Plasma-Enabled Dry Methane Reforming

*Zunrong Sheng, Seigo Kameshima, Kenta Sakata and Tomohiro Nozaki*

## **Abstract**

Plasma-enabled dry methane reforming is a promising technology for biogas upgrade and shows multiple benefits to provide additional energy and material conversion pathways. This chapter first presents the role of nonthermal plasma as a potential energy supply pathway in the low-temperature methane conversion: an appropriated combination of electrical energy provided by plasma (Δ*G*) and the low-temperature thermal energy (*T*Δ*S*) satisfies the overall reaction enthalpy (Δ*H*) with higher energy conversion efficiency. Moreover, plasma-enabled dry methane reforming could be operated at much lower temperature than thermal catalysis with sufficient material conversion. Three kinds of typical packed-bed plasma reactor were introduced to give a better understanding of the application of plasma and catalyst hybrid system. Subsequently, plasma-enabled dry methane reforming was diagnosed by pulsed reaction spectrometry compared with thermal catalysis, presenting a clear overview of gas component changes and significant promotion in reactant conversion and product yield. The interaction between plasma and catalyst was summarized based on two aspects: catalyst affects plasma, and plasma affects catalyst. We discussed the coke formation behavior of Ni/Al2O3 catalyst in the plasma-enabled and thermal dry methane reforming, followed by the oxidation behavior. The interaction between plasma and catalyst pellets was discussed toward deeper insight into the mechanism.

**Keywords:** plasma catalysis, dry methane reforming, dielectric barrier discharge, biogas, methane conversion

### **1. Introduction**

Dry methane reforming (DMR) has drawn keen attention as viable CO2 utilization technology because it may have one of the greatest commercial potentials [1, 2].

$$\text{CH}\_4 + \text{CO}\_2 \rightarrow 2\text{H}\_2 + 2\text{CO} \,\Delta H \,= \, 247 \,\text{kJ/mol} \tag{\text{R1}}$$

Moreover, products are the main components of syngas (H2 and CO), which can be converted to the synthetic fuels as well as H2 carrier via well-established C1 chemistry. Conventionally, the H2/CO ratio from DMR is more suitable for Fischer-Tropsch synthesis than other methane reforming reactions [3–5]. **Figure 1** shows the reaction enthalpy and Gibbs free energy of DMR (R1) with respect to temperature. According to the definition, reaction enthalpy (Eq. (1)) consists of two terms:

$$
\Delta H^\circ = \quad T\Delta S + \Delta G^\circ \tag{1}
$$

**Figure 1.** *Energy diagram of DMR.*

DMR is categorized as uphill (endothermic) reaction where energy input (Δ*H*) is indispensable in order to satisfy the conservation of energy. Moreover, the reaction does not occur spontaneously by the low-temperature thermal energy due to the large positive value of Δ*G* at low temperature. **Figure 1** shows that at least 900 K is required to have a negative value of Δ*G*, and all energy is supplied via hightemperature thermal energy. Such high-temperature heat is supplied by the combustion of initial feed that produces CO2 as well as NOx. Net CO2 utilization is partly canceled unless combustion-generated CO2 is utilized which is economically quite difficult. Moreover, heat transfer from the combustion gas flowing outside of the reactor to the catalyst bed governs the overall material throughput which is known as a *heat transfer-limiting* regime. Because the heat transport property of a fixed bed reactor is poor, excessively high-temperature operation beyond thermodynamic limitation (i.e., 900 K) is necessary.

To overcome the aforementioned problem, low-temperature DMR is demanded, pursuing a new technology, and potential use of nonthermal plasma is highlighted. Assume DMR is operated at a lower temperature than the thermodynamic limitation as schematically depicted in **Figure 1**. A part of the energy is supplied by a low-temperature thermal energy (*T*Δ*S*), while the rest of energy is supplied by the electricity (Δ*G*) under the nonthermal plasma environment so that *T*Δ*S* + Δ*G* satisfies reaction enthalpy (Δ*H*). Electrical energy is used to accelerate electrons; subsequently, the electron energy is transferred to the molecules to initiate DMR at much lower temperature than thermal catalysis. Electronic collision process is independent of reaction temperature if gas density does not change significantly. Meanwhile, a part of the electrical energy is converted to heat: electrical energy consumed by nonthermal plasma (*E*) is depicted in the dashed line in **Figure 1**: inevitably, *E* is greater than Δ*G* at a fixed temperature. Although heat generated by nonthermal plasma is considered as energy loss (i.e., *E−*Δ*G*), both excited species and heat are utilized via endothermic DMR, which enables efficient use of electrical energy without *heat transfer limitation*: electrification of reforming reaction, or chemical processes in general, has the greatest advantage that the energy transfer and the control are independent of temperature gradient.

**39**

single-state reactor.

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

**2. Packed-bed plasma reactor**

**2.1 Single-stage reactor**

Dielectric barrier discharge (DBD) is the most successful atmospheric pressure nonthermal plasma sources in industry applications [6] and is used exclusively for this purpose. DBD is combined with a catalyst bed reactor and generated at atmospheric pressure [7]. DBD is characterized as a number of transient discharge channels known as streamers with nanosecond duration. Because the streamer has a nature of propagation along the interface between two adjacent dielectric materials, namely, the catalyst pellet and the gas interface, excited species produced by DBD is transferred to the catalyst surface efficiently. Moreover, the heat generated by DBD is transferred directly to the catalysts; overall energy transfer from nonthermal plasma to the catalyst bed is efficient. If the electricity is supplied from the renewable energy such as photovoltaics and wind turbines, low-emission DMR is possible with free of combustion. Moreover, nonthermal plasma-assisted C1 chemistry enables renewable-to-chemical energy conversion, which provides an alternative and viable solution for the efficient renewable energy storage and transportation pathways.

