Abstract:The supercritical water gasification process is an alternative to both conventional gasification as well as anaerobic digestion as it does not require drying and the process takes place at much shorter residence times; a few minutes at most. The drastic changes in the thermo-physical properties of water from the liquid state to the supercritical state make it a promising technology for the efficient conversion of wet biomass into a product gas that after upgrading can be used as substitute natural gas. The earliest research goes back as far as the 1970s and since then, supercritical water has been the subject of many research works in the field of thermochemical conversion of wet biomass. This article reviews the state of the art of the supercritical water gasification technology starting from the thermophysical properties of water and the chemistry of reactions to the process challenges of such a biomass based supercritical water gasification process plant.
The stability over time of elemental mercury, methylmercury and inorganic mercury species was evaluated in heptane, toluene and mixed hydrocarbon solutions. Elemental mercury and inorganic mercury(ii) were determined using a specific extraction method followed by ICP-MS or CVAAS. Methylmercury and mercury(ii) were determined by GC-MIP-AES after derivatisation with Grignard reagent. The results show that significant losses of mercury species from solution can occur by two pathways: by adsorption on the container wall and by reactions forming mercury(i) compounds. For the latter pathway, rapid losses of dissolved elemental mercury and mercury(ii) chloride species occur when both are present in solution. For heptane solutions containing HgCl 2 , 80% of the HgCl 2 remains after 13 d in a pure standard compared with 11% in a standard containing Hg 0 . Mercury(i) compounds form a colloidal material, which is not soluble in these organic solvents at a detectable concentration. Mercury(i) compounds were butylated with Grignard reagent to form the organic mercury(i) compound (C 4 H 9 ) 2 Hg 2 that was measured specifically by GC-MIP-AES and GC-MS. This new compound was stable and appeared to precipitate from solution.
This
study aims to develop a model to predict the formation of
compounds during the supercritical water gasification of biomass with
a thermodynamic equilibrium approach. A Gibbs free energy minimization
routine has been developed using MatLab software to predict the equilibrium
state compounds of the system. Regarding all the calculations, pure
condensed phases (both liquids and solids), gases, and aqueous compounds
(both neutral and ionic species) have been taken into account in the
subcritical region, and in the supercritical region, aqueous hydrate
complexes have been taken into account instead of the aqueous compounds.
The model has been validated for subcritical and supercritical regions
separately. A case study for supercritical water gasification of a
microalgae feedstock sample has been performed. The effects of temperature,
pressure, and dry matter content on the phase behavior of elements
and main product gases have also been investigated. The results of
this work show that the model developed on the basis of Gibbs free
energy minimization involving multiphase compounds can be used to
predict the formation of equilibrium state compounds during supercritical
water gasification of biomass.
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