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Spent coffee grounds valorization through pyrolysis for energy and materials production in the concept of circular economy
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 27582–27588
www.materialstoday.com/proceedings
PSCCE_2017
Spent coffee grounds valorization through pyrolysis for energy and materials production in the concept of circular economy R. Ktori, P. Kamaterou, A. Zabaniotou* Biomass Group, School of Chemical Engineering, Faculty of Engineering, Aristotle University of Thessaloniki, Un. Box 455, Thessaloniki, 54124, Greece
Abstract The aim of the paper is to investigate the recovery of useful material and energy from waste, in the context of circular economy, by studying pyrolysis of spent coffee grounds, for the production of fuels and carbon materials such as biochar, to be further used as fertilizer in arid fields, thus closing loops in agriculture. Pyrolysis was carried out at temperature ranging from 400 to 700 ° C with a heating rate of 50 °C/s, at atmospheric pressure and inert atmosphere. The results have shown that a maximum yield of biooil can be achieved at 540° C (36 wt%) where the gas reached a yield 9 wt% and the char reached the 29 wt%. At 700 ° C, where oxidation reactions mainly take place against of cracking (gasification), gas yield reached 29 wt%, while biooil and char reached 20 wt% and 26 wt%, respectively. These preliminary data can challenge decision making in introducing sustainable food waste management strategies where pyrolysis can be the conversion pathway. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 11th Panhellenic Scientific Conference on Chemical Engineering. Keywords: spent coffee grounds; pyrolysis; valorization; circular economy
1. Introduction Coffee plays an important role in global economy, being one of the most important drinks in the western societies. It is believed that it is the second most traded commodity in the world after oil [1] and also the most consumed drink in the western world after water. Based on US Department of Agriculture (USDA) report, global
* Corresponding author. Tel.: +30-2310-996274, E-mail address: [email protected]
2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 11th Panhellenic Scientific Conference on Chemical Engineering.
Ktori et al. / Materials Today: Proceedings 5 (2018) 27582–27588
27583
Nomenclature SCG Spent Coffee Grounds ECR Exhausted coffee residue USDA US Department of Agriculture coffee industry reached an impressive estimated production of 9.34 million tons in 2016/17. .In the same time, its extraction and use creates massive quantities of solid residues. Consequently, there is a crucial need to manage and exploit spent coffee grounds (SCG), by using various technological pathways [2]. Exhausted coffee residue (ECR) or spent coffee ground (SCG), is a by-product of the soluble coffee production and extraction processes [2]. The disposal of these residues consists a significant environmental problem due to their toxicity and high content of organic matter [3,4]. However, because of their high organic compounds, spent coffee grounds are extremely attractive to be used as biomass to be converted towards obtaining biofuels and other valuable products [5]. Concurrently, spent coffee grounds show potential of becoming a feedstock for a coffee-based biorefinery [6]. Due to their total phenolic content, SCG can potentially be an exploitable resource for natural antioxidant production. The recovery of these compounds, if followed in a cascade way by thermochemical conversion of the remaining waste for energy production, is an interesting valorization approach [4,7]. It is worth mentioning that neither the phenolic content nor the type of phenolic compounds affect the potential these materials have for energy production [7]. Researchers have worked on extraction of various valuable compounds from SCG. Exhausted coffee extract was characterized as a source of antioxidants [8], with anti-tumor, anti-inflammatory and anti-allergic activities [9], and at the same time, as a source of probiotic compounds, with antimicrobial activity [10], very appealing applications for the food industry and pharmaceutical products [11]. Other researchers studied extensively SCG for its potential to adsorb different metals [12] and found that oxygen functional groups, mainly carboxylic groups, are responsible for the absorption of divalent metals [13]. Furthermore, they found that lignin is responsible