Operation of a 10 kWth chemical-looping combustor during 200 h with a CuO–Al2O3 oxygen carrier

Chemical-looping combustion (CLC) is an attractive technology to decrease greenhouse gas emissions affecting global warming, because it is a combustion process with inherent CO₂ separation and therefore without needing extra equipment for CO₂ separation and low penalty in energy demand. The CLC concept is based on the split of a conventional combustion of gas fuel into separate reduction and oxidation reactions. The oxygen transfer from air to fuel is accomplished by means of an oxygen carrier in the form of a metal oxide circulating between two interconnected reactors. A Cu-based material (Cu14Al) prepared by impregnation of γ-Al₂O₃ as support with two different particle sizes (0.1–0.3 mm, 0.2–0.5 mm) was used as an oxygen carrier for a chemical-looping combustion of methane. A 10 kWth CLC prototype composed of two interconnected bubbling fluidized bed reactors has been designed, built in and operated at 800 °C during 100 h for each particle size. In the reduction stage full conversion of CH₄ to CO₂ and H₂O was achieved using oxygen carrier-to-fuel ratios above 1.5. Some CuO losses as the active phase of the CLC process were detected during the first 50 h of operation, mainly due to the erosion of the CuO present in external surface of the alumina particles. The high reactivity of the oxygen carrier maintained during the whole test, the low attrition rate detected after 100 h of operation, and the absence of any agglomeration problem revealed a good performance of these CuO-based materials as oxygen carriers in a CLC process.     

Syngas combustion in a chemical-looping combustion system using an impregnated Ni-based oxygen carrier

Chemical-looping combustion (CLC) has emerged as a promising option for CO₂ capture because this gas is inherently separated from the other flue gas components and thus no energy is expended for the separation. This technology would have some advantages if it could be adapted for its use with coal as fuel. In this sense, a process integrated by coal gasification and CLC could be used in power plants with low energy penalty for CO₂ capture. This work presents the results obtained in the combustion of syngas as fuel with a Ni-based oxygen carrier prepared by impregnation in a CLC plant under continuous operation. The effect on the oxygen carrier behaviour and the combustion efficiency of several operating conditions was determined in the continuous CLC plant. High combustion efficiencies (～99%), close to the values limited by thermodynamics, were reached at oxygen carrier-to-fuel ratios higher than 5. The temperature in the FR had a significant influence, although high efficiencies were obtained even at 1073 K. The syngas composition had small effect on the combustion, obtaining high and similar efficiencies with syngas fuels of different composition, even in the presence of high CO concentrations. The low reactivity of the oxygen carrier with CO seemed to indicate that the water gas shift reaction acts as an intermediate step in the global reaction of the syngas in a continuous CLC plant. Neither agglomeration nor carbon deposition problems were detected during 50 h of continuous operation in the prototype. The obtained results showed that the impregnated Ni-based oxygen carrier could be used in a CLC plant for the combustion of syngas produced in an integrated gasification combined cycle (IGCC).     

The use of iron oxide as oxygen carrier in a chemical-looping reactor

Chemical-looping combustion (CLC) is a method for the combustion of fuel gas with inherent separation of carbon dioxide. This technique involves the use of two interconnected reactors, an air reactor and a fuel reactor. The oxygen demanded in the fuel combustion is supplied by a solid oxygen carrier, which circulates between both reactors. Fuel gas and air are never mixed and pure CO₂ can be obtained from the flue gas exit. This paper presents the results from the use of an iron-based oxygen-carrier in a continuously operating laboratory CLC unit, consisting of two interconnected fluidized beds. Natural gas or syngas was used as fuel, and the thermal power was between 100 and 300 W. Tests were performed at four temperatures: 1073, 1123, 1173 and 1223 K. The prototype was successfully operated for all tests and stable conditions were maintained during the combustion. The same particles were used during 60 h of hot fluidization conditions, whereof 40 h with combustion. The combustion efficiency of syngas was high, about 99% for all experimental conditions. However, in the combustion tests with natural gas, there was unconverted methane in the exit flue gases. Higher temperature and lower fuel flows increase the combustion efficiency, which ranged between 70% and 94% at 1123 K. No signs of agglomeration or mass loss were detected, and the crushing strength of the oxygen carrier particles did not change significantly. Complementary experiments in a batch fluidized bed were made to compare the reactivity of the oxygen carrier particles before and after the 40 h of operation, but the reactivity of the particles was not affected significantly.     

