Biogas to Syngas by Microwave-Assisted Reforming in the Presence

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Biogas to Syngas by Microwave-Assisted Reforming in the Presence of Char Mun Sup Lim and Young Nam Chun* Department of Environmental Engineering, Chosun University, Gwang-ju, Republic of Korea ABSTRACT: Methane (CH4) and carbon dioxide (CO2), the main components of biomass pyrolysis gasification gas and biogas, are byproduct gases that can be used as energy sources and are also greenhouse gases that contribute to global warming. This study on microwave reforming characteristics was conducted to convert the product gas into high-quality fuel energy. In the case of microwave reforming through carbon receptor application, the carbon was generated by a thermal decomposition, and then it attached on the surface of the receptor, interfering with the catalytic activity. Carbon dioxide reforming, however, cleaned the adsorbed carbon through carbon gasification and thus consistently maintained constant reforming conversion. Therefore, when the mixture of methane and carbon dioxide was reformed at the same time, the catalytic activity could be prevented from deteriorating because of carbon adsorption. When sludge char was used in this study as carbon receptor, the amount of hydrogen and carbon monoxide generated was higher than that generated by the commercial activated carbon receptor because of the former’s relatively high catalytic activity, yielding a gas product with higher heating value. It was also confirmed that the conversion and the product gas yield were high when the receptor bed reforming temperature was high and when the retention time was long.

1. INTRODUCTION Methane (CH4) and carbon dioxide (CO2) are the main components of biogas generated from an anaerobic digester. These two gases are greenhouse gases and are known to be the main causes of climate change.1 Therefore, there is a growing interest in developing reforming technologies for converting these biogas and greenhouse gases to high-quality fuel energy.2 Methane steam reforming has already been applied in the industrial field. However, methane dry reforming (i.e., CH4− CO2 reforming) is recognized as a more attractive method for greenhouse gas reduction and effective chemical conversion energy.3 The conversion of methane and carbon dioxide into hydrogen and carbon monoxide requires high-temperature reaction conditions or the help of a suitable catalyst. To address this problem, new catalysts with reforming conversion reaction and no carbon deposition have recently been under development by researchers. Catalysts made of noble metals or transition metal with excellent catalytic ability have been widely used.4−6 Noble metal catalysts, however, are too expensive, and transition-metal catalysts have the problem of catalytic ability degradation due to deactivation caused by carbon deposition. Basic research on methane−carbon dioxide reforming using different carbon materials is being conducted. These carbon materials are generally used as catalysts or catalyst carriers. Activated carbon,7 coal char,8 semicoke,9 and biochar10 are mainly being applied and used. These studies address the catalytic activity of the carbon materials themselves or impinged carbon-containing catalysts and the reforming reaction characteristics for different operating conditions. The carbonmaterial-based catalysts are relatively inexpensive, have high catalytic activity, and have little poisoning effect on sulfur components compared to existing catalysts. Biochar formed via biomass pyrolysis can be used for combustion and gasification or as the raw material for activated © XXXX American Chemical Society

carbon. Recently, there has been a great deal of interest in the production of syngas through the gasification of biochar.11,12 When biochar is used for methane reforming, it is also important to study the carbon gasification rate in the whole reforming reaction process because the gasification of biochar is a process not to be ignored. Nevertheless, there has been almost no in-depth research in this field. Two types of heating methods can be applied: a conventional hot gas heating or a microwave-induced heating. The microwave heating method has better energy efficiency than the existing hot gas or electric heating methods and has excellent thermal characteristics for rapid, selective, and uniform heating.11 Microwave heating has so far been widely applied to various fields, including environmental pollutant reduction, biomass pyrolysis/gasification, and material drying. The microwave heating method has been applied of late for the methane−carbon dioxide reforming of the already mentioned carbon-based catalysts.10,13 On the basis of these studies, it was confirmed that carbon materials are excellent microwave receptors and that their gas reforming conversion is superior to that of the existing heating methods. Furthermore, it has been reported that the selectivity of the product gas was improved and that the carbon deposition was also reduced. There have been few studies, however, on the reforming characteristics of sludge char when using it as a microwave receptor. Therefore, the reforming characteristics of the biogas were investigated in this study using the sludge char produced by the pyrolysis of dewatered sewage sludge as a microwave carbon receptor (MCR). The reforming characteristics for the biogas Received: September 17, 2017 Revised: November 28, 2017 Published: November 28, 2017 A

DOI: 10.1021/acs.energyfuels.7b02799 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 1. Experimental apparatus for the microwave heating reforming. The inset picture shows microplasma in the carbon receptor bed.

