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Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor
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Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor Zening Cheng, Hui Jin, Shanke Liu, Liejin Guo*, Jialing Xu, Di Su State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China
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abstract
Article history:
Semicoke powders with particle size less than 6 mm are by-products during the pyrolysis
Received 22 January 2016
of coal. Direct combustion of semicoke powders is difficult due to the low volatile content.
Received in revised form
Supercritical water gasification might provide an efficient conversion method for semicoke
6 June 2016
powders. In order to determine the optimum conditions of gasification of semicoke with
Accepted 8 June 2016
the supercritical water fluidized bed reactor, the influences of the main operating pa-
Available online xxx
rameters including temperature (540e660 C), feedstock concentration (10e30 wt%), flow rate of preheated water (40e80 g/min) and alkali catalysts (K2CO3, KOH, Na2CO3 and NaOH)
Keywords:
were systematically investigated in this study. The results showed that semicoke-water
Hydrogen production
slurry of 30 wt% was continuously transported into the reactor and stably gasified
Semicoke gasification
without plugging problems. Hydrogen yield of 85.90 mol/kg was obtained with the
Supercritical water
hydrogen molar fraction of 61.02%. In particular, carbon gasification efficiency of more
Alkali catalysts
than 95% was obtained under the conditions of 660 C, 60 g/min flow rate of preheated water and 10 wt% feedstock concentration with 5 wt% K2CO3. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Semicoke industry has become a characteristic industry to link coal production and coal chemical industry in China in recent 20 years. Semicoke powders with particle size less than 6 mm account for about 10% of the total amount of semicoke. Early, a lot of semicoke powders were thrown into rivers and farmlands due to lack of market demand, causing water and fertile farmlands to be contaminated. At present, in consideration of the constraints of rising energy prices as well as national environmental protection policies, the majority of semicoke powders are incinerated as the fuel of boiler [1,2]. However, direct combustion of semicoke powders is difficult because of the low volatile content and the high ignition point [3]. Thus it is extremely important to explore a new way to use semicoke powders reasonably.
Supercritical water can dissolve organic compounds because of its special physical and chemical properties such as very low dielectric constant, and reduced number and durability of hydrogen bonds [4,5]. In addition, gases are also miscible in supercritical water, so supercritical water provides a single fluid phase for chemical reaction process as reaction medium, which has the advantages of achieving higher concentrations of typical reactants, and omits the interphase heat/mass transfer [5]. Furthermore, the solubility and the diffusion coefficients of supercritical water can be easily controlled by adjusting the reaction temperature and pressure. Therefore, gasification of organic matters in supercritical water is considered to be a promising technology, which can not only use the supercritical water as the reaction medium, but also separate gas and/or liquid by simply reducing the reaction temperature and/or pressure [6]. Today, hydrogen
* Corresponding author. E-mail address: [email protected] (L. Guo). http://dx.doi.org/10.1016/j.ijhydene.2016.06.075 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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has become an important kind of raw materials in petrochemical and chemical industry, and there is a growing interest in the use of hydrogen as fuel [7]. Hydrogen is defined as a green attractive energy source and has attracted extensive attention worldwidely due to its potential higher energy efficiency and less generation pollutants, which may replace conventional fossil fuels in the future [8]. Due to the above advantages of supercritical water, many researches have conducted in the application of supercritical water to gasify waste [9,10], biomass [11e15] and coal [16e19] for hydrogen production. Vostrikov [9] studied municipal sewage sludge gasification in supercritical water conditions (T 750 C; P 30 MPa). The predominant gaseous products among volatile conversion included CO2, H2, CH4 and NH3 according to the massspectrometric data. They found that the rate of the conversion increased with temperature and mainly depended on the interaction of water molecules with sewage sludge carbon (T > 600 C). The influences of pressure, temperature, residence time, and alkali addition on the gasification of corn starch, clover grass and corn silage in supercritical water were studied by Pedro [11]. The results showed that increasing the temperature and residence time could improve the gas yield, but the changes in pressure had no effect on the gasification yield. Potassium addition affected the gasification yield of corn starch, but had no obvious influence on the gas yield of the potassium-containing natural products of clover grass and corn silage. Hui Jin [16] investigated the gasification of coal in supercritical water with a fluidized bed reactor. The coal-water slurry of 24 wt% could be continuously transported in the reactor and stably gasified without plugging problems; hydrogen yield of 32.26 mol/kg was obtained with the hydrogen fraction of 69.78%. Furthermore, the recycle of the liquid residual from the gasification system was also studied. Supercritical water gasification of waste, biomass or coal had been widely studied, however, there were limited studies on supercritical water gasification of semicoke. Semicoke gasification in supercritical water can not only achieve highefficiency utilization of semicoke powders but also obtain hydrogen. Gasification of semicoke by supercritical water gasification technology has many advantages. When semicoke is made into slurry with water as reaction materials, the reaction materials are not easy to coke and block the pipeline due to the low tar content of semicoke. In this case, slurry with high concentration can realize continuous transmission. Besides, the surface of semicoke particle has abundant pore structure [20], which can adsorb catalysts easily and accelerate the gasification reaction. Additionally, the carbon content of semicoke powders is high, but its price is cheap [21], so
using semicoke to produce hydrogen and carbon dioxide will bring immeasurable economic benefits. In order to make a better research of supercritical water gasification technology, our laboratory (State key Laboratory of Multiphase Flow in Power Engineering) developed the supercritical water fluidized bed reactor system independently [14]. Comparing to the plasma gasification and entrained gasification, the supercritical water fluidized bed gasification has many advantages. Firstly, in the supercritical water fluidized bed reactor, semicoke particles form a fluidization state, which ensures residence time of semicoke particles in the reactor long enough. Secondly, because of the special physical and chemical properties of supercritical water [5], the gaseous products can be diffused effectively, which benefits further gasification reactions. Thirdly, the plasma gasification and entrained gasification need relatively high temperature (about 1200 C or higher) in the gasification reactions [22e26], however, the supercritical water fluidized bed gasification can obtain good results at relatively low temperature (about from 500 to 700 C) [16]. Finally, the residues deposit in the bottom of the reactor during the gasification reactions, rather than carried away from the reactor top, which can reduce the investments of the fly ash capture equipment. In this study, a novel utilizing method for semicoke powders was proposed. This was the first time supercritical water was applied to gasify semicoke. The influences of the main operating parameters including temperature, feedstock concentration, flow rate of preheated water and alkali catalysts were systematically investigated in order to determine the optimum condition of gasification of semicoke with the supercritical water fluidized bed reactor. What's more, the higher feedstock concentration of slurry than the literature [16] was continuously transported in the reactor.
