Studies on over-bed start-up of bubbling fluidized combustors

The experimental study presented in this thesis has been done on a bubbling fluidized bed unit in order to reduce its start-up time. The start-up time is an important consideration in industries. As the start up time in the bubbling fluidized bed boilers increases, the cost of fuel also increases. This study relates to a fluidized bed combustor, fired by an over bed start-up burner at initial bubbling regime conditions. To reduce the start-up time and to gain a better understanding of the heat transfer between the flame and the fluidized bed, two studies have been undertaken.;The start-up can be facilitated by premixing different quantities of biomass (2.5, 5, 10 and 15%) with the bed materials. The heat generation from the biomass is found to contribute to the heating of the bed even at lower temperatures. The comparison of results obtained with the bench scale unit used in this study with those obtained from an industrial unit are presented. It is found that results from the bench scale unit have direct relevance and use in the design of industrial size units. A scale up (bench scale to industrial scale) approach on the basis of heat release from the burner per unit bed area as well as bed height has also been presented.;The convective heat transfer coefficient from flame to bubbling bed has been determined. The maximum value of the heat transfer coefficient by convection found in the study is 180+/-20 W/m2K for a 5.5kg bed inventory (bed height of 234 mm) after 1500 seconds from start-up, whereas for lower bed inventories such as 4kg (bed height of 173 mm) and 3kg (bed height of 127.8 mm), it reduces to 140 W/m2K and 83 W/m2K respectively. In the study, the bubbling bed is taken as a control volume. One of a conclusion found is that with decreasing bed height, it takes longer for the bed temperature to reach steady state.