Fe-based sorbent for hot coal gas under microwave irradiation

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Environmental and Carbon Dioxide Issues

Fe-based sorbent for hot coal gas under microwave irradiation: desulfurization performance and microwave effects Mengmeng Wu, Enhui Guo, Qiaochun Li, Hui-Ling Fan, and Jie Mi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b02117 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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Fe-based sorbent for hot coal gas under microwave irradiation: desulfurization performance and microwave effects Mengmeng Wu*, Enhui Guo, Qiaochun Li, Huiling Fan, Jie Mi* Key Laboratory of Coal Science and Technology of Shanxi Province and Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China ABSTRACT Microwave-assisted

chemical

process

is

environmentally

friendly

and

energy-saving. In this study, microwave irradiation was applied to enhance the hot coal gas desulfurization process with modified semi-coke-supported Fe2O3 as the sorbent. The results indicate that the sorbent with 20% Fe2O3 shows the greatest breakthrough sulfur capacity (9.0%) at 500 ℃ in the simulated coal gas. Besides Fe1-xS, sulfur was also produced during the desulfurization process. The deactivation model could well simulate the adsorption behavior of the sorbents for H2S. The activation energies of the sulfidation reaction by microwave and conventional techniques are 26.9 and 27.8 kJ mol−1, respectively. The sorbents adsorbing H2S under microwave irradiation show much larger initial rate constants (308 to 2071 m3 min-1 kg-1) for the sulfidation reaction. Compared to conventional technique, microwave desulfurization generally results in much better performance of H2S removal, and leads to less negative effects on the pore structure of the sorbent. The improvement of desulfurization properties may be attributed to the roles of microwave irradiation (enhanced mass transfer kinetics and intensified ion diffusions). Additionally, the adverse influence of H2, CO, and CO2 on the sulfur capacity of the sorbents under microwave irradiation is not significant when the adsorption time varies from 0 to 120 min. Keywords: H2S; Desulfurization; Kinetics; Deactivation model; Sorbent. *

Corresponding authors.

E-mail address: [email protected] (M. Wu), [email protected] (J. Mi).

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1. INTRODUCTION Coal-to-chemicals industry could realize higher value-added utilization of coal and coal gasification is the key technology for the coal conversion processes. During coal gasification process, the sulfur in coal can be transformed into mainly H2S in raw coal gas. To avoid the negative effects such as poisoning of catalyst used for syngas conversion, corrosion of gas pipeline or reactor, and harm to atmosphere, desulfurization should be done prior to the subsequent process 1. The desulfurization technology can be classified into two types. One is by wet removing method, while the

other

involves

dry

desulfurization

via

adsorption

(or

combined

adsorption/catalysis) of H2S using sorbents 2. The dry technology is more suitable for deep desulfurization

2,3.

In the case of dry removing method, hot (mid- and

high-temperature) coal gas desulfurization over the metal oxide with or without support is widely studied. The metal oxides3-18 include single (ZnO4,7,9,13,18, CuO1,3,14,16,17, Fe2O312, MnO25,8,10,, etc.) and mixed oxides (ZnFe2O43, LaFeO311, etc.), while the carbon-, Si-, and Al-based porous material are used for loading the active components

4,17,18.

Among different metal oxides, ZnO and CuO can reduce the

concentration of H2S to ppmv (or lower) levels

3,18.

Unfortunately, these metal oxide

sorbents are readily reduced when used in the hot coal gas with strong reductivity 3. Furthermore, the complete conversion of ZnS to ZnO for the sorbent regeneration requires higher reaction temperature (> 650 ℃) 3. As for MnO2 and Mo2O3, lower desulfurization depth and much higher regeneration (> 700 ℃) limits its application 3,7,12.

Rare-earth metal oxide can remove H2S at 800 ℃, but the high cost is the main

disadvantage

3,13.

