# Study of the Briquetting Process of Walnut Shells for Pyrolysis and Combustion
## 3 Results and Discussion
### Factors Affecting the Quality of Biomass Briquettes  
#### 3.3.1 Operating Temperature  
Figure 4 shows a linear relationship between the operating temperature and briquette density. The higher the temperature, the higher the density obtained, as expected. However, it must also be taken into account that the operating temperature is limited. This limit is the temperature at which biomass begins to decompose. Therefore, thermogravimetric analyses were carried out in order to determine these limiting temperatures. Figure 4: Briquette density vs. operating temperature for briquettes obtained at 66 MPa with walnut shells with 8% of walnuts and 6.83% of moisture. \begin{table}
\begin{tabular}{c c c c c} \hline
**Biomass** & **Temperature (\({}^{\circ}\)C)** & **Moisture (\%)** & **Density (kg/m\({}^{3}\))** & **Durability (\%)** \\ \hline Walnut shells with 8\% walnut & 21 & 6.83 & 951 \(\pm\) 6 & 41.5 \(\pm\) 1.2 \\ \hline Walnut shells with 8\% walnut & 42 & 6.83 & 970 \(\pm\) 6 & 82.8 \(\pm\) 0.7 \\ \hline Walnut shells with 8\% walnut & 80 & 6.83 & 989 \(\pm\) 6 & 90.3 \(\pm\) 0.4 \\ \hline Walnut shells with 8\% walnut & 95 & 6.83 & 1010 \(\pm\) 7 & 94.5 \(\pm\) 0.6 \\ \hline Walnut shells with 8\% walnut & 120 & 6.83 & 1023 \(\pm\) 7 & 95.9 \(\pm\) 0.3 \\ \hline Walnut shells with 8\% walnut & 140 & 6.83 & 1042 \(\pm\) 6 & 96.6 \(\pm\) 0.4 \\ \hline Walnut shells without walnut & 140 & 8.41 & 950 \(\pm\) 7 & 95.6 \(\pm\) 0.5 \\ \hline Walnut shells with 8\% walnut & 140 & 1 & 1056 \(\pm\) 4 & 99.0 \(\pm\) 0.1 \\ \hline Walnut shells without walnut & 140 & 1 & 963 \(\pm\) 4 & 98.3 \(\pm\) 0.4 \\ \hline \end{tabular}
\end{table} 
Table 3: Quality parameters of the briquettes obtained at a compaction pressure of 66 MPa (average value \(\pm\) standard deviation, \(n\) = 3). Figure 5 shows the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves from the thermal degradation of walnut shells and walnut shells with 8% of walnuts, under an air atmosphere. Table 4 summarizes the results obtained from the thermogravimetric analyses, where the main stages of thermal degradation of the biomass studied are detailed. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Thermal**} & \multirow{2}{*}{**T\({}_{\text{initial}}\) (\({}^{\circ}\)C)**} & \multirow{2}{*}{**T\({}_{\text{final}}\) (\({}^{\circ}\)C)**} & \multirow{2}{*}{**T\({}_{\text{max}}\) (\({}^{\circ}\)C)**} \\  & **Decomposition Stage** & & & \\ \hline \multirow{4}{*}{**Walnut shells with 8% of walnuts**} & Moisture and some & \multirow{2}{*}{29.5} & \multirow{2}{*}{145.7} & \multirow{2}{*}{65.9} \\  & light volatile loss & & & \\ \cline{1-1} \cline{3-5}  & Hemicellulose & 145.7 & 313.5 & 302.3 \\ \cline{1-1} \cline{3-5}  & Cellulose + lignin & 313.5 & 407.1 & 330.3 \\ \cline{1-1} \cline{3-5}  & Char oxidation & 407.1 & 504.4 & 469.7 \\ \hline \multirow{4}{*}{**Walnut shells without walnuts**} & Moisture loss & 30.1 & 156.5 & 73.2 \\ \cline{1-1} \cline{3-5}  & Hemicellulose & 156.5 & 254.3 & 243.3 \\ \cline{1-1} \cline{3-5}  & Cellulose & 254.3 & 353.9 & 321.3 \\ \cline{1-1} \cline{3-5}  & Lignin & 353.9 & 419.5 & 375.8 \\ \cline{1-1} \cline{3-5}  & Char oxidation & 419.5 & 485.1 & 442.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Stages of thermal decomposition of walnut shells and walnut shells with 8% of walnuts. Figure 5: TG and DTG curves from the thermal degradation, under an air atmosphere, of the studied biomass. Therefore, it can be concluded that briquettes obtained at a temperature lower than 140 \({}^{\circ}\)C do not suffer any thermal degradation. Consequently, briquettes obtained at the range of temperatures studied can only suffer moisture loss. Figure 6 shows durability (DU) vs. operating temperature for the briquettes obtained at 66 MPa with walnut shells with 8% of walnuts and 6.83% of moisture. From this figure, it can be observed that the higher the operating temperature, the higher the durability. However, no linear relationship was observed.