# Revalorization of _Posidonia oceanica_ Waste for the Thermochemical Production of Biochar
## 3 Results and Discussion  
### Characterisation of the Posidonia oceanica Samples  
The characterisation results of _Posidonia oceanica_ (ultimate and proximate analysis) show the main physic-chemical properties of the starting biomass (Table 1). _Posidonia oceanica_ had previously been characterised by other authors [3], and a comparison revealed very similar values to those already published. It is important to determine the H/C atom ratio and the O/C atom ratio because O/C ratios have been shown to correlate well with biochar stability [50] and H/C ratios change substantially after thermochemical treatment [43]. Finally, wt% of protein was also calculated. The protein fraction was estimated based on the nitrogen content (wt%) of the biomass using a nitrogen factor (NF) of 6.25 [51].  
To summarise the characterisation of _Posidonia oceanica_, the results obtained in the different thermogravimetric tests under different oxygenation conditions: combustion and pyrolysis are shown (Figure 1).  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Materials** & _Posidonia oceanica_ \\ \hline Elemental Analysis (wt\%) & \\ C & 29.28 \\ H & 3.62 \\ N & 0.77 \\ S & 0.87 \\ H/C atom ratio & 0.12 \\ O/C atom ratio & 0.72 \\ Protein (wt\%) a  & 4.81 \\ LCV (kcal/kg) & 2496.44 \\ HCV (kcal/kg) & 2686.57 \\ Proximate analysis (wt\%) & \\ Ash b  & 44.46 \\ Moisture content & 8.54 \\ Volatile matter & 51.61 \\ Fixed carbon & 12.07 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physicochemical characterization of the _Posidonia oceanica_ wastes (unwashed).  
Figure 1: _Cont._  
Regarding combustion (Figure 1a,b), there was a first stage of weight loss, which corresponded to the sample's moisture loss. From 250 \({}^{\circ}\)C onwards, the decomposition rate increased, which may be due to the loss of volatile components in the sample. According to the immediate analysis, the volatile components represented over 50% of the weight of the total dry mass (Table 1). In this phase, the organic macromolecular chains were broken, leading to the formation of smaller molecules. When in contact with oxygen at a temperature above their flash point, volatilised gases rapidly oxidise and burn. Above 500 \({}^{\circ}\)C, the resulting mass fraction corresponds to the oxidation of the carbonaceous residue formed or the biochar. Concerning differential thermogravimetry, there were two distinct peaks: one at 288 \({}^{\circ}\)C, which corresponds to the decomposition of hemicellulose; and another at 470 \({}^{\circ}\)C, which corresponds to the thermal decomposition of cellulose.  
The biomass pyrolysis results were presented in the same way as for combustion (Figure 1c,d). Unlike the case of combustion, the decomposition stages were not very abrupt. First, as the temperature increased, a drying process took place and, possibly, the release of some volatile compounds. From 250 \({}^{\circ}\)C onwards, the low molecular weight compounds began to volatilise, up to about 350 \({}^{\circ}\)C. The remaining mass fraction of the gas was then released. The rest of the mass fraction corresponded to ash and solid carbonaceous residues that were not oxidised by the atmospheric conditions under which the decomposition took place. The differential thermogravimetry revealed two other peaks: one at 273 \({}^{\circ}\)C, corresponding to the decomposition of hemicellulose, and the other at 319 \({}^{\circ}\)C, corresponding to the thermal decomposition of cellulose. Lignin decomposition was difficult to identify as it decomposes over a wide range of temperatures. The 660 \({}^{\circ}\)C peaks may have corresponded to the decomposition of this compound.