Xiuli Zhang, Zhengdong Gao, Yongzhuo Liu, Yuanhao Hou, Xiaoqing Sun, Qingjie Guo,2,*

1 Key Laboratory of Clean Chemical Processing of Shandong Province, College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China

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

Keywords:Caking coal Chemical looping combustion Optimized reaction conditions

ABSTRACT Under high-temperature batch fluidized bed conditions and by employing Juye coal as the raw material,the present study determined the effects of the bed material, temperature, OC/C ratio, steam flow and oxygen carrier cycle on the chemical looping combustion of coal.In addition, the variations taking place in the surface functional groups of coal under different reaction times were investigated, and the variations achieved by the gas released under the pyrolysis and combustion of Juye coal were analyzed.As revealed from the results, the carbon conversion ratio and rate were elevated significantly, and the volume fraction of the outlet CO2 remained more than 92% under the oxygen carriers.The optimized reaction conditions to achieve the chemical looping combustion of Juye coal consisted of a temperature of 900°C, an OC/C ratio of 2, as well as a steam flow rate of 0.5 g·min-1.When the coal was undergoing the chemical looping combustion, volatiles primarily originated from the pyrolysis of aliphatic -CH3 and-CH2,and CO and H2 were largely generated from the gasification of aromatic carbon.In the CLC process,H2O and CO2 began to separate out at 270°C,CH4 and tar began to precipitate at 370°C,and the amount of CO2 was continuously elevated with the rise of the temperature.

1.Introduction

The use of fossil fuels(e.g.,coal)has significantly expedited the development of human civilization, whereas it has also exerted some adverse effects.For instance, the greenhouse effect and global warming are being rigorously monitored.The main existing energy source in China refers to coal, and CO2emissions have surged.To satisfy the energy needs and tackle down the associated environmental problems,the comprehensive conversion of energy and the economical capture of CO2should be achieved.Chemical looping combustion technology represents a novel type of coalbased carbon capture technology in accordance with the nearzero emission concept [1].As impacted by the ‘‘oxygen release”and ‘‘oxygen absorption” of solid oxygen carriers, fuels undergo the indirect combustion.Chemical looping combustion effectively avoids direct contact between fuel and air,and an internal separation of CO2from the flue gas can be achieved under near-zero energy consumption, the principle of which is illustrated in Fig.1.Accordingly, chemical looping combustion is considered one of the most promising,clean and efficient CO2capturing technologies, and coal chemical looping combustion technology has become a hotspot over the past few years.

Fig.1.Schematic diagram of the chemical looping combustion mechanism.

Extensive studies have been conducted on the preparation and optimization of oxygen carriers,and the application of raw materials has been studied in depth.Guo et al.[2]synthesized CaAlNi and CaAlFe, and the reaction properties with Shenmu coal were investigated with the use of a fixed bed reactor.Cloires et al.[3]generated Ca-Fe/ben oxygen carriers with natural gypsum powder and bentonite and further studied the effects of the temperature,active component contents and cycle time of the oxygen carriers by adopting a small fluidized bed reactor.Song et al.[4]prepared Fe4-ATP6based on a range of methods,and the effect of the preparation methods on the structure and properties of oxygen carriers was investigated.Garderen et al.[5]prepared copper-based oxygen carriers by using the dipping method and then investigated the effect of the number of cycles on the oxygen load with a thermogravimetric analyzer.Fang et al.[6]performed several experiments in which natural copper ore absorbed and released oxygen in a small fluidized bed reactor and subsequently investigated its reaction performance with Gaoping coal.Ming et al.[7]synthesized Cu6Si4by adopting a mechanical mixing method,and the CLOU process of coal was initially explored based on a small intermittent fluidized bed reactor.Wang et al.[8]prepared Mn75Al25and a manganese copper composite oxygen carrier, and the effect of several factors(e.g., the temperature, the oxygen concentration and the heating rate) on oxygen absorption, and oxygen release was studied with a thermogravimetric analyzer.Tian et al.[9]investigated the effect of the combustion temperature,feed volume and water vapor volume on the gas product composition in a 10 kWthserial fluidized bed; as revealed from the results, high temperature and an increase in feed volume were harmful to CO2capture, and an increase in water vapor was beneficial to CO2capture.Guo et al.[10]adopted a thermogravimetric analyzer to delve into the reaction of copper-based, iron-based oxygen carriers and sawdust; as revealed from their experiments, the exploitation ratio of CLOU was higher than that of CLC, and coal and biomass mixing would benefit coal combustion.Xiao et al.[11]studied the effect of temperature and water vapor on the release of nitrogen oxides from husks in a fixed bed reactor; as revealed from the results, carbon conversion increased with temperature,and the maximum amount of water vapor reached 1 g·min-1.

