Method for removing carbon dioxide from biogas by calcined magnesium-aluminum hydrotalcite

Abstract

This invention belongs to the field of pollution control technology and discloses a method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite. The method includes the following steps: calcining the magnesium aluminum hydrotalcite, grinding, and sieving it to obtain calcined magnesium aluminum hydrotalcite; placing the calcined magnesium aluminum hydrotalcite and biogas in a container; and placing the container under constant temperature conditions to adsorb carbon dioxide from the biogas. This invention utilizes the high specific surface area, pore volume, and pore size of calcined hydrotalcite to efficiently remove CO2 gas from anaerobic digestion biogas. Furthermore, the calcined magnesium aluminum hydrotalcite does not cause secondary pollution during the adsorption process, exhibits mild reaction conditions, good adsorption effect, high CH4 recovery rate, good adsorbent regeneration performance, and can be reused, making it environmentally friendly.

Description

Technical Field

[0001] This invention relates to the field of pollution control technology, and in particular to a method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite. Background Technology

[0002] Anaerobic digestion technology can degrade organic matter in solid waste and kill pathogens. Simultaneously, the resulting biogas can be used in biogas boilers, biogas power generation, and other fields, thus meeting the needs for reduction, stabilization, harmlessness, and resource recovery of solid waste. However, biogas typically contains 50–70% CH4 and 25–45% CO2. This CO2 can reduce the calorific value of biogas and cause pipeline corrosion, thereby limiting the application scope of biogas.

[0003] To remove CO2 from anaerobic digestion biogas, common methods include high-pressure water washing, physical absorption, chemical adsorption, membrane separation, cryogenic methods, and pressure swing adsorption. However, these methods have significant drawbacks, such as high operating pressure, high cost, the need to add chemical agents, and the potential for secondary pollution.

[0004] Magnesium aluminum hydrotalcite (Mg / Al LDHs) is one of the most common layered metal hydroxides, possessing advantages such as mature preparation methods, simple preparation steps, wide application, stable properties, and no secondary pollution. After high-temperature calcination modification, the specific surface area, pore volume, and pore size of Mg / Al LDHs increase, as does the surface alkali potential, which is of great significance for the utilization of biogas resources.

[0005] However, there are currently no reports on suitable conditions for magnesium aluminum hydrotalcite to adsorb CO2 from anaerobic digestion biogas, and the CO2 adsorption capacity and CH4 recovery rate need to be studied, and the adsorption mechanism needs to be clarified. Summary of the Invention

[0006] The purpose of this invention is to provide a method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, and to determine the suitable conditions and adsorption capacity of magnesium aluminum hydrotalcite for adsorbing CO2 from anaerobic digestion biogas.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0008] This invention provides a method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, comprising the following steps:

[0009] (1) After calcination, magnesium aluminum hydrotalcite is ground and sieved to obtain calcined magnesium aluminum hydrotalcite.

[0010] (2) Roasted magnesium aluminum hydrotalcite and biogas are placed in a container and the container containing roasted magnesium aluminum hydrotalcite and biogas is placed under constant temperature conditions to adsorb carbon dioxide in biogas.

[0011] Furthermore, in the method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, in step (1), the calcination is carried out in an air atmosphere; the calcination temperature is 200-650°C, and the calcination time is 3-5 hours.

[0012] Furthermore, in the method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, in step (1), the sieving is performed through an 80-300 mesh sieve.

[0013] Furthermore, in the method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, in step (2), the mass-to-volume ratio of the calcined magnesium aluminum hydrotalcite to the biogas is 0.008–0.016 g: 1 mL; and the constant temperature condition is 25–55 °C.

[0014] Furthermore, in the method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, in step (2), before the biogas is loaded into the container, the container is further evacuated to a relative pressure of <-45 kPa.

[0015] Furthermore, in the method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, in step (2), the adsorption is carried out in a constant temperature shaker with a rotation speed of 40-50 rpm; the adsorption time is 3-5 hours.

