Carbon Deposition Using Various Solid Fuels for ... - ACS Publications

Apr 11, 2013 - Department of Chemical Engineering, Gadjah Mada University, Jalan ... Kulliyyah of Engineering, International Islamic University, 50728...
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Carbon Deposition Using Various Solid Fuels for Ironmaking Applications Rochim B. Cahyono,†,‡ Alya N. Rozhan,§ Naoto Yasuda,† Takahiro Nomura,† 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 Chemical Engineering, Gadjah Mada University, Jalan Grafika 2, Bulaksumur, Yogyakarta 55281, Indonesia § Department of Manufacturing and Materials Engineering, Kulliyyah of Engineering, International Islamic University, 50728 Kuala Lumpur, Malaysia ABSTRACT: In this paper, we describe an innovative process involving iron reduction through chemical vapor deposition for applications in the ironmaking industry. In our experiment, we produced tar vapors from pyrolysis of various solid fuels, including high-grade bituminous coal (HGC), low-grade lignite coal (LGC), and biomass palm kernel shell (PKS), and decomposed these vapors into gases, carbon, and light hydrocarbon. Carbon was deposited within the pores of pisolite ore (low-grade ore), which became porous during the dehydration process at 450 °C. We determined that the amount of tar produced during pyrolysis strongly affected carbon deposition, and HGC produced the highest carbon deposition because of its large tar product. In addition to tar amount, surface area and pore volume also played important roles in this process. PKS had the highest ratio of deposited carbon because it produced the smallest quantities of reacted tar and, consequently, the largest numbers of vacant pores. The amount of carbon deposition decreased at higher temperatures because tar was easily converted to a gaseous phase. The deposited carbon within iron ore showed potential as a reducing agent because it was highly reactive and reduced at lower temperatures. Carbon deposited within iron pores dramatically reduced the contact distance between the iron ore and carbon. Thus, these results show that our proposed methodology could have important applications as an alternative low-energy approach for producing metallic iron using low-grade materials.

1. INTRODUCTION The majority of metallic iron production processes typically rely on metallurgical cokes generated from high-grade bituminous coal.1 In addition to providing an energy source for blast furnaces, coke is important in reducing iron ore and maintaining bed permeability. Akiyama and Yagi reported that the Japanese ironmaking industry consumed approximately 525 kg of high-grade coal to produce 1 ton of hot metal.2 Recent rapid growth in the ironmaking industry has led to high demand for metallurgical coke as an energy source. However, because high-grade coal is expensive and a limited resource, industries have been forced to explore innovative alternatives to reduce their dependency on coke. Shui et al. applied a hydrothermal treatment using an autoclave (0.1 MPa) to enhance the properties of sub-bituminous coal for use as an alternative to coking coal. The enhanced coal product was used as blending material to reduce the amount of coking coal required for industrial processes.3 Other researchers have fabricated briquettes by combining carbon materials from steelmaking waste, tar sludges, oil from steel rolling mills, and deposits from coke oven gas pipelines.4 In addition, wooden biomass and low-grade coal (e.g., lignite, sub-bituminous coal), both of which are more plentiful and less expensive then highgrade coal, have been highlighted as promising alternatives to coke. Wooden biomass is of particular interest because it is a renewable resource and would reduce CO2 emissions in the ironmaking industry. © 2013 American Chemical Society

In the ironmaking process, coal was converted to coke, tar, and gas using the pyrolysis process in the coke oven unit. Coke is the main target product of this process for later use as a reducing agent, while tar vapor and gas are byproducts that contain high amounts of carbon and energy. Tar material is mainly composed of condensable organic materials and may cause operational problems such as pipe plugging, condensation, and tar aerosol formation.5 Catalytic tar decomposition has been investigated as a promising method for avoiding such problems and increasing the efficiency of the pyrolysis process.6,7 Researchers have explored several materials (e.g., Ni, Pt, Rh, and Pd) for use as catalysts in this reaction.8−11 Decomposition of CO2−CH4 over hot granulated slag containing high amounts of CaO and SiO2 was studied by Purwanto et al.12 Utilization of blast furnace slag as a catalyst for gasification of municipal solid waste was also evaluated by Zhao et al.13 Results of both studies showed that slag behaved as a strong catalyst for the decomposition process. In addition, nickel-based catalysts can also be very active in tar decomposition, but their uses are limited because they become deactivated as catalysts with increasing carbon deposition inherent to this process.14 Furthermore, disposal of toxic inactive nickel catalysts is a potential environmental problem. Another option for enhancing tar decomposition is to apply a Received: February 25, 2013 Revised: April 2, 2013 Published: April 11, 2013 2687

