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Co-pyrolysis behavior of fermentation residues with woody sawdust by thermogravimetric analysis and a vacuum reactor
Accepted Manuscript Co-pyrolysis behavior of fermentation residues with woody sawdust by thermogravimetric analysis and a vacuum reactor Yan Zheng, Yimin Zhang, Jingna Xu, Xiayang Li, Chunbao (Charles) Xu PII: DOI: Reference:
S0960-8524(17)31284-1 http://dx.doi.org/10.1016/j.biortech.2017.07.168 BITE 18588
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
4 June 2017 26 July 2017 27 July 2017
Please cite this article as: Zheng, Y., Zhang, Y., Xu, J., Li, X., (Charles) Xu, C., Co-pyrolysis behavior of fermentation residues with woody sawdust by thermogravimetric analysis and a vacuum reactor, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech.2017.07.168
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Co-pyrolysis behavior of fermentation residues with woody sawdust by thermogravimetric analysis and a vacuum reactor Yan Zhenga, Yimin Zhanga*, Jingna Xua, Xiayang Lia , Chunbao (Charles) Xub a
Key Laboratory for Green Chemical Technology of the State Ministry of Education, School
of Chemical Engineering and Technology, Tianjin University, Tianjin, 300350, PR China b
Institute for Chemicals and Fuels from Alternative Resource, Department of Chemical and
Biochemical Engineering, Western University, London, Ontario N6A5B9, Canada
Abstract: This study aimed at cost-effective utilization of fermentation residues (FR) from biogas project for bio-energy via co-pyrolysis of FR and woody sawdust (WS). In this study, a vacuum reactor was used to study the pyrolysis behaviors of individual and blend samples of FR and WS. Obvious synergistic effects were observed, resulting in a lower char yield but a higher gas yield. The presence of woody sawdust promoted the devolatilization of FR, and improved the syngas (H2 and CO) content in the gaseous products. Compared to those of the char from pyrolysis of individual feedstock, co-pyrolysis of FR and WS in the vacuum reactor promoted the cracking reactions of large aromatic rings, enlarged the surface area and reduced the oxygenated groups of the resulted char. Keywords: fermentation residues; woody sawdust; vacuum reactor; co-pyrolysis; synergistic effects
1
1. Introduction Because of the increasing concerns over efficiently and cleanly utilizing fossil resources, development advanced technologies for renewable energy and utilization of waste materials for alternative fuels are urgently demanded. Fermentation residues (FR) are generated as a byproduct in the biomass fermentation industry. Due to the presence of abundant harmful bacteria, organic matters and carcinogenic trace elements in FR (Koido et al., 2013), if not treated timely and effectively, FR would become a detrimental risk to the environment and human health. Some common disposal processes, e.g., use as food addition in the poultry industry (Li et al., 2013), farmland application and incineration (Wang et al., 2016; Zhu et al., 2015), are still used in many countries. However these methods with potential of causing serious heavy metals accumulation, groundwater pollution, greenhouse gas emissions, are being abandoned lately because of the increasingly restrictive environmental legislations (Ruiz-Gómez et al., 2016). Therefore safe treatment and resource recovery of FR have become a research focus and development trend for FR treatment and disposal globally. Pyrolysis offers advantage of large reduction of volume, complete elimination of pathogenic bacteria and promotion of heavy metal precipitation and complexation (Huang et al., 2012; Jin et al., 2016). Most importantly, the macromolecular organic matter in the FR is converted mainly to liquid bio-oils (Agrafioti et al., 2013). The resulting hydrogen and methane in gaseous products can be used as fuel gas, the 2
bio-oils can be upgraded into liquid bio-fuels or feedstocks for bio-based chemicals, and the solid residues can also be used as absorbents (Huang et al., 2012; Jin et al., 2016). However, direct pyrolysis of FR is significantly restricted due to its high-moisture and high-ash natures, low energy density and irregular morphology. Compared with FR, woody sawdust (WS) is a superior renewable energy resource with a higher heating value and lower ash content, and can be converted into high-quality pyrolysis products (Yan & He, 2017; Zhao et al., 2015). Thus utilizing FR together with woody biomass by co-pyrolysis might be a good solution. Due to highly heterogeneous and complex characteristics of these two materials, some interactive effects might exist between FR and wood biomass, which would accelerate or inhibit the thermal decomposition process during co-pyrolysis. Although there was some work on co-pyrolysis of coal and biomass or co-pyrolysis of biomass and another organic material, the reactivity and synergy of FR and biomass in co-pyrolysis has not been reported in the literature by far. Many studies have been focused on the characteristics of product distributions to investigate the possible synergetic effects during co-pyrolysis of coal and another feedstock (Emamitaba et al., 2013; Fermoso et al., 2009; Hernández et al., 2010). However, the findings were controversial. On one hand, some synergies were reported in co-pyrolysis in different types of reactors such as thermogravimetric analyses (TGA), fixed-bed reactor, fluidized-bed reactor and a pressurized entrained-bed reactor (Damartzis et al., 2011; Fermoso et al., 2009; Meng et al., 2016; Song et al., 2014). Sharypov et al., 2007 found coal promoted radical formation leading to the 3
production of higher hydrocarbons from polymer under hydrogen pressure during co-pyrolysis of brown coal and polyolefinic plastic. On the other hand, however, no synergistic effect during co-pyrolysis was also reported. Weiland et al., 2012 concluded that no interaction existed during the co-pyrolysis of biomass and coal in a drop tube reactor under atmospheric pressure. Similar findings were reported by Aigner et al (Aigner et al., 2011), who conducted co-gasification of coal and wood in a dual fluidized bed gasifier. As such, whether or not the synergistic effects existed in co-processing of two feedstocks might depend on different processing conditions such as reactor types, heating rate, temperature and type of feedstocks. So far, co-pyrolysis of FR and WS in a vacuum reactor that offers a long contract time and low pressure environment has not been reported. So it is of significance investigation on co-pyrolysis characteristics of these two feedstocks under vacuum condition. In this study, a systematic and comparative investigation on the pyrolysis characteristics of FR blended with WS was performed in a vacuum reactor. In addition, TGA was used to understand the pyrolysis reactivity and possible interactions between FR and WS during co-pyrolysis of these two feedstocks. The synergistic effects of co-pyrolysis on gas and char formation in the vacuum reactor were investigated.
2. Materials and Methods 2.1. Raw materials The FR used in this work was obtained from the ethanol fermentation industry of 4
Tianjin City, China, and the WS was collected from Taian district of Shandong Province, China. The samples were dried at 105C for 24 h, then milled and sieved to below 120 meshes. The blend of FR and WS (1:1, w/w) were mixed homogeneously. The ultimate (ASTM D3172) and proximate analysis (ASTM D5373) of the samples are shown in Table 1. The chemical compositions of the ashes derived from FR and WS were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES), and the results are also given in Table 1.
2.2. Experimental apparatus and methods The TG experiments were carried out with a thermal analyzer (NETZSCH TG 209F3) to investigate the thermal characteristics and thermal degradation kinetics of the samples. For each test, approximately 10 mg sample was placed into an alumina crucible and heated from ambient temperature to 900C at a heating rate of 10C/min in 30ml/min nitrogen flow. All samples were tested at the same condition. The devolatilization index (Di) (Lin et al., 2014; Wu et al., 2016) was used to evaluate the volatile release rates, the larger Di, the better pyrolysis performance. (1) Where
and
represent the maximum and the average mass loss
rate (%/min), respectively. M∞ (1-Mf/100) was the pyrolysis weight loss;
(C) was
the initial temperature; Tmax (C) and △T1/2 (C) represent the temperature at the (
)max and the (
)/
=0.5 (half peak), respectively.
