Production and Characterization of Low-ash Empty Fruit Bunches (EFB) Pellets as a Solid Biofuel

Fuel quality is among the major issues encountered for commercial production and utilisation of oil palm-based biomass pellets from empty fruit bunches (EFB). In this study, a physical treatment, namely sieving and water washing, was applied to EFB prior to densication using an industrial-scale plant. The physiochemical properties of the produced EFB pellets were compared with those without physical treatment as the control. The results showed that the physical treatment process had signicantly enhanced the fuel properties of EFB pellets, particularly in reducing ash content. Both the treatments; sieving alone and a combined sieving and water washing of the feedstock managed to reduce the ash content of EFB pellets to < 4 wt.% and < 2 wt.%, respectively compared to > 5 wt.% of typical commercial EFB pellets. The reduction in ash content improved the caloric value of pellets to > 17.5 MJ kg -1 which signied achieving a milestone for oil palm biomass-based heat and power generation from technical, operational, economic and environmental points of view. The ndings also indicated that the combined sieving and water washing treatment method could potentially reduce further the unwanted ash-inducing elementals in EFB pellets, in particular chlorine and potassium. This study demonstrated that the employed physical treatment is an immediate and cost-effective practical commercial approach to improve the fuel properties of EFB pellets.


Introduction
Renewable energy (RE) plays a signi cant role in shaping the total global primary energy supply, providing about 14% share in 2015 and would increase to 63% in 2050 [1]. The increasing use of biomass for energy exploitation as an alternative to fossil fuels is driven by environment bene ts as it is a natural carbon sequester [2]. Biomass contributes about 10% and 13% to the world primary energy supply and nal energy consumption, respectively, making it the fourth largest source of energy in the world, after oil, coal and natural gas [3]. According to Popp et al. [3], biomass accounts for 70% i.e. the biggest share of the total global renewable energy sources.
Biomass has been identi ed as one of the major resources for RE development in Malaysia. The country is blessed with an abundant amount of biomass, mainly as by-products from agro-commodity sectors such as oil palm and rice (paddy), forestry residues from wood and timber industries and municipal solid waste. The amount of biomass was estimated at 168 million tons (t) with potential renewable electricity of 2400 MW [4]. The oil palm industry is the single largest contributor of biomass resources in Malaysia, amounting to 90% of the country's total lignocellulosic biomass [5,6]. The oil palm lignocellulosic biomass is mainly generated from 5.90 million hectares of oil palm planted areas and 452 palm oil mills nationwide [7]. Assuming that the mean weight ratio of mesocarp bre, palm kernel shell and empty fruit bunches (EFB) to fresh fruit bunches (FFB) is 13%, 6% and 23% [8,9] respectively, the estimated total solid biomass production from the palm oil milling sector alone was 40.3 million t (wet basis) based on 98.28 million t FFB processed in 2019 [10].
Despite being used mainly as a fuel in palm oil mill biomass boiler, the remaining unutilized and disposed portion particularly EFB is huge. There are many issues and challenges in utilizing raw EFB or pressed EFB bers directly as a fuel, such as high moisture and oxygen contents, inherently inferior fuel properties, bulkiness and size heterogeneity [11]. All these contribute to a generally low bulk density and energy content, reduced thermal e ciency and increased pollutant emissions and operation cost of the fuel including biomass handling, processing and boiler maintenance [12].
Pelletisation is one of the promising options to improve physical and fuel properties of EFB, expanding its usage and marketability as a solid biomass fuel. The process can be part of integrated biore nery concept within existing palm oil mills. De ned as mechanical densi cation, the process involves raw material pre-treatment for size and moisture reduction, and compression of loose biomass into higher density and more uniform fuel using at die or ring die pelletizer [13]. Woody biomass pellet typically has a bulk density of 200 -650 kg m -3 , <3 cm in length (L) and 1 cm in diameter (D) [14]. Pellet size dimension, particularly diameter depends on individual country's solid fuel standard requirements, e.g. Austrian and German Standards, 4-10 mm; Swedish standard, <25 mm; Italian Standard, 6-10 mm (depending on pellet category) while the recently developed European (EN ISO 17225-2) and International (ISO 17225-6) Standards, 6-12 mm and 6-25 mm, respectively [15]. Densi ed biomass in pellet form provides numerous advantages, among others, improved solid fuel properties, better combustion e ciency, ease of handling, storage and logistic [16].
