Article pubs.acs.org/EF
Gasification Effect of Metallurgical Coke with CO2 and H2O on the Porosity and Macrostrength in the Temperature Range of 1100 to 1500 °C Soon-Mo Shin and Sung-Mo Jung* Graduate Institute of Ferrous Technology, POSTECH, Pohang, North Gyeongsang 790-784, Korea ABSTRACT: The gasification of metallurgical cokes with CO2 and H2O on their porosity and macrostrength were investigated in the current study. Cokes were reacted with CO2 or H2O in the temperature range of 1100 to 1500 °C. During the reaction, the compositional change of product gases were measured by quadruple mass spectrometry (QMS) for evaluating the gasification rate. Image analysis was carried out to measure the porosity according to the distance from the coke surface. The porosity at the surface of coke gasified with CO2 indicated low values due to its low reactivity, which resulted in the intraparticle reaction to diffuse into the pores at low temperatures while the coke gasified with H2O showed the tendency to react with the coke at the surface. This difference in the reaction behavior can be explained by Thiele modulus. Furthermore, fine powder formation and tensile strength were measured to evaluate macrostrength of cokes. According to the results, the different reaction mode caused an obvious difference in the tendency of macrostrength of the cokes of the same reaction degree. Tensile strength of cokes was strongly affected by their porosity distribution.
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INTRODUCTION The blast furnace (BF) ironmaking has been one of the most effective processes in the steel industry due to its high productivity and efficiency. Among the raw materials charged into the furnace, coke serves a great important purpose in the BF operation as a reducing agent, heat supply, and a producer of passage for the permeability. Although the BF produces pig iron as the principal unit in the ironmaking process, the process not only consumes a huge amount of energy and resources but also has great environmental impact. In 1997, the Kyoto protocol was adopted by UNFCCC (United Nations Framework Convention on Climate Change) and it became effective in 2005. Carbon dioxide is one of the targets to be reduced, and Korea will be forced to take the responsibility for reducing carbon dioxide emission in 2013−2017. In particular, the steel industry should find the solutions to carbon dioxide emission. In addition, the large consumption of iron ore and coke in China and India created the shortage of high quality ore and coke, which resulted in the abrupt price increase of raw materials. To cope with the aforementioned environmental problems, it is necessary to investigate the alternative methods of reducing carbon dioxide emission and coke consumption in the BF. Since hydrogen has been considered to be a much more favorable reducing agent than carbon monoxide, the effective injection of H2 into the BF has been extensively studied to meet the demand of CO2 issues and coke consumption.1−3 The injection of H2 will decrease the required amount of coke consumption; that is, the so-called low coke operation might be feasible in the BF ironmaking process. In the attempt of H2 injection into the BF, it is strongly demanded to enhance coke strength. Therefore, the gasification effect of metallurgical coke with H2O on coke strength should be clarified due to its great importance. © 2015 American Chemical Society
Several studies investigated the reaction mechanism and kinetics of steam reaction with various carbonaceous materials such as coal, char or coke in various reactors.4−14 In addition, several models have been proposed by thermogravimetric analyses (TGA) in order to predict the different nature of chars during gasification.15 Although numerous studies have been carried out to figure out the gasification kinetics of carbonaceous materials with H2O, the effect of coke gasification on its strength was rarely investigated. Several studies only dealt with the effect of annealing temperature on coke strength and pore structure.16−18 In addition, several investigations were carried out to clarify the effect of coke gasification with CO2 on the pore structure and strength.19−21 Iwanaga examined coke fine generation under various experimental conditions. On the basis of the results, reacted cokes with H2O generated coke fine at a higher rate, but there was not much difference at high temperature compared to reacted cokes with CO2.19 The fine coke rate and porosity of reacted cokes with CO2 by the tumbling test and the comparison of the fine coke rate between the calculation and observation were suggested by the research of Kashiwaya et al.20 According to the study of Kawakami et al., the C−CO2 reaction showed the change of the reaction mechanism at 1373 K by the measurement of porosity and the effect of reaction temperature on the tensile strength was suggested.21 From the previous research, it is clear that the reaction mode of coke is different, according to the type of reactant gas or reaction temperature. It is believed that the difference of the reaction mode would cause the difference of porosity distribution and macrostrength. The current research aims to study the effect of coke gasification with CO2 and H2O on coke strength in the Received: June 2, 2015 Revised: September 8, 2015 Published: September 8, 2015 6849
DOI: 10.1021/acs.energyfuels.5b01235 Energy Fuels 2015, 29, 6849−6857
Article
Energy & Fuels Table 1. Ultimate and Proximate Analysis of Cokes proximate analysis (wt %, db) Coke 1 Coke 2 Coke 3 a
ultimate analysis (wt %, daf)
FC
VM
Ash
C
H
N
S
Oa
CSR
CRI
87.63 88.58 86.92
1.02 0.88 2.87
11.35 10.54 10.21
97.05 97.52 96.45
0.63 0.65 0.73
0.94 1.05 1.04
0.52 0.48 0.56
0.86 0.30 1.22
67.5 67.9 44.7
24.3 23.7 41.2
By difference.
temperature range of 1100 to 1500 °C in terms of image analysis, tumbling, and tensile tests. Furthermore, it was undertaken to establish a relationship between the porosity distribution and macrostrength of reacted cokes by regression analysis.
