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Synergistic effect of mixing wheat straw and lignite in co-pyrolysis and steam co-gasification
Bioresource Technology 302 (2020) 122876
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Case Study
Synergistic effect of mixing wheat straw and lignite in co-pyrolysis and steam co-gasification
T
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Shuheng Zhao, Panbo Yang, Xiaofeng Liu, Quanguo Zhang , Jianjun Hu Collaborative Innovation Center of Biomass Energy, Key Laboratory of New Materials and Facilities for Rural Renewable Energy of Agricultural Ministry, Henan International Joint Laboratory of Biomass Energy and Nanomaterials, Henan Agricultural University, Zhengzhou 450002, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Co-pyrolysis Steam co-gasification Wheat straw Lignite Synergetic effect
Co-pyrolysis and steam co-gasification of wheat straw (WS) and lignite coal (LC) were studied in a tube furnace between 700 °C and 900 °C. Synergistic effect in co-pyrolysis is not always apparent. However, with the introduction of H2O vapor, synergetic effect is more obvious. Gas volume generated by co-gasification was higher than the prediction in all cases. Meanwhile, temperature played an important role and had a linear relationship with the excess gas volume when it exceeded 800 °C. These findings can be explained by that sufficient H2O vapor could enhance synergy according raising catalytic effect of alkali and alkaline earth metals (AAEMs), promoting free radical generated and increasing reactivity of half-chars. Moreover, co-gasification of WS and LC with several blending ratios were studied at 850 °C. It found H2O vapor could promote free radical formation stronger with higher ratio of WS during co-gasification, thus showing an enhancing effect on the reactivity of WS-derived chars.
1. Introduction
filed. On one hand, the partial replacement of coal by biomass behave more environment friendly, since it would reduce SOx, NOx and CO2 emissions (Emami-Taba et al., 2013; Hongrapipat et al., 2015; Mallick et al., 2017; Schwitalla et al., 2018). On the other hand, adding coal to biomass ensures reliable supply of feedstock and reduces operation costs for a commercial operation (Li et al., 2010; Masnadi et al., 2015). However, there would be some disagreements for previous researches when it comes to the synergetic effect in the co-utilization of biomass
As fuels, biomass and coal have similar characteristics, since they are both solid and mainly composed of carbon and hydrogen. The resource utilization approaches on them are also similar, including combustion, partial oxidation, pyrolysis, and gasification. Therefore, it would be naturally expected to comprehensively utilize biomass and coal as a mixture feedstock. There are two potential benefits on this ⁎
Corresponding author. E-mail address: [email protected] (Q. Zhang).
https://doi.org/10.1016/j.biortech.2020.122876 Received 18 December 2019; Received in revised form 18 January 2020; Accepted 20 January 2020 Available online 23 January 2020 0960-8524/ © 2020 Elsevier Ltd. All rights reserved.
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and coal. Many studies have claimed that a synergy is clear during co-pyrolysis or co-gasification of two feedstock, which could produce more excess gas volume and reduce the tar products simultaneously (Fermoso et al., 2010; Saw and Pang, 2013; Yang et al., 2018). The reason for this is generally associated with to alkali and alkaline earth metals (AAEMs) from the materials, or the reactions between volatile products of biomass and coal (Cabuk et al., 2019; Rizkiana et al., 2014; Schwitalla et al., 2018; Yang et al., 2018; Zhang et al., 2016b, 2017). However, some studies have reported that there was no interaction between biomass and coal during co-utilization processing (Aigner et al., 2011; Collot et al., 1999; Kastanaki et al., 2002). In these cases, the products of co-utilization can be precisely predicted using the blending ratio of the parent materials. Considering the variety of specific species, sources, and treatments of biomass and coal, it is possible to obtain different, even opposite results related to synergy in different studies. However, one trend across previous literatures is that the way that the two materials are brought into contact has a significant effect on whether or not a synergistic effect is observed (Miccio et al., 2012; Yuan et al., 2012; Zhang et al., 2016b). Specifically, a third participant in co-utilization, besides biomass and coal, which could enhance the contact between the two feedstock is conducive to generating synergy. Many studies about co-utilization of biomass and coal are concerned with co-pyrolysis and co-gasification. Although the target of processing is different, there are many comparable parameters during co-pyrolysis and co-gasification processing, including temperate, pressure and products. Moreover, the primary difference between co-pyrolysis and cogasification is whether a third participant exist (Collot et al., 1999; Zhang et al., 2016a). Air, CO2, or H2O vapor, which is introduced as the third participant, is often chose as gasification agent. As previous researches using those gasification agents, co-gasification was usually carried out between pyrolitic-chars from biomass or coal (Jeong et al., 2019; Massoudi Farid et al., 2016; Saw and Pang, 2013; Tursun et al., 2016). However, there are few examples in the literature comparing the co-pyrolysis and co-gasification of raw materials directly. In addition, the mechanism of how a third participant influences the synergy during co-gasification process needs further discussion. In this work, co-pyrolysis and steam co-gasification of raw material, biomass (wheat straw) and coal (lignite), were studied at different temperatures. The focus of this study is the gas products, including the total yield and the main composition of gas products. Furthermore, the effects of steam on the synergy of co-gasification is also considered. The results of this study are intended to support a more optimal method for the co-utilization of biomass and coal.
