Synergetic Sustainability Enhancement via Utilization of Carbon

Mar 23, 2015 - Considering mass balance under our experimental conditions, we confirmed reaction between CO2 and VOCs, which .... Email a Colleague...
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Synergetic Sustainability Enhancement via Utilization of Carbon Dioxide as Carbon Neutral Chemical Feedstock in the ThermoChemical Processing of Biomass Eilhann E. Kwon,*,† Seong-Heon Cho,† and Sungpyo Kim*,‡ †

Department of Environment and Energy, Sejong University, Seoul 143-747, South Korea Department of Environmental Engineering, Korea University, Sejong-City 339-700, South Korea

‡

ABSTRACT: This study investigated the utilization of CO2 as carbon neutral chemical feedstock in the thermo-chemical processing (i.e., pyrolysis and gasification) of biomass to enhance sustainability via modification of the composition of end products. To justify the universal function of CO2 in the thermochemical process, the biomass experimented on in this work was not limited to ligno-cellulosic biomass; seaweed (i.e., red macroalgae) was used to expand biofuel feedstock beyond terrestrial biomass. Our experimental results validated the achieved enhanced generation of ∼200% for H2 and ∼1000% for CO by means of adopting CO2 in the thermo-chemical process, as compared to the case in N2. This can be explained by the enhanced thermal cracking of volatile organic carbons (VOCs) evolved from the thermal degradation of biomass and the reaction between CO2 and VOCs. Considering mass balance under our experimental conditions, we confirmed reaction between CO2 and VOCs, which was universally observed in pyrolysis of all biomass samples used in this work. Thus, the identified influence of CO2 in the thermo-chemical process can be directly applied in a variety of research and industrial fields, which would be environmentally desirable.

1. INTRODUCTION The intergovernmental panel on climate change (IPCC) fourth assessment report indicated that CO2 is one of the main contributors of global warming. Thus, the public’s well-known awareness of the global environment has led to increased interest in alternative fuel research such as fuel cell technology,1,2 hydrogen fuel,3,4 and biofuel3,5,6 due to their high thermal efficiency and carbon neutrality.7,8 Nevertheless, it is currently estimated that in the next 10 years (i.e., 2015− 2025) fossil fuels (i.e., especially petroleum oil) will continue to be the predominant source of energy and chemical feedstocks.9 For this reason, efforts to restrict the utilization of petroleum oil would have an adverse effect on world economics.9 However, in the next 10−90 years (i.e., 2025−2105), in which the use of petroleum oil will be phased out due to the limited reserves and various economic reasons,9 our ability to transition to other sustainable energy sources including biomass, municipal solid waste (MSW), and other waste materials will spontaneously and/or continuously develop means for their use in an environmentally benign way.3,6,10−13 Thus, alternative fuels and research associated with the renewable energies will also alleviate the environmental burden and our concern for energy security.12−16 Other than biofuels3,4,6,11,17−21 (i.e., biogas, bioethanol, biodiesel, and biohydrogen, etc.), most renewable energies1,13−15,22 (i.e., wind mills, photovoltaic power, etc.) are not carbon based,6,9 which would be environmentally benign. However, paradoxically, our current chemical industry would be not only carbon based, but also the super ordinate concept of energy since the energy sector shares ∼25−30% of the chemical industry.9,13,23 Thus, having our energy policy and energy utilization © 2015 American Chemical Society

compatible with the chemical industry would be ideal, which would be the ultimate carbon management. Consequently, this suggests that the demand for a tremendous amount of carbon to sustain the chemical and energy sector should be supplied from various carbonaceous sources.23,24 In the near future, these carbonaceous materials will be mostly solid materials, such as coal,21,25−27 biomass,5,28,29 and MSW.11 To adopt these carbonaceous materials into the chemical industry, they must be transformed into the gaseous or liquid form, as the chemical industry employs gaseous or liquid feedstock as raw materials and intermediates.9,19,20 However, transforming these materials into gaseous or liquid feedstock compatible with the current chemical industry would be an energy-demanding step9 (i.e., generation of more CO2), as compared to the case of petroleum oil. Thus, utilizing carbon neutral sources, such as biomass and MSW, would be desirable in that these materials alleviate the environmental burden induced by transforming coal into gaseous or liquid feedstock.30 Thus, more efficient and environmentally benign technologies to transform carbonaceous material into gaseous or liquid feedstock need to be developed. The thermo-chemical process (i.e., pyrolysis and gasification) is one of the promising technologies for processing massive carbonaceous materials.3,21,25−28,30−37 For example, using a gasification process, in which a portion of the heating value of the carbonaceous material is transferred to gaseous energy Received: Revised: Accepted: Published: 5028

