化学链燃烧钙基载氧体CaSO4与CO在不同温度下的反应行为

基于热重和红外联用进行等温实验,探讨了化学链燃烧载氧体CaSO4在CO气氛下的还原反应特性.研究发现:温度对CaSO4还原反应历程和速率有显著的影响,在10%CO气氛下,温度低于900℃时,发生单一反应,CaSO4的还原产物只是CaS,气相产物为CO2;当温度高于950℃后,发生平行反应和连串反应组合成的多重反应,固体产物为CaS和CaO,而产物气中除了有CO2,还存在SO2和COS,且气相硫化物的析出以COS为主;随着反应温度的升高,CaSO4与CO反应速率显著增加,而目标产物CaS在固体产物中所占的摩尔分数呈下降趋势;基于钙基载氧体化学链燃烧中燃料反应器温度不宜高于950℃.     

10 kWth级串行流化床中木屑化学链燃烧试验

设计并建立了10 kW_th级串行流化床化学链燃烧反应器系统,以NiO/Al₂O₃为载氧体,在该系统上进行生物质（松木木屑）化学链燃烧分离CO₂的试验研究,探讨了燃料反应器温度T、水蒸气/生物质比率S/B对两个反应器（空气反应器和燃料反应器）气体产物组成以及燃烧效率的影响。试验结果表明,燃料反应器温度是影响生物质化学链燃烧过程的重要因素,随着温度的升高,燃料反应器气体产物中CO₂浓度不断上升,CH4浓度显著降低,CO浓度先升高而后迅速下降;较高的反应器温度有助于燃烧效率的提高。随着S/B的增加,燃料反应器气体产物中CO和CH₄浓度均会增大,CO₂浓度以及燃烧效率有所降低。在100 h的连续试验过程中,采用共沉淀法制备的NiO/Al₂O₃载氧体展现出良好的氧化-还原性能和较强的持续循环能力,是生物质化学链燃烧理想的载氧体。     

基于NiO载氧体的煤化学链燃烧实验

采用流化床反应器并以水蒸气作为气化-流化介质,研究了以NiO为载氧体在800～960℃内的煤化学链燃烧反应特性。实验结果表明,载氧体与煤气化产物在反应器温度高于900℃体现了高的反应活性。随着流化床反应器温度的提高,气体产物中CO₂的体积浓度(干基)呈单调递增;CO、H₂、CH₄的体积浓度(干基)呈单调递减;煤中碳转化为CO₂的比率逐渐递增,碳的残余率逐渐递减。反应器出口气体CO₂、CO、H₂、CH₄的生成率随反应时间呈单峰特性,H₂生成率的峰值远小于CO的峰值;且随反应器温度升高,CO₂生成率升高,CO、H₂、CH₄的生成率降低。反应温度高于900℃时,流化床反应器NiO载氧体煤化学链燃烧在9 min之内就基本完成,CO₂含量高于92%。     

CaSO4与CaS在N2气氛下反应动力学

利用热重分析仪研究了不同摩尔比下的硫酸钙和硫化钙混合物在N₂气氛下的反应特性，探讨了其化学反应动力学及反应机理.CaSO₄和CaS的固-固反应是CaS氧化生成CaO反应的速率控制反应.当CaSO₄过量时，CaSO₄与CaS的反应分两个阶段.在890～1120 ℃温度范围内CaSO₄与CaS发生固-固反应.在1160～1330 ℃温度范围内，剩余的CaSO₄发生自身分解.当CaSO₄与CaS的摩尔比为3∶1时，反应的活化能最大.CaSO₄和CaS的反应机理可以采用液态共熔体模型来解释.     

纳米钙基CO2吸附剂反应吸附与分解动力学

采用TGA测定纳米钙基CO₂吸附剂在500～650℃温度范围内,CO₂分压0.015～0.025 MPa氮气气氛中的吸附反应动力学。针对纳米钙基CO₂吸附剂吸附CO₂反应特征,提出以两倍最大吸附速率对应的时间点前后分别为快速反应段与慢速反应段。分别采用Boltzmann方程与Avrami-Erofeev方程拟合快速反应段与慢速反应段吸附反应动力学方程,得到纳米钙基CO₂吸附剂在快速反应段与慢速反应段的活化能分别为27.52、70.25 kJ·mol^-1。吸附率拟合与实验值平均相对误差分别为10.29％、4. 17%。研究测试了纳米钙基CO₂吸附剂在650～800℃温度范围内,N₂,0.02、0.04 MPa CO₂分压氮气气氛中的分解反应动力学。忽略反应过程中传热、传质影响,采用收缩核模型,分别求得吸附剂在N₂,0.02、0.04 MPa CO₂分压氮气气氛中的活化能为141.9、34.7、113.2 kJ·mol^-1。碳酸钙分解率与实验值比较平均相对误差分别小于5.66% 、7.82%、5.01%。     

