Pyrolysis characteristics, kinetics and evolved volatiles determination of rice-husk-based distiller's grains

Biomass and Bioenergy 135 (2020) 105525

Contents lists available at ScienceDirect

Biomass and Bioenergy journal homepage: http://www.elsevier.com/locate/biombioe

Research paper

Pyrolysis characteristics, kinetics and evolved volatiles determination of rice-husk-based distiller’s grains Zhao Zhang, Qiuju Wang, Luwei Li, Guoren Xu * National Engineering Laboratory for Sustainable Sludge Management & Resourcelization Technology, Harbin Institute of Technology, P.O. Box 2602, Harbin, 150090, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Pyrolysis Rice husks Distiller’s grains TG Py-GC/MS

Pyrolysis characteristics, kinetics and evolved volatiles of rice-husk-based distiller’s grains (RHDG) were determined by thermogravimetric analysis and pyrolysis (Py)-GC/MS with rice husks (RH) as a comparison. The activation energy of RHDG was 239.84 � 48.10 kJ/mol attained by Flynn-Wall-Ozawa method and 241.89 � 49.78 kJ/mol by Kissinger-Akahira-Sunose method, much higher than RH and other biomasses. Diesel range organics account for 59.40%, 54.44%, 52.77% of volatiles obtained from RHDG pyrolysis, and C18, C16 were the most abundant components including oleic acids, octadecanoic acid and n-hexadecanoic acid. Pyrolysis evolved volatiles from RHDG had higher atomic H/C and O/C ratios compared with those from RH. The highest content in the volatiles from RHDG was acids (55.66%, 39.29% and 37.04% at 300 � C, 400 � C and 500 � C), which were mainly the evaporation of fatty acids in fermentation residues. Pyrolysis kinetics and evolved volatiles deter­ mination provide an in-depth understanding of rice-husk-based distiller’s grains pyrolysis behaviors and basis for design and scale-up of pyrolysis process and reactor.

1. Introduction Rice-husk-based distiller’s grains (RHDG) is a byproduct of solid fermentation liquor production, contains 30%–40% rice husks (RH), and RH is used to improve the permeability and wetting environment of solid fermentation bed. Non-rice-husk-based distiller’s grains contain a high content of protein and other nutrients, often used as feed to achieve resource utilization [1–3]. Due to the high content of RH, RHDG is hard and rough, difficult for livestock to digest and thus not suitable to pro­ duce feed, treatment and disposal of RHDG is a tough problem for solid fermentation liquor factories. Pyrolysis is a promising technique to treat this kind of distiller’s grains to produce useful materials and bioenergy. As far as we know, there were few studies on RHDG pyrolysis, but there have many research on RH pyrolysis to produce biochar, carbon black, bio-silica, Si–C, and bio-oil. The specific BET surface area and total pore volume of black carbon could reach 1034 m2/g and 0.487 cm3/g [4]. Bio-silica with uniform spherical structure and particle sizes about 150 nm and a BET surface area of 179.3 m2/g was produced by pyrolysis of residues under enzymatic hydrolysis pretreatment [5]. RH pyrolysis under pressure could produce bio-oil with less oxygen and higher calorific values, and

high yields of acetic acid, phenol, cresol and xylenol and guaiacols [6]. Water washing and torrefaction remarkably increased the percentage of levoglucosan in bio-oil [7]. The bio-oil production yield increased with the final pyrolysis temperature, and 450 � C was found to be the optimum temperature for RH pyrolysis [8]. TG and Py-GC/MS have been widely applied to investigate weight loss, kinetics and evolved gases of biomass produced from pyrolysis [7,9–16]. We proposed a roadmap of utilization of rice-husk-based distiller’s grains in circular economy and green planting by pyrolysis process with biochar and bioenergy recovery & utilization (Fig. 1). The primary purposes of this research were to investigate bioenergy recovery po­ tential of rice-husk-based distiller’s grains in pyrolysis process of RHDG, by characterization of TG, carbon number, H/C and O/C ratios and compounds classification distributions of evolved volatiles from pyrol­ ysis of RHDG via Py-GC/MS. RH was used as reference material in this research as the matrix of RHDG was based on RH. The results of this investigation can provide a better understanding of rice-husk-based distiller’s grains pyrolysis behaviors and basis for the design and scaleup of pyrolysis reactor.

* Corresponding author. E-mail address: [email protected] (G. Xu). https://doi.org/10.1016/j.biombioe.2020.105525 Received 15 June 2019; Received in revised form 15 January 2020; Accepted 21 February 2020 Available online 29 February 2020 0961-9534/© 2020 Elsevier Ltd. All rights reserved.

Z. Zhang et al.

Biomass and Bioenergy 135 (2020) 105525

Fig. 1. Potential of rice-husk-based distiller’s grains in circular economy and green planting by pyrolysis process with bioenergy recovery & utilization. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

2. Materials and methods

� k ​ ðTÞ ¼ A exp

2.1. Materials

In TG analysis, the ramp rate β ¼ dT=dt, transforms Eq. (4) into: � � dα A Ea f ðαÞ (5) ¼ exp RT dt β Equation (5) was the key expression to calculated pyrolysis kinetic parameters using TG data. Flynn-Wall-Ozawa (FWO) method [18,19] shown in Eq. (6) and Kissinger-Akahira-Sunose (KAS) method [20] in Eq. (7) are two commonly used isoconversion model-free kinetic models and applicable to biomass pyrolysis [9,21,22]. � � AEa Ea In β ¼ In (6) 5:523 1:052 RGðαÞ RT

2.2. Pyrolysis characteristics of rice husks and rice-husk-based distiller’s grains Pyrolysis characteristics of RHDG and RH were analyzed in terms of global mass loss using a TG analyzer (STA 449 F5, Netzsch Instrument, Selb, Germany) in a pure N2 (99.999%) atmosphere. Samples (10 � 0.1 mg) were loosely and evenly distributed in open alumina (Al2O3) cru­ cible. The temperature program was from 50 � C to 1000 � C with ramp rates of 10 � C/min, 20 � C/min, 30 � C/min and 40 � C/min. A flow rate of 60 mL/min high purity N2 (99.999%) was continuously purged into the TG furnace to prevent oxidative reactions. Three replicates of TG anal­ ysis for RH and RHDG were performed, and thermogravimetric data for kinetic calculation were collected and processed following ICTAC Ki­ netics Committee recommendations [17].

