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CO2-assisted catalytic pyrolysis of digestate with steel slag
Energy xxx (xxxx) xxx
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CO2-assisted catalytic pyrolysis of digestate with steel slag Jung-Hun Kim a, 1, Jeong-Ik Oh b, 1, Yiu Fai Tsang c, Young-Kwon Park d, Jechan Lee e, *, Eilhann E. Kwon a, ** a
Department of Environment and Energy, Sejong University, Seoul, 05006, Republic of Korea Advanced Technology Development, Land & Housing Institute, Daejeon, 34047, Republic of Korea c Department of Science and Environmental Studies, The Education University of Hong Kong, New Territories, Tai Po, 999077, Hong Kong d School of Environmental Engineering, University of Seoul, Seoul, 02504, Republic of Korea e Department of Environmental and Safety Engineering, Ajou University, Suwon, 16499, 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 9 September 2019 Received in revised form 5 November 2019 Accepted 9 November 2019 Available online xxx
Digestate pyrolysis was investigated in this study. To establish a more sustainable pyrolysis platform, we particularly selected CO2 as a reactive gas medium. Thus, great emphasis was placed on clarifying the roles of CO2 in digestate pyrolysis. In addition, a series of thermo-gravimetric analysis (TGA) tests was conducted from 35 to 900 C (10 C min1) to characterize the thermolytic behaviors of digestate in CO2. The TGA tests demonstrated that the thermolytic patterns of digestate in both atmospheric environments (N2 and CO2) were the same at 780 C. However, the homogeneous reactions between volatile matter (VM) from digestate thermolysis and CO2 were not supported by the TGA tests. To confirm the homogeneous reactions, lab-scale digestate pyrolysis was conducted, which proved that the homogenous reactions initiated at 480 C in CO2 using one-stage and two-stage pyrolyzers. Pyrolytic products in three different phases were analyzed using micro-GC for pyrolytic gases, GC/MS for pyrolytic liquids, and ICP-OES and FE-SEM/EDX for solid residue. The homogeneous reactions resulted in enhanced CO generation while suppressing dehydrogenation. The identified CO2 role affected the compositional modifications of the pyrolytic oil, which was achieved via shifting the carbon distributions from the pyrolytic oil to gas. However, the reaction kinetics governing the CO2 roles was not rapid. To expedite the reaction kinetics of the CO2 roles, steel slag was used as a catalyst. Indeed, the use of steel slag enhanced the reaction kinetics of the homogeneous reactions. As a result, the non-catalytic pyrolysis conducted in the two-stage reactor evolved more gaseous products at comparable conditions. The use of CO2 and steel slag catalyst generated more pyrolytic gases for the pyrolysis of digestate. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Digestate Waste-to-energy Catalytic pyrolysis Carbon dioxide Biochar
1. Introduction Primary energy consumption worldwide has reached up to 535 EJ yr1 [1]. Based on the world’s population (7.6 billion in 2018), energy consumption per capita is approximately equivalent to 70 GJ yr1 [1]. This value (70 GJ yr1) implies that ~5.5 L of oil per capita is required daily [1]. Considering energy injustice [2], the amount of oil per capita in the developed countries may be much higher than ~5.5 L. Indeed, fossil fuels contribute ~80% of the global primary energy [3]. Despite the numerous socio-economic benefits from fossil fuel use [4], their massive consumption has inevitably
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Lee), [email protected] (E.E. Kwon). 1 These authors contributed equally to this study.
resulted in perturbation of the natural carbon cycle [5]. In short, CO2 emissions from fossil fuel use are exceeding the Earth’s full capacity to assimilate carbons, which poses global-scale environmental problems, such as global warming [6]. To abate the detrimental consequences from the carbon imbalance induced by the surplus carbon input (CO2), considerable research regarding renewable energy has been completed [7e11]. In particular, biogas (i.e. a mixture of CO2 and CH4) production from organic wastes via the anaerobic digestion (AD) process has been widely practiced because of its carbon neutrality and technical completeness relative to other renewable energy technologies [12,13]. As such, many countries have commercialized the AD process [12]; the total number of AD plants has noticeably increased [14]. In the European Union, the number of biogas plants dramatically increased from 10,433 in 2010 to 17,240 in 2014 [15]. The AD process is the fermentation process that converts organic wastes into biogas via four consecutive biological reactions
https://doi.org/10.1016/j.energy.2019.116529 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
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(i.e. hydrolysis, acidogenesis, acetogenesis, and methanogenesis) [16]. Various carbon substrates (livestock manure, food wastes, food waste leachate, sewage sludge, etc.) are used during the AD process [17e19]. In addition, the final digestion residue (i.e. digestate) has been used as a fertilizer in agricultural practices [20]. However, there many environmental concerns (odor control, transportation cost, pathogen, heavy metal (loid) contamination, etc.) remain in the further use of digestate as a fertilizer [21]. In detail, direct use of digestate as a fertilizer is problematic in terms of ammonia (NH3) emission and odor control [22]. Recognition of offensive odorants and the resulting nuisance have also been considered as among the most serious issues triggering public complaints [23]. Indeed, odors remain one of the top three complaints to air quality regulators and government bodies in different countries [24]. Moreover, 70% of the N source in digestate is emitted in the form of NH3 when used as a fertilizer [25], degrading air quality as NH3 plays a critical role in the formation of particulate matter (PM) via complex photo-induced reactions [26]. NH3 is among the well-known greenhouse gases [27]. In addition, heavy metal (loid)s, organic pollutants, and pathogens in digestate can be potential hazards in the direct use of digestate as a fertilizer [21,28]. Alternatively, digestate is also incinerated [29]; however, air pollution controls during incineration are difficult to implement because of the large amount of volatile matter (VM) in the digestate [30]. Thus, it is desirable to develop an environmentally benign technology for digestate disposal. Preferably, energy recovery during digestate disposal would be desirable [31]. As such, digestate pyrolysis offers an effective means of reducing its volume while recovering energy [32e34]. Pyrolysis is among the proven fuel processing technologies that reallocate carbons in carbonaceous materials into three pyrogenic products: syngas (H2 and CO), pyrolytic oil, and char [35,36]. Nevertheless, controlling the carbon distribution to modify the compositional matrix of three pyrogenic products is challenging in that it is sensitive to operational parameters [37,38] and the carbonaceous material physicochemical properties [37,39]. To seek a new means for manipulating the carbon distribution, we used CO2 as reactive gas medium. In a systematic investigation into the possible use of CO2 to manipulate the carbon distribution during digestate pyrolysis, the thermolysis of digestate in CO2 was thermo-gravimetrically characterized. To elucidate the effectiveness of CO2 during digestate pyrolysis, all the pyrogenic products from lab-scale pyrolysis were characterized using online gas chromatography (GC) for gaseous products, GC/mass spectrometry for liquid products, and inductively coupled plasma-optical emission spectrometry (ICP-OES) and field emission-scanning electron microscope/energy dispersive Xray spectroscope (FE-SEM/EDX) for char. For conducting the labscale non-catalytic and catalytic pyrolysis of digestate, one-stage and two-stage pyrolysis reactor setups [40] were used to more carefully elucidate the effects of CO2 and steel slag catalyst on the pyrolysis. All experimental results were referenced to those from the inert condition (i.e., N2) to elucidate and/or evaluate the effectiveness of CO2 and steel slag catalyst. Lastly, to enhance the CO2 effectiveness during digestate pyrolysis, the identified roles of CO2 were further evaluated by employing steel slag as a catalyst. 2. Materials and methods 2.1. Sample preparation and chemical agents Digestate from the AD process was collected from the Jungnang Water Reclamation Center (37.557650, 127.065363) in Korea. The collected digestate sample was dried at 80 C for 2 d. The ultimate analysis of the dried digestate sample was conducted using an organic elemental analyzer (Vario MACRO cube, Elementar
