THE INFLUENCE OF MICROWAVE PLASMA … INFLUENCE OF MICROWAVE PLASMA POWER ON THE DEGREE OF NATURAL...
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4. Carbon analysis THE INFLUENCE OF MICROWAVE PLASMA POWER ON THE DEGREE OF NATURAL GAS CONVERSION TO CARBON NANOFORMS 1. Introduction 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 CO 2 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. Carbon collected from runs at different powers and NG:N 2 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. 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 sp 2 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 (I D /I G ) 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. 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 m 2 g -1 for 1.5 and 2 kW. In all cases the pore sizes were in the 7-17 nm range. 002 100 010 102 004 103 104 110 112 Katarzyna L. Juda a , Aaron Clayton b , Bartek A. Glowacki a,c a Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK b GasPlas AS, Development Farm, Bluebell Road, Norwich, NR4 7AR, United Kingdom c Institute of Power Engineering, Augustówka 6, 02-981 Warszawa, Poland 2. Processing parameters The volumetric composition of NG used in the experiments is presented in the graph above. CH 4 and C 2 H 6 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 O 2 and CO 2 are negligible, and should not have any significant effect on hydrocarbon conversion. joined with a brass o-ring, were connected to the plasma nozzle. Carbon was collected from the inner walls of the shorter quartz tube directly connected to the plasma nozzle. Analytical techniques, as described in the table below, were employed. Research was conducted on the GasPlas double vortex plasma reactor (2.45 GHz), where different flow ratios of N 2 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 N 2 +NG. Each experiment was run for 3-20 min. Two quartz tubes, with an internal diameter of 17 mm, and lengths of 290 and 500 mm, Plasma nozzle Magnetron Tuning stubs Waveguide Products NG + N 2 Sliding short Gas chromatography (GC) BET surface area and porosity High resolution transition electron microscopy (HRTEM) X-ray diffractometry (XRD) Raman spectroscopy High resolution thermogravimetry (HRTGA) 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 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 (C n H 2n+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 (E d ) in the reacting system, figure B) presents the degree of absolute C n H 2n+2 conversion as a function of energy density. E d is calculated using the formula below, on the assumption that all the energy input goes into hydrocarbon cracking. E d = P · Q -1 From the data presented above it is clear that: · The degree of conversion increases with increasing power and decreasing NG:N 2 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 H 2 in the products, H 2 /(H 2 +C 2 H 2 +C 2 H 4 ) by flow, has an average value of 0.85 and varies little with energy density (standard deviation 0.014). · However, the proportion of H 2 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 CH 4 outlet flow rate with increasing energy density (fig. D) arises largely from the increase in volume associated with H 2 production. The main product is H 2 , 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 N 2 , unreacted methane and ethane (figure D), CO 2 below 0.28%, CO below 0.8% and traces of O 2 . Higher hydrocarbons (C3 up) were not detected. A) B) C) D) E) F) 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. 5 kW, 21.5 L min -1 NG 3 kW, 6 L min -1 NG 1.5 kW, 3 L min -1 NG 2 kW, 6 L min -1 NG 100 nm 100 nm 500 nm 100 nm 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. · 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 H 2 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. 5. Conclusions Katarzyna Juda acknowledges with gratitude the financial support of GasPlas AS. 6. Acknowledgments 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.
Transcript of THE INFLUENCE OF MICROWAVE PLASMA … INFLUENCE OF MICROWAVE PLASMA POWER ON THE DEGREE OF NATURAL...
4. Carbon analysis
THE INFLUENCE OF MICROWAVE PLASMA POWER ON THE DEGREE OF NATURAL GAS CONVERSION TO CARBON NANOFORMS
1. Introduction
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.
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.
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.
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.
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Katarzyna L. Judaa, Aaron Claytonb, Bartek A. Glowackia,c
aDepartment of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UKbGasPlas AS, Development Farm, Bluebell Road, Norwich, NR4 7AR, United KingdomcInstitute of Power Engineering, Augustówka 6, 02-981 Warszawa, Poland
2. Processing parameters
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.
joined with a brass o-ring, were connected to the plasma nozzle. Carbon was collected from the inner walls of the shorter quartz tube directly connected to the plasma nozzle. Analytical techniques, as described in the table below, were employed.
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 keptconstant at 30 L/min N2+NG. Each experiment was run for 3-20 min. Two quartz tubes, with an internal diameter of 17 mm, and lengths of 290 and 500 mm,
Plasma nozzle
Magnetron
Tuning stubs
Waveguide
Products
NG + N2
Sliding short
Gas chromatography (GC)
BET surface area and porosity
High resolution transition electron microscopy (HRTEM)
X-ray diffractometry (XRD)
Raman spectroscopy
High resolution thermogravimetry (HRTGA)
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
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.
Ed = P · Q-1
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.
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.
A) B)
C) D)
E) F)
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. 5 kW, 21.5 L min-1 NG3 kW, 6 L min-1 NG
1.5 kW, 3 L min-1 NG 2 kW, 6 L min-1 NG100 nm
100 nm
500 nm
100 nm
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.
· 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.
5. Conclusions
Katarzyna Juda acknowledges with gratitude the financial support of GasPlas AS.
6. Acknowledgments
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.
