1. Introduction
Crude
palm oil production is reaching 48.99 million metric tonnes per year
globally in 2011 and Southeast Asia is the main contributor, with
Indonesia accounting for 48.79%, Malaysia 36.75%, and Thailand 2.96%
(Palm Oil Refiners Association of Malaysia, 2011). Oil palm is a
multi-purpose plantation and it is also an intensive producer of
biomass. Accompanying the production of one kg of palm oil,
approximately 4 kg of dry biomass are produced. One third of the oil
palm biomass is oil palm empty fruit bunch (OPEFB) and the other two
thirds are oil palm trunks and fronds [1-3].
The supply of oil palm biomass and its processing by-products are found to be seven times that of natural timber [4].
Besides producing oils and fats, there are continuous interests in
using oil palm biomass as the source of renewable energy. Among the oil
palm biomass, OPEFB is the most often investigated biomass for biofuel
production. Traditionally, OPEFB is used for power and steam utilization
in the palm oil mills, and is used for composting and soil mulch.
Direct burning of OPEFB causes environmental problems due the incomplete
combustion and the release of very fine particles of ash. The
conversion of OPEFB to biofuels, such as syngas, ethanol, butanol,
bio-oil, hydrogen and biogas etc., might be a good alternative and have
less environmental footprint. The properties of OPEFB is listed in Table 1 [5].
Table 1.
Properties of oil palm empty fruit bunch
While
all the OPEFB components can be converted to biofuels, such as bio-oil
and syngas through thermo-chemical conversion, cellulose and
hemicellulose can be hydrolysed to sugars and subsequently be fermented
to biofuels such as ethanol, butanol, and biogas etc. Although many
scientists around the world are developing technologies to generate
biofuels from OPEFB, to-date, none of such technologies has been
commercialized. This is largely due to the recalcitrance of the OPEFB
and therefore the complexity of the conversion technologies making
biofuels from OPEFB less competitive than the fossil-based fuels.
Continual efforts in R&D are still necessary in order to bring such
technology to commercialization. The aim of this paper is to review the
progress and challenges of the OPEFB conversion technologies so as to
help expedite the OPEFB conversion technology development.
2. Pretreatment
Similar
to all other lignocellulosic biomass, OPEFB are composed of cellulose,
hemicellulose and lignin. Among the three components, lignin has the
most complex structure, making it recalcitrant to both chemical and
biological conversion. Pretreatment of OPEFB is therefore necessary to
open its structure and increase its digestibility and subsequently the
degree of conversion. Pretreatment of OPEFB can be classified as
biological pretreatment, physical pretreatment, chemical pretreatment,
and physical-chemical pretreatment.
For biological
pretreatment, oxidizing enzymes and white-rot fungi were used to degrade
the lignin content in OPEFB. For example, enzymes such as lignin
peroxidase (LiP) and manganese peroxidase (MnP) was used to pretreat
OPEFB for fast pyrolysis and the bio-oil yield was improved from 20% to
30% [6].
Syafwina et al. used white-rot fungi to pretreat OPEFB and the
saccharification efficiency was improved by 150% compared to that of the
untreated OPEFB [7].
Among all the pretreatment methods, chemical pretreatment is most often reported for OPEFB. Two-stage dilute acid hydrolysis [8], alkali pretreatment [9], sequential dilute acid and alkali pretreatment [10], alkali and hydrogen peroxide pretreatment [11], sequential alkali and phosphoric acid pretreatment [10], aqueous ammonia [12], and solvent digestion [5]
were used to increase the digestibility of OPEFB. Among all the
chemical methods investigated, alkali pretreatment seemed to be the most
effective. Umikalsom et al. autoclaved the milled OPEFB in the presence
of 2% NaOH and 85% hydrolysis yield was obtained [13]. Han and his colleagues investigated NaOH pretreatment of OPEFB for bioethanol production [9].
