3.1 Effect of Modification method on SO2 /NO x adsorption by AC
As shown in Fig. 2, the overall content of acidic functional groups in HP-AC modified by HNO3 oxidation increased substantially, accounting for 97% of the total content of acidic and basic functional groups. In contrast, compared with the P-AC not modified by peroxide reduction, the content of basic functional groups in the KP-AC modified by KOH reduction increased significantly, reaching about twice the content of acidic functional groups. It can be concluded that the modification of HNO3 and KOH can cause a significant increase in the content of acidic functional groups and alkaline metal functional groups on the surface of AC, respectively. In this experiment, the P-AC, HP-AC and KP-AC were subjected to simultaneous desulfurization and denitrification adsorption experiments under the same conditions, and the results obtained are shown in Table 3.
Table 3
Adsorption efficiency of surface reduction modified activated carbon(MAC) for NO and SO2
Adsorption efficiency | 0-AC | P-AC | HP-AC | KP-AC |
\({\text{η}}_{{N}{O}}\) | 27% | 27.70% | 28.80% | 36.90% |
\({\text{η}}_{{S}{O}₂}\) | 94% | 96.65% | 88.71% | 96.64% |
From the above results, it can be seen that after physical modification, the adsorption efficiency of P-AC was slightly increased, and the adsorption efficiency of KP-AC of AC modified by KOH increased the most significantly. Combined with the characterization results of Boehm titration method in Fig. 2, it can be concluded that the modification of HNO3 and KOH can cause a significant increase in the content of acidic functional groups and basic metal functional groups in the surface of AC, respectively. Xin (Xin H.2009) has been suggested that alkaline oxygen-containing functional groups can not only increase the adsorption sites and enhance the adsorption affinity of AC for the acid gases NO and SO2, but also act as a catalyst for the oxidation reaction, thus accelerating the overall reaction rate and increasing the adsorption capacity. The desulfurization and denitrification efficiency of MAC with nitric acid was significantly lower than that of MAC with potassium hydroxide. Hou et.al (Hou F et al.2011) has been shown that nitric acid modification leads to partial graphitization of the AC and destruction of the internal and surface void structures, which all lead to a decrease in the removal efficiency of the AC. Therefore, the redox modification stage was chosen to modify the AC by impregnation with KOH at a mass fraction of 3%, which is beneficial to improve the adsorption performance of the AC.
3.2 Effect of type of impregnating solution on SO 2 /NO x adsorption by AC
As can be seen from Fig. 3, the highest adsorption efficiency of AC for SO2 was achieved when cerium nitrate alone was modified; on the contrary, the adsorption efficiency of MAC for NO showed better performance under the conditions of cerium nitrate alone and cerium nitrate followed by copper nitrate modification.
By infrared characterization of different types of impregnated liquid-modified coconut shell AC, it can be seen that A and B showed characteristic peaks at 1560 cm− 1 from Fig. 4, corresponding to the group R-NO2 with the vibrational form Vas; while the characteristic peaks of C and D at 1390 cm− 1 indicated the presence of R-NO2 with the vibrational form Vs, and also the possible presence of amide groups. Meanwhile, the absorption peak of C at 1390 cm− 1 is the strongest, indicating that the content of nitrogen-containing groups in the AC increased after modification with 6% cerium nitrate compared with other modifications. At the wave number of 1720 cm− 1, only the A and C spectral lines have obvious absorption peaks, and the C spectral line peak is stronger than the A spectral line, where the corresponding group is ketone (RCOR), indicating that the ketone content of the pretreated AC after KOH modification decreases, but the ketone content of the AC increases after further modification with 6% cerium nitrate, and even higher than that of the unmodified AC. All four spectral lines show absorption peaks at 2350 cm− 1, where the corresponding group is O = C = O, and the absorption peak intensity of the C spectral line is the largest, indicating an increase in the content of the group O = C = O.
The above FT-IR detection analysis revealed a significant increase in the content of certain nitrogen-containing groups (R-NO2 and amide groups) and oxygen-containing groups (RCOR and O = C = O) in the AC modified with cerium nitrate. Sousa et al (Sousa JPS et al.2013) found through their study that surface nitrogen-containing groups can directly improve the catalytic activity of the carbon material and that the extra π-electrons of the nitrogen groups promote the formation of NO2 formation, thus improving the denitrification efficiency of the MAC. Meanwhile, the cerium nitrate MAC increased the content of oxygen-containing groups (RCOR and O = C = O), which also had an effect on the denitrification efficiency. The SO2 adsorption process of AC mainly occurred in the secondary micropores, and the phenolic and lactone oxygen-containing functional groups on the surface of AC could favorably influence the SO2 adsorption of AC. It was further speculated and analyzed that in the cerium nitrate followed by copper nitrate modification method, the addition of copper nitrate may block the micro-pores on the AC surface and affect the adsorption efficiency, so cerium nitrate impregnation alone is more beneficial to improve the adsorption efficiency of AC.
3.3 Effect of Impregnating solution concentration on SO2/NOx adsorption by AC
In this experiment, four activated carbons modified with cerium nitrate, 2-EKP-AC, 4-EKP-AC, 6-EKP-AC, 8-EKP-AC and surface reduction modified KP-AC, were examined by BET. The specific surface areas and pore capacities of the measured adsorbents are shown in Table 4. The specific surface area and pore volume of the AC increased substantially after being modified with 2% cerium nitrate by mass fraction alone, but the specific surface area of the MAC decreased gradually with the increase of the concentration of cerium nitrate in the impregnating solution, and the total pore volume and microporous pore volume of the above four cerium nitrate MAC also decreased gradually, and the microporosity also decreased with the increase of the concentration. The molecular diameter of NO is 0.32 nm and that of SO2 is about 0.23 nm, so the surface pores of AC with a diameter of 1 nm and 0.7 nm are more favorable for adsorption of NO and SO2.
