**7. Results and discussion**

### **7.1. Effect of temperature and pressure**

Table 2 summarizes the effect of temperature and pressure on sub- and supercritical hydro‐ thermal liquefaction of oil palm biomass at reaction time of 1 h and sample-to-water ratio = 1 : 10. From all the runs, the bio-oil yields of PMF were higher than that of EFB. This can be explained by the differences in composition of cellulose, hemicellulose and lignin of the biomass samples. PMF has relatively higher lignocellulosic content (75.9%) and lignin content (30.6%) compared to EFB (72.1% and 18.6, respectively).

At 25 and 30 MPa, the yield of bio-oil decreased with increasing temperature, most likely due to possible secondary decomposition of biomass components at higher temperatures, causing gasification and charring. At 35 MPa, increasing the temperature from 360°C to 390°C resulted into an increase in bio-oil yield. However, increasing the temperature further to 450°C decreased the yield. High pressure increases water density, hence enhancing solubility of the materials and its decomposition. However, higher temperature causes secondary decompo‐ sition of biomass and recombination of some free radicals, leading to gas and char formation.


**Table 2.** Effect of temperature and pressure on sub- and supercritical hydrothermal liquefaction of oil palm biomass (reaction time = 1 h, sample-to-water ratio = 1 : 10)

The increase in pressure has changing impacts on the bio-oil yield. Increasing the pressure increases the water density and also the solubility of the target compounds. These properties combined with inherent highly diffusive characteristics of supercritical fluids can enhance the ability of water to penetrate more easily into the sample matrix, hence improving extraction and decomposition of target compounds in biomass.

**6.4. Thermogravimetry-Differential Thermal Analysis (TG-DTA) analysis**

to 500°C at a heating rate of 20°C /min under N2 gas flow.

(30.6%) compared to EFB (72.1% and 18.6, respectively).

**7. Results and discussion**

466 Biofuels - Status and Perspective

**7.1. Effect of temperature and pressure**

**Reaction Conditions T (°C) P (MPa)**

(reaction time = 1 h, sample-to-water ratio = 1 : 10)

Subcritical water

Supercritical water

To determine the temperature-dependent mass behavior of biomass, thermogravimetrydifferential thermal analysis (TG-DTA) was performed using EXSTAR 6000 TG/DTA6300 apparatus (Seiko Instruments. Inc., Japan). The temperature range investigated was from 35

Table 2 summarizes the effect of temperature and pressure on sub- and supercritical hydro‐ thermal liquefaction of oil palm biomass at reaction time of 1 h and sample-to-water ratio = 1 : 10. From all the runs, the bio-oil yields of PMF were higher than that of EFB. This can be explained by the differences in composition of cellulose, hemicellulose and lignin of the biomass samples. PMF has relatively higher lignocellulosic content (75.9%) and lignin content

At 25 and 30 MPa, the yield of bio-oil decreased with increasing temperature, most likely due to possible secondary decomposition of biomass components at higher temperatures, causing gasification and charring. At 35 MPa, increasing the temperature from 360°C to 390°C resulted into an increase in bio-oil yield. However, increasing the temperature further to 450°C decreased the yield. High pressure increases water density, hence enhancing solubility of the materials and its decomposition. However, higher temperature causes secondary decompo‐ sition of biomass and recombination of some free radicals, leading to gas and char formation.

> **Bio-oil Yield (wt%) EFB PMF PKS**

 25 15.72 ± 1.05 16.40 ± 0.21 22.83 ± 0.56 30 16.96 ± 1.18 15.72 ± 0.81 25.62 ± 1.87 35 15.68 ± 0.68 14.79 ± 0.21 23.67 ± 0.27 25 25.31 ± 0.44 22.71 ± 0.83 26.55 ± 1.29 30 26.02 ± 0.44 23.22 ± 0.54 27.54 ± 0.70 35 22.76 ± 0.28 21.75 ± 0.01 23.44 ± 1.24

390 25 **37.39 ± 0.67 34.32 ± 1.87 38.53 ± 1.46** 390 30 28.17 ± 0.35 27.57 ± 1.03 31.16 ± 0.81 390 35 30.16 ± 0.98 24.07 ± 1.04 29.35 ± 0.71

**Table 2.** Effect of temperature and pressure on sub- and supercritical hydrothermal liquefaction of oil palm biomass

However, in some cases the increase in solvent density due to increase in pressure could result into cage effect for the C-C bonds present in the molecules of the target biomass components. This in effect inhibits the cleavage of C-C bonds, thus lowering hydrolysis rate resulting into low yields of liquefied products. Hence, as observed from the obtained data in this study, variation of pressure may or may not have positive results in sub- and supercritical hydro‐ thermal liquefaction of oil palm biomass. This observation merits more detailed investigation in the future.

