Study On The Direct Conversion Mechanism Of Methane To Nano-Carbon Black And Acetylene Via Thermal Plasma

In this paper the mechanism of methane pyrolysis into nano-carbon black and acetylene via thermal plasma has been studied theoretically and experimentally.The theoretical study includes two parts, one is the calculation of the components in the CH4 equilibrium systems with the Gibbs free energy minimization method at homogeneous region and equilibrium constant method at heterogeneous region between 1000-6000K, another is in terms of the concentrations of 5 stable substances of CH4,C2H4, C2H2, H2 and C varying with reaction times and temperatures , which have been investigated by simple kinetic mode and simulated by Terona Method at different pressure. The thermodynamic calculations show that methane almost can be completely decomposed over the temperature of 1200K,the maximal concentration of acetylene is 10.4% at 3750K . when C2H quenched reacts with H and converts into C2H2 completely the concentration of acetylene may reach to 21.3%,and its yield is 85.2% based on carbon. When temperature T >4500K the concentration and yield of carbon vapor are 19.7% and 98.5% respectively as well as the concentration of acetylene is 0.5%. When T < 2500K carbon black forms by aromatic intermediates; When temperatures range from 2500K to 4200K C2H, C2H2 and H2 are main productions and carbon black forms by C2H and C2H2 precursors; C and H radicals are the only productions when temperature T >4200K and carbon black forms by carbon free radical intermediates. The kinetics calculations prove that the pyrolysis of methane ,the synthesis of carbon black and acetylene are very sensitive to reaction temperature. Methane is almost completely decomposed at the reaction time of 0.1 us and 3000K while it is 1.0ms at 2000K. When reaction time ranges from 0.1 to 1 .0ms acetylene is the main product and carbon black dominates when reaction time excels 10ms. Acetylene yield decreases and carbon black yield increases withthe pressure increasing at the same reaction time. The reaction rate of acetylene and carbon black are proportional to reaction temperature. The thermodynamic and kinetics calculations prove that higher acetylene and carbon black yields can be achieved simultaneously when reaction time and reaction temperature are controlled within5.us-10.0u/s and 3500-4500K respectively. The pyrolysis of natural gas in the DC nitrogen plasma arc at latm has been studied in a 200KW plasma generator. The effect of methane flow, input power and nitrogen flow on the production has been investigated. The results show that natural gas undergoes very fast and complex reactions with highly reactive plasma species and converts to C2 hardrocarbons and carbon black completely. The maximal conversion of methane is 97.4%, in the optimal conditions of input power 120KW and the ratio of N2 /CH4 1.7, the conversion of methane is 88.4% and the yields of carbon black and C2 are 42.1% and 42.2% respectively, and the specific energy is 10.8KWh/kg(C2H2+C). The carbon black with a narrow size distribution have a small average size of 38nm and good adsorption capacity with a DBF value of 1.40ml/g and better structure with the volatiles of 16.8% and a PH value of 3.6. The FT-IR spectra reveals that the plasma carbon black has a characteristic of surface structure and that there exists some aromatic C-C bonds and nitrogen-containing functional groups such as -CN as well as -CH, -OH, -COOH groups in the carbon black.A conclusion has been drawn that C-H system do not reach thermodynamic equilibrium in the present processing because of sever distort between the calculated results and experimental ones. The process of conversion of methane to nana-carbon black and acetylene via thermal Plasma is under the control of mixing and diffusion rate.Finally, the analysis of economy and technology proves that the pyrolysis of natural gas in the plasma, which is a prospective method, has the advancement of less investment, lower cost and so on.