� In

(7)

The FTIR analyses of RH and RHDG were carried out by a Spectrum One B FTIR (PerkinElmer, Waltham, USA) in the wavenumber range of 400–4000 cm 1. The FTIR analysis of evolved volatiles was conducted by TG-FTIR (TG, 209F3, Netzsch, Germany; FTIR, Tensor 27, Thermo Scientific, USA). The pyrolysis temperature program was controlled from 35 � C to 800 � C with ramp rates of 10 � C/min, and High purity N2 (99.999%) of 60 mL/min.

(1)

In which k(T) is the reaction rate constant at T and fðαÞ is the py­ rolysis reaction model function, which was determined by the pyrolysis reaction mechanism. α is the extent of conversion, calculated by: ξ ξf

Ea RT

2.4. FTIR analysis of rice husks and rice-husk-based distiller’s grains and evolved volatiles

The fundamental reaction rate equation is generally described as:

ξi ξi

� � � β AEa ¼ In 2 T RGðαÞ

In which G(α) is usually constant, the most possible G(α) was determined by the method in previous research [23]. Kinetic computa­ tions on thermal analysis data were performed following ICTAC Kinetics Committee recommendations [24]. All the thermogravimetric data was processed and modelled on the platform of MATLAB (R2019a).

2.3. Pyrolysis kinetic models

α¼

(3)

In which Ea is the activation energy (kJ/mol) and A is the preexponential factor (min 1), R is the universal gas constant (8.314 J/ mol/K) and T is the thermodynamic temperature (K). The combination of Eqs. (1) and (3) gives: � � dα Ea (4) f ðαÞ ¼ A exp RT dt

RH were taken from a rice farm in Harbin (Heilongjiang, China) and RHDG were recovered from a distilled wine plant in Luzhou (Sichuan, China). This RHDG was fermented by-products of liquor produced by sorghum as the main raw material and RH as the auxiliary material. The RH and distiller’s grains were dehydrated at 105 � C to constant weight and ground to fine powders using an agate mortar over 100 mesh at 25000 rpm (BJ-150, Baijie, Zhejiang, China). The composition of C/H/ N/S in the dried RH and RHDG samples were analyzed with elemental analyzer (Vario EL cube, Elementar, Hanau, Germany), and other ele­ ments were analyzed with an X-ray fluorescence spectrometer (XRF, Axios PW4400, PANalytical, Eindhoven, Netherlands). Sample micro­ structure and element compositions were mapped with a scanning electron microscope with an energy dispersive spectrometer (SEM-EDS, S-3400, Hitachi, Tokyo, Japan).

dα ¼ kðTÞf ðαÞ dt

� Ea RT

2.5. Determination of evolved gases by Py-GC/MS

(2)

The volatiles from RHDG and RH pyrolyzed at 300 � C, 400 � C and 500 � C, with a ramp rate of 20 � C/ms and a dwelling time of 60 s at the target temperature was determined by a pyrolizer (CDS 5200 HPR, PA, USA) coupled with GC/MS (7890B–5977B, Agilent, USA). High purity

where ξ is the mass value at T, ξi is the initial weight and ξf is the final weight. The reaction rate constant is expressed by the Arrhenius equation: 2

Z. Zhang et al.

Biomass and Bioenergy 135 (2020) 105525

Table 1 The main element compositions in rice husks and distiller’s grains (%). Biomass

C

H

N

S

O

Mg

Si

P

K

Ca

C/H

C/N

Rice husks Distiller’s grains

33.30 38.98

5.36 6.32

0.68 3.02

0.15 0.42

20.59 25.23

0.07 0.16

7.84 2.91

0.09 0.36

0.91 0.58

0.45 4.71

6.22 6.17

48.97 12.93

Fig. 2. Macroscopic and microscopic images and elemental composition of rice husks (RH) and rice husk-based distiller’s grains (RHDG). a. RH macroscopic image, b. RH scanning electron microscope (SEM) image and c. Electron dispersive spectrometry (EDS) of a typical point. e. RHDG macroscopic image, f. RHDG SEM image and g. EDS of a typical point.

helium (99.999%) was operated at a continuous flow and purged the volatiles to a trap cooled and enriched by liquid N2 to 100 � C. And then the trap was heated rapidly to 300 � C. Meanwhile the volatiles was purged by high purity (99.999%) helium to the GC/MS. The evolved volatiles were analyzed by GC/MS, the transfer line connecting the pyrolyzer and GC/MS and the injector temperatures were kept at 325 � C; the chromatographic separation was performed with an HP-5ms capillary column (Agilent 19091S-433, 30 m � 250 μm � 0.25 μm, 5% phenyl methyl silox); the MS was operated in EI mode at 70 eV. Mass spectra were obtained with m/z from 45 to 500. Each compound yield was confirmed by the GC/MS spectra, by searching the NIST 2017 library. The relative percentage content of a product is semi-quantified by comparing the peak area of the product with the total peak area of all detected products [25].

