I. INTRODUCTION

CH4 is the main component of natural gas and biogas.

However, it has a poor energy density because it is gas in

the standard condition. Table I shows the energy density

of various energy sources. A carbon-containing liquid

fuel has higher volumetric energy density than other

forms of energy. Therefore CH4 should be converted into

liquid fuel via syngas (H2 and CO).

In commercialized CH4 reforming, initial CH4 is

partially combusted in order to obtain high temperature

thermal energy (> 800°C). Non-thermal plasma has low gas temperature and high electron temperature, then gas

molecules can be activated at low temperature by

colliding with high temperature electrons. It leads to

reactions without a limitation of thermal equilibrium.

Because of that, to lower the reaction temperature, many

studies for CH4 reforming using plasma have been

investigated [3–11] ([3–5]: steam methane reforming, [6–

8]: dry methane reforming, [9–11]: methane partial

oxidation).

Commercialized syngas production from CH4 is

steam methane reforming (SMR), as in (1). However,

SMR produces CO2 by water-gas shift (WGS) reaction,

as in (2). In order to reduce CO2 emission, dry methane

reforming (DMR), as in (3), is desired.

CH4 + H2O → CO + 3H2 (1)

CO + H2O → CO2 + H2 (2)

CH4 + CO2 → 2CO + 2H2 (3)

However, in DMR, solid carbon deposition (coking) is

inevitable as shown in Fig. 1 [12]. It describes that

carbon limit is clarified by O/C and H/C ratios of raw

material. Curves A and B represent the carbon limit

criteria, showing whisker (left side of B) and graphite

(left side of A) formation limit. Deposited carbon

deactivates and eventually destroys the catalysts in

several ways: surrounding and blocking access to the

active phase surface, encapsulating the active metal

particles, plugging the pores, accumulating as strong

Coking Characteristics of Dry Methane Reforming

by DBD-catalyst Hybrid Reaction with Cyclic Operation

S. Kameshima, K. Tamura, Y. Ishibashi, and T. Nozaki

Department of Mechanical Sciences and Engineering, Tokyo Institute of Technology, Japan

Abstract—Dry methane reforming is promising technology to reduce CO2 emission. However, high temperature

(> 800°C) is favor for the dry methane reforming by thermal reaction, and catalysts are deactivated and destroyed by

coking, which is the major issue in the dry methane reforming. In this work, we succeeded the long time operation at low

temperature by using DBD-catalyst hybrid reaction combined with cyclic operation, which conducted reforming and de-

coking reactions alternatively by pulsed methane supply. Plasma heating promoted reactions, and de-coking time was

reduced by 35% from thermal reaction. CO formation in reforming was dominated by the reactions of CO2-CHx and CO2-

H. In contrast, Boudouard reaction (C + CO2 → 2CO) eliminating solid carbon on catalysts has small contribution in

reforming. This is caused by the difference of the reaction rates.

Keywords—Dry methane reforming, methane, carbon dioxide, syngas, hybrid reaction, plasma

TABLE I

COMPARISON OF ENERGY DENSITY

Energy density

[kWh/L] Ref.

Hydrogen (207 bar) 0.50 [1]

Natural gas (207 bar) 2.8 [1]

Methanol 4.4 [1]

Li-ion battery 0.22-0.4 [2]

Corresponding author: Tomohiro Nozaki

e-mail address: tnozaki@mech.titech.ac.jp Presented at the 3rd Korea-Japan Conference on Plasma and

Electrostatics Technologies, in November 2014

Fig. 1. Carbon limit diagram [12], A: graphite, B: whisker carbon.

(Original figure adopted from [12] was modified.)

40 International Journal of Plasma Environmental Science & Technology, Vol.9, No.1, APRIL 2015

carbon filaments leading to catalyst disintegration, and

physically blocking the reactor [13]. Therefore,

countermeasures for coking are essential. We achieved to

carry out DMR while controlling coking by conducting

solid carbon removal (de-coking) through Boudouard

reaction (4) in CO2 plasma generated cyclically by pulsed

CH4 supply [14, 15].

C + CO2 → 2CO (4)

To avoid catalyst deactivation, the deposited carbon must

be removed before it grows thick layer of carbon

filaments. In this study, we investigated both of hybrid

and thermal reaction in order to discuss the coking issue

and the contribution of non-thermal plasma.

