Article pubs.acs.org/est
Effect of Carbon Dioxide on the Thermal Degradation of Lignocellulosic Biomass Eilhann E. Kwon,*,† Eui-Chan Jeon,† Marco J. Castaldi,‡ and Young Jae Jeon*,§ †
Department of Environment and Energy, Sejong University, Seoul 143-747, South Korea Department of Chemical Engineering, The City College of New York, New York, New York10031, U.S.A. § Department of Microbiology, Pukyong National University, Pusan, 608-747 South Korea ‡
ABSTRACT: Using biomass as a renewable energy source via currently available thermochemical processes (i.e., pyrolysis and gasification) is environmentally advantageous owing to its intrinsic carbon neutrality. Developing methodologies to enhance the thermal efficiency of these proven technologies is therefore imperative. This study aimed to investigate the use of CO2 as a reaction medium to increase not only thermal efficiency but also environmental benefit. The influence of CO2 on thermochemical processes at a fundamental level was experimentally validated with the main constituents of biomass (i.e., cellulose and xylan) to avoid complexities arising from the heterogeneous matrix of biomass. For instance, gaseous products including H2, CH4, and CO were substantially enhanced in the presence of CO2 because CO2 expedited thermal cracking behavior (i.e., 200−1000%). This behavior was then universally observed in our case study with real biomass (i.e., corn stover) during pyrolysis and steam gasification. However, further study is urgently needed to optimize these experimental findings.
1. INTRODUCTION Various energy applications using H2 as an energy source and hydrogen economy have been highlighted in response to increased awareness of environmental problems, such as global warming triggered by anthropogenic greenhouse gas.1−3 Unfortunately, the commercial scale of hydrogen production has been heavily oriented toward the catalytic reforming process of natural gas, which is petroleum based.4,5 Thus, exploiting or converting energy from a nonpetroleum source (i.e., lignocellulosic biomass) via thermochemical processes, such as pyrolysis and gasification is desirable owing to its intrinsic carbon neutrality.3,6−10 In addition, technical advances in thermochemical processes have been achieved and developed through long-standing efforts to optimize and maximize their thermal efficiency. Pyrolysis, the thermal decomposition of materials in the absence of an oxidant,8,11−13 has been used to convert various feedstocks including biomass,11,14−17 coal,18,19 polymer,20,21 and municipal solid waste22,23 into a blend of solid, liquid, and gaseous products depending on process variables. The complex chemical composition of pyrolytic oil depends on many factors,24,25 including biomass type, feedstock pretreatment, and pyrolysis conditions (e.g., temperature, heating rate, pressure, residence time, and gaseous environment). As a result, the fuel properties of various pyrolytic oils usually vary widely. Gasification,6,9,19,26 the transformation of solid fuel into a gaseous sustainable energy carrier, is also an attractive technology for the production of synthesis gas (H2 and CO). The production of hydrogen from biomass is attracting increased attention because using biomass to produce energy © 2013 American Chemical Society
may contribute significantly to sustainable development and reduce greenhouse gas emission. The pyrolysis and gasification of conventional fuel, such as coal, have been well documented.18,19 However, accessible information on lignocellulosic biomass6,7,11,13−15,27 is comparatively limited owing to the intrinsic heterogeneous matrix of biomass. Thus, fundamental investigation of this process is needed. In particular, the impact of CO2 on thermochemical processes has yet to be investigated fully. The main objective of this study was to investigate mechanistically the influence of CO2 on thermochemical processes. As a case study to validate the CO2 cofeed impact on these processes, the experiment was conducted with corn stover.
2. MATERIALS AND METHODS 2.1. Reagents. Powdered cellulose and xylan extracted from birch wood were purchased from Sigma Aldrich (St. Louis, U.S.A.). Corn stover was collected from Kyung-Nam Agricultural Research Institute in Korea. The samples were processed into a powder with a Wiley-Mill (Wiley, U.S.A.) using a 2-mm screening sieve. The ground corn stover was placed in a dry oven at 80 °C for 4 days before composition analysis. The composition of the corn stover was determined using the standard biomass analysis procedures proposed by Sluiter et al.28 Protein content was quantified according to Received: Revised: Accepted: Published: 10541
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Smith et al.,29 with bovine serum albumin (Promega, U.S.A.) as a standard reference. The basic composition analysis is summarized in Table 1.
