煤化学链燃烧载氧体研究进展

煤的化学链燃烧是清洁煤燃烧的重要技术之一。化学链中载氧体的使用可以避免煤和空气直接接触,从而避免氮氧化物等污染物的产生并提高能量转化效率。一般来说,煤的化学链燃烧有2种反应途径:煤气化化学链燃烧和氧解耦化学链燃烧;不同反应途径将极大影响载氧体组分以及结构设计。详细论述了2015-2020年煤化学链燃烧中固态金属载氧体的研究进展,包括铁基、锰基、铜基、镍基、硫酸钙以及其他复合金属载氧体。总结了不同金属载氧体的优缺点、反应路径、气-固和固-固反应机理、金属与载体的相互作用以及载氧体失活原理。铁基载氧体被广泛应用于气化化学链燃烧中,但单一铁基载氧体的反应速率较低。适量添加碱金属或碱土金属可以提升载氧体的反应活性。锰基载氧体在化学链燃烧中具有两面性:一方面可以在高温缺氧气氛中释放气态氧,另一方面也可以与还原性气体发生气-固反应。通过使用惰性载体以及碱金属添加剂可以提高锰基载氧体的机械强度和氧解耦能力。含铜载氧体具有出色的氧解耦能力和反应活性而被广泛关注,然而铜及其氧化物低熔点所带来的金属聚集导致载氧体的失活问题亟需克服。研究发现使用铁、锰和铜矿石制得的载氧体具有良好的反应性能。硫酸钙载氧体具有较好的反应活性,但煤的化学链燃烧时潜在的二氧化硫和硫化氢副产物需要引起重视。镍基载氧体虽然在煤的化学链燃烧中反应性能较好,但硫毒化、成本较高和环保性能不佳等缺点导致近年来镍基载氧体的研究较少。新型双金属或多金属载氧体可以同时结合2种金属的反应特性,从而显著提高载氧体的整体反应活性。基于载氧体的研究现状,对未来的发展方向提出了4点建议:结合2种煤的化学链燃烧机理设计新型氧解耦辅助化学链燃烧载氧体;发展新型材料和金属组分的载氧体;利用冶金工业废料制得载氧体;开发新型结构的载氧体。     

Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion

For combustion with CO₂ capture, chemical-looping combustion (CLC) with inherent separation of CO₂ is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g. metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized with air back to its original form before it is returned to the fuel reactor. The feasibility of using oxygen carrier based on oxides of iron, nickel, copper and manganese was investigated. Oxygen carrier particles were produced by freeze granulation. They were sintered at 1300 °C for 4 h and sieved to a size range of 125-180 μm. The reactivity of the oxygen carriers was evaluated in a laboratory fluidized bed reactor, simulating a CLC system by exposing the sample to alternating reducing and oxidizing conditions at 950 °C for all carriers except copper, which was tested at 850 °C. Oxygen carriers based on nickel, copper and iron showed high reactivity, enough to be feasible for a suggested CLC system. However, copper oxide particles agglomerated and may not be suitable as an oxygen carrier. Samples of the iron oxide with aluminium oxide showed signs of agglomeration. Nickel oxide showed the highest reduction rate, but displayed limited strength. The reactivity indicates a needed bed mass in the fuel reactor of about 80-330 kg/MW_th and a needed recirculation flow of oxygen carrier of 4-8 kg/s, MW_th.     

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。     

Oxygen carriers for chemical looping combustion of solid fuels

A thermal analyzer-differential scanning calorimeter-mass spectrometer (TG-DSC-MS) was used to study oxygen carriers (OC) for their potential use for the application of chemical looping combustion (CLC) to solid fuels. Reaction rates, changes in reaction rates with repeated oxidation-reductions, exothermic heats during oxidation, and the effect of changing reduction gas compositions were studied. Oxidation rates were greater than reduction rates and reaction rates were reproducible through multiple oxidation-reduction cycles except where agglomeration occurred with powders. Iron oxide (Fe₂O₃ powder) and iron-based catalysts were found suitable for CLC of solid fuels having rapid reduction rates which increased with higher reducing gas concentrations. Fe₂O₃ powder was used to oxidize a high carbon coal char in an inert gas removing 88% of the carbon from the char. Other properties such as cost and durability indicated iron oxide OCs potential use for CLC of solid fuels.     

