Catalytic reforming of pyrolysis tar over metallic nickel nanoparticles embedded in pyrochar

Fuel 159 (2015) 570–579

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Catalytic reforming of pyrolysis tar over metallic nickel nanoparticles embedded in pyrochar Yafei Shen a,⇑, Mindong Chen a, Tonghua Sun b,⇑, Jinping Jia b a b

School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, PR China School of Environmental Science and Engineering, Shanghai Jiaotong University, 800 Dongchuan Road, Shanghai 200240, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Rice husk char supported metallic

nickel can be carbothermally synthesized via one-step pyrolysis.  The pyrolysis tar was catalytically removed at a relatively lower temperature (500 °C).  The gasification rate is higher than the deposition rate resulting in the increase of RHC surface area.  RHC Ni can inhibit the formation of both heavy and light tars including poly-cyclic aromatic hydrocarbons (PAHs).

a r t i c l e

i n f o

Article history: Received 17 March 2015 Received in revised form 3 July 2015 Accepted 4 July 2015

Keywords: Biomass pyrolysis Tar catalytic reforming Metallic nickel nanoparticles Rice husk char Carbothermal reduction

a b s t r a c t Metallic nickel (Ni0) nanoparticles could be in situ generated in the carbon matrix of rice husk char (RHC) via a facile one-step pyrolysis. The synthesized RHC Ni has a considerable performance on tar reforming. Tar reforming efficiency is increased with the increases of the used catalyst weight and reforming temperature. In particular, tar reforming efficiency can reach up to 90.5% and 99.8% by using 5 g and 10 g of RHC Ni, respectively. Tar reforming efficiency can also keep stable after 5 cycles. Besides, RHC Ni showed high tar conversion efficiency, increasing from 92.3% to 100%, when the reforming temperature was increased from 500 °C to 900 °C. RHC Ni showed a high catalytic activity even at the relatively lower temperatures. Furthermore, the yield of liquid products was decreased from 30.2% to 10.7%, corresponding to tar reforming. Accordingly, the gas yield was increased from 37.5% to 58.0%, in which the main components of syngas are CO and H2. It is noted that the PAHs compounds in tar could be significantly reduced by using the RHC Ni. The surface areas of the used RHC Ni were increased due to the char gasification rate higher than deposition rate. The RHC Ni has a high potential to be used for tar catalytic reforming. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The application of thermal processes such as pyrolysis or gasification for the energy recovery from bio-wastes has attracted a ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Shen), [email protected] (T. Sun). http://dx.doi.org/10.1016/j.fuel.2015.07.007 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

growing attention. In general, pyrolysis can be optimized to obtain liquid oils and solid char, while gasification is more favor of syngas production at higher temperature [1–5]. Compared with the partial oxidative gasification, the high-temperature pyrolysis can produce fuel gas with a high heating value [4]. Also, pyrolysis-reforming is effective for the thermal conversion of biomass to improve gas yield, reduce tar contents, and enhance conversion rates [6–10].

Y. Shen et al. / Fuel 159 (2015) 570–579

In the two stage process, biomass is decomposed in the first stage, and then the derived vapors and tar are reformed in the second stage at higher temperatures (T P 800 °C). The addition of different catalysts with/without the steam in the second stage has shown a positive effect on tar reduction and syngas reformation. Biomass pyrolysis tar is a complex mixture of condensable hydrocarbons, including 1-ring to 5-ring aromatic organic compounds along with other oxygenated hydrocarbons and polycyclic aromatic hydrocarbons (PAHs). Aromatic compounds present in tars, such as benzene and PAHs, are toxic and represent environmental hazards. Tar can deposit on surfaces of filters, heat exchangers and engines. Moreover, tar could polymerize to form more complex structures. Catalytic reforming is the one of promising tar removal techniques from gaseous products by transformation of tar into H2 and CO in the presence or absence of steam [11]. A variety of catalysts such as Fe-, Co-, Ni-based catalysts, dolomites, olivine and catalyst-loaded zeolites have been widely used for tar catalytic conversion at 600–900 °C [12–14]. Among these supports, char, olivine, and dolomite appear particularly attractive, since they are all inexpensive and relatively abundant in different regions. Olivine is less active in gasification, but it shows the optimal hardness required for the fluidized-bed reactor. Dolomite has high catalytic activity in tar reforming and considerable CO2 capture performance. This characteristic in particular results in the gasification process possible at relative low temperatures (650– 700 °C), namely adsorption enhanced reforming process, than the more conventional range of 850–900 °C, without a significant increase in the downstream tar content. However, the catalytic activities of dolomite and olivine for tar conversion leave room for improvement, so the motivation is the search for catalytic additives [15]. Recently, chars derived from coal or biomass have been used as low-cost carbonaceous catalysts [16–20] and adsorbents [21–23] in tar elimination. Char itself exhibits a fair catalytic activity for tar reforming, which is often influenced by pore size, surface area and mineral contents. Moreover, char can act as a catalyst support to disperse the active clusters at nanoscale [24]. The deactivated char can be gasified without the need of frequent regeneration (e.g., (R1)) [25].

