Carbon Deposition of Biotar from Pine Sawdust by Chemical Vapor

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Carbon Deposition of Biotar from Pine Sawdust by Chemical Vapor Infiltration on Steelmaking Slag as a Supplementary Fuel in Steelworks Alya N. Rozhan,†,‡ Rochim B. Cahyono,†,§ Naoto Yasuda,† Takahiro Nomura,† Sou Hosokai,† 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, Kulliyyah of Engineering, International Islamic University, 50728 Kuala Lumpur, Malaysia § Department of Chemical Engineering, Gadjah Mada University, JI. Grafika 2, Bulaksumur, Yogyajarta 55281, Indonesia ABSTRACT: Steelmaking slag is an attractive material because it generally contains iron oxide and free lime, which makes it able to be regarded as flux-containing low-grade iron ore for steel production. In this study, tar vapor from the pyrolysis of pine sawdust was infiltrated within porous slag and carbon deposition occurred on the pore surface by chemical vapor infiltration. For preparation, the slag sample was dehydrated in an electric furnace to decompose hydrates in the sample, creating pores. Pine sawdust was charged from bowl feeder with flowing nitrogen gas into the reactor, where it was pyrolyzed at 500 °C, rapidly producing fuel gases, tar vapor, and char. Tar vapor was introduced into the dehydrated slag and trapped inside it, where tar decomposed and carbonized within the pores. The product distribution was analyzed after the experiment. Experiments were repeated by changing the temperature and time to obtain an optimum condition for this process. The purpose of this research is to examine the amount of carbon deposited within the steelmaking slag by this tar-carbonization process. The product of this process, which is carbon-containing slag, is useful for energy reduction in steelworks.

1. INTRODUCTION In our modern world, steel is important and the use of steel is vital to ensure a more sustainable future. Every year, over 1.3 billion tons of steel are manufactured and used worldwide. With this huge volume of steel production, it is possible to expect a strong continuing growth in steelworks, particularly in developing countries where more than 60% of steel will be used for making new infrastructure.1 The major challenge in steelworks is depletion of high-grade resources.2 When the amounts of these resources, which are mostly nonrenewable resources, are reduced, it is presumable that the price becomes rocketing high. Therefore, finding alternatives to these highgrade sources as the raw materials in steelwork is indeed crucial. Slag as the major byproduct in steelmaking is continuously and abundantly produced in this industry. It contains primarily silica and alumina from the original iron ore, combined with calcium oxide from the flux.3 Because slag has a high content of free lime and iron oxide,4 it is able to be regarded as a fluxcontaining low-grade iron ore. Using slag as one of the charging materials in steel production, the amount of iron ore and fluxing materials used can be reduced, making use of slag energy and cost efficient. Another important charging material for the steel-making process is coke, which is generally produced from coal. However, because of the increasing world coal consumption as a result of steelworks,5 a renewable energy source, which is biomass, is attractive to be used as an alternative.6−8 A study9 was performed to investigate the properties of biomass as a supplementary fuel in sintering machine. It shows a significant © 2012 American Chemical Society

reduction of CO2 emission. Therefore, biomass is a good candidate to be used as a supplementary fuel in sintering machine. At present, the most popular method to use biomass is by the pyrolysis process.6−8 Pyrolysis produces useful fuel gases, char and tar. Tar has been a problem in this process because it clogs fuel lines, filters, and engines, thus reducing the use efficiency of biomass. Even so, tar contains a mixture of naturally occurring compounds, including carbon, which makes it attractive to be collected and used as a fuel source.10 Tar vapor, which reacts under inert conditions, produces fuel gases containing carbon,11 which are sometimes referred to as pyrocarbon,12 carbon-rich dust, or soot.13 In this study, the tar-carbonization process was performed using slag and tar from pine sawdust to produce carboncontaining slag to be used in steelworks. Slag was dehydrated to remove hydrates and to create pores within it. Tar vapor was infiltrated into the pores, and carbon was deposited within the slag by the chemical vapor infiltration process. The product of this process, which is carbon-containing slag, is useful in steelwork because iron oxide in slag can be directly reduced to iron and also the carbonized tar within the slag can be a supplementary fuel in the sintering machine to reduce the amount of coal usage. The purpose of this research is to Received: March 22, 2012 Revised: May 21, 2012 Published: May 24, 2012 3196

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Table 1. Composition of the Slag Sample slag sample (mass %)

total Fe

Fe

FeO

CaO

SiO2

Al2O3

MgO

C

KR slag

21.03

14.05

5.38

40.02

8.42

2.75

2.45

2.67

examine the amount of carbon deposited within the steelmaking slag by the tar-carbonization process.