Table 1. Chemical Characteristics of the Sludge Chara ultimate analysisb,c (wt %)

proximate analysis (wt %) SC AC a

b

b

b

M

A

VM

FC

1.19 3.82

68.32 33.91

4.18 2.03

27.50 64.06

C

H

N

S

O

HHVb (MJ/kg)

28.13 89.66

0.62 0.19

2.86 ND

0 ND

6.18 5.55

9.32 29.69

SC, sludge char; AC, activated carbon; M, moisture; A, ash; VM, volatile matter; FC, fixed carbon. bDry basis. cAsh-free basis. instrument (CP-4900, Varian, Netherlands) was connected for the reforming product gas analysis. 2.2. Experimental Method. The microwave heating reforming experiments were performed by radiating microwave energy to the carbon receptor in the sample basket located in the quartz tube reactor. Then the simulated gas supplied into the quartz tube reactor passed through the receptor center so that it was reformed on the carbon receptor showing local microplasma discharge. The reformed gas came out of the exit and was sampled and analyzed by the gas analyzer. The carbon receptor was sieved to 1−3 mm, and 15 g of it was placed in the sample basket for every experiment so that it could be located in the center of the quartz tube reactor. The simulated gas was supplied at a constant flow rate of 75 mL/min so that the volumetric hourly space velocity (VHSV) could remain constant. The microwave power supply was initially started at 2 kW to allow the sample temperature to increase linearly from room temperature and then remain constant at the reference set temperature. The experiments were conducted for two cases. First, reforming experiments were performed for three different gases: CO2 25%, CH4 25%, and a mixture of CO2 12.5% and CH4 25% (all of them diluted in N2) to understand the reforming characteristics according to the different gas properties. Second, experiments were performed to investigate the effects of the changes in the main variables affecting reforming, such as the reforming temperature, space velocity, carbon receptor, and biogas. Sampling of the reformed gas for gas component analysis was performed using Tedlar bags for gas collection at each time interval (10 or 20 min) from the beginning of the experiment. The biogas and reformed gas were analyzed via a GC-TCD instrument. Molecular sieve 5A (80/100 mesh) was applied to the hydrogen, methane, carbon monoxide, oxygen, and nitrogen gases, and a PoraPlot-Q column was applied to the carbon dioxide, ethylene, and ethane gases.

according to the changes in the gas composition, type of carbon receptor, reforming temperature, and retention time were investigated.

2. EXPERIMENTAL APPARATUS AND METHOD 2.1. Experimental Apparatus. The microwave heating reforming system used in this study is a laboratory-scale test rig consisting of a microwave reformer, a gas supply line, monitoring and control equipment, and a sampling and analysis line (Figure 1).14 The structure of the microwave reformer (MW reformer) includes a quartz tube reactor (40 mm diameter and 320 mm length) vertically installed in a multimode-microwave cavity oven with a 2 kW power capacity. The reformer temperature could be set up to 1000 °C, and the temperature was finely controlled through a controller connected to a thermocouple (k-type, 2 mm diameter) in the microwave carbon receptor. The temperature change inside the carbon receptor was continuously monitored using a data logger (Model Hydra data logger 2625A, Fluke, United States). A sample basket of the carbon receptor was vertically shifted inside the quartz tube reactor so that the receptor sample could be moved into and out of the reactor. In the gas supply line, carbon dioxide and methane, the reforming gases, and nitrogen, the carrier gas, were supplied from their containers to the mixer via a mass flow controller (Bronkhorst, F201AC-FAC-22V, Netherlands), which controlled each gas flow. LabVIEW (LabVIEW 8.6, National Instrument, United States) was used as a monitoring and control device, and the gas flow control and temperature were continuously monitored. The sampling and analysis line consisted of a glass wool filter for removing soot and moisture, an impinger with injected calcium chloride, and a cooler (ECS-30SS, Eyela Co., Japan). A gas chromatography with thermal conductivity detection (GC-TCD) B


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Energy & Fuels Table 2. Selected Inorganic Composition of the Sludge Char (wt %) SiO2

