化学链燃烧技术的研究现状

化学链燃烧是一种新型的无火焰燃烧技术，它把金属氧化物作为载氧体，对能源进行了更为合理和高效的利用，介绍了化学链燃烧的基本概念，主要特点，概括了载氧体的研究进展，提出了若干问题。对化学链燃烧的前景进行了展望。     

化学链燃烧技术的研究进展

化学链燃烧技术是一种高效、清洁、经济的新型无火焰燃烧技术.介绍了化学链燃烧的基本概念及特点,总结了载氧体、化学链燃烧反应器及化学链燃烧系统分析的研究进展,并指出了化学链燃烧技术仍存在且亟待解决的问题.非金属载氧体、同体燃料化学链燃烧是该技术的最新研究热点,其中固体燃料化学链燃烧是未来研究的重要趋势.     

煤化学链燃烧技术的研究进展

介绍了国内外煤化学链燃烧技术的研究和发展状况,分析了其技术特点、限制环节、改进措施和发展方向.相对于以煤气化产物为燃料的煤间接化学链燃烧技术,直接以煤为燃料的化学链燃烧技术系统简单、运行成本低,具有显著的经济优势,但是过低的煤气化速率是其中的限制环节.为了提高煤的气化速率、促进煤的充分转化,需要从氧载体和煤质的选择及活化、运行参数的优化和燃料反应器结构的改进等方面进行全面考虑.高性能氧载体是煤化学链燃烧技术的基础,深入研究煤中硫组分和灰分的演化及其对氧载体活性的影响非常必要.另外,氧解耦煤化学链燃烧技术以及基于煤化学链制氢技术也需要重视.     

3 MWth化学链燃烧装置设计方法

反应器系统是以煤为燃料的化学链燃烧系统的基础组成部分,是提供载氧体反应的场所,可将载氧体以合适的速率在不同的反应器之间传输,实现气固分离和不同性质颗粒的分离。因此,设计、研究反应器系统是实现以煤为燃料的化学链燃烧的根本前提。本文对反应器系统中的空气反应器、燃料反应器、炭分离器及整体的循环特性进行研究,总结建立了以煤为燃料的化学链燃烧反应器系统的设计方法,在此基础之上设计了3 MW_th的化学链燃烧示范装置,为以煤为燃料的化学链燃烧热态系统的建造与运行奠定了基础。     

化学链燃烧中空气反应器内反应过程的模拟

化学链燃烧具有二氧化碳内分离和低NOx等特点,研究固体燃料的化学链燃烧将有利于实现固体燃料的高效、经济、清洁利用,具有广阔的应用前景。基于颗粒动力学和化学动力学理论,运用基于最小能量原理的气固相间作用力计算方法,结合双流体模型,对化学链燃烧中空气反应器内流动反应过程进行数值模拟,获得了反应器内流场特性以及温度场分布规律。计算结果表明:考虑颗粒团聚效应的模型能够很好地捕捉到空气反应器中颗粒呈现出的非均匀环核流动结构,模拟结果同时分析了操作速度,操作温度对反应器中反应过程的影响,氧气转换率随着操作速度的增加而降低,随着操作温度的增加,反应速率随之增大,为耦合反应器的设计优化提供了一定的依据。     

基于新型循环载氧体的天然气化学链燃烧热力学研究

以CH4为燃料对基于CaSO4载氧体的化学链燃烧的热力学性能进行了分析研究,计算了CaSO4在CH4氛围中的还原-氧化的热力学参数与温度的关系,分析结果显示,在一定的温度范围内,以CaSO4为载氧体实现化学链燃烧具有可行性,是一种理想的载氧体。基于Gibbs能最小化方法建立了化学链燃烧技术模型,模拟了温度,CH4与CaSO4摩尔比对燃料反应器和空气反应器的影响。结果表明,燃料反应器最佳反应温度850℃～900℃,空气反应器最佳反应温度为1000℃～1050℃,CH4与CaSO4的摩尔比最佳摩尔比为1。研究结果对燃煤化学链燃烧具有参考价值。     

