1. Introduction
In the agricultural plant world, both in the growing of agricultural plants and in the processing of them, an impressive amount of waste results, which could replace fossil fuels such as coal, natural gas or oil. All the more, there is a need for these agricultural plant wastes as they are renewable and sustainable year after year, having an ecological component by capturing carbon dioxide from the atmosphere and releasing an appreciable amount of oxygen through the phenomenon of photosynthesis.
The sunflower (
Helianhtus annuus) is an annual agricultural plant, which is native to America. It is part of the Asteraceae plant family and is cultivated worldwide for its seeds rich in edible oil [
1,
2,
3]. The seeds for which this plant is cultivated represent the smallest part of the plant (below 25%), the rest being the stem, root, leaves and seed husks, these constituting lignocellulosic biomass that could be used in combustion.
Figure 1 shows the beneficial ecological effect of sunflower cultivation on the environment [
4,
5,
6], an effect similar to all green plants that use photosynthesis for growth and development. Besides the other renewable energies (solar, wind, sea/ocean and geothermal), the energy obtained from biomass is found everywhere and has the lowest cost. The biomass torrefaction process brings a series of benefits, at only 7%–9% of the costs from the total pellet technology [
3,
5]. Lignocellulose waste resources are becoming more and more opportune nowadays.
By volume, sunflower seed shells represent about 50% of the total seeds, the other half is found in the core of the seeds from which the edible oil is extracted. The uses of sunflower seed wastes can be found in the composite board industry, in composting, but also in combustible products such as briquettes and pellets [
3,
7].
From an ecological point of view, pellets are friendly to the environment, their combustion balance being neutral, that is, the amount of carbon dioxide eliminated during combustion is equal to that absorbed by the plant during its growth (
Figure 1). When the amount of oxygen eliminated in the atmosphere through the process of photosynthesis is added to this environment aspect, as well as the fact that waste is recovered and recycled, it is clearly demonstrated that the manufacturing of native/torrefied pellets from agricultural resources such the husk of sunflower seeds is a sustainable process [
1,
3].
The calorific value of lignocellulosic materials depends on the lignin value (around 6000 kcal/kg, the equivalent of 25 MJ/kg) and cellulose and hemicelluloses calorific values (around 4150 kcal/kg) [
1,
7]. Calorific values of woody species are known to be 4000 kcal/kg for pine, 3725 for spruce, 3700 for birch, 3575 for acacia, 3550 for beech and cherry, 3450 for oak and 3200 kcal/kg for poplar [
1,
4].
Pellets obtained from vegetable waste (such as sunflower seed shells) are the most advanced combustible solid products because by compressing them toward a density of over 1100 kg/m
3, they concentrate a large amount of energy in a small volume. The low price and the simple technology of obtaining make these pellets very efficient on the market of renewable solid fuel. The fulminant development of pellet production in recent years [
8,
9,
10,
11] was possible due to the use of all categories of biomass (agricultural, wooden, forestry, food processes, etc.), but also the adaptation of pellet combustion to any type of heating furnace, fireplace or stove, with a suppling independence of at least 6–8 h. Among the many advantages of combustible lignocellulosic pellets, the following can be mentioned: they do not absorb moisture and humidity, they burn slowly, they do not emit harmful gases during combustion, they have an energetic efficiency 1.6–1.9 times higher than that of firewood, they do not contain chemicals or adhesives/additives, and they are environmentally friendly. As general characteristics, pellets obtained from sunflower seed husks can have a diameter of 8 mm, lengths between 10 and 30 mm, a moisture content of up to 10%, a calorific value over 18 MJ/kg and an ash content below 5% [
9,
10,
11,
12]. For comparison, the calorific values for other fuel products were centralized in
Table 1.
There were around 31 million tons of sunflower seeds worldwide [
8,
10] in 2007, which means a quantity of 7.7 million tons of shells that could have been turned into solid fuel (briquettes and pellets). Some authors [
14,
15]) have evaluated the chemical compounds of sunflower seed shells, finding an ash content of 3.8%. It was also found that an oil factory with a processing capacity of 400 t seeds/day, will produce about 100 t/day of sunflower seed husks, representing a ratio of 1:4. Other authors [
16] performed a proximate and ultimate analysis for sunflower seed shells, obtaining a volatile matter content of 72.2%, fixed carbon of 13.5%, ash content of 2.8%, hydrogen of 7%, carbon content of 41.8% and a calorific value of 18.03 MJ/kg.