2.4. Steam Gasifier Setup. Steam gasification with stainless tubing (25.4-mm inner diameter) and a continuous flow system was conducted using a TR with a screw feeder (Figure 1). The rate of sample loading into the gasifier was
Table 1. Composition of Corn Stover Used for the Study component cellulose xylan lignin extractives sucrose acetate ash arabinan protein unknown compounds
composition based on dry mass (wt/wt%) 34.1 21.2 18.6 9.8 3.2 3.9 1.4 1.2 1.6 5.0
± ± ± ± ± ± ± ± ± ±
0.7 1.2 0.4 0.8 1.2 0.2 0.3 0.7 0.2 0.7
2.2. Thermogravimetric Analysis (TGA). A Netzsch STA 499 F1 Jupiter TGA unit capable of differential temperature analysis (DTA) measurements was used with a temperature condition that increased from 20 to 1200 °C at a heating rate of 10−1500 °C min−1. Two embedded mass-flow controllers regulated the flow rates of the purge and protective gases in the TGA unit. The temperature ramp rates were provided as an input to the software, which then controlled the furnace to achieve the desired heating rates. All data were recorded digitally, and S-type thermocouple readings were compared simultaneously to the target temperature and time. The TGA unit has 2 inlet ports. One port was used for a protective inert gas (e.g., N2, He, and Ar), which shrouded the balance mechanism from heat or effluent gases and was set at a flow rate of 20 mL min−1. The other port was used to introduce the gas mixture of interest and provide the desired atmosphere during the experiment. The flow rate of this inlet was 80 mL min−1. The combination of the protective and atmosphere gas flows yielded a total flow past the test sample of 100 mL min−1, which was maintained for all experiments. The sample loading for each experiment was approximately 10 ± 0.3 mg. Morphological and structural information for the residuals was acquired using a Hitachi S4700 high-resolution scanning electron microscope in plane or cross-sectional views. In conjunction with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy was used to detect the elements present in a selected area of the SEM image, providing qualitative and quantitative information. 2.3. Tubular Reactor (TR) Setup. A batch type TR was used to conduct experiments under ambient pressure. A 25.4mm stainless Ultra-Torr vacuum fitting (Swagelok SS-4-UT400) was used to connect quartz tubing (25.4-mm outer diameter, Chemglass CGQ-0900T-13) and stainless tubing to ensure airtight conditions. The required experimental temperature was achieved with an external programmable split-hinged furnace (AsOne TMF-300N, Japan) over a temperature range of 400−1000 °C. Temperature was simultaneously compared with the thermocouple reading to ensure that the target temperature was reached. The sample loading for each batch was 10 g, and the sample temperature was measured using a Ktype thermocouple. Condensable hydrocarbons (i.e., tar) were collected with a condenser to establish the mass balance from the TR.
Figure 1. (a) Instrument layout for the tubular reactor, (b) screw feeder detail, and (C) tubular reactor detail.
