Review Hydrothermal conversion of biomass waste to activated carbon with high porosity: A review Akshay Jain a , Rajasekhar Balasubramanian b , M.P. Srinivasan c,⇑ a Department of Chemical and Biomolecular Engineering, National University of Singapore, 117585, Singapore b Department of Civil and Environmental Engineering, National University of Singapore, 117585, Singapore c School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia highlights  Enhanced resource and energy recovery from biomass waste by hydrothermal treatment.  Elucidation of the mechanism of hydrochar formation.  New insights in hydrothermal treatment for enhancing porosity of activated carbon.  Effect of processing conditions on hydrochar and activated carbon characteristics. article info Article history: Received 14 May 2015 Received in revised form 4 August 2015 Accepted 5 August 2015 Available online 8 August 2015 Keywords: Hydrothermal carbonization Activated carbons Biomass waste Chemical activation Biomass & hydrochar reactivity Surface area abstract With the awareness of the need for optimal and sustainable use of natural resources, hydrothermal treat- ment of biomass and biomass waste for energy and resource recovery has received increasing attention. In this context, hydrochars produced by hydrothermal treatment of biomass, which have been tradition- ally employed as catalyst supports or adsorbents, are now of considerable interest as precursors for acti- vated carbons. Activated carbon produced from hydrochars has been widely used in applications such as energy storage, environmental remediation and resource recovery. Apart from savings in energy and resource conservation, hydrochars are of interest in activated carbon production because they possess unique attributes such as high density of oxygenated functional groups (OFGs) and low degree of conden- sation which can be tailored to produce activated carbons with specific and desired characteristics. This review aims to integrate the current knowledge on hydrothermal carbonization of biomass waste in the context of resource and energy recovery. Specifically, we bring together recent advances made in this area through a systematic and critical review of the production of activated carbon from hydrochars. Mechanisms involved in the production of hydrochars with different surface characteristics in response to variations in the operating parameters are critically reviewed. The current knowledge gaps and chal- lenges involved in the hydrothermal carbonization of biomass waste are critically evaluated with sugges- tions for further research. Ó 2015 Elsevier B.V. All rights reserved. Contents 1. Introduction ......................................................................................................... 790 2. Hydrothermal carbonization of biomass................................................................................... 791 2.1. Formation of hydrochar .......................................................................................... 791 2.1.1. Carbohydrates to hydrochar................................................................................ 791 2.1.2. Cellulose to hydrochar .................................................................................... 792 2.1.3. Lignin to hydrochar ...................................................................................... 792 2.1.4. Biomass to hydrochar ..................................................................................... 792 http://dx.doi.org/10.1016/j.cej.2015.08.014 1385-8947/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Chemical Engineering Journal 283 (2016) 789–805 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej