Zhong Ma, Chuan Yuan, Shuai Zhang, Yonggang Lu, Junhui Xiong

1 School of Energy & Power Engineering, Jiangsu University, Zhenjiang 212013, China

2 State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan 750021, China

3 Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China

Keywords:Chemical looping Pyrite cinder Supports Fixed-bed Redox performance Waste treatment

ABSTRACT Chemical looping combustion (CLC) is a clean and efficient flame-free combustion technology, which combust the fuels by lattice oxygen from a solid oxygen carrier with inherent CO2 capture.The development of oxygen carriers with low cost and high redox performance is crucial to the whole efficiency of CLC process.As the solid by-product from the sulfuric acid production,pyrite cinder presented excellent redox performance as an oxygen carrier in CLC process.The main components in pyrite cinder are Fe2O3,CaSO4,Al2O3 and SiO2 in which Fe2O3 is the active component to provide lattice oxygen.In order to systematic investigate the functions of supports (CaSO4, Al2O3 and SiO2) in pyrite cinder, three oxygen carriers (Fe2O3-CaSO4, Fe2O3-Al2O3 and Fe2O3-SiO2) were prepared and evaluated in this study.The results showed that Fe2O3-CaSO4 displayed high redox activity and cycling stability in the multiple redox cycles.However, both Fe2O3-Al2O3 and Fe2O3-SiO2 experienced serious deactivation during redox reactions.It indicated that the inert Fe-Si solid solution (Fe2SiO4) was formed in the spent Fe2O3-SiO2 sample, which decreased the oxygen carrying capacity of this sample.The XPS results showed that the oxygen species on the surface of Fe2O3-CaSO4 could be fully recovered after the 20 redox cycles.It can be concluded that CaSO4 is the key to the high redox activity and cycling stability of pyrite cinder.

1.Introduction

Climate change as the core of global environmental problem is becoming more and more serious [1].Reducing carbon dioxide emissions from coal-fired power plants to mitigate global warming has become the focus of international community [2-4].Carbon capture and storage (CCS) technology is an important strategic measure to reduce CO2emission[5-7].Obtaining a high concentration of CO2stream from the flue gas is the most important part of the whole CCS process.Traditional CO2capture techniques include post-combustion, pre-combustion and oxy-fuel combustion[8-10], which exhaust a flue gas containing CO2diluted greatly by the presence of N2in air.Therefore, the above three traditional CO2capture technologies need high CO2separation energy consumption and cost, which will cause the decline in power generation performance.Chemical looping combustion (CLC) is a clean and efficient flame-free combustion technology [11].Compared with traditional combustion process, the biggest advantage of this technology is that it avoids direct contact between fuels and air by means of the circulation of solid oxygen carriers between air reactor and fuel reactor, which presents the essential characteristic of inherent CO2separation and advantage in reducing the energy consumption of CO2capture[12-14].In fuel reactor,the lattice oxygen released by oxygen carrier react with fuels to produce CO2and H2O.Then the oxygen-depleting carrier is re-oxidized to recover the lattice oxygen by gaseous oxygen in air reactor.The direct contact between air and fuels is avoided through using an oxygen carrier.As a result, the gas discharged from the fuel reactor ideally contains CO2and H2O without diluted by N2in the air.By condensing steam, high purity CO2is obtained, avoiding the high energy consumption required for CO2separation.

Oxygen carrier(OC)is the cornerstone in CLC process.The oxygen carrier transmits the oxygen required for fuels conversion from the air reactor to the fuel reactor,while providing the heat needed to maintain the reaction in the fuel reactor [15].Ideal oxygen carriers should possess the following characters: enough oxygen carrying capability, high activity in reduction-oxidation reactions,high cycling stability and mechanical strength, low cost and environmentally friendly [16,17].A variety of oxygen carriers were developed and evaluated, such as natural ores [18-20], synthetic oxygen carriers [21-23], industrial solid waste [24-27], etc.By adding the inert supports, the synthetic oxygen carriers (Fe, Cu,Ni, Mn and Co) present promising prospect in the CLC process[28,29].But the high cost limits the application of synthetic oxygen carriers.For reducing the cost of oxygen carriers preparation,lowcost and readily available oxygen carriers are more attractive to chemical looping applications.Based on this, many researchers have focused on finding natural minerals and industrial residues that act as oxygen carriers, such as iron, manganese, copperbased ores [18-20], and industrial by-products [24-27].Among the numerous natural minerals,ilmenite (FeTiO3) has been widely tested and has proven to be a promising oxygen carrier for CLC[17,20,30].However,serious problems with ilmenite are low reactivity and phase segregation during the redox reactions [31-33],which result in longer reaction times and large inventory to achieve high fuels conversion efficiency, thus increasing reactor size and investment costs.

