CO2-cofeeding catalytic pyrolysis of macadamia nutshell

Journal of CO₂ Utilization 37 (2020) 97–105

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CO2-cofeeding catalytic pyrolysis of macadamia nutshell a,1

Sungyup Jung a b c

, Dohee Kwon

a,1

b

c

T a,

, Yiu Fai Tsang , Young-Kwon Park , Eilhann E. Kwon *

Department of Environment and Energy, Sejong University, Seoul, 05006, Republic of Korea Department of Science and Environmental Studies, The Education University of Hong Kong, Tai Po, New Territories, Hong Kong School of Environmental Engineering, University of Seoul, Seoul, 02504, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Waste-to-energy Catalytic pyrolysis Macadamia nutshell Syngas Carbon dioxide

Here in this study, we laid great stress on a development of sustainable waste-to-energy (WtE) platform via CO2cofeeding catalytic pyrolysis of macadamia nutshell (MNS) at mild temperature region. To this end, one-stage and two-stage (non-catalytic/catalytic) pyrolysis of MNS was performed to establish the fundamental relationship of temperature effect on syngas formation. The formation of gaseous pyrolysates was substantially enhanced when two-stage pyrolysis of MNS was applied, and second heating zone isothermally ran at 700 °C. Such the enhanced generation of gaseous pyrolysates from two-stage MNS pyrolysis of MNS was likely due to temperature-driven cracking of volatile organic compounds (VOCs). Also, catalytic two-stage pyrolysis of MNS was performed at lower isothermal running temperature (500 °C) over Ni/SiO2 and Co/SiO2. The enhanced formation of syngas (H2 and CO) was observed from catalytic pyrolysis of MNS. Therefore, pyrolysis of MNS over Ni/SiO2 or Co/SiO2 could be a reliable platform for enhancing syngas formation at mild temperature (≤ 500 °C) under CO2 environment. In addition, all experimental findings suggested that the use of CO2 is beneficial in the WtE platform, and the use of CO2 could be a practical climate change mitigation measure.

1. Introduction Rapid urbanization in line with industrialization requires the enormous quantity of carbon [1]. To fulfill such the demand, the massive fossil resources (especially coal and petroleum) has been consumed world widely [2], which subsequently leads to anthropogenic CO2 emissions [3]. Hence, the surplus carbon inputs (CO2) from the use of the fossil resources are beyond the Earth’s capacity to sequester them via the spontaneous carbon cycle [4]. As such, the excess CO2 inputs into the atmosphere have been perceived as one of the main driving forces causing the catastrophic global environmental/socio-economic problems, namely global warming [5]. Thus, considerable efforts have been devoted to lower the atmospheric concentration of CO2 [6]. For example, carbon-free energies (wind power, solar (thermal) energy, geothermal energy, and tidal energy) have drawn considerable attention in both the academia and industrial sectors [7]. Indeed, harnessing the renewable resources into energy offers a principle strategy for lowering the obsessive reliance on fossil fuels [8]. Nevertheless, it is unable to replace carbon-based chemicals from the renewable resources (except biomass) because most of them are only transformed into electricity and heat energy [9]. Indeed, carbon is an essential element to modern society, of which the high reliance on carbon is likely

explained by our socio-economic carbon metabolisms [10]. In these contexts, employing biomass in production of chemicals (biorefinery) [11] and energy (biofuels) [12,13] offers a principle strategy for sating our massive carbon demand due to its intrinsic carbon neutrality [14]. Despite numerous socio-economic benefits from the use of chemicals and energy from biomass [15], insecure supply chain of it due to the regional/seasonal variations [16] has been realized as one of the main constraints restricting its practical implantation [17]. Also, ethical dilemma from converting edible crops into biofuels [18] has evoked the public resistance to use them (1st generation of biofuel) [19]. To circumvent the noted technical/ethical demerits, a great deal of researches on the production of energy has dealt with the valorization of inedible parts of biomass [20]. The use of 2nd and 3rd generation of biomasses (i.e., lignocellulosic biomass, municipal solid wastes, and algal biomass) have been considered viable options to produce biofuels [21]. However, most commercialized biofuels (i.e., bioethanol and biodiesel) are synthesized from the specific constituents in biomass [22]. In fact, the sugar-based compounds (cellulose and hemicellulose) and lipid in biomass as converted into bioethanol and biodiesel, respectively [23]. In these contexts, the use of the specific compounds in biomass implies that the significant amount of waste is inevitably generated after the biofuel conversion process [24]. To cope