and P in RHDG were higher but K was lower than those in RH, which could be attributed that, K is easily dissolved in liquor, but Mg and P are more likely to remain in the residues in the process of grain fermenta­ tion. In the process of drying and storing RHDG, lime was added for water absorption and deodorization, which led to the high content of Ca with the value of 4.71%. Si contents were higher than other biomass both in RHDG (2.91) and RH (7.84%), and Si mainly existed in the form of Si–O and Si–C which was revealed by Freitas et al. using 29Si-NMR [27]. While the RHDG and RH exhibited similar C/H ratios, the C/N ratio in RH was 48.97%, higher than 12.93% of RHDG. The difference of C/N ratios could be attributed to the high content of nitrogen-containing organics including proteins and amino acids in the distiller’s grains [26]. Fig. 2 showed the macroscopic and microscopic photographs as well as the EDS-determined element composition of RH and RHDG. As showed in the microscopic photographs the surface of the RH was dense and smooth, while the surface of the RHDG was coarse. The main ele­ ments are Si and O; meanwhile, the relative contents of Si and O in the RH surface (Fig. 2b and c) were much higher than the overall contents (Table 1). The high contents of surface Si and O indicated that although the RH mainly contains cellulose, hemicellulose, and lignin [28], these organics were covered by Si–O containing compounds on the surface. The main elements of the RHDG surface were C, O, S, P and Ca (Fig. 2e and f) and these were a mixture of fermentation residues and lime.

3. Results and discussion 3.1. Characterization of rice-husk-based distiller’s grains and rice husks The main elemental composition in RHDG and RH was given in Table 1. The total amount of C/H/N/S/O elements in the RHDG was 73.97% and higher than RH of 60.08%, indicating that the content of organic matter is higher in RHDG, which was caused by the sediment of residues and fermentation by-products on the RH [26]. Contents of Mg 3

Z. Zhang et al.

Biomass and Bioenergy 135 (2020) 105525

Fig. 3. Thermogravimetric characteristics of rice husks and distiller’s grains at different heating rates. a. Rice husks, b. Rice husk-based distiller’s grains.

3.2. Pyrolysis characteristics of rice husks and distiller’s grains

Table 2 Pyrolysis characteristics of rice husks and distiller’s grains at different heating rates. Samples

The first stage (50–250 � C) weight loss (%)

The second stage (250–400 � C) weight loss (%)

The third stage (400–600 � C) weight loss (%)

The fourth stage (600–800 � C) weight loss (%)

Residues (%)

RH

2.47 � 0.73 5.36 � 0.52

44.71 � 0.25 43.48 � 0.49

7.66 � 0.56

1.93 � 0.10

10.45 � 0.78

3.20 � 0.05

43.22 � 0.29 37.52 � 0.27

RHDG

Fig. 3 showed the thermogravimetric and differential thermal gravity of RH and RHDG at different ramp rates. Although the compositions of the RH and RHDG were different, they had similar TG and DTG curves. The whole pyrolysis process could be divided into 4 stages according to the DTG curves. The first stage was the dehydration stage (50 � C–250 � C) and in this stage, mass loss is mainly caused by the release of water and small molecule volatile organic compounds; the second stage was fast pyrolysis stage (250 � C–400 � C) where the samples lost about 44% of their total weight, hemicellulose, cellulose, proteins, and fatty acids decomposition were mainly in this stage; the third stage was slower pyrolysis stage (400 � C–600 � C) and lignin decomposition was mainly in this stage; and the final stage was the charring stage (600 � C–800 � C)

Fig. 4. The non-isothermal plot of RH and RHDGs by FWO and KAS methods (Rice husks: a. FWO, b. KAS; Distiller’s grains: c. FWO, d. KAS). 4

Z. Zhang et al.

Biomass and Bioenergy 135 (2020) 105525

Table 3 The kinetic parameters of RH and RHDG samples at different conversion rates. Samples

α

T(� C)

RH

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

279 303 318 330 340 349 359 380 464

RHDG

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

258 293 312 327 340 352 366 411 497

R2

Ea (kJ/mol)

A (min 1)

0.9734 0.9926 0.9926 0.9971 0.9982 0.9982 0.9995 0.9906 0.9767

163.78 180.97 190.59 193.00 183.82 189.29 194.84 245.69 327.27

5.46Eþ17 1.03Eþ19 4.81Eþ19 5.69Eþ19 6.51Eþ18 1.67Eþ19 4.22Eþ19 4.18Eþ23 5.09Eþ27

0.9963 0.9955 0.9972 0.9989 0.9978 0.9994 0.9994 0.9824 0.9794

255.07 222.60 227.83 213.11 209.25 199.95 209.04 268.72 353.02

4.72Eþ27 1.89Eþ23 1.91Eþ23 4.10Eþ21 1.09Eþ21 1.13Eþ20 4.21Eþ20 2.73Eþ24 2.51Eþ28

FWO method

R2

1

Ea (kJ/mol)

A (min

)

