Catalytic CO2 Gasification of Rice Husk Char for Syngas and Silica

Article pubs.acs.org/IECR

Catalytic CO2 Gasification of Rice Husk Char for Syngas and SilicaBased Nickel Nanoparticles Production Yafei Shen,*,† Ming Ding,‡ Xinlei Ge,† and Mindong Chen† †

Downloaded by STOCKHOLM UNIV on September 4, 2015 | http://pubs.acs.org Publication Date (Web): September 1, 2015 | doi: 10.1021/acs.iecr.5b02677

Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, PR China ‡ Jiangsu Environmental Monitoring, 241 Fenghuang West Street, Nanjing 210000, PR China ABSTRACT: The nickel-catalyzed CO2 gasification of rice husk char (RHC) was studied. In this work, carbon conversion is studied by thermogravimetric analysis (TGA) and a packed-bed reactor. The key influence parameters including nickel-loading amount and gasification temperature were investigated. In the absence of nickel, the carbon conversion efficiency of 100% was achieved in 40 min at 1000 °C, whereas this time was extended to 130 min at 700 °C. Although the use of high temperatures improved char reactivity, relatively high temperatures can shift the reaction rate toward the pore-diffusion-controlled regime. Also, the addition of nickel catalyst can improve the biochar reactivity by reducing the activation energy, in terms of the improvement of the carbon conversion and its rate. In the view of RHA Ni recycling, the silica-based nickel nanoparticles could be recycled in various catalytic applications. gasification rate.13 In general, the CO2 gasification rate of char is much slower than the steam gasification rate of chars because the steam reaction runs in parallel to the CO2 reaction. Consequently, the char−CO2 gasification rate is considered the rate-determining step in practical gasification processes.14−16 Compared with coal chars, biomass chars are relatively reactive because of their superior porosity and surface areas as well as availability of inherent catalytic elements. In a recent work, the characteristic of RHC gasification with steam was studied.17 The conversion rate of RHC increases with temperature and steam flow rate. Moreover, the reactivity of RHC prepared at low temperature is relatively high. When the temperature is greater than 850 °C, the diffusion through gas would control the overall reaction. Furthermore, the kinetics of RHC CO2 gasification was preliminarily studied.18 RHC gasification has a complex kinetic behavior, with reactivity being strongly dependent upon the temperature and char conversion. In addition, the reactivity of char could be enhanced in the presence of alkali, alkaline earth, and transition metal salts.19−23 However, the addition of various metal catalysts could increase the expense of the gasification process, so the recycling of metals in the residues become one of the significant obstacles for its application. So far, the catalytic effect of nickel on CO2 gasification of biomass char has been rarely reported. In our previous work, the nickel nanoparticles can be generated in situ in the carbon matrice of RHC via the pyrolysis process.24−26 RHC Ni shows good performance on tar catalytic reforming.26 The waste char catalysts can be directly converted into the syngas by the gasification process.27 In this work, the catalytic effect of nickel on CO2 gasification of RHC is preliminarily studied by thermogravimetric analysis

1. INTRODUCTION Rice husk (RH) is an agricultural residue abundantly available in rice-producing countries. According to estimations, the global production of rice is around 685 MT (including rice, straw, and husk), and RH is between 0.2 and 0.33 kg of each kilogram of rice harvested.1 RH is usually burnt in open fields, which involves energy waste and causes a serious environmental problem. Fast pyrolysis is a considerable option for the valorization of RH.2 The excellent performance of a bench-scale plant with a conical spouted-bed reactor was proven to be suitable for high bio-oil production (70%). However, the economic viability of the RH pyrolysis processes requires the valorization of the char fraction (continuously withdrawn from the reactor). The combined production of both amorphous silica and activated carbon from rice husk char (RHC) can be achieved by the pyrolysis process.2−5 In the view of energy, char from biomass pyrolysis could also be converted into value-added syngas or fuel gas by the gasification processes, such as H2O gasification and CO2 gasification (i.e., reactions R1−R3).6−10 The most important heterogeneous reactions that occur during the gasification are the solid carbon conversion.11,12 Water gas reaction: C + H 2O → CO + H 2 (R1) Boudouard reaction:

C + CO2 → 2CO

(R2)

Methanation reaction:

C + H 2 → CH4

(R3)

Gasification of char in the presence of CO2 would offer a potential solution for CO2 recycling as well as the alleviation of greenhouse gas emission. CO2 char gasification has been studied for many years, especially in coal gasification research. Char gasification is the controlling step because of its low © XXXX American Chemical Society

Received: July 22, 2015 Revised: August 20, 2015 Accepted: August 26, 2015

A

DOI: 10.1021/acs.iecr.5b02677 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Properties of RH, RHC and RHA ultimate analysis (wt %, dry and ash-free basis) RH RHC RHA


a

proximate analysis (wt %, dry basis)

C

H

Oa

N

37.9 64.8 9.5

6.3 2.4 0.3

55.3 35.1 90.2

0.4 0.1 0 Chemical Composition

VMb

FCc

ash

moisture

0.1 60.5 0 11.7 0 5.4 of RHA (wt %)

11.9 34.3 7.8

22.0 52.0 85.3

5.6 2.0 1.5

S

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

Zn (ppm)

Mn (ppm)

Cu (ppm)

94.64

0.06

0.23

1.88

0.96

0.39

0.58

18.20

52.24

32.17

By mass difference. bVM: volatile matters. cFC: fixed carbon.

(TGA). The key influence parameters including nickel-loading amount and gasification temperature on the carbon conversion of RHC are investigated. After that, the nickel-catalyzed CO2 gasification of RHC is conducted in a packed-bed (PB) reactor using conditions based on the optimal conditions determined by TGA. In addition, the solid residues derived from RHC gasification are characterized by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM) analysis.

Table 2. Properties of RHC loaded with different nickel amounts ultimate analysis (wt %, dry-basis)

2. MATERIALS AND METHODS 2.1. RH and RHC. RH was collected from Jinhu county in Jiangsu province of China. RH was ground and screened to less than 200 meshes. RH was pyrolyzed in a two-stage reactor at 500 °C in N2 atmosphere.24−26 Before adding the RH, the 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 500 °C. After that, RH was fed into the pyrolyzer, and it could be decomposed into solid (i.e., RHC), liquid, and gas. Before pyrolysis, the RH sample was dried in an oven for 2 days to remove moisture. Rice husk ash (RHA) used for ash composition analysis was obtained by calcination of RHC in a furnace at 650 °C in air. Table 1 lists the ultimate and proximate analyses as well as the ash composition. 2.2. Nickel Loading in RHC. Initially, 10 g of RHC from pyrolysis at 500 °C was added into 100 mL of Ni(NO3)2 solution (0.05, 0.08, and 0.16 mol/L for 3, 5, and 10% loading, respectively) as nickel precursor and subsequently dried in an oven at 105 °C overnight. Furthermore, the dry RHC Ni was pyrolyzed at 700 °C in N2 atmosphere for 30 min. In this step, metallic nickel catalysts could be in situ embedded in the char matrice (in terms of RHC Ni).26 According to the loading amount of nickel, the char samples are defined as 0% Ni RHC (raw RHC), 3% Ni RHC, 5% Ni RHC, and 10% Ni RHC, respectively. In addition, the residual volatiles in char can be further decomposed. Finally, the RHC Ni was obtained and dry-stored for the CO2 gasification process. Table 2 shows the properties of RHC Ni. It can be seen that the BET surface area of RHC was decreased with increasing nickel-loading amount. 2.3. CO2 Gasification of RHC. The overall in situ catalytic gasification process of carbon materials was shown in Figure 1, illustrating a simplified view of a cut of a porous grain with metal nanoparticles, which start moving under the reaction conditions.28 The diffusion of carbon atoms proceeds through the particle in one direction, and the particle itself moves in the opposite direction, keeping tight contact with carbon atom (into a step corresponding to a crystallographic a direction). RHC Ni is a nickel−carbon composite consisting of nickel (Ni0) nanoparticles highly dispersed in the carbon matrix. The char gasification reactions with CO2 or H2O are known to be

a

char samples

C

H

Oa

nickel conc. (mg/g)

SBET (m2/g)b

raw RHC 3% Ni RHC 5% Ni RHC 10% Ni RHC

82.60 63.85 62.33 60.18

2.30 3.72 3.98 4.50

15.10 32.43 33.69 35.32

0 30.08 35.20 62.86

115.20 93.18 85.12 63.25

Calculated by difference. bSBET refers to the specific surface area.

