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Supercritical water gasification of glycerol and glucose in different reactors: The effect of metal wall
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Supercritical water gasification of glycerol and glucose in different reactors: The effect of metal wall Chao Zhu, Runyu Wang, Hui Jin, Xiaoyan Lian, Liejin Guo*, Jianbing Huang State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi'an Jiaotong University, No.28, Xianning West Road, Xi'an 710049, Shaanxi, China
article info
abstract
Article history:
Supercritical water gasification of glycerol and glucose for hydrogen-rich gas was con-
Received 22 January 2016
ducted in two different reactors including quartz reactor and tubular reactor. The quartz
Received in revised form
reactors were batch reactors and they were made of SiO2 while the tubular reactor was a
2 June 2016
continuous reactor and the reactor was made of Hastelloy C-276. The feedstocks in quartz
Accepted 8 June 2016
reactors were gasified at two different conditions (without nickel wire in the reactors and
Available online 23 June 2016
with nickel wire in the reactors). The carbon gasification efficiency (CGE) of glycerol and glucose in quartz reactors with nickel wire were higher than that without nickel wire. The
Keywords:
feedstocks gasified in tubular reactor were also promoted by the catalytic effects of the
Supercritical water
reactor wall. The CGE of glycerol and glucose in the tubular reactor decreased with time
Different reactors
because the reactor wall was covered by char generated from these feedstocks and the
Catalytic wall
catalytic active of wall reduced with time. Analysis of the gasification results in these two
Gasification
reactors shows that the catalytic wall played an important role in supercritical water
Char
gasification and significantly improved the gasification efficiency. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen, as a clean, renewable and high efficient energy, is considered as a potential substitute for fossil energy. At present, hydrogen is mainly produced from fossil fuels, which causes a series of environment problems related to CO2 emissions and air pollution. Hydrogen produced from biomass is a clean, energy-saving process and zero CO2 net emission can be realized [1e3]. Glucose is always selected as a model compound to study the rules of biomass thermal chemistry conversion because glucose can be liberated from
cellulose, which is the most abundant organic renewable resource in nature [4,5]. In addition, renewable biomassderived sources can also be used to produce hydrogen-rich gas [6e8]. Gasification of glycerol for the production of hydrogen-rich gas is an appropriate way [9]. Glycerol is a byproduct from biodiesel manufacturing. 1 ton glycerol was obtained when 10 ton biodiesel was produced [10]. In recent years, supercritical water gasification (SCWG) technology has attracted wide attention due to its high efficient and clean approach of hydrogen-rich gas production [11]. Supercritical water can directly deal with high-moisture
* Corresponding author. Tel.: þ86 29 8266 3895; fax: þ86 29 8266 9033. E-mail address: [email protected] (L. Guo). http://dx.doi.org/10.1016/j.ijhydene.2016.06.085 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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biomass, thus avoiding the drying equipment [12]. The gas is produced under high pressure, thus leading to a small reactor volume and low energy expenditure to pressurize the gas [12,13]. Supercritical water provided a suitable environment for the interaction of the hydrocarbons with water due to its low dielectric constant [14,15]. Supercritical water gasification in a continuous or batch metal reactor could be catalyzed by the reactor wall. The metal reactors could be made of nickel-based alloy or stainless steel. Quartz tubes could be used as reactors to avoid the interference from the catalytic metal walls [16]. The metal wall plays an important role in supercritical water gasification and could improve the gasification efficiency [17,18]. Pairojpiriyakul et al. [19] investigated supercritical water gasification of glycerol in an empty Inconel 625 reactor. The results show that the active sites on Inconel 625 reactor wall could promote the conversion of glycerol to gas. Yu et al. [20] investigated supercritical water gasification of glucose in a Hastelloy tubular reactor and an Inconel 625 reactor at 600 C, 34.5 MPa and 30s without catalyst. The results show that different types of reactors could achieve obviously different gasification results. The gasification efficiency was about 68% in the Inconel reactor and 85% in the Hastelloy reactor. The Inconel reactor catalyzed the water gas shift reaction, thus increasing the yield of hydrogen and carbon dioxide. The Hastelloy C-276 reactor catalyzed the decomposition of acetic acid in supercritical water after the wall was corroded by salt solution. The effect of metal wall on SCWG has been studied in previous literature [19,20], but there were no analysis on the deactivation process of the metal wall in SCWG, especially the deactivation process in a continuous tubular reactor. In this study, the effect metal wall on SCWG in different reactors was systematically investigated and the deactivation process of the tubular reactor wall was analyzed. The effect of metal wall on carbon gasification efficiency (CGE), hydrogen gasification efficiency (HGE), gas compositions and organics in residual liquid were studied.
Materials and methods Materials The glycerol used in this study was produced by Tianjin Fuchen Chemical Reagent Factory. The glucose was provided by Sinopharm Chemical Reagent Co., Ltd. All these reagents were analytical pure. The quartz tubes were produced by Lianyungang Quartz Ceramics Co., Ltd with one end wellsealed.