Table 2 Ash composition analysis of wheat straw and lignite samples (wt. %). Main composition
Lignite ash
Wheat straw ash
SiO2 Al2O3 CaO Fe2O3 MgO Na2O K2O P2O5 TiO2 SO3
42.26 18.14 18.03 7.05 3.16 1.57 1.24 1.51 0.581 6.16
54.91 5.78 7.44 2.70 3.19 0.888 15.16 2.08 0.315 1.70
analyses of the coal and biomass samples are listed in Table 1. X-ray fluorescence (XRF, ARL Advant'X, Thermo Fisher Scientific, USA) analysis of ash composition from biomass and coal are list in Table 2. Blends of the two materials were prepared by directly mixing at biomass to coal weight ratios of 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, and 20:80. Among them, 50:50 was the ratio that focuses on under different temperature conditions. 2.2. Apparatus and procedure Pyrolysis experiments were performed in a lab-scale tube furnace, illustrated schematically in Fig. 1. The quartz tube (1000 mm in length, 50 mm inner diameter) was heated to a controlled temperature in an electric furnace. N2 was used as the carrier gas, with a flow rate about 100 mL/min. After the reactor was heated to the target temperature (700, 750, 800, 850, or 900 °C), 10 g of the sample was pushed from the inlet to the middle of the reactor, then held there for 30 min. In order to study the possible synergy during co-utilization, the gas products were collected by a gas bag at the end of the procedure after being cooled down. For steam gasification, a water pump was added upstream from the reactor to generate H2O vapor (dashed parts in Fig. 1) at a flow rate 0.3 mL/min, which is a sufficient steam supply according our previous research (Hu et al., 2018, 2019). The gas products were analyzed using a gas chromatograph (GC-6820, Agilent, USA) with a thermal conductivity detector (TCD) and packed column (Carbosieve SⅡ). During the analysis, 99.995% helium is taken as the carrier gas. Pyrolysis and gasification experiments were repeated at least three times under the same conditions. 2.3. Calculations The theoretical gas yield and product composition from the co-utilization experiments described in Section 2.2 can be calculated based assuming no synergetic effects. Therefore, the differences between these calculations and the actual experiment results could be the evidence of interactions occurred between biomass and coal during copyrolysis or co-gasification. The theoretical total produced gas volume (Vcal) and percent of each gaseous composition (Ccal) can be calculated by Eqs. (1) and (2), as:
2. Material and methods 2.1. Materials The lignite from Inner Mongolia, which is commonly used as a combustion fuel in power plants, was the coal sample in this study, and will be referred to as LC hereafter. Wheat straw (WS) from Henan Province in China, was selected as biomass sample. All samples were pulverized to below 120 μm in a hammer mill and were used for experimental tests in an air-dried base. The proximate and ultimate
Vcal = (VWS × RWS + VLC × RLC )/100
(1)
Table 1 Proximate analysis and ultimate analysis of wheat straw and lignite samples. Samples
Wheat straw Lignite
Proximate analysis wt. %
Ultimate analysis wt.%
Mad
Aad
Vad
FCad
Cad
Had
Nad
Oad*
Sad
7.57 1.42
9.70 63.27
66.56 11.12
16.17 24.19
47.3 59.90
6.30 4.12
1.01 0.85
45.28 34.78
0.11 0.35
M: Moisture; A: Ash; V: Volatile; FC: Fixed Carbon; O*: by difference; wt. %: mass fraction; ad: air-dried basis. 2
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Fig. 1. Schematic diagram of experimental system.
Ccal = (VWS × RWS × CWS + VLC × RLC × CLC )/100
(2)
700 °C, the experimental gas volume including H2 and CO percentage are higher than the calculation in Fig. 3b. It might because heat released by decomposition of WS promote the decomposition of LC and increased the H2 amount during co-pyrolysis (Ferrara et al., 2014). Increment of CO could be attribute to the catalytic effect of AAEMs from WS or LC as literatures (Ellis et al., 2015; Gong et al., 2009; Zhou et al., 2019). However, effect of heat transfer become weaken as the temperature rise. Besides, AAEMs might change valence and phase state, thus reduce even lose the catalytic effect during co-pyrolysis (Ellis et al., 2015; Zhou et al., 2019). Hence, the actual gas volume including the H2 and CO percent are both close to the calculation, which means the synergy effect is not obvious at 750 °C. With temperature rise higher, the activity of free radical increased dramatically. Thus above 800 °C as shown in Fig. 3a, the excess gas generated increase obviously could be mainly caused by the transfer of free radical. In addition, above 800 °C, both H2 and CO composition are higher than calculation during co-pyrolysis, except CO product at 900 °C. As known, there are less half-char as temperature rise higher, thus reactions between halfchars are reduced as temperature increase, which might be the reason why excess gas percent reduce above 800 °C temperature, accompanied CO percent reducing at 900 °C temperature. It should be noted that the percent of CO2 produced by co-pyrolysis at all temperatures are lower than the calculated prediction, as shown in Fig. 3b. As was discussed in previous work(Wu et al., 2019), the decline in CO2 percent could be caused by the reactions between gas products and half-chars, and demonstrates an environmental benefit of co-pyrolysis (Lv et al., 2019). However, unlike previous work (Li et al., 2013; Wei et al., 2011), the variations of CH4 percent are not obvious between the experimental data and the calculation. It might be caused by the essential distinction of materials.
where VWS and VLC are the gas volumes produced by 10 g of wheat straw and lignite individually. RWS and RLC represent the ratio of wheat straw and lignite respectively (RWS + RLC = 100). CWS or CLC is the percentage of a specific gaseous composition produced by wheat straw or lignite separately. In this study, four gas products were considered, H2, CO, CH4 and CO2, and the total amount of these four was specified as 100% for comparison. 3. Results and discussion 3.1. Gas products from co-pyrolysis at a range of 700–900 °C temperatures In order to explore the interactions in the co-pyrolysis process, the results of separate pyrolysis of biomass and coal need to be discussed first. Fig. 2 shows the gas volume and gas composition from pyrolysis of WS and LC at different temperatures. At lower temperature, WS generates more gas than LC because the volatile content of biomass is higher than coal refer to Table 1. However, the difference decreases with increasing temperature, since the fixed carbon content begins to take part in the reactions at higher temperature. At 900 °C, LC even generates more gas than WS in Fig. 2a. Based on gas component analysis in Fig. 2b & c, WS produces more CO and CO2 while LC produces more H2. In addition, CO2 produced by LC decrease as temperature increase. CH4 composition from LC is similar to WS with a small decrease as temperature growth. These gas distributions are determined by the characteristics of raw material. Fig. 3 shows the gas volume and gas composition from co-pyrolysis of WS and LC at a 50:50 ratio. Although the experimental data is close to the calculation at 750 °C, the actual produced gas volume is consistently higher than the calculation in Fig. 3a. However, the excess in gas volume generated by co-pyrolysis is not linear with temperature. The maximum percentage of excess is 40.59% at 800 °C and the minimum is 0.2% at 750 °C. As previous reports (Abnisa and Wan Daud, 2014), the synergy effect of co-pyrolysis caused by several reasons, such as the AAEMs catalytic effect(Ellis et al., 2015; Gong et al., 2009; Krerkkaiwan et al., 2013; Yan et al., 2016), the transfer of H and OH free radical from biomass to coal (Soncini et al., 2013; Sonobe et al., 2008; Yang et al., 2019; Yuan et al., 2012) and reactions between halfchars of biomass and coal(An et al., 2017; Quan et al., 2014). Based on the ash content analysis listed in Table 2, the components of AAEMs that may play a catalytic role in this study are highly similar to those in previous literatures (Mallick et al., 2017). But in this study, temperature plays an important role on the synergy effect according to Fig. 3. At
3.2. Gas products from co-gasification at a range of 700–900 °C temperatures As described above, several mechanisms are needed to explain the complex reactions during co-pyrolysis and why the synergy effect is not obvious at certain temperatures. In other words, there are several ways to increase synergy effect during co-processing of biomass and coal, such as increasing the catalytic effect of AAEMs from materials, increasing the reactivity of free radical and half-chars. Thus, the synergetic effects were studies in this work with the involvement of a third component, H2O vapor, which was supposed to increase the interactions between biomass and coal. Normally, the purpose of gasification of biomass or coal is to generate syngas, or gas mainly composed of H2 and CO. Steam is instrumental in this process (Siwal et al., 2020). 3
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Fig. 3. Gas volume (a) and gas composition (b) from co-pyrolysis of WS and LC at 50:50 ratio.
