Chemical looping catalytic gasification of biomass over active LaNixFe1-xO3 perovskites as functional oxygen carriers

Oxygen carriers(OCs) with perovskite structure are attracting increasing interests due to their redox tunability by introducing various dopants in the structure. In this study, LaNi_xFe_(1-x)O_3(x = 0, 0.1, 0.3, 0.5, 0.7,1.0) perovskite OCs have been prepared by a citric acid–nitrate sol–gel method, characterized by means of X-ray diffraction(XRD) analysis and tested for algae chemical looping gasification in a fixed bed reactor. The effects of perovskite types, OC/biomass mass ratio(O/B), gasification temperature and water injection rate on the gasification performance were investigated. Lower Ni-doped(0≤x≤0.5) perovskites crystalized in the rhombohedra system which was isostructural with LaNiO_3, while those with composition 0.5≤x≤1 crystalized in the orthorhombic system. Despite the high reactivity for LaNiO_3, LaNi_(0.5) Fe_(0.5) O_3(LN5 F5) was found to be more stable at a high temperature and give almost as good results as LaNiO_3 in the formation of syngas. The relatively higher syngas yield of 0.833 m~3·kg~(-1) biomass was obtained under the O/B of 0.4, water injection rate of 0.3 ml·min~(-1) and gasification temperature at 850 °C. Continuous high yield of syngas was achieved during the first 5 redox cycles, while a slight decrease in the reactivity for LN5 F5 after 5 cycles was observed due to the adhesion of small grains occurring on the surface of OCs. However, an obvious improvement in the gasification performance was attained for LN5 F5 compared to raw biomass direct gasification, indicating that LN5 F5 is a promising functional OC for chemical looping catalytic gasification of biomass.     

Methane dry reformer by application of chemical looping combustion via Mn-based oxygen carrier for heat supplying and carbon dioxide providing

In this study, the production of H₂ utilizing chemical looping combustion (CLC) in a methane dry reformer assisted by H₂ perm-selective membranes in a CLC-DRM configuration has been investigated. CLC via employment of a Mn-based oxygen carrier generates large amounts of heat in addition to providing CO₂ as the raw material for the dry reforming (DR) reaction. The main advantage of the CLC-DRM configuration is the simultaneous capturing and consuming of CO₂ as a greenhouse gas for H₂ production.A steady state one dimensional heterogeneous catalytic reaction model is applied to analyze the performance and applicability of the proposed CLC-DRM configuration. Simulation results show that CH₄ is completely consumed in the fuel reactor (FR) of the CLC-DRM and pure CO₂ is captured by condensation of H₂O. Also, CH₄ conversion and H₂ yield reach 73.46% and 1.459 respectively at the outlet of the DR side in the CLC-DRM. Additionally, 4562 kmol h⁻¹ H₂ is produced in the DR side of the CLC-DRM.Finally, results indicate that by increasing the FR feed temperature up to 880 K, CH₄ conversion and H₂ production are enhanced to 81.15% and 4790 kmol h⁻¹ respectively.     

钙基复合载氧体的制备及反应性能

采用湿式混合成粒法得到了一种复合型钙基载氧体,并分别在综合热分析仪和流化床上考察了其反应活性和循环反应性能。结果如下:浸渍Ni离子能够明显降低载氧体与煤反应的起始温度,加快反应速率,缩短反应时间。增加Ni离子浸渍量对反应速率的影响不明显,但反应时间略有缩短。选择CaAlNi10载氧体进行了10次还原-氧化循环实验,固体产物和气体产物分析表明NiO在循环过程中对S的释放有一定控制作用;CaAlNi载氧体具有较高的再生率和良好的持续循环反应能力。结果表明,制备的CaAlNi载氧体适用于工业生产。     

Techno‐Economic Comparison of a 7‐MWth Biomass Chemical Looping Gasification Unit with Conventional Systems

The chemical looping process is an alternative method to provide conventional gasification (CG) systems with the required oxygen. The syngas produced via chemical looping has a higher calorific value than that generated by a conventional process with air. For comparison, a conventional gasification unit with pure oxygen (CGPO) and a chemical looping gasification (CLG) system were simulated with Aspen Plus. The CGPO reactor consisted of a bubbling fluidized bed and sand as bed material with oxygen supplied via a pressure swing adsorption unit. The CLG comprised a bubbling fluidized‐bed gasifier working in parallel with a fast fluidized‐bed oxidizer. The total capital investment (TCI) of the CLG unit was higher than that of the CGPO unit but the annual operating cost of the former was less which repays the difference in TCI in less than six years.     

