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.     

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。     

Three‐stage well‐mixed reactor model for a pressurized coal gasifier

Three Canadian coals of different rank were gasified with air‐steam mixtures in a 0.1 m diameter spouted bed reactor at pressures to 292 kPa, average bed temperatures varying between 840 and 960°C, and steam‐to‐coal feed ratios between 0.0 and 2.88. In order to analyze gasifier performance and correlate data, a three‐stage model has been developed incorporating instantaneous devolatilization of coal, instantaneous combustion of carbon at the bottom of the bed, and steam/carbon gasification and water gas shift reaction in a single well mixed isothermal stage. The capture of H₂S by limestone sorbent injection is also treated. The effects of various assumptions and model parameters on the predictions were investigated. The present model indicates that gasifier performance is mainly controlled by the fast coal devolatilization and char combustion reactions, and the contribution to carbon conversion of the slow char gasification reactions is comparatively small. The incorporation of tar decomposition into the model provides significantly closer predictions of experimental gas composition than is obtained otherwise.     

CO2 uptake of modified calcium-based sorbents in a pressurized carbonation-calcination looping

This study focuses on enhancing CO₂ uptake by modifying limestone with acetate solutions under pressurized carbonation condition. The multicycle tests were carried out in an atmospheric calcination/pressurized carbonation reactor system at different temperatures and pressures. The pore structure characteristics (BET and BJH) were measured as a supplement to the reaction studies. Compared with the raw limestone, the modified sorbent showed a great improvement in CO₂ uptake at the same reaction condition. The highest CO₂ uptake was obtained at 700 °C and 0.5 MPa, by 88.5% increase over the limestone at 0.1 MPa after 10 cycles. The structure characteristics of the sorbents on N₂ absorption and SEM confirm that compared with the modified sorbent, the effective pores of limestone are greatly driven off by sintering, which hinders the easy access of CO₂ molecules to the unreacted-active sites of CaO. The morphological and structural properties of the modified sorbent did not reveal significant differences after multiple cycles. This would explain its superior performance of CO₂ uptake under pressurized carbonation. Even after 10 cycles, the modified sorbent still achieved a CO₂ uptake of 0.88.     

Modified CaO-based sorbent looping cycle for CO2 mitigation

CaO-based sorbent looping cycle, i.e. cyclic calcination/carbonation, is one of the most interesting technologies for CO₂ capture during coal combustion and gasification processes. In order to improve the durability of limestone during the multiple calcination/carbonation cycles, modified limestone with acetic acid solution was proposed as an CO₂ sorbent. The cyclic carbonation conversions of modified limestone and original one were investigated in a twin fixed bed reactor system. The modified limestone shows the optimum carbonation conversion at the carbonation temperature of 650 °C and achieves a conversion of 0.5 after 20 cycles. The original limestone exhibits the maximum carbonation conversion of 0.15 after 20 cycles. Conversion of the modified limestone decreases slightly as the calcination temperature increases from 920 °C to 1100 °C with the number of cycles, while conversion of the original one displays a sharp decay at the same reaction conditions. The durability of the modified limestone is significantly better than the original one during the multiple cycles because mean grain size of CaO derived from the modified limestone is lower than that from the original one at the same reaction conditions. The calcined modified limestone shows higher surface area and pore volume than the calcined original one with the number of cycles, and pore size distribution of the modified limestone is superior to the original one after the same number of calcinations.     

Increasing Porosity of Molded Calcium‐Based Sorbents by Glucose Templating for Cyclic CO2 Capture

With the aim to enhance the CO₂ capture capacity and anti‐attrition property of CaO‐based sorbents simultaneously, a novel CaO‐based sphere was prepared by extrusion‐spheronization using Ca(OH)₂ powder with glucose templating. The CO₂ capture characteristics and attrition resistance property of the sorbent were examined and the microstructure of the sorbents was analyzed. The results demonstrate that the obtained spherical sorbents exhibit an outstanding anti‐attrition performance compared to limestone sorbent. After 100 cycles, all of the templated sorbents hold a CO₂ capture capacity of more than one time higher than that of limestone. The optimum templating rate of glucose in the sorbent was 1–5 wt %.     

