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Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enhanced calcium looping gasification process
PTEC-14923; No of Pages 9 Powder Technology xxx (2019) xxx
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Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enhanced calcium looping gasification process Yijun Liu a, Zhao Sun a, Sam Toan b, Shiyi Chen a, Wenguo Xiang a,⁎ a b
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China Department of Chemical Engineering, University of Minnesota, Duluth, MN, USA
a r t i c l e
i n f o
Article history: Received 30 May 2019 Received in revised form 30 October 2019 Accepted 14 November 2019 Available online xxxx Keywords: Calcium looping gasification Cold flow model Scaling Segregation
a b s t r a c t This paper studies the mixing and movement of char in a dual fluidized bed for an enhanced calcium looping gasification process. The gasifier is a compact fluidized bed, consisting of a lower bubbling fluidized bed and an upper riser. The solid recirculation flux can be controlled by three loop seals. Quartz sand was used as a calcium-based sorbent and Chinese rice as the char. No flotsam behavior was observed in the bubbling fluidized bed. Experimental results reveal that the higher fluidization rate in the gasifier and the larger solid recirculation flux promoted char mixing. An increasing system char concentration decreased char mixing but led to higher char concentrations in the recirculation stream. The char holdup in the upper riser was primarily dependent on the fluidization rate in the gasifier. High solid recirculation flux with a low fluidization rate in the gasifier would benefit hot rig operation. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Climate change is a concern to people all over the world. The recent 2018 Intergovernmental Panel on Climate Change (IPCC) report encouraged the global community to limit warming to 1.5 °C above pre-industrial levels, lowering the previous target limit of 2 °C [1]. The transformation requirement to meet the new target is qualitatively similar to what was required for the previous target, but must be more pronounced, immediate and rapid. There is a consensus in the scientific community that the CO2 emissions use of carbonaceous energy is one of the prime drivers of climate change. Energy-saving, emissionsreduction and new energy sources are three ways to reduce CO2 emissions. Herein, emissions-reduction is a simple and effective way, which primarily refers to carbon capture, utilization and storage (CCUS). There are different approaches in the first step of carbon dioxide (CO2) capture: pre-combustion, post-combustion and oxycombustion. One means toward this processes intensification is chemical looping process. It offers a versatile platform to convert fuels and oxidizers in a clean and efficient manner [2–4]. Chemical looping process is not restricted to oxidation reactions, and if one uses other carrier materials, one can realize a broad reaction scope. Calcium looping process is a typical example to concentrate CO2 with CaO during precombustion and post-combustion capture. These includes fluidized bed processes for gas, solid and liquid fuels for heat, power, syngas or hydrogen production with inherent CO2 capture [5]. ⁎ Corresponding author. E-mail address: [email protected] (W. Xiang).
Calcium looping gasification process, as shown in Fig. 1, is a promising hydrogen generation technology [6–11]. CO2 sorption follows Baker's equation with calcium-based sorbent [12]. Baker's equation suggests that decreasing temperature at constant CO2 partial pressure or increasing CO2 partial pressure at constant temperature will promote further capture of CO2 by calcium oxide. For practical application, decreasing temperature is more economically feasible and more beneficial for hydrogen production than increasing pressure. The interconnected dual fluidized bed is the most common reactor type for this process [13]. Many researchers have studied different gasifier internals in an effort to form a more uniform and active gas-solid flow to enhance chemical reactions, and better heat and mass transfers to improve overall performance of fluidized bed reactors [14–20]. However, complex bed internals increase investment, operational difficulty and maintenance costs. Simple and efficient reactor design is necessary for the chemical looping process. Based on the characteristics of calcium-based sorbent, the compact fluidized bed is designed for enhanced calcium looping gasification, as shown in Fig. 2. The compact fluidized bed gasifier consists of a lower bubbling fluidized bed and an upper riser. The temperature of the lower bubbling fluidized bed (650–750 °C) is higher than that of the upper riser (550–650 °C). An evaporator wall can be installed around the upper riser to lower the temperature. Normally, CO2 is captured during the gasification process as high-purity hydrogen is generated as the resulting gas. Higher gasification temperatures in the lower bubbling fluidized bed is an advantage for fuel conversion and gas production, but is a disadvantage for CO2 capture. The exothermic process of CO2 sorption using calcium oxide benefits the endothermic gasification
https://doi.org/10.1016/j.powtec.2019.11.041 0032-5910/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Liu, Z. Sun, S. Toan, et al., Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enha..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.041
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Fig. 1. Calcium looping gasification process: Hydrogen generation with inherent carbon dioxide capture.
process. Lower temperature in the upper riser further enhance the sorption of CO2, and drive the water-gas shift reaction forward, further improving the hydrogen purity. The enhanced calcium looping coal/biomass gasification process uses solid fuel. The solid phases' in the gasifier mainly consist of char, ash and sorbent. The volatiles release rapidly during the gasification process, while the gasification of char is a slow process. This means that part of the residue char will reach the regenerator. Combustion of the unreacted char provides sufficient heat for calcium-based sorbent regeneration with high-purity CO2 capture [21]. In general, the sorbent is made up of widely used group B particles, which have a good fluidization characteristics [22]. Herein, the size of fuel particles must be determined. Small fuel particles have higher reactivity, but the milling process increases operating costs, separation difficulties and energy consumption. Large fuel particles have low reactivity. However, the solid circulation in the dual fluidized bed helps mill char particles to
smaller size. Since it is not clear if all the char is burnt in the regenerator, a certain amount may be transported back via the loop seal to the gasifier. Therefore, two type of particles would circulate in the system: larger and lighter char particles, and smaller and heavier sorbent particles. Many studies have been conducted with different type of reactors and binary bed materials and published [23–25]. A bubbling fluidized bed gasifier is widely used. It can provide high solid residence time to convert fuel. Different model particles in the cold flow model are also widely used [26–37]. For their configurations, concentration profiles, pressure profiles, segregation/separation behaviors, and particle circulation times with binary particles were investigated. The influence of different shapes of bubbling fluidized bed columns was also studied [38]. The configuration of the gasifier in this study is a compact fluidized bed, which is quite different from the gasifier described before, and it consists of a lower bubbling fluidized bed and an upper riser. Quartz sand is used as the model sorbent particle and Chinese rice is used as the model char particle. The selection of the model particles adheres to Glicksman scaling rules in the ratios of the densities and diameters of the solid particles. Various operation parameters of the dual fluidized bed reactor affect the movement of the char. The parameters include reactor fluidization rate, the solid recirculation rate, and the system char concentration. The separating and mixing characteristics of model sorbent and char particles under different operating conditions are studied in a cold flow model. The main purpose of the study was not to simulate the hydrodynamics of a hot rig, but rather to understand the operating rules of this system with binary particles. 2. Scaling and dimensionless parameters Cold flow models are widely used to study process fundamentals, including particle mixing and separating characteristics. Siddhartha et al. [39] reviewed studies on the cold flow model of dual fluidized beds. The philosophy of cold flow modeling utilizes non-dimensional analysis to provide scaling laws that can accurately represent dynamic similarity between a smaller-scale cold flow model and a corresponding larger reactor system. The cold flow model was designed and built by applying Glicksman scaling rules [40]. They non-dimensionalized the governing equations for a two-phase flow and proposed a full set of nondimensional numbers. Nevertheless, the full set is often not useful when the cold flow model is operated under ambient conditions. To allow more flexibility, Glicksman derived a simplified set of scaling parameters, with the goal of reducing the number of dimensionless parameters. The simplified set is as follow: u20 ρp Gs L1 ; ; ; ; bed geometry; ϕp ; PSD gL ρg ρp u0 L2
Fig. 2. Schematic of a compact fluidized bed designed for enhanced calcium looping gasification.