Analysensysteme GmbH, Germany). Percentages of 32.09 wt% of C, 5.25 wt% of H, 4.46 wt% of N, 17.01 wt% of O, and 1.51 wt% of S were determined via the ultimate analysis of digestate. Steel slag was obtained from POSCO in Korea and its metal compositions provided. The compositional matrix of metals in the slag is summarized in Table 1. Methanol (99.9% purity) (Prod. # 34860), dichloromethane (99.9% purity) (Prod. # 270997), nitric acid (70.0% purity) (Prod. #438073) and silica (Prod. # 243981) were purchased from Sigma Aldrich (St. Louis, MO, USA). Ultra-high purity gases (99.999% purity) of N2, CO2, and air were purchased from Green Gas Co. (Seoul, Republic of Korea). 2.2. Thermo-gravimetric analysis test of digestate Digestate thermolysis under various temperatures was monitored using a thermo-gravimetric analysis (TGA) unit (STA 449 F5 Jupiter, Netzsch, Germany). A total of 10 ± 0.01 mg of the dried digestate sample was loaded in an alumina crucible. A TGA test of the digestate was conducted at a heating rate of 10 C min1 from 20 to 900 C. To ensure reproducibility, the TGA test was performed in triplicate. The N2 or CO2 flow rates (70 mL min1) were controlled via the imbedded mass flow controllers (MFCs) in the TGA unit. Prior to each TGA test, a blank TGA run (i.e. with no sample loading) was conducted to eliminate the buoyancy effects arising from the density variations in the N2 or CO2. Note that the buoyancy effects were attributed to the density variations from the thermal expansion of the N2 or CO2 during the TGA test. 2.3. Laboratory-scale digestate pyrolysis using a batch-type tubular reactor A batch-type fixed-bed tubular reactor (TR) was assembled. Quartz tubing (Chemglass CGQ-0800T-68) was chosen as the main body of the TR reactor to avoid catalytic effects. The dimensions of the TR were 25.4 mm in outer diameter, 23 mm in inner diameter, and 1.2 m in length. To equip the gas inlet and outlet systems and monitor the inner temperature of the TR using a thermocouple (Ktype, Omega, USA), two stainless Ultra-Torr Vacuum Fittings (Swagelok SS-4-UT-6-600, USA) were connected to both ends of the TR. Two stainless step-down unions (25.4 mme6.35 mm) were also connected with two Ultra-Torr Vacuum fittings. The gas N2 or CO2 flow rates were set as 300 mL min1 and controlled using a mass flow controller (5850 Series E, Brook Instrument, USA). A tubular furnace (RD30/200/11, Nabertherm, USA) was used to control the
Table 1 Determination of metal species in the steel slag. Analyte
Composition (wt.%)
Na Mg Al Si P S Cl K Ca Ti V Cr Mn Fe Cu Zn Sr Ba
00.201 03.120 08.150 12.700 00.273 05.270 00.141 00.049 65.100 00.136 00.048 00.022 00.259 04.490 00.009 00.007 00.029 00.020
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experimental temperatures. The tubular furnace was comprised of two separated heating elements each was individually controlled. During the one-stage pyrolysis experiment, two heating zones were simultaneously controlled. A total of 2.0 ± 0.01 g of digestate was loaded at the center of the first heating zone in the furnace. During the two-stage pyrolysis experiment, two heating zones were individually controlled. In detail, the first heating zone was controlled at a heating rate of 10 C min1 and the isothermal mode was chosen in the second heating zone. Steel slag blended with silica was placed in the center of the second heating zone in the furnace. The gaseous effluents from the TR were quantified using an on-line GC system (3000 A micro-GC, Inficon, Switzerland). Prior to GC measurement, the condensable hydrocarbon species were separated using a cold trap cooled using liquid N2. The mass balance was established by measuring the biochar mass and the condensable hydrocarbon species. Mass for the gaseous effluents was estimated by subtracting the total mass of the biochar and the condensable hydrocarbon species. 2.4. Quantification, qualification, and characterization of pyrogenic products Quantification of the gaseous effluents from the TR, such as H2, CH4, CO, and C2-hydrocarbon species, was conducted using the micro-GC unit (3000 A micro-GC, Inficon, Switzerland). Multiple calibrations were performed using the gas standard (Scotty Analyzed Gases, INFICON, Switzerland). Identification of the condensable hydrocarbon species was conducted using a gas chromatography mass spectrometry (GC/ MS) unit (Clarus 500, PerkinElmer, USA). Note that an Elite-5MS (30 m 0.25 mm 0.25 mm) (Cat. N9316282, PerkinElmer, USA) column was used. The GC/MS operation conditions are follows. Oven temperature was started from 40 C (held for 10 min), followed by increasing at a ramping rate of 4 C min1 to 300 C (held for 25 min). Total analysis time was 35 min. Sample injection volume was 1 mL. Carrier gas flow was set to 21 mL min1. The NIST reference library (version 2.2) was used to identify each species. To determine the metal speciations in the digestate and biochar,
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an ICP-OES unit (Optima 5300 DV, PerkinElmer, USA) was used. Prior to the metal speciations, metal extraction in the digestate was conducted via a microwave digestion system (Ethos 1600, Milestone, Italy) using nitric acid (Table 2). In addition, a toxicity characteristic leaching procedure (TCLP) for biochar was completed at pH 4.88 and 30 rpm for 18 h [41]. To visualize the biochar surface morphology, a FE-SEM/EDX (Hitachi S-4700) was used. 3. Results and discussion 3.1. Thermo-gravimetric analysis of digestate in CO2 A series of TGA tests was conducted to characterize the digestate thermolysis providing fundamental information regarding the mass change of the analyte as a function of the thermolytic temperature. Note that a digestate TGA test in an inert condition (N2) was performed as a reference to discern any differences in the thermolytic behavior induced by CO2. Thus, the digestate mass decay as a function of the thermolytic temperature in the CO2 in reference to N2 is shown in Fig. 1a. In addition, the digestate thermal degradation rate depicted as a differential thermogram (DTG) was incorporated as shown in Fig. 1a to effectively discern the digestate mass decay patterns. Fig. 1a shows that the mass decay patterns of digestate in N2 and Table 2 Quantification and identification of digestate metal species. Analyte
Composition (wt.%)
Ba K Na Zn Ca Fe Al Cu Mn Mg
00.084 00.407 00.220 00.225 03.931 02.042 12.627 00.110 00.073 00.677
Fig. 1. (a) Digestate ass decay as a function of the thermolytic temperature in N2 (black color) and CO2 (red color) and its thermal degradation rate. (b) Digestate mass decay in air (blue color) and its thermal degradation rate.
