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Iron oxide reduction by graphite and torrefied biomass analyzed by TG-FTIR for mitigating CO2 emissions
Energy 180 (2019) 968e977
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Energy journal homepage: www.elsevier.com/locate/energy
Iron oxide reduction by graphite and torrefied biomass analyzed by TG-FTIR for mitigating CO2 emissions Aristotle T. Ubando a, b, Wei-Hsin Chen a, c, *, Hwai Chyuan Ong d a
Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan, 701, Taiwan Mechanical Engineering Department, De La Salle University, 2401 Taft Avenue, 0922, Manila, Philippines c Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan, 701, Taiwan d Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2019 Received in revised form 8 May 2019 Accepted 21 May 2019 Available online 23 May 2019
Biomass provides a sustainable source for iron oxide reduction and can replace coal for mitigating CO2 emissions. Torrefied biomass can act as a reducing agent in the iron oxide reduction to metallic iron which is important in chemical-looping combustion for lessening CO2 emissions. This study performs iron oxide reduction by graphite and torrefied biomass via thermogravimetric analysis (TGA), while the evolved gases from the reduction processes are analyzed using a Fourier transform infrared (FTIR) spectrometer. Iron ore reduction by graphite occurs at higher temperatures (>950 C), whereas iron oxide reduction using the torrefied biomass is more significant for low-to medium-range temperatures with an onset temperature of 300 C. The reduction extent is recognized from the comparison between theoretical and experimental TGA curves, and validated by the evolved gases. The reduction extent of the 2:1 ratio of hematite-to-torrefied biomass shows a lower onset reduction temperature compared to the 1:1 ratio. The TG-FTIR results confirm the direct reduction of iron oxides by carbon in graphite and torrefied biomass and the release of evolved CO2 instead of CO. A step-wise reduction procedure is observed which is triggered by the evolved gases released from torrefied biomass devolatilization at 370 C. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Torrefaction Iron oxide reduction Mechanism Hematite Graphite and biochar TG-FTIR
1. Introduction The ironmaking industry is essential in the growth of an economy as it provides a strong foundation for its infrastructure development. Metallic iron is utilized as a raw material in various industries such as manufacturing, automotive, and manufacturing [1], thus, the industry is considered as the backbone of other industrial sectors. Despite its economic contribution and industry presence, it constitutes significantly to greenhouse gas emissions, worsening the scenario of global warming [2,3]. The industry is considered as an energy- and carbon emitting-intensive sector as it consumes around one-fifth of the annual industrial fossil utilization while releasing almost 7% of the global carbon dioxide (CO2) emissions [4]. For every ton of steel produced, 1.9 tons of CO2 is
* Corresponding author. Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan, 701, Taiwan. E-mail addresses: [email protected], [email protected] (W.-H. Chen). https://doi.org/10.1016/j.energy.2019.05.149 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
emitted to the atmosphere [5]. In recent years, the industry has felt the impact of CO2 emission taxes as the awareness of the impacts of global warming intensifies [6]. The depleting source of fossil-based fuels and increasing demand for energy have led to the shift toward renewable energy resources [7]. Focusing on the reduction of fossilfuel based energy consumption and CO2 mitigation are strategic areas where sustainable iron manufacturing can be achieved. The blast furnace is considered as the heart of the iron making industry and requires a majority of the fossil-based fuels to convert iron ore to metallic iron [8]. The blast furnace contributes approximately 70% of the CO2 emissions of the iron and steel manufacturing [9]. In a blast furnace, high-quality coke is used as the carbon source for the direct reduction process to produce metallic iron [10]. The reduction process in converting iron ore to metallic iron in a blast furnace is shown in Table 1. The reduction of iron oxides can be categorized into direct reduction and indirect reduction. The direct reduction process describes the transformation of iron oxides from hematite-phase (Fe2O3) to magnetite-phase (Fe3O4), to wustite-phase (FeO), and finally to metallic iron (Fe) through direct contact with carbon (C). Past
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Fig. 1. The preparation and experimental procedures using (a) graphite and (b) biochar as reducing agents.
studies have explored the use of alternative carbon source aside from coke and coal. The direct reduction was demonstrated recently using a single sheet iron oxide together with a heterogeneous electro-Fenton catalyst at neutral pH [11]. The effect of CaCO3 as an additive to coal for the direct reduction iron has been investigated and has enhanced the reduction of wustite (FeO) to metallic iron by inhibiting the fayalite generation [12]. Briquetted waste materials together with blast furnace sludge were proposed for various iron oxide reduction to produce metallic iron while reducing industrial wastes [13]. Graphite used as a carbon source for the reduction of
iron oxides has been investigated at a medium temperature of 600 C [14] and a high temperature of 1400 C [15] to produce metallic iron. These studies are impressive and have the goal of potentially displacing fossil-based fuels such as coal and coke for the production of metallic iron. Nevertheless, one approach for the sustainable manufacturing of metallic iron is through the use of biomass. In addition, iron oxide reduction also plays a crucial role in the development of abating CO2 emissions in that iron oxide is an important oxygen carrier used in chemical-looping combustion [16]. Biomass is among the renewable energy sources which play a
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Table 1 Reduction processes in converting hematite to metallic iron [52]. Reaction,
Table 3 Proximate and calorific analyses of the torrefied biomass from forest residue. Eq.
Direct Reduction 3 Fe2O3 þ C / 2 Fe3O4 þ CO
(1)
Fe3O4 þ C / 3 FeO þ CO
(2)
FeO þ C / Fe þ CO Indirect Reduction 3 Fe2O3 þ CO / 2 Fe3O4 þ CO2
(3)
Fe3O4 þ CO / 3 FeO þ CO2
(5)
FeO þ CO / Fe þ CO2 Gasification of coke C þ CO2 / 2 CO
(6)
Proximate Analysis Moisture Volatile Fixed Carbon Ash Content HHV
(4)
(7)
major role as a clean energy source in the future. Biomass partially consists of fixed carbon and can potentially substitute pulverized coal or coke [17,18]. Biomass offers a renewable source of energy while significantly reducing CO2 emissions. The efficient use of biomass has been adopted for renewable energy generation and is essential in achieving a low carbon economy [19]. The use of biomass as a substitute for coal and coke in a blast furnace to produce iron has been found to reduce the CO2 emissions by about one-third while producing a comparable quality of metallic iron [20]. Biomass can be upgraded to various bio-products such as torrefied biomass and biochar using thermochemical conversion technologies [21]. Biochar has been found to be a good carbon source for the direct reduction of iron oxides. Biochar offers a higher carbon content with improved heating value through pyrolysis [22] or torrefaction [23]. Recent studies in the utilization of biochar as a reduction agent for iron ore reduction to metallic iron are shown in Table 2 with the corresponding preparation of biomass. As shown in the table, most of the iron ore reduction studies utilizing biochar employed pyrolysis as a preparation process for biomass upgrading. Among the works presented in the
Amount
Units
4.66 68.44 25.65 1.25 19.56
wt% wt% wt% wt% MJ/kg
table, Wang, Zhang [24] and Hosokai, Matsui [25] evaluated the iron ore reduction by biochar using a thermogravimetric analyzer (TGA). Fan, Ji [26] and Yunus, Ani [27] carried out both TGA and derivative thermogravimetric (DTG) analysis for iron ore reduction with biochar. To date, only the work of Najmi, Mohd Yunos [28] presented a Fourier-transform infrared (FTIR) spectroscopy analysis of the iron ore reduction with biochar from palm shell. Though such studies are comprehensive and specific on the biomass utilized, there have been limited studies understanding the iron ore reduction through TGA and DTG analysis with simultaneous analysis of the evolved gas from the reduction process through an FTIR. Torrefaction is a thermochemical process opted for upgrading solid biomass through heating at temperatures of 200e300 C in an inert atmosphere [23]. The torrefied biomass from torrefaction has lower moisture, improved carbon content, higher calorific value, and more stable and uniform properties compared to raw biomass [29]. The literature review suggests that, to date, there are limited studies on the use of torrefaction as a biomass conversion technology to produce torrefied biomass for the reduction of iron oxides, as shown in Table 2. The work of Robles [30] have blended torrefied biomass from forest residue and iron ore for the iron ore reduction by using a TG analyzer connected to a quadrupole mass spectrometer (QMS) which analyzed the off-gases. In addition, torrefied biomass from forest residue, saw dust, willow, and pine € wood has been used by El-Tawil, Okvist [31] to analyze the iron oxide reduction using a TG analyzer attached to a QMS for off-gas
Table 2 Recent literature review of various biomass products used as a reductant in iron oxide reduction. Biomass
Process
Carrier Gas
Time (min)
Temperature ( C)
Reference
Oak wood Palm shell Wood shavings Four biomass feedstocks - rice lemma - peanut hull - maize cob - pine sawdust Jujube wood Three biomass feedstocks - sugarcane bagasse - coconut husk - wooden dust Oak wood Corn straw
Pyrolysis Pyrolysis Pyrolysis Pyrolysis
N2 N2 N2 N2
120 120 30 90
550 450 550 1100
[53] Najmi, Mohd Yunos [28] Hu, Yao [43] Wang, Zhang [24]
Pyrolysis Pyrolysis
N2 N2
120 60
400 700
Tang, Fu [50] Suman and Gautam [54]
Pyrolysis Two-stage pyrolysis
N2 N2
Pyrolysis Fast pyrolysis Pyrolysis Pyrolysis
N2 N2 N2 N2
700 500 700 300, 350, 400, 450 500 700 360
Gan, Fan [55] Fan, Ji [26]
Wood Biomass tar (biotar) from pine wood Empty fruit bunch Bio-coal composing of coal and torrefied forest residuea Bio-coal from the following - forest residue - saw dust - willow - pine wood
30 30 30 15 0.17 30 168
6 6 6 14
286 297 330 350
a
€ El-Tawil, Okvist [31]
Not specified Torrefaction Torrefaction Pyrolysis Pyrolysis
Torrefaction process of the torrefied biomass from forest residue was not specified.
