Sulfation behavior of CaO from long-term carbonation/calcination cycles for CO2 capture at FBC temperatures

In this work, SO₂ capture behavior of CaO derived from the dolomite and the limestone during long-term carbonation/calcination cycles for CO₂ capture at fluidized bed combustion (FBC) temperatures was investigated. The cyclic carbonation/calcination of CaO was performed in a dual fixed-bed reactor and then the cycled CaO was sent for sulfation in a thermo-gravimetric analyzer. At the typical FBC temperatures (850–950 °C), SO₂ capture capacity of CaO from the different carbonation/calcination cycles increases with the increasing the temperature. The sulfation conversion of CaO derived from the dolomite (CaO-dolomite) decreases as the carbonation/calcination cycle number increases from 0 to 200. Although the sulfation conversion of CaO derived from the limestone (CaO-limestone) decreases with increasing the cycle number from 0 to 40, its conversion does not always decay with the number of cycles. The sulfation conversion of CaO-limestone shows a slight increase with increasing the cycle number from 40 to 150 and then exhibits a decrease with increasing the cycle number further. The sulfation conversions of CaO-limestone after different cycles are determined by the specific surface area and the volume of macropores >0.2 μm in diameter. The particle size and SO₂ concentration have important effect on sulfation behavior of CaO from various cycles.     

SO2 retention by highly cycled modified CaO-based sorbent in calcium looping process

The limestone modified by pyroligneous acid has been proved to have good CO₂ capture behavior in the calcium looping process. In this work, SO₂ retention of the highly cycled modified limestone in the carbonation/calcination cycles was investigated in a thermogravimetric analyzer (TG). The cyclic carbonation/calcination of the modified limestone was performed in a dual fixed-bed reactor and then the cycled modified limestone was sent for sulfation in TG. The effects of sulfation temperature, cycle number, and prolonged carbonation on SO₂ retention of the cycled modified limestone were discussed. The optimum temperature for sulfation of the cycled modified limestone should be in the range of 900–950 °C. The effect of sulfation temperature on SO₂ retention of the modified limestone drops with increasing cycle number. With increasing cycle number from 20 to 100, the sulfation conversion of the cycled modified limestone is stable and can reach ~0.4. The cycled modified limestone exhibits obviously higher SO₂ retention than the cycled raw one for the same number of cycles. The prolonged carbonation increases SO₂ retention of the modified limestone and the raw one after the subsequent cycles. The sulfation conversions of the modified limestone and the raw one at 118 min after 9-h carbonation in the 20th cycle increase 43 and 56 %, respectively. The cycled modified limestone shows a greater SO₂ retention than the cycled raw one after the same prolonged carbonation treatment. The prolonged carbonation increases the pores in 5–20 nm range which is considered the optimum pore size for sulfation of CaO-based sorbent, so it results in an improvement in SO₂ retention of the cycled sorbents.     

Multicyclic study on the carbonation of CaO using different limestones

Different samples of limestones, with small differences in their stoichiometry, have been studied comparatively. The carbonation reaction has been studied for a large area of isothermal temperatures. The conditions for the multicyclic experiments of calcination/carbonation were: isothermal temperature 670°C, heating time 60 min and carrier gas CO₂. The final carbonation conversion depends mainly on the isothermal temperature of the carbonation reaction and the heating time. The final temperature of the calcination reaction depends on the percentage of CaO that it has not been conversed to CaCO₃ in the repeated carbonation experiments. The quantity of CaO that has not been carbonated, in the same sample, affects the values of the coefficients of the kinetic model that fit the calcination reaction. In the multicyclic experiments the carbonation conversion for two of the four studied samples, was high enough in comparison to other samples of calcite. At sample A the reduction of the carbonation conversion during the first five cycles is less than it is at other samples from the literature. Under the above experimental conditions — isothermal temperature and heating time — specific samples consisted mainly of calcite can absorb larger quantities of CO₂ than samples consisted mainly of dolomite.     

