Reduced Oxygen Carrier Research Articles

The direct solid-solid reaction between coal char and iron-based oxygen carrier and its contribution to solid-fueled chemical looping combustion

Chemical looping combustion (CLC) is an advanced technology developed to achieve highly efficient fuel combustion with in-situ CO2 capture. In this process, metal oxide particles are used as an oxygen carrier (OC) to transport lattice oxygen for fuel combustion. In this process, a stream of CO2 and steam is produced by successful separation of atmospheric N2 and the gaseous product of combustion. In CLC of solid fuel, metal oxide particles are physically mixed and react with solid fuel at high temperature using gasification enhancer, such as steam, or CO2. A full understanding of the reaction mechanism between the OC and solid fuel is vital for OC development and the fuel reactor design. Several reactions may be involved in solid-fueled CLC when an iron-based OC is used, including (1) solid fuel devolatilization/gasification, (2) OC reduction with intermediate syngas, (3) the solid-solid reaction between OC and solid fuel via direct contact, and (4) the homogeneous water-gas shift reaction. The former two reactions have been extensively studied in recent years. This study focuses on the third reaction, the solid-solid reaction, which occurs thermodynamically at typical operational temperatures of CLC. The direct solid-solid reaction between coal char and two iron-based OCs via random particle collision in a fluidization bed regime was investigated and focuses on the reaction kinetics and the carbon conversion at different temperature. The contribution of the solid-solid reaction to the global carbon conversion was estimated for steam-gasified CLC at different temperature. The solid-solid reaction via static contact in a thermal-gravimetric analyzer (TGA) was also tested to evaluate the role of different OCs and to better understand the reaction mechanism between the two solid particles.

Chemical Looping for Energy Technology: A Special Issue

Chemical Looping for Energy Technology: A Special Issue

CaO/MgO modified perovskite type oxides for chemical-looping steam reforming of methane

Chemical-looping steam methane reforming (CL-SMR) is a novel method proposed on the base of chemical looping combustion (CLC) technology. In the CL-SMR scheme, methane is partially oxidized to syngas (H2/CO=2.0) by the lattice oxygen in reformer reactor in the absence of gaseous oxidant, and then the reduced oxygen carrier is oxidized by steam to produce hydrogen in steam reactor. The use of perovskite type oxide LaFeO3 as an oxygen carrier in CL-SMR was studied. While the basicity of CaO/MgO modified oxygen carriers, LaFeO3-CaO and LaFeO3-MgO, were also synthesized aiming to increase specific surface area, thermostability, and resistance to coke formation. The synthesized oxides were characterized by X-ray diffraction (XRD), H2-temperature-programmed reduction (H2-TPR), Brunauer-Emmett-Teller (BET) surface area and X-ray photoelectron spectroscopy (XPS). Three oxygen carriers exhibited high active and selective for syngas production from methane, and maintained perovskite type over cyclic redox operations. The LF-CaO sample is the best candidate for the CL-SMR of the three samples judging from the reactivity, selectivity, and resistance to carbon formation. It showed good regenerability during 5 redox reactions.

Reduction kinetics of hematite as oxygen carrier in chemical looping combustion

Chemical looping combustion (CLC), which has the characteristic of CO2 inherent separation, is a novel fossil fuel utilization technology for CO2 capture with low cost and low energy efficiency penalty. The reduction kinetics of oxygen carrier (OC) is one of the most important issues for CLC reactor design and computational fluid dynamics (CFD) simulation. In this work, well-organized isothermal experiments in a thermogravimetric analyzer (TGA) were first carried out to obtain the kinetics of a hematite as OC reduced by carbon monoxide. The reduction process was identified as two sequential stages, which correspond to the reduction of Fe2O3 to Fe3O4 and Fe3O4 to FeO, respectively. As obtained from the experimental result, the first stage is controlled by gas diffusion in the boundary layer on particle surface while the second stage is mainly controlled by heterogeneous chemical reaction. For the second stage, the activation energy, E, is 110.75kJ/mol and the reaction order, n, is 1.5. Then, the same batch of hematite was tested by reacting with the reducing gas of 10vol.% CO balanced by N2 in a fluidized bed reactor. The different reaction characteristics in TGA and fluidized bed reactor were analyzed. This work provides a deep understanding on the applicability of the OC reduction kinetics obtained by TGA to the reactor design and CFD simulation of a fluidized bed CLC rig.

