**Regeneration of Sodium Hydroxide from a Biogas Upgrading Unit through the Synthesis of Precipitated Calcium Carbonate: An Experimental Influence Study of Reaction Parameters**

**Francisco M. Baena-Moreno 1,2,\*, Mónica Rodríguez-Galán 1, Fernando Vega 1, T. R. Reina <sup>2</sup> , Luis F. Vilches <sup>1</sup> and Benito Navarrete <sup>1</sup>**


Received: 11 October 2018; Accepted: 22 October 2018; Published: 24 October 2018

**Abstract:** This article presents a regeneration method of a sodium hydroxide (NaOH) solution from a biogas upgrading unit through calcium carbonate (CaCO3) precipitation as a valuable by-product, as an alternative to the elevated energy consumption employed via the physical regeneration process. The purpose of this work was to study the main parameters that may affect NaOH regeneration using an aqueous sodium carbonate (Na2CO3) solution and calcium hydroxide (Ca(OH)2) as reactive agent for regeneration and carbonate slurry production, in order to outperform the regeneration efficiencies reported in earlier works. Moreover, Raman spectroscopy and Scanning Electron Microscopy (SEM) were employed to characterize the solid obtained. The studied parameters were reaction time, reaction temperature, and molar ratio between Ca(OH)2 and Na2CO3. In addition, the influence of small quantities of NaOH at the beginning of the precipitation process was studied. The results indicate that regeneration efficiencies between 53%–97% can be obtained varying the main parameters mentioned above, and also both Raman spectroscopy and SEM images reveal the formation of a carbonate phase in the obtained solid. These results confirmed the technical feasibility of this biogas upgrading process through CaCO3 production.

**Keywords:** carbon capture and utilization; biogas upgrading; calcium carbonate precipitation; chemical absorption

#### **1. Introduction**

Climate change is one of the major problems that has plagued humanity in recent times, consisting of a significant and lasting modification of local and global patterns of climate on the planet. The frequency and intensity of meteorological phenomena such as rainfall, hurricanes, storms, decreasing extent of ice, rising sea level and, above all, the increasing average temperature of the Earth's atmosphere are the main evidences found by scientists that corroborate climate change [1,2]. According to the Intergovernmental Panel on Climate Change (IPCC) [1], the main origin is the anthropogenic emissions of so-called greenhouse gases (GHG), due to the use of fossil fuels such as coal, oil and natural gas for the production of electricity, transportation or industrial uses, CO2 being the most relevant among the greenhouse gases. For this reason, the use of renewables energies which reduce CO2 emissions could be found as one of the fields most investigated in the last decade [3–11].

One of the most promising renewable energy sources is biomass [12]. Biomethane is obtained by upgrading biogas produced from anaerobic digestion of different types of biomass. There are several ways to use this biomethane as an energy resource, which depend on technical, economic and legislative factors of each country [13]. Biomethane is an improved biogas from landfills, farms, sewage treatment plants, agriculture or other sources [14]. For use, biomethane must be submitted to a process called upgrading, which separates the undesired compounds (mainly CO2) and adapts its composition to the standards set by the legislation corresponding to that suitable for a fuel gas [15]. CO2 content in biogas as produced in anaerobic digestion varies between 35%–45% [16,17]. To remove CO2, very diverse techniques have been studied: Pressure Swing Adsorption (PSA) [18,19], Water Scrubbing (WS) [20,21], Organic Physical Scrubbing (OPS) [22,23], Chemical Absorption Scrubbing (CAS) [13,24], Membrane Separation (MS) [25,26] and Cryogenic Separation (CS) [27,28]. CAS is one of the most promising technique [29,30], both with amines (monoethanolamine (MEA) or Piperazine (PZ)) and caustic solvents (NaOH or potassium hydroxide (KOH)). Previous studies have reported high capture yields and selectivity to CO2, achieving similar capture efficiencies from 90% to 99% [31–35]. In the case of amine solvents, a high regeneration efficiency of the solvent can be obtained via physical regeneration, with an acceptable energy consumption [34]. However, compared with the previous solvents explained, when employing caustic solvents, an elevated energy consumption is necessary in order to regenerate the solvent physically, which makes these solvents less usable than the conventional MEA [35]. In this reaction, Na2CO3 or potassium carbonate (K2CO3) is obtained as a consequence of the absorption step.

$$2\text{NaOH/KOH(aq)} + \text{CO}\_2(\text{g}) \rightarrow + \text{Na}\_2\text{CO}\_3/\text{K}\_2\text{CO}\_3 + \text{H}\_2\text{O} \tag{1}$$

An alternative path for CO2 utilization that avoids the energy penalty in the regeneration stage of the solvent would be the synthesis and separation of chemicals based on calcium (Ca+) by the precipitation processes into the solvent solution, as for example in Ca(OH)2 or residues with high Ca<sup>+</sup> content. This alternative is very attractive from an economic point of view in order to drastically reduce costs of CO2 capture and valorize CO2 as a commercial by-product. An interesting by-product to be taken into account when alkaline hydroxides are used as solvents is CaCO3. CaCO3 can be produced through chemical reaction with Ca(OH)2 and precipitated as a solid [33], according to the next reaction:

$$\mathrm{Na\_2CO\_3/K\_2CO\_3(aq)} + \mathrm{Ca(OH)\_2(s)} \rightarrow 2\mathrm{NaOH/KOH(aq)} + \mathrm{CaCO\_3(s)}\tag{2}$$

The type of CaCO3 obtained as a by-product is called Precipitated Calcium Carbonate (PCC). PCC is consumed in huge quantities and in variated applications for different industrial sectors, such as a filler for plastic materials, paper, foods, printing ink and medical necessities [36]. This synergy process between biogas upgrading, CO2 capture and PCC production is shown in Figure 1.

This process is much less energy intensive than physical regeneration previously studied by various authors [35,37,38], making the process economically attractive. Many researches focused on the carbonation of residues for storing CO2, as for instance steel slags [39], air pollution control residues [32,33], argon oxygen decarburization slags [23], incineration bottom ash [40] or basic oxygen furnace slags [41]. Baciocchi et al. [32,33] proposed an application of the process above explained using both NaOH and KOH as solvents, and Air Pollution Control residues as Ca<sup>+</sup> sources, focusing on the amount of CO2 that could be definitely stored by this residues [32,42]. However, their results showed a non-valuable by-product from a commercial point of view and solvent regeneration efficiencies from 50% to 60% due to the employment of the residues. Therefore, the purpose of this work was to study the main parameters that may affect NaOH regeneration using an aqueous Na2CO3 solution and Ca(OH)2 as a reactive for regeneration and carbonate slurry production, in order to achieve better regeneration efficiencies than previous works and obtaining PCC as a valuable by-product. With this, the foundations for future works are laid, in which valuable by-products would be obtained that would allow to achieve a more economical and sustainable process for carbon capture and utilization.

**Figure 1.** Biogas upgrading through Precipitated Calcium Carbonate (PCC) production.

#### **2. Materials and Methods**

#### *2.1. Materials*

Chemical compounds used in the experiments (Ca(OH)2, Na2CO3, CaCO3, NaOH) were provided by PanReac-AppliChem (Barcelona, Spain) (pure-grade or pharma-grade, 99% purity).