Experiments Materials The semicoke used in the experiments was obtained from Yulin, Shaanxi, China. The elemental and proximate analysis of it was listed in Table 1. K2CO3, KOH, Na2CO3 and NaOH were all anhydrous reagents and were supplied by the Tianli Chemical Reagent Co., Ltd. All these alkaline catalysts were analytical pure reagents. Xanthan gum was purchased from the Shandong Fufeng Fermentation Co., Ltd.
Experimental set up The experiments were carried out with the supercritical water fluidized bed reactor system, and the schematic
Table 1 e Elemental and proximate analysis of the semicoke. Elemental analysis [wt%] Species Semicoke a
C 69.12
H 1.35
N 0.89
S 0.711
Proximate analysis [wt%] Oa 10.329
Moisture 0.7
Ash 16.9
Volatiles 15.74
Fixed carbon 67.36
By difference.
Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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Fig. 1 e Scheme of system for hydrogen production from semicoke with a supercritical water fluidized bed reactor: 1 feedstock tank; 2, 3 feeder; 4 supercritical water fluidization bed reactor; 5 heat exchanger; 6 pre-heater; 7 cooler; 8, 9, 10 back-pressure regulator; 11 high pressure separator; 12 low pressure separator; 13, 14 wet test meter; 15, 16, 17, 18 high pressure metering pump; 19, 20, 21, 22 mass flow meter; 23 water tank.
diagram of the system was shown in Fig. 1. The system mainly consisted of the supercritical water fluidized bed reactor, pumps, feeding system, preheater, heat exchanger and cooler. The reactor was made of 316 stainless steel with the inner diameter of 40 mm and the total length of 1650 mm. The reactor's design temperature and pressure were 700 C and 30 MPa, respectively. The system pressure was measured by the pressure sensor. The fluid temperature in the reactor was monitored by five K-type thermocouples distributed along the reactor axis. The distributor was located in the bottom of the reactor, and the preheated water flowed into the reactor through the distributor. The feedstock nozzle was fixed on the reactor wall above the distributor. When the water-semicoke slurry flowed into reactor from the feedstock nozzle, the preheated water heated the water-semicoke slurry to the desired temperature quickly and formed a fluidization state. The feeding system contained a movable piston to separate water and water-semicoke slurry. The pump pushed the movable piston through water, and then the water-semicoke slurry could flow into the reactor continuously. More detail information of the system could be found in the literature [14].
Experimental procedure Firstly, semicoke powders were pulverized with particle size less than 100 mm. 0.1 wt% xanthan gum was used as the stabilizer to mix with a certain amount of semicoke uniformly. And then, the corresponding amount of deionized water and alkali catalysts were added into the mixture (semicoke and xanthan gum) and stirred uniformly. So that the uniform and stable water-semicoke slurry was obtained. At the start of the experiment, the water-semicoke slurry was added into the feeding system, and then the pump and back-pressure regulator were regulated to make the system pressure maintain at 23 MPa. When the system pressure was
stable, the fluid temperature in the reactor increased to the desired temperature by the heating equipment gradually. Finally, the water-semicoke slurry was added into the reactor by switching the valves when both the system pressure and fluid temperature were stable. The residues were collected from the bottom of the reactor after the temperature and pressure decreased to the room temperature and environmental pressure, respectively.
Analysis methods The volume flow of gaseous products in the experiments was the parameter by direct measurement, however, the fluid temperature, liquid mass flow rate and pressure of the system were the parameters by indirect measurement. Uncertainty analyses of the above four parameters were shown in Table 2. The molar fractions of the gaseous product were analyzed by Agilent Technologies 7890A GC System equipped with a thermal conductivity detector and capillary column C-2000 purchased from Lanzhou Institute of Chemical Physics in China. N2 adsorptionedesorption isotherms were conducted at 77 K using an Accelerated Surface Area and Porosimetry Analyzer (ASAP 2020, Micromeritics) after degassing the samples at 300 C for 3 h. Surface area and pore size distributions were determined using the BrunauereEmmetteTeller (BET) methods. The gaseous products were analyzed to be mainly H2, CO, CH4 and CO2 in the experiments. In this study, CE (carbon gasification efficiency) and gas yield were used to indicate the extent of the gasification. The content of each gas component was determined by the molar fraction. The expressions were as follows (Equations (1)e(3)):
CE ¼ (the mass of carbon in gaseous product/ the mass of carbon in feedstock) 100, %
(1)
Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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Table 2 e Uncertainty analysis of the experimental parameters. Parameters Volume flow of gaseous products Fluid temperature Liquid mass flow rate Pressure of the system
Instruments
Measurement range
Minimum scale
Accuracy
Wet test meter K-type thermocouples Mass flow meter Pressure Sensor
0e3 m3/h 0e900 C 0e300 g/min 0e40 MPa
0.1 L 0.01 C 0.01 g/min 0.01 MPa
±1.5% ±1.02% ±0.2% ±1.25%
Gas yield ¼ the mole of gaseous product/ the mass of dry matter in feedstock, mol/kg
(2)
Gas molar fraction ¼ (the mole of a certain gas product/ the summation of molar number of all the gaseous products) 100, %
(3)
Results and discussions Semicoke is composed of a variety of organic compounds with complicated structures. Semicoke gasification in supercritical water is a complex process. It is commonly recognized [27,28] that the chemical conversion can be made up from three simplified net reactions: steam reforming reaction (4), water gas shift reaction (5), and methanation reaction (6). C þ H2 O/CO þ H2 ; DH ¼ 132 kJ=mol
(4)
CO þ H2 O/CO2 þ H2 ; DH ¼ 41 kJ=mol
(5)
CO þ 3H2 /CH4 þ H2 O; DH ¼ 206 kJ=mol
(6)
Effect of catalyst Effect of catalyst type Multiple studies [11,29,30] demonstrated that the alkali catalysts played an important role in promoting supercritical water gasification. In this work, the effects of various alkali catalysts on the supercritical water gasification of semicoke were investigated. Fig. 2 is a plot of gasification with different
catalysts. CE, hydrogen fraction, and hydrogen yield improved significantly with the addition of the catalyst. CE increases significantly when catalysts were added, indicating that the catalysts can promote the steam reforming reaction (Equation (4)). Comparing with 4.66 mol/kg without catalyst, while the yield of hydrogen increases to 55.78, 54.43, 51.06 and 53.79 mol/kg after adding K2CO3, KOH, Na2CO3, and NaOH, respectively. The yield of hydrogen with the catalysts is in the following order: K2CO3 > KOH > NaOH > Na2CO3. Adding K2CO3 and KOH has a better gasification efficiency than that of NaOH and Na2CO3, which means the catalytic effect of Kþ is greater than that of Naþ [31]. Veraa et al. [32] showed that KOH had similar catalytic properties with K2CO3, because KOH transformed into K2CO3 during the gasification process.