C þ 2H2 O ! CO2 þ 2H2

ðR1Þ

In the previous work, it has been proved that rice husk char (RHC) supported nickel catalysts shows good catalytic activity on in-situ conversion of tar derived from biomass pyrolysis [26,27]. Catalysts placed in contact with the feedstock inside the pyrolysis reactor (in situ) can inhibit the nascent tars polymerization, reducing the macromolecular tar formation and condensation. However, the nickel catalysts in the RHC could be unfeasible to be frequently recycled from the solid residues. Likewise, catalysts can be also used in the downstream of the primary reactor (ex situ) for tar conversion and vapors upgrading [28,29]. In this work, the catalytic performances including reaction activity, cycle tests and service life (i.e., deactivation) of the RHC nickel catalysts will be preliminary studied for ex situ catalytic conversion of pyrolysis tar.

2. Materials and methods 2.1. Biomass and char characterization The biomass feedstock of RH was collected from Thailand. The as-received RH was dried in an oven at 105 °C overnight. After that, the dry-based RH was devolatilized by pyrolysis at 700 °C for 30 min for rice husk char (RHC) production. Furthermore, the rice husk ash (RHA) was obtained by burning the as-prepared RHC in a muffle furnace in air for 1 h. Table 1 shows the proximate and ultimate analyses of RH, RHC and RHA, which were determined

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by the elemental analyzer (Vario MICRO Cube, Elementar, Germany) and the thermogravimetric (TG) and differential TG (DTG) analysis (DTG-50, Shimadzu, Nakagyo-ku, Japan), respectively. The chemical composition of RHA was determined by the X-ray fluorescence analysis (XRF, Shimadzu, Rayny EDX 700, Japan). The total specific surface area (SBET) accordingly calculated by the Brunauer–Emmett–Teller (BET) method was measured by a Nova-2200e surface area and pore size analyzer (Quantachrome Instruments, USA). 2.2. Catalysts preparation RH has a highly anisotropic cellular structure, which is used as a natural biotemplate to generate the RHC with the ordered micro-, meso-, and macro-porous structures. The metallic Ni nanoparticles could be generated in the carbon matrix of RHC via one-step pyrolysis. Initially, RH (20 g) was added into the Ni(NO3)2 solution (0.1 mol/L) as a nickel precursor and then dried in an oven at 105 °C. In consideration of the economic efficiency, the nickel electroplating wastewater can be considered as an alternative. Subsequently, the dry-based RH Ni was pyrolyzed at 750 °C in an inert gas (e.g., N2) atmosphere for 10 min. After that, the RHC Ni was obtained and dry-stored for the further use. 2.3. Biomass pyrolysis-catalytic reforming The experimental setup (as shown in Fig. 1) is composed of a gas supplying system, a gas cleaning system and a pyrolysis-reforming facility, which was divided into three parts, an outer tube, a top cover with a feeding port and also a gas inlet and an inner tube. Two sintered quartz porous plates were fixed in each tube to support the biomass or the catalysts. The reactor was surrounded by a two-zone electric furnace. At first, the feedstock of RH was prepared by crushing and sieving with the particle size below 5.0 mm. Before adding the catalysts and biomass feedstock, the nitrogen (N2) with a flow rate of 1.0 L/min was continuously injected into the reactor to clear away the residual gases in the reactor. The pyrolyzer was heated up to 750 °C, while the temperature of reformer was controlled at a range of 500– 900 °C. The fresh RHC Ni catalyst was initially put in the reformer. After that, biomass feedstock (RH, 20 g) could be fed into the pyrolyzer, and the volatile matters were released mainly in the form of vapor, including the nascent tars, which could be cracked into the small molecular gases over the RHC Ni by catalytic reactions. The condensable tar was collected by isopropanol (IPA) in the gas cleaning unit. 2.4. Sampling and analysis The liquid products derived from biomass pyrolysis were determined by weighing, mainly including tar and water [30]. Biomass tar is a complex organic mixture composed of hundreds of condensable hydrocarbon compounds with other oxygenated hydrocarbons, which is determined by weighing [31]. The detectable composition of tar is determined by a gas chromatographymass spectrometer (GC–MS, Shimazu, GCMS-QP2010, Japan). Additionally, the yield of producer gas was estimated by the mass balance. The producer gas mainly composed of H2, CH4, CO, CO2, and light hydrocarbons (i.e., C2H4, C2H6) was measured by a micro gas chromatograph (Micro GC, Agilent, 3000A, USA), which is fitted with thermal conductivity detector (TCD). Each trial was kept for ten minutes to ensure a good mass balance. Meanwhile, the repeatability experiments were performed to ensure the reliability of the system. Therefore, the collected tar sample was the total quantity of tars generated from the repeatability experiments. The Ni concentration in the RHC Ni was determined by the