Apparatus and methods In our experiments, two different reactors were used to gasify glycerol and glucose in supercritical water. The operation procedures and methods are described as follows. The gasification of feedstocks in quartz tubes were conducted in a tube furnace, which has been described in detail earlier [21,22]. Quartz tubes (1.5 mm i.d., 3 mm o.d., 200 mm length) were used as batch quartz reactors. The mixed
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feedstock-water solution was injected into the quartz tube which was sealed by hydro-oxygen flame afterwards. In our experiment, the quartz tube reactors were located into the furnace at room temperature and heated to the set temperature with program. The feedstocks could be heat to 500 C in 30 min. The reaction time was 10 min and the quartz tube after reaction was taken out from the furnace and cooled in air. Then the cooled quartz tube was placed into a polyethylene pipe filled with argon at ambient pressure. The polyethylene pipe was clamped at two ends by clamps and the gas could be released and mixed with argon after crushing the quartz tube inside the polyethylene pipe by pincer pliers. Composition of the gaseous product was qualitatively and quantitatively determined by the gas chromatography. The chemical oxygen demand (COD) of residual liquid in the quartz tube after the reaction was measured by the potassium dichromate colorimetric method. The qualitative determination of the residual liquid was carried out by GC/MS (GC6890/MSD5973). The GC (Agilent 6890) is equipped with HP-INNOWAX (30 m 0.25 mm 0.25um) capillary column. A mass selective detector (MSD5973) was used to identify the organics in the organic solvent. The gasification of feedstocks was also conducted in a Hastelloy tubular reactor and the schematic diagram of the continuous tubular reactor was shown in Fig. 1. Water was fed into the system through a high performance liquid chromatography (HPLC) pump and heated to 500 C in a preheater. The feedstock solution was fed into the system by another HPLC pump and mixed with the heated water at the inlet of the reactor. Thus the feedstock could be quickly heated to the set temperature. The Hastelloy tubular reactor was heated in a vertical tube furnace which was developed with three heating zones to obtain a uniform temperature field. The feedstock reacted with supercritical water and generated gas in the tubular reactor. The products were cooled by a cooler after leaving the tubular reactor. Subsequently, the products flowed through a back pressure regulator and were divided into gas and liquid in a gaseliquid separator. The gas was analyzed by gas chromatography (Agilent 7890A) and the residual liquid was analyzed by GC/MS. Automatic and monitoring configuration software was used for system control and parameter recording.
Data interpretation The CGE, HGE and gas composition are defined as follows. CGE ¼
mol of carbon atom in gas product 100% mol of carbon atom in the feed
HGE ¼
mol of hydrogen atom in gas product 100% mol of hydrogen atom in the feed
Gas composition ¼
mol of gas product 100% sum of mol of gas product
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Fig. 1 e Schematic diagram of the continuous flow tubular reactor.
Results and discussion SCWG of glycerol and glucose in quartz reactors Different feedstocks (glycerol and glucose) were gasified in the quartz tube reactors. The quartz tubes were made of SiO2 and had no catalytic activity on SCWG process [23,24]. The products, such as tar and char, were clearly visible in the quartz tubes after reaction. The mass concentration of feedstock was 5wt.% and the density of water was about 0.08 g/ml at 500 C [17]. As Ni was the main component of the nickel-based alloy reactors, nickel wires (0.2 mm diameter and 150 mm length) were used as catalysts in the quartz reactors. Nickel could promote the water-gas shift reaction and methanation
Fig. 2 e The CGE and HGE of SCWG of glycerol and glucose in quartz reactors (500 C, 5 wt.%, 0.08 g/ml).
reaction [25,26]. Fig. 2 shows the CGE and HGE of SCWG of feedstocks in quartz reactor at two different conditions including without nickel wire and with nickel wire. As shown in Fig. 2, the CGE and HGE of glycerol and glucose gasified in quartz reactor with nickel wire were higher than that without nickel wire. The CGE and HGE of glycerol were 52.75% and 24.95% without catalyst, and they increased to 82.45% and 72.01% with nickel wire. Similarly, the CGE and HGE of glucose increased from 15.26% and 6.73% without catalyst to 24.59% and 23.82% with nickel wire. It can be concluded that nickel catalyst were helpful to improve the gasification efficiency of glycerol and glucose. Fig. 3 depicts the gas components of SCWG of glycerol and glucose. As shown in Fig. 3, when a nickel wire was used as catalyst in SCWG of glycerol and glucose, the percentage of H2 improved while the percentage of CO declined markedly, which indicates that nickel catalyst
Fig. 3 e The gas compositions of SCWG of glycerol and glucose in quartz reactors (500 C, 5wt.%, 0.08 g/ml).
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Table 1 e The COD analysis of residual liquid obtained at different reaction condition. Feed
COD (g/L) Without catalyst
With catalyst
25.2 3.3
9.2 2.9
Glycerol Glucose
could promote the water-gas shift reaction and increase the hydrogen production [27,28]. Table 1 shows the COD of residual liquid in the quartz tubes after reaction. As shown in Table 1, the COD of residual liquid of glycerol decreased from 25.2 g/L without catalyst to 9.2 g/L with catalyst. The COD value of residual liquid of glucose decreased from 3.3 g/L without catalyst to 2.9 g/L with catalyst. By observing the quartz reactors after reaction, it could be found that there was no char covered on the quartz
wall when glycerol was gasified while there was obvious char attached to the quartz tube wall when glucose was gasified in the quartz reactors. This is because a lot of furans and phenols were formed in SCWG of glucose and those cyclic compounds could easily further react with each other and produce char [29], which was also the reason why the COD value of glucose residual liquid was lower than that of glycerol residual liquid. Besides, char deposited on the reactor wall or on the surface of the catalyst could weaken the catalytic effect of wall or catalyst. The organic compounds in residual liquid after reaction were identified by GC/MS. As shown in Table 2, the main organic compounds in residual liquid of SCWG of glycerol without catalyst were phenols, alcohols, arenes and ketones. The number of organic compounds in residual liquid of SCWG of glycerol with nickel wire was lower than that without catalyst. Similarly, the main organic compounds in residual liquid of SCWG of glucose without catalyst were furans,
Table 2 e Identification of organic compounds in liquid products by GC/MS analysis. Feed
Glycerol
Glucose
a
“e”: not detected.
