Upgrading of physical and elemental quality of wheat straw pellets using additive blends

doi:10.21203/rs.3.rs-3610670/v1

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Upgrading of physical and elemental quality of wheat straw pellets using additive blends

https://doi.org/10.21203/rs.3.rs-3610670/v1

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Densification of biomass is typically done by pelletising to produce clean fuel from renewable resources. This study investigated the impact of additive blends for wheat straw pellet making and upgrading the physiochemical properties. Five additives, including sawdust (SD), bentonite clay (BC), corn starch (S), crude glycerol (CG), and biochar (BioC) were chosen for this study. Pellets were made from seven different combinations using a laboratory-scale pellet mill. The pellets' physical and elemental quality was evaluated and compared with ISO 17225-8 standards. The pellet’s mechanical durability, tensile strength and bulk density increased from 80 to 99%, 0.36 to 2.09 MPa, and 244 to 665.21 kg/m3, respectively, which met the requirements of the ISO standards. The additive blend pellets (T₃ ~ T₇) had a fines content of less than 2%, also meeting the ISO standard. Further, all pellets showed a water absorption capacity under 2%, except T₁ and T₅. Finally, the fixed carbon contents increased from 11.1–30.90% and energy contents from 17.02 to 20.36 MJ/kg, which showed a significant synergistic effect of blending additives. Overall, pellets with additives mixed met the pellet ISO standard specification requirements, except for inorganic ash content. The study demonstrated that wheat straw is a good biomass source for pellet-making for energy applications, but further research on suitable additive mixing is still needed.

Wheat straw

pellet quality

additives

physicochemical characterisation

bioenergy

pelletising

Global energy demand has continuously increased due to both population and industrial growth (Al-Hamamre et al. 2017). It is foreseen that there will be a 30% increase in energy demand between now and 2040 (OECD 2016). A total of 32,294 million tonnes of CO₂ will be also released into the earth to generate this large amount of energy (Ríos-Badrán et al. 2020). As a result, new alternative renewable energy generation with minimum environmental impact is gaining popularity day by day and must be developed.

Renewable energy options include biomass, solar, wind, and hydropower (Nanda et al. 2018). Solid biomass is the most demanding and promising renewable energy alternative to fossil fuels (Rajmohan, Ramya & Varjani 2021). Solid biomass is less location- and climate-dependent, inexpensive, abundant, and also globally available (Ribeiro & Junior 2023). Moreover, biomass is often emission negative (Quan, Jia & Gao 2020). Agricultural waste is the world's largest solid biomass by mass (Sarkar et al. 2012). This plentiful agro-waste is often highly accessible, representing both a management problem and a fossil-fuel-free alternative (Lu et al. 2014a). In addition, several technical hurdles are limiting the production of bioenergy from biomass on a larger scale (Sarwer et al. 2022). These barriers are connected to the physicochemical properties of biomass, such as low energy density, hydrophilicity, etc (Yu et al. 2019). To overcome the management problem of agro-residue and upgrade the raw materials quality, the densification process (pelleting) is often essential.

The interest in agricultural residues from straw (corn, sorghum, barley, wheat, rice, etc.) has recently increased, especially for wheat and rice straw, because both crops are widely grown globally (Singh et al. 2021). Worldwide total wheat and rice grain production in 2022/23 was 1286.78 million metric tonnes (https://www.statista.com/statistics/263977/world-grain-production-by-type/), which also produced approximately 1673.0 million metric tonnes of straw (grain and straw ration = 1:1.3). Therefore, managing this massive amount of straw is often quite challenging. To date, developing countries like India and Bangladesh typically burn straws on the field, resulting in raw material losses and also energy waste and pollution (Singh et al. 2021). However, direct use of straws as well as heating and cooking are often unjustified and impractical because of low energy density, unpredictable burning and hurdles in storage and transportation (Jian et al. 2019). In this situation, raw straw processing, like pelleting, is necessary for the purpose of easy management (handling, storage, transportation, and combustion) and obtaining sustainable combustion characteristics (High calorific value) (Guo et al. 2022).

Up to now, the majority of pellets are made from woody biomass because of their low ash contents (Mansuy et al. 2015). However, pellet production from wood-based biomass materials cannot totally meet the market demand. Also, some countries like Australia stopped pellet-making from forest biomass (https://happyeconews.com/australia-rejects-forest-biomass-in-first-blow-to-wood-pellet-industry/). On the other hand, Australia produced 45.0 million tonnes of wheat residue as crop waste. Therefore, wheat straw biomass could be a significant source for pellet making because of its low price and wide availability. Compared to woody pellets, agro-pellet, especially wheat straw pellets, are often regarded as low-grade pellet fuel, typically having a lower calorific value, a lower density, and a higher ash content (Yu et al. 2019). Shahram Emami (2014) stated that making standard-quality wheat straw pellets is challenging without additives. Moreover, manufacturing agricultural straw pellets is often more difficult due to lower lignin content (Liu et al. 2013; Brand et al. 2021). Therefore, there is a need of improved methods to increase the wheat straw pellets quality, which may be done by mixing different additives (Mehdi et al. 2021).

The volumetric densities, energy efficiency and mechanical qualities of agro-pellets are often incomparable with coal or wood (Guo et al. 2022). However, agro-pellet quality improvement could be possible through suitable treatment and pre-processing (Ashokkumar et al. 2022). However, pre-treatment often involves significant energy requirements (Nath et al. 2022). Complex and sophisticated steps involved in pre-treatment methods could increase the pellet production cost. For example, stream exploration needs a temperature of 180~230ºC for 2~10 minutes (Harmsen et al. 2010). This may not be realistic for farm-level pellet production, which should involve a relatively simple process and less cost. Therefore, this study is focused only on additive/biomass mixing and size reduction of wheat straw by milling machine for pellet production.

