Enhancement of Ca‐Based Sorbent Multicyclic Behavior in Ca Looping Process for CO2 Separation

The Ca‐based sorbent looping cycle represents an innovative way of CO₂ capture for power plants. However, the CO₂ capture capacity of the Ca‐based sorbent decays sharply with calcination/carbonation cycle number increasing. In order to improve the CO₂ capture capacity of the sorbent in the Ca looping cycle, limestone was modified with acetic acid solution. The cyclic carbonation behaviors of the modified and original limestones were investigated in a twin fixed‐bed reactor system. The modified limestone possesses better cyclic carbonation kinetics than the original limestone at each cycle. The modified limestone carbonated at 640–660 °C achieves the optimum carbonation conversion. The acetic acid modification improves the long‐term performance of limestone, resulting in directly measured conversion as high as 0.4 after 100 cycles, while the original limestone remains at a conversion of less than 0.1 at the same reaction conditions. Both the pore volume and pore area distributions of the calcines derived from the modified limestone are better than those derived from the original limestone. The CO₂ partial pressure for carbonation has greater effect on conversion of the original limestone than on that of the modified sorbent because of the difference in their pore structure characteristics. The carbonation conversion of the original limestone decreases with the increase in particle size, while the change in particle size of the modified sorbent has no clear effect on cyclic carbonation behavior.     

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.     

Natural Calcium‐Based Sorbents Doped with Sea Salt for Cyclic CO2 Capture

The calcium‐looping process for post‐combustion carbon dioxide capture, an economically and technically feasible method suitable for large‐scale use, has recently gained much attention. However, the capture capacity of calcium‐based sorbents rapidly decreases after only a few cycles. Herein, calcium‐based sorbents with enhanced cyclic CO₂‐capture capacity have been derived from cheap, natural raw materials by using a simple impregnation method. Limestone and shells were used as the calcium‐based raw materials, with sea salt as dopant. Modified limestone had the highest CO₂‐capture capacity after multiple carbonation‐calcination cycles. Sea‐salt‐doped sorbent showed a relatively stable porous surface during cycles, which resulted in a higher CO₂‐capture capacity.     

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     

Recent progress of amine modified sorbents for capturing CO2 from flue gas

Under the Paris agreement, China has committed to reducing CO₂ emissions by 60%–65% per unit of GDP by 2030. Since CO₂ emissions from coal-fired power plants currently account for over 30% of the total carbon emissions in China, it will be necessary to mitigate at least some of these emissions to achieve this goal. Studies by the International Energy Agency (IEA) indicate CCS technology has the potential to contribute 14% of global emission reductions, followed by 40% of higher energy efficiency and 35% of renewable energy, which is considered as the most promising technology to significantly reduce carbon emissions for current coal-fired power plants. Moreover, the announcement of a Chinese national carbon trading market in late 2017 signals an opportunity for the commercial deployment of CO₂ capture technologies.Currently, the only commercially demonstrated technology for post-combustion CO₂ capture technology from power plants is solvent-based absorption. While commercially viable, the costs of deploying this technology are high. This has motivated efforts to develop more affordable alternatives, including advanced solvents, membranes, and sorbent capture systems. Of these approaches, advanced solvents have received the most attention in terms of research and demonstration. In contrast, sorbent capture technology has less attention, despite its potential for much lower energy consumption due to the absence of water in the sorbent. This paper reviews recent progress in the development of sorbent materials modified by amine functionalities with an emphasis on material characterization methods and the effects of operating conditions on performance. The main problems and challenges that need to be overcome to improve the competitiveness of sorbent-based capture technologies are discussed.     

Enhanced cyclic stability of CO<Subscript>2</Subscript> adsorption capacity of CaO-based sorbents using La<Subscript>2</Subscript>O<Subscript>3</Subscript> or Ca<Subscript>12</Subscript>Al<Subscript>14</Subscript>O<Subscript>33</Subscript> as additives

To improve the stability of CaO adsorption capacity for CO₂ capture during multiple carbonation/calcination cycles, modified CaO-based sorbents were synthesized by sol-gel-combustion-synthesis (SGCS) method and wet physical mixing method, respectively, to overcome the problem of loss-in-capacity of CaO-based sorbents. The cyclic CaO adsorption capacity of the sorbents as well as the effect of the addition of La₂O₃ or Ca₁₂Al₁₄O₃₃ was investigated in a fixed-bed reactor. The transient phase change and microstructure were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FSEM), respectively. The experimental results indicate that La₂O₃ played an active role in the carbonation/calcination reactions. When the sorbents were made by wet physical mixing method, CaO/Ca₁₂Al₁₄O₃₃ was much better than CaO/La₂O₃ in cyclic CO₂ capture performance. When the sorbents were made by SGCS method, the synthetic CaO/La₂O₃ sorbent provided the best performance of a carbonation conversion of up to 93% and an adsorption capacity of up to 0.58 g-CO₂/g-sorbent after 11 cycles.     

