Numerical Simulation and Modeling of Downdraft Gasifier

Abstract

Gasification is a thermochemical process that converts carbon based material like coal, petroleum, biomass in to gaseous fuel. In gasification raw materials reacts with air, oxygen, stem or their mixture to produce syngas. The syngas produced has useful heating values and can be used as fuel for generating heat and power. Gasification is till today the most important process to generate energy. Along with experimental testing, computational work nowadays becoming more and more important due to lower cost and acceptable accuracy with minimum error. A fixed bed gasification method has been developed to contrive a novel clean gasification technology. The gasifier considered in this study is Downdraft throat type, fixed bed, 10 kW downdraft gasifier located at Nirma University's gasifier laboratory. Design data and material grades for modeling were taken from the same gasifier. In the present study the geometric model of the gasifier is created in ANSYS design modeler. The model of gasifier is discretized using unstructured hybrid mesh in ANSYS meshing modeler. The total no of elements used in the present study is nearly 5 lakh. The objective of this study is to develop a numerical model to investigate the Thermal-hydraulic and gasification process inside a Downdraft gasifier using the commercial CFD solver ANSYS/FLUENT 14.5. Turbulence model selected is standard k-ε model and combustion process has been displayed by species transport model to calculate global gasification reaction. In CFD of gasification chemical reactions considered are combustion, gasification, methanation, partial combustion and water gas shift reaction. Radiation model P1 is considered for radiation heat transfer. Discrete phase model is used to consider heat, mass transfer and momentum between solid phase and gas phase. Stochastic particle tracking method is used for considering turbulent dispersion. The governing equations are discretized using finite volume scheme. The pressure velocity coupling for momentum equation is done through SIMPLE algorithm. The convective terms in momentum, energy, and turbulence model are discretized by first order upwind while the diffusion terms are discretized by central difference scheme. Turbulence intensity is specified as 10% in turbulent model. The convergence criterion taken for energy and pressure is 0.45 and 0.55 respectively. The influence of variation in fuel composition, calorific value and Equivalence ratio (φ) on temperature distribution in various zone is evaluated. Finally, species concentration of the gases formed after gasification is discussed. It is observed that with Equivalence ratio 0.25 the outlet temperature is 960 K while that with 0.35 is 1024 K.