Method for preparing CNTs and hydrogen-rich gas through sludge pyrolysis

Abstract

The invention discloses a method for preparing CNTs and hydrogen-rich gas through sludge pyrolysis, sludge is subjected to pyrolysis after being conditioned by ferric salt, pyrolysis gas is collected and reduced under the action of a Ni-Fe / NF catalyst, the Ni-Fe / NF catalyst is prepared and obtained, and the Ni-Fe / NF catalyst is obtained by dipping foamed nickel in a mixed solution containing nickel nitrate and ferric nitrate and reducing the foamed nickel. Under the action of the Ni-Fe / NF catalyst, on the basis of not changing pyrolysis conditions and not influencing preparation of pyrolytic carbon and pyrolytic oil by sludge pyrolysis, the pyrolysis gas is synchronously converted into the Ni-Fe / NF catalyst with higher value, so that the economic values of the sludge and the pyrolysis gas are improved, the influence on the environment is reduced, and the engineering application of the sludge pyrolysis technology is favorably realized.

Description

Technical Field

[0001] The invention belongs to the technical field of sludge resource utilization, and specifically relates to a method for preparing CNTs and hydrogen-rich gas by pyrolysis of sludge. Background Art

[0002] Sludge is a by-product produced during sewage treatment. Studies have shown that about 50% of organic matter, 35% N, 90% P, and 90% heavy metals in sewage will eventually be enriched in sludge. Sludge has both resource attributes and pollution attributes, so we need to properly handle it. At present, common sludge treatment and disposal methods include land use, landfill, anaerobic digestion, thermal treatment, etc. Among them, sludge pyrolysis technology has been favored by more and more researchers in recent years because it can achieve efficient sludge reduction, energy and resource utilization.

[0003] At present, the large-scale application of sludge pyrolysis technology mainly faces two difficulties: one is how to reduce the energy consumption of the pyrolysis process, and the other is how to increase the value of the pyrolysis products. The moisture content of the residual sludge in the sludge plant is usually between 90-99%. Therefore, achieving deep dehydration of sludge is a key step to reduce the subsequent drying-pyrolysis energy consumption. On the other hand, the products of sludge pyrolysis mainly include pyrolytic carbon (CN113072267B A method for efficiently recovering phosphorus from municipal sludge and simultaneously preparing porous biochar), pyrolytic oil (CN114345359B A preparation method and application of a catalyst for efficient catalytic cracking of sludge pyrolysis tar and a real-time detection system) and pyrolysis gas. However, the current research on the high value of sludge pyrolysis products is more focused on pyrolytic carbon and pyrolysis oil. Pyrolysis gas faces problems such as low calorific value and secondary pollution when used as fuel during actual application. It is often ignored and discharged with the carrier gas, which not only wastes resources, but also easily pollutes the atmosphere, and needs further research and improvement.

[0004] The pyrolysis gas of sludge is rich in small - molecule hydrocarbons and carbon oxides, etc. Using it as a carbon source to simultaneously prepare CNTs and hydrogen - rich gas through chemical vapor deposition can not only further enhance the value of pyrolysis gas on the basis of existing sludge pyrolysis technology, reduce the environmental burden, but also reduce the economic cost of preparing CNTs by traditional chemical vapor deposition method. For example, Chinese Patent CN112408364B discloses a method for catalytic pyrolysis of waste thermosetting plastics to prepare carbon nanotubes. Using a core - shell catalyst, with waste thermosetting plastics as raw materials, the pyrolysis gas generated in a two - stage fixed - bed reactor forms carbon nanotubes on the surface of the catalyst. The whole process only needs to control the reactor temperature, with simple operation. Moreover, catalytically preparing carbon nanotubes from high - molecular polymers can not only recycle waste plastics but also produce high - value - added carbon nanotubes. However, it is only applicable to plastics and additionally adds a high - temperature condition, which is contrary to the value - added concept of sludge pyrolysis gas. In addition, due to the adverse effect of high moisture content of sludge on sludge pyrolysis, combining sludge deep dehydration with sludge pyrolysis technology has great practical significance and economic value.

