CO2 to fuel via pyrolysis of banana peel

Journal Pre-proofs CO2 to Fuel via Pyrolysis of Banana Peel Dohee Kwon, Sang Soo Lee, Sungyup Jung, Young-Kwon Park, Yiu Fai Tsang, Eilhann E. Kwon PII: DOI: Reference:

S1385-8947(19)33189-4 https://doi.org/10.1016/j.cej.2019.123774 CEJ 123774

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

4 October 2019 4 December 2019 9 December 2019

Please cite this article as: D. Kwon, S.S. Lee, S. Jung, Y-K. Park, Y.F. Tsang, E.E. Kwon, CO2 to Fuel via Pyrolysis of Banana Peel, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123774

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© 2019 Published by Elsevier B.V.

CO2 to Fuel via Pyrolysis of Banana Peel Dohee Kwon1‡, Sang Soo Lee2‡, Sungyup Jung1, Young-Kwon Park3, Yiu Fai Tsang4, and Eilhann E. Kwon1

1Department 2Department 3School

of Environment and Energy, Sejong University, Seoul 05006, Republic of Korea;

of Environmental Engineering, Yonsei University, Wonju 26493, Republic of Korea;

of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea;

4Department

of Science and Environmental Studies, The Education University of Hong Kong, Tai Po, New Territories, Hong Kong

ABSTRACT Considering the global production of banana, a large amount of banana peel (BP) waste is being generated world widely. Thus, pyrolysis of BP was investigated to develop a technically reliable platform for the simultaneous waste management and energy recovery. To synergistically increase the sustainability of BP pyrolysis, this study adopted carbon dioxide (CO2) as a raw material and examined the production of syngas as a function of temperature in reference to the N2 environment. CO2-cofeed pyrolysis expedited the thermal cracking of volatile pyrolysates from BP, resulting in the gas phase homogeneous reaction between CO2 and the pyrolysates at ≥ 420 ˚C. This promoted CO formation in the temperature region, and more than 20 times of CO formation was shown in comparison with ‡ Two



authors equally contributed to this study. Corresponding author Eilhann E. Kwon, Ph.D.; Tel: 82-2-3408-4166; Fax: 82-2-3408-4166; e-mail: [email protected]

pyrolysis in the N2 environment. These genuine mechanistic roles of CO2 offer a new mean for converting CO2 into CO. Also, CO2-cofeed pyrolysis of BP increased the aromaticity of biocrude (pyrolytic oil), expediting dehydrogenation of liquid pyrolysates without using any catalysts. Moreover, modification of biochar surface morphology under the CO2 condition was observed. Conclusively, this study informed a key clue for maximizing the carbon exploitation in the carbonaceous waste, which directly leads to the environmental benefits due to the use of CO2 as a reactant. CO2-cofeed pyrolysis can be an innovative and strategic thermo-chemical process to valorize food wastes.

KEYWORDS: CO2; CO2-to-fuel; banana peel; waste valorization; waste-to-energy; pyrolysis

1. INTRODUCTION Unilateral use of the fossil resources to exploit them as energies and chemicals has rapidly increased since the Industrial Revolution [1, 2]. In spite of numerous socio-economic benefits, CO2 emissions from the exploitation of the fossil resources have subsequently resulted in ecological perturbation [3, 4]. In fact, the International Energy Agency (IEA) reported that global CO2 emissions in 2018 reached up to 33.1 Gt. CO2 emissions from combustion of fossil fuels are beyond the planet’s capacity to regulate carbons, of which carbon imbalance triggers the global environmental burden, such as climate change [5, 6]. To seek an effective means to abate the global environmental issues in line with CO2, a great deal of researches on sustainable energies [7-10] and carbon capture and sequestration (CCS) [11-13] has gained unprecedented attention over the several decades. Regardless a low economic viability [14, 15], the public consensus in accordance with awareness of the detrimental consequences from global warming plays a crucial role for mandating the use of renewable energies [16]. Thus, renewable fuel standard (RFS) and renewable portfolio standard (RPS) have been legislatively mandated in most 2

