Non-oxidative conversion of methane in a DC plasma reactor.

Abstract

The interest in natural gas utilization has increased tremendously over the last decade. Natural gas is being looked at as an energy source, as well as a basis for the production of many chemicals. New technologies are needed for smaller scale, remote and niche applications to utilize natural gas resources that will not be suitable for large-scale production of distillate products via synthesis gas. Low temperature plasma reactors have been shown to be an effective method for the conversion of methane, and under some conditions have low power consumption. The advantage of low temperature processing is the reduction of the energy requirements and therefore the cost. Cold plasmas have energetic electrons that exist at high temperatures while the bulk gas temperature remains relatively low (∼ 65C). This allows for reactions that would not normally occur at these low temperatures, and removes the necessity to pre-heat feed streams.

The reduced electric field plays an important role in the conversion and selectivity of the system. The reduced electric field (E/P) is a function of the breakdown voltage, pressure, and gas gap. The lower the reduced field, the lower the average electron energy and the higher the fraction of energy going into methane excitation versus methane dissociation. The excitation of methane only requires around .2 eV, while the dissociation of methane requires near 10 eV. The excitation of methane via vibrational excitation is responsible for the high conversions and high selectivity toward acetylene. The excited molecules may proceed down a reaction path similar to a pyrolysis process resulting in acetylene production.

The dc plasma system utilizes a particle bed to assist in the stabilization of the discharge. Without the bed present, the breakdown voltage is much higher (10.5 kV versus 6.5 kV). In addition, without the bed present the discharge typically transforms from the stable streamer discharge to an arc discharge within 30 minutes. The material in the bed behaves similar to a dielectric barrier. A dielectric material may become polarized in an electric field, which reduces the overall electric field and in return the potential voltage difference. By reducing the voltage, the power input is reduced which lowers the temperature within the reactor. This in return reduces the tendency to form carbon. Carbon deposition on the walls and in the reactor bed result in a smaller effective gap between the electrodes, which causes the streamer discharge to convert to an arc discharge.