1. Introduction
The estimated extinction of fossil fuel has led to challenges in the energy industry in sourcing other powerful, yet environmentally friendly, potions with lower emission and abundant in nature to be equally standing on the world energy platform. According to the International Energy Agency (IEA), the global oil price has shot up to $80/barrel in 2018, yet the global energy demand is expected to increase up to 25% between 2017 and 2040, subsequently raising CO
2 emission by 1.6% in the year 2017, which were flat for the previous three consecutive years
[1]. Globally, greenhouse gas emissions (GHG) have increased from extreme human exploitation in finding ways to cope with the ever-increasing energy demand day by day due to the population growth and other human activities. The various sectors demanding energy are transportation, residential, commercial, industries, and agricultural practices, which indicate a heavy requirement for a sustainable fuel alternative.
Following the era of rapid development and advancement, the research effort and attention should be diverted to the discovery of renewable and sustainable fuel resources. The employment of biofuels in the energy and transportation sector should receive high research interest due to their several advantages such as cheaper production cost, cleaner production method, and reduced environmental issues
[2]. Biofuel has been perceived as a highly preferable replacement alternative to conventional fossil fuels. It is strongly favourable in the industries for their functionalities and advantages, which are comparable and compatible with fossil derived fuels. Notably, the distinguished advantage for biofuel is the abundance of available resources that are renewable and more sustainable in comparison to fossil fuel sources that are utilised until extinction. In addition, biofuel has shown its environmental friendliness as an alternative fuel with reduced environmental impacts, leading to the development of a greener environment.
Scholars and researchers in recent decades have selected biomass as a potential alternative fuel in the world. Biomass is primarily found in the form of living matters or recently living plants, as well as in waste. The term feedstock refers to whatever type of organic material that could be used to produce energy. Different feedstocks have different physical compositions, but generally, all feedstocks include varying amounts of carbon, water, and organic volatiles for energy production. This biomass feedstock is classified as “theoretically carbon neutral”. This is because the plants that change into biomass feedstock use carbon dioxide during the growing stage and repel the same amount of carbon to the atmosphere when they are incinerated. Sulaiman et al.
[3] revealed in their recent studies that the amount of CO
2 could be reduced by up to 0.8888% in 27 EU countries by an increment of 1% of wood biomass utilization. This statement is further substantiated by Gwan Seon Kim, Sun Ki Choi, and Jun Ho Seok
[4], revealing that an increase of 1% per capita biomass fuel reduces 0.65% emission of CO
2 per capita. Biomass has been readily used as an efficient fuel substance, which has varying benefits as well.
There are abundant biomass sources to be employed as fuel in the world. The biomass sources quality and characteristics vary regionally and are controlled by multidimensional components mechanism and drivers
[5], such as water, soil, climate, and agricultural science for sustainable farming practices
[6]. Palm oil is a potential biomass source abundantly available in Asia region, in which it requires less fertilizers and pesticides in comparison to other oil seeds
[7] and can even be grown in peat soil
[8] with less water needed for the cultivation. In addition, the palm oil tree utilizes nine times less land compared to any other oil crops
[9] and is disease-free with the recent discovery of GanoCare to cure
Ganoderma disease
[10]. Nevertheless, these elements have made palm oil, and the generated oil palm residues to become sustainable replacements of fossil fuels. However, the origin and characteristic, current utilisation, and environmental impact of palm oil industries in the world should be further investigated.
Therefore, a review on oil palm residues and their suitability as an alternative fuel is vital for a sustainable environmental prospect. Thus, this review focuses on the assessment and evaluation of palm oil and the usage of its agro-industrial residues as biofuel, bioenergy, and in the transportation sector. This is followed by an investigation of the possibilities of palm oil and its residues for sustainable development with reduced environmental-related issues. Thus, this review targets selected palm oil as a fossil fuel replacement to provide further insight into the current development in the palm oil industry. This development is achieved with the rapid advancement of both palm oil and palm residues conversion techniques and approaches, including the production of value-added products from the industries.