generate large amount of gas products. Table 3 lists the chemical reactions that may occur during the steam gasification of biomass or coal (Adanez-Rubio et al., 2019; Cabuk et al., 2019; Hu et al., 2019). Reactions (1) and (2) are first-order reactions, and reactions (3) to (7) are second-order reactions. In addition, reactions (3) and (4) are gas–solid reactions, and reactions (5) to (7) are reactions between intermediate gas products. As temperature increase, more gas volumes are generated from both WS and LC, except LC at 900 °C where generated gas volume is near to 850 °C. LC generates more H2 and WS generates more CO in Fig. 4b & c, which are more obvious than pyrolysis processing. However, a high H2 percent was produced by gasification of WS at 900 °C. Besides, gasification processing generates lower CO2 percent than pyrolysis. H2, CO and CH4 percent has changed as temperature raise. Despite fluctuations in some data, the overall trend of CH4 percent goes down as temperature increase both in WS and LC gasification. In addition, the general trend is that H2 and CO percent increase as temperature growth. It seems steam could promote the decomposition of LC more strongly. Considering that steam is supplied enough in this work, thus the gasification result is related to the essential of materials, which is same to the pyrolysis process. The contrast between experimental data and the calculated nonsynergetic gas production from Eqs. (1) & (2) for co-gasification is shown in Fig. 5. In all cases, the measured gas volume is higher than the calculated prediction. The excess of the experimental data versus calculation increases as temperature growth. The most important finding is the linear relationship between the excess in gas volume compared to
Fig. 2. Gas volume (a) and gas composition (b & c) from pyrolysis of WS and LC individually.
Similar to the study of co-pyrolysis process, the research of co-gasification requires the analysis of separate gasification of biomass and coal. Fig. 4 presents the gas volume and gas composition produced by steam gasification of WS and LC. Compared to pyrolysis, higher gas volumes are generated in gasification process as shown in Fig. 4a, especially for LC, since the reactions between half-char and H2O vapor 4
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Table 3 Reactions take place during steam gasification of solid fuels. Reactions
Number
Biomass / Coal → Tar + Char(mainly C) + Gaseous (H2, CO, CO2, CH4, others) Tar → Gas(H2, CO, CO2, CH4, others) Char(mainly C) + H2O → H2 + CO Char(mainly C) + 2H2O → 2H2 + CO2 Char(mainly C) + CO2 → 2CO CO + H2O → H2 + CO2 CH4 + H2O → 3H2 + CO CH4 + CO2 → 2CO + 2H2
(1) (2) (3) (4) (5) (6) (7)
Fig. 5. Gas volume (a) and gas composition (b) from co-gasification of WS and LC at 50:50 ratio.
amounts of individual gases, as shown in Fig. 5a & b, is needed to determine the mechanisms at work in co-gasification. At 700 °C and 750 °C shown in Fig. 5b, the H2 and CO2 percentage in the generated gas mixture are approach to the calculated prediction from Eq. (2), with a slightly higher CO and lower CH4 percentage. More CO product means the catalytic effect of AAEMs plays a key role on synergetic effect at 700 °C and 750 °C. Specifically, the AAEMs from WS might promotes decomposition of LC more intensely during co-gasification, since LC decomposition is strongly affected by steam comparing to the results from pyrolysis and co-pyrolysis. That is to say H2O vapor could prevent the crystal phase changing of AAEMs as temperature increasing, which would impede the synergy during co-processing of biomass and coal as described above. However, it is hard to impede the
Fig. 4. Gas volume (a) and gas composition (b & c) from gasification of WS and LC individually.
calculation and temperature above 800 °C. This implies that the synergetic effect is controlled by temperature. It is known that many reactions take place during steam gasification of solid fuels as shown in Table 3. The situation of steam co-gasification of biomass and coal is more complicated, since interactions between a large numbers of individual components are possible. A comprehensive comparison of the 5
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evaporation of AAEMs from materials as temperature growth higher. Above 800 °C, H2 percent produced by co-gasification increase while CO percent decrease. Although the catalytic effect from AAEMs become weak since less CO amount produced, the synergy is still distinct with temperature raise because the excess gas volume increase obviously. Both high temperature and adding H2O vapor could increase the amount of free radical (Gil et al., 1999; Minkova et al., 2001), thus more gas produced by steam co-gasification might be attribute to the transfer of free radical above 800 °C, specially the H free radical. Furthermore, many literatures showed steam could increase the reactivity of half-char through increasing the BET surface area(Alvarez et al., 2019; Chen et al., 2017; Ferreira et al., 2017; Liu et al., 2008). Since no one gas composition change evidently than calculation value shown in Fig. 5b, it is supposed both WS and LC decomposition are increased during co-gasification, which means the reactivity of both half-chars (WS and LC) is enhanced by H2O vapor to generating more gas products. All gas volumes are produced almost double by gasification compared to pyrolysis process, so the synergetic effect is more obvious in this regime of operation. The maximum excess gas yield over the calculated prediction in co-gasification is roughly 186.11 mL/g at 900 °C, shown in Fig. 5a. Many reactions take place during co-gasification according to Table 3, however, the water–gas shift reaction (CO + H2O → H2 + CO2, reaction (5)), might be one of the main reaction at 900 °C since a slightly higher H2 and CO2 percent but a little lower CO percent produced by experiment compared to the calculation value. Besides, the H2/CO ratio in the co-gasification products varied from 1.0 to 1.8 as previous work (Fermoso et al., 2010; Massoudi Farid et al., 2016). In summary, although the reactions involving H2O vapor are complicit, the mechanism of synergy might become simplification. First, H2O vapor improve the catalytic effect of AAEMs from materials at lower temperature. Second, adding H2O vapor enhance the transfer of free radical and reactivity of half-chars at higher temperature. On the other words, synergy effect is magnified by adding the third participant. 3.3. Effect of blending ratio on co-gasification at 850 °C Steam can affect the interactions between biomass and coal resulting in the generation of more gas products. In order to gather more information on the different effect of H2O vapor on biomass and coal, co-gasification of LC and WS with several blending ratios were studied at 850 °C. This temperature is in the range where the excess gas production has a linear relationship with temperature. The results of this experiment are shown in Fig. 6. The gas yield produced by experiment is higher than the calculation value at all ratios as shown in Fig. 6a, which means synergetic effect is obvious during co-gasification at temperature of 850 °C with different blending ratios. The largest excess gas yield over calculation value and largest total gas volume, is produced at the ratio of WS to LC 60:40. LC has a larger amount of gas product than WS during gasification, and accordingly the gas amount generated by mixture with higher LC ratio is a little larger than higher WC ratio, except for the WS to LC 60:40 ratio sample. The experimental and calculated percentage of the main gas composition are compared in Fig. 6b. It shows that a higher CO percent produced by experiment than calculation value when the WS ratio is less than LC, including the ratio of WS to LC 60:40, which seems caused by catalytic effect of AAEMs from materials under these conditions. In previous studies, a small ratio of biomass in mixed materials was usually beneficial for synergy during co-utilization of biomass and coal (Jeong et al., 2017, 2019). It is believed that the small amount of biomass plays a catalyst role, while higher amounts of biomass could hamper the synergetic effect. However, the catalytic effect of AAEMs from material is not the main cause of synergy at 850 °C in this study, as the excess gas volumes are large even the ratio of WS is higher companying with less CO product. Although H2 percent in gas product decrease, the excess H2 amount increase roughly as the WS ratio increase. It seems H2O vapor could
Fig. 6. Gas volume (a) and gas composition (b) from co-gasification of WS and LC at 850 °C with different blending ratios.