Pyrite cinder is a solid by-product from the industry of sulfuric acid production and consists mainly of Fe2O3, CaSO4, Al2O3and SiO2[34-36].In China, more than 10 million tons of pyrite cinder is discharged each year due to the high demand for sulphuric acid and sulphate products, accounting for about 30% of all chemical waste [36].For a long time, the rich pyrite cinder resources have not been reasonable used, not only pollute the environment, but also occupy enterprise funds.With the increase of consciousness for environmental protection, it is particularly important to develop low-cost pyrite cinder utilization process.Our group has proven that pyrite cinder works well as an oxygen carrier in the process of CLC [34-36].The redox performance of pyrite cinder was studied during CH4and coal CLC,respectively.In pyrite cinder, Fe2O3is the active component to provide lattice oxygen in redox reactions.The components of CaSO4, Al2O3and SiO2are supports or dopants for Fe2O3.The chemical components of pyrite cinder are affected largely by the process of sulfuric acid production and the chemical components of pyrite, which influence the reactivity of pyrite cinder in redox reactions.It is not clear which support of pyrite cinder play a major role in the cyclic reactions.In order to screen pyrite cinder suitable for CLC process, it is necessary to explore the effect of supports on the reactivity and cycling stability of pyrite cinder in CL processes.

To this end, this work studies the effect of different supports(CaSO4,Al2O3and SiO2)on the redox performance of pyrite cinder.Three oxygen carriers (Fe2O3-CaSO4, Fe2O3-Al2O3and Fe2O3-SiO2)with the same Fe2O3content were prepared through mechanical mixing method.The redox activity and cyclic stability of the oxygen carriers were determined in a small fixed-bed reactor.

2.Materials and Methods

2.1.Material preparation

In this study,the oxygen carriers(Fe2O3-CaSO4,Fe2O3-Al2O3and Fe2O3-SiO2)were synthesized by the method of mechanical mixing as shown in our previous study[15].According to the actual composition of pyrite cinder (Fig.1), the mass percent of Fe2O3in the prepared oxygen carriers was 70%.The powders of Fe2O3, CaSO4,γ-Al2O3and SiO2,provided by Aladdin Chemical,were used to prepare oxygen carriers.The calcination temperature of 950 °C was used to calcine the mixed oxides in air atmosphere for 2 hours.Then the calcined oxides were sieved to the needed particle size(0.180-0.250 mm) before the redox experiment.

Fig.1.Chemical components of pyrite cinder.

2.2.Characterization

The samples were analyzed using a X-ray diffraction (XRD)instrument (Ultima IV) with Cu Kα radiation (wavelength = 1.541 8 nm)from 20° to 80°.A ASAP 2020 instrument was used to characterize the pore volume and specific surface area of oxygen carriers.The surface morphology of oxygen carriers was compared on a Scanning Electron Microscope(SU3500).The chemical state of O 1s for oxygen carriers was analysed by X-ray photoelectron spectroscopy (XPS).

2.3.Experimental setup and procedure

A small fixed-bed reactor was applied in this CLC experiment(Fig.2),and the reaction system was presented in the previous literature [15].In the redox cycle, oxidation process was performed using 1000 ml·min-1flow of gas consisting of 5% (vol) O2/N2.The reduction half cycle used a 1000 ml·min-1flow of gas containing 5% (vol) CO/N2.In the experiment, the mass of oxygen carrier was 8.0 g.The reaction temperature was still kept at 900 °C.A MRU multicomponent analytical instrument (VarioPlus) was used to measure the gas compositions of the redox cycles.

In the study, CO conversion can be calculated by the following equation:

In the above equation, MCO,inis the CO moles in the feeding gas,MCO,outis the CO moles in the product gas.

3.Results and Discussion

3.1.Microscopic physicochemical properties of fresh OC samples

The microstructure of fresh OC samples was characterized by BET (Brunauer, Emmett and Teller) and XPS.

Table 1 showed the BET surface area and pore volume of the prepared oxygen carrier samples.It indicated that the BET surface areas of Fe2O3-CaSO4, Fe2O3-Al2O3and Fe2O3-SiO2were 3.70, 3.82 and 4.71 m2·g-1, respectively.The Fe2O3-SiO2sample presented the largest surface area and pore volume.

Table 1 BET surface area and pore volume of fresh oxygen carriers

Fig.2.Diagram of reaction device in this study.