Since pyrolysis is composed of a series of mass loss steps, Eq. (1) could be 5
represented by Eq. (2): (2) Where
is the mass loss percentage of each mass loss step (%); Di is the index D for
each step. In order to study whether or not there existed synergistic effects between the two individual samples during co-pyrolysis, a more intuitive method was introduced by estimating the deviation of the experimental and calculated values of TG curves. The equation of
is presented as below (Lin et al., 2014; Wu et al., 2016): (3) (4)
Where
is the relative mass loss deviation (%), representing the degree of
synergistic effects; Wcalculated is the calculated mass loss of the blend based on the TG results of individual samples. WFR and WWS are the experimental value during the pyrolysis of FR and WS, respectively. The schematic diagram of the vacuum reactor is displayed in Fig. 1. Approximately 15g sample was used in each experiment. Air in the reactor was displaced by N2 purge. A vacuum pump then pumped N2 out to provide a vacuum (around 5 kPa). The reactor was sealed and heated from ambient temperature to 900C and kept isothermally for 2 h to ensure enough long reaction/residence time. Then the volatile products were pumped out by vacuum pump through condensing traps. The bio-oils were washed out of the traps with hexane, dichloromethane and isopropanol. The non-condensable gas was measured by a cumulative volume flow meter and collected by gas bags. The 6
char was recovered after pyrolysis and directly weighted as solid residue fraction after cooling the reactor to a temperature below 50C. Most experiments and the product analyses were carried out in triplicate or duplicate to ensure the accuracy of experimental data and minimize the system errors of the reactors and instruments. The average relative deviations in gas volume and char yield between the experimental values and calculated values were determined based on by the follwing equations (Xiao et al., 2014; Yang et al., 2014): (5) (6) (7) Where Cyield,i : the calculated values of i including char yield and gas volume; Y1,i, Y2,i : the experimental yield of i for FR and WS, respectively; Cmfra,m: the calculated molar fraction of gas m including H2, CO2, CO, CH4; F1,gas,m, F2,gas,m: the experimental molar fraction of gas m for FR and WS, respectively; Y1,gas, Y2,gas: the experimental gas yield for FR and WS, respectively; Tchar: the calculated values of char characteristic values by Boehm titration, BET, FTIR and Raman analysis; E1 and E2: the experimental char characteristics for FR and WS, respectively; Y1,char, Y2,char : the experimental char yield for FR and WS, respectively. The average relative deviation can be calculated by Eq (8): (8)
2.3. Analysis of bio-gas and bio-char in vacuum reactor 7
GC was used for gas analysis, and the four main gases in the gaseous products: carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and hydrogen (H2) were detected with a thermal conductivity detector (TCD). The LabRAM HR800 Raman microspectroscopy system installed with an excitation laser of 514 nm was applied to characterize the char carbon structure variations. The laser power was 20 mW, and the spectra were collected from 800 to 2000 cm-1. The spectra obtained from the first ordered Raman spectra were curve-fitted using Origin.8 with 5 Gaussian bands. The surface morphology of the resulted chars was studied by SEM (JEOL JSM-6390). Surface areas of the chars were measured by nitrogen isothermal adsorption at 77K in an automatic apparatus ASAP 2020 Micrometrics. The functional groups of the chars were recorded on a FTIR spectrometer at room temperature in the 400-4000 cm-1 wavenumber range. Boehm titrations (Leng et al., 2015) were applied for quantifying the total acidic oxygenated groups on the bio-chars according to Boehm. Briefly, a given amount of bio-char was added to the 0.1 M sodium ethoxide (NaOC2H5) solution and the mixture was shaken for 48 h. Then the supernatant was drawn and back titrated with 0.1 M HCl.
3. Results and Discussion 3.1. Thermal behaviors The TG and DTG curves of TG of FR, WS and the Blend samples and main TG pyrolysis parameters are shown in Fig. 2a-2b and Table 2. Without considering the evaporation of moisture, the initial decomposition temperature of WS (305C) was 8
about 7C higher than that of FR (288C), while the end-pyrolysis temperature of FR (782C) was 203C higher than WS (579C), which indicated that FR contained more temperature-sensitive substances (Lin et al., 2014). The pyrolysis residue of WS and FR were 18.9 wt% and 50.5 wt%, mainly due to the differences of ash and volatile content in these two raw materials. The degradation of WS occurred mainly in a temperature range of 200-400C, characterized by a major weight loss with the maximum weight loss rate (-9.27 wt%/min) at 361.9C. Hemicellulose and cellulose are highly reactive and can completely pyrolyze in the temperature range of 285-390C. The decomposition of lignin could continue until nearly 800C (Damartzis et al., 2011; Yao et al., 2016). The main weight loss region for WS was centered at 250-400C with the weight loss of 61.6 wt%, which suggested that more hemicellulose and cellulose are contained in the WS (Yao et al., 2016). In the case of FR, the pyrolysis process involved two distinguished stages. The first stage extended from 200-560C with the maximum weight loss rate (-3.03 wt%/min) at approximately 341 C and the mass loss percentage of this stage in the total mass loss was 79.3 wt%. The second stage extended from 560-800C with the maximum weight loss rate (-0.75 wt%/min) occurring at 722C. The performance of the volatile matters release can be described by the devolatilization index (D) calculated from Eq. (1) and (2) (Cai et al., 2016), as listed in Table 3. The D of WS (11.47) was much higher than that of FR (0.95), indicating that the pyrolysis rate of WS was much faster than that of FR. As shown in Fig. 2a-2b, the initial decomposition temperature of the Blend was 9
277C, and the temperatures of first and second weight-loss peak shifted to lower temperatures, and the whole pyrolysis process was shortened with the addition of WS to FR. The intensity of the first peak of the Blend sample increased from -3.03 wt%/min at 341C to -5.34 wt%/min at 335C. The intensity of the second peak of the Blend sample was lower with -0.51 wt%/min at 642C for the Blend, compared with -0.75 wt%/min at 722C for FR. Furthermore, the final mass of the Blend decreased from 50.5 wt% to 37.3 wt%. Therefore, the Blend of FR and WS enhanced the pyrolysis process. The D of the Blend sample was 4.02, being over 4 times that of FR. As such, the Blend volatile release was promoted in the co-pyrolysis. In Fig. 2c, the experimental TG curves were below the calculated TG curves calculated by Eq. (4), indicating that some synergistic effects existed in the co-pyrolysis of FR and WS. In Fig. 2d, the values of △W were slightly positive at 100-223C. When the temperature reached in the range of 223 - 400C, the △W turned into negative, representing an obviously large gap. The deviation values were up to -9.97 wt% at 344C. This phenomenon indicated that the interaction between FR and WS at lower temperature contributed to the promoting effects of the co-pyrolysis. During co-pyrolysis of FR and WS, they devolatilized together at 250 -400C, and WS can act as a hydrogen donor for FR by transferring a plenty of H and OH radicals from WS to the surface of FR, suppressing the secondary reactions (condensation and cross-linking reactions, etc.), and promoting the cracking of the aromatic compounds as well as reducing the secondary reactions for char and tar formation (Alvarez et al., 2015; Wang et al., 2016). Besides, some researchers found 10
that the inorganic matters in FR might play catalytic roles in the reactions between vapors from pyrolysis of WS or FR (Ding & Hong, 2013). When the temperature reached 400-650C, the experimental values became higher than the calculated ones. The possible reason for this might be that high gas pressure accumulated in the reactor and the sticky liquid products started to coat on the sample particles, preventing the decomposition products from releasing.
