Supercritical water oxidation (SCWO) consists of the homogeneous oxidation above the critical point of water (374ºC and 22.12 MPa) and it is based on the unique properties of supercritical water as a solvent for organic substances and for being completely miscible with gases including oxygen and carbon dioxide. Thus, oxidation of the organics in supercritical water occurs in a homogeneous single phase and proceeds rapidly without interfacial mass transfer limitations. So far, this technology has been mainly applied to waste destruction. It has been successfully tested for different types of substances, obtaining total removals in residence times of a few seconds. Under these conditions, organic oxidation is completed to CO2, H2O, N2 and other salts or oxides and without producing by-products as dioxins or NOx.
Despite its advantages the process is not having the expected industrial development due to the harsh operational conditions, corrosion and salt deposition problems. To overcome these technical difficulties, research focused on solving these problems is necessary.
When the temperature of the mixture is higher than the autoignition temperature, supercritical water oxidation proceeds in the form of flames called hydrothermal flames. In these conditions ignition temperatures are reduced being possible to have stable flames in the temperature range 500-700ºC, much lower than combustion temperature that are higher than 1000ºC.
Supercritical Water Oxidation at hydrothermal flame regime present evident advantages over flameless oxidation, that can contribute to solve the difficulties in the SCWO process industrialization.
1) Residence times lower than 1 scond, which make possible to build smaller reactors.
2) It is possible to inject cold feeds over the hydrothermal flames avoiding preheating problems.
3) Improvement in the energy production due to the higher reaction temperatures.
The flame is defined as the surface where combustion is produced. This surface separates the oxidant from the fuel in the case of diffusion or non-premixed flames, in which fuel is injected into the oxidant. In the case of premixed flames, that is that the fuel and oxidant are injected already mixed, the flame is the surface separating the reagents from the reaction products.
In the High Pressure Process of the University of Valladolid several reactor designs working with a premixed hydrothermal flame as a heat source have been developed. Using these reactors total destructions efficiencies higher than 99.9% were obtained in the SCWO of several organic compounds as well as ammonia.
In premixed flames, the flame front moves towards the reagents with a certain velocity. This velocity must be equal than to the fluid flow velocity in order to have a steady flame.. It has been estimated that the flame front velocity in hydrothermal flames varied between 0.01-0.1 m/s, being much lower than the flame front velocity in conventional combustion (0.4-3 m/s). For this reason, SCWO in flame regime is favoured using vessel reactors in which fluid velocity is much lower than in tubular reactors.
New cooling wall reactor working at hydrothermal flame regime (Patent ES2381345)
The reactor consist of a pressure vessel made of AISI 316 stainless steel able to stand a maximum pressure of 30 MPa and a maximum wall temperature of 400ºC, containing a reaction chamber made of Ni-alloy 625 where the temperature be as high as 700ºC.
Wastewater feed and air are previously pressurized are injected by the bottom of the reactor and conducted to the top of the reactor chamber by means of a tubular injector. At the outlet of the injector the hydrothermal flame is formed. Cooling water, previously pressurized is circulating between the pressure vessel and the reaction chamber introduced by the top of the reactor in order to cool down the vessel at a temperature lower than 400ºC. This cooling water is entering in the reaction chamber through its lower part and leaving the reactor by the bottom together with a fraction of the products. The rest of the products leave the reactor by another outlet situated in the top of the reactor chamber.
This reactor was successfully tested at pilot plant scale obtaining TOC removal over 99.99% with feed injection temperatures as low as room temperature. The performance of the reactor was tested with the oxidation of a recalcitrant compound such as ammonia and with real waste such as sludge. Removals higher than 99% of N-NH4+ were achieved in both effluents, working with temperatures lower than 700ºC.
The behaviour of the reactor working with feeds with a high concentration of salts was also tested. Feeds containing up to 2.5% wt Na2SO4 could be injected in the reactor without plugging problems and a TOC removal of 99.7% was achieved in these conditions. In this conditions, the reactor was proved to be able to deliver an upper effluent at 600ºC with less than 100 ppm of inorganic salts-
In addition CFD modeling of this reactor has been developed allowing to get the temperature profiles, zones of recirculation and residence times, giving the tools for improving the design and operation of the reactor.