controlled using 2 screw feeders, as shown in Figure 1b. The required target temperature was controlled with a split-hinged furnace, with temperature simultaneously compared to the thermocouple reading to ensure that the experimental temperature was reached. The split-hinged furnace consisted of 3 furnace elements that could be individually controlled, as shown in Figure 1c. 2.5. Gas Steam Flow Control for the TR and Steam Gasifier. All gases used in the experimental work were of ultrahigh purity (i.e., 99.999%) and obtained from TechAir (Korea). All gas flow rates were controlled using a thermal mass flow meter (Brooks 5800S series, U.S.A.). The steam flow rate was controlled using a LabAlliance high-performance liquid chromatography pump (PN 210SFP01, U.S.A.), with steam generated using a heat tape (Omega STR201-060, U.S.A.) and a cartridge heater (CIR-1013/120, U.S.A.) at a temperature of 300 °C. To maximize heat transfer from the heating sources, we used 5 m stainless tubing with a 1.587-mm outer diameter. 2.6. Analysis for Identification and Quantification. The effluents from the TGA unit, TR, and steam gasifier were sent to either a microgas chromatographer (GC; Agilent 3000A, U.S.A.) or GC/mass spectrometer (GC/MS; Agilent 9890/ 5973, U.S.A.) for the identification and quantification of chemical species. A sampling pump (B19310TM5, Air Dimension, Inc., U.S.A.) capable of pumping 10−100 mL min−1 was used, and the lag times of the gaseous products from the TGA unit, TR, and steam gasifier were calculated to be less than 2 s based on the volume of the transfer line (3 mL). The injection block of the GC/MS unit contained both 10-port and 6-port valve assemblies (Valco Instrument, U.S.A.). The sampling system was maintained at 300 °C to minimize the condensation and adsorption of tar onto its surface.
3. RESULT AND DISCUSSIONS 3.1. Basic Characterization of the Thermal Degradation of Cellulose and Xylan. the initial experiment was 10542
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thermal degradation of cellulose is mainly limited to the reaction between the volatile compounds evolved from the thermal degradation of cellulose and CO2 because the DTA curves in N2 and CO2 were almost identical. However, SEM images in N2 and CO2 are quite different, suggesting that different thermal degradation pathways are followed in the presence of CO2. The SEM image of carbon residuals in Figure 2 cannot be explained if the influence of CO2 is limited to the volatile compounds from the cellulose sample. Thus, the influence of CO2 on the cellulose sample surface would be expected after most volatile compounds were devolatised, thereby leading to the differences in the morphology of carbon residuals in N2 and CO2. The reaction between carbon and CO2 can be explained by the Boudouard reaction. Specifically, the reaction between solid carbon and CO2 occurs and the turns into CO: C(s) +CO2 → 2CO . However, as evidenced in Figure 3, the Boudouard
conducted with cellulose and xylan as references to characterize the influence of CO2 during thermal degradation. Cellulose and xylan were used to avoid the complexities arising from the heterogeneous matrix of biomass. A series of TGA measurement with cellulose and xylan (i.e., pyrolysis of cellulose and xylan) was carried out at the heating rates of 10 and 500 °C min−1 over a temperature range of 25−900 °C in N2 and CO2 atmospheres. SEM images of residuals of the thermal degradation of cellulose are depicted in Figure 2.
Figure 3. ΔS and ΔG profiles of the Boudouard reaction calculated using HCB software.
reaction is only thermodynamically favorable at temperatures above 710 °C, which can be demonstrated by the Gibbs free energy calculation using the HCB software package. A negative Gibbs free energy indicates a spontaneous reaction. Although the residual has been identified as mostly carbon using energy dispersive X-ray spectroscopy analysis, the expected mass change via the Boudouard reaction cannot be identified in Figure 2. Two possible scenarios can explain this observation: (1) The temperature initiating the Boudouard reaction must be higher than the calculated temperature (i.e., theoretical temperature) of 710 °C or (2) the Boudouard reaction is slow because the mass difference in N2 and CO2 at the heating rates of 10 and 500 °C min−1 cannot be identified in Figure 2. To get more detailed information on the influence of CO2 during the thermal degradation of cellulose, we analyzed the effluent from the TGA unit with a micro-GC. One interesting observation was the substantial increase in H2 and CO generation in the presence of CO2 (data not shown). For example, the concentration of H2 and CO in the presence of CO2 was quantified by a factor of ∼2, and this enhancement was clearly observed under the high heating rate (i.e., 500 °C min−1). The enhanced generation of H2 and CO could not be identified owing to the significant dilution caused by the long residence time in the TGA unit. However, considering that the temperature is lower than 710 °C, this enhanced generation of H2 and CO in the presence of CO2 can be explained by the reaction between volatile chemical species evolved from the cellulose sample and CO2. Thus, the unknown reactions initiated by CO2 could be expected. This result is discussed in more detail below.
