MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

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Rate (gC /g C remaining /sec) 2.5x10 -3 2.0 1.5 1.0 0.5 0.0 0 20 40 160 60 %Burnout char #3 char #1 char #2 80 char #4 Figure A.6. TGA reactivities of Koonfontain chars (at 1” sampling height) obtained at 550 °C in 10 mole-% O 2. Rate (g C /g C remaining /sec) 2.5x10 -3 2.0 1.5 1.0 0.5 0.0 0 20 40 60 %Burnout char #1 char #3 char #2 80 char #4 Figure A.7. TGA reactivities of Middleburg chars (at 1” sampling height) obtained at 550 °C in 10 mole-% O 2. 100 100
It is interesting to compare Figures A.4 and A.6; the TGA reactivities (the steady part of the curve) of Koonfontain chars are proportional to their N 2 BET surface areas, but poorly correlated to their CO 2 surface areas. From Figure A.5 and A.7, the variations of TGA reactivities of Middleburg chars are also well correlated to their N 2 BET surface areas but are weakly correlated to their CO 2 surface areas. It is believed that the N 2 BET surface area represents the mesopore surface area while CO 2 surface area represents the micropore surface area (Gale et al., 1995). Internal surface area is not accessible to oxygen unless the feeder pore is big enough or the reaction is slow enough to allow oxygen to penetrate the pores before it is consumed (Laurendeau, 1978). It is generally accepted that at typical TGA temperatures, the burning rate of char particles is slow enough to allow complete penetration of oxygen into all the pore structures and onto all the surface area in the particles. The data produced by this project seem to suggest: 1. At the temperature of the TGA experiments, all the surface area contributed by the mesopores is accessible to oxygen, but not all the surface area contributed by micropores is accessible. 2. In Zone I, oxygen completely penetrates bigger pore structures so that from a macroscopic perspective, a particle burns uniformly throughout the whole particle. However, oxygen fails to penetrate smaller pore structures, so that from a microscopic perspective the burning of the particle is not uniform. That is to say, complete oxygen penetration is a relative term; it occurs only for pore structures bigger than a certain size. The relative nature of complete oxygen penetration, if further substantiated, would imply that the activation energies measured in TGA 161
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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
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 and 178: Apparent densities 1.00 0.75 0.50 0
Page 179: This observation is somewhat surpri
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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