No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
RT (min)
3.9 4.6 4.7 5.1 5.3 5.8 6.9 8.2 8.4 8.6 8.8 9.9 12.5 15.2 18.3 32.5 34.5 35.5 50.4 4.6 4.7 5.1 6.1 8.2 8.6 8.8 11.1 12 12.5 15.2 30.4 32.5 34.5 35.6 37 48.7 49.4 50.4 50.6
Peak area%a
Compounds
Benzene Methyl vinyl ketone 2-Pentanone 2,3-Butanedione 3-Buten-2-one, 3-methylToluene 1-Propanol Ethylbenzene 2-Propen-1-ol o-Xylene p-Xylene Cyclopentanone Benzene, 1,2,3-trimethylIndane Acetic acid Phenol Phenol, 2,3-dimethylPhenol, 4-methyl1,4-Benzenediol, 2-methylBenzene Furan, 2,5-dimethyl2-Pentanone Toluene Ethylbenzene o-Xylene p-Xylene Benzene, 1-ethyl-2-methylStyrene Benzene, 1,2,3-trimethylIndane Phenol, 3,4-dimethylPhenol Phenol, 4-methylPhenol, 2,3-dimethylPhenol, 3,4-dimethyl1,4-Benzenediol, 2,5-dimethyl2,5-Dimethylhydroquinone 1,4-Benzenediol, 2-methylHydroquinone
None
Ni
1.06 1.17 4.01 1.86 0.45 1.52 4.63 0.28 6.94 e 0.37 4.9 e e 9.45 56 3.61 e 3.75 1.88 0.94 0.91 8.93 1.22 1.21 1.62 1.3 1.05 1.18 0.92 4.06 27.5 21.04 3.33 3.68 1 1.44 9.56 7.23
3.13 e 1.32 e e 10.73 e 2.18 e 0.65 1.45 1.63 0.41 1.92 e 57.49 e 15.87 3.22 2.27 e 0.86 10.38 1.54 1.27 3.1 e e e e e 39.15 34.5 3.24 e e e 1.42 2.27
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useful tool to optimize the gasification result in supercritical water [30].
SCWG of glycerol and glucose in tubular Hastelloy reactor
Fig. 4 e The CGE and HGE of SCWG of glycerol and glucose with time in tubular reactor (500 C, 23 MPa, 5wt.%).
Fig. 5 e The gas compositions of SCWG of glycerol and glucose with time in tubular reactor (500 C, 23 MPa, 5wt.%).
ketones, arenes and phenols. The number of organic compounds in residual liquid of SCWG of glucose decreased when nickel wire was added in the quartz reactor. It can be concluded that nickel catalyst could reduce the yield of unwanted product and increased the gas yield. Catalyst were
Table 3 e SGWG of glycerol with and without the catalysis of Hastelloy wall (500 C, 23 MPa, 5wt.%). CGE HGE H2 CO CH4 CO2 (%) (%) (mol%) (mol%) (mol%) (mol%) with catalysis 77.13 101.72 of wall without catalysis 29.31 25.47 of wall
59.01
4.45
6.58
29.96
50.74
21.13
3.20
24.93
The tubular Hastelloy reactor is a continuous reactor and has high feedstock heating rate because the feedstocks could mix with supercritical water before entering the reactor. The tubular reactor wall is made of Hastelloy C-276 and SCWG of glycerol and glucose could be catalyzed by the reactor wall as the tubular reactor mainly composes of Ni. Nickel catalysts are active on the CeC bonds break, wateregas shift reaction and methanation reaction [31]. In our experiment, glycerol and glucose were gasified in the Hastelloy tubular reactor at 500 C and 23 MPa. The gas was collected and measured every 20 min. The Hastelloy reactor was fully cleaned before each experiment to ensure that no char, dust or tar deposited on the reactor wall. The feedstock solution (25wt.%, 3.5 ml/min) mixed with supercritical water (14 ml/min) before entering the reactor and the final mass concentration of the mixed feedstock solution in the Hastelloy reactor was 5wt.%. The reaction time of feedstock in the tubular reactor was about 30s. The gas yield of glycerol decreased along with the extending of time while the gas yield of glucose decreased first and then remained almost unaltered. This is because a small amount char deposition attached to the reactor wall in the process of SCWG of glycerol while a large amount of char attached to the reactor wall in the process of SCWG of glucose, leading to the failure of the catalytic effect of wall. After most of the active sites of the reactor wall were covered, the gas yield remained almost unaltered. Fig. 4 shows the CGE and HGE of feedstocks with time while Fig. 5 shows the gas compositions of SCWG of feedstocks with time. The CGE and HGE of glycerol were 77.13% and 101.72% at 20 min and they reduced to 71.08% and 94.67% at 120 min. The CGE and HGE of glucose were 36.52% and 22.24% at 20 min, 32.78% and 18.41% at 80 min, 32.31% and 17.83% at 120 min, respectively. Analysis of the gas compositions reveals that the percentage of CO significantly increased while the percentage of CH4 and CO2 decreased with time, because the Hastelloy reactor wall could catalyze the watergas shift reactions (CO þ H2O 4 CO2 þ H2) and CO methanation reaction (3H2 þ CO 4 CH4 þ H2O). As the reactor wall was covered by char, the catalysis of wall was suppressed and the percentage of CH4 and CO2 decreased along with the decrease of the CGE and HGE. To avoid char formation, loading an anticoking catalyst in the tubular reactor could be a suitable method to achieve the complete gasification. Elliott DC et al. reported that near-total conversion of the organics of the algae to gases was achieved in the presence of a Ru/C catalyst at 350 C and 20 MPa [32]. Ru catalysts could promote the gasification of furans, phenols and arenes and inhibited the formation of char [29]. As the catalytic activity of the Hastelloy wall almost disappeared after the process of SCWG of glucose, we were interested to know the gasification performance of glycerol in the tubular reactor without the impact of metal wall. To do this, the glycerol solution was fed into the tubular reactor system immediately after SCWG of glucose without washing the reactor which was covered with char generated from glucose. As shown in Table 3, SCWG of glycerol without the
