In this paper, steady state simulation model of air gasification has been developed in Aspen plus for fixed bed biomass gasifier using rice husk. Aspen plus is selected as a simulation tool due to its higher capability of handling solid feed using physical models. The fixed bed gasifier used here is an updraft type one with drying, pyrolysis, gasification, and combustion stages. The hydrodynamic process of the reactor was investigated using Ansys Fluent. The gasifier has been modeled based on thermodynamic equilibrium model by Gibbs free energy minimization in four stages. In the first stage moisture content of biomass feed is reduce through drying. In the second stage biomass is decomposed into its elements by specifying yield distribution. In the third and fourth stages gasification and combustion reactions has been modeled using Gibbs free energy minimization approach. Simulation result include; sensitivity analysis of Equivalence ratio(ER), gasification temperature, gasification pressure, biomass moisture content, steam injection have been varied over wide range and the effect of these parameters on syngas composition and heating value(LHV) has been investigated. The sensitivity analysis results indicated that temperature increases the production of CO and H2. Increase in Equivalence ratio decreases the heating value of the producer gas. Steam injection favors hydrogen production which results in increase of hydrogen content in the syngas, while increase of the steam to biomass ratio had negative effects on lower heating value of syngas. Based on the results, maximum lower heating value of syngas was found at the gasification temperature of 4000C, pressure of 1bar, Equivalence ratio of 0.1, steam to fuel ratio of 0.1 and 5% moisture content of biomass.