The effect of CO<Subscript>2</Subscript> or steam partial pressure in the regeneration of solid sorbents on the CO<Subscript>2</Subscript> capture efficiency in the two-interconnected bubbling fluidized-beds system

The effect of CO₂ or steam partial pressure in the regeneration of CO₂ solid sorbents was studied in the two-interconnected bubbling fluidized-beds system. Potassium-based dry solid sorbents, which consisted of 35 wt% K₂CO₃ for CO₂ sorption and 65 wt% supporters for mechanical strength, were used. To investigate the CO₂ capture efficiency of the regenerated sorbent after the saturated sorbent was regenerated according to the CO₂ or steam partial pressure in the regeneration, the mole percentage of CO₂ in the regeneration gas was varied from 0 to 50 vol% with N₂ balance and that of steam was varied from 0 to 100 vol% with N₂ balance, respectively. The CO₂ capture efficiency for each experimental condition was investigated for one hour steady-state operation with continuous solid circulation between a carbonator and a regenerator. The CO₂ capture efficiency decreased as the partial pressure of CO₂ in the fluidization gas of the regenerator increased, while it increased as that of steam increased. When 100 vol% of steam was used as the fluidization gas of the regenerator, the CO₂ capture efficiency reached up to 97% and the recovered CO₂ concentration in the regenerator was around 95 vol%. Those results were verified during 10-hour continuous experiment.     

Effect of water pretreatment on CO<Subscript>2</Subscript> capture using a potassium-based solid sorbent in a bubbling fluidized bed reactor

A bubbling fluidized bed reactor was used to study CO₂ capture from flue gas by using a potassium-based solid sorbent, sorbKX35 which was manufactured by the Korea Electric Power Research Institute. A dry sorbent, sorbKX35, consists of K₂CO₃ for absorption and supporters for mechanical strength. To increase initial CO₂ removal, some amount of H₂O was absorbed in the sorbent before injecting simulated flue gas. It was possible to achieve 100% CO₂ removal for more than 10 minutes at 60°C and a residence time of 2 s with H₂O pretreatment. When H₂O pretreatment time was long enough to convert K₂CO₃ of sorbKX35 into K₂CO₃ · 1.5H₂O, CO₂ removal was excellent. The results obtained in this study can be used as basic data for designing and operating a large scale CO₂ capture process with two fluidized bed reactors. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Experimental investigation of a circulating fluidized‐bed reactor to capture CO2 with CaO

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     

Simultaneous experiments of sulfidation and regeneration in two pressurized fluidized-bed reactors for hot gas desulfurization of IGCC

Hot Gas Desulfurizarion for IGCC is a new method to efficiently remove H2S in fuel gas with regenerable sorbents at high temperature and high-pressure conditions. The Korea Institute of Energy Research did operation of sulfidation in a desulfurizer and regeneration in a regenerator simultaneously at high pressure and high temperature conditions. The H₂S concentration at exit was maintained continuously below 50ppmv at 11,000 ppmv of inlet H₂S concentration. The sorbent had little effect on the reducing power in the inlet gas in the range from 11% to 33% of H₂. As inlet H₂S concentration was increased, H₂S concentration in the product gas was also increased linearly. The sorbent was maintained at low sulfur level by the continuous regeneration and the continuous solid circulation at the rate of 1.58× 10⁻³ kg/s with little mean particle size change.     

Investigation into a gas–solid–solid three-phase fluidized-bed carbonator to capture CO2 from combustion flue gas

