Ni-based Oxygen Carrier Research Articles

A Hydrodynamic Study of Chemical Looping Combustion System for Gaseous Fuel

Chemical looping combustion (CLC) is a promising technology for fossil fuel combustion with inherent CO2 capture and sequestration that is able to mitigate greenhouse gases (GHGs) emission. The CLC process employs metal oxide as an oxygen carrier circulating between fuel reactor, where the fuel is combusted to CO2 and H2O, and air reactor, where the oxygen carrier returned to its oxidized form by aeration. In order to demonstrate and investigate the CLC concept experimentally, several previous studies have been carried out with the aim of improving the performance of oxygen carrier. The performance of the oxygen carrier, which circulate between the air reactor and the fuel reactor to transfer oxygen and heat, is the most important factor that determines the efficiency of the CLC process. In order to evaluate the reactivity of the oxygen carrier, the fuel conversion and CO2 selectivity of oxygen carrier have been investigated using thermogravimetric analyzer, batch type bubbling fluidized bed, etc. However, the hydrodynamic study for the solid circulation of oxygen carrier should be preceded to ensure the steady state operation of CLC process. In this study, the characteristics of solid circulation between the air reactor and the fuel reactor was examined in the bench scale CLC unit made of Plexiglas, and the effects of change of fuel reactor configuration and cyclone on the solid circulation between the two reactors were investigated. Ni based oxygen carrier was used as the bed material and the optimal fluidizing gas flow rate capable of maintaining stable solid circulation were investigated.

Preparation and Characterization of Spray-Dried Ni-Based Oxygen Carriers Added with Ceo2 for Chemical Looping Combustion

Preparation and Characterization of Spray-Dried Ni-Based Oxygen Carriers Added with Ceo2 for Chemical Looping Combustion

Syngas chemical looping combustion using a highly performing fluidizable oxygen carrier

• A high performing 20 wt%Ni-1 wt%Co-5 wt%La/γ-Al 2 O 3 oxygen carrier is reported. • The preparation method secures a stable fluidizable OC with enhanced reactivity. • This OC carrier is suitable for biomass gasification derived syngas combustion. • This OC yields up to 92% CO 2 yields in the 550-650 º C range. • This OC is free from NiAl 2 O 4 formation in the targeted temperature operating range. Biomass steam gasification produces a blend of H 2 , CO, CH 4 and CO 2 , designated as syngas. Syngas can be further combusted using fluidizable oxygen carriers (OC). This operation which is known as Chemical Looping Combustion (CLC), is valuable for syngas combustion as it involves nickel oxidation and nickel reduction, which are both exothermic reactions. To improve syngas CLC and establish its application, a new Ni-based oxygen carrier was studied using a Co and La modified γ-Al 2 O 3 support. This type of OC considerably limits the NiAl 2 O 4 formation. A Highly Performing Oxygen Carrier (HPOC) was engineered using a special preparation methodology to exclude NiAl 2 O 4 species formation. The evaluation of this HPOC was developed using the fluidized CREC Riser Simulator reactor at mild CLC 550–650 °C temperatures. Reaction conditions employed ranged from 2 to 40 s and used a 0.5–1 fuel to oxygen stoichiometric supply ratio (ψ). It is shown that under these conditions, the HPOC demonstrates a stable performance, yielding 75–92% CO 2 , displaying a 1.84–2.75 wt% (gO 2 /gOC) oxygen transport capacity with 40–60% solid phase oxygen conversion.

Thermodynamic analysis of plant-wide CLC-SESMR scheme for H2 production: Studying the effect of oxygen carrier supports

