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Investigations on the synergistic effects of oxygen and CaO for bio-tars cracking during biomass gasification Fengkui Yin, Priscilla Tremain, Jianglong Yu, Elham Doroodchi, and Behdad Moghtaderi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02136 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 6, 2016
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Investigations on the synergistic effects of oxygen and CaO for bio-tars cracking during biomass gasification Fengkui Yin, Priscilla Tremain, Jianglong Yu, Elham Doroodchi, Behdad Moghtaderi* Priority Research Centre for Frontier Energy Technologies & Utilization, Discipline of Chemical Engineering, School of Engineering, Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan, NSW 2308, Australia. Abstract In this study, coupled thermogravimetric analysis with Fourier transform infrared spectroscopy (TG-FTIR) was primarily used to investigate the extent of bio-tar cracking apparent from biomass pyrolysis and partial oxidative gasification with and without the addition of CaO. TG kinetic analysis indicated that the gasification process reaction rate increased for increasing the oxygen and CaO content. The effect of oxygen in the gasification environment was more significant for bio-tars cracking in comparison to CaO addition but resulted in higher CO and CO2 yields in the syngas. However, the presence of CaO resulted in greater catalytic conversion of tars into higher H2/CO syngas ratios at lower temperatures. In the present study, syngas with a H2/CO ratio of ～1.5 can be achieved under conditions of Ca/B ratio of 3 and 5% O2 content. Therefore, it is crucial to control both the oxygen and CaO content in the biomass gasification process in order to achieve synergetic effects relating to bio-tar cracking and to ultimately produce syngas of the desired compositional requirement. Keywords: TG-FTIR, biomass, pyrolysis, partial oxidative gasification, bio-tars cracking. ∗ Corresponding author. E-mail address:
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1. Introduction Currently, biomass as a fuel is considered as an alternative and renewable energy resource.1 Biomass consumption is CO2 neutral in comparison to other fossil fuels and is abundantly available globally. The low cost and substantial quantities of biomass resources available, offer potential for it to replace fossil fuels as well as reduce greenhouse gas emissions.2-4 Thermochemical conversion technologies including pyrolysis, gasification and combustion are the most efficient way to convert biomass into power or biofuels such as bio-char and syngas.5 Bio-char, bio-tars and syngas are the three main products from biomass pyrolysis, while the target yield for biomass gasification is syngas production. The biomass gasification process is composed of three primary stages; biomass dehydration, pyrolysis and char gasification.1, 6, 7 A series of techniques 8-12 have been developed to improve gasification efficiency for the purpose of high quality syngas production. However, a major hurdle in the development of biomass gasification technologies are bio-tars, a byproduct formed during the gasification process, which can clog gasifiers, block pipes, deactivate catalysts and so on.13 In order to produce high quality bio-syngas without/with lower bio-tars content, a variety of metal based catalysts 14-20 such as alkali metals, Ni, Fe, Ce and Ca as well as activated carbon have been investigated in the literature to reduce tar formation in the gasification process. However, CaO rich minerals including stone dust and dolomite are not only cost effective, but are abundant natural resources that have been shown to perform well in tar destruction during biomass gasification. Moreover, CaO can enhance the water gas shift (WGS) reaction and produce H2 rich syngas via in situ CO2
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absorption, which greatly increases the lower heating value of syngas. 9, 14, 21-23 Therefore, CaO was considered as a potential catalyst for industrial application. In addition to catalytic tar cracking, several other methods are often employed to remove tars from syngas, such as thermal chemical conversion and mechanical separation methods. 