production of solid biofuel from agricultural wastes of the palm oil industry by hydrothermal...

ORIGINAL PAPER

Production of Solid Biofuel from Agricultural Wastes of the PalmOil Industry by Hydrothermal Treatment

Ahmad T. Yuliansyah • Tsuyoshi Hirajima •

Satoshi Kumagai • Keiko Sasaki

Received: 24 April 2010 / Accepted: 25 September 2010 / Published online: 21 October 2010

� Springer Science+Business Media B.V. 2010

Abstract In this study, upgrading of agricultural waste,

in the form of oil palm fronds and trunks, into solid biofuel

was investigated using hydrothermal treatment. A slurry of

300 mL of water and 30 g of material was treated in a

500-mL batch autoclave equipped with stirrer, thermo-

meter, and pressure sensor. Experiments were conducted in

the temperature range 200–350�C at an initial pressure of

2.0 MPa. The slurry was gradually heated to the target

temperature and held for a further 30 min. Approximately

35–65% of the original material was recovered as a solid

product with favorable solid fuel characteristics. The gross

calorific value ranged from 19.9 to 29.7 MJ/kg and the

equilibrium moisture content was 7.6–4.5 wt%. The carbon

content varied from 51.4 to 78.5 wt% and the oxygen

content was 42.1–16.1 wt% after upgrading. Changes in the

solid composition and carbon functional groups following

upgrading were identified by FTIR and 13C NMR. In

addition, analyses on the liquid product (by GC–MS) and

the gas product (by GC) were carried out to clarify the

decomposition behavior of material.

Keywords Upgrading � Agricultural waste � Biomass �Hydrothermal treatment � Solid biofuel

Introduction

The rapid increase in energy consumption within the last

few years, combined with the steady depletion of fossil fuel

reserves, has prompted a global search for alternative

energy resources. Biomass is a promising alternative owing

to its sustainability and environmental friendliness. Various

types of biomass products are available in large quantities

and have potential for further utilization.

One potential biomass source that is abundant in most

tropical countries is oil palm waste. This waste can be

categorized into two types: waste from harvesting and

replanting activity in plantation fields and waste from the

milling process to obtain palm oil. Numerous studies on

utilization of milling waste (oil palm fiber, shell, and

empty fruit bunches) have been conducted for energy

applications [1–3], pulp and papermaking [4–6], bio-

adsorbents [7–10], construction materials [11, 12], and

biocomposites [13–15]. However, most of the milling

waste is fully utilized by the palm oil industry either as an

additional energy source (oil palm fiber and shell) or as

fertilizer (empty fruit bunches). By contrast, only a few

studies have considered the utilization of harvesting

waste. Currently, most of this waste is used convention-

ally as an organic fertilizer in plantation fields. However,

unpleasant smells coupled with a slow release of CO2 and

CH4 gas on decomposition, which can last for up to

1 year, are common problems. Nevertheless, oil palm

fronds and trunks have great potential. In 2005, approxi-

mately 43.05 million ton of frond and 13.94 million ton

of trunk wastes were generated in Indonesia, the largest

crude palm oil producer in the world. This amount will

continue increase with the rapid growth in the Indonesian

palm oil industry. Thus, a better method to manage such

wastes is highly desired.

A. T. Yuliansyah—On leave from Department of Chemical

Engineering, Gadjah Mada University, Indonesia.

A. T. Yuliansyah � T. Hirajima (&) � K. Sasaki

Department of Earth Resources Engineering, Faculty of

Engineering, Kyushu University, 744 Motooka, Nishi-ku,

Fukuoka 819-0395, Japan

e-mail: [email protected]

