Article pubs.acs.org/EF
Valorization of Sugar Beet Pulp via Torrefaction with a Focus on the Effect of the Preliminary Extraction of Pectins Paola Brachi,*,§ Evelina Riianova,⊥ Michele Miccio,† Francesco Miccio,‡ Giovanna Ruoppolo,§ and Riccardo Chirone§ §
Institute for Research on Combustion, National Research Council, P.le Tecchio 80, 80125 Napoli, Italy Department of Production Safety and Industrial Ecology, Ufa State Aviation Technical University, K. Marks 12, 450077 Ufa, Russian Federation † Department of Industrial Engineering, University of Salerno, via Giovanni Paolo II 132, 84084 Fisciano (SA), Italy ‡ Institute of Science and Technology for Ceramics (ISTEC-CNR), via Granarolo 64, 48018 Faenza (RA), Italy ⊥
ABSTRACT: An agro-industrial residue, i.e., sugar beet pulp, was taken into consideration in this work as a feedstock for valorization as a solid fuel and, potentially, as a source of valuable biochemicals obtainable from the torgas condensable fraction. To this end, an experimental program based on torrefaction of such a residue after pectin extraction (PE-SBP) was performed. The alternative scenario of raw sugar beet pulp (raw-SBP) torrefaction was also investigated for comparison. Raw biomasses and torrefaction products were analyzed by different techniques including thermogravimetric analysis and derivative thermogravimetry (TGA-DTG), Fourier transform infrared spectroscopy (FTIR), gas chromatrography coupled to mass spectrometry (GC/MS), and proximate and ultimate analyses. This allowed the comparative investigation of the role played by the pectin extraction method and the torrefaction temperature on the process performance and main properties of the resulting solid products. Outcomes showed that light torrefaction (200−240 °C) is a suitable and more energy-efficient process for production of high quality solid fuels from SBP. Moreover, it resulted that PE-SBP is better than raw-SBP as a feedstock due to its lower nitrogen and ash content.
1. INTRODUCTION Sugar beet is the second largest source of sugar across the world, after sugar cane. In 2009, approximately 20% of the world’s sugar production (153.4 million tons) was obtained from sugar beet.1 Sugar beet pulp (SBP) is the main solid byproduct of the sugar beet industry. On a dry mass basis, about 130 kg of sugar and 50 kg of dried SBP can be obtained from 1 t. 2 Due to its high organic matter content (carbohydrates, protein, fat, oil, etc.), SBP represents a cheap (around 110 €/t dry matter) and abundant (e.g., the production in Europe is around 14 million t/y dry matter) source of valuable biomass and nutrients.3 In detail, its carbohydrate (cellulose, hemicellulose, pectin, and others) content has been reported to be as high as 75−85% (w/w, dry basis) and its lignin content as low as 1−4% (w/w, dry basis). SBP contains approximately 25% wt pectin, 24% wt cellulose, and 36% wt hemicellulose.4 At present SBP is mostly sold as animal feed at a relatively low price due to its relatively low protein content compared to the requirements of most ruminants, which entail the use of an extra protein source.5 However, alternative uses of such valuable byproducts of the sugar industry are currently being investigated to enhance its valorization. These mostly include the use of SBP as a carbohydrate source for the production of (i) food fibers5 to be incorporated into bread, cookies, spaghetti, and meat products and (ii) biofuels, especially liquid fuels such as bio-oil6 and bioethanol.7 Chen et al.8 also investigated the use of SBP as a filler in poly(lactic acid) composites, whereas Pavier and Gandini9 evaluated the possible © XXXX American Chemical Society
use of SBP as a source of polyol for the production of urethanes and polyurethanes. Due to its high content of pectic polysaccharides, SBP could also be a potential source of pectins, which are substances currently used in industry mainly for their gelling and thickening properties. Pectins from SBP have poor gelling properties compared with commercial pectins from apple pomace or citrus peels under classical conditions. However, SBP pectins have already proved to have great potential for alternative applications that justify its extraction. In particular, these include the use of SBP pectins as (a) cloud stabilizers in drinks;3 (b) water-absorbing agents in sanitary products;3 and (c) substrates for the biosorption of copper, cadmium, and lead.10 On the other side, pectin extraction from SBP leaves a solid residue, which has currently no known use and poses a disposal issue. In this context, the potential valorization of pectin-extracted sugar beet pulp (PE-SBP) as a solid fuel was taken into consideration in this work. The major challenge in using such a residue (PE-SBP) as an energy source lies in the operational and logistic limitations associated with its inherent chemical and physical properties. Pectin-free beet pulp, being of biological origin, has high oxygen content, low calorific value, low bulk density, high moisture content, pronounced hygroscopic behavior, and a strong tendency to biodegrade Received: June 21, 2017 Revised: August 5, 2017
A
DOI: 10.1021/acs.energyfuels.7b01766 Energy Fuels XXXX, XXX, XXX−XXX
