**2.1 Feedstock characterization**

The HDPE was supplied by Dow Chemical (Tarragona, Spain) in the form of chippings (4 mm), with the following properties: average molecular weight, 46.2 kg mol<sup>−</sup><sup>1</sup> ; polydispersity, 2.89; and density, 940 kg m<sup>−</sup><sup>3</sup> .

The biomass used in this study is forest pinewood waste (*Pinus insignis*). The sawdust has been sieved to obtain a particle size between 1 and 2 mm. This material has been dried at room temperature to a moisture content below 10 wt%. Ultimate and proximate analyses have been carried out in a LECO CHNS-932 elemental analyzer and in a TGA Q500IR thermogravimetric analyzer, respectively. The high heating value (HHV) for both biomass and HDPE was measured in a Parr 1356 isoperibolic bomb calorimeter. The main features of both the raw biomass and the HDPE are summarized in **Table 1**.
