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reactor for hydrogen production with biomass gasification in supercritical water
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Solar receiver/reactor for hydrogen production with biomass gasification in supercritical water Bo Liao, Liejin Guo*, Youjun Lu, Ximin Zhang State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710049, PR China
article info
abstract
Article history:
A novel receiver/reactor driven by concentrating solar energy for hydrogen production by
Received 29 December 2012
supercritical water gasification (SCWG) of biomass was designed, constructed and tested.
Received in revised form
Model compound (glucose) and real biomass (corncob) were successfully gasified under
13 March 2013
SCW conditions to generate hydrogen-rich fuel gas in the apparatus. It is found that the
Accepted 21 March 2013
receiver/reactor temperature increased with the increment of the direct normal solar
Available online 20 April 2013
irradiation (DNI). Effects of the DNI, the flow rates and concentration of the feedstocks as well as alkali catalysts addition were investigated. The results showed that DNI and flow
Keywords:
rates of reactants have prominent effects on the temperature of reactor wall and gasifi-
Solar receiver/reactor
cation results. Higher DNI and lower feed concentrations favor the biomass gasification for
Supercritical water
hydrogen production. The encouraging results indicate a promising approach for hydrogen
Biomass gasification
production with biomass gasification in supercritical water using concentrated solar
Hydrogen production
energy. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
With the rapid increase of world energy consumption and serious environmental pollutions caused by the utilization of fossil fuels, sustainable energy systems based on hydrogen as energy carrier coupled with renewable energy resources such as solar, biomass etc., are considered as an effective way to resolve issues of concern today including greenhouse gas emissions, national energy security, air pollution, and energy efficiency [1e3]. Solar energy with the characteristics such as clean and inexhaustible, is one of the most promising energy resources on Earth and in space, but it also have drawbacks such as dilution, intermission and unequal distribution, which limit its large-scale high-efficiency utilization. Concentrating solar power (CSP) has the potential to make major contributions to solar thermal utilization, which can be
used to supply heat for endothermic chemical reactions [4,5]. As we know, one of the drawbacks of biomass gasification systems is that the energy to power these reactors is typically drawn from a portion of the feedstock combustion with an oxidizing agent causing the gaseous products contamination [6]. However, solar energy heating can offer a truly carbonneutral, environmentally friendly alternative to conventional heating practices [7]. Solar-driven gasification, in which concentrated solar radiation is supplied as the energy source of high-temperature process heat to the endothermic reactions, is one of the most attractive candidates for the conversion process of solar high-temperature heat to chemical fuels [8e10]. Solar hydrogen production from biomass gasification in supercritical water (SCW) is one of the promising clean and renewable approaches for utilizing solar energy and high
* Corresponding author. Tel.: þ86 29 82663895; fax: þ86 29 82669033. E-mail address: [email protected] (L. Guo). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.03.113
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moisture content biomass [11], because the combination of SCWG of biomass with CSP technology to concentrate solar thermal as the energy source for high-temperature process heat offers several important advantages as follows [12e17]: (1) Concentrated solar thermal can be used to supply necessary energy, and low-grade solar heat and the calorific value of feedstock are upgraded to high-grade chemical energy for further utilization by solar-driven process [18]. (2) Compared to other conventional biomass gasification technologies, such as air gasification or steam gasification, SCWG not only can directly deal with the wet biomass with a natural water content of 80 wt% or more without energy-intensive drying process, but also has high gasification efficiency in lower temperatures (<700 C), higher molar fraction of hydrogen and lower CO content in the gas products, and little tar and char are produced [19,20]. (3) The required retention time for SCWG of biomass is in the order of a few seconds to 1 min. This means that the required reactor size is relatively small. (4) Energy recovery from SCWG process can occur directly by a compact and efficient high-pressure heat exchanger from the exit flow of reactor which has a strong effect on the thermal efficiency of the process [21]. (5) CO2 has high solubility in high pressurized water at room temperature, so it can be easily separated from H2 and off gas treatment can be neglected, in addition, high-pressure products are easy for future transportation and usage. So far, most of the solar thermochemical hydrogen production has focused on the utilization of solar thermal energy concentrated above 1000 C [22]. Based on above considerations for clean and efficient hydrogen production, in this paper, to lower the extremely high operating temperatures required and to eliminate the need for high-temperature gas separation in conventional gasification, a novel receiver/ reactor driven by concentrating solar energy for hydrogen production by SCWG of biomass was designed, fabricated and tested. A concentrated solar heat of around 500e750 C was utilized as process heat to drive SCWG of biomass. Model compound (glucose) and real biomass (corncob) were successfully gasified under SCW conditions to generate hydrogen-rich fuel gas with the solar receiver/reactor. The results of the solar-driven hydrogen production by SCWG of biomass are reported, and the thermal performance of the receiver/reactor is determined.
2.
Experimental sections
2.1.