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prevent undesired microbial degradation, SBP samples were sealed in a plastic bag and stored at −20 °C. Prior to use, SBP samples were placed in a ventilated fume hood for approximately 48 h of exposure to fresh air, which reduced its moisture content from about 73 wt % to about 6% wt. After air-drying and before any test or analysis, the moisture content of biomass samples was determined using a Kern DBS Halogen Moisture analyzer. When required, SBP samples were also ground in a batch knife mill (Grindomix GM 300 by Retsch) at 3200 rpm for 20 s, one or more times depending on the desired particle size, and then manually sieved in order to select the particle size range required for the specific use, which was specifically (i) 0− 400 μm for chemical analyses; (ii) 400−1000 μm for pectin extraction process; and (iii) 1−2 mm for torrefaction tests. It is worth noting that PE-SBP samples arising from pectin extraction process consisted of agglomerated particles having a size distribution larger than that originally used for the extraction process. Therefore, prior to be subjected to the torrefaction treatment PE-SBP samples were manually ground in a small batch grinder (Golden Bell grinder) and then sieved into the desired particle size range (i.e., 1−2 mm). In particular, pectins were isolated from raw-SBP according to the citric acid extraction method described by Ma et al.,20 which yielded approximately 25% wt crude pectin and 75% wt pectin-free solid residue (PE-SBP) on dry basis. More detailed information about extraction procedures and properties of pectins isolated from sugar beet pulp can be found elsewhere.21 The determination of moisture content (M), volatile matter (VM), fixed carbon (FC), and ashes (ASH) in raw and torrefied biomass samples was performed by using a TGA 701 LECO thermogravimetric analyzer by following the ASTM D5142 norm. Elemental composition (CHN) of samples was determined by using a CHN 2000 LECO analyzer. The oxygen content was calculated by subtraction of the ash and CHN content from the total. All these tests were made in duplicate at least. The higher heating value (HHV, kJ/kg, dry basis) of raw and torrefied SBP samples was evaluated based on ultimate analysis data by using the experimental correlation (eq 1) proposed by Channiwala and Parikh:22
(e.g., fungal attack). The high oxygen content of PE-SBP reduces the heating value and thereby makes it a lower-grade fuel. The fibrous nature of SBP not only increases its grinding cost but also is responsible for the inconsistency in the particle size. The pulverized particles from fibrous biomass are typically coarse and slender in nature. They typically have a low sphericity, which reduces flowability and fluidizability, thus reducing the performance of thermal conversion plants like entrained flow gasifier and fluidized bed reactors.11 Moreover, the hygroscopic nature makes PE-SBP reabsorb moisture from the surrounding atmosphere after drying, and accordingly, it cannot be stored outdoors. Torrefaction is a relatively new thermal pretreatment of biomass that, over the past 10 years, has been recognized as a technically feasible method for converting any lignocellulosic material into a high-energy density, hydrophobic, compactable, easily grindable, and biochemically stable coal-like solid, which is suitable for commercial and residential combustion and gasification applications.12 Basically, torrefaction is a thermochemical process where biomass is heated in an inert environment to a temperature ranging between 200 and 300 °C. It is traditionally characterized by low particle heating rate (typically less than 50 °C/min) and by a relatively long reactor residence time that typically ranges from 30 to 120 min depending on feedstock, technology, and temperature.11 So far, most of the research and development (R&D) work on torrefaction has been largely based on clean and dry biomass resources such as waste wood. Nevertheless, due to the lower price and the larger availability, the interest in residual biomass from agro-industry (e.g., tomato peels,12,13 orange skins,14 sunflowers,15 etc.) and waste streams (e.g., municipal solid waste,16 food waste,17 chicken litter, and digester sludge18) as a feedstock for torrefaction is currently increasing. However, due to the very dissimilar characteristics (i.e., composition and reactivity) of the different byproduct from agro-industry and waste streams, the potential benefits arising from torrefaction are hard to generalize and must be evaluated on a case-by-case basis. With this background, the main aim of this work was to assess the potential of torrefaction for upgrading low-value, exhausted sugar beet pulp after pectin extraction into a solid biofuel of higher quality. Accordingly, an experimental study was performed to investigate the effect of the most important process variable (i.e., temperature) on both the torrefaction performance parameters (i.e., mass yield, energy yield and energy densification index of torrefied solids) and the main properties of torrefied PE-SBP product as a solid fuel (i.e., the low heating value, the ratio of fixed carbon to volatile matter, the H/C and O/C ratios). The composition of the condensable fraction of the torgas evolved during torrefaction tests was also analyzed in this work, which may be a point of interest for the production of green chemicals.19 The alternative scenario posed by the direct torrefaction of raw sugar beet pulp (raw-SBP) was also investigated for comparison. This study is the first research work on torrefaction of both PE-SBP and raw-SBP. Therefore, the results of the present research work may be of great scientific and practical interest.