[0016] Furthermore, in the method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, step (1) involves the following steps in the preparation of the magnesium aluminum hydrotalcite:

[0017] Prepare a salt solution of magnesium nitrate and aluminum nitrate, and an alkaline solution of sodium hydroxide and sodium carbonate. Add the salt solution and alkaline solution dropwise to water simultaneously while stirring, and control the pH of the mixture to 9.5–10.5. After the addition is complete, continue stirring for 0.5–3 hours. Then, age the mixture at 60–65°C for 20–24 hours, wash it 7–10 times with water under vacuum filtration, and dry the resulting filter cake at 80–105°C to obtain magnesium aluminum hydrotalcite.

[0018] Furthermore, in the method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, the concentration of magnesium nitrate in the salt solution is 1-2 mol / L, and the molar ratio of magnesium in magnesium nitrate to aluminum in aluminum nitrate is 2:1-4:1.

[0019] Furthermore, in the method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, the concentration of sodium hydroxide in the alkaline solution is 2.5–3 mol / L, and the sodium hydroxide contains OH... -And the CO3 contained in the sodium carbonate 2- The molar ratio is 2:1 to 16:1.

[0020] Furthermore, in the method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, the calcination temperature in step (1) is 400°C; the mass-to-volume ratio of the calcined magnesium aluminum hydrotalcite to the biogas in step (2) is 0.016 g: 1 mL; and the constant temperature condition in step (2) is 55°C.

[0021] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) This invention prepares magnesium aluminum hydrotalcite by low supersaturation coprecipitation, and then carries out high-temperature calcination modification and intermittent adsorption reaction under suitable conditions. By utilizing the high specific surface area, pore volume and pore size of the calcined hydrotalcite, CO2 gas in anaerobic digestion biogas can be efficiently removed. Moreover, the preparation and modification methods of magnesium aluminum hydrotalcite are simple.

[0023] (2) The calcined magnesium aluminum hydrotalcite used in this invention to adsorb CO2 will not cause secondary pollution during the adsorption process. The reaction conditions are mild, the adsorption effect is good, the CH4 recovery rate is high, and the adsorbent has good regeneration performance and can be reused. It is an environmentally friendly adsorbent. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0025] Figure 1 The images show SEM images of the magnesium aluminum hydrotalcite obtained in Comparative Example 1 and the calcined magnesium aluminum hydrotalcite obtained in Example 2. In the images, (a) is Comparative Example 1 and (b) is Example 2.

[0026] Figure 2 The N2- adsorption-desorption isotherms of the calcined magnesium aluminum hydrotalcite obtained in Examples 1-3 and the magnesium aluminum hydrotalcite obtained in Comparative Example 1 are shown in the figure. In the figure, (a) is Comparative Example 1, (b) is Example 1, (c) is Example 2, and (d) is Example 3.

[0027] Figure 3 The figures show the pore size distribution curves of the calcined magnesium aluminum hydrotalcite obtained in Examples 1-3 and the magnesium aluminum hydrotalcite obtained in Comparative Example 1. In the figures, (a) is Comparative Example 1, (b) is Example 1, (c) is Example 2, and (d) is Example 3.

[0028] Figure 4 This is a SEM image of calcined magnesium aluminum hydrotalcite with added water after the reaction in Example 11.

[0029] Figure 5Line graph of CO2 removal rate after 6 cycles of adsorption-desorption of magnesium-aluminum hydrotalcite calcined at 400℃;

[0030] Figure 6 SEM image of magnesium-aluminum hydrotalcite calcined at 400℃ after six cycles of adsorption-desorption and water addition;

[0031] Figure 7 The figures show the XRD patterns of several magnesium-aluminum hydrotalcites. In the figures, a is the XRD pattern of the magnesium-aluminum hydrotalcite obtained in Comparative Example 1, b is the XRD pattern of calcined magnesium-aluminum hydrotalcite without water after the reaction in Example 11, c is the XRD pattern of calcined magnesium-aluminum hydrotalcite with water after the reaction in Example 11, d is the XRD pattern of calcined magnesium-aluminum hydrotalcite at 400℃ after 6 cycles of adsorption-desorption and then with water, e is the XRD pattern of calcined magnesium-aluminum hydrotalcite without water after the reaction in Example 7, f is the XRD pattern of calcined magnesium-aluminum hydrotalcite with water after the reaction in Example 7, g is the XRD pattern of calcined magnesium-aluminum hydrotalcite without water after the reaction in Example 9, and h is the XRD pattern of calcined magnesium-aluminum hydrotalcite with water after the reaction in Example 9. Detailed Implementation

[0032] This invention provides a method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, comprising the following steps:

[0033] (1) After calcination, magnesium aluminum hydrotalcite is ground and sieved to obtain calcined magnesium aluminum hydrotalcite.