3.2. Co-pyrolysis in a vacuum reactor 3.2.1. Pyrolysis products distribution Fig. 3 compares the experimental and calculated values of the char yields, the gas yields and gas composition distributions during co-pyrolysis of FR and WS or pyrolysis of individual feedstock. From Fig 3a, it could be observed that co-pyrolysis of FR with WS led to an obvious decrease in char yield. The average relative deviation (calculated by Eq. (8)) was 14.3%. While the yield of pyrolysis gas was enhanced with the average relative deviation of 54.7%. These deviations confirmed the presence of synergestic effects in co-pyrolysis of FR and WS. The BET and Raman analysis of the Blend char indicated that it has a larger surface area and offers more reaction sites than the other two chars from pyrolysis of individual feedstock, facilitating the secondary reactions of chars and promoting the devolatilization reactions (Wu et al., 2014; Eom et al., 2012). FR in this study has a high ash content of (Table 1). Vapors devolatilized from FR and WS particles had more opportunity to come into contact with the ash components, which 11
might catalyze the cracking reactions of the biomass. Moreover, char yield of the Blend sample could also be reduced through the enhanced char gasification by CO2 and H2O (Xiao et al., 2014; Yang et al., 2014).
3.2.2. Gas composition analysis The main gas compositions of these three experiments are shown in Fig. 3c. The average relative deviations between the gas compositions from the Blend sample and the calculated values were 34.04 %, 41.27 %, 53.77 % and 71.55 % for H2, CO2, CO, CH4, respectively. The gases from the Blend sample had higher hydrogen, carbon monoxide, and methane contents than the calculated values; while an obvious decrease in carbon dioxide content was detected. In the vacuum reactor operation, the vapor residence time was about 2h at 900C, when some secondary reactions could occur in the gaseous phase and more tar could crack into non-condensable gases, which account for more gases produced in the pyrolysis operation in the vacuum reactor. Gas derived from pyrolysis of FR contained lots of CO2 and H2O. The high temperature (900C) caused the consumption of CO2 via the Boudouard reaction, as expressed below (Emamitaba et al., 2013): (9) The increase of CO content with the Blend sample could be attributed to the decomposition of volatiles and decarboxylation reaction (Song et al., 2014). Moreover, the Boudouard reaction (Eq. (9)) resulted in more CO formation. In pyrolysis, CH4 is generally generated by hydrocarbon secondary cracking. WS has a higher H/C molar 12
ratio, and hence could form more H free radical that would react with highly unstable alkyl radicals from the pyrolysis of FR, leading to an increase in CH4. On the other hand, bio-oil could act as intermediate for secondary cracking at high temperature, producing small molecules such as methane. It was reported that H2 can be produced by two reactions during pyrolysis: radical polycondensation and dehydrogenation reaction (Song et al., 2014). Higher temperature favors the dehydrogenation reaction to form H2. FR could also produce more water vapor during the co-pyrolysis, which promotes the water-gas shift reaction, and hence contributes to more hydrogen formation (Yang et al., 2014). (10)
3.2.3. The structure evolution of char samples Raman analysis Fig. 4a illustrates the Raman spectra of different char samples in the range of 800-2000cm-1. Just like most of carbonaceous materials, all the spectra showed two obvious Raman vibration peaks appearing at 1336-1360 and 1591-1600 cm-1,which correspond to the D band (defect/disordered) and G band (aromatic layers graphite). Graphite structure (G) and disorder structure (D) are mainly adopted to investigate the characteristic structure of chars (Cai et al., 2016). Three bands exist in the char samples VR (~1380 cm-1), VL (~ 1465 cm-1) and GR (~1540 cm-1). It is believed that the combined intensity of VR+VL+GR represents small aromatics rings systems (Wu et al., 2014). The area ratios between different bands were used to investigate the char 13
structure in this paper. AG/AD, the area ratio of the G bands to the D bands was used to quantitatively characterize the aromatization degree of the char samples. A VR+VL+ GR/AD
represented the distribution of smaller to the larger aromatics in the char
samples (Wu et al., 2017). The total peak areas are shown in Fig. 4b. Relative researches reported that the Raman intensity of carbonaceous materials is mainly influenced by two factors: firstly, the O-containing functional groups as the sensitizer could enhance the total Raman area, especially through a resonance effect with the connected aromatics, and secondly, the big aromatics ring systems have higher light absorptivity, thereby weakening the total Raman intensity (Wu et al., 2014; Wu et al., 2017). The Blend char in Fig. 4b represents the largest total Raman intensity with the average relative deviation of about 15%. It indicated that it contains more functional groups to enhance the Raman intensity. During co-pyrolysis process at a high temperature (900C), the naturally catalyzed effect of alkali and alkaline earth metals (AAEM) in FR could annihilate the functional groups in WS and hence inhibit the formation of the larger aromatic ring molecules, resulting in the structure evolution of char with different degrees of graphitization. Although the oxygen contents (Table 1) in FR are greater than those in WS, its greater ash content especially the ion-exchangeable Na and Ca species in FR would reduce the Raman intensities of chars. In addition, the poor hydrogen content in the FR also led to less depolymerization and cracking reactions during its pyrolysis process, which would also weaken the Raman intensity. As shown Fig. 4c, WS char displayed the maximum AG/AD, whereas the value of 14
FR char is the lowest. This finding corresponds to an improvement in the degree of order in the graphitic crystallites and less concentration of structural defects. As studied by other researchers, the disordered carbon structure is more reactive than the high ordered structure. The addition of WS resulted in the consumption of the reactive functional groups in FR. Fig. 4d shows that AVR+VL+GR/AD values of the char samples are: Blend > FR > the calculated > WS, suggesting that some synergistic effects might exist between FR and WS during the co-pyrolysis. Some researchers have found that high processing temperatures could facilitate the transform from large to small aromatic ring system. Infrared spectra analysis FTIR spectra of the various pyrolysis chars at 900C were collected (presented in the Supplementary Data), in which the calculated spectra were constructed according to Eq (7). The peak at 3700-3000 cm-1 is assigned to the vibration of hydroxyl (-OH) functionalities, indicating the existence of water, alcohol or phenol or carboxylic acids. The peaks at 2920-2850 cm-1 correspond to the aliphatic –CH (CH, CH2, CH3) groups. The peak at near 1595 cm-1 belongs to C=O vibration. The peak at 1400 cm-1 can be attributed to the stretching vibration of C=C bonds in aromatics. The band at 1000 cm-1 is ascribed to the C-O stretch of alcohols, phenols or esters and minerals. The fingerprint IR absorption at 900-700 cm-1 is caused by the aromatic rings substituted functional groups (Guo et al., 2016; Xu et al., 2017). Comparing with that of the calculated spectra, the peak of C-O in the Blend char shows weak intensity. It is denoted that WS as hydrogen-rich donor can transfer 15
radicals such as OH to stabilize C-O during the FR pyrolysis, and so did the C=O in the Blend char (becoming less obvious in the Blend char) (Chen et al., 2015). Furthermore, the Car-H in the Blend char became less significant with the addition of WS, indicating that the addition of WS to FR facilitates the degradation of aromatics during the co-pyrolysis. The Boehm titration was performed to investigate the acidic oxygen surface functional groups of chars. The titration results were 5.28, 8.88, 6.40 meq/g for WS, FR and the Blend chars. The calculated value was 7.72 meq/g (Eq. (7) for the blended sample, higher than the experimental value, suggesting that synergetic effect between FR and WS could promote the release of acidic oxygen functional groups during the co-pyrolysis (Leng et al., 2015). Surface property analysis The BET specific surface area values (SBET) of char samples are 137, 51 and 221 m2/g for WS, FR and Blend, respectively. The SBET of the Blend char is much greater than that of the char from any individual feedstock, which might be attributed to the addition of WS to FR, which improved volatile release during the co-pyrolysis. The BET results were well explained by SEM images of char samples (presented in the Supplementary Data), from which, it can be observed that the surface of WS char still conserved the fibrous structure, smoother and flatter than other char samples. FR char exhibited an irregular and granular shape with smaller particles due to the higher content of inorganic matter in FR (Haykiri-Acma & Yaman, 2007). The surface morphology of co-pyrolysis char (Blend char) has a loose packed structure with a 16
higher specific surface area, which could result from faster devolatilization of the Blend sample (He et al., 2016).
