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Thermogravimetric characterization and pyrolysis of soybean hulls
Bioresource Technology Reports 6 (2019) 183–189
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Thermogravimetric characterization and pyrolysis of soybean hulls a
a,⁎
a
Jose Luis Toro-Trochez , Eileen Susana Carrillo-Pedraza , Diana Bustos-Martínez , Francisco José García-Mateosb, Ramiro Rafael Ruiz-Rosasb, José Rodríguez-Mirasolb, Tomás Corderob a b
T
Universidad autónoma de Nuevo León, Facultad de Ciencias Químicas, Av Universidad s/n, Cd. Universitaria, San Nicolás de Los Garza, Nuevo León C.P 64450, Mexico Universidad de Málaga, Facultad de Ciencias, Campus de Teatinos, 29010 Málaga, Spain
ARTICLE INFO
ABSTRACT
Keywords: Pyrolysis Soybean hulls Thermogravimetry GC–MS
This study focuses on the evaluation of soybean hulls as an alternative to obtain biofuels and bioproducts through pyrolysis. The study was carried out in three stages: 1) The physicochemical characteristics of the raw material were determined, 2) Yields of different pyrolysis products were obtained at 350, 400 and 600 °C; 3) The different obtained products (biochar, bio-oil and non-condensable gases) were characterized. The results obtained revealed that the soybean hulls used in this study present a 76% volatile mater and 2.3% ash. The highest pyrolysis yields obtained for bio-oil (40%) and non-condensable gases (36%) were achieved at 600 °C. The main compounds identified in the aqueous phase of bio-oil were acetic acid, furfural, furfural alcohol, 1,2-cyclopentadiene, among others. The organic phase of bio-oil presented a calorific power of 23.7 MJkg−1. Calorific power in the ranges 19–21 MJm−3 and 26.6–28.1 MJkg−1 were obtained for the biogas and the biochar obtained at different temperatures, respectively.
1. Introduction Biomass is a non-expensive, renewable source of energy with reduced environmental impacts. It can be treated through thermochemical and biochemical processes to obtain electrical, thermal or chemical energy and other chemical products (Bridgwater, 2003). The product distribution depends on the composition of principal biomass compounds and the type of thermal conversion process. The main components of biomass are cellulose (C6 polymers), hemicellulose (predominantly C5 polymers but including C6 species) and lignin (various phenylpropane units). These biopolymers have distinctive thermal behaviors and decompose at different temperature ranges, producing a variety of chemical compounds (aromatic and aliphatic hydrocarbons, organic alcohols, ethers and acids, between others) along with noncondensable gases (hydrogen, H2O, CO, CO2 and C2-C5 HCs) and solid residue (char) (Santos et al., 2010)(Collard and Blin, 2014). The chemical compounds can be recovered as added value products or processed to decrease oxygen content, enabling their use as fuel (Goyal et al., 2008). Biomass from agroindustry waste is an economic energy source through thermal conversion process. More specifically, the world production of soybean for the campaign 2015/2016 was 315.8 million tons (USDA, 2018). Soybean hulls represent 5% of the weight of the crop ⁎
and are currently managed as wastes. Nowadays, soybean hulls and straw are either landfilled, used as animal feed or burned in the fields after harvest, which represents environmental problems (Qing et al., 2017). Revalorization of soybean hulls waste through a thermochemical process like pyrolysis is proposed as an alternative use, at the same time reducing the negative effects of current uses on the environment (Giri et al., 2017). Pyrolysis is a well-known and widely used thermochemical conversion process that can be used to transform biomass and waste into valuable chemicals and energy using these techniques. The main pyrolysis products are biochar, biofuel and biogas (Yaman, 2004) (Mohan et al., 2006). The differences in structure and chemical composition of the raw biomass, temperature, heating rate and residence time are some of the key factors involved in the yields and the distribution of the final pyrolysis products (Guedes et al., 2018). In general, yield of the gas fraction increases with temperature, while bio-oil yield shows a maximum at intermediate temperatures, and the solid yield decreases with temperature. As temperature increases, the biopolymers decompose in a charred solid, a collection of organic macromolecules of varied size and functionalities, that can be recovered as condensable products, and light hydrocarbons, oxygenated hydrocarbons, and CO, CO2 and H2 that forms the non-condensable gaseous fraction. Further increase of the temperature degrades some of the macromolecules from the solid
Corresponding author. E-mail address: [email protected] (E.S. Carrillo-Pedraza).
https://doi.org/10.1016/j.biteb.2019.02.009 Received 7 November 2018; Received in revised form 11 February 2019; Accepted 12 February 2019 Available online 15 February 2019 2589-014X/ © 2019 Published by Elsevier Ltd.
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fraction, producing chemical compounds that enrich the bio-oil fraction. At even higher temperatures, the char experiences aromatic condensation and dehydrogenation reactions rendering the formation of H2O, H2 and CO, while the chemicals from the bio-oil fraction are thermally cracked through secondary reactions, producing non-condensable gases and a decrease in the liquid yield (Fabbri and Torri, 2016) (Di Blasi, 2008). Previous research has been focused on the bio-oil composition and yield obtained from the pyrolysis of industrial soybean waste (L. Zhang et al., 2017) (Oliveira et al., 2015) (Uzun et al., 2006), but little is known about the other bioproducts (i.e. solid and gas fractions). The characterization of all these products is the first step needed for setting up a biorefinery process based on pyrolysis of soybean. Therefore, this work aims to provide a full characterization of all the pyrolysis fractions of soybean hulls. Yields for the different fractions are calculated at different pyrolysis temperatures, while the physicochemical, compositional and calorimetric characterization of the bio-oil, biogas and biochar are reported.
to −30 °C), using limonene-dry CO2 mixture as a refrigerant, while the biochar was recovered from the tubular reactor. 1.4. Analysis of bio-oil composition by GC/MS Bio-oil was recovered from the condenser at the outlet of the pyrolysis reactors. The full bio-oil mixture was recovered and solved in acetone in the case of laboratory scale experiments, while the bio-oil produced from the pilot scale pyrolysis was segregated into aqueous and organic fractions that were extracted using acetone (analytical grade, Sigma-Aldrich) and dicloromethane (analytical grade, SigmaAldrich) in 1:10 and 1:5 volume ratios, respectively. All the resulting mixtures were analyzed by Gas Chromatography–Mass Spectrometry (GC/MS), using an 7000D GC/MS Triple Quad (Agilent Technologies, USA), equipped with an Agilent DB-624 column (60 m × 0.250 mm × 1.4 μm) and HP-5ms column (30 m × 0.250 mm × 0.25 μm), flame ionization detector (FID) and mass spectrometer detector (MS). The following settings were used: 250 °C injector temperature; carrier gas helium 1 mL/min; temperature program for DB-624: 125 °C for 10 min, heating rate of 10 °C min−1 up to 250 °C, holding time of 20 min; temperature program for HP-5ms: 50 °C, heating rate of 8 °Cmin−1 up to 200 °C for 2 min; FID temperature, 300 °C. Ions were detected in full scan mode (mass range from 15 to 400 m/z), electronic impact of 70 eV. Identification of compound was achieved by comparing the mass spectra with the NIST MS Search 2.0 mass spectral library. The separation of the two phases of the bio-oil was done by simple decanting Higher heating value (HHV) in organic phase bio-oil was also determined using a Parr oxygen bomb calorimeter (Fisher Scientific).
1.1. Materials preparation Soybean hulls currently managed as industrial waste were obtained from an oil company from the North of Mexico region. Prior to use, the biomass waste was milled and sieved to a particle size lower than 0.2 mm. The sieved samples were dried at 105 °C in an air-dry oven for 12 h to remove the moisture, and then stored in stopped bottles for further use. 1.2. Ultimate, proximate and thermogravimetric analyses The elemental analysis was recorded in a PerkinElmer 2400CHNS analyzer to determine the mass fractions of carbon, hydrogen, nitrogen and sulfur. The composition of the biopolymers was obtained by the Van Soest methodology (Loredo Medrano et al., 2016) (L. Zhang et al., 2017). The pyrolysis behaviour of soybean hulls was studied in a thermogravimetric analyzer (Q500, TA Instruments, USA). In the thermogravimetric analysis (TGA) experiment, 10 mg of sample (particle size between 250 and 425 μm) was loaded into the platinum container, then heated from ambient temperature to 900 °C at a heating rate of 20 °Cmin−1. The carrier gas of the test was nitrogen with a flow rate of 60 mL/min. The same equipment was employed for determining the proximate analysis of soybean hulls.
1.5. Analysis of the non-condensable gases Analysis of the gas fraction from the lab-scale pyrolysis experiments was performed at the condenser outlet. CO and CO2 evolution were determined by a non-dispersive infrared gas analyzer (NDIR) (Siemens Ultramat 22), whereas the evolution rate of hydrocarbons (methane, ethane, ethene, propane and propylene) were determined by GC in a Perkin Elmer Autosytem system equipped with a packed column (Hayasep-D 100–120 mesh, PE) Column and FID detector. The quantification of the hydrocarbons (methane, ethane, ethene, propane and propylene) in the non-condensable gas fraction of the labscale experiments were carried out in a Schimadzu GC-17A GC equipped with a Carboxen 1000 packed column (60/80) (4.57mx3.17 mm) with FID and TCD as detectors. Identification and absolute quantification were performed by external calibration with a standard commercial mixture of gases obtained from White Marines. Hydrogen evolution was quantified in a HP Hewlett Packard 5890 Series II GC equipped with a capillary column GS-GasPro 113-4362 J & M (60 m, 0.320 mm) and TCD detector. Identification and absolute quantification was performed by external calibration with a standard commercial of Infra mixture of Hydrogen 29.8% ± 1.0%.