[0034] (2) Roasted magnesium aluminum hydrotalcite and biogas are placed in a container and the container containing roasted magnesium aluminum hydrotalcite and biogas is placed under constant temperature conditions to adsorb carbon dioxide in biogas.

[0035] In this invention, in step (1), the roasting is preferably carried out in an air atmosphere; the roasting temperature is preferably 200-650°C, more preferably 400-600°C, and even more preferably 400°C; this invention does not limit the heating rate, and any scheme known to those skilled in the art is acceptable; the roasting time is preferably 3-5 hours, more preferably 3.5-4.5 hours, and even more preferably 4 hours.

[0036] In this invention, in step (1), the sieving is preferably through an 80-300 mesh sieve, more preferably through an 80-120 mesh sieve, and even more preferably through an 80 mesh sieve.

[0037] In this invention, the biogas in step (2) comes from the anaerobic digestion reactor of the remaining sludge. The initial concentration of CO2 in the biogas is 23.51%, and the remaining components are 50% CH4 and a small amount of N2, H2, H2S and water vapor.

[0038] In this invention, in step (2), the mass-to-volume ratio of the calcined magnesium aluminum hydrotalcite to the biogas is preferably 0.008-0.016 g:1 mL, more preferably 0.012-0.016 g:1 mL, and even more preferably 0.016 g:1 mL; the constant temperature condition is preferably 25-55°C, more preferably 40-55°C, and even more preferably 55°C.

[0039] In this invention, before the biogas is loaded into the container in step (2), it is preferable to evacuate the container to a relative pressure of <-45kPa.

[0040] In this invention, in step (2), the adsorption is preferably carried out in a constant temperature shaker; the rotation speed of the constant temperature shaker is preferably 40-50 rpm, more preferably 42-47 rpm, and even more preferably 45 rpm; the adsorption time is preferably 3-5 h, more preferably 4.5-5 h, and even more preferably 5 h.

[0041] In this invention, the adsorption in step (2) is preferably carried out intermittently, that is, calcined magnesium aluminum hydrotalcite and biogas are added to the headspace sample bottle, and the next batch of reaction is carried out after the adsorption reaches equilibrium.

[0042] In this invention, the internal pressure of the container is not controlled during the adsorption process described in step (2).

[0043] In this invention, step (1) of the preparation method of magnesium aluminum hydrotalcite preferably includes the following steps:

[0044] Prepare a salt solution of magnesium nitrate and aluminum nitrate, and an alkaline solution of sodium hydroxide and sodium carbonate. Add the salt solution and alkaline solution dropwise to water simultaneously while stirring, and control the pH of the mixture to 9.5–10.5. After the addition is complete, continue stirring for 0.5–3 hours. Then, age the mixture at 60–65°C for 20–24 hours, wash it 7–10 times with water under vacuum, and dry the resulting filter cake at 80–105°C to obtain magnesium aluminum hydrotalcite.

[0045] Further optimization includes: the same operation as described above, controlling the pH of the mixture to be 9.8-10.2; after the addition is complete, continue stirring for 0.5-1h, then age the mixture at 60-62℃ for 22-24h, wash it with water by vacuum filtration 8-9 times, and dry the resulting filter cake at 100-105℃;

[0046] More preferably, the following steps are performed: the same as described above, but the pH of the mixture is controlled to be 10; after the addition is complete, the mixture is stirred for 0.5 h, then aged at 60 °C for 24 h, washed 8 times by vacuum filtration with water, and the resulting filter cake is dried at 105 °C to obtain magnesium aluminum hydrotalcite.

[0047] In this invention, the concentration of magnesium nitrate in the salt solution is preferably 1-2 mol / L, more preferably 1-1.4 mol / L, and even more preferably 1.1 mol / L; the molar ratio of magnesium in magnesium nitrate to aluminum in aluminum nitrate is preferably 2:1-4:1, more preferably 2.5:1-3.5:1, and even more preferably 3:1.

