Sulfurization-Desulfurization of Iron-Calcium Oxygen Carriers during Chemical Looping Combustion of Syngas

Chemical looping combustion of SO 2 -containing syngas with various fabricated iron-calcium oxygen carriers was studied to comprehend the sulfurization and desulfurization of oxygen carriers during combustion. Experimental results indicated calcium-iron oxygen carriers fabricated with CaO:Fe 2 O 3 weight ratio of 3:7 and calcined at 800 ° C (CaFe-378) was suitable for chemical looping combustion of syngas. CaFe 2 O 4 was noticed to be the dominant crystalline phase in CaFe-378 even after 10 redox operations. Reduction kinetics for chemical looping combustion of both H 2 or CO in syngas with CaFe-378 were adequately described by the 2-dimension diffusion model. The activation energies of H 2 and CO with CaFe-378 were determined to be 35.4 kJ mol –1 and 46.0 kJ mol –1 , respectively. Calcium sulfide (CaS) was generated during the chemical looping combustion of SO 2 -containing syngas with CaFe-378. CaS was further oxidized to CaSO 4 on the surface of CaFe-378 during air regeneration, which might inhibit the complete regeneration of iron oxides. Satisfactory regeneration of sulfurized oxygen carriers was accomplished by introducing hydrogen after the air-regeneration because of the reduction of calcium sulfate to calcium oxide.


INTRODUCTION
Chemical looping process (CLP) is extensively studied recently on the combustion of various fuels and considered to be a novel alternative for fuel combustion to achieve efficient energy generation as well as to generate high purity carbon dioxide for inherent CO2 separation.Oxygen carriers utilized for chemical looping process plays an important role and is served as oxygen supplier for the oxidation of hydrogen, carbon and sulfur presented in the fuel during the combustion process.Thus, oxygen carriers should exhibit high reaction rate, high oxygen carrying capacity, great mechanical strength, and long-term recyclability for applications of chemical looping process.Because the presence of various components in the fuel, mainly sulfur and nitrogen, various air pollutants thus may be generated during chemical looping process that requires further concerns.The reactivity and recyclability of oxygen carriers might also be greatly reduced due to the poisoning by air pollutants in flue gas.Wang et al. (2019a) indicated that SO2 was always the dominant product during the chemical looping gasification process of petroleum coke by hematite.Chung et al. (2017) reported that sulfur emission was composed mainly of H2S and SO2 for a coal-direct chemical looping combustion (CLC) system.The ratio of H2S/SO2 in flue gas from the fuel reactor was about 0.5, while only SO2 was observed in flue gas from the air reactor.Some sulfur deposition on the oxygen carriers was also noticed.However, Adánez-Rubio et al. (2014) stated that most sulfur was exited as SO2 in flue gas from the fuel reactor; although only a small amount of SO2 was noticed in flue gas from the air reactor for a chemical looping with Oxygen Uncoupling (CLOU) process using lignite as fuel.Iron oxide, lime or limestone, copper oxide, manganese oxide and zinc oxide are commonly Volume 24 | Issue 2 | 220398 employed as oxygen carriers for chemical looping process (Mattisson et al., 2004); however, most of these metal oxides are also capable sorbents for sulfur compounds (such as H2S or SO2) in flue gas from the fuel reactor.Various kinetic models are developed for describing the behavior of different oxygen carriers during sulfurization and regeneration operations (Huang et al., 2008;Zeng et al., 2015).
Calcium oxide is a very common sorbent for flue gas desulfurization (Ishizuka et al., 2000;Ishizuka and Buryan, 2013;Yeo et al., 2019).It is rather cheap compared with other metal oxides, and can be employed for high temperature flue gas.Zheng et al. (2010) reported that CaS was generated during the reaction of calcium oxide with sulfur dioxide.Nevertheless, CaS might be further oxidized to form calcium sulfate (CaSO4) on the surface of CaS particle, which might block oxygen diffusion for further oxidation of CaS (Wang et al., 2015).Iron-Calcium based oxygen carriers have been proved to have excellent catalytic reactivity and reaction stability in the coal CLG process (Wei et al., 2019).Experimental results reported by Ismail et al. (2016) and Sun et al. (2018) indicated that CaFe2O4 or Ca2Fe2O5 composite crystal phases improved the reducibility, oxidation activity, and cyclic stability of oxygen carriers effectively for chemical looping combustion and hydrogen production.Xue et al. (2019) and Wang et al. (2019b) also employed the bimetallic CaO/Fe2O3 oxygen carriers to generate syngas through the chemical looping gasification process, and both reported that CO selectivity was enhanced when the composite iron and calcium oxide phases (CaFe2O4 or Ca2Fe2O5) were formed in the oxygen carriers.
In this study, various calcium-iron oxygen carriers were fabricated to combust with SO2-containing syngas in order to comprehend the sulfurization of the fabricated calcium-iron oxygen carriers with sulfur present in syngas during chemical looping combustion.Various kinetic models were employed for describing the reduction behavior of fabricated oxygen carrier for chemical looping combustion of syngas.The composition and morphological variations of fabricated oxygen carriers were examined.Furthermore, the reduced oxygen carriers after chemical looping combustion was regenerated and employed repeatedly.

