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Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts
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 x x x ( 2 0 1 7 ) 1 e1 0
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Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts Farid Safari a,*, Nader Javani b, Zehra Yumurtaci b a b
Department of Energy Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran Faculty of Mechanical Engineering, Yildiz Technical University, Besiktas, Istanbul, Turkey
article info
abstract
Article history:
Almond shell is one of the most abundant agricultural wastes in Kurdistan province of
Received 10 February 2017
Iran. Conversion of almond shell into hydrogen-rich gas via supercritical water gasification
Received in revised form
(SCWG) was investigated in this study using a tubular batch micro-reactor system. Non-
29 April 2017
catalytic tests were carried out in different conditions to determine the optimum condi-
Accepted 14 May 2017
tion for H2 production. Maximum hydrogen yield of 7.85 mmol/g, was observed in the
Available online xxx
temperature of 460 C, residence time (RT) of 10 min and feed/water ratio (F/W) of 0.01. Catalytic experiments were performed using hydrochars as solid residues remained after
Keywords:
SCWG of Cladophora glomerata (C. glomerata) macroalgae and wheat straw. Hydrochars
Supercritical water
were characterized by ICP-OES, FESEM and BET methods. For catalytic experiments,
Gasification
hydrochars were added to the almond shell by the weight ratio of 0.4. Conversion of
Almond shell
almond shell and hydrogen production, were more influenced by the presence of inorganic
Hydrochar
compounds in the hydrochars rather than the surface area and pore volume. The
Catalyse
maximum hydrogen yields of 10.77 and 11.63 mmol/g, were observed for catalytic experiments in the presence of wheat straw and C. glomerata hydrochars, respectively. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Development in all aspects of the energy, including production, conservation and utilization, is one of the most crucial concerns in the recent era [1,2]. Hydrogen is known for its versatility to be a suitable alternative for conventional fossil fuel [3,4]. Hydrogen is one of the most environmentally benign, efficient and promising energy carriers [5]. Nowadays, more than 90% of the hydrogen worldwide is made via steam reforming of methane which comes from fossil fuels [6]. Developing the advanced processes for production of
hydrogen from renewables can be a great leap forward toward the sustainable energy development [7]. Biomass is regarded by many scientists as a green source of energy with almost zero carbon emission in its carbon life cycle [8]. Unlike solar energy, biomass has no time limitation. This source of energy contains significant amount of carbon and hydrogen, makes it favorable for production of fuels and chemicals [9]. Almond shell as an agricultural waste of the Kurdistan province of Iran with a production of about 200,000 tons annually, is a valuable source of chemicals and energy carriers [10]. Therefore, due to the growing demand for energy especially renewable energy, efficient conversion of these bio-renewable resources can be a
* Corresponding author. Fax: þ98 21 44861681. E-mail address: [email protected] (F. Safari). http://dx.doi.org/10.1016/j.ijhydene.2017.05.102 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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Nomenclature Abbreviations AAEMs Alkali and alkaline earth metals CG Cladophora glomerata CCE Carbon conversion efficiency CGE Carbon gasification efficiency CLE Carbon liquefaction efficiency CSE Carbon solidification efficiency FID Flame ionization detector RT Residence Time SCWG Supercritical water gasification TOC Total organic carbon WGS Water gas shift WS Wheat straw Symbols F P T W
Feed Pressure Temperature Water
step forward toward sustainable hydrogen production as a clean fuel. Different types of biomass conversion have been developed while thermochemical conversion and biochemical conversion have been recommended for hydrogen production from biomass [11,12]. SCWG is a thermochemical conversion which biomass is treated with the high-pressure water to degrade into gaseous, liquid and solid bio-products named syngas, bio-oil and hydrochar, respectively [13]. Water in this process exists in its supercritical condition (T > 374 C, P > 22.1 MPa). Some unique properties of water are emerged with the transition of water from sub-critical to supercritical condition [14]. As seen in Fig. 1, at 250 bar, as temperature increases from 350 to 450 C, a drastic decrease in the density, ionic products and dielectric constant happens which in turn, along with free radical mechanism, make water a unique solvent for organic compounds in its supercritical condition [15].
Fig. 1 e Variation of water properties with temperature at 250 bars.
In this condition, water can easily hydrolyze natural polymers of biomass such as cellulose, hemicellulose and lignin. Cellulose and hemicellulose are the linear polymers made of C5 and C6 sugars subunits and are the main sources for gas production [16,17]. Lignin is an amorphous, highly branched resin consists of phenolic compounds [18]. It covers the cellulose and hemicellulose and prevents the plant from damages. A Lot of research has been done on SCWG of agricultural wastes. Model et al. observed the unique behavior of water in supercritical condition for gas production from saw dust [19]. After that, many researchers investigated the effect of different parameters on SCWG, including temperature, feed concentration and residence time (RT). Several studies have been carried out on the thermochemical conversion of almond residue. Rapagna and Latif, investigated the steam gasification of almond shell in a fluidized bed reactor [20]. Effects of particle size and temperature on product yield were investigated. Madenuglo et al., investigated the effect of temperature on the SCWG of agricultural residues including hazelnut, walnut and almond shells. The gas portion in the product was increased as a result of an increase in temperature. Also, the addition of Trona and Dolomite as alkali and alkaline earth metal (AAEMs) catalysts, promoted the total gas and hydrogen production. The maximum hydrogen yields of 4.41 and 7.09 mmol/g were observed for non-catalytic and catalytic tests, respectively [21]. Safari et al., investigated the noncatalytic SCWG of some agricultural wastes including almond shell, walnut shell and wheat straw through a range of temperatures between 400 C and 440 C. Maximum hydrogen yield of 4.1 mmol/g was reported in 440 C [15]. Various types of catalysts have been employed for the better conversion of biomass into gaseous products [22,23]. Among them, Nickel as metallic catalyst and some alkali catalysts have been broadly used. Nevertheless, there are some challenges in the application of Ni-based catalysts such as rapid deactivation, coke formation and contamination within the interactions between biomass content and the metal which results in lower selectivity and stability. Recently, some researchers investigate the effect of carbonaceous materials such as biochar and hydrochar on the thermochemical conversions of biomass [24,25]. Yao et al., tested the biochar derived from pyrolysis of wheat straw, rice husk and cotton stalk as a catalyst and catalyst support. Enhancement in hydrogen production was observed due to the porosity of the biochar and its mineral content [26]. Norouzi et al., studied the effect of the pyrolysis derived algal biochar as a catalyst on the hydrogen production from pyrolysis of macroalgae. Total gas and hydrogen production were favored by the addition of biochar [27]. In the previous study of the authors, algal hydrochar as a solid residue of the SCWG used as a catalyst for its own process which resulted in the higher hydrogen yield [28]. This study, investigates the effects of the algal and agricultural hydrochars (from SCWG of Cladophora glomerata and wheat straw) on the conversion of Almond shell as an agricultural waste into hydrogen-rich gas. To the best of our knowledge, although there are some reports on the SCWG of almond shell and catalytic effects of biochar, there is no study available in the literature on the conversion of agricultural waste using algal char as a catalyst. Moreover the comparison between the performance of the agricultural and algal
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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hydrochars has not been investigated. The main novel investigations of the current work are listed below: SCWG of almond shell for hydrogen production using hydrochar as a catalyst Characterization of wheat straw hydrochar (WS hydrochar) and C. glomerata hydrochar (CG hydrochar) Investigation of the relation between the characteristics and catalytic performances of hydrochars in hydrogen production
performed through feeding the almond shell into a thermogravimetric analyzer (TGA/SDTA851 and METTLER-TOLEDO compact) under the nitrogen atmosphere at the temperatures in the range of 30e800 C with a heating rate of 10 C min1. Also, Fixed carbon was calculated via Eq. (2) as follows Fixe carbon% ¼ 100 Ash% volatile matter% moisture% (2)
Hydrochar preparation Wheat straw hydrochar (WS hydrochar) and C. glomerata hydrochar (CG hydrochar) for macroalgae and prepared via non-catalytic gasification in supercritical water media. Operating conditions were: reaction time of 20 min, Temperature of 460 C, feedstock loading of 0.06 g and water loading of 6 g.