[0048] In this invention, the concentration of sodium hydroxide in the alkaline solution is preferably 2.5–3 mol / L, more preferably 2.7–3 mol / L, and even more preferably 2.9 mol / L; the sodium hydroxide contains OH... - And the CO3 contained in the sodium carbonate 2- The molar ratio is preferably 2:1 to 16:1, more preferably 12:1 to 16:1, and even more preferably 16:1.

[0049] In this invention, the roasting temperature in step (1) is preferably 400°C; the mass-to-volume ratio of the roasted magnesium aluminum hydrotalcite to the biogas in step (2) is preferably 0.016 g: 1 mL; and the constant temperature condition in step (2) is preferably 55°C.

[0050] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0051] Example 1

[0052] This embodiment provides a method for preparing calcined magnesium aluminum hydrotalcite, including the following steps:

[0053] (1) Prepare a salt solution of magnesium nitrate and aluminum nitrate mixed together using magnesium nitrate hexahydrate and aluminum nitrate nonahydrate, wherein the concentration of magnesium nitrate is 1.1 mol / L and the molar ratio of magnesium to aluminum is 3:1; prepare an alkaline solution of sodium carbonate and sodium hydroxide mixed together, wherein the concentration of sodium hydroxide is 2.9 mol / L and the OH group is 1.1 mol / L. - and CO3 2-The molar ratio was 16:1. The salt solution and alkali solution were simultaneously and slowly added dropwise to deionized water while vigorous stirring. The pH of the reaction system was controlled to be 10 by adjusting the dropping rates of the alkali and salt solutions (1.5 mL / min for the alkali solution and 3 mL / min for the salt solution). After titration, stirring was continued for 0.5 h. The mixture was then placed in an oven and aged at 60 °C for 24 h. It was then washed seven times by vacuum filtration with deionized water until the filtrate was nearly neutral. The filtrate was discarded, and the filter cake was dried in an oven at 105 °C to obtain magnesium aluminum hydrotalcite, which was stored in a vacuum desiccator for later use.

[0054] (2) The magnesium aluminum hydrotalcite (Mg / Al LDHs) prepared in step (1) was placed in a muffle furnace and calcined at 200°C for 4 hours in air atmosphere. After cooling, it was ground, passed through an 80-mesh sieve, and placed in a vacuum drying oven for later use to obtain calcined magnesium aluminum hydrotalcite, denoted as CLDHs. 200 .

[0055] Example 2

[0056] This embodiment provides a method for preparing calcined magnesium aluminum hydrotalcite, which differs from Example 1 in that: step (2) is calcined at 400℃, while other conditions are the same as in Example 1, to obtain calcined magnesium aluminum hydrotalcite, denoted as CLDHs. 400 .

[0057] Example 3

[0058] This embodiment provides a method for preparing calcined magnesium aluminum hydrotalcite, which differs from Example 1 in that: step (2) is calcined at 600℃, while other conditions are the same as in Example 1, to obtain calcined magnesium aluminum hydrotalcite, denoted as CLDHs. 600 .

[0059] Comparative Example 1

[0060] This comparative example provides a method for preparing magnesium aluminum hydrotalcite, which differs from Example 1 in that: step (2) is not calcined, and other conditions are the same as in Example 1, to obtain magnesium aluminum hydrotalcite, denoted as Mg / Al LDHs.

[0061] The pore size distribution curves of the calcined magnesium aluminum hydrotalcite obtained in Examples 1-3 and the magnesium aluminum hydrotalcite obtained in Comparative Example 1 are shown below. Figure 3 As shown in Table 1, the performance differences between the calcined magnesium-aluminum hydrotalcite obtained in Examples 1-3 and the magnesium-aluminum hydrotalcite obtained in Comparative Example 1 are shown in Table 1. With the increase of calcination temperature, the total specific surface area, total pore volume and average pore size of the magnesium-aluminum hydrotalcite continuously increase. This is due to the removal of interlayer water molecules and carbonate ions, surface crystal water and hydroxyl groups of the layers by high temperature.