[0005] Therefore, it is necessary to develop a resource utilization method applicable to sludge pyrolysis gas to improve the economic value and environmental significance of sludge pyrolysis technology. Summary of the Invention

[0006] In view of the above - mentioned technical problems, the present invention provides a method for preparing CNTs and hydrogen - rich gas by sludge pyrolysis. Using Ni - Fe / NF catalyst, without changing the pyrolysis conditions and without affecting the preparation of pyrolysis carbon and pyrolysis oil from sludge pyrolysis, the pyrolysis gas is converted into CNTs and hydrogen - rich gas, improving the economic value of sludge and pyrolysis gas and reducing the impact on the environment.

[0007] To achieve the above object, the present invention provides a method for preparing CNTs and hydrogen - rich gas by sludge pyrolysis, including the following steps: (1) Sludge pretreatment: Adding iron salt to the sludge for conditioning, and then dehydrating and drying to obtain iron - containing sludge; (2) Grinding, sieving and pyrolyzing the iron - containing sludge to obtain pyrolysis gas; (3) The pyrolysis gas flows through the catalyst, and CNTs and hydrogen - rich gas are deposited on the surface.

[0008] Preferably, the mass ratio of the sludge to the iron salt in step (1) is 100:1 - 5.

[0009] More preferably, the iron salt is any one of ferric chloride, ferric sulfate or ferric nitrate.

[0010] Preferably, the water content of the iron - containing sludge is ≤3%.

[0011] Preferably, the mesh number of the sieve used in step (2) is 60 - 100 meshes; the pyrolysis temperature is 600 - 800 °C, and the pyrolysis time is 60 - 120 min.

[0012] Preferably, the catalyst in step (3) is Ni-Fe / NF catalyst; the mass ratio of sludge to catalyst is 30 - 60:1.

[0013] More preferably, the preparation method of the Ni-Fe / NF catalyst includes the following steps: (1) Pretreatment of nickel foam: Ultrasonically clean the nickel foam with hydrochloric acid, absolute ethanol and water. (2) Mix and dissolve nickel salt and iron salt to obtain an impregnation solution. (3) Place the cleaned nickel foam in the impregnation solution, impregnate, dry and reduce to obtain the Ni-Fe / NF catalyst.

[0014] Even more preferably, the nickel salt in step (2) is nickel chloride or nickel sulfate; the iron salt is ferric chloride or ferric sulfate.

[0015] Even more preferably, the molar ratio of Ni:Fe in the impregnation solution in step (2) is 3:1 - 1:3, and the concentration of nickel is 0.1 - 0.5 M.

[0016] Even more preferably, the impregnation conditions in step (3) are rotary impregnation at 40 - 80 °C for 1 - 2 h, and the rotation speed is 20 - 40 rpm.

[0017] Even more preferably, the reduction conditions in step (3) are reduction at 600 - 800 °C for 40 - 60 min in a mixed atmosphere of hydrogen and inert gas.

[0018] Even more preferably, the volume percentage of hydrogen in the mixed atmosphere is 5 - 15%; the inert gas is nitrogen or argon.

[0019] Preferably, the pyrolysis gas in step (3) flows through the catalyst carried by the carrier gas, where the carrier gas is nitrogen or argon, and the flow rate is 40 - 100 mL / min.

[0020] Specifically, the principle of the present invention is as follows: The iron salt can be used as a catalyst for the conversion of organic matter and tar, changing the composition of the produced gas. The mechanism is as Figure 2 shown: During the pyrolysis gasification of sludge, metallic Fe can act as an in-situ catalyst for the consumption of organic matter and tar, changing the composition of the produced gas through various reactions to generate CH 4 , C 2 H 4 , CO and H 2Conditioner - The iron salt is evenly dispersed in the sludge cake, enabling in-situ catalytic pyrolysis gasification of the organic matter in the sludge, with a catalytic effect superior to that of externally added catalysts. The addition of iron salt promotes the content of small-molecule hydrocarbons in the sludge pyrolysis gas, which flows through the Ni-Fe / NF catalyst (heterogeneous catalysis) carried by the carrier gas. The nickel foam catalyst itself has a good 3D skeleton structure, which helps the carbon deposition to exist in a three-dimensional carbon nanostructure, significantly increasing the specific surface area and thus promoting catalytic reforming to produce CNTs. The presence of iron in the Ni-Fe / NF catalyst improves the carbon dissolution ability of the metal, and the formation of iron carbide will further promote the growth of CNTs as a catalytic active center.

