MODELLING AND SIMULATION OF FLUIDISED BED BIOMASS GASIFIER

Abstract

A two-phase one dimensional model capable of predicting the performance of fluidized bed biomass air gasification reactor during dynamic and steady state operations was developed based on the two phase theory of fluidisation. The fluidized bed was divided into two zones: (a) bed and (b) freeboard. Two phases were considered to be existing in the bed: (a) bubble and (b) emulsion phase, of which the emulsion phase takes both gas and solids into account, while the bubble phase consists of only gas. Reaction kinetics, material and energy balances for both phases were taken into consideration and the elemental mole balance technique was used to calculate the gas mole fractions after devolatilization. The mass transfer coefficient was calculated for interaction between bubble and emulsion phases. A quantitative estimation of bubble and particle properties is also included in the model along with the hydrodynamics properties of fluidized bed. The model assessed the effect of different parameters on gasifier performance and gaseous emissions. The model was solved numerically with MATLAB 9.0 platform to simulate two aspects of reaction process (bubble phase and emulsion phase) and the final gas composition. Effect of equivalence ratio, fluidisation velocity and bed height on the syngas composition was studied. It has been found that gas hydrodynamics plays a significant role, and the system can be optimized using these parameters. The product gas composition of the gasifier was influenced more by changes in the equivalence ratio than that of fluidisation velocity. It was found that the syngas composition increased in the product gas stream whereas the pyrolysis products' composition decreased with the increase in bed height for the selected operating conditions. The final inference drawn was that the best condition was least fluidisation velocity and highest equivalence ratio.