Migration and Redistribution of Sulfur Species during Chemical

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Migration and redistribution of sulfur species during chemical looping combustion of coal with CuFe2O4 combined oxygen carrier Baowen Wang, Yongmei Cao, Jun Li, Weishu Wang, Haibo Zhao, and Chuguang Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01446 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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Migration and redistribution of sulfur species during chemical looping combustion of coal with CuFe2O4 combined oxygen carrier Baowen Wang

1,2*

,Yongmei Cao1, Jun Li1, Weishu Wang1, Haibo Zhao2, Chuguang Zheng2

1

College of Electric Power, North China University of Water Resources and Electric Power, Zheng Zhou 450045, China. 2 State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China

Abstract: Chemical looping combustion (CLC) by direct use of coal as fuel has gained great recognition for the great advantage for CO2 capture, but sulfur occurrence and evolution in the CLC system is always a great concern. In order to gain a comprehensive insight into the migration and redistribution of various sulfur species in the CLC system, a typical Chinese coal (designated as LZ) with large size range around 180-400 µm (as frequently used in the real CLC system) was selected and its reaction with CuFe2O4 combined oxygen carrier (OC) was investigated using thermogravimetric analysis (TGA), which indicated that reaction behavior of CuFe2O4 with LZ coal of large size changed greatly due to the maceral enrichment and mineral segregation in the LZ coal. And at the two main reaction stages, the characteristic temperatures of CuFe2O4 reaction with LZ coal of large size range shifted to higher temperatures and the reaction rates increased in relative to reaction of CuFe2O4 with LZ coal of small size range (63-106µm). Furthermore, migration and redistribution of various sulfur species formed from reaction of LZ coal with CuFe2O4 at its oxygen excess number Φ =1.0 were studied through gaseous Fourier transform spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), which revealed that the SO2 mainly resulted from oxidation of H2S by CuFe2O4 combined with direct emission through pyrolysis of LZ coal at the peak temperature 413.6 oC, while the solid Cu2S was formed through the gaseous sulfur liberated out by LZ coal and further reaction with the reduced CuFe2O4. Finally, thermodynamical simulation of LZ coal reaction with CuFe2O4 OC was conducted, and among all the four factors considered such as the CuFe2O4 oxygen excess number Φ, reaction temperature, steam concentration and the system pressure, the CuFe2O4 oxygen excess 1

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number Φ was ascertained as the most significant to migrate and converting most of the sulfur involved in LZ coal to the solid Cu2S is suggested as a good option for its ensuing separation out of the reduced OC. In addition, in order to simultaneously realize the CO2 capture and effective in-situ desulfurization during reaction of LZ coal with CuFe2O4 OC, the optimized condition was preliminarily explored and the CuFe2O4 oxygen excess number Φ was suggested to fix around 1.5. Key words: CO2 capture; chemical looping combustion; CuFe2O4 combined OC; migration and redistribution of sulfur species; in-situ desulfurization. *Corresponding author. Tel/Fax: (371)69127630; E-mail: [email protected]. 1. Introduction CO2 capture and storage (CCS) has been acknowledged as one of the most effective options to mitigate the voluminous emission of CO2 from carbonaceous fuel combustion, especially coal 1. But the CO2 capture cost was estimated to exceed 80% of the total cost involved in CCS, and more attention should be directed to the cost-effective CO2 capture technologies 1, 2. Among various CO2 capture technologies available, as compared to the precombustion, post-combustion and oxy-fuel combustion, chemical looping combustion has received great interest for its inherent separation of CO2 without much energy penalty and system efficiency loss 3. Direct use of coal as fuel for CLC application is much advantageous due to the abundance, easy availability and price strength of coal. So far, various CLC systems with different capacities from lab-scale to a 1 MWth pilot plant have been operated with coal as fuel and various Fe2O3-based materials chosen as OC due to its low cost and environmental benignity 4-7. The operational results demonstrated that direct use of coal as fuel for CLC was feasible in the CLC system, where coal would experience a series of complex reaction processes, including pyrolysis/gasification and further oxidization of the devolatilization

2


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products by OC, but the slow gasification rate of the residual char left would affect the CO2 capture of the CLC system. In order to overcome the limitation of char gasification and to promote the full conversion of coal, an alternative CLC, i.e. chemical looping with oxygen uncoupling (CLOU), was developed 8 and received intensive researches 9, 10, where direct combustion of coal was realized through the O2 decomposed from some special OCs. Especially, CuO-based OCs were widely used 9, 10, but the low melting point, inferior resistance to sintering and high cost of CuO OC should be considered 11. Therefore, in order to combine the advantages of CuO and Fe2O3 OC and simultaneously overcome their disadvantages, a CuFe2O4 combined OC was first put forward in our previous research 12 and the superiority of CuFe2O4 over the single CuO and Fe2O3 OC was validated 12, 13, such as good resistance to sintering and satisfactory fuel adaptability to high-rank coal and even to the petroleum coke of quite low reactivity. Especially, the oxygen transfer pathways from CuFe2O4 to coal were flexible at the different reaction temperatures 12, 14. It not only had the capacity to direct transfer of the lattice oxygen involved to the coal, but also could decompose and emit O2 to promote the direct combustion of coal. Therefore, CuFe2O4 OC was considered as a competitive OC with great potential to apply in the realistic CLC system. Sulfur present in coal is always a great concern in the coal-fuelled CLC system. During reaction of coal with OC, sulfur species would migrate from coal and further reacted with OC to form solid sulfur compounds, which caused a series of deleterious effects, such as the OC poison and deactivation 15, possible agglomeration of the OC due to the low melting points of the formed solid sulfur compounds 16. Furthermore, if the solid sulfur compounds were entrained with the reduced OC from the fuel reactor (FR) to the air reactor (AR) and further oxidized to form and emit SO2 gas therein, an potential environmental pollution would occur. In addition, various gaseous sulfur species left in the fuel reactor would also affect the purity of the captured CO2 and incur great difficulty to the downstream CO2 sequestration 17. But 3


