Influence of torrefaction pretreatment on corncobs: A study on fundamental characteristics, thermal behavior, and kinetic

Journal Pre-proofs Influence of torrefaction pretreatment on corncobs: A study on fundamental characteristics, thermal behavior, and kinetic Xiaojie Tian, Leilei Dai, Yunpu Wang, Zihong Zeng, Shumei Zhang, Lin Jiang, Xiuhua Yang, Linqing Yue, Yuhuan Liu, Roger Ruan PII: DOI: Reference:

S0960-8524(19)31720-1 https://doi.org/10.1016/j.biortech.2019.122490 BITE 122490

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Bioresource Technology

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29 September 2019 21 November 2019 22 November 2019

Please cite this article as: Tian, X., Dai, L., Wang, Y., Zeng, Z., Zhang, S., Jiang, L., Yang, X., Yue, L., Liu, Y., Ruan, R., Influence of torrefaction pretreatment on corncobs: A study on fundamental characteristics, thermal behavior, and kinetic, Bioresource Technology (2019), doi: https://doi.org/10.1016/j.biortech.2019.122490

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Influence of torrefaction pretreatment on corncobs: A study on fundamental characteristics, thermal behavior, and kinetic

Xiaojie Tian a, #, Leilei Dai a, b, #, Yunpu Wang a, b*, Zihong Zeng a, Shumei Zhang a, Lin Jiang a, Xiuhua Yang a, Linqing Yue a, Yuhuan Liu a, b, Roger Ruan a, b

a

State Key Laboratory of Food Science and Technology, Engineering Research Center for Biomass

Conversion, Ministry of Education, Nanchang University, Nanchang 330047, China b Center

for Biorefining and Department of Bioproducts and Biosystems Engineering University of

Minnesota, 1390 Eckles Ave., St. Paul MN 55108, USA

# These

authors contributed equally.

* Corresponding

authors at: State Key Laboratory of Food Science and Technology, Nanchang

University, Nanchang, Jiangxi 330047, China. E-mail address: [email protected] (Yunpu Wang)

Abstract: The effects of torrefaction pretreatment on corncobs properties and its pyrolysis kinetic parameters

were investigated in this study. Proximate and ultimate analyses indicated that torrefaction increased the H/Ceff ratio and higher heating value of corncobs, and reduced its oxygen content. Although the mass yield was also reduced, the corresponding energy yield was relatively higher. The crystallinity index of biomass showed a first upward and then downward trend with the torrefaction temperature. Kinetic parameters obtained from three models indicated that both the activation energy and the pre-exponential factor increased with the elevated torrefaction temperature and it's better to calculate the activation energy by the OFW method and to use the KAS and DAEM methods to calculate the pre-exponential factor. In addition, it was found that the optimum pretreatment temperature of corncobs was 240 °C.

1. Introduction The rapid consumption of fossil fuels has driven researchers to search for sustainable energy sources recently (Shafiee & Topal, 2009; Wang et al., 2016). Biomass is considered to be the most promising alternative to fossil energy because of its abundant reserves and renewability (Dai et al., 2017; Li et al., 2016; Wang et al., 2018). Thermochemical pathways such as pyrolysis can ideally convert biomass into fuels or chemicals, which involves the thermal decomposition of biomass to form gaseous, liquid, and solid products. In terms of liquid product (also named bio-oil), some disadvantages including high oxygen content / water content, and low calorific value limit the application of bio-oil (Fan et al., 2014; Kan et al., 2016). Therefore, an upgrading technique needs to be developed to improve the economic potential of bio-oil. Torrefaction refers to the mild pyrolysis of biomass in absence of oxygen at lower temperatures (200 °C-300 °C), which is recognized as the useful way to improve the biomass properties and pyrolysis behavior (Dai et al., 2019b). The contents of oxygen and water in the biomass after torrefaction were significantly reduced, the energy density and grindability were enhanced to a large extent (Ciolkosz & Wallace, 2011; Medic et al., 2012; Tapasvi et al., 2012). Chen and his co-workers found that increasing the torrefaction temperature would reduce the

oxygen content in the bio-oil produced by biomass pyrolysis, and the relatively more cellulose was retained in the biomass (Chen et al., 2018b). Wang and his co-workers found that as the torrefaction temperature increased, the crystallinity of cellulose increased first and then decreased. Also, torrefaction increased the high calorific value of cellulose and reduced its oxygen content (Wang et al., 2017). Furthermore, pyrolysis of torrefied biomass produces higher yields of biochar, gas and furfural, lower yields of bio-oil and organic acids (Wang et al., 2017). In addition, torrefaction changes the structure of the biomass, thereby affecting the activation energy and pre-exponential factor of the pyrolysis process. Kinetic analysis using thermograms that are acquired from thermogravimetry technique is an effective method to determine the kinetic parameters (activation energy and pre-exponential factor) and reaction mechanism of torrefied biomass pyrolysis. The study of dynamics can provide information for a better understanding of the pyrolysis of torrefied biomass. In recent years, as an excellent pretreatment method of biomass, the mechanism and optimization of torrefaction have been studied by many researchers (Chen et al., 2018a; Zhang et al., 2018). However, to date, the kinetic calculation methods of the pyrolysis of torrefied corncobs has not been compared and optimized. Therefore, this study was designed to study the effects of different torrefaction temperatures on the thermodynamic properties and kinetic parameters of corncobs and to select the most appropriate kinetic calculation method and torrefaction condition. In this study, 210 °C, 240 °C, 270 °C and 300 °C were selected as the torrefaction temperature of corncobs. In addition, three kinetic calculation methods, Kissinger-Akahira-Sunose (KAS) method, Ozawa-Flynn-Wall (OFW) method and Distributed activation energy model (DAEM), were selected for analysis and optimization. The kinetic parameters before and after torrefaction were calculated and analyzed.

