1 Introduction

A large number of sulfur-containing flue gases will be produced in the process of non-ferrous metal smelting, which will not only directly cause air pollution, but also be transformed into new pollutants as precursors, which will cause serious pollution to the environment. The treatment methods of low concentration SO2 flue gas mainly include wet method (Valle-Zermeño et al. 2015), semi-dry method (Cheng et al. 2004), and dry method (Li et al., 2008; Mangun et al. 2001). Activated coke flue gas desulfurization, as a clean and environmentally friendly dry desulfurization technology, has the advantages of low water consumption, recycling of renewable raw materials, and utilization of sulfur resources, and is considered as an important development direction for the removal of sulfur dioxide (Wang et al. 2010; Zheng et al. 2010). Domestic and foreign researchers have done a lot of work on the properties of activated coke (Liu et al. 2003a; Shangguan et al., 2008) and the adsorption of activated coke on SO2 (Sun et al. 2011; Zheng et al. 2013).

Activated coke is the desulfurizer of activated coke desulfurization technology and is the key material affecting desulfurization performance and efficiency. In the process of desulfurization, the activated coke will lose its activity and some carbon will be lost during regeneration (Ania et al. 2004; Zhang et al. 2012b). Therefore, new activated coke needs to be continuously supplemented. It is found that the consumption of activated coke accounts for 50–70% of the total cost of activated coke flue gas desulfurization technology, which is the key factor affecting the economic performance of activated coke desulfurization technology. The traditional activated coke uses raw coal as the matrix and coal tar as the binder, so the production cost is high. Fine blue-coke and liquefaction residue are by-products in the coal chemical production process (Tian et al. 2015; Zhang et al. 2012a). Fine blue-coke is a product of low-temperature dry distillation of non-viscous or weakly viscous coal. The main components are carbon, ash, and volatiles. Because it has not been completely pyrolyzed, it contains more hydrogen and oxygen, richer pores, and surface structure, and is a high-quality raw material for preparing activated coke (Gao et al. 2017; Kang et al. 2017). Coal direct liquefaction residue (DCLR) is a by-product of coal hydro-liquefaction. Heavy oil and asphaltenes are the main components of liquefaction residue, and its content is as high as 50% (Liu et al. 2010; Ying et al. 2008). The colloid produced during pyrolysis of coal direct liquefaction residue has strong adhesion and can be used as a binder for molding activated coke (Ying et al. 2008; Zhang et al. 2013). Therefore, using fine blue-coke as main raw material and coal direct liquefaction residue as binder to prepare activated coke materials with good desulfurization, wear resistance, and compression resistance can improve economic benefits and realize rational utilization of resources.

Several studies have been reported on the literature on the usage of Fine blue-coke as an adsorbent. Adsorbents made from fine blue-coke have a good effect on the treatment of toxic gas-phase materials such as SOx, NOx (Kang et al. 2017), and Hg0, some wastewater such as phenol-containing wastewater (Gao et al. 2015; Wei et al. 2016), and other industrial wastewater (Wang et al., 2013). It is known that the activation of blue-coke can further improve the performance of activated coke. Activation can directly develop the internal pore structure of blue-coke and increase the surface area of activated coke. Zheng Yan et al. (Zheng et al. 2013) used four common reagents (including CO2, KOH, ZnCl2, and H3PO4) to activate blue-coke. It was found that the optimized pore structure and the increase of the number of active sites of activated blue-coke could contribute to the high desulfurization capacity. But most of these studies use expensive binders to prepare molded adsorbents or use powder adsorbents directly. In this study, activated coke was prepared by CO2 activation method using waste fine blue-coke as raw material, liquefied residue as the binder, and its desulfurization performance was investigated on the basis of characterization and analysis.

2 Experimental Section

2.1 Experimental Materials

The raw materials used in the experiment were DCLR and fine blue-coke. DCLR after ash removal was recorded as D-DCLR. The proximate analysis and elemental analysis are listed in Table 1.

Table 1 Proximate and ultimate analysis of experimental materials
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It can be seen from Table 1 that the DCLR is a high carbon, high ash, and high sulfur substance. The high ash content of DCLR will affect the performance and activity of activated coke (Yu et al., 2004). Therefore, the DCLR is subjected to ash removal pretreatment to reduce the ash content of the liquefaction residue.