ð1Þ
ρp u20 , the Froude number. The is the density ratio between the gas gL ρg Gs is for scaling the solid recirculation and the particle phase. The ρp u0 L1 flux, Gs. The is the scaling factor. The bed geometry generally means L2 the equivalent diameter and height of the reactor. The Φp is the sphericity of particles. The PSD is the particle size distribution. The dimensionless solid circulation flux Gs is not considered in the design because the investigated system is a freely circulating fluidized bed in which the solid circulation flux is not an independent variable. In this work, the objective is to learn about the dual fluidized bed with a focus on the qualitative behavior rather than on quantitative prediction of hot unit numbers. An exact match of all dimensionless groups cannot be achieved. Therefore, the objective is to arrive at a reasonable agreement of the dimensionless groups. The most important geometric data and other design parameters of the hot rig are summarized in Table 1.
Please cite this article as: Y. Liu, Z. Sun, S. Toan, et al., Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enha..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.041
Y. Liu et al. / Powder Technology xxx (2019) xxx Table 1 Design values of the hot rig.
Table 3 Comparison of relevant dimensionless numbers.
Parameter
Units
Value
Thermal power Fuel type Mean sorbent particle diameter Sorbent particle density Operating temperature in the upper riser Operating temperature in the bubbling fluidized bed Operating temperature in the regenerator Pressure Fluidization gas in the upper riser Fluidization gas in the bubbling fluidized bed Fluidization gas in the regenerator Sorbent Mean sorbent particle diameter Sorbent particle size distribution
kWth [−] m kg/m3 °C °C °C atm [−] [−] [−] [−] m m
Sorbent particle density Gas fluidization number in the upper riser (u/ut) Gas fluidization number in the bubbling fluidized bed (u/umf) Gas fluidization number in the regenerator (u/ut) Lower-gasifier diameter Lower-gasifier height Upper-gasifier diameter Upper-gasifier height Regenerator diameter Regenerator height
kg/m3 [−] [−]
8 Coal/Biomass 4.3 × 10−3 3350 600 700 900 1 Syngas Steam Oxygen Lime/Dolomite 0.6 × 10−3 (0.5–0.7) × 10−3 3350 2.3 33
[−] m m m m m m
2 0.28 1 0.045 4 0.045 6
dp uρg ; Ar ¼ μg
3 ρg ρp −ρg dp g μ 2g
; Fr ¼
u2 ; gdp
Re, Reynolds number. Ar, Archimedes number. Fr, Froude number. DR, density ratio. RTPD, reactor to particle diameter. dp∗, dimensionless velocity. U ∗, dimensionless particle diameter. In Table 3, all relative deviations are within 20%, except Fr and density ratio. Nevertheless, it is assumed that the fluid mechanics in the cold flow model are similar to those in the hot rig. All of the reactors and parts were built from acrylic glass, and the cyclones were designed according to the design formulas of Hoffmann and Stein [41]. In the cold flow model, quartz sand and air were chosen to keepthedensityratiooflimetogasclosetothatinthehotrig. umf ofparticles can be calculated using Eq. (3) [42]. ut can be calculated using Eq. (4) [43]. umf ¼
μg d p ρg
GAHR
REHR
GACFM
RECFM
Rel. dev. GA
Rel. dev. RE
Re Ar Fr DR RTPD dp∗ U∗
3.79 1305.15 67.84 6166.67 466.67 10.93 0.35
78.6 1274.57 29,181.69 6022.22 75 10.84 7.25
4 1182.76 27.65 2046.36 510.78 10.58 0.38
89.81 1182.76 13,948.85 2046.36 88 10.58 8.49
0.06 −0.09 −0.59 −0.67 0.09 −0.03 0.09
0.14 −0.07 −0.52 −0.66 0.17 −0.02 0.17
where C1 = 27.2, C2 = 0.0408. 31 2 g 2 d ρ −ρ p p g 4 5 ut ¼ 4 3 C D ρg
ρp;char dp;char and dp;bed material ρp;bed material
ð3Þ
ð5Þ
Chinese rice (CR) was used to simulate char particles, with the specifications shown in Table 4. 1.78 mm is selected as the equivalent diameter of a CR particle. The density of a CR particle is 1350 kg/m3, so it is a Geldart group D particle. umf of Chinese rice is 0.68 m/s and ut is 7.43 m/s in the atmosphere. In the cold flow model, bed materials are quartz sand and Chinese rice, as shown in Fig. 3. Air is supplied by a roots-type blower. Geldart particle classifications [44] of char and sorbent and their model particles are shown in Fig. 4. The specific positions of the quartz sand and Chinese rice employ in the context of their Geldart classifications. Different particle diameters and densities have different flow characteristics, such as minimum fluidization velocity (umf) and terminal velocity (ut). Thus, particle diameter has large effects on flow regime. According to the structure of the compact fluidized bed gasifier, the superficial velocity in the lower bubbling fluidized bed (uBFB) must be higher than the minimum fluidization velocity (umf_sor) and lower than the terminal velocity (ut_sor) of sorbent particles. The superficial velocity in the upper riser (uUR) must be higher than ut_sor. The specifications are shown in Eqs. (6)~(7). umf
0:5 C 21 þ C 2 Ar −C 1
ð4Þ
24 18:5 , Ret b 2; C D ¼ , Ret = 2~500; CD = 0.44, Ret = Ret Re0:6 t 500~20000. The mean particle size of quartz sand is 0.25 mm, and the density is 2251 kg/m3, which is a Geldart group B particle. umf of quartz sand is 0.057 m/s and ut is 1.48 m/s in the atmosphere. The scaling of the char particles was conducted as follows. It was assumed that the ratios of the densities and diameters of the solid particles remain constant:
ð2Þ
1 ρp D Re DR ¼ ; RTPD ¼ ; dp ¼ Ar3 ; U ¼ dp ρg dp
Parameters
where C D ¼
The relationships between the hot rig and the cold flow model are listed in Table 2. The formulas of each dimensionless number in the hot rig and cold flow model in Table 3 are as follow:
Re ¼
3
ut
sor buBFB but sor
ð6Þ
sor buUR
ð7Þ
Table 2 Used design values for the cold flow model. Parameters
Units
GAHR
REHR
URHR
GACFM
RECFM
URCFM
ηg ρg u ρp dp ф D
Pa/s kg/m3 m/s kg/m3 m [−] m
3.45·10−5 0.3 0.63 1850 0.0006 0.99 0.28
4.5·10−5 0.45 13.1 2710 0.0006 0.99 0.045
[−] [−] 24.45 3350 0.0006 0.99 0.045
1.79·10−5 1.1 0.26 2251 0.00025 0.99 0.128
1.79·10−5 1.1 5.85 2251 0.00025 0.99 0.022
1.79·10−5 1.1 8.15 2251 0.00025 0.99 0.022
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Table 4 Data for sorbent and coal char.
dp, HR [mm] dp, CFM [mm] ρp, HR [kg/m3] ρp, CFM [kg/m3]
Sorbent
Coal char
Ratio
0.6 0.25 3350 2251
4.3 1.78 1800 1350
7.2 7.12 0.6 0.6
under a base operating condition (See Table 5). Other experimental conditions were designed according to the base operating condition. All measurements were conducted under stable state. The solid recirculation flux (Gs) was determined by abruptly stopping the aerations of the upper loop seal when the system was operating under a stable state. Particles accumulated in the standpipe of the loop seal. The height of the particle accumulation (h1) was measured over time (t). With known values of the solids' bulk density (ρbulk), a rough approximation of the solids recirculation flux was determined with Eq. (8) [45]. Gs ¼
ρbulk h1 Asp t Ariser
ð8Þ
where Asp is the cross-sectional area of the standpipe of the loop seal, Ariser is the cross-sectional area of the riser. A mixing factor (M) is introduced [46]. The samples were taken at the standpipe after the upper riser and regenerator. For each operation point, five samples were taken at sampling point one (M1) and sampling point two (M2), as shown in Fig. 2. Then, the samples were sieved and the masses of the Chinese rice as well as quartz sand were recorded. Then the samples were put back into the system. The Chinese rice concentration for a sample i was calculated with the following formula: C CR;i ¼
Fig. 3. Particle species used: Chinese rice and quartz sand.
mCR;i mquartz sand;i
ð9Þ
where mCR, i and mquartz sand, i are the mass of the Chinese rice and the quartz sand in the sample, respectively. The mean of all of the samples was calculated. The M is calculated: M¼
cCR;i cCR;system
ð10Þ
When M equals zero, there is no char in the solid recirculation stream. When M = 1, Chinese rice load in the solid recirculation stream is the same as for the entire system. In this work, the air relative humidity in the reactor was kept constant at 60% to reduce electrostatic effects [47]. Steam was generated by a steam generator and was added into the air. The steam flow rate was controlled by an injection pump. Gas flow rates were measured by rotor flow sensors and were controlled by ball valves. Quartz sand and Chinese rice were mixed and put into the lower bubbling fluidized bed before the operation. 4. Results and discussions Fig. 4. Geldart particle classification of sorbent and char [44].