El-Tawil, Ahmed [47] Hosokai, Matsui [25] Yunus, Ani [27] Robles [30]
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(a) 100 Hematite Graphite Hematite-Graphite (1:1 ratio) Hematite-Graphite (2:1 ratio)
90
20
25
15
20
1153oC
60
10
15
DTG (wt% /min)
70
5
10
50
o 975 C
0 950
40
1000
1050
1100
1150
Temperature (oC)
5
1200
0 200
400
600
800
1000
Temperature (oC)
(b)
CO2 CO
CO2 Intensity
CO Intensity
Hematite-Graphite (2:1 ratio) Hematite-Graphite (1:1 ratio)
o
1153 C
Graphite
CO Intensity
1200
Hematite 200
400
600
800
1000
Temperature(oC)
1200
o
o
975 C
1111 C 1153oC
Hematite-Graphite (2:1 ratio)
CO2 Intensity
30
2.1. Materials
o
975oC
1100 C
Hematite-Graphite (1:1 ratio) 950
1000
1050
1100
1150
o
1200
Temperature( C)
(c) 100 2:1 ratio TGAExperimental TGATheoretical
90
1:1 ratio
80
100
90
90
60
TGA(wt%)
100
70 TGA(wt%)
TGA(wt%)
2.2. Proximate analysis The proximate analysis employed in the study adopted the ASTM D7582-15 method [34] which was also utilized by Lee, Lee [35] in analyzing various biomass feedstocks. A 15 mg sample of the torrefied biomass was evaluated using the following procedure. With a heating rate of 20 C/min and a flow rate of 100 mL/m of N2, the sample was heated from room temperature to 105 C using a thermogravimetric analyzer (Perkin Elmer Diamond TG/DTA). The sample was then held at 105 C for moisture evaluation, then heated to 800 C at a heating rate of 20 C/min. Upon reaching 800 C, the temperature was held constant for 30 min. Thereafter, combustion occurred when the air was introduced to the chamber at 100 mL/min. The temperature was held for an additional 5 min prior to the cool down to room temperature. The moisture, volatile matter, and fixed carbon contents are initially accounted through the abrupt changes of the TGA curves. Then, the sum of the portions of the three components are subtracted from 100% to quantify the ash content.
30
DTG (wt% /min)
1153oC
2. Materials and method
The hematite and graphite used in the study were sourced from Sinopharm Chemical Reagent Co., China, where both materials have a purity higher than 99.9%. The forest residue biomass, in the form of wood pellets, was sourced from Jinding Green Energy Technology Co. The pellets were dried in an oven for 24 h and were torrefied at 250 C for 60 min in an inert (N2) atmosphere. The proximate analysis of the torrefied biomass from forest residue is shown in Table 3. The duration effect of the mixing of ground samples on the experiment was evaluated by Kasai, Mae [15], and they found that the effect had a minimal influence if the mixing duration was controlled within 30 min. The present study mixing hematite with either graphite or torrefied biomass was controlled within 5 min prior to experiments to minimize the aforementioned effect. The preparation and the experimental procedure for the samples are shown in Fig. 1.
35
80
TGA (wt%)
analysis. However, both studies have not used a torrefaction temperature below 280 C for the pre-treatment of the biomass upgrading, have not used pure hematite (Fe2O3) to strictly limit the reactions to the iron oxide reductions, and have not utilized an FTIR to quantify the evolved-gases real-time. For this reason, forest residues are used as a biomass feedstock for torrefied biomass production to reduce iron oxides. The biomass feedstock presents a practical means to mitigate carbon emissions while addressing deforestation [32]. The production and use of bioenergy products such as biochar and torrefied biomass offer an efficient means of CO2 mitigation and potential for negative carbon emission [33]. In addition to torrefied biomass, the iron oxide reduction through graphite is also considered in this study to give reference and comparison to the results of torrefied biomass from forest residue since graphite is a known reductant from previous studies [15]. To figure out the direct reduction characteristics, the present study proposes a methodology which presents a transparent and realtime occurrence of iron ore reduction through thermal degradation using TGA/DTG and evolved gases using FTIR. The use of TGFTIR enables the description of the iron oxide reduction through the TGA/DTG curves to confirm the occurrence of iron oxide reduction based on the evolved gases absorbed by the FTIR. The comparative study gives an insight into the relative performance of torrefied biomass from forest residue with another alternative carbon source such as graphite for iron manufacturing.
971
80
70
60
80
70
60
1:1 ratio 50 950
50
200
1000
2:1 ratio
1050
1100
Temperature(oC)
400
1150
1200
600
50 950
1000
1050
800 o
1100
Temperature(oC)
1150
1000
1200
1200
Temperature( C) Fig. 2. (a) TGA and DTG curves and (b) CO and CO2 spectra of hematite, graphite, and their blends, and (c) the reduction extent.
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2.3. Thermogravimetric analysis The thermogravimetric (TG) analysis was conducted using the TG where the weights and the derivative weights of the samples were simultaneously acquired. The TG analysis was performed using N2 at a flow rate of 200 mL/min. A 15 mg fresh sample per treatment was used as a physical requirement of the TG equipment. The materials were heated from room temperature to 105 C. Then this temperature was kept for 10 min to remove moisture. After which, a constant heating rate of 20 C/min was implemented until 1200 C was reached. The instantaneous weight loss was monitored and recorded. Together with the TGA results, the derivative thermogravimetric (DTG) curves were also determined for all the samples. 2.4. Fourier transform infrared (FT-IR) analysis To understand the evolved gases from the reduction reactions, a Fourier transform infrared (FT-IR) spectrometer (Perkin Elmer Spectrum 100) was used which was in conjunction with the adopted Perkin Elmer Diamond TG/DTA. To gather the signals for the evolved gases, the TG/DTA and the FT-IR were connected by a thermally insulated pipe maintained at 260 C. The absorbance was then gathered every 5.25 s using the temperature profile discussed in the previous section from a wavenumber range of 4000-450 cm1 . The FTIR background was defined with an average of 16 background scans. Combining the results of the TGA and FT-IR, the formation of the generated gases during the reduction process provided a comprehensive insight into the reduction reactions of iron oxides such as hematite, magnetite, and wustite with the alternative carbon source such as graphite and torrefied biomass from forest residues as reduction agents. 3. Results and discussion The results are presented on a per reductant basis. The
phenomena using graphite as the reducing agent are discussed first, followed by the results using torrefied biomass. It is then followed by the comparison discussion between graphite and torrefied biomass as reductants. 3.1. Graphite 3.1.1. TGA and DTG curves The TGA and DTG curves of the sole hematite and the sole graphite are shown in Fig. 2a. The TGA and DTG curves depict that the thermal degradation and change of hematite are insignificant at temperatures below 1200 C. These characteristics agree with the work of Qu, Yang [36]. The onset of the thermal degradation of hematite occurs at temperatures greater than 1400 C [36]. Similar to hematite, the variation in TGA and DTG curves of graphite is insignificant. This can be explained by graphite being considered as the stable form of carbon. The results of graphite are in agreement with the work of Kasai, Mae [15] where the onset of the thermal degradation of graphite occurred at approximately 1200 C. The TGA and DTG curves for the blends of hematite and graphite at the mixing ratios of 1:1 and 2:1 are also shown in Fig. 2a. They suggest that the onset of the change for the two ratios occurs at a temperature higher than 900 C. Similar behavior was also observed in the study of Kasai et al. (1995) in which the temperature was around 975 C. However, for a ratio of 4.88:1 hematite to graphite, they observed that the mixture experienced a thermal degradation of up to 60% weight at a temperature between 1200 and 1300 C. These results suggest that the onset of thermal degradation temperature may vary dramatically for their mixing ratio between 2:1 and 4.88:1. The thermal degradation signifies the occurring iron oxide reduction. Based on the work of Qi, Murakami [37], the sharp weight loss and significant change in the DTG curve for hematite represents an endothermic reaction, leading to the occurrence of the reduction process. This has been confirmed by the work of Mckee [38] where the gasification of graphite started at a temperature of 700 C in the presence of oxides. With results of
Fig. 3. Three-dimensional FTIR spectra of (a) hematite, (b) graphite, and hematite-graphite mixtures at the ratios of (c) 1:1 and (d) 2:1.
