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Reduction Rate and Crushing Strength of Carbon-Containing Pellet Prepared by Impregnation Method of COG Tar Yuuki Mochizuki, Megumi Nishio, Naoto Tsubouchi, and Tomohiro Akiyama Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b03009 • Publication Date (Web): 04 Feb 2016 Downloaded from http://pubs.acs.org on February 20, 2016
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Reduction Rate and Crushing Strength of Carbon-Containing Pellet Prepared by Impregnation Method of COG Tar
Yuuki Mochizuki *, Megumi Nishio, Naoto Tsubouchi, Tomohiro Akiyama
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Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, Sapporo, 060-8628, Japan
*Corresponding author:
[email protected] Key word: Cold bonded pellet, Carbon, COG tar, Reduction rate, Crushing strength
Abstract The crushing strength and reduction rate of carbon-containing pellet (composite) prepared from cold-bonded pellet (CP) and coke oven gas (COG) tar are examined. The peak of the pore size distribution profile, at approximately 2 nm, observed in the as-prepared CP decreases with the increase in the mixture ratio of tar to CP, and completely disappear for the composites prepared above a mixture ratio of 1.0, and the SBET and VBJH values are < 1 m2/g and < 0.01 cm3/g, respectively. The crushing strengths of the composites increases with the increase in the mixture ratio of tar to CP and becomes 10 daN from 1.0 daN, above a mixture ratio of 2.0. Carbonaceous material derived from tar is detected on the surface of the composite particle, as well as inside the particle, and the C content in the composite is 22 mass%-C. When the composites are heated in He and 55%H2/He, the evolution of CO, CO2 and H2O starts at approximately 400 and 500 oC, respectively, and the formation profiles indicate a large peak at approximately 800-900 oC. The extent of reduction of the composites at 1000oC is 85-95 %. The crushing strengths of the dehydrated-CP decreases drastically up to a reduction extent of 50 %, whereas the strength of the composites is maintained at a reduction rate up to 50 % and then decreases with the increase in the reduction rate.
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1. Introduction The amount of CO2 emission from the iron and steel industry makes up 15% of the total amount of exhaust in Japan. 1 Therefore, decreasing the CO2 emission from the iron and steel industry is a very important challenge. In Japan, the COURSE50 (CO2 Ultimate Reduction in Steelmaking process by innovative technology for cool Earth 50) project has been developed to reduce CO2 emission by using modified coke oven gas (COG), which is a hydrogen-rich gas, instead of metallurgical coke in the blast furnace for the iron-making process. 2,3 However, the development of an innovative process will be necessary to decrease CO2 emission. An iron ore/carbonaceous material composite has been developed to solve this problem. This approach aims to lower the thermal reserve zone temperature in the blast furnace by decreasing the distance between the iron oxides and carbonaceous materials, resulting in a decrease in the reducing agent and leads to the possibility of decrease in the CO2 emission. Therefore, many researchers have investigated the preparation method of iron ore/carbonaceous materials composites and their reduction behavior.
4-11
However, these investigations were performed considering the application of the
composite in a conventional blast furnace, and there is no report about the reduction behavior and crushing strength of the composite when employed in a blast furnace using hydrogen gas. We have researched developing an iron oxide/carbonaceous materials composite having rapid reduction rate and
high
crushing
strength
from
sintered-iron-ore
and/or
inherent
carbon-containing
cold-bonded-pellet and COG tar. 12,13 In our previous work, the distribution of carbonaceous materials and their chemical form in the composite were not elucidated, and the mechanism of the decrease in the crushing strength of the composite with the reduction process was not obvious. In this study, we thus first prepared an iron oxide/carbonaceous materials composite (carbon-containing pellet) from COG tar and the carbon-less cold-bonded pellet, in which the carbonaceous material was completely filled into the pores of the pellets, and the distribution of carbonaceous material in the composite and chemical form were investigated. The reduction behavior and crushing strength of the composite were then examined, and the effect of changing the pore structure of the composite on the crushing strength during reduction process was discussed.
