Oxidative fast pyrolysis of banana leaves in fluidized bed reactor

Renewable Energy 96 (2016) 56e64

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Oxidative fast pyrolysis of banana leaves in fluidized bed reactor Noeli Sellin a, *, Diego Ricardo Krohl a, Cintia Marangoni b, Ozair Souza a a
rio, Zona Industrial, Master's Program in Process Engineering, University of Joinville RegioneUNIVILLE, Rua Paulo Malschitzki, 10, Campus Universita 89219710, Joinville, SC, Brazil b Federal University of Santa CatarinaeUFSC, Rua Pomerode, 710, Salto Norte, 89065300, Blumenau, SC, Brazil

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 March 2015 Received in revised form 1 March 2016 Accepted 10 April 2016

Dried banana leaves were previously chopped, characterized by proximate and elemental chemical analyses, thermogravimetric analysis (TGA), differential thermal analysis (DTA) and higher and lower heating value and submitted to oxidative fast pyrolysis in an auto-thermal fluidized bed reactor. The pyrolysis products were gases, bio-char and bio-oil (heavy and light phase). The gases were burnt in a combustion chamber and the energy was used for to heat the fluidization air in the reactor. The light biooil was analyzed by gas chromatography-mass spectrometry (GC/MS); the heavy bio-oil by infrared spectroscopy (FTIR/ATR) and higher and lower heating value; and the bio-char by elemental and proximate analysis, TGA, DTA and scanning electron microscopy (SEM). The mass yield and energy efficiency of the process were determined by mass and energy balances. The process produced 49.6% gases, 27.0% bio-oil and 23.3% bio-char. The light and heavy bio-oil presented complex chemical compounds and phenolic and acid nature. The heavy bio-oil showed elevated higher heating value of 25 MJ/kg. The biochar released high energy under combustion, enabling it to be used as fuel. The results suggest potential for generating fuel products and chemical inputs from fast pyrolysis of dried banana leaves. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Biomass Banana leaves Oxidative fast pyrolysis Fluidized bed reactor Energy

1. Introduction In the last decades, the production of banana expanded in most producing countries; rose from 35 million to 102 million tonnes between crops 1978 and 2012. This was due especially the more intensive use of technology, resulting in higher levels of productivity. Only six countries account for almost 65% of world production. India leads the world production (24.4%), followed by China (10.3%), the Philippines (9.1%), Ecuador (6.9%), Brazil (6.8%) and Indonesia (6.1%) [1]. In Brazil, bananas are cultivated throughout all regions of the federation and the country ranks fifth among the world's largest producers. In 2013, the national harvest of bananas recorded an area of 485,162 ha reaching 7,292,164 tons of produced fruit with average yield of 15,030 kg/ha. Santa Catarina state located in southern of Brazil participated with approximately 9.5% of the Brazilian banana production, being state ranked fourth in domestic production. In the past three years, banana production in Santa Catarina has alternated with apple production in socioeconomic importance. Banana cultivation has been a strong component in the income of a great number of small farmers [1]. According to

* Corresponding author. E-mail address: [email protected] (N. Sellin). http://dx.doi.org/10.1016/j.renene.2016.04.032 0960-1481/© 2016 Elsevier Ltd. All rights reserved.

Fernandes et al. [2], for each ton of harvested banana, 100 kg of fruit is rejected, and approximately four tons of lignocellulosic waste is generated (three tons pseudostem, 160 kg of stalks, 480 kg leaves and 440 kg peels). In 2013, 29,137 million tons of banana wastes (leaves, pseudostem and stalk) were generated [1]. Most of these wastes remain in the cultivation area until their decomposition by microorganisms producing greenhouse gases (methane gas and carbon dioxide). The use of such waste for the production of inputs, in addition to reducing environmental pollution by their removal from the field, provides added value to banana cultivation, which has been facing great challenges over recent years due to the product's fluctuation in the domestic market. Among the possibilities of enhancing the value of these wastes is to use it as biomass in the generation of renewable energy and to produce chemical inputs. The banana wastes can be compacted into briquettes [3e5], be biochemically converted to methane gas with anaerobic digestion [6] and fermented to ethanol [7e9]. Direct combustion of pseudostems and leaves can also generate power [6]. Various studies using different biomasses such as sawdust, banana wastes, rice husks, coffee wastes, sugarcane bagasse and corncobs for the production of energy and chemical inputs have been conducted and have demonstrated the great potential of these [2,10e12]. These studies generally use thermochemical conversion