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current researches on the migration and distribution of sulfur species in the CLC system were still quite limited. Jerndal et al.18 and Wang et al 19 adopted thermodynamic investigation of the H2S evolution involved in the syngas or natural gas. Wang et al. 10, García-Labiano et al. 20

and Foreso et al. 21 investigated the fate of H2S during reaction of different gaseous fuels

with NiO and CuO-based OC. While Dueso et al. 22, Gu et al. 23 and Cabello et al.24 evaluated the effect of H2S laden in CH4 or CO on the reactivity of NiO or iron ore OC. Furthermore, García-Labiano et al.25 and de Diego et al. 26 evaluated the reaction performance of CuO and Fe2O3-based OC with the sour gas with the highest concentration of H2S reaching 20%. As to the solid fuel coal, only Pérez-Vega et al.9, Adánez-Rubio et al. 16, Linderholm et al.27, Shen et al.28, and Mendiara et al. 29 studied the fate and redistribution of different sulfur species in the coal-fuelled CLC system. Therefore, it is of great necessity to conduct a systematic investigation on the migration and distribution of various sulfur species in the coal-fuelled CLC system to effectively reduce the potential sulfur harms. In this research, CuFe2O4 was chosen as the OC and prepared using a novel sol-gel combustion synthesis (SGCS) method. And a typical Chinese high sulfur coal (abbreviated as LZ) was adopted as fuel. Their reaction characteristics were investigated through thermogravimetric analysis (TGA). Considering the great harms of the sulfur occurring in the CLC system, migration and redistribution of the sulfur species during reaction of this high sulfur coal with CuFe2O4 was highlighted and systematically investigated. The gaseous sulfur species evolved from reaction of LZ coal with CuFe2O4 was in situ detected using a Fourier transform infrared specter (FTIR) coupled with the TGA, while the formed solid sulfur compounds were identified using X-ray photoelectron spectroscopy (XPS). Furthermore, thermodynamic simulation of CuFe2O4 with this high sulfur coal was conducted. In order to simultaneously realize CO2 capture and in situ desulfurization during LZ coal reaction with CuFe2O4 OC, a preliminary exploration of the most significant factor on the formation of the 4


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solid sulfur compounds was ascertained and the optimized condition was determined through thermodynamical investigation. 2. Experimental section 2.1 Materials and characterization CuFe2O4 combined OC and their two reference single oxides (CuO and Fe2O3) to be employed in this resarch were synthesized using our novel sol-gel combustion method. The details of this preparation method could be found out in our previous research 30. After mixing the precursor solutions of copper, iron and urea, preparation and dessication of the wet sol, ignition of the dried gel and calcination of the as-prepared OC, the desired OC was as received and further crushed, sieved with the particles of 63-106 µm collected for use. Meanwhile, a typical Chinese bituminous coal was collected from the LiuZhi district, Guizhou Province, one of the main areas in the south western region of China for high-sulfur coal reserve, and designated as LZ hereafter. Coal particle size was considered as one of the important parameters for coal utlization

31, 32

. According to our survey of the main coal

particle size frequently used in the real CLC system 8, 9, 16, 28, 29, 33-35, many of them fell within 180-400 µm. Therefore, the collected LZ coal was further dried, ground and sieved with the size range 180-400 µm collected for use in this research, instead of our previous coal particle size of 63-106 µm

36-38

. The ulitmate and proximate analysis of the collected LZ coal of the

large size in this research was analyzed according to GB/T212-2008 and GB/T 31391-2015 and preseneted in Table 1. From this Table, it could be observed that, as compared to the properties of LZ coal with small size in our previous research

36, 37

, the properties of larger

sized LZ coal changed greatly as analyzed in more detail below. Meanwhile, the total sulfur involved in the LZ coal of larger size was reduced to 4.52 wt% from 5.23% for LZ coal of small size around 63-106 µm 14. And according to the Chinese national standard GB/T 2152003, the relative contents of pyrite, sulfate and organic sulfur in LZ coal were further

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determined as 49.78%, 11.95% and 38.27%, respectively with pyrite dominated. Therefore, it is still necessary to investigate the reaction charactersitics of the LZ coal with CuFe2O4, though similar research with LZ coal of 63-106 µm was conducted before in our previous research 14. And then, after the OC and coal were prepared, stoichiometric amount of CuFe2O4 was evenly mixed with the prepared LZ coal at a certain mass ratio, which was determined according to the mass balance method described in our previous research

12

. Based on the

properties of the LZ coal (referred to the LZ coal of big size range around 180-400 µm hereafter without special specification) shown in Table 1, the relative chemical formula of 1kg LZ coal was repesented as C20.8H9.8N0.32S0.88O5.6 without the minerals involved, which were considered in the thermaldynamic simuation of LZ coal reaction with CuFe2O4 OC below. If the provided CuFe2O4 thereotically met the full conversion of the LZ coal, the CuFe2O4 oxygen excess number Φ could be defined as the unity. And thus, the mass ratio of CuFe2O4 to LZ coal was determined as 7.64. Similarly, mass ratios of the two reference oxides CuO and Fe2O3 to LZ coal could be calculated as 3.39 and 20.40, respectively. 2.2 Experimental methods Due to the main advantages of accuracy, convinence and the well controlled condition provided by TGA, reaction of LZ coal with CuFe2O4 was investigated in a simultaneous thermal analyzer (STA 449 F3, Netzsch, Germany). And the non-isothermal method was adopted in this research to overcome the disadvantage of the initial sample heating up and the inaccuracy incurred for the isothermal method

39, 40

. Of course, as to the non-isothermal

method, the limitation of the volatile matter emitted out in the lower temperature than the existing temperature in a CLC unit should be also noted and addressed in the future. And then, about 15 mg of the mixture between LZ and CuFe2O4 at Φ = 1 was loaded to the TGA sample pan and heated up from ambient to 900 oC at 20 oC/min and then soaked at this final

6


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temperature for 10 min for sufficient conversion of LZ coal. The pure N2 atmosphere was used and its flow rate was set as 100 ml/min after several pre-tests in advance to eliminate the effect of heat/mass trnansfer and to ensure the results reproducibile. Meanwhile, the gaseous products from reaction of LZ coal with CuFe2O4 in the TGA furnace was introduced to the FTIR spectrometer (EQUINOX 55, Bruker Corp., Germany). And the gas transmit line between the TGA and FTIR was heated to 180 oC to avoid condensation of the tar and steam. The scanning range of IR was 4000-500 cm-1. And the gaseous species were detected out in the spectrometer by their respective IR values. After TGA-FTIR analysis, the solid products formed from reaction of LZ coal with CuFe2O4 were carefully collected. The solid surface was further scanned in a narrow scanning mode and solid sulfur compositions of different chemical states were identified using an XPS spectrometer (VG MultiLab 2000, Thermo Electron Corp., U.S.). A monochromatic Mg Kα source (hv = 1253.6 eV) was used. The X-ray source power of this spectrometer was 300 W while the base pressure and pass energy were pre-set as 5×10-8 Pa and 25 eV, respectively for high scanning resolution. All the binding energies as obtained for the different elements were referenced to the C 1s peak at 284.6 eV. Meanwhile, the XPS spectra of the studied species were further deconvoluted using the commercial software affiliated to the XPS spectrometer. The relative contents of the obtained sulfur species were further quantified by their areas through curve-fitting. 2.3 Thermodynamic investigation of the reaction of CuFe2O4 OC with LZ coal Finally, in order to overcome the experimental analysis limitations and obtain the comprehensive knowledge on the migration and distribution of the different sulfur species, reaction of LZ coal with CuFe2O4 OC was investigated using the thermodynamic method, though this method owned limitations without considering the real kinetic process 19.