for the absorption of divalent metal ions with ion exchange between metal ion and light metals on the surface of the adsorbent [12]. Particular attention was paid to the adsorption of heavy metals from aqueous / industrial solutions such as Cr (VI), Cu (II) [12-15]. The high aromatic character and the low polarity index show their potential use as absorbents of hydrophobic pollutants [13]. The production of activated carbons was also investigated, for the adsorption of H2S, towards gas desulfurization [16]; the absorption is due to microporous structure of activated carbon and also to the presence of high hemicellulose and cellulose contents of SCG [3]. SCG is characterized chemically by a high content of carbon source [2] and oil yield of around 10-15% [17]. Many studies focused on lipids recovery through the process of extraction (hexane, or hexane/isopropanol mixture) [18], followed by transesterification (in two steps) for the production of biodiesel [17]. In-situ transesterification of wet spent coffee grounds (SCGs) for the production of biodiesel was also proposed [19]. By other researchers, direct transesterification (in one step) method was carried out to directly obtain biodiesel from SCGs without the steps of separate solvent extraction and esterification [20,21]. Moreover, due to their high content of lignocellulose, there is the potential of utilizing residues in the fermentation for bioethanol production that can be used as fuel [22,23]. Biodiesel by-products towards production of fuels in the context of biorefinery, was also investigated [24]. In addition, biodiesel wastes towards compost production was studies [25] i the case of co-production of biodiesel and bioethanol [26,27]. Most of SCG carbohydrates can be solubilized by two step hydrolysis due to the presence of high hemicellulose content of SCG, along with the relatively low amount of cellulose [28]. Calorific value measurements have shown that coffee residues have high energy content and in combination with low ash and low sulfur content, they are challenging alternatives for energy production or biofuels [1]. Bio-oil was tested for biodiesel production [5], as well as its insecticidal and bactericidal characteristics was examined [29]. As for the pyrolytic char, its use as a solid fuel in the industrial sector, was considered appropriate, due to its high calorific value [2]. Its use as a CO2 adsorbent was also examined [30].
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2. Materials and Methods 2.1. Raw Materials Spent coffee grounds (SCG) were carefully separated by exhausted coffee capsules of Nespresso brand, used in a Nespresso machine, for a cup of coffee production. Ultimate analysis of SCG was performed using a Thermofinnigan, CHNS, EA 1112 elemental analyzer. Oxygen content was calculated by difference (100- CHNS content). The moisture content of SCG was estimated using a laboratory furnace (Universal 2960 SDT V3.0F) at 105 °C, in triplicate. 2.2. Pyrolysis Pyrolysis of spent coffee grounds was performed using a laboratory scale, wire mesh captive sample type reactor. The experimental apparatus includes two electrodes, an electrical circuit, a water cooling coil, a moisture trap, two filters for liquid hydrocarbons a helium providing section, temperature controller and a gas sampling collection system. Each sample was placed in an envelope of stainless steel; it weighed around 0.5 g. The experiments were carried out within a range of pyrolysis temperature from 380-700°C, with a heating rate of approximately 50°C/s at atmospheric pressure and inert atmosphere. Fig. 1 shows the experimental set up. The pyrolytic products were char, liquid and non-condensable gas. Char was the solid product which remained on the screen and is determined gravimetrically. The liquid product consisted of tar and liquid hydrocarbons. Tar was considered as the material condensed within the reactor vessel, on the wall, flanges and on a paper filter at the exit of the reactor, at ambient temperature. It was removed by washing with C3H6O (acetone) soaked filter paper and measured gravimetrically. Hydrocarbons in the vapor phase at room temperature were collected in a lipophilic trap containing 80:100 mesh Porapaq Q chromatographic packing, placed at the exit of the reactor and then measured gravimetrically [31]. The collected gas was analyzed offline, in a gas chromatographic system, (Model 6890N, Agilent Technologies) fitted with two columns, HP-Plot Q and HP-Molsive type.
