- Email: [email protected]
Pyrolysis of aquatic carbohydrates using CO2 as reactive gas medium: A case study of chitin
Energy 177 (2019) 136e143
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Pyrolysis of aquatic carbohydrates using CO2 as reactive gas medium: A case study of chitin Gihoon Kwon a, Daniel C.W. Tsang b, Jeong-Ik Oh c, Eilhann E. Kwon a, **, Hocheol Song a, * a
Department of Environment and Energy, Sejong University, Seoul, 05005, Republic of Korea Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong c Advanced Technology Department, Land and Housing Institute, Daejeon 34047, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2019 Received in revised form 19 March 2019 Accepted 7 April 2019 Available online 9 April 2019
Here in this study, the thermolysis of aquatic biopolymer (i.e., chitin) was mainly investigated as a strategic means for reinforcing the insecure supply chains of terrestrial biomass. To maximize carbon utilization in the carbon substrate and establish a sustainable pyrolysis platform, this study particularly employed CO2 as reactive gas medium. To this end, this study laid great emphasis on elucidating the mechanistic role of CO2 in pyrolysis of chitin. For the fundamental study, the thermolysis of chitin in CO2 in reference to the case in N2 was characterized thermo-gravimetrically. A series of the TGA tests signified that the homogeneous reactions between solid-state chitin and CO2 should be excluded. However, a lab-scale pyrolysis of chitin in CO2 demonstrated that CO2 enhanced thermal cracking of the volatile hydrocarbon species from the thermolysis of chitin. In parallel, CO2 reacted with the volatile hydrocarbon species to form CO. To justify such genuine mechanistic roles of CO2, two-stage pyrolysis of chitin was conducted, and all experimental findings strongly supported the genuine mechanistic roles of CO2. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Carbohydrates Biopolymer Chitin Pyrolysis Syngas Carbon dioxide
1. Introduction The total CO2 emissions from utilization of fossil fuels (coal, natural gas, and petroleum) to satisfy enormous demand for transportation fuel, electricity, chemicals, and goods have increased continuously [1,2]. According to the statistical report from the International Energy Agency (IEA), the total CO2 emissions in 2017 reached up to 32.5 Gt [3]. Such CO2 emissions are far exceeding the Earth's full potential to sequester carbons through the complex natural carbon cycles [4]. Thus, the surplus carbon inputs (CO2) into the atmosphere have resulted in the global environmental problems, such as global warming and climate change [5]. Thus, the practical implementations of carbon-free/carbon-neutral renewable energies (i.e., wind power [6], photovoltaic [7], solar thermal energy [8], geothermal energy [9], biofuels [10], etc.) have been recommended as a strategic means for soothing the catastrophic consequences from the surplus carbon inputs (CO2) [11]. To spur
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (E.E. Kwon), [email protected] (H. Song). https://doi.org/10.1016/j.energy.2019.04.039 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
the further utilization of renewable energies, the legislative enactments such as renewable fuel standard (RFS) and renewable portfolio standard (RPS) have been made [12], and their practices have gained considerable public consensus. Nonetheless, global demand for carbons for producing chemicals and goods has not decreased [13]. Note that carbon demand is indeed proportional to the life quality index (LQI) [14]. Based on these rationales, synthesis of chemicals and goods from the carbon-neutral resources (i.e., biorefinery) has gained great attention [15]. Despite the diverse socio-economic benefits in line with utilization of biomass, the insecure supply chains for biomass has been recognized as one of the critical constraints for the further expansion of bio-refinery [16]. Indeed, biomass is one of the ubiquitous materials, but their production is highly dwindled by the regional and seasonal variations [16]. Furthermore, cultivating energy crops for biofuels requires a considerable amount of water [17]. Also, cultivating such energy crops bring forth the ethical dilemma in that edible crops are currently being used for biofuels [18]. Therefore, selecting an initial feedstock for bio-refinery and biofuels must be considered by the fully transparent manners. To fulfill such criteria, the practical use of waste materials and/or aquatic biomass is highly recommended [19]. Moreover, the technical approaches to
G. Kwon et al. / Energy 177 (2019) 136e143
maximize carbon utilization in the carbon substrates must be considered. Considering the fact that biofuels are an initial step for bio-refinery, the current conversion platforms for biofuels are disadvantageous in terms of the ultimate carbon utilization. Note that most biofuels are converted from the single nutritional component [20]. In detail, bioethanol and biodiesel are converted from carbohydrates and lipid (i.e., triglycerides: TGs), respectively [20,21]. Biogas production via the fermentation process (i.e., anaerobic digestion process) indeed takes advantage of all nutritional components in biomass [22]. Carbon deposit onto microbes and uncontrolled carbon loss by the metabolic mechanisms of microbes cannot be avoided [23]. Such microbial activities lead to the generation of sludge as well as the high volumetric ratio of CO2 in biogas [24]. Furthermore, the microbial activities during the fermentation process to produce biogas is highly sensitive to the compositional matrix and physico-chemical properties of the carbon substrates [25]. Thus, such microbial sensitivities are not suitable for mass production. Table 1. Considering the aforementioned technical problems, the thermo-chemical process (i.e., pyrolysis and gasification) can be a promising technical alternative in that all nutritional elements in the carbon substrates can be utilized via the thermo-chemical process [26e28]. However, as compared with the anaerobic digestion process, the thermo-chemical process is less sensitive to the compositional matrix and physico-chemical properties of the carbon substrates [26]. Thus, the thermo-chemical process is advantageous in terms of mass production. In particular, gasification is defined as “the chemical process for transferring heating value of carbonaceous materials in the gaseous products” [26], and its main product is referred as synthetic gas (i.e., syngas: mixture of H2 and CO) [29e31]. Syngas can be directly used as fuel and an initial feedstock for synthesizing chemicals and goods due to its high reactivity [32]. Pyrolysis (i.e., the thermolysis of carbonaceous materials in the oxygen free environment) has been known as an
137
intermediate step for the gasification process [33]. Such fact suggests that all experimental findings in the pyrolysis process can be directly applied in the gasification process. Based on these rationales, this study laid the particular concerns on the thermo-chemical process of chitin as a case study. Indeed, to date, few studies have been compiled from pyrolysis of chitin even though chitin is one of the abundant aquatic biopolymers [34]. Chitin is the supporting material of crustaceans [35]. Thus, to reach the fundamental level, the scope of this study is limited to pyrolysis of chitin. As declared above, pyrolysis is an intermediate step for gasification. Thus, all experimental findings in this study is directly applicable in the gasification process of chitin. To achieve the more sustainable pyrolytic platform, this study used CO2 as reactive gas medium in pyrolysis of chitin. To this end, the thermolytic behaviors of chitin were characterized thermo-gravimetrically, and a labscale pyrolysis of chitin was carried out in this study. To understand the mechanistic role of CO2, all experimental findings from the CO2 environment in reference to the case from an inert (N2) environment were compared systematically in this study.
2. Materials and methods 2.1. Sample preparation and chemical reagents Chitin (Poly-(1 / 4)-b-N-acetyl-D-glucosamine) powder from shrimp shells (Lot #: C7170) and dichloromethane (DCM) (Lot #: SLBS6470) were purchased from Sigma-Aldrich (St. Louis, USA). The size of chitin was 150 mm. Prior to the experimental work, chitin was pretreated at 70 C for 24 h using a dry oven to remove moisture. The ultra-high purity N2 and CO2 were purchased from Kukje Industrial Gases Inc. (Anyang, South Korea) and Green Gas (Gwangju, South Korea), respectively.
