Article pubs.acs.org/IECR
Design and Operation of a Fluidized Bed Hydrator for Steam Reactivation of Calcium Sorbent Alan Wang,† Dawei Wang,† Niranjani Deshpande,† Nihar Phalak,† William Wang,† and L.-S. Fan*,† †
William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 W. 19th Avenue, Columbus, Ohio 43210, United States ABSTRACT: The decreasing CO2 capture capacity of calcium sorbents over multiple reaction cycles poses a significant challenge to the large-scale cyclic carbonation-calcination process. Several approaches, including intermediate hydration, have been suggested to overcome this limitation. Until this study, most hydration studies have been performed at laboratory-scale using thermogravimetric techniques at conditions that may not be feasible for process scale-up. Moreover, data on the design of a steam hydrator suitable for the calcium looping process is not available. For the first time, this study reports the design of a bench-scale high-temperature steam hydrator for calcium sorbent reactivation. The hydrator, consisting of a fluidized-bed reactor with additional internals, was evaluated using cold-flow tests following which several reaction parameters were investigated in the hot unit. The results obtained from these high-temperature steam hydration tests (300−500 °C) are discussed here. Specifically, at an average temperature of 473 °C and PH2O = 1 atm, the 30 min hydration conversion exceeded 70%. In addition, the sorbent reactivity toward CO2 was appreciably recovered proportional to the extent of hydration. Lastly, the potential to extract the heat of reaction (>450 °C) for process heat integration further expands the marketability of this high-temperature steam hydrator as a viable option for a wide range of applications ranging from CO2 capture to chemical heat pump systems. reaction that occurs in a coal gasifier.12 Similarly, the Zero Emission Coal Alliance (ZECA) process and the sorptionenhanced steam-methane reforming (SE-SMR) process both introduce CaO into a WGS reactor to reactively remove the CO2 generated in order to generate a high-purity hydrogen (H2).13−16 More recently, the Calcium Looping Process (CLP) has been developed at The Ohio State University (OSU) for high-purity H2 and/or electricity production from coal-derived syngas and natural gas. Besides CO2, the CLP also removes other contaminants like hydrogen chloride (HCl), hydrogen sulfide (H2S), and carbonyl sulfide (COS).17−20 Calcium carbonate-calcium oxide cycles have also been investigated as a CO2 capture technology for post-combustion CO2 removal. All aspects of the process have been heavily researched, including sorbent reactivity, process integration, and process economics.21−25 The overall results have shown great promise to allow for scale-up and testing at a 1.7 MWth demonstration plant.26 The trend that pervades all results using naturally occurring limestone is its rapid decay in sorbent reactivity when cycled and is attributed to thermal sintering during the calcination step. The inability for limestone to maintain a high, stable reactivity over multiple cycles negatively affects the process efficiency and economics, and thus developing a consistently reactive sorbent through sorbent modification remains an area of active research.27,28 Calcium hydroxide (Ca(OH)2), in particular, has shown high reactivity toward the acid gases. This trend also applies to CO2, where commercial-grade Ca(OH)2 has shown superior initial
1. INTRODUCTION Chemisorption using solid sorbents, typically alkali-derived metal oxides, has been widely applied as an air pollution control technology for acid gas control. Specifically, calcium-based solid sorbents are used to remove acid gases, such as sulfur dioxide (SO2), sulfur trioxide (SO3), and hydrochloric acid (HCl), present in coal-fired flue gas.1−3 The use of calcium-based solid sorbents can be further extended to carbon dioxide (CO2) removal. With CO2 regulation being proposed at the local, national, and international level, the necessity to develop a viable, economical process to separate CO2 from sizable point sources, such as fossil-fuel power plants, becomes increasingly important. Currently, approximately two-thirds of the world’s electricity generation is obtained from fossil fuels but is also responsible for 31.8% of anthropogenic CO2 emissions.4,5 Both in the United States and worldwide, coal is currently the dominant source for electricity generation with pulverized coal combustion being the predominant option.6 However, a shift toward higher-efficiency, lower-emissions processes with product flexibility favors natural gas utilization and coal gasification.7−9 The development of a carbon capture technology that can be adapted as a retrofit either for existing coal-fired power plants as well as any future fossil-fueled power plant based on gasification or traditional combustion would provide flexibility and simplicity to a complex process since the exit gas properties and CO2 concentrations vary widely. Chemical looping using solid sorbents and oxygen-carrier particles offers a unique method of achieving high-efficiency energy conversion in different configurations in a carbonconstrained scenario.10,11 Beginning in the 1960s, calcium oxide (CaO) has been researched as a possible sorbent for CO2 capture with the CO2 Acceptor Process, in which CaO reacts with the CO2 formed from the steam-carbon reaction and water-gas shift (WGS) © 2013 American Chemical Society
Received: Revised: Accepted: Published: 2793
September 25, 2012 December 17, 2012 January 14, 2013 January 14, 2013 dx.doi.org/10.1021/ie302611z | Ind. Eng. Chem. Res. 2013, 52, 2793−2802
Industrial & Engineering Chemistry Research
Article
Figure 1. Photographs of the cold-flow model. (a) Reactor body. (b) Double-helical mixer. (c) Central gas distributor.
