# A Critical Review of SCWG in the Context of Available Gasification Technologies for Plastic Waste
## 5 The Problem of Tar: Formation and Removal  
One of the major problems in gasification is the formation of tar during the process. Tar is a complex mixture of condensable hydrocarbons, which includes aromatic, polyaromatic, and linear organic compounds some of which contain oxygen [46; 84]. The formation of tar gives rise to some important problems related to the formation of carcinogenic substances, the interruption of the process operations, and the deactivation of the catalyst [113]. Tar condensates in a wide temperature range from ca. 180 to 300 \({}^{\circ}\)C and in temperatures below zero, polymerizing in equipment such as engines and turbines [46]. As the tar condensates, it creates an adhesive liquid coating on cool surfaces which, together with dust, forms deposits that are very difficult to remove [114]. As explained in detail by [PERSON] et al. [20], in the case of plastic waste, tar formation depends on the nature of the polymers, as shown in Figure 5. The formation of primary aromatic tar occurs only in the degradation of polymers such as PS and PET, which contain aromatic rings in their structure. In the case of polyolefin such as PP and PE, the volatiles and the primary tar are made up of alkanes and alkenes of varying chain length. The components of the primary tar generally have an unstable nature and evolve into more stable structures which give rise to the so-called secondary and tertiary tar [115]. The removal of the tar can take place in two different ways: Primary methods, which consist of an internal treatment in the gasifier [116; 117] and secondary methods, which consist of the purification of the heated gas after gasification [118]. As described by [PERSON] et al. [84], secondary methods, including thermal or catalytic cracking of tar and mechanical methods such as cyclones or filters, are very effective but at the same time extremely expensive. Furthermore, with these methods it is difficult to obtain a syngas with a very low tar content [42; 119; 120]. Treatments inside the gasifier are gaining  
Figure 5: Tar formation from plastics of different nature (polyolefin, polystyrene (PS), polyethylene terephthalate (PET)) [20]. For each polymer we have represented only a small portion. With the symbol \"\" we indicate that the chain continues. much attention for solid waste gasification since they may eliminate or strongly reduce the need for downstream clean up [84]. In a primary treatment, gasification is optimized to produce a gas with the minimum concentration of tar as possible [46]. For this purpose, several factors must be taken into account such as:  
* A selection of specific operational parameters (temperature, gasifying agent, equivalence ratio, and residence time) [50; 121; 122]. * Modification of the gasifier. * The use of catalysts in the bed. Regarding the use of catalysts, [PERSON] et al. [123] identified three main groups. They are:
* Naturally occurring catalysts such as dolomite and olivine. * Alkali metals such as KOH, K\({}_{2}\)CO\({}_{3}\), KHCO\({}_{3}\), Na\({}_{2}\)CO\({}_{3}\), CaCO\({}_{3}\), CsCO\({}_{3}\), KCl, ZnCl\({}_{2}\), and NaCl. * Nickel-based catalysts, which have been evaluated for tar reduction in syngas. The dolomitic material is a double calcium and magnesium carbonate (MgCO\({}_{3}\)-CaCO\({}_{3}\)) and is the most common catalyst as it is readily available, inexpensive, disposable, and significantly reduces the tar content [124; 125]. However, dolomite undergoes rapid deactivation due to calcination inside the gasifier. When it calcined it decomposes into CaO and MgO that are highly basic [126]. Moreover, dolomite is a soft and fragile material that erodes easily, generating a raw gas with a high particulate content [127]. Olivine, natural mineral containing magnesium, iron, and silicon in the form of either ((Mg,Fe)\({}_{2}\)SiO\({}_{4}\)) [122; 128] and it is known for its effectiveness in the decomposition of tar.