In situ time-resolved laser diagnostics for plasma methane reforming

Abstract

There is significant interest in utilizing plasmas for reforming methane, a powerful greenhouse gas into larger hydrocarbons and useful chemicals using non-equilibrium plasmas powered by renewable electricity. However, the key reaction pathways and the plasma dynamics are not well understood due to lack of time-resolved in situ measurements in the literature for model validation. This demands the application and development of advanced laser diagnostics to provide critical experimental data for creating a quantitative understanding of the physics and chemistry of plasma CH4 reforming. In this dissertation, several different time-resolved laser diagnostics including Thomson scattering, electric field induced second harmonic generation (E-FISH), and spatially-resolved one-dimensional (1-D) hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) imaging of rotation-vibration non-equilibrium were developed and applied to identify the key physics and chemistry of CH4-containing pulsed plasmas. First, a sensitive Thomson/Raman scattering diagnostic was built to measure the time evolution of the electron properties in a CH4/He volumetrically uniform nanosecond-pulsed dielectric barrier discharge (ns-DBD) as well as vibrational temperature in a CH4/N2/He ns-DBD. The Thomson scattering instrument was able to measure electron temperatures of 0.5 eV and electron densities as low as 1×10^12 cm^(-3) Second, time-resolved multi-species laser absorption measurements were conducted to study the chemistry of plasma CH4/CO2 reforming. Both experimental species and electron properties data were used to develop and validate a 1-D plasma kinetic model of plasma CH4/CO2 reforming. Next, the plasma dynamics in the ns-DBD was studied through time-resolved electric field and electron properties measurements using E-FISH and Thomson scattering. Lastly, new fs/ps CARS diagnostics were developed for spatially resolved 1-D imaging of rotation-vibration non-equilibrium as well as 1-D thermometry using CH4 as the probe molecule. Time-domain fs/ps CARS modelling of the CH4 ν1 Q-branch enabled quantitative temperature measurements using CH4. Time and spatially-resolved fs/ps CARS measurements near the cathode of a CH4/N2 pin to pin discharge showed localized regions of high CH4 conversion and N2 vibrational temperature. Both of these fs/ps CARS diagnostics demonstrate significant potential in characterization of non-equilibrium molecular energy transfer near reacting gas-surface interfaces.