- Email: [email protected]
Solid circulation characteristics of the three-reactor chemical-looping process for hydrogen production
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Solid circulation characteristics of the three-reactor chemical-looping process for hydrogen production Doyeon Lee a, Myung Won Seo b,*, Thanh D.B. Nguyen c, Won Chul Cho c, Sang Done Kim a,** a
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea b Clean Fuel Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea c Hydrogen Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea
article info
abstract
Article history:
The three-reactor chemical-looping (TRCL) system for high purity hydrogen production
Received 4 April 2014
and intrinsic CO2 separation is an innovative concept using a circulating oxygen carrier.
Accepted 13 July 2014
The process employs iron oxide as an oxygen carrier, via which a redox reaction takes
Available online 6 August 2014
place alternately within three reactors, i.e. a fuel reactor (FR), where the natural gas is
Keywords:
and an air reactor (AR), where the oxygen carrier returned to its original form by aeration.
Three-reactor chemical-looping
As it consists of three reactors (AR, FR, and SR) and a riser, the TRCL system has compli-
process (TRCL)
cated hydrodynamic characteristics.
combusted to CO2 and H2O, a steam reactor (SR), where the steam is reduced to hydrogen,
Hydrogen production
In this study, a cold mode TRCL system with non-mechanical valve was designed and
CO2 separation
constructed to investigate the solid circulation characteristics. A series of hydrodynamic
Hydrodynamics
tests on the system was performed in which zirconia (dr ¼ 181 mm, rs ¼ 3850 kg/m3) was
Computational fluid dynamics (CFD)
used as a bed material. The solid flow rate increased up to a maximum value with increasing gas velocity into the loop-seal. The gas leakages between AR and FR, and between FR and SR due to solid circulation were negligible. Furthermore, a two-dimensional (2D) computational fluid dynamics (CFD) simulation using a commercial CFD code was carried out in order to better understand the flow behavior of the gas solid mixture inside the TRCL system. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As an environmentally friendly energy carrier, hydrogen could address issues related to global climate change, air
pollution, and energy security [1]. Currently, most hydrogen is produced via a steam methane reforming (SMR) process or processes that involve other fossil fuels [2]. The hydrogen production SMR process is considered an efficient process due
* Corresponding author. Fax: þ82 42 860 3428. ** Corresponding author. Fax: þ82 42 350 3910. E-mail addresses: [email protected] (M.W. Seo), [email protected] (S.D. Kim). http://dx.doi.org/10.1016/j.ijhydene.2014.07.059 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
to proven technology, low cost, and abundance of feedstock [3]. In the SMR process, however, an additional postpurification process, such as a pressure swing adsorption (PSA), is required to sequestrate CO2 for high-purity hydrogen production. Therefore, research to produce hydrogen that can enable CO2 removal from the product stream with low capital cost is necessary [4]. The three-reactor chemical looping (TRCL) process for high-purity hydrogen production with intrinsic CO2 separation is a novel concept derived from chemical looping combustion (CLC) [5]. It employs metal oxide as oxygen carrier particles. The redox reactions of the metal oxide take place alternately within three reactors. A common TRCL system consists of a fuel reactor (FR), a steam reactor (SR), and an air reactor (AR). In the FR, natural gas (CH4) is oxidized to CO2 and H2O by the oxygen-rich metal oxide (Eq. (1)). When the natural gas is completely converted, pure CO2 can be sequestrated for other applications after condensing the steam. The reduced metal oxide is transported to the SR, where steam is decomposed into H2 by the oxidation of the reduced metal oxide (Eq. (2)). In the AR, metal oxide is fully oxidized (Eq. (3)) and recirculated to the FR in order to allow the re-use of the oxygen-rich metal oxide and to make the system thermally independent (Eq. (4)) via a highly exothermic reaction (Eq. (3)). FR: (n þ m/2)MyOx þ CnH2m / (n þ m/2) MyOx2 þ mH2O þ nCO2
(1)
SR: (n þ m/2)MyOx2 þ mH2O / (n þ m/2)MyOx1 þ mH2
(2)
AR: (n þ m/2)MyOx1 þ 1/2O2 / (n þ m/2)MyOx
(3)
Overall reaction: CH4 þ 2/3H2O þ 2/3O2 / CO2 þ 8/3H2
(4)
DHo298K ¼ 157.6 kJ/mol In order to design the TRCL process efficiently and operate it sustainably, the characteristics of the oxygen carrier and reactors should be considered. Many studies on hydrogen production processes with intrinsic CO2 separation have been carried out in terms of feasibility of the chemical looping process and compatibility of the oxygen carrier. Fan et al. [6,7] confirmed the suitability of composite iron oxide particles as an oxygen carrier for the Syngas chemical looping (SCL) process by carrying out Thermo-gravimetric analysis (TGA) and a fixed bed experiment. The efficiencies of the syngas redox process were calculated using Aspen plus. Go et al. [8,9] studied two-step methane reforming with the redox of FeO/Fe3O4 in TGA and a fluidized bed reactor. Based on their experimental data, they proposed guidelines for a continuous process design. At Korea Institute of Energy Research (KIER), using natural gas and non-catalytic gasesolid reactions, the reactivity of oxygen carriers with support materials for the TRCL was studied [4,10,11]. The kinetic data were obtained from Fe2O3/ZrO2 prepared by the aerial oxidation method. A moving bed operation under counter-current
14547
flow condition was applied for efficient gasesolid contact. Temperature, gasesolid conversion profiles, and bed inventory were determined based on the kinetic model. Cormos et al. [12,13], at Barbes-Bolyai university, analyzed processes for producing hydrogen from fossil fuel conversion (co-gasification process of coal and biomass) based on chemical looping techniques. Moghtaderi [14] reviewed recent developments of chemical looping processes such as chemical looping reforming, syngas chemical looping, chemical looping combustion, etc. As mentioned above, many studies have focused on hydrogen production system evaluation and oxygen carrier development. However, limited research related to the hydrodynamics of the chemical looping process operation for hydrogen production has been reported. Xue et al. [15] proposed a compact fluidized-bed fuel reactor that integrates a bubbling fluidized bed and a riser. They confirmed stable pressure drop and solid circulation rate during system operation under a cold model. Hong et al. [16] studied the solid flow characteristics of a multistage circulating moving bed reactor for chemical looping hydrogen production. The effects of inlet gas velocity into the loop-seal and the bed height of the steam reactor on solid mass flux were investigated at various temperatures. In this study, compared to the common process, which utilizes only a single AR, a novel TRCL process design, as shown in Fig. 1(a), was developed for hydrogen production; in this new process, the partially reduced oxygen carrier from the SR can be fully oxidized in both the riser and the AR. By adding the riser, it is expected that the residence time of the bed materials in the AR can be controllable for the thermodynamic stability of the whole system. The cold mode TRCL system was then designed and constructed, and a series of hydrodynamic studies of the proposed system was performed. Computational fluid dynamics (CFD) simulations were also carried out using a commercial CFD code (Fluent Inc., USA) in order to better understand the flow behavior of the solidegas mixture inside the system. Due to this system's hydrodynamic complexity, this research is expected to be the first attempt to verify the solidegas flow using CFD simulations on a four-bed interconnected system.
Experiments Experimental set-up Experiments were carried out in a cold-mode TRCL system made of Plexiglas as shown in Fig. 1. It is composed of a riser, three interconnected reactors (AR, FR, and SR), cyclones and three non-mechanical valves (loop-seal). Each part was designed for optimal operation; the major design parameters are given in Table 1. A spider-shaped sparger was adopted as a distributor to ensure the moving bed conditions in the FR and SR. Two bubble-cap type distributors and a sintered metal plate distributor were installed for the AR, loop-seals, and riser, respectively. To measure the differential pressure, pressure taps were evenly installed across all parts. The pressure data were obtained using a data acquisition system (GL200, Graphtec, Japan) with 0.5 s intervals through the
14548
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Fig. 1 e Schematic diagram of TRCL system (a): 1. Riser, 2. Cyclone, 3. Air reactor (AR), 4. AR loop-seal, 5. Fuel reactor (FR), 6. FR loop-seal, 7. Steam reactor (SR), 8. SR loop-seal, 9. Distributor, initial solids load and gas injection points (b), and hot mode test unit (c).
pressure transducers (DPLH series, Sensys, Korea). Zirconia was used as a bed material due to its physical similarity to the oxygen carrier (20wt% Fe2O3/ZrO2). The physical properties of zirconia are summarized in Table 2. The minimum fluidization velocity (Umf) was determined by increasing the gas velocity in the fluidized bed. The terminal velocity (Ut) was calculated using the measured solid density, the particle diameter, and the estimated sphericity of particles. Using the emptying time method, the transport velocity (Utr) was found to be 3.19 m/s. The fluidizing gas was filtered air and all experiments were conducted at room temperature. The gas volumetric flow rate into the riser, reactors, and loop-seals was controlled by flow meters (RM Series, Dwyer, USA). The
Table 1 e Design parameters and reactor dimensions. Parameter Riser AR
FR, SR
Value Height [m] Diameter [m] Inventory [kg] Height [m] Diameter [m] Loop-seal length [m] Loop-seal width [m] Loop-seal weir height [m] Inventory [kg] Height [m] Diameter [m] Loop-seal length [m] Loop-seal width [m] Loop-seal weir height [m]
1.9 0.025 2.0 0.3 0.076 0.16 0.025 0.25 5.5 0.43 0.124 0.17 0.025 0.33
gas concentrations were analyzed using a gas chromatograph (7890A, Agilent Technologies, USA) with a Carboxen 1000 column for the gas leakage experiments.
