An approach to estimate the depth of oxidizing and reducing regions in a fluidized bed combustor with staged combustion

The effect of oxygen stoichiometry on SO₂ captured by limestone in a fluidized bed combustor was studied with a 0.3 m square, 2 m high fluidized bed combustor. The secondary air was injected 100 cm above the distributor. The levels of air staging investigated were 85 : 15, 70 : 30, and 60 : 40, equivalent to primary air/ coal ratios (PACR) of ∼1.02, 0.84 and 0.72. Experiments were carried out at ∼20% excess air, 1.5ms^-1 fluidizing velocity and 850°C bed temperature. The experimental investigation also covered the effect of fluidizing velocity and bed temperature. An existing model was used to estimate the depth of the oxidizing and reducing regions produced as a result of staged combustion. The model proposes that sulfur emissions increase as PACR is decreased, due to the existence of a sulfur reduction (oxygen-rich) region and sulfur generation due to a calcium sulfate decomposition (oxygen-starved) region in the bed.