循环捕集CO2后煅烧石灰石的硫化特性

ZEC(zero emission coal)系统中,粗煤气进入碳酸化/重整炉前需先脱除H₂S,提出利用经过多次碳酸化/煅烧捕集CO₂循环的煅烧石灰石(CaO)脱除H₂S,并研究循环碳酸化/煅烧次数、硫化温度、H₂S浓度和微观结构对循环CaO硫化特性的影响。结果表明,多次循环碳酸化/煅烧捕集CO₂后CaO仍具有较高H₂S吸收性能。前20次循环,CaO硫化转化率随循环次数增加迅速降低;20次循环后,CaO硫化转化率缓慢下降。硫化120 min后,未循环CaO的硫化转化率接近100%,而经历1、20和100次循环后CaO的硫化转化率分别为94%、81%和74%。H₂S浓度对循环CaO硫化性能影响较大。硫化温度(800～1000℃)对循环CaO的硫化性能影响较小,最佳硫化温度为900℃。随循环次数增加,CaO颗粒发生高温烧结,导致比表面积降低和20～150 nm内孔隙减少,而这是与H₂S吸收密切相关的孔隙,导致CaO硫化转化率降低。     

O_2/CO_2气氛下醋酸调质石灰石煅烧/硫化特性

研究了醋酸溶液调质对石灰石在O2/CO2气氛下煅烧/硫化特性的影响,并结合氮吸附法、压汞法和XRD技术进行了分析。结果表明,与原石灰石相比,调质石灰石在煅烧/硫化过程中表现出更高的钙转化率,XRD图谱及其相定量分析证明了这一点;随温度升高,调质石灰石具有最佳的硫化温度,此温度下其钙转化率最高;随SO2浓度增大,初始阶段煅烧/硫化反应速率升高,调质石灰石钙转化率增大;随醋酸浓度增大,调质石灰石的钙转化率提高,提高幅度因石灰石种类而异;氮吸附及压汞分析表明调质石灰石较未调质石灰石具有发达的孔结构,促进了硫化反应的进行。     

CO2载体CaO循环煅烧/碳酸化反应的分形特征

捕捉煤燃烧释放出的CO₂时,作为CO₂载体的CaO微观结构特性对其循环碳酸化性能具有显著影响。采用分形维数作为表征CaO微观结构的特征参数,研究在循环煅烧/碳酸化反应过程中CaO的分形特征及其对CO₂捕捉性能的影响规律。结果表明,随着循环次数的增加CaO分形维数逐渐下降,CaO孔道也由粗糙和不规则变得越来越平滑和有规则性。煅烧温度升高则CaO分形维数下降。分形维数较大的CaO具有较高的碳酸化速率。在碳酸化过程的前10 min内CaO的分形维数迅速减小,此后随时间变化缓慢。在分形维数D≤2.61的实验范围内,CaO分形维数与其循环碳酸化转化率呈线性正相关;当D＞2.61时,可能存在临界分形维数D_cr,当D＞D_cr时随着分形维数的进一步增大CaO转化率反而减小。     

CO2载体CaO循环煅烧/碳酸化反应的分形特征

捕捉煤燃烧释放出的CO₂时,作为CO₂载体的CaO微观结构特性对其循环碳酸化性能具有显著影响。采用分形维数作为表征CaO微观结构的特征参数,研究在循环煅烧/碳酸化反应过程中CaO的分形特征及其对CO₂捕捉性能的影响规律。结果表明,随着循环次数的增加CaO分形维数逐渐下降,CaO孔道也由粗糙和不规则变得越来越平滑和有规则性。煅烧温度升高则CaO分形维数下降。分形维数较大的CaO具有较高的碳酸化速率。在碳酸化过程的前10 min内CaO的分形维数迅速减小,此后随时间变化缓慢。在分形维数D≤2.61的实验范围内,CaO分形维数与其循环碳酸化转化率呈线性正相关;当D＞2.61时,可能存在临界分形维数D_cr,当D＞D_cr时随着分形维数的进一步增大CaO转化率反而减小。     