The optimal conditions were found to be 127.64°C, 22.08 min, and 2.89
mol/L NaOH. With a cellulase loading of 50 FPU /g cellulose a total
glucose conversion rate (TGCR) of 86.37% was obtained using the Changhae
Ethanol Multi Explosion (CHEMEX) facility. The effectiveness of alkali
pretreatment might be attributed to its capability in lignin
degradation. Mission et al. investigated the alkali treatment followed H2O2
treatment and found that almost 100% lignin degradation was obtained
when OPEFB was firstly treated with dilute NaOH and subsequently with H2O2 [11]. This confirmed the lignin degradation by NaOH and its enhancement by the addition of H2O2.
3. Thermo-chemical conversion
Thermo-chemical
conversion is one of the important routes to obtain fuels from
lignocellulosic biomass. Thermo-chemical conversion of biomass involves
heating the biomass materials in the absence of oxygen to produce a
mixture of gas, liquid and solid. Such products can be used as fuels
after further conversion or upgrading. Generally, thermo-chemical
processes have lower reaction time required (a few seconds or minutes)
and the superior ability to destroy most of the organic compounds. These
mainly include biomass pyrolysis and biomass gasification. Recently,
thermo-chemical pretreatment of biomass, such as torrefaction was
introduced to upgrade biomass for more efficient biofuel production [16-17].
3.1. OPEFB pyrolysis
Pyrolysis
is defined as the thermal degradation of the biomass materials in the
absence of oxygen. It is normally conducted at moderate temperature (400
– 600°C) over a short period of retention time. Its products comprise
of liquids (water, oil/tars), solids (charcoal) and gases (methane,
hydrogen, carbon monoxide and carbon dioxide). The efficiency of
pyrolysis and the amount of solid, liquid, and gaseous fractions formed
largely depend on the process parameters such as pretreatment condition,
temperature, retention time and type of reactors.
Misson et al investigated the effects of alkaline pretreatment using NaOH, Ca(OH)2, in conjunction with H2O2, on the catalytic pyrolysis of OPEFB [11]. They proved that consecutive addition of NaOH and H2O2 decomposed almost 100% of OPEFB lignin compared to 44% for the Ca(OH)2 and H2O2 system, while the exclusive use of NaOH and Ca(OH)2
could not alter lignin composition much. In addition, the pretreated
OPEFB was catalytically pyrolysed more efficiently than the untreated
OPEFB samples under the same conditions.
Fast pyrolysis
represents a potential route to upgrade the OPEFB waste to value-added
fuels and renewable chemicals. For woody feedstock, temperatures around
400-600°C together with short vapour residence times (0.5-2 s) are used
to obtain bio-oil yields of around 70%, along with char and gas yields
of around 15% each. Sulaiman and Abdullah investigated fast pyrolysis of
OPEFB using and bench top fluidized bed reactor with a nominal capacity
of 150 g/L [18].
After extensive feeding trials, it was found that only particles
between 250 and 355 m were easily fed. The maximum liquid and organics
yields (55% total liquids) were obtained at 450°C. Higher temperature
was more favourable for gas production and water content was almost
constant in the range of temperature investigated. The maximum liquids
yield and the minimum char yield were obtained at a residence time of
1.03 s. The pyrolysis liquids produced separated into two phases; a
phase predominated by tarry organic compounds (60%) and an aqueous phase
(40%). The phase separated liquid product would represent a challenging
fuel for boilers and engines, due to the high viscosity of the organics
phase and the high water content of the aqueous phase. These could be
overcome by upgrading. However, the by-product, charcoal, has been
commercialized for quite some time. It is worth noting that the first
pilot bio-oil plant by Genting Bio-oil has already started operation in
Malaysia [19].
3.2. OPEFB gasification
Gasification
process is an extension of the pyrolysis process except that it is
conducted at elevated temperature range of 800–1300 °C so that it is
more favourable for gas production [20].
The gas stream is mainly composed of methane, hydrogen, carbon
monoxide, and carbon dioxide. Biomass gasification offers several
advantages, such as reduced CO2 emissions, compact equipment
requirements with a relatively small footprint, accurate combustion
control, and high thermal efficiency. The main challenge in gasification
is enabling the pyrolysis and gas reforming reactions to take place
using the minimum amount of energy and gasifier design is therefore
important [21].