2. Methods 2.1. Material preparation

The corncobs used in this experiment were collected from the surrounding countryside of Nanchang, China. Prior to the experiment, the corncobs were ground to pass an 80-mesh sieve, and the sieved powder was then dried at 105 °C for 24 hours.

2.2. Torrefaction experiment The torrefaction process of corncobs was carried out in a tubular furnace (OTF-1200X) which was bought from Hefei Kejing Material Technology Co., Ltd. (Anhui, China). A quartz bottle containing about 15 g of dried corncobs were placed in the center of a quartz tube of the tubular furnace. After the equipment was assembled, nitrogen was used to exhaust the air inside the quartz tube, and nitrogen was continuously introduced to maintain an inert environment. Samples were heated at a heating rate of 15 °C/min until the desired torrefaction temperature (210 °C, 240 °C, 270 °C and 300 °C) was reached, which was maintained for 30 minutes. The torrefaction experiment at each temperature was repeated three times to ensure experimental repeatability. The residue after torrefaction was cooled to room temperature, weighed, and stored in a desiccator for subsequent use as the raw material of XRD, FTIR and Thermogravimetric (TG) analysis.

2.3. Biomass characterization The contents of C, H, and N were determined by an elemental analyzer (Vario EL III, Elementar, Germany), and the content of O was calculated according to the difference calculated. In addition, Mass yield and energy yield were calculated by the following equations. 𝐻𝐻𝑉 𝑜𝑓 𝑝𝑟𝑒𝑡𝑟𝑒𝑎𝑡𝑒𝑑 𝑠𝑎𝑚𝑝𝑙𝑒𝑠 × 100% 𝐻𝐻𝑉 𝑜𝑓 𝑟𝑎𝑤 𝑠𝑎𝑚𝑝𝑙𝑒𝑠 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑝𝑟𝑒𝑡𝑟𝑒𝑎𝑡𝑒𝑑 𝑠𝑎𝑚𝑝𝑙𝑒𝑠 𝑀𝑎𝑠𝑠 𝑦𝑖𝑒𝑙𝑑 = × 100% 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑟𝑎𝑤 𝑠𝑎𝑚𝑝𝑙𝑒𝑠 𝐸𝑛𝑒𝑟𝑔𝑦 𝑦𝑖𝑒𝑙𝑑 = 𝑚𝑎𝑠𝑠 𝑦𝑖𝑒𝑙𝑑 ×

(1) (2)

HHV means the Higher heating value, and the HHV of samples were listed in Table 1. The cellulose crystallinity of the samples was measured using a Bruker D8 ADVANCE X-ray diffractometer (XRD). The

scanning range was 5° to 50° at 2θ and the scanning rate is 2°/min. Calculate the cellulose crystallinity index (CrI) according to Eq. (3): 𝐶𝑟𝐼 =

∑𝐼c ∑𝐼c + ∑𝐼𝑎

× 100%

(3)

where ∑𝐼c is the total diffraction integral intensity of the crystalline portion, and ∑𝐼𝑎 is the scattered integral intensity of the amorphous portion. In this experiment, the FTIR spectra of the samples were determined using a fourier transform infrared spectrometer (Nicolet iS5, Thermo, USA) in the wave number range of 800-4000 cm-1. Set the resolution to 4cm-1 and the number of scans to 32.

2.4. Thermogravimetric (TG) analysis The samples (mass approximately 2.00-3.50 mg) were subjected to TG experiments using a thermogravimetric analyzer (TGA 4000, PerkinElmer, USA). In the experiment, an inert environment was created using high purity N2 at 100 ml/min. The samples were heated from 30 °C to 800 °C at the heating rates of 5 °C/min, 10 °C/min, 15 °C/min, and 20 °C/min. The data obtained were used for kinetic analysis.

2.5. Kinetic theory Since many reactions occur simultaneously in fractions of a second during pyrolysis, it is impossible to predict an accurate reaction mechanism. However, Sadhukhan and his co-workers (Sadhukhan et al., 2009) put forward an overall pyrolysis reaction mechanism: k

Biomass(Solid) volatile(gases + tar) + Char(Solid residue)

(4)

Converting raw materials into products was assumed to be the only one step process. Therefore, according to Arrhenius, the reaction rate constant (k) can be calculated by the following equation:

k = Ae

―

E RT

(5)

Where k, A, E, R, and T refer to reaction rate constant, pre-exponential factor, (min−1), activation energy, (kJ mol−1), Gas constant, (8.314 J mol−1 K− 1), and Absolute temperature, (K), respectively. This equation is proposed on the assumption that the activation energy is independent of the temperature. However, the activation energy is related to the temperature, so the error cannot be avoided. The process of converting biomass from solid state to volatile state by pyrolysis can be regarded as A→P (Sadhukhan et al., 2009), and the rate equation is: dx = kf(x) dt

