Syngas production from chemical looping reforming of ethanol over iron-based oxygen carriers:Theoretical analysis and experimental investigation

Atif Abdalazeez ,Wenju Wang,Siddig Abuelgasim

1 School of Energy and Power Engineering,Nanjing University of Science and Technology,Nanjing 210094,China

2 Department of Mechanical Engineering,University of Kassala,Kassala,Sudan

Keywords:Syngas Chemical looping reforming Iron oxygen carrier Ethanol Coke formation Thermodynamic analysis

ABSTRACT Chemical looping reforming (CLR) is a recent trend for syngas production,which has several merits compared to the conventional manner.One of the most important issues for CLR is to find low-cost material as oxygen carriers,so iron is a promising candidate.This paper contributes to testing the thermodynamic ability of iron-based oxygen carrier for chemical looping reforming of ethanol (CLRE).Iron thermodynamically investigated in temperature 100–1300 °C and excess oxygen number (φ) 0–4.It was found that the temperature and φ have an apparent effect on the gaseous composition produced from the process.Increases in temperature within the range of 100–1300 °C enhanced syngas generated and reduced coke formation and CH4.Whereas,increased φ,particularly at higher temperatures,had also enhanced syngas production as well as reduced coke formation.However,increasing φ for values beyond one had decreased syngas and not significantly reduced coke deposition.Moreover,an experimental investigation was carried out in a fixed bed reactor for more in-depth verification of iron ability as an oxygen carrier through using magnetite ore(mainly Fe3O4).It found that the effect of temperature on syngas production was consistent with that calculated thermodynamically,as syngas increased with raising the temperature through the CLRE.

1.Introduction

Syngas is a necessary chemical raw material,which can supply power and heat.Where it could produce from solid fuels through gasification or via the reforming of gaseous or liquid fuels;at all events,it mainly consists of hydrogen (H2) and carbon monoxide(CO) [1,2].The syngas is a combustible gas,as it can be directly used to produce energy,such as electric power in fuel cells and turbines [3].However,due to the current market situation,syngas is dominantly required as feedstock to produce familiar fuels such as naphtha,gasoline,diesel,and high value-added chemical inputs[4,5].Accordingly,syngas is mostly used as an intermediate material for producing various high value-added fuels via Fischer-Tropsch synthesis [6].Besides,several chemicals could be derived from syngas,include,olefins,alcohols,natural gas,ammonia,and many other aromatic compounds [7,8].

Bio-ethanol (C2H5OH) has been considered one of the ideal options for syngas production.There are several studies showed that C2H5OH could be converted into H2or syngas during steam reforming [9,10],partial oxidation [11],auto-thermal reforming(oxidative reforming) [12],and CO2reforming (dry reforming)[2,13].Although reforming is an effective way,it has suffered some drawbacks,such as CO2production from hydrocarbon combustion and highly endothermic nature of the reforming process,which have energy penalty.On the other hand,the partial oxidation of C2H5OH required an energy-consuming air separation unit to provide pure O2to the process;accordingly,chemical looping reforming (CLR) is a suitable way to address these concerns [14].

CLR is a partial oxidation process,where the fuel is partially oxidized by using oxygen carriers(OCs)to generate a stream of H2,CO,CO2,and H2O[15–20].The CLR conveys oxygen to the fuel through OCs particles that continuously circulates between the reformer and the air reactor,which provide sustainable oxygen and heat sources.Thus pure oxygen is no longer needed,so the cost is reduced and avoids N2dilution of the reforming gas.Furthermore,the OCs can play as a catalyst for cracking the tar during the syngas production[21].When ethanol is utilized as feedstock,the process is abbreviated by (CLRE).Up to date,only limited studies have investigated CLRE,whether experimentally [22–24] or theoretically[25,26].In the CLRE,oxygen carriers are typically transitional metal oxides such as Fe,Ni,Cu,Co,and Mn oxides [22,25,27].The oxygen carrier plays an important role in the chemical looping field,and its selection is sensitive because it depends on several factors such as cost,stability,environmental impact,etc.