Ogi et al. used an entrained-flow gasifier for OPEFB gasification at 900°C [22]. During gasification with H2O
alone, the carbon conversion rate was greater than 95% (C-equivalent),
and hydrogen-rich gas with a composition suitable for liquid fuel
synthesis ([H2]/[CO] = 1.8–3.9) was obtained. The gasification rate was improved to be greater than 99% when O2 was added to H2O; however, under these conditions, the gas composition was less suitable for liquid fuel synthesis due to the increase of CO2 amount. Thermogravimetric (TG) analysis suggested that OPEFB decomposed easily, especially in the presence of H2O and/or O2,
suggesting that OPEFB is an ideal candidate for biomass gasification.
Lahijani and Zainal investigated OPEFB gasification in a pilot-scale
air-blown fluidized bed reactor [23].
The effect of bed temperature (650–1050°C) on gasification performance
was studied and the gasification results were compared to that of
sawdust. Results showed that at 1050°C, OPEFB had almost equivalent gas
yield and cold gas efficiency compared with saw dust, however, with low
maximal heating values and higher carbon conversion. In addition, it was
realized that agglomeration was the major issue in OPEFB gasification
at high temperatures. This can be overcome by lowering the temperature
to 770 ± 20 °C. Mohammed et al. studied OPEFB gasification in a bench
scale fluidized-bed reactor for hydrogen-rich gas production [24].
The total gas yield was enhanced greatly with the increase of
temperature and it reached the maximum value (~92 wt.%) at 1000 °C with
big portions of H2 (38.02 vol.%) and CO (36.36 vol.%). The feedstock particle size of 0.3–0.5 mm, was found to obtain a higher H2 yield (33.93 vol.%), and higher LHV of gas product (15.26 MJ/m3). The optimum equivalence ratio (ER) (0.25) was found to attain a higher H2
yield (27.31 vol.%) at 850 °C. Due to the low efficiency of bench scale
gasification unit the system needs to be scaling-up. The cost analysis
for scale-up EFB gasification unit showed that the hydrogen supply cost
is $2.11/kg OPEFB. Recently, a characterization and kinetic analysis was
done by Mohammed et al. and it was found that a high content of
volatiles (>82%) increased the reactivity of OPEFB, and more than 90%
decomposed at 700 °C; however, a high content of moisture (>50%) and
oxygen (>45%) resulted in a low calorific value [25].
The fuel characteristics of OPEFB are comparable to those of other
biomasses and it can be considered a good candidate for gasification.
3.3. OPEFB torrefaction
Torrefaction
is a thermal conversion method of biomass in the low temperature range
of 200-300 °C. Biomass is pretreated to produce a high quality solid
biofuel that can be used for combustion and gasification [16-17].
It is based on the removal of oxygen from biomass to produce a fuel
with increased energy density. Different reaction conditions
(temperature, inert gas, reaction time) and biomass resources lead to
the differences in solid, liquid and gaseous products.
Uemura et al. [16]
studied the effect of torrefaction on the basic characteristics of oil
palm empty fruit bunches (EFB), mesocarp fibre and kernel shell as a
potential source of solid fuel. It was found that mesocarp fibre and
kernel shell exhibited excellent energy yield values higher than 95%,
whereas OPEFB, on the other hand, exhibited a rather poor yield of 56%.
Torrefaction can also be done in the presence of oxygen. Uemura and his
colleagues [17]
carried out OPEFB torrefaction in a fixed-bed tubular reactor in the
presence of oxygen at varied oxygen concentration. The mass yield
decreased with increasing temperature and oxygen concentration, but was
unaffected by biomass particle size. The energy yield decreased with
increasing oxygen concentrations, however, was still between 85% and
95%. It was found that the oxidative torrefaction process occurred in
two successive steps or via two parallel reactions, where one reaction
is ordinary torrefaction, and the other is oxidation.
3.4. Summary
The
analysis of thermo-chemical conversion of OPEFB suggests that
gasification is the most suitable thermo-chemical route for OPEFB
conversion to biofuels. It has the highest carbon conversion (>90%)
and biofuel yield. Due to the high viscosity and high water content of
pyrolysis products, application of bio-oil as a biofuel is still very
challenging. Compared to other oil palm residues, such as oil palm
kernel, due to its high water content, OPEFB may not be a good candidate
for solid fuels even after torrefaction pretreatment.