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Table 1 Properties of RH, RHC and RHA. Ultimate analysis (wt.%, dry & ash free basis)

RH RHC RHA

a

SBET (m2/g)

Proximate analysis (wt.%, dry basis/as received) b

c

C

H

O

N

S

VM

FC

Ash

Moisture

37.9 64.8 9.5

6.3 2.4 0.3

55.3 35.1 90.2

0.4 0.1 0

0.1 0 0

60.5/59.3 11.7 5.4

11.9/10.4 34.3 7.8

22.0/20.5 52.0 85.3

5.6/9.8 2.0 1.5

2.2 117.0 65.4

Chemical composition of RHA (wt.%)

a b c

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

Zn (ppm)

Mn (ppm)

Cu (ppm)

Cd (ppm)

94.64

0.06

0.23

1.88

0.96

0.39

0.58

18.20

52.24

32.17

0.48

By mass difference. VM – volatile matters. FC – fixed carbon.

Fig. 1. Schematic diagrams of experimental setup.

Inductively Coupled Plasma analysis (ICP, Iris Advantage 1000, Thermo king-cord Co.). In addition, the fresh and the used RHC Ni catalysts were characterized by the TG/DTG analysis, the X-ray diffraction analysis (XRD, Rigaku, XRD-DSC II, Japan), the scanning electron microscopy (SEM, JSM-6610, JEOL/EO, Japan), and the transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS, JEM-2010F, JEOL, Japan), respectively. 3. Results and discussions 3.1. Effect of catalyst weight The weight of spent catalyst is an important parameter for the catalytic reforming of tar. If excessive catalysts are used, it is

uneconomical for the tar catalytic reforming. Moreover, the gas percolation resistance to pass through the catalyst zone might be increased. In contrast, if insufficient catalysts are used, tar reforming efficiency is relative low. Moreover, the catalyst is easily deactivated due to the short retention time; accordingly, the excessive catalyst can increase the contact frequency and reaction time of active sites with tar molecules, enhancing the catalytic performance. Fig. 2 shows the effect of the used catalyst weight on tar reforming efficiency at 700 °C. Tar reforming efficiency increased with the increase of the weight of used RHC Ni. Tar reforming efficiency can reach about 50.1% by using 3 g of RHC Ni, while it significantly increased, up to 90.5% and 99.8% by using 5 g and 10 g of RHC Ni, respectively. It might be attributed to the enhancement of catalytic reaction and retention time resulted from the