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Table 4 e Identification of compounds in the liquid products of SCWG of glycerol with and without the catalysis of wall. Groups
Peak area%a
Compounds
With Without catalysis of catalysis of wall wall Alcohols, Aldehydes and Ketones
Acids
Phenols a
Propanal 2-Pentanone 2,3-Butanedione 1-Propanol 2-Propen-1-ol Cyclopentanone Cyclopentanone,2methyl2-Butanone,3hydroxy2-Propanone,1hydroxyAcetic acid Propanoic acid Prapanoic acid,2methylPhenol,3-methyl-
61.81 e e 19.93 8.25 2.9 e
13.34 2.32 1.13 1.71 15.55 2.05 1.35
e
6.85
e
1.98
7.11 e e e
17.44 21.95 5.69
Conclusion
catalysis of wall was obviously different from that with catalysis of wall. The CGE of glycerol decreased from 77.13% to 29.31% while the HGE of glycerol decreased from 101.72% to 25.47%. The percentage of CO in the gas significantly increased while the percentage of H2, CH4 and CO2 all decreased. It can be concluded from the comparison that the Hastelloy reactor wall could greatly promote the water-gas shift reaction, CO methanation reaction, CeC bond breaking and improve the CGE and HGE of feedstocks.
Table 5 e Identification of compounds in the liquid products of SCWG of glucose with the catalysis of wall.
Alcohols and Ketones
Acids Furans
Phenols
Tables 4 and 5 show the organic compounds in residual liquid of SCWG of glycerol and glucose identified by GC/MS. As shown in Table 4, the organic compounds in residual liquid of SCWG of glycerol with the catalysis of the reactor wall were propanal, 1-propanol, 2-propen-1-ol, cyclopentanone and acetic acid while that without catalysis of the wall mainly contained alcohols, aldehydes, ketones, acids and phenols. Thus it can be concluded that the Hastelloy wall promoted the small molecules to gas and inhibited the formation of phenols. As shown in Table 5, the main products of SCWG of glucose were alcohols, ketones, acids, furans and phenols. Small molecular intermediates, such as acids, ketones and alcohols, were detected by GC/MS. However, they were not detected in the residual liquid of SCWG of glucose in quartz reactors. This is because the feedstocks reacted with supercritical water in the tubular reactor only for about 30 s which was not enough for the complete gasification of the small molecule intermediates. Extending reaction time or adding catalysts could promote the gasification of those small molecule intermediates.
8.64
“e”: not detected.
Groups
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Compounds
Peak area%
Methyl vinyl ketone 3-Pentanone 2,3-Butanedione Cyclopentanone Cyclopentanone, 2-methyl2-Butanone, 3-hydroxy2-Propanone, 1-hydroxy3-Pentanol 2-Cyclopenten-1-one 2-Cyclopenten-1-one, 2-methyl1-Hydroxy-2-butanone Acetic acid Propanoic acid Furfural 2-Furancarboxaldehyde, 5-methyl2-Furancarboxaldehyde, 5(hydroxymethyl)Phenol Phenol, 2,4-dimethylPhenol, 3-methylPhenol, 2,5-dimethyl3,5-Dihydroxytoluene
1.13 0.60 1.65 1.71 0.81 5.05 7.92 0.96 2.17 1.46 2.77 15.12 17.24 10.5 5.12 7.19 13.08 1.12 1.48 1.41 1.51
Glycerol and glucose were gasified in supercritical water in quartz reactors and tubular reactor. Quartz reactors were batch reactors and the reactors wall had no catalytic activity on SCWG process. Feedstocks gasified in quartz reactors with nickel wire realized higher CGE and HGE than that without nickel wire. The percentage of H2 improved markedly with nickel wire in the quartz reactor compared to that without nickel wire. Tubular reactor was a continuous reactor and the reactor wall was made of Hastelloy C-276. Char attached to the reactor wall in SCWG of glycerol and glucose led to the decrease of the catalytic activity of the tubular reactor wall, which caused the decrease of the percentage of H2, CH4 and CO2 in gas as well as the CGE and HGE. This also proved reversely that the catalytic reactor wall could greatly promote the water-gas shift reaction, methanation reaction, CeC bond breaking and improve the CGE and HGE of feedstocks.
Acknowledgments We greatly acknowledge the financial supports from the National Key Project for Basic Research of China (973) No. 2012CB215303 and the National Natural Science Foundation of China (Contracts 51323011, 51527808 and 51306145).
