As in the palm oil industry, the consumption of palm oil has increased nearly two-fold over the past 20 years from the initial introduction of the oil into the world market. However, this issue has increased the accumulation of oil palm residue in both processing and transportation. Even though the palm oil industry satisfies the world’s needs, it negatively impacts the environment with the residues accumulated on the field and palm oil mills over the years of production. The oil palm residues are a definite matter of concern in the palm oil industry, whereby with every 10% of oil generated, approximately 90% [68
] remains as a waste biomass material. The waste comes in various form and shapes such as pressed fruit fibres (PFF), OPEFB, OPKS, palm oil mill effluent (POME), and it never stops, as the quantity of the waste produced by the trees is higher when they accomplish the optimal monetary lifespan through the OPT and OPF.
Today, the palm oil businesses are still rehearsing the conventional waste administration, with just little improvement. Back then, the disposal of palm oil fibre, mostly the OPEFB, is burnt in incinerators or discarded as mulch back to palm oil plants, creating environmental polluted emissions problems from the incineration and landfill gases. Currently, the typical disposal route of direct incineration has been announced illegal by the Malaysian government and found that no energy is recovered through this disposal mode. Indeed, some other defined route must be created and developed over the years of research to find the best suitable ways to dispose the oil palm residues, serving in energy reservation and eventually equipping the nation financially and environmentally.
Energy has been a vital concern over the years, and the development of energy versatility promising better energy with lower production prices is always be a topic of discussion among investors. Access to energy has been fundamental, reshaping billions of people’s daily life routines from lighting our homes to online transactions to even travelling, which relies on energy. According to a recent poll conducted by the United Nations on the Sustainable Development Goal (SDG), it was reported that almost 789 million people worldwide still lack electricity access. SDG further added that more affordable and reliable electricity is needed with the recent Covid-19 pandemic implication, especially in healthcare services. The solution to the above world scenarios can be performed in various ways, and oil palm residues can be used for a versatile energy generation. It is estimated that about 2400–7460 MW of potential installed capacity could be yielded from OPKS, OPEFB, and OPMF, whereas about 410–483 MW power could be produced from POME biogas with 7200hr/year of operating hours [69
]. Of all the different types of processes available globally, only two main methods are well known for their complete usability to transform waste into energy (WtE), such as thermochemical processes (gasification, pyrolysis, and direct combustion) and biochemical processes (anaerobic digestion, aerobic digestion) to produce solid, liquid, and gaseous biofuels.
3.1. Palm Oil as a Biofuel (Transportation)
The advanced technology has brought advancement in transportations (sea ships, aeroplanes, trains, cars, motorbike, and others), making movement possible with ease expeditiously for multiple facets, such as transferring of goods from one continental to another, travelling, and others. All these vehicles need fuel primarily derived from fossil fuel such as crude oil and natural gas. In some other cases, secondary fuel may be needed to run smoothly and efficiently, such as gasoline and diesel.
The main concern of transportation industries has corresponded to a long-term environmental impact and health issues through toxic emissions such as carbon monoxide (CO), carbon dioxide (CO2
), particulate matter (PM), hydrocarbon, and nitrogen oxides (NOX
]. Considerable study has been done and is still ongoing in search for the reduction of environmental impact by introducing the intelligent transport system (ITS) [71
] to track fuel consumption and emission estimation. However, fuel consumption is still primarily related to the type of vehicles used and the parameters of vehicles running on the road based on velocity, road condition, and volume of traffic and weather conditions [72
]. Therefore, it is crucial to find a potential alternative with similar power for vehicle engine start-up that also contributes to lesser pollution.