Fe2O3 is an ideal sorbent due to its low cost, high sulfur capacity,

and good desulfurization efficiency3,16,17. Consequently, iron oxide is suitable for application in the industrial desulfurization for hot coal gas. Sulfidation reaction over the metal oxide (denoted as MexOy) is a typical gas-solid reaction 3. It can be expressed by the following reaction (Eq. 1):

Mex O y  yH 2 S  yH 2 O  Mex S y


(1)

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The kinetic studies 1-3,10,24 indicate that the desulfurization process is controlled by the chemical reaction in the initial stage and the diffusion in the later stage. It is notable that the aforementioned diffusion is different from the gas diffusion as deduced from large diffusion activation energy1,3,15. Increasing temperature is beneficial for the reduction of diffusion resistance. However, it also leads to larger possibility of the sintering of the metal oxide grains and gives rise to an adverse effect on the exothermic sulfidation reaction (Eq. 1) from the viewpoint of the reaction thermodynamics. The composition of the coal gas is complex. The adsorption of H2S on MexOy is influenced by the other gases (CO, H2, CO2, etc.) 3. A large fraction of the reducing gas (H2 and CO) is generally presented in the raw coal gas

3-5.

Therefore, the

reduction of MexOy to the oxides with low valence in the hot coal gas is unavoidable3, giving a negative effect on the structure ability and sulfidation activity of the MexOy. In addition, E. Sasaoka 7 and B.S. Liu 11 observed that CO2 in the coal gas could have an inhibitory effect on the reaction between MexOy and H2S in the initial stage of the reaction. Microwave heating is environmentally friendly and can enhance energy conservation due to the uniform and rapid heating induced by volumetric heating 19,20. However, conventional heating is slower and relies on the heat transfer through surface contact. Therefore, microwave heating is very attractive for applications in chemical processes as non-conventional energy source

19-22.

It was reported that

microwave could results in the decline in the activation energy or Arrhenius pre-exponential factor of specific chemical reactions

20,21.

Additionally, mass transfer

in some reactions including solid reaction can be enhanced under microwave irradiation 22. It has also been found that microwave heating can alter the absorption selectivity of some gases during certain adsorption processes

19-21.

In view of the

microwave effects mentioned above, microwave desulfurization may exhibit an energy-efficiency advantage and have a positive effect on the sulfidation reaction over


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sorbents compared to conventional desulfurization process. The capital investment of the desulfurization technologies greatly depend on the cost of the desulfurization system. The microwave desulfurization system (MDS) consists of five basic components: power supply, reactor for providing space for the reaction between the sorbent and gaseous reactant, thermal insulation unit, heating unit (magnetron for generating microwaves, and waveguide for transporting microwaves from the generator to the reactor) and temperature controlling unit 20. The units of conventional desulfurization system (CDS) are the same as the MDS but the heat is generated by the heating elements such as heating wire or silicon carbide rod in the heat unit. Generally, the price of the heating unit involved in the MDS is higher than that in CDS. Therefore, the capital investment of MDS may be higher than that of CDS. However, the frequency of replacing the sorbent bed during the MDP should be reduced on the basis of favorable microwave effects on the sulfidation reaction. Thus, the operating cost may be lower due to longer operating time per desulfurization operation and the higher energy efficiency of the MDP. Herein, hot coal gas desulfurization under microwave irradiation was proposed to enhance the desulfurization performance of the sorbents. Fe2O3 supported on modified semi-coke with good microwave absorption ability was used as the sorbent. The microwave effects caused by microwave desulfurization in the simulated hot coal gas were clarified. The effects of Fe2O3 content and desulfurization temperature on the performance of sorbents were investigated. The kinetics analysis was performed. The disparity of desulfurization by conventional and microwave technologies was also compared. 2. EXPERIMENTAL 2.1. Preparation of Fe2O3/MS Sorbent The raw semi-coke (Shanxi Xinhua Chemical Co., Ltd, China) with 4.1 wt % ash content was modified by acid treatment 23 and then used as supporter for sorbents. The aqueous solution of Fe(NO3)3·9H2O was used as the immersion solution. The sample


