Characterization and Mechanisms of H2S and SO2 Adsorption by

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Characterization and Mechanisms of H2S and SO2 Adsorption by Activated Carbon Lei Shi, Ke Yang, Qiaopo Zhao, Haiyan Wang,* and Qun Cui* College of Chemical Engineering, Nanjing Tech University, Nanjing, Jiangsu 210009, P. R. China ABSTRACT: Coconut-shell-based activated carbon (ACS-1) was used as a sorbent to simultaneously remove H2S and SO2 from simulated Claus tail gas. Adsorption and regeneration tests were performed to systematically investigate the desulfurization performance, regenerability, and stability of the ACS-1 sorbent. The physicochemical properties of ACS-1 before and after adsorption were characterized by nitrogen adsorption, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The experimental results revealed that the ACS-1 sorbent exhibited good desulfurization performance under a feed gas of H2S (20 000 ppmv), SO2 (10 000 ppmv), and N2 (balance), and the concentrations of H2S and SO2 in the simulated Claus tail gas could be reduced to less than 10 mg/m3 by ACS-1. The breakthrough sulfur capacity of ACS1 is 64.27 mg of S/g of sorbent at an adsorption temperature of 30 °C and a gas hourly space velocity of 237.7 h−1. The micropores with sizes of around 0.5 nm in ACS-1 are the main active centers for adsorption of H2S and SO2, whereas mesopores have little desulfurization activity for deep removal of H2S and SO2. Both physical adsorption and chemical adsorption coexisted in the process of desulfurization. The majority of sulfides were removed by physical adsorption, and 11% of the sulfur compounds existing in the form of elemental sulfur (ca. 20 atom %) and sulfate (ca. 80 atom %) were derived from the chemical adsorption. The mechanism of H2S and SO2 adsorption on the ACS-1 sorbent is also discussed. H2S and SO2 are first adsorbed on ACS-1 by physical adsorption and then partially oxidized to elemental sulfur and sulfate, respectively, by the oxygen adsorbed on ACS-1. At the same time, the Claus reaction between H2S and SO2 occurs. In addition, the ACS-1 sorbent can be completely regenerated using water vapor at 450 °C with a stable breakthrough sulfur capacity during five adsorption−regeneration cycles.

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INTRODUCTION At present, the Claus tail gas still contains 1−4% sulfur species and other gaseous components such as CO, H2, CO2, water vapor, and N2 because of the thermodynamic limitations of the Claus equilibrium reaction. The sulfur species consist mainly of H2S, SO2, COS, CS2, and sulfur vapor. The volume fractions of H2S and SO2 in Claus tail gas are in the range of 0.3−1.99% and 0.15−0.89%, respectively.1,2 The Shell Claus off-gas treatment (SCOT) process is the most common tail gas treatment unit of the Claus process.3 In the SCOT process, all of the sulfur compounds and elemental sulfur are first reduced to hydrogen sulfide at 300 °C using typical hydrogenation catalysts such as cobalt−nickel or cobalt−molybdenum. The consequent reducing gas is composed of CO and H2.4,5 H2S is absorbed by a selective absorbent such as N-methyldiethanolamine (MDEA) and then stripped from the amine-rich solvent and recycled to the Claus unit.6 The tail gas of the SCOT unit contains rare H2S, which can be treated by a burner. This approach can achieve extremely high total sulfur recovery (>99.8%),7 but it has a high capital cost (about 50% of the total investment) and high CO2 emission because of its large energy requirement. In 2009, Schmidt developed a dry desulfurization process to simultaneously remove H2S and SO2 using a regenerable sorbent.8 This simplified process has the potential to reduce a four-stage process (SCOT or Cansolv) to a two-stage process. The adsorption process takes advantage of convenience in application, low energy demand, high desulfurization precision, and relatively low waste generation.9,10 Adsorbents for the removal of H2S or SO2 include activated carbons (ACs),11−14 zeolites,15−18 metal oxides,19−22 clays,23,24 and so on. The © XXXX American Chemical Society