The pellets have general presentation and classification standards as EN 14588, ENplus
® ST 1001, ENplus
® ST 1002, ENplus
® ST 1003 and ISO 17225-1 [
17,
18,
19,
20,
21], but they also have specific standards for each determination and its limiting value. For example, there are the European standards EN 16127 [
22,
23] for diameter and length, EN14774-1 and EN14774-2 [
24,
25] for moisture content determination, EN 14775 [
24,
25] and ASTM E-1755-01 (2003) [
26,
27] for ash content at 550 °C, EN 14918 [
28] and DIN 51900-1:2000 [
29] for net calorific value, EN 15148:2009 [
29] for volatiles, and EN 15103 [
30,
31] for bulk density. Other standards such as the Austrian standard ÖNORM M7135:2000 [
32] stipulate limitative values for all pellet and briquette characteristics. The diameter of the pellets is limited to 6 or 8 mm, the length is between 3 and 40 mm, the moisture content is less than 10%, the maximum ash content is 0.7%–3%, the net calorific value is between 16 and 19 MJ/kg, and the bulk density is minimal, limited at 600 kg/m
3. There is a limited variability of these values only in the case of the ash content (from 0.5% for pellets to 6% for agricultural briquettes) [
31] and a net calorific value (from 16.9 MJ/kg, up to 19.5 MJ/kg) as is stipulated by CTI—R 04/5 and DIN 51731 [
33,
34], when they are presented simultaneously by other authors [
35].
From an economic point of view, all pellets have an acceptable price of 1–1.1 Euro/kg, much lower than any other fossil energetic resources. Through torrefaction, due to the increase in the calorific power, hydrophilicity and natural durability, the investment in torrefaction is compensated, and the prices remain at the same level, at most an increase of 1%–2%. In fact, if a division of the costs is made for each technological operation, a proportion of about 9% of the pellet torrefaction costs is found [
1,
3].
The transition from laboratory to industrial production involves a manufacturing line and machines (silos, dryers, pelletizers, pellet coolers, sorters and bag-wrapping machines) with a high performance and capacity. Therefore, expenses for the purchase and maintenance of the equipment are added to the expenses at the laboratory level. Also, some characteristics of the pellets obtained at the laboratory level can change in an industrial production flow. The pellets have high mycological durability due to the high density and the fact that their surface is glossy and much denser than the inside one [
7].
Objectives. The main objective of this work is to demonstrate that sunflower seed shells can be used to obtain pellets, by evaluating some characteristics of the pellets obtained from the shells of sunflower seeds. For comparison, pellets from the sawdust obtained when cutting beech timber were considered. The physical and mechanical properties, the calorific value and other properties are intended to be evaluate. When the torrefying process was applied to the two types of pellets, they were compared with charcoal calorific properties.
2. Materials and Methods
The raw material. The material to be researched was made up of two parts, namely one part of sunflower seed husks and another part of circular sawdust obtained when cutting beech timber (
Fagus sylvatica L.). The moisture content of raw materials was about 10% [
36,
37,
38], being conditioned at air humidity of 55% and a temperature of 20 °C according to EN 1995-1-1:2004+A1, Eurocode 5 [
37] (
Figure 2). After that, so that the material was not influenced by the air humidity in the laboratory, it was cooled in a desiccator and kept in sealed polyethylene bags until it was transformed into pellets.
The pelletizing installation. The hulls of the sunflower seeds were ground in a laboratory mill, after which they were sorted with 4 × 4 mm and 1 × 1 mm sieves, taking the intermediate fraction. The sorting procedures were made using an electrical device that carried out the sorting by changing the electrical polarity of the device. The sawdust from beech wood was sorted with the same sieve, in order to use the same dimensional characteristics for both lignocellulosic species, respectively, to eliminate the influence of the granulometry of the shredded material on the properties of the pellets [
37,
38,
39,
40]. In order to obtain the pellets, a laboratory installation type Sarras 50 (Sarras, Brasov, Romania) with two pressing rollers and a capacity of 50 kg/h was used (
Figure 3). In order to identify/to differentiate the two categories of pellets, the pellets obtained from the husks of sunflower seeds (
Helianhtus annuus) were marked with “Pellet 1” and the pellets obtained when the beech (
Fagus sylvatica L.) timber was cut with circular discs were noted with “Pellet 2”. After cooling, the pellets were kept in sealed polyethylene bags, so that they would not absorb/desorb humidity from the laboratory atmosphere.
