Biomass and Coal Modification to Prepare Activated Coke for Desulfurization and Denitrification

Abstract

SO₂ and NOx in flue gas are serious environmental pollutants. As an excellent adsorbent, activated coke has good application potential in flue gas desulfurization and denitrification. In this study, different concentrations of NH₃ and K₂CO₃ solutions were used to modify the activated coke made from biomass and coal, and then separate and coordinated desulfurization and denitrification experiments were carried out. The adsorption efficiency of activated coke and the adsorption capacity of SO₂ and NO were compared, and the adsorption relationship between SO₂ and NO was clarified. The changes of the surface functional groups of activated coke before and after adsorption and the forms of SO₂ and NO after adsorption were analyzed by Fourier transform infrared spectrometer and X-ray diffractometer patterns. Results show that K₂CO₃ and NH₃ modification can promote the adsorption of SO₂ and NO in activated coke, and K₂CO₃ modification is more significant for improving the denitrification and desulfurization capacity. When the unmodified activated coke synergistically adsorbs SO₂ and NO, there is a competitive adsorption between NO and SO₂. However, when the activated coke modified with K₂CO₃ solution and NH₃ synergistically adsorbs SO₂ and NO, both have a promoting effect. After the adsorption of NH₃-modified activated coke, CaSO₄ and Na₂SO₄ crystals will appear. K₂SO₄ will exist after the adsorption of activated coke modified with K₂CO₃ solution.

1. Introduction

Flue gas pollutes the atmosphere around residential areas, and its components are very complex, including steam, SO₂, N₂, CO, CO₂, O₂, hydrocarbon compounds, and nitrogen oxides. In flue gas, SO₂ and NOx are the most harmful to human health and the environment [1,2]. Activated coke is an excellent adsorbent, and its specific surface area, pore structure abundance, and functional group types can all affect its physicochemical properties and adsorption performance [3,4,5,6]. Due to the difference in adsorption medium (gas phase and liquid phase) and adsorbate, the properties of activated coke are also different. Usually, activated coke with developed granular and microporous structure is mostly used for gas phase adsorption. The adsorption of pollutants by activated coke is divided into physical adsorption and chemical adsorption [7,8]. Physical adsorption relies on the van der Waals force combined with activated coke’s larger specific surface area and developed pore structure to adsorb pollutants in activated coke. Chemical adsorption relies on the defective carbon atoms and surface chemical functional groups on the surface of the activated coke to convert pollutants into non-polluting substances in a targeted manner. Activated coke desulfurization and denitrification have the advantages of strong reliability, low water consumption, high stability, no secondary pollution, and resource utilization, and has become the focus of research in recent years [9,10,11,12,13,14]. At present, the use of activated coke to desulfurize and denitrify flue gas is the most practical method for controlling the emission of polluting gases [10,15].

The essence of activated coke flue gas desulfurization is to use the abundant pore structure to adsorb and catalyze SO₂, O₂, H₂O, and other gases in flue gas, so as to convert SO₂ in the gas phase into H₂SO₄ [16,17], and store them in the pores of activated coke. Therefore, a flue gas desulfurization method using activated carbon to adsorb SO₂ was developed [10,11,18]. The essence of activated coke denitrification also mainly relies on the microporous structure and surface functional groups of activated coke for adsorption. First, NO is oxidized to NO₂, and then NO₂ is combined with H₂O and stored in the pore structure of activated coke in the form of HNO₃. Davini et al. [19] prepared activated coke with asphalt and polyacrylonitrile as raw materials, and carried out SO₂ adsorption experiments at an adsorption temperature of 360 °C to explore the effect of SO₂ concentration on desulfurization efficiency. They found that the basic functional groups on the surface of the activated coke were favorable for the adsorption of SO₂, and the adsorption amount was up to 87 mg/g. Raymundo-Pinero et al. [20] used ammonia to activate carbonaceous materials to prepare activated coke, and detected the functional groups on the surface of activated coke. It was found that nitrogen-containing functional groups can play a role in the desulfurization process. Shirahama N et al. [21] prepared activated coke to adsorb NO₂ using asphalt as raw material, and found that activated coke showed a higher adsorption efficiency (75%) at an adsorption temperature of 30 °C.