Solar receiver/reactor configuration
Solar reactors for concentrated solar systems usually feature a cavity-receiver type configuration, i.e. a well-insulated enclosure with a small aperture to let in concentrated solar radiation [23]. A cylindrical cavity-type solar receiver/reactor for SCWG of biomass constructed of spiral tube was developed, as shown in Fig. 1. The solar receiver/reactor consists of two cavities separated by insulation materials, with the upper one serving as the solar receiver/reactor and the lower one containing a deionized water pre-heater heated by crawlertype electric heaters as a backup when there are clouds in the sky. The receiver/reactor was constructed in 316 stainless
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Fig. 1 e Schematic diagram of the solar receiver/reactor for SCWG of biomass.
steel spiral tubes (5 mm i.d. and 18 m length) and heated using solar radiation concentrated by a 10 kW multi-dishes concentrator designed by IEE, CAS. The cavity-type geometry approaching a blackbody absorber was designed to effectively capture the incident solar radiation by a 25 cm diameter aperture equipped with a quartz window, through which the concentrated solar radiation irradiates the inner wall of the receiver/reactor. In order to achieve homogeneous temperature distribution and enhancement of heat and mass transfer, spiral tube was used as reactor simultaneously serving as the receiver for accepting and converting the concentrated solar energy to the reaction tube to drive the thermochemical reaction. To improve thermal efficiencies, the sensible heat of hot products exiting the reactor can be recovered by a countercurrent high-pressure coaxial spiral heat exchanger to preheat the preheating water. Type K-thermocouples were inserted into the center of the stainless steel tube to measure the fluid temperature, which were installed respectively in the outlet of feedstock pre-heater, outlet of water pre-heater, inlet and outlet of reactor, inlet and outlet of high-pressure spiral tube heat exchanger.
2.2.
Experimental apparatus and procedures
The schematic diagram of SCWG of biomass driven by solarethermal system is shown in Fig. 2. The biomass loading stream and deionized water were pressurized in two different lines by two high-pressure metering pumps and then separately preheated. To improve the heat ratio of reaction, feedstocks were preheated to 200e250 C depending on the
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 3 0 3 8 e1 3 0 4 4
Fig. 2 e System diagram of SCWG of biomass driven by solar thermal. 1: Feedstocks tank; 2,3: high-pressure feeder; 4: feedstocks pre-heater; 5: SCW reactor; 6: heat exchanger; 7: deionic water pre-heater; 8,9: cooler; 10e12: high-pressure hose; 13,14,15: back-pressure regulator; 16: high-pressure separator; 17: low pressure separator; 18,19: wet test meter; 20e23: high-pressure metering pump; 24e27: mass flow meter; 28: valve; 29: water tank; 30: multi-dishes concentrator.
thermal decomposition temperature of feedstocks and mixed with preheated water at the reactor inlet which recovered heat from the residue by an efficient countercurrent highpressure heat exchanger [19], so the inlet fluid temperature of the reactor reached to SCW state (374.3 C, 22.1 MPa) rapidly. The ratio of feedstocks flow rate and preheat water flow rate is 1/3 in our experiments, and this mixing method leads to essentially instantaneous heating of the biomass loading stream which eliminates any undesirable reaction such as polymerization or charring. When operating without the preheat stream of water, biomass stream has a much slower heating rate in the reactor resulting in a tendency to accumulate char and sometimes clog the reactor. After heat recovery, the reactor effluents were cooled rapidly in a cooler and the pressure was subsequently reduced using a backpressure regulator. Finally, the product stream was separated into gas and liquid phases by a gaseliquid separator.
2.3.
compound (glucose) was mixed directly with deionized water to the corresponding concentration, and real biomass (corncob) was mixed with sodium carboxymethyl cellulose (CMC) from Shanghai Shanpu Chemicals Ltd, and added desired amount of water, and then the feedstock was continually stirred for formation of suspension. Generally, the perfect concentration of CMC is about 2 wt%, which was used as suspending agent in the experiments. Table 1 shows the results of element and proximate analyses of corncob. This suspension is easily and reliably delivered to the SCW reactor by a high-pressure metering pump, and pressure is controlled by adjusting back-pressure regulator. Parts of the operational details of the SCWG system were given in our previous work in Ref. [16]. Direct solar radiation meter with the overall error less than 2% was used to measure the DNI. The gaseous effluent was analyzed by a gas chromatograph (Agilent Technologies 7890A) equipped with a thermal conductivity detector for the detection of hydrogen, carbon monoxide, methane, carbon dioxide, ethane and ethylene. High purity helium with a flow rate of 30 ml/min was used as the carrier gas. Data obtained from the gas chromatograph was used to calculate evaluation indicators of gasification results, for instance, H2 yield of gaseous products, gasification efficiency (GE, the total mass of product gas/the
Materials and analytical methods
In the experiments, glucose was from Tianjin Jingbei Fine Chemicals Ltd, and corncob from BEIPIAO BANGBANG CORNCOB PRESS CO., LTD were used as real biomass feedstocks, which was grinded to less than 120 mesh. Model
Table 1 e Proximate and ultimate analysis of corncob. Biomass
Corncob
Elemental analysis (wt%)
Proximate analysis (wt%) a
Calorific value (MJ/kg)
C
H
N
S
O
M
A
V
FC
45.53
6.27
0.35
0.63
37.94
7.92
1.37
73.40
17.30
a By difference.
19.22
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total mass of dry feedstock, %), carbon gasification efficiency (CE, the total carbon in the product gas/the total carbon in dry feedstock, %), and hydrogen yield potential (HYP ¼ YH2 þ YCO þ 4YCH4 þ 6YC2 H4 þ 7YC2 H6 , the sum of measured hydrogen and the hydrogen which could theoretically be formed by completely shifting carbon monoxide and completely reforming hydrocarbon species, mol/kg).
3.
Experimental results and discussions
3.1.
