Modified activated carbon for enhancing nitrogen oxide trapping performance based on in-hole in-situ carbonization and gradient activation as well as preparation method and application of modified activated carbon

Abstract

The invention discloses modified activated carbon for enhancing nitrogen oxide trapping performance based on in-hole in-situ carbonization and gradient activation as well as a preparation method and application of the modified activated carbon, and belongs to the technical field of flue gas purification. The problem that existing commercial activated carbon is poor in low-temperature denitration performance due to the fact that the micropore structure is insufficient is solved. The preparation method comprises the following steps: immersing activated carbon in a glucose solution to load glucose in the pore channel; carrying out hydrothermal carbonization treatment, and forming a carbonaceous layer rich in oxygen functional groups in situ in the pore channel; and COgradient activation treatment is carried out, new micropores are etched in a carbonaceous layer and near the carbonaceous layer through gasification reaction, a graded pore structure is constructed, and the method is green and economical. The micropore volume of the prepared modified activated carbon is increased by more than 50%, the specific surface area is 700-1200 m / g, and the new micropore aperture is concentrated at 0.5-0.8 nm. The NO adsorption capacity of the material at 30 DEG C is 2.6 times or above that of the raw material activated carbon, and the material is suitable for the field of low-temperature flue gas denitration of coal-fired power plants, industrial boilers and the like.

Description

Technical Field

[0001] This invention belongs to the field of atmospheric pollution control and functional carbon materials technology, specifically relating to a modified activated carbon for flue gas purification, and more particularly to an activated carbon with rich hierarchical microporous structure and enhanced surface chemical properties prepared by in-situ carbonization and gradient activation strategy, and its preparation method and application in the efficient capture of nitrogen oxides (NOx) in flue gas. Background Technology

[0002] Nitrogen oxides (NOx, mainly including NO and NO2) are key precursors to photochemical smog, acid rain, and fine particulate matter (PM2.5), posing a serious threat to the ecological environment and human health. Industrial flue gas, especially from coal-fired power plants and industrial boilers, is a major anthropogenic source of NOx emissions. With increasingly stringent environmental standards, developing efficient, low-consumption, and low-secondary-pollution denitrification technologies is crucial.

[0003] Activated carbon denitrification is a promising dry technology that removes NOx through a combination of physical adsorption and surface catalytic oxidation. Studies have shown that the adsorption and catalytic oxidation performance of activated carbon for NO is closely related to its pore structure, especially the micropores (<2 nm, particularly in the 0.5-0.8 nm range) that match the dynamic diameter of NO molecules (approximately 0.317 nm). These micropores not only provide a large specific surface area for enriching reactants, but their narrow channels also enhance the interaction between gas molecules and pore walls, promoting the reaction of NO with O2 to generate more easily trapped NO2. However, the micropore structure of ordinary commercial activated carbon (such as coal-based and coconut shell activated carbon) is essentially fixed during activation, with a limited number of micropores and a wide pore size distribution that is not well-suited for nitrogen oxide removal. This results in generally low-temperature denitrification performance, making it difficult to meet the requirements for high-efficiency purification.

[0004] To improve the denitrification performance of activated carbon, existing technologies mainly focus on surface chemical modification and pore structure control, such as acid / alkali treatment to introduce functional groups and loading metal oxides (such as V₂O₅ and CuO) to provide catalytic active sites. However, these methods have significant limitations: acid / alkali modification may damage the carbon skeleton, and the introduced functional groups may be unstable at high temperatures or during long-term operation; loading metal oxides involves complex preparation processes, easy sintering or loss of active components, high costs, and the potential risk of secondary heavy metal pollution. More importantly, these methods mostly focus on modifying the surface of activated carbon, making it difficult to "directionally" create a large number of new micropores of suitable size within the activated carbon itself, thus failing to fundamentally solve the bottleneck problem of insufficient effective adsorption and reaction sites.