HHV (MJ/kg) = 0.3491C + 1.1783H + 0.1005S − 0.1034O − 0.0151N − 0.0211ASH LHV (MJ/kg) = HHV (MJ/kg) − 2.442(8.936H/100)
(1) (2)
where C, H, S, O, N, and ASH are weight fraction (%) of carbon, hydrogen, sulfur, oxygen, nitrogen, and ash of samples on a dry basis, respectively. Since biomass typically contains a negligible amount of sulfur, it was assumed, with tolerable approximation, that S was equal to zero for both samples. The conversion of higher to lower heating values in megajoules per kilogram (dry basis) was performed according to eq 2. Thermogravimetric analyses were carried out on both air-dried rawand pectin-free SBP in order to investigate their thermal degradation behavior and, in particular, determine the onset decomposition temperature of each feedstock. In more details, dynamic runs at a heating rate of 5 °C/min were carried out over the temperature range from ambient to 700 °C by using a TA Instruments analyzer Q600 SDT. Nitrogen was used as the purge gas at a flow rate of 100 mL/min to ensure an inert atmosphere and to prevent secondary reactions by volatiles produced during the solid thermal decomposition. Low sample mass (about 12−13 mg) and small particle size (0−400 μm) were selected in order to reduce the effect of intraparticle mass and heat transport limitations during analyses. Spectroscopic characterization (FTIR) of raw and torrefied SBP samples was performed by using a Thermo Nicolet Nexus FTIR spectrometer. FTIR spectra were obtained using KBr discs containing about 10%wt. finely ground samples. Thirty-two scans were taken for each sample and recorded at wave numbers from 4000 to 400 cm−1 with a resolution of 4 cm−1. A background spectrum was obtained using a pure KBr disk prior to each test. The analysis of condensable volatile products, which were quantitatively recovered from the walls of the impinger bottles by
2. EXPERIMENTAL SECTION 2.1. Material Sampling, Characterization, and Pretreatments. Sugar beet pulp (raw-SBP) used in this work was obtained from a sugar beet processing plant located in Bologna (45° 27′ 56″ N, 9° 11′ 11″ E), Italy. In order to preserve its original quality and B
DOI: 10.1021/acs.energyfuels.7b01766 Energy Fuels XXXX, XXX, XXX−XXX
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(MYLIQUID), and gaseous (MYGAS) products were evaluated on an as-received basis (ar) through the following eqs 3−5. The energy densification index (IED) and the energy yield (EY) of torrefied solids were also evaluated on a dry basis by means of eqs 6 and 7.
washing them with acetone, was performed off-line with a gas chromatograph (HP 9600 series) equipped with a flame ionization detector (FID) using the same dilution ratio for all samples. 2.2. Experimental Apparatus and Test Procedures. A schematic representation of the bench-scale fixed bed apparatus used for torrefaction experiments is shown in Figure 1. The torrefaction
⎛m ⎞ MYSOLID (%, ar) = ⎜ torrefiedsolid ⎟ × 100 ⎝ mSBPfeedstock ⎠
(3)
⎛m ⎞ MYLIQUID (%, ar) = ⎜ condensable ⎟ × 100 ⎝ mSBPfeedstock ⎠
(4)
MYGAS (%, ar) = 100 − MYSOLID (%, ar) − MYLIQUID (%, ar) (5)
IED (− , db) =
LHVtorrefiedsolid LHVSBPfeedstock
(6)
db
EYSOLID (%, db) = MYSOLID (%, db)IED (− , db)
(7)
3. RESULTS AND DISCUSSION 3.1. Chemical Composition and Calorific Value of Raw-SBP and PE-SBP. Table 1 reports the results of Table 1. Chemical Composition and the Calorific Value of SBP raw-SBP moisture (wt %, as received) 73.15 Proximate Analysis (wt %, dry basis) volatile matter 79.04 fixed carbon 16.94 ash 4.24 Ultimate Analysis (wt %, daf basis) C 45.66 H 6.82 N 1.00 O (by diff) 46.52 Calorific Value Analysis (MJ/kg, dry basis) HHVa 18.12 LHVb 16.71
Figure 1. Schematic of fixed bed torrefaction experimental apparatus. reactor consists of a quartz tube (25 mm inner diameter and 150 mm length) surrounded by electrical heating tape (FGR-060/240 V-Rope Heater 250 W by Omegalux). The temperature of the reactor is regulated by means of an electronic PID controller (Gefran 600 PID), which reads the bed temperature by means of a K-type thermocouple inserted in the center of the reactor. A flow meter with 0.15−1.5 NL/ min flow range (Asameter Model E, by ASA) supplies nitrogen continuously during the test. Specifically, the carrier gas percolates the biomass bed downward and leaves the reactor from the bottom. A cold horizontal glass tubular trap followed by an impinger bottle is used to condensate torrefaction volatiles. The reactor was loaded with approximately 3 g of air-dried biomass particles in the size range 1− 2 mm, which were uniformly mixed with about 28 g of alumina spheres in the size range 400−600 μm (Sasol alumina spheres 0.6/ 170) to ensure a better temperature control throughout the packed bed and prevent the occurrence of localized hotspots. Biomass feedstock has a very low thermal conductivity and heat capacity. These properties coupled with the thermal effects involved in the torrefaction process may create temperature gradients, mostly in torrefaction unit where the heat transfer coefficient is low (e.g., fixed-bed reactor), which may affect the torrefaction performance.12 The large thermal inertia and the higher thermal conductivity of alumina spheres compared to SBP particles helped to overcome this drawback. Torrefaction tests were carried out at three different temperatures (i.e., 200, 250, and 300 °C) by keeping the reaction time constant at 30 min. In particular, after the evacuation of air from the system by flowing N2 through the bed at 1.5 NL/min for approximately 10 min, the reactor was heated up to the target temperature with a thermal ramp rate of about 15−20 °C/min. Once the prefixed test time was passed, the bed was cooled down as fast as possible by turning the electrical heater off and blowing cold compressed air onto the surface of the reactor. Finally, the solid and liquid products were recovered and weighted. The amount of torrefied biomass was calculated by subtracting the initial mass of alumina particles from the total bed mass. The weight of noncondensable gaseous products was calculated by difference. The solid product was separated from the inert bed component by sieving. Mass yields of solid (MYSOLID), liquid
PE-SBP 9.06c 78.10 19.32 2.57 45.44 6.98 0.79 46.79 18.69 17.21
a
HHV was estimated by means of the correlation (eq 1). bLHV was calculated based on eq 2. cAfter vacuum filtration.