The manufactured iron oxide has been considered a promising candidate for chemical looping due to its low cost,high thermal stability,non-toxicity,and environmental friendliness [28].Based on prices,iron is much lower than other competing transition metals(Ni,Co,Mn,and Cu) due to its abundance and matured mining and manufacturing techniques.This advantage would make the CLR for ethanol-containing organic wastewater over iron oxygen carrier possible;to obtain syngas with high quality (low tar content and high calorific value) and low cost [28].Therefore,CLRE by iron-based oxygen carrier is one of the target technologies for producing syngas in the next decades.Simultaneously,the environmental effect of using OCs should be evaluated due to the growing environmental preservation standards.Iron is considered a safe metal and is most commonly utilized in daily life compared to other transitional metals such as Co and Ni [29].Whoever,iron has multi-oxidation/reduction degrees states,where the Fe2O3-Fe3O4stage is quicker when it compared with the following stages of Fe3O4to FeO and FeO to Fe [30].

The industrialization cost of the OCs is fundamentally influenced by the cost of raw materials through its application on a large scale (an industrial scale) to produce syngas.Natural ores(manganese ores,ilmenite,hematite,and magnetite)are a promising candidate to apply as oxygen carriers in chemical looping,as reported in our recent work [31].The use of natural ores in the CLRE can reduce cost and avoid environmental pollution resulting in the oxygen carrier preparation.Besides,natural ores usually consist of metal oxides that are inherently bound to support materials like Al2O3,SiO2,and TiO2,which are similar to supported oxygen carriers[32].In particular,abundant iron ores could be applied as an oxygen carrier after only plain treatments.The iron content in magnetite ore is 72.4%,which is higher than that in limonite(37%–55%),siderite (48.2%),and hematite (70%) [33].Thus,the magnetite with a high content of Fe3O4may be an ideal oxygen carrier for CLRE.The reactivity of oxygen carrier is vital in chemical looping,as the thermodynamic analysis is important in the initial stage for iron-based OC investigation with CLRE to predict the product compositions.Which depending on a complex model of the process variables,particularly those related to temperature and excess oxygen number (φ).

Temperature and φ have significant importance on the syngas that produced from the CLR process.As the temperature importance on the syngas production is attributed to the endothermic nature of reactions in the reformer[34].Besides acting as a carrier of oxygen,the OCs acts as a heat carrier;thus,there is no need for an external heat source to provide heat for the endothermic reactions.The heat releases from the exothermic oxidation of the OCs in the air reactor can meet heat demand in the reformer.Furthermore,excessive temperature could lead to sintering oxygen carriers and agglomeration on the system,which highly appears when the oxygen carrier shows a low melting point[14,35].In particular,the melting point of iron oxygen carrier states is moderate(1597–1377 °C) [31],so it is important to study the effect of increasing temperature on syngas production while keeping away from the melting point.On the other hand,the importance of φ on syngas appears through the need of oxygen from the oxygen carrier for the occurrence of partial oxidation reactions of ethanol that produce syngas.Nevertheless,increasing the φ more than required would affect the syngas yields,as it can leads to oxidation of ethanol to CO2and H2O [28,36].Also,increasing φ means an increase in the amount of oxygen carrier,thus raise the cost of the process.In contrast,a decrease φ less than required can also affect syngas yields as well as lead to an increase in the carbon deposition in the reformer [28,34].Therefore,it is necessary to study more clearly and specifically the effect of φ on syngas production as well as gaseous by-products and carbon yields during the process.

Up to date,few studies about CLRE over iron can be found in previous researches.In this work,the thermodynamic ability of an iron-based oxygen carrier was tested for the CLRE process.It was thermodynamically analyzed to study the effect of temperature and excess oxygen number on syngas production as well as by-products.Furthermore,iron-based magnetite ore (mainly Fe3O4) was experimentally examined as an oxygen carrier in the CLRE,where the test has been performed in a fixed bed reactor to investigate the effect of temperature on the gaseous product composition.

