Agro-Pellets from Oil Palm Residues / Pine Sawdust Mixtures: Relationships of Their Physical, Mechanical and Energetic Properties, with the Raw Material Chemical Structure

: The need for biomass as an alternative source for energy purposes points toward oil palm fruit residues ( Elaeis guineensis Jacq.) as an attractive solution. Oil palm industry residue, such as oil palm empty fruit bunch (EFB) composites and mesocarp ﬁber (PMF), have a high gross caloriﬁc value and could help countries meet their energy demands. However, information concerning physical, mechanical, and energy characteristics of agro-pellets made from mixtures of oil palm residue with pine sawdust, is not available. In this research, oil palm residues were mixed at ratios of 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100 with pine sawdust, and were chemically analyzed. Agro-pellets from mixtures were tested on their physical and mechanical properties, and the relationship of these properties with the chemical composition of oil palm residue mixtures was determined. Normality distribution tests of the variables were performed using the Kolmogorov–Smirnov test. Normal variables were analyzed through one-way ANOVA tests. When di ﬀ erences were found, they were validated using the Tukey’s honestly signiﬁcant di ﬀ erence (HSD) test and were considered statistically signiﬁcant at p ≤ 0.05. Data not normally distributed were analyzed by the Kruskal–Wallis test. Bulk density, particle density, and gross caloriﬁc value of agro-pellets (from mixing 80:20 oil palm residues with pine sawdust) were statistically higher ( p < 0.05) to agro-pellets of 100% oil palm residues. Adding pine sawdust to oil palm residues increases some mechanical agro-pellet properties. Addition of pine sawdust oil palm residues some mechanical properties of agro-pellets.


Introduction
Biomass is versatile, having zero net CO 2 and less SO 2 emissions than fossil fuels. It is one of the most promising energy alternatives to face climate change and can lessen dependence on fossil fuels [1,2]. However, being a heterogeneous material, solid biomass for energy purposes has some volatile matter, and ash content of mixtures and agro-pellets were determined according to the international standards (The Spanish norm "UNE": Una Norma Española, and European Norm "EN"), UNE-EN 14774- 3 [22], UNE-EN 15,148 [23], and UNE-EN 14,775 [24], respectively. Fixed carbon was determined by mass difference using Equation (1), and gross calorific value was determined in a calorimetric bomb according to UNE-EN-14918 [25]. Agro-pellet bulk density and particle density were determined according to the International Standardization Organization, UNE-EN ISO 17,828 [26], the compression ratio (CR) and impact resistance index (IRI) were determined according to Equations (2) and (3), respectively.
FC is the fixed carbon (%) and Mv (%) is volatile material (%). CR = P pellet P biomass (2) CR is the compression ratio, P pellet (kg m −3 ) is the density of pellet, and P biomass (kg m −3 ) is the density of biomass mix.
IRI is the impact resistance index, N is the number of drops (2), and n is the total number of pieces after N drops [27].

Statistical Analysis
Since only two repetitions of chemical composition of each biomass mixture was obtained, just the mean and standard deviations were calculated. The percent data of immediate analysis was transformed by the arcsine square root function. Transformed data and calorific value of biomass mixtures, as well as energy, physical, and mechanical properties of pellets were examined to determine normality distribution by the Shapiro test [28]. Comparisons of chemical composition and bulk density among palm oil residues:pine sawdust mixes and bulk density, compression ratio, impact resistance index, and gross calorific values of agro-pellets were calculated through a one-way ANOVA. Where the F-test was significant (p ≤ 0.05), the differences were validated using the Tukey's honestly significant difference (HSD) test, and were considered statistically significant at p ≤ 0.05 [29]. Variables with no normal distribution were statistically analyzed by the Kruskal-Wallis test; the relationship among pellet bulk density, impact resistance index, and gross calorific values with mixtures of oil palm residues:pine sawdust were calculated with the Pearson correlation coefficient (r).

Chemical Characteristics, Proximate Analysis, and Bulk Density of Biomass Mixtures
Cellulose, hemicellulose, lignin, and extract content on mixtures of oil palm residues:pine sawdust ranged from 44.16 to 56.64%, 14.14 to 18.81%, 13.9 to 25.02%, and 4.20 to 22.17%, respectively ( Table 1). The highest value of cellulose and lignin was in the mixture 0:100; for hemicellulose, it was in the mixture 100:0; while with extract content, it was in the mixture 80:20.
ANOVA test values of volatile material and fixed carbon concentration on biomass mixtures showed significant statistical differences (p < 0.01); values ranged from 83.95 to 91.86% and 7.83 to 15.54%, respectively. Oil palm residue (100:0) showed the highest volatile material content (83.95%) as well as the highest fixed carbon content (15.54%). Kruskal-Wallis test of moisture content presented significant statistical differences (p < 0.01); values ranged from 7.58 to 10.14%. Ash did not show statistical differences (p > 0.05), values ranged from 0.31 to 0.58%. However, a non-linear behavior of these variables was recorded, with the highest values of 91.86 in the 80:20 mixture for volatile materials, 0.51 in the 100:0 mixture for the ash content, 10.14 (0:100) for the moisture content, and 15.54 (100:0) for fixed carbon. Regarding physical properties, bulk density showed statistical differences (p < 0.05) among biomass mixtures, and values ranged from 0.12 to 0.21 kg m −3 ( Table 1). The higher bulk density values obtained on mixtures were on pine sawdust; proportion increased, since 40:60 (0.20 g/cm 3 ), 20:80 (0.21 g/cm 3 ), and 0:100 (0.20 g/cm 3 ).
Bulk density of agro-pellets also showed a high degree of correlation coefficients (r) with oil palm residues:pine sawdust mixtures (R = 0.91, p < 0.001) and bulk density of oil palm residues:pine sawdust mixtures (R = 0.96, p < 0.01) (Figure 2).