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For the thermodynamic simulation, a complex reaction system for reaction of LZ coal with CuFe2O4 was established with 426 species involved, similar to our previous researches 12, 13

. Especially for LZ coal, according to its properties provided in Table 1, besides C, H, N, S

and O elements to compose the main organic matrix of LZ coal, various possible minerals were considered as well to represent the real coal with great heterogeneity, which would make the equilibrium simulation more realistic. 3. Results and discussion 3.1 Effect of particle size of LZ coal on its properties The particle size of coal used in the real CLC system changed greatly as mentioned above relative to our previous research 12, 14, which would have a great effect on the properties of coal and thus further on its reaction behavior in the CLC system. Therefore, in order to obtain the better understanding on the reaction characteristics of LZ coal with CuFe2O4 and the evolution of sulfur involved in the more realistic CLC system, the particle size range selected for LZ coal in this research was 180-400 µm instead of 63-106 µm in our previous investigations. The proximate and ultimate analysis of LZ coal around 180-400 µm was determined and shown in Table 1 and further compared with our previous LZ coal of small size range 63-106 µm. From Table 1, it could be observed that a little decrease of the volatile content but a great increase of the fixed carbon content for this large LZ coal occurred with the increase of the coal particle size. Similar results were also observed by others

41, 42

and attributed these

changes to the unavoidable maceral enrichment during grinding and sieving process. Meanwhile, due to the easy segregation of the mineral matter involved to the small particle size of coal during the sample preparation

32

, the ash content was greatly decreased from

41.88 wt% in the small size of LZ coal in our previous research 36, 37 to 37.86 wt% for the LZ coal of large size, which resulted in the ratios of both the volatile and fixed carbon contents to

8


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the ash content (i.e V/A and FC/V shown in Table 1) increased to 0.436 and 1.197 respectively with the increase in the particle size of LZ coal. Furthermore, the C/H ratio of LZ coal with large size also increased to 2.05, which reflected the variation of the chemical structure of LZ coal with the change of coal particle size

43

. In addition, the total sulfur

content determined according to the Chinese national standard GB/T215-2003 was 4.25 wt%, much lower than that of 5.23 wt% in our previous research for LZ coal of small size, which would be analyzed below. Overall, the particle size of LZ coal showed a remarkable effect on its properties and chemical sturcutre of LZ coal. Therefore, reaction charactersitics of LZ coal of large size with CuFe2O4 should be deeply investigated. 3.2 Reaction characteristics of LZ coal with CuFe2O4 OC Reaction characteristics of LZ coal around 180-400 µm with CuFe2O4 at its oxygen excess number Φ = 1 under the pure N2 atmosphere were studied through TGA analysis at the heating rate 20 oC/min. The weight loss (TG) and its loss rate (DTG) were presented in Figure 1(a)-1(d), respectively. Meanwhile, pyrolysis of LZ coal of large size under N2 atmosphere and its further reaction with the two reference oxides CuO and Fe2O3 at Φ = 1 were also included for reference. Firstly, LZ pyrolysis was conducted under the N2 atmosphere and both TG and DTG results were shown in Figure 1(a) and 1(b) for reference. It could be observed that the residual TG value left after reaction of CuFe2O4 with large size LZ coal was 82.5 wt%, a little less than 85.5 wt% for CuFe2O4 reaction with LZ coal of small size (63-106 µm) in our previous investigation 14, mainly resulting from the combined effect of variation in the mineral matter and carbon contents involved in LZ coal 44. After removal of the moisture involved in LZ coal below 200 oC, LZ coal was observed to experience two distinct reaction stages, as accompanied by the breakage of the weak bonds present in LZ coal and reorganization of the main carbon matrix 45, respectively. And the characteristic temperatures at the maximal DTG

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values of these two reaction stages centered around 565.1 and 872.2 oC, much higher than LZ coal of small size range around 63-106 µm in our previous research

14

, mainly due to the

increased fixed carbon content as shown in Table 1 and decreasing reactivity of LZ coal with the increased coal particle size

41

. Meanwhile, longer residence time of the volatiles to be

confined inside the coal particles should be also responsible for this temperature shift 31. Different from pyrolysis of LZ coal in the N2 atmosphere, reaction of LZ coal with CuFe2O4 changed greatly. As shown in Figure 1(a), the residual TG from reaction of LZ coal with CuO was 78.4 wt%, far less than that of LZ coal with Fe2O3 as 93.8 wt%, while the residual TG value for LZ coal reaction with CuFe2O4 was 88.9 wt% and fell within those TG values of LZ coal with Fe2O3 and CuO. Based on the weight loss behaviors of CuFe2O4 and its two reference oxides with LZ coal, the reactivity of CuFe2O4 and their two reference oxides with LZ coal was calculated and quantitatievely compared using the developed equation 14 below.

α=

W0 − W ( f /(1 + f ))∆WOC + (1 /(1 + f ))∆Wcoal

(1)

where α represented the conversion of CuFe2O4 OC and its two reference oxides CuO and Fe2O3 with LZ coal, W0 and W referred to the initial and instantaneous weight losses during CuFe2O4 or its two reference oxides with LZ coal (wt %) shown in Figure 1(a), respectively. While ∆WOC and ∆Wcoal meant the maximal weight losses of OC and LZ coal, respectively, and f represented the mass ratio of CuFe2O4, CuO and Fe2O3 to LZ coal at their oxygen excess number Φ = 1, as determined in Section 2.1. Based on the Eq.(1) above, the conversion of CuFe2O4 and its two reference oxides CuO, Fe2O3 during their reactions with LZ coal were calculated as 58.26%, 72.80% and 49.59%, respectively, which fully indicated that reactivity of CuFe2O4 was improved and much higher than that of Fe2O3 reaction with LZ coal, though still lower than that of CuO with LZ coal.

