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© 2020 Elsevier Ltd The increase in human consumption of plant and animal oils has led to the rise in waste cooking oil (WCO) production. Instead of disposing the used cooking oil as waste, recent technological advance has enabled the use of WCO as a sustainable feedstock for biofuels production, thereby maximising the value of biowastes via energy recovery while concomitantly solving the disposal issue. The current regulatory frameworks for WCO collection and recycling practices imposed by major WCO producing countries are reviewed, followed by the overview of the progress in biodiesel conversion techniques, along with novel methods to improve the feasibility for upscaling. The factors which influence the efficiency of the reactions such as properties of feedstock, heterogenous catalytic processes, cost effectiveness and selectivity of reaction product are discussed. Ultrasonic-assisted transesterification is found to be the least energy intensive method for producing biodiesel. The production of bio-jet fuels from WCO, while scarce, provide diversity in waste utilisation if problems such as carbon chain length, requirements of bio-jet fuel properties, extreme reaction conditions and effectiveness of selected catalyst-support system can be solved. Technoeconomic studies revealed that WCO biofuels is financially viable with benefit of mitigating carbon emissions, provided that the price gap between the produced fuel and commercial fuels, sufficient supply of WCO and variation in the oil properties are addressed. This review shows that WCO is a biowaste with high potential for advanced transportation fuel production for ground and aviation industries. The advancement in fuel production technology and relevant policies would accelerate the application of sustainable WCO biofuels.

Sustainable Aviation Fuel (SAF) has become an important measure in the aviation industry’s efforts to mitigate carbon emissions and reduce their overall environmental impacts. However, commercial usage is relatively stunted due to a plethora of drawbacks in the production process and economic feasibility of the fuel. In this study, the currently accepted technologies for producing synthetic jet fuels under the American Society for Testing Material (ASTM D7566) standard specification for aviation turbine fuel are reviewed. The emphasis is placed in terms of their reactions, type of catalysts used for the conversion pathways of Fisher-Tropsch (FT), Hydroprocessed Esters and Fatty Acids (HEFA) and Alcohol-to-Jet (ATJ), and the use of biomass resources as feedstock. The advancement in the production process and physicochemical properties of the uncertified biojet fuels are reviewed and discussed. Generally, Co- and Fe-based catalysts are commonly used for the FT process, while bimetallic catalysts consisting of Pt, Pd, Ni and Mo have shown excellent activities and selectivities for the HEFA process. For the ATJ process, zeolites such as HZSM-5, beta and SAPO have shown remarkable ethanol dehydration efficiency, while TiO2 and ferrierite have been studied for the combined iso-butanol dehydration and oligomerisation processes. Fundamental factors influencing the reaction efficiency including the feedstock properties, reaction conditions, catalytic reusability and catalyst supports are discussed. Finally, the key challenges and prospects for biojet fuel commercialisation are addressed.

Microalgae has been identified as a potential feedstock for biodiesel production since its cultivation requires less cropland compared to conventional oil crops and the high growth rate of microalgae. Research on microalgae oils often are focused on microalgae oil extraction and biomass harvesting techniques. However, energy intensive and costly lipid extraction methods are the major obstacles hampering microalgae biodiesel commercialisation. Direct biodiesel synthesis avoids such problems as it combines lipid extraction techniques and transesterification into a single step. In this review, the potential of direct biodiesel synthesis from microalgae biomass was comprehensively analysed. The various species of microalgae commonly used as biodiesel feedstock was critically assessed, particularly on high lipid content species. The production of microalgae biodiesel via direct conversion from biomass was systematically discussed, covering major enhancements such as heterogeneous catalysts, the use of ultrasonic and microwave- techniques and supercritical alcohols that focus on the overall improvement of biodiesel production. In addition, this review illustrates the cultivation conditions for biomass growth and lipid productivity improvement, the available harvesting and lipid extraction technologies, as well as the key challenges and future prospect of microalgae biodiesel production. This review serves as a basis for future research on direct biodiesel synthesis from modified microalgae biomass to improve profitability of microalgae biodiesel.

