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Effect of carbon dioxide on thermal treatment of food waste as a sustainable disposal method
Journal of CO₂ Utilization 36 (2020) 76–81
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Effect of carbon dioxide on thermal treatment of food waste as a sustainable disposal method
T
Younghyun Leea, Soosan Kima, Eilhann E. Kwonb,*, Jechan Leea,* a b
Department of Environmental Engineering, Ajou University, Suwon 16499, Republic of Korea Department of Environment and Energy, Sejong University, Seoul 05006, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Waste management Waste disposal Thermal treatment CO2 utilization
Herein, carbon dioxide (CO2) was applied to thermal treatment of real food waste to develop an environmentally benign way to dispose food waste. The food waste used in this study was collected from a food waste treatment plant. The application of CO2 to the thermal treatment of food waste affected the amount of non-condensable gases and condensable compounds produced from the thermal process, while it did not affect the amount of solid residue. When CO2 was supplied during the thermal treatment of food waste, less condensable compounds but more non-condensable gases such as H2, CO, and CH4 were generated during the thermal treatment of food waste at a range of temperatures from 400 to 700 °C. In addition to the change in product distribution, the generation of cyclic compounds was inhibited by applying CO2 to the thermal treatment. For example, approximately 30% less ring-structured compounds (e.g., benzene derivatives) were produced from the thermal treatment at 700 °C in CO2 condition than from the thermal treatment in inert condition. This was likely because CO2 inhibits gas phase free radical addition and/or dehydrogenation of linear compounds. This study suggests that the application of CO2 to thermally treating food waste would help develop a more environmentally friendly food waste treatment method.
1. Introduction Significant amounts of food waste are generated worldwide originating from various sectors such as residual waste of agricultural production, household, and restaurants [1]. According to Food and Agriculture Organization, food waste of 1.3 billion tons is generated annually in the US [2]. It was estimated that approximately 2.9 Gt CO2 per year can be mitigated by reducing food waste [3]. Food waste has been disposed by different ways such as landfill, composting, and anaerobic fermentation [4]. Food waste landfill releases landfill gas mainly consisting of methane (CH4) and carbon dioxide (CO2) that are potent greenhouse gases [5]. It also emits leachate and dusts that are harmful to the environment [6]. Composting food waste is a simple and well-established method to treat food waste; however, it requires a long reaction time to complete composting process [7] and needs additional transportation costs after making compost [8]. While anaerobic fermentation has recently received much attention to produce biogas from food waste [9], it needs high initial costs [10] and generates toxic compounds containing sulfur [11]. Hence, it is necessary to develop an effective clean method to dispose food waste. Thermal treatment of food waste would be an option for the food
⁎
waste disposal considering that it can ultimately reduce the volume of food waste [12]. Incineration is the most widely used thermal treatment process for various wastes [13] because it reduces the solid waste volume by over 90% [14] and produces direct heating and electrical energy to operate power generation or steam turbines [15]. However, incineration of food waste emits different air pollutants (e.g., particulate matters, dioxins, sulfur dioxide, nitrogen oxides, hydrogen fluoride, and hydrogen chloride) into the atmosphere [16]. Therefore, it is important to design a new class of thermal treatment technology to dispose food waste in an environmentally benign way. Various thermochemical conversion methods including pyrolysis and gasification have recently been used to treat and process food waste. For example, orange peel was converted into an activated carbon used for catalyst support by microwave pyrolysis, yielding ∼70 wt.% activated carbon [17]. Co-pyrolysis of used frying oil and plastic waste was conducted to produce bio-oil (81 wt.% yield) [18]. To simultaneously reduce waste and recovery energy from used cooking oil and waste plastic, vacuum pyrolysis heated was carried out using a microwave for heating [19]. Clean liquid fuel was obtained with 84 wt.% yield. The production cost of the pyrolysis process was estimated at $0.25 L−1.
Corresponding authors. E-mail addresses: [email protected] (E.E. Kwon), [email protected] (J. Lee).
https://doi.org/10.1016/j.jcou.2019.11.004 Received 5 October 2019; Received in revised form 1 November 2019; Accepted 2 November 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
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(Fig. S1). Dichloromethane was used as solvent for collecting condensable species generated during the pyrolysis. In addition to the in situ condensation process, the sticky matters (i.e., condensable compounds that are heavier than volatile species) remained inside the tube and lines after the thermal treatment were further collected by washing with dichloromethane. The collected condensable compounds were dried at 60 °C for 24 h to evaporate dichloromethane.
Recently, it has been tried to apply CO2 to thermochemical conversion of biomass [20–22]. For example, CO2 was used to enhance the production of hydrogen (H2) from pine sawdust via pyrolysis [23]. The use of CO2 at a pyrolysis medium improved thermal efficiency of algal biomass pyrolysis by 45% compared to pyrolysis conducted under typical inert condition [24]. It was also reported that CO2 helps thermal cracking of volatile species evolved during thermochemical process of different samples such as manure [25], algal biomass [26], and coal [27]. Thus, it was hypothesized that the application of CO2 to thermal treatment of food waste would be helpful to dispose food waste in a clean way. In this context, we researched effects of the supply of CO2 on the products generated from thermal treatment of food waste. In this work, real food waste was obtained from a food waste treatment plant (a part of a domestic wastewater treatment facility) and used for experiments to make the process more realistic. To the best of the authors’ knowledge, many studies about food waste treatment use model feedstocks that simulate food waste. The food waste was first characterized prior to thermal treatment, and non-condensable gases and condensable species produced from the thermal treatment of food waste were collected at different temperatures. The thermal treatment products were then identified and quantified to investigate the effect of CO2 on the process. The role of CO2 in changing constituents of the thermal treatment products of food waste is also discussed.
