- Email: [email protected]
Reduction of iron oxide by lignin: Characteristics, kinetics and superiority
Journal Pre-proof Reduction of Iron Oxide by Lignin: Characteristics, Kinetics and Superiority
Rufei Wei, Dongwen Xiang, Hongming Long, Chunbao (Charles) Xu, Jiaxin Li PII:
S0360-5442(20)30310-8
DOI:
https://doi.org/10.1016/j.energy.2020.117203
Reference:
EGY 117203
To appear in:
Energy
Received Date:
05 October 2019
Accepted Date:
19 February 2020
Please cite this article as: Rufei Wei, Dongwen Xiang, Hongming Long, Chunbao (Charles) Xu, Jiaxin Li, Reduction of Iron Oxide by Lignin: Characteristics, Kinetics and Superiority, Energy (2020), https://doi.org/10.1016/j.energy.2020.117203
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof
Reduction of Iron Oxide by Lignin: Characteristics, Kinetics and Superiority Rufei Wei a, Dongwen Xiang b, Hongming Long a, Chunbao (Charles) Xu a, c*, Jiaxin Lia
a. School of Metallurgical Engineering, Anhui University of Technology, Ma’anshan, Anhui 243002, China b. School of Metallurgy, Northeastern University, Shenyang, Liaoning, 110004 China c. Institute for Chemicals and Fuels from Alternative Resources (ICFAR), Department of Chemical and Biochemical Engineering, Western University, Ontario N6GA5B9, Canada.
Corresponding authors: [email protected] (D. Xiang), [email protected] (H. Long), [email protected] (C. Xu).
Journal Pre-proof Abstract: Reduction of iron oxide by biomass (a renewable energy) instead of fossil energy can greatly reduce greenhouse gas (carbon dioxide) emissions. In this work, the reduction characteristics and kinetics of iron oxide by lignin (a main component of biomass) were studied, aiming at efficient utilization of lignin as a renewable and highly reactive carbon substitute for coal. The reduction temperature range of iron oxide by lignin was found to be mainly 750-900C. It was also observed that the presence of iron could catalyze the pyrolysis of lignin, while the pyrolysis products of lignin promoted the reduction of iron oxide. An increase in the lignin-to-iron oxide mass ratio lowered the temperature at the maximum mass-loss rate determined by TGA. The activation energy varied, increasing first and then decreasing, while increasing the reaction fraction (α), with the turning point at α = 0.4. Compared with CO and coal, lignin appeared to be superior for reducing iron oxide, owing to the formation of nanometerthickness carbon film in the process. The temperature at the maximum reduction rate was 134C for lignin, much lower than that of coal.
Key words: Lignin; Iron oxide; Biomass; Reduction reaction; Kinetics, Pyrometallurgy
Journal Pre-proof 1 Introduction With the depletion of fossil energy and deterioration of ecological environment, development of sustainable resources for fuel and chemical products has attracted increased attention. Biomass is a renewable carbon source, and the fourth largest energy source, after coal, oil and gas. In addition, biomass has many superior characteristics such as its wide distribution, rich in resources, low contents of harmful elements and a low pyrolysis temperature. Therefore, it is promising for biomass as a heating agent or reducing agent for pyrometallurgy process, substituting for coal or coke. Meanwhile, biomass can be a sustainable feedstock for energy and chemical industries. According to statistics, biomass energy contributes to 10 ～ 14% in the global total energy consumption[1]. Iron and steel metallurgy consumes a lot of coal, nature gas and coke, in particular in ironmaking process, and emits an immense amount of carbon dioxide, from both blast furnace ironmaking [2-4] and non-blast furnace ironmaking processes [56].
Compared with blast furnace ironmaking process, non-blast furnace ironmaking
process is inferior in terms of comprehensive benefits, as it has higher energy consumption [7] and produces molten iron of low quality. Most of the previous studies on reduction of metal oxides by biomass were focused on iron oxide reduction. The reduction characteristics and mechanism of reducing iron oxide by biomass have been studied for both traditional blast furnace ironmaking [8-10] and direct reduction ironmaking technologies [11-15]. Udea and Watanabe [8] studied the reactivity and reduction behavior of bio-char, realizing a lower reducing temperature for iron ore block in blast furnace. Thomas et al.
[13]
tested agricultural residues as
substitutes for traditional fossil fuel reductants for iron oxides in a rotary hearth furnace, and the results showed that the reduction degree of iron oxides with the charcoal made from agricultural residues was higher than that of coke. Nurul et al.
[16]
studied
reduction of Malaysian low-grade ore by oil-palm shell. The output of iron was as high as 62.7%, indicating that the oil-palm shell char has great potential in substituting coke as a reductant for low-grade ores. Hu et al. [10] studied reducing the hematite by biochar powder by thermogravimetric analyzer (TGA). The results showed that the iron oxide
Journal Pre-proof reduction temperature with biomass was 100C lower than those of the pulverized coal and coke powder. The reaction rate and the final reaction extent increased owing to the high reactivity of the bio-char [17]. In addition, many studies have shown that iron oxide has catalytic effect on biomass pyrolysis. At 800 ℃ , the impregnation of Fe (III) led to almost complete conversion of the solid biomass into gas/liquid products, producing an extremely low char yield, and a very high yield of combustible gas[18]. Lahijani et al. [19] investigated the influence of alkali (Na, K), alkaline earth (Ca, Mg) and transition (Fe) metal nitrates on CO2 gasification reactivity of pistachio nut shell (PNS) char. The preliminary gasification experiments were performed by TGA and the results showed considerable improvement in carbon conversion. Moud et al.
[20]
found that iron-based materials
were potential candidates for application in a pyrolysis gas pre-conditioning step before further treatment or use, and a way of generating a pyrolysis gas enriched in hydrogen. Previous studies mainly focused on the effects of biomass on metallic oxide reduction, with less attention paid in the roles of the components of lignocellulosic biomass (cellulose, hemicellulose and lignin) in the metallic oxide reduction. Lignocellulosic biomass contains three main components: cellulose (30-50%), hemicelluloses (30-40%) and lignin (20-30%). In nature, the reserve of lignin is lower than those of cellulose and hemicellulose, and it regenerates at the rate of approx. 1500 million tons per year. About 50 million tons of lignin is produced as a by-product from the pulp and paper industry every year [21]. The complexity of the lignin structure unit (phenolics), the diversity of types of lignin and the complexity of chemicals derived from lignin pyrolysis, resulted in scarce in lignin value-added utilization, and by far lignin utilization ratio is less than 2%
[22-23].
So far, more than 95% of the technical
lignin obtained mainly from pulping process as Black-liquor is burned for energy and pulping chemical recovery
[24].
This work studied the iron oxide reduction
characteristics and mechanism by lignin, aiming at efficient utilization of lignin as a renewable and highly reactive carbon substitute for coal in pyrometallurgy.
Journal Pre-proof 2 Materials and methods 2.1 Materials The biomass material used in the experiments was organosolv lignin, derived from mixture of softwood and hardwood, and supplied by Lignol in British Columbia, Canada. The chemical compositions of the lignin are given in Table 1. The Hematite powder was provided by Sinopharm Chemical Reagent Beijing Co., Ltd., containing Fe2O3 of more than 99%. Before the experiments, these materials were dried in an oven at 378 K for 1 h and were subsequently mixed and stored in a desiccator before testing. Table1 Compositions of lignin. Ultimate analysis (%)a
Compositions (%)a
C
H
O
N
Ash
Lignin
Cellulose
Hemicellulose
71.6
6.3
21.9
0.2
≈0
>95.0b
n.a.c
n.a.c
a
% wt/wt, dry basis
b
Data from the supplier
c
Not analyzed
2.2 Experimental The reduction characteristics of iron oxide by lignin under nitrogen atmosphere were studied on a thermogravimetric analyzer (Setsys 2400 ℃, SETARAM Instrumentation, France). In this work, the mass ratios of lignin to hematite (L/H) powder were at 1:1, 1:2 and 1:3, respectively. For each test, the samples were heated from ambient temperature to 900C at heating rates of 8, 20 and 50 C/min under an N2 flow at 20 ml/min. In all tests, the amount of lignin used was ensured the same.
2.3 Kinetic Model The reduction of iron oxide by lignin is a complex process involving both the pyrolysis of lignin and the reduction of iron oxide by the pyrolysis products, so it cannot be modeled easily with a simple mechanism function. In contrast, the iso-conversional
Journal Pre-proof method can obtain relatively reliable activation energy without involving the actual kinetic models, so in this work, the iron oxide reduction process was studied by two iso-conversional methods, i.e., Flynn-Wall-Ozawa (FWO) and Kissinger-Akah-Sunose (KAS) [26-30]. The general kinetic equations are shown as follows: 𝑑𝛼 𝑑𝑡
(
𝑑𝛼
𝐸
)
(1)
= 𝛽𝑑𝑇 = 𝑘(𝑇)𝑓(𝛼) = 𝐴𝑒𝑥𝑝 ― 𝑅𝑇 𝑓(𝛼)
where β is the heating rate, k(T) is the rate constant at temperature T, f(α) is a function of α, A and E are the pre-exponential factor and the activation energy, respectively. 𝛼 1
(
𝑡
𝐸
)
(2)
𝑔(𝛼) = ∫0 𝑓(𝛼)𝑑𝛼 = ∫0𝐴𝑒𝑥𝑝 ― 𝑅𝑇 𝑑𝑡 where g(α) is the integral form of kinetic mechanism function.