Given the degree of coalification,coal falls to three categories,i.e., lignite, bituminous coal, and anthracite.Lignite exhibits the minimum degree of coalification and the maximum volatile content; anthracite displays the maximum degree of coalification and the minimum volatile content; the degree of coalification for bituminous coal ranks between those of the lignite and anthracite,and its degree of coalification ranks medium.When bituminous coal is heated in isolated air, the organic macromolecules in coal will be decomposed by the heat; colloids are formed, with three phases coexisting, i.e., gas, liquid and solid.The caking property of coal refers to the ability of colloids to bond inert substances,and it is a critical index to assess the softening,melting and coking of bituminous coal during the retorting process.In China,most coal pertains to the rank of bituminous coal,and the coals largely exhibit different degrees of caking.There are serious problems (e.g.,serious pollution and small application scope) in the application of bituminous coal.

The chemical looping combustion of caking coal avoids several problems (e.g., agglomeration and blocking), as well as leading to the internal separation of CO2.The study below was conducted to reasonably and completely investigate the use of existing caking coal resources, as an attempt to alleviate the harm to the environment and broaden its application area.With caking coal as the raw material, Fe4Al6and Cu4Al6were synthesized by the dipping method; the effect of oxygen carriers and operating conditions on the chemical looping combustion of caking coal was systematically explored, and its cycle performance was determined experimentally.The aim here was to achieve a safe and efficient use of caking coal.

2.Experimental

2.1.Materials and oxygen carrier preparation

The experimental fuel was Juye bituminous coal.After the coal was crushed with a pulverizer, coal particles 80-200 μm in size were determined and applied in subsequent experiments.The proximate analysis, ultimate analysis and caking index of the Juye coal are listed in Table 1.With Fe(NO3)3·9H2O and Cu(NO3)2·3H2O as the active components and γ-Al2O3as the inert carriers, Fe4Al6and Cu4Al6were prepared based on the dipping method.

Table 1 Proximate analysis, ultimate analysis and caking index of the Juye coal

2.2.Setup and process

The experimental device consisted of a fluidized bed reactor, a temperature control system, a feeding device, a gas distribution system, a steam generation system, a dust collection and condensation system, as well as a gas collection and testing system.The experimental device is illustrated in Fig.2.

By using a Fourier infrared analyzer (Broker Equinox55, Germany) and a thermogravimetric analyzer (German Netzsh STA 409PC, Germany), the evolution of the functional groups of coal and the change in the released gas were analyzed during chemical looping combustion.N2was the experimental atmosphere,and the materials were heated to 900°C at 30 K·min-1and held for 30 min.

2.3.Data assessment

(1) Carbon conversion ratio XC, %.

Where t denotes the reaction time,min;V is the total amount of gas collected per unit of time during the combustion reaction,L;C represents the average volume concentration of gas per unit time, %;W0denotes the quality of the coal added to the reactor, g; Ctotalis the content of carbon in the coal or coal char, %.

(2) Carbon conversion rate r(t), %·min-1.Such rate is defined as the differential of carbon conversion ratio XCto time.

(3) Instantaneous gasification rate R(t), %·min-1.Such rate is defined as the ratio of the carbon conversion rate of the gas product to the reaction carbon at some point.

(4) Concentration of the combustion product φiand average concentration Φi, %.

Fig.2.Schematic diagram of the experimental setup for coal chemical looping combustion.

Where Vi(i=CO2,CO,CH4and H2)denotes the volume flow of each component, L·min-1; V represents the total volume flow rate.