The fossil-fuel implications have made biofuel production an alternative fuel for mitigating crude-oil life-cycle emission. Mathew and Ardiyanto [73
] estimated that about 43.95% of greenhouse gas emissions could be restored in biodiesel production, and this greenhouse gas emission can be further recovered to 66.95% through methane capturing and flaring improvement. The counterattack created by MTBE addition in gasoline was entirely banned in 1996, and “zero” sulphur content and group of benzene in biodiesel [74
] have initiated vigorous biofuel modification and substitution as a vehicle’s fuel. Biofuels can be divided into first generation (food crops: sugar cane and soybean), second generation (waste and lignocellulosic crop waste), third generation (algae), and fourth generation based on their origin of biomasses [75
]. Apart from no engine modification needed in utilizing biofuel, biofuel reduces the dependency on oil imports and eventually increases the region’s economy.
Statistically over the decades, biofuel demand has grown substantially, and countries like Canada, U.S., Brazil, France, India, Malaysia, Indonesia, and Australia, were identified to be among the world’s largest biodiesel producers [76
]. These countries have proclaimed some major strategies through policies to expand their contribution of biodiesel globally. In line with biodiesel’s higher production by palm oil utilization, Malaysia is one of the countries that has taken the initial steps since late 2014 by mandating B7 to be implemented across the nation (Peninsular Malaysia and planning towards B10 implementation soon after the success of B7) [77
]. Indonesia has followed Malaysia’s footsteps in promoting B10 in biodiesel and E3 in ethanol production. Apart from these two producers, Canada has imparted a percentage of 5% ethanol in gasoline mix (E5) and 2% biodiesel in diesel and distillate oil production (B2) [78
]. Whereas the European Union has made it compulsory to substitute 5.75% and move towards 10% mixing by 2020 [80
The production of biodiesel through a series of reactions transforming triglycerides into fatty acids (ethyl or methyl esters) in the presence of methanol or ethanol with the catalyst is called the “transesterification process” [81
]. Transesterification process depends on the alcohol and glycerine used with catalyst (acid, enzymes, or alkaline) to promote biodiesel production. A catalyst such as sulphonated graphite [82
], hydrated lime-derived [83
], pig bone (hydroxyapatite) [84
], and enzymes such as lipase [85
] has been used in recent timeline with palm oil-based biodiesel. All these catalysts used in the experiments provide benefits in various ways, such as shortening the reaction time, and no alteration was found throughout the process of reusing the catalyst [82
]. In the alkaline-based catalyst reaction, the higher surface area created with the lower calcining temperature, higher reaction hours, and higher catalyst reused time has been identified [83
]. Among all these catalysts, pig bone has shown the highest recycling time even after eight recycling rounds with a biodiesel yield of about 90% [84
]. Whereas in the presence of an enzyme, alternating the temperature to 40 °C with the molar ratio of 3:1 (palm oil:enzyme) or equalizing both palm oil and enzyme molar ratio to (1:1) at 30 °C yields biodiesel in the range of 89–95% [85
The higher blending biodiesel could result in a drawback on the vehicle’s brake power. Nahian [86
] identified that an increase from 10 to 20% blending of palm oil biodiesel has shown a significant drop in vehicle’s brake performance, such as brake efficiency drop to less than 1% for B10 and 0.070 kg/kW/h more fuel consumed in comparison to 100% diesel with the same engine load. Moreover, a higher blending ratio of biodiesel exhibits a significant drop in the oxidative stability of the fuel, which results in sediment deposition at the bottom of storage tank, reduces the life cycle of engine fuel delivery components, and increases the viscosity of the fuel [87
]. J. Pullen and K. Saeed [87
] have used the Rancimat induction period (RIP) method twice for a period of 3 weeks apart from different biodiesel blends such as B2, B5, and B7 and found the RIP percentage decreases from 12.59, 12.39, and 8.13%, respectively. In addition, biodiesel with a higher than 40% blending ratio exhibits power loss due to a lower calorific value [89
]. However, Vieira da Silva et al. [90
] encountered a significant drop of NOx emission through an experiment conducted between palm-based biodiesel within the blending ratios of 20 to 50% compared to waste cooking oil. Moreover, the NOx emission is interrelated with the experimental setup, biodiesel content, type of engine, and operating conditions [91
]. The hydrocarbon emission in biodiesel was found to be slightly lower than fossil-fuel diesel.