Effect of catalyst loading amount K2CO3 exhibited the best effect on hydrogen production in Section effect of catalyst type, thus we analyzed the effect of catalyst loading amount to reflect the catalytic mechanism of K2CO3 in this section. The results by Mims et al. [33] suggested that increasing the number of the active sites on the surface of the semicoke was conducive to the gasification efficiency, and pointed out that the number of active sites could be increased by increasing the catalyst loading amount. The effect of catalyst loading amount is showed in Figs. 3 and 4. Fig. 4a shows the BET result of the residue by using different amounts of K2CO3. Fig. 4b indicates that K2CO3 may create a great amount of mesoporous structures on the surface of the semicoke. The mesoporous structures can increase the surface area of semicoke (Fig. 4a), leading to larger contact area between the semicoke particles and supercritical water molecules. Furthermore, K2CO3 can migrate into the interior of mesoporous, creating more mesoporous structures. With the increase of catalyst loading, the number of mesoporous
Fig. 2 e Effect of alkali catalyst type on semicoke gasification in supercritical water (600 C, 23 MPa, water flow rate 40 g/min, slurry flow rate 20 g/min, 10 wt% semicoke þ 0.1 wt% xanthan gum, 5 wt% catalyst). Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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Fig. 3 e Effect of catalyst loading on semicoke gasification in supercritical water (600 C, 23 MPa, water flow rate 40 g/min, slurry flow rate 20 g/min, 10 wt% semicoke þ 0.1 wt% xanthan gum).
Fig. 4 e Effect of catalyst loading on the BET and pore distribution of residue (600 C, 23 MPa, water flow rate 40 g/min, slurry flow rate 20 g/min, 10 wt% semicoke þ 0.1 wt% xanthan gum). structures and the surface area increase, leading to a higher gasification efficiency. As shown in Fig. 3a, when the catalyst loading amount increases from 0 to 5 wt%, CE and the yield of hydrogen increases from 7.19% to 79.04% and from 4.66 mol/ kg to 55.78 mol/kg, respectively, revealing that increasing the amount of K2CO3 can promote the steam reforming reaction (Equation (4)). As shown in Fig. 3b, with the catalyst loading amount increasing from 0 to 5 wt%, the fraction of hydrogen increases from 51.81% to 55.06%, while the fraction of methane and carbon monoxide decrease from 11.12% to 8.14% and from 2.56% to 2.28%, respectively, indicating that the water gas shift reaction and steam reforming reaction of methane can be improved by increasing the amount of K2CO3.
Effect of temperature The effect of temperature is showed in Fig. 5, it can be seen from the figure that the temperature has a significant influence on the semicoke gasification in supercritical water. As the fluid temperature in the reactor increases from 540 C to 660 C, CE and the yield of hydrogen increase from 27.83% to 95.26% and from 17.53 mol/kg to 85.90 mol/kg, respectively. The steam reforming reaction (Equation (4)) of semicoke is an endothermic reaction. The elevated temperature facilitates the steam reforming reaction, leading to the consumption of fixed carbon, which is a major reaction for gasification of semicoke in supercritical water. As shown in Fig. 4b, the fraction of hydrogen increases from 52.23% to 61.02% as the
temperature increases. The free radical degradation is dominated at lower pressures and/or higher temperatures in supercritical water [34]. It is well known the free radical reaction is the main way to produce hydrogen, so the high temperature favors to hydrogen production. As the temperature increases, the fraction of carbon monoxide increases from 1.84% to 2.75%, while the fraction of carbon dioxide reduces from 37.01% to 30.01%, because the higher temperature promotes the reverse of the water gas shift reaction (Equation (5)). In addition, because the steam reforming steam of methane is an endothermic reaction, which results in the decrease of methane and the increase of carbon monoxide [35]. The CE increases from 27.83% to 31.09% with the temperature increasing from 540 C to 570 C, but the CE increases significantly to 89.92% at 600 C. The pyrolysis is the initial step for semicoke gasification in supercritical water, which releases volatile. The pyrolysis can be conducted at a relatively lower temperature, while the fixed carbon gasification requires a higher temperature. Semicoke from the pyrolysis of coal contains less volatile but more fixed carbon than that of original coal. So 600 C may be the trigger temperature for the complete gasification of fixed carbon in the supercritical water fluidized bed reactor.