P2O5

Al2O3

CaO

Fe2O3

MgO

K2O

SO3

ZnO

CuO

BaO

MnO

SrO

NiO

28.35

24.76

13.09

12.06

8.98

4.55

2.64

2.05

0.56

0.35

0.20

0.16

0.07

0.04

Scanning electron microscopy (SEM; model S-4800, Hitachi Co., Japan) was performed to determine the physical properties of the microwave carbon receptor before and after the reforming test. Sludge char, which was produced after the microwave heating pyrolysis of a dewatered sewage sludge, was used as a carbon receptor for the reforming experiment. The conditions were 800 °C of pyrolysis temperature, 100 min of pyrolysis time, 60 °C/min of heating rate, and N2 atmosphere. Commercial activated carbon was also used as the carbon receptor for comparison with the result of the sludge char. Proximate analysis (Thermolyne Co., type 48000 furnace/Hansung Co., HS2140 electronic balance) and ultimate analysis (Thermofinnigan Co., EA2000/EA1112) were performed to identify the characteristics of the sludge char carbon receptor. Table 1 shows the results of the proximate and ultimate analyses of the sludge char and activated samples. The aluminum, silicon, iron, and manganese were measured using an X-ray fluorescence (XRF) spectrometer (Shimadzu Co., ED-720) to analyze the inorganic oxides be present in the sludge char. Table 2 shows the results. The conversions of carbon dioxide (XCO2) and methane (XCH4), the main gases to be reformed, are shown in eqs 1 and 2.

XCH4 =

FCH4in − FCH4out FCH4in

Figure 2. CO2 conversion and reformed-gas composition during carbon dioxide reforming.

energy. Therefore, a microplasma is generated inside the carbon receptor, which is a dielectric solid, so that a high temperature is maintained at specific local positions. As a result, the gasification reaction (eq 3), which is a heterogeneous reaction, is activated in the hot-spot portions. The microplasma phenomenon can be seen in the inset in Figure 1. 3.1.2. Methane Reforming. Figure 3 shows the case where only methane was supplied to the microwave carbon receptor.

× 100% (1)

where FCH4in and FCH4out are methane flows (mL min−1) at the inlet and outlet, respectively. XCO2 =

FCO2in − FCO2out FCO2in

× 100% (2)

where FCO2in and FCO2out are carbon dioxide flows (mL min−1) at the inlet and outlet, respectively.

3. RESULTS AND DISCUSSION The syngas conversion characteristics according to the simulated biogas composition and the effects of each reforming variable were investigated. 3.1. Effects of Biogas Composition. The reforming process of carbon dioxide, methane, and methane−carbon dioxide gas mixture was investigated in the presence of a microwave carbon receptor. These experiments for evaluating the impact of gas composition were carried out at a reforming temperature of 900 °C and a VHSV of 0.3 L/g·h using sludge char as the carbon receptor. 3.1.1. Carbon Dioxide Reforming. Figure 2 shows the reforming results when the only feed is carbon dioxide. As microwave reforming started, the carbon dioxide conversion increased sharply, reached the maximum conversion, and then gradually decreased. This is because the carbon dioxide reacted with solid carbon (Cchar) present in the carbon component and converted into carbon monoxide, as shown in eq 3, which is a carbon gasification reaction (Boudouard reaction). The reason for the slight CO2 conversion decrease after 40 min is the consumption of Cchar. Cchar + CO2 ⇔ 2CO

ΔH298 = + 173 kJ/mol

Figure 3. CH4 conversion and reformed-gas composition during methane reforming.

As the reforming time increased, the conversion of methane rapidly increased, reached its maximum value, and significantly decreased. This is because methane is converted to hydrogen and carbon through thermal decomposition reaction (eq 4). It is known that the produced carbon is adsorbed on the receptor surface, thus reducing the porosity. Through the partial oxidation reaction (due to the residual oxygen in sludge

(3)

Unlike the conventional heating method, in which the heat is transmitted from outside to inside the receptor, the microwave heating transfers microwave energy inside the receptor, and the kinetic energy due to the object vibration is converted to heat C


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Energy & Fuels char), CH4 was converted to hydrogen and carbon monoxide as shown in eq 5. As methane conversion increased, methane concentration decreased and hydrogen concentration increased in the outlet gas in a similar pattern, and a small amount of carbon monoxide increased in the first half of the reforming. CH4 ⇔ C + 2H 2

CH4 +

ΔH298 = +75 kJ/mol

1 O2 ⇔ 2H 2 + CO 2

(4)

ΔH298 = −8.5 kJ/mol (5)