Although biodiesel has brought several benefits in bringing down the emission and rising the economic prospects, the debate between food versus fuel and the challenges of the production cost within specific limitations are still significant factors influencing the biodiesel industry [92
]. According to Acevedo et al. [93
], the main contributor in the production cost would be the feedstock, which carries about three quarters of the overall production cost, followed by supply cost of about one quarter, and finally with the maintenance cost of less than 1%. However, the study mentioned that biodiesel production is still a profitable feasible option for about 22% of the overall production cost. Nevertheless, utilizing edible oil in biodiesel production has been the main factor contributing to the world’s food price increments, which is still a presumption [92
] as Malaysia and Indonesia have proved otherwise. Both leading producers have set a limitation towards the production of palm-based biodiesel, and the exportation of palm oil has been used widely in food production rather than biodiesel production in import countries.
3.2. Oil Palm Residues Valorisation into Biofuel and Bioenergy Production
A comparatively sufficient productivity of palm oil biomass is a significant non-conventional energy reserve compared to other renewable option such as hydropower, wind power, and solar energy. A wide range of by-products can be yielded through biomass waste to energy valorisation technologies. Different approaches to biomass waste valorisation have been used in converting waste into value-added products [94
] such as solid biofuel, gaseous biofuel, and liquid biofuel with respect to biochemical or thermochemical processes. In direct combustion, heat is generated (exothermic) to produce electricity, whereas in gasification and pyrolysis, heat is used (exothermic) in an oxygen-lean environment to produce bio-oil, char, chemical, and syngas, and in an anaerobic environment, digestion, biogas, and bioethanol are produced [94
In various scales, combustion can be used to obtain heat or electricity from oil palm residues even if the energy efficiency of the conversion is poor. Moreover, the lower calorific value, higher moisture content, and lower density of oil palm residues compared to conventional fuel have also contributed significant drawbacks in utilizing these biomass materials in biomass boiler. The introduction of solid-biomass densification techniques such as briquetting, pelletisation, etc., is discussed in upcoming sections of this manuscript. Additionally, the cocombustion of OPEFB and coal is an attractive method to mitigate the inefficiency of biomass fuel [95
]. Even though the energy-via-biomass boiler produces less emissions than conventional fossil-fuel boilers, the formation of crinkle and emission such as sulphur oxides (SOx
), nitrogen oxides (NOx
), hydrochloric acid, and particulate matter (PM) could still set other operational and maintenance-related issues due to the formation of fouling and slagging in biomass boiler [96
]. Lee et al. [97
] discovered that a mixing ratio of 1:3 of PKS with coal in cofiring compensates the formation of both CO and SOx
, ultimately produces a higher amount of NOX
. Lee et al. [97
] have further identified that reducing SOx
formation would be possible by reducing PKS mass from 10–15%. In addition, Hawari et al. [98
] have reviewed that PM could be further reduced by a few mechanical and instrument modifications in biomass boilers such as cyclone and multicyclone systems. Suheri and Kuprianov [99
] found that the overall usage of oil palm residue in generating electricity through direct combustion is still considered to be sustainable in terms of low emission. Furthermore, the by-product from this incineration can be reused to fertilize palm oil trees due to the higher potassium content found in the ash produced.
3.2.1. The Production of Solid Biofuel (Densification)
The considerable amount of processed waste from palm oil industries in the form of OPMF and other oil palm residues such as OPKS and OPEFB could be potentially harmful to the environmental deterioration over time if left untreated [100
]. Apart from that, high moisture content of up to 37% from OPKS and OPEFB incorporates high relative moisture content causing difficulties in both transportation and particle-size alteration [100
]. Furthermore, oil palm residues are lower in density than coal, which has further substantiated a pretreatment to increase the higher end utilization functionality. Sabil et al. [100
] have also stated that apart from being tough to grind, a higher loading rate is needed to produce the same amount of energy comparatively with coal due to the low carbon content and low energy density with a higher O/C ratio straining its fuel utilization. Several alternative possibilities (torrefaction, briquetting, and pelletisation) could attempt to compensate for the biomass material’s flaws.