Effect of concentration As seen in Fig. 6 that higher semicoke concentration is not conducive to semicoke gasification. Higher semicoke
Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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Fig. 5 e Effect of temperature on semicoke gasification in supercritical water (23 MPa, water flow rate 60 g/min, slurry flow rate 20 g/min, 10 wt% semicoke þ 0.1 wt% xanthan gum, 5 wt% K2CO3).
concentration means a lower water concentration. Thus, the solvolysis or hydrolysis reactions may be restricted, which leads to a higher yield of solid phases [36]. In addition, water is a reactant in the gasification process. Park K C et al. [37] used D2O instead of H2O as a reaction medium to conduct supercritical water gasification. The results showed that the hydrogenedeuterium exchange existed in the process and part of the deuterium atoms were transferred into the product, proving that water was a source of hydrogen in supercritical water gasification process. As the semicoke concentration increases from 10 to 30 wt %, the fraction of hydrogen reduces from 55.06% to 50.26% (Fig. 6b), while the fraction of methane increases from 8.14% to 12.57%. This seems to indicate that the competition of hydrogen element is existed between H2 and CH4 [16], which is mainly due to that the lower water concentration inhibits water gas shift reaction (Equation (5)) and the steam reforming reaction of methane [17]. Fig. 6a shows that when the semicoke concentration increases from 10 wt% to 30 wt%, hydrogen yield and CE decreases from 55.77 mol/kg to 36.28 mol/kg and from 79.04% to 62.35%, respectively. However, it's found that the feedstock concentration had a more significant effect on feedstock gasification in Zhiwei Ge's research [31]. Hydrogen yield and CE decreased from 57.7 mol/kg to 10 mol/kg and from 100% to 39.5%, respectively, as the feedstock concentration increased from 2 wt% to 20 wt% [31]. When coal slurry was heated in the
micro batch reactor, the volatile matter released from the coal char during pyrolysis process. The residual coal char gathered or even lumped at the bottom of the reactor. When the coal concentration increased, the contact area between coal and water decreased. And the steam reforming of coal char would be restricted, resulting in significantly reduced hydrogen yield and CE. Nevertheless, semicoke particles are kept in the fluidization state in the supercritical water fluidized bed reactor in this study. So heat and mass transfer between semicoke particles and water are greatly improved, which guarantee high gasification efficiency. High feedstock concentration means high processing capability, but high feedstock concentration has an adverse effect on gasification efficiency. Therefore, to find out the optimal concentration is essential for continuous and efficient operation of the experimental system.
Effect of fluidizing velocity As is shown in Fig. 1, the fluidization regime of the supercritical water fluidized bed is a bubbling bed, so it is need to ensure that the fluidization velocity is between the minimum fluidization velocity (umf) and terminal velocity (ut). Therefore, it is necessary to determine the minimum fluidization velocity and terminal velocity under the experimental conditions. The minimum fluidization velocity can be obtained by balancing the gravitational force and drag force from the fluid,
Fig. 6 e Effect of concentration on semicoke gasification in supercritical water (600 C, 23 MPa, water flow rate 40 g/min, slurry flow rate 20 g/min, 0.1 wt% xanthan gum, 5 wt% K2CO3). Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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Fig. 7 e Effect of temperature on umf and ut at 23 MPa.
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velocity firstly increases and then deceases, while the terminal velocity displays an increasing tread. And the minimum fluidization velocity and terminal velocity increases rapidly in the near-critical temperature, because the physical properties of water in the near-critical region change significantly. As shown in Fig. 8, when the flow rate of preheated water is between 40 and 80 g/min, the corresponding flow velocity is between 0.014 m/s and 0.023 m/s, and the values are between the minimum fluidization velocity and terminal velocity. As can be seen from Fig. 9a, with the flow rate of preheated water increasing from 40 g/min to 80 g/min, the CE and hydrogen yield firstly increases and then decreases. The higher flow rate of the preheated water means the higher fluidization velocity, leading to the stronger heat and mass transfer in the reactor, thus resulting in a better gasification efficiency. However, the higher flow rate of the preheated water always means the shorter residence time of the liquid intermediate product in the reactor, in reverse, causing a reduced gasification efficiency when the flow rate of preheated water is above 60 g/min. As can be observed from Fig. 9b, the fraction of hydrogen increases while the fraction of methane reduces with the flow rate of the preheated water increasing. Because when the concentration of the watersemicoke slurry and flow rate are kept constant, the increase of the flow rate of preheated water means the decrease of the feedstock concentration in the reactor, which contributes to the water gas shift reaction (Equation (5)) and the methane steam reforming reaction.
Conclusions Fig. 8 e Flow velocity at different flow rate of preheated water (600 C, 23 MPa).
and the equation to obtain terminal velocity is based on the laws of Stokes, Allen, and Newton. Equations to calculate the minimum fluidization velocity and terminal velocity can be found in the literature [13]. The results of the minimum fluidization velocity and terminal velocity are showed in Fig. 7. From Fig. 7, it can be found that as the temperature increases, the minimum fluidization
In this study, hydrogen generation from semicoke was conducted in a supercritical water fluidized bed reactor. Supercritical water gasification for hydrogen production by semicoke was proven to be an efficient and clean way for the use of semicoke powders. Main conclusions were as follows: 1. CE and hydrogen yield increased with alkali catalysts addition, increasing temperature, increasing catalyst loading, decreasing feedstock concentration, and the appropriate flow rate of preheated water. 2. K2CO3 showed the best catalytic effect for hydrogen production. The mesoporous structures could be formed on
Fig. 9 e Effect of flow rate of preheated water on semicoke gasification in supercritical water (600 C, 23 MPa, slurry flow rate 20 g/min, 10 wt% semicoke þ 0.1 wt% xanthan gum, 5 wt% K2CO3). Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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the semicoke particle surface by adding K2CO3. And the more K2CO3 was added, the more mesoporous structures were created when K2CO3 loading was less than 5 wt%. The mesoporous structures increased contact area between semicoke particle and supercritical water molecules, leading a higher gasification efficiency than without K2CO3. 3. CE improved significantly above 600 C. It indicated that 600 C might be the trigger temperature for the fixed carbon complete gasification. 4. CE of 95.26%, hydrogen yield of 85.90 mol/kg, and hydrogen molar fraction of 61.02% were obtained under the conditions of 660 C, 23 MPa, 60 g/min flow rate of preheated water and 10 wt% feedstock concentration with 5 wt% K2CO3. 5. High concentration of 30 wt% semicoke-water slurry could be fed into the reactor continuously without clogging problems.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Contract Nos. 51527808, 51323011 and 51236007).