developed countries [17]. In particular, the practical implementation of biofuels in reference to other renewable energies has readily been achieved owing to their compatibilities with fossil fuels [18]. As compared with other renewable energies, biofuels are only carbon-based because they are derived from the carbon-neutral resources, such as lignocellulosic and algal biomass [19]. Thus, they can share the distribution networks with fossil fuels by means of simple blending with gasoline and diesel [20]. They are also affordable to be used in the internal combustion (IC) engines without any mechanical modification [21]. Nonetheless, the insecure supply chain of biomass has been pointed out as one of the biggest constraints for the further expansion of biofuels [22]. Indeed, the annual production of biomass is highly depending on the regional and seasonal variations [23, 24]. Accordingly, it is desirable to exploit various waste materials as raw materials for biofuels [25] and to develop its reliable conversion platform consistently for the better efficiency [26]. The most biofuels are being transformed from the single nutrient in biomass [27]. For instance, bioethanol and biodiesel are converted from fermentation of sugar and chemicals reaction (transesterification) of lipid, respectively [28]. Indeed, biogas via the fermentation process offers the whole conversion of organic waste into fuel [29]. Nonetheless, the anaerobic digestion (AD) process are sensitive to the physico-chemical properties of carbon substrates, which greatly affects the production yield of biogas (i.e., methane) [30], and the control of carbon deposit onto microbes and loss (CO2) by the microbial metabolisms is also challenging [31]. In these respects, the thermo-chemical process (pyrolysis and gasification) of biomass offers an ideal solution to resolve most technical demerits in the AD process. In short, the thermo-chemical process is indeed invulnerable to the physico-chemical variations of carbon substrates and can offer a paramount means for mass production in a relatively short processing time [32]. Gasification is a process that fully converts solid carbonaceous materials into syngas (H2 and CO) with oxygen and/or steam condition at high temperature (≥ 700 ˚C) via oxidation reaction [33, 34]. This energy intensive process at high

3

temperature also accompanies mass production of greenhouse gas, CO2. Pyrolysis is one of the proven thermo-chemical processes to convert biomass into the three pyrolysates (flammable gas, biocrude, and biochar) in an oxygen-free environment at relatively lower temperature than gasification. Pyrolysis of biomass has been mainly adapted to produce biochar and biocrude for the further upgrading and various applications [11, 35]. However, energy recovery (syngas formation) from pyrolysis of biomass was less significantly highlighted, in comparison with a gasification, because thermolysis of biomass generally does not gasify as much as coal [36]. Thus, enhancement of energy recovery from pyrolysis of biomass wastes, consuming greenhouse gas, CO2, will be of great importance. In these rationales, CO2-cofeed pyrolysis of a biomass waste was mainly investigated to implement a sustainable thermolytic platform for energy recovery from biomass in reference to pyrolysis in N2 environment. In addition, low temperature pyrolysis was conducted at ≤ 700 ˚C for the syngas formation for more energy efficient syngas and biochar production. Banana peel (BP) was used as a case study, because banana (Musa paradisiaca L.) is one of tropical fruits consumed globally. The global production of banana was estimated to be 125 million tons in 2017 [37]. Considering 42 wt.% of a cluster of bananas, a large amount of banana peel (BP) waste is being generated world widely. To date, only the limited number of publications reported the recovery of energy from BP via the pyrolysis process, qualitatively [38]. Also, most of current researches for BP pyrolysis focused on the production of biochar and carbonaceous solid materials [38-40]. In this study, fundamental understanding of the mechanistic role of CO2 on pyrolysis of BP was also focused. To this end, the three pyrogenic products (solid biochar, liquid biocrude, and syngas) from lab-scale CO2-cofeed pyrolysis of BP were compared with those from the N2 environment. Also, the thermo-gravimetric analysis (TGA) tests were done to gain an insight on the mechanistic role of CO2. 2. MATERIALS AND METHODS

