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The influence of temperature on the physicochemical properties of products of pyrolysis of leather-tannery waste
Waste Management 88 (2019) 248–256
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The influence of temperature on the physicochemical properties of products of pyrolysis of leather-tannery waste Jacek Kluska ⇑, Mateusz Ochnio, Dariusz Kardas´, Łukasz Heda Institute of Fluid Flow Machinery, Polish Academy of Sciences, Fiszera 14, 80-231 Gdansk, Poland
a r t i c l e
i n f o
Article history: Received 25 September 2018 Revised 19 March 2019 Accepted 21 March 2019
Keywords: Pyrolysis Tannery waste Thermogravimetric analysis
a b s t r a c t The present paper examines the pyrolysis of waste from leather tanneries at 300–500 °C. These studies are important because of difficulties in the utilisation of this type of waste as well as its energy potential as fuel. The pyrolysis of tannery waste and data from the relevant literature showed that thermal degradation can be explained using tanned collagen as a reference. Moreover, the experimental results indicated that this process is highly non-linear, due to various mechanisms of heat transport which cause temperature differences in a laboratory pyrolysis reactor. Thermogravimetric analysis has shown that the greater part of mass loss is observed between 80 and 500 °C and that the most significant mass release occurs at 325 °C. Moreover, the proportions of CO2 and CO decrease along with increasing temperatures. The paper presents characteristics of the composition of solid, liquid, and gaseous products of leather-waste pyrolysis at various temperatures. The maximum heating value of gaseous products at 500 °C was 9.54 MJ/Nm3. An increase from 300 to 500 °C results in the dominant position of condensation polymerisation; the maximum value of the liquid phase yield is reached at 400 °C (42%). HHV analysis of the resulting char showed a maximum value of 21.18 MJ/kg at 450 °C. The results of oxidised component analysis showed that the major oxidised component of char was chromium oxide (Cr2O3), with a content of approximately 8.5% at all pyrolysis temperatures. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The leather industry is among the most dynamic industries operating in Poland. In 2016, the amount of leather processed by the tanning industry in Poland amounted to 21,000 tons. Importantly, one ton of raw material yields only about 200–250 kg of leather (Yılmaz et al., 2007). The remainder, i.e. about 75–80% of weight, generates solid and liquid wastes such as skins, shavings and trimmings, buffing dust, and fats. All of these wastes pose a serious problem for tanneries in terms of environmental pollution. One common solution involves their disposal at land sites or in landfills (Bañón et al., 2016). Another practical solution that should be taken into account is the energetic utilization of these wastes in thermochemical processes. Pyrolysis is a thermochemical process in which biomass is converted to solid, liquid, and gas fractions, and, compared with classic (and more popular) direct combustion, may be an alternative and promising method of leather-waste management. According to the relevant literature, pyrolysis can be applied to organic wastes such as sewage sludge, poultry litter (Fristak et al., 2018; Syed-Hassan et al., 2017; Kim et al., 2009; ⇑ Corresponding author. E-mail address: [email protected] (J. Kluska). https://doi.org/10.1016/j.wasman.2019.03.046 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
Agblevor et al., 2010), or others, including natural and synthetic rubbers and refuse-derived fuel (Kan et al., 2017; Miranda et al., 2013; Efika et al., 2015). Moreover, unlike pure combustion, in which heat is the key parameter, pyrolysis depends on process conditions and can be oriented towards the production of solid, liquid, or gaseous fuels. Experimental investigations of various forms of leather-waste pyrolysis have been presented in the literature, showing that leather waste is a useful recycling resource for the production of activated carbon and gaseous fuels. However, many of these papers are focused on the analytical aspect of thermal decomposition characteristics and are based on thermogravimetric or Py-GC/MS studies (Caballero et al., 1998; Marcilla et al., 2011a,b; Tian et al., 2017; Bañón et al., 2016). The experimental characteristics of leather-waste pyrolysis at temperatures of 450 and 600 °C with a heating rate of 5 °C/min were presented by Yılmaz et al. (2007). Increasing the final temperature from 450 to 600 °C caused an increase in the gas yield from 17 to 23% and a decrease in the liquid-phase yield from 40 to 29%. Moreover, the authors indicated that the pyrolysed leather waste, in the form of activated carbon, is desirable. Utilisation of leather waste in the pyrolysis process at temperatures ranging from 600 to 800 °C was also presented by Sethuraman et al. (2014), whereas Marcilla et al. (2012) conducted experimental
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research on rapid (450–550 °C) and slow (up to 750 °C) pyrolysis of bovine leather in an inert atmosphere. Taking the above considerations into account, the present paper thoroughly describes the pyrolysis of leather waste within a lower temperature range (300–500 °C) and provides detailed and critical analysis in terms of the successful and efficient utilisation of leather-tannery waste. This paper provides detailed information on the solid residue composition and caloric value generated at various pyrolysis temperatures, which may be of utilitarian significance in terms of chromium recovery, and the potential for catalytic effects involving chromium oxide, as well as the potential for utilisation of the char from leather-tannery pyrolysis. Moreover, this paper provides detailed information on the composition and higher heating values (HHV) of pyrolytic gases, as well as an analysis of heavy tar content in the liquid fraction, which is important in terms of the utilisation of these gases. In this paper, the specific energy of the solid and gas fraction is considered to qualify these products as sources of energy. One important aspect presented in this paper comprises the characteristics of heat transfer and phase transition phenomena in a fixed bed of leather waste in the reactor during pyrolysis. The heterogeneity of the relevant leather-tannery waste has a significant impact on the heating rate and dynamics of temperature change in the bed.
chLAB). The characteristics of the raw material are presented in Table 1. 2.2. Pyrolysis system and process The chamber of the laboratory scale pyrolysis reactor (Fig. 2) is cylindrical, with an inner diameter of 98 mm and a height of 480 mm. The active part of the chamber is enveloped by a heating induction coil and thermal insulation. The reactor is equipped with two ungrounded type K thermocouples: one fixed 21 cm from the bottom of the reactor and 1 cm from the reactor wall (T1), the other 16 cm from the bottom and 5 cm from the wall (in the core, T2). This arrangement enabled on-line measurements and recording of temperatures inside the reactor and inside the bed of the tested feed during the experiment. The sample was loaded from the top of the reactor and placed upon the sample holder (a stainless steel grate with holes 0.25 mm in diameter) located 12 cm from the bottom of the reactor. In each experiment, a 100-g sample of leather waste was loaded into the reactor, which is constructed of stainless steel, and heated to the chosen temperature. This temperature was then maintained for 30 min. In each experiment, the liquid/oil phase and char were determined by weight and the corresponding percentages were calculated. The gas yield was calculated as the remaining weight. The experiments were carried out at final temperatures of 300, 350, 400, 450, and 500 °C.
2. Materials and methods 2.3. Thermogravimetric analysis 2.1. Materials To conduct the proximate and ultimate analysis, waste material from leather tanneries situated in the north of Poland (Fig. 1) was dried at 105 °C and then placed in a centrifugal grinder equipped with a 0.2-mm perforated plate. The analysis was carried out on chromium-tanned waste (shavings). The elementary composition analysis was carried out using a CHNS-O Flash 2000 Analyzer (Thermo Scientific) and a wavelength dispersive X-ray fluorescence spectrometer (Bruker Scientific Instruments). The moisture content was determined using a MAC moisture analyzer (RADWAG), and the calorific value was determined using a calorimeter (Ekote-
Thermogravimetric analysis was conducted to investigate the rate of thermal decomposition of leather waste and the temperatures at which decomposition occurred. This analysis was carried out using a TA Instruments SDT Q600 thermogravimeter. The leather samples were heated from 30 to 750 °C at rates of 10, 15, 20, 30, 40, and 50 °C/min. The mass of each sample was 12 mg; the nitrogen (N2) flow rate was 100 ml/min. Moreover, in a separate analysis, the thermogravimetric analyser outlet (SDT Q600 thermogravimeter) was connected to an
Table 1 Proximate and ultimate analyses of tanning industry waste. Leather waste Heating value [MJ/kg] Moisture [wt.%, as delivered] Proximate Volatiles Fixed carbon Ash Ultimate [wt.%db]a C H O N S Crtotal Na Fe Ca Si Cl Mg K Al P Ag Ti Fig. 1. Waste from leather tanneries.
a
db = oven-dry basis.
