Energy Fuels 2010, 24, 5454–5463 Published on Web 09/28/2010
: DOI:10.1021/ef100989w
Simultaneous Removal of SO2 and NOx by Calcium Hydroxide at Low Temperature: Evolution of the Absorbent Surface Structure Jihui Gao,* Guoqing Chen,* Jiaxun Liu, Xiaolin Fu, Jianmin Gao, Qian Du, and Yukun Qin School of Energy Science and Technology, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, P. R. China Received July 29, 2010. Revised Manuscript Received September 3, 2010
The surface structure characteristics of the absorbent subjected to SO2 and NO2 for different reaction times were measured by using the N2 adsorption method. The pore surface and structure fractal dimensions (D1 and D2) calculated from the N2 adsorption isotherms data were applied to analyze the fractal characteristics of absorbent particles during reactions. On the basis of the measurement results and the reaction mechanism of SO2 and NO2 removal, the evolution mechanism of absorbent surface structure during reactions was discussed. The presence of reaction products on the surface of the absorbent can reshape the pore structure. With the comparison of the evolution of the surface structure in SO2 and NO2 single removal, the interaction of SO2 and NO2 on the absorbent surface has a great effect on the change of surface structure with simultaneous removal of SO2 and NO2. The decrement of the surface structure parameters values is lower than that in both SO2 and NO2 single removal. The change trend of the D1 and D2 can also not be well explained by their changes in SO2 and NO2 single removal. Because of the expansion rate discrepancy of the NO2 removal product and SO2 removal product, the pores from 7 to 30 nm were formed in SO2 and NO2 by simultaneous removal. The homogeneous reaction product and the same expansion rate make the product layer compact in both SO2 and NO2 single removal. On the basis of the analysis of both surface fractal dimension and structure fractal dimensions, it can be concluded that the higher surface fractal dimension as well as an appropriate pore structure fractal dimension D2 will helping in getting higher SO2 and NO2 removal.
such as KMnO4, O3, and methanol into the flue gas duct at an optimum temperature and then reacts with the conventional alkaline absorbent. With the comparison to the conventional desulfurization reaction process, the presence of NO2 in the flue gas not only makes the reactions on the surface of absorbent particle more complicated but also affects SO2 absorption. Therefore, development of the simultaneous removal of SO2 and NOx technology requires a good understanding of the kinetics of reactions between Ca-based absorbent with NO2 and SO2. The noncatalytic gas-solid reaction of Ca-based absorbent with SO2 at high temperature has been widely investigated. Many mathematical models have been proposed to describe the surface reaction process, such as grain models, pore models, volume reaction models, and deactivation models. However, the reaction rate of Ca(OH)2 with SO2 under low temperature and humid conditions is not only affected by the reaction conditions but also determined by the surface structure characteristics of the absorbent. The studies surveyed in the literature all agree with the great impact of H2O on both the solid conversion and the reaction rate. The presence of H2O in the flue gas makes the reaction paths on the absorbent surface more complicated. On one hand, the absorbent surface can be divided into the capillary condensation region and the multimolecules layer absorption region based on the H2O adsorption mechanism in different pore structures. On the other hand, the effect of product layer cannot be described by the means of the simplest form of the shrinking core model due to pore plugging. The effect of H2O on the absorbent surface reactions largely depends on the absorbent surface structure. Therefore, establishing the kinetic model of the simultaneous
1. Introduction SO2 and NOx emissions from the combustion of fossil fuels, such as coal and heavy oils, have caused serious environmental problems.1 Presently, many strategies and technologies have been developed to control the emission of precursors to acid deposition.2,3 Among these technologies, the combination of low NOx burner technology, wet flue gas desulfurization (WFGD), and selective catalytic reduction (SCR) was proved to be the most effective method for removing SO2 and NOx. However, higher expensive investment and operating cost makes it unsuitable for the developing countries such as China. Considering the high proportion of SO2 removal process with Ca-based absorbent in the old existing power plants, developing economical and effective technology for simultaneous removal of SO2 and NOx in the conventional dry or semidry FGD reaction cell has attracted many researchers’ attention.4,5 In the recent years, many studies have shown that oxidizing NO to more soluble NO2 is a promising method for enhancing NO removal in the conventional FGD processes. NO is first oxidized to NO2 by addition of oxidant *To whom correspondence should be addressed. Telephone: þ86451-86413231, ext. 816. Fax: þ86-451-86412528. E-mail: gaojh@hit. edu.cn (J.G.);
