Fig. 5 shows the ultimate analysis and higher heating values of the char fractions obtained at different pyrolysis temperatures. The calorific values of the chars have been calculated by means of the correlation proposed by Saldarriaga et al. [56]. As observed, carbon content in the char is enhanced by increasing temperature, from 76 wt.% to 86 wt.% in the studied temperature range. Conversely, oxygen and hydrogen contents decrease with pyrolysis temperature, indicating that the char formed at high temperatures has a more aromatic structure. Similar values have been obtained by other authors for poplar pyrolysis chars [40] and [42]. Ash content in the char is not very high due to the low amount of ash in the poplar wood.