Materials and methods Feedstock preparation and characterization The biomass particles used for the experiments were supplied from gardens and agriculture farms around Sanandaj, located in Kurdistan province, Iran. Considering the statistics of agricultural products of this province, wheat straw was selected as the most abundant feedstock in the province. Pretreatment for feedstock including washing, drying, grounding and sieving to achieve clean and dry biomass with particle sizes up to 150 mm were performed. A CHNS analyzer (Vario ELIII by Elementar, Germany) was used for the elemental analysis of the wheat straw. Also, the ash content was determined by the method of Sluiter et al. (2008) [29]. Briefly, after drying at 105 C for 5 h, the samples were placed in a porcelain crucible and heated in a muffle furnace at 575 ± 25 C for 24 ± 6 h to constant weight. Each sample was analyzed in triplicate. The weight percentage of oxygen was determined through the balance via Eq. (1). O% ¼ 100 C% H% N% S% Ash%
(1)
Moreover, the proximate analysis was carried out using thermogravimetric analysis (TGA). Characterization was
Hydrochar characterization Prepared hydrochars were characterized to determine their physiochemical properties. CHNS analyzer was employed for elemental analysis of hydrochars. The specific inorganic materials of hydrochars were measured using ICP-OES method. Also, a Field Emission Scanning Electro Microscope device (FESEMeMIRA3 LM, Tuscan), provided by Sharif university of technology, was used for scanning the surface of the char and observing its morphology. Coating of the samples is required in the field of electron microscopy to enable and improve the imaging of samples. Samples were coated by a thin gold layer for providing a conductive metal layer, and for inhibiting charging, reducing thermal damage and improving the secondary electron signal required for topographic characterization.
Reaction setup and experimental outline Schematic of the experimental setup is shown in Fig. 2. A batch micro reactor made of 316 stainless steel tube with
Fig. 2 e Schematic of reactor system: 1) molten salt bath, 2) tubular batch reactor, 3) electrical heater, 4) High pressure valves 5) Low pressure valve 6) Low pressure gauge 7) High pressure gauge 8) Mixer 9) k-type thermocouple 10) Water bath 11) Temperature controller 12) Flow meter 13) Argon gas bottle. Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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total volume of 26 mL was used in this study. Feedstock and deionized water were mixed to obtain feedstock to water ratio (F/W) of 0.01, 0.02 and 0.03. Argon as an inert gas stream was used for evacuation of the air inside the reactor in several minutes. Then, a syringe was used for injecting the mixture into the reactor. The reactor was immersed in a molten salt bath. The molten salt bath temperature was measured using a K-type thermocouple and was fixed at the desired temperature using a PID temperature controller. Also, a high pressure gauge was employed for measuring the pressure variation inside the reactor during the test. As seen in Fig. 3, at the temperature of 460 C and F/W of 0.01, the reactor's final pressure was close to 270 bars. After a determined RT, the reactor was pulled out of the molten salt bath and submerged into a water bath at the room temperature for cooling. For catalytic tests, a certain amount of each hydrochar was blended with almond shell and loaded into the reactor. A low pressure gauge was used for measuring the final reactor's pressure so that the total gas production could be calculated [30]. After SCWG, the water-insoluble part was retrieved by filtration. The aqueous products then transferred to a 10 mL centrifuge tube. After centrifugation, the top liquid phase was collected and stored at 4 C prior to analysis. The amount of carbon in aqueous phase of the products was measured using a TOC analyzer (Shimadzu, TOC-L). Total amount of solid, liquid and gaseous products were determined. Results for each test was the average of three consecutive runs in each condition.
was programmed by the following procedure: 40 C isothermal for 5 min, 2 C min1 to 75 C and isothermal in 75 C for 5 min. The injection port temperature was set at 120 C while for the detector, temperature was set at 380 C. When the column effluent mixes with the FID hydrogen supply and passes through the methanizer, CO and CO2 are converted to methane. The gas chromatograph was calibrated with standard gas mixture supplied by ROHAM Company in Tehran, Iran.
Product analysis method
CLE% ¼
ðCarbon in aqueous productsÞ 100 ðCarbon in almond shellÞ
(4)
CSE% ¼
ðCarbon in solid residueÞ 100 ðCarbon in almond shellÞ
(5)
CGE% ¼
ðCarbon in gaseous productsÞ 100 ðCarbon in Almond shellÞ
(6)
Process data interpretation Gas yield was measured to determine the mmoles of each gas produced by the conversion of a mass unit of the feedstock which is indicated by Eq. (3) as below: Gas yield ¼ ðmmoles of intended gaseous productÞ =ðgrams of used feedstockÞ
(3)
After hydrothermal conversion, the water-insoluble portion of the products was obtained by filtration. Total volumes of aqueous, gaseous and solid products were determined. Also, liquid and dried solid sample were weighted and analyzed to determine their carbon content. Moreover, carbon in the gas phase was calculated as the total moles of carbon in carbon-containing gaseous products in mass basis. Carbon liquefaction efficiency (CLE), Eq. (4), Carbon solidification efficiency (CSE), Eq. (5), and Carbon gasification efficiency (CGE), Eq. (6), were defined for calculation of Carbon Conversion Efficiency (CCE) via Eq. (7) as follows [31].
Gaseous product analysis A gas chromatograph (GC-TCD -Varian 3400 and TeyfgostarCompact) was used to identify the composition of gaseous products in a volume basis. The device was equipped with a methanizer, a Flame ionization detector (FID), and packed Proapak Q-S 80/100 column made of stainless steel with the length of 30 m and internal diameter of 0.53 mm 99.999% pure argon was used as a carrier gas. The temperature of the oven
Carbon balance ¼ CCE ¼ CGE þ CLE þ CSE
(7)
Also, hydrogen selectivity was calculated through Eq. (8) for performance analysis of hydrogen-rich gas production via SCWG of almond shell [32]. Hydrogen selectivity ¼ ðYield of H2 Þ=ðYield of other gassesÞ (8)
Results and discussion Feedstock characterization
Fig. 3 e Temporal variation of the pressure inside the reactor.
The results for ultimate, proximate and structural analysis of the prepared almond shell are mentioned in Table 1. Although wheat straw has considerable amount of cellulose which makes it favorable for gasification, the existence of the significant amount of lignin in the structure may barricade the full conversion of the hydrocarbons and hydrolyze of cellulose into monomers and lighter components. However, the low amount of ash content and high amount of volatile matter
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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Table 1 e Ultimate, proximate and structural analysis of almond shell sample.
Table 2 e The elemental analysis, inorganic content and the textured properties of prepared hydrochars.
Parameter
Parameter
Proximate analysis (wt.%) Moisture Ash Volatile Matter Fixed Carbon Ultimate analysis (wt.%) C H N S O Structural analysis (wt.%) Cellulose Hemicellulose Lignin Extractives
Almond shell 3.98 1.68 72.86 21.3 49.85 6.08 0.84 0.35 41.2 41.3 20.1 32.6 5.7
show a good potential of almond shell for gasification. As seen in Fig. 4, thermal decomposition of almond shell takes place in three steps. The first step is related to the drying and losing water in the sample which happens almost between 30 and 200 C. The second step is corresponds to devolatilization of the sample which the major loss in the weight happens. The final stage is related to the solid residue.
