Xin Li,Helong Hui,Songgeng Li,*,Lu He ,Lijie Cui*

1 State Key Laboratory of Multiphase Complex Systems,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 University of Chinese Academy of Sciences,Beijing 100049,China

1.Introduction

Iron and steel production is in continuous high demand as a result of the growth in the world economy.A large amount of coke is required as both reducing agent and fuel for iron production using the conventional blast furnace process.Metallurgy coke is produced from high grade bituminous coal.However,reserves of the high grade bituminous coal are limited,which account for only 6.2%of coal reserves in China[1].The limited reserves and the high demand cause a high price of coking coal,in turn leading to high costs of iron production.In addition,the environmental issues associated with the coke production impose serious health and ecological concerns.Thus,there is an increasing interest in exploring new iron making approaches[2–4].An alternative ironmaking route is direct iron reduction through either natural gas or coal-based technology[3,4].The coal-based technology has attracted considerable attention in China due to the lack of natural gas.In the coal-based technology,syngas(CO and H2)derived from coal gasification is used as a reducing agent for iron production.

Coal is the primary energy resource in China,which provides 67.5%of the total energy consumption in 2013[5].Low rank coals such as lignite and sub-bituminous coal are abundant,comprising up to 55%of coal reserves[6].These low rank coals contain high volatile matters.These volatiles can be released through pyrolysis and further converted into liquid fuels and gaseous products.In order to bridge the gaps between the demands and the supplies of oil and gas,various processes for the production of liquid fuel and gas from coal based on pyrolysis are being developed by many research institutions and coal companies in China[7,8].However,over 50%of heavy components in the obtained tar not only increase the difficulty in further processing the tar,but also cause serious operation problems in downstream equipment such as pipe blocking.One option to overcome this problem is in-situ catalytic upgrading of tar to increase its selectivity to light liquids.Catalytic upgrading of tar has been studied by a number of researchers[9–11].Many researches have shown that iron ore exhibits a high activity toward tar cracking/reforming[12–14].Compared with the conventional nickel-based catalysts,iron ore is cheaper and abundant.Moreover,the reactivation procedure could be avoided since the deactivated and reduced iron ore can be used as the feedstock of steel making industry.

In view of the abovementioned facts,an integrated coal pyrolysis with iron ore reduction was proposed in order to obtain high yields of light liquids and gas[15].In the proposed scheme,heavy components of tar in the gas are decomposed/reformed into light species and gas over the iron ore.Simultaneously,the tar-containing gas from pyrolysis of coal is used as a reductant in the production of iron.In the proposed scheme,the iron reduction behaviors could be different from those using natural gas or syngas(CO and H2)as a reductant[3]since the reductant employed in the proposed scheme contains a large amount of tar.It is known that low rank coal contains high volatile matters,which are released in the form of incondensable gases and tar during pyrolysis.The tar content is around 30 wt.%or even higher in the pyrolytic gas from low rank coal[8,16].However,research on the effects of tar on iron reduction especially for the case in the presence of a large amount of tar is rare.

As the first step,the objective of this work is to understand the reduction behavior of iron ore in the presence of a large amount of tar when using coal pyrolysis gas as a reduction agent.The reduction behavior of iron oxide is studied in a lab-scale fixed bed reactor using a simulated coal pyrolysis gas with benzene as a model compound of coal tar.Effects of reduction temperature,reduction time and benzene concentration on iron oxide reduction are discussed.The experiments using the simulated gas without the presence of benzene is also performed for comparison.Various characterization techniques such as X-ray diffraction(XRD)and scanning electron microscopy(SEM)are employed.

2.Experimental

2.1.Apparatus and operation conditions

The experiments were performed in a lab-scale fixed bed reactor as shown in Fig.1.The reactor is quartz glass tube with an inner diameter of 25 mmand a length of 800 mm,which is placed in an electric-heated oven consisting of four silica carbon rods.The reactor temperature is measured by a chromel–alumel thermocouple.Benzene is fed into the reactor through a syringe pump.The simulated coal pyrolysis gas is introduced into the reactor at the top where it is mixed with benzene and its flowrate is regulated by a mass flow controller.The exit of the reactor is connected to two cool traps immersed in an ice–water bath to recover the benzene residues.The exhausted gas is collected with a gas sample collection bag,which is analyzed with a micro-gas chromatograph(CP-4900,Varian).