In Situ Gasification Chemical Looping Combustion of Coal Using the Mixed Oxygen Carrier of Natural Anhydrite Ore and Calcined Limestone

Abstract The utilization of CaSO4-based oxygen carrier suffers the deactivation problem caused by sulphur loss. To capture the gas sulphides and to improve the stability of CaSO4 oxygen carrier, calcined limestone was introduced into the fuel reactor of Chemical Looping Combustion (CLC). Kinetic behaviors and thermodynamics of the combined process of coal gasification and oxygen carrier reduction using the mixed oxygen carrier CaSO4-CaO under different atmospheres were investigated. The effects of reaction temperature, gasification intermediate, and molar ratio of CaO to CaSO4 on gas sulphide emissions, CO2 generation, and distribution of other gas emissions and characterization of solid products are taken into account. It is found the CaO-based additive evidently suppressed the sulphur emissions, and improved both chemical reaction rate and CO2 generation efficiency. The sulfation products, both CaS and CaSO4, can be used as oxygen carrier later. The optimum reaction parameters are evaluated and obtained in terms of gas sulphide emissions, CO2 capture, other gas releases and maintenance of oxygen transfer capability.

Redox performance of Cu-based oxygen carrier used in chemical looping air separation combined oxy-fuel combustion technology

Chemical looping air separation offering the oxygen source to oxy-fuel combustion is an effective CO2 and NOx reduction technology in fossil fuel combustion. In the technology, oxygen carrier releases oxygen to flue gas and the reduced oxygen carrier absorbs oxygen from air in two separated reactors. And then the oxygen enriched flue gas returns to the combustor. The redox performance of oxygen carrier is essential to this technology. Thermodynamic calculations on CuO/Cu2O in flue gas atmosphere indicate that the redox reaction of copper oxides is not affected by the compositions in the flue gas. The optimization reduction temperatures are determined as 850~1050 °C. The redox performance of Cu-based oxygen carriers with 40wt%, 50wt%, 60wt% MgAl2O4 as inert binder was investigated in TGA and fixed-bed reactor. The reduction rate increases with temperature increasing and with oxygen concentration decreasing. The oxidation rate increases with temperature increasing when temperature is lower than 850 °C, while it begins to decrease when temperature is higher than 850 °C. The highest oxygen concentration in the reduction period is not limited to the gas flow but to the Pe at that temperature. The reduction and oxidation rates all increase with binder adding ratio decreasing. The average reaction rate of oxygen carrier begins to decrease when conversion is higher than 80%. The reduced oxygen carrier can be oxidized in low oxygen concentration.

Thermodynamic analysis of syngas generation from biomass using chemical looping gasification method

Chemical looping gasification (CLG) is a novel technology to convert carbon-containing feedstock into syngas. In the technology, lattice oxygen of oxygen carrier is used as oxygen source for char gasification, and then the reduced oxygen carrier is regenerated in air atmosphere. In this paper, the mechanism of CLG of biomass using Mn2O3 as oxygen carrier was discussed firstly and then thermodynamic analysis of syngas generation from biomass was performed by Gibbs free energy minimization method. Chemical equilibrium calculations were carried out to study the effects of Mn2O3/biomass, operation temperature, pressure, and steam/Mn2O3 on the gasification characteristics. The optimal thermodynamic conditions were determined to improve H2 and CO concentrations as high as possible. The optimal Mn2O3/corn cob is 0.18 to obtain the highest CO and H2 yields. The results suggest that the preferential conditions are achieved at 1000 °C and atmospheric pressure, under these conditions, the total dry concentration of CO and H2 is 98.8% in the syngas. Steam can be used in the CLG to modify the H2/CO in the syngas. The carbon conversion also increases under oxidizing atmosphere of steam.

