Modified iron-based oxygen carrier as well as preparation method and application thereof

Abstract

The invention discloses a modified iron-based oxygen carrier and a preparation method and application thereof.The preparation method of the modified iron-based oxygen carrier comprises the following steps that potassium hexatitanate and iron oxide are evenly mixed according to the weight ratio of (1-4): (9-6), then the mixture is heated to 1000 DEG C to be calcined for 6 h, and the modified iron-based oxygen carrier is obtained.By means of the modified iron-based oxygen carrier and the preparation method thereof, the defects in the prior art are overcome; by adding the potassium hexatitanate, the carbon conversion rate in the direct chemical continuous combustion process of the coal can be effectively improved, and the potassium fixation capability is relatively good.

Description

Technical Field

[0001] The present invention relates to the technical field of iron-based oxygen carriers, and in particular to a modified iron-based oxygen carrier and a preparation method and application thereof. Background Art

[0002] Carbon dioxide capture, utilization and storage technology can effectively reduce carbon dioxide emissions. It purifies the carbon dioxide emitted during the production process and then puts it into a new production process for recycling, rather than simply storing it. Chemical looping combustion technology (CLC) is considered to be one of the most promising carbon capture technologies, with the advantages of high energy conversion efficiency, low CO2 capture cost and coordinated control of pollutants. Chemical looping combustion technology (CLC) includes a fuel reactor and an air reactor, which uses an oxygen carrier (OC) to transfer oxygen in the air to the fuel, avoiding direct contact between air and fuel, so that the carbon dioxide produced by combustion is not diluted by the air, and can be captured efficiently and conveniently, and then utilized or stored. In this reaction process, the oxygen carrier plays the dual role of oxygen and heat carrier, and is the key to affecting the performance of the chemical looping combustion reaction.

[0003] During operation, fuel reactors still face the issue of slow coal / coal char reaction rates. As with most solid fuel gasification research, adding catalysts can achieve higher carbon conversion rates under the same conditions. Catalysts can also significantly reduce reaction temperatures. Previous research has shown that alkali metals (such as alkali carbonates, oxides, hydroxides, and alkaline earth metals) are effective catalysts for improving carbon conversion. However, potassium migration and conversion during the reaction can lead to potassium loss in the OC, thereby reducing its reactivity. Summary of the Invention

[0004] The purpose of the present invention is to provide a modified iron-based oxygen carrier and a preparation method and application thereof, which overcome the shortcomings of the prior art.

[0005] In order to solve the above problems, the technical solutions adopted by the present invention are as follows:

[0006] The preparation method of the modified iron-based oxygen carrier comprises the following steps: mixing potassium hexatitanate and iron oxide in a weight ratio of 1-4:9-6, then heating to 1000° C. and calcining for 6 hours, and cooling to obtain the modified iron-based oxygen carrier.

[0007] Among them, the preparation method of potassium hexatitanate is: grind and mix K2CO3 and TiO2 according to the molar ratio of 1:5.5, then heat to 800℃ at a rate of 3℃ / min, keep warm and calcine for 3h, then heat to 1000℃ at a rate of 5℃ / min, and keep warm and calcine for 3h to obtain the product potassium hexatitanate.

[0008] A modified iron-based oxygen carrier is prepared by the above method.

[0009] The modified iron-based oxygen carrier described above is used to improve the carbon conversion rate during direct chemical looping combustion of coal.

[0010] Compared with the prior art, the present invention has the following advantages:

[0011] (1) The addition of potassium hexatitanate can effectively improve the carbon conversion rate during the direct chemical combustion of coal, and Fe 3+ With Ti 4+ Ion exchange produces lattice distortion, thereby generating more oxygen vacancies to further improve the reactivity of the modified iron-based oxygen carrier.

[0012] (2) After repeated use, K 2+ The content is maintained at a stable level without loss of potassium, indicating that the modified iron-based oxygen carrier of the present application has a good potassium fixing ability. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 (a) X-ray diffraction pattern and (b) scanning electron micrograph of potassium hexatitanate.