Relationship of Agro-Pellets Energy with Mixtures and Proximate Elements
Gross calorific values showed a high degree of correlation coefficients (r) with oil palm residues:pine sawdust mixtures (R = 0.94, p < 0.0001); fixed carbon (R = 0.81, p < 0.05); and with volatile matter of oil palm residues:pine sawdust mixtures (R = 0.81, p < 0.05) (Figure 3b,c).

Relationship of Agro-Pellets Energy with Mixtures and Proximate Elements
Gross calorific values showed a high degree of correlation coefficients (r) with oil palm residues:pine sawdust mixtures (R = 0.94, p < 0.0001); fixed carbon (R = 0.81, p < 0.05); and with volatile matter of oil palm residues:pine sawdust mixtures (R = 0.81, p < 0.05) (Figure 3b,c).

Agro-Pellets Characterization
The effect of mixing pine sawdust with palm residues can explain the non-linear behavior of moisture content, which also affects the other variables. In the 100-0 proportion, there are only volatiles (84%) of the oil palm residues, while in 80-20, the volatiles increase (up to around 88) due to the volatiles contained in the pine sawdust. However, they drop to 87% in the 60-40 mixture due to the fact that the increase of the palm fiber "encapsulates" the volatiles, which is why they are released in a low proportion when the analysis is made. The same happens in the 20-80 mixture. However, in the 0-100 mixture, where there is more humidity, at the time of determining the immediate ones, it allows increasing the leaching of the volatiles at 100 • C; therefore, the percentage (%) of volatiles increases.
For the agro-pellets from all of the mixtures, moisture content values varied in the range 5.58-6.77%, meeting < 10% established by the standard ISO 17225-2 [30]. Values were lower than 9.0 to 9.2%, presented by Tenorio et al. [31]. Higher moisture content values (than 10%) affects the burning characteristics of biomass material and reduces agro-pellet strength and durability. Volatile material of agro-pellets presented the range 79.40-89.31%, similar to 79.58-85.11% reported by Onochie et al. [32].
The highest values were found in the mixtures 40:60, 20:80, and 0:100 when pine sawdust percent was high; this can be attributed to higher percent of lignin and volatile material pine sawdust in the mixtures (Table 1; Table 2), high volatile material on fuel pellets ease the ignition during combustion. Ash was significantly similar (p > 0.05) on all mixtures of agro-pellets and ranged from 0.44 to 0.58% (Table 2), meeting class A1, according to the European Committee for Standardization (CEN in spanish), CEN/TS EN 14961-2 [33]. However, combination 40:60 was in A0.5, while all other combinations were in class A0.7. Ash content values higher than 0.7% increase particle dispersion and allow particle adherence inside of boilers [34,35]. Fixed carbon showed the ranged 10.25-20.05%, with high values in mixtures of high oil palm residue proportion (100:0, 80:20, and 60:40). Values of these mixtures were higher than 16.87%, as reported by Forero Nunez et al. [12]. The distribution follows the same relationship found on oil palm residues:pine sawdust mixtures (Table 1). Fixed carbon, together with volatile material, increase the gross calorific value [36].
Bulk density values varied from 540 to 680 Kg m −3 , except for the mixture 100:0, all mixtures achieve quality parameters (>550 kg m −3 ) of ISO 17225-2 [30] ( Table 2). As was seen, increasing the proportion of pine sawdust in the mixture (100:20-0:100%) increased bulk density values from 610 to 670 kg m −3 . Bulk density is considered as a good quality indicator for pellets [37]. Particle density varied in the range 1090-1340 Kg m −3 , and were similar to those reported of agro-pellets produced from 12 types of agricultural and forest residues (1100-1290 kg m −3 ) [31]. Density values reduced when the pine sawdust proportion increased, similar to that reported by Hosseinizand et al. [38]. This is consistent with low density that typically characterizes lignocellulosic and fibrous materials, such as sawdust, as indicated by Ferero-Nuñez et al. [18]. Such materials present void spaces throughout their structures-full of air that increases the volume of the solid. After milling, the powder becomes finer, releasing the air inside and raising the density.
The highest compression ratios (4.75-4.13) of agro-pellets were obtained on mixtures of 100:0 and 80:20; the lowest (3.28-3.29) on mixtures of 40:60, 20:80, and 100:0. Compression ratio is a way to determine the effectiveness of densification; high compression values reduce cost during transportation, and handle management of biomass as a source of energy. In this concern, compression ratios found in this research were similar to 3.74-4.86 of Artemisia dubia Wall., presented by Zvicevičius et al. [39] when applying different pressure. Impact resistance indexes of mixtures varied in the range 108-157 and differed significantly (p < 0.05) among mixtures ( Table 2). Higher values were obtained on mixtures with high palm oil residues (100:0 and 80:20); however, values of all mixtures were considerably higher than 50; target considered by Richards [28]. Impact resistance is another important physical property for pellets, since high values reduce the dust production during handle and transport; this may produce irritation of the lungs, nasal, and respiratory system, with inhalation of an excessive amount of dust particles.
Gross calorific values varied in the range 18.84-22.31 (MJ/Kg) and were similar to 17.78-20.51 MJ kg −1 reported for pellets from oil palm frond [20]. The highest value was obtained in the mixture 80:20 and the lowest in the mixture 0:100, in accordance with 18.9 MJ kg−1, established for pine sawdust [40]. Increasing the proportion of oil palm residues increased the gross calorific value (GCV), which could be related to the highest amount of volatile material and fixed carbon, contrary to Obernberger and Thek [41], who stated that volatile matter provides low energy, because they vaporize before combustion reactions take place. For Forero-Nuñez et al. [12], volatile matter content and fixed carbon are parameters that affect the combustion process directly. In their study, these authors obtained that pine sawdust has more volatile compounds than palm oil shells, but less fixed carbon content. So that pine sawdust will release more volatile material at the first stage of the combustion process, the palm oil residues will add energy for char combustion, since materials with high fixed carbon content have more elemental carbon, and then, more energy power. In this way, the oil palm residues/pine sawdust mixture is necessary to establish the potential of these materials as energy sources for further processes, such as combustion or gasification and, above all, their palletization will increase their energy density.

