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.     

Development of Cu-based oxygen carriers for chemical-looping combustion

In a chemical-looping combustion (CLC) process, gas (natural gas, syngas, etc.) is burnt in two reactors. In the first one, a metallic oxide that is used as oxygen source is reduced by the feeding gas to a lower oxidation state, being CO₂ and steam the reaction products. In the second reactor, the reduced solid is regenerated with air to the fresh oxide, and the process can be repeated for many successive cycles. CO₂ can be easily recovered from the outlet gas coming from the first reactor by simple steam condensation. Consequently, CLC is a clean process for the combustion of carbon containing fuels preventing the CO₂ emissions to the atmosphere. The main drawback of the overall process is that the carriers are subjected to strong chemical and thermal stresses in every cycle and the performance and mechanical strength can decay down to unacceptable levels after enough number of cycles in use.In this paper the behaviour of CuO as an oxygen carrier for a CLC process has been analysed in a thermogravimetric analyser. The effects of carrier composition and preparation method used have been investigated to develop Cu-based carriers exhibiting high reduction and oxidation rates without substantial changes in the chemical, structural and mechanical properties for a high number of oxidation-reduction cycles. It has been observed that the carriers prepared by mechanical mixing or by coprecipitation showed an excellent chemical stability in multicycle tests in thermobalance, however, the mechanical properties of these carriers were highly degraded to unacceptable levels. On the other hand, the carriers prepared by impregnation exhibited excellent chemical stability without substantial decay of the mechanical strength in multicycle testing. These results suggest that copper based carriers prepared by impregnation are good candidates for CLC process.     

Chemical-looping combustion in a 300 W continuously operating reactor system using a manganese-based oxygen carrier

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. A solid oxygen carrier reacts with the oxygen in air in the air reactor and is then transferred to the fuel reactor, where the fuel gas is oxidized to carbon dioxide and water by the oxygen carrier. Fuel gas and air are never mixed and pure CO₂ can easily be obtained from the flue gas exit. The oxygen carrier is recycled between both reactors in a regenerative process. This paper presents the results from a continuously operating laboratory CLC unit, consisting of two interconnected fluidized beds. The feasibility of the use of a manganese-based oxygen carrier supported on magnesium stabilized zirconia was tested in this work. Natural gas or syngas was used as fuel in the fuel reactor. Fuel flow and air flow was varied, the thermal power was between 100 and 300 W, and the air ratio was between 1.1 and 5.0. Tests were performed at four temperatures: 1073, 1123, 1173 and 1223 K. The prototype was successfully operated at all conditions with no signs of agglomeration or deactivation of the oxygen carrier. The same particles were used during 70 h of combustion and the mass loss was 0.038% per hour, although the main quantity was lost in the first hour of operation. In the combustion tests with natural gas, methane was detected in the exit flue gases, while CO and H₂ were maintained at low concentrations. Higher temperature or lower fuel flows increases the combustion efficiency, which ranged from 0.88 to 0.99. On the other hand, the combustion of syngas was complete for all experimental conditions, with no CO or H₂ present in the gas from the fuel reactor.     

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.     

Integrating desulfurization with CO2-capture in chemical-looping combustion

Chemical looping combustion (CLC) is an emerging technology for clean combustion. We have previously demonstrated that the embedding of metal nanoparticles into a nanostructured ceramic matrix can result in unusually active and sinter-resistant nanocomposite oxygen carrier materials for CLC which maintain high reactivity and high-temperature stability even when sulfur contaminated fuels are used in CLC. Here, we propose a novel process scheme for in situ desulfurization of syngas with simultaneous CO₂-capture in chemical looping combustion by using these robust nanocomposite oxygen carriers simultaneously as sulfur-capture materials. We found that a nanocomposite Cu-BHA carrier can indeed strongly reduce the H₂S concentration in the fuel reactor effluent. However, during the process the support matrix is also sulfidized and takes part in the redox process of CLC. This results in SO₂ production during the reduction of the oxygen carrier and thus limits the degree of desulfurization attainable with this kind of carrier. Nevertheless, the results suggest that simultaneous desulfurization and CO₂ capture in CLC is feasible with Cu as oxygen carrier as long as appropriate carrier support materials are chosen, and could result in a novel, strongly intensified process for low-emission, high efficiency combustion of sulfur contaminated fuel streams.     

