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Design of micro interconnected fluidized bed for oxygen carrier evaluation
International Journal of Greenhouse Gas Control 90 (2019) 102806
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Design of micro interconnected fluidized bed for oxygen carrier evaluation Tianxu Shen, Xiao Zhu, Jingchun Yan, Laihong Shen
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T
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, Jiangsu, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Chemical looping combustion Micro interconnected fluidized bed Oxygen carrier evaluation
Oxygen carriers (OC), one of the key factors in chemical looping combustion (CLC), are regularly evaluated using thermogravimetric analysis (TGA) and batch fluidized beds with oxidizing and reducing gas streams. Nevertheless, the reaction status in these devices has a specific deviation from a real CLC process. It introduces difficulty when assessing attrition rate, thermal stability, and reactivity. In this work, a micro interconnected fluidized bed (MIFB) is designed to provide a real CLC environment to increase the accuracy of OC evaluation. The air reactor and fuel reactor are identical two-stage bubbling beds, which have 30 mm ID and 100 mm height. With stable and flexible fluidization, the amount of bed inventory in MIFB was minimized to reduce operational cost. 350 g bed inventory was required after size miniaturization, structure optimization and circulation path simplification. Synthetic NiO/Al2O3 and natural hematite are employed as OCs to give insight into MIFB performance. Reactivity of hematite was observed in MIFB with conversion efficiencies of 100%, 36.8%, and 16% for H2, CO and CH4 respectively at 900℃. Ni-based OC displayed excellent reactivity and thermodynamics, but also exhibited severe carbon deposition at low temperature. OC lifetimes of 457 h and 680 h were estimated for hematite and NiO/Al2O3, respectively. In addition, particle size distributions were employed to evaluate OC attrition behavior in MIFB. Based on this approach, the potential of MIFB as an OC evaluation device is demonstrated, accurately reflecting fluidization hydrodynamics, OC reactivity, mechanical behavior and lifetime, the influence of temperature and carbon deposition phenomenon.
1. Introduction Chemical looping combustion (CLC) has been recognized as the best technical scheme for commercial carbon capture when considering cost- and energy-efficiency (Fan et al., 2012). The basic strategy of chemical looping combustion is to utilize an oxygen agent to divide a one-step reaction into two stages. The oxygen demand for fuel conversion in CLC is met by using circulating metal oxides (oxygen carriers) between two interconnected reactors (fuel reactor and air reactor), with the benefit of an N2-free atmosphere in the fuel reactor (Adanez et al., 2018). The oxygen carrier (OC) provides lattice oxygen for combustion in the fuel reactor (FR) and is then oxidized back to its initial state by air in the air reactor (AR) (Lyngfelt et al., 2008). This strategy can provide a pure outlet stream of CO2 after condensing steam since combustion. Compared with other carbon capture technologies, the significant advantage of CLC is the low energy penalty without additional gas separation or pure oxygen production steps. The OC material, an essential section of CLC research, should have sufficient reactivity, favorable thermodynamics, excellent thermal stability, and high mechanical strength. More than 700 materials have
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been tested and improved for chemical looping processes (Adanez et al., 2012). Oxygen carrier development has been summarized in the review papers by Adanez (Adanez et al., 2012) and Lyngfelt (Lyngfelt, 2015), which explain OC characteristics such as reactivity, attrition, carbon deposition, agglomeration, and long-term suitability. The single metal oxide is unlikely to entirely fulfill CLC demands because of insufficient reactivity and severe performance degradation after long-term operation. Synthetic materials such as bimetallic OCs can overcome the above-mentioned technical hurdles and exhibit better thermal stability (Siriwardane et al., 2015). Although the development of complex synthetic materials has been successful under lab-scale conditions, high cost and short lifetime have restricted the further application to commercial-scale processes. A portion of OC particles may be elutriated during ash draining procedures (Ivan et al., 2018), and the OC lifetime may also decrease with the formation of complex ash components (Alberto et al., 2018). Accordingly, the low-cost requirement has inspired OC research into materials like minerals and waste materials, where iron and manganese ores are the favored candidates (Ivan et al., 2018). Intensive work in OC material development has been carried out,
Corresponding author. E-mail address: [email protected] (L. Shen).
https://doi.org/10.1016/j.ijggc.2019.102806 Received 8 January 2019; Received in revised form 3 August 2019; Accepted 4 August 2019 1750-5836/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Greenhouse Gas Control 90 (2019) 102806
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insights into the operational conditions and characteristics of MIFB.
but technical methods of evaluating OC performance have some inherent limitations. TGA and batch fluidized bed analysis are the popular configurations to characterize OC, where the OC circulation between two reactors is replaced by the switching of gas atmospheres. Nevertheless, several concerns are obvious with this concept of stationary OC particles in a single vessel (Mogahid et al., 2018). TGA is a common method of evaluation for OC reactivity and reaction kinetics within a controlled reaction atmosphere. However, composition and structure of solid samples can change during the heating period (Liu et al., 2008; Xu et al., 2015), introducing difficulty when attempting to characterizing OC performance at the target temperature. Another limitation of the TGA is the gas-solids contact conditions. The OC sample loaded inside a cylindrical cell, cannot be fluidized in TGA. The interfacial gas diffusion should be taken into consideration and mitigated through experimental evaluation (Yu et al., 2011). The drawbacks in TGA can be circumvented using the batch fluidized bed, but it imposes other hurdles. The major problem is that fluidization conditions in the batch fluidized bed have a specific deviation from the interconnected fluidized beds. The different gas-particle diffusions may cause deficient gas-solid contact (Liu et al., 2008), and further affect performance parameters such as carbon capture efficiency and oxygen demand. The other deficiencies in the batch fluidized bed include more pronounced carbon deposition, high thermo-chemical stresses on OC, and the requirement for reactor shutdown to replace the spent oxygen carrier material (Mogahid et al., 2018; Cloete et al., 2016). Besides, in the batch fluidized bed it is difficult to fully predict OC lifetime and attrition behavior, which cannot reflect the high mechanical stress applied in OC particles during the circulation between the two reactors. The interconnected fluidized bed with a continuous solids circulation is an accepted method to overcome the above deficiencies. According to the summaries of Adanez (Adanez et al., 2018) and Lyngfelt (Lyngfelt and Linderholm, 2017), 34 pilots have been described in literature. However, most of them employ mineral or industrial residue as the oxygen carrier, while only few pilot facilities can use synthetic OCs (Siriwardane et al., 2018; Jochen et al., 2018; Abad et al., 2012). The hurdles for applying synthetic OC in pilots include the complicated OC preparation process and massive demand for solid inventory. The popular OC manufacturing methods are mechanical mixing, spray drying and co-precipitation which are tedious processes requiring considerable time and workforce. In addition, expensive raw materials would be additional burdens for a laboratory. The average demand for bed inventory in CLC pilots ranges between 15 and 250 kg (Lyngfelt and Linderholm, 2017; Siriwardane et al., 2018; Jochen et al., 2018; Abad et al., 2012; Abad et al., 2006). It is a huge task to prepare OC with different elemental contents at such large scale. The minimum scales of CLC facilities using solid and gaseous fuels are 500Wth and 300Wth with the demand of 1.5 L and 300 mL bed inventory, respectively (Abad et al., 2006; Cuadrat et al., 2011). However, the bed inventory reduction in these two facilities is at the expense of flexible operation, in which standpipe structure is eliminated. The micro fluidized bed (MFB) with a few millimeters inner diameter was first put forward by Potic (Potic et al., 2005) with the purpose of lowering capital and operational costs. Xu (Liu et al., 2008) proposed to utilize MFB to accurately measure the kinetics of solid reactants, which is challenging to perform in TGA or the conventional fluidized bed. Based on the miniaturization principle of MFB, the micro interconnected fluidized bed (MIFB) is proposed and designed in this work for OC evaluation. The primary objective of MIFB is to reduce bed inventory with size miniaturization and structure optimization. In addition, stable and flexible fluidization is also required in MIFB. This study provides the first operation to test the feasibility and performance of MIFB under CLC atmosphere with gaseous fuels. Natural hematite and synthetic NiO/Al2O3 are selected as the oxygen carriers. It should be noted that OC optimization is beyond the scope of this study and efficient fuel conversion is not the purpose of MFIB. This work aims to demonstrate the applicability of MIFB for OC evaluation and achieve