Table 4
Specific surface area and pore capacity of MAC with different concentrations of cerium nitrate
MAC | SBET (m2·g− 1) | Vtotal (cm3·g− 1) | Vmicro (cm3·g− 1) | Microporosity (%) |
KP-AC | 154.92 | 0.08799 | 0.06525 | 74.16 |
2-EKP-AC | 270.72 | 0.15344 | 0.11278 | 73.50 |
4-EKP-AC | 200.77 | 0.12990 | 0.08238 | 63.42 |
6-EKP-AC | 137.74 | 0.09178 | 0.05593 | 60.93 |
8-EKP-AC | 118.44 | 0.08679 | 0.05066 | 58.37 |
As shown in Fig. 5, the pore surface area distribution of dp = 1nm micropores suitable for NO adsorption increased after modification with cerium nitrate, and the pore surface area distribution decreased with the increase of cerium nitrate concentration; the pore surface area distribution of dp = 0.7nm micropores suitable for SO2 adsorption increased only after modification with low concentration of cerium nitrate, but the pore area decreased significantly with the increase of cerium nitrate concentration. 8EKP-AC with dp = 0.7nm microporous surface area distribution decreased by nearly 50% compared to KP-AC. These results indicate that higher concentrations of impregnating solution adversely affect the surface structure of AC, presumably because excessive impregnating solution crystalline coverage clogs some of the pore channels on the AC surface.
In Fig. 6, all four spectral lines showed characteristic peaks at 1400 cm− 1-1600 cm− 1, indicating the presence of nitrogen-containing group R-NO2 and amide group, but the peak intensity of 6EKP-AC was significantly larger than that of 2EKP-AC, 4EKP-AC and 8EKP-AC, and the characteristic peak at wave number 1420 cm− 1 could be inferred that the different concentrations of cerium nitrate modified the order of the nitrogen-containing groups on the surface of AC was 6EKP-AC > 2EKP-AC > 4EKP-AC > 8EKP-AC.
The BET and FT-IR analyses of MAC with different concentrations of cerium nitrate showed that the increase of cerium nitrate concentration reduced the specific surface area and microporosity of AC, which was not conducive to the physical adsorption of AC using the surface structure; however, with the increase of impregnating solution concentration, the content of nitrogen-containing groups on the surface of AC that were favorable for the chemisorption of NO and SO2 increased, and the concentration of cerium nitrate was 6% when the maximum concentration of cerium nitrate was 6%. Considering the combination of physical and chemical adsorption, it can be assumed that the physical adsorption capacity gradually decreases and the chemical adsorption capacity gradually increases with the increase of impregnating solution concentration, and the maximum adsorption efficiency is reached at 6% of impregnating solution concentration.
With the change of concentration, the trend showed consistent, first decreasing, then increasing and then decreasing. The highest desulfurization efficiency of the MAC was achieved at a concentration of 6% in the impregnating solution; the highest denitrification efficiency of the MAC was achieved at a concentration of 2% in the impregnating solution, while the denitrification efficiency also increased significantly at a concentration of 6%.
3.4 Effect of SO2 concentration on SO2/NOx adsorption by AC
From Fig. 8, it can be seen that the MAC has the highest removal efficiency for NO and SO2 when the SO2 concentration is 1000 ppm. It can also be obtained that at the SO2 concentration of 1500–2500 ppm, as the removal efficiency of SO2 decreases and then increases, while the adsorption efficiency of NO increases and then decreases. This is because there is adsorption competition between SO2 and NO on the surface of AC, and the polarity of SO2 molecules is greater than that of NO molecules, and SO2 is more likely to react at the active sites on the surface of AC and take the advantage in the competitive adsorption with NO. Therefore, as the amount of desulfurization increases or decreases, the amount of denitrification decreases or increases (Wang Z.2017). To sum up, in this experiment, the highest efficiency of MAC desulfurization and denitrification was achieved at a flue gas SO2 concentration of 1000 ppm.
3.5 Effect of adsorption temperature on SO2/NOx adsorption by AC
It can be obtained that the adsorption temperature has different effects on both, and the adsorption efficiency of MAC for SO2 increases with the increase of temperature, while the adsorption efficiency for NO decreases with the decrease of temperature. The reason why the desulfurization efficiency of AC increases with increasing temperature is that the increase in temperature is not conducive to physical adsorption and favorable to chemisorption and chemical reaction. The reduced physical adsorption SO2 removal was smaller than the increased chemisorption and chemical reaction SO2 removal, so the desulfurization effect increased significantly with the increase of temperature.
When the temperature increased, the adsorption efficiency of MAC for NO decreased with the increase of temperature. The main reason is that NO molecules are mainly adsorbed on the surface of AC by van der Waals forces, when the temperature increases, it is not conducive to the physical adsorption of NO, and at the same time, the increase in temperature also accelerates the rate of NO desorption, which causes the adsorption efficiency to become smaller. In the conversion of NO to NO2, O2 is needed to be adsorbed at the active site, and the increase of temperature causes the reduction of oxygen active site, both NO and O2 adsorption are reduced, and the combined effect of these two makes the NO desorption rate of AC smaller. To sum up, in this experiment, 50°C is the optimum adsorption temperature.