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Table 4 The corresponding functional groups and compounds for the absorption of evolved gases in the FTIR. Reducing Agent
Ratio
Temperature Range (⁰C)
Wavenumber (cm-1)
Functional Groups
Compounds
Graphite
1:1
975 to 1100
2400e2240 1600e1420 2400e2240 2240e2180 1600e1420 2400e2240 1600e1420 2400e2240 2240e2180 1600e1420 2400e2240 1600e1420 4000e3500 3000e2700 2400e2240 2240e2180 1600e1420 1400e1000 1000e650 2400e2240 1600e1420 2400e2240 2240e2180 1600e1420 4000e3500 3000e2700 2400e2240 2240e2180 1600e1420 1400e1000 1000e650 2400e2240 1600e1420 2400e2240 2240e2180 1600e1420
O¼C]O C¼C O¼C]O CeO C¼C O¼C]O C¼C O¼C]O CeO C¼C O¼C]O C¼C OeH CeH O¼C]O CeO C¼C, C]O CeH CeO, OeH O¼C]O C¼C O¼C]O CeO C¼C OeH CeH O¼C]O CeO C¼C, C]O CeH CeO, OeH O¼C]O C¼C O¼C]O CeO C¼C
Carbon Dioxide Aromatic Compounds Carbon Dioxide Carbon Monoxide Aromatic Compounds Carbon Dioxide Aromatic Compounds Carbon Dioxide Carbon Monoxide Aromatic Compounds Carbon Dioxide Aromatic Compounds Water molecules Hydrocarbons Carbon Dioxide Carbon Monoxide Aromatic Compounds Methyl Group Alcohols and Phenols Carbon Dioxide Aromatic Compounds Carbon Dioxide Carbon Monoxide Aromatic Compounds Water molecules Hydrocarbons Carbon Dioxide Carbon Monoxide Aromatic Compounds Methyl Group Alcohols and Phenols Carbon Dioxide Aromatic Compounds Carbon Dioxide Carbon Monoxide Aromatic Compounds
1101 to 1200
2:1
975 to 1111 1112 to 1150
1112 to 1150 Torrefied Biomass
1:1
300 to 400
401 to 1100 Torrefied Biomass
1:1
1101 to 1200
Torrefied Biomass
2:1
300 to 400
401 to 1150 1151 to 1200
the sole hematite and sole graphite having insignificant TG degradation indicates that the TG degradation on the mixtures of hematite and graphite signifies iron oxide reduction. In Fig. 2a, the observed peaks at a temperature of 1153 C for the two mixing ratios reveal the potential occurrence of iron oxide reduction which can be validated by the FTIR and reduction extent results.
3.1.2. CO2 and CO FTIR spectra Chen, Dang [39] suggested that the transformation from one phase of iron oxides to another phase could happen simultaneously at a given range of temperature. This implies, in turn, the simultaneous shift from hematite to magnetite and from magnetite to wustite can occur at the same temperature. Similarly, it is possible for the simultaneous transformation of magnetite to wustite and wustite to metallic iron to occur at a given temperature range [39]. To proceed farther into an analysis of the reduction behavior, the FTIR spectra of CO and CO2 at the wavenumbers of 2210 cm-1 and 2384 cm-1 [40] are shown in Fig. 2b. The CO2 initially evolves at a temperature of 975 C which is earlier than CO. The work of Qi, Murakami [37] also found this behavior for iron ore-graphite composite. For the hematite-to-graphite ratios of 1:1 and 2:1, the onset temperatures of the evolved CO absorbance in the spectra are around 1100 C and 1111 C, respectively. This suggests that the reduction occurring from temperatures 975 Ce1100 C for the 1:1 ratio and from 975 C to 1111 C for the 2:1 ratio using C as a reductant follows the chemical reactions shown below instead of Eqs. (1)e(3): 6 Fe2O3 þ C / 4 Fe3O4 þ CO2
(8)
2 Fe3O4 þ C / 6 FeO þ CO2 2 FeO þ C / 2 Fe þ CO2
(9) (10)
It follows that, for the ratios of 1:1 and 2:1 with the temperature ranges of 1100e1200 C and 1111e1200 C, respectively, the chemical reactions consist of the direct and indirect reduction, as described in Eqs. (1)e(6) [15,37].
3.1.3. Reduction extent To account for the extent of iron ore reduction, the theoretical TGA curves is given below:
TGAtheoretical ¼ YH TGAH þ YG TGAG
(11)
where TGA and Y stand for the TGA curve and mass fraction, respectively, while the subscripts H and G denote hematite and graphite, respectively. It should be addressed that TGAH and TGAG represent the individual TGA curves obtained in Fig. 2a. The theoretical TGA curves and experimental TGA curves (TGAexperimental ) of the mixtures at the two ratios (1:1 and 2:1 ratios) are shown in Fig. 2c. The theoretical TGA curves show slight mass degradation at high temperatures; however, the experimental TGA curves decline obviously with increasing temperature. The gap between TGAtheoretical and TGAexperimental provide a measure of hematite reduction extent. Obviously, the higher the reaction temperature, the higher the reduction degree, stemming from the intensified gap.
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40
(a) 100
Hematite Biochar Hematite-Torrefied Biomass (1:1 ratio) Hematite-Torrefied Biomass (2:1 ratio)
35
50
25 20 1
0.8
0.6
0 0.4
0.2
15
DTG (wt% /min)
TGA (wt%)
o
370 C
DTG (wt% /min)
30
10
0
5
-0.2 750
800
850
900
950
1000
Temperature (oC)
1050
1100
1150
1200
o 830 C 1011-1138 C o
-50
0 200
400
600
800
1000
o
1200
Temperature ( C)
Hematite-Torrefied Biomass (2:1 ratio) CO2 Intensity
CO2 CO
CO Intensity
(b)
Hematite-Torrefied Biomass (1:1 ratio)
CO2 Intensity
CO Intensity
Torrefied Biomass Hematite 200
o
370 C
400
600
800
1000
Temperature(oC)
o
830 C
1200
o
1038 C
Hematite-Torrefied Biomass (2:1 ratio) 370oC
o
830 C
1011oC
Hematite-Torrefied Biomass (1:1 ratio) 200
400
600
800
1000
o
1200
Temperature( C)
(c) 100
TGAExperimental TGATheoretical
90
2:1 ratio
80
TGA(wt%)
70
1:1 ratio
60 50
100
100
30
90
85
TGA(wt%)
TGA(wt%)
90
40
2:1 ratio
1:1 ratio
95
80
3.1.5. Reduction mechanisms In light of the observations above, it can be summarized that two-stage reduction based on the temperature ranges is exhibited. For the composite with the ratio of 1:1, the first stage, ranging from 975 C to 1100 C, follows a direct reduction via Eqs. (8)e(10) as CO2 is the sole evolved gas observed. It was recently proposed that carbon is consumed in the process of direct reduction to generate the sole CO2 evolved gas [43]. Subsequently for the second stage at the temperature range of 1100e1200 C, a simultaneous direct reduction following Eqs. (1)e(3) and Eqs. (8)e(10) possibly transpire together with the occurrence of indirect reduction following Eqs. (4)e(6), as a consequence of both CO and CO2 observed (Fig. 2b). The concurrent reactions of direct reduction, Eqs. (1)e(3), and indirect reductions, Eqs. (4)e(6), and the char gasification, Eq. (7), can possibly occur [24]. The FTIR peaks for the CO evolved gas at the ratios of 1:1 and 2:1 are at 1153 C which is in line with the peaks in the DTG curves (Fig. 2a). Moreover, the high intensities of CO and CO2 in the spectra at 1153 C (Fig. 2b) can be explained by the simultaneous reactions from Eqs. (1)e(6) and Eqs. (8)e(10). Similarly, the composite with 2:1 ratio has almost the same results with the 1:1 ratio, except for the onset absorbance temperature of CO evolved gas observed at 1111 C instead of 1100 C for in the 1:1 ratio. At this temperature, the end of the first stage and the start of the second stage of the two-stage reduction of the composite with a 2:1 ratio is defined.
80
3.2. Torrefied biomass from forest residue
75 70
70
3.1.4. Three-dimensional FTIR spectra The three-dimensional (3D) FTIR spectra of the evolved gas from the heating of hematite, graphite, and the two mixtures are displayed in Fig. 3. The functional groups for the corresponding wavenumber ranges are given in Table 4. Fig. 3a and b depict a relatively flat profile in the heating processes of hematite and graphite, signifying no evolved gas produced. This is attributed to no mass degradation in the TGA and DTG curves in Fig. 2a. The elevated absorbance regions observed in Fig. 3 suggests the presence of CO, CO2, and some aromatic compounds (C]C at 16001420 cm-1) absorbed at a temperature range of 975e1200 C. The results reflect the enhanced gasification of carbon on these temperatures and reacting with various iron oxide phases to produce metallic iron and CO2. The well-known catalytic effect of iron on gasification has been established [41]. The peaks shown in Fig. 2a and b coincide with the absorbed gases of the spectra shown in Fig. 3. The 3D spectra also characterize the absorbed CO2 during the reduction processes occurring from 800 C to 1200 C. At a temperature of 1160 C, the absorption of evolved gases shown in Fig. 3c indicates that CO, CO2, and aromatic compounds (C]C at 1600-1420 cm-1) are present which have been confirmed by Amiri, Shanbedi [42]. The study of Qi, Murakami [37] quantified the presence of CO and CO2 for graphite using the chemical balance equations of oxides and carbon through the reduction and gasification processes. The overall results of the combined hematite and graphite suggest the occurrence of iron ore reduction at elevated temperatures of 950 C and beyond.
65
20 10
60
200
60
300
400
500
600
Temperature(oC)
400
700
800
600
300
400
500
600
o
700
800
Temperature( C)
800 o
1000
1200
Temperature( C) Fig. 4. (a) TGA and DTG curves and (b) CO and CO2 spectra of hematite, biochar, and their blends, and (c) the reduction extent.
