80℃ until neutral and dried. The mixture was then calcined in air at a temperature increased to 600℃ at a rate of 5℃ / min for 1.5 hours to obtain dry red mud. The red mud raw material contained 45% Fe₂O₃, 21% Al₂O₃, 13% SiO₂, and 5% Na₂O.

[0019] Biomass ash composite and structure-oriented: 100 g of dry red mud was physically mixed with rice husk ash with a K2O content of 8.6% at a mass ratio of 100:15 to obtain a composite substrate. An aqueous solution containing 3% polyvinylpyrrolidone (PVP) was prepared, with the amount of PVP being 3% of the total mass of the composite substrate. The substrate was impregnated with the PVP by an equal volume and aged for 18 hours to obtain the impregnated and aged material.

[0020] Drying and Calcination: After freeze-drying the impregnated and aged material, a programmed calcination process was performed. The first stage involved an air atmosphere, with the temperature increased to 400℃ at a rate of 3℃ / min and held for 1 hour. The second stage involved a nitrogen atmosphere, with the temperature increased to 900℃ at a rate of 8℃ / min and held for 1.5 hours. The third stage involved an air atmosphere, with the temperature increased to 1000℃ at a rate of 15℃ / min and held for 2 hours. Subsequently, a nitrogen mixture with a water vapor partial pressure of 20% was introduced at 400℃ and treated for 1 hour. This yielded a biomass ash-modified red mud oxygen carrier, designated RM1.

[0021] Comparative Example 1: Preparation and properties of unmodified red mud oxygen carrier: Using the same red mud raw material and pretreatment steps (mechanical activation, dilute acid washing, and thermal activation) as in Example 1, a dry red mud base was obtained. Without adding biomass ash or structure directing agents, the dry red mud base was directly calcined and steam-treated under the same conditions (calcination procedure and steam treatment parameters were the same as in Example 1) to obtain an unmodified red mud oxygen carrier, labeled RM2.

[0022] Performance comparison test and results: Gasification reaction performance (fixed bed reactor, bituminous coal fuel, gasification temperature 1050℃), as shown in Tables 1 and 2: Table 1 Results of a single gasification reaction .

[0023] Table 2 Cycle stability (%) (10 gasification-regeneration cycles) .

[0024] The embodiments proposed in this invention are preferred embodiments, but are not limited to the content described above. Those skilled in the art can easily replicate the above embodiments and further extend and modify them, but as long as they do not depart from the spirit of this invention, they are all within the protection scope of this invention.

Claims

1. A multi-stage construction method for a highly active red mud-biomass ash composite oxygen carrier, characterized in that, The specific steps are as follows: The first step involves mechanically activating, washing with dilute acid, and then thermally activating the red mud raw material in sequence: the mechanical activation is achieved by high-energy ball milling, controlling the milling intensity and time to increase the specific surface area of ​​the red mud to 3-8 times its original size; the dilute acid washing is performed using a 0.1-1.0 mol / L dilute nitric acid or dilute hydrochloric acid solution, with a liquid-to-solid ratio of 3:1-8:1, a temperature of 60-90℃, and a washing time of 1-3 hours. The thermal activation is achieved by calcining red mud at 500-700℃ for 1-2 hours in air atmosphere to obtain a dry red mud base. The second step involves physically mixing the red mud dry base obtained in the first step with biomass ash powder at a mass ratio of 100:5-100:30 to obtain a composite substrate; preparing a solution containing a structure-directing agent, wherein the structure-directing agent is a decomposable organic macromolecule or surfactant, and its amount is 1%-5% of the total mass of the composite substrate; and loading the solution containing the structure-directing agent onto the composite substrate using an equal-volume or over-volume impregnation method to obtain the impregnated and aged material. The third step involves freeze-drying or spray-drying the impregnated and aged material, followed by calcination under programmed temperature control, inert atmosphere, or oxidizing atmosphere. The calcination process is divided into three stages: the first stage involves heating at 2-5℃ / min to 300-500℃ and holding for 0.5-1 hour; the second stage involves heating at 5-10℃ / min to 800-950℃ and holding for 1-2 hours; and the third stage involves rapidly heating to 950-1100℃ under an oxidizing atmosphere and holding for 1-3 hours, ultimately forming a composite oxygen carrier with iron oxide as the main active phase, rich in alkaline active sites introduced by biomass ash, and possessing a multi-level porous structure. The fourth step involves subjecting the calcined composite oxygen carrier to short-term steam treatment at a temperature of 400-600℃ for 0.5-2 hours to further regulate its surface acidity / alkalinity and oxygen species properties, thereby obtaining a highly active red mud-biomass ash composite oxygen carrier.