For a long time, people have gradually realized that when SO₂ and NOx are treated separately, the investment and operating costs are high. In order to reduce the cost of flue gas purification and meet the needs of power plants, new technologies for synergistic desulfurization and denitrification need to be developed [22]. Lee et al. [23] used KOH modification to prepare activated coke to adsorb SO₂ and NO gas in flue gas. The study showed that the adsorption capacity of activated coke for NO was greater than that for SO₂ in the solo adsorption process, and SO₂ had more affinity than NO in the synergistic adsorption process.

The synergistic desulfurization and denitrification of activated coke has been widely studied, but the reaction mechanism of SO₂ and NO in the adsorption process remains unclear. The change of adsorption efficiency of activated coke obtained after modification with different modifying reagents and the change of activated coke before and after adsorption are also less studied. Biomass has low cost, large reserves, and less sulfur and nitrogen [24,25,26]. Coal has a high carbon content, is widely distributed and readily available, and is an excellent precursor material for the preparation of activated coke [27,28,29,30,31,32]. In this study, we used poplar bark and coal as raw materials to modify the activated coke and conducted solo and synergistic desulfurization and denitrification experiments. We compared the adsorption efficiency and adsorption capacity of activated coke and clarified the adsorption relationship between SO₂ and NO. The FTIR and XRD patterns were used to analyze the changes of the surface functional groups of the activated coke before and after the adsorption and the existing forms of SO₂ and NO after the adsorption, which provided a theoretical basis for the preparation and industrial application of the activated coke with specific adsorption properties.

2. Experimental

2.1. Preparation of Experimental Samples

The activated coke used in the experiment was the activated coke prepared in the previous modification experiments. The relevant data of modified activated coke are obtained from our previous experiments, see

Table 1. Physical properties of modified activated coke [5].

Modification Reagent	Total Surface Area (m2/g)	Micropore Specific Surface Area (m2/g)	Average Hole Diameter (nm)	Micropore Ratio (%)
unmodified	585	334	3.40	57.1
5% NH3	620	325	3.18	52.5
10% NH3	626	328	3.28	52.4
15% NH3	637	332	2.86	52.0
20% NH3	652	330	3.38	50.6
3% K2CO3	627	517	2.67	82.4
6% K2CO3	602	540	1.94	89.8
9% K2CO3	484	469	1.94	96.9
12%K2CO3	438	427	2.20	97.5

[5]. The specific surface area value and pore size distribution were all measured and calculated by using the BSD-PS1/2 high-performance specific surface area analyzer (Beijing Beishide Instrument Technology, Beijing, China).

2.2. Experiment Procedure

As shown in Figure 1, the system consists of three parts: simulated flue gas distribution system, fixed bed reaction system, and flue gas analysis system. The simulated flue gas used in the experiment was composed of SO₂, NO, N₂, O₂, and steam. The concentration of SO₂ and NO is controlled by a mass flow meter to be 400 ppm respectively, and the amount of activated coke is 0.5 g. The fixed bed reaction system is a quartz glass tube with an inner diameter of 18 mm and a length of 580 mm. After the gas entered the gas mixing bottle and was evenly mixed, it was mixed with the steam provided by the micro-injection pump and the heating cable and passed into the fixed bed reactor. A flue gas analyzer was used for gas distribution calibration. After the flue gas analyzer reading was stable, 0.5 g of activated coke was added into the middle of the fixed reaction bed for adsorption test. The fixed bed reaction temperature was set to 70 °C [33], the reaction time (1–2 h) was determined according to the adsorption efficiency, and the concentration of the gas at the outlet of the reactor was detected by a flue gas analyzer. The suction flow rate of the gas pump of the flue gas analyzer is 500 mL/min, so the total flow set for the adsorption experiment is 500 mL/min. When the SO₂ and NO outlet concentrations reached the expected value or reached the set time, the experiment was stopped, and the exhaust gas was completely treated by the gas washing bottle (containing NaOH solution) and then discharged into the air. After the experiment, the adsorbed activated coke was encapsulated for subsequent detection and use.

