MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

More documents

Recommendations

Info

Table 7.3. Conditions of the Large Particle Experiments Ptot PO2 Vg Tg (K) dpo x (atm) (atm) (m/s) (mm) Base 0.85 0.18 0.32 1050 8.00 21% 1 * 0.08 2 1.28 3 0.85 0.12 14% 4 0.85 0.06 7% 5 3 0.63 21% 6 5 1.05 21% 7 3 0.21 7% 8 5 0.21 4.2% 9 6.35 10 9.16 11 1200 12 † 825 * A blank cell means that the value is maintained at the baseline condition; † Condition excluded due to prolonged heating-up period. The particle center temperatures were measured for three conditions (base condition, and conditions 11 and 12) using a type-S thermocouple inserted in a small hole drilled approximately to the center of the particle and attached to the particle with a small amount of epoxy. For all of these three conditions, the temperature profiles all show a characteristic drop near the end of combustion (at about 85% daf burnout). This drop of temperature is consistent with the near-extinction behavior observed by Hurt and Davis (1994). The main interest of this study is the reaction rates before the near-extinction stage, and therefore average reactivities were determined for the 10-70% burnout region (the reaction rates before 10% burnout were also discarded since the particles were still heating up at early burnout). In addition, the temperature profile of condition 12 (T g = 825 K) showed that the particle was heating up continuously until the near-extinction behavior began. This prolonged heating period is due to the very low gas temperature and 98
the large size of the particle, and makes this condition different from all other conditions. Therefore, this one condition was excluded in this study. The particle reaction rates were originally reported as normalized mass rates (dU/dt) vs. burnout (B). The unburned fraction (U) and burnout (B) are related by: B = 1− U (7.5) Notice that both U and B are on a dry ash-free basis. The normalized mass rates (dU/dt) were converted to mass rates per unit external surface area as follows: 1) For each experimental condition, three values were obtained from the dU/dt curve at B = 20, 40, and 60%, respectively. 2) The mass release rates (dm/dt) were calculated from the normalized mass release rates (dU/dt) by: dm dt = m d(m/ mco ) dU co = mco dt dt where m co is the initial mass of the carbonaceous material in the char. Notice that there was 11.2% ash in the Pittsburgh coal. For the baseline condition, the coal particle had an 99 (7.6) initial mass of 0.2 g and thus had 0.0224 g ash. After devolatilization the char had a mass of 0.11 g and the mass of ash was assumed to be unchanged (0.0224 g). The initial mass of carbonaceous material could be obtained by subtraction: m co = 0.11 – 0.0224 = 0.0876 g. 3) The values of diameter at different burnouts were estimated assuming that the particle density remains constant with burnout. This assumption is supported by the measurements by Mathias (1996). The diameter can be calculated from (d/d o) 3 = (m + m ao)/(m o + m ao) (7.7) where m ao is the initial mass of ash. 4) The mass fluxes at the external surface were calculated from the mass release rates (dm/dt) and the external surface area ( d p 2 ). It was
Page 1 and 2:
MODELING CHAR OXIDATION AS A FUNCTI
Page 3 and 4:
BRIGHAM YOUNG UNIVERSITY As chair o
Page 5 and 6:
CBK model uses: 1) an intrinsic Lan
Page 7 and 8:
Table of Contents List of Figures..
Page 9:
Appendices.........................
Page 12 and 13:
Figure A.2. Mass releases of the Ko
Page 14 and 15:
Table 7.6. Parameters Used in Model
Page 16 and 17:
Ed activation energy of desorption,
Page 18 and 19:
vc the volume of combustible materi
Page 21 and 22:
Background 1. Introduction The rate
Page 23:
the CBK model developed at Brown Un
Page 26 and 27:
Zone III rate ∝ C og E obs → 0
Page 28 and 29:
coal-general kinetic rate constants
Page 30 and 31:
Boundary Layer Diffusion The molar
Page 32 and 33:
= q obs q max The factor can be use
Page 34 and 35:
where k 1 and K are two kinetic par
Page 36 and 37:
particle can therefore be convenien
Page 38 and 39:
This is the first time that the gen
Page 40 and 41:
Data of Mathias Mathias (1996) perf
Page 42 and 43:
urn with shrinking diameters, and t
Page 45 and 46:
3. Objectives and Approach The obje
Page 47 and 48:
Introduction 4. Analytical Solution
Page 49 and 50:
Task and Methodology Task One of th
Page 51 and 52:
2 [ (i +1) − (i − 1)] i b = −
Page 53 and 54:
Table 4.1. The Relative Error * (%)
Page 55 and 56:
The resulting observations regardin
Page 57 and 58:
correction. The values of functions
Page 59 and 60:
Table 4.6. The Relative Error* (%)
Page 61 and 62:
Table 4.8. The Relative Error* (%)
Page 63 and 64:
general asymptotic solution. An arc
Page 65 and 66:
5. Theoretical Developments The int
Page 67 and 68: order of a reaction is usually dete
Page 69 and 70: nobs = 1 (KCs ) 2 2 1 [KCs − ln(1
Page 71 and 72: The observed reaction order in Zone
Page 73 and 74: Bulk Diffusion vs. Knudsen Diffusio
Page 75 and 76: where D K is in cm 2 /sec, r p is t
Page 77 and 78: where T p is in K, P is in atm. The
Page 79 and 80: Both of these assumptions are argua
Page 81 and 82: 2 r obs ′ − [kD Pog + k d + kD
Page 83 and 84: oxygen partial pressure (Suuberg et
Page 85 and 86: Farrauto and Batholomew (1997) prop
Page 87: assumes a homogeneous, non-interact
Page 90 and 91: Single-Film Char Oxidation Submodel
Page 92 and 93: where and An energy balance is used
Page 94 and 95: where is the empirical burning mode
Page 96 and 97: calculation uses a 7 × 7 × 7 matr
Page 98 and 99: HP-CBK Model Development The HP-CBK
Page 100 and 101: Effective Diffusivity The major obs
Page 102 and 103: where r p1 and r p2 are the average
Page 104 and 105: where r p1 is the macro-pore radius
Page 107 and 108: 7. Model Evaluation and Discussion
Page 109 and 110: experiments are non-porous, the rat
Page 111 and 112: and 2850 K). For consistency with t
Page 113 and 114: The value of the roughness factor w
Page 115 and 116: = S int S ext D e r p a 2 2M C M O2
Page 117: Reactor Head Flow Straightener Reac
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 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 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
show all

MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

Create successful ePaper yourself

Delete template?

Save as template?