promote more H free radical generated through a higher ratio of WS material. Comparing unbalanced blending ratios of WS and LC, the mixture with close ratios of biomass and coal produce more gas volume. It could be concluded that the reactivities of both half-chars are enhanced by H2O vapor, which magnify the synergy between WS and LC consequently. With the ratio of WS to LC 60:40, the most amount of excess gas is generated at 850 °C. Therefore, H2O vapor might have a slightly stronger effect with wheat straw half-char compared to lignite. The actual CH4 and CO2 percentage are close to the calculated prediction, which indicate that less reactions between gas products as reactions (5) to (7) listed in Table 3 taking place during co-gasification. On the other words, the synergetic effect in steam co-gasification at different blending ratios are mainly associated with H2O vapor on increasing the reactivity of half-chars.
4. Conclusions Synergy with increasing gas volume between WS and LC in copyrolysis and steam co-gasification were measured at different temperatures. It found that, catalytic effect of AAEMs from material, transfer of free radical and reactivity of half-chars are enhanced with H2O vapor involvement, and synergetic effect between WS and LC is magnified consequently. Temperature plays an important role in the interactions among biomass, coal, and H2O vapor. The excess of gas produced over the calculation in co-gasification increases and exhibit a 6
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Technol. 175, 1–9. https://doi.org/10.1016/j.fuproc.2018.02.026. Siwal, S.S., Zhang, Q., Sun, C., Thakur, S., Gupta, V.K., Thakur, V.K., 2020... Energy production from steam gasification processes and parameters that contemplate in biomass gasifier - a review. Bioresour. Technol. 297. https://doi.org/10.1016/j. biortech.2019.122481. Soncini, R.M., Means, N.C., Weiland, N.T., 2013. Co-pyrolysis of low rank coals and biomass: product distributions. Fuel 112, 74–82. https://doi.org/10.1016/j.fuel. 2013.04.073. Sonobe, T., Worasuwannarak, N., Pipatmanomai, S., 2008. Synergies in co-pyrolysis of Thai lignite and corncob. Fuel Process. Technol. 89, 1371–1378. https://doi.org/10. 1016/j.fuproc.2008.06.006. Tursun, Y., Xu, S., Wang, C., Xiao, Y., Wang, G., 2016. Steam co-gasification of biomass and coal in decoupled reactors. Fuel Process. Technol. 141, 61–67. https://doi.org/ 10.1016/j.fuproc.2015.06.046. Wei, L.G., Zhang, L., Xu, S.P., 2011. Effects of feedstock on co-pyrolysis of biomass and coal in a free fall reactor. Ranliao Huaxue Xuebao/J. Fuel Chem. Technol. 39, 728–734. https://doi.org/10.1016/s1872-5813(11)60044-3. Wu, Z., Li, Y., Xu, D., Meng, H., 2019. Co-pyrolysis of lignocellulosic biomass with lowquality coal: optimal design and synergistic effect from gaseous products distribution.
linear relationship with temperature growth above 800 °C. And H2O vapor could increase reactivity of WS half-char more than LC. CRediT authorship contribution statement Shuheng Zhao: Writing - original draft, Writing - review & editing. Panbo Yang: Investigation; Formal analysis. Xiaofeng Liu: Methodology; Data curation. Quanguo Zhang: Project administration; Supervision. Jianjun Hu: Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported the National Natural Science Foundation of China (51676065 and 51576060); the Department of Science and Technology of Henan Province, China (192102310506); the Funding for scientific research in the Henan Agricultural University (30601661). References Abnisa, F., Wan Daud, W.M.A., 2014. A review on co-pyrolysis of biomass: an optional technique to obtain a high-grade pyrolysis oil. Energy Convers. Manag. 87, 71–85. https://doi.org/10.1016/j.enconman.2014.07.007. Adanez-Rubio, I., Perez-Astray, A., Abad, A., Gayan, P., De Diego, L.F., Adanez, J., 2019. Chemical looping with oxygen uncoupling: an advanced biomass combustion technology to avoid CO2 emissions. Mitig. Adapt. Strateg. Glob. Chang. 24, 1293–1306. https://doi.org/10.1007/s11027-019-9840-5. Aigner, I., Pfeifer, C., Hofbauer, H., 2011. Co-gasification of coal and wood in a dual fluidized bed gasifier. Fuel 90, 2404–2412. https://doi.org/10.1016/j.fuel.2011.03. 024. Alvarez, J., Lopez, G., Amutio, M., Bilbao, J., Olazar, M., 2019. Evolution of biomass char features and their role in the reactivity during steam gasification in a conical spouted bed reactor. Energy Convers. Manag. 181, 214–222. https://doi.org/10.1016/j. enconman.2018.12.008. An, Y., Tahmasebi, A., Yu, J., 2017. Mechanism of synergy effect during microwave copyrolysis of biomass and lignite. J. Anal. Appl. Pyrolysis 128, 75–82. https://doi.org/ 10.1016/j.jaap.2017.10.023. Cabuk, B., Duman, G., Yanik, J., Olgun, H., 2019. Effect of fuel blend composition on hydrogen yield in co-gasification of coal and non-woody biomass. Int. J. Hydrogen Energy. https://doi.org/10.1016/j.ijhydene.2019.02.130. Chen, H., Chen, X., Qin, Y., Wei, J., Liu, H., 2017. Effect of torrefaction on the properties of rice straw high temperature pyrolysis char: pore structure, aromaticity and gasification activity. Bioresour. Technol. 228, 241–249. https://doi.org/10.1016/j. biortech.2016.12.074. Collot, A.G., Zhuo, Y., Dugwell, D.R., Kandiyoti, R., 1999. Co-pyrolysis and co-gasification of coal and biomass in bench-scale fixed-bed and fluidized bed reactors. Fuel 78, 667–679. https://doi.org/10.1016/S0016-2361(98)00202-6. Ellis, N., Masnadi, M.S., Roberts, D.G., Kochanek, M.A., Ilyushechkin, A.Y., 2015. Mineral matter interactions during co-pyrolysis of coal and biomass and their impact on intrinsic char co-gasification reactivity. Chem. Eng. J. 279, 402–408. https://doi.org/ 10.1016/j.cej.2015.05.057. Emami-Taba, L., Irfan, M.F., Wan Daud, W.M.A., Chakrabarti, M.H., 2013. Fuel blending effects on the co-gasification of coal and biomass - a review. Biomass Bioenergy 57, 249–263. https://doi.org/10.1016/j.biombioe.2013.02.043. Fermoso, J., Arias, B., Gil, M.V., Plaza, M.G., Pevida, C., Pis, J.J., Rubiera, F., 2010. Cogasification of different rank coals with biomass and petroleum coke in a highpressure reactor for H2-rich gas production. Bioresour. Technol. 