Sorption enhanced steam methane reforming (SESMR) integrated with chemical looping combustion (CLC) is one of the most capable greener technologies that allows co-generation of H2 from natural gas together with CO2 capture. The performance of CLC mainly depends on the sustained reactivity, strength and durability of oxygen carriers (OCs). A suitable combination of active OCs with optimal inert composition is essential to meet the overall thermal demand of integrated CLC-SESMR process. In this work, the effect of inert OC supports on the performance of CLC-SESMR has been studied through thermodynamic analysis based on steady-state plant wide models developed using ASPEN plus. Ni-based OCs are considered with two different support materials, SiC/Al2O3 and MgAl2O4 at different inert compositions ranging from 0 to 70% by weight. The sensitivity analysis results revealed that H2 purity and CO2 captured are directly proportional to the inert composition while H2 yield is inversely proportional. The optimal inert compositions are found to be 30% and 40% by weight for the respective cases of SiC/Al2O3 and MgAl2O4. In both the cases, the overall performance of CLC-SESMR is found to be nearly same, i.e., with 97% overall methane conversion, 96% CO2 captured, 98.3% H2 purity, 2.24 H2 yield, and 71.4% net plant efficiency.

Sustainable iron-based oxygen carriers for Chemical Looping for Hydrogen Generation

We report a detailed reactor study of two sustainable Fe-based oxygen carriers (OC) prepared by spray drying. We compare Fe2O3 supported on Al2O3 and MgFeAlOx in isothermal Chemical Looping for Hydrogen Generation. In this process, CH4 is converted into syngas (H2 + CO) and H2O is used as the oxidant that reduces to pure H2. We noticed that the Al2O3 supported OC deactivates quickly while the MgFeAlOx supported OC withstands >60 cycles. We will show that the irreversible formation of an inactive FeAl2O4 phase is responsible for the deactivation of the alumina supported OC. We show detailed information on all the transient stages of the process and the different crystalline forms that are created, and combine this information into a full chemical looping cycle for both OC. Moreover, we evaluate the crushing strength of the oxygen carriers within a full chemical looping cycle and during long-term operation and we investigated the conversion of H2O and the selectivity to H2-production during long-term oxygen carrier regeneration. The MgFeAlOx support with an extra Fe-based active phase is a promising material to replace the typical Ni-based oxygen carriers, as it forms a sustainable non-toxic, stable and green alternative for hydrogen generation by chemical looping.

Kinetics of the reduction and oxidation of Mg added NiO/Al2O3 for chemical looping combustion

Reduction and oxidation kinetics of Mg added Ni-based oxygen carriers with different types of NiO and Mg contents were investigated for chemical looping combustion (CLC) using thermogravimetric analyzer (TGA) in the temperature range of 825–900°C. The Johnson–Mehl–Avrami (JMA) model was adopted for determination of kinetic parameters of both reduction and oxidation reaction of the three oxygen carriers. It was found that the reduction reaction consists of (1) fast surface reaction in which the reaction rate is independent of both the reaction temperature and the fuel gas concentration, (2) nucleation with short duration, and (3) kinetic controlled reaction under influence of gas diffusion rate. In oxidation reaction, the reaction rates were not heavily dependent on reaction temperature because of the low activation energies. Instead, the concentration of O2 strongly affected the pre-exponential factors, and the reaction rate constant k, consequently. Unlike the reduction reaction, the fast surface reaction was not observed in the oxidation reaction. The type of NiO used for preparation of the oxygen carriers was found to be more important factor than Mg contents for determination of the kinetic parameters for both reduction and oxidation.

Synthesis, characterization and application of Ni-based oxygen carrier supported on novel yttrium-incorporated SBA-16 for efficient hydrogen production via chemical looping steam methane reforming

Abstract In this study, yttrium-incorporated SBA-16 supported nickel-based oxygen carriers (OCs) were prepared and evaluated for syngas production in chemical looping steam methane reforming (CL-SMR) process. The mesoporous silica-based support was synthesized by pH-adjustment hydrothermal procedure and then it was impregnated with NiO particles. The influence of pH adjustment in the synthesis step, yttrium loading and nickel weight percentage are investigated on the physicochemical analysis and catalytic performances of OCs. The pH value exhibited significant effect on the formation of well-ordered mesostructure and substitution of yttrium ions within the structure of SBA-16. Thus, a zeolite like mesostructure of Si-Y is formed at the optimal pH value of 5.4. The results revealed that smaller dimensions and higher dispersion of nickel impregnated particles were formed by incorporation of Y2O3 species into the structure of SBA-16. The calcined OCs are tested in CL-SMR process in the temperature range of 500–750 °C. In addition, the performance of optimum OCs through 16 redox cycles are also investigated to study the activity, coke resistance and structural changes over the process time. The results demonstrated that 25.5NiO/Y(40)SBA-16(5.4) OC has the best performance in respect to catalytic activity and structural properties.