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.     

Interaction of mineral matter of coal with oxygen carriers in chemical-looping combustion (CLC)

The chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) processes are novel solutions for efficient combustion with direct separation of carbon dioxide. These processes use a metal oxide as an oxygen carrier to transfer oxygen from an air to a fuel reactor, where the fuel reacts with the solid oxygen carrier. When utilizing coal in CLC, the oxygen carrier particles could be affected through interaction with the ash-forming mineral matter found in coal, causing deactivation and/or agglomeration. In this work, possible interactions between minerals commonly encountered in coal and several promising oxygen carriers that are currently under investigation for their use in CLC are studied by both experiment and thermodynamic equilibrium calculations. Possible interaction was studied for both highly reducing and oxidizing conditions at 900 °C. Under highly reducing conditions pyrite was found to have by far the most deteriorating effect on the oxygen carrier particles, as the sulfur in the pyrite reacted with the oxygen carrier to form sulfides. Quartz and clay minerals were found to have a rather low influence on the oxygen carriers. Out of the oxygen carriers investigated, CuO/MgAl₂O₄ and the Mn₃O₄/ZrO₂ oxygen carriers tended to be quite reactive towards mineral matter whereas ilmenite has been shown to be the most robust oxygen carrier. Although sulfur can clearly deactivate Ni, Cu and Mn based oxygen carriers under sub-stoichiometric conditions, when the fuel is converted fully to CO₂ and H₂O, sulfides are only expected for Ni-based oxygen carriers.     

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.     

Carbon capture and utilization via chemical looping dry reforming

Chemical looping combustion (CLC) is a clean energy technology for CO₂ capture that uses periodic oxidation and reduction of an oxygen carrier with air and a fuel, respectively, to achieve flameless combustion and yield sequestration-ready CO₂ streams. While CLC allows for highly efficient CO₂ capture, it does not, however, provide a solution for CO₂ sequestration.Here, we propose chemical looping dry reforming (CLDR) as an alternative to CLC by replacing air with CO₂ as the oxidant. CLDR extends the chemical looping principle to achieve CO₂ reduction to CO, which opens a pathway to CO₂ utilization as an alternative to sequestration. The feasibility of CLDR is studied through thermodynamic screening calculations for oxygen carrier selection, synthesis and kinetic experiments of nanostructured carriers using cyclic thermogravimetric analysis (TGA) and fixed-bed reactor studies, and a brief model-based analysis of the thermal aspects of a fixed-bed CLDR process.Overall, our results indicate that it is indeed possible to reduce CO₂ to CO with high reaction rates through the use of appropriately designed nanostructured carriers, and to integrate this reaction into a cyclic redox (“looping”) process.     

On the evaluation of synthetic and natural ilmenite using syngas as fuel in chemical-looping combustion (CLC)

Chemical-looping combustion (CLC) is a combustion technique where the CO₂ produced is inherently separated from the rest of the flue gases with a considerably low energy penalty. For this reason, CLC has emerged as one of the more attractive options to capture CO₂ from fossil fuel combustion. When applying CLC with solid fuels, the use of a low cost oxygen carrier is highly important, and one such low cost oxygen carrier is the mineral ilmenite. The current work investigates the reactivity of several ilmenites, some which are synthetically produced by freeze granulation and two natural minerals, one Norwegian ilmenite and one South African ilmenite.     

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.     