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Fig. 2. Effect of the catalyst weight [3 g; 5 g; 10 g; 10 g (5 cycles)] on tar reforming.

increase of catalyst weight. Furthermore, tar reforming efficiency cannot decrease after 5 cycles, indicating a high stability performance on tar conversion. 3.2. Effect of catalytic temperature Catalytic temperature is another significant parameter, since it directly affects the reaction activity. During the thermo-chemical reactions, high temperature can enhance the catalytic reactivity. Fig. 3 shows the effect of the reforming temperature on tar conversion. Tar conversion efficiency generally increased with the increase of reforming temperature. If the RHC was used, tar conversion efficiency can significantly improve from 39.8% to 78.7% in a temperature range of 500–900 °C. However, tar conversion efficiency slightly improved from 92.3% to 100% in the same temperature range by using the RHC Ni. It means that the temperature can significantly influence the tar conversion efficiency, if no catalyst or only the RHC is used. In addition, it suggested that RHC Ni exhibited a higher catalytic activity for tar conversion even at the relatively lower temperatures (e.g., 500 °C). 3.3. Syngas yield and composition Gas, liquid and char are the predominate products derived from biomass pyrolysis or gasification. Liquid including water and tar

Fig. 3. Effect of the reforming temperature on tar reforming (10 g).

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are not considerable products during biomass gasification, while the char could be fabricated into the value-added carbonaceous materials. Fig. 4A shows the products composition from RH gasification at 750 °C. It can be found that the yield of pyrolysis char was not affected. The char yield from RH pyrolysis at 750 °C was around 31.3–32.5%. However, the yields of liquid and producer gas had been changed significantly. In particular, if the RHC Ni was used for catalytic reforming, the yield of liquid products reduced from 30.2% to 10.7%, most likely caused by a series of the thermo-chemical reactions, such as water–gas-shift reaction, char gasification, and tar reforming. Accordingly, the yield of product gas greatly increased from 37.5% to 58.0% (Fig. 4A), in which the main components of syngas are CO and H2. From Fig. 4B, it can be found that using the RHC Ni, the volume fractions of CO and H2 in syngas increased from 21.2% to 33.5% and from 18.5% to 24.3%, respectively. Besides, the volume fractions of CH4, CnHm (i.e., C2H4, C2H6) and CO2 reduced due to the catalytic reforming over the RHC Ni. In particular, RHC exhibited a stable catalytic activity for vapor conversion most likely due to the presence of minerals in it [17]. 3.4. GC–MS analysis of the condensed tar Tars can be classified by their solubility and condensability [32], categorized into five classes. Class 1 refers to the GC undetectable heavy tars, which can condense at relative high temperatures and very low concentrations; Class 2 refers to the heterocyclic aromatic compounds with high water solubility (e.g., phenol and cresol); Class 3 refers to the light hydrocarbons single-ring aromatic compounds (e.g., toluene and xylene); Class 4 refers to the light polycyclic aromatic hydrocarbons (2–3 rings), which can condense at the relatively high concentrations and intermediate temperatures (e.g., indene and naphthalene); Class 5 refers to the heavy polycyclic aromatic hydrocarbons (4–7 rings), which can condense at relatively high temperatures and low concentrations (e.g., pyrene and coronene). Fig. 5A shows the GC–MS spectra of the condensed tar derived from RH pyrolysis at 750 °C. It could be observed that more than 117 absorbance peaks were determined. Likewise, Fig. 5B shows the GC–MS spectra of the condensed tar derived from RH pyrolysis–catalytic reforming by the RHC Ni at the same temperature of 750 °C. In this case, the peak numbers are decreased to 37, indicating that the tar compounds was significantly reduced by the catalytic reforming. Meanwhile, these MS spectra were identified by the MS database. The most abundant and important organic components in the condensed tars were phenol, benzene, naphthalene, biphenylene, and their derivates. In particular, the naphthalene and its derivates occupied more than 30.26% in the tar sample 1 and 44.33% in the tar sample 2, respectively. However, the aromatic organic compounds consisting of PAHs, such as phenanthrene, pyrene, fluoranthene, existed in both of tar samples, indicating that it is difficult to convert them by the catalytic reforming. Furthermore, compared with the tertiary tars of PAHs, it is much easier to crack/reform the nascent tars by RHC Ni. Moreover, the tertiary tars except for naphthalene completely disappeared after the heterogeneous gasification process by the RHC and RHC Ni. More significantly, the total amounts of tar by-products consisting of the tertiary tars were significantly reduced. CO2 dry reforming can also reduce parts of nascent tar and convert them into thermally stable tertiary tar components such as toluene, naphthalene and styrene. Because of the corrosion, condensation, and deposition effects in syngas utilization, it is of great importance to reduce the PAHs content, especially in terms of these troublesome tars decomposed in the gasification zone by the char. In the reforming zone, the homogeneous partial oxidation and heterogeneous char conversion are two significant factors to ensure that the low-tar syngas is achieved. The selectively