Effect of Gasifying Medium on the Coal Chemical Looping Gasification with CaSO4 as Oxygen Carrier☆

The chemical looping gasification uses an oxygen carrier for solid fuel gasification by supplying insufficient lattice oxygen. The effect of gasifying medium on the coal chemical looping gasification w...     

Feasibility of a Co Oxygen Carrier for Chemical Looping Air Separation: Thermodynamics and Kinetics

Chemical looping air separation (CLAS) is based on the chemical looping principle: oxygen carriers release oxygen to carrier gas in a reduction reactor and absorb oxygen from air in an oxidation reactor. High oxygen transport capacity, high reactivity in reduction and oxidation reactions, and resistance to attrition and agglomeration are some of the criteria that feasible oxygen carrier materials should fulfill. Thermodynamic analysis was applied to prove the potential of Co₃O₄ as oxygen carrier. ZrO₂ served as binder to improve the anti‐sintering property and reactivity. Kinetic experiments were performed to determine the reaction rate and conversion of the oxygen carrier. Stability and durability of the oxygen carrier were characterized before and after cyclic experiments. The Co/Zr oxygen carrier proved to be a suitable candidate for the CLAS process.     

Hydrogen production through chemical looping using NiO/NiAl2O4 as oxygen carrier

A new autothermal route to produce hydrogen from natural gas via chemical looping technology was investigated. Tests were conducted in a micro-fixed bed reactor loaded with 200 mg of NiO/NiAl₂O₄ as oxygen carrier. Methane reacts with a nickel oxide in the absence of molecular oxygen at 800 °C for a period of time as high as 10 min. The NiO is subsequently contacted with a synthetic air stream (21% O₂ in argon) to reconstitute the surface and combust carbon deposited on the surface. Methane conversion nears completion but to minimize combustion of the hydrogen produced, the oxidation state of the carrier was maintained below 30% (where 100% represents a fully oxidized surface). Co-feeding water together with methane resulted in stable hydrogen production. Although the carbon deposition increased with time during the reduction cycle, the production rate of hydrogen remained virtually constant. A new concept is also presented where hydrogen is obtained from methane with inherent CO₂ capture in an energy neutral 3-reactors CFB process. This process combines a methane combustion step where oxygen is provided via an oxygen carrier, a steam methane reforming step catalyzed by the reduced oxygen carrier and an oxidizing step where the O-carrier is reconstituted to its original state.     

Experimental investigation of some metal oxides for chemical looping combustion in a fluidized bed reactor

Chemical looping combustion (CLC) is the process in which metal oxides, rather than air or pure oxygen, supply the oxygen required for combustion. In this process, different gaseous fuels can be burnt with the inherent separation of CO₂. The feasibility of the CLC system depends greatly on the selection of appropriate metal oxides as oxygen carriers (OC). In this study, NiO-NiAl₂O₄, Cu_0.95Fe_1.05AlO₄, and CuO-Cu_0.95Fe_1.05AlO₄ were tested experimentally in a fluidized bed reactor as a function of oxidation-reduction cycles, temperature, bed inventory and superficial gas velocity. The results showed that flue gases with a CO₂ concentration as high as 97% can be obtained. The flue gases should be suitable for transport and storage after clean-up and purification. With an increase in the bed inventory or a decrease in superficial gas velocity, the flue gas characteristics improved i.e. more CO₂ and fewer secondary components or less unreacted fuel were obtained. Carbon formation could occur during the reduction phase but it decreased with an increase in temperature and inventory and could be completely avoided by mixing steam with the fuel. The reactivity of NiO/NiAl₂O₄ was higher than the Cu- and Fe-based oxygen carriers. Increasing the CuO fraction in the oxygen carrier led to defluidization of the bed during the reduction and oxidation phases.     

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.     