The bulk density of the pellets. This test was made on the basis of EN 15103 [
31] and ISO 17828 [
22]. This type of density was determined with the help of a truncated bucket-type vessel with a fixed volume and a Kern-type analytical balance (Kaiser Kraft, Stuttgart, Germany) having the precision of 2 decimal places for weight determination. Before weighing, the bowl with pellets was vibrated in order to settle the pellets and obtain a plate surface at the level of the bowl [
38,
39,
40]. Considering the fact that the vessel had a frustoconical shape, the relationship for determining the bulk density of the pellets was the following (Equation (1)):
where
m—the mass of the pellets in the dish, in g;
h—height of the vessel, in mm;
R—large radius of the vessel, respectively, the one from the upper part, in mm; and
r—the small radius of the bowl in the truncated shape, respectively, the one at the bottom of the bowl, in mm.
Ten valid determinations were used for this test.
The unit density of the pellets [
40]. Before performing this test, the two types of pellets (Pellet 1 and Pellet 2) were kept in a conditioning room until the pellet moisture content of 10 ± 0.5% was obtained. This characteristic of the pellets had the role of finding the degree of compaction–compression of the crushed material during palletization and decisively influenced the mechanical properties of the pellets [
41,
42,
43,
44]. To facilitate a more precise determination of the length of the pellets subjected to this test, their ends were ground with a flat abrasive disc. The mass measurement was completed with the same electronic Kern scale, and the dimensions with the help of an electronic caliper with a precision of one decimal. Taking into account the cylindrical shape of the pellets, the relationship for determining the unit density of the pellets was the following (Equation (2)):
where
ρb—density of pellet, in kg/m
3;
m—mass of pellets, in g;
d—diameter of pellets, in mm; and
l—length of pellets, in mm.
Based on the unit density of the pellets, the compaction coefficient of the pellets could be determined, related to the density of the wood species used, and the compaction coefficient of the shredded material used for pelletizing was determined with the following two calculation relations (Equation (3)):
where
CCsd is the compaction coefficient compared to sawdust density, in decimals;
ρsd—sawdust density, in kg/m
3;
ρp—density of pellets, in kg/m
3;
CC—compaction coefficient compared to wood density, in decimals; and
ρw—wood density of the species used, in kg/m
3.
Shear strength. Due to the small dimensions of the pellets (diameter and length), the shear test is very difficult to be applied [
45,
46,
47,
48]. In the present research, the pellet shear test was carried out with the help of two metal blades fixed in a universal testing machine, which generated the shear plane for 5 pellets simultaneously (since the force on a single pellet would have been very small and difficult to record accurately), as seen in
Figure 4.
Torrefaction of pellets. The purpose of the pellet torrefaction procedure was to enrich pellets in carbon, respectively, to increase the calorific value. Additionally, the torrefied pellets are less prone to moisture, having high dimensional stability. The pellets were torrefied in a laboratory oven, with the air inlet valve closed (with low oxygen content). In this way, the torrefaction was possible at high temperatures of 210 °C without the danger of self-ignition or excessive carbonization of the pellets. Three torrefaction temperatures, 170, 190 and 210 °C, and 3 torrefaction periods of 1, 2 and 3 h were used. All pellets prepared for this test were weighed before and after the thermal treatment. Based on the weighed values, the mass loss during torrefaction was obtained (Equation (4)):
where
mi—initial mass of the pellets before torrefaction, in g; and
mf—final mass of pellets after torrefaction, in g.
At least 10 valid tests were performed for each type of pellets (Pellet 1 and Pellet 2) and torrefaction time/temperature.
Calorific value of pellets. The calorific value of beech pellets and sunflower seed shells was determined using an XRY-1C explosive combustion calorimeter (Shanghai Changji Geological Instrument Co., Shanghai, China) with pellet mass of 0.6–0.8 g (DIN 51731:1996 [
29]; CTI-R 04/5:2004; EN plus 2013 [
19,
20,
21]; ÖNORM M7135 2000 [
32]; and SS 18 71 20. 1998 [
35]). The actual test, with its three stages (before, main and after), was preceded by the preparation of the installation and the used materials and ended with the recording of the results and their interpretation. To eliminate the influence of moisture content on the calorific value, before the determination, the pellets were dried for 1 h in a Memmert-type electric oven at 105 °C (Memmert, Schwabach, Germany). Even in these conditions of dryness of the pellets, due to the fact that 3 mL of distilled water was arranged in the calorimetric bomb to capture the nitric acid eliminated during combustion, the calorimeter software (2022) offered two values (the low and high calorific values) corresponding to a moisture content of pellets of about 3.5% [
49,
50,
51,
52]. The formula used by the installation software (2017) to determine the high calorific value was the following (Equation (4)) [
3,
11]:
where
HCV is the high calorific value for a certain moisture content Mc, in MJ/kg;
K—coefficient of the calorimeter, in MJ/Celsius degrees;
Tf—temperature at the end of the test, in Celsius degrees;
Ti temperature at the beginning of the test before ignition, in Celsius degrees;
m—mass of the pellet, in kg; and
Qwc—the amount of heat provided by the nickel wire and the cotton wire burning, in MJ/Kg.