entering the hydrator instead of steam, the temperature range is lower than necessary for integration into either a steam turbine cycle or WGS reactor, the coupling effect between the hydrodynamics of cohesive CaO/Ca(OH)2 fluidization and the kinetics of the hydration reaction is not well understood, and utilization of reaction heat for steam generation has not been explored.61 In this study, a fluidized bed steam hydrator has been designed and operated to reactivate calcium sorbents for enhanced reactivity toward CO2. The fluidization of the fine calcium particles was realized with the assistance of a rotary mixer and proper gas distribution. Hydration conversion under different temperatures, steam partial pressures (PH2O), and steam flow rates was investigated to optimize the reactor performance.
reactivity over CaO along with a decreased rate of sorbent reactivity decay over multiple cycles.29−32 OSU has also pioneered the development of Ca(OH)2-based CarbonationCalcination Reaction (CCR) process for CO2 control, currently on its way to a 1.9 MWth pilot-scale demonstration.33−40 The challenge with CaO hydration resides within its integration into a carbon capture process, which occurs at intermediate temperatures between 400 and 700 °C. Commercially, Ca(OH)2 formation occurs with CaO and excess liquid water to absorb the heat of reaction to maintain the temperature around 95 °C. However, for integration into a CO2 capture process, either post-combustion CO2 flue gas or WGS reaction, the hydration reaction must occur at an intermediate temperature, where the high-quality heat of reaction can be utilized and the Ca(OH)2 sorbent should maintain its reactivity.41−43 Steam hydration of CaO has mainly been studied at lower temperatures, typically with wet gas instead of steam, for investigating sorbent properties such as carbonation extent and surface morphology.27,28,44−49 While the aforementioned steam hydration experiments have clearly shown to improve sorbent reactivity and stability, the reaction conditions are not an accurate reflection of the reactor design and conditions that are expected when integrated into either a pre-combustion or postcombustion CO2 capture system, with minimal existing data.50 In addition, applications of CaO steam hydration outside of CO2 control have been studied for use in chemical heat pumps, hydration of dolomite, and for production of a reactive, dry hydrate that does not agglomerate. In particular, several CaO/ Ca(OH)2 chemical heat pump systems have explored reactor design of a high-temperature hydrator with heat exchange, heat transfer requirements, and theoretical modeling, which are applicable to steam hydration for integration into a carbon capture process.51−55 CaO hydration has rapid kinetics and high conversions under ambient conditions, but dolomitic hydration requires both elevated temperature and pressure. The Corson pressure hydrator is the primary reactor used for dolomitic hydration and operates at 150 °C and 5 atm with a residence time around 30 min.56−60 The current status of steam hydration provides insight into the important fundamentals of the gas−solid reaction; however, the overall reactor design is incomplete since liquid water has always been the reactant
2. EXPERIMENTAL SECTION Considerations to both the gas−solid hydrodynamics and reaction kinetics are necessary for the design of a fluidized bed hydrator. A cold model was first designed to test the flow characteristics of calcium particles with external aids aimed to create uniform fluidization. The cold model study provided the operating conditions for satisfactory gas−solid mixing and was used as the basis for the design of the hot unit. The CaO hydration rate was investigated with respect to multiple parameters, such as PH2O, steam flow rate, and reaction temperature, using the hot unit. 2.1. Cold Model. Ca(OH)2 particles applicable for acid gas removal and CO2 capture processes are Geldart Group C particles that tend to agglomerate due to strong interparticle van der Waals forces and are very difficult to fluidize without the assistance of external forces. Channeling and spouting may occur where gas bypassing may lead to low conversion and poor reactor performance. Methods of external assistance, including vibration, centrifugal force, magnetic assistance, acoustic and electric fields, or addition of larger (inert) particles as a fluidization aid, can mitigate channel development and control the dynamic size of agglomerates such that the Group C particles can be fluidized.62−67 Mechanical agitation has shown relative success in fluidizing cohesive particles, thus it was chosen in this study for its simplicity and operability.68 2794
dx.doi.org/10.1021/ie302611z | Ind. Eng. Chem. Res. 2013, 52, 2793−2802
Industrial & Engineering Chemistry Research
Article
Figure 2. Schematic diagram of the hydrator apparatus.