4. Conclusions Co-pyrolysis of FR and WS were investigated using TGA and in a vacuum reactor. TGA results indicated that the pyrolysis reactivity of the Blend sample was improved. Co-pyrolysis of FR and WS in the vacuum reactor promoted the release of gas products, leading to higher volume yields of CO, H2, and CH4, reduced char yield. It was demonstrated that there were synergetic effects between FR and WS during the co-pyrolysis, which facilitated the degradation of aromatics and promoted the release of acidic oxygen functional groups, moreover, increased the ordering of the char with higher specific surface area.
Acknowledgements This work was supported by the Program of International Science and Technology Cooperation Program between China and Sweden (SQ2013ZOC600012). One author (C.X.) also acknowledges the funding (Discovery Grant) from Natural Science and Engineering Research Council of Canada (NSERC).
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on brown coal–polyolefinic plastic co-pyrolysis behavior. Journal of Analytical & Applied Pyrolysis. 78, 257-264. 24. Song, Y., Tahmasebi, A., Yu, J., 2014. Co-pyrolysis of pine sawdust and lignite in a thermogravimetric analyzer and a fixed-bed reactor. Bioresour. Technol. 174, 204-211. 25. Wang, X., Deng, S., Tan, H., Adeosun, A., Vujanović, M., Yang, F., Duić, N., 2016. Synergetic effect of sewage sludge and biomass co-pyrolysis: A combined study in thermogravimetric analyzer and a fixed bed reactor. Energy Convers. Manage. 118, 399-405. 26. Weiland, N.T., Means, N.C., Morreale, B.D. 2012. Product distributions from isothermal co-pyrolysis of coal and biomass. Fuel. 94, 563-570. 27. Wu, Z., Wang, S., Zhao, J., Chen, L., Meng, H., 2014. Thermal Behavior and Char Structure Evolution of Bituminous Coal Blends with Edible Fungi Residue during Co-Pyrolysis. Energy & Fuels. 28, 1792–1801. 28. Wu, Z., Wang, S., Zhao, J., Chen, L., Meng, H., 2016. Thermochemical behavior and char morphology analysis of blended bituminous coal and lignocellulosic biomass model compound co-pyrolysis: Effects of cellulose and carboxymethylcellulose sodium. Fuel. 171, 65-73. 29. Wu, Z., Yang, W., Chen, L., Meng, H., Zhao, J., Wang, S., 2017. Morphology and microstructure of co-pyrolysis char from bituminous coal blended with lignocellulosic biomass: effects of cellulose, hemicellulose and lignin. Applied Thermal Engineering. 116, 24-32. 21
30. Xiao, Y., Yuan, C., Jiao, X., Zhang, W. 2014. Co-pyrolysis of Chinese lignite and biomass in a vacuum reactor. Bioresour. Technol. 173, 1–5. 31. Xu, J., Bai, Z., Bai, J., Kong, L., Lv, D., Han, Y., Dai, X., Li, W., 2017. Physico-chemical structure and combustion properties of chars derived from co-pyrolysis of lignite with direct coal liquefaction residue. Fuel. 187, 103-110. 32. Yan, L., He, B. 2017. On a clean power generation system with the co-gasification of biomass and coal in a quadruple fluidized bed gasifier. Bioresour. Technol. 235, 113-121. 33. Yang, X., Yuan, C., Xu, J., Zhang, W., 2014. Potential method for gas production: high temperature co-pyrolysis of lignite and sewage sludge with vacuum reactor and long contact time. Bioresour. Technol. 179, 602-605. 34. Yao, Z., Ma, X., Wu, Z., Yao, T., 2016. TGA–FTIR analysis of co-pyrolysis characteristics of hydrochar and paper sludge. Journal of analytical and applied pyrolysis. 123, 40-48. 35. Zhao, B.W., Wang, X., Yang, X.Y., 2015. Co-pyrolysis characteristics of microalgae Isochrysis and Chlorella: kinetics, biocrude yield and interaction. Bioresour. Technol. 198, 332-339. 36. Zhu, X., Yang, S., Wang, L., Liu, Y., Qian, F., Yao, W., Zhang, S., Chen, J., 2015. Tracking the conversion of nitrogen during pyrolysis of antibiotic mycelial fermentation residues using XPS and TG-FTIR-MS technology. Environmental Pollution. 211, 20-27.
22
Figure Captions Fig. 1. Schematic diagram of experimental system (1) reactor; (2) electric furnace; (3) samples; (4) heating controller; (5) condenser; (6) ice bath; (7) hexane; (8) dichloromethane; (9) isopropanol; (10) activated carbon filter; (11) cumulative flow meter; (12) gas bag.
Fig. 2. TG pyrolysis curves of FR, WS and the Blend samples: (a) TG; (b) DTG; (c) comparison of experimental and calculated of TG; (d) △W. Fig. 3. Comparison of experimental and calculated values: (a) the char yields; (b) the gas yields; (c) gas composition distributions. Fig. 4. Raman analysis of char samples: (a) Raman spectra; (b) variation of the Aall; (c) variation of the AG/AD; (d) variation of the AGR+VR+VL/AD.
23
Tables and Figures
Table 1 Analysis of raw materials. Description
FR
Woody Sawdust
Moisture
6.53 ±0.02
6.49±0.02
Volatiles
50.94±0.02
66.39±0.02
Fixed carbon
21.95±0.02
24.62±0.02
Ash
20.58±0.02
2.50±0.02
Carbon (wt. % )
32.66±0.002
43.41±0.002
Hydrogen (wt. % )
3.68±0.002
8.95±0.002
Nitrogen (wt. % )
1.85±0.002
1.10±0.002
Sulfur (wt. % )
0.34±0.002
8.2±0.002
Oxygen (wt. % ) by difference
40.88±0.002
35.83±0.002
H/C
0.11±0.002
0.21±0.002
Na
2.81±0.002
0.18±0.002
Mg
6.77±0.002
0.35±0.002
Al
3.60±0.002
0.44±0.002
K
3.70±0.002
1.07±0.002
Ca
1.92±0.002
0.13±0.002
Fe
1.22±0.002
0.25±0.002
Th
0.25±0.002
-
Bi
0.22±0.002
-
Pb
0.06±0.002
-
Proximate analysis (dry basis, wt. % )
Ultimate analysis (dry basis)
ICP-AES (wt. % )
24
Table 2 Pyrolysis parameters of samples. Samples
Tia ℃
DTG1b %/min
T1c ℃
a1d %
DTG2b %/min
T2c ℃
a2d %
Tfe ℃
Mff %
WS
305.91
9.27
361.93
61.59
-
-
-
579.50
18.91
±0.02
±0.002
±0.02
±0.02
±0.02
±0.02
288.23
-3.03
341.55
39.41
-0.75
722.78
90.93
781.68
50.47
±0.02
±0.002
±0.02
±0.02
±0.002
±0.02
±0.02
±0.02
±0.02
277.21
-5.34
335.33
50.41
-0.51
642.73
91.24
738.73
37.26
±0.02
±0.002
±0.02
±0.02
±0.002
±0.02
±0.02
±0.02
±0.02
FR Blend
a
: Ti the initial decomposition temperature; b: DTG1, DTG2 the mass loss rate according to the first
peak and the second peak; c:T1, T2 the temeprature according to the first peak and the second peak; d
: a1, a2 the conversion rate according to the first peak and the second peak; e: Tf the terminated
temperature; f: Mf the pyrolysis residue mass
25
Table 3 The pyrolysis characteristics index of D for samples. △T11/2b ℃
DTGmean1a %/min
WS
-1.02
62.09
1.00
11.14
±0.002
±0.02
±0.002
±0.002
-0.83
86.74
0.80
1.17
-0.31
65.89
0.20
0.035
0.95
±0.002
±0.02
±0.002
±0.002
±0.002
±0.02
±0.002
±0.002
±0.002
-1.22
86.98
0.87
4.64
0.25
107.90
0.13
0.019
4.02
±0.002
±0.02
±0.002
±0.002
±0.002
±0.02
±0.002
±0.002
±0.002
FR Blend
η1
D1 10-7
DTGmean2a %/min
△T21/2b ℃
Sample
-
-
η2 -
D2 10-7
D 10-7
-
11.14 ±0.002
a
: DTGmean1, DTGmean2 the average mass loss rate according to the first peak and the second peak : △T11/2, △T21/2 the temerature range (half peak width) according to the first peak and the second
b
peak
26
Fig. 1.