#### *2.2. Regeneration Experiments*

In general, the regeneration experiments were carried out following the methodology exposed below, which will be explained in greater depth later. First, the solutions of the reactants were prepared, at the same time that the instruments needed for the precipitation reaction were tuned. After these steps, the reaction was produced, which, once finished, was filtered and separated quickly for analyzing. The main parameters considered for results were NaOH regeneration efficiency and carbonate phase reached. The three most important variables studied in these experiments were the reaction time, the reaction temperature and the molar ratio between Ca(OH)2 and Na2CO3, which according to previous works may have a considerable effect on the regeneration efficiency of the process [32,33]. The matrix of experiments carried out can be found in Table 1. In order to study how each parameter affects by itself, a standard value was set for each of them, according to the bibliography for similar studies [31–33,35], and later they were varied one by one. The standard value for temperature reaction was set at 50 ◦C, molar ratio at 1.2 mol Ca/Na2CO3 and reaction time at 30 min. Furthermore, the influence of an initial addition of NaOH in the Na2CO3 solution was tested, in order to analyze the effect on NaOH regeneration.

Lab scale batch precipitation experiments were carried out in a 600 mL beaker placed in a water bath to control temperature tests. The value of the pH gives some clues of the compounds that may be present in the solution. Therefore, to check that the NaOH regeneration reaction had the desired effect, the pH was measured and checked to be in the range 12–14, which is characteristic for hydroxides solutions [31]. During each whole experiment time, the solutions were stirred by an electromagnetic magnet at a constant speed of 1000 rpm. For temperature and pH measuremsent, a thermometer and a pH-meter by Trison Instrument (BANDELIN electronic GmbH & Co. KG, Berlin, Germany) were employed. Measures were continually carried out and recorded in a data logger. Reproducibility checks were conducted resulting in an overall experimental error of ±2% for the regeneration efficiency calculations. The first steps of the procedures followed were to prepare both

Na2CO3 and Ca(OH)2 solutions. In the case of Na2CO3, the aqueous solution was set at 20 g/100 mL according to the basis typical values expected after the absorption step [21,28], while the concentration of the Ca(OH)2 solution was stoichiometrically calculated for each test, as will be explained later. At the beginning of each experiment, a 200 mL distilled water slurry of Ca(OH)2 was poured into the beaker placed. After 15 min, 200 mL of Na2CO3 aqueous solution was added to start the reaction time. At the end of each experiment, the solution was vacuum filtered immediately and 50 mL sample was taken to determine the concentration of NaOH by inductively coupled plasma atomic emission spectroscopy. The solid obtained by filtration was dried at 105 ◦C to ensure a carbonate phase which was characterized by means of Scanning Electron Microscopy (SEM) and Raman spectroscopy.


**Table 1.** Matrix of the experiments carried out.

Raman measurements of the powders samples were recorded using a Thermo DXR2 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a Leica DMLM microscope (Thermo Fisher Scientific, Waltham, MA, USA). The wavelength of applied excitation line was 532 nm ion laser and 50× objective of 8-mm optical was used to focus the depolarized laser beam on a sport of about 3 μm in diameter.

A JEOL JSM6400 (JEOL Ltd., Tokyo, Japan) operated at 20 KV equipped with energy dispersive X-ray spectroscopy (EDX) and a wavelength dispersive X-ray spectroscopy (WDS) systems was used for the microstructural/chemical characterization (SEM with EDS and WDS).

#### **3. Results**

This section reports the experimental results of the different tests carried out. The results of NaOH regeneration efficiency are presented, with reference to every parameter studied. NaOH regeneration efficiency is defined has follow:

$$\text{NaOH regeneration efficiency} (\%) = \frac{\text{NaOH regenerated}}{\text{Maximum NaOH to regeneration}} \times 100$$

As it has been set previously, NaOH regenerated was determined by inductively coupled plasma atomic emission spectroscopy, while the maximum NaOH to be regenerated can be easily stochiometrically calculated from the concentration of the Na2CO3 initial solution.

Then, some Raman spectroscopies of PCC are shown to demonstrate the carbonate phase reached, which are accompanied by some SEM images that contribute to verify the results predicted by Raman.

#### *3.1. NaOH Regeneration*

Figures 2–4 show the regeneration efficiency curves of the filtered solutions resulting from regeneration experiments carried out with Ca(OH)2 at different reaction times, temperatures and molar ratios, respectively.

**Figure 2.** Evolution of NaOH regeneration with time at fixed Temperature 50 ◦C and R = 1.2.

#### 3.1.1. Reaction Time Effect

Figure 2 shows the effect of the reaction time in the regeneration phenomenon. As depicted in the plot, NaOH regeneration efficiency varies from 53% to 83% approximately from 5 min to 30 min of reaction time, and later, the slope of the curve changes drastically, passing through 91% regeneration efficiency at 60 min, until achieving a 95% of NaOH regeneration at 120 min. This means, in fact, that in a hypothetical real reactor, a duplication of its volume will be necessary to achieve an increase of 4% approximately (from 30 min to 60 min). As can be seen, from 60 min to 120 min, less than 0.001 mol NaOH is regenerated per minute. Thus, to operate at 120 min residence time is not worthy from a plant design point of view. The intersection of the two curves (regeneration rate and regeneration efficiency) indicates an interesting and very likely optimum operational point where a fair balance between both tendencies can be reached.

#### 3.1.2. Temperature Influence

As for the temperature influence, Figure 3 shows the effect of different temperatures in the regeneration studies. Temperature effect reflects a linear trend showcasing a direct correlation between NaOH regeneration efficiency and process temperature. Indeed, in the best case scenario (at 70 ◦C) it can reach 97% of NaOH regeneration efficiency. Normalizing the regeneration capacity by the incremental temperature (empty symbols in the Figure) maximum is obtained at around 50 ◦C which somehow indicates that the increment in temperature has a stronger impact on the regeneration efficiency in the low-medium temperature range. This is an important result to be highlighted from an energy consumption perspective, as a temperature of 50 ◦C could be easily achieved through low-cost and/or renewable energy sources such as solar.

**Figure 3.** Influence of temperature on NaOH regeneration. Experiments carried out at R = 1.2 and t = 30 min.

**Figure 4.** Influence of molar ratio (R) in the regeneration experiments. Runs conducted at t = 30 min and 50 ◦C.

#### 3.1.3. Molar Ratio Influence

Molar ratio inlet carbonate/precipitant agent is another important parameter to consider in the regeneration process. As can be seen in Figure 4, NaOH regeneration efficiency is favored by an increase of the molar ratio. Nevertheless, this increase of molar ratio promotes a higher quantity of Ca2+ ions that should be removed before recirculating the absorbent to the absorption tower in order to prevent accumulation of Ca(OH)2 which eventually may lead to fouling phenomenon in the tower. In parallel, as can be observed in Figure 4, NaOH mol regeneration per mol of Ca(OH)2 introduced decreases upon increasing the molar ratio above a threshold value of R = 1.1. This value set an optimum operational point beyond which no further benefits are envisaged from the process point of view.

