Within the scope of this study, it was determined that apple, orange and pomegranate fruit could be transformed into pellet form after dehumidification, milling and pressing processes in order to utilize their wastes. Test samples were quite rigid and their strength analysis were done but did not consider in the scope of this research. When the strength comparison of commercial pellets and test samples was made, samples obtained from wastes gave more durable results.
Table 2
The compressibility ratio of pellets
Sample type
|
ε0 (-)
|
Apple pulp
|
0.574
|
Pomegranate seed
|
0.560
|
Pomegranate peel
|
0.600
|
Orange peel
|
0.547
|
Commercial pellet
|
0.533
|
Eq. 1 and the measurement results in Table 2 were used to calculate bulk densities of test samples which are presented in Table 2. Bulk density and the porosity value affect the burning time, emission values after combustion and heat-based conversion events [46]. Relatively best porosity value was obtained from pomegranate peel among the organic wastes. However, similar results were seen in other wastes and commercial pellets. Remarkably, all considered wastes provide better mechanical pelletizability than commercial pellets due to compressibility. In this context, it is known that cellulosic structures add improvements to the binding effects and mechanical effects [47]. On the other hand, it is mentioned that the pellet particle density of woody structures should be at or above the limit value of 600 kg / m3 [48]. In this case, it is seen that the pellet density of all samples were almost twice this threshold value. In this case, it is believed that the pellets obtained can be mechanically effective in normal pelletizing processes.
Considering the bulk density of organic waste samples evaluated within the scope of the study, for apple pulp; 619.296785 kg / m3 for pomegranate seed; 515.358863 kg / m3 for pomegranate peel; 515.358863 kg / m3 for orange peel; 679.92724 kg / m3 for commercial pellet; It was found to be 651.777386 kg / m3. Similarly, in a study conducted by Acda, M.N in woody structures, the bulk densities of pellets obtained from tropical hardwoods using woody structures such as Gmelina arborea, Acacia mangium and Paraserianthes falcataria were 621.24 kg / m3, 732.45 kg / m3, 687.23 kg / m3, respectively [49]. It can be said that the orange peel had a resemblance to woody structures in terms of bulk densities than other organic wastes of this research.
Table 3
Chemical contents and fuel properties (Original: O, Dry: D)
|
Commercial pellet
|
Apple pulp
|
Pomegranate peel
|
Pomegranate seed
|
Orange peel
|
Measuring Devices
|
Standards
|
O
|
D
|
O
|
D
|
O
|
D
|
O
|
D
|
O
|
D
|
Moisture Content (%)
|
5.42
|
-
|
4.74
|
-
|
9.17
|
-
|
1.83
|
-
|
4.23
|
-
|
LECO - TGA 701
|
ASTM E914, ASTME1131, ASTM E 18 68,DIN 51 006, ISO 71 11, ISO 113 58
|
Ash Content (%)
|
1.02
|
1.08
|
2.58
|
2.71
|
3.75
|
4.13
|
3.88
|
3.96
|
3.46
|
3.62
|
LECO - TGA 701
|
ASTM E914, ASTME1131, ASTM E 18 68,DIN 51 006, ISO 71 11, ISO 113 58
|
Volatile Matter (%)
|
79.25
|
83.79
|
75.88
|
79.65
|
64.24
|
70.73
|
77.91
|
79.35
|
75.80
|
79.15
|
LECO - TGA 701
|
ASTM E914, ASTME1131, ASTM E 18 68,DIN 51 006, ISO 71 11, ISO 113 58
|
Fixed Carbon (%)
|
14.32
|
15.13
|
16.80
|
17.64
|
22.84
|
25.15
|
16.39
|
16.69
|
16.51
|
17.24
|
ELTRA - CS 580
|
ASTM D 4239, DIN EN 13137
|
Total Sulfur (S) (%)
|
0.02
|
0.03
|
0.05
|
0.05
|
0.06
|
0.06
|
0.26
|
0.26
|
0.14
|
0.14
|
ELTRA - CS 580
|
ASTM D 4239, DIN EN 13137
|
Lower Heating Value(kcal/kg)
|
4470
|
4759
|
3604
|
3812
|
3594
|
4016
|
4156
|
4244
|
3445
|
3623
|
IKA - C 6000
|
DIN 51900, ISO 1928, ASTM 5468, ASTM 5865
|
Higher Heating Value(kcal/kg)
|
4792
|
5066
|
3940
|
4135
|
3871
|
4262
|
4506
|
4589
|
3755
|
3921
|
IKA - C 6000
|
DIN 51900, ISO 1928, ASTM 5468, ASTM 5865
|
Hydrogen Content (%)
|
5.57
|
5.89
|
5.91
|
6.20
|
4.29
|
4.72
|
6.51
|
6.63
|
5.47
|
5.71
|
LECO - CHN 628
|
ASTM, ISO
|
Fuel characterization data obtained from the analysis are shown in Table 3 The chemical contents of the pellets and organic structures were included. In terms of moisture content, it is noteworthy that the driest sample structure is pomegranate seed and it is approximately 3 times drier than commercial pellets. The low water content here means that the amount of energy consumed for drying the raw material during pelletizing will be lower. On the other hand, it is reported that drying cost is considered the biggest expense in pellet processes [50].