[0005] Therefore, developing a modification method that is simple, low-cost, environmentally friendly, and can fundamentally optimize the internal microporous structure of activated carbon and simultaneously enhance its surface chemical activity is of great significance for promoting the large-scale application of activated carbon in the field of flue gas denitrification. Summary of the Invention

[0006] The technical problem this invention aims to solve is to overcome the shortcomings of existing commercial activated carbon, such as insufficient microporous structure, complex processes, or susceptibility to secondary pollution in existing modification methods. It provides an innovative "in-situ carbonization-gradient activation" modification method and the resulting high-performance modified activated carbon. This method aims to significantly improve the low-temperature capture capacity of activated carbon for NOx (especially NO) in flue gas by directionally constructing a large number of new, size-matched micropores within the existing pores of the activated carbon without introducing exogenous metals, while simultaneously enriching the surface oxygen-containing functional groups.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A method for preparing modified activated carbon to enhance the nitrogen oxide capture performance of flue gas is based on the synergistic effect of two key processes: "in-situ carbonization within pores" and "gradient activation".

[0009] S1. Impregnation and Loading: Commercially available activated carbon with initial macroporous and mesoporous structures was impregnated in an aqueous glucose solution. Glucose, as a liquid carbon precursor, leverages its small molecule properties and the fluidity of the aqueous solution to fully penetrate and adsorb within the macro- and mesoporous channels of the activated carbon, driven by capillary forces and concentration gradients. This step lays the spatial and material foundation for the subsequent in-situ construction of a carbonaceous layer within the pores.

[0010] S2. Hydrothermal Carbonization: Activated carbon precursors loaded with glucose are subjected to intermediate-temperature treatment in a hydrothermal environment. Under these closed, self-generated pressure conditions, glucose molecules within the pores undergo a series of reactions, including dehydration, condensation, and aromatization. This results in the in-situ formation of a dense carbonaceous polymer layer rich in oxygen-containing functional groups such as hydroxyl (-OH) and carboxyl (-COOH) groups (i.e., the hydrothermal carbon layer) on the pore wall surface of the activated carbon. This layer not only serves as a "sacrificial template" for subsequent pore formation, but its abundant oxygen-containing functional groups also provide active sites for NO adsorption and surface oxidation reactions.

[0011] S3. CO2 Gradient Activation: The hydrothermal carbonization intermediate is programmed to a high temperature under an inert atmosphere, then activated using a CO2 atmosphere. CO2, as a mild gasifying agent, preferentially reacts with the relatively disordered and highly reactive hydrothermal carbon layer generated during the hydrothermal carbonization step, as well as the surface layer of the activated carbon substrate, in a selective gasification reaction (C + CO2 → 2CO). By precisely controlling the activation temperature, time, and CO2 flow rate, a large number of new micropores can be "etched" into the interior of the hydrothermal carbon layer and the activated carbon-hydrothermal carbon interface region, and the previously small pore size (<0.6 nm) can be finely enlarged. Incompletely etched glucose microspheres can reduce the previously large pore size (>0.8 nm), improving the distribution of activated carbon pore size at 0.7 nm (suitable for nitrogen oxide treatment) through a "two-pronged" strategy. These newly formed micropores are interconnected with the original microporous network of the activated carbon, forming a more developed and interconnected hierarchical pore structure. Simultaneously, the CO2 activation process can further regulate surface chemical properties.

[0012] Further, in step S1, the concentration of the glucose aqueous solution is preferably 0.2 mol / L to 0.4 mol / L, and the dry basis mass ratio of the commercial activated carbon to glucose is preferably 1:2 to 1:4. This concentration and ratio range ensures that the glucose fully fills the pores without excessively clogging the pore openings.

[0013] Furthermore, in step S2, the temperature of the hydrothermal carbonization reaction is preferably 170°C to 190°C, and the time is preferably 4 to 6 hours. These mild conditions are sufficient to achieve effective carbonization of glucose while avoiding excessive damage to the structure of the activated carbon.