Table 1 Identification of chemical species in Fig. 5 and their comparison based on their peak area. Label number
Retention Time, [min]
Chemical species
Peak Area in N2 Peak Area in CO2
Peak Area difference, [%]
1 2 3 4 5 6 7 8 9 10 11
11.308 16.034 21.545 27.056 27.755 28.649 29.142 29.555 30.125 31.678 33.008
648,144,720 8,139,681 761,919,874 5394078 5,746,939 11,206,448 4,041,438 5,958,588 7,350,434 8,126,526 e
524,898,105 9,215,265 544,875,323 4,290,007 4,189,593 9,697,102 3,701,086 5,590,762 8,975,393 10,770,077 3,219,777
19.02 13.21 28.49 20.47 27.10 13.47 8.42 6.17 22.11 32.53 e
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
34.408 34.885 36.352 39.249 39.371 40.625 40.697 42.55 42.671 44.372 45.049 45.229 47.608 49.923 53.865
36,353,627 114,160,952 4,985,896 2,475,625 4,610,023 4,372,594 e e e e 16,963,611 e 7,315,142 8,253,593 2,863,271
36,434,037 100,147,668 8,730,488 2,892,169 4,478,650 e 18,365,328 3,592,437 3,227,832 3,115,832 16,379,090 5,129,088 8,013,864 9,184,768 3,261,021
0.22 12.28 75.10 16.83 2.85 e e e e e 3.45 e 9.55 11.28 13.89
27
64.724
Acetic acid Propanoic acid Acetamide 1H-Pyrrole-2-carboxaldehyde Bicyclo[3.2.0]hept-6-en-2-d-2-ol 10-Chloroisoborneol Cyclobutanol 2-METHYL-1-D1-AZIRIDINE 3-Hydroxy-2-methylene-4-hexenenitrile Furan-2-carbohydrazide, N2-(1-methylhexylideno)(S,RS)-3-Ethyl-7a-methyl-2,3-dihydropyrrolo [2,1-b] [1,3]oxazol-5(7aH)one 2H-Azepin-2-one, 1,5,6,7-tetrahydro2(5H)-Furanone, 5,5-dimethylIsoxazole, 5-methyl2,4-Pentanedione, 3-(1-methyl-2-propenyl)1H-Pyrazole-3-ethanamine 2-Propanone, 1-(2-piperidinyl)2-Methyl [1,3,4]oxadiazole Ethanone, 1-cyclopropyl7-Methoxypyrrolizin-3-one 2,3,4,5-Tetrahydropyridazine N-Vinylimidazole 4-Penten-2-one 1,3-Cyclohexanediamine Guanidine, 1,1'-[(methylethanediylidene)dinitrilo]di-, dihydrochloride .alpha.-D-Galactopyranoside, methyl 2-(acetylamino)-2-deoxy-6-OmethylAdenosine, 2-methyl-
e
2,420,526
e
138
G. Kwon et al. / Energy 177 (2019) 136e143
2.2. Characterization of the thermolysis of chitin using a thermogravimetric analysis (TGA) unit To characterize the thermolysis of chitin in various atmospheric conditions (i.e., N2, CO2, and air), the thermos-gravimetric analysis (TGA) test was carried out using STA 449 F5 Jupiter (Netzsch, Germany). 10 ± 0.01 mg of chitin was loaded in an alumina crucible, and the TGA test was conducted at a heating rate of 10 C min 1 from 20 to 900 C. The flow rate of the purge and protective gases was 20 and 50 mL min 1, respectively. Each flow rate (purge and protective gases) was controlled by the imbedded mass flow controllers (MFCs). Prior to each TGA test, the blank test was done to account for mass change arising from the buoyancy effects. Note that density variations of the purge and protectives gases lead to the buoyancy force.
2.3. A laboratory scale pyrolysis of chitin using a batch-type tubular reactor A laboratory scale pyrolysis of chitin (1.5 ± 0.01 g of chitin loading) was carried out using a batch-type tubular reactor. The flow rate of N2 or CO2 was set as 300 mL min 1, and its flow rate was controlled using a mass flow controller for N2 and CO2 (Brooks Instrument, USA). A quartz tubing was used for assembling a tubular reactor to exclude catalytic effects, and its dimension was 25 mm of outer diameter, 22 mm of inner diameter, and 0.6 m of length. Both ends of the quartz tubing were equipped with two Swagelok Ultra-Torr Vacuum fittings (SS-4-UT-6-600) to tap the gas inlet and out systems. To this end, a step-down Swagelok (1 in to 0.25 in) was directly hooked to the Swagelok Ultra-Torr Vacuum fitting. The experimental temperature was controlled using a tubular furnace (Nabertherm, Germany), and the lab-scale pyrolysis of chitin was conducted at a heating rate of 10 C min 1 from 20 to 720 C. The tubular furnace has two individual heating elements, and each heating element can be operated individually. Note that the overall scheme for the experimental setup was given in Fig. S1. (Supplementary Information).
2.4. Quantification and qualification of pyrogenic products Gas effluent from the lab-scale pyrolysis of chitin was quantified by on-line micro-GC (3000 A Inficon, Switzerland). Prior to the GC measurement, liquid N2 chilled condenser was employed to knock out the condensable hydrocarbons. Such collected condensable hydrocarbons were weighted to establish the overall mass balance. After measuring mass, the compositional matrix of the condensable hydrocarbon species was qualitatively characterized using a GC/ time-of-flight (TOF) mass spectrometer (Agilent 7850, USA and ALMSCO Bench-Top, UK). In detail, the DB-WAX column (length: 30 m, diameter: 250 mm, film thickness: 0.25 mm) purchased from Agilent Technology was used. The temperature of the injector and the detector was 240 and 300 C, respectively. The temperature programming for the GC analysis was started from 50 C (5 min) to 240 C by 4 C min 1 of a heating rate and hold for 17.5 min. Helium gas was used for the carrier gas, and N2 was used as the make-up gas. 3. Results and discussion 3.1. Characterization of thermal degradation of chitin in N2 and CO2 Prior to the TGA test in the CO2 environment, the TGA test in air was conducted at a heating rate of 10 C min 1 from 20 to 900 C to determine the ash content. Mass decay of chitin and its thermal degradation rate (i.e., differential thermogram: DTG) as a function of the reaction temperatures were presented in Fig. 1a. Mass loss (1 wt% of the original sample mass) observed at 150 C is likely attributed to moisture in the sample. The final residual mass (i.e., ash content) in Fig. 1a is equivalent to 2.5 wt% of the original sample mass. Also, Fig. 1a depicts two distinctive mass decay patterns. In detail, mass loss at 360 is rapidly achieved, but the thermal degradation rate induced by mass loss from 360 to 640 C is relatively slower as compared with the first one. Such observations suggest that the first mass decay in Fig. 1a is likely attributed to volatilization of each monomer by the direct bond scissions on the polymeric backbone of chitin [34]. The second mass decay from 360 to 640 C is attributed to oxidation of the remaining sample. Given
Fig. 1. Mass decay of chitin and its thermal degradation rate as a function of the experimental temperatures (from 20 to 900 C) in (a) air and (b) N2 (black color) and CO2 (red color) (Error bars were not given due to the small deviation less than 1%). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
G. Kwon et al. / Energy 177 (2019) 136e143
that oxidation mostly occurs in the surface of the sample in the TGA test, the thermal degradation rate from 360 to 640 C in Fig. 1a may be slower than the case of volatilization at 360 C. In an effort to provide the better understanding of chitin, the FTIR spectrum of chitin was given in Fig. S2. A series of the TGA tests of chitin in CO2 in reference to an inert (i.e., N2) condition was conducted to figure out the thermolysis features induced by CO2. Representative mass decay of chitin and its thermal degradation in N2 and CO2 were given in Fig. 1b. As well described in Fig. 1b, the overall thermolytic patterns of chitin in N2 and CO2 are identical at the entire experimental temperature ranges. Such observations indicate that CO2 does not influence the mass decay behaviors. Also, such observations imply that any heterogeneous reactions by CO2 should be excluded because any reactions between the sample surface of chitin and CO2 result in the subsequent change in the mass decay patterns [36]. Furthermore, the expected reaction, the Boudouard reaction was observed from the TGA test of chitin in CO2. Note that the Boudouard reaction can be thermodynamically initiated at 720 C because the Gibbs free energy of the Boudouard reaction is less than 0 at 720 C [37]. Interestingly, despite the existence of a carbon-rich (charring) material (i.e., biochar), no occurrence of the Boudouard reaction demonstrates that the reaction kinetics of the Boudouard reaction is very slow. One interesting observation in Fig. 1b is that the thermal
139
degradation rate of chitin relative to the case of lignocellulosic biomass is very fast. This claim is well supported by the TGA test in Fig. 1b in that the width of the DTG (i.e., the thermal degradation rate of chitin) relative to the case of lignocellulosic biomass, where reflects mass decay by means of volatilization of each monomer by the direct bond scissions on the polymeric backbone of chitin, is much narrower [38]. Such fast thermolytic kinetics of chitin explained by two factors. First, the thermal stability of chitin is inferior to that of lignocellulosic biomass. Second, the compositional matrix of chitin is much simpler than that of lignocellulosic biomass. Continuous mass decay at 460 C in Fig. 1b in N2 and CO2 is likely attributed to biochar formation. Also, such gradual mass decay at 460 C indirectly implies that H2 formation from the thermolysis of chitin through dehydrogenation is initiated at 460 C. Such hypothesis in terms of H2 formation at 460 C is not fully supported by a series of the TGA tests in Fig. 1b. 3.2. A lab-scale pyrolysis experimental work of chitin in CO2 To chase the mechanistic role of CO2, a lab-scale pyrolysis of chitin was conducted using a batch-type tubular reactor in that a series of the TGA tests in Fig. 1 did not reveal any mechanistic role of CO2. The concentration profiles of the major pyrolysis gases were plotted in Fig. 2. Except the sample loading and the gas flow rate of CO2, all experimental conditions were identical to the TGA test in
Fig. 2. CConcentration profiles of the major pyrolysis of gases from pyrolysis of chitin in N2 and O2 (a) H2, (b) CH4 (C) CO, (d) C2H2, (e) C2H4 and (f) C2H6 (Error bars were not given due to the small deviation less than 1%).
