Effect of Additives on the Performance of Monolithic Catalyst for Tar

The catalyst with Mo additive exhibited the best performance, with its tar ... were dried in a microwave oven for 15 min and calcined at 550 °C for 2...
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Effect of Additives on the Performance of Monolithic Catalyst for Tar Cracking Min Lu, Zuhong Xiong,* Pengmei Lu, Zhenhong Yuan, Genyu Fan, and Yong Chen Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China ABSTRACT: Five cordierite-supported monolithic catalysts with different additives were prepared by vacuum wetness impregnation. All catalysts were characterized by X-ray diffraction (XRD), Brunauer−Emmett−Teller (BET), temperatureprogrammed reduction (TPR), and Raman spectroscopy. These catalysts were also tested by the raw gas from biomass pyrolysis to contrast them in terms of catalytic performance and stability. Characterization results show the formation of Mo−Ni alloy in the reduced catalyst with Mo additive; however, the other additives in the reduced catalyst remain in oxidation state. TPR revealed a strong interaction between the active component and the additives, resulting in a decrease of reduction ability. The catalysts with additives were shown to perform more efficiently compared with the catalysts without additives. The catalyst with Mo additive exhibited the best performance, with its tar conversion and gas yield reaching 96.6% and 1.22 N m3/kg, respectively, at a weight hourly space velocity of 706 kg/(h·m3). The 1Mo3Ni1Co/Cor catalyst exhibited higher stability compared with 1Sm3Ni1Co/Cor because the tar conversion of the former remained higher than 90%, whereas that of the latter decreased to 73.1% after a 4 h catalysis. Coke deposits were formed on the surface of the 1Sm3Ni1Co/Cor catalyst, which mostly exhibited a graphite structure, as observed by Raman spectroscopy.

1. INTRODUCTION Problems on the high cost of energy, energy shortage, and environmental pollution have intensified with the exploitation of fossil energy. Biomass has drawn considerable attention owing to its high availability, low emission, and renewability. However, tar is an inevitable byproduct in biomass pyrolysis/ gasification, which can easily cause the downstream equipment blockage and energy waste.1,2 Therefore, tar removal from biomass pyrolysis/gasification is necessary. Catalytic tar cracking, the most effective and advanced method of tar removal, has received significant research interest among scholars. The most widely studied catalysts include ore catalysts,3−5 nickel-based catalysts,6−8 and novel metal catalysts.9−11 Whereas comparing with these granular catalysts, the monolithic catalysts show not only superior catalytic performance and minimal pressure reduction but also several technological advantages (large cross-sectional area, low heat capacity, low thermal expansion coefficient, and high thermal stability).12,13 In decade years, monolithic catalysts have been widely used in chemical product synthesis and environmental protection.14−16 However, studies on the use of monolithic catalysts for tar cracking are rarely reported. Corella et al.17 investigated the use of commercial nickel-based monolithic catalysts for the removal of tar in gasification raw gas. The commercial monolithic catalyst was shown to perform poorly because tar conversion was only 70% under most conditions and 90% under few conditions. The monolithic catalyst performed similarly with dolomite, but the monolithic catalyst could be adapted to a gas environment containing particles. Toledo et al.18 used a generation II reactor based on the work by Corella et al. to improve tar conversion. The monolithic catalysts were placed in two layers of the reactor. Air was fed between the two-layer catalyst to increase the gas temperature and reduce the difference in temperature between the two ends © 2013 American Chemical Society

of the monolithic catalysts by combustion. Tar conversion reached 91% ± 4% under this condition. Thus, the two-layer catalyst is more reasonable and effective than the one-layer catalyst. Zhao et al.19 discussed the steam reforming performance of Ni/cordierite under different conditions with toluene as the tar model compound. The results showed that toluene conversion was 94.1 wt % when steam/carbon atomic ratio was 2 and the catalytic temperature was 1173 K. The aforementioned studies on monolithic catalysts mainly focused on nickelbased catalysts. In the present study, different transition metal additives were introduced to Ni−Co bimetallic catalysts to improve the performance of the monolithic catalysts.