Hydrochars characterization Collected hydrochars from SCWG of C. glomerata macroalgae and wheat straw were characterized for better understanding of its effect on the process and determine their potential for promoting the gasification process toward hydrogen production. As shown in Table 2, Results from elemental analysis indicates that the weight percentage of carbon in the WS hydrochar is higher than that of CG hydrochar due to its higher ash content which mostly consists of minerals which are not oxidized during the thermal treatment. The amounts
CG hydrochar
Ultimate Analysis (wt.%) C H N S O Ash AAEMs (wt.%) Ca K Mg Na Physical Properties BET Surface area (m2/g) Total Pore volume (cm3/g)
WS hydrochar
44.2 2.5 3.4 3.1 10.6 36.2
66.9 2.05 2.4 0.65 18.2 9.8
12.2 29.4 8.2 17.5
5.42 4.11 0.57 1.8
57.6 0.0412
148.6 0.0962
of each AAEM (measured by ICP-OES analysis) which are important for understanding the catalytic effect of hydrochars, are presented in Table 2. As indicated, the amount of AAEMs in algal hydrochar is much higher than agricultural hydrochar. The place of growth can be the reason for such difference in content. Marine plants such as macroalgae absorb more minerals and salts than agricultural plants like wheat. That's why the amount of AAEMs in the C. glomerata is considerably more than that of wheat straw. On the other hand, as seen in Figs. 5 and 6, it can be inferred from FESEM analysis that the pores in the surface of the WS hydrochar are bigger in diameter (almost 5e20 mm) and there are many channels on the surface. Besides, the FESEM images of the CG hydrochar demonstrates the regular pore matrix on the surface with smaller pore sizes (almost 50e300 nm). Despite the regular matrix of the pores on the surface of the algal hydrochar, BET surface area and pore of the agricultural (wheat straw) hydrochar is much higher than that of algal hydrochar (CG hydrochar).
Non-catalytic H2 production
Fig. 4 e Thermogravimetric and differential thermogravimetric analysis of almond shell.
Non-catalytic test were performed at the conditions used for the previous study on SCWG of Cladophora glomerata (C. glomerata) macroalgae with this reactor by Safari et al. (2016). At these operating conditions mentioned in Table 3, feed to water ratio (F/W), temperature and residence time (RT) were considered as affecting parameters to identify the optimum condition in the term of hydrogen yield. As experimental tests at the higher temperatures are more energy intensive, the temperatures near to the critical condition (374 C) were considered. SCWG is a complex process which the natural polymers of biomass, including lignin, cellulose and hemicellulose cracks into the solid, liquid and gaseous products. Although there are many studies performed on the cracking of biomass model compounds, the exact pathway for conversion of these biopolymers into gaseous compounds remains unclear. However, the overall reactions for SCWG can be written as two steam reforming reactions (Eqs. (8) and (9)) as follows [33]:
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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Fig. 5 e FESEM images of CG hydrochar.
Fig. 6 e FESEM images of WS hydrochar.
Table 3 e Summary of experimental conditions and gaseous products for non-catalytic SCWG of almond shell. Test#
1 2 3 4 5 6 7 8 9 10 11 12
Temp ( C)
RT (min)
F/W
Pressure (MPa)
380 400 420 440 460 460 460 460 460 460 460 460
10 10 10 10 10 10 10 5 15 20 25 30
0.01 0.01 0.01 0.01 0.01 0.02 0.03 0.01 0.01 0.01 0.01 0.01
23.5 25 25.5 26.4 26.8 28.8 29.4 26.6 26.9 26.9 27 27
Gas Composition (mmol/g) CO
CO2
CH4
H2
C2H4
C2H6
Total
1.91 2.18 2.44 2.68 2,9 2.2 1.41 3.07 2.77 2.66 2.41 2.00
7.04 7.96 9.18 11.09 11.7 8.4 4.87 9.82 12.38 13.4 14.49 14.99
2.8 2.53 2.08 1.44 1.16 1.61 2.21 1.09 1.57 2.04 2.62 3.00
4.04 4.58 5.47 6.54 7.85 4.35 3.04 6.89 7.5 6.99 6.21 5.88
0.88 1.23 1.3 1,38 1.88 1.66 1.26 1.59 1.93 1.47 1.33 1.1
0.65 0.73 0.79 0.81 1.2 0.9 0.77 1.02 1.03 0.89 0.67 0.78
17.32 19.21 21.26 23.94 26.69 19.12 13.56 23.48 27.18 27.45 27.73 27.75
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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 x x x ( 2 0 1 7 ) 1 e1 0
x CHx Oy þ ð2 yÞH2 O/CO2 þ 2 y þ H2 2
DH[0
x CHx Oy þ ð1 yÞH2 O/CO þ 1 y þ H2 2
DH ¼ 310
7
(8) KJ mol
(9) Meanwhile, some important competing reactions occur in the gas phase as Eqs. (10) and (11): Wateregas shift: CO þ H2 O/CO2 þ H2
KJ DH ¼ 41 mol
(10)
Methanation: KJ CO þ 3H2 /CH4 þ H2 O DH ¼ 206 mol
(11)
It is obvious that for higher hydrogen production and for improving hydrogen yield, the methanation reaction should be suppresses while the WGS reaction should be accelerated. For studying the effect of different parameters on gasification and main gaseous components (CO, CO2, CH4, H2) was investigated. Effect of the variation in temperature from 380 to 460 C has been demonstrated in Fig. 7. Temperature has a direct effect on the both total gas and hydrogen yield. As seen in Fig. 7, by an increase in temperature, the total gas and hydrogen yields increase by the factors of 2.05 and 1.94, respectively. However, methane production declines. The decline in methane yield is in association with the exothermic reaction of methanation (Eq. (11)) inhibited by an increase in the temperature. The maximum hydrogen yield obtained in this study showed a 3.75 mmol/g increase compared to the work of Safari et al. [15]. This, mostly results from the higher temperature of the experiment in this study. Moreover, effect of feed concentration on main gas yields was investigated. According to Fig. 8 and Table 3, the more F/
Fig. 8 e Variation of main gaseous products with F/W ratio (T ¼ 460 C, RT ¼ 10 min).
W, the less H2 and CO2 yields and the more CH4 yield. Higher F/ W means less relative water portion which inhibits the steam reforming reactions (Eqs.(8) and (9)). Moreover, considering the equilibrium point of view, by an increase in F/W, the methanation reaction promotes to produce more H2O and CH4. On the other hand, low concentration of feedstock and less F/W allows the water molecules to easily colloid the significant ratio of biomass particles while in a high F/W, some particles can't meet the water molecules which results in less water-biomass collision frequency and less conversion of hydrocarbons [34]. Fig. 9, indicates the temporal variation of gas yields at 460 C. As seen, hydrogen yield increases at the first 10 min to 7.65 mmol/g and then declines to the 5.87 mmol/g. Fig. 9, clearly demonstrates that at the first 10 min, the steam reforming reactions (Eqs.(8) and (9)) are the dominant reactions of the process while methanation and wateregas shift reactions take place gradually. Hence, since methanation consumes 3 mol and WGS produces 1 mol of H2, overall Hydrogen yield declines as time goes on. Meanwhile. CO2 and CH4 increase. The total gas yield increased from 13.56 to 27.5 mmol/g by increasing the RT from 5 to 30 min.
Carbon balance
Fig. 7 e Variation of the main gaseous products with temperature (RT ¼ 10 min, F/W ¼ 0.01).