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2. Experimental 2.1. Samples and preparation of cold bonded pellet Cold bonded pellet (denoted as CP) of particle size fraction from 2.0 to 3.4 mm was used in this study. CP was prepared using an Australian Robe River (ROB) ore of particle size < 1mm, ordinary Portland cement (PC) and bentonite (BN), and these materials were received from the Japanese steel company, Taiheiyo cement co ltd. and Kanto chemical co ltd., respectively. First, ROB, PC and BN were mixed using a versatile mixing machine at 100 rpm for 30 s. Then distillated water was added to the mixture, and the mixtures were mixed at 100 rpm for 3 min. The mixture ratio ROB : PC : BN : H2O was 93 : 6 : 1 : 15. After the mixture was moved to the tray, it was aged in a sealed polyethylene bag at 45 oC for 10 days. The CP composition was as follows: Total-Fe; 46, FeO; 0.5, CaO; 3.7, SiO2; 5.5, Al2O3; 2.7, MgO; 0.2 mass%-dry, and the specific surface area (SBET) and pore volume (VBJH) values were 20 m2/g and 0.06 cm3/g, respectively. COG tar, which was recovered from the coke making process, was used without further purification as the carbon sources, and its elemental analysis showed the following composition: C, 91; H, 4.7; N, 1.1; S, 0.4 mass%-daf.
2.2. Preparation of carbon containing pellet Preparation of carbon containing pellet (denoted as composite) was carried out with a flow-type quartz made fixed-bed reactor. 12, 13 After the CP was charged into the alumina cell, tar was added to the cell. The mixture mass ratio of tar to CP was changed from 0 to 3.0. After physical mixing of CP and tar in the cell at room temperature, the cell was placed at the center of the reactor. The reactor was first vacuumed, high-purity He (>99.99995%) was then loaded at 200 cm3(STP)/min into the system, and the effluent was measured using a high-speed micro gas chromatograph (GC) (Inficon) to ensure that the remaining air was replaced with He. Finally, the mixture was pyrolyzed at 10 oC/min to 500 oC in He to obtain the composite.
2.3. Reduction experiment The reduction behavior of the prepared composite was investigated in the same manner as described in section 2.2. 12, 13 The composite was placed in an alumina cell and heated at 10 oC/min to 500-1000 3
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o
C in 200 cm3-STP/min of He and/or 55%H2/He, and then, immediately quenched in a He flow to
room temperature in the case of the 55%H2/He. The amounts of CO, CO2, CH4 and H2O evolved were on-line analyzed via a GC and gas-monitor (Innova) at intervals of 3 and 2 min, respectively. The extent of the reduction was estimated by the amount of O-containing gases evolved during the heat treatment.
2.4. Characterization The characterization of the samples before and after heat treatment was mainly performed using nitrogen adsorption (Quantachrome), powder X-ray diffraction (XRD, Shimadzu), scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS) and Renishaw InVia micro-Raman spectrometer (Renihaw, Wotton-under-Edge) equipped with a charge-coupled device (CCD) detector. The detailed analyses conditions of the N2 adsorption, XRD, SEM-EDS and Raman spectroscopy were described previously. 12-15. The crushing strength of a sample was determined using a tensile and compression testing machine (Minebea) according to the Japanese Industrial Standards (JIS M 8718). The test was performed 10-20 times per sample, and the average value was employed.
3. Result and discussion 3.1. Change in pore structure of CP during heat treatment in an inert gas atmosphere Figure 1 shows the change in the pore size distribution of the samples heat treated up to each temperature in He. The peak intensity of the pore size distribution observed at around 2 nm in the as-prepared CP (Figure 1a) increased slightly with increasing temperature between 300 and 400 oC (Figures 1b and d), whereas the intensity increased drastically at 500 oC and reached about 0.12 cm3/nm/g (Figure 1e), which is approximately three times that of CP. On the other hand, above 600 oC (Figures 1 f-h), the intensity of the pore size profile at around 2 nm decreased with an increase in the temperature, and it almost disappeared at 900 oC. Figure 2 presents the changes in the SBET and VBJH values of the samples with heat temperature corresponding to Figure 1. The SBET value (20 m2/g) of CP increased at 300-350 oC and achieved a maximum value of 60 m2/g at 350 oC, whereas it dropped linearly with an increase in temperature 4
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above 400 oC and was almost 0 m2/g at 900 oC. Although the VBJH value (0.06 cm3/g) of CP was a constant until 400 oC, it increased drastically over the temperature range of 400-500 oC and became 0.075 cm3/g. at 500 oC. At 500-700 oC, the decrease in VBJH was slight, whereas it decreased dramatically above 700 oC and almost disappeared at 900 oC. Thermogravimetric change was first investigated to clarify the change in the pore structure of CP during heat treatment in He. The derivative curve calculated from the thermogravimetric curve of CP provided the main peak of weight loss at around 300 oC, and the weight loss stopped at around 500 oC. The H2O formation from CP and the change in the iron form in the heated sample were then investigated to clarify the changes in SBET and VBJH values with temperature as shown in Figure 2. H2O formation started from 100 oC, and the profile of the formation rate gave the main peak at around 300-350 oC, which was observed as the maximum value of VBJH. Table 1 and Figure S1 of the Supporting Information shows the change in the Fe forms in the CP with temperature. The XRD signal intensity of α-FeOOH observed in the as-prepared CP, which is the main iron form, decreased until 300 oC and disappeared completely at to 400
o
C, whereas the peak attributed to Fe2O3 was observed. This Fe2O3 peak intensity tended to
increase with increasing temperature. The rate of H2O formation provided the main peak at around 300-350 oC in the case of heat treatment of only the ROB at same experimental conditions. This result suggests that H2O evolution from CP with the main peak at 300-350 oC was due to the de-hydration of α-FeOOH in CP. As is well known, the development of mesopore in limonite occurs by the de-hydration of α-FeOOH over the temperature range of 300-400 oC. 16 Therefore, it is estimated that the development of pore size distribution peaking at around 2 nm in the heated sample, is caused by the de-hydration reaction of α-FeOOH. Although an increasing SBET value was observed until 400 oC, the VBJH value was a constant over the same temperature range. This is why agglomeration of mesopores may occur below 400 oC, and pores start disappearing above 500 oC, because both the SBET and VBJH values tends to decrease with increasing temperature.
