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.     

CO2 Capture Performance of Calcium‐Based Sorbents in the Presence of SO2 under Pressurized Carbonation

In order to understand the effect of SO₂ on the CO₂ capture performance under pressurized carbonation conditions, tests by orthogonal design were carried out in a calcination/pressurized carbonation reactor system. The effects of variables such as carbonation temperature, carbonation pressure, SO₂ concentration, CO₂ concentration, and the number of cycles on carbonation and sulfation were investigated. A range method was employed for analysis. Phase structure and scanning electron microscopy images were measured as supplement for a reaction study. Temperature increase enhanced the SO₂ capture, leading to a rapid decay in CO₂ uptake. The carbonation pressure had a stronger effect on the CO₂ uptake than the temperature. SO₂ uptake increased rapidly with increasing pressure while CO₂ uptake decreased.     

Development of a Scalable Method for Manufacturing High‐Temperature CO2 Capture Sorbents

An engineered process for scalable manufacture of a calcium aluminum carbonate CO₂ sorbent with production amounts of about 1000 g per hour has been developed. The process includes mixing and heating, solid‐liquid separation, drying and extrusion, crushing and conveying, and calcined molding steps. The sorbent preparation involves the coprecipitation of Ca²⁺, Al³⁺, and CO₃^2– under alkaline conditions. By adjusting the Ca:Al molar ratio, a series of Ca‐rich materials could be synthesized for use as CO₂ sorbents at 750 °C. A calcium acetate‐derived sorbent exhibited better cyclic stability than sorbents originating from CaCl₂ and Ca(NO₃)₂. The initial sorption capacity increased with CaO concentration. High stability of more than 90 % was maintained by the Ca:Al sorbents after 40 looping tests.     

CO2 Capture for Fossil Fuel‐Fired Power Plants

An overview of technologies for fossil fuel‐fired power plants with drastically reduced CO₂ emissions is given. Post‐combustion capture, pre‐combustion capture, and oxyfuel technology are introduced and compared. Current research results indicate that post‐combustion capture may lead to slightly higher losses in power plant efficiency than the two other technologies. However, retrofitting of existing plants with oxyfuel technology is complex and costly, and retrofitting of pre‐combustion capture is not possible. On the other hand, post‐combustion capture can be retrofitted to existing power plants with only minimal effort. Based on the mature technology of reactive absorption, it can be implemented on a large scale in the near future. Therefore, post‐combustion capture using reactive absorption is discussed here in some detail.     

Innovative Internally Circulating Reactor Concept for Chemical Looping‐Based CO2 Capture Processes: Hydrodynamic Investigation

An experimental hydrodynamic investigation has been carried out for a novel internally circulating chemical looping (ICCL) reactor concept proposed to reduce the technical complexities encountered in conventional chemical looping combustion (CLC) and reforming (CLR) technologies. The concept consists of a single reactor with internal physical separations dividing it into two sections, i.e., the fuel and air sections. The trade‐off for this reduction in process complexity is increased gas leakage between the two reactor sections, so a pseudo‐2D cold‐flow experimental unit was designed. The ICCL concept remains highly efficient in terms of CO₂ separation while ensuring significant process simplifications. The solids circulation rate also proved easy to control by adjusting the fluidization velocity ratio and the bed loading. In the light of the excellent hydrodynamic performance, the ICCL concept appears to be well‐suited for further development as a CLC/CLR reactor model.     

Effect of Empty Fruit Bunch in Calcium Oxide for Cyclic CO2 Capture

Empty fruit bunch (EFB) is utilized to increase the CO₂ capture capacity and cyclic stability of calcium oxide (CaO) prepared from cockle shells (CS). The cyclic reaction of calcination and carbonation reaction was performed in pure N₂ environment and in the presence of CO₂ in N₂, respectively. The EFB in CS modified the pore structural properties, morphology, and composition of the pristine CaO. Higher EFB loading in CS reduced the CaO composition but improved the CO₂ capture capacity and cyclic stability during cyclic CO₂ capture.     

Synthesis of a Porous Nano‐CaO/MgO‐Based CO2 Adsorbent

A porous nano‐CaO/MgO‐based adsorbent was prepared using MgO as a support in order to increase the sorption capacity and durability. The magnesium sol prepared by reacting MgO slurry with citric acid was added to nano‐CaCO₃ slurry and the mixture was calcinated to obtain the nano‐CaO/MgO‐based adsorbent. The influence of MgO content on the structure and sorption performance of the resulting adsorbent was studied in detail. The pore radius and specific surface area of the adsorbent increased with higher MgO content. The adsorbent exhibited superior sorption performance during calcium looping and maintained a good durability at the calcination temperature, thus being an interesting candidate for future work.     

