Cyclic calcination/carbonation looping of dolomite modified with acetic acid for CO2 capture

The dolomite modified with acetic acid solution was proposed as a CO₂ sorbent for calcination/carbonation cycles. The carbonation conversions for modified and original dolomites in a twin fixed-bed reactor system with increasing the numbers of cycles were investigated. The carbonation temperature in the range of 630 °C–700 °C is beneficial to the carbonation reaction of modified dolomite. The carbonation conversion for modified dolomite is significantly higher than that for original sorbent at the same reaction conditions with increasing numbers of reaction cycles. The modified dolomite exhibits a carbonation conversion of 0.6 after 20 cycles, while the unmodified sorbent shows a conversion of 0.26 at the same reaction conditions, which is calcined at 920 °C and carbonated at 650 °C. At the high calcination temperature over 920 °C modified dolomite can maintain much higher conversion than unmodified sorbent. The mean grain size of CaO derived from modified dolomite is smaller than that from original sorbent with increasing numbers of reaction cycles. The calcined modified dolomite possesses greater surface area and pore volume than calcined original sorbent during the multiple cycles. The pore volume and pore area distributions for calcined modified dolomite are also superior to those for calcined unmodified sorbent during the looping cycle. The modified dolomite is proved as a new and promising type of regenerable CO₂ sorbent for industrial applications.     

Effect of rice husk ash addition on CO2 capture behavior of calcium-based sorbent during calcium looping cycle

Rice husk ash/CaO was proposed as a CO₂ sorbent which was prepared by rice husk ash and CaO hydration together. The CO₂ capture behavior of rice husk ash/CaO sorbent was investigated in a twin fixed bed reactor system, and its apparent morphology, pore structure characteristics and phase variation during cyclic carbonation/calcination reactions were examined by SEM-EDX, N₂ adsorption and XRD, respectively. The optimum preparation conditions for rice husk ash/CaO sorbent are hydration temperature of 75 °C, hydration time of 8 h, and mole ratio of SiO₂ in rice husk ash to CaO of 1.0. The cyclic carbonation performances of rice husk ash/CaO at these preparation conditions were compared with those of hydrated CaO and original CaO. The temperature at 660 °C–710 °C is beneficial to CO₂ absorption of rice husk ash/CaO, and it exhibits higher carbonation conversions than hydrated CaO and original CaO during multiple cycles at the same reaction conditions. Rice husk ash/CaO possesses better anti-sintering behavior than the other sorbents. Rice husk ash exhibits better effect on improving cyclic carbonation conversion of CaO than pure SiO₂ and diatomite. Rice husk ash/CaO maintains higher surface area and more abundant pores after calcination during the multiple cycles; however, the other sorbents show a sharp decay at the same reaction conditions. Ca₂SiO₄ found by XRD detection after calcination of rice husk ash/CaO is possibly a key factor in determining the cyclic CO₂ capture behavior of rice husk ash/CaO.     

Enhancement of reactivity in surfactant-modified sorbent for CO2 capture in pressurized carbonation

The calcination/carbonation loop of calcium-based (Ca-based) sorbents is considered as a viable technique for CO₂ capture from combustion gases. Recent attempts to improve the CO₂ uptake of Ca-based sorbents by adding calcium lignosulfonate (CLS) with hydration have succeeded in enhancing its effectiveness. The optimum mass ratio of CLS/CaO is 0.5 wt.%. The reduction in particle size and grain size of CaO appeared to be parts of the reasons for increase in CO₂ capture. The primary cause of increase in reactivity of the modified sorbents was the ability of the CLS to retard the sintering rate and thus to remain surface area and pore volume for reaction. The CO₂ uptake of the modified sorbents was also enhanced by elevating the carbonation pressure. Experimental results indicate that the optimal reaction condition of the modified sorbents is at 0.5 MPa and 700 °C and a high conversion of 0.7 is achieved after 10 cycles, by 30% higher than that of original limestone, at the same condition.     