2. Methodology 2.1. Design The design principle of micro interconnected fluidized beds is different with conventional CLC reactors. As an OC evaluation device, MIFB should follow three design criteria: (1) reduced bed inventory; (2) stable and flexible fluidization; (3) suitable capacity for gas and carbon conversion. Bed inventory reduction can be easily realized by reducing the reactor diameter. However, severe wall effects are inevitable for a reactor with small diameter (e.g. a few millimeters) (Liu et al., 2008), which would cause a deviation of gas-particle flow from the desirable conditions. The hydrodynamics of MIFB should resemble that of laboratoryscale CLC reactors. Hence the impact of adhesive forces should be diminished as much as possible. The relationship between the MIFB diameter and wall effects has been discussed in the previous work of cold model which will be published in the future. A 30 mm inner diameter is acceptable considering both limited wall effects and reduced bed inventory. In addition, structure optimization and circulation path simplification are also required in MIFB for the minimization of bed inventory. The carbon stripper is eliminated and conventional circulating fluidized bed was not used in the FR platform. Further reduction of the bed inventory in MIFB is realized by arranging internal gas distributors to occupy internal space. A suitable conversion efficiency of MIFB is chosen to present the difference among OCs and operating parameters. The combustion efficiency of CLC process is highly dependent on the bed inventory in the FR, solids circulation rate, particle residence time and gas-solid contact condition (Alberto et al., 2018; Johannes et al., 2018). According to the size miniaturization, the residence time and bed inventory in MIFB would be inadequate. Therefore, enhanced gas-solid contact is required in MIFB to promote fuel conversion, which is realized by employing gas distributors as bed internals. The previous experiments and modeling have proved that internal distributors can improve the homogeneity of solid particles from axial distribution and decrease the freeboard in FR (Sedor et al., 2008). Besides, internal distributors show a postive effect on suppressing gas escaping phenomenon, reducing bubble size, decreasing the rate of bubble coalescence and intensifing gas exchange between bubbles and the surrounding solids suspension (Christensen et al., 2008). An adequate interaction would be provided between gases and OC particles and further increase the conversion of a mass-transfer limited reaction. 2.2. Setup of the micro CLC reactor As shown in Fig.1, the micro reactor mainly consists of two interconnected fluidized beds – the air reactor and fuel reactor. The AR and FR are two-stage bubbling beds with 30 mm i.d. and 100 mm height, which are divided into two identical chambers by arranging an internal perforated plate in the middle. It is worth mentioning that there are dense and dilute phases in each chamber. In order to avoid particles back mixing and allow particles to fluidize through, the aperture ratios of the internal distributors in the AR and FR are designed as 22.9% and 20.9%, respectively. The smaller aperture ratio for the FR perforated plate was chosen to gain a longer particle residence time and shift solid distribution towards the fuel reactor. Too large of an aperture ratio was not recommended because particles in the upper chamber would fall into the lower one through the apertures. This would work against a homogeneous particle distribution in the reactors. Meawhile, high pressure drop and fluidization resistance are the issues when too small aperture ratio is applied, which would hinder the stable fluidization for internals. Two risers with smaller diameter can provide enough driving force 2
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Fig. 1. The 2D and 3D diagrammatic sketches of the micro interconnected fluidized bed.
2.3. Oxygen carrier
for the stable solids circulation. The typical L-valve is adopted as the structure of the loop-seal with consideration of operation adaptability of Geldart group B particles (oxygen carrier). Loop-seals are adjoined with the dense phase of the 1 st chamber in the AR and FR. Cyclones are arranged in the middle of the connection lines to prevent atmospheres mixing. In order to withstand high-temperature corrosion, 310S stainless steel is selected as the primary material of the MIFB. An electric heating device is arranged at two different heights to heat the MIFB to a maximum temperature of 1100 ℃. A PID temperature controller is used to control the temperature during heating and the hot operation period. Heat loss is reduced by a preservation furnace with an internal refractory lining, which is wrapped around most of the reactor. The MIFB can be heated from room temperature to 900 ℃ within 2 h. Temperature sensors (shown in Fig. 1) were installed along the height of the AR and FR. A relatively uniform temperature distribution is guaranteed in all parts of the system because of the small device scale. Four pressure gauges (illustrated in Fig. 1) are equipped in the two risers to measure pressure difference which can be further applied to calculate the upwards particles flow. During the heating period, the FR, AR, and two loop-seals were fluidized by air to completely oxidize OC. The flow rates of all gases were controlled by five calibrated rotameters. In order to maintain a constant gas flow rate, the air flows were automatically adjusted based on the real-time temperature. Upon reaching the target temperature, the fluidization agent of the FR and two loop-seals was quickly switched to nitrogen. The gaseous fuel was introduced from the FR bottom after the system remained stable for 1 h. The off gases of the AR and FR were led through several gas washing bottles to cool the gases and remove entrained fine particles. After the water removal process, the gas bags were used to collect flue gases for off-line analysis. The components and concentrations of the gases (O2, CO, CO2, CH4, and H2) were analyzed by the NGA-2000 type gas analyzer (EMERSON Company, USA).
Natural hematite and synthetic NiO/Al2O3 were employed as OCs. The hematite and NiO/Al2O3 are the most extensively used OC materials in natural minerals and synthetic OC, respectively. These two OCs serve as benchmarks to test the feasibility of MIFB for chemical looping combustion, as well as the adaptability for different OC materials. In addition, the pronounced difference of OC characteristics between hematite and Ni-based OC is helpful in proving the validity of using MIFB as an OC evaluation device. The NiO/Al2O3 with 14 wt.% NiO and 86 wt.% Al2O3 was prepared by the incipient-wetness impregnation method. With this weight percentage, most of the Ni-Al-O mixtures would transform into a spinel phase of NiAl2O4 (Adanez et al., 2012; Christensen et al., 2008). 99.99% Al2O3 powder was impregnated with Ni (NO3)2•6H2O with isometric solution for 8 h. It was then dried at 85 ℃ for 12 h and then sieved into the size range of 0.1-0.45 mm. The hematite, from South Africa, was supplied by Nanjing steel manufacturing company. The grinding and sieving process was also required for hematite to obtain the same size range of 0.1-0.45 mm. X-ray fluorescence (XRF) analysis was used to analyze the elemental composition of hematite, as illustrated in Table 1. The mechanical strength of NiO/Al2O3 and hematite OCs were improved by calcining them in a muffle oven at 950 ℃ for 6 h. The minimum fluidization velocities of hematite and NiO/Al2O3 particles were measured as 0.132 m/s and 0.137 m/s in MIFB. The bulk density of hematite OC was measured as 2286 kg/m3, and that of NiO/ Al2O3 was 2460 kg/m3. Based on 155 mL of total solids inventory in the MIFB, the NiO/Al2O3 and hematite bed materials were 381 g and 355 g, Table 1 Elemental Composition of Hematite (wt.%).
3
Fe2O3
SiO2
Al2O3
P2O5
CaO
SO3
others
83.21
7.06
5.13
0.38
0.24
0.21
3.77
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respectively. 2.4. Particle losses and attrition The data of particle loss provides a reliable estimation of the oxygen carrier lifetime. Some OC particles would be elutriated away from reactor system because of attrition behavior, system upsets and unstable fluidization (Siriwardane et al., 2015). The OC attrition rate (A ) is defined by the ratio of particle weight loss (Wl ) to total solid inventory (Wt ) during the operation period (Δt ), as shown in eq.1 (Pans et al., 2014). It should be noted that the OC attrition rate would fall in the industrial CLC facility in virtue of adopting the large-scale two-stage cyclones (Lindsley et al., 2006). A stainless steel tank is used as a particle filter to collect elutriated particles from the outlet of the AR and FR. The tank has a large diameter and height in which the majority of fine particles can be settled by gravity. The tank surface was polished to avoid fines adhesion. In addition, a filter equipped with three dense filter plates was installed to capture the fine particles. The mass of particle loss was calculated after experiments.
A= (
Wl ∙Δt) ∙100 Wt
Fig. 2. Pressure fluctuation of AR and FR risers under different temperature.
min. The gas flow of the AR and FR was 4 L/min and the injected aeration flow of two loop-seals was maintained at 0.8 L/min during all experimental campaigns. The critical gas velocity in the FR was estimated as 0.34-0.39 m/s based on the gas flow, cross-sectional area, and temperature correction. It should be noted that fuel decomposition and reduction reactions would change gaseous molecules. Therefore, the gas velocity in both stages of the FR may vary slightly. In addition, the temperature decrease with operation time can cause a reduction in the gas velocity of the FR and AR. The solids circulation rate (m˙ OC ) was acquired from the linear relationship between upwards particle flow and pressure drop. The bed inventory in the FR (mFR, OC ) was obtained from an experiment that repeated each experimental period and recovered all bed inventory in the FR after cooling to the ambient temperature. The solids residence time in the FR (τOC , FR ) was calculated using m˙ OC and mFR, OC . It is necessary to demonstrate the fluidization stability of MIFB under a reaction atmosphere. The pressure fluctuation in the AR and FR risers are shown in Fig.2 marked by red and blue, respectively. The pressure differences present the stable values during the whole operational time. No obvious change can be found in pressure profiles after switching gaseous fuels. The redox reactions had a limited influence on fluidization. Statistical analysis of the pressure signals is presented in Table 3. The variance of pressure difference showed a decreasing trend with temperature decrease, indicating a more stable upwards solid flow in the risers. The average values of pressure difference in the FR and AR risers also levels off with changing temperature. The average pressure of the AR riser was larger than that of the FR riser because of a longer distance between the two pressure gauges in the AR riser.