[email protected] (G.C.). (1) Zhao, Y.; Duan, L.; Xing, J.; Larssen, T.; Nielsen, C. P.; Hao, J. M. Environ. Sci. Technol. 2009, 43, 8021–8026. (2) Li, Z. Q.; Jing, J. P.; Chen, Z. C.; Ren, F.; Xu, B.; Wei, H. D.; Ge, Z. H. Combust. Sci. Technol. 2008, 180, 1370–1394. (3) Jing, J. P.; Li, Z. Q.; Liu, G. K.; Chen, Z. C.; Ren, F. Energy Fuels 2010, 24, 346–354. (4) Chu, H; Chien, T. W.; Li, S. Y. Sci. Total Environ. 2001, 275, 127– 135. (5) Zhang, H.; Tong, H. L.; Wang, S. J.; Zhuo, Y. Q.; Chen, C. H.; Xu, X. C. Ind. Eng. Chem. Res. 2006, 45, 6099–6103. r 2010 American Chemical Society
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: DOI:10.1021/ef100989w
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removal of NOx and SO2 with Ca-based absorbent under humid conditions at low temperature needs a good understanding of the evolutionary law of absorbent surface structure. Several reports have predicted a strong change in the absorbent surface structure property with reaction time, but only a few reports concerning the evolution process of the absorbent surface structure in SO2 removal process can be found in the previous literature. Oritiz et al.6 obtained the conclusion that the pores from 9 to 200 nm is the effective pore size for the desulfurization reaction and also found a linear decrease of the structural parameters with solid conversion. This result was later confirmed by Ho et al.7 Bausach et al.8 showed that the pore structure change and plugging was the crucial factor limiting absorbent utilization and led to the reaction rate decreasing rapidly at low temperature in the SO2 removal process. For the coexistence of SO2 and NO2 in the flue gas, the change of absorbent surface structure during reactions is never reported. Liu et al.9 found the presence NO2 in the flue gas could enhance SO2 removal. Neill and Rochelle10 reported that SO2 accelerated the reaction rate of NO2 and Ca(OH)2. Consequently, the presence of NO2 in the flue gas not only adds some complicated reaction steps but also may change the absorbent surface structure. However, the kinetic modeling of the simultaneous removal of SO2 and NO2 are almost based on the kinetic models of SO2 single removal. The effect of interaction of SO2 and NO2 on the evolution of the absorbent surface structure has rarely been studied. In this paper, the surface structure parameters of absorbents subjected to SO2 and NO2 for different time were measured by the N2 adsorption-desorption method. The change trend of the surface structure parameters was analyzed. The pore surface and structure fractal dimensions (D1 and D2) calculated from the N2 adsorption isotherms data were used to analyze the fractal characteristics of the absorbent during reactions. In order to further analyze the evolution mechanism of the absorbent surface structure in the simultaneous removal of SO2 and NO2, the changes of the surface structure parameters and the fractal dimensions in the SO2 and NO2 single removal processes were also obtained.
The experiments for the reaction of absorbent with SO2 and NO2 were carried out by using a fixed bed reactor system. The details of the experimental setup and procedure were described in our previous report.11 In this study, about 4.0 g of absorbent was used for each run. In order to prevent channeling and absorbent agglomeration in the reactor, about 14 g of silicon dioxide with particle size range of 250-380 μm mixed with absorbent was packed in the reactor. The simulated flue gas containing 1200 ppm of SO2, 400 ppm of NO2, 6% of O2, 20% of H2O, and balance N2 supplied by the high-pressure cylinder was passed through the absorbent at 70 °C. The total flow rate of the gas stream was controlled at 1500 mL/min (STP). After the reaction ended, the spent absorbents were separated from the silicon dioxide by using a sieve and were dried at 105 °C before they were subjected to analysis. The spent absorbent reacted with SO2 and NO2 for i h was named DeSN-0i (i = 0, 1, 2, 3, 4). 2.2. Measurement Analysis. In the experiment process, the concentration of SO2, NO, NO2, and H2O in the simulated flue gas were measured by an online Fourier transform infrared (FTIR) gas analyzer (GASMET-DX4000, Finland), with measurement errors of (2%. A Testo-335 gas analyzer was used to measure the concentrations of O2, with measurement errors of (0.8 vol %. The surface structure parameters were calculated from the N2 adsorption isothermals, which were obtained by using an automated surface area analyzer (ASAP-2020) at 77 K of liquid nitrogen temperature. The samples were first outgassed at 105 °C overnight under vacuum to a final pressure of 0.28 Pa and then measured for the relative pressure range from 0.01 to 0.991. The BET specific surface area and the volume of monolayer coverage were determined using the BrunauerEmmett-Teller (BET) equation. The pore surface, pore diameter, and pore size distribution were calculated by analyzing the desorption branch of the N2 adsorption isothermals in the pore range of 1.7-300 nm using the Barrett-Joyner-Halenda (BJH) method. The surface morphologies of the spent absorbents were observed by scanning electron microscopy (Hitachi S-4700, Japan).