[0014] Furthermore, in step S3, the CO2 activation temperature is preferably 750℃~850℃, the activation time is preferably 3 hours~5 hours, and the CO2 gas flow rate is preferably 150 mL / min~300 mL / min. This gradient activation condition is key to achieving the directional and controllable generation of micropores.

[0015] This invention also provides modified activated carbon prepared by the above method. The most significant feature of this modified activated carbon is the optimization of its microporous structure: its micropore volume (Vmicro, calculated by the NLDFT model, pore size <2 nm) is increased by more than 30% compared with the raw activated carbon, preferably by more than 50%; its BET specific surface area can reach 700 m² / g to 1200 m² / g; particularly important is that the pore size of the newly formed micropores is concentrated in a narrow range of 0.5 nm to 0.8 nm, which is highly matched with the molecular size of NO and O2, greatly enhancing the mass transfer efficiency and molecular-pore wall interaction.

[0016] This invention also provides the application of the above-mentioned modified activated carbon in capturing nitrogen oxides in flue gas. This modified activated carbon is particularly suitable for low-temperature (e.g., 20℃~150℃) flue gas conditions and can be applied to NOx purification treatment of stationary sources such as coal-fired power plants, industrial boilers, steel sintering plants, and glass kilns, as well as mobile sources such as vehicle exhaust.

[0017] Beneficial effects

[0018] Compared with the prior art, the present invention has the following outstanding substantive features and significant progress:

[0019] 1. Innovative "pore-forming within pores" mechanism: Unlike traditional surface modification or overall activation, this invention pioneers a "nanoreactor" mechanism using the large and medium pores of activated carbon itself. Glucose is introduced for in-situ hydrothermal carbonization, and then CO2 is used to selectively etch the carbon layer to achieve "pore-forming within pores" and "dual-sided etching." This strategy enables the directional and controllable construction of a large number of pores suitable for nitrogen oxide treatment within the activated carbon body, fundamentally increasing the density of effective adsorption and catalytic sites. Furthermore, the newly formed micropores are naturally connected to the existing pores, forming a highly efficient three-dimensional mass transfer network.

[0020] 2. Synergistic Optimization of Microporous Structure and Surface Chemistry: This invention introduces abundant oxygen-containing functional groups simultaneously through a hydrothermal carbonization step, and then combines this with CO2 activation for precise control of pore structure and surface chemistry, achieving a synergistic enhancement of the microporous physical structure (size, volume) and surface chemical properties (functional groups). This synergistic effect significantly improves the material's physical adsorption capacity for NO and the efficiency of its catalytic oxidation to NO2.

[0021] 3. The process is green, simple, and economical: The entire process uses only inexpensive, readily available, and environmentally friendly reagents such as glucose, water, and CO2, without involving strong acids, strong alkalis, or precious metals, thus avoiding secondary pollution and complex post-processing issues. Hydrothermal carbonization and CO2 activation are both mature unit operations in the field of carbon materials, requiring conventional equipment and easily enabling industrial-scale production.

[0022] 4. Significant improvement in denitrification performance: Experiments show that the modified activated carbon prepared in this invention can adsorb more than twice the amount of the raw activated carbon for NO in simulated flue gas at low temperatures (e.g., 25°C), and the breakthrough time is significantly extended, demonstrating excellent low-temperature denitrification performance. This provides a practical new material solution for solving the problem of low denitrification efficiency of existing activated carbon methods. Attached Figure Description

[0023] Figure 1 This is a flowchart illustrating the process of preparing modified activated carbon in an embodiment of the present invention.

[0024] Figure 2This is a flowchart of the fixed-bed experimental apparatus used to test the NOx capture performance of activated carbon in the embodiments and comparative examples of the present invention. Detailed Implementation

[0025] 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 specific embodiments. The following embodiments are for illustrative purposes only and should not be considered as limitations on the invention.

[0026] The commercially available activated carbon used in this invention can be coconut shell activated carbon, coal-based activated carbon, wood-based activated carbon, etc., with an initial specific surface area generally of 500-600 m² / g. Glucose is analytical grade or industrial grade. All gases (N2, CO2, simulated flue gas) are commercially available standard gases.

