Article pubs.acs.org/EF
Preparation of a Carbon-Containing Pellet with High Strength and High Reactivity by Vapor Deposition of Tar to a Cold-Bonded Pellet Yuuki Mochizuki,* Megumi Nishio, Naoto Tsubouchi, and Tomohiro Akiyama Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan S Supporting Information *
ABSTRACT: The optimum conditions for the preparation of carbon-containing pellets (composites) in which the carbonaceous materials derived from coke oven gas (COG) tar are completely filled into the pores of cold-bonded pellets (CPs) by vapor deposition are first investigated using a flow-type quartz fixed-bed reactor. The distribution of carbonaceous material and the crushing strength of the composites are then investigated against prepared composites by means of N2 adsorption measurements, scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy, and a tensile and compression testing machine. The maximum crushing strength of 10 daN is observed in the case of the composite prepared by a combination of tar pyrolysis at 700 °C and vapor deposition at 350 °C; this value matches the maximum crushing strength of commercial metallurgical coke with the same size fraction. Moreover, the 2 nm pores observed in the original CP are absent in the prepared composite. Carbonaceous material derived from tar is detected inside the particles and on the particle surface, and the C form is almost amorphous C. Furthermore, the reduction behavior and crushing strength of the composites are examined in a He or 55% H2/He atmosphere. The reduction rate of the prepared composite is greater than that of a mixture of dehydrated CP and coke in He. In addition, the extent of reduction in 55% H2/He is larger than that in He. The crushing strengths of the lump ore decrease drastically up to 50% reduction, whereas that of the composites is maintained up to a reduction rate of 50%.
1. INTRODUCTION The thermal reserve zone, which causes reduction rate control of iron ore, is exiting in the blast furnace for the ironmaking process. Decreasing the temperature of this zone is believed to reduce the CO2 emission and energy consumption in the blast furnace, owing to a decrease in the rate of reduction by reducing agents.1 Decreasing the CO2 emission from the ironand steelmaking industries is a very important challenge. In Japan, the CO2 Ultimate Reduction in Steelmaking Process by Innovative Technology for Cool Earth 50 (COURSE50) Project has been developed to reduce CO2 emission from ironmaking using modified coke oven gas (COG), which is a hydrogen-rich gas, instead of metallurgical coke in the blast furnace.2,3 However, the development of an innovative process is essential for reducing CO2 emission even during hydrogen reduction in the blast furnace. Therefore, the development of ironmaking materials with rapid reduction reactivity is crucial for solving this problem. On the other hand, iron materials with a low reduction disintegration index (RDI) are required for the blast furnace, because reduction disintegration occurs in the upper part of the blast furnace, owing to the reduction of hematite to magnetite in lump or sintered iron ore with volumetric expansion, and this causes the loss of permeability in the blast furnace.4 In general, a low-RDI lump or sintered iron ore, which has a high crushing strength, has a low reduction reactivity. Hence, developing iron materials with low RDI and high reduction reactivity for the blast furnace is extremely important. In an effort to simultaneously solve the above problems, lignite/biomass and iron ore have been used to produce carbon-containing iron ores by chemical vapor infiltration (CVI).5−7 Although such a CVI sample has rapid © XXXX American Chemical Society
reduction reactivity, the crushing strength has not been investigated because the C content of CVI samples is 3−5 wt % dry and the complete filling of pores with carbonaceous materials is very difficult.5−7 Our research group has been investigating the production of carbon-containing pellet/sinter by coke oven gas (COG) tar impregnation, and it has been clarified that the carbonaceous materials derived from COG tar could be completely filled into pores in samples.8−10 In addition, it has been found that composites have a high crushing strength and high reduction reactivity. If a composite has large amounts of C, high crushing strength, high reduction reactivity, and low RDI prepared by gaseous tar, it may be applicable for blast furnaces in a conventional or an advanced ironmaking process (COURSE50). In the case of the abovementioned composite preparation, it is considered that the utilization of the conventional process is effective. In other words, if gaseous tar containing COG produced from the cokemaking process and its sensible heat are possible to use the composite preparation, the production can be completed in the integrated iron and steel plants. Against this background, we first clarified the optimum conditions for preparing carbon-containing pellets with a high crushing strength and high reactivity; these pellets were prepared by completing filling the pores in a cold-bonded pellet (CP) with carbonaceous materials derived from COG tar by the vapor deposition method. The distribution of carbonaceous material and the crushing strength of composites were Received: March 7, 2017 Revised: July 20, 2017 Published: July 20, 2017 A
DOI: 10.1021/acs.energyfuels.7b00680 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
described in the literature.8−11 For the N2 adsorption analysis, the measurement was repeated to ensure that the results could be reproduced. The repeatability of SEBT and VBJH values were ±5 m2/g and ±0.005 cm3/g, respectively.
then investigated against prepared composites. In addition, the reduction behavior and crushing strength of the prepared composites were investigated.
2. EXPERIMENTAL SECTION
3. RESULTS AND DISCUSSION 3.1. Optimization of TPT and VDT. When as-prepared CP was heated to a predetermined temperature at the rate of 10 °C/min in He, the peak intensity at a pore size of approximately 2 nm increased with an increase in the temperature10 (Figure S1 of the Supporting Information shows the change in the N2 isothermal curve of as-received CP with the temperature). The maximum peak intensity was measured at 500 °C. On the other hand, above 500 °C, the peak decreased drastically with an increasing temperature and almost disappeared by 900 °C.10 Table 3 lists the SBET and VBJH
2.1. Samples. CPs were prepared using Portland cement and Australian low-grade iron ore (limonite) with a large α-FeOOH content (92 wt % dry). COG tar recovered from the actual coke manufacturing process was used in this study. These materials were supplied by a Japanese iron- and steelmaking company. The preparation method for CPs was reported in detail in our previous paper.10 The composition of the as-received CP (particle size of 2−3.5 mm) and the elemental analysis of the tar used in this study are listed in Tables 1 and 2, respectively. The specific surface area (SBET) and
Table 1. Composition of CP Used composition (wt % dry) total Fe
FeO
SiO2
Al2O3
CaO
MgO
46.0
0.5
5.5
2.7
3.7
0.2
Table 3. Summary of the BET Surface Area, BJH Pore Volume, and Fe Species with Heat Temperature of CP
Table 2. Analysis of Tar Used
temperature (°C)
BET surface areaa (m2/g)
BJH pore volumeb (cm3/g)
as received
20
0.06
200
20
0.06
300
30
0.06
350 400 500 600 700 800 900 1000
60 55 40 30 20 10