N. Sellin et al. / Renewable Energy 96 (2016) 56e64

processes, such as liquefaction, gasification, pyrolysis and combustion and wastes that are readily available in the region in which they are generated from harvesting and processing [13,14]. Within the biomass thermochemical conversion technologies, pyrolysis has been receiving special attention and has been investigated by various groups. It has become a common method of biomass conversion due to its simple operation and occurs in a continuous process. Pyrolysis is a process which breaks down the original molecular structure of a determined compound through the action of heat. In biomass pyrolysis, the particles are generally heated between 400 and 500
C, in inert atmosphere or with a low concentration of oxygen, causing the formation of a carbon (bio-char) rich waste and a volatile fraction composed of condensable organic gases and vapors (bio-oil) [11]. The proportions of these compounds depend on the biomass characteristics and the pyrolysis method used (slow pyrolysis or carbonization, rapid and ultrarapid pyrolysis), that depend on process parameters and the architecture of the reactor, such as temperature, residence time of the biomass particle, heating rate, pressure used, type of atmosphere and use of catalysts [15e17]. Fast pyrolysis is mainly used to maximize liquid product yield such as bio-oil (heavy and light), using moderate temperature of around 500
C, high heating rate (>100
C/s) and short vapor residence time (from 0.5 to 10 s). Very high heating and heat transfer rates generally require feeding by finely ground biomass with particle size smaller than 1 mm, carefully controlled reaction temperature and pyrolysis vapors and aerosols must be quickly cooled to generate the bio-oil [13,18]. The commercial operation currently used for bio-oil production based on vegetable biomass pyrolysis is reached in fluidized bed reactor and circulating fluidized bed. Fluidized beds have been extensively studied and plants already exist in Brazil, being manufactured and sold [19]. Various studies related to fast pyrolysis in fluidized bed reactors for the production of energy and inputs from biomasses have been presented. Fast pyrolysis of Eucalyptus grandis wood was conducted in a continuous-feed fluidized bed reactor by Heidari et al. [20], under temperatures between 450 and 600
C and in inert atmosphere. The maximum bio-oil yield (50.8%) with the minimum water content was obtained at 450
C. The gas yield was increased from 29.4 to 48.4 wt% when the reaction temperature increased from 450 to 600
C while the amount of bio-char yield decreased from 19.7 to 14.2 wt%. Four types of Canadian waste biomass including wheat straw, saw dust, flax straw and poultry litter were used by Azargohar et al. [11] for fast pyrolysis using a mobile unit. The pyrolysis products showed that mobile pyrolysis unit can operate for wide range of non-food biomass and its products have
rez great potential for fuel or agricultural applications. Mesa-Pe et al. [19] studied oxidative fast pyrolysis of sugar cane straw in an auto-thermal fluidized bed reactor, between 470 and 600
C. According authors, due to the pyrolysis plant configuration used, between 10 and 15 wt% of the biomass fed is burned with air to generate the heat necessary to warm the bed of inert material and achieve an adequate temperature for the beginning of the pyrolysis reaction of around 450 and 470
C. As a result, an auto-thermal regime is obtained, which facilitate energy integration and reduce operating costs, improving process feasibility. The maximum biooil yield was achieved at temperature of 470
C and the product yields of bio-oil and bio-char were up to 35.5 wt% and 48.2 wt%, respectively. In this type of process, the bio-oil yield is lower when compared to pyrolysis in an inert atmosphere, due to the combustion of part of the biomass with the air used for fluidization, however, the charcoal yield is high. Aimed at exploiting and adding value to the wastes produced by banana cultivation, which are generated in large quantities in Brazil, Fernandes et al. [2] evaluated the use of wet and semi-dried

57

banana leaves as a potential energy source. The chemical characteristics and the thermal behavior demonstrated by the semi-dried banana leaves indicate their potential for use as biomass, with results similar to other agro-industrial wastes currently used. In another study, Fernandes [21] carried out slow pyrolysis of wastes generated in banana cultivation (leaves and pseudostem) in a fixed bed reactor, under inert atmosphere, obtaining high bio-char yield (56.8% for the leaves and 58.4% for the pseudostem) and low yields for the bio-oil (9.4% for the leaves and 11.8% for the pseudostem), characteristic of slow pyrolysis. The pyrolysis products were characterized and the technical viability of the slow pyrolysis of banana leaves and pseudostem has been demonstrated on basis of the results obtained by the authors. In order to expand these studies, in this study, the thermochemical conversion of dried banana leaves in products such as bio-oil (light and heavy phase), charcoal and gases was evaluated. The thermoconversion process used was the oxidative fast pyrolysis in an auto-thermal pilot-scale plant similar
rez et al. [19] and under similar operating to that used by Mesa-Pe conditions. The characteristics of the biomass and products (bio-oil and bio-char) generated in the process were evaluated by chemical and thermal analysis and mass and energy yields were determined. The non-condensable gas were directly burned without any auxiliary fuel in a combustion chamber and used to heat the air used in fluidization contributing to the auto-thermal process and to reduce the energy consumption. The use of air as fluidizing agent instead of inert gases also causes reduction of operating costs in the pyrolysis plant. 2. Experimental study 2.1. Biomass preparation and characterization The banana leaves samples were obtained from the Musa cavendishii species on a property located in the municipality of Joinville/SC, Brazil. Only the leaves which were found in a dry state were picked directly from the banana tree or gathered from the ground. The banana leaves did not require drying because they presented 7.8% moisture, adequate level for the fast pyrolysis process. Once collected, the samples were chopped in a hammer mill, CID 125 mm model and submitted to granulometric analysis according to ASTM E828-81 (2004) using Tyler series sieves of different meshes, with shaking time of 15 min and 80 Hz frequency. Around 83% of the particles presented diameters of less than 1 mm and the rest between 1 and 2 mm. To evaluate the potential of the biomass for the fast pyrolysis process, the banana leaves particles were characterized by chemical and physical analyses. Moisture (%M), volatile matter (%VM) and ash were determined by proximate chemical analysis using thermogravimetry (dried in oven and burnt in muffle furnace) according to procedures described, respectively, in the ASTM E87182, ASTM E872-82 and ASTM E1755-01 standards. All procedures were carried out in triplicate and the crucibles used had been previously cleaned and dried. Fixed carbon (FC) was determinate using the data previously obtained in the proximate analysis using the formula % FC ¼ 100  (% Ash þ % VM). Ultimate analysis was performed in order to determine the elemental composition of biomass. The carbon (C), hydrogen (H) and nitrogen (N) contents were measured in the Perkin-Elmer CHN 2400 elemental analyzer. The method consists of burning the samples in oxidizing atmosphere, with fully developed combustion. The samples are reduced to a group of gases such as CO2, H2O and N2), which are continually measured and based on this, the C, H and N element percentages are calculated. The sulfur (S) content was determined by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) in the Spectro Ciros CCD equipment. The