environmentally friendly ACs are unique among the solid sorbents by virtue of their large surface areas, rich microporosity, broad range of surface functional groups, relatively high mechanical strength, and rapid adsorption velocity.25,26 In the past several years, the removal of H2S or SO2 gas individually by adsorption has been the subject of a wide variety of research. Bandosz’s group extensively studied AC-based materials as adsorbents to remove H2S. The effect of pH, surface chemistry, and pore structure on the desulfurization performance of various activated carbons were evaluated.27−30 Bagreev et al.31 studied the desulfurization performance of coconut-shell-based activated carbon used as a hydrogen sulfide adsorbent in four successive adsorption−regeneration cycles. They indicated that the most active adsorption sites are located in small pores. In addition, they also investigated the effects of nitrogen functionality and pore size on the adsorption of SO2. They believed that quaternary- and pyridinic-type nitrogen can significantly enhance the adsorption capacity and that the catalytic centers are located in the small pores, which is likely to help in achieving high dispersion of SO2.32 Schwartz and coworkers33 pointed out that a high volume of micropores and small mesopores in combination with a relatively narrow pore size distribution is crucial for the retention of SO2 in the activated carbons. In contrast to studies on removal of H 2 S and SO 2 individually, technologies for simultaneous removal of H2S Received: July 25, 2015 Revised: September 24, 2015

A

DOI: 10.1021/acs.energyfuels.5b01696 Energy Fuels XXXX, XXX, XXX−XXX

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4 h. The Brunauer−Emmett−Teller (BET) specific surface areas of ACS-1-Fresh and ACS-1-Adsorption were calculated using the BET equation. The total pore volume (Vtotal) was determined by the amount of adsorbed N2 at P/P0 = 0.99. The micropore volumes were calculated according to the t-plot method. The pore size distributions were obtained from analysis of the desorption branch using nonlocal density functional theory. The surface morphologies of ACS-1-Fresh and ACS-1-Adsorption were observed using a Philips Quanta 200 scanning electron microscope. The chemical elements on the surface of ACS-1-Fresh and ACS-1-Adsorption were analyzed using a Genisis energydispersive X-ray spectrometer attached to the Quanta 200 microscope. The surface composition of the ACS-1 sorbent was obtained using an ESCALAB 250 Xi spectrometer (Thermo Fisher Scientific) equipped with monochromatic Al Kα (1486.6 eV) X-rays. The Xray source was operated at 15 kV and 10 mA. The working pressure was lower than 2 × 10−8 Torr (1 Torr = 133.3 Pa). The core-level XPS spectra for C 1s, O 1s, and S 2p were measured at a step size of 0.1 eV. The binding energies were calibrated taking C 1s as a standard with a measured typical value of 248.6 eV. Each spectrum was fitted using a Voigt function (mixed Lorentzian−Gaussian) with Shirley backgrounds.

and SO2 by activated carbon have rarely been reported, and the underlying mechanisms for the adsorption of H2S and SO2 have not yet been fully elucidated. In this study, a coconut-shellbased activated carbon, ACS-1, was employed to simultaneously remove H2S and SO2 from simulated Claus tail gas, and the breakthrough sulfur capacity of the ACS-1 sorbent was determined. The changes in the pore structure and surface morphological properties of ACS-1 before and after adsorption were characterized by nitrogen adsorption, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). Next, the desulfurization mechanism of the ACS-1 sorbent was analyzed. Finally, the regeneration performance and stability of the ACS-1 sorbent in multiple adsorption−regeneration cycles were investigated.

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EXPERIMENTAL SECTION

Materials. ACS-1 was ground into particles with sizes of 0.45−0.30 mm (40−60 mesh), supplied by Mulinsen Activated Carbon Corporation (Nanjing, China). The fresh and exhausted ACS-1 are named ACS-1-Fresh and ACS-1-Adsorption, respectively. N2 with purity of 99.999% was purchased from Sanle Gas Corporation (Nanjing, China). H2S/N2 (40 000 ± 120 ppmv) and SO2/N2 (20 000 ± 120 ppmv) were supplied by Shangyuan Gas Plant (Nanjing, China). The simulated Claus tail gas was self-made by mixing equal volumes of H2S/N2 and SO2/N2. Adsorption and Regeneration Tests. The adsorption and regeneration tests of ACS-1 were carried out in a vertically oriented stainless steel tube reactor (16.0 mm × 45.0 mm). In each experiment, 4.0 ± 0.1 g of the ACS-1 sorbent was packed into the reactor. Before the adsorption tests, the activated carbon particles were pretreated in air at 150 ± 1 °C for at least 4 h using a conventional oven. The adsorption tests were conducted at atmospheric pressure with a temperature of 30 ± 0.5 °C and a gas hourly space velocity of 237.7 ± 10 h−1. The simulated Claus tail gas containing 2% (20 000 ppmv) H2S, 1% (10 000 ppmv) SO2, and balance N2 gas were prepared by mixing equal volumes of H2S/N2 and SO2/N2. The flow rates of H2S/ N2 (40 000 ± 120 ppmv) and SO2/N2 (20 000 ± 120 ppmv) were precisely controlled by mass flow controllers. The concentration of H2S and SO2 was measured by a Claus tail gas analyzer (model PGD3IR, supplied by Status Scientific Controls Ltd.). When the concentration of H2S and SO2 exceeded 10 mg/m3 in the outlet gas, the adsorption tests were terminated immediately. The breakthrough sulfur capacity of the ACS-1 sorbent (SC) was calculated using eq 1:

