Preparation method of oxygen vacancy-adjustable calcium-iron spinel oxygen carrier and application of oxygen vacancy-adjustable calcium-iron spinel oxygen carrier in sulfur-containing fuel conversion

Abstract

The invention discloses a preparation method of an oxygen vacancy-adjustable calcium-iron spinel oxygen carrier and application of the oxygen vacancy-adjustable calcium-iron spinel oxygen carrier in sulfur-containing fuel conversion, and belongs to the technical field of energy utilization, green conversion and treatment of sulfur-containing pollutants in the chemical looping combustion process. A CaFe2O4 spinel structure is formed through Ca element doping, and the CaFe2O4 spinel structure is synthesized at different temperatures so as to regulate and control the oxygen vacancy concentration of the CaFe2O4 spinel structure. According to the CaFe2O4 spinel oxygen carrier, by reducing the oxygen vacancy concentration, the lattice oxygen proportion can be increased at the same time so as to enhance the CH4 conversion activity, and the oxygen vacancy is reduced so as to inhibit H2S deposition and sulfur poisoning tendency. The calcium-iron spinel oxygen carrier has high reaction activity, excellent sulfur resistance and stability, can realize efficient conversion of sulfur-containing fuel and CO2 capture, can solve the problems of easy sulfur poisoning, poor reactivity and the like of a common metal oxide oxygen carrier, is suitable for the chemical looping combustion (CLC) process of fuels with different sulfur contents, and has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of energy efficiency utilization and green conversion and the treatment of sulfur-containing pollutants in chemical looping combustion. Specifically, it relates to a method for preparing a calcium-iron spinel oxygen carrier with adjustable oxygen vacancies, and also relates to the application of this oxygen carrier in the chemical looping combustion process of sulfur-containing fuel conversion. Background Technology

[0002] The massive emissions of greenhouse gases (such as CO2, CH4, N2O, and SF6) generated during the use of fossil fuels like coal, oil, and natural gas have led to the greenhouse effect and global warming, drawing increasing global attention. CO2 contributes the most to global warming, exceeding 60%. In recent years, energy-related CO2 emissions have increased rather than decreased, contradicting goals of mitigating global warming. Therefore, effectively controlling, storing, or capturing CO2 emissions is a crucial measure for addressing climate change.

[0003] Chemical Looping Combustion (CLC) is a highly efficient fossil fuel conversion technology that separates and captures CO2 while achieving efficient fuel utilization. The key to this technology is the selection of the oxygen carrier (OC), which transfers lattice oxygen and heat in the fuel reactor (FR) and air reactor (AR), maintaining the sustainability of the conversion. However, gaseous fuels, primarily natural gas, and solid fuels, primarily medium- and high-sulfur coal, release hydrogen sulfide (H2S) during combustion. This poses hazards to human health and the environment, and can corrode production equipment, reducing its lifespan. Furthermore, H2S competes with both the fuel and the oxygen carrier. The reaction of H2S with the oxygen carrier to form metal sulfides can lead to sulfur poisoning, reducing the reactivity of the oxygen carrier. H2S also occupies active sites in the oxygen carrier, weakening its selectivity for the main fuel and reducing combustion efficiency. Therefore, developing oxygen carrier materials that combine high activity and fuel selectivity in sulfur-rich environments is a key scientific issue for the green and healthy development of chemical looping combustion technology that enables efficient CO2 capture and conversion of sulfur-containing fuels.

[0004] The reaction of single-metal oxides with sulfur-containing fuels carries a significant risk of forming metal sulfides and poisoning the oxygen carrier. Compared to single-metal oxides such as iron, manganese, nickel, copper, and cobalt, multi-metallic iron spinel systems, with their multi-cation vacancy structure, allow multiple metal elements to leverage their respective strengths within a single crystal structure, compensating for their individual weaknesses. Their chemical chaining reactivity and cycling stability are superior to those of single-metal oxide oxygen carriers. Furthermore, studies have found that iron spinel formed from alkaline earth metal oxides such as Mg, Ca, and Ba with Fe₂O₃ exhibits better chemical chaining reactivity than transition metal iron spinel. Moreover, alkaline earth metal oxides (especially CaO) show advantages in terms of reserves and price compared to transition metal oxides. However, the application of calcium-iron spinel-type ferrites as multifunctional oxygen carriers with both high catalytic activity and sulfur tolerance in the chemical chaining combustion of CH₄ / H₂S mixed fuels remains to be studied.