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oxygen was obtained by subtraction. The analyses were carried out in duplicate. The higher heating value (HHV) and lower heating value (LHV) of the banana leaves was determined in a static and adiabatic bomb calorimeter, Parr model 1241, according to ASTM D2015-00 recommended for coal and coke, although adapted for other solid fuels, such as vegetable biomass. The analyses were carried out in duplicate. To examine the thermal decomposition behavior of the banana leaves and to provide a reference for operational parameters of fast pyrolysis, Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) were carried out. The samples were analyzed under N2 inert atmosphere, ambient temperature range up to 900
C, heating rate of 10
C/min, in simultaneous thermal analyzer, Netzsch model STA 449F3. 2.2. Fast pyrolysis of biomass The oxidative fast pyrolysis experiments of dried banana leaves were carried out in an auto-thermal pilot-scale plant, model PPR10, with fluidized bed reactor using air as fluidization agent, which belongs to the company Bioware Technology, Campinas/SP, Brazil. The flow diagram of the fluidized bed pyrolysis system used in this study is shown on Fig. 1. The assays were carried out in triplicate according to the stages: 7 kg silica sand (particle diameter of 0.164 mm) was added to the reactor to increase heat exchange with the biomass. Then, 10 Nm3/h air was injected into the reactor to fluidize the bed. Internal temperature was gradually increased to around 500
C and the time to reach this condition was approximately 30 min. The biomass was then continuously fed at a rate of 12 kg/h through a silo and conveyed by a screw feeder inside the reactor while air flow was increased to 15 Nm3/h to stabilize the bed. The biomass underwent continuous pyrolysis in the reactor generating condensable and non-condensable gaseous products, which were sent to a set of sequential cyclones to remove the fine bio-char particles. The gases were cooled in a condenser with water at room temperature, resulting in the formation of bio-oil, which was sent to a centrifugal system enclose to the pyrolysis plant for

separating the heavy bio-oil and light bio-oil (pyroligneous acid). This separation was carried out to obtain heavy bio-oil with low water content and high heating value. The non-condensable gases were conducted to a combustion chamber and the energy released in burning was used to heat the fluidizing air in the reactor. Depending on the stoichiometric amount of air fed, part of the biomass is combusted and releases the energy necessary to maintain the reactor internal temperature (auto-thermal) from 480
C to 500
C. In tests carried out the air flow fed to the reactor was about 0.6 kg air/kg biomass, or 10% relative to stoichiometric for complete combustion. The combustion took place in the lower part of fluidized bed and the thermal decomposition of biomass in higher part of the bed. These operating conditions were defined based on previous studies using a similar pilot-scale plant to obtain bio-oil and charcoal from sugar cane straw and elephant grass [19,22]. The temperature and pressure were registered using thermocouples and pressure transducers located along the reactor height. This fact allowed knowing the behavior of the profiles of temperature and static pressure during operation. At the end of the pyrolysis process, after cooling the system, the solid fraction (bio-char) generated in the bottom of the reactor was removed. 2.3. Mass yield and energy efficiency from pyrolysis The mass yield (%) of products was determined based on the ratio between the quantity of each product generated after fast pyrolysis and the quantity of fed biomass. The quantities of bio-char and liquid were determined by weighing the respective mass obtained after experiments. As described above, the flow rate of noncondensable gases was not quantified after its generation, because they were directly burned in the combustion chamber to heat the fluidizing air in a continuous process. Thus, the mass flow rate of non-condensable gases was calculated from the sum of the amounts of biomass and the air supplied less the amounts of bio-oil and ash produced. It was also considered that the air used for fluidization fully reacted with part of biomass fed and formed water which left in the bio-oil and non-condensable gases which were burnt in the combustion chamber. The energy balance was acquired

Fig. 1. Flow diagram of the fluidized bed pyrolysis system.

