## 1 Introduction
Hydrodynamic characteristics of dual fluidized bed gasifier have been investigated with cold model gasifier by [PERSON] et al. The authors revealed that the bed material circulation exceeded the design value indicating the system has a comfortable safety factor in its engineering design--however, the pressure distribution aligned with the published literature [16]. Gas distribution and bed materials are also prime parameters in a fluidized bed gasifier. Various geometric and operational parameters like the distributor design, gas flow rate and solid density were also evaluated via CFD study. Bed expansion ratio, gas and solid velocities distribution above the distributor plate, pressure drop across the distributor plate, distributor-to-bed pressure drop ratio and solid volume fraction have been examined. A distributor plate's perforated area and open area ratio greatly influenced uniform fluidization and overall bed pressure drop. It was also investigated in the early 90 s that the same open area ratio of different distributor plates did not influence bubble size [17]. However, different numbers and sizes of holes significantly alter the growth and size of bubbles [17]. Moreover, bed material properties and gas flow rate strongly impact the hydrodynamics of solid-gas fluidized beds [18]. Gas flow pattern primarily depends on different designsof gas flow distributors, which are mainly divided into static and rotating distributor plates. Based on the air entering direction, these distributor plates are categorized into perforated plates [19], slotted or inclined plates [20], and bubble cap distributors [21]. Previously, distributor plate rotation improved the radial dispersion by reducing high-concentration zones in fluidized beds [22]. It was concluded that uniform fluidization could be achieved by rotating distributors as compared with the static distributor. A comparative study of perforated and bubble cap distributor plates was carried out to analyze the flow pattern of the bottom area of a fluidized bed. The solid transfer rate from dense to free board area was higher at lower superficial gas velocity for perforated distributors [21]. In another study, it was found that the diameter of the reactor has a slighter effect on the performance of the gasifier. However, efficiency, heating value and carbon conversion were enhanced while increasing bed height [23]. Proper mixing reduces the operational cost of the industrial fluidization phenomenon. Air distributor plate design affects the mixing and performance of fluidized bed gasifiers. Enhancement in mixing was investigated using a slotted distributor plate. It was found that the distributor with 45\({}^{\circ}\) inclined slots improved the circulation rate, which critically enhanced the mixing [24]. In the case of a slotted distributor, entrained gas produces two velocity components: horizontal and vertical. The horizontal velocity component induces swirling motion in the bed, while the vertical component causes fluidization [25; 26]. Mixing of gas-solid was numerically analyzed and a comparative analysis of swirl distributor plate (SDP) with conventional distributor plate (CDP) was also conducted. CDP achieved better gas-solid mixing as compared with SDP [27]. Multizone circulatory bed reactors (MZCR) simulations have been carried out by [PERSON] et al. to investigate the oscillatory behavior of the reactor temperature, which is essential to design and install suitable temperature control within a reactor [28]. In another computational study, pressure drop and mixing patterns have been studied using three distributor plates: perforated plate, 90\({}^{\circ}\) slotted plate and 45\({}^{\circ}\) slotted plate. The pressure drop was the maximum in the case of the perforated distributor plate due to the smaller open area ratio. At the same time, it was found that the pressure drop in the case of a 90\({}^{\circ}\) slotted plate was 7% and 4% lesser than the perforated and 45\({}^{\circ}\) slotted plate, respectively.