Energy distribution and heat transfer mechanisms in atmospheric pressure non-equilibrium plasmas were investigated extensively through energy balance analysis, emission spectroscopy of the rotational band of CH (A2Δ→X2Π), and gas chromatographic analysis. Two plasma sources were examined: methane-fed dielectric barrier discharge (DBD) and atmospheric pressure glow-discharge (APG). The DBD features filamentary microdischarges accompanied by surface discharge along a dielectric barrier. As a result, 60% of the input power was measured as heat transfer to the dielectric electrode, whereas 20% was to the metallic electrode. Consequently, feed gas average temperature was increased only by 20-40 K. On the other hand, rotational temperature of the corresponding emission region exceeded average gas temperature by 100 K. In APG, heat transfer to electrodes was dominated by formation of negative glow regardless of whether the electrode was covered by a dielectric. However, negative glow tended to be thinner and more intense when it formed on a metallic electrode, leading to slightly higher metallic heating. Rotational temperature in APG was close to average gas temperature since APG does not show radial localization of plasma. Energy efficiency for methane decomposition process to produce ethane, ethylene, and hydrogen was about 1% regardless of the plasma source. Energy distribution and heat transfer mechanisms depend strongly on the plasma spatial structure rather than flow fields or feed gas physical properties.

The thermal structure of a methane-fed dielectric barrier discharge (DBD) and a atmospheric pressure glow-discharge (APG) has been extensively investigated in terms of time-averaged gas temperature profile between two parallel-plate electrodes separated by 1.0 mm. Emission spectroscopy of the rotational band of CH ((0, 0) A 2 → X 2 : 431 nm) was performed for this purpose. In order to minimize average temperature increase in the reaction field, DBD and APG were activated by 10 kHz with 2% duty cycle pulsed voltage (2 µs pulse width/100 µs interval). In DBD, temperature increase of a single microdischarge, on a time average, reached 200 K. It suddenly decreased below 100 K associated with the dark space formation near the dielectric barrier. Also, gas temperature in the surface discharge was fairly low because emission in these regions was limited within the initial stages of propagation (∼5 ns), whereas energy deposition would continue until microdischarge extinction; these facts implied that rotational temperature seemed to be far below the actual gas temperature in these regions. In APG, gas temperature was uniformly increased by positive column formation. In addition, a remarkable temperature increase due to negative glow formation was obtained only near the metallic electrode.For practical interest, we also investigated the net temperature increase with high frequency operations (AC-80 kHz), which depends not only on plasma properties, but also various engineering factors such as flow field, external cooling conditions, and total input power. In DBD, gas temperature in the middle of gas gap was significantly increased with increasing input power because of poor cooling conditions. In APG, in contrast, gas temperature near the electrodes was significantly increased associated with negative glow formation.

The gas temperature of reactive microdischarges has been extensively investigated in a methane-fed dielectric barrier discharge (DBD) configured by a two parallel plate reactor with 0.5 mm gap spacing. Emission spectroscopy of the rotational bands of CH(2Δ→2Π) coupled with heat transfer experiments have been employed for this purpose. Stationary and space-averaged gas temperature between the discharge gap was estimated from the heat transfer experiment; thus heat capacity and enthalpy gained by the feed gas stream resulted in a ≈20 K temperature increase. The rotational temperature showed fairly good sensitivity to the inlet gas temperature variation in the range 370-670 K. However, the local gas temperature increase inside the microdischarges represented an additional 100 K above the average gas temperature, indicating one order of magnitude higher value than the theoretically expected gas temperature increase (5-10 K) for a single microdischarge. High-frequency operation (80 kHz) is responsible for memory effect, thus such a high gas temperature increase was the result of multiple microdischarges rather than a single one.