10


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Furthermore, reaction behavior of CuFe2O4 and its two reference oxides CuO and Fe2O3 with LZ coal was studied. As shown in Figure 1(c) and 1(d), it could be observed that though two reaction stages occurred for reaction of LZ coal with CuFe2O4 and its two reference oxides, the maximal DTG value of LZ coal reaction with CuFe2O4 was greatly increased from 0.3077 wt%/min at the first stage to 0.5852 wt%/min at the second stage with nearly two times enhanced due to the beneficial synergistic effect of the combined CuFe2O4 and direct combustion of the residual LZ coal at this stage by O2 emitted from CuFe2O4, as found out in our previous research

12

. Different from LZ coal reaction with CuFe2O4, the maximal DTG

value of LZ coal reaction with Fe2O3 at the second reaction stage was decreased. As to CuO, though the maximal DTG vaule of LZ coal with CuO at the second stage was still increased, the net increase percentage was less than 37%. Overall, the maximal DTG values for reaction of LZ coal with CuFe2O4 at the second reaction stages was much higher than those at the first reaction stages, which was contrary to the pyrolysis of LZ coal shown in Figure 1(b) due to the beneficial effect of OC introduced on the disintegration and further oxidization of the main carbon matrix, as validated in our previous analysis

14

. In addition, the characteristic

tempeature of LZ coal with CuFe2O4 at the first reaction stage was 546.3 oC, much lower than 595.8 oC for LZ coal with CuO and 626.5 oC for LZ coal with Fe2O3, which reflected the better reactivity of CuFe2O4 to gaseous products emitted from LZ pyroysis at the first reaction stage. But at the second reaction stage, though the characteristic temperature for LZ coal reaction with CuFe2O4 reached 898.4 oC, higher than those temperatures for LZ coal reaction with CuO (856.6 oC) and Fe2O3 (837.3 oC), this reaction temperature still fell within the temperature range of realistic CLC (850-950 oC). Therefore, CuFe2O4 was a good OC candidate and should be preferred over CuO and Fe2O3 in a realistic CLC system.

3.3 FTIR analysis of the gaseous sulfur evolution

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Sulfur species occurring in the CLC system is quite detrimental either from the environmental or the operation of a CLC system aspect. In order to obtain the full knowledge of migration and redistribution of the different sulfur species in a CLC system, the gaseous sulfur species evolved from both pyrolysis of LZ coal under the pure N2 atmosphere and its reaction with CuFe2O4 OC was investigated through the FTIR analysis. The main gaseous sulfur species for pyrolysis of LZ coal under the pure N2 atmosphere were detected out as SO2, COS and CS2, as shown in Figure 2(a)-2(c). As to H2S, though generally considered as dominant through decomposition of pyrite (FeS2→FeS+S) hydrogen formed (S+(coal-H)→H2S+coal)

46

46

and further reaction with the

during pyrolysis of LZ coal with high pyrite

content involved, it could not be detected out by FTIR due to its weak IR absorbance

46, 47

.

While for reaction of LZ coal with CuFe2O4, only SO2 was detected out and shown in Figure 2(d). Furthermore, the gaseous sulfur species were integrated over the specific IR wavenumber regions and shown in Figure 2(a)-2(d) as a function of time. In addition, considering the gaseous sulfur species evolved from pyrolysis of LZ coal reaction with OC

37

47

and its further

were of different sources and quite complex, the FITR curves were

deconvoluted as Miura et al

48

conducted to illuminate the different sources of various sulfur

species involved and further quantified based on their integrated areas and listed in Table 2. As a baseline, gaseous sulfur species evolved from pyrolysis of LZ coal under the pure N2 atmosphere were firstly analyzed. As shown in Figure 2(a), the FTIR curve of SO2 was resolved into four peaks between 235-843 oC, among which the peak 1 of SO2 at 332.2 oC was assigned to oxidation of aliphatic sulfur either by the chemisorbed oxygen in the porous structure of LZ coal involved in LZ coal

46

or the oxygen-containing groups (such as ethers, carboxyls, etc.)

49

. While for peak 2 of SO2 at 472.3 oC, it mainly resulted from the

decomposition of the iron sulfate as validated by others

50-52

and contributed more than 60%

of the total SO2 yield from LZ coal pyrolysis under the N2 atmosphere as shown in Table 2, 12


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mainly due to oxidation of the pyrite present in LZ coal to iron sulfate 50, 53, as analyzed below. And then, peak 3 of SO2 at 577.8 oC was mainly attributed to thermal cracking of sulfone 54, as further validated in Figure 3(a) below for XPS analysis of the solid sulfur compounds involved in the original LZ coal. And as to peak 4 of SO2, it commenced at 540 oC and then extended to the 900 oC isothermal period with the peak temperature residing around 718.1 oC, which was mainly attributed to decomposition of calcium sulfate present in LZ coal due to its higher thermal stability than that of sulfone

55

.Though the temperature for direct

decomposition of CaSO4 under the N2 was above 1200 oC, the decomposition temperature of CaSO4 in the presence of coal was much lowered due to the catalytic function of the coal organic matrix 52. Different from SO2 emitted from pyrolysis of LZ coal under the N2 atmosphere, CS2 and COS evolved from pyrolysis of LZ coal were more complex. Besides the direct formation pathways from reaction of pyrite included in LZ coal with the pyrolysis gases emitted from LZ coal, many gaseous secondary reactions were involved as well. As shown in Figure 2(b) for FTIR curve of CS2, it was resolved into three peaks between 300-809 oC, among which the first peak of CS2 at 492.3 oC was attributed to direct reaction of CH4 (one of the main pyrolysis gas of LZ coal by our previous investigation through the pathway as 4FeS2+CH4→CS2+4FeS+2H2S

14, 36

) with pyrite present in LZ coal

53, 56

, and made up more than 77% of

the total CS2 formed from pyrolysis of LZ coal, as shown in Table 2. While for peak 2 of CS2 at 623.5 oC, it resulted from direct dissociation of COS (i.e. 2COS →CS2+CO2)56, especially in the presence of quartz, which was rich in the LZ coal and considered to act as the catalyst for CS2 formation

57

. And the peak 3 of CS2 at 701.7 oC was attributed to the gaseous

secondary reaction between H2S and COS through the pathway as H2S+COS →CS2+H2O 58

,where H2S was inferred to form from decomposition of pyrite and further combination of

the H involved in coal 46, 47.

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Similar to CS2, COS evolved from pyrolysis of LZ coal under the N2 atmosphere is shown in Figure 2(c). The three fitted FTIR curves of COS were assigned to gaseous secondary reactions between H2S and CO2 through H2S+CO2→COS+H2O for the peak 1 of COS at 396.5 oC, where the H2S mainly derived from decomposition of organic sulfur of thermally labile structures in LZ coal 46. And the peak 2 of COS at 495.1 oC mainly derived from the direct gas-solid reaction of the pyrolysis gas CO with pyrite (FeS2+CO→COS+FeS) 56

. While for the peak 3 of COS at 612.6 oC, it mainly resulted from the gaseous secondary

reaction between the pyrolysis gas CO emitted from LZ coal and the active sulfur derived from decomposition of pyrite (CO+S→COS)59. Among all these three main pathways for COS formed during pyrolysis of LZ under N2 atmosphere, the contribution from reaction of pyrite with CO was the most significant and reached up to 88%. Especially above 650 oC, CO2 atmosphere benefited the gasification of the residudal char to form CO and further reaction with the active sulfur left after pyrite decoposition 60. As such, a litte COS trace was still observed to form and increase with temperature, as shown in Figure 2(c). Yet for for reaction of LZ coal with CuFe2O4 OC, the main gaseous sulfur species detected out by FTIR was only SO2, as shown in Figure 2(d). As to H2S, though its yield was more than SO2 shown in Figure 4(d) below by the thermodynamic investigation, it could not be detected out by FTIR analysis as mentioned above and thus was not included in Figure 2(d). In order to deeply understand the evolution of SO2 from reaction of LZ coal with CuFe2O4, its FTIR curve was resolved into such six peaks, as included in Figure 2(d). As to the peak 1 at 307.6 oC, besides the direct contribution of SO2 evolved from LZ coal pyrolysis shown in Figure 2(a), a series of complex reactions were involved. Due to the possible gaseous secondary reactions occurring inside the LZ coal of large size, the H2S was formed from the secondary reaction of COS with H2O