### **7.2. Relation of obtained results with the lignocellulosic composition of samples**

As shown in the results of TG-DTA analysis of the samples in Fig. 5 and Fig. 6, respectively, hemicellulose degrades at a lower temperature range of 210–330°C due to the less uniform structure and low-degree crystallinity. Cellulose is a naturally occurring polymer consisting of linearly linked monomer units of glucose; hence, it has a higher degree of crystallinity and degrades at 300–375°C. Lignin is a highly crossed-linked polyphenolic aromatic polymer having no ordered repeating units. As such, lignin has the highest thermal stability and decomposed at 150–1000°C.

Due to higher lignin content, higher temperature and pressure (450°C and 30 MPa) are needed to degrade PMF and PKS compared to that of EFB (390°C and 25 MPa). PKS produces highest bio-oil yield at optimum condition, followed by PMF and EFB due to decreasing lignin content in the order of PKS > PMF > EFB.

**Figure 5.** Results of thermogravimetric analysis of various oil palm biomass samples.

**Figure 6.** Results of thermogravimetric analysis of various oil palm biomass samples.

### **7.3. Results of GC-MS analyses of the obtained bio-oils**

The typical GC-MS chromatograms of the bio-oil obtained from EFB, PMF and PKS at the optimum condition in this study (T=390°C, P=25 MPa) are shown in Fig. 7. The bio-oil obtained was typically dark brown, highly viscous liquid with distinct smoky odor. The chromatograms were almost identical, consisting of mostly phenolic and benzene compounds as depicted in Table 3. These compounds were likely obtained mainly from decomposition of lignin and cellulosic components of the biomass. Slight differences in composition for PKS as compared to other samples were observed as can be seen in the appearances of peaks 12 to 16. These peaks were identified as phenolic (peak 12), 1,2,3-trimethoxybenzene (peak 13), ester com‐ pound (peak 14), ketone (peak 15) and 1-(2,5-dimethoxyphenyl)-propanol (peak 16). These results agree with chemical compositions reported in literature consisting of several hundreds of organic compounds, such as acids, alcohols, aldehydes, esters, ketones, phenols and ligninderived oligomers [19]. It is also possible that some of these products are undesirable; thus, post-treatment of the obtained biocrude oil by removal of these unwanted compounds or by upgrading has been suggested. Production of these unwanted molecules can also be avoided by using catalysts suitable for selective degradation of the lignocellulosic components into the target compounds in addition to the enhancement of liquefaction rates.



**Table 3.** Major chemical components of the obtained bio-oil identified using GC-MS and NIST library

### **8. Use of catalysts to enhance liquefaction rates**

**Figure 6.** Results of thermogravimetric analysis of various oil palm biomass samples.

target compounds in addition to the enhancement of liquefaction rates.

1 18.44 Ethylbenzene 5.12 5.30 3.93 2 18.77 Benzene, 1,3-dimethyl- 5.55 - - 3 20.03 2-cyclopenten-1-one, 2-methyl 3.42 - - 4 22.38 Phenol 38.95 41.74 32.30

**Peak No.**

**Retention**

468 Biofuels - Status and Perspective

**Time (min) Compound**

The typical GC-MS chromatograms of the bio-oil obtained from EFB, PMF and PKS at the optimum condition in this study (T=390°C, P=25 MPa) are shown in Fig. 7. The bio-oil obtained was typically dark brown, highly viscous liquid with distinct smoky odor. The chromatograms were almost identical, consisting of mostly phenolic and benzene compounds as depicted in Table 3. These compounds were likely obtained mainly from decomposition of lignin and cellulosic components of the biomass. Slight differences in composition for PKS as compared to other samples were observed as can be seen in the appearances of peaks 12 to 16. These peaks were identified as phenolic (peak 12), 1,2,3-trimethoxybenzene (peak 13), ester com‐ pound (peak 14), ketone (peak 15) and 1-(2,5-dimethoxyphenyl)-propanol (peak 16). These results agree with chemical compositions reported in literature consisting of several hundreds of organic compounds, such as acids, alcohols, aldehydes, esters, ketones, phenols and ligninderived oligomers [19]. It is also possible that some of these products are undesirable; thus, post-treatment of the obtained biocrude oil by removal of these unwanted compounds or by upgrading has been suggested. Production of these unwanted molecules can also be avoided by using catalysts suitable for selective degradation of the lignocellulosic components into the