140
G. Kwon et al. / Energy 177 (2019) 136e143
Fig. 1. The concentration profiles of the major pyrolysis gases evolved from the N2 environment well reflect the general thermolytic behaviors of biomass. First, the concentration of H2 begins to increase as the pyrolysis temperature increase. Note that dehydrogenation is proportional to the reaction temperature. Interestingly, the initiation temperature for H2 by dehydrogenation in Fig. 2a is in good agreement with the hypothesis associated with continuous mass decay at 460 C in Fig. 1b. We assumed that such continuous mass decay is attributed to dehydrogenation of a carbon-rich material (biochar). As well demonstrated in Fig. 2a, H2 formation from pyrolysis of chitin is indeed initiated at 460 C, and the concentration of H2 is proportional to the pyrolysis temperature. Also, CH4 and CO formation from pyrolysis of chitin in N2 well reflects the general thermolytic behaviors of biomass. In detail, these chemicals are mostly created by thermal cracking of chitin. Thus, the formation of CH4 and CO is contingent on the degree of thermal cracking. Interestingly, the concentration of CH4 and CO begins to decrease at the temperatures where the concentration of H2 begins to increase. Such phenomena are likely due to the expedited cyclization by dehydrogenation. Note that H2 formation via dehydrogenation significantly increase aromaticity of pyrogenic products (pyrolysis oil & biochar). Thus, H2 formation restrict CH4 and CO formation. The concentration profiles of the C2-hydrocarbon species were also monitored, but their evolutions are nearly negligible as compared with the case of H2, CH4, and CO. The overall gas evolution patterns from pyrolysis of chitin in the CO2 environment are very similar with the case in the N2 environment. Indeed, the concentration profiles of H2 and CH4 exhibits the same evolution patterns, and their concentrations are similar. Despite the same evolution patterns for CO, the concentration of CO in the CO2 environment is indeed higher than the case in the N2 environment. Such observations are discrepant with the experimental findings from a series of the TGA tests. Note that the mass decay patterns of chitin in N2 and CO2 (Fig. 1b) were identical. Therefore, the enhanced formation of CO in CO2 is explained by the homogeneous reactions. Based on these rationales, the enhanced generation of CO in the presence of CO2 is likely explained by that CO2 provides a favorable condition for expediting thermal cracking of volatile hydrocarbons evolved from the thermolysis of chitin. Even at 630 C, Fig. 2c depicts that the concentration of CO begins to increase. Such genuine CO evolution patterns indirectly elucidate the mechanistic role of CO2 in pyrolysis of chitin. In detail, CO2 also reacts with the volatile hydrocarbon species to form CO. CO formation by the Boudouard reaction should be excluded since that the TGA test in the CO2 environment (Fig. 1b) did not reflect mass decay by the Boudouard reaction. Nevertheless, all experimental findings in Fig. 2 marginally support the genuine mechanistic role of CO2 claimed by the authors in that most mass decay via volatilization of each monomer by the direct bond scissions on the polymeric backbone of chitin was completed at 460 C. 3.3. Two-stage pyrolysis of chitin in CO2 For the further investigation, the experimental setup was modified. In detail, the experimental works in the section of 3.2 was carried out at the heating rate of 10 C min 1 from 20 to 720 C. As noted in the section of 2.3, the tubular furnace used in this study consists of two individual heating elements, and each heating element can be operated individually. So, the temperature conditions for the first heating zone was operated at the heating rate of 10 C min 1 from 20 to 720 C, and the second heating zone was isothermally operated at 650 C. The temperature of 650 C was intentionally chosen since the concentration of CO began to increase at 630 C in Fig. 2c. The concentration profiles of the major
pyrolysis gases were plotted in Fig. 3. As demonstrated in Fig. 3, the overall evolution patterns of the major pyrolysis gases (H2, CH4, and CO) are similar with all experimental results observed in Fig. 2. Interestingly, CO formation is substantially enhanced in the CO2 environment. The concentration of H2 and CH4 seems to decrease in CO2, but such observations are likely due to the dilution effects induced by the enhanced CO formation. Note that the GC measurement only provides a relative mole fraction. Thus, the enhanced generation of CO in CO2 in Fig. 3 is very consistent with all explanations given in the section of 3.2. In detail, CO2 expedites the thermal cracking of the volatile hydrocarbon species. Moreover, CO2 reacts the volatile hydrocarbon species to form CO. The concentration profiles of the major pyrolysis gases (H2, CH4, and CO) well reflect the aforementioned mechanistic features of CO2. The concentration of CH4 in CO2 well demonstrates the enhanced thermal cracking of the volatile hydrocarbon species. In Fig. 2b, the concentration of CH4 is much lower than that of H2. As depicted in Fig. 3b, the concentration of CH4 is higher than that of H2. Even under the diluted condition by the enhanced CO formation in CO2, the concentration of CH4 is higher than that of H2. Given that CH4 formation in the thermolysis of chitin is attributed thermal cracking, the concentration profiles of CH4 in Fig. 3b can be a key evidence for supporting the enhanced thermal cracking of the volatile hydrocarbon species by CO2. Also, the concentration of CO from 410 to 470 C in CO2 is much higher than that in the N2 environment. Such high CO formation cannot be explained without the additional source for C and O. Thus, such observations signify that CO2 plays a role for the C and O source via the reactions between CO2 and the volatile hydrocarbon species to form CO. Thus, the concentration levels of the C2-species (Fig. 3) in CO2 in reference to N2 is substantially lower in that carbons turn into CO by CO2. Unfortunately, the precise reaction pathways to form CO in CO2 are not elucidated at this current body of study. Considering the same residual mass from a series of the TGA tests in Fig. 1b, the proposed mechanistic roles of CO2 must decrease the mass fraction of pyrolysis oil. Note that pyrolysis is “the process for re-allocating carbon into three pyrogenic products” [39]. In other words, all experimental findings in Figs. 2 and 3 are only possible via shifting the carbon distribution from pyrolysis oil to pyrolysis gas. To confirm this, the overall mass balance for the pyrogenic products established from pyrolysis of chitin in Figs. 2 and 3 were plotted in Fig. 4. As depicted in Fig. 4, the mass fraction for biochar in N2 and CO2 is not much different, and such observations are in good agreement with the experimental results in Fig. 1. Also, Fig. 4 clearly shows that the more mass fraction of pyrolysis gas is being created in CO2. Such findings in Fig. 4 are in good agreement with the proposed mechanisms of CO2. In detail, the enhanced thermal cracking of the volatile hydrocarbon species by CO2 serves a role for reducing the mass fraction of pyrolysis oil. Furthermore, the homogeneous reactions of the volatile hydrocarbon species and CO2 to form CO serves a role to lower the mass fraction of pyrolysis oil. If the proposed mechanistic roles of CO2 are correct, the compositional matrix of pyrolysis oil in N2 and CO2 must be different. To confirm this, pyrolytic oil from two-stage pyrolysis of chitin in Fig. 3 was qualitatively compared using the representative chromatogram of pyrolytic oil from N2 and CO2 in Fig. 5, and their major constituents in pyrolytic oil were summarized with their peak area. Chromatogram from pyrolysis oil in Fig. 5 is in good agreement with the experimental findings in Figs. 2e4. In detail, the enhanced thermal cracking of the volatile hydrocarbon species and the reactions of CO2 and the volatile hydrocarbon species to form CO effectively lower mass of pyrolysis oil. Furthermore, the chemical species in pyrolysis oil in CO2 are different from those from N2. Thus, all experimental findings in this study suggests that the use of
G. Kwon et al. / Energy 177 (2019) 136e143
141
Fig. 3. Concentration profiles of the major pyrolysis of gases from two-stage pyrolysis of chitin in N2 and CO2 (a) H2, (b) CH4 (C) CO, (d) C2H2, (e) C2H4 and (f) C2H6 (Error bars were not given due to the small deviation less than 1%).
Fig. 4. Overall mass balance of the pyrogenic products from pyrolysis of chitin by (a) one-stage and (b) two-stage pyrolysis.
142
G. Kwon et al. / Energy 177 (2019) 136e143
[5] [6]
[7] [8] [9] [10]
[11]
[12] Fig. 5. Chromatogram of the pyrogenic oil from pyrolysis of chitin in N2 and CO2.