proximate and ultimate analyses performed on both raw- and pectin-free sugar beet pulp. In keeping with previous research findings,23,24 data show that the residue left after the acid extraction of pectin (PE-SBP) had a lower ash content compared to untreated sugar beet pulp (raw-SBP), namely 2.57 versus 4.24% wt on dry basis; this makes PE-SBP better suited than raw-SBP for torrefaction due to its lower ash content. It also results that the ratio of fixed carbon to volatile matter (i.e., the fuel ratio) of sugar beet pulp underwent a slight increase from 0.20 to 0.25 (dry ash free basis) after the acid extraction of pectin. Actually, the fuel ratio provides a measure of the difficulty with which the fuel can be devolatilized and subsequently gasified, or oxidized, depending on how the biomass is to be utilized as an energy source. No appreciable differences were observed in the elemental composition and the calorific value of raw-SBP and PE-SBP, except for a small decrease of the nitrogen content. 3.2. Thermogravimetric Analysis. Figure 2a and b shows the weight loss (TG) and the derivative weight loss (DTG) curves of both raw- and pectin-free sugar beet pulp samples, C
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respectively. The first region (25−135 °C) was attributed to the removal of water and light volatiles,26 the second decomposition stage (135−360 °C) was ascribed to the thermal degradation of polysaccharides (i.e., pectin, hemicelluloses and cellulose), whereas the third region located between 360 and 700 °C was ascribed both to the slow thermal degradation of lignin (aromatic polymer), which typically occurs without a characteristic peak over a wider temperature range,27 and to the mass losses of minor constituents, such as waxes, fats, etc. (e.g., f, g, and p peaks), which are typically present in SBP.5,25 In more detail, the c peak at 246 °C, the d peak at 283 °C, and the e peak at 337 °C in the DTG curve of raw-SBP were attributed to the thermal degradation of pectin, hemicelluloses, and cellulose, respectively. In fact, the DTG peak temperature of pure pectin isolated from sugar beet pulp is approximately 239 °C,28 whereas hemicelluloses and cellulose typically decompose over a temperature range 160−360 and 240−390 °C, respectively.29 Similarly, the i peak at 214 °C, the m peak at 242 °C, and the n peak at 333 °C detected in the DTG curve of PE-SBP were assumed to correspond to the thermal degradation of residual pectin, hemicelluloses and cellulose, respectively. An overall conclusion drawn from this analysis is that both the thermal stability and reactivity of hemicelluloses and residual pectin appeared notably changed after pectin extraction, while the thermal degradation behavior of cellulose and lignin was not affected by the adopted acid extraction method. In particular, hemicellulose and pectin components of PE-SBP were found to be less thermostable and less reactive than those of raw-SBP. Evidence of this was given by the fact that the peaks corresponding to pectin and hemicellulose components in the DTG curve of raw-SBP (see c and d peaks in Figure 2b) were found slightly shifted toward lower temperatures in the case of PE-SBP (see i and m peaks in Figure 2b), in addition to exhibiting lower heights (i.e., maximum decomposition rates). As a result, PE-SBP decomposed a little faster than raw-SBP and underwent a larger weight loss than raw-SBP under the same test conditions (see TG curves in Figure 2a). 3.3. Torrefaction Product Distribution and Properties of Solid Products. Table 3 shows the main results of the fixed bed torrefaction tests performed on raw-SBP and PE-SBP samples. The same data are also plotted in Figures 3−5 to comparatively show the influence of pectin extraction process and/or torrefaction severity on both the process performance parameters (i.e., mass yields, energy yields and energy densification index) and the properties of the resulting solid product (i.e., low heating value, the ratio of fixed carbon to volatile matter, H/C and O/C ratios). 3.3.1. Mass and Energy Yields. Figure 3 shows the effect of torrefaction temperature on the product distribution arising from the thermal treatment of raw and pectin-free sugar beet pulp samples at 200, 250, and 300 °C. For both feedstocks, the yield in solid product decreased when temperature increased, whereas the yield of volatile products (torgas), consisting of the condensable and the noncondensable fraction, consequently
Figure 2. TG (a) and DTG (b) curves of raw and pectin-free SBP recorded under nitrogen atmosphere at 5 °C/min heating rate from ambient temperature to 700 °C.
