Fig. 2b is the axial temperature distribution in the VFBC in FGR
combustion mode with different stoichiometric oxygen ratios. In
this figure, the bed temperature is 700 C. By comparing Fig. 2a
and b, the descending gradient of temperature in FGR combustion
is smaller than that of staged combustion. The study of Park et al.
(2003) clarified that the addition of CO2 showed the tendency towards
the reduction of bed temperature. However, due to the high
heat capacity of the chemical species itself and due to the radiation
heat loss by the high absorption coefficient of CO2 (Park et al.,
2003), thermal effects decreases the descending temperature gradient
between the bed zone and freeboard. Besides, the bed zone
combustion fraction increases with the increasing bed stoichiometric
oxygen (from 0.8 to 1), resulting in a higher peak temperature.