2.Methodology

2.1.Description of CLRE technology

The concept of CLRE is illustrated in Fig.1,as this technology mainly contains two reactors(air reactor and fuel reactor or reformer).Both reactors are typically worked in the fluidized scheme.The C2H5OH in the reformer is converted into syngas by a solid oxygen carrier (e.g.,Fe3O4) rather than O2.Whereas,Fe3O4is reduced to FeO and Fe by reforming of C2H5OH.FeO and Fe are then transferred into the air reactor for oxidizing to yield Fe3O4by air,which is subsequently sent another time to the fuel reactor.The reaction heat,which releases from the oxidation process of the Fe3O4oxygen carrier,can meet heat demand for the CLRE process.The syngas from the exit of a reformer is accompanying with H2O,which can be removed out easily in the condenser.Significantly,in the experimental mode,Fe3O4must be scattered on support.Moreover,the overall cost of CLRE is low due to cancel the need of energy requiring for the air separation process.The reactions in the air reactor and reformer reactor are summarized in detail below.In general,oxidation reactions that occur in the air reactor are described by Eqs.(1) and (2) [37]:

Fig.1.Schematic diagram of chemical looping reforming of ethanol (CLRE).

In the fuel reactor,oxygen carrier(Fe3O4)can oxidize C2H5OH to form H2,CO,H2O,CH4,C(s) and CO2.The overall reactions in the reformer have summarized in Eqs.(3)-(7) below.

In Eqs.(1)–(6),iron and its oxides act as a product or reactant.However,in the methanation reaction Eq.(7),iron and its oxides act as a catalyst material,instead of as a reactant or product.Eq.(7),mainly catalyzed by the metal(Fe)or metal oxide(FeO),is considered unfavorable and can be inhibited through setting reaction factors,mainly,temperature,pressure,and C2H5OH/oxygen carrier ratio in the fuel reactor.The excess oxygen number(φ)is known as the actual amount of oxygen added with metal oxide (na) to the amount of equivalents required for the complete conversion of ethanol (ns),as shown in Eq.(8).

2.2.Thermodynamic calculations method

Thermodynamic study for the equilibrium of product distribution has done by the Gibbs energy minimization scheme and the routine described in our earlier study [27,38,39].The summation of ith species of the system gives total Gibbs free energy:

where Gtstand for total of the Gibbs free energy;Gifor partial molar of Gibbs free energy for species i;Gi?is the standard Gibbs free energy;μiis the chemical potential;T is the temperature of system;R is the molar gas constant;P is the system pressure;fithe fugacity in system;fi?is the standard-state fugacity;and niis the mole of species i.For reaction equilibria in gas-phase,fi=yiφiP,fi?=P?,and Gi?=ΔGfi?are assumed.When using the Lagrange multiplier way,the minimum Gibbs free energy of each gaseous species and that of the total system can be expressed as in Eqs.(10) and (11),respectively.

where ΔGfi?is the standard Gibbs function of formation for species i;yithe gas phase mole fraction;P?the standard-state pressure of 101.3 kPa;φithe fugacity coefficient of species i;λkthe Lagrange multiplier;Akthe total mass of k element in the input;and aikthe number of atoms for the k element present in each molecule of species i.

When solids are involved in a system,the utilizing solid-vapor phase equilibrium is applied for the Gibbs energy of carbon as demonstrated in Eq.(13).Replacing Eq.(9) by Eq.(10) to gaseous species and by Eq.(13)to solid species gives the Gibbs-energy minimization function in Eq.(14) bellow:

where i include C2H5OH,H2,H2O,CO,CH4,and CO2;j involves C,Fe,FeO and Fe3O4.One assumption made is that all substances in the solid phase are in pure form and not mixtures.Gj(g),Gj(s)stand for the partial molar Gibbs free energy of gas j and solid j,respectively;while,ΔGfj(s)?and njfor standard Gibbs function for formation of solid j,and mole of j,respectively.

The predominant equilibrium compositions during CLRE over iron-based oxygen carrier are simulated at different temperatures(T,100–1300°C)and oxygen excess number(φ,0–4).With a note that thermodynamic analysis has not considered any kinetic limitations,such as mixing and temperature gradients occur in the real process.However,the thermodynamic calculation supplies importance guidance to determine the effect of temperature and oxygen excess number on predicting gaseous composition products (H2,CO,CH4,and CO2) from CLRE system.Which could lead to deep understanding to the complex reactions that occur during the CLRE;including interactions that contribute to the production of syngas(mainly H2and CO)and interactions that lead to the formation of the undesirable gaseous compounds (CH4and CO2).Also,the importance of thermodynamic calculation is demonstrated by an understanding of the complex reactions connected with coke formation during the CLRE process.Besides,in this work,we would study the effect of temperature on the composition of gas products thermodynamically and experimentally for comparison and determine the appropriate temperatures to increase syngas production and reduce unwanted products from the CLRE process over ironbased oxygen carrier.

