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Integration of coal pyrolysis process with iron ore reduction: Reduction behaviors of iron ore with benzene-containing coal pyrolysis gas as a reducing agent
Integration of Coal Pyrolysis Process with Iron Ore Reduction: Reduction Behaviors of Iron Ore with Benzene-containing Coal Pyrolysis Gas as a Reducing Agent Xin Li, Helong Hui, Songgeng Li, Lu He, Lijie Cui PII: DOI: Reference:
S1004-9541(15)00470-X doi: 10.1016/j.cjche.2015.12.020 CJCHE 460
To appear in: Received date: Revised date: Accepted date:
30 June 2015 5 October 2015 18 October 2015
Please cite this article as: Xin Li, Helong Hui, Songgeng Li, Lu He, Lijie Cui, Integration of Coal Pyrolysis Process with Iron Ore Reduction: Reduction Behaviors of Iron Ore with Benzene-containing Coal Pyrolysis Gas as a Reducing Agent, (2015), doi: 10.1016/j.cjche.2015.12.020
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ACCEPTED MANUSCRIPT Received 30 June 2015 Received in revised form 5 October 2015
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Accepted 18 October 2015
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Energy, Resource and Environmental Technology
Integration of Coal Pyrolysis Process with Iron Ore Reduction: Reduction Behaviors of Iron Ore with Benzene-containing Coal
Xin Li,1
Songgeng Li,1,* Lu He,1 Lijie Cui 2,*
State Key Laboratory of Multiphase Complex Systems, Institute of Process
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Helong Hui,1
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Pyrolysis Gas as a Reducing Agent
University of Chinese Academy of Sciences, Beijing 100049, China
ABSTRACT
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2
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Engineering, Chinese Academy of Sciences, Beijing 100190, China
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An integrated coal pyrolysis process with iron ore reduction is proposed in this article. As the first step, iron oxide reduction is studied in a fixed bed reactor using simulated coal pyrolysis gas with benzene as a model tar compound. Variables such as reduction temperature, reduction time and benzene concentration are studied. The carbon deposition of benzene results in the retarded iron reduction at low temperatures. At high temperatures over 800 oC, the presence of benzene in the gas can promote iron reduction. The metallization can reach up to 99% in 20 min at 900 o
C in the presence of benzene. Significant increases of hydrogen and CO/CO2 ratio
Supported by the joint program of the National Natural Science of Foundation of China and the Shenhua Group Cooperation Limited(51174284). *Corresponding author: [email protected] (S.Li); [email protected] (L. Cui)
ACCEPTED MANUSCRIPT are observed in the gas. It is indicated that iron reduction is accompanied by the reforming and decomposition of benzene. The degree of metallization and reduction
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increases with the increasing benzene concentration. Iron oxide can nearly completely
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be converted into cementite with benzene present in the gas under the experimental conditions. No sintering is found in the reduced sample with benzene in the gas.
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Key words: coal pyrolysis, iron reduction, integration, tar effect
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1. INTRODUCTION
Iron and steel production is in continuous high demand as a result of the growth
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in the world economy. A large amount of coke is required as both reducing agent and
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fuel for iron production using the conventional blast furnace process. Metallurgy coke
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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]
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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 iron-making 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.
ACCEPTED MANUSCRIPT 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
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coal are abundant, comprising up to 55% of coal reserves.[6] These low rank coals
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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
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production of liquid fuel and gas from coal based on pyrolysis are being developed by
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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
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processing the tar, but also cause serious operation problems in downstream
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equipment such as pipe blocking. One option to overcome this problem is in-situ
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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
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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 above mentioned 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
ACCEPTED MANUSCRIPT from pyrolysis of coal is used as a reductant in the production of iron. In the proposed scheme, the iron reduction behaviors could be deferent from those using natural gas or
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syngas (CO and H2) as a reductant [3] since the reductant employed in the proposed
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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
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low rank coal. [8,16] However, research on the effects of tar on iron reduction
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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
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behavior of iron ore in the presence of a large amount of tar when using coal pyrolysis
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gas as a reduction agent. The reduction behavior of iron oxide is studied in a lab-scale
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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
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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 Figure 1. The reactor is quartz glass tube with an inner diameter of 25 mm and a
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Benzene is fed into the reactor through a syringe pump. The simulated coal pyrolysis
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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
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exhausted gas is collected with a gas sample collection bag, which is analyzed with a
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micro-gas chromatograph (CP-4900, Varian).
