Influence of impregnated copper and zinc on the pyrolysis of rice husk in a micro-fluidized bed reactor: Characterization and kinetics

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Influence of impregnated copper and zinc on the pyrolysis of rice husk in a micro-fluidized bed reactor: Characterization and kinetics Feiqiang Guo*, Kuangye Peng, Xingmin Zhao, Xiaochen Jiang, Lin Qian, Chenglong Guo, Zhonghao Rao School of Electrical and Power Engineering, China University of Mining and Technology, 221116 Xuzhou, PR China

article info

abstract

Article history:

Catalytic performance of Cu and Zn catalysts was investigated during rice husk (RH) high-

Received 17 July 2018

temperature pyrolysis under isothermal conditions in a micro-fluidized bed reactor. The

Received in revised form

results showed that the presence of Cu and Zn evidently influenced the release charac-

26 September 2018

teristics and conversion of the gas components. The impregnated Cu promoted the con-

Accepted 1 October 2018

version of H2, CH4, CO and CO2, while Zn showed positive catalytic effect on the conversion

Available online xxx

of H2, CH4 and CO2 and negative effect on the conversion of CO. The X-ray diffraction patterns of the residue chars revealed that metallic copper nanoparticles (Cu0) were formed

Keywords:

during Cu impregnated biomass pyrolysis. Textural characterization and SEM images

Rice husk

showed that the impregnation of Cu and Zn, particularly Zn, promoted the generation of

Kinetics

micropores and mesopores, with the pore sizes predominantly at around 1.3 nm and

Micro-fluidized bed

3.9 nm. Reaction kinetics for generating these gases was studied based on model fitting

Copper

method, and the most probable reaction mechanism was obtained based on the relative

Zinc

error between experimental and calculated conversion data. The resulting apparent activation energies were 85.08, 12.56, 49.72 and 38.37 kJ/mol for the formation of H2, CO, CH4 and CO2 from pure RH pyrolysis. The presence of Cu decreased the forming activation energies of the four gases, and Zn decrease the forming activation energies of H2, CH4 and CO2 while increased the value for the formation of CO. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction At present, the energy crisis and related environmental pollution are serious global problems, which have provoked extensive research on renewable, environmentally friendly and economically viable energy sources [1,2]. Among various renewable resources such as wind and solar energy, biomass has received considerable attention since it is an organic

carbon resource in nature and carbon-neutral and lowemission fuels can be produced [3]. In China, biomass, particularly agricultural biomass, is naturally abundant and has the potential to be used as an alternative to fossil fuels. For example, China has the largest rice cultivation region in the world, and approximately 40 million tons of rice husk are produced annually [4]. As a renewable carbonaceous resource, rice husk can be converted into fuels, chemicals and other products by exploring appropriate methods.

* Corresponding author. E-mail address: [email protected] (F. Guo). https://doi.org/10.1016/j.ijhydene.2018.10.013 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

2

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

Thermochemical processes such as pyrolysis, gasification, combustion and hydrothermal liquefaction have been seen as a promising technology for producing renewable fuels from biomass. Among these thermochemical methods, pyrolysis which can be described as the direct thermal decomposition of the organic matrix in the absence of oxygen has been seen as the fundamental step of the other thermochemical technologies and a promising platform to obtain various fuels and chemicals from biomass [5]. Particularly, fast pyrolysis/gasification has been generally accepted as one of the most economical and feasible ways to convert biomass into valuable fuel gas and liquid oil from biomass at different temperature ranges [6]. However, if fuel gas is the target product, two serious issues in biomass pyrolysis and gasification is the generation of byproduct tar and low gas quality, which have become the major obstacle to the application of biomass pyrolysis and gasification [7,8]. For this problem, catalytic biomass pyrolysis/gasification has attracted ever-increasing attention since this process can improve the pyrolysis product quality and lower the tar yield during biomass decomposition process [9,10]. A number of catalysts have been developed for biomass pyrolysis, and the use of metal catalysts is considered as a promising option for biomass feedstock upgrading. A promising catalytic strategy is to impregnate biomass with metal salts, since the oxygenated groups present in the bio-macromolecules can act as adsorption sites for metal cations, leading to high metal precursor dispersion into the wood matrix [11]. Particularly, phytoremediation and biosorption have been widely used in the remediation of heavy-metal-polluted water (e.g., Zn, Cu, Cr, Ni, Pb and Cd), which are similar to the impregnation process and result in metal accumulation on the matrix of biomass [12,13]. Fast pyrolysis is also a preferable and environmentally friendly technology for converting biomass after heavy metal accumulation, because the emission of many toxic pollutants can be avoided in comparison with direct combustion and heavy metals are mainly enriched in residual char after pyrolysis [14,15]. Among these heavy metals, copper and zinc are two typical heavy metals in polluted water and often have favorable catalytic activity toward biomass pyrolysis [16,17]. Antonakou et al. [18] reported that they prepared Cu and Zn containing catalysts and used them for biomass pyrolysis, finding that the addition of these metals increased the yields of the main gas components (CO, CO2, CH4 and H2). Ozbay et al. [19] prepared Cu/Al2O3 catalyst for tomato waste pyrolysis in a fixed bed tubular reactor, and the results showed that the addition of catalysts have strong positive effect on decomposition of biomass. Catalytic gasification of biomass using MgO supported Cu and Zn oxides as catalysts was investigated by Mastuli et al. [20] for hydrogen production, showing that Cu and Zn enhanced the total H2 yield produced. Mesoporous alumina supported Cu and Zn based catalysts were also used for the catalytic reforming of acetic acid from biomass pyrolysis, finding that the presence of Cu and Zn has high activity in steam reforming of acetic acid and yield less coke [21]. Thus, the addition of Cu and Zn is an approach available to give good performance in biomass pyrolysis for gas production.

