Page 305 - Dust Explosions in the Process Industries

274 Dust Explosions in the Process Industries

Table 4.5 Summary of some experimental observationsfor laminar coal dust flames in air at atmo-
spheric pressure

1. Observed flame velocities depend on the burner used.
2. Peak burning velocities range mostly from 0.05 to 0.35 m/s, depending on burner design, coal type, and particle size.
An exception was the high value of 0.86 m/s measured by Ghosh, Basu, and Roy (1957), which was attributed to the
use of a furnace with preheated walls.
3. Peak burning velocities occur at higher fuel concentration than the stoichiometric, somewhere in the neighborhoodof
the stoichiometric concentrationfor combustion of the volatile matter.The peak flame velocity increases with the
specific surface area of the coal dust.
4. The rich flammability limit occurs at higher fuel concentrationsthan that giving the peak burning velocities, as
compared to gaseous flames.
5. Decreasing coal-dust particle size increases burning velocity on the lean side of the peak but may decrease it on the
rich side. Also, smaller particles shift the peak and rich flammability limit to a leaner concentration.
6. Increasing volatiles content increases the burning velocity and slightly shifts the peak to a leaner concentration.
7. Oxygen enrichment beyond the 21 vol% in air increases burning velocity, as does the addition of methane.
8. Thicknesses of steady, laminar, coal dust flames are usually on the order of 5 mm, but larger thicknesses have been
observed, especially for larger particles at high coal dust concentrations.
9. Measured peak flame temperatures range from 1000 to 1500 K and may be correlated with coal dust concentration.
These measured temperatures may be lower than the real temperatures due to inadequate measurement
techniques.
10. In the flame front, liberated volatile matter burns rapidly in the gas phase, while there is very little heterogeneous
combustion of the char.
1 11. In traversing the flame front, the irregularly shaped solid particlessoften and become rounded and filled with blow
holes but remain about the same size.
12. A considerable amount of volatile matter remains in the char leaving the flame front, the amount being a strong
function of coal dust concentration.
13. The extent of coal devolatilization is related especially to coal dust Concentration.
14. The volatile material liberated during rapid pyrolysis in this type of flame has a higher C/H ratio than the volatile
matter liberated during proximate analysis.
15. Only small amounts of H, or CH4 are observed in the flame.
Source: Smoot and Horton, 1977.

0.3 $* WITH 1% FLUIDIZING
::
E
L
c 0.2
U
0
1
W
>
L3
z AGENT
z
a
2 0.1
0.0
0 200 400 600 800 1000 1200
COAL DUST CONCENTRATION Ig/m31

Figure 4.10 Effect of very fine Si02 fluidizing agent (Acrosil) on the burning velocity of an air sus-
pension of 10 pm, 28% volatile content Sewell coal dust (From Smoot and Horton, 7 977).