钙循环捕集CO2后CaO的水合/脱水热化学储热性能

提出了基于CaO的钙循环捕集CO₂与CaO/Ca(OH)₂体系热化学储热耦合新工艺，在双固定床反应器上，研究了循环捕集CO₂中煅烧条件和碳酸化条件对CaO储热性能的影响，探究CaO循环捕集CO₂过程和循环水合/脱水储热过程的相互作用。研究表明，多次循环碳酸化/煅烧捕集CO₂后CaO仍具有较高储热性能，10次循环捕集CO₂后再经10次储热循环，CaO水合转化率可达0.66mol/mol。与苛刻煅烧条件相比，温和煅烧条件下经历多次循环捕集CO₂后CaO的储热性能更高。在碳酸化气氛中加入水蒸气对经历多次循环捕集CO₂后CaO储热性能的影响不大。钙循环捕集CO₂过程和水合/脱水循环储热过程能够相互促进。该工艺有望同时实现CO₂捕集和储热，具有一定的应用前景。     

纳米CaCO_3孔径调控及其对碳酸化反应性能的影响

研究了纳米CaCO_3颗粒间孔径调控对CaO与CO_2碳酸化反应性能的影响。通过有机模板法制备得到一系列比表面积相近、孔径分布不同的纳米CaCO_3,并考察其再生和碳酸化反应性能差异。结果表明:增大平均孔径能促进纳米CaCO_3的热分解反应,并降低分解温度约15℃。将平均孔径由15 nm增大至113 nm可显著提高碳酸化反应速率和转化率。研究认为平均孔径和比表面积对碳酸化反应转化率的影响存在交互作用;比表面积小的纳米CaCO_3,表现出碳酸化反应转化率受扩散控制影响较大,而比表面积较大的表现为碳酸化反应转化率以表面反应影响控制为主的规律。     

富氧燃烧气氛下石灰石煅烧/硫化特性及模型模拟

利用自制恒温热重装置,模拟循环流化床富氧燃烧气氛,进行了石灰石同时煅烧/硫化实验,并通过对煅烧/硫化产物孔结构以及硫化产物电导率的测量,探讨了硫化反应机理。相比石灰石先煅烧成CaO再硫化,吸收剂孔隙更容易堵塞且更早进入到产物层扩散控制阶段;产物层扩散控制阶段固态离子扩散率更高,可获得更快的硫化速率和更高的最终钙转化率。烧结会极大影响CaO的钙转化率,尤其当温度高于950℃时;粒径效应显著,随石灰石颗粒粒径减小最终钙转化率明显提高;SO₂浓度提高有助于最终钙利用率的提高。建立了晶粒-微晶粒模型,对不同温度、粒径、SO₂浓度条件下石灰石同时煅烧/硫化特性进行了数学模拟,模拟结果与实验结果较为吻合。     

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。     

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。     

H2O和SO2对CFB内石灰石同时煅烧/硫化反应中煅烧动力学的协同作用

采用自制恒温热重分析仪,研究了CFB工况下石灰石同时煅烧/硫化反应中H₂O和SO₂对石灰石煅烧动力学和孔结构的协同作用。煅烧环境中的H₂O能够促进石灰石的分解,但SO₂会减慢石灰石分解速度,且测试发现SO₂使煅烧后颗粒的孔容积下降,分解反应的效率因子减小。基于此提出SO₂减缓煅烧反应的机理：高温下,石灰石颗粒外层首先分解并生成多孔CaO层,其中的孔隙作为内部CaCO₃分解产生CO₂的外扩散通道,当煅烧气氛中含有SO₂时,颗粒的CaO层与SO₂反应生成CaSO₄,堵塞了CaO中的孔隙,增加了CO₂扩散的阻力,从而减缓了其分解速度。当石灰石在含有15% H₂O和0.3% SO₂的环境中分解时,其分解速度比不含二者的环境下快,而比含15% H₂O但不含SO₂的环境下慢,说明H₂O和SO₂对改变石灰石分解的速度有协同效应,但15% H₂O的作用比0.3% SO₂的作用更大。对效率因子的计算表明,该现象可能由于石灰石煅烧反应的速度控制步骤中本征反应速度的影响比扩散阻力的作用更大,而H₂O能够直接加速煅烧反应的本征速度。温度、粒径等均能够影响石灰石同时煅烧/硫化反应的中的煅烧速度。H₂O还能够促进CaO的烧结,并且H₂O和SO₂在降低石灰石煅烧产物的孔面积和孔容积上具有叠加效应。     