燃煤化学链燃烧技术的研究进展

燃煤化学链燃烧技术在实现煤炭高效利用的同时有效降低了CO_2的捕集能耗,是当前具有发展前景的第二代碳捕集技术。现有的研究主要从宏观层面评价燃煤化学链燃烧系统的反应性能,而从"结构与反应性"的角度进行综合评价的研究很少。围绕煤、载氧体和反应器三大系统核心,分析了煤种及其附属产物(煤灰、硫和氮)对燃煤化学链燃烧系统反应性能的影响;基于现有的载氧体开发类型及提高煤气化速率的载氧体改性方法,提出廉价、高效载氧体的规模化制备是未来载氧体研发的重点和难点;探讨了各种类型反应器的结构对煤、载氧体颗粒混合和反应的影响,指出反应器开发过程中存在的主要问题及未来的发展方向。     

燃料反应器中甲烷化学链燃烧的数值模拟

为了更好地了解化学链燃烧过程中气固流动的特点以及反应特性,基于化学动力学理论和颗粒动理学理论,考虑高颗粒浓度下摩擦应力的影响,运用双流体模型,对燃料反应器内化学链燃烧过程进行数值模拟,得到了燃料反应器内流场特性以及温度场分布规律。模拟结果同时得到了反应器内颗粒所形成的内循环流动结构。     

Fe2O3为载氧体的煤/秸秆化学链燃烧循环特性研究

对煤、秸秆与Fe2O3以不同质量掺混比混合后化学链燃烧过程中载氧体还原/再生的多循环反应特性进行了研究,重点分析了固体燃料带入的灰分对化学链反应速率的影响以及秸秆的掺入对化学链反应的改善.结果表明：载氧体Fe2O3质量掺混比的增大有利于化学链反应的进行,燃烧起始反应温度降低;Fe2O3作为载氧体受灰分积累的影响较大,其可持续循环能力较差;煤中掺入秸秆改善了煤的化学链燃烧特性,提高了燃烧反应速率和载氧体的再生反应速率.     

零排放天然气化学链置换燃烧仿真研究

以天然气为燃料,金属氧化物为载氧体,实现化学链置换燃烧(Chemical Looping Combustion-CLC)。“燃烧”气相产物H2O(汽)+CO2,冷凝水后,可分离出CO2。结合燃气蒸汽联合循环技术,实现能量的梯级利用,构成新型化学链置换燃烧联合循环,高效发电同时分离CO2。建立了化学链置换燃烧空气反应器(AR)和燃料反应器(FR)的质量平衡和能量平衡数学模型,对燃烧特性进行仿真计算。研究结果表明:载氧体氧化比率和还原比率增大,FR的出力及所需载氧体的最小量增加,使AR空气量减小;加大循环倍率或升高AR出口预设温度均使FR出口温度升高,AR空气量将更减少。这部分计算可为化学链置换燃烧技术的实验研究和系统概念设计提供基础数据。     

Hydrogen production with CO2 capture by coupling steam reforming of methane and chemical-looping combustion: Use of an iron-based waste product as oxygen carrier burning a PSA tail gas

In this work it is analyzed the performance of an iron waste material as oxygen carrier for a chemical-looping combustion (CLC) system. CLC is a novel combustion technology with the benefit of inherent CO₂ separation that can be used as a source of energy for the methane steam reforming process (SR). The tail gas from the PSA unit is used as fuel in the CLC system.The oxygen carrier behaviour with respect to gas combustion was evaluated in a continuous 500 W_th CLC prototype using a simulated PSA off-gas stream as fuel. Methane or syngas as fuel were also studied for comparison purposes. The oxygen carrier showed enough high oxygen transport capacity and reactivity to fully convert syngas at 880 °C. However, lower conversion of the fuel was observed with methane containing fuels. An estimated solids inventory of 1600 kg MW_th⁻¹ would be necessary to fully convert the PSA off-gas to CO₂ and H₂O. An important positive effect of the oxygen carrier-to-fuel ratio up to 1.5 and the reactor temperature on the combustion efficiency was found.A characterization of the calcined and after-used particles was carried out showing that this iron-based material can be used as oxygen carrier in a CLC plant since particles maintain their properties (reactivity, no agglomeration, high durability, etc.) after more than 111 h of continuous operation.     

