Partial discharging reactor correction 116

With the values of Ccell and Cdiel defined, cases where ζdiel < Cdiel can be examined. Shown in Figure 76 is the measured mean value for effective capacitance for 180 – 300 µm, and 300 –

500 µm BaTiO3 particles, in a 90% Ar – 10% CO2 reactor driven by a 5 kHz square wave, with

voltages ranging from 5 – 10 kV.

Figure 76: Measured mean effective capacitance of a 180 – 300 µm BaTiO3 packed reactor in

90% Ar – 10% CO2 at a range of applied voltages.

Figure 76 shows that with the 180 – 300 µm particles that the measured value for effective capacitance decreases with decreasing applied voltage almost linearly, from the point at 10 kV, where ζdiel is at ~32 pF, approximately ~ 3 pF less than Cdiel, to the point at 5 kV where ζdiel is almost equal to Ccell. This indicates a transition from almost full discharging of the reactor, with complete charge transfer from one electrode to the to other, to almost no discharge occurring. For comparison, the 300 – 500 µm BaTiO3 particles show ζdiel decreasing with applied voltage,

however in this case the minimum value is ~25.3 pF at 5kV, indicating that a significant proportion of charge is transferred from electrode to the other.

There are limitations to the Mathematica program that is used to analyse the data generated from the oscilloscope, namely that it is not able to determine all capacitances of the reactor when the reactor is partially discharging if the data obtained has a high level of noise (or is highly irregular in shape), even after data smoothing has been applied. The noisiest data happens to occur when Al2O3 is packed into the reactor, particularly with small particles. The

source of the noise is a point of discussion in itself, and its origins are discussed in section 6.1.5. Therefore, it is not possible with the current program to use this step decrease in voltage method to determine effective reactor capacitances at each different gas composition tested. However, the purpose of trying to determine effective capacitance is to correct the data obtained from the Lissajous figures, namely the burning voltage of the reactor. Theoretically, burning voltage for any individual void space in which a discharge can occur should be constant,

regardless of applied voltage. Once the applied voltage reaches a threshold voltage, an electrical breakdown will begin to occur in the void of the particle. Peeters and van de Sanden demonstrated that the burning voltage is almost constant as a function of applied voltage amplitude in a fixed geometry DBD plasma jet [95]. The reactor burning voltage of a packed bed reactor, being composed of a number of voids with a range of sizes, is likely to have an applied voltage dependent burning voltage. Before testing this hypothesis, the voltage dependent burning voltage of the unpacked reactor should be measured. The equation derived by Peeters and van de Sanden to determine burning voltage (Ub) based on the geometrically determined Δ

U is given by Equation 20:

𝑈_!=1 − 𝐶!"##/𝐶!"#$ 1 − 𝐶!"##/𝜁!"#$Δ𝑈

Equation 20

Using this equation, with Cdiel = 35 pF, and Ccell = 2.57 pF, the burning voltage of the empty reactor is determind for 90% Ar – 10% CO2 with a total flowrate of 100 ml/min, driven by a 5kHz

sine wave at voltages ranging from 5 – 10 kV, with the results presented in Figure 77.

Figure 77: Burning voltage of the empty reactor with a gas composition of 90% Ar – 10% CO2

at a total flowrate of 100 ml/min. Reactor is driven by a 5 kHz sine wave with voltages ranging between 5- 10 kV.

Figure 77 shows that as applied voltage decreases, that burning voltage increases. The burning voltage is seen to plateau with increasing applied voltage, up to 10 kV where it has a value of 2.07 kV +/- 0.1. This result contradicts that of Peeters and van de Sanden [95], who found the opposite trend to be true, i.e. burning voltage increases with applied voltage. A possible cause of this result is that with increasing applied voltage the reactor temperature increases up to 110 – 130 °C (as shown by Figure 68). An increase in temperature is known to decrease the breakdown strength of gases [113]. Using a 300 – 500 µm BaTiO3 packing material as an example the

change in burning voltage measured as a function of applied voltage to the reactor is presented in Figure 78.

Figure 78: Burning voltage and ΔU measured as a function of applied voltage for 300 – 500 µm BaTiO3 particles in 90 Ar – 10% CO2 driven by a 5kHz sine wave.

Figure 78 shows that with increasing applied voltage the burning voltage increases. As stated previously, this may be due to higher applied voltages causing localized electric field strengths to exceed the threshold field strength required to initiate an electrical breakdown in void spaces that happen to have a higher breakdown strength. This increased breakdown strength may be due to either the size of the void, or the shape of the particle on which it forms. This in turn raises the average burning voltage of the reactor, and consequently the measured burning voltage increases. Further studies of this burning voltage data at different applied voltages with a range of particle sizes and materials would be of interest, although the limitations of the Mathematica program prevent the ability to analyse all of the generated data with sufficient accuracy.

Determination of minimum burning voltage would be of academic interest, however this is very difficult to accurately measure. Optimal reactor performance from the perspective of CO2

conversion is obtained at the highest applied voltages. Therefore, measurements of reactor burning voltages are determined from the perspective of the highest possible applied voltage amplitude than can be generated by the HV amplifier, 10 kV.

An additional comparison that can be made is to determine whether wave shape has an effect on reactor burning voltage. Although the CO2 conversion experiments were performed using

square waves for the driving voltage, the majority of electrical analysis is carried out using sine waves, as the Lissajous figure analysis is derived for the diagnostics of sinusoidally driven reactors. The Lissajous figure derived from square waves can still effectively be applied for measurement of reactor burning voltages, however for determination of reactor capacitances the obtained values may differ; the problem of electrical diagnostics of square waves is beyond the scope of this thesis.

Figure 79: Reactor burning voltage in a 180 - 300 µm Al2O3 packed reactor with different

applied voltage wave shapes at 10 kV and 5 kHz.

Figure 79 shows that for both sine and square waves that the reactor burning voltage under the conditions tested is, within the margins of error, the same. This relationship has been tested with BaTiO3 packing as well as the unpacked reactor, and no change is observed when the applied

wave shape is changed.