28

inside the porous coal structue as shown in

14


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Eq.(2), which would emit out and react with CuFe2O4 OC to form SO2 as shown in Eq.(3) below. COS+H2O→CO2+H2S

(2)

2.25CuFe2O4+H2S→2.25Cu+1.5Fe3O4+H2O+SO2

(3)

Simiar to Peak 1, for the peak 2 of SO2 at 413.6 oC, besides the direct contribution from SO2 emitted from LZ coal pyrolysis shown in Figue 2(a), an extra gaseous secondary reaction between CS2 with steam confined in the LZ porous structure occurred to form H2S, as shown in Eq.(4) below. And the formed H2S was further oxidized by CuFe2O4 OC as shown in Eq.(3) above to form SO2. CS2+2H2O→CO2+2H2S

(4)

Wihle for the peak 3 at 541.65 oC, as shown in Table 2, though the relative fraction of SO2 emitted from pyrolysis of LZ coal was only 14.91%, but the SO2 produced from LZ coal reaction with CuFe2O4 reached up to 27.82%, which implied that oxidation of CuFe2O4 with extra H2S evolved from direct decomposition of pyrite occurred as shown in Eq.(3). And the temperature range around 500-600 oC was considered as the most relevant for pyrite decomposition and H2S formation 46, 47, 59. As to the peak 4 at 669.7 oC, its formation pathway was similar to that of the peak 2 with Eq.(2)-(4) involved, but the relative fraction of SO2 evolved from LZ coal reaction with CuFe2O4 was 22.56%, a little lower than that of the SO2 from the peak 3. And then, as to the peak 5 at 763.4 oC, the SO2 contributed from this peak was similar to the peak 2 and peak 4, besides SO2 evolved from LZ coal pyrolysis, the H2S was fromed through Eq.(4) and further oxidized by CuFe2O4. Finally, the last peak at 839.8 o

C was assigned to oxidization of the thiophene organic sulfur present in LZ coal as analyzed

in Figure 3(a) below. Though its contribution to the total SO2 yield was only 4.76%, it should be paid to enough attention due to its great thermal stability even over 900 oC 59.

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3.4 XPS analysis of the solid sulfur compounds formed from LZ coal reaction with CuFe2O4 In order to utilize coal more rationally and cleanly, it is of great necessity to understand the chemical transformation of sulfur in coal during CLC. After TGA-FTIR investigation of the reaction behavior of CuFe2O4 with LZ coal of large size and migration of the gaseous sulfur species as depicted above, the distribution of the solid sulfur compounds during LZ coal reaction with CuFe2O4 was analyzed using XPS through deconvoluting the S 2p of the related solid sulfur compounds and shown in Figure 3(b). Meanwhile, the solid sulfur compounds present in the original LZ coal was analyzed as well and provided in Figure 3(a) for reference. As a reference, the main sulfur compounds present in LZ original coal were identified out using XPS. From Figure 3(a), the S 2p XPS spectra of the LZ coal was resolved into five peaks, of which the peak 1 at 162.4 eV and peak 5 at 169.1 eV belonged to the inorganic sulfur and were assigned to pyrite and sulfate 61, respectively. And their relative contents were shown in Table 3 and determined as 44.96% and 22.64%, respectively. While the peak 2 at 163.3 eV, the peak 3 at 164.1 eV and the peak 4 at 168.3 eV shown in Figure 3(a) were attributed to aliphatic, thiophene and sulfone organic sulfurs

62

with their relative contents

10.71%, 13.15% and 8.54%, respectively. Obviously, LZ coal was pyrite dominated. But thiophene made up the largest fraction among all the three organic sulfur compounds in LZ coal and reached 40.6%, which was in accordance with Attar's findings 53, and he pointed out that thiophenes were generally considered as coal rank dependent and the relative content of thiophene was estimated around 40-70% for the bitumnious coals. Furthermore, considering the surface-sensitivity of XPS analysis and the heterogeneous distribution of the inorganic sulfur species, especially for pyrite 63, the relative contents of the main sulfur compounds present in the LZ original coal, including pyrite, sulfate and organic

16


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sulfur, were also tested and calculated using the routine wet chemical method on the basis of Chinese national standard GB/T215-2003. As shown in Table 3, the relative contents of pyrite and sulfate were determined as 49.78% and 11.95%. And the relative content of the organic sulfur present in LZ coal was calculated as 38.27% through subtraction of the total sulfur by the sum of pyritic and sulfate inorganic sulfur. Furthermore, from Table 3, as compared to the relative contents of pyrite and the total organic sulfur tested by the routine wet chemical method, these two values tested by XPS were acceptable with their errors below 15% 64. But the relative content of sulfate from XPS analysis was 22.64%, nearly two times of that tested by the routine wet chemical method, which indicated that pyrite distributed on the LZ coal surface was oxidized after being exposed to air 53. In addition, in comparison with the relative contents of pyrite and organic sulfur tested by the wet chemical method in Table 3, the LZ coal of large size owned the lower content of pyrite but higher content of organic sulfur in relative to LZ coal of small size range (63-106 µm), mainly due to the effect from the particle size of coal, just as Lytle et al. 65 found out that increase in the coal particle size would bring about more organic sulfurs enriched in the larger size coal while pyrite prone to the finer one. In relative to the solid sulfur compounds distributed in the LZ original coal, after its reaction with CuFe2O4 OC, the sulfur compounds changed greatly. As shown in Figure 3(b), the S 2p XPS spectra of the solid products formed from reaction of LZ coal with CuFe2O4 was mainly deconvoluted into three peaks. The binding energy (BE) value at the first S 2p peak centered at 160.1 eV, a little lower than that of the reported BE value 162.3±1.7 eV for Cu2S 66, 67. And the shift of S 2p BE value to a lower direction was mainly due to the different chemical environment of Cu during its sulfidization

68

. Especially in the presence of such

reducing gas as CO, the reduced Cu or the decomposed Cu2O from CuFe2O4 (as analyzed in our previous research12, 14) could further react with H2S liberated from LZ coal to form Cu2S (such as Cu+H2S → Cu2S+H2, Cu2O+H2S → Cu2S+H2O)

69,70

. Furthermore, based on the

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relative deconvoluted area of S 2p for Cu2S shown in Figure 3(b), its relative fraction was the largest and quantified as 77.16% among all the solid sulfur compounds formed from reaction of LZ coal with CuFe2O4. And the second S 2p peak at 161.6 eV was attributed to FeS 71 with its fraction up to 10.31%, which resulted from decomposition of the pyrite (FeS2) present in LZ coal (FeS2→FeS+S)

59

organic thiophene sulfur

62

. As to the third S 2p peak at 164.1 eV, it was assigned to the , which was reported as the most thermally stable organic sulfur

and even survived the temperature higher than 900 oC without an appreciable conversion 59, 72. Thefore, at the reaction temperature of CLC interest (~900-950 oC), conversion of the thiophene was still incompelte, which would bring about great environmental harms if the unreacted thiophenes embedded in the char was entrainded to the air reactor and oxidized with air to form SO2 theirein.

