Bioresource Technology 193 (2015) 185–191
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Pyrolysis of microalgal biomass in carbon dioxide environment Seong-Heon Cho a, Ki-Hyun Kim b, Young Jae Jeon c, Eilhann E. Kwon a,⇑ a
Department of Environment and Energy at Sejong University, Seoul 143-747, South Korea Department of Civil and Environmental Engineering at Hanyang University, Seoul 133-791, South Korea c Department of Microbiology at Pukyong National University, Pusan 608-737, South Korea b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Fast virtuous cycle of carbon via using
microalgae for energy recovery. CO2 assisted enhancement of syngas. Utilizing CO2 as chemical feedstocks
in the thermo-chemical process. Effectiveness of CO2 at temperature
higher than 530 °C.
a r t i c l e
i n f o
Article history: Received 29 May 2015 Received in revised form 22 June 2015 Accepted 23 June 2015 Available online 29 June 2015 Keywords: Microalgae Carbon dioxide Syngas Thermo-chemical process Pyrolysis
a b s t r a c t This work mechanistically investigated the influence of CO2 in the thermo-chemical process of microalgal biomass (Chlorella vulgaris and Microcystis aeruginosa) to achieve a fast virtuous cycle of carbon via recovering energy. This work experimentally justified that the influence of CO2 in pyrolysis of microalgal biomass could be initiated at temperatures higher than 530 °C, which directly led to the enhanced generation of syngas. For example, the concentration of CO from pyrolysis of M. aeruginosa increased up to 3000% at 670 °C in the presence of CO2. The identified universal influence of CO2 could be summarized by the expedited thermal cracking of VOCs evolved from microalgal biomass and by the unknown reaction between VOCs and CO2. This identified effectiveness of CO2 was different from the Boudouard reaction, which was independently occurred with dehydrogenation. Thus, microalgal biomass could be a candidate for the thermo-chemical process (pyrolysis and gasification). Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Global climate change triggering the detrimental disturbance of our ecosystem has been confirmed through the comprehensive and unprecedented investigations covering the scientific, engineering and social aspects (Chen et al., 2015). Among the numerous factors triggering global climate changes, the IPCC (international panel climate change) pointed out anthropogenic greenhouse gases (GHGs) as the main contributors, which was justified by fully transparent manners via longstanding scientific researches (Chen et al., 2015; Wang et al., 2014). Consequently, the potential environmental threats from anthropogenic GHGs have gained public awareness, ⇑ Corresponding author at: 209 Neungdong-Ro, Gwangjin-Gu, Seoul 143-747, South Korea (ROK). Tel.: +82 2 3408 4166; fax: +82 2 3408 4320. E-mail address: [email protected] (E.E. Kwon). http://dx.doi.org/10.1016/j.biortech.2015.06.119 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
which has led to a great deal of researches associated with carbon capture and storage (CCS) and various types of renewable energies to mitigate and to adapt the environmental impacts from anthropogenic GHGs (Kwon et al., 2014). Among various types of renewable energies, biofuels (bioethanol and biodiesel) transformed from edible agricultural resources (crops and oil-bearing biomass) have been firstly commercialized due to their intrinsic carbon neutrality relative to fossil fuels and their versatile compatibilities with the current energy infrastructure, system and applications (Kwon et al., 2013c; Papa et al., 2015). This commercialization of biofuels for public utilization has also been supported via our political reinforcement and enactment, such as renewable fuel standard (RFS) and renewable portfolio standard (RPS) (Kwon et al., 2013a). Nevertheless, the first generation of biofuels produced from the edible agricultural resources has led to ethical dilemmas and to unprecedented crop
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price increases (Park et al., 2015). In order to soothe these unwanted side effects, our research communities have actively developed the appropriate techniques that enable transformation of inedible bio-resources into biofuels (Fu et al., 2013). Therefore, biofuels from the inedible biomass (the second generation of biofuels) and from the aquatic biomass (the third generation of biofuels) were proposed as an alternative to the first generation of biofuels and the remarkable technical advances that have been achieved for the last 15 years (Do et al., 2015; Ma and Hanna, 1999; Park et al., 2015). In parallel, these technical advances have also brought force the concept of biorefinery to produce value added products and the substitution of petroleum-derived chemical products (Abdelaziz et al., 2015; Francavilla et al., 2015; Sarkar et al., 2015; Venkata Mohan et al., 2015). Unfortunately, in spite of the remarkable technical achievements, the concrete and optimum techniques for producing the second and the third generation of biofuels have yet to be fully implemented. However, our research communities positively expected the technical incompleteness for the biofuel industry would be overcome in the near future. Among various types of biomass, biomass from aquatic origins such as microalgae relative to terrestrial biomass (ligno-cellulosic biomass) was highly evaluated as the feasible feedstock for biofuel due to the extremely high carbon fixation efficiency (Posten and Schaub, 2009). As an aspect for energy recovery from microalgal biomass, three different routes for transforming microalgae into biofuels have been widely accepted and suggested (Quinn and Davis, 2015). For instance, the first route involves the conversion of microalgal lipid into biodiesel via the transesterification reaction (Cheng et al., 2014; Jin et al., 2014) and the second route involves pyrolysis of microalgal lipid in high temperature and pressure conditions, which is called the liquefaction process (Chen et al., 2015; Duan et al., 2015; Gai et al., 2015). Very similarly, the third route via the gasification process to produce syngas can be universally applied to all types of microalgal biomass (Onwudili et al., 2013; Ventura et al., 2013). This syngas can be the initial feedstock for the Fischer–Tropsch process to convert syngas into fuel range of alkanes. Therefore, there have been big initiatives to convert the microalgal lipid into biodiesel since some species contained a significant amount of lipid: the lipid content of some microalgal species reached up to 70 wt.% of dry basis. However, these microalgal species have shown slow growth kinetics (Cheng et al., 2014). Reversely, the microalgal species having low lipid content were reported to have fast growth kinetics. Fortunately, microalgae either showing high lipid content or fast growth kinetics have big advantages over terrestrial biomass for the aspect of energy recovery (Park et al., 2012). As a result, exploiting microalgal biomass for the purpose of energy recovery would be environmentally benign. In particular, the microalgal species having low lipid content can be exploited for carbon sources for the thermo-chemical process (gasification and pyrolysis), and then the produced syngas can be transformed into long-chained hydrocarbons via the Fischer–Tropsch process (Chen et al., 2015). Unfortunately, the significant carbon loss in the unit operational process to produce long-chained hydrocarbons could not be avoided. Therefore, increasing the efficiency of the thermo-chemical process is imperative to increase the sustainability associated with the utilization of microalgal biomass. In order to achieve this, this work mainly and fundamentally investigated the pyrolysis of Chlorella vulgaris (C. vulgaris) and Microcystis aeruginosa (M. aeruginosa) as a case study since pyrolysis has been known as the critical intermediate step for gasification. Thus in particular, our work was to systematically and mechanistically explore the influence of CO2 as a reaction medium in pyrolysis of microalgal biomass to increase the thermal efficiency of gasification: our mechanistic understanding of CO2 in pyrolysis of microalgal biomass will be a milestone for the