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Table 1. Properties of Solid Fuels Used in Experiments As Determined through Proximate and Ultimate Analyses of Raw Coal Materials proximate analysis [mass %, air-dried basis]

a

FCa

volatile

ash

C

H

N

Ob

S

bituminous, high-grade coal (HGC) lignite, low grade coal (LGC) palm kernel shell, biomass (PKS)

66.9 47.2 24.2

24.4 50.9 65.4

8.7 1.9 10.4

80.2 68.5 49.5

5.3 5.0 5.7

1.8 0.6 0.8

12.8 25.6 44.0

0.5 0.3 0.0

FC = fixed carbon. bCalculated by difference. utilization of this ore is currently limited to the ironmaking industry. We used large amounts of combined water to create porous material for carbon deposition. Ore was crushed and sieved to obtain a sample with similar particle sizes ranging from 0.95 to 2 mm. Tar decomposition and carbon deposition required high surface areas for reaction sites and carbon storage, respectively. In order to increase surface area and create porous material, we dehydrated the ore at 450 °C with a heating rate of 3 °C/min and a holding time of 1 h in the open atmosphere. Because thermogravity (TG) experiments of ore samples previously showed that CW decomposed at 350 °C, we assert that dehydration of ore at 450 °C was adequate to fully dissipate CW from the ore pores and the created porous materials. In order to evaluate the dehydration process, we measured total pore volume before and after the experiment using a Brunauer−Emmett−Teller (BET) method over Autosorb 6AG (Quantachrome Instruments). 2.2. Tar Decomposition and Carbon Deposition. During tar decomposition and carbon deposition experiments, we used a quartz reactor tube with an inner diameter and height of 30 mm and 550 mm, respectively (Figure 1). In order to maintain the correct constant experimental temperature, we used an electrical furnace equipped with six thermocouples and a temperature controller. Experiments were performed at atmospheric pressure with a total N2 flow of 250 mL/ min. Solid fuel was continuously added to the reactor using a bowl feeder at a rate of 0.1 g/min for 40 min after the temperature of the reactor became stable at 600−800 °C. Pyrolysis happened rapidly inside the reactor and produced char, tar vapor, and gases. We collected the char product using a thimble filter, SUS 404 mesh, while tar vapor and gases were poured directly to the reactor bed. The effect of iron ore on tar decomposition was evaluated by performing experiments without iron ore (case 1) and with 3 g of iron ore inside the reactor (case 2). In case 2, tar vapor and gases were introduced to iron ore in the reactor bed for tar decomposition and carbon deposition. Generated gas and remaining tar were removed from the bottom and allowed to flow through a cold trap, which was maintained at −73 °C by adding liquid N2 for separation between tar vapor and gases. We used a gas chromatography analyzer (GC 2014, Shimidzu) which was equipped with a flame ionization detector (FID) and both polymer and molecular sieve columns to detect CO, CO2, and light hydrocarbon gases using carrier gases N2, H2, and air. In addition, the amount of H2 was analyzed using gas chromatography (GC 323, Hitachi) that was prepared with a thermal conductivity detector (TCD) with N2 carrier gas. Ore structure and composition were characterized using XRD analysis (Miniflex, Rigaku), while Autosorb 6AG (Quantachrome Instruments) was used to examine changes in total pore volume. We measured the carbon content within the iron ore (i.e., carbon deposition) using an elemental analyzer (MT6, CHN corder). From these experimental results, we could evaluate the effect of solid fuels on tar decomposition and carbon deposition. 2.3. Reduction Reaction of Tar-Carbonized Ore for Ironmaking. In order to evaluate the reactivity of ore that infiltrated and was deposited by tar carbon (i.e., carbonized ore), we performed the reduction reaction using thermogravimetric methods. We heated 10 mg of the sample at room temperature at a rate of 50 °C/min until a temperature of 1200 °C was reached with an argon flow rate of 500 N mL/min. We used a high argon flow rate method to ensure that gas byproducts of the reduction reaction (e.g., CO2 and CO) never contributed to or affected the overall reduction process, allowing only direct reduction to occur in this step. XRD analysis (Miniflex, Rigaku) was utilized to investigate changes in ore compounds and structures.