2.2 Preparation of Molding Activated Coke

The raw material of the activated coke is fine blue-coke and D-DCLR. The mixture was uniformly placed in a custom cylindrical mold and pressed using a bench-top powder tablet press. Carbonization and activation were carried out in a tubular furnace. As shown in Fig. 1, the carbonization process was carried out at 600 °C for 60 min. The activation process was carried out at 850 °C for 90 min and the flow rate of CO2 was 60 mL/min.

Fig. 1
figure 1

Carbonization/activation device. 1—N2/CO2 cylinder; 2—Buffer bottle; 3—Rotor flowmeter; 4—Quartz reaction tube; 5—Pipe furnace; 6—Variable transformer; 7—Temperature controller; 8—Tail gas absorption device

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2.3 Characterization of Activated Coke

The specific surface area and pore size distribution of the samples were measured by JW-BK132F automatic physical adsorption instrument. The micro-crystal structure analysis adopts the Japanese Neo-Confucianism D/MAX-2400 type X-ray diffractometer. The surface structure analysis is based on the Fourier transform infrared spectrometer of the German ZEISS company.

2.4 Desulfurization Capacity of Activated Coke

The desulfurization capacity of activated coke was investigated by fixed bed equipment (shown in Fig. 2), which consists of three main parts: feeding system, reacting system (φ20 mm × 400 mm), and sampling system. The composition of the gas mixture was similar to flue-gas, including SO2, O2, H2O, and N2. The flow rate was controlled by the rotor flowmeter and mixed evenly in a buffer bottle before entering the desulfurization reactor. The concentration of SO2 at the inlet and outlet was measured by iodimetry. The initial concentration of SO2 in the experiment was from 500 to 1000 ppm, the adsorption temperature was from 60 to 120 °C, the oxygen concentration was from 3 to 12%, and the concentration of water vapor was from 2 to 12%.

Fig. 2
figure 2

Schematic diagram of the fixed bed equipment. 1—N2 cylinder; 2—O2 cylinder; 3—SO2 cylinder; 4—Buffer bottle; 5—Water vapor generator; 6—Muff; 7—Mixed gas cylinder; 8—Thermocouple; 9—Reactor; 10—Variable transformer; 11—Temperature controller; 12—Sampling

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3 Results and Discussion

3.1 Characterization of Activated Coke

3.1.1 BET Analysis of Activated Coke

Figure 3 shows the N2 adsorption-desorption isotherm curve of activated coke. As can be seen from Fig. 3, the adsorption-desorption isotherm of activated coke is a type I isotherm, which is a typical microporous material curve type. The adsorption curve rises almost perpendicular to the transverse axis in the low-pressure region, and the adsorption capacity of micropore increases rapidly. At the high-pressure region, the tail of the adsorption curve of the sample has an upward trend. This is due to the saturation of the microporous adsorption at high pressure and the filling of some mesoporous or macroporous materials, so it is judged that the activated coke contains a small amount of mesopores or macropores in addition to a large number of microporous structures.

Fig. 3
figure 3

N2 adsorption-desorption isotherm curve of activated coke

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Figure 4 is the pore size distribution curve of activated coke. Table 2 is the pore structure parameters of the activated coke. It can be seen from Fig. 4 and Table 2 that the specific surface area of activated coke is 239 m2·/g, the average pore size is 2.39 nm, and the micropore structure is rich. It shows that the small molecular substances of the raw materials are consumed to form micropore during carbonization and activation.

Fig. 4
figure 4

Pore size distribution of activated coke

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Table 2 Pore structure parameters of activated coke
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3.1.2 FT-TR Analysis of Activated Coke

The infrared spectrum analysis results of activated coke are shown in Fig. 5. It can be seen from Fig. 5 that the absorption peaks are mainly located in the range of 3500 cm−1, 1650 cm−1, and 1255 cm−1. The oscillation peaks at the range of 3500 cm−1 are the stretching oscillation peaks of O–H functional groups on the surfaces of alcohols, carboxylic acids, and phenols, which reflect the existence of chemisorbed water and surface hydroxyl groups (Li et al. 2017). These substances can be produced by decomposition reaction during pyrolysis. The characteristic peaks near 1650 cm−1 are the vibration peaks of carbon–oxygen double bonds in carbonyl group and carbon–carbon double bonds in benzene (Ding et al. 2015). The vibration peaks of oxygen-containing functional groups of ethers, epoxides, and non-epoxides are near 1255 cm−1. There is a relatively wide absorption peak near 604 cm−1, which is the absorption peak of minerals.