2.3. Material Characterization

Field emission scanning electron microscopy (Zeiss Sigma 300) produced by Zeiss, Germany was used to observe the microscopic morphological changes of the activated coke before and after adsorption. A Fourier transform infrared spectrometer (Thermo Scientific Nicolet iS50) produced by Thermo Fisher Scientific Co., Ltd. (Waltham, MA, USA) in the US was used to detect the changes of activated coke functional groups before and after adsorption. X-ray diffractometer (Ultima IV) produced by Shanghai Xinghe Instrument Co., Ltd. (Shanghai, China) was used to analyze the phase of activated coke to detect the phase change before and after adsorption of activated coke.

2.4. Performance Evaluation Method

The desulfurization and denitration performance of activated coke is mainly reflected by SO₂ adsorption efficiency, NO adsorption efficiency, SO₂ adsorption capacity, and NO adsorption capacity. The calculation method is as follows:

Desulfurization efficiency of activated coke:

$${\mathsf{\eta }}_{{\mathrm{SO}}_2}=\frac{{\mathrm{C}}_{\mathrm{in}}-{\mathrm{C}}_{\mathrm{out}}}{{\mathrm{C}}_{\mathrm{in}}}\times 100\%$$

(1)

Denitration efficiency of activated coke:

$${\mathsf{\eta }}_{\mathrm{NO}}=\frac{{\mathrm{C}}_{\mathrm{in}}-{\mathrm{C}}_{\mathrm{out}}}{{\mathrm{C}}_{\mathrm{in}}}\times 100\%$$

(2)

In Equations (1) and (2), η represents the adsorption efficiency (%), C_in is the gas concentration at the inlet of the fixed reaction bed (ppm), and C_out is the gas concentration at the outlet of the fixed reaction bed (ppm).

The SO₂ and NO adsorption capacity of activated coke refers to the SO₂ and NO adsorption capacity per unit mass of activated coke within a certain period of time.

SO₂ adsorption capacity of activated coke:

$${\mathrm{V}}_{{\mathrm{SO}}_2}={{\displaystyle \int }}_0^{\mathrm{t}}\frac{\left({\mathrm{C}}_{\mathrm{in}}-{\mathrm{C}}_{\mathrm{out}}\right)\times \mathrm{M}\times \mathrm{Q}\times {10}^{-6}}{22.4\times {\mathrm{m}}_{\mathrm{AC}}}\mathrm{dt}$$

(3)

NO adsorption capacity of activated coke:

$${\mathrm{V}}_{\mathrm{NO}}=={{\displaystyle \int }}_0^{\mathrm{t}}\frac{\left({\mathrm{C}}_{\mathrm{in}}-{\mathrm{C}}_{\mathrm{out}}\right)\times \mathrm{M}\times \mathrm{Q}\times {10}^{-6}}{22.4\times {\mathrm{m}}_{\mathrm{AC}}}\mathrm{dt}$$

(4)

In Equations (3) and (4), V represents the cumulative adsorption capacity of activated coke per unit mass (mg/g), M represents the molar mass of SO₂ and NO (g/mol), Q is the simulated flue gas flow (L/min), t represents the total time required for adsorption (min), and ${\mathrm{m}}_{\mathrm{AC}}$ is the mass of activated coke in the fixed bed (g).

3. Results and Discussion

3.1. Study on the Desulfurization and Denitrification Performance of NH₃-Modified Activated Coke

3.1.1. Effect of Modified Activated Coke with Different NH₃ Concentrations on SO₂ Adsorption Efficiency

Figure 2 shows that the process of SO₂ adsorption by activated coke can be divided into fast adsorption stage, transition adsorption, and slow adsorption stage [34]. The fast adsorption stage refers to the stage where the SO₂ adsorption efficiency is above 99%; the slow adsorption stage refers to the stage where the SO₂ adsorption efficiency is stable; the transition stage refers to stage between the slow adsorption stage and the fast adsorption stage. Figure 2 shows that the activated coke modified with 15% NH₃ has the longest fast adsorption stage, whereas the activated coke without NH₃ modification has no fast adsorption stage. In the transition and slow adsorption stages, the adsorption efficiency curves under these five modified conditions have similar linearity and changing trends, but the magnitude of the adsorption efficiency is obviously different. In the slow adsorption stage, the adsorption efficiency of activated coke modified with 5% NH₃ was stable at about 40%, and the adsorption efficiency of activated coke without NH₃ modification was the lowest at about 22%.