S. Kumagai

Research and Education Center of Carbon Resources, Kyushu

University, 744 Motooka, Nishiku, Fukuoka 819-0395, Japan

123

Waste Biomass Valor (2010) 1:395–405

DOI 10.1007/s12649-010-9045-3

Various treatment methods for several agricultural

wastes have been reviewed in several papers [16–19]. For

energy application purposes, the methods can be simply

divided into thermo-chemical and biological processes [20,

21]. Hydrothermal treatment is one of thermo-chemical

processes have attracted much attention recently. In this

process, biomass is treated in hot compressed water

yielding in gases, aqueous organics and upgraded solid [22,

23]. After filtered and dried, the obtained solid can be used

for solid fuel due to its high calorific value. Compared to

other thermo-chemical conversion methods such as pyro-

lysis and gasification, the temperature for hydrothermal

treatment is much lower (200–350�C for hydrothermal,

compared with 450–550�C for pyrolysis and 900–1200�C

for gasification) [16, 24]. In addition, biomass conversion

takes place in a wet environment so high moisture content

of feed biomass is not an issue. The role of water in the

treatment is not only as a medium, but also a chemical

reactant on decomposition. Therefore, such method is

suitable for treating biomass with high moisture content,

such as agricultural wastes which contain more than 50 wt. %

of moisture in fresh condition. A contrast situation is found

on pyrolysis and gasification which have a limitation on

moisture content of the feed [24, 25].

Many studies using hydrothermal treatment have been

conducted, but most of these used the method as a biomass

pretreatment step in bio-ethanol production [26–29]. Few

studies have considered benefits of the resulting solid. The

focus of the present study was upgrading of solid material

into solid biofuel by hydrothermal treatment. Therefore, we

evaluated the feasibility of upgrading oil palm fronds and

trunks and investigated their decomposition behavior dur-

ing hydrothermal treatment.

Experimental

Materials

Oil palm waste in the form of fronds and trunks was col-

lected from an oil palm plantation in southern Sumatra,

Indonesia. Both raw fronds and trunks were chipped into

pieces of approximately 2 cm in width of slabs. Prior to

use, the chips were ground using a cutting mill to form

powder with a maximum particle size of 1 mm. The

composition of the waste material is listed in Table 1.

Apparatus and Experimental Procedure

Experiments were carried out in a 500-mL batch-type auto-

clave (Taiatsu Techno MA 22) equipped with a stirrer and an

automatic temperature controller (Fig. 1). The autoclave had

a maximum temperature of 400�C and a maximum pressure

of 30 MPa. A slurry of 300 mL of water and 30 g of waste

material was loaded into the autoclave. A stream of N2 gas

was used to purge air from the autoclave and to maintain an

initial internal pressure of 2.0 MPa. With stirring at 200 rpm,

the autoclave was heated to the target temperature at an

average rate of 6.6�C/min. The target temperature, ranging

from 200 to 350�C, was automatically adjusted. Once the

target temperature was reached, the sample was held for a

further 30 min before the autoclave was cooled to ambient

Table 1 Composition of the raw materials

Component Frond Trunk

Cellulose (wt. %, d.b) 31.0 39.9

Hemicellulose (wt. %, d.b) 17.1 21.2

Klason lignin (wt. %, d.b) 22.9 22.6

Wax (wt. %, d.b) 2.0 3.1

Ash (wt. %, d.b) 2.8 1.9

Others 24.2 11.3

Control Board

PS

TS

Cooling water inCooling water out

Vessel

PS = Pressure SensorTS = Temperature Sensor

N2

Fig. 1 Schematic diagram of

the experimental apparatus

396 Waste Biomass Valor (2010) 1:395–405

123

conditions. Pressure and temperature profile for the experi-

ments was described in Fig. 2.

After cooling, the gas products were fed into a gasometer

(Shinagawa DC-1) to measure the volume. The gas was

sampled using a microsyringe (ITO MS-GANX00) and its

composition was determined by gas chromatography with

thermal conductivity detection (Shimadzu GC-4C). The

remaining slurry was filtered using an ADVANTEC 5C

filter and a water aspirator. The solid part was dried in an

oven at 105�C to yield the final solid product.

Analysis

The solid products were characterized using several tech-

niques. The elemental composition was measured using

Yanaco CHN Corder MT-5 and MT-6 elemental analyzer.