Many studies have reported positive impacts of biomass pellets on combustion process and emission. Several examples are: reduction of 80% (minimum) CO and SO 2 emissions using pellets of oil cake, rape straw and birch sawdust in a low-temperature biomass boiler [17], combustion e ciency of 92% using wood pellets [18], ~2% thermal e ciency increment relative to an increased combustion e ciency and heat exchanger areas [19], and a low-ash biomass pellets of 2.0 wt.% burnt without any ash agglomeration problem [20]. Besides, co-ring of coal with wood pellets could improve the combustion reaction, reduce gaseous emission of SOx and NOx, and slagging ratio of wood pellets [21]. Combustion of forest biomass pellets in a boiler with intelligently designed air-fuel ratio generates lower CO and dust emissions than woodchips and lignite i.e. less than 100 ppm and 0.05 g h -1 , respectively [22]. Similarly, combustion of high quality wood pellets emits lesser particulate matters and CO, compared to low-density pellet or solid fuel [23]. To date, biomass pellet is commercially used for heat and power generation, co-ring in coal power plant and as a potential feedstock for production of second generation biofuel, mainly via thermochemical process [9,24].
There have been numerous research and development (R&D) works on EFB pellets by many researchers with the majority focusing on production and characterisation of EFB pellets [25][26][27][28]9]. Although the potential of EFB pellets is manifold, these studies nevertheless showed that their quality is relatively lower compared to commercial wood pellets, mainly due to high ash content and presence of inorganic matters [9]. These disadvantages have rendered full commercialisation of EFB pellets impossible in existing market, creating operational implication and signi cant challenges during fuel combustion and ash management.
In order to exploit global demand opportunities for high quality biomass pellets and to stay competitive, much efforts are geared towards improving fuel properties of EFB pellets. These include pre-treatment of EFB using various methods such as steam explosion, hydrothermal, washing and leaching techniques [33][34][35]. These approaches have been found technically e cient in removing inorganic matters, ash and Cl from the feedstock, and improving mechanical durability, strength and combustion characteristics of the derived pellets [35,36,33]. Water leaching study on EFB is able to reduce ash content from 5.47 wt.% to 2.63 wt.%, and K content from 2.42 wt.% to 0.36 -0.69 wt.% [29]. Hydrothermal treatment applied on EFB has resulted in 55% reduction of ash content and lowered K and Cl contents to 0.84 wt.% and 0.18 wt.%, respectively [34]. Lam et al. [33] reported that a steam explosion treatment of EFB could increase gross calori c value (CV), density and breaking strength of EFB pellets by 20%, 7% and 63%, respectively. Most of these studies are laboratory-scale experiments which require more time and investigations before an e cient and cost-effective full-scale pre-treatment technologies for commercial EFB pellets plant can be established [36] [Hamzah, 2017 #164;Liu, 2017 #165].
As the demand for biomass pellets continues to grow worldwide, and wood pellets are technically more superior to the EFB pellets, commercial EFB pellets production which is on par with wood pellets must be fully exploited and developed for a competitive edge. Besides quality issue, other factors requiring attention are high EFB pellets production cost including operation and maintenance of the plant, and the associated environmental impacts.
Previously, our life cycle impact assessment showed that production of EFB pellets was mainly due to the heavy consumption of fossil fuels [9]. According to Ali, Koukoulas [37], current focus in the biomass pellet industry is to improve process and optimise pellet quality across the supply chain which ultimately poses cost reduction opportunities.
Building on current technical capacity, biomass resource availability and potential market opportunity for pellets from oil palm biomass as well as an emerging option for palm oil mill biore nery concept, some joint efforts between the Malaysian Palm Oil Board (MPOB) and an industry player, Global Green Synergy Sdn. Bhd. (GGS) were aimed at improving the process and quality of commercial EFB pellets. The study evaluated the impact of fuel pre-treatment on resulted fuel quality. It investigated the effect of physical treatment of raw material on fuel properties of EFB pellets, in complying with international solid biofuel standards. The physical treatment, namely sieving and water washing, was applied to the pressed EFB bre prior to pelletisation process using a newly developed commercial-scale plant. The resulted EFB pellets were characterised and their quality was compared with solid biofuel speci cations based on the European Standard (EN ISO 17225-2) for wood pellets (class B) and the International Standard (ISO 17225-6) for nonwood pellets (class B) requirements.

