Μοντελοποίηση αντιδραστήρα ψυχρού πλάσματος για την απορρύπανση εδαφών

Contaminated sites due to expanding industrialization are known to be potential sources of severe hazards to human health. Recent progress in soil remediation processes includes a promising plasma technology that uses a Dielectric Barrier Discharge (DBD) reactor to produce highly reactive oxidized species. The present work models plasma inside the DBD reactor and evaluates the production of oxidized species used for pollutant degradation and, eventually, for soil remediation. This investigation covers the plasma phenomena that take place inside the plasma reactor, and extends to the underlying transport and reaction phenomena, over a wide range of characteristic times, namely, from nanoseconds up to minutes. More specifically, the plasma working conditions are examined inside several geometries. 1D and 2D axisymmetric models are developed to simulate the electrical discharge of DBD at atmospheric pressure. Numerous key parameters were examined for the electrical conditions inside the reactor, such as the dielectric permittivities, the thickness of the dielectric parts, the distances between electrodes, and the applied voltages. These test case scenarios are investigated with sophisticated numerical models. Initially, a novel dual-time hierarchical model is developed to link the plasma process to the energy transport phenomena in the interior of an energetic DBD reactor. The generation of active species by the plasma reactions is simulated at the microseconds (μs) timescale, whereas convection and thermal conduction are simulated at macroscopic (minutes) timescales. The Effective Medium Theory is employed to provide estimates of the effective temporally-evolving and three-phasic transport properties of the soil sample. Model predictions considering the temporal evolution of the plasma remediation process are presented and compared with corresponding experimental data. The hierarchical model is modified for low temperature DBD plasma, addressing key mechanisms of soil remediation with characteristic time scales ranging from nanoseconds to minutes. Using appropriate modification, the simulated microscopic DBD plasma processes are linked to the macroscopic remediation modeling through the local species concentrations. A complex, 100-reaction set is implemented in the plasma simulator for the calculation of the concentrations of highly reactive species produced in pertinent operational conditions. These species concentrations are used as source terms for the solution of the macroscopic problem. In addition, an effective mobility term was used to capture the structural effect of the porous soil medium on the plasma process. A wide range of flow rate scenarios is examined, showing that the air velocity influences the remediation process significantly. Another case study involves the application of several sandy soil materials with varying porosity and permeability values, v and assesses their impact on the pollutant degradation rate. Improved operational conditions for the DBD reactor are suggested for the particular case of atrazine degradation.