3.Results and Discussion

3.1.Effect of bed materials on the combustion

When Al2O3acted as the bed material,the variations of the outlet gas over time during the CLC is shown in Fig.3(a).Juye coal was added to the fluidized bed reactor at high temperature,and it was instantly heated to the reactor temperature under the strong heat exchange [12].During this process, volatiles were separated out significantly.In addition, coke was vaporized by steam and generated H2and CO.After 6 min of reaction,CH4,CO and CO2exhibited their maximum concentrations of 1.84%, 1.08% and 4.92%, respectively.For the certain delay of the fluidized bed reactor, the outlet gas for the first 6 min of reaction originated from volatiles.After 20 min of reaction, H2reached its maximum concentration of 6.93%, primarily from the gasification reaction of coke and water vapor.

Fig.3(b)and(c)indicate that by employing Fe4Al6and Cu4Al6as the bed materials,the variations of the outlet gas over time in CLC was obviously inconsistent with that by using Al2O3.After the oxygen carrier was introduced to the fluidized bed reactor,the concentrations of H2, CH4and CO were lowered significantly to less than 0.4%.Such result demonstrated that the oxygen carriers exhibited a noticeably high selectivity, and they could react quickly with the reducing gas in the reactor.The oxygen carriers consumed most of the CO and H2, reduced the inhibitory effect of the gasification reaction and promoted the gasification of coke; thus, a favorable circulation was reported[13].When Fe4Al6acted as the bed material,CO2reached its maximum concentration of 16.72%after 8 min of reaction.When Cu4Al6acted as the bed material, CO2exhibited its maximum concentration of 23.26%after receiving 6 min of reaction.Thus, Cu4Al6turned out to be more reactive;it shortened the reaction time, as well as elevating the concentration of CO2.

Fig.4 illustrates the XRD patterns of the Fe4Al6and Cu4Al6oxygen carriers.As indicated from Fig.4(a),fresh Fe4Al6was primarily composed by Fe2O3as the active component and Al2O3as the inert support; Fe3O4turned out to be the predominant active component,demonstrating that Fe4Al6transferred lattice oxygen to react with coal during the reaction.First, the volatiles contained in coal were pyrolyzed and then reacted with Fe4Al6to synthesize CO2and H2O.Moreover,the coal reacted with steam to synthesize more CO and H2, and Fe4Al6acted as an oxygen carrier to convert combustible gases to CO2and H2O.Thus, according to Fig.4(b), fresh Cu4Al6largely consisted of CuO and CuAl2O4; after the reduction,Cu2O and Al2O3were reported as the main components,suggesting that Cu4Al6transferred lattice oxygen to convert coal and maintained a stable crystalline phase.Under a reducing atmosphere at high temperatures, CuO acted as the predominant active component involved in Cu4Al6uncoupling the oxygen to react with char and volatiles from devolatilization of the Juye coal; under such condition,more considerable amounts of CO2and H2O were generated.However,as impacted by the low residence time between the reactants and Cu4Al6,and since the oxygen release amount was not reached,char remained partially gasified to the intermediate products and then reacted with Cu4Al6to synthesize CO2and H2O.For this reason, it was concluded that the oxygen release of Cu4Al6at high temperatures was the major reason for the elevated conversion rate, shortened reaction time, as well as accelerated surface group assessment.

3.2.The effect of operating conditions on the combustion

As shown in Fig.5(a), at the temperature rising from 800 °C to 900 °C, the carbon conversion ratio was elevated from 60.46% to 92.63%, respectively, and the upward trend turned out to be increasingly obvious[14].Fig.5(b)indicates that with the increase in OC/C ratio, the carbon conversion ratio of Juye coal was continuously up-regulated.At the OC/C ratio increasing from 0 to 1.5,the carbon conversion ratio was up-regulated from 43.80% to 88.81%,respectively, and the increase was found to be highly significant.With the continuous increase in the OC/C ratio, the upward trend leveled off; at the OC/C ratio of 2 and 2.5, the carbon conversion ratios was nearly equal.According to Fig.6, with an elevation in the OC/C ratio, the carbon conversion rate continued to increase.With the OC/C ratio elevated from 0 to 2, the carbon conversion was significantly expedited; however, the carbon conversion rate at an OC/C ratio of 2 tended to comply with that at an OC/C ratio of 2.5[15].As revealed from this result meant that when the OC/C ratio was 2,Cu4Al6could provide sufficient oxygen for coal combustion,and it was not necessary to continue increase the amount of the oxygen carriers.

