## 1 Introduction
### Tar Treatments  
The methods of tar removal can be categorized into primary and secondary measures based on the place where tar is removed [11]. With the primary methods, the tar is removed by applying processes such as thermal or catalytic cracking in the gasifier itself, while the secondary methods, the tar is separated outside the gasifier [2]. Although the primary methods have some disadvantages such as the complex construction of the gasifier and the limited flexibility of feedstock, it promises high tar removal efficiency by promoting this technology with time [2]. However, Figure 1 shows that a combination of both methods can only achieve high tar removal efficiency. Currently, secondary methods are fitting for tar separation from the produced syngas because of their low cost and simple measures [12]. Wet scrubbing process is one of the secondary methods, which applied an absorber to exclude the tar. The absorber can be a plate or packed column. It is recommended to use the packed absorber because of its high capacity [13].  
Furthermore, the packed absorber can operate with lower overall pressure drops than the tray columns [14]. The packed absorber materials are categorized into random or structured packing. The modern random packings had a different wide range of geometries and shapes and are made from ceramic, metal, or plastics. The structured packings are ideal for lower pressures (i.e., less than 2 bar) and lower liquid rates (i.e., less than 50 m\({}^{3}\)/m\({}^{2}\)-h) [14].  
A suitable solvent must be appropriately selected for the absorption process since the solvent type has a significant influence on equipment sizing and operating costs [15]. [PERSON] et al. [16] summarized that the absorption process should concentrate on separating the components of the tar that cause the fouling problem. These components as per tar classification of [PERSON] et al. [17] are heterocyclic compounds, light polycyclic aromatic hydrocarbons such as naphthalene, and the heavier hydrocarbons that condensate easily. According to [PERSON] et al. [16], light aromatic hydrocarbon tares (one ring aromatic hydrocarbon) are not the reason for blocking and fouling  
Figure 1: Illustration of the need for primary and secondary measures [2].  
problems, they studied some scrubbing liquids such as diesel fuel, vegetable oil, engine oil, and water as a solvent to remove the tar. The removal efficiencies by using these solvents are shown in Table 1.  
From Table 1, it can be observed that diesel fuel is the most effective solvent used to remove naphthalene. However, diesel is considered an uneconomic solvent because of its simple evaporation, which raises the losses of the solvent [16]. The vegetable oil has proven to be efficient to separate naphthalene [12]. The water has comparatively high removal efficiency for phenol because the phenol is a hydrophilic component and it can lose H+ (ion) from a hydroxyl group, whereas the other components are nonpolar substances [12]. Applying water as a solvent to remove the tar achieves removal efficiency of about 31.8%, however water is not as effective solvent since the tar has a low solubility in water and the separation of the tar from the water is difficult and expensive [12]. [PERSON] et al. [16] placed the efficiency of the solvent as: vegetable oil \(>\) engine oil \(>\) water \(>\) diesel fuel. [PERSON] et al. [18] published the tar removal efficiency of vegetable oil as 89.8% and cooking oil as 81.4%. [PERSON] [19] investigated the effect of two kinds of vegetable oils, namely soybean and canola oil to separate the tar. The author summarized that there is no significant difference between the soybean and canola oils for all the conditions of absorbent like temperatures and volumetric flow rates. [PERSON] et al. [20] analyzed the relationship between the operating time and removal efficiency of some oils solvents like benzene and toluene. They concluded that the removal efficiency declines with time because of increasing the tar concentration in the absorbent.