# A Critical Review of SCWG in the Context of Available Gasification Technologies for Plastic Waste
## 6 Gasification of Plastic Waste: An Overview
### Polyolefin
#### 6.1.1 Polypropylene  
[PERSON] et al. [43] in 2006 conducted an experimental study on the gasification of spherical PP particles in a fluidized bed gasifier using air as the gasifying agent. They obtained a syngas with calorific value in the range of 5.2-11.4 MJ/Nm\({}^{3}\) and with a low tar content (the lowest tar content determined in the product gas was 250 mg/Nm\({}^{3}\)). Among the various operating parameters tested, the equivalence ratio is the one that most influences the reactor temperature, the distribution of the product yield, the gas composition, the calorific value of the gas, and the gas yield. The choice of the ER, strictly depends on the final use of the gas. [PERSON] et al. in 2007 [133] investigated the gasification of plastic compounds containing flame retardants. For this purpose, they prepared PP pellets with non-halogenated flame retardants (Al (OH)\({}_{3}\) and (NH\({}_{4}\)PO\({}_{3}\))\({}_{n}\)). The samples were decomposed in an image furnace at 1500 K, with a flow of gas (air or nitrogen) introduced from the bottom. Mass reduction during thermal decomposition was measured by an electric micro-balance. The presence of the flame retardant had a small effect on the thermal decomposition temperature of the polymer. However, the flame retardant ratio to the polymer influenced the gas composition and the mass of remains. [PERSON] et al. [127] in 2008 studied air gasification of PP waste supplied by a recycling company. The study compared the effectiveness of dolomite and olivine as additives in the gasifier bed for the elimination of tar. Working with the same amount of additive in the gasifier bed (30%), they found that dolomite is more active in the removal of tar than olivine. However, dolomite, unlike olivine, generated a high amount of particulate by obstructing the gas purification devices. Therefore, olivine is preferentially used as a gasifier bed material, leading to excellent results: A tar content of 2 g/Nm\({}^{3}\) at the exit, with an LHV (low heating value) of 6 MJ/Nm\({}^{3}\) and a gas yield of 6 Nm\({}^{3}\)/kg\({}_{\text{daf}}\). In 2009, [PERSON] and [PERSON] [134] investigated the suitability of different nickel-based catalysts for the production of hydrogen from the two-stage pyrolysis/steam gasification of virgin polymer pellets of PP. Pyrolysis was carried out working at a temperature of 500 \({}^{\circ}\)C. The pyrolysis gases were sent to the second reactor heated to 800 \({}^{\circ}\)C, where the gasification took place in the presence of the catalyst and steam. The catalyst/polpropylene ratio was 0.5. Among all the catalysts tested in the study Ni-Al (1:2) and Ni-Mg-Al (1:1:2) have showed good catalytic abilities in terms of both H\({}_{2}\) production and prevention of coke formation. The ZSM-5 zeolite with a higher surface area was also shown to be a good support for the nickel-based catalyst, since, the Ni/ZSM-5 catalyst showed a high rate of hydrogen production (the 44.3 wt% of the theoretical, where the maximum theoretical amount of H\({}_{2}\) produced from steam gasification of polypropylene was estimated to be 42.9 g per 100 g PP). The next year, [PERSON] and [PERSON] [88] tested a novel Ni-Mg-Al-CaO catalyst/sorbent, for hydrogen production from the pyrolysis-gasification of polypropylene by using always a two stage fixed-bed reaction system. Moreover, in this case they used virgin PP pellets. They observed that an increase in the amount of catalyst promoted the production of hydrogen from the pyrolysis-gasification of polypropylene. In particular, 70 wt% of the potential H\({}_{2}\) production was obtained, when the Ni-Mg-Al-CaO catalyst/polpropylene ratio was 5 and the steam gasification temperature was 800 \({}^{\circ}\)C.  
[PERSON] et al. [82] in 2011 continued the work of [PERSON] et al. [127] and studied air gasification of recycled PP in a fluidized-bed gasifier. In particular, they investigated several operating variables such as the amount of catalyst in the bed of the gasifier, the ER, and the temperature in the freeboard of the gasifier.