Second generation bioethanol production: A critical review H.B. Aditiya a,b,n , T.M.I. Mahlia a,e , W.T. Chong b , Hadi Nur c , A.H. Sebayang b,d a Department of Mechanical Engineering, Universiti Tenaga Nasional, 43009 Kajang, Selangor, Malaysia b Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia c Centre for Sustainable Nanomaterials, Ibnu Sina Institute for Scientific and Industrial Research, Universiti Teknologi Malaysia, Johor Bahru, Malaysia d Department of Mechanical Engineering, Medan State Polytechnic, 20155 Medan, Indonesia e Faculty of Integrated Technologies, Universiti Brunei Darussalam, JalanTungku Link, Gadong BE1410, Brunei Darussalam article info Article history: Received 1 April 2015 Received in revised form 12 February 2016 Accepted 4 July 2016 Keywords: Second generation Bioethanol production Agricultural waste Lignocellulosic biomass Starch biomass abstract It is a popular fact that the world's dependency on fossil fuel has caused unfavourable effects, including lessening crude oil reserve, decreasing air quality, rising global temperature, unpredictable weather change, and so on. As the effort to promote sustainability and independency from fossil fuel, bioethanol is now favoured as the blend or fossil petrol substitute. However, the feedstock functionality of first generation bioethanol production is restricted due to its edibleness since it would clash the feeding purpose. Second generation bioethanol production fulfils the impractical gap of first generation since it employs non-edible feedstock sourced from agriculture and forestry wastes. Lignocellulosic and starchy materials in them are convertible to fermentable sugars that are able to be further processed, resulting anhydrous bioethanol as the end product. This paper critically reviews the existing variance of second generation bioethanol production methodologies, namely pre-treatment, hydrolysis, fermentation and distillation, as well as the worth of second generation production for future reference. The discussions in this paper are also fit as the fundamental for feasible planning of second generation bioethanol pro- duction plant. & Published by Elsevier Ltd. Contents 1. Introduction ........................................................................................................ 632 2. Second generation bioethanol production ................................................................................ 633 2.1. Pre-treatment ................................................................................................ 633 2.1.1. Biological pre-treatment ................................................................................. 633 2.1.2. Chemical pre-treatment.................................................................................. 634 2.1.3. Mechanical pre-treatment ................................................................................ 635 2.1.4. Physicochemical pre-treatment ............................................................................ 636 2.2. Hydrolysis ................................................................................................... 636 2.2.1. Enzymatic hydrolysis of lignocellulosic biomass .............................................................. 636 2.2.2. Enzymatic hydrolysis of starchy biomass .................................................................... 638 2.3. Fermentation ................................................................................................. 639 2.4. Distillation ................................................................................................... 641 2.4.1. Adsorption distillation ................................................................................... 642 2.4.2. Azeotropic distillation ................................................................................... 642 2.4.3. Chemical dehydration ................................................................................... 644 2.4.4. Diffusion distillation .................................................................................... 645 2.4.5. Extractive distillation .................................................................................... 645 2.4.6. Membrane distillation ................................................................................... 647 Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/rser Renewable and Sustainable Energy Reviews http://dx.doi.org/10.1016/j.rser.2016.07.015 1364-0321/& Published by Elsevier Ltd. n Corresponding author at: Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail addresses: aditharjon@siswa.um.edu.my (H.B. Aditiya), chong_wentong@um.edu.my (W.T. Chong). Renewable and Sustainable Energy Reviews 66 (2016) 631–653