(721c) Abatement of Greenhouse Gases Into Methanol in a Ntp-DBD Reactor - Effects of Experimental Parameters

Abatement of Greenhouse
gases into Methanol in a NTP-DBD reactor

-
Effects of Experimental
Parameters

Abstract:
Direct conversion of natural gas to liquid products such as methanol
and formaldehyde has been receiving a great attention [1, 2].
However, converting methane (CH₄)
to methanol (CH₃OH)
with desired selectivity is difficult as in general, the oxidation
proceeds to complete to carbon oxides (CO or CO₂)
and water in conventional methods [2]. On the other hand, utilization
of nitrous oxide (N₂O),
as a co-reactant for CH₄
conversion appears to be very promising. Methane conversion by N₂O
may also beneficial due to potential green-house nature of these two
reactants (The GWP for CH₄,
N₂O
is 21 and 280 times more heat-trapping than that of CO₂).
In recent years, industrial applications of non-thermal plasma (NTP)
generated under ambient conditions has been receiving a great
interest [3-5]. NTP generated by electrical discharges under ambient
conditions has specific advantages like mild operating conditions, as
NTP is composed of energetic electrons that may initiate the
reaction, ions, excited atoms, radicals and meta stable ions. These
energetic electrons initiate the chemical reactions in gas phase.
With this back ground, the present study is aimed at co-processing of
two green house gases CH₄
and N₂O
for the production of oxygenated products like methanol and
formaldehyde.

In�uence of feed
gases proportion on oxidation of methane (CH₄)
with
nitrous oxide (N₂O)
has
been studied in dielectric barrier discharge (DBD) plasma operated
under ambient conditions (Fig.1). Plasma
reactor was powered by a high voltage AC generator (Yaskawa varispeed
F7 AC inverter upto 40 kV and 50 Hz). The
gas mixture was controlled by a set of mass �ow controllers
(Aalborg-USA).
CH₄
(10
vol% diluted in Ar) and N₂O
(5
vol% diluted in Ar) gases were introduced into the reactor.
At the reactor outlet gas
analysis was made with an online gas chromatograph (Varian 450-GC),
whereas
an on-line GC-MS (Thermo Fischer) was used to identify the
by-products. Typical
results indicated the formation of methanol (CH₃OH),
formaldehyde (HCHO), hydrogen (H₂),
carbon monoxide (CO) and carbon dioxide (CO₂).

The important
findings of the present study may be summarized as

1. NTP is effective in direct decomposition of N₂O into N₂ and O that will be alter converted to O₂ in the absence of a catalyst.

2. Increasing CH₄/N₂O molar ratio in the feed gases favors the reagents conversion and consequently promotes the formation of each by product.

3. Co-processing CH₄ and N₂O produces CH₃OH and HCHO, however, the selectivity to these products is rather low.

4. Water vapor has a positive effect on selectivity.

Figure 1. Schematic
representation of NTP-DBD reactor for oxidation
of methane (CH₄)
with
nitrous oxide (N₂O)

References:

1. [1] L.B. Han, S.
Tsubota, T. Kobayashi, M. Haruta, Formation of methanol by the
gas-phase partial oxidation of methane under normal pressures, J.
Chem. Soc. Chem. Commun. (1995) 93--94.

[2] L.M. Zhou, B.
Xue, U. Kogelschatz, B. Eliasson, Partial oxidation of methane to
methanol with oxygen or air in a non-equilibrium discharge plasma,
Plasma Chem. Plasma Process. 18 (3) (1998) 375--393.

[3] K. Okazaki, S.
Hirai, T. Nozaki, K. Ogawa, K. Hijikata, Plasma chemical reactions at
atmospheric pressure for high efficiency use of hydrocarbon fuels,
Energy 22 (2--3) (1997) 369--374.

[4] Sk.
Mahammadunnisa, E.L. Reddy, P. M. K. Reddy
and Ch. Subrahmanyam., A facile approach for direct decomposition of
nitrous oxide assisted by nonthermal plasma, Plasma Processes and
Polymers. (DOI: 10.1002/ppap.201200114)

[5] Sk.
Mahammadunnisa, P. M. K. Reddy, and Ch. Subrahmanyam., dry reforming
assisted non-thermal plasma catalytic reactor, ACS Energy & Fuels
(dx.doi.org/10.1021/ef302193e).