Figure 2. Thermograms of cellulose at a heating rate of 10 and 500 °C min−1 in N2 and CO2 and the scanning electron microscopy (SEM) images of the residuals.
As shown in Figure 2, the mass decay curves are almost identical when the heating rate is the same. For example, the initiation and end temperatures of the thermal degradation are almost identical. This observation suggests that CO2 does not influence the physical aspect (i.e., mass change) of thermal degradation at the same heating rate. However, more final mass conversion was achieved at the heating rate of 500 °C min−1 compared with that at 10 °C min−1. The residual masses at the heating rates of 10 and 500 °C min−1 were ∼17% and ∼8%, respectively. This observation suggests that volatilization actively occurs via bond scission of the cellulose polymer backbone at the high heating rate, which leads to the high yield of pyrolytic oil. This observation is consistent with previous studies by other authors.25,30−33 A more condensable oil yield can be achieved during the fast pyrolysis process. In addition, considering the main gas production (i.e., H2 and CO) during the pyrolysis process, the generation of H2 via thermal cracking leads directly to the formation of carbon residual (i.e., coke). Figure 2 does not clarify the influence of CO2 during thermal degradation. Furthermore, the DTA curves (data not shown) in N2 and CO2 were almost identical. This observation indirectly implies that the influence of CO2 on the cellulose sample is extremely limitedthat is, the influence of CO2 during the 10543
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The TGA experiment was conducted with xylan at the heating rate of 500 °C min−1 in N2 and CO2. The thermal degradation pattern was not differentiated in the cellulose experiment. Thus, the thermogram of xylan is not presented in this work. However, one interesting observation that occurred during the TGA experiments with xylan is shown in Figure 4.
Figure 4. Xylan sample at 500 °C at the heating rate of 500 °C min−1 in N2 (a) and CO2 (b).
The xylan sample examined at the rate of 500 °C min−1 in N2 and CO2 was inflated by thermal shock. This inflation did not occur at the low heating rate (i.e., 10 °C min−1). At this stage of research, we do not completely understand why this inflation accompanied only the high heating rates. 3.2. Influence of CO2 on Volatile Chemical Species during the Thermal Degradation of Cellulose and Xylan. As discussed in section 3.1, more gaseous products including H2 and CO were generated in the presence of CO2 at the high heating rate (i.e., 500 °C min−1) owing to the significant dilution in the TGA experiments. Thus, to obtain more detailed information on the influence of CO 2 during thermal degradation, we quantified the main gaseous products from the pyrolysis of cellulose and xylan. We used the batch type TR, into which a mixture of 5 g of cellulose and 5 g of xylan (total, 10 g) was loaded. The effluent from the TG unit was quantified after the condensation of the heavy hydrocarbons (i.e., condensable hydrocarbons). This experimental TR setup was expected to provide better understanding of the CO2 influence on the volatile chemical species evolved from the thermal degradation of cellulose and xylan compared with the experimental work conducted with the TGA unit. Our TGA unit was the top-loading type with a crucible, which impeded or limited the mass transport of CO2 during the experiments. The heating rate of the TR was 30 °C per 3.5 min (i.e., 8.57 °C min−1), which was controlled via a programmable tubular furnace because micro-GC analysis under the isothermal condition of 110 °C for each run took 3.5 min. The flow rate of N2 and CO2 was 150 mL min−1. The concentration profiles of H2, CH4, and CO in N2 and CO2 are depicted in Figure 5, which shows that the generation of these gaseous products was substantially enhanced in the presence of CO2. For example, the concentration of H2 and CH4 in the presence of CO2 was enhanced by factors of 4 and 7, respectively. As discussed earlier and evidenced in Figure 3, the Boudouard reaction is only thermodynamically favorable at temperatures higher than 710 °C. Considering the concentration of CO at 700 °C in the presence of CO2, the concentration of CO is enhanced by a factor of 10, which cannot be explained by the Boudouard reaction. Thus, CO2 likely expedites the thermal cracking of volatile compounds evolved from the mixture of cellulose and xylan. In addition, this CO2 influence can be applied at the low heating rate. This
Figure 5. Concentration profiles of H2, CH4, and CO from the effluent of the tubular reactor in N2 and CO2.