The precise understanding of the kinetic characteristics of biomass catalytic pyrolysis is essential to the design and optimization of biomass pyrolysis process. By far, thermogravimetric (TG) method has been generally used to characterize the non-isothermal biomass pyrolysis process and deduce its reaction kinetics. In our previous study, it was observed that the addition of a metal catalyst shifted the initial pyrolysis temperature and the peak temperature using a thermogravimetric analyzer (TGA) [22]. It has also been reported that the activity of biomass pyrolysis/gasification was attributed to the interaction between metal catalysts and biomass [23]. The activation energy represents essentially the energy barrier to start a reaction, and previous studies have also reported that the addition of metal catalysts changed the activation energy for biomass pyrolysis under non-isothermal conditions [22,24]. Nonetheless, TGA still suffers from some drawbacks due to its measurement principle, such as specified heating program, gas mixing and diffusion, especially for complex materials such as coal and biomass [25,26]. Recently, the micro-fluidized bed reaction analyzer (MFBRA) has been developed to study the kinetics of biomass pyrolysis under isothermal conditions based on gas releasing characteristics [25]. Compared with the TGA, the microfluidized bed reactor has a distinct advantage in better heat and mass transfer features, rapid heating and inhibition of external diffusion, which can make the obtained kinetics more close to the intrinsic chemical kinetics [27]. The MFBRA has been employed for kinetic studies of various gas-solid reactions, such as biomass pyrolysis [28,29], char gasification [26,30], tar cracking [31], etc. The use of the MFBRA allowed to analyze biomass pyrolysis process on the basis of the kinetic parameters of gas components, and thus can be employed to study the catalytic effect of metals on biomass pyrolysis in a novel manner. In this work, in order to better analyze the effect of copper and zinc on pyrolysis characteristics of rice husk, the MFBRA was employed to study the product gas generating characteristics under isothermal conditions. Kinetic parameters for the formation of four main gaseous products (H2, CO, CH4 and CO2) were investigated based on a model-fitting approach. The catalytic performance of Cu and Zn for biomass pyrolysis was investigated on the basis of the variation of gas releasing behaviors, the corresponding kinetic parameters and the structure of the residue chars. The results were hoped to offer a further understanding of the catalytic conversion of biomass into fuel gas.

Experimental section Biomass fuel The biomass feedstock of rice husk (RH) used in the experiment was collected from Xuzhou, Jiangsu Province. The received RH was crushed to a size range of 74e125 mm and then dried at 105
C for 24 h. The results of ultimate and proximate analyses of rice husk were presented in Table 1. The ultimate analysis was carried out using an elemental analyzer (VarioMicro Cube, Elementar, Germany) and the

Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

3

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

Table 1 e Proximate and ultimate analyses of RH. Sample

RH

Proximate analysis (wt%, db)

Ultimate analysis (wt%, daf)

V

FC

A

C

H

Oa

N

S

56.4

18.4

25.2

35.6

7.2

56.7

0.4

0.1

db, dry basis; daf, dry and ash free basis. a By difference.

proximate analysis was conducted according to GB/T287312012 standard using a muffle furnace (SX2-2.5e12A, China). Three types of samples including RH-Cu (Cu2þ: 0.5 mol/L), RH-Zn (Zn2þ: 0.5 mol/L) and RH-Cu/Zn (Cu2þ: 0.25 mol/L, Zn2þ: 0.25 mol/L) were prepared by the incipient wetness impregnation method using CuCl2$2H2O (99%, Qiangshun Chemical Industries, Ltd., China) and ZnCl2 (98%, Zhiyuan Chemical Industries, Ltd., China) as copper and zinc precursors, respectively. 3 g dried rice husk was immersed in 60 mL of CuCl2, ZnCl2 or CuCl2/ZnCl2 aqueous solution with a concentration of 0.5 mol/L. Then, the obtained samples after impregnation were stirred for 24 h at room temperature. Subsequently, the mixtures were filtered and dried at 105
C for about 24 h. Table 2 summaries the atom percentage of the samples, which was determined by the X-ray fluorescence (XRF, Shimadzu 1800, Japan). It can be seen that the amount of Cu, Zn and Cl in the biomass samples has a significant increase, indicating the successful impregnation of the samples. The corresponding chars after pyrolysis were referred to as RHC, RHC-Cu, RHC-Zn and RHC-Cu/Zn.

Apparatus and procedure Fig. 1 shows the principle and process of the adopted MFBRA, which consists of a sample injection system, a micro-fluidized bed (MFB) reactor, an electric furnace, a gas supply system (argon), a produced gas cleaning system and an online process mass spectrometer (Dycor ProLine, Ametek, American) connected to the gas outlet of the MFB reactor. The microfluidized bed reactor is 42 mm in height and 20 mm in inner diameter. The reactor was divided into three sections by two porous plates, from the bottom, which were a preheating section, a reaction section with fluidizing medium and a sedimentation section to reduce fine particle entrainment. The detailed schematic has been described in our previous work [32]. The sample feeding, temperatures and pressures as well as mass spectrometer are all monitored by a computer. Quartz sand (2 g) was used as fluidizing medium, which was filtered and calcined before each test to remove the possible impurities. Besides, 15 mg of sample was preloaded at the sample injection system which was driven by an