Organic wastes had lower moisture content than commercial pellet except for pomegranate peel. In line with these data, the inclusion of organic wastes in pellet applications may bring convenience during production. The hydrogen contents of the samples are also given in Table 3. It is seen here that there are similarities in the amount of hydrogen content of the test samples. Despite this similarity, it is also a striking factor that there is no linear or opposite ratio between hydrogen content and heating values. In a related study, it is mentioned that the hydrogen content or amount does not have much effect on pellet fuel [51]. However, in different studies on this point, it is also reported that the fixed carbon amount and hydrogen value are the factors that directly affect the calorific values of the samples. An increment of hydrogen ratio is thought to produce advantageous for fuel quality [52–54]. In this context, it is seen that the pomegranate seed and apple pulp wastes contain more hydrogen than the commercial pellet on average 14.6% and 5.6%, respectively. More particular investigations are needed in this respect, it is necessary to investigate the effects of hydrogen content in pellet applications.
Table 4
Parameter
|
Unit
|
ISO 17225-2
|
Ö-Norm
M7135
|
DIN
51731
|
DIN
Plus
|
Class 1
|
Class2
|
Class3
|
Diameter
|
mm
|
≤25
|
≤25
|
≤25
|
4-10
|
4-10
|
4-10
|
Length
|
mm
|
≤4×d
|
≤5×d
|
≤6×d
|
≤5×d
|
≤50
|
≤5×d
|
Density
|
kg/dm³
|
≥0.6
|
≥0.5
|
≥0.5
|
>1,12
|
1,0-1,4
|
>1,12
|
Moisture Content
|
%
|
≤10
|
≤10
|
≤12
|
≤10
|
≤12
|
≤10
|
Ash content
|
%
|
≤0.7
|
≤1.5
|
<1.5
|
≤0.5
|
≤1.5
|
≤0.5
|
Calorific value
|
kcal/kg
|
≥3955.2
|
≥4299.2
|
3702.1–4657.4
|
≥4299.2
|
Sulphur content
|
%
|
≤0.08
|
≤0.08
|
-
|
≤0.04
|
≤0.08
|
≤0.04
|
From the view of dimensional comparison of the pellets produced in this study with the standards, test samples were obtained with the aid of a custom die. Test samples were produced with slightly larger diameters than the common pellet size. Diameter and length were 28 mm and 30-35 mm, respectively. In terms of length, pellets were in accordance with all standards but in terms of diameter ISO 17225-2 standards have been approached up to a certain point (Table 4). On the other hand, it is seen that the produced pellets were within acceptable limits for all standards (Table 1) in terms of density. It can be seen that the same conditions are provided for the moisture content. In this context, considering that the emission effects worsen with the increase in moisture content [57], it is promising that the samples being within the limits of the standards. When the data obtained in terms of ash content are compared with the standards, it is seen that only commercial pellets comply with ISO 17225-2 second class pellet standard, third class pellet group and DIN 51731 standards. However, it has been determined that the ash content of pellets obtained from apple, pomegranate and orange wastes are not within acceptable limits. The low ash content, which has an important place in pellet quality in pellet applications, is often the desired factor [58–60]. It is desired to have a low ash ratio for many important factors such as high thermal efficiency [61], ensuring more environmentally friendly and cleaner fuel production [62–64], easier thermochemical and biochemical transformations [65–67]. Matúš, M. et al. conducted a study on wheat straw. High ash formations were reported in the preliminary analysis of wheat straw [47]. Considering organic waste, it is an expected output to have high ash rates in the wastes of apple, orange and pomegranate fruits according to the commercial pellet. Considering the amount of sulfur, it is seen that the commercial pellet complies with all standards. However, it is seen that apple pulp and pomegranate peel wastes comply with ISO 17225-2 and DIN 51731 standards. Nevertheless, orange peel and pomegranate seed waste could not be in any standard range in terms of sulfur content.
Table 5
LHV and HHV comparison of experimental samples
Dry Sample type
|
Lower Heating Value (kcal/kg)
|
Percentage change
|
Result
|
Commercial pellet
|
4759
|
%100
|
%0
|
Pomegranate peel
|
4016
|
%84.3
|
-%15.7
|
Pomegranate seed
|
4244
|
%89
|
-%11
|
Orange peel
|
3623
|
%76
|
-%24
|
Apple pulp
|
3812
|
%80
|
-%20
|
|
Higher Heating Value (kcal/kg)
|
Percentage change
|
Result
|
Commercial pellet
|
5066
|
%100
|
%0
|
Pomegranate peel
|
4262
|
%84.1
|
-%15.9
|
Pomegranate seed
|
4589
|
%90.5
|
-%9.5
|
Orange peel
|
3921
|
%77.3
|
-%22.7
|
Apple pulp
|
4135
|
%81.6
|
-%18.4
|
Table 5 shows the lower and higher heating values of the dry samples. Variations are determined by comparing with The commercial pellet results as the base point. Changes in the thermal values of pellets obtained from organic wastes were interpreted according to commercial pellets and analyzed. From the view of LHV, there was a thermal loss of 20% in apple pulp, 11% in pomegranate seed, 15% in pomegranate peel and 24% in orange peel compared to commercial pellet. Likewise, there was a thermal loss of 18% for apple pulp, 9.5% in pomegranate seed, 15.9% in pomegranate peel and 22% in orange peel on HHV. In general, fewer thermal losses occur when combustion occurs with full efficiency. However, considering waste recovery and reuse, organic waste-based pellet applications offer advantages despite these thermal losses. Leaving aside the comparison to commercial pellets to examine according to fuel standards, all organic wastes except the orange peel were found to be in the appropriate range from the view of ISO 17225-2 standards. In addition, it has been determined that pomegranate seeds and commercial pellets comply with Ö-Norm M7135 and DIN Plus standards. Finally, it is seen that all samples obtained from organic wastes comply with DIN 51731 standards.
