(690d) Numerical Simulation and Experimental Study of Fixed Bed Downdraft Gasifier

Authors: 
Yao, Z., National University of Singapore
Shen, Y., National University of Singapore
Yan, W. C., National University of Singapore
Dai, Y., Shanghai Jiaotong University
Tong, Y. W., National University of Singapore
Wang, C. H., National University of Singapore

The aim of this study is to understand the various aspects of interaction within a fixed bed downdraft gasifier and to provide researchers with a tool to overcome the need to conduct cost intensive experiments for improving gasification efficiencies. Both 3-D computational fluid dynamic (CFD) model and 1-D chemical reaction kinetics model have been developed to predict energy generation performance for a wide variety of feedstock in an industrial downdraft gasifier. In 3-D CFD model, the gas and solid phase were resolved using an Euler-Euler multiphase approach, with exchange terms of momentum, mass and energy. The standard k- turbulence model was used in the gas phase. In addition, some pilot scale problems, such as heat loss from wall surface and blockage inside the reactor, were also considered in the model.  The 1-D kinetic model described the variation of gasification reactions in the axial direction, including drying, pyrolysis, combustion and reduction reactions in the gasifier. All the gas components were assumed to be uniform and well mixed in the radial direction.  This 1-D kinetic model has the ability to predict the direct syngas composition with only initial feedstock compositions (based on proximate and ultimate analysis results) and uses minimum curve fittings.  In this study, both syngas and charcoal production were regarded as energy output. Experimental results were used to validate against simulation results and the comparison between 3-D CFD model and 1-D kinetic model were also discussed. The deviation between the numerical and experimental results obtained in this study was lower than 10%. Furthermore, according to the simulation results, the cool gas efficiency of co-gasification of sewage sludge and wood chips could reach about 86% at the optimum equivalence ratio, which is a reasonably high efficiency in industrial processes.