Application of the random pore model to the carbonation cyclic reaction

Calcium oxide has been proved to be a suitable sorbent for high temperature CO₂ capture processes based on the cyclic carbonation‐calcination reaction. It is important to have reaction rate models that are able to describe the behavior of CaO particles with respect to the carbonation reaction. Fresh calcined lime is known to be a reactive solid toward carbonation, but the average sorbent particle in a CaO‐based CO₂ capture system experiences many carbonation‐calcination cycles and the reactivity changes with the number of cycles. This study applies the random pore model (RPM) to estimate the intrinsic rate parameters for the carbonation reaction and develops a simple model to calculate particle conversion with time as a function of the number of cycles, partial pressure of CO₂, and temperature. This version of the RPM model integrates knowledge obtained in earlier works on intrinsic carbonation rates, critical product layer thickness, and pore structure evolution in highly cycled particles. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Mature <Emphasis Type="Italic">versus</Emphasis> emerging technologies for CO<Subscript>2</Subscript> capture in power plants: Key open issues in post-combustion amine scrubbing and in chemical looping combustion

Carbon capture and storage (CCS) have acquired an increasing importance in the debate on global warming as a mean to decrease the environmental impact of energy conversion technologies, by capturing the CO₂ produced from the use of fossil fuels in electricity generation and industrial processes. In this respect, post-combustion systems have received great attention as a possible near-term CO₂ capture technology that can be retrofitted to existing power plants. This capture technology is, however, energy-intensive and results in large equipment sizes because of the large volumes of the flue gas to be treated. To cope with the demerits of other CCS technologies, the chemical looping combustion (CLC) process has been recently considered as a solution for CO₂ separation. It is typically referred to as a technology without energy penalty. Indeed, in CLC the fuel and the combustion air are never mixed and the gases from the oxidation of the fuel (i.e., CO₂ and H₂O) leave the system as a separate stream and can be separated by condensation of H₂O without any loss of energy. The key issue for the CLC process is to find a suitable oxygen carrier, which provides the fuel with the activated oxygen needed for combustion. The aim of this work is to explore the feasibility of using perovskites as oxygen carriers in CLC and to consider the possible advantages with respect to the scrubbing process with amines, a mature post-combustion technology for CO₂ separation.

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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.     

Assessment of chemical looping-based conceptual designs for high efficient hydrogen and power co-generation applied to gasification processes

Carbon capture and storage (CCS) technologies are expected to play a significant role in the coming decades for curbing the greenhouse gas emissions and to ensure a sustainable development of power generation and other energy-intensive industrial sectors. Chemical looping systems are very promising options for intrinsically capture CO₂ with lower cost and energy penalties. Gasification offers significant advantages compared with other technologies in term of lower energy and cost penalties for carbon capture, utilization of wide range of fuels, poly-generation capability, plant flexibility, lower environmental impact, etc.     

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.     

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.     

烟气中水蒸气对钙基吸收剂碳酸化的影响特性

在循环煅烧/碳酸化反应系统上考察煅烧气氛和碳酸化气氛中水蒸气含量以及CO₂分压对钙基吸收剂成型颗粒碳酸化的影响,通过对钙基吸收剂微观结构分析(扫描电镜、氮吸附分析)以理解水蒸气影响碳酸化特性的机理。结果表明,煅烧气氛和碳酸化气氛中的水蒸气均可提高钙基吸收剂的碳酸化转化率,水蒸气含量分别为10%和5%时,吸收剂的碳酸化性能较好;水蒸气在碳酸化气氛中对高铝水泥改性吸收剂的改善作用较石灰石显著。煅烧气氛中的CO₂分压越高,烧结现象越严重,降低钙基吸收剂的捕集效率;碳酸化气氛CO₂分压提高,有利于提高钙基吸收剂的碳酸化转化率。烟气中水蒸气丰富了吸收剂的微观孔隙,使得吸收剂捕集CO₂性能得到改善。     

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.     

Simulation of a calcium looping CO2 capture process for pressurized fluidized bed combustion

The Canadian regulations on carbon dioxide emissions from power plants aim to lower the emissions from coal-fired units down to those of natural gas combined cycle (NGCC) units. Since coal is significantly more carbon intensive than natural gas, coal-fired plants must operate at higher net efficiencies and implement carbon capture to meet the new regulations. Calcium looping (CaL) is a promising post-combustion carbon capture (PCC) technology that, unlike other capture processes, generates additional power. By capturing carbon dioxide at elevated temperatures, the energy penalty that carbon capture technologies inherently impose on power plant efficiencies is significantly reduced. In this work, the CO₂ capture performance of a calcium-based sorbent is determined via thermogravimetric analysis under relatively high carbonation and low calcination temperatures. The results are used in an aspenONE™ simulation of a CaL process applied to a pressurized fluidized bed combustion (PFBC) system at thermodynamic equilibrium. The combustion of both natural gas and coal are considered for sorbent calcination in the CaL process. A sensitivity analysis on several process parameters, including sorbent feed rate and carbonator operating pressure, is undertaken. The energy penalty associated with the capture process ranges from 6.8–11.8 percentage points depending on fuel selection and operating conditions. The use of natural gas results in lower energy penalties and solids circulation rates, while operating the carbonator at 202 kPa(a) results in the lowest penalties and drops the solids circulations rates to below 1000 kg/s.     