3.5 Thermodynamic simulation of LZ coal reaction with CuFe2O4 Finally, in order to learn the migration and redistribution of sulfur species during reaction of LZ coal with CuFe2O4 more comprehensively and thus to effectively control the sulfur species emitted, thermodynamic simulation of LZ coal reaction with CuFe2O4 was conducted and studied in more detail.

3.5.1 Equilibrium components distribution of LZ coal reaction with CuFe2O4 Reaction of LZ coal with CuFe2O4 was thermodynamically simulated according to the method introduced above in the Section 2.3. The reference reaction conditions designed were CuFe2O4 oxygen excess number Φ = 1, system pressure P = 1 bar, concentration of the steam introduced R = 0, and the reaction temperature ranging from 400-1000 oC. And the conversion of coal, reduction of the CuFe2O4 and distribution of the different sulfur species during reaction of LZ coal with Cufe2O4 were presented in Figure 4(a)-4(d), respectively. From Figure 4(a) for conversion of LZ coal during its reaction with CuFe2O4, the main carbon matrix of LZ coal was found to gradually disintegrate as the reaction temperature 18


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increased from 400 to 1000 oC. As a result, the main reaction product CO2 from reaction of LZ coal with CuFe2O4 increased fast to 67.5% at 800 oC. And hereafter, a little decrease of CO2 was observed with a further increase of temperature. Meanwhile, as shown in Figure 4(b) and 4(c) for the reduced CuFe2O4, it was found out that during the reaction with LZ coal, CuFe2O4 was mainly reduced to Cu and Fe3O4. Yet, both CO and FeO were also observed to exist in Figure 4(a) and 4(c) and increased with temperature, which were mainly ascribed to the partially deactivated CuFe2O4 due to many side products formed, such as Cu2S and Fe2SiO4 formed and shown in Figure 4(b) and 4(c), though both insufficient oxidation of CO and deep reduction of Fe3O4 to FeO

73

were not desired for their detrimental effects in a real

CLC system 34. Furthermore, from Figure 4(b)-4(d), during reaction of LZ coal with CuFe2O4, many solid side products were found out to produce. As shown in Figure 4(b) and 4(d), the main solid sulfur compound formed was dominated with Cu2S, similar to our finding above from Figure 3(b) by XPS analysis. Furthermore, as observed in Figure 4(d), the fraction of Cu2S increased from ~85% at 700 oC to 92.16% at 1000 oC. As accompanied by the increase of Cu2S with temperature, the fraction of FeS declined from 13.75% at 700 oC to 6.58% at 1000 o

C. While CuFeS2 formed was generally stabilized around 0.80 %. In relative to the

predominant solid sulfur species formed from reaction of LZ coal with CuFe2O4, the gaseous sulfur species formed were far below 1%. As shown in Figure 4(d), even for the main H2S gas, it was increased with temperature and not completely oxidized to SO2 due to the deactivated CuFe2O4 OC by the formed side products as analyzed above. And the largest fraction of H2S at 1000 oC was still lower than 0.5%. But H2S could not be detected out in Figure 2(b), mainly due to its weak IR absorbance for gaseous FTIR analysis

46

. Much lower than the

fraction of H2S formed, the fraction of SO2 was below 0.1% even at 1000 oC. As to COS, though it was observed in Figure 4(d) and possibly arose from different gaseous secondary

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reactions, such as H2S+CO → COS+H2, H2S+CO2 → COS+H2O

Page 20 of 38

15

. But its fraction was far

lower than 0.001%, and thus COS was not detected out in Figure 2(d) above. In addition, similar to various solid sulfur compounds formed from LZ coal reaction with CuFe2O4 shown in Figure 4(d), Fe2SiO4, FeSiO3 and Fe2TiO4 were also found to form in Figure 4(c), which mainly resulted from reaction of the deeply reduced FeO with silicon and titanium minerals involved in LZ coal 37. Though they could be oxidized back to Fe2O3 in the air reactor again, complete regeneration of the reduced OC was not obtained and thus not desired 37.

3.5.2 Relative sensitivity analysis of the main factors on the solid sulfur products formed Sulfur species existing in the CLC system is always a great harm either from the economic operation of the CLC system or environmental perspective. Therefore, simultaneous realization of the inherent separation of CO2 as well as in-situ desulfurization in the CLC system should be greatly desired. As Solunke and Veser

69

and König et al.

74

demonstrated, simultaneous CO2 capture and in-situ desulfurization in a CLC system was feasible and the core process involved was conversion of the inputted sulfur to the solid sulfur compounds while minimized the gaseous sulfur species formed. Although König et al

74

applied Mn3O4-based OC in their chemical looping desulfurization process, Solunke and Veser

69

pointed out that Cu based OC should be preferred due to its largest combination

potential with S to form the solid sulfur compounds among various OCs available, including Cu, Co, Ni, Mn, Fe-based ones. Similar finding was also validated by our previous investigations on the reaction of LZ coal with different nickel/cobalt/manganese ferrites 36-38. According to the previous researches 9, 10, 16, 18-20, 22, 23, 28, 29, such four influencing factors were considered for evolution of the sulfur species in a CLC system, such as the oxygen excess number Φ, reaction temperature, system pressure and concentration of the steam introduced. Therefore, in order to maximize the solid sulfur compounds formed during reaction of LZ coal with CuFe2O4 and thus to simultaneously realize both CO2 capture and 20


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desulfurization, similar thermodynamic simulation of LZ coal reaction with CuFe2O4 was conducted again with sequentially changing one selected factor value while keeping the other three factors fixed at the designed base values as mentioned above. As such, the main solid sulfur compounds formed at the four different factors could be determined. And then, based on the simulation results obtained, effect of each factor on the formation of solid sulfur compounds was quantitatively evaluated using the relative sensitivity analysis method as we conducted before19 .