2. EXPERIMENTAL METHODS 2.1. Catalyst Preparation. M−Ni−Co/cordierite catalysts were prepared by vacuum impregnation (M represents La, Ce, Sm, Y, and Mo). Commercial cordierites were pretreated for 5 h in 30% boiling oxalic acid before vacuum impregnation. After acid treatment, boiling water was used to remove residual oxalic acid solution in cordierite. All cordierites were then dried at 100 °C for 2 h and calcined at 500 °C for 2 h. The impregnation solution was prepared by dissolving Ni(NO3)2·6H2O and Co(NO3)2·6H2O in deionized water (the mole ratio of Ni and Co is 3:1). The cordierite was impregnated in this mixed solution for 1 h. The samples after vacuum impregnation were dried in a microwave oven for 15 min and calcined at 550 °C for 2 h in air. Total loading of the active component was maintained at about 23 wt % after repeated loading. Several additive solutions such as (La(NO3)3, Ce(NO3)3, Sm(NO3)3, Y(NO3)3, and (NH4)6Mo7O24) at specific concentrations were prepared. The additives were loaded using the same method. The mole ratio of the transition metal atom and nickel atom is about 1:3. The prepared catalysts were named Received: November 28, 2012 Revised: April 9, 2013 Published: April 10, 2013 2599

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Table 1. Proximate and Ultimate Analyses of Miscellaneous Sawdust proximate analysis (wt%, dry basis)

ultimate analysis (wt%, dry basis)

higher heating value (MJ/kg)

moisture content (wt%, wet basis)

volatile matter

fixed carbon

ash

C

H

O

N

S

19.6

11.5%

80.45

17.83

1.82

48.88

5.38

42.73

0.24

0.95

Figure 1. Schematic of the equipment for biomass raw gas catalytic purification. xMmNinCo/Cor, with x, m, and n representing the mole ratios of M, Ni, and Co, respectively. 2.2. Catalyst Characterization. The X-ray diffraction (XRD) patterns of the samples before and after reduction were obtained on a D8 ADVANCE diffraction meter (Bruker Co. Germany) using Cu− Kα generated at 40 KV and 40 mA. Diffraction grams were registered in the range of 2θ between 5° and 90° with a scanning step of 0.02° and a step time of 17.7 s. Temperature-programmed reduction (TPR) with H2 was performed in a quartz tube reactor by using a CPB-1 automatic temperature programmed adsorption analyzer. Hydrogen consumption was measured with a thermal conductivity detector. About 0.1 to 0.2 g of catalyst was placed in the reactor, heated to 120 °C in argon gas with a flow rate of 30 mL/min, and purged for 30 min. After cooling down to 50 °C, the carrier gas was replaced by 10% H2/Ar at a flow rate of 20 mL/min. The temperature was increased from 50 to 850 °C at a heating rate of 5 °C/min. The surface area, pore volume, and average pore diameter of the catalysts were measured by physical adsorption of N2 by using a SIMP-10/PoreMaster 33 instrument (Quantachrome Co. USA). Prior to measurement of N2 adsorption, the catalysts were vacuumed for 5 h at 200 °C. N2 adsorption−desorption was performed in a liquid nitrogen environment. The surface area of the catalysts was measured by the Brunauer−Emmett−Teller method. The pore volume and the pore size distribution of the catalysts were measured by the Barrett− Joyner−Halenda method. Raman spectroscopy was performed using a HORIBA Jobin-Yvon HR800 spectrometer with a spectral resolution of less than 1 cm−1. The laser line at 325 nm of a He−Cd laser was used as an excitation source. 2.3. Feedstock. Miscellaneous sawdust (particle size ranging from 0.12 mm to 0.25 mm) obtained from a timber mill in Guangzhou, China, was used as feedstock for the experimental runs. Results of the proximate and ultimate analyses are reported in Table 1. 2.4. Catalytic Test. Catalytic activity tests were performed in a downdraft pyrolysis reactor (ID = 80 mm), as shown in Figure 1. Before the catalytic test, all catalysts were reduced at 650 °C for 5 h in H2/N2 atmosphere (1:8, vol%, 0.45 m3/h). Several pieces of activated

catalysts were placed in the catalytic reactor (ID = 43 mm), with a gap (about 40 mm) between each catalyst to allow full contact between the raw gas and the catalysts as well as to decrease pressure drop. When the temperature of pyrolysis and catalysis reached 800 °C, the miscellaneous sawdust was slowly conveyed to the pyrolysis reactor by a screw feeder. The carrier gas (N2-0.16 m3/h) was simultaneously conveyed to the paralysis reactor. Large carbon particles were removed by lowering the raw gas flow rate in a gravity dust remover (ID = 200 mm) which was connected to the pyrolysis reactor, and the temperature was maintained at 400 °C to prevent tar condensation. Silica wool was filled in the catalytic reactor entrance to remove the smaller carbon particles in the raw gas. After catalysis, the tar particles in the product gas were collected by acetone, the volume of reformed gas was measured with a wet gas flow meter, and the reformed gas was sampled to detect gas composition and content. 2.5. Gas and Tar Analyses. The contents of H2, O2, N2, CH4, CO, CO2, and C2 in the product gas were analyzed on a gas chromatograph (Model GC-20B-1, Shimadzu Co. Japan) equipped with a thermal conductivity detector. Ar was used as the carrier gas. Standard gas mixtures with 40.29 vol% N2 were used for quantitative calibration. After vacuum filtration, samples of the collected liquid were analyzed by gas chromatography (GC)−mass spectrometry (Model HP4890D, Thermo Quest CE Instrument Co., US). The column used was DB-5. The interface temperature, ion source temperature, and testing time were 250 °C, 200 °C, and 53 min. The quantity of tar was obtained by rotary distillation at a certain water bath temperature and under vacuum. Some parameters were calculated as follows: 1) Weight hourly space velocity (WHSV (kg/(h·m3)): WHSV = biomass feeding rate/total bulk volume of catalysts 2) Carbon conversion (%) = total number of mole of carbon in gas/ total number of mole of carbon in fed biomass × 100 3) Hydrogen yield:

YH2(g/kg, biomass) = 10 × Vg × H2 × M H2 /22.4 2600

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where Vg represents gas production per biomass (Nm3/kg, biomass); H2 represents volume content of H2 in product gas (%); and MH2 represents the molar mass of H2 (g/mol). 4) Hydrogen selectivity:

SH2 =

WH2 WH,biomass

(%)

where WH,biomass and WH2 represent H mass in biomass and in product gas (g). 5) Low heating value (LHV) of product gas LHV (MJ/Nm 3) = (107.98 × VH2 + 358.18 × VCH4 + 123.36 × VCO + 594.4 × VC2)/1000 where VH2, VCH4, VCO, and VC2 represent the volume content of H2, CH4, CO, and C2 in product gas (%) (excluding N2).

3. RESULTS AND DISCUSSION 3.1. Characterization of Catalysts. 3.1.1. XRD. Figure 2 shows the X-ray patterns of five kinds of catalysts before and after reduction. The calcined catalysts exhibit obvious NiO and CoO peaks, with major peaks at 2θ angle of 37.2°, 43.3°, and

Figure 3. (a) N2 adsorption−desorption isotherms and (b) pore size distribution of the as-prepared catalysts.

Table 2. BET Surface, Pore Volume, and Average Pore Diameter of the Catalysts before and after Reaction catalyst

SBETa (m2/g)

Vpb (cm3/g)

Dpc (nm)

1La3Ni1Co/Cor 1Ce3Ni1Co/Cor 1Sm3Ni1Co/Cor 1Y3Ni3Co/Cor 1Mo3Ni1Co/Cor 1Sm3Ni1Co/Cor after a 4 h reaction 1Mo3Ni1Co/Cor after a 4 h reaction

9.74 32.7 19.4 26.7 19.7 20.8 17.2

0.022 0.045 0.033 0.045 0.047 0.034 0.053

8.97 5.51 6.78 6.76 9.55 6.61 12.2

a

SBET, BET surface area. bVp, total pore volume of pores less than 160 nm. cDp, adsorption average pore diameter (4 V/A by BET).

62.8° as well as 36.9°, 42.8°, and 62.1°, respectively (Figure 2a). However, no characteristic peaks were observed for Ni(NO3)2, Co(NO3)2, NiAl2O4, and CoAl2O4. These findings suggest that the nitrate precursor decomposed completely, and no spinel formation occurred in all catalysts after calcination. The transition element compounds had different characteristic peaks that varied in intensity. This occurrence was due to the variation in size of transition metal oxide particles and variation in the distribution uniformity of the transition metal oxides in the catalysts. Figure 2b reveals the characteristic peaks of Ni and Co at 44.5°, 51.9°, and 76.4° as well as 44.4°, 51.6°, and

Figure 2. XRD patterns of the catalysts before and after reduction (a) before reduction and (b) after reduction). 2601

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Figure 4. TPR profiles of the as-prepared catalysts.

Table 3. Pyrolysis Operating Conditions and Raw Gas Composition pyrolysis operating conditions temperature (°C) carrier gas-N2 (m3/h) biomass feeding rate (kg/h wet basis) biomass water content (wt% wet basis) gas composition (vol.% dry basis) H2 CH4 CO CO2 C2H4 C2H6 C2H2 gas yield (Nm3/kg biomass) LHV (MJ/Nm3) carbon conversion (%) tar yield (g/kg)

800 0.16 0.16 11.5 23.85 14.55 45.17 12.03 4.06 0 0.34 0.70 15.77 69.76 67.63

Figure 5. Effect of the catalysts with different transition metals on (a) gas composition and (b) tar conversion, carbon conversion, and gas yield.