化学链燃烧中载氧体磨损和产生积碳的机制及抑制方法

磨损和积碳的产生都严重干扰化学链燃烧(CLC)的正常运行,磨损可造成载氧体的损耗,积碳可导致载氧体与燃料不能充分接触。载氧体的循环燃烧效率与CLC技术成本密切相关,是其进入商业化的关键。本工作针对载氧体的磨损和积碳抑制这两大主要问题进行了分析总结。阐述了CLC过程发生的详细反应及形成炭黑型积碳前驱物多环芳烃机制,总结了应对的可行方法,其中联合水蒸气和CO2喷射具有很好的前景。并在此基础上讨论了该领域的潜在研究需求。     

化学链燃烧中H2S对NiFe2O4氧载体活性的影响机理

采用密度泛函理论方法研究H₂S与NiFe₂O₄（001）完整表面和氧缺陷表面的相互作用机理。结果表明,H₂S在NiFe₂O₄氧载体表面Ni原子位的吸附能比其在Fe原子位的吸附能大。氧缺陷的形成会使H₂S在氧载体表面金属原子位的吸附能增大,并且Ni原子位吸附H₂S的吸附能增加更为明显。因而,NiFe₂O₄氧载体表面的Ni原子位是H₂S的主要吸附位。同时采用热力学方法进一步研究含H₂S的合成气与NiFe₂O₄氧载体之间的反应,发现H₂S与氧载体的反应产物与氧载体的还原程度密切相关。由于铁氧化物的深度还原过程受到热力学限制,H₂S与NiFe₂O₄氧载体反应的主要产物为Ni₃S₂。密度泛函理论方法与热力学方法研究结果均表明H₂S倾向于与NiFe₂O₄氧载体中Ni发生相互作用,这将对NiFe₂O₄氧载体的反应性能产生不利影响。     

温度对载氧体还原过程中汞的析出特性及形态分布的影响

化学链燃烧是近年来提出的一种具有高效、内分离CO₂特点的新型燃烧方式。本文在立式管式炉实验装置上研究了温度对基于Fe₂O₃载氧体的煤化学链燃烧载氧体还原过程中汞析出特性的影响,探讨了不同燃烧温度下燃料反应器（FR）出口烟气组分的变化及其对汞迁移变化的影响。结果表明：在高温条件下（≥ 800℃）,煤中的汞在载氧体还原过程中基本全部析出,180s时基本达到90%,并且随着温度升高而增加;FR出口烟气中的汞主要以单质态（Hg⁰）形式存在,各工况下的单质态汞占烟气中气态总汞比例都在88%以上,随着温度的升高,烟气中Hg⁰/Hg^T略有降低;温度对烟气组分具有影响,随着温度的升高,CO、NO和SO₂浓度上升;对于汞而言,SO₂会抑制Cl及Cl₂的形成从而抑制Hg⁰向Hg²⁺转化,NO会直接或间接促进汞的氧化过程,FR烟气中以CO为主的还原性气氛不利于汞的氧化。     

Syngas production from chemical looping reforming of ethanol over iron-based oxygen carriers:Theoretical analysis and experimental investigation

Chemical looping reforming (CLR) is a recent trend for syngas production,which has several merits compared to the conventional manner.One of the most important issues for CLR is to find low-cost mate-rial as oxygen carriers,so iron is a promising candidate.This paper contributes to testing the thermody-namic ability of iron-based oxygen carrier for chemical looping reforming of ethanol (CLRE).Iron thermodynamically investigated in temperature 100-1300 ℃ and excess oxygen number (Φ) 0-4.It was found that the temperature and Φ have an apparent effect on the gaseous composition produced from the process.Increases in temperature within the range of 100-1300 ℃ enhanced syngas generated and reduced coke formation and CH4.Whereas,increased Φ,particularly at higher temperatures,had also enhanced syngas production as well as reduced coke formation.However,increasing Φ for values beyond one had decreased syngas and not significantly reduced coke deposition.Moreover,an experimental investigation was carried out in a fixed bed reactor for more in-depth verification of iron ability as an oxy-gen carrier through using magnetite ore (mainly Fe3O4).It found that the effect of temperature on syngas production was consistent with that calculated thermodynamically,as syngas increased with raising the temperature through the CLRE.     

