THE INFLUENCE OF MICROWAVE PLASMA POWER ON THE DEGREE
OF NATURAL GAS CONVERSION TO CARBON NANOFORMS
Katarzyna L. Judaa, Aaron Claytonb, Bartek A. Glowackia,c
aDepartment
of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK
bGasPlas AS, Development Farm, Bluebell Road, Norwich, NR4 7AR, United Kingdom
cInstitute of Power Engineering, Augustówka 6, 02-981 Warszawa, Poland
1. Introduction
4. Carbon analysis
2. Processing parameters
Research was conducted on the GasPlas double vortex plasma reactor (2.45 GHz), where different flow ratios
of N2 to mains NG were subjected to power levels of 1.5, 2, 3, 4, 5, and 6 kW. Total incoming gas flow was kept
constant at 30 L/min N2+NG. Each experiment was run for
Tuning stubs
3-20 min. Two quartz tubes, with an internal diameter
of 17 mm, and lengths of 290 and 500 mm,
joined with a brass o-ring, were
connected to the plasma nozzle.
Products
Carbon was collected from the inner Sliding short
walls of the shorter quartz tube
Magnetron
directly connected to the plasma
Waveguide
nozzle. Analytical techniques, as
Plasma nozzle
described in the table below, were
NG + N2
employed.
Gas chromatography (GC)
BET surface area and porosity
High resolution transition
electron microscopy (HRTEM)
X-ray diffractometry (XRD)
Raman spectroscopy
High resolution
thermogravimetry (HRTGA)
Carbon collected from runs at different powers and NG:N2
flow ratio was analysed by XRD. All diffractograms were
almost the same as the one presented in the figure on the
right (for 1.5 kW, 3 L min-1 NG), indexed for graphite.
Diffractograms reveal:
· graphite-like structures;
· peaks slightly shifted in comparison to pattern;
· relative intensities not in good agreement with the
graphite reference pattern.
This means that the formed crystals lack 3D ordering,
possessing turbostatic character.
Philips X'Pert PW1820, Bragg Brentano parafocusing
geometry
Renishaw Ramascope-1000, 633 nm HeNe laser
TA Instruments TGA Q500, open platinum crucible,
flowing 40 mL/min air atmosphere
3. Gas analysis
The degree of conversion of hydrocarbons (CnH2n+2, where n є {1, 2}) at different power (P) levels and
different NG flows (Q) is presented in figure A) below. The maximum NG flow rate which could be used
without plasma instability increased with the power, and limited the range of flow rates which could be tested
at 1.5, 2 and 3 kW. The maximum tested NG flow rate of 21.5 L min-1 was limited by the mass flow controllers
used.
Taking into account that flow rate variations induce changes in the energy density (Ed) in the reacting system,
figure B) presents the degree of absolute CnH2n+2 conversion as a function of energy density. Ed is calculated
using the formula below, on the assumption that all the energy input goes into hydrocarbon cracking.
These findings are in agreement with HRTEM made
on the samples (on the right):
· Carbon from operating at 1.5 and 2 kW seems to be
almost entirely graphitic.
· At 3 kW, a greater complexity of structures
emerges. Graphitic sheets are still recognised, but
spheres indicating spherical nano-graphites or
amorphous carbon, as well as carbon nanotubes,
were noticed.
· At 4, 5, 6 kW, sharp ordered shapes are not in the
majority – carbon spheres appear in the
foreground.
112
104
110
102
004
103
010
500 nm
100 nm
B)
The main product is H2, however it is always accompanied by the production of small amounts of ethylene
and acetylene as presented in figure C). The rest of the output gas consists of N2, unreacted methane and
ethane (figure D), CO2 below 0.28%, CO below 0.8% and traces of O2. Higher hydrocarbons (C3 up) were not
detected.
1.5 kW, 3 L min-1 NG 100 nm
2 kW, 6 L min-1 NG
100 nm
5 kW, 21.5 L min-1 NG
3 kW, 6 L min-1 NG
Thermogravimetric analysis (on the left)
clearly reveals polymorphism of the
collected
samples.
Their
oxidation
temperatures are distributed widely
between
the
presented
standards:
amorphous carbon black and graphite (Alfa
Aesar). In some cases even five phases can
be distinguished. However, samples
collected at lower powers are closer to
graphite, and they have one dominant
phase. Samples collected at 4-6 kW have a
wide distribution of phases, of which some
portion is very close to carbon black.
Ed = P · Q-1
C)
100
Raman spectroscopy performed on the samples confirmed the XRD results. A typical spectrum (for 1.5 kW, 3
L min-1 NG) is presented in figure E, with specific features indicating the presence of graphitic sp2 planes:
· D-band at ~1330 cm-1: its intensity is related to the degree of disorder;
· G-band at ~1580 cm-1: its position and shape relate to the perfection of the graphene planes [7].
Following the procedure presented by Osipov et al. [7], the relative intensity of band D to G (ID/IG) versus full
width at half maximum (FWHM) of band G for carbon samples and three standards, are presented in figure F.
The FWHM of the G-band varies with both power and NG flow, but broadly seems to increase with increasing
MW plasma power. This trend suggests better crystallisation with lower MW plasma power.
Adopting the procedure of calculating particle size from Raman spectra [7], it was found that their average
size was in the range of 17-60 nm.
E)
F)
Perkin Elmer Clarus 580 with thermal conductivity
detector, Model Arnel 4017 analyser column set
Micromeritics Tristar II 3020; N2 adsorption at 77 K
200kV FEI Tecnai G220 Twin TEM
The volumetric composition of NG used in the experiments is presented in the graph above. CH4 and C2H6
constitute the main carbon and hydrogen sources. The total inlet and outlet flow rates of these two
components were used to calculate the degree of conversion. Amounts of O2 and CO2 are negligible, and
should not have any significant effect on hydrocarbon conversion.
A)
002
Natural gas (NG) is widely used in industry for hydrogen and synthesis gas production. The most common
processes employed for this production are steam methane reforming, hydrocarbon reforming, coal
gasification or water electrolysis [1]. These processes however have countless limitations, as they: require
specific catalysts which are susceptible to poisoning; are expensive for common applications; and usually
evolve the by-product CO2 which requires expensive sequestration. During the last years, reports on methane
reforming in plasmas, especially atmospheric pressure microwave (MW) plasma in oxygen-free conditions,
have been published [2-4], attracting the attention of industry. Another potential application of plasma
processing is the synthesis of highly ordered structures, such as carbon nanotubes, as presented in [5], [6].
This subject has attracted the attention of GasPlas AS, which developed its own microwave plasma reactor
working at atmospheric pressure. The GasPlas reactor system allows for efficient and continuous operation,
supplied by mains natural gas. Under the scope of GasPlas research, this cheap reactant is successfully
transformed into valuable gaseous products. This work reports on both gaseous and carbonaceous products
obtained from operating at different microwave powers in the range 1.5-6 kW, and different natural gas to
nitrogen ratios.
For the analysed set of samples, a clear correlation of
BET surface area measurements with energy density
(on the right) cannot be identified. However, the
surface area appears to vary systematically with MW
power. Lower processing power results in higher
surface area, reaching high values above 200 m2 g-1
for 1.5 and 2 kW. In all cases the pore sizes were in
the 7-17 nm range.
D)
5. Conclusions
·
·
·
·
·
·
From the data presented above it is clear that:
· The degree of conversion increases with increasing power and decreasing NG:N2 flow ratio.
· To get more than 90% conversion at lower MW power, the NG flow should not exceed 1 L min-1 at 2 kW, and
4 L min-1 at 3 kW (for 30 L min-1 total inlet flow rate).
· The proportion of H2 in the products, H2/(H2+C2H2+C2H4) by flow, has an average value of 0.85 and varies
little with energy density (standard deviation 0.014).
· However, the proportion of H2 in the products at constant power decreases almost linearly with increasing
NG flow.
· The degree of conversion is not directly correlated with energy density, but depends on both power and NG
flow. The apparently almost power-independent decrease in CH4 outlet flow rate with increasing energy
density (fig. D) arises largely from the increase in volume associated with H2 production.
Atmospheric pressure microwave plasma can be successfully applied for hydrogen and carbon generation
from non-purified natural gas.
A high conversion degree of natural gas to hydrogen, exceeding 90%, can be achieved.
An increase in the operating MW power and NG flow results in faster H2 and carbon generation.
Carbon obtained from plasma processing reveals a variety of morphologies: amorphous carbon, turbostatic
nanographite, spherical nanographite and carbon nanotubes, with the average size of 17-60 nm.
With increasing MW plasma power, a greater variety but lower selectivity of carbon nanoforms is obtained.
Carbon samples from higher operating power tend to have higher amorphicity and low BET surface area.
In order to achieve selectivity over the resulting carbon nanoforms, with the required physical properties,
the plasma processing parameters must be carefully controlled and also the application of a catalyst would
be required.
6. Acknowledgments
Katarzyna Juda acknowledges with gratitude the financial support of GasPlas AS.
7. References
[1] H. F. Abbas et al., International Journal of Hydrogen Energy, 35 (2010) 1160-1190.
[2] M. Jasinski et al., Journal of Physics D: Applied Physics, 44 (2011) 194002.
[3] G. Petitpas et al., International Journal of Hydrogen Energy, 32 (2007) 2848-2867.
[4] V. D. Rusanov et al., Doklady Chemistry, 395 (2004) 82-85.
[5] Y. C. Hong et al., Physics of Plasmas, 12 (2005) 053504.
[6] O. Jasek et al., Materials Science and Engineering: C, 26 (2006) 1189-1193.
[7] V. Y. Osipov et al., Diamond & Related Materials, 20 (2011) 205-209.