The composition of coal pyrolysis gas is related to coal type, operation
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conditions and reactor configuration. The coal pyrolysis gas obtained at low
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temperatures mainly consists of hydrogen, carbon monoxide, carbon dioxide and
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methane, which generally accounts for 85v%-95 v% [17,18], with the rest is C2 and C3 with a very low amount. The composition of the simulated coal pyrolysis gas
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prepared in this work is shown in Table 1, which is made by referring to the gas composition from sub-bituminous coal pyrolysis at low temperatures [19]. The iron 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 oC. 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 (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
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1. Nitrogen Cylinder 2. Simulative Gas Cylinder 3. Valve 4. Mass Flowmeter 5. Electric Furnace 6. Quartz Reactor 7. Cold Trap 8. Reservior 9. Syringe Pump 10. Gas Chromatograph
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Figure 1. Schematic diagram of the experimental set-up Table 1. Composition of the simulated gas H2 15
CO 18
CH4 30
CO2 25
C2H6 7
C2H4 5
total 100
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Gas Composition(by volume)/%
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: Degree
of
metallization
mass
of
metallic
iron ( free mass
iron
of
and
iron
combined
with
carbon )
×
total iron
100 (1) 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 extent of iron oxide reduction is the degree of reduction, expressed as
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reduction
oxygen removed by reduction 100 oxygen content before reduction
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Degree of
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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
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the weight loss caused by oxygen removal. Thus, the amount of deposited carbon
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should be taken into account in calculating the weight loss. The carbon content in the reduced sample was analyzed by a carbon–sulfur
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analyzer (LECO CS-344, USA). The phase compositions of the reduced samples were
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characterized using XRD (X' Pert MPD Pro, PANalytical, the Netherlands) with the
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Cu Kα radiation (λ = 0.15408 nm). SEM images were taken on a HITACHI H-8100
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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 Figure 2 shows the XRD patterns of the reduced iron oxides obtained at different reduction time 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 oC. In contrast, only Fe3O4 is 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
ACCEPTED MANUSCRIPT temperatures. At 800 oC, 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
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addition promotes the reduction reaction. The opposite effect could be attributed to
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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
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hindered the diffusion of the reducing gas to the particle surface, thus slowing down
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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:
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0.5C + 3Fe2O3 → 2Fe3O4 +0.5CO2
(3) (4)
0.5C + FeO → Fe + 0.5CO2
(5)
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0.5C + Fe3O4 → 3FeO + 0.5CO2
The reduction by carbon normally occurs at high temperatures (>1100 oC for the
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conventional mixture of metallurgy coke and iron ore).[20,21] However, this reduction reaction occurs at a relatively lower temperature of 800 oC. 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 oC 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
ACCEPTED MANUSCRIPT 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
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below:
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C6H6+CO2 → 12CO+3H2 C6H6 → 6C+3H2
(6) (7)
The dry reforming reaction results in a high ratio of CO/CO2 in the gas, which favors
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the carburization reaction. Larachi et al.[13] studied dry reforming reaction of
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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
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with the increasing CO2 concentration. The presence of CO can increase benzene
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conversion by preserving more active sites (metallic iron) for dry reforming of
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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
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pyrolysis gas used in this work, the concentration of CO2 is 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 CO2 content in the gas after the reduction (shown in Figure 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).
ACCEPTED MANUSCRIPT Fe3C Fe FeO Fe3O4 Fe2O3
700 FeO Fe3O4 Fe2O3
(a)
℃
20min
20min
10min
10min 20
30
40
50
60
70
80
20
o
2θ/( )
Fe3C Fe FeO Fe3O4 Fe2O3
Intensity[a.u]
10min 50
60
2θ[degree]
70
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20min
80
10min 20
30
40
50
60
2θ[degree]
70
80
Fe3C Fe FeO Fe3O4 Fe2O3
900℃ b
Intensity[a.u]
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Intensity[a.u]
b
℃
60min
20min
40min 20min
10min
10min 30
80
800
900℃ a
40min
20
70
20min
Fe3C Fe FeO Fe3O4 Fe2O3
60min
60
60min
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Intensity[a.u]
40min
30
50
2θ[degree]
40min
60min
20
40
Fe3C Fe FeO Fe3O4 Fe2O3
a
℃
30
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800
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Intensity
40min
40min
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700 60min
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Fe
Intensity[a.u]
Fe3C
b
℃
60min
40
50
60
70
80
20
30
40
50
60
2θ[degree]
2θ[degree]
Figure 2.
XRD patterns at different temperatures.
(a) without benzene; (b) with benzene.
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80
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H2
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CO
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Concentration /%
0
25
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CO2
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0
700
800
900
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Temperature /°C With benzene
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Without benzene
Figure 3. H2, CO and CO2 concentrations in the gas after reduction at 40 min
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Figure 4 presents a semi-quantitative analysis of phase composition after reduction, obtained by the normalized RIR (reference intensity ratio) method [24]
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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 oC in 40 min. At 900 oC, 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 oC. In the iron-carbon system, cementite is a metastable compound, which could partially decompose into metallic iron (Feo) and carbon over a long period of time at the temperature of 900 oC [25]: Fe3C→3Fe+C
(8)
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700C (a)
800C (a)
900C (a)
700C (b)
800C (b)
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T
60 40 20 0 100
900C (b)
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Phase composition /%
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80 60 40 0
10
20
40
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10
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20 20
40
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10
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40
60
Time /min Fe3O4
FeO
Fe
Fe3C
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Fe2O3
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Figure 4. Phase compositions of reduced iron oxide
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(a) without benzene; (b) with benzene.
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3.2 Degrees of reduction and metallization As expected, both reduction degree and metallization degree increase with
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temperature due to a relatively high reaction rate at high temperature (shown in Figure 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 oC, 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
ACCEPTED MANUSCRIPT metallization with benzene reaches up to 99% in 20 min at 900 oC, while it is only around 11% without benzene. Two reasons could cause this fact: (1) the composition
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of the reducing gas is altered due to the decomposition and reforming of benzene as
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indicated by R (6-7), as evidenced by a significant increase of hydrogen content and a decrease of CO2 content in the gaseous products measured at the outlet of the reactor shown in Figure 3; and (2) the deposited carbon as a reducing agent promotes the
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reduction reaction as indicated by R (3-5).
