3. Results and Discussion
The density, durability and water resistance of the briquettes produced from poplar wood fibres using a hydraulic briquetting machine were determined under different conditions. The temperature and feeding speed were varied, as specified in the experimental design, during densification to determine their effects on the quality of the briquettes. Since the degree of comminution of the raw material used also has a significant influence on the briquette properties, this was investigated in more detail first.

3.1. Properties of Poplar Fibre
The bulk density of the fibre after drying was 87.7 kg m−3 (moisture content 10%, wet based). The result of the particle size analysis is shown in Figure 5. According to this analysis, the raw material for briquetting had an average particle length (X50) of 0.55 mm and a content of fines (particles smaller than 0.5 mm) of 47%.

Figure 5. Particle size distribution of the raw material (poplar fibre) used for briquetting.

3.2. Effect of Temperature and Feeding Speed on Briquette Density
Table 3 shows the effects of the variables on density. From the table, the highest value for briquette density is 916.82 kg m−3, obtained at a feeding speed of 3.3 mm s−1 and a temperature of 140 °C. The lowest value was obtained at a temperature of 100 °C and a feeding speed of 2.4 mm s−1. Lindley and Vossoughi noted that, irrespective of the material type, the biomass feeding rate affects the density of the resulting briquettes. Coşereanu et al. used a hydraulic briquetting machine to densify corn stalks, corn cobs, staghorn sumac, apple tree pruning, vineyard pruning and pine cores. The densities obtained from the study by Coşereanu et al. were in the range of densities obtained in the present study for poplar wood.

Table 3. Results for average density (and standard deviation) of briquettes in kg m^−3.

For the statistical analysis, Figure 6 shows the residual plots for density. The plots show that the assumptions of ANOVA are met. These include the normal distribution of the population from which data samples are drawn, constant variance, and independence of cases. Table 4 presents the ANOVA for the density of the briquettes from poplar wood fibres. The table shows that the die temperature and feeding speed were statistically significant (p < 0.05) in determining the density. The interaction between the temperature and feeding speed on the density is also statistically significant (p < 0.05). This is in line with the study by [35], who noted that the combined effect of higher temperature and higher feeding rate results in higher briquette density. Figure 7 presents the main effects plot for density, which shows that density increases with increasing temperature from 100 to 120 °C, but remains almost constant for temperatures between 120 and 140 °C. For the speed, however, the density increases as the feeding speed is increased.

Figure 6. Residual plots for density: (a) normal probability plot; (b) versus fits; (c) histogram; (d) versus order.
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Figure 7. Main effects plot for density.

Table 4. ANOVA for density.

Although the effects of feeding speed and piston speed on briquette quality can be different, Voicea et al.  stated that the density of briquettes varies almost linearly with piston displacement speed. It was also noted that piston displacement speed is a weak parameter related to quality parameters but is, nevertheless, significant. Li and Liu found that increasing compaction speed up to 3 MPa s−1 decreased densities of briquettes beyond which the effect becomes negligible. Zafari et al. found that piston speed had significant effects on the pellet density, but there was a negative correlation between piston speed and the density for compost samples. The temperature was found to be statistically significant on density in studies by Orisaleye et al., Jekayinfa et al., Zhang et al., Lisowski et al., however, found that temperature did not have a statistically significant effect on the variations in the density of briquettes produced from walnut shells.

3.3. Effect of Temperature and Feeding Speed on Mechanical Durability
From Table 5, it is shown that the lowest mechanical durability is obtained at 2.4 mm s−1 and 100 °C and the highest at 3.3 mm s−1 and 140 °C. It is observed from the table that the mechanical durability increases with increasing temperature. This could be linked to the earlier reported higher density associated with a higher feeding rate and temperature. The average mechanical durability ranges between 97.4 and 98.43%.

Table 5. Results for average mechanical durability (and standard deviation) of briquettes in percentage.