EXPERIMENTAL
In this research, various CaO/Fe2O3 oxygen carriers were formulated with hematite (99.9% Fe2O3, China Steel) and calcium hydroxide (Ca(OH)2, Acros).Predetermined amounts of hematite and calcium hydroxide particles were mixed thoroughly in deionized water at room temperature.The weight percentage for CaO/Fe2O3 particles was determined from 30/70 to 70/30 (the first digital on the label of fabricated oxygen carriers designates the wt% of calcium hydroxide while the second digit designates the wt% of hematite, for instance, CaFe-37 indicates the oxygen carriers contains 30% calcium hydroxide and 70% hematite).The well-mixed slurry was desiccated at 130°C for 10 hours, and was subsequently crushed and partitioned for particles of size between 1.2 and 1.4 mm.The CaO/Fe2O3 particles were later sintered at temperature from 600 to 900°C (the final digital on the label of fabricated oxygen carriers designates the sintering temperature, for instance, CaFe-378 indicates the oxygen carriers was sintered at 800°C) in a muffle furnace for 2 hours.The fabricated oxygen carriers was subsequently characterized using a D2 PhASER X-Ray Diffractor by Bruker, a JSM-6500F Field Emission Scanning Electron Microscope (FE-SEM) by JOEL, and an Autosorb-1 Surface Area and Porosimetry System (BET) by Quantachrome.
The reactivity of fabricated oxygen carriers was analyzed by a Netzsch STA 449F3 thermogravimetric analyzer (TGA).A portion of 200 mg fabricated CaO/Fe2O3 oxygen carriers was loaded in an alumina crucible of the TGA, the temperature of the TGA chamber was elevated with a ramping rate of 30°C min -1 in air atmosphere and eventually kept at 900°C for 30 minutes.The 200 mL min -1 syngas composed of (10% H2 + 10% CO and 80%N2) was introduced for 20 minutes to reduce oxygen carriers in the period of reduction step.Continuously, 200 mL min -1 N2 was purged for 5 minutes to remove the fuel gas in the last part.After, the oxidation process was introduced 200 mL min -1 of air for 10 minutes.The reduction and oxidation procedures were replicated for up to 10 cycles to determine the recyclability of fabricated oxygen carriers.
For the study of sulfidation-regeneration of fabricated CaO/Fe2O3 oxygen carriers, syngas containing 1,000 ppm SO2 was introduced into the TGA chamber.The weight increase of fabricated oxygen carriers indicates the possibly formation of calcium sulfate.Thus, 20% H2 in nitrogen environment was introduced into the TGA chamber to remove sulfates before air was introduced for the regeneration of oxygen carriers.
The outlet gas from the reactor was passed through a cold trap to condense steam and was sampled at each intermittent period of operation time by a gas sampling valve.Oxygen, methane, carbon monoxide, carbon dioxide, and sulfur dioxide contents in the inlet and outlet gases were analyzed by a Non-dispersive Infrared Sensor (NDIR, Molecular Analysis 6000i).The hydrogen content was determined by a gas chromatograph equipped with a thermal conductivity detector (GC/TCD, China Chromatograph 2000).