Figure 1. Catalytic carbon in situ gasification. (A) Porous carbon cut showing catalyst nanoparticles (moving under reaction conditions). (B) Analogy with a woodworm and detail of a single “carbon worm” particle.28

highly endothermic and to occur at temperatures higher than 800 °C.15 Therefore, char gasification is one of the most energy-consuming steps in the overall gasification process. The presence of alkali and transition metals can enhance the CO2 gasification reactivity of biomass char.19 The presence of nickel inside the char matrix may enhance the reactivity of CO2 char gasification as well. CO2 gasification experiments were carried out under isothermal conditions in a thermogravimetric (TG) analyzer and a PB reactor, respectively. In each TG experiment, char powder (∼10 mg) was loaded in an Al2O3 pan and heated (20 °C/min) in N2 atmosphere (150 mL/min) to the preset gasification temperature. To make sure that the mass loss of RHC in the isothermal CO2 gasification process was only attributed to the char−gas reaction, the sample was kept at the gasification temperature under N2 for 10 min. Subsequently, N2 was switched to CO2 (150 mL/min) in order to initiate the isothermal gasification. In addition, the weight loss of char sample was recorded continuously as a function of the gasification time. The carbon conversion rate (XTGA) is calculated by eq E120,29 B


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Industrial & Engineering Chemistry Research


Figure 2. Scheme of a lab-scale packed-bed gasifier and experimental conditions.

Figure 3. Carbon conversion of CO2 gasification of RHC Ni at 700−1000 °C (A−D) by TGA.

X TGA =

mo − mt × 100% mo − mc − mash

the evolution of CO via the Boudouard reaction was detected by analyzing the outlet gas stream. Carbon conversion:

(E1)

where mo represents the initial mass of the char at the onset gasification, mt is the instantaneous mass of the char at time t, mc is the mass of catalyst, and mash is the mass of ash after completion of gasification. On the basis of the optimized conditions determined via TGA, the lab-scale CO2 gasification of RHC Ni is conducted in the PB reactor as shown in Figure 2, which is composed of the gasifier and the gas analyzer. The char sample was heated up under N2 at a flow rate of 50 mL/min to the desired temperature; then, the gas was shifted to CO2 (200 mL/min) to conduct the char gasification under the isothermal condition. The carbon conversion rate (XPB) is calculated by eq E2, and

XPB =

moles of carbon in the produced CO × 100% moles of carbon in the feed (E2)

Reaction equation:

C + CO2 → 2CO

(R4)

2.4. Analytical Methods. The ultimate and the proximate analyses were determined by the elemental analyzer (Vario MICRO Cube, Elementar, Germany) and the TG analyzer (DTG-50, Shimadzu, Nakagyo-ku, Japan), respectively. The chemical composition of RHA was determined by X-ray C


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Figure 4. (A) Carbon conversion and (B) CO generation rates of CO2 gasification of RHC Ni at 900 °C in a packed-bed (PB) reactor.

