H/C

First of all, you can identify the functional groups in biochar by FTIR analysis. If the spectrum reveals the evolution of aromatic functional groups replacing aliphatic groups of biomass, it shows the increase of aromatic Carbon in biochar. Also, you can use the Van Krevelen diagram. If H/C and O/C values decrease with the increase of pyrolysis temperature, it is an identification of increase of aromaticity in biochar.