2.4. Chemical analysis The non-condensable gases evolved from the thermal treatment of food waste were identified and quantified using an INFICON 3000A micro GC with a thermal conductivity detector (TCD) equipped with molecular sieve (10 m ×0.32 mm ×30 μm) and PLOTU (8 m × 0.32 mm × 30 μm) columns. A standard gas mixture (RIGAS, Daejeon, Republic of Korea) was used to calibrate the GC. The injector temperature was 100 °C. The oven temperature was set to 80 °C (analysis time for a sample: 3 min). The condensable compounds were analyzed by an Agilent 5975C gas chromatography–mass spectrometry (GC–MS) equipped with an Agilent DB-5MS column (30 m × 0.25 μm × 0.25 mm). Helium (≥99.999%) was used as a carrier gas with a flow rate of 1 mL min−1. The injector temperature was 250 °C. The oven temperature was set to: (1) 40 °C for 2 min; (2) 40–310 °C with a ramping rate of 8 °C min−1; (3) 310 °C for 5 min. The aux temperature was set to 300 °C. The scanning mass was ranged from 12 to 550 atomic mass unit. The GC–MS peaks were identified using the NIST mass spectral library. To quantify concentrations of chemical species corresponding to each GC–MS peak, each peak was integrated, and the actual concentrations of the chemical compounds were calculated using an internal standard. Considering that phenol was not detected in any product samples obtained from the thermal treatment of food waste, phenol was used as the internal standard (concentration: 10 ng μL−1).
2. Materials and methods 2.1. Materials and chemicals Food waste used as the feedstock in this work was obtained from a food waste treatment facility located in Seoul, Republic of Korea. The raw food waste just collected was dewatered using a screw press, followed by a separation of foreign materials using a magnetic separator. The dewatered food waste was dried at 90 °C for one day prior to any experiment. Phenol (purity: 99.5%) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Dichloromethane (purity: 99.9%) was purchased from Daejung Chemicals (Siheung, Republic of Korea). Nitrogen (N2) (≥99.999%) and CO2 (≥99.99%) gases were purchased from DK gas (Hwaseong, Republic of Korea).
3. Results and discussion The composition of the food waste used in this work was analyzed according to the method previously reported elsewhere [28]. The food waste characterization results are presented in Table 1. The food waste had high contents of glucan (23.1 wt.%) and protein (21.5 wt.%). It also contained fats and oils (36.5 wt.%), ash (16.6 wt.%), and polysaccharides such as galactan, mannan, and xylan (2.3 wt.%.). The TGA of food waste were conducted to characterize thermal behavior of the food waste in N2 and CO2 environments. Fig. 1 presents the change in weight of the food waste sample during the TGA. Even though the food waste was dried prior to the TGA, moisture (approximately 2 wt.%) still remained (i.e., weight loss at near 100 °C). The weight loss observed between 200 and 400 °C was likely due to thermal degradation of hemicellulose (200–300 °C) and cellulose (300–400 °C) [29] contained in the food waste. At temperatures lower than 820 °C, it was hard to differentiate weight loss of the food waste in N2 and CO2 conditions. However, a more weight loss of the food waste was observed under CO2 atmosphere at temperatures higher than 820 °C. This was likely ascribed to the Boudouard reaction (a reaction to form CO
2.2. Thermogravimetric analysis of food waste Thermogravimetric analysis (TGA) of the food waste under N2 and CO2 atmosphere was carried out from 25 to 900 °C (ramping rate: 10 °C min−1) using a Mettler Toledo TGA unit (Columbus, OH, USA). The total flow rate of purge (N2 or CO2; 40 mL min−1) and protective (N2; 20 mL min−1) gases was set to 60 mL min−1. A 10 mg of the food waste was loaded for the TGA. 2.3. Thermal treatment of food waste The thermal treatment of food waste was conducted in a system described in Supporting information (Fig. S1). Quartz tube was used as a reactor in which the thermal treatment was carried out. For an experiment, food waste (1 g) was loaded in an alumina boat. The food waste-loaded alumina boat was located at the center of the quartz tube. Total gas flow rate was controlled to maintain 300 mL min−1 using mass flow controllers (KOFLOC, Japan). A tube furnace equipped with a temperature controller (Tube Furnace-60, Hantech, Republic of Korea) was used to monitor and control thermal treatment temperature (ramping rate: 10 °C min−1). Gaseous product outlet was in situ connected to a micro gas chromatograph (GC). To effectively collect all condensable compounds generated during the thermal treatment, volatile species evolved from the thermal treatment of food waste passed through three consecutive cold traps which were placed in a cooling bath (temperature: −5 °C)
Table 1 Constituents of the food waste used as the feedstock in this study. Mean values of replicates (n = 3) are reported with ± standard deviations. Components
wt.%, dry basis
Glucan Polysaccharides (galactan, mannan, xylan) Protein Fats and oils Asha
23.1 2.3 21.5 36.5 16.6
a
77
Ash contains Na (1 wt.%), Ca (4.7 wt.%), K (0.7 wt.%), and Mg (0.1 wt.%).
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significantly changed as the temperature changed (Fig. 2c). The trend in changing the thermal treatment product distribution with changing thermal treatment temperature was same both under N2 and CO2 atmospheres. This indicates that solid residue was further converted into gaseous products at higher temperatures for the thermal treatment of food waste conduced in N2 and CO2 conditions. The use of CO2 in the thermal treatment of food waste increased the content of non-condensable gases such as H2, CO, and CH4 but decreased the content of condensable compounds (Fig. 2a and c). As seen in Fig. 2a and c, the extent of difference in the contents of non-condensable gases and condensable compounds between N2-thermal treatment and CO2-thermal treatment was similar at all temperatures tested. For instance, the employment of CO2 in the thermal treatment of food waste decreased the content of condensable compounds by 6.6% and by 6.2% at 600 °C and at 700 °C, respectively. This means that the carbon distribution is shifted from condensable compounds to noncondensable gases by using CO2 in the thermal treatment of food waste. In other words, the presence of CO2 during thermal treatment might help crack heavy molecules (i.e., condensable compounds) into light molecules (i.e., non-condensable gases). It would be favorable that more non-condensable gases are produced along with disposing food waste through thermal treatment because most non-condensable gases are combustible. Given that the thermal treatment is endothermic [31], supplying energy to the thermal treatment reactor by burning combustible gases produced during the waste disposal process would allow to improve its overall thermal efficiency. In Table 2, concentrations of the condensable compounds identified in the product produced via the thermal treatment of food waste performed in N2 and in CO2 were summarized. Molecular structure of each compound is given in Supporting information (Table S1). Approximately 44 wt.% N-containing compounds (e.g., pyrrolidines, nitriles, and amides) were contained in the condensable compounds at all temperatures tested for the thermal treatment. As found in Table 2, most N-containing compounds were pyrrolidine and its derivatives. Pyrrolidine is widely distributed in foodstuffs such as fatty fish, bread, coffee, milk, cheese, and celery stalks [32]. Sugar-derived chemicals such as allose and isomannide were identified, which represents expected characteristics of the food waste. Fatty acids (e.g., palmitic acid, linoleic acid, oleic acid, and stearic acid) and their derivatives should originate from fats and oils in the food waste (Table 1). Some chemical compounds contained in the condensable compounds generated from the thermal treatment of food waste are harmful to the environment and human beings. For instance, 1,4-benzendiol
Fig. 1. The change in weight as a function of temperature during the TGA of food waste under N2 and CO2 atmospheres.
