MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...
MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...
MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...
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asis behind the selection between the mole fraction and concentration needs to be<br />
explored.<br />
The data of Croiset et al. were reported in an Arrhenius plot [ln(k) vs. (1/T)]<br />
assuming the char/oxygen reaction is first order. The oxygen partial pressures associated<br />
with these values of k are required in order to: (1) convert these values of k into reaction<br />
rates, and (2) to evaluate other reaction rate forms, such as Langmuir-Hinshelwood rate<br />
forms. Unfortunately, these oxygen partial pressures were not reported. Therefore, this<br />
set of data was not considered in this project.<br />
Data of MacNeil and Basu<br />
MacNeil and Basu (1998) recently studied the combustion of char under<br />
simulated, pressurized, circulating fluidized bed (PCFB) conditions at furnace<br />
temperatures between 973 and 1123 K. A quartz wool matrix was used to disperse the<br />
char. The experiment was claimed to resemble the fluidized circulating bed conditions in<br />
that the voidage was very high. Kinetic rates of char combustion were measured at 1, 3,<br />
5, 7 atm, at oxygen concentrations of 5, 10 and 21%. The surface reaction rates increased<br />
with pressure up to 5 atm and further increase of pressure led to decrease of reaction<br />
rates. This trend was thought to be consistent with Monson’s observation (MacNeil and<br />
Basu, 1998). However, re-examination of Monson’s data in this study showed that<br />
Monson’s data at 1, 5, and 10 atm could be unified using a single rate expression, and the<br />
data at 15 atm were affected by ignition problems. The observation that the burning rate<br />
increased up to 5 atm and decreased beyond this point in Monson’s experiments was an<br />
artifact and was actually due to the lower wall temperatures and gas temperatures at high<br />
pressures.<br />
The unexpected effects of total pressure observed by MacNeil and Basu are<br />
difficult to explain and could also be an artifact. Therefore, this set of data was also<br />
excluded in this study.<br />
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