The source of empty fruit bunches was obtained from palm tree plantation area “Kilang Sawit FELCRA Berhad Nasaruddin, KM 37, Jalan Tronoh, 32600 Bota, Perak, Malaysia”. These empty fruit bunches were washed and dried in an oven at 100 °C for 3 days. The dried EFB was chopped and cut into smaller pieces. The sand and dust particles were cleaned by using water and then dried in the oven at 100 °C for 24 h. The dried EFB was then grinded with FRITSCH Cutting Mill to particle size below 500 µm. The grinded EFB was sieved to get the desired particle size of <500 µm. Table 2 shows the measured properties of EFB.

Ultimate analysis of the EFB was carried out using LECO 932 CHNS Analyzer and the Higher Heating Value (HHV) was measured using IKA C5000 Bomb Calorimeter. The zeolite catalyst used in the catalytic pyrolysis was purchased from Sigma-Aldrich with surface area of 780 m²/g, 30 wt% of SiO₂/Al₂O₃ and pore size of 7.4 × 7.4 Å [21]. The solvents (EMSURE^® ACS) acetone and ethyl acetate were obtained from Merck KgaA whereas ethanol of 95 vol% was purchased from HmbG chemicals.

Catalytic pyrolysis of Empty Fruit Bunch (EFB). In each run, about 15 gram of dried EFB was mixed with zeolite catalyst and loaded into the borosilicate glass tube. The glass tube was inserted inside the furnace. A glass wool was used to prevent the elutriation of reactant particles and blocked the gas inlet to the condenser. Nitrogen gas was allowed to flow through the tube for a few minutes to remove any oxygen presents before the reaction. The desired temperature of the reaction of 500 °C, the furnace heating rate of 20 °C/min and the nitrogen gas flow rate of 100 ml/min were set. During the pyrolysis, the pyrolysis vapor produced was carried out by the nitrogen gas and passed through an ice bath condenser, where the condensable vapor was condensed to form bio-oil as shown in Figure 1.

Figure 1.

Semi-batch reactor

The reaction required about 35 min. The weight of the bio-oil produced was determined by measuring the difference between the weight of the condenser before and after the experiment. The liquid yield was calculated using eq. (1) below [22]:

The bio-oil obtained from all the experimental runs was stored in a container and placed in a refrigerator in order to slow down the rate of aging.

Stabilization of EFB derived bio-oil. The aging of EFB derived bio-oil was studied at 80 °C for 7 days. 10 wt% of ethyl acetate, acetone and ethanol of 95 vol% were added to the EFB derived bio-oil in separate samples. The percentage change in viscosity and water content of the bio-oil with the selected solvents were calculated based on eq. (2) and eq. (3) respectively:

Characterization of EFB derived bio-oil. Metrohm 870 Karl Fischer Titrino Plus was used to determine the water content of bio-oil samples. It took into account of two dosinos with their dosing units mounted directly onto reagent bottles that are located on the instrument. On the other hand, the viscosity of the bio-oil in this study was measured with Brookfield CAP 2000+ Viscometer. The Gas Chromatography-Mass Spectrometry (GC-MS) analyses were conducted on samples diluted in acetone using GCMS-QP2010 Plus.

The chosen chemical solvents, ethanol of 95 vol%, acetone and ethyl acetate were mixed homogeneously with the bio-oil. The percentage changed in viscosity and water content of bio-oil, with and without solvents, after subjected to accelerate aging are presented in Figure 2 and Figure 3 respectively. Figure 2 shows that after the accelerated aging at 80 °C for 7 days, the percentage change in viscosity of bio-oil with solvents decreased compared to pure bio-oil. This indicated that the solvents were able to increase the stability of bio-oil. The percentages change of viscosity of bio-oil with ethyl acetate, acetone and ethanol were 64.60%, 79.09% and 38.31% respectively.

Figure 2.