(6)

x is the conversion value within the sample and is defined as: x=

α0 ― αt α0 ― αf

(7)

where α0, αt and αf are the weight of the sample at the beginning, a particular time and the end. Combining equations (5) and (6), we can get an equation as: E

dx ― = Ae RTf(x) dt

(8)

According to the n-level uniform kinetic reaction of A→P, 𝑓(𝑥) can be expressed by the following equation (Coats & Redfern, 1965): 𝑓(𝑥) = (1 ― 𝑥)𝑛

(9)

According to equations (8) and (9), we can get: E

dx ― = Ae RT(1 ― x)n dt

(10)

δ is the heating rate and can be defined as: dT dT dx = × dt dx dt dx dx =δ dt dT δ=

(11) (12)

According to equations (10) and (12), we can get: E

dx A ― RT = e (1 ― x)n dT δ

(13)

We can convert equation (13) into: E

A ― RT = e dT (1 ― x)n δ dx

(14)

Integrating both sides of equation (14) simultaneously we can get: x

dx

∫ (1 ― x) = ∫ n

0

E

T

A ― RT dT e T0 δ

(15)

Let the left side = g(x), we can get:

{

(1 ― x)1 ― n ― 1 , (n ≠ 1) g(x) = n―1 ―ln(1 ― x), (n = 1)

(16)

Let u = E/RT, then: AE δR

g(x) =

∫

u

u ―2e ―udu = u0

AE p(u) δR

(17)

However, p(u) has no exact solution; therefore, it needs to be solved by numerical approximation (Coats & Redfern, 1964; Kissinger, 1957; Takeo, 1965).

2.5.1. Kissinger-Akahira-Sunose (KAS) method Kissinger-Akahira-Sunose (KAS) method is an approximate modeless (equal conversion) method using p(u) = 𝑢 ―2𝑒 ―𝑢 in equation (17): ln

() [ ] δ

2

T

The kinetic curve between ln

= ln

() δ

T2

AR E ― Eg(x) RT

and

1 T

(18)

will give the slope and intercept used to calculate the activation

energy. In this calculation, x can be specified as a value, and the temperature Tt at this time can be obtained according to the thermogravimetric curve at different heating rates. Find ln

() δ

T2

and

1 T

with different temperatures Tt and

reaction rates. Different points can be obtained according to different heating rates and corresponding curves can be drawn by linear regression. The E can be obtained according to the slope and intercept of the curve.

2.5.2. Ozawa-Flynn-Wall (OFW) method The Ozawa-Flynn-Wall method is an integral conversion method that uses the Doyle approximation (Doyle, 1965), the accuracy of this equation is greater than 95%. The equation proposes an empirical equation for p(u): log (e ―uu ―2) = ―2.315 ― 0.4567u (u > 20)

(19)

𝐸

Substituting equation (19) and u = 𝑅𝑇, we will get: log (δ) = log

𝐴𝐸 [𝑅𝑔(𝑥) ] ― 2.315 ― 0.4567𝑅𝑇𝐸

(20)

Therefore, take x equal to a constant and obtain the temperature Tt at this time, bring different temperatures and reaction rates into and find the log (δ) and

1 T

at this time and draw the corresponding straight line, E can be

calculated based on the slope and intercept. In this article, u was verified to be greater than 20.

2.5.3. Distributed activation energy model (DEAM) According to the DAEM hypothesis, the reaction that occurs at a particular conversion value is determined, the reactions that occur at a particular conversion value are determined, even in two separate experiments at different heating rates. Applying the DAEM method to TGA data to calculate activation energy and pre-exponential factor: V 1― = Vt

∫

∞ ―A

e

T ― E RT

∫e 0

dT

h(E)dE

(21)

0

where V and Vt represent the effective volatile content and the volatile content at time t, respectively. The distribution curve h(E) is defined to satisfy:

∫

∞

h(E)dE = 1

(22)

0

Simplified DAEM is shown in equation (23): V =1― Vt

∫

∞

h(E)dE = 𝐸

𝐸

∫ h(E)dE

(23)

0

With appropriate integral calculations, a simplified DAEM can be expressed as an equation:

ln

() ( ) δ

2

T

= ln

AR E + 0.6075 ― E RT

(24)

Calculate the activation energy E and the pre-exponential factor A in the same way as the KAS method.

2.6. Pre-exponential factor and thermodynamic parameters The change in enthalpy ΔH, the change in Gibbs free energy ΔG, and the change in entropy ΔS (Kim et al., 2010) were calculated at T=Tmax from the thermogravimetric analysis data. (25)

∆H = E ― R𝑇𝑚𝑎𝑥 ∆G = E + R𝑇𝑚𝑎𝑥ln ( ∆S =

𝐾𝐵𝑇𝑚𝑎𝑥 ) 𝐴h

∆H ― ∆G 𝑇𝑚𝑎𝑥

(26) (27)

Where 𝑇𝑚𝑎𝑥 is the peak temperature, 𝐾𝐵 is the Boltzmann constant, and h is the Plank constant. In addition to this, the pre-exponential factor A can be calculated according to the following equation:

A=

δE𝑒

𝐸 𝑅𝑇𝑀𝐴𝑋

RT2𝑀𝐴𝑋

(28)