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after pore volume impregnation was dried successively at 110 ℃ for 6 h. Then calcination at 500 ℃ for 1 h was done for the treated sample. Consequently, the modified semi-coke supported Fe2O3 (Fe2O3/MS) with three loading contents (10, 15, 20%) was obtained. 2.2. Characterization of Sorbents X-ray diffraction (XRD) equipment (D/MAX-2500, Japan) using Cu Kα radiation was used for obtain XRD spectra. N2 adsorption (JW-BK122W, Beijing JWGB Sci. & Tech. Co., Ltd, China) conducted at −196 ℃ was used for analysis of pore structure. Brunauer-Emmett-Teller (BET) theory was selected to calculate the specific surface area of sorbents. The morphology of samples was analyzed by scanning electron microscopy (SEM, MAIA3 TESCAN, Czech Republic). The XPS spectra was obtained by HI5000C spectrometer (USA) using an Al Kα source operating at 250W and 93.9 eV passed energy. The H2-temperature programmed reduction (H2-TPR) was analyzed with a Micromeritics Auto Chem II 2920 apparatus. 100 mg sorbent was reduced in 10 vol% H2/Ar (30 mL min−1), and the reduction was performed in the range of 50–1000 ℃. 2.3. Adsorption Tests The adsorption tests were carried out in a fixed-bed microwave reactor. The Fe2O3/MS sorbent was loaded into quartz tube (14 mm in inner diameter) located in the middle of microwave heater (fabricated by Tangshan nano source microwave thermal instrument manufacturing co., rated power of furnace is 1.0 kW). The adsorption tests (1.5 or 2.0 g sorbent, 60-80 mesh) were carried out by flowing the simulated coal gas (0.49 vol% H2S, 10 vol % H2, 18 vol % CO, 5 vol % CO2, and N2 as balance gas) at a flow rate of 150 mL min-1 over the sorbent at 400–700 oC. The reactor was operated at atmospheric pressure. The temperature was measured by a special thermocouple (2 mm in diameter) placed inside the tube and controlled based on Proportional-Integral-Differential arithmetic. The pipeline was preheated to about 300 oC. Nitrogen with the same flow rate was flowed over the sorbent before the


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commencement of experiment. The H2S concentrations at the inlet and outlet of reactor were analyzed using a gas chromatograph (GC) with a flame photometric detector. For conventional way, same mass of sorbent and size of quartz tubes were used but without microwave irradiation for comparison. When investigating the effect of atmosphere, the concentrations of gases in other atmosphere are all same except for N2 compared to the simulated coal gas. For conventional way, same mass of sorbent and size of quartz tubes were used but without microwave irradiation for comparison. The breakthrough time (BT, min) is defined as the adsorption time when the exit concentration of H2S reaches 100 ppm. Breakthrough sulfur capacity (BS, %) was calculated according to the following equation (Eq. 2). The utilization rate of iron oxide (UR, %) is the ratio of experimental BS contributed by iron oxide only to theoretic sulfur capacity contributed by iron oxide, while the former is the BS of sorbents minus BS of MS with the same desulfurization temperature. BT

BS 

F  10-3  M   (C0  C )dt  10 6  100 gsorbent 0

Vm  msorbent

(2)

where F, M, Vm, and msorbent represent the flow rate of reaction gas (mL min-1), molar mass of sulfur, the molar volume (24.5 L mol-1) of H2S at 25 ℃ and 1 atm, and the weight of sorbents (g), respectively; and C0 and C correspond to the H2S concentration (ppmv) at the inlet and outlet, respectively. Each experiment was repeated for at least three times, and the average values were recorded. 2.4. Adsorption Modeling In previous study 16, Boltzmann function was used to fit the desulfurization curves of the sorbents for H2S removal in the mixed gas (N2+H2S). However, the adsorption curves do not belong to strictly Sigmoidal-type curves. Therefore, there may be large variation between the actual and simulated adsorption behavior. Deactivation model (DM) considering the decline in activity of the solid reactant due to textural changes is widely used to predict the breakthrough curves of gas-solid reactions


24-26.

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Consequently, DM is used for kinetics analysis in this study. The assumptions before application of DM are: 1) The reaction process is isothermal; 2) The external mass transfer resistance is neglected; 3) The reactor is approximate to a batch-solid reactor with plug constant flow of reaction gas; 4) A pseudo-steady state is valid; 4) The deactivation rate versus reaction activity obey first-order reaction and the concentration of H2S has no effect on the deactivation rate of solid reactant. Based on the related assumptions25, the equations (Eqs. 3 and 4) could be derived.



da  kd a dt

(3)

where a, t and kd represent activity of the sorbent, sorption time (min), and deactivation rate constant (min-1, a equals 1 at t=0), respectively. With the pseudo-steady state assumption, the isothermal H2S conservation equation (Eq. 4) is obtained.