N. Sellin et al. / Renewable Energy 96 (2016) 56e64

based on the mass balance and higher heating value (HHV) data of the biomass and products from pyrolysis. For the energy efficiency calculation, the energy consumption by equipment during the preparation (milling step) of the banana leaves and pyrolysis process, the primary energy of the biomass and the useful energy were considered. 2.4. Characterization of bio-oil and bio-char The liquid fraction obtained in pyrolysis is composed of light bio-oil (pyroligneous acid) and heavy bio-oil. The C, H and N percentage was determined by the ASTM D5291-92 and S by the ASTM D4294-10 in SC-LECO analyzer. The oxygen percentage was calculated by difference. The higher heating value of the two phases was measured using an IKA S200 oxygen bomb calorimeter, following ASTM D4809-00 standard method. Analysis of the chemical compounds present in the heavy bio-oil samples was carried out by Fourier Transform Infrared Spectroscopy (FTIR) in a Perkin Elmer One B brand spectrophotometer, in 12 scans of 4000 cm1 at 650 cm1, resolution of 4 cm1, using the Attenuated Total Reflection (ATR) accessory. The light bio-oil was characterized by Gas Chromatography coupled to Mass Spectrometry (GC/MS). The sample was injected directly into Agilent 7890A chromatograph, with HP-5 column. Compound identification was carried out by comparison with the mass spectra obtained with NIST 05 Mass Spectral Library, part of the equipment. The bio-char resulting from the pyrolysis process was characterized by proximate chemical analysis (moisture, volatile matter, ash and fixed carbon) according to ASTM D1762-84, and elemental chemical analysis (CHNS) and higher heating value (HHV) using the same procedures and standards as those used for the biomass (banana leaves). For evaluating thermal decomposition behavior of bio-char under combustion, Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) were carried out under oxidizing atmosphere, ambient temperature range up to 900
C, heating rate of 10
C/min, in simultaneous thermal analyzer, Netzsch model STA 449F3. The surface structure and morphology of the bio-char were analyzed by Scanning Electron Microscopy (SEM) in ZEISS DSM 940 microscope. The samples were previously metallized in gold and the surface micrographs were obtained using voltage of 20 kV. 3. Results and discussion 3.1. Biomass characteristics and properties The physical and chemical characteristics of dried banana leaves

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are presented in Table 1, together with some results determined for other biomasses described in literature. The dried banana leaves presented physical and chemical characteristics similar to those of other biomasses already studied in literature such as sugar cane straw, elephant grass and wheat straw as it can be seen in Table 1. Moisture in the banana leaves was 7.8%. This value is within the acceptable range for the pyrolysis process, between 7% and 15% [2,13]. Elevated moisture influences the heating value of the biomass, reducing the useful energy generated and also resulting in bio-oil production with high moisture content, reducing its potential for use as a fuel [2,20]. A high volatile material content was obtained in the banana leaves (78.2%). These materials are related to the lignocellulosic fractions of the biomass, which are thermally degraded generating the vapors and gases, inducing the formation of liquid and gaseous products in pyrolysis [22]. According previous studies of our research group [2], the dried banana leaves have average hemicellulose, cellulose and lignin contents of 25.8%, 26.7% and 17.0%, respectively. The ash content of the banana leaves was 6.2%. This value is similar to the contents generally found for vegetable biomasses which can vary between 0.4% and 22.6% [2,12]. The high percentage of ash in the raw material can have further influence on the product yields and on the characteristics of bio-oil and char obtained in the pyrolysis [19]. The fixed carbon was 15.6%, close to those found in vegetable biomasses, which are generally in the range of 7%e20% [12]. The banana leaves samples exhibited high carbon (43.5%) and hydrogen (6.2%) contents and low nitrogen (0.86%) and sulfur (0.95%) contents, which is characteristic of vegetal biomasses [12,19]. The low values for nitrogen and sulfur found in biomass are important characteristics that give rise to low generation of sulfur and nitrogen oxides, which are both toxic and corrosive [24]. The higher heating value of the banana leaves (17.1 MJ/kg) is similar to those obtained for other vegetable biomasses, according to that presented in Table 1. The greater the heating value of the biomass, the greater the useful energy generated in the pyrolysis process [25]. The TGA and DTA curves in inert atmosphere of the banana leaves are shown in Fig. 2. Thermal analysis is a useful method for studying thermal stability of materials, in order to understand the pyrolytic process of biomass. It is also used for biomass characterization [26]. As shown in Fig. 2, the banana leaves exhibited four thermal degradation and energy release stages of the lignocellulosic fractions. The water evaporation contained in the biomass and decomposition of extractives occurred from room temperature up to approximately 160
C. The degradation of hemicellulose occurred from

Table 1 Physical and chemical characteristics of dried banana leaves and of other biomasses described in literature.