2.3.Materials and experimental method

2.3.1.Oxygen carrier

In this work,the natural magnetite ore from Shanxi province in China was selected as the oxygen carrier.The magnetite ore was calcined at 800 °C to 5 h in Argon.The original magnetite ore is mainly composed of 85.14%(mass)Fe3O4as reactivity component,4.78% (mass) SiO2,and 8.01% (mass) Al2O3as inert components.The calcined ore was subsequently mashed and sieved to 0.125–0.180 mm to get the solid oxygen carrier particles.

2.3.2.Characterization

The surface observation of magnetite ore was performed through a scanning electron microscope (SEM,JSM-6380LV,JEOL Co.).Whereas,the syngas on dry basis was specified by using previously calibrated SIEMENS continuous analyzers.The concentrations of O2,CO,CO2and CH4were measured using the Ultramat 23 model,while H2concentration measured by the Calomat 6 model.

2.3.3.Fixed bed test

CLRE was performed in a fixed bed reactor (i.e.=10 mm;L=300 mm)at atmospheric pressure.In each run,0.5 g magnetite ore was applied.The C2H5OH(0.05 ml.min-1)was fed to the vaporizer held at 170°C.After that,the vaporized ethanol was pushed to the reformer.Furthermore,the liquid combinations in the exit stream were condensed by an ice-trap.In contrast,a gas analyzer was used to measure the gas product concentration of H2,CO,CO2,and CH4.

3.Results and Discussion

3.1.Thermodynamic results

3.1.1.Effect of temperature and φ

Figs.2 and 3 show the yields and concentrations on a dry basis of H2,CO,CO2,and CH4at atmospheric pressure.The effect of temperature and φ on syngas production from CLRE thermodynamically analyzed,since the individual trends of syngas composition(H2and CO) have been determined.Both yield and concentration of H2and CO increase with rising temperature within the range of 100–1300 °C,as shown in Fig.2(a) and Fig.3(a),and Fig.2(b)and Fig.3(b),respectively.This indicates that the catalytic reforming effect of iron-based oxygen carriers on syngas production has more active in high temperatures than in low temperatures [28].Also,the enhancement of H2and CO yield and concentration with an increase in the temperature is attributed to the endothermic nature of reactions in the reformer[26,34].Where,the syngas produced in Eqs.(3) and (4) through partial oxidation of C2H5OH by Fe3O4and FeO,respectively.The heat releases from the exothermic oxidation process of Fe-based oxygen carriers in the air reactor can meet the heat required for the endothermic reactions that produce syngas as in Eqs.(3) and (4) with critical adaptation for heat and mass balances in the reformer.In contrast,it must be more attention on the sintering of Fe-based oxygen carrier during CLR operation at a higher temperature.Since agglomeration may occur during reducing Fe3O4to FeO or Fe due to their relatively lower melting points and higher exothermic heat in the oxidation stage[29].Moreover,to reduce the impact of heat during OCs operation in higher temperature conditions,it can use supporting materials such as Al2O3and TiO2with an oxygen carrier to raise its melting point.Although the addition of these materials can decrease oxygen transport capacity,it enhances the mechanical strength and attrition resistance of the oxygen carrier [31].

Fig.2.The yield of H2 (a),CO (b),CH4 (c) and CO2 (d) as a function of temperature and φ at atmospheric pressure.

Fig.3.The concentration on a dry basis of H2 (a),CO (b),CH4 (c) and CO2 (d) as a function of temperature and φ at atmospheric pressure.