Carbon balance for non-catalytic tests has been demonstrated in Fig. 10. As seen, the amount of carbon in the gasification products including gaseous, aqueous and solid products are indicated in each column. Each measurement has been performed 3 times for more accuracy. Also, the red line corresponds to the variation of TOC within non-catalytic experiments. As shown, the TOC for aqueous products declines by an increase in the temperature from 380 to 460 C as a result of the conversion of aqueous products into the gas phase. Besides, as F/W increased within the tests 6 to 8, carbon conversion from solid to the liquid and from liquid to the gas phase declined and the CGE decreased from 51.61% to 29.56%. On the other hand, by an increase in the RT, TOC and CGE
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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Fig. 9 e Variation of the main gaseous products with RT (T ¼ 440 C, F/W ¼ 0.01).
increased slightly. The maximum CGE was equal to 55.92% and the carbon balance was closed within 94e99% for experiments. All the measurements were carried out in triplicate with the resulting accuracy of product yields about 3%. The error in carbon balance is associated to the solid residues sticking on the reactor's wall.
Catalytic H2 production. According to the non-catalytic experiments, the optimum condition for non-catalytic H2 production from wheat straw using this reactor was determined as: temperature of 460 C, F/W of 0.01 g and residence time of 10 min. The H2 yield of 7.85 mmol/g was obtained in this condition. Hence, the catalytic experiments performed in this condition in order to gain the maximum possible H2 yield. Moreover, the effects of hydrochar as bio-derived nonmetallic catalysts on the gaseous product's yield, CGE of the process and hydrogen selectivity were investigated. As seen in Fig. 11, higher amount of AAEMs existed in algal hydrochar, resulted in the higher cracking of the biopolymers of the almond shell and promoted the WGS reaction in the gas phase which in turn, elevated the hydrogen yield. This is clearly in
Fig. 10 e Carbon conversion into gaseous, aqueous and solid products for SCWG of almond shell.
Fig. 11 e Gas composition for the non-catalytic and catalytic SCWG of almond shell using agricultural and algal hydrochars as catalysts.
fair agreement with the previous studies on the effect of AAEMs on the gaseous products of SCWG [24,35]. Furthermore, as indicated in Fig. 11, CO2 yield increased by the addition of hydrochar catalysts as a result of the promotion of steam reforming and WGS reactions while the yield for light hydrocarbons including ethane and ethylene decreased due to the presence of AAEMs in catalytic tests and also high surface area of the hydrochars. Moreover, the total gas yields
Fig. 12 e CGE and HGE for non-catalytic and catalytic SCWG of almond shell.
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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for catalytic SCWG using WS hydrochar and CG hydrochar were equal to 29.2 and 31.1 mmol/g, respectively, which were greater than the 26.69 mmol/g total gas yield for non-catalytic experiment. Fig. 12, demonstrates the CGE and H2 selectivity for catalytic and non-catalytic tests. As seen, although the addition of hydrochars promoted the conversion by the addition of hydrochar catalysts CG increased slightly. According to Fig. 11, although the yield of CO2 and CH4 increased in catalytic tests, but the yield of C2 hydrocarbons and CO deceased. That's why CGE did not change significantly by the addition of hydrochar catalysts. Beside hydrogen selectivity increased from 0.41 in non-catalytic tests to 0.58 for the WS hydrochar added experiment and 0.6 for the CG hydrochar added experiment. Overall, the results for catalytic tests clearly implies the catalytic effect of agricultural and algal hydrochars on the promotion of total gas yield and hydrogen gas yields during SCWG of almond shell.
Conclusion Non-catalytic and catalytic conversion of almond shell as a lignocellulosic biomass into hydrogen-rich gas was in supercritical water media was investigated in this study using WS hydrochar and CG hydrochar as solid residues of gasification process. Characterization of solid hydrochars indicated the porous structure with high surface area for both agricultural and algal hydrochars and high AAEMs content for CG hydrochar as an algal hydrochar. The hydrogen yields were increased by the addition of WS hydrochar and CG hydrochar by the factors of 1.37 and 1.48, respectively. CG hydrochar showed more catalytic impact on hydrogen production than WS hydrochar which implied the significant effect of AAEMs on biomass decomposition and WGS reaction during SCWG.
Acknowledgement The authors would like to thank Mr. Omid Norouzi from University of Tehran, and the director of Nanoparticles and Coatings Laboratory (NCL) at Sharif University of Technology, for their kind help and support on this work.
references
[1] Najjar YSH. Hydrogen safety: the road toward green technology. Int J Hydrogen Energy 2013;38:10716e28. [2] Midilli A, Dincer I, Ay M. Green energy strategies for sustainable development. Energy Policy 2006;34:3623e33. [3] Das D, Veziroglu TN. Hydrogen production by biological processes: a survey of literature. Int J Hydrogen Energy 2001;26:13e28. [4] Midilli A, Dincer I. Hydrogen as a renewable and sustainable solution in reducing global fossil fuel consumption. Int J Hydrogen Energ 2008;33:4209e22. [5] Sheth PN, Babu BV. Production of hydrogen energy through biomass (waste wood) gasification. Int J Hydrogen Energy 2010;35:10803e10.
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[6] Stolten D, Emonts B. Hydrogen Science and Engineering: Materials, Processes, Systems and Technology, vol. 1. Wiley VCH; 2007. p. 419. [7] Dincer I, Acar C. Review and evaluation of hydrogen production methods for better sustainability. Int J Hydrogen Energy 2015;40:11094e111. [8] Safari F, Tavasoli A, Ataei A, Choi JK. Hydrogen and syngas production from gasification of lignocellulosic biomass in supercritical water media. Int J Recycl Org Waste Agric 2015;4:121e5. [9] Cohce MK, Dincr I, Rosen MA. Thermodynamic analysis of hydrogen production from biomass gasification. Int J Hydrogen Energy 2010;35:4970e80. [10] Faostat annual report 2014, available on (http://faostat3.fao. org/) [Accessed October 2016]. [11] Hepbasli A, Kalinci Y, Dincer I. Biomass-based hydrogen production: a review and analysis. Int J Hydrogen Energy 2009;34:8799e817. [12] Saxena RC, Seal D, Kumar S, Goyal HB. Thermo-chemical routes for hydrogen rich gas from biomass: a review. Renew Sust Energ Rev 2008;12:1909e27. [13] Reddy SN, Nanda S, Dalai AK, Kozinski JA. Supercritical water gasification of biomass for hydrogen production. Int J Hydrogen Energy 2014;39:6912e26. [14] Kruse A. Hydrothermal biomass gasification. J Supercritic Fluid 2009;47:391e9. [15] Safari F, Salimi M, Tavasoli A, Ataei A. Non-catalytic conversion of wheat straw, walnut shell and almond shell into hydrogen rich gas in supercritical water media. Chin J Chem Eng 2016;24:109e1103. [16] Kobayashi H, Fukuoka A. Synthesis and utilisation of sugar compounds derived from lignocellulosic biomass. Green Chem 2013;15:1740e63. [17] Safari F, Tavasoli A, Ataei A. Gasification of Iranian walnut shell as a bio-renewable resource for hydrogen rich gas production using supercritical water technology. Int J Ind Chem 2017;8:29e36. [18] Mohammad SHK, Dutta A, Chunbao Xu. Catalytic supercritical gasification of biocrude from hydrothermal liquefaction of cattle manure. Appl Catal B Environ 2016;189:119e32. [19] Modell M, Reid RC, Amin S (1978) Gasification process, U.S. Patent 4,113,446, September 1978. [20] Rapagana S, Latif A. Steam gasification of almond shells in a fluidised bed reactor: the influence of temperature and particle size on product yield and distribution. Biomass Bioenerg 1997;12:281e8. [21] Madenoglu TG, Yildirir E, Saglam M, Yuksel M, Ballice L. Improvement in hydrogen production from hard-shell nut residues by catalytic hydrothermal gasification. J Supercrit Fluid 2014;67:22e8. [22] Cao C, Zhang Y, Cao W, Jin H, Guo L, Huo Z. Transition metal oxides as catalysts for hydrogen production from supercritical water gasification of glucose. Catal Lett 2016;147:828e38. [23] Cao C, Xu L, He Y, Guo L, Jin H, Huo Z. High-efficiency gasification of wheat straw black liquor in supercritical water at high temperatures for hydrogen production. Energy Fuels 2017;31:3970e8. [24] Kamboo HS, Dutta A. A comparative review of biochar and hydrochar in terms of production, physico-chemical properties and applications. Renew Sustain Energy Rev 2015;45:359e78. [25] Klinghoffer NB, Castaldi MJ, Nzihou A. Influence of char composition and inorganics on catalytic activity of char from biomass gasification. Fuel 2015;157:37e47. [26] Norouzi O, Jafarian S, Safari F, Tavasoli A, Nejati B. Promotion of hydrogen-rich gas and phenolic-rich bio-oil production from. Bioresour Technol 2016;219:643e51.