[0021] The beneficial effects of the present invention are as follows: 1. Under the action of the Ni-Fe / NF catalyst, the influence of the reduction atmosphere and reduction temperature on the product performance and yield is reduced. At the same time, on the basis of not changing the pyrolysis conditions and not affecting the pyrolysis of sludge to prepare pyrolytic carbon and pyrolytic oil, the pyrolysis gas can be synchronously converted into CNTs and hydrogen-rich gas with higher value, improving the economic value of the sludge pyrolysis gas and reducing the impact on the environment, providing an effective way for the reduction, stabilization, harmlessness, and resource utilization of sludge, and having important theoretical significance for the engineering application of sludge pyrolysis technology.

[0022] 2. The Ni-Fe / NF catalyst is prepared only by using nickel salt, iron salt, and nickel foam. The 3D skeleton structure of nickel foam can increase the contact area between the catalyst and the pyrolysis gas, promoting the catalytic generation of CNTs with a three-dimensional carbon nanostructure from the carbon-containing gas components in the pyrolysis gas. In addition, after the Ni-Fe / NF catalyst catalytically generates CNTs from the carbon-containing gas components in the pyrolysis gas, hydrogen can be effectively separated to obtain hydrogen with higher purity, reducing the safety and environmental problems caused by impurity gases in the pyrolysis gas in applications such as hydrogen production and chemical synthesis, and at the same time reducing the gas treatment difficulty during the utilization of the pyrolysis gas. Description of the Drawings

[0023] Figure 1 It is a schematic diagram of the device for synthesizing CNTs and hydrogen-rich gas by one-step rapid pyrolysis of sludge according to the present invention.

[0024] Figure 2 It is a catalytic mechanism diagram of iron oxide for tar cracking according to the present invention.

[0025] Figure 3 It is an SEM image of the CNTs prepared in Example 2. Detailed Embodiments

[0026] The technical solution of the present invention will be further explained below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the following embodiments are only the preferred embodiments of the present invention and should not be construed as limitations on the present invention. The protection scope of the present invention shall be determined by the content recorded in the claims. Modifications and substitutions made by those skilled in the art to the technical solution of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] Example 1 To prepare the Ni-Fe / NF catalyst, the following steps are included: (1) Cut the nickel foam into rectangular sheets of 3.0 cm × 5.0 cm, and then ultrasonically clean it with 3M hydrochloric acid, absolute ethanol, and deionized water for 30 min in sequence, and then dry it. (2) Mix a 0.1M nickel chloride solution and a 0.3M iron chloride solution at a Ni:Fe molar ratio of 1:1 to obtain an impregnation solution. (3) Place the nickel foam obtained in step (1) in the impregnation solution and perform rotary impregnation at 60°C for 90 min, where the rotation speed during rotary impregnation is 20 rpm. (4) After the impregnation is completed, take out the nickel foam and place it in a mixed atmosphere of 700°C, 10% hydrogen and 90% argon for reduction for 60 min to obtain the Ni-Fe / NF catalyst.