Comparison of carbon capture IGCC with pre-combustion decarbonisation and with chemical-looping combustion

Carbon capture from conventional power cycles is accompanied by a significant loss of efficiency. One process concept with a potential for better performance is chemical-looping combustion (CLC). CLC uses a metal oxide to oxidize the fuel, and the reduced metal is then re-oxidized in a second reactor with air. The combustion products CO₂ and water remain unmixed with nitrogen, thereby avoiding the need for energy intensive air separation. In this paper, the performance of various configurations of CLC used in integrated gasification combined cycle power plants (CLC-IGCC) are analyzed and compared to a conventional IGCC design with pre-combustion carbon capture by physical absorption. The analysis is based on process simulation using Aspen Plus and GateCycle. Key design parameters are varied, and the results are interpreted using exergy analysis. The CLC-IGCC offers the advantages of higher plant efficiency and more complete carbon capture. The efficiency is very sensitive to changes in the gas turbine inlet temperature for both the CLC and the conventional IGCC designs. The development of oxygen carrier particles with a high thermal stability is therefore crucial for capitalizing on the potential efficiency advantage of CLC.     

Comparison of CuO and NiO as oxygen carrier in chemical looping combustion of a Victorian brown coal

Chemical looping combustion (CLC) is a novel process where an oxygen carrier, preferably oxides of metal, is used to transfer oxygen from the combustion air to the fuel. The outlet gas from the process reactor consists of CO₂ and H₂O, and concentrated stream of CO₂ is obtained for sequestration when water vapour is condensed. Chemical looping has been widely studied for combustion of natural gas; however its application to solid fuels, such as coal, is being studied relatively recently; no work has been done using Victorian brown coal which represents a very large resource, over 500 years at current consumption rate. In this study we carried out an experimental investigation pertaining to CLC of a Victorian brown coal from Loy Yang mine using NiO and CuO as oxygen carrier. The experiments were conducted using a thermogravimetric analyser (TGA) under CO₂ gasification environment with NiO and CuO. The reduction and re-oxidation of NiO in five repeated cycle operations were performed at 950 °C. However, the same cyclic operation for CuO was performed at 800 °C, as it was observed that at 950 °C CuO could not be re-oxidized to its original state due to sintering, which significantly altered the morphology. The extent of coal combustion and re-oxidation of metal oxides resulted in a 4.4-7.5% weight loss of NiO per cycle. No such weight loss was observed in case of CuO at 800 °C. The high reactivity of CuO was observed as compared to NiO during cyclic operation. The percentage of combustion at the end of the 5th cycle with CuO was 96% as compared to 67% with NiO. Fresh oxide particles and solid residues are characterized using SEM to understand surface morphological changes due to combustion. The energy dispersive X-rays (EDX) helped to get surface elemental information, albeit qualitative, of fresh and used metal oxide particles. The current study, for the first time, has generated practical information on the temperature range, approximate time, and percent combustion that can be achieved while using NiO and CuO as oxygen carriers during CLC with Loy Yang brown coal. Based on these results the ongoing work includes long duration experiments with Loy Yang and other Victorian brown coals.     

Reduction Kinetics of Fe-based Oxygen Carriers Using Syngas in a Honeycomb Fixed-Bed Reactor for Chemical-Looping Combustion

Chemical-looping combustion(CLC)is considered to be a vital method for utilizing hydrocarbon fuel with low carbon emissions.A honeycomb fixed-bed reactor is a new kind of reactor for CLC.However,the further application of the reactor is limited by the inadequacy of the kinetic equations for CLC.In this paper,the experimental studies on the kinetic of Fe-based oxygen carriers were carried out by the CLC experiments using syngas which was obtained from one typical type of coal gasification products.The experimental results show that there were two individual stages for the kinetic characteristics during the fuel reaction process.Therefore,the CLC fuel reaction process could be described by a two-stage unreacted-core shrinking model and the reaction rate equations for each of the two phases were provided.In both stages,the dominant resistances were analyzed.The activation energy and the reaction order in both stages were calculated respectively as well.Comparing the experimental results of reaction rate with the calculated results of the obtained rate equations,it could be clearly seen that the reaction kinetics model was appropriate for the CLC in the honeycomb reactor.This work is expected to provide a guideline for the future development and industrial design of the honeycomb CLC reactors from the perspective of kinetics.     