utilization of CO2 in the gasification process to enhance thermal efficiency, which will increase the sustainability for the utilization of microalgal biomass in the gasification process. In this regard, this work was intentionally limited to the investigation on the influence of CO2 in the pyrolysis process. Furthermore, the growth rate of C. vulgaris was monitored to justify the feasibility associated with the harness of microalgal biomass. 2. Methods 2.1. C. vulgaris cultivation C. vulgaris Beijerinck (KMMCC65) was obtained from the Korean Marine Microalgae Culture Center (Busan, South Korea), and C. vulgaris was cultured in filtered natural sea water containing a composition (i.e., L basis) of 750 mg NaNO3, 50 mg NaH2PO4H2O, 43.6 mg Na2EDTA, 31.5 mg FeCl36H2O with trace elemental solution including 1.8 mg MnCl24H2O, 0.1 mg CoCl26H2O, 0.1 mg CuSO45H2O, 0.23 mg ZnSO47H2O, 0.06 mg Na2MoO4, 1 mg vitamin B1, 5 lg vitamin B12 and 5 lg biotin (Guillard and Ryther, 1962). The unicellular alga C. vulgaris used in these experiments was cultivated in a 5 L photo-bioreactor containing a 3 L f/2 medium with a supply of 5% (v/v) CO2 mixed with air at 0.1 vvm (vessel volume per min), and the reactor was exposed to 2 36 W fluorescent daylight corresponding to 74 lmol m 2 s 1 in 12:12 circadian cycles at 25 °C. The culture of C. vulgaris was analyzed for biomass with a determination of optical density at 683 nm wavelength using a spectrophotometer (Pharmacia, USA). For the cell dry weight determination, the OD values from C. vulgaris cultures were using the equations Y = 1.841 X (R2 = 0.997), Y: where the optical density values at 683 nm; X: where cell dry weight (g L 1), which was previously published by Gonçalves et al. (2013). The maximum specific growth rate was determined during exponential growth (between interval t1–t2) using lmax = lnx2 lnx1/t2 t1. 2.2. Material preparations C. vulgaris obtained from the bioreactor was washed with distilled water to remove salt and then centrifuged at 8000 rpm for 15 min, and the microalgal sample was dried with a freeze dryer (TFD series, Seoul, Korea) at 80 °C and less than 1 Torr for 24 h. M. aeruginosa was obtained from the Korea Marine Microalgae Culture Center (Busan, South Korea) as dried form, and the same freeze dryer was employed to remove moisture. The dried microalgal biomass was stored at 20 °C in the desiccator filled with silica gel. The lipid in C. vulgaris and M. aeruginosa was separately extracted with the Soxhlet unit and the solvent for extracting lipid was n-hexane for 10 h. The rotary vacuum evaporator (Thermo-Scientific, USA) was used to recover n-hexane. All gases used for the experimental work were ultra-high purity (UHP) and were purchased from AirTech Korea. 2.3. Thermo-gravimetric analysis (TGA) A series of TGA tests were conducted with a STAR System TGA Unit (Mettler Toledo, Switzerland). The reactive and protective gases used in the TGA tests were controlled by the imbedded MFCs in the TGA unit. 10 (±0.1) mg of sample was loaded in each test. All TGA tests were conducted at a heating rate of 10 °C min 1 from 25 °C to 720 °C. The total flow rate of the reactive and protective gas was 150 mL min 1. 2.4. Tubular reactor setting for pyrolysis A tubular reactor made of a quartz tube (Chemglass CGQ-0900T-13, USA) and its dimension was 1 in (25.4 mm) of
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outer diameter and 24 in (0.61 m) of length. 1 in stainless Ultra Torr Vacuum Fitting (Swagelok SS-4-UT-6-400) was assembled to build the TR. The sample of 3 ± 0.01 g was loaded inside of the TR. The required experimental temperatures were provided using a programmable tube furnace (Wisetherm, Korea). The purge and reactive gases were controlled with mass flow controller (Brooks, 6850E series, USA). The condensable pyrolytic products were collected with a condenser, and the temperature of the condenser was maintained at 4 °C. 2.5. Analysis of the pyrolytic products The effluent from the TR was sent to a micro-GC (Inficon 3000A, USA) for the major pyrolytic gases. Two GC modules (i.e., molecular sieve (320 lm 10 m) module and Plot U (320 lm 3 m)) were equipped in the micro-GC, and multiple calibration was conducted before quantifying the major pyrolytic gases. 3. Result and discussions 3.1. Measurement of microalgal biomass (C. vulgaris) using a photo bioreactor Exploiting microalgal biomass as an initial feedstock for the thermo-chemical process (pyrolysis and gasification) is highly contingent on the possible recovery of microalgal biomass and its secure supply. In order to validate this, C. vulgaris was cultivated with a photo bioreactor (PBR) and the concentration profiles of dried microalgal biomass (C. vulgaris) were plotted in Figure SI-1 (Supplementary Information). In addition, the growth rate of C. vulgaris was compared with the literatures and summarized in Table 1. As depicted in Figure SI-1, the culture grown under autotrophic conditions reached the stationary phase with 0.8 g L 1 cell biomass yield that followed the typical pattern of the sigmoidal curve and a 0.30 h 1 specific growth rate indicating the ideal day for harvesting C. vulgaris. Similar results (Gonçalves et al., 2013) associated with a marine C. vulgaris with slightly high biomass productivity were found due to the different strengths of light intensity and circadian cycles. In addition to the growth rate of C. vulgaris, the cultivation of M. aeruginosa with the PBR under different light intensities, wavelengths via application of light emitting diode (LED), showed a similar sigmoidal pattern in the 6 days of our previous work, in which the concentration was 0.8 g L 1. The identified growth rate of M. aeruginosa in our previous work (Park et al., 2012) was relatively faster than that of C. vulgaris. The total lipid content of C. vulgaris was identified as 7.0 ± 0.2 wt.%, and the extracted microalgal lipid was transesterified via a non-catalytic way. The fatty acid profiles were very similar to edible vegetable oil, such as soybean and rapeseed oil: the major fatty acids of C. vulgaris were identified as C16–18 range and the representative chromatogram was depicted in Figure SI-2 (Supplementary Information). As illustrated in Figure SI-2, their identifications and quantifications were denoted in Figure SI-2.