Calcination and sintering characteristics of limestone under O2/CO2 combustion atmosphere

The O₂/CO₂ coal combustion technology is an innovative combustion technology that can control CO₂, SO₂ and NO_x emissions simultaneously. Calcination and sintering characteristics of limestone under O₂/CO₂ atmosphere were investigated in this paper. The pore size, the specific pore volume and the specific surface area of CaO calcined were measured by N₂ adsorption method. The grain size of CaO calcined was determined by XRD analysis. The specific pore volume and the specific surface area of CaO calcined in O₂/CO₂ atmosphere are less than that of CaO calcined in air at the same temperature. And the pore diameter of CaO calcined in O₂/CO₂ atmosphere is larger than that in air. The specific pore volume and the specific surface area of CaO calcined in O₂/CO₂ atmosphere increase initially with temperature, and then decline as temperature exceeds 1000 °C. The peaks of the specific pore volume and the specific surface area appear at 1000 °C. The specific surface area decreases with increase in the grain size of CaO calcined. The correlations of the grain size with the specific surface area and the specific pore volume can be expressed as L = 744.67 + 464.64 lg(1 / S) and L = − 608.5 + 1342.42 lg(1 / ε), respectively. Sintering has influence on the pore structure of CaO calcined by means of influencing the grain size of CaO.     

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.     

Experimental evaluation of Mg- and Ca-based synthetic sorbents for CO2 capture

Hydrogen with high purity can be directly derived from fossil and renewable energy sources, like natural gas, coal and biomass, by the so-called sorption-enhanced reforming (SER) and water gas shift (SEWGS) processes characterized by simultaneous CO₂ capture.     

Evaluation of solid sorbents as a retrofit technology for CO2 capture

Processes based upon solid sorbents are currently under consideration for post-combustion CO₂ capture. Twenty-four different sorbent materials were examined on a laboratory scale in a cyclic temperature swing adsorption/regeneration CO₂ capture process in simulated coal combustion flue gas. Ten of these materials exhibited significantly lower theoretical regeneration energies compared to the benchmark aqueous monoethanolamine, supporting the hypothesis that CO₂ capture processes based upon solids may provide cost benefits over solvent-based processes. The best performing materials were tested on actual coal-fired flue gas. The supported amines exhibited the highest working CO₂ capacities, although they can become poisoned by the presence of SO₂. The carbon-based materials showed excellent stability but were generally categorized as having low CO₂ capacities. The zeolites worked well under dry conditions, but were quickly poisoned by the presence of moisture. Although no one type of material is without concerns, several of the materials tested have theoretical regeneration energies significantly lower than that of the industry benchmark, warranting further development research.     

The rate and extent of uptake of CO2 by a synthetic, CaO-containing sorbent

A synthetic, Ca-based solid sorbent, showed a marked increase in its ultimate uptake for CO₂ at temperatures in excess of 750 °C when the concentration of CO₂ was increased during carbonation in a fluidised bed. In contrast, the uptakes by natural sorbents, e.g. dolomite, were relatively insensitive to the concentration of CO₂. It is apparent that the rate of reaction of the synthetic sorbent falls to zero once the small pores within the grains, of which the particle is composed, have largely filled and a thin layer of product has been deposited around each grain. To quantify this effect, theory has been developed in which the mechanical work required to disrupt the layer of product is taken into account. The resulting model for the overall uptake correlates experimental measurements well, except in circumstances where carbonation times are so long that sintering introduces gross changes in the morphology of the layer of product. The explanation for why the overall conversion of the synthetic sorbent is dependent on the concentration of CO₂, whilst the conversion of dolomite is insensitive to [CO₂], is attributed to differences in the yield stress, σ_Y, needed to disrupt the layer of product formed in the two materials. It was found that the value of σ_Y for dolomite is about an order of magnitude larger than that for the synthetic sorbent. The behaviour of the synthetic sorbent in response to increasing concentrations of CO₂ makes it an attractive solid for the separation of CO₂ from, e.g. the flue gases arising from combustion. By subsequently calcining the resulting solid, a pure stream of CO₂ can be produced whilst the sorbent is regenerated for further use.     

不同钙基吸附剂捕集CO2后的硫酸化反应特性研究

钙基吸附剂进行多次CO₂捕集后,碳酸化效率会大幅衰减,此时的吸附剂能否高效脱硫利用是值得重点关注的问题。鉴于此,筛选了高性能合成钙基吸附剂和天然石灰石吸附剂,通过热重分析仪分析对比其在多循环CO₂捕集后的碳酸化和硫酸化反应性能,采用微粒模型研究其硫酸化反应动力学特征。结果发现,高性能合成钙基吸附剂的碳酸化反应速率和CO₂吸附能力明显高于石灰石吸附剂。在长达500循环的CO₂捕集试验后,高性能合成钙基吸附剂的CO₂吸附能力比石灰石高10倍以上,其SO₂吸附能力相较于石灰石提升约40%。经历多次CO₂捕集反应循环后,2种吸附剂的硫酸化能力均有提升:其中,石灰石吸附剂的提升幅度更大,硫酸化转化率从26%提升到35%,而高性能合成钙基吸附剂的硫酸化转化率则从38%提升到43%。通过微粒模型计算发现,2种吸附剂的硫酸化反应均是与SO₂浓度相关的一级反应,多循环捕集CO₂反应后,石灰石吸附剂的硫酸化反应活化能下降接近30%,而高性能合成钙基吸附剂的硫酸化反应活化能只下降了5%。研究结果说明2种不同钙基吸附剂在进行循环CO₂捕集后,脱硫能力得到了不同程度的提高,且均可以较好地应用于SO₂的脱除。     