The composition of coal pyrolysis gas is related to coal type,operation conditions and reactor configuration.The coal pyrolysis gas obtained at low temperatures mainly consists of hydrogen,carbon monoxide,carbon dioxide and methane,which generally accounts for 85 vol%–95 vol%[17,18],with the rest is C2 and C3 with a very low amount.The composition of the simulated coal pyrolysis gas prepared in this work is shown in Table 1,which is made by referring to the gas composition from sub-bituminous coalpyrolysis at low temperatures[19].Theiron oxide sample used in the experiments is pure,purchased from Xilong Chemical Cooperation,Ltd.About 2.9 g sample of iron oxide are placed in the reactor for each test.The reduction temperature is set at 700,800 and 900°C.The simulated pyrolysis gas is fixed at a rate of 77 ml·min-1.The feeding rate of benzene is fixed at 0.05 ml·min-1(corresponding to 573 g·m-3)if not specified.The reduction time is controlled at 10,20,40 and 60 min,respectively.Nitrogen is used as purge gas during heat-up and cool-down to protect the sample from oxidation by air.The tests without benzene addition were carried out as a reference base for investigating benzene effect.

Table 1 Composition of the simulated gas

2.2.Analyses of iron oxide reduction

The quality of sponge iron is primarily ascertained by the degree of metallization,which is defined as follows:

Both the metallic iron and the total iron are determined through the titrimetric method based on National Standard GB 223.7-2002 of China.Another index normally used to measure the extentofiron oxide reduction is the degree of reduction,expressed as follows:

The amount of oxygen removed by reduction can be calculated through the weight loss of the sample after reduction.Due to the presence of benzene in the gas,a large amount of carbon may deposit on the sample during reduction,which partially offsets the weight loss caused by oxygen removal.Thus,the amount of deposited carbon should be taken into account in calculating the weight loss.

Fig.1.Schematic diagram of the experimental set-up.

The carbon content in the reduced sample was analyzed by a carbon–sulfur analyzer(LECO CS-344,USA).The phase compositions of the reduced samples were characterized using XRD(X'Pert MPD Pro,PANalytical,the Netherlands)with the CuKαradiation(λ =0.15408 nm).SEM images were taken on a HITACHI H-8100 microscope to characterize the micro-structures of the reduced samples.

3.Results and Discussion

3.1.Variations of phase compositions with reduction temperature and time

Fig.2.XRD patterns at different temperatures.(a)Without benzene;(b)with benzene.

Fig.2 shows the XRD patterns of the reduced iron oxides obtained at different reduction times and temperatures.For the case without benzene in the feed gas,the reduced product is mainly composed of FeO after experiencing 60 min reduction at 700°C.In contrast,only Fe3O4is observed in the reduced iron oxide sample at the same reduction time for the case with benzene.This indicates that benzene addition may retard iron oxide reduction.However,the situation is quite different at higher temperatures.At 800°C,metallic iron is observed at 10 min for the case with benzene,which is much earlier than without benzene(at 40 min).This implies that benzene addition promotes the reduction reaction.The opposite effect could be attributed to the carbon deposition due to benzene on metal surface during iron oxide reduction.The peak at 2θ=26°indicates the occurrence of graphite.No obvious graphite peak is found for the case without benzene.The carbon deposition over the iron oxide hindered the diffusion of the reducing gas to the particle surface,thus slowing down the reduction rate at low temperatures.At a higher temperature,the reduction reaction by the deposited carbon takes place,which can be expressed as follows:

The reduction by carbon normally occurs at high temperatures(＞1100 °C for the conventional mixture of metallurgy coke and iron ore)[20,21].However,this reduction reaction occurs at a relatively lower temperature of 800°C.Research[21]indicated that the carbonized iron ore is considered more reactive than the mixture of coke and iron ore due to the nanoscale contact of iron ore and carbon that enhances the reactivity.Thus,the presence of benzene in the feed gas exhibits a positive effect at high temperatures over 800°C in terms of iron oxide reduction.It is worth mentioning that cementite(Fe3C)is formed in the presence of benzene.Cementite is considered a premium quality feed for steelmaking,which enables steelmakers to make high quality steel more easily and at low costs[22].Carburization is known to lead to the formation of cementite.It is speculated that the reduction of iron ore is accompanied by the decomposition and reforming reactions of benzene as shown below:

