Investigation of Hydrodynamics and heat Transfer Characteristics with Biomass Blends in a Pressurized Circulating Fluidized Bed

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Pressurized circulating fluidized bed (PCFB) is gaining popularity among the scientific community in utilizing low grade fuel for the combustion and gasification applications due to its in-built capability of capturing of sulphur and NOx. Compactness, high heat release rate and less amount of sorbent requirement makes the PCFB system more attractive. However, the complexity in hydrodynamics and heat transfer phenomena associated with bed geometry, flow parameters as well as type of fuels etc., demands for extensive research so as to open an avenue for designing of a PCFB system. In the present investigation, two PCFB units (one cold bed and the other being hot bed) of similar dimensions have been designed and fabricated in order to investigate the hydrodynamics and heat transfer characteristics experimentally. The bed hydrodynamics along the height of the riser was investigated in the cold bed unit. The effect of superficial velocity, solid inventory, particle size and operating pressures were investigated on bed voidage and suspension density. A heat transfer probe was installed at the upper splash region of the riser to investigate the wall-to-bed heat transfer coefficient along the height of the probe. The radial variation of heat transfer coefficient was investigated at a height of 1.57 m from the distributor. The hydrodynamics and heat transfer characteristics for different blending ratios of sawdust with sand and different weight composition ratios were also investigated at different operating conditions. The hot PCFB unit has been developed to investigate the effect of temperature and pressure on bed-to-wall heat transfer and quality of product gas at different biomass blending ratios. Gas composition was evaluated with the help of a gas chromatography and a flue gas analyser. Two heat transfer probes were installed at the upper splash region of the riser to investigate the heat transfer coefficient. The heat transfer coefficient was calculated without and with twisted tapes at different solid inventories and the performance was compared. Results obtained in the present investigation were found to be well comparable with the published results. The blending of biomass used for the above investigation in the PCFB units has been characterized to understand the change of thermal property with the increase of rate of heating. To investigate the effect of heating rates on the degradation of biomass the experiments were performed at three different heating rates of 10, 30 and 80 ओC/min while performing thermogravimetric (TG) analysis. The degradation of mass with temperature was validated numerically. The kinetic parameters of biomass were evaluated for both first and second reaction zones. The thermal response of biomass undergoing decomposition has also been modelled by using one dimensional (1-D) transient thermal model. The model was tested by using transient conduction Heisler chart. This study is important to understand the requirement of optimum fluidizing air for a combustor and to maintain a temperature required for gasification when it operates below sub stoichiometric condition.
Supervisor: Pinakeswar Mahanta