Physical properties and pyrolysis characteristics of rice husks in different atmosphere

Accepted Manuscript Microarticle Physical Properties and Pyrolysis Characteristics of Rice Husks in Different Atmosphere Xinsheng Wang, Zhenlin Lu, Lei Jia, Jiangxian Chen PII: DOI: Reference:

S2211-3797(16)30147-4 http://dx.doi.org/10.1016/j.rinp.2016.09.011 RINP 366

To appear in:

Results in Physics

Received Date: Revised Date: Accepted Date:

27 August 2016 11 September 2016 19 September 2016

Please cite this article as: Wang, X., Lu, Z., Jia, L., Chen, J., Physical Properties and Pyrolysis Characteristics of Rice Husks in Different Atmosphere, Results in Physics (2016), doi: http://dx.doi.org/10.1016/j.rinp.2016.09.011

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Physical Properties and Pyrolysis Characteristics of Rice Husks in Different Atmosphere Xinsheng Wanga, Zhenlin Lua,*, Lei Jiaa, Jiangxian Chena a

School of Materials Science and Engineering, Xi'an University of Technology,

Xi'an,710048,China *Corresponding author：E-mail address: [email protected] Abstract This paper explores the physical properties and pyrolysis characteristics of rice husk combustion in air and in argon. The SEM results show that the outer epidermis of combusted RHIR(rice husk in air) is well organized with features that include papillae shapes and full, straight, high ridges. The inner epidermisof the RHIR has long rectangular furrow tissues. The results also show that the outer epidermis of pyrolyzed RHR(rice husk in argon) has ridges that are not as straight as for the RHIR and the top of the papillae have small holes. The inner surface of the RHR looks a some cracked. XPS analysis shows that the surfaces of RHIR and RHR contain carbon, oxygen, and silicon. The carbon was found to be in the elemental graphite form, the oxygen in the -2 oxidation state, and the silicon in the Si4+form as SiO2.The DSC graphs have "camel peaks", showing that an increase in rate of heating leads to an increase in the exothermic peaks. Calculations show that, initially, increased temperature leads to increased activation energy for pyrolysis, but as the temperature continues to increase, the activation energy decreases again. The frequency factor follows the same trend. In analysis of carbon content, rice husk volatile carbon content is the largest, it is about 33.94%, especially 700 ℃, the carbon content of

volatile minimum is about 0.33%. Keyword: Rice husk, Microstructure, Thermal analysis, Thermal properties , XPS 1.Introduction Rice husks are an important biomass resource in China. Up to 4×107 t are collected per year, and compared to other biomass, it is easier to get[1]. Compared to other agricultural bio wastes, such as bits of wood and corn straw, rice husks are known for the low utilization value of the husks themselves and high utilization value of their ash, which is mostly silica[2-3]. Rice husks are mainly composed of lignin, cellulose, hemicellulose, SiO2, and small amounts of metal oxides. Rice husks are used widely, because the SiO2 is a cheap source of silicon, which is often used for the preparation of materials such as concrete, filter aids, and silicides[4-5]. Organic matter of rice husk can be as an energy source, and the inorganic composition has high value-added utilization potential. Particularly, Rice husk ash contains more than 80% of the mass fraction of SiO2, the SiO2 are mainly composed of amorphous of cristobalite[6]. Husk removal of organic matter in the strict control of the combustion environment to get a high reaction activity of hydration of amorphous SiO2[7]. Rice husks can also be used to reinforce cement[8]. Because rice husk ash is very porous, it can be used as catalyst carrier[9], in the preparation of molecular sieve materials[10], and as a low-cost adsorbent[11]. A lot of research has been done on treating the rice husks with abundant air, slow heating rate, and low burning temperature to prepare rice husk ash[12], but this method wastes energy. This paper describes the physical properties and pyrolysis characteristics of rice husk treatment in different atmospheres,

which may eventually help find the best combination of rice husk energy and resource utilization. 2. Materials and methods The original rice husk (RH) was collected from Hubei province. To begin, RH were cleaned for 30 minutes in a deionized water and citric acid solution (10% concentration) to remove organic compounds. The RH were then dried in a stoving chest at 60℃. Table 1 Chemical composition analysis of the original rice husks. Table 1 Chemical composition analysis of the original rice husks Elemental analysis(wt%)

Proximate analysis(wt%)

O

C

H

N

Volatile

Carbon

Ash

H2 O

55.59

38.54

5.31

0.56

61.25

14.94

17.06

6.75

Rice husks were heated at 300ºC for 30min with an air flow rate of 100mL/min. This is called RHIR for rice husks in air(see Finger 1-left ). The rice husks were also heated at 300ºC for 30min with an argon flow rate of 100mL/min, This is called RHR （see Finger 1- right）. Microscopic image of RHIR and RHR were shown by SEM (Tescan VEGA II XMU, Czech Republic). X-ray photoelectron spectroscopy (XPS) (Phys-ical Electronics, USA) has been used in studies of the surface chemistry of RHIR and RHR. The thermal analysis of RH in mixed gas(argon and air) were measured using a differential scanning calorimeter (DSC) (METTLER TOLEDO, Switzerland) at a heating rate of (5℃,10℃,15℃,20℃)/min ranging from room temperature to 900℃ under a flow of argon at 100mL/min and air at 50mL/min.