The low calorific value was determined automatically by the calorimeter software, depending on the input data of the pellets. For the transformation of the calorific value from a certain moisture content of the pellets to that of absolutely dry pellets and vice versa, the following relationship was used (Equation (6)) [
3]:
where
CVMc—the calorific value of the pellets with a certain moisture content
Mc, in MJ/kg;
CV0—calorific value of absolutely dry pellets with a moisture content of 0%, in MJ/kg; and
Mc—moisture content, in %.
Energetic density. The energy obtained during pellet combustion inside bomb can be related to the mass of the sample (based on the calorific value) or to its volume (in the form of energetic density). In order to find the energetic density resulting from the pellets, the unit density of the pellets and the high calorific value must be taken into account by using the following relationship (Equation (7)):
where
HCV—high calorific value, in MJ/kg; and
ρ0—the unit density of the pellets at 0% moisture content, in kg/m
3.
In order to determine the density of the pellets at 0% moisture content, depending on the density at a certain moisture content (usually 10%), the following calculation formula was used (Equation (8)):
where
ρ0—density at 0% moisture content, in g/cm
3;
MC—moisture content, in decimals; and
ρMc—the density for a certain moisture content, in g/cm
3.
As a guideline, it is observed that the density of the pellets for a moisture content of 10% of 1100 g/cm
3 will decrease for a moisture content of 0% to about 0.909 g/cm
3, i.e., 909 kg/m
3. The difference in value determined the decision that the moisture of the pellets must be taken into consideration during the research [
53,
54,
55,
56,
57]. The dependence relationship (7) can be used in another way by finding the density at a certain moisture content, thus finding the following general relationship (Equation (9)).
where
ρMc—density of pellets at a certain moisture content, in g/cm
3;
ρ0—the density of absolutely dry pellets, in g/cm
3; and
Mc—moisture content, in decimals.
Rate of energy release. The speed of energy release is always different from one species of pellets to another and it helps to use the energy produced by pellets for stoves (with a gradual release of energy) or for thermal furnaces (with a faster release of energy) [
58,
59,
60,
61]. The rate of energy release from any fuel takes into account the calorific value, the actual burning time (main) in the calorimeter and the mass of the tested pellets. This parameter of the pellets was calculated with the following formula (Equation (10)):
where
HCV—high calorific value, in kJ/kg;
t—the effective burning period of the pellet in the calorimetric bomb, in min; and
m0—mass of dry pellet at 0% moisture content, in g.
Ash content. The ash content (ASTM E1755-01 2003 [
46]) was determined in the crushed and fine material, the part that passed through the 1 × 1 mm sieve. The fineness of the material used in this test is due to its easier burning in the calcination furnace at temperatures of 700 °C, leading to a reduction in the calcination time. Also, the test was performed using some crucibles made of nickel–chromium alloys, which were resistant to high temperatures. The relationship for determining the calcined ash content was as follows (Equation (11)):
where
Ac—ash content, in %;
ma+c—ash mass with crucible, in g;
mc—mass of crucible, in g; and
ms+c—mass of oven-dry shredded sample with crucible, in g.
Since a great quantity of smoke is released during calcination, the crushed material used to determine the ash content was first burned in an external environment, above a butane gas device, thus protecting the calcination furnace from soot deposits.
Volatile matter and fixed carbon. The volatile content was determined based on the EN 15148: 2009 standard [
30], using a ProTherm calcination furnace (Ploiesti, Romania) at a high temperature of over 700 °C. The fixed carbon is defined as the solid residue when a crushed wood sample of the biomass was burnt at a temperature of 750 °C for 7 min, or the residue obtained after eliminating the moisture content, ash content and volatile matters. The used crushed and dry material, identical to the case of determining the calcined ash content, was placed in a tall crucible with a tight lid so that the material would not oxidize during the test, this being the only way to obtain fixed carbon [
62,
63,
64,
65,
66]. The general relationship of dependence between the 3 elements, under the conditions of an absolutely dry material, is displayed in the following (Equation (12)):
where VC—volatile content, in %; FC—fixed carbon, in %; and AC—ash content, in %.