4

2.1. Chemical reagents and sample preparation BP was collected from a local restaurant in Korea. To avoid biodegradation, the collected BP sample was dried at 80 ˚C for 3 d. The sample was processed using a ball mill (Fritsch, PULVERISETTE 6 Mono Mill, Germany), with an average particle size of 45 mesh (345 µm). The milled BP sample was stored in a desiccator to avoid moisture absorption. The ultimate analysis of BP was done using the FlashSmart™ Elemental Analyzer (Thermo-Fisher Scientific, USA). The ultimate analysis of BP showed the empirical formula of C1H1.6O0.9N0.02S0.006. The GC calibration gas (Lot # 160-401257255-1) and dichloromethane (99.8 % purity of CH2Cl2) (Lot # MKBS5448V) were purchased from INFICON (Germany) and Sigma-Aldrich (St. Louis, USA), respectively. Ultra-high purity N2 and CO2 gases were also purchased from TechAir (Korea).

2.2. TGA of BP The mass change of BP in accordance with the thermolytic temperatures was determined using a TGA unit (NETZSCH STA 449 F5 Jupiter, Germany). The TGA test of BP (10.00 ± 0.01 mg, 345-µm particle size), loaded in a crucible, was done at a temperature ramp of 10 ˚C/min from 40 to 900 ˚C. To cancel out the buoyancy effect, the blank TGA run was done. Note that the buoyancy effect was induced by density change due to the thermal expansion of N2 and CO2.

2.3. Pyrolysis of BP A tubular reactor (TR) was assembled with the quartz tubing (1.2-m length, 23-mm inner dia., and 25.4-mm outer dia.) to exclude the catalytic influences. Except dimension of the TR, all parts to construct the TR are reported in our previous work [41]. 1.000 ± 0.001 g of BP was loaded for each pyrolysis run. The flow rate of gas medium was set as 300 mL/min, of which the flow rate was

5

controlled using a mass flow controller (Brooks, USA). A tubular furnace (RD 30/200/11, Nabertherm, Germany) was used. The tubular furnace had two heating zones. Accordingly, for one-stage pyrolysis of BP, two heating zones were controlled simultaneously. On the contrary, two heating zones were separately operated in two-stage pyrolysis of BP. In detail, the first and second heating zones were dynamic (a temperature ramp:10 ˚C/min, the operation temperature: 40-700 ˚C) and isothermal (600 ˚C) runs, respectively. The gaseous effluents from the TR were transferred to an on-line micro GC (3000A, INFICON, Switzerland). Note that condensable pyrolysates (biocrude) were trapped using liquid N2. Mass balance of all pyrolysates was determined by weighting mass of biochar and biocrude. Experimental setups for both one-stage and two-stage pyrolysis were described in the Scheme 1. Note that the mass fraction of the gaseous pyrolysates were estimated based on mass of biochar and biocrude. All the experiments were performed at least three times, and the errors were ≤ 1%.

2.4. Qualitative and quantitative determination of pyrolysates derived from pyrolysis of BP The concentrations of the gaseous pyrolysates (i.e., H2, CH4, CO, C2H6, C2H4, and C2H2) was determined using a micro-GC unit. The micro-GC unit was comprised of two GC modules (molecular sieve column and Plot U column). The GC calibration gas from INFICON and RIGAS were used to calibrate the micro-GC unit. To determine the constituents of biocrude qualitatively (i.e., condensable pyrolysates), GC/TOF-MS (Agilent 7850 B/Bench Top, ALMSCO, UK) was used, which was equipped with a capillary column (DB-Wax, Agilent, USA).

6

Scheme 1. Flow diagram of pyrolysis experiments

3. RESULTS AND DISCUSSION 3.1. One Stage Pyrolysis of BP in CO2 and N2 environments To gain an insight on the CO2-cofeed impact on pyrolysis of BP, 1.000 ± 0.001 g of BP was pyrolyzed in the one-stage reactor in the CO2 atmosphere. Given that the Boudouard reaction (𝐶 + 𝐶𝑂2 ⇌2𝐶𝑂) under 1 atm starts to be activated at 720 ˚C [42], the initiation of it leads to the enhanced generation of CO via a gasification reaction. Since the Boudouard reaction is well known and broadly applied gasification process, this work investigated the thermolysis of BP for syngas formation and mechanistic roles of CO2 at milder temperature region (between 300 and 700 ˚C) to exclude any effect from the Boudouard reaction. The same pyrolysis test of BP in N2 was performed to adopt it as the reference. To study the mechanistic role of CO2, the gaseous effluents were quantified as a function of pyrolysis temperature, and their concentration profiles (i.e., H2 and C1-2 gaseous species) were plotted (Fig. 1). The concentration profiles of the gaseous effluents in N2 demonstrate a typical thermolytic trend of biomass. As depicted, CO is created earlier than that of H2 in pyrolysis of BP [41]. This is ascribed to 7