[0014] Figure 2 is the X-ray diffraction pattern of the modified iron-based oxygen carrier.

[0015] Figure 3 XPS spectrum of the modified iron-based oxygen carrier (a), XPS spectrum of Fe2p (b) and XPS spectrum of O1s (c).

[0016] Figure 4 TG curve (a) and DTG curve (b) of the modified iron-based oxygen carrier.

[0017] Figure 5 Graph showing 10 stability cycle tests on the modified iron-based oxygen carrier.

[0018] Figure 6 The potassium content (a) during the 10-cycle test of the modified iron-based oxygen carrier and the O1s change curve after the 10-cycle test of the modified iron-based oxygen carrier.

[0019] Figure 7 X-ray diffraction pattern (a) and scanning electron microscopy image (b) of the modified iron-based oxygen carrier after 10 cycles of testing.

[0020] Figure 8 The graph shows the effects of temperature (a), fuel ratio (b) and reduction degree (c) on the occurrence form of potassium.

[0021] Figure 9Schematic diagram of the structure of the experimental device for direct chemical looping combustion experiment of coal. DETAILED DESCRIPTION

[0022] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0023] Example 1

[0024] First, K2Ti6O 13 The synthesis of K2CO3 and TiO2 was carried out by mixing them in a mass ratio of 1:5.5, grinding them thoroughly and drying them. Then, the mixture was placed in a muffle furnace and heated to 800°C at a rate of 3°C / min and kept at that temperature for 3 hours. Then, the mixture was heated to 1000°C at a rate of 5°C / min and kept at that temperature for 3 hours. After cooling, potassium hexatitanate (K2Ti6O 13 ). Prepared K2Ti6O 13 The mixture was mixed with Fe2O3 in a weight ratio of 3:7 and ground in a ball mill for 20 min to fully mix. The mixed powder was then pressed into tablets and calcined in a muffle furnace at 1000 ° C for 6 h to make K2Ti6O 13 Secondary crystallization and full fusion with iron oxide. The prepared oxygen carrier was placed in a drying oven and dried for 12 hours to obtain a modified iron-based oxygen carrier (KTO3Fe7).

[0025] Example 2

[0026] First, K2Ti6O 13 The synthesis of K2CO3 and TiO2 was carried out by mixing them in a mass ratio of 1:5.5, grinding them thoroughly and drying them. Then, the mixture was placed in a muffle furnace and heated to 800°C at a rate of 3°C / min and kept at that temperature for 3 hours. Then, the mixture was heated to 1000°C at a rate of 5°C / min and kept at that temperature for 3 hours. After cooling, potassium hexatitanate (K2Ti6O 13 ). Prepared K2Ti6O 13 The mixture was mixed with Fe2O3 in a weight ratio of 4:6 and ground in a ball mill for 20 min to fully mix. The mixed powder was then pressed into tablets and calcined in a muffle furnace at 1000 ° C for 6 h to make K2Ti6O 13 Secondary crystallization and full fusion with iron oxide. The prepared oxygen carrier was placed in a drying oven and dried for 12 hours to obtain a modified iron-based oxygen carrier (KTO4Fe6).

[0027] Example 3

[0028] First, K2Ti6O 13 The synthesis of K2CO3 and TiO2 was carried out by mixing them in a mass ratio of 1:5.5, grinding them thoroughly and drying them. Then, the mixture was placed in a muffle furnace and heated to 800°C at a rate of 3°C / min and kept at that temperature for 3 hours. Then, the mixture was heated to 1000°C at a rate of 5°C / min and kept at that temperature for 3 hours. After cooling, potassium hexatitanate (K2Ti6O 13 ). Prepared K2Ti6O 13 The mixture was mixed with Fe2O3 in a weight ratio of 2:8 and ground in a ball mill for 20 min to fully mix. The mixed powder was then pressed into tablets and calcined in a muffle furnace at 1000 ° C for 6 h to make K2Ti6O 13 Secondary crystallization and full fusion with iron oxide. The prepared oxygen carrier was placed in a drying oven and dried for 12 hours to obtain a modified iron-based oxygen carrier (KTO2Fe8).