Stabilized Performance of Al‐Decorated and Al/Mg Co‐Decorated Spray‐Dried CaO‐Based CO2 Sorbents

The sharp loss‐in‐capacity in CO₂ capture as a result of sintering is a major drawback for CaO‐based sorbents used in the calcium looping process. The decoration of inert supports effectively stabilizes the cyclic CO₂ capture performance of CaO‐based sorbents via sintering mitigation. A range of Al‐decorated and Al/Mg co‐decorated CaO‐based sorbents were synthesized via an easily scaled‐up spray‐drying route. The decoration of Al‐based and Al/Mg‐based supports efficiently enhanced the cyclic CO₂ capture capability of CaO‐based sorbents under severe testing conditions. The CO₂ capture capacity losses of Al‐decorated and Al/Mg co‐decorated CaO‐based sorbents were alleviated, representing more stable CO₂ capture performance. The stabilized CO₂ capture performance is mainly attributed to the formation of Ca₁₂Al₁₄O₃₃, MgAl₂O₄, and MgO that act as the skeleton structures to mitigate the sintering of CaCO₃ during carbonation/calcination cycles.     

Preparation and Application of the Sol-Gel Combustion Synthesis-Made CaO/CaZrO3 Sorbent for Cyclic CO2 Capture Through the Severe Calcination Condition

abstract Calcium looping method has been considered as one of the efficient options to capture CO2 in the combustion flue gas. CaO-based sorbent is the basis for application of calcium looping and shou...     

Thermodynamic Simulation of CO2 Capture for an IGCC Power Plant using the Calcium Looping Cycle

A CO₂ capture process for an integrated gasification combined cycle (IGCC) power plant using the calcium looping cycle was proposed. The CO₂ capture process using natural and modified limestone was simulated and investigated with the software package Aspen Plus. It incorporated a fresh feed of sorbent to compensate for the decay in CO₂ capture activity during long‐term cycles. The sorbent flow ratios have significant effect on the CO₂ capture efficiency and net efficiency of the CO₂ capture system. The IGCC power plant, using the modified limestone, exhibits higher CO₂ capture efficiency than that using the natural limetone at the same sorbent flow ratios. The system net efficiency using the natural and modified limestones achieves 41.7 % and 43.1 %, respectively, at the CO₂ capture efficiency of 90 % without the effect of sulfation.     

Enhancement of CO2 capture and microstructure evolution of the spent calcium-based sorbent by the self-reactivation process

The effect of self-reactivation on the CO_2 capture capacity of the spent calcium based sorbent was investigated in a dual-fixed bed reactor.The sampled sorbents from the dual-fixed bed reactor were sent for XRD,SEM and N_2 adsorption analysis to explain the self-reactivation mechanism.The results show that the CaO in the spent sorbent discharged from the calciner absorbs the vapor in the air to form Ca(OH)_2 and further Ca(OH)_2·2 H_2 O under environmental conditions,during which process the CO_2 capture capacity of the spent sorbent can be self-reactivated.The microstructure of the spent sorbent is improved by the self-reactivation process,resulting in more porous microstructure,higher BET surface area and pore volume.Compared with the calcined spent sorbent that has experienced 20 cycles,the pore volume and BET surface area are increased by 6.69 times and 56.3% after self-reactivation when φ=170%.The improved microstructure makes it easier for the CO_2 diffusion and carbonation reaction in the sorbent.Therefore,the CO_2 capture capacity of the spent sorbent is enhanced by self-reactivation process.A self-reactivation process coupled with calcium looping process was proposed to reuse the discharged spent calcium based sorbent from the calciner.Higher average carbonation conversion and CO_2 capture efficiency can be achieved when self-reactivated spent sorbent is used as supplementary sorbent in the calciner rather than fresh CaCO_3 under the same conditions.     

CO2 Capture Behavior of Shell during Calcination/Carbonation Cycles

The cyclic carbonation performances of shells as CO₂ sorbents were investigated during multiple calcination/carbonation cycles. The carbonation kinetics of the shell and limestone are similar since they both exhibit a fast kinetically controlled reaction regime and a diffusion controlled reaction regime, but their carbonation rates differ between these two regions. Shell achieves the maximum carbonation conversion for carbonation at 680–700 °C. The mactra veneriformis shell and mussel shell exhibit higher carbonation conversions than limestone after several cycles at the same reaction conditions. The carbonation conversion of scallop shell is slightly higher than that of limestone after a series of cycles. The calcined shell appears more porous than calcined limestone, and possesses more pores > 230 nm, which allow large CO₂ diffusion‐carbonation reaction rates and higher conversion due to the increased surface area of the shell. The pores of the shell that are greater than 230 nm do not sinter significantly. The shell has more sodium ions than limestone, which probably leads to an improvement in the cyclic carbonation performance during the multiple calcination/carbonation cycles.     

