SO2 (S) adsorption kinetics on the powdered activated carbons (AC) was performed in a laboratory-scale drop tube furnace. The reaction conditions affecting the adsorption of SO2, such as AC/S molar ratio, temperature, the concentration of SO2, O2, and H2O, and AC circulation rate, were studied. The powdered AC rapidly adsorbs SO2 in the initial 1.2 seconds and then the amount of SO2 adsorption slowly increases. The SO2 removal rate increases with the increasing of AC/S molar ratio, the decreasing of adsorption temperature, and inlet SO2 concentration. The O2 and H2O are beneficial to SO2 removal by powdered AC. SO2 removal rate drops to 6.83% after 14 cyclic adsorptions. The powdered AC circulation increases SO2 removal rate from 62.35% to 99.42% with AC circulation ratio = 7. The SO2 adsorption kinetics can be predicted by Bangham model.
1. Introduction
Emission of sulfur dioxide (SO2) has resulted in worldwide concerns in recent decades [1–3]. Adsorptive flue gas desulfurization (FGD) by activated carbon (AC) has been utilized in various industrial processes due to the advantage in recycling SO2 as sulfuric acid or sulfur and combined removal of other hazardous gasses [1–7]. Conventional adsorptive FGD uses pelletized AC with 5–9 mm diameter as adsorbent in the moving bed reactor. Efficient removal of SO2 is achievable with this method. However, high construction investment and operating consumption have always been a concern in some commercialization. The costs are mainly derived from the consumption of AC, which further results from the low utilization of inner part of pelletized AC and abrasion [8–10]. The powdered AC with diameter less than 100 μm, that was expected to have higher utilization ratio and lower production cost compared with the pelletized AC, is selected as the adsorbent in a novel method proposed by Ma [8, 11, 12]. The circulating fluidized bed (CFB) reactor is designed in the new method.
As for SO2 adsorption fundamentals over AC, the following processes have been widely accepted with researchers [7]. The SO2 in flue gas is first adsorbed on the surface of AC and then catalyzed to sulfuric acid (H2SO4) and H2SO4 is stored in pores of AC. The relevant reactions involved are as follows [7]:where (gas) is for gas phase, XV denotes the vacant sites, and (ad) represents the adsorbed species.
According to the adsorption mechanism above, the powdered AC would certainly show more rapid SO2 adsorption rate compared with the pelletized AC due to larger surface area exposed to the gas and shorter diffusion distance for adsorption process. When such powdered AC is utilized in a CFB reactor, the adsorption fundamentals are different with the pelletized AC in moving bed. These adsorption fundamentals over the powdered AC in fluidized bed reactor, especially for the adsorption dynamics, are required before further development of the new method.
The relative amount of SO2 and AC in the gas flow is an important value that would affect the adsorption dynamics. In order to simplify the value, the AC-SO2 (AC/S) molar ratio is proposed in this paper. With the assumption that the molar mass of activated carbon is 12 g/mol, AC/S molar ratio is defined as the molar ratio of AC to SO2.
Aimed at studying the adsorption dynamics in CFB, continuous supply of simulated flue gas and powdered AC in the experiment is essential. A drop tube reactor which can supply stable AC/S molar ratio with easy access as bench scale study is designed to simulate the adsorption process in CFB reactor.
In the paper, the SO2 adsorption kinetics on powdered AC is performed in a laboratory-scale drop tube furnace. The reaction conditions affecting the adsorption of SO2, such as AC/S molar ratio, temperature, the concentration of SO2, O2, and H2O, and AC circulation rate, are studied. The SO2 adsorption kinetics model is also proposed.
2. Experiments and Methods
2.1. Activated Carbon
A granular AC, which was made from coal by Shanghai Activated Carbon Co., Ltd., was used as raw material. The granular ACs were ground and screened and then 0.075 mm powdered ACs were selected as the research object. The N2 adsorption isotherms of AC were measured at 77 K by a Micromeritics ASAP2020 instrument. The Brunauer-Emmett-Teller (BET) equation and t-plot method were used to calculate the specific surface area (SBET) and the micropore specific surface area (Smic) of AC, respectively. The t-plot method and the Barrett-Joyner-Halenda (BJH) method were used to calculate the micropore volume (Vmic) and mesopore volume (Vmes), respectively. The Horvath-Kawazoe (HK) method was used to calculate the micropore size (Lmic). The HK method and BJH method were used to calculate the size distribution of micropore and mesopore, respectively.