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Fig. 4. (A) Mass fraction of pyrolyzed products, (B) volume fraction of gas products derived from RH pyrolysis at 750 °C (reforming temperature: 700 °C, catalyst weight: 10 g).

Fig. 5. GC–MS spectra identified for main organic compounds in the condensed tar from RH pyrolysis (A) combined with RHC Ni catalytic reforming (B).

properties of RHC Ni in eliminating tertiary tars greatly increased the application prospects of the gasifier [30]. In this work, the major tar compounds came from tar Class 4; sometimes referred as tertiary tars containing non-oxygenated organic compounds with light PAHs. Among the compounds grouped in tar Class 4, the major contribution came from the naphthalene and its derivates. It can be indicated that RHC Ni showed the promising tar conversion efficiency on the GC–MS detected tars. 3.5. Characterization of catalysts 3.5.1. XRD analysis The identification of crystal phases was performed by the XRD analysis using Rigaku D/Max 3400 powder diffraction system with Cu Ka radiation (k = 0.1542 nm) at 45 kV and 40 mA with a scanning rate of 5°/min. Fig. 6 presents the XRD patterns of the fresh and used RHC Ni catalysts. Silica in all samples is amorphous. Nevertheless, the typical silica characteristic is observed at a broad peak centered at around 2# = 22.5°, which can be attributed to the presence of disordered cristobalite [33,34]. The main chemical states of nickel in the RHC Ni catalysts were the nickel oxides (e.g., NiO, bunsenite) and metallic nickel (Ni0) by the reactions (R2)–(R5). Initially, the nickel cations (Ni2+) in the aqueous solution were transformed into the relative stable form of Ni(H2O)2+ 6 (i.e.,

Fig. 6. XRD patterns of the RHC, RHC Ni1 (fresh) and RHC Ni2 (used) catalysts.

octahedral water coordination complexes) [35], which was therefore decomposed into the bunsenite (NiO). With the increase of pyrolysis time, more nickel oxides (i.e., NiO) embedded in the carbon matrix of RHC were reduced into the metallic nickel (Ni0) by

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the carbothermal reduction ((R6) and (R7)) [27] along with the hydrogenation reduction (R8). As shown in Fig. 6, the crystalline Ni0 was appeared after the pyrolysis of RH Ni [36]. After used for catalytic reforming, the intensity of Ni0 characteristic peaks in the RHC Ni2 enhanced, indicating well crystallization. However, the particle size of Ni0 was unchanged. 2þ

Ni

þ 2ðSiOHÞ $ NiðSiOÞ2 þ 2Hþ 2þ

5NiðNO3 Þ2 þ 24H2 O ! Ni ½NiðH2 OÞ6 

2þ

þ 4½NiðH2 OÞ6 

ðR2Þ 2þ

þ 10NO3

ðR3Þ

! NiðOHÞ2 þ 2H3 Oþ þ 2H2 O ðthermal decompositionÞ

NiðOHÞ2 ! NiO þ H2 O

ðR4Þ ðR5Þ

Carbothermal reduction:

NiO þ C ! Ni þ CO

ðR6Þ

NiO þ CO ! Ni þ CO2

ðR7Þ

Hydrogenation reduction:

NiO þ H2 ! Ni þ H2 O

ðR8Þ

3.5.2. SEM analysis Rice is one of the most widespread food crops for human sustenance. The content of the RH reaches 20 wt% of the entire rice kernel, a very large amount, considering the massive scale of global rice production. In addition, RH contains a variety of components such as lignin (20–30 wt%), cellulose (55–65 wt%), and silica (15–20 wt%), which originates from monosilicic acid that is initially introduced into rice plants through their roots and is then moved to the rigid outer epidermal walls of the plants where it is converted into silica. Silicon (Si) exists along the outer rugged surfaces of RH [37]. RH has a typical globular structure, of which its main components are in the lemma/palea form, tightly interlocked with each other [38]. The corrugate structural outer epidermis is highly ridged, while its ridges are punctuated with the prominent globular protrusions. However, RH is assembled around the Si-O carcass, which is concentrated in the protuberances and hairs (trichomes) on the outer and inner epidermis, adjacent to the rice kernel. Many cavities having varying particle sizes were distributed within the char samples, as evidenced of the interconnected porous network and large specific surface area [39]. With the development of reaction, the surface texture of char becomes irregularity ascribed to the shrinkage of the globular structure, which might be caused by devolatilization [40]. Evaporation of the volatile materials could create new pores on the particle with rough surface and irregular outlet (Fig. 7A). The external surface is found covered mostly with smooth open pores of different sizes (Fig. 7B–D). Moreover, it appears that the possibility of fragmentation since several cracks passing through the particle can be observed clearly. From all of the used RHC Ni, small particles with globular shape could be observed by magnifying the above micrographs (Fig. 8). Some particles derived from dust attached on the parent RH. In addition, other formed larger clusters (Fig. 8B) was most likely attributed to the soot formation and carbon deposit resulting from hydrocarbons cracking [40,41]. 3.5.3. TEM analysis Metallic nickel (Ni0) nanoparticles could be dispersed uniformly in the carbon matrix of the wood char obtained at pyrolysis temperatures from 500 to 700 °C. However, the very wide dispersion of the monocrystalline metallic nickel (Ni0) particles with the particle sizes of 2–4 nm could form at the pyrolysis temperatures

ranging from 400 to 500 °C and their nanometric size confirmed the high dispersion of metal precursor in the wood obtained after the impregnation step [35]. It can also be seen that nickel nanoparticles were embedded and highly dispersed in the carbon matrix of RHC with the particle size of 10 nm (Fig. 9A); whereas these particle sizes became relative non-uniform after the reaction (Fig. 9B), in terms of the slight decrease of particle size. Herein, the agglutinated nickel nanoparticles with the size range of 10–20 nm might be attributed to the coagulation with the nanosized amorphous silica. Furthermore, carbon deposition on the surface of nickel particles could result in the increase of particle size. It was observed that some nanoparticles were encapsulated by the dark-colored matters after use (Fig. 9B). The elemental constitutes were the semi-quantitatively determined by EDS analysis. The main elements in the RHC Ni were carbon (C), silicon (Si) and nickel (Ni), etc. Also, a higher content of nickel (Ni) and a lower content of carbon (C) were detected in the used RHC Ni. Namely, the atomic ratio of Ni/C was increased after use. It might be caused by the carbothermal reduction, thereby enhancing the carbon consumption and the formation of nickel nanoparticles. Since C, Si, and Ni are the predominate elements inside the used RHC Ni, it can also be recycled for the fabrication of silica-based nickel after carbon conversion. 3.5.4. TG analysis The thermal behavior of the fresh and the used RHC Ni catalysts were characterized by TG analysis at a heating rate of 20 °C/min under the air flow of 150 mL/min. The DTG curve of RHC had one sharp peak at 400 °C due to devolatilization and the air combustion of volatiles and char (Fig. 10). Significantly, the fixed carbon combustion was the dominant combustion process for biochar [42]. A larger mass loss of the fresh RHC Ni occurred at the temperature range of 300–500 °C, suggesting the presence of volatile matters in it, detectable even at the high temperature of 600 °C. Still, the total mass loss in these three samples was only 4–6% at the temperature range of 100–800 °C, indicating the thermal stability of the fresh and used RHC Ni catalysts. In the process of biomass catalytic pyrolysis, the aromatic compounds deposited on the catalyst can be generally eliminated over 350 °C [43]. It showed a stable mass loss rate for the used RHC Ni from 400 to 500 °C, possibly ascribed to the fixed carbon burn off. Basically, the burnout temperature of char and coke (R9) was over 500 °C [44], where the mass loss rate of the used 10 g-RHC Ni decreased, compared to the used 3 g-RHC Ni and 5 g-RHC Ni. It is also indicated that hydrocarbons decomposition (R10) and carbon deposition (R11) are easier take place when a small amount of catalyst is employed.