[0029] Example 4

[0030] First, K2Ti6O 13 The synthesis of K2CO3 and TiO2 was carried out by mixing them in a mass ratio of 1:5.5, grinding them thoroughly and drying them. Then, the mixture was placed in a muffle furnace and heated to 800°C at a rate of 3°C / min and kept at that temperature for 3 hours. Then, the mixture was heated to 1000°C at a rate of 5°C / min and kept at that temperature for 3 hours. After cooling, potassium hexatitanate (K2Ti6O 13 ). Prepared K2Ti6O 13 The mixture was mixed with Fe2O3 in a weight ratio of 1:9 and ground in a ball mill for 20 min to make it fully mixed. The mixed powder was then pressed into tablets and calcined in a muffle furnace at 1000 ° C for 6 h to make K2Ti6O 13 Secondary crystallization and full fusion with iron oxide. The prepared oxygen carrier was placed in a drying oven and dried for 12 hours to obtain a modified iron-based oxygen carrier (KTO1Fe9).

[0031] The prepared K2Ti6O 13 XRD and SEM characterization were performed. Figure 1 As shown in the figure, it can be seen that the main XRD crystal phase is K2Ti6O 13 It is almost consistent with the standard PDF card (PDF#74-0275), indicating that well-crystalline K2Ti6O 13 , and from SEM it can be seen that it mainly presents a whisker-like structure, which is consistent with the characteristics of potassium hexatitanate. The reaction equation is: K2CO3(S)+6TiO2(S)→K2Ti6O 13 (S) + CO2 (g). The X-ray diffraction pattern of the modified iron-based oxygen carrier is as follows Figure 2As shown, the calcined oxygen carrier contains a strong Fe2O3 peak (PDF#73-2234) with a lattice parameter of and This indicates that the iron oxide has a relatively complete crystal phase under the preparation conditions. It is worth noting that there is no K2Ti6O 13 The characteristic peaks, diffraction peaks and K 1.46 Ti 7.2 Fe 0.8 O 16 (PDF#77-0990) and a series of structurally similar substances K 1.04 Ti8O 16 , K 1.28 Ti8O 16 The fit is good; this is because Ti 4+ (0.61A) ions and Fe 3+ (0.64A) ions are similar, making Fe 3+ Substituting K2Ti6O through ion diffusion 13 TiO6 8+ Ti in the mesooctahedral framework 4+ , thus forming K 1.46 Ti 7.2 Fe 0.8 O 16 and other substances.

[0032] The near-surface element concentration and binding energy of the modified iron-based oxygen carriers are shown in Figure 2. Figure 3 shown. Figure 3 The XPS spectrum in (a) shows the photoelectron peaks of Fe2p, O1s, Ti2p, and K2p at 709.9 eV, 529.2 eV, 457.6 eV, and 292.1 eV, respectively, indicating that the preparation of oxygen carriers was successful. Figure 3 (b) shows the high-resolution XPS spectra of Fe2p for each sample. It can be seen that the surface iron elements of the fresh oxygen carrier are mainly Fe 3+ Peaks corresponding to Fe2p3 / 2 and Fe2p1 / 2 appear around 709.8eV and 723.8eV, while satellite peaks are at 718.3eV and 732.2eV. Figure 3As shown in (c), the O1s spectrum is divided into lattice oxygen (OI), oxygen vacancies (OII) and physically adsorbed oxygen (OIII) by peak fitting, where the XPS peak at 529.2eV corresponds to OI, the peak at 531eV corresponds to OII, and the peak at 533.4eV corresponds to OIII. Among them, OI accounts for the largest proportion in the prepared modified iron-based oxygen carrier, which shows that it has excellent reaction potential in chemical looping combustion technology (CLC). OII is the main indicator for evaluating the active oxygen content. It can be seen from the figure that the OII content increases with the addition of potassium hexatitanate. 1.46 Ti 7.2 Fe 0.8 O 16 When Fe 3+ Replace K2Ti6O 13 TiO6 8+ Ti in the mesooctahedral framework 4+ The distortion results in a small amount of oxygen vacancies. The oxygen vacancies can increase the lattice oxygen transfer rate and effectively improve the carbon conversion rate.