3.2.1. TGA and DTG curves as well as CO2 and CO FTIR spectra The TGA and DTG curves of the torrefied biomass from forest residue are shown in Fig. 4a, and the result of hematite shown in Fig. 1a is used in Fig. 4a as a baseline for the comparison. The onset of the weight loss of the torrefied biomass shown in Fig. 4a develops at 250 C and is also observed in the varying ratios of the combined hematite and torrefied biomass mixtures. A peak at the DTG curve is observed at a temperature 370 C for the torrefied biomass and the mixtures at both the ratios of 1:1 and 2:1. The
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975
Fig. 5. Three-dimensional FTIR spectra of (a) torrefied biomass and hematite-torrefied biomass mixtures at the ratios of (c) 1:1 and (d) 2:1.
Fig. 4b, while the experimental and theoretical TGA curves of the mixtures at the hematite-to-torrefied biomass ratios of 1:1 and 2:1 are shown in Fig. 4c. It is of interest that two peaks at around 370 C and 830 C are observed in Fig. 4b. For the hematite-torrefied biomass mixture at the ratio of 1:1, Fig. 4c depicts the gap between TGAtheoretical and TGAexperimental starts to appear at around 600 C, and the gap increases with rising temperature. It should be addressed that even though the devolatilization of the torrefied biomass is drastic at 370 C, the gap is almost imperceptible therein. It follows that the hematite reduction is fairly slight. In contrast, the gap starts to develop at around 370 C for the mixture at the ratio of 2:1, implying that the onset of the mixture's reduction is triggered at a much lower temperature. In addition, at the end of the heating process (1200 C), the gap at the ratio of 2:1 is
onset weight loss of the samples observed in the TGA results and the peak identified from the DTG curve are in accordance to the results of Robles [30], which is mainly attributed to the devolatilization of the torrefied biomass. Compared to graphite (Fig. 2a), the reason causing the peaks triggered at lower temperatures is due to the higher volatile matter content (68.44 wt%) and lower fixed carbon content (25.65 wt%) in the torrefied biomass (Table 3) which € are in a good agreement with the results of El-Tawil, Okvist [31]. Specifically, the devolatilization process of the torrefied biomass is responsible for the peaks exhibited at 370 C in the DTG curves [44].
3.2.2. CO2 and CO FTIR spectra and reduction extent The FTIR spectra of the gases CO (2210 cm-1) and CO2 (2384 cm1 ) evolved from the heating of the four samples are shown in
CO2
CO2
CO2
CO2
Iron Ore
Iron Ore Extraction
Transportation
Composite Iron Ore-Biomass Iron Ore Sintering CO2
CO2
Torrefied Biomass
Biomass
Forest
Transportation
Iron
Torrefaction
Blast Furnace
Fig. 6. The process flow of ironmaking using torrefied biomass from forest residue with CO2 abatement.
976
A.T. Ubando et al. / Energy 180 (2019) 968e977
much greater than that at 1:1, suggesting the higher reduction extent of the former. Much of the degradation of the torrefied biomass at 370 C is attributed to the mass degradation and release of volatiles together with other aromatic compounds [45] based on the 3D FTIR results shown in Fig. 5. One of the most significant peaks was observed at the wavenumber range of 1600-1420 cm-1, representing C]C and C]O stretching vibration of aromatic compounds which is magnified by polar functional groups of alkenes [46]. The peaks at the wavenumber range of 1400-1000 cm-1 are accounted to the formation of the methyl group, while the peaks at the wavenumber range of 1000-350 cm-1 are responsible for the presence of alcohol and phenols corresponding to the stretching of CeO, and OeH [28]. The reduction occurring in low temperatures shown in Fig. 4c can mainly be governed by Eqs. (8)e(10) since the release of CO compared to CO2 is substantially lower, thus suggesting that the reduction is mainly owing to the carbon from the torrefied biomass. In addition, Hu, Yao [43] also proposed the possible direct reduction of iron ores using pyrolyzed biochar following Eqs. (8)e(10). For the torrefied biomass at 300 C, it is proposed that the reduction temperature of hematite to magnetite (Fe2O3 / Fe3O4) occurs from 365 to 555 C, magnetite to wustite (Fe3O4 / FeO) occurs from 595 to 799 C, and wustite to metallic iron (FeO / Fe) occurs from 799 to 1200 C [47]. Thus, the three peaks identified in Fig. 4b suggest the reduction of iron oxides from one phase to another with the corresponding temperatures. It was suggested by El-Tawil, Ahmed [47] that the reduction of iron oxides through pyrolyzed biochar occurred through a stepwise procedure (Fe2O3 / Fe3O4, Fe3O4 / FeO, and FeO / Fe) which was initiated due to the evolved gases from the thermal decomposition of volatiles observed at low temperatures of 370 C. The study of Robles [30] using torrefied biomass from forest residue suggested a stepwise reduction of Fe2O3 / Fe3O4 at 564 C, Fe3O4 / FeO at a range of temperature from 650 to 731 C, and FeO / Fe at a temperature range from 850 to 1200 C. The possible presence of oxygen in the aromatic compounds [46], alcohol, and phenols [28] in the torrefied biomass aided its devolatilization at this temperature range enabling the initial reduction to occur at this low temperature. 3.3. Comparison Comparing the thermogravimetric behavior using graphite and torrefied biomass as reducing agents, the important findings can be summarized as follows. Firstly, the reduction of iron oxides occurs at a higher temperature range (950e1200 C) when using graphite as the reductant, whereas it is triggered at a lower temperature range (300e1100 C) when using torrefied biomass. Consequently, the torrefied biomass from forest residue offers an alternative source of reductant for ironmaking, especially for the mixture with the hematite-to-torrefied biomass ratio of 2:1. Secondly, the FTIR spectra for the mixtures of hematite and graphite suggest that the reduction using carbon as a reductant occurs at 975e1111 C, following Eqs. (8)e(10). After that, the reduction follows Eqs. (1)e(6) until 1200 C. On the other hand, the FTIR spectra for the mixtures of hematite and torrefied biomass reveal that the iron oxide reduction is mainly governed by the reduction expressed in Eqs. (8)e(10) where the signal for CO is minimal compared to CO2. An increasing reduction behavior has been observed for the blends of hematite and torrefied biomass. This may be attributed to the volatiles liberated from the devolatilization of the torrefied biomass at temperatures higher than 350 C [44]. Thirdly, with the use of torrefied biomass from forest residues as the ironmaking reductant, carbon abatement is potentially realized. The CO2 emissions from the supply-chain of ironmaking such as the
iron ore extraction and preparation, the transportation of raw materials, the upgrading of biomass to torrefied biomass through torrefaction, and the iron manufacturing are absorbed by the forest and plants as shown in Fig. 6. Biomass is considered a renewable energy source and is known for its carbon neutral effect due to its capability to absorb CO2 during its growth phase [48]. Hence, the use of torrefied biomass from forest residues as a reductant for ironmaking offers a sustainable approach for the production of metallic iron which potentially offsets the CO2 flow [49]. In chemical-looping combustion, iron oxides are used as an oxygen carrier. Upgraded biomass can be used as a reducing agent for the oxygen carrier in the fuel-reactor for iron oxide reduction, thus, producing CO2 gas [50]. The rich CO2 gas can be then captured and stored to produce negative carbon flow. Torrefied biomass is considered as one of the future solutions in ironmaking plants to significantly reduce the carbon dioxide emissions while producing quality iron [51]. Lastly, it should be illustrated that the results of this study have provided a guide to the iron industry in the potential use of torrefied biomass from forest residues for ironmaking.
4. Conclusions Hematite reduced by graphite and torrefied biomass has been analyzed by a thermogravimetric analyzer accompanied by a Fourier transform infrared spectrometer. From the thermogravimetric and evolved gas analysis in association with introducing a measure of reduction extent, the reduction mechanisms of iron oxides have been recognized. The analysis shows that the reduction by graphite occurs at temperatures higher than 950 C, whereas the reduction by the torrefied biomass mainly develops at low-to medium-temperature with an onset reduction temperature of 300 C. The reduction process has been confirmed by the DTG peaks coinciding with the peaks of the absorbed gases in the FTIR where a step-wise reduction of iron oxides transpires, especially for torrefied biomass. Three peaks are identified for torrefied biomass which suggest the three step-wise reduction procedure of iron oxides (Fe2O3 / Fe3O4, Fe3O4 / FeO, and FeO / Fe) initiated from the evolved gases from the thermal decomposition of volatiles at the low temperature range of 300e400 C. The FTIR result indicates the possible direct reduction of the iron oxides using carbon from both graphite and torrefied biomass with an evolved carbon dioxide as opposed to the evolved CO as suggested by other studies. Comparing the reduction extent of the 2:1 and 1:1 ratios of hematite and torrefied biomass, 2:1 ratio has a lower onset temperature which possibly is attributed to the release of volatiles from the devolatilization of torrefied biomass. The utilization of the torrefied biomass as a reductant has the potential for carbonneutral flow enabling the sustainable metallic iron production. The results of the study provide an insight into the potential adoption of torrefied biomass from forest residues in chemicallooping combustion for carbon sequestration and clean energy production. Future studies may include kinetics analysis of using torrefied biomass for iron oxide reduction and the possible use of bench-type platform to accommodate larger amounts of sample leading to the potential analysis of the reduction residue.
Acknowledgments Acknowledgment is addressed to the Ministry of Science and Technology of Taiwan, R.O.C., for funding the research under the grant numbers MOST 106-2923-E-006-002-MY3 and MOST 1072811-E-006-529.