101, 3230–3235. https://doi.org/10.1016/j.biortech.2009.12.035. Ferrara, F., Orsini, A., Plaisant, A., Pettinau, A., 2014. Pyrolysis of coal, biomass and their blends: performance assessment by thermogravimetric analysis. Bioresour. Technol. 171, 433–441. https://doi.org/10.1016/j.biortech.2014.08.104. Ferreira, S.D., Lazzarotto, I.P., Junges, J., Manera, C., Godinho, M., Osório, E., 2017. Steam gasification of biochar derived from elephant grass pyrolysis in a screw reactor. Energy Convers. Manag. 153, 163–174. https://doi.org/10.1016/j.enconman. 2017.10.006. Gil, J., Corella, J., Aznar, M.P., Caballero, M.A., 1999. Biomass gasification in atmospheric and bubbling fluidized bed: effect of the type of gasifying agent on the product distribution. Biomass Bioenergy 17, 389–403. https://doi.org/10.1016/S09619534(99)00055-0. Gong, X., Guo, Z., Wang, Z., 2009. Effects of Fe2O3 on pyrolysis reactivity of demineralized higher rank coal and its char structure. Huagong Xuebao/CIESC J. 60, 2321–2326. Hongrapipat, J., Saw, W.L., Pang, S., 2015. Co-gasification of blended lignite and wood
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biortech.2012.01.019. Zhang, Y., Fan, D., Zheng, Y., 2016a. Comparative study on combined co-pyrolysis/gasification of walnut shell and bituminous coal by conventional and congruent-mass thermogravimetric analysis (TGA) methods. Bioresour. Technol. 199, 382–385. https://doi.org/10.1016/j.biortech.2015.07.120. Zhang, Y., Zheng, Y., Yang, M., Song, Y., 2016b. Effect of fuel origin on synergy during cogasification of biomass and coal in CO2. Bioresour. Technol. 200, 789–794. https:// doi.org/10.1016/j.biortech.2015.10.076. Zhang, Z., Pang, S., Levi, T., 2017. Influence of AAEM species in coal and biomass on steam co-gasification of chars of blended coal and biomass. Renew. Energy 101, 356–363. https://doi.org/10.1016/j.renene.2016.08.070. Zhou, L., Zhang, G., Reinmöller, M., Meyer, B., 2019. Effect of inherent mineral matter on the co-pyrolysis of highly reactive brown coal and wheat straw. Fuel 239, 1194–1203. https://doi.org/10.1016/j.fuel.2018.11.114.
Fuel 236, 43–54. https://doi.org/10.1016/j.fuel.2018.08.116. Yan, J., Shi, K., Pang, C., Lester, E., Wu, T., 2016. Influence of minerals on the thermal processing of bamboo with a suite of carbonaceous materials. Fuel 180, 256–262. https://doi.org/10.1016/j.fuel.2016.04.001. Yang, X., Liu, X., Li, R., Liu, C., Qing, T., Yue, X., Zhang, S., 2018. Co-gasification of thermally pretreated wheat straw with Shengli lignite for hydrogen production. Renew. Energy 117, 501–508. https://doi.org/10.1016/j.renene.2017.10.055. Yang, Z., Wu, Y., Zhang, Z., Li, H., Li, X., Egorov, R.I., Strizhak, P.A., Gao, X., 2019. Recent advances in co-thermochemical conversions of biomass with fossil fuels focusing on the synergistic effects. Renew. Sustain. Energy Rev. 103, 384–398. https:// doi.org/10.1016/j.rser.2018.12.047. Yuan, S., Dai, Z.H., Zhou, Z.J., Chen, X.L., Yu, G.S., Wang, F.C., 2012. Rapid co-pyrolysis of rice straw and a bituminous coal in a high-frequency furnace and gasification of the residual char. Bioresour. Technol. 109, 188–197. https://doi.org/10.1016/j.
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Case Study
Synergistic effect of mixing wheat straw and lignite in co-pyrolysis and steam co-gasification
T
⁎
Shuheng Zhao, Panbo Yang, Xiaofeng Liu, Quanguo Zhang , Jianjun Hu Collaborative Innovation Center of Biomass Energy, Key Laboratory of New Materials and Facilities for Rural Renewable Energy of Agricultural Ministry, Henan International Joint Laboratory of Biomass Energy and Nanomaterials, Henan Agricultural University, Zhengzhou 450002, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Co-pyrolysis Steam co-gasification Wheat straw Lignite Synergetic effect
Co-pyrolysis and steam co-gasification of wheat straw (WS) and lignite coal (LC) were studied in a tube furnace between 700 °C and 900 °C. Synergistic effect in co-pyrolysis is not always apparent. However, with the introduction of H2O vapor, synergetic effect is more obvious. Gas volume generated by co-gasification was higher than the prediction in all cases. Meanwhile, temperature played an important role and had a linear relationship with the excess gas volume when it exceeded 800 °C. These findings can be explained by that sufficient H2O vapor could enhance synergy according raising catalytic effect of alkali and alkaline earth metals (AAEMs), promoting free radical generated and increasing reactivity of half-chars. Moreover, co-gasification of WS and LC with several blending ratios were studied at 850 °C. It found H2O vapor could promote free radical formation stronger with higher ratio of WS during co-gasification, thus showing an enhancing effect on the reactivity of WS-derived chars.
1. Introduction
filed. On one hand, the partial replacement of coal by biomass behave more environment friendly, since it would reduce SOx, NOx and CO2 emissions (Emami-Taba et al., 2013; Hongrapipat et al., 2015; Mallick et al., 2017; Schwitalla et al., 2018). On the other hand, adding coal to biomass ensures reliable supply of feedstock and reduces operation costs for a commercial operation (Li et al., 2010; Masnadi et al., 2015). However, there would be some disagreements for previous researches when it comes to the synergetic effect in the co-utilization of biomass
As fuels, biomass and coal have similar characteristics, since they are both solid and mainly composed of carbon and hydrogen. The resource utilization approaches on them are also similar, including combustion, partial oxidation, pyrolysis, and gasification. Therefore, it would be naturally expected to comprehensively utilize biomass and coal as a mixture feedstock. There are two potential benefits on this ⁎
Corresponding author. E-mail address: [email protected] (Q. Zhang).
https://doi.org/10.1016/j.biortech.2020.122876 Received 18 December 2019; Received in revised form 18 January 2020; Accepted 20 January 2020 Available online 23 January 2020 0960-8524/ © 2020 Elsevier Ltd. All rights reserved.