3. Results and discussion 3.1. Fundamental characteristics of raw and torrefied corncobs Table 1 shows the mass yields, energy yields, and basic characteristics of the original and torrefied corncobs. CC refers to the original corncobs sample, CC210 refers to the corncobs after torrefaction at 210 °C for 30 mins, and so on. It can be seen that as the torrefaction temperature increased, the carbon content increased, the oxygen content remarkably decreased. At the same time, the hydrogen content decreased but the amplitude was relatively small. This indicated that the main reaction of torrefaction was deoxygenation. The H/C ratio and O/C ratio can be fitted by a linear function (Fig. 1) with a slope of 1.47. If the main occurrence of the torrefaction process is the deoxygenation reaction that generate H2O, the slope should be around 2 (Wang et al., 2017). Therefore, we can

determine that the removal of oxygen during the torrefaction process passed a more efficient deoxygenation reaction which the produced CO2 and CO (decarboxylation and demethylation). This allowed more hydrogen to be retained, so that the H/Ceff ratio of the torrefied corncobs increased with the elevated pretreatment temperature, which was beneficial to promote the formation of hydrocarbons in the pyrolysis (Dai et al., 2019a). The mass loss during the torrefaction of corncobs is mainly due to the decomposition of hemicellulose, followed by the slight decomposition of cellulose. Previous studies have pointed out that hemicellulose has two mass loss rate peaks at around 234 °C and 267 °C (Dai et al., 2019b; Shen et al., 2010). The first one was due to cleavage of glycosidic bonds and methylene decomposition on the side chains and removal of O-acetyl groups, and the second one was due to the fragmentation of structural units. Torrefaction at 300 °C made the hemicellulose to almost completely be decomposed. The glycosidic bonds of cellulose were slightly broke. The lignin is most stable, and a demethoxy reaction occurred during the torrefaction process and further polymerization between the benzene ring united (Dai et al., 2019b). In general, due to the relatively poor thermal stability of hemicellulose, torrefaction reduced the hemicellulose content of the corncobs and enhanced the aromaticity, thereby increasing the energy density, which was at the expense of mass and energy yield (Wannapeera et al., 2011). The product torrefied at 300 °C has the highest energy density, however it can only achieve a mass yield of 36.05% and an energy yield of 54.26%. Therefore, from these aspects, in order to achieve maximum economic benefits, 240 °C is the best torrefaction temperature of corncobs which has a mass yield of 62.36% and an energy yield of 76.04%.

3.2. XRD analysis The crystallinity (CrI) calculated from the XRD data is shown in Table 1. According to the calculation, it can be seen that as the pretreatment temperature increased, the CrI of the corncobs first increased, reaching a maximum of 76.46% at 240 °C, and then decreased, which was the same with the literature (Dai et al., 2019a; Zheng et al., 2015). Since only cellulose has a crystalline structure, when torrefaction affects the content of hemicellulose,

cellulose and lignin, the CrI of the corncobs would be affected accordingly. After torrefaction at 210 °C and 240 °C, due to the decrease in hemicellulose content, the percentage of cellulose content was relatively increased, thereby increasing the CrI of the sample. In addition, when the torrefaction temperature was lower than 250 °C, amorphous cellulose undergone recrystallization, and amorphous cellulose was more susceptible to thermal decomposition than crystalline cellulose (Wang et al., 2017), resulting in an increase in CrI. When the torrefaction temperature was further increased, the hydrogen bond in the crystal structure of cellulose broke or the spacing increased, causing the crystalline cellulose to become amorphous cellulose (Dai et al., 2019b), and the rate at which crystalline cellulose became amorphous cellulose was faster than that of fragmentation of amorphous cellulose (Wang et al., 2017). Therefore, the CrI was reduced and reached its lowest value of 21.55% at 300 °C.

3.3. FTIR analysis FTIR analysis was carried out to study the change of functional groups before and after corncobs torrefaction. The assignment of the characteristic signals was listed in the Table 2 and it was performed according to the literature (Ben & Ragauskas, 2012; Zheng et al., 2015). The FTIR spectra torrefied at 210 °C is very similar to the original sample, mainly due to its weak pretreatment conditions and less impact on the biomass structure. When the temperature was further increased, the O-H stretching vibration (3435-3402 cm-1) strength in the hydroxyl group and the carboxyl group was lowered due to dehydroxylation and condensation reaction (Segal et al., 1959). The stretching vibration of C=O (1739-1697 cm-1) also decreased as the elevated torrefaction temperature, indicating that decarboxylation and acetylation reactions occurred during torrefaction (Dai et al., 2019a). As the degree of pretreatment increased, syringyl ring and C−O stretch in lignin and xylan (1261-1250 cm-1), and C−O−C vibration of cellulose and hemicellulose (1170-1160 cm-1) also gradually decreased, indicating that hemicellulose and cellulose decomposed at higher torrefaction temperatures, and the cellulose and hemicellulose content decreased much at the torrefaction temperature of 300 °C. This can be mutually confirmed by changes in CrI. In summary, the

torrefaction process promotes the decomposition of oxygen-containing functional groups, which is consistent with the change in the content in Table 1.