[0005] Therefore, developing spinel-type oxygen carrier materials with adjustable oxygen vacancies suitable for chemical looping combustion of sulfur-containing fuels has become a key technical challenge that urgently needs to be overcome in this field. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes a method for preparing a calcium-iron spinel oxygen carrier with tunable oxygen vacancies and its application in the conversion of sulfur-containing fuels, solving or at least partially solving the problems existing in current applications. This invention synthesizes calcium-iron spinel oxygen carriers with different oxygen vacancy concentrations by equimolarly doping Ca into a spinel-type metallic iron-based oxygen carrier at different temperatures, thereby simultaneously enabling catalytic oxidation and sulfur resistance. Furthermore, the superior reactivity and cycle stability of the spinel-type oxygen carrier allow for efficient fuel conversion in chemical looping combustion (CLC).

[0007] To achieve the above objectives, according to one aspect of the present invention, a method for preparing an oxygen-vacancy-tunable calcium-iron spinel oxygen carrier is provided, the method comprising the following steps: (1) Mix calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and ferric nitrate nonahydrate (Fe(NO3)3·9H2O) in a molar ratio of 1:2, then add deionized water according to the solid-liquid ratio, stir to dissolve and obtain a mixed solution; (2) Under continuous stirring, ammonia solution was added dropwise to the resulting mixed solution to adjust the pH, and a brown precipitate was formed. Then the system temperature was raised to 80°C and stirred continuously until the material was converted into a brown solid. (3) The obtained brown solid was placed in an oven and dried to obtain a calcium-iron compound precursor; (4) The precursor is placed in a muffle furnace for full roasting, and after natural cooling, it is ground to obtain the calcium iron spinel oxygen carrier.

[0008] As a further explanation of the present invention, in step (1), the solid-liquid ratio is 0.2 g / mL.

[0009] As a further explanation of the present invention, in step (2), the pH is 9.

[0010] As a further explanation of the present invention, in step (3), the drying temperature is 105°C and the drying time is 12h.

[0011] As a further explanation of the present invention, in step (4), the calcination temperature is 850~1000℃, the heating rate is 10℃ / min, the calcination time is 1.42h-1.67h, and the holding time is 5h.

[0012] According to another aspect of the present invention, an oxygen vacancy-tunable calcium-iron spinel oxygen carrier is provided, which is prepared according to the above-described preparation method.

[0013] According to another aspect of the present invention, an application of an oxygen-vacancy-tunable calcium-iron spinel oxygen carrier is provided, the application being an application based on chemical looping combustion (CLC) technology to achieve efficient conversion using sulfur-containing gas as fuel, wherein the sulfur-containing gas is a mixture of 0.5% H2S, 5% CH4, and 94.5% N2; the efficient conversion refers to a CH4 conversion rate of over 70%.

[0014] As a further explanation of the present invention, the specific method of applying the above technical solution includes the following steps: (1) Fill the quartz partition plate of the vertical furnace quartz tube reactor with oxygen vacancy adjustable calcium iron spinel oxygen carrier, seal the reaction system, connect the gas circuit system and perform air tightness test; control the flow of nitrogen to purge the reaction system through the flow meter; set the target reaction temperature and reaction time through the K-type thermocouple temperature controller. (2) Start the fixed bed reactor. After the reactor is heated to the set temperature and stabilized, switch the inlet gas to sulfur-containing gaseous fuel and keep the flow rate constant. Connect a gas bag to the tail gas outlet of the fixed bed and continuously collect the tail gas generated by the reaction for 20 minutes. Quantitatively detect the concentration of CH4 in it and calculate the CH4 conversion rate based on the initial CH4 content. (3) After the reaction is complete, close the gas inlet valve; after the reactor cools to room temperature, remove the oxygen carrier particles on the quartz partition and calculate the amount of hydrogen sulfide deposited in the oxygen carrier.