2. MATERIALS AND METHODS 2.1. Dehydration of Steelmaking Slag. Table 1 shows the composition of the steelmaking slag sample used in this study, namely, KR slag. KR slag was collected after desulfurizing treatment of pig iron through hot metal stirring, before it is charged into an oxygen converter for further refining. This explains the high amount of carbon

Figure 2. Schematic diagram of the experimental apparatus for the pyrolysis of pine sawdust and tar deposition. Pine sawdust was fed from the bowl feeder into the pyrolyzer at 0.07 g/min together with flowing nitrogen. Char was collected in SUS mesh, and the generated tar vapor and reducing gas were introduced into the bed of dehydrated slag. T.C. = thermocouple, and GC = gas chromatograph. within the pores inside slag. 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. The temperature was sufficiently low for collecting tar but not CO2. After this carbonization process, the resultant carbonized slag was analyzed by N2 adsorption equipment to obtain the pore structure, such as BET surface area, average pore size, and pore volume. The state of resultant carbonized slag was also analyzed by X-ray diffraction (XRD). The carbon infiltrated within slag was examined using elemental analysis by combustion. The product distribution for each experiment was calculated.

Figure 1. Thermal analysis of KR slag was performed after heating the sample at 105 °C for more than 24 h. Decomposition between 350 and 450 °C was because of hydrate removal. content in the original sample, 2.67 mass %. Figure 1 shows the amount of hydrate content observed by thermogravimetry (TG) analysis, which indicated that KR slag contains a large amount of hydrates, 12.0 mass %, originated from Ca(OH)2. KR slag was sieved with a particle size ranging from 300 to 600 μm as a preparation. KR slag was dehydrated by being heated in an electric furnace to 450 °C at a heating rate of 1 °C/min and held there for 3 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 slag porous because hydrates are removed producing many nanopores.14 To confirm this effect, the pore structure of slag was measured by N2 adsorption equipment before and after dehydration. Specific surface area and pore size distribution were analyzed by the Brunauer−Emmett−Teller (BET) method and Barrett−Joyner−Halenda (BJH) method, respectively. 2.2. Infiltration and Carbonization of Biotar on Dehydrated Slag. Japanese pine sawdust was used as the biomass in this experiment. The elemental composition of the pine sawdust was C, 49.83 mass %; H, 6.18 mass %; and O, 43.99 mass %, by difference, and biomass was sieved with a particle size ranging from 210 to 350 μm. Figure 2 shows the schematic diagram of the experimental apparatus. The pine sawdust was fed into the reactor continuously at a predetermined feeding rate. In the case of fast pyrolysis, it is reported that it finishes at 500 °C and in 10 s.15 From this fact, the feeding rate was determined to be at 0.07 g/min and the temperature at all thermocouples was set to 500 °C. Pyrolysis and coking processes were both performed at this experimental temperature. Experiments were repeated by changing the experimental temperatures to 600 and 700 °C. The pyrolysis of fed biomass produced tar, char, and fuel gases. Char was collected in SUS mesh. Tar and fuel gases were introduced into the packed bed of dehydrated slag samples placed in the coker. The weight of the packed beds of the dehydrated slag was approximately 3.0 g. In the coker, tar vapor infiltrated and carbonized

3. RESULTS AND DISCUSSION 3.1. Effect of the Temperature on Carbon Deposition for KR Slag. Figure 3a shows the changes in the specific surface area of KR slag. After dehydration, the specific surface area increased significantly because hydrates were removed and formed nanopores. After carbonization, the specific surface area decreased significantly because of carbon infiltration within the pores. A small difference of the specific surface area after carbonization at different temperatures was observed. At 500 °C, the specific surface area was 7.6 m2/g, followed by 4.8 and 4.7 m2/g at 600 and 700 °C, respectively. The changes in pore size distribution were plotted and presented in Figure 3b. A large amount of nanopores was created after dehydration, and these nanopores were consumed after carbonization experiments. Figure 4 shows the XRD patterns throughout experiments at all three temperatures. The original sample contained mainly Ca(OH)2, which decomposed to CaO by reaction Ca(OH)2 → CaO + H2O after heating at 450 °C. After this dehydration process, the structures found were CaO and magnetite, Fe3O4. Magnetite was originated from the iron oxide content in the slag. After carbonization, dicalcium ferrite peaks, 2CaO·Fe2O3, appeared in the slag sample. This compound is commonly found in steelmaking slags16,17 because of the reaction of lime 3197

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Figure 5. Carbon content of KR slag before dehydration, after dehydration at 450 °C, and after tar-carbonization experiments at 500, 600, and 700 °C. H.C. = hydrocarbons.

