1
Char Oxidation
Char Oxidation Concepts
1.
2. CO/CO2ratio
3. nth order
4.
5. T dependence
6. dpdependence
7. CO CO2in boundary
layer (2-film model)
8. energy balance / iteration
9. Thiele modulus
10. Ian Smith reactivity
correlation
11. TGA rate vs high T rate
12. Catalytic effects at low T
13. Pressure effects
14. Correlations vs. chemistry
15. Late burnout ideas
16. N-release during char
oxidation
Order of Presentation
1. Basic concepts (film diffusion, surface
reaction), dp& T dependence, CO/CO2
ratio, , nth order, , Bob Hurt parameters
2. Intrinsic reactivities, Thiele modulus,
Reade-Hecker approach
3. Catalytic effects, pressure effects,
correlations vs elemental composition,
late burnout
Concepts
• Question 1 (definitions)
– film diffusion
– surface reaction
– pore diffusion
• Question 2 (rate expressions)
– pure film diffusion control
– pure surface reaction control
• Question 3 (1st order rate expression)
– surface rxn = diffusion rate through film
– Units must match (grams of C reacted/m2s)
Temperatur
e* Rxn controlled by What happens to
the particle
Zone 1 Low
(ca. 1100 -
1300K)
Intrinsic rxn rate inside
pores
Particle burns from
inside. Particle density
decreases while
particle size remains
the same.
Zone 2 Medium
(ca. 1300 –
1600K)
Both diffusion & reaction
kinetics. Consumption
of the reactant gas
exceeds the rate of
internal diffusion. The
reactant is consumed
before it reaches the
particle core.
Particle burns from
both inside and
outside. Particle size
and density both
decrease.
Zone 3 High
(ca.1600 –
2000K and
up)**
Film diffusion controls.
Reactant gas does not
have time to diffuse into
the particle before it
reacts at the particle
surface.
Particle burns from
outside. Particle
diameter decreases
and reactivity/porosity
remain constant.
Questions 3-4
3. Derivation of Eq. 6.19
4. What if n 1
docsity.com