Influence mechanism of supports on the reactivity of Ni-based oxygen carriers for chemical looping reforming: A DFT study

First-principle calculations based on the density functional theory (DFT) were employed to clarify the influence mechanism of supports on the reactivity of Ni-based oxygen carriers for chemical looping reforming. We firstly built the models of two supported Ni-based oxygen carriers (NiO/ZrO2 and NiO/MgAl2O4 composites). It was found that NiO cluster is easy to attach to surface of two supports and the forming composites NiO/ZrO2 and NiO/MgAl2O4 are stable. Then surface reaction between CH4 and two supported NiO including the steps of CH4 sequential dehydrogenation (S1), H2 formation (S2) and CO formation (S3) was investigated. Compared with that for unsupported NiO, both supports addition significantly decreases the energy barrier of S1 process. For both supported NiO, the energy barriers increased in the order of S1 < S2 < S3, suggesting that the S3 process is the rate-limiting step in the surface reaction. The energy barrier (2.77 eV) of S3 process for NiO/ZrO2 is lower than that (3.07 eV) for NiO/MgAl2O4, implying that ZrO2 support is more conducive to the surface reaction. Moreover, our calculation results also found that the Ni4O4/ZrO2 OC has a lower the oxygen vacancy formation energy (Evac), which showed that the ZrO2 support makes Ni-O bond weaker than MgAl2O4 support. The C reacts more readily with the oxygen of NiO supported on ZrO2. The lower energy barrier of S3 process for ZrO2 support could be attributed to its higher activity of surface oxygen in the NiO. Therefore, it was concluded that NiO/ZrO2 oxygen carrier demonstrates relatively better reactivity. Our DFT calculations could provide a detailed understanding of the effect of supports on the reactivity of Ni-based oxygen carrier.

Effect of solid residence time on CO2 selectivity in a semi-continuous chemical looping combustor

Chemical looping combustion (CLC) is a promising technology for fossil fuel combustion with inherent CO2 capture and sequestration, which is able to mitigate greenhouse gases (GHGs) emission. In this study, to design a 0.5MWth pressurized chemical looping combustor for natural gas and syngas the effects of solid residences time on CO2 selectivity were investigated in a novel semi-continuous CLC reactor using Ni-based oxygen carrier particle. The semi-continuous chemical looping combustor was designed to simulate the fuel reactor of the continuous chemical looping combustor. It consists of an upper hopper, a screw conveyor, a fluidized bed reactor, and a lower hopper. Solid circulation rate (G s ) was controlled by adjusting the rotational speed of the screw conveyor. The measured solid circulation rate increased linearly as the rotational speed of the screw increased and showed almost the same values regardless of temperature and fluidization velocity up to 800°C and 4 U mf , respectively. The solid circulation rate required to achieve 100% CH4 conversion was varied to change G s -fuel ratio (oxygen carrier feeding rate/fuel feeding rate, kg/Nm3). The measured CO2 selectivity was greater than 98% when the Gs-fuel ratio was higher than 78 kg/Nm3.

Power Generation Based on Chemical Looping Combustion: Will It Qualify To Reduce Greenhouse Gas Emissions from Life-Cycle Assessment?

The aim of this study is to disclose the relationship between the global warming impact (GWI) of chemical looping combustion (CLC) and four essential factors to investigate the environmental sustainability of this technology, namely, the types of oxygen carrier (OC), the lifetime of the OC, the global warming potential (GWP) of the OC, and thermodynamic performances of the CLC power facility. At designed conditions, the GWI of the CLC power plant is expected to be 63.4 kg of CO2 eq/(MW h) by using a Ni-based OC. The lifetime of the OC has a major influence on the GWI before it reaches 4000 h, and a further increment of OC lifetime presents a tender influence on reducing the GWI, whereas, at such a condition, the GWP of the OC presents obvious influences. A higher CLC system efficiency contributes to lower GWI; thus, integrating CLC with a more efficient combined cycle rather than the steam cycle and screening high-temperature resistance OCs should be focused on for future research needs.