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. tar → C + H 2 + CO + CO2 + CH4+other hydrocarbons (1)

Experiment performed at 500 °C gave the largest carbon deposition, 10.59 mass %, followed by experiments performed at 600 and 700 °C, with 9.48 and 6.43 mass %, respectively. For pyrolysis at higher temperatures, thermal cracking, which drives the product into the gas phase, is more likely to happen, decreasing tar formation.18 A decrease in tar formation gave lower tar decomposition during the chemical vapor infiltration process, reducing the amount of carbon deposit. In addition to that, at higher temperatures, the porosity of slag decreases, lowering the capability of the slag to capture tar effectively. Figure 6 shows the changes in the specific surface area with and without biomass charge.

Figure 3. (a) Changes in the BET specific surface area for sample KR slag before dehydration, after dehydration at 450 °C, and after tarcarbonization experiments at 500, 600, and 700 °C. (b) Changes in pore size distribution of samples before dehydration, after dehydration at 450 °C, and after tar-carbonization experiments at 500, 600, and 700 °C.

Figure 4. XRD patterns of KR slag before dehydration, after dehydration at 450 °C for 3 h, and after tar-carbonization experiments at 500, 600, and 700 °C.

Figure 6. Changes in the BET specific surface area for sample KR slag after experiments with and without biomass charge, which was just heated inside the reactor, having no carbon deposit. Experiments were performed at 500, 600, and 700 °C.

as a flux with iron oxides. The peaks of tar-carbonized slag were the same for 500, 600, and 700 °C. Figure 5 shows the carbon content of KR slag before dehydration, after dehydration, and after carbonization. Before dehydration, KR slag contained a small amount of carbon originated from processes in the industry. After the dehydration process, the carbon liberated as CO and/or CO2. The amount of carbon increased after the tar-carbonization process because of carbon deposition within the slag. During the process, the chemical vapor infiltration process occurred; tar vapor from the

Figure 7 shows the comparison between product distributions of experiments without slag, which were only pyrolysis experiments, and experiments with KR slag. This comparison was performed to investigate the ability of the slag to trap carbon from pine pyrolysis products. From the figure, it can be clearly seen that KR slag was able to trap carbon from volatile matters, which were tar vapor and gases. Experiment at 500 °C produced 23.98 mol of C/100 mol of C yield of coke, followed 3198

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The maximum and minimum carbon needed to directly reduce iron is 4.79 and 2.45 mass %, respectively. From the experiments, the amounts of carbon deposit in KR slag were more than the maximum carbon needed for iron reduction. Therefore, it is possible for the tar-carbonized KR slag to be directly reduced to iron. 3.4. Potential Coke Breeze Reduction in the Sintering Process. In the general process, steelmaking slag is often used in sintering machine to produce sinter, where coke breeze is widely used as the fuel. It is reported in a study19 that, in sinter mixtures, coke breeze used is 36 kg/ton of sinter and steelmaking slag used is 28 kg/ton of sinter. These values were used to calculate the potential coke breeze reduction in sintering machine if tar-carbonized slag is used in the process. Heating values were calculated using the Dulong equation by assuming that the heating value of carbonized tar inside slag is equal to that of char produced. The composition of char produced was C, 84.14 mass %; H, 3.30 mass %; and O, 12.56 mass %, by difference. The resulting heating value was 30.954 MJ/kg. Calculations were made based on these values and the experimental data from Figures 5 and 8. Slag, which was tar-carbonized at 500 °C for 80 min, gave the largest coke breeze reduction, 12.1%. For this reduction, the ratio of slag/biomass is 0.5. The rest of the data is reported in Figure 9.

Figure 7. Comparison of product distribution after experiments performed without and with slag. Volatile matters consist of gas and tar.

by 21.54 and 15.55 mol of C/100 mol of C yields for 600 and 700 °C, respectively. From these data, it can be concluded that 500 °C is a good temperature for the tar-carbonization process of KR slag. 3.2. Effect of the Time on Carbon Deposition within KR Slag. Experiments were repeated at 500 °C by increasing the experimental time. Results were observed for experiments performed for 40, 60, and 80 min. Figure 8 shows the specific

Figure 8. Changes in the BET specific surface area and carbon content for KR slag after experiments performed at 500 °C for 40, 60, and 80 min. Figure 9. Approximate reduction of coke breeze consumption in a sintering plant with the consumption of tar-carbonized slag, which was evaluated on the basis of experimental data in Figures 5 and 8 under the assumption that 28 kg/ton of slag is recycled into the sintering plant and 36 kg of coke breeze is consumed.

