Characterization of biomass char formation investigated by advanced solid state NMR

Char is produced during pyrolysis of biomass and it could be valorised as soil amendment, activated carbon or to produce syngas or heat by further oxidation. The mechanisms of biochars formation have to be better understood in order to tailor their properties. For this purpose, the pyrolysis of biomass was conducted in a fixed bed reactor allowing a good control of pyrolysis conditions. Miscanthus and the macromolecules extracted from this same biomass (holocellulose, cellulose, ethanol organosolv lignin) were pyrolysed to various final temperatures (200-500 degrees C). The chemical moieties formed in the chars were studied by solid state C-13 NMR. A Cross Polarization method at the Magic Angle Spinning with Adiabatic Passage though Hartmann-Hahn conditions (CP/MAS APHH) has been found to give similar spectra than the Direct Polarization quantitative method in much shorter acquisition times. A quantitative study on the formation of aromatic structures in the network of the macromolecules in miscanthus and in the fractionated macromolecules was conducted. It is shown that below 300 degrees C xylan is the main source of aromatic formation while above this temperature the aromatization of cellulose occurs. The interactions between lignin and carbohydrates inside the network of biomass for aromatic structures formation are discussed.