By becoming the energy crisis severely and the opportunity of using the palm oil industry residues as a renewable energy resources in Malaysia, the study of their exergy for decisions and investments is more crucial. The chemical exergy was nominated for the calculations and the comparison of the solid palm oil wastes biomass namely, empty fruit bunch (EFB), fiber and shell. By mean of the exergy factor β and the Lower Heating Value (LHV) of each residue, it was revealed that shell biomass has highest exergy of 24.60 MJ/Kg and EFB stands in the last place with 21.32 MJ/Kg. The combination of these two, which the former is less available and the later is more available, in the production of the briquettes and the pellets for burning in the specialized boilers was suggested. The amount of oxygen was identified as possessing direct influence on the exergy factor β due to the order of the polymers structure.
The rise of energy price combined with consumption of the limited fossil fuels such as oil and coal causes the need for utilizing renewable energy sources becomes crucial. By considering the second largest renewable energy source which is biomass in comparison with conventional fossil fuels it is revealed that biomass is more ecologically friendly fuel type and it is a clean sort of energy possessing approximately zero CO2 discharge to the environment, because the CO2 which was previously fixed through the photosynthesis of biomass during its growth, will be discharged throughout the utilization process of it (Lu et al., 2007). Energy production by utilizing oil palm wastes can be beneficial to avoid the use of fossil fuels. In the agricultural sector of Malaysia and Indonesia, from the early 1920s the palm oil production has been grown speedily particularly between 1960-2000 (Mahlia et al., 2001).
By utilizing the concept of exergy which is work potential of the considered system, the comparison of palm oil wastes such as EFB, shell and fiber can be implemented readily. Based on the first law of thermodynamics (FLT) for energy analyzing of any systems, the energy balance of that system should be considered. But the energy balance does not give any information on the energy reduction of the process and usefulness and quality of the resources. These weaknesses of the FLT can be prevailed by the mean of exergy analysis. The exergy concept is based on the FLT and the second law of thermodynamic (SLT) (Dincer and Rosen, 2007). The exergy analysis presents the energy losses of the system vividly due to its quantitative and qualitative demonstration of the variant losses.
The exergy analysis can be conducted on the process. The process based method is a useful tool for the analyzing and retrieving valuable results and therefore the method is well-known in this aspect. Nevertheless, the exergy analysis can be considered in accompanied with chemical exergy of the involved material. Chemical exergy of a material is considered as the maximum possible work which can be retrieved when it has been crossed from the relevant state to the reference environment through reversible processes with only heat and mass transfer to the environment (Tan et al., 2010). In this study, the chemical exergy of the palm oil milling process residues were calculated with respect to the air as the reference environment. Then the results have been compared with each other for finding the highest and lowest exegetic wastes.
2. Availability of palm oil biomass waste
Palm oil wastes are the main biomass resources in ASEAN countries. Malaysia is the world's second largest palm oil producer and largest exporter. It is stated that until June 2009, there had been 406 palm oil mills in Malaysia with a total capacity of 92.78 million tons of Fresh Fruit Bunches (FFB) per annum (Tan et al., 2010).
Table 1
Proximate an ultimate analyses of palm oil wastes (Yang et al., 2006)
Proximate analysis (wt.%)
Ultimate analysis (wt.%, dry basis)
LHV (MJ/Kg)
Molecular
Formula
Mad
Vad
Ad
FCad
C
H
N
S
Oa
Shell
5.73
73.74
2.21
18.37
53.78
7.20
0.00
0.51
36.30
22.14
CH1.61O0.51
Fiber
6.56
75.99
5.33
12.39
50.27
7.07
0.42
0.63
36.28
20.64
CH1.69O0.54
EFB
8.75
79.67
3.02
8.56
48.79
7.33
0.00
0.68
40.18
18.96
CH1.80O0.62
M: moisture content; V: volatile matters; A: ash; FC: fixed carbon; ad: on air dried basis; d: on dry basis
Aa The oxygen (O) content was determined by difference.