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Reduction (%)
900C
800C
700C 80
40
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20 0
900C
800C
60
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Metallization (%)
700C
80
40 20
0 0
20
40
60 0
20
40
60 0
20
40
60
Time (min) Without benzene
With benzene
Figure 5. Variations of metallization and reduction degrees with time
3.3 Effect of input benzene concentration on iron reduction The amount of tar generated during pyrolysis varies with coal types, pyrolysis
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temperature fast pyrolysis of low rank coals, the tar generated is much higher, varying
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between 340-590 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
reducing gas affects iron reduction.
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gas was tested in attempt to give a hint on how the content of tar contained in the
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As shown in Figure 6, degrees of reduction and metallization significantly increase with the increasing benzene concentration. When benzene concentration is
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beyond 573 g·m-3, no obvious effect is observed. Looking at the phase compositions
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shown in Figure 7, it is interesting to note that there is no cementite formation at low
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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
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further consolidate the fact that the formation of cementite is due to the decomposition and CO2 reforming of benzene, which result in a high ratio of CO to CO2.
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100
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Reduction Metallization
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60
40
20
Range of tar content from low temperature fast pyrolysis
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Percentage /%
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80
0 0
500
1000
1500
2000
3
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Benzene content (g/m )
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Figure 6. Effect of benzene concentration on iron reduction
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(reduction time: 40 min)
100
Phase compsition/%
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80
60
40
20
0
0
115
573
1146
1719
Benzene content /g·m Fe2O3
Fe3O4
FeO
2292
-3
Fe
Fe3C
Figure 7. Effect of benzene concentration on phase compositions
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3.4 Carbon content of the reduced samples
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Table 2 gives the carbon contents of the reduced samples obtained without the
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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 concentration of CO2 (25%) is high in the feed
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gas, which thermodynamically inhibits the Boudouard reaction.
Table 2. Carbon content of reduced iron oxide without benzene in 60 min 700℃
Carbon content/%
0.36
800℃
900℃
0.66
0.79
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Temperature
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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 Figure 8. It is
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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
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amount of element carbon in the reduced samples. As shown in Figure 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/CO2 ratio due to a great deal of CO2 consumption 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 minutes. 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
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temperatures of 700 and 800 oC. As previously mentioned, a large amount of iron
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carbide is formed at 20 min at 900 oC, which deactivates the metallic iron activity for carbon deposition.[29] Another possible reason is the gasification of carbon by CO2 at high temperatures. The gasification rate is accelerated at high temperatures. It could
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be the result of the competition between carbon deposition and gasification.
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700C 800C 900C
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40
20
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Carbon content /%
80
0
20
40
60
Time /min
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0
Figure 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 Figure 9. It can be seen that sintering occurred in the course of reduction without benzene in the gas as shown in figure 9(a). No obvious sintering is observed for the sample obtained in the presence of benzene shown in figure 9(b). Threadlike carbon attaching to iron carbide can be easily recognized in Figure 9 (b), which could play the role in preventing the reduced iron samples sintering.
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Figure 9. Images of the reduced samples obtained at 900 oC at 60 min (a) without benezene; (b) with benzene.
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4. CONCLUSIONS
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Major conclusions from the test results can be summarized as follows:
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(1) Benzene addition has a retarding effect on iron oxide reduction at low temperatures because the deposited carbon caused by benzene inhibits the
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diffusion of the reducing gas to the surface of iron oxide. At the temperature higher than 800 oC, benzene addition accelerates iron oxide reduction. This
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can be attributed to the significant increase of hydrogen content and the dramatic decrease of CO2 content 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 cenmentite with benzene present in the feed gas at temperature over 800 oC, which is considered a premium quality feed for steelmaking. No cenmentite 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
ACCEPTED MANUSCRIPT an increase of the benzene concentration in the feed gas under the experimental conditions.
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(4) During reduction, a large amount of carbon is deposited on the iron when
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benzene is present in the gas. At 900 oC, 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
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lower temperature, carbon deposition exhibits a monotonically increasing
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trend within the examined reduction time.
(5) Without benzene, particles sintering occurs during reduction. No sintering is
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observed in the reduced sample with benzene present in the gas, which could
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be attributed to the large amount of deposited carbon.
REFERENCES
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[1] Fan R. Coking Coal Resource and its Industrial Development. Chem. Ind. 26 (2008) 1-8.
[2] Hosokai S., Matsui K., Okinaka N., Ohno K., Shimizu M., Akiyama T. Kinetic Study on the Reduction Reaction of Biomass-tar-infiltrated Iron Ore. Energy Fuels. 26 (2012) 7274-7279. [3] Kopfle J., Robert H. Direct Reduction’s Role in the World Steel Industry. Ironmaking & Steelmaking. 35 (2008) 254-259. [4] Zhang T., Lei C., Zhu Q. Reduction of Fine Iron Ore via a Two-step Fluidized Bed Direct Reduction Process. Powder Technol. 254 (2014) 1-11.
ACCEPTED MANUSCRIPT [5] BP
Statistical
Review
of
World
Energy
2014.
http://www.bp.com/zh_cn/china/reports-and-publications/bp_2014.html.
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[6] Zhang G. Report on China’s Energy Development for 2010; Economic Science
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Press: Beijing, 2011. [7] Li A., Li X., Li S., Ren Y., Chi Y., Yan J., Chen K.
Pyrolysis of Solid Waste in
a Rotary Kiln: Influence of Final Pyrolysis Temperature on the Pyrolysis
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Products. J. Anal. Appl. Pyrolysis. 50 (1999) 149-162.
