MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

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as high as that of char #2, both on a per-gram-of-carbon and on a per-external-surface-area basis. This would indicate that the presence of steam decreases the high temperature reactivity of the Koonfontain char. The factor is the ratio of the observed burning rate to the maximum possible burning rate calculated under film diffusion limitations. The factor was calculated for char #2 and #4 in three different intervals: 1~2”; 2~4”; and 4~6”. The factor is less than 0.10 in most of the cases, as shown in Figure A.13, except for char #4 in the 2 to 4” interval. Therefore, in both conditions, film diffusion limitations are minimal. % of daf mass remaining (m/m o) 100 75 50 25 0 0 20 40 time (ms) 164 char #2 char #4 Figure A.10. Percentage of daf mass remaining of Koonfontain chars vs. residence time during char oxidation in condition #2 and #4. The TGA reactivities of the high temperature Koonfontain chars, collected from the FFB under condition #2 and #4 at 1”, 2”, 4” and 6”, were measured at 550 °C in 10% oxygen. These reactivities are shown in Figure A.14. It can be seen that the chars 60 80
collected in the #4 reactor condition at 2”, 4” and 6” have much lower TGA reactivities than chars collected in the #2 reactor condition at the same reaction height. Note that high temperature reactivities of chars from the #4 reactor condition were higher than observed for chars from the #2 reactor condition (see Figures A.11 and A.12), but that the opposite trend is observed in Figure A.14. This could be because char #4 has a higher activation energy than char #2, or because char #4 has a pore structure that enhances the accessibility of the internal surface area at high temperatures (e.g., the feeder pores are larger). Rate (gC /g C remaining /sec) 10.0 7.5 5.0 2.5 0.0 char #2 char #4 1" to 2" 2" to 4" 4" to 6" 1" to 6" overall Figure A.11. High temperature reactivities (based on carbon available) of Koonfontain chars from reactor conditions #2 and #4. 165
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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
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Reactor Head Flow Straightener Reac
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the large size of the particle, and
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taking into account convection, rad
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2.5x10 -4 2 /sec) 2.0 1.5 Rate (g/c
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Table 7.5. The Experimental Conditi
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The burnout and particle velocity d
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The HP-CBK was used to predict the
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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 and 168: times at heights of 1, 2, 4, and 6
Page 169 and 170: analysis. The char reactivities (in
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: The N 2 BET surfacea areas and H/C
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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MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

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