Procedure and measurement A series of hydrodynamic experiments was carried out in a cold-mode TRCL system to investigate the solid flow rate, pressure profile, and gas leakage. The solid flow rate (Fs) was determined by calculation from the bulk density of the particles and the pressure drop over time at the desired gas velocity within each reactor. Since the established pressure balance was not disturbed by this method, it was possible to conduct experiments continuously and accurately. The gas volumetric flow rate into each reactor and loop-seal recycle chamber was adjusted, while those into the riser and loop-seal supply chamber were maintained at 4.2 m/s and 0.02 m/s, respectively. The stable fast fluidization regime in the riser was obtained with a gas velocity above Utr þ 1 [m/s]. Usually, the loop-seal supply chamber velocity was operated in the range of 0.5 Umf to 1.5 Umf. The function of the gas injection into the loop-seal supply chamber was to control the solid flow from the packed bed to the moving bed [17]. To acquire a wide range of the solid flow rate as a function of gas velocity and to understand the variation of the flow behavior, the gas velocity to the loop-seal recycle chamber was regulated from 0 to 0.31 m/ s for the FR and SR and at 0.28 m/s for the AR. The effects of the degree of fluidization in each reactor (FR, SR, and AR) on the solid flow rate were also studied by changing the reactor flow regime from moving bed to fluidized bed. The suitable solid
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Table 2 e Physical properties of zirconia. Properties Mean diameter [㎛] Particle density [kg/m3] Bulk density [kg/m3] Minimum fluidization velocity (Calculated) [m/s] Terminal velocity (Calculated) [m/s] Transport velocity (Calculated) [m/s]
Zirconia 181 3850 2300 0.05 1.37 3.19
flow rate (Fs) was determined to be in a range from 120 g/s to 240 g/s based on the degree of solid reduction in the FR [10]. The experimental variables and ranges are summarized in Table 3. For the gas leakage measurements, a tracer gas method was adopted [18e20]. CO2 (99.99% purity) was introduced as a tracer gas into the AR and FR to determine the leakage from AR to FR and FR to SR, respectively. The outlet gas concentration from the cyclone was measured using a gas chromatograph while introducing N2 gas into all the rest of system and increasing gas velocity into the recycle chamber of the loopseal. The gas leakage is defined as the ratio of the amount of CO2 escaping out of the FR or SR, to the total amount of CO2 added to AR or FR, respectively. All experimental results were determined by repeated measurement and were averaged from a minimum of three experiments.
CFD simulation Solid-gas two-phase flows are often modeled based on two approaches that have been widely classified: (1) EulerianeEulerian (EeE), or the continuum approach and (2) Eulerian-Lagrangian (E-L), or the discrete particle approach. In the present study, the EeE approach is used to model both the gas and solid phases because several hydrodynamic regimes of fluidization can occur simultaneously in the whole TRCL system. In the EeE model, gas and solid are considered as continuous phases and an interpenetrating continuum assumption is applied to treat the phase interaction. The trajectory simulation and averaging of the dispersed phase particles were carried out at a hypothetical level [21]. The main assumptions of the CFD model used in the present work are: (1) the particles in the beds are spherical and uniform in size; (2) the particles are assumed to be inelastic and smooth; and (3) the kinetic theory of granular flow (KTFG) is used to
Table 3 e Experimental variables and operational ranges. Variables Gas velocity to riser [Ug,r,m/s] Gas velocity to AR [Ug,ar, m/s] Gas velocity to FR and SR [Ug,fr, Ug,sr, m/s] AR recycle chamber aeration velocity [Ug,arc, m/s] FR, SR recycle chamber aeration velocity [Ug,frc, Ug,src, m/s] AR, FR, SR supply chamber aeration velocity [Ug,sc, m/s]
Range 4.2 0.045e0.135 0.027e0.054 0e0.28 0e0.31 0.02
14549
describe the particle collisions and fluctuations in the beds. The governing equations of mass, momentum and kinetic energy conservations are summarized in Table 4, in which the thermal energy balance is not shown because the temperature effect is not considered in the cold-mode operation. The multiphase ke3 turbulence model is employed to take into account turbulent interactions between gas and solid phases. The continuity equations describing the mass conservation of the two phases are expressed as Eqs. (T1) and (T2). The momentum equation for each phase is given in Eqs. (T3) and (T4), in which the momentum transfer between the two phases is calculated in terms of the momentum exchange coefficient (Kgs) using Gidaspow's drag law (Eq. (T8)). The kinetic theory of granular flow, which considers conservation of the particle fluctuation energy, is written as Eq. (T5). The constitutive equations describing the phase stress-strain tensor (Eqs. (T6) and (T7)), solid pressure (Eq. (T9)), collisional solid viscosity (Eq. (T10)), and bulk solid viscosity (Eq. (T11)) are also given in Table 4. Details of the other drag functions and constitutive equations can be found elsewhere [22e24]. The governing equations of the granular Eulerian model are solved using a finite volume method with the commercial code Fluent. The simulation domain covers the whole 2D configuration of the TRCL system. The geometry is discretized into 196,000 cells in which, to improve the calculation accuracy, most of the cells are hexahedral and the maximum cell size is 0.002 m. Air is used as the gas phase; the density is set at 1.225 kg/m3 and the viscosity at 1.7897 105 kg/ms because the experiments are performed under ambient conditions. On the basis of the physical properties of the zirconia particles, the settings for the major physical properties of the solid phase in the simulation were summarized and are shown in Table 5. The initial and boundary conditions are given in Table 6; these data are based on the experimental operating conditions, along with the input locations depicted in Fig. 1(b). The three zones in red indicate the initial load of the solid beds. The height of each bed is taken from the solid inventory obtained from the experimental preparation step. Simulations are performed for 30 s of real time on a PC with Xeon 2.4 GHz and 4 GB of RAM. To avoid instability and to ensure that the solution converges at every time step, a time step size of 0.0001 s with 100 iterations/time step is used.
Results and discussion Solid flow rate Fig. 2 shows the effects of the gas flows injected into the loopseals and beds of the three reactors (AR, FR, and SR) on the solid flow rate. Velocity of the gas flows introduced to the loop-seals is presented in terms of fluidizing number, which is defined as the ratio between the operating velocity and the minimum fluidization velocity (Umf). As can be seen in Fig. 2(a), with increasing gas velocity into the AR loop-seal recycle chamber (Ug,arc) up to 4.0 Umf, the solid flow rate increases linearly. Then, the solid flow rate becomes constant with a maximum value of 300 g/s at each gas velocity into the AR (Ug,ar). This means that the change of the gas velocity into
14550
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Table 4 e Governing equations for CFD simulation. Continuity equation Gas phase
v vt v vt
Solid phase
Momentum equation Gas phase
v vt v vt
Solid phase
3 g rg
3 s rs
3 g rg
3 s rs
3 g rg
þ V:
þ V:
! vg
! vs
3 s rs
! vg
! vs
3 g rg
þ V:
3 s rs
¼0
(T1)
¼0
(T2)
þ V:
2 ! vg
2 ! vs
v g þ Kgs ! vs ¼ 3 g Vp þ V:tg þ 3 g rg ! vg !
(T3)
v s þ Kgs ! vs ¼ 3 s Vp Vps þ V:ts þ 3 s rs ! vg ! (T4)
Kinetic fluctuation energy
3 v rs 3 s Qs þ V: rs 3 s ! ¼ ps I þ ts : V:! v s þ V: kQs VQs gQs þ 4gs v s Qs 2 vt Phase stressestrain tensor equation Gas phase
Solid phase
(T5)
T v g þ V! tg ¼ ag mg V! vg
(T6)
T ts ¼ as ms V! v s þ as ls 2ms 3 V,! v s þ V! v sI
(T7)
Gidaspow drag law
Kgs ¼ 150
2 3 s mg 2 3 g ds
þ 1:75
3 s rs
ds
! vs ! v g ;
3g
< 0:8
3 3 s 3 g rg ! Kgs ¼ CD v g 3 2:65 ; 3 g 0:8 vs ! g 4 ds 0:678 where CD ¼ 3 g24 , Res ¼ Res 1 þ 0:15ð3 g Res Þ Solid pressure
! ps ¼ 2rs 1 þ ess
(T8) ! !