(E1)
OC attrition – a primary reason for particle losses – is generated from chemical stress of redox reactions, collisions, and fragmentation during the circulation process. At present, there are two prevailing methods to characterize OC attrition. The first approach is to predict attrition rate via characterizing attrition behavior in the principal sources of OC breakage (Kramp et al., 2011). However, tedious experimental work and large quantities of materials are required for a precise result. The secondary approach is to characterize the attrition potential of OC particles by ranking the mechanical resistance. The standard method ASTM D-5757 is one of the typical options applied in the secondary approach, which put particles under a defined mechanical stress similar to conditions in circulating fluidized beds (Cabello et al., 2016). Although this method is an excellent way to characterize mechanical strength, no strong correlation can be found from the actual attrition data (Rydén et al., 2014). The particle size distribution (PSD) method (Gayan et al., 2011) is applied in this work, where the particle size distribution of used and fresh OCs is compared to characterize OC attrition. All involved stresses are taken into consideration in this method, including thermal, transport disintegration, chemical reactions and fragmentation. 3. Result and discussion 3.1. Fluidization under CLC atmosphere
3.2. Chemical looping combustion with hematite OC 290 min of continuous operation was conducted with hematite OCs under the temperature range of 825–900 ℃. The detailed operational conditions and gas conversion efficiency ( Xgas, FR ) are shown in Table 2. A 12 min time interval between adjacent operational periods was used to cool the reactor to the next target temperature. Two kinds of gaseous fuel, H2 and CH4, were alternately introduced into the reactor at 70 mL/
An appropriate conversion efficiency is required in MIFB to clearly distinguish the influence of different experimental parameters. A temperature range of 825–900 ℃ is far away from the optimum temperature in CLC but is an excellent choice to detect the effect of temperature and fuel reactivity (Alberto et al., 2018). Fig.3 (A) and (B) present
Table 2 The operational conditions of 290 min continuous hot operation. Period (min)
Temperature (℃)
OC
Fuel (70 mL/min)
mFR, OC (g)
m˙ OC (kg/h)
τOC , FR (s)
Xgas, FR (%)
0-36 36-60 68-103 103-134 146-177 177-208 222-259 259-291
900 900 875 875 850 850 825 825
Hematite Hematite Hematite Hematite Hematite Hematite Hematite Hematite
CH4 H2 CH4 H2 CH4 H2 CH4 H2
105 105 112 112 109 109 106 106
9.3 9.3 9.2 9.2 9.1 9.1 9.1 9.1
40.6 40.6 43.8 43.8 43.1 43.1 41.9 41.9
16.0 100 7.7 100 3.4 89.1 2.6 84.8
4
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with CH4 was higher than that of H2. Around 11%–13% O2 was detected in the exhaust gas for H2 combustion, and increased to 13%–16% upon introducing CH4. The O2 concentration in MIFB can be used to qualitatively describe the reduction degree of the OCs. It should be noted that a minor amount of CO was also detected in the FR outlet with CH4 as the fuel. The influence of temperature on CO concentration was unclear, since the CO fraction showed an invariant trend with temperature increase. Two possible CO generation pathways are proposed in previous studies −CH4 decomposition (Gu et al., 2018) and CH4 reforming (Alberto et al., 2018). The CH4 decomposition reaction (R1) would produce solid carbon and gaseous H2, and then solid carbon is gasified by CO2 (R2) or H2O (R3). However, a 40 s solids residence time is too short for carbon gasification, and the low temperature (825–900 ℃) is not in favor of gasification. In addition, carbon from CH4 decomposition would also deposit on the OC surface and then be consumed by air in the AR. However, there is no detected CO2 or CO in the AR outlet, therefore, it is reasonable to speculate that CO originates primarily from CH4 reforming in MIFB.
Table 3 The statistical analysis of pressure difference in AR and FR risers under different temperature. Temperature (℃)
900
875
850
825
FR riser
0.83 0.212 0.86 0.132
0.82 0.144 0.87 0.110
0.81 0. 125 0.86 0. 071
0.81 0. 128 0.89 0. 064
AR riser
Average value (kPa) Variance Average value (kPa) Variance
reaction performance, in which the gas concentration of effluent gas was converted into gas flow of the effluent gas (mL/min) for a clear description. The gas conversion efficiency of H2 and CH4 is shown in Fig.3 (C). Hematite has an excellent reactivity with H2 but shows limited thermodynamic with CH4 (Hossain and Lasa, 2007). High H2 conversion was achieved under 900 ℃ and 875 ℃ such that almost no H2 could be detected at the outlet. The H2 conversion efficiency had a slight reduction from 91% to 86% when the temperature was decreased from 850 to 825 ℃. Although CH4 conversion showed a similar trend with decreasing temperature, the concentration of unconverted CH4 was significantly higher than H2. The CH4 conversion efficiency was only 16% at 900 ℃ with corresponding 59 mL/min stream of unconverted CH4 and a 7 mL/min stream of CO2 produced at the outlet. The conversion efficiency fell to 2% as temperature decreased to 825 ℃. The majority of CH4 cannot be oxidized by OCs in MIFB, which is reflected by the O2 concentration in the AR outlet. The oxygen demand for CH4 combustion should be four times larger than that of H2 combustion when the fuels are completely consumed. However, O2 concentration
CH4 → C+H2
(R1)
CO2+C → CO
(R2)
H2O + C →H2+CO2
(R3)
The capacity of gas conversion in MIFB was verified in this experiment. A significant difference of conversion efficiency was presented between highly reactive H2 and less reactive CH4. Meanwhile, the effect of temperature is reflected by an 8 times increase in CH4 conversion efficiency with a 75℃ increase in temperature. In addition,
Fig. 3. Detailed performance of H2 and CH4 as fuel with hematite OC as a function of the temperature. 5
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Fig. 4. Detailed performance of CO as fuel with hematite OC as a function of the temperature.
the internal gas distributor proved essential in MIFB. Without the enhanced gas diffusion, CH4 conversion would be insufficient to make a notable distinction under different temperatures.
3.3. Comparison between hematite and NiO/Al2O3 OCs The MFIB not only exhibits suitable conversion capability, but also flexibility of use with different oxygen carriers. Besides, other OC characteristics other than reactivity should be reflected in MIFB. Therefore, a comparative experiment with 70 mL/min of CO used as fuel was carried out by using hematite and NiO/Al2O3 as OCs. The hematite test was conducted under continuous operation for 2.6 h at a temperature of 825–900 ℃. The NiO/Al2O3 trial was performed with approximately 6 h of CLC operation, while the operating temperature started at 600℃ due to its excellent reactivity and thermodynamics (Hossain and Lasa, 2007). The testing data in this section was determined using average data from multi-sampling. Fig. 4 shows the effect of temperature on CO conversion with the hematite OC. As temperature increases, the unreacted CO at the outlet decreased from 62.1 to 43.5 mL/min, while the gas flow of CO2 increased from 7.8 to 25.8 mL/min. The conversion efficiency of CO presented an increasing trend with the temperature which was up to a peak of 36.8% at 900 ℃. A higher temperature can accelerate the Fe2O3−CO reactions by providing enough energy for activation in the reaction regions. In addition, carbonaceous gases including CO and CO2 were not detected in the AR. Thus, it is concluded that operating hematite OC in MIFB did not have any atmosphere mixture or carbon deposition issues. Fig.5 (A) presents the data of flue gases in the FR and AR with NiO/ Al2O3 OC. Comparing with hematite, an improved CO conversion appeared in NiO/Al2O3 even at a lower temperature of 600 ℃. With increasing temperature to 900 ℃, the CO2 gas flow increased sharply from 24.2 ml/min to 42.8 ml/min, and CO conversion efficiency was up to 70%. However, a large quantity of CO2 flow was detected in the AR outlet. The CO2 flow in a main reason for particle losses in the AR outlet which showed a decreasing trend as temperature increased, dropping from 22.6 mL/min at 600 ℃ to 5.8 mL/min at 900 ℃. It is reasonable to speculate that CO2 flow in the AR originated from carbon deposition, a fatal problem for Ni-based applications (Ryu et al., 2003). The carbon monoxide can decompose into carbon dioxide and graphite in the Boudouard reaction (R4). The attached solid carbon on the NiO/Al2O3 can cover the active site thus limiting the reduction rate. The source of CO2 in the FR was distinguished to evaluate the degree of carbon deposition, as Fig.5 (B) shows. A severe carbon deposition occurred at temperatures less than 700 ℃, in which the majority of CO2 was produced from the Boudouard reaction. Similarly, it was reported in previous research (Ryu et al., 2003) that carbon deposition decreased with
Fig. 5. Detailed performance of CO as fuel with NiO/Al2O3 OC as a function of the temperature.
