MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

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Bulk Density and True Density To obtain the bulk density of a coal or char, the coal or char sample was weighed using a microbalance and put into a graduated cylinder, which was then tapped until the volume of the sample did not change. The bulk density (also called tap density) is the weight of the sample divided by the volume displaced by the sample, where the volume includes micro- and macro-pores and inter-particle voids. Apparent densities were calculated from the bulk density, ρ b, by: app = b 1− bv where bv is the fraction of inter-particle void volume in the char bed, usually taken as 0.45 (White et al., 1995). True densities were measured in a Quanta Chrome helium micro-pycnometer (model MPY-2). High Temperature Reactivity The temperature profiles at the centerline of the FFB tower were measured for conditions #2 and #4 at extended sampling heights. These two measured temperature profiles were very similar at each sampling height. The difference between the two 148 (A.1) temperature profiles at any reaction height was usually about 7 K, and never greater than 50 K. Some of the differences observed in the measured temperature profiles were due to (a) the uncertainty of the thermocouple bead position (vertical and horizontal), and (b) the non-steady heat-up effects of the tower. Koonfontain chars were collected at heights of 1”, 2”, 4”, and 6” under these two conditions. The daf mass release data for these chars were determined by ICP tracer
analysis. The char reactivities (in gram C/gram C remaining/second) between 1” and 2”, 2” and 4”, 4” and 6” were calculated. These reactivities were then converted to reactivities based on external surface area using apparent densities calculated from the tap densities of these chars. Several assumption are made during the conversion of reactivities: 1) The sizes of the original coal particles range from 45 to 75 μm. The distribution of particle size is neglected and the diameter of coal particle is taken as the arithmetic mean of the lower and upper limits, 60 μm. 2) Fragmentation of coal or char particle during devolatilization or char oxidation is neglected. 3) In the calculation of the factor (actual reaction rate/the maximum possible rate dictated by diffusion), CO is assumed as the only surface product of the carbon-oxygen reaction, and the Sherwood number is taken as 2. 4) The average diameters of chars are calculated using the following equation: m m o = o ⎛ d ⎝ ⎜ d o 3 ⎞ ⎟ ⎠ 149 (A.2) where m, and d are the mass (including moisture and ash), apparent density and average diameter of a char, respectively, while m o, o and d o are the mass (including moisture and ash), apparent density and average diameter of its parent coal, respectively. Results The results of proximate and ICP tracer analysis are listed in Tables A.3-A.6. The results of elemental analysis are listed in Tables A.7-A.9. Bulk densities, true densities, and surface areas are listed in Table A.10.
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MODELING CHAR OXIDATION AS A FUNCTI
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BRIGHAM YOUNG UNIVERSITY As chair o
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CBK model uses: 1) an intrinsic Lan
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Table of Contents List of Figures..
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Appendices.........................
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Figure A.2. Mass releases of the Ko
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Table 7.6. Parameters Used in Model
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Ed activation energy of desorption,
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vc the volume of combustible materi
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Background 1. Introduction The rate
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the CBK model developed at Brown Un
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Zone III rate ∝ C og E obs → 0
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coal-general kinetic rate constants
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Boundary Layer Diffusion The molar
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= q obs q max The factor can be use
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where k 1 and K are two kinetic par
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particle can therefore be convenien
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This is the first time that the gen
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Data of Mathias Mathias (1996) perf
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urn with shrinking diameters, and t
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3. Objectives and Approach The obje
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Introduction 4. Analytical Solution
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Task and Methodology Task One of th
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2 [ (i +1) − (i − 1)] i b = −
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Table 4.1. The Relative Error * (%)
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The resulting observations regardin
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correction. The values of functions
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Table 4.6. The Relative Error* (%)
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Table 4.8. The Relative Error* (%)
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general asymptotic solution. An arc
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5. Theoretical Developments The int
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order of a reaction is usually dete
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nobs = 1 (KCs ) 2 2 1 [KCs − ln(1
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The observed reaction order in Zone
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Bulk Diffusion vs. Knudsen Diffusio
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where D K is in cm 2 /sec, r p is t
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where T p is in K, P is in atm. The
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Both of these assumptions are argua
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2 r obs ′ − [kD Pog + k d + kD
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oxygen partial pressure (Suuberg et
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Farrauto and Batholomew (1997) prop
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assumes a homogeneous, non-interact
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Single-Film Char Oxidation Submodel
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where and An energy balance is used
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where is the empirical burning mode
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calculation uses a 7 × 7 × 7 matr
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HP-CBK Model Development The HP-CBK
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Effective Diffusivity The major obs
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where r p1 and r p2 are the average
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where r p1 is the macro-pore radius
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7. Model Evaluation and Discussion
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experiments are non-porous, the rat
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and 2850 K). For consistency with t
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The value of the roughness factor w
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= S int S ext D e r p a 2 2M C M O2
Page 117 and 118: Reactor Head Flow Straightener Reac
Page 119 and 120: the large size of the particle, and
Page 121 and 122: taking into account convection, rad
Page 123 and 124: 2.5x10 -4 2 /sec) 2.0 1.5 Rate (g/c
Page 125 and 126: Table 7.5. The Experimental Conditi
Page 127 and 128: The burnout and particle velocity d
Page 129 and 130: The HP-CBK was used to predict the
Page 131 and 132: TGA and FFB Data-This Study The rea
Page 133 and 134: This equation can be derived as fol
Page 135 and 136: q = A 1p e − E 1 p / RT P os 1 +
Page 137 and 138: m obs = 0 at high temperatures) and
Page 139 and 140: Currently the correlations between
Page 141 and 142: 8. Summary and Conclusions The obje
Page 143 and 144: 0.5 due to the contribution from th
Page 145 and 146: Langmuir rate equation, the reactio
Page 147 and 148: II, in agreement with many observat
Page 149 and 150: 9. Recommendations The predictive c
Page 151 and 152: References Ahmed, S., M. H. Back an
Page 153 and 154: Essenhigh, R. H., D. Fortsch and H.
Page 155 and 156: Mehta, B. N. and R. Aris , “Commu
Page 157 and 158: Szekely, J. and M. Propster, "A Str
Page 159 and 160: Appendices 139
Page 161 and 162: Introduction Appendix A: Experiment
Page 163 and 164: detaching the flame from the burner
Page 165 and 166: To study the effects of steam, CO w
Page 167: times at heights of 1, 2, 4, and 6
Page 171 and 172: Table A.5. Moisture, Ash and ICP Ma
Page 173 and 174: Table A.9. Elemental Analyses of Fo
Page 175 and 176: temperature profile of the post-fla
Page 177 and 178: Apparent densities 1.00 0.75 0.50 0
Page 179 and 180: This observation is somewhat surpri
Page 181 and 182: It is interesting to compare Figure
Page 183 and 184: The N 2 BET surfacea areas and H/C
Page 185 and 186: collected in the #4 reactor conditi
Page 187 and 188: Rate (gC /g C remaining /sec) 1.6x1
Page 189 and 190: close to zero, the accumulated erro
Page 191: Appendix B: Errors and Standard Dev
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