which were obtained through nonisothermal runs performed at 5 °C/min heating rate and under a nitrogen atmosphere. Thermogravimetric data suggest that the thermal degradation of the SBP is a complex process, which occurs in several stages as evidenced by the presence of several peaks in the DTG curve in Figure 2b. This is likely a consequence of its rather complex chemical composition, which is characterized by the presence of several macrocomponents (i.e., pectin, cellulose, hemicelluloses, and lignin) and minor constituents (e.g., proteins, fats, residual sucrose, etc.) in varying proportions.5,25 In more detail, seven peaks were found in the DTG curve of raw-SBP (i.e., a, b, c, d, e, f, and g peaks) while only five peaks (i.e., h, i, m, n, p) were found in the DTG curve of PE-SBP. The characteristic temperatures, Tmax (°C), and heights, Wmax (%/min) of DTG peaks are listed in Table 2. Data show that the TGA and DTG curves of raw-SBP and PE-SBP samples exhibit three main decomposition regions at 25−135, 135−360, and 360−700 °C, Table 2. Characteristic Points of DTG Curves raw-SBP DTG peaks
PE-SBP DTG peaks
peak
a
b
c
d
e
f
Tmax (°C) Wmax (%/min)
60 0.71
131 0.20
246 1.85
283 1.73
337 3.28
480 0.27 D
g 635 0.15
h 65 0.47
i
m 214 1.24
242 1.25
n 333 2.88
p 486 0.25
DOI: 10.1021/acs.energyfuels.7b01766 Energy Fuels XXXX, XXX, XXX−XXX
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19.83 21.48 23.37 19.21 22.54 27.08
LHV HHV
21.14 22.71 24.53 20.45 23.69 28.07 37.71 33.10 26.89 39.32 30.22 16.67
O N
2.36 2.38 2.78 1.92 2.10 2.62 6.26 5.92 5.67 5.92 5.53 4.91
H C
53.67 58.61 64.67 52.85 62.15 75.80
FC
23.85 36.26 39.40 27.55 39.61 59.95
VM
76.15 63.75 60.59 72.46 60.39 40.06 4.28 4.83 6.68 3.72 5.06 7.40
ash content (% wt, db) EYSOLID (%, db)
83.23 63.31 61.19 77.73 66.10 52.57 1.19 1.29 1.40 1.12 1.31 1.57 raw-200 raw-250 raw-300 PE-200 PE-250 PE-300
IED (−, db) (% wt, ar)
69.50 49.07 44.46 66.30 48.06 31.81
8.01 16.57 16.43 16.11 16.96 15.82
22.49 34.35 39.11 17.59 34.98 52.38
solid product
increased. Data also show that the increase detected in the yield of volatile products was mainly driven by the release of permanent gases rather than condensable compounds. A probable impact of the presence of alumina on the torgas composition is not to be ruled out and, hence, it will be the subject of our future investigations. In keeping with thermogravimetric data in Figure 2a, which show that the conversion degree of PE-SBP was slightly higher than that of Raw-SBP under the same operating conditions, the torrefaction treatment of PE-SBP resulted in lower solid yields (MYSOLID 32−66% wt, ar; EYSOLID 53−78% wt, db) compared to that of raw-SBP (MYSOLID 44−70 wt %, ar; EYSOLID 61−83% wt, db), with the largest difference having occurred for tests performed at 300 °C (see Table 3). In line with previous research findings,12,19 experimental data also indicate that more mass than energy was lost to the gas phase during the torrefaction treatment of raw-SBP and PESBP samples, as evinced by the higher values of the energy yields compared to those of mass yield (see Table 3). The energy gain versus mass loss of torrefied solids is commonly ascribed to the fact that the latter predominantly arises from the release of volatiles that are richer in oxygen and hydrogen than in carbon.12 The decrease of the hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) elemental ratios in torrefied solids, which is clearly visible on the Van Krevelen diagram in Figure 4a, provide evidence of this. As a general trend, energy yields of torrefied raw-SBP samples were higher than that of torrefied PE-SBP, except for torrefaction test at 250 °C (Figure 5b). This can be ascribed to the higher thermal stability of hemicellulose in raw-SBP compared to that in PE-SBP (see section 3.2), which resulted in a slightly higher mass yield of torrefied raw-250 sample, the latter being richer in oxygen than the PE-250 sample. It is worth noting that a major objective of torrefaction is to increase the energy density of the biomass by increasing its
PE-SBP
T (°C)
200 250 300 200 250 300
ultimate analysis (% wt, daf) proximate analysis (% wt, daf) MYG MYL MYS
Table 3. Experimental Conditions and Results of Fixed-Bed Torrefaction Experiments Performed with the Same Residence Time of 30 min
Figure 3. Effect of torrefaction temperature on the distribution of products from torrefaction tests performed on raw and pectin-free SBP (30 min reaction time).
raw-SBP
calorific value (MJ/kg, db)
Energy & Fuels
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DOI: 10.1021/acs.energyfuels.7b01766 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 4. Van-Krevelen diagram (a) and fixed carbon to volatile matter ratio (fuel ratio) of torrefied solids (b) arising from torrefaction tests performed on raw and pectin-extracted sugar beet pulp (30 min reaction time).