Simulation of a calcium looping CO2 capture process for pressurized fluidized bed combustion

The Canadian regulations on carbon dioxide emissions from power plants aim to lower the emissions from coal-fired units down to those of natural gas combined cycle (NGCC) units. Since coal is significantly more carbon intensive than natural gas, coal-fired plants must operate at higher net efficiencies and implement carbon capture to meet the new regulations. Calcium looping (CaL) is a promising post-combustion carbon capture (PCC) technology that, unlike other capture processes, generates additional power. By capturing carbon dioxide at elevated temperatures, the energy penalty that carbon capture technologies inherently impose on power plant efficiencies is significantly reduced. In this work, the CO₂ capture performance of a calcium-based sorbent is determined via thermogravimetric analysis under relatively high carbonation and low calcination temperatures. The results are used in an aspenONE™ simulation of a CaL process applied to a pressurized fluidized bed combustion (PFBC) system at thermodynamic equilibrium. The combustion of both natural gas and coal are considered for sorbent calcination in the CaL process. A sensitivity analysis on several process parameters, including sorbent feed rate and carbonator operating pressure, is undertaken. The energy penalty associated with the capture process ranges from 6.8–11.8 percentage points depending on fuel selection and operating conditions. The use of natural gas results in lower energy penalties and solids circulation rates, while operating the carbonator at 202 kPa(a) results in the lowest penalties and drops the solids circulations rates to below 1000 kg/s.     

Auto-thermal chemical looping combustion of sulphur with Fe2O3 oxygen carriers for sulphuric acid production: Thermodynamics

Chemical looping is a novel fuel conversion and material separation technology. It can be applied to obtain sulphur through selective oxidation of H₂S. Further, chemical looping combustion (CLC) of sulphur can generate SO₂ with a high concentration without NO_x formation. The high SO₂ concentration is adjustable and facilitates large-scale H₂SO₄ production. In this study, we examined the thermodynamics of the CLC of sulphur for H₂SO₄ production, which has not been reported previously. We analyzed the effects of reactor temperature and sulphur to Fe₂O₃ oxygen carrier (OC) ratios on sulphur allotrope transformations and on the distributions of reaction products. Moreover, the reactors were operated auto-thermally. Based on this design, we examined the effects of fuel reactor (FR) and air reactor temperatures on the minimum recirculation of the OC, as well as the gas and solid products and heat released from the air reactor. Our results showed that the CLC of sulphur with Fe₂O₃ OC could occur through an auto-thermal process. The FR in a sulphur CLC system should be operated over a temperature range of 800–950°C, with an Fe₂O₃ OC recirculation between 45 and 143 kg/kg_S(s). Furthermore, when the FR was operated in the auto-thermal mode, we achieved 100% SO₂ conversion. The findings of this study may be applied to reactor design for large-scale H₂SO₄ production through CLC of sulphur.     

Kinetics of perovskite-like oxygen carriers for chemical looping air separation

Chemical looping air separation gives an energy-efficient choice for oxygen production. We performed kinetic analysis of YBaCo₄O_7+δ, Y_0.95Ti_0.05BaCo₄O_7+δ, Y_0.2Ti_0.05Dy_0.75BaCo₄O_7+δ, and Y_0.15Zr_0.1Dy_0.75BaCo₄O_7+δ oxygen carriers in a CLAS process. TG experiments were conducted with heating rates of 0.5, 1, and 2 °C/min in a thermogravimetric analyzer. Further exploration is required to develop an appropriate oxygen carrier. So, we used the model-free approach, Starink method, to evaluate the apparent activation energy. And, masterplots method was applied to determine the most probable mechanism function. The results show that the distributed activation energies of oxidation/reduction process are 189.42/286.22 kJ/mol, 197.70/324.87 kJ/mol, 195.41/310.4 kJ/mol, and 192.20/293.53 kJ/mol for YBaCo₄O_7+δ, Y_0.95Ti_0.05BaCo₄O_7+δ, Y_0.2Ti_0.05Dy_0.75BaCo₄O_7+δ, and Y_0.15Zr_0.1Dy_0.75BaCo₄O_7+δ oxygen carriers, respectively. Random nucleation and nuclei growth A model is the most suitable for oxidation process. The A model and D are the most suitable for the reduction process. Regarding YBaCo₄O_7+δ, Y_0.95Ti_0.05BaCo₄O_7+δ, Y_0.2Ti_0.05Dy_0.75BaCo₄O_7+δ, and Y_0.15Zr_0.1Dy_0.75BaCo₄O_7+δ kinetic, oxygen transfer materials are rate-determined by nucleation and nuclei growth. For reduction kinetic, the gas diffusion stage could also become a dominant step.     