low bonding energy of C-O. For instance, bonding energies of C-O and C-H are 358 and 416 kJ/mol, respectively. Accordingly, the formation of H2 by dehydrogenation of polymeric carbon structure of biomass is observed at ≥ 480 ˚C [43], of which the temperature initiating the formation of H2 in reference to CO is higher. Moreover, the formation of H2 increased as the thermolytic temperature increased (Fig. 1), which is consistent with the dehydrogenation mechanism (cleavage of C-H bond) that its reaction kinetics is proportional to the thermolytic temperatures [44]. As such, the concentration of H2 reached the maximum level at 630 ˚C. At ≥ 630 ˚C, the concentration of H2 starts to decrease. This is likely ascribed to the source depletion foe dehydrogenation. The concentrations of CH4, C2H6, C2H4, and C2H2 from pyrolysis of BP in N2 are negligible as compared with those of H2 and CO. Thus, the lower concentration levels of CH4, C2H6, C2H4, and C2H2 suggest that the pyrolysis of BP in N2 mainly follows direct bond scissions from the biopolymeric backbone of BP and dehydrogenation. CO formation is nearly negligible at the temperatures initiating H2 formation. Such the observation implies occurrence of cyclization, which results in the carbonization of biomass to produce biochar at ≥ 480 ˚C [11, 45].

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Fig. 1. Concentration of the gaseous effluents (H2 and C1-2 chemical species) from one-stage pyrolysis of BP

However, the evolution trend of the gaseous effluents in CO2 is different from that in N2. Especially, the evolution of CO from CO2-cofeed pyrolysis of BP is noticeably different. As depicted in Fig. 1, CO formation exponentially increases at ≥ 420 ˚C, and the highest molar concentration was 6.2 mol% at 700 ˚C. On the contrary, other gaseous effluents (H2, CH4, C2H6, C2H4, and C2H2) show the lower concentration levels as compared with those in N2. Thus, such the lower levels in Fig. 1 are likely due to the dilution induced by the CO enhancement in CO2. Note that the on-line GC measurement offers the molar ratio of analytes, and the relative molar concentration can be obtained. When CO level

9

exponentially increased at 420 ˚C, concentration of others, specifically H2, could be slightly decreased due to the dilution effect. Nonetheless, the concentration profiles (C2-hydrocarbon species) from both N2 and CO2 are different. In detail, the highest concentrations of C2-hydrocarbon species from CO2 appear earlier than those from N2. Such the observations provide a key clue in line with the mechanistic role of CO2. Given that the CO enhancement is only observed from the CO2 environment, the identified enhancement in Fig. 1 strongly suggests that there is an additional source of C and O. As such, the lower concentration levels of C2-hydrocarbon species imply that carbons in C2-hydrocarbon species and volatile pyrolysates from BP pyrolysis serve a role for the source of C. However, the additional source of O cannot be elucidated. Considering the experimental setup, CO2 is postulated as the feasible source of both C and O. More precisely, the CO enhancement can be interpreted as the result of the gas phase reaction between CO2 and volatile pyrolysates including C2-hydrocarbon species. Based on these rationales, depriving carbons from volatile pyrolysates by CO2 through the reaction to form CO leads to the lower concentration levels of C2-hydrocarbon species. The highest concentrations of C2-hydrocarbon species in CO2 appear earlier than those in N2 because C2-hydrocarbon species react with CO2 via the gas phase reaction. The concentration profile of H2 from the CO2 environment is interesting. As depicted, the concentration of H2 from the CO2 environment is nearly constant at ≥ 550 ˚C, where the concentration of CO is enhanced exponentially. The constant concentration levels for H2 in Fig. 1 cannot fully be explained by the dilution by the CO enhancement because exponential decrease of H2 formation is expected when CO2 dramatically increased at ≥ 600 ˚C. Accordingly, such the phenomena offer that the hypothesized gas phase reaction between volatile pyrolysates (including C2-hydrocarbon species) from BP pyrolysis and CO2 affects dehydrogenation. Considering the genuine mechanistic features of CO2 in Fig. 1, we assumed that CO2-cofeed impact on pyrolysis of BP likely influences the carbon distribution