The dry reforming reaction results in a high ratio of CO/CO2in the gas,which favors the carburization reaction.Larachi et al.[13]studied dry reforming reaction of benzene at varying syngas compositions(C6H6/CH4/CO2/CO/N2)with an iron bearing mineral as a catalyst.It was found that benzene conversion significantly increased with the increasing CO2concentration.The presence of CO can increase benzene conversion by preserving more active sites(metallic iron)for dry reforming of benzene on the iron-bearing minerals.It is known that the dry reforming and decomposition of benzene can be catalyzed by metallic iron[23].For the coal pyrolysis gas used in this work,the concentration of CO2is high(25%shown in Table 1).Also,there exists metallic iron due to the reduction atmosphere.The significant increase of hydrogen content and the decrease of CO2content in the gas after the reduction(shown in Fig.3)may evidence the occurrence of the dry reforming reaction of benzene to some extent.Decomposition of benzene(R7)on the iron ore may occur since there is significant carbon deposition observed in the presence of benzene(further discussion can be found in a later section).

Fig.3.H2,CO and CO2 concentrations in the gas after reduction at 40 min.

Fig.4 presents a semi-quantitative analysis of phase composition after reduction,obtained by the normalized RIR(reference intensity ratio)method[24]using X'Pert High Score.It can be seen that the iron sample is nearly completely reduced from hematite to cementite with benzene present at a temperature of 800 °C in 40 min.At 900 °C,this reduction is even faster.It takes only 20 min for the iron to completely be reduced to cementite.Note that the content of cementite decreases when the reduction time is further increased to 60 min at 900°C.In the iron–carbon system,cementite is a metastable compound,which could partially decompose into metallic iron(Fe0)and carbon over a long period of time at the temperature of 900°C[25]:

3.2.Degrees of reduction and metallization

As expected,both reduction degree and metallization degree increase with temperature due to a relatively high reaction rate at high temperature(shown in Fig.5).An increase in reduction time improves the degrees of reduction and metallization.The trends with reaction temperature and reduction time can be explained by the basic principles of chemical thermodynamics,kinetics and fundamental laws of diffusion.

At a temperature of 700°C,the degrees of reduction and metallization in the presence of benzene are lower than those without benzene,while it is opposite at elevated temperatures.This observation is consistent with the XRD analyses presented in the previous section.At elevated temperatures,the reduction reaction in the presence of benzene proceeds much faster than that without benzene.The metallization with benzene reaches up to 99%in 20 min at 900°C,while it is only around 11%without benzene.Two reasons could cause this fact:(1)the composition of the reducing gas is altered due to the decomposition and reforming of benzene as indicated by R(6-7),as evidenced by a significant increase of hydrogen content and a decrease of CO2content in the gaseous products measured at the outlet of the reactor shown in Fig.3;and(2)the deposited carbon as a reducing agent promotes the reduction reaction as indicated by R(3-5).

Fig.4.Phase compositions of reduced iron oxide.(a)Without benzene;(b)with benzene.

3.3.Effect of input benzene concentration on iron reduction

The amount of tar generated during pyrolysis varies with coal types,pyrolysis techniques employed and operation parameters.For the conventional coking process,the tar contained in raw coke oven gas ranges from 80 to 120 g·m-3[26].For low temperature fast pyrolysis of low rank coals,the tar generated is much higher,varying between 340 and 590 g·m-3[27].The amount of tar carried by the reducing gas may exert a significant effect on iron reduction.In this work,varied benzene content in the feed gas was tested in attempt to give a hint on how the content of tar contained in the reducing gas affects iron reduction.

As shown in Fig.6,degrees of reduction and metallization significantly increase with the increasing benzene concentration.When benzene concentration is beyond 573 g·m-3,no obvious effect is observed.Looking at the phase compositions shown in Fig.7,it is interesting to note that there is no cementite formation at low levels of benzene(＜115 g·m-3)under the current circumstances.Cementite becomes dominant when benzene concentration is higher than 573 g·m-3.This kind of variation further consolidates the fact that the formation of cementite is due to the decomposition and CO2reforming of benzene,which result in a high ratio of CO to CO2.