800 and 900 °C, respectively (Figure 3C). In Lahijani’s work,20 below 875 °C was considered a chemically controlled regime. Transition between the chemically controlled phase and porediffusion-controlled phase occurred at the temperature range of 875−900 °C. At temperatures above 900 °C, the reaction mechanism could be controlled by pore diffusion. The gasification temperature of 875 °C could be selected. This temperature was sufficiently low to keep the intrinsic reaction rate in the chemically controlled regime, which was the basis for calculating the activation energy.30 In this study, the nickelcatalyzed CO2 gasification of RHC was conducted at 900 °C in the further gasification experiments. Figure 4 shows the carbon conversion and CO generation rates originated from CO2 gasification of RHC Ni at 900 °C in a PB reactor. The carbon conversion rates are slightly lower than those in TGA, possibly influenced by the operating conditions, such as the CO2 flow rate. With regards to the 10% nickel-loaded RHC Ni, the carbon conversion efficiency of 100% could be achieved in almost 30 min at 900 °C for CO2 gasification. Both the carbon conversion and its rate in the PB experiment were much lower than those in the TGA analysis. This might be attributed to the different reaction conditions. However, it can be found that 3% and 5% Ni RHC showed a similar trend of carbon conversion rate under the same conditions, corresponding to the similar activation energies as below. It is suggested that the Ni loading amount can significantly influence char reactivity. 3.2. Kinetics of Nickel-Catalyzed CO2 Gasification. The reaction rate was sensitive to the gasification temperature, and the reactivity improved as the temperature increased. The experimentally acquired reactivity data were used for kinetics studies, and kinetic models were implemented to interpret the experimental results. Accordingly, one of the most reliable kinetic models to describe the evolution of biomass char gasification rate is the random pore model (RPM).31 This model assumes that the particles are comprised of cylindrical pores of the same radius and that the reaction is proportional to the internal surface area of the pores.32 The RPM reaction rate is represented as

fluorescence analysis (XRF, Shimadzu, Rayny EDX 700, Japan). The gas produced from RHC gasification was analyzed in a GC (Agilent 4890) equipped with a thermal-conductivity detector (TCD) and a packed column (Carboxene 1000 (15 ft. × 1/8 in., 80/100 mesh), Supelco, USA). The oven temperature was set at 150 °C and then increased to 210 °C at a rate of 20 °C/ min; the injector and detector temperatures were 150 and 220 °C, respectively. Nitrogen was used as the carrier gas at a flow rate of 45 mL/min. Samples of RHC Ni before and after CO2 gasification were characterized by XRD (Rigaku, XRD-DSC II, Japan) and the SEM-EDX (JSM-6610, JEOL/EO, Japan) analyses, respectively. The identification of crystal phases was carried out by XRD using a Rigaku D/Max 3400 powder diffraction system with Cu Kα radiation (λ = 0.1542 nm) at 45 kV and 200 mA with a scanning rate of 5°/min.

3. RESULTS AND DISCUSSION 3.1. Carbon Conversion. Gasification temperature is known to be one of the most influential parameters in controlling the gasification reaction rate. On the basis of this knowledge, the gasification temperature was varied in the range of 700−1000 °C. In addition, the nickel catalyst loading in the range of 0−10% was investigated in the isothermal gasification. Figure 3 shows the carbon conversion of CO2 gasification of RHC Ni. The results confirmed the pronounced effect of the gasification temperature on promoting char reactivity. In the absence of nickel, carbon conversion efficiency of 100% was achieved in 40 min at 1000 °C, whereas this time was prolonged to 130 min at 700 °C. Although the use of high temperatures improved the reactivity of biomass char, it was speculated that relatively high temperatures can shift the reaction rate toward the pore-diffusion-controlled regime.20 Furthermore, the addition of nickel catalyst could improve char reactivity by reducing the activation energy. As shown in Figure 3, the carbon conversion rate is remarkably increased by adding the nickel catalyst at the different gasification temperatures. In particular, the carbon conversion efficiency of 100% could be achieved in almost 10 min at 1000 °C by loading the nickel catalyst (Figure 3A). Moreover, it indicated that the loading amount of nickel catalyst had a trifling impact on the carbon conversion rate at very high temperatures (>1000 °C). As gasification temperature was decreased, the catalytic effect of nickel was significantly reduced. The carbon conversion rate was not obviously decreased for 10% loading of nickel catalyst, whereas it was decreased for 3 and 5% loading of nickel catalyst at 900 °C (Figure 3B). It is noted that when loading 3 and 5% of nickel catalysts, the carbon conversion rates were similar at

dX = kRPM⟨1 − X ⟩ (1 − Ψ) ln(1 − X ) dt

(E3)

where X is the carbon conversion, t represents the time, kRPM denotes the reaction rate constant (min −1), and Ψ is a structural constant. The applicability of the RPM to describe the gasification rate of nickel-loaded RHC at various temperatures was examined. D


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Figure 5. RPM fitting curves of CO2 gasification rates for the nickel-loaded RHC (A, 0%; B, 3%; C, 5%; and D, 10%).