⁎

Corresponding author. E-mail address: [email protected] (E.E. Kwon). 1 Two authors equally contributed to this study. https://doi.org/10.1016/j.jcou.2019.12.001 Received 14 November 2019; Received in revised form 2 December 2019; Accepted 3 December 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

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H2 gas (20 %, N2 balance) for catalyst reduction was obtained from RIGAS (Daejeon, Korea).

with the noted demerits, volatile fatty acids, C5-6 furanic and phenolic compounds extracted from wastes further processed through the consecutive unit operations [25], but its technical completeness has not been fully matured [13,26]. Thus, it is desirable to develop a new technical platform to directly convert raw biomass wastes into energy with a suppression of byproducts concurrently. Among the various fuel processing techniques, the thermo-chemical processes such as gasification and pyrolysis, can be viable options to redistribute biomass wastes into pyrogenic products such as syngas, biochar, and biocrude [27–31]. Since the gasification and pyrolysis at high temperature can be energy intensive processes, recent efforts have been made to convert biomass wastes into pyrogenic products with more energy efficient technologies such as microwave-assisted pyrolysis [28,29], pyrolysis under the vacuum environment [30,31], and catalytic pyrolysis [26,32,33]. Based on the above, pyrolysis of macadamia nutshell (MNS) was particularly investigated in this study. MNS is a hard-woody shell, considered a byproduct from macadamia nut processing, of which the annual production reaches up to 42,150 metric tons [34]. Nevertheless, a large portion of it is discarded. Accordingly, the production of activated carbons from MNS biochar has been proposed as an aspect of waste valorization [35]. However, energy recovery from MNS wastes via the thermo-chemical conversion process has been rarely investigated. To our best knowledge, CO2-cofeeding pyrolysis of MNS and product analysis at mild temperature region (200–720 °C) was not reported. To establish sustainable and environmentally benign energy conversion platform via thermo-chemical conversion of MNS, CO2 was employed as a reactive gas medium for MNS pyrolysis in reference to N2 condition. To enhance energy recovery from MNS pyrolysis, earth abundant/cheap metals (Ni and Co) catalysts were adopted because both the Ni-based [36] and Co-based [37–39] have been considered effective metals for bond scissions of complex organic materials. Accordingly, the present study laid great stress on shifting the carbon distributions of three phases of pyrolysates (syngas, biocrude, and biochar) in accordance with the use of metallic catalysts in the presence of CO2. Prior to MNS pyrolysis, the thermo-gravimetric analysis (TGA) test of MNS was performed to fundamentally understand thermolytic behavior of MNS in N2 and CO2 conditions.