Equation (4) below is obtained by substituting the β=dT/dt into equation (3):
(
𝑡
𝐸
)
𝐴 𝑇
(
𝐸
)
𝐴 𝑇
(
𝐸
)
𝐴𝐸 ∞𝑒𝑥𝑝 ( ― 𝜇) 𝑑𝜇 𝜇2
𝑔(𝛼) = ∫0𝐴𝑒𝑥𝑝 ― 𝑅𝑇 𝑑𝑡 = 𝛽∫𝑇 𝑒𝑥𝑝 ― 𝑅𝑇 𝑑𝑇 ≈ 𝛽∫0𝑒𝑥𝑝 ― 𝑅𝑇 𝑑𝑇 = 𝛽𝑅∫𝜇 𝑇 0
(4)
where μT is the value of μ (=E/(RT)) at temperature T. Equation (5) is obtained by taking the logarithm on both sides of the equation (4): 𝑙𝑛
[ ] = 𝑙𝑛[ 𝛽
𝑇2
] ― 𝑅𝑇𝐸
𝐴𝐸 𝑔(𝛼)𝑅
(5)
The Doyle method is used to obtain the equation (6): 𝐴𝐸 (𝑅𝑔(𝛼) ) ―5.331 ― 1.052𝑅𝑇𝐸
𝑙𝑛𝛽 = 𝑙𝑛
(6)
From equation (5), the plots of ln(β/T2) vs. 1/T should result in straight lines with a slope of –E/R. This mothed is known as the Kissinger-Akahira-Sunose (KAS) method. According to equation (11), plotting lnβ vs. 1/T should also yield a straight line, with a slope of –1.052E/R. This mothed is Flynn-Wall-Ozawa (FWO) method. Both methods are widely used to estimate the activation energy of a kinetic process.
3 Results and discussions 3.1 Comparison of lignin pyrolysis and iron oxide reduction by lignin To reveal the role of lignin in iron oxide reduction process, lignin alone and the mixture of lignin and iron oxide (1:1 wt/wt) were studied comparatively by
Journal Pre-proof thermogravimetric experiments (50 C/min). The TGA and DTG (derivative thermogravimetric analysis) curves of these samples are shown in Fig.1. As can be seen from Fig.1, the mass loss curves of lignin alone and lignin-iron oxide mixture are obviously different, particularly in high temperature range. The mass loss curve of the lignin alone decreased gradually with the increase of reaction temperature from 160C to 900C. Lignin is a complex amorphous aromatic polymer of three phenylpropane building blocks with -o-4 and -O-4 ether bonds and C-C bonds, so it has high thermal stability and is difficult to be pyrolyzed. The pyrolysis process of lignin mainly consists of three stages: (1) the volatilization stage of free water, (2) the fracture of functional groups on the benzene ring and recombination of volatile components, and (3) the volatilization of the compounds derived by cracking of lignin macromolecule, cracking of benzene ring and carbonization of polynuclear aromatics [25].
11
-0.07
Lignin Lignin+Fe2O3
10
-0.06 -0.05
Mass (mg)
8 -0.04 7 -0.03 6 385 ℃
5
402 ℃
-0.02
Mass loss rate (mg/℃)
810 ℃
9
-0.01
4 3 150
300
450
600
750
0.00 900
Temperature (℃)
Figure 1 Thermogravimetric curves of lignin and mixture of lignin/iron oxide.
The TG and DTG curves of lignin-iron oxide mixture are similar to those of lignin alone before 750 C, but the TG curve exhibits a sharp weight loss after 750 C. Before 750 C, dehydration of lignin and pyrolysis of lignin macromolecule to form smaller molecules would occur. As can be seen from the XRD patterns in Fig. 2, Fe2O3 was reduced to FeO at 750 C, indicating that the lignin-iron oxide reduction reactions could
Journal Pre-proof take place before 750 C. Comparing the two curves, the lignin pyrolysis is dominant over the reduction reaction in such temperature range. However, the mass-loss peak temperature for lignin-iron oxide mixture is 385C, slightly lower than that of lignin alone, implying that the existence of iron oxide catalyzed the pyrolysis of lignin [36]. After 750C, the mass-loss rate of the lignin-iron oxide mixture sharply increases to 0.055 mg/C at 825C. The XRD patterns of lignin-iron oxide mixture after reduction at 900C (Fig. 2) shows that the product of lignin-iron oxide mixture is mainly metallic Fe, suggesting almost complete reduction of iron oxide, occurring mainly after 750C. The morphology of raw materials and products after reduction is shown in Fig. 3. The morphology of the samples before and after reduction is basically the same, which shows that there is no large area growth after reduction of iron oxide to metallic iron due to the lower reduction temperature. From the energy spectrum point of view, the peak value of iron is large at high reduction temperature, indicating that the higher the metallization rate of the sample, which is consistent with the results of XRD.
Intensity (a.u.)
1
900 ℃
750 ℃
10
20
2
2
2
44
30
2 4 4
40
1
1
3 4
25℃
2
1-Fe 2-FeO 3-Fe3O4 4-Fe2O3
50
2
3
44
60
70
80
90
2θ (°)
Figure 2 XRD patterns of lignin-iron oxide mixture after reduction.
Journal Pre-proof
a.25℃
b.750℃
c.900℃
Figure 3 SEM and EDs of lignin-iron oxide mixture after reduction.
In the heating process, pyrolysis/gasification of lignin supplied carbon and hydrogen-containing reducing agents for iron oxide reduction, whereas the iron oxide could provide oxidant and the reduced iron could serve as a catalyst to promote lignin pyrolysis/gasification [36], via the following reactions: Lignin
FexOy
H2 + CO + CFixed carbon + Others
(7)
FexOy + yH2 → yH2O +xFe
(8)
FexOy + yCO → yCO2 + xFe
(9)
FexOy + 2y/3CFixed carbon → y/3CO2 + y/3CO + xFe
(10)
The overall reaction is: FexOy
Journal Pre-proof Lignin + 3FexOy
H2O + CO + CO2 + 3xFe + Others
(11)
3.2 Effects of L/H ratios on the reduction To reveal the effects of lignin content on the reduction of iron oxide, lignin-iron oxide mixture samples with different L/H mass ratios (1:1, 1:2 and 1:3) while maintaining the same total mass were studied at a heating rate of 50C/min, and the results are illustrated in Fig. 4.
b
100
Mass (%)
90
18.7%
80
31.1%
70 60
24.8%
50
1：3 1：2 1：1
40 150
300
Mass loss rate (%/℃)
a
1：3 1：2 1：1
-2.0 -1.5 -1.0 -0.5 0.0
450
600
Temperature (℃ )
750
900
150
300
450
600
750
900
Temperature (℃ )
Figure 4 Mass-loss curves (a) and mass-loss rate curves (b) lignin-iron oxide mixture samples at different L/H mass ratios.
As can be seen from Fig. 4(a), with all samples the mass decreased drastically when the temperature was higher than 750C, further implying that the reduction of iron oxide by lignin mainly occurred at above 750C. The mass loss % after 750C was 18.7%, 31.1%, and 24.8% at the L/H mass ratio of 1:3, 1:2 and 1:1, respectively. Thus, the mass loss due to iron oxide reduction depended strongly on the L/H ratio, but not in a linear relationship. At the L/H mass ratio of 1:3, the content of lignin in the mixture was low and hence not enough to completely reduce iron oxide, leading to the lowest mass loss % (18.7%,). The mass loss % increased to 31.1% at 1: 2 L/H mass ratio. However, surprisingly the mass loss % dropped to 24.8% when further increasing the L/H mass ratio to 1:1. A possible cause to such interesting results would be due to poorer contact between lignin and iron oxide powder when the lignin ratio was too high,
Journal Pre-proof when lignin could expand and foam during the heating process, as observed in our previous work [32]. The foaming of lignin prevented close contact between carbon and iron oxide, leading to the poorer reduction performance at a too high L/H ratio. So, the curve shape of three L/H samples is different. As shown in Fig. 4(b), the temperature at the maximum mass-loss rate decreased with the increase of the L/H ratio, as expected. When the content of lignin was high, the pyrolysis produced more carbon and hydrogen in the heating process, hence promoting the iron oxide reduction rate. Similarly, as observed and discussed previously in Fig. 3a, the maximum mass loss rates (approx. 0.5%/C) for both L/H ratios (1:1 and 1:3) was significantly lower than that (2.0 %/C) of the L/H ratio of 1:2. Again this result could be caused by the foaming of lignin performance at a too high L/H ratio, which prevented close contact between carbon and iron oxide. 3.3 Effects of hating rates on the reduction
100
50 ℃/min 20 ℃/min 8 ℃/min
Mass (%)
90 80 70 60 50 40
b Mass loss rate (%/℃)
a
50 ℃/min 20 ℃/min 8 ℃/min
-30 -25 -20 -15 -10 -5 0
150
300
450
600
Temperature (℃ )
750
900
5
150
300
450
600
750
900
Temperature (℃ )
Figure 5 Mass loss curves (a) and mass loss rate curves (b) of lignin-iron oxide mixture (L/H = 1:2, w/w) heated at different heating rates.
As shown from Fig. 5(a), the mass loss curve of lignin-iron oxide mixture (L/H = 1:2, w/w) shifted slightly to the high temperature zone when a higher heating rate was
employed, while the total mass loss was almost unchanged. This could be due to poorer heat transfer and greater temperature gradient between the internal and external of sample at a higher heating rate, which retarded the reduction reaction. As expected, the
Journal Pre-proof temperature at the maximum mass loss rate also shifted to a higher temperature while increasing the heating rate, shown in Fig. 5(b). The maximum mass loss rate was 7.5%/C at 8C/min heating rate and increased to 27.5%/C (by 266.7%) at 20C/min heating rate. However, when the heating rate continued to increase to 50C/min, the maximum mass loss rate decreased to 2%/C. Thus, a too high heating rate adversely effected the iron oxide reduction by lignin. Again, this could be due to poorer heat transfer and greater temperature gradient between the internal and external of sample at a too high heating rate, which would retard the reduction reaction. On one hand, the internal and external of lignin particles can be heated evenly at a lower heating rate, producing more carbon and hydrogen-containing reducing agents that are beneficial to the iron oxide reduction. On the one hand, a higher heating rate could enhance the formation of large pores and create more openings and larger specific surface areas for pyrolyzed char, and as a result promoted the reduction reaction. However, at too fast heating the reaction time of the lignin pyrolysis was too short to form enough reducing agents, hence resulted in a smaller value of the maximum mass loss rate. Moreover, the condensed volatiles from lignin pyrolysis (promoted at a higher heating rate) would condense as tar on the internal and external of char, which would block some pores and thereby reduce the surface area of the lignin-char and retard the iron-oxide reduction reactions. 3.4 Kinetics of the reduction of iron oxide by lignin In order to explore the mechanism and the rate limiting step of reducing iron oxide by lignin, we carried out reduction kinetics study. As can be seen from previous Figs. 4 and 5, the mass reduction for the lignin-iron oxide mixture can be divided into two stages: the first stage is mainly associated to lignin pyrolysis, and the second stage is mainly attributed to iron oxide reduction. To simplify the kinetics analysis, we assumed that the mass reduction occurred only in the second stage. According to the overall reaction (11), the mass loss of the mixture is mainly due to the release of C, H and O
Journal Pre-proof elements during the reduction of iron oxide by lignin. Therefore, the reaction fraction (α) of the samples is defined in the following equation: 𝑚 ― 𝑚𝑡
(12)
α = 𝑚𝑐 + 𝑚𝐻 + 𝑚𝑂 × 100%
where m, mt, mc, mH and mo denote the sample mass at the beginning of the reducing stage, the sample mass at a certain temperature, the carbon content of lignin, the hydrogen content of lignin, and the mass of oxygen in both lignin and iron oxide in the mixture, respectively. The data of the reaction fraction calculated are shown in Fig. 6. As discussed previously in Section 3.1, the reduction occurred mainly at the temperatures higher than 750C, so only α data at 0.1～0.95 were used in the kinetics study of this work.