Besides that,the syngas composition showed different trends depending on φ value at the various temperature ranges.Regarding H2,as shown in Fig.2(a)and Fig.3(a),the φ value has no effect on its yield and concentration in the temperature range 100–700 °C.However,H2decreases with increased φ at φ >1 for temperature above 700 °C,due to availability of more oxygen in the OC that favors the conversion of H2into H2O [28].In contrast,increases φ for φ<1 and temperature above 800°C has enhanced CO yield,as illustrated in Fig.2(b),which is attributed to low oxygen available for the partial oxidation of ethanol to CO.Whereas,the decrease in oxygen content leads to an increase in the deposition of carbon in the reformer [23].The maximum CO yield was found at φ approximately equal one in temperature higher than 950°C.Despite,no change in maximal CO yield occurred when rising φ in the range 1–1.8,an increased the φ beyond this range reduce CO yield.Since this increment of φ means more oxygen has existing in the process,which can convert CO into CO2.Furthermore,at increasing φ from 1 to 1.8,besides no enhancing in CO yield,this means an increase in the amount of oxygen carrier,which leads to a rise in the cost of the process.Therefore,it can be said that increasing φ value beyond one is not appropriate for syngas production.

On the other hand,the effect of temperature and φ on the byproducts during syngas production from CLRE over Fe-based oxygen carrier was thermodynamically analyzed,as the CH4and CO2considered the two unfavorable components in the composition of the product.The CH4reduces with increasing temperature of the reformer until it became insignificant at temperatures above 700 °C as shown in Fig.2(c) and Fig.3(c).This attributed the increasing temperature can decrease the CH4formation in gaseous products through inhibit methanation reaction (as in Eq.(7)).Which,highly exothermic reactions,where it favored at lower temperatures [40].The CH4can also be reduced through its endothermic decomposition,as in Eq.(15),which favors occurring at high temperatures.Therefore,the temperature effect was more apparent than φ on CH4yield,as shown in Fig.2(c).Furthermore,it is noted that both CO2yield and concentration from the reformer initially increased at a temperature range of 400–800 °C and φ >1 and later reduced with an increase in temperature above 800 °C as displayed in Fig.2(d) and Fig.3(d).The high value of φ can increase CO2yields simultaneously with decreased CO yields since CO2formation becomes thermodynamically favorable because excess oxygen is available from the oxygen carrier [6].

In summary,the results of the thermodynamic calculations showed that the temperature and φ had a significant influence on syngas composition and byproducts from CLRE on the ironbased oxygen carrier.The increasing temperature from 100 °C to 1300°C improved the syngas production due to the catalytic effect of the oxygen carrier at high temperatures,as well as the endothermic nature of reactions in reformer that required heat.Also,increasing temperature led to reducing CH4by inhibiting its formation besides stimulating its decomposition when formed.On the other hand,increased φ,particularly at higher temperatures,had enhanced syngas yield.However,the syngas yields reduced with increasing the φ for values beyond one at high temperatures,as syngas composition (H2and CO) could be oxidized into H2O and CO2,respectively.

3.1.2.Coke formation

Carbon deposition is an unfavorable product in CLRE as it causes the coke formation problems in reformer that can deactivate the active site of the Fe-based oxygen carrier.Hence,it is necessary to know the coke formation mechanisms,besides investigating the effect of temperature and φ on reducing or avoiding its deposition.Where,the coke formation is induced due to methane decomposition and Boudouard reaction presented in Eqs.(15)and (16),respectively [41].Eq.(15) is an endothermic reaction,which is thermodynamically favored at high temperatures.Conversely,the exothermic reaction in Eq.(16) is most probable to occur at low temperatures.As known kinetically,both reactions have limited significance at the non-attendance of the catalyst.The transition metals as Fe,Co,and Ni can stimulate coke formation.However,its formation also depends on operation conditions as fuel conversion,oxygen available,temperature,and pressure[42].

Methane decomposition:

Boudouard reaction:

As illustrated in Fig.4,the carbon yield reduced through rising reaction temperature and φ,more clearly at an increasing temperature above 700 and φ <1.That is due to the endothermic reduction of iron by coke as in Eqs.(17)and(18)since the oxygen carrier performance can be enhanced by raising the reaction temperature above 850 °C,where higher temperatures had induced the formation of carbon oxides[43].In CLRE,to utilize more lattice oxygen in the Fe-based oxygen carrier,a deeper oxygen carrier reduction is suitable.At the same time,carbon deposition should be avoided due to the methane decomposition reaction takes place as a result of overdrawn oxygen consumption in the reduction stage.In other words,the methane decomposition will inhibit when φ becomes high because there is no enough methane to decompose [44].The reactions of production CO2from carbon formation on the oxygen carrier during syngas yields are considered undesirable in the oxidation process [43].