2.2. Catalyst preparation and characterization Ni(NO3)2∙6H2O and Co(NO3)2∙6H2O metal precursors were used to fabricate 5 wt.% metals on a silica support (Ni/SiO2 and Co/SiO2) by the wetness incipient impregnation methods [40]. Each metal precursor was dissolved in DI water, and droplets of mixed solution were wetted on SiO2 until impregnated. The prepared samples were dried at 105 °C for 1 d, and then the samples were calcined at 500 °C for 4 h. The samples were further reduced at 450 °C for 4 h (a ramp rate of 2 °C min−1 from room temperature) under 20 % H2 (N2 balanced). X-ray diffraction (XRD) was performed using a Rigaku D/MAX-III diffractometer to determine crystal structures of catalysts with a scanning rate of 8˚/min (Ni and Co) from 10 to 80˚. To confirm the Ni and Co metal loading on the SiO2, inductively coupled plasma optical emission spectrometry (Perkin-Elmer, Optima 5300 DV) was utilized. Each Ni or Co metal loading on the support was 5.1 ± 0.1 wt.%. 2.3. Thermo-gravimetric analysis test To characterize the thermal decomposition behavior of MNS as a function of temperature, thermo-gravimetric analysis (TGA, STA 499 F1 Jupiter, Netzsch, Germany) tests were carried out in CO2, N2 and air environments. The gas flow rates were controlled using an embedded mass flow controller in the TGA unit (70 mL min−1). Temperature range was from 35 to 900 °C at a heating rate of 10 °C min−1 with a 10.0 ± 0.1 mg of sample loading. To compensate buoyancy effect arising from the density change of flow gases, blank tests were performed before each TGA running. Proximate analysis of MNS was conducted by comparing the mass decay in both the N2 and air conditions: moisture (1.1 wt.%), volatile matters (74.5 wt.%), fixed carbon (17.3 wt.%), and ash (7.1 wt.%). 2.4. MNS pyrolysis in a lab-scale reactor For a lab-scale pyrolysis of MNS, a batch-type tubular reactor (TR) was installed. A 90 cm long quartz tube was used as a reactor body placed within an external furnace, and inner and outer diameters of the quartz tube were 2.5 and 2.2 cm, respectively. To install gas inlet and outlet ports, Ultra-Torr vacuum fittings were connected to both ends of quartz tube. Then, step-down unions (from 2.54 cm to 0.635 cm) were directly connected to the vacuum fitting. The assembled TR was placed into a tubular furnace (RD 30/200/11, Nabertherm, Germany), which was used as an external heating source. The furnace consisted of two heating zones, and temperature of each heating zone was operated independently. Mass flow controller (5850E series, Brooks Instrument, USA) was used to set the flow rate of CO2 and N2 gases (300 mL min−1) during all the pyrolysis experiments. For one-stage MNS pyrolysis, MNS (1.00 ± 0.01 g) on the first heating stage center was pyrolyzed until 720 °C with a constant heating rate (10 °C min−1) under each N2 or CO2 gas flow. In two-stage non-catalytic pyrolysis of MNS, setup for the first stage was identical with the one-stage pyrolysis, while the second stage was placed next to the first heating zone, isothermally running at 500 or 700 °C under CO2 condition. The ex-situ catalytic pyrolysis of MNS was carried out with two-stage pyrolysis setup. Ni/SiO2 or Co/SiO2 catalyst (1.00 ± 0.01 g) was loaded into the center of second heating zone, packing within quartz wools, and the second heating stage ran at 500 °C isothermally. The gaseous effluents from MNS pyrolysis were monitored using an on-line micro-GC unit equipped with two GC modules having a thermal conductivity detector. Two Micro-GC units had molecular sieve column and plot U column, respectively. A standard calibration gas was used for quantification of gaseous pyrolysates. Liquid pyrolysates from the TR was further flowed to a condenser filled with a dichloromethane, which

2. Materials and methods 2.1. Sample preparation and chemical reagents Macadamia nutshell (MNS) was purchased from a macadamia processing company in Korea. Prior to all the experiments, MNS was washed with deionized (DI) water to remove nut residue and dust, and then dried in a convection oven at 80 °C for 72 h. The dried MNS was ground to adjust particle size below than 355 μ m using a ball mill (PULVERISETTE 6 Mono Mill, Fritsch, Germany). To determine the elemental composition of MNS, the ultimate analysis (Table 1) was performed with an elemental analyzer. Metal precursors, Ni (NO3)2∙6H2O (97 %) and Co(NO3)2∙6H2O (97 %), were purchased from Daejung Chemical (Korea). Silica (Lot # MKCG0862) and dichloromethane (≥ 99.9 % purity) were purchased from Sigma-Aldrich (USA). Ultra-high purity grade N2 and CO2 were purchased from Green Gas (Gwangju, Korea). A calibration gas mixture (Lot # 160401257255-1) for micro-GC was purchased from INFICON (Germany). Table 1 Ultimate analysis of MNS. Elemental Component N (wt.%)

C (wt.%)

S (wt.%)

H (wt.%)

O (wt.%)

0.541

50.48

0.377

6.375

42.227

98

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Scheme 1. Schematic diagram of experimental setup for MNS pyrolysis using the TR.