Sulfation behavior of white mud from paper manufacture as SO<Subscript>2</Subscript> sorbent at fluidized bed combustion temperatures

The calcination characteristics, sulfation conversion, and sulfation kinetics of a white mud from paper manufacture at fluidized bed combustion temperatures were investigated in a thermogravimetric analyzer. Also, the comparison between the white mud and the limestone in sulfation behavior and microstructure was made. Although the white mud and the limestone both contain lots of CaCO₃, they are different in the alkali metal ions content and microstructure. It results in a marked difference in sulfation behavior between the white mud and the limestone. The CaO derived from white mud achieves the maximum sulfation conversion of 83% at about 940 °C which is 1.7 times higher than that derived from limestone at about 880 °C. The shrinking unreacted core model is appropriate to analyze the sulfation kinetics of the white mud. The chemical reaction activation energy E _a and the activation energy for product layer diffusion E _p for the sulfation of the white mud are 44.94 and 55.61 kJ mol⁻¹, respectively. E _p for the limestone is 2.8 times greater than that for the white mud. The calcined white mud possesses higher surface area than the calcined limestone. Moreover, the calcined white mud has more abundant pores in 4–24 nm range which is almost optimum pore size for sulfation. It indicates that the microstructure of the white mud is beneficial for SO₂ removal.     

The effect of sintering on the maximum capture efficiency of CO<Subscript>2</Subscript> using a carbonation/calcination cycle of carbonate rocks

The effect of sintering on the maximum capture efficiency of CO₂ is studied, using a carbonation/calcination cycle for a series of samples with different stoichiometries of dolomite and calcite. For the materials that belong to the categories of limestone and dolomitic limestone, sintering decreases the extent of carbonation significantly at the two different highest temperatures studied. The extent of carbonation for the same maximum heating temperature depends mainly on the percentage of dolomite. Sintering is negligible in the dolomitic rocks, especially at the maximum heating temperature of 1005°C. The composition of the carrier gas does not seem to play a significant role. The reduction of the extent of carbonation at the second heating /cooling cycle in limestone, and the durability after enough successive cycles of calcination/carbonation in the dolomitic rocks, does not seem to be affected by the maximum temperatures of calcination that were used at the experiments.     

电石渣在煅烧/氯化反应中的HCl脱除特性研究

采用钙基废弃物———电石渣煅烧后脱除HCl。在煅烧/氯化反应器上研究脱氯反应温度、HCl体积分数、颗粒粒径和煅烧温度对电石渣脱氯性能的影响。结果表明,电石渣在700℃时取得最高氯化转化率;脱氯反应温度高于650℃后,电石渣氯化转化率均高于石灰石,电石渣高温脱氯更有优势。电石渣氯化转化率随反应气氛中HCl体积分数提高呈线性增长。随电石渣颗粒粒径增大,氯化转化率缓慢降低。高于900℃的煅烧温度不利于电石渣脱除HCl。煅烧后电石渣分布在2~10 nm的孔隙较多,氯化后分布在此孔径范围内的孔容和孔面积分别降低了56.2%和62.2%,2~10 nm孔隙是煅烧后电石渣吸收HCl的主要区域。     

Use of Taguchi approach for synthesis of calcite particles from calcium carbide slag for CO2 fixation by accelerated mineral carbonation

This study is focused on the conversion of harmful materials (calcium carbide slag [CCS] and flue gas) into CaCO₃ particles through an accelerated mineral carbonation process. The influences of reaction temperature, amount of Na-oleate, solid-to-liquid ratio, and stirring speed on the properties of CaCO₃ particles were determined using XRF, XRD, SEM, FTIR, TG, and contact angle measurements. Experiments were designed based on an orthogonal array L9 (3⁴) of the Taguchi approach. The gas mixture of CO₂/N₂ (16.3% of CO₂ cons.) gas was used to represent the flue gas for each experiment. The formation of CaCO₃ particles from CCS depending on time was monitored via SEM. Experiments showed that the presence of Na-oleate in the slurry played a curial role in the carbonation process, and the conversion ratio of CO₂ into a solid carbonate phase was higher than that in the experiments conducted without Na-oleate. The crystallite size of CaCO₃ particles varied between 11.55 and 38.11 nm depending on the production conditions. Each obtained CaCO₃ particles were identified as calcite (cubic-like rhombohedral), which is in high demand in many industrial applications.     