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.     

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.     

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 different organic compounds on the preparation of CaO-based CO2 sorbents derived from wet mixing combustion synthesis

CaO based sorbents have great potential for commercial use to capture CO_2 of power plants. In the demand of producing sorbents with better cyclic performance, CaO-based sorbents derived from different kinds of calcium precursors, containing calcium carbonate(CC-CaO), calcium gluconate monohydrate(CG-CaO), calcium citrate(CCi-CaO) and calcium acetate monohydrate(CA-CaO), were tested cyclically and compared using simultaneous thermal analyzer(STA). And further study was conducted on the sorbents modified with citric acid monohydrate and 50% gluconic acid solution by wet mixing combustion synthesis. The modified sorbents showed better performance and higher pore parameters as well as porous microstructure with more organic acid added. After 20 cycles of carbonation and calcination, the C2CCi8(CaO: citric acid = 2:8 by mass ratio) and C2G8(CaO: gluconic acid = 2:8 by mass ratio) sorbent possess CO_2 capture capacity of 0.45 g·g~(-1)(g CO_2 per g sorbents) and 0.52 g·g~(-1) respectively. The citric acid was more effective for modification than gluconic acid for extended 50 cycles. Furthermore, good linear relationship between CaO conversion and specific surface area as well as pore volume were determined, of which the specific surface area showed closer correlation with CaO conversion.     

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.     

Enhanced CO2 adsorption capacity and stability using CaO‐based adsorbents treated by hydration

The synthesis of highly efficient CaO‐based sorbents using Ca(Ac)₂ as a precursor and ethanol as a modification agent for CO₂ capture is described. This adsorbent has several characteristics such as large surface area and pore volume and small particle size. The influence of ratio of ethanol and water on CO₂ adsorption capacity was evaluated considering that the ethanol concentration could affect the pore structure of sorbents. The results showed that CaO modified by ethanol solution had a higher carbonization and better stability. Particularly, when the volume ratio of ethanol and water was 3, a performance of adsorption capacity of 74% and conversion of 94% was observed. CaO modified by ethanol solution had a superior performance due to the decrease of grain size and the formation of loose porous structure. The influence of steam on stability of adsorbents at high temperatures was examined, and it was found that with the existence of steam diffusion, the capacity of the sorbent could remain at a higher level and the stability was evidently improved. After 18 cycles of adsorption/desorption process, the capacity remained as high as 65%. It was proposed that dynamic and cyclic steam injection was favorable for preventing the sintering of sorbents and facilitating the diffusion of CO₂. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3586–3593, 2013     

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.     

钙基吸收剂微观结构特性及其反应性能

在小型流化床反应器中对5种钙基吸收剂的脱硫反应性能进行了实验研究,并利用扫描电镜和压汞分析等方法对吸收剂反应前后的微观结构进行了分析.结果发现石灰石吸收剂的比表面积大约是贝壳颗粒的4～5倍,但平均孔径却呈现出相反的趋势,石灰石吸收剂的最佳脱硫温度为900 ℃左右,而贝壳吸收剂最终转化率随着温度的升高而增大,在950 ℃下表现出了较好的硫化性能.研究发现吸收剂的孔结构参数是影响脱硫反应性能的主要因素.     

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.     

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.