24, 25 Of these, thermal chemical conversion, i.e. partial oxidation, inside the gasifier has been considered as a more effective and economical method to remove tars. Partial oxidative gasification can provide heat from partial combustion of pyrolysis gases for tar cracking. Partial oxidation is a type of chemical reaction which occurs under a sub-stoichiometric fuel-air mixture, creating a hydrogen-rich syngas. Partial oxidation for biomass tar cracking has been extensively investigated in the literature. Zhao et al. 13 investigated biomass tar reduction under a partial oxidative environment (O2 concentrations were 0%, 1% and 5%, respectively) and found that a tar conversion rate of 86% could be reached at 700 °C. Moreover, the coupling of char and a certain amount of oxygen benefitted the decomposition of larger polycyclic aromatic hydrocarbon (PAH) tar compounds, and it was a feasible method for bio-tar reduction. Su et al. 26 carried out tar destruction experiments under partial oxidative conditions via a fixed bed reactor and advocated that certain amounts of oxygen enhance H2 yield due to cracking of secondary tar. However, among the above studies, limited research attention was paid to the cracking of individual tar components such as aliphatic and aromatic compounds during partial oxidative gasification in the presence of CaO. In order to gain a fundamental understanding of the catalytic cracking abilities of different catalysts or experimental conditions, continuous evolved gas analysis as well as kinetic analysis is critical. Thermogravimetric (TG) analysis coupled with Fourier transformed infrared spectroscopy (FTIR) is an effective method to gain both a kinetic
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understanding as well as analyze gas species evolution over the course of a reaction. In this work, a comprehensive investigation on bio-tars cracking from the synergy of CaO and oxygen was conducted. In addition, kinetic analysis of biomass partial oxidative gasification was performed, utilizing TG data, based on the volumetric reaction model (VM) to study the catalytic effect of oxygen on the gasification process. Through this research, knowledge on the cracking of volatile matter catalyzed by CaO during partial oxidative gasification was obtained, which may be used in the improvement and design of lower temperature (600 °C) was the result of calcination 11 ACS Paragon Plus Environment
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of CaCO3 formed during the process. The reversible reaction (Eq. 10) between CaO and CaCO3 during this process is shown as follows: CaO + CO2 ↔ CaCO3
(10)
The comparison of CO2 emission profiles for different Ca/B ratios during gasification under both 1% and 5% oxygen content are presented in Fig. 2d. It can be seen that the amount of CO2 released at the primary stage (first peak) was proportional to the oxygen content. Moreover, the amount of CO2 released at the char gasification stage (second peak) was reduced as Ca/B ratio increased due to capture of CO2 by CaO to form CaCO3. Meanwhile, it confirmed that CaO had dual functions which are both catalysis and CO2 absorption during the gasification process. 4, 35, 36 3.3.2. Emission of H2 Fig. 3a and 3b compare the H2 release profile during biomass pyrolysis and partial gasification at different Ca/B ratios and oxygen contents. It can be seen in Fig. 3 that the majority of H2 was evolved in the temperature range of 400-600 °C during pyrolysis, which was mainly due to thermal cracking of tar residue. Fig. 3 also shows that the H2 concentration significantly reduced with increasing oxygen content in the same temperature range, indicating that some H2 was combusted with O2. However, with addition of CaO, two stages for H2 release (250-400 °C and 400-600 °C, respectively) during partial oxidative gasification were observed. This suggests that CaO can greatly catalyse the conversion of volatiles into H2 in the temperature range of 250-400 °C. Moreover, it can also be seen that the H2 concentration also increased for the temperature range of 400-600 °C. It can be concluded that H2 yield can be significantly increased with increasing Ca/B ratio under the same oxygen content.
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3.3.3. Emission of CO CO is the major gas produced from the pyrolysis of lignin and is mainly released as a result of breaking C-O-C and C-O bonds. 32, 33As Fig. 3c and 3d show, the CO emission profiles showed two zones during pyrolysis. The highest CO emissions occurred at approximately 350 °C, due to the decarboxylation reactions of alkyl and unsaturated side-chains containing the carbonyl functional groups (-CHO) of biomass. The subsequent CO emissions in the second zone from 400-600 °C during pyrolysis were attributed to thermal cracking of tar residue, such as phenol-type aromatic rings,37 in the solid sample (secondary pyrolysis). In comparison, the CO release profile during partial oxidative gasification was generally larger than that during pyrolysis. A similar CO evolution trend to pyrolysis can be seen in Fig. 3c and 3d, for the addition of 1% O2 whereas at 5% O2 significantly more CO was released below 400 °C. For treatments in which CaO was added, the major release of CO occurred in the second zone (400-650 °C). The main reactions that may occur for CO formation during partial oxidative gasification are shown in Eq. (11) and Eq. (12): CxHy +