3. Results and Discussion 3.1. Change of Pore Shape on the Absorbent Surface. The isotherms data for N2 gas adsorption-desorption obtained from the spent absorbents subjected to the mixed gas of SO2 and NO2 for 0, 1, 3, and 4 h are illustrated in Figure 1. It can be seen that the adsorption branches of the isotherms gradually increase with the relative pressure increasing from 0 to 0.8, then sharply ascend over P/P0 = 0.8 and do not reach the stable status even as the relative pressure is close to 1.0. This indicates that N2 vapor confined in the pore of the absorbent particle condenses at a pressure lower than saturation pressure (P0), which is defined as capillary condensation. According to the International Union of Pure and Applied Chemistry (IUPAC) classifications, the adsorption isotherms of all absorbents are type II. The typical characteristic of this type of adsorption is that the monolayer adsorption and the transition of monolayer to multilayer adsorption occur at low relative pressure, and the multilayer adsorption process and capillary condensation are present at the higher relative pressures. In addition, the hysteresis loop of the fresh absorbent formed by the adsorption and desorption branches at the relative pressure over 0.8 is very vertical and the adsorption branch is almost parallel to the desorption isotherms. On the basis of the IUPAC classifications of hysteresis loops, the hysteresis loop of the fresh absorbent is similar to the
2. Experimental Section 2.1. SO2 and NO2 Removal Experiments. The raw material used to prepare absorbent is calcium oxide, which is of reagent grade. The raw calcium oxide was first milled and sieved to ensure that almost all calcium oxide particles were in the size range 60-80 μm. The powder-type calcium oxide was then added into the deionized water at a H2O/CaO weight ratio of 5 at a temperature of 70 °C. The slurry was heated up to 90 °C and maintained for 3 h with continuous stirring. After stirring, the resulting slurry was dried at 110 °C until there was no weight change to produce the dry absorbent. The dry absorbent was crushed and sieved into the required particle size range of 125-180 μm with a specific BET surface area of 7.26 m2/g. (6) Ortiz, M. I.; Garea, A.; Irabien, A.; Cortabitarte, F. Powder Technol. 1993, 75, 167–172. (7) Ho, C.; Shih, S.; Liu, C.; Chu, H.; Lee, C. Ind. Eng. Chem. Res. 2002, 41, 3357–3364. (8) Bausach, M.; Pera-Titus, M.; Fite, C.; Cunill, F.; Izquierdo, J.-F.; Tejero, J.; Iborra, M. AIChE J. 2005, 51, 1455. (9) Liu, C. F.; Liu; Shih, S. M. Ind. Eng. Chem. Res. 2006, 45, 8765– 8769. (10) Nelli, C.; Rochelle, G. T. J. Air Waste Manage. Assoc. 1998, 48, 918–828.
(11) Chen, G. Q.; Gao, J. H.; Wang, S.; Fu, X. L.; Xu, L. L.; Qin, Y. K. Ind. Eng. Chem. Res. 2010, 49, 1450–1456.