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CO2 at 780 C are identical. The digestate thermal degradation under both atmospheric conditions as shown in Fig. 1a starts at 50 C, likely because of moisture volatilization in the sample. However, as noted, the digestate sample was dried at 80 C for 2 d. Thus, digestate mass decay at 50 C is also indicative of its inferior thermal stability relative to terrestrial biomass, such as lignocellulosic biomass [42]. Such a data interpretation is plausible in that the thermal stability of a cell membrane is inferior to that of a cell wall [43]. For example, lignocellulosic biomass thermolysis starts at 250 C [44]. Note that digestate is a bacterial colony and bacteria does not have a cell wall. Identical mass decay patterns under both atmospheric conditions offer several implications. First, the identical mass decay patterns shown in Fig. 1a imply no reaction between the digestate and CO2. Any reactions between the digestate and CO2 result in differences in the digestate thermolytic kinetics. Indeed, different thermolytic patterns resulting from the interactions between CO2 and digestate were not observed as shown in Fig. 1a. Second, no differences in the TGA result also suggest that any influence of CO2 is not initiated at 780 C. However,
homogeneous reactions between volatile matter (VM) from digestate thermolysis and CO2 cannot be elucidated via the TGA test because it only provides the digestate mass change under various temperatures. The thermal degradation rates of digestate begin to decrease at 400 C, indirectly suggesting that carbonization (biochar formation) via dehydrogenation is initiated [45,46]. Unfortunately, the digestate TGA test does not offer a key clue of dehydrogenation [47]. At 780 C, the digestate mass decay patterns in CO2 in reference to N2 are different. As shown in Fig. 1a, greater mass conversion (i.e. the less residual mass) via digestate thermolysis was achieved in the CO2 environment. The greater mass conversion shown in Fig. 1a is attributed to the Boudouard reaction (C þ CO2 $ 2CO) because the Gibbs free energy of the Boudouard reaction is less than zero at 720 C [48]. As shown, the Boudouard reaction initiates at 780 C and mass decay via the reaction continuously occurs up to 900 C. This observation implies that the conversion of carbon into CO via the Boudouard reaction is not complete. Accordingly, the continuous mass loss shows that the Boudouard
Fig. 2. H2, CH4, CO, C2H6, C2H4, and C2H2 concentration profiles evolved from lab-scale digestate pyrolysis in N2 (black color) and CO2 (red color) (Digestate pyrolysis (2.0 ± 0.01 g) is conducted at a heating rate of 10 C min1 from 30 to 720 C).
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reaction kinetics is slow [48]. Considering a 10 C min1 heating rate, mass loss via the Boudouard reaction for 12 min is equivalent to 4.3 wt% of the original sample mass. To gain an insight into the slow reaction kinetics of the Boudouard reaction, a digestate TGA test in air was conducted. As shown in Fig. 1b, the final residual mass in air is 39 wt% of the original sample mass. Fig. 1a shows that the final residual masses from the TGA test in N2 and CO2 are 45.5 and 41.2 wt% of the original sample mass, respectively. Fig. 1a and b suggest that ~66% of the carbon in the charring material (i.e. biochar) are converted into CO via the Boudouard reaction. Such a low conversion of carbon in biochar strongly suggests that the reaction kinetics of the Boudouard reaction is slow. As shown in Fig. 1, the snapshots of the final residues in CO2 and air differ. The final residue from the TGA test in air was identified as inorganic substances, such as soil. Thus, biochar obtained from the digestate TGA test in CO2 can be referred to as the carbon-soil composite. Lastly, the TGA test suggests that most carbon in the digestate can be recovered as syngas or pyrolytic oil through the pyrolysis process. 3.2. Lab-scale digestate pyrolysis in CO2 As discussed, the digestate TGA test did not confirm the occurrence of homogeneous reactions between VM and CO2. For further investigation, lab-scale digestate pyrolysis was conducted. Except for the sample loading (2.0 ± 0.01 g of digestate) and flow rate (300 mL min1) of N2 or CO, the other experimental parameters were identical to those of the TGA test. The gaseous effluents from the TR were monitored and their concentration profiles as a function of thermolytic temperature were plotted as shown in Fig. 2. To avoid any influence on the compositional modifications of the gaseous effluents via the Boudouard reaction, the final temperature for the lab-scale digestate pyrolysis was intentionally limited to 720 C. As noted, the digestate TGA test shown in Fig. 1a confirmed that the Boudouard reaction initiated at 780 C in the CO2
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environment. The interesting observations shown in Fig. 2 are that the gas evolution of the major pyrolytic gases starts at 270 C. As reported, mass decay from the digestate TGA test starts at 50 C. Mass loss from 50 to 270 C from the digestate TGA test is equivalent to 15 wt% of the original sample mass. Thus, no evolution of the major pyrolytic gases suggests that the digestate contains a large number of VM. In addition, the gas evolutionary patterns from the digestate pyrolysis at 270 C in the N2 well reflect the typical biomass thermolytic behavior. CH4 and CO began to form at 300 and 270 C, respectively. Their formations are likely because of bond scissions from the digestate polymeric backbone [49]. Indeed, the same data interpretations (i.e. random bond scissions) for the formation of C2-hydrocarbon species have been made [50], but their concentrations in reference to H2, CH4, and CO are not comparable. In addition, the H2 evolutionary patterns show the typical biomass thermolytic patterns in that dehydrogenation results in H2 formation and the degree of dehydrogenation is proportional to the thermolytic temperature [48,51]. Such evolutionary trends are well reflected in Fig. 2. In detail, H2formation starts at 400 C. Fig. 2 shows that the H2 concentration began to increase as the thermolytic temperature increased. However, the dehydrogenation reaction kinetics at 600 C began to decrease. Such an observation is likely because of H source depletion during a batch test. The temperature initiating dehydrogenation is consistent with the aforementioned discussions as shown in Fig. 1a. As noted, the digestate thermolytic rate began to decrease at 400 C. We assumed that such observations indirectly show carbonization (i.e. biochar formation) via dehydrogenation. However, the gas evolutionary patterns in the digestate pyrolysis in the CO2 environment are different from those in the N2 environment. Specifically, CO formation during the digestate pyrolysis at 480 C sharply increased. At 480 C, H2 formation was also restricted. Such observations are discrepant with the general
Fig. 3. Identification of the chemical species in pyrolytic oil derived from digestate pyrolysis in N2 (black color) and CO2 (red color) using one-stage reactor. The information of each species shown in the figure is given in Supporting Information (Table S1). Similarity with the GC/MS library was higher than 80% for all identified compounds.
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thermolytic phenomena of biomass. As noted, dehydrogenation is proportional to the thermolytic temperature. However, Fig. 2 shows that CO2 suppresses dehydrogenation while enhancing t CO formation. Under these circumstances, the mass decay patterns from
the digestate TGA tests (Fig. 1a) at 780 C in N2 and CO2 were nearly the same. Therefore, such gas evolutionary patterns in CO2 are likely attributed to the hypothesized homogeneous interactions between VM and CO2. In other words, the enhanced CO generation
Fig. 4. Soil-carbon composite morphology derived from digestate pyrolysis at 640 C in (a) N2 and (b) CO2.
Fig. 5. H2, CH4, CO, C2H6, C2H4, and C2H2 concentration profiles from two-stage digestate pyrolysis in N2 (black color) and CO2 (red color).