Figure 8 shows that the assumptions of normal distribution of the population from which data samples are drawn, constant variance and independence of cases for ANOVA are met. The ANOVA for mechanical durability is given in Table 6. It is observed from the table that the temperature is statistically significant (p < 0.05) in the determination of the mechanical durability. Although the feeding speed is not statistically significant, the interaction of the temperature with the feeding speed is shown to be statistically significant in determining the mechanical durability. This agrees with, who observed that the strength of wheat straw and flax straw briquettes was not significantly affected by temperature or feeding rate. The main effects plot in Figure 9 shows that varying the temperature between 100 and 140 °C has a greater effect on the mechanical durability than the feeding speed. It is also shown that increasing the temperature increases the mechanical durability of the briquettes from poplar wood fibre.

Figure 8. Residual plots for mechanical durability: (a) normal probability plot; (b) versus fits; (c) histogram; (d) versus order.

Figure 9. Main effects plot for mechanical durability.

Table 6. ANOVA for mechanical durability.

According to Gilvari et al., some research has shown that a compression temperature higher than room temperature is crucial for making pellets with high durability. In line with the findings in this study, Nurek et al. stated that an increase in temperature improves the durability of briquettes from shredded logging residues. Other studies with similar results on the significant effect of temperature on briquette durability include Zhang et al. and Zafari and Kianmehr. Zafari and Kianmehr noted that low piston speed had a significant effect on increasing pellet durability. From Figure 6, however, no particular trend in briquette durability with the feeding speed was observed in this study. The mean durability increases from a feeding speed of 2.4 to 2.9 mm s−1 but begins to reduce afterwards. Contrary to the observations in this study, Voicea et al. noted that piston speed is significant to the durability of briquettes.

3.4. Effect of Temperature and Feeding Speed on Water Resistance
Results presented in Table 7 show that the water resistance of the briquettes ranged between 91.60 and 96.12%. The mean water resistance was highest at temperature of 120 °C and at feeding speed of 5 mm s−1. Figure 10 shows that the assumptions of ANOVA have been met, whilst Table 8 shows the ANOVA for water resistance. From Table 8, it is observed that temperature and feeding speed were statistically significant (p < 0.05) in determining the water resistance. The interaction between the temperature and feeding speed in determining the water resistance was also statistically significant (p < 0.05). While temperature and feeding rate were observed to significantly affect the water resistance of poplar wood briquettes, reported a significant effect on flax straw briquettes, but no effect on the water resistance of sunflower stalk briquettes. Orisaleye et al. and Jekayinfa et al. have shown that die temperature is significant to the water resistance with higher temperature resulting in better water resistance of briquettes. Figure 11 shows the main effects plot for water resistance.

Figure 10. Residual plots for water resistance: (a) normal probability plot; (b) versus fits; (c) histogram; (d) versus order.

Figure 11. Main effects plot for water resistance index.

Table 7. Results for average water resistance (and standard deviation) of briquettes in percentage.

Table 8. ANOVA for water resistance.

4. Conclusions
In this study, the effects of die temperature and feeding speed of a hydraulic biomass briquette machine on the quality of briquettes from poplar wood were determined. The temperatures considered were 100, 120 and 140°C. The feeding speed was varied to 2.4, 2.9 and 3.3 mm s−1. The density of briquettes ranged from 746.7 to 916.8 kg m−3. The mechanical durability ranged from 97.4 to 98.4%. The water resistance index was between 91.6 and 96.1%. The temperature was statistically significant (p < 0.05) on all investigated quality parameters and briquette quality increased with rising die temperature. The feeding speed was statistically significant (p < 0.05) on the density and water resistance. The interaction of temperature and feeding speed was statistically significant (p < 0.05) on the density, mechanical durability and water resistance of biomass briquettes.

This study particularly established that high feeding rates and high die temperatures were beneficial to enhancing the physical and mechanical properties of the briquette. Subsequent studies could expand the work to optimise the biomass properties by including other machine parameters, such as the geometry of the die, and biomass material variables, such as particle size, moisture content and biomass type. In addition to this, the effects of the variables on the thermal properties also need to be investigated.

Orisaleye, J. I., Jekayinfa, S. O., Dittrich, C., Obi, O. F., & Pecenka, R. Effects of Feeding Speed and Temperature on Properties of Briquettes from Poplar Wood Using a Hydraulic Briquetting Press. Resources, 12(1), 12. https://doi.org/10.3390/resources12010012

© 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).