4.1. Visual observations The flow characteristics of char particles under these operating conditions will be investigated. In the upper riser, the sorbent should be entirely fluidized up. Nevertheless, char would drop down to the lower bubbling fluidized bed. Therefore, the mixing and separating behaviors of sorbent and char under different operating conditions deserve to be studied. 3. Experiment procedure and data processing All experiments were performed at room temperature and atmospheric pressure. The system can operate with long-term stability
The construction of the cold flow model with acrylic glass allowed visual observation of fluidization behavior. Some of the model char particles accumulated on the air distribution board. The accumulation height was lower than the height of the blast cap (20 mm). No char layer was observed in any of the experiments on the bed surface in the bubbling fluidized bed. This is different from other studies [48–50], probably because of the small density difference between particles of the model char and the model sorbent. Some of the model char particles also accumulated on the air distribution board of the U-type loop seals. In each experiment, data was obtained under stable solid
Table 5 Specifications of the designed operation condition.
Aeration in supply chamber [L/h] Aeration in recycle chamber [L/h] Air flow in gasifier [m3/h] Air flow in regenerator [m3/h]
Loop seal 1
Loop seal 2
Loop seal 3
150 400 8 8
50 600 Bed height in bubbling fluidized bed [mm] Solid recirculation rate [kg/(m2s)]
100 500 400 19.7
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recirculation conditions. It is reported that the total sum of the pressure drops in the whole pressure loop should be equal to zero if the system is stable [51,52]. Therefore, pressure drops were also measured during the operation to ensure the stable state. The solid height in the standpipes of the two upper loop seals was kept within a certain range. It is noteworthy that, the particles in the recycle chamber of all loop seals were fluidized by the air, and that the particles in the standpipe were in a moving bed condition. 4.2. The influence of the fluidization rate in the reactors In this section, the influence of the fluidization rate in the gasifier on the mixing factor was investigated, as shown in Fig. 5. Regarding the effect of the fluidization rate in the regenerator, it should be noted that the system cannot operate for a long time when the superficial velocity in uRE the regenerator is lower than the terminal velocity of char b1 . ut char Otherwise, char particles accumulate, suspend and/or block the regenerator. A back-mixing and plug flow emerged in the regenerator, resulting in the failure formation of continuous solid recirculation stream in the system. Therefore, the fluidization rate in the regenerauRE tor was fixed at ¼ 1.5 in each experiment. It is also noteworthy ut char that the solid recirculation flux was adjusted primarily by three loop seals in the system [53]. The pressure drop in the bubbling fluidized bed is constant under stable state according to Eq. (11) [54]. h i ΔP BFB ¼ ρp 1−εmf þ ρ f εmf gH
ð11Þ
The pressure drop was related to the height of the bubbling fluidized bed, independent of the superficial velocity in the bubbling fluidized bed. The total solid inventory in the system is determined. The solid flux can be effectively adjusted by aeration in the supply section of the lower loop seal. However, the increase of the solid recirculation flux decreased bed height and pressure drop in the bubbling fluidized bed. In accordance to the Ergun equation [55], the increase in aeration increased pressure drops in the standpipe of the loop seals, and the increase of solid recirculation flux increased pressure drops in the riser [56–58]. The increased pressure drops in the standpipe and riser compensated for the decreased pressure drop in the bubbling fluidized bed and kept the operation stable. Variation of the fluidization rate in the gasifier changed the backpressure at the outlet of loop seal, which affected the characteristics of the loop seals' solid discharge. To maintain a stable solid recirculation
Fig. 5. Influence of the fluidization rate in the gasifier.
5
stream across the system, the aerations in the loop seals were tunable. Increasing the fluidization rate in the gasifier did not change the solid recirculation flux, due to the use of high resistance loop seals [53]. Therefore, in this work, the different solid recirculation flux was achieved by adjusting the loop seals. Model char particle agglomeration occurred with low fluidization rate in the bubbling fluidized bed and the agglomeration sank down to the bottom. The agglomeration flowed to the lower loop seal, and accumulated at the connection between supply chamber and recycle chamber, and obstructed the solid flow. Herein, the exit at the bottom of the bubbling fluidized bed was encircled by a wall (100 mm height is compared with the air distribution board), to prevent the agglomeration flowing to the lower loop seal. In Fig. 5, the increase of the fluidization rate in the gasifier increased the mixing factor in the regenerator (M1) and upper riser (M2). For low fluidization numbers, M1 and M2 had a value of about 50% and 20%, respectively, indicating aggregation of char in the bubbling fluidized bed. The bubble formation and eruption improved the mixing of the char and sorbent particles. No flotsam behavior was observed in the bubbling fluidized bed during the experiments. This was probably due to the small density difference between model char and sorbent particles. Low superficial velocity decreased formation and motion of bubbling, and therefore also decreased the mixing of the char particles in the bed. There was a large difference between M2 and M1, which indicated a separation of char and sorbent in the upper riser. As observed, some of char particles in the upper riser dropped to the bubbling fluidized bed. Both M1 and M2 have a value of about 80% at a higher fluidization rate in the gasifier. A high superficial velocity increased the bubbling motion, promoting a homogenous mixture of char and sorbent particles and flow to the regenerator. The curves in the figure come closer and closer, indicating that the particles were well-mixed in the bubbling fluidized and that there were less separations in the upper riser. For reference, in a hot rig system, a high fluidization rate in the gasifier increased the amount of char moving to the regenerator, and the char was more evenly distributed over the bed height in the bubbling fluidized bed. However, high fluidization rates obstructed the separation of char and sorbent particles in the upper riser. Large char particles were fluidized up in the upper riser and lower temperature was a disadvantage for conversion of char particles. It is also expensive to generate more steam for fluidization as well due to the high evaporation enthalpy of water. 4.3. The influence of the solid recirculation flux In this section, the effect of the solid recirculation flux on the mixing factor is shown. For each of the three char concentrations in the system (3%, 6%, 9%), a diagram with varying fluidization rates and solid
Fig. 6. Influence of the solid recirculation flux with 3% char concentration in the system.
Please cite this article as: Y. Liu, Z. Sun, S. Toan, et al., Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enha..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.041
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recirculation flux are plotted, as shown in Figs. 6–8. As discussed above, the three loop seals can control the solid recirculation flux. When uGA b3:5, M1 got higher with an increasing solid recirculation flux umf qs while M2 decreased. But the tendencies of M2 were reversed when uGA N3:5. With low superficial velocity in the upper riser, the drag umf qs force on the char particles was not great enough to elevate them up. The char particles separated with sorbent particles in the upper riser and dropped down to the bubbling fluidized bed. Therefore, the char mass flow in the upper riser was limited while the sorbent mass flow increased with an increasing solid recirculation flux. The char concentration in the upper riser fell. This resulted in M2 decreasing with an increasing solid recirculation flux and a low fluidization rate in the gasifier. It indicates that the major effect on char concentration in the upper riser is the fluidization rate in the gasifier. In above-mentioned paragraph, low fluidization rate in the gasifier was more beneficial for operating the system and saving energy. Higher solid recirculation flux leads more sorbent to the upper riser, but the char mass would be limited. In all three figures, M2 is lower than M1. The mixing factor did not vary much between the lowest and highest solid recirculation flux. The mixing factor decreased with an increasing char concentration in the system, indicating that the influence of the char concentration in the system cannot be ignored. As can be inferred from previous results, in the hot rig system, high solid recirculation flux benefits heat transfer and CO2 capture, which is an advantage for hydrogen production. High solid recirculation flux with low fluidization rate in the gasifier helped the separation of bed materials in the upper riser and conversion of char.