ωi =

∆mi / m ∆χ i / χ

(5)

where i represented each influencing factors determined above, including the oxygen excess number of CuFe2O4 OC Φ, reaction temperature T, system pressure P and concentration of the steam introduced R. χ i denoted the factors with the corresponding base values as designed before, i.e. Φ = 1, T = 900 oC, P = 1 bar, and R = 0. And ∆χ i was the difference of each factor value selected by the corresponding base factor value. Similarly, m was the fraction of the solid sulfur compounds formed from reaction of LZ coal with CuFe2O4 at the base factor values and calculated using Eq.(6) below. m =1−

[ H 2 S ] + [ SO2 ] + [COS ] St

(6)

And in Eq.(6), St was the total sulfur moles involved in LZ original coal. [H2S], [SO2], [COS] were the moles of H2S, SO2 and COS formed during reaction of LZ coal with CuFe2O4. And then, ∆mi could be obtained through subtraction of the solid sulfur compounds formed from reaction of LZ coal with CuFe2O4 at the different factor value by that fraction formed at the corresponding base value. Based on the relative sensitivity analysis method introduced above, reaction of LZ coal with CuFe2O4 at the different factors was simulated. And different relative sensitivity values

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ωi of the four selected factors on the formation of the solid sulfur compounds were calculated, as shown in Figure 5. Firstly, it could be observed that the sensitivity values of CuFe2O4 oxygen excess number Φ, reaction temperature T and concentration of the steam introduced R on the formation of solid sulfur compounds were negative, which implied that increase of these parameters would inhibit the formation of solid sulfur compounds during reaction of LZ coal with CuFe2O4 and thus was not desired for effective desulfurization in the CLC system. But as observed by other researchers

75, 76

, increase of these three factors were favorable to the

conversion of coal and CO2 capture in the CLC system. Therefore, the optimized conditions for effective CO2 capture as well as simultaneous desulfurization should be explored for the real CLC system. In addition, the sensitivity value of the system pressure on the formation of solid sulfur compounds was positive, which meant increasing pressure was beneficial to the solid sulfur formation, which was the case with the previous analysis 18, 19 though the reaction system differed. Meanwhile, the system pressure was found out to benefit the conversion of coal and CO2 capture

35

. Therefore, different from the former three factors, including the

oxygen excess number, reaction temperature and concentration of the steam introduced, relevant increase of the system pressure was beneficial for both CO2 capture and desulfurization in a CLC system. Furthermore, the sensitivity values of these four factors were compared and their significance effects on the formation of solid sulfur compounds during reaction of LZ coal with CuFe2O4 were quantitatively evaluated. The bigger sensitivity value ωi meant the higher significant effect of the selected factor on the formation of solid sulfur compounds. As shown in Figure 5, the highest sensitivity value of the CuFe2O4 oxygen excess number Φ was up to 0.6. As followed to Φ was the reaction temperature with its sensitivity coefficient approaching 0.35. But both the sensitivity coefficients of the system pressure and the concentration of the 22


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steam introduced was below 0.15. Therefore, the oxygen excess number Φ was ascertained as the most significant factor to form the solid sulfur species and thus should be preferably considered and evaluated in the realistic CLC system to simultaneously realize CO2 capture and in-situ desulfurization.

3.5.3 Optimized conditions determined for simultaneous CO2 capture and in-situ desulfurization Finally, based on the relative sensitivity analysis of the four different factors on the solid sulfur compounds formation above, the optimized reaction conditions were further explored so as to simultaneously realize CO2 capture and effective desulfurization in the CLC system. Since the most significant factor on the solid sulfur formation was CuFe2O4 oxygen excess number Φ, the research focus here was concentrated upon the CuFe2O4 oxygen excess number Φ with its values changing from 0 (i.e pyrolysis of LZ coal without CuFe2O4 added) to 2.0. According to the previous researches from the realistic CLC system, the other three factors were fixed at the values of the realistic CLC system of interest, including the reaction temperature T = 900 oC 9, 28, system pressure P = 5 bar 33, 35 and the concentration of the steam introduced R = 20% 76. And distribution of various sulfur species during reaction of LZ coal with CuFe2O4 and the percentage of the solid sulfur compounds formed were presented in Figure 6. It could be observed from Figure 6 that, as the oxygen excess number Φ from 0 to 0.50, the solid sulfur fraction increased fast from 76.98 % to 98.67%, where more Cu2S was formed, but the residual FeS formed from pyrolysis of pyrite in LZ coal was decreased. And then, with the further increase of the oxygen excess number Φ from 0.5 to 1.50, the fraction of the solid sulfur compounds increased much slowly as shown in Figure 6. But at this reaction period, the solid sulfur compounds changed greatly, where most of the sulfur involved in LZ coal was decomposed and liberated out and further reaction with CuFe2O4 to form Cu2S. As the 23


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increase of the oxygen excess number Φ is stabilized around 1.5, the fraction of the solid sulfur formed was increased to above 99.73%, where the sulfur present in LZ coal was completely converted and fixed as Cu2S. And then, according to the process for cyclic syngas desulfurization and regeneration using high temperature Cu-based sorbent

77

, if the formed

Cu2S was separated out of the reduced CuFe2O4 OC and further regenerated using post oxygen polishing, either SO2 formed in the FR or AR could be avoided, and thus the potential harms either to the purity of CO2 in the flue gas of FR or the environmental harm in the AR could be eliminated. Meanwhile, after oxidation of Cu2S, the formed SO2 could be recovered for other industrial use, such as sulfuric acid production78, while the formed CuO could be used for later CLC cycle. But further increase of the oxygen excess number Φ to 2.0, SO2 was found out and increase with the oxygen excess number Φ, while the formed Cu2S was decreased due to its further oxidation to form SO2, where the formed SO2 would not only mix with CO2 and influence the purity of the sequestrated CO2 17, but also caused great corrosion to the pipeline during CO2 transformation 79. In addition, due to the close critical constants of SO2 to that of CO2, separation of SO2 out of the CO2 stream was energy intensive

80

.

Therefore, in order to simultaneously realize CO2 capture and in-situ desulfurization during reaction of LZ coal with CuFe2O4, the CuFe2O4 oxygen excess number Φ should be strictly controlled around 1.5, and related research in a more realistic CLC system would be conducted in the following project. Overall, through FTIR analysis of the evolution of the gaseous species, XPS characterization of the solid sulfur compounds combined with thermodynamical simulation of the LZ coal reaction with CuFe2O4, insightful knowledge on the migration and redistribution of sulfur species in the CLC system was provided and effective in situ desulfurization in the CLC system was preliminarily explored, which was much beneficial to abate the harms from the sulfur species in the CLC systems.

