Chapter 6: RESULTS AND DISCUSSION
6.4 Inter-electrode gap variation study
95
96 An increase in the inter-electrode gap denoted an increase in the arc length and volume of syngas being treated. The concentrations of methane (22 424 ppm), ethane (517 ppm), ethylene (101 ppm), propane (79 ppm) and propylene (19 ppm) for the 6 wt% Co catalyst at an inter-electrode gap of 2 mm were approximately 22, 10, 6, 26 and 5 times greater, respectively, than the hydrocarbon yields obtained at 0.5 mm.
In contrast, the methane and ethane yields for pure plasma FTS increased from 0.5 to 2 mm by factors of only 5 and 2 respectively. In addition, the pure plasma ethylene and C3 hydrocarbon yields remained below 1 ppm (not shown in Figure 6.14). These results indicated that the inter-electrode gap had a more significant influence on hydrocarbon production in Co-based plasma-catalytic FTS.
The considerable differences in the pure plasma and plasma-catalytic concentration-gap yields were attributed to their active arc discharge volume differences. For pure plasma FTS, increasing the inter- electrode gap by a factor of 4 (from 0.5 to 2 mm) corresponded to a fourfold increase in the discharge volume assuming a linear relationship. The same increase in the inter-electrode gap for plasma-catalytic systems not only caused a fourfold increase in the arc discharge volume, but also a fourfold increase in the catalyst surface area exposed to the discharge, leading to activation of more Co particles. This behaviour was verified at the widest discharge gap of 2 mm, where methane, ethane, ethylene and propane concentrations for the 6 wt% Co catalyst were 226, 210, 278 and 1353 times greater, respectively, than that obtained for pure plasma. For pure plasma between 250 and 450 mA, the ethane concentrations were between 0.2 and 2.5 ppm, whereas ethylene and propane concentrations were lower than 1 ppm and propylene was not produced (note that these concentration values are not presented in Figures 6.14.a to 6.14.d above). Furthermore, similar product yields were obtained for the blank catalyst.
The hydrocarbon yields obtained for the 6 wt% Co catalyst exceeded those of the 2 wt% Co catalyst for most discharge gaps investigated. However, the opposite trend was seen for methane, ethane and ethylene at 0.5 mm, which was especially evident for ethylene between 1 and 2 mm. The lower 6 wt% Co catalyst’s ethylene yields at the 1 and 2 mm discharge gaps could be due to these larger discharge volumes causing an increase in the catalyst surface temperature, probably triggering ethylene readsorption followed by secondary reactions such as hydrogenation to ethane or reinsertion into propane or propylene chains (phenomena described for the pressure variation study in Section 6.2.1.6). These reaction phenomena describing the lower ethylene yields at 1 and 2 mm for the 6 wt% Co catalyst, were verified by the 38 and 71% higher ethane and propane concentrations, respectively, and the exclusive production of propylene (not detected for the 2 wt% Co catalyst system). In addition, the mentioned ethylene secondary reactions
97 occurring for the 6 wt% Co catalyst seemed to be much more prevalent at higher discharge gaps than at higher pressures in the pressure variation study (discussed in Section 6.2.1.6).
The maximum product yields for the plasma-catalytic systems, discussed above, were related to the energy consumption, discussed below, in order to determine the optimum operating conditions for the inter- electrode gap variation study.
6.4.2 The influence of the inter-electrode gap on energy consumption
An increase in the inter-electrode gap from 0.5 to 2 mm at a fixed current of 350 mA caused an increase in the voltage as shown in Figure 6.15. The voltage increase was necessary for sustaining the arc discharge.
Figure 6.15: The influence of inter-electrode gap on rms voltage for pure plasma and plasma-catalytic FTS (NTP + Blank, 2 or 6 wt% Co catalyst) at a discharge time of 60 s. Legend: ■ – 6 wt% Co; ▲ – 2 wt% Co;
Δ – Blank; □ – Pure plasma. Operating conditions: Syngas ratio: 2.2:1; pressure: 2 MPa; current: 350 mA;
wall temperature: 25oC.
This increasing voltage-gap trend could be explained in terms of the various regions constituting the arc discharge, namely the near-cathode and near-anode border zones (more non-equilibrium in nature) and positive arc column (less non-equilibrium in nature due to its higher inherent electrical resistivity). The thicknesses of these arc regions usually vary with the inter-electrode gap [43, 60].