Cyclic CO2 capture behavior of KMnO4-doped CaO-based sorbent

This study examines the CO₂ capture behavior of KMnO₄-doped CaO-based sorbent during the multiple calcination/carbonation cycles. The cyclic carbonation behavior of CaCO₃ doped with KMnO₄ and the untreated CaCO₃ was investigated. The addition of KMnO₄ improves the cyclic carbonation rate of the sorbent above carbonation time of 257 s at each carbonation cycle. When the mass ratio of KMnO₄/CaCO₃ is about 0.5-0.8 wt.%, the sorbent can achieve an optimum carbonation conversion during the long-term cycles. The carbonation temperature of 660-710 °C is beneficial to cyclic carbonation of KMnO₄-doped CaCO₃. The addition of KMnO₄ improves the long-term performance of CaCO₃, resulting in directly measured conversion as high as 0.35 after 100 cycles, while the untreated CaCO₃ retains conversion less than 0.16 at the same reaction conditions. The addition of KMnO₄ decreases the surface area and pore volume of CaCO₃ after 1 cycle, but it maintains the surface area and pores between 26 nm and 175 nm of the sorbent during the multiple cycles. Calculation reveals that the addition of KMnO₄ improves the CO₂ capture efficiency significantly using a CaCO₃ calcination/carbonation cycle and decreases the amount of the fresh sorbent.     

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.     

Simultaneous easy CO2 recovery and drastic reduction of SOx and NOx in O2/CO2 coal combustion with heat recirculation

Coal combustion with O₂/CO₂ is promising because of its easy CO₂ recovery, extremely low NO_x emission and high desulfurization efficiency. Based on our own fundamental experimental data combined with a sophisticated data analysis, its characteristics were investigated. It was revealed that the conversion ratio from fuel-N to exhausted NO in O₂/CO₂ pulverized coal combustion was only about one fourth of conventional pulverized coal combustion. To decrease exhausted NO further and realize simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x, a new scheme, i.e. O₂/CO₂ coal combustion with heat recirculation, was proposed. It was clarified that in O₂/CO₂ coal combustion, with about 40% of heat recirculation, the same coal combustion intensity as that of coal combustion in air could be realized even at an O₂ concentration of as low as 15%. Thus exhausted NO could be decreased further into only one seventh of conventional coal combustion. Simultaneous easy CO₂ recovery and drastic reduction of SO_x and NO_x could be realized with this new scheme.     

Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming

The production of hydrogen from methane via two chemical looping reforming (CLR) processes was simulated and thermodynamically analysed, one process being the conventional CLR process, the other being a CO₂ sorption enhanced process. The aim of the work was to identify suitable operating conditions for obtaining an optimum hydrogen gas purity and yield, whilst operating auto-thermally, at atmospheric pressure and with no carbon formation. In both simulations, the reactors were simulated using the Gibbs minimisation technique. NiO was used as the oxygen storing species, whilst CaO was used as the CO₂ adsorbent.     

Thermodynamic performance of O2/CO2 gas turbine cycle with chemical recuperation by CO2-reforming of methane

This paper proposes a novel power cycle system composed of chemical recuperative cycle with CO₂–NG (natural gas) reforming and an ammonia absorption refrigeration cycle. In which, the heat is recovered from the turbine exhaust to drive CO₂–NG reformer firstly, and then lower temperature heat from the turbine exhaust is provided with the ammonia absorption refrigeration system to generate chilled media, which is used to cool the turbine inlet gas except export. In this paper, a detailed thermodynamic analysis is carried out to reveal the performance of the proposed cycle and the influence of key parameters on performance is discussed. Based on 1 kg s⁻¹ of methane feedstock and the turbine inlet temperature of 1573 K, the simulation results shown that the optimized net power generation efficiency of the cycle rises up to 49.6% on the low-heating value and the exergy efficiency 47.9%, the new cycle system reached the net electric-power production 24.799 MW, the export chilled load 0.609 MW and 2.743 kg s⁻¹ liquid CO₂ was captured, achieved the goal of CO₂ and NO_x zero-emission.     