具有自载体功能的CoFeAlO4载氧体化学链燃烧反应特性

采用燃烧法合成了具有尖晶石结构的CoFeAlO₄载氧体材料,通过表征手段和实验研究考察了不同温度下CoFeAlO₄载氧体的化学链燃烧反应特性和循环稳定性,并对CoFeAlO₄载氧体晶相结构和表观形貌的变化规律进行了分析。结果表明,温度升高有利于提高CoFeAlO₄载氧体转化还原性气体CO的能力,使得还原反应速率更快,但高温下经“还原-氧化”会造成CoFeAlO₄载氧体相态分离,难以保持稳定的自载体尖晶石结构。对反应前后CoFeAlO₄载氧体晶相结构的分析表明,高温条件下经过“还原-氧化”后生成的CoFe₂O₄和CoAl₂O₄是导致CoFeAlO₄载氧体烧结和循环稳定性下降的主要原因。     

Co-Fe2O3纳米载氧体作用下CO化学链燃烧富集CO2

制备了不同量级Co掺杂Fe₂O₃载氧体Co-Fe₂O₃,利用BET和TEM对载氧体结构进行表征。通过在不同温度下Co-Fe₂O₃与气体燃料CO的还原反应,考察Co-Fe₂O₃对CO化学链燃烧特性。结果表明,同一温度条件下,掺杂量越高,还原反应转化率越高;掺杂量不变的情况下,温度升高促使还原程度加深,缩短了载氧体完全还原转化的时间。根据TGA曲线进行了化学动力学分析,发现Co_0.2Fe还原反应过程在344.7～391.0℃和414.7～472.5℃温度范围反应动力学对应扩散控制的Jander方程模型,607.6～681.5℃温度范围对应二维扩散反应模型,并分别计算出相应模型的表观活化能和频率因子。结果可为化学链燃烧技术应用提供理论指导。     

Co-Fe2O3[104]铁基载氧体优化体系作用下褐煤化学链燃烧特性

前期研究发现高弥勒指数晶面载氧体Fe₂O₃[104]具有高的化学链燃烧反应特性,且Co对煤及其热解中间产物具有催化气化和催化转化作用。通过正交实验优化制备Co-Fe₂O₃[104]/Al₂O₃载氧体体系结构,开展Co-Fe₂O₃[104]/Al₂O₃与褐煤的化学链燃烧,揭示载氧体与褐煤发生化学链燃烧的特性。结果表明:形貌控制制备的高弥勒指数晶面铁基载氧体Co-Fe₂O₃[104]/Al₂O₃(质量分数10%)促进了褐煤化学链燃烧过程中氧的迁移速率以及载氧体的还原程度,进而显著提高了载氧体与褐煤化学链燃烧的反应速率及反应效率。进一步通过CO多循环化学链燃烧反应、XRD和TEM表征了Co-Fe₂O₃[104]/Al₂O₃(10%)的可再生性及反应稳定性。     

反应条件对甲烷化法去除重整氢气中CO的影响

研究了在采用甲烷化法去除重整氢气中CO的过程中,反应温度、CO浓度和CO₂浓度对3个竞争反应即CO甲烷化反应、CO₂甲烷化反应、逆变换反应（RWGS）的影响。实验结果表明,随着温度升高,CO、CO₂甲烷化反应速率均增大,但CO甲烷化的选择性降低。CO浓度对CO甲烷化反应速率的影响在高温时较为明显,反应速率随CO浓度的升高而增大;CO对CO₂甲烷化反应的影响在较低温度下较为显著,CO₂甲烷化反应速率随CO浓度的升高而减小,表明CO对CO₂的甲烷化具有抑制作用,因而随着CO浓度的升高,选择性增大。另一方面,CO₂浓度对CO甲烷化反应几乎没有影响,而CO₂甲烷化反应速率和RWGS反应速率均随CO₂浓度的升高而增大,该趋势在高温下更加显著,并对3个竞争反应的宏观动力学进行了初步研究。     