The use of petroleum coke as fuel in chemical-looping combustion

Chemical-looping combustion is a novel technique used for CO₂ separation that previously has been demonstrated for gaseous fuel. This work demonstrates the feasibility of using solid fuel (petroleum coke) in chemical-looping combustion (CLC). Here, the reaction between the oxygen carrier and solid fuel occurs via the gasification intermediates, primarily CO and H₂. A laboratory fluidized-bed reactor system for solid fuel, simulating a CLC-system by exposing oxygen-carrying particles to alternating reducing and oxidizing conditions, has been developed. In each reducing period, 0.2 g of petroleum coke was added to 20 g of oxygen carrier composed of 60% active material of Fe₂O₃ and 40% inert MgAl₂O₄. The effect of steam and SO₂ concentration in the fluidizing gas was investigated as well as effect of temperature. The rate of reaction was found to be highly dependent on the steam and SO₂ concentration as well as the temperature. Also shown was that the presence of a metal oxide enhances the gasification of petroleum coke. A preliminary estimation of the oxygen carrier inventory needed in a real CLC system showed that it would be below 2000 kg/MW_th.     

Comparison of nickel- and iron-based oxygen carriers in chemical looping combustion for CO2 capture in power generation

In chemical looping combustion (CLC), a solid oxygen carrier circulates between two fluidised bed reactors and transports oxygen from the combustion air to the fuel; thus, the fuel is not mixed with air and an inherent CO₂ separation occurs. In this paper, CLC is integrated in a natural gas fired combined cycle (NGCC). In this system, nickel- and iron-based oxygen carriers are compared regarding the system's electrical and exergy efficiencies. Furthermore, the feasibility of CLC in two interconnected pressurised fluidised bed reactors (IPFBR) is studied for both oxygen carriers. The hypothetical layout plus dimensions of the IPFBR is presented for a capacity of 800 MW input of natural gas. Finally, top-firing is proposed as an option to overcome the apparent limitation in operating temperature of the reactor equipment and/or the oxygen carriers. The results indicate that there is no significant difference in the system's efficiency if both oxygen carriers could operate at the same temperature. However, CLC seems easier to be technically realised in an IPFBR with a nickel-based oxygen carrier.     

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.     

Multiphase CFD Modeling for a Chemical Looping Combustion Process (Fuel Reactor)

There are growing concerns about increasing emissions of greenhouse gases and a looming global warming crisis. CO₂ is a greenhouse gas that affects the climate of the earth. Fossil fuel consumption is the major source of anthropogenic CO₂ emissions. Chemical looping combustion (CLC) has been suggested as an energy‐efficient method for the capture of carbon dioxide from combustion. A chemical‐looping combustion system consists of a fuel reactor and an air reactor. The air reactor consists of a conventional circulating fluidized bed and the fuel reactor is a bubbling fluidized bed. The basic principle involves avoiding direct contact of air and fuel during the combustion. The oxygen is transferred by the oxygen carrier from the air to the fuel. The water in combustion products can be easily removed by condensation and pure carbon dioxide is obtained without any loss of energy for separation. With the improvement of numerical methods and more advanced hardware technology, the time required to run CFD (computational fluid dynamic) codes is decreasing. Hence, multiphase CFD‐based models for dealing with complex gas‐solid hydrodynamics and chemical reactions are becoming more accessible. To date, there are no reports in the literature concerning mathematical modeling of chemical‐looping combustion using FLUENT. In this work, the reaction kinetics models of the (CaSO₄ + H₂) fuel reactor is developed by means of the commercial code FLUENT. The effects of particle diameter, gas flow rate and bed temperature on chemical looping combustion performance are also studied. The results show that the high bed temperature, low gas flow rate and small particle size could enhance the CLC performance.     

