Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Liquid biofuels utilization for gas turbines: A review Ibrahim I. Enagi a , K.A. Al-attab b , Z.A. Zainal c, ⁎ a Department of Mechanical Engineering, School of Engineering Technology, Federal Polytechnic, P.M.B 55, Bida, Niger State, Nigeria b Department of Mechanical Engineering, Faculty of Engineering, Sana’a University, Sana’a, Yemen c School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia ARTICLE INFO Keywords: Liquid biofuels Gas turbine Colourless distributed combustion High temperature air combustion Moderate or intensive low oxygen dilution Catalytic combustion ABSTRACT The global demand for utilization of renewable fuels in gas turbines has been on the increase to secure a sus- tainable, and pollution free environment. In this paper, the reviewed studies were on different liquid biofuels production methods such as the catalytic conversion of biomass, gasification, pyrolysis and transesterification. The review also included different studies on directly fired gas turbine (DFGT) and externally fired gas turbine (EFGT) utilizing biomass and liquid biofuels. Furthermore, this study elucidated the use of biofuels in clean combustion methods scalable to gas turbines such as colourless distributed combustion (CDC), high temperature air combustion (HiTAC), moderate or intensive low oxygen dilution (MILD) combustion and catalytic com- bustion. The discussion included the effect of different input parameters associated with the clean combustion systems that have influence on the attainment of ultra-low emissions of NOx and CO under premixed and non- premixed modes. As for the fuel types, biodiesel is one of the most studied biofuel in gas turbine especially in small-scale micro gas turbine (MGT) engines. The materials for the path of hot gas, types of fuels, heat recovery and cogeneration techniques are the variables, found to be affecting the performance of the DFGT. As for the EFGT, the high temperature heat exchanger with its lower turbine inlet temperature of 700–900 °C is generally the main limiting factor for this technology. The paper concluded by highlighting relevant and recent findings, thereby proposes a further research to improve the versatility in the utilization of liquid biofuels in gas turbines. 1. Introduction The high global demand of energy has increased expeditiously in the past decade with a reported ratio of annual increment of 2.3% in 2013 [1]. The depletion of fossil fuels and global warming concerns encouraged the development of new combustion technologies for al- ternative fuels utilization. These technologies should not only cover the demand for power, but also maintain high performance, conversion and efficiency without any environmental impacts [2]. The significance of biomass resources to energy production has shown that 75% of global renewable energy and 13% of world primary energy are from biomass, whereas up to 30% of global energy supply by 2050 is estimated to be from bio-energy contributions especially liquid biofuels [3,4] Due to the increased diffusion of renewable energy sources in recent years, biomass has gained a growing interest in the combined heat and power (CHP) applications [5]. Using different energy conversion tech- nologies, biomass is storable, programmable and can be utilised to meet a wide range of energy needs [5]. There are several thermo-chemical conversion processes of biomass into different biofuels that include gasification, pyrolysis, liquefaction and transesterification. Focusing on the thermochemical conversion systems, the process technologies for microalgae-to-biofuel production systems were extensively discussed [6] with the benefits of exploiting upstream microalgae biomass de- velopment for bioremediation. In addition, there is a recent progress in gasification techniques including important pathways for production of biofuels, socio-economic impacts of biofuel generation and process design [7]. The first generation or advanced biofuels in existing com- bustion engines are performing well as pure or blended additives. In addition, oxygenated biofuels produces lower NO and sooth emissions than hydrocarbon fuels. However, in order to improve fuel efficiency and reduce engine emissions several novel technologies are being de- veloped [8]. High efficiency, fuel flexibility and ultra-low emission heat engines and fuel cell technologies will in future enable customers to switch to the cleanest fuel available at the lowest cost [8]. One of the major power generation technologies with a significant share in global carbon footprint is gas turbine. However, it is still lag- ging behind when it comes to renewable resources utilization. One of the European Union targets [9] for micro gas turbine (MGT) power generation is the CHP small scale distributed generation. Some of the advantages of gas turbine include; low pollutant emission, high https://doi.org/10.1016/j.rser.2018.03.006 Received 11 April 2017; Received in revised form 13 October 2017; Accepted 9 March 2018 ⁎ Corresponding author. E-mail address: mezainal@yahoo.com (Z.A. Zainal). Renewable and Sustainable Energy Reviews 90 (2018) 43–55 1364-0321/ © 2018 Elsevier Ltd. All rights reserved. T