A mathematical model has been developed 10 predict the combustion of liquid fuel in a fluidized bed. From the fluid-dynamic point of view, the model is based on the modified two-phase theory of fluidization where the gas in the bed is assumed to have three phases. A jet-bubble phase which is fuel rich bubbles formed at fuel feeding jet, a distributor-bubble phase which is fuel free bubbles generated at distributor plate and an emulsion phase. Each phase has its composition and exchanges masses with the other two ones.
The bubble is assumed to have a constant size throughout the bed. Gas mixing due to bubbles coalescence is taken into consideration based on an equivalent mass interchange coefficient between the two-bubble phases. The coefficient has been derived based on bubble size growth and probability of coalescence among jet and distributor bubbles.
Two-step fuel reaction has been applied. The fuel firstly reacts to carbon monoxide and water vapor. In the second step, carbon monoxide reacts with oxygen to form carbon dioxide. Mass balance differential equations have been derived for various reacting species (CxHy, O2, CO, H2O and CO2) in the three phases. The heavy liquid fuel has been considered as a base case whereas gasoline and diesel fuel have been studied for comparison.
The numerical solution of the model yields the axial concentration profiles of different gaseous species within the three phases. The in-bed fuel combustion has been estimated. The relative importance of gas mixing due to bubble coalescence has been assessed. The influences of different parameters on the combustion performance have been studied and discussed. Among the later parameters, jet velocity has a major effect on combustion performance.
Experimental tests have been carried out to validate the model using a bubbling fluidized bed combustor of 300 mm inner diameter. The comparison between the model and experimental results shows a good agreement, especially, at higher jet velocity.