SC =

vspVbed m

⎡ M ×⎢ × ⎣ Vmol

∫0

t

⎤ (C in − Cout) dt ⎥ ⎦

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RESULTS AND DISCUSSION Dynamic Desulfurization Performance of ACS-1. The coconut-shell-based activated carbon ACS-1 was used as a sorbent to investigate the performance in the desulfurization of simulated Claus tail gas. During the adsorption tests, the temperature inside the fixed bed was maintained at 30 °C. The gas hourly space velocity of simulated Claus tail gas (20 000 ppmv H2S, 10 000 ppmv SO2, and balance N2 gas) was maintained at 237.7 h−1. The breakthrough curves of H2S and SO2 on ACS-1 at 30 °C are shown in Figure 1. The ACS-1

(1)

where SC is the breakthrough sulfur capacity of ACS-1 (mg of S/g of sorbent), vsp is the gas hourly space velocity (STP) of the simulated Claus tail gas (h−1), Vbed is the volume of sorbent in the reactor (mL), m is the weight of sorbent in the reactor (g), M is the atomic weight of sulfur (g mol−1), Vmol is the molar volume of gas (22.4 L mol−1 under standard conditions), Cin and Cout are the inlet and outlet total H2S and SO2 concentrations (%), and t is the breakthrough time (BT) (h). During regeneration, water vapor was used to regenerate ACS-1Adsorption in the same reactor. Water vapor was obtained from the steam tank. The temperature of the steam tank was maintained at 100 ± 1 °C, and the flow rate of water entering the steam tank was controlled at 60 ± 0.1 mL/h using a microcapacity metering pump. The regeneration of the used adsorbents was performed at a regeneration temperature of 450 ± 2 °C for 60 min. The adsorption−regeneration cycle was repeated five times, and the breakthrough sulfur capacity of each cycle was determined. Characterization of the Adsorbents. Nitrogen adsorption− desorption isotherms were measured at 77 K with a BEL-SORP max volumetric analyzer (BEL Japan, Inc.). Before the measurements, ACS1-Fresh and ACS-1-Adsorption were degassed in vacuum at 150 °C for

Figure 1. Breakthrough curves of H2S and SO2 on ACS-1 at 30 °C.

sorbent presents very high precision in simultaneously removing H2S and SO2, as no hydrogen sulfide or sulfur dioxide were detected in the outlet gas before the breakthrough point was reached. The breakthrough time of H2S and SO2 on the ACS-1 sorbent is 186 min, and the breakthrough sulfur capacity calculated with eq 1 is 64.27 mg of S/g of sorbent. According to the literature,34,35 the breakthrough sulfur capacities of coconut-shell-based activated carbons for adsorption of H2S and SO2 gas individually are 53 and 21.21 mg/g, respectively. Both of them are lower than the breakthrough sulfur capacity of the ACS-1 sorbent. The high B

DOI: 10.1021/acs.energyfuels.5b01696 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels breakthrough sulfur capacity may be attributed to the larger surface area and favorable surface morphological properties of the ACS-1 sorbent. In addition, SO2 was subsequently detected after the concentration of H2S in the outlet gas reached over 10 mg/m3. This implies that the adsorption of SO2 on the ACS-1 sorbent is also weakened when the ACS-1 sorbent gradually loses the ability to remove H2S. Characterization of the Pore Structures of ACS-1Fresh and ACS-1-Adsorption. To investigate the effect of the pore volume and pore structure on the breakthrough sulfur capacity of the ACS-1 sorbent, ACS-1-Fresh and ACS-1Adsorption were characterized by nitrogen adsorption to identify the changes in pore volume and pore structure of the ACS-1 sorbent before and after adsorption. Nitrogen adsorption−desorption isotherms of ACS-1-Fresh and ACS-1Adsorption at 77 K are shown in Figure 2. The nitrogen