Reduction degree (%)
100
8 ℃/min 20 ℃/min 50 ℃/min
80 60 40 20 0 600
675
750
825
900
Temperature (℃)
Figure 6 Reaction fraction vs. temperature for the lignin-iron oxide mixture (L/H = 1:1) heated at different heating rates. In this work, ten different reduction fractions (α=0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 0.95) were selected for the kinetic analysis with both KAS and FWO methods. In accordance with the FWO method, lnβ and 1/T for each α are marked in Fig. 7. The kinetic parameters obtained from the fitting curves in Fig. 7 are shown in Table 2. In accordance with the KAS method, ln(β/T2) and 1/T for each α are marked in Fig. 8. The kinetic parameters obtained from the fitting curves in Fig. 8 are shown in Table 3. These results indicate that the both methods selected for kinetic analysis for reducing iron oxide by lignin are appropriate and the activation energy values obtained are relatively accurate.
Journal Pre-proof
0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 0.95
4.0
3.5
ln
3.0
2.5
2.0
=0.10 =0.95
0.00088
0.00090
0.00092
0.00094 -1
T /K
0.00096
0.00098
0.00100
-1
Figure 7 Linear fitting of lnβ vs. 1/T at different reaction fractions.
Table 2 Kinetic parameters at different reaction fractions (FWO method). Correlation coefficient α
Fitting equation
Activation energy (E) (R2)
0.1
y = 30260.69x+32.03
239.15
0.9278
0.2
y =-39271.07x+39.62
310.36
0.9761
0.3
y = 42178.47x+42.14
333.34
0.9879
0.4
y = 43273.26x+43.04
341.99
0.9786
0.5
y = 43061.54x+42.78
340.32
0.9750
0.6
y = 42168.28x+41.91
333.26
0.9843
0.7
y = 40503.94x+40.31
320.10
0.9845
0.8
y = 39429.38x+39.22
311.61
0.9861
0.9
y = 37688.76x+37.46
297.86
0.9741
0.95
y = 34924.66x+34.74
276.01
0.9159
Journal Pre-proof
0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 0.95
-10.0
ln(/T2)
-10.5
-11.0
-11.5
=0.10
-12.0
=0.95
0.00088
0.00090
0.00092
0.00094
0.00096
0.00098
0.00100
T-1 / K-1
Figure 8 Linear fitting of ln(β/T2) vs. 1/T at different reaction fractions.
Table 3 Kinetic parameters at different reaction fractions (KAS method) Correlation coefficient α
Fitting equation
Activation energy (E) (R2)
0.1
y = 28185.26x+16.13
234.33
0.9177
0.2
y = 37131.01x+23.67
308.71
0.9734
0.3
y = 40026.26x+26.17
332.78
0.9867
0.4
y = 41115.95x+27.07
341.84
0.9775
0.5
y = 40901.23x+26.81
340.05
0.9725
0.6
y = 40004.41x+25.93
332.60
0.9827
0.7
y = 38336.28x+24.33
318.73
0.9828
0.8
y = 37256.00x+23.24
309.75
0.9846
0.9
y = 35506.62x+21.47
295.20
0.9710
0.95
y = 32734.21x+18.74
272.15
0.9052
Comparison of the linear correlation coefficients and activation energy obtained by the two methods are shown in Fig. 9 and Fig. 10, respectively. As can be seen from Fig.9, the linear correlation of the fitting lines obtained by the FWO method is better than that of KAS method, so the FWO method was chosen in this work. From Fig. 10,
Journal Pre-proof the activation energy of the iron oxide reduction by lignin varies from 239.15kJ·mol-1 to 341.99kJ·mol-1 during the whole reaction process. The activation energy increases initially from 239.15kJ·mol-1 to 341.99kJ·mol-1 with increasing α (α≤0.4), indicating that the reaction of a lower extent becomes gradually more difficult. While at α>0.4, activation energy decreases with α, suggesting that the reaction at later stage becomes easier. 1.00
FWO KAS 0.98
R2
0.96
0.94
0.92
0.90 0
20
40
60
80
100
/%
Figure 9 Comparison of the linear correlation coefficients (R2) obtained by the FWO and KAS methods. 360
FWO KAS
340
E / (kJmol-1)
320 300 280 260 240 220 0
20
40
60
80
100
Figure 10 Comparison of the activation energy obtained by the FWO and KAS methods.
Journal Pre-proof The variation in the activation energy values during the reduction of iron oxide by lignin at 600 ～ 900C implies that the reduction reaction is a complex multi-step reaction process involving different reaction mechanisms in different temperature ranges. Previous studies have shown that the process of lignin pyrolysis includes fracturing, volatilization and recombination of fractured compounds. At temperatures lower than 750C, the final products include mainly H2O, CO2, CO and CH4. In the process of lignin pyrolysis, free water volatilization takes place at 80-110C, CH4 evolves at 300-600C, and CO2 and CO begin to appear at 320C and reach a maximum yield at approx. 750C. Therefore, the carbon and hydrogen-containing reducing agents produced by the pyrolysis of lignin will be consumed gradually by the iron oxide reduction reactions, leading to the deterioration of kinetic conditions for the iron oxide reduction, as evidenced by the gradually increased activation energy. When the reaction temperature is above 750℃, the benzene rings in lignin degrade and condense to form char with increasing the temperature, resulting in favorable thermo and kinetic conditions for the iron oxide reduction, hence resulting in lowered activation energy. According to Fig. 5 and Table 3, the activation energy of iron oxide reduction by lignin varies from 310 kJ·mol-1 to 340 kJ·mol-1 when the reaction fraction is 0.20.8. However, the temperature difference is insignificant, being only about 20℃, given the same heating rate (e.g., the temperature range is 771-788℃ at 8℃/min, 803-817℃ at 20℃/min and 824-842℃ at 50℃/min, respectively), suggesting that temperature is not the main factor affecting activation energy. Moreover, it can be seen from Fig. 2 that the main reduction reaction is FeO→Fe when the reaction fraction is greater than 0.2, and the corresponding activation energy of this reaction is also high, suggesting that reduction of FeO to Fe may be the rate limiting step for the reduction of iron oxide by lignin. Thus, to avoid the FeO→Fe reduction by altering the reducing conditions would enhance the overall reduction of iron ore. This would point out the future direction of the relevant research, and hence benefit the research society with new knowledge.
Journal Pre-proof In addition, as shown previously from Fig. 2, the reaction of Fe3O4→FeO and FeO→Fe occur in the lignin-iron oxide mixture at 750-900C. Wei [28] found that the activation energy of reduction of Fe3O4→FeO and FeO→Fe by graphite are 628kJ·mol1
and 648kJ·mol-1, respectively. However, the maximum activation energy is only
341.99kJ·mol-1 during the reduction of iron oxide by lignin, much lower than that by graphite. Besides, many studies have demonstrated that iron could play a catalytic role in lignin pyrolysis. For instance, Yang[29] found that the presence of Fe2O3 catalyzed lignin pyrolysis at 500-800C. Wang
[30]
studied the pyrolysis of biomass tar with
different dolomites and found that the pyrolysis of tar was promoted with the dolomite containing a higher content of Fe2O3. FeS was also found to be effective for catalyzing the cleavage of ether bonds in lignin
[31].
The above analysis shows that there is a
coupling synergistic effect in the reduction of iron oxide by lignin: the presence of iron catalyzes the pyrolysis of lignin, meanwhile, the pyrolysis products of lignin promotes the reduction of iron oxide.
3.5 Superiority of lignin as a reducing agent
552
H2 835
Reduction rate(a.u)
CO 918
Coal 784
Lignin
200
400
600
800
Temperature (℃)
1000
1200
Journal Pre-proof Figure 11 Comparison of reduction rates of iron oxide with different reductants.
As we known that the reduction of iron oxide by carbon, or CO, or H2 proceeds in two
or
three
steps;
it
occurs
in
accordance
with
the
process
of
Fe2O3→Fe3O4→FeO→Fe or Fe2O3→Fe3O4→Fe. In the past, many controlling mechanisms have been suggested for the reduction of iron oxide and various ores containing iron oxide, including a random nucleation mechanism, phase boundary mechanism, a mixed reaction mechanism, and nucleation and growth kinetics model. Because of the different experimental conditions of each study, the results are different. The performance of lignin as a reducing agent for iron oxide was compared with that of other conventional reductants including H2, CO and pulverized coal, using TGA [35]
under the following conditions: 50 C/min heating rate, 10 mg mass of iron oxide,
10 mg mass of pulverized coal or lignin (if applicable), 10 ml/min flow rate of N2, H2 or CO if applicable). The results of the comparative study are shown in Fig. 11. The order of the temperature at the maximum reduction rate from low to high is H2 (552C) ＜ lignin (784C) ＜ CO (835C) ＜ pulverized coal (918C). Fig. 11 shows that the reduction rate of iron oxide reduced by lignin is relatively high compared with that of coal, CO and H2. In the process of reducing iron oxide by lignin, the temperature corresponding to the maximum reduction rate is 784℃, which is 51℃ lower than that of carbon monoxide and 134℃ lower than that of coal, but 232℃ higher than that of hydrogen. Large-scale acquisition of H2, CO and coal will be accompanied by pollution and consumption of fossil fuels. lignin is a renewable source of energy while significantly reducing CO2 emissions. The efficient use of lignin is very beneficial to the realization of low carbon economy, so the reduction of iron oxide by lignin has demonstrated superiority. The mic-morphology of lignin char (the residue removed from heating lignin in N2 to 750 C) is shown in Fig.12(a). The char appears in the form of nanometerthickness carbon film
[36].