Materials
Pressed EFB bers were used as the feedstock for the study. It was obtained from a palm oil mill located in Rantau, Negeri Sembilan. The pressed EFB bers (2 -3 t) were produced by shredding and pressing the raw EFB using a dual cutter shredder and screw-type press machine installed at the mill. EFB juice was also generated from this process as a by-product which can be further recovered as a technical-grade crude oil. These approaches are adopted by the mill as part of the strategies to reuse EFB volume for various value-added products and applications. The pressed EFB bers were then transported to the site for production trials.

Physical treatment of feedstock and pelletizing process
Physical treatment of the pressed EFB bers and the subsequent pelletization were conducted at a demonstration plant (1 t hr -1 ) located in Mambau, Negeri Sembilan. Two types of physical treatment, namely sieving and a combined sieving and water washing, were applied to the feedstock. The sieving process of pressed EFB bers was carried out using a 25-mm mesh size rotary sieving drum (D = 1200 mm, L = 6000 mm). The dirt passing through the sieving mesh was collected and weighed. For the combined treatment, pressed EFB bers were sieved and then washed directly with water using spraying techniques for less than 30 seconds. Figure 1 depicts the process ow used in physical treatment and production trials of EFB pellets. The pressed EFB bers (without physical treatment) were used as a control (Test 1), while the physical treatment employed yielded 2 types of feedstocks for production of EFB pellets, namely sieved EFB bers (Test 2) and sieved cum water-washed EFB bers (Test 3). Tap water was used for washing purpose. The pelletisation involved three main processes; a prepelletizing step for size and moisture reduction, a pelletizing process and a post treatment process. Size and moisture reduction of the pressed EFB bers were carried out through a sequence of steps using shredder, rotary drying unit and grinder. A 30-kW downdraft biomass gasi er was used to supply hot gases for drying purpose. Hot gases for drying was about 200°C to 250°C for 5 to 6 minutes. All these processes was carried out to ensure that the size and moisture content of the feeding materials were less than 10 mm and 15% (wt./wt.), respectively prior to pelletising process. The pelletizing process was carried out using a ring die technology and the produced pellets were cooled and dried in the pellet silo using an air blower before packaging. Each test was replicated twice. Table 1 summarises type of machineries used for pelletizing process. The machineries were manufactured by GGS.

Analysis
The pressed EFB bers (feedstock), pulverised EFB bers (after grinding) and EFB pellets from each test were randomly selected and analysed for their fuel properties. Proximate and ultimate analyses as described by Loh [6] were performed. The proximate analysis to determine moisture, ash, volatile matter and xed carbon contents was carried out using a thermogravimetric analyser (LECO TGA 701) according to ASTM 5142. The temperature and time The density of the pellets was determined using the density kit (Mettler Toledo ML-DNY43) according to ASTM D792-08. It was calculated by dividing the mass of the pellets by its volume [31]. An one-liter (l) container was used to determine the bulk density of the pellets according to the method described by Theerarattananoon et al. [38]. The weights of the container lled full with pellets and the empty container were determined, and the weight difference obtained was divided by the volume of the container to get the bulk density. The feedstock and EFB pellets produced from Test 2 (sieving) and Test 3 (sieving and water washing) were further analysed for trace elements and Cl contents.  [39,40]. Pellet durability was determined using a Holmen pellet durability tester (model NHP 100) with a 100 g sample for a 60 seconds, in accordance to ISO 17831-1:2015 as described by Brunerová et al. [31]. All the analyses were carried out in triplicate. Table 2 summarises the analyses employed in this study.

Results And Discussion
Physical treatment of raw material The pressed EFB bers used for this study were physically treated to obtain the sieved EFB bers (Test 2) and the sieved and water-washed bers (Test 3), prior to size and moisture reduction via pre-pelletizing process. The unwanted materials, less than 25 mm in size, passing through the sieving mesh were dirt including soils and stones, short and pulverized bers which contributed to approximately 1% (wt/wt) of total raw pressed EFB bers fed to the sieving drum. The wastewater generated from the washing process of EFB bers was collected and analysed. Table 3 shows the characteristics of the wastewater generated. It appeared that the wastewater had much lower characteristics, in particular BOD and COD values compared to the raw palm oil mill e uent (POME) and hydrocyclone wastewater typically generated during the palm oil milling process. This nding indicates that it is possible to treat the EFBwashed wastewater using currently practiced POME treatment system if the EFB pellets production plant is part of the integrated activity in palm oil extraction.