16.6 35.3 [wt.%db]a 67.9 21.2 10.9 41.71 7.12 28.46 11.01 3.43 3.16 1.77 0.99 0.63 0.59 0.57 0.21 0.14 0.11 0.05 0.02 0.02
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Fig. 2. Test stand for a pyrolysis reactor.
adsorption tube 125 mm in length and 4 mm in diameter in order to characterise thermal decomposition of compounds as a function of temperature. The tube was filled with a sorbent in the form of 20 mm of poly(2,6-diphenylphenylene oxide) (Tenax TA) and 20 mm of carboxen. The leather samples were placed in a thermogravimeter furnace, which had previously been heated to temperatures of 300, 400, or 500 °C; the chosen temperature was maintained for 10 min. The mass of each sample was about 17 mg and the nitrogen (N2) flow rate was 100 ml/min. Following TGA analysis, the adsorption tube was connected to a thermal desorber (Eclipse 4660 Purge-and-Trap Sample Concentrator) and heated to 300 °C. After reaching the required temperature, nitrogen was passed through the tube prior to carrying the analytes to the sorption trap (column), which was heated to 210 °C. Then, hydrogen was passed through the tube prior to carrying the analytes to a GC/MS analyzer (GCMS-QP2010 Plus, Shimadzu). The pyrolysis experiments were carried out at temperatures of 300, 400, and 500 °C. 2.4. Development of the method for tar/oil sampling and analysis Sampling of the tar from leather waste pyrolysis was carried out using the ‘European Tar Protocol’ (Good et al., 2005; Tchapda et al., 2017). After each experiment, the contents of all impinger bottles were combined. Isopropanol was then added, to a total volume of 500 ml. Subsequently, about 45 g of the sample was placed in a beaker and dried at 105 °C for 3 h, then weighed. The residue mass indicated heavy tar content in the pyrolytic gas (Hernández et al., 2013; Kihedu et al., 2016). The tar composition analyses were performed using a gas chromatograph (Shimadzu GC-2012) coupled with a mass spectrometer (Shimadzu GCMS-QP2010 Plus) with an Optima 5 MS 0.5 lm column (dimensions: 60 m, 0.25 mm) (Macherey-Nagel GmbH). The elution was performed according to the following temperature programme: the initial temperature of 35 °C was maintained for 10 min, then raised to 270 °C at a rate of 20 °C/min; finally, the temperature of 270 °C was maintained for 5 min. The mobile phase was helium with a flow rate of 1 ml/min. 2.5. Gas composition analysis The obtained pyrolysis gas was collected in Tedlar bags. Prior to sampling, the pyrolysis gas was first directed to the tar sampling
stand and cleaned. Analysis of gas composition was performed using an SRI Instruments 310 Gas Chromatograph with a thermal conductivity detector (TCD). The gas analyser was pre-calibrated using a standard mixture of gas to determine CO, CO2, CH4, and H2. Argon was used as the carrying medium. Calculations of the higher heating value (HHV) of the gas was based on an equation presented by Wang et al. (2011):
HHV g ¼ Y CO HHV CO þ Y CH4 HHV CH4 þ Y H2 HHV H2 þ Y N2 HHV N2 þ Y CO2 HHV CO2
where HHVi [MJ/Nm3] is the heating value and Yi is the volume fraction of the gas component.
3. Results and discussion 3.1. TGA analysis According to the proximate and ultimate analysis (Table 1), the leather waste contains about 68% volatiles, more than sewage sludge (Magdziarz and Wilk, 2013; Phuphuakrat et al., 2010; Kijo-Kleczkowska et al., 2016) and approximately the same as poultry litter (Hussein et al., 2017). It should also be noted that leather waste is characterised by nitrogen content higher than that of lignocellulose biomass (Hernández et al., 2013; Kihedu et al., 2016). The thermogravimetric analysis of leather waste samples for a heating rate of 15 °C/min show that the first mass loss is observed between 0 and 100 °C and is related to the release of inherent moisture within the sample (Fig. 3). The greater part of the mass loss is observed between 80 and 500 °C. The most significant mass release in leather sample decomposition occurred at 325 °C (0.47%/ °C). This effect is caused by the thermal decomposition of volatile organic compounds in the material. Moreover, the mass loss curves obtained for various heating rates indicate the significant impact of heat transport on the maximum temperature peak (Fig. 4). Increasing the heating rate from 10 to 50 °C/min caused a shift of 30 °C in the temperature peak, from 322 (0.49%/°C) to 352 °C (0.47%/°C).
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Fig. 3. Thermogravimetric curves for leather waste in a nitrogen atmosphere at a heating rate of 15 °C/min.
Fig. 4. Derivative thermogravimetric curves for various heating rates.
Fig. 5. Temperature as a function of time at points T1 and T2 during the pyrolysis of tannery waste.
3.2. Heating rate of leather-tannery waste in the reactor The results of the experimental investigation indicate that the rate of heating of the fuel bed depends on time and temperature (Fig. 5). Within two time intervals, for measurement points T1 (16 min < t1 < 20 min) and T2 (11 min < t1 < 14 min), a decrease in the heating rate of the bed can be observed. In the case of measurement point T2, the temperature in the bed reached about 90 °C, close to the temperature at which water evaporates. The increase in temperature was retarded by the evaporation of moisture. Water absorbs heat and uses it to evaporate, and simultaneously delays devolatilisation (Pham et al., 2018; Arni et al., 2010). The slowdown in the rate of heating at T1, which occurs at 200 °C, may be caused by the flow of steam through the upper part of the reactor and fuel bed movement caused by a decrease in its volume during pyrolysis and evaporation of light volatiles (Chen et al., 2015). Furthermore, the heating rate in the fuel bed in the reactor is not constant; the dynamics of temperature change (dT/dt) are
much higher closer to the wall, reaching 50 °C/min at T1 (Fig. 6). Within a range up to 200 °C, the average rate of heating of the bed was 8.91 °C/min at measuring point T1 and 8.85 °C/min at T2. Within the range from 200 to 500 °C, the average rate of heating of the bed equalled 27.56 and 23.94 °C/min at T1 and T2, respectively.
3.3. Pyrolysis of leather waste 3.3.1. Pyrolysis yields According to the data available in the literature, for different types of biomass, such as pine nuts, shell, sewage sludge, chicken or turkey litter, or leather waste, the mass balance of pyrolysis products depends on the final temperature of the process (Yılmaz et al., 2007; Kim et al., 2009; Xiao and Yang, 2013; Chen et al., 2016a). The effect of pyrolysis temperature on the pyrolysis products of leather-tannery waste is shown in Fig. 7.
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2013). However, in this case, secondary tar reactions also increase. A balance exists between the rate of tar formation and secondary tar reactions near the equilibrium point at 400 °C; therefore the tar yield was highest near this point.
Fig. 6. Heating rate of the bed during pyrolysis of tannery waste.
Fig. 7. Effect of the temperature of leather-tannery pyrolysis on product yields.