The mic-morphology of coal residue obtained at the same
condition is comparatively shown in Fig. 12(b). The coal char is in the form of particles with rough and different size. In general, reduction of iron oxides is generally considered to be achieved through indirect reduction via reducing gas/vapor. The
Journal Pre-proof formation of char of a thin film structure with a high surface area would promote the carbon gasification and hence facilitate iron oxide reduction reactions. This discovery will provide a possibility for the subsequent development of renewable hyperreactive carbon for pyrometallurgy of metal oxides.
a
b
Figure 12 SEM images of lignin char (a) and coal char (b).
4. Conclusions In this work, the process and reaction kinetics of reduction of iron oxide by lignin were studied. The presence of iron catalyzes the pyrolysis of lignin, meanwhile the pyrolysis products of lignin promote the reduction of iron oxide. Iron oxide could be significantly reduced by lignin at temperatures of 750 - 900C. The best reduction performance was obtained when mixing lignin and iron oxide at the lignin-to-iron oxide (L/H) mass ratio of 1:2. With a too high content of lignin in the lignin-iron oxide mixture, the efficiency of lignin as a reducing agent would decrease due to its quickly foaming that prevented a close contact between carbon and iron oxide. The reduction of iron oxide by lignin is a complex multi-step reaction process with different mechanisms. At the reduction fraction α≤0.4, the activation energy increased rapidly from approx. 240kJ·mol-1 to 342kJ·mol-1, while it gradually dropped to 276kJ·mol-1 at α = 0.95. Lignin as a renewable reducing agent demonstrated to be superior to CO and pulverized coal for the reduction of iron oxide, as it has a lower temperature at the
Journal Pre-proof maximum reduction rate, owing to the formation of nanometer-thickness carbon film in the process.
Acknowledgements This study was financially supported by the National Natural Science Foundation of China (No. U1860113), Research Fund for Young Teachers of Anhui University of Technology (No. QZ201604), as well as Natural Science and Engineering Council of Canada Discovery Grant awarded to C. Xu (NSERC DG).
References [1] Saxena R C, Adhikari D K, Goyal H B. Biomass-based energy fuel through biochemical routes: A review[J]. Renewable & Sustainable Energy Reviews, 2009, 13(1):167-178. [2] Liu G S, Strezov V, Lucas J A, et al. Thermal investigations of direct iron ore reduction with coal[J]. Thermochimica Acta, 2004, 410(1):133-140. [3] Pang J M, Guo P M, Zhao P. Reduction of 1-3 mm Iron Ore by CO on Fluidized Bed[J]. J Iron Steel Res Int, 2011, 18(3):1-5. [4] Shi J, Donskoi E, Mcelwain D L S, et al. Modelling the reduction of an iron orecoal composite pellet with conduction and convection in an axisymmetric temperature field *[J]. Mathematical & Computer Modelling, 2005, 42(1):45-60. [5] Xu G Q. The latest progress and development prospect of CORE[J]. Ironmaking, 2004, 23(2): 50 [6] Zhang S Q, Qiang W H, Li Q M. FINEX melting reduction ironmaking[J]. Ironmaking, 2005, 24(4): 50 [7] Wang W X. Energy consumption comparison of iron-making in blast furnace and non-blast furnace [J]. Ironmaking, 2011, 30（1）：59-61. [8] Watanabe K, Ueda S, Inoue R, et al. Enhancement of Reactivity of Carbon Iron Ore Composite Using Redox Reaction of Iron Oxide Powder[J]. ISIJ International, 2010, 50(4):524-530.
Journal Pre-proof [9] Zandi M, Martinez-Pacheco M, Fray T A T. Biomass for iron ore sintering[J]. Minerals Engineering, 2010, 23(14):1139-1145. [10]Ueda S, Yanagiya K, Watanabe K, et al. Reaction Model and Reduction Behavior of Carbon Iron Ore Composite in Blast Furnace[J]. Isij International, 2009, 49(6):827-836. [11]Konishi H, Ichikawa K, Usui T. Effect of Residual Volatile Matter on Reduction of Iron Oxide in Semi-charcoal Composite Pellets[J]. Transactions of the Iron & Steel Institute of Japan, 2010, 50(3):386-389. [12]Strezov V. Iron ore reduction using sawdust: Experimental analysis and kinetic modelling[J]. Renewable Energy, 2006, 31(12):1892-1905. [13]Hu Z W, Zhang J L, Zuo H B, et al. Thermogravimetric analysis of the reduction of hematite with biomass char[A]; National Iron Production Technology Conference and Iron Academic Conference Proceedings [C]. Wuxi, 2012:636 [14]Wang Y B ， Zhu G C ， Chi R A ， et al ． An investigation on reduction and magnetization of limonite using biomass．Chin J Process Eng，2009，9( 3): 508 [15]Griessacher T, Antrekowitsch J, Steinlechner S, et al. Charcoal from agricultural residues as alternative reducing agent in metal recycling[J]. Biomass & Bioenergy, 2012, 39(39):139-146. [16]Geng W W, Zuo H B and Wang G W. Study on the application of biomass in iron making raw materials [A]. National Iron Production Technology Conference and Iron Academic Conference Proceedings [C]. Zhengzhou, 2014:1119-1123. [17]R. Wei, D. Cang, Y. Bai, et al. Reduction characteristics and kinetics of iron oxide by carbon in biomass[J]. Ironmaking & Steelmaking, 2016, 43(2):144-152. [18]Hurley S, Li H N and Xu C B. Effects of impregnated metal irons on air/CO2gasification of woody biomass[J]. Bioresource Technology, 2010(101), 93019307. [19]Lahijani P, Zainal Z A, Mohamed A R, Mohammadi M, CO2 gasification reactivity of biomass char: Catalytic influence of alkali, alkaline earth and transition metal salts[J], Bioresource Technology, 2013(144),288-295. [20]Moud P H, Kantarelis E, Andersson K J and Engvall K. Biomass pyrolysis gas conditioning over an iron-based catalyst for mild deoxygenation and hydrogen production [J]. Fuel, 2018(211),149-158.
Journal Pre-proof [21]Cheng X S. Development and Application of lignin, an important component of Biomass [J]. Biological industry technology, 2013(2):25-45. [22]Chen F, Dixon R A. Lignin modification improves fermentable sugar yields for biofuel production[J]. Nature Biotechnology, 2007, 25(7):759. [23]Fu C, Mielenz J R, Xiao X, et al. Genetic manipulation of lignin reduces recalcitrance and improves ethanol production from switchgrass[J]. Proceedings of the National Academy of Sciences of the United States of America, 2011, 108(9):3803-3808. [24]Lu Yao, Wei Xianyong, Zong Zhimin, et al. Structural Investigation and Application of Ligins [J]. Progress in Chemistry, 2013,25(5):838-858. [25]Chen H, Yu J, Yao M Q, et al. Mechanism Analysis of slow Pyrolysis of lignin [J]. CIESC Journal, 2013, 64(5):1757-1765. [26]F. R. Huang, X.Q. Wang, S.J. Li. The thermal stability of polyetherimide [J]. Polymer Degradation and Stability. 1987, 18(3):247-259. [27]Y. G. Liang, B.J. Cheng, Y.B. Si, et al. Thermal Decomposition Kinetics and Characteristics of Spartina Alterniflora via Thermogravimetric Analysis [J]. Renewable Energy. 2014, 68(7): 111-117. [28]Sava, A. Burescu, G. Lisa. Study of Thermal Behavior of Polyimides Containing Pendent-Substituted Azobenzene Units [J]. Polymer Bulletin. 2014, 21:385. [29]G. Pitchaimari, C.T. Vijayakumar. Functionalized N-(4-Hydroxy Phenyl) Maleimide Monomers: Kinetics of Thermal Polymerization and Degradation Using Model-Free Approach [J]. Journal of Applied Polymer Science. 2014, DOI: 10.1002/APP.39935 [30]Starink M J. The determination of activation energy from linear heating rate experiments: a comparison of the accuracy of isoconversion methods[J]. Thermochimica Acta, 2003, 404(1):163-176. [31]Ru-feiWei, Da-qiangCang, Ling-lingZhang, et al. Staged reaction kinetics and characteristics of iron oxide direct reduction by carbon [J]. International Journal of Minerals, Metallurgy and Materials, 2015, 22(10):1025-1032. [32]Yang Haiping. Study on the pyrolysis of oil-brown waste and its mechanism [D]. Huazhong University of Science and Technology, Wuhan, 2005. [33]Wang T, Chang J, Lv P, et al. Novel Catalyst for Cracking of Biomass Tar[J]. Energy & Fuels, 2005, 19(1):22-27.
Journal Pre-proof [34]Sui X, Wu S. Preparation of phenols using thermal chemical conversion of industrial kraft lignin [J].CIESC Journal, 2011,62(6):1763-1769. [35]Wei R. Fundamental studies of iron ore reduction by different reductants under mechanical activation. Beijing: University of Science and Technology Beijing; 2016. [36]Wei R, Feng S, Long H, et al. Coupled biomass (lignin) gasification and iron ore reduction: a novel approach for biomass conversion and application[J]. Energy, 2017, 140.
Journal Pre-proof
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Journal Pre-proof
Highlight
Lignin has great superiority compared with the reduction of iron oxide by coal.
The high reduction rate temperature of lignin is 134℃ lower than that of coal.