Figure 1: The macro picture of samples : RHIR(left) and RHR(right) 3.Results and discussions 3.1 Analysis of physical properties

Figure 2:Scanning electron micrographs of samples: (a) outer epidermis of RHIR, (b)inner epidermis of RHIR, (c) outer epidermis of RHR, and(d) inner epidermis of RHR. Figure 2(a) shows scanning electron micrograph of the outer epidermis of RHIR. It has organized features, including papillae shapes and tall, wide, straight ridges. Figure 2(b) shows the inner surface of RHIR, which has ridges and troughs and

contains long rectangular furrow tissues attached to the surface of the inner epidermis. Figure 2(c) shows the outer epidermis of RHR. The ridges are not as fully straight and high as the RHIR, and the top of papillae haves mall holes. Figure 2(d) shows the inner surface of RHR, which is a little cracked, this may be due to the increase in temperature resulting from the thermal cracking of rice hulls in Figure 2(c) and (d) . In nature, rice husks have a globular structure, for which the main components are in the lemma or palea form, tightly interlocking with one another[13]. 3.2 Analysis of XRD 250

（002）

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Figure 3. XRD patterns of RHR and RHIR Figure 3 is shown the X-ray diffractogram of the RHR and RHIR, There is no difference in XRD patterns of the RHR and RHIR and yielded amorphous patterns the characteristic of silica with a diffraction peak around 2θ=22.5°, no other impurities were detected. This peak confirmed that silica are amorphous in nature, whereas the broadness of the XRD peaks revealed that the prepared biogenic silica was nanoscale in size [14]. Earlier studies have proven that RH silica nanoparticles have an amorphous nature [15,16]. 3.3 Analysis of XPS

C1s

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Figure 4:X-ray photoelectron spectroscopy (XPS) patterns of RHIR(A) and RHR(B) Figure 4 displays theX-ray photoelectron spectroscopy (XPS) patterns of RHR and RHIR, showing that their surfaces contain carbon, oxygen, and silicon. Figure 4 also shows that RHIR’s C1s peak is 283.4eV, and the RHR C1s peak is 284.7eV. This corresponds to the graphite morphology, suggesting that the rice husk contains pure carbon. The RHIR and RHR O1s peaks are 531.1eV and 532.6eV, respectively, suggesting that the oxygen atoms are found in the -2 oxidation state. The lower binding energy of 531.1eV is from Al2O3 and the higher binding energy of 532.6eV is from SiO2. RHIR and RHR Si2p peaks show binding energies of 101.65eV and 103.45eV, respectively, confirming that silicon is in the Si4+state as SiO2. The appearance of the peak and the test results of XRD are consistent with amorphous SiO2. 3.4 Analysis of pyrolysis

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900

T(℃)

Figure 5:Differential scanning calorimetry (DSC) and thermogravimetic analysis(TGA) curves of rice husks in mixed gases In Figure 5, the differential scanning calorimetry(DSC) curve shows that the process of pyrolysis of rice husks is divided into four stages. The first stage is a drying stage that takes place below 100ºC where water and small molecules evaporate. Although the rice husks were already dry, there was still a loss of mass, indicating water loss of about 8%. The second stage from 100-250ºC is when the rice husks began to change their chemical composition. Small amounts of volatile components decompose into low molecular mass compounds. The third stage is the pyrolysis stage that takes place between250-600ºC. The rice husks decompose and generate a lot of heat. The TGA indicates a decrease in mass of about 53%. DSC curve shows a wide exothermic peak that is mainly due to the decomposition of rice hull cellulose, leaving a carbon residue. Condensation and dehydrogenation reactions convert aromatic rings into thermally stable pairs of hexagonal ring structures. The last step is the carbide phase that takes place at about 600ºC. It includes lignin carbonization of the rice husks. The TGA curve shows that the rate of massloss is about 23%, and the DSC curve shows anexothermic combustion peak staggered and overlapping the broad exothermic peak. In the DSC curve, every curve has a "camel peak" indicating that an

increase in rate of heating leads to an rise in the exothermic peak. At the heating rate of 10ºC/min, the rice husk pyrolysis properties are different at different temperatures range. The common activation energy of chemical reaction in the range of 40 ~ 400kJ/mol, the differential form of the reaction mechanism as a function of 1-a, the integral form of the corresponding mechanism function is -ln(1-a) and the rice husk biomass pyrolysis reaction involves the conversion of solid materials to solids and gases ,so it is defined as first-order reaction kinetic mode. The following two formul as describe the pyrolysis reactions of rice husks.