Several investigations have used various methods and considered different types of blending or additive materials for straw or non-weedy pellet production and their quality improvement. For example, Park et al. (2020) and Ríos-Badrán et al. (2020) have used a wood/agricultural residue mixture for pellet production. Siyal et al. (2021) and Jiang, L. et al. (2016) have belnded agro-wastes with other residues, such as sewage sludge, pyrolysis oil and hydrolysis lignin for co-pelleting. Previous research has demonstrated that biomass mixing can considerably enhance the physiochemical character by mechanical interlocking among the particles (Li et al. 2015), ultimately improving pelleting performance. For instance, mixing spruce wood with corn stover increases their pellet mechanical qualities (Stasiak et al. 2017; Yub Harun, Parvez & Afzal 2018). García et al. (2019) also found that adding pine sawdust (high calorific value) with wheat straw boosts the heating value by 15%. Additionally, energy additives like glycerol, molasses, cole, biochar, and rapeseed cake help reduce the ash percentage and increase the pellet heating value (Kaliyan & Vance Morey 2009; Yang, Hanna & Sun 2012). Furthermore, lubricant oil can improve the biomass pellet's bulk density and flowability (Adapa 2007). The sawdust and biomass mixtures are usually structural additives that are environmentally friendly (Ståhl & Berghel 2011). Therefore, the present study considered sawdust (works as structural formation), glycerol and biochar (acts as energy additives), corn starch and bentonite (are binding agents) for additive/blending materials.

It is noted that previous work mainly focused on pellet fuel properties (Kaliyan & Vance Morey 2009), for different combinations of biomass (Bilal et al. 2017), varied binders use (Shahram Emami 2014), and pellet durabilities (Shaw 2008; Kumar, Jones & Hanna 2009), among others (Table 1). Until now, less attention has been paid to the quality improvement of wheat straw pellets. An attempt has been made to quantify oat, wheat straw, barley and canola straw's densification properties (Adapa, Tabil & Schoenau 2009). Lu et al. (2014b) investigated the additives admixing and densification load for wheat straw pelletisation. They found that binding materials and compact load (4000 N) positively impacted density and tensile strength. However, as noted previously, the improvement of wheat straw pellets quality has only been limited. In addition, there have only been limited studies which attempts to manufacturing pellet at on-farm conditions.

The objective of this work was to make quality fuel pellets using several additives combinations for the upgradation of chemical and physical characteristics. In addition, the impact of additives on fuel properties (calorific values, strength, durability, fines content and wettability index) was explored in this research. Finally, the fuel properties were compared with the standard value specified by ISO 17225-8 (ISO/TS 2016).

Table 1

List of different biomass blends/additives for wheat straw pellet production.

Material	Objectives	Analysis/Outcomes	References
Rice husk and wheat straw pellets and their blends	Physicochemical and energetic characterisation	Ric husk exhibits lower calorific value and higher ash content Moisture, ashes, and nitrogen content did not match ISO 17225-6 standard but diameter, length, and durability were compiled	(Ríos-Badrán et al. 2020)
Wheat straw pellet blended with wood residues, pre-treated wood residues, lignosulfonate, glycerol, and bentonite clay	Investigate the binder effects on pellet quality Study of specific energy consumption and pellet properties	Binders significantly decrease the specific energy consumption Additives increase the tensile strength, higher heating value, and reduce the ash content	(Lu et al. 2014)
Wheat straw pellet	Effect of pelletisation process and densification parameters on the properties of the wheat straw powder and 40% epoxy 1092 mixture	Increase the fixed carbon and heating value, bulk density Improved combustion characteristic	(El-Sayed & Elsaid Mohamed 2018)
Wheat straw pellet making	Identification of the key factors affecting the pelletising pressure in biomass pelletisation processes	Pelletising pressure increased the pellet length Increasing the temperature resulted in a decrease in the pelletising pressure	(StelteClemons, et al. 2011)
Pine sawdust and wheat/ rapeseed straw blends and pellet production	Mechanical and combustion properties of pellets made of pine sawdust mixed with straws	Pellet density, strength and calorific capacity increased with the addition of pine sawdust	(Stasiak et al. 2017)
Wheat straw pellet manufacturing	Investigation of the effects of molasses on wheat straw pellet physical quality	Temperature is a key factor for good pellet quality Exceeding the lignin glass transition temperature leads to better pellet quality Molasses strengthens pellets production at temperatures below the lignin glass transition	(Mišljenović et al. 2016)
Wheat straw pellet	Investigation of biological pre-treatment to improve the pellet quality	Temperature and biological pre-treatment could improve the physical quality	(Gao et al. 2017)
Pellet from torrefied and raw wheat straw	Thermo kinetic properties study	The bulk density is higher in brown torrefied pellets Pellet properties satisfy the ISO 17225-6 standards Temperature is the potential pre-treatment application	(Azocar et al. 2019)

2.1. Materials

The study was conducted at the University of Southern Queensland, Toowoomba, Australia. This research used wheat (Triticum aestivum L.) straw to produce pellets. A total of five different additives/binding materials (sawdust, bentonite clay, glycerol, corn starch, and activated carbon/biochar) were used in this study.

2.2. Chopping and milling of wheat straw

Wheat straw is uneven in shape and size. Furthermore, wheat straw needs to be mixed with other materials to produce a uniform dimension for pelletisation. For size reduction, wheat straw was chopped and ground by a hammermill with an electric motor power of 4.0 kW (MKHM 198, Meelko CO-United States, Agricultural Machinery, China). Sixteen swinging knives were used for chopping purposes, and the blade rotational speed was 1500 rpm. An exchangeable screen (3.2 mm) was used for milling the straw. The sample was milled to less than 7 mm ground particles (Fig. 1). However, fines powders were also produced in this process.

The ground materials' particle size distribution was determined using a sieve analysis with the opening (sizes) of 4.75, 2.36, 1.8, and 0.7 mm. The particle size distribution of ground raw materials is presented in Table 2. All blending materials' particle size was less than 1.8 mm.

Table 2

Particle size (%) distribution of raw materials.
Raw materials	≥ 6.7 mm	≥ 4.75 mm	≥ 2.36 mm	≥ 1.8 mm	≥ 0.7 mm	< 0.7 mm
Wheat straw	13.40	12.60	27.20	16.70	7.80	22.30
Sawdust	0.00	0.00	0.00	16.50	22.27	61.23
Bentonite clay	0.00	0.00	0.00	0.00	32.30	67.70
Biochar	0.00	0.00	0.00	7.21	28.63	64.16
Starch	0.00	0.00	0.00	0.00	0.00	100.00

2.3. Blending material selection and proportion

Different combinations of blenders or additives were used to develop a suitable wheat straw (WS) pellet (Table 3). In this work, seven different types (treatments) of pellets were used for qualitative analyses. This research followed the published article regarding the used blenders' proportion and material characteristics (moisture content and particle size). According to Pradhan, Mahajani and Arora (2018), the moisture range is 15 to 23%, and a particle size of less than 6.5 mm is required for pellet processing. In addition, Nazemi et al. (2022) suggested that 20% of sawdust with wheat straw can enhance pellet strength. Also, additives proportion of 0.5 to 5% (total mass fraction) can improve pellet quality (Tabil 1996). Therefore, the present research considered the raw material moisture content to be around 20%, with particle size not bigger than 2 mm. Moreover, the bentonite clay, sawdust, starch, glycerol, and biochar ratio were all set at 10% (weight basis) level.