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4. Conclusions Occurrence of sulfur in the CLC system is always a great concern. In order to gain a comprehensive insight into the migration and redistribution of sulfur species and thus to control the sulfur harms, reaction of LZ coal of large size range with CuFe2O4 was investigated using TGA. And the evolution and redistribution of both gaseous and solid sulfur species were further systematically studied through both gaseous FTIR analysis, solid XPS characterization and thermodynamic investigation. In addition, in order to realize the simultaneous CO2 capture and in-situ desulfurization, the optimized reaction condition for reaction of LZ coal with CuFe2O4 was explored. The main conclusions as such reached were listed below. (1) The properties of LZ coal with large size around 180-400 µm changed greatly due to the maceral enrichment and mineral segregation during grinding and sieving. (2) TGA experiment for CuFe2O4 reaction with LZ coal of large size indicated that though two main reaction stages were underwent similar to that of CuFe2O4 with LZ coal of small size (63-106 µm) in our previous research, the reaction characteristic temperatures at these two stages were shifted to higher direction and the reaction rates were improved. (3) Gaseous FTIR analysis, XPS characterization combined with thermodynamic investigation provided the comprehensive knowledge on the migration and redistribution of sulfur species during reaction of LZ coal, which indicated that most of the sulfur liberated from LZ coal were converted to solid Cu2S. Meanwhile, the thiophene organic sulfur should be paid to enough attention due to its strong thermal stability and potential harm. (4) Finally, in order to realize the simultaneous CO2 capture and desulfurization during LZ coal reaction with CuFe2O4, formation of the solid Cu2S was desired and its most significant factor was determined as CuFe2O4 oxygen excess number Φ, which should be strictly controlled around 1.5.

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Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 51276210, 50906030), the Foundations of Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Key R&D program of Henan Province (Nos.162102210233, 142100210459), Innovative Research Team in S&T

in University of Henan Province

(No.16IRTSTHN017) and in Fluid hydromachine & transportation engineering, Scientific Innovation Talent of Henan Province(No.154100510011), North China University of Water Resources and Electric Power (No.70481). Meanwhile, the support provided by the China Scholarship Council (CSC 201508410060) is appreciated.

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List of Tables and Figures: Table 1. Properties of LZ coal sample studied; Table 2. FTIR peaks for the characteristic temperature of the main gaseous sulphur species evolved from pyrolysis of LZ coal under the N2 atmosphere and its reaction with CuFe2O4 OC

Table 3. Sulphur forms present in LZ original coal determined by XPS and Chinese national standard method (GB).

Figure 1. Reaction of LZ coal with CuFe2O4 OC; Figure 2. Gaseous sulfur species evolved from reaction of LZ coal with CuFe2O4; Figure 3. XPS analysis of the solid sulfur compounds formed from reaction of LZ coal with CuFe2O4;

Figure 4. Equlibrium distribution of various species for the reaction of LZ coal with CuFe2O4;

Figure 5. Sensitivity analysis of effect from main influencing factors on the distribution of sulfur species during reaction of LZ coal with CuFe2O4.

Figure 6. Effect of the CuFe2O4 oxygen excess number Φ on the sulfur distribution during its reaction with LZ coal.

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Table 1. Properties of LZ coal sample studied Ultimate analysis (wt.%, adb) Proximate analysisa (wt.%) Mad

Vad

Aad

FCad

C

H

N

S

Oc

LHVd (MJ/kg)

0.31 16.51 37.86 45.32 40.16 1.63 0.71 4.52 14.81 24.235 Variation of the ratios for LZ coal properties LZ particle size (µm) V/A FC/A C/H (atomic ratio) 63-106e 0.402 0.973 1.891 180-400 0.436 1.197 2.053 a M: moisture content; V: volatile matters; A: ash content; FC: fixed carbon; ad: air-dried basis; b: dry basis; c: the O content was determined by difference; d: lower heating value; e: On the basis of the properties of smaller size LZ coal in our previous researches, as shown in references [36-38].

Table 2. FTIR peaks for the characteristic temperature of the main gaseous sulphur species evolved from pyrolysis of LZ coal under the N2 atmosphere and its reaction with CuFe2O4 OC. Sample Gaseous Characteristic temperature Tx a (oC) sulphur Peak 1 Peak 2 Peak 3 Peak 4 SO2 332.2 472.5 577.8 718.1 12.57 62.60 14.91 9.93 LZ Peak 1 Peak 2 Peak 3 CS 2 pyrolysis COS

Peak 1 396.59.46 Peak 1 307.63.53

492.377.04 Peak 2 495.184.84 Peak 2 413.535.67

623.517.24 Peak 4 612.65.70 Peak 4 669.722.56

701.75.72

Peak 3 Peak 5 Peak 6 LZSO2 541.725.92 763.47.55 839.84.76 CuFe2O4 a : subscript numbers below the peak characteristic temperature T represented the relative percentage content of related sulfur sources.

Table 3. Relative sulphur contents of different sulphur compounds present in LZ original coal determined by XPS and Chinese national standard method (GB). Different test method Relative contents of the different solid sulphur compounds (%) Sp Ss So GB 180-400 µm 49.78 11.95 38.27 63-106 µm a 65.81 5.87 28.32 XPS (180-400 µm) 44.96 22.64 Saliphatic Sthiophene Ssulfone 10.71 13.15 8.54 a : On the basis of the properties of smaller size LZ coal in our previous researches, as shown in references [36-38].

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Figure 1. Reaction of LZ coal with CuFe2O4 at Φ=1.0: (a) weight loss; (b) weight loss rate of LZ pyrolysis under the N2 atmosphere; (c) weight loss rate of LZ coal reaction with reference oxides CuO and Fe2O3; (d) weight loss rate of LZ coal reaction with CuFe2O4 combined OC.

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Figure 2. Gaseous sulphur species evolotion as a function of time: (a) SO2 for LZ pyrolysis; (b) CS2 for LZ pyrolysis; (c) COS for LZ pyrolysis; (d) SO2 for LZ reaction with CuFe2O4.

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Figure 3. XPS analysis of the solid sulfur species formed during reaction of LZ coal with CuFe2O4. (a) LZ original coal; (b) solid products fromed from reaction of LZ coal with CuFe2O4 OC at Φ=1.

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Figure 4. Thermodynamic simulation of LZ coal with CuFe2O4 OC (Ф = 1, P = 1 bar, R = 0): (a) Equilibrium distribution of C-containing species, (b) Equilibrium distribution of Cucontaining species (Ф=1), (c) Equilibrium distribution of Fe-containing species, (d) Equlibrium distribution of various sulfur species.

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Figure 5. Sensitivity analysis of effect from the main influencing factors on the distribution of solid sulfur compounds during reaction of LZ coal with CuFe2O4.

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Figure 6. Effect of CuFe2O4 oxygen excess number Φ on the sulfur distribution during its reaction with LZ coal (Reaction temperature T: 900 oC; system pressure P 5 bar; steam cocentration R: 20%).

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Migration and Redistribution of Sulfur Species during Chemical

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