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coke was calculated by the ratio of pore total areas. Only pores with a size above 1000 μm2 were measured to calculate the porosity because it is believed that smaller pores have a marginal effect on the mechanical properties of cokes.16 The porosity of each bulk coke was measured at a distance from the surface of coke, d, to evaluate the pore geometry. Evaluation of Fine Powder Formation. The tumbler test of the coke sample was carried out to investigate the fine powder formation in terms of the tumbling time by Turbula-mixer (WAB Turbula T2F Mixer). One coke was put into a steel crucible (40 mm-ID and 125 mm-height) to evaluate the fine powder formation. The tumbler was rotated 3-dimensionally, and the coke was broken by the impact. After an interval of 5 min, the coke and fine powder were sieved with a 10 mm-sieve and the weights of residue and fine powder were measured. Tumbler testing was repeated for the tumbling time of 30 min. The fine powder formation was evaluated by the fraction of coke particle with size smaller than 10 mm after the tumbler test. The tumbler test was performed several times in an experimental condition to reduce the error resulting from the inhomogeneous properties of a sample. The average value measured for 12 samples in an experimental condition was used to evaluate the degree of fine powder formation. Evaluation of Tensile Strength. The tensile strength was measured by testing 12 cylindrical cokes on an Instron universal testing machine. It lays down the coke sample and presses it across a diagonal direction. The rate of machine head speed was 3 mm/min. The tensile strength of the cylindrical material was calculated using eq 4 below:
EXPERIMENTAL DETAILS
Materials Preparation. Cokes used in the current study were obtained from the coking plant of POSCO for BF operation, and their analyses of constituents are shown in Table 1. Two industrial cokes for actual operation (Coke 1 and Coke 2) and one coke of high reactivity (Coke 3) were used to test in this study. Experimental Procedure. Cokes weighing approximately 5 g with a particle size of 20 mm were used in the current experiments. Cokes were charged into an alumina tube (22 mm-ID), and the gas mixture was directly injected onto the cokes through an alumina tube (16 mmID). A horizontal tube furnace (Lenton furnace, LTF-17) was employed to heat the cokes at the heating rate of 5 °C/min up to 1100, 1200, 1300, 1400, and 1500 °C in an Ar atmosphere. Then, the reaction gas mixture (Ar + CO2 or Ar + H2O) was introduced into the furnace at the flow rate of 5 L/min because the critical gas flow rate excluding the effect of boundary layer diffusion on the surface of coke to the gasification reaction was determined to be higher than 5 L/min according to the preliminary experiment. In the experiments, each partial pressure of CO2 and H2O was 0.1. After the temperature indicated a stable value, the reactant gas was introduced to react with coke for 10 min. While the gasification reaction was performed, the product gas was analyzed by quadruple mass spectrometry (QMS) to evaluate the gasification rate. In order to perform the experiments of coke gasification with H2O, Ar gas was passed through a water bath for hydration. The saturation vapor pressure was evaluated using the relation between water vapor pressure and temperature.22 The rate of carbon gasification (RCS) can be calculated using the difference of carbon concentration between the reactant and product gases as described by eq 1:14 RCS (1/s) = ([CO]out − [CO]in + [CO2 ]out − [CO2 ]in )
τ=
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RESULTS AND DISCUSSION Effect of Gasification on Macroporosity of Coke. In order to evaluate the change of pore geometry by the gasification reaction of coke, the cross sectional images (50×) of coke were scanned before and after the gasification reaction for 10 min for the parts of coke which are about 2−4 mm apart from the surface of coke as shown in Figure 1. Black areas represent pore, and white areas indicate carbon. In the case of coke gasification with CO2, pores generally grew and the boundary of pores formed a round shape in the progress of gasification. However, in the case of coke gasification with H2O, the pores of coke look similar to that of the original coke despite some differences resulting from the inhomogeneity of coke. The images showed that the pores rarely grew with increasing temperature. In order to quantitatively analyze the change of porosity, the average porosities of reacted coke at a distance from the surface of coke were investigated in the temperature range of 1100 to 1500 °C as shown in Figure 2 and the standard deviation was shown in Table 2. The porosity of the original cokes was in the range of 26% to 38% and was averaged to be 33.2%. In the case of Coke 1 and Coke 2, the porosity showed similar values regardless of the distance from the surface of coke which was reacted CO2 at low temperatures (