attributed to moisture within MNS samples in both N2 and CO2 atmospheres. Following rapid thermal degradation is shown at the higher temperature (206–396 °C), and the mass loss reached to 63 wt.% by 396 °C. This thermal degradation originated from the depletion of the most volatile organic matters (VOCs) of biomass [41–43]. In this temperature region, two peaks are overlapped and shown at 294 °C and 362 °C in DTG curves, respectively. These indicate that there may be more than one pyrolysis mechanism. Further thermal degradation is shown at higher temperature, but the degradation rate began to substantially decrease from 396 °C. Mass decay in the temperature range between 396 and 745 °C was 12 wt.%, and the residual mass at 745 °C was 25 wt. % in both conditions. The slower thermolysis of MNS between 396 and 745 °C implies the biochar formation, which should be derived from both dehydrogenation and deoxygenation of the carbon substrate [41,42]. Note that deoxygenation and dehydrogenation increase C to O and C to H ratio and accelerates carbonization. Since bonding energies for CeO (358 kJ mol−1) is lower than and CeH (416 kJ mol−1), respectively [44,45], it is expected that deoxygenation may occur prior to dehydrogenation. Further studies will be discussed in one-stage and two-stage pyrolysis studies. At ≥ 745 °C, the only slight mass decay of MNS is shown in N2 condition, while additional decomposition occurred from CO2 atmosphere. This mass decay at 745 °C is attributed to the Boudouard reaction (BR: C (s ) + CO2 ⇌ 2CO ). [46], because the BR is thermodynamically favorable at ≥720 °C [14]. Under CO2 environment, continuous thermal degradation of MNS from TGA study was shown beyond 900 °C, describing that the BR is incomplete due to their slow kinetics. To find an evidence of such the slow kinetics of BR and possibly other reactions in both environments, TGA test was further conducted in air condition to estimate remaining carbon at 900 °C. The final residual mass of MNS was 7.1 wt.% at 600 °C, and there was no further mass decay up to 900 °C in the air condition, while that in both CO2 (19.7 wt.%) and N2 (24.4 wt.%) conditions was much higher at 900 °C. These results strongly show that the ash content in MNS is about 7.1 wt.%, and thermolysis of MNS in both conditions is incomplete due to remaining organic contents there. Such the series of TGA tests estimated a depletion of moisture and VOCs, and biochar formation from thermolysis of MNS, but the tests were not able to prove the reaction mechanism and products distribution during thermo-chemical conversion of MNS. To understand the thermolysis of MNS in depth, quantitative analysis of gas and liquid pyrolysates during the pyrolysis of MNS was necessary in a wide range of temperature.

was immersed in a cold water. Then, the diluted solutions in a dichloromethane were injected into a GC/MS-TOF (ALMSCO, UK), equipped with a DB-WAX column (Agilent). Experimental setup using the TR is depicted in Scheme 1.

3. Results and discussion 3.1. The thermolytic behaviors of MNS To understand thermal profiles of MNS in an oxygen-free condition, MNS pyrolysis was performed with a TGA unit under two gas (N2 and CO2) conditions from 40 to 900 °C. Mass decay and thermal degradation rate (differential thermogram: DTG) of MNS are depicted as a function of temperature in Fig. 1. Thermolytic behaviors of MNS in both conditions are identical at ≤745 °C. Small mass decay below 200 °C is

Fig. 1. Mass decay and differential thermogram (DTG) curves of MNS as a function of temperature in the N2 (black) and CO2 (red) atmospheres. 99

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Fig. 2. Gaseous effluents evolution profile (H2 and C1-2 chemical species) from one-stage MNS pyrolysis in N2 (black) and CO2 (red) atmospheres.

depletion of MNS resulting from evolution of gaseous pyrolysates (Fig. 1) in both conditions. Although thermolytic behavior of MNS in both conditions from TGA experiments is identical at ≤ 745 °C, CO evolution profiles in both environments are different in the temperature range of 600–720 °C (Fig. 2). TGA experiments confirmed that no heterogeneous reaction between MNS and CO2 occurred at ≤ 745 °C. In the presence of CO2, however, CO generation continuously increased as temperature increased at ≥ 600 °C unlike N2 condition. This result is likely due to the homogeneous reaction between volatiles pyrolysates from MNS pyrolysis and CO2. Note that there is no BR in this temperature region in the presence of CO2. CH4 evolution is initiated at 330 °C, and continuous evolution of CH4 is shown in both environments until source depletion (Fig. 2). Corresponding mass decay patterns of MNS from TGA study indicate that CH4 formation results from cleavage of polymeric backbone on biomass compounds [14,42]. H2 formation is initiated from 510 °C, and its maximum concentration was achieved at the highest pyrolysis temperature (720 °C) in N2 condition (Fig. 2). Continuous mass decrease of MNS at ≥ 510 °C corresponds to H2 formation according to TGA studies under both conditions (Fig. 1). In this temperature region, H2 generation is likely