rs ds j v s v g j mg
" a2s g0;ss qs
where g0;ss ¼ 1
1=3 #1
as
(T9)
as;max
Collisional solid viscosity
4 ms;col ¼ as rs ds g0;ss 1 þ ess 5
! pffiffiffiffiffiffiffi 2 1=2 qs 10rs ds qs p 4 þ 1 þ g0;ss as 1 þ ess 5 p 96as 1 þ ess g0;ss
(T10)
Bulk solid viscosity
4 ls ¼ as rs ds g0;ss 1 þ ess 3
! 1=2 qs p
(T11)
Table 5 e CFD simulation settings. Properties
Setting 3
Density [kg/m ] Diameter [m] Granular temperature Granular viscosity Granular bulk viscosity Granular temperature Solids pressure Radial distribution Packing limit
3850 1.81 104 Phase property Syamlal-O'brien Lun et al. Algebraic Lun et al. Lun et al. 0.63
the AR in the range of 1.0 Umf to 3.0 Umf does not affect the carrying capacity of the loop-seal As the structural configuration of the FR and SR are identical, the solid flow characteristics in the FR and SR are expected to be similar. In the FR and SR, the contacting mode of the oxygen carrier and the fluidizing gas should be that of counter-current flow, i.e. a moving bed condition for high selectivity of the solid product in the FR and high conversion of steam to hydrogen in the SR [11]. The fluidized bed condition is not favored because the presence of axial mixing leads to decrease in selectivity and conversion. In the FR, as can be seen in Fig. 2(b), the solid flow rate increases with increasing
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Table 6 e Initial and boundary conditions for CFD simulation. Variables Ug,r, ① Ug,ar, ② Ug,fr, ③ Ug,sr, ④ Ug,sc, ⑤ Ug,arc, ⑥ Ug,sc, ⑦ Ug,frc, ⑧ Ug,sc, ⑨ Ug,src, ⑩
Value [m/s] 4.2 0.08 0.04 0.04 0.02 0.13 0.02 0.1 0.02 0.08
of the gas velocity into the FR loop-seal recycle chamber (Ug,frc); solid flow rate reaches a maximum value of about 850 g/s with a fluidizing number (Ug,frc/Umf) of 6.0. In the SR, under the same gas flow conditions as those in the FR, about 700 g/s of maximum solid flow rate is found near 5.0 Ug,src/Umf; the characteristics of the solid flow rate with respect to the gas velocity into the SR bed are similar to those of the FR bed, as shown in Fig. 2(c). However, higher solid flow rate with high gas velocity into the SR (Ug,sr ¼ 1.2 Umf) is observed due to the downward flow of solid in the SR. At gas velocities into the loop-seal recycle chamber corresponding to desirable solid flow rate range (120e240 g/s), the
14551
solid flow rate can be flexibly controlled when the fluidizing condition of the FR and SR is that of moving bed (0.6 Umf, 0.8 Umf) or fluidized bed (1.0 Umf, 1.2 Umf). With increasing of the gas velocity into the loop-seal recycle chamber, the maximum solid flow rates reach values of 300 g/s, 850 g/s, and 700 g/s for the AR, FR, and SR, respectively. Moderate and wide ranges of solid flow rate are also obtained.
Gas leakage measurement Air, methane, and steam are introduced into the AR, FR, and SR beds, respectively, in the TRCL system. The reactant gas injected into each bed has to be ejected through a cyclone after the reaction. If unreacted gas of a noticeable concentration moves to the following reactor along with the circulating solid through the loop-seal, the efficiency of the TRCL process is affected, decreasing because an undesired side reaction occurs that causes a change in the whole set of reaction conditions. In particular, a reaction between oxygen gas transferred abnormally from the AR and methane injected into the FR can, under high temperature, cause an explosion. Experiment for gas leakage measurement should be conducted to confirm that the injected gas is being carried to the next reactor with the circulating solid. The gas leakage measurement configuration and measured CO2 concentration with respect to the gas velocity injected into the loop-seal recycle chamber, and the experimental conditions for gas
Fig. 2 e Loop-seal calibration curves as a function of fluidizing number, (a) AR, (b) FR, and (c) SR.
14552
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Fig. 3 e Gas leakage measurement configuration and CO2 concentration versus loop-seal recycle chamber gas velocity.
leakage measurement, are shown in Fig. 3 and in Table 7, respectively. The optimum condition in the AR requires a moderate mixing for oxidation of the reduced oxygen carrier. Therefore, it is essential to observe whether unreacted air enters the FR via the AR loop-seal when the gas velocity into the AR bed is increased. If unreacted methane or CO2, a reaction product, were to enter the SR, the purity of the hydrogen produced in the SR would drop. As shown in Fig. 4, the CO2 concentration with increasing of the gas velocity injected into the loop-seal recycle chamber is very low and constant in the four case; the effects of the gas velocity into the AR bed on the CO2 concentration measured at the FR cyclone are negligible. This was also confirmed through continuous hot bed operation [25]. Under steady state operating conditions, the gas from the FR had an average CO2 yield of 94.15%; the average hydrogen concentration from the SR was 99.95%. It can be concluded that scarcely any gas leakage occurs when the injected gas moves into the following reactor, thereby proving that this is an efficient reactor that minimizes side reactions.
Overall pressure profile Fig. 4 shows the overall pressure profile around the TRCL system under hot (800 C) and cold-mode operating conditions. The solid flow rate of the system is fixed at 120 g/s and the absolute pressure values are recorded. The pressure distribution values of the TRCL system for the two operating modes showed similar trends for the conventional circulating fluidized bed [26]. The absolute pressure reaches a maximum value at the loop-seal bottom of each reactor and gradually decreases toward the top of the riser. The overall pressure profile of the hot-mode operation shifted to the right, especially in the FR compared to the AR or the SR, as shown in Fig. 4. The iron oxides are reduced to an Fe/FeO mixture in the FR, in which the particles can agglomerate [27]. Thus it is assumed that the right shift of the pressure profile in the FR is due to the agglomeration of particles across the FR. Hong et al. [16] also reported that solid mass flux and pressure drop in the loop-seal increase as the temperature increased. This is attributed mainly to
Table 7 e Gas leakage experimental conditions.
Qar Qfr Qsr Qsc Qarc Qfrc Measuring port
Case 1
Case 2
Case 3
Case 4
CO2, 1.0 Umf N2, 1.0 Umf e N2, 0.5 Umf N2, 0e4.0 Umf e P1
CO2, 2.0 Umf N2, 1.0 Umf e N2, 0.5 Umf N2, 0e4.0 Umf e P1
CO2, 3.0 Umf N2, 1.0 Umf e N2, 0.5 Umf N2, 0e4.0 Umf e P1
e CO2, 1.0 Umf N2, 0.5 Umf N2, 0.5 Umf e N2, 0e4.0 Umf P2
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
14553
Solidegas flow pattern
Fig. 4 e Overall static pressure profile in the hot and coldmode TRCL system at a given solid flow rate (Fs ¼ 120 g/s).
the decrease in the viscosity of the gas. The pressure measurements of the cold-mode operation can be different from those of the hot-mode operation due to several variables that are affected by temperature, such as the gas viscosity, volumetric expansion due to the reaction, particle agglomeration, etc. The pressure profile provides a significant implication for the circulating solid distribution. It is confirmed that the proposed TRCL system maintains a stable and continuous solid circulation; this will be discussed more with the simulation results in the following section.
Simulation results In this section, simulation results at a given solid flow rate as indicated in Table 6 are provided after 30 s of real time. The results are presented both visually and quantitatively in order to allow an investigation of the hydrodynamics of the TRCL system.
The distribution of the solid particles over the whole system at certain specific times (t ¼ 0, 3, 6, 9, and 12 s) are shown in Fig. 5. From the simulation results, it can be seen that the circulation of the solid phase occurs after 12 s of real time. During the first circulation, the solid particles transport over the SR loop-seal to the riser; then, they are flown up to the higher part of the riser, leading to accumulation at the higher part and at the top of the riser. This accumulation also induces dense solidegas flow to enter the cyclone of the riser, and some part of the solid phase exits through the vortex finder of the riser cyclone. However, after the first loop, the solid phase accumulation along the upper part of the riser and riser cyclone decreases sharply and a macroscopically steady state flow is established. This phenomenon is similar to the solidegas flow pattern in a circulating fluidized bed system [28]. It should be noted that undesired bubble formation is found through the bed in the FR and SR, as can be seen in Fig. 5(c)e(e), while a bubbling fluidized bed is established in the AR as expected. Gas bubbles formed at the loop-seal moves up to the bottom of the adjacent bed, where minimum fluidized bed conditions were already established. It is also confirmed that gas bypassing due to bubble formation causes a decrease of the reaction conversion under hot mode operation [25]. When methane was introduced into the FR at a sub-stoichiometric ratio, nearly 100% of methane was converted to CO2. However, with increasing of the methane flow rate around Umf, unreacted methane was detected. The steam conversion rate also decreased in the SR even though the bed was under moving bed condition. Fig. 6 shows the relative pressure drop signals in the AR, FR, and SR under steady state hot mode operation. As expected, pressure fluctuation in the AR is much higher than that in the FR and SR due to the formation of bubbles or voids. The movement of bubbles enhances the gasesolid mixing, as can be seen in Fig. 6(a), with higher frequency and amplitudes of signal. On the other hand, pressure fluctuation signals of lower amplitude and standard deviation are detected in the FR and SR, which are supposed to be operated under moving bed conditions. These signals can be evidence of the existence of bubbles or voids
Fig. 5 e Instantaneous solidegas flow pattern in the TRCL system at certain specific real time of operation (Fs ¼ 120 g/s).
14554
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Fig. 6 e Pressure drop signals in (a) AR, (b) FR, and (c) SR under steady state operation (Hot mode, Fs ¼ 120 g/s).
that were invisible in the experiment. According to the agreement between the simulation and experimental results, moderate operating conditions can be selected to achieve complete conversion.
Simulated solid hold-up The simulated solid volume fraction values for the TRCL system for Fs ¼ 120 g/s are shown in Fig. 7, together with the experimental data (Fs ¼ 60, 120, 180 g/s). Solid hold-up (3 s ) in
the reactor can be calculated using the pressure difference; the expression, ignoring the gas density, is written as. DP=DL ¼ rs 3 s g
(5)
DL is the distance between the measured port in the reactor, rs is the density of solid particles, and g is the acceleration due to gravity. It can be observed that the axial solid hold-up distribution of the system is similar to that of a typical circulating fluidized bed, indicating the decrease of solid
Fig. 7 e Axial solid hold-up along the reactors in the TRCL system: (a) Riser, (b) AR, and (c) FR.
