Investigation of Partial Oxidation of Methane in a Cold Plasma Reactor with Detailed Product Analysis

Abstract

Cold plasma is a partially ionized state of matter that unites high reactivity and mild conditions. Therefore, cold plasma reactors are intriguing for reaction engineering. In this work, a laboratory scale dielectric barrier discharge (DBD) cold plasma reactor was designed, set up, and used for studying the influence of the specific energy input (SEI) on the product spectrum of the partial oxidation of methane. In total, 23 experiments were carried out near ambient conditions with a molar reactant ratio of methane to oxygen of 2:1 at SEI between 0.3 and 6.0 J cm−3. The feed also contained argon at a mole fraction of 0.75 mol mol−1. The product stream was split into a fraction that was condensed in a cold trap and the remaining gaseous fraction. The latter was analyzed at-line in a gas chromatograph equipped with a dual column and two carrier gases. The condensed fraction was analyzed by qualitative and quantitative 1H and 13C NMR spectroscopy, Karl Fischer titration, and sodium sulfite titration. In the product stream, 16 components were identified and quantified: acetic acid, acetone, carbon dioxide, carbon monoxide, ethanol, ethane, ethene, ethylene glycol, formaldehyde, formic acid, hydrogen, methanol, methyl acetate, methyl hydroperoxide, methyl formate, and water. A univariant influence of the SEI on the conversions of methane and oxygen and the selectivities to the products was observed. The experimental results provided here are an asset for developing reaction kinetic models of the partial oxidation of methane in DBD plasma reactors.

Conversions and selectivities in cold plasma partial oxidation of methane

“…The volumetric feed flow rate was calculated from the mass flow rates and the feed temperature and pressure using the ideal gas law. The relative uncertainties of the mole fractions of the feed gas components are 0.8% of the reported value, as in Müller et al [1] Further information on the uncertainties is provided in the Supporting Information. The residence time τ was calculated from the active reactor volume V of 26.3 cm³ and the volumetric feed flow rate V ˙in :…”

“…Both electrodes are connected to a high-voltage generator that yields a sinusoidal, high-frequency high-voltage and has a maximal effective output power of 700 W (averaged over the period). The effective electrical power P fed to the reactor is calculated from Equation (1).…”

“…where i is the instantaneous current, u the instantaneous voltage, t the time, and t the time of one period. The three feed gases, that is, CH 4 , O 2 , and Ar, were supplied from high-pressure cylinders and their flow rates were controlled individually by thermal mass flow controllers, which also recorded the inlet temperature of the feed gases, which was near ambient in a range from 291 to 298 K. The suppliers and purities were the same as in Müller et al [1] The reactor was thermostatted by cooling water with a temperature of 291 K flowing through a jacket and operated at ambient pressure, which was in a…”

“…Figure 1). The condensable fraction was analyzed by wet-chemistry (Karl Fischer and sodium sulfite titration method) and NMR spectroscopy (quantitative 1 H NMR spectroscopy and quantitative 13 C inverse-gated 1 H-decoupled NMR spectroscopy). In total, the quantitative analysis was performed for 12 components in the condensable fraction and 8 components in the gaseous fraction as listed in Table 1.…”

“…In a recent study, [1] we have therefore set up a DBD cold plasma reactor and have applied it to systematically study the partial oxidation of methane. The results of 23 experiments were presented that were all carried out at a molar reactant ratio CH 4 :O 2 in the feed of 2:1 and an argon mole fraction in the feed of 0.75 mol mol −1 , varying only the residence time τ and the electrical power input P. The following 16 product components were quantified: acetic acid (HAc), acetone (Ace), carbon dioxide (CO 2 ), carbon monoxide (CO), ethane (C 2 H 6 ), ethene (C 2 H 4 ), ethanol (EtOH), ethylene glycol (EG), formaldehyde (FA), formic acid (FAc), hydrogen (H 2 ), methyl acetate (MeAc), methyl formate (MeFo), methanol (MeOH), methyl hydroperoxide (MeOOH), and water (H 2 O).…”