In most of the torrefaction process, the end torrefied biomass feedstock could gain thermal efficiency benefits and increase the overall calorific value. Torrefied biomass feedstock can be used for the pelletisation process as well where the advantages are compressibility strength and improved adhesiveness of the material, but with the drawback of low calorific value due to the presence of higher binding element [101
]. In most of the torrefaction process, the end torrefied biomass feedstock could gain thermal efficiency benefits and increase the overall calorific value. In addition to torrefied biomass, especially OPEFB, a higher yield of lignin (28%) with higher extractive concentration (22.65%) is produced and only starts to disintegrate after reaching a maximum temperature of 300 °C [102
]. Moreover, it has been suggested by Nurdiawati et al. [103
] that wet torrefaction followed by densification could be a potential future innovation in preparing palm oil-based biomass fuels to compensate for the booming energy demands.
Pretreatment can be categorized under several groups such as physical, thermal, chemical, biological, and combinatorial methods determined by the characteristic of the biomass residues and the characteristic of product yield. In a recent study by Wattana et al. [104
], it was discovered that mixed biomass pellets from oil palm residues (palm leaves and OPF) with rubber tree branches and leaves improve the feedstock’s combustion characteristic. Wattana et al. [104
] further discovered that the ultimate pelletisation mixing of palm leaves and rubber leaves shows a significant drop in ash percentage of 7.18% compared to pure palm leaves where the ash content is 9.64%. Furthermore, self-produced electricity has proven to be efficient through polygeneration in palm oil mills using the pelletisation method [105
The low energy density of oil palm residues has made possible today’s advancement in the briquetting process to produce better fuel for future power generation. Even though the briquetting has been commonly used in the power generation field, these briquettes are commonly used to replace firewood or coal for heat generation purposes, too [106
]. Briquettes are better than pellets in various ways, such as no preprocessing needed, lower price value, and correlative production location, such as palm mills (production decentralization). However, higher moisture content and smaller particle size of the biomass feedstock could lower mechanical strength and lower energy density [108
]. Therefore, fundamental presetting factors such as an additional binder, feedstock mixing ratios, and ultimate pressure and temperature setting are essential to produce a better briquette pellet. Oke et al. [109
] have discovered particle size of less than 0.6 mm causes a significant drop in calorific value by approximately 6.18% of briquette made from PKS compared to the average calorific value of 18.41 MJ/kg with the particle size ranges in between 0.6 to 4.76 mm. Sing and Aris [110
] found that 60% of PKS with 40% of OPF and additional paper binder would produce a better briquette with higher calorific value as compared to other ratio’s binder. In addition, oil palm residue could be mixed with other biomass material to produce higher strength hybrid briquette. Kpalo et al. [111
] mixed corn cobs with OPT under low-pressure densification of less than 7 MPa and found that the hybrid briquette has better compressibility strength and exhibited lower moisture content, lower ash content, and prevents agglomeration with the addition of paper pulp as a binder.
