## 1 Introduction
Tar is a product of the thermal decomposition process of biomass, including condensed oils such as olefins, phenols, aromatics, etc. Few Aspen Plus models of biomass gasification have been reported on tar and its kinetics. For example, a mathematical model of biomass gasification in a bubbling bed reactor was developed in Aspen Plus with a sub-model for tar generation and cracking [30]. This study has defined tar and its cracking kinetics to improve the model performance and its credibility. [PERSON] et al. [31] proposed a process model for biomass gasification using external FORTRAN subroutines for both hydrodynamic and reaction kinetic calculations simultaneously in Aspen Plus. A process model was developed to simulate the air-stream gasification of biomass in a bubbling fluidized bed reactor [17]. This model was based on chemical reaction rates, empirical correlations of pyrolysis mass yields, and hydrodynamic parameters. A simulation was performed in the Aspen program for steam gasification of rice husk to evaluate the influence of gasification temperature and steam-to-biomass ratio on product gas composition [32]. The model was developed based on the chemical equilibrium to predict the gas composition of the process. A model is developed based on Gibbs free energy minimization applying the restricted equilibrium method to study the influence of key parameters on the performance of steam gasification of biomass [33]. The high amount of volatile matter in biomass and the complexity of biomass reaction kinetics in fluidized beds have hindered the simulation of biomass gasification. Many studies ignored the kinetics of char gasification and developed their gasification model based on Gibbs equilibrium. Additionally, in a typical fluidized bed gasifier, solid fuels and bed material are fluidized by a mixture of gases resulting in good solid-gas heat and mass transfer. Consequently, hydrodynamic behavior is a crucial factor in a fluidized bed gasifier influencing strongly the performance of the gasification process. Therefore, reliable process simulation studies on biomass gasification are still limited, resulting in a lack of understanding of the fundamentals of the biomass-based gasification process. This study has been developed in Aspen Plus based on the previous studies with some improvements to provide a good understanding of biomass gasification in terms of the effect of operating parameters on the process performance and the phenomena occurring in a bubbling fluidized bed gasifier. The proposed model based on a combination of both hydrodynamic and reaction kinetic calculations simultaneously is capable of predicting the steady-state performance of a negative gauge pressure bubbling fluidized bed gasifier. Due to the complexity of biomass characteristics, the mass yields of pyrolytic products released from torrefied wood chips were determined by the model of [PERSON] et al. [34] according to the biomass proximate and ultimate analyses. Char gasification kinetics obtained from experimental investigations are to determine the reaction rate of char gasification. Three chemical reactions, i.e., char gasification with steam and CO2 and water-gas shift reactions are taken into consideration in the model to calculate variations of components in biomass gasification. Due to the lack of a library model to simulate fluidized bed units in the Aspen Plus simulator, external FORTRAN codes are implemented with input data to simulate an operation of a bubbling fluidized bed gasifier. The validity and accuracy of the model are evaluated by comparing the numerical result obtained with the experimental data of biomass steam gasification.