The research aim was pellet production at the farm level. Based on this objective, locally available and less costly additives were considered for pellet making. Therefore, this research selects additives from bentonite clay, sawdust, starch, glycerol, and biochar. During the pellet production, it was found that the (pure) wheat straw pellet (without additive) production capacity was very low. However, the other pellets (T₂ ~ T₇) production rate was satisfactory.

Table 3

Pellet production with different blends of materials.
Treatment and mixing materials	Composition, %	Pellet production capacity, kg/h
T₁: Wheat straw	100	32
T₂: Wheat straw: Sawdust: Bentonite clay	80: 10: 10	131
T₃: Wheat straw :Sawdust: Corn starch	80:10:10	107
T₄: Wheat straw: Sawdust: Bentonite clay: Glycerol	70: 10: 10: 10	149
T₅: Wheat straw: Sawdust: Bentonite clay: Biochar	70: 10: 10: 10	196
T₆: Wheat straw :Sawdust: Corn starch: Biochar	70: 10: 10: 10	163
T₇: Wheat straw :Sawdust: Corn starch: Bentonite clay: Biochar	60: 10: 10: 10 : 10	185

Note, % = fraction of total mass basis

T₂ and T₃: wheat straw with binding additives

T₄ and T₅: wheat straw with energy materials

T₆: wheat straw with both binding and energy additive

T₇: all combinations with wheat straw.

2.4. Pelleting process and pellet production

The wheat straw pellet production method follows the raw materials collection, size reduction of wheat straw, mixing additives with ground wheat straw, homogenisation (mixing all materials), pelleting, and drying steps. The pelleting process adopted in this research is presented as a block diagram in Fig. 2.

A Roller-Turned Flat Die pellet mill (GEMCO-China, Model: ZLSP200B R-Type) was used for pelleting, with a production capacity (wood pellet) of 80 ~ 220 kg/hour. This pellet mill consisted of a feeding hopper, a barrel, a cylindrical steel roller, a plate-type flat die with a hole, and an electrical motor (3-phase, 7.5 kW). The pellet ejection hole’s inner length and diameter were 40 and 8 mm, respectively.

The rotating (50 rpm) roller's weight produces friction, compresses the biomass sample, and feeds them through a die. The roller slipped on the die plate surface, which helped to discharge the densified sample. In this study, the produced pellets were allowed to cool and dry in the open air under ambient conditions. The pellets were then stored in the laboratory for drying (< 10% moisture content). Their moisture content reached approximately 12% after one day, and pellets became stabilised 14 days after extrusion (Fig. 3).

2.5. Standard for biomass and pellets physiochemical characterisation

The Association of Official Analytical Chemists (AOAC) and the American Society for Testing and Materials (ASTM E 873 − 82: bulk density of densified particulates biomass fuel) standard methods were used to determine biomass and pellet's physicochemical characteristics.

2.6. Measurement of elemental composition

The chemical analysis of wheat straw, sawdust, biochar, bentonite clay, and produced pellet's was performed in the Feed Central Laboratory, Toowoomba, Queensland, Australia. Triplicate measurements were carried out for each sample to ensure accurate and reliable results.

2.6.1. Component analysis

The primary components of biomass are lignin, cellulose, and hemicellulose. These were determined from acid detergent fibre (ADF), acid detergent lignin (ADL) and neutral detergent fibre (NDF). ADL and ADF were determined with the Association of Official Analytical Chemists (AOAC) standard technique 973.18 - Fiber (Acid detergent) (AOAC 1990b). NDF was determined using AOAC standard method 992.16 (AOAC 1990c). Cellulose and Hemicellulose content was determined from ADF and lignin as: % Hemicellulose= % NDF - % ADF and % Cellulose = % ADF - % ADL (Mani, Tabil & Sokhansanj 2006).

2.6.2. Proximate analysis

The Hach method determined moisture content and the volatile compound (AOAC 2002). Also, the AOAC standard method 942.05 was used to determine the total ash, where a 2 ~ 3 gm sample was burned in a furnace above 700°C in an oxygen environment (AOAC 1990a). Next, it took the remaining material (after VM loss). Finally, the fixed carbon (FC) content was considered with Eq. 1 (dry basis) (ASTM 2013).

FC = 1 – M – Ash – VM ………….………………….………………(1)

Whereas: FC = Fixed carbon, %; M = Mass, %; VM = Volatiles matter, %

2.6.3. Thermal analysis

Solid material's gross energy (gross heating value) is expressed as calories per gram. Following the ASTM D5865-03 standard (ASTM 2003), the heating value was quantified by the IKA C2000 basic oxygen bomb calorimeter for thermal analysis. This procedure fixed the instrument to IKA's dynamic mode through an outer vessel temperature of 25ºC. In addition, 1.0 gm of Parr standard benzoic acid was used to calibrate the calorimeter. For gross energy determination, around 0.50 gm of the sample was deposited in the combustion capsule and kept in the sample holder head of the bomb calorimeter.

An essential criterion for a material's combustibility is its gross calorific value. Gross calorific value (GCV) is the quantity of energy released per unit of fuel burned. Based on analytical techniques, three types of energy were determined from biomass. In this case, a higher heating value (at constant volume - dry basis), a lower heating value (constant pressure - dry basis), and a gross calorific value (constant pressure - wet basis or as received) are all applicable (Telmo & Lousada 2011)). The gross calorific value was considered for all pellets in this investigation because it is measured most practically.

2.6.4. Minor elemental analysis

CN628 Carbon/Nitrogen was used to quantify total organic carbon (TOC) and nitrogen. In addition, AOAC 990.03 - the determinator follows protein (Crude) in Animal Feed (AOAC 2006). In addition, the sulphur was determined by the CEM Application Notes for Acid Digestion method (ASTM 2008). Finally, the difference in elements decided the oxygen.