0.9705 0.9919 0.9919 0.9968 0.9980 0.9980 0.9995 0.9897 0.9750

163.12 180.79 190.65 192.99 183.17 188.77 194.43 247.54 331.92

4.6Eþ11 4.35Eþ12 1.24Eþ13 8.73Eþ12 6.21Eþ11 1.09Eþ12 1.82Eþ12 1.16Eþ16 6.62Eþ19

0.9961 0.9952 0.9969 0.9988 0.9976 0.9994 0.9994 0.9810 0.9779

259.4298 224.7085 229.8897 214.1823 209.8912 199.9132 209.2478 271.2633 358.4878

1.06Eþ22 1.26Eþ17 7.22Eþ16 8.67Eþ14 1.35Eþ14 8.22Eþ12 2.05Eþ13 9.35Eþ16 3.99Eþ20

KAS method

during which the charring reactions was very slow and gradually stopped [29–31]. With the increase in heating rate from 10 � C/min to 40 � C/min, devolatilization stage shifted obviously to the higher tem­ perature. Other research showed the same phenomenon and attributed it to heat and mass transfer limitations of materials in pyrolysis process, temperature gradients could exist in the sample and the devolatilization rate was faster than volatile release rate, which led to the devolatiliza­ tion stage shifts [32]. In Table 2, the weight loss of RH and RHDG in different pyrolysis stages were compared in detail and the average treatment of weightlessness under different heating rates was presented. When the final temperature was 800 � C, the residue of RHDG was 37.52% lower than 43.22% of the RH, which could be due to the higher total amount of C/H/N/S/O elements in the RHDG.

of RH was close to that of cellulose, α ¼ 0.1 was mainly semi-cellulose pyrolysis, α ¼ 0.2–0.8 were mainly cellulose pyrolysis, and α ¼ 0.9 was residue pyrolysis charring. Due to the low content, lignin had less effect on the activation energy of RH. The activation energy of RHDG was 239.84 � 48.10 kJ/mol attained by FWO method and 241.89 � 49.78 kJ/mol by KAS method. When α ¼ 0.2–0.7 the activation energies varied between 200 and 223 kJ/mol; when α ¼ 0.1, 0.8 and 0.9, the activation energies were relatively high with the values of 255.07 kJ/mol, 245.69 kJ/mol and 327.27 kJ/mol. In addition to cellulose, hemicellulose and lignin, distiller’s grains with also contains fatty acids and proteins, the proportion of fatty acids and proteins is about 10% and 30%, respectively [26]. Pyrolysis of vegetable oil can be divided into 3 stages, activation energies of the first stage (200–400 � C), the second stage (400–500 � C) and the third stage (500–600 � C) were about 90–100 kJ/mol, 200–300 kJ/mol and over 300 kJ/mol, respectively [31]. The pyrolysis temperature range of proteins was mainly 295–399 � C, and the activation energy was about 210 kJ/mol [37]. It can be inferred that α ¼ 0.2–0.7 were mainly cel­ lulose, proteins and vegetable oil pyrolysis, and α ¼ and 0.9 were residue pyrolysis charring. When α ¼ 0.1, the activation energy was much higher than common biomass due to the higher surface Ca content, which hindered heat and mass transfer, affected the heat transfer effi­ ciency and made the pyrolysis gases difficult to evolve. The Ea of RHDG was higher than RH, other dry distiller’s grains with solubles (DDGS) and common biomass as follows. V. Pasangulapati et al. reported the activation energy of DDGS was 31.67 kJ/mol [38], similar to 27.5–36.1 kJ/mol of that by Wang et al. [39]. C. Branca et al. attained a higher activation energy of DDGS with the value of 94.7–152.8 kJ/mol [40], close to the activation energy of wheat straw (100.67 kJ/mol), switch grasses (103.70 kJ/mol) and eastern redcedar (90.16 kJ/mol), and pine wood wastes (83.9 kJ/mol) [38,41]. By comparison, it can be concluded that the activation energy of the RHDG is about 32 kJ/mol higher than that of the RH, which is 100 kJ/mol higher than those of DDGS and other biomass, which may be caused by the following rea­ sons. The Si–O bonds in the surface of RH a dense protective layer which hindered the diffusion of pyrolysis gases, result in a higher activation energy of the RH and RHDG; different ways of fermentation (solid fermentation residues in this research and liquid fermentation residues in the references) and the adding of lime during RHDG drying process made the activation energy of residues higher than that of DDGS in previous literatures.

3.3. Pyrolysis kinetics of RH and RHDG by the FWO and KAS models Pyrolysis kinetics of RH and RHDG can be used for in-depth under­ standing pyrolysis behaviors and a reference for pyrolysis process and reactors design and optimization. Fig. 4 shows the relationship between 1/T, log(β) and 1/T, ln (β/T2) at the conversion rates from 0.1 to 0.9 in the FWO and KAS models. The lines at different conversion rates were nearly parallel, indicating that the calculation methods were reliable. Table 3 showed the pyrolysis kinetic parameters of RH and RHDG with different conversion rates. The linear relationship coefficients (R2) were all higher than 0.97, indicating the kinetics parameters of the samples calculated by the FWO and KAS models were suitable. A was used to characterize the pyrolysis reaction rate, while A of RH and RHDG changed at different conversion rates [9]. The activation energies of RH calculated by FWO method (207.69 � 49.96 kJ/mol) and KAS method (208.15 � 51.68 kJ/mol) were very close (Table 3), indicated that the activation energies values were relatively accurate. Loy et al. reported a close result that the activation energy of RH was 190.8 kJ/mol [10]. The conversion rates (α) of 0.2–0.7 were all located in the temperature interval of 300–360 � C, and the activation energies were floating between 181 and 195 kJ/mol. When α ¼ 0.1, the activation energy was the lowest of 163.78 kJ/mol, and when α ¼ 0.8 and 0.9, the activation energies were relatively high with the values of 245.69 kJ/mol and 327.27 kJ/mol. RH consists of 33% cel­ lulose, 26% hemicellulose, and 7% lignin [28]. The activation energies of cellulose, hemicellulose, and lignin were around 187–260 kJ/mol, 100–170 kJ/mol and 45–125 kJ/mol, respectively [30,33–36], the temperature ranges were 220–315 � C predominantly for hemicellulose decomposition, 315–400 � C for cellulose decomposition and over 400 � C for lignin [30]. Based on the previous studies, we concluded that the Ea 5

Z. Zhang et al.

Biomass and Bioenergy 135 (2020) 105525

Fig. 5. FTIR spectra of RH, RHDG and residuals at different pyrolysis temperatures.