Proximate analysis Moisture Volatile mattera Asha Fixed carbona Elemental analysisa C H N S O Higher heating value (MJ/kg) Lower heating value (MJ/kg) a

wt % on dry basis.

Dried banana leaves

Sugar cane straw [19]

Elephant grass [22]

Wheat straw [23]

wt%

wt%

wt%

wt%

7.8 78.2 6.2 15.6

± ± ± ±

0.38 0.90 0.43 0.47

10.4 74.0 16.4 13.0

12.2 ± 0.37 67.3 ± 1.26 4.9 ± 1.23 15.5 ± 0.90

4.60 79.9 4.9 15.2

43.5 6.2 0.86 0.95 42.3 17.1 15.6

± ± ± ± ± ± ±

0.36 0.01 0.03 0.05 0.14 0.30 0.1

43.2 6.70 0.30 0.20 33.2 18.0 17.0

41.2 ± 0.52 5.5 ± 0.08 1.8 ± 0.15 e 46.6 ± 0.50 14.7 ± 0.50 e

44.9 5.71 0.63 e 43.8 17.3 16.0

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N. Sellin et al. / Renewable Energy 96 (2016) 56e64

Fig. 2. Thermal behavior of banana leaves by TGA and DTA.

approximately 175
C to 302
C. From approximately 302 to 380
C the greatest mass loss and energy release occurred (maximum peak at 340
C), attributed to the degradation cellulose and part of the lignin constituting the biomass. The degradation of the remaining lignin occurred until 700
C. From the TGA and DTA curves of the banana leaves, it was observed that the degradation was significant up to 500
C. Therefore, the process temperature for bio-oil production would be at about 500
C in order to attain the maximum degradation and volatilization of biomass. The thermal behavior observed for the banana leaves was also similar to that obtained by Fernandes et al. [2] for banana leaves, who also found three thermal degradation stages, with similar temperature profiles to those observed in Fig. 2. Yang et al. [10] found four steps of degradations for cedar sawdust, coffee bean residue, and rice straw, namely, start of decomposition of extractives (below 217
C), hemicellulose decomposition (217e377
C), cellulose and lignin decomposition (377e507
C), and lignin decomposition (above 507
C). 3.2. Mass yield of pyrolysis products The mass yield of pyrolysis products of dried banana leaves and products determined by other authors based on different biomasses are presented in Table 2. The liquid fraction yield was 27.0% (10% for light bio-oil and 17% for heavy bio-oil). The lowest liquid fraction yield obtained for banana leaves compared to the other biomasses can be attributed to differences in the reactor type, experimental setup, processing capacity and the feedstock source. Based on the thermal analysis results, the pyrolysis of banana leaves was carried out at 500
C, and air was used to fluidize the bed in the reactor and to maintain the auto-thermal process. Mesa
rez et al. [19] studied the influence of the temperature (470, 550 Pe and 600
C) on the bio-oil yield from oxidative fast pyrolysis of sugar cane straw in an experimental plant (auto-thermal process) similar to that used in this work. The authors have concluded that

the bio-oil yield (of 35.5%, as observed on Table 2) was optimized in temperature of 470
C and under higher temperature the bio-oil yield decreased, resulting in an increasing of the gas yield. The same authors reported similar yields (between 31 and 40%) of biooil from oxidative fast pyrolysis of several wastes such as sugar cane bagasse, orange bagasse and tobacco waste using the same operational conditions. According to Heidari et al. [20], the bio-oil yield decreases with increasing reaction temperature and it is maximized at pyrolysis temperature around 450
C. The gas yield of pyrolyzed banana leaves was higher than observed for the other biomasses. The reduction of liquid yield and increase in the gas yield occur due to secondary cracking of pyrolysis vapors and liquid product in high temperature. The oxidative fast pyrolysis also contributes to the decrease in the yield of bio-oil because part of the biomass is burned with air in the reactor and volatilized thereby producing water and non-condensable gases. The water and the non-condensable gases will leave in the bio-oil and pyrolytic gas product, respectively. Another explanation for the low bio-oil yield can be due to separation stages (condensation and centrifugation) of bio-oil in fast pyrolysis plant, since gases and liquids are formed in the process and part of the liquid can have been dragged in flow of non-condensable gases. A further study of the air-biomass ratio used in oxidative fast pyrolysis process and the improvement of bio-oil phase separation process, can improve the bio-oil yield as well as their properties. The solid fraction (bio-char) presented the lowest yield (23.3%) among the products generated in fast pyrolysis of banana leaves, however, it was higher than that obtained for other biomass. The bio-char yield is directly linked to the ash, fixed carbon and lignin amounts present in the material. As observed in Table 1, sugar cane straw presents higher ash content than the others biomass and higher solid fraction in the products.