Synthesis and Application of Cerium-Incorporated SBA-16 Supported Ni-Based Oxygen Carrier in Cyclic Chemical Looping Steam Methane Reforming

Hydrogen, as a clean energy carrier, could be produced aided by cyclic oxidation-reduction of oxygen carriers (OCs) in contact with carbonaceous fuel in chemical looping steam methane reforming (CL-SMR) process. In this study, the cerium was incorporated into the SBA-16 support structure to synthesize the Ni/Ce-SBA-16 OC. The supports were synthesized using hydrothermal method followed by impregnation of Ni and characterized via low and wide angle X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), coupled with energy dispersive X-ray (EDX) spectroscopy, and transmission electron micrograph (TEM) techniques. In addition, the effect of various Si/Ce molar ratios (20–60) in the support structure, Ni loading (10–30 wt %), reaction temperature (500–750 °C), and life time of optimal oxygen carrier over 16 cycles were investigated. The results of wide angle XRD and SEM revealed that the incorporation of CeO2 in the channels of SBA-16 caused the formation of nickel metallic particles with smaller size and prevents the coke formation. The results showed that OC with 15 wt % Ni and Si/Ce molar ratio of 40 (15Ni/Ce-SBA-16(40)) has the best performance when compared with other OCs in terms of catalytic activity and structural properties. The methane conversion of about 99.7% was achieved at 700 °C using 15Ni/Ce-SBA-16(40) OC. We anticipate that the strategy can be extended to investigate a variety of novel modified mesoporous silica as the supporting material for the Ni based OCs.

A Multi-scale model for CO2 capture: A Nickel-based oxygen carrier in Chemical-looping Combustion

In this work, we present a multi-scale modelling framework for the Ni-based oxygen carrier (OC) particle that can explicitly account for the complex reaction mechanism taking place on the contacting surface between gas and solid reactants in Chemical Looping Combustion (CLC). This multi-scale framework consists of a gas diffusion model and a surface reaction model. Continuum equations are used to describe the gas diffusion inside OC particles, whereas Mean-field approximation method is adopted to simulate the micro-scale events, such as molecule adsorption and elementary reaction, occurring on the contacting surface. A pure CO stream is employed as the fuel gas whereas the NiO is used as the metal oxide because it is one of the mostly used material in laboratory and pilot-scale plants. Rate constants for the micro-scale events considered in the present work were obtained from a systematic Density Functional Theory (DFT) analysis, which provides a reasonable elementary reaction kinetics and lays a solid foundation for multi-scale calculations. A sensitivity analysis on the size of intra-particle pore and the adsoprtion rate constant was conducted to assess the mass transport effects on the porous particle. The proposed multi-scale model shows reasonable tendencies and responses to changes in key modelling parameters.

Hydrogen Production from Cyclic Chemical Looping Steam Methane Reforming over Yttrium Promoted Ni/SBA-16 Oxygen Carrier

In this work, the modification of Ni/SBA-16 oxygen carrier (OC) with yttrium promoter is investigated. The yttrium promoted Ni-based oxygen carrier was synthesized via co-impregnation method and applied in chemical looping steam methane reforming (CL-SMR) process, which is used for the production of clean energy carrier. The reaction temperature (500–750 °C), Y loading (2.5–7.4 wt. %), steam/carbon molar ratio (1–5), Ni loading (10–30 wt. %) and life time of OCs over 16 cycles at 650 °C were studied to investigate and optimize the structure of OC and process temperature with maximizing average methane conversion and hydrogen production yield. The synthesized OCs were characterized by multiples techniques. The results of X-ray powder diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) of reacted OCs showed that the presence of Y particles on the surface of OCs reduces the coke formation. The smaller NiO species were found for the yttrium promoted OC and therefore the distribution of Ni particles was improved. The reduction-oxidation (redox) results revealed that 25Ni-2.5Y/SBA-16 OC has the highest catalytic activity of about 99.83% average CH4 conversion and 85.34% H2 production yield at reduction temperature of 650 °C with the steam to carbon molar ratio of 2.