76.1°, respectively. The characteristic peaks of NiO and CoO disappeared, indicating a marked reduction in NiO and CoO. For the 1Mo3Ni1Co/Cor catalyst, a strong interaction occurred between the metals Mo and Ni, and a Mo−Ni alloy was formed with the 2θ angle at 40.9°, 43.2°, 43.9°, 44.4°, and 45.4°. However, the transition metals, La, Ce, Sm, and Y, were in the form of oxides. 3.1.2. BET. The N2 adsorption−desorption isotherm and the pore size distribution curve of five kinds of catalysts are shown in Figure 3. Figure 3a reveals the mesoporous structure of the catalysts. All N2 adsorption−desorption isotherms were IV isotherms and exhibited N2 hysteresis loops caused by capillary condensation, indicating the presence of numerous disorder particles or agglomeration and sintering of several particles.20 The pore size of all catalysts ranged from 1 to 5 nm, and most pores were nearly 4 nm in size, as shown in Figure 3b. The pores with a size ranging from 1 to 3 nm in catalyst showed a marked change in quantity. The 1Ce3Ni1Co/Cor and 1La3Ni1Co/Cor catalysts had the largest and smallest quantities of mesoporous, respectively. Table 2 presents the specific surface area, pore volume, and average pore diameter of all catalysts. They were different because the surface active components lacked uniformity or had different degree of

Figure 6. Effect of catalysts with different transition metals on the H2/ CO ratio and V(H2+CO) in product gas.

sintering. The specific surface areas of the 1Sm3Ni1Co/Cor and 1Mo3Ni1Co/Cor catalysts after reaction were 20.8 m2/g 2602

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ranged from 750 to 800 °C and 950 to 1000 °C, respectively. For the 1Mo3Ni1Co/Cor catalyst, the reduction temperature of MoO3 was lower than 700 °C in the presence of NiO, which indicates that the existence of NiO favors the decrease in the reduction temperature of MoO3.23 However, the cleavage of the Ni−O bond in the 1Mo3Ni1Co/Cor catalyst was more difficult to achieve, and the reduction temperature in NiO increased to 470 °C for the interaction between NiO and MoO3. For the other four catalysts, the transition metals were in oxide form (shown in Figure 2b) resulting in the reduction temperatures higher than 600 °C for some NiO. In the range above 600 °C, the H2 consumption peak of the 1Sm3Ni1Co/Cor catalyst had the least high-reduction peak area, followed by the 1Y3Ni1Co/ Cor, 1La3Ni1Co/Cor, and 1Ce3Ni1Co/Cor catalysts. Therefore, the reducibility of all catalysts can be presented in the following order: 1Mo3Ni1Co/Cor > 1Sm3Ni1Co/Cor > 1Y3Ni1Co/Cor > 1La3Ni1Co/Cor ≈ 1Ce3Ni1Co/Cor. 3.2. Raw Gas. To test the catalytic activity, the biomass pyrolysis gas with high tar concentration was selected as the raw gas, with its components present in Table 3. The contents of the components were as follows: H2, less than 24%; CH4, 15%; CO2, 12%; C2H4, 4%; and C2H6, zero. Dai et al.24 suggested that these results are caused by the cracking of saturated hydrocarbons to unsaturated hydrocarbons. The test time was 1 h, WHSV was 706 kg/(h·m3), and all tars were collected. 3.3. Effect of Different Transition Metals on Catalytic Activity. Figure 5a indicates that the gas composition improved after transition metals were added. The CH4 and CO2 contents in the product gas were 3.24% and 4.11%, respectively, for the 3Ni1Co/Cor catalyst. After the addition of transition metal, the CH4 content decreased markedly to almost zero, and the CO2 content decreased to a different degree. These observations suggest that the steam reforming reaction rate accelerates in the presence of transition metals. Addition of transition metals enhanced biomass gas yield and tar conversion (Figure 5b). The effects on the 1Sm3Ni1Co/Cor and 1Mo3Ni1Co/Cor catalysts were more apparent. The corresponding gas yield and tar conversion of the 1Sm3Ni1Co/Cor catalyst were 1.18 N m3/kg and 94.3%, whereas those of the 1Mo3Ni1Co/Cor catalyst were 1.22 N m3/kg and 96.6%, respectively. Comparing with 3Ni1Co/Cor catalyst, its tar conversion increased by nearly 8%. The H2/CO ratio and V(H2+CO) in the product gas increased significantly after catalytic reforming reaction (Figure 6), whereas those in the raw gas were only 0.53 and 69%, respectively. After catalysis, the H2/CO ratio was about 1. A slight change was observed after different transition metals were added. V(H2+CO) exceeded 90% and even reached 97% for the 1Mo3Ni1Co/Cor catalyst. These results indicate that the addition of transition metal favors the formation of H2 and CO and that the formation rates of H2 and CO remain the same. The chemical equilibrium of water−gas reaction can be

