the coking properties of coal at elevated pressures. - Argonne ...

from Figure 5, due to the scale, is that the time for total burnaway is the same
for each distribution since they all have the same maximum particle size of 3m.
Evaluation of C
P
Equation 27) was numericatly integrated to give the particulate oxygen
concentration C for the three distributions. The results are shown in
Figure 6. An igteresting feature of these results is the value of Cp at t=o.
According to Avedesian and Davidson 17) the value of 5 should be almost zero
for the batch fed system but it is clear from Figure 6 that this is not the case.
Equations 25), 26) and 27) are valid only for constant temperature and
pressure. These conditions are usually met in FBC's except that the particle
temperature Tp can vary during burnaway and can be appreciably higher than
the bed temperature (4,5,6). The effect of increasing Tp would be to increase
the value of k2 relative to kl and the controlling mechanism would tend
towards that of diffusion. Increasing the pressure of the system would only
have a significant effect if the combustion rate was initially dominated by
chemical kinetics.
Conclusions
A theoretical model has been developed which predicts the change in size
distribution during burnaway, burn-out times and particulate phase oxygen
concentrations as a function of original particle size distribution in a batch
fed fluidised bed.
batch fed experiment could be devised to determine the role of chemical kinetics
for a given type of coal and would thus be an aid to modelling of fluidised bed
combus tion.
Acknowledgements
If the original size distribution is accurately known a
The work described here form part of the activities of the Sheffield
Coal Research Unit sponsored by Shell Coal International and the N.C.B.. The
authors are grateful for the financial assistance of the Sponsors and wish to
point out that the views expressed here are those of the authors and not
necessarily those of the Sponsors. We are also grateful to Dr. R.G. Siddall and
Dr. P.J. Foster for many illuminating discussions.
Nomenclature
Constant defined by equation 24)
Area of fluidised bed (m2)
Rosin-Rammler constant - equation 21)
Oxygen exchange parameter - equation 12)
Oxygen concentration of fluidising air (mole/m 3 )
3
Particulate phase oxygen concentration (mole/rn )
Gas diffusion coefficient (m2/s)
constant (= l/k2)
constant (= l/kl)
Defined by equation 12)
Function of Oxygen concentration (= 2M C /~p)
O P
302