2.7. Measurement of physical properties

Pellet quality assessment checks and compares with the standard and commercially prescribed levels (ISO 17225-8). The density, strength, water absorption index, and fines are commonly used to evaluate pellet quality (Zafari & Kianmehr 2014). First, all samples were handled and chosen to ensure a certain homogeneity of the samplings. Then, the laboratory tests, measuring elements such as weight, diameter, length, bulk density, water absorption, and dust content, were undertaken to evaluate the samples (Mani, Tabil & Sokhansanj 2006; Jiang, L.-b. et al. 2016; Pampuro, Busato & Cavallo 2018).

2.7.1 Pellet dimension

Typically, pellets were cylindrical. This study used 33 pellets for each treatment to measure physical properties such as diameter, length, weight, and densities (individual and bulk). Then, the pellet weights were measured with a digital balance while a vernier calliper recorded the length and diameter.

2.7.2. Unit density

The pellet's unit density (\({\rho }_{u})\) was calculated from each pellet's weight and volume (depending on length and diameter), using the following equation.

Unit density (\({\rho }_{u}\)) = \(\frac{\pi {m}_{u}}{4{D}^{2}L}\) …………………………………………………………..(2)

Where: D = diameter of an individual pellet, mm; L = length of an individual pellet, mm; \({\rho }_{u}\) = density of an individual pellet, kg/m³ and \({m}_{u}\) = mass of an individual pellet, g.

2.7.3. Bulk density

The norm ASTM E 873 − 82 was used to calculate the bulk density of densified particulates biomass fuels (ASTM 2013). The material's mass-to-volume ratio was used to calculate the bulk density (\({\rho }_{b}\)). First, a digital balance was used to weigh the pellets after the container top surface was levelled. Then, the volume was estimated by measuring the container's length and diameter (Eq. 3).

Bulk density (\({\rho }_{b}\)) = \({{m}_{b}/V}_{b}\) ……………………………………………………….(3)

Where: \({\rho }_{b}\) = bulk density, kg/m³; \({V}_{b}\) = volume of a container, m³, and

\({m}_{b}\) = total mass of pellets, kg.

2.7.4. Mechanical durability

The mechanical durability test in this work followed ASTM D3038 with minor modifications. First, a digital scale was used to weigh the pellet, which was released on a steel pan from a 1.85 m height. Subsequently, the large fragment weight was recorded as a pellet’s final mass and repeated three times. Then, Eq. 4 was used to calculate the impact resistance/mechanical durability (Gilvari, De Jong & Schott 2020).

Pellet durability (%) = \(\frac{weight of biggest broken pellet piece}{weight of the original pellet} \times 100\)…………….………(4)

2.7.5. Tensile strength

The pellet diametric compression test is often represented by tensile strength (Horabik et al. 2023). Therefore, in this study, pellets were initially diametrically sliced into a 2 mm thick tablet using a diamond cutter wheel bit in an automatic precision cutting machine (Struers Minitom, Pederstrupvej 84, Denmark) (Shaw, Karunakaran & Tabil 2009). Then, the Instron plunger inserted the pellet tablet lengthwise in the base plate centre and pressed at a 1 mm/min speed. First, the force-displacement information and fracture load data were noted at the pellet tablet’s failure point, such as when the tablet was divided into two semi-circular segments. Next, the pellet tensile strength was determined with the following Eq. 5 (Farsi et al. 2020). The test was repeated ten times for every treatment.

Tensile strength (σ) = \(\frac{2F}{\varPi dl}\) …………..…………………..…………………..……..(5)

Where: σ = Tensile (horizontal) stress, Pa

F = load at fracture, N

d = Compact diameter/ Pellet diameter, m

l = Compact thickness, m

2.7.6. Fines content

The amount of fines materials was calculated following the ASABE 269.4 standard procedures for solid biofuels using tumbling methods (ASABE 2010). According to the process, 100 g of pellet sample was weighed, and then 10 minutes tumbled at 50 rpm in a dust-tight enclosure chamber. A sieve with 5.70 mm mesh was ed to calculate the particles created in the pellets tumble operation. In addition, the sieve opening sizes: 5.6, 3.15, and 1.0 mm were used to grade particles. The technique was performed threefold. The small particles were determined with Eq. 6:

% of small particles = \(\frac{({W}_{i}-{W}_{f})}{{W}_{f}}\times 100\) ……………………………………….……….(6)

Where: W_i = pellet initial weight before tumbling, g and

W_f = pellet final weight after tumbling, g

Three types of sieves were considered for analysing the particle size distribution of small particles (Table 4).

Table 4

Sieve for particle size distribution.
Sieve with screen sizes	Particle category
Pan	dust (< 1 mm)
1 mm (square holes)	fines (1 mm < fines < 3.15 mm)
3.15 mm (round holes)	lumps (3.15 mm < lumps < 5.6 mm)
5.6 mm (square holes)	whole pellets > 5.6 mm

All samples were weighed first and then manually sieved. The dust, particles, and lumps were then measured, recorded, and balanced. Finally, the fines were calculated for each particle group using the following method (Eq. 7).

% of particle type = \(\frac{{M}_{i}}{{M}_{t}}\times 100\) ……………………………………………….(7)

Where: \({\text{M}}_{\text{t}}\) = total mass of pellet/sample before tumbling, g and

\({\text{M}}_{\text{i}}\) = final mass of particles after sieve, g

2.7.7. Wettability index

During transportation and storage, short-term experiences with rain and moisture conditions could harm pellet quality (Kaliyan & Vance Morey 2009). Therefore, the moisture or water resistance of the pellet is an essential parameter that is evaluated using the wettability index (Papandrea et al. 2021). For the moisture resistance assessment test, 50 g pellets for each treatment were immersed in water for 30 seconds in ambient conditions (26°C and RH 51%) (Kaliyan & Vance Morey 2009; Papandrea et al. 2021; Yoshida et al. 2021). After removing the water, the pellet was placed in ambient conditions for one hour and weighed again. The wettability index was expressed as the percentage of weight variation between the pellet before and after water immersion (Papandrea et al. 2021), and repeated the test was five times for each sample (Eq. 8).

Wettability index (WI) = \(\frac{({W}_{f}-{W}_{i})}{{W}_{f}}\times 100\)…………………………….…..………..(8)

Where: W_i = pellet initial weight before submerging in water, g

W_f = pellet final weight after (30 s) submerging in water, g

2.8. Statistical analyses

The one-way analysis of variance (ANOVA) was used to determine the effect of different additive mixing levels on size, density, durability, and strength. In addition, Duncan's multiple range tests were used to examine significant variations in means at a 5% significance level. IBM SPSS software version 27 was used for statistical analysis (IBM crop., Armonk, New York).