Gas generation during Cypris clay expansion

Blast furnace slag was leached using HCl to prepare lithium-based sorbents for CO₂ capture, and chemical composition and phase of the acid leaching slag were determined by X-ray fluorescence analysis. The microstructure and morphology of both sorbents were characterized by scanning electron microscope and X-ray diffraction. The absorption capacity of both sorbents was observed non-isothermally and isothermally using thermogravimetric analysis, and 12 carbonation and calcination cycles were conducted to observe cycling stability. Controlling step of absorption process was determined by fitting the isothermal graphs using a double exponential model. The results show that 98.33% amorphous SiO₂ can be obtained when the blast furnace slag was treated at 373 K for 10 h. Purified lithium-based sorbent by acid leaching slag (LBS-ALS) shows dense polyhedral particles with particle size between 25 and 120 μm. LBS-ALS shows similar absorption capacity with pure Li₄SiO₄ (P-Li₄SiO₄), but narrower absorption temperature range at non-isothermal absorption condition. The double exponential model fits well with the isothermal graphs for LBS-ALS and P-Li₄SiO₄, and diffusion of CO₂ is the controlling step of the absorption process at lower temperature. LBS-ALS shows different controlling mechanism for desorption process compared with P-Li₄SiO₄. LBS-ALS maintains higher absorption capacity after 12 cycles in 100% CO₂ flow.     

SGCS-made ultrafine CaO/Al_2O_3 sorbent for cyclic CO_2 capture

Multi-cyclic CaO carbonation/calcination is an attractive method for CO₂ capture during coal combustion.However,the capture capacity of CaO sharply decreases with increasing carbonation/calcination cycles.In order to improve the stability of CO₂ capture capacity of CaO during carbonation/calcination cycles,synthetic CaO/Al₂O₃ sorbents were synthesized by two methods:wet chemistry and sol-gel-combustion-synthesis（SGCS） to make a further comparison.The results indicate that the SGCS-made CaO/ Al₂O₃ = 80:20 wt%sorbent provides a competitive performance of a capture capacity of 0.43 g-CO₂/g-sorbent after 20 cycles.     

Decarbonization of Natural Lime-containing Materials and Reactivity of Calcined Products Towards SO2 and CO2

The results obtained by studying decarbonization of different samples of Estonian limestone and dolomite and the following sulphation or carbonation of calcined products to estimate their SO2 and CO2 binding ability were presented. Experiments were carried out with thermogravimetric equipment(Q-Derivatograph, MOM and Labsys™, SETARAM) – calcination of the samples in the atmosphere of air with the heating rate 10 K per minute using multiplate crucibles, the following sulphation or carbonation of the calcined products after cooling to the fixed temperature (temperature range 400–900°C) under isothermal conditions in the flow of air-SO₂ or air-CO₂ mixture. Chemical, X-ray, BET nitrogen dynamic desorption, etc. methods for the characterization of the initial samples, intermediate and final products were used. In addition, the possibilities of recurrent use of oil shale ashes taken from different technological points at operating thermal power plants (Estonian and Baltic TTPs, Estonia) as sorbents for SO₂ binding from gaseous phase were studied, as well as the possibilities of activation of these ashes towards SO₂ binding. The results of these studies confirmed the high reactivity of Estonian limestone and dolomite towards SO₂ and CO₂. Dependence of SO₂ binding mechanism on the SO₂ concentration has been established. Modelling of SO₂ capture of dolomite and limestone was carried out to establish the kinetic parameters of these processes. The possibilities of activation of oil shale ashes and their effective recurrent use for binding SO₂ and CO₂ from gaseous phase were confirmed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect exerted by texture of calcined calcium oxide on its sorption capacity in the CO2 sorption-regeneration cycles