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of three pyrolysates (i.e., gas, biocrude, and biochar) in line with the overall mass balance. CO2 likely affects the compositional matrix of biocrude (as pyrolytic oil) condensed from volatile pyrolysates. To confirm these, mass balance was established in Fig. 2(a). In addition, the constituents of the condensable pyrolysate were determined qualitatively. Since the volatile pyrolysates produced from BP pyrolysis were gas phase products through the tubular furnace, they were condensed using a solvent trap to make liquid products (biocrude) in dichloromethane. To visualize each constituent in the biocrude (condensable pyrolysate or pyrolytic oil), the chromatogram was shown in Fig. 2(b). The major constituents in biocrude were labelled and their details were summarized in Table 1.

Fig. 2. (a) Mass balance of three pyrogenic products from N2 and CO2, and (b) chromatogram of biocrude derived from one-stage pyrolysis of BP in N2 (black color) and CO2 (red color) Since biomass consisted of complex polymeric structure such as cellulose, hemicellulose, and lignin with impurities, pyrolysis of BP resulted in the formation of various carbon-based constituents. In Fig. 2(a), the mass portions of three pyrolysates from N2 and CO2 are not much different. Nonetheless, the compositional matrix of biocrude is indeed different. Such the differences are evidenced by the peak 11

area of the chemical species in Table 2. All experimental findings (Figs.1 and 2, and Table 1) experimentally support that CO2 plays a crucial role to modify the compositional matrix of the gaseous effluents and biocrude. Interestingly, most chemical species in Table 1 are the oxygenated hydrocarbons. Identification of the oxygenated chemical compounds in biocrude is consistent with the ultimate analysis of BP. Note that the chemical formula of BP was determined as C1H1.6 O

0.9N0.02S0.006.

On the

assumption that there is no additional source of O, the high content of the oxygenated compounds in biocrude from pyrolysis of BP in CO2 must result in the less formation of CO. On the contrary, the CO enhancement was observed in CO2 (Fig. 2). Thus, it is pertinent that the identified CO enhancement in Fig. 2 is a result of the gas phase reaction between CO2 and volatile pyrolysates. For the fundamental investigation, it is desirable to clarify the hypothesized gas phase reaction.

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Table 1. Identified chemical species in biocrude from BP pyrolysis in N2 and CO2 conditions (Peak area in N2 and CO2 are scaledowned as 106) No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Retention Time, [min] 11.04 11.30 14.74 15.65 16.21 17.87 18.97 19.76 21.42 22.19 23.57 24.98 25.43 26.40 27.16 28.45 30.14 30.35 30.68 31.68 32.30 33.20 33.71 34.20 35.55 35.77 36.29 38.08 38.51 39.35 40.96

Compounds butan-2-one 2,3-dimethylbutane 2-pentanone acetic acid oxiran-2-ylmethanol 4-methoxybutane-1,2-diol 3-methylpentan-2-one propanoic acid 3,4-dimethylfuran-2,5-dione furan-3-carbaldehyde D-limonene 1-(furan-2-yl)ethanone dihydrofuran-2(3H)-one guanidine 4-pentenal 2-methylpenta-1,4-diene cyclotene phenol 2-methoxyphenol m-cresol cis-3-hexenal p-cresol 3-Hydroxydihydrofuran-2(3H)-one cyclobutanol 4-ethylphenol 2,4-dimethylphenol 3,5-dihydroxy-6-methyl-2H-pyran-4(3H)-one cyclobutane 1,4:3,6-dianhydro-A-D-glucopyranose 1H-indole 5-acetyloxolan-2-one 13