[0028] Comparative Example 1 To prepare Ni-Fe (1) Cut the nickel foam into rectangular sheets of 3.0 cm × 5.0 cm, and then ultrasonically clean it with 3M hydrochloric acid, absolute ethanol, and deionized water for 30 min in sequence, and then dry it. (2) Mix a 0.1M nickel chloride solution, a 0.3M iron chloride solution, a 0.2M urea solution, and a 0.08M ammonium fluoride solution to obtain an impregnation solution; the Ni:Fe molar ratio in the impregnation solution is 1:1. (3) Place the nickel foam obtained in step (1) in the impregnation solution and perform rotary impregnation at 60°C for 90 min, where the rotation speed during rotary impregnation is 20 rpm. (4) After the impregnation is completed, take out the nickel foam and place it in a mixed atmosphere of 700°C, 10% hydrogen and 90% argon for reduction for 60 min to obtain the Ni-Fe catalyst.

[0029] Example 2 (1) Add iron chloride to the sludge, stir evenly, then perform mechanical dehydration, drying, and grinding, and then pass through an 80-mesh sieve to obtain iron-containing sludge with a water content ≤ 3%; the mass ratio of sludge to iron chloride is 100:3. (2) Place the iron-containing sludge at the front end of the pyrolysis reactor, and place the Ni-Fe / NF catalyst prepared in Example 1 at the rear end of the pyrolysis reactor. Using nitrogen as the carrier gas, place the reactor containing the iron-containing sludge in a reaction furnace at 700 °C and hold for 60 min for pyrolysis. The pyrolysis gas generated by pyrolysis is carried by the carrier gas and flows through the Ni-Fe / NF catalyst, and CNTs ( Figure 3 ) and hydrogen-rich gas are deposited on its surface; the flow rate of nitrogen is 70 mL / min.

[0030] Example 3 (1) Add ferric sulfate to the sludge, stir evenly, then mechanically dehydrate, dry, grind, and pass through a 60-mesh sieve to obtain iron-containing sludge with a water content ≤ 3%; the mass ratio of sludge to ferric sulfate is 100:3; (2) Place the iron-containing sludge at the front end of the pyrolysis reactor, and place the Ni-Fe / NF catalyst at the rear end of the pyrolysis reactor. Using nitrogen as the carrier gas, place the reactor containing the iron-containing sludge in a reaction furnace at 650 °C and hold for 90 min for pyrolysis. The pyrolysis gas generated by pyrolysis is carried by the carrier gas and flows through the Ni-Fe / NF catalyst, and CNTs and hydrogen-rich gas are deposited on its surface; in the preparation process of the Ni-Fe / NF catalyst, the concentrations of nickel chloride and ferric chloride solutions are both 0.2 M, and the molar ratio of Ni:Fe in the impregnation solution is 3:2; the flow rate of nitrogen is 70 mL / min.

[0031] Example 4 (1) Add ferric chloride to the sludge, stir evenly, then mechanically dehydrate, dry, grind, and pass through a 100-mesh sieve to obtain iron-containing sludge with a water content ≤ 3%; the mass ratio of sludge to ferric chloride is 100:3; (2) Place the iron-containing sludge at the front end of the pyrolysis reactor, and place the Ni-Fe / NF catalyst prepared in Example 1 at the rear end of the pyrolysis reactor. Using nitrogen as the carrier gas, place the reactor containing the iron-containing sludge in a reaction furnace at 800 °C and hold for 60 min for pyrolysis. The pyrolysis gas generated by pyrolysis is carried by the carrier gas and flows through the Ni-Fe / NF catalyst, and CNTs and hydrogen-rich gas are deposited on its surface; in the preparation process of the Ni-Fe / NF catalyst, the concentrations of nickel chloride and ferric chloride solutions are both 0.4 M, and the molar ratio of Ni:Fe in the impregnation solution is 2:1; the flow rate of nitrogen is 70 mL / min.

[0032] Example 5 (1) Add ferric nitrate to the sludge, stir evenly, then mechanically dehydrate, dry, grind, and pass through a 60-mesh sieve to obtain iron-containing sludge with a water content ≤ 3%; the mass ratio of sludge to ferric nitrate is 100:1; (2) Prepare the Ni-Fe / NF catalyst according to the method described in Example 1, only replace nickel chloride with nickel sulfate; (3) Place the iron-containing sludge at the front end of the pyrolysis reactor, and place the Ni-Fe / NF catalyst prepared in step (2) at the rear end of the pyrolysis reactor. Using argon as the carrier gas, place the reactor containing the iron-containing sludge in a reaction furnace at 700 °C and hold for 100 min for pyrolysis. The pyrolysis gas generated by pyrolysis is carried by the carrier gas and flows through the Ni-Fe / NF catalyst, and CNTs and hydrogen-rich gas are deposited on its surface; the flow rate of nitrogen is 40 mL / min.