CO2 Capture Using CaO Modified with Ethanol/Water Solution during Cyclic Calcination/Carbonation

The calcium‐based sorbent cyclic calcination/carbonation reaction is an effective technique for capturing CO₂ from combustion processes. The CO₂ capture capacity for CaO modified with ethanol/water solution was investigated over long‐term calcination/carbonation cycles. In addition, the SEM micrographs and pore structure for the calcined sorbents were analyzed. The carbonation conversion for CaO modified with ethanol/water solution is greater than that for CaO hydrated with distilled water and is much higher than that for calcined limestone. Modified CaO achieves the highest conversion for carbonation at the range of 650–700 °C. Higher values of ethanol concentration in solution result in higher carbonation conversion for modified CaO, and lead to better anti‐sintering performance. After calcination, the specific surface area and pore volume for modified CaO are higher than those for hydrated CaO, and are much greater than those for calcined limestone. The ethanol molecule enhances H₂O molecule affinity and penetrability to CaO in the hydration reaction so that the pores in CaO modified are obviously expanded after calcination. CaO modified with ethanol/water solution can act as a new and promising type of calcium‐based regenerable CO₂ sorbent for industrial applications.     

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.     

CO2 capture capacity of CaO in long series of pressure swing sorption cycles

One promising method for the capture of CO₂ from point sources is through the usage of a lime-based sorbent. Lime (CaO) acts as a CO₂ carrier, absorbing CO₂ from the flue gas (carbonation) and releasing it in a separate reactor (calcination) to create a pure stream of CO₂ suitable for sequestration. One of the challenges with this process is the decay in calcium utilization (CO₂ capture capacity) during carbonation/calcination cycling. The reduction in calcium utilization of natural limestone over large numbers of cycles (>250) was studied. Cycling was accomplished using pressure swing CO₂ adsorption in a pressurized thermogravimetric reactor (PTGA). The effect of carbonation pressure on calcium utilization was studied in CO₂ with the reactor operated at 1000 °C. The pressure was cycled between atmospheric pressure for calcination, and 6, 11 or 21 bar for carbonation. Over the first 250 cycles, the calcium utilization reached a near-asymptotic value of 12.5-27.7%, depending on the cycling conditions. Pressure cycling resulted in improved long-term calcium utilization compared to temperature swing or CO₂ partial pressure swing adsorption under similar conditions. An increased rate of de-pressurization caused an increase in calcium utilization, attributed to fracturing of the sorbent particle during the rapid calcination, as observed via SEM analysis.     

Influence of calcination conditions on carrying capacity of CaO-based sorbent in CO2 looping cycles

This study examines the loss of sorbent activity caused by sintering under realistic CO₂ capture cycle conditions. The samples tested here included two limestones: Havelock limestone from Canada (New Brunswick) and a Polish (Upper Silesia) limestone (Katowice). Samples were prepared both in a thermogravimetric analyzer (TGA) and a tube furnace (TF). Two calcination conditions were employed: in N₂ at lower temperature; and in CO₂ at high temperature. The samples obtained were observed with a scanning electron microscope (SEM) and surface compositions of the resulting materials were analyzed by the energy dispersive X-ray (EDX) method. The quantitative influence of calcination conditions was examined by nitrogen adsorption/desorption tests, gas displacement pycnometry and powder displacement pycnometry; BET surface areas, BJH pore volume distributions, skeletal densities and envelope densities were determined. The SEM images showed noticeably larger CaO sub-grains were produced by calcination in CO₂ during numerous cycles than those seen with calcination in nitrogen. The EDX elemental analyses showed a strong influence of impurities on local melting at the sorbent particle surface, which became more pronounced at higher temperature. Results of BET/BJH testing clearly support these findings on the effect of calcination/cycling conditions on sorbent morphology. Envelope density measurements showed that particles displayed densification upon cycling and that particles calcined under CO₂ showed greater densification than those calcined under N₂. Interestingly, the Katowice limestone calcined/cycled at higher temperature in CO₂ showed an increase of activity for cycles involving calcination under N₂ in the TGA. These results clearly demonstrate that, in future development of CaO-based CO₂ looping cycle technology, more attention should be paid to loss of sorbent activity caused by realistic calcination conditions and the presence of impurities originating from fuel ash and/or limestone.     