Figure 5. Effect of torrefaction temperature on the energy densification index and yield in torrefaction tests performed on raw and pectin-free SBP (30 min reaction time).
carbon content while decreasing its oxygen and hydrogen content. This objective is similar to that of carbonization that produces charcoal, but with an important difference, namely that torrefaction unlike carbonization aims to retain as much as possible the amount of energy of the biomass in the solid product, and thereby gives high-energy yield.30 This suggests that, when raw and pectin-free sugar beet pulp feedstocks are to be subjected to torrefaction treatment, it may be preferable to proceed via light torrefaction (200−240 °C) rather than medium (240−260 °C) or severe torrefaction (260−300 °C) in order to ensure a more effective energy-saving process. 3.3.2. Ultimate Analysis, Proximate Analysis, and Calorific Values. The elemental composition of raw- and pectin-free SBP, before the torrefaction treatment (Table 1) and after it (Table 3), is shown using the Van Krevelen diagram in Figure 4a, as expressed in terms of O/C and H/C elemental ratios, on a dry ash free (daf) basis. In more details, these data show that the higher the torrefaction temperature, the lower the hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) elemental ratios in torrefied solids and, consequently, the longer is the distance that the composition of torrefied SBP samples takes from the characteristic region of biomass moving toward that typical of low- and medium-rank fossil fuels (lignite and coal). This occurred mainly as a consequence of
dehydration reactions rather than decarboxylation reactions (Figure 4a). This effect was more marked in the case of PE-SBP compared to raw-SBP, although prior to torrefaction treatment both samples had similar hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) elemental ratios (i.e., approximately 1.8 and 0.8). In particular, it was found that after a thermal treatment at 300 °C for 30 min, the H/C and O/C elemental ratios in torrefied raw-SBP dropped to about 1.0 and 0.3, respectively, which reflect the typical chemical composition of lignite, whereas the same ratios in torrefied PE-SBP decreased further down to 0.8 and 0.2, respectively, which reflect the typical composition of coal (Figure 4a). It is worth noting that the decrease in the H/C and O/C elemental ratios induced by torrefaction makes the biomass feedstock more suitable for fuel application, resulting in less smoke and water vapor formation, and reduced energy loss during subsequent combustion and gasification processes.31 An increase in the nitrogen content of torrefied solids (Table 3) with respect to that of the untreated feedstocks (Table 1) was also detected by increasing torrefaction temperature. Table 3 shows the proximate composition of torrefied samples. As a general trend, it results that volatile matter (VM) decreased with an increase in the torrefaction temperature, while fixed carbon (FC) and ash contents increased. However, F
DOI: 10.1021/acs.energyfuels.7b01766 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels while only a slight effect on the proximate composition was observed in the case of torrefied raw-SBP samples, more significant changes occurred in PE-SBP torrefied at 300 °C, the fixed carbon-to-volatile matter ratio of which was significantly higher than 1. It is worth noting that generally biomass has a significantly higher content of volatile matter compared to coal and so its fuel ratio (i.e., the fixed carbon-to-volatile matter ratio) is typically below unity. However, the fact that PE-SBP torrefied at 300 °C had a fuel ratio higher than 1 is fully consistent with its elemental composition, which reflects that typical of coals (Figure 4a). The chemical composition changes induced by the torrefaction treatment gave rise to an increase in the calorific value of torrefied solids with respect to the parent ones (Table 3). For both investigated feedstocks, the higher the torrefaction temperature, the higher the energy densification index (IED, db) of the solid products (Figure 5a). In particular, the more substantial energy densification was detected for PE-SBP treated at 300 °C for 30 min (PE-300), whose lower heating value (LHV) increased by a factor 1.6, changing from 17.21 to 27.08 MJ/kg, dry basis. The value detected for the calorific value of PE-300 is fully consistent with its elemental composition, which reflects that typical of coals (Figure 4a). The higher heating values (HHVs) estimated by means of the correlation of Channiwala and Parikh22 (eq 1) were compared with those obtained using correlations proposed by Sheng and Azevedo32 as well as Friedl et al.33 A good agreement was found between the HHVs estimated as according to correlations by Channiwala and Parikh22 and Friedl et al.;33 conversely, data determined by means of Sheng and Azevedo32 correlations appeared slightly underestimated. 3.4. FTIR Analysis of Solids. FT-IR spectroscopy was used in this work to obtain information on the chemical and structural changes occurring in sugar beet pulp components after the pectin extraction (Figure 6) and/or in relation to the
Figure 7. FT-IR spectra of torrefied raw-SBP and PE-SBP samples. All spectra have been separated to ease the comparison.