One interesting observation was that the fatty acid profiles were very different from our previous work (Kwon et al., 2013b), which can be explained by the different strains. For example, our previous work transesterified C. vulgaris sp-13 into fatty acid methyl esters (FAMEs) and the profiles of fatty acids were much more complex. Unlike C. vulgaris, the lipid content of M. aeruginosa was much lower than that of C. vulgaris, which was less than 1 ± 0.31 wt.% of dry basis. The growth rate expressed as dried cell concentration in Figure SI-1 and the comparative result in Table 1 suggested that exploiting microalgal biomass as an initial feedstock for the thermo-chemical process (i.e., pyrolysis and gasification) is promising even though the net productivity is highly contingent on a combination of geographic factors and the microalgal strains: the productivity described as dried cell concentration in Figure SI-1 and Table 1 is at least 20 times bigger than the terrestrial biomass such as the representative ligno-cellulosic biomass (i.e., oak wood) (Park et al., 2012; Posten and Schaub, 2009). This can be explained by higher energy efficiencies for converting solar energy into chemical energy via the photosynthesis, which reached up to 5% of sunlight energy to biomass (Posten and Schaub, 2009). Furthermore, a great deal of researches reported that wastewater could be used as the nutrient source for cultivating microalgae. Thus, harnessing microalgal biomass for the raw material in the thermo-chemical process (i.e., pyrolysis and gasification) can be an environmentally benign option. In addition, the fast growth kinetics relative to terrestrial biomass leads to the fastest carbon circulation as compared to other biomass.
3.2. Characterization of the thermal degradation of C. Vulgaris in N2 and CO2 In order to characterize the influence of CO2, a series of thermo-gravimetric analysis (TGA) tests at 10 °C min 1 from 25 °C to 720 °C were conducted with C. vulgaris and M. aeruginosa. The final experimental temperature was intentionally limited to 720 °C to exclude the possible influence of the Boudouard reaction (Kwon and Castaldi, 2012) since the Boudouard reaction is thermodynamically favorable at temperatures higher than 710 °C based on the theoretical calculation of DG 6 0. As evidenced in Fig. 1(a), the thermal degradation of C. vulgaris in N2 and in CO2 was exactly identical in terms of the initial and end temperature of mass decay. This is indicative that there is a lack of influence of CO2 associated with the physical aspects, which were shown as the identical mass decay pattern in the thermograms of C. vulgaris in N2 and CO2 in Fig. 1(a): temperature regime showing major devolatilization up to 480 °C is very similar. This was clearly justified in the differential thermogram (DTG) in Fig. 1(b) since the maximum achievable thermal decomposition rates of C. vulgaris in N2 and CO2 at 340 °C was identical. This similar thermal degradation was identified in the case of M. aeruginosa (i.e., no influence of CO2 associated with physical aspects that were the same as the thermal degradation of C. vulgaris), thus the comparison of the thermogram of M. aeruginosa in N2 and in CO2 was
Table 1 Lipid content and productivity of the different strains of C. vulgaris. Strains
Culture conditions
C. vulgaris Beijerinck (KMMCC65) C. vulgaris #259
Autotrophic F/2 medium, 5% (v/v) CO2 mixed air, 74 lmol m s 1, 12:12 circadian cycles, 25 °C Autotrophic
C. vulgaris (ATCC)
Autotrophic F/2 medium, 14:10 circadian cycles, air supply, irradiance: 126 lmol m 2 s 1, 22 °C
2
Lipid content
Biomass productivity (mg L 1 day 1)
References
7.0 ± 0.2 wt.%
67
This study
N/A
10
N/A
75
Liang et al. (2009) (Liang et al., 2009) Gonçalves et al. (2013) (Gonçalves et al., 2013)
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Fig. 1. (a) Representative thermograms of C. vulgaris in N2 and in CO2, (b) Representative DTG curves of C. vulgaris in N2 and in CO2, (c) Comparison of the representative thermograms of C. vulgaris and M. aeruginosa in CO2.
not shown in this work. Alternatively, the thermal degradation of C. vulgaris and M. aeruginosa in CO2 was compared in Fig. 1(c). One interesting observation in Fig. 1(c) was the thermal stability of M. aeruginosa, which was superior to that of C. vulgaris. This thermal stability was expressed as the wide window of temperature in thermogram in Fig. 1(c): the thermal degradation of M. aeruginosa was initiated at 330 °C and the thermal degradation via volatilization was not completed even at 720 °C. For further investigation on the influence of CO2, the effluent from the TGA unit was monitored: in particular, the major pyrolytic syngas was monitored. Interestingly, the concentration of syngas from the thermal degradation of C. vulgaris in CO2 was increased, as compared to the case in N2 (data not shown in this work). However, for the case of M. aeruginosa, the concentration of syngas evolved from the thermal degradation of C. vulgaris in CO2 was very similar to that in N2 (data not shown in this work). This discrepancy with respect to the influence of CO2 is not clearly explained at this stage work due to the experimental limitations that arose from the small sample loading (10 mg) and from the significant dilution (150 mL min 1 of purge and reactive gas in the TGA unit). However, a series of TGA tests fully justified that the influence of CO2 associated with the physical aspects was indeed negligible. Moreover, monitoring the effluent from the TGA unit marginally and/or partially evidenced the influence of CO2 associated with the chemical aspects in the TGA test with M. aeruginosa, but the fundamental and systematic investigation on the chemical aspects is necessary. 3.3. Influence of CO2 in pyrolysis of microalgal biomass In order to overcome the experimental limitations identified and stated in Sec. 3.2, a tubular (TR) was employed to test the large amount of microalgal biomass: the sample loading was increased from 10 mg to 3.0 ± 0.01 g, the flow rate of N2 and CO2 was set at 600 mL min 1 each, and the heating rate was the same as TGA tests (i.e., 10 °C min 1). 3.0 g of M. aeruginosa was pyrolyzed in N2 and CO2, and the effluent from the TR was monitored. The major pyrolytic gases (i.e., H2, CH4, and CO) were measured at temperatures from 330 °C to 700 °C and their concentration profiles were plotted in Fig. 2(a). The concentration profiles of the major pyrolytic gases in the N2 and CO2 environment in this work were used as the reference and the control, respectively.
