# Study of the Briquetting Process of Walnut Shells for Pyrolysis and Combustion
## 2 Materials and Methods
### Briquetting Process and Evaluation of the Quality of the Briquettes Obtained  
A hydraulic piston press briquette machine (Mega KCK-50, SEYSU Hidraulica S.L., Madrid, Spain) was used for briquette production. A 53.0 mm diameter cylindrical mold (die) was used to make the briquettes, with height depending on the initial mass.  
This briquetting system was located inside a furnace with a temperature controller in order to be able to operate at different temperatures, from room temperature up to  
\begin{table}
\begin{tabular}{c c c} \hline  & **Walnut Shells with Small** & **Walnut Shells** \\  & **Fractions of** & **without Walnut (\%)** \\ \hline
**Moisture (\%)** & 6.83 \(\pm\) 0.02 & 8.41 \(\pm\) 0.04 \\ \hline  & **Analysis on dry basis (d.b.)** & \\  & **Proximate analysis** & \\ \hline Ash content (wt.\%) & 1.2 \(\pm\) 0.2 & 1.1 \(\pm\) 0.1 \\ \hline Volatile matter (wt.\%) & 75.2 & 73.6 \\ \hline Fixed carbon by difference (\%) & 23.6 & 25.3 \\ \hline  & **Elemental analysis (d.b.)** & \\ \hline C (\%) & 50.3 \(\pm\) 0.1 & 49.4 \(\pm\) 0.3 \\ \hline H (\%) & 5.88 \(\pm\) 0.12 & 5.48 \(\pm\) 0.06 \\ \hline N (\%) & 0.70 \(\pm\) 0.07 & 0.70 \(\pm\) 0.10 \\ \hline O (\%) by difference & 42.1 \(\pm\) 0.3 & 43.2 \(\pm\) 0.4 \\ \hline
**Net Calorific Value (d.b.) (MJ/kg)** & 17.08 \(\pm\) 0.05 & 16.85 \(\pm\) 0.06 \\ \hline
**Net Calorific Value (with moisture)** & 15.75 \(\pm\) 0.04 & 15.23 \(\pm\) 0.05 \\ \hline \end{tabular}
\end{table}
Table 1: Characterization of the two samples studied (average value \(\pm\) standard deviation, \(n\) = 3).  
\begin{table}
\begin{tabular}{c c c} \hline  & **Walnut Shell + Walnut (\%)** & **Only Walnut Shell** \\  & **(\% Dry Weight)** & **(\% Dry Weight)** \\ \hline C1 & 0.016 & 0.031 \\ \hline Mg & 0.024 & 0.027 \\ \hline Al & 0.016 & 0.025 \\ \hline Si & 0.042 & 0.059 \\ \hline P & 0.014 & 0.018 \\ \hline S & 0.009 & 0.008 \\ \hline K & 0.365 & 0.364 \\ \hline Ca & 0.660 & 0.565 \\ \hline Fe & 0.013 & nd \\ \hline Cu & 0.008 & nd \\ \hline Sr & 0.005 & 0.003 \\ \hline
**W** & 0.028 & nd \\ \hline Na & nd & nd \\ \hline \end{tabular}
\end{table}
Table 2: Minor elements of the materials studied by X-Ray fluorescence analysis.  
140 \({}^{\circ}\)C. The compaction pressure was 66 MPa and the pressing process kept the material under this pressure for 1 s twice. This pressure was selected after a previous series of runs carried out at different pressures and observing that the best briquettes were obtained at this pressure. On the other hand, this value was similar to that used with almond shells, furniture waste, and polyurethane waste in previous studies, obtaining in all cases good briquettes [12; 13; 17; 18]. The material was preheated at the selected temperature for 10 min before applying pressure. Experiments were carried out at different values of temperatures (21-140 \({}^{\circ}\)C), moisture content (1-8.4%), and presence of walnut particles (0% and 8%).  
Determining mechanical durability (DU) for evaluating the quality of briquettes was carried out by subjecting the briquette to controlled shocks by collision against the walls of a dustproof rotating prototype. Durability was calculated from the mass of the sample remaining after the separation of eroded and broken fine particles, following the standard UNE-EN ISO 17831-2:2016 [29]. However, the briquettes studied are smaller than the commercial briquettes indicated by the standard method, so a modification of this method was used to measure the durability of the briquettes obtained. The drum used had an internal diameter and depth of 120 mm and the speed rotation was 50 rpm (see Figure 1).  
On the other hand, the densities of the briquettes were measured immediately after ejection from the mold by weighing them and measuring their diameter and length. After stabilization for 24 h, the densities were measured once more. During the stabilization time, the size and parameters of the briquettes could change due to dilatation, as stated in the literature [15].