The curve fitting results for the temperatures of 700, 800, 900, and 1000 °C are presented in Figure 5. From the curve fitting results, it is concluded that the RPM could well describe the gasification reaction rate at all implemented temperatures (regression coefficient, R2 > 0.99). The RPM, which considers both the evolution and destruction of pores, predicts a maximum reactivity with the increase of conversion. For example, the maximum reactivity (rmax) of 0.18 min−1 was achieved at carbon conversion of 0.26 and the temperature of 900 °C. Table 3 shows the kinetic parameters of the RPM and

Thus, RPM is preferred as the kinetic model for nickelcatalyzed CO2 gasification of biomass char (e.g., RHC). The activation energy of CO2 char gasification was determined by employment of the Arrhenius plot. Upon the obtainment of the gasification rate constant (kRPM), the plot of ln kRPM was developed against 1/T. The rate of the gas−solid reaction in a chemical reaction could generally be expressed by dX = k(1 − x)n dt

where k is the reaction rate constant (min−1) and n is the reaction index. The Arrhenius equation can be expressed as follows:

Table 3. Kinetic Parameters of the RPM and Its Regression Coefficient T (°C)

r0 (1/min)

Ψ

R2

⎤ ⎡ Ea k = A exp⎢ − ⎥ ⎣ R(T + 273.15) ⎦

Raw RHC 700 800 900 1000

2.63 4.82 4.20 1.52

× × × ×

700 800 900 1000

2.25 1.26 4.80 1.55

× × × ×

700 800 900 1000

2.38 1.21 2.28 1.56

× × × ×

700 800 900 1000

5.12 5.80 1.32 1.78

× × × ×

10−5 0.3329 1.1520 10−3 10−2 2.5785 10−1 4.9028 3% Ni RHC 10−5 1.3513 10−2 3.6825 10−2 5.5236 10−1 8.7508 5% Ni RHC 10−5 2.1314 10−2 4.2096 8.8363 10−2 10−1 11.0825 10% Ni RHC 10−2 8.3379 12.2536 10−2 10−1 13.8709 10−1 14.1533

(E4)

0.9985 0.9972 0.9954 0.9937

(E5) −1

where A is a frequency factor (min ), Ea is the activation energy (kJ/mol), R is a gas constant (8.314 J/(mol °C)), and T is the gasification temperature (°C). The Arrhenius plots used for calculation of activation energy were developed for the conversion values of 0.1−0.9 and temperatures of 700−1000 °C, as illustrated in Figure 6. No shift was observed in the data points, and linear relationships were also obtained between ln(dX/dt) and 1/T, which implies that the reactions followed the Arrhenius law and that the studied temperature range (i.e., 800−900 °C) was suitable to keep the reaction in the chemically controlled regime. The Arrhenius curve fitting resulted in relatively parallel lines, which is an indication of almost constant activation energy. The results of linear fittings and activation energies are shown in Table 4. Comparing the activation energies of RHC and 10% nickel-loaded RHC indicated that the activation energy of noncatalytic CO2 gasification reaction was around 169.2 kJ/ mol, which decreased to 73.3 kJ/mol in the presence of nickel catalyst. 3.3. XRD Analysis. RHC, the fresh and used RHC Ni, and the RHA Ni from CO2 gasification at 900 °C were characterized by XRD analysis. Figure 7 shows the XRD patterns of RHC and RHC Ni before and after CO 2 gasification. As shown in Figure 7A, an obvious strong peak at 2θ = 22.5° in the RHC was a typical characteristic peak of