from carbon and CO2 or reverse) now that the Boudouard reaction is favorable at temperatures higher than 800 °C [30]. The weight loss of the food waste in N2 and CO2 conditions was nearly identical at temperatures lower than 820 °C. As shown in Fig. 1, the apparent effect of CO2 on the thermal treatment of food waste was not significant. Therefore, the composition of the products generated from the food waste via thermal treatment and their contents were further analyzed. Fig. 2 shows the contents of non-condensable gases, solid residue, and condensable compounds in the products produced from the thermal treatment of food waste in N2 and in CO2 at a range of temperatures from 400 to 700 °C. As presented in Fig. 2a, the content of non-condensable gases in the thermal treatment product increased as the thermal treatment temperature increased. However, the content of solid residue in the thermal treatment product decreased with an increase in the thermal treatment temperature (Fig. 2b). For example, the noncondensable gas content was increased from 45.1 to 51.3 wt.%, but the solid residue content was decreased from 37.3 to 28.3 wt.% as the temperature was increased from 400 to 700 °C for the thermal treatment of food waste in N2 condition. Unlike the case of non-condensable gases and solid residue, the content of condensable compounds was not
Fig. 2. Content of (a) non-condensable gases, (b) solid residue, and (c) condensable compounds in the product produced from the thermal treatment of food waste in N2 and in CO2 as a function of temperature. Mean values of replicates (n = 3) are reported with standard deviations given as error bars. 78
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Table 2 Condensable species identified in the products generated via the thermal treatment of food waste in N2 and in CO2 as a function of thermal treatment temperature. Classification
Chemical name
Chemical formula
MW
Concentration (ppmw) 400 °C
Cyclic compound
Non-cyclic compound
Pyrrolidine Imidazolidine-2,4-dione Pyrrole-2-carboxamide 1,4-Benzendiol 5-Isopropyl-2,4-imidazoldione Isomannide 4-Hydroxy-benzenepropanenitrile Pyrrolo[1,2-a]pyrazine-1,4-dione Benzenepropanamide Pyrrolo[1,2-a]piperazine-3,6-dione (3R,8aS)-3-Methylhexahydropyrrolo[1,2-a]pyrazine-1,4-dione Allose Octahydrodipyrrolo[1,2-a:1',2'-d]pyrazine-5,10-dione 3-Benzylhexahydropyrrolo[1,2-b][1,4,2,5]dioxadiazine 2-Dodecen-1-ylsuccinic anhydride Ergotamine Palmitic acid Palmitonitrile Palmitamide Heptadecanenitrile Linoleic acid Oleic acid Oleonitrile Oleamide Stearic acid Stearamide
C4H9N C3H4N2O2 C5H6N2O C6H6O2 C6H8N2O2 C6H10O4 C9H9NO C7H4N2O2 C9H11NO C7H6N2O2 C8H12N2O2 C6H12O6 C10H14N2O2 C12H16N2O2 C16H26O3 C33H35N5O5 C16H32O2 C16H31N C16H33NO C17H33N C18H32O2 C18H34O2 C18H33N C18H35NO C18H36O2 C18H37NO
71.1 100.1 110.1 110.1 140.1 146.1 147.2 148.1 149.2 150.1 168.2 180.2 194.2 220.3 266.4 581.7 256.4 237.4 255.4 251.5 280.5 282.5 263.5 281.5 284.5 283.5
500 °C
600 °C
700 °C
N2
CO2
N2
CO2
N2
CO2
N2
CO2
130 46 200 180 98 110 96 800 100 180 57 110 700 46 150 70 2400 500 1000 450 420 2100 580 170 1100 77
160 51 150 210 120 100 38 1100 110 240 320 560 840 44 190 99 4600 660 2100 800 700 4000 900 390 2300 160
140 150 190 120 110 150 180 1000 110 240 400 270 870 57 170 87 3600 640 1500 630 650 3400 740 260 1600 100
220 130 290 370 140 290 79 950 130 270 250 740 840 260 270 70 4600 1000 1900 860 700 4000 1000 390 2100 260
190 170 310 220 150 310 130 990 140 230 470 540 760 99 140 58 2800 830 1500 580 500 2500 710 230 1300 180
110 180 290 260 150 310 190 1100 130 260 320 290 850 170 77 87 5100 880 2000 910 760 4200 1100 420 2300 180
140 210 300 260 150 220 180 1000 130 230 370 520 800 57 250 76 3100 690 1700 580 590 2800 690 240 1500 170
240 180 320 350 110 290 220 1300 160 300 350 610 890 200 220 93 5900 1100 1500 930 900 5100 1100 480 2400 200
Boudouard reaction to form CO by reacting CO2 and carbon contained in the food waste. To check this, the CO distribution was monitored at a wider range of temperatures (Fig. 3b). As shown in Fig. 3b, the CO production began to be enhanced at temperatures higher than 500 °C. Considering that the Boudouard reaction is thermodynamically favorable at > 720 °C and kinetically favorable at > 850 °C [30], the enhancement of CO production shown in Fig. 3 was not due to the Boudouard reaction. The results shown in Table 2 and Fig. 3a indicate that CO2 allowed to crack heavy carbon-containing compounds to CO. These also support the claim about shifting the carbon distribution from condensable compounds to non-condensable gases (Fig. 2). The results discussed above indicate that the application of CO2 to the thermal treatment of food waste leads to enhancing a generation of non-condensable gaseous products. In addition to the enhancement of evolving non-condensable gases, this might suggest another key role of
may constitute a danger to human health due to its carcinogenicity [33]. Other benzene derivatives such as 4-hydroxy-benzenepropanenitrile and benzenepropanamide and 2-dodecen-1-ylsuccinic anhydride are irritant and toxic chemicals that may damage human health [34–36]. Ergotamine, a natural ergot alkaloid, is classified as an acute toxic chemical that causes a health hazard [37]. Nitriles can be harmful to the environment. For example, oleonitrile is very toxic to aquatic life with long lasting effects [38]. Fig. 3a shows the effect of the amount of CO2 existing during the thermal treatment of food waste on product distribution of non-condensable gases generated from the thermal treatment of food waste at 700 °C at which most non-condensable gases were produced among all tested temperatures. As more CO2 was supplied during the thermal treatment, more CO was produced while less H2 and CH4 were evolved. At first, it was suspected that the enhanced CO production is due to the
Fig. 3. (a) Product distribution of noncondensable gases generated from the thermal treatment of food waste at 700 °C as a function of volumetric percent of CO2 (N2 balance). Mean values of replicates (n = 3) are reported standard deviations of the mean values of around 5%; (b) Product distribution of CO generated from the thermal treatment of food waste as a function of temperature. Mean values of replicates (n = 3) are reported with standard deviations given as error bars.