hydrogen-bonded O−H groups in SBP components, such as phenolic OH in lignin,34 carboxylic acid O−H in pectin35 and extractives,36 and aliphatic hydroxyl (−OH) group in cellulose and hemicelluloses;36 (ii) a narrow C−H stretching absorption band at approximately 2900 cm−1 that, in keeping with the work of Shang et al.,37 was ascribed to the aliphatic fraction of wax present in SBP. The band for C−H stretching appeared to not change significantly neither due to the acid treatment of pectin extraction nor due to the thermal treatment of torrefaction, even though a small decrease of this band was observed for the highest investigated torrefaction temperature (300 °C). This suggests that higher molecular weight waxes are still present in the samples torrefied at 300 °C. This is consistent with the presence of decomposition peaks (i.e., f, g, and p) at temperature higher than 300 °C in the DTG curves of raw-SBP and PE-SBP shown in Figure 2. However, the assignment of the absorption band at 2930 cm−1 also to the stretching of the O−CH3 bond arising from methyl esters of galacturonic acids present in residual pectin cannot to be completely ruled out.38 In contrast to the high degree of similarities in the functional group area of the spectra, changes that are more significant were observed in the fingerprint region comprised between 1800 and 650 cm−1. In particular, by comparing the FT-IR spectra of raw-SBP and PE-SBP shown in
Figure 6. FT-IR spectra of raw-SBP and PE-SBP samples. All spectra have been separated to ease the comparison.
severity of torrefaction treatment (Figure 7). In particular, Figures 6 and 7 show the FT-IR spectra of untreated and torrefied raw-SBP and PE-SBP samples. All the investigated samples were found to exhibit the same absorption bands in the functional group region of the spectrum, which extends from 4000 to 1800 cm−1, in particular: (i) a broad absorption band near 3400 cm−1, which is related to the stretching vibration of G
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Table 4. Identified Components of Torgas Condensable Fractions Arising from the Thermal Treatment of Raw-SBP and PESBP Samples group
sample torrefaction temperature (°C)
alkenes alcohols
aldehydes
ketones
esters and carboxylic acids
D-limonene phenols ethanol, 2,2′-oxybismaltol 3-pyridinol p-cresol benzaldehyde, 2hydroxyfurfurals 5hydroxymethylfurfural 2-furancarboxaldehyde, 5-methyl2-cyclopenten-1-one 2-cyclopenten-1-one, 2hydroxy2-cyclopenten-1-one, 3methyl2,5-furandione, 3methyl1,2-cyclopentanedione, 3-methyllevoglucosenone 1 n-hexadecanoic acid 9-octadecenoic acid, (E)-12 cis-vaccenic acid 12 9,12-octadecadienoic acid (Z,Z)-1 1,2-benzenedicarboxylic acid, bis
raw-SBP 200 250 × ×
×
× × ×
×
group
PE-SBP 300 200 × ×
× × ×
×
250 × × × × ×
300 × × × × ×
aromatics
× × × × × ×
×
× ×
alkanes
× × × ×
sample torrefaction temperature (°C)
× × ×
× × × sugar others
4-4-methyl-[1,3,2] dioxaborinan-2-6 benzene, (1-methyl-2cyclopropen-1 1 1H-indene, 1,3dimethyl-2 1,2,3-trimethylindene 2 naphthalene,1,2,3,4tetrahydro-2,5 benzothiazole 2(3H)-benzothiazolone 1H-indole, 2,3dimethylNaphthalene, 1,6,7trimethylbenzenesulfonamide, N-butyl- 7 imidazole-4carboxamide heneicosane docosane tricosane tetracosane eicosane heptadecane octacosane octadecane cyclohexane, isocyanatononadecane D-allose levoglucosan
raw-SBP 200
250
PE-SBP 300 200
250 300 ×
×
×
×
×
×
×
× ×
× × × ×
× × ×
× ×
× × × ×
×
× ×
× × × × × × ×
× × × × ×
× ×
× ×
×
× × × ×
that thermal decomposition of cellulose polymer in SBP only starts at about 300 °C. By analyzing the FTIR spectra of untreated and torrefied solids in Figure 7a and b, the reduction was particularly noteworthy in the intensity of the absorption band at 1160 cm−1, which corresponds to the antisymmetric stretching of C−O−C glycosidic linkages in polysaccharides, such as cellulose, hemicellulose, pectin.37 This suggests that the depolymerization degree of polysaccharides increases as torrefaction temperature increases and reaches the completion point at about 300 °C, as evidenced by the absence of this band in the spectrum of raw-300 and PE-300 samples. FT-IR data in Figure 7a and b also highlighted a decrease in the intensity of the absorption band at 1742 cm−1, which corresponds to the vibrations of free carboxyl groups in pectins and hemicelluloses, and the simultaneous appearance of a new degradation product band at 1700 cm−1 as the torrefaction temperature increases. The absence of this band in the spectra of solids torrefied at 250 and 300 °C denotes the complete removal of hemicelluloses and pectins. These observations are in line with the earlier research findings of Shang et al.37 Finally, FT-IR spectra of torrefied and untreated solids in Figure 7a and b also pointed out that there were no major structural changes in samples torrefied at 200 °C, apart from the already discussed ones. Conversely, changes that are more significant can be observed by comparing the FT-IR spectra of untreated solids with those of samples torrefied at 250 and 300 °C. In particular, as far as
