Gas exchange between the bubble and emulsion phases during bubbling fluidized bed coal combustion elucidated by conditional gas sampling

Abstract

A method for gas composition measurements in bubble and emulsion phases has been developed and applied to bubbling fluidized bed coal combustion. Resolution of the bubbles from the emulsion phase is based on detection of the time change of the pressure difference between two points along the path of bubbles. Cold model experiments confirm that the conditional sampling performs as expected. The probe isdemonstrated to resolve bubbles from the emulsion phase correctly. In addition, with single CO 2 bubbles formed in the fluidized bed under the minimum fluidization condition by air, gas exchange between the bubble and emulsion phases is characterized by using this sampling system. Gas in a bubble tends to remain in the bubble provided that the ratio of bubble rising velocity to gas velocity in the emulsion phase exceeds a certain value. This sampling method is also applied to coal combustion conditions in order to elucidate the roles ofbubble and emulsion phase in the combustion process. For fluidizing material of smaller particle diameter, the combustion of volatile matter mainly takes place in the bubble phase. NO concentration in the bubble phase becomes lower than that in the emulsion phase, while N 2 O shows the opposite tendency. This might be caused by homogeneous reactions of NO to form N 2 O in the bubble phase.