Hydrothermal conversion of biomass to fuels and energetic materials Andrea Kruse 1,4 , Axel Funke 2 and Maria-Magdalena Titirici 3 Available biomass, preferentially residues, can be divided in two groups: biomass with a high or natural water content (‘wet’ or ‘green’ biomass) and biomass with low water content such as wood and straw. In ‘dry’ biomass gasification processes, originating in most coal processing technologies, biomass of low water content is necessary to avoid the energy loss by water evaporation. In contrast, hydrothermal processes need water as reaction medium; therefore, these processes are preferentially used for wet or ‘green’ biomass.In this review paper we will describe the main research directions in the hydrothermal conversion of biomass into fuels and carbon throughout gasification to produce H 2 or CH 4 , liquefaction to produce crude oils and phenols from lignin as well as carbonization to produce carbonaceous materials which can be either used as fuels (carbon negative chars) or interesting energetic materials (hydrothermal carbons). Addresses 1 Kahrlsruhe Institute of Technology, Karlsruhe, Germany 2 Leibniz-Institut fu ¨r Agrartechnik Potsdam, Germany 3 Queen Mary University of London, School of Materials Science and Engineering, London, UK 4 University of Hohenheim, Stuttgart, Germany Corresponding author: Titirici, Maria-Magdalena (m.m.titirici@qmul.ac.uk) Current Opinion in Chemical Biology 2013, 17:515–521 This review comes from a themed issue on Energy Edited by Michael D Burkart and Stephen P Mayfield For a complete overview see the Issue and the Editorial Available online 23rd May 2013 1367-5931/$ – see front matter, # 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cbpa.2013.05.004 Introduction Dry thermochemical conversion processes have been used by mankind since centuries and recent develop- ments increased the quality of these processes to make them available for the urgent required shift from fossil to renewable resources. They are readily applicable to pro- duce high quality gaseous, liquid, and solid fuels. How- ever, dry conversion processes can only work with high efficiency when dry feedstock is used, which in general represents a fuel with a comparable good quality. In order to increase the energetic use of moist biomass, which is often regarded as waste biomass because it is difficult to be processed with good efficiency, hydrothermal pro- cesses are investigated. These processes are capable of producing high quality fuels and materials with different characteristics than their dry counterparts. Hydrothermal processes take place in liquid water at elevated temperatures, that is, the pressure in the system must be at or above saturated pressure. They can be classified by different regions above the vapor–pressure curve and the critical point in the phase diagram of water (see Figure 1). Hydrothermal gasification dominates at high temperatures in near and supercritical conditions whereas carbonization takes place at comparably mild temperatures. Under subcritical hydrothermal conditions, increasing temperature always means an increasing pres- sure due to the conditions set by the vapor–pressure curve. In contrast to hydrothermal processes, steam assisted pyrolysis is represented by different conditions below the vapor–pressure curve, that is, the state of aggregation of water is dry steam. Chemistry of hydrothermal processes The dramatic change of the properties of water at elev- ated temperatures, even in the subcritical region, is well known. Water becomes an excellent reaction environ- ment, reactant and solvent for a diverse range of reactions, for example, the increasing ion product in subcritical conditions favors reactions that are typically catalyzed by acids or bases. The initial reaction taking place when biomass is heated up in water is the hydrolysis of cellulose to glucose, which is the decisive difference to dry thermo- chemical conversion [2] (see Figure 2). The hydrolysis gives way to homogeneous reactions in an aqueous solution, which are not limited by heat and mass transfer. Same holds true for the destruction of lignin to its main product phenol. Further dehydration of glucose to a wide range of potentially interesting substances has been the topic of many excellent reviews and will not be discussed in detail here. Some key substances are indicated in Figure 2 as representatives. This mixture of organic acids, different ketones, and phenols represents the liquid product ‘biocrude’ from hydrothermal liquefaction. As free radical reactions become more important toward the critical point of water, gasification for the production of hydrogen and methane is favored: C 6 H 12 O 6 þ 6H 2 O ! 6CO 2 þ 14H 2 (1) C 6 H 12 O 6 ! 3CH 4 þ 3CO 2 (2) The formation of hydrogen is endothermic and that of methane slightly exothermic [3]. Therefore, hydrogen for- mation predominates over methane formation at high tem- peratures according to the LE CHATELIER principle. Available online at www.sciencedirect.com www.sciencedirect.com Current Opinion in Chemical Biology 2013, 17:515–521