Fig.1. Experimental set up for fast pyrolysis (Biomass group, AUTh Greece)
Ktori et al. / Materials Today: Proceedings 5 (2018) 27582–27588
27585
3. Results and Discussion 3.1. Spent coffee grounds characteristics SCG moisture content was estimated at 33 wt%. This value is high due to the fact that SCG is a water-extraction product. Ultimate analysis of SCG, in comparison with other SCG feedstock found in bibliography, is presented in Table 1. Their compositions in C-N-H-S were found almost similar with the results reported in the literature. SCG of the current study contained 54.9 wt% C, whereas the corresponding values of H, N and were 7.9 wt% and 3.5 wt% respectively. The S content of SCG was negligible and the containing O was calculated by difference at 33.8 wt% (100-C-H-N-S). Table 1. Ultimate Analysis of SCG C%
H%
N%
S%
O%
References
54.9
7.9
48.9
7.9
3.51
0
33.75
Present study
1.5
0.1
40.1
[32]
52.54±0.43 57.56
6.95±0.03
3.46±0.01
0.1±0.0
34.82±0.1
[2]
7.86
2.48
0
32.1
[14]
54.5
7.1
2.4
0.1
34.2
[5]
3.2. Spent coffee grounds pyrolysis yields The yields of char, biooil and gas pyrolysis products and the gas Low Heating Value in MJ/m3 are presented in Fig. 2, 3 and 4, respectively. Char yields asymptotically decreased with pyrolysis temperature starting from 43 wt% at 400 °C and reaching the value of 26 wt%, at 700 °C pyrolysis temperature (Fig. 2). Concerning biooil yields, they showed a maximum value of 36 wt% at 540 °C pyrolysis temperature (Fig. 3).
Fig.2. Char yields as a function of pyrolysis temperature
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Ktori et al. / Materials Today: Proceedings 5 (2018) 27582–27588
Fig.3. Biooil yields as a function of pyrolysis temperature
Contrary to char yields, gas yields followed an increasing trend with pyrolysis temperature (Fig. 4), reaching the value of 28 wt% at 700 °C pyrolysis temperature. Pyrolysis gas was mainly composed of H2, CO, CO2, CH4, C2H6 and C2H4 and its LHV was calculated by the following equation (1) [33]: LHV [30 v / v%CO 25.7 v / v% H 2 85.4 v / v%CH 4 151.3 v / v%(C2 H 4 C2 H 6 )] 4.2 / 1000
(1)
The LHV of biogas produced at 540 °C is 13.3 MJ/m3 (Fig.5) and belongs to the medium level gas fuels, thus it can be directly used in engines, turbines and boilers for power production [34].
Fig.4. Biogas yields as a function of pyrolysis temperature
Ktori et al. / Materials Today: Proceedings 5 (2018) 27582–27588
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Fig.5. LHV of produced gas as a function of pyrolysis temperature
It must be noted that these values are indicative and presented in order to show trends and potential towards testing at pilot scale, and then application at industrial scale. In order to assess char applicability in soils as fertiliser, closing thus the loop in agriculture, its further physicochemical characterization is proposed. Pyrolysis oil could possibly lead to biodiesel production, thus its role as a potential biofuel candidate should be investigated. 4. Conclusions The ever-increasing coffee consumption generates remarkable quantities of waste which constitutes a challenge of our modern times. Spent coffee grounds pyrolysis could be the solution to this problem, by achieving not only to handle but also to valorize the secondary solid waste streams generated by coffee consumption. More specifically, spent coffee grounds can be converted into biooil (36 wt%) when pyrolyzed at 540 °C, whereas the corresponding values for biochar and biogas product yields can reach 29 wt% and 9 wt%, respectively. The LHV of biogas produced at 540 °C is 13.3 MJ/m3 and belongs to the medium level gas fuels; thus it can be used in engines, turbines and boilers for power production. Biooil and char characteristics need further characterization in order to be assessed as biofuel and biochar respectively.