A comparative simulation study of power generation plants involving chemical looping combustion systems

This work presents a simulation study on both energy and economics of power generation plants with inherent CO₂ capture based on chemical looping combustion technologies. Combustion systems considered include a conventional chemical looping system and two extended three-reactor alternatives (exCLC and CLC3) for simultaneous hydrogen production. The power generation cycles include a combined cycle with steam injected gas turbines, a humid air turbine cycle and a simple steam cycle. Two oxygen carriers are considered in our study, iron and nickel. We further analyze the effect of the pressure reaction and the turbine inlet temperature on the plant efficiency. Results show that plant efficiencies as high as 54% are achieved by the chemical looping based systems with competitive costs. That value is well above the efficiency of 46% obtained by a conventional natural gas combined cycle system under the same conditions and simulation assumptions.     

Integration of a Ca looping system for CO2 capture in existing power plants

This work analyses a Ca looping system that uses CaO as regenerable sorbent to capture CO₂ from the flue gases generated in power plants. The CO₂ is captured by CaO in a CFB carbonator while coal oxycombustion provides the energy required to regenerate the sorbent. Part of the energy introduced into the calciner can be transferred to a new supercritical steam cycle to generate additional power. Several case studies have been integrated with this steam cycle. Efficiency penalties, mainly associated with the energy consumption of the ASU, CO₂ compressor and auxiliaries, can be as low as 7.5% p. of net efficiency when working with low‐CaCO₃ make‐up flows and integrating the Ca looping with a cement plant that makes use of the spent sorbent. The penalties increase to 8.3% p. when this possibility is not available. Operation conditions aiming at minimum calciner size result in slightly higher‐efficiency penalties. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Migration of sulfur in in-situ gasification chemical looping combustion of Beisu coal with iron-and copper-based oxygen carriers

Chemical looping combustion (CLC) is an energy conversion technology with high efficiency and inherent separation of CO2.The existence of sulfur in coal may affect the CO2 purity and the performance of oxygen carrier due to the interactions between sulfur contaminants and oxygen carrier.The migration of sulfur in Beisu coal during the in-situ gasification chemical looping combustion (iG-CLC) process using two oxygen carriers (iron ore and CuO/SiO2) was investigated respectively.The thermodynamic analysis results showed the formation of metal sulfides was thermodynamically favored at low temperatures and low oxygen excess coefficients,while they were obviously inhibited and the production of SO2 was signifi-cantly promoted with an increase in temperature and oxygen excess coefficient.Moreover,part of sulfur was captured and fixed in the forms of alkali/alkaline earth metal sulfate due to the high amount of alkali/alkaline earth metal oxides in the coal ash or/and oxygen carrier.The experimental results showed that the sulfur in coal mainly released in the form of SO2,and the sulfur conversion efficiency (Xs) in the reduction stage were 51.04％ and 48.24％ when using iron ore and CuO/SiO2 respectively.The existence of metal sulfides was observed in the reduced oxygen carriers.The values of Xs in the reoxidation process reached 3.80％ and 7.64％ when using iron ore and CuO/SiO2 respectively.The residue and accumulation of sulfur were also found on the surfaces of two oxygen carriers.     

Layered Mg-Al spinel supported Ce-Fe-Zr-O oxygen carriers for chemical looping reforming

A series of layered Mg-Al spinel supported Ce-Fe-Zr-O oxygen carriers were prepared for co-production of syngas and pure hydrogen via chemical looping steam reforming (CLSR). The presence of magnesium-aluminum layered double oxides (MgAl-LDO) significantly increases the specific surface area of the mixed oxides, reduces the particle size of CeO₂-based solid solution and promotes the dispersion of free Fe₂O₃. When reacting with methane, MgAl-LDO supported oxygen carrier shows much lower temperature for methane oxidation than the pure CeFe-Zr-O sample, indicating enhanced low-temperature reactivity. Among different Ce-Fe-Zr-O(x)/MgAl-LDO samples, the Ce-Fe-Zr-O(40 wt%)/MgAl-LDO sample shows the best performance for the selective oxidation of methane to syngas and the H₂ production by water splitting. After a long period of high temperature redox experiment, the Ce-Fe-Zr-O(40 wt%)/MgAl-LDO oxygen carrier still shows high activity for syngas generation. The comparison on the morphology of the fresh and cycled oxygen carriers indicates that the Mg-Al spinel support still forms a stable skeleton structure with high dispersion of active components on the surface after the long-term cycling, which contributes to excellent redox stability of the Ce-Fe-Zr-O(40 wt%)/MgAl-LDO oxygen carrier.     