[0033] The TG and DTG curves of the modified iron-based oxygen carrier are as follows: Figure 4 As shown. Figure 4 As shown in (a), the total weight loss of the pure iron oxide oxygen carrier and the iron-based oxygen carrier with potassium hexatitanate added differed significantly, while the homemade oxygen carriers with varying amounts of potassium hexatitanate showed no significant difference. The pure iron oxide oxygen carrier exhibited the greatest weight loss of 14.4%, while the other homemade oxygen carriers with potassium hexatitanate added all lost approximately 17%. To rule out the possibility that the weight loss of the oxygen carrier was due to the properties of potassium hexatitanate itself, TG analysis was also performed on potassium hexatitanate. The results showed that the potassium hexatitanate structure was relatively stable and did not exhibit any weight loss. Figure 4 (b) The DTG curve primarily exhibits a peak at the maximum reaction rate. The temperature at which the pure iron oxide oxygen carrier reaches its maximum reaction rate is 948°C. As the addition of potassium hexatitanate increases, the temperature at which the maximum reaction rate is reached decreases to varying degrees. KTO4Fe6 exhibits the greatest decrease, reaching 880°C, a decrease of nearly 68°C. This gradual decrease in the temperature at the maximum reaction rate can be attributed to the catalytic effect of K2Ti6O13 on the Fe-O bond, which reduces the energy required for Fe-O fracture. Furthermore, as the addition of K2Ti6O13 increases, oxygen vacancies gradually increase. Furthermore, the maximum reaction rate decreases with the addition of potassium hexatitanate. This is primarily due to the fact that the ratio of oxygen carrier to coal char in the TG test is 10:1. With the addition of potassium hexatitanate, the iron oxide decreases, reducing the number of active centers and leading to a continuous decrease in the maximum reaction rate.

[0034] Cyclic isothermal CLC experiments were conducted at 900 °C to understand the effect of potassium hexatitanate on carbon conversion. The redox cycles were performed for 10 cycles and the performance of the oxygen carrier is summarized as follows: Figure 5 As shown. It can be seen that in the first redox process, the carbon conversion rate of pure iron oxide is about 20%, and the carbon conversion rate increases with the addition of potassium hexatitanate, and the carbon conversion rate can reach 50% when the addition amount is 40%. As the number of cycles increases, the carbon conversion rate of oxygen carriers with potassium hexatitanate addition amounts of 20%, 30%, and 40% shows an increasing trend, and the maximum carbon conversion rate can reach 84%. This may be caused by two aspects. On the one hand, it may be due to the Ti in potassium hexatitanate. 4+ As the ions continue to oxidize and reduce with Fe 3+ Ion exchange was performed to produce more K 1.46 Ti 7.2 Fe 0.8 O 16 , thereby creating more oxygen vacancies and increasing carbon conversion. Another possible explanation is that the octahedral tunnel structure of potassium hexatitanate is disrupted during redox reactions, releasing potassium ions. These ions enable more effective catalysis, but are also associated with higher volatility. ICP and XPS analyses of the oxygen carriers were performed. Figure 6 (a) is the K in oxygen carrier with cycle number of 0, 2, 4, 6, 8, and 10 2+ From the content distribution diagram, it can be seen that the content of potassium ions increases with the addition of potassium hexatitanate, and the ion concentration remains at a stable level with the increase in the number of cycles, without any serious volatilization, which indicates that potassium hexatitanate has a good potassium fixing ability. Figure 6 (b) shows the XPS spectrum of the oxygen carrier after 10 cycles. The peak at 531 eV corresponds to an increase in the OII content compared to the fresh oxygen carrier, reaching 29.54%, 31.75%, 33.39%, and 38.65%, respectively. This increase in oxygen vacancies and active sites leads to an increase in carbon conversion rate with continued redox reactions.