RESULTS AND DISCUSSION
Various CaO/Fe2O3 oxygen carriers with the size of 8 to 18 mesh were fabricated in this study to study the reactivities with syngas.Experimental results indicate that the size of fabricated oxygen carriers only slightly influenced the reactivities in this study.Fig. 1 compares the highest reactivities achieved for various fabricated CaO/Fe2O3 oxygen carriers with syngas examined in a TGA.Syngas was introduced at 50 minutes of operating time after the temperature of TGA was maintained constant at 900°C.As demonstrated in Fig. 1, the first-order differential thermogravimetric (DTG) values indicated the weight loss was constant first 9 minutes for all fabricated CaO/Fe2O3 oxygen carriers during reduction.A sudden weight loss occurred decrease at 10 minutes after fuel gas was introduced; subsequently, oxygen carriers with higher Fe2O3 contents exhibited higher oxygen releases during reduction.Analyses of flue gas revealed most hydrogen and carbon monoxide were oxidized by oxygen released from fabricated oxygen carriers.
The reactivity of various fabricated CaO/Fe2O3 oxygen carriers were examined in the TGA operated at 900°C using syngas as reducing gas for ten successive redox cycles.The conversions of fabricated oxygen carriers during the oxidation and reduction were calculated as: where m o is weight of fully-oxidized oxygen carriers; m r is weight of fully-reduced oxygen carriers; m(t) is weight of oxygen carriers at period of t of each stage.
As depicted in Fig. 2, CaO modified Fe2O3 oxygen carriers demonstrated better reactivity and prolonged stability during redox cycles compared to unmodified Fe2O3.The conversions of fabricated CaO/Fe2O3 oxygen carriers remained between 40 and 70% for ten redox cycles.The conversion of pure Fe2O3 was 55% for first cycle and reduced to roughly 30% for 10 th cycle.indicating that the fabricated CaO/Fe2O3 oxygen carriers are capable of providing reasonable reactivity and recyclability for continuous redox operation of syngas combustion.
Experimental results revealed in Fig. 2 suggest that CaFe-378 and CaFe-737 oxygen carriers exhibited better stability and recyclability by delivering more than 60% conversions over multiple redox cycles, while conversions decreased evidently for some fabricated oxygen carriers after replicated operations.However, more serious attrition of CaFe-737 was observed during the redox cycles, compared to CaFe-378; therefore, CaFe-378 oxygen carriers were selected over CaFe-737 for following experiments.
The weight variation of CaFe-378 oxygen carriers reduced by syngas in the TGA is illustrated in Fig. 3.The slope of the curve (weight loss rate) shown in Fig. 3 indicates the phase transformations occurred at 3 and 50 minutes.Roughly 20 wt% of oxygen contained in CaFe-378 was rapidly released in first 3 minutes, indicating Fe2O3 was reduced to Fe3O4 in this step.Approximately 72 wt% of oxygen was released during 3 to 50 minutes while Fe3O4 was reduced to FeO.The remaining 8 wt% of oxygen was released during 50 to 150 minutes suggesting the reduction of FeO to Fe.The conversions of hydrogen and carbon monoxide by CaFe-378 oxygen carriers reducing with syngas at 800-950°C were calculated.By fitting with different reaction models, the 2-dimension diffusion (D2) model is adequate to illustrate the conversions and to describe the reduction behavior of hydrogen and carbon monoxide by CaFe-378 oxygen carriers.The D2 diffusion model assumes a cylindrical shape for the reactants and considers reactions occurring between the lattices or with molecules that need to penetrate into the lattice, which occured radially through the cylindrical shell with an increased reaction area.The CaFe-378 oxygen carrier had a primarily cylindrical structure.It was observed that the D2 model provides a high correlation coefficient of over 98% when fitting to all experimental data.Hence, the D2 model is suitable for describing the reduction behavior of CaFe-378 during hydrogen and carbon monoxide reduction.The calculated   1.The reduction activation energy of H2 and CO by CaFe-378 oxygen carriers was therefore calculated by Arrhenius Equation and determined to be 35.4kJ mol -1 and 46.0 kJ mol -1 , respectively.The fabricated CaFe-378 oxygen carriers reduced by syngas were sampled at various reacting times for phase characterization by XRD.As shown in Fig. 4, CaFe2O4 was noticed the dominant crystalline phase of the fresh fabricated CaO/Fe2O3 oxygen carriers, while Ca2Fe2O5 and Fe2O3 phases were also observed.After first 3 minutes of reduction, CaFe2O4 was still the dominant crystalline phase; however, it was gradually decomposed to Fe2O3 and CaO while Fe2O3 served as the oxygen carriers for this chemical looping operation.After reduction for 20 minutes, Ca2Fe2O5 phase was becoming a major crystalline phase possibly because of the combination of CaO with remaining CaFe2O4.For oxygen carriers reduced for more than 40 minutes, Fe and CaO were noticeably observed while the presence of Ca2Fe2O5 and CaFe2O4 were markedly decreased.For oxygen carriers reduced for 150 minutes, only Fe and CaO crystalline phases were identified indicating that CaFe-378 oxygen carriers were completely reduced.Therefore, the reduction of CaFe-378 oxygen carriers is suggested by following reactions: The phase characterization by XRD for oxygen carriers sampled after 10 redox cycles indicated as demonstrated in Fig. 5, CaFe2O4 was still the dominant crystalline phase in regenerated oxygen carriers even after 10 redox cycles, implying the excellent recyclability of fabricated CaFe-378 oxygen carriers.However, the fraction of CaFe2O4 was somewhat declined, indicating the oxygen carriers was not completely regenerated.
Fig. 6 depicted the XRD patterns of CaFe-378 oxygen carriers at various reduction times for chemical looping combustion of syngas containing 1,000 ppm SO2.CaS was identified at very early stage of reaction, and the amount of CaS was gradually increased during the reduction; thus, calcium oxide was reacted with SO2 to form CaS, as expressed by Reaction (9).
Fig. 7 illustrates the weight variation of CaFe-378 oxygen carriers during three sulfurizationregeneration cycles by introducing SO2-containing syngas.A slight weight loss was noticed whenever the gas flow was switched.The reason for the slight weight loss might be resulting from that the TGA system with 500 mg oxygen carriers was very sensitive to gas flow variation resulted from the operational switch.Sharp weight loss was observed in the first 15 minutes for each sulfurization cycle because of the reduction of oxygen carriers by SO2-containing syngas.The weight of oxygen carriers was noticed to be slightly increased after first cycle, also demonstrating the formation of heavier calcium sulfide molecules.Reduced CaFe-378 oxygen carriers were then regenerated under the air atmosphere, most iron compounds (Fe3O4, FeO, and Fe) are oxidized to Fe2O3 for further fuel combustion.However, calcium sulfide was also oxidized during regeneration:  As demonstrated in Fig. 7, the mass of regenerated oxygen carriers were markedly decreased with increasing operation cycle, indicating the conversions for regenerated oxygen carriers were also noticeably reduced, similar to the observations reported by Wang et al. (2015).This occurrence suggests the possible formation of CaSO4 on the surface of regenerated oxygen carriers during CaS oxidation, which might inhibit oxygen diffusion for the complete regeneration of iron oxides.