Within the range 300–450 °C, the evolution of the solid-phase fraction of pyrolysis is quasi-constant and remains at the level of 33%, after which it decreases to 29% at 500 °C. It is noteworthy that molecular components could not be fully decomposed at lower temperatures, and that only weak chemical bonds could be broken (He et al., 2018), as indicated as well by the high yield of the gas fraction and the relatively low yield of the liquid fraction at 300 °C. The congruent yield of the solid phase was obtained by Marcilla et al. (2012), who presented experimental research on rapid (450–550 °C) and slow (up to 750 °C) bovine leather pyrolysis at 750 °C at a heating rate of 10 °C/min. A similar effect of temperature on char yield was observed by Marculescu and Stan (2014) for the pyrolysis of chicken feathers with traces of blood, whereby a temperature increase from 350 to 450 °C at a heating rate of 30 °C/min resulted in decreased char yields, from about 35 to 21%. Moreover, in the pyrolysis of leather waste, a temperature increase from 300 to 500 °C leads to the dominant position of condensation polymerisation, in which the liquid phase yield increases from 21 to 40%, with a maximum value of 42% at 400 °C. This may indicate an optimum temperature reaction for tar yield (Chen et al., 2016a, 2016b; He et al., 2018). It should be noted that, within the presented temperature range, the tar formation rate should increase with an increase in pyrolysis temperature (Baniasadi et al., 2016; He et al., 2018; Morgano et al., 2018; Xiao and Yang,
3.3.2. Influence of pyrolysis temperature on gaseous products The composition measurements of the gas obtained from the pyrolysis of leather waste are presented in Fig. 8. The results of experimental investigation showed that at lower temperatures, up to 450 °C, carbon dioxide was the dominant compound (above 40%). Subsequently, its content decreased to 28.7% at 500 °C. According to the literature, this aspect can be explained by the cracking-reforming process of functional groups related to carbonyl (CAOAC), carboxyl (C@O), and carboxylic acid (COOH) (Chen et al., 2016a, 2016b). During pyrolysis, the production of CO, whose content in the pyrolysis gas increased from 11.8% at 300 °C to 18.4% at 500 °C, can be explained by the decomposition of oxygen-containing functional groups such as carbonyl or hydroxyl (Zhang et al., 2010). The concentration of hydrogen increased from 1.7% at 300 °C to 33.2% at 500 °C, whereas the concentration of methane increased from 0.9% to 10.1%, respectively. Higher temperatures favour the thermal cracking of hydrocarbon chains (Zhang et al., 2010; Morgano et al., 2018; Sadrameli, 2015). Analysis of the gas composition showed that the higher heating value (HHV) increased from 2 MJ/Nm3 at 300 °C to about 10 MJ/Nm3 at 500 °C, which was caused by greater amounts of methane and hydrogen at higher temperatures as well as by carbon monoxide, the content of which amounted to approximately 18% at 500 °C. A similar heating value of pyrolytic gas at 450 °C (about 11 MJ/Nm3) was reported by Marculescu and Stan (2014) for the pyrolysis of chicken feathers. Morgano et al. (2018) recently showed a similar influence of temperature on the caloric value of gas from the pyrolysis of sewage sludge, recording a heating value of approximately 13.5 MJ/Nm3 at 500 °C. In addition, energy yield may be related to the weight of the feedstock (specific gas energy) and may determine the energy efficiency of the pyrolysis process (Taupe et al., 2016). Despite the fact that, at 300 °C, the gas yield reached 45%, a large amount of carbon dioxide and low hydrogen and methane contents in the obtained gas resulted in a low gas calorific value and low specific gas energy, amounting to 0.64 MJ/kgfuel. Pyrolysis at 500 °C caused a reduction in the gas fraction (30%); however, large amounts of methane and hydrogen led to a high calorific value for the obtained gas and a much higher specific energy, reaching 3.34 MJ/kgfuel.
Fig. 8. Influence of temperature on gas composition from leather waste pyrolysis.
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It is noteworthy that during the experiments, ammonium carbonate accumulated on cold connections between impingers, which, in the long term, led to a reduction of the diameters of cross sections of the tubes through which the produced gas passed. According to the literature, this effect may have been caused by the reaction between NH3 (as a product of the degradation of polypeptide) and CO2 (Yılmaz et al., 2007).
3.3.3. Influence of pyrolysis temperature on solid products The hydrogen and nitrogen concentrations decreased along with increasing temperatures, from 2.91% at 300 °C to 1.26% at 500 °C for hydrogen, and from 12.46 to 10.73%, respectively, for nitrogen (Table 2). Meanwhile, the total content of carbon in the char increased along with pyrolysis temperatures, from 47% to a maximum value of 52% at 450 °C. Analysis of the higher heating value of the obtained char indicated that this value did not change significantly; initially it decreased to 18.55 MJ/kg at 350 °C, then increased to 21.18 at 450 °C, which is higher than the calorific value reported by Morgano et al. (2018) for char from sewage sludge pyrolysis at 500 °C (8.6 MJ/kg) and lower than the calorific value reported by Marculescu and Stan (2014) for char from poultry litter pyrolysis at 450 °C (26 MJ/kg). Another important aspect is the inclusion of the char yield in the energy yield obtained from one kilogram of feedstock (specific char energy). Between 300 and 450 °C, the pyrolysis solid-phase fraction remained at the level of 33%, then decreased to 29% at 500 °C, resulting in a reduction in the specific energy of solid residue, from 7.1 MJ/kgfuel at 300 °C to 5.9 MJ/kgfuel at 500 °C. Char generated from leather-tannery residues can be used in the metallurgical sector. Filho et al. (2016) reported that carbonised leather residues with a higher heating value of 20.8 MJ/kg can partially replace mineral coal in the metallurgical process. Experiments were carried out for fixed-carbon basic content values in the prepared mixtures of 10 and 25%, demonstrating the potential for utilisation of the prepared charcoal as an energy source, with a recovery rate of up to 76% of chromium in the final production of iron ore pellets. Moreover, Yılmaz et al. (2007) showed that carbonised leather residues may be used in the
production of activated carbon (activated by CO2) and as an adsorbant of dyes from aqueous solutions. The results derived from wavelength dispersive X-ray fluorescence spectroscopy (Table 3) showed that the major oxidised component of char was chromium oxide (Cr2O3), with a content of approximately 8.5% in all cases. Furthermore, the char contained 7.5–8.25% of sulphur trioxide (SO3), 4.3–5.4% of sodium oxide (Na2O), and 1.2–1.7% of calcium oxide (CaO). The chlorine content was 0.7–1.2%. Chromium oxide is widely used in the industrial sector in processes such as leather tanning, finishing of metals, or wood preservation (Godinho et al., 2017). The topic of chromium recovery has been thoroughly described by Wionczyk et al. (2011), Godinho et al. (2017), and Dias et al. (2018). Chromium oxide is also commonly used as a catalyst (Chen et al., 2018; Song et al., 2018; Tian et al., 2018; Wang et al., 2014; Rotaru et al., 2005; Jia et al., 2011). Chen et al. (2018) demonstrated the potential for and efficiency of toluene combustion over mesoporous Cr2O3 catalysts; Song et al. (2018) presented the application of ZnO/Cr2O3 catalysts in the high-temperature synthesis of methanol. Moreover, Table 3 presents the composition of ash from leathertannery waste combustion (PN-EN ISO 18122:2016-01), indicating about 14.7% content of sulphur trioxide. Taking into account the facts that leather contains about 11% ash, and that the pyrolysis solid phase remains at the level of about 30%, the ash from 1 kg of tannery waste contains about 1.6% sulphur trioxide, whereas the corresponding content in the char reached about 2.5%. This
Table 2 Influence of pyrolysis temperature on elemental composition of char. T [°C]
C [%]
N [%]
H [%]
HHV [MJ/kg]
300 350 400 450 500
47.08 49.81 48.44 52.21 49.75
12.46 13.31 12.89 13.03 10.73
2.91 3.12 3.01 1.85 1.26
21.50 18.55 19.46 21.18 19.92 Fig. 9. Heavy tar yields at various pyrolysis temperatures.
