Supercritical Water Oxidation Energy – SCWOe

For operation at temperatures above the ignition, Supercritical Water Oxidation (SCWO) with hydrothermal flame as internal heat source allows to use air or oxygen and the faster kinetics minimizes the reactor volume. The operation under hydrothermal flames allows total oxidation of the waste within milliseconds residence times, which opens the possibility of developing small combustors to produce high-pressure gas/vapor streams. The application of hydrothermal flames opens a wide field for the production of energy from wastes [1], biomass [2] or low grade fuels. The cooled wall reactor developed at University of Valladolid is the only reactor prototype currently in operation with hydrothermal flame as internal heat source that produces a reduced liquid effluent with dissolved solids and a high-pressure and high-temperature effluent at 600-650 ºC and 23 MPa, that is able to produce work and thermal energy in a more efficient way that the below ignition tubular reactors effluent [3].

One of the principal challenges in the SCWO field is the improvement of the energy balance in order to make processes energetically advantageous. Enthalpy content of SCWO reactors effluents is often higher than this of the feeds, including compressions and preheatings, so energy balance improvement necessarily involves power and heat recovery from effluents [4]. Efficient thermal and shaftwork energy recovery would open the opportunity to use SCWO as efficient and clean energy production (SCWOe) processes from wastes, biomass or low grade fuels.

Most of the practical development has followed a concept line based on recovering the heat released by effluents in order to generating steam. Reactor effluent energy can then be recovered by a Closed Rankine Cycle using a conventional steam turbine, but the indirect heat transfer to the working fluid penalizes this process, that remains still highly energy demanding [5], mainly due to the process being unable to recover the pressure energy of effluents. On the other hand, direct expansion of the effluents, which would recover the pressure energy, entails costly development of specific, efficient expansion equipment that has not been tackled.

yoana_diagramaAs a workaround, the proposal of our research group [6] has been the injection of the top SCWO reactor effluent into the combustor or the expanding path of commercial gas turbines (GT) serving heat and power to a main, large facility where the SCWO reactor resides as a process unit, even though a smaller, dedicated GT can be considered. Enthalpy from the SCWO effluent would be recovered by joining it to the GT flue gases at the maximum possible pressure and temperature and following the same power recovery path through the expanding section of the GT and heat recovery path after the GT flue gases outlet. This way com- mercial, ready available, efficient equipment could be employed, avoiding the costs of the grass-root design of a custom turbine. Energy recovery from the effluent would take place within the GT flue gases stream. Heat and power from the GT offer also energy integration opportunities to the SCWO unit, allowing this unit to use in situ generated, less costly energy flows for preheating and compression. Careful balancing of fuels and loads allows to consider the SCWO reactor/GT as a net, clean energy producing system.

References

[1] J.P.S. Queiroz, On the development of computational tools for the modeling and simulation of SCWO process intensified by hydrothermal flames., PhD dissertation, University of Valladolid, 2014.

[2] M.D. Bermejo, Á. Martín, J. Queiroz, P. Cabeza, F. Mato, M.J. Cocero, Supercritical Water Oxidation (SCWO) of Solid, Liquid and Gaseous Fuels for Energy Generation, in: Z. Fang, C. Xu (Eds.) Near-critical and Supercritical Water and Their Applications for Biorefineries, Springer Netherlands, 2014, pp. 401-426.

[3] P. Cabeza, Studies in the development of supercritical water oxidation vessel reactors with hydrothermal flame as an internal heat source., in, University of Valladolid, 2012.

[4] J.P.S. Queiroz, M.D. Bermejo, F. Mato, M.J. Cocero, Supercritical water oxidation with hydrothermal flame as internal heat source: Efficient and clean energy production from waste, The Journal of Supercritical Fluids, 96 (2015) 103-113.

[5] E.D. Lavric, H. Weyten, J. De Ruyck, V. Pleşu, V. Lavric, Delocalized organic pollutant destruction through a self-sustaining supercritical water oxidation process, Energy Convers. Manage., 46 (2005) 1345-1364.

[6] Yoana García-Rodríguez, Fidel A. Mato , Alexandra Martín, M. Dolores Bermejo, M. José Cocero, Energy recovery from effluents of supercritical water oxidation reactors, J. of Supercritical Fluids, 104 (2015) 1–9

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