3.4. TG/DTG analysis Corncobs and their thermal behavior after torrefaction were studied based on data from thermogravimetric analysis at 10 °C/min. According to Fig. 2, as the elevated pretreatment temperature, the peak value between 30-150 °C became smaller and smaller, mainly due to the removal of the constituent water. In addition, the DTG peak of hemicellulose between 200 °C and 300 °C gradually became smaller, and the DTG peak almost disappeared after torrefaction at 240 °C, which confirmed the large loss of hemicellulose content after torrefaction. It can be seen from Table 3 that as the torrefaction temperature increased, the initial decomposition temperature (Ti) gradually increased. Since previous studies have shown that torrefaction reduces the thermal stability of cellulose, resulting in an increase in Ti (Wang et al., 2017). In addition, during the torrefaction of the corncobs, the lignin content increased because of the continuous decomposition of hemicellulose and cellulose, thereby increasing the thermal stability and causing the gradual increase of Ti. The maximum weight loss rate (DTGmax) represents the decomposition of the main structure of cellulose, which increased first as the torrefaction temperature increased, which is mainly due to the relative increase of cellulose content and the elimination of heat-labile functional groups of cellulose (Wang et al., 2017), and obtained the highest value of 9.74%/min at 210 °C. As the pretreatment temperature increased further, the hydrogen bond in the ordered structure of cellulose broke and the structure became more complicated (Dai et al., 2019b), the temperature range of decomposition of the main structure became wider, which led to a rapid decrease in DTGmax. This can be confirmed by the change in CrI. In addition, as the torrefaction temperature increased, the content of lignin continued to increase relatively, and its benzene ring unit was further polymerized, thereby enhancing the aromaticity of the corncobs and increasing the content of residue.

3.5. Kinetic analysis The thermogravimetric data of 10 °C/min was analyzed by Kissinger-Akahira-Sunose (KAS) method, Ozawa-Flynn-Wall method (OFW) and distributed activation energy model (DEAM) for the calculation of the kinetic parameters of the original and torrefied corncobs pyrolysis. The kinetic parameters in this paper were calculated using the reaction order n=1 recognized by many researchers (Liang et al., 2017; Shang et al., 2014; Tong et al., 2019). Since the KAS, FWO, and DAEM methods do not involve the reaction order when calculating the activation energy E, this assumption would not affect the analysis of it. As can be seen from Fig. 3, the fitting of the thermogravimetric data is very good. The activation energy values obtained using the KAS and DAEM methods are the same, which is consistent with the findings of previous researchers (Tong et al., 2019). The activation energy value calculated by the OFW method is slightly different, because it was developed from the same program with KAS methods, except for the approximate value of the temperature integral term. The fitting performance calculated by the OFW method is better, so the OFW method is more accurate in calculating the activation energy of corncobs. In addition, according to Table 4, as the elevated pretreatment temperature, the activation energy gradually increased. This is mainly due to the large decomposition of hemicellulose and the partial decomposition of cellulose, which led to lignin becoming its main component, and the activation energy of the three components in descending order is lignin, cellulose and hemicellulose (Tong et al., 2019), resulting in an increasing activation energy. In addition, the increase in activation energy represents an increase in the thermal stability of the corncobs, which is mutually proved with the Ti which increased as the elevated pretreatment temperature. Moreover, with the increase of conversion value x, the activation energy increased continuously, which indicates that the condensation reaction of lignin becomes more serious when the thermogravimetry proceeds to the later stage of the reaction (Wang et al., 2017), thereby improving its aromaticity and activation energy. The pre-exponential factor was calculated and analyzed by the above three methods (method A) and equation

28 (method B), and the results are shown in Table 4. Since the calculation of the DAEM method does not need to involve reaction order n, the calculated pre-exponential factor A can be used as a reference to judge the results calculated by other methods. It can be seen that the KAS method is more accurate by using the method A, and the OFW method using the method B to calculate the pre-exponential factor A is more accurate. The comparison between the two methods shows that the calculation results of the KAS method is closer to that of the DAEM method. In addition, since the larger pre-exponential factor (>6 × 1010min ―1) represents a more complicated reaction (Ye et al., 2018), as the torrefaction temperature and conversion value x increased, it tended to have a more complex reaction, which can be mutually confirmed with changes in activation energy. In summary, the OFW method is more accurate when calculating the activation energy, and the results obtained by the DAEM method and the KAS method are more accurate when calculating the pre-exponential factor. In addition, since the activation energy increases the difficulty of the reaction, it can be concluded from the above factors that 240 °C is the optimum torrefaction temperature of corncobs, with an activation energy of 188.08 kJ / mol and a pre-exponential factor of 2.94 × 1015min ―1.