Comparison of natural ilmenites as oxygen carriers in chemical-looping combustion and influence of water gas shift reaction on gas composition

Chemical-looping combustion (CLC) is a promising technology for CO₂-capture for storage or reuse as a method to mitigate CO₂ emissions from the use of fossil fuels. In a CLC system the oxygen carrier is of great importance. Environmentally sound and low cost materials seem to be preferable especially for CLC of solid fuels. The natural occurring ore ilmenite has already been the target of different studies in order to work out its feasibility as oxygen carrier for different fuels. The initial part of this work is a screening of five commercial available ilmenite minerals as oxygen carrier, crushed and sieved to 125–180 μm. The screening includes an examination of the sulfur released during the first heat up and the activation of the oxygen carrier, indicated by the fuel conversion using alternating reduction (syngas 50 vol.% CO in H₂) and oxidation conditions (10 vol.% O₂ in N₂). The five first cycles were carried out at 850 °C to avoid initial agglomeration whereas the main activation cycles have been performed at 950 °C in a tubular quartz reactor under fluidized bed conditions. From these experiments it is concluded that rock ilmenites are preferable as oxygen carriers since they revealed an improved fuel conversion, although offering a higher sulfur content, which is released during the initial heat up.     

Gas leakage measurements in a cold model of an interconnected fluidized bed for chemical-looping combustion

In chemical-looping combustion (CLC) a gaseous fuel is burnt with inherent separation of the greenhouse gas carbon dioxide. The oxygen is transported from the combustion air to the fuel by means of metal oxide particles acting as oxygen carriers. A CLC system can be designed similar to a circulating fluidized bed, but with the addition of a bubbling fluidized bed on the return side. Thus, the system consists of a riser (fast fluidized bed) acting as the air reactor. This is connected to a cyclone, where the particles and the gas from the air reactor are separated. The particles fall down into a second fluidized bed, the fuel reactor, and are via a fluidized pot-seal transported back into the riser. The gas leaving the air reactor consists of nitrogen and unreacted oxygen, while the reaction products, carbon dioxide and water, come out from the fuel reactor. The water can easily be condensed and removed, and the remaining carbon dioxide can be liquefied for subsequent sequestration.The gas leakage between the reactors must be minimized to prevent the carbon dioxide from being diluted with nitrogen, or to prevent carbon dioxide from leaking to the air reactor decreasing the efficiency of carbon dioxide capture. In this system, the possible gas leakages are: (i) from the fuel reactor to the cyclone and to the pot-seal, (ii) from the cyclone down to the fuel reactor, (iii) from the pot-seal to the fuel reactor. These gas leakages were investigated in a scaled cold model. A typical leakage from the fuel reactor was 2%, i.e. a CO₂ capture efficiency of 98%. No leakage was detected from the cyclone to the fuel reactor. Thus, all product gas from the air reactor leaves the system from the cyclone. A typical leakage from the pot-seal into the fuel reactor was 6%, which corresponds to 0.3% of the total air added to the system, and would give a dilution of the CO₂ produced by approximately 6% air. However, this gas leakage can be avoided by using steam, instead of air, to fluidize the whole, or part of, the pot-seal. The disadvantages of diluting the CO₂ are likely to motivate the use of steam.     

Carbon deposition characteristics and regenerative ability of oxygen carrier particles for chemical-looping combustion

For gaseous fuel combustion with inherent CO₂ capture and low NO_x emission, chemical-looping combustion (CLC) may yield great advantages for the savings of energy to CO₂ separation and suppressing the effect on the environment. In a chemical-looping combustor, fuel is oxidized by metal oxide medium (oxygen carrier particle) in a reduction reactor. Reduced particles are transported to the oxidation reactor and oxidized by air and recycled to the reduction reactor. The fuel and the air are never mixed, and the gases from the reduction reactor, CO₂ and H₂O, leave the system as separate streams. The H₂O can be easily separated by condensation and pure CO₂ is obtained without any loss of energy for separation. In this study, NiO based particles are examined from the viewpoints of reaction kinetics, carbon deposition, and cyclic use (regenerative ability). The purpose of this study is to find appropriate reaction conditions to avoid carbon deposition and achieve high reaction rate (e.g., temperature and maximum carbon deposition-free conversion) and to certify regenerative ability of NiO/bentonite particles. In this study, 5.04% methane was used as fuel and air was used as oxidation gas. The carbon deposition characteristics, reduction kinetics and regenerative ability of oxygen carrier particles were examined by TGA (Thermal Gravimetrical Analyzer).     