2. The multi-stage construction method of a highly active red mud-biomass ash composite oxygen carrier according to claim 1, characterized in that: In the first step, mechanical activation and high-energy ball milling are performed with a ball-to-material ratio of 8:1-15:1, a rotation speed of 300-500 rpm, and a milling time of 30-120 minutes, which refines the particles, creates lattice defects, and significantly increases the specific surface area to 3-8 times that of the original sample; the acid concentration for the dilute acid washing is 0.3-0.6 mol / L.

3. The multi-stage construction method of a highly active red mud-biomass ash composite oxygen carrier according to claim 1, characterized in that: In the second step, the biomass ash is one or more of rice husk ash, straw ash, and wood chip ash, and its K2O content is not less than 5% by mass; the structure guiding agent is selected from one or more of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and hexadecyltrimethylammonium bromide; the impregnation and aging process specifically involves stirring thoroughly and then allowing it to stand for aging for 12-24 hours.

4. The multi-stage construction method of a highly active red mud-biomass ash composite oxygen carrier according to claim 1, characterized in that: In the third step, freeze drying or spray drying is used; during the second stage of calcination, the active components in the biomass ash and the iron components in the red mud undergo a preliminary solid-phase reaction in an inert or weakly reducing atmosphere to form a precursor. The inert atmosphere for calcination is nitrogen, argon, or a weakly reducing atmosphere, with the weakly reducing atmosphere being a mixture of hydrogen and nitrogen with a volume fraction of 5%-10%. In the third stage, the crystal phase is reconstructed and activated, and the calcination heating rate is 10-20℃ / min, with the temperature rising to 980-1050℃.

5. The multi-stage construction method of a highly active red mud-biomass ash composite oxygen carrier according to claim 1, characterized in that: In the fourth step, the short-term steam treatment involves a steam partial pressure of 10%-30%.

6. A highly active red mud-biomass ash composite oxygen carrier prepared by any one of the multi-stage construction methods according to claims 1-5, characterized in that: The highly active red mud-biomass ash composite oxygen carrier contains iron oxides and composite oxides or silicate phases of potassium and calcium introduced from the biomass ash. The highly active red mud-biomass ash composite oxygen carrier has a bimodal hierarchical porous structure, in which mesopores with a pore size of 2-50 nm account for 60%-85% of the total pore volume, and the BET specific surface area is 15-200 m² / m³. 2 / g.

7. The highly active red mud-biomass ash composite oxygen carrier according to claim 6, characterized in that: The highly active red mud-biomass ash composite oxygen carrier was characterized by H2-TPR, and the proportion of the low-temperature reduction peak area (<600℃) to the total reduction peak area was not less than 40%. Raman spectroscopy characterization showed that the intensity of its oxygen vacancy-related defect peak was increased by more than 50% compared with that of the unmodified red mud oxygen carrier.

8. The application of a highly active red mud-biomass ash composite oxygen carrier prepared by any one of the multi-stage construction methods described in claims 1-5 in a chemical looping gasification process, characterized in that: The highly active red mud-biomass ash composite oxygen carrier is suitable for the gasification of carbonaceous fuels, which are one or more of coal, biomass, and organic solid waste. The gasification reaction temperature is 950-1150℃.

9. A chemical looping gasification system, characterized in that, The chemical looping gasification system uses a highly active red mud-biomass ash composite oxygen carrier prepared by any of the multi-stage construction methods described in claims 1-5 as an oxygen carrier. The system is equipped with an online activation or regeneration unit for the highly active red mud-biomass ash composite oxygen carrier. The regeneration unit can introduce air containing 5%-15% water vapor by volume to periodically treat the deactivated oxygen carrier.