The density and the radial compressive strength values of the briquettes depending on pressure and moisture content for various medium chopped materials are listed in Table 7.1. The materials were pressed mechanically in press Ø 30 mm at 10% moisture content. The influence of material on density and stability of briquette was studied on the basis of these data. When comparing the density values of the pressed materials at 140 MPa it was found that the materials have a limited effect on the density of briquettes. As shown in the table, the density values of poplar and cotton stalk briquettes are relatively higher than those of hemp and straw briquettes. When chopping cotton stalks it was found that the pressed materials contained fibres in a ratio of about 27% of the total weight. Hemp shives used in this work also have fibres in a ratio of about 10%. These fibres are more compressible than the woody particle of the other materials. As a result, the density values of the cotton stalk briquettes were higher than the relevant values for the hemp and straw briquettes. The density values of hemp briquettes also were higher than the density values of straw briquettes. This leads us to confirm that the pressed materials that contain fibres produce briquettes of high density values. Increasing the fibre percentage in these materials would increase the briquette density.

Moisture content %

Pressure MPa

Dry matter density,

g/cm³

Radial compressive strength, N/mm²

Poplar

Hemp

Straw

Cotton stalks

Poplar

Hemp

Straw

Cotton stalks

10

14

-

0.42

0.41

0.51

-

0.09

0.04

-

42

0.59

0.60

0.64

0.65

0.09

0.17

0.09

0.11

70

0.72

0.73

0.73

0.72

0.16

0.25

0.17

0.21

98

0.81

0.75

0.77

0.82

0.25

0.37

0.17

0.22

126

0.88

0.79

0.82

0.90

0.37

0.46

0.20

0.30

140

0.91

0.83

0.79

0.90

0.47

0.55

0.22

0.33

Table 7.1: Density and stability of briquettes depending on pressure and moisture content for various medium chopped materials and a stamp diameter of 30 mm

The woody particles of poplar are heavier than the hollow stems of straw and the fibres. This affects the briquette density positively, so that density values of poplar briquettes were higher than the others. Straw consists of hollow stalks. This means that the void ratio while pressing is higher than in the other materials. Straw also elongates after pressing and thus negatively affects its briquette density.

On the other hand there is an observable effect of the material on briquette stability measured as radial compressive strength. Generally, the woody materials have greater stability than straw. The lignin contained in all woody-cellulose materials begins to flow and acts as a natural glue to bind the compressed materials (FAO, 1990). The increase of lignin content in cotton stem (about 25.3%) compared with straw (about 18-19%) illustrates the high stability value of cotton briquettes.

The influence of pressure on the density and stability of briquettes was studied as shown in Figure 7.1. It is clear that there is a correlation between pressure and briquette density. As mentioned by Lindley and Vossoughi (1989), increasing the pressure up to 30 MPaleads to closely packed briquettes due to the plastic deformation of the particles. Furthermore, increasing the pressure leads to further compaction and reduction of the void ratio. Clauß (2002) stated that increasing the press pressure raises the briquette density and its stability As shown in Figure 7.1, increasing the pressure up to 70 MPa increases the briquette density digressively. In addition, increasing the pressure above 70 MPa for cotton, straw and hemp briquettes has a limited effect on density. The briquettes probably reach the plastic stage and resist further pressing. This agrees with Lindley and Vossoughi (1989). After 100 MPa, increasing the pressure increased the briquette density further. This increase is limited for straw and hemp. For cotton, there is an observable digressive increase. The results show also that increasing the moisture content to 15% decreases the briquette density. This leads us to conclude that 70 MPa can be the marginal pressure that produces a well-compacted briquette of cotton and straw, but pressed at 10% moisture content.

As a consequence, the influence of pressure on stability of briquettes was also studied as shown in Figure 7.2. It was found that there is also a correlation between the pressure and briquette stability measured as radial compressive strength of briquettes. For cotton and straw it was found that increasing the pressure up to 70 MPa tends to increase the radial compressive strength up to 70 MPa. From MPa to 100 MPa, the increase in pressure would have a very limited effect on the radial compressive strength.

Fig. 7.1: Influence of the pressure on density of medium chopped materials

Increasing the pressure above 100 MPa would increase the radial compressive strength digressively again as presented by Lindley and Vossoughi (1989). For poplar and hemp there was always a digressive increase. This agreed with the results of the effect of pressure on density. This means that increasing the pressure would increase briquette density and its stability simultaneously, thus confirming that there is a correlation between briquette density and stability.

Generally, increasing the moisture content to 15% would also decrease the briquette stability. As shown in Figure 7.2, there is an observable reduction in the briquette stability for all the materials used according to the increase in the moisture content. It can be summarized that 70 MPa is the marginal pressure required to produce a stable briquette for cotton stalks and straw. The optimal moisture content in this case is 10%. Increasing the pressure above 70 MPa would increase the briquette density in a limited range, but would raise its stability.