[13] [14]
CO2 as a reactive gas medium offers a strategic way to modify the pyrogenic products without using catalysts. Note that all experimental findings in this study are not optimized in that the main goal is to elucidate the mechanistic role of CO2 in pyrolysis of chitin. To maximize the identified mechanistic features of CO2, the further study should be done in the near future. Nonetheless, all experimental findings in this study will provides a great venue for maximize the carbon utilization in the carbon substrates.
[15]
[16]
[17]
[18]
4. Conclusions The mechanistic role of CO2 in pyrolysis of aquatic biopolymer (chitin) was fundamentally investigated in this study. To this end, a series of the TGA tests of chitin in N2 and CO2 was carried out. The TGA test confirmed that the mechanistic role of CO2 was limited to the homogenous reactions between CO2 and the volatilized hydrocarbons species evolved from the thermolysis of chitin. All findings in this study suggested that CO2 expedited thermal cracking of the volatilized hydrocarbons species and reacted with the volatilized hydrocarbons species to form CO. Note that the maximum concentration of CO reached up to 27 mol%. Such genuine mechanistic features of CO2 led to shifting the carbon distribution between pyrogenic products even without using catalysts (i.e., 30% reduction of pyrolytic oil in CO2). Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1A09000800).
[19] [20] [21]
[22] [23]
[24]
[25]
[26] [27]
[28] [29]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.04.039.
[30] [31] [32]
References [33] [1] Gurney KR, Mendoza DL, Zhou Y, Fischer ML, Miller CC, Geethakumar S, et al. High resolution fossil fuel combustion CO2 emission fluxes for the United States. Environ Sci Technol 2009;43(14):5535e41. re C, Canadell JG, Klepper G, et al. [2] Raupach MR, Marland G, Ciais P, Le Que Global and regional drivers of accelerating CO2 emissions. Proc Natl Acad Sci Unit States Am 2007;104(24):10288. [3] Hüser N, Schmitz O, Kenig EY. A comparative study of different amine-based solvents for CO2-capture using the rate-based approach. Chem Eng Sci 2017;157:221e31. [4] Ro KS, Libra JA, Bae S, Berge ND, Flora JRV, Pecenka R. Combustion behavior of
[34] [35]
[36]
[37]
animal-manure-based hydrochar and pyrochar. ACS Sustainable Chem Eng 2019;7(1):470e8. Davis SJ, Caldeira K, Matthews HD. Future CO2; emissions and climate change from existing energy infrastructure. Science 2010;329(5997):1330. lez F, Sumper A, Gomis-Bellmunt O, Villafa fila-Robles R. A review Díaz-Gonza of energy storage technologies for wind power applications. Renew Sustain Energy Rev 2012;16(4):2154e71. Parida B, Iniyan S, Goic R. A review of solar photovoltaic technologies. Renew Sustain Energy Rev 2011;15(3):1625e36. Tian Y, Zhao CY. A review of solar collectors and thermal energy storage in solar thermal applications. Appl Energy 2013;104:538e53. Lund JW, Freeston DH, Boyd TL. Direct application of geothermal energy: 2005 Worldwide review. Geothermics 2005;34(6):691e727. Naik SN, Goud VV, Rout PK, Dalai AK. Production of first and second generation biofuels: a comprehensive review. Renew Sustain Energy Rev 2010;14(2):578e97. Asdrubali F, Baldinelli G, D'Alessandro F, Scrucca F. Life cycle assessment of electricity production from renewable energies: review and results harmonization. Renew Sustain Energy Rev 2015;42:1113e22. Huh S-Y, Lee J, Shin J. The economic value of South Korea׳s renewable energy policies (RPS, RFS, and RHO): a contingent valuation study. Renew Sustain Energy Rev 2015;50:64e72. Zhang Y. Urban metabolism: a review of research methodologies. Environ Pollut 2013;178:463e73. Kuo N-W, Chen P-H. Quantifying energy use, carbon dioxide emission, and other environmental loads from island tourism based on a life cycle assessment approach. J Clean Prod 2009;17(15):1324e30. Bozell JJ, Petersen GR. Technology development for the production of biobased products from biorefinery carbohydratesdthe US Department of Energy's “Top 10” revisited. Green Chem 2010;12(4):539e54. Yue D, You F, Snyder SW. Biomass-to-bioenergy and biofuel supply chain optimization: overview, key issues and challenges. Comput Chem Eng 2014;66:36e56. Yang H, Zhou Y, Liu J. Land and water requirements of biofuel and implications for food supply and the environment in China. Energy Policy 2009;37(5): 1876e85. Jung J-M, Lee J, Kim J, Kim K-H, Kwon EE. Pyrogenic transformation of oilbearing biomass into biodiesel without lipid extraction. Energy Convers Manag 2016;123:317e23. Fatih Demirbas M, Balat M, Balat H. Biowastes-to-biofuels. Energy Convers Manag 2011;52(4):1815e28. Harahap F, Silveira S, Khatiwada D. Cost competitiveness of palm oil biodiesel production in Indonesia. Energy 2019;170:62e72. Firouzi S, Nikkhah A, Aminpanah H. Resource use efficiency of rice production upon single cropping and ratooning agro-systems in terms of bioethanol feedstock production. Energy 2018;150:694e701. Wang H, Xu J, Sheng L, Liu X. Effect of addition of biogas slurry for anaerobic fermentation of deer manure on biogas production. Energy 2018;165:411e8. Mirmasoumi S, Ebrahimi S, Saray RK. Enhancement of biogas production from sewage sludge in a wastewater treatment plant: evaluation of pretreatment techniques and co-digestion under mesophilic and thermophilic conditions. Energy 2018;157:707e17. Huttunen S, Manninen K, Leskinen P. Combining biogas LCA reviews with stakeholder interviews to analyse life cycle impacts at a practical level. J Clean Prod 2014;80:5e16. Sun Q, Li H, Yan J, Liu L, Yu Z, Yu X. Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation. Renew Sustain Energy Rev 2015;51:521e32. € m O, Brink A. Energy conversion of agriPrestipino M, Galvagno A, Karlstro cultural biomass char: steam gasification kinetics. Energy 2018;161:1055e63. Benedikt F, Schmid JC, Fuchs J, Mauerhofer AM, Müller S, Hofbauer H. Fuel flexible gasification with an advanced 100 kW dual fluidized bed steam gasification pilot plant. Energy 2018;164:329e43. Kim Y, Lee J, Yi H, Fai Tsang Y, Kwon EE. Investigation into role of CO2 in twostage pyrolysis of spent coffee grounds. Bioresour Technol 2019;272:48e53. Kun Z, He D, Guan J, Zhang Q. Thermodynamic analysis of chemical looping gasification coupled with lignite pyrolysis. Energy 2019;166:807e18. Oyedun AO, Lam KL, Hui CW. Charcoal production via multistage pyrolysis. Chin J Chem Eng 2012;20(3):455e60. Cheung K-Y, Lee K-L, Lam K-L, Chan T-Y, Lee C-W, Hui C-W. Operation strategy for multi-stage pyrolysis. J Anal Appl Pyrolysis 2011;91(1):165e82. € pke M, Simpson SD, Humphreys C, Minton NP, Dürre P. Syngas De Tissera S, Ko € tter N, editors. biorefinery and syngas utilization. In: Wagemann K, Tippko Biorefineries. Cham: Springer International Publishing; 2019. p. 247e80. Bulushev DA, Ross JRH. Catalysis for conversion of biomass to fuels via pyrolysis and gasification: a review. Catal Today 2011;171(1):1e13. Stolarek P, Ledakowicz S. Pyrolysis kinetics of chitin by non-isothermal thermogravimetry. Thermochim Acta 2005;433(1):200e8. Hong S, Yuan Y, Yang Q, Zhu P, Lian H. Versatile acid base sustainable solvent for fast extraction of various molecular weight chitin from lobster shell. Carbohydr Polym 2018;201:211e7. Kwon E, Castaldi MJ. Fundamental understanding of the thermal degradation mechanisms of waste tires and their air pollutant generation in a N2 atmosphere. Environ Sci Technol 2009;43(15):5996e6002. Jang W-J, Jeong D-W, Shim J-O, Kim H-M, Roh H-S, Son IH, et al. Combined
G. Kwon et al. / Energy 177 (2019) 136e143 steam and carbon dioxide reforming of methane and side reactions: thermodynamic equilibrium analysis and experimental application. Appl Energy 2016;173:80e91. [38] Xing S, Yuan H, Huhetaoli, Qi Y, Lv P, Yuan Z, et al. Characterization of the decomposition behaviors of catalytic pyrolysis of wood using copper and
143
potassium over thermogravimetric and Py-GC/MS analysis. Energy 2016;114: 634e46. [39] McClelland DJ, Motagamwala AH, Li Y, Rover MR, Wittrig AM, Wu C, et al. Functionality and molecular weight distribution of red oak lignin before and after pyrolysis and hydrogenation. Green Chem 2017;19(5):1378e89.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Pyrolysis of aquatic carbohydrates using CO2 as reactive gas medium: A case study of chitin Gihoon Kwon a, Daniel C.W. Tsang b, Jeong-Ik Oh c, Eilhann E. Kwon a, **, Hocheol Song a, * a