Table 4. Effect of Catalysts with Different Transition Metals on Hydrogen Yield, Hydrogen Selectivity, and LHV catalyst

hydrogen yield (g/kg)

hydrogen selectivity (%)

LHV (MJ/ Nm3)

14.9 44.6 49.7 49.1

24.7 73.8 82.3 81.3

15.77 12.01 11.21 11.39

52.3

86.6

11.07

48.4 54.5

80.1 90.2

11.35 11.36

no catalyst 3Ni1Co/Cor 1La3Ni1Co/Cor 1Ce3Ni1Co/ Cor 1Sm3Ni1Co/ Cor 1Y3Ni1Co/Cor 1Mo3Ni1Co/ Cor

and 17.2 m2/g, respectively. The results were similar with their specific surface areas before reaction, suggesting that few carbon particles are deposited on the catalyst surface and few mesopores were blocked after a 4 h reaction. 3.1.3. TPR. The reducibility of the five catalysts was investigated by H2-TPR. The H2-TPR profiles are presented in Figure 4. All catalysts showed three H2 consumption peaks after the addition of transition metals, and their temperature ranged from 250 to 350 °C (low-temperature reduction peaks), 350 to 500 °C (medium-temperature reduction peaks), and 500 to 750 °C (high-temperature reduction peaks). Lowtemperature reduction peaks corresponded to the reduction peaks of free NiO21 and CoO; medium-temperature reduction peaks corresponded to the reduction peaks of NiO and CoO, exhibiting a weakened interaction with the transition metal compounds or support; high-temperature reduction peaks corresponded to the reduction peaks of NiO and CoO, showing a strong interaction with the transition metal compound or support, except for the 1Mo3Ni1Co/Cor catalyst. The high-temperature reduction peak of the 1Mo3Ni1Co/Cor catalyst included the reduction in MoO3. Spinels of NiAl2O4 and CoAl2O4 were not observed in all catalysts. Thus, the interaction among NiO and CoO and the transition metal compound has a higher chance of occurrence compared with that among NiO and CoO and support. The low-temperature reduction peaks of the five catalysts (1La3Ni1Co/Cor, 1Ce3Ni1Co/Cor, 1Sm3Ni1Co/Cor, 1Y3Ni1Co/Cor, and 1Mo3Ni1Co/Cor), were observed at 269, 275, 270, 261, and 310 °C, respectively; the mediumtemperature peaks were at 406, 396, 391, 394, and 470 °C, respectively; and the high-temperature peaks were at 640, 664, 609, 623, and 571 °C, respectively. The low-temperature reduction peak of 1Mo3Ni1Co/Cor was minimal, and its medium- and high-temperature reduction peaks largely overlapped. Figure 2b shows that the reduction in NiO, CoO, and MoO3 is complete. Haber et al.22 reported that the reduction of pure MoO3 involves two steps: (1) MoO3MoO2 and (2) MoO2Mo, and the corresponding reduction temperatures Table 5. Results of 1Mo3Ni1Co/Cor Catalyst Stability Test gas composition (vol.%) catalysts 1Mo3Ni1Co/Cor 1 2 3 4

h h h h

H2

CH4

CO

CO2

C2H4

C2H6

C2H2

gas yield (Nm3/kg)

carbon conversion (%)

tar conversion (%)

50.08 45.08 44.65 43.02

0.43 0.13 1.32 2.48

47.02 51.76 48.60 47.25

2.47 2.88 5.27 6.99

0 0.15 0.16 0.26

0 0 0 0

0 0 0 0

1.22 1.12 1.10 1.06

75.4 76.4 75.6 75.1

96.6 92.1 91.7 90.3

2603

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Table 6. Results of 1Sm3Ni1Co/Cor Catalyst Stability Test gas composition (vol.%) catalysts: 1Sm3Ni1Co/Cor 1 2 3 4

h h h h

H2

CH4

CO

CO2

C2H4

C2H6

C2H2

gas yield (Nm3/kg)

carbon conversion (%)

tar conversion (%)

49.65 44.64 38.43 37.93

0.37 1.33 3.71 4.89

45.03 48.60 51.15 47.35

4.92 5.27 6.26 9.20

0.03 0.16 0.45 0.61

0 0 0 0.02

0 0 0 0

1.18 1.10 1.00 0.98

73.6 75.6 76.8 76.1

94.3 85.2 81.3 73.1

Figure 7. Effect of reaction time on gas composition over the (a) 1Mo3Ni1Co/Cor and (b) 1Sm3Ni1Co/Cor catalyst.

