Plasma pyrolysis offers several advantages over traditional catalytic pyrolysis.
Plasma pyrolysis of methane has been investigated for utility as a process for producing hydrogen. This process was conceived as a means of recovering hydrogen from methane produced as a byproduct of operation of a life-support system aboard a spacecraft. On Earth, this process, when fully developed, could be a means of producing hydrogen (for use as a fuel) from methane in natural gas.
The most closely related prior competing process — catalytic pyrolysis of methane — has several disadvantages:
- The reactor used in the process is highly susceptible to fouling and deactivation of the catalyst by carbon deposits, necessitating frequent regeneration or replacement of the catalyst.
- The reactor is highly susceptible to plugging by deposition of carbon within fixed beds, with consequent channeling of flow, high pressure drops, and severe limitations on mass transfer, all contributing to reductions in reactor efficiency.
- Reaction rates are intrinsically low.
- The energy demand of the process is high.
In contrast, because the plasma pyrolysis process does not involve either a catalyst or fixed beds, it is inherently amenable to long-term, continuous operation without fouling of a catalyst or plugging of beds. Also, because this process involves only minimal heating of non-reactive components, it offers potential advantages of operation at relatively low power with high energy efficiency, plus enhanced safety (because of lower power levels and fewer hot surfaces).
The apparatus used in the investigation includes a 0.75-in. (1.9-cm)-diameter quartz reactor tube that contains the methane feed gas. Part of the length of the reactor tube lies in a horizontal WR-284 rectangular waveguide (see figure), through which microwave power is supplied to excite the methane to the plasma state. The reactor tube is inserted vertically through the middle of the horizontal waveguide via vertical metal tubes, attached to the waveguide, that serve as microwave chokes. Apertures defined by two additional, horizontally oriented microwave chokes enable direct visual observation of the plasma. The microwave system delivers variable microwave power levels, monitors delivered and reflected power, and enables matching of microwave-source, waveguide, and load impedances (the main load being the plasma) for maximum power-transmission efficiency. The microwave source is a water-cooled magnetron that operates at the fixed frequency of 2.45 GHz and can deliver between 100 and 1,200 Watts of power under manual or computer control. Reflected microwave power not absorbed by the plasma is absorbed by a circulating-water load.
In operation, a process gas that consists of or includes methane is fed into the reactor through a mass flow controller that has a range from 0 to 1,000 standard cubic centimeters per minute. The microwave plasma is created and confined within the portion of the quartz tube that lies inside the waveguide. Early experiments were conducted using a 1:9 methane:argon feed-gas mixture, followed by experiments using pure methane. Operating conditions were identified under which methane-to-hydrogen conversion efficiencies approached 100 percent. Long-term tests were conducted to demonstrate continuous production of hydrogen without loss of reactor efficiency. Chemical analyses of the reaction products revealed that generation of hydrogen through decomposition of methane is accompanied by a combination of cracking, oligomerization, and aromatization reactions, which tend to minimize the formation of elemental carbon. Further research is planned to refine understanding of these reactions and to determine whether and how they might be exploited.