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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[27] Yao D, Hu Q, Wang D, Yang H, Wu C, Wang X, et al. Hydrogen production from biomass gasification using biochar as a catalyst/support. Bioresour Technol 2016;216:159e64. [28] Safari F, Norouzi O, Tavasoli A. Hydrothermal gasification of Cladophora glomerata as a green macroalgae for hydrogenrich gas production over its hydrochar as a catalyst. Bioresour Technol 2016;222:232e41. [29] Sluiter A, Hames B, Ruiz R, Scarlata C, Sluiter J, Templeton D. Determination of ash in biomass. National Renewable EnergyLaboratory; 2008. Technical Report NREL/TP510e42622. [30] Azadi P, Khodadadi AA, Mortazavi Y, Farnood R. Hydrothermal gasification of glucose using Raney nickel and homogeneous organometallic catalysts. Fuel Proc Technol 2009;90:145e51. [31] Norouzi O, Safari F, Jafarian S, Tavasoli A, Karimi A. Hydrothermal gasification performance of Enteromorpha intestinalis as an algal biomass for hydrogen-rich gas
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production using Ru promoted FeeNi/g-Al2O3 nanocatalysts. Energy Convers Manag 2016;141:63e71. Azadi P, Afif E, Azadi F, Farnood R. Screening of nickel catalysts for selective hydrogen production using supercritical water gasification of glucose. Green Chem 2012;14:1766e77. Salimi M, Safari F, Tavasoli A, Shakeri A. Hydrothermal gasification of different agricultural wastes in supercritical water media for hydrogen production: a comparative study. Int J of Ind Chem 2016;7:277e85. Cao W, Cao C, Guo L, Jin H, Dargusch M, Bernhardt D, et al. Hydrogen production from supercritical water gasification of chicken manure. Int J Hydrogen Energy 2016;41:22722e31. Muangrat R, Onwudili JA, Williams PT. Alkali-promoted hydrothermal gasification of biomass food processing waste: a parametric study. Int J Hydrogen Energy 2010;35:7405e515.
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Available online at www.sciencedirect.com
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Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts Farid Safari a,*, Nader Javani b, Zehra Yumurtaci b a b
Department of Energy Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran Faculty of Mechanical Engineering, Yildiz Technical University, Besiktas, Istanbul, Turkey
article info
abstract
Article history:
Almond shell is one of the most abundant agricultural wastes in Kurdistan province of
Received 10 February 2017
Iran. Conversion of almond shell into hydrogen-rich gas via supercritical water gasification
Received in revised form
(SCWG) was investigated in this study using a tubular batch micro-reactor system. Non-
29 April 2017
catalytic tests were carried out in different conditions to determine the optimum condi-
Accepted 14 May 2017
tion for H2 production. Maximum hydrogen yield of 7.85 mmol/g, was observed in the
Available online xxx
temperature of 460 C, residence time (RT) of 10 min and feed/water ratio (F/W) of 0.01. Catalytic experiments were performed using hydrochars as solid residues remained after
Keywords:
SCWG of Cladophora glomerata (C. glomerata) macroalgae and wheat straw. Hydrochars
Supercritical water
were characterized by ICP-OES, FESEM and BET methods. For catalytic experiments,
Gasification
hydrochars were added to the almond shell by the weight ratio of 0.4. Conversion of
Almond shell
almond shell and hydrogen production, were more influenced by the presence of inorganic
Hydrochar
compounds in the hydrochars rather than the surface area and pore volume. The
Catalyse
maximum hydrogen yields of 10.77 and 11.63 mmol/g, were observed for catalytic experiments in the presence of wheat straw and C. glomerata hydrochars, respectively. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Development in all aspects of the energy, including production, conservation and utilization, is one of the most crucial concerns in the recent era [1,2]. Hydrogen is known for its versatility to be a suitable alternative for conventional fossil fuel [3,4]. Hydrogen is one of the most environmentally benign, efficient and promising energy carriers [5]. Nowadays, more than 90% of the hydrogen worldwide is made via steam reforming of methane which comes from fossil fuels [6]. Developing the advanced processes for production of
hydrogen from renewables can be a great leap forward toward the sustainable energy development [7]. Biomass is regarded by many scientists as a green source of energy with almost zero carbon emission in its carbon life cycle [8]. Unlike solar energy, biomass has no time limitation. This source of energy contains significant amount of carbon and hydrogen, makes it favorable for production of fuels and chemicals [9]. Almond shell as an agricultural waste of the Kurdistan province of Iran with a production of about 200,000 tons annually, is a valuable source of chemicals and energy carriers [10]. Therefore, due to the growing demand for energy especially renewable energy, efficient conversion of these bio-renewable resources can be a
* Corresponding author. Fax: þ98 21 44861681. E-mail address: [email protected] (F. Safari). http://dx.doi.org/10.1016/j.ijhydene.2017.05.102 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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Nomenclature Abbreviations AAEMs Alkali and alkaline earth metals CG Cladophora glomerata CCE Carbon conversion efficiency CGE Carbon gasification efficiency CLE Carbon liquefaction efficiency CSE Carbon solidification efficiency FID Flame ionization detector RT Residence Time SCWG Supercritical water gasification TOC Total organic carbon WGS Water gas shift WS Wheat straw Symbols F P T W
Feed Pressure Temperature Water
step forward toward sustainable hydrogen production as a clean fuel. Different types of biomass conversion have been developed while thermochemical conversion and biochemical conversion have been recommended for hydrogen production from biomass [11,12]. SCWG is a thermochemical conversion which biomass is treated with the high-pressure water to degrade into gaseous, liquid and solid bio-products named syngas, bio-oil and hydrochar, respectively [13]. Water in this process exists in its supercritical condition (T > 374 C, P > 22.1 MPa). Some unique properties of water are emerged with the transition of water from sub-critical to supercritical condition [14]. As seen in Fig. 1, at 250 bar, as temperature increases from 350 to 450 C, a drastic decrease in the density, ionic products and dielectric constant happens which in turn, along with free radical mechanism, make water a unique solvent for organic compounds in its supercritical condition [15].
Fig. 1 e Variation of water properties with temperature at 250 bars.