November 24, 2014 March 23, 2015 March 23, 2015 March 23, 2015 DOI: 10.1021/es505744n Environ. Sci. Technol. 2015, 49, 5028−5034

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Environmental Science & Technology

Figure 1. Representative thermogram of four different biomass samples in N2 and CO2.

achieve the desired heating rates, and the setting error was less than ±0.5 °C. Mass change, time, flow rates, and temperature were recorded digitally; S-type thermocouple readings were compared simultaneously to the experimental temperature. A total flow during the experimental work was 150 mL min−1. The sample loading for each experiment was 10 ± 0.1 mg. 2.3. Reactor Setup. A batch-type tubular reactor (TR) was assembled to conduct a series of experimental works under ambient pressure (i.e., 1 bar). A 25.4 mm (i.e., outer diameter, OD) stainless Ultra-Torr vacuum fitting (Swagelok SS-4-UT400, USA) was used to connect the quartz tubing (25.4 mm OD, Chemglass CGQ-0900T-13, USA) and stainless tubing. The required experimental temperature was achieved with an external programmable split-hinged furnace (AsOne TMF300N, Japan) over a temperature range of 400−850 °C. The temperature was simultaneously compared with the thermocouple readings (i.e., S-type thermocouple) to ensure that the target temperature was reached. The sample loading for each batch was 5−10 g, and the sample temperatures measured using a K-type thermocouple (Omega, USA). Condensable hydrocarbons (i.e., tar) were collected with a condenser that was chilled with a water circulation jacket (4 °C) to establish the mass balance from the TR. A drop tube reactor (DTR) made of 2-m long, 25.4-mm OD, and 19-mm inner diameter (ID) quartz tubing (Chemglass CGQ-0207) was used. A 25.4-mm OD stainless Ultra Torr Vacuum fitting (Swagelok SS-16-UT-6) was used for airtight connections. The DTR was vertically secured at the center of a furnace using a 25.4-mm OD bulkhead union (Swagelok SS1610-61). An insulation collar (Duraboard high-temperature insulation) at the top and bottom of the furnace was used in order to block heat transfers from the exposed end portions of the tube and to secure the quartz tubing. Required experimental temperatures were achieved using a split-hinged vertical furnace having 5 temperature zones (SV Furnace MA 100087, Mellen Inc.), and the temperature was simultaneously compared with S-type thermocouples implemented in each zone of the furnace to maintain the target temperature. The biomass sample was continuously introduced into the DTR using a screw feeder (WLS-0.3, China). 2.4. Gas and Steam Flow Control for the TR and DTR. All gases used in the experimental work were of ultrahigh purity and obtained from TechAir (Korea). All gas flow rates were controlled using a thermal mass flow meter (Brooks 5800S series, USA). The steam flow rate was controlled using a LabAlliance high-performance liquid chromatography pump

carriers, can be a viable means of addressing energy security, as gases are easy to clean, transport, and combust efficiently with less excess air and lower levels of a variety of pollutants.26,27,37 In addition, the main products of gasification, called syngas (i.e., H2 and CO), can be easily employed to the current chemical industry platform.26,27,35−37 In addition, pyrolysis is one of the three main thermal routes, along with gasification and combustion, offering advantages of producing a liquid that can be readily stored and transported, and used as fuel, energy carrier, or as a source of chemicals and chemical feedstock. The pyrolysis and gasification of conventional fuels such as coal has been well documented.13,21,25−27,34,36 But information on ligno-cellulosic biomass is comparatively limited due to the intrinsic heterogeneous matrix of biomass. 5,32,38 Thus, fundamental investigation of the thermo-chemical processing of biomass is necessary. Particularly, the influence of CO2 on the thermo-chemical process has yet been fully investigated.39,40 Thus, the main goal of this study is to investigate the fundamental influence of CO2 on the thermo-chemical processing of ligno-cellulosic biomass. In addition, the influence of CO2 on the thermo-chemical process with aquatic biomass (i.e., seaweed) was investigated not only to expand biofuel beyond terrestrial biomass, but also to test the universal CO2 influence on the thermo-chemical process.