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[8] Jin L., Zhou X., He X., Hu H., Integrated Coal Pyrolysis with Methane Aromatization over Mo/HZSM-5 for Improving Tar Yield. Fuel. 44 (2013)
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187-190.
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[9] Zhang H., Xiao R., Wang D., Zhong Z., Song M., Pan Q., He G. Catalytic Fast
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Pyrolysis of Biomass in a Fluidized Bed with Fresh and Spent Fluidized Catalytic Cracking Catalysts. Energy Fuels. 23 (2009) 6199-6206.
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[10] Matsuhara T., Hosokai S., Norinaga K., Matasuoka K., Li C., Hayashi J. In-situ Reforming of Tar from the Rapid Pyrolysis of a Brown Coal over Char. Energy Fuels. 24 (2010) 76-83. [11] Gong X., Wang Z., Li S., Song W., Lin W. Coal Pyrolysis in a Laboratory-scale Two-stage Reactor: Catalytic Upgrading of Pyrolytic Vapors. Chem. Eng. Technol. 37 (2014) 2135-2142. [12] Li L., Morishita K., Takarada T. Light Fuel Gas Production from Nascent Coal Volatiles Using a Natural Limonite Ore. Fuel. 86 (2007) 1570-1576. [13] Sarvaramini A., Larachi F. Catalytic Oxygenless Steam Cracking of
ACCEPTED MANUSCRIPT Syngas-containing Benzene Model Tar Compound over Natural Fe-bearing Silicate Minerals. Fuel. 97 (2012) 741-750.
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[14] Min Z., Yimsiri P., Zhang S., Wang Y., Asadullah M., Li C. Catalytic Reforming
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of Tar during Gasification. Part III. Effects of Feedstock on Tar Reforming Using Ilmenite as a Catalyst. Fuel. 103 (2013) 950-955.
[15] Li S., Song W., Hao L., Lin W. Method and Device for Coupling of Pyrolysis of
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Solid Fuels with Iron Ore Reduction. Patent CN 2012 1.0353477.9.
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[16] Dong P., Wang Z., Li Z., Li S., Lin W., Song W. Experimental Study on Pyrolysis Behaviors of Coal in a Countercurrent Downer Reactor. Energy Fuels.
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26 (2012) 5193-5198.
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[17] Yang C., Li S., Song W., Lin W. Pyrolysis Behavior of Large Coal Particles in a
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Lab-scale Bubbling Fluidized Bed. Energy Fuels. 27 (2013) 126-132. [18] Qu X., Liang P., Wang Z., Zhang R., Sun D., Gong X., Gan Z., Bi J. Pilot
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Development of a Polygeneration Process of Circulating Fluidized Bed Combustion combined with Coal Pyrolysis. Chem. Eng. Technol. 34 (2011) 61-68.
[19] Li S., Song W., Lin W. Coal Topping Pyrolysis Process: Fundamentals and Its Application. 2014 AIChE annual meeting, Atlanta, USA, 2014. [20] Cahyono R., Saito G., Yasuda N., Nomura T., Akiyama T. Porous Ore Structure and Deposited Carbon Type during Integrated Pyrolysis−Tar Decomposition. Energy Fuels. 28 (2014) 2129-2134. [21] Cahyono R., Rozhan A., Yasuda Y., Nomura T., Hosokai S., Kashiwaya Y.,
ACCEPTED MANUSCRIPT Akiyama T. Catalytic Coal-tar Decomposition to Enhance Reactivity of Low-grade Iron Ore. Fuel Process. Technol. 113 (2013) 84-89.
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[22] Shoji H., Yoshiaki I. Production of Iron Carbide from Iron Ores in a Fluidized
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Bed. ISIJ. Int. 38 (1998) 1053-1061.
[23] Xu M., Brown J. Mechanism of Iron Catalysis of Carbon Monoxide Decomposition in Refractories. J. Am. Ceram. Soc. 72 (1989) 110-115.
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[24] Chung F H. Quantitative Interpretation of X-ray Diffraction Patterns:I.
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Matrix-flushing Method of Quantitative Multicomponent Analysis. J. Appl. Crys. 7, 513−519 (1974).
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[25] Wang G., Jiang M., Wang W. The Preparation of Iron Carbide by Gas Reduction.
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Acta Metallurgica Sinica. 34 (1998) 769-773.
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[26] Yang J., Wang X., Li L., Shen K., Lu X., Ding W. Catalytic Conversion of Tar from Hot Coke oven Gas Using 1-methylnaphthalene as a Tar Model Compound.
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Appl. Catal. B. 96 (2010) 232-237. [27] Cui L., Lin W., Yao J. Influences of Temperature and Coal Particle Size on the Flash Pyrolysis of Coal in a Fast-entrained bed. Chem．Res．Chinese U． 22 (2006) l03-110. [28] Krauss G. Steels: Processing, Structure, and Performance, first ed. ASM International, 2005. [29] Yang K., Yang R. The Accelerating and Retarding Effects of Hydrogen on Carbon Deposition on Metal Surfaces. Carbon. 24 (1986) 687-693.
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Graphical abstract
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Range of tar content from low temperature fast pyrolysis
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1000
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2000
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Benzene content (g/m )
An integrated coal pyrolysis process with iron reduction was proposed. Opposite effects on iron reduction were observed at low and high temperatures due to the
ACCEPTED MANUSCRIPT presence of a large amount of coal tar in coal pyrolysis gas. Iron oxide can nearly completely be reduced to cementite with coal tar present in the gas. The content of
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coal tar in the pyrolysis gas has a significant effect on iron reduction.
