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.