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ScienceDirect journal homepage: www.elsevier.com/locate/he
Supercritical water gasification of glycerol and glucose in different reactors: The effect of metal wall Chao Zhu, Runyu Wang, Hui Jin, Xiaoyan Lian, Liejin Guo*, Jianbing Huang State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi'an Jiaotong University, No.28, Xianning West Road, Xi'an 710049, Shaanxi, China
article info
abstract
Article history:
Supercritical water gasification of glycerol and glucose for hydrogen-rich gas was con-
Received 22 January 2016
ducted in two different reactors including quartz reactor and tubular reactor. The quartz
Received in revised form
reactors were batch reactors and they were made of SiO2 while the tubular reactor was a
2 June 2016
continuous reactor and the reactor was made of Hastelloy C-276. The feedstocks in quartz
Accepted 8 June 2016
reactors were gasified at two different conditions (without nickel wire in the reactors and
Available online 23 June 2016
with nickel wire in the reactors). The carbon gasification efficiency (CGE) of glycerol and glucose in quartz reactors with nickel wire were higher than that without nickel wire. The
Keywords:
feedstocks gasified in tubular reactor were also promoted by the catalytic effects of the
Supercritical water
reactor wall. The CGE of glycerol and glucose in the tubular reactor decreased with time
Different reactors
because the reactor wall was covered by char generated from these feedstocks and the
Catalytic wall
catalytic active of wall reduced with time. Analysis of the gasification results in these two
Gasification
reactors shows that the catalytic wall played an important role in supercritical water
Char
gasification and significantly improved the gasification efficiency. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen, as a clean, renewable and high efficient energy, is considered as a potential substitute for fossil energy. At present, hydrogen is mainly produced from fossil fuels, which causes a series of environment problems related to CO2 emissions and air pollution. Hydrogen produced from biomass is a clean, energy-saving process and zero CO2 net emission can be realized [1e3]. Glucose is always selected as a model compound to study the rules of biomass thermal chemistry conversion because glucose can be liberated from
cellulose, which is the most abundant organic renewable resource in nature [4,5]. In addition, renewable biomassderived sources can also be used to produce hydrogen-rich gas [6e8]. Gasification of glycerol for the production of hydrogen-rich gas is an appropriate way [9]. Glycerol is a byproduct from biodiesel manufacturing. 1 ton glycerol was obtained when 10 ton biodiesel was produced [10]. In recent years, supercritical water gasification (SCWG) technology has attracted wide attention due to its high efficient and clean approach of hydrogen-rich gas production [11]. Supercritical water can directly deal with high-moisture
* Corresponding author. Tel.: þ86 29 8266 3895; fax: þ86 29 8266 9033. E-mail address: [email protected] (L. Guo). http://dx.doi.org/10.1016/j.ijhydene.2016.06.085 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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biomass, thus avoiding the drying equipment [12]. The gas is produced under high pressure, thus leading to a small reactor volume and low energy expenditure to pressurize the gas [12,13]. Supercritical water provided a suitable environment for the interaction of the hydrocarbons with water due to its low dielectric constant [14,15]. Supercritical water gasification in a continuous or batch metal reactor could be catalyzed by the reactor wall. The metal reactors could be made of nickel-based alloy or stainless steel. Quartz tubes could be used as reactors to avoid the interference from the catalytic metal walls [16]. The metal wall plays an important role in supercritical water gasification and could improve the gasification efficiency [17,18]. Pairojpiriyakul et al. [19] investigated supercritical water gasification of glycerol in an empty Inconel 625 reactor. The results show that the active sites on Inconel 625 reactor wall could promote the conversion of glycerol to gas. Yu et al. [20] investigated supercritical water gasification of glucose in a Hastelloy tubular reactor and an Inconel 625 reactor at 600 C, 34.5 MPa and 30s without catalyst. The results show that different types of reactors could achieve obviously different gasification results. The gasification efficiency was about 68% in the Inconel reactor and 85% in the Hastelloy reactor. The Inconel reactor catalyzed the water gas shift reaction, thus increasing the yield of hydrogen and carbon dioxide. The Hastelloy C-276 reactor catalyzed the decomposition of acetic acid in supercritical water after the wall was corroded by salt solution. The effect of metal wall on SCWG has been studied in previous literature [19,20], but there were no analysis on the deactivation process of the metal wall in SCWG, especially the deactivation process in a continuous tubular reactor. In this study, the effect metal wall on SCWG in different reactors was systematically investigated and the deactivation process of the tubular reactor wall was analyzed. The effect of metal wall on carbon gasification efficiency (CGE), hydrogen gasification efficiency (HGE), gas compositions and organics in residual liquid were studied.
Materials and methods Materials The glycerol used in this study was produced by Tianjin Fuchen Chemical Reagent Factory. The glucose was provided by Sinopharm Chemical Reagent Co., Ltd. All these reagents were analytical pure. The quartz tubes were produced by Lianyungang Quartz Ceramics Co., Ltd with one end wellsealed.
