The mechanism of char ignition in fluidized bed combustors

Knowledge about ignition processes of coal in fluidized beds is of importance for the start-up and dynamic control of these combustors. Initial experiments in a transparent fluidized bed scale model showed the existence of a considerable induction period for the ignition of char, especially at low bed temperatures (e.g., 800–950K for bituminous coal). This paper focuses on char-ignition delay at these low temperatures. It is shown that temperature rise during ignition is not caused by coal particle diameter shrinkage but rather by an increase in reactivity. Analysis of the thermal ignition process leads to the conclusion that the process is noncritical, causing a gradual temperature rise without Semenov Jump. Consequences of this result for future research are expounded. Calculated values of the maximum inclination of the heat generation curve (heat release versus coal particle temperature) may be of use to the development of an adequate experimental device.