Percentage change in viscosity of bio-oil with and without solvents

Based on Figure 3, the water content of bio-oil was reduced with the addition of solvents where the percentage changed of water content of bio-oil with ethyl acetate, acetone and ethanol of 95 vol% were 4.10%, 6.63% and 0.75% respectively. The results were comparable to the work of Osama et al. [23] as he investigated that by adding 2 wt%, 5%, 10%, and 20% of ethanol to a hardwood bio-oil and observed that the initial viscosity of the bio-oil at 50 °C decreased from 50 cost with absence of ethanol to 10 cost with presence of 20% ethanol. After aging at 50 °C for 7 days, the increased in viscosity of the neat bio-oil was 3.5 cost/day compared to 0.4 cost/day for a mixture of 5 wt% ethanol in bio-oil.

Figure 3.

Percentage change in water content of bio-oil with and without solvents

The effectiveness of solvent increased in the following order: acetone, ethyl acetate, and ethanol of 95 vol%. For bio-oil blended with solvents, no phase separation was observed after 7 days of accelerated aging. It showed that addition of ethanol to pure bio-oil inhibited phase separation. This was particularly important for bio-oil storage at high temperatures. Gas Chromatography-Mass Spectrometry (GC-MS) analysis was conducted on bio-oil with ethanol to identify the compounds that presence in the bio-oil.

Diebold [2] investigated that bio-oil with high molecular weight of hemiacetals, acetals, and esters would shift the equilibrium composition to a mixture with a lower molecular weight and viscosity by adding a low molecular weight alcohol. This could be important in stabilizing the bio-oil by four mechanisms which are reducing the concentration of reactive aldehydes, by converting more of them to less reactive, relatively low to moderate molecular-weight hemiacetals and acetals, transacetalizing large hemiacetals and acetals to form lower molecular weight hemiacetals and acetals, converting organic acids to low molecular weight esters and transesterifying large esters to form lower molecular weight esters. The second and fourth mechanisms could be considered to depolymerize the original high molecular weight hemiacetals, acetals, and esters. The chemical compositions of pure bio-oil and bio-oil with ethanol of 95 vol% were plotted in pie charts as shown in Figure 4 below.

Figure 4.

Chemical composition of pure bio-oil (a); and bio-oil with ethanol of 95 vol% (b)

Figure 4 shows that adding ethanol of 95 vol% to bio-oil shifts the equilibrium composition of bio-oil towards more esters, acetals and water. The formation of esters and water from the reactions between carboxylic acids and ethanol was shown in the eq. (4) as follows:

(4)

Water was the most abundant compound present in pure bio-oil. Water was miscible with many of the oligomeric lignin derived components due to the solubilising effect of hydrophilic compounds. In addition, the water could not be removed by distillation as there was an azeotrope system formed between the carboxylic acid and water. Thus, conversion of low molecular weight acids into their corresponding esters allowed a large quantity of the water to be removed by distillation.

On the other hand, increased water content encouraged the hydrolysis of ester linked polymers to form lower-molecular-weight products such as furfural and furan. It also converted the reactants to inactive forms. The ethanol could react with the reactive sites in the oligomers that terminated polymerization. Likewise, polymeric esters were transesterified with alcohol to form a lower-molecular-weight methyl ester. The increased in molecular weights of these methyl esters and acetals were relatively low compared to the possible molecular weights that occurs in a real bio-oil with oligomeric hydroxy compounds reacting with oligomeric acids and aldehydes. This proved that addition of low molecular weight reactants reacts with oligomeric bio-oil components to form medium low molecular weight oligomers instead of high molecular weight polymers. Besides, aldehyde groups could also be converted to acetals which reduce the availability for polymerization. The results were analogous with the work of Etzkorn et al. [24]. He explored that unsaturated aldehydes such as acrolein could react with alcohol to form the alkoxy aldehyde, unsaturated acetal or alkoxy acetal according to the reaction shown in eq. (5) as follows:

(5)

where R is an alkyl group.

Addition of ethanol to bio-oil also inhibited phase separation as there was an absence of low-viscosity water-rich component on the top and a high-viscosity tar-rich layer at the bottom of the bio-oil which confirmed similar findings with the work of Oasmaa et al. [23]. He stated that the stability of bio-oil during storage increased by adding proper solvents.

The estimated bio-oil required for the study of stabilization was produced in 29 runs of experiment. The weight percentage of liquid yield for each run was calculated and tabulated in Table 3. The average liquid yield for catalytic pyrolysis of EFB in this research was 25.73 wt%.