4.4. The influence of char concentration in the system In this section, the influence of char concentration on the mixing factor is investigated with a constant solid recirculation flux, as shown in Figs. 9–10 and 12. In each figure, the mixing factor in the regenerator or upper riser decreased with increasing char concentration in the system. The increase of mixing factor with an increasing char concentration was b30%. For a char concentration of 6% in the system, the largest M1 and M2 had values of about 80% and 50%, respectively. The lowest M1 had a value of about 40% and M2 approached to zero. This indicates that an increasing system char concentration cannot improve mixing of particles. Most of increasing char particles stayed in the bubbling
Fig. 7. Influence of the solid recirculation flux with 6% char concentration in the system.
Fig. 8. Influence of the solid recirculation flux with 9% char concentration in the system.
fluidized bed. Interestingly, for M2, the influence of char concentration in the system increased with an increasing fluidization rate in the gasifier. From the results in Fig. 10, the increase of char concentration cannot promote the mixing of char and sorbent particles in the regenerator and upper riser. Instead, it decreased the mixing. However, the char concentration in the regenerator increased with an increasing char concentration in the system, as shown in Fig. 11. The char concentration in the upper riser remained essentially unchanged with u an increasing char concentration when GA b3:5. Although larger umf qs char mass flow comes from the regenerator, mass of char particles in the upper riser is limited. Most char particles dropped down to u the bubbling fluidized bed. When GA N3:5, the tendency of the umf qs mixing factor in the upper riser is similar to the mixing factor in the regenerator. In the hot rig system, system load increases with an increasing char concentration in the system. However, the system char concentration cannot improve mixing of char and sorbent particles. Interestingly, the increase of char concentration in the system simultaneously increases the char holdup in the gasifier and regenerator. Therefore, the Ca/C/
Fig. 9. Influence of the char concentration in the system at a solid recirculation rate of 19.7 kg/(m2 s).
Please cite this article as: Y. Liu, Z. Sun, S. Toan, et al., Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enha..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.041
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7
Fig. 10. Influence of the char concentration in the system at a solid recirculation rate of 32.2 kg/(m2 s).
Fig. 12. Influence of the char concentration in the system at a solid recirculation rate of 47.8 kg/(m2 s).
steam ratio should be considered to select a suitable fuel ratio in the hot rig system.
• The fluidization rate in the regenerator should be high in order to fluidize all particles upward. Otherwise the system cannot operate for a long time. The large char particles' accumulation and blocking would destroy the stable solid recirculation stream in the system. • High fluidization rate in the gasifier promotes the mixing of particles and enhances the char flow to the regenerator. However, this obstructs the separation of char and sorbent in the upper riser and leads char to flow up to the upper riser. High fluidization rate in the gasifier means that more steam has to be generated using additional energy. • High solid recirculation flux improves the mixing of particles, but the effect is limited. The effect of solid recirculation flux on the mixing factor is reversed with an increasing fluidization rate in the gasifier. • Higher system char concentration enhances char holdup in the regenerator. However, it cannot improve the mixing of particles, and most of the char stays in the bubbling fluidized bed.
5. Conclusions This work discussed char movement in the dual fluidized bed system. The cold flow model was scaled with Glicksman's simplified set of similarity rules. Two model particles were used to investigate the mixing and segregation behavior of char and sorbent particles. It cannot give an accurate quantitative predication of hydrodynamic, but can give a similar qualitative behavior in the system. Quartz sand was used as the model sorbent particles and Chinese rice was used as the model char particles. The gasifier consists of a lower bubbling fluidized bed and an upper riser. There are three loop seals to control the solid recirculation flux, a riser to pneumatically convey the solids upward, two cyclones for gas-solid separations. During the operations, solid samples were taken to investigate the amount of char in the solid recirculation stream. According to the results, the following conclusions can be reached for the hot rig:
In conclusion, the fluidization rate in the gasifier is the main influence on the mixing of particles. Increase of solid recirculation flux benefits hydrogen production, CO2 capture and heat transfer. Therefore, high solid recirculation flux with a relatively low fluidization rate in the gasifier is optimal. Variables
Fig. 11. Char concentration at sampling points at a solid recirculation rate of 32.2 kg/(m2 s).
Symbol
Meaning (SI units)
A Ar CD C D dp dp∗ Fr g Gs h L m M Q Re t U∗
Area (m2) Archimedes number ([−]) Drag coefficient ([−]) Concentration ([−]) Equivalent diameter (m) Particle diameter (m) Dimensionless particle diameter ([−]) Froude number ([−]) Gravitational acceleration (m/s2) Solid circulation flux (kg/(m2s)) Height (m) Characteristic length (m) Mass (g) Mixing factor ([−]) Gas flow (m3/h) Particle Reynolds number ([−]) Time (s) Dimensionless velocity ([−])
Please cite this article as: Y. Liu, Z. Sun, S. Toan, et al., Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enha..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.041
8
Y. Liu et al. / Powder Technology xxx (2019) xxx
u umf ut utf
Superficial velocity (m/s) Minimum fluidization velocity (m/s) Terminal velocity (m/s) Fast fluidization velocity (m/s)
Greek symbols εs
Voidage ([−])
ηg ρp ρg ρbulk Φp
Gas viscosity (kg/(m·s)) Particle density (kg/m3) Gas density (kg/m3) Solid bulk density (kg/m3) Sphericity ([−])
Acknowledgements The authors gratefully acknowledge the National Key Research and Development Program of China (2018YFB0605403) and National Natural Science Foundation of China (51576042), for financial support of this work. Declarations of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] J. Rogelj, D. Shindell, K. Jiang, S. Fifita, P. Forster, V. Ginzburg, C. Handa, H. Kheshgi, S. Kobayashi, E. Kriegler, L. Mundaca, R. Séférian, M.V. Vilariño, The Intergovernmental Panel on Climate Change (IPCC), 2018. [2] S.D. Kenarsari, D.L. Yang, G.D. Jiang, S.J. Zhang, J.J. Wang, A.G. Russell, Q. Wei, M.H. Fan, Review of recent advances in carbon dioxide separation and capture, RSC Adv. 3 (2013), 22739. [3] L. Zeng, Z. Cheng, J.A. Fan, L.S. Fan, J.L. Gong, Metal oxide redox chemistryfor chemical looping processes, Nat. Rev. Chem. 2 (2018) 349–364. [4] L.S. Fan, Chemical Looping Systems for Fossil Energy Conversions, A John Wiley & Sons, Inc., New York, 2010. [5] T. Mattisson, M. Keller, C. Linderholm, P. Moldenhauer, M. Ryden, H. Leion, A. Lyngfelt, Chemical-looping technologies using circulating fluidized bed systems: status of development, Fuel Process. Technol. 172 (2018) 1–12. [6] L.S. Fan, F.X. Li, S. Ramkumar, Utilization of chemical looping strategy in coal gasification processes, Particuology 6 (2008) 131–142. [7] N. Phalak, W. Wang, L.S. Fan, Ca(OH)2-based calcium looping process development at the Ohio State University, Chem. Eng. Technol. 