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and coal. Many studies have claimed that a synergy is clear during co-pyrolysis or co-gasification of two feedstock, which could produce more excess gas volume and reduce the tar products simultaneously (Fermoso et al., 2010; Saw and Pang, 2013; Yang et al., 2018). The reason for this is generally associated with to alkali and alkaline earth metals (AAEMs) from the materials, or the reactions between volatile products of biomass and coal (Cabuk et al., 2019; Rizkiana et al., 2014; Schwitalla et al., 2018; Yang et al., 2018; Zhang et al., 2016b, 2017). However, some studies have reported that there was no interaction between biomass and coal during co-utilization processing (Aigner et al., 2011; Collot et al., 1999; Kastanaki et al., 2002). In these cases, the products of co-utilization can be precisely predicted using the blending ratio of the parent materials. Considering the variety of specific species, sources, and treatments of biomass and coal, it is possible to obtain different, even opposite results related to synergy in different studies. However, one trend across previous literatures is that the way that the two materials are brought into contact has a significant effect on whether or not a synergistic effect is observed (Miccio et al., 2012; Yuan et al., 2012; Zhang et al., 2016b). Specifically, a third participant in co-utilization, besides biomass and coal, which could enhance the contact between the two feedstock is conducive to generating synergy. Many studies about co-utilization of biomass and coal are concerned with co-pyrolysis and co-gasification. Although the target of processing is different, there are many comparable parameters during co-pyrolysis and co-gasification processing, including temperate, pressure and products. Moreover, the primary difference between co-pyrolysis and cogasification is whether a third participant exist (Collot et al., 1999; Zhang et al., 2016a). Air, CO2, or H2O vapor, which is introduced as the third participant, is often chose as gasification agent. As previous researches using those gasification agents, co-gasification was usually carried out between pyrolitic-chars from biomass or coal (Jeong et al., 2019; Massoudi Farid et al., 2016; Saw and Pang, 2013; Tursun et al., 2016). However, there are few examples in the literature comparing the co-pyrolysis and co-gasification of raw materials directly. In addition, the mechanism of how a third participant influences the synergy during co-gasification process needs further discussion. In this work, co-pyrolysis and steam co-gasification of raw material, biomass (wheat straw) and coal (lignite), were studied at different temperatures. The focus of this study is the gas products, including the total yield and the main composition of gas products. Furthermore, the effects of steam on the synergy of co-gasification is also considered. The results of this study are intended to support a more optimal method for the co-utilization of biomass and coal.
Table 2 Ash composition analysis of wheat straw and lignite samples (wt. %). Main composition
Lignite ash
Wheat straw ash
SiO2 Al2O3 CaO Fe2O3 MgO Na2O K2O P2O5 TiO2 SO3
42.26 18.14 18.03 7.05 3.16 1.57 1.24 1.51 0.581 6.16
54.91 5.78 7.44 2.70 3.19 0.888 15.16 2.08 0.315 1.70
analyses of the coal and biomass samples are listed in Table 1. X-ray fluorescence (XRF, ARL Advant'X, Thermo Fisher Scientific, USA) analysis of ash composition from biomass and coal are list in Table 2. Blends of the two materials were prepared by directly mixing at biomass to coal weight ratios of 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, and 20:80. Among them, 50:50 was the ratio that focuses on under different temperature conditions. 2.2. Apparatus and procedure Pyrolysis experiments were performed in a lab-scale tube furnace, illustrated schematically in Fig. 1. The quartz tube (1000 mm in length, 50 mm inner diameter) was heated to a controlled temperature in an electric furnace. N2 was used as the carrier gas, with a flow rate about 100 mL/min. After the reactor was heated to the target temperature (700, 750, 800, 850, or 900 °C), 10 g of the sample was pushed from the inlet to the middle of the reactor, then held there for 30 min. In order to study the possible synergy during co-utilization, the gas products were collected by a gas bag at the end of the procedure after being cooled down. For steam gasification, a water pump was added upstream from the reactor to generate H2O vapor (dashed parts in Fig. 1) at a flow rate 0.3 mL/min, which is a sufficient steam supply according our previous research (Hu et al., 2018, 2019). The gas products were analyzed using a gas chromatograph (GC-6820, Agilent, USA) with a thermal conductivity detector (TCD) and packed column (Carbosieve SⅡ). During the analysis, 99.995% helium is taken as the carrier gas. Pyrolysis and gasification experiments were repeated at least three times under the same conditions. 2.3. Calculations The theoretical gas yield and product composition from the co-utilization experiments described in Section 2.2 can be calculated based assuming no synergetic effects. Therefore, the differences between these calculations and the actual experiment results could be the evidence of interactions occurred between biomass and coal during copyrolysis or co-gasification. The theoretical total produced gas volume (Vcal) and percent of each gaseous composition (Ccal) can be calculated by Eqs. (1) and (2), as:
2. Material and methods 2.1. Materials The lignite from Inner Mongolia, which is commonly used as a combustion fuel in power plants, was the coal sample in this study, and will be referred to as LC hereafter. Wheat straw (WS) from Henan Province in China, was selected as biomass sample. All samples were pulverized to below 120 μm in a hammer mill and were used for experimental tests in an air-dried base. The proximate and ultimate
Vcal = (VWS × RWS + VLC × RLC )/100
(1)
Table 1 Proximate analysis and ultimate analysis of wheat straw and lignite samples. Samples
Wheat straw Lignite
Proximate analysis wt. %
Ultimate analysis wt.%
Mad
Aad
Vad
FCad
Cad
Had
Nad
Oad*
Sad
7.57 1.42
9.70 63.27
66.56 11.12
16.17 24.19
47.3 59.90
6.30 4.12
1.01 0.85
45.28 34.78
0.11 0.35
M: Moisture; A: Ash; V: Volatile; FC: Fixed Carbon; O*: by difference; wt. %: mass fraction; ad: air-dried basis. 2
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Fig. 1. Schematic diagram of experimental system.