Table 3 The oxidized components of char from leather tannery pyrolysis. Compound
Cr2O3 SO3 Na2O CaO Cl Fe2O3 SiO2 MgO P2O5 K2O
Content [%] 300 °C
350 °C
400 °C
450 °C
500 °C
Ash
8.66 8.25 4.68 1.49 0.93 0.34 0.22 0.21 0.18 0.02
8.09 7.78 4.33 1.55 1.09 0.33 0.29 0.19 0.16 0.03
8.62 8.19 4.57 1.66 0.98 0.34 0.24 0.22 0.16 0.03
8.66 7.88 5.39 1.18 0.73 0.43 0.3 0.24 0.17 0.03
8.52 7.5 5.21 1.54 1.2 0.29 0.24 0.24 0.17 –
34.97 14.73 9.78 5.38 0.11 1.30 0.73 0.68 0.68 0.42
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Table 4 The identified compounds in the gas phase at different temperatures. Ret. Time
Formula
Compounds
% Area
9.806 9.979 11.145 12.375 12.465 12.611 12.702 12.786 13.575 14.769 14.831 15.423 16.512 16.580 16.751 16.792 16.860 15.906 17.050 17.215 17.556 17.708 17.952 17.990 18.183 18.509 18.629 18.742 18.995 19.043 19.253 19.441 19.533 19.601 19.858 19.903 20.028 20.266 20.373 20.666 20.926 20.975 21.002 21.080 21.422 21.678
C4H8O C6H14 C4H8O C4H7N C6H6 C5H10O C7H14 C8H18NO C4H9NO4 C4H9Cl C7H8 C4H4NCH3 C8H10 C5H10 C6H11N C4H5N7 C5H7N C6H7N C6H18 C7H14O C6H10O C7H9N C7H9NO C6H10O C8H14 C6H5CHO C8H16O C6H11N C7H7NO C6H5(CN) C12H26 C7H14O C8H14O C6H11N C9H18O C19H36O C6H8O2 C6H12 C13H2 C12H26S C8H7N C7H12O C4H8O C8H16 C5H9NO C11H22
Propanal, 2-methyln-Hexane 2-Butanone Isobutyronitrile 1,3-Hexadien-5-yne Butanal, 3-methyl(Z)-2-Heptene Nitroxide, bis(1,1-dimethylethyl) 2-Nitro-2-methyl-1,3-propanediol Propane, 1-chloro-2-methylTolune Pyrrole 1,3-Cyclopentadiene, 5-(1-methylethylidene)Cyclobutane, methylIsoamyl cyanide 6-Hydrazinotetrazolo(b)pyridazine 1H-Pyrrole, 3-methylPyridine, 4-methyl1,3,5,7-Cyclooctatetraene Heptanal Cyclohexanone Pyridine, 3,4-dimethyl3-Pyridinol, 2,6-dimethyl2,4-Hexadien-1-ol 6-Methyl-2-heptyne Benzaldehyde Octanal Aziridine, 1-(2-buten-2-yl)Benzaldehyde, oxime, (E)Benzonitrile 2-Undecanethiol, 2-methylOxetane, 2-methyl-4-propyl2-Octenal, (E)Aziridine, 1-(2-buten-1-yl)-, (E)2-Nonen-1-ol 5-Cyclopropylcarbonyloxypentadecane 1,3-Butadien-1-ol, acetate Propane, 2-cyclopropylBenzene, (2,4-dimethylpentyl)2-Undecanethiol, 2-methyl2,4,6-Cycloheptatriene-1-carbonitrile 3-Hepten-2-one, (E)3-Buten-1-ol 1-Hexene, 4,5-dimethyl2-Piperidinone 3-Undecene, (Z)-
400 °C
500 °C
0.10 0.28 0.11 0.51 – 0.39 – 0.36 0.51 15.86 – 67.75 – 0.76 0.66 – – – 0.17 0.40 0.20 0.24 0.41 0.11 0.12 4.99 0.34 0.23 0.20 – – 0.23 0.36 0.25 0.92 – 0.35 0.16 0.61 0.12 0.24 0.06 – 0.12 – 0.07
– – – – 0.12 – 0.07 – – – 4.0 75.47 0.12 – 0.53 0.17 – 0.16 0.22 – 0.10 0.10 – – – 3.08 – – – 12.70 0.10 – – – – 0.11 – – – 0.16 – – 0.19 – 0.11 –
– – – – 0.19 – 0.13 – – – 5.52 69.77 0.25 – – 0.23 0.74 0.26 0.37 – 0.14 0.18 – 0.14 – 3.98 – – – 15.11 0.11 – – – – 0.16 – – – 0.34 – – 0.47 – 0.20 –
the amount of heavy tars from 0.45 to approximately 7% (g tars/100 g leather). Subsequently, the amount of tars decreased to about 6% at 500 °C (Fig. 9).
Table 5 Classification and % area of compounds identified. % Area
Aldehydes Ketone Alcohol Ester Heterocyclic aromatic compounds Aromatic Nitriles and derivatives Chloroalkanes Others
300 °C
300 °C
400 °C
500 °C
6.58 0.37 1.54 0.35 68.4 – 1.89 15.86 5.01
3.08 0.10 0.19 0.11 75.73 4.0 12.52 – 4.27
3.98 0.14 0.61 0.16 70.95 5.52 15.11 – 3.53
comparison shows that about half of the sulphur contained in the leather-tannery waste may be released in gaseous form into the generated sulphur compounds in the flue gas. 3.3.4. Influence of pyrolysis temperature on liquid products The results of analysis of heavy tar content showed that an increase in temperature from 300 to 450 °C caused an increase in
3.3.5. TGA-GC/MS gas analysis Table 4 shows the list of compounds identified by the GC/MS analyzer (GCMS-QP2010 Plus, Shimadzu). Due to the linear relationship between the total area and the area of the pyrolysed sample, all of the detected peaks were normalised with reference to the total area of the analysed chromatogram. The results show that the majority of the detected compounds correspond to nitrogen derivatives; this is related to the protein content in the leather waste (Sethuraman et al., 2014; Font et al., 1999; Marcilla et al., 2011a, 2011b). Table 5, based on identification of gas phase composition, shows eight groups of compounds, characterised as follows: aldehydes, ketones, alcohols, esters, heterocyclic aromatic compounds, including pyrrole and pyridine derivatives; aromatic compounds, represented by toluene, nitriles, and nitrile derivatives, chloroalkanes, and others. The main group comprises heterocyclic aromatic compounds, which are associated with the thermal degradation
J. Kluska et al. / Waste Management 88 (2019) 248–256 Table 6 Amino acid composition of type I collagen from calf-skin (Friess, 1998; Choi and Ko, 2011). Amino acid
a1(I)-chain
a2(I)-chain
Alanine Arginine Asparagine Aspartic acid Glutamic acid Glutamine Glycine Histidine Hydroxylysine Hydroxyproline Isoleucine Leucine Lysine Methionine Phenylalanine Proline Serine Threonine Tyrosine Valine Total
124 (2) 53 (2) 13 33 (3) 52 (2) 27 (3) 345 (6) 3 (1) 4 114 9 (1) 22 (3) 34 (2) 7 13 (1) 127 (4) 37 (5) 17 (1) 5 (5) 17 (1) 1056 (42)
111 (3) 56 (1) 23 24 (2) 46 (2) 24 (1) 346 (6) 8 9 99 18 33 21 (1) 4 15 (3) 108 (1) 35 (1) 20 4 (3) 34 1038 (24)
of proteins. The dominant percentage of aromatic compounds was also demonstrated by Marcilla et al. (2011a). Moreover, at 300 °C a significant number of chloroalkane compounds were observed. According to the relevant literature, the product obtained via leather tanning from tanned collagen fibre networks of various sizes consists of 99% by mass (Li et al., 2009; Šteˇpánová et al., 2017), including pyrolysed waste considered to represent tanned collagen. Of the approximately 28 types of collagen (Bhagwat and Dandge, 2018), the main type found in animal skin is represented by Type I (Friess, 1998). The amino acid composition of Type 1 collagen is presented in Table 6 (Friess, 1998; Choi and Ko, 2011). Considering these aspects, it can be concluded that the high content of organic nitrogen derivatives (mainly pyrrole) refers to the structure of collagen-building amino acids. Proline and hydroxyproline contain pyrrole rings, which constitute the main element of the heterocyclic aromatic compounds (Choi and Ko, 2011), in their structures. Moreover, amino acids such as asparagine, arginine, glutamine, and lysine contain more than one amino group that may be released as amine or that may constitute part of the structures of products obtained as a result of the cyclisation and aromatisation reactions (Wei et al., 2018). Most of the main amino acids – alanine, arginine, glutamic acid, and glycine – are characterised by a linear structure, which, as a result of the pyrolysis process, can lead to the formation of aliphatic compounds such as 2-Nonen-1-ol, isoamyl cyanide, isobutyronitrile, or propane, 1-chloro-2-methyl- (Zhao et al., 2016). 4. Summary Proximate and ultimate analysis showed that leather waste is similar to poultry-derived waste. The thermal characteristics of leather tannery-waste pyrolysis indicates that this process is highly non-linear due to various mechanisms of heat transport which cause differences in heating rate inside a reactor. This is related to the evaporation of moisture in the core of the fuel bed, steam flow through the upper part of the reactor, and movement of the fuel bed caused by reductions in its volume during pyrolysis. The results presented here, obtained within a temperature range of 300–500 °C, show that pyrolysis for energetic application is potentially feasible above 400–450 °C. This is the result of a greater amount of hydrogen and methane in the gas mixture,
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which, despite the lower yield of gases, leads to higher specific gas energy values. HHV analysis of the obtained char indicated that this value does not change significantly. However, the decrease in the solid fraction yield results in a noticeable decrease in the specific energy of char. Analysis of char composition showed that the contents of oxidised components do not change significantly along with pyrolysis temperature in cases where the major component is chromium oxide (Cr2O3). The main group of tar components comprises heterocyclic aromatic compounds, which are associated with the thermal degradation of proteins. The results and the data in the relevant literature indicate that the pyrolysis of leather waste can be explained using tanned collagen as a reference material.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
The influence of temperature on the physicochemical properties of products of pyrolysis of leather-tannery waste Jacek Kluska ⇑, Mateusz Ochnio, Dariusz Kardas´, Łukasz Heda Institute of Fluid Flow Machinery, Polish Academy of Sciences, Fiszera 14, 80-231 Gdansk, Poland
a r t i c l e
i n f o
Article history: Received 25 September 2018 Revised 19 March 2019 Accepted 21 March 2019