Hydrogen production and reduction of Ni-based oxygen carriers during chemical looping steam reforming of ethanol in a fixed-bed reactor

Hydrogen production and reduction of Ni-based oxygen carriers (OCs) during chemical looping steam reforming (CLSR) of ethanol were studied in a fixed-bed reactor using four OCs with different supports including NiO/SBA-15, NiO/MCM-41, NiO/MMT and NiO/Al2O3. The OCs prepared were characterized by N2 adsorption-desorption, TPR, TPO, XRD, TEM, FTIR and TGA-DSC. The results demonstrated that NiO component in all the OCs was first reduced by ethanol and the reduced OCs were responsible of catalytic steam reforming and water gas shift for hydrogen production. Mesoporous NiO/SBA-15 presented increasing conversion of NiO reduction and the highest selectivity of hydrogen production. The conversion of ethanol increased with reactions proceeding until the highest value is reached after about ∼300s, and the negative steam conversion obtained was resulted from H2O formation from ethanol oxidation by OCs. Compared with MMT and Al2O3 supports, the oxidization of NiO with MCM-41 and SBA-15 supports was very fast and less carbon was formed and deposited. Enhancement in hydrogen production from CLSR process was achieved by in-situ CO2 removal. Shrinking Core model (SCM) based on the constant pattern model for fixed-bed reactor indicated the rates of OCs reduction with ethanol were mainly controlled by surface chemical reaction and product layer diffusion. The reduction process was found to undergo three different rate-limiting stages, and the critical times for changes in the rate-limiting steps were determined.

Study of reaction mechanism of methane conversion over Ni-based oxygen carrier in chemical looping reforming

First-principle calculations based on the density functional theory (DFT) were used to clarify the reaction mechanism of methane conversion over Ni-based oxygen carrier for chemical looping reforming. A systematic investigation about the methane sequential dehydrogenation process was firstly performed. Our calculation results showed that CH3→CH2+H is the rate-limiting step with the activation energy of 2.0eV. Then the H2 formation was studied to understand the pathways of H2 production on NiO surface. It was revealed that the H atom on O-top site could facilely migrate to Ni-top site. We proposed two possible pathways for H2 formation, path (a) and path (b). The activation energy (2.79eV) of path (a) is largely higher than that (0.54eV) of path (b). For path (b), the H diffusion process is identified to the rate-limiting step with the activation energy of 0.59eV. Finally, we also explored oxygen anion diffusion process in NiO crystal structure. The activation energy (2.23eV) is very high and this process significantly limits the reaction rate between methane and Ni-based oxygen carrier in CLR process.

Study of the reactivity by pulse of CH4 over NiO/Fe-doped MgAl2O4 oxygen carriers for hydrogen production

The reactivity of Ni-based oxygen carrier (OC) was studied by CH4 pulse test. The MgAl2O4 spinel was synthesized by microwave assisted combustion method and Ni and Fe were added by wet impregnation method. The results of CH4 pulse test revealed that the OCs were more reactive for partial oxidation reaction. The XRD analysis of OCs after the test confirms the presence of NiO and MgAl2O4 without the secondary phases like NiAl2O4 and FeAl2O4. Among the OCs, Ni15Fe2MA was the most reactive producing the highest amounts of H2 and exhibiting good re-oxidation capacity, illustrating its potential for use in Chemical Looping Reforming (CLR). The high reactivity is associated to a change on the NiO-support interaction.

Combustion and Reforming of Liquid Fossil Fuels through Chemical Looping Processes: Integration of Chemical Looping Processes in a Refinery

