Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Review article Significant parameters and technological advancements in biodiesel production systems M. Erdem Günay a , Lemi Türker b , N. Alper Tapan c, ⁎ a Department of Energy Systems Engineering, Istanbul Bilgi University, 34060 Eyup, Istanbul, Turkey b Department of Chemistry, Middle East Technical University, 06800 Çankaya, Ankara, Turkey c Department of Chemical Engineering, Gazi University, 06570 Maltepe, Ankara, Turkey ARTICLE INFO Keywords: Catalyst type Reaction temperature Calcination temperature Mechanical stirring Supercritical transesterification Microwave transesterification Hydrodynamic cavitation Ultrasound technology Future directions ABSTRACT Biodiesel is a mixture of fatty acid esters formed by transesterification of vegetable oil, animal fat, algae oil or waste oil with an alcohol like methanol (CH 3 OH), ethanol (C 2 H 5 OH) or higher alcohols. There are many im- portant catalytic variables like catalyst type and composition, support type and pretreatment conditions (i.e. calcination temperature and time) which are utilized to achieve high yields for the transesterification reaction. In addition, operational conditions such as reaction temperature, alcohol type, alcohol to oil molar ratio and stirring speed have also quite high significance. Moreover, all these variables can be optimized under super- critical conditions by novel techniques like ultrasonic and microwave irradiation or hydrodynamic cavitation. In this work, significant catalytic and operational variables for biodiesel production are reviewed. In addition, dominant parameters together with their limitations during the application of advanced technologies are in- vestigated in detail. Then, it has been concluded that, for better control and higher yields of biodiesel pro- duction, future research works should focus on addition of co-solvents, use of longer chain alcohols, bulky structures or ionic liquids, adjustment of mode of irradiation and modification of the instrumentation or the equipment. 1. Introduction The world fuel consumption has been increasing since the industrial revolution and this increase is even sharper especially in the recent years due to the increased movement of goods, services and technology. Although most of the fuel demand for transportation is supplied by petroleum-based fuels, their resources are depleted year by year. Moreover, the recovery of petroleum from new reservoirs in extreme locations is sometimes also too difficult being too costly. Besides, ex- cessive use of petroleum-based fuels causes air and water pollution leading to global warming. However, biofuel production from renew- able sources is a sustainable way to maintain the ever-growing fuel demand, causing negligible harm to the environment due to the fast bioenergy cycle. As a result, the world biofuel production has an in- creasing trend in the recent years as shown in Fig. 1; such that, annual bioethanol production increased from 13.7 billion metric ton in the year 2000 to 78.8 billion metric ton in the year 2016 (multiplied by a factor of 5.8) while annual biodiesel production increased from 0.77 billion metric ton to 29.8 billion metric ton in the same period of time (mul- tiplied by a factor of 38.6) [1]. Although biodiesel is an alternative fuel that can be produced from any edible plant-based oil or animal-based fat, this kind of 1st gen- eration feedstocks are not preferred anymore due to the requirement of large farm areas, competition with the food market and thus high costs in the raw material [2]. Instead, biodiesel is recently preferred to be made from non-edible plant-based oils with (high oil yield), waste cooking oils and algae oil (2nd or 3rd generation feedstocks) [3]. The basic pictorial representation of biodiesel production is shown in Fig. 2, where the main steps for the process are given. It should be noted that the process may have some minor (or sometimes major) differences especially in the case of application of advanced techniques. The main reaction in biodiesel production is transesterification of any oil with a short chain alcohol like methanol or ethanol. In fact, vege- table oil can straightly be used as a fuel but its low fuel quality due to high viscosity, incomplete combustion, coking etc. make it insufficient for combustion engines. Therefore, transesterification of these types of oils by lower boiling alcohols convert them to high quality fuels by lowering viscosity, boiling and flash point. The transesterification of oil (Eq. (1)) is an endothermic equilibrium reaction requiring 3 mol of a short chain alcohol per 1 mol of oil in https://doi.org/10.1016/j.fuel.2019.03.147 Received 27 September 2018; Received in revised form 26 March 2019; Accepted 28 March 2019 ⁎ Corresponding author. E-mail address: atapan@gazi.edu.tr (N. Alper Tapan). Fuel 250 (2019) 27–41 0016-2361/ © 2019 Published by Elsevier Ltd. T