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is only possible in the presence of an additional O source. Considering the experimental setup, the possible O source seems to be CO2. Accordingly, the homogeneous reactions between VM and CO2 results in the enhanced CO generation. Thus, CO2 plays a key role in consuming carbon while providing O to form CO via homogeneous reactions between VM and CO2. As a result, carbon consumption by CO2 to form CO subsequently leads to suppressed dehydrogenation in that carbon vacancies via the homogeneous reactions play a critical role in decreasing opportunities for dehydrogenation. The same explanations are applicable to the low C2-hydrocarbon species concentrations in the CO2 environment. Restricting dehydrogenation via CO2 while enhancing CO formation offers a new means for shifting the carbon distribution between the pyrogenic products. Note that the final residual mass from the digestate TGA test in N2 and CO2 at 780 C was the same. Accordingly, dehydrogenation restriction offers a means for lowering aromaticity, which subsequently results in pyrolytic oil compositional modifications. To confirm this, collected pyrolytic oil was diluted with the same amount of dichloromethane; identification of the chemical species in the pyrolytic oil is shown in chromatograms (Fig. 3). Also shown in Fig. 3 is that the compositional matrix of the pyrolytic oil from N2 in reference to CO2 is more complex, and thus their concentrations are substantially higher. Therefore, Fig. 3 shows that the pyrolytic oil compositional matrix is influenced by CO2, confirming the aforementioned hypothesis in line with shifting the carbon distribution between pyrolytic oil and pyrolytic gas. All experimental findings from the lab-scale digestate pyrolysis confirmed that CO2 played a key role in modifying the compositional matrix of the pyrolytic oil and syngas through the homogeneous reactions between VM and CO2. In addition, the digestate TGA tests at 780 C in N2 and CO2 were identical. Therefore, the biochar morphology (i.e. the carbon-soil composite) from the N2 and CO2 environments should be similar. To this end, the biochar
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morphology is shown in Fig. 4. As illustrated, the surface morphology of the carbon-soil composites from the digestate pyrolysis at 640 C in the N2 and CO2 environments are not different. To further investigate, BrunauereEmmetteTeller/BarretteJoynereHalenda (BET/BJH) measurements were conducted, but the surface area and the average pore distribution of the both the composites were similar. Accordingly, the very similar morphology of the carbon-soil composite also confirms the aforementioned homogeneous reactions between VM and CO2. However, given that most VM from digestate thermolysis evolves at 500 C, it is desirable to seek reasonable explanations for the lack of homogeneous reactions at 480 C as shown in Fig. 2. 3.3. Two-stage digestate pyrolysis in the CO2 environment As noted, VM evolution from digestate thermolysis was mostly complete at 500 C. Nevertheless, enhanced CO generation through the homogeneous reactions shown in Fig. 2 was initiated at 480 C. Such observations strongly suggest that the reaction kinetics leading the CO2 roles is highly contingent on the experimental temperature. To gain an insight on the reaction kinetics initiating these roles, two-stage lab-scale digestate pyrolysis was conducted. As noted, the tubular furnace used in this study was comprised of two separate heating elements. During the two-stage digestate pyrolysis, two heating zones were individually controlled. In detail, the first heating zone was controlled at a 10 C min1 heating rate from 30 to 720 C and an isothermal (640 C) condition was maintained in the second heating zone. A temperature of 640 C was chosen as the homogeneous reactions between VMs and CO2 to form CO initiate at 480 C. The gaseous effluents from the TR were monitored and their concentration profiles as a function of thermolytic temperature were plotted as shown in Fig. 5. As shown in Fig. 5, enhanced CO generation in the CO2 environment was observed, but the expected enhancement of CO by
Fig. 6. Identification of chemical species in the pyrolytic oil derived from digestate pyrolysis in N2 (black color) and CO2 (red color) using two-stage reactor. The information of each species shown in the figure is given in Supporting Information (Table S2). Similarity with the GC/MS library was higher than 80% for all identified compounds.
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CO2 is not discernible. Such observations suggest two implications in line with the CO2 roles during digestate pyrolysis. First, the expected CO2 roles are not initiated because of a heat transfer delay. Second, the reaction kinetics for initiating the expected roles are not sufficiently rapid to form CO. Despite the similar CO evolutionary patterns at 440 C in both environments, one of the interesting observations shown in Fig. 5 is the H2 concentration. For example, dehydrogenation suppression in the CO2 environment is indeed discernible. Thus, such observations suggest that the reaction kinetics for suppressing dehydrogenation via CO2 is faster than that for enhancing CO formation. To further investigate, collected pyrolytic oil was diluted with the same amount of dichloromethane, and identification of the chemical species in the pyrolytic oil derived from two-stage digestate pyrolysis was visualized using chromatograms. As shown in Fig. 6, the concentration of each chemical species is substantially reduced in the CO2 environment. Interestingly, Fig. 6 shows that polycyclic aromatic hydrocarbon (PAH) formation is substantially restricted in the CO2 environment. Considering that the PAH formation is closely related to aromaticity, dehydrogenation suppression by CO2 likely impedes PAH
formation [52e54]. To enhance the reaction kinetics of the expected CO2 roles, steel slag was loaded in the second heating zone. As reported in Table 1, steel slag contains a large number of metal species, such as Fe, Mn, etc. We assumed that the catalytic capabilities are being imparted by such metal species. All experimental parameters are identical in the case of Fig. 5. The gaseous effluents from the TR were monitored and their concentration profiles as a function of the thermolytic temperature were plotted as shown in Fig. 7. As shown in Fig. 7, the CO2 roles are remarkably enhanced in the presence of steel slag. Thus, Fig. 7 shows that the reaction kinetics governing the CO2 roles seems to be catalytically expedited. In parallel, all experimental findings shown in Fig. 7 strongly suggest that the insignificant CO enhancement shown in Fig. 5 is mainly attributed to the slow reaction kinetics governing the CO2 roles. In reference to H2 and CO, the C2-hydrocarbon species concentrations are not substantial even in the CO2 environment. To further investigate the CO2 roles, the overall mass balance from digestate pyrolysis (Figs. 2, 5 and 6) is shown in Fig. 8. As well reflected in Fig. 8, the char mass portion (i.e., carbon-soil composite) is very
Fig. 7. H2, CH4, CO, C2H6, C2H4, and C2H2 concentration profiles from two-stage digestate pyrolysis using steel slag as a catalyst in N2 (black color) and CO2 (red color).
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Fig. 8. Overall mass balance for pyrogenic products from (a) one-stage, (b) two-stage, and (c) two-stage with steel slag digestate pyrolysis in N2 and CO2 atmospheres.
