ARTICLE pubs.acs.org/EF
Effect of Pyrolysis and CO2 Gasification Pressure on the Surface Area and Pore Size Distribution of Petroleum Coke Maryam Malekshahian and Josephine M. Hill* Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive, Northwest, Calgary, Alberta T2N 1N4, Canada ABSTRACT: During pyrolysis and gasification, the pore structure of the feed material will change, and these changes affect the reaction rates. In this study, the surface area and reactivity of petroleum coke char prepared at different pyrolysis pressures were studied. In addition, the surface area and pore size distribution of petroleum coke have been monitored during gasification at various pressures. The change in these properties has been related to reactivity to determine how the physical structure of the petroleum coke influences the gasification rate. The samples were characterized using nitrogen and carbon dioxide physisorption and X-ray diffraction, while the reactivity was measured in a thermogravimetric analysis unit at 1173 K and total pressures of CO2 between 0.1 and 2.1 MPa. The surface area of char, prepared at higher pressures, is slightly higher than that prepared at 0.1 MPa. The corresponding increase in reactivity with an increasing surface area suggested that the different reactivities of chars prepared at different pressures were a result of the effect of the pressure on the surface area. In contrast to the minor effect of the pyrolysis pressure on the char surface area, an increase in pressure during gasification significantly increased pore development and, hence, surface area. Normalization of the reaction rate by the surface area indicated that the effect of the pressure on the physical characteristics of the petcoke was the main but not sole factor in the change of the reaction rate with the gasification pressure.
1. INTRODUCTION As refiners are pushed toward producing cleaner transportation fuels from poorer quality crudes, the production of petroleum coke (petcoke) is increasing as a byproduct of heavy oil upgrading units.1,2 The majority of petcoke produced in Canada is currently stockpiled on the site of the plant.1,3 Gasification, however, can be an effective way to use petcoke to produce syngas. The gasification process generally involves several steps, including pyrolysis of the feed material to produce a char, heterogeneous reactions of the produced char with the surrounding gases, and homogeneous gas-phase reactions. During the gasification process, the reactant gases must diffuse through the pores in the solid to the active sites. Reaction at these sites produces product gases that must then diffuse back through the pores to the gas phase. The pore structure of the solid affects the accessibility of the active sites and, thus, the reaction rate. The pore structure of the feed material changes during the pyrolysis step because of the release of volatile components and the effect of heat treatment on the crystal structure of the char. In addition, the internal structure (porosity, pore size, and pore volume) and transport properties (diffusivity and tortuosity) of the solid change continually during gasification. Knowledge of the variation in the pore structure during pyrolysis and gasification may be helpful for understanding the gasification kinetics, which are required for the design and analysis of gasification processes. The variation in surface area during gasification has been used to explain changes in the reaction rate with conversion.47 For example, some reactivity profiles have a maximum rate at a specific conversion that corresponds to a maximum in the surface area.6,7 As the reaction proceeds, the pore surface area increases as carbon is consumed. Eventually, however, the pores merge, and pore walls collapse, r 2011 American Chemical Society
resulting in a decrease in the pore surface area.4,7 The overall change in the surface area is a balance between the pore creation and pore collapse. Although the change of the reaction rate with conversion can be qualitatively explained by the variation of the surface area during gasification, the reactivity is not necessarily constant after normalization by the total surface area, measured by physisorption. Several groups have suggested that heterogeneous gasification reactions occur on the active sites at the edges of carbon crystallites.812 These edge carbon atoms are more reactive than basal plane carbon atoms and, thus, form stronger bonds with chemisorbed oxygen. In addition, mineral material from the solid, such as alkalis, tend to accumulate near the edge sites, and these materials can catalyze the gasification reactions.8 If the change in the total surface area, as