27
Fig. 2.
28
Fig. 3.
29
Fig. 4.
30
Highlights
Behavior of co-pyrolysis of fermentation residues and woody sawdust was studied. Woody sawdust was found to promote the devolatilization of the fermentation residues. Obvious synergistic effect was observed in the products yields from the co-pyrolysis. The char characteristics were comprehensively studied using various techniques.
31
S0960-8524(17)31284-1 http://dx.doi.org/10.1016/j.biortech.2017.07.168 BITE 18588
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
4 June 2017 26 July 2017 27 July 2017
Please cite this article as: Zheng, Y., Zhang, Y., Xu, J., Li, X., (Charles) Xu, C., Co-pyrolysis behavior of fermentation residues with woody sawdust by thermogravimetric analysis and a vacuum reactor, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech.2017.07.168
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Co-pyrolysis behavior of fermentation residues with woody sawdust by thermogravimetric analysis and a vacuum reactor Yan Zhenga, Yimin Zhanga*, Jingna Xua, Xiayang Lia , Chunbao (Charles) Xub a
Key Laboratory for Green Chemical Technology of the State Ministry of Education, School
of Chemical Engineering and Technology, Tianjin University, Tianjin, 300350, PR China b
Institute for Chemicals and Fuels from Alternative Resource, Department of Chemical and
Biochemical Engineering, Western University, London, Ontario N6A5B9, Canada
Abstract: This study aimed at cost-effective utilization of fermentation residues (FR) from biogas project for bio-energy via co-pyrolysis of FR and woody sawdust (WS). In this study, a vacuum reactor was used to study the pyrolysis behaviors of individual and blend samples of FR and WS. Obvious synergistic effects were observed, resulting in a lower char yield but a higher gas yield. The presence of woody sawdust promoted the devolatilization of FR, and improved the syngas (H2 and CO) content in the gaseous products. Compared to those of the char from pyrolysis of individual feedstock, co-pyrolysis of FR and WS in the vacuum reactor promoted the cracking reactions of large aromatic rings, enlarged the surface area and reduced the oxygenated groups of the resulted char. Keywords: fermentation residues; woody sawdust; vacuum reactor; co-pyrolysis; synergistic effects
1
1. Introduction Because of the increasing concerns over efficiently and cleanly utilizing fossil resources, development advanced technologies for renewable energy and utilization of waste materials for alternative fuels are urgently demanded. Fermentation residues (FR) are generated as a byproduct in the biomass fermentation industry. Due to the presence of abundant harmful bacteria, organic matters and carcinogenic trace elements in FR (Koido et al., 2013), if not treated timely and effectively, FR would become a detrimental risk to the environment and human health. Some common disposal processes, e.g., use as food addition in the poultry industry (Li et al., 2013), farmland application and incineration (Wang et al., 2016; Zhu et al., 2015), are still used in many countries. However these methods with potential of causing serious heavy metals accumulation, groundwater pollution, greenhouse gas emissions, are being abandoned lately because of the increasingly restrictive environmental legislations (Ruiz-Gómez et al., 2016). Therefore safe treatment and resource recovery of FR have become a research focus and development trend for FR treatment and disposal globally. Pyrolysis offers advantage of large reduction of volume, complete elimination of pathogenic bacteria and promotion of heavy metal precipitation and complexation (Huang et al., 2012; Jin et al., 2016). Most importantly, the macromolecular organic matter in the FR is converted mainly to liquid bio-oils (Agrafioti et al., 2013). The resulting hydrogen and methane in gaseous products can be used as fuel gas, the 2
bio-oils can be upgraded into liquid bio-fuels or feedstocks for bio-based chemicals, and the solid residues can also be used as absorbents (Huang et al., 2012; Jin et al., 2016). However, direct pyrolysis of FR is significantly restricted due to its high-moisture and high-ash natures, low energy density and irregular morphology. Compared with FR, woody sawdust (WS) is a superior renewable energy resource with a higher heating value and lower ash content, and can be converted into high-quality pyrolysis products (Yan & He, 2017; Zhao et al., 2015). Thus utilizing FR together with woody biomass by co-pyrolysis might be a good solution. Due to highly heterogeneous and complex characteristics of these two materials, some interactive effects might exist between FR and wood biomass, which would accelerate or inhibit the thermal decomposition process during co-pyrolysis. Although there was some work on co-pyrolysis of coal and biomass or co-pyrolysis of biomass and another organic material, the reactivity and synergy of FR and biomass in co-pyrolysis has not been reported in the literature by far. Many studies have been focused on the characteristics of product distributions to investigate the possible synergetic effects during co-pyrolysis of coal and another feedstock (Emamitaba et al., 2013; Fermoso et al., 2009; Hernández et al., 2010). However, the findings were controversial. On one hand, some synergies were reported in co-pyrolysis in different types of reactors such as thermogravimetric analyses (TGA), fixed-bed reactor, fluidized-bed reactor and a pressurized entrained-bed reactor (Damartzis et al., 2011; Fermoso et al., 2009; Meng et al., 2016; Song et al., 2014). Sharypov et al., 2007 found coal promoted radical formation leading to the 3
production of higher hydrocarbons from polymer under hydrogen pressure during co-pyrolysis of brown coal and polyolefinic plastic. On the other hand, however, no synergistic effect during co-pyrolysis was also reported. Weiland et al., 2012 concluded that no interaction existed during the co-pyrolysis of biomass and coal in a drop tube reactor under atmospheric pressure. Similar findings were reported by Aigner et al (Aigner et al., 2011), who conducted co-gasification of coal and wood in a dual fluidized bed gasifier. As such, whether or not the synergistic effects existed in co-processing of two feedstocks might depend on different processing conditions such as reactor types, heating rate, temperature and type of feedstocks. So far, co-pyrolysis of FR and WS in a vacuum reactor that offers a long contract time and low pressure environment has not been reported. So it is of significance investigation on co-pyrolysis characteristics of these two feedstocks under vacuum condition. In this study, a systematic and comparative investigation on the pyrolysis characteristics of FR blended with WS was performed in a vacuum reactor. In addition, TGA was used to understand the pyrolysis reactivity and possible interactions between FR and WS during co-pyrolysis of these two feedstocks. The synergistic effects of co-pyrolysis on gas and char formation in the vacuum reactor were investigated.