3.2. Crushing strength of the prepared composite The changes in the pore structure in the composite prepared with a mixture of tar and CP were first investigated to clarify the development of the crushing strength. Figure 3 shows the changes in pore 5
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size distribution, SBET and VBJH values with the mixture ratio of tar to CP. Although the peak intensity of the pore size distribution on the CP around 2 nm increased to 0.12 cm3/nm/g until 500 oC, as mentioned above, it dramatically decreased to 0.01 cm3/nm/g in the case of a mixture ration of tar/CP = 0.25 (Figure 3a). Moreover, this peak around 2 nm tended to decrease with increasing tar to CP ratio, and almost disappeared above tar/CP = 1.0. Figure 3b represents the changes in the SBET and VBJH values corresponding to Figure 3a. When the CP was heated to 500 oC (denoted as CP/500), the SBET value was 40 m2/g. On the other hand, this value dropped drastically to 5 m2/g at a tar to CP ratio of 0.15 and decreased below 1 m2/g above tar mixture to CP ratio of 1.0. When only tar was heated in He during thermogravimetric measurement, weight loss occurs above 100 oC. The derivative curve obtained from the thermogravimetric curve of tar showed a main peak at around 300 oC, and the weight loss ended at 500 oC. This result shows that the volatilization of tar occurs at 100-500 oC. As mentioned above, the weight loss rate of tar showed a main peak at around 300 oC, which corresponds to the temperature range of the main peak observed for H2O formation. These results suggests that a part of tar added to CP at room temperature was transformed to the gas phase during heat treatment, and the gaseous-tar vapor infiltrated the mesopores formed in the heated CP sample, when carbonaceous materials derived from tar remained in these pores. Figure 4 illustrates the changes in the carbon content and crushing strength of the prepared composite corresponding to Figure 3. The carbon content in the composite (Figure 4a) increased dramatically even at a tar to CP ratio of 0.25 and was 10 mass%-dry. Beyond this, the C content increased with increasing tar to CP ratio. This increase in the C-% was almost constant above a ratio of 2.0, when it was 22-25 mass%-dry. On the other hand, the crushing strength of the prepared composite was only 2.5 daN at a tar to CP ratio of 0.25 (Figure 4b), which was observed to dramatically decrease the peak intensity of the pore size distribution at around 2 nm seen in Figure 3a. When the tar to CP ratio increases to 1.0, the pore size distribution peak at around 2 nm almost disappears, and the crushing strength dramatically increased to 7 daN. Although this strength reached 10 daN for a tar to CP ratio of 2.0, an increase in the strength was not observed beyond this ratio. This crushing strength of the composite matched that of commercial metallurgical coke of the same size fraction (Drum index : DI1506 = 87.1, 97 mass%-daf C). Figure 5 shows the relationship between carbon content and 6
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crushing strength of the composite seen in Figures 4. The crushing strength increased linearly with increasing carbon content above 12 mass%-dry in the composite, and the value was a constant above 22 mass%-dry. This result suggests that carbonaceous material derived from tar needs to fill into the mesopore of the CP to develop the crushing strength. Although a slight increase in the carbon content was observed beyond tar to CP ratio of 2.0, as shown in Figure 4a, the crushing strength was almost constant. This phenomenon suggests that carbonaceous material derived from tar deposited onto the CP particle surface above a tar mixture to CP ratio of 2.0. The carbon content in the composite prepared from a tar mixture to CP ratio of greater than 2.0 is larger than that of the theoretical carbon content (15 mass%-C), which is hypothesized to be amorphous carbon, calculated by the VBJH value (0.075 cm3/g) of CP/500. This result also supports the hypothesis that carbonaceous materials are deposited onto the surface of CP particles. From these results, a tar mixture to CP ratio of 2.0 is used for further investigations. Distribution of carbonaceous materials in the prepared composite was observed using SEM-EDS where the cross-section of the particle was polished using sand paper after the particle was filled with a low-melting alloy. Figure 6 represents the SEM images, C and/or Fe mapping and line analyses of