The Fourier transformed infrared spectroscopy (FT-IR) spectra of the powdered AC was measured by Vertex-70 from Brook Company in Germany. The spectrum was recorded from 4000 to 400 cm−1 at a resolution of 4 cm−1.
2.2. SO2 Adsorption
The SO2 adsorption kinetics on the powdered AC was performed in a laboratory-scale drop tube furnace, and the system diagram of the drop tube furnace is shown in Figure 1. The stainless steel reactor of the drop tube furnace is 2 meters in length and 40 mm in diameter. The reaction temperature is controlled by an electric heating device. There are five gas sampling holes, which were evenly arranged along the reactor, to measure SO2 concentration. The MFEV-1VO microfeeder continuously and uniformly delivers the powdered AC into the reactor.
SO2, O2, H2O, and N2 constitute the simulated flue gas and the concentration of SO2, O2, and N2 is controlled by mass flowmeters (Sevenstar CS200). The peristaltic pump transports deionized water to the evaporator to generate water steam. The electric heating device heats the simulated flue gas to the reaction temperature to avoid condensation of water vapor before entering the reactor. The flow rate of the simulated flue gas is 20 L/min. The SO2 concentration is between 0.015% and 0.1%, the O2 concentration is between 0% and 6%, the H2O concentration is between 0% and 8%, and the rest is N2. The reaction temperature is between 65°C and 95°C.
The FT-IR gas analyzer (Gasmet Dx4000) was used to measure the SO2 concentration in flue gas. The SO2 removal rate by the powdered AC was determined by the following equation:where η is desulfurization efficiency, is the SO2 concentration at the reactor inlet, and is the SO2 concentration at the sampling port.
The adsorption amount of SO2 on the powdered AC was determined by the following equation:where q is the adsorption amount of SO2 on the powdered AC, mg/g, V is the flue gas flow rate, L/min, and mAC is the feeding rate of the powdered AC, g/min.
3. Results and Discussions
3.1. Characterization of the Powdered AC
The N2 adsorption isotherms of the granular AC and powdered AC are shown in Figure 2, the shape of which is type I according to IUPAC, indicating that the powdered AC mainly contains micropores [13].
Table 1 shows the pore structure parameters of the granular AC and powdered AC. Figure 3 shows the pore-size distribution of the granular AC and powdered AC. The value of Smic of the powdered AC is 541 m2/g, which is larger than that of granular AC. The value of Lmic of the powdered AC is 0.699 nm, which is larger than the molecular dynamics diameter of SO2, O2, and H2O, meaning that SO2, O2, and H2O can diffuse into the micropores of the powdered AC [8].
(a)
(b)
Because the particle mass transfer and pore diffusional resistances of the granular AC are larger than those of the powdered AC, N2 molecules cannot diffuse into some micropores inside the granular AC; therefore, the values of SBET, Smic, and Vmic of the granular AC are lower than that of the powdered AC.
The FT-IR spectrum of the powdered AC is shown in Figure 4. The sample had adsorption bands at around 1560, 1414, and 1107 cm−1. The one at 1560 cm−1 could be associated with quinone and carbonyl groups. The 1414 cm−1 can be attributed either to carboxyl groups or to phenolic hydroxyl groups. The adsorption band at 1107 cm−1 is related to C–O stretching in ethers, lactones, and phenols [8].
3.2. Influence of AC/S Molar Ratio on SO2 Adsorption
The influence of AC/S molar ratio on the adsorption of SO2 by the powdered AC is shown in Figure 5. The flow rate of the simulated flue gas is 20 L/min, and therefore the gas residence time in the reactor is about 6 seconds. In the first 1.2 seconds, the powdered AC quickly adsorbs SO2, and the amount of SO2 adsorbed rapidly increases. And then the SO2 adsorption rate by the powdered AC decreases and the amount of SO2 adsorbed slowly increases. As shown in the reaction equations (1)–(5), the SO2 in flue gas is first adsorbed on the surface of AC and then catalyzed to H2SO4 and H2SO4 is stored in pores of AC. The concentration difference between SO2 in flue gas and SO2 on the surface of the powdered AC is the driving force for the powdered AC to adsorb SO2. As the adsorption progresses, the active sites on the surface of the powdered AC are occupied by SO2, and the number of the vacant active sites decreases. The concentration of SO2 on the surface of the powdered AC is increased while the concentration of SO2 in flue gas is decreased. The concentration difference between SO2 in flue gas and SO2 on the surface of the powdered AC is therefore decreased and then the driving force for the powdered AC to adsorb SO2 is also decreased. Therefore, the SO2 removal rate by powdered AC increases slowly after 1.2 seconds.