#### 3.1.4. Effect of NaOH Spark in the Regeneration Efficiency

The addition of small quantities of NaOH at the beginning of the precipitation process may promote the recovery of NaOH (initially entering the precipitation reactor in the form of Na2CO3). Also, it should be taken into account that, in a real industrial plant, 100% conversion from NaOH to Na2CO3 would hardly be reached in the absorption stage. In this sense, small amounts of NaOH as "sparking species" were added to investigate its effect on the process. The impact exerted by the addition of an initial concentration of NaOH (1 M) in the Na2CO3 solution in terms of NaOH regeneration efficiency is reported in Figure 5. Analyzing this Figure, it may be noted that the NaOH regeneration was slower than the results obtained without an initial NaOH concentration. It seems that the presence of alkaline compounds do not benefit the regeneration process—a fact that can be related to the poorer solubility of Ca(OH)2 due to the ion common effect as previously observed elsewhere [32,36,43].

**Figure 5.** Comparison curves of NaOH regeneration efficiency with and without initial addition of NaOH.

#### *3.2. Physicochemical Characterization of the PCC*

Aiming to determine the purity of the carbonates obtained during the recovery process, a combined Raman-SEM study was conducted on selected samples. Figure 6 shows the Raman spectra of the recovered carbonated after 30 min of reaction at 50 ◦C using a R = 1.2 in comparison with standards samples of pure CaCO3 and pure Ca(OH)2. CaCO3 typically presents a monoclinic structure belonging to the P21/c group [44]. The main characteristic band of CaCO3 polymorphs is a strong and narrow feature which appears at around 1100 cm<sup>−</sup>1. Also, another band ca. 700 cm−<sup>1</sup> is typically ascribed to this type of structure [44]. As can be seen, these two peaks are presented in PCC spectra, confirming the successful precipitation process. In fact, the spectrum of our PCC sample resembles that of the CaCO3 standard as shown in Figure 6. Nevertheless, it must be highlighted that a certain amount of Ca(OH)2 remains present in our solid sample as intended by the Raman vibration mode at ca. 400 cm−<sup>1</sup> which matches well with the most intense band on the Ca(OH)2 standard. In fact, these data correlate well with the regeneration efficiency data discussed above where 100% regeneration is never reached. In this sense, Raman experiments indicate that despite the fact that the regeneration process is highly effective, there is still some room for further improvements.

**Figure 6.** Raman spectra of the PCC obtained (time = 30 min, T = 50 ◦C, R = 1.2) and the Ca(OH)2 and CaCO3 standards.

Scanning Electron Microscopy images are useful to gain further insights on the samples structures. Selected SEM images of different sections of the sample studied by Raman are presented in Figure 7. SEM images again confirmed the presence of CaCO3 with the typical morphology of calcite as previously observed by Altiner et al. [45]. In the case of SEM, it is hard to distinguish between CaCO3 and Ca(OH)2—especially when the amount of Ca(OH)2 is just a minor contribution in the overall sample composition. In general terms, our SEM study confirms that the successful carbonate precipitation is in good agreement with the regeneration efficiency studies and the Raman results.

**Figure 7.** SEM images of the PCC obtained (time = 30 min, T = 50 ◦C, R = 1.2).

#### **4. Conclusions**

The results obtained from this lab scale work have confirmed the technical feasibility of this biogas upgrading process through PCC production. In general, the majority of the tests have shown better regeneration efficiencies than previous studies identified in the first section of this work (53%–97% vs. 50%–60%) [32,33]. The multiple reaction parameters have a different impact on the overall process performance. For instance, it was identified that the ideal reaction time would be around 30–60 min for T = 50 ◦C and R = 1.2, leading to compact reactor units. As for the temperature effect, a maximum NaOH mol regenerated per grade is reached at 50 ◦C for t = 30 min and R = 1.2; this would be an advisable value for a real process since it could be reached easily through the employment of a renewable energy source. The molar ratio Ca(OH)2/Na2CO3 also influences the process, 1.1 being an ideal ratio to be implemented for realistic operations, for t = 30 min and T = 50 ◦C. This result has been chosen taking into account the maximum in the curve of NaOH mol regeneration per mol of Ca(OH)2. The presence of small quantities of NaOH do not benefit the regeneration process and in fact it produces a decrease in the regeneration efficiency due to the ion common effect; this would suggest an effort to get the maximum percentage of NaOH conversion to Na2CO3 in the absorption stage.

Raman and SEM studies confirm the large majority presence of CaCO3 on the recovered material. Interestingly, although the obtained solid is mainly composed by calcite type CaCO3, some traces of Ca(OH)2 are still present. This opens some room for further research to improve the regeneration process.

**Author Contributions:** Conceptualization, F.M.B.-M., L.F.V. and B.N.; Methodology, F.M.B.-M., M.R.-G., F.V. and T.R.R.; Data curation, F.M.B.-M. and T.R.R.; Writing—original draft preparation, F.M.B.-M., T.R.R., M.R.-G., B.N.; Writing—review and editing, F.M.B.-M., T.R.R., M.R.-G., B.N.; Supervision, T.R.R., M.R.-G., B.N.; Funding acquisition, B.N., L.F.V. and T.R.R.

**Funding:** This work was supported by University of Seville through its V PPIT-US. Financial support for this work was also provided by the EPSRC grant EP/R512904/1 as well as the Royal Society Research Grant RSGR1180353. This work was also partially sponsored by the CO2Chem UK through the EPSRC grant EP/P026435/1.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **E**ff**ect of Physical and Mechanical Activation on the Physicochemical Structure of Coal-Based Activated Carbons for SO2 Adsorption**

**Dongdong Liu <sup>1</sup> , Zhengkai Hao 1, Xiaoman Zhao 1, Rui Su 1, Weizhi Feng 1, Song Li <sup>1</sup> and Boyin Jia 2,\***


Received: 9 September 2019; Accepted: 26 September 2019; Published: 5 October 2019

**Abstract:** The SO2 adsorption efficiency of activated carbons (ACs) is clearly dependent on its physicochemical structure. Related to this, the effect of physical and mechanical activation on the physicochemical structure of coal-based ACs has been investigated in this work. In the stage of CO2 activation, the rapid decrease of the defective structure and the growth of aromatic layers accompanied by the dehydrogenation of aromatic rings result in the ordered conversion of the microstructure and severe carbon losses on the surfaces of Char-PA, while the oxygen content of Char-PA, including C=O (39.6%), C–O (27.3%), O–C=O (18.4%) and chemisorbed O (or H2O) (14.7%), is increased to 4.03%. Char-PA presents a relatively low SBET value (414.78 m2/g) owing to the high value of Non-*V*mic (58.33%). In the subsequent mechanical activation from 12 to 48 h under N2 and dry ice, the strong mechanical collision caused by ball-milling can destroy the closely arranged crystalline layers and the collapse of mesopores and macropores, resulting in the disordered conversion of the microstructure and the formation of a defective structure, and a sustained increase in the SBET value from 715.89 to 1259.74 m2/g can be found with the prolonging of the ball-milling time. There is a gradual increase in the oxygen content from 6.79 to 9.48% for Char-PA-CO2-12/48 obtained by ball-milling under CO2. Remarkably, the varieties of physicochemical parameters of Char-PA-CO2-12/48 are more obvious than those of Char-PA-N2-12/48 under the same ball-milling time, which is related to the stronger solid-gas reactions caused by the mechanical collision under dry ice. Finally, the results of the SO2 adsorption test of typical samples indicate that Char-PA-CO2-48 with a desirable physicochemical structure can maintain 100% efficiency within 30 min and that its SO2 adsorption capacity can reach 138.5 mg/g at the end of the experiment. After the 10th cycle of thermal regeneration, Char-PA-CO2-48 still has a strong adsorptive capacity (81.2 mg/g).