as-prepared CP and composite prepared. From the result of the mapping and line analyses of the as-prepared CP (Figures 6b-d), it was observed that Fe was present uniformly in the particle without any presence of C. On the other hand, in the case of the composite (Figures 6f-h), by comparing the line analyses of the as-prepared CP (Figure 6b-d) and the composite (Figure 6f-h), significant amount of C was detected on the particle surface, as well as inside the particle. This observation shows that carbonaceous material derived from tar was filled inside the particle via vapor infiltration of the gaseous-tar that was generated during the pyrolysis of the mixture of CP and tar. From the XRD measurement of the prepared composite, a weak peak of Fe2O3 and medium peak of Fe3O4 were observed. A part of Fe2O3 was reduced to Fe3O4 during preparation of the composite because iron was present only as Fe2O3 in CP/500, as mentioned above. Raman scattering spectra for the prepared composite was obtained to clarify the chemical form of the carbonaceous material deposited. The Raman spectra provided a broad profile peaking at around 1300-1400 and 1600 cm-1. According to a previous study, the two peaks are attributed to the G and D 7
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band, respectively.
14
From the result of curve-fitting of the observed spectra, the proportion of
partially graphitized-C and amorphous-C was estimated as 40 and 60 %, respectively, and it was found that a large amount of the deposited carbonaceous material consisted of amorphous-C with high reactivity.
3.3. Reduction reactivity of as-prepared CP and composite during heat treatment in He or 55%H2/He The formation of O-containing gases was investigated to understand the reduction reactivity of prepared composite in an inert and/or reduction atmosphere with a flow-type quartz made fixed-bed reactor, and the results are shown in Figure 7. In the case of heat treatment in He (Figure 7a), evolution of CO and CO2 started at around 400 oC, and the former showed distinct main and shoulder peaks of the formation rate at 650 and 800 oC, respectively, and the latter provided the two peaks at 450 and 650 oC, respectively. The formation of H2O began from about 400 oC, and the main peak of formation rate was observed at around 800 oC with weak and shoulder peaks at around 500 and 650 oC, respectively. Although CH4 formation was also observed at around 400 oC, the amount was smaller than those of the O-containing gases. On the other hand, in 55%H2/He (Figure 7b), which simulated COG modified gas, CO and CO2 evolution from the composite were measured from around 350 oC, and the former gave distinct large and small peaks at 650 and 800 oC, respectively, and the latter gave only weak peak at around 650 oC. The H2O formation also started from around 350 oC and provided weak and distinct large peaks of formation rate at 450 and 700 oC, respectively. The formation of CH4 began above 300 oC, and showed a main peak at around 550 oC. From the comparison between Figures 7a and b, CO and CO2 formation rates in He and 55%H2/He were similar, but the amounts of these gases generated in 55%H2/He was smaller than that in He. On the other hand, the formation rates and amounts of H2O and CH4 in 55%H2/He were greater than those in He. These results suggest that the reduction mechanism of iron oxides in the composite is different in He and 55%H2/He. The formation of O-containing gases from CP/500 during heat treatment in 55%H2/He was investigated to compare with the results of the composite. For CP/500 heated in 55%H2/He, H2O was the main gas produced. H2O evolved beyond 300 oC, and the formation 8
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rate showed small and very large peaks at around 350 and 600 oC, respectively, and the evolution was terminated above 800 oC. This profile of the formation rate was quite difference from that of the composite in He and 55%H2/He. This implies that the reduction of iron oxide in CP/500 completed by H2 until 800 oC, and the reduction mechanism is different from that of the composite. The proportion of O-containing gases evolved from the composite for 60 min at 1000oC in He and 55%H2/He was investigated based on the formation rate of the O-containing gases shown in Figure 7. Although the O-containing gases evolved in He followed the order CO2 (15%) < H2O (29%) < CO (36%), which are shown based on the amount of O in the prepared composite, the order in 55%H2/He was CO2 (7%)