temperature. However, excellent regenerative ability was found in NiO/ Al2O3 OC such that little carbon deposition was observed with temperature increase. It seems that the regenerative ability of NiO/ Al2O3 OC was not influenced by multiple cyclic redox reactions and carbon deposition. Besides, it should be noted that carbon formation on OC particles in MIFB was more severe than the general CLC reactors. The low bed inventory is the reason that carbon deposition can be largely avoided if enough metal oxide is supplied in fuel reactor (Mattisson et al., 2006). CO → C + CO2
(R4)
3.4. Characterization of oxygen carrier Along with reactivity and oxygen transport capacity, OC lifetime related to particle attrition is another key parameter for process development. It has a strong impact on economic cost and flow hydrodynamics. The particle losses during the entire hot operation were collected in particle filters to estimate OC lifetime. The appearance of undesirable fluidization was avoided during the whole operation such 6
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Al2O3. Scanning electron microscopy (SEM) was used to probe the microstructure of sampled OC particles, as is shown in Fig.7. The hematite had a smoother surface than that of NiO/Al2O3. Several small grains were found to be covered with a looser structure in NiO/Al2O3. As Fig.8 shows, XRD patterns reveal that the main crystalline phases of NiO/Al2O3 OC in the AR are the spinel phase of NiAl2O4 and Al2O3. The NiO phase cannot be detected since all of the NiO was converted into NiAl2O4 through interaction with Al2O3. The XRD analysis also illustrates good cyclic stability and regenerative ability as the oxygen carriers always reached their full oxidation state after long-term operation. A specific C phase detected in the samples of the FR verified the presence of carbon deposition in the Ni-based OC. For the hematite OC, no Fe3O4 was detected in the AR illustrating that the AR design could fulfill the demand of OC complete oxidation. The oxygen carrier from the FR mainly contained Fe3O4 but also had some unreacted Fe2O3 phase. The reacted Fe3O4 would cover the surface and then restrict contact between inner Fe2O3 and gaseous fuel. (Fig. 9)
that no obvious agglomeration was observed during the operation (including a drastically decreasing pressure drop or defluidization). Therefore, the elutriated OC was only from abrasive attrition, particle fragmentation, and transport disintegration. Identical fluidization conditions were applied with hematite and NiO/Al2O3 as OCs. The critical velocities of the FR and AR were the same at 0.34-0.38 m/s corresponding to around 1.62–1.72 m/s gas velocity in the distributor plates. The inlet gas velocities of the two risers varied between 3.8 and 4.2 m/ s. The generation of fine particles (below 40 μm) was defined from attrition behaviors in the classical method, which cannot fully describe OC breakage in CLC process since the particle size generated by attrition is higher (Amblard et al., 2015). Therefore, the total amount of elutriated particles over the whole size range was used to calculate OC lifetime. 3.681 g of hematite was collected after 7.4 h of CLC operation, including 2.6 h of CO operation, and 4.8 h of CH4 and H2 operation. The attrition rate of hematite OC was 0.219 wt.%/h corresponding to 457 h lifetime. The attrition rate is not constant with time, which is high at the beginning and decreases towards the end. Gayan et al. (Gayan et al., 2011) found that the generation of OC fines was high at the beginning of the fluidization process due to (1) the rounding effects on the particle irregularities and (2) fines stuck to the particles during preparation. The abrasion behavior was enhanced at the beginning until reaching a certain threshold, and abrasion also lessened after a long operational duration where particle breakage tended to stop when reaching a certain size (Amblard et al., 2015). The attrition rate of NiO/Al2O3 was 0.147 wt.%/h corresponding to an OC lifetime of 680 h after 6.8 h CO operation. The mechanical strength of synthetic OCs was higher than that of mineral OCs in this experiment. After high-temperature operation, the MIFB was cooled down to the ambient temperature with an inert N2 atmosphere. All OC particles were collected for particle size analysis. The diffraction particle size analyzer (Master-sizer 2000) was used, which can determine the size distribution of particles from 0.04 to 1800 μm. Fig. 6 presents the particle size distribution of fresh and used OCs samples. The green curve corresponds to the difference between the particle size distribution of fresh and used OCs. The maximum diameter of particle breakage is indicated by the intersection point between the difference curve and x-axis, which is around 0.215 mm and 0.23 mm for hematite and NiO/ Al2O3 OCs, respectively. It can be concluded that OC attrition was not dominant during OC breakage, where only fine particles were generated (Amblard et al., 2015). Therefore, the fragmentation behavior was dominant in OC circulation meaning that several particles with intermediate size would be generated from the splitting of a mother particle. In addition, the higher mechanical strength of NiO/Al2O3 can also be found from the result of the PSD. The abrasion in hematite was severe generating fine particles of hematite that were larger than that of NiO/
4. Conclusion This study aims to analyze the potential of the micro interconnected fluidized bed (MIFB) as an oxygen carrier (OC) evaluation device. In order to reduce operational cost and increase evaluation accuracy, the MIFB incorporates characteristics of lab-scale CLC pilots, while eliminating the complex structure and miniaturizing diameter size. The design criteria followed by MIFB are bed inventory reduction, stable and flexible fluidization, and suitable conversion capacity. The air reactor (AR) and fuel reactor (FR) are two-stage bubbling beds, which each have a 30 mm i.d. and 100 mm height. A perforated plate as the internals is arranged in the middle of the AR and FR to divide them into two identical chambers. After structure optimization, total bed inventory is only 350 g. NiO/Al2O3, produced by an impregnation method, and natural hematite were employed as OCs to investigate the performance of the MIFB and to obtain insights into configuration and operation conditions. The fluidization experiment was carried out under a 290 min CLC operation where stable pressure fluctuation with regular waves was obtained in both the AR and FR risers. Notable distinctions were present between influencers including different temperatures, OCs, and fuels. The conversion efficiencies of H2, CO, and CH4 with hematite OCs were 100%, 36.8%, and 16%, respectively at 900 ℃. The reactivity of gaseous fuels with hematite was indicated clearly in MIFB. Meanwhile, all the fuels showed obvious trends with temperature increase. The NiO/ Al2O3 has a higher reactive activity such that CO conversion efficiency was greater than 61% at 900 ℃. Severe carbon deposition was also found in Ni-based OCs such that 90% CO2 in the FR off-gas was sourced from the Boudouard reaction under low temperature. The attrition rates of hematite and NiO/Al2O3 were 0.219 wt.%/h and 0.147 wt.%/h, respectively. The analysis of particle size distribution showed that OC attrition behavior focused on particles with size around 0.25 mm. The XRD patterns revealed that NiAl2O4 and Al2O3 were the main crystalline phases of the Ni-based OC without the existence of the NiO phase. The experimental campaigns have proven the qualification of MIFB as an OC evaluation device that can reflect the differences of reactivity between fuels and OCs, the influence of reaction temperature, and the phenomenon of CO decomposition and carbon deposition. Declaration of Competing Interest The authors declare no competing financial interest. Acknowledgments
Fig. 6. Particle size distributions of the fresh and used OCs in the MIFB (Amblard et al., 2015).
The authors gratefully acknowledge the support of this research work by National Key R&D Program of China (2018YFB0605404), 7
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Fig. 7. The SEM images of microstructure of used oxygen carriers: (A) NiO/Al2O3 OC; (B) Hematite OC.
Fig. 8. The XRD spectra of NiO/Al2O3 OC in AR and FR.
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Fig. 9. The XRD spectra of hematite OC in AR and FR.
