**1. Introduction**

Gasification technology makes the biomass a vital source of energy. It converts the biomass into raw syngas that is a fuel gas mixture consisting primarily of hydrogen, carbon monoxide, tar, and other gases which are considered contaminants such as hydrogen chloride, carbon dioxide, hydrogen sulfide. The tar in gasifier may create fouling and soot accumulation in downstream processes, moreover, tar solubility in the water may generate wastewater di fficulties [1]. In literature, many definitions for tar have been reported. All the definitions seek to present a view about the nature of the tar. Besides, these definitions are influenced by the gas quality specifications required for a particular end-use application and how the tar is assembled and analyzed [2]. One of the definitions of the tar was described by Milne et al. [3] as follows: It is the organics components that are created under thermal or partial-oxidation process (gasification) and are supposed to be mostly aromatic. Devi et al. [4] defined the tar as a complex blend of condensable hydrocarbons, which comprises single to multiple ring aromatic compounds along with other oxygen-containing hydrocarbons and complex polycyclic aromatic hydrocarbons. The Energy Research Centre of the Netherlands (ECN) considers that the tar consists of organic molecules, which have a higher molecular weight than benzene (benzene is not considered to be tar) [5]. According to Unger et al. [6], tar is a mix of the hydrocarbons that can form liquid or highly viscous to solid accumulation by dropping the temperature of the gaseous phase down to ambient temperature; it consists of carbon, hydrogen, and other organic linked elements such as oxygen (O), nitrogen (N), or sulfur (S). The tar can be classified based on di fferent criteria. Li et al. [7] listed tar into five classes: GC-undetectable, heterocyclic aromatics, light aromatic (one ring), light PAH compounds (two to three rings), and heavy PAH compounds (four to seven rings).

Several studies [3,8–10] published that the tar can also be classified into primary, secondary, and tertiary tar. Wolfesberger et al. [10] described how the tar components are created and what is

the effect of temperature on the nature of the formed tar components. The primary tar components begin to appear during the pyrolysis process, the complex polymers that make the main parts of biomass (cellulose, hemicellulose and lignin) are broken down from cellulose and hemicellulose, tar components like alcohols, ketones, aldehydes, or carbon acids are formed, whereas bi-and trifunctional monoaromatics mostly substituted phenols are derived from lignin [10]. By growing temperature and attendance of an oxidant, a portion of the cellulose-contributed primary tars convert to small gaseous molecules, the remaining primary tar creates secondary tar, examples for secondary tar components are like alkylated mono- and diaromatics including heteroaromatics such as pyridine, furan, dioxin, and thiophene [10]. At a temperature above 800 ◦C, components such as benzene, naphthalene, phenanthrene, pyrene, and benzopyrene (polynuclear aromatic hydrocarbons (PAH)) are created; these components form the tertiary tars components [10].