3.1. Pellet composition

Cellulose, hemicellulose, and lignin are the basic components of biomass (Lu et al. 2014a). Hemicelluloses and lignin are amorphous polymers that act as binders. It is activated (softening) with temperature and moisture to create strong particle-to-particle bonding. The glass transition temperature of lignin is usually 60–200ºC (Olsson & Salmén 1997). StelteHolm, et al. (2011) however noted that the wheat straw's lignin-softening glass transition was about 53ºC. It was found in this study that the pellet development temperature reached around 80ºC because of particle friction in the pelletising machine.

The proximate analysis examined the major elements, whereas minor features were found in the ultimate analysis. Major and minor chemical components comprise the biomass element characteristics (Table 5). Many research studies have noted that increasing lignin is essential for densification and pellet quality improvement. Wheat straw contains low lignin content and may not bind the particles well (Thomas, Van Vliet & Van der Poel 1998). In this research, the lignin content of sawdust (26.66%) was significantly higher than wheat straw (7.30%) (Table 5).

Table 6 shows that the hemicellulose content for different treatments was moderately varied. The physical process (temperature change) could not change the hemicellulose amount. Instead, the cellulose content decreased through additive mixing. Hence, the pelletising temperature might break down the structure and change the cellulose content. Therefore, different additives were mixed with wheat straw to increase the lignin content and help it bond with particles. Among the treatments, the lignin and cellulose content have an inverse relationship. Overall, cellulose decreased by 41–28%, and lignin varied from 7 to 13.10% (Table 6).

Table 5

Physiochemical analysis of raw materials used for pellet production.
Sample	Proximate analysis					Ultimate analysis					Density	Composition
Sample	MC, %, db	VM, %, db	FC, %, db	Ash, %, db	GCV, MJ/kg	C, %	H, %	N, %	S, %	O^a, %	BD, kg/m³	HC, %	C, %	L, %
Wheat straw	9.62	72.78	10.30	7.30	17.6	46.06	5.00	0.53	0.11	48.3	107.26	23.00	40.5	7.30
Sawdust	6.60	76.80	16.27	0.33	20.95	51.8	6.14	0.26	0.02	41.78	208.09	13.66	45.38	26.66
Coconut shell biochar	7.40	7.40	81.30	3.90	30.75	83.8	0.90	0.50	0.10	14.70	583.48	-	-	-
Bentonite clay	7.30	-	-	89.63	0.00	-	-	-	-	-	890.00	-	-	-

Note; MC: Moisture content; VM: volatile matter, FC: Fixed carbon; GCV: Gross calorific value; C: carbon;

H: Hydrogen; N: Nitrogen; S: Sulfur; O: Oxygen; BD: Bulk density, HC: Hemicellulose; C: cellulose;

L: Lignin; wb: Wet basis; db: Dry basis; ar: As received

^a Determined by a difference

Table 6

The elemental composition of pellets.
Treatment	Dry wt, %
Treatment	Hemicellulose	Cellulose	Lignin
T₁	22.40^a	41.30^a	7.0^a
T₂	27.30^b	30.70^b	8.10^b
T₃	29.00^c	28.80^c	10.50^c
T₄	24.80^d	30.20^b	9.60^d
T₅	23.30^e	30.0^b	10.60^c
T₆	20.0^f	31.10^b	13.10^e
T₇	20.6^g	28.20^d	12.10^f

Note; N = 3 replications

Superscript letters of alphabets indicate that means followed by the same letter do not significantly differ at p = 0.05

3.2. Chemical configuration

The chemical analysis of the elemental composition of biomass and pellets is useful for thermochemical conversion modeling, predicting the solid and gas phase and reactant ratio. Table 7 shows the proximate analysis to identify pellets' moisture content, volatile components, fixed carbon, ash, and gross calorific energy.

Table 7

Proximate analysis of wheat straw pellets.
Treatments	MC, %	VM, %	FC, %	Ash, %	GCV, MJ/kg
T₁	6.20^a	75.61^a	11.10^a	7.09^a	17.02^a
T₂	4.20^b	58.9^b	20.70^b	16.20^b	18.67^b
T₃	3.45^c	61.88^c	21.80^c	12.87^c	18.89^c
T₄	7.20^d	60.30^d	18.60^d	13.90^d	18.49^d
T₅	3.50^c	53.03^e	31.60^e	11.87^e	19.06^e
T₆	5.70^e	53.50^f	32.60^f	8.20^f	19.12^f
T₇	3.70^f	56.60^g	30.90^g	8.80^g	20.36^g

Note; MC: Moisture content; VM: Volatile materials; FC: Fixed carbon; HV: Heating value; GCV = Gross calorific value

N = 3 replications

Superscript letters of alphabets indicate that means followed by the same letter do not significantly differ at p = 0.05

3.3. Moisture content

The moisture content of pellet’s directly impacts the combustion efficiency (Gil et al. 2010). Initially, the raw material moisture content was maintained at around 20% for pellet production. However, the generated pellet's moisture content was significantly lower when compared to pelletising moisture because water evaporated due to the temperature produced by the die and roller frictions (Lisowski et al. 2019). Moreover, the produced pellet's moisture content variation was statistically significant at 95% (p 0.05) among the different treatments (Table 7). Huangfu et al. (2014) found that lowering the biomass moisture content from 20% (initial) to 5% (final) boosted 50% the combustion efficiency. This study also found that additives improve pellet quality by minimising moisture content.

3.4. Inorganic ash content

The wheat straw and sawdust ash contents were 7.30% and 0.33%, but the bentonite clay ash was extremely high (89.63%) (Table 5). It was observed that the pellet's inorganic ash presence was significantly influenced by additives composition, fuel source and elements (Dick et al. 2007). In Table 7, pellets ash content varied from 7.09 to 16.20%. Among the treatments, the ash percentage was highest in T₂ pellets (16.20%), possibly due to the addition of bentonite clay. Also, biochar positively impacted ash reduction which could be the synergetic effect (T₆). In addition, treatment T₇ had the second lowest ash content (8.80%). Overall, all produced pellet's ash content had not met the ISO pellet fuels standard (ISO/TS 2016). As a result, further research is needed to minimise the ash content in pellets by adding alternative additives and applying suitable technologies.