3.2. Non-catalytic pyrolysis of MNS One-stage MNS pyrolysis was conducted in a fixed-bed TR under N2 or CO2 flow (300 mL min−1) to quantify and compare the gaseous and liquid pyrolysates in both conditions. Pyrolysis temperature was from 90 to 720 °C at a heating rate of 10 °C min−1. The final temperature, 720 °C, was chosen, to elucidate mechanistic roles of CO2 in comparison with N2 environment when no BR occurs (Fig. 1). Gaseous effluents from the TR were quantitatively monitored using an on-line micro-GC system, and the concentration profiles of the gas products are depicted in Fig. 2. Because MNS pyrolysis in the one-stage reactor has a similar experimental setting with TGA experiment, the concentration profiles of gaseous pyrolysates in the one-stage pyrolysis should well reflect the thermal profiles of MNS from TGA tests (Fig. 1). From the one-stage pyrolysis, CO generation is initiated at 300 °C and reached to maximum concentration at 360 °C (Fig. 2), followed by the continuous decrease of CO evolution until 600 °C in both environments. Such the CO concentration profiles are well matched with the thermal profiles of MNS discussed in Fig. 1. The biggest DTG peaks in TGA tests are shown at 294 and 362 °C, corresponding to a significant 100

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(0.22 mol% at 450 °C) in the two-stage pyrolysis, employing 700 °C second heating zone, whereas C2H2 is not produced from both one-stage pyrolysis and two-stage pyrolysis, ran at 500 °C. This is likely attributed to the reaction mechanism only occurred in the two-stage pyrolysis setup. Overall evolution patterns commonly showed dramatic enhancement of gaseous effluents between 300 and 600 °C in two-stage pyrolysis, isothermally running at 700 °C. Also, C2H4 was only shown in the two-stage pyrolysis setup, meaning that additional reactions in the second heating zone promoted evolution of gaseous pyrolysates. It is expected that VOCs generated from MNS pyrolysis in the first heating zone is delivered to the second heating zone, and additional thermal cracking of VOCs in the second heating zone could significantly contribute to the formation of gaseous effluents at 700 °C. Such the great improvement of gaseous effluents only showed at 700 °C of the second heating zone, while the gas evolution is not enhanced at 500 °C. Therefore, it is hypothesized that VOCs seem to be rapidly converted into gaseous effluents at ≥ 700 °C, but one-stage pyrolysis or two-stage pyrolysis, isothermally running at 500 °C, are not able to promptly convert VOCs into permanent gases. To confirm the conversion of VOCs (i.e., liquid tar) to gaseous pyrolysates, mass balance of solid char, liquid tar and gas products is constructed in both one-stage pyrolysis and two-stage pyrolysis (700 °C) (Fig. 4-a). Mass content of solid char in the two-stage pyrolysis is consistent with one-stage pyrolysis because solid char formed in the first heating zone is remained in there. The second heating zone could not have influence on the gas or liquid formation from the solid char. As hypothesized, liquid tar content significantly dropped to 2.2 wt.% in two-stage pyrolysis from 13.3 wt.% in one-stage pyrolysis. This confirmed that VOCs is converted into permanent gaseous effluents at 700 °C. It can be further estimated that the remaining liquid compounds after thermal cracking at 700 °C should lose functional groups having oxygen and hydrogen due to the formation of CO, H2 and hydrocarbons. From the analysis of GC/MS chromatograms, composition of the liquid tar is further analyzed to investigate their functional groups (Table 2 and Fig. 4-b). Since lignocellulosic biomass is composed of cellulose, hemicellulose and lignin parts, the liquid tar produced from one-stage MNS pyrolysis mainly consisted of oxygenates and heterocyclic compounds (Table 2). However, significant loss of oxygen-containing functional groups with increased aromaticity is shown after two-stage pyrolysis at 700 °C from GC/MS results. This is likely contributed to the enhancement of CO, H2 and hydrocarbons in the two-stage pyrolysis (Fig. 3).