The thermal decomposition reaction (eq 4) and the partial oxidation reaction (eq 5) mentioned above are homogeneous reactions in which carbon receptors containing catalytic components have an active role in microwave-absorbing dielectric solids.10 As mentioned earlier, as the microplasma is formed in the receptor bed and the high temperature is maintained, the reactivity of the peripheral gas is improved because of the high temperature. This is particularly true in the thermal decomposition shown in eq 4.10 Methane conversion was reduced after reaching the maximum value. This is because the carbon generated in the thermal decomposition (eq 4) was attached on the surface of the receptor, thereby preventing inlet methane from penetrating the catalytic activated pores of the carbon receptor and interfering with the catalytic activity that boosts the thermal decomposition reaction. As a result, the problem of microwave heating methane reforming using a catalyst-containing receptor is that the carbon produced in the methane decomposition reaction is adsorbed on the surface of the catalyst receptor, thereby inhibiting the catalytic activity of the receptor. Similar results have been reported by other studies on various carbonaceous catalysts, such as char,15 activated carbon,16 and carbon black.17 3.1.3. Reforming of the Carbon Dioxide−Methane Mixture Gas. The problem of the deposition of the carbon generated during methane reforming (see eq 4) on the receptor catalytic active center can be solved by carbon gasification with carbon dioxide, as shown in eq 6. It has been reported that the cleaning of the catalytic active center is also possible through steam gasification.18 In addition, the microwave heating method, in which microplasma is generated in the carbon receptor bed, is more effective than the conventional heating method.16

CCH4 + CO2 ⇔ 2CO

Figure 4. CO2−CH4 conversions and reformed-gas composition during mixed-gas reforming.

reaction shown in eq 7, which is a homogeneous reaction of methane and carbon dioxide gases. Therefore, in the case of the methane−carbon dioxide mixture gas, the conversion was higher than that in the carbon dioxide reforming. The conversion was somewhat lower than that in the methane reforming but remained constant. CH4 + CO2 ⇔ 2CO + 2H 2

ΔH298 = 260.5 kJ/mol (7)

The reason for the better conversion of carbon dioxide than methane in the case of the mixture gas is that the adsorbed carbon generated by the methane thermal decomposition reaction (eq 4) was converted by eq 6, while the attached carbon promoted the generation of microplasma as a microwave receptor, thereby favoring the carbon gasification reaction (eq 3). The above-mentioned conversion of the mixture gas could be analyzed from the syngas composition that the concentration of methane and carbon dioxide decreased and the concentration of hydrogen and carbon monoxide increased as the microwave reforming progressed. Panels a and b of Figure 5 show the SEM microphotographs (1000× magnification) of the sludge char carbon receptor before and after the microwave reforming process, respectively. Through the microwave heating reforming of the carbon dioxide−methane mixture gas, microwave energy transferred inside the carbon receptor, resulting in volumetric heating and local microplasma formation. As a result, local high-temperature parts were generated, and the texture of the carbon receptor’s surface became vitreous-like. Furthermore, it can be seen that some residual adsorbed carbon generated by the thermal decomposition reaction (eq 4) was not cleaned by the carbon gasification reaction (eq 6) and that some of the carbon adsorbed near the active center was converted to carbon nanofibers, as can be seen in Figure 5b. Similar results (i.e., that carbon nanotubes were generated only in the microwave reforming method) were obtained in a study on the carbon receptor reforming through the microwave heating method.19 Figure 6 shows the energy-dispersive X-ray (EDX) results for the semiquantitative chemical analysis of the sludge char receptor. As already mentioned, inorganic and metallic

(6)

To confirm this, an experiment was conducted in this study to investigate the reforming process of a gas mixture composed of methane and carbon dioxide. The results are shown in Figure 4. In the case of the mixed gas, after the start of reforming, methane and carbon dioxide conversions significantly increased until 60 min and then remained consistent. Methane was thermally decomposed (eq 4), and hydrogen was generated. Carbon was absorbed on the surface of the receptor, and carbon monoxide was generated while the adsorbed carbon was reduced through carbon gasification (eq 6) by carbon dioxide. Therefore, unlike the above-mentioned case, where only methane was supplied, it can be seen that in the case of the mixture gas, the CH4 conversions did not decrease significantly after the lapse of time. In addition to this heterogeneous solid−gas reaction, carbon monoxide and hydrogen are produced by the dry reforming D


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Energy & Fuels

Table 3 shows the surface properties of the sludge char carbon receptor before and after reforming. After reforming, Table 3. Surface Properties of the Sludge Char sample BET specific surface area (m2 g−1) microporous specific surface area (m2 g−1) mesoporous specific surface area (m2 g−1) total pore volume (cm3 g−1) average pore size (nm)

sludge char

used sludge char after reforming

13.32 6.31

109.06 55.34

7.01

53.72

0.04 20.59

0.11 11.12

the micropores and mesopores increased, and as such, the specific surface area also increased. On the other hand, the average pore size decreased. This is because, as already mentioned, the micropores increased because of the carbon gasification reaction shown in eq 3. 3.2. Effect of the Reforming Temperature. Figure 7 shows the results of the reforming process of the carbon

Figure 5. SEM images for the sludge char: (a) before MW reforming and (b) after MW reforming.