1.3. Pyrolysis experiments The analytical pyrolysis experiments were carried at lab and pilot plant scales. Lab scale pyrolysis were performed under nitrogen atmosphere (flow rate: 150 mL/min STP) using a vertical laboratory scale tubular reactor (diameter: 2.5 cm) loaded with 5 g of samples from the Section 1.1. The experiments were carried out under atmospheric pressure at five different temperatures (300, 350, 400, 500 and 600 °C) at heating rate of 10 °Cmin−1. The bed temperature was tracked using an additional thermocouple. The solid residue (i.e. Biochar) is recovered from the tubular reactor at the end of the experiment, whereas the bio-oil is obtained at the reactor outlet by condensation using refrigerated acetone (e15C) as coolant. The yields of the solid and liquid fractions are obtained by weight, while that of the non-condensable fraction is obtained by difference. All yields were expressed dry basis. The pyrolysis experiments at pilot scale were carried out under nitrogen atmosphere (flow rate: 25 Lmin−1 STP) in a 4 L tubular reactor (4 cm radius x 80 cm long) with 500 g of samples from the Section 1.1. The reactor was heated in an electric oven brand THERMOLYNE, model 79,400. Temperature measurements were acquired from the bed for a accurately controlling the reactor temperature. The experiments were carried out at final temperature of 600 °C using a heating rate of 25 °Cmin−1. The bio-oil is obtained by condensation in a range of (−20 °C
1.6. Analysis of the solid fraction by infrared spectroscopy The FTIR spectra were recorded in a spectrophotometer (Spectrum One, Perkin Elmer, USA) to characterize the functional groups of the soybean hulls and biochars obtained in the pyrolysis at different temperatures (300 °C, 400 °C, 500 °C and 600 °C). Prior to the FTIR experiment, the samples were pelletized using KBr as binder. The spectrum scan frequency was 10 times per minute at a resolution of 4 cm−1 and with spectrum range from 4000 cm−1 to 500 cm−1. The gross calorific power was calculated by Dulong formula (Pehlivan et al., 2016):
QGCV = 33.83C + 144.3 H 184
O 8
(MJkg 1)
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2. Results and discussion
region due to the loss of oxygen functional groups from the cellulose and hemicellulose and the side chains of lignin (L. Zhang et al., 2017). The chemical decomposition of these functional groups are likely to produce CO and CO2 (Zhao et al., 2016). The largest changes in these bands were observed at temperatures above 400 °C, probably as a result of the complete decomposition of the cellulose and hemicellulose, which own > 70% of the composition of the raw material. From 500 °C, only small changes are observed in these bands. The bands at 1649 and 1429 cm−1 do not decrease throughout the temperature range in Fig. S2, possibly because these bands are associated with the stretching of highly stable functional groups in lignin (Zhao et al., 2016)(Z. Wang et al., 2016), a biopolymer that is known to be thermally degraded in a wide range of temperatures, reaching temperatures up to 900 °C (S. Wang et al., 2017). Changes on the bands of FTIR analysis as temperature increases confirmed the thermal degradation of diverse functional groups with temperature. Along with the TG analysis and the proximate and Van Soest analyses, this information provide that the most suitable operation conditions for the pyrolysis process should be in the range of 300 °C to 600 °C, where temperature should have a critical effect in obtaining different products and yields.
2.1. Characterization of the feedstock The proximate analysis determined that the soybean hulls contain high contents of volatile material (M.V) with 82.2 ± 0.6% and low concentration of inorganic material (ash) 2.5 ± 0.1%, which makes it a feasible material for obtaining energy through a thermal pyrolysis process (Parascanu et al., 2017). According to the Van Soest method, the composition of the soybean hulls was 18.5 ± 0.3% hemicellulose, 52.3 ± 0.5% cellulose and 3.7 ± 0.4% lignin. The values obtained from the elemental analysis and composition of biopolymer are similar to those reported by other researches (Liu and Li, 2017), (Oliveira et al., 2015). The small deviations observed in the composition of the feedstock can be attributed to the properties and characteristics of the soil, the amount of rainfall among other factors typical of each harvest (De la Cruz, 2012). The concentrations of the biopolymers that form the soybean hulls and the physicochemical characteristics of the material (highly volatile material and low percentage of fixed coal) make it an attractive raw material to obtain bio products by thermal degradation by pyrolysis processes. Fig. S1 shows the TG and DTG profiles of the thermal decomposition of the soybean hulls in inert atmosphere. Three main weight loss stages are observed. The first weight loss (8%) located between 30 and 120 °C is attributed to the humidity of the material. In the second stage, hemicellulose and cellulose decomposition takes part from 200 to 414 °C. According to the DTG profile, where a main peak at 364 °C preceded by a small hump at 305 °C can be observed, two different decomposition subregions have been identified, i.e. the first one ranging from 200 °C to 315 °C (weight loss of 18.3%) and the second one starting at 315 °C up to 415 °C (weight loss of 44%), that have been attributed in the past to the thermal decomposition of hemicellulose and cellulose (El-Sayed and Mostafa, 2014) (Johar et al., 2012). The large contribution of cellulose to the weight loss during pyrolysis is in agreement with the cellulose composition in the soybean hulls. The last stage starts at 415 °C, where the weight changes are associated to the thermal decomposition of the lignin happened throughout the temperature in the range of 200–700 °C (Pehlivan et al., 2016). In accordance to this measurement, the pyrolysis of the biomass waste was performed at temperatures ranging from 300 to 600 °C, a temperature range where the pyrolysis temperature is expected to have a large impact on the yield and composition of the different fractions. Fig. S2 shows the FTIR spectra of the soybean hulls and the biochar obtained at different pyrolysis temperatures. Absorption bands are found at several region owing to the presence of different functional groups, even though the intensity of these bands is affected by the temperature of the pyrolysis process. The absorption bands dominant in the region of 3430–3050 cm−1 are caused by OH stretch vibrations, associated with phenolic compounds, alcohols and acids functionalities present in the three biopolymers (Zhao et al., 2016) (Sequeiros and Labidi, 2017) and also water in the biomass (Johar et al., 2012). The absorption bands observed from 3000 to 2800 cm−1 comes from the CeH stretching vibration in methyl groups and aliphatic carbon (Zhao et al., 2016). The absorption in 1740 cm−1 is assigned to the stretching of the C] O in the functional groups in organic acids, ketones, aldehydes and esters, that are present in cellulose and hemicellulose (Z. Wang et al., 2016) (Oun and Rhim, 2016). The band in 1649 cm−1 is produced by stretching of the C]C associated with vibrations of aromatic compounds that conform the lignin (Yang et al., 2007). The band at 1429 cm−1 is associated with methoxyl functional group in the lignin by CeH (Lopez-Velazquez et al., 2013) (Pino, 2013) and the intense band at 1045 is caused to the bond eCeOeH or eCeOeR of alcohols or esters, present in the three biopolymers. As previously mentioned, Fig. S2 shows that increase in pyrolysis temperature produce a decrease of the bands in the 3430–1740 cm−1
2.2. Evolution of the distribution of pyrolysis products with temperature Laboratory scale pyrolysis experiments were conducted at different temperatures, with the yield of each pyrolysis fraction being determined at the end of each pyrolysis run. The relationship between temperature and the distribution of pyrolysis products is shown in the Fig. 1. In overall, a good agreement is found between the yield of the solid fraction and the TGA results (see Fig. S1). At low temperature, the solid pyrolysis fraction was the prevailing product, and non-condensable gases were the main decomposition product of biomass at this temperature, showing a weight ratio of non-condensable to condensable (i.e. bio-oil) gases of 3.6:1. At 350 °C, cellulose and hemicellulose decomposition takes part in a large extend, producing a large drop of the biochar yield. As a result of the decomposition of these biopolymers, bio-oil is obtained as the main product, increasing in 22% the share of bio-oil in the product distribution. At this point, weight ratio of non-condensable gases to bio-oil is around 1:1. Further increase of pyrolysis temperature to 400 °C enlarged these tendencies, i.e. increase of the bio-oil and non-condensable fractions at the cost of the solid fraction. Changes in the distribution of products at this temperature range are small, since all the homocellulose have been decomposed at lower temperatures and the amount of lignin in soybean hulls is low. The yields obtained in the pilot scale pyrolysis were 39.4% for the bio-oil, 26.6% for the biochar and 34.0% for the non-condensable gases, showing only minor deviations from the results attained during the lab scale pyrolysis at 600 °C. 2.3. Composition of the non-condensable gas fraction The evolution of non-condensable gases was analyzed during the course of a pyrolysis run up to 650 °C. These gases consisted mainly in methane (CH4), hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and other light hydrocarbons such as ethane (C2H6), ethene (C2H4), propane (C3H8) and propene (C3H6). Fig. 2 shows the evolution profiles of these gases. It can be seen that CO and CO2 are formed at low temperatures, between 220 and 400 °C, the same temperature range for the thermal decomposition of hemicellulose and cellulose. These biopolymers have large amounts of oxygenated compounds (Loredo Medrano et al., 2016), in the form of carbonyl and carboxyl groups which by thermal degradation are mostly decomposed releasing CO and CO2, respectively. Zhao et al (Zhao et al., 2016) obtained similar results by performing cellulose pyrolysis. The presence of CO and CO2 at temperatures above 400 °C is possibly related to the breakdown of phenolic and 185
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Fig. 1. Product performance pyrolysis soybean hulls.