Relationships of Agro-Pellets Characteristics and Raw Material Properties
The relationship of pellet characteristics and some chemical and physical properties of oil palm residues:pine sawdust mixtures was contrasting. Bulk density of agro-pellets was related with the chemical components cellulose, hemicellulose, lignin, and extracts on oil palm residues:pine sawdust biomass mixtures, as was seen (Figure 1a,b) when increasing the amount of cellulose and lignin, increasing pellets bulk density, both are considered as natural binderies and responsible for bonding [42]. On the one hand, cellulose is the main (and the strongest) mass compound on lignocellulosic matrix; this increases friction, providing density of pellets during pelletizing. On the other hand, lignin provides hardness after pelletizing, where biomass is heated at high temperature and pressure, its low melting point becomes soft and melts with the particles; after that, pellets cool [13,43]. In contrast to the previous, hemicellulose considered as the weakest parties on polymer structure in wood matrix reduces the bulk density; while extracts, as a mixture of different compounds, migrate during pelletizing to the pellet surface and reduce friction, producing weak pellets [44].
The increment of bulk density on agro-pellets was also seen when the proportion of pine sawdust increased in the mixture, the same was observed when increasing the bulk density of raw material (Figure 1a,b). This is consistent with the study carried out by Li and Liu [45], attributing it to the bonding strength between small and long particles that stick together strongly.
Gross calorific values of pellets varied in the range 18.84-22.31 MJ Kg −1 ; values showed a reduction when increasing the pine sawdust proportion in the mixture (Figure 3a). The same trend (17.02-22.72 MJ/kg) was presented by Sirrajudin et al. [46] after add glycerin on percentage of 15, 30, and 45% to oil palm fronds, they ascribed this to major biochemical components, such as cellulose, hemi-cellulose, lignin, extractives, and other ash forming minerals [47].

Conclusions
Agro-pellets elaborated from mixtures of oil palm residues and pine sawdust show quality parameters that meet international standards, completely made from oil palm residues. The addition from 20 to 40% of pine sawdust to oil palm residues increases gross calorific value, impact resistance index, and compression ratio values. However, they present lowest bulk and particle densities values. Bulk density of oil palm pellets was positively correlated with pine sawdust content; the same relationship was observed with the amount of cellulose and lignin on mixes, but negative with hemicellulose and extracts. Compression ratio was reduced while adding pine sawdust; the most suitable combinations were found at 80:20 and 60:40. Funding: This research was funded by the Energy Sustainability Fund through the project SENER CONACYT 2014 (No 246911): Cluster of Solid Biofuels for Thermal and Electric Generation. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.