Carbon Deposition from Biotar by Fast Pyrolysis Using the Chemical

Nov 7, 2012 - This paper presents a technology for iron-making using biomass and a low grade iron ore by implementing chemical vapor infiltration (CVI...
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Carbon Deposition from Biotar by Fast Pyrolysis Using the Chemical Vapor Infiltration Process within Porous Low-Grade Iron Ore for IronMaking Alya N. Rozhan,†,‡ Rochim B. Cahyono,†,§ Naoto Yasuda,† Takahiro Nomura,† Sou Hosokai,∥ Hadi Purwanto,‡ and Tomohiro Akiyama*,† †

Center for Advanced Research of Energy Conversion Materials, Hokkaido University, North 13 West 8, Kita-ku, Sapporo, 060-8628, Japan ‡ Department of Manufacturing and Materials Engineering, Faculty of Engineering, International Islamic University, 50728 Kuala Lumpur, Malaysia § Department of Chemical Engineering, Gadjah Mada University, JI. Grafika 2, Bulaksumur, Yogyakarta 55281, Indonesia ∥ Energy Technology Research Institute, Advanced Industrial Science and Technology (AIST), AIST Tsukuba West 16-1, Onogawa, Tsukuba, Ibaraki, 305-8569, Japan ABSTRACT: This paper presents a technology for iron-making using biomass and a low grade iron ore by implementing chemical vapor infiltration (CVI) for the tar carbonization process. In this process, tar vapor from pyrolysis of biomass was infiltrated within a porous ore and carbon deposition occurred on the pore surface. For preparation, ore sample was heated in an electric furnace to decompose combined water in the sample, creating nanosized pores. In the experiments, the traditional slow pyrolysis was compared with fast pyrolysis to determine which condition is better for maximizing carbon deposition. Tar vapor from the pyrolysis process was introduced into the porous ore and trapped inside it, where tar decomposed and carbonized within the pores. The product of this process which is a carbon−magnetite composite with close arrangement of iron ore and carbon is useful for reduction of iron by carbon and is able to lower the temperature needed for reduction of iron to occur, as compared to that in steelworks. The purpose of this research is to compare the effects of slow pyrolysis and fast pyrolysis processes on the amount of carbon deposited within iron ore by the tar carbonization process and to observe the reduction reactivity of the carbon-deposited iron ore.

1. INTRODUCTION Steel has been produced for centuries, and many improvements in its production have been made to date. In our modern world, steel utilization is vital to ensure a more sustainable future. With over 1.3 billion tons of steel being produced each year, it is possible to expect a strong continuing growth in steelworks, particularly in developing countries where more than 60% of steel is used to make new infrastructure.1 Today, the major challenge in steelworks is depletion of high-grade resources and finding alternatives to these high-grade sources as the raw materials in steel work is indeed crucial.2 Due to depletion of high grade resources, it is necessary to use low grade iron ores extensively in the iron-making process. As compared to high grade iron ores, low grade iron ores contain more gangue minerals and combined water since the major component is goethite, FeO·OH.3 Because of that, when heated, the specific surface area of ore increases due to thermal decomposition which leaves the iron ore porous.4−7 However, this requires additional thermal energy in steel works which makes the utilization of low grade iron ore less energy efficient.8 Therefore, alternatives to utilize low grade iron ore efficiently are essential. In the iron-making process, a blast furnace is the most important reactor and is expected to remain being so for many years to come. In this industry, coal is widely used in a blast furnace as a fuel. It is a nonrenewable fuel, which is being © 2012 American Chemical Society

mined at coal mines. So, it is important to understand that the use of renewable resources is necessary as we know that the fossil reserve is limited at present.9 Biomass being an important renewable energy source has the highest potential toward sustainable development in the near future.10 Generally, biomass is utilized by thermochemical processing with the use of heat and catalysts to be transformed into fuels, chemicals, or even electric power. Recently, studies have been made to utilize agricultural waste11 and polymeric materials12 in the electric arc furnace steelmaking process. Both studies show the efficiency of these agricultural waste and polymeric materials to be used as a partial replacement of coke in electric arc furnace steelmaking. One of the promising thermochemical processes to utilize biomass is by the pyrolysis process. Pyrolysis produces useful fuel gases, char, and tar.13 Tar, which has been an unwanted constituent in this process, causes a problem since it clogs fuel lines, filters, and engines, thus reducing the utilization efficiency of biomass. Even so, decomposition of tar gives carbon deposition which is useful and attractive to be collected and used as a fuel source.14,15 This carbon is sometimes being referred to as pyrocarbon,16 carbon-rich dust, or soot.17 Received: August 28, 2012 Revised: November 7, 2012 Published: November 7, 2012 7340