CO2 effect was not clearly defined in the experiments described in section 3.1. The concentration profiles of CH4 in the presence of CO2 are interesting in that the maximum achievable concentration is reached at relatively lower temperatures compared to those for H2 and CO. Furthermore, the concentration of CH4 begins to decrease when the concentration of H2 and CO increases. This reaction pattern is commonly observed in the gas phase reaction, which usually represents the thermal degradation hierarchy.20,21 For instance, the decreased chemical species is used as the substrate. In other words, bond dissociation from the thermal energy of cellulose and xylan forms CH4, and then CH4 is used as the substrate to form H2 and CO or other reaction mechanisms for creating H2, and CO blocks the pathway for CH4 formation. Thus, CO2 expedites thermal cracking from cellulose and xylan to form CH4 at temperatures lower than 520 °C compared with thermal degradation in N2. In addition, CO2 expedites thermal cracking to form hydrogen and CO at temperatures higher than 520 °C. At this stage of our research, a precise reaction mechanism remains unconfirmed. However, the enhanced generation of the gaseous products discussed above directly decreases the formation of condensable hydrocarbons (i.e., tar) in that the enhanced generation of CH4 and CO in the presence of CO2 requires the condensable hydrocarbons as a carbon source. The condensable hydrocarbons collected during the experimental work with the TR 10544
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were substantially decreased: approximately 67% based on the mass balance. However, the composition of these condensable hydrocarbons in N2 and CO2 were not differentiated in the GC/MS analysis. 3.3. Influence of CO2 on the Thermal Degradation of Corn Stover. Validating identified CO2 effects on real biomass with a heterogeneous matrix is imperative. As summarized in Table 1, real biomass displays complex composition. Corn stover contains a significant amount of lignin (18.6 ± 0.4 wt %) and other compounds (extractives, sucrose, acetate, arabinan, protein, and unknown compounds). In addition to its complex composition, corn stover has a much more complex structure than that of experimental reagent we used for the experiments described in sections 3.1 and 3.2. Results of the experimental investigation of corn stover under the same conditions as those depicted in Figure 3 and representative thermograms are shown in Figure 6.
Figure 6. Thermograms of corn stover at a heating rate of 500 °C min−1 in N2 and CO2 and the scanning electron microscopy (SEM) image of the residual.
Figure 7. Concentration profiles of H2, CH4, and CO from the effluent of the tubular reactor in N2 and CO2.
The thermal degradation pattern of corn stover was similar to that of cellulose even though corn stover has a heterogeneous matrix. For example, the thermal degradation in N2 and CO2 was not differentiated, and the SEM images of residuals in N2 and CO2 were different. Similarly, the concentration of the main gaseous products (i.e., H2, CH4, and CO) was higher in the presence of CO2. Thus, we conclude that CO2 expedites the thermal cracking of volatile chemical species, which directly reduces tar. To get more detailed information about this phenomenon, we quantified the main gaseous products released from the thermal degradation of corn stover in the TR (Figure 7). The experimental conditions with the TR for the corn stover experiment were different from those used in the previous experiment. The flow rates of N2 (i.e., 200 mL min−1) and CO2 (i.e., 200 mL min−1) were set, because we do not know the optimal flow rate of CO2. The heating rate was 20 °C per 3.5 min (i.e., 5.7 °C min−1). This lower heating rate (compared with that depicted in Figure 5 [8.57 °C min−1]) was selected to study the temperature effect in detail. The trend of concentration profiles shows a pattern similar to that shown in Figure 5. However, the quantified gaseous products plotted in Figure 7 have a relatively lower magnitude compared to those in Figure 5 owing to a dilution factor that arose from the experimental conditions (i.e., heating and flow rates of N2 and CO2). Thus, the observation in Figure 7 verifies that the influence of CO2 can be applied to real biomass. This