Oil refining processes demand and use vast quantities of energy and thus are responsible for the emission of a great deal of CO2. In addition, hydrogen is used in oil refineries for hydrodesulfurization and hydrocraking processes. In this sense, the integration of Chemical Looping technology in an oil refinery using vacuum residues as fuel could drive to significant reductions in CO2 emissions. In this work, Chemical Looping Combustion (CLC) and Chemical Looping Reforming (CLR) experiments have been carried out in a continuously operated 1 kWth unit using a Cu- and Ni-based oxygen carrier, respectively. Diesel, synthetic and mineral lubricant oil were used as fuels as a previous step to the use of low grade residues. Regarding Chemical Looping Combustion conditions, almost 100% of combustion efficiency and full carbon capture were obtained at low oxygen carrier-to-fuel molar ratios (ϕ≥1.6). Regarding Chemical Looping Reforming conditions, a syngas containing a H2 concentration over 50 vol.% in dry basis was obtained with the additional advantage of reaching 100% CO2 capture efficiency in the process. In all cases, syngas composition obtained was close to the given by the thermodynamic equilibrium. These results provide a basis for concluding that the integration of Chemical Looping processes for heat/steam and hydrogen production in an oil refinery is feasible and could lead to significant environmental advantages.

Autothermal operation of a pressurized Gas Switching Combustion with ilmenite ore

This paper presents further experimental testing of the Gas Switching Combustion (GSC) concept where ilmenite is used as oxygen carrier and CO as fuel for integration into an IGCC plant. In contrast to conventional chemical looping combustion (CLC), GSC keeps the oxygen carrier in a single fluidized bed reactor where it is alternately exposed to fuel and air. This reactor configuration greatly simplifies scale-up under the pressurized conditions required for competitive efficiency in gas-fuelled chemical looping applications. Autothermal operation of the lab-scale GSC reactor was achieved for ilmenite and a synthetic Ni-based oxygen carrier. Even though the synthetic oxygen carrier clearly outperformed the ilmenite ore in terms of reactivity and oxygen transport capacity, ilmenite performance was found to be good enough for efficient application in GSC-IGCC power plants. The experiments also suggest that ilmenite kinetics is not negatively affected by increases in pressure as has been reported for other synthetic oxygen carriers, thereby further increasing the attractiveness of ilmenite as an oxygen carrier for the high-pressure GSC-IGCC application.

Steam, dry, and steam-dry chemical looping reforming of diesel fuel in a 1 kWth unit

Chemical looping reforming (CLR) is a process that enables the production of syngas/H2 with CO2 capture by using oxygen carriers that prevent direct contact between the fuel and air. This work presents the experimental results obtained in a 1kWth CLR unit using a Ni-based oxygen carrier and diesel as fuel, as a first trial for further advancement with heavier liquid fuels. The influence of the main operating conditions, such as oxygen-to-diesel molar ratio and the H2O and/or CO2 feed into the system, was analysed in both steam and dry CLR processes. In addition, the combined steam-dry CLR process enabled the production of syngas with any H2/CO molar ratio between 0.2 and 3, which resulted in a wide variety of final products during its use in Fischer-Tropsch processes. In all cases, the syngas composition was close to that given by the thermodynamic equilibrium. These results demonstrate the technical feasibility of steam, dry, and combined steam-dry reforming processes for liquid fossil fuels in a chemical looping system.

Synthesis and characterization of cerium promoted Ni/SBA-16 oxygen carrier in cyclic chemical looping steam methane reforming

Abstract Chemical-looping steam methane reforming (CL-SMR) is based on oxidation–reduction cycles through a solid–gas reaction with an oxygen carrier (OC) for producing high purity hydrogen or synthesis gas. In this study, Ce promoted Ni-based OC was synthesized via co-impregnation method and applied in this process. The presence of CeO2 nanoparticles in the framework of nickel oxide doped SBA-16 oxygen carrier could significantly improve the uniformity and distribution of nickel oxide nanoparticles duo to the restrictional influence of the SBA-16 framework and the strong interaction of nickel and cerium. The reaction temperature (500–750 °C), Ce loading percentage (3.9–23.3 wt. %) and Ni loading percentage (10–30 wt. %) were studied in order to investigate and optimize the catalyst structure and process temperature with maximizing the average CH4 conversion and H2 production yield in this process. The synthesized oxygen carriers were characterized by X-ray powder diffraction (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) techniques. The redox results revealed that 20Ni-11.6Ce/S16 oxygen carrier had the high catalytic activity of about 100% average CH4 conversion and 86.98% H2 production yield at reduction temperature of 700 °C.