Effect of DNI
DNI is the indicator of solar power input, which has a significant effect on hydrogen production by SCWG of biomass, and various experiments were carried out to determine the effect of DNI and temperature on gasification. Experimental conditions and gasification results of SCWG of biomass driven by solar energy are shown in Table 2. According to the gas chromatography analysis, gaseous products are mainly composed of H2, CH4, CO2, and trace CO. Temperature and gasification characteristics in solar receiver/reactor with DNI fluctuations are shown in Fig. 3. It can be seen from Fig. 3(a) that the mix inlet temperature of the fluid and reactor wall temperature are greatly affected with the solar irradiance fluctuation, and the receiver/reactor temperature increased with the increment of DNI. Fig. 3(b) shows variation of product gas composition and temperature versus time in run #1. With the decrease of reactor temperature, it can be seen that molar fraction of H2 decreases, CO2 increases, and CH4 content have a smaller range increase, nearly remain at about 10%, and CO, C2H6, C2H4 are less than 2%. Reactor temperature increased mainly due to the DNI increase, and the increase of reaction temperature helps to improve the generation of hydrogenrich gas. The yield of hydrogen among gaseous products increases sharply with increasing temperature especially above
Fig. 3 e Temperature and gasification characteristics in solar receiver/reactor with DNI fluctuations: (a) changes of DNI and temperature versus time (run #12). (b) Changes of temperature and product gas composition versus time (run #1).
Table 2 e Experimental conditions and gasification results of SCWG of biomass driven by solar energy. Runa,b
1 2 3 4 5 6 7 8 9 10 11 12
DNIc (W/m2)
571 656 616 380 463 564 605 578 533 615 705 715
Flow ratesd (g/min)
41.4 57.4 43.2 42.8 43.1 43.6 119.0 82.0 64.9 90.1 98.3 100.6
Outlet temperature ( C)
636 676 651 543 589 649 584 593 615 574 585 634
Mole fraction of gas products (%) H2
CO
CH4
CO2
48.6 51.7 22.6 54.0 51.9 27.5 26.9 27.9 41.6 36.3 43.9 31.7
0.8 0.1 0.1 1.2 0.1 0.8 15.3 10.1 0.2 0.6 0.1 0.5
12.4 12.6 9.5 5.5 6.9 16.2 8.1 8.2 12.3 11.5 12.6 10.7
35.0 32.5 65.1 37.8 37.8 51.0 47.2 51.6 43.2 49.0 41.0 54.4
GE (%)
CE (%)
HYP (mol/kg)
H2 yield (mol/kg)
109.7 100.6 91.8 77.0 61.0 86.9 55.9 62.2 81.6 59.1 48.9 77.2
93.1 83.8 70.2 56.7 46.5 69.0 45.9 49.4 62.7 43.4 36.8 56.3
69.2 60.0 22.7 37.2 28.6 25.8 18.5 18.9 39.2 23.5 25.0 27.3
27.2 25.2 6.6 21.1 15.7 5.9 5.5 6.2 15.0 8.6 9.9 9.2
a Pressure of all conditions: 24 MPa. b Feedstocks (1: 0.1 M glucose; 2: 0.1 M glucose; 3: 0.4 M glucose; 4: 0.2 M glucose; 5: 0.3 M glucose; 6: 0.4 M glucose; 7: 0.4 M glucose; 8: 0.5 M glucose; 9: 2 wt% CMC þ 1.5 wt% corncob; 10: 2 wt% CMC þ 3 wt% corncob; 11: 2 wt% CMC þ 3 wt% corncob þ 0.5 wt% KOH; 12: 2 wt% CMC þ 5wt% corncob). c DNI is the mean direct normal solar irradiation during the time on stream. d Total flow rates, the feedstocks flow rate/preheat water flow rate ¼ 1:3.
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600 C. The reason is that higher DNI results in higher temperatures, which are in favor of high reaction rate and freeradical reaction thus promoting the formation of gaseous products such as H2 or CH4 but inhibiting ionic reaction [24]. On the other hand, the yield of carbon monoxide decreases with temperature increase, most probably due to the role of a wateregas shift reaction. Although reaction temperature determines reaction rate, it is also hypothesized that heating rate is an important parameter in SCW gasification, since high heating rates are positive for the chemical process [15]. The higher furnace temperature allows for higher mixing temperatures and thus higher heating rates of the feed stream, as well as a higher reaction temperature.
3.2.
Effect of flow rates
The effect of flow rates (residence time) on the gas composition from glucose gasification in SCW with solar receiver/ reactor was shown in Table 2. It can be seen from run #6 and #7 that flow rates increase induced temperature reduction significantly from 649 C to 584 C, and CO content substantially increased from 0.8% to 15.3%, but CH4 dropped from 16.2% to 8.1%, this means that conducive to the reforming reaction of methane, and similarity to run #9, #10, GE and CE decreased sharply. The reason is the higher the flow rate, the lower the temperature and the shorter the residence time, thus hydrogen content and GE decrease. However, as shown in Fig. 4(a), run #8, when the total flow rate increased from 80.0 g/min to 100.0 g/min at 13:55, it can be seen that H2, CO in the gas composition increased from 27.9%, 10.1% to 32.6%, 19.6%, respectively, but CO2, CH4 in the gas composition decreased from 51.6%, 8.2% to 40.4%, 5.6%, respectively. This is contrary to the conclusions of the above, and this was mainly due to the characteristics of spiral reactor, and the experiment results showed that it is not always the lower the flow rate, the higher the temperature. In a certain flow range, convection is enhanced in the spiral reactor and heat exchanger with the flow rate increase, thereby the fluid temperature inside the tube increased and gasification results improved, thus indicating that when the reaction temperature and residence time affected simultaneously, the reaction temperature is the dominant role of conditions. SCWG of biomass is more sensitive to changes of DNI than changes in residence time. Therefore, the gasification results showed that there are different optimal flow rates for the experimental apparatus according to different DNI, and this requires further study.