Figure 6. Element compounds of the sludge char measured via EDX.

components among the sludge char components that act as catalysts for gas reforming were observed. In particular, magnesium and calcium, which are alkaline earth metals, and iron and magnesium, which are common metals, have a great effect on reforming.19 After the reforming, the carbon in the sludge char receptor was consumed and reduced by the carbon gasification reaction (eq 3). The concentrations of the catalytic metal components were increased, which was not due to an increase in the amount of metals after reforming but was a result of the simple increase in the weight ratio due to the carbon reduction.

Figure 7. Comparison of the different reforming temperatures (VHSV = 0.3 L·g−1·h−1): (a) carbon dioxide and methane conversions and (b) reformed-gas composition. E


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Energy & Fuels dioxide−methane gas mixture when the temperature of the carbon receptor bed was changed while the VHSV was constantly maintained at 0.3 L·g−1·h−1. The temperature was measured by the thermocouple which is known to generate microwave irradiation interference and catalytic effect during the microwave heating process.20 This could be expected to cause slight reforming data fluctuations according to the test time lapse, but the reforming data for each reforming temperature was not affected by the data fluctuations. Figure 7a shows the separate conversions of carbon dioxide and methane. The three reforming temperatures showed nearly constant conversions after a sharp increase. Both conversions were found to be higher when the reforming temperature was higher. This is because the reactivity of the reforming reaction was enhanced as the set reforming temperature was increased, and the carbon gasification reaction (eq 3) for carbon dioxide reforming and the thermal decomposition reaction (eq 4) for methane reforming were enhanced. Figure 7b shows the composition and the higher heating value of the product gases according to the different reforming temperatures. As the reforming temperature increased, the concentration of carbon dioxide and methane decreased because of an increase of both conversions, whereas the concentration of hydrogen and carbon monoxide increased because of the dry reforming reaction (eq 7). In this case, the concentrations of carbon dioxide and carbon monoxide showed little trends because the attached carbon reaction (eq 6) was sensitive to the local bed temperatures. In this case, the molar ratio of the two gases (H2/CO) was 1.58 at 1000 °C, and this ratio decreased when reducing the reforming temperature. In the case of the heating value, however, the highest value was found at 16 MJ/m3 when the reforming temperature was 800 °C. This is due to the lowest methane conversion. 3.3. Effect of Retention Time on the Carbon Receptor Bed. Figure 8 shows the results of the reforming conversions according to the change in the volumetric hourly space velocity in which the processed gas passes through the carbon receptor bed in the quartz tube reactor. As shown in Figure 8a, the overall reforming pattern was similar, and the carbon dioxide and methane conversions were higher when the space velocity was 0.3 L·g−1·h−1 and were lower when the space velocity was 0.8 L·g−1·h−1 or higher, because of the short retention time. This is because, as already mentioned, the retention time affected the carbon gasification reaction (eq 3) for carbon dioxide and the thermal decomposition reaction (eq 4) for methane. Particularly, in the case of carbon dioxide conversion, it was necessary for carbon dioxide to penetrate the pores of the sludge char receptor; thus, carbon dioxide conversion was relatively high at a low space velocity of 0.3 L·g−1·h−1 because that provided a longer retention time. Figure 8b shows the composition and higher heating value of the reformed gas. As the volumetric space velocity increased, the concentrations of hydrogen and carbon monoxide gradually decreased, while the ratio between both gases (H2/CO) remained almost the same. Methane and carbon dioxide concentrations were lower when the VHSV was lower, as can be seen from the conversions. The heating value increased with lower retention time because of the increase of combustible gases.

Figure 8. Comparison of the different retention times; reforming temperature: 900 °C. (a) Carbon dioxide and methane conversions (b) Reformed-gas composition.