0.9992 0.9975 0.9960 0.9926 0.9987 0.9980 0.9933 0.9915 0.9983 0.9971 0.9952 0.9908

its regression coefficient. In this study, the instantaneous char reactivity (r0) was generally increased with increasing temperature. In addition, the structural constant of Ψ increased with the increase of temperature, and the nickel-loading amount is considered an indication of char porosity development and the progress of reactions taking place inside the pores of char. E


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Figure 6. Arrhenius plots of CO2 gasification at temperatures of 700−1000 °C for the nickel-loaded RHC (A, 0%; B, 3%; C, 5%; and D, 10%).

crystallization, whereas the characteristic peak of crystalline graphitic carbon disappeared possibly because of the carbon conversion. It is noteworthy that the characteristic peaks of Ni0 became weakened or disappeared, whereas more characteristic sharp peaks of bunsenite (NiO) appeared at 37.17, 43.24, 62.87, and 75.37° (Figure 7B). This suggests that the metallic nickel (Ni0) is oxidized to nickel oxide (NiO) after CO2 gasification at relatively high temperatures. The mechanisms can be concluded as reactions R5−R7). Initially, the CO2 concentration is much higher than the CO concentration in this system, which thereby promotes the reversible reactions R5 and R6 to transform Ni0 into NiO and Ni2O3.35 Furthermore, the crystal components in RHA Ni are very pure, mainly composed of NiO and SiO2. On the basis of the reported works, there exists the potential to reuse a silica-based nickel catalyst,36,37 especially in terms of tar catalytic reforming.38,39

Table 4. CO2 Gasification Kinetic Parameters of NickelLoaded RHC char samples

A (1/min)

Ea (kJ/mol)

R2

raw RHC 3% Ni RHC 5% Ni RHC 10% Ni RHC

2.78 × 10 6.02 × 103 8.54 × 103 28.4

169.2 119.1 124.7 73.3

0.9855 0.9629 0.9858 0.8845

6

amorphous silica (SiO2) corresponding to the presence of cristobalite. In addition, a weak characteristic peak at 2θ = 44.3° could indicate the formation of a turbostratic structure of amorphous carbon.33,34 The characteristic peaks of amorphous SiO2 were clearly observed in RHC Ni and the RHA Ni (Figure 7B). Furthermore, it could be found that RHC Ni1 and RHC Ni2 exhibited similar patterns with four sharp characteristic peaks. The first peak at 22° is ascribed to the presence of cristobalite silica, and the second weak peak at 26.6° can be assigned to the crystalline graphitic carbon. Also, three sharp peaks appeared at 44.43, 51.8, and 76.31°, respectively, that are the characteristic peaks of crystalline metallic nickel (Ni0) (Figure 7B). After CO2 gasification was completed, the peak intensity of cristobalite silica became very strong, indicating its good

Ni + CO2 → NiO + CO

(R5)

2NiO + CO2 → Ni 2O3 + CO

(R6)

Ni 2O3 + C → 2NiO + CO

(R7)

3.4. SEM Analysis. RH particles with a typical globular structure are tightly interlocked with each other.40 The corrugate structural outer epidermis is highly ridged, whereas

Figure 7. XRD patterns of (A) RHC and (B) RHC Ni1 (fresh), RHC Ni2 (used), and RHA Ni. F


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Figure 8. SEM micrographs of RHC (A, 50×; B, 150×; and C, 300×).

Figure 9. SEM micrographs of RHA (A, 100×; B, 200×; and C, 450×).

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 with varying particle sizes were distributed within the char, as evidenced by the interconnected porous network and large surface area.41 With the development of reaction, the surface texture of char becomes irregularity G


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Figure 10. SEM micrographs of RHC Ni (A, 50× and B, 400×).

Figure 11. SEM micrographs of RHA Ni (A, 100×; B, 350×; and C, 650×).