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Fig. 4. Composition of cyclic and non-cyclic compounds identified in the condensable compounds generated via the thermal treatment of food waste conducted under N2 and CO2 atmospheres at (a) 400 °C; (b) 500 °C; (c) 600 °C; and (d) 700 °C. Mean values of replicates (n = 3) are reported standard deviations of the mean values of around 5%.
thermal treatment would be higher than what to be shown in Fig. 4.
CO2 in the thermal treatment of food waste: CO2 likely provides a favorable condition to form chemical compounds having low molecular weight. This could be attributed to an enhanced thermal cracking of hydrocarbons evolved from food waste that is thermally decomposed under CO2 atmosphere. In Fig. 4, it is compared the contents of cyclic compounds and noncyclic compounds that were found in the products produced from the thermal treatment of food waste as a function of the thermal treatment temperature. Cyclic compounds (e.g., benzene derivatives) can be produced via dehydrogenation of linear compounds [39] and/or gas phase free radical addition [40]. During pyrolysis, radicals are formed via a mechanism involving initiation, secondary radical formation via depolymerization, monomer formation, hydrogen transfer, isomerization, and disproportionation of radicals [41,42]. As presented in Fig. 4, ring-structured compounds were reduced at all temperatures tested for the thermal treatment, while non-cyclic compounds were generated more under CO2 atmosphere than under N2 atmosphere. Therefore, the decrease in the formation of cyclic compounds and the increase in the formation of non-cyclic compounds were most likely because CO2 inhibits free radical reactions, which resulted in impeding cyclization reactions. Furthermore, the compositions in Fig. 4 are relative values obtained at different masses of condensable compounds generated from the thermal treatment of food waste in N2 and CO2 (Fig. 2c); hence, the extent of the decrease in the cyclic compounds by applying CO2 to the
4. Conclusions In this work, real food waste obtained from a food waste treatment plant was thermally treated under N2 and CO2 atmospheres. The results indicate that the employment of CO2 decreased the contents of cyclic compounds such as benzene derivatives in the condensable compounds produced during the thermal treatment of food waste. The use of CO2 as a reaction medium did not affect the amount of solid residue after the thermal treatment of food waste. However, it affected the amount of non-condensable gases and condensable compounds. The decrease in the formation of condensable compounds led to the increase in the formation of non-condensable gases such as H2, CO, and CH4. This shows that carbon distribution of products produced from the thermal treatment of food waste was shifted from condensable compounds to non-condensable gases. In addition, the composition of cyclic compounds (e.g., benzene derivatives) was decreased when using CO2 in the thermal treatment of food waste at a range of temperatures from 400 to 700 °C. This means that CO2 inhibited the formation of cyclic compounds (which are potentially harmful to the environment and human health) during the thermal treatment of food waste. This work experimentally proves that the application of CO2 to thermal treatment of food waste (and potentially municipal solid wastes) would contribute to 80
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the development of an environmentally friendly waste disposal process.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by a National Research Foundation of Korea (NRF) Grant funded by the Korean Government (Ministry of Education) (No. 2018R1D1A1A09082841). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2019.11.004. References [1] C.S.K. Lin, L.A. Pfaltzgraff, L. Herrero-Davila, E.B. Mubofu, S. Abderrahim, J.H. Clark, A.A. Koutinas, N. Kopsahelis, K. Stamatelatou, F. Dickson, S. Thankappan, Z. Mohamed, R. Brocklesby, R. Luque, Food waste as a valuable resource for the production of chemicals, materials and fuels. Current situation and global perspective, Energy Environ. Sci. 6 (2) (2013) 426–464. [2] J. Gustavsson, C. Cederberg, U. Sonesson, R. Otterdijk, A. Meybeck, Global Food Losses and Food Waste — Extent, Causes and Prevention, Food and Agricultural Organization of the United Nations, Rome, Italy, 2011. [3] S. Roe, C. Streck, P.H. Weiner, M. Obersteiner, S. Frank, How Improved Land Use Can Contribute to the 1.5 °C Goal of the Paris Agreement, Working Paper prepared by Climate Focus and the International Institute for Applied Systems Analysis, 2017. [4] Y.F. Tsang, V. Kumar, P. Samadar, Y. Yang, J. Lee, Y.S. Ok, H. Song, K.-H. Kim, E.E. Kwon, Y.J. Jeon, Production of bioplastic through food waste valorization, Environ. Int. 127 (2019) 625–644. [5] A. Johari, S.I. Ahmed, H. Hashim, H. Alkali, M. Ramli, Economic and environmental benefits of landfill gas from municipal solid waste in Malaysia, Renew. Sustain. Energy Rev. 16 (5) (2012) 2907–2912. [6] M. Palmiotto, E. Fattore, V. Paiano, G. Celeste, A. Colombo, E. Davoli, Influence of a municipal solid waste landfill in the surrounding environment: toxicological risk and odor nuisance effects, Environ. Int. 68 (2014) 16–24. [7] C.R. Sudharmaidevi, K.C.M. Thampatti, N. Saifudeen, Rapid production of organic fertilizer from degradable waste by thermochemical processing, Int. J. Recycl. Org. Waste Agric. 6 (1) (2017) 1–11. [8] S. Elkhalifa, T. Al-Ansari, H.R. Mackey, G. McKay, Food waste to biochars through pyrolysis: A review, Resources, Conserv. Recycl. 144 (2019) 310–320. [9] J.-I. Oh, J. Lee, K.-Y.A. Lin, E.E. Kwon, Y. Fai Tsang, Biogas production from food waste via anaerobic digestion with wood chips, Energy Environ. 29 (8) (2018) 1365–1372. [10] L. De Baere, Will anaerobic digestion of solid waste survive in the future? Water Sci. Technol. 53 (8) (2006) 187–194. [11] Y. Chen, J.J. Cheng, K.S. Creamer, Inhibition of anaerobic digestion process: a review, Bioresour. Technol. 99 (10) (2008) 4044–4064. [12] T.P.T. Pham, R. Kaushik, G.K. Parshetti, R. Mahmood, R. Balasubramanian, Food waste-to-energy conversion technologies: current status and future directions, Waste Manag. 38 (2015) 399–408. [13] E. Autret, F. Berthier, A. Luszezanec, F. Nicolas, Incineration of municipal and assimilated wastes in France: assessment of latest energy and material recovery performances, J. Hazard. Mater. 139 (3) (2007) 569–574. [14] K.W. Man, Implementation of incineration for efficient waste reduction, 2015 International Conference on Advances in Environment Research 87 (2015) 77–80. [15] J.-F. Perrot, A. Subiantoro, Municipal waste management strategy review and waste-to-energy potentials in New Zealand, Sustainability 10 (9) (2018) 3114. [16] A. Bernstad, J. la Cour Jansen, A life cycle approach to the management of household food waste–a Swedish full-scale case study, Waste Manag. 31 (8) (2011) 1879–1896. [17] S.S. Lam, R.K. Liew, Y.M. Wong, E. Azwar, A. Jusoh, R. Wahi, Activated carbon for catalyst support from microwave pyrolysis of orange peel, Waste Biomass Valorization 8 (6) (2017) 2109–2119.