**Keywords:** activated carbons; physical activation; mechanical activation; physicochemical structure; SO2 adsorption

#### **1. Introduction**

For a long time, SO2 emission from large coal-fired power plants has seriously polluted the environment and threatened human health [1]. The traditional wet desulfurization technology using calcium-based absorbent is unable to satisfy sustainable development, owing to its ecological destruction and the production of massive low-value by-products [2]. Dry desulfurization technology using porous carbon materials (such as activated carbons, ACs) as adsorbents has a better application prospect owing to its low price and ability to produce valuable by-products (such as sulfuric acid) [3]. ACs are usually prepared via a physical activation method and chemical activation method. In the process of chemical activation, the substantial water/acid is consumed to remove a large number of residual reagents (such as KOH [4,5], K2CO3 [6,7], ZnCl2 [8], and H3PO4 [9]), which not only increases the preparation costs but also causes environmental pollution. Thus, it is highly desirable to apply physical activation as a green and low-cost method for the preparation of ACs. In the process of physical activation, activation agents including steam, CO2 and their mixtures can partially etch the carbon network to produce some porosity and functional groups, and some researchers [10–14] have found that CO2 activation can make it easier to generate pores inside the particles than steam activation can. In addition, the preparation of ACs using coal as raw material can also meet desulfurization requirements in coal-fired power plants. Furthermore, the desulfurization performance of ACs is closely related to its physicochemical structure, such as a lot of active sites and a high specific surface area (SBET) in the presence of the hierarchical pore [15,16]. In the process of physical activation, the pore development follows a branched model. First, the micropore is formed on the surface of particles at the initial stage, after which the successive diffusion of activated agents from surface to core helps the formation of a new micropore; meanwhile, the formation of mesopores and macropores originates from the enlargement of the original micropores [17,18]; this branched model inevitably leads to a low specific surface area (SBET) and a high carbon loss on the surface of the particles, even under various activation conditions (such as the activation temperature, activation time and activated gas species, etc.) [13,14,19,20]. In addition, the active sites of porous carbon materials usually include oxygen/nitrogen-containing functional groups and defects at the edge of the aromatic layers. Zhu et al. [13,14] have found that the surface chemical properties of ACs are also significantly changed to form some oxygen-containing functional groups with the carbon loss at the stage of CO2 activation. Some researchers and our previous work [21–23] have found that the dehydrogenation of aromatic rings accompanied by the rapid decrease of the defect structure helps the vertical condensation and the transverse growth of aromatic layers during the physical activation, thus resulting in the removal of a large number of active sites. In summary, it is difficult to obtain the ideal ACs with a desirable porosity and more active sites in the stage of physical activation.

A ball-milling technique is a novel method to synthesize materials by mechanical activation without producing hazardous products (which can destroy the chemical bonds of the macromolecular structure), finally resulting in the molecular rearrangement, amorphization and recrystallization of the crystal structure [24,25]. Ong et al. [26] have found that the specific surface area (SBET) and the oxygen content of the sample increase rapidly from 6 m2/g to 450 m2/g and from 3.6% to 5.0% within 12 h of milling in the oxidizing atmosphere. Zhang et al. [27] also found that the size distribution of the sample ends up being narrower and its average particle size decreases with the increase of the milling time from 0.25 to 8 h because of the high collision strength between the agate balls during the ball-milling process. Salver-Disma et al. [28] have demonstrated that the mechanical milling of natural graphite is one possibility for producing disordered carbons with large intercalation capacities. The milled graphite contains large amounts of defects and present anisotropies [29]. Nevertheless, information regarding the changes in the physicochemical structure of coal-based ACs during physical and mechanical activation is still limited.

In this work, a precursor with a stable carbon-based framework obtained by pyrolysis could be treated first by CO2 activation, which ensures the formation of the original pores (such as some micropores and the hierarchical pores) and some oxygen-containing functional groups. In order to further increase the specific surface area and quantities of the active sites, the samples mentioned above continued to be activated via the ball-milling method under different times (12 h and 48 h, respectively) and different atmospheres (dry ice or N2) to further increase the specific surface area and quantities of the active sites. In addition, the physicochemical structure of all the samples were measured by a D/max-rb X-ray diffractometer (XRD), Raman spectroscopy, Nitrogen adsorption, X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM) and high-resolution scanning electron microscope (SEM). Finally, to verify the application potentials of the ACs with the

ideal physicochemical structure, an SO2 removal test was performed to further explore the relationship between the physicochemical structure and SO2 adsorption of ACs.

#### **2. Materials and Methods**

#### *2.1.* Materials

In this work, Jixi bituminous coal with particle sizes of 200–350 μm was obtained from the northeast of China. In order to eliminate the interference of minerals in the raw material, Jixi bituminous coal was treated sequentially using 30 wt % hydrofluoric acid (HF) and 5 mol·L−<sup>1</sup> hydrochloric acid (HCl) following the steps in the literature [30]. Then, the acid-treated samples were washed using deionized water and dried in an oven at 90 ◦C for 12 h. The proximate analysis and elemental analysis of the acid-treated samples (JX) were given in Table 1.

**Table 1.** The proximate analyses and elemental analyses of JX (wt %).


<sup>\*</sup> By difference; ad (air-dried basis): the coal in dry air was used as a benchmark; daf (dry ash free basis): the remaining component after the removal of water and ash in coal was used as a benchmark; *V*: volatile; *FC*: fixed carbon; *A*: ash; *M*: moisture; *C*: carbon element; *H*: hydrogen element; *O*: oxygen element; *N*: nitrogen element; *S*: sulfur element.

#### *2.2.* Experimental Process

#### 2.2.1. CO2 activation

10 g of JX were heated to 900 ◦C at a constant rate of 5 ◦C/min in an argon atmosphere flow of 600 mL/min and held for 60 min, and this sample was marked as Char. Then, argon atmosphere was converted to CO2 (99.999%) at 600 mL/min and held for 60 min, before being finally cooled down to room temperature under an argon atmosphere and being marked as Char-PA.