(a)
(b)
Increasing the AC/S molar ratio increases the desulfurization efficiency of the powdered AC. When the AC/S molar ratio is increased from 100 to 345, the desulfurization rate by the powdered AC is increased from 28.11% to 70.24% at 1.2 seconds and is increased from 37.41% to 73.64% at 6.0 seconds, respectively. Increasing AC/S molar ratio indicates that the feed rate of the powdered AC is increased, the concentration of the powdered AC in flue gas is increased, more active sites are provided to adsorb SO2, and thereby the desulfurization efficiency is increased.
Increasing the AC/S molar ratio reduces the adsorption amount of SO2 on the powdered AC. When the AC/S molar ratio is increased from 100 to 345, the adsorption amount of SO2 on the powdered AC is decreased from 12.87 mg/g to 9.23 mg/g at 1.2 seconds and is decreased from 17.13 mg/g to 10.11 mg/g at 6.0 seconds, respectively. When the AC/S molar ratio is 100, the concentration of the powdered AC in flue gas is lower and the concentration of SO2 relative to the unit mass of powdered AC in flue gas is higher. The concentration difference between SO2 in flue gas and SO2 on the surface of the powdered AC is therefore increased and then the driving force for the powdered AC to adsorb SO2 is also increased. When the AC/S molar ratio is 345, the concentration of the powdered AC in flue gas is higher and the competitive adsorption of SO2 between the powdered ACs is increased, which reduces the adsorption quantity of SO2 on the powdered AC. Zhang et al. have studied SO2 removal by powdered AC in a CFB reactor and found that when the AC/S molar ratio increased, SO2 removal efficiency increased rapidly, but SO2 adsorption quantity decreased [11]. The research results of this paper are consistent with those of Zhang.
3.3. Influence of the Reaction Temperature on SO2 Adsorption
The influence of the reaction temperature on the adsorption of SO2 by powdered AC is shown in Figure 6. Increasing the reaction temperature reduces the desulfurization efficiency and the amount of SO2 adsorbed by the powdered AC. When the reaction temperature is increased from 65°C to 95°C, the desulfurization rate by the powdered AC is decreased from 58.24% to 29.06% at 1.2 seconds and is decreased from 74.35% to 36.03% at 6.0 seconds, respectively. The amount of SO2 adsorbed by the powdered AC is decreased from 10.74 mg/g to 5.36 mg/g at 1.2 seconds and is decreased from 13.71 mg/g to 6.64 mg/g at 6.0 seconds, respectively.
(a)
(b)
The AC/S molar ratio is 245 and SO2, O2, and H2O concentration is 0.085%, 6%, and 8%.
As shown in reaction equations (1)–(5), the powdered AC is used as adsorbent and catalyst to convert SO2 into H2SO4. The adsorption of SO2 and the catalytic oxidation of SO2 to H2SO4 by the powdered AC are all affected by reaction temperature. From the perspective of adsorption, the adsorption of SO2 in the gas phase on the solid surface is an instantaneous process. With the reduction of the free energy of the system, it also loses its degree of freedom, resulting in a reduction in enthalpy. Regardless of physical adsorption or chemical adsorption, the adsorption process is always exothermic. Therefore, increasing temperature is not conducive to the adsorption of SO2 by the powdered AC. From the perspective of surface catalytic oxidation reaction, there is the breakage and formation of chemical bonds, which requires certain temperature conditions. Therefore, increasing temperature is conducive to the catalytic oxidation reaction of SO2 by the powdered AC. The temperature has a great influence on the physical adsorption of SO2 by powdered AC. As the temperature increases, it reduces the physical adsorption of SO2, which in turn affects the catalytic oxidation of SO2 by the powdered AC. In addition, H2O adsorption decreases with increasing temperature, and H2SO4 cannot be desorbed from the active sites in time [14].
3.4. Influence of SO2 Concentration on SO2 Adsorption
The influence of the concentration of SO2 on the adsorption of SO2 by powdered AC is shown in Figure 7. Increasing the concentration of SO2 reduces the desulfurization efficiency of the powdered AC. When the concentration of SO2 is increased from 0.015% to 0.05%, the desulfurization rate of the powdered AC is decreased from 86.02% to 74.13% at 1.2 seconds and is decreased from 94.67% to 77.25% at 6.0 seconds, respectively. When the concentration of SO2 is increased, a part of SO2 could not be adsorbed by the powdered AC and therefore the desulfurization efficiency is decreased.