O2 → xCO + H2O
(11)
C + O2 → CO
(12)
For gasification at the same level of O2 and increasing Ca/B ratio, CO release during the first zone (250-400 °C) decreased. This is possibly due to the excess CaO, in relation to biomass, allowing for a greater catalytic effect, which leads to the conversion of volatile matters to CO2 instead of CO in the presence of O2 (see Fig. S2). In the char gasification zone (Eq. (12)) in the temperature range of 500-650 °C, it can be seen in Fig. 3c and 3d that the catalytic effect of CaO greatly increased the evolution of CO. It was also observed that for treatments in the presence of CaO, CO evolution decreased 13 ACS Paragon Plus Environment
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as the O2 content was increased, primarily due to increased CO2 production. Hence, with increasing O2 content, more CO and CO2 were formed due to cracking and oxidation of tars at a relatively faster rate. 3.3.4. H2/CO ratio H2/CO ratio is regarded as a benchmark for evaluation of syngas quality produced from gasification processes.38, 39 Fig. 4 presents the variation of H2/CO ratio as a function of temperature during pyrolysis and partial gasification at different Ca/B ratios. During pyrolysis, the H2/CO ratio initially decreased for temperatures below 350 °C, which was possibly attributed to the formation of more CO, CO2 and CH4 (see Fig. S3 and Fig. S4) in this temperature range. However, the H2/CO ratio dramatically increased as temperature increased from 400 °C to 550 °C due to secondary pyrolysis, and then significantly decreased until all tars were completely decomposed by 650 °C. With the addition of O2, similar H2/CO profiles were obtained, however in comparison to pyrolysis, the H2/CO ratio dropped with increasing oxygen content. This is possibly due to the H2 preferably reacting with O2 to form water, while the volatile matter was converted into CO/CO2. When CaO was added, it can be clearly seen in Fig. 4a that the H2/CO ratio during the devolatilization stage of gasification (310-390 °C) was much higher than that without the addition of CaO and the H2/CO ratio was proportional to CaO content. This suggests that tars can be catalytically converted by CaO into syngas of higher H2/CO ratios. When the temperature was above 550 °C, the H2/CO ratio was lower than 2 mainly due to gasification of heavy tar species and/or char. The highest H2/CO ratio was observed for a Ca/B ratio of 3 and 1% O2. When the O2 content was increased to 5%, a similar H2/CO ratio profile was obtained, but the maximum ratio reached was lower than that of 1% (see Fig. 4b). Generally it was
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observed that CaO can greatly catalyze the gasification process to produce high H2/CO ratio syngas at partial oxidative conditions at lower temperatures. 3.3.5. Emission of CH4 Fig. 5 shows there were two observable peaks for CH4 emissions during the pyrolysis of biomass. For pyrolysis, the primary emission of CH4 occurred at 420 °C and a second peak occurred at around 580 °C. These results were consistent with a study by Parshettiet al. 40. According to the literature,34 methane formation primarily occurs from the cracking of methoxy (-OCH3) and methylene groups during pyrolysis. The major release of CH4 can be attributed to the decomposition of all three main components of biomass (hemicellulose, cellulose and lignin). The second peak in CH4 release during pyrolysis may be attributed to the secondary pyrolysis of hemicellulose at higher temperature as well as the decomposition of the -OCH3 content in lignin.31 It can also be seen in Fig. 5 that the CH4 yield was dramatically lower during partial oxidative gasification in comparison to pyrolysis. Furthermore, the methane evolution almost disappeared when oxygen content was increased to 5% (shown in Fig. 5b). The significant fall in the evolution of CH4 was attributed to it being instantly combusted in the presence of increasingly oxidative conditions (see Eq. 13 and 14), which was also confirmed by the variation of CH4/CO2 ratio and CO/CH4 ratio (see Fig. S5 and Fig. S6, respectively) presented in the supporting information. CH4 + 2O2 → CO2 + 2H2O CH4 +
(13)
O2 → CO + 2H2O
(14)
In the presence of CaO, the emission of CH4 was greatly increased during partial oxidative gasification when the temperature was lower than 400 °C, indicating that CaO can greatly catalyze the conversion of tars into CH4 under oxidative conditions during