In Malaysia and Indonesia, which is being the two main largest palm oil producing countries in the world, there were about 30 million tons and 8.2 million tons of solid palm oil wastes which are such as Empty Fruit Bunch (EFB), fiber and shell generated respectively in year 2000 and to cater for the rapidly expanding of food and manufacturing industries. In general, the processing of FFB in palm oil mills also generates other biomass residues such as palm kernel, fiber and shell. FFB comprises 21% of palm oil, 7% of palm kernel, fiber, shell and 23% of EFB (Tan et al., 2010). Table 1 shows the proximate and ultimate analyses of palm oil shell, fiber and EFB based on Yang et al. (2006) analysis.
Empty fruit bunch is the major component of all solid wastes. Large quantities of EFB are available from processing FFB in palm oil mills. EFB's are the residual bunch followed by removal of fruits from FFB in a thresher. Based on Table 1, from EFB has lower heating value of 18.96 MJ/Kg. On dry solid basis, it contains 18.1 wt.% of lignin, 59.7 wt.% cellulose and 22.1 wt.% hemi cellulose. Moreover, the ultimate analysis of EFB indicated the content of carbon to be 48.79 %, hydrogen 7.33 %, sulfur 0.68 %, and oxygen 36.30 %. From oil palm biomass, EFB found to be used to produce steam for processing activities and for generating electricity.
However, a few countermeasures has to be taken into consideration before the direct usage of EFB, among those are reducing the size of the EFB using shedding machines, and drying the EFB as it contains moisture as shown in Table 1 which is about 8.75 wt.% which is relatively higher compared to all three biomass wastes. Hence, EFB is used in agricultural activities such for vegetative growth as it helps to retain moisture and returns organic matter to the soil and found to be a very good fertilizer/soil conditioner. However, due to the "white smoke" problem, from incineration of EFB to be used as ash for agricultural purpose, the activity of incineration is discouraged. The "white smoke" is mainly contributed from the presence of high moisture content of the EFB (Yusoff, 2006).
Palm Kernel is obtained from palm fruitlet after the removal of the Mesocarp and shell. It can be divided into two categories which are Palm Kernel Cake (PKC) and Palm Kernel Shell (PKS). Palm kernel cake is a by-product from the kernel extraction process. It is used as a raw material for animal feed, especially for cattle feed. The PKC is obtained from the palm fruit within two stages of oil extraction. The primary stage is implemented by extraction of the palm oil in the pericarp part of the crushed kernel of the fruit. PKC is produced after the oil extraction of crushed kernel and is considered as by-product which comprises of conventional mechanical screw press method and solvent extraction method which the solvent extracted type is resulted.
PKS is an energy intensive substance and most difficult waste to decompose. Palm kernel shell is the waste kernel shell from Crude Palm Oil (CPO) processing. Due to the characteristic of PKS with high heating value compared to other biomass fuels and in terms of recycled waste matter as fuel, PKS is used widely used as fuels to generate heat for boilers or furnace in industries such as in the manufacturing plant, factories, etc. At present, there are 110,550 tons of PKS available annually. With the heating value of palm kernel shell about 17.4MJ/Kg, it is widely used mainly in power generation industries due to high demand from population growth (Prasertsan and Prasertsan, 1996).
Based on the statement above, it shows that PKS along with the other palm oil based biomass, being a renewable biomass has great application potential in many industries as highlighted above. Numerous R&D have been carried out to develop different applications of palm based biomass by Malaysian Palm Oil Board (MPOB), SIRIM, Forest Research Institute of Malaysia (FRIM) and other private companies.
Oil palm fiber is non-hazardous material resulted from oil palm's EFB from decortation process. The oil palm fiber is light and can absorb a lot of water without congealing. It has the capabilities of withstanding extreme temperatures and moisture conditions. EFB fibers can also be used for cushion filling material by adding binding agent such as later latex (Prasertsan and Prasertsan, 1996). The oil palm fibers are also used by manufacturer to make various fiber composite such furniture, infrastructures, mattress, erosion control and also landscaping.