3.6. Thermodynamic parameters The change in enthalpy (ΔH), the change in Gibbs free energy (ΔG), and the change in entropy (ΔS) were calculated from kinetic parameters of 10 °C/min and x = 0.6 (near the average), and the corresponding results are shown in Table 5. With the elevated pretreatment temperature, the values of ΔH, ΔG, and ΔS increased, but the causes were not the same. Torrefaction caused a decrease in hemicellulose content which is less thermally stable and an increase in cellulose and lignin content. Since hemicellulose has a lower endothermic heat in the pyrolysis process than lignin and cellulose (Dai et al., 2019b) , the endothermic component in the corncobs

increased,

which led to an increase in ΔH. In addition, due to the torrefaction process, the degree of aromatization of lignin was more serious, which made the structure more complicated and more difficult to be decomposed (Chen et al.,

2018c), so ΔH increased with the elevated torrefaction temperature. The increase in ΔG also represented an increase in the difficulty of the reaction, which was consistent with the kinetic analysis. The increase in ΔH and ΔG would lead to an increase in the difficulty of the reaction, which can be mutually confirmed by the increase in activation energy. The ΔS of original corncobs was less than 0, which means that the order degree of the material particles in the original corncobs was lower than that of the product. The ΔS of torrefied corncobs was greater than 0 and gradually increased with the elevated torrefaction temperature, indicating that torrefaction made the structure in the corncobs more well-organized (Its lignocellulose composition was slowly biased towards a single lignin), which was consistent with the description in the literature (He et al., 2019). The increase in ΔS would lead to an increase in the difficulty of the reaction, which was consistent with the previous description.

4. Conclusions The effects of torrefaction on the chemical structure and thermodynamic properties of corncobs were investigated. It was found that torrefaction increased the H/Ceff ratio, the activation energy and pre-exponential factor of corncobs and reduced its oxygen content, but the mass and energy yields was also reduced. Moreover, it was found that the calculation of activation energy by the OFW method, and the calculation of the pre-exponential factor by the KAS and DAEM methods are relatively the best choices. In addition, after comprehensive consideration, the optimum pretreatment condition of corncobs was found to be torrefied at 240 °C.

Acknowledgement This project is financially supported by the National Natural Science Foundation of China (No. 21766019), Natural Science Foundation of Jiangxi Province, China (20181BAB206030), Key Research and Development Program of Jiangxi Province, China (20171BBF60023), Innovation Fund for Graduate of Jiangxi Province, China (YC2019-B005), Science and Technology Program of Guangzhou, China (Grant No. 201904010342).

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of biomass wastes and their correlations with torrefaction severity index. Appl. Energy 220, 598-604. [34] Zheng, A., Zhao, Z., Chang, S., Huang, Z., Zhao, K., Wei, G., He, F., Li, H. 2015. Comparison of the effect of wet and dry torrefaction on chemical structure and pyrolysis behavior of corncobs. Bioresour. Technol. 176, 15-22. [35] Zheng, A., Zhao, Z., Chang, S., Huang, Z., Zhao, K., Wei, G., He, F., Li, H. 2015. Comparison of the effect of wet and dry torrefaction on chemical structure and pyrolysis behavior of corncobs. Bioresour Technol, 176, 15-22.

Figure captions Fig. 1. Variations in O/C, H/C of original and torrefied corncobs. Fig. 2. TG and DTG curves for original and torrefied corncobs. Fig. 3. Kinetic plot for original and torrefied corncobs: (a) Calculation of CC kinetic parameters using KAS/DAEM method. (b) Calculation of CC kinetic parameters using OFW method. (c) Calculation of CC210 kinetic parameters using KAS/DAEM method. (d) Calculation of CC210 kinetic parameters using OFW method. (e) Calculation of CC240 kinetic parameters using KAS/DAEM method. (f) Calculation of CC240 kinetic parameters using OFW method. (g) Calculation of CC270 kinetic parameters using KAS/DAEM method. (h) Calculation of CC270 kinetic parameters using OFW method. (i) Calculation of CC300 kinetic parameters using KAS/DAEM method. (j) Calculation of CC300 kinetic parameters using OFW method.

Fig. 1. Variations in O/C, H/C of original and torrefied corncobs.

Fig. 2. TG and DTG curves for original and torrefied corncobs.

Fig. 3. Kinetic plot for original and torrefied corncobs: (a) Calculation of CC kinetic parameters using KAS/DAEM method. (b) Calculation of CC kinetic parameters using OFW method. (c) Calculation of CC210 kinetic parameters using KAS/DAEM method. (d) Calculation of CC210 kinetic parameters using OFW method. (e) Calculation of CC240 kinetic parameters using KAS/DAEM method. (f) Calculation of CC240 kinetic parameters using OFW method. (g) Calculation of CC270 kinetic parameters using KAS/DAEM method. (h) Calculation of CC270 kinetic parameters using OFW method. (i) Calculation of CC300 kinetic parameters using KAS/DAEM method. (j) Calculation of CC300 kinetic parameters using OFW method.

Tables Table 1 Characteristics of original and torrefied corncobs. Table 2 Peak assignment of FTIR spectrum. Table 3 TG-DTG characteristics parameters of original and torrefied corncobs. Table 4 Kinetic parameters of the thermal degradation of original and torrefied corncobs. Table 5 Thermodynamic parameters of original and torrefied corncobs at 10°C/min.