In this condition, water can easily hydrolyze natural polymers of biomass such as cellulose, hemicellulose and lignin. Cellulose and hemicellulose are the linear polymers made of C5 and C6 sugars subunits and are the main sources for gas production [16,17]. Lignin is an amorphous, highly branched resin consists of phenolic compounds [18]. It covers the cellulose and hemicellulose and prevents the plant from damages. A Lot of research has been done on SCWG of agricultural wastes. Model et al. observed the unique behavior of water in supercritical condition for gas production from saw dust [19]. After that, many researchers investigated the effect of different parameters on SCWG, including temperature, feed concentration and residence time (RT). Several studies have been carried out on the thermochemical conversion of almond residue. Rapagna and Latif, investigated the steam gasification of almond shell in a fluidized bed reactor [20]. Effects of particle size and temperature on product yield were investigated. Madenuglo et al., investigated the effect of temperature on the SCWG of agricultural residues including hazelnut, walnut and almond shells. The gas portion in the product was increased as a result of an increase in temperature. Also, the addition of Trona and Dolomite as alkali and alkaline earth metal (AAEMs) catalysts, promoted the total gas and hydrogen production. The maximum hydrogen yields of 4.41 and 7.09 mmol/g were observed for non-catalytic and catalytic tests, respectively [21]. Safari et al., investigated the noncatalytic SCWG of some agricultural wastes including almond shell, walnut shell and wheat straw through a range of temperatures between 400 C and 440 C. Maximum hydrogen yield of 4.1 mmol/g was reported in 440 C [15]. Various types of catalysts have been employed for the better conversion of biomass into gaseous products [22,23]. Among them, Nickel as metallic catalyst and some alkali catalysts have been broadly used. Nevertheless, there are some challenges in the application of Ni-based catalysts such as rapid deactivation, coke formation and contamination within the interactions between biomass content and the metal which results in lower selectivity and stability. Recently, some researchers investigate the effect of carbonaceous materials such as biochar and hydrochar on the thermochemical conversions of biomass [24,25]. Yao et al., tested the biochar derived from pyrolysis of wheat straw, rice husk and cotton stalk as a catalyst and catalyst support. Enhancement in hydrogen production was observed due to the porosity of the biochar and its mineral content [26]. Norouzi et al., studied the effect of the pyrolysis derived algal biochar as a catalyst on the hydrogen production from pyrolysis of macroalgae. Total gas and hydrogen production were favored by the addition of biochar [27]. In the previous study of the authors, algal hydrochar as a solid residue of the SCWG used as a catalyst for its own process which resulted in the higher hydrogen yield [28]. This study, investigates the effects of the algal and agricultural hydrochars (from SCWG of Cladophora glomerata and wheat straw) on the conversion of Almond shell as an agricultural waste into hydrogen-rich gas. To the best of our knowledge, although there are some reports on the SCWG of almond shell and catalytic effects of biochar, there is no study available in the literature on the conversion of agricultural waste using algal char as a catalyst. Moreover the comparison between the performance of the agricultural and algal
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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hydrochars has not been investigated. The main novel investigations of the current work are listed below: SCWG of almond shell for hydrogen production using hydrochar as a catalyst Characterization of wheat straw hydrochar (WS hydrochar) and C. glomerata hydrochar (CG hydrochar) Investigation of the relation between the characteristics and catalytic performances of hydrochars in hydrogen production
performed through feeding the almond shell into a thermogravimetric analyzer (TGA/SDTA851 and METTLER-TOLEDO compact) under the nitrogen atmosphere at the temperatures in the range of 30e800 C with a heating rate of 10 C min1. Also, Fixed carbon was calculated via Eq. (2) as follows Fixe carbon% ¼ 100 Ash% volatile matter% moisture% (2)
Hydrochar preparation Wheat straw hydrochar (WS hydrochar) and C. glomerata hydrochar (CG hydrochar) for macroalgae and prepared via non-catalytic gasification in supercritical water media. Operating conditions were: reaction time of 20 min, Temperature of 460 C, feedstock loading of 0.06 g and water loading of 6 g.
Materials and methods Feedstock preparation and characterization The biomass particles used for the experiments were supplied from gardens and agriculture farms around Sanandaj, located in Kurdistan province, Iran. Considering the statistics of agricultural products of this province, wheat straw was selected as the most abundant feedstock in the province. Pretreatment for feedstock including washing, drying, grounding and sieving to achieve clean and dry biomass with particle sizes up to 150 mm were performed. A CHNS analyzer (Vario ELIII by Elementar, Germany) was used for the elemental analysis of the wheat straw. Also, the ash content was determined by the method of Sluiter et al. (2008) [29]. Briefly, after drying at 105 C for 5 h, the samples were placed in a porcelain crucible and heated in a muffle furnace at 575 ± 25 C for 24 ± 6 h to constant weight. Each sample was analyzed in triplicate. The weight percentage of oxygen was determined through the balance via Eq. (1). O% ¼ 100 C% H% N% S% Ash%
(1)
Moreover, the proximate analysis was carried out using thermogravimetric analysis (TGA). Characterization was
Hydrochar characterization Prepared hydrochars were characterized to determine their physiochemical properties. CHNS analyzer was employed for elemental analysis of hydrochars. The specific inorganic materials of hydrochars were measured using ICP-OES method. Also, a Field Emission Scanning Electro Microscope device (FESEMeMIRA3 LM, Tuscan), provided by Sharif university of technology, was used for scanning the surface of the char and observing its morphology. Coating of the samples is required in the field of electron microscopy to enable and improve the imaging of samples. Samples were coated by a thin gold layer for providing a conductive metal layer, and for inhibiting charging, reducing thermal damage and improving the secondary electron signal required for topographic characterization.
Reaction setup and experimental outline Schematic of the experimental setup is shown in Fig. 2. A batch micro reactor made of 316 stainless steel tube with
Fig. 2 e Schematic of reactor system: 1) molten salt bath, 2) tubular batch reactor, 3) electrical heater, 4) High pressure valves 5) Low pressure valve 6) Low pressure gauge 7) High pressure gauge 8) Mixer 9) k-type thermocouple 10) Water bath 11) Temperature controller 12) Flow meter 13) Argon gas bottle. Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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total volume of 26 mL was used in this study. Feedstock and deionized water were mixed to obtain feedstock to water ratio (F/W) of 0.01, 0.02 and 0.03. Argon as an inert gas stream was used for evacuation of the air inside the reactor in several minutes. Then, a syringe was used for injecting the mixture into the reactor. The reactor was immersed in a molten salt bath. The molten salt bath temperature was measured using a K-type thermocouple and was fixed at the desired temperature using a PID temperature controller. Also, a high pressure gauge was employed for measuring the pressure variation inside the reactor during the test. As seen in Fig. 3, at the temperature of 460 C and F/W of 0.01, the reactor's final pressure was close to 270 bars. After a determined RT, the reactor was pulled out of the molten salt bath and submerged into a water bath at the room temperature for cooling. For catalytic tests, a certain amount of each hydrochar was blended with almond shell and loaded into the reactor. A low pressure gauge was used for measuring the final reactor's pressure so that the total gas production could be calculated [30]. After SCWG, the water-insoluble part was retrieved by filtration. The aqueous products then transferred to a 10 mL centrifuge tube. After centrifugation, the top liquid phase was collected and stored at 4 C prior to analysis. The amount of carbon in aqueous phase of the products was measured using a TOC analyzer (Shimadzu, TOC-L). Total amount of solid, liquid and gaseous products were determined. Results for each test was the average of three consecutive runs in each condition.
was programmed by the following procedure: 40 C isothermal for 5 min, 2 C min1 to 75 C and isothermal in 75 C for 5 min. The injection port temperature was set at 120 C while for the detector, temperature was set at 380 C. When the column effluent mixes with the FID hydrogen supply and passes through the methanizer, CO and CO2 are converted to methane. The gas chromatograph was calibrated with standard gas mixture supplied by ROHAM Company in Tehran, Iran.