[0027] Example 1

[0028] This embodiment provides a modified activated carbon preparation and performance evaluation under preferred conditions.

[0029] S1. Impregnation and Loading: Weigh 10.0 g of coconut shell activated carbon (particle size 20-40 mesh) with an iodine value of 950 mg / g and impregnate it in 300 mL of a 0.3 mol / L glucose aqueous solution. Impregnate for 24 hours in a constant temperature water bath at 30℃ with mechanical stirring at 150 rpm. After impregnation, filter and separate the solid product. Dry the solid product in a forced-air drying oven at 80℃ for 12 hours to obtain the glucose-loaded activated carbon precursor.

[0030] S2. Hydrothermal carbonization: The dried precursor was uniformly placed in a 100 mL polytetrafluoroethylene-lined container and sealed in a stainless steel hydrothermal reactor. The reactor was placed in an oven and heated to 180°C, and the reaction was maintained at this temperature for 5 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. The product was then removed and repeatedly washed with 60°C deionized water until the filtrate was neutral and colorless. It was then dried at 105°C for 6 hours to obtain the hydrothermal carbonization intermediate, denoted as AC-G-180H.

[0031] S3. CO2 Gradient Activation: Place approximately 5.0 g of AC-G-180H sample in a quartz boat of a tubular furnace. Under a protective atmosphere of 100 mL / min high-purity N2 (99.999%), heat to 800℃ at a programmed heating rate of 5℃ / min. After reaching the target temperature, switch the gas flow to 200 mL / min high-purity CO2 and maintain the temperature at 800℃ for 4 hours. After activation, switch the gas flow back to N2 (100 mL / min) and allow it to cool naturally to room temperature (<50℃) while maintaining N2 flow. The resulting product is the modified activated carbon, denoted as AC-G-180H-800C4.

[0032] Structural characterization: The N2 adsorption-desorption isotherm of the samples was determined at 77 K using a physical adsorption analyzer (Micromeritics ASAP 2460). The specific surface area was calculated using the BET model, and the pore size distribution and micropore volume were calculated using the NLDFT (carbon slit pore model). The results showed that the BET specific surface area of ​​the raw activated carbon was 780 m² / g, the total pore volume was 0.42 cm³ / g, and the micropore volume (pore size <2 nm) was 0.28 cm³ / g. The modified activated carbon AC-G-180H-800C4 showed an increased BET specific surface area of ​​850 m² / g, a total pore volume of 0.58 cm³ / g, and a significantly increased micropore volume of 0.43 cm³ / g (an increase of approximately 53.6%). A distinct new peak appeared in its pore size distribution curve near 0.7 nm, indicating the successful formation of a large number of micropores of the target size.

[0033] Denitrification performance test: in such Figure 2 Dynamic adsorption tests were conducted on the fixed-bed reactor shown. 0.50 g (accurate to 0.001 g) of modified activated carbon sample (20-40 mesh) was weighed and placed into a quartz reaction tube with an inner diameter of 10 mm, and both ends were fixed with quartz wool. The simulated flue gas composition was: NO 500 ppm, O2 5%, equilibrium gas was N2, and the total gas flow rate was 500 mL / min (corresponding to a space velocity of approximately 6000 h⁻¹). The reaction was carried out at atmospheric pressure and 30℃. The NO and NO2 concentrations at the reactor inlet and outlet were continuously monitored using an online Fourier transform infrared gas analyzer (FTIR, Gasmet DX4000). The breakthrough point was defined as the outlet NOx (NO+NO2) concentration reaching 10% of the inlet NO concentration, and the breakthrough adsorption capacity was calculated.

[0034] Test results: The NOx breakthrough adsorption capacity of the raw activated carbon was 0.45 mmol / g. The modified activated carbon AC-G-180H-800C4, however, achieved a breakthrough adsorption capacity of 1.18 mmol / g, which is 2.62 times that of the raw material. Furthermore, significant NO to NO2 conversion (conversion rate exceeding 40%) was observed in the initial stages of the test, indicating that the material possesses excellent catalytic oxidation activity.