3.3.
Effect of feedstocks concentration
Table 2 clearly shows that glucose concentration has prominent effect on biomass gasification. The concentration of glucose switched in real time successfully from 0.1 mol/L to 0.4 mol/L at 13:30 in run #2, #3 and the compositions of the produced gases are illustrated in Fig. 4(b). Obviously, molar fraction of gas composition was changed greatly before and after switching glucose concentration. As the concentration increased, gasification efficiency and the average mole fraction of H2, CH4 in the gas products decreased rapidly, while mole fraction of CO2 increased sharply, and CO is less than 1%, which illustrated that higher concentration was not conducive to the
Fig. 4 e Real time switching experiment: (a) effect of flow rates on gasification results (run #8). (b) Effect of feedstocks concentration on gas composition (run #2, run # 3).
generation of hydrogen-rich gas. Table 2, run #2, #3 shows that the GE reduced from 100.6% to 91.8%, and the CE decreased from 83.8% to 70.2%, despite the decline is not large, but in fact, as 0.4 mol/L glucose gasification, the mole fraction of CO2 in the gas products is relatively large, hydrogen yield decreased from 25.2 mol/kg to 6.6 mol/kg, and HYP decreased from 60.0 mol/kg to 22.7 mol/kg. Therefore, increasing the feed concentration, GE declined, in particular, H2 yield and HYP declined rapidly, and high concentrations of biomass have a negative effect on the gas yield. At higher concentrations, it is possible that polymerization reactions resulting in formation of recalcitrant species reduce the number of glucose molecules available for decomposition or gasification [25]. Therefore, these results suggest that lower feed concentrations favor the biomass gasification for hydrogen production. Particularly, Table 2 shows that the GE can have values higher than 100%, because water is a reactant in this system either through steam reforming or the wateregas shift reactions, so the gas products can have a higher mass than the biomass alone [26].
3.4.
Effect of alkali catalysts
The effect of alkali catalysts on corncob gasification efficiency can be observed by comparing the results of run #10 and #11 in
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 3 0 3 8 e1 3 0 4 4
Table 2. In the case of without and with addition of KOH, H2 yield increased from 8.6 mol/kg to 9.9 mol/kg and molar fraction of H2 in the gas products increased significantly from 36.32% to 43.93%, whereas CO2 concentration decreased. The use of alkali catalysts led to an increase in gas yield, that is due to the carbonecarbon scission as free-radical reactions occurring at higher temperatures were improved and the gasification reaction, hydrocarbon reforming reaction as well as the wateregas shift reaction were catalyzed and the dehydration and polymerization pathway were suppressed in the presence of KOH [27,28]. However, since alkaline solutions are highly corrosive at supercritical temperatures, noncorrosive transition metal catalysts have been developed widely to moderate the violent reaction condition and improve gasification efficiency [29].
3.5.
Challenges
In order to solve the contradiction of reaction temperature stability and solar irradiance fluctuations due to cloud passages and wind speed [30], the thermal storage and heat transfer technology of molten salts may be an effective way to reduce reaction temperature fluctuations due to solar irradiance changes [12]. From the overall heat balance, intensive heat exchange to recover waste heat between feedstock and products by highly efficient heat exchanger is essential for system thermal efficiency [31]. Furthermore, to achieve complete gasification of tar and char, use of catalysts in the process is a good solution. In addition, some technical breakthroughs, especially on practical difficulties of operation in harsh temperature and pressure conditions, the need of high-pressure reactors, material requirement and its associated high cost and potential corrosion problems, use of highpressure piping, might be bottlenecks for the technology to become widely commercially available [32]. Therefore, new reactor materials and reactor configurations are needed to explore and develop, in order to avoid reactor wall causticity of alkaline catalyst and improve operational stability, process performance and efficiencies, development of non-corrosive efficient catalysts, optimization of process operating conditions as well as solar receiver/reactor design and system integration including rapid heating methods, heat storage and efficient energy recovery management are also attractive options for our next job.
4.
Conclusion
In this paper, a novel solar thermochemical receiver/reactor for hydrogen production by biomass gasification in SCW with multi-dishes concentrator was designed and constructed, and a series of performance testing on the system were carried out. Model compound (glucose) and real biomass (corncob) were continuously gasified under SCW conditions to generate hydrogen-rich fuel gas in the apparatus. 0.5 M glucose and 5 wt% corncob þ 2 wt% CMC feedstock were continually and stably gasified and reactor plugging was not observed. Among the parameters varied, DNI and feed concentration are found to have the most significant effect on the SCWG of biomass. The maximum reaction temperature of the solar receiver/
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reactor enclosed by a quartz glass window reached 650 C, which was sufficiently high enough to realize biomass gasify completely in SCW. As to 2 wt% CMC þ 1.5 wt% corncob gasification, the average volume percentage of H2 is more than 40%, mean H2 yield reached 15 mol/kg, and GE more than 80% was reached. As DNI (reaction temperature), residence time increase and feed concentration decrease, gas yield, CE and GE increase. The encouraging results indicated that hydrogen production with SCWG of biomass using concentrated solar energy is a promising approach.
