Various solid-state 1 3C NMR techniques, such as CP/MAS and occasionally DP technique, have been employed to identify carbon functionality and aromaticity of biochars [1, 3, 7, 8, 10, 11, 13, 19, 23, 26]. In these studies, chemical structure of biochars to a greater extent depends upon the pyrolysis temperature, but is not much affected by heating rate and the nature of biomass [1, 13]. Biochars prepared at rela­tively low temperatures up to ~350°C retain spectral features of the original ligno — cellulosic composition of biomass [25]. For example, characteristic peaks of cellulose (0-alkyl carbons around 62, 72, 84 ppm, and di-O-alkyl carbons around 103 ppm), as well as those of lignin (methoxyl carbons ~57 ppm, aromatic carbons ~130 ppm, and aromatic C-O ~150 ppm) can still be observed in spectra of biochars within this temperature range (<350°C). For higher heat treatment temperature (HTT) biochars, a well-defined aromatic resonance evolves simultaneously with the decrease of the lignocellulosic resonances and signals of aliphatic, carboxyl, and carbonyl carbons. Lignin structures are more thermally stable than cellulose struc­tures because characteristic peaks of lignin such as the phenolic shoulders around 150 ppm can survive even at temperatures about ~550°C [25]. For HTT about 600°C upwards, the general shapes of the 13C NMR spectra of biochars are very alike, with a strong broad resonance line near 128 ppm in the aromatic region [1, 8, 22, 25].

Chemical transformation of lignocellulosic materials into graphitic structures with the temperatures between 800 and 1,000°C is well documented [1, 13, 25]. Aromatic cluster typical of chars is also reported to grow with increasing HTT [21].