Table 1 Characteristics of original and torrefied corncobs. Samples

CC

CC210

CC240

CC270

CC300

Proximate analysis (wt%), db Ash

4.87

5.76

7.15

10.84

12.22

Volatile

85.72

81.04

81.33

57.18

37.80

Fixed carbon

9.41

13.20

11.52

31.98

49.98

Ultimate analysis (wt%), db C

44.31

47.76

54.16

65.12

68.38

H

6.38

6.26

6.03

5.21

4.96

Oa

44.44

40.22

32.66

18.83

14.44

N

0.36

0.41

0.48

0.67

0.75

O/C ratio

0.75

0.63

0.45

0.22

0.16

H/C ratio

1.73

1.57

1.34

0.96

0.87

H/Ceff ratio b

0.22

0.31

0.43

0.53

0.55

HHV c (MJ/kg)

18.66

20.11

22.75

26.85

28.09

Mass yield (%)

-

81.28

62.36

42.50

36.05

-

87.58

76.04

61.15

54.26

40.80

66.68

76.46

44.80

21.55

Energy

yield

(%) CrI (%)

db: dry basis. a The oxygen content was calculated by difference. Oxygen content = 100% ― carbon content ― hydrogen content ― nitrogen content ― ash content ; b H/Ceff ratio =

Moles of hydrogen ― 2 × Moles of oxygen 𝑀𝑜𝑙𝑒𝑠 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑛

; c Higher heating value,

HHV(MJ/kg) = 34.1C + 123.9H ― 9.85O + 6.3N + 19.1S.

Table 2 Peak assignment of FTIR spectrum. Peak

Wavenumbers (cm-1)

Description of the main cause of the vibration

1

3435-3402

O-H stretching vibration in hydroxyl and carboxyl

2

2928-2919

C−H vibration of cellulose, hemicellulose and lignin

3

1739-1697

C=O stretching vibration of acetyl of xylan (hemicellulose)

4

1626-1603

Skeletal vibration of aromatic in lignin

5

1521-1515

Skeletal vibration of aromatic in lignin

6

1381-1371

C−H deformation of cellulose, hemicellulose and lignin

7

1261-1250

Syringyl ring and C−O stretch in lignin and xylan

8

1170-1160

C−O−C vibration of cellulose and hemicellulose

9

1060-1039

C–O stretch of cellulose and hemicellulose

Table 3 TG-DTG characteristics parameters of original and torrefied corncobs. Characteristic parameter

CC

CC210

CC240

CC270

CC30 0

Ti (°C) a

79.89

249.90

280.44

297.48

348.90

Tmax (°C) b

329.89

329.90

327.94

325.98

399.40

DTGmax (%/min) c

8.70

9.74

9.12

3.80

2.10

Residue (%)

18.58

28.84

39.93

53.69

66.43

a

Corresponding temperature of a weight loss of 5%; loss rate; c Maximum weight loss rate.

b

Corresponding temperature of the maximum weight

Table 4 Kinetic parameters of the thermal degradation of original and torrefied corncobs. KAS/DAEM

OFW D

S

x

ample

E

A

A

AEM

E

A

A

(kJ

(m

(m

-A

(kJ

(m

(m

/ mol)

in -1) a

in -1) b

(m

/ mol)

in -1) a

in -1) b

in -1) 0 .40

8.22 0

.45 .50 C .55 .60

17 7.09

0 .65

17 6.90

0 .70

17 9.67

0 .75

18 1.62

0 .40

19 2.58

0 .45

18 6.61

0 .50

18 8.43

0 .55

18 5.47

0 .60

17 3.06

0

C210

17 1.94

0

C

17 2.04

0

C

16

18 3.60

7.6

2.0

8.1

5E+14

7E+14

6E+14

1.3

4.5

1.2

3E+15

4E+14

2E+15

9.9

4.4

7.8

7E +14

6E+14

4E+14

9.7

5.6

6.6

8E+14

0E+14

7E+14

1.8

1.2

1.0

2E+15

8E+15

8E+15

1.4

1.2

7.6

7E+15

3E+15

2E+14

2.2

2.1

1.0

4E+15

7E+15

1E+15

2.8

3.2

1.1

8E+15

4E+15

3E+15

6.2

3.0

6.6

0E+16

6E+16

1E+16

1.3

9.0

1.2

4E+16

1E+15

2E+16

1.5

1.3

1.2

8E+16

1E+16

4E+16

7.3

7.1

5.0

5E+15

3E+15

1E+15

4.4

4.8

2.6

5E+15

6E+15

5E+15 29

16 9.04 17 2.77 17 2.78 17 3.93 17 7.85 17 7.74 18 0.44 18 2.37 19 2.33 18 6.76 18 8.57 18 5.83 18 4.12