Table 1. Pore Structure Parameters of ACS-1-Fresh and ACS-1-Adsorption Calculated from the N2 Adsorption− Desorption Isotherms sample

SBET/m2 g−1

Vtotal/cm3 g−1

Vmic/cm3 g−1

Vmes/cm3 g−1

ACS-1-Fresh ACS-1-Adsorption

1854 1661

0.97 0.83

0.90 0.75

0.07 0.08

Figure 2. Nitrogen adsorption−desorption isotherms of ACS-1-Fresh and ACS-1-Adsorption at 77 K.

adsorption isotherms of ACS-1 exhibit the typical type-I feature according to the Brunauer classification, which indicates that the ACS-1 sorbent mainly contains micropores.36 The hysteresis loops of ACS-1-Fresh and ACS-1-Adsorption are observed at high relative pressures, indicating the presence of mesopores. These results are fully supported by the pore size distributions of the ACS-1 sorbents (see Figure 3). In addition, the N2 adsorption capacity of ACS-1-Adsorption was lower than that of ACS-1-Fresh. It is well-known that the N2 adsorption capacity is proportional to the pore volume. Therefore, ACS-1-Adsorption has a smaller surface area and pore volume than ACS-1-Fresh. These decreases might be attributed to the fact that H2S and SO2 molecules were adsorbed in the pores of the ACS-1 sorbent during the process of desulfurization. The pore structure parameters of ACS-1-Fresh and ACS-1Adsorption were calculated from the N 2 adsorption− desorption isotherms. The Brunauer−Emmett−Teller (BET) method was utilized to calculate the specific surface areas of ACS-1-Fresh and ACS-1-Adsorption. The total pore volume (Vtotal) was determined from the nitrogen uptake at a relative pressure of 0.99. The micropore volume was calculated according to the t-plot method. The detailed pore structure parameters, namely, the BET surface area (SBET), Vtotal, the micropore volume (Vmic), and the mesopore volume (Vmes), are summarized in Table 1. Figure 3 shows the pore size

Figure 3. Pore size distribution curves of (a) ACS-1-Fresh and (b) ACS-1-Adsorption.

distribution curves for ACS-1-Fresh and ACS-1-Adsorption derived from the desorption branches of the isotherms using nonlocal density functional theory (NL-DFT). As observed in Table 1, the pore structure of the ACS-1 sorbent was significantly changed after the adsorption of H2S and SO2. SBET decreased from 1854 m2 g−1 for ACS-1-Fresh to 1661 m2 g−1 for ACS-1-Adsorption, corresponding to a loss of about 10.4% of the total surface area. Meanwhile, Vmic decreased from 0.90 to 0.75 cm3 g−1 after the desulfurization, sustaining only 83.3% capability of the fresh form. In contrast, Vmes was nearly the same for ACS-1-Fresh and ACS-1Adsorption. These facts indicate that the micropores of the ACS-1 sorbent are the main active centers for the adsorption of H2S and SO2, whereas the mesopores have little desulfurization activity for deep removal of H2S and SO2 (H2S and SO2 contained less than 10 mg/m3). These conclusions are fully supported by the pore size distributions calculated using NLC

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Figure 4. SEM-EDX analyses of (a, b) ACS-1-Fresh and (c, d) ACS-1-Adsorption.

DFT. As can be seen from Figure 3, the only visible change is in the pores with sizes of around 0.5 nm. This result indicates that the deactivation of the ACS-1 sorbent is mainly attributed to the accumulation of H2S and SO2 inside the micropores with sizes of around 0.5 nm in the ACS-1 sorbent. This is consistent with the experimental results of Wang et al,37 who found that the breakthrough sulfur capacity (H2S) is proportional to the volume of micropores and independent of the volume of mesopores. In addition, Raymundo-Pinero et al.38 and Wang et al.39 pointed out that there is an optimum size of pores where the adsorption of SO2 is enhanced, and this size should be less than 0.7 nm. An increase in the micropore width leads to a decrease in the differential adsorption energy in SO2 micropore filling as a result of a decrease in the enhanced micropore field. SEM-EDX Analyses of ACS-1-Fresh and ACS-1Adsorption. It was found that the adsorption of H2S and SO2 not only changed the specific surface area and pore size distribution but also altered the surface morphology and elemental composition of the ACS-1 sorbent. SEM was used to observe the changes in surface morphology of ACS-1 before and after adsorption, and the EDX detector attached to the SEM instrument was employed to estimate the amounts of specific elements on the surface of ACS-1-Fresh and ACS-1Adsorption. The SEM images and the maps of elements for (a, b) ACS-1-Fresh and (c, d) ACS-1-Adsorption are presented in Figure 4. The contents of elements (C, O, S, Si, P, and K) on the surface of ACS-1 in weight percent are collected in Table 2. The EDX analysis of ACS-1-Fresh (Figure 4b) shows that the major elements on the surface of the fresh activated carbon are C and O, accompanied by small amounts of Si, P, and K. The contents of C and O are 92.26 and 6.05 wt %, respectively. As observed in Table 2, the content of sulfur in the ACS-1 sorbent