As shown in Fig. 2(a), the concentration profiles of major pyrolytic gases in N2 followed a typical pyrolytic pattern of pyrolysis phenomena. For example, the generation of H2 was proportional to the pyrolytic temperatures due to the thermal cracking of VOCs, also known as dehydrogenation, thus pyrolytic temperature achieving the highest concentration of CH4 was significantly lower than that of H2 since dehydrogenation is the major chemical degradation mechanisms at temperature higher than 400 °C. Furthermore, the concentrations of CO in N2 were significantly lower than those of H2. This could be also explained by dehydrogenation since dehydrogenation expedited carbonization, which generally leads to more carbon residue. However, the evolution patterns of major pyrolytic gases in the CO2 environment were very different from that in the N2 environment. The enhanced generation of CO in the CO2 environment was initiated at 530 °C. This observation was consistent with the previous discussions associated with the enhanced generation of syngas in Sec. 3.2. This could be the particular effect induced by CO2 used as reaction medium and could fall into two possible explanations: expedition thermal cracking of VOCs evolved from the thermal degradation of M. aeruginosa and the unknown reaction between VOCs and CO2. This enhanced generation of CO in the presence of CO2 would be discrepant from the general effect of dehydrogenation since dehydrogenation triggers and/or expedites the carbonization, which would be reflected through the increased carbon residue in thermogram. However, as stated in Sec. 3.2, the final mass differences were not identified. Furthermore, the concentration profiles of H2 in the CO2 environment followed a very similar trend with those in the N2 environment. Thus, the identified differences induced by CO2 indirectly suggested not only that unknown reaction triggered by VOCs evolved from M. aeruginosa and that CO2 is only effective at temperatures higher than 530 °C, but also that the influence of CO2 simultaneously and/or independently occurred with dehydrogenation. Fig. 2(a) shows that the concentration of H2 in the presence of CO2 was significantly lower than that in the N2 environment. This phenomenon can be explained by the dilution from the enhanced generation of CO since the quantification of pyrolytic gases via the GC analysis only indicated the relative mole fraction: this can be easily validated by adding a small amount of N2 as an internal standard. For example, the concentration of CO from pyrolysis of M. aeruginosa increased up to 3000% at 670 °C in
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Fig. 2. Concentration profiles of H2, CH4, and CO evolved from the thermal decomposition of M. aeruginosa (a) and C. vulgaris (b) at various pyrolytic temperatures (3 g of sample loading).
the presence of CO2. Considering the additional source of carbon and oxygen, this enhanced generation of CO significantly implied that CO2 was a possible source for carbon and oxygen. In other words, CO2 was used as the substrate for providing carbon and oxygen source and the unknown reaction with CO2 and VOCs evolved from the thermal degradation of M. aeruginosa was initiated at temperatures higher than 530 °C. The same experimental work in Fig. 2(a) was conducted with C. vulgaris, and the concentration profiles of the major pyrolytic gases were plotted in Fig. 2(a). Interestingly, the concentration profiles in Fig. 2(b) were discrepant from the observation in Fig. 2(b). For example, the concentration of H2 in various environmental conditions was identical. In particular, the enhanced generation of CO showed a very similar pattern, but its magnitude in terms of concentration of CO relative to the experimental result of M. aeruginosa was not apparent even in the presence of CO2. However, this can be fully explained by the previous explanations associated with not only the thermal stability of C. vulgaris, but also the effective influence of CO2 at temperatures higher than 530 °C. As
evidenced in Fig. 1, the major thermal degradation of C. vulgaris via devolatilization was ended at temperatures lower than 500 °C, thus the same influence of CO2 could not expected. This suggested that there was the insufficient amount of VOCs evolved from the thermal degradation of C. vulgaris. In order to check the identified CO2 influence at temperatures at higher than 530 °C, the ratio of CO/H2 based on the experimental results in Fig. 2 under various temperatures was plotted in Fig. 3. Interestingly, the experimental temperature showing the effectiveness of CO2 was similarly aligned at 530 °C, which was very consistent with our discussions. Thus, it would be imperative to investigate the universal influence of CO2 on VOCs evolved from the thermal degradation of C. vulgaris. In order to validate the universal influence of CO2, similar experimental work with C. vulgaris was conducted with a furnace capable of controlling two temperature zones. The first heating zone of the furnace was set, which was identical to the experimental setup in Fig. 2, and the second heating zone was isothermally controlled at 550 °C. Similar to Fig. 2, the concentration profiles
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Fig. 3. Ratio of CO to H2 evolved from the thermal degradation of M. aeruginosa and C. vulgaris in N2 and in CO2 under various temperatures.
Fig. 4. Concentration H2, CH4, and CO evolved from the thermal decomposition of C. vulgaris in CO2 at various pyrolytic temperatures (3 g of sample loading).
of H2, CH4, and CO under various temperatures were plotted in Fig. 4. In order to compare the result, their concentrations were compared with the experimental results in Fig. 2(b). Fig. 4 justifies not only the influence of CO2 on VOCs evolved from the thermal decomposition of C. vulgaris, but also our previous discussions associated with the effective temperature for initiating the reaction between CO2 and VOCs. Therefore, this observation significantly suggested that microalgal biomass having different thermal stability could be efficiently converted into syngas by applying CO2. Considering pyrolysis as a crucial intermediate step for gasification, this will directly enhance the thermal efficiency of gasification, but further study will be followed.
4. Conclusions This work validated the role of CO2 in the thermal degradation of microalgal biomass: unknown reaction induced by CO2 was
initiated at temperatures higher than 530 °C. This can be explained by reactions between VOCs from the thermal degradation of microalgal biomass and CO2. Moreover, the enhanced thermal cracking in the presence of CO2 could not be excluded. The identified influence of CO2 directly led to the enhanced generation of syngas evolved from the thermal degradation of M. aeruginosa, which reached up to 3000% at 670 °C, as compared to the pyrolysis process of M. aeruginosa in N2.
Acknowledgements This work was supported by a National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. 2914RA1A004893). The second author acknowledged partial support made by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No. 2009-0093848).
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Kwon, E.E., Jeon, Y.J., Yi, H., 2013b. Noncatalytic transformation of the crude lipid of Chlorella vulgaris into fatty acid methyl ester (FAME) with charcoal via a thermo-chemical process. Bioresour. Technol. 129, 672–675. Kwon, E.E., Yi, H., Jeon, Y.J., 2013c. Sequential co-production of biodiesel and bioethanol with spent coffee grounds. Bioresour. Technol. 136, 475–480. Kwon, E.E., Yi, H., Jeon, Y.J., 2014. Boosting the value of biodiesel byproduct by the non-catalytic transesterification of dimethyl carbonate via a continuous flow system under ambient pressure. Chemosphere 113, 87–92. Liang, Y., Sarkany, N., Cui, Y., 2009. Biomass and lipid productivities of Chlorella vulgaris under autotrophic, heterotrophic and mixotrophic growth conditions. Biotechnol. Lett. 31 (7), 1043–1049. Ma, F., Hanna, M.A., 1999. Biodiesel production: a review 1. Bioresour. Technol. 70 (1), 1–15. Onwudili, J.A., Lea-Langton, A.R., Ross, A.B., Williams, P.T., 2013. Catalytic hydrothermal gasification of algae for hydrogen production: composition of reaction products and potential for nutrient recycling. Bioresour. Technol. 127, 72–80. Papa, G., Rodriguez, S., George, A., Schievano, A., Orzi, V., Sale, K.L., Singh, S., Adani, F., Simmons, B.A., 2015. Comparison of different pretreatments for the production of bioethanol and biomethane from corn stover and switchgrass. Bioresour. Technol. 183, 101–110. Park, J., Seo, J., Kwon, E.E., 2012. Microalgae production using wastewater: effect of light-emitting diode wavelength on microalgal growth. Environ. Eng. Sci. 29 (11), 995–1001. Park, J.-Y., Park, M.S., Lee, Y.-C., Yang, J.-W., 2015. Advances in direct transesterification of algal oils from wet biomass. Bioresour. Technol. 184, 267–275. Posten, C., Schaub, G., 2009. Microalgae and terrestrial biomass as source for fuels: a process view. J. Biotechnol. 142 (1), 64–69. Quinn, J.C., Davis, R., 2015. The potentials and challenges of algae based biofuels: a review of the techno-economic, life cycle, and resource assessment modeling. Bioresour. Technol. 184, 444–452. Sarkar, O., Agarwal, M., Naresh Kumar, A., Venkata Mohan, S., 2015. Retrofitting hetrotrophically cultivated algae biomass as pyrolytic feedstock for biogas, biochar and bio-oil production encompassing biorefinery. Bioresour. Technol. 178, 132–138. Venkata Mohan, S., Rohit, M.V., Chiranjeevi, P., Chandra, R., Navaneeth, B., 2015. Heterotrophic microalgae cultivation to synergize biodiesel production with waste remediation: progress and perspectives. Bioresour. Technol. 184, 169– 178. Ventura, J.-R.S., Yang, B., Lee, Y.-W., Lee, K., Jahng, D., 2013. Life cycle analyses of CO2, energy, and cost for four different routes of microalgal bioenergy conversion. Bioresour. Technol. 137, 302–310. Wang, H.-Y., Bluck, D., Van Wie, B.J., 2014. Conversion of microalgae to jet fuel: process design and simulation. Bioresour. Technol. 167, 349–357.
