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Converting biomass into hydrogen can be accomplished in supercritical water. State of the art experiments are thoroughly presented. Chars and tars formation may be the most significant technological problem. However, catalysis should be the solution to obtain higher yields of hydrogen and to decrease the amounts of chars and tars. As conversion yield reaches 98% with a proportion of hydrogen higher than 50% in the gaseous phase, the next step to consider in a successful transfer to the industrial scale is to evaluate the energy cost of the process. We made an evaluation of the energy efficiency of biomass gasification, basing our work on data of the literature. Our results show that the energy efficiency from thermodynamic calculation reaches 60% when considering hydrogen, carbon monoxide and methane as valuable species in the ideal case. Including energy recovery from the water at 280 bar and 740 °C, the overall energy yield reaches 90%. These calculations are overestimated because no heat losses are taken into account. However, this study shows that the key point of the process is energy recovery as the chemical reaction is endothermic and needs high temperature and a rather large ratio of water/biomass.

We developed a new apparatus dedicated to Supercritical Water Oxidation in order to overcome the two well-known problems of this process, which are corrosion and salt plugging. This reactor has both a double shell in titanium, which prevents corrosion, and a stirrer creating a turbulent flow, which prevents the sedimentation of inorganic compounds and enables better heat transfers. We first tested our reactor with easily oxidized effluents, i.e. pure organic compounds like PEG and dodecane. Then we oxidized two different kinds of effluent: one forming a corrosive phosphoric acid, and another one containing an inorganic salt in significant amount. We get conversion yields over 99.8% with organic and salt concentrations up to 6 and 4%, respectively, without production of chars or undesired gases like CO and CH4. © 2003 Elsevier B.V. All rights reserved.

Ion-pairing and dehydration phenomena occurring in ZnBr2 aqueous solutions from normal to critical T, P conditions were investigated by x-ray absorption spectroscopy. The respective influences of temperature, pressure, and concentration were studied. The evolution of the density of solute ions, probed by the height of the absorption edge, allowed us to get information on phase diagrams and salt precipitation. The average structural evolution deduced from extended x-ray absorption fine structure was related to the formation of complexes identified from x-ray absorption near edge structure analysis. Consequently, in noncritical conditions, an increase of temperature or concentration produces dehydration and ion-pairing, while a rise of pressure destroys the ion-pairs. In contrast, concentration and pressure have weaker effects on the local order in high P, T conditions. Moreover, ion pairing formation is found not to be specifically enhanced when the fluid is close to supercritical conditions as it also occ...

Local-order evolution around ions in aqueous solutions has been investigated between normal and hydrothermal conditions. The behavior of cations and anions in aqueous ZnBr2 solution were studied by performing x-ray absorption spectroscopy experiments at both Br and Zn edges. Extended x-ray absorption fine structure analyses are made on account of anharmonic treatment and multiple scattering contributions involving H atoms at the Br edge. The extended x-ray absorption fine structure results are coupled to x-ray absorption near-edge structure simulations in order to identify the complexes formed under normal and hydrothermal conditions. It appears that both Zn and Br ions are largely hydrated under normal conditions and that Zn–Br pairs are formed in hydrothermal conditions. This is related to an octahedral-to-tetrahedral evolution of the Zn local environment, the majority of Zn atoms being surrounded by water octahedra in normal conditions and by distorted tetrahedra involving Br and O atoms in hydrotherma...

International audience; The aim of this work is to validate experimentally the results of the Monte Carlo Recoil Energy Determination (MC-RED) nuclear physics code used to determine the deposited energy in a silicon volume taking into account the probabilistic approach of the physical phenomenon. A silicon sensor has been used to measure the deposited energy spectrum after an irradiation with a 14 MeV neutron source. Neutrons of 14 MeV were produced by the SAMES accelerator of the Valduc CEA research center. The experimental results were compared with the one obtained by the MC-RED code in the same silicon volume. To compare experiment and simulations, it requires that the complete neutron field be specified. We have to take into account the facility cell environment. Nuclear simulations were performed with the MCNP code to determine with accuracy the energy spectra of neutrons and gamma rays produced by this facility on the sensor. It is shown that the contribution of secondary neutrons is not negligible

X-ray absorption spectroscopy (XAS) experiments have been performed on 1.0 and 1.6 M ZnBr2 aqueous solutions under ambient to supercritical conditions. Both the X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) show striking evolutions, which indicate major structural changes, with a strong reduction in hydration and the increasing formation of ZnBr pairs. A quantitative analysis of the evolution of the Zn environment is presented that supports the existence of a transition from an octahedral to a tetrahedral environment.