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ABSTRACT: Calcium looping processes for capturing CO2 from large emissions sources are based on the use of CaO particles as sorbent in circulating fluidized-bed (CFB) reactors. A continuous flow of CaO from an oxyfired calciner is fed into the carbonator and a certain inventory of active CaO is expected to capture the CO2 in the flue gas. The circulation rate and the inventory of CaO determine the CO2 capture efficiency. Other parameters such as the average carrying capacity of the CaO circulating particles, the temperature, and the gas velocity must be taken into account. To investigate the effect of these variables on CO2 capture efficiency, we used a 6.5 m height CFB carbonator connected to a twin CFB calciner. Many stationary operating states were achieved using different operating conditions. The trends of CO2 capture efficiency measured are compared with those from a simple reactor model. This information may contribute to the future scaling up of the technology. © 2010 American Institute of Chemical Engineers AIChE J, 57: 000–000, 2011
AIChE Journal 04/2011; 57(5):1356 - 1366. · 2.26 Impact Factor
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ABSTRACT: Steam hydration has been proposed as a suitable technique for improving the performance of CaO as a regenerable sorbent in CO2-capture systems. New hydration experiments conducted in this study confirm the reported improvements in the capacity of sorbents to carry CO2. An examination of the textural properties of the sorbent after hydration and mild calcination revealed a large increase in the area of the reaction surface and the formation of a fraction of pores ≈20 nm diameter that enhance the CO2 carrying capacity and increase the carbonation reaction rate. However, these changes in textural properties also lead to lower values of crushing strength as measured in the reactivated particles. Experiments conducted with a high hydration level of the sorbent [Ca molar conversion to Ca(OH)2 of 0.6] in every cycle produced a 6-fold increase in the sorbent residual CO2 carrying capacity. This improvement has been estimated to be achieved at the expense of a very large consumption of steam in the system (about 1.2 mol of steam/mol of captured CO2). The trade off between the improvements in CO2 capture capacity and steam consumption is experimentally investigated in this work, with it being concluded that there is need to design a comprehensive sorbent reactivation test that takes into account all of the hydration reactivation processes.
03/2011;
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ABSTRACT: This work analyzes the capacity of a novel CaCO3 precalcination method to reduce the CO2 emissions from an industrial cement plant by means of CO2 capture and storage. The precalciner consists of a circulating fluidized-bed combustor (CFBC) operating at about 1050 °C connected with a fluidized-bed calciner. A stream of CaO at about 940 °C coming from the CFBC transfers the heat required for calcination to the fluidized-bed calciner. The thermal integration between the main units in the capture system (the CFBC, the calciner, the CO2 compressor, and a reference cement plant) was fully analyzed using Aspen HYSYS. Optimum management of the heat fluxes between the units is essential to minimize the heat requirements for the CO2 capture system. A subcritical steam cycle that can be integrated into the new cement plant is proposed to obtain the energy necessary to drive the CO2 compressor and generate electricity, which can either be used in the cement plant itself or be exported. It is shown that, with this level of integration, it is possible to avoid about 38% of the CO2 emissions produced by the cement manufacturing process. Furthermore, an estimation of the additional cost of implementing the precalcination system in the cement plant shows that a competitive cost of about $12 per ton of CO2 avoided can be achieved.
01/2011;
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ABSTRACT: This paper presents a new solids looping process for capturing CO2 while generating hydrogen and/or electricity from natural gas. The process is based on the sorption enhanced reforming of CH4, employing CaO as a high temperature CO2 sorbent, combined with a second chemical loop of CuO/Cu. The exothermic reduction of CuO with CH4 is used to obtain the heat necessary for the decomposition of the CaCO3 formed in the reforming step. The main part of the process is completed by the oxidation of Cu to CuO, which is carried out with air diluted with a product gas recycle of this reactor at sufficiently low temperatures and high pressures to avoid the decomposition of a substantial fraction of CaCO3.
Environmental Science & Technology 09/2010; 44(17):6901-4. · 4.80 Impact Factor
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ABSTRACT: There is an emerging postcombustion capture technology that uses CaO to capture CO2 from combustion flue gases in a circulating fluidized bed reactor. This paper summarizes recent work conducted at CSIC to
understand and develop this technology. The paper includes experimental results at conditions close to those expected in the
real system, carried out in continuous mode in a 30kW test facility made up of two interconnected circulating fluidized bed
reactors. In one of the reactors, CO2 is captured from the gas phase by the CaO continuously circulating from a calciner. In the second reactor, the CaCO3 formed in the carbonator is regenerated to CaO and CO2 by calcination. Modeling of the system at process level, at reactor level (in particular the CFB carbonator), and at particle
level (decay in capture capability of CaO) is also outlined. The work carried out so far confirms that the carbonator reactors
can be designed to attain capture efficiencies between 70–90%, operating at fluid dynamic conditions close to those present
in circulating fluidized bed combustors.