#### 2.2.2. Mechanical activation

Char-PA was prepared by a planetary ball mill (Pulverisette 6, FRITSCH, Idar-Oberstein, Germany,) by applying a rotation speed of 400 rpm in dry ice or N2 atmosphere for 12 h and 48 h, respectively. Additionally, a ball-to-powder weight ratio of 15:1 and 12 stainless steel balls with a diameter of 10 mm were used in the process of ball-milling, and these treated samples were marked as Char-PA-N2/CO2-different activation times. In addition, thermal annealing of a typical sample was processed at 800 ◦C for 60 min in 5% H2/Ar atmosphere; this treated sample was marked as Char-PA-N2/CO2-different activation times-H2.

#### *2.3. Measurement Analysis*

The surface topography of the samples was obtained via a scanning electron microscope (SEM, Quanta 200, FEI, Oregon, OR, USA) at 200 kV. The microstructure of the samples was tested by high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30, FEI, Oregon, OR, USA) at 300 kV. The crystal information of the samples was received by a D/max-rb X-ray diffractometer (XRD, D8 ADVANCE, Brooke, Karlsruhe, Germany) at a fixed scanning speed of 3◦/min from 5◦ to 85◦. The hybrid carbon information of the samples was received by Raman spectroscopy at a stable scanning scope from 1000–1800 cm−<sup>1</sup> using a 532 nm wavelength laser. The elemental composition, chemical state and relative concentration on the surface of the samples were obtained by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA) with an Al Kα X-ray at 14 kV and 6 mA [31]. The pore parameters of the samples were received by a micromeritics adsorption apparatus (ASAP2020, Micromeritics, Norcross, GA, USA) at 77 K and a relative pressure (P/P0) range

from 10−<sup>7</sup> to 1 [32]. The vacuum degassing pretreatment of the tested samples was carried out at 473 K for 12 h. Moreover, the specific surface area (SBET) of the samples was calculated using a BET model in a relative pressure range of 0.05–0.2 [33]; the total pore volume (Vtot) caused by the adsorption value of liquid nitrogen at a relative pressure of 0.98 was obtained using the *t*-plot method [34]; the micropore volume (Vmic) of the samples was calculated using the Horvath–Kawazoe (HK) method [35]; the nonlocal density functional theory (NLDFT) was used to obtain the pore size distribution of the micropore and mesopore, and the relative pressure range was 10<sup>−</sup>7~0.9 [13].

#### *2.4. SO2 Adsorption Test*

A fixed-bed experimental system was used to investigate the SO2 adsorption performance of ACs. This system included four parts: the gas distribution system, the fixed-bed reactor, the heating and insulation system, and the gas analysis system, as shown in Figure 1. First, 250 mL of deionized water were added to the humidifying tank, which was placed in a constant temperature water bath to maintain it at 90 ◦C. Simulated flue gas was prepared by mixing a certain amount of N2 with SO2, O2 and other gases. The water vapor content in the simulated flue gas was controlled by adjusting the N2 flow rate.

**Figure 1.** Schematic figure of the fixed-bed reactor system for SO2 adsorption.

The experimental process is as follows: 5 g of tested sample were placed in a glass tube reactor that consisted of a glass tube and sand core. Furthermore, this sand core not only supported AC particles, but also played the role of current sharing. The filling height and diameter of the adsorbent in the glass tube reactor were 25 mm and 20 mm, respectively. The SO2 adsorption test was performed at 80 ◦C for 210 min. The gas volumetric composition used in the experiments was: SO2, 1500 ppm; O2, 5%; water vapor, 10%; and N2, balance, total flow rate 250 mL·min<sup>−</sup>1. The SO2 concentrations at the entrance and exit were measured by an on-line Fourier transform infrared gas analyzer (FTIR, Dx4000, Gasmet, Vantaa, Finland) for calculating the SO2 removal efficiency and the change of the removal rate with time. According to the integral area and reaction time on the removal curve, the SO2 removal capacity of the coal-based ACs was obtained [36]. In addition, the SO2 removal efficiency, SO2 removal rate and cumulative sulphur capacity were calculated with the following formula:

SO2 removal efficiency (*DeSO*2):

$$\text{DecSO}\_2(\%) = \frac{\text{SO}\_2(in) - \text{SO}\_2(out)}{\text{SO}\_2(in)} \times 100\% \tag{1}$$

In the formula, *SO*<sup>2</sup> (*in*) and *SO*<sup>2</sup> (*out*) are the SO2 concentrations at the reactor inlet and outlet measured by FTIR, respectively.

SO2 removal rate (*RSO*2):

$$RSO\_2(\text{mg}\cdot \text{g}^{-1}\cdot \text{min}^{-1}) = \frac{DeSO\_2 \cdot SO\_2(in) \cdot 0.3 \cdot 64}{22400} \tag{2}$$

In the formula, *DeSO*<sup>2</sup> is the SO2 removal efficiency and *SO*<sup>2</sup> (*in*) is the SO2 concentration at the reactor inlet measured by FTIR,

The accumulated sulfur capacity (*ASO*2) refers to the cumulative removal capacity of SO2 from samples varying with time and is obtained by integrating the removal rate of SO2 with the time.

$$ASO\_2(\text{mg}\cdot\text{g}^{-1}) = \int\_0^t RSO\_2dt\tag{3}$$

#### *2.5. Thermal Regeneration*

For the thermal regeneration, desulfurized ACs were placed in a glass tube, and afterwards were settled in a vertical furnace (Figure 1). The ACs were heated in a flow of nitrogen (200 mL/min), at a heating rate of 10 ◦C min−<sup>1</sup> and maintained at 400 ◦C for 30 min, before being cooled for 30 min while the nitrogen purge was continued.

#### **3. Results and Discussion**

#### *3.1. Microstructure and Surface Morphology Analysis by HRTEM and SEM*

Figure 2 shows several HRTEM and SEM images of samples produced under pyrolysis and different activation conditions, including (a) HRTEM and (h) SEM of Char; (b) HRTEM and (i) SEM of Char-PA; (c) HRTEM and (j) SEM of Char-PA-N2-12; (d) HRTEM and (k) SEM of Char-PA-N2-48; (e) HRTEM and (l) SEM of Char-PA-CO2-12; (f) HRTEM and (m) SEM of Char-PA-CO2-48; (g) HRTEM and (n) SEM of Char-PA-CO2-48H2.There are some crystalline layers with different orientations near small quantities of amorphous carbon for Char in Figure 2a. In addition, a smooth surface and compact texture of Char can be found in Figure 2h. He et al. [37] found that metaplast material that was formed via the combination of transferable hydrogen and aliphatic hydrocarbons during pyrolysis can not only promote the ordered arrangement of aromatic layers but also reshape the particle surface. After CO2 activation, a large amount of long and multi-layer crystallite with a consistent orientation can be found in Char-PA, as shown in Figure 2b; the highly ordered crystalline layers of Char-PA can hinder the further diffusion of activated gas into the interior of particles, leading to severe carbon losses on the particle surfaces, as shown in Figure 2i. After 12 h of high-energy ball milling, some short and thin crystallite layers of Char-PA-N2/CO2-12 with a granular structure and rough surface can be found in Figure 2c,e,j,l. Furthermore, as the time of ball milling increases from 12 h to 48 h, blurred boundaries and a disordered arrangement of crystalline layers of Char-PA-N2/CO2-48 with a rougher surface and some smaller particles are also found in Figure 2d,f,k,m. The above results show that the mechanical collision caused by ball milling can significantly promote the disordered conversion of the microstructure and improve the surface morphology of particles with the increase of the ball-milling times from 12 to 48 h. In the process of mechanical activation, the high-energy ball milling can reduce the reaction temperature of the solid state and cause the increase of the local temperature on the material [24,25]. Based on this fact, a strong mechanical collision under dry ice can rapidly promote the solid-gas reactions between CO2 and the carbon matrix, as a result of which Char-PA-CO2-12/48 shows more obvious changes in its microcrystalline structure and surface morphology than Char-PA-N2-12/48 under the same ball-milling time.