2. Materials and Methods 2.1. Raw materials The FR used in this work was obtained from the ethanol fermentation industry of 4
Tianjin City, China, and the WS was collected from Taian district of Shandong Province, China. The samples were dried at 105C for 24 h, then milled and sieved to below 120 meshes. The blend of FR and WS (1:1, w/w) were mixed homogeneously. The ultimate (ASTM D3172) and proximate analysis (ASTM D5373) of the samples are shown in Table 1. The chemical compositions of the ashes derived from FR and WS were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES), and the results are also given in Table 1.
2.2. Experimental apparatus and methods The TG experiments were carried out with a thermal analyzer (NETZSCH TG 209F3) to investigate the thermal characteristics and thermal degradation kinetics of the samples. For each test, approximately 10 mg sample was placed into an alumina crucible and heated from ambient temperature to 900C at a heating rate of 10C/min in 30ml/min nitrogen flow. All samples were tested at the same condition. The devolatilization index (Di) (Lin et al., 2014; Wu et al., 2016) was used to evaluate the volatile release rates, the larger Di, the better pyrolysis performance. (1) Where
and
represent the maximum and the average mass loss
rate (%/min), respectively. M∞ (1-Mf/100) was the pyrolysis weight loss;
(C) was
the initial temperature; Tmax (C) and △T1/2 (C) represent the temperature at the (
)max and the (
)/
=0.5 (half peak), respectively.
Since pyrolysis is composed of a series of mass loss steps, Eq. (1) could be 5
represented by Eq. (2): (2) Where
is the mass loss percentage of each mass loss step (%); Di is the index D for
each step. In order to study whether or not there existed synergistic effects between the two individual samples during co-pyrolysis, a more intuitive method was introduced by estimating the deviation of the experimental and calculated values of TG curves. The equation of
is presented as below (Lin et al., 2014; Wu et al., 2016): (3) (4)
Where
is the relative mass loss deviation (%), representing the degree of
synergistic effects; Wcalculated is the calculated mass loss of the blend based on the TG results of individual samples. WFR and WWS are the experimental value during the pyrolysis of FR and WS, respectively. The schematic diagram of the vacuum reactor is displayed in Fig. 1. Approximately 15g sample was used in each experiment. Air in the reactor was displaced by N2 purge. A vacuum pump then pumped N2 out to provide a vacuum (around 5 kPa). The reactor was sealed and heated from ambient temperature to 900C and kept isothermally for 2 h to ensure enough long reaction/residence time. Then the volatile products were pumped out by vacuum pump through condensing traps. The bio-oils were washed out of the traps with hexane, dichloromethane and isopropanol. The non-condensable gas was measured by a cumulative volume flow meter and collected by gas bags. The 6
char was recovered after pyrolysis and directly weighted as solid residue fraction after cooling the reactor to a temperature below 50C. Most experiments and the product analyses were carried out in triplicate or duplicate to ensure the accuracy of experimental data and minimize the system errors of the reactors and instruments. The average relative deviations in gas volume and char yield between the experimental values and calculated values were determined based on by the follwing equations (Xiao et al., 2014; Yang et al., 2014): (5) (6) (7) Where Cyield,i : the calculated values of i including char yield and gas volume; Y1,i, Y2,i : the experimental yield of i for FR and WS, respectively; Cmfra,m: the calculated molar fraction of gas m including H2, CO2, CO, CH4; F1,gas,m, F2,gas,m: the experimental molar fraction of gas m for FR and WS, respectively; Y1,gas, Y2,gas: the experimental gas yield for FR and WS, respectively; Tchar: the calculated values of char characteristic values by Boehm titration, BET, FTIR and Raman analysis; E1 and E2: the experimental char characteristics for FR and WS, respectively; Y1,char, Y2,char : the experimental char yield for FR and WS, respectively. The average relative deviation can be calculated by Eq (8): (8)
2.3. Analysis of bio-gas and bio-char in vacuum reactor 7
GC was used for gas analysis, and the four main gases in the gaseous products: carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and hydrogen (H2) were detected with a thermal conductivity detector (TCD). The LabRAM HR800 Raman microspectroscopy system installed with an excitation laser of 514 nm was applied to characterize the char carbon structure variations. The laser power was 20 mW, and the spectra were collected from 800 to 2000 cm-1. The spectra obtained from the first ordered Raman spectra were curve-fitted using Origin.8 with 5 Gaussian bands. The surface morphology of the resulted chars was studied by SEM (JEOL JSM-6390). Surface areas of the chars were measured by nitrogen isothermal adsorption at 77K in an automatic apparatus ASAP 2020 Micrometrics. The functional groups of the chars were recorded on a FTIR spectrometer at room temperature in the 400-4000 cm-1 wavenumber range. Boehm titrations (Leng et al., 2015) were applied for quantifying the total acidic oxygenated groups on the bio-chars according to Boehm. Briefly, a given amount of bio-char was added to the 0.1 M sodium ethoxide (NaOC2H5) solution and the mixture was shaken for 48 h. Then the supernatant was drawn and back titrated with 0.1 M HCl.
3. Results and Discussion 3.1. Thermal behaviors The TG and DTG curves of TG of FR, WS and the Blend samples and main TG pyrolysis parameters are shown in Fig. 2a-2b and Table 2. Without considering the evaporation of moisture, the initial decomposition temperature of WS (305C) was 8
about 7C higher than that of FR (288C), while the end-pyrolysis temperature of FR (782C) was 203C higher than WS (579C), which indicated that FR contained more temperature-sensitive substances (Lin et al., 2014). The pyrolysis residue of WS and FR were 18.9 wt% and 50.5 wt%, mainly due to the differences of ash and volatile content in these two raw materials. The degradation of WS occurred mainly in a temperature range of 200-400C, characterized by a major weight loss with the maximum weight loss rate (-9.27 wt%/min) at 361.9C. Hemicellulose and cellulose are highly reactive and can completely pyrolyze in the temperature range of 285-390C. The decomposition of lignin could continue until nearly 800C (Damartzis et al., 2011; Yao et al., 2016). The main weight loss region for WS was centered at 250-400C with the weight loss of 61.6 wt%, which suggested that more hemicellulose and cellulose are contained in the WS (Yao et al., 2016). In the case of FR, the pyrolysis process involved two distinguished stages. The first stage extended from 200-560C with the maximum weight loss rate (-3.03 wt%/min) at approximately 341 C and the mass loss percentage of this stage in the total mass loss was 79.3 wt%. The second stage extended from 560-800C with the maximum weight loss rate (-0.75 wt%/min) occurring at 722C. The performance of the volatile matters release can be described by the devolatilization index (D) calculated from Eq. (1) and (2) (Cai et al., 2016), as listed in Table 3. The D of WS (11.47) was much higher than that of FR (0.95), indicating that the pyrolysis rate of WS was much faster than that of FR. As shown in Fig. 2a-2b, the initial decomposition temperature of the Blend was 9
277C, and the temperatures of first and second weight-loss peak shifted to lower temperatures, and the whole pyrolysis process was shortened with the addition of WS to FR. The intensity of the first peak of the Blend sample increased from -3.03 wt%/min at 341C to -5.34 wt%/min at 335C. The intensity of the second peak of the Blend sample was lower with -0.51 wt%/min at 642C for the Blend, compared with -0.75 wt%/min at 722C for FR. Furthermore, the final mass of the Blend decreased from 50.5 wt% to 37.3 wt%. Therefore, the Blend of FR and WS enhanced the pyrolysis process. The D of the Blend sample was 4.02, being over 4 times that of FR. As such, the Blend volatile release was promoted in the co-pyrolysis. In Fig. 2c, the experimental TG curves were below the calculated TG curves calculated by Eq. (4), indicating that some synergistic effects existed in the co-pyrolysis of FR and WS. In Fig. 2d, the values of △W were slightly positive at 100-223C. When the temperature reached in the range of 223 - 400C, the △W turned into negative, representing an obviously large gap. The deviation values were up to -9.97 wt% at 344C. This phenomenon indicated that the interaction between FR and WS at lower temperature contributed to the promoting effects of the co-pyrolysis. During co-pyrolysis of FR and WS, they devolatilized together at 250 -400C, and WS can act as a hydrogen donor for FR by transferring a plenty of H and OH radicals from WS to the surface of FR, suppressing the secondary reactions (condensation and cross-linking reactions, etc.), and promoting the cracking of the aromatic compounds as well as reducing the secondary reactions for char and tar formation (Alvarez et al., 2015; Wang et al., 2016). Besides, some researchers found 10
that the inorganic matters in FR might play catalytic roles in the reactions between vapors from pyrolysis of WS or FR (Ding & Hong, 2013). When the temperature reached 400-650C, the experimental values became higher than the calculated ones. The possible reason for this might be that high gas pressure accumulated in the reactor and the sticky liquid products started to coat on the sample particles, preventing the decomposition products from releasing.
