Air Reactivity of Petroleum Cokes: Role of Inaccessible Porosity Kien N. Tran and Suresh K. Bhatia* DiVision of Chemical Engineering, The UniVersity of Queensland, St. Lucia, QLD 4072, Australia Alan Tomsett Rio Tinto Aluminium, Thomastown, VIC 3074, Australia This paper presents a detailed study of the air reactivity of petroleum cokes measured at temperatures between 400 and 600 °C using a combination of characterization techniques and reactivity measurements. The microstructure of the coke was found to comprise an essentially inaccessible pore system at low temperatures of 77-273 K used in characterization, and it is more accessible to oxygen at higher temperatures of about 773 K used in oxidation. The correlation of reactivity data using the random pore model suggests that the true micropore area is significantly larger than that measured using physical gas adsorption methods. The difference in surface area can be attributed to the low kinetic energy of gas molecules at the lower temperatures of characterization; as a result, they are unable to overcome the pore mouth energy barrier. By examining the variation of coke structure with burnoff level, we find that most of the internal reaction occurs in pores in the narrow pore width range 1-2 nm. For pores greater than 2 nm, however, the surface areas remains essentially constant with burnoff level. The apparent activation energy of the coke-air reaction derived from the extracted rate constants falls in the range 145-160 kJ/mol. 1. Introduction The air reactivity of petroleum coke is known to depend on the microstructure and impurity content. 1-7 Hume has shown that metal impurities such as sodium, vanadium, iron, and nickel strongly catalyze the coke-air reaction, while sulfur decreases the reactivity by binding with the free sodium to form inactive complexes. 4 On the other hand, Rey Boero 6 and Tyler 7 showed the air reactivity is directly proportional to the available surface area of the coke. However, these studies did not distinguish between oxygen attacks on different parts of the internal surface of the coke structure during the gasification process. In a study of coal char gasification with air, Feng and Bhatia found that the micropore surfaces, particularly in pores smaller than 1 nm, are underutilized during the gasification process. 8 This behavior was partly attributed to blockage of reactive edge sites by functional groups or molecules composed of disorga- nized matter that cross-links the crystallites. These smaller micropores consist of stable basal plane sites in crystallites, which are essentially unreactive. 9 This is not the case for micropores greater than 1 nm, which are more likely to be intercrystallite spaces composed largely of reactive edge sites. There is some literature that details the modeling of the impact of coke structure on reactivity. 6 The standard air reactivity measurement employed by the aluminum smelting industry measures the ignition temperature of the coke, then converts this temperature to an air reactivity value expressed in percent- age per minute. 4,5 This method provides no information about the relationship between the structure and reactivity. The aim of this study was to investigate air reactivity of high sulfur petroleum cokes within a temperature range 400-600 °C. A number of different experimental techniques were employed to characterize the structure of reacted and unreacted cokes. The results show that the pore network is initially largely inaccessible, a problem which is overcome after about 5% conversion of the solid (also termed burnoff). In addition, the random pore model 10 is applied to correlate the reactivity with the microstructure of the coke, and it yields a consistent interpretation once the pore network becomes accessible (con- version >5%). 2. Experimental Section 2.1. Materials. A high sulfur petroleum coke (HSC) (∼3 wt % S) was used. The coke was calcined at five different temperatures, 1000, 1100, 1150, 1225, and 1300 °C, using a two-step laboratory calcination process. The green coke was first calcined using a rotary kiln to 900 °C and then flash calcined using a high-temperature oven at the final heat treatment temperature for 30 min. The physical and chemical properties of these five coke samples are listed in Table 1. Each coke sample was named as HSC-temperature. For example, HSC-1000 stands for high sulfur coke calcined at 1000 °C. In addition, a high sulfur industrial coke calcined in a gas-fired rotary kiln to 1225 °C was also included in the study. This industrial coke sample was named HSC-1225p. Sample HSC- 1225p was crushed and sieved into eight different particle size fractions, 355-500, 250-355, 212-250, 180-212, 106-180, 90-106, 75-90, and 45-75 μm, to study the variation of reactivity with particle size as well as the variation of structure with burnoff level. The particle size range 180-212 μm was used for the air reactivity measurements of the laboratory calcined cokes. 2.2. Air Reactivity Measurements. Reactivity measurements were conducted using a SETARAM (SETSYS 16/18) thermo- gravimetric analyzer (TGA). In each experiment, approximately 1-5 mg of sample was used, spread evenly as a single layer in a crucible. The reaction experiments were conducted under isothermal conditions in flowing air at 100 mL/min. During the heating period (20 K/min), the sample was protected from oxidation using high purity nitrogen. When the desired temperature was reached, the sample was held at temperature for 10 min and * To whom correspondence should be addressed. Tel: +61-7-3365- 4263. Fax: +61-7-3365-4199. E-mail: sureshb@cheque.uq.edu.au. 3265 Ind. Eng. Chem. Res. 2007, 46, 3265-3274 10.1021/ie061017e CCC: $37.00 © 2007 American Chemical Society Published on Web 11/24/2006