CO2 capture from syngas via cyclic carbonation/calcination for a naturally occurring limestone: Modelling and bench-scale testing

The intrinsic rate constants of the CaO-CO₂ reaction, in the presence of syngas, were studied using a grain model for a naturally occurring calcium oxide-based sorbent using a thermogravimetric analyzer. Over temperatures ranging from 580 to 700 °C, it was observed that the presence of CO and H₂ (with steam) during carbonation caused a significant increase in the initial rate of carbonation, which has been attributed to the CaO surface sites catalyzing the water-gas shift reaction, increasing the local CO₂ concentration. The water-gas shift reaction was assumed to be responsible for the increase in activation energy from 29.7 to 60.3 kJ/mol for limestone based on the formation of intermediate complexes. Changes in microporosity due to particle sintering during calcination have been credited with the rapid initial decrease in cyclic CaO maximum conversion for limestone particles, whereas the presence of steam during carbonation has been shown to improve the long-term maximum conversion in comparison to previous studies without steam present.     

Sequential SO2/CO2 capture enhanced by steam reactivation of a CaO-based sorbent

The steam hydration reactivation characteristics of three limestone samples after multiple CO₂ looping cycles are presented here. The CO₂ cycles were performed in a tube furnace (TF) and the resulting samples were hydrated by steam in a pressure reactor (PR). The reactivation was performed with spent samples after carbonation and calcination stages. The reactivation tests were done with a saturated steam pressure at 200 °C and also at atmospheric pressure and 100 °C. The characteristics of the reactivation samples were examined using BET and BJH pore characterization (for the original and spent samples, and samples reactivated under different conditions) and also by means of a thermogravimetric analyzer (TGA). The levels of hydration achieved by the reactivated samples were determined as well as the conversions during sulphation and multiple carbonation cycles. It was found that the presence of a CaCO₃ layer strongly hinders sorbent hydration and adversely affects the properties of the reactivated sorbent with regard to its behavior in sulphation and multiple carbonation cycles. Here, hydration of calcined samples under pressure is the most effective method to produce superior sulphur sorbents. However, reactivation of calcined samples under atmospheric conditions also produces sorbents with significantly better properties in comparison to those of the original sorbents. These results show that separate CO₂ capture and SO₂ retention in fluidized bed systems enhanced by steam reactivation is promising even for atmospheric conditions if the material for hydration is taken from the calciner.     

Biomass Gasification Integrated with Chemical Looping System for Hydrogen and Power. Coproduction Process – Thermodynamic and Techno‐Economic Assessment

Three biomass gasification‐based hydrogen and power coproduction processes are modeled with Aspen Plus. Case 1 is the conventional biomass gasification coupled with a shift reactor, cases 2 and 3 involve integration of biomass gasification with iron‐based and calcium‐based chemical looping systems. The effects of important process parameters on the performance indicators such as hydrogen yield and efficiencies are evaluated by sensitivity analyses. These parameters include gasification temperature, molar ratios of steam to biomass in the gasifier, Fe₂O₃ to syngas in the fuel reactor, Fe/FeO to steam in the steam reactor, CaO to CO, and steam to CO in the carbonator. The energy and exergy balance distributions for the above three cases are comprehensively discussed and compared. Furthermore, techno‐economic assessments are performed to evaluate the three cases in terms of capital cost, operating cost, and leveled cost of energy.     

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.     