Product analysis method
CLE% ¼
ðCarbon in aqueous productsÞ 100 ðCarbon in almond shellÞ
(4)
CSE% ¼
ðCarbon in solid residueÞ 100 ðCarbon in almond shellÞ
(5)
CGE% ¼
ðCarbon in gaseous productsÞ 100 ðCarbon in Almond shellÞ
(6)
Process data interpretation Gas yield was measured to determine the mmoles of each gas produced by the conversion of a mass unit of the feedstock which is indicated by Eq. (3) as below: Gas yield ¼ ðmmoles of intended gaseous productÞ =ðgrams of used feedstockÞ
(3)
After hydrothermal conversion, the water-insoluble portion of the products was obtained by filtration. Total volumes of aqueous, gaseous and solid products were determined. Also, liquid and dried solid sample were weighted and analyzed to determine their carbon content. Moreover, carbon in the gas phase was calculated as the total moles of carbon in carbon-containing gaseous products in mass basis. Carbon liquefaction efficiency (CLE), Eq. (4), Carbon solidification efficiency (CSE), Eq. (5), and Carbon gasification efficiency (CGE), Eq. (6), were defined for calculation of Carbon Conversion Efficiency (CCE) via Eq. (7) as follows [31].
Gaseous product analysis A gas chromatograph (GC-TCD -Varian 3400 and TeyfgostarCompact) was used to identify the composition of gaseous products in a volume basis. The device was equipped with a methanizer, a Flame ionization detector (FID), and packed Proapak Q-S 80/100 column made of stainless steel with the length of 30 m and internal diameter of 0.53 mm 99.999% pure argon was used as a carrier gas. The temperature of the oven
Carbon balance ¼ CCE ¼ CGE þ CLE þ CSE
(7)
Also, hydrogen selectivity was calculated through Eq. (8) for performance analysis of hydrogen-rich gas production via SCWG of almond shell [32]. Hydrogen selectivity ¼ ðYield of H2 Þ=ðYield of other gassesÞ (8)
Results and discussion Feedstock characterization
Fig. 3 e Temporal variation of the pressure inside the reactor.
The results for ultimate, proximate and structural analysis of the prepared almond shell are mentioned in Table 1. Although wheat straw has considerable amount of cellulose which makes it favorable for gasification, the existence of the significant amount of lignin in the structure may barricade the full conversion of the hydrocarbons and hydrolyze of cellulose into monomers and lighter components. However, the low amount of ash content and high amount of volatile matter
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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Table 1 e Ultimate, proximate and structural analysis of almond shell sample.
Table 2 e The elemental analysis, inorganic content and the textured properties of prepared hydrochars.
Parameter
Parameter
Proximate analysis (wt.%) Moisture Ash Volatile Matter Fixed Carbon Ultimate analysis (wt.%) C H N S O Structural analysis (wt.%) Cellulose Hemicellulose Lignin Extractives
Almond shell 3.98 1.68 72.86 21.3 49.85 6.08 0.84 0.35 41.2 41.3 20.1 32.6 5.7
show a good potential of almond shell for gasification. As seen in Fig. 4, thermal decomposition of almond shell takes place in three steps. The first step is related to the drying and losing water in the sample which happens almost between 30 and 200 C. The second step is corresponds to devolatilization of the sample which the major loss in the weight happens. The final stage is related to the solid residue.
Hydrochars characterization Collected hydrochars from SCWG of C. glomerata macroalgae and wheat straw were characterized for better understanding of its effect on the process and determine their potential for promoting the gasification process toward hydrogen production. As shown in Table 2, Results from elemental analysis indicates that the weight percentage of carbon in the WS hydrochar is higher than that of CG hydrochar due to its higher ash content which mostly consists of minerals which are not oxidized during the thermal treatment. The amounts
CG hydrochar
Ultimate Analysis (wt.%) C H N S O Ash AAEMs (wt.%) Ca K Mg Na Physical Properties BET Surface area (m2/g) Total Pore volume (cm3/g)
WS hydrochar
44.2 2.5 3.4 3.1 10.6 36.2
66.9 2.05 2.4 0.65 18.2 9.8
12.2 29.4 8.2 17.5
5.42 4.11 0.57 1.8
57.6 0.0412
148.6 0.0962
of each AAEM (measured by ICP-OES analysis) which are important for understanding the catalytic effect of hydrochars, are presented in Table 2. As indicated, the amount of AAEMs in algal hydrochar is much higher than agricultural hydrochar. The place of growth can be the reason for such difference in content. Marine plants such as macroalgae absorb more minerals and salts than agricultural plants like wheat. That's why the amount of AAEMs in the C. glomerata is considerably more than that of wheat straw. On the other hand, as seen in Figs. 5 and 6, it can be inferred from FESEM analysis that the pores in the surface of the WS hydrochar are bigger in diameter (almost 5e20 mm) and there are many channels on the surface. Besides, the FESEM images of the CG hydrochar demonstrates the regular pore matrix on the surface with smaller pore sizes (almost 50e300 nm). Despite the regular matrix of the pores on the surface of the algal hydrochar, BET surface area and pore of the agricultural (wheat straw) hydrochar is much higher than that of algal hydrochar (CG hydrochar).
Non-catalytic H2 production
Fig. 4 e Thermogravimetric and differential thermogravimetric analysis of almond shell.
Non-catalytic test were performed at the conditions used for the previous study on SCWG of Cladophora glomerata (C. glomerata) macroalgae with this reactor by Safari et al. (2016). At these operating conditions mentioned in Table 3, feed to water ratio (F/W), temperature and residence time (RT) were considered as affecting parameters to identify the optimum condition in the term of hydrogen yield. As experimental tests at the higher temperatures are more energy intensive, the temperatures near to the critical condition (374 C) were considered. SCWG is a complex process which the natural polymers of biomass, including lignin, cellulose and hemicellulose cracks into the solid, liquid and gaseous products. Although there are many studies performed on the cracking of biomass model compounds, the exact pathway for conversion of these biopolymers into gaseous compounds remains unclear. However, the overall reactions for SCWG can be written as two steam reforming reactions (Eqs. (8) and (9)) as follows [33]:
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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Fig. 5 e FESEM images of CG hydrochar.
Fig. 6 e FESEM images of WS hydrochar.
