Hydrogen Recovery by Methane Decomposition in a Microwave Plasma Reactor

In the Sabatier reactor, oxygen is recovered (as water) by hydrogenation of carbon dioxide. Half of the reacted hydrogen is contained within the product water, the other half forms methane (CH₄). To close the hydrogen loop, we are investigating methods for the efficient recovery of hydrogen from CH₄. This paper describes microwave plasma-based methods for the thermal decomposition (cracking) of methane to produce hydrogen, elemental carbon, and related carbonaceous substances. Two primary reactor configurations have been employed in this work: 1) a quartz tube vertically oriented within a section of rectangular waveguide, and 2) waveguide transmission through a quartz window into a cylindrical vacuum chamber based multimode cavity. Hydrogen recoveries of up to 98% have been obtained. Three primary mechanisms of methane decomposition have been identified: methane pyrolysis, methane oligomerization, and methane aromatization. In the multimode plasma chamber methane pyrolysis experiments, we have observed the formation of a highly dendritic form of elemental carbon which may be beneficial in the minimization of potential problems associated with carbon deposition. Alternatively, by optimization of the reactor to promote volatile oligomerization products, carbon formation is minimized.