3.5. Gross calorific value

The pellet base materials (wheat straw) has a gross calorific value 17.6 MJ/kg, while the biochar heating value was the highest (30.75 MJ/kg) among the raw materials and pellets (Table 5). Telmo and Lousada (2011) found that the composition of materials significantly impacts the pellet's gross calorific value. From this study, it was evident that pellets with additives (T₂ ~ T₇) had a higher gross calorific value than pellets without additives (T₁) (Table 7). Due to additive blending, the gross calorific value (GCV) varied from 17.02 to 20.36 MJ/kg. The analysis data showed that the additives combination positively impacted the GCV and was the highest in T₇ (20.36 MJ/kg ). Moreover, all pellets had a GCV of more than 17.0 MJ/kg, which satisfies the ISO standard's minimum value for commercial/domestic pellet production (ISO/TS 2016). Mixing additives with wheat straw are worthwhile for the vital combustion parameters (GCV).

3.6. Minor chemical elements of pellets

Table 8 shows the minor components of the pellets, while Table 5 denotes the raw materials components, according to the ultimate analysis. The treatments showed slight differences in carbon, hydrogen, nitrogen, and oxygen. These chemical characteristics help promote clean combustion (Vamvuka & Sfakiotakis 2011). A higher oxygen level in biomass will be more thermally reactive but reduce the heating value (Haykiri-Acma & Yaman 2008). As observed in Table 8, the oxygen content variation levels were not remarkable (43 ~ 50%).

In contrast, higher carbon content is essential for good combustion (Munir et al. 2009). In Table 8, the carbon concentration level of the pellets was similar to the oxygen. The hydrogen and nitrogen variation amount ranges from 4.40 to 6.30% and 0.56 to 0.81%, respectively. In addition, the pellets had varying sulphur content (~ 0.21%), which has little influential role in combustion.

Table 8

Ultimate analysis of pellets.
Treatments	Carbon, %	Hydrogen, %	Nitrogen, %	Sulphur, %	Oxygen, %
T₁	43.20^a	4.90^a	0.56^a	0.11^a	50.11^a
T₂	44.60^bh	5.20^b	0.81^b	0.21^bg	48.30^b
T₃	47.27^c	5.00^a	0.66^c	0.17^c	47.60^cg
T₄	43.87^di	6.20^cg	0.68^d	0.13^dh	47.68^dg
T₅	44.51^eh	6.30^dg	0.72^e	0.21^eg	46.90^e
T₆	43.83^fi	4.40^e	0.59^f	0.12^ah	50.00^a
T₇	50.37^g	5.60^f	0.77^g	0.19^f	43.00^f

Note; ^a Determined by a difference

N = 3 replications

Superscript letters of alphabets indicate that means followed by the same letter do not significantly differ at p = 0.05

3.7. Measurement of pellet quality attributes

Pellet quality is an important consideration in handling, transportation, and storage. In this work, pellet quality was indicated by the following six criteria: dimension, bulk density, durability, tensile strength, fines content, and wettability index.

3.7.1. Pellet dimension

The pellet's dimensions (diameter and length) are important aspects of combustion. It is well known that, especially in small furnaces, the thinner pellet enables a more uniform burning rate than the thicker one (Liu et al. 2013). The pellet length also exaggerated the fuel-feeding qualities. It is easier to facilitate a continuous flow with shorter particles (Lehtikangas 2001).

Table 9 shows the pellet's length, diameter, and weight from 33 pellets. The lowest length was 22.66 mm of the wheat straw pellet (T₁). The highest length had 39.77 mm for the T₇ treatment (combination of all three additives with wheat straw). Therefore, the additive mixing significantly increased the pellet length.

The pellet's diameter varied slightly from 8.0 to 8.20 mm (Table 9). The variation may be because water movement among the particles disrupts the pellet formation bond (Mahapatra et al. 2010). It seemed that additives had no considerable impact on pellet diameter.

Pellets' apparent/unit density is a significant reactor design and modeling factor. The unit density depends upon individual weight, length, and diameter influence. The unit weight varied from 1.33 to 2.34 gm but was inconsistent with additive adding (Table 9). In this work, T₄ had the highest unit weight (2.36 gm) as well as the greatest apparent density at 1266.51 kg/m³. The second highest apparent density was 1181.37 kg/m³ for T₁, and the lowest was for T₅ (713.38 kg/m³). The T₅ pellet's apparent density was lowest due to the individual weight of the pellet being higher. A similar trend was found for the apparent density, indicating that there was no impact from additive adding.

Table 9

Physical characteristics of pellet.
Treatment	Length, mm	Diameter, mm	Unit mass, gm	Apparent density, kg/m³
T₁	22.66 ± 7.34^a	8.20 ± .16^a	1.33 ± .22^a	1181.37 ± 229.26^a
T₂	35.26 ± 3.46^b	8.15 ± .15^ab	1.64 ± .24^ad	893.86 ± 103.99^b
T₃	29.26 ± 7.64^c	8.12 ± .13^b	1.40 ± .52^b	911.37 ± 213.72^cb
T₄	34.97 ± 7.05^b	8.25 ± .10^a	2.36 ± .54^aeg	1266.51 ± 176.06^d
T₅	37.44 ± 3.67^bd	8.10 ± .10^ac	1.36 ± .18^c	713.38 ± 104.31^e
T₆	34.01 ± 5.58^bd	8.02 ± .13^d	1.85 ± .38^af	1072.17 ± 123.79^f
T₇	39.77 ± 2.25^be	8.14 ± .06^ae	2.34 ± .25^aeg	1131.27 ± 98.35^ag

Note; The values are expressed as means ± standard deviation

N = 33 replications

Superscript letters of alphabets indicate that means followed by the same letter do not significantly differ at p = 0.05

3.7.2. Pellet bulk density

Particle density has an effect on densified fuels such as pellets and briquettes (Filbakk et al. 2011). The pellet's bulk density impacts transportation and storage space necessities. Higher bulk density means better transportation efficiency and less storage space (Liu et al. 2016). The pellets' average density (bulk and unit) from 33 repetitions/treatments is shown in Table 10. The pellets from the pure wheat straw (T₁) had a low bulk density (244.79 kg/m³), probably because of being more diffictlt to compact. In comparison, the densities of T₃ ~ T₇ pellets were higher, possibly because of a strong bond among the particles. The significantly highest density found in T₇ (665.21 kg/m³) could be due to additive combinations. Based on statistical analysis, the density were veried significantly with additive admixing (Table 10).