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Iron oxide reduction by graphite and torrefied biomass analyzed by TG-FTIR for mitigating CO2 emissions Aristotle T. Ubando a, b, Wei-Hsin Chen a, c, *, Hwai Chyuan Ong d a
Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan, 701, Taiwan Mechanical Engineering Department, De La Salle University, 2401 Taft Avenue, 0922, Manila, Philippines c Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan, 701, Taiwan d Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2019 Received in revised form 8 May 2019 Accepted 21 May 2019 Available online 23 May 2019
Biomass provides a sustainable source for iron oxide reduction and can replace coal for mitigating CO2 emissions. Torrefied biomass can act as a reducing agent in the iron oxide reduction to metallic iron which is important in chemical-looping combustion for lessening CO2 emissions. This study performs iron oxide reduction by graphite and torrefied biomass via thermogravimetric analysis (TGA), while the evolved gases from the reduction processes are analyzed using a Fourier transform infrared (FTIR) spectrometer. Iron ore reduction by graphite occurs at higher temperatures (>950 C), whereas iron oxide reduction using the torrefied biomass is more significant for low-to medium-range temperatures with an onset temperature of 300 C. The reduction extent is recognized from the comparison between theoretical and experimental TGA curves, and validated by the evolved gases. The reduction extent of the 2:1 ratio of hematite-to-torrefied biomass shows a lower onset reduction temperature compared to the 1:1 ratio. The TG-FTIR results confirm the direct reduction of iron oxides by carbon in graphite and torrefied biomass and the release of evolved CO2 instead of CO. A step-wise reduction procedure is observed which is triggered by the evolved gases released from torrefied biomass devolatilization at 370 C. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Torrefaction Iron oxide reduction Mechanism Hematite Graphite and biochar TG-FTIR
1. Introduction The ironmaking industry is essential in the growth of an economy as it provides a strong foundation for its infrastructure development. Metallic iron is utilized as a raw material in various industries such as manufacturing, automotive, and manufacturing [1], thus, the industry is considered as the backbone of other industrial sectors. Despite its economic contribution and industry presence, it constitutes significantly to greenhouse gas emissions, worsening the scenario of global warming [2,3]. The industry is considered as an energy- and carbon emitting-intensive sector as it consumes around one-fifth of the annual industrial fossil utilization while releasing almost 7% of the global carbon dioxide (CO2) emissions [4]. For every ton of steel produced, 1.9 tons of CO2 is
* Corresponding author. Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan, 701, Taiwan. E-mail addresses: [email protected], [email protected] (W.-H. Chen). https://doi.org/10.1016/j.energy.2019.05.149 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
emitted to the atmosphere [5]. In recent years, the industry has felt the impact of CO2 emission taxes as the awareness of the impacts of global warming intensifies [6]. The depleting source of fossil-based fuels and increasing demand for energy have led to the shift toward renewable energy resources [7]. Focusing on the reduction of fossilfuel based energy consumption and CO2 mitigation are strategic areas where sustainable iron manufacturing can be achieved. The blast furnace is considered as the heart of the iron making industry and requires a majority of the fossil-based fuels to convert iron ore to metallic iron [8]. The blast furnace contributes approximately 70% of the CO2 emissions of the iron and steel manufacturing [9]. In a blast furnace, high-quality coke is used as the carbon source for the direct reduction process to produce metallic iron [10]. The reduction process in converting iron ore to metallic iron in a blast furnace is shown in Table 1. The reduction of iron oxides can be categorized into direct reduction and indirect reduction. The direct reduction process describes the transformation of iron oxides from hematite-phase (Fe2O3) to magnetite-phase (Fe3O4), to wustite-phase (FeO), and finally to metallic iron (Fe) through direct contact with carbon (C). Past
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Fig. 1. The preparation and experimental procedures using (a) graphite and (b) biochar as reducing agents.
studies have explored the use of alternative carbon source aside from coke and coal. The direct reduction was demonstrated recently using a single sheet iron oxide together with a heterogeneous electro-Fenton catalyst at neutral pH [11]. The effect of CaCO3 as an additive to coal for the direct reduction iron has been investigated and has enhanced the reduction of wustite (FeO) to metallic iron by inhibiting the fayalite generation [12]. Briquetted waste materials together with blast furnace sludge were proposed for various iron oxide reduction to produce metallic iron while reducing industrial wastes [13]. Graphite used as a carbon source for the reduction of
iron oxides has been investigated at a medium temperature of 600 C [14] and a high temperature of 1400 C [15] to produce metallic iron. These studies are impressive and have the goal of potentially displacing fossil-based fuels such as coal and coke for the production of metallic iron. Nevertheless, one approach for the sustainable manufacturing of metallic iron is through the use of biomass. In addition, iron oxide reduction also plays a crucial role in the development of abating CO2 emissions in that iron oxide is an important oxygen carrier used in chemical-looping combustion [16]. Biomass is among the renewable energy sources which play a
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Table 1 Reduction processes in converting hematite to metallic iron [52]. Reaction,
Table 3 Proximate and calorific analyses of the torrefied biomass from forest residue. Eq.
Direct Reduction 3 Fe2O3 þ C / 2 Fe3O4 þ CO
(1)
Fe3O4 þ C / 3 FeO þ CO
(2)
FeO þ C / Fe þ CO Indirect Reduction 3 Fe2O3 þ CO / 2 Fe3O4 þ CO2
(3)
Fe3O4 þ CO / 3 FeO þ CO2
(5)
FeO þ CO / Fe þ CO2 Gasification of coke C þ CO2 / 2 CO
(6)
Proximate Analysis Moisture Volatile Fixed Carbon Ash Content HHV
(4)
(7)
major role as a clean energy source in the future. Biomass partially consists of fixed carbon and can potentially substitute pulverized coal or coke [17,18]. Biomass offers a renewable source of energy while significantly reducing CO2 emissions. The efficient use of biomass has been adopted for renewable energy generation and is essential in achieving a low carbon economy [19]. The use of biomass as a substitute for coal and coke in a blast furnace to produce iron has been found to reduce the CO2 emissions by about one-third while producing a comparable quality of metallic iron [20]. Biomass can be upgraded to various bio-products such as torrefied biomass and biochar using thermochemical conversion technologies [21]. Biochar has been found to be a good carbon source for the direct reduction of iron oxides. Biochar offers a higher carbon content with improved heating value through pyrolysis [22] or torrefaction [23]. Recent studies in the utilization of biochar as a reduction agent for iron ore reduction to metallic iron are shown in Table 2 with the corresponding preparation of biomass. As shown in the table, most of the iron ore reduction studies utilizing biochar employed pyrolysis as a preparation process for biomass upgrading. Among the works presented in the
Amount
Units
4.66 68.44 25.65 1.25 19.56
wt% wt% wt% wt% MJ/kg
table, Wang, Zhang [24] and Hosokai, Matsui [25] evaluated the iron ore reduction by biochar using a thermogravimetric analyzer (TGA). Fan, Ji [26] and Yunus, Ani [27] carried out both TGA and derivative thermogravimetric (DTG) analysis for iron ore reduction with biochar. To date, only the work of Najmi, Mohd Yunos [28] presented a Fourier-transform infrared (FTIR) spectroscopy analysis of the iron ore reduction with biochar from palm shell. Though such studies are comprehensive and specific on the biomass utilized, there have been limited studies understanding the iron ore reduction through TGA and DTG analysis with simultaneous analysis of the evolved gas from the reduction process through an FTIR. Torrefaction is a thermochemical process opted for upgrading solid biomass through heating at temperatures of 200e300 C in an inert atmosphere [23]. The torrefied biomass from torrefaction has lower moisture, improved carbon content, higher calorific value, and more stable and uniform properties compared to raw biomass [29]. The literature review suggests that, to date, there are limited studies on the use of torrefaction as a biomass conversion technology to produce torrefied biomass for the reduction of iron oxides, as shown in Table 2. The work of Robles [30] have blended torrefied biomass from forest residue and iron ore for the iron ore reduction by using a TG analyzer connected to a quadrupole mass spectrometer (QMS) which analyzed the off-gases. In addition, torrefied biomass from forest residue, saw dust, willow, and pine € wood has been used by El-Tawil, Okvist [31] to analyze the iron oxide reduction using a TG analyzer attached to a QMS for off-gas
Table 2 Recent literature review of various biomass products used as a reductant in iron oxide reduction. Biomass
Process
Carrier Gas
Time (min)
Temperature ( C)
Reference
Oak wood Palm shell Wood shavings Four biomass feedstocks - rice lemma - peanut hull - maize cob - pine sawdust Jujube wood Three biomass feedstocks - sugarcane bagasse - coconut husk - wooden dust Oak wood Corn straw
Pyrolysis Pyrolysis Pyrolysis Pyrolysis
N2 N2 N2 N2
120 120 30 90
550 450 550 1100
[53] Najmi, Mohd Yunos [28] Hu, Yao [43] Wang, Zhang [24]
Pyrolysis Pyrolysis
N2 N2
120 60
400 700
Tang, Fu [50] Suman and Gautam [54]
Pyrolysis Two-stage pyrolysis
N2 N2
Pyrolysis Fast pyrolysis Pyrolysis Pyrolysis
N2 N2 N2 N2
700 500 700 300, 350, 400, 450 500 700 360
Gan, Fan [55] Fan, Ji [26]
Wood Biomass tar (biotar) from pine wood Empty fruit bunch Bio-coal composing of coal and torrefied forest residuea Bio-coal from the following - forest residue - saw dust - willow - pine wood
30 30 30 15 0.17 30 168
6 6 6 14
286 297 330 350
a
€ El-Tawil, Okvist [31]
Not specified Torrefaction Torrefaction Pyrolysis Pyrolysis
Torrefaction process of the torrefied biomass from forest residue was not specified.
