MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...
MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ... MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...
ABSTRACT MODELING CHAR OXIDATION AT ATMOSPHERIC AND ELEVATED PRESSURES USING AN INTRINSIC LANGMUIR RATE EQUATION AND AN EFFECTIVENESS FACTOR Jianhui Hong Chemical Engineering Department Doctor of Philosophy A global n-th order rate equation is often used to model char oxidation rates at atmospheric pressure. However, it was recently shown that this approach was inadequate for modeling char oxidation rates as a function of total pressure. It is generally thought that in order to model the effects of total pressure, an intrinsic modeling approach (i.e., pore diffusion effects are accounted for explicitly) is required, and a Langmuir- Hinshelwood type expression is needed. The objective of this project was to develop a model that can be used to explain and unify char oxidation rates over wide ranges of experimental conditions (including temperature, total pressure, oxygen mole fraction, particle size, etc.) without excessive computational efforts. In this project a new High Pressure Carbon Burnout Kinetics (HP-CBK) model was developed on the basis of the CBK model by Hurt and his co-workers. The HP-
CBK model uses: 1) an intrinsic Langmuir rate equation rather than global n-th order kinetics; 2) an analytical solution of the effectiveness factor for the Langmuir rate equation with a correction function (developed in this project) to improve its accuracy; 3) a pore structure model for calculation of the effective diffusivity, taking into account both Knudsen diffusion and molecular diffusion; and 4) general correlations for Nusselt and Sherwood numbers, which allow the HP-CBK model to be used for both entrained-flow, pulverized char oxidation and large-particle combustion in fixed beds. The HP-CBK model was evaluated by comparison with five sets of experimental measurements: 1) graphite flake oxidation data; 2) rough sphere combustion data; 3) large particle oxidation data; 4) pulverized char drop-tube data, and 5) TGA and FFB data from this study. Results showed that the HP-CBK model was able to quantitatively explain: 1) the effects of temperature, total gas pressure, oxygen mole fraction, particle size and gas velocity on reaction rates, and 2) the change of reaction order with temperature and oxygen partial pressure. Therefore, the Langmuir rate equation, when used with the appropriate effectiveness factor, seems to be satisfactory for modeling char oxidation over wide ranges of experimental conditions.
- Page 1 and 2: MODELING CHAR OXIDATION AS A FUNCTI
- Page 3: BRIGHAM YOUNG UNIVERSITY As chair o
- 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 * (%)
CBK model uses: 1) an intrinsic Langmuir rate equation rather than global n-th order<br />
kinetics; 2) an analytical solution of the effectiveness factor for the Langmuir rate<br />
equation with a correction function (developed in this project) to improve its accuracy; 3)<br />
a pore structure model for calculation of the effective diffusivity, taking into account both<br />
Knudsen diffusion and molecular diffusion; and 4) general correlations for Nusselt and<br />
Sherwood numbers, which allow the HP-CBK model to be used for both entrained-flow,<br />
pulverized char oxidation and large-particle combustion in fixed beds. The HP-CBK<br />
model was evaluated by comparison with five sets of experimental measurements: 1)<br />
graphite flake oxidation data; 2) rough sphere combustion data; 3) large particle oxidation<br />
data; 4) pulverized char drop-tube data, and 5) TGA and FFB data from this study.<br />
Results showed that the HP-CBK model was able to quantitatively explain: 1) the effects<br />
of temperature, total gas pressure, oxygen mole fraction, particle size and gas velocity on<br />
reaction rates, and 2) the change of reaction order with temperature and oxygen partial<br />
pressure. Therefore, the Langmuir rate equation, when used with the appropriate<br />
effectiveness factor, seems to be satisfactory for modeling char oxidation over wide ranges<br />
of experimental conditions.
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