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Pyrolysis of microalgal biomass in carbon dioxide environment Seong-Heon Cho a, Ki-Hyun Kim b, Young Jae Jeon c, Eilhann E. Kwon a,⇑ a
Department of Environment and Energy at Sejong University, Seoul 143-747, South Korea Department of Civil and Environmental Engineering at Hanyang University, Seoul 133-791, South Korea c Department of Microbiology at Pukyong National University, Pusan 608-737, South Korea b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Fast virtuous cycle of carbon via using
microalgae for energy recovery. CO2 assisted enhancement of syngas. Utilizing CO2 as chemical feedstocks
in the thermo-chemical process. Effectiveness of CO2 at temperature
higher than 530 °C.
a r t i c l e
i n f o
Article history: Received 29 May 2015 Received in revised form 22 June 2015 Accepted 23 June 2015 Available online 29 June 2015 Keywords: Microalgae Carbon dioxide Syngas Thermo-chemical process Pyrolysis
a b s t r a c t This work mechanistically investigated the influence of CO2 in the thermo-chemical process of microalgal biomass (Chlorella vulgaris and Microcystis aeruginosa) to achieve a fast virtuous cycle of carbon via recovering energy. This work experimentally justified that the influence of CO2 in pyrolysis of microalgal biomass could be initiated at temperatures higher than 530 °C, which directly led to the enhanced generation of syngas. For example, the concentration of CO from pyrolysis of M. aeruginosa increased up to 3000% at 670 °C in the presence of CO2. The identified universal influence of CO2 could be summarized by the expedited thermal cracking of VOCs evolved from microalgal biomass and by the unknown reaction between VOCs and CO2. This identified effectiveness of CO2 was different from the Boudouard reaction, which was independently occurred with dehydrogenation. Thus, microalgal biomass could be a candidate for the thermo-chemical process (pyrolysis and gasification). Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Global climate change triggering the detrimental disturbance of our ecosystem has been confirmed through the comprehensive and unprecedented investigations covering the scientific, engineering and social aspects (Chen et al., 2015). Among the numerous factors triggering global climate changes, the IPCC (international panel climate change) pointed out anthropogenic greenhouse gases (GHGs) as the main contributors, which was justified by fully transparent manners via longstanding scientific researches (Chen et al., 2015; Wang et al., 2014). Consequently, the potential environmental threats from anthropogenic GHGs have gained public awareness, ⇑ Corresponding author at: 209 Neungdong-Ro, Gwangjin-Gu, Seoul 143-747, South Korea (ROK). Tel.: +82 2 3408 4166; fax: +82 2 3408 4320. E-mail address: [email protected] (E.E. Kwon). http://dx.doi.org/10.1016/j.biortech.2015.06.119 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
which has led to a great deal of researches associated with carbon capture and storage (CCS) and various types of renewable energies to mitigate and to adapt the environmental impacts from anthropogenic GHGs (Kwon et al., 2014). Among various types of renewable energies, biofuels (bioethanol and biodiesel) transformed from edible agricultural resources (crops and oil-bearing biomass) have been firstly commercialized due to their intrinsic carbon neutrality relative to fossil fuels and their versatile compatibilities with the current energy infrastructure, system and applications (Kwon et al., 2013c; Papa et al., 2015). This commercialization of biofuels for public utilization has also been supported via our political reinforcement and enactment, such as renewable fuel standard (RFS) and renewable portfolio standard (RPS) (Kwon et al., 2013a). Nevertheless, the first generation of biofuels produced from the edible agricultural resources has led to ethical dilemmas and to unprecedented crop
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price increases (Park et al., 2015). In order to soothe these unwanted side effects, our research communities have actively developed the appropriate techniques that enable transformation of inedible bio-resources into biofuels (Fu et al., 2013). Therefore, biofuels from the inedible biomass (the second generation of biofuels) and from the aquatic biomass (the third generation of biofuels) were proposed as an alternative to the first generation of biofuels and the remarkable technical advances that have been achieved for the last 15 years (Do et al., 2015; Ma and Hanna, 1999; Park et al., 2015). In parallel, these technical advances have also brought force the concept of biorefinery to produce value added products and the substitution of petroleum-derived chemical products (Abdelaziz et al., 2015; Francavilla et al., 2015; Sarkar et al., 2015; Venkata Mohan et al., 2015). Unfortunately, in spite of the remarkable technical achievements, the concrete and optimum techniques for producing the second and the third generation of biofuels have yet to be fully implemented. However, our research communities positively expected the technical incompleteness for the biofuel industry would be overcome in the near future. Among various types of biomass, biomass from aquatic origins such as microalgae relative to terrestrial biomass (ligno-cellulosic biomass) was highly evaluated as the feasible feedstock for biofuel due to the extremely high carbon fixation efficiency (Posten and Schaub, 2009). As an aspect for energy recovery from microalgal biomass, three different routes for transforming microalgae into biofuels have been widely accepted and suggested (Quinn and Davis, 2015). For instance, the first route involves the conversion of microalgal lipid into biodiesel via the transesterification reaction (Cheng et al., 2014; Jin et al., 2014) and the second route involves pyrolysis of microalgal lipid in high temperature and pressure conditions, which is called the liquefaction process (Chen et al., 2015; Duan et al., 2015; Gai et al., 2015). Very similarly, the third route via the gasification process to produce syngas can be universally applied to all types of microalgal biomass (Onwudili et al., 2013; Ventura et al., 2013). This syngas can be the initial feedstock for the Fischer–Tropsch process to convert syngas into fuel range of alkanes. Therefore, there have been big initiatives to convert the microalgal lipid into biodiesel since some species contained a significant amount of lipid: the lipid content of some microalgal species reached up to 70 wt.% of dry basis. However, these microalgal species have shown slow growth kinetics (Cheng et al., 2014). Reversely, the microalgal species having low lipid content were reported to have fast growth kinetics. Fortunately, microalgae either showing high lipid content or fast growth kinetics have big advantages over terrestrial biomass for the aspect of energy recovery (Park et al., 2012). As a result, exploiting microalgal biomass for the purpose of energy recovery would be environmentally benign. In particular, the microalgal species having low lipid content can be exploited for carbon sources for the thermo-chemical process (gasification and pyrolysis), and then the produced syngas can be transformed into long-chained hydrocarbons via the Fischer–Tropsch process (Chen et al., 2015). Unfortunately, the significant carbon loss in the unit operational process to produce long-chained hydrocarbons could not be avoided. Therefore, increasing the efficiency of the thermo-chemical process is imperative to increase the sustainability associated with the utilization of microalgal biomass. In order to achieve this, this work mainly and fundamentally investigated the pyrolysis of Chlorella vulgaris (C. vulgaris) and Microcystis aeruginosa (M. aeruginosa) as a case study since pyrolysis has been known as the critical intermediate step for gasification. Thus in particular, our work was to systematically and mechanistically explore the influence of CO2 as a reaction medium in pyrolysis of microalgal biomass to increase the thermal efficiency of gasification: our mechanistic understanding of CO2 in pyrolysis of microalgal biomass will be a milestone for the