The cellulose, hemicellulose, and lignin contents were

measured using a procedure recommended by the US

National Renewable Energy Laboratory [30] that is sub-

stantially similar to ASTM E1758-01. The mineral com-

position in ash was determined using X-ray fluorescence

(XRF) on Rigaku ZSX Primus II equipment. In addition,

proximate, total sulfur and gross calorific value (GCV)

analyses were carried out according to JIS M 8812, JIS M

8819, and JIS M 8814, respectively. The equilibrium

moisture content (EMC) of raw frond and trunk and the

corresponding solid products was determined according to

JIS M 8811. An aliquot of the sample was placed in a

desiccator containing saturated salt solution at a constant

relative humidity (75% RH). After equilibrium was reached,

the moisture content of the solid was quickly measured

using a Sartorius MA 150 analyzer. Identification of the

chemical structure and functional groups was performed on

a Fourier transform infrared (FTIR) spectrometer (JASCO

670 Plus) using the KBr disk technique. Cross polarization/

magic angle spinning (CP/MAS) 13C NMR spectra were

measured on a solid-state spectrophotometer (JEOL CMX-

300) with the following conditions: 10,000 scans; contact

time, 2 ms; spinning speed,[12 kHz; pulse repetition time,

7 s. The spectrum was calibrated using hexamethyl ben-

zene. Curve fitting analysis of the spectrum was performed

using Grams/AI 32 ver. 8.0 software. Composition of the

liquid products were analyzed by gas chromatography—

mass spectrophotometer (GC–MS) Agilent 6890 N equip-

ped with Jeol JMS-Q1000GC(A) system. Additionally, total

organic carbon content of liquid was determined on a Shi-

madzu TOC-5000A instrument. Solid yield, energy densi-

fication ratio, and energy yield are three important

parameters in this study which are defined as [31]:

Solid yield ¼�mass of dried solid product=

mass of dried feed material�� 100%

Energy densification ratio ¼ GCV of product=

GCV of feed material

Energy yield ¼ solid yield � energy densification ratio

.

Results and Discussion

Product Distribution

Hydrothermal treatment led to thermal degradation of the

feed material. Physical and chemical bonds in the material

were broken, so that large long-chain compounds such as

cellulose, hemicellulose and lignin were broken down into

smaller and simpler molecules. Furthermore, some of the

molecules were dissolved into liquid part and some others

were degraded to gases. The remainder of feed material

was recovered as a solid residue. Filtration and drying

yielded a solid product from this residue.

Figure 3 shows the distribution of organic compounds,

represented by percentage carbon, in the gas, liquid, and

solid phases. However, a small amount of material was not

recovered during filtration and drying. The results indicate

that most of the carbon remained in the solid phase and that

the relative amount gradually decreased as the temperature

increased. Conversely, the proportion of carbon in the gas

phase steadily increased with temperature. For the liquid

phase, a unique characteristic was observed. The relative

amount of carbon increased with temperature to a maxi-

mum at *270�C and then decreased owing to polymeri-

zation of soluble compounds to produce solid precipitates.

Solid products

The properties of solid products for different temperature

treatments are described in Table 2. Reaction temperature

Fig. 2 Pressure and temperature profile for experiments (1 200�C;

2 240�C; 3 270�C; 4 300�C; 5 330�C; 6 350�C)

Waste Biomass Valor (2010) 1:395–405 397

123

had a significant effect to the solid yield, energy densifi-

cation ratio and energy yield of the solid products.

Increasing temperature led to a decrease in solid yield and

energy yield, while energy densification ratio increased.

For the same reaction temperature, the solid product of

trunk had a higher solid yield and energy yield than frond.

However, the differences were smaller at higher tempera-

ture. At 200�C, the solid yields were 58.3 and 67.8 wt%,

while the energy yields were 63.5 and 71.7%, for frond and

trunk, respectively. The solid yield reduced to 35.1 and

35.3 wt%, while the energy yield decreased to 55.4 and

55.7% at 350�C treatment.

Like other biomass materials, both fronds and trunks

have very high volatile content of 82.5 and 83.9 wt%, in

contrast to the low fixed carbon of 17.5 and 16.1 wt%,

respectively. Progressive decomposition reactions occurred

at higher temperature, leading to an increase in fixed carbon

content and a decrease in volatile content. Treatment at

350�C increased the fixed carbon content to 54.8 and

55.0 wt% and decreased the volatile content to 45.2 and

45.0 wt% for fronds and trunks, respectively. This led to an

increase in gross calorific value of the solid product.