electromagnetic valve. High-purity argon (99.999%) was used as the carrier gas in the experiment. A stream of argon at 500 mL/min was continuously fed into the reactor to fully fluidize the quartz sand and remove the gaseous products from the reactor, thus minimizing any secondary vapor-phase reactions. The temperatures chosen for this work were 700, 750, 800 and 850
C, which is also the main operating temperature range for biomass pyrolysis or gasification in industrial-scale fluidized bed. After reaching the preset reaction temperature, the sample was rapidly injected into the reactor within 0.1 s by compressed gas, which in turn initiated the pyrolytic reactions and the formed gaseous products were quickly carried out of the reactor by the fluidizing gas. Then, the gaseous products were filtered, condensed and dried for the analysis by an on-line mass spectrometer. When the mass spectrometer was adopted, a heated capillary tube (around 350
C) was employed to connect the mass spectrometer and the filter. The capillary decreases the pressure of the sample gas to the pressure required by the spectrometer. Also, the heated capillary is available to prevent sample gas condensation for the analysis. The composition of the gas was monitored until no more gas was detected via the mass spectrometer. Before each experiment, 2 h of preheating is necessary to ensure that the mass spectrometer has been in a stable state. In the tests, the intensity of the gas components which is mainly composed of H2, CH4, CO and CO2 was measured and recorded at around 0.9 s intervals by the mass spectrometer. To assure the reliability of the test results, each test is repeated for three times. The scanning electron microscopy with energy-dispersive X-ray (SEM-EDX, JEOL 7800, Japan) was employed to observe the surface properties of the residual chars and measure the distribution of Cu and Zn on the residual chars. The identification of crystal phases of the char samples was performed by the X-ray diffraction analysis (XRD, Bruker D8 Advance, Germany) at the 2q range of 5 to 80
with a step size of 0.02
. Textural properties of the residue char samples were investigated via N2 adsorption-desorption isotherms at 77 K using a Rapid Surface Area and Porosity Analyzer (ASAP2020, American). Surface area of the char samples was analyzed by applying the BrunauerEmmettTeller (BET) equation. The functional groups on the surface of the char samples were characterized by FTIR (Bruker, VERTEX 80v, Germany).

Pyrolysis kinetics analysis During the fast pyrolysis experiment, the release characteristics of four main gas components (H2, CH4, CO and CO2) were continuously measured by an online mass spectrometer, and the gas conversion rate was calculated by Eq. (1)

Table 2 e Element compositions of RH, RH-Cu, RH-Zn and RH-Cu/Zn (wt %). Sample RH RH-Cu RH-Zn RH-Cu/Zn

O

Mg

Si

S

Cl

K

Ca

Fe

Cu

Zn

90.74 87.64 92.16 89.89

0.08 0.04 0.01 0.01

8.38 3.54 2.23 3.12

0.09 0.46 0.03 0.04

0.09 3.85 3.47 4.41

0.26 0.41 0.06 0.09

0.18 0.17 0.19 0.03

0.10 0.08 0.13 0.03

y 3.09 y 1.09

y 0.10 2.02 1.25

Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

4

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

Fig. 1 e A schematic diagram of the micro-fluidized bed reactor analyzer.

Z

t

x¼Z

t0 te

4i  qv dt (1) 4i  qv dt

t0

where, x is the conversion degree of pyrolysis gas; t0 is represents the initial reaction time; t is the reaction time; te denotes to the reaction end time; 4i denotes to the concentration of gas component i, vol %; qv represents the flow rate of the gaseous products from the reactor, L/min. As a heterogeneous gas-solid reaction, the global kinetics of the biomass pyrolysis can be described as Eqs. (2) and (3) dx ¼ kðTÞf ðxÞ dt

(2)

  Ea kðTÞ ¼ A exp  RT

(3)

where T is the absolute reaction temperature, K; k(T) is the reaction constant which depends on temperature. k(T) is a constant in isothermal process, and it can be separated from f(x). f(x) is the differential reaction function depending on the reaction mechanism. A is the apparent pre-exponential factor, 1/s; Ea represents the apparent activation energy, kJ/mol, and R is the idea gas constant, 8.314 J/(mol$K). The modelfitting method was used on the basis of reaction models to deduce the activation energy and pre-exponential factor (Eqs. (4) and (5)), Zx GðxÞ ¼ 0

dx ¼ kðTÞt f ðxÞ

Ea lnðkðTÞÞ ¼  þ lnðAÞ RT

(4)

(5)

where, G(x) is the integral reaction model. Generally, nineteen reaction models have been widely used for the determination of the kinetic parameters of gas-solid reactions [33], as shown Table S1 in the Supporting Information. Then, the activation energies determined by Eq. (4) using different reaction models were tested on the basis of the proximity of calculated conversion (x) to the experimental values to select the suitable reaction function model.

Results and discussion Pyrolysis gases releasing characteristics During biomass pyrolysis, gas products were formed via different chemical reaction routes and mechanisms. As the four major gas components from biomass pyrolysis, the forming characteristics of H2, CH4, CO and CO2 were tested by the online mass spectrometer. Fig. 2 shows the evolution of these four major gas components from fast pyrolysis of RH, RH-Cu, RH-Zn and RH-Cu/Zn at 700 and 850
C. It can be seen that the biomass pyrolysis in the micro fluidized bed reactor generally finished within 20 s. It has been reported that the heating rate in MFBRA is up to 1000e10000
C/s for fine particles in mm [28]. When the desired temperature was reached in each test, about 15 mg fuel sample was injected into the reactor for pyrolysis. The fuel sample was heated quickly under isothermal conditions in the micro fluidized bed, which caused in turn the lignin, hemicellulose and cellulose in the fuel to pyrolyze almost simultaneously. Thus, it was observed that the intensity of each gas component increased quickly after injection and reached a peak within 5 s. Then, the release of the gases became slower due to that the release of volatile components was generally completed, which in turn appeared as the decease of gas intensity with further

Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

5

Fig. 2 e Releasing characteristics of gaseous components.

Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

6

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

increasing the pyrolysis time [34]. The release of H2 and CH4 was a little later and their release intensity was weaker in comparison with CO2 and CO. However, as the pyrolysis temperature rose to 850
C, the difference in gas release sequence was significantly reduced, showing that CO2, CH4 and H2 were generated at almost the same time. The different release time of various gases indicated that the gases were produced via different chemical routes and mechanism [25]. For H2, its generation mainly originates from the free radical polycondensation reaction and dehydrogenation reaction during biomass pyrolysis process which was favored at higher temperature [35]. The generation of CH4 is mainly due to the degradation of eOCH3e and eCH2e groups during lignin pyrolysis [36]. In comparison, the formation of CO2 mainly originated from the cracking of carbonyl (C]O) and carboxyl (COOH) groups at a lower temperature, while it was mostly attributed to lignin decomposition via cracking of C]O and COOH groups on the carbohydrate backbone at a higher temperature [36,37]. The release of CO is generally originated from the degradation of CeOeC and C]O functional groups of hemicellulose and cellulose at a relatively low temperature [38,39]. Since the formation of CO2 and CO related to various reactions occurs likely at lower temperatures, their release was generally earlier during biomass pyrolysis under isothermal conditions. It can also be seen from Fig. 2 that the intensity of the gases, particularly H2, was influenced by the addition of metal compared to pure RH under the 850
C. This is mainly due to that the addition of Zn and Cu may change the forming processes of the gases. In general, apparent

activation energy indicates the difficulty to start a chemical reaction in essence, and the frequency factor represents the occurring effective collision of reactant molecules. Consequently, the effect of the metals on biomass pyrolysis can be investigated from the kinetic parameters for the formation of the gases.