utilization of CO2 in the gasification process to enhance thermal efficiency, which will increase the sustainability for the utilization of microalgal biomass in the gasification process. In this regard, this work was intentionally limited to the investigation on the influence of CO2 in the pyrolysis process. Furthermore, the growth rate of C. vulgaris was monitored to justify the feasibility associated with the harness of microalgal biomass. 2. Methods 2.1. C. vulgaris cultivation C. vulgaris Beijerinck (KMMCC65) was obtained from the Korean Marine Microalgae Culture Center (Busan, South Korea), and C. vulgaris was cultured in filtered natural sea water containing a composition (i.e., L basis) of 750 mg NaNO3, 50 mg NaH2PO4H2O, 43.6 mg Na2EDTA, 31.5 mg FeCl36H2O with trace elemental solution including 1.8 mg MnCl24H2O, 0.1 mg CoCl26H2O, 0.1 mg CuSO45H2O, 0.23 mg ZnSO47H2O, 0.06 mg Na2MoO4, 1 mg vitamin B1, 5 lg vitamin B12 and 5 lg biotin (Guillard and Ryther, 1962). The unicellular alga C. vulgaris used in these experiments was cultivated in a 5 L photo-bioreactor containing a 3 L f/2 medium with a supply of 5% (v/v) CO2 mixed with air at 0.1 vvm (vessel volume per min), and the reactor was exposed to 2 36 W fluorescent daylight corresponding to 74 lmol m 2 s 1 in 12:12 circadian cycles at 25 °C. The culture of C. vulgaris was analyzed for biomass with a determination of optical density at 683 nm wavelength using a spectrophotometer (Pharmacia, USA). For the cell dry weight determination, the OD values from C. vulgaris cultures were using the equations Y = 1.841 X (R2 = 0.997), Y: where the optical density values at 683 nm; X: where cell dry weight (g L 1), which was previously published by Gonçalves et al. (2013). The maximum specific growth rate was determined during exponential growth (between interval t1–t2) using lmax = lnx2 lnx1/t2 t1. 2.2. Material preparations C. vulgaris obtained from the bioreactor was washed with distilled water to remove salt and then centrifuged at 8000 rpm for 15 min, and the microalgal sample was dried with a freeze dryer (TFD series, Seoul, Korea) at 80 °C and less than 1 Torr for 24 h. M. aeruginosa was obtained from the Korea Marine Microalgae Culture Center (Busan, South Korea) as dried form, and the same freeze dryer was employed to remove moisture. The dried microalgal biomass was stored at 20 °C in the desiccator filled with silica gel. The lipid in C. vulgaris and M. aeruginosa was separately extracted with the Soxhlet unit and the solvent for extracting lipid was n-hexane for 10 h. The rotary vacuum evaporator (Thermo-Scientific, USA) was used to recover n-hexane. All gases used for the experimental work were ultra-high purity (UHP) and were purchased from AirTech Korea. 2.3. Thermo-gravimetric analysis (TGA) A series of TGA tests were conducted with a STAR System TGA Unit (Mettler Toledo, Switzerland). The reactive and protective gases used in the TGA tests were controlled by the imbedded MFCs in the TGA unit. 10 (±0.1) mg of sample was loaded in each test. All TGA tests were conducted at a heating rate of 10 °C min 1 from 25 °C to 720 °C. The total flow rate of the reactive and protective gas was 150 mL min 1. 2.4. Tubular reactor setting for pyrolysis A tubular reactor made of a quartz tube (Chemglass CGQ-0900T-13, USA) and its dimension was 1 in (25.4 mm) of
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outer diameter and 24 in (0.61 m) of length. 1 in stainless Ultra Torr Vacuum Fitting (Swagelok SS-4-UT-6-400) was assembled to build the TR. The sample of 3 ± 0.01 g was loaded inside of the TR. The required experimental temperatures were provided using a programmable tube furnace (Wisetherm, Korea). The purge and reactive gases were controlled with mass flow controller (Brooks, 6850E series, USA). The condensable pyrolytic products were collected with a condenser, and the temperature of the condenser was maintained at 4 °C. 2.5. Analysis of the pyrolytic products The effluent from the TR was sent to a micro-GC (Inficon 3000A, USA) for the major pyrolytic gases. Two GC modules (i.e., molecular sieve (320 lm 10 m) module and Plot U (320 lm 3 m)) were equipped in the micro-GC, and multiple calibration was conducted before quantifying the major pyrolytic gases. 3. Result and discussions 3.1. Measurement of microalgal biomass (C. vulgaris) using a photo bioreactor Exploiting microalgal biomass as an initial feedstock for the thermo-chemical process (pyrolysis and gasification) is highly contingent on the possible recovery of microalgal biomass and its secure supply. In order to validate this, C. vulgaris was cultivated with a photo bioreactor (PBR) and the concentration profiles of dried microalgal biomass (C. vulgaris) were plotted in Figure SI-1 (Supplementary Information). In addition, the growth rate of C. vulgaris was compared with the literatures and summarized in Table 1. As depicted in Figure SI-1, the culture grown under autotrophic conditions reached the stationary phase with 0.8 g L 1 cell biomass yield that followed the typical pattern of the sigmoidal curve and a 0.30 h 1 specific growth rate indicating the ideal day for harvesting C. vulgaris. Similar results (Gonçalves et al., 2013) associated with a marine C. vulgaris with slightly high biomass productivity were found due to the different strengths of light intensity and circadian cycles. In addition to the growth rate of C. vulgaris, the cultivation of M. aeruginosa with the PBR under different light intensities, wavelengths via application of light emitting diode (LED), showed a similar sigmoidal pattern in the 6 days of our previous work, in which the concentration was 0.8 g L 1. The identified growth rate of M. aeruginosa in our previous work (Park et al., 2012) was relatively faster than that of C. vulgaris. The total lipid content of C. vulgaris was identified as 7.0 ± 0.2 wt.%, and the extracted microalgal lipid was transesterified via a non-catalytic way. The fatty acid profiles were very similar to edible vegetable oil, such as soybean and rapeseed oil: the major fatty acids of C. vulgaris were identified as C16–18 range and the representative chromatogram was depicted in Figure SI-2 (Supplementary Information). As illustrated in Figure SI-2, their identifications and quantifications were denoted in Figure SI-2.