The lignin-derived char appears in the form of nanometer level carbon film.
The reduction temperature range of iron oxide by lignin was 750～900℃.
Rufei Wei, Dongwen Xiang, Hongming Long, Chunbao (Charles) Xu, Jiaxin Li PII:
S0360-5442(20)30310-8
DOI:
https://doi.org/10.1016/j.energy.2020.117203
Reference:
EGY 117203
To appear in:
Energy
Received Date:
05 October 2019
Accepted Date:
19 February 2020
Please cite this article as: Rufei Wei, Dongwen Xiang, Hongming Long, Chunbao (Charles) Xu, Jiaxin Li, Reduction of Iron Oxide by Lignin: Characteristics, Kinetics and Superiority, Energy (2020), https://doi.org/10.1016/j.energy.2020.117203
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof
Reduction of Iron Oxide by Lignin: Characteristics, Kinetics and Superiority Rufei Wei a, Dongwen Xiang b, Hongming Long a, Chunbao (Charles) Xu a, c*, Jiaxin Lia
a. School of Metallurgical Engineering, Anhui University of Technology, Ma’anshan, Anhui 243002, China b. School of Metallurgy, Northeastern University, Shenyang, Liaoning, 110004 China c. Institute for Chemicals and Fuels from Alternative Resources (ICFAR), Department of Chemical and Biochemical Engineering, Western University, Ontario N6GA5B9, Canada.
Corresponding authors: [email protected] (D. Xiang), [email protected] (H. Long), [email protected] (C. Xu).
Journal Pre-proof Abstract: Reduction of iron oxide by biomass (a renewable energy) instead of fossil energy can greatly reduce greenhouse gas (carbon dioxide) emissions. In this work, the reduction characteristics and kinetics of iron oxide by lignin (a main component of biomass) were studied, aiming at efficient utilization of lignin as a renewable and highly reactive carbon substitute for coal. The reduction temperature range of iron oxide by lignin was found to be mainly 750-900C. It was also observed that the presence of iron could catalyze the pyrolysis of lignin, while the pyrolysis products of lignin promoted the reduction of iron oxide. An increase in the lignin-to-iron oxide mass ratio lowered the temperature at the maximum mass-loss rate determined by TGA. The activation energy varied, increasing first and then decreasing, while increasing the reaction fraction (α), with the turning point at α = 0.4. Compared with CO and coal, lignin appeared to be superior for reducing iron oxide, owing to the formation of nanometerthickness carbon film in the process. The temperature at the maximum reduction rate was 134C for lignin, much lower than that of coal.
Key words: Lignin; Iron oxide; Biomass; Reduction reaction; Kinetics, Pyrometallurgy
Journal Pre-proof 1 Introduction With the depletion of fossil energy and deterioration of ecological environment, development of sustainable resources for fuel and chemical products has attracted increased attention. Biomass is a renewable carbon source, and the fourth largest energy source, after coal, oil and gas. In addition, biomass has many superior characteristics such as its wide distribution, rich in resources, low contents of harmful elements and a low pyrolysis temperature. Therefore, it is promising for biomass as a heating agent or reducing agent for pyrometallurgy process, substituting for coal or coke. Meanwhile, biomass can be a sustainable feedstock for energy and chemical industries. According to statistics, biomass energy contributes to 10 ～ 14% in the global total energy consumption[1]. Iron and steel metallurgy consumes a lot of coal, nature gas and coke, in particular in ironmaking process, and emits an immense amount of carbon dioxide, from both blast furnace ironmaking [2-4] and non-blast furnace ironmaking processes [56].
Compared with blast furnace ironmaking process, non-blast furnace ironmaking
process is inferior in terms of comprehensive benefits, as it has higher energy consumption [7] and produces molten iron of low quality. Most of the previous studies on reduction of metal oxides by biomass were focused on iron oxide reduction. The reduction characteristics and mechanism of reducing iron oxide by biomass have been studied for both traditional blast furnace ironmaking [8-10] and direct reduction ironmaking technologies [11-15]. Udea and Watanabe [8] studied the reactivity and reduction behavior of bio-char, realizing a lower reducing temperature for iron ore block in blast furnace. Thomas et al.
[13]
tested agricultural residues as
substitutes for traditional fossil fuel reductants for iron oxides in a rotary hearth furnace, and the results showed that the reduction degree of iron oxides with the charcoal made from agricultural residues was higher than that of coke. Nurul et al.
[16]
studied
reduction of Malaysian low-grade ore by oil-palm shell. The output of iron was as high as 62.7%, indicating that the oil-palm shell char has great potential in substituting coke as a reductant for low-grade ores. Hu et al. [10] studied reducing the hematite by biochar powder by thermogravimetric analyzer (TGA). The results showed that the iron oxide
Journal Pre-proof reduction temperature with biomass was 100C lower than those of the pulverized coal and coke powder. The reaction rate and the final reaction extent increased owing to the high reactivity of the bio-char [17]. In addition, many studies have shown that iron oxide has catalytic effect on biomass pyrolysis. At 800 ℃ , the impregnation of Fe (III) led to almost complete conversion of the solid biomass into gas/liquid products, producing an extremely low char yield, and a very high yield of combustible gas[18]. Lahijani et al. [19] investigated the influence of alkali (Na, K), alkaline earth (Ca, Mg) and transition (Fe) metal nitrates on CO2 gasification reactivity of pistachio nut shell (PNS) char. The preliminary gasification experiments were performed by TGA and the results showed considerable improvement in carbon conversion. Moud et al.
[20]
found that iron-based materials
were potential candidates for application in a pyrolysis gas pre-conditioning step before further treatment or use, and a way of generating a pyrolysis gas enriched in hydrogen. Previous studies mainly focused on the effects of biomass on metallic oxide reduction, with less attention paid in the roles of the components of lignocellulosic biomass (cellulose, hemicellulose and lignin) in the metallic oxide reduction. Lignocellulosic biomass contains three main components: cellulose (30-50%), hemicelluloses (30-40%) and lignin (20-30%). In nature, the reserve of lignin is lower than those of cellulose and hemicellulose, and it regenerates at the rate of approx. 1500 million tons per year. About 50 million tons of lignin is produced as a by-product from the pulp and paper industry every year [21]. The complexity of the lignin structure unit (phenolics), the diversity of types of lignin and the complexity of chemicals derived from lignin pyrolysis, resulted in scarce in lignin value-added utilization, and by far lignin utilization ratio is less than 2%
[22-23].
So far, more than 95% of the technical
lignin obtained mainly from pulping process as Black-liquor is burned for energy and pulping chemical recovery
[24].
This work studied the iron oxide reduction
characteristics and mechanism by lignin, aiming at efficient utilization of lignin as a renewable and highly reactive carbon substitute for coal in pyrometallurgy.
Journal Pre-proof 2 Materials and methods 2.1 Materials The biomass material used in the experiments was organosolv lignin, derived from mixture of softwood and hardwood, and supplied by Lignol in British Columbia, Canada. The chemical compositions of the lignin are given in Table 1. The Hematite powder was provided by Sinopharm Chemical Reagent Beijing Co., Ltd., containing Fe2O3 of more than 99%. Before the experiments, these materials were dried in an oven at 378 K for 1 h and were subsequently mixed and stored in a desiccator before testing. Table1 Compositions of lignin. Ultimate analysis (%)a
Compositions (%)a
C
H
O
N
Ash
Lignin
Cellulose
Hemicellulose
71.6
6.3
21.9
0.2
≈0
>95.0b
n.a.c
n.a.c
a
% wt/wt, dry basis
b
Data from the supplier
c
Not analyzed
2.2 Experimental The reduction characteristics of iron oxide by lignin under nitrogen atmosphere were studied on a thermogravimetric analyzer (Setsys 2400 ℃, SETARAM Instrumentation, France). In this work, the mass ratios of lignin to hematite (L/H) powder were at 1:1, 1:2 and 1:3, respectively. For each test, the samples were heated from ambient temperature to 900C at heating rates of 8, 20 and 50 C/min under an N2 flow at 20 ml/min. In all tests, the amount of lignin used was ensured the same.
2.3 Kinetic Model The reduction of iron oxide by lignin is a complex process involving both the pyrolysis of lignin and the reduction of iron oxide by the pyrolysis products, so it cannot be modeled easily with a simple mechanism function. In contrast, the iso-conversional
Journal Pre-proof method can obtain relatively reliable activation energy without involving the actual kinetic models, so in this work, the iron oxide reduction process was studied by two iso-conversional methods, i.e., Flynn-Wall-Ozawa (FWO) and Kissinger-Akah-Sunose (KAS) [26-30]. The general kinetic equations are shown as follows: 𝑑𝛼 𝑑𝑡
(
𝑑𝛼
𝐸
)
(1)
= 𝛽𝑑𝑇 = 𝑘(𝑇)𝑓(𝛼) = 𝐴𝑒𝑥𝑝 ― 𝑅𝑇 𝑓(𝛼)
where β is the heating rate, k(T) is the rate constant at temperature T, f(α) is a function of α, A and E are the pre-exponential factor and the activation energy, respectively. 𝛼 1
(
𝑡
𝐸
)
(2)
𝑔(𝛼) = ∫0 𝑓(𝛼)𝑑𝛼 = ∫0𝐴𝑒𝑥𝑝 ― 𝑅𝑇 𝑑𝑡 where g(α) is the integral form of kinetic mechanism function.