Acknowledgments This work is financially supported by the National Key Project for Basic Research Program of China (Contracted No. 2009CB220000, No. 2012CB215303) and National Natural Science Foundation of China (Grant No. 51121092).
references
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Solar receiver/reactor for hydrogen production with biomass gasification in supercritical water Bo Liao, Liejin Guo*, Youjun Lu, Ximin Zhang State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710049, PR China
article info
abstract
Article history:
A novel receiver/reactor driven by concentrating solar energy for hydrogen production by
Received 29 December 2012
supercritical water gasification (SCWG) of biomass was designed, constructed and tested.
Received in revised form
Model compound (glucose) and real biomass (corncob) were successfully gasified under
13 March 2013
SCW conditions to generate hydrogen-rich fuel gas in the apparatus. It is found that the
Accepted 21 March 2013
receiver/reactor temperature increased with the increment of the direct normal solar
Available online 20 April 2013
irradiation (DNI). Effects of the DNI, the flow rates and concentration of the feedstocks as well as alkali catalysts addition were investigated. The results showed that DNI and flow
Keywords:
rates of reactants have prominent effects on the temperature of reactor wall and gasifi-
Solar receiver/reactor
cation results. Higher DNI and lower feed concentrations favor the biomass gasification for
Supercritical water
hydrogen production. The encouraging results indicate a promising approach for hydrogen
Biomass gasification
production with biomass gasification in supercritical water using concentrated solar
Hydrogen production
energy. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
With the rapid increase of world energy consumption and serious environmental pollutions caused by the utilization of fossil fuels, sustainable energy systems based on hydrogen as energy carrier coupled with renewable energy resources such as solar, biomass etc., are considered as an effective way to resolve issues of concern today including greenhouse gas emissions, national energy security, air pollution, and energy efficiency [1e3]. Solar energy with the characteristics such as clean and inexhaustible, is one of the most promising energy resources on Earth and in space, but it also have drawbacks such as dilution, intermission and unequal distribution, which limit its large-scale high-efficiency utilization. Concentrating solar power (CSP) has the potential to make major contributions to solar thermal utilization, which can be
used to supply heat for endothermic chemical reactions [4,5]. As we know, one of the drawbacks of biomass gasification systems is that the energy to power these reactors is typically drawn from a portion of the feedstock combustion with an oxidizing agent causing the gaseous products contamination [6]. However, solar energy heating can offer a truly carbonneutral, environmentally friendly alternative to conventional heating practices [7]. Solar-driven gasification, in which concentrated solar radiation is supplied as the energy source of high-temperature process heat to the endothermic reactions, is one of the most attractive candidates for the conversion process of solar high-temperature heat to chemical fuels [8e10]. Solar hydrogen production from biomass gasification in supercritical water (SCW) is one of the promising clean and renewable approaches for utilizing solar energy and high
* Corresponding author. Tel.: þ86 29 82663895; fax: þ86 29 82669033. E-mail address: [email protected] (L. Guo). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.03.113
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moisture content biomass [11], because the combination of SCWG of biomass with CSP technology to concentrate solar thermal as the energy source for high-temperature process heat offers several important advantages as follows [12e17]: (1) Concentrated solar thermal can be used to supply necessary energy, and low-grade solar heat and the calorific value of feedstock are upgraded to high-grade chemical energy for further utilization by solar-driven process [18]. (2) Compared to other conventional biomass gasification technologies, such as air gasification or steam gasification, SCWG not only can directly deal with the wet biomass with a natural water content of 80 wt% or more without energy-intensive drying process, but also has high gasification efficiency in lower temperatures (<700 C), higher molar fraction of hydrogen and lower CO content in the gas products, and little tar and char are produced [19,20]. (3) The required retention time for SCWG of biomass is in the order of a few seconds to 1 min. This means that the required reactor size is relatively small. (4) Energy recovery from SCWG process can occur directly by a compact and efficient high-pressure heat exchanger from the exit flow of reactor which has a strong effect on the thermal efficiency of the process [21]. (5) CO2 has high solubility in high pressurized water at room temperature, so it can be easily separated from H2 and off gas treatment can be neglected, in addition, high-pressure products are easy for future transportation and usage. So far, most of the solar thermochemical hydrogen production has focused on the utilization of solar thermal energy concentrated above 1000 C [22]. Based on above considerations for clean and efficient hydrogen production, in this paper, to lower the extremely high operating temperatures required and to eliminate the need for high-temperature gas separation in conventional gasification, a novel receiver/ reactor driven by concentrating solar energy for hydrogen production by SCWG of biomass was designed, fabricated and tested. A concentrated solar heat of around 500e750 C was utilized as process heat to drive SCWG of biomass. Model compound (glucose) and real biomass (corncob) were successfully gasified under SCW conditions to generate hydrogen-rich fuel gas with the solar receiver/reactor. The results of the solar-driven hydrogen production by SCWG of biomass are reported, and the thermal performance of the receiver/reactor is determined.
2.
Experimental sections
2.1.