methoxy groups present in the lignin structure (Parascanu et al., 2017), and also to the catalytic gasification of the biochar by alkali compounds that are naturally present in all biomass (Bensakhria, 2012) (Shadman, 1986) (Feng et al., 2018). In any case, the evolution of these species decreases with the temperature. Methane evolution starts being relevant at temperatures higher than 300 °C, being the main gaseous product at 500 °C. Methane release is followed by the evolution of some other light C2-C3 hydrocarbons (HCs). These hydrocarbons are only formed at temperatures between 400 and 550 °C, whereas methane is still being released at temperatures higher than 600 °C. The formation of C2-C3 HCs is attributed to the secondary reactions in gaseous phase of organic compounds coming
from the pyrolysis of cellulose and lignin (Parascanu et al., 2017). The formation of CH4 can be related to thermal degradation of aromatic compounds and to the rupture of methoxy groups (O-CH3) associated of lignin, that mainly occurs at temperatures below 600 °C (Parascanu et al., 2017). At higher temperatures > 600 °C, the presence of CH4 is associated with the decomposition of lignin, which is known to take part at a wide degradation range of temperature (200 °C – 900 °C)(RuizRosas et al., 2010). In accordance to these findings, the total gas released during pyrolysis was collected at 400, 500 and 600 °C (where lower concentrations of CO and CO2 are expected) and the composition was analyzed by GC. For these measurements, hydrogen was also determined. The
Fig. 2. Evolution rate of non-condensable gases during the pyrolysis of soybean hulls. 186
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compositional results confirm that methane is the main product at 500 °C. Further increase of the pyrolysis temperature to 600 °C produces a decrease in the formation of C2-C3 HCs, while H2 evolution starts to be relevant. Since aromatic dehydrogenation of char usually proceeds at higher temperatures, the hydrogen evolution observed at this point is likely to be produced as a result of the thermal cracking of light HCs (Ly et al., 2016). The higher heating value (HHV) of the gas mixture increase in temperature range of 500 and 600 °C, reaching values of 20.3 and 24.3 MJm−3, respectively. The calorific power of the pyrolytic gases in the soybean hulls increases at 500 °C and also at 600 °C as a consequence of the predominance of methane (32.0 and 38.1% v/v, respectively), the light HCs in the mixture (32.0 and 38.1% v/v), and because of the formation of H2.(0.4 and 10.2% v/v). These compositions are reported discounting the nitrogen share and in dry basis. HHV in the range of 19–25 MJm−3 could be employed as synthesis gas and as fuel for gas engines and turbines (Pino, 2013).
Table 2 Main compounds and composition (%) as determined by GC/MS. Classification
Acids Furans
Alcohols Ketones
2.4. Proximate analysis of biochar Phenols
The biochar obtained at different pyrolysis temperatures was recovered and analyzed by FTIR (reported in Section 2.3.), ultimate and proximate analysis. The decrease of volatile matter in the solid as pyrolysis temperature rises is in accordance with the increase in the amount of non-condensable gases and oils observed in Section 2.4. analyzed in previous sections. Ash content increases because of the high thermal stability of the inorganic matter found in the soybean hulls. Ultimate analysis confirmed the loss of oxygen due to the formation of CO, CO2 and other oxidative compounds. As expected, the main oxygen loss is detected at 350 °C, a temperature high enough to decompose most of the homocellulose in soybean hulls. Hydrogen content remains constant up to 600 °C, where a noticeable drop is observed. The decrease in hydrogen amount agrees with the results obtained in the analysis of the non-condensable fraction, where a higher concentration of H2 was obtained. The HHV associated to the composition of the biochar has been estimated and reported in Table 1. The HHV increases dramatically at 350 °C. The maximum heating value is achieved at 400 °C. From that point, negligible changes in HHV are observed.
Aldehydes Carbohydrates
Bio-oils from the pyrolysis experiments at lab scale were recovered, solved in acetone and analyzed by GC–MS. Table 2 shows the identified compounds with > 1% of total area and a reliability index during their identification > 80%. The analysis has been able to identify 24 compounds. Acetic acid, furfural alcohol, furfural and the family of methylciclopentones where the main components of bio-oil, accounting together for ca. 60% of the total composition. Given the large number of compounds, they have been grouped into acids, furans, alcohols, ketones, phenols, aldehydes and carbohydrates. The composition of biooil in all these families of compounds and how their relative abundance change with temperature have been summarized in Fig. 3. Between these family of compounds, acidic ones were those Table 1 Characterization of biochar soybean hulls.
Ultimate analysis C H O N HHVa a
wt% wt% wt% wt% MJ/kg
Temperature (°C) 300
350
400
600
54.3 5.8 32.5 1.9 20.1
66.1 5.1 17.4 2.6 26.6
69.5 5.0 10.3 2.7 28.9
73.5 3.0 6.1 2.4 28.1
Acetic acid Propenoic acid Furfural 3-Methylfuran Fulfural alcohol 2(3H)-Furanone 2-Furaldehyde, 5-methyl2-Hexadecanol Cyclopentanol 2-Propanone, 1-acetoxy2-Cyclopentenone, 2-methyl1,2-Cyclopentadione Cyclopentone, 1-methyl2-Cyclopenten-1-one, 2,3-dimethyl2-Cyclopenten-1-one, 2-hydroxy- 3ethyl 2-butanone,1-hydroxy Phenol Phenol, 2-methylPhenol, 3-methylGuaiacol Butanedial Glucose Levoglucosan 1,4,3,6-Dianhydro-D-glucopyranose
Temperature °C 300
350
400
600
28.7 0.6 9.9 1.0 12.2 1.3 2.4 0.1 0.8 3.2 0.5 3.5 2.5 1.1 2.4
23.3 1.7 4.0 1.9 11.7 1.9 0.9 0.1 0.8 2.2 1.1 3.9 4.5 1.0 2.7
22.9 1.3 3.9 2.1 11.4 1.8 0.9 0.1 0.7 2.2 1.2 4.1 4.8 0.9 2.7
23.6 1.1 4.1 2.2 11.7 1.6 1.0 0.1 1.1 2.4 1.5 3.9 4.2 1.0 2.5
1.2 0.6 0.5 0.0 1.5 4.1 1.2 0.3 0.1
1.1 0.9 0.3 0.9 1.9 3.6 1.0 1.1 0.7
1.1 1.2 0.3 0.9 1.6 3.5 1.3 1.4 1.5
1.2 1.3 0.4 1.6 1.0 3.8 1.2 1.0 0.8
showing the highest concentrations. Acid compounds formation in pyrolysis is attributed mainly to acetyl group present in Hemicellulose (Pino, 2013). Acetic acid is the specie with the highest concentrations across the temperature range of lab scale pyrolysis. Other species such as furans and ketones are also attributed to the degradation of hemicellulose, which explains why the higher concentrations of acids and furans found at 300 °C (see Fig. 3). When temperature increases, the percentage of these groups decreases owing to a large extend of pyrolysis liquids being produced from cellulose and lignin decomposition. It is important to highlight that acid groups maintain their highest share in concentration through the entire temperature range, probably as a result of the thermal decomposition of levoglucosan, a product generated during the depolymerization and thermal decomposition of cellulose (Oliveira et al., 2015). Substantial concentrations of aliphatic aldehydes were also obtained, especially butanedial, which is attributed to the decomposition of the open rings of the cellulose and hemicellulose. The production of similar acids and aliphatic aldehydes compounds have been reported from the pyrolysis of hemicellulose and cellulose mixtures and other biomasses (Wu et al., 2016) (Li et al., 2017). As temperature increases, higher percentages of phenols were obtained. This large share of phenolic compounds is attributed to thermal decomposition of lignin, whose structure is based in the combination of different phenylpropane units as building blocks (Rosas et al., 2014). Concentration of carbohydrates have an increasing share in bio-oil composition until temperature of 400 °C. Beyond this temperature, these carbohydrates are thermally degraded to simpler species (Dong et al., 2015)(B. Zhang et al., 2015). In addition, a similar degradation tendency is observed for acids and furans at the highest pyrolysis temperature, Fig. 3, confirming that oxygenated and large organic compounds are thermally cracked at high temperatures. As the composition of the pbtained pyrolytic oils are rich in acidic and oxygenated compounds, it would be necessary previous processing through hydrodeoxidation to improve their properties as fuel (CorderoLanzac et al., 2017) (Oliveira et al., 2015). However, compounds such as acetic acid, furfural alcohol, furfural, and 1,2 cyclopentadione have already been found in pyrolysis of soyben stalk and considered as added-value chemicals (L. Zhang et al., 2017).
2.5. Analysis of bio-oil generated from the pyrolysis
Units
Compound
Higher Heating Value. 187
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Fig. 3. Relative abundance of added value organic compounds in the bio-oil obtained at different temperatures during the laboratory scale experiments.
