# Study of the Briquetting Process of Walnut Shells for Pyrolysis and Combustion
## 4 Conclusions  
In this work, good-quality briquettes were obtained from walnut shells through a simple, versatile, and easy-to-maintain process. Since the process does not require adding extra binders or previous grinding, the cost is reduced compared to other densification techniques. The moderate temperature used could be achieved by using the residual heat generated from other processes. The briquettes obtained can be considered as a clean material suitable for special combustion processes where pollution is not allowed. The effect of some parameters on the quality of the briquettes has been studied and it can be concluded that:  
There is a positive and linear correlation between the operating temperature and the density obtained. The higher the operating temperature, the higher the durability and, therefore, the briquette quality. However, in this case, no linear correlation is observed. A higher increase in durability is observed between briquettes obtained at room temperature and those obtained at 80 \({}^{\circ}\)C. Operating temperatures higher than 80 \({}^{\circ}\)C generate briquettes of higher durability, although these durability increases are not as significant as those obtained between 20-80 \({}^{\circ}\)C.  
Increasing operating temperature causes the viscosity of natural binders, such as water, to decrease and consequently, the mobility of the natural binders increases. A linear correlation has been determined between water dynamic viscosity and the durability of the briquettes. Briquettes obtained from biomass with 1% moisture show better durabilities than those from biomasses with higher moisture content (6.8% and 8.4%). Water is necessary for the densification of the biomass because it acts as a binder and a lubricant, although briquettes with a high moisture content show worse mechanical durability results, probably due to water re-expansion after the compaction process. The presence of small amounts of walnuts with the walnut shells (around 8% weight, similar to the percentage obtained in the walnut shelling process) improved the durability of the briquettes. This is attributed to the high content of natural binders (such as lipids, proteins, and carbohydrates) found in walnuts. As a final conclusion, walnut shells can be used to obtain good-quality briquettes according to the specifications of solid biofuels established in the standards, without the need for any pretreatment. A hydraulic piston press briquette machine with a compaction pressure of 66 MPa and temperatures of 80-140 \({}^{\circ}\)C allowed us to obtain briquettes with diameters of 53 mm, varying heights depending on the initial mass, densities around 1040 kg/m\({}^{3}\), and durabilities around 94%. Thus, a waste product is suitable to be valorized by combustion to obtain energy without the need for fossil fuels. \begin{table}
\begin{tabular}{c c} \hline \hline  & **Walnuts**[40] \\ \hline
**Total lipid (fat) (wt.\%)** & 65.2 \\ \hline
**Protein (wt.\%)** & 15.2 \\ \hline
**Carbohydrate (wt.\%)** & 13.7 \\ \hline
**Fiber (wt.\%)** & 6.7 \\ \hline
**Sugars (wt.\%)** & 2.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Major components of walnuts. **Author Contributions:** Conceptualization, R.F. ; Formal analysis, E.V., M.A.G., A.I.M., M.F.G.-R. and N.O. ; Funding acquisition, R.F. ; Investigation, E.V., M.A.G. and A.I.M. ; Methodology, R.F., M.A.G. and A.I.M. ; Supervision, R.F. ; Validation, M.A.G. and A.I.M. ; Visualization, E.V., M.F.G.-R. and N.O. ; Writing--original draft, R.F. ; Writing--review & editing, E.V., M.A.G., A.I.M., M.F.G.-R. and N.O. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the Agencia Estatal de Investigacion (Spain) [grant number AEI/10.13039/501100011033], by the University of Alicante [grant number UAUSTI21-03] and by the Spanish Ministerio de Economia y Competividad [Research Project CTQ2016-76608-R].