The calorific value is correlated with the elemental

composition of a solid. The data in Table 2 show that an

increase in calorific value is correlated with an increase in

carbon content and a decrease in oxygen content. Com-

pared to the raw material, the solid produced at 350�C has

*59% higher carbon content and *58% lower oxygen

content. These results suggest that components degraded

and removed from the material were mainly oxygen-rich

compounds. Dramatic changes in the calorific value and

elemental composition took place in the temperature range

200–270�C.

Due to thermal degradation, the content of cellulose,

hemicellulose, and lignin in the solid decreased. Table 3

shows the percentage of these components in the solid

products after treatment at 200–300�C. The data suggest

that hemicellulose and cellulose were relatively easier to

degrade than lignin. The treatment significantly degraded

both hemicellulose and cellulose to produce a more lig-

neous solid. Slightly different behavior was observed for

hemicellulose decomposition between frond and trunk

material. The frond solid produced at 200�C still had a

small amount of hemicellulose, which completely vanished

on treatment at 240�C. On the other hand, no hemicellulose

was found for the trunk products, even for treatment

200�C. This suggests that hemicellulose decomposition

started at temperatures\200�C. By contrast, cellulose was

gradually degraded at higher temperature and \0.5 wt%

(on a solid product basis) remained after treatment at

270�C. This behavior is in agreement with previous reports

[22, 32].

The obtained solids contained a small amount of ash

ranging from 0.7 to 2.2 wt% (Table 2). The use of these

low-ash products as fuel will be beneficial because it will

reduce potential for solid deposition on burner equipment

that commonly found on combustion of high-ash fuel.

Table 4 shows ash analysis of frond and trunk solid prod-

ucts. As can be seen from this table, CaO and SiO2 were

two major oxides accounted for 28.80–51.30 and

16.40–39.10 wt% for frond, and 15.20–27.30 and

22.80–44.30 wt% for trunk, respectively. The data in

table 4 also indicates that some minerals such as CaO,

MgO and K2O were selectively leached from material

during treatment.

Coal Band

Under hydrothermal treatment, frond and trunk materials

undergo a coalification-like process, as demonstrated in the

Van Krevelen diagram in Fig. 4. The raw materials have

high atomic H/C and O/C ratios, which both gradually

decreased during treatment. The slope of the trajectories

suggests that the O content decreased in proportion to the H

content, probably due to dehydration. It is clear that the

Fig. 3 Distribution of carbon in the hydrothermal product a frond;

b trunk


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decrease in O and H content occurred mainly in the range

200–270�C. Less significant changes were observed at

higher temperature. The solids resulting from higher tem-

perature treatment had comparable compositions with

typical solid fuels such as sub-bituminous coal. Although

fronds and trunks have different raw compositions, the

products after treatment at C300�C had almost identical

compositions.

Fourier Transform Infrared (FTIR) Analysis

To understand changes in functional groups during

hydrothermal treatment, FTIR analysis of the products was

Table 2 Properties of the raw materials and the solid products

Properties Raw Treated temperature (�C)

200 240 270 300 330 350

(a) Frond

Proximate analysis (wt.%)

Fixed carbon (d.a.f) 17.5 20.5 29.7 45.9 48.1 52.3 54.8

Volatile matter (d.a.f) 82.5 79.5 70.3 54.1 51.9 47.7 45.2

Ash (d.b) 1.8 1.3 1.3 1.2 1.0 0.7 0.8

Equilibrium moisture 14.7 7.6 6.4 5.5 5.2 5.0 5.2

Ultimate analysis (wt.%) (d.a.f)

C 47.2 53.6 58.6 69.4 71.1 73.9 75.1

H 5.9 5.7 5.4 4.9 4.9 4.9 4.8

N 0.2 0.2 0.3 0.4 0.4 0.4 0.4

O (diff) 46.6 40.4 35.7 25.3 23.5 20.7 19.5

S 0.1 0.1 0.1 0.1 0.1 0.1 0.1

Yield of solid product (wt.%) (d.b) 58.3 52.0 42.5 38.4 36.7 35.1

Gross calorific value (MJ/kg) (d.a.f) 18.8 20.5 23.0 26.7 27.3 29.0 29.7

Energy densification ratio 1.09 1.22 1.42 1.45 1.54 1.58

Energy yield (%) 63.5 63.6 60.3 55.9 56.5 55.4

(b) Trunk

Proximate analysis (wt.%)