The results of XPS analysis of the O 1s peaks for fresh oxygen carriers were displayed in Fig.3.In Fig.3, the O 1s peaks were belonged to two species: lattice oxygen (OⅠ, 529.3 eV) and surface adsorbed or defective oxygen (OⅡ, 532.0 eV) [37,38].OⅠis beneficial to the complete oxidation of fuels.OⅡcan promote the partial oxidation in CL processes[37].Table 2 was the oxygen species percentages of fresh oxygen carriers based on the XPS calculation in Fig.3.By comparison, it can be seen that the oxygen species content in Fe2O3-CaSO4and Fe2O3-Al2O3are similar.The oxygen species in Fe2O3-SiO2were significantly different with that in the other two samples.The content of lattice oxygen and surface adsorbed or defective oxygen for Fe2O3-SiO2were 42.78% and 57.22%, respectively.The difference in oxygen species for the OC samples was due to the interaction between active component and support.

Table 2 Oxygen species percentages of fresh oxygen carriers based on the XPS calculation

3.2.Redox performance of OC samples

Fig.4 presented the redox activity of different oxygen carriers in the continuous cycle reactions.The CO conversion of Fe2O3-Al2O3and Fe2O3-SiO2were decreased rapidly with the increasing of redox cycle,which indicated that the two samples experienced significant deactivation in the continuous redox reactions.The 1st cycled CO conversion of Fe2O3-Al2O3and Fe2O3-SiO2were 99.97%and 83.94%, respectively.After the 20th cycle, the corresponding CO conversion of the two samples were decreased to 67.30% and 39.68%, respectively.For Fe2O3-CaSO4sample, the CO conversion in the 1st cycle was 80.92%.During the 2nd cycle, the reactivity of Fe2O3-CaSO4sample presented obvious increase, and the CO conversion was increased to 97.88%.In the following redox cycles,the CO conversion of Fe2O3-CaSO4showed some fluctuations without distinct decrease.The Fe2O3-CaSO4sample displayed a CO conversion at around 90% in the whole cycling experiment, which meant that this sample possessed higher redox performance in CO chemical looping combustion.

Through continuous cyclic experiments, it showed that Fe2O3-CaSO4had the best cyclic reaction performance among the three OC samples.A combined with a variety of characterization analysis was conducted to investigate the interaction mechanism between doping components and Fe2O3in redox reactions.

3.3.Microscopic physicochemical properties of spent OC samples

Fig.5 was the characterization results of microscopic morphology for the fresh and spent OC samples after the 20th cycle.From Fig.5a, all the three OC samples presented porous structure with different grain sizes.Some large grains were formed on the surface of fresh Fe2O3-CaSO4, which was due to the grains agglomeration in calcining process.The Fe2O3-SiO2sample had a flourishing pore structure, which was the reason this sample presented the largest values of surface area and pore volume.Compared with the SEM results of fresh samples in Fig.5a,significant change was occurred for the surface morphology of the spent OC samples.The 20th cycled Fe2O3-CaSO4sample presented obvious grain aggregation,which demonstrated that thermal sintering was occurred for this sample.However, a porous structure was still existed for the 20th cycled Fe2O3-CaSO4sample.The presence of rich porous structure is conducive to the diffusion of reaction gas into the inside of OC particles and improving the gas-solid reactions.The spent Fe2O3-Al2O3sample experienced a severe surface sintering,leading to material densification and formation of a gas-blocking layer.It increases the resistance of reaction gas diffusion and reduces the reaction activity of OC sample.Based on the available results,Fe2O3-Al2O3is prone to deactivation due to thermal sintering during CLC process[39-41].The content of Al2O3in the Fe2O3-Al2O3sample was only 30% (mass), which made this sample more prone to sintering deactivation.In addition, Fe cations enrichment on the surface of spent Fe2O3-Al2O3sample often appeared after multiple redox cycles[41,42],which further aggravated the surface sintering.In contrast, the surface structure of spent Fe2O3-SiO2sample was basically unchanged,which presented small grain size and obvious pores.However, the results in Fig.4 showed that the Fe2O3-SiO2sample experienced serious deactivation during continuous cycling reactions.The SEM results indicated that surface sintering was not the cause of Fe2O3-SiO2deactivation.

Fig.3.XPS analysis of the O 1s peaks for fresh oxygen carriers.(a) Fe2O3-CaSO4, (b) Fe2O3-Al2O3, (c) Fe2O3-SiO2.

Fig.4.CO conversion for the oxygen carriers during successive redox cycles.