Evaluation of Cu-based oxygen carrier for chemical looping air separation in a fixed-bed reactor

Chemical looping air separation (CLAS) is a novel technology to separate oxygen from air. In CLAS process, oxygen carrier releases oxygen in steam atmosphere and the reduced oxygen carrier is regenerated in air atmosphere. The performance of oxygen carrier is essential to CLAS technology. In this study, the feasibility of CuO in steam atmosphere was evaluated by thermodynamics. Steam is not involved in any reaction. The production of oxygen is achieved by the decomposition of CuO to Cu2O and the optional reduction temperatures should be 850–1050°C. In order to inhibit the agglomeration of copper oxides, oxygen carriers were prepared with 60wt.% SiO2, TiO2, ZrO2 and MgAl2O4 as support materials. The oxygen release and absorb rates of the four oxygen carriers were determined under reduction temperatures of 950, 975 and 1000°C and oxidation temperatures of 800, 850, 900°C in a fixed-bed reactor. Both the reduction and oxidation include quick and slow reaction stages. The reduction rate increases and the oxidation rate decreases with the investigated temperature increasing. The reduction and oxidation macro-kinetic models of the four oxygen carriers were established based on the reactivity data. Binders have great effect on reduction kinetic parameters. The Cu4Mg6 oxygen carrier has the highest reduction and oxidation rates. The stability of Cu4Mg6 was investigated by multi-cycles test. The oxygen release and absorb rates keep stable in each cycle. The chemical and physical characteristics of the oxygen carriers prepared and reacted were also measured. The chemical phases are only copper oxides and binder. The surface area becomes larger after cycles. There is no obvious agglomeration observed after multi-cycles test.

Evaluation of multi-cycle performance of chemical looping dry reforming using CO2 as an oxidant with Fe–Ni bimetallic oxides

Chemical looping dry reforming (CLDR) is an innovative technology for CO2 utilization using the chemical looping principle. The CLDR process consists of three stages, i.e. CH4 reduction, CO2 reforming, and air oxidation. Spinel nickel ferrite (NiFe2O4) was prepared and its multi-cycle performance as an oxygen carrier for CLDR was experimentally investigated. X-ray diffraction (XRD) and Laser Raman spectroscopy showed that a pure spinel crystalline phase (NiFe2O4) was obtained by a parallel flow co-precipitating method. NiFe2O4 was reduced into Fe–Ni alloy and wustite (FexO) during the CH4 reduction process. Subsequent oxidation of the reduced oxygen carrier was performed with CO2 as an oxidant to form an intermediate state: a mixture of spinel Ni1−xFe2+xO4, Fe2+yO4 and metallic Ni. And CO was generated in parallel during this stage. Approximate 185 mL of CO was generated for 1 g spinel NiFe2O4 in a single cycle. The intermediate oxygen carrier was fully oxidized in the air oxidation stage to form a mixture of Ni1+xFe2−xO4 and Fe2O3. Although the original state of oxygen carrier (NiFe2O4) was not fully regenerated and agglomeration was observed, a good recyclability was shown in 10 successive redox cycles.

Kinetic determination of chemical looping combustion reactions in a continuous stirred tank reactor : Reduction of the oxygen carrier

This paper deals with the study of methane combustion with NiO/NiAl2O4 as oxygen carrier for a better understanding of Chemical Looping Combustion. Partial combustion of methane with NiO/NiAl2O4 tablets has been investigated in a lab scale continuous perfectly auto-stirred tank reactor (CASTR) where reacting gases methane and steam were diluted in argon and temperature ranged from 700°C to 833°C. Influence of methane and water concentrations as well as temperature are studied. Experimental results show that total oxidized compounds are firstly produced, followed by the production of hydrogen, carbon monoxide and coke, covering the particles and limiting reactions. Increasing the temperature or water concentration enable reducing the coke formation. Analysis of reaction products helps defining a reaction mechanism and ensuring absence of any mass and heat transfer limitations. A model for the reduction of the oxygen carrier, based on the grainy pellet model and taking into account the diffusion of gas species through the tablet and their reactions at the surface of each grain is developed. Gas-solid reaction rates are calculated using the shrinking core model and catalytic reaction rates are defined according to the Langmuir–Hinshelwood mechanism. The kinetic parameters of the catalytic and redox reactions involved in the oxygen carrier reduction are optimized by comparison between simulated and experimental results in the CASTR. The obtained parameters give a very good description of the observed phenomena and are in agreements with former published results.