A two-dimensional (2D) transient model was developed to simulate the local hydrodynamics of a gas (flue gas)–solid (CaO)–solid (CaCO₃) three-phase fluidized-bed carbonator using the computational fluid dynamic method, where the chemical reaction model was adopted to determine the molar fraction of CO₂ at the exit of carbonator and the partial pressure of CO₂ in the carbonator. This investigation was intended to improve an understanding of the chemical reaction effects of CaO with CO₂ on the CO₂ capture efficiency of combustion flue gases. For this purpose, we had utilized Fluent 6.2 to predict the CO₂ capture efficiency for different operation conditions. The adopted model concerning the reaction rate of CaO with CO₂ is joined into the CFD software. Model simulation results, such as the local time-averaged CO₂ molar fraction and conversion of CaO, were validated by experimental measurements under varied operating conditions, e.g., the fraction of active CaO, chemical reaction temperature, particle size, and cycle number at different locations in a gas–solid–solid three-phase fluidized bed carbonator. Furthermore, the local transient hydrodynamic characteristics, such as gas molar fraction and partial pressure were predicted reasonably by the chemical reaction model adopted for the dynamic behaviors of the gas–solid–solid three-phase fluidized bed carbonator. On the basis of this analysis, capture CO₂ strategies to reduce CO₂ molar fraction in exit of carbonator reactor can be developed in the future. It is concluded that a fluidized bed of CaO can be a suitable reactor to achieve very effective CO₂ capture from combustion flue gases.     

Modeling and analysis of circulation variables of continuous sorbent loop cycling for CO<Subscript>2</Subscript> capture

Carbon capture and storage (CCS) technologies are a cornerstone for reducing CO₂ emissions from energy and energy-intensive industries. Among the various CCS technologies, solid sorbent looping systems are considered to be potentially promising solutions for reducing CO₂ capture energy penalty. We present an evaluation module for a carbonator with sorbent looping cycle to calculate the carbonation efficiency. The module incorporates a simple sorbent activity model, and the solid/gas balances are constructed by assuming simple reactor mixing quality. By conducting simulations, we examine the variation in the carbonation efficiencies as a function of the sorbent looping operation factors and discuss an optimum operating strategy.     

CO2 capture from flue gases using a fluidized bed reactor with limestone

The CO₂ capture from flue gases by a small fluidized bed reactor was experimentally investigated with limestone. The results showed that CO₂ in flue gases could be captured by limestone with high efficiency, but the CO₂ capture capacity of limestone decayed with the increasing of carbonation/calcination cycles. From a practical point of view, coal may be required to provide the heat for CaCO₃ calcination, resulting in some potential effect on the sorbent capacity of CO₂ capture. Experiment results indicated that the variation in the capacity of CO₂ capture by using a limestone/coal ash mixture with a cyclic number was qualitatively similar to the variation of the capacity of CO₂ capture using limestone only. Cyclic stability of limestone only undergoing the kinetically controlled stage in the carbonation process had negligible difference with that of the limestone undergoing both the kinetically controlled stage and the product layer diffusion controlled stage. Based on the experimental data, a model for the high-velocity fluidized bed carbonator that consists of a dense bed zone and a riser zone was developed. The model predicted that high CO₂ capture efficiencies (>80%) were achievable for a range of reasonable operating conditions by the high-velocity fluidized bed carbonator in a continuous carbonation and calcination system.     

Hydrodynamic analysis of a 10 kWth Calcium Looping Dual Fluidized Bed for post-combustion CO2 capture

Calcium Looping (CaL) in a Dual Fluidized Bed (DFB), utilizing a carbonator and a regenerator, is a post-combustion CO₂ capture technology currently under development. At IFK, University of Stuttgart, a 10 kW_th CaL DFB system has been built consisting of a carbonator riser and a Bubbling Fluidized Bed (BFB) regenerator. A major novelty of this facility is the implementation of a cone valve to control the sorbent looping rate between the two beds. This study presents detailed results of tests conducted on a hydrodynamically scaled cold model of the 10 kW_th CaL DFB facility. The performance of the cold model was compared with CaL process boundary conditions in order to determine the suitability of the 10 kW_th CaL DFB system. The resulting qualitative conclusions regarding DFB hydrodynamics may be of aid to other DFB processes, such as Chemical Looping Combustion (CLC) and Fast Internally Circulating Fluidized Bed (FICFB) gasification. All important operational parameters of the cold model DFB system, namely the Total Solid Inventory (TSI), riser superficial velocity, loop seal aeration, BFB overpressure, cone valve opening and mean particle size were varied in order to fully characterize the DFB operation. A stable operating region, bordered by two unstable regions, has been identified for the cold model riser. The cold model riser pressure drop profile, solid fraction profile, solid flow structure and their variation with respect to operational parameters have been analyzed in order to draw conclusions regarding axial inventory allocation and gas–solid contacting which are important criteria for the CFB carbonator's CO₂ capture efficiency. Finally, empirical correlations regarding the cold model riser entrainment and the solid looping rate have been derived.     