3.3. Analysis of pyrolysis products 3.3.1. Bio-oil characteristics and properties The bio-oil consisted of two phases, namely, the heavy bio-oil and light bio-oil (pyroligneous acid), as shown in Fig. 3. The former was a viscous and opaque black liquid (Fig. 3a), and the latter had lower viscosity (Fig. 3b). Different condensation temperatures oil production by biomass fast pyrolysis lead to different oil composition, as discussed later. Many chemical compounds found in pyroligneous acid are related to the degradation of polymeric carbohydrates (cellulose and hemicellulose) and lignin which contain guaiacyl and syringyl units, among others, in biomass [27e29]. The presence of complex chemical compounds possessing varied chemical functions such as organic acids, nonaromatic ketones, furans, furfural, lignin-derived phenols, guaiacols (methoxy-phenols) and syringols (dimethoxy-

Table 2 Mass yield (%) for products generated in pyrolysis of banana leaves and other biomasses described in literature. Biomass

Liquids (%)

Solids (%)

Gases (%)

Banana leaves Sugar cane straw [19] Wheat straw [23] Switch grass [23] Beech wood [23] Miscanthus giganteus [27]

27.0 35.5 44.9 63.4 72.7 64.0

23.3 48.2 28.0 20.0 14.4 13.7

49.6 20.4 26.9 16.6 13.0 22.2

Fig. 3. Heavy (a) and light (b) bio-oil from banana leaves.

N. Sellin et al. / Renewable Energy 96 (2016) 56e64

phenols) can be observed in Table 3. The presence of organic acids, as well as aldehydes and phenols compounds confers an acidic character to bio-oil. The pH of bio-oils generally ranged between 2.0 and 3.7. However, it also can be higher [19,30]. The water contents of heavy and light bio-oil were not determined in this study, however, as the pyroligneous acid or acid water was separated from the heavy bio-oil by centrifugation during the pyrolysis process, it is expected that the water content is low in heavy bio-oil. The water content depends on the type of biomass, operational conditions and technology used. In previous studies, the same separation steps were used and the moisture content of heavy bio-oil from sugar cane straw was less than 3 wt% [19]. FTIR/ATR spectrum of heavy bio-oil from banana leaves pyrolysis is shown in Fig. 4. Table 4 shows the main functional chemical groups and corresponding wave number observed in FTIR/ATR spectrum of heavy bio-oil. Compounds such as ethers, alcohols, aliphatic hydrocarbons, aromatic hydrocarbons, phenols, and others are present. These compounds were also observed for others biomasses as cotton seed [31], safflower seed [32], apricot stone, hazelnut shell, grapeseed and chestnut shell [33]. Light and heavy bio-oil contain similar constituents regardless of the composition of the biomass, differing only in the amounts of components. Differences in the two bio-oil phases resulted from differences in the condensation temperatures, which influenced the water content, compounds, and bio-oil properties. The heavy bio-oil is collected mainly at higher condensation temperatures and contain less water and higher-molecular-weight compounds. In contrast, the light bio-oil requires lower condensation temperatures and therefore contain more water and low-molecular-weight compounds [10,19,34]. Among the compounds, hydrocarbons are valuable components in bio-oil for fuel application. Specifically, aromatic hydrocarbons serve as important industrial chemicals [30,35]. Table 5 shows the results of the CHNS analyses and higher heating value for the two bio-oil (heavy and light) phases obtained from pyrolysis of banana leaves. The heavy bio-oil has a higher heating value of 25.0 MJ/kg. This value is higher than the HHV obtained by Greenhalf et al. [23], of 22.0 MJ/kg for the wheat straw, 22.3 MJ/kg for switch straw and 18.8 MJ/kg for beech wood, and by

61

Fig. 4. FTIR/ATR spectrum of heavy bio-oil from banana leaves pyrolysis.

Table 4 Main functional chemical groups of heavy bio-oil obtained from FTIR/ATR spectrum. Class of compounds

Groups

Wave number (cm1)

Alcohols, phenols Alkanes (aliphatic) Aldehydes, ketones Alkenes, aromatics Alkanes (aliphatic) Aromatic ethers Alcohols, phenols, esters Aromatic groups

OeH CeH C]O C]C CeH CeO CeO OeH

3400e3000 3000e2800 1750e1650 1600e1450 1450e1350 1275 1300e1050 900e690


rez et al. of 13.0 MJ/kg for the sugar cane straw [19]. The Mesa-Pe high carbon content (55.96%) in heavy bio-oil corroborates the great potential for its use as fuel, as do the low sulfur and nitrogen contents. The light phase showed a low higher heating value due to the chemical characteristics described above. It can be concluded of these results that heavy bio-oil and pyroligneous acid can be used, respectively, as an alternative biofuel and chemical feedstock after some purifying and improving processes (such as Fischer-Tropsch synthesis, cracking, hydrogeneration, deoxygenation, etc.), or directly, as additives in the

Table 3 Main chemical compounds determined by GC/MS of light bio-oil from fast pyrolysis of banana leaves, their respective condensed formulas, retention time in column (tR) and relative area. #

Identified compound

Condensed formula

tR (min)

