Introduction

\cite{Jeguirim_2005}

Materials and methods

Beech wood supplied by the SPPS company (France) was used as precursor for biomass char production. The raw sample was finely ground with particles having sizes ranging from tens of microns to a few millimetres. After sieving, particles having a mean size of 370 µm were retained for the pyrolysis experiments in entrained fixed reactor (EFR).  In each pyrolysis test, a mass flow 1g/min was introduced in the reactor heated at temperature between 500 and 1400°C. The residual char was weighted, stored and characterized using different analytical techniques. 
The chemical composition of  the different chars was determined by means of a CHONS-NA2100 protein CE apparatus according to XP CEN/TS 15104 standard. The structural properties of the biomass chars were assessed through Raman spectroscopy  analysis. Raman spectra were recorded with a BX40 LabRam, Jobin Yvon/Horiba spectrometer. Several particles were sampled and deposited on a rectangular glass slide for the Raman analysis. Raman spectra were obtained in a backscattered configuration with an excitation laser at 635 nm. The Raman spectra at each position give average structural information of a large number of carbon micro-crystallites. The Raman spectra were recorded at 6 locations of the char sample. An average spectrum is afterward calculated.

Results and discussion

Raman spectroscopy of the biomass chars

Raman spectra of the biomass chars are shown in figure \ref{931080}. The Raman spectra exhibit two main overlapping bands around 1350-1370 cm-1 and 1580-1600 cm-1, commonly called the "D" and "G" bands. These two bands corrspond respectively to the in-plane vibrations of sp2 -bonded carbon structures with structural defects (D band) and to the in-plane vibrations of the sp2 -bonded graphitic carbon structures (G band). In the case where a high proportion of amorphous carbon structures is present -which is the case of biomass chars- these two bands overlap. This overlap is due to the contribution of the amorphous carbon forms to the Raman signal in the region between 1400 and 1550 cm-1 called the valley region "V". 
In figure \ref{931080}, the Raman signal intensity is normalized to the intensity of the G band. The impact of the pyrolysis treatment severity is reflected in the Raman spectra of the different chars. The structure of the chars appears to be highly affected. For instance, it can be seen that the intensity in the wavenumber ranges of 800 to 1100 cm-1 and 1700 to 2000 cm-1 strongly decreases with increasing pyrolysis temperature. This is related to the decrease of the highly reactive carbonaceous structures contributing to the Raman signal in these regions due to  higher pyrolysis and char cracking reactions extents \cite{Smith_2016}. The decrease of the Raman signal in these regions (together with the valley region) is reflected in a decrease of the TRA with temperature which was also denoted in \cite{Keown_2007} for cane trash chars, in \cite{Asadullah_2010}for mallee wood chars as well as in \cite{Elmay_2015} for miscanthus chars.