Solar receiver/reactor configuration
Solar reactors for concentrated solar systems usually feature a cavity-receiver type configuration, i.e. a well-insulated enclosure with a small aperture to let in concentrated solar radiation [23]. A cylindrical cavity-type solar receiver/reactor for SCWG of biomass constructed of spiral tube was developed, as shown in Fig. 1. The solar receiver/reactor consists of two cavities separated by insulation materials, with the upper one serving as the solar receiver/reactor and the lower one containing a deionized water pre-heater heated by crawlertype electric heaters as a backup when there are clouds in the sky. The receiver/reactor was constructed in 316 stainless
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Fig. 1 e Schematic diagram of the solar receiver/reactor for SCWG of biomass.
steel spiral tubes (5 mm i.d. and 18 m length) and heated using solar radiation concentrated by a 10 kW multi-dishes concentrator designed by IEE, CAS. The cavity-type geometry approaching a blackbody absorber was designed to effectively capture the incident solar radiation by a 25 cm diameter aperture equipped with a quartz window, through which the concentrated solar radiation irradiates the inner wall of the receiver/reactor. In order to achieve homogeneous temperature distribution and enhancement of heat and mass transfer, spiral tube was used as reactor simultaneously serving as the receiver for accepting and converting the concentrated solar energy to the reaction tube to drive the thermochemical reaction. To improve thermal efficiencies, the sensible heat of hot products exiting the reactor can be recovered by a countercurrent high-pressure coaxial spiral heat exchanger to preheat the preheating water. Type K-thermocouples were inserted into the center of the stainless steel tube to measure the fluid temperature, which were installed respectively in the outlet of feedstock pre-heater, outlet of water pre-heater, inlet and outlet of reactor, inlet and outlet of high-pressure spiral tube heat exchanger.
2.2.
Experimental apparatus and procedures
The schematic diagram of SCWG of biomass driven by solarethermal system is shown in Fig. 2. The biomass loading stream and deionized water were pressurized in two different lines by two high-pressure metering pumps and then separately preheated. To improve the heat ratio of reaction, feedstocks were preheated to 200e250 C depending on the
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Fig. 2 e System diagram of SCWG of biomass driven by solar thermal. 1: Feedstocks tank; 2,3: high-pressure feeder; 4: feedstocks pre-heater; 5: SCW reactor; 6: heat exchanger; 7: deionic water pre-heater; 8,9: cooler; 10e12: high-pressure hose; 13,14,15: back-pressure regulator; 16: high-pressure separator; 17: low pressure separator; 18,19: wet test meter; 20e23: high-pressure metering pump; 24e27: mass flow meter; 28: valve; 29: water tank; 30: multi-dishes concentrator.
thermal decomposition temperature of feedstocks and mixed with preheated water at the reactor inlet which recovered heat from the residue by an efficient countercurrent highpressure heat exchanger [19], so the inlet fluid temperature of the reactor reached to SCW state (374.3 C, 22.1 MPa) rapidly. The ratio of feedstocks flow rate and preheat water flow rate is 1/3 in our experiments, and this mixing method leads to essentially instantaneous heating of the biomass loading stream which eliminates any undesirable reaction such as polymerization or charring. When operating without the preheat stream of water, biomass stream has a much slower heating rate in the reactor resulting in a tendency to accumulate char and sometimes clog the reactor. After heat recovery, the reactor effluents were cooled rapidly in a cooler and the pressure was subsequently reduced using a backpressure regulator. Finally, the product stream was separated into gas and liquid phases by a gaseliquid separator.
2.3.
compound (glucose) was mixed directly with deionized water to the corresponding concentration, and real biomass (corncob) was mixed with sodium carboxymethyl cellulose (CMC) from Shanghai Shanpu Chemicals Ltd, and added desired amount of water, and then the feedstock was continually stirred for formation of suspension. Generally, the perfect concentration of CMC is about 2 wt%, which was used as suspending agent in the experiments. Table 1 shows the results of element and proximate analyses of corncob. This suspension is easily and reliably delivered to the SCW reactor by a high-pressure metering pump, and pressure is controlled by adjusting back-pressure regulator. Parts of the operational details of the SCWG system were given in our previous work in Ref. [16]. Direct solar radiation meter with the overall error less than 2% was used to measure the DNI. The gaseous effluent was analyzed by a gas chromatograph (Agilent Technologies 7890A) equipped with a thermal conductivity detector for the detection of hydrogen, carbon monoxide, methane, carbon dioxide, ethane and ethylene. High purity helium with a flow rate of 30 ml/min was used as the carrier gas. Data obtained from the gas chromatograph was used to calculate evaluation indicators of gasification results, for instance, H2 yield of gaseous products, gasification efficiency (GE, the total mass of product gas/the
Materials and analytical methods
In the experiments, glucose was from Tianjin Jingbei Fine Chemicals Ltd, and corncob from BEIPIAO BANGBANG CORNCOB PRESS CO., LTD were used as real biomass feedstocks, which was grinded to less than 120 mesh. Model
Table 1 e Proximate and ultimate analysis of corncob. Biomass
Corncob
Elemental analysis (wt%)
Proximate analysis (wt%) a
Calorific value (MJ/kg)
C
H
N
S
O
M
A
V
FC
45.53
6.27
0.35
0.63
37.94
7.92
1.37
73.40
17.30
a By difference.
19.22
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total mass of dry feedstock, %), carbon gasification efficiency (CE, the total carbon in the product gas/the total carbon in dry feedstock, %), and hydrogen yield potential (HYP ¼ YH2 þ YCO þ 4YCH4 þ 6YC2 H4 þ 7YC2 H6 , the sum of measured hydrogen and the hydrogen which could theoretically be formed by completely shifting carbon monoxide and completely reforming hydrocarbon species, mol/kg).
3.
Experimental results and discussions
3.1.
