(555g) Nondimensional Specific Energy Index for Methane Pyrolysis in Thermal Plasma

Acetylene is the raw material for several important chemical products including polyvinyl chloride (PVC), vinyl acetate, 1,4-Butylene glycol (BDO) and so on. Partial oxidation of natural gas is the main access to acetylene in Europe and America, yet the introduction of oxygen reduces the carbon selectivity to hydrocarbons. Methane pyrolysis in thermal plasma is an efficient way to produce acetylene. In this process, ultra-high temperature in reductive plasma allows for more preferred reactor performance such as methane conversion at above 90% and selectivity to acetylene at above 70%. A large amount of researches have been done on methane pyrolysis on various lab-scale plasma devices. However, due to the diversity of experimental conditions, it is hard to summarize the conclusions from different researches.

In this work, we introduced a nondimensional specific energy index, defined as the ratio of supplied power and enthalpy flow needed for plasma weighted with inert gas and methane content, to better evaluate the influences of supplied power and methane content in feed on the methane pyrolysis. This nondimensional value stands for the excess of energy needed for plasma and includes both the influences of energy provided for per unit of gas and methane content. It is worth noted that compared with enthalpy change of gas in plasma, reaction heat of methane to acetylene is negligible. In an ideal process, the index value should be 1, which means that the supplied power is precisely sufficient to maintain the plasma and the reaction. A series of experiments in a 2 kW DC plasma reactor were carried out to testify the applicability of the index. The experimental results showed that the conversion of methane increased from around 10% when the index was 0.5~0.6, and reached 90% when the index was 1.0~1.2, then slowly increased to a maximum around 95%~97% when the index kept increasing. This means that this nondimensional index positively correlates to the conversion and implies the energy excess state of the system as expected.