S1004-9541(15)00470-X doi: 10.1016/j.cjche.2015.12.020 CJCHE 460
To appear in: Received date: Revised date: Accepted date:
30 June 2015 5 October 2015 18 October 2015
Please cite this article as: Xin Li, Helong Hui, Songgeng Li, Lu He, Lijie Cui, Integration of Coal Pyrolysis Process with Iron Ore Reduction: Reduction Behaviors of Iron Ore with Benzene-containing Coal Pyrolysis Gas as a Reducing Agent, (2015), doi: 10.1016/j.cjche.2015.12.020
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ACCEPTED MANUSCRIPT Received 30 June 2015 Received in revised form 5 October 2015
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Accepted 18 October 2015
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Energy, Resource and Environmental Technology
Integration of Coal Pyrolysis Process with Iron Ore Reduction: Reduction Behaviors of Iron Ore with Benzene-containing Coal
Xin Li,1
Songgeng Li,1,* Lu He,1 Lijie Cui 2,*
State Key Laboratory of Multiphase Complex Systems, Institute of Process
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Helong Hui,1
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Pyrolysis Gas as a Reducing Agent
University of Chinese Academy of Sciences, Beijing 100049, China
ABSTRACT
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2
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Engineering, Chinese Academy of Sciences, Beijing 100190, China
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An integrated coal pyrolysis process with iron ore reduction is proposed in this article. As the first step, iron oxide reduction is studied in a fixed bed reactor using simulated coal pyrolysis gas with benzene as a model tar compound. Variables such as reduction temperature, reduction time and benzene concentration are studied. The carbon deposition of benzene results in the retarded iron reduction at low temperatures. At high temperatures over 800 oC, the presence of benzene in the gas can promote iron reduction. The metallization can reach up to 99% in 20 min at 900 o
C in the presence of benzene. Significant increases of hydrogen and CO/CO2 ratio
Supported by the joint program of the National Natural Science of Foundation of China and the Shenhua Group Cooperation Limited(51174284). *Corresponding author: [email protected] (S.Li); [email protected] (L. Cui)
ACCEPTED MANUSCRIPT are observed in the gas. It is indicated that iron reduction is accompanied by the reforming and decomposition of benzene. The degree of metallization and reduction
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increases with the increasing benzene concentration. Iron oxide can nearly completely
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be converted into cementite with benzene present in the gas under the experimental conditions. No sintering is found in the reduced sample with benzene in the gas.
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Key words: coal pyrolysis, iron reduction, integration, tar effect
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1. INTRODUCTION
Iron and steel production is in continuous high demand as a result of the growth
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in the world economy. A large amount of coke is required as both reducing agent and
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fuel for iron production using the conventional blast furnace process. Metallurgy coke
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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]
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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 iron-making 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.
ACCEPTED MANUSCRIPT 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
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coal are abundant, comprising up to 55% of coal reserves.[6] These low rank coals
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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
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production of liquid fuel and gas from coal based on pyrolysis are being developed by
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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
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processing the tar, but also cause serious operation problems in downstream
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equipment such as pipe blocking. One option to overcome this problem is in-situ
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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
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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 above mentioned 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
ACCEPTED MANUSCRIPT from pyrolysis of coal is used as a reductant in the production of iron. In the proposed scheme, the iron reduction behaviors could be deferent from those using natural gas or
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syngas (CO and H2) as a reductant [3] since the reductant employed in the proposed
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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
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low rank coal. [8,16] However, research on the effects of tar on iron reduction
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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
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behavior of iron ore in the presence of a large amount of tar when using coal pyrolysis
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gas as a reduction agent. The reduction behavior of iron oxide is studied in a lab-scale
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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
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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 Figure 1. The reactor is quartz glass tube with an inner diameter of 25 mm and a
ACCEPTED MANUSCRIPT 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.
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Benzene is fed into the reactor through a syringe pump. The simulated coal pyrolysis
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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
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exhausted gas is collected with a gas sample collection bag, which is analyzed with a
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micro-gas chromatograph (CP-4900, Varian).