Ccal = (VWS × RWS × CWS + VLC × RLC × CLC )/100
(2)
700 °C, the experimental gas volume including H2 and CO percentage are higher than the calculation in Fig. 3b. It might because heat released by decomposition of WS promote the decomposition of LC and increased the H2 amount during co-pyrolysis (Ferrara et al., 2014). Increment of CO could be attribute to the catalytic effect of AAEMs from WS or LC as literatures (Ellis et al., 2015; Gong et al., 2009; Zhou et al., 2019). However, effect of heat transfer become weaken as the temperature rise. Besides, AAEMs might change valence and phase state, thus reduce even lose the catalytic effect during co-pyrolysis (Ellis et al., 2015; Zhou et al., 2019). Hence, the actual gas volume including the H2 and CO percent are both close to the calculation, which means the synergy effect is not obvious at 750 °C. With temperature rise higher, the activity of free radical increased dramatically. Thus above 800 °C as shown in Fig. 3a, the excess gas generated increase obviously could be mainly caused by the transfer of free radical. In addition, above 800 °C, both H2 and CO composition are higher than calculation during co-pyrolysis, except CO product at 900 °C. As known, there are less half-char as temperature rise higher, thus reactions between halfchars are reduced as temperature increase, which might be the reason why excess gas percent reduce above 800 °C temperature, accompanied CO percent reducing at 900 °C temperature. It should be noted that the percent of CO2 produced by co-pyrolysis at all temperatures are lower than the calculated prediction, as shown in Fig. 3b. As was discussed in previous work(Wu et al., 2019), the decline in CO2 percent could be caused by the reactions between gas products and half-chars, and demonstrates an environmental benefit of co-pyrolysis (Lv et al., 2019). However, unlike previous work (Li et al., 2013; Wei et al., 2011), the variations of CH4 percent are not obvious between the experimental data and the calculation. It might be caused by the essential distinction of materials.
where VWS and VLC are the gas volumes produced by 10 g of wheat straw and lignite individually. RWS and RLC represent the ratio of wheat straw and lignite respectively (RWS + RLC = 100). CWS or CLC is the percentage of a specific gaseous composition produced by wheat straw or lignite separately. In this study, four gas products were considered, H2, CO, CH4 and CO2, and the total amount of these four was specified as 100% for comparison. 3. Results and discussion 3.1. Gas products from co-pyrolysis at a range of 700–900 °C temperatures In order to explore the interactions in the co-pyrolysis process, the results of separate pyrolysis of biomass and coal need to be discussed first. Fig. 2 shows the gas volume and gas composition from pyrolysis of WS and LC at different temperatures. At lower temperature, WS generates more gas than LC because the volatile content of biomass is higher than coal refer to Table 1. However, the difference decreases with increasing temperature, since the fixed carbon content begins to take part in the reactions at higher temperature. At 900 °C, LC even generates more gas than WS in Fig. 2a. Based on gas component analysis in Fig. 2b & c, WS produces more CO and CO2 while LC produces more H2. In addition, CO2 produced by LC decrease as temperature increase. CH4 composition from LC is similar to WS with a small decrease as temperature growth. These gas distributions are determined by the characteristics of raw material. Fig. 3 shows the gas volume and gas composition from co-pyrolysis of WS and LC at a 50:50 ratio. Although the experimental data is close to the calculation at 750 °C, the actual produced gas volume is consistently higher than the calculation in Fig. 3a. However, the excess in gas volume generated by co-pyrolysis is not linear with temperature. The maximum percentage of excess is 40.59% at 800 °C and the minimum is 0.2% at 750 °C. As previous reports (Abnisa and Wan Daud, 2014), the synergy effect of co-pyrolysis caused by several reasons, such as the AAEMs catalytic effect(Ellis et al., 2015; Gong et al., 2009; Krerkkaiwan et al., 2013; Yan et al., 2016), the transfer of H and OH free radical from biomass to coal (Soncini et al., 2013; Sonobe et al., 2008; Yang et al., 2019; Yuan et al., 2012) and reactions between halfchars of biomass and coal(An et al., 2017; Quan et al., 2014). Based on the ash content analysis listed in Table 2, the components of AAEMs that may play a catalytic role in this study are highly similar to those in previous literatures (Mallick et al., 2017). But in this study, temperature plays an important role on the synergy effect according to Fig. 3. At
3.2. Gas products from co-gasification at a range of 700–900 °C temperatures As described above, several mechanisms are needed to explain the complex reactions during co-pyrolysis and why the synergy effect is not obvious at certain temperatures. In other words, there are several ways to increase synergy effect during co-processing of biomass and coal, such as increasing the catalytic effect of AAEMs from materials, increasing the reactivity of free radical and half-chars. Thus, the synergetic effects were studies in this work with the involvement of a third component, H2O vapor, which was supposed to increase the interactions between biomass and coal. Normally, the purpose of gasification of biomass or coal is to generate syngas, or gas mainly composed of H2 and CO. Steam is instrumental in this process (Siwal et al., 2020). 3
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Fig. 3. Gas volume (a) and gas composition (b) from co-pyrolysis of WS and LC at 50:50 ratio.
generate large amount of gas products. Table 3 lists the chemical reactions that may occur during the steam gasification of biomass or coal (Adanez-Rubio et al., 2019; Cabuk et al., 2019; Hu et al., 2019). Reactions (1) and (2) are first-order reactions, and reactions (3) to (7) are second-order reactions. In addition, reactions (3) and (4) are gas–solid reactions, and reactions (5) to (7) are reactions between intermediate gas products. As temperature increase, more gas volumes are generated from both WS and LC, except LC at 900 °C where generated gas volume is near to 850 °C. LC generates more H2 and WS generates more CO in Fig. 4b & c, which are more obvious than pyrolysis processing. However, a high H2 percent was produced by gasification of WS at 900 °C. Besides, gasification processing generates lower CO2 percent than pyrolysis. H2, CO and CH4 percent has changed as temperature raise. Despite fluctuations in some data, the overall trend of CH4 percent goes down as temperature increase both in WS and LC gasification. In addition, the general trend is that H2 and CO percent increase as temperature growth. It seems steam could promote the decomposition of LC more strongly. Considering that steam is supplied enough in this work, thus the gasification result is related to the essential of materials, which is same to the pyrolysis process. The contrast between experimental data and the calculated nonsynergetic gas production from Eqs. (1) & (2) for co-gasification is shown in Fig. 5. In all cases, the measured gas volume is higher than the calculated prediction. The excess of the experimental data versus calculation increases as temperature growth. The most important finding is the linear relationship between the excess in gas volume compared to
Fig. 2. Gas volume (a) and gas composition (b & c) from pyrolysis of WS and LC individually.