Table 6
Theoretical calorific value calculations
Sample type
|
Original Sample type
|
Dry Sample type
|
Theoretical Heating Value
|
MJ/kg
|
kcal/kg
|
MJ/kg
|
kcal/kg
|
Commercial pellet
|
17.41
|
4158.3
|
18.4
|
4394.7
|
Pomegranate peel
|
18.06
|
4313.5
|
19.88
|
4748.2
|
Pomegranate seed
|
17.91
|
4277.7
|
18.24
|
4356.5
|
Orange peel
|
17.62
|
4208.4
|
18.4
|
4394.7
|
Apple pulp
|
17.75
|
4239.5
|
18.63
|
4449.6
|
The theoretical heating value results are given in Table 6 were calculated by using Eq. 2. Measured fixed carbon, ash content and volatile matter were the factors that affect the outcome. According to experimental data, commercial pellet provided the best calorific value result while pomegranate peel sample was the best for theoretical calculations. Another remarkable point was that all samples complied with standards according to theoretical calorific value calculations.
Figure 4 presents the experimental and theoretical LHV analysis of the samples for dry and original forms. According to the data obtained from organic wastes, it is seen that the LHV increases from the experimental result of the original form to the theoretical results of the dry form. The same situation is not seen in commercial pellets. In addition, while the calorific values obtained in the theoretical calculations for all waste samples were higher while this situation occurred in the opposite way for the commercial pellet. Among all waste samples, the pomegranate seeds sample performed the most similar experimental and theoretical results.
Figure 5. presents the experimental and theoretical HHV analysis of the samples for dry and original forms. Results and characteristics of graphs are similar to LHV results except for the pomegranate seed sample (Fig. 5.). In addition, the sample in which all calorific value results are closest to each other was pomegranate seed. A notable point of Fig. 5. was, unlike other samples, in pomegranate seeds and commercial pellets, the calorific value results obtained in the experimental results were higher than theoretical calculations. In general, the occurrence of these differences between experimental results and theoretical calculations in the calorific value parameter might be hydrogen amounts or other parameters that not taken into account. In this context, it is thought that elements such as nitrogen, oxygen, ash content and hydrogen might have an effect on the calorific value [24].
According to both theoretical calculation and experimental measurement results, it has been determined that there is a relationship between the moisture content of the samples and their calorific values. The percentage increase in the calorific values of the samples is almost equal to the moisture content removed by the applied drying process.
Numerically; the moisture content of commercial pellet, apple pulp, pomegranate peel, pomegranate seed and orange peel are 5.42%, 4.74%, 9.17%, 1.83% and 4.23%, respectively. Theoretical LHV increase rates of these samples in their dry form are 5.68%, 4.96%, 10.07%, 1.84% and 4.42%, respectively. When these increments are examined for LHV values based on experimental analysis, it is 6.46%, 5.77%, 11.75%, 2.11% and 5.16% for commercial pellets, apple pomace, pomegranate peel, pomegranate seeds and orange peel, respectively. The positive variation of HHV with the moisture content removal is higher than LHV. However, it is important to have the optimum value for the amount of moisture. It is stated that the energy spent for compaction in pellet production increases as the moisture content decreases [68]. In this context, optimum moisture content affects both the calorific value and the energy consumed in pellet production.
In addition, it was determined that the number of volatile substances increased with the dryness value of the samples which positively affects the calorific value. A 3-7% volatile matter variation was observed between the dry and original forms of the test samples. It is thought that this situation affects calorific value calculations. Wichianphong, N et al., in their study on the pelletization of water hyacinth and coffee waste, reported that the maximum calorific value of 4,275,341 cal/kg could be obtained in the pellet sample with 5.61% moisture, 73.20% volatile matter, 10.11% fixed carbon content [69]. An evaluation can be made about pellet quality by considering the volatile matter number and moisture content.
As the last, there is fixed carbon amount difference between the original and dry samples at a range of 0.3% and 0.84%. This difference creates an effective difference in calorific value calculations. It is also reported that the amount of fixed carbon is among the factors that increase the calorific value [70, 71].