Regenerable potassium-based alumina sorbents prepared by CO<Subscript>2</Subscript> thermal treatment for post-combustion carbon dioxide capture

Potassium carbonate supported on alumina is used as a solid sorbent for CO₂ capture at low temperatures. However, its CO₂ capture capacity decreases immediately after the first cycle. This regeneration problem is due to the formation of the by-product [KAl(CO₃)(OH)₂] during CO₂ sorption. To overcome this problem, a new regenerable potassium-based sorbent was fabricated by CO₂ thermal treatment of sorbents prepared by the impregnation of δ-alumina with K₂CO₃ in the presence of 10 vol% CO₂ and 10 vol% H₂O. The CO₂ capture capacities of the new regenerable sorbents were maintained over multiple CO₂ sorption tests. These results can be explained by the fact that the sorbent prepared by CO₂ thermal treatment did not form any by-product during CO₂ sorption. Based on these results, we suggest that the regeneration properties of potassium-based sorbents using δ-alumina could be significantly improved by the use of the CO₂ thermal treatment developed in this study.     

Effect of solid residence time on CO<Subscript>2</Subscript> selectivity in a semi-continuous chemical looping combustor

Chemical looping combustion (CLC) is a promising technology for fossil fuel combustion with inherent CO₂ capture and sequestration, which is able to mitigate greenhouse gases (GHGs) emission. In this study, to design a 0.5MW_th pressurized chemical looping combustor for natural gas and syngas the effects of solid residences time on CO₂ selectivity were investigated in a novel semi-continuous CLC reactor using Ni-based oxygen carrier particle. The semi-continuous chemical looping combustor was designed to simulate the fuel reactor of the continuous chemical looping combustor. It consists of an upper hopper, a screw conveyor, a fluidized bed reactor, and a lower hopper. Solid circulation rate (G _s ) was controlled by adjusting the rotational speed of the screw conveyor. The measured solid circulation rate increased linearly as the rotational speed of the screw increased and showed almost the same values regardless of temperature and fluidization velocity up to 800°C and 4 U _mf , respectively. The solid circulation rate required to achieve 100% CH₄ conversion was varied to change G _s -fuel ratio (oxygen carrier feeding rate/fuel feeding rate, kg/Nm³). The measured CO₂ selectivity was greater than 98% when the Gs-fuel ratio was higher than 78 kg/Nm³.     

Carbon capture from stationary power generation sources: A review of the current status of the technologies

The world will need greatly increased energy supply in the future for sustained economic growth, but the related CO₂ emissions and the resulting climate changes are becoming major concerns. CO₂ is one of the most important greenhouse gases that is said to be responsible for approximately 60% of the global warming. Along with improvement of energy efficiency and increased use of renewable energy sources, carbon capture and sequestration (CCS) is expected to play a major role in curbing the greenhouse gas emissions on a global scale. This article reviews the various options and technologies for CO₂ capture, specifically for stationary power generation sources. Many options exist for carbon dioxide capture from such sources, which vary with power plant types, and include post-combustion capture, pre-combustion capture, oxy fuel combustion capture, and chemical looping combustion capture. Various carbon dioxide separation technologies can be utilized with these options, such as chemical absorption, physical absorption, adsorption, and membrane separation. Most of these capture technologies are still at early stages of development. Recent progress and remaining challenges for the various CO₂ capture options and technologies are reviewed in terms of capacity, selectivity, stability, energy requirements, etc. Hybrid and modified systems hold huge future potentials, but significant progress is required in materials synthesis and stability, and implementations of these systems on demonstration plants are needed. Improvements and progress made through applications of process systems engineering concepts and tools are highlighted and current gaps in the knowledge are also mentioned. Finally, some recommendations are made for future research directions.     

Kinetic study on carbonation of crude Li<Subscript>2</Subscript>CO<Subscript>3</Subscript> with CO<Subscript>2</Subscript>-water solutions in a slurry bubble column reactor

Investigations were conducted to purify crude Li₂CO₃ via direct carbonation with CO₂-water solutions at atmospheric pressure. The experiments were carried out in a slurry bubble column reactor with 0.05 m inner diameter and 1.0 m height. Parameters that may affect the dissolution of Li₂CO₃ in the CO₂-water solutions such as CO₂-bubble perforation diameter, CO₂ partial pressure, CO₂ gas flow rate, Li₂CO₃ particle size, solid concentration in the slurry, reaction temperature, slurry height in the column and so on were investigated. It was found that the increases of CO₂ partial pressure, and CO₂ flow rate were favorable to the dissolution of Li₂CO₃, which had the opposite effects with Li₂CO₃ particle size, solid concentration, slurry height in the column and temperature. On the other hand, in order to get insight into the mechanism of the refining process, reaction kinetics was studied. The results showed that the kinetics of the carbonation process can be properly represented by 1−3(1−X)^2/3+2(1–X)=kt+b, and the rate-determining step of this process under the conditions studied was product layer diffusion. Finally, the apparent activation energy of the carbonation reaction was obtained by calculation. This study will provide theoretical basis for the reactor design and the optimization of the process operation.