catalyst or a catalyst-like solid that can be used as fuel even after loss of activity, such as, for example, tar decomposition over charcoal. After reaction, charcoal and deposited carbon can be utilized as fuels or reducing agents.15 In this study, we proposed an integrated system that involves tar decomposition and deposition of carbon (from biomass and low-grade and high-grade coals) over low-grade iron ore. Udin et al. previously reported that iron oxide is a strong catalyst in the tar decomposition process and produces valuable gases. In this reaction, the surface area of the ore is important for maintaining catalytic activity.16 Low-grade iron ores (e.g., goethite (FeOOH) and limonite (FeOOH·nH2O)) are abundantly available but rarely used because they contain high combined water (CW) and require large amounts of energy for the dehydration process. In contrast, dehydration of CW is an attractive method for creating porous material within iron ore; nanocracks are created and propagated during the dehydration process, and the created nanopores can be used as valuable reaction sites.17 Utilization of low-grade iron ore would solve problems related to shortages and overuse of high-grade iron ore in the steelmaking industry. Hata et al. reported that decomposition of biomass tar over goethite ore could produce deposited carbon within the ore, which could act as a reduction agent. Tar carbon infiltrate and deposit within the pores and increase the carbon content by approximately 4 mass %.18 This procedure is also applied by Alya et al. to utilize steelmaking slag as a supplementary fuel in a sinter machine.19 The proposed system could simultaneously solve problems related to overuse of coking coal as a reducing agent and production of clogging tar materials during the pyrolysis process. Our objectives in this research were (1) to evaluate the effect of raw fuel material on carbon deposition, (2) to examine the reactivity of deposited carbon during the reduction reaction, and (3) to investigate the effect of temperature on carbon deposition within iron ore.

2. EXPERIMENTAL SECTION 2.1. Materials. In these experiments, we examined the qualities of solid fuels used as carbon sources for tar decomposition (Table 1). On the basis of proximate and ultimate analyses, bituminous coal acted as the highest quality fuel because of its large carbon content. To avoid the effect of particle size, we crushed and sieved each solid fuel to a granular size of approximately 250−500 μm. Table 2 shows the main properties of low-grade pisolite ore used in these experiments. Because of the small amount of total iron,

Table 2. Properties of pisolite iron ore

pisolite ore a

ultimate analysis [mass %, dry basis]

sample

TFea [mass %]

FeO [mass %]

C [mass %]

CWb [mass %]

surfaces area [m2/g]

49.73

0.12

0.14

5.90

11.83

TFe: Total iron. bCW: combined water. 2688

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Figure 1. Experimental apparatus for pyrolysis, tar decomposition, and carbon deposition: case 1, no iron ore; case 2, 3 g of iron ore. Solid fuels were charged at a flow rate of 0.1 g/min for 40 min under N2 flow rate of 250 mL/min (T.C., temperature controller; PC, personal computer; GC, gas chromatography).

3. RESULTS AND DISCUSSION 3.1. Tar Decomposition and Carbon Deposition. We determined pyrolysis characteristics of each solid fuel type using TG-DTG in an N2 atmosphere (Figure 2) for understanding

pyrolysis temperatures, PKS produced the smallest levels of char and largest levels of volatile matter compared to the other fuels. Figure 3 shows the effect of iron ore on carbon product distribution for each solid fuel during the pyrolysis process

Figure 2. TG/DTG profiles of various solid fuels studied during pyrolysis process at N2 atmosphere: sample weight, 10 mg; particle size, 250−500 μm; heating rate, 50 °C/min.

Figure 3. Product distribution of the pyrolysis of various solid fuels at 600 °C: case 1, no iron ore; case 2, 3 g of iron ore.

where total carbon was calculated based on the total mass input of fuel to the reactor. HGC produced the highest total amount of carbon because of the large amount of fixed carbon in this raw material. In case 1, PKS produced the smallest amount of tar while HGC produced the highest. The large quantity of volatile matter in PKS raw material was more easily converted into a gaseous form due to its low decomposition temperature as reported in Figure 2 . The effect of iron ore on the pyrolysis product could be examined by comparing case 1 and case 2. Introduction of iron ore in the pyrolysis process decreased the amount of tar that was converted into gas and carbon through catalytic tar decomposition (reaction 1).

the pyrolysis behavior. All fuels experienced a small weight loss as temperatures approached 200 °C because of moisture evaporation. We observed that HGC had the smallest total weight loss compared to the other fuel types due to its high content of fixed carbon, suggesting that HGC will produce high amounts of char and small amounts of volatile matter (e.g., tar and gas). On the other hand, PKS exhibited large weight losses because of its high oxygen and moisture content. Thermal decomposition began at temperatures approaching 240, 300, and 420 °C for PKS, LGC, and HGC, respectively. At this point, high levels of weight loss occurred as a result of volatile matter decomposition. Thermal decomposition of PKS consisted of two main peaks corresponding, first, to decomposition of hemicelluloses−cellulose at 250−310 °C and, second, to decomposition of lignin at 330−370 °C. Meanwhile, LGC and HGC exhibited only a single peak because these fuels were predominantly composed of carbon. These results were important for optimization of pyrolysis temperature to achieve high carbon deposition during the integrated pyrolysis−tar decomposition process. At equal

tar → H 2 + CO + CO2 + CH4 + other light hydrocarbons + C

(1)