Similar to the study of co-pyrolysis process, the research of co-gasification requires the analysis of separate gasification of biomass and coal. Fig. 4 presents the gas volume and gas composition produced by steam gasification of WS and LC. Compared to pyrolysis, higher gas volumes are generated in gasification process as shown in Fig. 4a, especially for LC, since the reactions between half-char and H2O vapor 4
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Table 3 Reactions take place during steam gasification of solid fuels. Reactions
Number
Biomass / Coal → Tar + Char(mainly C) + Gaseous (H2, CO, CO2, CH4, others) Tar → Gas(H2, CO, CO2, CH4, others) Char(mainly C) + H2O → H2 + CO Char(mainly C) + 2H2O → 2H2 + CO2 Char(mainly C) + CO2 → 2CO CO + H2O → H2 + CO2 CH4 + H2O → 3H2 + CO CH4 + CO2 → 2CO + 2H2
(1) (2) (3) (4) (5) (6) (7)
Fig. 5. Gas volume (a) and gas composition (b) from co-gasification of WS and LC at 50:50 ratio.
amounts of individual gases, as shown in Fig. 5a & b, is needed to determine the mechanisms at work in co-gasification. At 700 °C and 750 °C shown in Fig. 5b, the H2 and CO2 percentage in the generated gas mixture are approach to the calculated prediction from Eq. (2), with a slightly higher CO and lower CH4 percentage. More CO product means the catalytic effect of AAEMs plays a key role on synergetic effect at 700 °C and 750 °C. Specifically, the AAEMs from WS might promotes decomposition of LC more intensely during co-gasification, since LC decomposition is strongly affected by steam comparing to the results from pyrolysis and co-pyrolysis. That is to say H2O vapor could prevent the crystal phase changing of AAEMs as temperature increasing, which would impede the synergy during co-processing of biomass and coal as described above. However, it is hard to impede the
Fig. 4. Gas volume (a) and gas composition (b & c) from gasification of WS and LC individually.
calculation and temperature above 800 °C. This implies that the synergetic effect is controlled by temperature. It is known that many reactions take place during steam gasification of solid fuels as shown in Table 3. The situation of steam co-gasification of biomass and coal is more complicated, since interactions between a large numbers of individual components are possible. A comprehensive comparison of the 5
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evaporation of AAEMs from materials as temperature growth higher. Above 800 °C, H2 percent produced by co-gasification increase while CO percent decrease. Although the catalytic effect from AAEMs become weak since less CO amount produced, the synergy is still distinct with temperature raise because the excess gas volume increase obviously. Both high temperature and adding H2O vapor could increase the amount of free radical (Gil et al., 1999; Minkova et al., 2001), thus more gas produced by steam co-gasification might be attribute to the transfer of free radical above 800 °C, specially the H free radical. Furthermore, many literatures showed steam could increase the reactivity of half-char through increasing the BET surface area(Alvarez et al., 2019; Chen et al., 2017; Ferreira et al., 2017; Liu et al., 2008). Since no one gas composition change evidently than calculation value shown in Fig. 5b, it is supposed both WS and LC decomposition are increased during co-gasification, which means the reactivity of both half-chars (WS and LC) is enhanced by H2O vapor to generating more gas products. All gas volumes are produced almost double by gasification compared to pyrolysis process, so the synergetic effect is more obvious in this regime of operation. The maximum excess gas yield over the calculated prediction in co-gasification is roughly 186.11 mL/g at 900 °C, shown in Fig. 5a. Many reactions take place during co-gasification according to Table 3, however, the water–gas shift reaction (CO + H2O → H2 + CO2, reaction (5)), might be one of the main reaction at 900 °C since a slightly higher H2 and CO2 percent but a little lower CO percent produced by experiment compared to the calculation value. Besides, the H2/CO ratio in the co-gasification products varied from 1.0 to 1.8 as previous work (Fermoso et al., 2010; Massoudi Farid et al., 2016). In summary, although the reactions involving H2O vapor are complicit, the mechanism of synergy might become simplification. First, H2O vapor improve the catalytic effect of AAEMs from materials at lower temperature. Second, adding H2O vapor enhance the transfer of free radical and reactivity of half-chars at higher temperature. On the other words, synergy effect is magnified by adding the third participant. 3.3. Effect of blending ratio on co-gasification at 850 °C Steam can affect the interactions between biomass and coal resulting in the generation of more gas products. In order to gather more information on the different effect of H2O vapor on biomass and coal, co-gasification of LC and WS with several blending ratios were studied at 850 °C. This temperature is in the range where the excess gas production has a linear relationship with temperature. The results of this experiment are shown in Fig. 6. The gas yield produced by experiment is higher than the calculation value at all ratios as shown in Fig. 6a, which means synergetic effect is obvious during co-gasification at temperature of 850 °C with different blending ratios. The largest excess gas yield over calculation value and largest total gas volume, is produced at the ratio of WS to LC 60:40. LC has a larger amount of gas product than WS during gasification, and accordingly the gas amount generated by mixture with higher LC ratio is a little larger than higher WC ratio, except for the WS to LC 60:40 ratio sample. The experimental and calculated percentage of the main gas composition are compared in Fig. 6b. It shows that a higher CO percent produced by experiment than calculation value when the WS ratio is less than LC, including the ratio of WS to LC 60:40, which seems caused by catalytic effect of AAEMs from materials under these conditions. In previous studies, a small ratio of biomass in mixed materials was usually beneficial for synergy during co-utilization of biomass and coal (Jeong et al., 2017, 2019). It is believed that the small amount of biomass plays a catalyst role, while higher amounts of biomass could hamper the synergetic effect. However, the catalytic effect of AAEMs from material is not the main cause of synergy at 850 °C in this study, as the excess gas volumes are large even the ratio of WS is higher companying with less CO product. Although H2 percent in gas product decrease, the excess H2 amount increase roughly as the WS ratio increase. It seems H2O vapor could
Fig. 6. Gas volume (a) and gas composition (b) from co-gasification of WS and LC at 850 °C with different blending ratios.
promote more H free radical generated through a higher ratio of WS material. Comparing unbalanced blending ratios of WS and LC, the mixture with close ratios of biomass and coal produce more gas volume. It could be concluded that the reactivities of both half-chars are enhanced by H2O vapor, which magnify the synergy between WS and LC consequently. With the ratio of WS to LC 60:40, the most amount of excess gas is generated at 850 °C. Therefore, H2O vapor might have a slightly stronger effect with wheat straw half-char compared to lignite. The actual CH4 and CO2 percentage are close to the calculated prediction, which indicate that less reactions between gas products as reactions (5) to (7) listed in Table 3 taking place during co-gasification. On the other words, the synergetic effect in steam co-gasification at different blending ratios are mainly associated with H2O vapor on increasing the reactivity of half-chars.
4. Conclusions Synergy with increasing gas volume between WS and LC in copyrolysis and steam co-gasification were measured at different temperatures. It found that, catalytic effect of AAEMs from material, transfer of free radical and reactivity of half-chars are enhanced with H2O vapor involvement, and synergetic effect between WS and LC is magnified consequently. Temperature plays an important role in the interactions among biomass, coal, and H2O vapor. The excess of gas produced over the calculation in co-gasification increases and exhibit a 6
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