CaS +2O2 → CaSO4
(11) Considering Reactions ( 10) and ( 11), the presence of excessive oxygen may enhance the formation of CaSO4 instead of CaO; therefore, adequate oxygen input is vital for the air regeneration of oxygen carriers.
In this research, further regeneration (desulfurization) of oxygen carriers was performed by introducing hydrogen to reduce CaSO4 formed during air regeneration, possibly as described by following reaction: The CaSO4-CO-H2 reaction system within the temperature range of 800 and 1,000°C was thoroughly studied by previous researchers (Zeng et al., 2017), the main sulfur-containing gases released were reported to be SO2 and H2S, while COS generation was much smaller.Weight variations of CaFe-378 oxygen carriers during ten sulfurization-regeneration operation with hydrogen is shown in Fig. 8, indicating that the conversion of hydrogen-desulfurized oxygen carriers was maintained relatively consistent and suggesting CaSO4 was reduced removed effectively by hydrogen.Fig. 9 exhibits the 5000x SEM surface morphology images of fresh CaFe-378 oxygen carriers, oxygen carriers after sulfurization (CaFe-378-S), and oxygen carriers after sulfurization/hydrogen regeneration (CaFe-378-S-H), for chemical looping combustion of syngas containing SO2.As demonstrated in Fig. 9, the observed size of granules was significantly increased for regenerated oxygen carriers, indicating the occurrence of severe sintering of the oxygen carriers during redox operation.Some smaller fragments were also noticed in Fig. 9(b) for oxygen carriers after sulfurization (CaFe-378-S), indicating the formation of CaSO4 particles which might inhibit the regeneration of oxygen carriers.After hydrogen regeneration, the small fragments were disappeared from regenerated oxygen carriers (CaFe-378-S-H) and transformed to more globular crystalline, as shown in Fig. 9(c).Table 2 showed the determined specific surface area (as), total pore volume (Vp), and mean pore diameter (Dp) of fabricated CaFe-378 oxygen carriers obtained at different operating stages.The specific surface area of fresh CaFe-378 was demonstrated to be highest, which was decreased obviously because of the sintering occurred during the redox operations, similar to the SEM observations.The specific surface area, total pore volume, and mean pore diameter of regenerated oxygen carriers (CaFe-378-S and CaFe-378-S-H) were fairly comparable after the redox operations.