Integration of a Ca looping system for CO2 capture in existing power plants

This work analyses a Ca looping system that uses CaO as regenerable sorbent to capture CO₂ from the flue gases generated in power plants. The CO₂ is captured by CaO in a CFB carbonator while coal oxycombustion provides the energy required to regenerate the sorbent. Part of the energy introduced into the calciner can be transferred to a new supercritical steam cycle to generate additional power. Several case studies have been integrated with this steam cycle. Efficiency penalties, mainly associated with the energy consumption of the ASU, CO₂ compressor and auxiliaries, can be as low as 7.5% p. of net efficiency when working with low‐CaCO₃ make‐up flows and integrating the Ca looping with a cement plant that makes use of the spent sorbent. The penalties increase to 8.3% p. when this possibility is not available. Operation conditions aiming at minimum calciner size result in slightly higher‐efficiency penalties. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Ca-based sorbent looping combustion for CO2 capture in pilot-scale dual fluidized beds

To demonstrate process feasibility of in situ CO₂ capture from combustion of fossil fuels using Ca-based sorbent looping technology, a flexible atmospheric dual fluidized bed combustion system has been constructed. Both reactors have an ID of 100 mm and can be operated at up to 1000 °C at atmospheric pressure. This paper presents preliminary results for a variety of operating conditions, including sorbent looping rate, flue gas stream volume, CaO/CO₂ ratio and combustion mode for supplying heat to the sorbent regenerator, including oxy-fuel combustion of biomass and coal with flue gas recirculation to achieve high-concentration CO₂ in the off-gas. It is the authors' belief that this study is the first demonstration of this technology using a pilot-scale dual fluidized bed system, with continuous sorbent looping for in situ CO₂ capture, albeit at atmospheric pressure. A multi-cycle test was conducted and a high CO₂ capture efficiency (> 90%) was achieved for the first several cycles, which decreased to a still acceptable level (> 75%) even after more than 25 cycles. The cyclic sorbent was sampled on-line and showed general agreement with the features observed using a lab-scale thermogravimetric analysis (TGA) apparatus. CO₂ capture efficiency decreased with increasing number of sorbent looping cycles as expected, and sorbent attrition was found to be another significant factor to be limiting sorbent performance.     

Economical assessment of competitive enhanced limestones for CO2 capture cycles in power plants

CO₂ capture systems based on the carbonation/calcination loop have gained rapid interest due to promising carbonator CO₂ capture efficiency, low sorbent cost and no flue gases treatment is required before entering the system. These features together result in a competitively low cost CO₂ capture system. Among the key variables that influence the performance of these systems and their integration with power plants, the carbonation conversion of the sorbent and the heat requirement at calciner are the most relevant. Both variables are mainly influenced by CaO/CO₂ ratio and make-up flow of solids. New sorbents are under development to reduce the decay of their carbonation conversion with cycles. The aim of this study is to assess the competitiveness of new limestones with enhanced sorption behaviour applied to carbonation/calcination cycle integrated with a power plant, compared to raw limestone. The existence of an upper limit for the maximum average capture capacity of CaO has been considered. Above this limit, improving sorbent capture capacity does not lead to the corresponding increase in capture efficiency and, thus, reduction of CO₂ avoided cost is not observed. Simulations calculate the maximum price for enhanced sorbents to achieve a reduction in CO₂ removal cost under different process conditions (solid circulation and make-up flow). The present study may be used as an assessment tool of new sorbents to understand what prices would be competitive compare with raw limestone in the CO₂ looping capture systems.     