3. Methods
3.1. Exergy definition
Recently, in the thermodynamics engineering the definition of exergy is introduced in order to finding out that how much work potential exists in a system according to natural environment (Sato, 2004). The exery and its definition are the tool for engineers to evaluate usefulness and quantitative of new or current resources of energy with respect to the reference environment. Alternatively saying, a system with more deviation from the condition of reference environment has more exergy as a consequence. In contrast, the aforementioned system in the environmental condition has no exergy because of no from it.
The concept of energy transfer to or from a system through its system boundary is also valid for exergy transfer. It means that exergy can cross the system boundary to the adjacent environment of system or vice versa. However, in contrast to the energy, exergy conservation is only valid for reversible processes and base on the second law of thermodynamics (SLT) the exergy destruction generates through real processes (Tan et al., 2010). The exergy destruction is due to losses or imperfectness of the system which as a consequence exergy is considered as part of energy that is useful for the society and has worth for investing. Hence, seeking for energy resources with higher exergy is desirable with respect to lower exergy resources of energy. For example, valuable fossil fuel has more exergy but waste heat at the environment condition has not high exergy although it can possess much energy (Dincer and Rosen, 2007).
3.2. Reference Environment
Conducting an accurate and precise exergy analysis for any system under consideration requires being base on a convenient environmental reference. As a result, in theory the reference environment is called to a time when the system is in equilibrium to the environment from three points of view, namely, Mechanical, Thermal and Chemical. The Reference Environment can be resembled to a thermodynamically dead planet where all materials have reacted, dispersed and mixed (Szargut et al., 2005). Reference environment has two type, environmental state and dead state. Environmental state refers to the system which is in balance to the nature mechanically and thermally and dead state refers to the system which is in balance to the nature mechanically, thermally and chemically (Tan et al., 2010). There are many variety of choice between substances and models for considering as reference environment (Dincer and Rosen, 2007). The air is selected in this study as the most common and standard reference environment. The temperature and pressure for the air are 25ËšC and 1atm, respectively. The reference environment could also be solid components of the external layer of the Earth's crust or ionic or molecular components of seawater (Dincer and Rosen, 2007). Also, in the similar way to the exergy, the chemical exergy of a substance is defined as the maximum retrievable work when the substance comes to the equilibrium with the environment in a reversible process (Szargut et al., 2005).
3.3. Exergy Analysis
The Eq.1 (Dincer and Rosen, 2007) might be used for computing the overall exergy of a system. From this equation it is understandable that the total exergy comprises of physical (Exph), chemical (Exo), kinetic (Exkin) and potential (Expot) exergies. The kinetic and potential exergies resemble to the corresponding kinetic and potential energies, respectively. Examples for kinetic exergy are analyzing of a flywheel or turbine. Examples for potential exergy are analyzing of electrical or hydraulic systems. For this study and most of the industrial systems with the aim of analyzing, these two exergies can be ignored without affecting the accuracy of total exergy significantly (Gleich et al., 2009). Physical exergy possesses vital role in the optimization of mechanical and thermal processes like heat engines and power plants. However, in dealing with substances, including palm oil wastes biomass this term also can be neglected (Gleich et al., 2009, Cohce, 2010). Consequently, for the purpose of this study only the chemical exergy was considered and the results of chemical exergy of EFB, fiber and shell were compared in respect to each other.
Ex = Exph + Exo + Exkin + Expot (1)
Chemical exergy of chemical elements or their compounds can be retrieved from tables at the aforementioned standard conditions for reference environment which is T0 and P0 equals to 25ËšC and 1 atm, respectively or any standard models (Tan et al., 2010). Szargut (2005) suggested the tabulated chemical exergy of various substances.