The composition of coal pyrolysis gas is related to coal type, operation
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conditions and reactor configuration. The coal pyrolysis gas obtained at low
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temperatures mainly consists of hydrogen, carbon monoxide, carbon dioxide and
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methane, which generally accounts for 85v%-95 v% [17,18], with the rest is C2 and C3 with a very low amount. The composition of the simulated coal pyrolysis gas
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prepared in this work is shown in Table 1, which is made by referring to the gas composition from sub-bituminous coal pyrolysis at low temperatures [19]. The iron 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 oC. 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 (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
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1. Nitrogen Cylinder 2. Simulative Gas Cylinder 3. Valve 4. Mass Flowmeter 5. Electric Furnace 6. Quartz Reactor 7. Cold Trap 8. Reservior 9. Syringe Pump 10. Gas Chromatograph
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Figure 1. Schematic diagram of the experimental set-up Table 1. Composition of the simulated gas H2 15
CO 18
CH4 30
CO2 25
C2H6 7
C2H4 5
total 100
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Gas Composition(by volume)/%
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: Degree
of
metallization
mass
of
metallic
iron ( free mass
iron
of
and
iron
combined
with
carbon )
×
total iron
100 (1) 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 extent of iron oxide reduction is the degree of reduction, expressed as
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reduction
oxygen removed by reduction 100 oxygen content before reduction
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Degree of
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(2)
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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
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the weight loss caused by oxygen removal. Thus, the amount of deposited carbon
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should be taken into account in calculating the weight loss. The carbon content in the reduced sample was analyzed by a carbon–sulfur
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analyzer (LECO CS-344, USA). The phase compositions of the reduced samples were
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characterized using XRD (X' Pert MPD Pro, PANalytical, the Netherlands) with the
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Cu Kα radiation (λ = 0.15408 nm). SEM images were taken on a HITACHI H-8100
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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 Figure 2 shows the XRD patterns of the reduced iron oxides obtained at different reduction time 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 oC. In contrast, only Fe3O4 is 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
ACCEPTED MANUSCRIPT temperatures. At 800 oC, 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
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addition promotes the reduction reaction. The opposite effect could be attributed to
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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
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hindered the diffusion of the reducing gas to the particle surface, thus slowing down
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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:
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0.5C + 3Fe2O3 → 2Fe3O4 +0.5CO2
(3) (4)
0.5C + FeO → Fe + 0.5CO2
(5)
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0.5C + Fe3O4 → 3FeO + 0.5CO2
The reduction by carbon normally occurs at high temperatures (>1100 oC for the
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conventional mixture of metallurgy coke and iron ore).[20,21] However, this reduction reaction occurs at a relatively lower temperature of 800 oC. 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 oC 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
ACCEPTED MANUSCRIPT 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
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below:
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C6H6+CO2 → 12CO+3H2 C6H6 → 6C+3H2
(6) (7)
The dry reforming reaction results in a high ratio of CO/CO2 in the gas, which favors
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the carburization reaction. Larachi et al.[13] studied dry reforming reaction of
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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
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with the increasing CO2 concentration. The presence of CO can increase benzene
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conversion by preserving more active sites (metallic iron) for dry reforming of
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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
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pyrolysis gas used in this work, the concentration of CO2 is 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 CO2 content in the gas after the reduction (shown in Figure 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).
ACCEPTED MANUSCRIPT Fe3C Fe FeO Fe3O4 Fe2O3
700 FeO Fe3O4 Fe2O3
(a)
℃
20min
20min
10min
10min 20
30
40
50
60
70
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20
o
2θ/( )
Fe3C Fe FeO Fe3O4 Fe2O3
Intensity[a.u]
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60
2θ[degree]
70
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20min
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10min 20
30
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2θ[degree]
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Fe3C Fe FeO Fe3O4 Fe2O3
900℃ b
Intensity[a.u]
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Intensity[a.u]
b
℃
60min
20min
40min 20min
10min
10min 30
80
800
900℃ a
40min
20
70
20min
Fe3C Fe FeO Fe3O4 Fe2O3
60min
60
60min
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Intensity[a.u]
40min
30
50
2θ[degree]
40min
60min
20
40
Fe3C Fe FeO Fe3O4 Fe2O3
a
℃
30
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800
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Intensity
40min
40min
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Fe
Intensity[a.u]
Fe3C
b
℃
60min
40
50
60
70
80
20
30
40
50
60
2θ[degree]
2θ[degree]
Figure 2.
XRD patterns at different temperatures.
(a) without benzene; (b) with benzene.
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H2
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CO
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Concentration /%
0
25
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CO2
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700
800
900
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Temperature /°C With benzene
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Without benzene
Figure 3. H2, CO and CO2 concentrations in the gas after reduction at 40 min
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Figure 4 presents a semi-quantitative analysis of phase composition after reduction, obtained by the normalized RIR (reference intensity ratio) method [24]
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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 oC in 40 min. At 900 oC, 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 oC. In the iron-carbon system, cementite is a metastable compound, which could partially decompose into metallic iron (Feo) and carbon over a long period of time at the temperature of 900 oC [25]: Fe3C→3Fe+C
(8)
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700C (a)
800C (a)
900C (a)
700C (b)
800C (b)
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60 40 20 0 100
900C (b)
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Phase composition /%
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80 60 40 0
10
20
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10
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10
20
40
60
Time /min Fe3O4
FeO
Fe
Fe3C
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Fe2O3
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Figure 4. Phase compositions of reduced iron oxide
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(a) without benzene; (b) with benzene.
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3.2 Degrees of reduction and metallization As expected, both reduction degree and metallization degree increase with
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temperature due to a relatively high reaction rate at high temperature (shown in Figure 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 oC, 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
ACCEPTED MANUSCRIPT metallization with benzene reaches up to 99% in 20 min at 900 oC, while it is only around 11% without benzene. Two reasons could cause this fact: (1) the composition
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of the reducing gas is altered due to the decomposition and reforming of benzene as
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indicated by R (6-7), as evidenced by a significant increase of hydrogen content and a decrease of CO2 content in the gaseous products measured at the outlet of the reactor shown in Figure 3; and (2) the deposited carbon as a reducing agent promotes the
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reduction reaction as indicated by R (3-5).
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Reduction (%)
900C
800C
700C 80
40
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20 0
900C
800C
60
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Metallization (%)
700C
80
40 20
0 0
20
40
60 0
20
40
60 0
20
40
60
Time (min) Without benzene
With benzene
Figure 5. Variations of metallization and reduction degrees with time
3.3 Effect of input benzene concentration on iron reduction The amount of tar generated during pyrolysis varies with coal types, pyrolysis
ACCEPTED MANUSCRIPT 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
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temperature fast pyrolysis of low rank coals, the tar generated is much higher, varying
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between 340-590 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
reducing gas affects iron reduction.