C þ O2 ! CO2

ðcoke combustionÞ

ðR9Þ

Cn Hm ! C þ H2 þ CH4 þ Cx Hy

ðR10Þ

CH4 ! C þ 2H2

ðR11Þ

ðcoke formationÞ

3.5.5. Surface characteristics The fresh RHC Ni (72.75 m2/g) has a lower BET surface area (117.08 m2/g) compared with the RHC, indicating that metal catalyst (i.e., nickel) loading decrease the surface area of char. The isotherms suggest the significant formation of mesopores during the reactions with the RHC Ni, because the isotherm curves resemble Type II isotherms after catalytic reforming. In contrast, the RHC Ni1 shows the Type I isotherm, indicating a more typical microporous structure. Moreover, an increase in the hysteresis elbow is observed in the RHC Ni2, indicating the widened mesopores and the possibility of deeper pore formation (Fig. 11); accordingly, its BET surface area and pore diameter increased from 72.75 m2/g to 200.50 m2/g and 0.12 m2/g to 0.15 cm3/g, respectively (Fig. 12).

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Fig. 7. SEM images of the RHC catalysts [RHC (A), fresh (B) and used (C & D) RHC Ni particles].

Fig. 8. Magnified SEM images of the fresh (A) and used (B & C) RHC Ni catalysts.

In general, the increase of surface areas for the used RHC Ni catalysts is most likely attributed to the formation of new pores by RHC and soot gasification, corresponding to the soot/char gasification rate higher than deposition rate.

3.6. Integrated catalytic pyrolysis and gasification The integrated strategy of catalytic biomass pyrolysis/gasification includes different key reaction steps: (step 1) metal precursor

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577

Fig. 9. TEM-EDS analysis of (A) the fresh and (B) used RHC Ni catalysts.

tar yields after the different conditions. Besides tertiary tar, noncondensable gases such as CO, CO2, H2O, CH4, CnHm, and H2 are also yielded. As for the important syngas components, these products formation will increase the conversion efficiency of the gasifier. Reforming effects of CO2 on tar evolution is likely due to the hydroxy (OH) radicals produced from CO2 during the thermal conversion of syngas. A higher concentration of OH radicals can enhance the oxidation process of tar [45]. Over the RHC Ni, dry reforming is significantly enhanced (R13). In this study, less water (H2O) from the as-received biomass can enhance the nascent tar reforming and inhibit the polymerization reactions, which are ascribed to more hydrogen produced by the reactions of

Fig. 10. DTG curves of the fresh and used RHC Ni.

(e.g., Ni2+) insertion into solid biomass, (step 2) catalytic pyrolysis of biomass, (step 3) the catalytic active nanoparticles (e.g., Ni0) in situ generated and highly dispersed in the carbon matrix, (step 4) catalytic conversion of nascent tars by the formed nanocomposites, (step 5) catalytic gasification of the char residue, and (step 6) recycling and reuse of the metal species in the ash (e.g., Ni/SiO2). Each of these reaction steps requires a thorough fundamental understanding to develop a high-efficiency biomass pyrolysis– gasification process at nanoscales. In this work, the metallic nickel (Ni0) nanoparticles had been successfully embedded into the carbon matrix of RHC, which was employed for catalytic conversion of tar derived from biomass gasification. The reactions in the second stage mainly include tar cracking and reforming. The tertiary tar in (R12) implies the total

Fig. 11. N2 adsorption–desorption curves of RHC, RHC Ni1 and RHC Ni2.