Catalytic effect of copper and zinc Based on Eq. (1), the conversion degree (x) of the four main gas components under different conditions can be obtained. Fig. 3 shows the relationship between conversion degree of H2, CH4, CO2 and CO and reaction time from pyrolysis of different samples in the micro-fluidized bed reactor at 700 and 850
C. At 850
C, the corresponding forming reactions of all gas components from the four samples required much shorter reaction time, indicating the higher reaction rate at higher pyrolysis temperature. In addition, there is a significant difference in the time span for different gas components release, which appeared the longest for H2, shortest for CO, and equivalent for CH4 and CO2. At a certain temperature, the addition of Cu, Zn and Cu/Zn in RH showed different catalytic effects on the conversion of the four gas components. For H2, the realization of a given conversion required a shorter time for RH-Cu, RH-Zn and RHCu/Zn compared to pure RH, and the effect of Zn was even stronger than Cu and Cu/Zn. Similar results were obtained for the release of CH4, while RH-Cu showed higher reactivity. Also, the conversion of CO2 was promoted with the existence

Fig. 3 e Effect of Cu and Zn on conversion degree of the product gases. Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

of Cu and Zn, and Zn showed a little stronger effect in comparison with Cu. The results indicated that Cu and Zn had catalytic effect on the generation of H2, CH4 and CO2, while their catalytic performance differed for different gas components. However, the conversion of CO became slower with impregnation of Zn and Cu/Zn, indicating that the addition of Zn is unfavorable to CO formation. X-ray diffraction (XRD) measurements were performed to investigate the crystal phases of the obtained residue char samples at 800
C and the results were shown in Fig. 4. A broad characteristic peak centered at around 2q ¼ 22.5
was observed in all of the four char samples, standing for the crystal structure of amorphous silica, which can be attributed to the presence of disordered cristobalite [40,41]. The obtained RHC-Cu char appeared three diffraction peaks, which was consistent with the standard XRD data for the metallic copper (Cu0). Similar metallic copper (Cu0) phrases were also observed in RHC-Cu/Zn. However, for RHC-Zn, it was observed that no XRD signal related to zinc was detected due to sublimation of ZnCl2 which occurred at above 450
C [42]. The observed results are in good accordance with those obtained by Terakado et al. [43], who reported that the impregnated copper salt with biomass led to the formation of metallic copper nanoparticles (Cu0). As suggested by Eibner et al. [44], CuCl2 was first thermally degraded to CuO during the pyrolysis process, and then the char might act as a reducing agent and led to the reduction of CuO to Cu0. It also has been reported that the generation of H2 and CO might strengthen the reduction process. SEM-EDX images of RHC, RHC-Cu, RHC-Zn and RHC-Cu/Zn at 800
C are presented in Fig. 5, and the distribution of Cu and Zn is shown in Table 3. The surface of RHC appeared irregular structure (Fig. 5 (a)), which is attributed to the release of volatile matter, leading to the shrinkage of the globular structure [45]. In Fig. 5 (c) and (g), some white particles on the surface of RHC-Cu and RHC-Cu/Zn were observed which might correspond to the existence of copper nanoparticles, suggesting that the formation of well-dispersed copper particles covering these two char samples. Additionally, Fig. 5(d) and (h) show

Fig. 4 e XRD patterns of the RHC, RHC-Cu, RHC-Zn and RHC-Cu/Zn at 800
C.

7

that the characteristic peaks of copper in Cu-RHC were enhanced. Elemental compositions of the chars were listed in Table 3, showing that the relative contents of copper in RHCCu and RHC-Cu/Zn increased to 10.10% and 4.30%, respectively, indicating the metallic matter was successfully inserted into char framework. Similar features were also observed by Eibner et al. [44], and the presence of these metal particles can be used as active centers to influence the pyrolysis reaction, which may in turn promote the formation of gas components. For the ZnCl2 impregnated biomass samples, no obvious zinc increase was observed in the RHC-Zn and RHCCu/Zn chars, which further confirmed the sublimation of ZnCl2 [42]. From SEM-EDX images, it is noteworthy that no chlorine was observed on the four char samples. After impregnation, chlorine in biomass mainly exists in the form of inorganic compounds, and chlorine release is generally in the form of tar associated Cl or HCl [46e48]. Then, the chlorine may possibly be recaptured in the char by secondary reactions with available metals [49]. Thus, the release may affected the existence of other metals, particular potassium. As a result, obvious potassium content observed in pure RHC (Fig. 5b), while no potassium was observed in RHC-Cu, RHC-Zn and RHC-Cu/Zn. However, the evolution of chlorine plays a nondominant role in comparison with metal elements. It has been reported that the changes in volatiles and char yields are similar impregnating carbonate or chlorides, indicating that the effect of salt impregnation is mainly due to the cation loading [50]. Textural properties of the four char samples were characterized by N2 adsorption-desorption at 77 K, and resulting structure parameters are compiled in Table 3. As illustrated in Fig. 6, these samples contain both micropores and mesopores, and the pore sizes of them were predominantly at round 1.3 nm and 3.9 nm. As can be seen in Table 3, the average pore size of RHC, RHC-Cu, and RHC-Cu/Zn was around 3.9 nm, while the value of RHC-Zn was around 1.3 nm, indicating that the addition of Zn was beneficial to the formation of micropores. Additionally, the RHC sample prepared with Zn impregnation has the highest surface area which is attributed to stronger activation effect of ZnCl2 during biomass pyrolysis [51]. Generally, the addition of ZnCl2 enhanced the chemical attack on the matrix of the precursor, breaking cellulose, hemicellulose and lignin. As a consequence, the breakdown of these components resulted in the reorganization of the precursor's matrix, swelling of the particles, dehydration, gradual development of a porous structure and the increase in surface area [52]. Furthermore, the addition of CuCl2 also has activation effect during biomass pyrolysis, leading to an increase in the surface area [53]. It has been reported that the loose carbon atoms in mesopore and macropore reacted with copper ions at the melting point of CuCl2 (498.8
C), causing the formation of new microporous structure. Liu et al. [54], also reported that the impregnation of copper affected the evolution of many functional groups such as eOH, eCOOH and CeO, which may in turn affect the formation of small molecular gases. The surface functional groups of RHC, RHC-Cu, RHC-Zn, and RHC-Cu/Zn were characterized by a FTIR spectrometer. As shown in Fig. 7, a wide band around 3444 cm1 is assigned to

Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

8

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

Fig. 5 e SEM-EDX images of the RHC (a) and (b), RHC-Cu (c) and (d), RHC-Zn (e) and (f), and RHC-Cu/Zn (g) and (h) at 800
C. Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

9

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

Table 3 e Porous structure parameters by BET and Cu and Zn distribution by SEM-EDX analyses of RHC, RHC-Cu, RHC-Zn and RHC-Cu/Zn at 800
C. Samples RHC RHC-Cu RHC-Zn RHC-Cu/Zn

Total pore volume (cm3/g)

Average pore diameter (nm)

BET surface area (m2/g)

Cu (wt.%)

Zn (wt.%)

0.07 0.05 0.06 0.06

3.90 3.90 1.32 3.91

238.62 283.55 334.01 316.90

y 10.10 y 4.30

y y y y

was found that there were no obvious changes in the types of surface functional groups for the Zn or Cu impregnated char samples. However, the intensity of CeO and OeH groups showed obvious difference for these four char sample, indicating that the impregnation of Zn and Cu influenced the evolution of the functional groups in biomass, which in turn influenced the releasing characteristics of the gas components.

Analysis of pyrolysis kinetics

Fig. 6 e Pore size distribution obtained by BJH method for RHC, RHC-Cu, RHC-Zn and RHC-Cu/Zn at 800
C.

OeH stretching of hydrogen-bonded hydroxyl groups. The band at 1622 cm1 was attributed to the carbonyl and carboxyl C]O groups. The peaks at 1091 and 796 cm1 of the samples represent the stretching vibration of CeO and aromatic CH, respectively. The band at 462 cm1 was attributed to the eOH out-of-plane bending vibrations. The above results indicate that the main functional groups present in the chars are carboxyl, carbonyl and hydroxyl on the aromatic carbon skeleton [45]. Compared to the FTIR spectroscopy of RHC, it

Considering isothermal nature of the micro-fluidized bed reactor, pyrolysis kinetics of biomass was studied using the model-fitting method on the basis of Eqs. (4) and (5) to deduce the kinetic parameters. In this work, three mechanism models which have better fitting accuracy for each gas component were chosen from the 19 model functions in Table S1. The resulting fittings of the three mechanism models for RH-Cu pyrolysis were shown in Fig. 8, indicating that the three chosen mechanism functions enabled good description for the isothermal testing data in the micro-fluidized bed. The models enabled good linear fitting in the range wider that x ¼ 0.2e0.85 and all of the linear correlation factors R2 exceeded 0.95, which ensured that the biomass pyrolysis behavior was reasonably described. The obtained kinetic parameters were presented in Table S2 in the Supporting Information including apparent activation energy, pre-exponential factor and R2. It is observed that the activation energies for a certain gas using three different mechanism models are very close for all the four biomass samples, also suggesting that the models provide good description for the catalytic pyrolysis of biomass under isothermal conditions. Based on the obtained kinetic parameters in Table S2, the calculated values of the conversion degree can be obtained, and the mechanism models can be further evaluated based on the comparison of experimental and calculated values. Thus, the average deviation (d) between the experimental and calculated conversion degree was used to evaluate the precision of the model fitting, d¼

Fig. 7 e FTIR spectrums of RHC, RHC-Cu, RHC-Zn and RHCCu/Zn at 800
C.

1 X jx  xCal j  100% n x

(6)

where d is the average deviation value; n denotes the number of data in the experiment and xcal is the calculated conversion degree based on the obtained kinetic parameters. The values of d at different conditions were also presented in Table S2. In this work, the mechanism function corresponding to the minimum d value is considered to be the most probable mechanism function for the formation of gases. The obtained most probable reaction model for H2 and CO2 formation from

Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

10

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

Fig. 8 e Fitting of three typical mechanism functions with time using Cu as catalyst.

the pyrolysis of RH, RH-Cu, RH-Zn and RH-Cu/Zn was G(3) (3-D diffusion(Jander), [1-(1-x)1/3]2). In terms of the generation of CO, G(16) (Order based (n ¼ 1), -ln(1-x)) showed a better description for the four samples. For the formation of CH4, although the lowest d values for the four samples were obtained using different mechanism functions, G(3) (3-D diffusion(Jander), [1-(1-x)1/3]2) showed low d values for all the samples, and therefore G(3) was satiable to description of release of CH4. As the conversion fraction increases during pyrolysis, the solid matter will decompose from surface to center in various ways, such as shrinking spheres/cylinders, nucleation, and growth [55]. During this process, the release of gases originated from different reaction route and influenced by the complex physical and chemical interactions, and thus the formation of the gases may be described by different model functions [56]. The corresponding values of d obtained by the above functions are all less than 5%, and comparison between experimental and calculated data by the most probable reaction models for pyrolysis of RH-Cu was shown in Fig. 9. Consequently, it can be concluded that the mechanism models obtained in this study can be used to predict the formation of these four gas components during biomass pyrolysis. Fig. 10 shows the apparent activation energies of the four gas components obtained according to their most probable mechanism functions. For the pyrolysis of pure RH, H2 showed the highest forming activation energy of 85.08 kJ/mol, corresponding to the fact that H2 was more difficult to release during biomass pyrolysis. The forming activation energies

