This note is prompted by the recent publication of Donsi et al. [1] concerning the calculation of carbon hold-up in a fluidized bed combustor. They tested the sensitivity of bed carbon content predictions to cross-flow ratio under the assumption that char particles burn to CO2 in the dense phase and that coal volatiles burn in the freeboard. It was found that bed carbon content is sensitive to the rate of oxygen transfer from the bubble phase.
In the period 1980-1982 we made measurements and predictions of carbon hold-up in support of an investigation into possible fuel quality effects in fluidized bed combustion. In agreement with Donsi et al., we found bed-carbon contents to depend on oxygen transfer from the bubble phase; however, we also found it important to take account of bubble phase oxygen concentration depletion by the combustion of coal volatile matter and carbon monoxide evolved in the particulate phase.
The original model of Donsi et al. [2] was modified according to the following observations of bubble phase combustion made using an 150 mm i.d. fluid bed combustor contained in a quartz tube [31.
i. Slow motion video analysis clearly showed volatile flames attached to freshly injected coal particles as they fell through rising bubbles. However, the volatile flames remained inside the bubble as the particles reentered the dense phase and were swept away. No volatile flames were seen in the dense phase, although cold