[0015] As a further explanation of the present invention, in step (1), the purity of nitrogen is 99.9% and the flow rate is 100 mL / min.

[0016] As a further explanation of the present invention, in step (1), the amount of oxygen carrier particles is determined by calculation based on the oxygen supply coefficient Φ=1, the set heating rate is 10℃ / min, the target reaction temperature is 900℃, and the reaction time is 60min.

[0017] As a further explanation of the present invention, in step (2), the sulfur-containing gaseous fuel is a mixture of 0.5% H2S, 5% CH4 and 94.5% N2, with a constant flow rate of 100 mL / min.

[0018] As a further illustration of the present invention, step (2) is carried out in a fuel reactor (FR) consisting of a vertical furnace quartz tube reactor quartz partition.

[0019] Advantages of this invention: (1) The oxygen vacancy adjustable calcium iron spinel oxygen carrier of the present invention has stable physicochemical properties, rich lattice oxygen concentration, high oxygen mobility, and simple preparation process.

[0020] (2) The present invention provides a calcium-iron spinel oxygen carrier with adjustable oxygen vacancy, which has different oxygen vacancy concentrations at different synthesis temperatures. The presence of oxygen vacancy causes changes in the local electronic structure of the oxygen carrier, thereby changing the adsorption strength of gas molecules. The high synthesis temperature promotes the release of lattice oxygen, which will help improve fuel conversion.

[0021] (3) The present invention provides an oxygen vacancy-tunable calcium-iron spinel oxygen carrier. By adjusting its oxygen vacancy concentration, it can achieve differentiated sulfur resistance, thereby effectively suppressing oxygen carrier poisoning and deactivation caused by the formation of metal sulfides. This oxygen carrier exhibits excellent performance in terms of fuel conversion efficiency and stability, indicating that it has good application potential and development prospects in chemical looping combustion (CLC) technology. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a calcium-iron spinel oxygen carrier with adjustable oxygen vacancy, applying the present invention, during the chemical looping combustion process. Figure 2 These are the XRD patterns of the calcium-iron spinel oxygen carriers obtained in Examples 1-3; Figure 3 These are XPS test images of the calcium-iron spinel oxygen carriers obtained in Examples 1-3. Detailed Implementation

[0023] The study focuses on CaFe2O4 oxygen carrier with a spinel structure, which has significant advantages in terms of oxygen carrying capacity, reaction rate, and stability. Therefore, its application in chemical looping combustion (CLC) of sulfur-containing gaseous fuels is also the focus of this invention.

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0025] The equipment and raw materials used in this invention are all commercially available or commonly used in the field. Unless otherwise specified, the methods in the following embodiments are conventional methods in the field.

[0026] Example 1 An oxygen-vacancy-tunable calcium-iron spinel oxygen carrier of this embodiment is prepared by the following method: Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and ferric nitrate nonahydrate (Fe(NO3)3·9H2O) were mixed in a molar ratio of 1:2. Deionized water was then added at a solid-liquid ratio of 0.2 g / mL, and the mixture was stirred to dissolve and obtain a mixed solution. Ammonia solution was added dropwise to adjust the pH of the mixed solution to 9 and form a brown precipitate. The precipitate was stirred and evaporated at 80℃ until it became a brown solid. The precipitate was then dried in an oven at 105℃ for 12 h. The dried sample was placed in a muffle furnace and calcined at 850℃ for 1.42 h, followed by holding at that temperature for 5 h. After grinding, the calcium-iron spinel oxygen carrier was obtained and designated as 850CaFe2O4.

