Design and Development of Fluidized Bed Reactor

Abstract

Fluidized bed reactor works on the principle of fluidization. Fluidization is an industrially important and highly complex phenomenon. Its design and development requires sound fundamentals and understanding of hydrodynamics and reaction engineering. In this work, process of coal to fuel is considered. Due to energy and environmental issues, there is a high interest to produce synthesis gas (syngas) , basically a mixture of H2, CO and CO2, which opens up to making a variety of downstream energy carriers. The syngas may be used as a fuel in integrated gasification combined cycles (IGCC) or as a feedstock for producing H2 or a synthetic natural gas (SNG). Coal gasification – and virtually gasification of other carbon-based resources such as biomass or refinery residues - is a versatile conversion technology adding flexibility to the energy systems. The syngas produces from the gasification process can be used to produce the variety of synthetic fuels like diesel,naphtha and LPG by conducting fischer tropsch synthesis. Gasification is an important route for conversion of coal or solid wastes materials i.e. biomass to useful gaseous products for direct firing in thermal applications and as well as raw gas for production of fuels or chemicals. Gasification with O2, H2O, CO2 and H2 produces combustibles such as CH4 and CO/H2 mixtures for use as gaseous fuels or chemical feed stocks. Among the coal-gasification processes, the fluidized-bed process with inherent advantages of high heat transfer and easy handling of solids is a natural choice. Coal gasification with O2 and H2O in a fluidized-bed reactor involves pyrolysis, combustion and steam gasification. Gasification in fluidized bed offers advantages, since fluidized beds are capable of being scaled up to medium and large scale, overcoming limitations found in smaller scale, fixed-bed designs. In this work the modeling of a fluidized bed coal gasifier is discussed which uses a coal as its feed. A two phase model consisting of the bubble phase and the emulsion phase is used to describe the coal gasification process.A non-isothermal model has been considered taking into account the effect of heterogeneous reactions and homogeneous reactions. Homogeneous reactions involve gas-gas reactions which incorporates the gasification reactions taking place in the fluidized bed gasifier and heterogeneous reactions involve gas-solid reactions that incorporates the combustion reactions. For gases in both emulsion and bubble phase, it can predict concentration profiles , gas composition, velocities and other fluid dynamic parameters.The fluid dynamics parameters like minimum fluidization velocity (Umf), bubble diameter (db), bubble velocity (Ub) have been studied as a function of bed height. Model has been solved numerically using MATLAB. MATLAB program has been developed to study the fluid dynamics parameters as a function of bed height. Also the changes in the molar fraction of product gases H2, CO and CO2 has been studied along the different bed height values. Finally, the mathematical model is validated with the experimental data taken from the literature. Simulation of coal gasification process and fischer tropsch synthesis is being carried out using ASPEN PLUS.