Equation (4) below is obtained by substituting the β=dT/dt into equation (3):
(
𝑡
𝐸
)
𝐴 𝑇
(
𝐸
)
𝐴 𝑇
(
𝐸
)
𝐴𝐸 ∞𝑒𝑥𝑝 ( ― 𝜇) 𝑑𝜇 𝜇2
𝑔(𝛼) = ∫0𝐴𝑒𝑥𝑝 ― 𝑅𝑇 𝑑𝑡 = 𝛽∫𝑇 𝑒𝑥𝑝 ― 𝑅𝑇 𝑑𝑇 ≈ 𝛽∫0𝑒𝑥𝑝 ― 𝑅𝑇 𝑑𝑇 = 𝛽𝑅∫𝜇 𝑇 0
(4)
where μT is the value of μ (=E/(RT)) at temperature T. Equation (5) is obtained by taking the logarithm on both sides of the equation (4): 𝑙𝑛
[ ] = 𝑙𝑛[ 𝛽
𝑇2
] ― 𝑅𝑇𝐸
𝐴𝐸 𝑔(𝛼)𝑅
(5)
The Doyle method is used to obtain the equation (6): 𝐴𝐸 (𝑅𝑔(𝛼) ) ―5.331 ― 1.052𝑅𝑇𝐸
𝑙𝑛𝛽 = 𝑙𝑛
(6)
From equation (5), the plots of ln(β/T2) vs. 1/T should result in straight lines with a slope of –E/R. This mothed is known as the Kissinger-Akahira-Sunose (KAS) method. According to equation (11), plotting lnβ vs. 1/T should also yield a straight line, with a slope of –1.052E/R. This mothed is Flynn-Wall-Ozawa (FWO) method. Both methods are widely used to estimate the activation energy of a kinetic process.
3 Results and discussions 3.1 Comparison of lignin pyrolysis and iron oxide reduction by lignin To reveal the role of lignin in iron oxide reduction process, lignin alone and the mixture of lignin and iron oxide (1:1 wt/wt) were studied comparatively by
Journal Pre-proof thermogravimetric experiments (50 C/min). The TGA and DTG (derivative thermogravimetric analysis) curves of these samples are shown in Fig.1. As can be seen from Fig.1, the mass loss curves of lignin alone and lignin-iron oxide mixture are obviously different, particularly in high temperature range. The mass loss curve of the lignin alone decreased gradually with the increase of reaction temperature from 160C to 900C. Lignin is a complex amorphous aromatic polymer of three phenylpropane building blocks with -o-4 and -O-4 ether bonds and C-C bonds, so it has high thermal stability and is difficult to be pyrolyzed. The pyrolysis process of lignin mainly consists of three stages: (1) the volatilization stage of free water, (2) the fracture of functional groups on the benzene ring and recombination of volatile components, and (3) the volatilization of the compounds derived by cracking of lignin macromolecule, cracking of benzene ring and carbonization of polynuclear aromatics [25].
11
-0.07
Lignin Lignin+Fe2O3
10
-0.06 -0.05
Mass (mg)
8 -0.04 7 -0.03 6 385 ℃
5
402 ℃
-0.02
Mass loss rate (mg/℃)
810 ℃
9
-0.01
4 3 150
300
450
600
750
0.00 900
Temperature (℃)
Figure 1 Thermogravimetric curves of lignin and mixture of lignin/iron oxide.
The TG and DTG curves of lignin-iron oxide mixture are similar to those of lignin alone before 750 C, but the TG curve exhibits a sharp weight loss after 750 C. Before 750 C, dehydration of lignin and pyrolysis of lignin macromolecule to form smaller molecules would occur. As can be seen from the XRD patterns in Fig. 2, Fe2O3 was reduced to FeO at 750 C, indicating that the lignin-iron oxide reduction reactions could
Journal Pre-proof take place before 750 C. Comparing the two curves, the lignin pyrolysis is dominant over the reduction reaction in such temperature range. However, the mass-loss peak temperature for lignin-iron oxide mixture is 385C, slightly lower than that of lignin alone, implying that the existence of iron oxide catalyzed the pyrolysis of lignin [36]. After 750C, the mass-loss rate of the lignin-iron oxide mixture sharply increases to 0.055 mg/C at 825C. The XRD patterns of lignin-iron oxide mixture after reduction at 900C (Fig. 2) shows that the product of lignin-iron oxide mixture is mainly metallic Fe, suggesting almost complete reduction of iron oxide, occurring mainly after 750C. The morphology of raw materials and products after reduction is shown in Fig. 3. The morphology of the samples before and after reduction is basically the same, which shows that there is no large area growth after reduction of iron oxide to metallic iron due to the lower reduction temperature. From the energy spectrum point of view, the peak value of iron is large at high reduction temperature, indicating that the higher the metallization rate of the sample, which is consistent with the results of XRD.
Intensity (a.u.)
1
900 ℃
750 ℃
10
20
2
2
2
44
30
2 4 4
40
1
1
3 4
25℃
2
1-Fe 2-FeO 3-Fe3O4 4-Fe2O3
50
2
3
44
60
70
80
90
2θ (°)
Figure 2 XRD patterns of lignin-iron oxide mixture after reduction.
Journal Pre-proof
a.25℃
b.750℃
c.900℃
Figure 3 SEM and EDs of lignin-iron oxide mixture after reduction.
In the heating process, pyrolysis/gasification of lignin supplied carbon and hydrogen-containing reducing agents for iron oxide reduction, whereas the iron oxide could provide oxidant and the reduced iron could serve as a catalyst to promote lignin pyrolysis/gasification [36], via the following reactions: Lignin
FexOy
H2 + CO + CFixed carbon + Others
(7)
FexOy + yH2 → yH2O +xFe
(8)
FexOy + yCO → yCO2 + xFe
(9)
FexOy + 2y/3CFixed carbon → y/3CO2 + y/3CO + xFe
(10)
The overall reaction is: FexOy
Journal Pre-proof Lignin + 3FexOy
H2O + CO + CO2 + 3xFe + Others
(11)
3.2 Effects of L/H ratios on the reduction To reveal the effects of lignin content on the reduction of iron oxide, lignin-iron oxide mixture samples with different L/H mass ratios (1:1, 1:2 and 1:3) while maintaining the same total mass were studied at a heating rate of 50C/min, and the results are illustrated in Fig. 4.
b
100
Mass (%)
90
18.7%
80
31.1%
70 60
24.8%
50
1：3 1：2 1：1
40 150
300
Mass loss rate (%/℃)
a
1：3 1：2 1：1
-2.0 -1.5 -1.0 -0.5 0.0
450
600
Temperature (℃ )
750
900
150
300
450
600
750
900
Temperature (℃ )
Figure 4 Mass-loss curves (a) and mass-loss rate curves (b) lignin-iron oxide mixture samples at different L/H mass ratios.
As can be seen from Fig. 4(a), with all samples the mass decreased drastically when the temperature was higher than 750C, further implying that the reduction of iron oxide by lignin mainly occurred at above 750C. The mass loss % after 750C was 18.7%, 31.1%, and 24.8% at the L/H mass ratio of 1:3, 1:2 and 1:1, respectively. Thus, the mass loss due to iron oxide reduction depended strongly on the L/H ratio, but not in a linear relationship. At the L/H mass ratio of 1:3, the content of lignin in the mixture was low and hence not enough to completely reduce iron oxide, leading to the lowest mass loss % (18.7%,). The mass loss % increased to 31.1% at 1: 2 L/H mass ratio. However, surprisingly the mass loss % dropped to 24.8% when further increasing the L/H mass ratio to 1:1. A possible cause to such interesting results would be due to poorer contact between lignin and iron oxide powder when the lignin ratio was too high,
Journal Pre-proof when lignin could expand and foam during the heating process, as observed in our previous work [32]. The foaming of lignin prevented close contact between carbon and iron oxide, leading to the poorer reduction performance at a too high L/H ratio. So, the curve shape of three L/H samples is different. As shown in Fig. 4(b), the temperature at the maximum mass-loss rate decreased with the increase of the L/H ratio, as expected. When the content of lignin was high, the pyrolysis produced more carbon and hydrogen in the heating process, hence promoting the iron oxide reduction rate. Similarly, as observed and discussed previously in Fig. 3a, the maximum mass loss rates (approx. 0.5%/C) for both L/H ratios (1:1 and 1:3) was significantly lower than that (2.0 %/C) of the L/H ratio of 1:2. Again this result could be caused by the foaming of lignin performance at a too high L/H ratio, which prevented close contact between carbon and iron oxide. 3.3 Effects of hating rates on the reduction
100
50 ℃/min 20 ℃/min 8 ℃/min
Mass (%)
90 80 70 60 50 40
b Mass loss rate (%/℃)
a
50 ℃/min 20 ℃/min 8 ℃/min
-30 -25 -20 -15 -10 -5 0
150
300
450
600
Temperature (℃ )
750
900
5
150
300
450
600
750
900
Temperature (℃ )
Figure 5 Mass loss curves (a) and mass loss rate curves (b) of lignin-iron oxide mixture (L/H = 1:2, w/w) heated at different heating rates.
As shown from Fig. 5(a), the mass loss curve of lignin-iron oxide mixture (L/H = 1:2, w/w) shifted slightly to the high temperature zone when a higher heating rate was
employed, while the total mass loss was almost unchanged. This could be due to poorer heat transfer and greater temperature gradient between the internal and external of sample at a higher heating rate, which retarded the reduction reaction. As expected, the
Journal Pre-proof temperature at the maximum mass loss rate also shifted to a higher temperature while increasing the heating rate, shown in Fig. 5(b). The maximum mass loss rate was 7.5%/C at 8C/min heating rate and increased to 27.5%/C (by 266.7%) at 20C/min heating rate. However, when the heating rate continued to increase to 50C/min, the maximum mass loss rate decreased to 2%/C. Thus, a too high heating rate adversely effected the iron oxide reduction by lignin. Again, this could be due to poorer heat transfer and greater temperature gradient between the internal and external of sample at a too high heating rate, which would retard the reduction reaction. On one hand, the internal and external of lignin particles can be heated evenly at a lower heating rate, producing more carbon and hydrogen-containing reducing agents that are beneficial to the iron oxide reduction. On the one hand, a higher heating rate could enhance the formation of large pores and create more openings and larger specific surface areas for pyrolyzed char, and as a result promoted the reduction reaction. However, at too fast heating the reaction time of the lignin pyrolysis was too short to form enough reducing agents, hence resulted in a smaller value of the maximum mass loss rate. Moreover, the condensed volatiles from lignin pyrolysis (promoted at a higher heating rate) would condense as tar on the internal and external of char, which would block some pores and thereby reduce the surface area of the lignin-char and retard the iron-oxide reduction reactions. 3.4 Kinetics of the reduction of iron oxide by lignin In order to explore the mechanism and the rate limiting step of reducing iron oxide by lignin, we carried out reduction kinetics study. As can be seen from previous Figs. 4 and 5, the mass reduction for the lignin-iron oxide mixture can be divided into two stages: the first stage is mainly associated to lignin pyrolysis, and the second stage is mainly attributed to iron oxide reduction. To simplify the kinetics analysis, we assumed that the mass reduction occurred only in the second stage. According to the overall reaction (11), the mass loss of the mixture is mainly due to the release of C, H and O
Journal Pre-proof elements during the reduction of iron oxide by lignin. Therefore, the reaction fraction (α) of the samples is defined in the following equation: 𝑚 ― 𝑚𝑡
(12)
α = 𝑚𝑐 + 𝑚𝐻 + 𝑚𝑂 × 100%
where m, mt, mc, mH and mo denote the sample mass at the beginning of the reducing stage, the sample mass at a certain temperature, the carbon content of lignin, the hydrogen content of lignin, and the mass of oxygen in both lignin and iron oxide in the mixture, respectively. The data of the reaction fraction calculated are shown in Fig. 6. As discussed previously in Section 3.1, the reduction occurred mainly at the temperatures higher than 750C, so only α data at 0.1～0.95 were used in the kinetics study of this work.