References [1] J.P. Bok, H.S. Choi, Y.S. Choi, H.C. Park, S.J. Kim, Energy 47 (2012) 17-24. [2] W.T. Tsai, S.C.Liu, C.H. Hsieh, J. Anal. Appl. Pyrolysis 93 (2012) 63-67. [3] M.G. Plaza, A.S. Gonzalez, C. Pevida, J.J. Pis, F. Rubiera, Applied Energy 99 (2012) 273-279. [4] R.N.M.J. Páscoa, L.M. Magalhães, J.A. Lopes., Food Res. Int. 51 (2013) 579-586. [5] X. Li, V. Strezov, T. Kan, J. Anal. Appl. Pyrolysis 110 (2014) 79-87. [6] M.V.P. Rocha, L.J.B.L. De Matos, L.P. De Lima, P.M. Da Silva Figueiredo, I.L. Lucena, F.A.N. Fermandes, L.R.B. Gonçalvesa, Bioresour. Technol. 167 (2014) 343-348. [7] A. Zuorro, R. Lavecchia, J. Clean. Prod. 34 (2012) 49-56. [8] A. Panusa, A. Zuorro, R. Lavecchia, G. Marrosu, R. Petrucci, J. Agric. Food Chem. 61 (2013) 4162-4168. [9] K. Ramalakshmi, L.J.M. Rao, Y. Takano-Ishikaw, M. Goto, Food Chemistry 115 (2009) 79-85. [10] A. Jiménez-Zamora, S. Pastoriza, J.A. Rufián-Henares, LWT- Food Science and Technology 61 (2015) 12-18. [11] S.I. Mussatto, L.F. Ballesteros, S. Martins, J.A. Teixeira, Sep. Purif. Technol. 83 (2011) 173-179. [12] G.Z. Kyzas, Materials 5 (2012) 1826-1840.
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[13] C. Liu, Metal ions removal from polluted waters by sorption onto exhausted Coffee Waste. Application to metal finishing industries waste water treatment. PhD Thesis, University of Girona, 2014. http://dugi-doc.udg.edu/bitstream/handle/10256/9641/tcl.pdf?sequence=1. [14] C. Liu, D. Pujol, M.A. Olivella, F. de la Torre, N. Fiol, J. Poch, I. Villaescusa, Water Air Soil Pollut. 226 (2015) 289. [15] C. Jeon, Korean J. Chem. Eng. 34 (2016) 384-391. [16] K. Kante, C. Nieto-Delgado, J.R. Rangel-Mendez, T.J. Bandosz, J. Hazard. Mater. 201-202 (2012) 141-147. [17] Z. Al-Hamamre, S. Foerster, F. Hartmann, M. Kröger, M. Kaltschmitt, Fuel 96 (2012) 70-76. [18] N.S. Caetano, V.F.M. Silva, A.C. Melo, A.A. Martins, T.M. Mata, Clean Technol. Envir. Policy 16 (2014) 1423-1430. [19] J. Park, B. Kim, Y.K. Chang, J.W Lee, Bioresour. Technol. 221 (2016) 55-60. [20] Y. Liu, Q. Tu, G. Knothe, M. Lu, Fuel 199 (2017) 157-161. [21] F. Calixto, J. Fernandes, R. Couto, E.J. Hernández, V. Najdanovic-Visak, P.C.Simões, Green Chem. 13 (2011) 1196. [22] N.S. Caetano, V.F.M. Silvaac, T.M. Matab, AIDIC 26 (2012) 267-272. [23] S.I. Mussatto, E.M.S. Machado, L.M. Carneiro, J.A.Teixeira, Appl. Energy 92 (2012) 763-768. [24] M. Haile, Biofuel Res. J. 2 (2014) 65-69. [25] N. Kondamudi, S.K. Mohapatra, M. Misra, J. Agric. Food Chem. 56 (2008) 11757-11760. [26] E.E. Kwon, H. Yi, Y.J. Jeon, Bioresour. Technol. 136 (2013) 475-80. [27] M.V.P. Rocha, L.J.B.L. de Matos, L.P.D. Lima, P.M.D.S. Figueiredo, I.L. Lucena, F.A.N. Fernandes, L.R.B. Gonçalves, Bioresour. Technol. 167 (2014) 343-348. [28] H.D. Wang, Y.S. Cheng, C.H. Huang, C.W. Huang, Appl. Biochem. Biotechnol. 180 (2016) 753-765. [29] R. Bedmutha, C.J. Booker, L. Ferrante, C. Briens, F. Berruti, K.K.-C. Yeung, I. Scott, K. Conn., J. Anal. Appl. Pyrolysis 90 (2011) 224-231. [30] D.W. Cho, S.H. Cho, H. Song, E.E. Know, Bioresour. Technol. 189 (2015) 1-6. [31] P. Manara, A. Zabaniotou, J. Anal. Appl. Pyrolysis 100 (2013) 166–172. [32] D. Kim, K. Lee, D. Bae, K.Y. Park, Characterizations of biochar from hydrothermal carbonization of exhausted coffee residue, J Mater Cycles Waste Manag, 3rd 3R International Scientific Conference (3rd 3RINCs 2016), DOI 10.1007/s10163-016-0572-2. [33] H. Yang, R. Yan, H. Chen, D.H. Lee, D.T. Liang, C. Zheng, Fuel Process. Technol. 87 (2006) 935–42. [34] A. Zabaniotou, O. Ioannidou, Fuel 87 6 (2008) 834-843.