4. Bioconversion
Bioconversion
of lignocellulosic biomass to fuels involves three major steps: 1)
pretreatment- to effectively broken the biomass structure and release
the biomass components i.e. cellulose, hemicellulose, and lignin, and
therefore increase the digestibility of the biomass; 2) enzymatic
hydrolysis – to hydrolyse cellulose and hemicellulose and produce
fermentable sugar, such as glucose, xylose etc.; 3) fermentation – to
convert the biomass hydrolysate sugars to the desired products. OPEFB
was intensively investigated as a potential substrate for the production
of biofuels, such as ethanol, butanol, and biogas etc. Among the
biofuels produced through bioconversion of OPEFB, cellulosic ethanol is
the most intensively studied.
Two stage dilute acid
hydrolysis was applied for OPEFB bioconversion to ethanol, 135.94 g
xylose/kg OPEFB and 62.70 g glucose/kg OPEFB were produced in the first
stage and 2nd stage, respectively [8]. They were then fermented to ethanol using Mucor indicus and Saccharomyces cerevisiae, respectively, and the corresponding ethanol yields were 0.45 and 0.46 g ethanol/g sugar.
Alkali
is the most often used pretreatment chemical for cellulosic ethanol
production from OPEFB. Kassim et al. pretreated OPEFB using 1% NaOH
followed by mild acid (0.7% H2SO4) hydrolysis and enzymatic saccharification [26].
A total of 16.4 g/L of glucose and 3.85 g/L of xylose were obtained
during enzymatic saccharification. The OPEFB hydrolysate was fermented
with Saccharomyces cerevisiae and an ethanol yield of 0.51 g/g
yield was obtained, suggesting that OPEFB is a potential substrate for
cellulosic ethanol production. Han and his colleagues investigated
ethanol production through pilot scale alkali pretreatment and
fermentation [9].
The best pretreatment condition was 127.64 °C, 22.08 min, and 2.89
mol/L NaOH. Enzyme loading of 50 FPU/g cellulose resulted in 86.37%
glucose conversion in their Changhae Ethanol Multi Explosion (CHEMEX)
facility. An ethanol concentration of 48.54 g/L was obtained at 20%
(w/v) pretreated biomass loading, along with simultaneous
saccharification and fermentation (SSF) processes. This is so far the
highest reported ethanol titre from OPEFB. Overall, 410.48 g of ethanol
were produced from 3 kg of raw OPEFB in a single run, using the CHEMEX
50 L reactor.
Jung and his colleagues tried aqueous ammonia soaking for the pretreatment of OPEFB and its conversion to ethanol [12].
Pretreated OPEFB at 60°C, 12 h, and 21% (w/w) aqueous ammonia, showed
19.5% and 41.4% glucose yields after 96h enzymatic hydrolysis using 15
and 60 FPU of cellulase per gram of OPEFB, respectively. An ethanol
concentration of 18.6 g/L and a productivity of 0.11 g/L/h were obtained
with the ethanol yield of 0.33 g ethanol/glucose.
Lau et al. successfully applied ammonia fibre expansion (AFEX) pretreatment for cellulosic ethanol production from OPEFB [14].
The sugar yield was close to 90% after enzyme formulation optimization.
Post-AFEX size reduction is required to enhance the sugar yield
possibly due to the high tensile strength (248 MPa) and toughness (2,000
MPa) of palm fibre compared to most cellulosic feedstock.
Interestingly, the water extract from AFEX-pretreated OPEFB at 9% solids
loading is highly fermentable and up to 65 g/L glucose can be fermented
to ethanol within 24 h without the supplement of nutrients.
OPEFB was also used for butanol production. Noomtim and Cheirsilp (2011) studied butanol production from OPEFB using Clostridium acetobutylicum [27].
Again, the pretreatment by alkali was found to be the most suitable
method to prepare OPEFB for enzymatic hydrolysis. 1.262 g/L ABE
(acetone, butanol and ethanol) was obtained in RCM medium containing 20
g/L sugar obtained from cellulase hydrolysed OPEFB. Ibrahim et al also
investigated OPEFB as the potential substrate for ABE production [28]. Higher ABE yield was obtained from treated OPEFB when compared to using a glucose-based medium using Clostridium butyricum
EB6. A higher ABE level was obtained at pH 6.0 with a concentration of
3.47 g/L. The accumulated acid (5 to 13 g/L) had inhibitory effects on
cell growth.