**Figure 2.** The HRTEM and SEM images of the samples produced at different treatment conditions. (**a**) HRTEM and (**h**) SEM of Char; (**b**) HRTEM and (**i**) SEM of Char-PA; (**c**) HRTEM and (**j**) SEM of Char-PA-N2-12; (**d**) HRTEM and (**k**) SEM of Char-PA-N2-48; (**e**) HRTEM and (**l**) SEM of Char-PA-CO2-12; (**f**) HRTEM and (**m**) SEM of Char-PA-CO2-48; (**g**) HRTEM and (**n**) SEM of Char-PA-CO2-48H2.

#### *3.2. Crystal Structure Analysis by XRD*

The XRD profiles of the samples produced under pyrolysis and different activation conditions are given in Figure 3. There are two obvious broad diffraction peaks at 2θ = 16–32◦ and 36–52◦ in all of the samples; the information on two diffraction peaks (such as positions and half-peak width) can be obtained using the peak fitting treatment, as shown in Figure 4. Some crystal parameters, including the interlayer distance (*d*002), stacking height (*Lc*), and the size (*La*) and numbers (*N*) of the aromatic layers, are calculated via the following formulas [38]:

$$d\_{002} = \frac{\lambda}{2\sin\theta} \tag{4}$$

$$L\_c = \frac{0.89\lambda}{\beta \cos \theta} \tag{5}$$

$$L\_4 = \frac{1.84\lambda}{\beta \cos \theta} \tag{6}$$

$$N = \frac{L\_c}{d\_{002}}\tag{7}$$

**Figure 3.** The XRD profiles from the samples produced at different treatment conditions.

**Figure 4.** The fitting curve of the peaks for Char-PA in the 2θ range (**a**) 15–32◦ and (**b**) 35–53◦.

In the above formulas, λ: the wavelength of the X-ray, λ = 1.54 Å; θ: peaks' positions (◦); and β: half peak width. The results of the crystal parameters of all the samples are given in Table 2.


**Table 2.** The crystal parameters of the samples produced at different treatment conditions.

First, the rapid increase in the numbers (*N*), stacking height (*Lc*) and size (*La*) of the aromatic layers and the obvious reduction in the layer spacing (*d*002) can be found in the Char-PA during CO2 activation. The longitudinal condensation and transversal growth of the aromatic layers are related to the rapid consumption of the side chains and bridge bonds within the aromatic layers and defective structure on the edge of the aromatic layers [14,39]. Then, with the ball-milling time increases from 12 to 48 h, there is a sustained decrease in the *La*, *Lc* and *N* values and a persistent increase in the d002 value for Char-PA-CO2/N2-12/48, indicating the disordered conversion of the microcrystalline structure. These changes are closely related to the breakdown and distortion of the aromatic layers caused by a strong mechanical collision, which can further destroy the parallelism of the layer and the constancy of the interlayer spacing. Finally, the changes of the crystal parameters of Char-PA-CO2-12/48 are more obvious than those of Char-PA-N2-12/48 under the same ball-milling time, which may be related to the fact that the local high temperature caused by ball milling promotes CO2 etching on the aromatic layers. In this process, the more defective structures on the edge of the aromatic layers can also be formed as active sites.

#### *3.3. Carbon Structure Analysis by Raman*

The Raman spectra of the samples produced under pyrolysis and different activation conditions are shown in Figure 5. There are two obvious broad diffraction peaks at 1230–1450 cm−<sup>1</sup> (D peak) and 1450–1580 cm−<sup>1</sup> (G peak) in all of the samples. Some researchers [40,41] have found that the widening of the D and G peaks is serious for incomplete graphitized materials in Raman spectra, indicating the existence of many sp2-hybridized structures and sp2–sp3 hybridized structures. Thus, it is necessary to resolve overlapped peaks using a fitting treatment, including for the D1 peak (1300 cm<sup>−</sup>1), D3 peak (1520 cm−1), D4 peak (1200 cm−1) and G peak (1550 cm−1). Figure 6 shows the fitting curve of the Raman spectrum for Char-PA-N2-48. Furthermore, the D1 peak is represented as the defective sp2 bonding carbon atoms; the D3 peak is represented as the amorphous sp2 bonding carbon atoms; the D4 peak is represented as the sp2–sp3 bonding carbon atoms; the G peak is represented as the crystalline sp<sup>2</sup> bonding carbon atoms; furthermore, the relative quantities of different hybridized structures is as follows: (1) AD1/AG represents the relative quantity of the big aromatic rings, including C–C between aromatic rings and aromatics with no fewer than 6 rings with a defective structure; (2) AD3/AG represents the relative quantity of the small aromatic rings including aromatics with 3–5 rings and the semi-circle breathing of aromatic rings; and (3) AD4/AG represents the relative quantity of the cross-linking structure, including Caromatic–Calkyl, aromatic (aliphatic) ethers and C–C on hydroaromatic rings [42]. The results of the different hybrid carbons in the form of area ratios of the samples are shown in Table 3.

**Figure 5.** The Raman spectra from the samples produced at different treatment conditions.

**Figure 6.** The fitting curve of the Raman spectrum for Char-PA-N2-48.

**Table 3.** Hybrid carbon in the form of the area ratio of the samples produced at different treatment conditions.


First, the values of AD1/AG, AD3/AG and AD4/AG of Char-PA obviously decrease when compared to those of Char. In the process of CO2 activation, the defective structure at the edge of the aromatic layers and some small aromatic rings are consumed preferentially [43].The consumption of the defective structure can promote the dehydrogenation of the big aromatic rings; and the removal of the small aromatic rings can help the inner reactivation of the big aromatic rings accompanied by the breakdown of the cross-linking structure [44]; these changes finally lead to a substantial increase in the quantities of crystalline sp<sup>2</sup> bonding carbon atoms, which further promotes the stability of the carbon structure. Then, with the increase of the ball-milling time from 12 to 48 h, the values of AD1/AG and AD4/AG of Char-PA-N2/CO2-12/48 increase gradually; but there is a slight increase in the value of AD3/AG. It can be inferred that a strong mechanical collision caused by the ball milling has partly destroyed the crystalline sp2 bonding carbon structure, decomposing it into the big aromatic rings, which is accompanied by the formation of new crosslinking structures. In particular, the variety of the hybrid carbon parameters of Char-PA-CO2-12/48 is more obvious than that of Char-PA-N2-12/48 under the same ball-milling time. Some oxygen atoms from dry ice are easily bonded and fixed to the carbon matrix in the form of cross-linking bonds (such as -COO- and -O-), and some oxygen-containing heterocycles under a local high temperature are caused by a strong mechanical collision; furthermore, the presence of O-containing structures can also promote the reorganization of aromatic fragments to form more big aromatic rings.