F

dC  k0Ca  0 dW

(4)

where F, C, W, and k0 represent the flow rate of reaction gas (m3 min-1), the H2S concentrations (ppmv) at the outlet, the weight of sorbent bed (kg), and initial (a equals 1) adsorption (sulfidation reaction) rate constant (m3 min-1 kg-1), respectively. 3. RESULTS AND DISCUSSION 3.1. Desulfurization Performance of Sorbents with Different Fe2O3 Contents As listed in Table 1, higher (10–20%) iron oxide loading leads to the decrease in specific surface area (by 13.8–18.5%), pore volume (by 24.3–35.5%), and average pore size (by 17.9–18.7%) of the modified semi-coke. This is mainly attributed to the plugging of some pores in the supporter induced by Fe2O3. Generally, the replacement of S2− (radius: 0.184 nm) by O2− (radius: 0.140 nm) during desulfurization can lead to negative effects on the pore structure of sorbents 7,9. Interestingly, the used Fe2O3/MS has larger specific surface area and pore volume than the fresh sorbents. It is attributed to the fact that reactions (Eqs. 5 and 6) could occur at high temperatures 27-29,

which facilitates expansion of the pore structure of sorbents during

desulfurization process.


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C  H 2 O  CO  H 2

(5)

C  CO2  2CO

(6)

As can calculated based on Eq. 2 and the data shown in Figure 1, with increasing Fe2O3 content from 10 to 15%, the values of BT and BS increase by 29.7 and 33.5%, respectively. Further increase of the loading content to 20% results in much longer BT and greater BS values. The XRD patterns of the fresh and used sorbents are presented in Figure S1. Very weak peaks assigned to Fe2O3 are observed in the diffraction patterns of fresh sorbents. The peak broadening can be due to nanosized particles (shown later). There are strong peaks (2θ = 29.9°, 33.8°, 43.7°, and 53.1°) belonging to Fe1-xS (0 < x < 1; [PDF# 29-0726]) after desulfurization. Furthermore, the Fe2O3/MS with 20% loading content shows the strongest intensity of XRD peaks after adsorption of H2S. The XRD results agree with the desulfurization properties of sorbents. As hot coal gas is highly reductive, the reduction of metal oxide in sorbents is inevitable during the desulfurization process. Figure 1b presents the H2-TPR curves of two sorbents with higher loading contents. The peaks at 332 (or 344 ℃) correspond to the reduction of Fe2O3 to Fe3O4, while peaks located at above 500 oC are attributed to further reduction of Fe3O4 to FeO and/or Fe

30,31.

It is notable that the peak

temperature of the second type of TPR peaks is higher for sorbents with 20% Fe2O3. In addition, the integration area (corresponding to the amount of H2 consumed by reduction reaction) calculated from TPR curves of Fe2O3/MS with 20% Fe2O3 is lower than that of the other sorbent. The results indicate that the sorbent with 20% Fe2O3 is less reactive when exposed to the reducing gas containing H2, which is favorable for its application in the reducing coal gas atmosphere. Meanwhile, this sorbent has much higher BS. Then the sorbent containing 20% Fe2O3 was used in the following experiments to investigate the effect of temperature, desulfurization way and atmosphere. 3.2. Desulfurization Performance of Sorbents at Different Temperatures


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The sorbents for removing H2S under microwave irradiation at 400, 500, 600, and 700 ℃ are denoted as M400, M500, M600 and M700, respectively. As shown in Figures. 2a and 2b, the BT values of these Fe2O3/MS sorbents range from 103 to 176 min, significantly longer than that of single modified semi-coke (BT 100%). Another

reason is that the reaction of H2S with CO3,7 also consumes H2S. However, a further increase of temperature from 500 to 600 ℃ results in the decrease of BT, BS, and UR by 22.2, 22.3, and 22.3%, respectively. It is mainly attributed to the fact that increasing temperature is adverse to the exothermic sulfidation reaction according to the Le Chatelier's principle although kinetically beneficial. M700 performs worse than M600, which may be due to unfavorable reaction (desulfurization) thermodynamics and over-reduction of iron oxide at 700 ℃. Another plausible explanation is the sintering (shown later) of Fe1-xS grain resulting from local overheating at 700 ℃, which is unfavorable for the exposure of unreacted Fe2O3. Table 2 lists the pore structure properties of the fresh and used sorbents. There is much less decrease (by 1.5 to 5%) in the specific surface area but an increase (by 6.7 to 18.0%) of pore volume of sorbents after removing H2S at 500, 600 and 700 ℃. This indicates that replacement of O2− by S2− does not lead to significant blocking of pore structure. It may be attributed to the reactions of modified semi-coke with CO2 and/or H2O (Eqs. 5 and 6). In addition, the XRD patterns (Figure 4a) of the used sorbents show that the absence of diffraction peaks belonging to Fe2O3 but appearance of strong peaks assigned to Fe1-xS. It agrees with the desulfurization performance of sorbents. 3.3. Comparison of Microwave and Conventional Desulfurization C400, C500, C600, and C700 shown in Figure 2c represent the sorbents removing H2S by the conventional technique at 400, 500, 600, and 700 ℃, respectively. In