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Table 1. Composition of M-Ore

a

composition [mass %]

total Fe

Fe2+

Fe3+

Si

Al

C

Mn

CWa

M-ore

49.73

0.12

49.61

6.05

2.19

0.14

2.42

5.90

Combined water. pyrolysis, it is reported that fast pyrolysis finishes at 500 °C in 10 s.18 From this fact, the temperature at all thermocouples was set to 500 °C before the experiment. The figure of the apparatus is reported somewhere else.19 Biomass was fed into the preheated reactor continuously for 60 min at a feeding rate of 0.1 g/min. The reactor was held at experimental temperature for another 60 min to allow complete tar vapor infiltration within ore. Pyrolysis and coking processes were both done at this experimental temperature. Experiments were repeated for all biomass samples. For the second part, slow pyrolysis was done to get tar vapor for carbonization process. Figure 1 shows the schematic diagram of the

In this study, the tar carbonization process was done by utilizing tar vapor from different types of biomass and low grade iron ore to produce carbon-containing iron ore. Low grade iron ore was heated to remove combined water and to create nanosized pores within it. Porous iron ore acted as a “tar filter” for tar decomposition to occur. Carbon was deposited within the ore by the chemical vapor infiltration process, where tar vapor from pyrolysis process infiltrated into the pores and decomposed. The product of this process which is a carbon− magnetite composite with nanoscale arrangement of iron ore and carbon is useful for reduction of iron by carbon at a lower temperature than that used in steelwork. The purpose of this research is to compare the effects of slow pyrolysis and fast pyrolysis processes on the amount of carbon deposited within iron ore by the tar carbonization process and to observe the reduction reactivity of the carbon-deposited iron ore.

2. MATERIALS AND METHOD 2.1. Materials Preparation. Table 1 shows the composition of a Malaysian low-grade iron ore used in this study, namely M-ore. The amount of combined water content was observed by thermogravimetry (TG) analysis, which indicated that M-ore contains 5.90 mass % combined water. M-ore was sieved with a particle size ranging from 1− 2 cm as a preparation. Before each experiment, 5.0g of M-ore was heated in an electric furnace in air atmosphere to 500 °C at a heating rate of 3 °C/min and held there for 10 h. The heating conditions were determined in preliminary experiments by observing the conditions at which a high porosity can be obtained. It is reported that this heat treatment makes ore porous because combined water were removed producing many nanopores.8 To confirm this effect, the pore structure of M-ore was measured by N2 adsorption equipment before and after heating. Specific surface area and pore size distribution were analyzed by the Brunauer−Emmet−Teller (BET) method and Barrett−Joyner− Halenda (BJH) method, respectively. Malaysian agricultural residues from the palm industry, palm kernel shell (PKS), palm f iber (PF), and empty f ruit bunch (EFB), were used as the biomass in this experiment. The elemental composition of the biomass is presented in Table 2, and biomass was sieved with a particle size ranging from 350 to 510 μm.

Figure 1. Schematic diagram of the experimental apparatus for the slow pyrolysis of biomass and tar deposition. Biomass was put inside SUS mesh in the reactor with nitrogen atmosphere. Temperatures of the pyrolyzer and coker were increased from room temperature to 500 °C at a heating rate of 30 °C/min. The generated tar vapor and reducing gas was introduced into the bed of porous M-ore sample: (TC) thermocouple; (GC) gas chromatograph; (MFC) mass flow controller.

experimental apparatus. Biomass was put inside the stainless steel mesh in the reactor in a nitrogen atmosphere. Considering the fact that slow pyrolysis is normally done at temperatures between 300 and 400 °C,11 it is possible to use the same temperature with the fast pyrolysis process, which was 500 °C. In the experiments, the temperature of the reactor was increased from room temperature to 500 °C at a heating rate of 30 °C/min for pyrolysis to occur. The reactor was held at this temperature to complete 120 min of experimental time. Pyrolysis and coking processes were both done at this experimental temperature. Experiments were repeated for all biomass samples. In both cases, the pyrolysis processes produced tar, char, and fuel gases. Char was collected in stainless steel mesh. Tar and fuel gases were introduced into the packed bed of dehydrated ore placed in the coker. The weight of the packed beds of the dehydrated ore was approximately 3.0 g. In the coker, tar vapor infiltrated and carbonized within the pores inside ore. The residual reducing gas and tar vapor were collected by a cold trap filled with glass beads of 1 mm in diameter and cooled in an ethanol bath at −75 °C. This temperature is low enough to collect tar but not CO2. After this carbonization process, the resultant carbonized ore was analyzed by N2 adsorption equipment to get the pore structure. The state of resultant carbonized ores was analyzed by X-ray diffraction,