Activated coke plays a dual role of adsorbent and catalytic oxidant in the process of adsorbing SO₂. The process is to first adsorb O₂, SO₂, and steam in the flue gas on the surface of the activated coke and then react O₂ (adsorbed state) and SO₂ (adsorbed state) to oxidize SO₂ to SO₃. Finally, SO₃ reacts with steam (adsorbed state) to form H₂SO₄. The basic steps of the activated coke desulfurization reaction are as follows [35]:

$$\frac{1}{2}{\mathrm{O}}_2+\mathsf{\sigma }\rightleftharpoons {\mathrm{O}}^{*}$$

(5)

$${\mathrm{SO}}_2+\mathsf{\sigma }\rightleftharpoons {\mathrm{SO}}_2^{*}$$

(6)

$${\mathrm{SO}}_2^{*}+{\mathrm{O}}^{*}\to {\mathrm{SO}}_3^{*}+\mathsf{\sigma }$$

(7)

$${\mathrm{H}}_2\mathrm{O}+\mathsf{\sigma }\rightleftharpoons {\mathrm{H}}_2{\mathrm{O}}^{*}$$

(8)

$${\mathrm{H}}_2{\mathrm{O}}^{*}+{\mathrm{SO}}_3^{*}\rightleftharpoons {\mathrm{H}}_2{\mathrm{SO}}_4+\mathsf{\sigma }$$

(9)

where σ is the adsorption activated site and the superscript * on the right represents the adsorption state. Activated coke adsorption is further divided into physical adsorption and chemical adsorption. Physical adsorption is caused by the balance of intermolecular attractive and repulsive forces; chemical adsorption is mainly achieved by chemical reactions between gas molecules and the activated sites on the adsorbent interface. Equations (5), (6) and (8) are chemical adsorption, in which gas molecules react with the activated sites on the surface of the activated coke to form gas molecules (adsorbed state). Equations (7) and (8) show that when SO₂ is oxidized to SO₃, it reacts with steam to form H₂SO₄, which will release new activated sites.

3.1.2. Effect of Modified Activated Coke with Different NH₃ Concentrations on NO Adsorption Efficiency

Figure 3 shows the denitration process curves of activated coke modified with different NH₃ concentrations. There is almost no fast adsorption stage in the denitration process of activated coke, and the transition stage enters the slow adsorption stage soon after the experiment starts. The NO adsorption efficiency of activated coke prepared under different modification conditions showed the same trend, but the final adsorption efficiency was different. The final adsorption efficiency of the activated coke without NH₃ modification treatment is the lowest at about 10%, whereas the final adsorption efficiency of the activated coke modified with 15% NH₃ concentration is about 17%. From the whole denitration process, the adsorption efficiency of activated coke to NO has been improved after NH₃ modification.

Under aerobic conditions, most NO is oxidized to NO₂. The polarity of NO₂ is stronger than that of NO, which makes it easier to adsorb on the active center of activated coke and to be reduced more easily. O₂ and NO in the gas phase react with NH₃ originally adsorbed on the surface of activated coke to generate N₂ and H₂O [36]. In addition to physical adsorption, the denitration process of activated coke has the following reactions:

$${\mathrm{NH}}_3+\mathsf{\sigma }\rightleftharpoons {\mathrm{NH}}_3^{*}$$

(10)

$${\mathrm{NH}}_3^{*}+\mathrm{NO}+{\mathrm{O}}_2\to {\mathrm{N}}_2+{\mathrm{H}}_2{\mathrm{O}}^{*}$$

(11)

In the denitration process of activated coke, part of NO is removed by physical adsorption, and part of it is catalytically reduced with NH₃, so as to achieve the purpose of removal. The physical adsorption of NO occupies a small part, so there is no obvious fast adsorption stage in the activated coke denitrification process. The pore structure of activated coke is only one of the factors affecting NO adsorption, whereas its surface chemical properties have a greater impact on NO adsorption.