Reduction kinetics analysis of sol–gel-derived CuO/CuAl2O4 oxygen carrier for chemical looping with oxygen uncoupling

Chemical looping with oxygen uncoupling (CLOU) is a promising technology due to its potential to reduce energy efficiency penalty and cost associated with CO2 capture. In this work, a CuO/CuAl2O4 oxygen carrier (OC) prepared by sol–gel was investigated in its oxygen release kinetics (4CuO → 2Cu2O + O2). Based on several well-organized temperature-programmed reduction experiments which were conducted in a thermogravimetric analyzer, the activation energy E (343.7 kJ mol−1) and pre-exponential factor A (3.78 × 1012 s−1) were determined and the Avrami–Erofeev random nucleation and subsequence growth model fitted well with the reduction experimental data. The enhancement of OC reduction rate in real fluidized bed CLOU reactor using different types of solid fuels (petroleum coke, anthracite, bituminous, and lignite) was identified in terms of the chemical kinetics and thermodynamics for the first time. It was found that the CuO reduction rate is more sensitive to the local temperature change than the oxygen concentration driving force. The results could contribute to the design, operation, and performance prediction of real CLOU reactors.

Reaction characteristics research of coal char chemical looping gasification for hydrogen production with an Fe-Zr oxygen carrier modified by K2CO3

An Fe-Zr oxide (Fe2O3 and ZrO2) modified by K2CO3 was prepared as the oxygen carrier (denoted as K3-Fe70Zr30) to examine the effects of temperature and char mass/oxygen carrier (OC) ratio on gas yield and composition during coal char chemical looping gasification for hydrogen production. The results of temperature-programmed experiments show that the reaction of oxygen carrier and coal char starts at 500°C, and the reaction rate increases sharply above 750°C. The reduced oxygen carrier begins to react with steam at 400°C, and the concentration of hydrogen significantly increases above 500°C. The results of isothermal experiments indicate that the reaction rate is accelerated with increasing reaction temperature. However, the H2 production yield decreases as the temperature rises when the ratio of CO/CO2 volume ratio increases. In addition, with the increase of char/OC ratio, the ratio of CO/CO2 in the outlet gas increases. As a result, the hydrogen production firstly increases and reaches a maximum value of 1.734 L/g, and then decreases. The activity of oxygen carrier can keep stable during the first 2 redox cycles, but decreases in the 3rd one. The activity can be renewed by the supplement of K2CO3, suggesting that the decreased activity of oxygen carrier can ascribe to the loss of K2CO3.

Thermodynamic analysis and thermogravimetric investigation on chemical looping gasification of biomass char under different atmospheres with Fe2O3 oxygen carrier

Chemical looping gasification (CLG) is viewed as a promising gasification technology because gas phase oxygen of gasifying medium can be replaced by lattice oxygen of oxygen carrier. In the present work, the reactivity of biomass char with Fe2O3 oxygen carrier under different atmospheres was detailed investigated through the thermodynamic and thermo-gravimetric analysis. The Fe2O3 was almost completely reduced into metallic iron when the reaction temperature exceeded 1000°C, but the carbon conversion of char only attained to 40.80% due to the lack of oxygen source under inert atmosphere. However, the carbon conversion of char apparently increased and the oxygen conversion of oxygen carrier dramatically decreased under oxidizing atmosphere (CO2, or H2O). It is attributed to the fact that oxidizing agents can supply oxygen sources to promote char conversion as well as to alleviate the reduction of oxygen carrier. These thermodynamic predictions were confirmed by TGA tests, where the char was fully converted into gas under CO2 or H2O atmosphere because of enough oxygen sources but a low carbon conversion was obtained under inert atmosphere. TGA results further indicated that the reactivity of char with oxygen carrier under steam atmosphere is much higher than that under CO2 atmosphere and the Fe2O3 phase is mainly reduced into Fe3O4 phase in all the tests. Additionally, several other different carriers (Al2O3, NiO-modified iron ore and NiO) were employed to compare the reactivity of iron ore as well. The order of reactivity is speculated as follows: Pure oxygen≈NiO>H2O>Iron ore>CO2>Al2O3. The reduced oxygen carrier can recover entire lattice oxygen [O] to its initial state under air atmosphere. Therefore, it suggests that the Fe2O3 material used as an oxygen carrier can be recycled in the process of char CLG.