Heavy and light hydrocarbons of tar decomposed over the iron ore surface to produce carbon, gases, and other light hydrocarbons. In addition to its ability for carbon storage, iron ore played an important role as a catalyst in the tar decomposition reaction. Each solid fuel produced a specific 2689

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amount of tar that acted as raw material for tar decomposition, and consequently, each fuel resulted in different levels of carbon deposition. Because carbon deposition requires large amounts of tar generated in the pyrolysis process, we concluded that solid fuels which produced the highest amounts of tar would be most suitable for carbon deposition. This conclusion was supported by the fact that HGC produced the highest quantities of tar, which resulted in the largest amount of carbon deposition compared to other fuels. Figure 4 shows the change in total pore volume of iron ore in each experimental step. Tar decomposition and carbon Figure 5. Effect of various fuels on reacted tar and ratio of deposited-C within porous iron ore: case 1, no iron ore; case 2, 3 g of iron ore.

consumed available area and pore volume for the deposition process. Therefore, the decreasing surface area and pore volume of iron ore during tar decomposition reduced the probability of the carbon deposition and resulted in a small ratio of deposited carbon. 3.2. Effect of Temperature on Carbon Deposition and Gas Composition. Previous studies have shown that pyrolysis products (e.g., tars and gases) are strongly affected by temperature.20 In this study, we found that the amount of tar produced decreased while the amount of gaseous products increased at high temperature (Figure 6). Thermal cracking,

Figure 4. Change in total pore volume of iron ore, and the amount of deposited carbon during experiments at 600 °C.

deposition required large surface areas and pore volumes for reaction sites and carbon storage. Dehydration of CW at 450 °C was effective for increasing pore volume around twice from the original ore. CW within iron ore decomposed and evaporated during the heating process according to reaction 2. The remaining site of CW was vacant and enhanced pore volume. 2FeOOH → Fe2O3 + H 2O

Figure 6. Effect of temperatures on the tar amount and total gas volume in pyrolysis of low-grade lignite coal, LGC.

(2)

Tar carbon was deposited within pores and reduced the total pore volume after the decomposition process. In general, the decrease in pore volume was proportional to the amount of carbon deposited. Thus, tar carbon deposited effectively within the iron pore which was revealed with a large decrease of pore volume at high carbon deposition. However, we observed that the remaining pore volume after decomposition was still onehalf that of the dehydrated ore. This observation prompted us to investigate ways of optimizing pyrolysis and tar decomposition in order to increase the amount of carbon deposition. In order to evaluate the effectiveness of carbon deposition through tar decomposition, we calculated the ratio of deposited carbon to reacted tar which could describe the possibility of carbon deposition. Figure 5 shows the correlation between the amount of reacted tar and the ratio of deposited carbon for each solid fuel. We observed that HGC had the highest level of reacted tar but smallest ratio of deposited carbon. Tar was converted into gas and carbon through the catalytic process which was affected by several factors such as available area, catalyst components, as well as tar composition. During tar decomposition, carbon deposited within pore ore and

decarboxylation, and depolymerization were preferred at high temperatures.21 We also determined that carbon deposition within ore pores declined at higher temperatures. The limited amount of tar produced in the pyrolysis process ultimately led to a decrease in the amount of carbon deposition. The tar components were more active and easier to decompose into a gaseous phase at high temperature. Therefore, the total quantity of deposited carbon was proportional to the amount of tar that was produced by pyrolysis. In addition, we noted that tar carbon was preferable to deposit within pore ore at high temperature. The pore volume was still vacant at high temperatures owing to less tar decomposition and carbon deposition. Figure 7 shows the effect of iron ore on gas composition during the LGC pyrolysis process at varying temperatures. In case 2, several reactions occurred simultaneously inside the reactor, including tar decomposition (reaction 1), indirect reactions (reactions 3 and 4), and gas reforming (reaction 5) 3Fe2O3 + CO → 2Fe3O4 + CO2 2690

(3)