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comparison with conventional desulfurization, microwave irradiation result in the significant increase in longer BT (by 33.2–37.1%), BS (by 33.2–37.3%) and UR (by 33.2–37.3%), except that there is a minor increase (by 3.5, 3.3 and 3.3% in BT, BS, and UR, respectively) in the case of removing H2S at 700 ℃. It indicates that microwave could generally results in better desulfurization performance. In order to understand microwave effects involved in desulfurization, the reaction kinetics using deactivation model was investigated. Decrease in activity of the solid reactant due to textural changes is taken into consideration in this model

24,25.

The

reactions (Eqs. 5 and 6) between modified semi-coke and CO2 (or H2O) cannot be ignored at 700 ℃ 27-29. It can be inferred from much higher (12.0–18.0%) pore volume of C700 and M700 than fresh sorbents. Therefore, only the desulfurization data at 400−600 ℃ were chosen for reaction dynamics modeling. The following equation for adsorption curves can be derived by integration of Eqs. 3 and 4.

ln[ ln(C / C 0 )]  ln(

k 0W )  kd t F

(7)

Figure 5 presents linear plots of ln[-ln(C/C0)] versus time. The fitting parameters are listed in Table 3. High correlation coefficient (R2, ≥0.967) suggests deactivation mode could be used to simulate the desulfurization behavior of sorbents in the simulated hot coal gas. The initial reaction rate constant (k0) of Fe2O3/MS ranges from 308 to 2071 m3 min-1 kg-1. As expected, the initial reaction rate constant (k0) increases with the elevated desulfurization temperature. Furthermore, the k0 values of sorbents in the case of microwave desulfurization are at least two times larger than that of the conventional way. Curves of lnk0 versus 1/RT are given in Figure 6. Activation energy (Ea) of the sulfidation reaction can be obtained from the slope of plots. Consequently, Ea of the sulfidation reaction by microwave and conventional ways are 26.9 and 27.8 kJ mol−1, respectively. It indicates that microwave irradiation does not result in significant decrease of the energy barrier for the desulfurization reaction. The intensified intraparticle mass transfer has been found in some reactions

21,22,32,33.

The

adsorption of H2S and desorption of H2O vapor can occur during the desulfurization


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process. Considering the microwave effects

19-22

mentioned in Section 1, the

acceleration of initial reaction rate under microwave irradiation may be due to the quicker diffusion/desorption rate of the gaseous product (H2O) during the desulfurization process. As the desulfurization proceeds, the FeS1-x layer forms and covers the surface of metal oxide. Consequently, the activity of sorbents declines with the increase of coverage by the FeS1-x layer due to coverage/shielding of the underlying active sites. According to previous studies 2,3,10,24, the diffusion of ion (O2-, S2-) through the metal oxide/metal sulfide interface becomes the main resistance in the later stage of desulfurization process. As listed in Table 3, the deactivation rate constant (kd) in the case of microwave desulfurization is generally smaller than that of the conventional desulfurization. Janney 22 stated that microwave heating can intensify the diffusion of solid-state ions. On the basis of the heating mechanism of the microwave heating, microwave could produce efficient internal heating (in-core volumetric heating) by direct coupling of the microwaves with ions

20.