attributed to dehydrogenation of MNS. Although the identical thermal decomposition of MNS is shown from TGA experiments in both environments, H2 evolution under CO2 is lower than N2 condition at ≥ 600 °C (Fig. 2). Because this temperature region is where CO formation began to substantially increase when CO2 applied (Fig. 2), relative H2 concentration in CO2 condition became lower than N2 condition due to dilution effect, not suppression of H2 formation. The formation of CO, CH4 and H2 is expected from CeO, CH and CCee bond cleavages on polymeric back bones of MSN at ≤ 600 °C in both conditions. As discussed in the TGA studies (Fig. 1), a formation of biochar should result from deoxygenation and dehydrogenation of biomass compounds, which mainly consisted of organic C, O, and H elements. During the thermolysis of MNS, CO evolution could be from deoxygenation of MNS, while a formation of CH4 and H2 could be attributed to dehydrogenation of it. Evolution patterns of both CO and H2/CH4 confirmed that CO formation was occurred prior to the generation of CH4 and H2 (Fig. 2). This means that deoxygenation is proceeded earlier than dehydrogenation, and the result agrees with the aforementioned discussion that CeO (358 kJ mol−1) has lower bonding energy than CeH (416 kJ mol−1) [44,45], leading to deoxygenation prior to dehydrogenation. In detail, the most dramatical mass decay by 362 °C is mainly attributed to deoxygenation of MNS, and the following mass decay is possibly contributed to the dehydrogenation. Interestingly, additional CO evolution occurred in the presence of CO2, though TGA confirmed the no heterogeneous reaction between CO2 and MNS at ≥ 600 °C. This tells that CO2 could have homogeneous reaction with VOCs derived from MNS pyrolysis, resulting in the production of CO at the high temperature region. In N2 condition, the homogeneous reaction did not occur due to an absence of O source. Two C2 gaseous effluents, C2H6 and C2H4 are also observed from one-stage MNS pyrolysis at ≥ 360 °C, though their quantities (≤0.03 mol%) produced from one-stage pyrolysis are negligible, comparing to the amount of CO and H2. The formation of C2 or long-chain hydrocarbons probably requires additional reaction mechanism beyond CeC, CO or CHee cleavages. Two-stage MNS pyrolysis is also conducted to examine the product distribution in a different reactor setup and thermolysis condition in CO2 environment. In detail, additional second heating zone is operated isothermally at 500 or 700 °C, and the effect of second heating zone temperatures on MNS pyrolysis is investigated (Fig. 3). When the second heating zone temperature is 500 °C, concentration profiles of gaseous products are similar with one-stage pyrolysis. However, substantial improvement of gaseous effluents (CO, H2, CH4, C2H4 and C2H2) is shown when the second heating zone ran at 700 °C isothermally (Fig. 3). As noted in both the TGA and one-stage pyrolysis experiments (Figs. 1 and 2), deoxygenation of biomass is responsible for CO generation at ≤ 600 °C, and the maximum evolution of CO was shown around 360 °C. In two stage pyrolysis, the maximum peak in terms of CO evolution is shown at 420 °C, and the concentration increase more than 7 times at the maximum when the second heating zone ran at 700 °C. However, CO concentration is same with one-stage pyrolysis at ≥ 600 °C. This indicates that the substantial improvement of CO generation around 420 °C is not originated from homogeneous reaction between CO2 and VOCs. H2 evolution in one-stage pyrolysis is initiated at ≥510 °C, and the maximum generation is at the maximum experiment temperature (Fig. 2). However, two-stage pyrolysis initiated H2 generation much earlier at ≥ 360 °C, leading to maximum H2 evolution at 420 °C when the second heating zone isothermally ran at 700 °C (Fig. 3). There is also remarkable production of CH4 and C2H4 at ≥ 360 °C in two-stage pyrolysis, while they are negligibly produced in the one-stage pyrolysis and two-stage pyrolysis ran at 500 °C. The starting temperatures for CH4 and C2H4 formation in one-stage pyrolysis are identical with twostage pyrolysis, the evolution of CH4 and C2H4 in two-stage pyrolysis (700 °C) jumped more than two orders of magnitude higher, comparing with one-stage pyrolysis. In addition, a formation of C2H2 is shown

3.3. Catalytic pyrolysis of MNS Non-catalytic MNS pyrolysis showed the syngas formation, originated from the direct production of gaseous effluents from MNS pyrolysis or thermal cracking of VOCs. When two-stage pyrolysis is employed, isothermally running at 700 °C, substantial improvement of syngas formation is shown, suppressing liquid tar formation. However, two stage pyrolysis, running at 500 °C, was not able to enhance the syngas formation. Elevation of pyrolysis temperature is suggested to produce more gaseous pyrolysates, but employment of higher temperature to promote syngas formation may consume excessive energy to generate energy-related products. To make the two-stage pyrolysis more promising, operating at mild temperature should be necessary. Catalyst is a substance that can enable to proceed a chemical reaction with lower activation energy in different conditions (desirable for lower temperature and pressure). In this regard, earth abundant and inexpensive metal catalysts are prepared to conduct catalytic pyrolysis in two-stage reactor at lower pyrolysis temperature (500 °C). Metal catalysts are fabricated on SiO2 support, and crystal orientation of the Ni and Co catalysts is examined with XRD (Fig. 5). Such the metal catalysts showed corresponding peaks of each metal and a SiO2 support; those peaks are identified with the JCPDS library (Ni: 65-0380, 101

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Fig. 3. Gaseous effluents evolution profile (H2 and C1-2 chemical species) from MNS pyrolysis in CO2. Non-catalytic MNS pyrolysis in both two-stage and one-stage setups are compared.