[0035] Example 2

[0036] This embodiment explores the effect of hydrothermal carbonization temperature.

[0037] Except for changing the hydrothermal carbonization temperature in step S2 to 160°C, all other steps and conditions were the same as in Example 1. The resulting sample was designated AC-G-160H-800C4.

[0038] Characterization and Testing: The sample has a BET specific surface area of ​​810 m² / g and a micropore volume of 0.38 cm³ / g. Its NOx breakthrough adsorption capacity under the same denitrification test conditions is 0.92 mmol / g. The results indicate that the hydrothermal carbonization at 160℃ is slightly weaker, and the resulting hydrothermal carbon layer may be thinner or have a lower degree of cross-linking. This leads to a slightly fewer micropores generated by subsequent CO2-activated etching compared to the 180℃ condition. The performance is still significantly better than the raw material but slightly lower than that of Example 1.

[0039] Example 3

[0040] This embodiment explores the effect of CO2 activation time.

[0041] Except for changing the CO2 activation time in step S3 to 2 hours, all other steps and conditions were the same as in Example 1. The resulting sample was designated AC-G-180H-800C2.

[0042] Characterization and Testing: The sample has a BET specific surface area of ​​780 m² / g and a micropore volume of 0.35 cm³ / g. Its NOx breakthrough adsorption capacity under the same denitrification test conditions is 0.81 mmol / g. The results indicate that shortening the activation time leads to insufficient gasification reaction, insufficient number of newly formed micropores, and limited performance improvement. Extending the activation time (such as 4 hours in Example 1) is beneficial for the full development of micropores.

[0043] Example 4

[0044] This embodiment explores the effect of glucose loading.

[0045] Except for changing the concentration of the glucose aqueous solution in step S1 to 0.1 mol / L, all other steps and conditions were the same as in Example 1. The resulting sample was denoted as AC-G(0.1)-180H-800C4.

[0046] Characterization and Testing: The sample has a BET specific surface area of ​​760 m² / g and a micropore volume of 0.32 cm³ / g. Its NOx breakthrough adsorption capacity under the same denitrification test conditions is 0.70 mmol / g. The results indicate that a low glucose concentration leads to insufficient carbon precursor content within the pores, limiting the available "template" for subsequent etching. The "two-sided approach" strategy was not effectively implemented, resulting in a weakened pore-forming effect.

[0047] Comparative Example 1

[0048] The denitrification performance was tested directly using the raw coconut shell activated carbon (without any modification) from Example 1, under the same conditions as in Example 1. Its NOx breakthrough adsorption capacity was 0.45 mmol / g.

[0049] Comparative Example 2

[0050] Hydrothermal carbonization was performed only, without CO2 activation. That is, sample AC-G-180H was prepared according to steps S1 and S2 of Example 1, without step S3. Its denitrification performance was tested, and the NOx breakthrough adsorption capacity was 0.52 mmol / g. This indicates that introducing only oxygen-containing functional groups without significantly increasing the matching micropores resulted in very limited performance improvement.

[0051] Comparative Example 3

[0052] Comparison with traditional KOH activation method. 10.0 g of activated carbon of the same raw material was weighed and mixed with solid KOH at a mass ratio of 1:3. A small amount of deionized water was added for wetting, and after drying, the mixture was placed in a tube furnace and activated at 800℃ for 1.5 hours under N2 protection at a rate of 5℃ / min. After cooling, it was washed with hydrochloric acid and water until neutral, and then dried to obtain KOH-activated activated carbon. Its BET specific surface area is as high as 1850 m² / g, but the micropore size distribution is relatively wide. Under the same denitrification test conditions, its NOx breakthrough adsorption capacity is 0.88 mmol / g. Although its adsorption capacity is higher than that of the raw material, it is lower than that of the optimal embodiment of this invention (1.18 mmol / g). More importantly, the KOH activation process is highly corrosive, the waste liquid is difficult to treat, and it over-activates all carbonaceous materials, failing to achieve "directional pore formation within the pores" as in this invention.