Table 2. Contents of Elements (C, O, S, Si, P, and K) in ACS-1-Fresh and ACS-1-Adsorption element

ACS-1-Fresh (wt %)

ACS-1-Adsorption (wt %)

C O S Si P K

92.26 6.05 0 0.32 0.47 0.90

93.19 5.10 0.70 0.13 0.12 0.76

increased from 0 to 0.70 wt % after the adsorption of H2S and SO2. Meanwhile, the content of oxygen decreased from 6.05 to 5.10 wt %, indicating that a chemical reaction occurred between H2S and oxygen in the process of adsorption that not only consumed some of the oxygen on the surface of the activated carbon but also formed elemental sulfur in the ACS-1 sorbent. These results confirm that chemical adsorption occurred in the process of desulfurization in addition to physical adsorption. This is consistent with the changes in the specific surface area and pore volume of ACS-1 before and after adsorption. XPS Analyses of ACS-1-Fresh and ACS-1-Adsorption. In order to further reveal the changes in surface morphological properties of ACS-1 before and after adsorption and ascertain the sulfur composition derived from the chemical adsorption in ACS-1-Adsorption, XPS was employed to investigate the chemical valences of C, O, and S. Figure 5 shows the XPS spectra of C 1s core levels of (a) ACS-1-Fresh and (b) ACS-1Adsorption recorded in high-resolution mode. The calculated percentages of carbon species are listed in Table 3. According to the literature,40−42 the carbon spectra were well-fitted with three peaks (Table 3), which were assigned to carbon atoms bonded to carbon atoms (binding energy (BE) = 284.6 eV, C1.1 D

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Figure 6. High-resolution fitted O 1s XPS spectra of (a) ACS-1-Fresh and (b) ACS-1-Adsorption.

Figure 5. High-resolution fitted C 1s XPS spectra of (a) ACS-1-Fresh and (b) ACS-1-Adsorption.

Table 4. XPS Spectra of O 1s Electrons of ACS-1-Fresh and ACS-1-Adsorption

Table 3. XPS Spectra of C 1s Electrons in ACS-1-Fresh and ACS-1-Adsorption binding energy (eV)

binding energy (eV)

relative peak area (%)

sample

C1.1

C1.2

C1.3

C1.1

C1.2

C1.3

ACS-1-Fresh ACS-1-Adsorption

284.6 284.6

286.2 286.2

287.6 287.6

87.4 85.0

7.7 10.6

4.9 4.4

relative peak area (%)

sample

O1.1

O1.2

O1.3

O

ACS-1-Fresh ACS-1-Adsorption

532.0 532.0

533.2 533.2

537.2 537.2

50.2 53.8

1.1

O

1.2

48.1 46.0

O

1.3

1.7 0.2

to be closely related to the adsorption of SO2 and the formation of sulfate on the surface of the ACS-1 sorbent. On the other hand, the intensity of the O1.3 peak at 537.2 eV decreased from 1.7% to 0.2% after adsorption, which is attributed to the reaction between H2S/SO2 and oxygen adsorbed on the ACS-1 surface during the process of desulfurization. The high-resolution S 2p XPS spectrum of ACS-1Adsorption is shown in Figure 7. The S 2p spectrum of ACS-1-Adsorption was fitted with two peaks, which were assigned to elemental sulfur (BE = 164.1 eV, S2.1 peak) and sulfate (BE = 169.1 eV, S2.2 peak).45,46 For the fresh activated carbon, no sulfur species were detected. After desulfurization, the weight percent of S in the ACS-1 sorbent increased remarkably, and the sulfur species are mainly sulfate (ca. 80 atom %) with a small quantity of elemental sulfur (ca. 20 atom %). The results testify to the occurrence of a chemical reaction between the H2S and SO2 adsorbed on the surface of ACS-1. These observations correspond to the BET and EDX results, which showed a decrease in the amount of micropores with sizes of around 0.5 nm and an increase in the sulfur content in the ACS-1 sorbent after desulfurization. Mechanisms of H2S and SO2 Adsorption by ACS-1. The structural characterization of the ACS-1 sorbent proves that sulfides are mostly adsorbed in the micropores with sizes of