2. MATERIALS AND METHODS 2.1. Reagents. Powder type of cellulose (i.e., 20 μm) extracted from cotton linters was purchased from SigmaAldrich (St. Louis, MO). Corn stover and oak wood were obtained from the National Institute of Horticulture and Herbal Science (Korea), and red seaweed (i.e., Gelidium amansii) was obtained from the National Institute of Ocean Science and Technology (Korea). Before a series of experimental works, the biomass samples were ground to a powder form with a Wiley Mill (Thomas Scientific, Swedesboro, NJ) using a 2-mm screen sieve. Thus, the sample size was less than 2 mm. These ground samples were dried at 95 °C for 3 days at 0.1 bar. 2.2. Thermogravimetric Analysis (TGA). A Netzsch STA 499 F1 Jupiter TGA unit was used with an experimental temperature condition from ambient temperature (i.e., 25 °C) to 900 °C at a heating rate of 300 °C min−1. Three embedded mass flow controllers regulated the experimental flow rates (i.e., purge and protective gases) in the TGA unit. The temperature ramp rates were provided as an input to the software (Proteus, Netzsch, Germany), which then controlled the furnace to 5029

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Environmental Science & Technology (PN 210SFP01, USA). Steam was generated using heating tape (Omega STR201-060, USA) and a cartridge heater (CIR-1013/ 120, USA) at a temperature of 300 °C. To maximize the heat transfer from the heating source, 10 m of stainless tubing with 1.587 mm OD was used. 2.5. Effluent Analysis from the TGA Unit and TR and DTR. The effluent from the TGA unit, TR, and DTR was sent to either a micro gas chromatograph (GC; Agilent 3000A, USA) or GC/mass spectrometer (GC/MS; Agilent 9890/5973, USA) for identification and quantification of the chemical species.39,41,42 A sample pump (B 19310TM5, Air Dimension, Inc., USA) capable of pumping 10−100 mL min−1 was used, and the lag times of the gaseous products from the TGA unit, TR, and DTR were calculated to be less than 2 s based on the volume of the transfer line (3 mL).42,43

Figure 2. Thermogram of oak wood at the heating rate of 5 °C min−1 in N2 and CO2.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Thermal Decomposition of Biomass in N2 and CO2. A series of TGA tests at the heating rate of 300 °C min−1 from 25 to 900 °C was conducted with four different biomass samples (i.e., cellulose, corn stover, oak wood, and red seaweed) to fundamentally characterize and compare the thermal degradation of the biomass in N2 and CO2. The representative thermograms obtained from four different samples are depicted in Figure 1. The mass decay curves at temperatures lower than 130 °C are not shown because the samples were dried prior to the experiment for 72 h (i.e., negligible mass change). The mass decay curves obtained from the thermal degradation of each biomass sample in the presence of CO2 are very similar to those in N2. For example, the initial and end temperatures of thermal degradation in N2 and CO2 are almost identical. This observation suggests that CO2 does not influence the physical aspect (i.e., mass change) of the thermal degradation of biomass. As evidenced in Figure 1, the expected Boudouard reaction (i.e., C(s) + CO2 → 2CO) was not observed at experimental temperatures higher than 710 °C, where the Boudouard reaction is thermodynamically favorable (i.e., ΔG ≤ 0).30 As indicated in Figure 1, except for the cellulose sample, the thermal decomposition (i.e., mass change) of corn stover, oak wood, and red seaweed occurred at temperatures higher than 710 °C. As compared to other biomass samples, the extent of thermal decomposition depicted as the mass change at temperatures higher than 710 °C was more apparent. However, this observed mass decay of corn stover, oak wood, and red seaweed at temperatures higher than 710 °C would not be attributed to the Boudouard reaction in that the mass decay was observed even in N2. Thus, this observation implies that the kinetics of the Boudouard reaction are very slow. In other words, the Boudouard reaction is thermodynamically favorable, but not kinetically favorable in our experimental conditions (i.e., heating rate of 300 °C min−1). To justify the discussion associated with the slow kinetics of the Boudouard reaction, similar TGA tests were conducted with oak wood at the heating rate of 5 °C min−1 in N2 and CO2, and their thermograms are shown in Figure 2. Unlike Figure 1, the complete final mass conversion was achieved in the presence of CO2, and the initiation of the Boudouard reaction occurred at 720 °C. As evidenced in Figure 2, the complete mass conversion of the carbon residue was achieved at 852 °C. Considering the experimental heating rate (i.e., 5 °C min−1), the total elapsed time to complete the mass conversion

