MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

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concentration on N 2 BET surface area and CO 2 surface area is not obvious, except that the Middleburg char #3 has a 20% higher N 2 BET surface area than the Middleburg char #1. Surface area (m 2 /g) 500 400 300 200 100 0 N2 BET surface area CO2 surface area char #1 char #2 char #3 char #4 Figure A.4. Surface areas of Koonfontain chars obtained in four different reactor conditions Surface area (m 2 /g) 500 400 300 200 100 0 N2 BET surface area CO2 surface area char #1 char #2 char #3 char #4 Figure A.5. Surface areas of Middleburg chars obtained in four different reactor conditions. 158
This observation is somewhat surprising, since Gale et al. (1995) performed experiments to show that the presence of steam in high heating rate experiments (drop tubes and FFB's) significantly increased the N 2 BET surface areas of chars prepared in such reactors. Reasons for this seeming discrepancy are not clear at this time. TGA Reactivities: Effects of Reactor Conditions The reactivities of Koonfontain chars and Middleburg chars prepared in these four conditions were measured using a thermogravimetric analyzer (TGA) at 550 º C in 10% oxygen. The results are shown in Figure A.6 and Figure A.7. Figure A.6 indicates that the Koonfontain char #2 and char #4 collected at 1 inch have similar TGA reactivities; char #1 and char #3 collected at 1 inch also have similar TGA reactivities. However, chars #1 and #3 have higher TGA reactivities than chars #2 and #4. This means that the presence of oxygen in the preparation environment reduces the TGA reactivity of the char. The effect of reduced steam concentration in the char preparation environment is not significant for the Koonfontain chars. Similar trends with post-flame O 2 environment are observed for the Middleburg chars, except that char #3 has notably higher reactivity than that of char #1, indicating an effect of post-flame steam concentration for this coal. This is consistent with the fact that char #3 has higher N 2 BET surface area than char #1. The reason why the reduced steam concentration affects the N 2 BET surface area and TGA reactivity of the Middleburg char, but not the same properties of the Koonfontain char, is not clear at this point. 159
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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
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 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: Apparent densities 1.00 0.75 0.50 0
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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MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

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