Hydrothermal Conversion of Biomass: I, Glucose Conversion in Hot Compressed Water D. Knez ˇevic ´, W. P. M. van Swaaij, and S. R. A. Kersten* UniVersity of Twente, Faculty of Science and Technology, Research Institute Impact, P.O. Box 217, 7500 AE Enschede, The Netherlands In this paper, hydrothermal conversion of biomass is investigated. Part I deals with glucose and part II focuses on woody biomass and pyrolysis oil. Hydrothermal conversion of glucose (250-350 °C) has been studied in batch quartz capillary reactors. Kinetics of the overall glucose decomposition was determined and was in agreement with the majority of literature data. Attention was paid to the initial glucose decomposition: primary glucose decay products were identified from literature and used in experiments. It was found that all primary decay components of glucose, with the exception of formaldehyde, produce a kind of char (acetone insoluble product). Characteristic gas (primarily CO 2 ) formation reactions are discussed on the basis of separate tests with primary and other known initial glucose degradation products. Complete mass and elemental balances were obtained for two different temperatures, 300 and 350 °C, and various residence times from 10 s to 10 days. It was clearly observed that the product formation as a function of residence time occurs in two distinctly different regimes. The first 5-10 min are characterized by fast changes, whereas after the initial 10 min the changes occur at a much lower rate. It was found that water production, which occurred predominantly in the first 5 min of residence time, was constant (3 mol/mol glucose) and unaffected by temperature or glucose concentration. The yield of the oil product, called here water-acetone soluble (WSS) yield exhibited a maximum at ca. 5 min residence time. After 5 min it was reduced in favor of gas and char, called here water-acetone insoluble (WSIS). However, a certain quantity of WSS is stable even after 10 days residence time. The elemental composition of WSS and WSIS was found to be very similar, which indicates that they are essentially the same product. The highest molecular weight fraction of this product does not dissolve in acetone, whereas the lower molecular weight part does. Compositions of gas, WSS, and WSIS were used as a basis for estimation of the overall reaction enthalpy, which was calculated to be ΔH r )-1.5 ( 0.5 MJ/kg. It was found that higher glucose concentrations resulted in more WSIS and less WSS, whereas the gas and water yield did not change. All findings were incorporated into a lumped engineering reaction path and kinetic model of glucose hydrothermal decomposition. Introduction Hot compressed water is proposed and used as reaction medium in liquefaction, 1-12 gasification, 13-26 and combustion 27-32 processes of biomass. It is an excellent solvent for organic components and gases, and near the critical point it is an acid/ base catalyst because of its high ionic product (K w ) 10 -11 ). 33,34 Another benefit of working at high pressure is the marginal heat of vaporization of water (ΔH v decreases from 2.26 MJ/kg at ambient pressure to zero above the critical point). This will keep shell-to-tube temperature differences in a counter-current heat exchanger, operating between the reactor effluent and the feed stream, finite. As a result of this, high thermal efficiencies can be achieved despite a low dry matter content of the feedstock. 24,25 Depending on the process conditions and application of catalysis, the main reaction product can be a gas or a liquid (oil). Hydrothermal gasification experiments at laboratory and pilot scale are reported over the whole temperature range of 250 to 650 °C. 13-17,19-26 A catalyst is necessary to achieve complete gasification. 17,24,26 By choice of temperature and catalyst design/ selection, it has been possible to steer the gas composition, at least partly, toward methane or hydrogen. Methane production from biomass in the temperature range of 350 to 400 °C has been demonstrated at pilot scale and is now being commercial- ized, 15 whereas the production of hydrogen is more difficult and still in an embryonic stage of development. 20,24 Production of hydrogen-rich gas is difficult because a high CH 4 yield is dictated by thermodynamics for low temperature and also for high temperature in combination with realistic feedstock concentrations of >10 wt % organics. Only for diluted feed streams (less than 2 wt % carbohydrates in water) and high temperature (T > 600 °C) the production of high concentration of hydrogen is thermodynamically possible. 35 In the present contribution, hydrothermal conversion (HTC) aiming at oil production is investigated. HTC is performed in the temperature range between 240 °C and the critical point of water (374 °C) with or without catalyst. 1-10 Under these conditions, biomass is converted, in a complex sequence of chemical reactions, into various components, which, upon cooling the reactor effluent, constitute three different phases: a water phase, a hydrophobic phase, and a gas phase. By extraction, the hydrophobic reaction product can be further separated into a solvent soluble (oil) and a solvent insoluble (char) part. The hydrophobic product has a considerably lower oxygen content (typically 20 vs 45 wt % of the feedstock) and, consequently, a higher heating value (HHV) than the feedstock (typically 30, 6 vs 19 36 MJ/kg). The whole hydrophobic product can be used as fuel in furnaces and boilers. 2,6 It is reported that the solvent soluble fraction (oil) of the hydrophobic product can be upgraded into transportation fuel by catalytic hydrotreat- ment. 2,6 The present paper deals with the measurement of kinetic and yield data necessary for process development. Despite the fact that noncatalytic hydrothermal conversion of biomass has been a research subject of numerous studies, only a limited * To whom correspondence should be addressed. Tel.: +31 534894430. Fax: +31 534894738. E-mail: S.R.A.Kersten@utwente.nl. Ind. Eng. Chem. Res. 2009, 48, 4731–4743 4731 10.1021/ie801387v CCC: $40.75  2009 American Chemical Society Published on Web 04/17/2009