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At 750 °C, the weight change ratio of carbonized ore drastically declined, indicating that the reduction reaction was initiated. Reduction of the reference mixture, Fe3O4, and coke began at 1100 °C.18 Therefore, the carbonized ore was more reactive than the reference mixture, Fe3O4, and coke. Tar carbon was deposited within the ore pores and minimized the distance between iron and carbon to the nanoscale, which enhanced the contact area and resulted in higher reactivity. The carbonized ore was also effective in the reduction reaction; it was denoted by the largest reduction degree of HGC that contained the highest carbon content. Small differences in carbon content between PKS and HGC could result in diverse total reduction degrees. We performed XRD analysis to investigate the transformation of the iron compound in each experimental step (Figure 9). We were able to convert all FeOOH to Fe2O3

Figure 7. Gas composition produced during pyrolysis of low-grade lignite coal (LGC) at different temperatures: case 1, no iron ore; case 2, 3 g of iron ore.

3Fe2O3 + H 2 → 2Fe3O4 + H 2O

(4)

CH4 + CO2 → 2H 2 + 2CO

(5)

The main reaction in each temperature condition could be predicted based on gas composition. All gases increased with introduction of iron ore (case 2) with the exception of CO at 600 °C. Tar decomposition that produced gases such as H2, CO2, CO, and CH4 was the main reaction compared to the indirect reaction and gas reforming at 600 °C. A small indirect reduction occurred simultaneously and reduced the amount of CO. At 800 °C, the amount of each gas declined due to the presence of the iron ore with the exception of CO2; thus, indirect reduction was prominent in this temperature. The decrease in CH4 signaled that gas reforming occurred as the second main reaction. 3.3. Reactivity of Carbonized Ore in Reduction Reaction. Figure 8 shows the variation in weight change

Figure 9. Change in XRD patterns of iron ore for each experimental step in pyrolysis of high-grade bituminous coal (HGC).

through the dehydration process (reaction 2). This process created porous material that was suitable for tar decomposition and carbon deposition. All Fe2O3 was reduced and converted to Fe3O4 after tar carbonization. Simultaneously with tar decomposition, reducing gases (e.g., H2 and CO) were generated during the pyrolysis process and initiated indirect reduction through reactions 3 and 4. Therefore, the ore product of tar decomposition contained deposited carbon and semireduced ore (Fe3O4). During the reduction process, deposited carbon was used effectively to produce FeO and Fe through reactions 6−9. However, we observed that an incomplete reduction reaction occurred because of the small quantity of available carbon. On the basis of stoichiometric calculations, 5.2 mass % C was required for complete reduction of pisolite ore sample.

Figure 8. Weight change and reduction degree at varying temperatures for carbon-deposited ore during reduction process.

4. CONCLUSIONS In this study, we proposed a new ironmaking process, carbon was produced from tar decomposition during pyrolysis, deposited within the pores of low-grade iron ore, and ultimately used as a reducing agent for iron. We also investigated the effect of several solid fuels on the behavior of carbon deposition and the reactivity of deposited carbon. Our main results can be summarized as follows. 1. Carbon deposition was strongly dependent on the amount of tar that was produced during the pyrolysis process. HGC produced more deposited carbon than other solid fuels because of its large tar product. The ratio of deposited carbon during tar decomposition was also highly affected by the availability of surface area. PKS

ratio and temperature during the reduction reaction, which was characterized by a heating rate of 50 °C/min and an argon flow rate of 500 N mL/min. We used a high argon flow rate to ensure that gas products of the reduction reaction (e.g., CO2 and CO) never affected the overall reduction process; thus, only direct reduction occurred through reactions 6−9 2Fe3O4 + C → 6FeO + CO2

(6)

Fe3O4 + C → 3FeO + CO

(7)

2FeO + C → 2Fe + CO2

(8)

FeO + C → Fe + CO

(9) 2691

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had the highest ratio of deposited carbon because only small amounts of tar reacted, and consequently, large amounts of surface area remained vacant. 2. Deposited carbon within iron ore showed potential as a reducing agent because it had higher reactivity and required lower reduction temperatures. Larger quantities of deposited carbon resulted in a greater reduction degree in the reduction process. The reactivity of carbondeposited ore increased because carbon deposited within iron pores decreased the contact distance between iron ore and carbon. 3. Deposited carbon was reduced at high temperatures due to smaller quantities of tar. Thermal cracking of solid fuels at high temperature was easier to transfer into the gas phase and decreased tar amount. Our results have important implications for iron reduction in the ironmaking industry, chiefly because our proposed process utilizes inexpensive low-grade source materials, leads to reduced energy consumption, and enhances overall pyrolysis efficiency.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +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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REFERENCES

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dx.doi.org/10.1021/ef400322w | Energy Fuels 2013, 27, 2687−2692