peak) and carbon atoms bonded to oxygen and/or sulfur atoms by one bond (BE = 286.2 eV, C1.2 peak) or two bonds (BE = 287.6 eV, C1.3 peak). The data shown in Table 3 suggest that the adsorption of H2S and SO2 on the ACS-1 sorbent decreased the intensities of the C1.1 and C1.3 peaks and increased the relative area of the peak assigned to C−O and/or C−S groups, indicating that some C−C bonds were broken and some C−O and C−S groups were formed on the surface of the ACS-1 sorbent during the adsorption of H2S and SO2. The O 1s XPS spectra of (a) ACS-1-Fresh and (b) ACS-1Adsorption were fitted with three peaks (Figure 6), and the binding energies are reported in Table 4. On the basis of published data,40−42 the O1.1 peak with BE = 532.0 eV was assigned to CO- and SO-type oxygen (CO, COOR, SO2, and SO42−). The second peak, O1.2 (BE = 533.2 eV), corresponds to C−O- and S−O-type oxygen (C−OH, COOR, and S−OH). According to the literature, the binding energy of adsorbed water O 1s electrons was determined to be in the range of 535.0−536.0 eV, while for adsorbed oxygen a binding energy of 537.2−537.6 eV was observed.43,44 Therefore, the O1.3 peak should be assigned to oxygen adsorbed on the surface of the ACS-1 sorbent. After desulfurization, the relative area of the O1.1 peak increased from 50.2% to 53.8%, which is thought E

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sulfur, water, and sulfate, respectively, as the end products of the chemical adsorption. Regeneration and Stability of the ACS-1 Adsorbent. Regeneration of any sorbent is very necessary for industrial applications so that sorbents can be reused in successive adsorption−regeneration cycles. To investigate the regeneration and stability of the ACS-1 sorbent, multiple adsorption− regeneration cycles were performed in this study. The ACS-1 sorbent was regenerated using water vapor at 450 °C for 60 min. The breakthrough sulfur capacities of the ACS-1 sorbent over five adsorption−regeneration cycles are shown in Figure 8.

Figure 7. High-resolution fitted S 2p XPS spectrum of ACS-1Adsorption.

around 0.5 nm in ACS-1, whereas mesopores have little desulfurization activity for deep removal of H2S and SO2. According to the SEM-EDX and XPS analyses, both physical adsorption and chemical adsorption occur in the process of desulfurization. The majority of sulfides were sucked into the pores of ACS-1 by means of physical adsorption, and 11% of the sulfur compounds in the form of elemental sulfur (ca. 20 atom %) and sulfate (ca. 80 atom %) were fixed in the ACS-1 sorbent by chemical adsorption. The formation of elemental sulfur and sulfate is attributed to the oxidation of adsorbed H2S and SO2 and the Claus reaction between H2S and SO2. The following equations indicate the desulfurization mechanism of the ACS-1 sorbent. Physical Adsorption. H2S (eq 2) and SO2 (eq 3) molecules are first adsorbed in the micropores with sizes of around 0.5 nm in the ACS-1 sorbent: H 2S(g) → H 2S(ads)

(2)

SO2 (g) → SO2 (ads)

(3)

Figure 8. Breakthrough sulfur capacities of the ACS-1 sorbent over five adsorption−regeneration cycles (adsorption: 30 °C, 20 000 ppmv H2S, 10 000 ppmv SO2, balance N2, GHSV = 237.7 h−1; regeneration: 450 °C, water vapor, 60 min).

