The process of CO2 dissociation was studied in a non-equilibrium gliding arc plasmatron (GAP). The GAP was designed not only for efficient reforming but also to ensure significant variability of reactor parameters. The effect of vortex flow configuration on efficiency was also studied in the reactor by comparing forward vortex flow and reverse vortex flow. The maximum thermodynamic efficiency of the dissociation process was determined to be approximately 43%. The high level of efficiency may be attributed to non-equilibrium vibrational excitation of CO2 and a high-temperature gradient between gliding arc and the surrounding gas that results in fast quenching.
Thermal plasma technology can be used in the production of hydrogen and
hydrogen-rich gases
from a variety of fuels. This paper describes experiments and
calculations of high-temperature
conversion of methane using homogeneous and heterogeneous processes.
The thermal plasma
is a highly energetic state of matter that is characterized by
extremely high temperatures (several
thousand degrees Celsius) and high degree of ionization. The high
temperatures accelerate the
reactions involved in the reforming process. Plasma reformers can
be operated with a broad
range of fuels, are very compact and are very light (because of high
power density), have fast
response time (fraction of a second), can be manufactured with minimal
cost (they use simple
metallic or carbon electrodes and simple power supplies), and have high
conversion efficiencies.
Hydrogen-rich gas (50−75% H2, with 25−50% CO for
steam reforming) can be efficiently made
in compact plasma reformers. Experiments have been carried out in
a small device (2−3 kW)
and without the use of efficient heat regeneration. For partial
oxidation it was determined that
the specific energy consumption in the plasma reforming processes is 40
MJ/kg H2 (without the
energy consumption reduction that can be obtained from heat
regeneration from an efficient
heat exchanger). Larger plasmatrons, better reactor thermal
insulation, efficient heat regeneration, and improved plasma catalysis could also play a major role in
specific energy consumption
reduction. With an appropriate heat exchanger to provide a high
degree of heat regeneration,
the projected specific energy consumption is expected to be ∼15−20
MJ/kg H2. In addition, a
system has been demonstrated for hydrogen production with low CO
content (∼2%) with power
densities of ∼10 kW (H2 HHV)/L of reactor, or ∼4
m3/h H2 per liter of reactor. Power
density
should increase further with power and improved
design.
The long history of plasma application for fuel conversion shows that reasonably low specific energy requirement has been achieved in most cases using non-equilibrium systems with relatively high local temperature (‘warm’ plasmas). Analysis of reasons for this trend presented in this paper indicates that transitional warm plasma discharge systems are optimal for large-scale fuel processing. This analysis also reveals one specific feature of warm discharges that was not discussed earlier: warm discharge-based plasma-chemical systems are very sensitive to gas temperature and chemical reactions. When temperature reaches the level that is high enough to support chemical reactions in a particular system (ignition temperature), chemical reactions produce high concentration of excited molecules, and these molecules form a basis for stepwise ionization. This results in a significant drop in the energy necessary to support electric discharge in the system for two reasons. First, stepwise ionization that requires relatively low electron energy overcomes direct ionization that is typical for low-temperature non-equilibrium plasmas and requires much higher ionization energy. Second, high temperature of surrounding gas reduces heat losses from the discharge channel, while a significant portion of the discharge energy in warm plasma systems should be spent to compensate these losses. Thus, an intensive chemical reaction, e.g. combustion, supports the existence of a warm electric discharge.
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