3.1.3. SO₂/NO Adsorption Capacity per Unit Mass of Activated Coke after Modification with Different NH₃ Concentrations

Table 2. SO₂/NO adsorption capacity per unit mass of activated coke.

Sample	SO2 Adsorption Capacity (mg/g)	NO Adsorption Capacity (mg/g)
unmodified	7.25	4.15
5% NH3 modified	14.2	6.21
10% NH3 modified	13.3	5.08
15% NH3 modified	14.4	6.58
20% NH3 modified	14.1	6.33
3% K2CO3 modified	19.8	8.24
6% K2CO3 modified	18.8	13.2
9% K2CO3 modified	36.6	16.1
12% K2CO3 modified	68.0	18.1

shows the adsorption capacity of SO₂ and NO per unit mass of activated coke. As can be seen, the activated coke modified with 15% NH₃ concentration has the highest SO₂ and NO adsorption capacity (14.4 and 6.58 mg/g, respectively). On the whole, the desulfurization performance of activated coke is doubled after NH₃ modification, whereas the concentration of NH₃ modification has little effect on the desulfurization performance of activated coke. NH₃ catalyzes the reduction of NO and promotes the conversion of NO into N₂, so the adsorption capacity will also increase. The values in Table 2 were also validated from recent studies. Wang et al. [33] also prepared activated coke by ammonia modification and carried out SO₂ adsorption experiments. They found that the adsorption capacity of the ammonia-modified activated coke was significantly improved compared with that of the non-ammonia-modified activated coke.

3.2. Study on the Desulfurization and Denitrification Performance of K₂CO₃-Modified Activated Coke

3.2.1. Effect of Modified Activated Coke with Different K₂CO₃ Concentrations on SO₂ Adsorption Efficiency

Figure 4 shows the effect of activated coke modified with different K₂CO₃ concentrations on the SO₂ adsorption efficiency. As can be seen, the unmodified activated coke has almost no fast adsorption stage. With the increase of K₂CO₃ concentration, the fast adsorption stage of activated coke was prolonged. The SO₂ fast adsorption stage of activated coke modified with 9% and 12% K₂CO₃ concentrations lasted for 60 min, and then the SO₂ adsorption efficiency gradually decreased, indicating that its microporous pore structure is rich. The adsorption efficiency of activated coke SO₂-modified with 3% and 6% K₂CO₃ concentrations is basically maintained at about 60%. Given that K₂CO₃ solution modifies the activated coke to load K metal, which plays an important role in the adsorption of SO₂, SO₂ (adsorbed state) is oxidized and combined with H₂O to form H₂SO₄, which in turn reacts with K metal. Therefore, the higher concentration of K₂CO₃ solution will promote the SO₂ adsorption performance of modified activated coke.

3.2.2. Effect of Modified Activated Coke with Different K₂CO₃ Concentrations on NO Adsorption Efficiency

Figure 5 shows that the higher the concentration of K₂CO₃ solution, the higher the denitration performance of the modified activated coke, which has a great relationship with the microporous structure and surface functional groups of the activated coke. When the modified activated coke adsorbs NO, physical adsorption and chemical adsorption exist simultaneously [37]. The role of the modifier is mainly to improve the chemical adsorption performance of the activated coke for NO by adjusting the physical and chemical properties of the activated coke surface. The adsorption efficiency of activated coke modified with 12% K₂CO₃ concentration remained at about 60% in the first 15 min. In this process, there were both physical adsorption of NO by pore structure and chemical adsorption of NO after oxidation to NO₂. The oxidized nitrogen oxides react with the metal oxides in the pore structure and are adsorbed on the surface of the activated coke. With the increase of time, the denitration performance of activated coke gradually decreased, because the activated sites of the pore structure were occupied, resulting in the gradual saturation of micropores. On the whole, the modification of K₂CO₃ solution greatly improves the denitration performance of activated coke, and the effect of 3% and 6% K₂CO₃ concentrations is more significant. With the continuous increase of K₂CO₃ concentration, this promoting effect became no longer obvious.