Fig.4.The effect of temperature and φ on the carbon yield (%) at atmospheric pressure.

3.2.Experimental results

In this section,the effect of temperature on the composition of gaseous products in CLRE over magnetite ore oxygen carrier is presented.Fig.5 showed the results of concentrations on a dry basis of H2,CO,CH4,and CO2at various reaction temperatures.The trends of individual product gases under the effect of temperature have been analyzed separately.Fig.5(a)illustrated the effect of temperature on the concentration of H2in the product components.The H2is produced from the partial oxidation of ethanol via the reduction of Fe3O4and FeO as in Eqs.(3) and (4),respectively.Where these reactions are producing a high concentration of H2with is increasing temperatures in CLRE over iron-based oxygen carrier[28].Accordingly,the concentration of H2was minimal (5%) at 400 °C.While,the H2increased relatively with reaction time in 500 °C until it reached 32% at the end of the reaction (60 min).In addition,we find that at 600 °C,H2reaches the maximum peak within 8 min and then decreases slightly until the end of the reaction,as a result of the reaction of H2with oxygen carrier(FeO)as in Eq.(6)which is occur in the temperature over 570 °C[45].In contrast,the H2has reached its maximum peak (39%) at 700 °C in a short time (5 min),but it did not drop until the reaction end.Which may attribute to the H2production via the methane decomposition in Eq.(15).Where,the iron-based catalysts shown good performance for methane decomposition at the high operating temperatures [27,46].

Similarly,the effect of temperature on the CO concentration in the product components was showed in Fig.5(b).Firstly,at a temperature of 400 °C,no CO is produced.While with the increased temperature to 500 °C,the CO concentration starts to rise until the end of the reaction time.As similar to H2,the concentration of CO has enhanced with increasing temperatures during partial oxidation of ethanol by iron oxides in Eqs.(3) and (4) [28].Thus,at 600 °C,the CO reached a peak (12%) at 10 min of the reaction time;however,it began to decrease until it reached 10% at the end of the reaction.This due to the reaction of CO with oxygen carrier (FeO) in Eq.(5),as the rate of CO reduction is relatively increased in the presence of iron-based oxygen carriers at high temperatures [47].Also,at 700 °C,the CO reaches a peak (14%)in a short time (5 min);after that decreases slightly and then increases and remains constant until the end of the reaction,as a result of the reaction between oxygen carrier and coke.Where the iron-based oxygen carriers (Fe3O4and FeO) at high temperatures can enhance CO production through their reactions with coke as in Eqs.(17)and(18)[48].In general,the increasing temperature can directly enhance the syngas composition,where higher contents of H2and CO were produced from CLRE over Fe-based oxygen carriers at 700°C than those at 600°C,500°C,and 400°C.It should be noted that the H2concentration in the product composition at 700 °C is approximately three times of CO concentration until the end of the reaction,this indicates that the magnetite ore oxidizes the ethanol to syngas (H2and CO) with the H2/CO ratio of about 3.0.Moreover,we find that the experiment results are consistent with those achieved by thermodynamic calculation regarding the syngas,as the increase in temperature enhanced its production.This enhancement was due to the catalytic reforming effect of magnetite ore(mainly Fe3O4),which showed significant activity in high temperatures than those at low temperatures,as the high temperature could promote the reactivity of Fe3O4for CLRE to achieve maximum ethanol conversion.Besides,the higher temperature facilitates the endothermic reactions in the reformer.

Fig.5.The concentration on a dry basis (without Ar) of H2,CO,CH4 and CO2 at different temperatures of 400 °C (a),500 °C (b),600 °C (c) and 700 °C (d).