5.5

2.4

3E+22

5E+14

1.5

5.2

1E+23

8E+14

6.3

5.2

6E+22

9E+14

4.9

6.7

5E+22

0E+14

1.6

1.5

0E+23

0E+15

8.2

1.4

3E+22

6E+15

1.7

2.5

2E+23

5E+15

2.4

3.7

7E+23

9E+15

8.8

2.9

4E+26

1E+16

2.3

9.2

6E+25

8E+15

2.7

1.3

5E+25

5E+16

4.1

7.6

1E+24

8E+15

1.1

5.4

2E+24

1E+15

0 .65

2.29 0

.70 .75 .40

17 3.54

0 .45

17 0.82

0 .50

17 7.72

0 C .55

18 0.14

0 .60

18 8.08

0 .65

20 7.77

0 .70

21 2.58

0 .75

21 5.28

0 .40

21 9.82

0 .45

21 4.36

0 C .50

21 4.14

0 .55

21 6.60

0 .60

23 3.32

0 .65

18 8.58

0

C270

18 4.28

0

C240

18

23 8.88

3.0

3.7

1.5

6E+15

2E+15

9E+15

4.0

5.5

1.8

5E+15

9E+15

3E+15

7.8

1.3

3.0

7E+15

5E+16

9E+15

3.7

6.9

4.0

6E+14

7E+14

1E+14

2.0

3.9

1.8

7E+14

8E+14

9E+14

7.9

1.6

6.2

6E+14

5E+15

5E+14

1.1

2.7

7.9

7E+15

1E+15

9E+14

4.9

1.3

2.9

5E+15

9E+16

4E+15

1.7

7.8

8.8

1E+17

7E+17

8E+16

1.9

2.1

8.7

3E+17

1E+18

3E+16

9.2

3.6

3.6

4E+16

6E+18

3E+16

1.4

1.0

1.5

6E+18

8E+19

5E+18

2.8

3.5

2.5

2E+17

1E+18

7E+17

1.3

3.3

1.0

1E+17

6E+18

3E+17

1.0

5.5

6.9

2E+17

7E+18

7E+16

1.0

1.7

6.1

3E+18

2E+20

2E+17

1.2

5.3

6.6

8E+18

8E+20

2E+17

30

18 2.94 18 4.90 18 9.08 17 4.53 17 1.99 17 8.62 18 0.97 18 8.60 20 7.45 21 2.25 21 5.17 21 8.79 21 3.75 21 3.76 21 6.32 23 2.44 23 7.96

4.0

4.2

5E+23

5E+15

6.2

6.3

1E+23

4E+15

2.2

1.4

2E+24

9E+16

9.7

8.5

4E+21

4E+14

2.1

5.0

1E+21

6E+14

3.3

1.9

9E+22

8E+15

6.5

3.2

6E+22

1E+15

1.3

1.5

2E+24

4E+16

2.7

7.3

9E+27

7E+17

2.8

1.9

5E+27

7E+18

4.1

3.5

7E+26

8E+18

8.2

8.7

4E+29

2E+18

1.6

3.1

5E+28

0E+18

2.3

3.1

4E+27

1E+18

1.0

5.2

6E+27

5E+18

1.4

1.4

2E+29

4E+20

1.8

4.4

0E+29

5E+20

0 .70

8.21 0

.75 .40

22 2.33

0 .45

21 3.26

0 .50

23 7.47

0 C .55

25 4.81

0 .60

26 0.83

0 .65

27 8.47

0 .70

27 6.72

0 .75

24 3.20

0

C300

27

32 9.38

5.4

1.6

2.4

1E+20

8E+24

5E+20

4.3

1.3

1.7

2E+17

0E+21

0E+17

4.1

1.1

4.4

9E+16

0E+17

7E+16

5.3

2.0

4.8

4E+15

8E+16

6E+15

2.5

1.7

2.0

8E+17

6E+18

2E+17

3.1

4.1

2.1

2E+18

9E+19

3E+18

4.6

1.2

2.7

4E+18

6E+20

6E+18

4.7

3.1

2.4

8E+19

5E+21

8E+19

1.6

2.2

7.2

0E+19

9E+21

2E+18

2.8

3.3

1.1

9E+22

5E+25

4E+22

a

27 5.61 24 2.62 22 2.09 21 3.61 23 6.80 25 3.47 25 9.38 27 6.36 27 4.93 32 5.30

b

1.0

9.8

3E+35

9E+23

9.7

1.1

3E+27

6E+21

2.2

1.0

3E+26

5E+17

1.8

2.2

1E+24

2E+16

7.9

1.5

1E+27

5E+18

1.6

3.2

3E+30

8E+19

3.1

9.6

4E+30

6E+19

4.5

2.1

7E+32

4E+21

3.1

1.6

2E+31

5E+21

4.6

1.6

6E+38

0E+25

Calculated by the formulas 18, 20, and 24 (method A); Calculated by the formula 28 (method B).

31

Table 5 Thermodynamic parameters of original and torrefied corncobs at 10°C/min. KAS S amp le

C C C C21 0 C C24 0 C C27 0 C C30 0

Δ H

OFW

Δ

ΔS

G (

(

kJ∙mo

kJ·m

l−1)

ol-1) 1

1

72.07

72.67

1

1

78.58

74.69

1

1

83.09

78.67

2

1

28.34

97.32

2

2

55.24

12.66

(J· mol−1 K−1) -0. 99 6.4 5

7.3 5

51. 77

63. 31

Δ H

DAEM

Δ

ΔS

G (

(

kJ∙mo kJ∙mo l−1)

l−1) 1

1

72.83

74.41

1

1

79.10

75.20

1

1

83.61

73.50

2

1

27.46

71.85

2

1

53.79

94.23

32

(J· mol−1 K−1) -2. 62 6.4 8

16. 81

92. 82

88. 56

Δ H

Δ

ΔS

G (

(

kJ∙mo kJ∙mo l−1)

l−1) 1

1

72.07

75.28

1

1

78.58

77.30

1

1

83.09

81.27

2

1

28.34

99.91

2

2

55.24

15.57

(J· mol−1 K−1) -5. 32 2.1 3

3.0 3

47. 44

58. 99

Highlights: 

The most suitable method (OFW method) for activation energy calculation was found.



The most suitable methods (KAS and DAEM methods) for pre-exponential factor calculation were found.



The optimal torrefaction condition of corncob (240°C) was determined. 

Torrefaction enhanced the aromaticity of corncob.

33