appeared the lowest for CO of 12.56 kJ/mol, and equivalent for CH4 and CO2 of 49.72 and 38.37 kJ/mol, respectively. Since the activation energy represents essentially the difficulty to start a reaction, lower Ea value for the gas component was in favor of its generation [28]. The obtained results indicated that the generation of gas components should be the easiest for CO, and in succession are CO2, CH4 and H2, which were in accord with the release order. Similar studies were carried out on the kinetics characteristics of biomass pyrolysis under isothermal conditions, and the apparent activation energies for the formation of the main gas components produced were of the same order of magnitude as the results in this work [28,32]. Similar order for gas release and the corresponding variation in the activation energy were also observed for the four main gas components. Additionally, the results obtained here were far lower than values obtained under the non-isothermal conditions by the thermogravimetric analysis method [57,58]. Compared with TGA, the micro-fluidized bed has high-rate heating to fuel particles and the diffusion inhibition on the reaction can be effectively minimized. Benefiting from these advantages, the obtained kinetics in the micro fluidized bed reactor under isothermal conditions should be closer to the intrinsic chemical kinetics of the gas release reactions during fast biomass pyrolysis. The addition of Cu and Zn has catalytic effect on the formation of gas components, leading to the variation of activation energies. For H2, the apparent activation energies decreased for RH-Cu, RH-Zn and RH-Cu/Zn in comparison

Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

11

Fig. 9 e Comparison between experimental data and those calculated by the models using Cu as catalyst. 100

RH RH-Cu RH-Zn RH-Cu/Zn

E / KJ ·mol -1

80 60 40 20 0

H2

CH4

CO2

CO

Fig. 10 e Apparent activation energies for major gas components based on most probable mechanism functions.

with pure RH, suggesting the positive effect of Cu and Zn on the generation of H2 during biomass pyrolysis. The apparent activation energies for the formation of CH4 and CO2 decreased as well, while the addition of Cu and Zn showed different effect. The CH4 generating Ea decreased to 30.02 kJ/ mol with the impregnation of Cu, while the value was much

higher of 39.93 kJ/mol using Zn as catalyst. In comparison, the addition of Zn and Cu decreased the generating Ea of CO2 to 17.05 and 35.98 kJ/mol, respectively, implying better catalytic performance of Zn. It was also found that the obtained Ea values using the combined catalyst of Cu and Zn values ranged between values of using only Cu or Zn. For the generation of CO, opposite effect of Cu and Zn was found, showing that the Ea value decreased to 8.30 kJ/mol with the addition of Cu while the values increased to 17.72 and 17.82 kJ/ mol respectively for RHC-Zn and RHC-Cu/Zn. The obtained results were consistent with the variation in gas release order and conversion degree observed in Section Catalytic effect of copper and zinc as well. On the basis of the variation of apparent activation energies, it can be concluded that the presence of Cu can promote the generation of the four gas components, and Zn has a positive effect on H2, CH4 and CO while has a negative effect on the formation of CO. This conclusion would be expected to help improving the gas quality from biomass pyrolysis, where metal catalysts can provide catalytic effect and promote higher production of combustible gases.

Conclusion The pyrolysis of rice husk using Cu and Zn as catalysts was studied in the micro-fluidized bed reaction analyzer to

Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

12

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

characterize the release of gaseous products and investigate the kinetics of the reactions under isothermal conditions at 700e850
C. Results showed that the release order of pyrolysis gas and the conversion degree differed for different gas components, implying different generating mechanisms and chemical routes. It was found that the Cu and Zn had a catalytic effect on the release rate of H2 which promoted its conversion and the effect of Zn was the stronger. For CH4 and CO2, both Cu and Zn showed positive catalytic effect on their generation, while the catalytic performance of Cu was much better. However, the impregnation of Zn showed negative effect on CO formation, which slowed down its conversion. The impregnated copper with biomass led to the formation of metallic copper nanoparticles, while no XRD signal related to zinc was detected due to sublimation of ZnCl2. The relative contents of copper in RHC-Cu and RHC-Cu/Zn increased to 10.10% and 4.30% according to EDX results, while no obvious zinc increase was observed in the RHC-Zn and RHC-Cu/Zn chars. The char samples contained both micropores and mesopores, and the pore sizes of them were predominantly at round 1.3 nm and 3.9 nm. The char sample prepared with Zn impregnation had the highest surface area of 334.011 m2/g due to stronger activation effect of ZnCl2. The impregnation of Cu and Zn did not change in the types of surface functional groups, while the intensity of these functional groups was influenced. Kinetic results indicated that the impregnation of Cu decreased the forming apparent activation energies of H2, CH4, CO2 and CO due to its catalytic performance. The addition of Zn and Cu/Zn also reduced the apparent activation energies required for the generation of H2, CH4 and CO2, whereas the apparent activation energy of CO is increased from 12.56 kJ/ mol to 17.72 and 17.82 kJ/mol, respectively.

Acknowledgement This work was financially supported by the Fundamental Research Funds for the Central Universities (No. 2017XKZD05).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.10.013.

references

[1] Liu WJ, Jiang H, Yu HQ. Development of biochar-based functional materials: toward a sustainable platform carbon material. Chem Rev 2015;115:12251e85. [2] Wang SR, Dai GX, Yang HP, Luo ZY. Lignocellulosic biomass pyrolysis mechanism: a state-of-the-art review. Prog Energ Combust 2017;62:33e86. [3] Li CZ, Zhao XC, Wang AQ, Huber GW, Zhang T. Catalytic transformation of lignin for the production of chemicals and fuels. Chem Rev 2015;115:11559e624. [4] Fu P, Hu S, Xiang J, Yi WM, Bai XY, Sun LS, et al. Evolution of char structure during steam gasification of the chars produced from rapid pyrolysis of rice husk. Bioresour Technol 2012;114:691e7.