Modeling the carbonator of a Ca-looping process for CO2 capture from power plant flue gas

The calcium-looping process is a promising technique for CO₂ capture from coal-fired power plants and for reducing GHG emissions from the power generation sector. This paper presents a calculation model of the carbonator, the key reactor of the Ca-looping process, where CO₂ is captured as a result of its reaction with CaO. The model presented is based on the Kunii–Levenspiel theory for circulating fluidized bed and on the recent findings on the properties of CaO as a CO₂ sorbent, while taking into account the effects of coal ash and sulfur species.This model can be used for process optimization and for the prediction of the performance of power plants based on the Ca-looping process. Also presented in this paper are the results of a sensitivity analysis of the primary parameters that influence the performance of the carbonator. These results confirm the feasibility of the Ca-looping process with reactors of reasonable size for industrial applications and highlight the importance of the properties of the Ca-based sorbent as they highly affect the carbonator's performance.     

Modelling of a fluidized bed carbonator reactor to capture CO2 from a combustion flue gas

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.     

Effect of bed height on the carbon dioxide capture by carbonation/regeneration cyclic operations using dry potassium-based sorbents

The effect of bed height on CO₂ capture was investigated by carbonation/regeneration cyclic operations using a bubbling fluidized bed reactor. We used a potassium-based solid sorbent, SorbKX35T5 which was manufactured by the Korea Electric Power Research Institute. The sorbent consists of 35% K₂CO₃ for absorption and 65% supporters for mechanical strength. We used a fluidized bed reactor with an inner diameter of 0.05 m and a height of 0.8 m which was made of quartz and placed inside of a furnace. The operating temperatures were fixed at 70 °C and 150 °C for carbonation and regeneration, respectively. The carbonation/regeneration cyclic operations were performed three times at four different L/D (length vs diameter) ratios such as one, two, three, and four. The amount of CO₂ captured was the most when L/D ratio was one, while the period of maintaining 100% CO₂ removal was the longest as 6 minutes when L/D ratio was three. At each cycle, CO₂ sorption capacity (g CO₂/g sorbent) was decreased as L/D ratio was increased. The results obtained in this study can be applied to design and operate a large scale CO₂ capture process composed of two fluidized bed reactors. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Sorbent-enhanced/membrane-assisted steam-methane reforming

Thermodynamic equilibrium and kinetic reactor models are used to simulate a fluidized bed membrane reactor with in situ or ex situ hydrogen and/or CO₂ removal for production of pure hydrogen by steam methane reforming. In the equilibrium model, the membranes and CO₂ removal are located in separate vessels downstream of the reformer. As the recycle ratio increases, the overall performance approaches that where membranes are located inside the reactor. Whether located in situ or ex situ, hydrogen removal by membranes and CO₂ capture by sorbents both enhance hydrogen production. In the kinetic reactor model, a circulating fluidized bed membrane reformer is coupled with a catalyst/sorbent regenerator. Sorbent enhancement combined with membranes could provide very high hydrogen yields. In addition, since carbonation is exothermic, with its heat of reaction similar in magnitude to the endothermic heat of reaction of the net reforming reactions, sorbent enhancement can provide much of the heat needed in the reformer. The overall heat needed for the process would then be provided in a separate calciner, acting as a sorbent regenerator. While the technology is promising, several practical issues need to be examined.     

Lime enhanced gasification of solid fuels: Examination of a process for simultaneous hydrogen production and CO2 capture

The lime enhanced gasification (LEGS) process uses CaO as a CO₂ carrier and consists of two coupled reactors: a gasifier in which CO₂ absorption by CaO produces a hydrogen-rich product gas, and a regenerator in which the sorbent is calcined producing a high purity CO₂ gas stream suitable for storage. The LEGS process operates at a pressure of 2.0 MPa and temperatures less than 800 °C and therefore requires a reactive fuel such as brown coal. The brown coal ash and sulfur are purged from the regenerator together with CaO which is replaced by fresh limestone in order to maintain a steady-state CaO carbonation activity (a_ave). Equilibrium calculations show the influence of process conditions and coal sulfur content on the gasifier carbon capture (>95% is possible). Material balance calculations of the core process show that the required solid purge of the sorbent cycle is mainly attributed to the necessary removal of ash and CaSO₄ if the solid purge is used as a pre-calcined feedstock for cement production. The decay in the CaO capture capacity over many calcination–carbonation cycles demands a high sorbent circulation ratio but does not dictate the purge fraction. A thermodynamic analysis of a LEGS-based combined power and cement production process, where the LEGS purge is directly used in the cement industry, results in an electric efficiency of 42% using a state of the art combined cycle.     