CaSO4载氧体在煤气化化学链燃烧中的脱汞

为了研究载氧体在煤气化化学链燃烧中的脱汞机理,选择CaSO₄载氧体作为研究对象,900℃的反应温度下,在还原反应器中通入CO₂气体和水蒸气作为气化介质进行实验。结果表明：以CaSO₄作为载氧体的煤气化化学链燃烧中,CaSO₄载氧体本身促进Hg⁰的氧化,但CaSO₄分解产生的SO₂抑制Hg⁰的氧化。CaSO₄促进煤气化化学链燃烧产生S单质,会进一步与Hg⁰反应生成多种复杂的HgS_n,降低了烟气中Hg⁰含量,提高了脱汞效率。同时CaSO₄载氧体在还原-氧化的循环反应中具有良好的循环特性,是一种优良的化学链载氧体。     

Thermodynamic investigation into carbon deposition and sulfur evolution in a Ca-based chemical-looping combustion system

Chemical-looping combustion is a promising technology that concentrates CO₂ and separates it during combustion. In this study, both the carbon deposition and sulfur evolution in the reduction of a calcium sulfate (CaSO₄) oxygen carrier with a typical syngas were investigated using thermodynamic simulations. The effects of reaction temperature, operating pressure and the oxygen ratio number (defined in this paper) on the amount of deposited carbon and released sulfurous gases are discussed. A reaction temperature from 750 to 950 °C, an operating pressure from 1 to 15 bars and an oxygen ratio number between 0.4 and 0.8 were determined to be the most favorable operating conditions. In addition, the amounts of released sulfurous gases were found to be largely dependent on the partial pressures of H₂ and CO based on the thermo-gravimetric analyzer (TGA) tests. When the partial pressure of H₂ or CO was above 40 kPa, the release of sulfurous gases could be prevented in the reaction between CaSO₄ and syngas, even if the reaction temperature was as high as 1000 °C. The XRD profiles of the products also demonstrated that the mole fraction of CaS in the products increased gradually with an increasing partial pressure of H₂ or CO, until the products were almost pure CaS.     

Reactivity of a CaSO<Subscript>4</Subscript>-oxygen carrier in chemical-looping combustion of methane in a fixed bed reactor

Chemical-looping combustion (CLC) is a promising technology for the combustion of gas or solid fuel with efficient use of energy and inherent separation of CO₂. A reactivity study of CaSO₄ oxygen carrier in CLC of methane was conducted in a laboratory scale fixed bed reactor. The oxygen carrier particles were exposed in six cycles of alternating reduction methane and oxidation air. A majority of CH₄ reacted with CaSO₄ to form CO₂ and H₂O. The oxidation was incomplete, possibly due to the CaSO₄ product layer. The reactivity of CaSO₄ oxygen carrier increased for the initial cycles but slightly decreased after four cycles. The product gas yields of CO₂, CH₄, and CO with cycles were analyzed. Carbon deposition during the reduction period was confirmed with the combustible gas (CO+H₂) in the product gas and slight CO₂ formed during the early stage of oxidation. The mechanism of carbon deposition and effect was also discussed. SO₂ release behavior during reduction and oxidation was investigated, and the possible formation mechanism and mitigation method was discussed. The oxygen carrier conversion after the reduction decreased gradually in the cyclic test while it could not restore its oxygen capacity after the oxidation. The mass-based reaction rates during the reduction and oxidation also demonstrated the variation of reactivity of CaSO₄ oxygen carrier. XRD analysis illustrated the phase change of CaSO₄ oxygen carrier. CaS was the main reduction product, while a slight amount of CaO also formed in the cyclic test. ESEM analysis demonstrated the surface change of particles during the cyclic test. The reacted particles tested in the fixed bed reactor were not uniform in porosity. EDS analysis demonstrated the transfer of oxygen from CaSO₄ to fuel gas while leaving CaS as the dominant reduced product. The results show that CaSO₄ oxygen carrier may be an interesting candidate for oxygen carrier in CLC. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, China, June 26–28, 2008.     

DECOMPOSITION BEHAVIOR AND MECHANISM OF CALCIUM SULFATE UNDER THE CONDITION OF O2/CO2 PULVERIZED COAL COMBUSTION

The decomposition behavior and mechanism of calcium sulfate in O₂/CO₂ pulverized coal combustion were studied in an entrained flow reactor. A reaction rate expression correlating the influence of various factors was proposed for CaS0₄ decomposition and it is able to predict CaS0₄ decomposition satisfactorily. Under the conditions investigated, the decomposition of CaS0₄ was found to be a regime of chemically controlled shrinking core reaction. A CO₂-rich atmosphere enhances CaSO₄ decomposition in absence of oxygen. CaSO₄ particles have catalytic effect on formation of CO from CO₂. A high SO₂ concentration inhibits CaSO₄ decomposition. The kinetics of CaSO₄decomposition has obvious dependence on experimental facilities and conditions, whereas the activation energy has much lower dependence. The kinetics derived in this work is more appropriate for investigating desulfurization in O₂/CO₂ pulverized coal combustion because an entrained flow reactor has a much closer condition to that in O₂/CO₂ pulverized coal combustion than a TGA.     