Apparatus and methods In our experiments, two different reactors were used to gasify glycerol and glucose in supercritical water. The operation procedures and methods are described as follows. The gasification of feedstocks in quartz tubes were conducted in a tube furnace, which has been described in detail earlier [21,22]. Quartz tubes (1.5 mm i.d., 3 mm o.d., 200 mm length) were used as batch quartz reactors. The mixed
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feedstock-water solution was injected into the quartz tube which was sealed by hydro-oxygen flame afterwards. In our experiment, the quartz tube reactors were located into the furnace at room temperature and heated to the set temperature with program. The feedstocks could be heat to 500 C in 30 min. The reaction time was 10 min and the quartz tube after reaction was taken out from the furnace and cooled in air. Then the cooled quartz tube was placed into a polyethylene pipe filled with argon at ambient pressure. The polyethylene pipe was clamped at two ends by clamps and the gas could be released and mixed with argon after crushing the quartz tube inside the polyethylene pipe by pincer pliers. Composition of the gaseous product was qualitatively and quantitatively determined by the gas chromatography. The chemical oxygen demand (COD) of residual liquid in the quartz tube after the reaction was measured by the potassium dichromate colorimetric method. The qualitative determination of the residual liquid was carried out by GC/MS (GC6890/MSD5973). The GC (Agilent 6890) is equipped with HP-INNOWAX (30 m 0.25 mm 0.25um) capillary column. A mass selective detector (MSD5973) was used to identify the organics in the organic solvent. The gasification of feedstocks was also conducted in a Hastelloy tubular reactor and the schematic diagram of the continuous tubular reactor was shown in Fig. 1. Water was fed into the system through a high performance liquid chromatography (HPLC) pump and heated to 500 C in a preheater. The feedstock solution was fed into the system by another HPLC pump and mixed with the heated water at the inlet of the reactor. Thus the feedstock could be quickly heated to the set temperature. The Hastelloy tubular reactor was heated in a vertical tube furnace which was developed with three heating zones to obtain a uniform temperature field. The feedstock reacted with supercritical water and generated gas in the tubular reactor. The products were cooled by a cooler after leaving the tubular reactor. Subsequently, the products flowed through a back pressure regulator and were divided into gas and liquid in a gaseliquid separator. The gas was analyzed by gas chromatography (Agilent 7890A) and the residual liquid was analyzed by GC/MS. Automatic and monitoring configuration software was used for system control and parameter recording.
Data interpretation The CGE, HGE and gas composition are defined as follows. CGE ¼
mol of carbon atom in gas product 100% mol of carbon atom in the feed
HGE ¼
mol of hydrogen atom in gas product 100% mol of hydrogen atom in the feed
Gas composition ¼
mol of gas product 100% sum of mol of gas product
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Fig. 1 e Schematic diagram of the continuous flow tubular reactor.
Results and discussion SCWG of glycerol and glucose in quartz reactors Different feedstocks (glycerol and glucose) were gasified in the quartz tube reactors. The quartz tubes were made of SiO2 and had no catalytic activity on SCWG process [23,24]. The products, such as tar and char, were clearly visible in the quartz tubes after reaction. The mass concentration of feedstock was 5wt.% and the density of water was about 0.08 g/ml at 500 C [17]. As Ni was the main component of the nickel-based alloy reactors, nickel wires (0.2 mm diameter and 150 mm length) were used as catalysts in the quartz reactors. Nickel could promote the water-gas shift reaction and methanation
Fig. 2 e The CGE and HGE of SCWG of glycerol and glucose in quartz reactors (500 C, 5 wt.%, 0.08 g/ml).
reaction [25,26]. Fig. 2 shows the CGE and HGE of SCWG of feedstocks in quartz reactor at two different conditions including without nickel wire and with nickel wire. As shown in Fig. 2, the CGE and HGE of glycerol and glucose gasified in quartz reactor with nickel wire were higher than that without nickel wire. The CGE and HGE of glycerol were 52.75% and 24.95% without catalyst, and they increased to 82.45% and 72.01% with nickel wire. Similarly, the CGE and HGE of glucose increased from 15.26% and 6.73% without catalyst to 24.59% and 23.82% with nickel wire. It can be concluded that nickel catalyst were helpful to improve the gasification efficiency of glycerol and glucose. Fig. 3 depicts the gas components of SCWG of glycerol and glucose. As shown in Fig. 3, when a nickel wire was used as catalyst in SCWG of glycerol and glucose, the percentage of H2 improved while the percentage of CO declined markedly, which indicates that nickel catalyst
Fig. 3 e The gas compositions of SCWG of glycerol and glucose in quartz reactors (500 C, 5wt.%, 0.08 g/ml).
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Table 1 e The COD analysis of residual liquid obtained at different reaction condition. Feed
COD (g/L) Without catalyst
With catalyst
25.2 3.3
9.2 2.9
Glycerol Glucose
could promote the water-gas shift reaction and increase the hydrogen production [27,28]. Table 1 shows the COD of residual liquid in the quartz tubes after reaction. As shown in Table 1, the COD of residual liquid of glycerol decreased from 25.2 g/L without catalyst to 9.2 g/L with catalyst. The COD value of residual liquid of glucose decreased from 3.3 g/L without catalyst to 2.9 g/L with catalyst. By observing the quartz reactors after reaction, it could be found that there was no char covered on the quartz
wall when glycerol was gasified while there was obvious char attached to the quartz tube wall when glucose was gasified in the quartz reactors. This is because a lot of furans and phenols were formed in SCWG of glucose and those cyclic compounds could easily further react with each other and produce char [29], which was also the reason why the COD value of glucose residual liquid was lower than that of glycerol residual liquid. Besides, char deposited on the reactor wall or on the surface of the catalyst could weaken the catalytic effect of wall or catalyst. The organic compounds in residual liquid after reaction were identified by GC/MS. As shown in Table 2, the main organic compounds in residual liquid of SCWG of glycerol without catalyst were phenols, alcohols, arenes and ketones. The number of organic compounds in residual liquid of SCWG of glycerol with nickel wire was lower than that without catalyst. Similarly, the main organic compounds in residual liquid of SCWG of glucose without catalyst were furans,
Table 2 e Identification of organic compounds in liquid products by GC/MS analysis. Feed
Glycerol
Glucose
a
“e”: not detected.