similar. However, the pyrolytic oil mass portion in the CO2 environment is significantly reduced in the three pyrolytic cases. Furthermore, the CO2 roles (i.e. shifting the carbon distribution from pyrolytic oil to pyrolytic gas via the homogeneous reactions between VM evolved from digestate thermolysis and CO2 while suppressing dehydrogenation) are quite consistent with the experimental results shown in Fig. 8. Lastly, TCLP tests of carbonsoil composites were also conducted to determine metal leaching. However, metal leaching was not identified from the TCLP tests of carbon-soil composites. Note that the metal species in the digestate are summarized in Table 2.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2018R1A2B2001121). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.116529. References
4. Conclusions This study investigated the role of CO2 as a reactive gas medium during digestate pyrolysis. All experimental observations confirmed that CO2 roles occurred via a gas phase reaction (i.e. homogeneous reactions between VM evolved from digestate pyrolysis and CO2). The main CO2 roles during digestate pyrolysis resulted in enhanced CO generation while suppressing dehydrogenation. Such an observation also suggests shifting the carbon distribution from pyrolytic oil to pyrolytic gas was only achieved at 480 C by CO2. However, the reaction kinetics governing the CO2 roles was not sufficiently rapid. To enhance the reaction kinetics associated with CO2 in digestate pyrolysis, we employed steel slag as a catalyst. In addition, the non-catalytic pyrolysis carried out in the two-stage reactor generated more pyrolytic gas at comparable conditions. The use of CO2 and steel slag as the catalyst also increased the evolution of gaseous products for the pyrolysis of digestate. This study experimentally demonstrated that steel slag imparted catalytic capability, which played a key role in expediting the reaction kinetics ruling the CO2 roles.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
CO2-assisted catalytic pyrolysis of digestate with steel slag Jung-Hun Kim a, 1, Jeong-Ik Oh b, 1, Yiu Fai Tsang c, Young-Kwon Park d, Jechan Lee e, *, Eilhann E. Kwon a, ** a
Department of Environment and Energy, Sejong University, Seoul, 05006, Republic of Korea Advanced Technology Development, Land & Housing Institute, Daejeon, 34047, Republic of Korea c Department of Science and Environmental Studies, The Education University of Hong Kong, New Territories, Tai Po, 999077, Hong Kong d School of Environmental Engineering, University of Seoul, Seoul, 02504, Republic of Korea e Department of Environmental and Safety Engineering, Ajou University, Suwon, 16499, 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 9 September 2019 Received in revised form 5 November 2019 Accepted 9 November 2019 Available online xxx
Digestate pyrolysis was investigated in this study. To establish a more sustainable pyrolysis platform, we particularly selected CO2 as a reactive gas medium. Thus, great emphasis was placed on clarifying the roles of CO2 in digestate pyrolysis. In addition, a series of thermo-gravimetric analysis (TGA) tests was conducted from 35 to 900 C (10 C min1) to characterize the thermolytic behaviors of digestate in CO2. The TGA tests demonstrated that the thermolytic patterns of digestate in both atmospheric environments (N2 and CO2) were the same at 780 C. However, the homogeneous reactions between volatile matter (VM) from digestate thermolysis and CO2 were not supported by the TGA tests. To confirm the homogeneous reactions, lab-scale digestate pyrolysis was conducted, which proved that the homogenous reactions initiated at 480 C in CO2 using one-stage and two-stage pyrolyzers. Pyrolytic products in three different phases were analyzed using micro-GC for pyrolytic gases, GC/MS for pyrolytic liquids, and ICP-OES and FE-SEM/EDX for solid residue. The homogeneous reactions resulted in enhanced CO generation while suppressing dehydrogenation. The identified CO2 role affected the compositional modifications of the pyrolytic oil, which was achieved via shifting the carbon distributions from the pyrolytic oil to gas. However, the reaction kinetics governing the CO2 roles was not rapid. To expedite the reaction kinetics of the CO2 roles, steel slag was used as a catalyst. Indeed, the use of steel slag enhanced the reaction kinetics of the homogeneous reactions. As a result, the non-catalytic pyrolysis conducted in the two-stage reactor evolved more gaseous products at comparable conditions. The use of CO2 and steel slag catalyst generated more pyrolytic gases for the pyrolysis of digestate. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Digestate Waste-to-energy Catalytic pyrolysis Carbon dioxide Biochar
1. Introduction Primary energy consumption worldwide has reached up to 535 EJ yr1 [1]. Based on the world’s population (7.6 billion in 2018), energy consumption per capita is approximately equivalent to 70 GJ yr1 [1]. This value (70 GJ yr1) implies that ~5.5 L of oil per capita is required daily [1]. Considering energy injustice [2], the amount of oil per capita in the developed countries may be much higher than ~5.5 L. Indeed, fossil fuels contribute ~80% of the global primary energy [3]. Despite the numerous socio-economic benefits from fossil fuel use [4], their massive consumption has inevitably
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Lee), [email protected] (E.E. Kwon). 1 These authors contributed equally to this study.
resulted in perturbation of the natural carbon cycle [5]. In short, CO2 emissions from fossil fuel use are exceeding the Earth’s full capacity to assimilate carbons, which poses global-scale environmental problems, such as global warming [6]. To abate the detrimental consequences from the carbon imbalance induced by the surplus carbon input (CO2), considerable research regarding renewable energy has been completed [7e11]. In particular, biogas (i.e. a mixture of CO2 and CH4) production from organic wastes via the anaerobic digestion (AD) process has been widely practiced because of its carbon neutrality and technical completeness relative to other renewable energy technologies [12,13]. As such, many countries have commercialized the AD process [12]; the total number of AD plants has noticeably increased [14]. In the European Union, the number of biogas plants dramatically increased from 10,433 in 2010 to 17,240 in 2014 [15]. The AD process is the fermentation process that converts organic wastes into biogas via four consecutive biological reactions
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(i.e. hydrolysis, acidogenesis, acetogenesis, and methanogenesis) [16]. Various carbon substrates (livestock manure, food wastes, food waste leachate, sewage sludge, etc.) are used during the AD process [17e19]. In addition, the final digestion residue (i.e. digestate) has been used as a fertilizer in agricultural practices [20]. However, there many environmental concerns (odor control, transportation cost, pathogen, heavy metal (loid) contamination, etc.) remain in the further use of digestate as a fertilizer [21]. In detail, direct use of digestate as a fertilizer is problematic in terms of ammonia (NH3) emission and odor control [22]. Recognition of offensive odorants and the resulting nuisance have also been considered as among the most serious issues triggering public complaints [23]. Indeed, odors remain one of the top three complaints to air quality regulators and government bodies in different countries [24]. Moreover, 70% of the N source in digestate is emitted in the form of NH3 when used as a fertilizer [25], degrading air quality as NH3 plays a critical role in the formation of particulate matter (PM) via complex photo-induced reactions [26]. NH3 is among the well-known greenhouse gases [27]. In addition, heavy metal (loid)s, organic pollutants, and pathogens in digestate can be potential hazards in the direct use of digestate as a fertilizer [21,28]. Alternatively, digestate is also incinerated [29]; however, air pollution controls during incineration are difficult to implement because of the large amount of volatile matter (VM) in the digestate [30]. Thus, it is desirable to develop an environmentally benign technology for digestate disposal. Preferably, energy recovery during digestate disposal would be desirable [31]. As such, digestate pyrolysis offers an effective means of reducing its volume while recovering energy [32e34]. Pyrolysis is among the proven fuel processing technologies that reallocate carbons in carbonaceous materials into