3.2.3. SO₂/NO Adsorption Capacity per Unit Mass of Activated Coke after Modification with Different K₂CO₃ Concentrations

It can be seen from Table 2 that, the SO₂ and NO adsorption of the unmodified activated coke are the smallest (7.25 and 4.15 mg/g, respectively). With the increase of modifier concentration, the adsorption capacity of SO₂ and NO in activated coke showed an upward trend, indicating that K₂CO₃ could improve the adsorption capacity of activated coke for SO₂ and NO at the same time. The desulfurization and denitration capacities per unit mass of activated coke modified with 12% K₂CO₃ concentration are 67.95 and 18.11 mg/g, respectively, which are 10 and 5 times higher than that of unmodified activated coke. In addition to physical adsorption, SO₂ in the simulated flue gas will react with O₂ to form SO₃, which will then combine with water molecules to form H₂SO₄, and H₂SO₄ will continue to react with metal oxides. NO will also react with O₂ adsorbed on the activated coke to form NO₂ (adsorbed state), and NO₂ (adsorbed state) will react with H₂O (adsorbed state) to form HNO₃ (adsorbed state) [38]. A portion of HNO₃ (adsorbed state) reacts with the metal oxides on the activated coke. After H₂SO₄ and HNO₃ react with metal oxides, more activated sites will be generated for physical and chemical adsorption of SO₂ and NO.

3.3. Study on the Solo/Synergistic Performance of Modified Activated Coke for Desulfurization and Denitrification

3.3.1. Solo and Synergistic Adsorption EFFICIENCIES of Activated Coke

The solo and synergistic desulfurization and denitrification efficiencies of activated coke prepared under different modification conditions are shown in Figure 6. As can be seen from the figure, when the activated coke adsorbs SO₂ solo, the adsorption efficiency of SO₂ by the activated coke modified with NH₃ is not significantly improved, which is basically the same as that of the unmodified activated coke, and the SO₂ adsorption efficiency is finally stabilized at about 22%. However, the adsorption efficiency of SO₂ by the activated coke modified with K₂CO₃ solution was significantly improved and maintained at about 50%. This is related to the pore structure and surface functional groups of activated coke. In the initial stage of SO₂ adsorption, activated coke is mainly physically adsorbed; however, the proportion of micropores of activated coke modified with K₂CO₃ solution can reach 90%, and the micropore structure is quite rich. When SO₂ adsorption was in the slow and stable adsorption stage, chemical adsorption played the main role at this time, and K₂CO₃ solution modification not only developed the microporous structure of activated coke, but also loaded K₂O in the pore structure of activated coke. K₂O reacts with SO₂, O₂, and steam to form sulfate, which also improves the adsorption efficiency of SO₂. When the activated coke was synergistically desulfurized and denitrified, the adsorption efficiency of SO₂ by the activated coke modified with K₂CO₃ solution reached 60%, which is higher than that of solo desulfurization. This indicates that synergistic desulfurization and denitrification (in the presence of NO) contribute to the adsorption of SO₂. The denitration effect of activated coke modified with K₂CO₃ is better than that of activated coke under other modified conditions, and the synergistic adsorption efficiency is higher than that of solo adsorption.

3.3.2. Capacity of Activated Coke per Unit Mass to Adsorb SO₂ and NO Solo/Synergistically

The adsorption capacities of SO₂ and NO by the activated coke prepared under different modification conditions are shown in Figure 7. As can be seen from the figure, when the unmodified activated coke adsorbs NO and SO₂ solo, the total adsorption capacity is greater than that in the synergistic adsorption, which indicates that competitive adsorption occurs when SO₂ and NO coexist. According to the adsorption theory, when the multi-component gas passes through the adsorption bed, the high boiling point will replace the low boiling point, and the high molecular weight will replace the small molecular weight. The boiling point and molecular weight of SO₂ are higher than those of NO, so SO₂ is more competitive in the adsorption process. The synergistic adsorption capacity of SO₂ and NO of activated coke modified with K₂CO₃ solution and NH₃ is higher than that of solo adsorption. This is because in the synergistic adsorption process, only the physically adsorbed NO will be replaced, whereas the chemically adsorbed NO is not easily replaced. Under the action of surface functional groups, chemically adsorbed NO reacts with adsorbed SO₂ to generate new intermediate products NO₂ (adsorbed state) and SO₃ (adsorbed state), and H₂SO₄ and HNO₃ can be generated under the action of water [39]. The presence of NO will promote the adsorption of SO₂. Therefore, the modified activated coke has higher adsorption capacity and adsorption efficiency when it is synergistically adsorbing SO₂ and NO.