El-Tawil, Ahmed [47] Hosokai, Matsui [25] Yunus, Ani [27] Robles [30]
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40
(a) 100 Hematite Graphite Hematite-Graphite (1:1 ratio) Hematite-Graphite (2:1 ratio)
90
20
25
15
20
1153oC
60
10
15
DTG (wt% /min)
70
5
10
50
o 975 C
0 950
40
1000
1050
1100
1150
Temperature (oC)
5
1200
0 200
400
600
800
1000
Temperature (oC)
(b)
CO2 CO
CO2 Intensity
CO Intensity
Hematite-Graphite (2:1 ratio) Hematite-Graphite (1:1 ratio)
o
1153 C
Graphite
CO Intensity
1200
Hematite 200
400
600
800
1000
Temperature(oC)
1200
o
o
975 C
1111 C 1153oC
Hematite-Graphite (2:1 ratio)
CO2 Intensity
30
2.1. Materials
o
975oC
1100 C
Hematite-Graphite (1:1 ratio) 950
1000
1050
1100
1150
o
1200
Temperature( C)
(c) 100 2:1 ratio TGAExperimental TGATheoretical
90
1:1 ratio
80
100
90
90
60
TGA(wt%)
100
70 TGA(wt%)
TGA(wt%)
2.2. Proximate analysis The proximate analysis employed in the study adopted the ASTM D7582-15 method [34] which was also utilized by Lee, Lee [35] in analyzing various biomass feedstocks. A 15 mg sample of the torrefied biomass was evaluated using the following procedure. With a heating rate of 20 C/min and a flow rate of 100 mL/m of N2, the sample was heated from room temperature to 105 C using a thermogravimetric analyzer (Perkin Elmer Diamond TG/DTA). The sample was then held at 105 C for moisture evaluation, then heated to 800 C at a heating rate of 20 C/min. Upon reaching 800 C, the temperature was held constant for 30 min. Thereafter, combustion occurred when the air was introduced to the chamber at 100 mL/min. The temperature was held for an additional 5 min prior to the cool down to room temperature. The moisture, volatile matter, and fixed carbon contents are initially accounted through the abrupt changes of the TGA curves. Then, the sum of the portions of the three components are subtracted from 100% to quantify the ash content.
30
DTG (wt% /min)
1153oC
2. Materials and method
The hematite and graphite used in the study were sourced from Sinopharm Chemical Reagent Co., China, where both materials have a purity higher than 99.9%. The forest residue biomass, in the form of wood pellets, was sourced from Jinding Green Energy Technology Co. The pellets were dried in an oven for 24 h and were torrefied at 250 C for 60 min in an inert (N2) atmosphere. The proximate analysis of the torrefied biomass from forest residue is shown in Table 3. The duration effect of the mixing of ground samples on the experiment was evaluated by Kasai, Mae [15], and they found that the effect had a minimal influence if the mixing duration was controlled within 30 min. The present study mixing hematite with either graphite or torrefied biomass was controlled within 5 min prior to experiments to minimize the aforementioned effect. The preparation and the experimental procedure for the samples are shown in Fig. 1.
35
80
TGA (wt%)
analysis. However, both studies have not used a torrefaction temperature below 280 C for the pre-treatment of the biomass upgrading, have not used pure hematite (Fe2O3) to strictly limit the reactions to the iron oxide reductions, and have not utilized an FTIR to quantify the evolved-gases real-time. For this reason, forest residues are used as a biomass feedstock for torrefied biomass production to reduce iron oxides. The biomass feedstock presents a practical means to mitigate carbon emissions while addressing deforestation [32]. The production and use of bioenergy products such as biochar and torrefied biomass offer an efficient means of CO2 mitigation and potential for negative carbon emission [33]. In addition to torrefied biomass, the iron oxide reduction through graphite is also considered in this study to give reference and comparison to the results of torrefied biomass from forest residue since graphite is a known reductant from previous studies [15]. To figure out the direct reduction characteristics, the present study proposes a methodology which presents a transparent and realtime occurrence of iron ore reduction through thermal degradation using TGA/DTG and evolved gases using FTIR. The use of TGFTIR enables the description of the iron oxide reduction through the TGA/DTG curves to confirm the occurrence of iron oxide reduction based on the evolved gases absorbed by the FTIR. The comparative study gives an insight into the relative performance of torrefied biomass from forest residue with another alternative carbon source such as graphite for iron manufacturing.
971
80
70
60
80
70
60
1:1 ratio 50 950
50
200
1000
2:1 ratio
1050
1100
Temperature(oC)
400
1150
1200
600
50 950
1000
1050
800 o
1100
Temperature(oC)
1150
1000
1200
1200
Temperature( C) Fig. 2. (a) TGA and DTG curves and (b) CO and CO2 spectra of hematite, graphite, and their blends, and (c) the reduction extent.
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2.3. Thermogravimetric analysis The thermogravimetric (TG) analysis was conducted using the TG where the weights and the derivative weights of the samples were simultaneously acquired. The TG analysis was performed using N2 at a flow rate of 200 mL/min. A 15 mg fresh sample per treatment was used as a physical requirement of the TG equipment. The materials were heated from room temperature to 105 C. Then this temperature was kept for 10 min to remove moisture. After which, a constant heating rate of 20 C/min was implemented until 1200 C was reached. The instantaneous weight loss was monitored and recorded. Together with the TGA results, the derivative thermogravimetric (DTG) curves were also determined for all the samples. 2.4. Fourier transform infrared (FT-IR) analysis To understand the evolved gases from the reduction reactions, a Fourier transform infrared (FT-IR) spectrometer (Perkin Elmer Spectrum 100) was used which was in conjunction with the adopted Perkin Elmer Diamond TG/DTA. To gather the signals for the evolved gases, the TG/DTA and the FT-IR were connected by a thermally insulated pipe maintained at 260 C. The absorbance was then gathered every 5.25 s using the temperature profile discussed in the previous section from a wavenumber range of 4000-450 cm1 . The FTIR background was defined with an average of 16 background scans. Combining the results of the TGA and FT-IR, the formation of the generated gases during the reduction process provided a comprehensive insight into the reduction reactions of iron oxides such as hematite, magnetite, and wustite with the alternative carbon source such as graphite and torrefied biomass from forest residues as reduction agents. 3. Results and discussion The results are presented on a per reductant basis. The
phenomena using graphite as the reducing agent are discussed first, followed by the results using torrefied biomass. It is then followed by the comparison discussion between graphite and torrefied biomass as reductants. 3.1. Graphite 3.1.1. TGA and DTG curves The TGA and DTG curves of the sole hematite and the sole graphite are shown in Fig. 2a. The TGA and DTG curves depict that the thermal degradation and change of hematite are insignificant at temperatures below 1200 C. These characteristics agree with the work of Qu, Yang [36]. The onset of the thermal degradation of hematite occurs at temperatures greater than 1400 C [36]. Similar to hematite, the variation in TGA and DTG curves of graphite is insignificant. This can be explained by graphite being considered as the stable form of carbon. The results of graphite are in agreement with the work of Kasai, Mae [15] where the onset of the thermal degradation of graphite occurred at approximately 1200 C. The TGA and DTG curves for the blends of hematite and graphite at the mixing ratios of 1:1 and 2:1 are also shown in Fig. 2a. They suggest that the onset of the change for the two ratios occurs at a temperature higher than 900 C. Similar behavior was also observed in the study of Kasai et al. (1995) in which the temperature was around 975 C. However, for a ratio of 4.88:1 hematite to graphite, they observed that the mixture experienced a thermal degradation of up to 60% weight at a temperature between 1200 and 1300 C. These results suggest that the onset of thermal degradation temperature may vary dramatically for their mixing ratio between 2:1 and 4.88:1. The thermal degradation signifies the occurring iron oxide reduction. Based on the work of Qi, Murakami [37], the sharp weight loss and significant change in the DTG curve for hematite represents an endothermic reaction, leading to the occurrence of the reduction process. This has been confirmed by the work of Mckee [38] where the gasification of graphite started at a temperature of 700 C in the presence of oxides. With results of
Fig. 3. Three-dimensional FTIR spectra of (a) hematite, (b) graphite, and hematite-graphite mixtures at the ratios of (c) 1:1 and (d) 2:1.