The carbon conversion ratio of Juye coal at different steam flows is presented in Fig.5(c).Fig.5(c) indicates the effect of the water vapor flow rate on the carbon conversion ratio was slight.With the steam flow rate elevated from 0 to 0.5 g·min-1,the carbon conversion ratio was elevated slightly.When the steam flow rate continued to increase to 0.75 g·min-1, the carbon conversion ratio decreased slightly.In brief,the optimal reaction conditions for the gas coal consisted of a temperature of 900°C,an OC/C ratio of 2,as well as a steam flow rate of 0.5 g·min-1.

Fig.3.Variations of outlet gas over time during the chemical looping combustion of Juye coal.

Fig.4.XRD patterns of the Fe4Al6 and Cu4Al6 oxygen carriers.

3.3.Effect of the oxygen carrier cycle on the combustion

The effect of cycle time on carbon conversion is illustrated in Fig.7.Notably, the carbon conversion of Juye coal remained high at 10 cycles.From the 1st cycle to the 3rd cycle,the carbon conversion ratio was elevated from 92.63%to 98.53%,respectively.Subsequently, the carbon conversion was decelerated slightly.Such phenomenon was because the first cycle experiment to some extent activated the oxygen carriers, and the internal holes of the oxygen carriers turned out to be more developed after the experiment, thereby increasing the reactivity [16].With the number of cycles increasing continuously, the high temperature and coal ash led to partial sintering of the oxygen carriers.Accordingly,the reactivity of the oxygen carriers decreased slightly.

Figs.8 and 9 illustrate the effect of cycle times on the carbon conversion rate and the volume fraction of outlet gas,respectively.The curve of the carbon conversion rate tended to be consistent during the 10 cycles of the experiment,and the carbon conversion rate was not down-regulated significantly.Moreover, the volume fraction of CO2remained above 92%, and the volume fractions of CO, CH4and H2remained below 5%.Thus, the reactivity of Cu4Al6was stable and could remain at a high level during 10 cycles of the experiment.The oxygen carriers could rapidly release lattice oxygen and molecular oxygen for the combustion of coal,and the coke released was overall oxidized to CO2.During the process, the reduction rate of the oxygen carriers was constantly higher than the gasification rate of char.

Fig.5.Effect of the operating conditions on carbon conversion.

Fig.6.The change in the carbon conversion rate over time.

Fig.7.Effect of cycle times on the carbon conversion.

Fig.8.Effect of cycle times on the carbon conversion rate.

3.4.The change in the surface functional groups of coal during the CLC process

Fig.9.Effect of cycle times on the volume fraction of gas.

The variations taking placed in the surface functional groups of Juye coal during the CLC under the different reaction times were analyzed by performing FT-IR.The experimentally achieved results are presented in Fig.10,and the analysis of the peak information is illustrated in Table 2.

Table 2 FT-IR absorption peak assignment of functional groups of coal with different reaction times [17]

Table 3 The absorption spectrum of some gases [20]

As shown in Fig.10,3694 cm-1and 3620 cm-1are the vibration peaks of the free OH groups; 3470 cm-1to 3300 cm-1represent the vibration peaks of self-associated OH; near 2918 cm-1and 2853 cm-1refer to the stretching vibration peaks of -CH2and-CH3, respectively; both 1597 cm-1and 1435 cm-1are the stretching vibration peaks of aromatic C=C;near 1250 cm-1represents the stretching and bending vibration peak of C=O; near 1080 cm-1is the vibration peak of C-O in phenols,alcohols,ethers and esters.