Table 2. Composition of Biomass Used in This Study biomass (mass %)

C

H

N

O

palm kernel shell, PKS palm fiber, PF empty fruit bunch, EFB

49.48 45.84 45.64

5.69 6.18 6.19

0.47 0.96 0.35

44.36 47.02 47.82

2.2. Infiltration and Carbonization of Biotar on Dehydrated ore. Experiments were done to compare the effects of slow pyrolysis and fast pyrolysis on the tar carbonization process. A general description on both processes is presented in Table 3. In the case of carbonization experiments using tar vapor produced from fast

Table 3. General Description on Slow and Fast Pyrolysis22 pyrolysis

heating rate (°C/s)

main product

slow fast

0.1−2 >2

char tar 7341

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XRD. The carbon infiltrated within ore was examined using elemental analysis by combustion. 2.3. Iron Reduction for Iron Making. After the tar carbonization process, the carbonized ores were further analyzed by using thermogravimetry analysis to observe the reduction reactivity. They were heated in an argon atmosphere to 1000 °C with a heating rate of 50 °C/min, and the argon flow rate was set to 500 N mL/min. The resultant samples were analyzed by XRD, and the carbon content remaining was examined.

3. RESULTS AND DISCUSSION 3.1. Tar Carbonization Experiments by Slow Pyrolysis and Fast Pyrolysis. Figure 2 shows the changes in the specific Figure 3. Carbon content and BET surface area of M-ore after tarcarbonization experiments with slow pyrolysis and fast pyrolysis of palm kernel shell, PKS; palm fiber, PF; and empty fruit bunch, EFB. Carbon content was examined using elemental analysis by combustion.

In the slow pyrolysis process, biomass was heated slowly from room temperature to the desired temperature. The main product of slow pyrolysis is char, which results in low tar vapor production.20,21 In the tar carbonization process within iron ore, besides high porosity of dehydrated ore, it is important to maximize the amount of tar vapor produced from the pyrolysis process in order to get more carbon deposit within the ore. Fast pyrolysis which occurs in less than 1 s at about 500 °C promises more tar vapor since the char produced is very little, resulting a larger amount of tar vapor.22,23 In addition to that, tar produced from fast pyrolysis contains a higher amount of carbon than that of slow pyrolysis.24 Therefore, in carbonization experiments by fast pyrolysis, more carbon was deposited within the ore after the decomposition of the tar. The changes in pore size distribution of carbonized PKS by fast pyrolysis and slow pyrolysis were plotted and presented in Figure 4. A large amount of nanosized pores were created after dehydration, and the pores of pore size less than 4 nm were consumed after carbonization experiments. Figure 4a shows the pore size distribution for experiments using slow pyrolysis, which indicates small porosity consumption after carbonization. As for experiments using fast pyrolysis, Figure 4b beautifully shows the consumption of nanosized pores after carbonization. During tar carbonization, the chemical vapor infiltration process occurred; tar vapor from the pyrolysis of biomass infiltrated inside the pores, adsorbed onto the pore surface, and decomposed into carbon and gases. The carbon deposited on the pore surface while the gases diffused out of the pores. Figure 5 shows the XRD patterns throughout experiments for all conditions as stated above. The original sample contained mainly goethite, FeO·OH, and hematite, Fe2O3. After dehydration at 500 °C, combined water was removed and the structure found was Fe2O3. After carbonization experiments, magnetite Fe3O4 peaks appeared in all the carbonized ores. During the experiments, CO and H2 generated from the pyrolysis process became the iron reduction agent for indirect reduction of Fe2O3 into Fe3O4.8 From these data, it can be concluded in tar carbonization process within iron ore, besides a high porosity of iron ore8 and a large amount of tar vapor19 produced from pyrolysis process, a high amount of carbon content in the tar is also important to get a large amount of carbon deposit. 3.2. Reduction of Iron Ore. Iron reduction reactivity was observed by thermogravimetry analysis. The weight change curves of carbonized ores from experiments using slow

Figure 2. Changes in BET specific surface area and total pore volume for sample M-ore before heat treatment, after heating at 500 °C for 10 h, and after tar-carbonization experiments with slow pyrolysis and fast pyrolysis of palm kernel shell, PKS.