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Table 4 The corresponding functional groups and compounds for the absorption of evolved gases in the FTIR. Reducing Agent
Ratio
Temperature Range (⁰C)
Wavenumber (cm-1)
Functional Groups
Compounds
Graphite
1:1
975 to 1100
2400e2240 1600e1420 2400e2240 2240e2180 1600e1420 2400e2240 1600e1420 2400e2240 2240e2180 1600e1420 2400e2240 1600e1420 4000e3500 3000e2700 2400e2240 2240e2180 1600e1420 1400e1000 1000e650 2400e2240 1600e1420 2400e2240 2240e2180 1600e1420 4000e3500 3000e2700 2400e2240 2240e2180 1600e1420 1400e1000 1000e650 2400e2240 1600e1420 2400e2240 2240e2180 1600e1420
O¼C]O C¼C O¼C]O CeO C¼C O¼C]O C¼C O¼C]O CeO C¼C O¼C]O C¼C OeH CeH O¼C]O CeO C¼C, C]O CeH CeO, OeH O¼C]O C¼C O¼C]O CeO C¼C OeH CeH O¼C]O CeO C¼C, C]O CeH CeO, OeH O¼C]O C¼C O¼C]O CeO C¼C
Carbon Dioxide Aromatic Compounds Carbon Dioxide Carbon Monoxide Aromatic Compounds Carbon Dioxide Aromatic Compounds Carbon Dioxide Carbon Monoxide Aromatic Compounds Carbon Dioxide Aromatic Compounds Water molecules Hydrocarbons Carbon Dioxide Carbon Monoxide Aromatic Compounds Methyl Group Alcohols and Phenols Carbon Dioxide Aromatic Compounds Carbon Dioxide Carbon Monoxide Aromatic Compounds Water molecules Hydrocarbons Carbon Dioxide Carbon Monoxide Aromatic Compounds Methyl Group Alcohols and Phenols Carbon Dioxide Aromatic Compounds Carbon Dioxide Carbon Monoxide Aromatic Compounds
1101 to 1200
2:1
975 to 1111 1112 to 1150
1112 to 1150 Torrefied Biomass
1:1
300 to 400
401 to 1100 Torrefied Biomass
1:1
1101 to 1200
Torrefied Biomass
2:1
300 to 400
401 to 1150 1151 to 1200
the sole hematite and sole graphite having insignificant TG degradation indicates that the TG degradation on the mixtures of hematite and graphite signifies iron oxide reduction. In Fig. 2a, the observed peaks at a temperature of 1153 C for the two mixing ratios reveal the potential occurrence of iron oxide reduction which can be validated by the FTIR and reduction extent results.
3.1.2. CO2 and CO FTIR spectra Chen, Dang [39] suggested that the transformation from one phase of iron oxides to another phase could happen simultaneously at a given range of temperature. This implies, in turn, the simultaneous shift from hematite to magnetite and from magnetite to wustite can occur at the same temperature. Similarly, it is possible for the simultaneous transformation of magnetite to wustite and wustite to metallic iron to occur at a given temperature range [39]. To proceed farther into an analysis of the reduction behavior, the FTIR spectra of CO and CO2 at the wavenumbers of 2210 cm-1 and 2384 cm-1 [40] are shown in Fig. 2b. The CO2 initially evolves at a temperature of 975 C which is earlier than CO. The work of Qi, Murakami [37] also found this behavior for iron ore-graphite composite. For the hematite-to-graphite ratios of 1:1 and 2:1, the onset temperatures of the evolved CO absorbance in the spectra are around 1100 C and 1111 C, respectively. This suggests that the reduction occurring from temperatures 975 Ce1100 C for the 1:1 ratio and from 975 C to 1111 C for the 2:1 ratio using C as a reductant follows the chemical reactions shown below instead of Eqs. (1)e(3): 6 Fe2O3 þ C / 4 Fe3O4 þ CO2
(8)
2 Fe3O4 þ C / 6 FeO þ CO2 2 FeO þ C / 2 Fe þ CO2
(9) (10)
It follows that, for the ratios of 1:1 and 2:1 with the temperature ranges of 1100e1200 C and 1111e1200 C, respectively, the chemical reactions consist of the direct and indirect reduction, as described in Eqs. (1)e(6) [15,37].
3.1.3. Reduction extent To account for the extent of iron ore reduction, the theoretical TGA curves is given below:
TGAtheoretical ¼ YH TGAH þ YG TGAG
(11)
where TGA and Y stand for the TGA curve and mass fraction, respectively, while the subscripts H and G denote hematite and graphite, respectively. It should be addressed that TGAH and TGAG represent the individual TGA curves obtained in Fig. 2a. The theoretical TGA curves and experimental TGA curves (TGAexperimental ) of the mixtures at the two ratios (1:1 and 2:1 ratios) are shown in Fig. 2c. The theoretical TGA curves show slight mass degradation at high temperatures; however, the experimental TGA curves decline obviously with increasing temperature. The gap between TGAtheoretical and TGAexperimental provide a measure of hematite reduction extent. Obviously, the higher the reaction temperature, the higher the reduction degree, stemming from the intensified gap.
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40
(a) 100
Hematite Biochar Hematite-Torrefied Biomass (1:1 ratio) Hematite-Torrefied Biomass (2:1 ratio)
35
50
25 20 1
0.8
0.6
0 0.4
0.2
15
DTG (wt% /min)
TGA (wt%)
o
370 C
DTG (wt% /min)
30
10
0
5
-0.2 750
800
850
900
950
1000
Temperature (oC)
1050
1100
1150
1200
o 830 C 1011-1138 C o
-50
0 200
400
600
800
1000
o
1200
Temperature ( C)
Hematite-Torrefied Biomass (2:1 ratio) CO2 Intensity
CO2 CO
CO Intensity
(b)
Hematite-Torrefied Biomass (1:1 ratio)
CO2 Intensity
CO Intensity
Torrefied Biomass Hematite 200
o
370 C
400
600
800
1000
Temperature(oC)
o
830 C
1200
o
1038 C
Hematite-Torrefied Biomass (2:1 ratio) 370oC
o
830 C
1011oC
Hematite-Torrefied Biomass (1:1 ratio) 200
400
600
800
1000
o
1200
Temperature( C)
(c) 100
TGAExperimental TGATheoretical
90
2:1 ratio
80
TGA(wt%)
70
1:1 ratio
60 50
100
100
30
90
85
TGA(wt%)
TGA(wt%)
90
40
2:1 ratio
1:1 ratio
95
80
3.1.5. Reduction mechanisms In light of the observations above, it can be summarized that two-stage reduction based on the temperature ranges is exhibited. For the composite with the ratio of 1:1, the first stage, ranging from 975 C to 1100 C, follows a direct reduction via Eqs. (8)e(10) as CO2 is the sole evolved gas observed. It was recently proposed that carbon is consumed in the process of direct reduction to generate the sole CO2 evolved gas [43]. Subsequently for the second stage at the temperature range of 1100e1200 C, a simultaneous direct reduction following Eqs. (1)e(3) and Eqs. (8)e(10) possibly transpire together with the occurrence of indirect reduction following Eqs. (4)e(6), as a consequence of both CO and CO2 observed (Fig. 2b). The concurrent reactions of direct reduction, Eqs. (1)e(3), and indirect reductions, Eqs. (4)e(6), and the char gasification, Eq. (7), can possibly occur [24]. The FTIR peaks for the CO evolved gas at the ratios of 1:1 and 2:1 are at 1153 C which is in line with the peaks in the DTG curves (Fig. 2a). Moreover, the high intensities of CO and CO2 in the spectra at 1153 C (Fig. 2b) can be explained by the simultaneous reactions from Eqs. (1)e(6) and Eqs. (8)e(10). Similarly, the composite with 2:1 ratio has almost the same results with the 1:1 ratio, except for the onset absorbance temperature of CO evolved gas observed at 1111 C instead of 1100 C for in the 1:1 ratio. At this temperature, the end of the first stage and the start of the second stage of the two-stage reduction of the composite with a 2:1 ratio is defined.
80
3.2. Torrefied biomass from forest residue
75 70
70
3.1.4. Three-dimensional FTIR spectra The three-dimensional (3D) FTIR spectra of the evolved gas from the heating of hematite, graphite, and the two mixtures are displayed in Fig. 3. The functional groups for the corresponding wavenumber ranges are given in Table 4. Fig. 3a and b depict a relatively flat profile in the heating processes of hematite and graphite, signifying no evolved gas produced. This is attributed to no mass degradation in the TGA and DTG curves in Fig. 2a. The elevated absorbance regions observed in Fig. 3 suggests the presence of CO, CO2, and some aromatic compounds (C]C at 16001420 cm-1) absorbed at a temperature range of 975e1200 C. The results reflect the enhanced gasification of carbon on these temperatures and reacting with various iron oxide phases to produce metallic iron and CO2. The well-known catalytic effect of iron on gasification has been established [41]. The peaks shown in Fig. 2a and b coincide with the absorbed gases of the spectra shown in Fig. 3. The 3D spectra also characterize the absorbed CO2 during the reduction processes occurring from 800 C to 1200 C. At a temperature of 1160 C, the absorption of evolved gases shown in Fig. 3c indicates that CO, CO2, and aromatic compounds (C]C at 1600-1420 cm-1) are present which have been confirmed by Amiri, Shanbedi [42]. The study of Qi, Murakami [37] quantified the presence of CO and CO2 for graphite using the chemical balance equations of oxides and carbon through the reduction and gasification processes. The overall results of the combined hematite and graphite suggest the occurrence of iron ore reduction at elevated temperatures of 950 C and beyond.
65
20 10
60
200
60
300
400
500
600
Temperature(oC)
400
700
800
600
300
400
500
600
o
700
800
Temperature( C)
800 o
1000
1200
Temperature( C) Fig. 4. (a) TGA and DTG curves and (b) CO and CO2 spectra of hematite, biochar, and their blends, and (c) the reduction extent.