36 (9) (2013) 1451–1459. [8] S. Ramkumar, L.S. Fan, Calcium looping process (CLP) for enhanced noncatalytic hydrogen production with integrated carbon dioxide capture, Energ. Fuel 24 (2010) 4408–4418. [9] D.P. Connell, D.A. Lewandowski, S. Ramkumar, N. Phalak, R.M. Statnick, L.S. Fan, Process simulation and economic analysis of the calcium looping process (CLP) for hydrogen and electricity production from coal and natural gas, Fuel 105 (2013) 383–396. [10] S.Y. Lin, M. Harada, Y. Suzuki, H. Hatano, Process analysis for hydrogen production by reaction integrated novel gasification (HyPr-RING), Energ. Convers. Manage. 46 (2005) 869–880. [11] T. Weimer, R. Berger, C. Hawthorne, J.C. Abanades, Lime enhanced gasification of solid fuels: examination of a process for simultaneous hydrogen production and CO2 capture, Fuel 87 (2008) 1678–1686. [12] J.R. Li, Y. Ma, M.C. McCarthy, J. Sculley, J. Yu, H.K. Jeong, P.B. Balbuena, H.C. Zhou, Carbon dioxide capture-related gas adsorption and separation in metal-organic frameworks, Coordin. Chem. Rev. 255 (2011) 1791–1823. [13] T. Song, L.H. Shen, Review of reactor for chemical looping combustion of solid fuels, Int. J. Greenh. Gas Con. 76 (2018) 92–110. [14] Y. Jin, F. Wei, Y. Wang, Effects of internal tubes and baffles, Handbook of Fluidization and Fluid-Particle Systems, Marcel Dekker Inc 2003, pp. 171–199. [15] A. Tong, S. Bayham, M.V. Kathe, L. Zeng, S. Luo, L.S. Fan, Iron-based syngas chemical looping process and coal-direct chemical looping process development at Ohio state university, Appl. Energ. 113 (2014) 1836–1845. [16] T. Pröll, H. Hofbauer, A dual fluidized bed system for chemical looping combustion of solid fuels, 10th AIChE Annual Meeting, Salt Lake City, Utah, 2010. [17] A. Abad, T. Mendiara, P. Gayán, F. García-Labiano, L.F. de Diego, J.A. Bueno, R. PérezVega, J. Adánez, Comparative evaluation of the performance of coal combustion in 0.5 and 50 kWth chemical looping combustion units with ilmenite, red mud or iron ore as oxygen carrier, Energy Procedia 114 (2017) 285–301. [18] L. Shen, J. Wu, J. Xiao, Experiments on chemical looping combustion of coal with a NiO based oxygen carrier, Combust. Flame 156 (2009) 721–728.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enhanced calcium looping gasification process Yijun Liu a, Zhao Sun a, Sam Toan b, Shiyi Chen a, Wenguo Xiang a,⁎ a b
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China Department of Chemical Engineering, University of Minnesota, Duluth, MN, USA
a r t i c l e
i n f o
Article history: Received 30 May 2019 Received in revised form 30 October 2019 Accepted 14 November 2019 Available online xxxx Keywords: Calcium looping gasification Cold flow model Scaling Segregation
a b s t r a c t This paper studies the mixing and movement of char in a dual fluidized bed for an enhanced calcium looping gasification process. The gasifier is a compact fluidized bed, consisting of a lower bubbling fluidized bed and an upper riser. The solid recirculation flux can be controlled by three loop seals. Quartz sand was used as a calcium-based sorbent and Chinese rice as the char. No flotsam behavior was observed in the bubbling fluidized bed. Experimental results reveal that the higher fluidization rate in the gasifier and the larger solid recirculation flux promoted char mixing. An increasing system char concentration decreased char mixing but led to higher char concentrations in the recirculation stream. The char holdup in the upper riser was primarily dependent on the fluidization rate in the gasifier. High solid recirculation flux with a low fluidization rate in the gasifier would benefit hot rig operation. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Climate change is a concern to people all over the world. The recent 2018 Intergovernmental Panel on Climate Change (IPCC) report encouraged the global community to limit warming to 1.5 °C above pre-industrial levels, lowering the previous target limit of 2 °C [1]. The transformation requirement to meet the new target is qualitatively similar to what was required for the previous target, but must be more pronounced, immediate and rapid. There is a consensus in the scientific community that the CO2 emissions use of carbonaceous energy is one of the prime drivers of climate change. Energy-saving, emissionsreduction and new energy sources are three ways to reduce CO2 emissions. Herein, emissions-reduction is a simple and effective way, which primarily refers to carbon capture, utilization and storage (CCUS). There are different approaches in the first step of carbon dioxide (CO2) capture: pre-combustion, post-combustion and oxycombustion. One means toward this processes intensification is chemical looping process. It offers a versatile platform to convert fuels and oxidizers in a clean and efficient manner [2–4]. Chemical looping process is not restricted to oxidation reactions, and if one uses other carrier materials, one can realize a broad reaction scope. Calcium looping process is a typical example to concentrate CO2 with CaO during precombustion and post-combustion capture. These includes fluidized bed processes for gas, solid and liquid fuels for heat, power, syngas or hydrogen production with inherent CO2 capture [5]. ⁎ Corresponding author. E-mail address: [email protected] (W. Xiang).
Calcium looping gasification process, as shown in Fig. 1, is a promising hydrogen generation technology [6–11]. CO2 sorption follows Baker's equation with calcium-based sorbent [12]. Baker's equation suggests that decreasing temperature at constant CO2 partial pressure or increasing CO2 partial pressure at constant temperature will promote further capture of CO2 by calcium oxide. For practical application, decreasing temperature is more economically feasible and more beneficial for hydrogen production than increasing pressure. The interconnected dual fluidized bed is the most common reactor type for this process [13]. Many researchers have studied different gasifier internals in an effort to form a more uniform and active gas-solid flow to enhance chemical reactions, and better heat and mass transfers to improve overall performance of fluidized bed reactors [14–20]. However, complex bed internals increase investment, operational difficulty and maintenance costs. Simple and efficient reactor design is necessary for the chemical looping process. Based on the characteristics of calcium-based sorbent, the compact fluidized bed is designed for enhanced calcium looping gasification, as shown in Fig. 2. The compact fluidized bed gasifier consists of a lower bubbling fluidized bed and an upper riser. The temperature of the lower bubbling fluidized bed (650–750 °C) is higher than that of the upper riser (550–650 °C). An evaporator wall can be installed around the upper riser to lower the temperature. Normally, CO2 is captured during the gasification process as high-purity hydrogen is generated as the resulting gas. Higher gasification temperatures in the lower bubbling fluidized bed is an advantage for fuel conversion and gas production, but is a disadvantage for CO2 capture. The exothermic process of CO2 sorption using calcium oxide benefits the endothermic gasification
https://doi.org/10.1016/j.powtec.2019.11.041 0032-5910/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Liu, Z. Sun, S. Toan, et al., Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enha..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.041
2
Y. Liu et al. / Powder Technology xxx (2019) xxx
Fig. 1. Calcium looping gasification process: Hydrogen generation with inherent carbon dioxide capture.