[0035] In order to understand the effects of multiple cycle experiments on the crystal phase, morphology and surface element distribution of oxygen carriers, oxygen carriers after ten cycles were used for research. Figure 7 (a) shows the addition of different proportions of K2Ti6O 13 The XRD spectrum of the oxygen carrier after ten cycles shows that the composition of the oxygen carrier before and after the cycle does not change much, and the main components are still Fe2O3 and K 1.46 Ti 7.2 Fe 0.8 O 16 But K 1.46 Ti 7.2 Fe0.8 O 16 The content of K is slightly reduced, which may be due to the fact that iron oxide is more frequently oxidized and reduced as the active center, while K 1.46 Ti 7.2 Fe 0.8 O 16 Relatively weak, thus forming a surface of iron oxide and a center of K 1.46 Ti 7.2 Fe 0.8 O 16 The package structure. Figure 7 (b) and Figure 7 (c) shows the morphological changes and elemental distribution of the fresh oxygen carrier and the oxygen carrier after ten cycles. After ten cycles, the diameter of the whisker-like structures in the oxygen carrier has significantly decreased and is interspersed between the iron oxides, preventing the oxygen carrier from experiencing severe sintering. This demonstrates that the addition of potassium hexatitanate facilitates the stability of the oxygen carrier.

[0036] Taking KTO3Fe7 as an example, the occurrence form of potassium was studied. The influence of temperature on the distribution of potassium existence form was simulated by thermodynamic simulation software (Factsage). Figure 8 (a) shows that below 570°C, potassium exists primarily as K2CO3, with virtually no gaseous potassium produced. Between 780 and 1100°C, it transforms into K2Ti2O5, and the gaseous potassium content gradually increases with temperature, reaching 0.0035% at 1100°C, though this amount remains at a trace level.

[0037] Effect of the ratio of fuel to modified iron-based oxygen carrier on the form of potassium generation Figure 8 (b). During the calculation process, the effect of the mass ratio of Fe2O3 to C on the form of potassium was studied when a fixed modified iron-based oxygen carrier was input. Between Fe2O3:C=1-5, it mainly exists in the form of potassium hexatitanate. As the Fe2O3 / C ratio increases, potassium hexatitanate gradually transforms into K2Ti3O7, and is completely converted when Fe2O3:C=7. Thereafter, as the Fe2O3 / C ratio increases, K2Ti3O7 transforms into K2Ti2O5, and no longer changes with the increase in the Fe2O3 / C ratio. Throughout the process, potassium volatilization remains in a relatively low range (<0.0003).

[0038] Effect of the reduction degree of modified iron-based oxygen carrier on the appearance of potassium Figure 8 (c) When the reduction degree is less than 11.11% (the oxygen carriers are mainly Fe2O3 and Fe3O4), almost all potassium exists in the form of potassium hexatitanate. When the reduction degree increases, FeO appears and K2Ti6O 13It begins to transform into K2Ti3O7, and then gradually decreases to produce K2Ti2O5 after the reduction degree reaches 15%. After that, K2Ti2O5 remains almost stable with the increase of reduction degree until the reduction degree reaches 60%. In the stable range of K2Ti2O5, a small amount of K volatilizes (<0.02%). After that, K2Ti2O5 gradually decreases, while K2Ti3O7 and K2Ti6O 13 The content of K2Ti3O7 gradually increases until the reduction degree reaches 80% and reaches the maximum value, but K2Ti6O 13 The content continues to increase gradually until the reduction degree reaches 100%.