Layered Mg-Al spinel supported Ce-Fe-Zr-O oxygen carriers for chemical looping reforming

A series of layered Mg-Al spinel supported Ce-Fe-Zr-O oxygen carriers were prepared for co-production of syngas and pure hydrogen via chemical looping steam reforming (CLSR). The presence of magnesium-aluminum layered double oxides (MgAl-LDO) significantly increases the specific surface area of the mixed oxides, reduces the particle size of CeO₂-based solid solution and promotes the dispersion of free Fe₂O₃. When reacting with methane, MgAl-LDO supported oxygen carrier shows much lower temperature for methane oxidation than the pure Ce-Fe-Zr-O sample, indicating enhanced low-temperature reactivity. Among different Ce-Fe-Zr-O(x)/MgAl-LDO samples, the Ce-Fe-Zr-O(40 wt%)/MgAl-LDO sample shows the best performance for the selective oxidation of methane to syngas and the H₂ production by water splitting. After a long period of high temperature redox experiment, the Ce-Fe-Zr-O(40 wt%)/MgAl-LDO oxygen carrier still shows high activity for syngas generation. The comparison on the morphology of the fresh and cycled oxygen carriers indicates that the Mg-Al spinel support still forms a stable skeleton structure with high dispersion of active components on the surface after the long-term cycling, which contributes to excellent redox stability of the Ce-Fe-Zr-O(40 wt%)/MgAl-LDO oxygen carrier.     

CO2气氛下准东煤化学链燃烧特性研究

我国准东煤储量丰富,钠含量高。以高钠准东煤为燃料,CO₂为气化介质,铁矿石为载氧体,基于鼓泡流化床反应器开展准东煤化学链燃烧特性的实验研究,考察了煤粒径、温度、流化风速和煤焦粒径对煤及煤焦化学链燃烧过程中可燃气体逃逸规律的影响;同时研究了煤中矿物质对煤焦气化过程的影响。结果表明,在基于鼓泡流化床实施的煤化学链燃烧过程中,由于煤颗粒和载氧体床料流化特性差异大,存在离析现象;离析影响煤化学链燃烧过程中挥发分和焦炭的转化;较高流化风速可显著增强载氧体与煤/焦炭颗粒的混合,有效改善离析对可燃气体转化的影响,降低可燃气体逃逸,并加快焦炭气化速率;煤焦中的矿物质能够维持煤焦较快的气化速率。     

MnFeO3和MnFe2O4氧载体在稻草化学链气化中的应用

通过溶胶凝胶燃烧法合成了MnFeO₃和MnFe₂O₄两种锰铁复合氧载体。通过原位红外实验探究其与稻草的化学链气化过程,发现其加速了稻草热解产物的析出,并通过气化反应促进CO和CO₂的产生,提高了碳转化率。固定床实验结果表明MnFeO₃和MnFe₂O₄在与水蒸气耦合气化的条件下大幅提高了合成气中H₂和CO的产率,气化效率分别达到94.49%和92.76%。并通过XRD分析,发现MnFeO₃和MnFe₂O₄在气化过程主要还原为(Fe,Mn)O,且在氧化反应后能回到初始晶相。在固定床的10次循环实验以及SEM的结果表明,MnFeO₃在循环反应中逐渐形成的颗粒状多孔结构有利于维持稳定的气化效率,而MnFe₂O₄由于团聚和烧结作用形成了块状结构,气化效率呈缓慢下降趋势。因此,认为MnFeO₃在生物质化学链气化中具有更好的适用性。     

Fe‐substituted Ba‐hexaaluminates oxygen carrier for carbon dioxide capture by chemical looping combustion of methane

Fe‐substituted Ba‐hexaaluninates (BFA‐x (x = 1–3), x indicates Fe content) oxygen carrier (OC) were found to exhibit excellent sintering‐resistance under cyclic redox atmosphere at 800°C thanks to the reservations of the structure during the CH₄ reduction step, thus preventing the agglomeration of particles during the subsequent reoxidation step. Lattice oxygen highly active for the total combustion of CH₄ was observed in the hexaaluminate structure and its chemical state was influenced by Fe content. The highest amount of active O coordinated with Fe³⁺ in the mirror plane (O‐Fe³⁺(M)) for the total combustion was reacted (0.77 mmol/g) for BaFe₃Al₉O₁₉ hexaaluminate OC. As a result, it exhibited the best reactivity with the CH₄ conversion of 83% and CO₂ selectivity of 100%. Moreover, superior regeneration and recyclability was also obtained, which originated from the fully recovery of O‐Fe³⁺(M) in the hexaaluminate structure. © 2015 American Institute of Chemical Engineers AIChE J, 62: 792–801, 2016