process. Lower temperature in the upper riser further enhance the sorption of CO2, and drive the water-gas shift reaction forward, further improving the hydrogen purity. The enhanced calcium looping coal/biomass gasification process uses solid fuel. The solid phases' in the gasifier mainly consist of char, ash and sorbent. The volatiles release rapidly during the gasification process, while the gasification of char is a slow process. This means that part of the residue char will reach the regenerator. Combustion of the unreacted char provides sufficient heat for calcium-based sorbent regeneration with high-purity CO2 capture [21]. In general, the sorbent is made up of widely used group B particles, which have a good fluidization characteristics [22]. Herein, the size of fuel particles must be determined. Small fuel particles have higher reactivity, but the milling process increases operating costs, separation difficulties and energy consumption. Large fuel particles have low reactivity. However, the solid circulation in the dual fluidized bed helps mill char particles to
smaller size. Since it is not clear if all the char is burnt in the regenerator, a certain amount may be transported back via the loop seal to the gasifier. Therefore, two type of particles would circulate in the system: larger and lighter char particles, and smaller and heavier sorbent particles. Many studies have been conducted with different type of reactors and binary bed materials and published [23–25]. A bubbling fluidized bed gasifier is widely used. It can provide high solid residence time to convert fuel. Different model particles in the cold flow model are also widely used [26–37]. For their configurations, concentration profiles, pressure profiles, segregation/separation behaviors, and particle circulation times with binary particles were investigated. The influence of different shapes of bubbling fluidized bed columns was also studied [38]. The configuration of the gasifier in this study is a compact fluidized bed, which is quite different from the gasifier described before, and it consists of a lower bubbling fluidized bed and an upper riser. Quartz sand is used as the model sorbent particle and Chinese rice is used as the model char particle. The selection of the model particles adheres to Glicksman scaling rules in the ratios of the densities and diameters of the solid particles. Various operation parameters of the dual fluidized bed reactor affect the movement of the char. The parameters include reactor fluidization rate, the solid recirculation rate, and the system char concentration. The separating and mixing characteristics of model sorbent and char particles under different operating conditions are studied in a cold flow model. The main purpose of the study was not to simulate the hydrodynamics of a hot rig, but rather to understand the operating rules of this system with binary particles. 2. Scaling and dimensionless parameters Cold flow models are widely used to study process fundamentals, including particle mixing and separating characteristics. Siddhartha et al. [39] reviewed studies on the cold flow model of dual fluidized beds. The philosophy of cold flow modeling utilizes non-dimensional analysis to provide scaling laws that can accurately represent dynamic similarity between a smaller-scale cold flow model and a corresponding larger reactor system. The cold flow model was designed and built by applying Glicksman scaling rules [40]. They non-dimensionalized the governing equations for a two-phase flow and proposed a full set of nondimensional numbers. Nevertheless, the full set is often not useful when the cold flow model is operated under ambient conditions. To allow more flexibility, Glicksman derived a simplified set of scaling parameters, with the goal of reducing the number of dimensionless parameters. The simplified set is as follow: u20 ρp Gs L1 ; ; ; ; bed geometry; ϕp ; PSD gL ρg ρp u0 L2
Fig. 2. Schematic of a compact fluidized bed designed for enhanced calcium looping gasification.
ð1Þ
ρp u20 , the Froude number. The is the density ratio between the gas gL ρg Gs is for scaling the solid recirculation and the particle phase. The ρp u0 L1 flux, Gs. The is the scaling factor. The bed geometry generally means L2 the equivalent diameter and height of the reactor. The Φp is the sphericity of particles. The PSD is the particle size distribution. The dimensionless solid circulation flux Gs is not considered in the design because the investigated system is a freely circulating fluidized bed in which the solid circulation flux is not an independent variable. In this work, the objective is to learn about the dual fluidized bed with a focus on the qualitative behavior rather than on quantitative prediction of hot unit numbers. An exact match of all dimensionless groups cannot be achieved. Therefore, the objective is to arrive at a reasonable agreement of the dimensionless groups. The most important geometric data and other design parameters of the hot rig are summarized in Table 1.
Please cite this article as: Y. Liu, Z. Sun, S. Toan, et al., Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enha..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.041
Y. Liu et al. / Powder Technology xxx (2019) xxx Table 1 Design values of the hot rig.
Table 3 Comparison of relevant dimensionless numbers.
Parameter
Units
Value
Thermal power Fuel type Mean sorbent particle diameter Sorbent particle density Operating temperature in the upper riser Operating temperature in the bubbling fluidized bed Operating temperature in the regenerator Pressure Fluidization gas in the upper riser Fluidization gas in the bubbling fluidized bed Fluidization gas in the regenerator Sorbent Mean sorbent particle diameter Sorbent particle size distribution
kWth [−] m kg/m3 °C °C °C atm [−] [−] [−] [−] m m
Sorbent particle density Gas fluidization number in the upper riser (u/ut) Gas fluidization number in the bubbling fluidized bed (u/umf) Gas fluidization number in the regenerator (u/ut) Lower-gasifier diameter Lower-gasifier height Upper-gasifier diameter Upper-gasifier height Regenerator diameter Regenerator height
kg/m3 [−] [−]
8 Coal/Biomass 4.3 × 10−3 3350 600 700 900 1 Syngas Steam Oxygen Lime/Dolomite 0.6 × 10−3 (0.5–0.7) × 10−3 3350 2.3 33
[−] m m m m m m
2 0.28 1 0.045 4 0.045 6
dp uρg ; Ar ¼ μg
3 ρg ρp −ρg dp g μ 2g
; Fr ¼
u2 ; gdp
Re, Reynolds number. Ar, Archimedes number. Fr, Froude number. DR, density ratio. RTPD, reactor to particle diameter. dp∗, dimensionless velocity. U ∗, dimensionless particle diameter. In Table 3, all relative deviations are within 20%, except Fr and density ratio. Nevertheless, it is assumed that the fluid mechanics in the cold flow model are similar to those in the hot rig. All of the reactors and parts were built from acrylic glass, and the cyclones were designed according to the design formulas of Hoffmann and Stein [41]. In the cold flow model, quartz sand and air were chosen to keepthedensityratiooflimetogasclosetothatinthehotrig. umf ofparticles can be calculated using Eq. (3) [42]. ut can be calculated using Eq. (4) [43]. umf ¼
μg d p ρg
GAHR
REHR
GACFM
RECFM
Rel. dev. GA
Rel. dev. RE
Re Ar Fr DR RTPD dp∗ U∗
3.79 1305.15 67.84 6166.67 466.67 10.93 0.35
78.6 1274.57 29,181.69 6022.22 75 10.84 7.25
4 1182.76 27.65 2046.36 510.78 10.58 0.38
89.81 1182.76 13,948.85 2046.36 88 10.58 8.49
0.06 −0.09 −0.59 −0.67 0.09 −0.03 0.09
0.14 −0.07 −0.52 −0.66 0.17 −0.02 0.17
where C1 = 27.2, C2 = 0.0408. 31 2 g 2 d ρ −ρ p p g 4 5 ut ¼ 4 3 C D ρg
ρp;char dp;char and dp;bed material ρp;bed material
ð3Þ
ð5Þ
Chinese rice (CR) was used to simulate char particles, with the specifications shown in Table 4. 1.78 mm is selected as the equivalent diameter of a CR particle. The density of a CR particle is 1350 kg/m3, so it is a Geldart group D particle. umf of Chinese rice is 0.68 m/s and ut is 7.43 m/s in the atmosphere. In the cold flow model, bed materials are quartz sand and Chinese rice, as shown in Fig. 3. Air is supplied by a roots-type blower. Geldart particle classifications [44] of char and sorbent and their model particles are shown in Fig. 4. The specific positions of the quartz sand and Chinese rice employ in the context of their Geldart classifications. Different particle diameters and densities have different flow characteristics, such as minimum fluidization velocity (umf) and terminal velocity (ut). Thus, particle diameter has large effects on flow regime. According to the structure of the compact fluidized bed gasifier, the superficial velocity in the lower bubbling fluidized bed (uBFB) must be higher than the minimum fluidization velocity (umf_sor) and lower than the terminal velocity (ut_sor) of sorbent particles. The superficial velocity in the upper riser (uUR) must be higher than ut_sor. The specifications are shown in Eqs. (6)~(7). umf
0:5 C 21 þ C 2 Ar −C 1
ð4Þ
24 18:5 , Ret b 2; C D ¼ , Ret = 2~500; CD = 0.44, Ret = Ret Re0:6 t 500~20000. The mean particle size of quartz sand is 0.25 mm, and the density is 2251 kg/m3, which is a Geldart group B particle. umf of quartz sand is 0.057 m/s and ut is 1.48 m/s in the atmosphere. The scaling of the char particles was conducted as follows. It was assumed that the ratios of the densities and diameters of the solid particles remain constant:
ð2Þ
1 ρp D Re DR ¼ ; RTPD ¼ ; dp ¼ Ar3 ; U ¼ dp ρg dp
Parameters
where C D ¼
The relationships between the hot rig and the cold flow model are listed in Table 2. The formulas of each dimensionless number in the hot rig and cold flow model in Table 3 are as follow:
Re ¼
3
ut
sor buBFB but sor
ð6Þ
sor buUR
ð7Þ
Table 2 Used design values for the cold flow model. Parameters
Units
GAHR
REHR
URHR
GACFM
RECFM
URCFM
ηg ρg u ρp dp ф D
Pa/s kg/m3 m/s kg/m3 m [−] m
3.45·10−5 0.3 0.63 1850 0.0006 0.99 0.28
4.5·10−5 0.45 13.1 2710 0.0006 0.99 0.045
[−] [−] 24.45 3350 0.0006 0.99 0.045
1.79·10−5 1.1 0.26 2251 0.00025 0.99 0.128
1.79·10−5 1.1 5.85 2251 0.00025 0.99 0.022
1.79·10−5 1.1 8.15 2251 0.00025 0.99 0.022
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Y. Liu et al. / Powder Technology xxx (2019) xxx
Table 4 Data for sorbent and coal char.