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Fig. 8 e Effect of solid flow rate on solid hold-up in the riser, AR, and FR.
concentration from the bottom to the top of the reactors [28,29]. In the riser, applied pressure increases and pressure drop decreases along the solid flow stream with increasing of solid flow rate. However, pressure drop slightly increases around the cyclone exit duct due to the end effect. The solid volume fraction suddenly increases from the height of 1.5 m in the riser for the case of Fs ¼ 180 g/s. This is caused by solid backflow having struck the riser top. The profound change of the solid hold-up in the AR indicates that the flow behaviors are closed to the mild bubbling fluidized bed regime, while the solid hold-up across the FR bed is quite constant, meaning that moving bed conditions are established. Fig. 8 shows the effect of the solid flow rate on the solid hold-up in the riser, AR, and FR. With increasing of the solid flow rate, the solid hold-up increases in the riser and decreases in the AR and FR. It is thought that the carrying capacity of the riser at the cyclone exit duct is lower than that of bed loopseals. Compared to the experimental data, the solid volume fraction values along the riser and the FR, obtained from the simulation, are under-predicted; on the other hand, overprediction of this value is observed in the AR. As a matter of fact, a 2D simplification of a complicated system cannot capture all the geometrical aspects of the real system well. Due to the 2D simplification used in this study, the swirl flow in the cyclone and the vortex finder part of the cyclone are neglected, which causes the under-prediction of the pressure profile in the riser and the over-prediction of the pressure distribution along AR. Also, the deviations of the pressure profiles between the experimental data and the simulation results are attributed to the simplification of the spider shaped spargers in the FR and SR. 3D simulations would be required in the future for better prediction.
aeration in the loop-seal; the moderate gas velocities to the loop-seal recycle chamber were determined in order to obtain a desirable solid flow rate (120e240 g/s). The pressure profile along the system is considered to be suitable for stable CFB operation. Gas leakage experiments using CO2 as a tracer indicate that no gas leakage occurs under the proposed operational conditions. A two-dimensional CFD simulation using the multiphase Eulerian model incorporating the kinetic theory of the granular phase and a drag coefficient model was used to simulate the flow behavior of the gasesolid phase in the TRCL system. The simulation results capture the pressure profile over the whole system well, as can be observed in both the experimental and the computational results. Furthermore, agreement on the solid flow patterns between the simulation and the experimental data was obtained as well. The CFD simulation results support the hydrodynamic behaviors observed experimentally in the cold-mode TRCL system; the results from both are expected to provide guidelines for scale-up in the near future.
Acknowledgments This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B4-2434-05,B4-2481-14). Additionally, this work was supported by the project “Development of the design technology of a Korean 300 MWclass IGCC demonstration plant,” funded by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), affiliated with the Ministry of Knowledge Economy of the Korean Government (No.2011951010001B).
Nomenclature Cd dp Fs g g0 kQs Kgs L P,p Re Q t Ug,ar Ug,fr Ug,r Ug,sr Ug,sc Ug,arc
Conclusion Ug,frc A cold-mode three-reactor chemical looping system for hydrogen production was constructed; the operating parameters were determined according to the hydrodynamic characteristics. It was possible to control the solid flow rate by
14555
Ug,src Umf
drag coefficient mean diameter of particle, m solid flow rate, g/s acceleration of gravity, m/s2 radial distribution function diffusion coefficient for granular energy, kg/ms gas/solid momentum exchange coefficient length, m pressure, Pa Reynolds number gas velocity for leakage measurement, m/s time, s gas velocity into the air reactor, m/s gas velocity into the fuel reactor, m/s gas velocity into the riser, m/s gas velocity into the steam reactor, m/s gas velocity into the loop-seal supply chamber, m/s gas velocity into the air reactor loop-seal recycle chamber, m/s gas velocity into the fuel reactor loop-seal recycle chamber, m/s gas velocity into the steam reactor loop-seal recycle chamber, m/s minimum fluidization velocity, m/s
14556
Ut Utr v
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
terminal velocity, m/s transport velocity, m/s velocity, m/s
Greek symbols collision dissipation of energy, kg/s3 m gQs D difference 3 g ; ag volume fraction of gas, gas hold-up 3 s ; as ; q volume fraction of solid, solid hold-up Q granular temperature, m2/s2 I unity matrix V gradient or lambda m viscosity, kg/ms r density, kg/m3 t stress tensor, Pa transfer rate of kinetic energy, kg/s3 m fgs Subscript g gas s solids
references
[1] Turner JA. Sustainable hydrogen production. Science 2004;305:972e4. [2] Koroneos C, Dompros A, Roumbas G, Moussiopoulos N. Life cycle assessment of hydrogen fuel production processes. Int J Hydrogen Energ 2004;29:1443e50. [3] Kothari R, Buddhi D, Sawhney RL. Comparison of environmental and economic aspects of various hydrogen production methods. Renew Sust Energ Rev 2008;12:553e63. [4] Cho WC, Seo MW, Kim SD, Kang KS, Bae KK, Kim CH, et al. Reactivity of iron oxide as an oxygen carrier for chemicallooping hydrogen production. Int J Hydrogen Energ 2012;37:16852e63. [5] Chiesa P, Lozza G, Malandrino A, Romano M, Piccolo V. Three-reactors chemical looping process for hydrogen production. Int J Hydrogen Energ 2008;33:2233e45. [6] Gupta P, Velazquez-Vargas LG, Fan LS. Syngas redox (SGR) process to produce hydrogen from coal derived syngas. Energ Fuel 2007;21:2900e8. [7] Fan LS, Li FX. Chemical looping technology and its fossil energy conversion applications. Ind Eng Chem Res 2010;49:10200e11. [8] Go KS, Son SR, Kim SD. Reaction kinetics of reduction and oxidation of metal oxides for hydrogen production. Int J Hydrogen Energ 2008;33:5986e95. [9] Go KS, Son SR, Kim SD, Kang KS, Park CS. Hydrogen production from two-step steam methane reforming in a fluidized bed reactor. Int J Hydrogen Energ 2009;34:1301e9. [10] Kang KS, Kim CH, Bae KK, Cho WC, Kim SH, Park CS. Oxygencarrier selection and thermal analysis of the chemicallooping process for hydrogen production. Int J Hydrogen Energ 2010;35:12246e54. [11] Kang KS, Kim CH, Bae KK, Cho WC, Jeong SU, Kim SH, et al. Modeling a counter-current moving bed for fuel and steam reactors in the TRCL process. Int J Hydrogen Energ 2012;37:3251e60.
[12] Cormos CC. Evaluation of iron based chemical looping for hydrogen and electricity co-production by gasification process with carbon capture and storage. Int J Hydrogen Energ 2010;35:2278e89. [13] Cormos CC. Hydrogen production from fossil fuels with carbon capture and storage based on chemical looping systems. Int J Hydrogen Energ 2011;36:5960e71. [14] Moghtaderi B. Review of the recent chemical looping process developments for novel energy and fuel applications. Energ Fuel 2012;26:15e40. [15] Xue ZP, Chen SY, Wang D, Xiang WG. Design and fluid dynamic analysis of a three-fluidized-bed reactor system for chemical-looping hydrogen generation. Ind Eng Chem Res 2012;51:4267e78. [16] Hong YS, Kang KS, Park CS, Kim SD, Bae JW, Nam JW, et al. Solid mass flux in a chemical-looping process for hydrogen production in a multistage circulating moving bed reactor. Int J Hydrogen Energ 2013;38:6052e8. [17] Seo MW, Nguyen TDB, Lim YI, Kim SD, Park S, Song BH, et al. Solid circulation and loop-seal characteristics of a dual circulating fluidized bed: experiments and CFD simulation. Chem Eng J 2011;168:803e11. [18] Ryu HJ, Park YC, Jo SH, Park MH. Development of novel twointerconnected fluidized bed system. Korean J Chem Eng 2008;25:1178e83. [19] Johansson E, Lyngfelt A, Mattisson T, Johnsson F. Gas leakage measurements in a cold model of an interconnected fluidized bed for chemical-looping combustion. Powder Technol 2003;134:210e7. [20] Kronberger B, Johansson E, Loffler G, Mattisson T, Lyngfelt A, Hofbauer H. A two-compartment fluidized bed reactor for CO2 capture by chemical-looping combustion. Chem Eng Technol 2004;27:1318e26. [21] Vejahati F, Mahinpey N, Ellis N, Nikoo MB. Cfd simulation of gas-solid bubbling fluidized bed: a new method for adjusting drag law. Can J Chem Eng 2009;87:19e30. [22] Almuttahar A, Taghipour F. Computational fluid dynamics of a circulating fluidized bed under various fluidization conditions. Chem Eng Sci 2008;63:1696e709. [23] Zhang N, Lu B, Wang W, Li JH. Virtual experimentation through 3D full-loop simulation of a circulating fluidized bed. Particuology 2008;6:529e39. [24] Nguyen TDB, Seo MW, Lim YI, Song BH, Kim SD. CFD simulation with experiments in a dual circulating fluidized bed gasifier. Comput Chem Eng 2012;36:48e56. [25] Cho WC, Lee DY, Seo MW, Kim SD, Kang KS, Bae KK, et al. Continuous operation characteristics of chemical looping hydrogen production system. Appl Energ 2014;113:1667e74. [26] Basu P, Butler J. Studies on the operation of loop-seal in circulating fluidized bed boilers. Appl Energ 2009;86:1723e31. [27] Cho P, Mattisson T, Lyngfelt A. Defluidization conditions for a fluidized bed of iron oxide-, nickel oxide-, and manganese oxide-containing oxygen carriers for chemical-looping combustion. Ind Eng Chem Res 2006;45:968e77. [28] Wang S, Yang YC, Lu HL, Xu PF, Sun LY. Computational fluid dynamic simulation based cluster structures-dependent drag coefficient model in dual circulating fluidized beds of chemical looping combustion. Ind Eng Chem Res 2012;51:1396e412. [29] Kaiser S, Loffler G, Bosch K, Hofbauer H. Hydrodynamics of a dual fluidized bed gasifier. Part II: simulation of solid circulation rate, pressure loop and stability. Chem Eng Sci 2003;58:4215e23.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Solid circulation characteristics of the three-reactor chemical-looping process for hydrogen production Doyeon Lee a, Myung Won Seo b,*, Thanh D.B. Nguyen c, Won Chul Cho c, Sang Done Kim a,** a
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea b Clean Fuel Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea c Hydrogen Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea
article info
abstract
Article history:
The three-reactor chemical-looping (TRCL) system for high purity hydrogen production
Received 4 April 2014
and intrinsic CO2 separation is an innovative concept using a circulating oxygen carrier.