Evaluation of different oxygen carriers for biomass tar reforming (I): Carbon deposition in experiments with toluene

In this work, an innovative method for gas conditioning in biomass gasification is analyzed. The objective is to remove tar by selectively reforming the unwanted hydrocarbons in the product gas with a chemical looping reformer (CLR), while minimizing the carbon formation during the process. Toluene, in a concentration of 600-2000 ppmv, was chosen as a tar model compound. Experiments were performed in a TGA apparatus and a fixed bed reactor. Four oxygen carriers (60% NiO/MgAl₂O₄ (Ni60), 40% NiO/NiAl₂O₄ (Ni40), 40% Mn₃O₄/Mg-ZrO₂ (Mn40) and FeTiO₃ (Fe)) were tested under alternating reducing/oxidizing cycles. Several variables affecting the reducing cycle were analyzed: temperature, time for the reduction step and H₂O/C₇H₈ molar ratio. Ni40 and Mn40 presented interesting characteristics for CLR of biomass tar. Both showed stable reactivity to C₇H₈ after a few cycles. Ni40 showed a high tendency to carbon deposition compared to Mn40, specially at high temperatures. Carbon deposition could be controlled by decreasing the temperature and the time for the reduction step. The addition of water also reduced the amount of carbon deposited, which was completely avoided working with a H₂O/C₇H₈ molar ratio of 26.4.     

生物质化学链气化氧载体的研究进展

生物质化学链气化（chemical looping gasification, CLG）为生物质能源的利用开辟了新途径,氧载体在CLG过程中具有重要作用,其性能是影响CLG反应过程的关键因素。本文重点阐述了CLG技术中氧载体的性能评价指标、类型、制备方法及其对CLG过程中产生焦油的影响。通过对比分析目前研究成果,指出Fe基氧载体在生物质CLG过程中应用最广泛,而Ni基氧载体具有较高的活性和较大的载氧能力,且对于CLG副产物焦油具有较高的催化转化效率。未来该领域研究的重点方向是开发高活性且环境友好的氧载体,推进CLG工艺的工业应用。本文为今后生物质CLG氧载体的开发与优化提供了借鉴。     

K_2CO_3修饰钙基载氧体的煤化学链气化反应特性

以膨润土(Bentonite)为载体,采用机械混合-浸渍法制备了Ca SO4-K2CO3/Bentonite(Ca KBen)复合载氧体。在流化床反应器中,以水蒸气作为气化、流化介质,考察复合载氧体Ca KBen与不同煤种的化学链气化反应特性,并对该载氧体的循环特性、作用机理及气化动力学方程进行了研究。结果表明:复合载氧体Ca KBen适用于不同煤种的化学链气化过程。900℃时,与纯煤气化相比,Ca KBen化学链气化的碳转化率提高17.88%,反应时间缩短10 min,复合载氧体Ca KBen表现出良好的反应活性和催化性能。十次循环实验过程中,冷煤气效率稳定在85%以上,表明复合载氧体Ca KBen具有良好的循环特性。XRD、SEM分析表明十次循环实验后,钙基载氧体的晶相结构稳定,主要以CaS O4形式存在,K+主要以K2CO3形式存在,载氧体表面变得疏松多孔,该结构有利于化学链气化反应的进行。活性位扩展模型很好地体现了复合载氧体Ca KBen与煤化学链气化的动力学规律,验证了气化过程中CaS O4与K2CO3具有协同作用。     

Evaluation of different oxygen carriers for biomass tar reforming (II): Carbon deposition in experiments with methane and other gases

This work is a continuation of a previous paper by the authors [1] which analyzed the suitability of the Chemical Looping technology in biomass tar reforming. Four different oxygen carriers were tested with toluene as tar model compound: 60% NiO/MgAl₂O₄ (Ni60), 40% NiO/NiAl₂O₄ (Ni40), 40% Mn₃O₄/Mg-ZrO₂ (Mn40) and FeTiO₃ (Fe) and their tendency to carbon deposition was analyzed in the temperature range 873-1073 K. In the present paper, the reactivity of these carriers to other compounds in the gasification gas is studied, also with special emphasis on the tendency to carbon deposition. Experiments were carried out in a TGA apparatus and a fixed bed reactor. Ni-based carriers showed a tendency to form carbon in the reaction with CH₄, especially Ni60. The addition of water in H₂O/CH₄ molar ratios of 0.4-2.3 could decrease the carbon deposited, but not in the case of Ni60. Mn-based sample reacted with CH₄ almost completely and with low tendency to carbon deposition, while the Fe-based sample showed low reactivity. Ni40 showed more reactivity to CO than Mn40, although in both cases carbon was deposited, especially at 873 K. When H₂ was present, it reacted rapidly with both carriers, decreasing the amount of carbon deposited. The presence of CO₂ could also decrease the carbon deposited on Ni40 at 1073 K. According to both these and the previous results [1], it can be concluded that Mn40 is the most adequate for minimization of carbon deposition in Chemical Looping Reforming (CLR).     

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%。