[0053] The above examples and comparative examples fully demonstrate that the present invention, through the synergistic process of "in-situ carbonization in pores-gradient activation", can directionally and efficiently construct modified activated carbon rich in micropores of specific sizes and active sites. It exhibits significantly better performance than raw activated carbon and products obtained by traditional modification methods in low-temperature flue gas denitrification, and the process is more green and environmentally friendly.

Claims

1. A method for preparing modified activated carbon for enhancing the nitrogen oxide capture performance of flue gas, characterized in that, Includes the following steps: S1. Impregnation and loading: Commercial activated carbon is impregnated in a glucose aqueous solution at 20℃~40℃ for 12~36 hours to allow glucose molecules to fully diffuse and adsorb into the macropores and mesopores of the activated carbon; after impregnation, solid-liquid separation is performed, and the carbon is dried at 60℃~100℃ to obtain glucose-loaded activated carbon precursor. S2. Hydrothermal carbonization: The activated carbon precursor obtained in step S1 is placed in a hydrothermal reactor and subjected to hydrothermal carbonization reaction at 160℃~220℃ for 4~8 hours, so that glucose is converted in situ into a carbonaceous layer rich in oxygen-containing functional groups in the pores; after the reaction is completed, it is washed and dried to obtain the hydrothermal carbonization intermediate. S3. CO2 gradient activation: Under inert gas protection, the hydrothermal carbonization intermediate obtained in step S2 is heated to 700℃~900℃ at a heating rate of 2℃ / min~10℃ / min, and then CO2 gas is introduced to perform microporous directional activation treatment for 2~5 hours under CO2 atmosphere; after activation, it is cooled to room temperature under inert atmosphere to obtain the modified activated carbon.

2. The preparation method according to claim 1, characterized in that, In step S1, the concentration of the glucose aqueous solution is 0.1 mol / L to 0.5 mol / L; the mass ratio of the commercial activated carbon to glucose is 1:1 to 1:

5.

3. The preparation method according to claim 1, characterized in that, In step S1, the impregnation is carried out under stirring or ultrasonic assistance to promote the penetration and diffusion of the glucose solution.

4. The preparation method according to claim 1, characterized in that, In step S2, the hydrothermal carbonization reaction is carried out under sealed conditions, and the reaction medium is deionized water or the liquid phase remaining in the impregnation step.

5. The preparation method according to claim 1, characterized in that, In step S2, the preferred temperature for the hydrothermal carbonization reaction is 180°C, and the preferred reaction time is 5 hours.

6. The preparation method according to claim 1, characterized in that, In step S3, the preferred temperature for CO2 activation treatment is 800℃, the preferred activation time is 4 hours, and the CO2 gas flow rate is 100 mL / min to 500 mL / min.

7. A modified activated carbon prepared by the preparation method according to any one of claims 1 to 6, characterized in that, The modified activated carbon has a micropore volume that is more than 30% larger for NO oxidation compared to the raw activated carbon, and its low-temperature adsorption capacity for nitric oxide (NO) in flue gas is increased by more than 100%.

8. The modified activated carbon according to claim 7, characterized in that, The modified activated carbon has a BET specific surface area of ​​700 m² / g to 1200 m² / g, and the micropore volume with a pore size in the range of 0.5 nm to 0.8 nm accounts for more than 60% of the total micropore volume.

9. The application of the modified activated carbon as described in claim 7 or 8 in the capture of nitrogen oxides in flue gas.

10. The application according to claim 9, characterized in that, The flue gas originates from coal-fired power plants, industrial boilers, steel sintering, or vehicle exhaust; the nitrogen oxides include nitric oxide and nitrogen dioxide; the collection process is carried out under low-temperature conditions of 20℃ to 150℃.

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