Figure 6, an expected slight decrease emerged in the intensity of the absorption band at 1742 cm−1, which corresponds to the vibrations of free carboxyl groups of pectin.38 This reduction in intensities was also accompanied by a slight shift of the same band to lower wavenumbers. The band at 1645 cm−1, which corresponds to the vibrations of esterified carboxyl groups of pectin,38 instead, just exhibited a shift to lower wavenumbers. Moreover, the complete disappearance of bands at 1437 and 1373 cm−1 and the appearance of a new band at 1400 cm−1 were also detected. On the basis of previous literature data,39 this phenomenon was ascribed to structural changes occurring in the cellulose fraction of SBP as a result of the acid treatment adopted for pectin extraction; absorption bands at 1437 and 1373 cm−1 are, in fact, typically observed in the FTIR spectra of crystalline cellulose, while the same are either absent in amorphous cellulose or replaced by a strong peak shifted at 1400 cm−1. A similar phenomenon can also be observed by comparing the FT-IR spectra of untreated and torrefied rawSBP samples in Figure 7a, suggesting that similar structural changes in the cellulose fraction of SBP could arise from both pectin extraction and torrefaction thermal treatment. Moreover, the fact that the absorption band at 1400 cm−1, although with less intensity, was still present in FT-IR spectra of samples torrefied at 300 °C also suggests that the cellulose in SBP is stable up to about 300 °C. This finding is in agreement with thermogravimetric data shown in Figure 2b, which highlighted H
DOI: 10.1021/acs.energyfuels.7b01766 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels torrefied raw-SBP samples, these changes involved mostly the disappearance of the absorption bands at about 1518 cm−1 (CC aromatic stretching), 1248 cm−1 (asymmetric C−O−C stretching vibration from asymmetrical aryl alkyl ethers), and 1034 cm−1 (asymmetric C−O−C stretching vibration in asymmetrical aryl alkyl ethers) mostly arising from lignin. This suggests that the lignin was degraded to large extent already at 250 °C. A similar behavior was also exhibited by torrefied PE-SBP, except that the disappearance of these bands occurred only in the FT-IR spectrum of the sample treated at 300 °C, which is indicative of a higher thermal stability of the lignin fraction of PE-SBP compared to that of raw-SBP. This may be most likely due to structural changes occurred in lignin as a result of acid extraction of pectin.40 3.5. GC-MS Analysis of Condensable Fraction. The condensable fraction of torrefaction gases (torgas) arising from the thermal treatment of raw-SBP and PE-SBP samples was analyzed by means of gas-chromatography coupled to mass spectrometry (GC-MS). Table 4 reports the main components identified in the investigated samples, which were grouped into nine categories based on their functionality, namely alkenes, alcohols, aldehydes, ketones, esters and carboxylic acids, aromatics, alkanes, sugar, and others. By comparing the relative areas of peaks in each chromatogram (not shown here), it results that the condensable fraction of torgas obtained through the thermal treatment of pectin-free sugar beet pulp (PE-SBP) was richer in carbonyl compounds (aldehydes, ketones, esters, and carboxylic acids) and alcohols and poorer in alkanes when compared to that obtained through the torrefaction of RawSBP. In particular, the greatest release of carbonyl compounds and alcohols was obtained during the torrefaction treatment of PE-SBP at 250 and 300 °C. Among the detected carbonyl compounds, there were some important biomass-derived platform molecules like furfural,41 levoglucosenone,42 and levoglucosan (1,6-anhydro-β-D-glucopyranose) as well as chemicals like D-limonene of relatively high commercial value as a solvent or biocide.43 This suggests that chemical and structural changes occurring in SBP after pectin extraction have a significant impact on improving torrefaction process performance thanks to the possibility it might offer to couple recovery of energy and chemicals in the same process. However, since the economic feasibility of the isolation of biobased chemicals depends on their concentrations in the mixture of condensable products, the quantitative assessment of the condensable species generated by the torrefaction of pectin-free SBP under different operating conditions (i.e., temperature and time) deserves further evaluation. Torrefaction operating conditions which aim at maximizing the recovery of highvalue chemicals are preferred.
Results show that the conversion degree of PE-SBP was slightly higher than that of Raw-SBP under the same operating conditions, but the energy yields of torrefied raw-SBP samples were generally higher than those of torrefied PE-SBP. FTIR analysis shows that both the acid treatment adopted for pectin extraction and the torrefaction treatment produce structural changes in the cellulose fraction of SBP. The most significant findings of this work can be summarized as follows: (i) torrefaction is a suitable process for the valorization of PE-SBP as a high quality solid fuel (i.e., the torrefied solid product) and, potentially, as a source of valuable biobased chemicals (e.g., D-limonene, furfural and levoglucosan) provided from the condensable fraction of the released torgas; (ii) PE-SBP is a better feedstock than raw-SBP due to the lower nitrogen and ash content; and (iii) a light torrefaction treatment (200−240 °C) ensures a more energy-saving process due to the high reactivity of raw-SBP and even more so for PESBP.
■
AUTHOR INFORMATION
Corresponding Author
*Tel.: +39 081 5931567. E-mail address:
[email protected]. ORCID
Paola Brachi: 0000-0003-2584-1394 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors are grateful to the Prof. Vincenzo Palma and Dr. Daniela Barba (University of Salerno) for providing expertise and access to their TGA facilities, as well as Dr. Luigi Vertuccio (University of Salerno) for the valuable support to FT-IR analyses. The financial support from the Russian Ministry of Education is acknowledged (Grant No. 6444280-4403ES/1 of September 2, 2016). Special thanks are given to COPROB (Cooperativa Produttori Bieticoli) for the feedstock supply.
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REFERENCES
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