**2. Packed-bed plasma reactor**

Based on the location and number of plasma zone and catalyst bed, the combination of heterogeneous catalysts with plasma can be operated in three configurations: single-, two- and multistage, which are shown in **Figure 2**.

### **2.1 Single-stage reactor**

In a single-stage reactor (**Figure 2(a)**), the catalyst is packed inside the plasma zone, where the interaction of plasma and catalysts occurs. Because thermal plasma (gas bulk temperature >3000°C [9]) could damage catalyst, single-stage reactor is, therefore, suitable for nonthermal plasma sources. The single-stage reactor is widely applied in CH4 reforming [10–14], direct conversion of CO2 [15–19], VOCs abatement [20–22], exhaust matter removal [23], formaldehyde removal [24], NOx synthesis [25], and ozone synthesis [6]. There are two significant merits of single-state reactor: (I) great flexibility exists in terms of electrode and reactor configurations that the reactor can be constructed using inexpensive materials such as glass and polymers and (II) reactive species, ions, electrons, etc. generated by nonthermal plasma could modify the gas composition, which affects the surface reactions with catalyst synergistically. However, the interaction between plasma and catalyst is complex when the catalyst is placed directly in the plasma zone. The synergy of plasma and catalyst will be discussed further in Section 4 based on the single-state reactor.

#### **Figure 2.**

*Schematic diagram of single-stage (a), two-stage (b), and multistage reactor (c). Catalyst is depicted as orange circle; plasma is depicted as purple "lightning" symbol.*

#### **Figure 3.**

*Single-stage DBD reactor system for DMR: (a) overview of the reactor system, (b) cross-sectional view, (c) overview of the catalyst bed, (d) DBD generated in the catalyst bed, and (e) temperature distribution during reforming reaction.*

**Figure 3** depicts a single-stage DBD reactor for CH4 reforming [26], mainly including a quartz tube reactor, high voltage (HV) electrode at the center, and ground electrode outside of the tube. Catalyst pellets are packed in the plasma zone between two electrodes, and both ends of the catalyst bed are fixed by metallic supports. The high voltage is applied between the HV centered electrode and ground electrode to generate dielectric barrier discharge over the pellet surface. Discharge power was measured by voltage-charge Lissajous analysis. The discharge gap, which is the distance between HV electrode and the ground electrode, is usually less than 10 mm [27]. Catalyst temperature is controlled by a furnace. The temperature distribution of the catalyst bed is measured by thermography through the observation window. **Figure 3(c)**–**(e)** shows an overview of the catalyst bed, DBD generated in the catalyst bed, and the temperature distribution during reforming reaction. The

**41**

*Plasma-Enabled Dry Methane Reforming DOI: http://dx.doi.org/110.5772/intechopen.80523*

**2.2 Two-stage reactor**

[32–37].

**2.3 Multistage reactor**

was suppressed.

**3. DBD-enabled dry methane reforming**

changes in the entire hybrid reforming.

consecutively, while total flow rate was fixed at 1000 cm3

temperature matched within a measurement error.

catalyst bed temperature was clearly decreased because of the endothermic nature of DMR. In addition, gas temperature was estimated by optical emission spectroscopy (OES) of CO(B-A) transition [28], showing that catalyst temperature and gas

In the two-stage reactor, the catalyst is located at the downstream of plasma (**Figure 2(b)**). The gas is first addressed by the plasma and subsequently interacts with the catalyst [29]. Due to the separation of plasma and catalyst, both thermal and nonthermal plasma can be utilized. Because excited species generated in plasma have very short lifetimes, plasma mainly plays the role to preconvert the gas composition and then feed it into the catalyst reactor, e.g., in NOx removal process, due to the pretreatment of plasma, NO and NO2 were coexisted, which enhanced the following selective catalytic reduction in catalyst bed [30]; the other example is the benzene removal process where ozone (O3) was formed from background O2 by plasma, which promoted the decomposition of benzene in the next stage [31]. However, compared with the single-stage reactor, application of a two-stage reactor is limited in plasma catalysis and shows a lower performance for a given catalyst