dp, HR [mm] dp, CFM [mm] ρp, HR [kg/m3] ρp, CFM [kg/m3]
Sorbent
Coal char
Ratio
0.6 0.25 3350 2251
4.3 1.78 1800 1350
7.2 7.12 0.6 0.6
under a base operating condition (See Table 5). Other experimental conditions were designed according to the base operating condition. All measurements were conducted under stable state. The solid recirculation flux (Gs) was determined by abruptly stopping the aerations of the upper loop seal when the system was operating under a stable state. Particles accumulated in the standpipe of the loop seal. The height of the particle accumulation (h1) was measured over time (t). With known values of the solids' bulk density (ρbulk), a rough approximation of the solids recirculation flux was determined with Eq. (8) [45]. Gs ¼
ρbulk h1 Asp t Ariser
ð8Þ
where Asp is the cross-sectional area of the standpipe of the loop seal, Ariser is the cross-sectional area of the riser. A mixing factor (M) is introduced [46]. The samples were taken at the standpipe after the upper riser and regenerator. For each operation point, five samples were taken at sampling point one (M1) and sampling point two (M2), as shown in Fig. 2. Then, the samples were sieved and the masses of the Chinese rice as well as quartz sand were recorded. Then the samples were put back into the system. The Chinese rice concentration for a sample i was calculated with the following formula: C CR;i ¼
Fig. 3. Particle species used: Chinese rice and quartz sand.
mCR;i mquartz sand;i
ð9Þ
where mCR, i and mquartz sand, i are the mass of the Chinese rice and the quartz sand in the sample, respectively. The mean of all of the samples was calculated. The M is calculated: M¼
cCR;i cCR;system
ð10Þ
When M equals zero, there is no char in the solid recirculation stream. When M = 1, Chinese rice load in the solid recirculation stream is the same as for the entire system. In this work, the air relative humidity in the reactor was kept constant at 60% to reduce electrostatic effects [47]. Steam was generated by a steam generator and was added into the air. The steam flow rate was controlled by an injection pump. Gas flow rates were measured by rotor flow sensors and were controlled by ball valves. Quartz sand and Chinese rice were mixed and put into the lower bubbling fluidized bed before the operation. 4. Results and discussions Fig. 4. Geldart particle classification of sorbent and char [44].
4.1. Visual observations The flow characteristics of char particles under these operating conditions will be investigated. In the upper riser, the sorbent should be entirely fluidized up. Nevertheless, char would drop down to the lower bubbling fluidized bed. Therefore, the mixing and separating behaviors of sorbent and char under different operating conditions deserve to be studied. 3. Experiment procedure and data processing All experiments were performed at room temperature and atmospheric pressure. The system can operate with long-term stability
The construction of the cold flow model with acrylic glass allowed visual observation of fluidization behavior. Some of the model char particles accumulated on the air distribution board. The accumulation height was lower than the height of the blast cap (20 mm). No char layer was observed in any of the experiments on the bed surface in the bubbling fluidized bed. This is different from other studies [48–50], probably because of the small density difference between particles of the model char and the model sorbent. Some of the model char particles also accumulated on the air distribution board of the U-type loop seals. In each experiment, data was obtained under stable solid
Table 5 Specifications of the designed operation condition.
Aeration in supply chamber [L/h] Aeration in recycle chamber [L/h] Air flow in gasifier [m3/h] Air flow in regenerator [m3/h]
Loop seal 1
Loop seal 2
Loop seal 3
150 400 8 8
50 600 Bed height in bubbling fluidized bed [mm] Solid recirculation rate [kg/(m2s)]
100 500 400 19.7
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Y. Liu et al. / Powder Technology xxx (2019) xxx
recirculation conditions. It is reported that the total sum of the pressure drops in the whole pressure loop should be equal to zero if the system is stable [51,52]. Therefore, pressure drops were also measured during the operation to ensure the stable state. The solid height in the standpipes of the two upper loop seals was kept within a certain range. It is noteworthy that, the particles in the recycle chamber of all loop seals were fluidized by the air, and that the particles in the standpipe were in a moving bed condition. 4.2. The influence of the fluidization rate in the reactors In this section, the influence of the fluidization rate in the gasifier on the mixing factor was investigated, as shown in Fig. 5. Regarding the effect of the fluidization rate in the regenerator, it should be noted that the system cannot operate for a long time when the superficial velocity in uRE the regenerator is lower than the terminal velocity of char b1 . ut char Otherwise, char particles accumulate, suspend and/or block the regenerator. A back-mixing and plug flow emerged in the regenerator, resulting in the failure formation of continuous solid recirculation stream in the system. Therefore, the fluidization rate in the regenerauRE tor was fixed at ¼ 1.5 in each experiment. It is also noteworthy ut char that the solid recirculation flux was adjusted primarily by three loop seals in the system [53]. The pressure drop in the bubbling fluidized bed is constant under stable state according to Eq. (11) [54]. h i ΔP BFB ¼ ρp 1−εmf þ ρ f εmf gH
ð11Þ
The pressure drop was related to the height of the bubbling fluidized bed, independent of the superficial velocity in the bubbling fluidized bed. The total solid inventory in the system is determined. The solid flux can be effectively adjusted by aeration in the supply section of the lower loop seal. However, the increase of the solid recirculation flux decreased bed height and pressure drop in the bubbling fluidized bed. In accordance to the Ergun equation [55], the increase in aeration increased pressure drops in the standpipe of the loop seals, and the increase of solid recirculation flux increased pressure drops in the riser [56–58]. The increased pressure drops in the standpipe and riser compensated for the decreased pressure drop in the bubbling fluidized bed and kept the operation stable. Variation of the fluidization rate in the gasifier changed the backpressure at the outlet of loop seal, which affected the characteristics of the loop seals' solid discharge. To maintain a stable solid recirculation
Fig. 5. Influence of the fluidization rate in the gasifier.