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Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor Zening Cheng, Hui Jin, Shanke Liu, Liejin Guo*, Jialing Xu, Di Su State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China
article info
abstract
Article history:
Semicoke powders with particle size less than 6 mm are by-products during the pyrolysis
Received 22 January 2016
of coal. Direct combustion of semicoke powders is difficult due to the low volatile content.
Received in revised form
Supercritical water gasification might provide an efficient conversion method for semicoke
6 June 2016
powders. In order to determine the optimum conditions of gasification of semicoke with
Accepted 8 June 2016
the supercritical water fluidized bed reactor, the influences of the main operating pa-
Available online xxx
rameters including temperature (540e660 C), feedstock concentration (10e30 wt%), flow rate of preheated water (40e80 g/min) and alkali catalysts (K2CO3, KOH, Na2CO3 and NaOH)
Keywords:
were systematically investigated in this study. The results showed that semicoke-water
Hydrogen production
slurry of 30 wt% was continuously transported into the reactor and stably gasified
Semicoke gasification
without plugging problems. Hydrogen yield of 85.90 mol/kg was obtained with the
Supercritical water
hydrogen molar fraction of 61.02%. In particular, carbon gasification efficiency of more
Alkali catalysts
than 95% was obtained under the conditions of 660 C, 60 g/min flow rate of preheated water and 10 wt% feedstock concentration with 5 wt% K2CO3. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Semicoke industry has become a characteristic industry to link coal production and coal chemical industry in China in recent 20 years. Semicoke powders with particle size less than 6 mm account for about 10% of the total amount of semicoke. Early, a lot of semicoke powders were thrown into rivers and farmlands due to lack of market demand, causing water and fertile farmlands to be contaminated. At present, in consideration of the constraints of rising energy prices as well as national environmental protection policies, the majority of semicoke powders are incinerated as the fuel of boiler [1,2]. However, direct combustion of semicoke powders is difficult because of the low volatile content and the high ignition point [3]. Thus it is extremely important to explore a new way to use semicoke powders reasonably.
Supercritical water can dissolve organic compounds because of its special physical and chemical properties such as very low dielectric constant, and reduced number and durability of hydrogen bonds [4,5]. In addition, gases are also miscible in supercritical water, so supercritical water provides a single fluid phase for chemical reaction process as reaction medium, which has the advantages of achieving higher concentrations of typical reactants, and omits the interphase heat/mass transfer [5]. Furthermore, the solubility and the diffusion coefficients of supercritical water can be easily controlled by adjusting the reaction temperature and pressure. Therefore, gasification of organic matters in supercritical water is considered to be a promising technology, which can not only use the supercritical water as the reaction medium, but also separate gas and/or liquid by simply reducing the reaction temperature and/or pressure [6]. Today, hydrogen
* Corresponding author. E-mail address: [email protected] (L. Guo). http://dx.doi.org/10.1016/j.ijhydene.2016.06.075 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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has become an important kind of raw materials in petrochemical and chemical industry, and there is a growing interest in the use of hydrogen as fuel [7]. Hydrogen is defined as a green attractive energy source and has attracted extensive attention worldwidely due to its potential higher energy efficiency and less generation pollutants, which may replace conventional fossil fuels in the future [8]. Due to the above advantages of supercritical water, many researches have conducted in the application of supercritical water to gasify waste [9,10], biomass [11e15] and coal [16e19] for hydrogen production. Vostrikov [9] studied municipal sewage sludge gasification in supercritical water conditions (T 750 C; P 30 MPa). The predominant gaseous products among volatile conversion included CO2, H2, CH4 and NH3 according to the massspectrometric data. They found that the rate of the conversion increased with temperature and mainly depended on the interaction of water molecules with sewage sludge carbon (T > 600 C). The influences of pressure, temperature, residence time, and alkali addition on the gasification of corn starch, clover grass and corn silage in supercritical water were studied by Pedro [11]. The results showed that increasing the temperature and residence time could improve the gas yield, but the changes in pressure had no effect on the gasification yield. Potassium addition affected the gasification yield of corn starch, but had no obvious influence on the gas yield of the potassium-containing natural products of clover grass and corn silage. Hui Jin [16] investigated the gasification of coal in supercritical water with a fluidized bed reactor. The coal-water slurry of 24 wt% could be continuously transported in the reactor and stably gasified without plugging problems; hydrogen yield of 32.26 mol/kg was obtained with the hydrogen fraction of 69.78%. Furthermore, the recycle of the liquid residual from the gasification system was also studied. Supercritical water gasification of waste, biomass or coal had been widely studied, however, there were limited studies on supercritical water gasification of semicoke. Semicoke gasification in supercritical water can not only achieve highefficiency utilization of semicoke powders but also obtain hydrogen. Gasification of semicoke by supercritical water gasification technology has many advantages. When semicoke is made into slurry with water as reaction materials, the reaction materials are not easy to coke and block the pipeline due to the low tar content of semicoke. In this case, slurry with high concentration can realize continuous transmission. Besides, the surface of semicoke particle has abundant pore structure [20], which can adsorb catalysts easily and accelerate the gasification reaction. Additionally, the carbon content of semicoke powders is high, but its price is cheap [21], so
using semicoke to produce hydrogen and carbon dioxide will bring immeasurable economic benefits. In order to make a better research of supercritical water gasification technology, our laboratory (State key Laboratory of Multiphase Flow in Power Engineering) developed the supercritical water fluidized bed reactor system independently [14]. Comparing to the plasma gasification and entrained gasification, the supercritical water fluidized bed gasification has many advantages. Firstly, in the supercritical water fluidized bed reactor, semicoke particles form a fluidization state, which ensures residence time of semicoke particles in the reactor long enough. Secondly, because of the special physical and chemical properties of supercritical water [5], the gaseous products can be diffused effectively, which benefits further gasification reactions. Thirdly, the plasma gasification and entrained gasification need relatively high temperature (about 1200 C or higher) in the gasification reactions [22e26], however, the supercritical water fluidized bed gasification can obtain good results at relatively low temperature (about from 500 to 700 C) [16]. Finally, the residues deposit in the bottom of the reactor during the gasification reactions, rather than carried away from the reactor top, which can reduce the investments of the fly ash capture equipment. In this study, a novel utilizing method for semicoke powders was proposed. This was the first time supercritical water was applied to gasify semicoke. The influences of the main operating parameters including temperature, feedstock concentration, flow rate of preheated water and alkali catalysts were systematically investigated in order to determine the optimum condition of gasification of semicoke with the supercritical water fluidized bed reactor. What's more, the higher feedstock concentration of slurry than the literature [16] was continuously transported in the reactor.