Fig. 5
figure 5

The FT-IR analysis of activated coke

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3.1.3 XRD Analysis of Activated Coke

Figure 6 shows the XRD analysis of activated coke. The amorphous nature of carbons can be confirmed by the peaks obtained by X-ray diffraction analysis. As can be seen from Fig. 6, the diffraction peaks of (002) and (100) crystal planes in graphite-like structure appear near the diffraction angles of 2θ = 26° and 2θ = 43° respectively. It can be concluded that a graphite-like structure appears in activated coke.

Fig. 6
figure 6

XRD analysis of activated coke

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3.2 Effect of Operating Conditions on Desulfurization Capacity of Activated Coke

3.2.1 Effect of Initial SO2 Concentration on Desulfurization Capacity of Activated Coke

The relationship between the initial concentration of SO2 and the outlet concentration of SO2 is shown in Fig. 7. The total flow rate was 500 mL/min. The height of activated coke bed was 12 cm. It can be seen from Fig. 7 that the initial SO2 concentrations have a great influence on the desulfurization performance of activated coke, and the concentration of SO2 at the outlet becomes larger as the concentration of the initial SO2 increases. This is because with the increase of initial SO2 concentration, some SO2 molecules have penetrated into the outlet before reaching the surface of activated coke, leading to the increase of SO2 concentration in the outlet. In summary, the high initial concentration of SO2 is not conducive to the removal of SO2 from the flue gas by activated coke.

Fig. 7
figure 7

Effect of initial SO2 concentration on outlet SO2 concentration

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3.2.2 Effect of Adsorption Temperature on Desulfurization Capacity of Activated Coke

The relationship between the adsorption temperatures and desulfurization efficiency of activated coke is shown in Fig. 8. The total flow rate was 500 mL/min, the initial concentration of SO2 was 700 ppm, and the height of activated coke bed was 12 cm. As can be seen from Fig. 8, when the adsorption temperature is 60 °C, 80 °C, 100 °C, and 120 °C, the time for desulfurization rate to remain above 95% is 20 min, 26 min, 15 min, and 10 min. With the increase of adsorption temperature, the time for activated coke to maintain high desulfurization efficiency first increases and then decreases. This is because when the temperature rises to 80 °C, the water on the surface of the activated coke evaporates, which reduces the resistance of the water film on the surface. It is beneficial to the adsorption of SO2 on the surface of activated coke. However, when the adsorption temperature continues to rise, the desulfurization rate of activated coke gradually decreases, which is due to the Brownian motion intensified by the increase of temperature, that is, the probability of SO2 molecules escaping from the surface of activated coke increases, resulting in a decrease in desulfurization efficiency. In summary, the optimum temperature obtained in this paper is 80 °C, lower than Liu’s 90 °C (Liu et al. 2003b). This may be due to differences in raw materials and modification methods.

Fig. 8
figure 8

Effect of adsorption temperature on desulfurization capacity of activated coke

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3.2.3 Effect of Oxygen Concentration on Desulfurization Capacity of Activated Coke

The relationship between oxygen concentration and desulfurization capacity of activated coke is shown in Fig. 9. The total flow rate was 500 mL/min, the initial concentration of SO2 was 700 ppm, the height of activated coke bed was 12 cm, and the adsorption temperature was 80 °C. As can be seen from Fig. 9, with the increase of oxygen concentration, the desulfurization performance of activated coke increases first and then decreases. When the oxygen concentration increases from 3 to 9%, the initial desulfurization rate of activated coke increases from 94 to 97%. However, when the oxygen concentration is increased from 9 to 12%, the desulfurization performance of the activated coke is lowered. In other words, there is synergy and competition between SO2 and O2 in the desulfurization process. When the oxygen concentration is in a certain range, the presence of O2 can promote the desulfurization of activated coke. In the presence of O2, it participates in the SO2 oxidation reaction. The oxidation product SO3 is first adsorbed on the active site. In the subsequent desulfurization process, part of the active site will desorb and release the active site, thus ensuring the continuous adsorption and oxidation of SO2, In this case, the desulfurization performance will be greatly improved. However, if an excess of O2 is introduced, too much O2 molecules will occupy a limited active site on the activated coke surface, which will prevent the adsorption of SO2, resulting in a decrease in desulfurization efficiency. In this case, O2 competes with SO2. In summary, the optimum oxygen concentration was 9% in this experiment.