[0062] Table 1. Performance differences of magnesium-aluminum hydrotalcites obtained in Examples 1-3 and Comparative Example 1

[0063]

[0064] SEM images of the calcined magnesium aluminum hydrotalcite obtained in Example 2 and the magnesium aluminum hydrotalcite obtained in Comparative Example 1 are shown below. Figure 1 As shown, by Figure 1 It can be seen that the uncalcined magnesium-aluminum hydrotalcite exhibits a flower-like microsphere structure assembled from a large number of nanosheets, while the layered structure of the hydrotalcite obtained after calcination at 400℃ is destroyed. The N2- adsorption-desorption isotherms of the calcined magnesium-aluminum hydrotalcite obtained in Examples 1-3 and the magnesium-aluminum hydrotalcite obtained in Comparative Example 1 are shown below. Figure 2 As shown, by Figure 2 It can be seen that the nitrogen adsorption-desorption curves of all four materials conform to the form of type IV isotherms, and all exhibit H3-type hysteresis loops over a wide pressure ratio range, indicating that all four materials possess mesoporous structures with uneven pore size distribution. The XRD pattern of the magnesium-aluminum hydrotalcite obtained in Comparative Example 1 is shown below. Figure 7 As shown in a, by Figure 7 As can be seen from a, the XRD pattern of the uncalcined LDHs is consistent with the structure of Mg / Al LDHs (PDF#00-054-1030). There are three relatively strong diffraction peaks with gradually decreasing diffraction intensity at the low 2θ angle, which correspond to the (003), (006) and (012) crystal planes, respectively.

[0065] Example 4

[0066] This embodiment provides a method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, including the following steps:

[0067] (1) The magnesium aluminum hydrotalcite (Mg / Al LDHs) prepared in step (1) of Example 1 was placed in a muffle furnace and calcined at 400°C for 4 hours in an air atmosphere. After cooling, it was ground, passed through an 80-mesh sieve, and placed in a vacuum drying oven for later use to obtain calcined magnesium aluminum hydrotalcite.

[0068] (2) Add 0.3g of calcined magnesium aluminum hydrotalcite to a 12mL headspace vial and evacuate to a relative pressure below -45kPa; press in 25mL of biogas at a relative pressure of 0Pa at 25℃, shake the headspace vial thoroughly to ensure that the calcined magnesium aluminum hydrotalcite and biogas are in full contact, corresponding to an addition amount of 0.012g / mL of calcined magnesium aluminum hydrotalcite, and an initial relative pressure of 80kPa in the headspace vial; place the headspace vial containing calcined magnesium aluminum hydrotalcite and biogas in a constant temperature shaker at a speed of 45rpm and react at 40℃ for 5h. After the reaction, measure the relative pressure in the headspace vial and the content of residual CH4 and CO2 in the biogas, and calculate the CO2 adsorption capacity according to formulas 1 and 2.

[0069]

[0070]

[0071] In Formula 1, V 沼气STP V is the volume (mL) of biogas under standard conditions (0℃, 1 atm); 顶空 This is the headspace volume (mL) of the headspace vial after the calcined hydrotalcite is added; P 顶空 It is the headspace pressure (kPa) inside the headspace vial, measured using a barometer; P STP This is the pressure under standard conditions (101.3 kPa), T STP T is the temperature under standard conditions (273.15 K), and T is the reaction temperature (K).

[0072] In formula 2, q t It is the adsorption capacity (mmol / g); V 始STP and V 末STP , respectively, represent the initial and final volumes (mL) of CO2 in the biogas; 22.4 indicates that 1 mmol of gas has a volume of 22.4 mL under standard conditions; m is the mass of the adsorbent (g).

[0073] The difference between Examples 5-12 and Example 4 is that the calcination temperature of magnesium aluminum hydrotalcite in step (1), the mass of magnesium aluminum hydrotalcite added in step (2), and the reaction temperature in step (2) are different. The specific parameters are shown in Table 2. All other conditions are the same as in Example 4. The content of residual CH4 and CO2 in biogas, the percentage reduction of CO2 content, and the adsorption capacity of calcined magnesium aluminum hydrotalcite for CO2 are shown in Table 2.