The bio-oil composition obtained at laboratory scale is similar to those reported in the pyrolysis of soybean hulls (Oliveira et al., 2015) and soybean crop residues (L. Zhang et al., 2017). The differences can possibly be attributed to longer reaction times, which produce higher concentrations of short-chain species, to the type of biomass and to the analytical conditions of the method utilized to solve the bio-oil prior to the GC–MS measurement. The bio-oil obtained from the pilot scale pyrolysis was also analized by means of GC–MS. factors such as faster heating rates and different nitrogen flow rates can affect the final composition of the product (Oliveira et al., 2015). In fact, the bio-oil obtained in pilot scale showed two phases, an aqueous ones. Both phases were recovered separately and analyzed by GC/MS. Tables 2 and 4 shows the most abundant compounds that has been identified with a reliability index > 90%. The aqueous phase of bio-oil showed as main components Furfural, acetone, phenols and C11-C14 hydrocarbons, Table 3. Aqueous phase of bio-oils is reported to be composed of water, oxygenated compounds with high and low molecular results obtained from GC/MS reported in Table 3. The absence of acetic acid and aldehydes on Table 3 is related to the use of dichloromethane as solvent in the GC/MS analysis (Oliveira et al., 2015). The main compounds of the organic phase of bio-oil are compiled in
Table 4 Main compounds identified in the organic phase of bio-oil.
Compound
Area (%)
Furfural
Furfural 2-Furaldehyde, 5-methyl Cyclopentanone, 4,4-dimethyl2-Cyclopenten-1-one, 2,3-dimethylPhenol, 2-methylPhenol, 3-methylPhenol, 3,4-dimethylPhenol, 2-methoxyUndecane Dodecane Tridecane Tetradecane
5.3 2.3 6.2 0.6 3.6 0.7 0.8 0.4 2.7 1.2 1.1 1.2
Acetone Phenols
Hydrocarbons
Compound
Area (%)
Ketones Furans Phenols
2-Cyclopentanone, 4,4-dimethyl Furfural Phenol Phenol, 3-methylp-Cresol Phenol, 2-methoxyTricosane Dodecene Tridecene Tetradeceno Docosane
5.7 6.6 10.8 0.6 1.4 1.4 6.6 6.6 5.3 3.1 2.1
Hydrocarbons
Table 4. The organic phase of bio-oil is considered to be a promising high-grade fuel candidate after proper upgrading, and can also be used for obtaining chemical compounds (Dhyani and Bhaskar, 2017). Larger composition in hydrocarbons is found in this phase, followed by phenols and furans. The appearance of a larger share of hydrocarbons in the bio-oil obtained at pilot scale pyrolysis could be related to the longer residence time of the gas, which leads to higher secondary reactions [6]. The phenols detected in the organic bio-oil can be used as precursors in different processes like the production of resins (Effendi et al., 2008), medicaments and additives (Vu et al., 2018). Obtaining phenol from biomass waste contributes to the environmental protection, given that the production of phenols nowadays involves processes where fossil fuels are utilized as raw material. The caloric power of the organic phase has been determined to be 23.7 MJkg−1, owing to the presence of aliphatic hydrocarbons obtained from the deoxygenation reactions, that seems to be favoured at the higher pyrolysis temperature and longer residence time of gaseous products.
Table 3 Main compounds identified in the polar fraction of bio-oil. Classification
Classification
3. Conclusions Soybean hulls pyrolysis yields on bioproducts have been obtained using laboratory scale and pilot scale pyrolysis at 600 °C, with only a 4% of difference, indicate the potential of the process scalability. A 76% 188
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volatile mater and 2.3% were observed at pyrolysis distribution products as bio-oil (40%) and non-condensable gases (36%) highest yields achieved at 600 °C. The aqueous phase of bio-oil were acetic acid, furfural, furfural alcohol, 1,2-cyclopentadiene. The organic phase of bio-oil presented a calorific value of 23.7 MJkg−1. Whiting 19–21 kJmol−1 and 26.6–28.1 kJkg−1 obtained for the biogas and the biochar obtained at different temperatures, respectively. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biteb.2019.02.009.
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Acknowledgements The authors would like to acknowledge the financial support of CONACYT and Facultad de Ciencias Químicas Universidad Autónoma de Nuevo León, The authors also acknowledge to the Universidad de Malaga for allowing the use of its facilities and equipment and the Spanish Ministry of Science, Innovation and Universities and FEDER (Project CTQ2015-68654-R) for the finantial support. References Bensakhria, Ammar, 2012. Influence of impregnated metal on the pyrolysis conversion of biomass constituents. J. Anal. Appl. Pyrolysis 95, 213–226. https://doi.org/10.1016/ j.jaap.2012.02.009. Bridgwater, A.V., 2003. Renewable fuels and chemicals by thermal processing of biomass. Chem. Eng. J. 91 (2–3), 87–102. https://doi.org/10.1016/S1385-8947(02)00142-0. Collard, François Xavier, Blin, Joël, 2014. A review on pyrolysis of biomass constituents: mechanisms and composition of the products obtained from the conversion of cellulose, hemicelluloses and lignin. Renew. Sust. Energ. Rev. 38, 594–608. https://doi. org/10.1016/j.rser.2014.06.013. (Elsevier). Cordero-Lanzac, Tomás, Palos, Roberto, Arandes, José M., Castaño, Pedro, RodríguezMirasol, José, Cordero, Tomás, Bilbao, Javier, 2017. stability of an acid activated carbon based bifunctional catalyst for the raw bio-oil hydrodeoxygenation. Appl. Catal. B Environ. 203, 389–399. https://doi.org/10.1016/j.apcatb.2016.10.018. De la Cruz, Miguel G., 2012. Fertilización foliar con potacio, calcio y silicio en fresa. In: Universidad Autonoma Chapingo. Dhyani, Vaibhav, Bhaskar, Thallada, 2017. A comprehensive review on the pyrolysis of lignocellulosic biomass. Renew. Energy 129, 695–716. https://doi.org/10.1016/j. renene.2017.04.035. (Elsevier Ltd). Di Blasi, Colomba, 2008. Modeling chemical and physical processes of wood and biomass pyrolysis. Prog. Energy Combust. Sci. 34 (1), 47–90. https://doi.org/10.1016/j.pecs. 2006.12.001. Dong, Qing, Zhang, Shuping, Li, Zhang, Ding, Kuan, Xiong, Yuanquan, 2015. Effects of four types of dilute acid washing on moso bamboo pyrolysis using Py-GC/MS. Bioresour. Technol. 185, 62–69. https://doi.org/10.1016/j.biortech.2015.02.076. (Elsevier Ltd). Effendi, A., Gerhauser, H., Bridgwater, A.V., 2008. Production of renewable phenolic resins by thermochemical conversion of biomass: a review. Renew. Sust. Energ. Rev. 12 (8), 2092–2116. https://doi.org/10.1016/j.rser.2007.04.008. El-Sayed, Saad A., Mostafa, M.E., 2014. Pyrolysis characteristics and kinetic parameters determination of biomass fuel powders by differential thermal gravimetric analysis (TGA/DTG). Energy Convers. Manag. 85, 165–172. https://doi.org/10.1016/j. enconman.2014.05.068. (Elsevier Ltd). Fabbri, Daniele, Torri, Cristian, 2016. Linking pyrolysis and anaerobic digestion (Py-AD) for the conversion of lignocellulosic biomass. Curr. Opin. Biotechnol. 38, 167–173. https://doi.org/10.1016/j.copbio.2016.02.004. (March. Elsevier Ltd). Feng, Dongdong, Zhang, Yu, Zhao, Yijun, Sun, Shaozeng, 2018. Catalytic effects of ionexchangeable K+ and Ca2+on rice husk pyrolysis behavior and its gas–liquid–solid product properties. Energy 152, 166–177. https://doi.org/10.1016/j.energy.2018. 03.119. (Elsevier Ltd). Giri, Germán F., Viarengo, Gastón, Furlán, Ricardo L.E., Suárez, Alejandra G., Véscovi, Eleonora Garcia, Spanevello, Rolando A., 2017. Soybean hulls, an alternative source of bioactive compounds: combining pyrolysis with bioguided fractionation. Ind. Crop. Prod. 105, 113–123. https://doi.org/10.1016/J.INDCROP.2017.05.005. ((October). Elsevier). Goyal, H.B., Seal, Diptendu, Saxena, R.C., 2008. Bio-fuels from thermochemical conversion of renewable resources: a review. Renew. Sust. Energ. Rev. 12 (2), 504–517. https://doi.org/10.1016/j.rser.2006.07.014. Guedes, Raquel Escrivani, Luna, Aderval S., Torres, Alexandre Rodrigues, 2018. Operating parameters for bio-oil production in biomass pyrolysis: a review. J. Anal. Appl. Pyrolysis 129, 134–149. https://doi.org/10.1016/j.jaap.2017.11.019. (November 2017. Elsevier). Johar, Nurain, Ahmad, Ishak, Dufresne, Alain, 2012. Extraction, preparation and characterization of cellulose fibres and nanocrystals from rice husk. Ind. Crop. Prod. 37 (1), 93–99. https://doi.org/10.1016/j.indcrop.2011.12.016. (Elsevier B.V.). Li, Kai, Zhang, Liqiang, Zhu, Liang, Zhu, Xifeng, 2017. Bioresource technology comparative study on pyrolysis of lignocellulosic and algal biomass using pyrolysis-gas chromatography/mass spectrometry. Bioresour. Technol. 234, 48–52. https://doi. org/10.1016/j.biortech.2017.03.014. (Elsevier Ltd). Liu, Hua-Min, Li, Hao-Yang, 2017. Application and conversion of soybean hulls. In: Soybean - The Basis of Yield, Biomass and Productivity, pp. 111–132. https://doi.