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Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Effect of carbon dioxide on thermal treatment of food waste as a sustainable disposal method
T
Younghyun Leea, Soosan Kima, Eilhann E. Kwonb,*, Jechan Leea,* a b
Department of Environmental Engineering, Ajou University, Suwon 16499, Republic of Korea Department of Environment and Energy, Sejong University, Seoul 05006, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Waste management Waste disposal Thermal treatment CO2 utilization
Herein, carbon dioxide (CO2) was applied to thermal treatment of real food waste to develop an environmentally benign way to dispose food waste. The food waste used in this study was collected from a food waste treatment plant. The application of CO2 to the thermal treatment of food waste affected the amount of non-condensable gases and condensable compounds produced from the thermal process, while it did not affect the amount of solid residue. When CO2 was supplied during the thermal treatment of food waste, less condensable compounds but more non-condensable gases such as H2, CO, and CH4 were generated during the thermal treatment of food waste at a range of temperatures from 400 to 700 °C. In addition to the change in product distribution, the generation of cyclic compounds was inhibited by applying CO2 to the thermal treatment. For example, approximately 30% less ring-structured compounds (e.g., benzene derivatives) were produced from the thermal treatment at 700 °C in CO2 condition than from the thermal treatment in inert condition. This was likely because CO2 inhibits gas phase free radical addition and/or dehydrogenation of linear compounds. This study suggests that the application of CO2 to thermally treating food waste would help develop a more environmentally friendly food waste treatment method.
1. Introduction Significant amounts of food waste are generated worldwide originating from various sectors such as residual waste of agricultural production, household, and restaurants [1]. According to Food and Agriculture Organization, food waste of 1.3 billion tons is generated annually in the US [2]. It was estimated that approximately 2.9 Gt CO2 per year can be mitigated by reducing food waste [3]. Food waste has been disposed by different ways such as landfill, composting, and anaerobic fermentation [4]. Food waste landfill releases landfill gas mainly consisting of methane (CH4) and carbon dioxide (CO2) that are potent greenhouse gases [5]. It also emits leachate and dusts that are harmful to the environment [6]. Composting food waste is a simple and well-established method to treat food waste; however, it requires a long reaction time to complete composting process [7] and needs additional transportation costs after making compost [8]. While anaerobic fermentation has recently received much attention to produce biogas from food waste [9], it needs high initial costs [10] and generates toxic compounds containing sulfur [11]. Hence, it is necessary to develop an effective clean method to dispose food waste. Thermal treatment of food waste would be an option for the food
⁎
waste disposal considering that it can ultimately reduce the volume of food waste [12]. Incineration is the most widely used thermal treatment process for various wastes [13] because it reduces the solid waste volume by over 90% [14] and produces direct heating and electrical energy to operate power generation or steam turbines [15]. However, incineration of food waste emits different air pollutants (e.g., particulate matters, dioxins, sulfur dioxide, nitrogen oxides, hydrogen fluoride, and hydrogen chloride) into the atmosphere [16]. Therefore, it is important to design a new class of thermal treatment technology to dispose food waste in an environmentally benign way. Various thermochemical conversion methods including pyrolysis and gasification have recently been used to treat and process food waste. For example, orange peel was converted into an activated carbon used for catalyst support by microwave pyrolysis, yielding ∼70 wt.% activated carbon [17]. Co-pyrolysis of used frying oil and plastic waste was conducted to produce bio-oil (81 wt.% yield) [18]. To simultaneously reduce waste and recovery energy from used cooking oil and waste plastic, vacuum pyrolysis heated was carried out using a microwave for heating [19]. Clean liquid fuel was obtained with 84 wt.% yield. The production cost of the pyrolysis process was estimated at $0.25 L−1.
Corresponding authors. E-mail addresses: [email protected] (E.E. Kwon), [email protected] (J. Lee).
https://doi.org/10.1016/j.jcou.2019.11.004 Received 5 October 2019; Received in revised form 1 November 2019; Accepted 2 November 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
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(Fig. S1). Dichloromethane was used as solvent for collecting condensable species generated during the pyrolysis. In addition to the in situ condensation process, the sticky matters (i.e., condensable compounds that are heavier than volatile species) remained inside the tube and lines after the thermal treatment were further collected by washing with dichloromethane. The collected condensable compounds were dried at 60 °C for 24 h to evaporate dichloromethane.