3.2.2. The Production of Liquid Biofuel
The adaptability of modern applications in power generation, liquid fuel production, and chemical production from biomass is possible today by converting biodegradable material into liquid fuel. Pyrolysis has been one way to convert biomass material thermochemically to produce the end product consisting of liquid biofuel, char, and biogas. Unlike direct combustion and gasification, pyrolysis is an endothermic process in the absence of oxygen with relatively low temperatures. The variation of the end product depends on the prepyrolytic condition such as pyrolysis mechanism (percentage of lignin, cellulose, and hemicellulose), type of feedstocks, pretreatment (physical, thermal, and biological), and condition of reactions (heating rate and residence time) [112
]. In fast pyrolysis, it is proven that char, tar, and gas production depend on the reduction and increment of both lignin and cellulose content [113
]. Moreover, the pyrolysis rate slows down with the increase of lignin and a decrease in cellulose content of any biomass material [113
Pyrolysis of oil palm residues can be conducted using the proven feasible microwave technique by Abas et al. [114
] which yields a pyrolysis liquid oil of OPMF via a catalytic base through microwave heating. However, the percentage of biochar and pyrolytic liquid oil contributed mainly to the percentage of lignin, cellulose, and hemicellulose contents in the biomass feedstock. According to Safana et al. [57
] and Lee et al. [97
], the percentage of char yield is quite prominent in feedstock which contains a higher amount of lignin as compared to the cellulose and hemicellulose content, and it is vice-versa for a higher yield of pyrolytic liquid, which is proven by Sukiran et al. [115
]. Their experiment reported that OPEFB has the highest yield of pyrolytic oil (47 wt.%), and OPKS has the highest yield of char (55 wt.%) in comparison to other palm oil biomasses [115
]. Apart from that, a higher percentage of ash yield as contaminants during fast pyrolysis reduced the produced bio-oil [116
]. Therefore, water-prewashed OPEFB resulted in the reduction of ash to 2.39% from the initial percentage of 5.9%, with the overall bio-oil yield in between the range of about 28 to 48.4% and decreased for higher reaction temperature, which was produced through an experiment conducted by Yoo et al. [116
]. The yield of either char or bio-liquid through a pyrolysis process is also determined by the application of prewashing methods. OPEFB treated with sulphuric acid produces the highest yield of pyrolytic oil of about 56% due to a decrease in the ash content of up to 56% with the highest fixed carbon percentage of 0.51% and with the highest calorific value of 18.14 MJ/kg contrary from OPEFB treated with distilled water and sodium hydroxide [117
]. Overall, fast pyrolysis has been selected as an efficient method with few drawbacks by most scholars and researchers, which produces better bio-oil yield than slow and flash pyrolysis. However, this depends solely on the overall production cost with the type of prewashed method employed for the maximum yield of pyrolytic liquid.
3.2.3. The Production of Gaseous Biofuel
In palm oil mills, for every ton of processed fresh fruit bunches, it produces 0.7 m3
of POME [118
] and is kept either in a ponding system or tank system as a final treatment before dischargement to a nearby water source. Palm oil mill effluent (POME) still causes a principal amount of pollution to the surrounding from its final discharge to natural water sources, which can be life-threatening to the aquatic life ecosystem as it halts the dissolved oxygen [119
]. Besides that, POME can produce an enormous amount of methane gas by almost 600 million m3
per year, which could contribute to global warming up to 25 times more than carbon dioxide [120
]. There are two types of biochemical digestion practiced widely in palm oil mills, such as aerobic digestion (open lagoon) and anaerobic digestion (closed with high-density polyethylene, HDPE).
In anaerobic digestion, the resulting gas consists mainly of 55–77% methane (CH4
), 30–45% carbon dioxide (CO2
), a small number of other gases such as 1–2% of hydrogen sulphide (H2
S), 0–1% of nitrogen (N2
), and 0–1% of hydrogen (H2
), and traces of carbon monoxide (CO) and oxygen (O2
]. Even though anaerobic digestion is suggested to be the most fundamental method due to the high volume of carbon content in the palm oil mill effluent, this factor could lead to unstable C/N ratio in anaerobic digestion as the optimal C/N ratio should be in the range of 20–30/1 [74
]. Irregularity in C/N ratios resulted in a more significant release of total ammonia nitrogen or high build-up of volatile fatty acids (VFA), which also inhibits the AD processes [123
]. Besides that, some other factors could still halt the overall reaction and reduce biogas production efficiencies, such as pH, organic loading rate (OLR), ambient temperature, and hydraulic retention time (HRT) [74
]. These problems were addressed by Shakib and Rashid [120
] in an experimental analysis between all these three elements, in which it was discovered that the optimum pH of 6.9 with 30/1 C/N ratio and 6 g/L.d VSS is needed to produce 3.8L/d of biogas.