Department of Environment and Energy, Sejong University, Seoul, 05005, Republic of Korea Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong c Advanced Technology Department, Land and Housing Institute, Daejeon 34047, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2019 Received in revised form 19 March 2019 Accepted 7 April 2019 Available online 9 April 2019
Here in this study, the thermolysis of aquatic biopolymer (i.e., chitin) was mainly investigated as a strategic means for reinforcing the insecure supply chains of terrestrial biomass. To maximize carbon utilization in the carbon substrate and establish a sustainable pyrolysis platform, this study particularly employed CO2 as reactive gas medium. To this end, this study laid great emphasis on elucidating the mechanistic role of CO2 in pyrolysis of chitin. For the fundamental study, the thermolysis of chitin in CO2 in reference to the case in N2 was characterized thermo-gravimetrically. A series of the TGA tests signified that the homogeneous reactions between solid-state chitin and CO2 should be excluded. However, a lab-scale pyrolysis of chitin in CO2 demonstrated that CO2 enhanced thermal cracking of the volatile hydrocarbon species from the thermolysis of chitin. In parallel, CO2 reacted with the volatile hydrocarbon species to form CO. To justify such genuine mechanistic roles of CO2, two-stage pyrolysis of chitin was conducted, and all experimental findings strongly supported the genuine mechanistic roles of CO2. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Carbohydrates Biopolymer Chitin Pyrolysis Syngas Carbon dioxide
1. Introduction The total CO2 emissions from utilization of fossil fuels (coal, natural gas, and petroleum) to satisfy enormous demand for transportation fuel, electricity, chemicals, and goods have increased continuously [1,2]. According to the statistical report from the International Energy Agency (IEA), the total CO2 emissions in 2017 reached up to 32.5 Gt [3]. Such CO2 emissions are far exceeding the Earth's full potential to sequester carbons through the complex natural carbon cycles [4]. Thus, the surplus carbon inputs (CO2) into the atmosphere have resulted in the global environmental problems, such as global warming and climate change [5]. Thus, the practical implementations of carbon-free/carbon-neutral renewable energies (i.e., wind power [6], photovoltaic [7], solar thermal energy [8], geothermal energy [9], biofuels [10], etc.) have been recommended as a strategic means for soothing the catastrophic consequences from the surplus carbon inputs (CO2) [11]. To spur
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (E.E. Kwon), [email protected] (H. Song). https://doi.org/10.1016/j.energy.2019.04.039 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
the further utilization of renewable energies, the legislative enactments such as renewable fuel standard (RFS) and renewable portfolio standard (RPS) have been made [12], and their practices have gained considerable public consensus. Nonetheless, global demand for carbons for producing chemicals and goods has not decreased [13]. Note that carbon demand is indeed proportional to the life quality index (LQI) [14]. Based on these rationales, synthesis of chemicals and goods from the carbon-neutral resources (i.e., biorefinery) has gained great attention [15]. Despite the diverse socio-economic benefits in line with utilization of biomass, the insecure supply chains for biomass has been recognized as one of the critical constraints for the further expansion of bio-refinery [16]. Indeed, biomass is one of the ubiquitous materials, but their production is highly dwindled by the regional and seasonal variations [16]. Furthermore, cultivating energy crops for biofuels requires a considerable amount of water [17]. Also, cultivating such energy crops bring forth the ethical dilemma in that edible crops are currently being used for biofuels [18]. Therefore, selecting an initial feedstock for bio-refinery and biofuels must be considered by the fully transparent manners. To fulfill such criteria, the practical use of waste materials and/or aquatic biomass is highly recommended [19]. Moreover, the technical approaches to
G. Kwon et al. / Energy 177 (2019) 136e143
maximize carbon utilization in the carbon substrates must be considered. Considering the fact that biofuels are an initial step for bio-refinery, the current conversion platforms for biofuels are disadvantageous in terms of the ultimate carbon utilization. Note that most biofuels are converted from the single nutritional component [20]. In detail, bioethanol and biodiesel are converted from carbohydrates and lipid (i.e., triglycerides: TGs), respectively [20,21]. Biogas production via the fermentation process (i.e., anaerobic digestion process) indeed takes advantage of all nutritional components in biomass [22]. Carbon deposit onto microbes and uncontrolled carbon loss by the metabolic mechanisms of microbes cannot be avoided [23]. Such microbial activities lead to the generation of sludge as well as the high volumetric ratio of CO2 in biogas [24]. Furthermore, the microbial activities during the fermentation process to produce biogas is highly sensitive to the compositional matrix and physico-chemical properties of the carbon substrates [25]. Thus, such microbial sensitivities are not suitable for mass production. Table 1. Considering the aforementioned technical problems, the thermo-chemical process (i.e., pyrolysis and gasification) can be a promising technical alternative in that all nutritional elements in the carbon substrates can be utilized via the thermo-chemical process [26e28]. However, as compared with the anaerobic digestion process, the thermo-chemical process is less sensitive to the compositional matrix and physico-chemical properties of the carbon substrates [26]. Thus, the thermo-chemical process is advantageous in terms of mass production. In particular, gasification is defined as “the chemical process for transferring heating value of carbonaceous materials in the gaseous products” [26], and its main product is referred as synthetic gas (i.e., syngas: mixture of H2 and CO) [29e31]. Syngas can be directly used as fuel and an initial feedstock for synthesizing chemicals and goods due to its high reactivity [32]. Pyrolysis (i.e., the thermolysis of carbonaceous materials in the oxygen free environment) has been known as an
137
intermediate step for the gasification process [33]. Such fact suggests that all experimental findings in the pyrolysis process can be directly applied in the gasification process. Based on these rationales, this study laid the particular concerns on the thermo-chemical process of chitin as a case study. Indeed, to date, few studies have been compiled from pyrolysis of chitin even though chitin is one of the abundant aquatic biopolymers [34]. Chitin is the supporting material of crustaceans [35]. Thus, to reach the fundamental level, the scope of this study is limited to pyrolysis of chitin. As declared above, pyrolysis is an intermediate step for gasification. Thus, all experimental findings in this study is directly applicable in the gasification process of chitin. To achieve the more sustainable pyrolytic platform, this study used CO2 as reactive gas medium in pyrolysis of chitin. To this end, the thermolytic behaviors of chitin were characterized thermo-gravimetrically, and a labscale pyrolysis of chitin was carried out in this study. To understand the mechanistic role of CO2, all experimental findings from the CO2 environment in reference to the case from an inert (N2) environment were compared systematically in this study.
2. Materials and methods 2.1. Sample preparation and chemical reagents Chitin (Poly-(1 / 4)-b-N-acetyl-D-glucosamine) powder from shrimp shells (Lot #: C7170) and dichloromethane (DCM) (Lot #: SLBS6470) were purchased from Sigma-Aldrich (St. Louis, USA). The size of chitin was 150 mm. Prior to the experimental work, chitin was pretreated at 70 C for 24 h using a dry oven to remove moisture. The ultra-high purity N2 and CO2 were purchased from Kukje Industrial Gases Inc. (Anyang, South Korea) and Green Gas (Gwangju, South Korea), respectively.