3.7.3. Pellet durability

Several factors can affect the durability of pellets, such as pressure, pelleting temperature, volume reduction, and material composition (Tilay et al. 2015). This research produced the pellet through the rollers' constant spinning and friction between the plate die and the rollers. As a result, the production pressure was fixed. As previously mentioned, the excellent quality pellet results from the barrel's operating temperature were 60 ~ 80°C.

Table 10 shows the drop test results for each treatment. Pellet durability increased when the binder was increased, and the maximum (99.66%) was observed in T₆. According to the statistical study, there was however no significant variation in durability, excluding the pellets from wheat straw (T₁). In this study, the pellet’s durability ranged from 97–99% for T₃ to T₇ (pellets with additives), meeting the ISO standard. Iroba et al. (2014) explored the manufacture of ground barley straw pellets via pre-treatment radio-frequency and temperature change during compaction compared to other non-woody biomass pellets. They found the durability was 99.17% (Iroba et al. 2014), similar to this research. This demonstrated that the pellet's durability could be improved through additives mixed with wheat straw. Inversely, wheat straw pellet (T₁) production is moew challenging without additives, and quality may fall short of expectations.

3.7.4. Pellet strength

Tensile strength reflects pellet quality when it comes to pellet strength and hardness. High tensile strength resists breaking and dust generation through transportation and handling. In this work, the pellets tensile strength ranged from 0.36 to 2.09 MPa (Table 10). The highest strength was T_7, while the lowest was T₁ (pure wheat straw pellet without additives). Moreover, T₂ to T₆ pellet's strength was almost similar but significantly different from T₇. The T₇ pellet strength was highest (2.09 MPa), probably a better combination of corn starch and biochar, and made a strong particle bond. Kashaninejad and Tabil (2011) noted that wheat straw pellets had a tensile strength of 0.81 MPa. Additives also help to improve pellet tensile strength.

Table 10

Physical properties of wheat straw pellets.
Treatment	BD, kg/m³	DU, %	GCV, MJ/Kg	TS, MPa	FC, %	WI, %
T₁	244.79^a	85.22^a	17.02^a	0.36^a	8.58^a	78.63^a
T₂	204.69^b	92.20^bg	18.67^b	1.07^b	5.13^b	54.84^b
T₃	567.04^c	97.17^b	18.89^c	1.08^b	1.92^c	22.32^c
T₄	513.97^d	97.27^cf	18.49^d	1.15^b	1.97^c	21.61^c
T₅	607.40^e	97.06b^efg	19.06^e	1.09^b	1.93^c	28.17^c
T₆	610.12^f	99.66b^g	19.12^f	1.25^b	1.91^c	15.57^c
T₇	665.21^g	97.20^d	20.36^g	2.09^c	1.82^c	24.29^c

Note; BD = Bulk density; Du = Durability; GCV = Gross calorific value; TS = Tensile strength;

FC: Fines content and Wi = Wettability index

Superscript letters of alphabets indicate that means followed by the same letter do not significantly differ at p = 0.05

3.7.5. Pellet wettability

Moisture absorption characteristics of pellets influences the physical deformation encountered throughout storage, transit, and transportation. Yılmaz et al. (2021) reported that the formation of loose pellets is caused by increased moisture, followed by a decline in bulk density.

Table 10 shows the pellet wettability index. It can be seen that the binders have a significant effect on the water absorption rates. Among the different pellets, T₁ (without additive) and T₂ (bentonite clay) pellets absorbed the highest amount of water, i.e., 78.63 and 54.84%, respectively. The causes might be wheat straw and bentonite clay, which may have enhanced the water-holding ability of pellets. In contrast, T₃ to T₇ pellets had a superior water-proof capacity, leading to advantages for storage in small-scale usage. Kubojima and Yoshida (2015) and Ghiasi et al. (2014) have also examined the various pellet’s water absorption capacities. They have shown that the pellet with blending materials (post-torrefied pellet) protects moisture absorption, supporting a similar result in this research. Pellets produced from binders generally showed less water intake than those without binders. Therefore, the results demonstrated that avoiding water uptake needs additives like biochar or a combination.

3.7.6. Pellet fines content

Small particles created directly from the pellet are one kind of quality indicator. Due to the fragile nature of pellets, fines/small particles are generated during transportation, handling, and storage by attrition and breakage (Boac, Casada & Maghirang 2008; Oveisi et al. 2013). This has implications, including increased dust explosions, equipment fouling, inhaling problems, and the loss of a large amount of the material (Ilic et al. 2018); (Ramírez-Gómez 2016).

As part of the sieving analysis, the sieved particle size distribution is presented in Table 11. The fines produced from the abrasive test were in the subsequent order T₁ > T₂ > T₆ > T₄ > T₅>T₃>T₇> (Table 10). The T₁ and T₂ pellets produce more fines than others because of less tensile strength and probably loose particle bonding. The fines from T₃ to T₇ pellet were nearly 2%, which could be due to the better structural formation of the pellet. The additiveshave reduced the fines generation.

Table 11

Produced particle classification.
Treatment	Small particles
Treatment	3.15 mm < Lump ≤ 5.6mm, %	1 mm < Fines ≤ 3.15 mm, %	Dust ≤ 1 mm, %
T1	30.88	8.58	4.86
T2	4.43	5.13	2.75
T3	6.68	1.92	0.95
T4	7.89	1.97	0.60
T5	4.52	1.93	1.07
T6	4.03	2.01	2.17
T7	5.13	1.82	2.74

3.8. Pellet quality comparison with ISO 17225-8: 2016

Table 12 presents a comparison of various pellets with the ISO standards for non-woody/herbaceous biomass/straw biomass, specifically focusing on factors such as ash content, heating value, durability, moisture content, length, diameter, and bulk density. The ISO standards differentiate between A (household) and B (commercial settings) quality types (ISO/TS 2016). The ash content should be no more than 6% for A-class pellets, while for B-class pellets, it should be no more than 10%. The heating value requirements are ≥ 18 MJ/Kg for A-class and ≥ 17 MJ/Kg for B-class. The durability standards are ≥ 97.5% for A-class and ≥ 96.5% for B-class, while the moisture content should be 10% for both.

