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CO2 Capture by CaO in a Sound Assisted Fluidized Bed at Ca-Looping Conditions
| Content Provider | Semantic Scholar |
|---|---|
| Author | Valverde, Jose Manuel Raganati, Federica Quintanilla, Miguel Angel Sanchez Ebri, J. M. P. Ammendola, Paola Chirone, Riccardo |
| Copyright Year | 2013 |
| Abstract | The Ca-Looping process is a viable technology to achieve high CO2 postcombustion capture efficiencies (Blamey et al. (1)). This involves the separation of CO2 by means of the carbonation reaction of CaO to capture CO2 in a fluidized bed at high temperature, and the subsequent calcination of limestone (CaCO3) to regenerate the sorbent. In this paper we show a study on the capture performance of a fluidized bed of CaO at Ca-looping conditions as affected by acoustic vibrations, which serve to enhance the fluidization quality and the mass/heat gas-solids transfer. As a consequence, it is shown that the CO2 capture capacity during the fast phase of practical interest is notably increased for sound intensities and vibration frequencies of typically around 140 dB and 100 Hz, respectively. INTRODUCTION The Ca-Looping (CaL) process for postcombustion capture is realized in practice by means of two interconnected fluidized beds. CaO powder in a fluidized bed reacts with the CO2 present in the gas to form CaCO3 at temperatures typically around 650C. The spent sorbent is then regenerated by calcining it at high temperatures (typically around 900C) in a second fluidized bed reactor interconnected with the carbonator. In the calciner, CaCO3 decomposes to yield CaO and a concentrated stream of CO2 ready to be stored. In practice, the sorbent particles must react with CO2 at low volume concentrations (of around 15%) during short contact times while ideally maintaining a high CO2 capture capacity with the number of cycles (Blamey et al. (1), Manovic and Anthony (2)). The CaL process finds also a precombustion application in the sorption enhanced steam methane reforming (SE-SMR) process. Higher methane to hydrogen conversion and improved energy efficiency are achieved by on-line capture of CO2 while steam methane reforming and water gas shift reactions occur (Romano et al. (3)). Carbonation of CaO particles occurs in two phases (Grasa et al. (4)). A first fast carbonation stage is characterized by the sorption of CO2 on the free surface of the particles. After a thin layer of CaCO3 (between 30 and 50 nm thick (Grasa et al. (4)) covers the free surface of the sorbent particles, CO2 sorption turns to be controlled by a much slower phase characterized by the diffusion of CO2 through the solid CaCO3 layer. It must be taken into account that carbonation of CaO during the fast phase proceeds under mass/heat transfer control. Thus, the rate of CO2 capture in the fast phase by a fluidized bed is not just controlled by the kinetics of the chemical reaction itself, but also by the transport of CO2 and heat to the particles' surface. In this regard, carbonation can be hindered by poor and heterogeneous gas/solids contact and mass/heat transfer (Blamey et al. (1)). We describe in the present manuscript the use of a noninvasive physical method to enhance the CO2 capture performance of a fluidized bed of CaO at CaL conditions consisting of the application of acoustic vibration. Acoustic vibrations are useful to promote gas/solids mixing uniformity in fluidized beds by forcing particle vibrations, which reduces aggregation and disrupts gas channels thus homogenizing fluidization and increasing the gas/solids contact efficiency (Ammendola et al. (8)). On the other hand, sound waves induce a number of phenomena taking place at the gas/solids interface, such as acoustic streaming, which contribute to enhance mass/heat transfer rates in gas/solids reactors (Yarin et al. (5), Komarov et al. (6), Al Zaitone (7)). The fundamental principle is that attenuation of acoustic energy flux makes momentum flux available to force streaming motions around the solids. Accordingly, it is seen that the imposition of acoustic vibrations on a steady gas flow yields remarkable improvements in the efficiency of industrial processes such as fuel combustion, pyrometallurgical, and pollutant removal processes involving gas-solid reactions at high temperatures (Komarov et al. (6)). Altogether, the improvement of gas/solids contact efficiency in a heterogeneous fluidized bed and the enhancement of mass/heat transfer at the gas/solids interface, would serve to enhance fast capture of CO2 in a fluidized bed carbonator at CaL conditions as it is confirmed by our experimental results. Figure 1: Particle size distribution of CaO samples suspended in isopropanol (stirred and ultrasonicated) taken before and after 1 carbonation/calcination cycle. Measurements performed by means of laser based diffractometry using a Mastersizer 2000 (Malvern Instruments). EXPERIMENTAL SETUP AND PROCEDURE CaO from Sigma-Aldrich (see Fig. 1 for particle size distribution measurements) has been used as CO2 sorbent. The experimental setup used is schematized in Fig. 2. The material (100 g corresponding to a bed height of 5 cm) is placed in a 50 mm i.d. quartz reactor where it rests over a porous quartz plate that serves as gas