[0074] Table 2. Reaction conditions and test results of Examples 4-12

[0075]

[0076]

[0077] The test results above show that when the reaction temperature is constant, the CO2 removal gradually increases with the increase of the calcined hydrotalcite dosage. The CO2 removal efficiency first increases and then decreases with increasing calcination temperature, with the optimal calcination temperature around 400℃. When the calcination temperature exceeds 450℃, the adsorption effect begins to deteriorate. When the calcined hydrotalcite dosage is constant, the CO2 removal gradually increases with increasing reaction temperature, indicating that increasing the temperature is beneficial for CO2 adsorption. However, the effect of reaction temperature on CO2 removal is smaller than that of calcination temperature and dosage. When the calcination temperature is constant, the CO2 removal increases with increasing reaction temperature and dosage. The most suitable conditions for calcined magnesium aluminum hydrotalcite to adsorb CO2 from biogas are: calcination at 400℃, magnesium aluminum hydrotalcite dosage of 0.016 g / mL, and reaction temperature of 55℃, i.e., Example 11.

[0078] As shown in the test results of Examples 7, 9, and 12 in Table 2, magnesium aluminum hydrotalcite calcined at 200℃, 400℃, and 600℃ can all be used for anaerobic digestion biogas decarbonization. However, the CO2 removal efficiency and CH4 recovery rate are the highest under the calcination condition of 400℃. Combined with the data in Table 1, this is due to its appropriately high specific surface area, pore volume, pore size, and interlayer CO3. 2- The problem stems from the vacancy created by the removal of adsorption sites: the excessively high specific surface area, pore volume, and pore size of 600℃-calcined magnesium-aluminum hydrotalcite cause competition between CH4 and CO2 for adsorption sites, resulting in poor biogas decarbonization; CO3 between the layers of 200℃-calcined magnesium-aluminum hydrotalcite... 2- Although CO2 was not removed, it can still adsorb CO2, indicating that its decarbonization mainly relies on surface adsorption. Therefore, it is inferred that the CO2 adsorption of 400℃ calcined magnesium aluminum hydrotalcite exhibits the best adsorption effect through both surface adsorption and interlayer anion exchange.

[0079] Example 11: After the reaction was completed, the calcined magnesium aluminum hydrotalcite with added water was subjected to SEM analysis. (The results are as follows...) Figure 4 As shown, by Figure 4 It can be seen that the calcined magnesium aluminum hydrotalcite material still exhibits a nanosheet structure, but the sheets are thicker and irregular in shape, which may be due to the adsorption of CH4 and CO2 on the surface.

[0080] After the reaction in Example 7 was completed, the calcined magnesium aluminum hydrotalcite with and without added water was subjected to XRD analysis. Figure 7 e Figure 7 As shown in f; XRD patterns of calcined magnesium aluminum hydrotalcite with and without water after the reaction in Example 11 are shown in f. Figure 7 b、 Figure 7 As shown in c; the XRD patterns of calcined magnesium aluminum hydrotalcite with and without water after the reaction in Example 9 are shown in Figure c. Figure 7 g、 Figure 7 As shown in h. The results of the XRD characterization analysis above show that after calcination at 200–600℃, magnesium aluminum hydrotalcite still has a "memory effect" and can recover its layered structure.

[0081] Comparative Example 2

[0082] This comparative example provides a method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite. The difference from Example 11 is that after grinding in step (1), the material is passed through a 200-mesh sieve; other conditions are the same as in Example 11. Testing showed that the biogas from Comparative Example 2 contained 46.56% CH4 and 0% CO2, with a CO2 reduction rate of 100% and a CH4 recovery rate of 80%.

[0083] Comparative Example 3

[0084] This comparative example provides a method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite. The difference from Example 11 is that after grinding in step (1), the material is passed through a 300-mesh sieve; other conditions are the same as in Example 11. Testing showed that the biogas from Comparative Example 3 contained 45.01% CH4 and 0% CO2 after adsorption, with a CO2 reduction rate of 100% and a CH4 recovery rate of 78%.

[0085] The test results of Example 11 and Comparative Examples 2-3 show that although calcined magnesium aluminum hydrotalcite passing through 200-mesh and 300-mesh sieves can remove all CO2 from biogas, the CH4 recovery rate is less than 80%, far lower than the 94.7% CH4 recovery rate when passing through an 80-mesh sieve. This is because the particle size of magnesium aluminum hydrotalcite passing through 200-mesh and 300-mesh sieves is smaller than that of the material passing through an 80-mesh sieve, resulting in a larger specific surface area and more CH4 occupying adsorption sites.