Accepted 13 July 2014
The process employs iron oxide as an oxygen carrier, via which a redox reaction takes
Available online 6 August 2014
place alternately within three reactors, i.e. a fuel reactor (FR), where the natural gas is
Keywords:
and an air reactor (AR), where the oxygen carrier returned to its original form by aeration.
Three-reactor chemical-looping
As it consists of three reactors (AR, FR, and SR) and a riser, the TRCL system has compli-
process (TRCL)
cated hydrodynamic characteristics.
combusted to CO2 and H2O, a steam reactor (SR), where the steam is reduced to hydrogen,
Hydrogen production
In this study, a cold mode TRCL system with non-mechanical valve was designed and
CO2 separation
constructed to investigate the solid circulation characteristics. A series of hydrodynamic
Hydrodynamics
tests on the system was performed in which zirconia (dr ¼ 181 mm, rs ¼ 3850 kg/m3) was
Computational fluid dynamics (CFD)
used as a bed material. The solid flow rate increased up to a maximum value with increasing gas velocity into the loop-seal. The gas leakages between AR and FR, and between FR and SR due to solid circulation were negligible. Furthermore, a two-dimensional (2D) computational fluid dynamics (CFD) simulation using a commercial CFD code was carried out in order to better understand the flow behavior of the gas solid mixture inside the TRCL system. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As an environmentally friendly energy carrier, hydrogen could address issues related to global climate change, air
pollution, and energy security [1]. Currently, most hydrogen is produced via a steam methane reforming (SMR) process or processes that involve other fossil fuels [2]. The hydrogen production SMR process is considered an efficient process due
* Corresponding author. Fax: þ82 42 860 3428. ** Corresponding author. Fax: þ82 42 350 3910. E-mail addresses: [email protected] (M.W. Seo), [email protected] (S.D. Kim). http://dx.doi.org/10.1016/j.ijhydene.2014.07.059 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
to proven technology, low cost, and abundance of feedstock [3]. In the SMR process, however, an additional postpurification process, such as a pressure swing adsorption (PSA), is required to sequestrate CO2 for high-purity hydrogen production. Therefore, research to produce hydrogen that can enable CO2 removal from the product stream with low capital cost is necessary [4]. The three-reactor chemical looping (TRCL) process for high-purity hydrogen production with intrinsic CO2 separation is a novel concept derived from chemical looping combustion (CLC) [5]. It employs metal oxide as oxygen carrier particles. The redox reactions of the metal oxide take place alternately within three reactors. A common TRCL system consists of a fuel reactor (FR), a steam reactor (SR), and an air reactor (AR). In the FR, natural gas (CH4) is oxidized to CO2 and H2O by the oxygen-rich metal oxide (Eq. (1)). When the natural gas is completely converted, pure CO2 can be sequestrated for other applications after condensing the steam. The reduced metal oxide is transported to the SR, where steam is decomposed into H2 by the oxidation of the reduced metal oxide (Eq. (2)). In the AR, metal oxide is fully oxidized (Eq. (3)) and recirculated to the FR in order to allow the re-use of the oxygen-rich metal oxide and to make the system thermally independent (Eq. (4)) via a highly exothermic reaction (Eq. (3)). FR: (n þ m/2)MyOx þ CnH2m / (n þ m/2) MyOx2 þ mH2O þ nCO2
(1)
SR: (n þ m/2)MyOx2 þ mH2O / (n þ m/2)MyOx1 þ mH2
(2)
AR: (n þ m/2)MyOx1 þ 1/2O2 / (n þ m/2)MyOx
(3)
Overall reaction: CH4 þ 2/3H2O þ 2/3O2 / CO2 þ 8/3H2
(4)
DHo298K ¼ 157.6 kJ/mol In order to design the TRCL process efficiently and operate it sustainably, the characteristics of the oxygen carrier and reactors should be considered. Many studies on hydrogen production processes with intrinsic CO2 separation have been carried out in terms of feasibility of the chemical looping process and compatibility of the oxygen carrier. Fan et al. [6,7] confirmed the suitability of composite iron oxide particles as an oxygen carrier for the Syngas chemical looping (SCL) process by carrying out Thermo-gravimetric analysis (TGA) and a fixed bed experiment. The efficiencies of the syngas redox process were calculated using Aspen plus. Go et al. [8,9] studied two-step methane reforming with the redox of FeO/Fe3O4 in TGA and a fluidized bed reactor. Based on their experimental data, they proposed guidelines for a continuous process design. At Korea Institute of Energy Research (KIER), using natural gas and non-catalytic gasesolid reactions, the reactivity of oxygen carriers with support materials for the TRCL was studied [4,10,11]. The kinetic data were obtained from Fe2O3/ZrO2 prepared by the aerial oxidation method. A moving bed operation under counter-current
14547
flow condition was applied for efficient gasesolid contact. Temperature, gasesolid conversion profiles, and bed inventory were determined based on the kinetic model. Cormos et al. [12,13], at Barbes-Bolyai university, analyzed processes for producing hydrogen from fossil fuel conversion (co-gasification process of coal and biomass) based on chemical looping techniques. Moghtaderi [14] reviewed recent developments of chemical looping processes such as chemical looping reforming, syngas chemical looping, chemical looping combustion, etc. As mentioned above, many studies have focused on hydrogen production system evaluation and oxygen carrier development. However, limited research related to the hydrodynamics of the chemical looping process operation for hydrogen production has been reported. Xue et al. [15] proposed a compact fluidized-bed fuel reactor that integrates a bubbling fluidized bed and a riser. They confirmed stable pressure drop and solid circulation rate during system operation under a cold model. Hong et al. [16] studied the solid flow characteristics of a multistage circulating moving bed reactor for chemical looping hydrogen production. The effects of inlet gas velocity into the loop-seal and the bed height of the steam reactor on solid mass flux were investigated at various temperatures. In this study, compared to the common process, which utilizes only a single AR, a novel TRCL process design, as shown in Fig. 1(a), was developed for hydrogen production; in this new process, the partially reduced oxygen carrier from the SR can be fully oxidized in both the riser and the AR. By adding the riser, it is expected that the residence time of the bed materials in the AR can be controllable for the thermodynamic stability of the whole system. The cold mode TRCL system was then designed and constructed, and a series of hydrodynamic studies of the proposed system was performed. Computational fluid dynamics (CFD) simulations were also carried out using a commercial CFD code (Fluent Inc., USA) in order to better understand the flow behavior of the solidegas mixture inside the system. Due to this system's hydrodynamic complexity, this research is expected to be the first attempt to verify the solidegas flow using CFD simulations on a four-bed interconnected system.
Experiments Experimental set-up Experiments were carried out in a cold-mode TRCL system made of Plexiglas as shown in Fig. 1. It is composed of a riser, three interconnected reactors (AR, FR, and SR), cyclones and three non-mechanical valves (loop-seal). Each part was designed for optimal operation; the major design parameters are given in Table 1. A spider-shaped sparger was adopted as a distributor to ensure the moving bed conditions in the FR and SR. Two bubble-cap type distributors and a sintered metal plate distributor were installed for the AR, loop-seals, and riser, respectively. To measure the differential pressure, pressure taps were evenly installed across all parts. The pressure data were obtained using a data acquisition system (GL200, Graphtec, Japan) with 0.5 s intervals through the
14548
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Fig. 1 e Schematic diagram of TRCL system (a): 1. Riser, 2. Cyclone, 3. Air reactor (AR), 4. AR loop-seal, 5. Fuel reactor (FR), 6. FR loop-seal, 7. Steam reactor (SR), 8. SR loop-seal, 9. Distributor, initial solids load and gas injection points (b), and hot mode test unit (c).
pressure transducers (DPLH series, Sensys, Korea). Zirconia was used as a bed material due to its physical similarity to the oxygen carrier (20wt% Fe2O3/ZrO2). The physical properties of zirconia are summarized in Table 2. The minimum fluidization velocity (Umf) was determined by increasing the gas velocity in the fluidized bed. The terminal velocity (Ut) was calculated using the measured solid density, the particle diameter, and the estimated sphericity of particles. Using the emptying time method, the transport velocity (Utr) was found to be 3.19 m/s. The fluidizing gas was filtered air and all experiments were conducted at room temperature. The gas volumetric flow rate into the riser, reactors, and loop-seals was controlled by flow meters (RM Series, Dwyer, USA). The
Table 1 e Design parameters and reactor dimensions. Parameter Riser AR
FR, SR
Value Height [m] Diameter [m] Inventory [kg] Height [m] Diameter [m] Loop-seal length [m] Loop-seal width [m] Loop-seal weir height [m] Inventory [kg] Height [m] Diameter [m] Loop-seal length [m] Loop-seal width [m] Loop-seal weir height [m]
1.9 0.025 2.0 0.3 0.076 0.16 0.025 0.25 5.5 0.43 0.124 0.17 0.025 0.33
gas concentrations were analyzed using a gas chromatograph (7890A, Agilent Technologies, USA) with a Carboxen 1000 column for the gas leakage experiments.