Experiments Materials The semicoke used in the experiments was obtained from Yulin, Shaanxi, China. The elemental and proximate analysis of it was listed in Table 1. K2CO3, KOH, Na2CO3 and NaOH were all anhydrous reagents and were supplied by the Tianli Chemical Reagent Co., Ltd. All these alkaline catalysts were analytical pure reagents. Xanthan gum was purchased from the Shandong Fufeng Fermentation Co., Ltd.
Experimental set up The experiments were carried out with the supercritical water fluidized bed reactor system, and the schematic
Table 1 e Elemental and proximate analysis of the semicoke. Elemental analysis [wt%] Species Semicoke a
C 69.12
H 1.35
N 0.89
S 0.711
Proximate analysis [wt%] Oa 10.329
Moisture 0.7
Ash 16.9
Volatiles 15.74
Fixed carbon 67.36
By difference.
Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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Fig. 1 e Scheme of system for hydrogen production from semicoke with a supercritical water fluidized bed reactor: 1 feedstock tank; 2, 3 feeder; 4 supercritical water fluidization bed reactor; 5 heat exchanger; 6 pre-heater; 7 cooler; 8, 9, 10 back-pressure regulator; 11 high pressure separator; 12 low pressure separator; 13, 14 wet test meter; 15, 16, 17, 18 high pressure metering pump; 19, 20, 21, 22 mass flow meter; 23 water tank.
diagram of the system was shown in Fig. 1. The system mainly consisted of the supercritical water fluidized bed reactor, pumps, feeding system, preheater, heat exchanger and cooler. The reactor was made of 316 stainless steel with the inner diameter of 40 mm and the total length of 1650 mm. The reactor's design temperature and pressure were 700 C and 30 MPa, respectively. The system pressure was measured by the pressure sensor. The fluid temperature in the reactor was monitored by five K-type thermocouples distributed along the reactor axis. The distributor was located in the bottom of the reactor, and the preheated water flowed into the reactor through the distributor. The feedstock nozzle was fixed on the reactor wall above the distributor. When the water-semicoke slurry flowed into reactor from the feedstock nozzle, the preheated water heated the water-semicoke slurry to the desired temperature quickly and formed a fluidization state. The feeding system contained a movable piston to separate water and water-semicoke slurry. The pump pushed the movable piston through water, and then the water-semicoke slurry could flow into the reactor continuously. More detail information of the system could be found in the literature [14].
Experimental procedure Firstly, semicoke powders were pulverized with particle size less than 100 mm. 0.1 wt% xanthan gum was used as the stabilizer to mix with a certain amount of semicoke uniformly. And then, the corresponding amount of deionized water and alkali catalysts were added into the mixture (semicoke and xanthan gum) and stirred uniformly. So that the uniform and stable water-semicoke slurry was obtained. At the start of the experiment, the water-semicoke slurry was added into the feeding system, and then the pump and back-pressure regulator were regulated to make the system pressure maintain at 23 MPa. When the system pressure was
stable, the fluid temperature in the reactor increased to the desired temperature by the heating equipment gradually. Finally, the water-semicoke slurry was added into the reactor by switching the valves when both the system pressure and fluid temperature were stable. The residues were collected from the bottom of the reactor after the temperature and pressure decreased to the room temperature and environmental pressure, respectively.
Analysis methods The volume flow of gaseous products in the experiments was the parameter by direct measurement, however, the fluid temperature, liquid mass flow rate and pressure of the system were the parameters by indirect measurement. Uncertainty analyses of the above four parameters were shown in Table 2. The molar fractions of the gaseous product were analyzed by Agilent Technologies 7890A GC System equipped with a thermal conductivity detector and capillary column C-2000 purchased from Lanzhou Institute of Chemical Physics in China. N2 adsorptionedesorption isotherms were conducted at 77 K using an Accelerated Surface Area and Porosimetry Analyzer (ASAP 2020, Micromeritics) after degassing the samples at 300 C for 3 h. Surface area and pore size distributions were determined using the BrunauereEmmetteTeller (BET) methods. The gaseous products were analyzed to be mainly H2, CO, CH4 and CO2 in the experiments. In this study, CE (carbon gasification efficiency) and gas yield were used to indicate the extent of the gasification. The content of each gas component was determined by the molar fraction. The expressions were as follows (Equations (1)e(3)):
CE ¼ (the mass of carbon in gaseous product/ the mass of carbon in feedstock) 100, %
(1)
Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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Table 2 e Uncertainty analysis of the experimental parameters. Parameters Volume flow of gaseous products Fluid temperature Liquid mass flow rate Pressure of the system
Instruments
Measurement range
Minimum scale
Accuracy
Wet test meter K-type thermocouples Mass flow meter Pressure Sensor
0e3 m3/h 0e900 C 0e300 g/min 0e40 MPa
0.1 L 0.01 C 0.01 g/min 0.01 MPa
±1.5% ±1.02% ±0.2% ±1.25%
Gas yield ¼ the mole of gaseous product/ the mass of dry matter in feedstock, mol/kg
(2)
Gas molar fraction ¼ (the mole of a certain gas product/ the summation of molar number of all the gaseous products) 100, %
(3)
Results and discussions Semicoke is composed of a variety of organic compounds with complicated structures. Semicoke gasification in supercritical water is a complex process. It is commonly recognized [27,28] that the chemical conversion can be made up from three simplified net reactions: steam reforming reaction (4), water gas shift reaction (5), and methanation reaction (6). C þ H2 O/CO þ H2 ; DH ¼ 132 kJ=mol
(4)
CO þ H2 O/CO2 þ H2 ; DH ¼ 41 kJ=mol
(5)
CO þ 3H2 /CH4 þ H2 O; DH ¼ 206 kJ=mol
(6)
Effect of catalyst Effect of catalyst type Multiple studies [11,29,30] demonstrated that the alkali catalysts played an important role in promoting supercritical water gasification. In this work, the effects of various alkali catalysts on the supercritical water gasification of semicoke were investigated. Fig. 2 is a plot of gasification with different
catalysts. CE, hydrogen fraction, and hydrogen yield improved significantly with the addition of the catalyst. CE increases significantly when catalysts were added, indicating that the catalysts can promote the steam reforming reaction (Equation (4)). Comparing with 4.66 mol/kg without catalyst, while the yield of hydrogen increases to 55.78, 54.43, 51.06 and 53.79 mol/kg after adding K2CO3, KOH, Na2CO3, and NaOH, respectively. The yield of hydrogen with the catalysts is in the following order: K2CO3 > KOH > NaOH > Na2CO3. Adding K2CO3 and KOH has a better gasification efficiency than that of NaOH and Na2CO3, which means the catalytic effect of Kþ is greater than that of Naþ [31]. Veraa et al. [32] showed that KOH had similar catalytic properties with K2CO3, because KOH transformed into K2CO3 during the gasification process.