destroyed when the formation rate of one is faster than the other. Thus, inletting steam is a feasible technique for enhancing the H2/CO ratio in the product gas.6 Table 4 indicates that the hydrogen yield after catalysis is more than three times of that before catalysis. The hydrogen yield of the raw gas from biomass pyrolysis was only 14.9 g/kg, whereas that of the product gas after catalysis exceeded 40 g/kg. The maximum hydrogen yield could reach 54.5 g/kg, and the corresponding hydrogen selectivity was 90.2% for the 1Mo3Ni1Co/Cor catalyst. The LHV of the product gas ranged from 10 MJ/Nm3 to 16 MJ/Nm3, which belongs to the medium-heating value gas. The LHV of the product gas after catalytic reforming was lower than that of the raw gas, because during the process of catalytic reforming between hydrocarbon and CO2 or steam, the increased energy cannot compensate for the energy reduction per unit volume gas for its expansion. The data above reveal that the performances of all catalysts with transition metals are improved. With gas yield and tar conversion considered, the 1Mo3Ni1Co/Cor catalyst per-

Figure 8. Effect of reaction time on (a) hydrogen yield and (b) hydrogen selectivity.

formed most efficiently, followed by the 1Sm3Ni1Co/Cor, 1Y3Ni1Co/Cor, 1La3Ni1Co/Cor, and 1Ce3Ni1Co/Cor catalysts. Combining the above results with the catalyst characterization, the catalyst performances are therefore close to their reducibility, and Mo−Ni alloy is beneficial to the improvement of catalytic performance. 3.4. Catalyst Stability Test. In accordance with the aforementioned results, two kinds of catalyst (1Mo3Ni1Co/ Cor and 1Sm3Ni1Co/Cor) were selected to test stability. The test results are listed in Tables 5 and 6. As shown in these tables, the gas yield decreased from 1.22 N m3/kg to 1.06 N m3/kg, and tar conversion remained above 90% after a 4 h reaction over the 1Mo3Ni1Co/Cor catalyst. However, for the 1Sm3Ni1Co/Cor catalyst, gas yield and tar conversion decreased significantly, and the tar conversion decreased by 2604

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Figure 10. XRD patterns of the catalysts before and after catalysis over the 1Mo3Ni1Co/Cor and 1Sm3Ni1Co/Cor catalysts.

Figure 11. Raman spectra of the spent catalysts (1Mo3Ni1Co/Cor and 1Sm3Ni1Co/Cor).

Cor catalyst, H2 content decreased from 50% to 40%. CH4 and CO2 contents increased from zero and 1.6% to 3.5% and 10%, respectively. Despite the similarity in behavior of 1Sm3Ni1Co/ Cor and 1Mo3Ni1Co/Cor catalysts in terms of gas content, the H2 reduction and the CH4 growth after a 4 h reaction over the 1Sm3Ni1Co/Cor catalyst were larger than those over 1Mo3Ni1Co/Cor (Figure 7b). These observations suggest that the presence of Mo can inhibit carbon deposition and decrease the catalyst deactivation rate. As shown in Figure 8, the 1Mo3Ni1Co/Cor and 1Sm3Ni1Co/Cor catalysts had similar hydrogen yield and hydrogen selectivity in the initial reaction stage. After a 4 h reaction over the 1Mo3Ni1Co/Cor catalyst, hydrogen yield and hydrogen selectivity were 40.7 g/kg and 67.54%, respectively, which were greatly higher than those for the 1Sm3Ni1Co/Cor catalysts. Figure 9a shows that the LHV of the product gas exhibited almost no change with the increase in reaction time over the 1Mo3Ni1Co/Cor catalyst. However, the LHV of the product gas increased significantly and exceeded 12 MJ/Nm3 after a 4 h reaction over the 1Sm3Ni1Co/Cor catalyst because of the increase in CH4 content. As shown in Figure 9b, the H2/CO ratio decreased initially and then increased. When

Figure 9. Effect of reaction time on (a) LHV of the product gas, (b) H2/CO, and (c) V(H2+CO).