= Kfa



ln 

 = Aepx   ，β = 







(1)

 = ln

 

−



(2)



，ga = − ln1 − a ，fa = 1 − a

decomposing and transforming a =

"#"$ "#"%

n

,a is rate of

, m1 and m3 represents the initial mass

and end point of the sample，m2 represents a point of temperature of rice hull quality at a specified temperature ,A = frequency factor（min-1 ）E = activation energy （KJ/mol）, T = temperature (ºC), The reaction mechanism of the pyrolysis of rice husk is random nucleation and as first-order reaction kinetic mode, so n=1. A graph can be drawn with ln

&' # 



on the vertical axis and

#



on the



horizontal axis. The slope of the curve is−  , and ln  is the y-intercept. Table 2 shows the calculated activation energies (E) and frequency factors(A) for different temperature ranges.

Table 2:Calculated activation energies and frequency factors for various temperature ranges. Heating rate (ºC/min)

10

Temperature range (ºC) 150-300 300-450 450-700

kinetic equation

Y=-253.33X-14.39 Y=-1470X-9.22 Y=-483.33X-13.12

Activation energy(kJ·mol-1)

Frequency factor (min-1)

2.11 12.22 4.02

0.89 1.44 1.21

Initially, an increase in temperature causes the activation energy to increase, but it decreases with a further increase in temperature. The same trend is seen for the frequency factor. This suggests a "kinetic compensation effect". The decreased activation energy intensifies the rice husk pyrolysis reaction. 3.5 Analysis of carbon content 40

C(%)

30

20

10

0

0

200

400

600

800

T(℃)

Figure 6.the carbon content change by differert temperature stage of the rice husk in air Figure 6 is shown the carbon content change by differert temperature stage of the rice husk, it can be seen that with the increase of heating temperature, carbon content increases gradually, but at 300℃, rice husk volatile carbon content is the largest, it is about 33.94%, this stage is the preheating solution stage of rice husk, rice husk caused by a large number of decarburization of organic matter, the resulting temperature rise in carbon content is lower, especially 700℃, the carbon content of volatile minimum

is about 0.33%, it is carbide stage of rice husk, main material is white SiO2, the DSC curve is consistent with the change of carbon content. 4.Conclusion In this study, it explored physical properties and pyrolysis characteristics of rice husks. Results obtained from this study provided the following conclusions: (1) The SEM results show that the outer epidermis of combusted RHIR is well organized with features that include papillae shapes and full, straight, high ridges. The inner epidermisof the RHIR has long rectangular furrow tissues. The results also show that the outer epidermis of pyrolyzed RHR has ridges that are not as straight as for the RHIR and the top of the papillae have small holes. The inner surface of the RHR looks a little fuzzy and cracked. (2) In analysis of XRD, There is no difference in XRD patterns of the RHR and RHIR and yielded amorphous patterns the characteristic of silica with a diffraction peak around 2θ=22.5°; no other impurities were detected (3) The XPS analysis shows that the surfaces of RHIR and RHR contain carbon, oxygen, and silicon. The carbon was found to be in the elemental graphite form, the oxygen in the -2 oxidation state, and the silicon in the Si4+form as SiO2. (4) The DSC analysis shows that graphs have "camel peaks", showing that an increase in rate of heating leads to an increase in the exothermic peaks. Calculations show that, initially, increased temperature leads to increased activation energy for pyrolysis, but as the temperature continues to increase, the activation energy decreases again. The frequency factor follows the same trend. (5) In analysis of carbon content, when RH is upto 300℃, rice husk volatile carbon

content is the largest, it is about 33.94%, especially 700 ℃, the carbon content of volatile minimum is about 0.33%. Acknowledgments The author would like to thank the financial support from National Science Foundation of China (51601143) and the Pivot Innovation Team of Shaanxi Electric Materials and Infiltration Technique (2012KCT-25).

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Highlights Physical Properties and Pyrolysis Characteristics of Rice Husks in different atmosphere, this highlights are rarely discussed in the now research. (1)The SEM results show that the outer epidermis of combusted RHIR is well organized with features that include papillae shapes and full, straight, high ridges. The inner epidermisof the RHIR has long rectangular furrow tissues. The results also show that the outer epidermis of pyrolyzed RHR has ridges that are not as straight as for the RHIR and the top of the papillae have small holes. The inner surface of the RHR looks a little fuzzy and cracked. (2)The XPS analysis shows that the surfaces of RHIR and RHR contain carbon, oxygen, and silicon. The carbon was found to be in the elemental graphite form, the oxygen in the -2 oxidation state, and the silicon in the Si4+form as SiO2. (3) The DSC analysis shows that graphs have "camel peaks", showing that an increase in rate of heating leads to an increase in the exothermic peaks. Calculations show that, initially, increased temperature leads to increased activation energy for pyrolysis, but as the temperature continues to increase, the activation energy decreases again. The frequency factor follows the same trend.