5
stream across the system, the aerations in the loop seals were tunable. Increasing the fluidization rate in the gasifier did not change the solid recirculation flux, due to the use of high resistance loop seals [53]. Therefore, in this work, the different solid recirculation flux was achieved by adjusting the loop seals. Model char particle agglomeration occurred with low fluidization rate in the bubbling fluidized bed and the agglomeration sank down to the bottom. The agglomeration flowed to the lower loop seal, and accumulated at the connection between supply chamber and recycle chamber, and obstructed the solid flow. Herein, the exit at the bottom of the bubbling fluidized bed was encircled by a wall (100 mm height is compared with the air distribution board), to prevent the agglomeration flowing to the lower loop seal. In Fig. 5, the increase of the fluidization rate in the gasifier increased the mixing factor in the regenerator (M1) and upper riser (M2). For low fluidization numbers, M1 and M2 had a value of about 50% and 20%, respectively, indicating aggregation of char in the bubbling fluidized bed. The bubble formation and eruption improved the mixing of the char and sorbent particles. No flotsam behavior was observed in the bubbling fluidized bed during the experiments. This was probably due to the small density difference between model char and sorbent particles. Low superficial velocity decreased formation and motion of bubbling, and therefore also decreased the mixing of the char particles in the bed. There was a large difference between M2 and M1, which indicated a separation of char and sorbent in the upper riser. As observed, some of char particles in the upper riser dropped to the bubbling fluidized bed. Both M1 and M2 have a value of about 80% at a higher fluidization rate in the gasifier. A high superficial velocity increased the bubbling motion, promoting a homogenous mixture of char and sorbent particles and flow to the regenerator. The curves in the figure come closer and closer, indicating that the particles were well-mixed in the bubbling fluidized and that there were less separations in the upper riser. For reference, in a hot rig system, a high fluidization rate in the gasifier increased the amount of char moving to the regenerator, and the char was more evenly distributed over the bed height in the bubbling fluidized bed. However, high fluidization rates obstructed the separation of char and sorbent particles in the upper riser. Large char particles were fluidized up in the upper riser and lower temperature was a disadvantage for conversion of char particles. It is also expensive to generate more steam for fluidization as well due to the high evaporation enthalpy of water. 4.3. The influence of the solid recirculation flux In this section, the effect of the solid recirculation flux on the mixing factor is shown. For each of the three char concentrations in the system (3%, 6%, 9%), a diagram with varying fluidization rates and solid
Fig. 6. Influence of the solid recirculation flux with 3% char concentration in the system.
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Y. Liu et al. / Powder Technology xxx (2019) xxx
recirculation flux are plotted, as shown in Figs. 6–8. As discussed above, the three loop seals can control the solid recirculation flux. When uGA b3:5, M1 got higher with an increasing solid recirculation flux umf qs while M2 decreased. But the tendencies of M2 were reversed when uGA N3:5. With low superficial velocity in the upper riser, the drag umf qs force on the char particles was not great enough to elevate them up. The char particles separated with sorbent particles in the upper riser and dropped down to the bubbling fluidized bed. Therefore, the char mass flow in the upper riser was limited while the sorbent mass flow increased with an increasing solid recirculation flux. The char concentration in the upper riser fell. This resulted in M2 decreasing with an increasing solid recirculation flux and a low fluidization rate in the gasifier. It indicates that the major effect on char concentration in the upper riser is the fluidization rate in the gasifier. In above-mentioned paragraph, low fluidization rate in the gasifier was more beneficial for operating the system and saving energy. Higher solid recirculation flux leads more sorbent to the upper riser, but the char mass would be limited. In all three figures, M2 is lower than M1. The mixing factor did not vary much between the lowest and highest solid recirculation flux. The mixing factor decreased with an increasing char concentration in the system, indicating that the influence of the char concentration in the system cannot be ignored. As can be inferred from previous results, in the hot rig system, high solid recirculation flux benefits heat transfer and CO2 capture, which is an advantage for hydrogen production. High solid recirculation flux with low fluidization rate in the gasifier helped the separation of bed materials in the upper riser and conversion of char.
4.4. The influence of char concentration in the system In this section, the influence of char concentration on the mixing factor is investigated with a constant solid recirculation flux, as shown in Figs. 9–10 and 12. In each figure, the mixing factor in the regenerator or upper riser decreased with increasing char concentration in the system. The increase of mixing factor with an increasing char concentration was b30%. For a char concentration of 6% in the system, the largest M1 and M2 had values of about 80% and 50%, respectively. The lowest M1 had a value of about 40% and M2 approached to zero. This indicates that an increasing system char concentration cannot improve mixing of particles. Most of increasing char particles stayed in the bubbling
Fig. 7. Influence of the solid recirculation flux with 6% char concentration in the system.
Fig. 8. Influence of the solid recirculation flux with 9% char concentration in the system.
fluidized bed. Interestingly, for M2, the influence of char concentration in the system increased with an increasing fluidization rate in the gasifier. From the results in Fig. 10, the increase of char concentration cannot promote the mixing of char and sorbent particles in the regenerator and upper riser. Instead, it decreased the mixing. However, the char concentration in the regenerator increased with an increasing char concentration in the system, as shown in Fig. 11. The char concentration in the upper riser remained essentially unchanged with u an increasing char concentration when GA b3:5. Although larger umf qs char mass flow comes from the regenerator, mass of char particles in the upper riser is limited. Most char particles dropped down to u the bubbling fluidized bed. When GA N3:5, the tendency of the umf qs mixing factor in the upper riser is similar to the mixing factor in the regenerator. In the hot rig system, system load increases with an increasing char concentration in the system. However, the system char concentration cannot improve mixing of char and sorbent particles. Interestingly, the increase of char concentration in the system simultaneously increases the char holdup in the gasifier and regenerator. Therefore, the Ca/C/
Fig. 9. Influence of the char concentration in the system at a solid recirculation rate of 19.7 kg/(m2 s).
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Fig. 10. Influence of the char concentration in the system at a solid recirculation rate of 32.2 kg/(m2 s).
Fig. 12. Influence of the char concentration in the system at a solid recirculation rate of 47.8 kg/(m2 s).
steam ratio should be considered to select a suitable fuel ratio in the hot rig system.
• The fluidization rate in the regenerator should be high in order to fluidize all particles upward. Otherwise the system cannot operate for a long time. The large char particles' accumulation and blocking would destroy the stable solid recirculation stream in the system. • High fluidization rate in the gasifier promotes the mixing of particles and enhances the char flow to the regenerator. However, this obstructs the separation of char and sorbent in the upper riser and leads char to flow up to the upper riser. High fluidization rate in the gasifier means that more steam has to be generated using additional energy. • High solid recirculation flux improves the mixing of particles, but the effect is limited. The effect of solid recirculation flux on the mixing factor is reversed with an increasing fluidization rate in the gasifier. • Higher system char concentration enhances char holdup in the regenerator. However, it cannot improve the mixing of particles, and most of the char stays in the bubbling fluidized bed.
5. Conclusions This work discussed char movement in the dual fluidized bed system. The cold flow model was scaled with Glicksman's simplified set of similarity rules. Two model particles were used to investigate the mixing and segregation behavior of char and sorbent particles. It cannot give an accurate quantitative predication of hydrodynamic, but can give a similar qualitative behavior in the system. Quartz sand was used as the model sorbent particles and Chinese rice was used as the model char particles. The gasifier consists of a lower bubbling fluidized bed and an upper riser. There are three loop seals to control the solid recirculation flux, a riser to pneumatically convey the solids upward, two cyclones for gas-solid separations. During the operations, solid samples were taken to investigate the amount of char in the solid recirculation stream. According to the results, the following conclusions can be reached for the hot rig:
In conclusion, the fluidization rate in the gasifier is the main influence on the mixing of particles. Increase of solid recirculation flux benefits hydrogen production, CO2 capture and heat transfer. Therefore, high solid recirculation flux with a relatively low fluidization rate in the gasifier is optimal. Variables
Fig. 11. Char concentration at sampling points at a solid recirculation rate of 32.2 kg/(m2 s).
Symbol
Meaning (SI units)
A Ar CD C D dp dp∗ Fr g Gs h L m M Q Re t U∗
Area (m2) Archimedes number ([−]) Drag coefficient ([−]) Concentration ([−]) Equivalent diameter (m) Particle diameter (m) Dimensionless particle diameter ([−]) Froude number ([−]) Gravitational acceleration (m/s2) Solid circulation flux (kg/(m2s)) Height (m) Characteristic length (m) Mass (g) Mixing factor ([−]) Gas flow (m3/h) Particle Reynolds number ([−]) Time (s) Dimensionless velocity ([−])
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Y. Liu et al. / Powder Technology xxx (2019) xxx
u umf ut utf
Superficial velocity (m/s) Minimum fluidization velocity (m/s) Terminal velocity (m/s) Fast fluidization velocity (m/s)
Greek symbols εs
Voidage ([−])
ηg ρp ρg ρbulk Φp
Gas viscosity (kg/(m·s)) Particle density (kg/m3) Gas density (kg/m3) Solid bulk density (kg/m3) Sphericity ([−])
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