[0033] Example 6 (1) Add ferric nitrate to the sludge, stir evenly, then mechanically dehydrate, dry, grind, and pass through 60 meshes to obtain iron-containing sludge with a water content ≤ 3%; the mass ratio of sludge to ferric nitrate is 100:5. (2) Prepare the Ni-Fe / NF catalyst according to the method described in Example 1, replacing nickel chloride with nickel sulfate and ferric chloride with ferric sulfate; during the reduction process, the volume percentage of hydrogen in the mixed atmosphere is 5%. (3) Place the iron-containing sludge at the front end of the pyrolysis reactor, and place the Ni-Fe / NF catalyst prepared in step (2) at the rear end of the pyrolysis reactor. Using argon as the carrier gas, place the reactor containing the iron-containing sludge in a reaction furnace at 700 °C and hold for 120 min for pyrolysis. The pyrolysis gas generated by pyrolysis is carried by the carrier gas and flows through the Ni-Fe / NF catalyst, and CNTs and hydrogen-rich gas are deposited on its surface; the flow rate of nitrogen is 100 mL / min.

[0034] Comparative Example 2 The method and steps are the same as those in Example 2, except that ferric chloride is not added, and CNTs and hydrogen are prepared.

[0035] Comparative Example 3 The method and steps are the same as those in Example 2, except that nickel foam is directly used as the catalyst, and CNTs and hydrogen are prepared.

[0036] Comparative Example 4 The method and steps are the same as those in Example 2, except that the Ni-Fe catalyst prepared in Comparative Example 1 is used for pyrolysis, and CNTs and hydrogen are prepared.

[0037] Comparative Example 5 The method and steps are the same as those in Example 2, except that in the preparation process of the Ni-Fe / NF catalyst, the concentration of nickel chloride in the impregnation solution is changed to 0.6 M, the concentration of ferric chloride solution is changed to 0.3 M, and the Ni:Fe molar ratio is 2:1, and CNTs and hydrogen are prepared.

[0038] Comparative Example 6 The method and steps are the same as those in Example 2, except that in the preparation process of the Ni-Fe / NF catalyst, the Ni:Fe molar ratio in the impregnation solution is changed to 4:1, and CNTs and hydrogen are prepared.

[0039] Comparative Example 7 The method and steps are the same as those in Example 2, except that the molar ratio of Ni:Fe in the impregnation solution during the preparation of the Ni-Fe / NF catalyst is changed to 1:4, and CNTs and hydrogen are prepared.

[0040] Comparative Example 8 The method and steps are the same as those in Example 2, except that the pyrolysis temperature is changed to 500 °C, and CNTs and hydrogen are prepared.

[0041] Comparative Example 9 The method and steps are the same as those in Example 2, except that the pyrolysis temperature is changed to 900 °C, and CNTs and hydrogen are prepared.

[0042] Comparative Example 10 The method and steps are the same as those in Example 2. The pyrolysis temperature is changed to 500 °C, and nickel foam is directly used as the catalyst to prepare CNTs and hydrogen.

[0043] Comparative Example 11 The method and steps are the same as those in Example 2, except that the nickel foam in the preparation process of the Ni-Fe / NF catalyst is changed to a nickel sheet with a thickness of 2 mm, and CNTs and hydrogen are prepared.

[0044] Comparative Example 12 The method and steps are the same as those in Example 2, except that the mixed atmosphere in the preparation process of the Ni-Fe / NF catalyst is replaced with an air atmosphere, and CNTs and hydrogen are prepared.