Fig. 6. 3D FTIR spectra and FTIR spectra of functional groups and small molecules at different temperatures of evolved volatiles from RH (a, c) and RHDG (b, d).

3000–3200 cm 1; –CH3, 1365–1380 cm 1, 2870 cm 1; – C,1620–1680 cm 1; ¼C–H, 3000–3100 cm 1; –COOH, 3560-3500 C– cm 1; Si–O–Si, 467 cm 1, 799 cm 1, 1093 cm 1; CO, 2178 cm 1; CO2, 2349 cm 1; NH3, 965 cm 1; HCN, 715 cm 1 [23,42]. As shown in Fig. 5, RHDG had more –CH3 and –COOH groups than RH, and both the two

3.4. FTIR analysis of rice husks and rice-husk-based distiller’s grains and evolved volatiles Typical wave numbers of organic and inorganic functional groups – O, 1740–1770 cm 1; –OH, and small molecules are as follows: C– 6

Z. Zhang et al.

Biomass and Bioenergy 135 (2020) 105525

Fig. 7. Carbon number distribution of evolved organic volatiles from rice husks and rice-husk-based distiller’s grains pyrolysis at 300 � C, 400 � C and 500 � C (a. Rice husks, b. Rice-husk-based distiller’s grains.).

groups in RH and RHDG obviously decreased after pyrolyzed at 300 � C, 400 � C and 500 � C, and their pyrolysis products were mainly hydro­ – O and –OH groups carbons and carbon dioxide. RHDG also had more C– than RH, this can be attributed to more acid in the RHDG. Both RH and RHDG had obvious absorption peak around 1093 cm 1, which indicated high SiO2 contents in the two biomasses, and the SiO2 were stable during pyrolysis. As shown in Fig. 6, evolved volatiles from RH and RHDG mainly occurred in the temperature range from 300 � C to 500 � C, which had a good correspondence with the thermogravimetric curves and the mass mainly lost in the temperature range of 300–500 � C. When the tem­ – O, –OH, –CH3, C– – C, perature was around 350 � C, functional groups C– ¼C–H, –COOH, small molecules CO and CO2 had the largest absorption peak for both RH and RHDG, which indicated that at this temperature, the pyrolysis rate of RH and RHDG reached the fastest. Generally, two nitrogen oxide precursors NH3 and HCN had lower production during both RH and RHDG pyrolysis. There was a small peak of CO2 at around 700 � C for RHDG pyrolysis, which was caused by the thermal decom­ position of calcium carbonate in RHDG.

atomization effects of biofuel sprays, leading to injector operation problems, such as injector coking, oil ring attachment and thickening, and carbon deposition increasing [44]. Organics with carbon numbers of C6–C12 are gasoline range organics (GROs) and those with C10–C28 are diesel range organics (DROs) [45–47]. As shown in Fig. 7, when the pyrolysis temperature was 300 � C, 400 � C and 500 � C, the carbon number distribution of volatiles obtained from RH pyrolysis was from C3 to C31, GROs accounts for 75.86%, 71.64% and 77.09% of the total volatiles, and DROs accounts for 33.55%, 35.76% and 31.95%. GRO mainly included compounds such as C8, C10, C9 and C5 that had a higher content than 10%. The highest content in C8 was 2,3-dihydro-Benzofuran (20.25% at 300 � C, 17.26% at 400 � C and 17.92% at 500 � C), which was the product of lignin pyrolysis [48–50]. Except for C11 and C12, C16 was the main heavy component in DROs, and mainly included n-Hexadecanoic acid (2.60% at 300 � C, 2.42% at 400 � C and 1.99% at 500 � C). The heavier components include C20 (Vanillin lactoside, 0% at 300 � C, 1.51% at 400 � C and 2.33% at 500 � C), C27 (Cholesta-4,6-dien-3-ol, 0.53% at 300 � C, 0.43% at 400 � C and 0.49% at 500 � C) and C33 (Sitosterol acetate, 0% at 300 � C, 0% at 400 � C and 0.69% at 500 � C), in which C20 was incomplete pyrolysis products of cellulose, C27 (Cholesta-4,6-dien-3-ol, (3.beta.)-) and C33 were incomplete pyrolysis products of plant Steroids. Although the content of heavy components in pyrolysis products in volatiles obtained from RH pyrolysis was low, it had a great influence on fuel quality and might also bring clogging problems to the long-term operation of the pyrolysis system. When the pyrolysis temperature was 300 � C, 400 � C and 500 � C, the carbon number distribution of volatiles obtained from RHDG pyrolysis was from C3 to C31, GROs accounts for 32.60%, 45.47% and 44.90% of the total volatiles, and DROs accounts for 59.40%, 54.44%, 52.77%. C18

3.5. Py-GC/MS analysis of pyrolysis evolved volatiles from rice husks and distiller’s grains 3.5.1. Carbon number distribution of pyrolysis evolved volatiles The number of carbon atoms in an organic compound molecule is defined as the carbon number [43]. Carbon number distribution of evolved volatiles affect combustion performance and biofuels with higher carbon number easily form heavier tar and clog the system. Compared with diesel, biofuels with carbon numbers greater than or equal to C16 have poor atomization characteristics, resulting in poor 7