measured by nitrogen physisorption, during the reaction is proportional to the change in the number of edge sites, then the gasification rate normalized by the total surface area will be a constant value.5,6,13 Normalization by the total surface area will not result in a constant value if the surface area probed by nitrogen at 77 K is different from the surface area accessible to the reactant gas at the reaction temperature.810 The change in the surface area and pore volume during gasification depends upon the structure of the original feedstock and the conditions of pyrolysis and gasification. For instance, the pyrolysis pressure influences the swelling properties, morphology, and thermal-plastic properties of the char,14,15 which in turn influence the reactivity of the char.1419 The relationship between the pyrolysis pressure and reactivity is not universal. Received: August 12, 2011 Revised: October 2, 2011 Published: October 03, 2011 5250
dx.doi.org/10.1021/ef201231w | Energy Fuels 2011, 25, 5250–5256
Energy & Fuels
ARTICLE
That is, some studies have reported a decrease in reactivity with an increasing pyrolysis pressure,14,19 while other studies have reported a maximum17,18 or minimum16,17 in reactivity with the pyrolysis pressure. Although many studies have investigated the effect of the pyrolysis pressure on char (from materials other than petcoke) reactivity, there are limited studies that investigate how the pressure during gasification affects the pore structure.20 Our previous work21 has shown that the rate of CO2 gasification of petcoke char, prepared at atmospheric pressure, increased with an increasing CO2 partial pressure. In this work, we investigated whether the increase in reaction rate was due to an increase in the surface area and pore volume of the feed at an increasing pressure. Both the pressure during pyrolysis and the pressure during gasification were varied, while the physical properties of the feed (either petcoke as received or char) were monitored. The gasification rate was normalized by both the total surface area, measured by N2 adsorption, and the micropore surface area, measured by CO2 adsorption, to determine if the increase in the surface area was the sole factor in the increased reactivity at higher pressures.
Table 1. Proximate, Ultimate, and Mineral Analyses of Petcoke (300600 μm) and Surface Area Analysis of Petcoke and Char, Prepared at 1173 K and 0.1 MPa
2. EXPERIMENTAL SECTION
rather than char. Because a larger sample size (∼100 mg) was required for the physisorption analysis, these experiments were performed in a flow-through platinum crucible to ensure uniform char conversion through the bed. To obtain reaction rate data, additional experiments were performed under the same conditions but with ∼15 mg of petcoke to ensure that mass-transfer limitations do not govern the rate. 2.3. Characterization. Nitrogen adsorption at 77 K and CO2 adsorption at 273 K were performed with a Micromeritics micropore analyzer (Tristar 3000). The samples were pretreated by heating to 523 K in a vacuum degassing unit for 3 h. The surface area, and pore size distribution of the petcoke char samples were determined by applying the non-local density functional theory (NLDFT) method for carbon with slit-shaped pores to the N2 and CO2 adsorption data. A detailed description of this method can be found elsewhere.22,23 Briefly, NLDFT is a molecular-based statistical thermodynamic theory, relating the adsorption isotherm to the microscopic properties of the system.22 In this method, for a specific adsorbateadsorbent system, theoretical isotherms are calculated for individual pores of different sizes. These isotherms are combined to provide the best fit to the experimental data.24 Unlike traditional adsorption theories, such as the Brunauer EmmettTeller (BET) theory, the NLDFT uses data from the complete isotherm, and thus, the resulting surface area incorporates contributions from micropores to macropores.22 X-ray diffraction (XRD) analysis of the petcoke as received and char samples was conducted in a Rigaku Multiflex X-ray diffractometer. Cu Kα1 radiation (λ = 1.540 56 Å) at a 40 kV tube voltage and a 40 mA tube current with a scanning speed of 2°/min was used. The petcoke was characterized for volatile matter, ash, and fixed carbon in the TGA using the American Society for Testing and Materials (ASTM) D5142 standard. The ultimate analysis was determined in an elemental analyzer (Perkin-Elmer 2400 Series II CHNS/O). The mineral analysis of ash (Loring Laboratories Ltd.) was performed by an inductively coupled plasma (ICP) spectrometer (Thermo Scientific 6000 ICAP), after multiacid digestion of the ash, prepared at 1023 K.