II. METHODOLOGY

A. DBD-catalyst Hybrid Reactor

Experimental setup is shown in Fig. 2. The inner

diameter of the reactor was 20 mm, and the high voltage

electrode with 3 mm diameter was fixed in an axis of the

quartz tube. The grounded electrode length was 20 mm,

and it had a 10 mm width slit to measure the catalyst bed

temperature by infrared camera (TH5104, NEC San-ei

Instruments, Ltd.). 12wt%/Al2O3 catalysts with 3 mm

mean diameter was used for the experiments. Catalysts

were reduced for 90-min in H2/N2 = 50/450 cm3/min with

600°C prior to the experiment.

B. Cyclic Operation and Analysis

Feed gas was changed periodically by turning CH4

flow on and off with duty ratio of 25%. During cyclic

operation, CO2 was supplied continuously. Feed gas for

the reforming reaction was CH4/CO2 = 2 and 10300 h-1

GHSV (gaseous hourly space velocity, as in (5)). For the

de-coking reaction, GHSV decreased to one third

because only CO2 was supplied. Discharge power and

frequency were 71 W and 12.5 kHz, respectively.

GHSV [h−1] =60 × (𝑡𝑜𝑡𝑎𝑙 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 [cm3/min])

(𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑣𝑜𝑙𝑢𝑚𝑒 [cm3])

(5)

Within the product gas, H2, CH4, CO, O2 and CO2

were analyzed by QMS (quadrupole mass spectrometer,

Prisma, Pfeiffer Vacuum Technology) after trapping

liquid components by cold trap (ca. -40C). Pressure was

decreased to 5 kPa because DBD was extinguished when

solid carbon, which is electrically conductive material,

was formed on the catalyst surface.

III. RESULTS

Long Time Operation

Both hybrid and thermal operations were conducted

for 56-min, 14 cycles. Feed gas was supplied with

CH4/CO2 = 2, duty ratio = 25% and cycle time = 4-min.

The result of hybrid reaction is shown in Fig. 3. It shows

that the cyclic operation made long time stable operation

possible. Fig. 4 (a) focuses on the last 3 cycles of Fig. 3.

Fig. 4 (b) is the last 3 cycles of full operation by thermal

reaction. Before the reaction, catalyst bed temperature

was elevated in N2 flow until steady state temperature

distribution was established. This temperature is defined

as initial temperature as shown in Fig. 4.

Fig. 2. DBD-catalyst hybrid reactor.

Fig. 3. Long time operation (14 cycles, 56-min) by hybrid reaction.

Kameshima et al. 41

IV. DISCUSSION

A. Plasma heating

Table II shows conversions, solid carbon selectivity

and yield defined as (6-9).

CH4 𝑐𝑜𝑛𝑣. =𝑖𝑛𝑝𝑢𝑡 𝐶𝐻4 − 𝑜𝑢𝑡𝑝𝑢𝑡 CH4

𝑖𝑛𝑝𝑢𝑡 CH4

(6)

CO2 𝑐𝑜𝑛𝑣. =𝑖𝑛𝑝𝑢𝑡 𝐶𝑂2 − 𝑜𝑢𝑡𝑝𝑢𝑡 CO2

𝑖𝑛𝑝𝑢𝑡 CO2

(7)

C(s) 𝑠𝑒𝑙. =C(s)

𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑 C

=C(s)

𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑 CH4 + 𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑 CO2

(8)

C(s) 𝑦𝑖𝑒𝑙𝑑 =C(s)

𝑖𝑛𝑝𝑢𝑡 C

=C(s)

𝑖𝑛𝑝𝑢𝑡 CH4 + 𝑖𝑛𝑝𝑢𝑡 CO2

(9)

The moles of deposited carbon is same as the moles of

converted CO2 during the de-coking process because

carbon and CO2 react 1:1 molar ratio according to

Boudouard reaction (4). Deposited carbon is estimated by

(10), which is denoted by the area Ah or At in Fig. 4.