Fixed carbon (d.a.f) 16.1 16.2 26.6 45.1 48.8 52.8 55.0

Volatile matter (d.a.f) 83.9 83.8 73.4 54.9 51.2 47.2 45.0

Ash (d.b) 2.2 1.8 1.8 2.2 2.1 1.9 2.1

Equilibrium moisture 13.6 7.5 6.5 5.1 4.8 4.6 4.5

Ultimate analysis (wt.%) (d.a.f)

C 47.5 51.4 57.5 69.3 71.4 73.4 75.3

H 5.9 5.9 5.6 5.1 5.0 4.9 4.9

N 0.5 0.4 0.6 0.8 0.8 1.0 1.0

O (diff) 45.9 42.1 36.2 24.6 22.6 20.6 18.6

S 0.1 0.1 0.1 0.1 0.1 0.1 0.2

Yield of solid product (wt.%) (d.b) 67.8 56.9 41.7 38.7 36.9 35.3

Gross calorific value (MJ/kg) (d.a.f) 18.8 19.9 22.6 27.0 28.0 29.4 29.7

Energy densification ratio 1.06 1.20 1.44 1.49 1.57 1.58

Energy yield (%) 71.7 68.3 60.0 57.8 57.9 55.7

d.b dry basis, d.a.f dry ash free basis, diff. differences

Table 3 Percent component in the solid products

Product Cellulose Hemicellulose Lignin & other

Frond

200�C 55.1 3.2 41.7

240�C 42.3 0.0 57.7

270�C 0.2 0.0 99.8

300�C 0.0 0.0 100.0

Trunk

200�C 62.9 0.0 37.1

240�C 44.8 0.0 55.1

270�C 0.4 0.0 99.6

300�C 0.0 0.0 100.0


123

performed. Peaks were assigned based on literature data [3,

33]. Figure 5 shows spectra of the raw materials and the

corresponding solid products. The spectra for frond and

trunk materials were similar. The intensity of the peak

*3500 cm-1 attributed to –OH groups decreased at ele-

vated temperature, indicating that water molecules within

the solids were gradually expelled. In other words, dehy-

dration of the feed material occurred. The peak at

*2900 cm-1 attributed to aliphatic CHn groups also

weakened, indicating that several long aliphatic chains were

broken down. More distinctive peaks were observed in the

region below 2000 cm-1. The peak at *1700–1740 cm-1

represents carbonyl (C=O) stretching vibrations. The peak

at *1515 cm-1 reveals to aromatic skeletal vibration

derived from lignin. The peak at *1050 cm-1 attributed to

glycosidic bonds, indicating the presence of cellulose,

steadily weakened and completely disappeared for tem-

peratures [270�C, indicating that cellulose was totally

degraded at this temperature. The decrease in intensity for

C–O–C aryl–alkyl ether linkages at *1230 cm-1 suggests

lignin decomposition. Conversely, solids derived from

polymerization of intermediate compounds in the liquid

phase increased the aromatic content, particularly at tem-

peratures[300�C, as indicated by the increase in intensity

for the peak at 1600 cm-1 attributed to aromatic skeletal

vibrations and CO stretching.

