Causes of changes to CO 2 conversion with packing material and size 133

The final questions posed, “why does CO2 conversion increase when using small particle sizes in

packed bed reactors?” and “why does Al2O3 show a significant increase in CO2 conversion

compared with BaTiO3?” are perhaps the most difficult questions to attempt to answer.

There are a number of parameters that change as a consequence of increasing or decreasing particle size. These parameters will have either a positive, negative or neutral effect on CO2

conversion. It has been demonstrated in section 6.1.3 and 6.1.4 that a decrease in particle size leads to an increase in reactor burning voltage, and an increase in the tendency of the reactor towards partial discharging. Despite this, under high argon concentrations, where the reactor is generally shown to be discharging effectively, small particle sizes yield the highest CO2

conversions. Table 8 shows a summary of the effect of changing packing particle size or material. This table includes a “resultant variable”, that is a property of the reactor that changes as a result of the input variable, as well as the probable net effect on CO2 conversion and a note

on whether or not the effect is experimentally verifiable from the data obtained for this thesis.

Table 8

Input

Variable Resultant Parameter

Probable Net Effect on CO2 Conversion Effect Experimentally Verifiable? Decrease in particle size

Increased surface area to volume ratio for

heterogeneous catalytic reactions é  /  ?  

Surface  area  could  be   quantified  with  further  

experimentation   Decrease

in particle size

Increased number density of contact points between particles, leading to an increase in the number of plasma discharges simultaneously occurring

é Possibly  –  from   oscilloscope  data   Decrease in particle size

Greater number density of sharp edges – these may result in a strong localised electric field and lead to formation of discharges é Weak   Contribution   Further   experimentation   required   Decrease in particle size

Improved gas – plasma contacting. Presuming that the reactor is fully discharging, decreasing particle size is likely to improve gas - plasma contacting. This is linked to the increased number density of contact points between particles, as well as the number density of sharp edges, and the possibility of surface discharges.

éé  /  é   The  net  effect  

of  multiple   variables   Possibly  –  From   Oscilloscope  data   Decrease in particle

Reduced residence time – In the reactor

tested at a flowrate of 100 ml /min, space ê Weak  

Known  to  have  an   effect.  In  this  instance  

size time varies between ~ 0.39 and 0.49s. Although residence time is known to be an important parameter, in this circumstance the contribution to decrease in CO2 conversion is likely to be weak due

to the small range of space times between particle sizes.

Contribution   net  contribution  to  CO2   conversion  is  likely  to  

be  weak.  

Decrease in particle

size

Reduction in void size – This leads to an increase in reactor burning voltage, as well as the possibility of reactor partial discharging êê Experimentally   demonstrated  in   sections  6.1.3  and  6.1.4   Decrease in particle size

Increased rate of loss of electrons and exited species – leads to lower electron and ion density in packed bed reactors

ê Further   experimentation   required   Decrease in particle size

Behaviour of the plasma discharge is very likely to change with a change in particle size. Plasma may change from point-to- point discharges, to a surface discharge or another unidentified mechanism

?  

Yes  –  From  oscilloscope   data  

Decrease in particle

size

Change in local electric field strength ?  

Possibly  too  complex  to   ever  be  elucidated  

experimentally.   Modeling  may  help.   Change in

material Possible catalytic activity é  

Requires  further   experimentation   Change in

material Change in dielectric constant ?  

Requires  further   experimentation   Change in

material Change in material work function ?  

Requires  further   experimentation   Change in

material Material acidity / basicity ?  

Requires  further   experimentation   Change in

material

Increase in porosity / surface area / pore

size é  

Requires  further   experimentation  

Clearly there are many parameters that change as a result of simply changing packing particle size and packing material. Therefore it is not possible to isolate any particular parameter as being the main cause of the change in CO2 conversion observed in the reactor.

Of the resultant parameters presented in Table 8, the change in the behavior of the reactor is the only parameter that can be further investigated from the data collected. Oscilloscope data can be used to provide an insight into the nature of the discharge in the reactor, specifically

providing information regarding the magnitude, number and duration of current pulses. Further work would have to be carried out in order to quantify these parameters; consequently the oscilloscope data presented here is used solely for qualitative analysis.

Figure 89: Oscilloscope data showing applied voltage and current data for reactors driven by a 10 kV, 5 kHz square wave. The data presented here is for the experiments where the feed gas composition is 90% Ar – 10% CO2.

Figure 89 shows oscilloscope data comparing reactors packed with BaTiO3 or Al2O3 with particle

sizes of either 180 – 300 µm, or 1400 – 2000 µm, as well as an empty reactor. The data is collected for an applied voltage of 10 kV, with a 5 kHz square wave in a gas composition of 90% Ar – 10% CO2. On each graph the Y-axis is scaled identically, with a range from -500 to +500

mA.