#### *3.4. Pore Structure Analysis by N2 Adsorption*

The N2 adsorption isotherm and pore-size distribution of the samples under pyrolysis and different activation conditions are given in Figure 7, and the corresponding pore parameters are shown in Table 4.


**Table 4.** The pore structure parameters of the samples at different treatment conditions.

<sup>a</sup> Specific surface area determined by the BET method for P/P0 from 0.05 to 0.24. <sup>b</sup> Total pore volume calculated at P/P0 <sup>1</sup> <sup>4</sup> 0.98. <sup>c</sup> Volume of micropores (<sup>&</sup>lt; 2 nm) calculated by the *<sup>t</sup>*-plot method. <sup>d</sup> *<sup>V</sup>*<sup>t</sup> minus *<sup>V</sup>*mic (>2 nm).

First, the N2 adsorption isotherm of Char is attributed to a type I according to the IUPAC classification, and its N2 adsorption capacity is very small, showing small amounts of pores. The metaplast formed by the combination of transferable hydrogen with free radicals during pyrolysis can plug the pores, thus leading to an SBET value of 48.45 m2·g−1, Vmic value of 0.034 m3·g−<sup>1</sup> and non-Vmic value of 29.17% for Char, with a narrow size distribution of less than 2 nm. Then, the N2 adsorption isotherm of Char-PA exhibits a typical characteristic of type IV, with the increase of the relative pressure from 0 to 1. This isotherm has begun to branch, and a hysteresis loop has also been formed with the increase of the relative pressure, indicating the formation of hierarchical pores. In the process of CO2 activation, some micropores can be formed at the initial stage of activation, after which these micropores can further be enlarged into mesopores and macropores with the prolongation of the activation time; finally, these mesopores and macropores, as channels, can promote the diffusion of activated gas to help the production of new micropores [17,18]. Therefore, the N2 adsorption capacities of Char-PA are higher than those of Char with the increase of the relative pressure, presenting an SBET value of 414.78 m2·g−<sup>1</sup> and Vmic value of 0.1 m3·g<sup>−</sup>1. However, the non-Vmic value of 58.33% of

Char-PA with a wide pore distribution indicates the rapid development of mesopores and macropores instead of that of micropores during CO2 activation.

**Figure 7.** (**a**,**c**,**e**,**g**) The N2 adsorption isotherm and (**b**,**d**,**f**,**h**) pore-size distribution of the samples at different treatment conditions.

After 12 h of high-energy ball milling, the N2 adsorption isotherms of Char-PA-CO2/N2-12 exhibit the typical characteristic of type IV, with an increase of the relative pressure from 0 to 1. With the increase of the ball-milling time from 12 to 48 h, the flat development of adsorption isotherms and a small hysteresis loop can also be found for Char-PA-CO2/N2-48, with high N2 adsorption capacities at a low pressure and presenting a rapid increase of the SBET and Vmic values and an obvious decrease of the non-Vmic value of Char-PA-CO2/N2-12/48, accompanied by a gradual narrowing of the pore size distribution. These changes indicate a sustained formation of new micropores and a rapid decrease of mesopores and macropores during the ball milling, which are related to the fact that the collapse of the mesopores and macropores caused by the strong mechanical collision brings about the production of new micropores. Furthermore, a stronger solid-gas reaction caused by the mechanical collision under dry ice can favor the development of the porous structure. Therefore, the variety of pore structures of Char-PA-CO2-12/48 is more obvious than that of Char-PA-N2-12/48 under the same ball-milling time.

#### *3.5. Surface Chemical Structure Analysis of XPS*

Figure 8 shows the broad scanning energy spectrum of all the samples, determined by XPS in the range of 10–1200 eV binding energy to obtain the strongest peak for most elements. There are two obvious peaks (C1s and O1s peaks) in Char-PA and Char-PA-N2/CO2-12/48, indicating the dominant position of C and O in the element composition; however, the disappearance of the O1s peak of Char is related to the release of the oxygen elements in the form of small molecules (such as CO, CO2) during pyrolysis. With the increase of the ball-milling time from 12 to 48 h, there is a slight increase in the oxygen content of Char-PA-N2-12/48 from 4.09% to 4.11%. A ball milling under dry ice can rapidly promote the combination of surface unsaturated carbon atoms with CO2 to fix a large number of O atoms in the form of oxygen-containing functional groups; thus, the oxygen content of Char-PA-CO2-12/48 increases rapidly from 6.79% to 9.48%.

**Figure 8.** The survey XPS spectra of the samples at different treatment conditions.

In carbon materials, surface oxygen-containing functional groups are the most important functional groups that have been identified as affecting the surface chemical properties of carbon materials. In order to further explore the types and contents of oxygen functional groups on the surface of Char-PA and Char-PA-N2/CO2-12/48 quantitatively, the O1s peak of five samples is fitted and analyzed

according to different binding energies, as follows: carboxyl or ester carbon (C=O) at 530.9 eV, phenolic hydroxyl or ether (C–O) at 532.4 eV, carboxyl or ester carbon (O–C=O) at 533.8 eV and chemisorbed O2 (or H2O) at 535.2 eV. The results of the curve fitting of the five samples are given in Table 5. In addition, the XPS spectra of Char and Char-PA-CO2-48H2 are not treated further using the fitting method due to a minor oxygen content. After CO2 activation, the carbonyl and quinone group (C=O) of Char-PA accounts for the most part (39.6%); its phenol and ether group (C–O) takes second place (27.3%), and the carboxyl group or ester group (O–C=O) and chemisorbed O (or H2O) come last (18.4% and 14.7%) out of all the oxygen-containing functional groups. After ball milling under N2, the proportions of C=O, C–O and O–C=O of Char-PA-N2-12/48 are gradually consistent with each other with the prolongation of the ball-milling time. After ball milling under dry ice, the proportion of O–C=O of Char-PA-CO2-12/48 increases and its proportion of C–O, C=O decreases gradually, indicating the existence of oxidation phenomena. The above results illustrate that the content and type of oxygen-containing groups in the samples can be controlled by the ball-milling treatment in different atmospheres.