Procedure and measurement A series of hydrodynamic experiments was carried out in a cold-mode TRCL system to investigate the solid flow rate, pressure profile, and gas leakage. The solid flow rate (Fs) was determined by calculation from the bulk density of the particles and the pressure drop over time at the desired gas velocity within each reactor. Since the established pressure balance was not disturbed by this method, it was possible to conduct experiments continuously and accurately. The gas volumetric flow rate into each reactor and loop-seal recycle chamber was adjusted, while those into the riser and loop-seal supply chamber were maintained at 4.2 m/s and 0.02 m/s, respectively. The stable fast fluidization regime in the riser was obtained with a gas velocity above Utr þ 1 [m/s]. Usually, the loop-seal supply chamber velocity was operated in the range of 0.5 Umf to 1.5 Umf. The function of the gas injection into the loop-seal supply chamber was to control the solid flow from the packed bed to the moving bed [17]. To acquire a wide range of the solid flow rate as a function of gas velocity and to understand the variation of the flow behavior, the gas velocity to the loop-seal recycle chamber was regulated from 0 to 0.31 m/ s for the FR and SR and at 0.28 m/s for the AR. The effects of the degree of fluidization in each reactor (FR, SR, and AR) on the solid flow rate were also studied by changing the reactor flow regime from moving bed to fluidized bed. The suitable solid
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Table 2 e Physical properties of zirconia. Properties Mean diameter [㎛] Particle density [kg/m3] Bulk density [kg/m3] Minimum fluidization velocity (Calculated) [m/s] Terminal velocity (Calculated) [m/s] Transport velocity (Calculated) [m/s]
Zirconia 181 3850 2300 0.05 1.37 3.19
flow rate (Fs) was determined to be in a range from 120 g/s to 240 g/s based on the degree of solid reduction in the FR [10]. The experimental variables and ranges are summarized in Table 3. For the gas leakage measurements, a tracer gas method was adopted [18e20]. CO2 (99.99% purity) was introduced as a tracer gas into the AR and FR to determine the leakage from AR to FR and FR to SR, respectively. The outlet gas concentration from the cyclone was measured using a gas chromatograph while introducing N2 gas into all the rest of system and increasing gas velocity into the recycle chamber of the loopseal. The gas leakage is defined as the ratio of the amount of CO2 escaping out of the FR or SR, to the total amount of CO2 added to AR or FR, respectively. All experimental results were determined by repeated measurement and were averaged from a minimum of three experiments.
CFD simulation Solid-gas two-phase flows are often modeled based on two approaches that have been widely classified: (1) EulerianeEulerian (EeE), or the continuum approach and (2) Eulerian-Lagrangian (E-L), or the discrete particle approach. In the present study, the EeE approach is used to model both the gas and solid phases because several hydrodynamic regimes of fluidization can occur simultaneously in the whole TRCL system. In the EeE model, gas and solid are considered as continuous phases and an interpenetrating continuum assumption is applied to treat the phase interaction. The trajectory simulation and averaging of the dispersed phase particles were carried out at a hypothetical level [21]. The main assumptions of the CFD model used in the present work are: (1) the particles in the beds are spherical and uniform in size; (2) the particles are assumed to be inelastic and smooth; and (3) the kinetic theory of granular flow (KTFG) is used to
Table 3 e Experimental variables and operational ranges. Variables Gas velocity to riser [Ug,r,m/s] Gas velocity to AR [Ug,ar, m/s] Gas velocity to FR and SR [Ug,fr, Ug,sr, m/s] AR recycle chamber aeration velocity [Ug,arc, m/s] FR, SR recycle chamber aeration velocity [Ug,frc, Ug,src, m/s] AR, FR, SR supply chamber aeration velocity [Ug,sc, m/s]
Range 4.2 0.045e0.135 0.027e0.054 0e0.28 0e0.31 0.02
14549
describe the particle collisions and fluctuations in the beds. The governing equations of mass, momentum and kinetic energy conservations are summarized in Table 4, in which the thermal energy balance is not shown because the temperature effect is not considered in the cold-mode operation. The multiphase ke3 turbulence model is employed to take into account turbulent interactions between gas and solid phases. The continuity equations describing the mass conservation of the two phases are expressed as Eqs. (T1) and (T2). The momentum equation for each phase is given in Eqs. (T3) and (T4), in which the momentum transfer between the two phases is calculated in terms of the momentum exchange coefficient (Kgs) using Gidaspow's drag law (Eq. (T8)). The kinetic theory of granular flow, which considers conservation of the particle fluctuation energy, is written as Eq. (T5). The constitutive equations describing the phase stress-strain tensor (Eqs. (T6) and (T7)), solid pressure (Eq. (T9)), collisional solid viscosity (Eq. (T10)), and bulk solid viscosity (Eq. (T11)) are also given in Table 4. Details of the other drag functions and constitutive equations can be found elsewhere [22e24]. The governing equations of the granular Eulerian model are solved using a finite volume method with the commercial code Fluent. The simulation domain covers the whole 2D configuration of the TRCL system. The geometry is discretized into 196,000 cells in which, to improve the calculation accuracy, most of the cells are hexahedral and the maximum cell size is 0.002 m. Air is used as the gas phase; the density is set at 1.225 kg/m3 and the viscosity at 1.7897 105 kg/ms because the experiments are performed under ambient conditions. On the basis of the physical properties of the zirconia particles, the settings for the major physical properties of the solid phase in the simulation were summarized and are shown in Table 5. The initial and boundary conditions are given in Table 6; these data are based on the experimental operating conditions, along with the input locations depicted in Fig. 1(b). The three zones in red indicate the initial load of the solid beds. The height of each bed is taken from the solid inventory obtained from the experimental preparation step. Simulations are performed for 30 s of real time on a PC with Xeon 2.4 GHz and 4 GB of RAM. To avoid instability and to ensure that the solution converges at every time step, a time step size of 0.0001 s with 100 iterations/time step is used.
Results and discussion Solid flow rate Fig. 2 shows the effects of the gas flows injected into the loopseals and beds of the three reactors (AR, FR, and SR) on the solid flow rate. Velocity of the gas flows introduced to the loop-seals is presented in terms of fluidizing number, which is defined as the ratio between the operating velocity and the minimum fluidization velocity (Umf). As can be seen in Fig. 2(a), with increasing gas velocity into the AR loop-seal recycle chamber (Ug,arc) up to 4.0 Umf, the solid flow rate increases linearly. Then, the solid flow rate becomes constant with a maximum value of 300 g/s at each gas velocity into the AR (Ug,ar). This means that the change of the gas velocity into
14550
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Table 4 e Governing equations for CFD simulation. Continuity equation Gas phase
v vt v vt
Solid phase
Momentum equation Gas phase
v vt v vt
Solid phase
3 g rg
3 s rs
3 g rg
3 s rs
3 g rg
þ V:
þ V:
! vg
! vs
3 s rs
! vg
! vs
3 g rg
þ V:
3 s rs
¼0
(T1)
¼0
(T2)
þ V:
2 ! vg
2 ! vs
v g þ Kgs ! vs ¼ 3 g Vp þ V:tg þ 3 g rg ! vg !
(T3)
v s þ Kgs ! vs ¼ 3 s Vp Vps þ V:ts þ 3 s rs ! vg ! (T4)
Kinetic fluctuation energy
3 v rs 3 s Qs þ V: rs 3 s ! ¼ ps I þ ts : V:! v s þ V: kQs VQs gQs þ 4gs v s Qs 2 vt Phase stressestrain tensor equation Gas phase
Solid phase
(T5)
T v g þ V! tg ¼ ag mg V! vg
(T6)
T ts ¼ as ms V! v s þ as ls 2ms 3 V,! v s þ V! v sI
(T7)
Gidaspow drag law
Kgs ¼ 150
2 3 s mg 2 3 g ds
þ 1:75
3 s rs
ds
! vs ! v g ;
3g
< 0:8
3 3 s 3 g rg ! Kgs ¼ CD v g 3 2:65 ; 3 g 0:8 vs ! g 4 ds 0:678 where CD ¼ 3 g24 , Res ¼ Res 1 þ 0:15ð3 g Res Þ Solid pressure
! ps ¼ 2rs 1 þ ess
(T8) ! !
rs ds j v s v g j mg
" a2s g0;ss qs
where g0;ss ¼ 1
1=3 #1
as
(T9)
as;max
Collisional solid viscosity
4 ms;col ¼ as rs ds g0;ss 1 þ ess 5
! pffiffiffiffiffiffiffi 2 1=2 qs 10rs ds qs p 4 þ 1 þ g0;ss as 1 þ ess 5 p 96as 1 þ ess g0;ss
(T10)
Bulk solid viscosity
4 ls ¼ as rs ds g0;ss 1 þ ess 3
! 1=2 qs p
(T11)
Table 5 e CFD simulation settings. Properties
Setting 3
Density [kg/m ] Diameter [m] Granular temperature Granular viscosity Granular bulk viscosity Granular temperature Solids pressure Radial distribution Packing limit
3850 1.81 104 Phase property Syamlal-O'brien Lun et al. Algebraic Lun et al. Lun et al. 0.63
the AR in the range of 1.0 Umf to 3.0 Umf does not affect the carrying capacity of the loop-seal As the structural configuration of the FR and SR are identical, the solid flow characteristics in the FR and SR are expected to be similar. In the FR and SR, the contacting mode of the oxygen carrier and the fluidizing gas should be that of counter-current flow, i.e. a moving bed condition for high selectivity of the solid product in the FR and high conversion of steam to hydrogen in the SR [11]. The fluidized bed condition is not favored because the presence of axial mixing leads to decrease in selectivity and conversion. In the FR, as can be seen in Fig. 2(b), the solid flow rate increases with increasing
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Table 6 e Initial and boundary conditions for CFD simulation. Variables Ug,r, ① Ug,ar, ② Ug,fr, ③ Ug,sr, ④ Ug,sc, ⑤ Ug,arc, ⑥ Ug,sc, ⑦ Ug,frc, ⑧ Ug,sc, ⑨ Ug,src, ⑩
Value [m/s] 4.2 0.08 0.04 0.04 0.02 0.13 0.02 0.1 0.02 0.08
of the gas velocity into the FR loop-seal recycle chamber (Ug,frc); solid flow rate reaches a maximum value of about 850 g/s with a fluidizing number (Ug,frc/Umf) of 6.0. In the SR, under the same gas flow conditions as those in the FR, about 700 g/s of maximum solid flow rate is found near 5.0 Ug,src/Umf; the characteristics of the solid flow rate with respect to the gas velocity into the SR bed are similar to those of the FR bed, as shown in Fig. 2(c). However, higher solid flow rate with high gas velocity into the SR (Ug,sr ¼ 1.2 Umf) is observed due to the downward flow of solid in the SR. At gas velocities into the loop-seal recycle chamber corresponding to desirable solid flow rate range (120e240 g/s), the
14551
solid flow rate can be flexibly controlled when the fluidizing condition of the FR and SR is that of moving bed (0.6 Umf, 0.8 Umf) or fluidized bed (1.0 Umf, 1.2 Umf). With increasing of the gas velocity into the loop-seal recycle chamber, the maximum solid flow rates reach values of 300 g/s, 850 g/s, and 700 g/s for the AR, FR, and SR, respectively. Moderate and wide ranges of solid flow rate are also obtained.