Numerical simulation of fuel reactor for a methane-fueled chemical looping combustion using bubbling fluidized bed with internal particle circulation

Chemical-looping combustion (CLC) is recognized as a promising technique to efficiently and economically capture emitted carbon dioxide in common combustion processes. In this study, the bubbling fluidized bed (BFB) fuel reactor performance of the CLC system was examined through numerical simulation. The reduction reaction performance obtained from conventional BFB fuel reactor and BFB fuel reactor incorporated with internal particle circulation denoted as internal circulation bubbling fluidized bed reactor (ICBFB), were compared under the same fuel flow rate and operating conditions. By using CH₄ as fuel and ilmenite as the oxygen carrier, it was found the reduction reaction can be enhanced by using the ICBFB fuel reactor due to particle circulation. The particle circulation increased the mixing and contact time between fuel and oxygen carrier that produced reduction reaction enhancement. Moreover, the simulation results indicated that higher reduction reaction performance can be achieved by higher reduction reaction temperature and initial oxygen carrier volume fraction.     

Investigation of Coal Fueled Chemical Looping Combustion Using Fe_3O_4 as Oxygen Carrier：Influence of Variables

Chemical-looping combustion (CLC) is a novel combustion technique with inherent CO2 separation.Magnetite (Fe3O4) was selected as the oxygen carrier.Shenhua coal (Inner Mongolia,China),straw coke and natural coke were used as fuels for this study.Influences of operation temperatures,coal to Fe3O4 mass ratios,and different kinds of fuels on the reduction characteristics of the oxygen carrier were investigated using an atmosphere thermogravimetric analyzer (TGA).Scanning electron microscopy (SEM) was used to analyse the characteristic of the solid residues.Experimental results shown that the reaction between the coal and the oxygen carrier become strong at a temperature of higher than 800℃.As the operation temperature rises,the reduction conversion rate increases.At the temperatures of 850oС,900℃,and 950℃,the reduction conversion rates were 37.1%,46.5%,and 54.1% respectively.However,SEM images show that at the temperature of higher than 950℃,the iron oxides become melted and sintered.The possible operation temperature should be kept around 900℃.When the mass ratios of coal to Fe3O4 were 5/95,10/90,15/85,and 20/80,the reduction conversion rates were 29.5%,40.8%,46.5%,and 46.6% respectively.With the increase of coal,the conversion rate goes up.But there exist an optimal ratio around 15/85.Comparisons based on different kinds of fuels show that the solid fuel with a higher volatile and a more developed pore structure is conducive to the reduction reactivity of the oxygen carrier.     

Exergy analysis of chemical-looping combustion systems

A power plant based on chemical-looping combustion offers both a possibility of high net power efficiency and separation of the greenhouse gas CO₂. This is due to the way the oxidation of the fuel takes place. Instead of oxidizing the fuel with oxygen from the combustion air, the fuel is oxidized by an oxygen carrier, i.e., an oxygen-containing compound. The oxygen carriers that have been suggested in previous studies are metal oxides like NiO, Fe₂O₃ and Mn₃O₄. The reduced oxygen carrier is in the next step reoxidized by air in a second reactor and then recirculated to the first reactor. In this way, fuel and air are never mixed and the fuel oxidation products CO₂ and water leave the system undiluted by air. All that is needed to get an almost pure CO₂ product is to condense the water vapour and remove the liquid water.Chemical-looping combustion (CLC) is also claimed to reduce the fuel exergy destruction in the overall reaction of combustion of the fuel. This gives a possibility to increase the net power efficiency.This paper gives an introduction to chemical-looping combustion. Results from simulations and a detailed exergy analysis of two different CLC gas turbine (GT) systems are also presented. The first system utilizes methane as a fuel and NiO as oxygen carrier. The second system utilizes a fuel gas mixture consisting mainly of CO and H₂, simulating a fuel gas from for instance coal gasification. Results for this system are given for simulations with both NiO and Fe₂O₃ as oxygen carrier. The two systems are compared to comparable simulated systems with conventional combustion of the same fuel. The exergy analysis shows that the irreversibilities generated upon combustion of the fuel are reduced. The net power efficiency of the CLC–GT systems is similar or higher than for the corresponding GT systems with conventional combustion. The net power efficiency of CLC systems could be even further increased if the exergy remaining in the exhaust could be utilized in an efficient way.     