Production trials and physical characteristics of raw material and EFB pellets The feedstocks (untreated and physically-treated EFB bers) were shredded, dried and pulverized to meet the required size and moisture content as a feeding material for pelletizing process. Moisture content and particle size of the feedstocks are two important parameters affecting pellet durability and bulk density [13]. For e cient pelletisation, many studies have suggested the allowable maximum limit of moisture content and particle size to be < 15% wt.% and 5 mm [31,13]. Table 4 summarizes the characteristics of pressed EFB bers (raw material) and pulverized EFB bers employing different physical treatment. The raw material appears to have heterogeneous characteristics, particularly with high variation of moisture content i.e. from 46 wt.% to > 65 wt.%. The pre-pelletizing process employed in this study managed to reduce signi cantly the moisture and size of EFB bers to < 14 wt.% and < 10 mm in their pulverised forms, respectively, which are comparable to those in the literature. The bulk density of EFB bers was therefore increased signi cantly from 62 to 152 kg m -3 , thus able to facilitate better material ow throughout the production process.
The production trials conducted showed that binderless EFB pellets were successfully produced from untreated and physically-treated feedstock. The presence of lignin in EFB bers, about 18%, acts as a natural binder [43]. The high pressure and temperature applied during the pelletisation had softened the lignin to act as an intrinsic resin for improving the particles binding during pelletisation [44,45]. The size and moisture content of the feeding material were also reduced to a suitable level for effective pelletization to produce EFB pellets of desirable quality without affecting the overall production process. Based on the production trials, it was estimated that about 2.35 -2.50 t of pressed EFB bers and approximately 160 kWhr of electricity were required to produce 1 t of EFB pellets. The relatively lower energy consumption in this study compared to those in the literature i.e. > 200 kWhr t -1 pellet [43,9] was due to the use of partially-treated pressed EFB bers, e cient machineries and biomass gasi er for EFB drying process.
Three types of EFB pellets were produced from this study, namely a typical EFB pellet using untreated EFB bers (Test 1), and a low-ash EFB pellet from Test 2 and Test 3 using 2 different physical treatments. The insigni cantly different physical properties of the resulted EFB pellets from Test 1 to Test 3 in Table 5 indicated that the different treatments applied did not affect the physical properties of the pellets. The EFB pellets were relatively uniform in terms of diameter and bulk density although the varying length could affect the weight of individual pellet. On average, the diameter, length and bulk density of the produced EFB pellets were 8.4 mm, 40.0 mm and 661.7 kg m -3 , which complied with the requirement as stipulated in the European (EN ISO 17225-2) and International (ISO 17225-6) standards.
Pellets with high bulk density are preferred to facilitate logistics arrangement, storage and cost reduction. Pelletizing process could increase the bulk density of EFB pellet to > 650 kg m -3 , far better than those of the raw and pulverised feeding materials in the range of 62 -152 kg m -3 (Table 4). Therefore, higher bulk density relates to an increased energy density i.e. more energy per unit volume of the resulted EFB pellets as a solid biofuel [46]. Moisture reduction, downsizing and densi cation process at the employed pressure and temperature in this study had largely in uenced and compacted the loose EFB bers to a much denser pellet form [44]. The average speci c density (1306 kg m -3 ) was higher than that reported by Tenorio et al. [28] and within the range required by the German standard, DIN 51731. Table 5 also shows the durability index of EFB pellets produced. There was no differences in durability index of the pellets produced from untreated (Test 1) and physically-treated (Tests 2 and 3) EFB bers. The mean value of 94.62% exceeding 80% can be regarded as high and acceptable although the value is slightly lower than those by the European and International standards [13]. Pellet durability index is an important parameter to describe capability of the produced pellets to tolerate shock and vibration during handling and transportation. Moisture content is a major factor affecting pellet durability level. An increasing amount of moisture beyond 14 wt.% causes pellet durability to drop signi cantly [38]. Other factors affecting pellet durability are particles size and type of pre-treatment of the raw material. Table 6 shows the proximate and ultimate analyses of the three types of EFB pellets produced from this study. There were signi cant differences on the moisture and ash contents and the CV of the EFB pellets compared to the raw material (Table 4). Huge improvement was made on moisture content and CV of the pellets, at >79.2% and 65.0%, compared to 46.33 wt.% and 10566 kJ kg -1 , respectively from the pressed EFB bers. The pre-pelletising step to reduce EFB's size and moisture and the pelletising process to densify the loose and pulverised bers had proven to improve the two important fuel properties of the EFB pellets as a solid fuel.