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Thermogravimetric characterization and pyrolysis of soybean hulls a
a,⁎
a
Jose Luis Toro-Trochez , Eileen Susana Carrillo-Pedraza , Diana Bustos-Martínez , Francisco José García-Mateosb, Ramiro Rafael Ruiz-Rosasb, José Rodríguez-Mirasolb, Tomás Corderob a b
T
Universidad autónoma de Nuevo León, Facultad de Ciencias Químicas, Av Universidad s/n, Cd. Universitaria, San Nicolás de Los Garza, Nuevo León C.P 64450, Mexico Universidad de Málaga, Facultad de Ciencias, Campus de Teatinos, 29010 Málaga, Spain
ARTICLE INFO
ABSTRACT
Keywords: Pyrolysis Soybean hulls Thermogravimetry GC–MS
This study focuses on the evaluation of soybean hulls as an alternative to obtain biofuels and bioproducts through pyrolysis. The study was carried out in three stages: 1) The physicochemical characteristics of the raw material were determined, 2) Yields of different pyrolysis products were obtained at 350, 400 and 600 °C; 3) The different obtained products (biochar, bio-oil and non-condensable gases) were characterized. The results obtained revealed that the soybean hulls used in this study present a 76% volatile mater and 2.3% ash. The highest pyrolysis yields obtained for bio-oil (40%) and non-condensable gases (36%) were achieved at 600 °C. The main compounds identified in the aqueous phase of bio-oil were acetic acid, furfural, furfural alcohol, 1,2-cyclopentadiene, among others. The organic phase of bio-oil presented a calorific power of 23.7 MJkg−1. Calorific power in the ranges 19–21 MJm−3 and 26.6–28.1 MJkg−1 were obtained for the biogas and the biochar obtained at different temperatures, respectively.
1. Introduction Biomass is a non-expensive, renewable source of energy with reduced environmental impacts. It can be treated through thermochemical and biochemical processes to obtain electrical, thermal or chemical energy and other chemical products (Bridgwater, 2003). The product distribution depends on the composition of principal biomass compounds and the type of thermal conversion process. The main components of biomass are cellulose (C6 polymers), hemicellulose (predominantly C5 polymers but including C6 species) and lignin (various phenylpropane units). These biopolymers have distinctive thermal behaviors and decompose at different temperature ranges, producing a variety of chemical compounds (aromatic and aliphatic hydrocarbons, organic alcohols, ethers and acids, between others) along with noncondensable gases (hydrogen, H2O, CO, CO2 and C2-C5 HCs) and solid residue (char) (Santos et al., 2010)(Collard and Blin, 2014). The chemical compounds can be recovered as added value products or processed to decrease oxygen content, enabling their use as fuel (Goyal et al., 2008). Biomass from agroindustry waste is an economic energy source through thermal conversion process. More specifically, the world production of soybean for the campaign 2015/2016 was 315.8 million tons (USDA, 2018). Soybean hulls represent 5% of the weight of the crop ⁎
and are currently managed as wastes. Nowadays, soybean hulls and straw are either landfilled, used as animal feed or burned in the fields after harvest, which represents environmental problems (Qing et al., 2017). Revalorization of soybean hulls waste through a thermochemical process like pyrolysis is proposed as an alternative use, at the same time reducing the negative effects of current uses on the environment (Giri et al., 2017). Pyrolysis is a well-known and widely used thermochemical conversion process that can be used to transform biomass and waste into valuable chemicals and energy using these techniques. The main pyrolysis products are biochar, biofuel and biogas (Yaman, 2004) (Mohan et al., 2006). The differences in structure and chemical composition of the raw biomass, temperature, heating rate and residence time are some of the key factors involved in the yields and the distribution of the final pyrolysis products (Guedes et al., 2018). In general, yield of the gas fraction increases with temperature, while bio-oil yield shows a maximum at intermediate temperatures, and the solid yield decreases with temperature. As temperature increases, the biopolymers decompose in a charred solid, a collection of organic macromolecules of varied size and functionalities, that can be recovered as condensable products, and light hydrocarbons, oxygenated hydrocarbons, and CO, CO2 and H2 that forms the non-condensable gaseous fraction. Further increase of the temperature degrades some of the macromolecules from the solid
Corresponding author. E-mail address: [email protected] (E.S. Carrillo-Pedraza).
https://doi.org/10.1016/j.biteb.2019.02.009 Received 7 November 2018; Received in revised form 11 February 2019; Accepted 12 February 2019 Available online 15 February 2019 2589-014X/ © 2019 Published by Elsevier Ltd.
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fraction, producing chemical compounds that enrich the bio-oil fraction. At even higher temperatures, the char experiences aromatic condensation and dehydrogenation reactions rendering the formation of H2O, H2 and CO, while the chemicals from the bio-oil fraction are thermally cracked through secondary reactions, producing non-condensable gases and a decrease in the liquid yield (Fabbri and Torri, 2016) (Di Blasi, 2008). Previous research has been focused on the bio-oil composition and yield obtained from the pyrolysis of industrial soybean waste (L. Zhang et al., 2017) (Oliveira et al., 2015) (Uzun et al., 2006), but little is known about the other bioproducts (i.e. solid and gas fractions). The characterization of all these products is the first step needed for setting up a biorefinery process based on pyrolysis of soybean. Therefore, this work aims to provide a full characterization of all the pyrolysis fractions of soybean hulls. Yields for the different fractions are calculated at different pyrolysis temperatures, while the physicochemical, compositional and calorimetric characterization of the bio-oil, biogas and biochar are reported.
to −30 °C), using limonene-dry CO2 mixture as a refrigerant, while the biochar was recovered from the tubular reactor. 1.4. Analysis of bio-oil composition by GC/MS Bio-oil was recovered from the condenser at the outlet of the pyrolysis reactors. The full bio-oil mixture was recovered and solved in acetone in the case of laboratory scale experiments, while the bio-oil produced from the pilot scale pyrolysis was segregated into aqueous and organic fractions that were extracted using acetone (analytical grade, Sigma-Aldrich) and dicloromethane (analytical grade, SigmaAldrich) in 1:10 and 1:5 volume ratios, respectively. All the resulting mixtures were analyzed by Gas Chromatography–Mass Spectrometry (GC/MS), using an 7000D GC/MS Triple Quad (Agilent Technologies, USA), equipped with an Agilent DB-624 column (60 m × 0.250 mm × 1.4 μm) and HP-5ms column (30 m × 0.250 mm × 0.25 μm), flame ionization detector (FID) and mass spectrometer detector (MS). The following settings were used: 250 °C injector temperature; carrier gas helium 1 mL/min; temperature program for DB-624: 125 °C for 10 min, heating rate of 10 °C min−1 up to 250 °C, holding time of 20 min; temperature program for HP-5ms: 50 °C, heating rate of 8 °Cmin−1 up to 200 °C for 2 min; FID temperature, 300 °C. Ions were detected in full scan mode (mass range from 15 to 400 m/z), electronic impact of 70 eV. Identification of compound was achieved by comparing the mass spectra with the NIST MS Search 2.0 mass spectral library. The separation of the two phases of the bio-oil was done by simple decanting Higher heating value (HHV) in organic phase bio-oil was also determined using a Parr oxygen bomb calorimeter (Fisher Scientific).
1.1. Materials preparation Soybean hulls currently managed as industrial waste were obtained from an oil company from the North of Mexico region. Prior to use, the biomass waste was milled and sieved to a particle size lower than 0.2 mm. The sieved samples were dried at 105 °C in an air-dry oven for 12 h to remove the moisture, and then stored in stopped bottles for further use. 1.2. Ultimate, proximate and thermogravimetric analyses The elemental analysis was recorded in a PerkinElmer 2400CHNS analyzer to determine the mass fractions of carbon, hydrogen, nitrogen and sulfur. The composition of the biopolymers was obtained by the Van Soest methodology (Loredo Medrano et al., 2016) (L. Zhang et al., 2017). The pyrolysis behaviour of soybean hulls was studied in a thermogravimetric analyzer (Q500, TA Instruments, USA). In the thermogravimetric analysis (TGA) experiment, 10 mg of sample (particle size between 250 and 425 μm) was loaded into the platinum container, then heated from ambient temperature to 900 °C at a heating rate of 20 °Cmin−1. The carrier gas of the test was nitrogen with a flow rate of 60 mL/min. The same equipment was employed for determining the proximate analysis of soybean hulls.
1.5. Analysis of the non-condensable gases Analysis of the gas fraction from the lab-scale pyrolysis experiments was performed at the condenser outlet. CO and CO2 evolution were determined by a non-dispersive infrared gas analyzer (NDIR) (Siemens Ultramat 22), whereas the evolution rate of hydrocarbons (methane, ethane, ethene, propane and propylene) were determined by GC in a Perkin Elmer Autosytem system equipped with a packed column (Hayasep-D 100–120 mesh, PE) Column and FID detector. The quantification of the hydrocarbons (methane, ethane, ethene, propane and propylene) in the non-condensable gas fraction of the labscale experiments were carried out in a Schimadzu GC-17A GC equipped with a Carboxen 1000 packed column (60/80) (4.57mx3.17 mm) with FID and TCD as detectors. Identification and absolute quantification were performed by external calibration with a standard commercial mixture of gases obtained from White Marines. Hydrogen evolution was quantified in a HP Hewlett Packard 5890 Series II GC equipped with a capillary column GS-GasPro 113-4362 J & M (60 m, 0.320 mm) and TCD detector. Identification and absolute quantification was performed by external calibration with a standard commercial of Infra mixture of Hydrogen 29.8% ± 1.0%.