Effect of DNI
DNI is the indicator of solar power input, which has a significant effect on hydrogen production by SCWG of biomass, and various experiments were carried out to determine the effect of DNI and temperature on gasification. Experimental conditions and gasification results of SCWG of biomass driven by solar energy are shown in Table 2. According to the gas chromatography analysis, gaseous products are mainly composed of H2, CH4, CO2, and trace CO. Temperature and gasification characteristics in solar receiver/reactor with DNI fluctuations are shown in Fig. 3. It can be seen from Fig. 3(a) that the mix inlet temperature of the fluid and reactor wall temperature are greatly affected with the solar irradiance fluctuation, and the receiver/reactor temperature increased with the increment of DNI. Fig. 3(b) shows variation of product gas composition and temperature versus time in run #1. With the decrease of reactor temperature, it can be seen that molar fraction of H2 decreases, CO2 increases, and CH4 content have a smaller range increase, nearly remain at about 10%, and CO, C2H6, C2H4 are less than 2%. Reactor temperature increased mainly due to the DNI increase, and the increase of reaction temperature helps to improve the generation of hydrogenrich gas. The yield of hydrogen among gaseous products increases sharply with increasing temperature especially above
Fig. 3 e Temperature and gasification characteristics in solar receiver/reactor with DNI fluctuations: (a) changes of DNI and temperature versus time (run #12). (b) Changes of temperature and product gas composition versus time (run #1).
Table 2 e Experimental conditions and gasification results of SCWG of biomass driven by solar energy. Runa,b
1 2 3 4 5 6 7 8 9 10 11 12
DNIc (W/m2)
571 656 616 380 463 564 605 578 533 615 705 715
Flow ratesd (g/min)
41.4 57.4 43.2 42.8 43.1 43.6 119.0 82.0 64.9 90.1 98.3 100.6
Outlet temperature ( C)
636 676 651 543 589 649 584 593 615 574 585 634
Mole fraction of gas products (%) H2
CO
CH4
CO2
48.6 51.7 22.6 54.0 51.9 27.5 26.9 27.9 41.6 36.3 43.9 31.7
0.8 0.1 0.1 1.2 0.1 0.8 15.3 10.1 0.2 0.6 0.1 0.5
12.4 12.6 9.5 5.5 6.9 16.2 8.1 8.2 12.3 11.5 12.6 10.7
35.0 32.5 65.1 37.8 37.8 51.0 47.2 51.6 43.2 49.0 41.0 54.4
GE (%)
CE (%)
HYP (mol/kg)
H2 yield (mol/kg)
109.7 100.6 91.8 77.0 61.0 86.9 55.9 62.2 81.6 59.1 48.9 77.2
93.1 83.8 70.2 56.7 46.5 69.0 45.9 49.4 62.7 43.4 36.8 56.3
69.2 60.0 22.7 37.2 28.6 25.8 18.5 18.9 39.2 23.5 25.0 27.3
27.2 25.2 6.6 21.1 15.7 5.9 5.5 6.2 15.0 8.6 9.9 9.2
a Pressure of all conditions: 24 MPa. b Feedstocks (1: 0.1 M glucose; 2: 0.1 M glucose; 3: 0.4 M glucose; 4: 0.2 M glucose; 5: 0.3 M glucose; 6: 0.4 M glucose; 7: 0.4 M glucose; 8: 0.5 M glucose; 9: 2 wt% CMC þ 1.5 wt% corncob; 10: 2 wt% CMC þ 3 wt% corncob; 11: 2 wt% CMC þ 3 wt% corncob þ 0.5 wt% KOH; 12: 2 wt% CMC þ 5wt% corncob). c DNI is the mean direct normal solar irradiation during the time on stream. d Total flow rates, the feedstocks flow rate/preheat water flow rate ¼ 1:3.
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600 C. The reason is that higher DNI results in higher temperatures, which are in favor of high reaction rate and freeradical reaction thus promoting the formation of gaseous products such as H2 or CH4 but inhibiting ionic reaction [24]. On the other hand, the yield of carbon monoxide decreases with temperature increase, most probably due to the role of a wateregas shift reaction. Although reaction temperature determines reaction rate, it is also hypothesized that heating rate is an important parameter in SCW gasification, since high heating rates are positive for the chemical process [15]. The higher furnace temperature allows for higher mixing temperatures and thus higher heating rates of the feed stream, as well as a higher reaction temperature.
3.2.
Effect of flow rates
The effect of flow rates (residence time) on the gas composition from glucose gasification in SCW with solar receiver/ reactor was shown in Table 2. It can be seen from run #6 and #7 that flow rates increase induced temperature reduction significantly from 649 C to 584 C, and CO content substantially increased from 0.8% to 15.3%, but CH4 dropped from 16.2% to 8.1%, this means that conducive to the reforming reaction of methane, and similarity to run #9, #10, GE and CE decreased sharply. The reason is the higher the flow rate, the lower the temperature and the shorter the residence time, thus hydrogen content and GE decrease. However, as shown in Fig. 4(a), run #8, when the total flow rate increased from 80.0 g/min to 100.0 g/min at 13:55, it can be seen that H2, CO in the gas composition increased from 27.9%, 10.1% to 32.6%, 19.6%, respectively, but CO2, CH4 in the gas composition decreased from 51.6%, 8.2% to 40.4%, 5.6%, respectively. This is contrary to the conclusions of the above, and this was mainly due to the characteristics of spiral reactor, and the experiment results showed that it is not always the lower the flow rate, the higher the temperature. In a certain flow range, convection is enhanced in the spiral reactor and heat exchanger with the flow rate increase, thereby the fluid temperature inside the tube increased and gasification results improved, thus indicating that when the reaction temperature and residence time affected simultaneously, the reaction temperature is the dominant role of conditions. SCWG of biomass is more sensitive to changes of DNI than changes in residence time. Therefore, the gasification results showed that there are different optimal flow rates for the experimental apparatus according to different DNI, and this requires further study.