Z. Zhang et al.

Biomass and Bioenergy 135 (2020) 105525

atomic oxygen to carbon (O/C) ratio, and when the atomic hydrogen to carbon (H/C) ratio increases, the effective heating value of the bio-fuel reduces [51]. As shown in Table 4, RHDG had higher total H/C and O/C ratios (0.485 and 1.945) than RH (0.463 and 1.931), and as a result, the effective heat value and higher heating value of RHDG were lower than RH. Pyrolysis evolved volatiles had lower total atomic H/C and O/C ratios than both RHDG and RH at different temperatures, and the total atomic H/C and O/C ratios of volatiles decreased as the pyrolysis tem­ perature increased from 300 � C to 500 � C. In general, pyrolysis evolved volatiles from RHDG had higher atomic H/C and O/C ratios compared with those from RH, which indicated that pyrolysis evolved volatiles from RHDG had lower higher heating value and effective heating value. The total atomic H/C and O/C ratios of sunflower oil were 1.719 and 0.134, for castor oil the values were 1.734 and 0.187 [52]. Pyrolysis evolved volatiles from RHDG had a close value of H/C and higher O/C value than sunflower oil and castor oil, and volatiles from RH had lower H/C and higher O/C values. Fig. 8 illustrated the atomic O/C and H/C ratios distribution of evolved volatiles from RH and RHDG pyrolysis at a different temperature, and the size of the bubble indicated the area percent content of volatiles. With the increase of pyrolysis temperature, the distribution of H/C and O/C was more concentrated, and the values tended to decrease, but the overall change was not noticeable. The distribution of H/C and O/C of volatiles from RH and RHDG was quite different, and the proportion of volatiles of H/C > 1.5 and O/C > 0.4 was higher, which indicated that RHDG pyrolysis volatiles contains more saturated C–C bonds and oxygen-containing functional groups.

Table 4 Total O/C and H/C ratios of RH, RHDG and evolved volatiles of RH, RHDG pyrolysis at 300, 400, 500 � C. Atomic O/C ratios

Atomic H/C ratios

RH RH300 RH400 RH500

0.463 0.221 0.208 0.207

1.931 1.371 1.404 1.341

RHDG RHDG300 RHDG400 RHDG500

0.485 0.258 0.227 0.199

1.945 1.766 1.680 1.612

(30.06% at 300 � C, 24.47% at 400 � C and 24.96% at 500 � C) and C16 (14.86% % at 300 � C, 12.64% at 400 � C and 11.63% at 500 � C) were the most abundant components in the pyrolysis volatiles of RHDG, C18 mainly included oleic acid and octadecanoic acid, and C16 mainly included n-hexadecanoic acid. Oleic acids, octadecanoic acid and nhexadecanoic acid were originated from RHDG as they were compo­ nents of distiller’s grains [26]. Volatiles obtained from RHDG pyrolysis had much higher molecular weight (carbon number�16) organic com­ pounds than RH pyrolysis, resulting in poor fuel flammability of the pyrolysis gases of the RHDG, which was more likely to clog the pyrolysis system and the combustion system [44]. 3.5.2. Atomic O/C and H/C ratios distribution of pyrolysis evolved volatiles The atomic ratios could help us to evaluate the heating value of a biofuel, the higher heating value (HHV) of bio-fuel correlates with the

Fig. 8. O/C and H/C ratios distribution of evolved volatiles from RH and RHDG pyrolysis at different temperature (RH300, RH400, RH500 were evolved volatiles of RH pyrolysis at 300, 400, 500 � C; DG300, DG400, DG500 were evolved volatiles of DG pyrolysis at 300, 400, 500 � C). 8

Z. Zhang et al.

Biomass and Bioenergy 135 (2020) 105525

Fig. 9. Organic compounds classification of evolved volatiles from RH and RHDG pyrolysis at 300 � C, 400 � C, and 500 � C. (RH300, RH400, RH500 have evolved volatiles of RH pyrolysis at 300, 400, 500 � C; DG300, DG400, DG500 were evolved volatiles of DG pyrolysis at 300, 400, 500 � C).

3.5.3. Organic compounds distribution of pyrolysis evolved volatiles Organics classification of evolved volatiles from RH and RHDG py­ rolysis at 300 � C, 400 � C, and 500 � C were illustrated in Fig. 9, and pyrolysis volatiles was divided into 11 categories according to their characteristics of functional groups. The most abundant of the pyrolysis products of the RH was Phenols (29.95%, 27.15% and 30.43% at 300 � C,400� Cand 500 � C) and furans (22.24%,18.61% and 17.92% at 300 � C,400� Cand 500 � C). Phenols were mainly the product of lignin py­ rolysis, and furan includes benzofurans and furans, benzofurans are mostly lignin pyrolysis products, but furan is primarily the product of cellulose and hemicellulose pyrolysis [48–50]. The highest content in the pyrolysis volatiles of RHDG was acids (55.66%,39.29% and 37.04% at 300 � C,400 � C and 500 � C), which were mainly caused by the evap­ oration of fatty acids in fermentation residues, and the acid content decreased with the increase of pyrolysis temperature, might be attrib­ uted to the pyrolysis of hydrocarbon generation and carbon dioxide. The contents of alcohols and esters in the pyrolysis products of RHDG were also higher than those of RH pyrolysis products because these two kinds of organics were also by-products in the fermentation process of distilled liquor. Hydrocarbons in evolved volatiles increased for both RHDG and RH with the increase of pyrolysis temperature, and hydrocarbons in products from RHDG (0.00%, 1.82% and 3.45% at 300 � C,400� Cand 500 � C) was relatively higher than those from RH (0.00%, 0.76% and 1.01% at 300 � C,400� Cand 500 � C), because RHDG contains more fatty acids, which include longer C–C bonds than cellulose and hemicellulose.