Recently, it has been tried to apply CO2 to thermochemical conversion of biomass [20–22]. For example, CO2 was used to enhance the production of hydrogen (H2) from pine sawdust via pyrolysis [23]. The use of CO2 at a pyrolysis medium improved thermal efficiency of algal biomass pyrolysis by 45% compared to pyrolysis conducted under typical inert condition [24]. It was also reported that CO2 helps thermal cracking of volatile species evolved during thermochemical process of different samples such as manure [25], algal biomass [26], and coal [27]. Thus, it was hypothesized that the application of CO2 to thermal treatment of food waste would be helpful to dispose food waste in a clean way. In this context, we researched effects of the supply of CO2 on the products generated from thermal treatment of food waste. In this work, real food waste was obtained from a food waste treatment plant (a part of a domestic wastewater treatment facility) and used for experiments to make the process more realistic. To the best of the authors’ knowledge, many studies about food waste treatment use model feedstocks that simulate food waste. The food waste was first characterized prior to thermal treatment, and non-condensable gases and condensable species produced from the thermal treatment of food waste were collected at different temperatures. The thermal treatment products were then identified and quantified to investigate the effect of CO2 on the process. The role of CO2 in changing constituents of the thermal treatment products of food waste is also discussed.
2.4. Chemical analysis The non-condensable gases evolved from the thermal treatment of food waste were identified and quantified using an INFICON 3000A micro GC with a thermal conductivity detector (TCD) equipped with molecular sieve (10 m ×0.32 mm ×30 μm) and PLOTU (8 m × 0.32 mm × 30 μm) columns. A standard gas mixture (RIGAS, Daejeon, Republic of Korea) was used to calibrate the GC. The injector temperature was 100 °C. The oven temperature was set to 80 °C (analysis time for a sample: 3 min). The condensable compounds were analyzed by an Agilent 5975C gas chromatography–mass spectrometry (GC–MS) equipped with an Agilent DB-5MS column (30 m × 0.25 μm × 0.25 mm). Helium (≥99.999%) was used as a carrier gas with a flow rate of 1 mL min−1. The injector temperature was 250 °C. The oven temperature was set to: (1) 40 °C for 2 min; (2) 40–310 °C with a ramping rate of 8 °C min−1; (3) 310 °C for 5 min. The aux temperature was set to 300 °C. The scanning mass was ranged from 12 to 550 atomic mass unit. The GC–MS peaks were identified using the NIST mass spectral library. To quantify concentrations of chemical species corresponding to each GC–MS peak, each peak was integrated, and the actual concentrations of the chemical compounds were calculated using an internal standard. Considering that phenol was not detected in any product samples obtained from the thermal treatment of food waste, phenol was used as the internal standard (concentration: 10 ng μL−1).
2. Materials and methods 2.1. Materials and chemicals Food waste used as the feedstock in this work was obtained from a food waste treatment facility located in Seoul, Republic of Korea. The raw food waste just collected was dewatered using a screw press, followed by a separation of foreign materials using a magnetic separator. The dewatered food waste was dried at 90 °C for one day prior to any experiment. Phenol (purity: 99.5%) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Dichloromethane (purity: 99.9%) was purchased from Daejung Chemicals (Siheung, Republic of Korea). Nitrogen (N2) (≥99.999%) and CO2 (≥99.99%) gases were purchased from DK gas (Hwaseong, Republic of Korea).
3. Results and discussion The composition of the food waste used in this work was analyzed according to the method previously reported elsewhere [28]. The food waste characterization results are presented in Table 1. The food waste had high contents of glucan (23.1 wt.%) and protein (21.5 wt.%). It also contained fats and oils (36.5 wt.%), ash (16.6 wt.%), and polysaccharides such as galactan, mannan, and xylan (2.3 wt.%.). The TGA of food waste were conducted to characterize thermal behavior of the food waste in N2 and CO2 environments. Fig. 1 presents the change in weight of the food waste sample during the TGA. Even though the food waste was dried prior to the TGA, moisture (approximately 2 wt.%) still remained (i.e., weight loss at near 100 °C). The weight loss observed between 200 and 400 °C was likely due to thermal degradation of hemicellulose (200–300 °C) and cellulose (300–400 °C) [29] contained in the food waste. At temperatures lower than 820 °C, it was hard to differentiate weight loss of the food waste in N2 and CO2 conditions. However, a more weight loss of the food waste was observed under CO2 atmosphere at temperatures higher than 820 °C. This was likely ascribed to the Boudouard reaction (a reaction to form CO
2.2. Thermogravimetric analysis of food waste Thermogravimetric analysis (TGA) of the food waste under N2 and CO2 atmosphere was carried out from 25 to 900 °C (ramping rate: 10 °C min−1) using a Mettler Toledo TGA unit (Columbus, OH, USA). The total flow rate of purge (N2 or CO2; 40 mL min−1) and protective (N2; 20 mL min−1) gases was set to 60 mL min−1. A 10 mg of the food waste was loaded for the TGA. 2.3. Thermal treatment of food waste The thermal treatment of food waste was conducted in a system described in Supporting information (Fig. S1). Quartz tube was used as a reactor in which the thermal treatment was carried out. For an experiment, food waste (1 g) was loaded in an alumina boat. The food waste-loaded alumina boat was located at the center of the quartz tube. Total gas flow rate was controlled to maintain 300 mL min−1 using mass flow controllers (KOFLOC, Japan). A tube furnace equipped with a temperature controller (Tube Furnace-60, Hantech, Republic of Korea) was used to monitor and control thermal treatment temperature (ramping rate: 10 °C min−1). Gaseous product outlet was in situ connected to a micro gas chromatograph (GC). To effectively collect all condensable compounds generated during the thermal treatment, volatile species evolved from the thermal treatment of food waste passed through three consecutive cold traps which were placed in a cooling bath (temperature: −5 °C)
Table 1 Constituents of the food waste used as the feedstock in this study. Mean values of replicates (n = 3) are reported with ± standard deviations. Components
wt.%, dry basis
Glucan Polysaccharides (galactan, mannan, xylan) Protein Fats and oils Asha
23.1 2.3 21.5 36.5 16.6
a
77
Ash contains Na (1 wt.%), Ca (4.7 wt.%), K (0.7 wt.%), and Mg (0.1 wt.%).