The fuel properties of EFB pellets deriving from different physical treatment (Test 1 to Test 3) were relatively comparable except for ash content. The employed Test 2 involving sieving process had managed to reduce the ash content of EFB pellets to 3.63 wt.%, compared to 4.66 wt.% that of an untreated normal EFB pellet. A combined sieving and water washing of EFB bers (Test 3) had signi cantly reduced the ash content of EFB pellets to 1.58 wt.%. This means physical treatment of raw material is necessary and should be incorporated at any commercial EFB pellets production line, in order to produce EFB pellets low in ash content for niche markets. As an agro-based by-product, EFB is naturally high in ashes plus easily contaminated by soil and dirt due to poor material handling, storage and transportation either at the mill or the pellet production plant compared to wood-based counterpart [9]. The use of an integrated gasi er system for drying EFB bers provides smokeless cleaner hot gases compared to existing biomass burner utilised in pellets production site.
Besides creating ash-related problem, a higher ash content also decreases the CV of the biomass pellets [47]. An increase of 1 wt.% of ash content has resulted in CV reduction of 0.11 -0.2 MJ kg -1 [48,49]. In such situation, more biomass fuel sources are required to generate the same amount of energy. Previous studies reported a 1.18 to 1.39 % higher biomass fuel consumption for every 1 wt.% increase in ash content [48,50]. By doing so, a higher ash production and management cost is anticipated due to frequent cleaning and machinery maintenance. High-ash content solid fuel will also in uence NOx emission, in particular agro-based pellets, causing negative environmental impact [51]. Verma et al. [52] reported that the NOx emitted from burning of straw pellet with 9 -10 wt.% ash content was 7 times higher than that of wood pellet containing just 0.65 wt.% ash. This phenomenon is due to catalytic effect of the ash present; higher amount of ashes tend to provide more active sites to catalyse N into NOx during the combustion [53,52]. Therefore, minimizing ash content in biomass fuel provides signi cant advantages from technical, operation, economic and environmental points of view for biomass-based heat and power generation. A combined sieving and washing pre-treatment in this study was able to solve the above-mentioned shortcomings. All the EFB pellets produced exhibited < 10 wt.% moisture content ( Table 6). As a result, the EFB pellets had energy value signi cantly higher than the raw EFB as large initial water content would affect the recoverable heat. Similarly, lower moisture content had contributed to a higher CV leading to better combustion temperature and e ciency.
The elemental contents of the EFB pellets produced (Table 6) and their initial physically-treated feeding materials (Table 4) were insigni cantly different but differed signi cantly from those of the raw material mainly due to high moisture content. The Van Krevelen plot (Figure 2) shows that the H/C and O/C atomic ratios of EFB pellets produced from different physical treatment (Tests 1-3) were closer to those torre ed EFB, coal and lignite [11] and signi cantly better than their raw material (pressed EFB bers). These ndings postulate that the pelletizing process largely involved both the mechanical and thermal (drying) processes to improve the physicochemical properties of the raw EFB bers, in particular the signi cantly reduced moisture content, leading to an increased C but reduced O contents of the EFB pellets. Like other biomass [54], the EFB pellets contained > 99 wt.% of O, C and H with the latter two totallỹ 50 wt.%. CV of a biomass fuel is much dependant on the amount of C and H present where higher contents of these two elements contribute to better energy content. As biomass combustion is an exothermic reaction, combustion of higher amount of C and H with O 2 would release more heat while generate carbon dioxide (CO 2 ) and water [55,56].
Comparing the C and H contents of the following: torre ed EFB, 58.89 wt.% and 5.12 wt.% [11]; coal, 64.34 wt% and 4.06 wt.% [57]; lignite, 58.5 wt.% and 3.0 wt.% [58] and pelletized EFB, 42.86 -43.98 wt.% and 6.38 -6.63 wt.% (this study), it is thus apparent that though the EFB pellets has a much lower CV amongst the solid fuels, its CV has been greatly improved compared to the raw material used. The results indicated that simple physical removal of moisture coupled with different sieving and washing co-treatment methods were su cient to improve the CV of EFB pellets which then could perform much better as a fuel compared to normal EFB pellets.