One interesting observation was that the fatty acid profiles were very different from our previous work (Kwon et al., 2013b), which can be explained by the different strains. For example, our previous work transesterified C. vulgaris sp-13 into fatty acid methyl esters (FAMEs) and the profiles of fatty acids were much more complex. Unlike C. vulgaris, the lipid content of M. aeruginosa was much lower than that of C. vulgaris, which was less than 1 ± 0.31 wt.% of dry basis. The growth rate expressed as dried cell concentration in Figure SI-1 and the comparative result in Table 1 suggested that exploiting microalgal biomass as an initial feedstock for the thermo-chemical process (i.e., pyrolysis and gasification) is promising even though the net productivity is highly contingent on a combination of geographic factors and the microalgal strains: the productivity described as dried cell concentration in Figure SI-1 and Table 1 is at least 20 times bigger than the terrestrial biomass such as the representative ligno-cellulosic biomass (i.e., oak wood) (Park et al., 2012; Posten and Schaub, 2009). This can be explained by higher energy efficiencies for converting solar energy into chemical energy via the photosynthesis, which reached up to 5% of sunlight energy to biomass (Posten and Schaub, 2009). Furthermore, a great deal of researches reported that wastewater could be used as the nutrient source for cultivating microalgae. Thus, harnessing microalgal biomass for the raw material in the thermo-chemical process (i.e., pyrolysis and gasification) can be an environmentally benign option. In addition, the fast growth kinetics relative to terrestrial biomass leads to the fastest carbon circulation as compared to other biomass.
3.2. Characterization of the thermal degradation of C. Vulgaris in N2 and CO2 In order to characterize the influence of CO2, a series of thermo-gravimetric analysis (TGA) tests at 10 °C min 1 from 25 °C to 720 °C were conducted with C. vulgaris and M. aeruginosa. The final experimental temperature was intentionally limited to 720 °C to exclude the possible influence of the Boudouard reaction (Kwon and Castaldi, 2012) since the Boudouard reaction is thermodynamically favorable at temperatures higher than 710 °C based on the theoretical calculation of DG 6 0. As evidenced in Fig. 1(a), the thermal degradation of C. vulgaris in N2 and in CO2 was exactly identical in terms of the initial and end temperature of mass decay. This is indicative that there is a lack of influence of CO2 associated with the physical aspects, which were shown as the identical mass decay pattern in the thermograms of C. vulgaris in N2 and CO2 in Fig. 1(a): temperature regime showing major devolatilization up to 480 °C is very similar. This was clearly justified in the differential thermogram (DTG) in Fig. 1(b) since the maximum achievable thermal decomposition rates of C. vulgaris in N2 and CO2 at 340 °C was identical. This similar thermal degradation was identified in the case of M. aeruginosa (i.e., no influence of CO2 associated with physical aspects that were the same as the thermal degradation of C. vulgaris), thus the comparison of the thermogram of M. aeruginosa in N2 and in CO2 was
Table 1 Lipid content and productivity of the different strains of C. vulgaris. Strains
Culture conditions
C. vulgaris Beijerinck (KMMCC65) C. vulgaris #259
Autotrophic F/2 medium, 5% (v/v) CO2 mixed air, 74 lmol m s 1, 12:12 circadian cycles, 25 °C Autotrophic
C. vulgaris (ATCC)
Autotrophic F/2 medium, 14:10 circadian cycles, air supply, irradiance: 126 lmol m 2 s 1, 22 °C
2
Lipid content
Biomass productivity (mg L 1 day 1)
References
7.0 ± 0.2 wt.%
67
This study
N/A
10
N/A
75
Liang et al. (2009) (Liang et al., 2009) Gonçalves et al. (2013) (Gonçalves et al., 2013)
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Fig. 1. (a) Representative thermograms of C. vulgaris in N2 and in CO2, (b) Representative DTG curves of C. vulgaris in N2 and in CO2, (c) Comparison of the representative thermograms of C. vulgaris and M. aeruginosa in CO2.
not shown in this work. Alternatively, the thermal degradation of C. vulgaris and M. aeruginosa in CO2 was compared in Fig. 1(c). One interesting observation in Fig. 1(c) was the thermal stability of M. aeruginosa, which was superior to that of C. vulgaris. This thermal stability was expressed as the wide window of temperature in thermogram in Fig. 1(c): the thermal degradation of M. aeruginosa was initiated at 330 °C and the thermal degradation via volatilization was not completed even at 720 °C. For further investigation on the influence of CO2, the effluent from the TGA unit was monitored: in particular, the major pyrolytic syngas was monitored. Interestingly, the concentration of syngas from the thermal degradation of C. vulgaris in CO2 was increased, as compared to the case in N2 (data not shown in this work). However, for the case of M. aeruginosa, the concentration of syngas evolved from the thermal degradation of C. vulgaris in CO2 was very similar to that in N2 (data not shown in this work). This discrepancy with respect to the influence of CO2 is not clearly explained at this stage work due to the experimental limitations that arose from the small sample loading (10 mg) and from the significant dilution (150 mL min 1 of purge and reactive gas in the TGA unit). However, a series of TGA tests fully justified that the influence of CO2 associated with the physical aspects was indeed negligible. Moreover, monitoring the effluent from the TGA unit marginally and/or partially evidenced the influence of CO2 associated with the chemical aspects in the TGA test with M. aeruginosa, but the fundamental and systematic investigation on the chemical aspects is necessary. 3.3. Influence of CO2 in pyrolysis of microalgal biomass In order to overcome the experimental limitations identified and stated in Sec. 3.2, a tubular (TR) was employed to test the large amount of microalgal biomass: the sample loading was increased from 10 mg to 3.0 ± 0.01 g, the flow rate of N2 and CO2 was set at 600 mL min 1 each, and the heating rate was the same as TGA tests (i.e., 10 °C min 1). 3.0 g of M. aeruginosa was pyrolyzed in N2 and CO2, and the effluent from the TR was monitored. The major pyrolytic gases (i.e., H2, CH4, and CO) were measured at temperatures from 330 °C to 700 °C and their concentration profiles were plotted in Fig. 2(a). The concentration profiles of the major pyrolytic gases in the N2 and CO2 environment in this work were used as the reference and the control, respectively.