National Natural Science Foundation of China (51761135119), and Scientific Research Foundation of Graduate School of Southeast University (YBPY1857). References Fan, L., Zeng, L., Wang, W., et al., 2012. Chemical looping processes for CO2 capture and carbonaceous fuel conversion – prospect and opportunity. Energy Environ. Sci. 5 (6), 7254–7280. Adanez, J., Abad, A., Mendiara, T., et al., 2018. Chemical looping combustion of solid fuels. Prog. Energy Combust. Sci. 65. Lyngfelt, A., Johnansson, M., Mattison, T., 2008. Chemical looping combustion, status & development. 9th International Conference on Circulating Fluidized Beds. Adanez, J., Abad, A., Garcia-Labiano, F., et al., 2012. Progress in Chemical-Looping combustion and reforming technologies. Prog. Energy Combust. Sci. 38 (2), 215–282. Lyngfelt, A., 2015. In: Fennell, P., Anthony, E.J. (Eds.), Calcium and Chemical Looping Technology for Power Generation and Carbon Dioxide (CO2) Capture. Woodhead Publishing, pp. 221–254. Siriwardane, R., Tian, H., Miller, D., et al., 2015. Fluidized bed testing of commercially prepared MgO-promoted hematite and CuO–Fe2O3, mixed metal oxide oxygen carriers for methane and coal chemical looping combustion. Appl. Energy 157, 348–357. Ivan, G., Carl, L., Dan, G., 2018. Chemical-looping combustion in a 100 kW unit using a mixture of synthetic and natural oxygen carriers - operational results and fate of biomass fuel alkali. 5th International Conference on Chemical Looping [C]. Alberto, A., Raul, P., Teresa, M., 2018. Improving the performance of the chemical looping combustion process with coal in a 50kWth unit. 5th International Conference on Chemical Looping [C]. Mogahid, O., Abdelghafour, Z., Schalk, C., 2018. Experimental study of chemical looping combustion in internally circulating reactor. 5th International Conference on Chemical Looping [C]. Liu, X., Xu, G., Gao, S., 2008. Micro fluidized beds: wall effect and operability. Chem. Eng. J. 137 (2), 302–307. Xu, X., Wang, Y., Chen, Z., et al., 2015. Variations in char structure and reactivity due to the pyrolysis and in-situ gasification using Shengli brown coal[J]. J. Anal. Appl. Pyrolysis 115, 233–241. Yu, J., Yao, C., Zeng, X., et al., 2011. Biomass pyrolysis in a micro-fluidized bed reactor:
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Design of micro interconnected fluidized bed for oxygen carrier evaluation Tianxu Shen, Xiao Zhu, Jingchun Yan, Laihong Shen
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T
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, Jiangsu, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Chemical looping combustion Micro interconnected fluidized bed Oxygen carrier evaluation
Oxygen carriers (OC), one of the key factors in chemical looping combustion (CLC), are regularly evaluated using thermogravimetric analysis (TGA) and batch fluidized beds with oxidizing and reducing gas streams. Nevertheless, the reaction status in these devices has a specific deviation from a real CLC process. It introduces difficulty when assessing attrition rate, thermal stability, and reactivity. In this work, a micro interconnected fluidized bed (MIFB) is designed to provide a real CLC environment to increase the accuracy of OC evaluation. The air reactor and fuel reactor are identical two-stage bubbling beds, which have 30 mm ID and 100 mm height. With stable and flexible fluidization, the amount of bed inventory in MIFB was minimized to reduce operational cost. 350 g bed inventory was required after size miniaturization, structure optimization and circulation path simplification. Synthetic NiO/Al2O3 and natural hematite are employed as OCs to give insight into MIFB performance. Reactivity of hematite was observed in MIFB with conversion efficiencies of 100%, 36.8%, and 16% for H2, CO and CH4 respectively at 900℃. Ni-based OC displayed excellent reactivity and thermodynamics, but also exhibited severe carbon deposition at low temperature. OC lifetimes of 457 h and 680 h were estimated for hematite and NiO/Al2O3, respectively. In addition, particle size distributions were employed to evaluate OC attrition behavior in MIFB. Based on this approach, the potential of MIFB as an OC evaluation device is demonstrated, accurately reflecting fluidization hydrodynamics, OC reactivity, mechanical behavior and lifetime, the influence of temperature and carbon deposition phenomenon.
1. Introduction Chemical looping combustion (CLC) has been recognized as the best technical scheme for commercial carbon capture when considering cost- and energy-efficiency (Fan et al., 2012). The basic strategy of chemical looping combustion is to utilize an oxygen agent to divide a one-step reaction into two stages. The oxygen demand for fuel conversion in CLC is met by using circulating metal oxides (oxygen carriers) between two interconnected reactors (fuel reactor and air reactor), with the benefit of an N2-free atmosphere in the fuel reactor (Adanez et al., 2018). The oxygen carrier (OC) provides lattice oxygen for combustion in the fuel reactor (FR) and is then oxidized back to its initial state by air in the air reactor (AR) (Lyngfelt et al., 2008). This strategy can provide a pure outlet stream of CO2 after condensing steam since combustion. Compared with other carbon capture technologies, the significant advantage of CLC is the low energy penalty without additional gas separation or pure oxygen production steps. The OC material, an essential section of CLC research, should have sufficient reactivity, favorable thermodynamics, excellent thermal stability, and high mechanical strength. More than 700 materials have
⁎
been tested and improved for chemical looping processes (Adanez et al., 2012). Oxygen carrier development has been summarized in the review papers by Adanez (Adanez et al., 2012) and Lyngfelt (Lyngfelt, 2015), which explain OC characteristics such as reactivity, attrition, carbon deposition, agglomeration, and long-term suitability. The single metal oxide is unlikely to entirely fulfill CLC demands because of insufficient reactivity and severe performance degradation after long-term operation. Synthetic materials such as bimetallic OCs can overcome the above-mentioned technical hurdles and exhibit better thermal stability (Siriwardane et al., 2015). Although the development of complex synthetic materials has been successful under lab-scale conditions, high cost and short lifetime have restricted the further application to commercial-scale processes. A portion of OC particles may be elutriated during ash draining procedures (Ivan et al., 2018), and the OC lifetime may also decrease with the formation of complex ash components (Alberto et al., 2018). Accordingly, the low-cost requirement has inspired OC research into materials like minerals and waste materials, where iron and manganese ores are the favored candidates (Ivan et al., 2018). Intensive work in OC material development has been carried out,
Corresponding author. E-mail address: [email protected] (L. Shen).
https://doi.org/10.1016/j.ijggc.2019.102806 Received 8 January 2019; Received in revised form 3 August 2019; Accepted 4 August 2019 1750-5836/ © 2019 Elsevier Ltd. All rights reserved.
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insights into the operational conditions and characteristics of MIFB.
but technical methods of evaluating OC performance have some inherent limitations. TGA and batch fluidized bed analysis are the popular configurations to characterize OC, where the OC circulation between two reactors is replaced by the switching of gas atmospheres. Nevertheless, several concerns are obvious with this concept of stationary OC particles in a single vessel (Mogahid et al., 2018). TGA is a common method of evaluation for OC reactivity and reaction kinetics within a controlled reaction atmosphere. However, composition and structure of solid samples can change during the heating period (Liu et al., 2008; Xu et al., 2015), introducing difficulty when attempting to characterizing OC performance at the target temperature. Another limitation of the TGA is the gas-solids contact conditions. The OC sample loaded inside a cylindrical cell, cannot be fluidized in TGA. The interfacial gas diffusion should be taken into consideration and mitigated through experimental evaluation (Yu et al., 2011). The drawbacks in TGA can be circumvented using the batch fluidized bed, but it imposes other hurdles. The major problem is that fluidization conditions in the batch fluidized bed have a specific deviation from the interconnected fluidized beds. The different gas-particle diffusions may cause deficient gas-solid contact (Liu et al., 2008), and further affect performance parameters such as carbon capture efficiency and oxygen demand. The other deficiencies in the batch fluidized bed include more pronounced carbon deposition, high thermo-chemical stresses on OC, and the requirement for reactor shutdown to replace the spent oxygen carrier material (Mogahid et al., 2018; Cloete et al., 2016). Besides, in the batch fluidized bed it is difficult to fully predict OC lifetime and attrition behavior, which cannot reflect the high mechanical stress applied in OC particles during the circulation between the two reactors. The interconnected fluidized bed with a continuous solids circulation is an accepted method to overcome the above deficiencies. According to the summaries of Adanez (Adanez et al., 2018) and Lyngfelt (Lyngfelt and Linderholm, 2017), 34 pilots have been described in literature. However, most of them employ mineral or industrial residue as the oxygen carrier, while only few pilot facilities can use synthetic OCs (Siriwardane et al., 2018; Jochen et al., 2018; Abad et al., 2012). The hurdles for applying synthetic OC in pilots include the complicated OC preparation process and massive demand for solid inventory. The popular OC manufacturing methods are mechanical mixing, spray drying and co-precipitation which are tedious processes requiring considerable time and workforce. In addition, expensive raw materials would be additional burdens for a laboratory. The average demand for bed inventory in CLC pilots ranges between 15 and 250 kg (Lyngfelt and Linderholm, 2017; Siriwardane et al., 2018; Jochen et al., 2018; Abad et al., 2012; Abad et al., 2006). It is a huge task to prepare OC with different elemental contents at such large scale. The minimum scales of CLC facilities using solid and gaseous fuels are 500Wth and 300Wth with the demand of 1.5 L and 300 mL bed inventory, respectively (Abad et al., 2006; Cuadrat et al., 2011). However, the bed inventory reduction in these two facilities is at the expense of flexible operation, in which standpipe structure is eliminated. The micro fluidized bed (MFB) with a few millimeters inner diameter was first put forward by Potic (Potic et al., 2005) with the purpose of lowering capital and operational costs. Xu (Liu et al., 2008) proposed to utilize MFB to accurately measure the kinetics of solid reactants, which is challenging to perform in TGA or the conventional fluidized bed. Based on the miniaturization principle of MFB, the micro interconnected fluidized bed (MIFB) is proposed and designed in this work for OC evaluation. The primary objective of MIFB is to reduce bed inventory with size miniaturization and structure optimization. In addition, stable and flexible fluidization is also required in MIFB. This study provides the first operation to test the feasibility and performance of MIFB under CLC atmosphere with gaseous fuels. Natural hematite and synthetic NiO/Al2O3 are selected as the oxygen carriers. It should be noted that OC optimization is beyond the scope of this study and efficient fuel conversion is not the purpose of MFIB. This work aims to demonstrate the applicability of MIFB for OC evaluation and achieve