[0027] Example 2

[0028] An oxygen-vacancy-tunable calcium-iron spinel oxygen carrier of this embodiment is prepared by the following method: Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and ferric nitrate nonahydrate (Fe(NO3)3·9H2O) were mixed in a molar ratio of 1:2. Deionized water was then added at a solid-liquid ratio of 0.2 g / mL, and the mixture was stirred to dissolve and obtain a mixed solution. Ammonia solution was added dropwise to adjust the pH of the mixed solution to 9 and form a brown precipitate. The precipitate was stirred and evaporated at 80℃ until it became a brown solid. The precipitate was then dried in an oven at 105℃ for 12 h. The dried sample was placed in a muffle furnace and calcined at 950℃ for 1.58 h, followed by holding at that temperature for 5 h. After grinding, the calcium-iron spinel oxygen carrier was obtained and designated as 950CaFe2O4.

[0029] Example 3 An oxygen-vacancy-tunable calcium-iron spinel oxygen carrier of this embodiment is prepared by the following method: Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and ferric nitrate nonahydrate (Fe(NO3)3·9H2O) were mixed in a molar ratio of 1:2. Deionized water was then added at a solid-liquid ratio of 0.2 g / mL, and the mixture was stirred to dissolve and obtain a mixed solution. Ammonia solution was added dropwise to adjust the pH of the mixed solution to 9 and form a brown precipitate. The precipitate was stirred and evaporated at 80°C until it became a brown solid. The precipitate was then dried in an oven at 105°C for 12 h. The dried sample was placed in a muffle furnace and calcined at 1000°C for 1.67 h, followed by holding at 1000°C for 5 h. After grinding, the calcium-iron spinel oxygen carrier was obtained and designated as 1000CaFe2O4.

[0030] The test results are analyzed as follows: XRD testing was performed on the oxygen carriers prepared in Examples 1-3. The test results are as follows: Figure 2 As shown.

[0031] (1) such as Figure 2 As shown, all synthesized spinel samples belong to the orthorhombic crystal system with space group Pnam. Their X-ray diffraction patterns are consistent with the corresponding standard cards (PDFs) obtained from the Inorganic Crystal Structure Database (ICSD), and no impurity phase diffraction peaks were observed. This indicates that a well-crystallized calcium-iron spinel oxygen carrier was successfully prepared by the co-precipitation method. In particular, the diffraction peaks of the samples obtained by calcination at 950℃ and 1000℃ are sharp, and the patterns are in excellent agreement with the standard cards, indicating that they have high phase purity.

[0032] XPS testing was performed on the oxygen carriers prepared in Examples 1-3. The test results are as follows: Figure 3 As shown.

[0033] (1) Chemical adsorption of oxygen (O) γ Generally composed of OH - O - O 2- Composed of components such as oxygen, it can completely oxidize fuel to produce CO2 and H2O. (The text also mentions lattice oxygen (O2)...) α Oxygen vacancies (O2) typically have low activity and tend to partially oxidize fuels to CO and H2. β Oxygen ions (OIs) are channels for oxygen ion migration and active sites for reactions. Their concentration and formation can directly affect the reaction kinetics and oxygen transport capacity of oxygen carriers.

[0034] (2) Based on XPS analysis results Figure 3 As shown, the relative contents of different oxygen species in 850CaFe2O4, 950CaFe2O4, and 1000CaFe2O4 differ to some extent. Among them, adsorbed oxygen (O γ The proportions were 15.20%, 14.49%, and 12.16%, respectively; lattice oxygen (O) αThe percentages were 44.91%, 44.17%, and 40.60%, respectively; oxygen vacancies (O2) β The proportions were 39.89%, 41.34%, and 47.24%, respectively. These results indicate that all three types of calcium-iron spinel oxygen carriers possess relatively abundant oxygen vacancy concentrations, especially the samples calcined at 1000℃. As active sites for lattice oxygen migration, the high proportion of oxygen vacancies significantly enhances the oxygen mobility and reactivity of the oxygen carrier; therefore, it can be concluded that these calcium-iron spinel oxygen carriers all possess excellent oxygen transport capabilities.

[0035] Application Examples A schematic diagram illustrating the application of the present invention's oxygen-vacancy-tunable calcium-iron spinel oxygen carrier in the chemical looping combustion of sulfur-containing fuels is shown below. Figure 1 .

