ARTICLE pubs.acs.org/EF
Kinetic Analysis of CO2 Gasification of Petroleum Coke at High Pressures Maryam Malekshahian and Josephine M. Hill* Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada. T2N 1N4 ABSTRACT: Determining the intrinsic gasification rates at high pressures is essential for designing and modeling high-pressure technologies. In this study, the intrinsic kinetics of CO2 gasification of petroleum coke was investigated. The kinetic experiments were performed in a high-pressure thermogravimetric analysis unit at different temperatures (11731248 K) and pressures (0.12.4 MPa). Grain, volume, and random-pore models were applied to the experimental results to determine the kinetic parameters. The random-pore model provided the best fit to the data and was used to determine the kinetic parameters such that the activation energies and reaction orders could be determined. The reaction rates were described by a power law model that was halforder (0.49 ( 0.02) in CO2 pressure. Similar activation energies, 260 ( 24 and 254 ( 12 kJ/mol, were determined for pressures of 0.1 and 1.4 MPa, respectively, indicating similar reaction mechanisms for these pressures.
1. INTRODUCTION There is a substantial advantage to gasifying under pressure because of savings in compression energy, reduction in the equipment size,1 and an increase in the intensity of the reaction and feed throughput.2 The pressure in a gasifier is selected based on the requirements of the downstream processes. For instance, in an integrated gasification combined cycle plant, a pressure range of 1.52.5 MPa is used based on the requirements of the downstream turbines.2 A good understanding of the intrinsic kinetics of heterogeneous reactions is the key factor in the design and analysis of gasification processes. To determine the intrinsic reaction rates, experiments must be performed at conditions relevant to industrial processes and, as such, an investigation of the effect of the operating pressure on the gasification rate was undertaken in this study. The effect of the pressure on CO2 gasification of different types of chars has been studied by changing the total pressure with constant CO2 compositions37 or by changing the gas composition at a constant total pressure.810 On the basis of these studies, the behavior of the reaction rates at high pressures appears to depend on the type of char. Therefore, there is no unified interpretation of gasification kinetics of various chars, and it is not always possible to predict the reactivity behavior of a particular char from that of other chars. For instance, several studies35 showed a linear increase of the reaction rate with increasing pressure up to a certain pressure, at which a plateau was reached. These studies used the LangmuirHinshelwood equation, assuming an oxygen-exchange mechanism for CO2 gasification,11 to describe this behavior. However, some other studies6,7 observed a different rate variation at higher pressures, such that no saturation asymptotes were obtained in the studied pressure range. Therefore, these authors6,7 suggested new reaction mechanisms for CO2 gasification at high pressures. It was also reported12,13 that the response to the pressure variation was different among different coal ranks. This variation in the reactivity behavior at high pressures was explained as being the result of a balance between the influences of the reactivity of the base char, the formation of secondary char deposition, and the melting of char at higher pressures.13 Although high-pressure char gasification has been studied for a wide range of coal char types, kinetic studies of gasification with r 2011 American Chemical Society
petroleum coke (petcoke) at high pressures are scarce in the literature. The aim of the present work is to study the effect of the total pressure, CO2 concentration, and temperature on the reaction rate of CO2 gasification of petcoke chars. Experiments were performed with petcoke char in a thermogravimetric analysis (TGA) unit at temperatures between 1173 and 1248 K and pressures between 0.1 and 2.4 MPa with various partial pressures of CO2. Three structural models were applied to describe the experimental data, and the intrinsic kinetic parameters were evaluated using the model of best fit.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Petcoke, supplied by Suncor Energy, was crushed in a ball mill and sieved to obtain petcoke particles with sizes of less than 90 μm. The char was prepared by heating petcoke in a fixedbed reactor to 1248 K at a heating rate of 20 K/min under N2 at atmospheric pressure and holding it at 1248 K for 1 h before cooling to room temperature over 2 h. 2.2. Characterization Methods. The petcoke and char were characterized for volatile matter, ash, and fixed carbon in a TGA unit (Cahn Thermax 500) using standard ASTM D5142. The ultimate analysis was determined in an elemental analyzer (Perkin-Elmer 2400 series II CHNS/O). The surface areas were measured by N2 physisorption at 77 K and CO2 physisorption at 273 K using the Brunauer EmmettTeller (BET) and DubininAsthakov (DA) methods, respectively. Although BET is the most common method to determine the surface area, it is not suitable for microporous materials. Therefore, CO2 physisorption analyzed by the DA method was used to determine the micropore surface area. The physisorption experiments were performed on a Micromeritics Tristar Instrument. 2.3. Reactivity Measurements. A high-pressure TGA unit (Cahn Thermax 500) was used to measure the weight loss of each sample during gasification. The kinetic studies were done isothermally at different temperatures (11731248 K) and pressures (0.12.4 MPa). In each experiment, approximately 15 mg of the char (particle size