2. Methodology 2.1. Design The design principle of micro interconnected fluidized beds is different with conventional CLC reactors. As an OC evaluation device, MIFB should follow three design criteria: (1) reduced bed inventory; (2) stable and flexible fluidization; (3) suitable capacity for gas and carbon conversion. Bed inventory reduction can be easily realized by reducing the reactor diameter. However, severe wall effects are inevitable for a reactor with small diameter (e.g. a few millimeters) (Liu et al., 2008), which would cause a deviation of gas-particle flow from the desirable conditions. The hydrodynamics of MIFB should resemble that of laboratoryscale CLC reactors. Hence the impact of adhesive forces should be diminished as much as possible. The relationship between the MIFB diameter and wall effects has been discussed in the previous work of cold model which will be published in the future. A 30 mm inner diameter is acceptable considering both limited wall effects and reduced bed inventory. In addition, structure optimization and circulation path simplification are also required in MIFB for the minimization of bed inventory. The carbon stripper is eliminated and conventional circulating fluidized bed was not used in the FR platform. Further reduction of the bed inventory in MIFB is realized by arranging internal gas distributors to occupy internal space. A suitable conversion efficiency of MIFB is chosen to present the difference among OCs and operating parameters. The combustion efficiency of CLC process is highly dependent on the bed inventory in the FR, solids circulation rate, particle residence time and gas-solid contact condition (Alberto et al., 2018; Johannes et al., 2018). According to the size miniaturization, the residence time and bed inventory in MIFB would be inadequate. Therefore, enhanced gas-solid contact is required in MIFB to promote fuel conversion, which is realized by employing gas distributors as bed internals. The previous experiments and modeling have proved that internal distributors can improve the homogeneity of solid particles from axial distribution and decrease the freeboard in FR (Sedor et al., 2008). Besides, internal distributors show a postive effect on suppressing gas escaping phenomenon, reducing bubble size, decreasing the rate of bubble coalescence and intensifing gas exchange between bubbles and the surrounding solids suspension (Christensen et al., 2008). An adequate interaction would be provided between gases and OC particles and further increase the conversion of a mass-transfer limited reaction. 2.2. Setup of the micro CLC reactor As shown in Fig.1, the micro reactor mainly consists of two interconnected fluidized beds – the air reactor and fuel reactor. The AR and FR are two-stage bubbling beds with 30 mm i.d. and 100 mm height, which are divided into two identical chambers by arranging an internal perforated plate in the middle. It is worth mentioning that there are dense and dilute phases in each chamber. In order to avoid particles back mixing and allow particles to fluidize through, the aperture ratios of the internal distributors in the AR and FR are designed as 22.9% and 20.9%, respectively. The smaller aperture ratio for the FR perforated plate was chosen to gain a longer particle residence time and shift solid distribution towards the fuel reactor. Too large of an aperture ratio was not recommended because particles in the upper chamber would fall into the lower one through the apertures. This would work against a homogeneous particle distribution in the reactors. Meawhile, high pressure drop and fluidization resistance are the issues when too small aperture ratio is applied, which would hinder the stable fluidization for internals. Two risers with smaller diameter can provide enough driving force 2
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Fig. 1. The 2D and 3D diagrammatic sketches of the micro interconnected fluidized bed.
2.3. Oxygen carrier
for the stable solids circulation. The typical L-valve is adopted as the structure of the loop-seal with consideration of operation adaptability of Geldart group B particles (oxygen carrier). Loop-seals are adjoined with the dense phase of the 1 st chamber in the AR and FR. Cyclones are arranged in the middle of the connection lines to prevent atmospheres mixing. In order to withstand high-temperature corrosion, 310S stainless steel is selected as the primary material of the MIFB. An electric heating device is arranged at two different heights to heat the MIFB to a maximum temperature of 1100 ℃. A PID temperature controller is used to control the temperature during heating and the hot operation period. Heat loss is reduced by a preservation furnace with an internal refractory lining, which is wrapped around most of the reactor. The MIFB can be heated from room temperature to 900 ℃ within 2 h. Temperature sensors (shown in Fig. 1) were installed along the height of the AR and FR. A relatively uniform temperature distribution is guaranteed in all parts of the system because of the small device scale. Four pressure gauges (illustrated in Fig. 1) are equipped in the two risers to measure pressure difference which can be further applied to calculate the upwards particles flow. During the heating period, the FR, AR, and two loop-seals were fluidized by air to completely oxidize OC. The flow rates of all gases were controlled by five calibrated rotameters. In order to maintain a constant gas flow rate, the air flows were automatically adjusted based on the real-time temperature. Upon reaching the target temperature, the fluidization agent of the FR and two loop-seals was quickly switched to nitrogen. The gaseous fuel was introduced from the FR bottom after the system remained stable for 1 h. The off gases of the AR and FR were led through several gas washing bottles to cool the gases and remove entrained fine particles. After the water removal process, the gas bags were used to collect flue gases for off-line analysis. The components and concentrations of the gases (O2, CO, CO2, CH4, and H2) were analyzed by the NGA-2000 type gas analyzer (EMERSON Company, USA).
Natural hematite and synthetic NiO/Al2O3 were employed as OCs. The hematite and NiO/Al2O3 are the most extensively used OC materials in natural minerals and synthetic OC, respectively. These two OCs serve as benchmarks to test the feasibility of MIFB for chemical looping combustion, as well as the adaptability for different OC materials. In addition, the pronounced difference of OC characteristics between hematite and Ni-based OC is helpful in proving the validity of using MIFB as an OC evaluation device. The NiO/Al2O3 with 14 wt.% NiO and 86 wt.% Al2O3 was prepared by the incipient-wetness impregnation method. With this weight percentage, most of the Ni-Al-O mixtures would transform into a spinel phase of NiAl2O4 (Adanez et al., 2012; Christensen et al., 2008). 99.99% Al2O3 powder was impregnated with Ni (NO3)2•6H2O with isometric solution for 8 h. It was then dried at 85 ℃ for 12 h and then sieved into the size range of 0.1-0.45 mm. The hematite, from South Africa, was supplied by Nanjing steel manufacturing company. The grinding and sieving process was also required for hematite to obtain the same size range of 0.1-0.45 mm. X-ray fluorescence (XRF) analysis was used to analyze the elemental composition of hematite, as illustrated in Table 1. The mechanical strength of NiO/Al2O3 and hematite OCs were improved by calcining them in a muffle oven at 950 ℃ for 6 h. The minimum fluidization velocities of hematite and NiO/Al2O3 particles were measured as 0.132 m/s and 0.137 m/s in MIFB. The bulk density of hematite OC was measured as 2286 kg/m3, and that of NiO/ Al2O3 was 2460 kg/m3. Based on 155 mL of total solids inventory in the MIFB, the NiO/Al2O3 and hematite bed materials were 381 g and 355 g, Table 1 Elemental Composition of Hematite (wt.%).
3
Fe2O3
SiO2
Al2O3
P2O5
CaO
SO3
others
83.21
7.06
5.13
0.38
0.24
0.21
3.77
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respectively. 2.4. Particle losses and attrition The data of particle loss provides a reliable estimation of the oxygen carrier lifetime. Some OC particles would be elutriated away from reactor system because of attrition behavior, system upsets and unstable fluidization (Siriwardane et al., 2015). The OC attrition rate (A ) is defined by the ratio of particle weight loss (Wl ) to total solid inventory (Wt ) during the operation period (Δt ), as shown in eq.1 (Pans et al., 2014). It should be noted that the OC attrition rate would fall in the industrial CLC facility in virtue of adopting the large-scale two-stage cyclones (Lindsley et al., 2006). A stainless steel tank is used as a particle filter to collect elutriated particles from the outlet of the AR and FR. The tank has a large diameter and height in which the majority of fine particles can be settled by gravity. The tank surface was polished to avoid fines adhesion. In addition, a filter equipped with three dense filter plates was installed to capture the fine particles. The mass of particle loss was calculated after experiments.