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gas was tested in attempt to give a hint on how the content of tar contained in the
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As shown in Figure 6, degrees of reduction and metallization significantly increase with the increasing benzene concentration. When benzene concentration is
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beyond 573 g·m-3, no obvious effect is observed. Looking at the phase compositions
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shown in Figure 7, it is interesting to note that there is no cementite formation at low
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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
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further consolidate the fact that the formation of cementite is due to the decomposition and CO2 reforming of benzene, which result in a high ratio of CO to CO2.
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100
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Reduction Metallization
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60
40
20
Range of tar content from low temperature fast pyrolysis
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Percentage /%
T
80
0 0
500
1000
1500
2000
3
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Benzene content (g/m )
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Figure 6. Effect of benzene concentration on iron reduction
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(reduction time: 40 min)
100
Phase compsition/%
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80
60
40
20
0
0
115
573
1146
1719
Benzene content /g·m Fe2O3
Fe3O4
FeO
2292
-3
Fe
Fe3C
Figure 7. Effect of benzene concentration on phase compositions
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3.4 Carbon content of the reduced samples
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Table 2 gives the carbon contents of the reduced samples obtained without the
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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 concentration of CO2 (25%) is high in the feed
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gas, which thermodynamically inhibits the Boudouard reaction.
Table 2. Carbon content of reduced iron oxide without benzene in 60 min 700℃
Carbon content/%
0.36
800℃
900℃
0.66
0.79
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Temperature
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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 Figure 8. It is
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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
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amount of element carbon in the reduced samples. As shown in Figure 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/CO2 ratio due to a great deal of CO2 consumption 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 minutes. 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
ACCEPTED MANUSCRIPT carbon content reaches the maximum at 20 min for the reduction at a temperature of 900 oC, whereas the carbon content still increases for the reduction at relatively lower
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temperatures of 700 and 800 oC. As previously mentioned, a large amount of iron
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carbide is formed at 20 min at 900 oC, which deactivates the metallic iron activity for carbon deposition.[29] Another possible reason is the gasification of carbon by CO2 at high temperatures. The gasification rate is accelerated at high temperatures. It could
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be the result of the competition between carbon deposition and gasification.
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700C 800C 900C
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60
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40
20
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Carbon content /%
80
0
20
40
60
Time /min
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0
Figure 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 Figure 9. It can be seen that sintering occurred in the course of reduction without benzene in the gas as shown in figure 9(a). No obvious sintering is observed for the sample obtained in the presence of benzene shown in figure 9(b). Threadlike carbon attaching to iron carbide can be easily recognized in Figure 9 (b), which could play the role in preventing the reduced iron samples sintering.
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Figure 9. Images of the reduced samples obtained at 900 oC at 60 min (a) without benezene; (b) with benzene.
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4. CONCLUSIONS
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Major conclusions from the test results can be summarized as follows:
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(1) Benzene addition has a retarding effect on iron oxide reduction at low temperatures because the deposited carbon caused by benzene inhibits the
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diffusion of the reducing gas to the surface of iron oxide. At the temperature higher than 800 oC, benzene addition accelerates iron oxide reduction. This
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can be attributed to the significant increase of hydrogen content and the dramatic decrease of CO2 content 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 cenmentite with benzene present in the feed gas at temperature over 800 oC, which is considered a premium quality feed for steelmaking. No cenmentite 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
ACCEPTED MANUSCRIPT an increase of the benzene concentration in the feed gas under the experimental conditions.
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(4) During reduction, a large amount of carbon is deposited on the iron when
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benzene is present in the gas. At 900 oC, 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
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lower temperature, carbon deposition exhibits a monotonically increasing
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trend within the examined reduction time.
(5) Without benzene, particles sintering occurs during reduction. No sintering is
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observed in the reduced sample with benzene present in the gas, which could
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be attributed to the large amount of deposited carbon.
REFERENCES
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[1] Fan R. Coking Coal Resource and its Industrial Development. Chem. Ind. 26 (2008) 1-8.
[2] Hosokai S., Matsui K., Okinaka N., Ohno K., Shimizu M., Akiyama T. Kinetic Study on the Reduction Reaction of Biomass-tar-infiltrated Iron Ore. Energy Fuels. 26 (2012) 7274-7279. [3] Kopfle J., Robert H. Direct Reduction’s Role in the World Steel Industry. Ironmaking & Steelmaking. 35 (2008) 254-259. [4] Zhang T., Lei C., Zhu Q. Reduction of Fine Iron Ore via a Two-step Fluidized Bed Direct Reduction Process. Powder Technol. 254 (2014) 1-11.
ACCEPTED MANUSCRIPT [5] BP
Statistical
Review
of
World
Energy
2014.
http://www.bp.com/zh_cn/china/reports-and-publications/bp_2014.html.
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T
[6] Zhang G. Report on China’s Energy Development for 2010; Economic Science
SC R
Press: Beijing, 2011. [7] Li A., Li X., Li S., Ren Y., Chi Y., Yan J., Chen K.
Pyrolysis of Solid Waste in
a Rotary Kiln: Influence of Final Pyrolysis Temperature on the Pyrolysis
NU
Products. J. Anal. Appl. Pyrolysis. 50 (1999) 149-162.
MA
[8] Jin L., Zhou X., He X., Hu H., Integrated Coal Pyrolysis with Methane Aromatization over Mo/HZSM-5 for Improving Tar Yield. Fuel. 44 (2013)
D
187-190.