1.3. Pyrolysis experiments The analytical pyrolysis experiments were carried at lab and pilot plant scales. Lab scale pyrolysis were performed under nitrogen atmosphere (flow rate: 150 mL/min STP) using a vertical laboratory scale tubular reactor (diameter: 2.5 cm) loaded with 5 g of samples from the Section 1.1. The experiments were carried out under atmospheric pressure at five different temperatures (300, 350, 400, 500 and 600 °C) at heating rate of 10 °Cmin−1. The bed temperature was tracked using an additional thermocouple. The solid residue (i.e. Biochar) is recovered from the tubular reactor at the end of the experiment, whereas the bio-oil is obtained at the reactor outlet by condensation using refrigerated acetone (e15C) as coolant. The yields of the solid and liquid fractions are obtained by weight, while that of the non-condensable fraction is obtained by difference. All yields were expressed dry basis. The pyrolysis experiments at pilot scale were carried out under nitrogen atmosphere (flow rate: 25 Lmin−1 STP) in a 4 L tubular reactor (4 cm radius x 80 cm long) with 500 g of samples from the Section 1.1. The reactor was heated in an electric oven brand THERMOLYNE, model 79,400. Temperature measurements were acquired from the bed for a accurately controlling the reactor temperature. The experiments were carried out at final temperature of 600 °C using a heating rate of 25 °Cmin−1. The bio-oil is obtained by condensation in a range of (−20 °C
1.6. Analysis of the solid fraction by infrared spectroscopy The FTIR spectra were recorded in a spectrophotometer (Spectrum One, Perkin Elmer, USA) to characterize the functional groups of the soybean hulls and biochars obtained in the pyrolysis at different temperatures (300 °C, 400 °C, 500 °C and 600 °C). Prior to the FTIR experiment, the samples were pelletized using KBr as binder. The spectrum scan frequency was 10 times per minute at a resolution of 4 cm−1 and with spectrum range from 4000 cm−1 to 500 cm−1. The gross calorific power was calculated by Dulong formula (Pehlivan et al., 2016):
QGCV = 33.83C + 144.3 H 184
O 8
(MJkg 1)
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2. Results and discussion
region due to the loss of oxygen functional groups from the cellulose and hemicellulose and the side chains of lignin (L. Zhang et al., 2017). The chemical decomposition of these functional groups are likely to produce CO and CO2 (Zhao et al., 2016). The largest changes in these bands were observed at temperatures above 400 °C, probably as a result of the complete decomposition of the cellulose and hemicellulose, which own > 70% of the composition of the raw material. From 500 °C, only small changes are observed in these bands. The bands at 1649 and 1429 cm−1 do not decrease throughout the temperature range in Fig. S2, possibly because these bands are associated with the stretching of highly stable functional groups in lignin (Zhao et al., 2016)(Z. Wang et al., 2016), a biopolymer that is known to be thermally degraded in a wide range of temperatures, reaching temperatures up to 900 °C (S. Wang et al., 2017). Changes on the bands of FTIR analysis as temperature increases confirmed the thermal degradation of diverse functional groups with temperature. Along with the TG analysis and the proximate and Van Soest analyses, this information provide that the most suitable operation conditions for the pyrolysis process should be in the range of 300 °C to 600 °C, where temperature should have a critical effect in obtaining different products and yields.
2.1. Characterization of the feedstock The proximate analysis determined that the soybean hulls contain high contents of volatile material (M.V) with 82.2 ± 0.6% and low concentration of inorganic material (ash) 2.5 ± 0.1%, which makes it a feasible material for obtaining energy through a thermal pyrolysis process (Parascanu et al., 2017). According to the Van Soest method, the composition of the soybean hulls was 18.5 ± 0.3% hemicellulose, 52.3 ± 0.5% cellulose and 3.7 ± 0.4% lignin. The values obtained from the elemental analysis and composition of biopolymer are similar to those reported by other researches (Liu and Li, 2017), (Oliveira et al., 2015). The small deviations observed in the composition of the feedstock can be attributed to the properties and characteristics of the soil, the amount of rainfall among other factors typical of each harvest (De la Cruz, 2012). The concentrations of the biopolymers that form the soybean hulls and the physicochemical characteristics of the material (highly volatile material and low percentage of fixed coal) make it an attractive raw material to obtain bio products by thermal degradation by pyrolysis processes. Fig. S1 shows the TG and DTG profiles of the thermal decomposition of the soybean hulls in inert atmosphere. Three main weight loss stages are observed. The first weight loss (8%) located between 30 and 120 °C is attributed to the humidity of the material. In the second stage, hemicellulose and cellulose decomposition takes part from 200 to 414 °C. According to the DTG profile, where a main peak at 364 °C preceded by a small hump at 305 °C can be observed, two different decomposition subregions have been identified, i.e. the first one ranging from 200 °C to 315 °C (weight loss of 18.3%) and the second one starting at 315 °C up to 415 °C (weight loss of 44%), that have been attributed in the past to the thermal decomposition of hemicellulose and cellulose (El-Sayed and Mostafa, 2014) (Johar et al., 2012). The large contribution of cellulose to the weight loss during pyrolysis is in agreement with the cellulose composition in the soybean hulls. The last stage starts at 415 °C, where the weight changes are associated to the thermal decomposition of the lignin happened throughout the temperature in the range of 200–700 °C (Pehlivan et al., 2016). In accordance to this measurement, the pyrolysis of the biomass waste was performed at temperatures ranging from 300 to 600 °C, a temperature range where the pyrolysis temperature is expected to have a large impact on the yield and composition of the different fractions. Fig. S2 shows the FTIR spectra of the soybean hulls and the biochar obtained at different pyrolysis temperatures. Absorption bands are found at several region owing to the presence of different functional groups, even though the intensity of these bands is affected by the temperature of the pyrolysis process. The absorption bands dominant in the region of 3430–3050 cm−1 are caused by OH stretch vibrations, associated with phenolic compounds, alcohols and acids functionalities present in the three biopolymers (Zhao et al., 2016) (Sequeiros and Labidi, 2017) and also water in the biomass (Johar et al., 2012). The absorption bands observed from 3000 to 2800 cm−1 comes from the CeH stretching vibration in methyl groups and aliphatic carbon (Zhao et al., 2016). The absorption in 1740 cm−1 is assigned to the stretching of the C] O in the functional groups in organic acids, ketones, aldehydes and esters, that are present in cellulose and hemicellulose (Z. Wang et al., 2016) (Oun and Rhim, 2016). The band in 1649 cm−1 is produced by stretching of the C]C associated with vibrations of aromatic compounds that conform the lignin (Yang et al., 2007). The band at 1429 cm−1 is associated with methoxyl functional group in the lignin by CeH (Lopez-Velazquez et al., 2013) (Pino, 2013) and the intense band at 1045 is caused to the bond eCeOeH or eCeOeR of alcohols or esters, present in the three biopolymers. As previously mentioned, Fig. S2 shows that increase in pyrolysis temperature produce a decrease of the bands in the 3430–1740 cm−1
2.2. Evolution of the distribution of pyrolysis products with temperature Laboratory scale pyrolysis experiments were conducted at different temperatures, with the yield of each pyrolysis fraction being determined at the end of each pyrolysis run. The relationship between temperature and the distribution of pyrolysis products is shown in the Fig. 1. In overall, a good agreement is found between the yield of the solid fraction and the TGA results (see Fig. S1). At low temperature, the solid pyrolysis fraction was the prevailing product, and non-condensable gases were the main decomposition product of biomass at this temperature, showing a weight ratio of non-condensable to condensable (i.e. bio-oil) gases of 3.6:1. At 350 °C, cellulose and hemicellulose decomposition takes part in a large extend, producing a large drop of the biochar yield. As a result of the decomposition of these biopolymers, bio-oil is obtained as the main product, increasing in 22% the share of bio-oil in the product distribution. At this point, weight ratio of non-condensable gases to bio-oil is around 1:1. Further increase of pyrolysis temperature to 400 °C enlarged these tendencies, i.e. increase of the bio-oil and non-condensable fractions at the cost of the solid fraction. Changes in the distribution of products at this temperature range are small, since all the homocellulose have been decomposed at lower temperatures and the amount of lignin in soybean hulls is low. The yields obtained in the pilot scale pyrolysis were 39.4% for the bio-oil, 26.6% for the biochar and 34.0% for the non-condensable gases, showing only minor deviations from the results attained during the lab scale pyrolysis at 600 °C. 2.3. Composition of the non-condensable gas fraction The evolution of non-condensable gases was analyzed during the course of a pyrolysis run up to 650 °C. These gases consisted mainly in methane (CH4), hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and other light hydrocarbons such as ethane (C2H6), ethene (C2H4), propane (C3H8) and propene (C3H6). Fig. 2 shows the evolution profiles of these gases. It can be seen that CO and CO2 are formed at low temperatures, between 220 and 400 °C, the same temperature range for the thermal decomposition of hemicellulose and cellulose. These biopolymers have large amounts of oxygenated compounds (Loredo Medrano et al., 2016), in the form of carbonyl and carboxyl groups which by thermal degradation are mostly decomposed releasing CO and CO2, respectively. Zhao et al (Zhao et al., 2016) obtained similar results by performing cellulose pyrolysis. The presence of CO and CO2 at temperatures above 400 °C is possibly related to the breakdown of phenolic and 185
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Fig. 1. Product performance pyrolysis soybean hulls.