3.4. Changes of Surface Microstructure before and after Adsorption of Modified Activated Coke

Scanning electron microscopy and infrared spectroscopy were performed on the activated coke before and after adsorption to understand the surface morphology and functional group changes of the activated coke adsorbing SO₂ and NO. This section takes 5% NH₃- and 3% K₂CO₃-modified activated coke as an example.

3.4.1. Study on the Microstructure Change of Activated Coke Modified with 5% NH₃

Figure 8a shows that the pore structure of the NH₃-modified activated coke is relatively developed, exists in a layered structure, and has a large number of slits and pores. After the adsorption of SO₂ and NO, the surface becomes more disordered, the pore structure of the activated coke does not change much, and the layered structure still exists; however, a large number of crystalline particles are scattered on the surface of the activated coke. This is because the SO₂ and NO adsorbed by the activated coke react with it and exist in the form of crystals on its surface. In order to further verify the adsorption forms of SO₂ and NO by activated coke, infrared spectroscopy analysis was performed on activated coke before and after adsorption. Figure 9 shows that the activated coke mainly shows strong absorption peaks at 3400–3700, 1400, and 1100 cm⁻¹ and weaker absorption peaks at 2800 and 2400 cm⁻¹. The peak at 3430 cm⁻¹ is due to OH stretching vibrations generated by hydroxyl functional groups and adsorbed moisture [40,41,42]. The peak at 1400 cm⁻¹ is formed by C-H bending vibration, which proves the existence of aliphatic structure. Comparing the infrared spectra before and after adsorption, the types of surface functional groups of activated coke have not changed, but an obvious absorption peak appears around 3680 cm⁻¹, indicating the existence of a trace amount of NH₄⁺. The peak at 1100 cm⁻¹ also increases significantly, indicating that SO₄²⁻ appears after the activated coke adsorbs SO₂ [33]. Therefore, after the activated coke adsorbs SO₂ and NO at the same time, SO₂ mainly exists on the surface of the activated coke in the form of H₂SO₄ and (NH₄)₂SO₄. The peaks around 870 cm⁻¹ are mainly related to the presence of C-H groups at the edge of the aromatic ring. The peak around 650 cm⁻¹, the out-of-plane bending vibration peak of hydroxy −OH is located when hydrogen bond association occurs.

3.4.2. Study on the Microstructure Change of Activated Coke Modified with 3% K₂CO₃

Part of the pore structure of the activated coke modified with K₂CO₃ solution merges and collapses to form super-large pores, and the originally occluded pore structure will be activated and opened to generate cracks and pore structures. Comparing the SEM images of the activated coke before and after adsorption of SO₂ and NO, the surface of the activated coke in Figure 10a is relatively smooth, and there are a large number of small block structures. The activated coke in Figure 10b also has a large number of small block structures, and small particles appear on the surface, because SO₂ and NO exist in the form of crystals on the surface of the activated coke after being adsorbed. Chemisorption involves two factors: one is the surface functional group, and the other is the metal oxide supported on the surface of the activated coke. Figure 11 shows the FTIR spectra of the activated coke before and after adsorption of SO₂ and NO. As can be seen, the types of functional groups on the surface of the activated coke do not change between the wavenumbers of 4000 and 400 cm⁻¹, but the content of some functional groups increases. There is a strong peak at 3430 cm⁻¹, which is formed by the stretching vibration of the O-H bond in carboxylic acid, and there are a large number of hydroxyl functional groups in the adsorbed activated coke. The peak at 1127 cm⁻¹ is also significantly increased, indicating that SO₄²⁻ appears after the activated coke adsorbs SO₂. The peak at 1427 cm⁻¹ is caused by NO₃⁻ stretching vibration, indicating the presence of KNO₃, and it also corresponds to the pyridine functional group [43,44]. K₂CO₃ modification does not change the chemical surface functional groups, but loads K₂O in the microporous structure, which provides activated sites for SO₂ and NO adsorption, thereby playing a role in promoting desulfurization and denitrification.