Combined the analysis above, the

interaction between microwave and ions (O2-, S2) may enhance the cross-diffusion of O2- and S2- between the product layer and unreacted metal oxide, which facilitates the exposure of the active sites. Compared to C500, much stronger XRD peak (Figure 4) of M500 is mainly attributed to the fact that the latter has much greater sulfur capacity and the Fe1-xS grains grow faster under microwave irradiation

16,20

due to uniform heating. Another

reason is that microwave can improve the crystallinity of the material

16,20,21,

giving

more intense XRD peaks. Although the BS of M700 is lower than that of C700, the intensity of characteristic peaks (assigned to Fe1-xS) for C700 is not stronger than that for M700. Faster growth rate of Fe1-xS grain under microwave irradiation 16,20 may be responsible for this phenomenon. As listed in Table 2, compared to conventional way, microwave desulfurization at the same temperature results in less decrease in the specific surface area of sorbents, and the pore volumes of the used sorbents are larger.


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In order to evaluate the effect of the sulfidation reaction on the pore structure of sorbents, DS (m2 g−1) and DV (cm3 g−1), representing the decline in specific surface area (S) and pore volume (V) per unit BS, respectively, are calculated. In comparison with conventional way, desulfurization under microwave irradiation causes a decrease of the DS and DV by 15.7–85.0%, and 50.0–330%, respectively. It reveals that microwave desulfurization leads to less adverse impact on the pore structure of sorbents. XPS analysis was conducted to obtain the information of chemical composition on the surface of sorbents. The results are shown in Figs. 7–9 and Table 4. The asymmetric peaks in the Fe 2p3/2 spectra of used sorbents can be deconvoluted into two peaks located at 711.3–711.6 eV and 707.1–707.3 eV, respectively. They are assigned to Fe1-xS (Fea) and Fe (Feb), respectively34. The percentages of Feb among Fe-containing products in used sorbents are 3.6–8.0%. The presence of elemental Fe is due to the reduction of Fe-containing compounds by hot coal gas containing reductive gases (H2 and CO). Compared to conventional way, microwave desulfurization contributes to a higher increase of Feb percentages by 8.1–85.0%. It is attributed to the fact that even heating and much longer exposure time (obtained from adsorption curves) under microwave irradiation are favorable for the reduction of Fe2O3 and Fe3O4. Fortunately, the reduction reaction has no significant effect on the performance of the sorbent as deduced from the better microwave desulfurization properties of sorbents. The O 1s XPS spectra of sorbents could be deconvoluted into three types of peaks, belonging to Oa (absorbed oxygen), Ob (oxygen vacancy), and Oc (lattice oxygen), respectively35-37. Except for C600, the concentration of lattice oxygen on the surface of sorbents decrease by 29.1 to 75.4% due to the replacement of O2− by S2− in the Fe-containing oxide. Furthermore, compared to conventional way, smaller percentages of lattice oxygen on the surface of the sorbents removing H2S by microwave technique at the same temperature (400–600 ℃) are observed. This agrees with greater sulfur capacity of M400, M500, and M600. Three type of peaks assigned


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to Sa, Sb and Sc, respectively, were obtained by Gaussian-Lorentz fitting of XPS patterns of S 2p (Figure 9). They are representing Fe1-xS, elemental sulfur, and S6+, respectively

38-40.

Compared to conventional way, the concentrations of Fe1-xS on the

surface of sorbents increase by 16.7–138.9% (except for M700) under microwave irradiation at the same desulfurization temperature. It agrees with higher sulfur capacity of sorbents removing H2S by microwave way at 400–600 oC. Additionally, the proportions of elemental sulfur reach 35.3−52.1% of sulfur-containing species on the surface of used sorbents. Actually, yellow solid is observed on the wall of quarz tube during the desulfurization process. The appearance of sulfur is ascribed to the catalytic conversion of H2S to elemental sulfur over Fe1-xS 3,17,41,42. On the basis of the catalytic mechanism for the H2S decomposition reaction proposed by Al-Shamma 42, the possible mechanism for catalytic decomposition of H2S by Fe1-xS in our experiments may be presented as follows: Fe1- x S  mH 2 S  Fe1- x S m 1  mH 2 Fe1- x S m 1  Fe1- x S n 

m 1- n S2 2

(8) (9)

It is notable that S6+ is also detected for the used sorbents. It may be caused by the oxidation of sulfur when samples were grinded for XPS tests. The morphology of fresh and used sorbents is shown in Figure 10. The assembled iron oxide composed of small nanoparticles (about 10 nm) appears on the surface of fresh sorbents (Figure 10a). The structure of Fe1-xS (0

Fe-based sorbent for hot coal gas under microwave irradiation

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