At least 8 valid determinations were made for this test.
Statistical analysis. For the statistical interpretation of the obtained results, the arithmetic mean and the standard deviation of the values were used as main statistical parameters. By using the facilities of the Microsoft Excel program, some influencing graphs were made and the Pearson R2 correlation coefficient was determined. Also, based on the statistical program Minitab 18 (Minitab LLC, State College, PA, USA), some statistical graphs and specific statistical parameters were obtained, all of these for a data error of 0.05.
4. Discussion
In order to find some elements of differentiation or proximity compared to the research of other authors, based on the experimental data obtained and/or collected,
Table 6 was made for the 11 main characteristics of the pellets.
Regarding the unit density of the pellets, it was clearly observed that the pellets made of sunflower seed shells have a higher density than those obtained from beech sawdust. Even if analyzing the two categories of pellets at the same moisture content of 8.1% (by using Equation (9)), the beech sawdust pellets would still have a slightly lower density (1023 kg/m
3) than the existing one from
Table 6, that is, the difference between the two categories of pellets would be accentuated. The two moisture levels of the pellets in the work fall within the limitations of European standards, which propose a limitation lower than 12% such as ÖNORM M7135 [
32] and DIN 51731 [
34], lower than 10% such as SS 18 71 20 [
35] and less than 11% like CTI—R 04/5—Italy [
33]. The unit density falls within the limits of the Austrian standard [
32], with a minimum of 1000 kg/m
3, or the German DIN 51731 standard [
34], with a value range of 1000–1400 kg/m
3. Lower values of the bulk density were also observed compared to those specified by the European standards due to non-compliance with the dimensional conditions, namely the existence of a minimum ratio of 5:1 between the length and diameter of the pellets. Other authors [
3,
8] have identified similar or close moisture and unit densities of the studied pellets, regardless of the wood species or the nature of the material used.
Regarding the mechanical properties of the pellets, the shear resistance of the beech sawdust pellets was higher than that of the sunflower seed husk pellets due to the fringing of the respective chips, which led to better packing and compaction within the pellets. Therefore, it can be seen that beech pellets are a little better from this point of view because they will crumble and grind less during transport and storage. The shear resistance values obtained in this research are not limited by European standards, but are in accordance with those obtained by other authors [
1,
35,
42].
Regarding the calorific value, it was found that the two values of the investigated pellets agree with the values of the European standards in the field; they are higher than 16.9 MJ/kg (SS 18 71 20/Sweden) [
35] and 16.9 MJ/kg (ÖNORM M7135/Austria) [
32], are between 17.5 and 19.3 MJ/kg (DIN 51731/DIN plus/Germany) [
29] and are higher than 16.9 MJ/kg (CTI—R 04/5/Italy) [
33]. Other authors [
50] found values of the calorific value of 17.91 MJ/kg for the wood of Norway spruce, or similar values for other vegetable species [
64,
65].
Regarding the ash content, European standards have made a clear distinction between the ash content of pellets based on wood (the values are below 1.5% according to Sweden, Italian and German standards) [
32,
36] and the ash content of pellets based on agricultural resources (the values are much higher, but lower than 6%, according to the Austrian standard ÖNORM M7135) [
32]. The two values obtained in this research fall within the above limits. Also, many authors found similar values of the ash content [
49,
50,
52].
Regarding the fixed carbon and volatile matters, the values obtained in this work were similar to those found by other authors. For example, some research [
49,
50,
52] found values of volatile matter between 47 and 69% for wood and other plant resources and between 7 and 15% of fixed carbon.
Regarding the torrefaction process of pellets, two research directions have been identified, namely torrefaction in a nitrogen environment with the help of special torrefaction installations, and torrefaction in a low-oxygen environment with the help of classic torrefaction furnaces. The torrefied products obtained by the two methods are different, namely torrefaction in a weakly oxygenated environment generates products with a lower degree of torrefaction under 20% [
38,
53,
56]. Added to the protection of the environment and the capture of carbon dioxide by this type of biomass [
66], all the characteristics of the pellets obtained from the shells of sunflower seeds showed that the biomass of sunflower seeds lends itself to transformation into pellets of good quality.
Future research directions may be about comparisons with other agricultural pellets, the comparative total quality of pellets, or torrefaction beyond 210 °C. Other new features such as the time of combustion should be highlighted.