Synthesis gas generation by chemical-looping reforming in a continuously operating laboratory reactor

Chemical-looping reforming is a technology that can be used for partial oxidation and steam reforming of hydrocarbon fuels. This paper describes continuous chemical-looping reforming of natural gas in a laboratory reactor consisting of two interconnected fluidized beds. Particles composed of 60 wt% NiO and 40 wt% MgAl₂O₄ are used as bed material, oxygen carrier and reformer catalyst. There is a continuous circulation of particles between the reactors. In the fuel reactor, the particles are reduced by the fuel, which in turn is partially oxidized to H₂, CO, CO₂ and H₂O. In the air reactor the reduced oxygen carrier is reoxidized with air. Complete conversion of natural gas was achieved and the selectivity towards H₂ and CO was high. In total, 41 h of reforming were recorded. Formation of solid carbon was noticed for some cases. Adding 25 vol% steam to the natural gas reduced or eliminated the carbon formation.     

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。     

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.     

RECENT ADVANCES IN CaSO4 OXYGEN CARRIER FOR CHEMICAL-LOOPING COMBUSTION (CLC) PROCESS

Chemical-looping combustion (CLC) is a novel combustion technology with inherent separation of the greenhouse gas CO₂ and low NOx (NO, NO₂, N₂O) emissions. In CLC, the solid oxygen carrier supplies the stoichiometric oxygen needed for CO₂ and water formation, resulting in a free nitrogen mixture. The performance of oxygen carrier is the key to CLC's application. A good oxygen carrier for CLC should readily react with the fuel (fuel reactor) and should be re-oxidized upon being contacted with oxygen (air reactor). In this case, the behavior of CaSO₄ as an oxygen carrier for a CLC process, reacting with gas fuels (e.g., CO, H₂, and CH₄) and solid fuels (e.g., coal and biomass), has been analyzed. The performance of the oxygen carrier can be improved by changing the preparation method or by making mixed oxides. Generally, Al₂O₃, SiO₂, etc., which act a porous support providing a higher surface area for reaction, are used as the inert binder to increase the reactivity, durability, and fluidizability of the oxygen carrier particles. Further, simulation analysis of a CLC process based on CaSO₄ oxygen carrier was also analyzed. Finally, some important tendencies related to CaSO₄ oxygen carrier in CLC technology are put forward.     

Low-pressure DME synthesis with Cu-based hybrid catalysts using temperature-gradient reactor

Dimethyl ether (DME), the target product of this study, has many advantages as diesel fuel. The aim of this study is to develop a catalytic process in which 90% CO conversion to DME and CO₂ from syngas (3CO+3H₂→DME+CO₂) is attained at 1–3 MPa. In such a process, both recycling loop and compression of syngas can be omitted resulting in an economic process based on unused, dispersed and small-scale carbon resources. To overcome the equilibrium conversion limit we designed temperature-gradient reactor (TGR). In TGR, the temperature of the catalyst bed decreases along with the down flow of reaction gas. The performance of the catalyst in TGR was much higher than that in a conventional isothermal fixed bed reactor. For example, 90% CO conversion and high STY (1.1 kg MeOH eqiv./kg cat./h) was attained at the same time in TGR at 550–510 K, 3 MPa.     