2. Palm Oil Origin, Products and By-Products with Their Characteristics and Current Utilisation
Palm oil is one of the major ingredients in up to 50% of all daily-used products, everything from cosmetics, such as lipsticks, shampoo, and deodorants, to food ingredients, like margarine, chocolate, pastries, and baby food, including instant noodles, mentioned as either hydrogenated vegetable fats or plant fats. Palm oil (
Elaeis guinneensis)
[6] is considered as a versatile oil crop as palm oil trees are highly productive and have multiple uses in each different tree parts, from the fruits to the oil palm residues, including the ability to produce bioenergy or biofuels
[11].
Palm oil originating from West Africa, Malaysia, and Indonesia with conducive tropical climate contributes to potentially healthy growth of palm oil trees, substantially making these countries the prime producers of palm oil in the world
[12]. Initially, the palm oil production was led by Malaysia until the end of 2007; it was then led by Indonesia in the beginning of 2008 with sturdy growth until the present time
[13]. In 2019, the Malaysian Palm Oil Council (MPOC)
[13] reported that Malaysia has exported 18.47 million metric tonnes of palm oil, while Indonesia has exported 29.52 million metric tonnes, making Malaysia the leading secondary palm oil producers in the world. The world’s third-largest palm oil producers include Colombia, Thailand, and Nigeria, with a total production of 7% in the world
[14].
The oil demand has bloomed, outracing the productivity of the significant oil crop market over the past decades, which is in parallel with the human population growth. Palm oil seems to surpass other major oil-producing commodities such as sunflower oil and soybean oil with an average consumption of 71.48 million metric tonnes a year
[15], mainly due to the high-yield factors of palm oil in contrast to other oil-producing crops. Palm oil could yield about an average of 4 to 4.5 tonnes of oil per hectare
[15], which is the highest in comparison to soybean oil yield of only about 1.5 to 3.5 tonnes/ha, 1 to 1.5 tonnes/ha oil from sunflower seeds, and about 1.57 tonnes/ha oil from rapeseed oil
[16] as shown in . It is also anticipated that the breakthrough via microbial oil production and palm oil species could also increase the palm oil industry’s yield
[17]. In addition, the demand for palm oil has increased due to its affordable price compared to other plant-based alternatives and is free from genetically modified organisms (GMO) or trans-fatty acids (TFA)
[18]. Despite the proportional demand for palm oil for its reasonable price, unintended rapid economic growth, alongside the elevation of biodiesel production and labour cost, has massively increased the palm oil price in recent years
[19].
Figure 1. Illustrated significant plant-based oil yields per hectares.
The primary global consumer of palm oil is India, which was expected to consumed nearly 9.3 million tonnes from 2016 to 2017, and the consumption is expected to increase by two-fold by 2030, followed by Europe and China with the consumption of 6.7 and 5 million tonnes, respectively
[20]. Apart from that, the Balkans (Bulgaria, Croatia, Greece, and etc.) have also shown an increase in demand for edible oil consumption especially palm oil from the year 2010 to 2016, Kalsom et al.
[21]. It has been discovered that these regions have also imported palm oil from nonpalm oil producers such as the Netherlands, Germany, and Italy, aside from the prime palm oil producers like Malaysia and Indonesia
[21].
Palm oil fruit is unique in comparison to other vegetable oil producers worldwide as it could grow up to 25 m tall, produce fruit throughout the year, and expire after 25 years
[22]. After the oil is squeezed out, the leftover mesocarp fibre from it is a lignocellulosic material, which consists of exocarp (the outer layer of the fruit), mesocarp (the inner layer of the fruit), and endocarp (seed cap of the fruit), as shown in . The volume of oil obtained from the inner, middle, and outer layers of palm oil fruits vary from 14.2% to 46.9%
[23]. Additionally, both parthenocarpy fruit and fruits’ oil compositions are indistinguishable in terms of palmitic acid, oleic, linoleic acid, and linolenic
[23].
Figure 2. Palm oil fruits and their processed residues.
Various usages or applications of palm oil fruits and its processed residues can be seen in several industries such as pastries, confectioneries, medicines, and supplements for general health. The debate against the consumption of palm oil is always a matter of discussion among researchers and scientists. It was also reported that heart-related disease contributors might be due to the unhealthy diet commonly practiced in the western countries with less vegetable oil consumption
[24]. Furthermore, Zhang et al.
[25] have illustrated that palm oil intake is proven to reduce cholesterol level in comparison to lard fats or other vegetable oil (soybean and peanut) intake. In addition, palm oil utilization is quite prominent even in nonfood applications, which were proven to be about 20% of the overall palm oil usage
[26]. shows the common usage of palm oil in food industries, medical industries, and other industries.