The dynamic capacity of a set of sorbents prepared by calcination of different precursors was studied in multiple CO₂ sorption-regeneration cycles. The effect exerted by type of a precursor and calcination temperature on the steady state value of the dynamic capacity attained after several tens of cycles was determined. A model was suggested for estimate of the sorption capacity of CaO sintered above the Tammann temperature from data on the mercury porosimetry.     

Reaction and transport effects in the heterogeneous systems for lean gas purification

Sorption of hydrogen chloride gas by active soda and that of hydrogen sulfide gas by calcium oxide are explored by experiment as promising means of removing these detrimental contaminants from fuel gas: active Na₂CO₃ was prepared by the calcination of commercial NaHCO₃ at 200 °C; reactive CaO was formed by decomposing a fine-grained, high-calcium limestone at 830 °C. Techniques with a differential reactor were employed to explore the rate of reaction of HCl with Na₂CO₃ at 500 °C and that of H₂S with CaO at 800 °C. Time-resolved data on the sorbents’ conversion were collected as a function of mean particle size in the range between 0.285 and 1.12 mm. The surface reaction constants, deduced via the tractable model from the initial reaction rates of the two reactions, slightly increase with the increasing particle size. The proposed correlations enable to interpolate or cautiously extrapolate to other isotropic, irregularly shaped solids. The effective diffusivities educed by means of the model from the experimental curves decrease significantly with the increasing conversion and are affected by the particle size in both sorptions. The developed reaction rate equations can conveniently be applied to the design and simulation of the deep dechloridization and the bulk desulfurization of hot producer gas.     

Enhancement of CO2 Conversion Rate and Conversion Efficiency by Homogeneous Discharges

The CO₂ conversion rate and conversion efficiency were greatly enhanced by homogeneous dielectric barrier discharges generated in our experiment. Influence of CaO?CB₂O₃?CSiO₂ glass addition on dielectric properties and microstructures of Ca_0.8Sr_0.2TiO₃ were investigated for the purpose of discerning the effect of dielectric barrier material on the dielectric barrier discharge performance so as to improve the CO₂ conversion rate and conversion efficiency. It was found that considerable grain boundaries on the dielectric barrier surface serving as charge trapping sites contribute a great many charges during plasma generation. And low resistance of the dielectric barrier surface distributes the charges effectively. More importantly, when the gap of the discharge is narrowed down, the surface charges on the dielectric barrier will play a dominant role during the discharge. As a result, for the 5.0 wt% glass addition, the CO₂ conversion rate and conversion efficiency reached the maximum values of 48.71?% and 1.14?W/%, respectively.     

New kinetic model for the rapid step of calcium oxide carbonation by carbon dioxide

Carbonation of solid calcium oxide by gaseous carbon dioxide was monitored by thermogravimetry. A kinetic model of CaO carbonation is proposed in order to interpret the first rapid step of the reaction. By taking into account, the existence of large induction period as well as the sigmoidal shape of the kinetic curves in this kinetic-controlled region, a surface nucleation and isotropic growth kinetic model based on a single nucleus per particle is proposed, and the expressions of the fractional conversion and the reaction rate versus time are detailed. The induction period is found to have a linear variation with respect to temperature and to follow a power law with respect to CO₂ partial pressure. The areic reactivity of growth decreases with temperature increase, and increases with CO₂ partial pressure increase. A mechanism of CaCO₃ growth is proposed to account for these results and to determine a dependence of the areic reactivity of growth on the temperature and the CO₂ partial pressure.     