3.2. Co-pyrolysis in a vacuum reactor 3.2.1. Pyrolysis products distribution Fig. 3 compares the experimental and calculated values of the char yields, the gas yields and gas composition distributions during co-pyrolysis of FR and WS or pyrolysis of individual feedstock. From Fig 3a, it could be observed that co-pyrolysis of FR with WS led to an obvious decrease in char yield. The average relative deviation (calculated by Eq. (8)) was 14.3%. While the yield of pyrolysis gas was enhanced with the average relative deviation of 54.7%. These deviations confirmed the presence of synergestic effects in co-pyrolysis of FR and WS. The BET and Raman analysis of the Blend char indicated that it has a larger surface area and offers more reaction sites than the other two chars from pyrolysis of individual feedstock, facilitating the secondary reactions of chars and promoting the devolatilization reactions (Wu et al., 2014; Eom et al., 2012). FR in this study has a high ash content of (Table 1). Vapors devolatilized from FR and WS particles had more opportunity to come into contact with the ash components, which 11
might catalyze the cracking reactions of the biomass. Moreover, char yield of the Blend sample could also be reduced through the enhanced char gasification by CO2 and H2O (Xiao et al., 2014; Yang et al., 2014).
3.2.2. Gas composition analysis The main gas compositions of these three experiments are shown in Fig. 3c. The average relative deviations between the gas compositions from the Blend sample and the calculated values were 34.04 %, 41.27 %, 53.77 % and 71.55 % for H2, CO2, CO, CH4, respectively. The gases from the Blend sample had higher hydrogen, carbon monoxide, and methane contents than the calculated values; while an obvious decrease in carbon dioxide content was detected. In the vacuum reactor operation, the vapor residence time was about 2h at 900C, when some secondary reactions could occur in the gaseous phase and more tar could crack into non-condensable gases, which account for more gases produced in the pyrolysis operation in the vacuum reactor. Gas derived from pyrolysis of FR contained lots of CO2 and H2O. The high temperature (900C) caused the consumption of CO2 via the Boudouard reaction, as expressed below (Emamitaba et al., 2013): (9) The increase of CO content with the Blend sample could be attributed to the decomposition of volatiles and decarboxylation reaction (Song et al., 2014). Moreover, the Boudouard reaction (Eq. (9)) resulted in more CO formation. In pyrolysis, CH4 is generally generated by hydrocarbon secondary cracking. WS has a higher H/C molar 12
ratio, and hence could form more H free radical that would react with highly unstable alkyl radicals from the pyrolysis of FR, leading to an increase in CH4. On the other hand, bio-oil could act as intermediate for secondary cracking at high temperature, producing small molecules such as methane. It was reported that H2 can be produced by two reactions during pyrolysis: radical polycondensation and dehydrogenation reaction (Song et al., 2014). Higher temperature favors the dehydrogenation reaction to form H2. FR could also produce more water vapor during the co-pyrolysis, which promotes the water-gas shift reaction, and hence contributes to more hydrogen formation (Yang et al., 2014). (10)
3.2.3. The structure evolution of char samples Raman analysis Fig. 4a illustrates the Raman spectra of different char samples in the range of 800-2000cm-1. Just like most of carbonaceous materials, all the spectra showed two obvious Raman vibration peaks appearing at 1336-1360 and 1591-1600 cm-1,which correspond to the D band (defect/disordered) and G band (aromatic layers graphite). Graphite structure (G) and disorder structure (D) are mainly adopted to investigate the characteristic structure of chars (Cai et al., 2016). Three bands exist in the char samples VR (~1380 cm-1), VL (~ 1465 cm-1) and GR (~1540 cm-1). It is believed that the combined intensity of VR+VL+GR represents small aromatics rings systems (Wu et al., 2014). The area ratios between different bands were used to investigate the char 13
structure in this paper. AG/AD, the area ratio of the G bands to the D bands was used to quantitatively characterize the aromatization degree of the char samples. A VR+VL+ GR/AD
represented the distribution of smaller to the larger aromatics in the char
samples (Wu et al., 2017). The total peak areas are shown in Fig. 4b. Relative researches reported that the Raman intensity of carbonaceous materials is mainly influenced by two factors: firstly, the O-containing functional groups as the sensitizer could enhance the total Raman area, especially through a resonance effect with the connected aromatics, and secondly, the big aromatics ring systems have higher light absorptivity, thereby weakening the total Raman intensity (Wu et al., 2014; Wu et al., 2017). The Blend char in Fig. 4b represents the largest total Raman intensity with the average relative deviation of about 15%. It indicated that it contains more functional groups to enhance the Raman intensity. During co-pyrolysis process at a high temperature (900C), the naturally catalyzed effect of alkali and alkaline earth metals (AAEM) in FR could annihilate the functional groups in WS and hence inhibit the formation of the larger aromatic ring molecules, resulting in the structure evolution of char with different degrees of graphitization. Although the oxygen contents (Table 1) in FR are greater than those in WS, its greater ash content especially the ion-exchangeable Na and Ca species in FR would reduce the Raman intensities of chars. In addition, the poor hydrogen content in the FR also led to less depolymerization and cracking reactions during its pyrolysis process, which would also weaken the Raman intensity. As shown Fig. 4c, WS char displayed the maximum AG/AD, whereas the value of 14
FR char is the lowest. This finding corresponds to an improvement in the degree of order in the graphitic crystallites and less concentration of structural defects. As studied by other researchers, the disordered carbon structure is more reactive than the high ordered structure. The addition of WS resulted in the consumption of the reactive functional groups in FR. Fig. 4d shows that AVR+VL+GR/AD values of the char samples are: Blend > FR > the calculated > WS, suggesting that some synergistic effects might exist between FR and WS during the co-pyrolysis. Some researchers have found that high processing temperatures could facilitate the transform from large to small aromatic ring system. Infrared spectra analysis FTIR spectra of the various pyrolysis chars at 900C were collected (presented in the Supplementary Data), in which the calculated spectra were constructed according to Eq (7). The peak at 3700-3000 cm-1 is assigned to the vibration of hydroxyl (-OH) functionalities, indicating the existence of water, alcohol or phenol or carboxylic acids. The peaks at 2920-2850 cm-1 correspond to the aliphatic –CH (CH, CH2, CH3) groups. The peak at near 1595 cm-1 belongs to C=O vibration. The peak at 1400 cm-1 can be attributed to the stretching vibration of C=C bonds in aromatics. The band at 1000 cm-1 is ascribed to the C-O stretch of alcohols, phenols or esters and minerals. The fingerprint IR absorption at 900-700 cm-1 is caused by the aromatic rings substituted functional groups (Guo et al., 2016; Xu et al., 2017). Comparing with that of the calculated spectra, the peak of C-O in the Blend char shows weak intensity. It is denoted that WS as hydrogen-rich donor can transfer 15
radicals such as OH to stabilize C-O during the FR pyrolysis, and so did the C=O in the Blend char (becoming less obvious in the Blend char) (Chen et al., 2015). Furthermore, the Car-H in the Blend char became less significant with the addition of WS, indicating that the addition of WS to FR facilitates the degradation of aromatics during the co-pyrolysis. The Boehm titration was performed to investigate the acidic oxygen surface functional groups of chars. The titration results were 5.28, 8.88, 6.40 meq/g for WS, FR and the Blend chars. The calculated value was 7.72 meq/g (Eq. (7) for the blended sample, higher than the experimental value, suggesting that synergetic effect between FR and WS could promote the release of acidic oxygen functional groups during the co-pyrolysis (Leng et al., 2015). Surface property analysis The BET specific surface area values (SBET) of char samples are 137, 51 and 221 m2/g for WS, FR and Blend, respectively. The SBET of the Blend char is much greater than that of the char from any individual feedstock, which might be attributed to the addition of WS to FR, which improved volatile release during the co-pyrolysis. The BET results were well explained by SEM images of char samples (presented in the Supplementary Data), from which, it can be observed that the surface of WS char still conserved the fibrous structure, smoother and flatter than other char samples. FR char exhibited an irregular and granular shape with smaller particles due to the higher content of inorganic matter in FR (Haykiri-Acma & Yaman, 2007). The surface morphology of co-pyrolysis char (Blend char) has a loose packed structure with a 16
higher specific surface area, which could result from faster devolatilization of the Blend sample (He et al., 2016).