Keywords: Pyrolysis Tannery waste Thermogravimetric analysis
a b s t r a c t The present paper examines the pyrolysis of waste from leather tanneries at 300–500 °C. These studies are important because of difficulties in the utilisation of this type of waste as well as its energy potential as fuel. The pyrolysis of tannery waste and data from the relevant literature showed that thermal degradation can be explained using tanned collagen as a reference. Moreover, the experimental results indicated that this process is highly non-linear, due to various mechanisms of heat transport which cause temperature differences in a laboratory pyrolysis reactor. Thermogravimetric analysis has shown that the greater part of mass loss is observed between 80 and 500 °C and that the most significant mass release occurs at 325 °C. Moreover, the proportions of CO2 and CO decrease along with increasing temperatures. The paper presents characteristics of the composition of solid, liquid, and gaseous products of leather-waste pyrolysis at various temperatures. The maximum heating value of gaseous products at 500 °C was 9.54 MJ/Nm3. An increase from 300 to 500 °C results in the dominant position of condensation polymerisation; the maximum value of the liquid phase yield is reached at 400 °C (42%). HHV analysis of the resulting char showed a maximum value of 21.18 MJ/kg at 450 °C. The results of oxidised component analysis showed that the major oxidised component of char was chromium oxide (Cr2O3), with a content of approximately 8.5% at all pyrolysis temperatures. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The leather industry is among the most dynamic industries operating in Poland. In 2016, the amount of leather processed by the tanning industry in Poland amounted to 21,000 tons. Importantly, one ton of raw material yields only about 200–250 kg of leather (Yılmaz et al., 2007). The remainder, i.e. about 75–80% of weight, generates solid and liquid wastes such as skins, shavings and trimmings, buffing dust, and fats. All of these wastes pose a serious problem for tanneries in terms of environmental pollution. One common solution involves their disposal at land sites or in landfills (Bañón et al., 2016). Another practical solution that should be taken into account is the energetic utilization of these wastes in thermochemical processes. Pyrolysis is a thermochemical process in which biomass is converted to solid, liquid, and gas fractions, and, compared with classic (and more popular) direct combustion, may be an alternative and promising method of leather-waste management. According to the relevant literature, pyrolysis can be applied to organic wastes such as sewage sludge, poultry litter (Fristak et al., 2018; Syed-Hassan et al., 2017; Kim et al., 2009; ⇑ Corresponding author. E-mail address: [email protected] (J. Kluska). https://doi.org/10.1016/j.wasman.2019.03.046 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
Agblevor et al., 2010), or others, including natural and synthetic rubbers and refuse-derived fuel (Kan et al., 2017; Miranda et al., 2013; Efika et al., 2015). Moreover, unlike pure combustion, in which heat is the key parameter, pyrolysis depends on process conditions and can be oriented towards the production of solid, liquid, or gaseous fuels. Experimental investigations of various forms of leather-waste pyrolysis have been presented in the literature, showing that leather waste is a useful recycling resource for the production of activated carbon and gaseous fuels. However, many of these papers are focused on the analytical aspect of thermal decomposition characteristics and are based on thermogravimetric or Py-GC/MS studies (Caballero et al., 1998; Marcilla et al., 2011a,b; Tian et al., 2017; Bañón et al., 2016). The experimental characteristics of leather-waste pyrolysis at temperatures of 450 and 600 °C with a heating rate of 5 °C/min were presented by Yılmaz et al. (2007). Increasing the final temperature from 450 to 600 °C caused an increase in the gas yield from 17 to 23% and a decrease in the liquid-phase yield from 40 to 29%. Moreover, the authors indicated that the pyrolysed leather waste, in the form of activated carbon, is desirable. Utilisation of leather waste in the pyrolysis process at temperatures ranging from 600 to 800 °C was also presented by Sethuraman et al. (2014), whereas Marcilla et al. (2012) conducted experimental
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research on rapid (450–550 °C) and slow (up to 750 °C) pyrolysis of bovine leather in an inert atmosphere. Taking the above considerations into account, the present paper thoroughly describes the pyrolysis of leather waste within a lower temperature range (300–500 °C) and provides detailed and critical analysis in terms of the successful and efficient utilisation of leather-tannery waste. This paper provides detailed information on the solid residue composition and caloric value generated at various pyrolysis temperatures, which may be of utilitarian significance in terms of chromium recovery, and the potential for catalytic effects involving chromium oxide, as well as the potential for utilisation of the char from leather-tannery pyrolysis. Moreover, this paper provides detailed information on the composition and higher heating values (HHV) of pyrolytic gases, as well as an analysis of heavy tar content in the liquid fraction, which is important in terms of the utilisation of these gases. In this paper, the specific energy of the solid and gas fraction is considered to qualify these products as sources of energy. One important aspect presented in this paper comprises the characteristics of heat transfer and phase transition phenomena in a fixed bed of leather waste in the reactor during pyrolysis. The heterogeneity of the relevant leather-tannery waste has a significant impact on the heating rate and dynamics of temperature change in the bed.
chLAB). The characteristics of the raw material are presented in Table 1. 2.2. Pyrolysis system and process The chamber of the laboratory scale pyrolysis reactor (Fig. 2) is cylindrical, with an inner diameter of 98 mm and a height of 480 mm. The active part of the chamber is enveloped by a heating induction coil and thermal insulation. The reactor is equipped with two ungrounded type K thermocouples: one fixed 21 cm from the bottom of the reactor and 1 cm from the reactor wall (T1), the other 16 cm from the bottom and 5 cm from the wall (in the core, T2). This arrangement enabled on-line measurements and recording of temperatures inside the reactor and inside the bed of the tested feed during the experiment. The sample was loaded from the top of the reactor and placed upon the sample holder (a stainless steel grate with holes 0.25 mm in diameter) located 12 cm from the bottom of the reactor. In each experiment, a 100-g sample of leather waste was loaded into the reactor, which is constructed of stainless steel, and heated to the chosen temperature. This temperature was then maintained for 30 min. In each experiment, the liquid/oil phase and char were determined by weight and the corresponding percentages were calculated. The gas yield was calculated as the remaining weight. The experiments were carried out at final temperatures of 300, 350, 400, 450, and 500 °C.
2. Materials and methods 2.3. Thermogravimetric analysis 2.1. Materials To conduct the proximate and ultimate analysis, waste material from leather tanneries situated in the north of Poland (Fig. 1) was dried at 105 °C and then placed in a centrifugal grinder equipped with a 0.2-mm perforated plate. The analysis was carried out on chromium-tanned waste (shavings). The elementary composition analysis was carried out using a CHNS-O Flash 2000 Analyzer (Thermo Scientific) and a wavelength dispersive X-ray fluorescence spectrometer (Bruker Scientific Instruments). The moisture content was determined using a MAC moisture analyzer (RADWAG), and the calorific value was determined using a calorimeter (Ekote-
Thermogravimetric analysis was conducted to investigate the rate of thermal decomposition of leather waste and the temperatures at which decomposition occurred. This analysis was carried out using a TA Instruments SDT Q600 thermogravimeter. The leather samples were heated from 30 to 750 °C at rates of 10, 15, 20, 30, 40, and 50 °C/min. The mass of each sample was 12 mg; the nitrogen (N2) flow rate was 100 ml/min. Moreover, in a separate analysis, the thermogravimetric analyser outlet (SDT Q600 thermogravimeter) was connected to an
Table 1 Proximate and ultimate analyses of tanning industry waste. Leather waste Heating value [MJ/kg] Moisture [wt.%, as delivered] Proximate Volatiles Fixed carbon Ash Ultimate [wt.%db]a C H O N S Crtotal Na Fe Ca Si Cl Mg K Al P Ag Ti Fig. 1. Waste from leather tanneries.
a
db = oven-dry basis.