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Figure 1. Nitrogen adsorption-desorption isotherms of the absorbents.
also observed that the shape of the hysteresis loop changes with reaction time and can be present at a lower relative pressure. It is well-known that the hysteresis loop is generally related to the pore shape. Therefore, the change of the hysteresis loop means the change of the pore shape. However, it is difficult to find a typical hysteresis loop classified by IUPAC similar to the hysteresis loops produced by the spent absorbent DeSN-01, DeSN-03, and DeSN-04. Therefore, it is supposed that the SO2 and NO2 removal products reshape the pore shape of the absorbent and make pores shape more complicated and heterogeneous. The hysteresis loop is considered to be composed of many typical hysteresis loops.12 3.2. Change of Absorbent Surface Structure Parameters during SO2 and NO2 Simultaneous Removal. The surface structure parameter values of the samples subjected to SO2 and NO2 for different times are shown in Figure 3. It can be seen that the surface structure parameters undergo a complex history during reaction. In the initial 3 h, there is no significant change in the values of the surface structure parameters. However, the BET specific surface area, pore area, and volume decrease markedly as the reaction time exceeds 3 h. For the purpose to well explain the above staged phenomenon, the evolution of SO2 and NOx concentrations at the reactor outlet are displayed in Figure 4. It can be found that the absorbent has no reactivity to SO2 as the reaction time is above 3 h but can still remove NO2 at a high efficiency. This result suggests that the reactions occurring in the initial 3 h are different from the reactions taking place after 3 h.
Figure 2. SEM image of the fresh absorbent.
H1 type, which suggests that the pores shape of the fresh absorbent is spheric agglomerate and compact shapes. The SEM image of the fresh absorbent in Figure 2 can also confirm this conclusion. As can be seen from Figure 2, an absorbent particle is composed of some small particles and some nanometer scale particles are found to agglomerate on the small particle surface. The agglomeration of particles builds up the pores on the absorbent surface. Moreover, it is
(12) Han, X. X.; Jiang, X. M.; Yu, L. J.; Cui, Z. G. Energy Fuels 2006, 20, 2408–2412.
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Figure 3. Evolution of pore structure parameters of absorbent during reactions.
Figure 4. Evolution of NOx and SO2 concentration at the reactor outlet.
Figure 5. Pore size distribution of spent absorbent for different reaction times.
The discrepancy of reactions on the absorbent surface before and after 3 h can account for the staged phenomenon. As Figure 3 indicated, the reactions of SO2 and NO2 removal have a negligible effect on the structural parameters. However, from the results of Figure 1, we know that the change of pore shape does take place during reactions. Therefore, it is presumed that some new pores may be formed to replace the pores packed by the reaction products. The shape of the new pores is different from that of the pores packed by reaction products, but the decrement of the structure parameters values due to the plugging of the original pores can be offset by the presence of the new pores. In order to analyze the change of pore size distribution during the reactions, and confirm the above presumption, the pore size distribution of the spent absorbents are shown in Figure 5. It is seen that the distribution of pore size changes greatly with reaction time, and the pores were classified into three types based on the change trend of the contribution of the pores to the total pore volume, i.e., the small pores (d < 7 nm), the middle pores (7 nm< d 30 nm). The contribution of the small pores (d30 nm) to the total pore volume decreases with the increase of reaction time, while the contribution of the middle pore (7 nm < d < 30 nm) increases in the beginning 3 h and then decreases. For a quantification illustration of the changes of these three types of pores, the pore area and pore volume of these
three types of pores for different samples are listed in Table 1. The pore area and pore volume of the small pores and wide pores decrease with reaction time, which means that these two types of pores disappear during the reactions, and SO2 and NO2 removal reactions must be present in these two types of pores. The change trend of the middle pores shows an opposite trend, which is in agreement with the results of Figure 5. The increase in the middle pores can confirm the presumption that some new pores may be formed to replace the pores packed by reaction products. However, it is difficult to estimate whether the reaction occurs in the middle pores. 3.3. Change of Fractal Characteristics of Pore during SO2 and NO2 Simultaneous Removal. On the basis of the abovementioned analysis, the presence of reaction products in the pores not only can form some middle pores but also can affect the pore shape. However, the surface structure parameters calculated from N2 adsorption isotherms data can only describe the changes from macroscopic states. The irregularities of the surface and pore structure, such as surface roughness and pore complexity, are hard to be quantitatively described by Euclidean geometry, nevertheless these parameters in large part serve as important factors affecting the surface reaction activity. Hence, the fractal theory was applied to illustrate the changes of surface roughness and pore complexity during the SO2 and NO2 removal process. 5457
