(17d) Non-Thermal Plasma Assisted Decomposition of Light Hydrocarbons to Hydrogen-Rich Gas And Carbon

Single-step decomposition (or pyrolysis, cracking) of light hydrocarbon fuels, such as natural gas (NG) and liquefied petroleum gas (LPG), to hydrogen and carbon is an attractive CO/CO2-free alternative to steam methane reforming (SMR) [1]. The process is moderately endothermic, e.g., in case of methane, the energy input requirement per mole of hydrogen produced (37.8 kJ/mole H2) is considerably less than that for the SMR process (63.3 kJ/mole H2). Furthermore, the process produces a valuable byproduct-clean carbon. However, a major problem with the practical realization of the NG pyrolysis process is that methane is one of the most thermally stable organic molecules, thus, thermal methane decomposition requires very high temperatures (>1200oC). Recently, Kværner Co. (Norway) has developed process for thermal plasma-assisted pyrolysis of NG to hydrogen and carbon black [2]; however, the process is very energy intensive, because electrical energy is used to heat the feedstock to very high temperatures (>5000oC).

In this work, non-thermal plasma is utilized to decompose pure methane, pipeline NG and commercial propane to H2 and carbon at near ambient conditions. In contrast to thermal plasma, non-thermal (or non-equilibrium) plasma is characterized by very high electron temperatures, but low bulk gas temperatures (as a result, the feedstock is not heated to high temperature, and the energy consumption is relatively low). Electronically-excited methane molecules CH4* play a major role in non-thermal plasma assisted conversion of methane to hydrogen and carbon:

(e >10 eV) + CH4 => (CH4)* => ?CH3, ?CH2, ?CH, H? => H2, C

Non-thermal plasma reactors with stationary and moving electrodes for continuous decomposition of light hydrocarbons were designed, fabricated and tested in this study. Different electrode materials (e.g., graphite, stainless steel, Ni, Ni-Cu, Fe) were used in the reactor, and their effect on the reactor performance was evaluated. The main products of plasma-assisted NG decomposition were H2 and carbon, along with smaller amounts of C2+ hydrocarbons. It was found that the distribution of the reaction products was mostly affected by the residence time in the plasma reactor. During plasma decomposition of NG, H2 concentration in the effluent gas reached 40-50 vol.% (the balance: mostly CH4 and C2+ hydrocarbons). The carbon product was characterized by SEM, TEM, XRD, Raman, FTIR methods, and it was shown to be amorphous carbon. Advantageously, the process is not affected by sulfur impurities, which would allow utilizing commercial sulfurous hydrocarbon feedstocks without an expensive pre-treatment stage. After a gas purification stage, the hydrogen-rich gas can be used as a fuel in a PEM fuel cell.

References

[1] N.Muradov, Int. J. Hydrogen Energy 18 (1993) 211.

[2] S. Lynum et al. Proc. 12th World Hydrogen Energy Conf., Buenos Aires, Argentina, 1998, p.637.