The aforementioned thermodynamic analysis (**Figure 1**) implies that the temperature-benign and low-emission chemical processes are possible with the appropriate combination of nonthermal plasma and the heterogeneous catalysts. Meanwhile, such hybrid system does not work at room temperature simply because the overall reaction rate is *kinetically controlled* at much low temperature: Nevertheless, we would like to highlight that nonthermal plasma technology solves many technological obstacles such as the elimination of combustion as well as heat transfer limitation. Moreover, low-temperature operation suppresses coke formation which is one of the big issues in DMR. In this book chapter, we focus on low-temperature DMR and compare thermal and plasma catalysis. Plasma catalysis of DMR was diagnosed by pulsed reaction spectrometry [8], and results were compared with thermal catalysis to highlight the benefit of DBD and catalyst combination. Subsequently, the interaction between DBD and catalyst pellets was discussed toward deeper insight into the mechanism. Finally, future prospects of plasma catalysis of DMR are provided.

Based on the location and number of plasma zone and catalyst bed, the combination of heterogeneous catalysts with plasma can be operated in three configura-

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

tions: single-, two- and multistage, which are shown in **Figure 2**.

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

*Plasma Chemistry and Gas Conversion*

limitation (i.e., 900 K) is necessary.

and the control are independent of temperature gradient.

DMR is categorized as uphill (endothermic) reaction where energy input (Δ*H*) is indispensable in order to satisfy the conservation of energy. Moreover, the reaction does not occur spontaneously by the low-temperature thermal energy due to the large positive value of Δ*G* at low temperature. **Figure 1** shows that at least 900 K is required to have a negative value of Δ*G*, and all energy is supplied via hightemperature thermal energy. Such high-temperature heat is supplied by the combustion of initial feed that produces CO2 as well as NOx. Net CO2 utilization is partly canceled unless combustion-generated CO2 is utilized which is economically quite difficult. Moreover, heat transfer from the combustion gas flowing outside of the reactor to the catalyst bed governs the overall material throughput which is known as a *heat transfer-limiting* regime. Because the heat transport property of a fixed bed reactor is poor, excessively high-temperature operation beyond thermodynamic

To overcome the aforementioned problem, low-temperature DMR is demanded, pursuing a new technology, and potential use of nonthermal plasma is highlighted. Assume DMR is operated at a lower temperature than the thermodynamic limitation as schematically depicted in **Figure 1**. A part of the energy is supplied by a low-temperature thermal energy (*T*Δ*S*), while the rest of energy is supplied by the electricity (Δ*G*) under the nonthermal plasma environment so that *T*Δ*S* + Δ*G* satisfies reaction enthalpy (Δ*H*). Electrical energy is used to accelerate electrons; subsequently, the electron energy is transferred to the molecules to initiate DMR at much lower temperature than thermal catalysis. Electronic collision process is independent of reaction temperature if gas density does not change significantly. Meanwhile, a part of the electrical energy is converted to heat: electrical energy consumed by nonthermal plasma (*E*) is depicted in the dashed line in **Figure 1**: inevitably, *E* is greater than Δ*G* at a fixed temperature. Although heat generated by nonthermal plasma is considered as energy loss (i.e., *E−*Δ*G*), both excited species and heat are utilized via endothermic DMR, which enables efficient use of electrical energy without *heat transfer limitation*: electrification of reforming reaction, or chemical processes in general, has the greatest advantage that the energy transfer

**38**

**Figure 1.**

*Energy diagram of DMR.*

Dielectric barrier discharge (DBD) is the most successful atmospheric pressure nonthermal plasma sources in industry applications [6] and is used exclusively for this purpose. DBD is combined with a catalyst bed reactor and generated at atmospheric pressure [7]. DBD is characterized as a number of transient discharge channels known as streamers with nanosecond duration. Because the streamer has a nature of propagation along the interface between two adjacent dielectric materials, namely, the catalyst pellet and the gas interface, excited species produced by DBD is transferred to the catalyst surface efficiently. Moreover, the heat generated by DBD is transferred directly to the catalysts; overall energy transfer from nonthermal plasma to the catalyst bed is efficient. If the electricity is supplied from the renewable energy such as photovoltaics and wind turbines, low-emission DMR is possible with free of combustion. Moreover, nonthermal plasma-assisted C1 chemistry enables renewable-to-chemical energy conversion, which provides an alternative and viable solution for the efficient renewable energy storage and transportation pathways.

The aforementioned thermodynamic analysis (**Figure 1**) implies that the temperature-benign and low-emission chemical processes are possible with the appropriate combination of nonthermal plasma and the heterogeneous catalysts. Meanwhile, such hybrid system does not work at room temperature simply because the overall reaction rate is *kinetically controlled* at much low temperature: Nevertheless, we would like to highlight that nonthermal plasma technology solves many technological obstacles such as the elimination of combustion as well as heat transfer limitation. Moreover, low-temperature operation suppresses coke formation which is one of the big issues in DMR. In this book chapter, we focus on low-temperature DMR and compare thermal and plasma catalysis. Plasma catalysis of DMR was diagnosed by pulsed reaction spectrometry [8], and results were compared with thermal catalysis to highlight the benefit of DBD and catalyst combination. Subsequently, the interaction between DBD and catalyst pellets was discussed toward deeper insight into the mechanism. Finally, future prospects of plasma catalysis of DMR are provided.