16.6 35.3 [wt.%db]a 67.9 21.2 10.9 41.71 7.12 28.46 11.01 3.43 3.16 1.77 0.99 0.63 0.59 0.57 0.21 0.14 0.11 0.05 0.02 0.02
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Fig. 2. Test stand for a pyrolysis reactor.
adsorption tube 125 mm in length and 4 mm in diameter in order to characterise thermal decomposition of compounds as a function of temperature. The tube was filled with a sorbent in the form of 20 mm of poly(2,6-diphenylphenylene oxide) (Tenax TA) and 20 mm of carboxen. The leather samples were placed in a thermogravimeter furnace, which had previously been heated to temperatures of 300, 400, or 500 °C; the chosen temperature was maintained for 10 min. The mass of each sample was about 17 mg and the nitrogen (N2) flow rate was 100 ml/min. Following TGA analysis, the adsorption tube was connected to a thermal desorber (Eclipse 4660 Purge-and-Trap Sample Concentrator) and heated to 300 °C. After reaching the required temperature, nitrogen was passed through the tube prior to carrying the analytes to the sorption trap (column), which was heated to 210 °C. Then, hydrogen was passed through the tube prior to carrying the analytes to a GC/MS analyzer (GCMS-QP2010 Plus, Shimadzu). The pyrolysis experiments were carried out at temperatures of 300, 400, and 500 °C. 2.4. Development of the method for tar/oil sampling and analysis Sampling of the tar from leather waste pyrolysis was carried out using the ‘European Tar Protocol’ (Good et al., 2005; Tchapda et al., 2017). After each experiment, the contents of all impinger bottles were combined. Isopropanol was then added, to a total volume of 500 ml. Subsequently, about 45 g of the sample was placed in a beaker and dried at 105 °C for 3 h, then weighed. The residue mass indicated heavy tar content in the pyrolytic gas (Hernández et al., 2013; Kihedu et al., 2016). The tar composition analyses were performed using a gas chromatograph (Shimadzu GC-2012) coupled with a mass spectrometer (Shimadzu GCMS-QP2010 Plus) with an Optima 5 MS 0.5 lm column (dimensions: 60 m, 0.25 mm) (Macherey-Nagel GmbH). The elution was performed according to the following temperature programme: the initial temperature of 35 °C was maintained for 10 min, then raised to 270 °C at a rate of 20 °C/min; finally, the temperature of 270 °C was maintained for 5 min. The mobile phase was helium with a flow rate of 1 ml/min. 2.5. Gas composition analysis The obtained pyrolysis gas was collected in Tedlar bags. Prior to sampling, the pyrolysis gas was first directed to the tar sampling
stand and cleaned. Analysis of gas composition was performed using an SRI Instruments 310 Gas Chromatograph with a thermal conductivity detector (TCD). The gas analyser was pre-calibrated using a standard mixture of gas to determine CO, CO2, CH4, and H2. Argon was used as the carrying medium. Calculations of the higher heating value (HHV) of the gas was based on an equation presented by Wang et al. (2011):
HHV g ¼ Y CO HHV CO þ Y CH4 HHV CH4 þ Y H2 HHV H2 þ Y N2 HHV N2 þ Y CO2 HHV CO2
where HHVi [MJ/Nm3] is the heating value and Yi is the volume fraction of the gas component.
3. Results and discussion 3.1. TGA analysis According to the proximate and ultimate analysis (Table 1), the leather waste contains about 68% volatiles, more than sewage sludge (Magdziarz and Wilk, 2013; Phuphuakrat et al., 2010; Kijo-Kleczkowska et al., 2016) and approximately the same as poultry litter (Hussein et al., 2017). It should also be noted that leather waste is characterised by nitrogen content higher than that of lignocellulose biomass (Hernández et al., 2013; Kihedu et al., 2016). The thermogravimetric analysis of leather waste samples for a heating rate of 15 °C/min show that the first mass loss is observed between 0 and 100 °C and is related to the release of inherent moisture within the sample (Fig. 3). The greater part of the mass loss is observed between 80 and 500 °C. The most significant mass release in leather sample decomposition occurred at 325 °C (0.47%/ °C). This effect is caused by the thermal decomposition of volatile organic compounds in the material. Moreover, the mass loss curves obtained for various heating rates indicate the significant impact of heat transport on the maximum temperature peak (Fig. 4). Increasing the heating rate from 10 to 50 °C/min caused a shift of 30 °C in the temperature peak, from 322 (0.49%/°C) to 352 °C (0.47%/°C).
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Fig. 3. Thermogravimetric curves for leather waste in a nitrogen atmosphere at a heating rate of 15 °C/min.
Fig. 4. Derivative thermogravimetric curves for various heating rates.
Fig. 5. Temperature as a function of time at points T1 and T2 during the pyrolysis of tannery waste.
3.2. Heating rate of leather-tannery waste in the reactor The results of the experimental investigation indicate that the rate of heating of the fuel bed depends on time and temperature (Fig. 5). Within two time intervals, for measurement points T1 (16 min < t1 < 20 min) and T2 (11 min < t1 < 14 min), a decrease in the heating rate of the bed can be observed. In the case of measurement point T2, the temperature in the bed reached about 90 °C, close to the temperature at which water evaporates. The increase in temperature was retarded by the evaporation of moisture. Water absorbs heat and uses it to evaporate, and simultaneously delays devolatilisation (Pham et al., 2018; Arni et al., 2010). The slowdown in the rate of heating at T1, which occurs at 200 °C, may be caused by the flow of steam through the upper part of the reactor and fuel bed movement caused by a decrease in its volume during pyrolysis and evaporation of light volatiles (Chen et al., 2015). Furthermore, the heating rate in the fuel bed in the reactor is not constant; the dynamics of temperature change (dT/dt) are
much higher closer to the wall, reaching 50 °C/min at T1 (Fig. 6). Within a range up to 200 °C, the average rate of heating of the bed was 8.91 °C/min at measuring point T1 and 8.85 °C/min at T2. Within the range from 200 to 500 °C, the average rate of heating of the bed equalled 27.56 and 23.94 °C/min at T1 and T2, respectively.
3.3. Pyrolysis of leather waste 3.3.1. Pyrolysis yields According to the data available in the literature, for different types of biomass, such as pine nuts, shell, sewage sludge, chicken or turkey litter, or leather waste, the mass balance of pyrolysis products depends on the final temperature of the process (Yılmaz et al., 2007; Kim et al., 2009; Xiao and Yang, 2013; Chen et al., 2016a). The effect of pyrolysis temperature on the pyrolysis products of leather-tannery waste is shown in Fig. 7.
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2013). However, in this case, secondary tar reactions also increase. A balance exists between the rate of tar formation and secondary tar reactions near the equilibrium point at 400 °C; therefore the tar yield was highest near this point.
Fig. 6. Heating rate of the bed during pyrolysis of tannery waste.
Fig. 7. Effect of the temperature of leather-tannery pyrolysis on product yields.