When coal was introduced to the reactor for 1 min,the absorption peak of free OH and self-associated OH disappeared, where free OH evaporated, and self-associated OH dehydrated to form the acid anhydride or decarboxylated to form CO2[18].Under the reaction for 1 min, the peak intensity of aliphatic -CH3and-CH2became weak.Since aliphatic -CH3and -CH2were significantly active,-CH3and-CH2at the endpoint and side chain of aliphatic hydrocarbons broke after the absorption of heat and formed CH4, C2H6, etc.[19].During the chemical looping combustion of Juye coal, the volatile matter mainly originated from the pyrolysis of aliphatic-CH3and-CH2,which also acted as the sources of gas during the previous period.Over the reaction time, the absorption peak of aromatic C=C tended to decline.As the reaction time was prolonged, aromatic carbon was gradually graphitized and then vaporized by steam into CO and H2.When Juye coal was undergoing chemical looping combustion,most of the CO and H2originated from the gasification of aromatic carbon.During the burning process, a robust absorption peak was identified at 1100 cm-1and became increasingly stronger in intensity.Such phenomenon might occur since considerable stable phenols, alcohols, ethers and esters were formed when coal was burning.

Fig.10.FT-IR spectra of Juye coal at different reaction time.

Fig.11.FT-IR spectra of gas released from Juye coal at the different temperatures.

3.5.Variations of the gas released from Juye coal during the CLC process

To investigate the variations of the gas during the combustion of Juye coal, the gas released from Juye coal at different temperatures was analyzed by performing TG-FTIR.The achieved results are presented in Fig.11, and the absorption peaks of the gas product are listed in Table 3.

Fig.11 indicates that the Juye coal began to separate water and CO2at 270 °C,and the water and CO2were respectively generated from the free OH and self-associated OH in coal.At 370 °C, the amount of CO2was elevated, and a small amount of CH4and tars began to be emitted and separate out, respectively.Such CO2was primarily produced by the cleavage of carboxylic acids and oxycarbonyls of coal[21],and the CH4was synthesized from the cleavage of aliphatic hydrocarbon and side chain of aromatic hydrocarbon with methyl groups.With the temperature rising to 480 °C, CO2was intensely released, and the amounts of CH4and tar slightly increased.The mentioned result might be attributed to the rise in the temperature, which facilitated the thermal cracking of coal[22].From 580 °C to 900 °C, more CO2was precipitated, and the CO2mainly originated from the reaction of coke and oxygen carriers.After the coal reacted at 900 °C for 15 min, all the gases displayed the reduced absorption peak intensity, suggesting that the coal reacted completely.

4.Conclusions

By using Fe4Al6and Cu4Al6as the bed materials, the maximum carbon conversion ratio of Juye coal increased by 2.35 and 3.28 times, respectively, and the volume fraction of CO2in the outlet gas was elevated by 3.40 and 4.73 times, respectively.The oxygen carriers consumed considerable CO and H2and oxidized them to CO2and H2O, respectively, thereby facilitating the gasification of coal.In addition, the carbon conversion ratio was noticeably elevated.When Cu4Al6acted as the bed material,the optimized reaction conditions for gas coal consisted of a temperature of 900°C,an OC/C ratio of 2, as well as a steam flow rate of 0.5 g·min-1.Under the mentioned reaction conditions, the carbon conversion ratio reached 98.53%, the volume fraction of CO2in the outlet gas took up 97.5%,and the volume fractions of H2,CH4and CO were overall less than 5%.After 10 cycles of the experiment with the use of Cu4-Al6, the carbon conversion ratio and rate of coal remained at relatively high levels.Thus, Cu4Al6was proved to be active and stable and could be applied cyclically to the chemical looping combustion of coal.

As revealed from the results achieved by performing TG-FTIR,the evolved gases varied during the chemical looping combustion.Water and CO2started to be synthesized from the Juye coal at 270 °C.Subsequently, at 370 °C, partial tar and CH4began to precipitate.At the temperature rising to 470 °C, the amount of CO2was obviously elevated, and the CO2mainly originated from the cleavage of carboxylic acids and oxycarbonyls in coal.At 900 °C,the precipitated CO2might originate from the reaction of coke and oxygen carriers,and all the gas was significantly reduced after 15 min, suggesting that the coal had reacted thoroughly.

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

The authors gratefully acknowledge the support from the National Key Research and Development Program of China(2018YFB06050401), Key Research and Development Program of the Ningxia Hui Autonomous Region (2018BCE01002), and the Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (2019-KF30, 2019-KF33).