Char combustion kinetics using a micro fluidized bed reactor

The char combustion kinetics of low sodium Rhenish lignite (RB) in synthetic air (N2/O2) and oxyfuel atmosphere (CO2/O2) are investigated experimentally and using a 1D modeling approach. Combustion experiments are carried out in an externally heated micro fluidized bed reactor (mFBR) at temperatures of 773 - 1123K with 25 vol% oxygen. At 873K, the oxygen volume fraction is varied from 15 - 30 vol%. Depending on the reaction rate variance, experiments are repeated 5 - 50 times, resulting in a total of 556 data sets. Time-resolved gas analysis (≤10 Hz) of the reaction products CO2 and CO is performed ex situ with a Fourier-transform infrared (FTIR) spectrometer. Combined with information on the gas mass flow and fuel sample mass per batch, a carbon balance is closed around the reactor. Using this balance, the time-resolved carbon conversion rate dX/dt(t) and total conversion X(t) are computed for each experiment. Modeling of char combustion is done based on the Carbon Burnout Kinetic model version 8 (CBK8) by Hurt et al. [1998], adapted to fluidized bed combustion conditions. Using the adapted model, named CBK/FB, different correlations for fluidized bed heat- and mass transfer, as well as fuel heat capacity and CO/CO2 production ratio at the particle are compared. Using the best-fit combination of heat and mass transfer sub-models, the char kinetic parameters of the CBK/FB model are determined for experiments conducted in synthetic air. The resulting kinetic parameters are plausible and compare favorably to results from literature. Overall, the adapted model predicts carbon conversion times with good precision in synthetic air. A test of the model against experiments in oxyfuel atmosphere does not reveal any systematic deviation