3.5. Integrated Strategy. The integrated strategy of biomass pyrolysis with char gasification includes different key reaction steps. As illustrated in Figure 12, biomass (e.g., RH) can be decomposed to bio-oil, biochar, and a small amount of gas product via the pyrolysis process (e.g., devolatilization and carboninzation) at moderate temperatures (e.g., 500 °C).4 After that, the nickel precursor (Ni2+) was introduced to the char (e.g., RHC) matrix. The residual volatiles could be further removed from RHC by high-temperature pyrolysis (e.g., 700 °C). During this process, the Ni2+ could be transformed to the metallic nickel (Ni0) via the carbothermal reduction. Of note is the fact that the produced RHC Ni can be used as a carbonbased catalyst especially for tar reforming. Furthermore, the nickel nanoparticles embedded in the carbon matrix can catalytically promote CO2 gasification of char (e.g., RHC). Finally, in the presence of sufficient CO2, RHA NiO was generated. The chemical state of nickel (Ni2+ or Ni0) was most likely controlled by the reversible reactions R5 and R6. In the view of RHA Ni recycling, the silica-based nickel could be recycled in various catalytic applications.43−46

ascribed to the shrinkage of the globular structure, which might be caused by devolatilization.42 Evaporation of the volatile materials could create new pores on the particle with rough surfaces and irregular outlets. Figures 8−11 shows the SEM micrographs of the RHC and RHC Ni before and after CO2 gasification. After pyrolysis and gasification processes, silicon (Si) remained along the outer rugged surfaces. It could be found that the initial morphological structures of RHC, corresponding to that of RH, remained after CO2 gasification at 900 °C for 30 min (Figures 8 and9). On the contrary, the morphological structures of RHC Ni could be significantly destroyed by CO2 gasification under the same conditions (Figures 10 and 11). This suggests that the metal nickel embedded in RHC can significantly enhance the carbon conversion via of CO2 gasification. Furthermore, loose flake structures were observed on the surface of RHA Ni (Figure 11B). In addition, nickel nanoparticles dispersed uniformly on the surfaces of RHC and RHA. It showed that metal actives could be inclined to form intercalation compounds with carbon, which increases the interlayer distance and causes volume expansion. In this case, the C−C bonds existing between layers were weakened and the gasification reaction was enhanced. H


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Figure 12. Integrated strategy of biomass (RH) pyrolysis with nickel-catalyzed CO2 char gasification.

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4. CONCLUSIONS

ACKNOWLEDGMENTS We thank the editors and anonymous referees for their helpful comments.

CO2 catalytic gasification of RHC was studied for carbon conversion. In the absence of nickel, the carbon conversion efficiency of 100% was achieved in 40 min at 1000 °C, whereas the reaction time was extended to 130 min at 700 °C. Although the use of high temperatures improved char reactivity, the relatively high temperatures could shift the reaction rate toward the pore-diffusion-controlled regime. The addition of nickel catalyst can improve the reactivity of biochar by reducing the activation energy. Accordingly, the carbon conversion rate is significantly increased by addition of nickel catalyst at different gasification temperatures. In particular, the carbon conversion efficiency of 100% was achieved in almost 10 min at 1000 °C by loading the nickel catalyst. The loading amount of nickel had trifling impact on the carbon conversion rate at very high temperatures (>1000 °C). Decreasing gasification temperature, the catalytic effect of nickel was remarkably reduced. The carbon conversion rate was not obviously decreased for 10% loading of nickel catalyst, whereas it was decreased for 3 and 5% loading of nickel catalyst at 900 °C. As for 3 and 5% of nickel catalysts, the carbon conversion rates were similar at 800 and 900 °C, respectively. This might be attributed to the approximate activation energy. The RPM can describe the gasification reaction rate well. In addition, the metal nickel embedded in RHC can significantly enhance the carbon conversion and its rate of CO2 gasification, thereby destroying the biochar porous structures completely. In the view of metal catalysts recycling in ash, the silica-based nickel nanoparticles can be reused in catalytic applications.

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REFERENCES

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*E-mail: [email protected], [email protected]. Notes

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Catalytic CO2 Gasification of Rice Husk Char for Syngas and Silica

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