There are four different stages of anaerobic digestion (AD) (hydrolysis, acidogenesis, acetogenesis, and methanogenesis) associated with the presence of multicomplex microorganisms to break down complex biomass material into the end product of biogas. The growth of these microbes depends on the pH, temperature, agitation ratio, and HRT. These complex conditions and factors inhibit microorganisms’ growth rate, causing separation of acidogenesis and methanogenesis to ease the microbial conversion activities in later years [124
]. In all of these stages, methanogenesis is the most sensitive to temperature alteration and is the most time-consuming process. There are two types of temperature conditions present in the methanogenesis stage: thermophilic (50-60 °C) and mesophilic (25–40 °C) [118
]. According to Trisakti et al. [125
], an increase in temperature raises the microbial activity of microorganisms in both mesophilic and thermophilic conditions in POME, which significantly maximizes the yield of biogas, CO2,
and methane production, as well as reduces the maximum HRT. Trisakti et al. [125
] have further stated that an increase in temperature reduces the chemical oxygen demand (COD) and volatile solid (VS) in a thermophilic condition, which differs slightly in mesophilic condition [125
Nevertheless, a further increase of temperature of more than 60 °C eventually alleviates the microorganism’s microbial activity in less than six days [126
]. In addition, the presence of higher volatile fatty acids in the digester would reduce pH, which eventually inhibits the growth of methanogenesis microbes and leads to an overall decrease of methane gas production in both ultrasonic and normal POME [126
]. Wong et al. [127
] further added that pH of (6.8–7.8) would be important for the overall production of biogas in AD and suggested stretching HRT to 20 days and adding calcium carbonate CaCO3
to neutralize the VFA acidity and facilitate the production of methane. Suksong et al. [123
] discovered that the yield of methane from POME could be enhanced by mixing sewage sludge. They yielded the highest methane of 56 mL CH4 g−1
VS through mixing ratios of 99:1 from codigestion POME and sewage chemical sludge [123
]. Suksong et al. [123
] further added that the methane yield from liquid AD is 20–25 times higher than solid AD due to lower water content inhibiting the microbial conversion.
In general, biogas produced via POME could be utilized in many ways: heat, electricity, or both. However, the principal matter that constrain the production is the complexity of the overall production, which elevates the cost of the biogas. Hosseini et al. [128
] introduced H2
with POME biogas, which visibly stretches the formation of flame and improved the low calorific biogas nature by increasing the percentage of H2
from 5 to 10%. The only drawback of this method is that it produces NOX
, enhanced via the temperature [128
]. Bukhari et al. identified that cofiring of biogas in palm oil mill boilers could potentially reduce the formation of PM by up to 50% and provide fuel saving by up to 80–90% [129
]. Of all these significant contributions of POME, the most prominent and essential aspect is the economic analysis. A typical biogas plant’s overall installation is estimated at around RM 4–6 million, subjected to the mill’s capacity [130
]. The return on investment (ROI) would be fruitful after completing 2–4 years with revenues from grid-connected electricity generation and reduced diesel usage [130
]. Biogas from POME could be a solution for both environments and generate income in rural or remote areas whereby natural gas infrastructure does not reach.
With the types of valorisation used like thermochemical and biochemical processes and factors affecting the biofuel yield production and bioenergy production, it was concluded that all these processes (e.g., combustion, gasification, pyrolysis, and anaerobic digestion) contributed to different benefits in the form of solid, liquid, and gaseous biofuel production from various types of palm oil feedstocks. Therefore, it is essential to perform further research on the employment of different processes in the optimisation and advancement of biofuel or bioenergy production in a cost-effective way.