Copper-based oxygen carriers supported with alumina/lime for the chemical looping conversion of gaseous fuels

Copper(Ⅱ) oxide in varying ratios was combined with either an alumina-based cement(Al300), or CaO derived from limestone as support material in a mechanical pelletiser. This production method was used to investigate its influence on possible mechanical and chemical improvements for oxygen carriers in chemical looping processes. These materials were tested in a lab-scale fluidised bed with CO or CH_4 as a reducing gas at 950 °C. As expected, the oxygen carriers containing a greater ratio of support material exhibited an enhanced crushing strength. Oxygen carriers comprised of a 1:3 ratio of support material to active CuO exhibited increased crushing strength by a minimum of 280% compared to pure CuO pellets.All oxygen carriers exhibited a high CO conversion yield and were fully reducible from CuO to Cu. For the initial redox cycle, Al300-supported oxygen carriers showed the highest fuel and oxygen carrier conversion. The general trend observed was a decline in conversion with an increasing number of redox cycles.In the case of CaO-supported oxygen carriers, all but one of the oxygen carriers suffered agglomeration.The agglomeration was more severe in carriers with higher ratios of CuO. Oxygen carrier Cu25Al75(75 wt% aluminate cement and 25 wt% CuO), which did not suffer from agglomeration, showed the highest attrition with a loss of approximately 8% of its initial mass over 25 redox cycles. The reducibility of the oxygen carriers was limited with CH_4 in comparison to CO. CH_4 conversion were 15%-25% and 50% for Cu25Ca75(25 wt% CuO and 75 wt% CaO) and Cu25Al75, respectively. Cu25Ca75 demonstrated improved conversion, whereas Cu25Al75 exhibited a trending decrease in conversion with increasing redox cycles.     

Hydration mechanisms of supersulfated cement

Considerable attention has been given to special cements, capable of reducing CO₂ emissions, energy and limestone consumption. Supersulfated cements are made of blast furnace slag (GBFS), calcium sulfate (CS), and small quantities of activator, but achieving their optimal proportions is complex. In this paper, the effects of the both CS and alkali activator (KOH) contents were studied. The main results showed that the compressive strength, heat of hydration, and consumption of anhydrite phase were strongly influenced by the alkaline content, while low calcium sulfate or alkaline content increased the formation of CSH. The instability of ettringite was verified: with low CS, the probable hypothesis was its conversion into monosulfate due to the scarcity of sulfate; with high CS, it was associated with intense, rapid consumption of anhydrite with high KOH content, followed by the precipitation of ettringite on the surface of slag grains and its conversion into monosulfate.     

The calcination of limestone — Studies on the past, the presence and the future of a crucial industrial process

The calcination of limestone is one of the oldest technical processes and it is still of actual interest. Very early calcitic mortars from Turkey have been investigated and compared with materials of other early civilisations i.e. with Egyptian mortars containing gypsum as well as medieval dolomite-based mortars from alpine regions. Contemporary calcination procedures, in particular the cement production, range among the most important global industrial processes causing non neglectable environmental problems. Sustainable, solar energy assisted calcination technologies and the conversion of product CO₂ into useful commodities are discussed.The authors like to thank Prof. K. von Salis Perch-Nielson, Institute of Geology, ETH Zürich, for assistance in sedimentological investigations.     

Indirect mineral carbonation of blast furnace slag with(NH_4)_2SO_4 as a recyclable extractant