Formula C4H8O C6H14 C5H10O C2H4O2 C3H6O2 C5H12O3 C6H12O C3H6O2 C6H6O3 C5H4O2 C10H16 C6H6O2 C4H6O2 CH5N3 C6H10 C6H10 C6H8O2 C6H6O C7H8O2 C7H8O C6H10O C7H8O C4H6O3 C4H8O C8H10O C8H10O C6H8O4 C4H8 C6H8O4 C8H7N C6H8O3

Area in N2 13.79 21.03 5.41 278.72 84.30 9.01 6.30 17.69 18.40 34.54 79.57 10.18 37.98 10.79 18.17 17.13 4.35 29.48 6.08 10.75 5.33 19.03 6.59 77.00 8.01 4.51 9.58 6.92 7.08 6.75 6.52

Area in CO2 16.66 26.79 6.79 326.60 72.02 8.27 10.97 18.05 26.92 48.30 108.52 14.69 59.47 9.80 27.32 17.55 4.43 47.56 9.52 18.78 6.26 26.91 8.83 104.59 11.65 5.72 16.33 5.30 7.13 5.79 4.01

Variation*, [%] 20.84 27.42 25.62 17.18 -14.58 -8.19 74.17 2.02 46.35 39.81 36.39 44.28 56.59 -9.19 50.39 2.42 1.87 61.32 56.50 74.68 17.40 41.43 34.06 35.83 45.33 26.76 70.43 -23.39 0.75 -14.27 -38.49

32

46.47

3-methylcyclopentane-1,2-dione

C6H8O2

14

4.41

5.55

25.87

3.2. Characterization of the thermal degradation of BP in CO2 and N2 environments To confirm the hypothesized gas phase reaction, a series of the TGA tests was carried out in various atmospheric environments. To align all experimental findings (Fig.1- 2, and Table 1) with the TGA result, the TGA tests of BP was done at a temperature ramp of 10 ˚C/min from 40 to 900 ˚C. Mass change of BP and its thermal degradation rate were presented (Fig. 3).

Fig. 3. (a) Thermogram (TG) and differential thermogram (DTG) curves of BP from N2 and CO2, (b) TG and DTG curves of BP from air

As well reflected in Fig. 3(a), mass changes upon the thermolytic temperatures in N2 and CO2 are very similar at ≤ 715 ˚C, such the similarities are well evidenced by the mass decay and differential thermogram (DTG) curves. On the contrary to the results in Fig. 3(a), Fig. 2 already showed that the evolution trends of the gaseous effluents from pyrolysis of BP in CO2 were different from those in N2. Thus, such the discrepancies in Fig. 2 and 3(a) can be a key clue for the gas phase reaction between 15

volatile pyrolysates and CO2 because any heterogeneous interactions between the solid sample (BP) and CO2 results in the subsequent difference in the TG and DTG curves. Note that the TGA test only offers mass change in accordance with the thermolytic temperature. However, at ≥ 715 ˚C, the more mass conversion was achieved only from CO2, and the identified mass change is mainly attributed to the Boudouard reaction. To confirm the completion of Boudouard reaction, the same TGA test was done in air. Because the residual component after BP pyrolysis in the air is ash, which does not have a carbon source for the Boudouard reaction, the residual mass from BP pyrolysis in CO2 condition should be identical when the Boudouard reaction is done. As evidenced, the final residual masses in both the CO2 (11.2 wt.%) and air (11.6 wt.%) environments are similar, which informs that most carbon in BP biochar is converted in CO through the Boudouard reaction.

3.3. Two-stage pyrolysis of BP in in CO2 and N2 environments As demonstrated in Fig. 3(a), 60 wt.% of the original sample mass was thermally decomposed at ≤ 420 ˚C. Nonetheless, the CO enhancement by the gas phase reaction only occurred at ≥ 420 ˚C. Such the observation clarifies that the gas phase reaction was not activated at ≤ 420 ˚C. Thus, it is desirable to prove the temperature region that CO2 reacts with volatile pyrolysates from pyrolysis of BP below 720 ˚C, where no Boudouard reaction occurs for a gasification reaction. To confirm this, two-stage pyrolysis of BP was done. As noted, the tubular furnace has two heating zones. The first heating zone was operated dynamically, and the second heating zone was operated isothermally at 600 ˚C. The gaseous effluents (H2 and C1-2 chemical species) in accordance with the thermolytic temperatures were quantified (Fig. 4).