Gas leakage measurement Air, methane, and steam are introduced into the AR, FR, and SR beds, respectively, in the TRCL system. The reactant gas injected into each bed has to be ejected through a cyclone after the reaction. If unreacted gas of a noticeable concentration moves to the following reactor along with the circulating solid through the loop-seal, the efficiency of the TRCL process is affected, decreasing because an undesired side reaction occurs that causes a change in the whole set of reaction conditions. In particular, a reaction between oxygen gas transferred abnormally from the AR and methane injected into the FR can, under high temperature, cause an explosion. Experiment for gas leakage measurement should be conducted to confirm that the injected gas is being carried to the next reactor with the circulating solid. The gas leakage measurement configuration and measured CO2 concentration with respect to the gas velocity injected into the loop-seal recycle chamber, and the experimental conditions for gas
Fig. 2 e Loop-seal calibration curves as a function of fluidizing number, (a) AR, (b) FR, and (c) SR.
14552
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Fig. 3 e Gas leakage measurement configuration and CO2 concentration versus loop-seal recycle chamber gas velocity.
leakage measurement, are shown in Fig. 3 and in Table 7, respectively. The optimum condition in the AR requires a moderate mixing for oxidation of the reduced oxygen carrier. Therefore, it is essential to observe whether unreacted air enters the FR via the AR loop-seal when the gas velocity into the AR bed is increased. If unreacted methane or CO2, a reaction product, were to enter the SR, the purity of the hydrogen produced in the SR would drop. As shown in Fig. 4, the CO2 concentration with increasing of the gas velocity injected into the loop-seal recycle chamber is very low and constant in the four case; the effects of the gas velocity into the AR bed on the CO2 concentration measured at the FR cyclone are negligible. This was also confirmed through continuous hot bed operation [25]. Under steady state operating conditions, the gas from the FR had an average CO2 yield of 94.15%; the average hydrogen concentration from the SR was 99.95%. It can be concluded that scarcely any gas leakage occurs when the injected gas moves into the following reactor, thereby proving that this is an efficient reactor that minimizes side reactions.
Overall pressure profile Fig. 4 shows the overall pressure profile around the TRCL system under hot (800 C) and cold-mode operating conditions. The solid flow rate of the system is fixed at 120 g/s and the absolute pressure values are recorded. The pressure distribution values of the TRCL system for the two operating modes showed similar trends for the conventional circulating fluidized bed [26]. The absolute pressure reaches a maximum value at the loop-seal bottom of each reactor and gradually decreases toward the top of the riser. The overall pressure profile of the hot-mode operation shifted to the right, especially in the FR compared to the AR or the SR, as shown in Fig. 4. The iron oxides are reduced to an Fe/FeO mixture in the FR, in which the particles can agglomerate [27]. Thus it is assumed that the right shift of the pressure profile in the FR is due to the agglomeration of particles across the FR. Hong et al. [16] also reported that solid mass flux and pressure drop in the loop-seal increase as the temperature increased. This is attributed mainly to
Table 7 e Gas leakage experimental conditions.
Qar Qfr Qsr Qsc Qarc Qfrc Measuring port
Case 1
Case 2
Case 3
Case 4
CO2, 1.0 Umf N2, 1.0 Umf e N2, 0.5 Umf N2, 0e4.0 Umf e P1
CO2, 2.0 Umf N2, 1.0 Umf e N2, 0.5 Umf N2, 0e4.0 Umf e P1
CO2, 3.0 Umf N2, 1.0 Umf e N2, 0.5 Umf N2, 0e4.0 Umf e P1
e CO2, 1.0 Umf N2, 0.5 Umf N2, 0.5 Umf e N2, 0e4.0 Umf P2
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
14553
Solidegas flow pattern
Fig. 4 e Overall static pressure profile in the hot and coldmode TRCL system at a given solid flow rate (Fs ¼ 120 g/s).
the decrease in the viscosity of the gas. The pressure measurements of the cold-mode operation can be different from those of the hot-mode operation due to several variables that are affected by temperature, such as the gas viscosity, volumetric expansion due to the reaction, particle agglomeration, etc. The pressure profile provides a significant implication for the circulating solid distribution. It is confirmed that the proposed TRCL system maintains a stable and continuous solid circulation; this will be discussed more with the simulation results in the following section.
Simulation results In this section, simulation results at a given solid flow rate as indicated in Table 6 are provided after 30 s of real time. The results are presented both visually and quantitatively in order to allow an investigation of the hydrodynamics of the TRCL system.
The distribution of the solid particles over the whole system at certain specific times (t ¼ 0, 3, 6, 9, and 12 s) are shown in Fig. 5. From the simulation results, it can be seen that the circulation of the solid phase occurs after 12 s of real time. During the first circulation, the solid particles transport over the SR loop-seal to the riser; then, they are flown up to the higher part of the riser, leading to accumulation at the higher part and at the top of the riser. This accumulation also induces dense solidegas flow to enter the cyclone of the riser, and some part of the solid phase exits through the vortex finder of the riser cyclone. However, after the first loop, the solid phase accumulation along the upper part of the riser and riser cyclone decreases sharply and a macroscopically steady state flow is established. This phenomenon is similar to the solidegas flow pattern in a circulating fluidized bed system [28]. It should be noted that undesired bubble formation is found through the bed in the FR and SR, as can be seen in Fig. 5(c)e(e), while a bubbling fluidized bed is established in the AR as expected. Gas bubbles formed at the loop-seal moves up to the bottom of the adjacent bed, where minimum fluidized bed conditions were already established. It is also confirmed that gas bypassing due to bubble formation causes a decrease of the reaction conversion under hot mode operation [25]. When methane was introduced into the FR at a sub-stoichiometric ratio, nearly 100% of methane was converted to CO2. However, with increasing of the methane flow rate around Umf, unreacted methane was detected. The steam conversion rate also decreased in the SR even though the bed was under moving bed condition. Fig. 6 shows the relative pressure drop signals in the AR, FR, and SR under steady state hot mode operation. As expected, pressure fluctuation in the AR is much higher than that in the FR and SR due to the formation of bubbles or voids. The movement of bubbles enhances the gasesolid mixing, as can be seen in Fig. 6(a), with higher frequency and amplitudes of signal. On the other hand, pressure fluctuation signals of lower amplitude and standard deviation are detected in the FR and SR, which are supposed to be operated under moving bed conditions. These signals can be evidence of the existence of bubbles or voids
Fig. 5 e Instantaneous solidegas flow pattern in the TRCL system at certain specific real time of operation (Fs ¼ 120 g/s).
14554
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Fig. 6 e Pressure drop signals in (a) AR, (b) FR, and (c) SR under steady state operation (Hot mode, Fs ¼ 120 g/s).
that were invisible in the experiment. According to the agreement between the simulation and experimental results, moderate operating conditions can be selected to achieve complete conversion.
Simulated solid hold-up The simulated solid volume fraction values for the TRCL system for Fs ¼ 120 g/s are shown in Fig. 7, together with the experimental data (Fs ¼ 60, 120, 180 g/s). Solid hold-up (3 s ) in
the reactor can be calculated using the pressure difference; the expression, ignoring the gas density, is written as. DP=DL ¼ rs 3 s g
(5)
DL is the distance between the measured port in the reactor, rs is the density of solid particles, and g is the acceleration due to gravity. It can be observed that the axial solid hold-up distribution of the system is similar to that of a typical circulating fluidized bed, indicating the decrease of solid
Fig. 7 e Axial solid hold-up along the reactors in the TRCL system: (a) Riser, (b) AR, and (c) FR.