Sulphur dioxide removal using South African limestone/siliceous materials

This study presents an investigation into the desulfurization effect of sorbent derived from South African calcined limestone conditioned with fly ash. The main aim was to examine the effect of chemical composition and structural properties of the sorbent with regard to SO₂ removal in dry-type flue gas desulfurization (FGD) process. South African fly ash and CaO obtained from calcination of limestone in a laboratory kiln at a temperature of 900 °C were used to synthesize CaO/ash sorbent by atmospheric hydration process. The sorbent was prepared under different hydration conditions: CaO/fly ash weight ratio, hydration temperature (55-75 °C) and hydration period (4-10 h). Desulfurization experiments were done in the fixed bed reactor at 87 °C and relative humidity of 50%. The chemical composition of both the fly ash and calcined limestone had relatively high Fe₂O₃ and oxides of other transitional elements which provided catalytic ability during the sorbent sorption process. Generally the sorbents had higher SO₂ absorption capacity in terms of mol of SO₂ per mol of sorbent (0.1403-0.3336) compared to hydrated lime alone (maximum 0.1823). The sorbents were also found to consist of mesoporous structure with larger pore volume and BET specific surface area than both CaO and fly ash. X-ray diffraction (XRD) analysis showed the presence of complex compounds containing calcium silicate hydrate in the sorbents.     

Effect of sulfidation/oxidative regeneration cycle on the solid structural changes of sorbent for high-temperature removal of H2S

In order to investigate the effects of sulfidation/oxidative regeneration cycle on the change of structural properties and removal capacity of sorbent, sulfidation/regeneration cycle was carried out up to 15 times in a fixed-bed reactor. The effluent gases from the fixed-bed reactor were analyzed by gas chromatography, and XRD, SEM, and liquid nitrogen physisorption method were used to characterize the reacted sorbents. The sorbent treated first sulfidation/regeneration cycle exhibited maximum specific surface area and the highest H₂S removal capacity. Hysteresis of adsorption isotherm of the regenerated sorbent reflected the growth of pores of fresh sorbent and pore size distribution confirmed this fact. Furthermore constant H₂S removal capacity was maintained up to 15 times of sulfidation/regeneration cycle.     

Sorbent development for continuous regenerative H2S removal in a rotating monolith reactor

A high capacity and regenerable manganese based sorbent for desulfurization of hot dry fuel gas from coal gasification has been developed. Pure γ-Al₂O₃ and washcoated cordierite monoliths impregnated with manganese acetate and calcined at 973 K resulted in highly dispersed Mn₃O₄ on γ-Al₂O₃. MnS was formed during sulfidation and MnAl₂O₄ during subsequent regeneration with steam. The optimal operation temperature was found to be between 1123 and 1223 K. The maximum capacity of the acceptor was 17 mass% sulfur which was obtained for a 32 mass% manganese loading. A deactivation test of 65 subsequent sulfidation and regeneration cycles showed minor deactivation during the first cycles followed by a stable performance. This sorbent will be used in a rotating monolith reactor in which absorption and regeneration takes place simultaneously in separate sections, which enables a continuous operation.     

Natural Calcium‐Based Residues for Carbon Dioxide Capture in a Bubbling Fluidized‐Bed Reactor

Used clamshells (Paphia undulata), as a precursor of calcium oxide (CaO) sorbents, were employed for carbon dioxide (CO₂) adsorption in a bubbling fluidized‐bed reactor. To find the optimal calcination conditions, a 2^k experimental design was used to vary the ground clamshell particle size, heating rate, and calcination time at 950 °C under a nitrogen atmosphere. The heating rate was the most significant factor affecting the CO₂ adsorption capacity of the obtained CaO sorbent. The maximum CO₂ adsorption capacity of the CaO obtained under these study conditions was higher than that of commercial CaO.     

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.     

High temperature desulphurization by fine limestone during staged fluidized‐bed combustion

This paper reviews the SO₂ emission from a 0.3 m² stainless‐steel fluidized‐bed combustor. Fine coal was premixed with fine limestone and fed pneumatically under the bed. The SO₂ emission was found to depend largely on air staging ratio and bed temperature, which agrees with previous observations. The SO₂ emission observed in sorbent‐free tests (reported earlier by Khan and Cibbs, 1995) was found to be proportional to the sulphur content of the fuel when limestone was added, the sulphur capture at a fixed Ca/S molar ratio was dependent on oxygen stoichiometry and bed temperature. Finely sized limestone enhanced the effectivity of the sorbent at low bed temperature and air staging ratio. During staged combustion, the combustion efficiency depended largely on primary air to coal ratio. Around 90% combustion efficiency was observed at 1 m/s fluidizing velocity which was reduced when fluidizing velocity was increased to 1.5 and 2 m/s. This reduction is due to increased elutriation of finer coal particles from the combustor.