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CO þ H2 O $ CO2 þ H2

ðR18Þ

Char gasification (char present):

C þ CO2 $ 2CO

ðR19Þ

C þ H2 O $ CO þ H2

ðR20Þ

4. Conclusions

Fig. 12. Surface characteristics of RHC, the fresh and used RHC Ni.

(R14–R19). Additionally, the presence of CO2 or H2O agents could influence the formation of other noncondensable gases. Nevertheless, gases in the outlet of gasifier are present in a relatively stable composition, possibly attributed to these reversible reactions (R14–R18). In the reformer, char heterogeneous gasification is the dominant endothermic reaction, which is of benefit to the char conversion efficiency (R19 and R20) normally occurring at relatively high temperatures. Without sufficient energy, the proceeding of char/carbon gasification reactions such as Boudouard reaction would be inhibited or occurred with low-efficiency. Moreover, the pyrolysis–catalytic reforming could not significantly consume the RHC supported catalysts. However, the RHC Ni could be deactivated by carbon deposition on the surfaces of metal actives. Furthermore, the deposited carbon is directly gasified into the additional syngas by steam or CO2, thereby inhibiting its deactivation by producing new pores. Tar decomposition:

Nascent tar þ CO2 =H2 O=RHC ! Tertiary tar þ CO þ H2 þ CO2 þ H2 O þ Cn Hm þ CH4

ðR12Þ

Nascent tar þ CO2 =H2 O=RHCNi ! CO þ H2 þ CO2 þ H2 O þ Cn Hm þ CH4

Metallic nickel (Ni0) nanoparticles could be successfully in situ generated in the rice husk char via one-step pyrolysis. Ni0 nanoparticles are embedded and highly dispersed in the carbon matrix with the particle size of 5–10 nm. The synthesized RHC Ni exhibited a high activity on the tar reforming. In particular, tar conversion efficiency increased with the increase of catalyst weight and reforming temperature. Tar reforming efficiency can reach 50.1% by using 3 g of RHC Ni, while it significantly increased, up to 99.8% by using 10 g of RHC Ni, maintaining a high value of 99.3% after 5 cycles. It might be attributed to the enhancement of catalytic reaction and residence time, which is caused by the increase of catalyst weight. Besides, if 10 g of RHC Ni employed, tar conversion efficiency could be improved from 92.3% to 100% in the temperature range of 500–900 °C. It is indicated that RHC Ni showed higher catalytic activity for tar reforming even at relatively lower temperatures (>500 °C). More importantly, by using the RHC Ni, less PAHs can be formed from the nascent tars. Compared to the tertiary tar compounds of PAHs, it is much easier to crack/reform the nascent tar compounds by RHC Ni. If the RHC Ni was used for the catalytic reforming, the yield of liquid products reduced from 30.2% to 10.7%, possibly caused by a series of thermo-chemical reactions, such as water–gas-shift reaction, char gasification, and steam reforming of tar. Accordingly, the yield of gas product greatly increased from 37.5% to 58.0%. Among these gas molecules, the volume fractions of CO and H2 increased from 21.2% to 33.5% and from 18.5% to 24.3%, respectively, attributed to the catalytic reforming over the RHC Ni. Moreover, the RHC Ni could be deactivated by coke deposition on the metal actives. After catalytic reforming, the surface areas of the used RHC Ni were increased due to the char gasification rate higher than deposition rate, contributing to the prolong of its service life. Consequently, the waste RHC Ni could be directly gasified into the additional syngas by gasification agents, such as steam and CO2, accompanied by recycling of the silica-based nickel nanocomposites. All these results indicated that the RHC Ni has a potential to be used for catalytic reforming of tar during biomass pyrolysis or gasification. Acknowledgements The author would like to acknowledge the China Scholarship Council (CSC) for the financial support under the Grant No. 201206230168. The authors also thank the editors and anonymous referees for their helpful comments. References

ðR13Þ

Gas reforming:

nCO2 þ Cn Hm $ 2n CO þ m=2 H2

ðR14Þ

n H2 O þ Cn Hm $ n CO þ ðn þ m=2ÞH2

ðR15Þ

CH4 þ H2 O $ CO þ 3H2

ðR16Þ

CH4 þ CO2 $ 2CO þ 2H2

ðR17Þ

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