Fig. 9
figure 9

Effect of oxygen concentration on desulfurization capacity of activated coke

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3.2.4 Effect of Water Vapor Content on Desulfurization Capacity of Activated Coke

The relationship between water vapor concentration and activated coke desulfurization performance is shown in Fig. 10. The total flow rate was 500 mL/min, the initial SO2 concentration was 700 ppm, the activated coke bed height was 12 cm, and the adsorption temperature is 80 °C and the O2 concentrations was 9%. It can be seen from Fig. 10 that with the gradual increase of the concentration of water vapor, the desulfurization efficiency of the activated coke first increases and then decreases.

Fig. 10
figure 10

Effect of the concentration of water vapor desulfurization capacity of activated coke

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According to the SO2 removal mechanism on carbon materials (Lisovskii et al., 1997), water vapor in the flue gas plays a crucial role in the desulfurization process. When part of the water vapor is present in the feed stream, the water vapor forms a thin film on the surface of the activated coke. The oxidized SO2 is easily dissolved in the film and reacts with the H2O molecule to form H2SO4, that is, H2O plays a synergistic role in the adsorption of SO2. The effect is to improve the desulfurization efficiency of the activated coke. However, if the water vapor content exceeds 8%, the water film will become too thick, so that a large amount of H2O molecules will prevent the adsorption of SO2. At this time, the desulfurization activity of the activated coke is lowered. In conclusion, the concentration of water vapor in subsequent experiments was 8%.

3.3 The Mechanism of Activated Coke Adsorption Process

The adsorption of SO2 by activated coke includes both physical adsorption and chemical adsorption. When only SO2 exists in flue gas, only physical adsorption occurs, and when both O2 and H2O are present, physical adsorption and chemical adsorption occur simultaneously, and chemical adsorption is dominant (Gaur et al. 2006; Tsuji and Shiraishi 1997). Figure 11 shows the pore structure of activated coke and the adsorption-transformation process of SO2 in the pores. As can be seen from Fig. 11, the adsorption of SO2 by activated coke can be divided into three stages. Firstly, SO2, H2O, and O2 in flue gas diffuse from the gas phase to the active sites on the surface of activated coke and are adsorbed. Then a chemical reaction takes place on the active center, SO2 is catalytically oxidized to form SO3 (Raymundo-Piñero et al. 2001). Finally, SO2 reacts with H2O to form sulfuric acid, which is stored in the pores of the activated coke (Zawadzki 1987a, 1987b).

Fig. 11
figure 11

Pore structure and desulfurization process of activated coke

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When the flue gas does not contain O2 and H2O, only the physical adsorption occurs in the process of adsorbing sulfur dioxide by the activated coke, the adsorption occurs rapidly, and the adsorption amount is low (Davini 1999; Sun et al. 2015). When the adsorption temperature is low, the initial SO2 concentration has a great influence on the desulfurization performance of activated coke, and simple physical adsorption is prone to desorption, which makes the adsorbed SO2 desorbed into the flue gas again. When oxygen and water vapor are present in the flue gas, SO2 adsorbed on the surface of activated coke is oxidized to SO3 due to the catalytic oxidation of the oxygen-containing group on the surface of the activated coke. Rubio et al. (Rubio and Izquierdo 1998) thought that the oxidation is caused by the O2− on the surface of the carbon. However, how to oxidize SO2 to SO3 in the absence of water is not explicitly stated. Zawadzi (Zawadzki 1987a, 1987b) found that in the absence of water vapor, SO2 cannot be oxidized to SO3 even at high oxygen concentrations. Kaixi Li et al. (Kaixi et al. 2000) thought that both oxygen and water vapor can promote the SO2 adsorption of activated coke (carbon). The specific desulfurization mechanism is as follows:

$$ S{O}_2+{C}_1\to {C}_1-S{O}_2 $$
(1)
$$ {O}_2+{C}_2\to 2{C}_2-O $$
(2)
$$ {C}_1-S{O}_2+{C}_2-O\to {C}_1-S{O}_3+{C}_2 $$
(3)