It was observed that the produced pellets met the ISO standards for durability and moisture content. The pellet dimensions (length and diameter) also fell within the ISO-specified range. The heating values were found to increase with the addition of additives, and all pellets met the ISO standard. The ISO standard also specifies a minimum bulk density of 600 kg/m3. Treatments T1 to T4 did not meet this requirement, but pellets were produced with biochar (T5 ~ T7). Furthermore, T2, T3, T4, and T5 pellets failed to meet the ISO ash content standards for both A and B categories. On the other hand, T1, T6, and T7 pellets (with ash content percentages of 7.09%, 8.20%, and 8.80%, respectively) complied with the B-class standards. It shows that T1 and T2 also fell short of meeting the fines content standard, according to Table 12.

Table 12

Comparison of the produced pellets by the *ISO 17225-8:2016* classification (cereal straw).
Parameter	Standard value		T₁	T₂	T₃	T₄	T₅	T₆	T₇
Parameter	A	B	T₁	T₂	T₃	T₄	T₅	T₆	T₇
Length, mm	3.15 ~ 40		**	**	**	**	**	**	**
Diameter, mm	6 ~ 25		**	**	**	**	**	**	**
Moisture content, %	≤ 10		**	**	**	**	**	**	**
Bulk density, kg/m³	≥ 600		X	X	X	X	**	**	**
Ash content, %	≤ 6	≤ 10	X	X	X	X	X	X	X
Heating value, MJ/Kg	≥ 18	≥ 17	**	**	**	**	**	**	**
Durability, %	≥ 97.5	≤ 96.5	X	**	**	**	**	**	**
^a Fines, %	≤ 2.0		X	X	**	**	**	**	**

Note; A: **; B:*, Out of standard: X

A = Household use; B = Commercial use, ^a Fines content (< 3.15 mm)

Examining and identifying the factors affecting the quality of pellets is challenging because it is affected by many factors, such as moisture content, additives addition, and pelleting conditions (Sultana & Kumar 2012).

Figure 10 provides a comprehensive overview of how additives impact various pellet parameters. It highlights the significance of enhanced binding in boosting pellet quality, but the abundance of additives does not necessarily contribute to durable pellet production (Kaliyan & Morey 2009). While additives positively affect specific parameters, their influence on others remains marginal.

Researchers generally agreed that additives contribute to improved tensile strength, durability, heating value, and length of pellets (Carone, Pantaleo & Pellerano, 2011). The value of fines and water absorption capability also decreased, indicating that additives strengthen these characteristics. However, the moisture content and diameter have shown minimal alterations with increasing additive content. In summary, it is possible to produce durable wheat straw pellets incorporating additives like sawdust, biochar, and bentonite clay (Whittaker & Shield 2017).

Ash content in biomass plays a pivotal role in the performance of various systems such as gasifiers, boilers, etc. Cereal crop residues tend to have relatively higher nitrogen, sulphur, chlorine and potassium levels in their ash composition, potentially leading to complications like slagging and corrosion in boilers (Pastre 2002). The study faces a notable challenge in substantially reducing ash content, which is the main limitation. Consequently, further research is essential to manage the ash content in wheat straw pellets effectively.

A large volume of agricultural residues and straw is produced from crop production chains every year. Many countries are looking for alternative options for managing and disposing of field crop residues. The study used wheat straw with additives to improve pellet quality for environmentally friendly waste management and also renewable energy production. This research has found that the pellets from wheat straw and a mixture of additives could effectively meet the pellet ISO standard specification requirements, except for inorganic ash content and fines content in some treatments. The following conclusions can be made:

Wheat straw pellets without additives did not fulfil most of the ISO solid fuel standard.
Adding additives to wheat straw for pellet production was an effective way to improve the solid biomass fuel properties and is a viable option.
The pellet's physical properties (diameter, length, density, durability and tensile strength), significantly improved by adding additives to wheat straw.
Additives also improves the pellets' composition and chemical characteristics, particularly for lignin, cellulose content, fixed carbon, etc.
The combustion parameter, such as decreased moisture content, and increased heating value were obtained from additive mixing.
The fines content and water absorption decreased in pellet making with additives. This promotes safe and secure storage and transportation. However, T₁ to T₂ pellets failed to comply with ISO solid fuel standard for fines content.
T₁ to T₅ pellets also did not comply with ISO solid fuel standard for inorganic ash content. This was in comparison with T₆ and T₇ pellets which have met all the requirements for commercial uses.
It is recommended that there is research need to minimise the ash content by considering alternative additives or other methods.

Author Contribution

Bidhan Nath: Conceptualization, Methodology, Investigation, Writing – original draft. Guangnan Chen: Supervision, Conceptualization, Writing – review & editing. Les Bowtell: Writing – review & editing. Thong Nguyen-Huy: Data analysis, review and editing

Data Availability Statement: The data presented in this study are available on request from the corresponding author.

Funding Decleration: The author(s) received no financial support for the research, authorship, and/or publication of this article.

Acknowledgments: The authors thank the University of Southern Queensland, Australia, for its research facility. They also acknowledge the financial support from the National Agricultural Technology Program phase-2 project of the Bangladesh Agricultural Research Council, Dhaka, Bangladesh.

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Upgrading of physical and elemental quality of wheat straw pellets using additive blends

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1. Materials

2.2. Chopping and milling of wheat straw

2.3. Blending material selection and proportion

2.4. Pelleting process and pellet production

2.5. Standard for biomass and pellets physiochemical characterisation

2.6. Measurement of elemental composition

2.6.1. Component analysis

2.6.2. Proximate analysis

2.6.3. Thermal analysis

2.6.4. Minor elemental analysis

2.7. Measurement of physical properties

2.7.1 Pellet dimension

2.7.2. Unit density

2.7.3. Bulk density

2.7.4. Mechanical durability

2.7.5. Tensile strength

2.7.6. Fines content

2.7.7. Wettability index

2.8. Statistical analyses

3. Results and discussions

3.1. Pellet composition

3.2. Chemical configuration

3.3. Moisture content

3.4. Inorganic ash content

3.5. Gross calorific value

3.6. Minor chemical elements of pellets

3.7. Measurement of pellet quality attributes

3.7.1. Pellet dimension

3.7.2. Pellet bulk density

3.7.3. Pellet durability

3.7.4. Pellet strength

3.7.5. Pellet wettability

3.7.6. Pellet fines content

3.8. Pellet quality comparison with ISO 17225-8: 2016

4. Study observations and limitations

5. Conclusion and recommendations

Declarations

Author Contribution

References

Additional Declarations

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