three pyrogenic products: syngas (H2 and CO), pyrolytic oil, and char [35,36]. Nevertheless, controlling the carbon distribution to modify the compositional matrix of three pyrogenic products is challenging in that it is sensitive to operational parameters [37,38] and the carbonaceous material physicochemical properties [37,39]. To seek a new means for manipulating the carbon distribution, we used CO2 as reactive gas medium. In a systematic investigation into the possible use of CO2 to manipulate the carbon distribution during digestate pyrolysis, the thermolysis of digestate in CO2 was thermo-gravimetrically characterized. To elucidate the effectiveness of CO2 during digestate pyrolysis, all the pyrogenic products from lab-scale pyrolysis were characterized using online gas chromatography (GC) for gaseous products, GC/mass spectrometry for liquid products, and inductively coupled plasma-optical emission spectrometry (ICP-OES) and field emission-scanning electron microscope/energy dispersive Xray spectroscope (FE-SEM/EDX) for char. For conducting the labscale non-catalytic and catalytic pyrolysis of digestate, one-stage and two-stage pyrolysis reactor setups [40] were used to more carefully elucidate the effects of CO2 and steel slag catalyst on the pyrolysis. All experimental results were referenced to those from the inert condition (i.e., N2) to elucidate and/or evaluate the effectiveness of CO2 and steel slag catalyst. Lastly, to enhance the CO2 effectiveness during digestate pyrolysis, the identified roles of CO2 were further evaluated by employing steel slag as a catalyst. 2. Materials and methods 2.1. Sample preparation and chemical agents Digestate from the AD process was collected from the Jungnang Water Reclamation Center (37.557650, 127.065363) in Korea. The collected digestate sample was dried at 80 C for 2 d. The ultimate analysis of the dried digestate sample was conducted using an organic elemental analyzer (Vario MACRO cube, Elementar
Analysensysteme GmbH, Germany). Percentages of 32.09 wt% of C, 5.25 wt% of H, 4.46 wt% of N, 17.01 wt% of O, and 1.51 wt% of S were determined via the ultimate analysis of digestate. Steel slag was obtained from POSCO in Korea and its metal compositions provided. The compositional matrix of metals in the slag is summarized in Table 1. Methanol (99.9% purity) (Prod. # 34860), dichloromethane (99.9% purity) (Prod. # 270997), nitric acid (70.0% purity) (Prod. #438073) and silica (Prod. # 243981) were purchased from Sigma Aldrich (St. Louis, MO, USA). Ultra-high purity gases (99.999% purity) of N2, CO2, and air were purchased from Green Gas Co. (Seoul, Republic of Korea). 2.2. Thermo-gravimetric analysis test of digestate Digestate thermolysis under various temperatures was monitored using a thermo-gravimetric analysis (TGA) unit (STA 449 F5 Jupiter, Netzsch, Germany). A total of 10 ± 0.01 mg of the dried digestate sample was loaded in an alumina crucible. A TGA test of the digestate was conducted at a heating rate of 10 C min1 from 20 to 900 C. To ensure reproducibility, the TGA test was performed in triplicate. The N2 or CO2 flow rates (70 mL min1) were controlled via the imbedded mass flow controllers (MFCs) in the TGA unit. Prior to each TGA test, a blank TGA run (i.e. with no sample loading) was conducted to eliminate the buoyancy effects arising from the density variations in the N2 or CO2. Note that the buoyancy effects were attributed to the density variations from the thermal expansion of the N2 or CO2 during the TGA test. 2.3. Laboratory-scale digestate pyrolysis using a batch-type tubular reactor A batch-type fixed-bed tubular reactor (TR) was assembled. Quartz tubing (Chemglass CGQ-0800T-68) was chosen as the main body of the TR reactor to avoid catalytic effects. The dimensions of the TR were 25.4 mm in outer diameter, 23 mm in inner diameter, and 1.2 m in length. To equip the gas inlet and outlet systems and monitor the inner temperature of the TR using a thermocouple (Ktype, Omega, USA), two stainless Ultra-Torr Vacuum Fittings (Swagelok SS-4-UT-6-600, USA) were connected to both ends of the TR. Two stainless step-down unions (25.4 mme6.35 mm) were also connected with two Ultra-Torr Vacuum fittings. The gas N2 or CO2 flow rates were set as 300 mL min1 and controlled using a mass flow controller (5850 Series E, Brook Instrument, USA). A tubular furnace (RD30/200/11, Nabertherm, USA) was used to control the
Table 1 Determination of metal species in the steel slag. Analyte
Composition (wt.%)
Na Mg Al Si P S Cl K Ca Ti V Cr Mn Fe Cu Zn Sr Ba
00.201 03.120 08.150 12.700 00.273 05.270 00.141 00.049 65.100 00.136 00.048 00.022 00.259 04.490 00.009 00.007 00.029 00.020
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experimental temperatures. The tubular furnace was comprised of two separated heating elements each was individually controlled. During the one-stage pyrolysis experiment, two heating zones were simultaneously controlled. A total of 2.0 ± 0.01 g of digestate was loaded at the center of the first heating zone in the furnace. During the two-stage pyrolysis experiment, two heating zones were individually controlled. In detail, the first heating zone was controlled at a heating rate of 10 C min1 and the isothermal mode was chosen in the second heating zone. Steel slag blended with silica was placed in the center of the second heating zone in the furnace. The gaseous effluents from the TR were quantified using an on-line GC system (3000 A micro-GC, Inficon, Switzerland). Prior to GC measurement, the condensable hydrocarbon species were separated using a cold trap cooled using liquid N2. The mass balance was established by measuring the biochar mass and the condensable hydrocarbon species. Mass for the gaseous effluents was estimated by subtracting the total mass of the biochar and the condensable hydrocarbon species. 2.4. Quantification, qualification, and characterization of pyrogenic products Quantification of the gaseous effluents from the TR, such as H2, CH4, CO, and C2-hydrocarbon species, was conducted using the micro-GC unit (3000 A micro-GC, Inficon, Switzerland). Multiple calibrations were performed using the gas standard (Scotty Analyzed Gases, INFICON, Switzerland). Identification of the condensable hydrocarbon species was conducted using a gas chromatography mass spectrometry (GC/ MS) unit (Clarus 500, PerkinElmer, USA). Note that an Elite-5MS (30 m 0.25 mm 0.25 mm) (Cat. N9316282, PerkinElmer, USA) column was used. The GC/MS operation conditions are follows. Oven temperature was started from 40 C (held for 10 min), followed by increasing at a ramping rate of 4 C min1 to 300 C (held for 25 min). Total analysis time was 35 min. Sample injection volume was 1 mL. Carrier gas flow was set to 21 mL min1. The NIST reference library (version 2.2) was used to identify each species. To determine the metal speciations in the digestate and biochar,
3
an ICP-OES unit (Optima 5300 DV, PerkinElmer, USA) was used. Prior to the metal speciations, metal extraction in the digestate was conducted via a microwave digestion system (Ethos 1600, Milestone, Italy) using nitric acid (Table 2). In addition, a toxicity characteristic leaching procedure (TCLP) for biochar was completed at pH 4.88 and 30 rpm for 18 h [41]. To visualize the biochar surface morphology, a FE-SEM/EDX (Hitachi S-4700) was used. 3. Results and discussion 3.1. Thermo-gravimetric analysis of digestate in CO2 A series of TGA tests was conducted to characterize the digestate thermolysis providing fundamental information regarding the mass change of the analyte as a function of the thermolytic temperature. Note that a digestate TGA test in an inert condition (N2) was performed as a reference to discern any differences in the thermolytic behavior induced by CO2. Thus, the digestate mass decay as a function of the thermolytic temperature in the CO2 in reference to N2 is shown in Fig. 1a. In addition, the digestate thermal degradation rate depicted as a differential thermogram (DTG) was incorporated as shown in Fig. 1a to effectively discern the digestate mass decay patterns. Fig. 1a shows that the mass decay patterns of digestate in N2 and Table 2 Quantification and identification of digestate metal species. Analyte
Composition (wt.%)
Ba K Na Zn Ca Fe Al Cu Mn Mg
00.084 00.407 00.220 00.225 03.931 02.042 12.627 00.110 00.073 00.677
Fig. 1. (a) Digestate ass decay as a function of the thermolytic temperature in N2 (black color) and CO2 (red color) and its thermal degradation rate. (b) Digestate mass decay in air (blue color) and its thermal degradation rate.
