Production of hydrogen and carbon black by Methane decomposition using DC-RF hybrid thermal plasmas

Recently, thermal plasma technology has been getting popular among energy and materials industries in the conversion of hydrocarbons into hydrogen (H ₂) and carbon black (CB) since its overall process with DC and/or RF plasma torches has turned out to be very clean without carbon dioxide emission in opposition to existing processes. In our experimental work, H₂ and CB are produced from decomposition of methane (CH₄) by using DC-RF hybrid thermal plasmas. The DC-RF hybrid plasma offers a larger volume of hot core region and lower velocity of thermal plasma flow compare to DC plasma jets. Consequently, it provides a longer residence time for the reactant gas flowing along the high temperature region with relatively uniform plasma fields across the reaction chamber. In addition, an easy ignition and stable operation of the RF torch at atmospheric pressure condition are possible due to high enthalpy fluxes supplied from the combined DC plasma jet. In order to find an effective synthesis process for producing pure H₂ and high quality CB by this thermal plasma method, appropriate operation conditions and reactor geometries are firstly predicted from theoretical considerations and numerical simulations. Plasma temperatures and equilibrium compositions of CH ₄/Ar mixture are obtained using thermodynamic data and by Gibbs free energy minimization method, respectively. The temperature and velocity distributions inside the reactor are also computed to get reactor design data and examine the heat balance over the reaction chamber. Lastly, one-dimensional gas phase kinetic simulations on the methane decomposition process are performed to estimate major species and their mole fraction by considering detailed reaction mechanism. For the synthesis experiments on continuous production of H₂ and CB, a stainless steel reactor and a DC-RF hybrid torch are fabricated, and all experiments are conducted under an atmospheric pressure environment with the reactor pre-heated for 20 minutes by applying a 3-MHz RF plate power of 30 kW and a DC power of 7.6 kW. Once the steady conditions are established, CH₄ gas is radially injected into the hybrid plasma jet. During its decomposition process, off-gas compositions are continuously monitored by a quadruple mass spectrometer and the temperature variations inside the reactor are recorded from thermocouples. The injected methane is converted mostly into H₂ with a small volume fraction of C₂H₂, and the fine carbon particles of 50-200 nm are identified from their TEM images, The detailed results from the CB property test will be presented at the conference.