Biodiesel is considered as a potential substitute for petroleum-based diesel fuel owing to its comparable properties to diesel. Biodiesel is generally produced from renewable sources such as agricultural products and microalgae in the presence of a suitable catalyst. Recently ionic liquid (IL) catalyzed synthesis of biodiesel has become a promising pathway to an eco-friendly production route for biodiesel. This review focuses on the use of ILs both as solvents as well as catalysts for sustainable biodiesel production from agricultural feedstocks and microalgae with high free fatty acid content. Reactions catalyzed by ILs are known to render high reactivity under the mild condition and high selectivity of ester product with simple separation steps. The article first discusses the state of the art of biodiesel production using ILs along with the physicochemical properties of the produced biodiesel. Then, current IL technologies were elucidated in terms of the categories such as acidic and basic ILs. The use of more advanced ILs such as supported ionic liquids and ionic liquid-enzyme catalysts on different biodiesel feedstocks were also discussed. Furthermore, the role of IL catalyst in intensified biodiesel production methods such as microwave and ultrasound technologies were also discussed. Finally, the prospects and challenges of IL catalyzed biodiesel production are discussed in this article. The review shows that ILs with bronsted acidity or basicity not only pose a low risk to the environment but also result in high biodiesel yields with mild reaction conditions in a short time. Bronsted acidic ILs can convert free fatty acids as well as triglycerides to biodiesel without the need for pretreatment, which facilitates in reducing the production cost of biodiesel. From the review, it can be concluded that ILs present great potential as catalysts for biodiesel production.

Spent Coffee Ground (SCG) was studied as a potential source of oil feedstock for biodiesel production as an alternative waste utilisation instead of being disposed as municipal waste. This study evaluated the ultrasonic assisted oil extraction from SCG, which was followed by biodiesel conversion via transesterification. Hexane was found to be the most effective extractant for SCG oil. Soxhlet extraction could obtain maximum SCG oil yield of 12.5% within 3 h whereas the highest SCG oil yield (14.52%) using ultrasonic extraction was obtained at hexane to SCG ratio of 4 mL g−1 and at 30% ultrasonic amplitude for 30 min. The improved oil yield with shorter extraction time was due to the ultrasonic fragmentation on SCG cells, which enhanced the interactions between oil and solvents. Fourier transform infrared analysis showed that the SCG oil possessed suitable functional groups for biodiesel conversion. Then, the SCG oil was successfully converted to biodiesel via ultrasonic assisted transesterification. The optimal FAME yield (97.11%) was achieved with molar ratio of methanol to SCG oil of 30:1, 4 wt% of catalyst concentration, at 30% ultrasonic amplitude and for 3 h. The produced SCG biodiesel has promising properties which adhere to the biodiesel standards but acid value was beyond the permissible limit which could be overcome by utilising as blend feedstock with other commercially available biodiesel. The high calorific value along with low viscosity, density and corrosion properties suggested SCG biodiesel as an interesting and viable option for biodiesel blending.

A two-step catalytic reactive extraction and transesterification process was adopted to synthesize biodiesel from solid Jatropha seeds, a non-edible source with high free acid content, directly using ultrasound irradiation. From the intensification of reactive extraction process coupled with ultrasound, esterified oil with satisfactory extraction efficiency of 84.0 ± 0.5%, FAME purity of 38.6 ± 1.3% and esterification efficiency of 71.1 ± 1.3% were obtained. High acid value (AV) of 18.2 ± 0.5 mg KOH/g was successfully reduced to 5.3 ± 0.2 mg KOH/g. In the subsequent transesterification of esterified Jatropha oil, FAME purity of 99.0 ± 1.3% and biodiesel yield of 85.2 ± 1.3% were attained at KOH loading of 1.5 wt.%, methanol to oil molar ratio of 12:1 and ultrasonic amplitude of 60% after 15 min reaction time. From the artificial neural network (ANN) modelling, coefficient of determination (R2) for FAME purity and biodiesel yield were found to be 0.9926 and 0.9845, respectively, which proved that the model had good fit with the experimental data. Under the same optimized reaction conditions, this process intensification could achieve higher FAME purity (99.0% vs. 91.8%), biodiesel yield (85.2% vs. 75.6%) and AV reduction efficiency (88.5% vs. 88.2%) than conventional magnetic stirring.