surface area and total pore volume for sample M-ore before heating, after heating at 500 °C, and after tar-carbonization experiments with slow pyrolysis and fast pyrolysis of PKS. After heating, combined water was removed, leaving nanosized pores behind. This justifies the dramatic increase in the specific surface area of M-ore from 11.8 before the heat treatment to 31.3 m2/g after heating. After carbonization experiments, specific surface areas of all carbonized ores dropped because of carbon deposition within the ores by chemical vapor infiltration process. As shown in the figure, the changes in surface area affected the changes in total pore volume of the ore as well. By comparing the slow and fast pyrolysis processes for this tar carbonization process, it was clearly observed that the specific surface areas of carbonized ores from experiments using slow pyrolysis were higher than those of experiments using fast pyrolysis. Specific surface areas for carbonized ores by experiments using slow pyrolysis of PKS, PF, and EFB were 17.9, 17.6, and 17.1 m2/g, respectively. For experiments using fast pyrolysis of PKS, PF, and EFB, the specific surface areas of the carbonized ores were 11.5, 10.0, and 10.4 m2/g, respectively. This is because experiments using slow pyrolysis gave much lower carbon deposition as compared to carbon deposit in carbonized ore by experiments using fast pyrolysis, which can be seen in Figure 3. Carbon contents of carbonized ores were 1.42, 1.73, and 2.09 mass percent after tar carbonization using slow pyrolysis of PKS, slow pyrolysis of PF, and slow pyrolysis of EFB, respectively. For carbonized ores after tar carbonization using fast pyrolysis of PKS, fast pyrolysis of PF, and fast pyrolysis of EFB, the carbon contents were higher: 4.33, 3.52, and 3.92 mass %, respectively. 7342

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pyrolysis and fast pyrolysis of EFB are shown in Figure 6. For carbonized ore by slow pyrolysis, decomposition occurring from 220 to 720 °C was related to re-evaporation of tar components, and decomposition after 720 °C was due to reduction of iron which finished at 950 °C. For carbonized ore by fast pyrolysis, re-evaporation of tar components occurred from 220 to 780 °C, and reduction of iron started at 780 °C and finished at 960 °C. While cooling down the reactor after carbonization experiment, a small amount of tar was entrapped within the ores without having enough time to decompose; which explains re-evaporation of the tar components at the beginning of the heat-up process of carbonized ores. From the data, it can be seen that decompositions of carbonized ores completed before 1000 °C. This temperature is lower than that of blast furnace operation temperature which requires more than 1000 °C for iron reduction to occur. This analysis proved that for carbonized ore, Fe3O4 with carbon being inserted within the pores has a nanoscale contact between iron ore and carbon, thus higher reactivity and lower starting temperature of the reaction.25,26 Considering this idea, it was predicted that carbonized ore with larger amount of carbon deposit gives higher reduction degree because of larger surface contact between carbon and iron ore. In reference to Table 1, the total iron content in original More was 49.73 mass %. From this data, the amount of carbon needed to completely reduce all iron species in carbonized More to metal iron, Fe, was calculated using equations below. By considering the furthest route of iron reduction, Fe2O3− Fe3O4−FeO−Fe, an assumption made was that all iron species in carbonized M-ore was hematite, Fe2O3.

Figure 4. Changes in pore size distribution of M-ore before heat treatment, after heating at 500 °C for 10 h, and after tar carbonization. (a) S-PKS is carbonized M-ore by slow pyrolysis of palm kernel shell, PKS. (b) F-PKS is carbonized M-ore by fast pyrolysis of palm kernel shell, PKS.

Figure 5. XRD patterns of M-ore before heat treatment, after heating at 500 °C for 10 h, and after tar-carbonization with slow pyrolysis and fast pyrolysis of PKS, PF, and EFB. S-PKS, S-PF, and S-EFB are carbonized M-ore by slow pyrolysis of PKS, PF, and EFB, respectively. F-PKS, F-PF, and F-EFB are carbonized M-ore by fast pyrolysis of PKS, PF, and EFB, respectively. 7343

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Figure 7. XRD patterns of carbonized ores after heated to 1000 °C. Heating rate: 50 °C/min. Atmosphere: Ar flow of 500 N mL/min. SPKS, S-PF, and S-EFB are carbonized M-ore by slow pyrolysis of PKS, PF, and EFB, respectively. F-PKS, F-PF, and F-EFB are carbonized More by fast pyrolysis of PKS, PF, and EFB, respectively.

Figure 6. Temperature dependence of percentage mass for carbonized ores. S-EFB and F-EFB are carbonized M-ore by slow pyrolysis and fast pyrolysis of EFB, respectively. Mass loss occurrences for both carbonized ores from 220 °C until the starting points of iron reduction was because of re-evaporation of tar components. The gray region indicates the reduction of iron. Decomposition of ores completed before 1000 °C.

