# 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
The highest increase in durability is observed between briquettes obtained at room temperature and those obtained around 80 \({}^{\circ}\)C. Operating temperatures higher than 80 \({}^{\circ}\)C generate briquettes with higher durability, although the increase is not as significant as that observed between 20-80 \({}^{\circ}\)C. This could be due to the softening of some biomass components in this range of temperatures. For instance, lignin, which is known as a natural binder in densification processes, shows a glass transition temperature range from 60 to 90 \({}^{\circ}\)C [33]. Consequently, densification carried out at these temperatures causes the glass transition of lignin (and other natural components), and they can act as natural binders [34]. Therefore, the most significant changes in durability occur at temperatures between the glass transition ranges of some natural components of biomass. Initially, the water content of the biomass was found in the vapor phase. However, when compaction pressure was applied, the biomass structure was broken, opening new pores due to the breakage of the biomass fibers. Then, water vapor that was in equilibrium with relative air humidity (40-60% humidity) condensed into liquid water. This liquid water probably redistributed itself across the biomass net according to water viscosity. Figure 7 shows the relationship between the dynamic viscosity of water and temperature [35; 36] and it can be observed that the higher the temperature, the lower the dynamic viscosity of water. This decrease is faster at lower temperatures. From Figure 8, it seems that the dynamic viscosity of water could be an important factor that affects the durability of the biomass briquettes. This can be explained because the lower the value of the water viscosity, the further the water will reach. This water will remain adhered through Van der Waals forces, acting as a binder. Consequently, as can be observed in Figure 8, the lower the water dynamic viscosity, the higher the durability of the obtained briquettes. Figure 6: Durability vs. operating temperature for briquettes obtained at 66 MPa with walnut shells with 8% of walnuts and 6.83% of moisture. On the other hand, water is attracted to pores due to the attraction of water to other surfaces and due to capillarity. Figure 9 shows the relationship between the surface tension of the water in contact with air and temperature [35; 37], and it can be observed that the higher the temperature, the lower the surface tension. Therefore, capillarity also decreases if the temperature increases. However, the results of durability cannot be explained by the possible variation of the surface tension of water with temperature because, as observed in Figure 9, the variation in the surface tension due to temperature is small and could be considered negligible. The temperature increase between experiments is not very high. Greater differences would be observed at higher working temperatures. Figure 8: Durability of briquettes obtained at 66 MPa with walnut shells with 8% of walnuts and 6.83% of moisture at different operating temperatures vs. their corresponding water dynamic viscosity. Figure 7: Relationship between water dynamic viscosity and temperature based on data from [PERSON] et al. [36].