(2018YFC1901102). The authors declare no conflict of interest. References [1] Y. Kim, et al., Effect of compositional variability of distillers’ grains on cellulosic ethanol production, Bioresour. Technol. 101 (14) (2010) 5385–5393. [2] D.J. Cookman, C.E. Glatz, Extraction of protein from distiller’s grain, Bioresour. Technol. 100 (6) (2009) 2012–2017. [3] Y. Kim, et al., Composition of corn dry-grind ethanol by-products: DDGS, wet cake, and thin stillage, Bioresour. Technol. 99 (12) (2008) 5165–5176. [4] L. Wang, et al., Preparation of carbon black from rice husk by hydrolysis, carbonization and pyrolysis, Bioresour. Technol. 102 (17) (2011) 8220–8224. [5] X. Yu, et al., The integrated production of microbial lipids and bio-SiO2 from rice husks by an organic electrolytes pretreatment technology, Bioresour. Technol. 153 (2014) 403–407. [6] Y. Qian, J. Zhang, J. Wang, Pressurized pyrolysis of rice husk in an inert gas sweeping fixed-bed reactor with a focus on bio-oil deoxygenation, Bioresour. Technol. 174 (2014) 95–102. [7] S. Zhang, et al., Effects of water washing and torrefaction on the pyrolysis behavior and kinetics of rice husk through TGA and Py-GC/MS, Bioresour. Technol. 199 (2016) 352–361. [8] B. Biswas, et al., Pyrolysis of agricultural biomass residues: comparative study of corn cob, wheat straw, rice straw and rice husk, Bioresour. Technol. 237 (2017) 57–63. [9] F. Liang, et al., Investigating pyrolysis characteristics of moso bamboo through TGFTIR and Py-GC/MS, Bioresour. Technol. 256 (2018) 53–60. [10] A.C.M. Loy, et al., Thermogravimetric kinetic modelling of in-situ catalytic pyrolytic conversion of rice husk to bioenergy using rice hull ash catalyst, Bioresour. Technol. 261 (2018) 213–222. [11] X. Kai, et al., Study on the co-pyrolysis of rice straw and high density polyethylene blends using TG-FTIR-MS, Energy Convers. Manag. 146 (2017) 20–33. [12] T. Chen, J. Zhang, J. Wu, Kinetic and energy production analysis of pyrolysis of lignocellulosic biomass using a three-parallel Gaussian reaction model, Bioresour. Technol. 211 (2016) 502–508. [13] D. Chen, et al., Bamboo pyrolysis using TG–FTIR and a lab-scale reactor: analysis of pyrolysis behavior, product properties, and carbon and energy yields, Fuel 148 (2015) 79–86. [14] J. Zhao, et al., Thermal degradation of softwood lignin and hardwood lignin by TGFTIR and Py-GC/MS, Polym. Degrad. Stabil. 108 (2014) 133–138. [15] N. Gao, et al., TG–FTIR and Py–GC/MS analysis on pyrolysis and combustion of pine sawdust, J. Anal. Appl. Pyrol. 100 (2013) 26–32. [16] S. Wang, et al., Mechanism research on cellulose pyrolysis by Py-GC/MS and subsequent density functional theory studies, Bioresour. Technol. 104 (2012) 722–728. [17] S. Vyazovkin, et al., ICTAC Kinetics Committee recommendations for collecting experimental thermal analysis data for kinetic computations, Thermochim. Acta 590 (2014) 1–23. [18] T. Ozawa, A new method of analyzing thermogravimetric data, Bull. Chem. Soc. Jpn. 38 (11) (1965) 1881–1886. [19] J.H. Flynn, L.A. Wall, General treatment of the thermogravimetry of polymers, J. Res. Nat. Bur. Stand. Sect. A Phys. Chem. 70A (6) (1966) 487. [20] T. Sunose, T. Akahira, Method of determining activation deterioration constant of electrical insulating materials, Res. Rep. Chiba Inst. Technol. 16 (1971) 22–31. [21] Z. Ma, et al., Determination of pyrolysis characteristics and kinetics of palm kernel shell using TGA–FTIR and model-free integral methods, Energy Convers. Manag. 89 (2015) 251–259. [22] E. Apaydin-Varol, S. Polat, A.E. Putun, Pyrolysis kinetics and thermal decomposition behavior of polycarbonate - a TGA-FTIR study, Therm. Sci. 18 (3) (2014) 833–842.

4. Conclusions RHDG in this study is biomass that has RH as their raw material, but the physicochemical properties, thermal decomposition kinetics and components of evolved volatiles are entirely different. RHDG had higher activation energy than RH and other biomasses, evolved volatiles from RHDG pyrolysis contained more diesel range organics and higher H/C and O/C ratios, while the rice husk pyrolysis volatiles contained more gasoline range organic compounds and lower H/C and O/C ratios. The difference between RH and RHDG should be fully considered when considering the utilization of RHDG and rice husks through pyrolysis techniques. In the future research, efforts should be focused on the catalysis or co-pyrolysis methods to reduce the pyrolysis activation en­ ergy of RHDG, increase the content of gasoline range products and reduce H/C and O/C ratios to improve the in-site bio-energy utilization and conversion efficiency of RHDG. Acknowledgments The authors greatly acknowledge the financial support from the National Key Research and Development Program of China 9