Within the range 300–450 °C, the evolution of the solid-phase fraction of pyrolysis is quasi-constant and remains at the level of 33%, after which it decreases to 29% at 500 °C. It is noteworthy that molecular components could not be fully decomposed at lower temperatures, and that only weak chemical bonds could be broken (He et al., 2018), as indicated as well by the high yield of the gas fraction and the relatively low yield of the liquid fraction at 300 °C. The congruent yield of the solid phase was obtained by Marcilla et al. (2012), who presented experimental research on rapid (450–550 °C) and slow (up to 750 °C) bovine leather pyrolysis at 750 °C at a heating rate of 10 °C/min. A similar effect of temperature on char yield was observed by Marculescu and Stan (2014) for the pyrolysis of chicken feathers with traces of blood, whereby a temperature increase from 350 to 450 °C at a heating rate of 30 °C/min resulted in decreased char yields, from about 35 to 21%. Moreover, in the pyrolysis of leather waste, a temperature increase from 300 to 500 °C leads to the dominant position of condensation polymerisation, in which the liquid phase yield increases from 21 to 40%, with a maximum value of 42% at 400 °C. This may indicate an optimum temperature reaction for tar yield (Chen et al., 2016a, 2016b; He et al., 2018). It should be noted that, within the presented temperature range, the tar formation rate should increase with an increase in pyrolysis temperature (Baniasadi et al., 2016; He et al., 2018; Morgano et al., 2018; Xiao and Yang,
3.3.2. Influence of pyrolysis temperature on gaseous products The composition measurements of the gas obtained from the pyrolysis of leather waste are presented in Fig. 8. The results of experimental investigation showed that at lower temperatures, up to 450 °C, carbon dioxide was the dominant compound (above 40%). Subsequently, its content decreased to 28.7% at 500 °C. According to the literature, this aspect can be explained by the cracking-reforming process of functional groups related to carbonyl (CAOAC), carboxyl (C@O), and carboxylic acid (COOH) (Chen et al., 2016a, 2016b). During pyrolysis, the production of CO, whose content in the pyrolysis gas increased from 11.8% at 300 °C to 18.4% at 500 °C, can be explained by the decomposition of oxygen-containing functional groups such as carbonyl or hydroxyl (Zhang et al., 2010). The concentration of hydrogen increased from 1.7% at 300 °C to 33.2% at 500 °C, whereas the concentration of methane increased from 0.9% to 10.1%, respectively. Higher temperatures favour the thermal cracking of hydrocarbon chains (Zhang et al., 2010; Morgano et al., 2018; Sadrameli, 2015). Analysis of the gas composition showed that the higher heating value (HHV) increased from 2 MJ/Nm3 at 300 °C to about 10 MJ/Nm3 at 500 °C, which was caused by greater amounts of methane and hydrogen at higher temperatures as well as by carbon monoxide, the content of which amounted to approximately 18% at 500 °C. A similar heating value of pyrolytic gas at 450 °C (about 11 MJ/Nm3) was reported by Marculescu and Stan (2014) for the pyrolysis of chicken feathers. Morgano et al. (2018) recently showed a similar influence of temperature on the caloric value of gas from the pyrolysis of sewage sludge, recording a heating value of approximately 13.5 MJ/Nm3 at 500 °C. In addition, energy yield may be related to the weight of the feedstock (specific gas energy) and may determine the energy efficiency of the pyrolysis process (Taupe et al., 2016). Despite the fact that, at 300 °C, the gas yield reached 45%, a large amount of carbon dioxide and low hydrogen and methane contents in the obtained gas resulted in a low gas calorific value and low specific gas energy, amounting to 0.64 MJ/kgfuel. Pyrolysis at 500 °C caused a reduction in the gas fraction (30%); however, large amounts of methane and hydrogen led to a high calorific value for the obtained gas and a much higher specific energy, reaching 3.34 MJ/kgfuel.
Fig. 8. Influence of temperature on gas composition from leather waste pyrolysis.
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It is noteworthy that during the experiments, ammonium carbonate accumulated on cold connections between impingers, which, in the long term, led to a reduction of the diameters of cross sections of the tubes through which the produced gas passed. According to the literature, this effect may have been caused by the reaction between NH3 (as a product of the degradation of polypeptide) and CO2 (Yılmaz et al., 2007).
3.3.3. Influence of pyrolysis temperature on solid products The hydrogen and nitrogen concentrations decreased along with increasing temperatures, from 2.91% at 300 °C to 1.26% at 500 °C for hydrogen, and from 12.46 to 10.73%, respectively, for nitrogen (Table 2). Meanwhile, the total content of carbon in the char increased along with pyrolysis temperatures, from 47% to a maximum value of 52% at 450 °C. Analysis of the higher heating value of the obtained char indicated that this value did not change significantly; initially it decreased to 18.55 MJ/kg at 350 °C, then increased to 21.18 at 450 °C, which is higher than the calorific value reported by Morgano et al. (2018) for char from sewage sludge pyrolysis at 500 °C (8.6 MJ/kg) and lower than the calorific value reported by Marculescu and Stan (2014) for char from poultry litter pyrolysis at 450 °C (26 MJ/kg). Another important aspect is the inclusion of the char yield in the energy yield obtained from one kilogram of feedstock (specific char energy). Between 300 and 450 °C, the pyrolysis solid-phase fraction remained at the level of 33%, then decreased to 29% at 500 °C, resulting in a reduction in the specific energy of solid residue, from 7.1 MJ/kgfuel at 300 °C to 5.9 MJ/kgfuel at 500 °C. Char generated from leather-tannery residues can be used in the metallurgical sector. Filho et al. (2016) reported that carbonised leather residues with a higher heating value of 20.8 MJ/kg can partially replace mineral coal in the metallurgical process. Experiments were carried out for fixed-carbon basic content values in the prepared mixtures of 10 and 25%, demonstrating the potential for utilisation of the prepared charcoal as an energy source, with a recovery rate of up to 76% of chromium in the final production of iron ore pellets. Moreover, Yılmaz et al. (2007) showed that carbonised leather residues may be used in the
production of activated carbon (activated by CO2) and as an adsorbant of dyes from aqueous solutions. The results derived from wavelength dispersive X-ray fluorescence spectroscopy (Table 3) showed that the major oxidised component of char was chromium oxide (Cr2O3), with a content of approximately 8.5% in all cases. Furthermore, the char contained 7.5–8.25% of sulphur trioxide (SO3), 4.3–5.4% of sodium oxide (Na2O), and 1.2–1.7% of calcium oxide (CaO). The chlorine content was 0.7–1.2%. Chromium oxide is widely used in the industrial sector in processes such as leather tanning, finishing of metals, or wood preservation (Godinho et al., 2017). The topic of chromium recovery has been thoroughly described by Wionczyk et al. (2011), Godinho et al. (2017), and Dias et al. (2018). Chromium oxide is also commonly used as a catalyst (Chen et al., 2018; Song et al., 2018; Tian et al., 2018; Wang et al., 2014; Rotaru et al., 2005; Jia et al., 2011). Chen et al. (2018) demonstrated the potential for and efficiency of toluene combustion over mesoporous Cr2O3 catalysts; Song et al. (2018) presented the application of ZnO/Cr2O3 catalysts in the high-temperature synthesis of methanol. Moreover, Table 3 presents the composition of ash from leathertannery waste combustion (PN-EN ISO 18122:2016-01), indicating about 14.7% content of sulphur trioxide. Taking into account the facts that leather contains about 11% ash, and that the pyrolysis solid phase remains at the level of about 30%, the ash from 1 kg of tannery waste contains about 1.6% sulphur trioxide, whereas the corresponding content in the char reached about 2.5%. This
Table 2 Influence of pyrolysis temperature on elemental composition of char. T [°C]
C [%]
N [%]
H [%]
HHV [MJ/kg]
300 350 400 450 500
47.08 49.81 48.44 52.21 49.75
12.46 13.31 12.89 13.03 10.73
2.91 3.12 3.01 1.85 1.26
21.50 18.55 19.46 21.18 19.92 Fig. 9. Heavy tar yields at various pyrolysis temperatures.