50 75 100 125 150 175 200 225 250 275 300
0,00 0,50 1,00 1,50 2,00
Vrms/ V
Inter-electrode gap / mm 6wt% Co
2wt% Co Blank Pure plasma
98 For wide inter-electrode gaps (2 mm in this work), the positive arc column was long and was therefore more influential in controlling the discharge behaviour. The long arc column had a high electrical resistivity leading to a more thermal plasma (greater convective losses). In order to sustain these thermal processes, comparatively high input voltages were required (275 V for pure plasma as shown in Figure 6.15 above).
In contrast to the wide discharge gaps, for short discharge gaps (0.5 mm in this work), the non-equilibrium border zones governed the electrical behaviour of the discharge, leading to a lower voltage requirement (67 V for pure plasma as shown in Figure 6.15 above). This arc zone-gap behaviour also extended to the plasma- catalytic systems. In addition to these arc zones, the higher specific input energy (SIE) values at wider inter- electrode gaps (Figure 6.16) also indicated that the plasma was further from non-equilibrium.
Figure 6.16: Specific input energy (kJ/molsyngas) as a function of inter-electrode gap for pure plasma and plasma-catalytic FTS (NTP + Blank, 2 or 6 wt% Co catalyst) at a discharge time of 60 s. Legend: ■ – 6 wt% Co; ▲ – 2 wt% Co; Δ – Blank; □ – Pure plasma. Operating conditions: Syngas ratio: 2.2:1; pressure:
2 MPa; current: 350 mA; wall temperature: 25oC.
The specific required energy (SRE) values, shown in Figure 6.17 below, decreased significantly from 0.5 to 2 mm for the 2 and 6 wt% Co catalysts, revealing that 1.5 to 2 mm was the optimum inter-electrode gap range in terms of energy efficiency and hydrocarbon yields. At 2 mm, the SRE values for the 2 wt% Co catalyst (224 MJ/molmethane,prod) and 6 wt% Co catalyst (265 MJ/molmethane,prod) were factors of approximately 126 and 107, respectively, lower than that of pure plasma.
0 1 000 2 000 3 000 4 000 5 000 6 000 7 000
0,00 0,50 1,00 1,50 2,00
SIE / kJ/molsyngas
Inter-electrode gap / mm 6wt% Co
2wt% Co Blank Pure plasma
99 Although the SRE value for both Co catalysts were similar between 1.5 and 2 mm, the 6 wt% Co catalyst produced significantly greater quantities of paraffins and propylene (not produced by the other three systems) in this gap range. Therefore, this catalyst was again shown to be the most suitable for promoting chain growth. However, if higher ethylene yields were desired, then the 2 wt% Co catalyst operating between 1.5 and 2 mm would be favoured.
Figure 6.17: Specific required energy (MJ/molmethane,prod) as a function of inter-electrode gap for pure plasma and plasma-catalytic FTS (NTP + Blank, 2 or 6 wt% Co catalyst) at a discharge time of 60 s. Legend: ■ – 6 wt% Co; ▲ – 2 wt% Co; Δ – Blank; □ – Pure plasma. Operating conditions: Syngas ratio: 2.2:1; pressure:
2 MPa; current: 350 mA; wall temperature: 25oC.
In conclusion, for the pressure, current and inter-electrode gap variation studies (discussed in Sections 6.2 to 6.4), it was shown that the 2 and 6 wt% Co catalysts systems unanimously improved the C1-C3
hydrocarbon product yields and energy consumption compared to the blank catalyst and pure plasma FTS systems. The optimum operating parameters for chain growth and energy efficiency for the 2 and 6 wt%
Co catalysts were: (i) 10 MPa at 10 s and 2 MPa at 60 s for the pressure variation study (0.5 to 10 MPa);
(ii) 250 mA for the current variation study (200 to 450 mA) and; (iii) 2 mm for the inter-electrode gap variation study (0.5 to 2 mm). At these optimum conditions, the 6 wt% Co catalyst was generally the most effective for catalysing FTS reactions, thus encouraging the use of higher cobalt loadings in future plasma- catalytic FTS investigations.
100 1 000 10 000 100 000 1 000 000
0,00 0,50 1,00 1,50 2,00
SRE / MJ/molmethane,prod
Inter-electrode gap / mm 6wt% Co
2wt% Co Blank Pure plasma
100 In order to better understand the influence of plasma on the catalyst and their related interactions, the catalyst was characterised using various microscopic probing tools. These findings are presented in Section 6.5.