Properties of char particles obtained under O2/N2 and O2/CO2 combustion environments

Pulverized coal combustion in O₂/N₂ and O₂/CO₂ environments was investigated with a drop tube furnace. Results present that the reaction rate and burn-out degree of O₂/CO₂ chars (obtained in O₂/CO₂ environments) are lower than that of O₂/N₂ chars (obtained in O₂/N₂ environments) under the same experimental condition. It indicates that a higher O₂ concentration in O₂/CO₂ environment is needed to achieve the similar combustion characteristic to that in O₂/N₂ environment. The main differences between O₂/N₂ and O₂/CO₂ chars rely on the pore structure determined by N₂ adsorption and chemical structure measured by FT-IR. For O₂/CO₂ char, the surface is thick and the pores are compact which contribute to the fragmentation reduction of particles burning in O₂/CO₂ environment. The organic functional group elimination rate from the surface of O₂/CO₂ chars is slower or delayed. The present research results might have important implications for further understanding the intrinsic kinetics of pulverized coal combustion in O₂/CO₂ environment.     

The maximum capture efficiency of CO2 using a carbonation/calcination cycle of CaO/CaCO3

The use of natural calcium carbonates as regenerable CO₂ sorbents in industrial processes is limited by the rapid decay of the carbonation conversion with the number of cycles carbonation/calcination. However, new processes are emerging to capture CO₂ using these cycles, that can take advantage of the intrinsic benefits of high temperature separations in energy systems. This work presents an analysis of a general carbonation/calcination cycle to capture CO₂, incorporating a fresh feed of sorbent to compensate for the decay in activity during sorbent re-cycling. A general design equation for the maximum CO₂ capture efficiency is obtained by incorporating to the cycle mass balances a simple but realistic equation to estimate the decay in sorbent activity with the number of cycles.     

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.     

Impact of SO2 concentration on the corrosion rate of X70 steel and iron in water-saturated supercritical CO2 mixed with SO2

The corrosion behavior of X70 steel and iron in water-saturated supercritical CO₂ mixed with SO₂ was investigated using weight-loss measurements. As a comparison, the instantaneous corrosion rate in the early stages for iron in the same corrosion environment was measured by resistance relaxation method. Surface analyzes using SEM/EDS, XRD and XPS were applied to study the morphology and chemical composition of the corroded sample surface. Weight-loss method results showed that the corrosion rate of X70 steel samples increased with SO₂ concentration, while the corrosion rate increased before decreasing with SO₂ concentration for iron sample. Comparing resistance relaxation method results with weight-loss method results, it is found that the instantaneous corrosion rate of iron is much higher than the uniform corrosion rate of the iron tablet specimens which are covered with thick corrosion product films after a long period of corrosion. The corrosion product films were mainly composed of FeSO₄ and FeSO₃ hydrates. The possible reaction mechanism under such environment was also analyzed, and the electrochemical reaction between the dissolved SO₂ in the condensed water film with iron is the critical reaction step.     

Development of new alumina-modified sorbents for CO2 sorption and regeneration at temperatures below 200 °C

A new regenerable alumina-modified sorbent was developed for CO₂ capture at temperatures below 200 °C. The CO₂ capture capacity of a potassium-based sorbent containing Al₂O₃ (KAlI) decreased during multiple CO₂ sorption (60 °C) and regeneration (200 °C) tests due to the formation of the KAl(CO₃)(OH)₂ phase, which could be converted into the original K₂CO₃ phase above 300 °C. However, the new regenerable potassium-based sorbent (Re-KAl(I)) maintained its CO₂ capture capacity during multiple tests even at a regeneration temperature of 130 °C. In particular, the CO₂ capture capacity of the Re-KAl(I)60 sorbent which was prepared by the impregnation of Al₂O₃ with 60 wt.% K₂CO₃ was about 128 mg CO₂/g sorbent. This excellent CO₂ capture capacity and regeneration property were due to the characteristics of the Re-KAl(I) sorbent producing only a KHCO₃ phase during CO₂ sorption, unlike the KAlI30 sorbent which formed the KHCO₃ and KAl(CO₃)(OH)₂ phases even at 60 °C. This result was explained through the structural effect of the support containing the KAl(CO₃)(OH)₂ phase which was prepared by impregnation of Al₂O₃ with K₂CO₃ in the presence of CO₂.