HYDROGEN PRODUCTION VIA METHANE REFORMING USING VARIOUS MICROWAVE PLASMA SOURCES

Methane or natural gas reforming is widely used in industry to obtain hydrogen or synthesis gas (H2+CO), which are utilized in industry, for example as source materials for the production of raw chemicals (e.g. methanol and ammonia), as well as hydrogenation agents in oil refinery and reducing gases in steel industry. Recently hydrogen gains in importance as fuel in fuel cell applications, combustion engines or gas turbines with the goal to achieve more efficient exploitation of energy sources and to reduce noxious emissions. The conventional reforming of methane is carried out thermally using water steam and oxygen. In this process, oxidation of methane provides heat that is necessary for performing the endothermic methane reforming reaction using steam. The steam methane reforming yields a mixture of hydrogen and carbon monoxide, as follows: CH4 + H2O ↔ CO + 3 H2, ∆H298 = 206 kJ mol (1) where ∆H298 is a standard reaction enthalpy. Similar process, named the partial oxidation, occurs when carbon dioxide (CO2) is used instead of water steam: CH4 + CO2 ↔ 2 CO + 2 H2, ∆H298 = 247 kJ mol (2) Hydrogen may be also produced in the process of methane pyrolysis, which is, however, often accompanied with acetylene production: CH4 ↔ C + 2 H2, ∆H298 = 75 kJ mol (3) 2 CH4 → C2H2 + 3 H2 (4) Since reactions (1) and (2) are highly endothermic, to decrease the activation energy, a catalyst (usually Ni/Al2O3 working at temperature 1100−1150 K) is used. This methane reforming method is sensible to impurities in substrates which deactivate the catalyst. The other conventional technologies of hydrogen production, i.e. coal gasification, hydrocarbon reforming and water electrolysis, are too expensive or not applicable for specific applications (e.g. for fuel cells) due to technical reasons. Thus, new methods for hydrogen production are under development, such as water photolysis, and biological and plasma methods. One of attracting methods for reforming hydrocarbons to produce hydrogen is the use of plasmas. The plasma contains reactive radicals, ions and high-energetic electrons. High reactivity shown by these species enhances the chemical reaction rates whereby expensive and impurity vulnerable atalysts can be avoided. This plasma capability as well as its high energy density ensure the compactness of the plasma system. Besides, the plasma system can be adapted for reforming various hydrocarbons, such as natural gas, gasoline, heavy oils and biofuels. Another advantage is the fast response time that can be achieved because the plasma is operated by electricity. When water steam is used as the plasma supporting gas, reductive and oxidative radicals such as H, OH, and O, are produced in the plasma, enabling the plasma to be effective for reforming different hydrocarbons. Recently developed microwave plasma sources (MPSs) operated at atmospheric pressure seem to have a high potential for hydrogen production via hydrocarbon reforming. The microwave plasma at atmospheric pressure is one of the plasma techniques providing the electron temperature of 4000−10 000 K, and the heavy particle temperature of 2000−6000 K (ref.). Such properties of microwave plasma provide proper conditions for methane reforming. In this paper results of investigations on the usefulness of several microwave plasma methods for hydrogen production via methane reforming are presented. The microwave plasma methods employed are: − steam reforming of methane using a low flow rate nozzle-type MPS; − reforming of methane using high flow rate coaxial-type MPSs (three versions A, B, and C) supplied through either a rectangular waveguide WR 284 or rectangular waveguide WR 430. In both high flow rate coaxial-type MPSs an additional swirl flow, N2 or CO2, was used; − reforming of methane using high flow rate cylinder-type MPS supplied through a rectangular waveguide WR 430. The only gas used in this MPS was methane delivered as a swirl.