Table 1 Identification of chemical species in Fig. 5 and their comparison based on their peak area. Label number
Retention Time, [min]
Chemical species
Peak Area in N2 Peak Area in CO2
Peak Area difference, [%]
1 2 3 4 5 6 7 8 9 10 11
11.308 16.034 21.545 27.056 27.755 28.649 29.142 29.555 30.125 31.678 33.008
648,144,720 8,139,681 761,919,874 5394078 5,746,939 11,206,448 4,041,438 5,958,588 7,350,434 8,126,526 e
524,898,105 9,215,265 544,875,323 4,290,007 4,189,593 9,697,102 3,701,086 5,590,762 8,975,393 10,770,077 3,219,777
19.02 13.21 28.49 20.47 27.10 13.47 8.42 6.17 22.11 32.53 e
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
34.408 34.885 36.352 39.249 39.371 40.625 40.697 42.55 42.671 44.372 45.049 45.229 47.608 49.923 53.865
36,353,627 114,160,952 4,985,896 2,475,625 4,610,023 4,372,594 e e e e 16,963,611 e 7,315,142 8,253,593 2,863,271
36,434,037 100,147,668 8,730,488 2,892,169 4,478,650 e 18,365,328 3,592,437 3,227,832 3,115,832 16,379,090 5,129,088 8,013,864 9,184,768 3,261,021
0.22 12.28 75.10 16.83 2.85 e e e e e 3.45 e 9.55 11.28 13.89
27
64.724
Acetic acid Propanoic acid Acetamide 1H-Pyrrole-2-carboxaldehyde Bicyclo[3.2.0]hept-6-en-2-d-2-ol 10-Chloroisoborneol Cyclobutanol 2-METHYL-1-D1-AZIRIDINE 3-Hydroxy-2-methylene-4-hexenenitrile Furan-2-carbohydrazide, N2-(1-methylhexylideno)(S,RS)-3-Ethyl-7a-methyl-2,3-dihydropyrrolo [2,1-b] [1,3]oxazol-5(7aH)one 2H-Azepin-2-one, 1,5,6,7-tetrahydro2(5H)-Furanone, 5,5-dimethylIsoxazole, 5-methyl2,4-Pentanedione, 3-(1-methyl-2-propenyl)1H-Pyrazole-3-ethanamine 2-Propanone, 1-(2-piperidinyl)2-Methyl [1,3,4]oxadiazole Ethanone, 1-cyclopropyl7-Methoxypyrrolizin-3-one 2,3,4,5-Tetrahydropyridazine N-Vinylimidazole 4-Penten-2-one 1,3-Cyclohexanediamine Guanidine, 1,1'-[(methylethanediylidene)dinitrilo]di-, dihydrochloride .alpha.-D-Galactopyranoside, methyl 2-(acetylamino)-2-deoxy-6-OmethylAdenosine, 2-methyl-
e
2,420,526
e
138
G. Kwon et al. / Energy 177 (2019) 136e143
2.2. Characterization of the thermolysis of chitin using a thermogravimetric analysis (TGA) unit To characterize the thermolysis of chitin in various atmospheric conditions (i.e., N2, CO2, and air), the thermos-gravimetric analysis (TGA) test was carried out using STA 449 F5 Jupiter (Netzsch, Germany). 10 ± 0.01 mg of chitin was loaded in an alumina crucible, and the TGA test was conducted at a heating rate of 10 C min 1 from 20 to 900 C. The flow rate of the purge and protective gases was 20 and 50 mL min 1, respectively. Each flow rate (purge and protective gases) was controlled by the imbedded mass flow controllers (MFCs). Prior to each TGA test, the blank test was done to account for mass change arising from the buoyancy effects. Note that density variations of the purge and protectives gases lead to the buoyancy force.
2.3. A laboratory scale pyrolysis of chitin using a batch-type tubular reactor A laboratory scale pyrolysis of chitin (1.5 ± 0.01 g of chitin loading) was carried out using a batch-type tubular reactor. The flow rate of N2 or CO2 was set as 300 mL min 1, and its flow rate was controlled using a mass flow controller for N2 and CO2 (Brooks Instrument, USA). A quartz tubing was used for assembling a tubular reactor to exclude catalytic effects, and its dimension was 25 mm of outer diameter, 22 mm of inner diameter, and 0.6 m of length. Both ends of the quartz tubing were equipped with two Swagelok Ultra-Torr Vacuum fittings (SS-4-UT-6-600) to tap the gas inlet and out systems. To this end, a step-down Swagelok (1 in to 0.25 in) was directly hooked to the Swagelok Ultra-Torr Vacuum fitting. The experimental temperature was controlled using a tubular furnace (Nabertherm, Germany), and the lab-scale pyrolysis of chitin was conducted at a heating rate of 10 C min 1 from 20 to 720 C. The tubular furnace has two individual heating elements, and each heating element can be operated individually. Note that the overall scheme for the experimental setup was given in Fig. S1. (Supplementary Information).
2.4. Quantification and qualification of pyrogenic products Gas effluent from the lab-scale pyrolysis of chitin was quantified by on-line micro-GC (3000 A Inficon, Switzerland). Prior to the GC measurement, liquid N2 chilled condenser was employed to knock out the condensable hydrocarbons. Such collected condensable hydrocarbons were weighted to establish the overall mass balance. After measuring mass, the compositional matrix of the condensable hydrocarbon species was qualitatively characterized using a GC/ time-of-flight (TOF) mass spectrometer (Agilent 7850, USA and ALMSCO Bench-Top, UK). In detail, the DB-WAX column (length: 30 m, diameter: 250 mm, film thickness: 0.25 mm) purchased from Agilent Technology was used. The temperature of the injector and the detector was 240 and 300 C, respectively. The temperature programming for the GC analysis was started from 50 C (5 min) to 240 C by 4 C min 1 of a heating rate and hold for 17.5 min. Helium gas was used for the carrier gas, and N2 was used as the make-up gas. 3. Results and discussion 3.1. Characterization of thermal degradation of chitin in N2 and CO2 Prior to the TGA test in the CO2 environment, the TGA test in air was conducted at a heating rate of 10 C min 1 from 20 to 900 C to determine the ash content. Mass decay of chitin and its thermal degradation rate (i.e., differential thermogram: DTG) as a function of the reaction temperatures were presented in Fig. 1a. Mass loss (1 wt% of the original sample mass) observed at 150 C is likely attributed to moisture in the sample. The final residual mass (i.e., ash content) in Fig. 1a is equivalent to 2.5 wt% of the original sample mass. Also, Fig. 1a depicts two distinctive mass decay patterns. In detail, mass loss at 360 is rapidly achieved, but the thermal degradation rate induced by mass loss from 360 to 640 C is relatively slower as compared with the first one. Such observations suggest that the first mass decay in Fig. 1a is likely attributed to volatilization of each monomer by the direct bond scissions on the polymeric backbone of chitin [34]. The second mass decay from 360 to 640 C is attributed to oxidation of the remaining sample. Given
Fig. 1. Mass decay of chitin and its thermal degradation rate as a function of the experimental temperatures (from 20 to 900 C) in (a) air and (b) N2 (black color) and CO2 (red color) (Error bars were not given due to the small deviation less than 1%). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
G. Kwon et al. / Energy 177 (2019) 136e143
that oxidation mostly occurs in the surface of the sample in the TGA test, the thermal degradation rate from 360 to 640 C in Fig. 1a may be slower than the case of volatilization at 360 C. In an effort to provide the better understanding of chitin, the FTIR spectrum of chitin was given in Fig. S2. A series of the TGA tests of chitin in CO2 in reference to an inert (i.e., N2) condition was conducted to figure out the thermolysis features induced by CO2. Representative mass decay of chitin and its thermal degradation in N2 and CO2 were given in Fig. 1b. As well described in Fig. 1b, the overall thermolytic patterns of chitin in N2 and CO2 are identical at the entire experimental temperature ranges. Such observations indicate that CO2 does not influence the mass decay behaviors. Also, such observations imply that any heterogeneous reactions by CO2 should be excluded because any reactions between the sample surface of chitin and CO2 result in the subsequent change in the mass decay patterns [36]. Furthermore, the expected reaction, the Boudouard reaction was observed from the TGA test of chitin in CO2. Note that the Boudouard reaction can be thermodynamically initiated at 720 C because the Gibbs free energy of the Boudouard reaction is less than 0 at 720 C [37]. Interestingly, despite the existence of a carbon-rich (charring) material (i.e., biochar), no occurrence of the Boudouard reaction demonstrates that the reaction kinetics of the Boudouard reaction is very slow. One interesting observation in Fig. 1b is that the thermal
139
degradation rate of chitin relative to the case of lignocellulosic biomass is very fast. This claim is well supported by the TGA test in Fig. 1b in that the width of the DTG (i.e., the thermal degradation rate of chitin) relative to the case of lignocellulosic biomass, where reflects mass decay by means of volatilization of each monomer by the direct bond scissions on the polymeric backbone of chitin, is much narrower [38]. Such fast thermolytic kinetics of chitin explained by two factors. First, the thermal stability of chitin is inferior to that of lignocellulosic biomass. Second, the compositional matrix of chitin is much simpler than that of lignocellulosic biomass. Continuous mass decay at 460 C in Fig. 1b in N2 and CO2 is likely attributed to biochar formation. Also, such gradual mass decay at 460 C indirectly implies that H2 formation from the thermolysis of chitin through dehydrogenation is initiated at 460 C. Such hypothesis in terms of H2 formation at 460 C is not fully supported by a series of the TGA tests in Fig. 1b. 3.2. A lab-scale pyrolysis experimental work of chitin in CO2 To chase the mechanistic role of CO2, a lab-scale pyrolysis of chitin was conducted using a batch-type tubular reactor in that a series of the TGA tests in Fig. 1 did not reveal any mechanistic role of CO2. The concentration profiles of the major pyrolysis gases were plotted in Fig. 2. Except the sample loading and the gas flow rate of CO2, all experimental conditions were identical to the TGA test in
Fig. 2. CConcentration profiles of the major pyrolysis of gases from pyrolysis of chitin in N2 and O2 (a) H2, (b) CH4 (C) CO, (d) C2H2, (e) C2H4 and (f) C2H6 (Error bars were not given due to the small deviation less than 1%).