ScienceDirect Materials Today: Proceedings 5 (2018) 27582–27588
www.materialstoday.com/proceedings
PSCCE_2017
Spent coffee grounds valorization through pyrolysis for energy and materials production in the concept of circular economy R. Ktori, P. Kamaterou, A. Zabaniotou* Biomass Group, School of Chemical Engineering, Faculty of Engineering, Aristotle University of Thessaloniki, Un. Box 455, Thessaloniki, 54124, Greece
Abstract The aim of the paper is to investigate the recovery of useful material and energy from waste, in the context of circular economy, by studying pyrolysis of spent coffee grounds, for the production of fuels and carbon materials such as biochar, to be further used as fertilizer in arid fields, thus closing loops in agriculture. Pyrolysis was carried out at temperature ranging from 400 to 700 ° C with a heating rate of 50 °C/s, at atmospheric pressure and inert atmosphere. The results have shown that a maximum yield of biooil can be achieved at 540° C (36 wt%) where the gas reached a yield 9 wt% and the char reached the 29 wt%. At 700 ° C, where oxidation reactions mainly take place against of cracking (gasification), gas yield reached 29 wt%, while biooil and char reached 20 wt% and 26 wt%, respectively. These preliminary data can challenge decision making in introducing sustainable food waste management strategies where pyrolysis can be the conversion pathway. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 11th Panhellenic Scientific Conference on Chemical Engineering. Keywords: spent coffee grounds; pyrolysis; valorization; circular economy
1. Introduction Coffee plays an important role in global economy, being one of the most important drinks in the western societies. It is believed that it is the second most traded commodity in the world after oil [1] and also the most consumed drink in the western world after water. Based on US Department of Agriculture (USDA) report, global
* Corresponding author. Tel.: +30-2310-996274, E-mail address: [email protected]
2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 11th Panhellenic Scientific Conference on Chemical Engineering.
Ktori et al. / Materials Today: Proceedings 5 (2018) 27582–27588
27583
Nomenclature SCG Spent Coffee Grounds ECR Exhausted coffee residue USDA US Department of Agriculture coffee industry reached an impressive estimated production of 9.34 million tons in 2016/17. .In the same time, its extraction and use creates massive quantities of solid residues. Consequently, there is a crucial need to manage and exploit spent coffee grounds (SCG), by using various technological pathways [2]. Exhausted coffee residue (ECR) or spent coffee ground (SCG), is a by-product of the soluble coffee production and extraction processes [2]. The disposal of these residues consists a significant environmental problem due to their toxicity and high content of organic matter [3,4]. However, because of their high organic compounds, spent coffee grounds are extremely attractive to be used as biomass to be converted towards obtaining biofuels and other valuable products [5]. Concurrently, spent coffee grounds show potential of becoming a feedstock for a coffee-based biorefinery [6]. Due to their total phenolic content, SCG can potentially be an exploitable resource for natural antioxidant production. The recovery of these compounds, if followed in a cascade way by thermochemical conversion of the remaining waste for energy production, is an interesting valorization approach [4,7]. It is worth mentioning that neither the phenolic content nor the type of phenolic compounds affect the potential these materials have for energy production [7]. Researchers have worked on extraction of various valuable compounds from SCG. Exhausted coffee extract was characterized as a source of antioxidants [8], with anti-tumor, anti-inflammatory and anti-allergic activities [9], and at the same time, as a source of probiotic compounds, with antimicrobial activity [10], very appealing applications for the food industry and pharmaceutical products [11]. Other researchers studied extensively SCG for its potential to adsorb different metals [12] and found that oxygen functional groups, mainly carboxylic groups, are responsible for the absorption of divalent metals [13]. Furthermore, they found that lignin is responsible for the absorption of divalent metal ions with ion exchange between metal ion and light