KeywordsCO2 capture-carbonation-calcination-reactor modeling-pilot testing
12/2009: pages 549-554;
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ABSTRACT: The use of an internal circulating fluidized bed (ICFB) is proposed for the oxidative dehydrogenation of butane. The reactor consists of two adjacent zones, separated by a wall. In the oxidizing zone the catalyst is oxidized and in the reducing zone the butane is dehydrogenated by reaction with the oxygen from the catalyst lattice. The circulation between both zones is produced by the different bed porosity, which originates a pressure difference between the two zones at the communication orifice at the bottom of the vessel. The design of this reactor is studied in a cold model. An image analysis technique was applied to measure solid circulation rates between the different regions in the reactor, by using a solid coated with a long afterglow phosphor as a tracer. The solid circulation rates through the orifices were found to correlate reasonably well with a previously developed correlation for the orifice drag coefficient. The model for solid circulation between fluidized regions was then integrated in the overall mathematical model for the ICFB reactor system to incorporate the effect of design and operating conditions on the catalyst circulation rates between compartments. A good correlation between the experimentally obtained values of conversion and selectivity and those predicted by the model is obtained. Improvements in selectivity to olefins are obtained, compared with conventional fluidized-bed reactors with cofeeding of reactants. © 2004 American Institute of Chemical Engineers AIChE J, 50: 1510–1522, 2004
AIChE Journal 06/2004; 50(7):1510 - 1522. · 2.26 Impact Factor
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ABSTRACT: There is a growing interest in developing high temperature CO2 capture looping systems using CaO as a regenerable sorbent. The evolution of the sorbent properties with the number of cycles plays an important role in the design of these systems. One of the key variables to be determined is the sorbent's CO2 carrying capacity at the end of the so called fast carbonation stage. This is the only useful conversion for practical purposes and it is known to decrease with the number of cycles. It is obviously important to obtain experimental CaO deactivation curves from sorbent laboratory tests in conditions as close as possible those to an industrial system. This paper reviews previously reported results and investigates the effect of carbonation conditions on the CO2 carrying capacities of CaO. An upgraded version of an existing deactivation model is used to provide a better interpretation of the sorbent deactivation trends observed. The inclusion in the deactivation model of an additional diffusion controlled carbonation stage may help to explain some important discrepancies and observations reported in the literature. This review highlights the need for an improved methodology to avoid the distortional diffusional effects in the determination of CaO deactivation curves.
Chemical Engineering Journal. 167(1):255-261.
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ABSTRACT: Two widely used models to describe axial solid mixing in fluidised beds (the dispersion model and the countercurrent backmixing (CCBM) model) are evaluated against identical sets of experimental data. Experimental work has been obtained at different conditions (gas velocity, particle properties and two column diameters) using an image analysis technique. Previously published data by other authors are also compiled to enlarge the experimental database for model development and validation. It is shown that both models are capable to fit the majority of experiments well, in agreement with a well-known relation between the models in some extreme conditions. This relation is further explored by incorporating independent measurements of the tracer rise velocities during the mixing experiments. It is concluded that, although a simple correlation for the solid dispersion coefficients compiled in this work is useful, the CCBM model is a much more reliable idealisation in describing and scaling up axial solid mixing in fluidised beds.
Chemical Engineering Science 57(14):2791-2798. · 2.43 Impact Factor
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ABSTRACT: Experiments in a cold model have been conducted to determine lateral solid diffusion coefficients in a narrow fluidised bed. The experimental data have been obtained using an image analysis technique that traces the dispersion of phosphor coated particles in the bed. A diffusion model has been used to describe lateral solids mixing in the configuration proposed for the narrow fluidised bed. The equation proposed by Shi and Fan [1984. Lateral mixing of solids in batch gas–solids fluidized beds. Industrial and Engineering Chemistry, Process Design and Development 23, 337–341] to calculate the diffusion coefficient in the radial direction fits well the available experimental data. The solid diffusion coefficients obtained have been used to solve a heat transfer problem and discuss the feasibility of a fluidised bed system that makes use of narrow fluidised beds arranged to transfer heat from bed to bed through a separating wall between them. This novel configuration of fluidised bed reactors could find application in some emerging systems that make use of solid regenerable sorbents to capture CO2.