N and S embedded in biomass fuel create undesirable pollutants, namely NOx and SOx during thermal combustion. Their concentration limits are important, as evidenced by the EN ISO 17225-2 (for wood pellets) and the ISO 17225-6 (non-woody pellets) standards demanding for environmental-friendly biomass pellets [15]. The N and S contents in the EFB pellets ranged 0.24-0.54 wt.% and 0.03-0.07 wt.%, respectively are notably and relatively lower compared to the standard limits as stipulated in the EN ISO 17225-2 (≤ 1.0 wt.%) and the ISO 17225-6 (≤ 2.0 wt.%), except for S content in the EFB pellets derived from Tests 1 and 2 which was 0.01-0.02 wt.% higher than the limit (0.05 wt.%) speci ed in the EN ISO 17225-2, hence are of advantageous for cleaner energy generation. Tenorio et al. [28] reported that a combustion study of EFB pellets generated a very low emission of NOx (142.33 ppm) and none for SOx, with ame temperature varies between 500 -700°C. During combustion, the N and S contained in biomass are oxidised and volatised to generate NOx and SOx. The physical co-treatment used in this study i.e. a combined sieving and water washing had further reduced the S and N contents of EFB pellets, and lowering further the potential of air pollutant emissions. Table 7 shows major and minor trace elements exhibited in the pressed EFB bers (raw material) and EFB pellets produced from Test 2 (sieving) and Test 3 (combined sieving and water washing). The trace elements present are mainly alkali and heavy metals, and their concentrations are relatively low and comparable to woody biomass pellets, except for K, Cl, Fe, Zn and Na. This difference may be due to the nature of the plant and their ability to uptake speci c compounds and nutrients from the ecosystem i.e. soil, water as well as fertilizers and pesticides applied [59]. As agrobased residues pellets, high contents of Zn and Fe of EFB pellets in particular is attributed to other natural factors concerning oil palm cultivation practices such as irrigation with treated or semi-treated e uent (wastewater from palm oil mill), chemical-based fertiliser, and organic sludge that is typically used in the soil environment and agriculture plantations for improving soil fertility [60]. In addition, their presence serve as micro-nutrients bene cial as an alternative feed material and for soil amendment to enhance plant/animal growth [61].
These easily vaporized high amounts of alkali metals (K and Cl) together with the present S and silicates in the EFB pellets would result in severe ash-related problems to boilers such as slagging, fouling and corrosion [62,6]. These elements would also have adverse effects on boiler operation, ash quality, particulate matters and pollutants emission [63]. The ndings indicated that the applied sieving and water washing co-treatment could potentially reduce K, Cl and some heavy metals from the raw EFB feedstock at a minimum of 40% removal e ciency. This result corresponded with some of the previous studies, in particular concerning removal of K and Cl from EFB via other methods such as leaching and hydrothermal-washing co-treatment [29,35,34].

Conclusion
Two types of physical treatment of the pressed EFB bers i.e. sieving and a combined sieving and water washing were assessed to study their effects on the fuel properties of the derived EFB pellets. The ndings showed potential of these treatments as immediate and cost-effective means to reduce ash content of EFB pellets at commercial production plant. The sieving and water washing co-treatment employed could also signi cantly reduce some of the alkali metals, heavy metals, ash, Cl, N and S contents from the EFB pellets. It further enhanced environmental bene ts particularly in reducing pollutant emissions and ash generation during combustion of the resulted EFB pellets. The fuel properties such as moisture and ash contents, bulk density and CV of the resulted EFB pellets were comparable to the woody biomass pellets and complied to the minimum speci cations of the EN 14961-2 (for wood pellets) and the ISO 17225-6 (for non-woody pellets). The integrated physical treatment cum pelletisation of the pressed EFB bers is technically viable, as was evidenced from the production trials in producing EFB pellets with improved quality to fetch better commercial values as an environmental-friendly solid biofuel and for development of second generation biofuel.      Table 5 Physical properties and durability index of empty fruit bunches (EFB) pellets  Figure 1 Process ow diagram for the production of low-ash empty fruit bunches (EFB) pellets Van Krevelen plot of pressed empty fruit bunches (EFB) bers (raw material) and EFB -pellets produced from untreated (Test 1), sieving (Test 2), sieving and water washing (Test 3) and physical treatment compared to torre ed EFB, lignite and coal [11]