Large quantities of CO_2 and blast furnace slag are discharged in the iron and steel industry. Mineral carbonation of blast furnace slag can offer substantial CO_2 emission reduction and comprehensive utilisation of the solid waste. In this study, a recyclable extractant,(NH_4)_2SO_4, was used to extract calcium and magnesium from blast furnace slag(main phases of gehlenite and akermanite) by using low-temperature roasting to fix CO_2 through aqueous carbonation. The process parameters and efficiency of the roasting extraction, mineralisation, and Al recovery were investigated in detail. The results showed that the extractions of Ca, Mg, and Al can reach almost 100% at an(NH4)_2SO_4-to-slag mass ratio of 3:1 and at 370°C in 1 h. Adjusting the p H value of the leaching solution of the roasted slag to 5.5 with the NH_3 released during the roasting resulted in 99% Al precipitation, while co-precipitation of Mg was lower than 2%. The Mg-rich leachate after the depletion of Al and the leaching residue(main phases of CaSO_4 and SiO_2) were carbonated using(NH_4)_2CO_3 and NH_4HCO_3 solutions, respectively, under mild conditions. Approximately 99% of Ca and 89% of Mg in the blast furnace slag were converted into CaCO_3 and(NH_4)_2 Mg(CO_3)_2·4 H_2O,respectively. The latter can be selectively decomposed to magnesium carbonate at 100-200 °C to recover the NH_3 for reuse. In the present route, the total CO_2 sequestration capacity per tonne of blast furnace slag reached up to 316 kg, and 313 kg of Al-rich precipitate, 1000 kg of carbonated product containing CaCO_3 and SiO_2, and 304 kg of carbonated product containing calcium carbonate and magnesium carbonate were recovered simultaneously. These products can be used, respectively, as raw materials for the production of electrolytic aluminium, cement, and light magnesium carbonate to replace natural resources.     

Pure and Metal-confining Carbon Nanotubes through Electrochemical Reduction of Carbon Dioxide in Ca-based Molten Salts

To be successfully implemented, an efficient conversion, affordable operation and high values of CO₂-derived products by electrochemical conversion of CO₂ are yet to be addressed. Inspired by the natural CaO-CaCO₃ cycle, we herein introduce CaO into electrolysis of SnO₂ in affordable molten CaCl₂-NaCl to establish an in situ capture and conversion of CO₂. In situ capture of anodic CO₂ from graphite anode by the added CaO generates CaCO₃. The consequent co-electrolysis of SnO₂ and CaCO₃ confines Sn in carbon nanotube (Sn@CNT) in cathode and increases current efficiency of O₂ evolution in graphite anode to 71.9 %. The intermediated CaC₂ is verified as the nuclei to direct a self-template generation of CNT, ensuring a CO₂-CNT current efficiency and energy efficiency of 85.1 % and 44.8 %, respectively. The Sn@CNT integrates confined responses of Sn cores to external electrochemical or thermal stimuli with robust CNT sheaths, resulting in excellent Li storage performance and intriguing application as nanothermometer. The versatility of the molten salt electrolysis of CO₂ in Ca-based molten salts for template-free generation of advanced carbon materials is evidenced by the successful generation of pure CNT, Zn@CNT and Fe@CNT.     

Thermal transformation of water-dispersible magnetite nanoparticles

This paper presents a study on rapid hardening behaviors of β-C₂S by accelerated carbonation curing. β-C₂S cubes compacted at various molding pressures were subjected to different CO₂ concentration for accelerated carbonation curing. The CO₂ uptake and microstructure changes were analyzed by thermogravimetric analysis (TG), QXRD, FT-IR and MAS-NMR. The results indicated that CO₂ uptake was affected by molding pressure and CO₂ concentration seriously. TG analysis indicated that the carbonation reaction was rapid in the first hour. The carbonation degree reached 21.6% and giving a compressive strength of 85.7 MPa after 6 h carbonation in 99.9% CO₂ concentration. And it showed a much less carbonation degree in 20.0% CO₂. Calcite, vaterite and amorphous silica-rich phase formed in the carbonation progress. The FT-IR and NMR analysis indicated β-C₂S was decalcified to C–S–H gel and further decalcified to formation of an amorphous silica gel composed of Q ³ and Q ⁴ silicate tetrahedral. The chain length of C–S–H gel increased from to 2.67 to 6.36 with prolonged carbonation time, showing a lower C/S ratio and higher polymerization and also resulting in a lower C–S–H content.