A Feasibility Study for Synthesis Gas Production by Considering Carbon Dioxide Capturing in an Industrial-Scale Methanol Synthesis Plant

In this paper, a feasibility study for the application of chemical-looping combustion (CLC) instead of fired furnace for synthesis gas production during methanol synthesis in an industrial-scale conventional steam reformers (CSR) has been considered. The aims are the prevention of large emission of CO2 to atmosphere and enhancement of synthesis gas production. For this purpose, employment of Ni18–Al2O3, Ni40–Al2O3, and Fe45–Al2O3 oxygen carrier (OC) has been investigated. Simulation results show that complete oxidation and reduction of OC occurs in air reactor and fuel reactor (FR), respectively. Also, combustion efficiency reaches to 1 in the FR part of CLC-SR with all types of OCs. Utilizing CLC instead of fired furnace enhances CH4 conversion and H2 yield in SR side of CLC-SR. Results indicate that in CLC-SR, CH4 conversion is equal to 26.4, 27.96, and 26.33 % via Ni18–Al2O3, Ni40–Al2O3, and Fe45–Al2O3 OC, respectively, in comparison with 26 % in CSR. Slightly higher conversion is observed with Ni40–Al2O3, i.e., from 26 % to almost 28 %. Synthesis gas production increases from 3633 kmol h−1 in CSR to 3639, 3868, and 3673 kmol h−1 in CLC-SR via Ni18–Al2O3, Ni40–Al2O3, and Fe45–Al2O3 OC, respectively. Results illustrate that by increasing FR feed temperature from 800 to 1000 K, CH4 conversion in SR side increases 4.99, 7.93, and 4.57 % by using Ni18–Al2O3, Ni40–Al2O3, and Fe45–Al2O3 OC, respectively, in comparison with CSR. Also, synthesis gas production enhances 15 and 26.01 % via Ni18–Al2O3, Ni40–Al2O3, and Fe45–Al2O3 OC, respectively, in comparison with CSR.

Effect of hematite addition to CaSO4 oxygen carrier in chemical looping combustion of coal char

Catalytic mechanism of synergy effect of iron oxide in gasification of char and in the reduction of CaSO4 oxygen carrier.

Cu-Modified Manganese Ore as an Oxygen Carrier for Chemical Looping Combustion

A new type of Mn-based oxygen carrier was prepared by impregnating manganese ore with copper nitrate solution. Cyclic reduction and oxidation reactivity of the materials was investigated in a fluidized-bed reactor. The potential use of these oxygen carriers for chemical looping combustion (CLC) was examined. The reactivity of the manganese ore can be highly improved by impregnation of copper. The reactivity of the oxygen carrier reduction is higher with a larger amount of copper impregnated. However, the degree of the reactivity enhancement is not proportional to the amount of copper doped on the oxygen carriers. An important finding is that, even with low Cu loading, such as 0.5 wt % copper impregnated on manganese, the period with full CO conversion can be enhanced 6 times. A very interesting phenomenon is that the Cu-modified manganese ore can completely convert CO, even at a low temperature, such as 500 °C. This study proves that the reactivity of the manganese ore could be significantly improved by impregnating copper, even with a small amount. The copper impregnation method could be very promising to improve the reactivity of the manganese ore as oxygen carriers for CLC.