A= (
Wl ∙Δt) ∙100 Wt
Fig. 2. Pressure fluctuation of AR and FR risers under different temperature.
min. The gas flow of the AR and FR was 4 L/min and the injected aeration flow of two loop-seals was maintained at 0.8 L/min during all experimental campaigns. The critical gas velocity in the FR was estimated as 0.34-0.39 m/s based on the gas flow, cross-sectional area, and temperature correction. It should be noted that fuel decomposition and reduction reactions would change gaseous molecules. Therefore, the gas velocity in both stages of the FR may vary slightly. In addition, the temperature decrease with operation time can cause a reduction in the gas velocity of the FR and AR. The solids circulation rate (m˙ OC ) was acquired from the linear relationship between upwards particle flow and pressure drop. The bed inventory in the FR (mFR, OC ) was obtained from an experiment that repeated each experimental period and recovered all bed inventory in the FR after cooling to the ambient temperature. The solids residence time in the FR (τOC , FR ) was calculated using m˙ OC and mFR, OC . It is necessary to demonstrate the fluidization stability of MIFB under a reaction atmosphere. The pressure fluctuation in the AR and FR risers are shown in Fig.2 marked by red and blue, respectively. The pressure differences present the stable values during the whole operational time. No obvious change can be found in pressure profiles after switching gaseous fuels. The redox reactions had a limited influence on fluidization. Statistical analysis of the pressure signals is presented in Table 3. The variance of pressure difference showed a decreasing trend with temperature decrease, indicating a more stable upwards solid flow in the risers. The average values of pressure difference in the FR and AR risers also levels off with changing temperature. The average pressure of the AR riser was larger than that of the FR riser because of a longer distance between the two pressure gauges in the AR riser.
(E1)
OC attrition – a primary reason for particle losses – is generated from chemical stress of redox reactions, collisions, and fragmentation during the circulation process. At present, there are two prevailing methods to characterize OC attrition. The first approach is to predict attrition rate via characterizing attrition behavior in the principal sources of OC breakage (Kramp et al., 2011). However, tedious experimental work and large quantities of materials are required for a precise result. The secondary approach is to characterize the attrition potential of OC particles by ranking the mechanical resistance. The standard method ASTM D-5757 is one of the typical options applied in the secondary approach, which put particles under a defined mechanical stress similar to conditions in circulating fluidized beds (Cabello et al., 2016). Although this method is an excellent way to characterize mechanical strength, no strong correlation can be found from the actual attrition data (Rydén et al., 2014). The particle size distribution (PSD) method (Gayan et al., 2011) is applied in this work, where the particle size distribution of used and fresh OCs is compared to characterize OC attrition. All involved stresses are taken into consideration in this method, including thermal, transport disintegration, chemical reactions and fragmentation. 3. Result and discussion 3.1. Fluidization under CLC atmosphere
3.2. Chemical looping combustion with hematite OC 290 min of continuous operation was conducted with hematite OCs under the temperature range of 825–900 ℃. The detailed operational conditions and gas conversion efficiency ( Xgas, FR ) are shown in Table 2. A 12 min time interval between adjacent operational periods was used to cool the reactor to the next target temperature. Two kinds of gaseous fuel, H2 and CH4, were alternately introduced into the reactor at 70 mL/
An appropriate conversion efficiency is required in MIFB to clearly distinguish the influence of different experimental parameters. A temperature range of 825–900 ℃ is far away from the optimum temperature in CLC but is an excellent choice to detect the effect of temperature and fuel reactivity (Alberto et al., 2018). Fig.3 (A) and (B) present
Table 2 The operational conditions of 290 min continuous hot operation. Period (min)
Temperature (℃)
OC
Fuel (70 mL/min)
mFR, OC (g)
m˙ OC (kg/h)
τOC , FR (s)
Xgas, FR (%)
0-36 36-60 68-103 103-134 146-177 177-208 222-259 259-291
900 900 875 875 850 850 825 825
Hematite Hematite Hematite Hematite Hematite Hematite Hematite Hematite
CH4 H2 CH4 H2 CH4 H2 CH4 H2
105 105 112 112 109 109 106 106
9.3 9.3 9.2 9.2 9.1 9.1 9.1 9.1
40.6 40.6 43.8 43.8 43.1 43.1 41.9 41.9
16.0 100 7.7 100 3.4 89.1 2.6 84.8
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with CH4 was higher than that of H2. Around 11%–13% O2 was detected in the exhaust gas for H2 combustion, and increased to 13%–16% upon introducing CH4. The O2 concentration in MIFB can be used to qualitatively describe the reduction degree of the OCs. It should be noted that a minor amount of CO was also detected in the FR outlet with CH4 as the fuel. The influence of temperature on CO concentration was unclear, since the CO fraction showed an invariant trend with temperature increase. Two possible CO generation pathways are proposed in previous studies −CH4 decomposition (Gu et al., 2018) and CH4 reforming (Alberto et al., 2018). The CH4 decomposition reaction (R1) would produce solid carbon and gaseous H2, and then solid carbon is gasified by CO2 (R2) or H2O (R3). However, a 40 s solids residence time is too short for carbon gasification, and the low temperature (825–900 ℃) is not in favor of gasification. In addition, carbon from CH4 decomposition would also deposit on the OC surface and then be consumed by air in the AR. However, there is no detected CO2 or CO in the AR outlet, therefore, it is reasonable to speculate that CO originates primarily from CH4 reforming in MIFB.
Table 3 The statistical analysis of pressure difference in AR and FR risers under different temperature. Temperature (℃)
900
875
850
825
FR riser
0.83 0.212 0.86 0.132
0.82 0.144 0.87 0.110
0.81 0. 125 0.86 0. 071
0.81 0. 128 0.89 0. 064
AR riser
Average value (kPa) Variance Average value (kPa) Variance
reaction performance, in which the gas concentration of effluent gas was converted into gas flow of the effluent gas (mL/min) for a clear description. The gas conversion efficiency of H2 and CH4 is shown in Fig.3 (C). Hematite has an excellent reactivity with H2 but shows limited thermodynamic with CH4 (Hossain and Lasa, 2007). High H2 conversion was achieved under 900 ℃ and 875 ℃ such that almost no H2 could be detected at the outlet. The H2 conversion efficiency had a slight reduction from 91% to 86% when the temperature was decreased from 850 to 825 ℃. Although CH4 conversion showed a similar trend with decreasing temperature, the concentration of unconverted CH4 was significantly higher than H2. The CH4 conversion efficiency was only 16% at 900 ℃ with corresponding 59 mL/min stream of unconverted CH4 and a 7 mL/min stream of CO2 produced at the outlet. The conversion efficiency fell to 2% as temperature decreased to 825 ℃. The majority of CH4 cannot be oxidized by OCs in MIFB, which is reflected by the O2 concentration in the AR outlet. The oxygen demand for CH4 combustion should be four times larger than that of H2 combustion when the fuels are completely consumed. However, O2 concentration
CH4 → C+H2
(R1)
CO2+C → CO
(R2)
H2O + C →H2+CO2
(R3)
The capacity of gas conversion in MIFB was verified in this experiment. A significant difference of conversion efficiency was presented between highly reactive H2 and less reactive CH4. Meanwhile, the effect of temperature is reflected by an 8 times increase in CH4 conversion efficiency with a 75℃ increase in temperature. In addition,
Fig. 3. Detailed performance of H2 and CH4 as fuel with hematite OC as a function of the temperature. 5
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Fig. 4. Detailed performance of CO as fuel with hematite OC as a function of the temperature.
the internal gas distributor proved essential in MIFB. Without the enhanced gas diffusion, CH4 conversion would be insufficient to make a notable distinction under different temperatures.