(a)
(b)
The temperature is 65°C, AC feeding rate is 2.634 g/min, and O2 and H2O concentration is 6% and 8%.
Increasing the concentration of SO2 increases the amount of SO2 adsorbed by the powdered AC. When the concentration of SO2 is increased from 0.015% to 0.05%, the amount of SO2 adsorbed by the powdered AC is increased from 2.80 mg/g to 8.03 mg/g at 1.2 seconds and is increased from 3.08 mg/g to 8.35 mg/g at 6.0 seconds, respectively. Increasing the concentration of SO2 increases the concentration difference between SO2 in flue gas and SO2 on the surface of the powdered AC, and then the driving force for the powdered AC to adsorb SO2 is also increased [15].
3.5. Influence of O2 Concentration on SO2 Adsorption
The influence of the concentration of O2 on the adsorption of SO2 by the powdered AC is shown in Figure 8. Increasing the concentration of O2 increases the desulfurization efficiency and the amount of SO2 adsorbed by the powdered AC. When the concentration of O2 is increased from 0% to 6%, the desulfurization rate by the powdered AC is increased from 22.32% to 58.24% at 1.2 seconds and is increased from 25.76% to 74.35% at 6.0 seconds, respectively. The amount of SO2 adsorbed by the powdered AC is increased from 4.06 mg/g to 10.74 mg/g at 1.2 seconds and is increased from 4.75 mg/g to 13.71 mg/g at 6.0 seconds, respectively.
(a)
(b)
There are two forms of adsorption of SO2 on AC. One is weakly bonded SO2, which is easy to be desorbed at low temperature, corresponding to physically adsorbed SO2 and the other is strongly bonded SO2 which can only be desorbed at high temperature, namely, SO3, which is related to the active sites of catalytic oxidation on AC surface [16]. Pinero et al. have found that, in the absence of O2, SO2 was almost physical adsorption, and activated carbon materials could not oxidize SO2 to SO3 [16]. When the concentration of O2 is 0%, SO2 adsorbed on the surface of the powdered AC could not be oxidized to SO3 and generated to H2SO4, which cannot empty the SO2 adsorption center and hinder the adsorption of SO2. At this time, SO2 mainly exists in the form of physical adsorption, with lower adsorption amount and lower desulfurization efficiency. When the concentration of O2 increases, SO2 adsorbed on the surface of the powdered AC is easily oxidized to SO3, which interacts with H2O to generate H2SO4, and H2SO4 could be desorbed from the active center regenerated to further adsorb SO2, which increases the desulfurization efficiency and the amount of SO2 adsorption.
3.6. Influence of H2O Concentration on SO2 Adsorption
The influence of the concentration of H2O on the adsorption of SO2 by the powdered AC is shown in Figure 9. Increasing the concentration of H2O increases the desulfurization efficiency and the amount of SO2 adsorbed by the powdered AC. When the concentration of H2O is increased from 0% to 8%, the desulfurization rate by the powdered AC is increased from 23.18% to 58.24% at 1.2 seconds and is increased from 26.94% to 74.35% at 6.0 seconds, respectively. The amount of SO2 adsorbed by the powdered AC is increased from 4.27 mg/g to 10.74 mg/g at 1.2 seconds and is increased from 4.97 mg/g to 13.71 mg/g at 6.0 seconds, respectively.
(a)
(b)
Mochida et al. have studied the continuous adsorption of SO2 by AC fiber at low temperature, and the product was aqueous H2SO4. Mochida has found that larger H2O and O2 concentrations in flue gas were conducive to the continuous removal of SO2 in the form of aqueous H2SO4, and the rate control step was the aqueous H2SO4 desorption from the surface of AC fiber [2]. When the volume fraction of H2O is 0%, SO2 is oxidized to SO3 by O2 and SO3 cannot be desorbed, which makes the active center invalid and cannot continue to adsorb SO2. When the volume fraction of H2O increases, SO3 is easily combined with H2O to form H2SO4 after SO2 oxidized to SO3 by O2. Excess H2O can elute H2SO4 from the active center. The eluted H2SO4 is stored in the micropores of the powdered AC, and the active center is regenerated to continue to adsorb and oxidize SO2. The increase of the volume fraction of H2O in flue gas has two main effects: on the one hand, the product SO3 after SO2 oxidation is hydrated to H2SO4, and on the other hand, H2SO4 is eluted from the active center by excessive H2O to regenerate active centers to facilitate the continuous adsorption of SO2 [2, 7].