3.2.1. TGA and DTG curves as well as CO2 and CO FTIR spectra The TGA and DTG curves of the torrefied biomass from forest residue are shown in Fig. 4a, and the result of hematite shown in Fig. 1a is used in Fig. 4a as a baseline for the comparison. The onset of the weight loss of the torrefied biomass shown in Fig. 4a develops at 250 C and is also observed in the varying ratios of the combined hematite and torrefied biomass mixtures. A peak at the DTG curve is observed at a temperature 370 C for the torrefied biomass and the mixtures at both the ratios of 1:1 and 2:1. The
A.T. Ubando et al. / Energy 180 (2019) 968e977
975
Fig. 5. Three-dimensional FTIR spectra of (a) torrefied biomass and hematite-torrefied biomass mixtures at the ratios of (c) 1:1 and (d) 2:1.
Fig. 4b, while the experimental and theoretical TGA curves of the mixtures at the hematite-to-torrefied biomass ratios of 1:1 and 2:1 are shown in Fig. 4c. It is of interest that two peaks at around 370 C and 830 C are observed in Fig. 4b. For the hematite-torrefied biomass mixture at the ratio of 1:1, Fig. 4c depicts the gap between TGAtheoretical and TGAexperimental starts to appear at around 600 C, and the gap increases with rising temperature. It should be addressed that even though the devolatilization of the torrefied biomass is drastic at 370 C, the gap is almost imperceptible therein. It follows that the hematite reduction is fairly slight. In contrast, the gap starts to develop at around 370 C for the mixture at the ratio of 2:1, implying that the onset of the mixture's reduction is triggered at a much lower temperature. In addition, at the end of the heating process (1200 C), the gap at the ratio of 2:1 is
onset weight loss of the samples observed in the TGA results and the peak identified from the DTG curve are in accordance to the results of Robles [30], which is mainly attributed to the devolatilization of the torrefied biomass. Compared to graphite (Fig. 2a), the reason causing the peaks triggered at lower temperatures is due to the higher volatile matter content (68.44 wt%) and lower fixed carbon content (25.65 wt%) in the torrefied biomass (Table 3) which € are in a good agreement with the results of El-Tawil, Okvist [31]. Specifically, the devolatilization process of the torrefied biomass is responsible for the peaks exhibited at 370 C in the DTG curves [44].
3.2.2. CO2 and CO FTIR spectra and reduction extent The FTIR spectra of the gases CO (2210 cm-1) and CO2 (2384 cm1 ) evolved from the heating of the four samples are shown in
CO2
CO2
CO2
CO2
Iron Ore
Iron Ore Extraction
Transportation
Composite Iron Ore-Biomass Iron Ore Sintering CO2
CO2
Torrefied Biomass
Biomass
Forest
Transportation
Iron
Torrefaction
Blast Furnace
Fig. 6. The process flow of ironmaking using torrefied biomass from forest residue with CO2 abatement.
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much greater than that at 1:1, suggesting the higher reduction extent of the former. Much of the degradation of the torrefied biomass at 370 C is attributed to the mass degradation and release of volatiles together with other aromatic compounds [45] based on the 3D FTIR results shown in Fig. 5. One of the most significant peaks was observed at the wavenumber range of 1600-1420 cm-1, representing C]C and C]O stretching vibration of aromatic compounds which is magnified by polar functional groups of alkenes [46]. The peaks at the wavenumber range of 1400-1000 cm-1 are accounted to the formation of the methyl group, while the peaks at the wavenumber range of 1000-350 cm-1 are responsible for the presence of alcohol and phenols corresponding to the stretching of CeO, and OeH [28]. The reduction occurring in low temperatures shown in Fig. 4c can mainly be governed by Eqs. (8)e(10) since the release of CO compared to CO2 is substantially lower, thus suggesting that the reduction is mainly owing to the carbon from the torrefied biomass. In addition, Hu, Yao [43] also proposed the possible direct reduction of iron ores using pyrolyzed biochar following Eqs. (8)e(10). For the torrefied biomass at 300 C, it is proposed that the reduction temperature of hematite to magnetite (Fe2O3 / Fe3O4) occurs from 365 to 555 C, magnetite to wustite (Fe3O4 / FeO) occurs from 595 to 799 C, and wustite to metallic iron (FeO / Fe) occurs from 799 to 1200 C [47]. Thus, the three peaks identified in Fig. 4b suggest the reduction of iron oxides from one phase to another with the corresponding temperatures. It was suggested by El-Tawil, Ahmed [47] that the reduction of iron oxides through pyrolyzed biochar occurred through a stepwise procedure (Fe2O3 / Fe3O4, Fe3O4 / FeO, and FeO / Fe) which was initiated due to the evolved gases from the thermal decomposition of volatiles observed at low temperatures of 370 C. The study of Robles [30] using torrefied biomass from forest residue suggested a stepwise reduction of Fe2O3 / Fe3O4 at 564 C, Fe3O4 / FeO at a range of temperature from 650 to 731 C, and FeO / Fe at a temperature range from 850 to 1200 C. The possible presence of oxygen in the aromatic compounds [46], alcohol, and phenols [28] in the torrefied biomass aided its devolatilization at this temperature range enabling the initial reduction to occur at this low temperature. 3.3. Comparison Comparing the thermogravimetric behavior using graphite and torrefied biomass as reducing agents, the important findings can be summarized as follows. Firstly, the reduction of iron oxides occurs at a higher temperature range (950e1200 C) when using graphite as the reductant, whereas it is triggered at a lower temperature range (300e1100 C) when using torrefied biomass. Consequently, the torrefied biomass from forest residue offers an alternative source of reductant for ironmaking, especially for the mixture with the hematite-to-torrefied biomass ratio of 2:1. Secondly, the FTIR spectra for the mixtures of hematite and graphite suggest that the reduction using carbon as a reductant occurs at 975e1111 C, following Eqs. (8)e(10). After that, the reduction follows Eqs. (1)e(6) until 1200 C. On the other hand, the FTIR spectra for the mixtures of hematite and torrefied biomass reveal that the iron oxide reduction is mainly governed by the reduction expressed in Eqs. (8)e(10) where the signal for CO is minimal compared to CO2. An increasing reduction behavior has been observed for the blends of hematite and torrefied biomass. This may be attributed to the volatiles liberated from the devolatilization of the torrefied biomass at temperatures higher than 350 C [44]. Thirdly, with the use of torrefied biomass from forest residues as the ironmaking reductant, carbon abatement is potentially realized. The CO2 emissions from the supply-chain of ironmaking such as the
iron ore extraction and preparation, the transportation of raw materials, the upgrading of biomass to torrefied biomass through torrefaction, and the iron manufacturing are absorbed by the forest and plants as shown in Fig. 6. Biomass is considered a renewable energy source and is known for its carbon neutral effect due to its capability to absorb CO2 during its growth phase [48]. Hence, the use of torrefied biomass from forest residues as a reductant for ironmaking offers a sustainable approach for the production of metallic iron which potentially offsets the CO2 flow [49]. In chemical-looping combustion, iron oxides are used as an oxygen carrier. Upgraded biomass can be used as a reducing agent for the oxygen carrier in the fuel-reactor for iron oxide reduction, thus, producing CO2 gas [50]. The rich CO2 gas can be then captured and stored to produce negative carbon flow. Torrefied biomass is considered as one of the future solutions in ironmaking plants to significantly reduce the carbon dioxide emissions while producing quality iron [51]. Lastly, it should be illustrated that the results of this study have provided a guide to the iron industry in the potential use of torrefied biomass from forest residues for ironmaking.
4. Conclusions Hematite reduced by graphite and torrefied biomass has been analyzed by a thermogravimetric analyzer accompanied by a Fourier transform infrared spectrometer. From the thermogravimetric and evolved gas analysis in association with introducing a measure of reduction extent, the reduction mechanisms of iron oxides have been recognized. The analysis shows that the reduction by graphite occurs at temperatures higher than 950 C, whereas the reduction by the torrefied biomass mainly develops at low-to medium-temperature with an onset reduction temperature of 300 C. The reduction process has been confirmed by the DTG peaks coinciding with the peaks of the absorbed gases in the FTIR where a step-wise reduction of iron oxides transpires, especially for torrefied biomass. Three peaks are identified for torrefied biomass which suggest the three step-wise reduction procedure of iron oxides (Fe2O3 / Fe3O4, Fe3O4 / FeO, and FeO / Fe) initiated from the evolved gases from the thermal decomposition of volatiles at the low temperature range of 300e400 C. The FTIR result indicates the possible direct reduction of the iron oxides using carbon from both graphite and torrefied biomass with an evolved carbon dioxide as opposed to the evolved CO as suggested by other studies. Comparing the reduction extent of the 2:1 and 1:1 ratios of hematite and torrefied biomass, 2:1 ratio has a lower onset temperature which possibly is attributed to the release of volatiles from the devolatilization of torrefied biomass. The utilization of the torrefied biomass as a reductant has the potential for carbonneutral flow enabling the sustainable metallic iron production. The results of the study provide an insight into the potential adoption of torrefied biomass from forest residues in chemicallooping combustion for carbon sequestration and clean energy production. Future studies may include kinetics analysis of using torrefied biomass for iron oxide reduction and the possible use of bench-type platform to accommodate larger amounts of sample leading to the potential analysis of the reduction residue.
Acknowledgments Acknowledgment is addressed to the Ministry of Science and Technology of Taiwan, R.O.C., for funding the research under the grant numbers MOST 106-2923-E-006-002-MY3 and MOST 1072811-E-006-529.
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