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Table 1. Pore Area and Pore Volume of the Absorbent Subjected to Flue Gas for Different Reaction Time pore diameter < 7 nm
7 nm < pore diameter < 30 nm
30 nm < pore diameter
sample
pore area (m2/g)
pore volume 102(cm3/g)
pore area (m2/g)
pore volume 102 (cm3/g)
pore area (m2/g)
pore volume 102 (cm3/g)
DeSN-00 DeSN-01 DeSN-02 DeSN-03 DeSN-04
2.37 2.12 1.72 1.57 0.84
0.29 0.23 0.20 0.18 0.10
1.96 2.48 2.35 2.75 1.74
0.72 1.23 1.16 1.28 0.81
2.61 2.59 2.54 2.66 2.02
3.92 3.96 3.97 3.60 2.86
Various methods have been employed to investigate the fractal characteristics of the porous materials.13-15 Among these methods, N2 adsorption analysis has been proved to be an effective method in characterizing the pore structure of porous materials.16,17 The fractal dimension is calculated from the analysis of N2 absorption isotherms by the following equation: 2 3 ! V P 0 5 þ constant ¼ A4ln ln ln ð1Þ Vmono P
The adsorbed layer n can be calculated from the following equation. n ¼ ðV=Vmono Þ1=ð3 - DÞ
ð2Þ
In the multiple layer adsorption region, the fractal dimension cannot describe the surface irregularity but can illustrate the adsorbed gas molecule agglomerates. Yao et al.21 investigated the discrepancies of the fractal dimensions calculated in the low and high relative pressure regions, respectively, and maintained that the fractal dimension calculated from eq 1 in the capillary condensation regions and monolayer adsorption can characterize the pore structure fractal dimension and surface fractal dimension. Liu et al.22 also obtained the pore structure fractal dimension of the super fine pulverized coal particle in the capillary condensation region. Hence, the fractal dimension D2 calculated in the linear region at the high relative pressure regions occurring capillary condensation was used to characterize the pore structure fractal characteristics. The slopes (A1 and A2) of the lines fitted through the data point in the first and third linear regions were substituted into expressions A = (D - 3)/3 and A = D - 3, respectively, and obtained the value of the fractal dimension D1 and D2. It is interesting that the values of D1 and D2 calculated by the expression A = (D - 3)/3 were less than 2, which is unrealistic. Therefore, the fractal dimensions of all sample were calculated from the expression A = D - 3. Figure 7 shows the evolution of D1 and D2 during the SO2 and NO2 removal process. It can be seen that the values of D1 and D2 belong to the range from 2 to 3, which coincides with the definition of the surface and pore structure fractal dimensions.23 It is obvious that the D2 values first increase and then decrease with the reaction time. The maximum value presents at 2 h of reaction time. However, the D1 values slightly decrease in the initial 3 h and then markedly decrease as the reaction time increases up to 4 h. In general, the surface fractal dimension characterizes the pore surface irregularity. Generally speaking, the larger is the value of the surface fractal dimension, the more irregular and rougher is the pore surface. The result of Figure 7 reveals that the reactions of SO2 and NO2 removal do not affect the roughness of the pore surface. Therefore, the BET specific surface area, pore volume, and area displayed in Figure 3 show no apparent change with reaction time. As the reaction time increases from 3 to 4 h, the pores on the absorbent surface were almost filled by NO2 removal products. The sharp decrease of pore volume of three types of pores in Table 1 can account for the decrease of the D1 value. After reaction for 4 h, the pores on