Chemical Engineering Science 62:619-626. · 2.43 Impact Factor
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ABSTRACT: Post-combustion carbonate looping processes are based on the capture of CO2 from the flue gases of an existing power plant in a circulating fluidized bed (CFB) reactor of CaO (the carbonator) at around 650 ∘C. The calcination of CaCO3 in a new oxy-fired experimental results from a small test facility (30 kWt) operated in continuous mode using two interconnected CFB reactors as carbonator and calciner. Capture efficiencies between 70 and 97% have been obtained under realistic flue gas conditions in the carbonator reactor. The similarity between process conditions and those existing in CFBC power plants should allow a rapid scaling up of this technology.
Energy Procedia 1(1):1147-1154.
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ABSTRACT: It is well known that the solid sorbents used in calcium looping CO2 capture systems experience a reduction in carrying capacity with the number of cycles. Several sorbent reactivation schemes have been proposed as means of overcoming this deactivation process. This work analyzes the integration of a reactivation process in a Ca-looping cycle by means of a hydration reactor. The mass balances involved in this three-reactor systems must then be solved in order to evaluate the effect of the different variables on the average activity of the sorbent. The positive impact of reactivation by hydration (i.e. average increase in activity of the sorbent arriving at the carbonator) is discussed in conjunction with the negative impacts on the overall operation of the system (e.g. steam consumption, etc.) and on the large reactivation reactors. Two different scenarios employing different degrees of hydration have been evaluated. The results obtained show that steam is used more efficiently when only a small fraction of the circulating solids is hydrated to a high degree. Moreover, the performance of the sorbent reactivation step is better when low make up flows of limestone are used.
Chemical Engineering Journal. 163(3):324-330.
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ABSTRACT: In recent years several processes incorporating a carbonation–calcination loop in an interconnected fluidized bed reactor have been proposed as a way to capture CO2 from flue gases. This paper is a first approximation to the modelling of a fluidized bed carbonator reactor. In this reactor the flue gas comes into contact with an active bed composed of particles with very different activities, depending on their residence time in the bed and in the carbonation–calcination loop. The model combines the residence time distribution functions with existing knowledge about sorbent deactivation rates and sorbent reactivity. The fluid dynamics of the solids (CSTR) and gases (PF) in the carbonator are based on simple assumptions. The carbonation rates are modelled defining a characteristic time for the transition between a fast reaction regime to a regime with a zero reaction rate. On the basis of these assumptions the model is able to predict the CO2 capture efficiency for the flue gas depending on the operating and design conditions. Operating windows with high capture efficiencies are discussed, as well as those conditions where only modest capture efficiencies are possible.
Chemical Engineering Science.
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ABSTRACT: Calcium looping is an energy-efficient CO2 capture technology that uses CaO as a regenerable sorbent. One of the advantages of Ca-looping compared with other postcombustion technologies is the possibility of operating with flue gases that have a high SO2 content. However, experimental information on sulfation reaction rates of cycled particles in the conditions typical of a carbonator reactor is scarce. This work aims to define a semiempirical sulfation reaction model at particle level suitable for such reaction conditions. The pore blocking mechanism typically observed during the sulfation reaction of fresh calcined limestones is not observed in the case of highly cycled sorbents (N > 20) and the low values of sulfation conversion characteristic of the sorbent in the Ca-looping system. The random pore model is able to predict reasonably well, the CaO conversion to CaSO4 taking into account the evolution of the pore structure during the calcination/carbonation cycles. The intrinsic reaction parameters derived for chemical and diffusion controlled regimes are in agreement with those found in the literature for sulfation in other systems. © 2011 American Institute of Chemical EngineersAIChE J, 2011
AIChE Journal · 2.26 Impact Factor
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ABSTRACT: Post-combustion carbonate looping processes are based on the capture of carbon dioxide from the flue gases of an existing power plant in a circulating fluidized bed reactor (CFB) of calcium oxide (the carbonator) particles. The calcination of calcium carbonate in a new oxy-fired CFBC power plant regenerates the sorbent (calcium oxide particles) and obtains high purity carbon dioxide. This communication presents experimental results from a small test facility (30 kWt) operated in continuous mode using two interconnected CFB reactors as carbonator and calciner. Capture efficiencies between 70 and 97% have been obtained under realistic flue gas conditions in the carbonator reactor (temperatures around 650 °C). The similarity between process conditions and those existing in CFBC power plants should allow a rapid scaling up of this technology. The next steps for this process development are also outlined.
International Journal of Greenhouse Gas Control.