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CO2 at 780 C are identical. The digestate thermal degradation under both atmospheric conditions as shown in Fig. 1a starts at 50 C, likely because of moisture volatilization in the sample. However, as noted, the digestate sample was dried at 80 C for 2 d. Thus, digestate mass decay at 50 C is also indicative of its inferior thermal stability relative to terrestrial biomass, such as lignocellulosic biomass [42]. Such a data interpretation is plausible in that the thermal stability of a cell membrane is inferior to that of a cell wall [43]. For example, lignocellulosic biomass thermolysis starts at 250 C [44]. Note that digestate is a bacterial colony and bacteria does not have a cell wall. Identical mass decay patterns under both atmospheric conditions offer several implications. First, the identical mass decay patterns shown in Fig. 1a imply no reaction between the digestate and CO2. Any reactions between the digestate and CO2 result in differences in the digestate thermolytic kinetics. Indeed, different thermolytic patterns resulting from the interactions between CO2 and digestate were not observed as shown in Fig. 1a. Second, no differences in the TGA result also suggest that any influence of CO2 is not initiated at 780 C. However,
homogeneous reactions between volatile matter (VM) from digestate thermolysis and CO2 cannot be elucidated via the TGA test because it only provides the digestate mass change under various temperatures. The thermal degradation rates of digestate begin to decrease at 400 C, indirectly suggesting that carbonization (biochar formation) via dehydrogenation is initiated [45,46]. Unfortunately, the digestate TGA test does not offer a key clue of dehydrogenation [47]. At 780 C, the digestate mass decay patterns in CO2 in reference to N2 are different. As shown in Fig. 1a, greater mass conversion (i.e. the less residual mass) via digestate thermolysis was achieved in the CO2 environment. The greater mass conversion shown in Fig. 1a is attributed to the Boudouard reaction (C þ CO2 $ 2CO) because the Gibbs free energy of the Boudouard reaction is less than zero at 720 C [48]. As shown, the Boudouard reaction initiates at 780 C and mass decay via the reaction continuously occurs up to 900 C. This observation implies that the conversion of carbon into CO via the Boudouard reaction is not complete. Accordingly, the continuous mass loss shows that the Boudouard
Fig. 2. H2, CH4, CO, C2H6, C2H4, and C2H2 concentration profiles evolved from lab-scale digestate pyrolysis in N2 (black color) and CO2 (red color) (Digestate pyrolysis (2.0 ± 0.01 g) is conducted at a heating rate of 10 C min1 from 30 to 720 C).
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reaction kinetics is slow [48]. Considering a 10 C min1 heating rate, mass loss via the Boudouard reaction for 12 min is equivalent to 4.3 wt% of the original sample mass. To gain an insight into the slow reaction kinetics of the Boudouard reaction, a digestate TGA test in air was conducted. As shown in Fig. 1b, the final residual mass in air is 39 wt% of the original sample mass. Fig. 1a shows that the final residual masses from the TGA test in N2 and CO2 are 45.5 and 41.2 wt% of the original sample mass, respectively. Fig. 1a and b suggest that ~66% of the carbon in the charring material (i.e. biochar) are converted into CO via the Boudouard reaction. Such a low conversion of carbon in biochar strongly suggests that the reaction kinetics of the Boudouard reaction is slow. As shown in Fig. 1, the snapshots of the final residues in CO2 and air differ. The final residue from the TGA test in air was identified as inorganic substances, such as soil. Thus, biochar obtained from the digestate TGA test in CO2 can be referred to as the carbon-soil composite. Lastly, the TGA test suggests that most carbon in the digestate can be recovered as syngas or pyrolytic oil through the pyrolysis process. 3.2. Lab-scale digestate pyrolysis in CO2 As discussed, the digestate TGA test did not confirm the occurrence of homogeneous reactions between VM and CO2. For further investigation, lab-scale digestate pyrolysis was conducted. Except for the sample loading (2.0 ± 0.01 g of digestate) and flow rate (300 mL min1) of N2 or CO, the other experimental parameters were identical to those of the TGA test. The gaseous effluents from the TR were monitored and their concentration profiles as a function of thermolytic temperature were plotted as shown in Fig. 2. To avoid any influence on the compositional modifications of the gaseous effluents via the Boudouard reaction, the final temperature for the lab-scale digestate pyrolysis was intentionally limited to 720 C. As noted, the digestate TGA test shown in Fig. 1a confirmed that the Boudouard reaction initiated at 780 C in the CO2
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environment. The interesting observations shown in Fig. 2 are that the gas evolution of the major pyrolytic gases starts at 270 C. As reported, mass decay from the digestate TGA test starts at 50 C. Mass loss from 50 to 270 C from the digestate TGA test is equivalent to 15 wt% of the original sample mass. Thus, no evolution of the major pyrolytic gases suggests that the digestate contains a large number of VM. In addition, the gas evolutionary patterns from the digestate pyrolysis at 270 C in the N2 well reflect the typical biomass thermolytic behavior. CH4 and CO began to form at 300 and 270 C, respectively. Their formations are likely because of bond scissions from the digestate polymeric backbone [49]. Indeed, the same data interpretations (i.e. random bond scissions) for the formation of C2-hydrocarbon species have been made [50], but their concentrations in reference to H2, CH4, and CO are not comparable. In addition, the H2 evolutionary patterns show the typical biomass thermolytic patterns in that dehydrogenation results in H2 formation and the degree of dehydrogenation is proportional to the thermolytic temperature [48,51]. Such evolutionary trends are well reflected in Fig. 2. In detail, H2formation starts at 400 C. Fig. 2 shows that the H2 concentration began to increase as the thermolytic temperature increased. However, the dehydrogenation reaction kinetics at 600 C began to decrease. Such an observation is likely because of H source depletion during a batch test. The temperature initiating dehydrogenation is consistent with the aforementioned discussions as shown in Fig. 1a. As noted, the digestate thermolytic rate began to decrease at 400 C. We assumed that such observations indirectly show carbonization (i.e. biochar formation) via dehydrogenation. However, the gas evolutionary patterns in the digestate pyrolysis in the CO2 environment are different from those in the N2 environment. Specifically, CO formation during the digestate pyrolysis at 480 C sharply increased. At 480 C, H2 formation was also restricted. Such observations are discrepant with the general
Fig. 3. Identification of the chemical species in pyrolytic oil derived from digestate pyrolysis in N2 (black color) and CO2 (red color) using one-stage reactor. The information of each species shown in the figure is given in Supporting Information (Table S1). Similarity with the GC/MS library was higher than 80% for all identified compounds.
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thermolytic phenomena of biomass. As noted, dehydrogenation is proportional to the thermolytic temperature. However, Fig. 2 shows that CO2 suppresses dehydrogenation while enhancing t CO formation. Under these circumstances, the mass decay patterns from
the digestate TGA tests (Fig. 1a) at 780 C in N2 and CO2 were nearly the same. Therefore, such gas evolutionary patterns in CO2 are likely attributed to the hypothesized homogeneous interactions between VM and CO2. In other words, the enhanced CO generation
Fig. 4. Soil-carbon composite morphology derived from digestate pyrolysis at 640 C in (a) N2 and (b) CO2.
Fig. 5. H2, CH4, CO, C2H6, C2H4, and C2H2 concentration profiles from two-stage digestate pyrolysis in N2 (black color) and CO2 (red color).