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[9] Zhang H., Xiao R., Wang D., Zhong Z., Song M., Pan Q., He G. Catalytic Fast
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Pyrolysis of Biomass in a Fluidized Bed with Fresh and Spent Fluidized Catalytic Cracking Catalysts. Energy Fuels. 23 (2009) 6199-6206.
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[10] Matsuhara T., Hosokai S., Norinaga K., Matasuoka K., Li C., Hayashi J. In-situ Reforming of Tar from the Rapid Pyrolysis of a Brown Coal over Char. Energy Fuels. 24 (2010) 76-83. [11] Gong X., Wang Z., Li S., Song W., Lin W. Coal Pyrolysis in a Laboratory-scale Two-stage Reactor: Catalytic Upgrading of Pyrolytic Vapors. Chem. Eng. Technol. 37 (2014) 2135-2142. [12] Li L., Morishita K., Takarada T. Light Fuel Gas Production from Nascent Coal Volatiles Using a Natural Limonite Ore. Fuel. 86 (2007) 1570-1576. [13] Sarvaramini A., Larachi F. Catalytic Oxygenless Steam Cracking of
ACCEPTED MANUSCRIPT Syngas-containing Benzene Model Tar Compound over Natural Fe-bearing Silicate Minerals. Fuel. 97 (2012) 741-750.
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T
[14] Min Z., Yimsiri P., Zhang S., Wang Y., Asadullah M., Li C. Catalytic Reforming
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of Tar during Gasification. Part III. Effects of Feedstock on Tar Reforming Using Ilmenite as a Catalyst. Fuel. 103 (2013) 950-955.
[15] Li S., Song W., Hao L., Lin W. Method and Device for Coupling of Pyrolysis of
NU
Solid Fuels with Iron Ore Reduction. Patent CN 2012 1.0353477.9.
MA
[16] Dong P., Wang Z., Li Z., Li S., Lin W., Song W. Experimental Study on Pyrolysis Behaviors of Coal in a Countercurrent Downer Reactor. Energy Fuels.
D
26 (2012) 5193-5198.
TE
[17] Yang C., Li S., Song W., Lin W. Pyrolysis Behavior of Large Coal Particles in a
CE P
Lab-scale Bubbling Fluidized Bed. Energy Fuels. 27 (2013) 126-132. [18] Qu X., Liang P., Wang Z., Zhang R., Sun D., Gong X., Gan Z., Bi J. Pilot
AC
Development of a Polygeneration Process of Circulating Fluidized Bed Combustion combined with Coal Pyrolysis. Chem. Eng. Technol. 34 (2011) 61-68.
[19] Li S., Song W., Lin W. Coal Topping Pyrolysis Process: Fundamentals and Its Application. 2014 AIChE annual meeting, Atlanta, USA, 2014. [20] Cahyono R., Saito G., Yasuda N., Nomura T., Akiyama T. Porous Ore Structure and Deposited Carbon Type during Integrated Pyrolysis−Tar Decomposition. Energy Fuels. 28 (2014) 2129-2134. [21] Cahyono R., Rozhan A., Yasuda Y., Nomura T., Hosokai S., Kashiwaya Y.,
ACCEPTED MANUSCRIPT Akiyama T. Catalytic Coal-tar Decomposition to Enhance Reactivity of Low-grade Iron Ore. Fuel Process. Technol. 113 (2013) 84-89.
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T
[22] Shoji H., Yoshiaki I. Production of Iron Carbide from Iron Ores in a Fluidized
SC R
Bed. ISIJ. Int. 38 (1998) 1053-1061.
[23] Xu M., Brown J. Mechanism of Iron Catalysis of Carbon Monoxide Decomposition in Refractories. J. Am. Ceram. Soc. 72 (1989) 110-115.
NU
[24] Chung F H. Quantitative Interpretation of X-ray Diffraction Patterns:I.
MA
Matrix-flushing Method of Quantitative Multicomponent Analysis. J. Appl. Crys. 7, 513−519 (1974).
D
[25] Wang G., Jiang M., Wang W. The Preparation of Iron Carbide by Gas Reduction.
TE
Acta Metallurgica Sinica. 34 (1998) 769-773.
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[26] Yang J., Wang X., Li L., Shen K., Lu X., Ding W. Catalytic Conversion of Tar from Hot Coke oven Gas Using 1-methylnaphthalene as a Tar Model Compound.
AC
Appl. Catal. B. 96 (2010) 232-237. [27] Cui L., Lin W., Yao J. Influences of Temperature and Coal Particle Size on the Flash Pyrolysis of Coal in a Fast-entrained bed. Chem．Res．Chinese U． 22 (2006) l03-110. [28] Krauss G. Steels: Processing, Structure, and Performance, first ed. ASM International, 2005. [29] Yang K., Yang R. The Accelerating and Retarding Effects of Hydrogen on Carbon Deposition on Metal Surfaces. Carbon. 24 (1986) 687-693.
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Graphical abstract
100
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Percentage /%
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Reduction Metallization
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Range of tar content from low temperature fast pyrolysis
0 0
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Benzene content (g/m )
An integrated coal pyrolysis process with iron reduction was proposed. Opposite effects on iron reduction were observed at low and high temperatures due to the
ACCEPTED MANUSCRIPT presence of a large amount of coal tar in coal pyrolysis gas. Iron oxide can nearly completely be reduced to cementite with coal tar present in the gas. The content of
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coal tar in the pyrolysis gas has a significant effect on iron reduction.