3.5. Phase Analysis of Activated Coke after Adsorption

This section takes unmodified activated coke, 5% NH₃-modified activated coke, and 3% K₂CO₃-modified activated coke as examples. Infrared spectroscopy and X-ray diffraction analysis were performed on the activated coke after synergistic adsorption of SO₂ and NO to determine the changes in the surface functional groups and crystallite structure of the activated coke.

Figure 12 shows that there are many high-intensity graphite characteristic diffraction peaks in the standard graphite X-ray diffraction spectrum. The appearance of characteristic peaks can represent the degree of graphitization. The more diffraction peaks in the spectrum, the higher the degree of graphitization and the better the order of carbon materials. On the whole, the characteristic peaks of the three activated cokes are similar, and there are four characteristic peaks at about 23°, 44°, 65° and 82°of 2θ, which correspond to the (002), (100), (110) and (006). The intensities of the diffraction peaks decrease in turn, indicating that the graphitization intensity of the activated coke decreases, and the crystallinity decreases, which promotes the development of the pore structure. Figure 12a shows that when 2θ is about 29°, there is an obvious diffraction peak, indicating that the activated coke contains CaCO₃ crystals, and the diffraction peak at about 31° indicates that there are CaSO₄ and Na₂SO₄ crystals. Figure 12b shows that besides CaCO₃, CaSO₄, and Na₂SO₄ crystals, there are also a large number of NaHSO₄ crystals. Figure 12c shows that there is a large amount of K₂SO₄ in the 2θ range of 20–40°. XRD analysis found that SO₂ is adsorbed and combined with the substances in the activated coke in the form of SO₄²⁻ to form sulfate minerals.

4. Conclusions

The desulfurization and denitrification performance of the activated coke prepared by different modification methods was analyzed through solo and synergistic desulfurization and denitrification experiments of activated coke. The active coke before and after adsorption was characterized and analyzed. The specific conclusions are as follows:

• NH₃ modification has a promoting effect on the adsorption of SO₂ and NO in activated coke. With the continuous increase of NH₃ concentration, the promoting effect is continuously enhanced, but the increase is not large. The presence of NH₃ causes NO to undergo a reduction reaction to generate N₂. A large number of crystalline particles appeared in the adsorbed activated coke. Combined with the FTIR spectrum after adsorption, SO₂ was oxidized and combined with steam to form sulfate, which existed on the surface of activated coke in the form of crystals.
• K₂CO₃ solution modification greatly improves the desulfurization performance of activated coke. The fast adsorption stage of activated coke modified with 9% and 12% K₂CO₃ solutions was as long as 60 min, and the adsorption efficiency decreased slowly. The desulfurization efficiency of activated coke modified with 3% and 6% K₂CO₃ concentrations is approximately the same, and both are stable at about 60%. SO₂ and NO are oxidized and combined with steam to form H₂SO₄ (adsorbed state) and HNO₃ (adsorbed state), which regenerate activated sites. Through FTIR analysis, KNO₃ was formed on the surface of the adsorbed activated coke.
• The K₂CO₃ solution modification did not change the surface functional groups of the activated coke, but the developed microporous structure and the loading of metal oxides enhanced the SO₂ and NO adsorption capacity of the activated coke. When the unmodified activated coke synergistically adsorbs SO₂ and NO, there is a competitive adsorption between NO and SO₂; however, when the activated coke treated with K₂CO₃ solution and NH₃ synergistically adsorbs SO₂ and NO, both have a promoting effect. The vibration of different bonds will lead to the generation of absorption peaks in the same wavenumber range. With the adsorption of SO₂ and NO on the activated coke, the functional groups on the surface of the activated coke will be reacted to generate SO₄²⁻ and NO₃⁻. The XRD patterns showed that the intensity of the diffraction peaks of the three decreased in turn, and the degree of graphitization decreased, indicating that the microporous structure was more developed. CaSO₄ and Na₂SO₄ crystals appeared after the adsorption of NH₃-modified activated coke, and K₂SO₄ existed after the adsorption of K₂CO₃ solution-modified activated coke. After SO₂ is adsorbed, it combines with the substances in the activated coke in the form of SO₄²⁻ to form sulfate minerals.