Fig.5.Variations of metallization and reduction degrees with time.

Fig.6.Effect of benzene concentration on iron reduction.(Reduction time:40 min).

Fig.7.Effect of benzene concentration on phase compositions.

3.4.Carbon content of the reduced samples

Table 2 gives the carbon contents of the reduced samples obtained without the presence of benzene in the feed gas.It is found that the contents of carbon are less than 0.8%.It is reasonable since the concentrationof CO2(25%)is high in the feed gas,which thermodynamically inhibits the Boudouard reaction.

Table 2 Carbon content of reduced iron oxide without benzene in 60 min

For the case with the presence of benzene,the carbon content of the samples is extremely high,which can reach up to 60%at 60 min as shown in Fig.8.It is known that the content of carbon in iron carbide is around 6.67%[28],which is much lower than those in the reduced samples.This indicates that there exists a large amount of element carbon in the reduced samples.As shown in Fig.8,the carbon content increases at high temperatures.However,the Boudouard reaction is thermodynamically favored at low temperatures.This phenomenon can be explained by the increased CO/CO2ratio due to a great deal of CO2consumption by benzene reforming at high temperatures.The decompositions of benzene and other hydrocarbons at high temperatures also make a contribution.It is worth mentioning that the carbon deposition rate is low during the initial 10 min.Subsequently,the carbon deposition rate significantly increases.This can be attributed to the catalytic effect of the large amount of iron phase formed during reduction[23].Note that the carbon content reaches the maximum at 20 min for the reduction at a temperature of900°C,whereas the carbon content still increases for the reduction at relatively lower temperatures of 700 and 800°C.As previously mentioned,a large amount of iron carbide is formed at 20 min at 900°C,which deactivates the metallic iron activity for carbon deposition[29].Another possible reason is the gasification of carbon by CO2at high temperatures.The gasification rate is accelerated at high temperatures.It could be the result of the competition between carbon deposition and gasification.

Fig.8.Carbon contents of the reduced samples in the presence of benzene.

3.5.SEM analysis of the reduced samples

SEM images of the reduced iron oxides are shown in Fig.9.It can be seen that sintering occurred in the course of reduction without benzene in the gas as shown in Fig.9(a).No obvious sintering is observed for the sample obtained in the presence of benzene shown in Fig.9(b).Threadlike carbon attaching to iron carbide can be easily recognized in Fig.9(b),which could play the role in preventing the reduced iron sample sintering.

4.Conclusions

Major conclusions from the test results can be summarized as follows:

Fig.9.Images of the reduced samples obtained at 900°C at 60 min.(a)Without benzene;(b)with benzene.

(1)Benzene addition has a retarding effect on iron oxide reduction at low temperatures because the deposited carbon caused by benzene inhibits the diffusion of the reducing gas to the surface of iron oxide.At the temperature higher than 800°C,benzene addition accelerates iron oxide reduction.This can be attributed to the significant increase of hydrogen content and the dramatic decrease of CO2content due to benzene reforming at high temperature,as well as the reduction by the deposited carbon.

(2)The iron oxide can nearly completely be reduced to cementite with benzene present in the feed gas at temperature over 800°C,which is considered a premium quality feed for steelmaking.No cementite is found without benzene present in the simulated gas under the experimental conditions.

(3)The concentration of benzene in the gas has a significant effect on iron reduction.Degrees of metallization and reduction significantly increase with an increase of the benzene concentration in the feed gas under the experimental conditions.

(4)During reduction,a large amount of carbon is deposited on the iron when benzene is present in the gas.At 900°C,the amount of deposited carbon increases with reduction time at first and then levels off,which could be the result of the balance between carbon gasification and deposition reactions.At lower temperature,carbon deposition exhibits a monotonically increasing trend within the examined reduction time.

(5)Without benzene,particle sintering occurs during reduction.No sintering is observed in the reduced sample with benzene present in the gas,which could be attributed to the large amount of deposited carbon.

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