methoxy groups present in the lignin structure (Parascanu et al., 2017), and also to the catalytic gasification of the biochar by alkali compounds that are naturally present in all biomass (Bensakhria, 2012) (Shadman, 1986) (Feng et al., 2018). In any case, the evolution of these species decreases with the temperature. Methane evolution starts being relevant at temperatures higher than 300 °C, being the main gaseous product at 500 °C. Methane release is followed by the evolution of some other light C2-C3 hydrocarbons (HCs). These hydrocarbons are only formed at temperatures between 400 and 550 °C, whereas methane is still being released at temperatures higher than 600 °C. The formation of C2-C3 HCs is attributed to the secondary reactions in gaseous phase of organic compounds coming
from the pyrolysis of cellulose and lignin (Parascanu et al., 2017). The formation of CH4 can be related to thermal degradation of aromatic compounds and to the rupture of methoxy groups (O-CH3) associated of lignin, that mainly occurs at temperatures below 600 °C (Parascanu et al., 2017). At higher temperatures > 600 °C, the presence of CH4 is associated with the decomposition of lignin, which is known to take part at a wide degradation range of temperature (200 °C – 900 °C)(RuizRosas et al., 2010). In accordance to these findings, the total gas released during pyrolysis was collected at 400, 500 and 600 °C (where lower concentrations of CO and CO2 are expected) and the composition was analyzed by GC. For these measurements, hydrogen was also determined. The
Fig. 2. Evolution rate of non-condensable gases during the pyrolysis of soybean hulls. 186
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compositional results confirm that methane is the main product at 500 °C. Further increase of the pyrolysis temperature to 600 °C produces a decrease in the formation of C2-C3 HCs, while H2 evolution starts to be relevant. Since aromatic dehydrogenation of char usually proceeds at higher temperatures, the hydrogen evolution observed at this point is likely to be produced as a result of the thermal cracking of light HCs (Ly et al., 2016). The higher heating value (HHV) of the gas mixture increase in temperature range of 500 and 600 °C, reaching values of 20.3 and 24.3 MJm−3, respectively. The calorific power of the pyrolytic gases in the soybean hulls increases at 500 °C and also at 600 °C as a consequence of the predominance of methane (32.0 and 38.1% v/v, respectively), the light HCs in the mixture (32.0 and 38.1% v/v), and because of the formation of H2.(0.4 and 10.2% v/v). These compositions are reported discounting the nitrogen share and in dry basis. HHV in the range of 19–25 MJm−3 could be employed as synthesis gas and as fuel for gas engines and turbines (Pino, 2013).
Table 2 Main compounds and composition (%) as determined by GC/MS. Classification
Acids Furans
Alcohols Ketones
2.4. Proximate analysis of biochar Phenols
The biochar obtained at different pyrolysis temperatures was recovered and analyzed by FTIR (reported in Section 2.3.), ultimate and proximate analysis. The decrease of volatile matter in the solid as pyrolysis temperature rises is in accordance with the increase in the amount of non-condensable gases and oils observed in Section 2.4. analyzed in previous sections. Ash content increases because of the high thermal stability of the inorganic matter found in the soybean hulls. Ultimate analysis confirmed the loss of oxygen due to the formation of CO, CO2 and other oxidative compounds. As expected, the main oxygen loss is detected at 350 °C, a temperature high enough to decompose most of the homocellulose in soybean hulls. Hydrogen content remains constant up to 600 °C, where a noticeable drop is observed. The decrease in hydrogen amount agrees with the results obtained in the analysis of the non-condensable fraction, where a higher concentration of H2 was obtained. The HHV associated to the composition of the biochar has been estimated and reported in Table 1. The HHV increases dramatically at 350 °C. The maximum heating value is achieved at 400 °C. From that point, negligible changes in HHV are observed.
Aldehydes Carbohydrates
Bio-oils from the pyrolysis experiments at lab scale were recovered, solved in acetone and analyzed by GC–MS. Table 2 shows the identified compounds with > 1% of total area and a reliability index during their identification > 80%. The analysis has been able to identify 24 compounds. Acetic acid, furfural alcohol, furfural and the family of methylciclopentones where the main components of bio-oil, accounting together for ca. 60% of the total composition. Given the large number of compounds, they have been grouped into acids, furans, alcohols, ketones, phenols, aldehydes and carbohydrates. The composition of biooil in all these families of compounds and how their relative abundance change with temperature have been summarized in Fig. 3. Between these family of compounds, acidic ones were those Table 1 Characterization of biochar soybean hulls.
Ultimate analysis C H O N HHVa a
wt% wt% wt% wt% MJ/kg
Temperature (°C) 300
350
400
600
54.3 5.8 32.5 1.9 20.1
66.1 5.1 17.4 2.6 26.6
69.5 5.0 10.3 2.7 28.9
73.5 3.0 6.1 2.4 28.1
Acetic acid Propenoic acid Furfural 3-Methylfuran Fulfural alcohol 2(3H)-Furanone 2-Furaldehyde, 5-methyl2-Hexadecanol Cyclopentanol 2-Propanone, 1-acetoxy2-Cyclopentenone, 2-methyl1,2-Cyclopentadione Cyclopentone, 1-methyl2-Cyclopenten-1-one, 2,3-dimethyl2-Cyclopenten-1-one, 2-hydroxy- 3ethyl 2-butanone,1-hydroxy Phenol Phenol, 2-methylPhenol, 3-methylGuaiacol Butanedial Glucose Levoglucosan 1,4,3,6-Dianhydro-D-glucopyranose
Temperature °C 300
350
400
600
28.7 0.6 9.9 1.0 12.2 1.3 2.4 0.1 0.8 3.2 0.5 3.5 2.5 1.1 2.4
23.3 1.7 4.0 1.9 11.7 1.9 0.9 0.1 0.8 2.2 1.1 3.9 4.5 1.0 2.7
22.9 1.3 3.9 2.1 11.4 1.8 0.9 0.1 0.7 2.2 1.2 4.1 4.8 0.9 2.7
23.6 1.1 4.1 2.2 11.7 1.6 1.0 0.1 1.1 2.4 1.5 3.9 4.2 1.0 2.5
1.2 0.6 0.5 0.0 1.5 4.1 1.2 0.3 0.1
1.1 0.9 0.3 0.9 1.9 3.6 1.0 1.1 0.7
1.1 1.2 0.3 0.9 1.6 3.5 1.3 1.4 1.5
1.2 1.3 0.4 1.6 1.0 3.8 1.2 1.0 0.8
showing the highest concentrations. Acid compounds formation in pyrolysis is attributed mainly to acetyl group present in Hemicellulose (Pino, 2013). Acetic acid is the specie with the highest concentrations across the temperature range of lab scale pyrolysis. Other species such as furans and ketones are also attributed to the degradation of hemicellulose, which explains why the higher concentrations of acids and furans found at 300 °C (see Fig. 3). When temperature increases, the percentage of these groups decreases owing to a large extend of pyrolysis liquids being produced from cellulose and lignin decomposition. It is important to highlight that acid groups maintain their highest share in concentration through the entire temperature range, probably as a result of the thermal decomposition of levoglucosan, a product generated during the depolymerization and thermal decomposition of cellulose (Oliveira et al., 2015). Substantial concentrations of aliphatic aldehydes were also obtained, especially butanedial, which is attributed to the decomposition of the open rings of the cellulose and hemicellulose. The production of similar acids and aliphatic aldehydes compounds have been reported from the pyrolysis of hemicellulose and cellulose mixtures and other biomasses (Wu et al., 2016) (Li et al., 2017). As temperature increases, higher percentages of phenols were obtained. This large share of phenolic compounds is attributed to thermal decomposition of lignin, whose structure is based in the combination of different phenylpropane units as building blocks (Rosas et al., 2014). Concentration of carbohydrates have an increasing share in bio-oil composition until temperature of 400 °C. Beyond this temperature, these carbohydrates are thermally degraded to simpler species (Dong et al., 2015)(B. Zhang et al., 2015). In addition, a similar degradation tendency is observed for acids and furans at the highest pyrolysis temperature, Fig. 3, confirming that oxygenated and large organic compounds are thermally cracked at high temperatures. As the composition of the pbtained pyrolytic oils are rich in acidic and oxygenated compounds, it would be necessary previous processing through hydrodeoxidation to improve their properties as fuel (CorderoLanzac et al., 2017) (Oliveira et al., 2015). However, compounds such as acetic acid, furfural alcohol, furfural, and 1,2 cyclopentadione have already been found in pyrolysis of soyben stalk and considered as added-value chemicals (L. Zhang et al., 2017).
2.5. Analysis of bio-oil generated from the pyrolysis
Units
Compound
Higher Heating Value. 187
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Fig. 3. Relative abundance of added value organic compounds in the bio-oil obtained at different temperatures during the laboratory scale experiments.