where V is the volume of adsorbed gas molecules at the equilibrium pressure P, Vmono is the volume of the monolayer coverage, A is the power-law exponent depending on D and the adsorption mechanism, and P0 represents the saturation pressure of the gas. According to the fractal Frenkel-Halsey-Hill (FHH) model, the slope of the straight line on the plot of ln(V/Vmono) and ln[ln(P/P0)] should be equal to A, and the fractal dimension D depends on the value of A. Concerning the relation between A and D, there are two expressions: A = (D - 3)/3 and A = D - 3. Figure 6 depicts the relation between ln(V/Vmono) and ln[ln(P0/P)] for the samples DeSN-01, DeSN-02, DeSN-03, and DeSN-04. It can be found that there are more than two linear regions for all the samples, and all of them have good linear fits. Hence, choosing the appropriate region is an important step for obtaining the correct fractal dimension. In general, two typical fractal dimensions, the pore structure fractal dimension and the surface fractal dimension, are defined to illustrate the fractal characteristics of porous materials.18 Wu et al.19 pointed out that the interface of the adsorbate with the adsorbed gas molecules becomes smooth in the multilayer adsorption regions, which makes it difficult to measure the true interface surface fractal dimension. Tang et al.20 maintained that as the adsorbed layer (n) varying from 1.0 ( 0.5 to 2.0 ( 0.5, the adsorbed layer could be regarded as monolayer coverage, and the fractal dimensions calculated in this region could represent the surface irregularity. Hence, the first linear region in the low relative pressure region was chosen to calculate the surface fractal dimension D1. This region is the transition of the monolayer coverage to the multiple layer coverage, but before the smoothing effect of the multiple layer coverage presents. (13) Cuerda-Correa, E. M.; Dı´ az-Dı´ ez, M. A.; Macı´ as-Garcı´ a, A.; Ga~ n an-G omez, J. Appl. Surf. Sci. 2006, 252, 6102–6105. (14) Diduszko, R.; Swiatkowski, A.; Trznadel, B. J. Carbon 2000, 38, 1153–1162. (15) Drake, J. M.; Yacullo, L. N.; Levita, P.; Klafter, J. J. Phys. Chem. 1994, 98, 380–382. (16) Mahnke, M.; M ogel, H. J. Colloids Surf., A 2003, 216, 215–228. (17) Shafei, G. M. S. E.; Philip, C. A.; Moussa, N. A. J. Colloid Interface Sci. 2004, 277, 410–416. (18) Pyun, S. i.; Rhee, C. K. Electrochim. Acta 2004, 49, 4171–7180. (19) Wu, M. K. Aerosol Sci. Technol. 1996, 25, 392–398. (20) Tang, P.; Che, N. Y. K.; Chan, H. K.; Raper, J. A. Langmuir 2003, 19, 2632–2638.
(21) Yao, Y. B.; Liu, D. M.; Tang, D. Z.; Tang, S. H.; Huang, W. H. Int. J. Coal Geol. 2008, 73, 27–42. (22) Liu, J. X.; Jiang, X. M.; Huang, X. Y.; Wu, S. H. Energy Fuels 2010, 24, 3072–3085. (23) Pfeifer, P.; Avnir, D. J. Phys. Chem. 1983, 79, 3369–3558.
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Figure 6. Plots of ln(V/Vmono) vs ln(ln(P0/P)) obtained from absorption data of spent absorbent.
volume in Figure 5 are responsible for the increase of the D2 value in the beginning. Moreover, the reaction product building up in the vicinity of the external or internal entrances to the pores also makes the pore structure more complex. This suggests that the changes of the pore shape and pore size distribution take place during SO2 and NO2 simultaneous removal, although the surface structure parameters show no obvious change. 3.4. Evolution Mechanism of Pore Structure in SO2 and NOx Simultaneous Removal. 3.4.1. Change of Surface Structure Parameters in SO2 and NO2 Single Removal Processes. In order to analyze the evolution mechanism of the absorbent surface structure in the simultaneous removal of SO2 and NO2, the values of BET specific surface area and pore volume of the samples subjected to SO2 and NO2, respectively, are given in Figure 8, and the breakthrough curves of SO2 and NO2 concentration are shown in Figure 9. The samples subjected to SO2 and NO2, respectively, for i hours (i = 0, 1, 2, 3, 4) were named DeS-0i and DeN-0i. From Figure 8, we can find that the surface structure parameters values also decrease with reaction time in both SO2 and NO2 single removal, which is similar to the change trend in the simultaneous removal of SO2 and NO2 process. With the comparison of the evolution of the absorbent surface structure parameters in SO2 and NO2 single removal, it can be seen that the decrement of the surface structure parameters value in NO2 single removal was much greater than that in SO2 single removal, i.e., the NO2 removal products are more helpful to the reshaping of the absorbent. However, in
Figure 7. Evolutions of fractal dimensions (D1 and D2) in SO2 and NO2 removal process.
the surface of absorbent are only wide pores, and consequently the surface becomes smoother. The pore structure fractal dimension represents the pore structure irregularity, which belongs to the range 2-3. The values of the pore structure fractal dimension are equal to 2 and 3 and mean that the pore size on the porous absorbent is homogeneous and inhomogeneous, respectively. The larger is the value of the pore fractal dimension, the narrower is the pore size distribution. The plugging of the wide pore and the increase in the contribution of the middle (7-30 nm) to total pore 5459
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pores >10 nm decrease obviously, and the pores 30 nm in Figure 10 results from both SO2 and NO2 removal reactions. The decrease of pores