is 25.4 min. Therefore, the observation in Figures 1 and 2 implies that utilizing CO2 via the Boudouard reaction would be very limited due to slow kinetics. To obtain more detailed information on the influence of CO2, the biochar (i.e., carbon residue obtained from the TGA experiment in Figure 1) of red seaweed was visualized with scanning electron microscopy (SEM), and the SEM images of biochar obtained from red seaweed are shown in Figure 3. One interesting observation in Figure 3 is that the SEM images of biochar in the presence of CO2 were different from those obtained in N2, which explains that thermal degradation mechanisms of red seaweed in CO2 are different from those in N2. Like red seaweed, the SEM images (data not shown) of biochar obtained from other biomass samples in CO2 (i.e., cellulose, corn stover, and oak tree) were very distinctive, as compared to the case in N2. As evidenced in the thermogram in Figure 1, the residual mass was not different in N2 and CO2. However, the surface area (i.e., BET) of biochar in CO2 was double, as compared to the case in N2. These observations are indicative of the unidentified influence of CO2. However, these dissimilarities triggered by CO2 cannot be explained at this stage of work. Furthermore, some physical and chemical properties of biochar44,45 were not considered in this stage of work. To investigate the effect of CO2, we monitored the effluent from the TGA unit and quantified the effluent with GC. Considering our experimental conditions in Figure 1 (i.e., heating rate of 300 °C min−1) and GC analysis time (i.e., 2.5 min), only one GC measurement was allowed; the major pyrolytic gases (i.e., H2, CH4, and CO) were measured at 600 °C. The major pyrolytic gases from the thermal degradation in N 2 and CO2 were very distinctive in terms of their concentrations. For example, the enhanced generation of H2 (∼70%), CH4 (∼450%), and CO (∼550%) occurred during the thermal degradation of cellulose in CO2, as compared to the case in N2. As with the case of cellulose, other biomass samples showed similar enhanced generation of the major pyrolytic gases. This suggests that the influence of CO2 is universally employed to biomass. To investigate the effect of CO2 at various experimental temperatures, TGA tests with four biomass samples were conducted at a heating rate of 5 °C min−1. The effluent from the TGA unit was then monitored and quantified. As with the previous discussion, the enhanced generation of syngas in the presence of CO2 (data not shown) at the temperature range of 400 to 700 °C was marginally observed owing to significant dilution. 5030

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Figure 3. SEM images of biochar of red seaweed in N2 and CO2.