2.1. Effect of the Pyrolysis Pressure. Petcoke, supplied by Suncor Energy, was crushed in a ball mill and sieved to obtain petcoke particles with sizes between 300 and 600 μm. Char was prepared by heating these sieved petcoke particles in a high-pressure thermogravimetric analyser (TGA, Cahn Thermax 500) to 1173 K at a heating rate of 25 K/min under N2 at different pressures of 0.1, 0.5, 1.0, 1.4, and 2.1 MPa. The char was held at 1173 K for 1 h before being cooled in N2 over 2 h. Particle sizes of 300600 μm were required to be able to use a flowthrough platinum crucible (hole size of 149 μm) in the TGA. Part of the char samples prepared at different pressures was used for characterization, while part was used for reactivity measurements. The reactivity measurements were performed in the TGA at 0.1 MPa with approximately 15 mg of char in a quartz closed-bottom crucible. The system was heated at 25 K/min to 1173 K in flowing N2 (350 mLN/min; all flows were controlled by mass flow controllers). The gas was switched from N2 to CO2 once the desired temperature was reached and the mass signal had stabilized (typically 30 min after reaching 1173 K). The mass of sample was recorded during the reaction and the carbon conversion (X) and reaction rate (Rm) were calculated by m0 m m0 mash
ð1Þ
1 dX ð1 XÞ dt
ð2Þ
XðtÞ ¼
Rm ¼
where m is the mass at time t, m0 is the initial mass, and mash is the mass of ash in the sample. The reaction rates of the chars prepared at different pyrolysis pressures were compared at 5% conversion, which is close to the initial reactivity before the physical properties of the chars may have changed substantially but at a sufficiently high conversion to avoid the mass fluctuations that resulted from switching the gas from N2 to CO2. 2.2. Effect of the Gasification Pressure. For this study, the petcoke was again sieved to sizes between 300 and 600 μm and then gasified in the TGA with 100% CO2 at 1173 K and at pressures of 0.1, 0.5, 1, and 1.4 MPa to several different conversions up to 40%. The petcoke was gasified in CO2 after it was heated in N2 to 1173 K. After the desired conversion was reached, the sample was cooled in N2 over 2 h and then characterized with physisorption, as described in section 2.3. For clarity from the first set of experiments in which the pyrolysis pressure was studied, these samples will be referred to as gasified petcoke
proximate analysis (wt %, dry basis) volatile matter
9.8 ( 0.4
SiO2
36.67
ash
3.1 ( 0.8
Al2O3
26.06
fixed carbon
86.8 ( 0.5
TiO2
4.19
ultimate analysis (wt %, dry basis)
Fe2O3
7.66 4.70 2.02
C (%wt) H (%wt)
84.1 ( 0.1 3.8 ( 0.1
CaO MgO
N (%wt)
1.8 ( 0.1
Na2O
1.87
S (%wt)
6.5 ( 0.2
K2O
3.52
surface area (m2/g)
a
mineral analysis of ash (wt %)
P2O5
0.35
petcoke, N2 adsorption
0.5
SO3
2.20
char, N2 adsorption
0.4
V2O5
6.38
petcoke, CO2 adsorption
82
NiO
1.53
char, CO2 adsorption
7
undetecteda
2.85
The fraction of the ash that could not be identified.
3. RESULTS AND DISCUSSION 3.1. Effect of the Pyrolysis Pressure. Initially experiments were performed to determine how the pyrolysis pressure impacted the pore structure and reactivity of the char. The proximate, ultimate, and ash analyses of petcoke (300600 μm) and surface area analysis of petcoke and char, prepared at 1173 K and 5251
dx.doi.org/10.1021/ef201231w |Energy Fuels 2011, 25, 5250–5256
Energy & Fuels
Figure 1. XRD patterns of petcoke and char, prepared at 1173 K and 0.1 MPa.
0.1 MPa, are presented in Table 1. For petcoke, the surface area measured by N2 adsorption was significantly lower than that measured by CO2 adsorption, suggesting that the petcoke contains mainly micropores. N2 adsorption generally probes mesopores (pore diameters between 2 and 50 nm) and macropores (>50 nm) because the slow diffusion of N2 at 77 K limits the access of N2 to micropores (