metals on the surface of the adsorbent [12]. Particular attention was paid to the adsorption of heavy metals from aqueous / industrial solutions such as Cr (VI), Cu (II) [12-15]. The high aromatic character and the low polarity index show their potential use as absorbents of hydrophobic pollutants [13]. The production of activated carbons was also investigated, for the adsorption of H2S, towards gas desulfurization [16]; the absorption is due to microporous structure of activated carbon and also to the presence of high hemicellulose and cellulose contents of SCG [3]. SCG is characterized chemically by a high content of carbon source [2] and oil yield of around 10-15% [17]. Many studies focused on lipids recovery through the process of extraction (hexane, or hexane/isopropanol mixture) [18], followed by transesterification (in two steps) for the production of biodiesel [17]. In-situ transesterification of wet spent coffee grounds (SCGs) for the production of biodiesel was also proposed [19]. By other researchers, direct transesterification (in one step) method was carried out to directly obtain biodiesel from SCGs without the steps of separate solvent extraction and esterification [20,21]. Moreover, due to their high content of lignocellulose, there is the potential of utilizing residues in the fermentation for bioethanol production that can be used as fuel [22,23]. Biodiesel by-products towards production of fuels in the context of biorefinery, was also investigated [24]. In addition, biodiesel wastes towards compost production was studies [25] i the case of co-production of biodiesel and bioethanol [26,27]. Most of SCG carbohydrates can be solubilized by two step hydrolysis due to the presence of high hemicellulose content of SCG, along with the relatively low amount of cellulose [28]. Calorific value measurements have shown that coffee residues have high energy content and in combination with low ash and low sulfur content, they are challenging alternatives for energy production or biofuels [1]. Bio-oil was tested for biodiesel production [5], as well as its insecticidal and bactericidal characteristics was examined [29]. As for the pyrolytic char, its use as a solid fuel in the industrial sector, was considered appropriate, due to its high calorific value [2]. Its use as a CO2 adsorbent was also examined [30].
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2. Materials and Methods 2.1. Raw Materials Spent coffee grounds (SCG) were carefully separated by exhausted coffee capsules of Nespresso brand, used in a Nespresso machine, for a cup of coffee production. Ultimate analysis of SCG was performed using a Thermofinnigan, CHNS, EA 1112 elemental analyzer. Oxygen content was calculated by difference (100- CHNS content). The moisture content of SCG was estimated using a laboratory furnace (Universal 2960 SDT V3.0F) at 105 °C, in triplicate. 2.2. Pyrolysis Pyrolysis of spent coffee grounds was performed using a laboratory scale, wire mesh captive sample type reactor. The experimental apparatus includes two electrodes, an electrical circuit, a water cooling coil, a moisture trap, two filters for liquid hydrocarbons a helium providing section, temperature controller and a gas sampling collection system. Each sample was placed in an envelope of stainless steel; it weighed around 0.5 g. The experiments were carried out within a range of pyrolysis temperature from 380-700°C, with a heating rate of approximately 50°C/s at atmospheric pressure and inert atmosphere. Fig. 1 shows the experimental set up. The pyrolytic products were char, liquid and non-condensable gas. Char was the solid product which remained on the screen and is determined gravimetrically. The liquid product consisted of tar and liquid hydrocarbons. Tar was considered as the material condensed within the reactor vessel, on the wall, flanges and on a paper filter at the exit of the reactor, at ambient temperature. It was removed by washing with C3H6O (acetone) soaked filter paper and measured gravimetrically. Hydrocarbons in the vapor phase at room temperature were collected in a lipophilic trap containing 80:100 mesh Porapaq Q chromatographic packing, placed at the exit of the reactor and then measured gravimetrically [31]. The collected gas was analyzed offline, in a gas chromatographic system, (Model 6890N, Agilent Technologies) fitted with two columns, HP-Plot Q and HP-Molsive type.