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Fig. 4. Concentration of the gaseous effluents (H2 and C1-2 chemical species) from two-stage pyrolysis (isothermal run at 600 ˚C) of BP in N2 and CO2 The overall evolution trends of the gaseous effluents are similar with those in Fig. 1. However, the more CO was formed from 360 to 450 ˚C from CO2. At ≤ 360 ˚C, the expected CO enhancement was not observed, which is due to the heat transfer delay. Nonetheless, Fig. 4 marginally evidences that the gas phase reaction can universally be applied by modifying the experimental setup in pyrolysis of BP. Thus, the CO enhancement suggests that CO2 can be converted into fuel. In detail, the enhanced production of CO by CO2-cofeed can further produce H2 via the water-gas-shift (WGS: CO + H2O ⇌ CO2 + H2) reaction [46]. Interestingly, Fig. 4 demonstrates that the concentrations of H2 (from 300 to

17

450 ˚C) and C1-2-hydrocarbon species are enhanced. Considering the dilution from the CO enhancement, the enhanced formation of H2 and C1-2-hydrocarbons offers another mechanistic feature associated with CO2. For instance, CO2-cofeed pyrolysis of BP likely expedite the thermal cracking of volatile pyrolysates evolved from BP and dehydrogenation while consuming carbons by CO2. These particular mechanistic roles may greatly affect the compositional matrix of pyrolysates. For the further study, the overall mass balance from pyrolysis of BP was established in Fig. 5(a), and the constituents of the condensable pyrolysates were qualitatively specified. Like Fig. 2(b), the chromatogram was presented in Fig. 5(b) to visualize each constituent in biocrude. The constituents in biocrude were labelled, and their details were summarized in Table 2.

Fig. 5. (a) Overall mass balance of three pyrogenic products from two-stage pyrolysis (isothermal run at 600 ˚C) of BP, (b) Chromatogram of biocrude from two-stage pyrolysis of BP in N2 (black color) and CO2 (red color)

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Table 2. Identified chemical species in biocrude from two-stage BP pyrolysis in N2 and CO2 conditions (Peak area in N2 and CO2 are scale-downed as 106) No.

Retention Time, [min]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

11.67 16.12 19.26 19.51 19.81 20.47 21.93 25.65 26.00 26.91 28.14 29.01 30.89 32.55 33.80 36.09 36.50 37.24 40.24 41.59 44.56 50.61 53.48

Compounds benzene toluene 1H-pyrrole pyridine benzo[d]oxazol-2(3H)-one m-xylene styrene allylbenzene benzofuran 1-ethyl-4-methylbenzene benzonitrile phenyl acetate phenol naphthalene o-cresol 2-methylnaphthalene 1-methylnaphthalene 1H-indole acenaphthylene 9H-fluorene anthracene phenanthrene 6,8-dimethoxyquinoline-5-amine

19

Formula

Area in N2

Area in CO2

Variation*, [%]

C6H6 C7H8 C4H5N C5H5N C7H5NO2 C8H10 C8H8 C9H10 C8H6O C9H12 C7H5N C8H8O2 C6H6O C10H8 C7H8O C11H10 C11H10 C11H10 C8H7N C12H8 C13H10 C14H10 C14H10

100.50 198.71 19.47 8.75 N.D. 17.64 54.99 3.79 7.32 34.37 7.55 N.D. 21.83 164.82 N.D. 29.75 15.41 13.27 14.46 10.17 39.50 7.19 6.06

105.84 204.57 24.94 N.D. 4.92 26.06 70.89 8.52 7.97 51.50 5.48 7.02 28.94 112.90 8.39 21.78 16.87 7.42 6.40 4.69 14.34 2.24 N.D.