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the devolatilization stage. When the temperature was above 500 °C, the CH4 release profile almost disappeared, primarily due to the char gasification (see Fig. S5 and Fig. S6). It can be concluded that high concentration of CH4 can be expected with high Ca/B mass ratios at the temperatures less than 500 °C during partial oxidative gasification of biomass. 3.3.6. Emission of CH3OH Methanol (CH3OH) was reported as one of major gases released during pyrolysis of lignin.41 Methanol is easily condensed at ambient temperature, resulting in a decrease in the LHV of syngas. In this work, CH3OH was considered as a light tar component and was detected to compare the release of one of light bio-tars during pyrolysis and partial oxidative gasification. In Fig. 6 an initial peak can be seen between 250-300°C for all treatments, attributed to the decomposition of hemicellulose.41 Fig. 6a shows the main emission of CH3OH occurred around 350 °C for pyrolysis and partial oxidative gasification. This primary evolution was dominantly caused by the thermal decomposition of lignin.41 Chen et al. 42 proposed that the evolution of CH3OH was possibly as by-product of the rearrangement of –OCH3. It can be seen in Fig. 6a and Fig. 6b that the yield of CH3OH significantly decreased with increasing oxygen content from 1% to 5%. Moreover, it was observed that CaO can greatly catalyze the cracking of CH3OH in a partial oxidative environment. Specifically, when biomass was gasified under 1% oxygen content, CH3OH yield decreased gradually with increasing Ca/B ratio. However, when the oxygen content increased to 5%, the CH3OH release almost disappeared with increasing Ca/B ratio (i.e. from 1 to 3), showing much greater efficiency than the 1% O2 treatment. Therefore, it can be concluded that CaO can greatly catalyze the decomposition reaction of CH3OH under low oxygen contents.
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3.3.7. Emission of C-H compounds The integral area of C-H group was obtained for the FT-IR spectrum ranges of 30002650 cm-1. Symmetrical and asymmetrical C-H stretching, vibration of aliphatic CH3 and CH2 were all assigned as part of the C-H group representing aliphatic hydrocarbons. The C-H compounds were mainly produced by the thermal decomposition of cellulose and hemicellulose.43 It can be seen in Fig. 6c and Fig. 6d that the major emission of aliphatic hydrocarbons occurred at about 360 °C during pyrolysis. Unlike pyrolysis, the C-H group yield, under partial oxidative gasification conditions, reduced with increasing oxygen content. It was also observed that the peak emission temperature shifted to lower temperatures for gasification as oxygen content increased (350 °C for 1% O2 and 340 °C for 5% O2, respectively). This is due to a reduction in the activation energy of the process with the introduction of oxygen and especially in presence of CaO, causing the depolymerization and cleavage of bonds to be more favorable. For the 1% and 5% O2 conditions there also appeared to be an increase in C-H group release at lower temperatures when CaO was present, evident in the broad peak area for the Ca/B treatments. This signifies the catalytic effect of CaO in the breakdown of the biomass structure at lower temperatures resulting in sooner release of the C-H group and a reduction in the intensity of peak emission. It also can be seen in Fig. 6c and Fig. 6d that the C-H group yield greatly reduced when the oxygen content increased from 1% to 5% during gasification without CaO, and it also reduced with increasing Ca/B ratio at the same oxygen content. 3.3.8. Emission of C=O groups The C=O functional group in acetic acid, ketones, aldehydes, carboxylic acids, primary amides and esters contributed to the 1660-1820 cm-1 IR absorption band. The C=O group is an important group of volatiles as the carbonyl group is considered one of 17 ACS Paragon Plus Environment
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the functional groups which have a diverse array of reaction pathways. 44 C=O compounds were possibly produced from the thermal decomposition of cellulose and hemicellulose during pyrolysis. 43 The yield of C=O compounds in pyrolysis were much higher than under gasification conditions. As can be seen in Fig. 7a and Fig. 7b, the release of C=O compounds significantly reduced with the addition of oxygen. With increasing O2 present the C=O compounds further reduced as there was more O2 in the reaction atmosphere to break the C=O bond. Meanwhile, with increasing Ca/B ratio, the release of C=O compounds slightly reduced, which suggests that CaO also played a role in decomposing C=O compounds into lighter gases such as CO and CO2. 