20%. The results indicate that the 1Mo3Ni1Co/Cor catalyst has higher stability compared with the 1Sm3Ni1Co/Cor catalyst. With the increase in reaction time, the CH4 and CO2 contents gradually increased, whereas the H2 decreased. The changes in the CO exhibited no trend, and its values were about 50% (Figure 7a). After a 4 h reaction over the 1Mo3Ni1Co/ 2605

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Table 7. Effect of the Catalytic Reaction Time on Tar Components after reaction (area%) retention time (min)

name

chemical formula

before reaction (area%)

1h

2h

3h

4h

2.51 3.69 4.15 6.86 7.30 11.94 16.25 19.38 19.79 21.64 22.96 23.43 25.11 26.70 31.14 31.37 33.88 35.04 36.71 37.69 39.76 42.44 43.32 43.45 43.60 48.20 49.24 49.44

benzene pyridine toluene phenylethyne styrene indene naphthalene naphthalene, 2-methylnaphthalene, 1-methylbiphenyl acenaphthene acenaphthylene dibenzofuran fluorene anthracene phenanthrene 4H-cyclopenta[def]phenanthrene 2-phenylnaphthalene pyrene fluoranthene 11H-benzo[b]fluorene cyclophenta[cd]pyrene benzo[ghi]fluoranthene triphenylene chrysene benzo[a]pyrene perylene benz[e]acephenanthrylene

C6H6 C5H5N C7H8 C8H6 C8H8 C9H8 C10H8 C11H10 C11H10 C12H10 C12H10 C12H8 C12H8O C13H10 C14H10 C14H10 C15H10 C16H12 C16H10 C16H10 C17H12 C18H10 C18H10 C18H12 C18H12 C20H12 C20H12 C20H12

10.71 0.56 5.25 0.40 3.80 4.04 19.16 0.70 0.28 1.41 0.64 6.84 1.25 3.77 10.10 3.61 1.41 0.71 7.27 7.01 0.31 0.32 0.99 1.33 1.40 1.15 0.54 1.28

41.80 0.19 16.38 0 0.7 1.05 22.53 0 0 0 0 0.62 0 0.35 3.15 0.28 0 0 0.73 1.79 0 0 0 0 0 0 0 0

43.44 0.05 14.47 0 0.42 1.13 23.96 0 0 0.05 0 0.72 0.04 0 3.24 0.34 0 0 0.63 1.72 0 0 0 0 0.04 0 0 0

37.30 0.13 14.45 0 2.49 1.97 25.38 0.07 0.01 0.01 0 0.62 0.05 0.25 2.32 0.22 0 0 0.41 0.93 0 0 0 0 0 0 0 0

38.52 0 12.09 0 1.08 2.20 30.41 0.15 0.04 0.24 0 1.75 0.12 0.51 3.29 0.41 0 0 0.58 1.23 0 0 0 0 0.01 0 0 0

CH4. Arkatova et al.26 also indicated that Mo2C formation can considerably influence hydrocarbon reforming reaction. However, no obvious change was observed in the XRD patterns before and after reaction for the 1Sm3Ni1Co/Cor catalyst, which may indicate that the carbon deposit has high dispersion. In addition, the 1Mo3Ni1Co/Cor and 1Sm3Ni1Co/Cor catalysts were characterized by Raman spectroscopy (Figure 11). Two obvious peaks at 1400 cm−1 (referred to as D peak) and 1581 cm−1 (referred to as G peak) could be observed between 1200 cm−1 to 1700 cm−1 for the 1Sm3Ni1Co/Cor catalyst. The D and G peaks represent an amorphous carbon structure and a CC graphite structure, respectively. The valley bottom between two peaks is called V valley at 1500 cm−1. The IV/IG ratio is commonly used to represent the degree of carbon structural disorder. The 1Sm3Ni1Co/Cor catalyst had a IV/IG ratio of 0.27, which was lower than that of coke from biomass pyrolysis.27 This result indicates that the degree of disorder of the carbon deposit is relatively low and that the carbon deposit is mainly in the form of a graphite structure. For the 1Mo3Ni1Co/Cor catalyst, no obvious D and G peaks were observed, which suggests that the carbon deposit on the catalyst surface is minimal because most carbon deposits react with Mo and form a Mo2C structure. The main components of tar at different reaction times are listed in Table 7. With the increase in reaction time, the contents of benzene, methylbenzene, pyrene, and fluoranthene decreased gradually, whereas the contents of naphthalene and acenaphthylene increased gradually. The contents of anthracene and phenanthrene showed no apparent regularity. These components are difficult to crack because most exhibit