140
G. Kwon et al. / Energy 177 (2019) 136e143
Fig. 1. The concentration profiles of the major pyrolysis gases evolved from the N2 environment well reflect the general thermolytic behaviors of biomass. First, the concentration of H2 begins to increase as the pyrolysis temperature increase. Note that dehydrogenation is proportional to the reaction temperature. Interestingly, the initiation temperature for H2 by dehydrogenation in Fig. 2a is in good agreement with the hypothesis associated with continuous mass decay at 460 C in Fig. 1b. We assumed that such continuous mass decay is attributed to dehydrogenation of a carbon-rich material (biochar). As well demonstrated in Fig. 2a, H2 formation from pyrolysis of chitin is indeed initiated at 460 C, and the concentration of H2 is proportional to the pyrolysis temperature. Also, CH4 and CO formation from pyrolysis of chitin in N2 well reflects the general thermolytic behaviors of biomass. In detail, these chemicals are mostly created by thermal cracking of chitin. Thus, the formation of CH4 and CO is contingent on the degree of thermal cracking. Interestingly, the concentration of CH4 and CO begins to decrease at the temperatures where the concentration of H2 begins to increase. Such phenomena are likely due to the expedited cyclization by dehydrogenation. Note that H2 formation via dehydrogenation significantly increase aromaticity of pyrogenic products (pyrolysis oil & biochar). Thus, H2 formation restrict CH4 and CO formation. The concentration profiles of the C2-hydrocarbon species were also monitored, but their evolutions are nearly negligible as compared with the case of H2, CH4, and CO. The overall gas evolution patterns from pyrolysis of chitin in the CO2 environment are very similar with the case in the N2 environment. Indeed, the concentration profiles of H2 and CH4 exhibits the same evolution patterns, and their concentrations are similar. Despite the same evolution patterns for CO, the concentration of CO in the CO2 environment is indeed higher than the case in the N2 environment. Such observations are discrepant with the experimental findings from a series of the TGA tests. Note that the mass decay patterns of chitin in N2 and CO2 (Fig. 1b) were identical. Therefore, the enhanced formation of CO in CO2 is explained by the homogeneous reactions. Based on these rationales, the enhanced generation of CO in the presence of CO2 is likely explained by that CO2 provides a favorable condition for expediting thermal cracking of volatile hydrocarbons evolved from the thermolysis of chitin. Even at 630 C, Fig. 2c depicts that the concentration of CO begins to increase. Such genuine CO evolution patterns indirectly elucidate the mechanistic role of CO2 in pyrolysis of chitin. In detail, CO2 also reacts with the volatile hydrocarbon species to form CO. CO formation by the Boudouard reaction should be excluded since that the TGA test in the CO2 environment (Fig. 1b) did not reflect mass decay by the Boudouard reaction. Nevertheless, all experimental findings in Fig. 2 marginally support the genuine mechanistic role of CO2 claimed by the authors in that most mass decay via volatilization of each monomer by the direct bond scissions on the polymeric backbone of chitin was completed at 460 C. 3.3. Two-stage pyrolysis of chitin in CO2 For the further investigation, the experimental setup was modified. In detail, the experimental works in the section of 3.2 was carried out at the heating rate of 10 C min 1 from 20 to 720 C. As noted in the section of 2.3, the tubular furnace used in this study consists of two individual heating elements, and each heating element can be operated individually. So, the temperature conditions for the first heating zone was operated at the heating rate of 10 C min 1 from 20 to 720 C, and the second heating zone was isothermally operated at 650 C. The temperature of 650 C was intentionally chosen since the concentration of CO began to increase at 630 C in Fig. 2c. The concentration profiles of the major
pyrolysis gases were plotted in Fig. 3. As demonstrated in Fig. 3, the overall evolution patterns of the major pyrolysis gases (H2, CH4, and CO) are similar with all experimental results observed in Fig. 2. Interestingly, CO formation is substantially enhanced in the CO2 environment. The concentration of H2 and CH4 seems to decrease in CO2, but such observations are likely due to the dilution effects induced by the enhanced CO formation. Note that the GC measurement only provides a relative mole fraction. Thus, the enhanced generation of CO in CO2 in Fig. 3 is very consistent with all explanations given in the section of 3.2. In detail, CO2 expedites the thermal cracking of the volatile hydrocarbon species. Moreover, CO2 reacts the volatile hydrocarbon species to form CO. The concentration profiles of the major pyrolysis gases (H2, CH4, and CO) well reflect the aforementioned mechanistic features of CO2. The concentration of CH4 in CO2 well demonstrates the enhanced thermal cracking of the volatile hydrocarbon species. In Fig. 2b, the concentration of CH4 is much lower than that of H2. As depicted in Fig. 3b, the concentration of CH4 is higher than that of H2. Even under the diluted condition by the enhanced CO formation in CO2, the concentration of CH4 is higher than that of H2. Given that CH4 formation in the thermolysis of chitin is attributed thermal cracking, the concentration profiles of CH4 in Fig. 3b can be a key evidence for supporting the enhanced thermal cracking of the volatile hydrocarbon species by CO2. Also, the concentration of CO from 410 to 470 C in CO2 is much higher than that in the N2 environment. Such high CO formation cannot be explained without the additional source for C and O. Thus, such observations signify that CO2 plays a role for the C and O source via the reactions between CO2 and the volatile hydrocarbon species to form CO. Thus, the concentration levels of the C2-species (Fig. 3) in CO2 in reference to N2 is substantially lower in that carbons turn into CO by CO2. Unfortunately, the precise reaction pathways to form CO in CO2 are not elucidated at this current body of study. Considering the same residual mass from a series of the TGA tests in Fig. 1b, the proposed mechanistic roles of CO2 must decrease the mass fraction of pyrolysis oil. Note that pyrolysis is “the process for re-allocating carbon into three pyrogenic products” [39]. In other words, all experimental findings in Figs. 2 and 3 are only possible via shifting the carbon distribution from pyrolysis oil to pyrolysis gas. To confirm this, the overall mass balance for the pyrogenic products established from pyrolysis of chitin in Figs. 2 and 3 were plotted in Fig. 4. As depicted in Fig. 4, the mass fraction for biochar in N2 and CO2 is not much different, and such observations are in good agreement with the experimental results in Fig. 1. Also, Fig. 4 clearly shows that the more mass fraction of pyrolysis gas is being created in CO2. Such findings in Fig. 4 are in good agreement with the proposed mechanisms of CO2. In detail, the enhanced thermal cracking of the volatile hydrocarbon species by CO2 serves a role for reducing the mass fraction of pyrolysis oil. Furthermore, the homogeneous reactions of the volatile hydrocarbon species and CO2 to form CO serves a role to lower the mass fraction of pyrolysis oil. If the proposed mechanistic roles of CO2 are correct, the compositional matrix of pyrolysis oil in N2 and CO2 must be different. To confirm this, pyrolytic oil from two-stage pyrolysis of chitin in Fig. 3 was qualitatively compared using the representative chromatogram of pyrolytic oil from N2 and CO2 in Fig. 5, and their major constituents in pyrolytic oil were summarized with their peak area. Chromatogram from pyrolysis oil in Fig. 5 is in good agreement with the experimental findings in Figs. 2e4. In detail, the enhanced thermal cracking of the volatile hydrocarbon species and the reactions of CO2 and the volatile hydrocarbon species to form CO effectively lower mass of pyrolysis oil. Furthermore, the chemical species in pyrolysis oil in CO2 are different from those from N2. Thus, all experimental findings in this study suggests that the use of
G. Kwon et al. / Energy 177 (2019) 136e143
141
Fig. 3. Concentration profiles of the major pyrolysis of gases from two-stage pyrolysis of chitin in N2 and CO2 (a) H2, (b) CH4 (C) CO, (d) C2H2, (e) C2H4 and (f) C2H6 (Error bars were not given due to the small deviation less than 1%).
Fig. 4. Overall mass balance of the pyrogenic products from pyrolysis of chitin by (a) one-stage and (b) two-stage pyrolysis.
142
G. Kwon et al. / Energy 177 (2019) 136e143
[5] [6]
[7] [8] [9] [10]
[11]
[12] Fig. 5. Chromatogram of the pyrogenic oil from pyrolysis of chitin in N2 and CO2.
[13] [14]
CO2 as a reactive gas medium offers a strategic way to modify the pyrogenic products without using catalysts. Note that all experimental findings in this study are not optimized in that the main goal is to elucidate the mechanistic role of CO2 in pyrolysis of chitin. To maximize the identified mechanistic features of CO2, the further study should be done in the near future. Nonetheless, all experimental findings in this study will provides a great venue for maximize the carbon utilization in the carbon substrates.