4. Conclusions Co-pyrolysis of FR and WS were investigated using TGA and in a vacuum reactor. TGA results indicated that the pyrolysis reactivity of the Blend sample was improved. Co-pyrolysis of FR and WS in the vacuum reactor promoted the release of gas products, leading to higher volume yields of CO, H2, and CH4, reduced char yield. It was demonstrated that there were synergetic effects between FR and WS during the co-pyrolysis, which facilitated the degradation of aromatics and promoted the release of acidic oxygen functional groups, moreover, increased the ordering of the char with higher specific surface area.
Acknowledgements This work was supported by the Program of International Science and Technology Cooperation Program between China and Sweden (SQ2013ZOC600012). One author (C.X.) also acknowledges the funding (Discovery Grant) from Natural Science and Engineering Research Council of Canada (NSERC).
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Figure Captions Fig. 1. Schematic diagram of experimental system (1) reactor; (2) electric furnace; (3) samples; (4) heating controller; (5) condenser; (6) ice bath; (7) hexane; (8) dichloromethane; (9) isopropanol; (10) activated carbon filter; (11) cumulative flow meter; (12) gas bag.
Fig. 2. TG pyrolysis curves of FR, WS and the Blend samples: (a) TG; (b) DTG; (c) comparison of experimental and calculated of TG; (d) △W. Fig. 3. Comparison of experimental and calculated values: (a) the char yields; (b) the gas yields; (c) gas composition distributions. Fig. 4. Raman analysis of char samples: (a) Raman spectra; (b) variation of the Aall; (c) variation of the AG/AD; (d) variation of the AGR+VR+VL/AD.
23
Tables and Figures
Table 1 Analysis of raw materials. Description
FR
Woody Sawdust
Moisture
6.53 ±0.02
6.49±0.02
Volatiles
50.94±0.02
66.39±0.02
Fixed carbon
21.95±0.02
24.62±0.02
Ash
20.58±0.02
2.50±0.02
Carbon (wt. % )
32.66±0.002
43.41±0.002
Hydrogen (wt. % )
3.68±0.002
8.95±0.002
Nitrogen (wt. % )
1.85±0.002
1.10±0.002
Sulfur (wt. % )
0.34±0.002
8.2±0.002
Oxygen (wt. % ) by difference
40.88±0.002
35.83±0.002
H/C
0.11±0.002
0.21±0.002
Na
2.81±0.002
0.18±0.002
Mg
6.77±0.002
0.35±0.002
Al
3.60±0.002
0.44±0.002
K
3.70±0.002
1.07±0.002
Ca
1.92±0.002
0.13±0.002
Fe
1.22±0.002
0.25±0.002
Th
0.25±0.002
-
Bi
0.22±0.002
-
Pb
0.06±0.002
-
Proximate analysis (dry basis, wt. % )
Ultimate analysis (dry basis)
ICP-AES (wt. % )
24
Table 2 Pyrolysis parameters of samples. Samples
Tia ℃
DTG1b %/min
T1c ℃
a1d %
DTG2b %/min
T2c ℃
a2d %
Tfe ℃
Mff %
WS
305.91
9.27
361.93
61.59
-
-
-
579.50
18.91
±0.02
±0.002
±0.02
±0.02
±0.02
±0.02
288.23
-3.03
341.55
39.41
-0.75
722.78
90.93
781.68
50.47
±0.02
±0.002
±0.02
±0.02
±0.002
±0.02
±0.02
±0.02
±0.02
277.21
-5.34
335.33
50.41
-0.51
642.73
91.24
738.73
37.26
±0.02
±0.002
±0.02
±0.02
±0.002
±0.02
±0.02
±0.02
±0.02
FR Blend
a
: Ti the initial decomposition temperature; b: DTG1, DTG2 the mass loss rate according to the first
peak and the second peak; c:T1, T2 the temeprature according to the first peak and the second peak; d
: a1, a2 the conversion rate according to the first peak and the second peak; e: Tf the terminated
temperature; f: Mf the pyrolysis residue mass
25
Table 3 The pyrolysis characteristics index of D for samples. △T11/2b ℃
DTGmean1a %/min
WS
-1.02
62.09
1.00
11.14
±0.002
±0.02
±0.002
±0.002
-0.83
86.74
0.80
1.17
-0.31
65.89
0.20
0.035
0.95
±0.002
±0.02
±0.002
±0.002
±0.002
±0.02
±0.002
±0.002
±0.002
-1.22
86.98
0.87
4.64
0.25
107.90
0.13
0.019
4.02
±0.002
±0.02
±0.002
±0.002
±0.002
±0.02
±0.002
±0.002
±0.002
FR Blend
η1
D1 10-7
DTGmean2a %/min
△T21/2b ℃
Sample
-
-
η2 -
D2 10-7
D 10-7
-
11.14 ±0.002
a
: DTGmean1, DTGmean2 the average mass loss rate according to the first peak and the second peak : △T11/2, △T21/2 the temerature range (half peak width) according to the first peak and the second
b
peak
26
Fig. 1.
27
Fig. 2.
28
Fig. 3.
29
Fig. 4.
30
Highlights
Behavior of co-pyrolysis of fermentation residues and woody sawdust was studied. Woody sawdust was found to promote the devolatilization of the fermentation residues. Obvious synergistic effect was observed in the products yields from the co-pyrolysis. The char characteristics were comprehensively studied using various techniques.
31