Effect of catalyst loading amount K2CO3 exhibited the best effect on hydrogen production in Section effect of catalyst type, thus we analyzed the effect of catalyst loading amount to reflect the catalytic mechanism of K2CO3 in this section. The results by Mims et al. [33] suggested that increasing the number of the active sites on the surface of the semicoke was conducive to the gasification efficiency, and pointed out that the number of active sites could be increased by increasing the catalyst loading amount. The effect of catalyst loading amount is showed in Figs. 3 and 4. Fig. 4a shows the BET result of the residue by using different amounts of K2CO3. Fig. 4b indicates that K2CO3 may create a great amount of mesoporous structures on the surface of the semicoke. The mesoporous structures can increase the surface area of semicoke (Fig. 4a), leading to larger contact area between the semicoke particles and supercritical water molecules. Furthermore, K2CO3 can migrate into the interior of mesoporous, creating more mesoporous structures. With the increase of catalyst loading, the number of mesoporous
Fig. 2 e Effect of alkali catalyst type on semicoke gasification in supercritical water (600 C, 23 MPa, water flow rate 40 g/min, slurry flow rate 20 g/min, 10 wt% semicoke þ 0.1 wt% xanthan gum, 5 wt% catalyst). Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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Fig. 3 e Effect of catalyst loading on semicoke gasification in supercritical water (600 C, 23 MPa, water flow rate 40 g/min, slurry flow rate 20 g/min, 10 wt% semicoke þ 0.1 wt% xanthan gum).
Fig. 4 e Effect of catalyst loading on the BET and pore distribution of residue (600 C, 23 MPa, water flow rate 40 g/min, slurry flow rate 20 g/min, 10 wt% semicoke þ 0.1 wt% xanthan gum). structures and the surface area increase, leading to a higher gasification efficiency. As shown in Fig. 3a, when the catalyst loading amount increases from 0 to 5 wt%, CE and the yield of hydrogen increases from 7.19% to 79.04% and from 4.66 mol/ kg to 55.78 mol/kg, respectively, revealing that increasing the amount of K2CO3 can promote the steam reforming reaction (Equation (4)). As shown in Fig. 3b, with the catalyst loading amount increasing from 0 to 5 wt%, the fraction of hydrogen increases from 51.81% to 55.06%, while the fraction of methane and carbon monoxide decrease from 11.12% to 8.14% and from 2.56% to 2.28%, respectively, indicating that the water gas shift reaction and steam reforming reaction of methane can be improved by increasing the amount of K2CO3.
Effect of temperature The effect of temperature is showed in Fig. 5, it can be seen from the figure that the temperature has a significant influence on the semicoke gasification in supercritical water. As the fluid temperature in the reactor increases from 540 C to 660 C, CE and the yield of hydrogen increase from 27.83% to 95.26% and from 17.53 mol/kg to 85.90 mol/kg, respectively. The steam reforming reaction (Equation (4)) of semicoke is an endothermic reaction. The elevated temperature facilitates the steam reforming reaction, leading to the consumption of fixed carbon, which is a major reaction for gasification of semicoke in supercritical water. As shown in Fig. 4b, the fraction of hydrogen increases from 52.23% to 61.02% as the
temperature increases. The free radical degradation is dominated at lower pressures and/or higher temperatures in supercritical water [34]. It is well known the free radical reaction is the main way to produce hydrogen, so the high temperature favors to hydrogen production. As the temperature increases, the fraction of carbon monoxide increases from 1.84% to 2.75%, while the fraction of carbon dioxide reduces from 37.01% to 30.01%, because the higher temperature promotes the reverse of the water gas shift reaction (Equation (5)). In addition, because the steam reforming steam of methane is an endothermic reaction, which results in the decrease of methane and the increase of carbon monoxide [35]. The CE increases from 27.83% to 31.09% with the temperature increasing from 540 C to 570 C, but the CE increases significantly to 89.92% at 600 C. The pyrolysis is the initial step for semicoke gasification in supercritical water, which releases volatile. The pyrolysis can be conducted at a relatively lower temperature, while the fixed carbon gasification requires a higher temperature. Semicoke from the pyrolysis of coal contains less volatile but more fixed carbon than that of original coal. So 600 C may be the trigger temperature for the complete gasification of fixed carbon in the supercritical water fluidized bed reactor.