Reduction degree (%)
100
8 ℃/min 20 ℃/min 50 ℃/min
80 60 40 20 0 600
675
750
825
900
Temperature (℃)
Figure 6 Reaction fraction vs. temperature for the lignin-iron oxide mixture (L/H = 1:1) heated at different heating rates. In this work, ten different reduction fractions (α=0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 0.95) were selected for the kinetic analysis with both KAS and FWO methods. In accordance with the FWO method, lnβ and 1/T for each α are marked in Fig. 7. The kinetic parameters obtained from the fitting curves in Fig. 7 are shown in Table 2. In accordance with the KAS method, ln(β/T2) and 1/T for each α are marked in Fig. 8. The kinetic parameters obtained from the fitting curves in Fig. 8 are shown in Table 3. These results indicate that the both methods selected for kinetic analysis for reducing iron oxide by lignin are appropriate and the activation energy values obtained are relatively accurate.
Journal Pre-proof
0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 0.95
4.0
3.5
ln
3.0
2.5
2.0
=0.10 =0.95
0.00088
0.00090
0.00092
0.00094 -1
T /K
0.00096
0.00098
0.00100
-1
Figure 7 Linear fitting of lnβ vs. 1/T at different reaction fractions.
Table 2 Kinetic parameters at different reaction fractions (FWO method). Correlation coefficient α
Fitting equation
Activation energy (E) (R2)
0.1
y = 30260.69x+32.03
239.15
0.9278
0.2
y =-39271.07x+39.62
310.36
0.9761
0.3
y = 42178.47x+42.14
333.34
0.9879
0.4
y = 43273.26x+43.04
341.99
0.9786
0.5
y = 43061.54x+42.78
340.32
0.9750
0.6
y = 42168.28x+41.91
333.26
0.9843
0.7
y = 40503.94x+40.31
320.10
0.9845
0.8
y = 39429.38x+39.22
311.61
0.9861
0.9
y = 37688.76x+37.46
297.86
0.9741
0.95
y = 34924.66x+34.74
276.01
0.9159
Journal Pre-proof
0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 0.95
-10.0
ln(/T2)
-10.5
-11.0
-11.5
=0.10
-12.0
=0.95
0.00088
0.00090
0.00092
0.00094
0.00096
0.00098
0.00100
T-1 / K-1
Figure 8 Linear fitting of ln(β/T2) vs. 1/T at different reaction fractions.
Table 3 Kinetic parameters at different reaction fractions (KAS method) Correlation coefficient α
Fitting equation
Activation energy (E) (R2)
0.1
y = 28185.26x+16.13
234.33
0.9177
0.2
y = 37131.01x+23.67
308.71
0.9734
0.3
y = 40026.26x+26.17
332.78
0.9867
0.4
y = 41115.95x+27.07
341.84
0.9775
0.5
y = 40901.23x+26.81
340.05
0.9725
0.6
y = 40004.41x+25.93
332.60
0.9827
0.7
y = 38336.28x+24.33
318.73
0.9828
0.8
y = 37256.00x+23.24
309.75
0.9846
0.9
y = 35506.62x+21.47
295.20
0.9710
0.95
y = 32734.21x+18.74
272.15
0.9052
Comparison of the linear correlation coefficients and activation energy obtained by the two methods are shown in Fig. 9 and Fig. 10, respectively. As can be seen from Fig.9, the linear correlation of the fitting lines obtained by the FWO method is better than that of KAS method, so the FWO method was chosen in this work. From Fig. 10,
Journal Pre-proof the activation energy of the iron oxide reduction by lignin varies from 239.15kJ·mol-1 to 341.99kJ·mol-1 during the whole reaction process. The activation energy increases initially from 239.15kJ·mol-1 to 341.99kJ·mol-1 with increasing α (α≤0.4), indicating that the reaction of a lower extent becomes gradually more difficult. While at α>0.4, activation energy decreases with α, suggesting that the reaction at later stage becomes easier. 1.00
FWO KAS 0.98
R2
0.96
0.94
0.92
0.90 0
20
40
60
80
100
/%
Figure 9 Comparison of the linear correlation coefficients (R2) obtained by the FWO and KAS methods. 360
FWO KAS
340
E / (kJmol-1)
320 300 280 260 240 220 0
20
40
60
80
100
Figure 10 Comparison of the activation energy obtained by the FWO and KAS methods.
Journal Pre-proof The variation in the activation energy values during the reduction of iron oxide by lignin at 600 ～ 900C implies that the reduction reaction is a complex multi-step reaction process involving different reaction mechanisms in different temperature ranges. Previous studies have shown that the process of lignin pyrolysis includes fracturing, volatilization and recombination of fractured compounds. At temperatures lower than 750C, the final products include mainly H2O, CO2, CO and CH4. In the process of lignin pyrolysis, free water volatilization takes place at 80-110C, CH4 evolves at 300-600C, and CO2 and CO begin to appear at 320C and reach a maximum yield at approx. 750C. Therefore, the carbon and hydrogen-containing reducing agents produced by the pyrolysis of lignin will be consumed gradually by the iron oxide reduction reactions, leading to the deterioration of kinetic conditions for the iron oxide reduction, as evidenced by the gradually increased activation energy. When the reaction temperature is above 750℃, the benzene rings in lignin degrade and condense to form char with increasing the temperature, resulting in favorable thermo and kinetic conditions for the iron oxide reduction, hence resulting in lowered activation energy. According to Fig. 5 and Table 3, the activation energy of iron oxide reduction by lignin varies from 310 kJ·mol-1 to 340 kJ·mol-1 when the reaction fraction is 0.20.8. However, the temperature difference is insignificant, being only about 20℃, given the same heating rate (e.g., the temperature range is 771-788℃ at 8℃/min, 803-817℃ at 20℃/min and 824-842℃ at 50℃/min, respectively), suggesting that temperature is not the main factor affecting activation energy. Moreover, it can be seen from Fig. 2 that the main reduction reaction is FeO→Fe when the reaction fraction is greater than 0.2, and the corresponding activation energy of this reaction is also high, suggesting that reduction of FeO to Fe may be the rate limiting step for the reduction of iron oxide by lignin. Thus, to avoid the FeO→Fe reduction by altering the reducing conditions would enhance the overall reduction of iron ore. This would point out the future direction of the relevant research, and hence benefit the research society with new knowledge.
Journal Pre-proof In addition, as shown previously from Fig. 2, the reaction of Fe3O4→FeO and FeO→Fe occur in the lignin-iron oxide mixture at 750-900C. Wei [28] found that the activation energy of reduction of Fe3O4→FeO and FeO→Fe by graphite are 628kJ·mol1
and 648kJ·mol-1, respectively. However, the maximum activation energy is only
341.99kJ·mol-1 during the reduction of iron oxide by lignin, much lower than that by graphite. Besides, many studies have demonstrated that iron could play a catalytic role in lignin pyrolysis. For instance, Yang[29] found that the presence of Fe2O3 catalyzed lignin pyrolysis at 500-800C. Wang
[30]
studied the pyrolysis of biomass tar with
different dolomites and found that the pyrolysis of tar was promoted with the dolomite containing a higher content of Fe2O3. FeS was also found to be effective for catalyzing the cleavage of ether bonds in lignin
[31].
The above analysis shows that there is a
coupling synergistic effect in the reduction of iron oxide by lignin: the presence of iron catalyzes the pyrolysis of lignin, meanwhile, the pyrolysis products of lignin promotes the reduction of iron oxide.
3.5 Superiority of lignin as a reducing agent
552
H2 835
Reduction rate(a.u)
CO 918
Coal 784
Lignin
200
400
600
800
Temperature (℃)
1000
1200
Journal Pre-proof Figure 11 Comparison of reduction rates of iron oxide with different reductants.
As we known that the reduction of iron oxide by carbon, or CO, or H2 proceeds in two
or
three
steps;
it
occurs
in
accordance
with
the
process
of
Fe2O3→Fe3O4→FeO→Fe or Fe2O3→Fe3O4→Fe. In the past, many controlling mechanisms have been suggested for the reduction of iron oxide and various ores containing iron oxide, including a random nucleation mechanism, phase boundary mechanism, a mixed reaction mechanism, and nucleation and growth kinetics model. Because of the different experimental conditions of each study, the results are different. The performance of lignin as a reducing agent for iron oxide was compared with that of other conventional reductants including H2, CO and pulverized coal, using TGA [35]
under the following conditions: 50 C/min heating rate, 10 mg mass of iron oxide,
10 mg mass of pulverized coal or lignin (if applicable), 10 ml/min flow rate of N2, H2 or CO if applicable). The results of the comparative study are shown in Fig. 11. The order of the temperature at the maximum reduction rate from low to high is H2 (552C) ＜ lignin (784C) ＜ CO (835C) ＜ pulverized coal (918C). Fig. 11 shows that the reduction rate of iron oxide reduced by lignin is relatively high compared with that of coal, CO and H2. In the process of reducing iron oxide by lignin, the temperature corresponding to the maximum reduction rate is 784℃, which is 51℃ lower than that of carbon monoxide and 134℃ lower than that of coal, but 232℃ higher than that of hydrogen. Large-scale acquisition of H2, CO and coal will be accompanied by pollution and consumption of fossil fuels. lignin is a renewable source of energy while significantly reducing CO2 emissions. The efficient use of lignin is very beneficial to the realization of low carbon economy, so the reduction of iron oxide by lignin has demonstrated superiority. The mic-morphology of lignin char (the residue removed from heating lignin in N2 to 750 C) is shown in Fig.12(a). The char appears in the form of nanometerthickness carbon film
[36].