the H2/CO ratio is comparatively small, the water gas reaction can shift to a positive direction, resulting in an increase in the H2/CO ratio because the CO content is higher than the H2 content. However, the H2/CO ratio has difficulty returning to its initial value because the catalyst surface active centers are reduced by the carbon deposits. Consequently, the contact frequency between the active center and the H2 or CO is reduced. This occurrence does not favor water gas reaction. After a 4 h reaction over the 1Mo3Ni1Co/Cor and 1Sm3Ni1Co/Cor catalysts, their corresponding H2/CO ratios were 0.91 and 0.8, respectively, which decreased by 0.16 and 0.3, respectively. These behaviors indicate that the coverage rate of carbon deposit in the 1Mo3Ni1Co/Cor catalyst is lower than that in the 1Sm3Ni1Co/Cor catalyst. A similar result is shown in Figure 9c, which indicates that the effect of the 1Mo3Ni1Co/Cor catalyst on V(H2+CO) is weaker than that of the 1Sm3Ni1Co/Cor catalyst. After a 4 h reaction over the 1Mo3Ni1Co/Cor catalyst, V(H2+CO) still exceeded 90%. After a 4 h reaction, the 1Mo3Ni1Co/Cor and 1Sm3Ni1Co/ Cor catalysts were characterized by XRD (Figure 10). The active component phase changed significantly for the 1Mo3Ni1Co/Cor catalyst. The Mo−Ni alloy was destroyed, but Mo2C were formed with peaks at 34.4°, 37.9°, 39.4°, 52.1°, 61.5°, 69.6°, 74.6°, and 75.5°. A reaction occurred between Mo and hydrocarbon, which may be the reason that the 1Mo3Ni1Co/Cor catalyst performs most efficiently among all catalysts. Lee et al.25 suggested that the Mo2C can form simultaneously with the penetration of methane to MoO3 by topotactic reaction and that if MoO3 is reduced to Mo, the Mo2C can form more easily by the reaction between Mo and 2606

dx.doi.org/10.1021/ef301936z | Energy Fuels 2013, 27, 2599−2607

Energy & Fuels

Article

(16) Liu, Y.; Wang, H.; Li, J. F.; Lu, Y.; Wu, H. H.; Xue, Q. S.; Chen, L. Appl. Catal., A 2007, 328, 77−82. (17) Corella, J.; Toledo, M.; Padilla, R. Ind. Eng. Chem. Res. 2004, 43, 2433−2445. (18) Toledo, J. M.; Corella, J.; Molina, G. Ind. Eng. Chem. Res. 2006, 45, 1389−1396. (19) Zhao, B. F.; Zhang, X. D.; Sun, L.; Meng, G. F.; Chen, L.; Xiaolu, Y. Int. J. Hydrogen Energy 2010, 35, 2606−2611. (20) Rezaei, M.; Alavi, S. M.; Sahebdelfar, S.; Bai, P.; Liu, X. M.; Yan, Z. F. Appl. Catal., B 2007, 77, 346−354. (21) Zieliński, J. J. Catal. 1982, 76, 157−163. (22) Haber, J.; Braithwaite, E. R. Stud. Inorg. Chem. 1994, 19, 477− 485. (23) Borowiecki, T.; Gac, W.; Denis, A. Appl. Catal., A 2004, 270, 27−36. (24) Dai, X. W.; Yin, X. L.; Wu, C. Z.; Zhang, W. N.; Chen, Y. Energy 2001, 26, 385−399. (25) Lee, J. S.; Oyama, S. T.; Boudart, M. J. Catal. 1987, 106, 125− 133. (26) Arkatova, L. A. Catal. Today 2010, 157, 170−176. (27) Okumura, Y.; Hanaoka, T.; Sakanishi, K. Proc. Combust. Inst. 2009, 32, 2013−2020.

symmetric molecular structures and belong to nonpolar substances. Thus, for the decrease of active centers on the catalyst surface, the contact frequency between these benzene series materials and these active centers is reduced, resulting in incomplete cracking of polymer benzenes and an increase in the content of naphthalene.

4. CONCLUSIONS In catalytic biomass pyrolysis, five kinds of monolithic catalysts were characterized and tested. The 1Mo3Ni1Co/Cor catalyst exhibited the best catalytic performance for the formation of Mo−Ni alloy. Tar conversion and gas yield were 96.6% and 1.22 N m3/kg, respectively, at a WHSV of 706 kg/(h·m3). Meanwhile, compared with the 1Sm3Ni1Co/Cor, the 1Mo3Ni1Co/Cor catalyst exhibited higher stability for the formation of Mo2C effectively reducing carbon deposits catalyst. After a 4 h reaction over the 1Sm3Ni1Co/Cor catalyst, the carbon deposits adopted a CC graphite structure, preventing these benzene series materials from cracking to permanent small molecular gases.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 20 87013240. Fax: +86 20 87013240. E-mail: [email protected] (Z.X.), [email protected] (M.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial supports of National Science & Technology Pillar Program of China No. NC2010MB0021.

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dx.doi.org/10.1021/ef301936z | Energy Fuels 2013, 27, 2599−2607