MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

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