The mic-morphology of coal residue obtained at the same
condition is comparatively shown in Fig. 12(b). The coal char is in the form of particles with rough and different size. In general, reduction of iron oxides is generally considered to be achieved through indirect reduction via reducing gas/vapor. The
Journal Pre-proof formation of char of a thin film structure with a high surface area would promote the carbon gasification and hence facilitate iron oxide reduction reactions. This discovery will provide a possibility for the subsequent development of renewable hyperreactive carbon for pyrometallurgy of metal oxides.
a
b
Figure 12 SEM images of lignin char (a) and coal char (b).
4. Conclusions In this work, the process and reaction kinetics of reduction of iron oxide by lignin were studied. The presence of iron catalyzes the pyrolysis of lignin, meanwhile the pyrolysis products of lignin promote the reduction of iron oxide. Iron oxide could be significantly reduced by lignin at temperatures of 750 - 900C. The best reduction performance was obtained when mixing lignin and iron oxide at the lignin-to-iron oxide (L/H) mass ratio of 1:2. With a too high content of lignin in the lignin-iron oxide mixture, the efficiency of lignin as a reducing agent would decrease due to its quickly foaming that prevented a close contact between carbon and iron oxide. The reduction of iron oxide by lignin is a complex multi-step reaction process with different mechanisms. At the reduction fraction α≤0.4, the activation energy increased rapidly from approx. 240kJ·mol-1 to 342kJ·mol-1, while it gradually dropped to 276kJ·mol-1 at α = 0.95. Lignin as a renewable reducing agent demonstrated to be superior to CO and pulverized coal for the reduction of iron oxide, as it has a lower temperature at the
Journal Pre-proof maximum reduction rate, owing to the formation of nanometer-thickness carbon film in the process.
Acknowledgements This study was financially supported by the National Natural Science Foundation of China (No. U1860113), Research Fund for Young Teachers of Anhui University of Technology (No. QZ201604), as well as Natural Science and Engineering Council of Canada Discovery Grant awarded to C. Xu (NSERC DG).
References [1] Saxena R C, Adhikari D K, Goyal H B. Biomass-based energy fuel through biochemical routes: A review[J]. Renewable & Sustainable Energy Reviews, 2009, 13(1):167-178. [2] Liu G S, Strezov V, Lucas J A, et al. Thermal investigations of direct iron ore reduction with coal[J]. Thermochimica Acta, 2004, 410(1):133-140. [3] Pang J M, Guo P M, Zhao P. Reduction of 1-3 mm Iron Ore by CO on Fluidized Bed[J]. J Iron Steel Res Int, 2011, 18(3):1-5. [4] Shi J, Donskoi E, Mcelwain D L S, et al. Modelling the reduction of an iron orecoal composite pellet with conduction and convection in an axisymmetric temperature field *[J]. Mathematical & Computer Modelling, 2005, 42(1):45-60. [5] Xu G Q. The latest progress and development prospect of CORE[J]. Ironmaking, 2004, 23(2): 50 [6] Zhang S Q, Qiang W H, Li Q M. FINEX melting reduction ironmaking[J]. Ironmaking, 2005, 24(4): 50 [7] Wang W X. Energy consumption comparison of iron-making in blast furnace and non-blast furnace [J]. Ironmaking, 2011, 30（1）：59-61. [8] Watanabe K, Ueda S, Inoue R, et al. Enhancement of Reactivity of Carbon Iron Ore Composite Using Redox Reaction of Iron Oxide Powder[J]. ISIJ International, 2010, 50(4):524-530.
Journal Pre-proof [9] Zandi M, Martinez-Pacheco M, Fray T A T. Biomass for iron ore sintering[J]. Minerals Engineering, 2010, 23(14):1139-1145. [10]Ueda S, Yanagiya K, Watanabe K, et al. Reaction Model and Reduction Behavior of Carbon Iron Ore Composite in Blast Furnace[J]. Isij International, 2009, 49(6):827-836. [11]Konishi H, Ichikawa K, Usui T. Effect of Residual Volatile Matter on Reduction of Iron Oxide in Semi-charcoal Composite Pellets[J]. Transactions of the Iron & Steel Institute of Japan, 2010, 50(3):386-389. [12]Strezov V. Iron ore reduction using sawdust: Experimental analysis and kinetic modelling[J]. Renewable Energy, 2006, 31(12):1892-1905. [13]Hu Z W, Zhang J L, Zuo H B, et al. Thermogravimetric analysis of the reduction of hematite with biomass char[A]; National Iron Production Technology Conference and Iron Academic Conference Proceedings [C]. Wuxi, 2012:636 [14]Wang Y B ， Zhu G C ， Chi R A ， et al ． An investigation on reduction and magnetization of limonite using biomass．Chin J Process Eng，2009，9( 3): 508 [15]Griessacher T, Antrekowitsch J, Steinlechner S, et al. Charcoal from agricultural residues as alternative reducing agent in metal recycling[J]. Biomass & Bioenergy, 2012, 39(39):139-146. [16]Geng W W, Zuo H B and Wang G W. Study on the application of biomass in iron making raw materials [A]. National Iron Production Technology Conference and Iron Academic Conference Proceedings [C]. Zhengzhou, 2014:1119-1123. [17]R. Wei, D. Cang, Y. Bai, et al. Reduction characteristics and kinetics of iron oxide by carbon in biomass[J]. Ironmaking & Steelmaking, 2016, 43(2):144-152. [18]Hurley S, Li H N and Xu C B. Effects of impregnated metal irons on air/CO2gasification of woody biomass[J]. Bioresource Technology, 2010(101), 93019307. [19]Lahijani P, Zainal Z A, Mohamed A R, Mohammadi M, CO2 gasification reactivity of biomass char: Catalytic influence of alkali, alkaline earth and transition metal salts[J], Bioresource Technology, 2013(144),288-295. [20]Moud P H, Kantarelis E, Andersson K J and Engvall K. Biomass pyrolysis gas conditioning over an iron-based catalyst for mild deoxygenation and hydrogen production [J]. Fuel, 2018(211),149-158.
Journal Pre-proof [21]Cheng X S. Development and Application of lignin, an important component of Biomass [J]. Biological industry technology, 2013(2):25-45. [22]Chen F, Dixon R A. Lignin modification improves fermentable sugar yields for biofuel production[J]. Nature Biotechnology, 2007, 25(7):759. [23]Fu C, Mielenz J R, Xiao X, et al. Genetic manipulation of lignin reduces recalcitrance and improves ethanol production from switchgrass[J]. Proceedings of the National Academy of Sciences of the United States of America, 2011, 108(9):3803-3808. [24]Lu Yao, Wei Xianyong, Zong Zhimin, et al. Structural Investigation and Application of Ligins [J]. Progress in Chemistry, 2013,25(5):838-858. [25]Chen H, Yu J, Yao M Q, et al. Mechanism Analysis of slow Pyrolysis of lignin [J]. CIESC Journal, 2013, 64(5):1757-1765. [26]F. R. Huang, X.Q. Wang, S.J. Li. The thermal stability of polyetherimide [J]. Polymer Degradation and Stability. 1987, 18(3):247-259. [27]Y. G. Liang, B.J. Cheng, Y.B. Si, et al. Thermal Decomposition Kinetics and Characteristics of Spartina Alterniflora via Thermogravimetric Analysis [J]. Renewable Energy. 2014, 68(7): 111-117. [28]Sava, A. Burescu, G. Lisa. Study of Thermal Behavior of Polyimides Containing Pendent-Substituted Azobenzene Units [J]. Polymer Bulletin. 2014, 21:385. [29]G. Pitchaimari, C.T. Vijayakumar. Functionalized N-(4-Hydroxy Phenyl) Maleimide Monomers: Kinetics of Thermal Polymerization and Degradation Using Model-Free Approach [J]. Journal of Applied Polymer Science. 2014, DOI: 10.1002/APP.39935 [30]Starink M J. The determination of activation energy from linear heating rate experiments: a comparison of the accuracy of isoconversion methods[J]. Thermochimica Acta, 2003, 404(1):163-176. [31]Ru-feiWei, Da-qiangCang, Ling-lingZhang, et al. Staged reaction kinetics and characteristics of iron oxide direct reduction by carbon [J]. International Journal of Minerals, Metallurgy and Materials, 2015, 22(10):1025-1032. [32]Yang Haiping. Study on the pyrolysis of oil-brown waste and its mechanism [D]. Huazhong University of Science and Technology, Wuhan, 2005. [33]Wang T, Chang J, Lv P, et al. Novel Catalyst for Cracking of Biomass Tar[J]. Energy & Fuels, 2005, 19(1):22-27.
Journal Pre-proof [34]Sui X, Wu S. Preparation of phenols using thermal chemical conversion of industrial kraft lignin [J].CIESC Journal, 2011,62(6):1763-1769. [35]Wei R. Fundamental studies of iron ore reduction by different reductants under mechanical activation. Beijing: University of Science and Technology Beijing; 2016. [36]Wei R, Feng S, Long H, et al. Coupled biomass (lignin) gasification and iron ore reduction: a novel approach for biomass conversion and application[J]. Energy, 2017, 140.
Journal Pre-proof
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Journal Pre-proof
Highlight
Lignin has great superiority compared with the reduction of iron oxide by coal.
The high reduction rate temperature of lignin is 134℃ lower than that of coal.
The lignin-derived char appears in the form of nanometer level carbon film.
The reduction temperature range of iron oxide by lignin was 750～900℃.