Table 3 The oxidized components of char from leather tannery pyrolysis. Compound
Cr2O3 SO3 Na2O CaO Cl Fe2O3 SiO2 MgO P2O5 K2O
Content [%] 300 °C
350 °C
400 °C
450 °C
500 °C
Ash
8.66 8.25 4.68 1.49 0.93 0.34 0.22 0.21 0.18 0.02
8.09 7.78 4.33 1.55 1.09 0.33 0.29 0.19 0.16 0.03
8.62 8.19 4.57 1.66 0.98 0.34 0.24 0.22 0.16 0.03
8.66 7.88 5.39 1.18 0.73 0.43 0.3 0.24 0.17 0.03
8.52 7.5 5.21 1.54 1.2 0.29 0.24 0.24 0.17 –
34.97 14.73 9.78 5.38 0.11 1.30 0.73 0.68 0.68 0.42
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Table 4 The identified compounds in the gas phase at different temperatures. Ret. Time
Formula
Compounds
% Area
9.806 9.979 11.145 12.375 12.465 12.611 12.702 12.786 13.575 14.769 14.831 15.423 16.512 16.580 16.751 16.792 16.860 15.906 17.050 17.215 17.556 17.708 17.952 17.990 18.183 18.509 18.629 18.742 18.995 19.043 19.253 19.441 19.533 19.601 19.858 19.903 20.028 20.266 20.373 20.666 20.926 20.975 21.002 21.080 21.422 21.678
C4H8O C6H14 C4H8O C4H7N C6H6 C5H10O C7H14 C8H18NO C4H9NO4 C4H9Cl C7H8 C4H4NCH3 C8H10 C5H10 C6H11N C4H5N7 C5H7N C6H7N C6H18 C7H14O C6H10O C7H9N C7H9NO C6H10O C8H14 C6H5CHO C8H16O C6H11N C7H7NO C6H5(CN) C12H26 C7H14O C8H14O C6H11N C9H18O C19H36O C6H8O2 C6H12 C13H2 C12H26S C8H7N C7H12O C4H8O C8H16 C5H9NO C11H22
Propanal, 2-methyln-Hexane 2-Butanone Isobutyronitrile 1,3-Hexadien-5-yne Butanal, 3-methyl(Z)-2-Heptene Nitroxide, bis(1,1-dimethylethyl) 2-Nitro-2-methyl-1,3-propanediol Propane, 1-chloro-2-methylTolune Pyrrole 1,3-Cyclopentadiene, 5-(1-methylethylidene)Cyclobutane, methylIsoamyl cyanide 6-Hydrazinotetrazolo(b)pyridazine 1H-Pyrrole, 3-methylPyridine, 4-methyl1,3,5,7-Cyclooctatetraene Heptanal Cyclohexanone Pyridine, 3,4-dimethyl3-Pyridinol, 2,6-dimethyl2,4-Hexadien-1-ol 6-Methyl-2-heptyne Benzaldehyde Octanal Aziridine, 1-(2-buten-2-yl)Benzaldehyde, oxime, (E)Benzonitrile 2-Undecanethiol, 2-methylOxetane, 2-methyl-4-propyl2-Octenal, (E)Aziridine, 1-(2-buten-1-yl)-, (E)2-Nonen-1-ol 5-Cyclopropylcarbonyloxypentadecane 1,3-Butadien-1-ol, acetate Propane, 2-cyclopropylBenzene, (2,4-dimethylpentyl)2-Undecanethiol, 2-methyl2,4,6-Cycloheptatriene-1-carbonitrile 3-Hepten-2-one, (E)3-Buten-1-ol 1-Hexene, 4,5-dimethyl2-Piperidinone 3-Undecene, (Z)-
400 °C
500 °C
0.10 0.28 0.11 0.51 – 0.39 – 0.36 0.51 15.86 – 67.75 – 0.76 0.66 – – – 0.17 0.40 0.20 0.24 0.41 0.11 0.12 4.99 0.34 0.23 0.20 – – 0.23 0.36 0.25 0.92 – 0.35 0.16 0.61 0.12 0.24 0.06 – 0.12 – 0.07
– – – – 0.12 – 0.07 – – – 4.0 75.47 0.12 – 0.53 0.17 – 0.16 0.22 – 0.10 0.10 – – – 3.08 – – – 12.70 0.10 – – – – 0.11 – – – 0.16 – – 0.19 – 0.11 –
– – – – 0.19 – 0.13 – – – 5.52 69.77 0.25 – – 0.23 0.74 0.26 0.37 – 0.14 0.18 – 0.14 – 3.98 – – – 15.11 0.11 – – – – 0.16 – – – 0.34 – – 0.47 – 0.20 –
the amount of heavy tars from 0.45 to approximately 7% (g tars/100 g leather). Subsequently, the amount of tars decreased to about 6% at 500 °C (Fig. 9).
Table 5 Classification and % area of compounds identified. % Area
Aldehydes Ketone Alcohol Ester Heterocyclic aromatic compounds Aromatic Nitriles and derivatives Chloroalkanes Others
300 °C
300 °C
400 °C
500 °C
6.58 0.37 1.54 0.35 68.4 – 1.89 15.86 5.01
3.08 0.10 0.19 0.11 75.73 4.0 12.52 – 4.27
3.98 0.14 0.61 0.16 70.95 5.52 15.11 – 3.53
comparison shows that about half of the sulphur contained in the leather-tannery waste may be released in gaseous form into the generated sulphur compounds in the flue gas. 3.3.4. Influence of pyrolysis temperature on liquid products The results of analysis of heavy tar content showed that an increase in temperature from 300 to 450 °C caused an increase in
3.3.5. TGA-GC/MS gas analysis Table 4 shows the list of compounds identified by the GC/MS analyzer (GCMS-QP2010 Plus, Shimadzu). Due to the linear relationship between the total area and the area of the pyrolysed sample, all of the detected peaks were normalised with reference to the total area of the analysed chromatogram. The results show that the majority of the detected compounds correspond to nitrogen derivatives; this is related to the protein content in the leather waste (Sethuraman et al., 2014; Font et al., 1999; Marcilla et al., 2011a, 2011b). Table 5, based on identification of gas phase composition, shows eight groups of compounds, characterised as follows: aldehydes, ketones, alcohols, esters, heterocyclic aromatic compounds, including pyrrole and pyridine derivatives; aromatic compounds, represented by toluene, nitriles, and nitrile derivatives, chloroalkanes, and others. The main group comprises heterocyclic aromatic compounds, which are associated with the thermal degradation
J. Kluska et al. / Waste Management 88 (2019) 248–256 Table 6 Amino acid composition of type I collagen from calf-skin (Friess, 1998; Choi and Ko, 2011). Amino acid
a1(I)-chain
a2(I)-chain
Alanine Arginine Asparagine Aspartic acid Glutamic acid Glutamine Glycine Histidine Hydroxylysine Hydroxyproline Isoleucine Leucine Lysine Methionine Phenylalanine Proline Serine Threonine Tyrosine Valine Total
124 (2) 53 (2) 13 33 (3) 52 (2) 27 (3) 345 (6) 3 (1) 4 114 9 (1) 22 (3) 34 (2) 7 13 (1) 127 (4) 37 (5) 17 (1) 5 (5) 17 (1) 1056 (42)
111 (3) 56 (1) 23 24 (2) 46 (2) 24 (1) 346 (6) 8 9 99 18 33 21 (1) 4 15 (3) 108 (1) 35 (1) 20 4 (3) 34 1038 (24)
of proteins. The dominant percentage of aromatic compounds was also demonstrated by Marcilla et al. (2011a). Moreover, at 300 °C a significant number of chloroalkane compounds were observed. According to the relevant literature, the product obtained via leather tanning from tanned collagen fibre networks of various sizes consists of 99% by mass (Li et al., 2009; Šteˇpánová et al., 2017), including pyrolysed waste considered to represent tanned collagen. Of the approximately 28 types of collagen (Bhagwat and Dandge, 2018), the main type found in animal skin is represented by Type I (Friess, 1998). The amino acid composition of Type 1 collagen is presented in Table 6 (Friess, 1998; Choi and Ko, 2011). Considering these aspects, it can be concluded that the high content of organic nitrogen derivatives (mainly pyrrole) refers to the structure of collagen-building amino acids. Proline and hydroxyproline contain pyrrole rings, which constitute the main element of the heterocyclic aromatic compounds (Choi and Ko, 2011), in their structures. Moreover, amino acids such as asparagine, arginine, glutamine, and lysine contain more than one amino group that may be released as amine or that may constitute part of the structures of products obtained as a result of the cyclisation and aromatisation reactions (Wei et al., 2018). Most of the main amino acids – alanine, arginine, glutamic acid, and glycine – are characterised by a linear structure, which, as a result of the pyrolysis process, can lead to the formation of aliphatic compounds such as 2-Nonen-1-ol, isoamyl cyanide, isobutyronitrile, or propane, 1-chloro-2-methyl- (Zhao et al., 2016). 4. Summary Proximate and ultimate analysis showed that leather waste is similar to poultry-derived waste. The thermal characteristics of leather tannery-waste pyrolysis indicates that this process is highly non-linear due to various mechanisms of heat transport which cause differences in heating rate inside a reactor. This is related to the evaporation of moisture in the core of the fuel bed, steam flow through the upper part of the reactor, and movement of the fuel bed caused by reductions in its volume during pyrolysis. The results presented here, obtained within a temperature range of 300–500 °C, show that pyrolysis for energetic application is potentially feasible above 400–450 °C. This is the result of a greater amount of hydrogen and methane in the gas mixture,
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which, despite the lower yield of gases, leads to higher specific gas energy values. HHV analysis of the obtained char indicated that this value does not change significantly. However, the decrease in the solid fraction yield results in a noticeable decrease in the specific energy of char. Analysis of char composition showed that the contents of oxidised components do not change significantly along with pyrolysis temperature in cases where the major component is chromium oxide (Cr2O3). The main group of tar components comprises heterocyclic aromatic compounds, which are associated with the thermal degradation of proteins. The results and the data in the relevant literature indicate that the pyrolysis of leather waste can be explained using tanned collagen as a reference material.
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