The breakthrough sulfur capacity of fresh ACS-1 was 64.27 mg of S/g of sorbent. After the first regeneration, the breakthrough sulfur capacity was 66.16 mg of S/g of sorbent, and no deterioration in desulfurization performance was observed. This means that the ACS-1 sorbent can be fully regenerated by water vapor. Elemental sulfur (boiling point = 444 °C) and sulfate were eluted from the ACS-1 sorbent by water vapor at 450 °C, allowing the pore structure and surface properties of the ACS-1 sorbent to be completely restored. It can be obviously observed that a small amount of sulfur was evolved during the regeneration, which evidenced that elemental sulfur was actually formed in the process of desulfurization. This is consistent with the XPS results. It was also confirmed that chemical adsorption occurred during the adsorption of H2S and SO2 in addition to physical adsorption. In addition, the breakthrough sulfur capacities, maintained at 63.88−66.16 mg of S/g of sorbent, were showed quite a stable trend of variation in five adsorption−regeneration cycles. The results suggest that the ACS-1 sorbent has excellent regenerability and stable desulfurization performance. Therefore, it can be concluded that the ACS-1 sorbent is effective in the simultaneous removal of H2S and SO2 in Claus tail gas and suitable for successive adsorption−regeneration cycles.

where (g) and (ads) denote the gaseous and the adsorbed states, respectively, of H2S and SO2. Chemical Adsorption. The adsorbed H2S dissociates into HS− and H+ (eq 4). HS− is oxidized to elemental sulfur and OH− (eq 5) by adsorbed oxygen on ACS-1, and then H+ and OH− combine to form water (eq 6). In addition, SO2 is also oxidized to SO42− and H+ by adsorbed oxygen in the presence of water (eq 7). At the same time, the reaction between HS− and SO2 occurs (eq 8). H 2S(ads) → HS− + H+

(4)

HS− + O*(ads) → S(ads) + OH−

(5)

H+ + OH− → H 2O(ads)

(6)

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CONCLUSIONS The coconut-shell-based activated carbon ACS-1 exhibits good desulfurization performance for simulated Claus tail gas (20 000 ppmv H2S, 10 000 ppmv SO2, balance N2). H2S and SO2 in simulated Claus tail gas can be completely removed by ACS-1, and the breakthrough sulfur capacity of the ACS-1 sorbent is 64.27 mg of S/g of sorbent at a temperature of 30 °C

SO2 (ads) + O*(ads) + H 2O(ads) → SO4 2 −(ads) + 2H+ (7) −

2HS + SO2 (ads) → 3S(ads) + 2OH

−

(8)

where O*(ads) denotes oxygen adsorbed on the ACS-1 sorbent and S(ads), H2O(ads), and SO42−(ads) represent elemental F

DOI: 10.1021/acs.energyfuels.5b01696 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels and gas hourly space velocity of 237.7 h−1. The BET results indicate that the adsorption of H2S and SO2 caused decreases both in surface area and pore volume of the ACS-1 sorbent. The micropores with sizes of around 0.5 nm in ACS-1 are considered as the main active centers for the adsorption of H2S and SO2, whereas mesopores have little desulfurization activity for the deep desulfurization (H2S and SO2 contained less than 10 mg/m3). SEM-EDX analysis shows that both physical adsorption and chemical adsorption occurred in the process of removing H2S and SO2. Most sulfides were sucked into the pores of ACS-1 by physical adsorption, and 11% of the sulfur compounds were fixed in ACS-1 by chemical adsorption. The XPS results reveal that the chemisorbed sulfur compounds are composed of elemental sulfur (ca. 20 atom %) and sulfate (ca. 80 atom %), which were derived from the oxidation of adsorbed H2S and SO2 and the Claus reaction between H2S and SO2. In addition, a desulfurization mechanism was proposed as follows: H2S and SO2 are first adsorbed in micropores with sized of around 0.5 nm in ACS-1. Then a small amount of adsorbed H2S and SO2 are oxidized to elemental sulfur and sulfate by the oxygen adsorbed on ACS-1, and the Claus reaction between H2S and SO2 occurs simultaneously. Finally, ACS-1-Adsorption can be fully regenerated using water vapor at 450 °C. After five successive adsorption−regeneration cycles, the ACS-1 sorbent exhibits high desulfurization efficiency, excellent regenerability, and good stability and is suitable for the simultaneous removal of H2S and SO2 in Claus tail gas.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86 025 83587188-0. *E-mail: [email protected]. Tel: +86 025 83587188-0. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China under Contract 51476074, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Project of Sinopec Yangzi Petrochemical Company Ltd. named Research on Desulfurization Technology of One Hundred Thousand Tons Tail Gas in Sulfur Recovery Unit (30600000-12-Z-ZC060725400006-0-*).

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