C(s) = 𝐴ℎ (𝑜𝑟 𝑡) = ∫ (𝑄𝐶𝑂2 × CO2 𝑐𝑜𝑛𝑣. )𝑑𝑡𝑑𝑒−𝑐𝑜𝑘𝑖𝑛𝑔

(10)

As shown in Fig. 4, the time for de-coking by thermal

reaction was 1.5 times long as hybrid reaction even

though the amount of deposited carbon were almost same,

as in Table II. It might be caused by the temperature

difference. Operating temperature of hybrid reaction was

higher than that of thermal reaction. However, initial

temperature of hybrid reaction was lower than that of

thermal reaction. Plasma generates not only radicals but

also heat internally. Plasma enables internal heating, so

efficient heat supply was possible and it resulted in high

conversions and short de-coking time.

B. CO formation and coking

In Fig. 4, at the moment of turning CH4 flow on and

off, the increase of CO mass spectrum intensity, as

described by arrows in Fig. 4, were different. Namely, it

Fig. 4. Last 3 cycles of cyclic operation (14 cycles, 56-min). (a): hybrid reaction, (b): thermal reaction.

TABLE II

MEAN CONVERSIONS, SOLID CARBON SLECTIVITY AND YIELD

IN REFORMING OF LAST CYCLE

CH4 conv. CO2 conv. C(s) sel. C(s) yield

Hybrid 0.60 0.77 0.17 0.11

Thermal 0.46 0.60 0.24 0.12

TABLE III

RATE CONSTANTS [17]

T [°C] k [s−1]

(CO2 – adsorbed CHx)

k [s−1]

(CO2 – C)

440 1.8 × 10−2 2.5 × 10−3

484 5.0 × 10−2 4.1 × 10−3

560 − 6.7 × 10−3

42 International Journal of Plasma Environmental Science & Technology, Vol.9, No.1, APRIL 2015

increased sharply when CH4 was supplied, but it increase

gradually when CH4 was stopped. It indicates that the CO

formation mechanism during the reforming and the de-

coking process are different.

As DMR mechanism on the nickel catalysts, following

series of reactions is widely accepted [16].

CH4 + (5 – x) * → CHx* + (4 – x) H* (11)

CO2 + H* → CO + OH* (12)

CHx* + OH* → CHxO* + H* (13)

CHxO* → CO* + x/2 H2 (14)

CO* → CO + * (15)

2H* → H2 + 2* (16)

Here, * denotes adsorbed species on active sites of

catalysts. First, CH4 is adsorbed and dissociated to CHx

and H on the catalysts (11). Subsequently, the adsorbed

species react with CO2 (12)-(15) and desorption of H2

occur (16). (12)-(15) can be summarized into (17). It is

the CO formation mechanism in the reforming process.

CO2 + CHx* → 2CO + x/2 H2 + * (17)

On the other hand, in the de-coking process, CO is

produced from solid carbon and CO2 via Boudouard

reaction (4). According to Osaki et al, the rate constant

on Ni/Al2O3 catalyst of (17) is faster than that of (4) [17],

as shown in Table III. Due to the above, CO2-CHx and

CO2-H reactions are dominant pathway for CO formation,

so CO mass spectrum intensity increased sharply when

CH4 supply was started. However, when CH4 supply was

stopped, only Boudouard reaction (4) whose rate is slow

compared with (17) was possible, so the increase of CO

mass spectrum intensity is slow.

CH4 adsorption (11) finally forms solid carbon (18).

CH4 → (CH3* → CH2* → CH* →) C* (18)

Intermediates and final product of (17), CHx and C, are

converted into CO via (17) and (4), respectively. If the

rate of CO formation was faster than that of (18), coking

didn’t occur, however, solid carbon was formed. It

suggests that the rate of coking (18) is fast.

V. CONCLUSION

In this work, long time (56-min) operation of DMR

was succeeded, and it was implied that the CO formation

rate in the reforming process was faster than in the de-

coking process. This suggests that reactions by CO2-CHx

and CO2-H are dominant in CO formation of reforming.

However, CO formation reactions, CO2-CHx, CO2-H and

CO2-C reactions, are slow compared with CH4

decomposition, then carbon is accumulated. Heat

generated by DBD could heat up the catalysts, so

reforming and de-coking reactions were promoted,

thereby CH4 and CO2 conversions increased to 0.60 from

0.46 and 0.77 from 0.60 in reforming, respectively, and

de-coking time was shortened by 35%, even when the

initial temperature was low.

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