Gasification inhibition in chemical-looping combustion with solid fuels

Chemical-looping combustion (CLC) is a novel technology that can be used to meet growing demands on energy production without CO₂ emissions. The CLC process includes two reactors, an air and a fuel reactor. Between these two reactors oxygen is transported by an oxygen carrier, which most often is a metal oxide. This arrangement prevents mixing of N₂ from the air with CO₂ from the combustion giving combustion gases that consist almost entirely of CO₂ and H₂O. The technique reduces the energy penalty that normally arises from the separation of CO₂ from other flue gases, hence, CLC could make capture of CO₂ cheaper. For the application of CLC to solid fuels, the char remaining after devolatilization will react indirectly with the oxygen carrier via steam gasification. It has been suggested that H₂, and possibly CO, has an inhibiting effect on steam gasification in CLC. In this work experiments were conducted to investigate this effect. The experiments were conducted in a laboratory fluidized-bed reactor that was operating cyclically with alternating oxidation and reduction periods. Two different oxygen carriers were used as well as an inert sand bed. During the reducing period varying concentrations of CO or H₂ were used together with steam while the oxidation was conducted with 10% O₂ in N₂. The temperature was constant at 970 °C for all experiments. The results show that CO does not directly inhibit the gasification whereas the partial pressure of H₂ had a significant influence on fuel conversion. The results also suggest that dissociative hydrogen adsorption is the predominant hydrogen inhibition mechanism under the laboratory conditions, thus explaining why char conversion is much faster in a bed of oxygen carrying material, compared to an inert sand bed.     

Pressurized chemical-looping combustion of coal with an iron ore-based oxygen carrier

Chemical-looping combustion (CLC) is a new combustion technology with inherent separation of CO₂. Most of the previous investigations on CLC of solid fuels were conducted under atmospheric pressure. A pressurized CLC combined cycle (PCLC-CC) system is proposed as a promising coal combustion technology with potential higher system efficiency, higher fuel conversion, and lower cost for CO₂ sequestration. In this study pressurized CLC of coal with Companhia Valedo Rio Doce (CVRD) iron ore was investigated in a laboratory fixed bed reactor. CVRD iron ore particles were exposed alternately to reduction by 0.4 g of Chinese Xuzhou bituminous coal gasified with 87.2% steam/N₂ mixture and oxidation with 5% O₂ in N₂ at 970 °C. The operating pressure was varied between 0.1 MPa and 0.6 MPa. First, control experiments of steam coal gasification over quartz sand were performed. H₂ and CO₂ are the major components of the gasification products, and the operating pressure influences the gas composition. Higher concentrations of CO₂ and lower fractions of CO, CH₄, and H₂ during the reduction process with CVRD iron ore was achieved under higher pressures. The effects of pressure on the coal gasification rate in the presence of the oxygen carrier were different for pyrolysis and char gasification. The pressurized condition suppresses the initial coal pyrolysis process while it also enhances coal char gasification and reduction with iron ore in steam, and thus improves the overall reaction rate of CLC. The oxidation rates and variation of oxygen carrier conversion are higher at elevated pressures reflecting higher reduction level in the previous reduction period. Scanning electron microscope and energy-dispersive X-ray spectroscopy (SEM-EDX) analyses show that particles become porous after experiments but maintain structure and size after several cycles. Agglomeration was not observed in this study. An EDX analysis demonstrates that there is very little coal ash deposited on the oxygen carrier particles but no appreciable crystalline phases change as verified by X-ray diffraction (XRD) analysis. Overall, the limited pressurized CLC experiments carried out in the present work suggest that PCLC of coal is promising and further investigations are necessary.