No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
RT (min)
3.9 4.6 4.7 5.1 5.3 5.8 6.9 8.2 8.4 8.6 8.8 9.9 12.5 15.2 18.3 32.5 34.5 35.5 50.4 4.6 4.7 5.1 6.1 8.2 8.6 8.8 11.1 12 12.5 15.2 30.4 32.5 34.5 35.6 37 48.7 49.4 50.4 50.6
Peak area%a
Compounds
Benzene Methyl vinyl ketone 2-Pentanone 2,3-Butanedione 3-Buten-2-one, 3-methylToluene 1-Propanol Ethylbenzene 2-Propen-1-ol o-Xylene p-Xylene Cyclopentanone Benzene, 1,2,3-trimethylIndane Acetic acid Phenol Phenol, 2,3-dimethylPhenol, 4-methyl1,4-Benzenediol, 2-methylBenzene Furan, 2,5-dimethyl2-Pentanone Toluene Ethylbenzene o-Xylene p-Xylene Benzene, 1-ethyl-2-methylStyrene Benzene, 1,2,3-trimethylIndane Phenol, 3,4-dimethylPhenol Phenol, 4-methylPhenol, 2,3-dimethylPhenol, 3,4-dimethyl1,4-Benzenediol, 2,5-dimethyl2,5-Dimethylhydroquinone 1,4-Benzenediol, 2-methylHydroquinone
None
Ni
1.06 1.17 4.01 1.86 0.45 1.52 4.63 0.28 6.94 e 0.37 4.9 e e 9.45 56 3.61 e 3.75 1.88 0.94 0.91 8.93 1.22 1.21 1.62 1.3 1.05 1.18 0.92 4.06 27.5 21.04 3.33 3.68 1 1.44 9.56 7.23
3.13 e 1.32 e e 10.73 e 2.18 e 0.65 1.45 1.63 0.41 1.92 e 57.49 e 15.87 3.22 2.27 e 0.86 10.38 1.54 1.27 3.1 e e e e e 39.15 34.5 3.24 e e e 1.42 2.27
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useful tool to optimize the gasification result in supercritical water [30].
SCWG of glycerol and glucose in tubular Hastelloy reactor
Fig. 4 e The CGE and HGE of SCWG of glycerol and glucose with time in tubular reactor (500 C, 23 MPa, 5wt.%).
Fig. 5 e The gas compositions of SCWG of glycerol and glucose with time in tubular reactor (500 C, 23 MPa, 5wt.%).
ketones, arenes and phenols. The number of organic compounds in residual liquid of SCWG of glucose decreased when nickel wire was added in the quartz reactor. It can be concluded that nickel catalyst could reduce the yield of unwanted product and increased the gas yield. Catalyst were
Table 3 e SGWG of glycerol with and without the catalysis of Hastelloy wall (500 C, 23 MPa, 5wt.%). CGE HGE H2 CO CH4 CO2 (%) (%) (mol%) (mol%) (mol%) (mol%) with catalysis 77.13 101.72 of wall without catalysis 29.31 25.47 of wall
59.01
4.45
6.58
29.96
50.74
21.13
3.20
24.93
The tubular Hastelloy reactor is a continuous reactor and has high feedstock heating rate because the feedstocks could mix with supercritical water before entering the reactor. The tubular reactor wall is made of Hastelloy C-276 and SCWG of glycerol and glucose could be catalyzed by the reactor wall as the tubular reactor mainly composes of Ni. Nickel catalysts are active on the CeC bonds break, wateregas shift reaction and methanation reaction [31]. In our experiment, glycerol and glucose were gasified in the Hastelloy tubular reactor at 500 C and 23 MPa. The gas was collected and measured every 20 min. The Hastelloy reactor was fully cleaned before each experiment to ensure that no char, dust or tar deposited on the reactor wall. The feedstock solution (25wt.%, 3.5 ml/min) mixed with supercritical water (14 ml/min) before entering the reactor and the final mass concentration of the mixed feedstock solution in the Hastelloy reactor was 5wt.%. The reaction time of feedstock in the tubular reactor was about 30s. The gas yield of glycerol decreased along with the extending of time while the gas yield of glucose decreased first and then remained almost unaltered. This is because a small amount char deposition attached to the reactor wall in the process of SCWG of glycerol while a large amount of char attached to the reactor wall in the process of SCWG of glucose, leading to the failure of the catalytic effect of wall. After most of the active sites of the reactor wall were covered, the gas yield remained almost unaltered. Fig. 4 shows the CGE and HGE of feedstocks with time while Fig. 5 shows the gas compositions of SCWG of feedstocks with time. The CGE and HGE of glycerol were 77.13% and 101.72% at 20 min and they reduced to 71.08% and 94.67% at 120 min. The CGE and HGE of glucose were 36.52% and 22.24% at 20 min, 32.78% and 18.41% at 80 min, 32.31% and 17.83% at 120 min, respectively. Analysis of the gas compositions reveals that the percentage of CO significantly increased while the percentage of CH4 and CO2 decreased with time, because the Hastelloy reactor wall could catalyze the watergas shift reactions (CO þ H2O 4 CO2 þ H2) and CO methanation reaction (3H2 þ CO 4 CH4 þ H2O). As the reactor wall was covered by char, the catalysis of wall was suppressed and the percentage of CH4 and CO2 decreased along with the decrease of the CGE and HGE. To avoid char formation, loading an anticoking catalyst in the tubular reactor could be a suitable method to achieve the complete gasification. Elliott DC et al. reported that near-total conversion of the organics of the algae to gases was achieved in the presence of a Ru/C catalyst at 350 C and 20 MPa [32]. Ru catalysts could promote the gasification of furans, phenols and arenes and inhibited the formation of char [29]. As the catalytic activity of the Hastelloy wall almost disappeared after the process of SCWG of glucose, we were interested to know the gasification performance of glycerol in the tubular reactor without the impact of metal wall. To do this, the glycerol solution was fed into the tubular reactor system immediately after SCWG of glucose without washing the reactor which was covered with char generated from glucose. As shown in Table 3, SCWG of glycerol without the