On the other hand,regarding the effect of the reaction temperature on the formation of the undesirable products during syngas production from the CLRE,the CH4and CO2are considered the two unfavorable components that have been found in the products mixture.Fig.5 (c) showed the relation between temperature and formation of CH4.In the low temperature at 400 °C,CH4has not shown but its formed in 500 °C via the conversion of H2and CO to CH4(as in Eq.(7)) and increasing with raising temperature.While,this is opposite to thermodynamics calculation,therein the CH4production is favored in lower temperature.This difference may be due to the performance of the methanation reaction is limited by other parameters at the lower temperature,resulting in a weak dependence on temperature that below 500 °C [49].Although the concentration of CH4reaches its maximum peak at 700 °C through the first minute,it remains almost constant until the end of the reaction due to the methane decomposition in Eq.(15),which is a moderately endothermic reaction that favored thermodynamically at the relatively higher temperatures [43,46].Furthermore,the effect of the temperature on the CO2content in product composition during the CLRE process is illustrated in Fig.5 (d).It was found that the CO2content in the composition does not exceed 1% at 400 °C and 500 °C.However,CO2content reaches 2% and 3% at temperatures of 600 °C and 700 °C,respectively,where the production of CO2in CLRE is due to the reaction between the oxygen carrier and CO.The first stage of producing CO2corresponds to weight loss of the oxygen carrier.Since the FeO is reduced by CO gradually with the formation of CO2as in Eq.(5),where this reaction is favored at high temperatures.After FeO was reduced,more Fe atoms have appeared on the FeO surface,which could act as active sites to catalyze the decomposition of the CO as in Eq.(16),that is resulting in coke deposition and CO2production [47,50].

Fig.6.SEM images of magnetite ore before reaction (a) and after reaction (b).

Fig.6 illustrated the scanning electron microscope (SEM)images of the magnetite ore before and after reaction with ethanol.The external morphology of ore particles size that calcined in argon has been captured from the SEM (Fig.6 (a)).It notes that the particles have a relatively regular shape and smooth surface with a compact structure covered by slight grains of different sizes.Besides,the high temperature of calcination generally can enhance the growth of crystals,which may increase the diffusion of different elements during the reaction [33].The morphology of magnetite ore particles significantly changed after the reduction step,as shown in Fig.6(b).This change because of the lattice oxygen release from active Fe-based and the accompanying phase transformations that occur during the reduction,where numbers of pores will be formed when the magnetite ore is reacted with ethanol [32,51].Which is matched with the study of De Vos et al.[51],they reported that the morphology of iron particles significantly changed after reduction reactions due to oxygen transfer and accompanied phase transformations.Besides that,coke deposition on the iron particles surface was also noted,which may form due to the methane decomposition or Boudouard reaction.As it also shown in the other study for partial oxidation of methane over iron-based oxygen carrier [52].Moreover,no sintering was observed for iron particles after reduction as the size of grains had reduced,which may ascribed to the existence of inert components such as Al2O3and SiO2in magnetite ore,since these materials act to improve the resistance to sintering and agglomeration.This is also consistent with previous studies that indicated the use of the support materials could limit the sintering of the oxygen carrier [53,54].

4.Conclusions

Iron has several advantages basically regarding cost and environment,so,in this work,its thermodynamic capability as oxygen carrier was tested for the CLRE process by minimizing Gibbs free energy.Iron thermodynamically studied in temperature 100–1300 °C and oxygen excess number 0–4.

The thermodynamic results showed that temperature and φ have a significant effect on product composition as follows.A higher syngas could generate with the increase in temperature within the range 100–1300°C due to the enhancing catalytic effect of the iron oxygen carrier at high temperatures,as well as the endothermic nature of reactions in reformer that required heat.Also,the increasing temperature reduced CH4by inhibiting its formation through methanation reaction or decomposing after formation via methane decomposition reaction.Besides that,increases temperature decreased coke through its oxidation by iron-based oxygen carrier.In contrast,increases φ has enhanced syngas production as well as reduced coke formation.However,increasing φ for values that beyond one at high temperatures had no significant effect on reducing coke,besides led to oxidation syngas composition H2and CO into H2O and CO2,respectively,as a result of more oxygen available from the oxygen carrier.On the other hand,an experimental investigation was carried out in a fixed bed reactor for more in-depth verification of iron ability as an oxygen carrier through using magnetite ore (mainly Fe3O4).The experiment results proved that syngas production has increased with raising the reaction temperature,which is consistent with thermodynamic calculation results.In conclusion,iron-based oxygen carrier proved its good ability for syngas production from CLRE both in the thermodynamic calculation and experimental investigation.

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 is supported financially by the National Natural Science Foundation of China (Grant No.21676148).