Hydrodynamic Design and Optimization of Bubbling Fluidized Bed Gasifier through Comparative Parametric Analysis

Abstract

This research work includes geometric and operational parametric studies to improve the bubbling fluidized bed gasifier’s (BFBG) design and operational efficiency. The first stage of the study uses computational analysis to investigate the mixing pattern and pressure drop across different configurations of air distributor plates, such as perforated, 90° slotted, and 45° angular slotted plates. The results reveal that the perforated distributor plate has the highest pressure drop due to its small open area ratio. In contrast, the 90° slotted plate has a lower pressure drop due to the larger open area of the straight slotted plates, while the 45° slotted plate exhibits a higher pressure drop than the 90° slotted plate due to the longer path length of the slot. The CFD simulations also show that the swirling flow at the bottom region of the bed improves the radial and axial mixing of solids for the 45° slotted plate scenario. The numerical pressure drop results show reasonable agreement with available experimental results. Another CFD study has been carried out that establishes a novel computational technique to incorporate the effect of rotating air distributor plates on the hydrodynamics of bubbling fluidized bed gasifiers. The study uses static and rotating perforated distributor plates and applies dynamic mesh methodology coupled with the sliding mesh technique of ANSYS FLUENT®. The study reveals that the impact of tangential and radial velocities on the fluidized bed due to the plate rotation is highest for lower superficial velocities and shallow initial depth, and the maximum rotational velocity of the distributor plate. A comparative evaluation of static and rotating distributor plates is also carried out, revealing that the pressure drop across the bed is increased by about 6-7% and Umf is decreased by up to 10% compared to the static distributor plate. Bed expension ratio is also higher for static plate distributors, while the pressure variations inside the fluidized bed are reduced by up to 88% when using a rotating distributor instead of a static distributor plate. After fabricating a laboratory-scale BFBG, the experimental setup was finalized with the help of initial CFD studies. A series of experiments were conducted with various operational and geometric parameters using different distributor plates. The first distributor plate used was a commonly used perforated plate to predict several parameters, such as Umf, pressure drop, bed height rise, and bubble rise velocity. A 45° slotted distributor plate was then used for similar experiments, resulting in a slightly iv increased pressure drop and Umf due to the two velocity components caused by the 45° entrance of fluidizing air. The third distributor plate was a novel hybrid plate, which had eight 45° angular slots and open perforated holes. This plate had the highest open area ratio, resulting in the lowest pressure drop across the fluidized bed, and Umf was between the perforated and 45° slotted plates due to the combination of smaller and larger bubbles. The mixing properties of binary solid mixtures in a fluidized bed gasifier were also investigated by introducing rice husk biomass and inert sand particles. To understand the impact of different distributor plates on the mixing pattern, samples were taken from various axial cross-sections and radial planes of the gasifier. The axial mass fraction profile for the perforated distributor plate was found to be more uniform than the 45° slotted and hybrid distributor plates. Radial dispersion of rice husk biomass was highest in the upper section of the fluidized bed due to the lighter biomass particles. In contrast, the 45° slotted distributor plate produced more mixing at the lower portion of the bed. The novel hybrid distributor plate showed significantly improved mixing in the middle and upper regions of the bed due to a mixed flow pattern. A mixing index was calculated to investigate the mixing efficiency of rice husk/sand particles with different distributor plates and superficial air velocities. The hybrid plate exhibited the best mixing performance at a range of velocities with a mixing index closer to unity.