3.4. Effect of the Reforming Carbon Receptor. Figure 9 shows the results of the reforming experiments for the carbon dioxide−methane gas mixture using a commercial activated carbon under the same conditions as the reference test (VHSV = 0.3 L·g−1·h−1 s, 900 °C) to compare the conversion characteristics of the sludge char receptor used in this study and of the existing commercial carbon receptor. In Figure 9a, the activated carbon receptor showed the conversion patterns of carbon dioxide and methane to be similar to those of the sludge char receptor. In the case of the carbon dioxide conversion, however, the final conversion was smaller than that of the sludge char receptor. This is because the sludge char receptor contained the catalyst components seen in Table 2, and the carbon gasification reaction (eq 3) on the surface of the micropores in the sludge char receptor was more reactive than the commercial activated carbon receptor, thereby promoting the reaction. The conversion pattern and the final conversion value of methane were almost the same in both cases, so it is considered that methane conversion was not significantly affected by the catalyst content. F


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Figure 10. Effect of the simulated biogas: H2, 40%; CO, 10%; CO2, 15%; CH4, 15%; N2, 20%.

dioxide, thus trading off and reducing the carbon dioxide conversion. Hydrogasification C + 2H 2 ⇔ CH4

−75 MJ/kmol

(8)

Methanation CO + 3H 2 ⇔ CH4 + H 2O

−227 MJ/kmol

(9)

Water-gas shift reaction CO + H 2O ⇔ CO2 + H 2

−41 MJ/kmol

(10)

Figure 9. Comparison of the different receptors (VHSV = 0.3 L·g−1· h−1; reforming temperature = 900 °C): (a) carbon dioxide and methane conversions and (b) reformed-gas composition.

As can be seen from the hydrogen and carbon monoxide concentrations, although certain amounts of these gases were supplied when the simulated gas was introduced to the reforming reactor, the increased concentrations of these gases were smaller than those of the gas mixture in Figure 4 that were newly generated by reforming.

Figure 9b shows the gas composition and higher heating values. The sludge char receptor generated relatively higher concentrations of hydrogen and carbon monoxide, but the ratio of both gases (H2/CO) was not significantly different. The heating values were also almost the same at 14.3 MJ/m3 in both cases. 3.5. Reforming Characteristics of the Simulated Biogas. Figure 10 shows the reforming results of the biogas. As the reforming time increased, the conversions of carbon dioxide and methane showed patterns similar to experiment results obtained with the gas mixture (shown in Figure 4), but the initial increases of both conversions were relatively low, and the conversions were also rather low according to the lapse of time. This is due to the secondary reactions caused by the addition of hydrogen and carbon monoxide in the case of the simulated gas as compared with the case of the mixture of carbon dioxide and methane (shown in Figure 4). The main secondary reactions involve the hydrogen gasification reaction (eq 8) and the methanation reaction (eq 9), where the added hydrogen produces methane and thus the methane conversion is traded off and reduced, and the water−gas shift reaction (eq 10), where the added carbon monoxide produces carbon

4. CONCLUSIONS In this study, microwave heat reforming was investigated to convert methane and carbon dioxide, the main components of biogas, to high-quality fuel energy and to address the greenhouse gas problem. As a result of the analysis of the carbon monoxide and methane reforming characteristics using the microwave heating along with carbon receptors, the carbon dioxide produced carbon monoxide through the gasification of carbon present in the solid, and the methane produced hydrogen and carbon through thermal decomposition. The carbon that had been produced was adsorbed by the activated center of the carbon receptor and reduced the gas reforming rate. In the case of the microwave reforming of the carbon monoxide−methane mixture, while the carbon that was generated during methane decomposition was adsorbed by the receptor and interfered with the catalytic activity, the cleaning of carbon through gasification with carbon dioxide consistently maintained constant reforming conversion. The use of sludge char as a carbon receptor increased the amounts of hydrogen and carbon monoxide through a relatively high catalyst activity compared to the commercial activated G


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Energy & Fuels carbon. It was confirmed that the conversion and the product gas yield were high when the receptor bed reforming temperature was high and the bed retention time was long.

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AUTHOR INFORMATION

Corresponding Author

*Department of Environmental Engineering, Chosun University, #375, Seosuk-dong, dong-gu, Gwangju 501-729, Rep. of Korea. E-mail: [email protected]. Phone: 82-62-230-7156. Fax: +82-62-232-2474. ORCID

Young Nam Chun: 0000-0002-7617-7705 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the South Korean government (MSIP) (No. 2015R1A2A2A03003044).

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REFERENCES

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Biogas to Syngas by Microwave-Assisted Reforming in the Presence

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