The bio-oil composition obtained at laboratory scale is similar to those reported in the pyrolysis of soybean hulls (Oliveira et al., 2015) and soybean crop residues (L. Zhang et al., 2017). The differences can possibly be attributed to longer reaction times, which produce higher concentrations of short-chain species, to the type of biomass and to the analytical conditions of the method utilized to solve the bio-oil prior to the GC–MS measurement. The bio-oil obtained from the pilot scale pyrolysis was also analized by means of GC–MS. factors such as faster heating rates and different nitrogen flow rates can affect the final composition of the product (Oliveira et al., 2015). In fact, the bio-oil obtained in pilot scale showed two phases, an aqueous ones. Both phases were recovered separately and analyzed by GC/MS. Tables 2 and 4 shows the most abundant compounds that has been identified with a reliability index > 90%. The aqueous phase of bio-oil showed as main components Furfural, acetone, phenols and C11-C14 hydrocarbons, Table 3. Aqueous phase of bio-oils is reported to be composed of water, oxygenated compounds with high and low molecular results obtained from GC/MS reported in Table 3. The absence of acetic acid and aldehydes on Table 3 is related to the use of dichloromethane as solvent in the GC/MS analysis (Oliveira et al., 2015). The main compounds of the organic phase of bio-oil are compiled in
Table 4 Main compounds identified in the organic phase of bio-oil.
Compound
Area (%)
Furfural
Furfural 2-Furaldehyde, 5-methyl Cyclopentanone, 4,4-dimethyl2-Cyclopenten-1-one, 2,3-dimethylPhenol, 2-methylPhenol, 3-methylPhenol, 3,4-dimethylPhenol, 2-methoxyUndecane Dodecane Tridecane Tetradecane
5.3 2.3 6.2 0.6 3.6 0.7 0.8 0.4 2.7 1.2 1.1 1.2
Acetone Phenols
Hydrocarbons
Compound
Area (%)
Ketones Furans Phenols
2-Cyclopentanone, 4,4-dimethyl Furfural Phenol Phenol, 3-methylp-Cresol Phenol, 2-methoxyTricosane Dodecene Tridecene Tetradeceno Docosane
5.7 6.6 10.8 0.6 1.4 1.4 6.6 6.6 5.3 3.1 2.1
Hydrocarbons
Table 4. The organic phase of bio-oil is considered to be a promising high-grade fuel candidate after proper upgrading, and can also be used for obtaining chemical compounds (Dhyani and Bhaskar, 2017). Larger composition in hydrocarbons is found in this phase, followed by phenols and furans. The appearance of a larger share of hydrocarbons in the bio-oil obtained at pilot scale pyrolysis could be related to the longer residence time of the gas, which leads to higher secondary reactions [6]. The phenols detected in the organic bio-oil can be used as precursors in different processes like the production of resins (Effendi et al., 2008), medicaments and additives (Vu et al., 2018). Obtaining phenol from biomass waste contributes to the environmental protection, given that the production of phenols nowadays involves processes where fossil fuels are utilized as raw material. The caloric power of the organic phase has been determined to be 23.7 MJkg−1, owing to the presence of aliphatic hydrocarbons obtained from the deoxygenation reactions, that seems to be favoured at the higher pyrolysis temperature and longer residence time of gaseous products.
Table 3 Main compounds identified in the polar fraction of bio-oil. Classification
Classification
3. Conclusions Soybean hulls pyrolysis yields on bioproducts have been obtained using laboratory scale and pilot scale pyrolysis at 600 °C, with only a 4% of difference, indicate the potential of the process scalability. A 76% 188
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volatile mater and 2.3% were observed at pyrolysis distribution products as bio-oil (40%) and non-condensable gases (36%) highest yields achieved at 600 °C. The aqueous phase of bio-oil were acetic acid, furfural, furfural alcohol, 1,2-cyclopentadiene. The organic phase of bio-oil presented a calorific value of 23.7 MJkg−1. Whiting 19–21 kJmol−1 and 26.6–28.1 kJkg−1 obtained for the biogas and the biochar obtained at different temperatures, respectively. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biteb.2019.02.009.
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Acknowledgements The authors would like to acknowledge the financial support of CONACYT and Facultad de Ciencias Químicas Universidad Autónoma de Nuevo León, The authors also acknowledge to the Universidad de Malaga for allowing the use of its facilities and equipment and the Spanish Ministry of Science, Innovation and Universities and FEDER (Project CTQ2015-68654-R) for the finantial support. References Bensakhria, Ammar, 2012. Influence of impregnated metal on the pyrolysis conversion of biomass constituents. J. Anal. Appl. Pyrolysis 95, 213–226. https://doi.org/10.1016/ j.jaap.2012.02.009. Bridgwater, A.V., 2003. Renewable fuels and chemicals by thermal processing of biomass. Chem. Eng. J. 91 (2–3), 87–102. https://doi.org/10.1016/S1385-8947(02)00142-0. Collard, François Xavier, Blin, Joël, 2014. A review on pyrolysis of biomass constituents: mechanisms and composition of the products obtained from the conversion of cellulose, hemicelluloses and lignin. Renew. Sust. Energ. Rev. 38, 594–608. https://doi. org/10.1016/j.rser.2014.06.013. (Elsevier). Cordero-Lanzac, Tomás, Palos, Roberto, Arandes, José M., Castaño, Pedro, RodríguezMirasol, José, Cordero, Tomás, Bilbao, Javier, 2017. stability of an acid activated carbon based bifunctional catalyst for the raw bio-oil hydrodeoxygenation. Appl. Catal. B Environ. 203, 389–399. https://doi.org/10.1016/j.apcatb.2016.10.018. De la Cruz, Miguel G., 2012. Fertilización foliar con potacio, calcio y silicio en fresa. In: Universidad Autonoma Chapingo. Dhyani, Vaibhav, Bhaskar, Thallada, 2017. A comprehensive review on the pyrolysis of lignocellulosic biomass. Renew. Energy 129, 695–716. https://doi.org/10.1016/j. renene.2017.04.035. (Elsevier Ltd). Di Blasi, Colomba, 2008. Modeling chemical and physical processes of wood and biomass pyrolysis. Prog. Energy Combust. Sci. 34 (1), 47–90. https://doi.org/10.1016/j.pecs. 2006.12.001. Dong, Qing, Zhang, Shuping, Li, Zhang, Ding, Kuan, Xiong, Yuanquan, 2015. Effects of four types of dilute acid washing on moso bamboo pyrolysis using Py-GC/MS. Bioresour. Technol. 185, 62–69. https://doi.org/10.1016/j.biortech.2015.02.076. (Elsevier Ltd). Effendi, A., Gerhauser, H., Bridgwater, A.V., 2008. Production of renewable phenolic resins by thermochemical conversion of biomass: a review. Renew. Sust. Energ. Rev. 12 (8), 2092–2116. https://doi.org/10.1016/j.rser.2007.04.008. El-Sayed, Saad A., Mostafa, M.E., 2014. Pyrolysis characteristics and kinetic parameters determination of biomass fuel powders by differential thermal gravimetric analysis (TGA/DTG). Energy Convers. Manag. 85, 165–172. https://doi.org/10.1016/j. enconman.2014.05.068. (Elsevier Ltd). Fabbri, Daniele, Torri, Cristian, 2016. Linking pyrolysis and anaerobic digestion (Py-AD) for the conversion of lignocellulosic biomass. Curr. Opin. Biotechnol. 38, 167–173. https://doi.org/10.1016/j.copbio.2016.02.004. (March. Elsevier Ltd). Feng, Dongdong, Zhang, Yu, Zhao, Yijun, Sun, Shaozeng, 2018. Catalytic effects of ionexchangeable K+ and Ca2+on rice husk pyrolysis behavior and its gas–liquid–solid product properties. Energy 152, 166–177. https://doi.org/10.1016/j.energy.2018. 03.119. (Elsevier Ltd). Giri, Germán F., Viarengo, Gastón, Furlán, Ricardo L.E., Suárez, Alejandra G., Véscovi, Eleonora Garcia, Spanevello, Rolando A., 2017. Soybean hulls, an alternative source of bioactive compounds: combining pyrolysis with bioguided fractionation. Ind. Crop. Prod. 105, 113–123. https://doi.org/10.1016/J.INDCROP.2017.05.005. ((October). Elsevier). Goyal, H.B., Seal, Diptendu, Saxena, R.C., 2008. Bio-fuels from thermochemical conversion of renewable resources: a review. Renew. Sust. Energ. Rev. 12 (2), 504–517. https://doi.org/10.1016/j.rser.2006.07.014. Guedes, Raquel Escrivani, Luna, Aderval S., Torres, Alexandre Rodrigues, 2018. Operating parameters for bio-oil production in biomass pyrolysis: a review. J. Anal. Appl. Pyrolysis 129, 134–149. https://doi.org/10.1016/j.jaap.2017.11.019. (November 2017. Elsevier). Johar, Nurain, Ahmad, Ishak, Dufresne, Alain, 2012. Extraction, preparation and characterization of cellulose fibres and nanocrystals from rice husk. Ind. Crop. Prod. 37 (1), 93–99. https://doi.org/10.1016/j.indcrop.2011.12.016. (Elsevier B.V.). Li, Kai, Zhang, Liqiang, Zhu, Liang, Zhu, Xifeng, 2017. Bioresource technology comparative study on pyrolysis of lignocellulosic and algal biomass using pyrolysis-gas chromatography/mass spectrometry. Bioresour. Technol. 234, 48–52. https://doi. org/10.1016/j.biortech.2017.03.014. (Elsevier Ltd). Liu, Hua-Min, Li, Hao-Yang, 2017. Application and conversion of soybean hulls. In: Soybean - The Basis of Yield, Biomass and Productivity, pp. 111–132. https://doi.
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