5.31 2.95 28.06 47.73 28.91 124.68 8.84 49.87 -27.42 32.54 -31.50 -26.80 9.47 -44.04 -55.74 -53.85 -63.69 -68.78 -

The overall mass balances from the two-stage pyrolysis of BP in both the atmospheric environments are not much different, which is consistent with the findings in the one-stage pyrolysis (Fig. 2(a)). Nonetheless, Fig. 5(b) demonstrates that the compositional matrix of biocrude is greatly affected by CO2. In detail, the low molecular benzene derivatives, such as benzene, toluene, xylene, and styrene, were generated more in the CO2 environment. On the contrary, the high molecular benzene derivatives, such as polycyclic aromatic carbons (PAHs: naphthalene, anthracene, and phenanthrene), were generated less in the CO2 environment. In general, the enhanced cracking of volatile hydrocarbon species and dehydrogenation expedites the formation of PAHs [47], which is discrepant with the general knowledge associated with the PAH formation mechanisms [47]. However, considering carbon conversion into CO by the gas phase reaction, consuming carbons by CO2 likely reduce a chance to form PAHs. Thus, the mechanistic roles of CO2 identified in this study offers a favorable condition to control aromaticity of pyrogenic products. Accordingly, the further endeavors should be given to advance the mechanistic role of CO2 in the near future.

Fig. 6. FE-SEM images of biochar from pyrolysis of BP in N2 and CO2 at 600 ˚C

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Pyrolysis of BP is achieved by re-distribution carbons into three pyrolysates. Thus, it is desirable to seek any differences in biochar. Accordingly, the surface morphology of two biochar samples generated from BP in both the N2 and CO2 environments was further examined, and the SEM snapshots of two biochar samples were presented in Fig. 6. As depicted, the surface morphology of BP biochar from CO2 is different from that from N2. Considering the thermolytic temperature (600 ˚C), the effect of the Boudouard reaction must not be counted. Thus, the noted mechanistic roles of CO2 likely result in the different surface morphology of BP biochar. However, unfortunately, the precise mechanistic explanation in line with the morphologic modification of biochar is not been given at this stage of study.

4. CONCLUSIONS To establish a sustainable platform for the simultaneous waste management and energy recovery, CO2-cofeed pyrolysis of BP was mainly investigated as a case study. The gas phase reaction between CO2 and volatile pyrolysates was confirmed in both the one-stage and two-stage pyrolysis, which led to more than 20 times of CO generation in CO2-cofeed pyrolysis, comparing to the pyrolysis in N2 between 300 and 700 ˚C. Conclusively, the CO enhancement implied CO2 could be converted into fuels (H2, CO and C1-2 hydrocarbons) at significantly lower temperature region than the gasification temperature without oxygen and steam. Moreover, CO2 expedited thermal cracking of volatile pyrolysates evolved from BP and dehydrogenation. Nonetheless, the aromaticity of biocrude from pyrolysis of BP in CO2 was significantly reduced because the formation of the high molecular benzene derivatives, such as PAHs was non-catalytically restricted. The use of CO2 in pyrolysis of BP offered a new way to modify the surface morphology of biochar. As a case study for CO2-cofeed pyrolysis of heavily produced biomass waste, BP, it was proven that CO2 enhanced the CO production, manipulated biocrude, and modified surface morphology of biochar, which may apply to various biomass feedstocks. Controlling H 21

to C ratio from biomass to pyrogenic products is a promising strategy that can be feasible and potentially useful to various applications. For the practical use of this platform, further elucidated CO2-cofeed BP pyrolysis studies have to be conducted. Long-term pyrolysis studies at various isothermal temperature and use of different reactor setups will be promising future works. Further parametric optimizations also should be established in the near future.

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Declaration of interest: None

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Highlights 

Valorization of banana peel to energy as a form of syngas was achieved by pyrolysis

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Banana peel pyrolysis in CO2 produced more than 20 times of CO comparing to N2

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CO2-cofeed pyrolysis manipulated biocrude composition and biochar morphology

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CO2 was changed into fuel via a sustainable thermolytic platform of food waste

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