CONCLUSIONS
Experimental results indicated fabricated CaFe-378 was suitable calcium-iron oxygen carriers for chemical looping operation, considering reactivity, recyclability and mechanical strength.CaFe2O4 was the dominant crystalline phase in CaFe-378 oxygen carriers even after 10 redox cycles, implying the excellent recyclability of fabricated oxygen carriers.However, the fraction of CaFe2O4 was somewhat declined, indicating the oxygen carriers was not completely regenerated.The reduction of CaFe-378 with both hydrogen or carbon monoxide in syngas could be adequately expressed by 2-dimension diffusion model because of the structure of CaFe-378 oxygen carriers is basically cylindrical.The reduction activation energies of hydrogen or carbon monoxide with fabricated CaFe-378 oxygen carriers were determined to be 35.4kJ mol -1 and 46.0 kJ mol -1 , respectively.
Calcium sulfide was identified to be generated in the fabricated CaFe-378 oxygen carriers at very early stage of chemical looping combustion of SO2-containing syngas.Calcium sulfide might be further oxidized to generate calcium sulfate because of the presence of excessive oxygen during the air regeneration of oxygen carriers, which would inhibit the complete regeneration of iron oxides.Therefore, adequate oxygen input during air regeneration of oxygen carriers is critical.Satisfactory regeneration of sulfurized oxygen carriers was accomplished by introducing hydrogen after air-regeneration because of the reduction of calcium sulfate to calcium oxide.The observed size of granules, specific surface areas, total pore volume, and mean pore diameter of regenerated oxygen carriers were observed to be fairly comparable after the redox operations.

Fig. 3 .
Fig. 3. Weight variation for the reduction of CaFe-378 conducted with syngas.

Fig. 4 .
Fig. 4. XRD patterns of CaFe-378 oxygen carriers at various reduction time for chemical looping combustion with syngas.

Fig. 5 .
Fig. 5. XRD patterns of fresh CaFe-378 oxygen carriers, and oxygen carriers after 1 and 10 redox cycles for chemical looping combustion with syngas.

Fig. 6 .
Fig. 6.XRD patterns of CaFe-378 oxygen carriers at various reduction time for chemical looping combustion with SO2-containing syngas.

Table 1 .
Rate constants based on D2 model for hydrogen and carbon monoxide conversions by CaFe-378 oxygen carriers.

Table 2 .
The specific surface area (as), total pore volume (Vp), and mean pore diameter (Dp) of fabricated oxygen carriers at various stages.