## 1 Introduction  
Fluidized bed gasification is an appropriate technique to convert various biomass wastes into valuable energy. Therefore, it is essential to understand the mass transfer and heat distribution between the phases to make an efficient design of a fluidized bed gasifier. Bubbling fluidized bed gasifiers are known for better heat and mass transfer characteristics between phases. However, there are challenges related to properly mixing biomass within the fluidization zone to eliminate the dead zones, which may lead to hot spot formation during gasification [1]. As a result of the removal of hotspots, the reaction rate within the fluidized bed increased manifold, resulting in the improved efficiency of the gasifier [2]. Mixing is a critical factor that significantly influences heat and mass transfer in fluidized bed gasifiers. Mixing quality in the fluidized bed is greatly influenced by geometric parameters such as gas distributor plate design, the fluidization column's size and shape, the gasifying agent's entry point, biomass (feed) and secondary air [3]. Moreover, operational factors such as gas flow distribution, particle size, particle velocity distribution and bed aspect ratio also influence the operational efficiency of the gasifier as a whole [4; 5; 6; 7]. In order to enhance the operational efficiency of the gasifier, a thorough understanding of independent or input operational variables such as particle size, initial bed height ratio, and superficial air velocity range, as well as dependent/output variables such as pressure drop, minimum fluidization velocity, bubble rise velocity, bubble dynamics within fluidization zone, and bed height rise, on overall hydrodynamics of gasifier is essential. Moreover, the parameters mentioned above have a coupled effect on the gasifier's hydrodynamics with the geometric parameter variations. Therefore, geometric parameters must be included in the optimization process to understand the influence of operational parameters on overall hydrodynamics. In this regard, it was studied that particle size affected pressure drop, which eventually influenced solid-gas fluidization behavior [8]. However, it is not practical to study only the effect of variation in particle size since geometric factors also affect the fluidization behavior, like distributor plate design. For example, the earlier slugging and higher pressure drops were promoted by coarse particles as compared to smaller particles, producing larger bubbles. It was also observed that fluidization quality is enhanced when the average particle size and segregation are reduced [9; 10]. These observations were also perceived by variation in the distributor plate's hole size and shape, stating that the emerging bubble size increases with bigger holes of the distributor plate [11]. Therefore, changing the distributor plate along with particle size might significantly affect the hydrodynamic characteristics of a gasifier. Similarly, changing the initial bed height ratio alone increased pressure drop and minimum fluidization velocity [12]. However, variations in the distributor plate design and changes in the initial bed height ratio may result in drastic changes in biomass/inert material mixing behavior. Hence, the overall hydrodynamics of the gasifier may be optimized by effectively using operational and geometric characteristics. Heat and mas transfer within a fluidized bed of bubbling fluidized bed is also affected by operational parameters such as biomass particle diameter, initial bed temperature, steam to biomass (S/B) ratio, inlet gas flow rate, and feed position, which influence the production of H\({}_{2}\) and CH\({}_{4}\)[13]. More computational efforts have also been carried out to investigate the drag model, equivalence ratio (ER), and fluidization number (FN) effect on the gas-solid flow dynamics of biomass gasification using the CFD-DEM model. The parametric study revealed slight CO and H\({}_{2}\) concentration decreases by increasing FN by fixing the Er value at 0.27 [14; 15].