Article pubs.acs.org/EF
Mutual Inhibition between Catalytic Impurities of Sulfur and Those of Calcium in Coke during Carbon−Air and Carbon−CO2 Reactions Jin Xiao, Qifan Zhong,* Fachuang Li, Jindi Huang, Yanbin Zhang, and Bingjie Wang School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, People’s Republic of China ABSTRACT: The effect of sulfur and calcium impurities on coke reaction was investigated by simulating petroleum coke with low-impurity pitch coke and via impurity doping. Its mechanism has often been discussed via X-ray powder diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive spectrometry. The results show that sulfur has a strong catalytic effect on both the reactivity of coke in air and CO2 in the case with no other impurity interference. Its catalysis is probably realized by triggering a reaction system. However, during the carbon−air and carbon−CO2 reactions, an obvious mutual inhibition between the catalytic impurities of sulfur and those of calcium occurred when calcium was added. Inert CaSO4 and weak catalytic CaS are likely to be the key to producing this mutual inhibition.
1. INTRODUCTION In the petroleum industry, petroleum coke is the main source of both the raw material and impurities of the carbon anode. Thus, the quality of petroleum coke has an extremely important effect on the quality of the carbon anode as well as several economic and technological indexes in aluminum production. Since the beginning of the 21st century, an increasing amount of petroleum has been contained in the resource material of sulfur in many regions of the world. In China, the S content in petroleum coke ranged mainly between 0.8 and 1.5 wt % in 2003. However, through the years, the amount has risen to 2.0−4.5 wt %.1 In view of such drastic changes in the quality of petroleum coke, the effect of S impurities on the properties of petroleum coke and carbon anode has aroused much attention and controversial discussions in the past few decades. Houston and Oye,2 on the basis of their extensive data gathered from aluminum production, summarized that the effect of S on anode reactivity is hard to define because it often changes significantly together with the varying content of impurities in the anode. Hardin and Beilharz3 and Tran et al.4 stated that an increase in the impurities in S can bring about an obvious decrease in CO2 reactivity in petroleum coke, which is used as the raw material for anodes but a dramatic increase in the air reactivity in S. Sorlie et al.5 reported that, with an increasing S content, the air reactivity of the carbon anode undergoes a changing tendency of decreasing and then increasing, whereas the CO2 reactivity of the anode experiences a consistent drop. Franca et al.,6 on the basis of a series of massive scale experiments, pointed out that, with an increasing S content, both the air and CO2 reactivities of the anode can be inhibited. Hume et al.7 said that the inhibition of S on both the S and CO2 reactivities of the anode may be performed through a particular stable structure with Na, which is achieved by suppressing the catalytic reaction of Na. Eidet et al.8 and Zhou et al.9 found that, in the process of the carbon−CO2 reaction, S has the same inhibitive effect on the reactive catalysis of Fe impurities because S can combine with Fe to produce relatively stable FeS. Engvoll,10 on the basis of his study of the effect of S on the reactive catalysis of Ca in dibenzothiophene (DBT), which was performed by substituting petroleum coke with pitch © XXXX American Chemical Society
coke and adding DBT and Ca species, concluded that S displays an obvious inhibitive effect on the reactive catalysis of CO2 of Ca. Engvoll proposed that this finding may be related to CaS formation, but no direct evidence was found. On the basis of the study by Engvoll,10 the present study conducts a systematic and profound exploration into the independent or coordinative effect of S on the reactivity of Ca to coke in pitch coke by adopting the same study method of substituting petroleum coke with pitch coke, by adding various chemical compounds that contain Ca and S to low-impurity pitch coke and combining several testing devices, such as X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS).
