The char  chemical composition changes with the pyrolysis conditions. The char get richer in C and poorer in H and O as the pyrolysis reactor temperature increases. Taking the beech wood as a reference, a noticeable decrease of the molar O/C and H/C ratios is observed for the char-500 due to the high extent of the pyrolysis reaction. A higher departure of O and H atoms compared to C atom is behind this decrease of the two ratios. For instance, the emissions of H2O, H2, CO2, alkanes (CH4, C3H8) and alkenes (C3H6), which constitutes a big proportion of the pyrolysis gases induces an enrichment of the char in C element.
Up to 800°C, a systematical linear decrease of O/C and H/C with the pyrolysis temperature is observed. Above, 1000°C, the H/C ratio decreased further while O/C was similar to the value at 800°C. Such behaviour is in agreement with H2 emission at higher temperature,  This decomposition leads to the formation of richer carbon char.