3.7. Influence of the Powdered AC Circulation Ratio on SO2 Adsorption
The adsorption quantity of SO2 is lower than the saturated adsorption capacity of SO2 by the powdered AC under various working conditions [6, 15], so the recycling of the powdered AC plays a very important role in improving the desulfurization rate, reducing the amount of the powdered AC, and improving the utilization rate of the powdered AC. In order to study the cyclic adsorption performance of the powdered AC, a material receiving device is arranged at the outlet of the reactor to collect the powdered AC, which is defined as the first adsorption. And then the collected powdered AC is sent to the reactor according to a certain AC/S molar ratio to measure the desulfurization performance, and the powdered AC is collected again at the material receiving device, which is defined as the second adsorption, in turn until the adsorption of SO2 is saturated. The influence of cyclic adsorption of the powdered AC on the adsorption of SO2 is shown in Figure 10. When the cyclic adsorption is increased from the first to the eighth, the desulfurization rate by the powdered AC is decreased slowly from 62.35% to 45.53%; after that, the decreasing trend of the desulfurization rate is accelerated, and the desulfurization rate is decreased to 6.83% at the fourteenth.
(a)
(b)
With the increase of cyclic adsorption, SO2 adsorption amount increases, and then H2SO4 generated gradually increases. Because H2SO4 is stored in AC pores, H2SO4 occupies the active sites of AC, and the desulfurization rate decreases with cyclic adsorption increasing until the adsorption of SO2 is saturated.
The circulation ratio refers to the ratio of the amount of material captured by the material separator and returned to the reactor to the amount of material supplied. In this paper, the influence of the circulation of the powdered AC on the desulfurization efficiency is simulated under the condition that the molar ratio of fresh powdered AC to SO2 in flue gas is certain. The results are shown in Figure 10(b). Increasing the circulation ratio of the powdered AC increases the desulfurization efficiency. The desulfurization efficiency of the powdered AC is 62.35% when the powdered AC is not recycled. The desulfurization efficiency is increased to 99.42% when the circulation ratio of the powdered AC is 7. When the AC/S mole ratio is 300 and the powdered AC circulating ratio is 7, SO2 can be removed effectively.
Increasing the circulation ratio of the powdered AC increases the concentration of AC in reactor and the number of active sites, which is conducive to the removal of SO2.
3.8. The SO2 Adsorption Kinetics
The Bangham model can be used to fit the adsorption process where multiple adsorption mechanisms exist, such as gas film diffusion, surface reaction, and intraparticle diffusion. This model was obtained by time compensated pseudo-first-order kinetics model, which made it have good adaptability in various adsorption processes. The Bangham model is shown in the following equation [17–19]:where qt is the adsorption amount of SO2 on the powdered AC at time t, qe is the saturation adsorption capacity of SO2 on the powdered AC, mg/g, and k and n are constants.
Figures 5(b), 6(b), 7(b), 8(b), and 9(b) show the fitting results of the Bangham model. Tables 2–6 show the parameters of the Bangham model. The calculated values of the Bangham adsorption kinetics model agree well with the experimental values, and the fitting correlation coefficient R2 is higher, all of which are above 0.98. This indicates that SO2 adsorption on the powdered AC in the drop tube furnace is controlled by multiple mechanisms, such as gas film diffusion, intragranule diffusion, and surface reaction. The SO2 adsorption kinetics can be predicted by Bangham model.
4. Conclusions
The SO2 adsorption kinetics on powdered AC was performed in a laboratory-scale drop tube furnace. The results show that the powdered AC rapidly adsorbs SO2 in the initial 1.2 seconds and then the amount of SO2 adsorption slowly increases. The SO2 adsorption kinetics can be predicted by Bangham model.
The SO2 removal rate increases with the increasing of AC/S molar ratio, the decreasing of adsorption temperature, and inlet SO2 concentration. The adsorption amount of SO2 by the powdered AC increases with the increasing of the inlet SO2 concentration, the decreasing of AC molar ratio, and adsorption temperature. The O2 and H2O are beneficial to SO2 removal by the powdered AC.
The recycling of the powdered AC plays a very important role in improving the desulfurization rate, reducing the amount of the powdered AC and improving the utilization rate of the powdered AC. SO2 removal rate drops to 6.83% after 14 cyclic adsorptions. The powdered AC circulation increases SO2 removal rate from 62.35% to 99.42% with AC circulation ratio = 7.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work has been supported by China Huadian Corporation Ltd. 2019 Science and Technology Project (CHDKJ19-02-206) and China Huadian Corporation Ltd. 2020 Science and Technology Project (CHDKJ20-02-80).