[15]
[16]
[17]
[18]
4. Conclusions The mechanistic role of CO2 in pyrolysis of aquatic biopolymer (chitin) was fundamentally investigated in this study. To this end, a series of the TGA tests of chitin in N2 and CO2 was carried out. The TGA test confirmed that the mechanistic role of CO2 was limited to the homogenous reactions between CO2 and the volatilized hydrocarbons species evolved from the thermolysis of chitin. All findings in this study suggested that CO2 expedited thermal cracking of the volatilized hydrocarbons species and reacted with the volatilized hydrocarbons species to form CO. Note that the maximum concentration of CO reached up to 27 mol%. Such genuine mechanistic features of CO2 led to shifting the carbon distribution between pyrogenic products even without using catalysts (i.e., 30% reduction of pyrolytic oil in CO2). Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1A09000800).
[19] [20] [21]
[22] [23]
[24]
[25]
[26] [27]
[28] [29]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.04.039.
[30] [31] [32]
References [33] [1] Gurney KR, Mendoza DL, Zhou Y, Fischer ML, Miller CC, Geethakumar S, et al. High resolution fossil fuel combustion CO2 emission fluxes for the United States. Environ Sci Technol 2009;43(14):5535e41. re C, Canadell JG, Klepper G, et al. [2] Raupach MR, Marland G, Ciais P, Le Que Global and regional drivers of accelerating CO2 emissions. Proc Natl Acad Sci Unit States Am 2007;104(24):10288. [3] Hüser N, Schmitz O, Kenig EY. A comparative study of different amine-based solvents for CO2-capture using the rate-based approach. Chem Eng Sci 2017;157:221e31. [4] Ro KS, Libra JA, Bae S, Berge ND, Flora JRV, Pecenka R. Combustion behavior of
[34] [35]
[36]
[37]
animal-manure-based hydrochar and pyrochar. ACS Sustainable Chem Eng 2019;7(1):470e8. Davis SJ, Caldeira K, Matthews HD. Future CO2; emissions and climate change from existing energy infrastructure. Science 2010;329(5997):1330. lez F, Sumper A, Gomis-Bellmunt O, Villafa fila-Robles R. A review Díaz-Gonza of energy storage technologies for wind power applications. Renew Sustain Energy Rev 2012;16(4):2154e71. Parida B, Iniyan S, Goic R. A review of solar photovoltaic technologies. Renew Sustain Energy Rev 2011;15(3):1625e36. Tian Y, Zhao CY. A review of solar collectors and thermal energy storage in solar thermal applications. Appl Energy 2013;104:538e53. Lund JW, Freeston DH, Boyd TL. Direct application of geothermal energy: 2005 Worldwide review. Geothermics 2005;34(6):691e727. Naik SN, Goud VV, Rout PK, Dalai AK. Production of first and second generation biofuels: a comprehensive review. Renew Sustain Energy Rev 2010;14(2):578e97. Asdrubali F, Baldinelli G, D'Alessandro F, Scrucca F. Life cycle assessment of electricity production from renewable energies: review and results harmonization. Renew Sustain Energy Rev 2015;42:1113e22. Huh S-Y, Lee J, Shin J. The economic value of South Korea׳s renewable energy policies (RPS, RFS, and RHO): a contingent valuation study. Renew Sustain Energy Rev 2015;50:64e72. Zhang Y. Urban metabolism: a review of research methodologies. Environ Pollut 2013;178:463e73. Kuo N-W, Chen P-H. Quantifying energy use, carbon dioxide emission, and other environmental loads from island tourism based on a life cycle assessment approach. J Clean Prod 2009;17(15):1324e30. Bozell JJ, Petersen GR. Technology development for the production of biobased products from biorefinery carbohydratesdthe US Department of Energy's “Top 10” revisited. Green Chem 2010;12(4):539e54. Yue D, You F, Snyder SW. Biomass-to-bioenergy and biofuel supply chain optimization: overview, key issues and challenges. Comput Chem Eng 2014;66:36e56. Yang H, Zhou Y, Liu J. Land and water requirements of biofuel and implications for food supply and the environment in China. Energy Policy 2009;37(5): 1876e85. Jung J-M, Lee J, Kim J, Kim K-H, Kwon EE. Pyrogenic transformation of oilbearing biomass into biodiesel without lipid extraction. Energy Convers Manag 2016;123:317e23. Fatih Demirbas M, Balat M, Balat H. Biowastes-to-biofuels. Energy Convers Manag 2011;52(4):1815e28. Harahap F, Silveira S, Khatiwada D. Cost competitiveness of palm oil biodiesel production in Indonesia. Energy 2019;170:62e72. Firouzi S, Nikkhah A, Aminpanah H. Resource use efficiency of rice production upon single cropping and ratooning agro-systems in terms of bioethanol feedstock production. Energy 2018;150:694e701. Wang H, Xu J, Sheng L, Liu X. Effect of addition of biogas slurry for anaerobic fermentation of deer manure on biogas production. Energy 2018;165:411e8. Mirmasoumi S, Ebrahimi S, Saray RK. Enhancement of biogas production from sewage sludge in a wastewater treatment plant: evaluation of pretreatment techniques and co-digestion under mesophilic and thermophilic conditions. Energy 2018;157:707e17. Huttunen S, Manninen K, Leskinen P. Combining biogas LCA reviews with stakeholder interviews to analyse life cycle impacts at a practical level. J Clean Prod 2014;80:5e16. Sun Q, Li H, Yan J, Liu L, Yu Z, Yu X. Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation. Renew Sustain Energy Rev 2015;51:521e32. € m O, Brink A. Energy conversion of agriPrestipino M, Galvagno A, Karlstro cultural biomass char: steam gasification kinetics. Energy 2018;161:1055e63. Benedikt F, Schmid JC, Fuchs J, Mauerhofer AM, Müller S, Hofbauer H. Fuel flexible gasification with an advanced 100 kW dual fluidized bed steam gasification pilot plant. Energy 2018;164:329e43. Kim Y, Lee J, Yi H, Fai Tsang Y, Kwon EE. Investigation into role of CO2 in twostage pyrolysis of spent coffee grounds. Bioresour Technol 2019;272:48e53. Kun Z, He D, Guan J, Zhang Q. Thermodynamic analysis of chemical looping gasification coupled with lignite pyrolysis. Energy 2019;166:807e18. Oyedun AO, Lam KL, Hui CW. Charcoal production via multistage pyrolysis. Chin J Chem Eng 2012;20(3):455e60. Cheung K-Y, Lee K-L, Lam K-L, Chan T-Y, Lee C-W, Hui C-W. Operation strategy for multi-stage pyrolysis. J Anal Appl Pyrolysis 2011;91(1):165e82. € pke M, Simpson SD, Humphreys C, Minton NP, Dürre P. Syngas De Tissera S, Ko € tter N, editors. biorefinery and syngas utilization. In: Wagemann K, Tippko Biorefineries. Cham: Springer International Publishing; 2019. p. 247e80. Bulushev DA, Ross JRH. Catalysis for conversion of biomass to fuels via pyrolysis and gasification: a review. Catal Today 2011;171(1):1e13. Stolarek P, Ledakowicz S. Pyrolysis kinetics of chitin by non-isothermal thermogravimetry. Thermochim Acta 2005;433(1):200e8. Hong S, Yuan Y, Yang Q, Zhu P, Lian H. Versatile acid base sustainable solvent for fast extraction of various molecular weight chitin from lobster shell. Carbohydr Polym 2018;201:211e7. Kwon E, Castaldi MJ. Fundamental understanding of the thermal degradation mechanisms of waste tires and their air pollutant generation in a N2 atmosphere. Environ Sci Technol 2009;43(15):5996e6002. Jang W-J, Jeong D-W, Shim J-O, Kim H-M, Roh H-S, Son IH, et al. Combined
G. Kwon et al. / Energy 177 (2019) 136e143 steam and carbon dioxide reforming of methane and side reactions: thermodynamic equilibrium analysis and experimental application. Appl Energy 2016;173:80e91. [38] Xing S, Yuan H, Huhetaoli, Qi Y, Lv P, Yuan Z, et al. Characterization of the decomposition behaviors of catalytic pyrolysis of wood using copper and
143
potassium over thermogravimetric and Py-GC/MS analysis. Energy 2016;114: 634e46. [39] McClelland DJ, Motagamwala AH, Li Y, Rover MR, Wittrig AM, Wu C, et al. Functionality and molecular weight distribution of red oak lignin before and after pyrolysis and hydrogenation. Green Chem 2017;19(5):1378e89.