The XRD patterns of fresh and the 20th cycled samples were given in Fig.6.From Fig.6a, both Fe2O3and CaSO4were showed in the fresh Fe2O3-CaSO4.After the 20 redox cycles, the diffraction peaks of CaSO4not appeared in the XRD pattern of spent Fe2O3-CaSO4.The crystal phases of Ca-Fe solid solutions (Ca4Fe14O25,CaFe2O4) were formed in the spent Fe2O3-CaSO4, which was because CaSO4was unstable in the redox reactions [36], resulting in producing Ca-Fe solid solutions by the solid reaction between iron oxides and CaO.Although CaSO4was decomposed after multiple cycles,no significant change in the cycling performance of this OC sample was occurred.For Fe2O3-Al2O3sample,the XRD patterns were stable for the samples before and after cycling reactions,which was attributed to the difficult solid reactions between iron oxides and Al2O3[41].From the XRD results, fresh Fe2O3-SiO2was constituted by Fe2O3and SiO2.Nonetheless, the crystal phase of Fe2SiO4was generated in the 20th cycled Fe2O3-SiO2, which could be found in some other studies [16,43,44].Solid-solid reaction (Eq.(6)) occurs between FeO and SiO2, which decrease the oxygen carrying capacity and redox performance of OC.It can ascribe the deactivation of Fe2O3-SiO2in redox cycles to the generation of inert Fe2SiO4.

Fig.5.SEM characterization of the fresh and 20th cycled oxygen carriers.

Fig.6.XRD patterns of fresh and the 20th cycled samples.(a) Fe2O3-CaSO4, (b) Fe2O3-Al2O3, (c) Fe2O3-SiO2.

The main disadvantage of CaSO4as oxygen carrier is the existence of side reactions releasing gaseous sulfur (SO2) and producing CaO in fuel reactor or in air reactor [45], as shown below(Eqs.(2)-(5)).Under the condition of CLC, the solid-solid reaction(Eq.(3))is dependent on the operating conditions,such as reaction temperature and gas composition.According to the previous study[46],the main sulfur release was in the oxidation half cycle of CLC.When CaSO4is decomposed to CaO, the solid-solid reactions (Eqs.(7) and (8)) between calcium oxide and iron oxide is easy to form Ca-Fe solid solutions.

Fig.7 showed the O 1s peaks of spent samples from XPS analysis.Table 3 was the oxygen species percentages of spent oxygen carriers based on the XPS calculation in Fig.7.Compared with the results in Table 2, the oxygen species percentages in Fe2O3-CaSO4were almost no change after 20 redox cycles.The increasing of OⅡand decreasing of OⅠwere observed for spent Fe2O3-Al2O3and Fe2O3-SiO2samples, which meant that the oxygen species on the surfaces of Fe2O3-Al2O3and Fe2O3-SiO2were changed significantly after cycling tests.The results of XPS characterization indicated that the surface oxygen species of Fe2O3-CaSO4could be recovered after reduction-oxidation reactions.However,the lattice oxygen was not fully recovered for both Fe2O3-Al2O3and Fe2O3-SiO2in the redox reactions.The loss of lattice oxygen weakens the surface reaction of gas-solid reactions, thereby reducing the redox performance of oxygen carrier.

Table 3 Oxygen species percentages of the 20th cycled oxygen carriers based on the XPS calculation

Combined with the above results, it can be seen that the presence of CaSO4is conducive to the redox activity and cycling stability of pyrite cinder.It indicated that CaSO4was the key to the high redox performance of pyrite cinder.In future studies, the pyrite cinder rich in CaSO4can be selected as an oxygen carrier in the processes of chemical looping.

4.Conclusions

In this study, the effect of supports on the cycling activity of pyrite cinder was investigated deeply by preparing Fe2O3-based oxygen carriers based on the actual composition of pyrite cinder.Among the studied oxygen carrier samples, the Fe2O3-CaSO4sample presented high CO conversion and cycling stability in the multiple redox reactions.Both Fe2O3-Al2O3and Fe2O3-SiO2samples experienced obvious deactivation,which was due to severe sintering and the generation of Fe-Si solid solution,respectively.Surface sintering also occurred for the spent Fe2O3-CaSO4sample.But this sample still kept higher redox performance,which was attribute to the stable content of surface oxygen species.It can be concluded that CaSO4is the key to the excellent cycling activity of pyrite cinder in chemical looping combustion.This work provide guidance for selecting pyrite cinders with high redox performance in the applications of chemical looping.

Fig.7.XPS analysis of the O 1s peaks for the 20th cycled samples.(a) Fe2O3-CaSO4, (b) Fe2O3-Al2O3, (c) Fe2O3-SiO2.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Program for High-Level Entrepreneurial and Innovative Talents Introduction of Jiangsu Province,Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (2021-K56), Foundation of Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education and Senior Talent Foundation of Jiangsu University (20JDG40).