In summary, Figure 1 shows that the mass decay curves of each biomass sample in N2 and CO2 were almost identical; the carbon residual masses are almost identical due to slow kinetics of the Boudouard reaction. Moreover, Figure 3 indicates that morphology of the carbon residue in N2 and CO2 was very distinctive. These observations suggest that the thermal degradation rate is not affected by the unknown reaction triggered by CO2. Moreover, the enhanced generation of pyrolytic gases in the presence of CO2 appears to reveal that the influence of CO2 can be selectively affected to volatile organic compounds (VOCs) evolved from thermal degradation. 3.2. New Reaction Triggered by CO2. In Section 3.1, we identified that CO2 led to the enhanced generation of pyrolytic gas and the morphological modification of biochar. However, the influence of CO2 has not been fully investigated due to experimental limitations (i.e., small amount sample loadings, significant dilution triggered by the retention time, high heating rate, etc.). Thus, a batch type tubular reactor (TR) was used to resolve the experimental limitations that arose in Section 3.1 and to obtain more realistic data. Furthermore, the experimental temperature was limited up to 700 °C to exclude the Boudouard reaction and to establish the mass balance in that the Boudouard reaction is thermodynamically favorable at the temperatures higher than 710 °C. Ten g of biomass sample was loaded into the TR, and then 200 mL min−1 of N2 and CO2 were fed into the TR to investigate and compare the influence of CO2. In addition, the heating rate was set at 10 °C min−1 due to the online GC analysis (i.e., total elapsed time of GC analysis: 2.5 min) with the TR. Prior to the GC analysis, the effluent from the TR unit was condensed to establish the mass balance. For example, the mass balance was established via measuring mass of carbon residue and condensable hydrocarbons (i.e., tar). The mass of pyrolytic gases was assumed by subtracting the mass of carbon residue (i.e., biochar) and condensable hydrocarbons. The established mass balance of four samples obtained from pyrolysis of four biomass samples in N2 and CO2 is shown in Figure 4. The mass difference of biochar was not apparent as the experimental work with the TR was conducted at temperature regime lower than 710 °C to exclude the effect of the Boudouard reaction. However, as shown in Figure 4, the mass portion of condensable hydrocarbons (i.e., tar) substantially deceased in the presence of CO2, as compared to the case in N2, which directly led to the increase of the mass portion for pyrolytic gases. The enhanced generation of pyrolytic gases was universally observed in four biomass samples. For example, a ∼57, ∼51, ∼46, and ∼68% of the mass portion of condensable hydrocarbons (i.e., tar) decrease in the presence of CO2 was

Figure 4. Mass balance of the thermal decomposition products in N2 and CO2.

observed in pyrolysis of cellulose, corn stover, oak wood, and red seaweed, respectivelyu. The high fraction of unwanted side products of thermo-chemical conversion (i.e., tar) has been recognized as a problem.45 Thus, utilizing CO2 in the thermochemical process will lead to environmental benefits. Thus, the mass balance in Figure 4 elucidates one of the key roles of CO2 during the thermal degradation of biomass: the role of CO2 is to expedite the thermal cracking of VOCs evolved from the thermal degradation of biomass. This is well consistent with the previous discussion in Section 3.1, in that the expedited thermal cracking can partially explain the enhanced generation of syngas. To gain further detailed information on the function of CO2 during the pyrolysis process, the major pyrolytic gases evolving from the thermal degradation of cellulose were monitored and quantified. The cellulose sample was intentionally considered first, as the chemical composition of cellulose was well-known as (C6H10O5)n. Unlike the experimental setup in Figure 4, the experimental temperature was limited up to 780 °C to investigate the effect of the Boudouard reaction. The concentration profiles of the major pyrolytic gases (i.e., H2, CH4, and CO) are constructed in Figure 5. Figure 5 shows the enhanced generation of the major pyrolytic gases at all of the experimental temperatures, which is quite consistent with the previous discussions. The concentration profiles of CH4 showed the typical gas evolution pattern in the thermal degradation. For example, the concentration of CH4 began to decrease as H2 and CO began to increase. This can be explained by the thermal degradation of CH4 into H2 and CO (i.e., CH4 used as substrate for H2 and CO). One of 5031

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expressed as C25.64H28.52O17.26 (i.e., normalized with H can be expressed as C0.9H1O0.6). Considering the chemical formula of cellulose as (C0.6H1O0.5)n, the pyrolytic gases at 660 °C showed the exceeded C (carbon) and O (oxygen atom), which suggests that the exceeded amount of C and O is attributed to unknown sources. The experimental data justifies that the source of C and O can be attributed to CO2 (i.e., VOCs + CO2 → CO + H2). Thus, we can conclude that CO2 can expedite the thermal cracking of VOCs and can react with VOCs. These observations provide a new methodology to utilize CO2 as the chemical feedstock, which would be environmentally benign. However, the optimal conditions associated with the required quantity of CO2, experimental temperature, and so on, have not been fully investigated at this stage of work. The same experimental work was conducted with red seaweed to expand biomass beyond terrestrial source and to test the universal influence of CO2 during the pyrolysis process. As with the experimental result in Figure 5, the concentration profiles of the major pyrolytic gases are constructed in Figure 6. The concentration profiles of the major pyrolytic gases evolving from pyrolysis of red seaweed showed a pattern similar to that of cellulose, and the same explanation in Figure 5 is possible. This identified and justified the effect of CO2 as observed in pyrolysis of corn stover and oak wood (data not