3.3.
Effect of feedstocks concentration
Table 2 clearly shows that glucose concentration has prominent effect on biomass gasification. The concentration of glucose switched in real time successfully from 0.1 mol/L to 0.4 mol/L at 13:30 in run #2, #3 and the compositions of the produced gases are illustrated in Fig. 4(b). Obviously, molar fraction of gas composition was changed greatly before and after switching glucose concentration. As the concentration increased, gasification efficiency and the average mole fraction of H2, CH4 in the gas products decreased rapidly, while mole fraction of CO2 increased sharply, and CO is less than 1%, which illustrated that higher concentration was not conducive to the
Fig. 4 e Real time switching experiment: (a) effect of flow rates on gasification results (run #8). (b) Effect of feedstocks concentration on gas composition (run #2, run # 3).
generation of hydrogen-rich gas. Table 2, run #2, #3 shows that the GE reduced from 100.6% to 91.8%, and the CE decreased from 83.8% to 70.2%, despite the decline is not large, but in fact, as 0.4 mol/L glucose gasification, the mole fraction of CO2 in the gas products is relatively large, hydrogen yield decreased from 25.2 mol/kg to 6.6 mol/kg, and HYP decreased from 60.0 mol/kg to 22.7 mol/kg. Therefore, increasing the feed concentration, GE declined, in particular, H2 yield and HYP declined rapidly, and high concentrations of biomass have a negative effect on the gas yield. At higher concentrations, it is possible that polymerization reactions resulting in formation of recalcitrant species reduce the number of glucose molecules available for decomposition or gasification [25]. Therefore, these results suggest that lower feed concentrations favor the biomass gasification for hydrogen production. Particularly, Table 2 shows that the GE can have values higher than 100%, because water is a reactant in this system either through steam reforming or the wateregas shift reactions, so the gas products can have a higher mass than the biomass alone [26].
3.4.
Effect of alkali catalysts
The effect of alkali catalysts on corncob gasification efficiency can be observed by comparing the results of run #10 and #11 in
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Table 2. In the case of without and with addition of KOH, H2 yield increased from 8.6 mol/kg to 9.9 mol/kg and molar fraction of H2 in the gas products increased significantly from 36.32% to 43.93%, whereas CO2 concentration decreased. The use of alkali catalysts led to an increase in gas yield, that is due to the carbonecarbon scission as free-radical reactions occurring at higher temperatures were improved and the gasification reaction, hydrocarbon reforming reaction as well as the wateregas shift reaction were catalyzed and the dehydration and polymerization pathway were suppressed in the presence of KOH [27,28]. However, since alkaline solutions are highly corrosive at supercritical temperatures, noncorrosive transition metal catalysts have been developed widely to moderate the violent reaction condition and improve gasification efficiency [29].
3.5.
Challenges
In order to solve the contradiction of reaction temperature stability and solar irradiance fluctuations due to cloud passages and wind speed [30], the thermal storage and heat transfer technology of molten salts may be an effective way to reduce reaction temperature fluctuations due to solar irradiance changes [12]. From the overall heat balance, intensive heat exchange to recover waste heat between feedstock and products by highly efficient heat exchanger is essential for system thermal efficiency [31]. Furthermore, to achieve complete gasification of tar and char, use of catalysts in the process is a good solution. In addition, some technical breakthroughs, especially on practical difficulties of operation in harsh temperature and pressure conditions, the need of high-pressure reactors, material requirement and its associated high cost and potential corrosion problems, use of highpressure piping, might be bottlenecks for the technology to become widely commercially available [32]. Therefore, new reactor materials and reactor configurations are needed to explore and develop, in order to avoid reactor wall causticity of alkaline catalyst and improve operational stability, process performance and efficiencies, development of non-corrosive efficient catalysts, optimization of process operating conditions as well as solar receiver/reactor design and system integration including rapid heating methods, heat storage and efficient energy recovery management are also attractive options for our next job.
4.
Conclusion
In this paper, a novel solar thermochemical receiver/reactor for hydrogen production by biomass gasification in SCW with multi-dishes concentrator was designed and constructed, and a series of performance testing on the system were carried out. Model compound (glucose) and real biomass (corncob) were continuously gasified under SCW conditions to generate hydrogen-rich fuel gas in the apparatus. 0.5 M glucose and 5 wt% corncob þ 2 wt% CMC feedstock were continually and stably gasified and reactor plugging was not observed. Among the parameters varied, DNI and feed concentration are found to have the most significant effect on the SCWG of biomass. The maximum reaction temperature of the solar receiver/
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reactor enclosed by a quartz glass window reached 650 C, which was sufficiently high enough to realize biomass gasify completely in SCW. As to 2 wt% CMC þ 1.5 wt% corncob gasification, the average volume percentage of H2 is more than 40%, mean H2 yield reached 15 mol/kg, and GE more than 80% was reached. As DNI (reaction temperature), residence time increase and feed concentration decrease, gas yield, CE and GE increase. The encouraging results indicated that hydrogen production with SCWG of biomass using concentrated solar energy is a promising approach.
Acknowledgments This work is financially supported by the National Key Project for Basic Research Program of China (Contracted No. 2009CB220000, No. 2012CB215303) and National Natural Science Foundation of China (Grant No. 51121092).
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