Figure 1 shows the cold-flow model with an inner diameter (ID) of 12.7 cm and a height of 30.5 cm. Compressed air was used for fluidization at room temperature. The plenum section evenly distributes the fluidizing gas prior to entering the solid bed through a perforated plate gas distributor. For better gas− solid mixing, a cylindrical gas distributor, with a 2.54 cm ID and a 7.62 cm height, was installed through the perforated plate at the centerline of the fluidized bed. The central to main gas flow ratio could be adjusted in the range of 1:3 to 1:2. A double helical agitator, with a height of 8.89 cm and a 0.32 cm clearance from the reactor wall, was installed to provide additional fluidization assistance. The agitator was powered by a Leeson variable speed motor operating at its maximum speed of 28 rpm. Fixed vertical baffles, suspended from the top of the bed and penetrating downward into the bed, were positioned inside the fluidized bed. The relative motion between the fixed baffles and rotating mixer prevented formation of large agglomerates. The cold-flow model fluidized bed was operated in batch mode where calcium particles were added to form a loosely packed bed with a height of 10.16 cm. Airflow to the reactor was gradually increased, with or without agitation, until channeling, spouting, or smooth fluidization was observed. 2.2. Hot Unit. As shown in Figure 2, a bench-scale stainless steel (SS 304) reactor was designed and fabricated based on the cold model. The reactor has a height of 34.3 and 12.7 cm ID. Mixtures of steam and air at different ratios and temperatures were utilized as the fluidizing medium. The steam generator, consisting of a 60 cm tall stainless steel heating coil surrounded by two high-temperature ceramic heaters (OMEGA Engineering Inc.), is capable of producing superheated steam at 200−300 °C. Water feed to the steam generator was controlled by a high-precision Optos 3HM liquid metering pump (Eldex Laboratories) that can deliver up to 80 mL/min. The steam flow rate was calculated based on the water feed rate and output steam temperature. The air was first preheated by an air torch preheater (OMEGA) to 350−400 °C and then mixed with steam to obtain the desired steam:air ratio, temperature, and flow rate for the combined gas feed. Prior to gas injection, the reactor was heated to the initial set point temperature using a pair of high-temperature ceramic
heaters (Watlow Inc.) controlled by PID controllers (OMEGA i-series). The reactor body and heaters were insulated to minimize heat loss. Multiple Type K thermocouples continuously monitored the temperatures of the steam generator outlet, solid bed, steam inlet, and steam outlet. The temperature data were recorded on a local computer by a data-acquisition box (Measurement Computing) using DAQFactory software (AzeoTech, Inc.). 2.3. High Temperature Hydration Procedure. To demonstrate the effectiveness of this bench-scale steam hydrator for sorbent reactivation, the sorbent was first subjected to thermal sintering at high calcination temperatures. Graymont high-calcium pulverized limestone, whose composition is listed in Table 1, was calcined in a rotary calciner at 900−950 °C for over 2 h until the extent of decomposition exceeded 90%. The hydrator was operated as a semibatch fluidized bed reactor with a continuous steam feed. For each test, a predetermined amount of solid sorbent was fed into the reactor to achieve the desired steam to calcium mol ratio Table 1. Solid Sorbent Composition compound CaCO3 MgO SiO2 Fe2O3+Al2O3 S LOI solid property mean particle size (μm), dp density (kg/m3), ρp particle size distribution diameter (μm) 50+ 20−50 10−20 2−10 0−2 2795
wt % >90.0%