13C Nuclear Magnetic Resonance (NMR) Results

13C NMR measurements were conducted to complement

FTIR in characterizing the molecular structure of the solid

products. NMR is useful for making comparisons without the

need for peak ratios. Each resonance peak can be measured

relative to the total resonance intensity to give the relative

amount of individual molecular groups. Typical 13C NMR

spectra for raw biomass with peak assignment can be found

in the literature [34–36]. In brief, resonance peaks in spectra

for raw frond and trunk material were assigned to CH3 in

acetyl groups (21 ppm), methoxyl groups in lignin

(56 ppm), C-6 carbon atoms in cellulose (62–65 ppm), C-2/

C-3/C-5 atoms in cellulose (72–75 ppm), C-4 atoms in cel-

lulose (84–89 ppm), C-1 atoms in hemicellulose (102 ppm),

Table 4 Mineral composition of ash in the raw materials and the solid products

Solid products Oxide (wt.%)

CaO SiO2 MgO K2O Fe2O3 Al2O3 ZnO SO3 P2O5 Others

(a) Frond

Raw 52.70 12.20 16.00 8.29 0.39 0.21 0.05 7.21 2.43 0.52

200�C 51.30 28.70 6.04 3.66 1.41 0.79 0.06 5.78 1.10 1.16

240�C 39.50 32.90 7.52 4.36 1.97 1.49 0.07 8.31 2.54 1.34

270�C 28.80 39.10 7.60 4.64 1.22 1.61 0.05 12.40 2.95 1.63

300�C 30.80 29.60 6.95 5.50 2.19 2.17 0.05 15.20 5.36 2.18

330�C 37.10 16.60 8.49 6.03 2.54 2.62 0.07 16.30 8.14 2.11

350�C 36.50 16.40 8.51 5.71 2.96 2.67 0.08 16.00 7.83 3.34

(b) Trunk

Raw 23.50 14.90 15.60 24.50 0.71 0.25 0.12 10.70 8.71 1.00

200�C 18.80 39.80 8.37 13.20 1.14 0.67 0.11 9.54 7.27 1.10

240�C 15.20 44.30 5.83 9.21 1.43 0.76 0.10 9.23 13.10 0.84

270�C 16.40 44.00 4.64 7.38 1.21 0.83 0.06 9.95 14.80 0.73

300�C 18.20 41.40 4.64 5.38 1.69 1.49 0.06 7.97 18.20 0.97

330�C 27.30 22.80 5.05 6.76 2.57 1.22 0.15 9.64 23.00 1.51

350�C 22.10 22.80 11.80 5.16 3.13 0.76 0.12 9.33 18.40 6.41

Fig. 4 Van Krevelen diagram for the solid products obtained at

different temperatures in comparison with other solid fuel (1 raw

material; 2 200�C; 3 240�C; 4 270�C; 5 300�C; 6 330�C; 7 350�C;

a frond; b trunk)


123

C-1 atoms in cellulose (105 ppm), unsubstituted olefinic or

aromatic carbon atoms (110–127 ppm), quaternary olefinic

or aromatic carbon atoms (127–143 ppm), olefinic or aro-

matic carbon atoms with OH or OR substituents

(143–167 ppm), esters and carboxylic acids (169–195 ppm)

including acetyl groups in hemicellulose (173 ppm), and

carbonyl groups in lignin (195–225 ppm). Despite the vari-

ous resonance peaks observed, for semi-quantitative analysis

the spectra can be simply classified into aliphatic

(0–59 ppm), carbohydrate (59–110 ppm), aromatic

(110–160 ppm), carboxyl (160–188 ppm), and carbonyl

regions (188–225 ppm) [35, 36].

Data for raw and treated fronds reveal that the peak

resonance for hemicellulose and cellulose progressively

decreased (Fig. 6). A similar pattern was observed for

trunk spectra (Fig. 7). Products resulting from treatment at

200 to 240�C exhibited identical spectra to that of the raw

material. However, treatment at 270�C led to extreme

spectral changes to a more aromatic nature. The relative

aromatic content, which correlates with the lignin content,

consistently increased in the range 200–270�C, whereas the

carbohydrate content (hemicellulose and cellulose)

decreased. This is in good agreement with the component

analysis, which suggested that lignin was the predominant

component for treatment at C270�C (Table 3).

Equilibrium Moisture Content (EMC) Analysis

Hydrothermal treatment greatly reduced the EMC of

materials. Treatment at 200�C reduced the EMC from 14.7

to 7.6 wt% for fronds and from 13.6 to 7.5 wt% for trunks.

Further treatment at 350�C led to EMC as low as 5.2 and

4.5 wt% for fronds and trunks, respectively. However, the

decrease in EMC mainly occurred in the range 200–270�C,

with only small changes observed at higher temperatures.