Table 1. Palm oil utilization in various industries.
Confectionery, Baking, and Food |
Usage |
Year |
References |
Baking fats |
2020 |
Meng, Xiaoyu, et al. [27], |
Shortening |
2019 |
Goh et al. [28] |
Imitation meat emulsifier |
2013 |
Rafidah Abd Hamid [29] |
Fat substitution in a chicken nugget |
2009 |
Alina et al. [30] |
Margarine |
2019 |
Makeri, Mohammad, et al. [31] |
Medicinal properties |
Replacement of Vitamin E (α−tocotrienol) |
2010, 2015 |
Sen et.al [32], Ali and Woodman [33] |
Lubricant in human joints |
2019 |
Sapawe, Norzahir, Muhammad Farhan Hanafi, and Syahrullail Samion [34] |
Suppression of cancer cell |
2013 |
Loganathan et al. [35] |
Antioxidant for diabetes mellitus |
2020 |
Alabi, Olabiyi, and Oguntibeju [36] |
Industries |
Hydrogenated biofuel |
2020 |
Boonrod, Bulin, et al. [37], |
Mix biodiesel |
2017, 2018 |
Khalil et al. [38], Cordero-Ravelo and Schallenberg-Rodriguez [39]. |
Structural and Chemical Properties
The chemical composition oil palm residues such as oil palm mesocarp fibre (OPMF), OPEFB, oil palm kernel shell (OPKS), OPT, and OPF were outlined by previous studies as illustrated in . Overall, the cellulose content is quite prominent in comparison to the lignin and hemicellulose contents from oil palm residue. Another chemical substance present in oil palm residues apart from these two components is ash
[48]. The cellulose, hemicellulose, and lignin of oil palm residues were compared with other biomasses such as banana pseudostem, corn straw, rice husk, sugarcane bagasse, and jatropha based on prevalent literature. The cellulose content of oil palm residues, especially OPF, has a higher percentage of cellulose in contrast with corn straw (31.7%)
[49] and rice husk (37.1%)
[50], respectively. Besides that, the lignin content of oil palm residues, ranging from 12 to 50% is significantly high, especially in OPKS as compared to banana pseudostem (17.26%)
[51], sugarcane bagasse (20%)
[52], jatropha (29.6%)
[53], and other oil palm residues. The removal of lignin content can contribute to value-added fuels or chemicals in a later stage of fuel production
[54].
Table 2. Composition of oil palm residues.
Composition |
Cellulose |
Hemicellulose |
Lignin |
Ash |
Ref. |
I (%) |
25.0 ± 1.7 |
25.7 ± 3.3 |
25.5 ± 0.5 |
5.8 ± 0.2 |
[55] |
II (%) |
34.4 ± 0.7 |
26.7 ± 0.2 |
12.45 ± 0.45 |
4.85 ± 0.15 |
[56] |
III (%) |
20.7 |
23.3 |
49.5 |
|
[57] |
IV (%) |
34.5 |
31.8 |
25.7 |
4.3 |
[58] |
V (%) |
40.7 |
26.1 |
26.2 |
|
[59] |
The possible utilization of multiple agricultural solid biomass feedstocks as an energy substitution has been evaluated through energy content, proximate and ultimate analysis, as well as associating with the characteristic requirement to run optimal operation for thermochemical processes
[60]. The principal constituents of oil palm residues in the ultimate analysis are carbon, hydrogen, oxygen, nitrogen, chlorine, and sulphur. The volatile matter, moisture content, ash content, and fixed carbon of the feedstocks which are known as proximate analysis has been illustrated in . The higher percentage of moisture content in both OPMF and OPEFB resulted in low bulk density and transportation difficulties, which could be altered by pretreatment methods and stretches the feedstock’s usability and market value
[61]. However, OPEFB still has proven to produce approximately one third of the power from the direct combustion system as compared to a similar amount of methane used to produce electricity
[62]. The application of OPKS and OPMF in thermochemical processes is relatively significant in comparison with OPEFB due to its low moisture and high nutrient content as well as possessing the value-added potential for other material production
[63].
Table 3. Proximate and ultimate analysis.