The most apparent feature of this figure is the magnitude of the current pulses in the 180 – 300 µm Al2O3 packed bed. Compared with every other reactor set-up the magnitude of the pulses is

oscilloscope with the settings used). This reactor shows the highest conversion of CO2 with a

value of 26.9%. In addition to this, the power used in the plasma is much higher, 17.5 W, compared with 13.2 W for 180 – 300 µm BaTiO3 and 14.3 W with 1400 – 2000 µm Al2O3. The

amplitude of these pulses is much larger than in the non-packed reactor. This shows that the instantaneous charge transferred is much larger during one of these pulses, than can be transferred from one microdiscarge in the unpacked reactor.

Further observations can be made by comparing the behavior of pulse clusters for the 180 – 300 µm Al2O3 packed bed and the unpacked reactor, as shown by Figure 90 and Figure 91.

Figure 90: Pulse clusters observed with increasing applied voltage in a 180 – 300 µm Al2O3

Figure 91: Pulse clusters observed with increasing applied voltage in a non-packed reactor driven by a 5 kHz square wave at 10 kV in 90% Ar – 10% CO2.

On both Figure 90 and Figure 91, the X-axis is scaled identically for easy comparison, the Y-axis is scaled proportionately to the magnitude of the pulses. Comparing the two figures, there are 3 main differences:

1. The amplitude of the pulses is larger in the Al2O3 packed bed than the non-packed

reactor

2. There are more pulses during one half discharge cycle in Al2O3 packed bed than the

non-packed reactor

3. The decay time of pulses in the non-packed bed is longer than those in the Al2O3 packed

bed

The reduced decay time of the pulses in the packed bed can be explained by the very large plasma – particle interface area very rapidly neutralising charged species present in the plasma. This hypothesis is supported by the pulse cluster data for 1400 – 2000 µm Al2O3, shown in

Figure 92, where a large fraction of the current pulses observed have a long decay time, similar in time scale to those observed in the non-packed reactor. Indicating that the large voids of the bed allow for longer pulse decay times, due to a lower rate of charge neutralisation.

Figure 92: Pulse clusters observed with increasing applied voltage in a 1400 – 2000 µm Al2O3

packed bed driven by a 5 kHz square wave at 10 kV in 90% Ar – 10% CO2

The large magnitude of the current pulses is more challenging to explain. Especially when comparing the data obtained to that of the 180 – 300 µm BaTiO3 packed reactor, shown in

Figure 93. The current data from 180 – 300 µm BaTiO3 packed bed features very few current

pulses, the current appears as a continuously drawn current by the reactor. This may be due to the cumulative effect of many very small microdischarges occurring between particles each drawing a small amount of current, as well as a displacement current due to polarisation switching in BaTiO3.

The data presented in section 6.1.4 indicated the tendency of the BaTiO3 packed reactor with

small particles towards partial reactor discharging. It is likely that under the conditions used in this experiment that the plasma discharge is contained to the regions where particles are touching. It may be that if a larger driving potential was applied to the reactor, that the discharges would expand into the void spaces between particles and the behavior of the reactor may be more similar to the 180 – 300 µm Al2O3 packed reactor. Plasma discharge transition from

a point-to-point plasma, to a discharge expansion into the void spaces of a BaTiO3 packed

reactor has been observed by Tu et al [117] when plasma power is increased. It may be that in the conditions used in this thesis that the potential difference applied is insufficient to cause this transition in discharge behavior to occur. The operational regimes under which this transition in behavior occurs requires further investigation.

In the 180 – 300 µm Al2O3 packed bed given the small size of the particles used, coupled with

the low dielectric capacity of Al2O3 resulting in a low charge storage capacity per particle, the

large amplitude current pulses may be the cumulative contribution of the stored charge of many particles dissipating their energy into one high power pulse propagating through the packed bed. This hypothesis is supported by observations made of the reactor whilst it is operating in this condition, where intense filamentary discharges can be seen appearing at different locations in the packed bed. These intense pulses are responsible for the seemingly “noisy” Lissajous figures that were previously shown in Figure 33, indicating that very large amounts of charge are transferred during one of these current pulses. In order to gain a better understanding of discharge phenomena in packed beds, the Lissajous figures and oscilloscope data generated from these experiments requires further analysis.

To return to the questions at the start of this section, there is no certain answer as to why small particles are seemingly beneficial for CO2 reduction in a packed bed, however it is likely to be

caused by improved gas - plasma contacting and a change in plasma discharge behavior. If the material happens to be catalytic in a plasma system, there may be further benefits due to the increased plasma – catalyst interface area.

Addressing the cause of Al2O3 packing performance exceeding that of BaTiO3 under conditions

when the reactor is discharging well (i.e. α values are low) the mechanism remains uncertain. The BaTiO3 packing needs to be tested with higher applied potential differences in order to

determine if the discharge expands more into the void spaces in the reactor. In addition to this, the Al2O3 packing should be tested for possible catalytic activity as a plasma activated CO2