[5] Lim JS, Manan ZA, Alwi SRW, Hashim H. A review on utilisation of biomass from rice industry as a source of renewable energy. Renew Sust Energy Rev 2012;16:3084e94. [6] Mohan D, Pittman CU, Steele PH. Pyrolysis of wood/biomass for bio-oil: a critical review. Energy Fuel 2006;20:848e89. [7] Liu HB, Chen TH, Chang DY, Chen D, He HP, Frost RL. Catalytic cracking of tar derived from rice hull gasification over palygorskite-supported Fe and Ni. J Mol Catal Chem 2012;363:304e10. [8] Abu El-Rub Z, Bramer EA, Brem G. Review of catalysts for tar elimination in Biomass gasification processes. Ind Eng Chem Res 2004;43:6911e9. [9] Lee J, Kim KH, Kwon EE. Biochar as a catalyst. Renew Sustain Energy Rev 2017;77:70e9. [10] Iliopoulou EF, Stefanidis SD, Kalogiannis KG, Delimitis A, Lappas AA, Triantafyllidis KS. Catalytic upgrading of biomass pyrolysis vapors using transition metal-modified ZSM-5 zeolite. Appl Catal B Environ 2012;127:281e90. [11] Richardson Y, Blin J, Volle G, Motuzas J, Julbe A. In situ generation of Ni metal nanoparticles as catalyst for H2 - rich syngas production from biomass gasification. Appl Catal A Gen 2010;382:220e30. [12] Hsu LC, Wang SL, Lin YC, Wang MK, Chiang PN, Liu JC, et al. Cr(VI) removal on fungal biomass of neurospora crassa: the importance of dissolved organic carbons derived from the biomass to Cr(VI) reduction. Environ Sci Technol 2010;44:6202e8. [13] Wang JL, Chen C. Biosorbents for heavy metals removal and their future. Biotechnol Adv 2009;27:195e226. [14] Liu WJ, Tian K, Jiang H, Zhang XS, Ding HS, Yu HQ. Selectively improving the bio-oil quality by catalytic fast pyrolysis of heavy-metal-polluted biomass: take copper (Cu) as an example. Environ Sci Technol 2012;46:7849e56. [15] Bridgwater AV. Review of fast pyrolysis of biomass and product upgrading. Biomass Bioenergy 2012;38:68e94. [16] Di Blasi C, Branca C, Galgano A. Products and global weight loss rates of wood decomposition catalyzed by zinc chloride. Energy Fuel 2008;22:663e70. [17] Liu WJ, Tian K, Jiang H, Yu HQ. Harvest of Cu NP anchored magnetic carbon materials from Fe/Cu preloaded biomass: their pyrolysis, characterization, and catalytic activity on aqueous reduction of 4-nitrophenol. Green Chem 2014;16:4198e205. [18] Antonakou E, Lappas A, Nilsen MH, Bouzga A, Stocker M. Evaluation of various types of Al-MCM-41 materials as catalysts in biomass pyrolysis for the production of bio-fuels and chemicals. Fuel 2006;85:2202e12. [19] Ozbay N, Yargic AS, Sahin RZY. Tailoring Cu/Al2O3 catalysts for the catalytic pyrolysis of tomato waste. J Energy Inst 2018;91:424e33. [20] Mastuli MS, Kamarulzaman N, Kasim MF, Sivasangar S, Saiman MI, Taufiq-Yap YH. Catalytic gasification of oil palm frond biomass in supercritical water using MgO supported Ni, Cu and Zn oxides as catalysts for hydrogen production. Int J Hydrogen Energy 2017;42:11215e28. [21] Karaman BP, Cakiryilmaz N, Arbag H, Oktar N, Dogu G, Dogu T. Performance comparison of mesoporous alumina supported Cu & Ni based catalysts in acetic acid reforming. Int J Hydrogen Energy 2017;42:26257e69. [22] Guo FQ, Liu Y, Wang Y, Li XL, Li TT, Guo CL. Pyrolysis kinetics and behavior of potassium-impregnated pine wood in TGA and a fixed-bed reactor. Energy Convers Manag 2016;130:184e91. [23] Lv DZ, Xu MH, Liu XW, Zhan ZH, Li ZY, Yao H. Effect of cellulose, lignin, alkali and alkaline earth metallic species on biomass pyrolysis and gasification. Fuel Process Technol 2010;91:903e9. [24] Saddawi A, Jones JM, Williams A. Influence of alkali metals on the kinetics of the thermal decomposition of biomass. Fuel Process Technol 2012;104:189e97.

Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 3

[25] Yu JA, Yue JR, Liu ZE, Dong L, Xu GW, Zhu JH, et al. Kinetics and mechanism of solid reactions in a micro fluidized bed reactor. AIChE J 2010;56:2905e12. [26] Zeng X, Wang F, Wang YG, Li AM, Yu J, Xu GW. Characterization of char gasification in a micro fluidized bed reaction analyzer. Energy Fuel 2014;28:1838e45. [27] Yu J, Zeng X, Zhang JW, Zhong M, Zhang GY, Wang Y, et al. Isothermal differential characteristics of gas-solid reaction in micro-fluidized bed reactor. Fuel 2013;103:29e36. [28] Yu J, Yao CB, Zeng X, Geng S, Dong L, Wang Y, et al. Biomass pyrolysis in a micro-fluidized bed reactor: characterization and kinetics. Chem Eng J 2011;168:839e47. [29] Liu Y, Guo FQ, Li XL, Li TT, Peng KY, Guo CL, et al. Catalytic effect of iron and nickel on gas formation from fast biomass pyrolysis in a microfluidized bed reactor: a kinetic study. Energy Fuel 2017;31:12278e87. [30] Guo YZ, Zhao YJ, Gao DY, Liu P, Meng S, Sun SZ. Kinetics of steam gasification of in-situ chars in a micro fluidized bed. Int J Hydrogen Energy 2016;41:15187e98. [31] Guo FQ, Dong YP, Fan PF, Lv ZC, Yang S, Dong L. Detailed kinetic study of phenol decomposition under isothermal conditions to understand tar catalytic cracking process. J Anal Appl Pyrol 2016;118:155e63. [32] Liu Y, Wang Y, Guo FQ, Li XL, Li TT, Guo CL, et al. Characterization of the gas releasing behaviors of catalytic pyrolysis of rice husk using potassium over a micro-fluidized bed reactor. Energy Convers Manag 2017;136:395e403. [33] Guo FQ, Dong YP, Fan PF, Lv ZC, Yang S, Dong L. Catalytic decomposition of biomass tar compound by calcined coal gangue: a kinetic study. Int J Hydrogen Energy 2016;41:13380e9. [34] Di Blasi C. Modeling chemical and physical processes of wood and biomass pyrolysis. Prog Energ Combust 2008;34:47e90. [35] Porada S. The reactions of formation of selected gas products during coal pyrolysis. Fuel 2004;83:1191e6. [36] Lopez MCB, Blanco CG, Martinez-Alonso A, Tascon JMD. Composition of gases released during olive stones pyrolysis. J Anal Appl Pyrol 2002;65:313e22. [37] Park DK, Kim SD, Lee SH, Lee JG. Co-pyrolysis characteristics of sawdust and coal blend in TGA and a fixed bed reactor. Bioresour Technol 2010;101:6151e6. [38] Yang HP, Yan R, Chen HP, Lee DH, Zheng CG. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 2007;86:1781e8. [39] Du ZY, Wang X, Qin YH, Zhang ZH, Feng J, Li WY. Effects of secondary reactions on the destruction of cellulose-derived volatiles during biomass/coal Co-gasification. Energy Fuel 2016;30:1145e53. [40] Liou TH. Preparation and characterization of nanostructured silica from rice husk. Mat Sci Eng A Struct 2004;364:313e23. [41] Liou TH. Evolution of chemistry and morphology during the carbonization and combustion of rice husk. Carbon 2004;42:785e94. [42] Hayashi J, Kazehaya A, Muroyama K, Watkinson AP. Preparation of activated carbon from lignin by chemical activation. Carbon 2000;38:1873e8.

13

[43] Terakado O, Amano A, Hirasawa M. Explosive degradation of woody biomass under the presence of metal nitrates. J Anal Appl Pyrol 2009;85:231e6. [44] Eibner S, Broust F, Blin J, Julbe A. Catalytic effect of metal nitrate salts during pyrolysis of impregnated biomass. J Anal Appl Pyrol 2015;113:143e52. [45] Hu S, Xiang J, Sun LS, Xu MH, Qiu JR, Fu P. Characterization of char from rapid pyrolysis of rice husk. Fuel Process Technol 2008;89:1096e105. [46] Johansen JM, Jakobsen JG, Frandsen FJ, Glarborg P. Release of K, Cl, and S during pyrolysis and combustion of highchlorine biomass. Energy Fuels 2011;25:4961e71. [47] van Lith SC, Jensen PA, Frandsen FJ, Glarborg P. Release to the gas phase of inorganic elements during wood combustion. Part 2: influence of fuel composition. Energy Fuels 2008;22:1598e609. [48] Hamilton JTG, McRoberts WC, Keppler F, Kalin RM, Harper DB. Chloride methylation by plant pectin: an efficient environmentally significant process. Science 2003;301:206e9. [49] Knudsen JN, Jensen PA, Lin W, Dam-Johansen K. Secondary capture of chlorine and sulfur during thermal conversion of biomass. Energy Fuels 2005;19:606e17. [50] Raveendran K, Ganesh A, Khilar KC. Influence of mineral matter on biomass pyrolysis characteristics. Fuel 1995;74:1812e22. [51] Kalderis D, Bethanis S, Paraskeva P, Diamadopoulos E. Production of activated carbon from bagasse and rice husk by a single-stage chemical activation method at low retention times. Bioresour Technol 2008;99:6809e16. [52] Molina-Sabio M, Rodriguez-Reinoso F. Role of chemical activation in the development of carbon porosity. Colloid Surface A 2004;241:15e25. [53] Liu B, Gu J, Zhou JB. High surface area rice husk-based activated carbon prepared by chemical activation with ZnCl2-CuCl2 composite activator. Environ Prog Sustain 2016;35:133e40. [54] Liu S, Xu WH, Liu YG, Tan XF, Zeng GM, Li X, et al. Facile synthesis of Cu(II) impregnated biochar with enhanced adsorption activity for the removal of doxycycline hydrochloride from water. Sci Total Environ 2017;592:546e53. [55] Gai C, Dong YP, Lv ZC, Zhang ZL, Liang JC, Liu YT. Pyrolysis behavior and kinetic study of phenol as tar model compound in micro fluidized bed reactor. Int J Hydrogen Energy 2015;40:7956e64. [56] Font R, Garcı´a AN. Application of the transition state theory to the pyrolysis of biomass and tars. J Anal Appl Pyrol 1995;35:249e58. [57] Loy ACM, Gan DKW, Yusup S, Chin BLF, Man KL, Shahbaz M, et al. Thermogravimetric kinetic modelling of in-situ catalytic pyrolytic conversion of rice husk to bioenergy using rice hull ash catalyst. Bioresour Technol 2018;261:213e22. [58] Mallick D, Poddar MK, Mahanta P, Moholkar VS. Discernment of synergism in pyrolysis of biomass blends using thermogravimetric analysis. Bioresour Technol 2018;261:294e305.

Please cite this article in press as: Guo F, et al., Influence of impregnated copper and zinc on the pyrolysis of rice husk in a microfluidized bed reactor: Characterization and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.10.013