observation reflects a very important rationale with respect to the enhancement of thermal efficiency at relatively low temperatures in thermochemical processes by means of using CO2 as a reaction medium. The consecutive thermal degradation of corn stover from the same batch represented in Figure 7 was conducted. The heating rate was changed to 50 °C per 3.5 min (i.e., 14.29 °C min−1) in the temperature range of 700−1000 °C, and then the temperature was isothermally maintained for 35 min to investigate how fast the Boudouard reaction is achieved with char derived from corn stover. The experimental results are illustrated in Figure 8. Concentrations of H2 and CH4 are not shown owing to their negligible amounts compared to that of CO. As expected, the generation of CO in CO2 was expedited as the experimental temperature increased; the maximum achievable concentration of CO was nearly 80%. The concentration of CO during the isothermal run was gradually decreased owing to the consumption of the carbon source (i.e., batch reaction) by means of the Boudouard reaction. As evidenced in Figure 8, the final mass conversion by the Boudouard reaction took nearly 34 min. Considering the time for the isothermal run, the reaction rate of the Boudouard reaction seems to be quite slower than what would be expected given the identified CO2 influence on the volatile compounds. Thus, the identified CO2 influence directly leads to high thermal efficiency and environmental benefits of gasification because the pyrolysis process is its intermediate step. Furthermore, the generation of condensable hydrocarbons 10545
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Figure 8. Concentration profiles of CO in the effluent from the tubular reactor in N2 and CO2.
(i.e., tar) is one of the operational problems in gasification. Similar to the results with cellulose and xylan, the condensable hydrocarbons derived from corn stover were decreased approximately 63%. This well-known problem would be resolved or mitigated if CO2 were used during gasification. 3.4. Steam Gasification of Corn Stover. Owing to the incapability for continuous feeding of biomass, the identified CO2 influence on thermal degradation does not reflect the real nature of the thermochemical process. Therefore, steam gasification was conducted with the instrument illustrated in Figure 1. The flow rate of corn stover and steam was 1 g min−1, respectively. The flow rate of N2 and 50% N2/50% CO2 was 3 L min−1. The generation of H2 and CO was enhanced in the presence of CO2, which was consistent with previous observations. The concentration of H2 and CO at 700 °C in the presence of CO2 was increased by factors of ∼1.8 and ∼1.5, respectively, which related directly to the thermal cracking behavior induced by CO2. H2 generation was enhanced at 900 °C in N2 and 50% N2/50% CO2, which is explained by the water-gas shift (WGS) reaction. The WGS reaction is a reversible chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen. However, Figure 9 shows that the enhanced generation of H2 and CO in the presence of CO2 enabled the generation of additional H2 via the WGS reaction. The influence of CO2 on the WGS reaction could not be confirmed at this stage of our research. Despite finding clear evidence of the influence of CO2, we will need to conduct further experimentation to determine optimal reaction conditions, such as the amount of CO2. In summary, this study experimentally validated the influence of CO2 in thermochemical processes at a fundamental level. The main constituents of biomass, cellulose and xylan, were used to show the CO2 cofeed impact on thermochemical processes. The thermal cracking of volatile chemical species evolved from cellulose and xylan is expedited in the presence of CO2. Our case study with real biomass (i.e., corn stover) with a heterogeneous matrix yielded results similar to those obtained with cellulose and xylan. Furthermore, this study showed that the influence of CO2 on the steam gasification of corn stover via a continuous feeding system is universal. Thus, this study validated that the use of CO2 as a reaction medium leads to high thermal efficiency and environmental benefits.
Figure 9. Concentration profiles of syngas (H2 and CO) in N2 and 50% N2/50% CO2.
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AUTHOR INFORMATION
Corresponding Authors
*Tel: 82-2-3408-3320. E-mail:
[email protected]. *Tel: 82-51-629-5619. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Human Resources Development program (No.20094010200030) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry, and Energy.
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REFERENCES
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dx.doi.org/10.1021/es402250g | Environ. Sci. Technol. 2013, 47, 10541−10547