Z. Zhang et al.

Biomass and Bioenergy 135 (2020) 105525 [37] P.L.M. Barreto, A.T.N. Pires, V. Soldi, Thermal degradation of edible films based on milk proteins and gelatin in inert atmosphere, Polym. Degrad. Stabil. 79 (1) (2003) 147–152. [38] V. Pasangulapati, et al., Effects of cellulose, hemicellulose and lignin on thermochemical conversion characteristics of the selected biomass, Bioresour. Technol. 114 (2012) 663–669. [39] L. Wang, et al., Thermal degradation kinetics of distillers grains and solubles in nitrogen and air, Energy Sources, Part A Recovery, Util. Environ. Eff. 31 (10) (2009) 797–806. [40] C. Branca, C. Di Blasi, Thermogravimetric analysis of the combustion of dry distiller’s grains with solubles (DDGS) and pyrolysis char under kinetic control, Fuel Process. Technol. 129 (2015) 67–74. [41] S. Singh, C. Wu, P.T. Williams, Pyrolysis of waste materials using TGA-MS and TGA-FTIR as complementary characterisation techniques, J. Anal. Appl. Pyrol. 94 (2012) 99–107. [42] Y. Qiao, et al., Thermal behavior, kinetics and fast pyrolysis characteristics of palm oil: analytical TG-FTIR and Py-GC/MS study, Energy Convers. Manag. 199 (2019), 111964. [43] N. Dayan, L. Kromidas, Formulating, Packaging, and Marketing of Natural Cosmetic Products, John Wiley & Sons, 2011. [44] Q. Shu, et al., Predicting the viscosity of biodiesel fuels based on the mixture topological index method, Fuel 86 (12–13) (2007) 1849–1854. [45] C.J. Paradis, Characterization of Anaerobic Natural Attenuation of Petroleum Hydrocarbons, University of California, Davis, 2013. [46] L. Patil, et al., Thermocatalytic degradation of high density polyethylene into liquid product, J. Polym. Environ. 26 (5) (2018) 1920–1929. [47] D.P. Serrano, et al., Hierarchical ZSM-5 zeolites synthesized by silanization of protozeolitic units: mediating the mesoporosity contribution by changing the organosilane type, Catal. Today 227 (2014) 15–25. [48] R. Lou, S. Wu, G. Lv, Effect of conditions on fast pyrolysis of bamboo lignin, J. Anal. Appl. Pyrol. 89 (2) (2010) 191–196. [49] X. Lin, et al., Fast pyrolysis of four lignins from different isolation processes using py-GC/MS, Energies 8 (6) (2015) 5107–5121. [50] C. Zhao, E. Jiang, A. Chen, Volatile production from pyrolysis of cellulose, hemicellulose and lignin, J. Energy Inst. 90 (6) (2017) 902–913. [51] P. Basu, Biomass Gasification, Pyrolysis and Torrefaction: Practical Design and Theory, Academic press, 2018. [52] G. Teri, L. Luo, P.E. Savage, Hydrothermal treatment of protein, polysaccharide, and lipids alone and in mixtures, Energy Fuels 28 (12) (2014) 7501–7509.

[23] Z. Zhang, et al., Pyrolysis characteristics, kinetics, and evolved gas determination of chrome-tanned sludge by thermogravimetry–Fourier-transform infrared spectroscopy and pyrolysis gas chromatography-mass spectrometry, Waste Manag. 93 (2019) 130–137. [24] S. Vyazovkin, et al., ICTAC Kinetics Committee recommendations for performing kinetic computations on thermal analysis data, Thermochim. Acta 520 (1–2) (2011) 1–19. [25] J. Wang, et al., Catalytic fast co-pyrolysis of bamboo residual and waste lubricating oil over an ex-situ dual catalytic beds of MgO and HZSM-5: analytical PY-GC/MS study, Energy Convers. Manag. 139 (2017) 222–231. [26] K. Liu, Chemical composition of distillers grains, a review, J. Agric. Food Chem. 59 (5) (2011) 1508–1526. [27] J.C.C. Freitas, F.G. Emmerich, T.J. Bonagamba, High-resolution solid-state NMR study of the occurrence and thermal transformations of silicon-containing species in biomass materials, Chem. Mater. 12 (3) (2000) 711–718. [28] M.G. Jackson, The alkali treatment of straws, Anim. Feed Sci. Technol. 2 (2) (1977) 105–130. [29] M.E.A.J. Lee, Adsorption characteristics of arsenic and phosphate onto iron impregnated biochar derived from anaerobic granular sludge, Kor. J. Chem. Eng. 35 (7) (2018) 1409–1413. [30] H. Yang, et al., In-depth investigation of biomass pyrolysis based on three major Components: hemicellulose, cellulose and lignin, Energy Fuels 20 (1) (2006) 388–393. [31] J.C.O. Santos, et al., Thermal stability and kinetic study on thermal decomposition of commercial edible oils by thermogravimetry, J. Food Sci. 67 (4) (2002) 1393–1398. [32] D. Chen, J. Zhou, Q. Zhang, Effects of heating rate on slow pyrolysis behavior, kinetic parameters and products properties of moso bamboo, Bioresour. Technol. 169 (2014) 313–319. [33] H. Yu, et al., Thermogravimetric analysis and kinetic study of bamboo waste treated by Echinodontium taxodii using a modified three-parallel-reactions model, Bioresour. Technol. 185 (2015) 324–330. [34] L. Burhenne, et al., The effect of the biomass components lignin, cellulose and hemicellulose on TGA and fixed bed pyrolysis, J. Anal. Appl. Pyrol. 101 (2013) 177–184. [35] W. Chen, P. Kuo, Isothermal torrefaction kinetics of hemicellulose, cellulose, lignin and xylan using thermogravimetric analysis, Energy 36 (11) (2011) 6451–6460. [36] P.T. Williams, S. Besler, Thermogravimetric analysis of the components of biomass, in: Advances in Thermochemical Biomass Conversion, Springer, 1993, pp. 771–783.

10