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significantly changed as the temperature changed (Fig. 2c). The trend in changing the thermal treatment product distribution with changing thermal treatment temperature was same both under N2 and CO2 atmospheres. This indicates that solid residue was further converted into gaseous products at higher temperatures for the thermal treatment of food waste conduced in N2 and CO2 conditions. The use of CO2 in the thermal treatment of food waste increased the content of non-condensable gases such as H2, CO, and CH4 but decreased the content of condensable compounds (Fig. 2a and c). As seen in Fig. 2a and c, the extent of difference in the contents of non-condensable gases and condensable compounds between N2-thermal treatment and CO2-thermal treatment was similar at all temperatures tested. For instance, the employment of CO2 in the thermal treatment of food waste decreased the content of condensable compounds by 6.6% and by 6.2% at 600 °C and at 700 °C, respectively. This means that the carbon distribution is shifted from condensable compounds to noncondensable gases by using CO2 in the thermal treatment of food waste. In other words, the presence of CO2 during thermal treatment might help crack heavy molecules (i.e., condensable compounds) into light molecules (i.e., non-condensable gases). It would be favorable that more non-condensable gases are produced along with disposing food waste through thermal treatment because most non-condensable gases are combustible. Given that the thermal treatment is endothermic [31], supplying energy to the thermal treatment reactor by burning combustible gases produced during the waste disposal process would allow to improve its overall thermal efficiency. In Table 2, concentrations of the condensable compounds identified in the product produced via the thermal treatment of food waste performed in N2 and in CO2 were summarized. Molecular structure of each compound is given in Supporting information (Table S1). Approximately 44 wt.% N-containing compounds (e.g., pyrrolidines, nitriles, and amides) were contained in the condensable compounds at all temperatures tested for the thermal treatment. As found in Table 2, most N-containing compounds were pyrrolidine and its derivatives. Pyrrolidine is widely distributed in foodstuffs such as fatty fish, bread, coffee, milk, cheese, and celery stalks [32]. Sugar-derived chemicals such as allose and isomannide were identified, which represents expected characteristics of the food waste. Fatty acids (e.g., palmitic acid, linoleic acid, oleic acid, and stearic acid) and their derivatives should originate from fats and oils in the food waste (Table 1). Some chemical compounds contained in the condensable compounds generated from the thermal treatment of food waste are harmful to the environment and human beings. For instance, 1,4-benzendiol
Fig. 1. The change in weight as a function of temperature during the TGA of food waste under N2 and CO2 atmospheres.
from carbon and CO2 or reverse) now that the Boudouard reaction is favorable at temperatures higher than 800 °C [30]. The weight loss of the food waste in N2 and CO2 conditions was nearly identical at temperatures lower than 820 °C. As shown in Fig. 1, the apparent effect of CO2 on the thermal treatment of food waste was not significant. Therefore, the composition of the products generated from the food waste via thermal treatment and their contents were further analyzed. Fig. 2 shows the contents of non-condensable gases, solid residue, and condensable compounds in the products produced from the thermal treatment of food waste in N2 and in CO2 at a range of temperatures from 400 to 700 °C. As presented in Fig. 2a, the content of non-condensable gases in the thermal treatment product increased as the thermal treatment temperature increased. However, the content of solid residue in the thermal treatment product decreased with an increase in the thermal treatment temperature (Fig. 2b). For example, the noncondensable gas content was increased from 45.1 to 51.3 wt.%, but the solid residue content was decreased from 37.3 to 28.3 wt.% as the temperature was increased from 400 to 700 °C for the thermal treatment of food waste in N2 condition. Unlike the case of non-condensable gases and solid residue, the content of condensable compounds was not
Fig. 2. Content of (a) non-condensable gases, (b) solid residue, and (c) condensable compounds in the product produced from the thermal treatment of food waste in N2 and in CO2 as a function of temperature. Mean values of replicates (n = 3) are reported with standard deviations given as error bars. 78
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Table 2 Condensable species identified in the products generated via the thermal treatment of food waste in N2 and in CO2 as a function of thermal treatment temperature. Classification
Chemical name
Chemical formula
MW
Concentration (ppmw) 400 °C
Cyclic compound
Non-cyclic compound
Pyrrolidine Imidazolidine-2,4-dione Pyrrole-2-carboxamide 1,4-Benzendiol 5-Isopropyl-2,4-imidazoldione Isomannide 4-Hydroxy-benzenepropanenitrile Pyrrolo[1,2-a]pyrazine-1,4-dione Benzenepropanamide Pyrrolo[1,2-a]piperazine-3,6-dione (3R,8aS)-3-Methylhexahydropyrrolo[1,2-a]pyrazine-1,4-dione Allose Octahydrodipyrrolo[1,2-a:1',2'-d]pyrazine-5,10-dione 3-Benzylhexahydropyrrolo[1,2-b][1,4,2,5]dioxadiazine 2-Dodecen-1-ylsuccinic anhydride Ergotamine Palmitic acid Palmitonitrile Palmitamide Heptadecanenitrile Linoleic acid Oleic acid Oleonitrile Oleamide Stearic acid Stearamide
C4H9N C3H4N2O2 C5H6N2O C6H6O2 C6H8N2O2 C6H10O4 C9H9NO C7H4N2O2 C9H11NO C7H6N2O2 C8H12N2O2 C6H12O6 C10H14N2O2 C12H16N2O2 C16H26O3 C33H35N5O5 C16H32O2 C16H31N C16H33NO C17H33N C18H32O2 C18H34O2 C18H33N C18H35NO C18H36O2 C18H37NO
71.1 100.1 110.1 110.1 140.1 146.1 147.2 148.1 149.2 150.1 168.2 180.2 194.2 220.3 266.4 581.7 256.4 237.4 255.4 251.5 280.5 282.5 263.5 281.5 284.5 283.5
500 °C
600 °C
700 °C
N2
CO2
N2
CO2
N2
CO2
N2
CO2
130 46 200 180 98 110 96 800 100 180 57 110 700 46 150 70 2400 500 1000 450 420 2100 580 170 1100 77
160 51 150 210 120 100 38 1100 110 240 320 560 840 44 190 99 4600 660 2100 800 700 4000 900 390 2300 160
140 150 190 120 110 150 180 1000 110 240 400 270 870 57 170 87 3600 640 1500 630 650 3400 740 260 1600 100
220 130 290 370 140 290 79 950 130 270 250 740 840 260 270 70 4600 1000 1900 860 700 4000 1000 390 2100 260