[0086] Example 13

[0087] Evaluation of the regeneration performance of calcined magnesium-aluminum hydrotalcite: The desorption condition for CO2 by calcined magnesium-aluminum hydrotalcite was calcination in a muffle furnace at 400℃ for 1 hour. Under the adsorption conditions of Example 11 and the above desorption conditions, the 400℃ calcined magnesium-aluminum hydrotalcite was subjected to 6 adsorption-desorption tests. The CO2 removal rate was as follows: Figure 5 As shown in the image, after six cycles of adsorption-desorption, calcined magnesium aluminum hydrotalcite reduced the CO2 content in biogas from 23.51% to 1.5%. Although the adsorption efficiency decreased slightly, the CO2 removal rate remained as high as 92%. This indicates that calcined magnesium aluminum hydrotalcite at 400℃ has good regeneration performance. The SEM image of calcined magnesium aluminum hydrotalcite at 400℃ after six cycles of adsorption-desorption with added water is shown in the image. Figure 6 As shown, the XRD pattern is as follows Figure 7 As shown in d. Figure 6 It can be seen that the calcined magnesium-aluminum hydrotalcite material, after being recycled six times, still exhibits a nanosheet structure, but the sheets become thicker and more irregular in shape, possibly due to the adsorption of CH4 and CO2 on the surface. Figure 7 As can be seen from d, the calcined magnesium aluminum hydrotalcite material, after being recycled 6 times, can recover its layered structure well after being added to water due to the "memory effect", indicating that it has good regeneration performance.

[0088] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite, characterized in that, Includes the following steps: (1) After calcination, magnesium aluminum hydrotalcite is ground and sieved to obtain calcined magnesium aluminum hydrotalcite. (2) Roasted magnesium aluminum hydrotalcite and biogas are put into a container, and the container containing roasted magnesium aluminum hydrotalcite and biogas is placed under constant temperature conditions to adsorb carbon dioxide in biogas. In step (1), the preparation method of magnesium aluminum hydrotalcite includes the following steps: Prepare a salt solution of magnesium nitrate and aluminum nitrate, and an alkaline solution of sodium hydroxide and sodium carbonate. Add the salt solution and alkaline solution dropwise to water simultaneously while stirring, and control the pH of the mixture to 9.5-10.

5. After the addition is complete, continue stirring for 0.5-3 h. Then, age the mixture at 60-65 °C for 20-24 h, wash it 7-10 times with water under vacuum filtration, and dry the resulting filter cake at 80-105 °C to obtain magnesium aluminum hydrotalcite. In the salt solution, the concentration of magnesium nitrate is 1~2 mol / L, and the molar ratio of magnesium in magnesium nitrate to aluminum in aluminum nitrate is 2:1~4:

1. In the alkaline solution, the concentration of sodium hydroxide is 2.5~3 mol / L, and the sodium hydroxide contains OH groups. - and the CO3 contained in the sodium carbonate 2- The molar ratio is 2:1 to 16:1; In step (1), the roasting is carried out in an air atmosphere; the roasting temperature is 200~600 ℃, and the roasting time is 3~5 h; In step (2), the mass-to-volume ratio of the calcined magnesium aluminum hydrotalcite to the biogas is 0.008~0.016 g:1 mL; the constant temperature condition is 25~55 ℃.

2. The method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite as described in claim 1, characterized in that, In step (1), the sieving is performed through an 80-300 mesh sieve.

3. A method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite as described in claim 1 or 2, characterized in that, In step (2), before the biogas is loaded into the container, the container is evacuated to a relative pressure of <-45kPa.

4. The method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite as described in claim 3, characterized in that, In step (2), the adsorption is carried out in a constant temperature shaker with a rotation speed of 40-50 rpm; the adsorption time is 3-5 h.

5. A method for removing carbon dioxide from biogas using calcined magnesium aluminum hydrotalcite as described in claim 1 or 2, characterized in that, The roasting temperature in step (1) is 400 ℃; the mass-to-volume ratio of the roasted magnesium aluminum hydrotalcite to the biogas in step (2) is 0.016 g: 1 mL; the constant temperature condition in step (2) is 55 ℃.

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