On the development of computational tools for the modelling and simulation of SCWO process intensified by hydrothermal flames

Abstract

Hydrothermal flames used as internal heat source contribute to overcome many of the challenges present in supercritical water oxidation. The aim of this thesis is to improve the understanding of the hydrothermal flames in the supercritical water oxidation process, developing tools for their modeling and simulation. This knowledge is necessary for the advances on the utilization of SCWO as power generation process. In the chapter 1, the state of the art of the technology, focusing on energy production is reviewed. This revision makes evident the necessities of research in several fields related to the SCWO, besides technical solutions on pumping and pretreatment steps of real biomass feed. Modeling is essential when hydrothermal flames are present, since their behavior can not always be observed directly, and these models require better sub-models on kinetics, turbulence, equations of state and transport properties. Finally, expansion devices appropriate to streams nature and conditions, and efficient enough should be developed and engineered. In chapter 2, the property estimation methods used in this thesis are presented. A simple cubic equation of state shows good accuracy and fast calculation for thermodynamic properties of supercritical mixtures. Densities are calculated with Peng-Robinson EoS with the modification of volume translation, while enthalpies are calculated with the original Peng-Robinson EoS. Transport properties could be predicted with reasonable accuracy at high temperature. However, over the low temperature range (at high densities), the methods have show poor results and mass-weighted mixing laws of tabulated values of pure species are used. In the third part of the thesis a study of the kinetic description of SCWO at hydrothermal flame regime is made. In chapter 3, a method for kinetic determination using easily available experimental data is presented. The method consists of adjusting kinetic parameters in order to match the temperature profiles obtained experimentally. Using this method, a new global reaction rate for the oxidation of isopropyl alcohol in hydrothermal regime is fitted: r = k0*exp(-Ea/RT)*C_IPA*C_O2, with k0 = (9.308±3.989)·10^7(m3*s-1*kmol-1) and Ea = 89.441±2.457(kJ*mol-1). The least square error of the fitting is 10.8%. This kinetic model is applied in a parametric analysis of flame formation, and it is used to analyze the behavior of a supercritical water oxidation vessel reactor. The kinetic model is able to describe the behavior of the vessel reactor when working in steady state hydrothermal flame regime at subcritical injection temperatures. The model predicts both flameless and hydrothermal flame regimes. In chapter 4 the interactions of turbulence with the reactive flow are studied with a turbulent combustion method based on quantifying turbulence fluctuations through probabilities. It has been found that the inclusion of these turbulent fluctuations, even though affects the flame structure, has no influence on final efficiency on waste elimination or energy generation. This can be explained by the usual oversized volume of reactors, which gives residence times much higher than reaction times. In chapter 5 the influence of the internal configuration of vessel reactors for the SCWO process is evaluated by simulation and compared to experimental data. The CFD-model developed provides a good prediction of the experimental results (deviations of 14% for temperature predictions) and can be used for designing reactors working under hydrothermal flame looking at performance and flame stabilization. Residence time distribution curves are obtained providing additional information about non-ideal behavior of the reactor. The methodologies developed in this work are applied to a new reactor configuration especially designed for energy generation in chapter 6. The experimental results obtained are briefly described and model is applied to study the internal behavior of the reactor and how it is affected by the existence of two outlets. Finally the energetic integration is studied, and it is shown that heat integration, generation of high pressure steam and generation of electricity by products expansion, are theoretically feasible in SCWO.