Article pubs.acs.org/EF
Reduction and Desulfurization of Petroleum Coke in Ammonia and Their Thermodynamics Jin Xiao,†,‡ Yanbing Zhang,*,† Qifan Zhong,† Fachuang Li,† Jindi Huang,† and Bingjie Wang† †
School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, People’s Republic of China National Engineering Laboratory of Efficient Utilization of Refractory Nonferrous Metal Resources, Changsha, Hunan 410083, People’s Republic of China
‡
ABSTRACT: NH3-reducing desulfurization was demonstrated using a high-sulfur coke calcining desulfurization experiment at 1000 °C, and the sulfur chemical reactions of the three aromaticities of thiophene were studied during desulfurization. Results showed that NH3-reducing desulfurization could significantly remove sulfur in the coke, and the best operation temperature was approximately 800 °C, at which more than 80% of organic sulfur could be removed. The physical and chemical indicators of petroleum coke after desulfurization were not affected. Thermodynamic calculation results showed that the desulfurization reaction was more favorable at higher temperatures. However, the reaction was also affected by other factors; thus, the desulfurization efficiency decreased when the desulfurization temperature exceeded 800 °C. This paper presents a specific explanation of this phenomenon.
1. INTRODUCTION Petroleum coke is the residue of heavy oil cracking during oil refining. It is the main raw material in the process of carbon products, especially carbon anode for aluminum electrolysis.1 In recent years, the quality of petroleum coke has been significantly decreased by increasing sulfur content, which consequently negatively affected the use of carbon products. For example, carbon anodes with high sulfur contents not only reduce anode quality (increased resistivity, decreased antioxidant activity, etc.) but also etch anode production equipment and anode steel claw, eventually increasing electrolytic cost. Moreover, coke sulfur appears in the form of SO2 during calcination and electrolysis, and this form is a serious air pollutant.2 Therefore, reducing the sulfur content in the aluminum electrolysis industry to an acceptable level is of great economic and environmental significance. Sulfur in petroleum coke appears in two forms: organic and inorganic. The content of organic sulfur is up to 90%, which presents in the form of the five-membered ring structure thiophene. Organic sulfur has three main types of structures, namely, thiophene, benzothiophene, and dibenzothiophene.3 Thiophene is difficult to remove, even when heated to 1300 °C, because of its high thermal stability. El-Kaddah and Ezz4 have shown that the desulfurization efficiency of petroleum coke reaches 80% at 1400 °C. Meanwhile, Hassan5 reported that the desulfurization efficiency of petroleum coke reaches 80% at 1650 °C and further increases to 88% at 1700 °C. Gillot et al.6 adopted hydrogenation desulfurization and significantly decreased the temperature; results show that the desulfurization efficiency is approximately 50% at 800 °C without heat preservation and 92% when the temperature rises to 1300 °C. Saha and Tollefson7 heated petroleum coke to 800 °C for 90 min and determined that the desulfurization efficiency reaches 86.6% after hydrodesulfurization. Mochida et al.8 found that the hydrodesulfurization efficiency is 83% at 950 °C and later revealed9 that grinding the raw material to a small particle size, © XXXX American Chemical Society
pretreating petroleum coke, and prolonging pretreatment time can further increase the desulfurization efficiency. These studies indicated that thermal desulfurization and reductive desulfurization are effective methods of desulfurization, but the former requires a high temperature (i.e., 1600 °C), which not only consumes a large amount of heat but also increases the equipment cost. The latter has a larger potential safety hazard with H2 as a reducing agent. Recent research has focused on petroleum coke desulfurization, but limited thermodynamic research concentrated on the desulfurization reaction of petroleum coke. Several studies have investigated the thermodynamics of the desulfurization of coal, which is another important raw material for carbon materials. Attar10 studied the reaction of sulfur-containing groups in coal under a hydrogen atmosphere and the secondary reaction between product gas desulfurization and fat compounds and calculated the thermodynamic data of some desulfurization reactions. Xing11 studied the mechanism of thiophene during coal pyrolysis through a series of chemical reactions producing thiophenic sulfur. Chong et al.12 applied density functional theory to study the pyrolysis mechanism of coal thiophene and used quantum chemical methods to calculate the formation of free groups. In the current paper, NH3-reducing desulfurization was adopted to desulfurize high-sulfur coke at low temperatures (