Fig. 7.2: Influence of the pressure on briquette stability

As reported by Lindley and Vossoughi, (1989) water is particularly suitable as an aid in briquetting. The moisture content is one of the parameters used to determine the quality of the briquette. They added that strength and density of the briquettes increase with increasing moisture content until an optimum level is reached.

The relationship investigated in this study between density of briquette and moisture content is shown in Figure 7.3. It was found that increasing the moisture content above 10% results in a decrease in the density of briquettes. This means that the briquette stability will also be reduced. It was also found that briquettes of 15 % and 20% moisture content were elongated and their volume increased directly after removing the pressure effect. This negatively affects the briquette density. Because the moisture works as a binding material, decreasing the moisture of the materials below a certain limit negatively affects the briquette stability. On the other hand, increasing the moisture over the suitable range also negatively affects the briquette stability. As presented in Figure 7.4, it is generally clear that 10% moisture content is the optimum moisture ratio to produce a stable briquette with the materials used. The high-density and more stable briquettes are produced at this moisture level.

Fig. 7.3: Density of briquette versus moisture content

Fig. 7.4: Density of briquette versus moisture content for medium chopped materials

The influence of particle size on the briquette density and stability was investigated. Lindley and Vossoughi (1989) described the path of pressing operations and illustrated the role of particle size in this process. They mentioned that in briquetting without binding agents, when external compressive forces are applied and transmitted through the aggregate of the particles, a sequence of changes of the particles occurs. Under low pressure (< 1.4 MPa), the particles slide over each other into a tighter packing arrangement and void space between them decreases. With an increase in pressure (up to 30 MPa), closer packing of the particles can only be achieved by fracturing them into smaller particles and by plastic deformation. Further compaction and reduction of void space can then occur by plastic deformation of very small particles by increasing the press pressure over 30 MPa.

As shown in Figure 7.5, the small particle sizes (x50 < 3mm) are more able to fill the interval spaces of the briquettes and that leads to a decrease in briquette volume compared with similar briquettes of coarse particles that resist compression. Therefore this has a limited effect on the briquette density. Consequently the exchange in density values is quite low for the coarse particle size as shown in Figure 7.5.

According to the results for straw it was found that there was a limited effect of particle size on briquette density. Generally, it was observed that the briquette density of the small particles pressed in a press of diameter 42.5 mm was higher. The limited effect of the particle size on the briquette density also leads to a limited effect on the briquette stability for the range of particle sizes studied.

Fig. 7.5: Influence of particle size and stamp diameter on briquette density

The influence of the press diameter on density depending on moisture content is presented in Figure 7.6. It was found that the press diameter has an observable influence on the briquette density. The press stamp of diameter 30 mm has a higher effect on the relation (surface area / volume) than a press stamp of diameter 42.5 mm. This means it has higher values of density and radial compressive strength. The major parts of the pressed materials are found in the outer surface. Because of the friction between these parts and the press wall, they are compacted more than the inner parts. In Figure 7.6 it is clear that the density values of the briquettes pressed in a press stamp of diameter 30 mm are higher than the values of poplar briquettes pressed in a press of diameter 42.5 mm. On the other hand, the density values of the hemp and straw briquettes pressed in a press stamp of diameter 42.5 mm are the highest in the maximum range of press pressure studied in a press of diameter 42.5 mm (70 MPa). These results may change if the curve is continued.

Fig. 7.6: Influence of the press diameter on briquette density of middle chopped materials

The influence of the press diameter on the stability of briquettes was also studied for the medium chopped hemp shives by way of example. The results presented in Figure 7.7 show that briquette stability (measured as radial compressive strength) increases with increasing briquetting pressure. Furthermore, they also show that the briquettes pressed in a press of diameter 30 mm are stronger than those pressed in a press of diameter 42.5 mm with the same moisture content. This confirms the effect of the relation (surface area / volume) illustrated above.

Fig. 7.7: Influence of the press diameter on briquette stability of middle chopped hemp

It was found that the equation developed by Clauß (2002) is suitable for use in this work to compare the experimental results and the theoretical models. According to this equation (2.2), the theoretical diagrams increase digressively in the low press pressure range up to 70 MPa. Then increasing the pressure has a very limited effect on the briquette density until the end density (ρ_max) is reached as presented in Figure 7.8. This density is equal to the boundary density ρ_G. That means increasing the pressure will not increase the density of briquette. The diagram in this stage is flat.

These theoretical results agreed with the experimental results for poplar, hemp and straw particles pressed in a press diameter of 30 mm at 10% moisture content. It is clear from this figure that there is a limited difference between the experimental and the theoretical results. The experimental results increase following the same trend, and thus confirm the theoretical results of Clauß (2002).

Fig. 7.8: Comparison of experimental and theoretical results for poplar, hemp and straw (10% moisture content)