Figure 5. Concentration profiles of the major pyrolytic gases evolving from the thermal degradation of cellulose.

the most interesting observations in Figure 5 is the enhanced generation of CO in the presence of CO2 in that this observed enhanced generation of CO was nearly negligible in pyrolysis of cellulose in N2. Thus, investigating the origin of the carbon source of the enhanced generation of CO in the presence of CO2 is key to discovering the function of CO2 in the pyrolysis process. For instance, the concentration of CO could not exceed the concentration of H2 during the pyrolysis process of biomass because a significant amount of carbon would remain in biochar. Furthermore, more H2 could be generated as the pyrolysis process proceeds due to the thermal cracking (i.e., dehydrogenation), which could be a reasonable explanation for the high content of aromatic compounds in the general pyrolytic oil (i.e., condensable hydrocarbons or tar). However, our experimental results showed that the concentration of CO is higher than that of N2 in the entire experimental temperature. The experimental results in Figure 4 and 5 indirectly clarify the carbon source of CO. For example, the enhanced generation of CO is derived from the reaction between VOCs (i.e., condensable hydrocarbons) and CO 2 . To investigate the influence of CO2, the concentration of the major pyrolytic gases obtained at 660 °C in the presence of CO2 was considered. The concentrations of H2, CH4, and CO were 14.26, 8.38, and 17.26%, respectively. Considering the molecular formula of the cellulose known as (C6H10O5)n, the most interesting observation was the total carbon content in the major pyrolytic gases. For example, the chemical formula obtained from the major pyrolytic gases at 660 °C can be

Figure 6. Concentration profiles of the major pyrolytic gases evolving from the thermal degradation of red seaweed. 5032

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*Tel: 82-44-860-1457; fax: 82-44-860-1588; e-mail: [email protected].

shown). Thus, we can conclude that the influence of CO2 is universal. However, except for biomass considered in this work, the extent of the CO2 impact associated with VOCs evolving from various carbonaceous materials possibly used as chemical feedstocks for the thermo-chemical process could not be elucidated at this stage of work. So, further study is needed to fully investigate the influence of CO2. The identified influence of CO2 can be directly applied to the gasification process to increase the thermal efficiency and reduce the generation of tar in that pyrolysis is the intermediate step of the gasification process. So, steam gasification of red seaweed was conducted with a drop tube reactor (DTR). The flow rates of red seaweed and steam were both 1 g min−1. The flow rates of N2 and CO2 were 1 L min−1. Concentration profiles of syngas in N2 and CO2 are shown in Figure 7.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIP) (2914RA1A004893)

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Figure 7. Concentration profiles of syngas in N2 and CO2.

Enhanced generation of syngas was observed in the presence of CO2, which was consistent with previous discussions. However, as compared to the concentration of CO, the enhanced generation of H 2 was almost negligible, and further investigation to understand this observation is necessary. In summary, our work experimentally validated the influence of CO2 in thermo-chemical processes at a fundamental level. Our experimental data justified that CO2 not only expedited the thermal cracking of VOCs, but also reacted with VOCs. The identified influence of CO2 was universally observed in all biomass samples used in our experimental work. Thus, this study validated that the use of CO2 as a reaction medium leads to high thermal efficiency and environmental benefits.

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*Tel: 82-2-3408-4166; fax: 82-2-3408-4320; e-mail: ekwon74@ sejong.ac.kr; mail: 209 Neudong-Ro, Gwangjin-Gu, South Korea. 5033

DOI: 10.1021/es505744n Environ. Sci. Technol. 2015, 49, 5028−5034

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DOI: 10.1021/es505744n Environ. Sci. Technol. 2015, 49, 5028−5034