CFD Modeling of a Carbonation Reactor with a K‐Based Dry Sorbent for CO2 Capture

A 2D CFD simulation of the carbonation reactor is carried out to evaluate the performance of potassium‐based dry sorbent during the CO₂ capture process. A multiscale drag coefficient model is incorporated into the two‐fluid model to take the effects of clusters into account. The influence of several parameters on CO₂ removal is investigated. The results indicate that increasing the reactor height and reducing the gas velocity can lengthen the residence time of particles and enhance the CO₂ removal. The operating pressure has a significant influence on the performance of solid sorbents. A higher pressure will decrease the CO₂ removal efficiency.     

Enhanced hydrogen production from biomass with in situ carbon dioxide capture using calcium oxide sorbents

The steam gasification of biomass, in the presence of a calcium oxide (CaO) sorbent for carbon dioxide (CO₂) capture, is a promising pathway for the renewable and sustainable production of hydrogen (H₂). In this work, we demonstrate the potential of using a CaO sorbent to enhance hydrogen output from biomass gasifiers. In addition, we show that CaO materials are the most suitable sorbents reported in the literature for in situ CO₂ capture. A further advantage of the coupled gasification-CO₂ capture process is the production of a concentrated stream of CO₂ as a byproduct. The integration of CO₂ sequestration technology with H₂ production from biomass could potentially result in the net removal of CO₂ from the atmosphere.Maximum experimental H₂ concentrations reported for the steam gasification of biomass, without CO₂ capture, range between 40%-vol and 50%-vol. When CaO is used to remove CO₂ from the product gas, as soon as it is formed, we predict an increase in the H₂ concentrations from 40%-vol to 80%-vol (dry basis), based on thermodynamic modelling and previously published data.We examine the effect of key variables, with a specific focus on obtaining fundamental data relevant to the design and scale-up of novel biomass reactors. These include: (i) reaction temperature, (ii) pressure, (iii) steam-to-biomass ratio, (iv) residence time, and (v) CO₂ sorbent loading. We report on operational challenges related to in situ CO₂ capture using CaO-based sorbents. These include: (i) sorbent durability, (ii) limits to the maximum achievable conversion and (iii) decay in reactivity through multiple capture and release cycles. Strategies for enhancing the multicycle reactivity of CaO are reviewed, including: (i) optimized calcination conditions, and (ii) sorbent hydration procedures for reactivation of spent CaO. However, no CaO-based CO₂ sorbent, with demonstrated high reactivity, maintained through multiple CO₂ capture and release cycles, has been identified in the literature. Thus, we argue that the development of a CO₂ sorbent, which is resistant to physical deterioration and maintains high chemical reactivity through multiple CO₂ capture and release cycles, is the limiting step in the scale-up and commercial operation of the coupled gasification-CO₂ capture process.     

Sulfation Rates of Particles in Calcium Looping Reactors

The sulfation reaction rate of CaO particles in three reactors comprising a post‐combustion calcium looping system is discussed: a combustion chamber generating flue gases, a carbonator reactor to capture CO₂ and SO₂, and an oxy‐fired calciner to regenerate the CO₂ sorbent. Due to its strong impact on the pore size distribution of CaO particles, the number of carbonation/calcination cycles arises as a new important variable to understand sulfation phenomena. Sulfation patterns change as a result of particle cycling, becoming more homogeneous with higher number of cycles. Experimental results from thermogravimetric tests demonstrate that high sulfation rates can be measured under all conditions tested, indicating that the calcium looping systems will be extremely efficient in SO₂ capture.     

Sulfation rates of cycled CaO particles in the carbonator of a Ca‐looping cycle for postcombustion CO2 capture

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, 2012