**Percentage Peak Area (%)**

**EFB PMF PKS**

**7.3. Results of GC-MS analyses of the obtained bio-oils**

The use of catalysts is indispensable and important to enhancement of liquefaction rates of biomass even under hydrothermal conditions. This will shorten reaction time even under mild conditions, thus reducing the required energy and processing cost. Some of the catalysts used for this purpose includes both homogeneous (alkali, alkali salts, etc.) and heterogeneous catalysts (metal oxides, etc.).

Akhtar et al. [20] has investigated liquefaction of empty palm fruit bunch (EPFB) under subcritical water conditions using various alkalis (such as NaOH, KOH and K2CO3). Catalytic performance and biomass to water ratio suitable for high EPFB conversion, liquid hydrocar‐ bons yield and lignin degradations were screened in a batch reactor operating at 270°C and 2 MPa in a reaction time of 20 min. Results showed that addition of alkali had positive effect on conversion, liquefaction and lignin degradation, increasing the rates by almost two-folds compared to that without any catalysts. The reactivity of the alkalis was in the order of K2CO3 > KOH > NaOH. The composition of bio-oil also depended on the type of catalysts, obtaining the highest yield of phenols using K2CO3 (1.0 M), while much of ester compounds were obtained using 1.0 M NaOH.

**7.3 Results of GC-MS analyses of the obtained bio-oils** 

**Figure 7.** Typical GC-MS chromatograms of bio-oil obtained from EFB, PMF and PKS at the optimum condition (T = 390°C, P=25 MPa).

Various catalytic processes for production of biofuels from palm oil and oil palm biomass were also summarized by Chew and Bhatia [21]. This includes reviews on catalytic processes of palm oil to produce biodiesel, and cracking to produce high-grade biofuels. This review also discusses biomass gasification to produce hydrogen and syngas, and its conversion to liquid fuels by Fischer-Tropsch synthesis (FTS), including upgrading of liquid/gas fuels obtained from liquefaction/pyrolysis of biomass. Several catalysts including zeolites such as ZSM-5 and aluminosilicates were reported to enhance biomass conversion and upgrading; however, application to hydrothermal liquefaction was not reported.

The use of rare earth modified zeolite, Ce/HZSM-5 has been proposed by Xu et al. [22] for hydrothermal liquefaction of microalgae. They found that this novel type of catalysts exhibited good potential and benefits for the preparation of bio-oil from microalgae (e.g. *Chlorella pyrenoidosa*) with high efficiency.

Several other catalysts have been screened for hydrothermal catalytic processing of pretreated algal oil that was produced from the hydrothermal liquefaction of Chlorella pyrenoidosa [23]. The activities of 5% Pt/C, 5% Pd/C, 5% Ru/C, 5% Pt/C (sulfided), Mo2C, MoS2, alumina, CoMo/ γ-Al2O3 (sulfided), Ni/SiO2-Al2O3, HZSM-5, activated carbon, and Raney-Ni for hydrothermal hydrodeoxygenation and hydrodenitrogenation of the pretreated algal oil at 400°C were investigated. It was reported that Ru/C showed the best performance for deoxygenation, and Raney-Ni was the most suitable catalyst for denitrogenation. The upgraded oil from the Ru/C catalyzed reaction contained the highest hydrocarbon content, the highest fraction of material boiling below 400°C, and the highest higher heating value (45.1 MJ/kg). All of the metal catalysts produced freely flowing upgraded oil. The combination of Ru/C and Raney Ni, which showed very good deoxygenation and denitrogenation of the oil, produced upgraded oil that retained 86% of the heating value in the original pretreated oil. This upgraded oil was also produced in a higher yield (77.2 wt.%) and with lower gas (9.4 wt.%), coke (15.6 wt.%) and water-soluble products yields (2.1 wt.%) than from either catalyst alone. The authors also suggested a two-step biocrude treatment strategy (non-catalytic treatment followed by catalytic upgrading) and a two-component catalyst bed in the second step for the hydrothermal catalytic upgrading of algal biocrude.