These results are in agreement with the changes in solid

components shown in Fig. 3.

Based on the component characteristic on water

adsorption, hemicellulose exhibits the strongest water

adsorption, followed by cellulose and lignin [37]. Since

hemicellulose was removed first from the solid at low

temperature, it is reasonable that the EMC of the material

dramatically decreased in this range. By contrast, solids

with high lignin content adsorb only a small amount of

moisture. The EMC results were confirmed by NMR

Fig. 5 FTIR spectra for the raw materials and the corresponding

products a frond; b trunk

Fig. 6 13C NMR spectra with curve fitting for raw frond and products

obtained at different temperatures


123

results demonstrating an increase in aromatic content in the

solid material. The presence of aromatic compounds,

which are hydrophobic, results in resistance to humidity

and water adsorption from air. Therefore, a higher aromatic

content is correlated with lower EMC. The relationship

between the relative amount of aromatic carbon and the

EMC is presented in Fig. 8.

EMC and calorific value are two important properties of

solid fuels. When material is burned, some of the energy

released by combustion is consumed to vaporize the water

contained in the material. Material with a higher EMC will

require more energy for moisture evaporation. Thus, a

good solid fuel should have a high calorific value and a low

EMC. Our experiments demonstrated that both properties

were improved by hydrothermal treatment.

Although combustion of the solid fuel products totally

produces less energy than that of raw materials, the fuels

offer other benefits. They are more resistant to undesired

biological decomposition when they are kept in storage. In

addition, as solid fuel which may be transported from one

location to other, their higher energy density will affect on

reduction of storage cost, as well as transportation cost.

Liquid Product

Under hydrothermal treatment, materials were degraded

into numerous low-molecular weight compounds that

subsequently leached into liquid. Figure 9 shows GC–MS

chromatogram of the liquid product of frond and trunk,

respectively. As indicated by those chromatograms,

hydrothermal liquid product contained various organic

compounds. Table 5 lists peaks for several major

Fig. 7 13C NMR spectra with curve fitting for raw trunk and products

obtained at different temperatures

Fig. 8 Relationship between percentage aromatic carbon and equi-

librium moisture content

Fig. 9 GC-MS spectrogram for the liquid products obtained at

various temperatures a frond; b trunk


123

compounds identified on GC/MS chromatogram. The

chromatograms show that acetic acid and furfural were two

predominant organics obtained at 200�C, followed by 5-

HMF, phenol, and 1-hydroxy 2-propanone. The intensity of

acetic acid and phenol tended to increase at elevated

temperature. However, peaks revealed furfural and 5-HMF,

secondary decomposition products of hemicellulose and

cellulose, were present only at 200–270�C. These data

confirmed the results of chemical analysis and 13C NMR of

solid that suggest a progressive decomposition of hemi-

cellulose and cellulose took place within 200–270�C. On

the other hand, phenol and other phenolic compounds such

as 2,6-dimethoxy phenol and 1,2 benzenediol, indicating

decomposition of lignin, were observed along the temper-

ature range of 200–350�C. Due to their composition which

mainly contained organic acids and phenolic compounds,

the liquid product could be considered for disinfectant and

organic preservatives. However, such applications need a

further investigation.

Gas Product

Hydrothermal reactions involve the formation of gases

derived from degradation of water-soluble compounds [38,

39]. A small amount of gas was produced at low temper-

ature and steadily increased as the temperature was

increased. However, CO2 was the predominant gas

observed (C80 vol.%), followed by CO and H2. As shown

in Fig. 10, the gas proportion strongly depended on the

temperature. At 200�C, only CO2 was produced. The per-

centage of CO2 gradually decreased, accompanied by

increases in CO and H2 at elevated temperature. A slight

amount of CH4, indicating a methanation, was found at

350�C. The gas formed at this temperature comprised

5.3–5.5 vol.% H2, 3.5–5.3 vol.% CO, 83.5–85.2 vol.% CO2

and 5.6–6.0 vol.% CH4. Application of this gas product

seems less essential. Although the gas product contained

H2 and CH4, clean and high calorific gases, presence of

very high of CO2 makes its application for fuel gas

Table 5 Peak for identified

compounds in GC-MS

chromatograms

No.