Effect of fuel particle size on reaction rate in chemical looping combustion

Chemical looping combustion (CLC) uses an oxygen carrier circulating between an air and a fuel reactor to replace direct burning of fuels in air. The very low energy penalty for CO₂ separation in CLC gives it the potential to become an important technology on the way to a CO₂ neutral energy supply. In this work, the influence of the particle size of coal on the rate of reaction of the coal was investigated in a bed of oxygen carrier. In order to do this, a method to quench the reaction of coal with oxygen carriers at a specified time and measure the particle size distribution of the remaining coal was developed. Three size fractions of coal were used in the experiments: 90–125, 180–212 and 250–355 μm. Particle size distributions of the fuel show a decrease in particle size with time. The influence of devolatilisation of the coal on the coal particle size was measured, showing that coal particles do not break in the fluidized bed reactor used for the experiments. Reaction rates based on measurements of gas phase concentrations of CO₂, CO and CH₄ showed that the reaction rate is independent of the particle size. These results are in line with literature findings, as studies have shown that carbon gasification is size-independent at conditions similar to those in the performed CLC experiments.     

Effect of temperature on reduction reactivity of oxygen carrier particles in a fixed bed chemical-looping combustor

In a chemical-looping combustor (CLC), gaseous fuel is oxidized by metal oxide particle, e.g. oxygen carrier, in a reduction reactor (combustor), and the greenhouse gas CO₂ is separated from the exhaust gases during the combustion. In this study, NiO/bentonite particle was examined on the basis of reduction reactivity, carbon deposition during reduction, and NO_x formation during oxidation. Reactivity data for NiO/bentonite particle with methane and air were presented and discussed. During the reduction period, most of the CH₄ are converted to CO₂ with small formation of CO. Reduction reactivity (duration of reduction) of the NiO/bentonite particle increased with temperature, but at higher temperature, it is somewhat decreased. The NiO/bentonite particle tested showed no agglomeration or breakage up to 900 ‡C, but at 1,000 ‡C, sintering took place and lumps of particles were formed. Solid carbon was deposited on the oxygen carrier during high conversion region of reduction, i.e., during the end of reduction. It was found that the appropriate temperature for the NiO/bentonite particle is 900 ‡C for carbon deposition, reaction rate, and duration of reduction. We observed experimentally that NO, NO₂, and N₂O gases are not generated during oxidation.     

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.     

Thermodynamic study of combining chemical looping combustion and combined reforming of propane

Existing energy generation technologies emit CO₂ gas and are posing a serious problem of global warming and climate change. The thermodynamic feasibility of a new process scheme combining chemical looping combustion (CLC) and combined reforming (CR) of propane (LPG) is studied in this paper. The study of CLC of propane with CaSO₄ as oxygen carrier shows thermodynamic feasibility in temperature range (400-782.95 °C) at 1 bar pressure. The CO₂ generated in the CLC can be used for combined reforming of propane in an autothermal way within the temperature range (400-1000 °C) at 1 bar pressure to generate syngas of ratio 3.0 (above 600 °C) which is extremely desirable for petrochemical manufacture. The process scheme generates (a) huge thermal energy in CLC that can be used for various processes, (b) pure N₂ and syngas rich streams can be used for petrochemical manufacture and (c) takes care of the expensive CO₂ separation from flue gas stream and CO₂ sequestration. The thermoneutral temperature (TNP) of 702.12 °C yielding maximum syngas of 5.98 mol per mole propane fed, of syngas ratio 1.73 with negligible methane and carbon formation was identified as the best condition for the CR reactor operation. The process can be used for different fuels and oxygen carriers.     

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.     

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.