That is, there are two active sites C1 and C2 on the surface of the activated coke. When the active site C1 is occupied by the SO2 molecule, if there is C2–O near C1–SO2, the adsorbed sulfur dioxide can react with oxygen to form SO3; otherwise, C2–O will tend to be stable. When O2 and H2O are present at the same time, it is possible to carry out physical adsorption and chemical adsorption simultaneously, which will facilitate the continuous adsorption and removal of SO2 in the flue gas by activated coke (Small et al. 2012). Lizzio et al. (Lizzio and DeBarr 1996a; Lizzio and DeBarr 1996b) proposed that there is a competitive active site in SO2 and O2, that is, the presence of oxygen will occupy some active sites, which is not conducive to the adsorption and removal of sulfur dioxide in flue gas by activated coke, that is, the SO2 removal by activated coke occurs as follows reaction:

$$ S{O}_2+{X}_v\to X-S{O}_2 $$
(4)
$$ {O}_2+2{X}_v\to 2X-O $$
(5)
$$ {H}_2O+{X}_v\to X-{H}_2O $$
(6)
$$ X-S{O}_2+{O}_2+{X}_v\to X-S{O}_3+X-O $$
(7)
$$ X-S{O}_2+X-O\to X-S{O}_3+{X}_v $$
(8)
$$ S{O}_2+X-O\to X-S{O}_3 $$
(9)
$$ X-S{O}_3+X-{H}_2O\to X-{H}_2S{O}_4+{X}_v $$
(10)
$$ X-{H}_2S{O}_4+{H}_20\to {H}_2S{O}_4(aq)+{X}_v $$
(11)

where Xv is active site, X is adsorption state, and aq is aqueous.

As for the role of water molecule in desulfurization process, researchers believe that the presence of water molecule can help SO3 dissociate from the active site, release the active site, and then make the adsorption continue. In addition, Zawadzi (Zawadzki 1987a, 1987b) believes that only when oxygen and water vapor exist at the same time, SO2 will be oxidized to SO3. He believes that it is necessary to provide protons to O2− so that O2− can have the ability to oxidize. The existence of water satisfies this condition. In conclusion, the existence of water molecule has two functions: one is to provide the protons needed for the reaction; the other is that water molecule participates in the reaction to dissociate SO3 from the active site and release the active site for the subsequent adsorption process.

In the process of activated coke desulfurization, the presence of H2O and O2 in the flue gas is indispensable. In the low temperature state, if only oxygen is present, it is not conducive to SO2 adsorption and removal by the activated coke. SO2 and O2 have a competitive adsorption relationship on the surface of activated coke. When the oxygen concentration is too high, a large number of active sites will be occupied during the adsorption process, and the ability of activated coke to SO2 remove will be reduced. If there is water vapor in the flue gas, the probability of SO2 adsorption and conversion to sulfuric acid will be greatly improved. But not the higher the content of water vapor, the better. If the H2O concentration exceeds a certain concentration, the SO2 adsorption efficiency of the activated coke decreases as the H2O concentration increases. The concentration of sulfur dioxide in flue gas also affects the removal of sulfur dioxide by activated coke. Although high concentration of sulfur dioxide increases the amount of sulfur dioxide adsorbed by activated coke, the desulfurization efficiency decreases accordingly. In summary, under different temperature stages and different gas compositions, the activated coke has different adsorption processes for SO2 removing from the flue gas.

4 Conclusion

Blue-coke-based activated coke was prepared by CO2 activation method. Through BET analysis, FT-TR analysis, and XRD analysis, the activated coke has abundant pore structure and surface functional groups, which is beneficial to the adsorption of SO2.

When the concentration of O2 in the simulated flue gas is too high, SO2 and O2 will compete for adsorption, and the presence of O2 will occupy some active sites, which is not conducive to the adsorption and removal of sulfur dioxide by activated coke. If the water vapor concentration is too high, a water film will be formed on the surface of activated coke, which will hinder the adsorption of SO2 by activated coke. The optimum desulphurization operating conditions are as follows: the initial concentration of SO2 is 700 ppm, the adsorption temperature is 80 °C, the oxygen concentration is 9%, and the concentration of water vapor is 8%.