MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

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H/C mole ratio 1.00 0.75 0.50 0.25 0.00 Koonfontain Middleburg coal char #1 char #2 char #3 char #4 Figure A.15. H/C mole ratios of Koonfontain and Middleburg coals and chars prepared at four different conditions (at the 1” sampling height). A problem was found in calculating the oxygen composition in some chars. The elemental analyzer only analyzes elemental compositions of C, H, N, and S. The value of ash content from the proximate analysis is used to convert the C, H, N, and S compositions to daf basis. The oxygen content is then calculated by difference: O% = 100% - (C%+H%+N%+S%). However, negative values were sometimes obtained for the oxygen content of these chars. Several possible reasons are listed here: A. The ash content from the proximate analysis is too high. In the proximate analysis the ashing temperature is 750 °C but in the CHNS analysis the ashing temperature is 900 °C. Therefore, the ash content may be overestimated in the proximate analysis. However, the proximate analysis was repeated using an ashing temperature of 900 °C, but no change in the value of ash content was observed. B. The deviations associated with the C, H, N, S contents are accumulated when oxygen content is calculated by difference. When the true value of oxygen content is very 168
close to zero, the accumulated error can easily cause the calculated value of oxygen content to become slightly negative. C. The standard coal sample (with known composition) may have a carbon content that is too low. The CHNS analyzer works best when the coal or char sample to be analyzed has a similar composition with the standard coal sample. Additional standards of coals and cokes with higher carbon contents will be used in the future to better calibrate the instrument. D. The sulfur measured in the CHNS analysis is the total sulfur, including both inorganic and organic sulfur. Therefore, the inorganic sulfur contributes to inaccuracies in the mass balance. Regardless of the slight negative oxygen concentrations for some chars, the H/C ratio can be used with confidence since it is independent of the O 2 concentration. Conclusions The presence of oxygen in the char preparation environment was examined by performing experiments in fuel-rich and fuel-lean FFB conditions. The following effects were observed for both the Koonfontain and Middleburg coals: 1) The mass release of chars obtained in conditions with post-flame O 2 exhibited 5-7% more mass release (on an absolute daf basis) than corresponding chars produced in fuel-rich environments. This was likely due to the initiation of char oxidation in the fuel-lean reactor conditions. 2) The H/C ratio is lower in the chars produced with O 2 present, i.e., more hydrogen is lost during pyrolysis in the presence of O 2. 169
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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
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This equation can be derived as fol
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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 and 184: The N 2 BET surfacea areas and H/C
Page 185 and 186: collected in the #4 reactor conditi
Page 187: Rate (gC /g C remaining /sec) 1.6x1
Page 191: Appendix B: Errors and Standard Dev
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MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

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