2. EXPERIMENTAL SECTION 2.1. Materials. In the experiment, a low-impurity coal tar pitch is chosen as the raw material in producing coke. Table 1 shows the basic characteristics of the coke sample without any dopant. The S impurities in petroleum coke exist mainly in the form of organic S asthiophenes.11 At the same time, DBT is deduced as organic S impurities and particular non-organic S impurities, such as ammonium sulfate [(NH4)2SO4] and dilute sulfuric acid (H2SO4, 30 wt %), are investigated on the basis of the method by Engvoll.10 The Ca impurities (CaX) used in the experiment include CaO, CaCO3, Ca(OH)2, CaF2, CaSO4, and CaCl2, all of which are approximately of reagent grade. 2.2. Sample Preparation. The pitch coke samples added with impurities were prepared as follows: (1) Approximately 100 g of coal tar pitch was melted using an oil bath at 200 °C. (2) Appropriate amounts of dopants were added, and the solution was mixed evenly. (3) The container with the melted pitch was placed in a furnace reactor kept at 550 °C, and the pitch was carbonized for 1 h to obtain the sample precursor. (4) The spongy parts above and below the precursor were cut (5 mm), and the rest was crushed into particles. (5) The sample precursors were calcined at 1100 °C. The temperature was kept for 1 h, and the coke samples were obtained. 2.3. Measuring the Reactivity of the Samples. 2.3.1. Reactivity Measurements. In this study, reactivity refers to the chemical Received: October 28, 2014 Revised: January 27, 2015
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DOI: 10.1021/ef502415g Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Basic Characteristics of the Pitch Coke coking value (%)
Na (ppm)
Ca (ppm)
V (ppm)
S (%)
Fe (ppm)
Si (ppm)
59.0
13
21
10
0.15
27
35
reactivity of carbon gas. The gas refers to air and CO2. The reactivity is usually determined by the method involving loss in mass. The mass of the samples were determined using an analytical balance with ±0.1 mg analytical precision. The air and CO2 reactivity of the coke samples were measured using the equipment at a constant temperature with 50 L/h air or CO2 volume, as shown in Figure 1. In the experiment,
S remained the same in the three groups of samples designated as DBTC3, NHSOC, and HSOC. The findings suggest that DBT added a specific amount of S impurity into the coke, with the S content reaching a maximum of approximately 2 wt %. Engvoll10 pointed out that such a phenomenon might be related to DBT dispersion during the process of making pitch coke. In contrast, (NH4)2SO4 and H2SO4 did not increase the S content in coke because the S that they added into the coke may have been dispersed in gaseous forms, such as SO2 or H2S, during the carbonization process. In addition, a comparative analysis of EC, DBTC1, and DBTC2 showed that, with an increasing S content, both its air and CO2 reactivities displayed an obvious increasing tendency. When the S content in the coke was increased from 0.15 to 2.05 wt %, the weight loss rates of both the carbon−air and carbon−CO2 reactions increased from 18.9 and 7.1 wt % to 26.5 and 35.0 wt %, respectively. Furthermore, a comparison of the S contents of the various samples in the air and CO2 reactivity tests revealed the following findings: No obvious variation in the S contents in the various samples was found, indicating that, during the process of air and CO2 testing, no falling off of impure S elements occurred from within the samples. The variation in the reactivity of sulfur may have taken place only in a small amount of S impurities, which were removed from the carbon chain with the consumption of the coke. As shown in Figure 2a, the dispersibility of S doped in the samples is good. The EDS analysis result shows that the S doped in the coke by adding DBT was thoroughly dispersed in the coke. As shown in panels b and c of Figure 2, given the similar electron voltage (b, 163.98 eV; c, 163.80 eV),12 the existence of S in the samples and the high-sulfur petroleum coke were the same in the XPS analysis. 3.2. Synergistic Effects of Sulfur and Calcium (CaO) on the Coke Reactivity with Different Calcium Contents. As shown in Table 3, low-sulfur (CaO-doped) and high-sulfur (CaO + DBT-doped) pitch coke samples with different Ca contents were prepared by adding different amounts of CaO and DBT into the pitch. During the preparation, when the quantity of DBT added was kept constant, the final S content in the pitch coke increased slightly with an increasing amount of added CaO. Ca in the coke seems to produce a special “fixed sulfur” effect. As shown in Figure 3, the dispersibilities of Ca and S doped in the samples are good. The air reactivity test results of the samples are shown in Figure 4a. With an increasing Ca content,
Figure 1. Structure of the testing device of reactivity. testing samples with 5 g weight and 1−1.4 mm width were placed in the reactors at 600 °C (to measure air reactivity) or 1000 °C (to measure CO2 reactivity) for 1 h. The weight loss rate of the samples indicates a high or low change in their reactivity. 2.3.2. Other Analyses. The S content in coke was measured using a coulomb HDS3000 model AI sulfur detector (Hua De Electronic, Ltd., Hunan, China), with