[0045] Comparative Example 13 The method and steps are the same as those in Example 2, except that the volume percentage of hydrogen in the mixed atmosphere in the preparation process of the Ni-Fe / NF catalyst is changed to 20%, and CNTs and hydrogen are prepared.

[0046] Comparative Example 14 The method and steps are the same as those in Example 2, except that the flow rate of nitrogen is changed to 30 mL / min, and CNTs and hydrogen are prepared.

[0047] Comparative Example 15 The method and steps are the same as those in Example 2, except that the flow rate of nitrogen is changed to 110 mL / min, and CNTs and hydrogen are prepared.

[0048] Result detection: Take the CNTs and hydrogen prepared in the above examples and comparative examples, and count their CNTs yields and hydrogen production amounts. The results are shown in the following table:

[0049] As can be seen from the above table, when nickel sheets, nickel foam, or Ni-Fe / NF catalysts containing urea and ammonium fluoride are used, high yields of CNTs and high yields of hydrogen cannot be obtained during the pyrolysis of sludge. Moreover, when the nickel content in the Ni-Fe / NF catalyst is either too high or too low, the yields of CNTs and hydrogen will also be significantly reduced.

Claims

1. A method for preparing CNTs and hydrogen-rich gas by pyrolysis of sludge, characterized in that: The following steps are involved: (1) Sludge pretreatment: Add iron salt to the sludge for conditioning, and then dehydrate and dry it to obtain iron-containing sludge; (2) Grinding, screening and pyrolyzing the iron-containing sludge to obtain pyrolysis gas; (3) The pyrolysis gas flows through the catalyst, where CNTs and hydrogen-rich gas are deposited on the surface.

2. The method for preparing CNTs and hydrogen-rich gas by pyrolysis of sludge according to claim 1, characterized in that: The mass ratio of the sludge to the iron salt in step (1) is 100:1-5.

3. The method for preparing CNTs and hydrogen-rich gas by pyrolysis of sludge according to claim 2, characterized in that: The iron salt is any one of ferric chloride, ferric sulfate or ferric nitrate.

4. The method for preparing CNTs and hydrogen-rich gas by pyrolysis of sludge according to claim 1, characterized in that: The mesh size of the sieve in step (2) is 60-100 mesh; the pyrolysis temperature is 600-800°C, and the pyrolysis time is 60-120 min.

5. The method for preparing CNTs and hydrogen-rich gas by pyrolysis of sludge according to claim 1, characterized in that: The catalyst described in step (3) is a Ni-Fe / NF catalyst.

6. The method for preparing CNTs and hydrogen-rich gas by pyrolysis of sludge according to claim 5, characterized in that: The preparation method of the Ni-Fe / NF catalyst comprises the following steps: (1) Pretreatment of nickel foam: ultrasonically clean the nickel foam with hydrochloric acid, anhydrous ethanol and water; (2) mixing and dissolving the nickel salt and the iron salt to obtain an impregnation solution; (3) The cleaned nickel foam is placed in an impregnation solution, and after impregnation, it is dried and reduced to obtain a Ni-Fe / NF catalyst.

7. The method for preparing CNTs and hydrogen-rich gas by pyrolysis of sludge according to claim 6, characterized in that: The nickel salt in step (2) is nickel chloride or nickel sulfate; the iron salt is ferric chloride or ferric sulfate.

8. The method for preparing CNTs and hydrogen-rich gas by pyrolysis of sludge according to claim 6, characterized in that: The molar ratio of Ni:Fe in the impregnation solution of step (2) is 3:1-1:3, and the concentration of nickel is 0.1-0.5M.

9. The method for preparing CNTs and hydrogen-rich gas by pyrolysis of sludge according to claim 6, characterized in that: The immersion conditions in step (3) are 40-80°C and 20-40 rpm for 1-2 hours.

10. The method for preparing CNTs and hydrogen-rich gas by pyrolysis of sludge according to claim 6, characterized in that: The reduction conditions in step (3) are reduction in a mixed atmosphere of hydrogen and inert gas at 600-800° C. for 40-60 min.

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