Peak

Retention time

(min)

Compounds Chemical

formula

Molecular

weight

1 00:04:50 Methanol CH4O 32

2 00:05:45 Ethanol C2H6O 46

3 00:06:16 Aceton C3H6O 58

4 00:09:24 Acetic acid C2H4O2 60

5 00:10:21 1-hydroxy 2-propanon C3H6O2 74

6 00:10:59 Propanoic acid C3H6O2 74

7 00:12:11 Cyclopentanone C5H8O 84

8 00:12:57 Furfural C5H4O2 96

9 00:13:00 2-cyclopenten-1-one C5H6O 82

10 00:13:55 2-cyclopenten-1-one, 2-methyl C6H8O 96

11 00:14:25 4-oxopentanethioic acid C5H8O2S 132

12 00:15:25 Phenol C6H6O 94

13 00:16:16 2-methoxy phenol C7H8O2 124

14 00:17:15 3-pyridinol C5H5NO 95

15 00:18:20 1,2 benzenediol C6H6O2 110

16 00:18:21 5-HMF C6H6O3 126

17 00:19:04 2,6-dimethoxy phenol C8H10O3 154

18 00:19:43 2-methyl 1,3-benzenediol C7H8O2 124

19 00:20:03 4-methoxy-3-(methoxymethyl)

phenol

C9H12O3 168

Fig. 10 Gas produced form hydrothermal treatment at various

temperatures


123

inappropriate. On the other hand, its utilization for high

purity CO2 source will be costly since an additional sepa-

ration process is needed. Meanwhile, conversion into the

gas products is less than 10%, as shown in the carbon

balance (Fig. 3).

Conclusions

Upgrading of oil palm frond and trunk was investigated by

hydrothermal treatment at 200–350�C for 30 min.

Approximately 35–65 wt% of the original material was

recovered after the process as solid fuel. The very high

oxygen and volatile matter content of the original material

were significantly reduced. By contrast, the fixed carbon

content increased sharply due to carbonization. The van

Krevelen diagram revealed that solids resulting from

treatment at C330�C have a composition comparable to

that of sub-bituminous coal. FTIR analysis confirmed that

oxygen elimination due to dehydration in conjunction with

decomposition of hemicellulose and cellulose occurred at

200–270�C. At temperatures [270�C, the structure of the

solid dramatically changed and was dominated by lignin.

This was indicated by an increase in aromatic compounds,

as determined by 13C NMR spectroscopy.

Hydrothermal treatment progressively changes the cal-

orific value and EMC of materials. Treatment at 350�C

produced solid with a gross calorific value as high as

29.7 MJ/kg (for both materials) and EMC of *5.2 and

*4.5 wt% for frond and trunk material, respectively.

However, significant changes in the calorific value and the

EMC was observed in 200–270�C range, which can be

attributed to progressive removal of hemicellulose and

cellulose. This was in agreement with the GC–MS analysis

results, showing that decomposition products of hemicel-

lulose and cellulose (furfural and 5-HMF) were observed

only at 200–270�C. The GC–MS result also indicated

degradation of lignin took place along the temperature

range of 200–350�C.

After treatment, liquid containing various organic

compounds, mainly organic acids and phenolic com-

pounds, was produced. A small amount of gas was formed

at low temperature and steadily increased as the tempera-

ture was increased. However, CO2 was the predominant

gas observed (C80 vol.%).

Although total energy produced from combustion of this

solid product was less than that of raw material, the use of

this fuel offers other benefits. The fuel had higher energy

density and lower EMC which may improve its handling

and storage properties. Thus, it is proposed that hydro-

thermal treatment could become an advantageous tech-

nology for producing solid fuel from biomass wastes.

Acknowledgments The authors are grateful for support of this

research by a Grant-in-Aid for Scientific Research No. 21246135

from the Japan Society for the Promotion of Science (JSPS) and the

Global COE program (Novel Carbon Resources Sciences, Kyushu

University).

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