Hydrogen production from methane using an RF plasma source in total nonambipolar flow

Abstract

A radio-frequency (RF) helicon plasma reaction chamber (HPRC) is developed and used to decompose methane gas into high-purity hydrogen gas and solid carbon in the form of graphite. A single-turn (m = 0) helicon antenna, operated at 13.56 MHz, and a 100 G dipole magnetic field are used to excite a helicon mode in a nonthermal plasma, creating plasma densities exceeding 1013 cm−3 using 8–20 SCCM methane gas at up to 1300 W of RF power. The HPRC device takes advantage of a uniform large amplitude electron sheath across the exit aperture. At this aperture, all of the incident electron flux from the plasma is extracted and all ions are reflected back into the source. In this way, only neutrals and electrons are allowed out of the reaction chamber, enhancing the breakdown of methane into deposited carbon and hydrogen gas that escapes. A methane decomposition percentage of 99.99 ± 0.06% is demonstrated using 1300 W of RF power and a methane gas flow rate of 8 SCCM. A total nonambipolar flow of particles maximizes the recirculation of ions, and leads to the very high degree of molecular decomposition achieved in this proof-of-concept device. The HPRC in its present proof-of-concept form requires 37× more energy per kg of H2 produced, compared with steam-methane reformation, though this energy comparison does not include the energy required to sequester the emitted CO2 during the steam–methane reformation cycle.