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
Fig. 8 e Effect of solid flow rate on solid hold-up in the riser, AR, and FR.
concentration from the bottom to the top of the reactors [28,29]. In the riser, applied pressure increases and pressure drop decreases along the solid flow stream with increasing of solid flow rate. However, pressure drop slightly increases around the cyclone exit duct due to the end effect. The solid volume fraction suddenly increases from the height of 1.5 m in the riser for the case of Fs ¼ 180 g/s. This is caused by solid backflow having struck the riser top. The profound change of the solid hold-up in the AR indicates that the flow behaviors are closed to the mild bubbling fluidized bed regime, while the solid hold-up across the FR bed is quite constant, meaning that moving bed conditions are established. Fig. 8 shows the effect of the solid flow rate on the solid hold-up in the riser, AR, and FR. With increasing of the solid flow rate, the solid hold-up increases in the riser and decreases in the AR and FR. It is thought that the carrying capacity of the riser at the cyclone exit duct is lower than that of bed loopseals. Compared to the experimental data, the solid volume fraction values along the riser and the FR, obtained from the simulation, are under-predicted; on the other hand, overprediction of this value is observed in the AR. As a matter of fact, a 2D simplification of a complicated system cannot capture all the geometrical aspects of the real system well. Due to the 2D simplification used in this study, the swirl flow in the cyclone and the vortex finder part of the cyclone are neglected, which causes the under-prediction of the pressure profile in the riser and the over-prediction of the pressure distribution along AR. Also, the deviations of the pressure profiles between the experimental data and the simulation results are attributed to the simplification of the spider shaped spargers in the FR and SR. 3D simulations would be required in the future for better prediction.
aeration in the loop-seal; the moderate gas velocities to the loop-seal recycle chamber were determined in order to obtain a desirable solid flow rate (120e240 g/s). The pressure profile along the system is considered to be suitable for stable CFB operation. Gas leakage experiments using CO2 as a tracer indicate that no gas leakage occurs under the proposed operational conditions. A two-dimensional CFD simulation using the multiphase Eulerian model incorporating the kinetic theory of the granular phase and a drag coefficient model was used to simulate the flow behavior of the gasesolid phase in the TRCL system. The simulation results capture the pressure profile over the whole system well, as can be observed in both the experimental and the computational results. Furthermore, agreement on the solid flow patterns between the simulation and the experimental data was obtained as well. The CFD simulation results support the hydrodynamic behaviors observed experimentally in the cold-mode TRCL system; the results from both are expected to provide guidelines for scale-up in the near future.
Acknowledgments This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B4-2434-05,B4-2481-14). Additionally, this work was supported by the project “Development of the design technology of a Korean 300 MWclass IGCC demonstration plant,” funded by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), affiliated with the Ministry of Knowledge Economy of the Korean Government (No.2011951010001B).
Nomenclature Cd dp Fs g g0 kQs Kgs L P,p Re Q t Ug,ar Ug,fr Ug,r Ug,sr Ug,sc Ug,arc
Conclusion Ug,frc A cold-mode three-reactor chemical looping system for hydrogen production was constructed; the operating parameters were determined according to the hydrodynamic characteristics. It was possible to control the solid flow rate by
14555
Ug,src Umf
drag coefficient mean diameter of particle, m solid flow rate, g/s acceleration of gravity, m/s2 radial distribution function diffusion coefficient for granular energy, kg/ms gas/solid momentum exchange coefficient length, m pressure, Pa Reynolds number gas velocity for leakage measurement, m/s time, s gas velocity into the air reactor, m/s gas velocity into the fuel reactor, m/s gas velocity into the riser, m/s gas velocity into the steam reactor, m/s gas velocity into the loop-seal supply chamber, m/s gas velocity into the air reactor loop-seal recycle chamber, m/s gas velocity into the fuel reactor loop-seal recycle chamber, m/s gas velocity into the steam reactor loop-seal recycle chamber, m/s minimum fluidization velocity, m/s
14556
Ut Utr v
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 9 ( 2 0 1 4 ) 1 4 5 4 6 e1 4 5 5 6
terminal velocity, m/s transport velocity, m/s velocity, m/s
Greek symbols collision dissipation of energy, kg/s3 m gQs D difference 3 g ; ag volume fraction of gas, gas hold-up 3 s ; as ; q volume fraction of solid, solid hold-up Q granular temperature, m2/s2 I unity matrix V gradient or lambda m viscosity, kg/ms r density, kg/m3 t stress tensor, Pa transfer rate of kinetic energy, kg/s3 m fgs Subscript g gas s solids
references
[1] Turner JA. Sustainable hydrogen production. Science 2004;305:972e4. [2] Koroneos C, Dompros A, Roumbas G, Moussiopoulos N. Life cycle assessment of hydrogen fuel production processes. Int J Hydrogen Energ 2004;29:1443e50. [3] Kothari R, Buddhi D, Sawhney RL. Comparison of environmental and economic aspects of various hydrogen production methods. Renew Sust Energ Rev 2008;12:553e63. [4] Cho WC, Seo MW, Kim SD, Kang KS, Bae KK, Kim CH, et al. Reactivity of iron oxide as an oxygen carrier for chemicallooping hydrogen production. Int J Hydrogen Energ 2012;37:16852e63. [5] Chiesa P, Lozza G, Malandrino A, Romano M, Piccolo V. Three-reactors chemical looping process for hydrogen production. Int J Hydrogen Energ 2008;33:2233e45. [6] Gupta P, Velazquez-Vargas LG, Fan LS. Syngas redox (SGR) process to produce hydrogen from coal derived syngas. Energ Fuel 2007;21:2900e8. [7] Fan LS, Li FX. Chemical looping technology and its fossil energy conversion applications. Ind Eng Chem Res 2010;49:10200e11. [8] Go KS, Son SR, Kim SD. Reaction kinetics of reduction and oxidation of metal oxides for hydrogen production. Int J Hydrogen Energ 2008;33:5986e95. [9] Go KS, Son SR, Kim SD, Kang KS, Park CS. Hydrogen production from two-step steam methane reforming in a fluidized bed reactor. Int J Hydrogen Energ 2009;34:1301e9. [10] Kang KS, Kim CH, Bae KK, Cho WC, Kim SH, Park CS. Oxygencarrier selection and thermal analysis of the chemicallooping process for hydrogen production. Int J Hydrogen Energ 2010;35:12246e54. [11] Kang KS, Kim CH, Bae KK, Cho WC, Jeong SU, Kim SH, et al. Modeling a counter-current moving bed for fuel and steam reactors in the TRCL process. Int J Hydrogen Energ 2012;37:3251e60.
[12] Cormos CC. Evaluation of iron based chemical looping for hydrogen and electricity co-production by gasification process with carbon capture and storage. Int J Hydrogen Energ 2010;35:2278e89. [13] Cormos CC. Hydrogen production from fossil fuels with carbon capture and storage based on chemical looping systems. Int J Hydrogen Energ 2011;36:5960e71. [14] Moghtaderi B. Review of the recent chemical looping process developments for novel energy and fuel applications. Energ Fuel 2012;26:15e40. [15] Xue ZP, Chen SY, Wang D, Xiang WG. Design and fluid dynamic analysis of a three-fluidized-bed reactor system for chemical-looping hydrogen generation. Ind Eng Chem Res 2012;51:4267e78. [16] Hong YS, Kang KS, Park CS, Kim SD, Bae JW, Nam JW, et al. Solid mass flux in a chemical-looping process for hydrogen production in a multistage circulating moving bed reactor. Int J Hydrogen Energ 2013;38:6052e8. [17] Seo MW, Nguyen TDB, Lim YI, Kim SD, Park S, Song BH, et al. Solid circulation and loop-seal characteristics of a dual circulating fluidized bed: experiments and CFD simulation. Chem Eng J 2011;168:803e11. [18] Ryu HJ, Park YC, Jo SH, Park MH. Development of novel twointerconnected fluidized bed system. Korean J Chem Eng 2008;25:1178e83. [19] Johansson E, Lyngfelt A, Mattisson T, Johnsson F. Gas leakage measurements in a cold model of an interconnected fluidized bed for chemical-looping combustion. Powder Technol 2003;134:210e7. [20] Kronberger B, Johansson E, Loffler G, Mattisson T, Lyngfelt A, Hofbauer H. A two-compartment fluidized bed reactor for CO2 capture by chemical-looping combustion. Chem Eng Technol 2004;27:1318e26. [21] Vejahati F, Mahinpey N, Ellis N, Nikoo MB. Cfd simulation of gas-solid bubbling fluidized bed: a new method for adjusting drag law. Can J Chem Eng 2009;87:19e30. [22] Almuttahar A, Taghipour F. Computational fluid dynamics of a circulating fluidized bed under various fluidization conditions. Chem Eng Sci 2008;63:1696e709. [23] Zhang N, Lu B, Wang W, Li JH. Virtual experimentation through 3D full-loop simulation of a circulating fluidized bed. Particuology 2008;6:529e39. [24] Nguyen TDB, Seo MW, Lim YI, Song BH, Kim SD. CFD simulation with experiments in a dual circulating fluidized bed gasifier. Comput Chem Eng 2012;36:48e56. [25] Cho WC, Lee DY, Seo MW, Kim SD, Kang KS, Bae KK, et al. Continuous operation characteristics of chemical looping hydrogen production system. Appl Energ 2014;113:1667e74. [26] Basu P, Butler J. Studies on the operation of loop-seal in circulating fluidized bed boilers. Appl Energ 2009;86:1723e31. [27] Cho P, Mattisson T, Lyngfelt A. Defluidization conditions for a fluidized bed of iron oxide-, nickel oxide-, and manganese oxide-containing oxygen carriers for chemical-looping combustion. Ind Eng Chem Res 2006;45:968e77. [28] Wang S, Yang YC, Lu HL, Xu PF, Sun LY. Computational fluid dynamic simulation based cluster structures-dependent drag coefficient model in dual circulating fluidized beds of chemical looping combustion. Ind Eng Chem Res 2012;51:1396e412. [29] Kaiser S, Loffler G, Bosch K, Hofbauer H. Hydrodynamics of a dual fluidized bed gasifier. Part II: simulation of solid circulation rate, pressure loop and stability. Chem Eng Sci 2003;58:4215e23.