**Table 5.** The results of the curve fitting of Char-PA and Char-PA-N2/CO2-12/48 in O1s from the XPS spectra.


#### *3.6. Study of SO2 Adsorption*

In order to further explore the effect of the physicochemical structure of ACson SO2 adsorption, an SO2 adsorption test of Char-PA, Char-PA-N2-48, Char-PA-CO2-48 and Char-PA-CO2-48-H2 from a simulated flue gas is performed at 80 ◦C for 210 min. In these tested samples, Char-PA-CO2-48-H2 is obtained via a thermal annealing treatment of Char-PA-CO2-48 at 800 ◦C for 1 h in 5% H2/Ar atmosphere, and the results of the physicochemical structure and corresponding parameters are given in Figures 2–8 and Tables 2–4. It can be found that the thermal annealing process has removed almost all of oxygen-containing functional groups (the oxygen content is only 0.89% in Figure 8) but cannot further change its porosity, microstructure and surface morphology. The results of the SO2 removal of Char-PA, Char-PA-N2-48, Char-PA-CO2-48 and Char-PA-CO2-48-H2 are shown in Figure 9.

**Figure 9.** SO2 removal of typical ACs: (**a**) SO2 breakthrough curve and (**b**) SO2 adsorption quantity.

First, the efficient adsorption of Char-PA is mainly presented within only 60 min, while the SO2 concentrations of the gas outlet arrive quickly 1500 ppm; this result indicates that Char-PA has already been penetrated by SO2. In addition, Char-PA can only achieve 21.2 mg/g at the end of the experiment. Therefore, Char-PA, with undeveloped pores and fewer activated sites, can only maintain an efficient SO2 adsorption and conversion at the initial stage. Then, the SO2 adsorption of Char-PA-CO2-48 can be maintained at 100% within 30 min. After that, there is a slow increase from 70 to 1500 ppm in the SO2 concentrations of the gas outlet, and its SO2 adsorption capacity can reach 138.5 mg/g at the end of the experiment. Remarkably, Char-PA-N2-48, with relatively developed pores and some active sites, also has a relatively high SO2 adsorptive capacity (92.2 mg/g) at the end of the experiment, and its SO2 adsorption curve is similar to the curve of Char-PA-CO2-48. In the presence of O2 and H2O, the SO2 removal of porous carbon is a multi-step heterogeneous reaction. First, SO2, O2 and H2O can be rapidly absorbed by a lot of micropores, inside of which the oxidation and hydration of SO2 with O2 and H2O is further catalyzed by active sites to form H2SO4 [45]; after that, the active sites can continue to migrate H2SO4 from micropores to meso-/macro-pores to release the microporous space [46]. Therefore, Char-PA-CO2-48, with a large quantity of micropores and active sites, presents a high adsorption capacity as compared to that of samples in the previous literatures [47–49]. In addition, Char-PA-CO2-48-H2, with a large number of micropores and fewer active sites, has a limited adsorptive capacity (48.7 mg/g) at the end of the experiment, and its SO2 adsorption curve is similar to the curve of Char-PA. In the desulphurization process of Char-PA-CO2-48-H2, SO2 can be adsorbed effectively by its micropores in the initial stage, but the adsorbed SO2 within the micropores cannot be further catalyzed and migrated into the mesopores and macropores due to its limited active sites.

In order to measure the cyclic desulfurization performance of the prepared adsorbent, a thermal regeneration of Char-PA-CO2-48 is performed at 400 ◦C for 30 min in N2 atmosphere, and the corresponding result is given in Figure 10. The SO2 removal capacities of Char-PA-CO2-48 exhibit a general decreasing trend from 133.3 mg/g in the first-time desulfurization to 81.2 mg/g in the 10th cycle. Pi et al. [47] found that the pore structure and chemically active sites were damaged during a long-time (30 min), high-temperature (400 ◦C) and repeated thermal-treatment process, thus leading to rapidly decreased SO2 removal capacities.

**Figure 10.** SO2 removal capacities vs. cycling number of Char-PA-CO2-48.

#### **4. Conclusions**

The effect of physical and mechanical activation on the physicochemical structure of coal-based activated carbons (ACs) for SO2 adsorption has been investigated in this work. Char, using Jixi bituminous coal as raw materials obtained by pyrolysis, is activated sequentially via physical and mechanical methods. The results of the physicochemical structure of a series of AC samples indicate that a substantial reduction in the defective structure at the edge of the aromatic layers and the rapid growth of the aromatic layers accompanied by the dehydrogenation of the aromatic rings result in the order transformation of microstructures of Char-PA and its severe carbon losses on the particle surfaces in the stage of CO2 activation. Furthermore, the oxygen content of Char-PA is increased to 4.03%, and the proportions of the different oxygen-containing functional groups in Char-PA are as follows: C=O (39.6%), C–O (27.3%), O–C=O (18.4%) and chemisorbed O (or H2O) (14.7%). The pore development of Char-PA follows a hierarchical model, leading to a relatively low SBET value (414.78 m2/g) and a high value of Non-*V*mic (58.33%). Char-PA with undeveloped pores and fewer activated sites can only maintain an efficient SO2 adsorption and conversion within 60 min and achieve 21.2 mg/g at the end of the experiment. In the subsequent mechanical activation under N2 and dry ice from 12 to 48 h, the strong mechanical collision can improve the surface morphology and destroy the parallelism of the aromatic layers and the constancy of the interlayer spacing, resulting in the disordered conversion of the microstructure and the formation of more defective structures with the prolonging of the ball-milling time. In addition, the collapse of mesopores and macropores caused by a strong ball milling facilitates the formation of more micropores, leading to a sustained increase in the SBET value from 715.89 to 1259.74 m2/g and of the micropore volume from 0.22 to 0.34 m3/g, as well as a sustained decrease in Non-Vmic from 33.33 to 19.05% with the prolonging of the ball-milling time. However, the oxygen content of Char-PA-N2-12/48 increases slowly from 4.09 to 4.11%, presenting a similar distribution proportion, whereas the oxygen content of Char-PA-CO2-12/48 increases rapidly from 6.79 to 9.48%, presenting an increased proportion of O–C=O and a decreased proportion of C–O, C=O. It is worth noting that the varieties of physicochemical parameters of Char-PA-CO2-12/48 are more obvious than those of Char-PA-N2-12/48 under the same ball-milling time, which is related to the strong solid-gas reactions between CO2 and the carbon matrix caused by the mechanical collision under dry ice. The desulfurization efficiency of Char-PA-CO2-48 with a desirable physicochemical structure can be maintained at 100% within 30 min and reached 138.5 mg/g. Char-PA-N2-48 has a similar structure to Char-PA-CO2-48, thus presenting a relatively high SO2 adsorptive capacity (92.2 mg/g). Char-PA-CO2-48-H2, with fewer active sites obtained by the thermal annealing treatment, has a limited adsorptive capacity (48.7 mg/g) at the end of the experiment. After the 10th cycle of thermal regeneration, Char-PA-CO2-48 still has a strong adsorptive capacity (81.2 mg/g).

**Author Contributions:** D.L., S.L. and W.F. conceived and designed the experiments; X.Z., R.S. and Z.H. carried out the experiments; D.L. wrote the paper; D.L. and B.J. reviewed the paper.

**Funding:** This research was funded by National Natural Science Foundation of China, grant number 51806080, and Scientific Research Fund Project of Jilin Agricultural University, grant number 201801, and Jilin Province Education Department Science and Technology Program during the Thirteenth Five-year Plan Period, grant number JJKH20190940KJ.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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