surface area and carbon content for tar-carbonized slag after experiments. The specific surface areas for experiments performed for 40, 60, and 80 min were 7.6, 5.5, and 4.7 m2/ g, and the carbon deposit was 10.59, 11.55, and 13.19 mass % C, respectively. More carbon deposit on the pore surface gave a decrease in the specific surface area of the slag. The carbon deposit increased with experimental time because of the increasing amount of biomass charged. 3.3. Carbon Needed for Iron Reduction in KR Slag. In reference to the data above, the total iron content in original KR slag was 21.03 mass %. From this value, the amount of carbon needed for iron reduction in KR slag was calculated using the equations below, assuming that all iron content in the slag was hematite, Fe2O3. maximum C needed:

Fe2O3 + 3C → 2Fe + 3CO

minimum C needed:

Fe2O3 + 3/2C → Fe + 3/2CO2

4. CONCLUSION Dehydrated KR slag was heated in the reactor at temperatures of 500, 600, and 700 °C, with pine biomass being charged into it at a rate of 0.07 g/min. Pyrolysis gas from pine biomass produced biotar, which was then decomposed and carbonized within the porous slag. Finally, the carbon deposit was analyzed. The results obtained are as follows: (1) The results for the experiment performed at 500 °C gave the highest amount of carbon content, followed by 600 and 700 °C. This is because, at higher temperatures, the production of tar decreases, resulting in a low carbon deposition on the pore surface within the slag. (2) At 500 °C, most carbon was found deposited within KR slag in the experiment for 80 min compared to experiments for

(2) (3) 3199

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(10) Mermelstein, J.; Millan, M.; Brandon, N. P. The interaction of biomass gasification syngas components with tar in a solid oxide fuel cell and operational conditions to mitigate carbon deposition on nickel-gadolinium doped ceria anodes. J. Power Sources 2011, 196 (11), 5027−5034. (11) Vreugdenhill, B. J.; Zwart, R. W. R. Tar Formation in Pyrolysis and Gasification. Energy Research Centre of the Netherlands: Petten, The Netherlands, 2009; ECN-E--08-087. (12) Tesner, P. A.; Rafal’kes, I. S.; Zhedeneva, O. B. Influence of hydrogen on the kinetics of the formation of pyrocarbon in the thermal decomposition of aromatic hydrocarbons. Solid Fuel Chem. 1984, 18 (4), 110−113. (13) Jess, A. Mechanisms and kinetics of thermal reactions of aromatic hydrocarbons from pyrolysis of solid fuels. Fuel 1996, 75 (12), 1441−1448. (14) Hata, Y.; Purwanto, H.; Hosokai, S.; Hayashi, J.; Kashiwaya, Y.; Akiyama, T. Biotar ironmaking using wooden biomass and nanoporous iron ore. Energy Fuels 2009, 23, 1128−1131. (15) Okuno, T.; Sonoyama, N.; Hayashi, J.; Li, C. Z.; Sathe, C.; Chiba, T. Primary release of alkali and alkaline earth metallic species during the pyrolysis of pulverized biomass. Energy Fuels 2005, 19 (5), 2164−2171. (16) Waligora, J.; Bulteel, D.; Degrugilliers, P.; Damidot, D.; Potevin, J. L.; Measson, M. Chemical and mineralogical characterizations of LD converter steel slags: A multi-analytical techniques approach. Mater. Charact. 2010, 61, 39−48. (17) Mikhail, S. A.; Turcotte, A. M. Thermal behaviour of basic oxygen furnace waste slag. Thermochim. Acta 1995, 263, 87−94. (18) Samsudin, A.; Zainal, Z. A. Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: A review. Renewable Sustainable Energy Rev. 2011, 15 (5), 2355−2377. (19) Shatokha, V. I.; Gogenko, O. O.; Kripak, S. M. Utilizing of the oiled rolling mills scale in iron ore sintering process. Resour., Conserv. Recycl. 2011, 55, 435−440.

40 and 60 min. This shows that carbon content increases with the experimental time because the longer experimental time indicates more biomass being charged, thus more tar production for carbon deposition. By calculations, it was proven that it is possible for this tarcarbonized slag to be directly reduced to iron. The carbon deposit amounts within the slag for all experiments were more than the minimum and/or maximum carbon needed for reduction of iron, which was 2.45 and 4.79 mass %, respectively. Other calculations were performed to determine the heating value of these products. From these calculations, we can conclude that, if the use of steelmaking slag in sintering machine is replaced by tar-carbonized slag, coke breeze consumption in the sintering machine can be reduced up to 12%. Overall, this research suggests the usability of steelmaking slag by the tar-carbonization process, in which carbon from pine biotar was able to be infiltrated and deposited within the slag. Carbon deposit within the slag makes this tar-carbonized slag useful in steelworks, specifically for direct reduction of iron and as a supplementary fuel in sintering machine.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +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. We thank Dr. Takazo Kawaguchi for meaningful discussions.

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REFERENCES

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