Effect of concentration As seen in Fig. 6 that higher semicoke concentration is not conducive to semicoke gasification. Higher semicoke
Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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Fig. 5 e Effect of temperature on semicoke gasification in supercritical water (23 MPa, water flow rate 60 g/min, slurry flow rate 20 g/min, 10 wt% semicoke þ 0.1 wt% xanthan gum, 5 wt% K2CO3).
concentration means a lower water concentration. Thus, the solvolysis or hydrolysis reactions may be restricted, which leads to a higher yield of solid phases [36]. In addition, water is a reactant in the gasification process. Park K C et al. [37] used D2O instead of H2O as a reaction medium to conduct supercritical water gasification. The results showed that the hydrogenedeuterium exchange existed in the process and part of the deuterium atoms were transferred into the product, proving that water was a source of hydrogen in supercritical water gasification process. As the semicoke concentration increases from 10 to 30 wt %, the fraction of hydrogen reduces from 55.06% to 50.26% (Fig. 6b), while the fraction of methane increases from 8.14% to 12.57%. This seems to indicate that the competition of hydrogen element is existed between H2 and CH4 [16], which is mainly due to that the lower water concentration inhibits water gas shift reaction (Equation (5)) and the steam reforming reaction of methane [17]. Fig. 6a shows that when the semicoke concentration increases from 10 wt% to 30 wt%, hydrogen yield and CE decreases from 55.77 mol/kg to 36.28 mol/kg and from 79.04% to 62.35%, respectively. However, it's found that the feedstock concentration had a more significant effect on feedstock gasification in Zhiwei Ge's research [31]. Hydrogen yield and CE decreased from 57.7 mol/kg to 10 mol/kg and from 100% to 39.5%, respectively, as the feedstock concentration increased from 2 wt% to 20 wt% [31]. When coal slurry was heated in the
micro batch reactor, the volatile matter released from the coal char during pyrolysis process. The residual coal char gathered or even lumped at the bottom of the reactor. When the coal concentration increased, the contact area between coal and water decreased. And the steam reforming of coal char would be restricted, resulting in significantly reduced hydrogen yield and CE. Nevertheless, semicoke particles are kept in the fluidization state in the supercritical water fluidized bed reactor in this study. So heat and mass transfer between semicoke particles and water are greatly improved, which guarantee high gasification efficiency. High feedstock concentration means high processing capability, but high feedstock concentration has an adverse effect on gasification efficiency. Therefore, to find out the optimal concentration is essential for continuous and efficient operation of the experimental system.
Effect of fluidizing velocity As is shown in Fig. 1, the fluidization regime of the supercritical water fluidized bed is a bubbling bed, so it is need to ensure that the fluidization velocity is between the minimum fluidization velocity (umf) and terminal velocity (ut). Therefore, it is necessary to determine the minimum fluidization velocity and terminal velocity under the experimental conditions. The minimum fluidization velocity can be obtained by balancing the gravitational force and drag force from the fluid,
Fig. 6 e Effect of concentration on semicoke gasification in supercritical water (600 C, 23 MPa, water flow rate 40 g/min, slurry flow rate 20 g/min, 0.1 wt% xanthan gum, 5 wt% K2CO3). Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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Fig. 7 e Effect of temperature on umf and ut at 23 MPa.
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velocity firstly increases and then deceases, while the terminal velocity displays an increasing tread. And the minimum fluidization velocity and terminal velocity increases rapidly in the near-critical temperature, because the physical properties of water in the near-critical region change significantly. As shown in Fig. 8, when the flow rate of preheated water is between 40 and 80 g/min, the corresponding flow velocity is between 0.014 m/s and 0.023 m/s, and the values are between the minimum fluidization velocity and terminal velocity. As can be seen from Fig. 9a, with the flow rate of preheated water increasing from 40 g/min to 80 g/min, the CE and hydrogen yield firstly increases and then decreases. The higher flow rate of the preheated water means the higher fluidization velocity, leading to the stronger heat and mass transfer in the reactor, thus resulting in a better gasification efficiency. However, the higher flow rate of the preheated water always means the shorter residence time of the liquid intermediate product in the reactor, in reverse, causing a reduced gasification efficiency when the flow rate of preheated water is above 60 g/min. As can be observed from Fig. 9b, the fraction of hydrogen increases while the fraction of methane reduces with the flow rate of the preheated water increasing. Because when the concentration of the watersemicoke slurry and flow rate are kept constant, the increase of the flow rate of preheated water means the decrease of the feedstock concentration in the reactor, which contributes to the water gas shift reaction (Equation (5)) and the methane steam reforming reaction.
Conclusions Fig. 8 e Flow velocity at different flow rate of preheated water (600 C, 23 MPa).
and the equation to obtain terminal velocity is based on the laws of Stokes, Allen, and Newton. Equations to calculate the minimum fluidization velocity and terminal velocity can be found in the literature [13]. The results of the minimum fluidization velocity and terminal velocity are showed in Fig. 7. From Fig. 7, it can be found that as the temperature increases, the minimum fluidization
In this study, hydrogen generation from semicoke was conducted in a supercritical water fluidized bed reactor. Supercritical water gasification for hydrogen production by semicoke was proven to be an efficient and clean way for the use of semicoke powders. Main conclusions were as follows: 1. CE and hydrogen yield increased with alkali catalysts addition, increasing temperature, increasing catalyst loading, decreasing feedstock concentration, and the appropriate flow rate of preheated water. 2. K2CO3 showed the best catalytic effect for hydrogen production. The mesoporous structures could be formed on
Fig. 9 e Effect of flow rate of preheated water on semicoke gasification in supercritical water (600 C, 23 MPa, slurry flow rate 20 g/min, 10 wt% semicoke þ 0.1 wt% xanthan gum, 5 wt% K2CO3). Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075
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the semicoke particle surface by adding K2CO3. And the more K2CO3 was added, the more mesoporous structures were created when K2CO3 loading was less than 5 wt%. The mesoporous structures increased contact area between semicoke particle and supercritical water molecules, leading a higher gasification efficiency than without K2CO3. 3. CE improved significantly above 600 C. It indicated that 600 C might be the trigger temperature for the fixed carbon complete gasification. 4. CE of 95.26%, hydrogen yield of 85.90 mol/kg, and hydrogen molar fraction of 61.02% were obtained under the conditions of 660 C, 23 MPa, 60 g/min flow rate of preheated water and 10 wt% feedstock concentration with 5 wt% K2CO3. 5. High concentration of 30 wt% semicoke-water slurry could be fed into the reactor continuously without clogging problems.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Contract Nos. 51527808, 51323011 and 51236007).
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Please cite this article in press as: Cheng Z, et al., Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.075