CFD analysis of bubble hydrodynamics in a fuel reactor for a hydrogen-fueled chemical looping combustion system

This study investigates the temporal development of bubble hydrodynamics in the fuel reactor of a hydrogen-fueled chemical looping combustion (CLC) system by using a computational model. The model also investigates the molar fraction of products in gas and solid phases. The study assists in developing a better understanding of the CLC process, which has many advantages such as being a potentially promising candidate for an efficient carbon dioxide capture technology.The study employs the kinetic theory of granular flow. The reactive fluid dynamic system of the fuel reactor is customized by incorporating the kinetics of an oxygen carrier reduction into a commercial computational fluid dynamics (CFD) code. An Eulerian multiphase treatment is used to describe the continuum two-fluid model for both gas and solid phases. CaSO4 and H2 are used as an oxygen carrier and a fuel, respectively. The computational results are validated with the experimental and numerical results available in the open literature. The CFD simulations are found to capture the features of the bubble formation, rise and burst in unsteady and quasi-steady states very well. The results show a significant increase in the conversion rate with higher dense bed height, lower bed width, higher free board height and smaller oxygen carrier particles which upsurge an overall performance of the CLC plant.

Effect of steam reforming on methane-fueled chemical looping combustion with Cu-based oxygen carrier

The reduction characteristics of Cu-based oxygen carrier with H2, CO and CH4 were investigated using a fixed bed reactor, TPR and TGA. Results showed that temperatures for the complete reduction of Cu-based oxygen carrier with H2 and CO are 300 °C and 225 °C, respectively, while the corresponding temperature with CH4 is 650 °C. The carbon deposition from CH4 occurred at over 550 °C. CO-chemisorption experiments were also conducted on the oxygen carrier, and it was indicated that Cu-based oxygen carrier sinter seriously at 700 °C. In order to lower the required reduction temperature of oxygen carriers, a new chemical looping combustion (CLC) process with CH4 steam reforming has been presented in this paper. The basic feasibility of the process was illustrated using CuO–SiO2. The new CLC process has the potential to replace the conventional gas-fired middle- and low-pressure steam and hot water boilers.

Experimental Study of Chemical-Looping Reforming in a Fixed-Bed Reactor: Performance Investigation of Different Oxygen Carriers on Al2O3 and TiO2 Support

This study examines the hydrogen production by the steam reforming of methane integrated to chemical-looping reforming (CLR) as a novel technology in a fixed-bed reactor at 700–1200 °C. The particles are present in two consecutive oxidation and reduction steps. In the reduction step, the oxygen carrier is reduced with the fuel, which, in turn, is partially oxidized to H2 and CO (synthesis gas), and in the oxidation step, the reduced oxygen carrier is reoxidized with oxygen (O2 + argon). The oxygen carriers Fe, Mn, Co, and Cu using inert materials Al2O3 and TiO2 as a support are prepared by the precipitation method. The samples are analyzed using energy-dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), and X-ray diffraction (XRD) to check the carrier specifications before and after the process. The main goal of this study is investigation of the reactivity of different metal oxides on Al2O3 and TiO2 support. The conversion of fuel into products depends upon the type of oxygen carriers an...

Mechanism study of reaction between CO and NiO(0 0 1) surface during chemical-looping combustion: Role of oxygen

First-principle calculations based on density function theory (DFT) were used to clarify the reactivity mechanism of CO with NiO(0 0 1) surface, including the effect of the oxygen species during the Ni-based oxygen carrier reduction process in chemical-looping combustion. The results indicate that the SGGA (spin-polarized generalized gradient approximation) + U (a Hubbard-type on-site Coulomb repulsion) method could give an accurate description of the reaction in the CO–NiO(0 0 1) system by comparing with experimentally determined parameters. A systematic investigation of the adsorption of CO on the NiO(0 0 1) perfect and defective surfaces was performed. It was found that CO could be only adsorbed on both surfaces, but no oxidation reactions were observed. The role of O species for the reaction between CO and NiO(0 0 1) surface was studied in detail. Our calculations clearly reveal that the oxygen vacancy exhibits a high surface reactivity toward the dissociation of O2 to O atoms and a reaction between oxygen adsorbed on the surface of NiO(0 0 1) and CO is the major reaction pathway in forming CO2.