Fig. 4. (a) Mass balance of pyrolysates from MNS pyrolysis in the CO2 and (b) GC/MS chromatograms of liquid tar in one-stage and two-stage pyrolysis (isothermally run at 700 °C). 102

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Table 2 Chemical composition of liquid tar in both one-stage and two-stage pyrolysis (isothermally run at 700 °C) in the CO2 condition. No.

RT (min)

Chemical Composition

Formula

1stage Pick Areaa

2stage Pick Areaa

1 2 3 4

11.5641 12.4629 14.0758 15.6216

C6H6 C4H8O2 C3H8O C5H8O

N.D. 16601390 79869941 N.D.

169414947 N.D. N.D. 3681890

5 6 7 8 9

16.1592 16.7557 17.0833 20.7376 20.9812

C2H4O2 C7H8 C4H10O C3H6O2 C7H14O

483150864 N.D. 94990746 44649274 14345591

N.D. 31860325 N.D. N.D. N.D.

10 11

22.1153 22.3757

C4H6O2 C4H6O3

14415260 43484755

N.D. N.D.

12 13 14 15 16 17 18 19

22.4598 23.0058 24.7363 24.9967 25.2235 26.4753 27.3742 27.9202

C8H8 C5H4O2 C5H8O3 C5H6O2 C4H8O C8H6O C9H8 C6H6O2

N.D. 173959248 18319891 48007561 15607594 N.D. N.D. 70670241

45050940 N.D. N.D. N.D. N.D. 3239788 46366231 N.D.

20 21

28.7351 30.0372

C7H5N C5H6O3

N.D. 32973378

1845350 N.D.

22

C6H6O

69265652

1025162

23 24 25 26 27

31.1545 (N2) 31.1797 (CO2) 31.5829 32.6246 33.0279 34.0527 34.4811

benzene ethyl acetate propan-1-ol 3-methylbut-3-en-2one acetic acid toluene 2-methylpropan-1-ol 3-hydroxypropanal 2,4-dimethylpentan-3one succinaldehyde methyl 2oxopropanoate styrene furan-2-carbaldehyde 2-oxopropyl acetate furan-2-ylmethanol butan-2-one benzofuran 1H-indene 5-methylfuran-2(3 H)one benzonitrile 4-hydroxy-5,6-dihydro2H-pyran-2-one phenol

C7H8O2 C7H8O C10H8 C7H8O C8H10O2

140753853 16247453 N.D. 40083138 147794362

N.D. N.D. 195889582 N.D. N.D.

28 29 30

35.0776 35.3044 36.5813

C4H8O C8H10O C9H12O2

83030560 27654167 41329123

N.D. N.D. N.D.

31 32 33

36.7914 37.1862 38.1186

C11H10 C11H10 C5H6O2

N.D. N.D. 11797778

3390404 1294351 N.D.

34 35

38.5808 39.1267

C12H10 C9H10O2

N.D. 15668395

7031145 N.D.

36 37

39.3115 39.992

C6H10O2 C6H8O3

15999457 15660128

N.D. N.D.

38 39

40.6473 41.4033

C8H7N C6H6O3

N.D. 28934548

664467 N.D.

40 41

42.0923 42.2434

C12H8 C10H12O2

N.D. 29399401

16323328 N.D.

42 43

43.6296 44.4192

C12H8O C8H8O3

N.D. 13900757

4169912 N.D.

44 45 46 47 48

45.0493 51.0894 51.6102 52.383 54.0464

C13H10 C14H10 C14H10 C4H8O2 C15H10

N.D. N.D. N.D. 32076425 N.D.