3.3.9. Emission of aromatic compounds Aromatic compounds including PAHs are the toughest bio-tar species to be cracked during gasification and severely affect the development of biomass gasification processes. Similar to CO and CH4, Fig. 7c and Fig. 7d show that the temperature range for the emissions of aromatic compounds was relatively broad. During pyrolysis, the major aromatic compound release commenced at 200 °C and reached a maximum at around 350 °C. As the temperature increased above 450 °C, the evolution of aromatic compounds decreased and almost disappeared by 800 °C. Generally, it can be seen in Fig. 7c that more aromatic compounds were evolved during pyrolysis than partial oxidative gasification, mainly due to the formation of primary oxygenated oils during pyrolysis below 500 °C. 45 The broad range at which aromatics were observed during pyrolysis was attributed to the broad temperature range in which lignin decomposition occurs (200 – 900 °C). This is in good agreement with the TG results which showed that after 500 °C there was still a small amount of release. Moreover, the aromatic compound release profile was in agreement with the research performed by Cao et al. 5 who showed that new groups were formed as the cleavage of benzene bonds and phenyl 18 ACS Paragon Plus Environment
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chains transformed to aromatic rings at temperatures ranging from 500 to 800 °C during lignin pyrolysis. It was also reported that the aromatic structure can be formed and released during char pyrolysis due to the variation of char structure. 46 With addition of 1% O2, there was not a significant reduction in aromatic release at different Ca/B ratios (shown in Fig. 7c). However, aromatic compound release significantly reduced when the oxygen content increased to 5%, indicating that increasing oxygen can significantly reduce the yield of aromatic compounds during partial oxidative gasification. It was concluded that oxygen plays a more important role in cracking aromatic compounds than CaO and it is necessary to optimize the amount of O2 during partial oxidative gasification according to syngas requirements. 3.4. Characteristics of gas production analysis Table 5 compares the cumulative total of the main syngas components evolved over the entire temperature range during pyrolysis and partial oxidative gasification at various Ca/B ratios and oxygen contents. These results were compiled according to the integration of each individual gas emission profile. Overall, it can clearly be seen in Table 5 that the yields of H2 and CO significantly increased with increasing CaO content during partial oxidative gasification compared to pyrolysis, while bio-tars such as CH3OH, C-H, C=O and aromatic compounds greatly decreased. As the oxygen content increased the decomposition of various bio-tar components, such as very heavy bio-tars (aromatic compounds) and relatively light bio-tars (CH3OH, C-H group and C=O compounds) was greatly promoted, resulting in a decrease in the yield of all components examined with the exception of CO2 which increased with increased O2. Further it was observed that the addition of CaO may enhance the decomposition of heavy bio-tars into light hydrocarbons (C=H group) and increase CO2 capture capacity. While the yield of CH4 was sensitive to oxygen content and to a lesser extent CaO 19 ACS Paragon Plus Environment
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content. As derived from Table 5, it was found that the H2/CO ratio of syngas can reach around 1.5 at a Ca/B ratio of 3 during gasification under 5% O2 content and it is possible to achieve syngas of higher H2/CO ratios (2 or above) by adjusting the gasification temperature, i.e. less than 700 °C. Table 6 presents a comparison of the measured amounts of bio-tars released during pyrolysis as well as at 1% O2 biomass gasification and 5% O2 biomass gasification with a Ca/B ratio of 3. The reduction in these values observed as the O2 and CaO content increased is also presented. It can be clearly seen that relative quantities of all of the bio-tar compounds were greatly reduced with an increasing amount of oxygen and CaO, highlighting the synergistic effect of the combination of O2 and CaO in the gasification process. Therefore, it was concluded that a combination of O2 and CaO can efficiently improve the cracking of bio-tars formed during biomass gasification as well as providing potential for the syngas composition to be adjusted by optimizing the Ca/B ratio and oxygen content utilized in the gasification process.
4. Conclusions A kinetic study determined that increasing the oxygen content (from 1% to 5%), increased the reaction rate of biomass partial oxidative gasification, as did CaO addition. When considered separately, the oxygen content present in the gasification process was found to have a more significant effect on bio-tar cracking than CaO addition. While a combination of increasing oxygen content and CaO addition resulted in a synergistic decrease in CO2 concentration in syngas and increase in bio-tar cracking, H2/CO ratio and the yields of H2 and CO. Syngas with a H2/CO ratio of ～1.5 was obtained in the present study and results indicate that it is possible to achieve a syngas with a higher H2/CO ratio at lower gasification temperatures (