Fig.1. Experimental set up for fast pyrolysis (Biomass group, AUTh Greece)
Ktori et al. / Materials Today: Proceedings 5 (2018) 27582–27588
27585
3. Results and Discussion 3.1. Spent coffee grounds characteristics SCG moisture content was estimated at 33 wt%. This value is high due to the fact that SCG is a water-extraction product. Ultimate analysis of SCG, in comparison with other SCG feedstock found in bibliography, is presented in Table 1. Their compositions in C-N-H-S were found almost similar with the results reported in the literature. SCG of the current study contained 54.9 wt% C, whereas the corresponding values of H, N and were 7.9 wt% and 3.5 wt% respectively. The S content of SCG was negligible and the containing O was calculated by difference at 33.8 wt% (100-C-H-N-S). Table 1. Ultimate Analysis of SCG C%
H%
N%
S%
O%
References
54.9
7.9
48.9
7.9
3.51
0
33.75
Present study
1.5
0.1
40.1
[32]
52.54±0.43 57.56
6.95±0.03
3.46±0.01
0.1±0.0
34.82±0.1
[2]
7.86
2.48
0
32.1
[14]
54.5
7.1
2.4
0.1
34.2
[5]
3.2. Spent coffee grounds pyrolysis yields The yields of char, biooil and gas pyrolysis products and the gas Low Heating Value in MJ/m3 are presented in Fig. 2, 3 and 4, respectively. Char yields asymptotically decreased with pyrolysis temperature starting from 43 wt% at 400 °C and reaching the value of 26 wt%, at 700 °C pyrolysis temperature (Fig. 2). Concerning biooil yields, they showed a maximum value of 36 wt% at 540 °C pyrolysis temperature (Fig. 3).
Fig.2. Char yields as a function of pyrolysis temperature
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Ktori et al. / Materials Today: Proceedings 5 (2018) 27582–27588
Fig.3. Biooil yields as a function of pyrolysis temperature
Contrary to char yields, gas yields followed an increasing trend with pyrolysis temperature (Fig. 4), reaching the value of 28 wt% at 700 °C pyrolysis temperature. Pyrolysis gas was mainly composed of H2, CO, CO2, CH4, C2H6 and C2H4 and its LHV was calculated by the following equation (1) [33]: LHV [30 v / v%CO 25.7 v / v% H 2 85.4 v / v%CH 4 151.3 v / v%(C2 H 4 C2 H 6 )] 4.2 / 1000
(1)
The LHV of biogas produced at 540 °C is 13.3 MJ/m3 (Fig.5) and belongs to the medium level gas fuels, thus it can be directly used in engines, turbines and boilers for power production [34].
Fig.4. Biogas yields as a function of pyrolysis temperature
Ktori et al. / Materials Today: Proceedings 5 (2018) 27582–27588
27587
Fig.5. LHV of produced gas as a function of pyrolysis temperature
It must be noted that these values are indicative and presented in order to show trends and potential towards testing at pilot scale, and then application at industrial scale. In order to assess char applicability in soils as fertiliser, closing thus the loop in agriculture, its further physicochemical characterization is proposed. Pyrolysis oil could possibly lead to biodiesel production, thus its role as a potential biofuel candidate should be investigated. 4. Conclusions The ever-increasing coffee consumption generates remarkable quantities of waste which constitutes a challenge of our modern times. Spent coffee grounds pyrolysis could be the solution to this problem, by achieving not only to handle but also to valorize the secondary solid waste streams generated by coffee consumption. More specifically, spent coffee grounds can be converted into biooil (36 wt%) when pyrolyzed at 540 °C, whereas the corresponding values for biochar and biogas product yields can reach 29 wt% and 9 wt%, respectively. The LHV of biogas produced at 540 °C is 13.3 MJ/m3 and belongs to the medium level gas fuels; thus it can be used in engines, turbines and boilers for power production. Biooil and char characteristics need further characterization in order to be assessed as biofuel and biochar respectively.
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