As observed in Table 2, the results showed a reasonable mass balance closure in the range of 92e95 wt.% in all cases. The mass of the sample (mixture of biomass and plastic) and that of the water injected are considered in the mass balance. The total liquid retained in the condensers was a mixture of mostly water and a brown coloured oil. Table 2 shows that the addition of polypropylene in the non-catalytic process appears to increase the gas and hydrogen yields. Polypropylene, as a polyolefinic plastic, represents a significant source of hydrogen. The gas yield based on the mass of sample and water injected increased from 15.5 to 17.9 wt.% when the amount of polypropyelene was increased from 0 wt.% to 20 wt.% when the reaction was carried out without the catalyst. This increase may be due to the higher H/C molar ratio caused by the higher amount of plastic in the feed. Thus, more H and OH radicals are released and they act as hydrogen donor species, promoting the cracking of the aromatic compounds in the biomass [32]. In addition, the char and the liquid generated decreased with increasing plastic content, suggesting that the suppression of secondary reactions, such as condensation and repolymerization reactions, leads to a reduction in char and oil formation [33]. The hydrogen released by the polyolefinic polymer in the copyrolysis reaction (first pyrolysis stage) is responsible for the inhibition of recondensation reactions forming the char [34].