Experimental and Computational Studies on Fluidized Bed Biomass Gasifier for Production of Clean Energy

Abstract

An energy efficient approach to hydrogen rich syn-gas production from biomass using a fluidized bed Gasifier is presented. A fluidized bed gasifier is designed and installed in the laboratory by fabricating outside in parts. The effects of different biomass materials, temperature, steam to biomass ratio (S/B) and Equivalence Ratio (ER) on gas yield, gas composition, and carbon conversion efficiency have been studied. Catalytic effects are also studied by changing the bed materials (viz. sand, dolomite, red mud and their mixtures). Different biomass samples such as rice husk, rice straw, saw dust, wood chips, sugarcane bagasse and coconut coir have been gasified in the present work with different bed materials. Temperature during gasification was varied with 500-10000C. ER was varied within 0.15 to 0.35 and steam to biomass ratio was varied within 1.35 to 2.5. Attempt is made to develop correlation for the yield of hydrogen on the basis of dimensional analysis by relating different system parameters for all the biomass feed samples. Carbon conversion efficiency was observed to vary within 70 – 97%. Experimental results show hydrogen yields to vary within 56-74 gm per kg of feed sample for different biomass samples. The calculated values of H2yieldare compared against the experimentally observed data. It is observed that higher temperature contributes to higher gas yield and higher carbon conversion. CFD simulations have also been carried out for optimization of process parameters. The gas-solid interaction, the thermal-flow behavior and gasification process inside a fluidized-bed biomass gasifier are studied using the commercial CFD solver ANSYS/FLUENT15.0. A 2-D and 3-D model based on Eulerian-Eulerian approach coupled with granular kinetic theory has been developed to simulate the bed hydrodynamics and heat transfer for the FBG where volume fraction, bed pressure drop, temperature profile have been focused using FLUENT software. The influences of particle properties viz. gas velocity and temperature of bed material within the gasifier have been investigated comprehensively for simulation which provides a powerful basis for accurate design of FBG. Simulation and experimental observations are found to have very good approximation in most of the cases thereby validating the results against each other. Performance of fluidized bed gasifier is found to be satisfactory for ER = 0.25, S/B = 0. 5 and temperature=700oC with the use of red mud sand mixture as bed material. Little deviation among carbon conversion efficiency and thermal efficiency for all samples ensure that this technology can be used successfully for clean energy production and the developed correlations can be used for other biomass samples over a wide range of parameters.