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     

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     

CO2 Capture Using Monoethanolamine in a Bubble‐Column Scrubber

A continuous bubble‐column scrubber, capturing CO₂ gas by monoethanolamine (MEA) solution in a pH‐stat operation, is used to search for optimum process parameters by means of the Taguchi method. The process variables are the pH of the solution, gas flow rate, concentration of CO₂ gas, and temperature. From the measured outlet CO₂ gas concentrations, the absorption rate and overall mass transfer coefficient can be determined with the support of a steady‐state material balance equation as well as a two‐film model. According to the signal‐to‐noise ratio, the significance sequence influencing the parameters and optimum conditions can be determined. CO₂ concentration and pH value proved to be decisive parameters, while temperature and gas flow rate were minor. Five sets of optimum conditions were obtained and could be further verified by empirical equations.     

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.     

NO removal performance of CO in carbonation stage of calcium looping for CO2 capture

Calcium looping realizes CO2 capture via the cyclic calcination/carbonation of CaO.The combustion of fuel supplies energy for the calciner.It is unavoidable that some unburned char in the calciner flows into the carbonator,generating CO due to the hypoxic atmosphere in the carbonator.CO can reduce NO in the flue gases from coal-fired power plants.In this work,NO removal performance of CO in the carbonation stage of calcium looping for CO2 capture was investigated in a bubbling fluidized bed reactor.The effects of carbonation temperature,CO concentration,CO2 capture,type of CaO,number of CO2 capture cycles and presence of char on NO removal by CO in carbonation stage of calcium looping were discussed.CaO possesses an efficient catalytic effect on NO removal by CO.High temperature and high CO concen-tration lead to high NO removal efficiency of CO in the presence of CaO.Taking account of better NO removal and CO2 capture,the optimal carbonation temperature is 650 ℃.The carbonation of CaO reduces the catalytic activity of CaO for NO removal by CO due to the formation of CaCO3.Besides,the catalytic performance of CaO on NO removal by CO gradually decreases with the number of CO2 capture cycles.This is because the sintering of CaO leads to the fusion of CaO grains and blockage of pores in CaO,hin-dering the diffusion of NO and CO.The high CaO content and porous structure of calcium-based sorbents are beneficial for NO removal by CO.The presence of char promotes NO removal by CO in the carbonator.CO2/NO removal efficiencies can reach above 90％.The efficient simultaneous NO and CO2 removal by CO and CaO in the carbonation step of the calcium looping seems promising.     

Efficient CO2 Capture by Ultra‐high Microporous Activated Carbon Made from Natural Coal

Efficient CO₂ capture capabilities of activated carbons prepared from natural coal are presented. The preparation method involved simple chemical activation using wet impregnation or dry physical mixing of the raw sample with activating agents like KOH or NaOH. The activated materials were characterized for their structural and textural properties by different analysis techniques. The activated samples exhibited well‐developed porosity, large surface area, and high pore volumes and had other active elements like oxygenated functional groups. These groups modified the surface energy of the resultant samples. The superior performance of the activated carbons was attributed to several factors, including large surface area, presence of narrow micropores, and oxygenated functional groups on the surface.     

Acidification Optimization and Granulation of a Steel‐Slag‐Derived Sorbent for CO2 Capture

Steel slag was used as a low‐cost feedstock to prepare CaO‐based sorbents for CO₂ capture by acidification treatment, and the acidification process was optimized. Four main acidification parameters (i.e., extraction time, extraction temperature, acetic acid concentration, and solid/liquid ratio) were investigated. The solid/liquid ratio and extraction time are the most important factors that affect the CO₂ capture capacity and stability of the sorbents. The CO₂ sorption performance of optimal steel‐slag‐derived sorbent is more stable than that of naturally occurring limestone, due to the low Si/Ca ratio and the presence of MgO with high anti‐sintering ability. CaO‐based pellets with high resistance to attrition and compression were produced by extrusion of the steel‐slag‐derived sorbent powders.     

Application of polyethylenimine‐impregnated solid adsorbents for direct capture of low‐concentration CO2

A systematic study of CO₂ capture on the amine‐impregnated solid adsorbents is carried out at CO₂ concentrations in the range of 400–5000 ppm, relating to the direct CO₂ capture from atmospheric air. The commercially available polymethacrylate‐based HP2MGL and polyethylenimine are screened to be the suitable support and amine, respectively, for preparation of the adsorbent. The adsorbents exhibit an excellent saturation adsorption capacity of 1.96 mmol/g for 400 ppm CO₂ and 2.13 mmol/g for 5000 ppm CO₂. Moisture plays a promoting effect on CO₂ adsorption but depends on the relative humidity. The presence of O₂ would lead to the decrease of adsorption capacity but do not affect the cyclic performance. The diffusion additive is efficient to improve the adsorption capacity and cyclic performance. Moreover, the adsorbents can be easily regenerated under a mild temperature. This study may have a positive impact on the design of high‐performance adsorbents for CO₂ capture from ambient air. © 2014 American Institute of Chemical Engineers AIChE J, 61: 972–980, 2015