The chemical exergy calculations for solid fuels like biomass can be implemented by using Eq.2 and Eq.3, according to Szargut (2005). For biomass such as palm oil waste biomass which the value of exergy is not available in tables or in the literature, the correlation factor of β which is based on statistical correlations was being used (Cohce, 2010).
(2)
Valid until the condition of is satisfied (Tan et al., 2010).
Exch,biomass=β. LHVbiomass (3)
In the above relation LHVbiomass stands for Lower Heating Value of the biomass and weight fraction of Hydrogen, Oxygen, Carbon and Nitrogen components denoted by Z. The exergy factor or alternatively called exergy quality factor is the ratio of exergy and energy and for new energy resources like palm oil wastes which their exergy is neither in tables nor in literature can be used in accompany with lower heating value in order to calculate the value of exergy of that particular substance (Tan et al., 2010, Hovelius and Hansson, 1999).
4. Results and Discussions
The exergy calculation for palm oil waste biomass was conducted and their corresponding chemical exergy and exergy factor β in relative to lower heating value was calculated. The results from Eq. (2) and Eq. (3) are depicted in the Table 2. Shell possesses the highest value of chemical exergy, 26.60 MJ/Kg, and EFB has the minimum value of chemical exergy, 21.32 MJ/Kg. Also, as it can be seen from Table 2 the value for exergy factor β for EFB is 1.12 and weight percentage of oxygen equals to 40.18% which is the highest among other palm oil waste biomass, shell and fiber. Ptasinski et al. (2007) explained this by that polymers structures like cellulose and hemi cellulose are highly ordered, so the decomposition of them able to deliver work. Saidur et al. (2007) suggested the quantity of chemical exergy mainly depends on the lower heating value and merely on exergy factor.
Table 2
Chemical exergy of the palm oil wastes biomass
Oil Palm Biomass Waste
(wt.%, dry basis)â€
β
LHVâ€
(MJ/Kg)
Exo
(MJ/Kg)
H
C
O
N
Shell
7.20
53.78
36.30
0.00
1.11
22.14
24.60
Fiber
7.07
50.27
36.28
0.42
1.11
20.64
23.03
EFB
7.33
48.79
40.18
0.00
1.12
18.96
21.32
†(Yang et al., 2006)
The availability of EFB compare to Shell is significant, 22 Mtpa to 4 Mtpa, respectively (MPOB). In the production of the briquettes and pellets which are utilized in the specially designed boilers for the purpose of biomass fuels is suggested that the shell which has higher LHV in combination with EFB which is more available in palm oil milling process might be used in order to produce more efficient briquettes or pellets. In Malaysia, Global Green Synergy Sdn Bhd, has conducted the fabrication of such briquettes and pellets and the results for the evaluation of their corresponding heating values are shown in the Fig. 1.
Fig. 1 reveals that by increasing the ratio of shell with respect to EFB the heating value of the product will increase. The significant increase is for values of shell to EFB ratio between 20% and 30%.
Further researches shall be implemented to determine the optimized values of shell to EFB ratio to be used in the production of briquettes and pellets from palm oil milling wastes.
5. Conclusions
In this study, the exergy calculations for three different biomass wastes from palm oil milling process have been conducted. It was pointed out that the chemical exergy of a substance can be estimated from the corresponding LHV directly without altering the final results significantly. From the results it can be seen that the combination of shell and empty fruit bunch (EFB) produces the higher heating valued briquettes and pellets for utilizing in boilers. A significant advantage from the usage of palm oil wastes biomass is that they are more environmentally friendly in compared to the convention fossil fuels, since they discharge less pollutant substance which are dangerous. Despite of being feasible, especially in Malaysia, the main disadvantage of utilizing wastes biomass, particularly palm oil residues, is the small percentage coverage of energy part needed for corresponding demands.
The investments for the adequate facilities such as biomass fuel boilers and generators and more effort for optimization of process and the production are recommended.