As shown in Fig. 2(a), the concentration profiles of major pyrolytic gases in N2 followed a typical pyrolytic pattern of pyrolysis phenomena. For example, the generation of H2 was proportional to the pyrolytic temperatures due to the thermal cracking of VOCs, also known as dehydrogenation, thus pyrolytic temperature achieving the highest concentration of CH4 was significantly lower than that of H2 since dehydrogenation is the major chemical degradation mechanisms at temperature higher than 400 °C. Furthermore, the concentrations of CO in N2 were significantly lower than those of H2. This could be also explained by dehydrogenation since dehydrogenation expedited carbonization, which generally leads to more carbon residue. However, the evolution patterns of major pyrolytic gases in the CO2 environment were very different from that in the N2 environment. The enhanced generation of CO in the CO2 environment was initiated at 530 °C. This observation was consistent with the previous discussions associated with the enhanced generation of syngas in Sec. 3.2. This could be the particular effect induced by CO2 used as reaction medium and could fall into two possible explanations: expedition thermal cracking of VOCs evolved from the thermal degradation of M. aeruginosa and the unknown reaction between VOCs and CO2. This enhanced generation of CO in the presence of CO2 would be discrepant from the general effect of dehydrogenation since dehydrogenation triggers and/or expedites the carbonization, which would be reflected through the increased carbon residue in thermogram. However, as stated in Sec. 3.2, the final mass differences were not identified. Furthermore, the concentration profiles of H2 in the CO2 environment followed a very similar trend with those in the N2 environment. Thus, the identified differences induced by CO2 indirectly suggested not only that unknown reaction triggered by VOCs evolved from M. aeruginosa and that CO2 is only effective at temperatures higher than 530 °C, but also that the influence of CO2 simultaneously and/or independently occurred with dehydrogenation. Fig. 2(a) shows that the concentration of H2 in the presence of CO2 was significantly lower than that in the N2 environment. This phenomenon can be explained by the dilution from the enhanced generation of CO since the quantification of pyrolytic gases via the GC analysis only indicated the relative mole fraction: this can be easily validated by adding a small amount of N2 as an internal standard. For example, the concentration of CO from pyrolysis of M. aeruginosa increased up to 3000% at 670 °C in
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Fig. 2. Concentration profiles of H2, CH4, and CO evolved from the thermal decomposition of M. aeruginosa (a) and C. vulgaris (b) at various pyrolytic temperatures (3 g of sample loading).
the presence of CO2. Considering the additional source of carbon and oxygen, this enhanced generation of CO significantly implied that CO2 was a possible source for carbon and oxygen. In other words, CO2 was used as the substrate for providing carbon and oxygen source and the unknown reaction with CO2 and VOCs evolved from the thermal degradation of M. aeruginosa was initiated at temperatures higher than 530 °C. The same experimental work in Fig. 2(a) was conducted with C. vulgaris, and the concentration profiles of the major pyrolytic gases were plotted in Fig. 2(a). Interestingly, the concentration profiles in Fig. 2(b) were discrepant from the observation in Fig. 2(b). For example, the concentration of H2 in various environmental conditions was identical. In particular, the enhanced generation of CO showed a very similar pattern, but its magnitude in terms of concentration of CO relative to the experimental result of M. aeruginosa was not apparent even in the presence of CO2. However, this can be fully explained by the previous explanations associated with not only the thermal stability of C. vulgaris, but also the effective influence of CO2 at temperatures higher than 530 °C. As
evidenced in Fig. 1, the major thermal degradation of C. vulgaris via devolatilization was ended at temperatures lower than 500 °C, thus the same influence of CO2 could not expected. This suggested that there was the insufficient amount of VOCs evolved from the thermal degradation of C. vulgaris. In order to check the identified CO2 influence at temperatures at higher than 530 °C, the ratio of CO/H2 based on the experimental results in Fig. 2 under various temperatures was plotted in Fig. 3. Interestingly, the experimental temperature showing the effectiveness of CO2 was similarly aligned at 530 °C, which was very consistent with our discussions. Thus, it would be imperative to investigate the universal influence of CO2 on VOCs evolved from the thermal degradation of C. vulgaris. In order to validate the universal influence of CO2, similar experimental work with C. vulgaris was conducted with a furnace capable of controlling two temperature zones. The first heating zone of the furnace was set, which was identical to the experimental setup in Fig. 2, and the second heating zone was isothermally controlled at 550 °C. Similar to Fig. 2, the concentration profiles
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Fig. 3. Ratio of CO to H2 evolved from the thermal degradation of M. aeruginosa and C. vulgaris in N2 and in CO2 under various temperatures.
Fig. 4. Concentration H2, CH4, and CO evolved from the thermal decomposition of C. vulgaris in CO2 at various pyrolytic temperatures (3 g of sample loading).
of H2, CH4, and CO under various temperatures were plotted in Fig. 4. In order to compare the result, their concentrations were compared with the experimental results in Fig. 2(b). Fig. 4 justifies not only the influence of CO2 on VOCs evolved from the thermal decomposition of C. vulgaris, but also our previous discussions associated with the effective temperature for initiating the reaction between CO2 and VOCs. Therefore, this observation significantly suggested that microalgal biomass having different thermal stability could be efficiently converted into syngas by applying CO2. Considering pyrolysis as a crucial intermediate step for gasification, this will directly enhance the thermal efficiency of gasification, but further study will be followed.
4. Conclusions This work validated the role of CO2 in the thermal degradation of microalgal biomass: unknown reaction induced by CO2 was
initiated at temperatures higher than 530 °C. This can be explained by reactions between VOCs from the thermal degradation of microalgal biomass and CO2. Moreover, the enhanced thermal cracking in the presence of CO2 could not be excluded. The identified influence of CO2 directly led to the enhanced generation of syngas evolved from the thermal degradation of M. aeruginosa, which reached up to 3000% at 670 °C, as compared to the pyrolysis process of M. aeruginosa in N2.
Acknowledgements This work was supported by a National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. 2914RA1A004893). The second author acknowledged partial support made by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No. 2009-0093848).
S.-H. Cho et al. / Bioresource Technology 193 (2015) 185–191
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