[0036] Application Example 1 (1) Fill the 850CaFe2O4 spinel oxygen carrier particles onto the quartz partition of the vertical furnace quartz tube reactor, seal the reaction system, connect the gas path system and perform airtightness test; control the flow of nitrogen to purge the reaction system through the flow meter; set the target reaction temperature (900℃) and reaction time (60min) through the K-type thermocouple temperature controller.

[0037] (2) Start the fixed bed reactor. After the reactor is heated to the set temperature and stabilized, switch the inlet gas to sulfur-containing gaseous fuel (0.5% H2S, 5% CH4, 94.5% N2) and keep the flow rate constant (100 mL / min). Connect a gas bag to the tail gas outlet of the fixed bed and continuously collect the tail gas generated by the reaction for 20 min. Quantitatively detect the concentration of CH4 in it and calculate the CH4 conversion rate based on the initial CH4 content.

[0038] (3) After the reaction is complete, close the gas inlet valve; after the reactor cools to room temperature, remove the oxygen carrier particles on the quartz partition and calculate the amount of hydrogen sulfide deposited in the oxygen carrier.

[0039] Application Example 2 The difference between this application example and application example 1 is that the oxygen carrier is different; the oxygen carrier is 950CaFe2O4 prepared in example 2.

[0040] Application Example 3 The difference between this application example and application example 1 is that the oxygen carrier is different; the oxygen carrier is 1000CaFe2O4 prepared in example 3.

[0041] Application Example 4 The difference between this application example and application example 1 is that the fuel composition is different; the fuel composition is 5% CH4 and 95% N2.

[0042] Application Example 5 The difference between this application example and application example 2 is that the fuel composition is different; the fuel composition is 5% CH4 and 95% N2.

[0043] Application Example 6 The difference between this application example and application example 3 is that the fuel composition is different; the fuel composition is 5% CH4 and 95% N2.

[0044] After the test reaction was completed, the CH4 conversion rate and H2S deposition amount are summarized in Table 1.

[0045] Table 1 project <![CDATA[H2S deposition amount (mg / g)]]> <![CDATA[CH4 conversion rate %]]> Example 1 7 77.2 Example 2 15 72.6 Example 3 17 72.5 Example 4 - 83.1 Example 5 - 82.4 Example 6 - 80.7 Discussion and Conclusion (1) After preparing spinel oxygen carriers by co-precipitation and characterizing them, the 950CaFe2O4 and 1000CaFe2O4 spinel oxygen carriers exhibited high crystallinity and purity. The high lattice oxygen and oxygen vacancy content of the calcium-iron spinel oxygen carriers indicates that the oxygen carriers tend to undergo a more complete redox reaction with the fuel during chemical looping combustion.

[0046] (2) Under the conditions of 0.5% H2S, 5% CH4, and 94.5% N2, the H2S deposition amounts of the three oxygen carriers were 7 mg / g, 15 mg / g, and 17 mg / g, respectively. All three spinel oxygen carriers exhibited excellent fuel conversion efficiencies, reaching 77.2%, 72.6%, and 72.5%, respectively. This difference in conversion may be due to the different adsorption strengths of H2S by different oxygen vacancy concentrations. Higher surface adsorption oxygen on the oxygen carrier facilitates fuel conversion, while higher oxygen vacancy concentrations tend to combine with H2S to form metal sulfides, inhibiting oxygen migration and thus reducing conversion. In addition, the conversion rates of the three oxygen carriers under the conditions of 5% CH4 and 95% N2 reached 83.1%, 82.4%, and 80.7%, respectively, indicating that sulfides have a certain inhibitory effect on the performance of the oxygen carrier. 850CaFe2O4 can still maintain a good CH4 conversion effect.

[0047] In summary, by reducing the oxygen vacancy concentration of the calcium-iron spinel oxygen carrier, the dual effects of simultaneously increasing the lattice oxygen content to enhance reactivity and reducing the oxygen vacancy content to suppress sulfide deposition can be achieved. Accordingly, by controlling the synthesis temperature, a series of oxygen carriers with different oxygen vacancy concentrations and thus differentiated sulfur resistance and reactivity can be obtained to meet the specific performance requirements of oxygen carriers in chemical looping combustion processes of fuels with different sulfur contents. Therefore, this technology has promising development prospects in the field of chemical looping combustion (CLC) and is expected to stand out from other chemical looping technologies.