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Table 4 e Identification of compounds in the liquid products of SCWG of glycerol with and without the catalysis of wall. Groups
Peak area%a
Compounds
With Without catalysis of catalysis of wall wall Alcohols, Aldehydes and Ketones
Acids
Phenols a
Propanal 2-Pentanone 2,3-Butanedione 1-Propanol 2-Propen-1-ol Cyclopentanone Cyclopentanone,2methyl2-Butanone,3hydroxy2-Propanone,1hydroxyAcetic acid Propanoic acid Prapanoic acid,2methylPhenol,3-methyl-
61.81 e e 19.93 8.25 2.9 e
13.34 2.32 1.13 1.71 15.55 2.05 1.35
e
6.85
e
1.98
7.11 e e e
17.44 21.95 5.69
Conclusion
catalysis of wall was obviously different from that with catalysis of wall. The CGE of glycerol decreased from 77.13% to 29.31% while the HGE of glycerol decreased from 101.72% to 25.47%. The percentage of CO in the gas significantly increased while the percentage of H2, CH4 and CO2 all decreased. It can be concluded from the comparison that the Hastelloy reactor wall could greatly promote the water-gas shift reaction, CO methanation reaction, CeC bond breaking and improve the CGE and HGE of feedstocks.
Table 5 e Identification of compounds in the liquid products of SCWG of glucose with the catalysis of wall.
Alcohols and Ketones
Acids Furans
Phenols
Tables 4 and 5 show the organic compounds in residual liquid of SCWG of glycerol and glucose identified by GC/MS. As shown in Table 4, the organic compounds in residual liquid of SCWG of glycerol with the catalysis of the reactor wall were propanal, 1-propanol, 2-propen-1-ol, cyclopentanone and acetic acid while that without catalysis of the wall mainly contained alcohols, aldehydes, ketones, acids and phenols. Thus it can be concluded that the Hastelloy wall promoted the small molecules to gas and inhibited the formation of phenols. As shown in Table 5, the main products of SCWG of glucose were alcohols, ketones, acids, furans and phenols. Small molecular intermediates, such as acids, ketones and alcohols, were detected by GC/MS. However, they were not detected in the residual liquid of SCWG of glucose in quartz reactors. This is because the feedstocks reacted with supercritical water in the tubular reactor only for about 30 s which was not enough for the complete gasification of the small molecule intermediates. Extending reaction time or adding catalysts could promote the gasification of those small molecule intermediates.
8.64
“e”: not detected.
Groups
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Compounds
Peak area%
Methyl vinyl ketone 3-Pentanone 2,3-Butanedione Cyclopentanone Cyclopentanone, 2-methyl2-Butanone, 3-hydroxy2-Propanone, 1-hydroxy3-Pentanol 2-Cyclopenten-1-one 2-Cyclopenten-1-one, 2-methyl1-Hydroxy-2-butanone Acetic acid Propanoic acid Furfural 2-Furancarboxaldehyde, 5-methyl2-Furancarboxaldehyde, 5(hydroxymethyl)Phenol Phenol, 2,4-dimethylPhenol, 3-methylPhenol, 2,5-dimethyl3,5-Dihydroxytoluene
1.13 0.60 1.65 1.71 0.81 5.05 7.92 0.96 2.17 1.46 2.77 15.12 17.24 10.5 5.12 7.19 13.08 1.12 1.48 1.41 1.51
Glycerol and glucose were gasified in supercritical water in quartz reactors and tubular reactor. Quartz reactors were batch reactors and the reactors wall had no catalytic activity on SCWG process. Feedstocks gasified in quartz reactors with nickel wire realized higher CGE and HGE than that without nickel wire. The percentage of H2 improved markedly with nickel wire in the quartz reactor compared to that without nickel wire. Tubular reactor was a continuous reactor and the reactor wall was made of Hastelloy C-276. Char attached to the reactor wall in SCWG of glycerol and glucose led to the decrease of the catalytic activity of the tubular reactor wall, which caused the decrease of the percentage of H2, CH4 and CO2 in gas as well as the CGE and HGE. This also proved reversely that the catalytic reactor wall could greatly promote the water-gas shift reaction, methanation reaction, CeC bond breaking and improve the CGE and HGE of feedstocks.
Acknowledgments We greatly acknowledge the financial supports from the National Key Project for Basic Research of China (973) No. 2012CB215303 and the National Natural Science Foundation of China (Contracts 51323011, 51527808 and 51306145).
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