3.3. Comparison between hematite and NiO/Al2O3 OCs The MFIB not only exhibits suitable conversion capability, but also flexibility of use with different oxygen carriers. Besides, other OC characteristics other than reactivity should be reflected in MIFB. Therefore, a comparative experiment with 70 mL/min of CO used as fuel was carried out by using hematite and NiO/Al2O3 as OCs. The hematite test was conducted under continuous operation for 2.6 h at a temperature of 825–900 ℃. The NiO/Al2O3 trial was performed with approximately 6 h of CLC operation, while the operating temperature started at 600℃ due to its excellent reactivity and thermodynamics (Hossain and Lasa, 2007). The testing data in this section was determined using average data from multi-sampling. Fig. 4 shows the effect of temperature on CO conversion with the hematite OC. As temperature increases, the unreacted CO at the outlet decreased from 62.1 to 43.5 mL/min, while the gas flow of CO2 increased from 7.8 to 25.8 mL/min. The conversion efficiency of CO presented an increasing trend with the temperature which was up to a peak of 36.8% at 900 ℃. A higher temperature can accelerate the Fe2O3−CO reactions by providing enough energy for activation in the reaction regions. In addition, carbonaceous gases including CO and CO2 were not detected in the AR. Thus, it is concluded that operating hematite OC in MIFB did not have any atmosphere mixture or carbon deposition issues. Fig.5 (A) presents the data of flue gases in the FR and AR with NiO/ Al2O3 OC. Comparing with hematite, an improved CO conversion appeared in NiO/Al2O3 even at a lower temperature of 600 ℃. With increasing temperature to 900 ℃, the CO2 gas flow increased sharply from 24.2 ml/min to 42.8 ml/min, and CO conversion efficiency was up to 70%. However, a large quantity of CO2 flow was detected in the AR outlet. The CO2 flow in a main reason for particle losses in the AR outlet which showed a decreasing trend as temperature increased, dropping from 22.6 mL/min at 600 ℃ to 5.8 mL/min at 900 ℃. It is reasonable to speculate that CO2 flow in the AR originated from carbon deposition, a fatal problem for Ni-based applications (Ryu et al., 2003). The carbon monoxide can decompose into carbon dioxide and graphite in the Boudouard reaction (R4). The attached solid carbon on the NiO/Al2O3 can cover the active site thus limiting the reduction rate. The source of CO2 in the FR was distinguished to evaluate the degree of carbon deposition, as Fig.5 (B) shows. A severe carbon deposition occurred at temperatures less than 700 ℃, in which the majority of CO2 was produced from the Boudouard reaction. Similarly, it was reported in previous research (Ryu et al., 2003) that carbon deposition decreased with
Fig. 5. Detailed performance of CO as fuel with NiO/Al2O3 OC as a function of the temperature.
temperature. However, excellent regenerative ability was found in NiO/ Al2O3 OC such that little carbon deposition was observed with temperature increase. It seems that the regenerative ability of NiO/ Al2O3 OC was not influenced by multiple cyclic redox reactions and carbon deposition. Besides, it should be noted that carbon formation on OC particles in MIFB was more severe than the general CLC reactors. The low bed inventory is the reason that carbon deposition can be largely avoided if enough metal oxide is supplied in fuel reactor (Mattisson et al., 2006). CO → C + CO2
(R4)
3.4. Characterization of oxygen carrier Along with reactivity and oxygen transport capacity, OC lifetime related to particle attrition is another key parameter for process development. It has a strong impact on economic cost and flow hydrodynamics. The particle losses during the entire hot operation were collected in particle filters to estimate OC lifetime. The appearance of undesirable fluidization was avoided during the whole operation such 6
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Al2O3. Scanning electron microscopy (SEM) was used to probe the microstructure of sampled OC particles, as is shown in Fig.7. The hematite had a smoother surface than that of NiO/Al2O3. Several small grains were found to be covered with a looser structure in NiO/Al2O3. As Fig.8 shows, XRD patterns reveal that the main crystalline phases of NiO/Al2O3 OC in the AR are the spinel phase of NiAl2O4 and Al2O3. The NiO phase cannot be detected since all of the NiO was converted into NiAl2O4 through interaction with Al2O3. The XRD analysis also illustrates good cyclic stability and regenerative ability as the oxygen carriers always reached their full oxidation state after long-term operation. A specific C phase detected in the samples of the FR verified the presence of carbon deposition in the Ni-based OC. For the hematite OC, no Fe3O4 was detected in the AR illustrating that the AR design could fulfill the demand of OC complete oxidation. The oxygen carrier from the FR mainly contained Fe3O4 but also had some unreacted Fe2O3 phase. The reacted Fe3O4 would cover the surface and then restrict contact between inner Fe2O3 and gaseous fuel. (Fig. 9)
that no obvious agglomeration was observed during the operation (including a drastically decreasing pressure drop or defluidization). Therefore, the elutriated OC was only from abrasive attrition, particle fragmentation, and transport disintegration. Identical fluidization conditions were applied with hematite and NiO/Al2O3 as OCs. The critical velocities of the FR and AR were the same at 0.34-0.38 m/s corresponding to around 1.62–1.72 m/s gas velocity in the distributor plates. The inlet gas velocities of the two risers varied between 3.8 and 4.2 m/ s. The generation of fine particles (below 40 μm) was defined from attrition behaviors in the classical method, which cannot fully describe OC breakage in CLC process since the particle size generated by attrition is higher (Amblard et al., 2015). Therefore, the total amount of elutriated particles over the whole size range was used to calculate OC lifetime. 3.681 g of hematite was collected after 7.4 h of CLC operation, including 2.6 h of CO operation, and 4.8 h of CH4 and H2 operation. The attrition rate of hematite OC was 0.219 wt.%/h corresponding to 457 h lifetime. The attrition rate is not constant with time, which is high at the beginning and decreases towards the end. Gayan et al. (Gayan et al., 2011) found that the generation of OC fines was high at the beginning of the fluidization process due to (1) the rounding effects on the particle irregularities and (2) fines stuck to the particles during preparation. The abrasion behavior was enhanced at the beginning until reaching a certain threshold, and abrasion also lessened after a long operational duration where particle breakage tended to stop when reaching a certain size (Amblard et al., 2015). The attrition rate of NiO/Al2O3 was 0.147 wt.%/h corresponding to an OC lifetime of 680 h after 6.8 h CO operation. The mechanical strength of synthetic OCs was higher than that of mineral OCs in this experiment. After high-temperature operation, the MIFB was cooled down to the ambient temperature with an inert N2 atmosphere. All OC particles were collected for particle size analysis. The diffraction particle size analyzer (Master-sizer 2000) was used, which can determine the size distribution of particles from 0.04 to 1800 μm. Fig. 6 presents the particle size distribution of fresh and used OCs samples. The green curve corresponds to the difference between the particle size distribution of fresh and used OCs. The maximum diameter of particle breakage is indicated by the intersection point between the difference curve and x-axis, which is around 0.215 mm and 0.23 mm for hematite and NiO/ Al2O3 OCs, respectively. It can be concluded that OC attrition was not dominant during OC breakage, where only fine particles were generated (Amblard et al., 2015). Therefore, the fragmentation behavior was dominant in OC circulation meaning that several particles with intermediate size would be generated from the splitting of a mother particle. In addition, the higher mechanical strength of NiO/Al2O3 can also be found from the result of the PSD. The abrasion in hematite was severe generating fine particles of hematite that were larger than that of NiO/
4. Conclusion This study aims to analyze the potential of the micro interconnected fluidized bed (MIFB) as an oxygen carrier (OC) evaluation device. In order to reduce operational cost and increase evaluation accuracy, the MIFB incorporates characteristics of lab-scale CLC pilots, while eliminating the complex structure and miniaturizing diameter size. The design criteria followed by MIFB are bed inventory reduction, stable and flexible fluidization, and suitable conversion capacity. The air reactor (AR) and fuel reactor (FR) are two-stage bubbling beds, which each have a 30 mm i.d. and 100 mm height. A perforated plate as the internals is arranged in the middle of the AR and FR to divide them into two identical chambers. After structure optimization, total bed inventory is only 350 g. NiO/Al2O3, produced by an impregnation method, and natural hematite were employed as OCs to investigate the performance of the MIFB and to obtain insights into configuration and operation conditions. The fluidization experiment was carried out under a 290 min CLC operation where stable pressure fluctuation with regular waves was obtained in both the AR and FR risers. Notable distinctions were present between influencers including different temperatures, OCs, and fuels. The conversion efficiencies of H2, CO, and CH4 with hematite OCs were 100%, 36.8%, and 16%, respectively at 900 ℃. The reactivity of gaseous fuels with hematite was indicated clearly in MIFB. Meanwhile, all the fuels showed obvious trends with temperature increase. The NiO/ Al2O3 has a higher reactive activity such that CO conversion efficiency was greater than 61% at 900 ℃. Severe carbon deposition was also found in Ni-based OCs such that 90% CO2 in the FR off-gas was sourced from the Boudouard reaction under low temperature. The attrition rates of hematite and NiO/Al2O3 were 0.219 wt.%/h and 0.147 wt.%/h, respectively. The analysis of particle size distribution showed that OC attrition behavior focused on particles with size around 0.25 mm. The XRD patterns revealed that NiAl2O4 and Al2O3 were the main crystalline phases of the Ni-based OC without the existence of the NiO phase. The experimental campaigns have proven the qualification of MIFB as an OC evaluation device that can reflect the differences of reactivity between fuels and OCs, the influence of reaction temperature, and the phenomenon of CO decomposition and carbon deposition. Declaration of Competing Interest The authors declare no competing financial interest. Acknowledgments
Fig. 6. Particle size distributions of the fresh and used OCs in the MIFB (Amblard et al., 2015).
The authors gratefully acknowledge the support of this research work by National Key R&D Program of China (2018YFB0605404), 7
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Fig. 7. The SEM images of microstructure of used oxygen carriers: (A) NiO/Al2O3 OC; (B) Hematite OC.
Fig. 8. The XRD spectra of NiO/Al2O3 OC in AR and FR.
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Fig. 9. The XRD spectra of hematite OC in AR and FR.
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