Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc Review Waste materials as potential catalysts for biodiesel production: Current state and future scope Akshey Marwaha a , Pali Rosha b , Saroj Kumar Mohapatra a , Sunil Kumar Mahla c , Amit Dhir b, ⁎ a Department of Mechanical Engineering, Thapar Institute of Engineering & Technology, Patiala 147004, India b School of Energy and Environment, Thapar Institute of Engineering & Technology, Patiala 147004, India c Department of Mechanical Engg., I.K. Gujral Punjab Technical University Campus, Hoshiarpur 146001, India ARTICLE INFO Keywords: Waste material Calcium-based catalyst Biodiesel Transesterifcation Environment Economical ABSTRACT Recent studies on the exploration of eco-friendly approach by utilizing large-scale waste materials as potential catalyst in biodiesel production have attracted much attention. The development of heterogeneous catalysts especially from calcium has gained much awareness due to the large availability of calcium-rich waste materials and their corresponding high catalytic activity in the transesterifcation of oil. Most of the waste materials employed as heterogeneous catalysts have an abundance of natural Ca content and they have high catalyst activity and selectivity despite being environment-friendly and cost-efective. Heterogeneous catalysts with high activity can be produced from Ca based waste materials when calcined at high temperatures. This review gives a brief overview of the developments of various Ca based catalysts derived from waste materials as an efcient catalyst for biodiesel production with signifcant yield. Industrial wastes (red mud, slag, ash) and biological catalysts (chicken eggshells, mollusk shells, animal bones) possess enormous potential towards developing an economical catalyst and subsequently, low-cost biodiesel generation. However, future challenges await a better utilization of useless wastes into a useful resource to satisfy human needs. 1. Introduction The continuous exigency for fossil fuels for industrial, transporta- tion and domestic purposes has led to the exploration of alternative energy sources. Coal, oil, and gas are the main contributors in the en- ergy sector. The literature studies reveal that oil, coal, and gas may last for a further 40, 200 and 70 years, respectively, as per the ongoing consumption of fossil fuel reserves [1]. Sky-high prices of gasoline and diesel have forced the researchers and scientists to develop liquid bio- fuels as alternative fuels [2]. Bio-diesel, a non-renewable and biode- gradable fuel, is a suitable alternative fuel whose properties match the specifcations of the ASTM and EN standards [3]. Transesterifcation process is a widely used method, which involves the conversion of triglycerides to methyl esters (with methanol) or ethyl esters (with ethanol) along with the use of a suitable catalyst. Fig. 1 depicts the transesterifcation mechanism for biodiesel production. Homogeneous base catalysts (KOH and NaOH) are not capable to convert used cooking oils and inedible oils due to their high FFA con- tent causing the formation of soaps in the product. Moreover, the se- paration of biodiesel from the solution is difcult and thus costly. Acid- catalyzed reactions for pre-esterifcation of high FFA are not suitable due to the corrosive nature of acid catalysts. Therefore, the studies on the development of solid heterogeneous catalysts have escalated [4,5]. The higher molar ratios, catalyst amount and reaction temperatures required for the heterogeneous catalysis when compared to homo- geneous catalysis is a complication for production of low-cost biodiesel. Solid base heterogeneous catalysts relatively require lower reaction conditions when compared to those for solid acid catalysts. Waste materials derived from industries and surroundings can assist in the development of an economical solid base catalyst. Awareness towards an exploration of more waste materials can help in the development of catalysts promoting a sustainable and environment-friendly approach towards biodiesel production [6]. Rice husk as by-product constitutes about 20% in terms of weight of rice when milled. Around 151 mil- lion tonnes of rice husk was produced as reported in July 2017 [7]. Calcium oxide (CaO) is one of the most active catalysts in the ca- tegory of solid base catalysts. Due to its low cost, easy availability and high regenerability. CaO is a widely used catalyst for transesterifcation of feedstocks. Also, various waste products contain Ca content in huge amounts and are easily available at low cost. CaO as catalyst support is ideal due to its high surface area and a large number of pores available on the surface. CaO is a non-toxic catalyst possessing high basicity and https://doi.org/10.1016/j.fuproc.2018.09.011 Received 20 June 2018; Received in revised form 10 September 2018; Accepted 14 September 2018 ⁎ Corresponding author. E-mail address: amit.dhir@thapar.edu (A. Dhir). Fuel Processing Technology 181 (2018) 175–186 0378-3820/ © 2018 Published by Elsevier B.V. T