Relative area (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Acetic acid Propanone Propanoic acid Pyridine 1-Hidroxy-2-butanone 2-Furaldehyde Acetol acetate 2-Cyclopenten-1-one, 2-methyl 2(5H)-Furanone 2,5-Hexanedione 2(5H)-Furanone,5-methyl 2-Formyl-5-methylfuran Phenol 1,2-Cyclopentanedione,3-methyl 2-Cyclopenten-1-one, 2,3-dimethyl Phenol, 4-methyl o-Methoxyphenol 2-Cyclopenten-1-one, 3-ethyl-2-hydroxy p-Ethylphenol p-Cresol, 2-methoxy 4-Ethylguaicol Phenol, 2,6-dimethoxy-eugenol

C2H4O2 C3H6O C3H6O2 C5H5N C4H8O2 C5H4O2 C5H8O3 C6H8O C4H4O2 C6H10O2 C5H6O2 C6H6O2 C6H6O C6H8O2 C7H10O C7H8O C7H8O2 C7H10O2 C8H10O C10H12O3 C9H12O2 C8H10O3

1.92 2.09 2.31 2.76 2.94 3.22 4.00 4.33 4.50 4.64 4.76 4.95 5.10 5.62 5.71 5.99 6.17 6.43 6.78 7.04 7.70 8.27

21.6 35.1 0.3 0.4 2.9 12.9 4.8 1.4 2.0 0.4 0.5 1.0 1.0 2.5 0.7 1.0 4.9 0.4 0.4 2.1 0.6 0.1

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Table 5 CHNS and higher heating value of light and heavy bio-oil. Sample

Carbon (%)

Hydrogen (%)

Nitrogen (%)

Sulfur (%)

Oxygen (%)

HHV (MJ/kg)

Bio-oil (heavy) Bio-oil (light)

55.9 16.9

7.8 8.8

0.87 e

0.08 0.01

35.3 74.3

25.0 1.2

formulation of emulsifying, foaming, and extraction agents, among other applications [19,33]. 3.3.2. Bio-char characteristics and properties Proximate and elemental chemical analyses and the higher heating value of vegetable bio-char obtained from fast pyrolysis of dried banana leaves and other biomasses described in literature are presented in Table 6. The physical and chemical characteristics of bio-char depend on the biomass and the experimental conditions of the fast pyrolysis process. The bio-char generated in pyrolysis of banana leaves presented low moisture and fixed carbon contents and high volatile matter and elemental carbon compared to other biomasses presented on Table 6. The high volatile matter and low fixed carbon suggest that part of the material was not thermally degraded in fast pyrolysis and this also explains the low liquid fraction yield in the products generated. The ash content of bio-char was higher than the biomass due to the mineral matter which form ash remains in the bio-char after pyrolysis. The higher heating value of banana leaves bio-char, 18.2 MJ/kg, is in the range found for vegetal biomasses as showed in Table 6 and described in literature. The low moisture content and high carbon and volatile matter contributes to high higher heating value. Just like the banana leaves, the bio-char presented low nitrogen and sulfur contents. These results show the potential application as replacement for combustible solid fuels. In Fig. 5 the thermogravimetric analysis (TGA) and differential thermal analysis (DTA) curves in oxidizing atmosphere of bio-char obtained from pyrolysis of banana leaves are presented. It was verified in the TGA and DTA curves that the banana leaves bio-char presents four main stages of thermal degradation of lignocellulosic fractions and three thermal events. From ambient temperature up to approximately 150
C there was a mass loss related to the moisture present on bio-char. The hemicellulose degradation occurred from 150 to 215
C. From 215 to 530
C, there was a greater mass loss related to the degradation of cellulosic and part of lignin. Above 530
C, degradation of the remaining lignin occurred. The exothermic peak with a high flow of released heat observed in the second stage (DTA curve) of sample degradation suggests that there is still non pyrolyzed solid material, such as volatile and carbon compounds as seen in the proximate and

Fig. 5. Thermal behavior of bio-char in TGA and DTA under oxidizing atmosphere.

elemental analyses. In this case, the material in a combustion process has significant heating value to be used as fuel, alone or mixed with other fuels in energy generation. SEM micrographs in different magnifications of bio-char obtained from pyrolysis of banana leaves are shown in Fig. 6. The bio-char presented particles with very irregular shapes and sizes (Fig. 6a). This difference can be due to the size and structure of biomass after grinding. From Fig. 6b, pores with an elongated shapes and with sizes varying of 10e20 mm can be observed. With a magnification of 1000 times, Fig. 6c, it is evident the hexagonal shape of the pores, which is very similar to the honeycomb structure. These structures indicate that there is still lignocellulosic material that can be pyrolyzed or used as fuel. The pores and holes formed in bio-char structure is because of the evolution of volatile € matter during pyrolysis process. According Ozçimen et al. [33], the extent of devolatilization has a significant effect on the characteristics of the produced biochar. Higher volatile matter release produces biochars with lower densities, higher porosities and significantly different pore structure. Thus, it is possible to use the biochar as a carbon feedstock for producing different carbon

Table 6 Physical and chemical characteristics of bio-char from pyrolysis of dried banana leaves and other biomasses.