Multiphase CFD-based models for chemical looping combustion process: Fuel reactor modeling

Chemical looping combustion (CLC) is a flameless two-step fuel combustion that produces a pure CO₂ stream, ready for compression and sequestration. The process is composed of two interconnected fluidized bed reactors. The air reactor which is a conventional circulating fluidized bed and the fuel reactor which is a bubbling fluidized bed. The basic principle is to avoid the direct contact of air and fuel during the combustion by introducing a highly-reactive metal particle, referred to as oxygen carrier, to transport oxygen from the air to the fuel. In the process, the products from combustion are kept separated from the rest of the flue gases namely nitrogen and excess oxygen. This process eliminates the energy intensive step to separate the CO₂ from nitrogen-rich flue gas that reduce the thermal efficiency.Fundamental knowledge of multiphase reactive fluid dynamic behavior of the gas-solid flow is essential for the optimization and operation of a chemical looping combustor.Our recent thorough literature review shows that multiphase CFD-based models have not been adapted to chemical looping combustion processes in the open literature. In this study, we have developed the reaction kinetics model of the fuel reactor and implemented the kinetic model into a multiphase hydrodynamic model, MFIX, developed earlier at the National Energy Technology Laboratory. Simulated fuel reactor flows revealed high weight fraction of unburned methane fuel in the flue gas along with CO₂ and H₂O. This behavior implies high fuel loss at the exit of the reactor and indicates the necessity to increase the residence time, say by decreasing the fuel flow rate, or to recirculate the unburned methane after condensing and removing CO₂.     

The effect of carbon dioxide during the desulfurization of flue gas with Mardin–Mazıdagı phosphate rock

The effects of temperature, CO₂ concentration and particle size on simultaneous calcination/sulfation of Mardin–Maz?dag? phosphate rock in fluidized-bed reactor were investigated. For this, a raw sample was exposed to calcination and sulfation processes in a fluidized-bed reactor to determine the effects of parameters by using a model gas mixture similar to the flue gas composition. The calcination ratio increased with increasing temperature and decreasing particle size, but decreased with increasing CO₂ concentration. In sulfation process, however, sulphate conversion ratio increased with increasing CO₂ ratio and decreased with decreasing particle size. The sulfation reaction is well represented by the shrinking core model and can be divided into two regions with different rate controlling step. For low conversions, the controlling step was found to be chemical reaction at the interface, but the diffusion through the product layer for high conversion. The activation energies for the chemical reaction at the interface and diffusion through the product layer cases were calculated as 100 and 296 kJ mol^?1, respectively.     

Fuel conversion from oxy-fuel combustion in a circulating fluidized bed

The study of the combustion process carried out in an oxygen-enriched atmosphere in a circulating fluidized bed (CFB) combustor is presented. The experiments were focused on fuel behavior in the conditions of increased oxygen concentration, at a different temperature and a different fuel load in the combustion chamber. The tests were performed in a laboratory-scale CFB combustor. Brown coal was used as the fuel. The values of variable parameters were in the following ranges: the oxygen concentration in the delivered stream of gas substrates (mixtures of O₂ + N₂ and O₂ + CO₂): 21 ÷ 60%; the combustor's temperature: 973 ÷ 1133 K; the mass of fuel portions: 4 ÷ 8 g. Based on the obtained data, carbon, sulfur and nitrogen conversion ratios were calculated.     

The reaction of NiO/NiAl2O4 particles with alternating methane and oxygen

The reduction and oxidation behaviour of oxygen carrier particles of NiO and NiAl₂O₄ has been investigated in a fluidized bed reactor as well as a thermogravimetric analyzer (TGA). The particles showed high reactivity and gas yield to CO₂ with methane in the temperature interval 750–950°C. In the fluidized bed the yield to CO₂ was between 90 and 99% using bed masses corresponding to 16–57 kg/MW_fuel. Complementary experiments in a TGA at 750 and 950°C showed a clear reaction of the NiAl₂O₄ with CH₄ at the higher temperature. There was methane released from the reactor at high degrees of solid oxidation, which is likely associated with the lack of Ni‐sites on the particles which can reform the methane. There was some carbon formation during the reduction, although the amount was minor when the gas yield to carbon dioxide and degree of oxidation of the solid was high. A simple reactor model using kinetic data from a previous study predicted the gas yield during the reduction in the fluidized bed experiments with reasonable accuracy. The oxygen carrier system investigated in this work shows high promise for use in a real CLC system, provided that the particle manufacturing process can be scaled up with reasonable cost.     