Claims

1. A method for preparing a calcium-iron spinel oxygen carrier with tunable oxygen vacancies, characterized in that: Includes the following steps: (1) Mix calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and ferric nitrate nonahydrate (Fe(NO3)3·9H2O) in a molar ratio of 1:2, then add deionized water according to the solid-liquid ratio, stir to dissolve and obtain a mixed solution; (2) Under continuous stirring, ammonia solution was added dropwise to the resulting mixed solution to adjust the pH, and a brown precipitate was formed. Then the system temperature was raised to 80°C and stirred continuously until the material was converted into a brown solid. (3) The obtained brown solid was placed in an oven and dried to obtain a calcium-iron compound precursor; (4) The precursor is placed in a muffle furnace for full roasting, and after natural cooling, it is ground to obtain the calcium iron spinel oxygen carrier.

2. The method for preparing the oxygen-vacancy-tunable calcium-iron spinel oxygen carrier according to claim 1, characterized in that: In step (1), the solid-liquid ratio is 0.2 g / mL.

3. The method for preparing the oxygen-vacancy-tunable calcium-iron spinel oxygen carrier according to claim 1, characterized in that: In step (2), the pH of the solution is adjusted to 9.

4. The method for preparing the oxygen-vacancy-tunable calcium-iron spinel oxygen carrier according to claim 1, characterized in that: In step (3), the drying temperature is 105°C and the drying time is 12 hours.

5. The method for preparing the oxygen-vacancy-tunable calcium-iron spinel oxygen carrier according to claim 1, characterized in that: In step (4), the calcination parameters are set as follows: calcination temperature 850~1000℃, heating rate 10℃ / min, calcination time 1.42~1.67h, and holding time 5h; the grinding is dry grinding, and the residue on a 100-mesh sieve is 1%.

6. A calcium-iron spinel oxygen carrier with tunable oxygen vacancies, characterized in that, It is prepared by the preparation method described in any one of claims 1-5.

7. The application of the oxygen-vacancy-tunable calcium-iron spinel oxygen carrier as described in claim 6 as a catalyst in the chemical looping combustion process of sulfur-containing gaseous fuel conversion.

8. The application of the oxygen-vacancy-tunable calcium-iron spinel oxygen carrier as described in claim 7 as a catalyst in the chemical looping combustion process of sulfur-containing gaseous fuel conversion, characterized in that: The specific method for applying the method includes the following steps: (1) Fill the quartz partition plate of the vertical furnace quartz tube reactor with oxygen vacancy adjustable calcium iron spinel oxygen carrier, seal the reaction system, connect the gas circuit system and perform air tightness test; control the flow of nitrogen to purge the reaction system through the flow meter; set the target reaction temperature and reaction time through the K-type thermocouple temperature controller. (2) Start the vertical furnace reactor. After the reactor is heated to the set temperature and stabilized, switch the gas inlet to sulfur-containing gaseous fuel and keep the flow rate constant. Connect a gas bag to the tail gas outlet of the fixed bed and continuously collect the tail gas generated by the reaction for 20 minutes. Quantitatively detect the concentration of CH4 in it and calculate the CH4 conversion rate based on the initial CH4 content. (3) After the reaction is complete, close the gas inlet valve; after the reactor cools to room temperature, remove the oxygen carrier particles on the quartz partition and calculate the amount of hydrogen sulfide deposited in the oxygen carrier.

9. The application according to claim 8, characterized in that: The loading amount of the oxygen-vacancy-adjustable calcium-iron spinel oxygen carrier mentioned in step (1) is determined by calculating the oxygen supply coefficient Φ=1.

10. The application according to claim 8, characterized in that: In step (2), the target temperature is set to 900℃, the heating rate is 10℃ / min, and the reaction time is 60min.

Images