Proximate analysis Moisture Volatile mattera Asha Fixed carbona Elemental analysisa C H N S Higher heating value (MJ/kg) a

wt.% on dry basis.

Banana leaves bio-char

Sugar cane bio-char [19]

Eucalyptus grandis bio-char [36]

Corn stover bio-char [36]

wt%

wt%

wt%

wt%

1.68 53.2 23.5 23.2

± ± ± ±

0.12 1.01 0.27 0.01

e 36.4 33.8 30.1

4.0 15.7 5.0 75.3

4.0 17.7 30.4 47.9

48.0 3.2 1.2 0.33 18.2

± ± ± ± ±

1.36 0.12 0.07 0.05 0.1

41.3 2.9 0.40 0.10 14.15

79.5 2.77 0.17 0.27 30.7

56.7 1.89 0.72 0.22 19.8

N. Sellin et al. / Renewable Energy 96 (2016) 56e64

63

Fig. 6. SEM micrographs in different magnifications (a) 50, (b) 350 and (c) 1k of bio-char obtained from pyrolysis of banana leaves.

materials such as activated carbon, carbon nanotubes and carbon fibres. The use of bio-char offers great advantages compared to the use of mineral charcoal, due to not containing mercury, sulfur and lead, and also for its low ash index. Such aspects make bio-char a potential alternative for iron ore processing and various other sectors such as sugar refining, as a fertilizer, adsorbent, or in the production of synthesis gases with low tar index [18,37,38]. It also used as cheap adsorbent for some pollutants such as arsenic, cadmium and lead. Furthermore, biochar can added to soil to enhance soil quality and replace carbon, nitrogen and other nutrients that is important for plant growth [39,40].

3.4. Energy consumption and energy yield of pyrolysis products Table 7 presents the energy consumption by equipment during the preparation (milling step) of the banana leaves and pyrolysis process and in Table 8 is presented the energy efficiency for products from oxidative fast pyrolysis. The total useful energy obtained from pyrolysis process was 35,270.8 kcal/h for the total biomass fed at the plant. The higher energy efficiencies were obtained for gases and bio-char. The

Table 7 Energy consumed in biomass preparation and fast oxidative pyrolysis process. Energy consumed (EC) in the process

Consumption (kWh)

Crusher Blower motor Thread feed motor Motor recovery Feeding unit Combustion chamber Total energy consumed (TEC)

2.49 0.38 0.38 0.38 0.31 1.64 5.58 kWh (¼4798.8 kcal/h)

variations found in the results point to the need for specific analysis under reactor operating conditions in order to adapt the parameters to better exploit the banana leaves biomass. The results attained show the potential of using the banana leaves in the fast oxidative pyrolysis process, and considering this a pioneering study, greater liquid fraction yields could be attained if process conditions were adapted, thus improving the energy yield. 4. Conclusions The oxidative fast pyrolysis of banana leaves in an auto-thermal fluidized bed reactor was investigated. The product yields from pyrolysis of banana leaves were 27.0% for the liquids, 23.3% for the solids and 49.6% for the gases. Bio-oil was separated in two phases, light (pyroligneous acid) and heavy. Within the chemical compounds found in light bio-oil, acetic acid, propanone and furfural, phenol, 4-methyl-phenol, o-methoxyphenol and p-ethylphenol offer industrial applications and high market value. The heavy biooil showed a higher heating value of 25 MJ/kg, and low nitrogen and elemental sulfur contents, presenting characteristics for use as fuel after application of some purifying and improving process. The bio-char presented a porous structure, rich in carbon, with high heating value and relatively pollution-free, enabling it to be applied as solid fuel. A useful power generated from products, 41.0 kWh, was determined in the energy balance, showing energy feasibility in carrying out the process. The dried banana leaves showed potential for generating chemical inputs and fuel products by oxidative fast pyrolysis. The study and development of this technology, in the specific case of Brazil and other less developed countries, will generate the conditions needed to produce, in addition to bio-oil, primary charcoal from low market prices inputs but that have high levels of production of dry matter per hectare per year. The markets for charcoal of fast pyrolysis can be the domestic sector, the industrial sector and as an input for the activated charcoal production.

Table 8 Energy efficiency for products from pyrolysis of banana leaves. Products

Energy of products (kcal/h)

Energy efficiency - based on TPEa (%)

Energy efficiency e based on TUEb (%)

Bio-oil (heavy) Bio-oil (light) Bio-char Gases

7704.5 561.9 13,244.6 20,630.9

20.3 1.48 34.9 54.3

21.8 1.59 37.5 58.5

a b

Total primary energy (TPE) ¼ 37,927.8 kcal/h. Total useful energy (TUE) ¼ TPE  TEC ¼ 35,270.8 kcal/h ¼ 41.0 kWh.

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