12057580 43878774 2912045 N.D. 2403237

a

2-methoxyphenol m-cresol naphthalene p-cresol 4-methoxy-3methylphenol cyclopropylmethanol 3,4-dimethylphenol 4-ethyl-2methoxyphenol 2-methylnaphthalene 1-methylnaphthalene cyclopentadiene dioxide biphenyl 2-methoxy-4vinylphenol (Z)-ethyl but-2-enoate (E)-4-oxobut-2-enyl acetate 1H-indole 5-(hydroxymethyl) furan-2-carbaldehyde acenaphthylene (E)-2-methoxy-4-(prop1-enyl)phenol dibenzo[b,d]furan 4-hydroxy-3methoxybenzaldehyde 9H-Fluorene phenanthrene anthracene butyric acid 4H-cyclopenta[def] phenanthrene

Fig. 5. XRD patterns of 5 wt.% Ni and Co catalysts supported on SiO2.

When Ni/SiO2 and Co/SiO2 catalysts are placed on the second heating zone, running isothermally at 500 °C, CO evolution increased approximately three times more than non-catalytic pyrolysis (Fig. 6) at ≤ 600 °C. The initiation temperature for CO generation over metal catalysts is close to that non-catalytic pyrolysis, but more exponential increase of CO formation is shown due to catalytic thermal cracking of VOCs over metal catalysts at 500 °C of the second heating zone. This tells that the Ni and Co metal catalysts helped to lower thermal cracking temperature for cleavage of various carbon networks in VOCs. Non-catalytic pyrolysis in both one-stage and two-stage setups (isothermally ran at 500 °C) did not generate H2 gas at ≤ 510 °C, while catalytic pyrolysis promoted H2 generation between 300 and 500 °C (Fig. 6). In catalytic pyrolysis, another H2 evolution at ≥ 510 °C is identical with non-catalytic pyrolysis. Although catalytic pyrolysis enhanced H2 and CO generation, both Ni and Co catalysts did not promote the generation of hydrocarbons. From these results, it can be interpreted that both Ni and Co catalysts promote dehydrogenation and deoxygenation of VOCs at 500 °C. Non-catalytic pyrolysis, employing 500 °C in the second stage, did not affect to the production of gaseous pyrolysates, while H2 and CO generation greatly enhanced by the addition of Ni and Co catalysts.

4. Conclusions This work investigated CO2-cofeeding catalytic pyrolysis of MNS to examine the formation of syngas fuels under various pyrolysis conditions and catalysts. CO2 expedited CO generation at ≥ 610 °C due to homogeneous reaction with VOCs. Two-stage pyrolysis greatly enhanced the formation of gaseous pyrolysates (H2, CO, CH4, and C2H4) at ≤ 610 °C when the second heating zone ran at high temperature (700 °C). Such the enhanced generation of syngas and fuels in two-stage pyrolysis stemmed from the thermal cracking of VOCs at the second heating zone, losing oxygen-containing functional groups with increase of aromaticity. Catalytic two-stage pyrolysis was carried out over earth abundant metal catalysts (Ni and Co) to lower second heating zone operating temperature (500 °C). Significant enhancement of H2 and CO formation was shown over Ni and Co catalysts because the catalysts promoted deoxygenation and dehydrogenation of VOCs. These results greatly offer a sustainable approach to produce syngas and fuels from biomass-derived wastes at mild operating temperature with greenhouse gas, CO2. For the practical application of CO2-cofeeding MNS pyrolysis for syngas and biochar formation, process optimization should be followed to find a proper reactor setup and operating condition. This work would contribute to helping the development of more practical systems under the CO2 condition for the future works.

N.D.: Not detected.

Co: 15-0806, and SiO2: 29-0085). On Ni/SiO2, 44.3˚, 51.7˚, 76.2˚ peaks corresponding to Ni (111), Ni (200), and Ni (220) were observed with a peak of SiO2 support at 22.5˚. SiO2 peaks also appeared on Co/SiO2 at the same position, Co (111) peak was shown at 44.1˚. 103

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Fig. 6. Gaseous effluents evolution profiles from two-stage catalytic pyrolysis of MNS over Ni (red) and Co (blue) catalysts, comparing to non-catalytic pyrolysis (black). Isothermal running temperature at the second heating zone was 500 °C.

Authors contribution statement

References

S. Jung: Experiment design and data interpretation, original draft preparation. D. Kwon: Leading experiment and data interpretation, partial draft writing. Y.F. Tsang: Partial data interpretation, manuscript review and editing. Y.-K. Park: Partial data collection and interpretation, manuscript review and editing. E.E. Kwon: Supervision, Writing - Review & Editing.

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Declaration of Competing Interest The authors have no conflicts of interest to declare.

Acknowledgements This work was also supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIT) (NRF2019R1A4A1027795 and NRF-2019H1D3A1A01070644). 104

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