Table 3 e Summary of experimental conditions and gaseous products for non-catalytic SCWG of almond shell. Test#
1 2 3 4 5 6 7 8 9 10 11 12
Temp ( C)
RT (min)
F/W
Pressure (MPa)
380 400 420 440 460 460 460 460 460 460 460 460
10 10 10 10 10 10 10 5 15 20 25 30
0.01 0.01 0.01 0.01 0.01 0.02 0.03 0.01 0.01 0.01 0.01 0.01
23.5 25 25.5 26.4 26.8 28.8 29.4 26.6 26.9 26.9 27 27
Gas Composition (mmol/g) CO
CO2
CH4
H2
C2H4
C2H6
Total
1.91 2.18 2.44 2.68 2,9 2.2 1.41 3.07 2.77 2.66 2.41 2.00
7.04 7.96 9.18 11.09 11.7 8.4 4.87 9.82 12.38 13.4 14.49 14.99
2.8 2.53 2.08 1.44 1.16 1.61 2.21 1.09 1.57 2.04 2.62 3.00
4.04 4.58 5.47 6.54 7.85 4.35 3.04 6.89 7.5 6.99 6.21 5.88
0.88 1.23 1.3 1,38 1.88 1.66 1.26 1.59 1.93 1.47 1.33 1.1
0.65 0.73 0.79 0.81 1.2 0.9 0.77 1.02 1.03 0.89 0.67 0.78
17.32 19.21 21.26 23.94 26.69 19.12 13.56 23.48 27.18 27.45 27.73 27.75
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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 x x x ( 2 0 1 7 ) 1 e1 0
x CHx Oy þ ð2 yÞH2 O/CO2 þ 2 y þ H2 2
DH[0
x CHx Oy þ ð1 yÞH2 O/CO þ 1 y þ H2 2
DH ¼ 310
7
(8) KJ mol
(9) Meanwhile, some important competing reactions occur in the gas phase as Eqs. (10) and (11): Wateregas shift: CO þ H2 O/CO2 þ H2
KJ DH ¼ 41 mol
(10)
Methanation: KJ CO þ 3H2 /CH4 þ H2 O DH ¼ 206 mol
(11)
It is obvious that for higher hydrogen production and for improving hydrogen yield, the methanation reaction should be suppresses while the WGS reaction should be accelerated. For studying the effect of different parameters on gasification and main gaseous components (CO, CO2, CH4, H2) was investigated. Effect of the variation in temperature from 380 to 460 C has been demonstrated in Fig. 7. Temperature has a direct effect on the both total gas and hydrogen yield. As seen in Fig. 7, by an increase in temperature, the total gas and hydrogen yields increase by the factors of 2.05 and 1.94, respectively. However, methane production declines. The decline in methane yield is in association with the exothermic reaction of methanation (Eq. (11)) inhibited by an increase in the temperature. The maximum hydrogen yield obtained in this study showed a 3.75 mmol/g increase compared to the work of Safari et al. [15]. This, mostly results from the higher temperature of the experiment in this study. Moreover, effect of feed concentration on main gas yields was investigated. According to Fig. 8 and Table 3, the more F/
Fig. 8 e Variation of main gaseous products with F/W ratio (T ¼ 460 C, RT ¼ 10 min).
W, the less H2 and CO2 yields and the more CH4 yield. Higher F/ W means less relative water portion which inhibits the steam reforming reactions (Eqs.(8) and (9)). Moreover, considering the equilibrium point of view, by an increase in F/W, the methanation reaction promotes to produce more H2O and CH4. On the other hand, low concentration of feedstock and less F/W allows the water molecules to easily colloid the significant ratio of biomass particles while in a high F/W, some particles can't meet the water molecules which results in less water-biomass collision frequency and less conversion of hydrocarbons [34]. Fig. 9, indicates the temporal variation of gas yields at 460 C. As seen, hydrogen yield increases at the first 10 min to 7.65 mmol/g and then declines to the 5.87 mmol/g. Fig. 9, clearly demonstrates that at the first 10 min, the steam reforming reactions (Eqs.(8) and (9)) are the dominant reactions of the process while methanation and wateregas shift reactions take place gradually. Hence, since methanation consumes 3 mol and WGS produces 1 mol of H2, overall Hydrogen yield declines as time goes on. Meanwhile. CO2 and CH4 increase. The total gas yield increased from 13.56 to 27.5 mmol/g by increasing the RT from 5 to 30 min.
Carbon balance
Fig. 7 e Variation of the main gaseous products with temperature (RT ¼ 10 min, F/W ¼ 0.01).
Carbon balance for non-catalytic tests has been demonstrated in Fig. 10. As seen, the amount of carbon in the gasification products including gaseous, aqueous and solid products are indicated in each column. Each measurement has been performed 3 times for more accuracy. Also, the red line corresponds to the variation of TOC within non-catalytic experiments. As shown, the TOC for aqueous products declines by an increase in the temperature from 380 to 460 C as a result of the conversion of aqueous products into the gas phase. Besides, as F/W increased within the tests 6 to 8, carbon conversion from solid to the liquid and from liquid to the gas phase declined and the CGE decreased from 51.61% to 29.56%. On the other hand, by an increase in the RT, TOC and CGE
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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Fig. 9 e Variation of the main gaseous products with RT (T ¼ 440 C, F/W ¼ 0.01).
increased slightly. The maximum CGE was equal to 55.92% and the carbon balance was closed within 94e99% for experiments. All the measurements were carried out in triplicate with the resulting accuracy of product yields about 3%. The error in carbon balance is associated to the solid residues sticking on the reactor's wall.
Catalytic H2 production. According to the non-catalytic experiments, the optimum condition for non-catalytic H2 production from wheat straw using this reactor was determined as: temperature of 460 C, F/W of 0.01 g and residence time of 10 min. The H2 yield of 7.85 mmol/g was obtained in this condition. Hence, the catalytic experiments performed in this condition in order to gain the maximum possible H2 yield. Moreover, the effects of hydrochar as bio-derived nonmetallic catalysts on the gaseous product's yield, CGE of the process and hydrogen selectivity were investigated. As seen in Fig. 11, higher amount of AAEMs existed in algal hydrochar, resulted in the higher cracking of the biopolymers of the almond shell and promoted the WGS reaction in the gas phase which in turn, elevated the hydrogen yield. This is clearly in
Fig. 10 e Carbon conversion into gaseous, aqueous and solid products for SCWG of almond shell.
Fig. 11 e Gas composition for the non-catalytic and catalytic SCWG of almond shell using agricultural and algal hydrochars as catalysts.
fair agreement with the previous studies on the effect of AAEMs on the gaseous products of SCWG [24,35]. Furthermore, as indicated in Fig. 11, CO2 yield increased by the addition of hydrochar catalysts as a result of the promotion of steam reforming and WGS reactions while the yield for light hydrocarbons including ethane and ethylene decreased due to the presence of AAEMs in catalytic tests and also high surface area of the hydrochars. Moreover, the total gas yields
Fig. 12 e CGE and HGE for non-catalytic and catalytic SCWG of almond shell.
Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102
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 x x x ( 2 0 1 7 ) 1 e1 0
for catalytic SCWG using WS hydrochar and CG hydrochar were equal to 29.2 and 31.1 mmol/g, respectively, which were greater than the 26.69 mmol/g total gas yield for non-catalytic experiment. Fig. 12, demonstrates the CGE and H2 selectivity for catalytic and non-catalytic tests. As seen, although the addition of hydrochars promoted the conversion by the addition of hydrochar catalysts CG increased slightly. According to Fig. 11, although the yield of CO2 and CH4 increased in catalytic tests, but the yield of C2 hydrocarbons and CO deceased. That's why CGE did not change significantly by the addition of hydrochar catalysts. Beside hydrogen selectivity increased from 0.41 in non-catalytic tests to 0.58 for the WS hydrochar added experiment and 0.6 for the CG hydrochar added experiment. Overall, the results for catalytic tests clearly implies the catalytic effect of agricultural and algal hydrochars on the promotion of total gas yield and hydrogen gas yields during SCWG of almond shell.
Conclusion Non-catalytic and catalytic conversion of almond shell as a lignocellulosic biomass into hydrogen-rich gas was in supercritical water media was investigated in this study using WS hydrochar and CG hydrochar as solid residues of gasification process. Characterization of solid hydrochars indicated the porous structure with high surface area for both agricultural and algal hydrochars and high AAEMs content for CG hydrochar as an algal hydrochar. The hydrogen yields were increased by the addition of WS hydrochar and CG hydrochar by the factors of 1.37 and 1.48, respectively. CG hydrochar showed more catalytic impact on hydrogen production than WS hydrochar which implied the significant effect of AAEMs on biomass decomposition and WGS reaction during SCWG.
Acknowledgement The authors would like to thank Mr. Omid Norouzi from University of Tehran, and the director of Nanoparticles and Coatings Laboratory (NCL) at Sharif University of Technology, for their kind help and support on this work.
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Please cite this article in press as: Safari F, et al., Hydrogen production via supercritical water gasification of almond shell over algal and agricultural hydrochars as catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.102