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is only possible in the presence of an additional O source. Considering the experimental setup, the possible O source seems to be CO2. Accordingly, the homogeneous reactions between VM and CO2 results in the enhanced CO generation. Thus, CO2 plays a key role in consuming carbon while providing O to form CO via homogeneous reactions between VM and CO2. As a result, carbon consumption by CO2 to form CO subsequently leads to suppressed dehydrogenation in that carbon vacancies via the homogeneous reactions play a critical role in decreasing opportunities for dehydrogenation. The same explanations are applicable to the low C2-hydrocarbon species concentrations in the CO2 environment. Restricting dehydrogenation via CO2 while enhancing CO formation offers a new means for shifting the carbon distribution between the pyrogenic products. Note that the final residual mass from the digestate TGA test in N2 and CO2 at 780 C was the same. Accordingly, dehydrogenation restriction offers a means for lowering aromaticity, which subsequently results in pyrolytic oil compositional modifications. To confirm this, collected pyrolytic oil was diluted with the same amount of dichloromethane; identification of the chemical species in the pyrolytic oil is shown in chromatograms (Fig. 3). Also shown in Fig. 3 is that the compositional matrix of the pyrolytic oil from N2 in reference to CO2 is more complex, and thus their concentrations are substantially higher. Therefore, Fig. 3 shows that the pyrolytic oil compositional matrix is influenced by CO2, confirming the aforementioned hypothesis in line with shifting the carbon distribution between pyrolytic oil and pyrolytic gas. All experimental findings from the lab-scale digestate pyrolysis confirmed that CO2 played a key role in modifying the compositional matrix of the pyrolytic oil and syngas through the homogeneous reactions between VM and CO2. In addition, the digestate TGA tests at 780 C in N2 and CO2 were identical. Therefore, the biochar morphology (i.e. the carbon-soil composite) from the N2 and CO2 environments should be similar. To this end, the biochar
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morphology is shown in Fig. 4. As illustrated, the surface morphology of the carbon-soil composites from the digestate pyrolysis at 640 C in the N2 and CO2 environments are not different. To further investigate, BrunauereEmmetteTeller/BarretteJoynereHalenda (BET/BJH) measurements were conducted, but the surface area and the average pore distribution of the both the composites were similar. Accordingly, the very similar morphology of the carbon-soil composite also confirms the aforementioned homogeneous reactions between VM and CO2. However, given that most VM from digestate thermolysis evolves at 500 C, it is desirable to seek reasonable explanations for the lack of homogeneous reactions at 480 C as shown in Fig. 2. 3.3. Two-stage digestate pyrolysis in the CO2 environment As noted, VM evolution from digestate thermolysis was mostly complete at 500 C. Nevertheless, enhanced CO generation through the homogeneous reactions shown in Fig. 2 was initiated at 480 C. Such observations strongly suggest that the reaction kinetics leading the CO2 roles is highly contingent on the experimental temperature. To gain an insight on the reaction kinetics initiating these roles, two-stage lab-scale digestate pyrolysis was conducted. As noted, the tubular furnace used in this study was comprised of two separate heating elements. During the two-stage digestate pyrolysis, two heating zones were individually controlled. In detail, the first heating zone was controlled at a 10 C min1 heating rate from 30 to 720 C and an isothermal (640 C) condition was maintained in the second heating zone. A temperature of 640 C was chosen as the homogeneous reactions between VMs and CO2 to form CO initiate at 480 C. The gaseous effluents from the TR were monitored and their concentration profiles as a function of thermolytic temperature were plotted as shown in Fig. 5. As shown in Fig. 5, enhanced CO generation in the CO2 environment was observed, but the expected enhancement of CO by
Fig. 6. Identification of chemical species in the pyrolytic oil derived from digestate pyrolysis in N2 (black color) and CO2 (red color) using two-stage reactor. The information of each species shown in the figure is given in Supporting Information (Table S2). Similarity with the GC/MS library was higher than 80% for all identified compounds.
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CO2 is not discernible. Such observations suggest two implications in line with the CO2 roles during digestate pyrolysis. First, the expected CO2 roles are not initiated because of a heat transfer delay. Second, the reaction kinetics for initiating the expected roles are not sufficiently rapid to form CO. Despite the similar CO evolutionary patterns at 440 C in both environments, one of the interesting observations shown in Fig. 5 is the H2 concentration. For example, dehydrogenation suppression in the CO2 environment is indeed discernible. Thus, such observations suggest that the reaction kinetics for suppressing dehydrogenation via CO2 is faster than that for enhancing CO formation. To further investigate, collected pyrolytic oil was diluted with the same amount of dichloromethane, and identification of the chemical species in the pyrolytic oil derived from two-stage digestate pyrolysis was visualized using chromatograms. As shown in Fig. 6, the concentration of each chemical species is substantially reduced in the CO2 environment. Interestingly, Fig. 6 shows that polycyclic aromatic hydrocarbon (PAH) formation is substantially restricted in the CO2 environment. Considering that the PAH formation is closely related to aromaticity, dehydrogenation suppression by CO2 likely impedes PAH
formation [52e54]. To enhance the reaction kinetics of the expected CO2 roles, steel slag was loaded in the second heating zone. As reported in Table 1, steel slag contains a large number of metal species, such as Fe, Mn, etc. We assumed that the catalytic capabilities are being imparted by such metal species. All experimental parameters are identical in the case of Fig. 5. The gaseous effluents from the TR were monitored and their concentration profiles as a function of the thermolytic temperature were plotted as shown in Fig. 7. As shown in Fig. 7, the CO2 roles are remarkably enhanced in the presence of steel slag. Thus, Fig. 7 shows that the reaction kinetics governing the CO2 roles seems to be catalytically expedited. In parallel, all experimental findings shown in Fig. 7 strongly suggest that the insignificant CO enhancement shown in Fig. 5 is mainly attributed to the slow reaction kinetics governing the CO2 roles. In reference to H2 and CO, the C2-hydrocarbon species concentrations are not substantial even in the CO2 environment. To further investigate the CO2 roles, the overall mass balance from digestate pyrolysis (Figs. 2, 5 and 6) is shown in Fig. 8. As well reflected in Fig. 8, the char mass portion (i.e., carbon-soil composite) is very
Fig. 7. H2, CH4, CO, C2H6, C2H4, and C2H2 concentration profiles from two-stage digestate pyrolysis using steel slag as a catalyst in N2 (black color) and CO2 (red color).
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Fig. 8. Overall mass balance for pyrogenic products from (a) one-stage, (b) two-stage, and (c) two-stage with steel slag digestate pyrolysis in N2 and CO2 atmospheres.
similar. However, the pyrolytic oil mass portion in the CO2 environment is significantly reduced in the three pyrolytic cases. Furthermore, the CO2 roles (i.e. shifting the carbon distribution from pyrolytic oil to pyrolytic gas via the homogeneous reactions between VM evolved from digestate thermolysis and CO2 while suppressing dehydrogenation) are quite consistent with the experimental results shown in Fig. 8. Lastly, TCLP tests of carbonsoil composites were also conducted to determine metal leaching. However, metal leaching was not identified from the TCLP tests of carbon-soil composites. Note that the metal species in the digestate are summarized in Table 2.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2018R1A2B2001121). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.116529. References
4. Conclusions This study investigated the role of CO2 as a reactive gas medium during digestate pyrolysis. All experimental observations confirmed that CO2 roles occurred via a gas phase reaction (i.e. homogeneous reactions between VM evolved from digestate pyrolysis and CO2). The main CO2 roles during digestate pyrolysis resulted in enhanced CO generation while suppressing dehydrogenation. Such an observation also suggests shifting the carbon distribution from pyrolytic oil to pyrolytic gas was only achieved at 480 C by CO2. However, the reaction kinetics governing the CO2 roles was not sufficiently rapid. To enhance the reaction kinetics associated with CO2 in digestate pyrolysis, we employed steel slag as a catalyst. In addition, the non-catalytic pyrolysis carried out in the two-stage reactor generated more pyrolytic gas at comparable conditions. The use of CO2 and steel slag as the catalyst also increased the evolution of gaseous products for the pyrolysis of digestate. This study experimentally demonstrated that steel slag imparted catalytic capability, which played a key role in expediting the reaction kinetics ruling the CO2 roles.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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