Investigation into sulfur release in reductive decomposition of calcium sulfate oxygen carrier by hydrogen and carbon monoxide

Chemical looping combustion (CLC) is a promising technology with the inherent property of separating CO₂ from flue gas. For calcium sulfate (CaSO₄) oxygen carrier, the inhibition of the produced sulfurous gases in the reduction of CaSO₄, including sulfur dioxide (SO₂), hydrogen sulfide (H₂S) and carbonyl sulfide (COS), is the key for a CLC system. In this paper, the sensitivities of reacting temperature, oxygen ratio number (defined in this paper) and the mole fraction of both carbon monoxide (CO) and hydrogen(H₂) in the syngas to the sum of the amounts of released SO₂, H₂S and COS are discussed respectively. Thermo-gravimetric analysis (TGA) tests demonstrated that the amount of the produced sulfurous gases is greatly dependent on the partial pressure of H₂ or CO in the reduction of CaSO₄. When the partial pressure of H₂ or CO is higher than 40 kPa, the production of sulfurous gases, indicating the deterioration of the recyclability of CaSO₄, can be prevented completely even if the reacting temperature is as high as 1000 °C. A new kind of CaSO₄/CaCO₃ oxygen carrier is prepared using a mechanical mixing method. The addition of CaCO₃ nanoparticles largely improves the recyclability of the oxygen carrier in comparison with the fresh CaSO₄ oxygen carrier, without CaCO₃ nanoparticles, in a multi-cycle TGA test.     

加压下硫化钙氧化反应动力学和模型

在加压下研究了硫化钙在730～970 ℃的氧化行为及动力学和模型,结果表明：硫酸钙是硫化钙氧化的惟一产物,增大压力或提高温度均能使硫化钙的转化率增加,加压下可抑制硫化钙和硫酸钙的固固反应,动力学和模型分析表明硫化钙的氧化基本上受扩散控制.     

焦炉煤气化学链置换燃烧模拟研究

以焦炉煤气(COG,coke oven gas)为燃料,研究了CaSO4载氧体在燃料反应器(FR,fuel reactor)内的还原性,从原子层面对反应路径进行了探讨分析。当反应温度较低时(100~300℃),主要发生CO-H2甲烷化反应和CH4-CaSO4热化学硫酸盐还原反应,CaCO3与H2S是该温度范围内的主要产物。当反应温度较高时(400~1000℃),CaSO4与CO、H2和CH4之间的还原反应占据了主导地位,CaS、H2O和CO2是该温度范围内的主要产物。当反应温度进一步升高时(1000~1400℃),CaSO4与CaS发生固固反应生成大量的副产物SO2和CaO。温度和压力对产物中硫化物的分布有很大影响,在反应温度为1000℃,压力为0.1 MPa时,焦炉煤气的燃烧反应进行的很充分,但是反应在加压条件下进行时,CaSO4、CaS和H2S含量会有明显的下降,而SO2含量有一定程度的增加。     

Ni载体整体煤气化链式燃烧联合循环性能

本文将具有分离CO₂的链式燃烧技术与整体煤气化联合循环(IGCC)技术结合，构成整体煤气化链式燃烧联合循环系统，对系统性能进行了模拟研究。结果表明，采用德士古气化工艺、空气反应器出口温度1200℃，NiO/NiAl₂O₄作载氧体，压气机压比17、补燃后透平初温(TIT)1350℃、冷却空气量12％时，系统净效率39.61% HHV（41.55%LHV），CO₂排放量126 g·kW^-1·h^-1。补燃温度1350℃，空气反应器温度由1000℃升高到1200℃，CO₂的回收率提高约23％，系统效率由40.3%降低到39.61％；补燃温度由1200℃提高到1500℃，系统净效率由37.4%增加到40.8%，CO₂的排放量从3g·kW^-1·h^-1增加到202 g·kW^-1·h^-1；补燃温度一定，压比增大，系统比功减小，CO₂排放量增加，效率先增大后减小，存在最佳压比.