190 170 310 220 150 310 130 990 140 230 470 540 760 99 140 58 2800 830 1500 580 500 2500 710 230 1300 180
110 180 290 260 150 310 190 1100 130 260 320 290 850 170 77 87 5100 880 2000 910 760 4200 1100 420 2300 180
140 210 300 260 150 220 180 1000 130 230 370 520 800 57 250 76 3100 690 1700 580 590 2800 690 240 1500 170
240 180 320 350 110 290 220 1300 160 300 350 610 890 200 220 93 5900 1100 1500 930 900 5100 1100 480 2400 200
Boudouard reaction to form CO by reacting CO2 and carbon contained in the food waste. To check this, the CO distribution was monitored at a wider range of temperatures (Fig. 3b). As shown in Fig. 3b, the CO production began to be enhanced at temperatures higher than 500 °C. Considering that the Boudouard reaction is thermodynamically favorable at > 720 °C and kinetically favorable at > 850 °C [30], the enhancement of CO production shown in Fig. 3 was not due to the Boudouard reaction. The results shown in Table 2 and Fig. 3a indicate that CO2 allowed to crack heavy carbon-containing compounds to CO. These also support the claim about shifting the carbon distribution from condensable compounds to non-condensable gases (Fig. 2). The results discussed above indicate that the application of CO2 to the thermal treatment of food waste leads to enhancing a generation of non-condensable gaseous products. In addition to the enhancement of evolving non-condensable gases, this might suggest another key role of
may constitute a danger to human health due to its carcinogenicity [33]. Other benzene derivatives such as 4-hydroxy-benzenepropanenitrile and benzenepropanamide and 2-dodecen-1-ylsuccinic anhydride are irritant and toxic chemicals that may damage human health [34–36]. Ergotamine, a natural ergot alkaloid, is classified as an acute toxic chemical that causes a health hazard [37]. Nitriles can be harmful to the environment. For example, oleonitrile is very toxic to aquatic life with long lasting effects [38]. Fig. 3a shows the effect of the amount of CO2 existing during the thermal treatment of food waste on product distribution of non-condensable gases generated from the thermal treatment of food waste at 700 °C at which most non-condensable gases were produced among all tested temperatures. As more CO2 was supplied during the thermal treatment, more CO was produced while less H2 and CH4 were evolved. At first, it was suspected that the enhanced CO production is due to the
Fig. 3. (a) Product distribution of noncondensable gases generated from the thermal treatment of food waste at 700 °C as a function of volumetric percent of CO2 (N2 balance). Mean values of replicates (n = 3) are reported standard deviations of the mean values of around 5%; (b) Product distribution of CO generated from the thermal treatment of food waste as a function of temperature. Mean values of replicates (n = 3) are reported with standard deviations given as error bars.
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Fig. 4. Composition of cyclic and non-cyclic compounds identified in the condensable compounds generated via the thermal treatment of food waste conducted under N2 and CO2 atmospheres at (a) 400 °C; (b) 500 °C; (c) 600 °C; and (d) 700 °C. Mean values of replicates (n = 3) are reported standard deviations of the mean values of around 5%.
thermal treatment would be higher than what to be shown in Fig. 4.
CO2 in the thermal treatment of food waste: CO2 likely provides a favorable condition to form chemical compounds having low molecular weight. This could be attributed to an enhanced thermal cracking of hydrocarbons evolved from food waste that is thermally decomposed under CO2 atmosphere. In Fig. 4, it is compared the contents of cyclic compounds and noncyclic compounds that were found in the products produced from the thermal treatment of food waste as a function of the thermal treatment temperature. Cyclic compounds (e.g., benzene derivatives) can be produced via dehydrogenation of linear compounds [39] and/or gas phase free radical addition [40]. During pyrolysis, radicals are formed via a mechanism involving initiation, secondary radical formation via depolymerization, monomer formation, hydrogen transfer, isomerization, and disproportionation of radicals [41,42]. As presented in Fig. 4, ring-structured compounds were reduced at all temperatures tested for the thermal treatment, while non-cyclic compounds were generated more under CO2 atmosphere than under N2 atmosphere. Therefore, the decrease in the formation of cyclic compounds and the increase in the formation of non-cyclic compounds were most likely because CO2 inhibits free radical reactions, which resulted in impeding cyclization reactions. Furthermore, the compositions in Fig. 4 are relative values obtained at different masses of condensable compounds generated from the thermal treatment of food waste in N2 and CO2 (Fig. 2c); hence, the extent of the decrease in the cyclic compounds by applying CO2 to the
4. Conclusions In this work, real food waste obtained from a food waste treatment plant was thermally treated under N2 and CO2 atmospheres. The results indicate that the employment of CO2 decreased the contents of cyclic compounds such as benzene derivatives in the condensable compounds produced during the thermal treatment of food waste. The use of CO2 as a reaction medium did not affect the amount of solid residue after the thermal treatment of food waste. However, it affected the amount of non-condensable gases and condensable compounds. The decrease in the formation of condensable compounds led to the increase in the formation of non-condensable gases such as H2, CO, and CH4. This shows that carbon distribution of products produced from the thermal treatment of food waste was shifted from condensable compounds to non-condensable gases. In addition, the composition of cyclic compounds (e.g., benzene derivatives) was decreased when using CO2 in the thermal treatment of food waste at a range of temperatures from 400 to 700 °C. This means that CO2 inhibited the formation of cyclic compounds (which are potentially harmful to the environment and human health) during the thermal treatment of food waste. This work experimentally proves that the application of CO2 to thermal treatment of food waste (and potentially municipal solid wastes) would contribute to 80
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the development of an environmentally friendly waste disposal process.
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