[0039] The direct chemical looping combustion experiment of coal was carried out in a fixed bed reactor. Figure 9 The experimental setup consists of a fixed-bed reactor, a temperature controller, a mass flow controller, and a gas chromatograph. During the experiment, the sample was placed in a crucible, and its position within the reaction tube was adjusted by moving an external magnetic ring to eliminate air interference and allow for rapid heating and cooling of the sample. The ratio of Fe₂O₃ to coal char in the oxygen carrier represents the ideal ratio for complete reduction of Fe₂O₃ to Fe by carbon in the coal char. First, a reduction reaction was performed. A cylindrical corundum crucible loaded with 2.5 g of the modified iron-based oxygen carrier sample was placed in a cooling zone, and the fixed-bed reactor was heated to 900°C at 15°C / min. During the reduction step, N₂ was introduced at a flow rate of 1000 mL / min for 10 minutes to exclude air. After heating to 900°C, the N₂ flow rate was reduced to 300 mL / min, and the corundum crucible was rapidly lowered to the constant temperature zone. After a 30-minute reaction, the crucible was returned to the cooling zone. Gases from the reactor during the reduction reaction were analyzed by gas chromatograph. During the oxidation step, air was introduced at a flow rate of 500 mL / min for 30 minutes to completely reoxidize the oxygen carrier. The oxygen carrier then underwent 10 redox reactions in the fixed-bed reactor to test the stability of the direct chemical looping combustion of coal. After an even number of redox reactions, 0.5 g of the modified iron-based oxygen carrier was sampled for potassium content measurement via digestion combined with inductively coupled plasma emission spectrometry. The digestion process employed a stepwise microwave digestion method: 0.05 g of the modified iron-based oxygen carrier was weighed, added to 10 ml of concentrated HNO₃, and microwave-digested at 140°C for 30 minutes. Then, 2 ml of HF was added and microwave-digested at 160°C for 40 minutes. Finally, 30 ml of a 4% H₃BO₃ solution was added and microwave-digested at 130°C for 20 minutes.

[0040] In addition, the test conditions of the present invention are described as follows: the scanning range of the X-ray diffractometer (XRD) is 10-80° and the scanning speed is 2° / min. 13The surface morphology was observed by field emission scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was used to obtain the chemical state of the oxygen carrier surface. Thermochemical behavior analysis was performed using a thermogravimetric analyzer (TG / DTG). The modified iron-based oxygen carrier and the coal sample were evenly mixed in a ratio of 10:1. About 10 mg of the sample was placed in the crucible of the thermogravimetric analyzer. Under a N2 atmosphere, the temperature was raised to 1000°C at a heating rate of 20°C / min. The weight loss of the sample was measured and recorded during the whole process.

[0041] The carbon conversion rate is determined as follows: The product gases in the experiment include CO, CO2, and N2. The content of other gases is quite small and can be ignored. The gas concentration during the reaction is determined by the nitrogen conservation method. The carbon conversion rate is calculated as follows:

[0042]

[0043] Wherein, X is carbon conversion rate (%); and are the concentrations of CO and CO2 in the product gas (%); m C，char is the mass of element C in coal (g); is the molar flow rate of nitrogen (mol / min).

[0044] When calculating the temperature and reduction depth of the modified iron-based oxygen carrier, the amount of carbon is the amount required for complete reduction of the iron oxide. The variables considered in the chemical thermodynamic calculations include the reaction temperature, the C / Fe2O3 molar ratio, and the degree of reduction of the iron oxide. The formula for calculating the degree of reduction of the iron oxide is as follows:

[0045]

[0046] in is the amount of O element in the iron oxide compound in the initial state, It is the amount of O element in the iron oxide compound after reduction.

[0047] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a modified iron-based oxygen carrier, characterized in that: The method comprises the following steps: mixing potassium hexatitanate and iron oxide uniformly in a weight ratio of 1-4:9-6, then heating to 1000° C. and calcining for 6 hours, and cooling to obtain a modified iron-based oxygen carrier.

2. The preparation method according to claim 1, characterized in that The preparation method of potassium hexatitanate is as follows: K2CO3 and TiO2 are ground and mixed according to the molar ratio of 1:5.5, then heated to 800℃ at a rate of 3℃ / min, kept warm and calcined for 3 hours, and then heated to 1000℃ at a rate of 5℃ / min, and kept warm and calcined for 3 hours to obtain the product potassium hexatitanate.

3. A modified iron-based oxygen carrier, characterized in that Prepared by the method of claim 1 or 2.

4. The modified iron-based oxygen carrier according to claim 3, characterized in that Used to improve the carbon conversion rate during direct chemical looping combustion of coal.

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