Minimum C needed: Fe2O3 + 3/2C → Fe + 3/2CO2

(1)

Maximum C needed: Fe2O3 + 3C → 2Fe + 3CO

(2)

The minimum and maximum C needed to directly reduce iron is 5.48 and 10.38 mass %, respectively. From the data presented in Figure 3, the amounts of carbon deposit in M-ore from tar carbonization using fast pyrolysis was closer to the minimum C needed to reduce all iron species in the ore to metal iron. Therefore, carbonized ores by fast pyrolysis was expected to be reduced further than carbonized ores by slow pyrolysis. Figure 7 shows the XRD peaks of the reduced iron ores from all experiments. For reduced iron ores from experiments with slow pyrolysis, the main structure was FeO. As for reduced iron ores from experiments with fast pyrolysis, the peaks appeared were FeO and interestingly metal iron, Fe. The weight changes of carbon in the carbonized ores during heating were observed by plotting the carbon content against temperature. Figure 8 shows the changes in ore weight and carbon content with temperature; for carbonized ore by slow pyrolysis and fast pyrolysis of EFB, respectively. The plots denote that the iron reduction was initiated by direct reduction with carbon. By referring to Figure 8, sample ores were analyzed by XRD as a function of temperature. Figure 9 shows XRD peaks for carbonized ore by slow pyrolysis of EFB being heated in an argon atmosphere. As temperature increases, the changes can be observed clearly, with the as-carbonized sample having mainly Fe3O4 peaks. The same peaks appeared for the next three points which were 400, 700, and 800 °C. After 900 °C, FeO peaks appeared together with Fe3O4 peaks. Finally, only

Figure 8. Percentage mass and carbon content of the carbonized ore by (top) slow or (bottom) fast pyrolysis of EFB, during the heating process: (A) as-carbonized, (B) 400, (C) 700, (D) 800, (E) 900, and (F) 1000 °C. Carbon content for each point was determined using elemental analysis by combustion.

FeO peaks appeared at 1000 °C. Figure 9b shows the peaks for the carbonized ore by fast pyrolysis of EFB being heated to 1000 °C. From the as-carbonized ore up to 800 °C, the main structure was Fe3O4. After that, FeO and Fe peaks appeared at 900 °C. This continued as temperature increased to 1000 °C.

4. CONCLUSION Slow pyrolysis and fast pyrolysis were compared to observe the effect of both processes on the tar carbonization process. The 7344

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larger amount of tar containing a larger amount of carbon, which allows more tar carbonization to occur. 2. For carbonized ores from experiments using fast pyrolysis process, after heating to 900 °C, samples were reduced to FeO and interestingly Fe. As for carbonized ores from experiments using slow pyrolysis, after 900 °C, samples were direct reduced by carbon to only FeO. This was due to the carbonized ores from slow pyrolysis having smaller amount of carbon deposit than that of fast pyrolysis. Overall, this research suggests the usability of low grade iron ore by the tar carbonization process, in which carbon from biotar was able to be infiltrated and deposited within the porous ore. Carbon deposits within the ore makes this product able to be reduced to FeO and/or Fe at a lower temperature than that used in steelworks.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81 11 706 6842. Fax: +81 11 726 0731. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Sumitomo Metal Industries, Ltd. for partial support in this research. REFERENCES

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Figure 9. XRD patterns of carbonized ore by (top) slow or (bottom) fast pyrolysis of EFB, during the heating process. Heating rate: 50 °C/ min. Atmosphere: Ar flow of 500 N mL/min. (A) As-carbonized, (B) 400, (C) 700, (D) 800, (E) 900, and (F) 1000 °C.

heating rates were 30 °C/min and more than 500 °C/s for the slow pyrolysis process and fast pyrolysis, respectively. Tar vapor produced from pyrolysis process infiltrated into the porous ore, decomposed, and carbonized within the ore. Carbonized ore was analyzed by TG analysis to observe the reduction reactivity. The results obtained are as follows: 1. Experiments done by fast pyrolysis process gave a higher carbon content than those of the slow pyrolysis process. Carbon content values for carbonized ores from fast pyrolysis of palm kernel shell, palm fiber, and empty fruit bunch were 4.33, 3.52, 3.92 mass %. In contrast, carbon contents for carbonized ores from slow pyrolysis of palm kernel shell, palm fiber, and empty fruit bunch were 1.42, 1.73, and 2.09 mass %, respectively. This is because slow pyrolysis mainly produces char, which gives a low tar production, resulting in a low carbon deposition on the pore surface within the slag. Fast pyrolysis produces a 7345

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