The multistage reactor can be described as a combination of more than one single-stage bed/reactor (**Figure 2(c)**). The multistage reactor gives a more flexible option in the industrialization of the plasma catalysis, attributing to the combination of catalysts with a different function for the expected reaction [38]. Chavade et al. [39] used a four-stage plasma and catalytic reactor system for oxidation of benzene. The results showed that the increase in stage number enhanced benzene conversion and CO2 selectivity. The same result can be found in biogas reforming process using a multistage gliding arc discharge system without catalyst [40]. Harling et al. [41] developed a three-stage reactor for VOCs removal. The combination of plasma and catalyst in series could significantly improve the efficiency of VOCs decomposition. At the same time, the formation of by-product such as NOx

The pulsed reaction spectrometry using DBD with Ni/Al2O3 catalysts was investigated to develop a reforming diagnostic method [10]. Pulsed reforming enables the transient analyses of both CH4/CO2 consumption and H2 and CO generation. Furthermore, carbon formation was analyzed quantitatively without serious catalyst deactivation. The varied CH4/CO2 ratios between 0.5 and 1.5 were investigated at a fixed catalyst temperature near 600°C. The CH4/CO2 ratio was initially set to 0.5, and then the CH4/CO2 ratio was incremented stepwise until CH4/CO2 = 1.5,

(R2) was followed up after every pulsed reaction. System pressure was kept at 5 kPa during the reforming process. Discharge power was 85−93 W where specific energy input was ca. 1.2 eV/molecule. Commercially available catalyst pellets (11 wt% Ni-La/Al2O3, Raschig ring type: 3 mm) was packed for 40 mm length (total weight ca. 12 g; Ni 1.36 g; La 0.35 g). **Figure 4** provides an overview of gas component

/min. De-coking process

### *Plasma-Enabled Dry Methane Reforming DOI: http://dx.doi.org/110.5772/intechopen.80523*

catalyst bed temperature was clearly decreased because of the endothermic nature of DMR. In addition, gas temperature was estimated by optical emission spectroscopy (OES) of CO(B-A) transition [28], showing that catalyst temperature and gas temperature matched within a measurement error.

## **2.2 Two-stage reactor**

*Plasma Chemistry and Gas Conversion*

*circle; plasma is depicted as purple "lightning" symbol.*

**40**

**Figure 3.**

**Figure 2.**

*reforming reaction.*

**Figure 3** depicts a single-stage DBD reactor for CH4 reforming [26], mainly including a quartz tube reactor, high voltage (HV) electrode at the center, and ground electrode outside of the tube. Catalyst pellets are packed in the plasma zone between two electrodes, and both ends of the catalyst bed are fixed by metallic supports. The high voltage is applied between the HV centered electrode and ground electrode to generate dielectric barrier discharge over the pellet surface. Discharge power was measured by voltage-charge Lissajous analysis. The discharge gap, which is the distance between HV electrode and the ground electrode, is usually less than 10 mm [27]. Catalyst temperature is controlled by a furnace. The temperature distribution of the catalyst bed is measured by thermography through the observation window. **Figure 3(c)**–**(e)** shows an overview of the catalyst bed, DBD generated in the catalyst bed, and the temperature distribution during reforming reaction. The

*Single-stage DBD reactor system for DMR: (a) overview of the reactor system, (b) cross-sectional view, (c) overview of the catalyst bed, (d) DBD generated in the catalyst bed, and (e) temperature distribution during* 

*Schematic diagram of single-stage (a), two-stage (b), and multistage reactor (c). Catalyst is depicted as orange* 

In the two-stage reactor, the catalyst is located at the downstream of plasma (**Figure 2(b)**). The gas is first addressed by the plasma and subsequently interacts with the catalyst [29]. Due to the separation of plasma and catalyst, both thermal and nonthermal plasma can be utilized. Because excited species generated in plasma have very short lifetimes, plasma mainly plays the role to preconvert the gas composition and then feed it into the catalyst reactor, e.g., in NOx removal process, due to the pretreatment of plasma, NO and NO2 were coexisted, which enhanced the following selective catalytic reduction in catalyst bed [30]; the other example is the benzene removal process where ozone (O3) was formed from background O2 by plasma, which promoted the decomposition of benzene in the next stage [31]. However, compared with the single-stage reactor, application of a two-stage reactor is limited in plasma catalysis and shows a lower performance for a given catalyst [32–37].