distributor. The inlet gas flow is set to 2000 cm/min (which is about 5 times the minimum fluidization gas velocity) and can be switched to dry air (used for calcination) or to a mixture of 15% CO2/85% N2 in volume (used for carbonation) by means of mass flow controllers. The material is firstly subjected to a calcination step (T=900C) for 15 min during which Ca(OH)2 and CaCO3 present as impurities decompose to CaO. A carbonation step then proceeds while the vol % of CO2 in the effluent gas is continuously registered by a gas analyzer. A subsequent calcination step is carried out up to complete decarbonation. A digital signal generator produces an electric sine wave of fixed frequency which is amplified by a power audio amplifier and excites a 8 W woofer loudspeaker. The acoustic vibration is driven to the reactor by means of a wave guide. The Sound Pressure Level (SPL) is sampled by a 1/4'' condenser microphone. Figure 2: Sketch of the experimental setup. 1: Compressed gas used for carbonation (15% CO2/85% N2 vol/vol). 2: Compressed gas used for calcination (dry air). 3: Mass flow controllers. 4: Temperature controller. 5: Oven. 6: Quartz reactor. 7: Sound wave guide. 8: Elastic membrane. 9: Microphone. 10: Loudspeaker. 11: Differential pressure transducer. 12: Particle filter. 13: Mass flow meter. 14: Gas analyzer. 15: Signal amplifier. 16: Signal generator. 17: Oscilloscope. Measurements of the differential gas pressure ∆p across the powder at ambient temperature (taken between a point just above the gas distributor plate and atmospheric pressure) were performed by using a 40 mm i.d. polycarbonate cell. The purpose of these tests was to assess the fluidization behavior of the powder as affected by acoustic vibrations. Typically, it is ∆p ~ W, where W is the material weight per unit area, for uniform fluidization. EXPERIMENTAL RESULTS AND DISCUSSION Figure 3 shows ∆p vs. gas flow for fresh CaO, which exhibits in ordinary conditions a cohesive heterogeneous fluidization (Geldart C behavior). Once the gas velocity reaches a value sufficiently large to fluidize the bed, ∆p takes a maximum value notably smaller than W and displays strong oscillations due to the development of channels through which the gas finds a bypass indicating very poor gas/solids contact efficiency. On the other hand, Fig. 3 demonstrates that acoustic vibrations serve to smooth the fluctuations of ∆p, which now reaches a plateau that approaches W as either the sound intensity level (SPL) or frequency f are increased being the effect of sound intensity more marked. Visually, it is observed that gas channels are destabilized, the bed is expanded and gas-solids mixing uniformity is improved. Figure 3: Differential gas pressure between the bottom of the bed and ambient atmosphere (made nondimensional with the material weight per unit area W=513 Pa) vs. gas velocity as affected by acoustic vibrations applied of different intensities and frequencies (indicated). In a) the SPL is fixed to 140 dB. In b) the frequency is fixed to 100 HZ. CO2 breakthrough curves as affected by acoustic vibration are plotted in Fig. 4. As can be observed, application of sound causes a significant drop of the %CO2 measured in the effluent gas during the first minutes of carbonation (fast phase). Moreover, there is a clear correlation between the enhancement of CO2 capture in the fast phase, which is determined by the gas/solids contact efficiency, and the improvement of fluidization uniformity (as seen in Fig. 3). In both cases the main effect is observed when the sound intensity is increased while the effect of frequency is less relevant. Note also that the CO2 breakthrough curves tend to converge at t ~ 50 min when %CO2 ~ 10%. This suggests that from this point, CO2 sorption becomes ruled by a slower process in which the gas-solids contact efficiency is not a determinant factor. Figure 4: a) CO2 vol % in the effluent gas of the fludized bed (gas flow 2000 cm 3 /min) measured during carbonation as affected by acoustic vibration applied of different intensities and frequencies (indicated). Results from blank tests (empty cell) are also shown. In b) CO2 breakthrough curves obtained by turning on/off the acoustic vibration (140 dB, 130 Hz) are plotted. Figure 4b shows CO2 breakthrough curves obtained by turning on/off the acoustic vibration during carbonation. Turning on acoustic vibration at t ~ 6 min yields a marked drop of the %CO2 while turning it off gives rise to an increase of the %CO2. Note, however, that in this latter case the CO2 breakthrough curve keeps well below the curve obtained for the ordinary test in spite that the sound had been turned off. This indicates that the acoustic vibration applied during the first minutes of carbonation preconditions the material behavior likely by irreversibly disrupting particle aggregates and improving fluidization uniformity. Figure 5: a) Differential gas pressure between the bottom of the bed and ambient atmosphere (made nondimensional with the material weight per unit area W=513 Pa) |
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| Alternate Webpage(s) | http://dc.engconfintl.org/cgi/viewcontent.cgi?article=1033&context=fluidization_xiv |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