Nieves et al. investigated biogas production using OPEFB. OPEFB was pre-treated using NaOH and phosphoric acid [29].
When 8% NaOH (60 min) was used for the pretreatment, 100% improvement
in the yield of methane production was observed and 97% of the
theoretical value of methane production was achieved under such
pretreatment condition. The results showed that the carbohydrate content
of OPEFB could be efficiently converted to methane under the anaerobic
digestion process. O-Thong et al. investigated the effect of
pretreatment methods for improved biodegradability and biogas production
of oil palm empty fruit bunches (EFB) and its co-digestion with palm
oil mill effluent (POME) [30]. The maximum methane potential of OPEFB was 202 mL CH4/g VS-added corresponding to 79.1 m3 CH4/ton
OPEFB with 38% biodegradability. Co-digestion of treated OPEFB by NaOH
presoaking and hydrothermal treatment with POME resulted in 98%
improvement in methane yield comparing with co-digesting untreated
OPEFB. The maximum methane production of co-digestion treated OPEFB with
POME was 82.7 m3 CH4/ton of mixed treated OPEFB and POME (6.8:1), corresponding to methane yield of 392 mL CH4/g VS-added. The study showed that there was a great potential to co-digestion treated OPEFB with POME for bioenergy production.
In
summary, OPEFB has been frequently investigated as a substrate for
biofuel production through bioconversion. Cellulosic ethanol production
was most intensively investigated and the highest ethanol titre of 48.54
g/L was obtained through alkali pretreatment in a pilot scale reactor [9].
Although not much research has been done for ABE and biogas production,
the few reports summarized in this paper suggest that OPEFB is also
potential substrate for butanol and biogas production. Throughout the
reports reviewed, alkali-based pretreatment methods, such as NaOH alone,
NaOH followed by acid, and ammonium fibre expansion (AFEX) pretreatment
are the most effective in enhancing OPEFB digestibility.
5. Conclusion
In
conclusion, OPEFB is the most potential renewable resource for biofuel
production in Southeast Asia. It can be converted to biofuels through
thermo-chemical or biological conversion. Pretreatment of OPEFB is
necessary for both routes of conversion and alkali pretreatment is the
most effective. A summary of OPEFB conversion technology is shown in Fig. 2.
Among
the studies on OPEFB thermo-chemical conversion, it seems that
gasification is the most suitable approach to obtain bioenergy from
OPEFB and has potential in commercialization. Pyrolysis, on the other
hand, produced very complex bio-oil with high viscosity and water
content, making it challenging for commercialization. However, charcoal
from OPEFB pyrolysis can be a potential commercial product. Compared to
other palm oil residues, such as oil palm kernel, OPEFB may not be a
good candidate for solid fuel production, even after torrefaction
pretreatment due to its high water content and low energy capacity.
Biological
conversion of OPEFB is another route to obtain biofuels from OPEFB.
Cellulosic ethanol production was most intensively studied and around 50
g/L titre was obtained with 20% (w/v) biomass loading through NaOH
pretreatment. AFEX also showed potential in OPEFB pretreatment and a
glucose yield of 90% was obtained with 9% biomass loading. The water
extract of the AFEX pretreated OPEFB was highly fermentable. OPEFB also
showed some promising preliminary results in ABE (acetone, butanol and
ethanol) and biogas production; however, further investigation is
necessary to enhance OPEFB conversion potentials in these areas.
For
both thermo-chemical and biological conversion of OPEFB, pretreatment
technology is the key for the process cost. Although alkali pretreatment
is effective, scaling-up the process requires huge amount of acid to
neutralize the base in the pretreatment solution. In addition, before
alkali pretreatment, OPEFB should be milled to reduce its size, which is
energy-consuming. Steam explosion is effective for a lot of
lignocellulosic biomass, however not much research was found on its
pretreatment of OPEFB. A cost-effective pretreatment is the key for the
successful commercialization of OPEFB conversion technologies for
biofuel production.
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