Simulation of coal pyrolysis by solid heat carrier in a moving-bed pyrolyzer

A one-dimensional, steady state, numerical model for coal pyrolysis by solid heat carrier in moving-bed has been developed. The multiple-reaction model of coal pyrolysis and the gas–solid–solid three phases heat transfer theory in packed bed have been applied to account for the pyrolysis process. The results show that the axial temperature distribution of the coal particles increase with a heating rate more than 600 K/min. Coal particle size has significant influence on the heating rate, while blending ratio is the determinant factor of pyrolysis temperature. Given the main operating parameters, product distributions (H₂, CO, CH₄, tar, etc) are calculated by the model. The modeling results are found to agree the experimental data using a moving-bed pyrolyzer with processing capacity 10 kg h^?1 of coal.     

Light fuel gas production from nascent coal volatiles using a natural limonite ore

Basic research on fuel gas production from nascent coal volatiles was conducted in a two-stage fixed-bed reactor. The volatiles from coal carbonization decomposed in the reactor. Indonesian natural limonite ore dramatically promoted coal volatile decomposition at 873–1023 K in ambient pressure and the carbon yields, except in tar, were above 97.8%. We compared these results to experiments without a catalyst. Subsequently, coal tar converted more completely and more than twice the amount of methane-rich product gases were obtained using the limonite catalyst. In addition, catalyst sulphur poisoning was scarcely observed in the H₂S–H₂–N₂ (H₂S/H₂: 0.002–0.004) system at 923 K.     

Continuous experiment regarding hydrogen production by Coal/CaO reaction with steam (II) solid formation

Integration of coal gasification and CO₂ separation reactions in one reactor may produce a high concentration of hydrogen. To design a reactor for this new reaction system, it is necessary to know all reaction behaviors in the integrated reaction system. In our previous study, we performed a continuous reaction experiment of coal/CaO under high steam pressure and confirmed that H₂ concentration higher than 80 vol% with little CH₄ was produced; and that almost all CO₂ was fixed by adding CaO. In this study, the behaviors of solid products during the continuous experiment were investigated. It was found that, CaO first reacted with high-pressure steam to form Ca(OH)₂ (hydration), then the Ca(OH)₂ absorbed the CO₂ generated by coal gasification to form CaCO₃. The hydration of CaO restored sorbent reactivity. Eutectic melting of Ca(OH)₂/CaCO₃ was found to occur in the experiment at 973 K, and this eutectic melting led to the growth of large particles of solid materials. However, at the relatively low temperature of 923 K, eutectic melting could be avoided. Carbon conversion of the coal in the continuous reaction of the coal/CaO mixture with steam was high as 60–80%, even at the lower temperature of 923 K.     

Dynamic behaviour of sewage sludge incineration in a large-scale bubbling fluidised bed in relation to feeding-rate variations

In this paper, a mathematical model is developed for the simulation of a large-scale sewage sludge incineration plant. The model assumes the bed to consist of a fast gas phase, an emulsion phase and a fuel particle phase with specific consideration for thermally-thick fuel particles. The developed model is employed to predict the dynamic response of the bed combustion to fluctuations in sludge feeding-rate. Calculation results indicate that the bed combustion is sensitive to fluctuations with response times greater than 30 min, but severe delays exist for both outlet oxygen level and bed temperatures; from 6 to 13 min for O₂ and 22–45 min for temperatures. Depending on the fluctuation frequency, the corresponding phase shifts are 39–96° for outlet O₂, 138–336° for bed temperature and 80–336° for freeboard temperature.